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视网膜功能启发的边缘检测层级模型

视网膜功能启发的边缘检测层级模型郑程驰 1范影乐1摘要基于视网膜对视觉信息的处理方式, 提出一种视网膜功能启发的边缘检测层级模型. 针对视网膜神经元在周期性光刺激下产生适应的特性, 构建具有自适应阈值的Izhikevich 神经元模型; 模拟光感受器中视锥细胞、视杆细胞对亮度的感知能力, 构建亮度感知编码层; 引入双极细胞对给光−撤光刺激的分离能力, 并结合神经节细胞对运动方向敏感的特性, 构建双通路边缘提取层; 另外根据神经节细胞神经元在多特征调控下延迟激活的现象, 构建具有脉冲延时特性的纹理抑制层; 最后将双通路边缘提取的结果与延时抑制量相融合, 得到最终边缘检测结果. 以150张来自实验室采集和AGAR 数据集中的菌落图像为实验对象对所提方法进行验证, 检测结果的重建图像相似度、边缘置信度、边缘连续性和综合指标分别达到0.9629、0.3111、0.9159和0.7870, 表明所提方法能更有效地进行边缘定位、抑制冗余纹理、保持主体边缘完整性. 本文面向边缘检测任务, 构建了模拟视网膜对视觉信息处理方式的边缘检测模型, 也为后续构建由视觉机制启发的图像计算模型提供了新思路.关键词边缘检测, 视网膜, Izhikevich 模型, 神经编码, 方向选择性神经节细胞引用格式郑程驰, 范影乐. 视网膜功能启发的边缘检测层级模型. 自动化学报, 2023, 49(8): 1771−1784DOI 10.16383/j.aas.c220574Multi-layer Edge Detection Model Inspired by Retinal FunctionZHENG Cheng-Chi 1 FAN Ying-Le 1Abstract Based on the processing of visual information by the retina, this paper proposes a multi-layer model of edge detection inspired by retinal functions. Aiming at the adaptive characteristics of retinal neurons under periodic light stimulation, an Izhikevich neuron model with adaptive threshold is established; By simulating the perception ability of cones and rods for luminance and color in photoreceptors, the luminance perception coding layer is con-structed; By introducing the ability of bipolar cells for separating light stimulation, and combining with the charac-teristics of ganglion cells sensitive to the direction of movement, a multi-pathway edge extraction layer is constructed;In addition, according to the phenomenon of delayed activation of ganglion cell neurons under multi-feature regula-tion, a texture inhibition layer with pulse delay characteristics is constructed; Finally, by fusing the result of multi-pathway edge extraction with the delay suppression amount, the final edge detection result is obtained. The 150colony images from laboratory collection and AGAR dataset are used as experimental objects to test the proposed method. The reconstruction image similarity, edge confidence, edge continuity and comprehensive indicators of the detection results are 0.9629, 0.3111, 0.9159 and 0.7870, respectively. The results show that the proposed method can better localize edges, suppress redundant textures, and maintain the integrity of subject edges. This research is oriented to the task of edge detection, constructs an edge detection model that simulates the processing of visual information by the retina, and also provides new ideas for the construction of image computing model inspired by visual mechanism.Key words Edge detection, retina, Izhikevich model, neural coding, direction-selective ganglion cells (DSGCs)Citation Zheng Cheng-Chi, Fan Ying-Le. Multi-layer edge detection model inspired by retinal function. Acta Automatica Sinica , 2023, 49(8): 1771−1784边缘检测作为目标分析和识别等高级视觉任务的前级环节, 在图像处理和工程应用领域中有重要地位. 以Sobel 和Canny 为代表的传统方法大多根据相邻像素间的灰度跃变进行边缘定位, 再设定阈值调整边缘强度和冗余细节[1]. 虽然易于计算且快速, 但无法兼顾弱边缘感知与纹理抑制之间的有效性, 难以满足复杂环境下的应用需要. 随着对生物视觉系统研究的进展, 人们对视觉认知的过程和视觉组织的功能有了更深刻的了解. 许多国内外学者在这些视觉组织宏观作用的基础上, 进一步考虑神经编码方式与神经元之间的相互作用, 并应用于边缘检测中. 这些检测方法大多首先会选择合适的神经元模型模拟视觉组织细胞的群体放电特性, 再关联例如视觉感受野和方向选择性等视觉机制, 以不收稿日期 2022-07-14 录用日期 2022-11-29Manuscript received July 14, 2022; accepted November 29,2022国家自然科学基金(61501154)资助Supported by National Natural Science Foundation of China (61501154)本文责任编委张道强Recommended by Associate Editor ZHANG Dao-Qiang1. 杭州电子科技大学模式识别与图像处理实验室杭州 3100181. Laboratory of Pattern Recognition and Image Processing,Hangzhou Dianzi University, Hangzhou 310018第 49 卷第 8 期自动化学报Vol. 49, No. 82023 年 8 月ACTA AUTOMATICA SINICAAugust, 2023同的编码方式将输入的图像转化为脉冲信号, 经过多级功能区块处理和传递后提取出图像的边缘. 其中, 频率编码和时间编码是视觉系统编码光刺激的重要方式, 在一些计算模型中被广泛使用. 例如,文献[2]以HH (Hodgkin-Huxley)神经元模型为基础, 使用多方向Gabor滤波器模拟神经元感受野的方向选择性, 实现神经元间连接强度关联边缘方向,将每个神经元的脉冲发放频率作为边缘检测的结果输出, 实验结果表明其比传统方法更有效; 文献[3]在 LIF (Leaky integrate-and-fire) 神经元模型的基础上进行改进, 引入根据神经元响应对外界输入进行调整的权值, 在编码的过程中将空间的脉冲发放转化为时序上的激励强度, 实现强弱边缘分类, 对梯度变化幅度小的弱边缘具有良好的检测能力. 除此之外, 也有关注神经元突触间的相互作用, 通过引入使突触的连接权值产生自适应调节的机制来提取边缘信息的计算方法. 例如, 文献 [4] 构建具有STDP (Spike-timing-dependent plasticity) 性质的神经元模型, 根据突触前后神经元首次脉冲发放时间顺序来增强或减弱突触连接, 对真伪边缘具有较强的辨别能力; 文献 [5] 则在构建神经元模型时考虑了具有时间不对称性的STDP机制, 再融合方向特征和侧抑制机制重建图像的主要边缘信息, 其计算过程对神经元突触间的动态特性描述更加准确.更进一步, 神经编码也被应用于实际的工程需要.例如, 文献 [6]针对现有的红外图像边缘检测算法中存在的缺陷, 构建一种新式的脉冲神经网络, 增强了对红外图像中弱边缘的感知; 文献 [7] 则通过模拟视皮层的处理机制, 使用包含左侧、右侧和前向3条并行处理支路的脉冲神经网络模型提取脑核磁共振图像的边缘, 并将提取的结果用于异常检测,同样具有较好的效果. 上述方法都在一定程度上考虑了视觉组织中神经元的编码特性以及视觉机制,与传统方法相比, 在对复杂环境的适应性更强的同时也有较高的计算效率. 但这些方法都未能考虑到神经元自身也会随着外界刺激产生适应, 从而使活动特性发生改变. 此外, 上述方法大多也只选择了频率编码、时间编码等编码方式中的一种, 并不能完整地体现视觉组织中多种编码方式的共同作用.事实上, 在对神经生理实验和理论的持续探索中发现, 视觉组织(以视网膜为例)在对视觉刺激的加工中就存在着丰富的动态特性和编码机制[8−9]. 视网膜作为视觉系统中的初级组织结构, 由多种不同类型的细胞构成, 共同组成一个纵横相连、具有层级结构的复杂网络, 能够针对不同类型的刺激性选择相应的编码方式进行有效处理. 因此, 本文面向图像的边缘检测任务, 以菌落图像处理为例, 模拟视网膜中各成分对视觉信息的处理方式, 构建基于视网膜动态编码机制的多层边缘检测模型, 以适应具有多种形态结构差异的菌落图像边缘检测任务.1 材料和方法本文提出的算法流程如图1所示. 首先, 根据视网膜神经元在周期性光刺激下脉冲发放频率发生改变的特性, 构建具有自适应阈值特性的Izhikevich 神经元模型, 改善神经元的同步发放能力; 其次, 考虑光感受器对强弱光和颜色信息的不同处理方式编码亮度信息, 实现不同亮度水平目标与背景的区分;然后, 引入固视微动机制, 结合神经节细胞的方向选择性和给光−撤光通路的传递特性, 将首发脉冲时间编码的结果作为双通路的初级边缘响应输出;随后, 模拟神经节细胞的延迟发放特性, 融入对比度和突触前后偏好方向差异, 计算各神经元的延时抑制量, 对双通路的计算结果进行纹理抑制; 最后,整合双通路边缘信息, 将二者融合为最终的边缘检测结果.1.1 亮度感知编码层构建神经元模型时, 本文综合考虑对神经元生理特性模拟的合理性和进行仿真计算的高效性, 以Izhikevich模型[10]为基础构建神经元模型. Izhike-vich模型由Izhikevich在HH模型的基础上简化而来, 在保留原模型对神经元放电模式描述的准确性的同时, 也具有较低的时间复杂度, 适合神经元群体计算时应用, 其表达式如下式所示v thv th 其中, v为神经元的膜电位, 其初始值设置为 −70; u为细胞膜恢复变量, 设置为14; I为接收的图像亮度刺激; 为神经元脉冲发放的阈值, 设置为30; a描述恢复变量u的时间尺度, b描述恢复变量u 对膜电位在阈值下波动的敏感性, c和d分别描述产生脉冲发放后膜电位v的重置值和恢复变量u的增加程度, a, b, c, d这4个模型参数的典型值分别为0.02、0.2、−65和6. 若某时刻膜电位v达到,则进行一次脉冲发放, 同时该神经元对应的v被重置为c, u被重置为u + d.适应是神经系统中广泛存在的现象, 具体表现为神经元会根据外界的刺激不断地调节自身的性质. 其中, 视网膜能够适应昼夜环境中万亿倍范围的光照变化, 这种适应能够帮助其在避免饱和的同时保持对光照的敏感性[11]. 研究表明, 视网膜持续1772自动化学报49 卷接受外界周期性光刺激时, 光感受器会使神经元细胞的活动特性发生改变, 导致单个神经元的发放阈值上升, 放电频率下降; 没有脉冲发放时, 对应阈值又会以指数形式衰减, 同时放电频率逐渐恢复[12].因此, 本文在Izhikevich 模型的基础上作出改进,加入根据脉冲发放频率对阈值进行自适应调节的机制, 如下式所示τ1τ2τ1τ2v th τ1v th τ2其中, 和分别为脉冲发放和未发放时阈值变化的时间常数, 其值越小, 阈值变化的幅度越大, 神经元敏感性变化的过程越快; 反之, 则表示阈值变化的幅度越小, 神经元敏感性变化的过程也就越慢.生理学实验表明, 在外界持续光刺激下, 神经元对刺激产生适应导致放电频率降低后, 这种适应衰退的过程比产生适应的过程通常要长数倍[13]. 因此,为了在准确模拟生理特性的同时保证计算模型的性能, 本文将和分别设置为20和40. 这样, 当某时刻某个神经元产生脉冲发放时, 则对应阈值根据的值升高, 神经元产生适应, 活跃度降低; 反之, 对应阈值根据的值下降, 神经元的适应衰退, 活跃度提升. 实现限制活跃神经元的脉冲发放频率, 促进不活跃神经元的脉冲发放, 改善神经元群体的同步发放能力, 减少检测目标内部冗余. 图2边缘检测结果图 1 边缘检测算法原理图Fig. 1 Principle of edge detection algorithm8 期郑程驰等: 视网膜功能启发的边缘检测层级模型1773显示了改进前后的Izhikevich 模型对图像进行处理后目标内部冗余情况.0∼255为了规范检测目标图像的亮度范围, 本文将输入的彩色图像Img 各通路的亮度映射到区间内, 如下式所示Img (;i )I (;i )其中, 和表示经亮度映射前和映射后的R 、G 、B 三种颜色分量图像; max(·) 和min(·)分别计算对应分量图像中的最大和最小像素值.光感受器分两类, 分别为视锥细胞和视杆细胞[14], 都能将接收到的视觉刺激转化为电信号, 实现信息的编码和传递. 其中, 视锥细胞能够根据外界光刺激的波长来分解为三个不同的颜色通道[15].考虑到人眼对颜色信息的敏感性能有效区分离散目标与背景, 令图像中的每个像素点对应一个神经元,将R 、G 、B 三种颜色分量图像分别输入上文构建的神经元模型中, 在一定时间范围内进行脉冲发放,如下式所示fires (x,y ;i )其中, 为每个神经元的脉冲发放次数,函数Izhikevich(·)表示式(2)给出的神经元模型.视杆细胞对光线敏感, 主要负责弱光环境下的外界刺激感知. 当光刺激足够强时, 视杆细胞的感知能力达到饱和, 视觉系统转为使用视锥细胞负责亮度信息的处理[16]. 因此, 除了对颜色信息敏感外,视锥细胞对强光也有高度辨别能力. 考虑到作为检测对象的图像中, 目标与背景具有不同的亮度水平,本文构建一种综合视锥细胞和视杆细胞亮度感知能力的编码方法, 以适应目标与背景不同亮度对比的多种情况, 如下式所示I base I base (x,y )fires Res (x,y )其中, var(·) 计算图像亮度方差; ave(·) 计算图像亮度均值. 本文取三种颜色分量图像中方差最大的一幅作为基准图像 , 对于其中的像素值 ,将其中亮度低于平均亮度的部分设置为三种颜色分量脉冲发放结果的最小值, 反之设置为最大值, 最终得到模型的亮度编码结果 , 实现在图像局部亮度相对较低的区域由视杆细胞进行弱光感知, 亮度较高区域由视锥细胞处理, 强化计算模型对不同亮度目标和背景的区分能力, 凸显具有弱边缘的对象. 图3显示了亮度感知编码对存在弱边缘的对象的感知能力.1.2 基于固视微动的多方向双通路边缘提取层Img gray 人眼注视目标时, 接收的图像并非是静止的,而是眼球以每秒2至3次的微动使投射在视网膜上的图像发生持续运动, 不断地改变照射在光感受器上的光刺激[17]. 本文考虑人眼的固视微动机制,在原图像的灰度图像上构建大小为3×3的微动作用窗口temp , 使窗口接收到的亮度信息朝8个方向进行微动, 如下式所示p i q i θi temp θi d x d y 其中, 和是用于决定微动方向的参数, 其值被设置为 −1、0或1, 通过计算反正切函数能够得到以45° 为单位、从0° 到315° 的8个角度的微动方向, 对应8个微动结果窗口 ; 和分别表示水平和竖直方向的微动尺度; Dir 为计算得到(a) 原图(a) Original image (b) Izhikevich 模型(b) Izhikevich model (c) 改进的 Izhikevich 模型(c) Improved Izhikevich model图 2 改进前后的Izhikevich 模型对图像进行脉冲发放的结果对比图Fig. 2 Comparison of the image processing results of the Izhikevich model before and after improvement1774自动化学报49 卷Dir (x,y )的微动方向矩阵, 其中每个像素点的值为 ;sum(·) 计算窗口中像素值的和. 本文取每个微动窗口前后差异最大的方向作为该点的偏好方向, 分别用数字1 ~ 8表示.视网膜存在一类负责对运动刺激编码、具有方向选择性的神经节细胞 (Direction-selective gangli-on cells, DSGCs)[18]. 经过光感受器处理, 转化为电信号的视觉信息, 通过双极细胞处理后传递给神经节细胞. 双极细胞可分为由光照增强 (ON) 激发的细胞和由光照减弱 (OFF) 激发的细胞[19], 分别将信号输入给光通路 (ON-pathway)和撤光通路 (OFF-pathways) 两条并行通路[20], 传递给光运动和撤光运动产生的刺激. 而神经节细胞同样包括ON 和OFF 两种, 会对给光和撤光所产生的运动方向做出反应[21]. 因此, 本文构造5×5大小的对特定方向微动敏感的神经节细胞感受野窗口, 将其对偏好方向和反方向微动所产生的响应分别作为给光通路和撤光通路的输入. 以偏好方向为45° 的方向选择性神θi fires Res S xy ∗通过上述定义, 可以形成以45° 为单位、从0°到315° 的8个方向的感受野窗口, 与上文的8个方向对应. 之后本文在亮度编码结果上构筑与感受野相同大小的局部窗口 , 根据最优方向矩阵Dir 对应窗口中心点的方向, 取与其相同和相反方向的感受野窗口和亮度编码结果进行卷积运算 (本文用符号表示卷积运算), 分别作为ON 和OFF 通道的输入, 如下式所示T ON T OFF 考虑到眼球微动能够将静止的空间场景转变为视网膜上的时间信息流, 激活视网膜神经元的发放,同时ON 和OFF 两通路也只在光刺激的呈现和撤去的瞬时产生电位发放, 因此本文采用首发脉冲时间作为编码方式, 将和定义为两通路首次脉冲发放时间构成的时间矩阵, 并作为初级边缘响应的结果. 将1个单位的发放时间设置为0.25, 当总发放时间大于30时停止计算, 此时还未进行发放的神经元即被判断为非边缘.1.3 多特征脉冲延时纹理抑制层视网膜神经节细胞在对光刺激编码的过程中,外界刺激特征的变化会显著影响神经元的反应时间. 研究发现, 当刺激对比度增大时, 神经元反应延时会减小, 更快速地进行脉冲发放; 反之, 则反应延时增大, 抑制神经元的活性[22]. 除此之外, 方向差异也会影响神经元活动, 突触前后偏好方向相似的神经元更倾向于优先连接, 在受到外界刺激时能够更快被同步激活[23]. 因此, 本文引入视网膜的神经元延时发放机制, 考虑方向和对比度对神经元敏感性的影响, 构造脉冲延时抑制模型. 首先结合局部窗口权重函数计算图像对比度, 如下式所示ω(x i ,y i )其中, 为窗口权重函数, L 为亮度图像, Con(a) 原图(a) Original image (b) Izhikevich 模型(b) Izhikevich model (c) 改进的 Izhikevich 模型(c) Improved Izhikevich model (d) 亮度感知编码(d) Luminance perception coding图 3 不同方式对存在弱边缘的菌落图像的处理结果Fig. 3 Different ways to process the image of colonies with weak edges8 期郑程驰等: 视网膜功能启发的边缘检测层级模型1775S xy x i y i µ=∑x i ,y i ∈S xy ω(x i ,y i )为对比度图像, 为以(x , y )为中心的局部窗口,( , ) 为方窗中除中心外的周边像素, ws 为局部方窗的窗长, . 之后考虑局部方窗中心神经元和周边神经元方向差异, 同时用高斯函数模拟对比度大小与延时作用强度之间的关系, 构建脉冲延时抑制模型, 如下式所示D Dir (x,y )D Con (x,y )D (x,y )∆Dir (x i ,y i )min {|θ(x i ,y i )−θ(x,y )|,2π−|θ(x i ,y i )−θ(x,y )|}δ其中, 和分别表示方向延时抑制量和对比度延时抑制量; 为计算得到的综合延时抑制量; 为突触前后神经元微动方向的差异, 被定义为 ; 用于调节对比度延时抑制量.T ON T OFFRes ON Res OFF 将上文计算得到的两个时间矩阵和中进行过脉冲发放的神经元与综合延时抑制量相加, 同样设置1个单位的发放时间为0.25, 将经延时作用后总发放时间大于30的神经元设置为不发放, 即判定为非边缘, 反之则判定为边缘. 根据式(19)和式(20) 得到两通道边缘检测结果和. 最后, 将两通道得到的结果融合, 得到最终边缘响应结果Res ,如下式所示2 算法流程基于视网膜对视觉信息的处理顺序和编码特性, 本文构建图4所示的算法流程, 具体步骤如下:1) 根据视网膜在外界持续周期性光刺激下产生的适应现象, 在式(1)所示的Izhikevich 模型上作出改进, 构建如式(2)所示的具有自适应阈值的Izhikevich 模型.2) 根据式(3)将作为检测目标的图像映射到0 ~ 255区间规范亮度范围, 接着分离3种通道的颜色分量, 根据式(4)输入到改进的Izhikevich 模型中进行脉冲发放.3) 根据式(5)的方差计算提取出基准图像, 再结合基准图像根据式(6)对三通道脉冲发放的结果进行亮度感知编码, 得到亮度编码结果.4) 考虑人眼的固视微动机制, 根据式(7)和式(8)通过原图的灰度图像提取每个神经元的偏好方向, 得到微动方向矩阵, 接着根据式(9)和式(10)构筑8个方向的方向选择性神经节细胞感受野窗口.5) 根据式(11)和式(12), 将感受野窗口与亮度编码图像作卷积运算, 并输入Izhikevich 模型中得到ON 和OFF 通路的首发脉冲时间矩阵, 作为两通道的初级边缘响应.6) 根据式(13) ~ 式 (15), 结合局部窗口权重计算图像对比度.7) 考虑对比度和突触前后偏好方向对脉冲发放的延时作用, 根据式(16) ~ 式 (18)构建延时纹理抑制模型, 并根据式(19)和式(20)将纹理抑制模型和两通道的初级边缘响应相融合.8) 根据式(21)将两通路纹理抑制后的结果在神经节细胞处进行整合, 得到最终边缘响应结果.3 结果为了验证本文方法用于菌落边缘检测的有效性, 本文选择Canny 方法和其他3种同样基于神经元编码的边缘检测方法作为横向对比, 并进行定性、定量分析. 首先, 选择文献[4]提出的基于神经元突触可塑性的边缘检测方法(Synaptic plasticity model, SPM), 用于对比本文方法对弱边缘的增强效果; 其次, 选择文献[24]提出的基于抑制性突触的多层神经元群放电编码的边缘检测方法 (Inhibit-ory synapse model, ISM), 验证本文的延时抑制层在抑制冗余纹理方面的有效性; 然后, 选择文献[25]提出的基于突触连接视通路方向敏感的分级边缘检测方法(Orientation sensitivity model, OSM), 对比本文方法在抑制冗余纹理的同时保持边缘提取完整性上的优势; 最后, 还以本文方法为基础, 选择去除亮度感知编码后的方法(No luminance coding,NLC)作为消融实验, 以验证本文方法模拟光感受器功能的亮度感知编码模块的有效性.本文使用实验室在微生物学实验中采集的菌落图像和AGAR 数据集[26]作为实验对象. 前者具有丰富的颜色和形态结构, 用于检验算法对复杂检测环境的适应性; 后者则存在更多层次强度的边缘信息, 菌落本身与背景的颜色和亮度水平也较为相近,用于检测算法对颜色、亮度特征和弱边缘的敏感性.本文通过局部采样生成150张512×512像素大小的测试图像, 其中38张来自实验室采集, 112张来自AGAR 数据集. 然后分别使用上文的6种边缘1776自动化学报49 卷检测算法提取图像边缘, 使每种算法得到150张边缘检测结果, 其中部分检测结果如图5所示.定性分析图5可知, Canny 、SPM 和ISM 方法在Colony4和Colony5等存在弱边缘的图像中往往会出现大面积的边缘丢失. OSM 方法对弱边缘的敏感性强于以上3种方法, 但仍然会出现不同程度的边缘断裂, 且在调整阈值时难以均衡边缘连续性和目标菌落内部冗余. NLC 方法同样丢失了Colony4和Colony5中几乎所有的边缘, 对于Colony3也只能检出其中亮度较低的菌落内部, 对于梯度变化不明显的边缘辨别力差. 与其他方法相比, 本文方法检出的边缘更加显著且完整性更高, 对于弱边缘也有很强的检测能力, 在Colony3、Colony4和Colony5等存在多层次水平强弱边缘的菌落图像中能够取得较好的检测结果. 为了对检测结果进行定量分析并客观评价各方法的优劣, 计算边缘图像重建相似度MSSIM [27]对检测结果进行重建, 并计算重建图像与原图像的相似度作为边缘定位的准确性RGfires (R)fires (G)亮度编码结果Luminance codingresult方差计算Variance1 2 3ON-result对比度Contrast脉冲延时抑制量Neuron spiking delay感受野窗口感受野窗口DSGC templateOFF-通路输出OFF-result 5)6)7)图 4 边缘检测算法流程图Fig. 4 The procedure of edge detection algorithm8 期郑程驰等: 视网膜功能启发的边缘检测层级模型1777图 5 Colony1 ~ Colony5的边缘检测结果(第1行为原图; 第2行为Canny 检测的结果; 第3行为SPM 检测的结果; 第4行为ISM 检测的结果; 第5行为OSM 检测的结果; 第6行为NLC 检测的结果; 第7行为本文方法检测的结果)Fig. 5 Edge detection results of Colony1 to Colony5 (The first line is original images; The second line is the results of Canny; The third line is the results of SPM; The fourth line is the results of ISM; The fifth line is the results of OSM;The sixth line is the results of NLC; The seventh line is the results of the proposed method)1778自动化学报49 卷指标. 首先对检测出的边缘图像做膨胀处理, 之后将原图像上的像素值赋给膨胀后边缘的对应位置,得到的图像记为ET , 则边缘重建如下式所示T k ET d k 其中, 为图像上3×3窗口中8个方向的周边像素, 为窗口中心像素点与周边像素的距离, 计算得到重建图像R . 重建图像的相似度指标如下式所示µA µB σA σB σAB 其中, 和为原图像和重建图像的灰度均值, 和为其各自的标准差, 为原图像与重建图像之间的协方差. 将原图像和重建图像各自分为N 个子图, 并分别计算相似度指标SSIM , 得到平均相似度指标MSSIM . 除此之外, 为了验证边缘检测方法检出边缘的真实性和对菌落内部冗余纹理的抑制能力, 本文计算边缘置信度BIdx [28], 根据边缘两侧灰度值的跃变程度判断边缘的真伪. 边缘置信度指标如下式所示σij E (x i k ,y ik )(x i ,y i )d i其中, 为边缘像素在原图像对应位置的邻域标准差, EdgeNum 为边缘像素数量. 另外, 本文进一步计算边缘连续性 CIdx [29]来验证检出目标的边缘完整性. 首先将得到的边缘图像E 分割为m 个区域, 分别计算每个区域中的边缘像素到其空间中心的距离 ,则连续性指标如下式所示c i k C i n i 其中, 为边缘连续性的贡献值, D 为阈值, 为第i 个区域的像素点的连续性贡献值之和,为第i 个区域边缘像素点数量. 最后, 将计算得到的3个指标根据下式融合, 得到综合评价指标EIdx [21]其中, row 和col 分别为原图像的行数和列数. 于是, 检测图像的各项性能指标如表1 ~ 表5所示, 图像重建的结果如图6所示.表 1 不同检测方法下的重建相似度MSSIM Table 1 MSSIM of different methodsSerial number MSSIMCanny SPMISMOSMNLC本文方法Colony10.74520.77250.83570.92650.91750.9371Colony20.79510.79710.84900.95280.94470.9725Colony30.85760.86620.83140.91490.83370.9278Colony40.96900.98270.98380.98870.98930.9972Colony50.96340.97580.97800.97710.98830.9933表 2 不同检测方法下的边缘置信度BIdx Table 2 BIdx of different methodsSerial number BIdxCanny SPMISMOSMNLC本文方法Colony10.49880.46180.43070.58010.50580.6026Colony20.18210.15370.15530.33650.46150.4479Colony30.19830.15100.16100.26340.12630.3257Colony40.16310.14880.19060.14370.15210.2016Colony50.16200.18960.19020.18820.17350.1654表 3 不同检测方法下的边缘连续性CIdxTable 3 CIdx of different methodsSerial numberCIdxCanny SPMISMOSMNLC本文方法Colony10.83770.85300.86010.86760.97490.9652Colony20.80690.86550.85330.82930.91770.9518Colony30.80640.74080.72930.82690.77640.9406Colony40.81430.86110.90440.84300.90150.9776Colony50.90470.84480.86320.85920.87090.95718 期郑程驰等: 视网膜功能启发的边缘检测层级模型1779。

乙烯基三氯硅烷的醇解研究_李建勇

图 1 N 2 流速不同对产物收率的影响 [ 溶剂 :正己烷 , 反应温度 50 ℃, 原料配比(mol):3.2 ∶1]
由图 1 可见 , N 2 流速为 100ml/ min 时产物收率最高。
2.4 醇解温度不同对产物收率的影响
一般来讲 , 提高反应温度可加快醇解反应速度 , 但提高醇解温度 , 副反应的速度也可能增加 , 反而降低了目的产物的收率。为获得最高产物收率需要寻找最佳醇解温度。实验结果见下图 2 。
Abstract A lcoho ly sis reactio n of vinyltrichlo rosilane and me thanol o r alcohol w as studied.V inyltrimethox ysilane
and viny ltrie tho xy silane w ere sy nthe sized.T he effects of different so lvents and reacting co nditio ns such as temper ature , N2 flo w , adding ma te rial were resea rched .
乙烯基三氯硅烷醇解反应中影响因素较多 , 溶剂的种类、 N2 流速、反应温度、原料配比以及投料方式等均对产物的收率产生影响。
乙烯基三氯硅烷醇解反应的最佳条件是 :以正己烷为溶剂 , N 2 流速为 100mL/ min 左右 , 反应温度 50℃左右 , 甲醇(或乙醇)与乙烯基三氯硅烷的摩尔比约为 3 .2 ∶1, 将乙烯基三氯硅烷滴入醇(甲醇或乙醇)中 , 乙烯基三甲氧基硅烷和乙烯基三乙氧基硅烷的收率分别为 95.5 %和 97.5%, 产品经精馏后纯度分别为 99.2 %和 99.3 %。

Photocatalysis A to Z — What we know and what we do not know in a scientific sense

2.1. Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158 2.2. Band structure (see also “Junction”, “Thermodynamics” and “Visible light”) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 2.3. Crystallinity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 2.4. Doping (see also “Visi

透射电子显微镜TEM

现代分析方法与技术
透射电子显微镜
Transmission Electron Microscopy
目录 CONTENTS
1 2 3 4
历史 History 原理 Background 结构 Components
制备 Preparation
应用 Application
5
2 透射电子显微镜
历史 HISTORYBiblioteka 2形制 Shapes
field will cause electrons to move according to the left hand rule
热电子发射或场电子发射 thermionic or field electron emission
• Hairpin-style filament
5 透射电子显微镜
目录 CONTENTS
1 2 3 4
历史 History 原理 Background 结构 Components
制备 Preparation
应用 Application
5
6 透射电子显微镜
原理 BACKGROUND
电子 ELECTRON
光学显微镜所能达到的最大分辨率受到以下条件的限制
IG Farben-Werke
The De Broglie wavelength of electrons was many orders of magnitude smaller than that for light, theoretically allowing for imaging at atomic scales 1936 改进 TEM 的成像效果，尤其是对生物样品的成像
• Small spike-shaped •使用磁场可以形成不同聚焦能力的磁透镜

环境因素对水体中四环素光催化降解行为的影响

化工进展Chemical Industry and Engineering Progress2024 年第 43 卷第 1 期环境因素对水体中四环素光催化降解行为的影响徐诗琪1，朱颖1，陈宁华2，陆彩妹1，江露莹1，王俊辉1，覃岳隆2，张寒冰1（1 广西大学资源环境与材料学院，广西南宁 530004；2 广西环境科学保护研究院，广西南宁 530022）摘要：为探索实际水体中四环素（tetracycline ，TC ）的降解规律，以ZnO 作为光催化剂研究四环素在复杂的自然环境条件（曝气、重金属、光照）下反应时间、pH 、腐殖酸（humic acid ，HA ）浓度及四环素浓度对光催化降解过程的影响。

结果表明，3种环境条件均促进了四环素的降解：曝气情况下大量的溶解氧会和催化剂协同促进超氧自由基和羟基自由基的生成，使TC 达到99%的光催化降解率；重金属Cu(Ⅱ)的加入使溶液中形成TC-Cu(Ⅱ)-ZnO 络合物，显著提高了ZnO 对TC 的降解效率，在30min 时达到89%的降解率；自然光拥有全光谱，相比可见光展现出更强的TC 降解作用，TC 降解率达到86%，比可见光下降解率提高了14%。

三因素协同作用可以有效降低TC 的降解时间，在75min 时达到降解平衡，降解率为99%。

通过动力学分析比较了不同环境状态下的光催化活性，结果为：曝气>重金属>光照。

关键词：水体；四环素；光催化；曝气；Cu(Ⅱ)共存；自然光中图分类号：X522 文献标志码：A 文章编号：1000-6613（2024）01-0551-09Effect of environmental factors on the photocatalytic degradationbehavior of tetracycline in waterXU Shiqi 1，ZHU Ying 1，CHEN Ninghua 2，LU Caimei 1，JIANG Luying 1，WANG Junhui 1，QIN Yuelong 2，ZHANG Hanbing 1(1 School of Resources, Environment and Materials, Guangxi University, Nanning 530004, Guangxi, China;2Scientific Research Academy of Guangxi Environmental Protection, Nanning 530022, Guangxi, China)Abstract: To explore the degradation pattern of tetracycline (TC) in actual water, ZnO was used as a photocatalyst to investigate the effects of reaction time, pH, humic acid (HA) concentration and tetracycline concentration on the photocatalytic degradation process under complex natural environmental conditions (aeration, heavy metals and light). The results showed that all three environmental conditions promoted the degradation of tetracycline. The large amount of dissolved oxygen under aeration wouldcollaboratively promote the generation of superoxide radicals and hydroxyl radicals with the catalyst, enabling TC to reach 99% photocatalytic degradation efficiency. The addition of the heavy metal Cu(Ⅱ) caused the formation of TC-Cu(Ⅱ)-ZnO composite in solution, which significantly improved the degradation efficiency of TC by ZnO, reaching 89% degradation efficiency at 30min. Natural light possessed a full spectrum and exhibited stronger TC degradation compared to visible light, with a TC研究开发DOI ：10.16085/j.issn.1000-6613.2023-0217收稿日期：2023-02-17；修改稿日期：2023-05-04。

光催化降解除草剂途径和动力学

Photocatalytic process begins when the TiO2 particles absorb light at a wavelength smaller than 384 nm. Conse-
quently, valence band electrons are promoted through the
* Corresponding author. Tel.: +33 4 72 44 54 83; fax: +33 4 72 44 53 99. E-mail address: marion.carrier@rs.fr (M. Carrier).
0926-3373/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2005.11.014
This study dealt with Imazapyr which was used for vegetation control in forest and rights of way, rubber plantations and oil palm plantations. Imazapyr was manufactured by American Cyanamid Co. and sold under the trade names Arsenal, Chopper and Assault. It was ﬁrst registered in the United States in 1984.
a Laboratoire d’Application de la Chimie a` l’Environnement (LACE), Universite´ Claude Bernard Lyon 1, 43 Bd du 11 Novembre 1918, 69100 Villeurbanne, France

光催化降解抗生素废水的研究

1引言抗生素是一种低分子量的微生物代谢产物，在低浓度时（一般低于1g/L）即能抑制或杀死其他微生物，是世界上用量最大、使用最广泛的药物之一，农业上广泛应用于粮食储藏、动物饲养、农业增产等方面。

2011年加拿大和美国的抗生素使用总量分别为250吨、3290吨；2013年英国抗生素的使用总量为640吨；同年中国的抗生素使用量为77760吨。

在中国抗生素药物主要用于人体医疗和畜禽养殖。

因抗生素类药物分子结构的稳定性，其在生物体内一般不会完全代谢，以代谢活性产物甚至原结构形式排出生物体。

抗生素制药废水、城市污水、畜禽、水产养殖废水都是潜在的抗生素污染源。

有文献报道发现[1]，国内主要河流中深圳河和珠江（广州段）抗生素污染最为严重，枯水期浓度达1340ng/L。

目前，国内300多家药企共生产70多种的抗生素，年产量占全世界产量的一半。

抗生素类药物分子结构中通常含有氮元素和环状结构，这些分子进入环境后，经过一系列的硝基化反应，可形成含亚硝基的化合物，特别是N-亚硝基化合物，具有较大的生物毒性、致突变和致癌性。

抗生素生产过程中产生的高浓度废水一直是污水治理领域的一个难题。

对于这种成分复杂、色度高、生物毒性大、难降解高浓度有机废水处理光催化降解抗生素废水的研究Study on Photocatalytic Degradation of Antibiotic Wastewater卜聃（中铁上海设计院集团有限公司，上海，200070）BU Dan(ChinaRailwayShanghai DesignInstituteGroupCo.Ltd.,Shanghai200070,China)【摘要】抗生素是世界上用量最大的药物之一，农业上广泛应用于粮食储藏、动物饲养、农业增产等。

虽然抗生素的半衰期较短,但是用量大。

在环境中表现出“假”持久性，可诱导环境自然菌产生耐药性。

传统废水处理工艺对抗生素废水的处理难度较大。

光催化技术因氧化的无选择性可有效降解抗生素废水。

怎样培养摄影能力英语作文

怎样培养摄影能力英语作文Title: How to Develop Photography Skills。

Photography is a captivating art form that allows individuals to express their creativity and capture the beauty of the world around them. Whether you're an aspiring photographer or someone looking to enhance your photography skills, there are several steps you can take to improve your craft. In this essay, we will explore various methods to develop your photography skills.First and foremost, practice is essential for improving any skill, including photography. Make it a habit to take your camera with you wherever you go and capture moments that catch your eye. Experiment with different angles, lighting conditions, and compositions. The more you practice, the more you will understand how to effectively use your camera settings and techniques to achieve the desired results.In addition to practice, studying the work of other photographers can provide valuable insights and inspiration. Take the time to explore photography books, magazines, and online galleries to discover different styles and techniques. Analyze the composition, lighting, and subject matter of various photographs to gain a deeperunderstanding of what makes a compelling image.Furthermore, consider taking photography classes or workshops to learn from experienced professionals. These courses can provide structured guidance and feedback tohelp you improve your skills more rapidly. Additionally, interacting with other photography enthusiasts in alearning environment can be both motivating and educational.Another valuable tool for developing photography skills is constructive criticism. Share your work with friends, family, or online photography communities and ask for feedback. Constructive criticism can help you identifyareas for improvement and gain valuable perspectives from others.Moreover, don't be afraid to step out of your comfort zone and experiment with new techniques and subjects. Challenge yourself to photograph unfamiliar environments or try your hand at different genres of photography, such as portrait, landscape, or macro photography. Pushing yourself to explore new avenues can lead to unexpected discoveries and growth as a photographer.Furthermore, don't underestimate the importance ofpost-processing in photography. Learning how to edit your photos effectively can significantly enhance their visual impact. Experiment with editing software such as Adobe Lightroom or Photoshop to refine your images and bring out their full potential.Lastly, patience and persistence are key virtues in the journey of developing photography skills. Improvement takes time, and setbacks are inevitable. Stay committed to your craft, stay curious, and never stop learning.In conclusion, developing photography skills requires a combination of practice, study, feedback, experimentation,and perseverance. By following these steps and dedicating yourself to continual improvement, you can unlock your full potential as a photographer and capture the world in stunning detail.。

g-C3N4_Ag2O Heterostructured Photocatalysts with Enhanced Visible-Light Photocatalytic Activity

Received: September 5, 2013 Accepted: November 8, 2013 Published: November 8, 2013
12533
/10.1021/am4038307 | ACS Appl. Mater. Interfaces 2013, 5, 12533−12540
© 2013 American Chemical Society
modiﬁcation,18 heterostructured composite fabrication,19−23 and so forth. In particular, the fabrication of heterostructured composites by combining g-C3N4 with other semiconductors can not only restrict eﬃciently the recombination of photogenerated charge carriers, but also endow the composites novel characteristics or some enhanced properties by the synergistic eﬀects or antagonistic eﬀects. On the other hand, Ag2O as a p-type semiconductor has been found to be a self-stable and highly eﬃcient visible-light photocatalyst with a narrow energy bandgap of ∼1.3 eV.24 And it has been widely used as sensitizer to tune the light response of some wide bandgap semiconductors into the visible region and enhance their photocatalytic activity, such as Ag2O/TiO2 nanobelts,25 Ag2O/ZnO,26 Ag2O/Bi2WO6,27 and so forth. The combination of g-C3N4 and Ag2O that possesses well matched overlapping band structure can easily fabricate a p−n heterojunction, which will bring more eﬀective interface transfer of photogenerated electrons and holes in comparison with the traditional composites. Furthermore, Ag2O nanoparticles are dispersed on the surface of g-C3N4, which can greatly reduce the usage of Ag for enhanced photocatalytic activity.

Photocatalytic Degradation of Methylene Blue in ZIF-8

a
can act as photocatalysts, are gave increasing attentions.22–27 These emerging researches not only demonstrate porous MOFs materials to be a potential new class of photocatalysts for their catalytic degradation of organic pollutants under UV-visible irradiation, but also trigger extensive research on MOFs materials as photocatalysts. Compared to conventional semiconductor photocatalysts, the photoactive MOFs have some advantages in degrading organic pollutants: (i) the well-dened crystalline structures of MOFs are benecial in the characterization and study of structure–property relationship of these solid photocatalysts; (ii) the modular nature of the MOFs synthesis allows the rational design and ne tuning of these catalysts at the molecular level, making the electronic structure of the MOFs catalysts to be easily tailored; (iii) the structural features of tunable active sites (i.e., metal-oxoclusters and organic linkers) in MOFs lead to more eﬃciency of solar harnessing; (iv) diﬀerent from typical TiO2-based catalysts, the visible light photocatalytic activity can be easily introduced via the linker substitutions of organic chromophores in MOF structures, such as amino group.22–27 Methylene blue (MB) with molecular formula of C16H18N3SCl (FW 319.85 g molÀ1), is commonly used as a representative of widespread organic dyes that contaminate textile eﬄuents and that are lowering light penetration, photosynthesis and damage the aesthetic nature of the water surface. The molecular structure of MB is illustrated in Scheme 1. Methylene blue (MB) is a heterocyclic aromatic chemical compound, which has many uses in a range of diﬀerent elds, like biology and chemistry. At doses of 2–4 mg kgÀ1, hemolytic anemia and skinຫໍສະໝຸດ bScheme 1

p区金属氧化物Ga_(2)O_(3)和Sb_(2)O_(3)光催化降解盐酸四环素性能差异

收稿日期：2020⁃09⁃29。

收修改稿日期：2020⁃12⁃28。

国家自然科学基金(No.21875037，51502036)和国家重点研发计划(No.2016YFB0302303，2019YFC1908203)资助。

＊通信联系人。

E⁃mail ：***************.cn ，***************第37卷第3期2021年3月Vol.37No.3509⁃515无机化学学报CHINESE JOURNAL OF INORGANIC CHEMISTRYp 区金属氧化物Ga 2O 3和Sb 2O 3光催化降解盐酸四环素性能差异毛婧芸1黄毅玮2黄祝泉1刘欣萍1薛珲＊,1肖荔人＊,3(1福建师范大学环境科学与工程学院，福州350007)(2福建师范大学生命科学学院，福州350007)(3福建师范大学化学与材料学院，福州350007)摘要：对沉淀法合成的p 区金属氧化物Ga 2O 3和Sb 2O 3紫外光光催化降解盐酸四环素的性能进行了研究，讨论了制备条件对光催化性能的影响。

最佳制备条件下得到的Ga 2O 3⁃900和Sb 2O 3⁃500样品光催化性能存在巨大差异，通过X 射线粉末衍射、傅里叶红外光谱、N 2吸附-脱附测试、荧光光谱、拉曼光谱、电化学分析及活性物种捕获实验等对样品进行分析，研究二者光催化降解盐酸四环素的机理，揭示影响光催化性能差异的本质因素。

结果表明，Ga 2O 3和Sb 2O 3光催化性能差异主要归结于二者不同的电子和晶体结构、表面所含羟基数量及光催化降解机理。

关键词：p 区金属；氧化镓；氧化锑；光催化；盐酸四环素中图分类号：O643.36；O614.37+1；O614.53+1文献标识码：A文章编号：1001⁃4861(2021)03⁃0509⁃07DOI ：10.11862/CJIC.2021.063Different Photocatalytic Performances for Tetracycline Hydrochloride Degradation of p ‑Block Metal Oxides Ga 2O 3and Sb 2O 3MAO Jing⁃Yun 1HUANG Yi⁃Wei 2HUANG Zhu⁃Quan 1LIU Xin⁃Ping 1XUE Hun ＊,1XIAO Li⁃Ren ＊,3(1College of Environmental Science and Engineering,Fujian Normal University,Fuzhou 350007,China )(2College of Life and Science,Fujian Normal University,Fuzhou 350007,China )(3College of Chemistry and Materials Science,Fujian Normal University,Fuzhou 350007,China )Abstract:The UV light photocatalytic performances of p ⁃block metal oxides Ga 2O 3and Sb 2O 3synthesized by a pre⁃cipitation method for the degradation of tetracycline hydrochloride were explored.The effects of synthesis conditions on the photocatalytic activity were discussed.The Ga 2O 3⁃900and Sb 2O 3⁃500samples prepared under optimal condi⁃tions exhibited a remarkable photocatalytic activity difference,which were characterized by X⁃ray diffraction,Fouri⁃er transform infrared spectroscopy,N 2adsorption⁃desorption tests,fluorescence spectrum,Raman spectrum,electro⁃chemical analysis and trapping experiment of active species.The photocatalytic degradation mechanisms of tetracy⁃cline hydrochloride over the photocatalysts were proposed and the essential factors influencing the difference of pho⁃tocatalytic performance were revealed.The results show that the different photocatalytic activities observed for Ga 2O 3and Sb 2O 3can be attributed to their different electronic and crystal structures,the amount of hydroxyl groupin the surface and the photocatalytic degradation mechanisms.Keywords:p ⁃block metal;Ga 2O 3;Sb 2O 3;photocatalysis;tetracycline hydrochloride无机化学学报第37卷0引言盐酸四环素(TC)作为一种四环素类广谱抗生素，被广泛应用于治疗人体疾病及预防畜禽、水产品的细菌性病害，其在世界范围的大量使用致使其在环境中积累[1]。

39光催化结合AC已经影响因素

The effect of activated carbon adsorption on the photocatalytic removal of formaldehydeYuanwei Lu *,Dinghui Wang,Chongfang Ma,Hongchang YangKey Laboratory of Enhanced Heat Transfer and Energy Conservation,Ministry of Education and Key Laboratory of Heat Transfer and Energy Conversion,Pingleyuan 100,Chaoyang District,Beijing Municipality,Beijing University of Technology,Beijing 100022,Chinaa r t i c l e i n f oArticle history:Received 24March 2009Received in revised form 13July 2009Accepted 31July 2009Keywords:Photocatalytic oxidation Formaldehyde Titanium dioxide Adsorption Mass transfera b s t r a c tPhotocatalysis is an emerging and promising technology for indoor air puriﬁcation.This photocatalytic method is effective in the case of a higher pollutant concentration,but its wide application in indoor air puriﬁcation is limited due to the low level of indoor air contaminants.In order to improve the removal of pollutants in indoor air,we evaluated the photocatalytic performance over the nanosized TiO 2particles immobilized on the surface of activated carbon (AC)ﬁlter for the removal of formaldehyde (HCHO).It is shown that the photocatalytic reaction rate increased because the AC could adsorb the pollutants from the diluted air stream to generate a high concentration of the pollutants on the catalyst surfaces.The photocatalytic reaction took place from the diffusion control process to the photocatalytic reaction control process with the rise in ﬂow velocity.In the former process,the photocatalytic reaction rate increased,whereas in the later process photocatalytic reaction rate changed little with increasing ﬂow velocity.The ﬂow velocity was lower over the TiO 2/AC catalyst than over the TiO 2/glass catalyst when the photocatalytic reaction was switched from the diffusion control process to the photocatalytic reaction control process.It is also observed that the indoor low-concentration HCHO could be photocatalytically degraded over TiO 2/AC,with the HCHO concentration in the product mixture falling into the standard range that is speciﬁed by the indoor air quality standard of China.Ó2009Elsevier Ltd.All rights reserved.1.IntroductionPeople are paying more and more attention on the indoor environment along with the improvement of living standard,and the indoor decoration is popular.As a result,indoor air pollutants become a serious problem especially in urban areas.Pollutants,such as NO x ,SO 2,and VOCs (volatile organic compounds),which come from indoor decoration and vehicular emissions,cause adverse health impacts on occupants.Indoor air quality has attracted immense attention [1–3].Formaldehyde (HCHO)is a typical pollutant,which is a colorless,strong-smelling gas and can cause nausea,chest tightness,wheezing,skin rashes,and allergic reaction at 0.1mg/m 3concentration or higher [1,4–9].This pollutant can be found in many indoor products,such as pressed wood,coated paper products,paints,insulation,and combustible materials.The International Agency for Research on Cancer (IARC),which is part of World Health Organization,has approved that HCHO can bring cancer to the mankind [1].So the removal of indoorHCHO is of widespread interest.Activated carbon (AC)adsorption is a traditional method for cleaning air contaminants.However,the use of adsorbent merely transfers pollutants from the gaseous phase to the solid phase and causes a disposal and regeneration problem.Improper maintenance of these ﬁlters may even create a new source of pollutants.Photocatalysis is an emerging and promising technology for indoor air puriﬁcation by using TiO 2as a photocatalyst under the illumination of UV light,which can oxidize the VOCs into CO 2and H 2O at room temperature and atmospheric pressure [3,5–10].Studies show that HCHO at high concentration,for example,above 10ppmv (1ppmv ¼1.25mg/m 3),can be photocatalytically decomposed without problem,but when the concentration drops below 1ppmv,the decomposition rapidly slows down and then almost terminates on the way [3,6–14].However,indoor HCHO usually exists at a concentration level below 1ppmv,which makes the photocatalytic decomposition of HCHO much harder and limits the application of this technology in indoor air puriﬁcation.It is reported that by immobilizing TiO 2on an adsorbent material the pollutant removal can be largely improved [3,6–8,15–17].Ao and coworkers have investigated the photodegradation of indoor air pollutants BTEX (benzene,toluene,ethylbenzene,and xylenes)*Corresponding author.Tel.:þ861067391612/8317;fax:þ861067392774.E-mail address:luyuanwei@ (Y.Lu).Contents lists available at ScienceDirectBuilding and Environmentjournal homepage:/locate/buildenv0360-1323/$–see front matter Ó2009Elsevier Ltd.All rights reserved.doi:10.1016/j.buildenv.2009.07.019Building and Environment 45(2010)615–621and binary pollutants using the TiO 2catalysts loaded on AC ﬁlter [5,16–19],the results showed that the combination of TiO 2with AC increased the photocatalytic degradation of pollutants.Our research group has prepared the catalyst ﬁlm by loading TiO 2on AC and glass plate,respectively,and has analyzed the photocatalytic removal of HCHO with the initial concentration below 1ppmv.The experimental results showed that a HCHO removal of 79.4%could be achieved over the TiO 2/AC catalyst,while the removal was only 25.7%over the TiO 2/glass catalyst [7,8].However,the effects of diffusion enhanced by adsorption of AC and ﬂow velocity on the photocatalytic removal of HCHO are still scarce.The objective of this study is to investigate the effect of combination of TiO 2with AC on the photocatalytic removal of indoor HCHO.The photocatalytic reaction rate [10]and HCHO removal [17]were used as two parameters to evaluate photo-catalytic removal efﬁciency of HCHO.2.Experiment2.1.Reagents and catalyst preparationThe reactant gas HCHO was acquired from a HCHO penetration equipment which was made up of a conical ﬂask and Teﬂon tube [19,20],as shown in Fig.1.The Teﬂon tube was placed in a conical ﬂask containing liquid HCHO and the conical ﬂask was immerged in a constant temperature water bath,so the HCHO gas penetrated into the Teﬂon tube at a constant concentration and then was brought into the reactor by high-pressure cylinder gas.TiO 2(Degussa p-25)was used as the photocatalyst.Water suspension with 5wt%of TiO 2was coated on a piece of glass by the dipping method,and then was calcined at 180 C for 1h with a ramp of 3 C/min to form the TiO 2-coated ﬁlm (i.e.,TiO 2/glass ﬁlm).The same procedure was followed to form the TiO 2/netlike AC ﬁlm by loading TiO 2on a honeycomb activated carbon ﬁlter,or to form the TiO 2/granular AC ﬁlm by loading it on a granular activated carbon ﬁlter.The granular activated carbon ﬁlter was generated by gluing granular activated carbon on the surface of glass-cloth.The surface area of the glass plate was identical to those of the netlike AC ﬁlter and granular AC ﬁlter with the size of 75mm Â25mm.Theamount of TiO 2loaded was determined by the weight difference before and after the coating procedure.In all of the experiments,the weight of TiO 2loaded was about 0.12g.Figs.2–4showed the TiO 2ﬁlms coated on the glass plate,honeycomb AC ﬁlter,and granular AC ﬁlter,respectively.2.2.Experimental systemThe schematic experimental system is shown in Fig.5,and the conﬁguration of the experimental section is illustrated in Fig.6.The route I in Fig.5was used for generating the HCHO gas with constant concentration from the HCHO penetration equipment.The route II was used to form the diluted gas,i.e.,zero gas which comes from a zero air generator (Thermo Environmental Inc.Model 111)to dilute the HCHO gas of route I,and a dynamic gas calibrator (Thermo Environmental Inc.Model 146c)was cooperated with the zero air generator to control the mass ﬂow ﬂux of zero gas.The route III was used to humidify the reactant gas,which was controlled by passing the high-pressure cylinder gas through a humidiﬁcation chamber.Three routes entered the reactor to form the reactant gas with a constant concentration,humidity,and reaction ﬂow velocity,which was adjusted by the ﬂow meters in three routes.The reactor in experimental section was made of stainless steel with a volume of 195ml (3H Â13L Â5W cm),as shown in Fig.6.Because the photocatalytic technology used in air puriﬁer is usually loading the TiO 2particles onto the surface of through-type honeycomb ﬁlter [21]which has low pressure drop,we analyzed the one-pass removal effect of HCHO by TiO 2combination with AC.The upper wall of the reactor is made of quartz for light entrance.Illumination was provided by an 8W UV lamp which emits light at a primary wavelength of 365nm and was horizontally placed on the upper of the quartz window of the reactor.The UV light intensity was determined by a UV meter (UV-A).UV intensity used in all of the experiments was 1180m W/cm 2.The TiO 2ﬁlm with different substrates was placed horizontally in the bottom of the reactor,respectively.There is a fully developed region before the reactor to form a stable air stream and a mixer was placed inside of it to mix the reactant gas well.The shape of the fully developed region is round,while the shape of reactor is rectangular,so both of which was connected by a round-to-rectangular adaptor.The sampling port was set at the end of reactor.After the inlet concentration equaled to the outlet concentration,that is to say,theFig.1.HCHO penetrationequipment.Fig.2.TiO 2/glassﬁlm.Fig.3.TiO 2/netlike ACﬁlm.Fig.4.TiO 2/granular AC ﬁlm.Y.Lu et al./Building and Environment 45(2010)615–621616experimental system reached equilibrium (about 1–2h),the UV lamp was turned on and the photocatalytic reaction was initiated.After the HCHO concentration at the reactor outlet did not change with time,the photocatalytic degradation of HCHO was end and the reaction product mixture was collected.3.Results and discussion 3.1.Adsorption of HCHOIn order to analyze the effect of the coated TiO 2particles on the adsorption of AC ﬁlter,the HCHO adsorptive capacities of netlike AC ﬁlter,TiO 2/netlike AC ﬁlm,granular AC ﬁlter,and TiO 2/granular AC ﬁlm were tested with UV lamp turned off.Fig.7shows the adsorption proﬁles of different ﬁlms with a HCHO concentration of 1.18mg/m 3at the humidity level of 10.81g/m 3(relative humidity,30%)and the ﬂow velocity of 0.82cm/s.The humidity and ﬂow velocity adopted are the optimal values for the efﬁcient removal of HCHO using TiO 2/glass ﬁlm in the same experimental system [12–14].From Fig.7,we can see that the HCHO adsorption capacity of the AC ﬁlter was higher than that of the TiO 2/AC ﬁlm due to the partial occupation of the adsorption sites of netlike AC or granular AC by TiO 2particles,although the speciﬁc surface area of AC was higher than that of the TiO 2particles and the HCHO adsorption capacity of the TiO 2could be ignored [17].Prior to the experiment,the HCHO concentration had reached equilibrium,as seen from the steady concentration within 1h of HCHO introduction (Fig.7)in the absence of any ﬁlters in the reactor.After 1h,the AC ﬁlter or TiO 2/AC ﬁlm was put in the reactor,respectively,and the outlet concentration of HCHO dropped sharply owing to the adsorption of HCHO by the AC.When the HCHO concentration reached point A(Fig.7),the HCHO adsorption capacity of the AC ﬁlter or TiO 2/AC ﬁlm decreased,causing the outlet concentration of HCHO to augment gradually until the HCHO adsorption capacity of the AC ﬁlter or TiO 2/AC ﬁlm reached saturation.From the above results,one can see that the activated carbon adsorbent existed adsorption saturation and needed to be regenerated after practical use for some time.However,if the UV light was turned on at the time of point A ,the HCHO concentration would further decrease due to the photo-catalytic degradation (Fig.8).It implies that the photocatalytic removal of HCHO could delay the adsorption saturation of AC.The reason is that the HCHO adsorbed on the external surface of AC was ﬁrst degraded photocatalytically,resulting in the HCHO concen-tration around the TiO 2lower than that on the internal surface of AC.The driving force deriving from the concentration difference would lead to the migration of HCHO from the internal surface to the external surface (around the TiO 2particles)of AC and the HCHO was hence decomposed,giving rise to the in situ regeneration of the activated paring with the removal of HCHO over the TiO 2/AC ﬁlm in Fig.8,the photocatalytic degradation of HCHO over the TiO 2/glass ﬁlm slowed down rapidly with reactiontimeFig.5.The Schematic diagram of the experimentalsystem.Fig.6.Experimentalsection.Fig.7.The adsorption capacity of differentﬁlters.Fig.8.The photocatalytic degradation of HCHO by different TiO 2ﬁlms.Y.Lu et al./Building and Environment 45(2010)615–621617and then almost remained constant.It is obvious that the photo-catalytic removal of the pollutant increased over the TiO2/ACﬁlm catalysts and the adsorption saturation of AC was delayed.Fig.9shows the HCHO removal(%)over different TiO2-coated ﬁlms.The removal of HCHO is deﬁned as:HCHO removalð%Þ¼100ðC inÀC outÞ=C in(1) Where C in and C out are the initial HCHO concentration and the steady outlet HCHO concentration(mg/m3),respectively.It can be seen that the removal of HCHO was only26%over the TiO2/glass ﬁlm,but nearly67%over the TiO2/granular ACﬁlm,and about80% over the TiO2/netlike ACﬁlm.Apparently,the combination of TiO2 with AC enhanced the photocatalytic removal of HCHO signiﬁ-cantly.Therefore,the coupling of TiO2with AC might be a good route for the practical application of photocatalytic technology in indoor air puriﬁcation.3.2.Effect ofﬂow velocity on the photocatalytic reaction rateIn order to analyze the effect ofﬂow velocities on the photo-catalytic removal of HCHO,we investigated the photocatalyticreaction rates of HCHO over the TiO2/AC or TiO2/glassﬁlm catalysts under the same conditions.The reaction rate(mg/(m3min))is used as a dependent parameter and can be deﬁned as[10]:r¼ðC inÀC outÞQ=V(2) where Q is theﬂow rate of air stream(ml/min)and V is the volume of photoreactor(ml).3.2.1.Photocatalytic reaction rate at high concentrationThe indoor air quality standard(GB/T1883-2002)in China speciﬁes that the indoor HCHO concentration should be below 0.1mg/m3.However,the equivalent accumulation or total VOC concentrations are generally in the range of0.5–2.0mg/m3.In order to conveniently evaluate the removal efﬁciency of pollutant over the differentﬁlm catalysts,the initial pollutant concentration is usually set5–10times health standard concentration.In this study, the HCHO concentration was1.18mg/m3that exceeded more than 10-fold of the speciﬁed one(0.1mg/m3).The effect ofﬂow velocity on the photocatalytic reaction rate of HCHO at a humidity of 10.81mg/m3was examined,and the results are shown in Fig.10.It is observed that the photocatalytic reaction rate over the TiO2/ netlike ACﬁlm or TiO2/granular ACﬁlm was higher than that over the TiO2/glassﬁlm,a result is that the enriched HCHO adsorption of AC creates a higher concentration level around TiO2,and the higher speciﬁc surface area of AC creates a larger TiO2area on AC surface. Because the TiO2/netlike ACﬁlm has the stronger adsorption capacity and the lager reaction area than the granular counterpart, the photocatalytic reaction rate over the TiO2/netlike ACﬁlm was higher than that over the TiO2/granular ACﬁlm.From Fig.10,one canﬁnd similar features over the three different TiO2ﬁlms for the addressed photocatalytic reaction:the photocatalytic reaction rateﬁrst increased and then changed small with the increase inﬂow velocity,which could be explained according to Fig.11.If the concentration of the A ingredient(HCHO) in host gas is CA g and that on the surface of the ball-shaped catalyst is CA s,one can know a sequence of CA g>CA s.The concentration difference(CA g–CA s)is the driving force for the diffusion of A ingredient from the host gas to the surface of catalyst.If CA g>>CA s,the external diffusion resistance is the biggest and the reaction is controlled byﬁlm diffusion.In order to enhance the reaction rate,one can increase theﬂow velocity to decrease the diffusion resistance.If CA g z CA s,the reaction is controlledby Fig.9.The photocatalytic removal of HCHO by different TiO2ﬁlms.Fig.10.The effect ofﬂow velocity on the reaction rate of HCHO at highconcentration.Fig.11.Pollutant concentration distribution on the surface of ball-shaped catalyst.Y.Lu et al./Building and Environment45(2010)615–621618the surface reaction.In order to increase the reaction rate,one can improve the catalyst performance,or raise the UV light intensity. Therefore the photocatalytic reaction is controlled by HCHO diffu-sion at lowerﬂow velocities and by photocatalytic surface reaction at higherﬂow velocities.Zhang and Mo et al.[22–24]reported similar results in a photocatalytic air cleaner.The only difference in photocatalytic reaction rates over the TiO2/ACﬁlm and TiO2/glassﬁlm was theﬂow velocity at which the control process was from diffusion to surface reaction.Over the TiO2/glassﬁlm catalyst,the switch point velocity was about 0.82cm/s;over the TiO2/netlike ACﬁlm,the switch point velocity was0.59cm/s.The lower switch point velocity is due to the adsorption of HCHO over the AC that reduced the HCHO diffusion resistance and shortened theﬂow velocity range in the diffusion control process,thus showing the best photocatalytic performance at lowerﬂow velocities.Because the adsorption capacity of a granular AC was lower than that of a netlike AC,but higher than that of a glass plate,the switch point velocity(0.65cm/s)in the case of the TiO2/granular ACﬁlm was between0.59and0.82cm/s.The effect ofﬂow velocity on the removal of HCHO is shown in Fig.12.It is observed that the removal of HCHO increased with the rise inﬂow velocity and reached the maximum at the switch pointvelocity and decreased hereinafter.The reason is that,after the switch point velocity,the HCHO diffusion rate was much higher than the photocatalytic reaction rate and the removal of HCHO was controlled by the photocatalytic reaction process.Under this circumstance,employing a high-performance photocatalyst or increasing the UV light intensity may obviously improve the HCHO removal.When the UV light intensity and catalyst performance remains unchanged in the photocatalytic reaction control process, the pollutant removal will decrease with increasingﬂow velocity. However,the removal of HCHO could be enhanced by increasing the path length of channels in air puriﬁer(i.e.,increasing residence time within the channels)during the photocatalytic reaction control ually,the capacity of indoor air to be cleaned is large and indoor air cleaner is made with honeycomb-like TiO2-coatedﬁlms[21],theﬂow velocity for the photocatalytic pollutant removal is in the range required for the photocatalytic reaction control process.In order to improve the pollutant removal efﬁ-ciency of photocatalytic air cleaner,the TiO2-coated single-channel length or the section surface made of the TiO2-coatedﬁlm should be increased.Therefore,one should consider the effect of air treatment capacity on the removal of pollutants in the practical application(conﬁguration design of air puriﬁer)of photocatalytic air puriﬁcation technology if the photocatalyst performance and UV light intensity was unchanged.3.2.2.Photocatalytic reaction rate at low HCHO concentrationsThe above results show that the indoor HCHO with a concen-tration around1ppmv could be removed considerably over the TiO2/netlike(or granular)ACﬁlm.However,the indoor HCHO concentration is usually below0.5ppmv.It is worth discussing whether the low level of indoor HCHO can be photocatalytically decreased to a value below0.1mg/m3(speciﬁed in the indoor air quality standard of China)over the TiO2/ACﬁlter.Figs.13and14 show the photocatalytic reaction rate and removal of HCHO at an initial HCHO concentration of0.37mg/m3.It is observed that the photocatalytic reaction process at lower HCHO concentrations was the same as that at higher concentrations over the TiO2/netlike (or granular)ACﬁlm;no similar trend,however,was observed over the TiO2/glassﬁlm.It might be a result due to the predominant inﬂuence of residence time at low HCHO concentrations.Forthe Fig.12.The effect ofﬂow velocity on the removal of HCHO at highconcentration.Fig.13.The effect ofﬂow velocity on the reaction rate of HCHO at highconcentration.Fig.14.The effect ofﬂow velocity on the removal of HCHO at high concentration.Y.Lu et al./Building and Environment45(2010)615–621619TiO 2/AC ﬁlm,the adsorption of HCHO over the activated carbon prolonged the residence time of HCHO on the surface of TiO 2and enhanced the diffusion of HCHO to the surface of TiO 2,so the photocatalytic reaction rate exhibited a feature same as that at high concentrations.But for the TiO 2/glass ﬁlm,the low adsorption of HCHO caused the residence time and diffusion of HCHO to exert little impacts on the HCHO removal,hence showing a behavior different from that at high concentrations.Fig.14shows the removal of HCHO as a function of ﬂow velocity.It can be seen that the removal of HCHO over the three ﬁlms at low concentrations decreased signiﬁcantly with the increase in ﬂow velocity,but a different scenario appeared at high concentrations.This phenomenon probably resulted from the fact that,at low HCHO concentrations,the photocatalytic reaction was mainly controlled by the residence time of HCHO on the surface of TiO 2.When ﬂow velocity increased,the inﬂuence of HCHO residence time on the removal of HCHO decreased signiﬁcantly.As shown in Fig.14,the removal of HCHO over the TiO 2/netlike (or granular)AC ﬁlm could reach nearly 100%at the lower ﬂow velocity,at which the inﬂu-ences of residence time and the diffusion due to the HCHO adsorption of AC were signiﬁcant.The increase in ﬂow velocity may shorten the HCHO residence time,resulting in the drop in photo-catalytic removal of HCHO.From the above results,one can realize that,at the indoor HCHO concentration levels,the residence time of pollutants in air cleaner should be taken into account for the photocatalytic removal of HCHO.The combination of TiO 2with an adsorbent material is likely to be a good choice for the removal of indoor contaminants.The photocatalytic degradation of HCHO over the TiO 2/netlike AC ﬁlm and TiO 2/glass ﬁlm at a HCHO concentration of 0.37mg/m 3with the switch point velocity of 0.59cm/s is shown in Fig.15.It is observed that the HCHO decomposed photocatalytically over the TiO 2/netlike AC ﬁlm and reached a concentration below 0.1mg/m 3.However,the photocatalytic removal of HCHO over the TiO 2/glass ﬁlm was few.The above results indicate that the combination of photocatalytic technology with AC adsorptive technology might be a valid pathway to purify indoor HCHO.4.Conclusions1.The TiO 2ﬁlms coated on activated carbon ﬁlters performed better than the TiO 2ﬁlm coated on glass.The combination ofTiO 2with AC could enhance the removal of indoor HCHO and delay the adsorption saturation of HCHO over the activated carbon.2.The photocatalytic reaction took place from the diffusion control process to the photocatalytic reaction control process with the rise in ﬂow velocity at higher HCHO paring with photocatalytic removal of HCHO over the TiO 2/glass ﬁlm,the switch point of photocatalytic reaction changing from the diffusion control process to the reaction control process was advanced over the TiO 2/netlike (or granular)AC ﬁlm since the adsorption of HCHO over the activated carbon enhanced the diffusion.3.The effects of ﬂow velocity on the photocatalytic reaction rate and removal of HCHO at lower concentrations were mainly associated with the residence time when other reaction conditions (catalyst performance,UV light intensity,etc)remain constant.So the combination of photocatalytic tech-nology with AC adsorptive technology might be a valid pathway to purify indoor HCHO.AcknowledgmentsThis project was supported by the National Natural Science Foundation (50476036),and the Foundation of New Star of –Bei-jing Municipal Science &Technology (2005A10),and the Open Foundation of the Key Laboratory of Heat Transfer and Energy Conversion,Beijing Municipality,Beijing University of Technology.References[1]/iaq/formalde.html#Health%20Effects .[2]Lee SC,Kwok NH,Guo H.The effect of wet ﬁlm thickness on VOC emissionsfrom a ﬁnishing varnish.The Science of the Total Environment 2003;302:75–84.[3]Ao CH,Lee SC.Indoor air puriﬁcation by photocatalyst TiO 2immobilized on anactivated carbon ﬁlter installed in an air cleaner.Chemical Engineering Science 2005;60:103–9.[4]Chin P,Yang LP,Ollis DF.Formaldehyde removal from air via a rotatingabsorbent combined with a photocatalyst reactor:kinetic modeling.Journal of Catalysis 2006;237:29–37.[5]Ao CH,Lee SC,Yu JC.Photodegradation of formaldehyde by photocatalyst TiO 2:effects on the presences of NO,SO 2and VOCs.Applied Catalysis B:Environ-mental 2004;54:41–50.[6]Lu YW,Chf Ma,Xia GD,et al.Research on the photocatalytic decomposition ofindoor formaldehyde for air puriﬁcation.Acta Energiae Solaris Sinica 2004;25:542–6[in Chinese].[7]Lu YW,Li WC,Wang W,et bination of Activated carbon with TiO 2forthe photodegradation of indoor formaldehyde (I).Acta Energiae Solaris Sinica 2008;29:114–8[in Chinese].[8]Lu YW,Li WC,Wang bination of activated carbon with TiO 2for thephotodegradation of indoor formaldehyde (II).Acta Energiae Solaris Sinica 2008;29:550–4[in Chinese].[9]Yang JJ,Li DX,Zh Zhang,et al.A study of the photocatalytic oxidation offormaldehyde on Pt/Fe 2O 3/TiO 2.Journal of Photochemistry and Photobiology A:Chemistry 2000;137:197–202.[10]Zhang PY,Liu J.Photocatalytic degradation of trace hexane in the gas phasewith and without ozone addition:kinetic study.Journal of Photochemistry and Photobiology A:Chemistry 2004;167:87–94.[11]Lu YW,Li WC,Sheng JP,et al.Enhancement of photocatalytic reaction rate ofHCHO under the action of mass transfer.Beijing,China.Journal of Beijing University of Technology 2007;33:858–63[in Chinese].[12]Shiraishi F,Yamaguchi S,Ohbuchi Y.A rapid treatment of formaldehyde ina highly tight room using a photocatalytic reactor combined with a continuous adsorption and desorption apparatus.Chemical Engineering Science 2003;58:929–34.[13]Kim SB,Hong S.Kinetic study for photocatalytic degradation of volatileorganic compounds in air using thin ﬁlm TiO 2photocatalyst.Applied Catalysis B:Environmental 2002;35:305–15.[14]Kim SB,Hwang HT,Hong S.Photocatalytic degradation of volatile organiccompounds at the gas–solid interface of a TiO 2photocatalyst.Chemosphere 2002;48:437–44.[15]Sopyan I,Watanabe M,Murasawa S.A ﬁlm-type photocatalyst incorporatinghighly active TiO2powder and ﬂuororesin binder:photocatalytic activity and long-term stability.Journal of Electroanalytical Chemistry 1996;415:183–6.Fig.15.The photocatalytic removal of HCHO by different ﬁlms.Y.Lu et al./Building and Environment 45(2010)615–621620[16]Ao CH,Lee SC.Enhancement effect of TiO2immobilized on activated carbonﬁlter for the photodegradation of pollutants at typical indoor air level.Applied Catalysis B:Environmental2003;44:191–205.[17]Ao CH,Lee bination effect of activated carbon with TiO2for the pho-todegradation of binary pollutants at typical indoor air level.Journal of Photochemistry and Photobiology A:Chemistry2004;161:131–40.[18]Ao CH,Lee SC,Yu JC.Photocatalyst TiO2supported on glassﬁber for indoor airpuriﬁcation:effect of NO on the photodegradation of CO and NO2.Journal of Photochemistry and Photobiology A:Chemistry2003;156:171–7.[19]Ao CH,Lee SC,Mak CL,Chan LY.Photodegradation of volatile organiccompounds(VOCs)and NO for indoor air puriﬁcation using TiO2:promotion versus inhibition effect of NO.Applied Catalysis B:Environmental 2003;42:119–29.[20]Li WC.Experimental research on the immobilization of TiO2applying tophotocatalytic air cleaner,master thesis.Beijing University of Technology, Beijing,China;2006;[in Chinese].[21]Hossain MM,Raupp GB,Hay SO,Obee TN.Three-dimensional developingﬂow model for photocatalytic monolith reactors.AICHE Journal1999;45(6):1309–21.[22]Zhang YP,Yang R,Zhao R.A model for analyzing the performance of photo-catalytic air cleaner in removing volatile organic compounds.Atmospheric Environment2003;37:3395–9.[23]Mo JH,Zhang YP,Yang R.Novel insight into VOC removal performance ofphotocatalytic oxidation reactors.Indoor Air2005;15:291–300.[24]Mo JH,Zhang YP,Yang R,Xu QJ.Inﬂuence ofﬁns on formaldehyde removal inannular photocatalytic reactors.Building and Environment2008;43:238–45.Y.Lu et al./Building and Environment45(2010)615–621621。

doping TiO2 review

Journal of Alloys and Compounds 509 (2011) 1648–1660Contents lists available at ScienceDirectJournal of Alloys andCompoundsj o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /j a l l c omReviewRoles of titanium dioxide and ion-doped titanium dioxide on photocatalytic degradation of organic pollutants (phenolic compounds and dyes)in aqueous solutions:A reviewChao Min Teh,Abdul Rahman Mohamed ∗School of Chemical Engineering,Engineering Campus,University Science Malaysia,Seri Ampangan,Nibong Tebal 14300,Penang,Malaysiaa r t i c l e i n f o Article history:Received 19July 2010Received in revised form 28October 2010Accepted 28October 2010Available online 9 November 2010Keywords:PhotocatalysisTitanium dioxide (TiO 2)Phenolic compounds Dyes Dopantsa b s t r a c tWater pollution by organic pollutants is an ever increasing problem for the global concerns.This paper presents a critical review on the abatement of organic pollutants,dyes and phenolic compounds in par-ticular,using photocatalytic reaction by titanium dioxide (TiO 2).Mechanism of photocatalytic reaction is brieﬂy discussed.A detailed search of published reports on the advancement in photocatalytic degrada-tion of organic pollutants in wastewater by doping titanium dioxide with foreign species such as metal and non-metal component has also been carried out and analyzed in this paper.© 2010 Elsevier B.V. All rights reserved.Contents 1.Introduction..........................................................................................................................................16492.Organic pollutants in wastewater ...................................................................................................................16492.1.Dyes...........................................................................................................................................16492.2.Phenolic compounds .........................................................................................................................16493.Decomposition of organic pollutants in wastewater by photocatalytic reactions..................................................................16503.1.Titanium dioxide (TiO 2)......................................................................................................................16503.2.Mechanism of photocatalytic reaction.......................................................................................................16504.Photocatalytic degradation of organic pollutants using undoped TiO 2.............................................................................16514.1.Dyes...........................................................................................................................................16514.2.Phenolic compounds .........................................................................................................................16515.Photocatalytic degradation of organic pollutants using single-doped TiO 2.........................................................................16515.1.TiO 2doped with rare earth metals...........................................................................................................16515.2.TiO 2doped with transition metals...........................................................................................................16535.3.TiO 2doped with noble metals ...............................................................................................................16565.4.TiO 2doped with poor metals ................................................................................................................16565.5.TiO 2doped with non-metals.................................................................................................................16565.6.TiO 2doped with metalloid...................................................................................................................16576.Drawbacks of metal doping..........................................................................................................................16577.Photocatalytic degradation of organic pollutants using TiO 2with multiple dopants ..............................................................16588.Conclusions ..........................................................................................................................................1659Acknowledgement...................................................................................................................................1659References . (1659)∗Corresponding author.Tel.:+6045996410;fax:+6045941013.E-mail address:chrahman@m.my (A.R.Mohamed).0925-8388/$–see front matter © 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.jallcom.2010.10.181C.M.Teh,A.R.Mohamed/Journal of Alloys and Compounds509 (2011) 1648–166016491.IntroductionThe world’s most precious and important natural resource, water,is under threat from various contaminants,causing a water contamination crisis.While the world’s population has been tripled in the20th century,use of renewable water resources has grown six-fold,and global population is expected to increase by another 40–50%within the next50years.The increasing demand for water generated by this population growth will cause serious conse-quences on the environment[1].According to the World Health Organization(WHO),in2002more than one out of six people lacked access to safe drinking water,namely1.1billion people,represent-ing17%of the global population.Moreover,in the same year,2.6 billion people,i.e.42%of the world’s populations were without even the most basic sanitation facilities.A Lack of clean drinking water and sanitation kills about4500children each day and condemns their parents,siblings,and neighbors to sickness,pollution,and enduring poverty[2].It should be noted that theseﬁgures repre-sent only those people living in very poor conditions.In reality,the overallﬁgures are expected to be much higher.Humans are generating and disposing more wastewater today than any other time.The disposal of toxic contaminants,such as dyes and phenolic compounds which are harmful to the environ-ment,hazardous to humans,and difﬁcult to degrade by natural means,is pervasively associated with industrial development and these contaminants are frequently found in the industrial efﬂuents[3].Chemical precipitation,ﬁltration,electro-deposition, ion-exchange adsorption,and membrane systems are some of the conventional methods for water treatment and have found cer-tain practical applications.However,these methods may not be very effective,because they are either slow or non-destructive to some or most persistent organic pollutants.Besides,large scale implementations of these methods have some limitations,owing to the expensive equipments involved in these processes[4].It is therefore essential to investigate the use of efﬁcient catalytic mate-rials to remove highly toxic compounds from potential sources of drinking water.Semiconductor heterogeneous photocatalysis is a popular technique that has the great potential to control the organic contaminants in water or air[5].This process which is also known as“Advanced Oxidation Process(AOP)”is suitable for the oxidation of recalcitrant contaminants such as dyes and phenolic compounds[6].Heterogeneous photocatalytic oxidation,devel-oped in the1970s,has attracted considerable attention particularly when used under solar light[7].In the past decades,numerous studies have been carried out by researchers from all over the world on the application of heterogeneous photocatalytic oxidation process to decompose and mineralize certain recalcitrant contam-inants.The photocatalytic activity of various forms of TiO2,such as TiO2ﬁlm[8],TiO2powders[9],TiO2nanotubes[10],supported TiO2[11–13]and doped TiO2[14,15]have been evaluated through degradation of dyes and/or phenolic compounds under light irra-diation.Results of these studies showed that TiO2was effective for removing dyes and phenolic compounds from aqueous solutions. In the present review,some advances in photocatalytic removal of organic pollutants,particularly dyes and phenolic compounds from ﬂuid streams have been presented.anic pollutants in wastewater2.1.DyesThe widespread disposal of industrial wastewater containing dyes on land and in bodies of water often has received the most attention in studies because of the color and toxicity of some of the raw materials used to synthesize dyes,such as certain aro-matic amines used to produce azo dyes.Their disposal has led to serious contamination in many countries worldwide[16,17].Dyes are usually theﬁrst contaminant to be recognized in wastewater because they are highly visible and undesirable in water,even in very small amounts(<1ppm for some dyes)[18,19].Nevertheless, dyes are still widely used in many contemporaryﬁelds of technol-ogy[20].Over100,000commercially available dyes exist,and more than7×105tonnes of dyes are produced annually[21].Dyes play a very important role in various branches of the tex-tile industry;dyes used in this industry are often synthetic,usually derived from two sources:coal tar and petroleum-based inter-mediates[20–25].Synthetic dyes have become common water pollutants and are usually found in trace quantities in industrial wastewater owing to their good solubility in water[26,27].It has been estimated that approximately15%of the total world produc-tion of dyes is lost during the dyeing process and then released to the environment through textile efﬂuents[17].Apart from the textile industry,leather tanning industry[28–30],paper industry [31],food technology[32,33],hair colorings[34–36],photoelec-trochemical cells[37–39]and light-harvesting arrays[40–42]also contribute to the presence of dyes in wastewater.Many of the dyes used in industry are toxic and carcinogenic, and this poses a serious hazard to aquatic living organisms.The toxicity and impact of dyes released to the environment have therefore been extensively studied[43,44].Furthermore,because of the increasingly strict restrictions on the organic composi-tion of industrial efﬂuents,it is essential to eliminate dyes from wastewater before they can be discharged into the environment. However,many dyes are difﬁcult to decolorize,owing to their synthetic origin and complex structure.These dyes usually have many structural varieties,for example,acidic,basic,azo,diazo,dis-perse,anthroquinone-based,and metal complex dyes.As a result, traditional wastewater treatment technologies are markedly inef-fective in handling synthetic dye-contained wastewater,because of the chemical stability of these pollutants[45].A wide range of technologies have been developed to remove synthetic dyes from wastewaters so as to reduce their impact on the environ-ment.Chemical precipitation,adsorption on organic or inorganic matrices,and decolorization by photocatalysis and/or by chemical oxidation processes are some of the technologies currently being used for the removal of synthetic dyes[46].2.2.Phenolic compoundsBesides dyes,presence of phenols and their derivatives in water supplies and industrial efﬂuents is another problem attracting global concern.These hazardous water-soluble phenolic com-pounds are continuously released to the environment through domestic and industrial activities,representing a severe toxicolog-ical risk to the earth as well as all living creatures on it.Phenolic compounds are aromatic compounds with one or more hydroxyl groups attached to the aromatic ring.These compounds are usually found in wastewater discharged from a variety of indus-tries,such as petroleum reﬁneries,chemical synthesis,plastics, coke plants,dyes,pulp and paper,textiles,detergents,pharma-ceutics,as well as pesticides and herbicides synthesis[47,48]. Phenolic compounds can also arise from natural sources in the aquatic environment,such as algal secretion,lignin transformation, hydrolysable tannins andﬂavanoids,and humidiﬁcation processes at low concentration[49].Humans are potentially exposed to phenols in all places,as they are found in tea,fruits,and vegetables and are widely used in industrial processes,pharmaceuticals,and consumer products[50]. Exposure to phenols poses hazards to humans as the compounds are corrosive to the respiratory tract,eyes,and skin.Repeated or long-term exposure of skin to phenols will cause dermatitis,1650 C.M.Teh,A.R.Mohamed/Journal of Alloys and Compounds509 (2011) 1648–1660or even second-and third-degree burns because of the phenol’s defatting and caustic properties[51,52].Phenol derivatives such as bisphenol A(BPA),chlorophenol,and nitrophenol are also known to be hazardous to humans.BPA is an organic compound with two phenol functional groups widely used in the plastics indus-try for the production of polycarbonate plastics and epoxy resins. Dental composites/sealants,baby bottles,the lining of food cans, and drinking-water bottles are some of the consumer products developed from BPA.The presence of BPA in industrial efﬂuents has received wider attention since it was listed as an endocrine-disrupting chemical(EDC),which can inﬂuence the generative function of humans and other living creatures by mimicking the body’s own hormones and leads to negative health effects[53–56].The next phenol derivative,chlorophenol,is an organochloride of phenol,usually consisting of one or more covalently bonded chlorine atoms.It is also a known endocrine disruptor that is toxic and non-biodegradable.This compound is an important xenobiotic micropollutant of aquatic environments and is usually present in wastewater as by-products of the pulp and paper,dyestuff,phar-maceutical,and agrochemical industries[57,58].Biodegradation of chlorophenol is slow and incomplete,eventually generating by-products that are more toxic and hazardous than chlorophenol to the environment as well as to human health[59,60].In addi-tion,nitrophenols,another family of common phenolic compounds found in industrial efﬂuents,are continually detected in urban and agricultural waste as they are among the most widely used and ver-satile industrial organic compounds.These compounds are usually used in the manufacture of pharmaceuticals,pesticides,explo-sives,dye,pigments,wood preservatives,and rubber chemicals. Nitrophenols have been proven to be carcinogenic and may pose signiﬁcant health risks to humans and other organisms.Among the nitrophenols,2-nitrophenol,4-nitrophenol,and2,4-dinitrophenol have been listed as“Priority Pollutants”by the US Environmental Protection Agency,which recommended that their concentrations in natural waters should be restricted to below10ng/L[61–64]. 3.Decomposition of organic pollutants in wastewater by photocatalytic reactionsFrom the perspective of environmental science,regulatory laws, and human health,it is urgent that the release of toxic chemi-cals from industrial processes and commercial products must be restricted.In fact,many processes and technologies for destroy-ing these toxins have been proposed over the years,and some are currently employed in a number of wastewater treatment plants.Among the various methods for decomposition of these toxic compounds,a more promising technology based on an advanced oxidation process has been extensively studied.This process involves the degradation of pollutants by irradiating sus-pensions of metal oxide semiconductor particles such as TiO2or zinc oxide(ZnO)with light.It is a promising method because it not only degrades the pollutants but also completely mineralizes them to carbon dioxide(CO2),water(H2O),and mineral acids[65–67]. The low-cost and mild operating conditions(mild temperature and pressure)of this photocatalytic degradation process are also factors in its popularity in wastewater treatment[68].A semiconductor is a material with electrical resistivity between that of an insulator and a conductor and is usually characterized by an electronic band structure in which the lowest empty energy bands,called the conduction band(CB),and the highest occupied energy band,called the valence band(VB),are separated by a band-gap.The ability of a photocatalytic reaction to degrade organic and inorganic pollutants arises from the redox environment generated from the photoactivation of a semiconductor such as titanium diox-ide(TiO2).In general,three components must be present so that the heterogeneous photocatalytic reaction can occur:an emitted photon(in the appropriate wavelength),a strong oxidizing agent (usually oxygen),and a catalyst surface(a semiconductor material) [69].3.1.Titanium dioxide(TiO2)Titanium dioxide(TiO2)is a natural occurring oxide of titanium; it is also named as titania or titanium(IV)oxide.In nature,TiO2 exists inﬁve different forms,i.e.rutile,anatase,brookite,mono-clinic and orthorhombic.However,monoclinic and orthorhombic phase of TiO2are two exceptions found only in shocked granet gneisses from Ries crater in Germany[70,71].Rutile appears to be the most common form of TiO2,while anatase and brookite forms of TiO2tend to convert into rutile form upon heating at high tem-perature.Calcinated TiO2,especially in rutile form is very stable and insoluble in water;it is also insoluble or only moderately soluble in concentrated and hot acids[72].TiO2is well known for its widespread applications in paints, sunscreens,environmental treatment and puriﬁcation purposes [73–76].These widespread applications of TiO2are credited to its high level of photoconductivity,ready availability,low toxicity, inertness,low cost as well as high photoefﬁciency and activity.TiO2 has drawn great attentions of researchers in photovoltaic and pho-tocatalysisﬁelds since Fujishima and Hondaﬁrst discovered the ability of TiO2in splitting of water under ultraviolet(UV)light[77]. Crystalline structure of TiO2has been reported as one of the factors affecting its photocatalytic activity.Anatase form of TiO2has the best photocatalytic activity,followed by rutile form[78].TiO2can utilize natural UV radiation from sunlight for pho-tocatalysis because it has suitable energetic separation between its conduction and valence band[79].Band-gap energy of TiO2 (3.2eV for anatase;3.03for rutile)is relatively smaller compared to other semiconductors,such as ZnO(3.35eV)and SnO2(3.6eV) [80].Therefore,TiO2is able to absorb photons energy in the near UV range( <387nm).Photocatalytic reaction is initiated with the suf-ﬁcient input of radiation equal or higher than the band-gap energy of the target semiconductor which causes molecular excitation and charge separation.As a result,mobile electrons and holes will be generated and migrate to the surface of the semiconductor to take part in the photocatalytic reaction[81].Mechanism of the pho-tocatalytic reaction will be further discussed in later part of this article.3.2.Mechanism of photocatalytic reactionThe heterogeneous photocatalytic reaction is initiated with the absorption of radiation equal to or higher than the band-gap energy (E bg)of the target semiconductor.E bg is deﬁned as the difference between theﬁlled VB and the empty CB;in this case TiO2has a band-gap of3.2eV in the form of anatase or3.0eV as rutile.When photons with energy equal to or higher than E bg reach the sur-face of the photocatalyst,they will cause molecular excitation.As a result,mobile electrons will be generated in the higher-energy CB simultaneously with the generation of positive holes in the lower-energy VB of the photocatalyst.After the initiation of pho-togenerated electron–hole pairs,the photocatalytic reaction will proceed through a series of chemical events.The photogener-ated holes and electrons can either recombine and dissipate the absorbed energy as heat or be available for use in the redox reaction. Photogenerated holes and electrons that do not recombine migrate to the surface of catalyst for redox reaction.The redox reaction will utilize both the electron and hole,with the positive holes(h+)for oxidation processes and the electrons(e−)for reduction processes on the surface of the photocatalysts.The positive holes break apart the water molecule to form hydron(positive hydrogen cation,H+)C.M.Teh,A.R.Mohamed /Journal of Alloys and Compounds 509 (2011) 1648–16601651Table 1General mechanism of the photocatalytic reaction on illuminated TiO 2.ProcessReaction stepsPhoto-excited TiO 2generates electron–hole pairs (h v ≥E G )TiO 2h v−→e −+h+Photogenerated holes,h +migrate to catalyst surface and react with water molecules adsorbed on the catalyst surface H 2O adTiO 2(h +)+h 2O ad →TiO 2+HO ·+h +Photogenerated electrons,e −migrate to catalyst surface and molecular oxygen acts as an acceptor species in the electron-transfer reaction TiO 2(e −)+O 2→TiO 2+O ·−2Reactions of superoxide anions,O 2−O ·−2+H +→HO ·2O ·−2+3HO ·2→HO ·+3O 2+H 2O +e −2HO ·2→O 2+H 2O 2Photoconversion of hydrogen peroxide to give more HO •free-radical groupsH 2O 2+TiO 2(e −)→TiO 2+HO −+HO ·Oxidization of organic adsorbed pollutants (S ad )by HO •onto the surface of the TiO 2HO ·2+S ad →IntermediatesOverall reaction Organic Pollutant TiO 2/h v−→Intermediates →CO 2+H 2Oand the hydroxyl radical (OH −).This OH −will then lead to the pro-duction of strong oxidizing HO •radicals.Meanwhile,the negative electrons react with the oxygen molecule to form a superoxide anion (O 2•−).This superoxide anion also produces HO •radicals via the formation of HO 2•radicals and H 2O 2.The electron–hole recombination step is undesirable as it will result in process inef-ﬁciencies and waste the energy supplied by the photon.Therefore,it is often considered as one of the major factors limiting the efﬁ-ciency of the photocatalytic processes.Besides,it is found that HO •is the most plentiful radical species in TiO 2aqueous suspension and the reaction of HO •with organic pollutants is the most impor-tant step that leads to the mineralization of organic pollutants.The heterogeneous photocatalytic reaction can basically be represented by a number of mechanistic steps.The general mechanism of the photocatalytic reaction on light-illuminated TiO 2is summarized in Table 1[79,82,83].4.Photocatalytic degradation of organic pollutants using undoped TiO 24.1.DyesMany studies have been carried out to examine the photocat-alytic degradation of dyes in wastewater in the presence of TiO 2as photocatalyst.Table 2summarizes these studies.It is seen that TiO 2is able to photodegrade various types of dyes in the presence of light.Light intensity,catalyst concentration,solution pH,dye con-centration,and the presence of electron acceptor were found to be factors affecting the rate and efﬁciency of the photocatalysis.4.2.Phenolic compoundsApart from dyes,the feasibility and efﬁciency of the photode-composition of phenol and its derivatives in water using TiO 2as photocatalyst have also been investigated.The results of these stud-ies are summarized in Table 3.As with dyes,the efﬁciency and rate of the photodegradation of phenol and its derivatives were found to be dependent on light intensity,catalyst concentration,solution pH,substrate (phenol and its derivatives)concentration,and the presence of electron acceptor.5.Photocatalytic degradation of organic pollutants using single-doped TiO 2TiO 2is well known as a promising photocatalyst owing to its non-toxicity,low cost,and capability of degrading a wide range of both gaseous and liquid pollutants.However,this semiconductor displays its photoactivity only under ultraviolet (UV)light exci-tation.The relatively wide band-gap of TiO 2(3.2eV)means that it can be stimulated only by UV radiation with a wavelength of about 387nm.As a result,only about 3–5%of incoming solar energyon the earth’s surface can be utilized and is therefore not practi-cal for wastewater treatment applications [113,115].In addition,the photocatalytic activity of TiO 2is limited by the low interfa-cial charge-transfer rates of photogenerated carriers as well as its high charge carrier recombination rate [116].In this regard,sev-eral strategies have been developed to increase the efﬁciency of the photocatalytic process in TiO 2[117,118].One route for sensitizing TiO 2to visible light so as to permit the use of the main part of the solar spectrum and also forming charge traps to keep electron–hole pairs separate is by doping pure TiO 2with foreign ions [119].For instance,TiO 2doped with metals or metallic cations as a photocata-lyst,such as transition metals,rare earth metals,and noble metals,have been widely investigated.Generally,metal-ion-doped TiO 2could improve the redox potential of the photogenerated radicals,widen the light absorption range,and enhance quantum efﬁciency via inhibiting the recombination of photogenerated electrons and holes as the ions act as electron traps [120].5.1.TiO 2doped with rare earth metalsRare earth metals or rare earth elements are a group of 17chemical elements in the periodic table and include scandium (Sc),yttrium (Y),and the 15lanthanoids.These metals having incom-pletely occupied 4f and empty 5d orbitals often serve as catalysts or promote catalysis.Therefore,the incorporation of rare earth metal ions into the TiO 2matrix could provide a mean to increase the concentration of organic pollutants at the semiconductor surface,hence improving the photoactivity of TiO 2[121].Doping TiO 2with rare earth metals that have incompletely occupied 4f orbitals,such as neodymium (Nd)has been investi-gated by ˇStengl et al.[122]and Xu et al.[123]to show its betterphotocatalytic activity.It was inferred that the increase in photo-catalytic activity was due to the transition of 4f electrons in rare earth ions,which led to the enforcement of optical adsorption of the photocatalysts and supported the separation of photogenerated electron–hole pairs [122].It was also concluded that the presence of a 4f level in Nd ions helped to decrease the TiO 2energy band-gap by allowing charge transfer between the TiO 2valence/conduction band and the Nd ion 4f level [123].Research done by El-Bahy et al.[121]showed that the photo-catalytic activity of TiO 2depends on its band-gap,surface area,and pore volume.Thus,gadolinium (Gd)-doped TiO 2prepared by El-Bahy et al.,a doped TiO 2with high surface area,large pore volume,small particle size,and small band-gap,presented the highest pho-tocatalytic activity.In addition,TiO 2doped with cerium (Ce)and holmium (Ho)were found to be able to retard grain growth of TiO 2[124]as well as decrease its crystallite size while increasing its spe-ciﬁc surface area [125].Shi et al.[125]claimed that Ho-doped TiO 2with a smaller crystallite size favored the shifting of photogener-ated carriers to the surface of the photocatalysts and interaction with the reactants.Increased surface area,on the other hand,was1652 C.M.Teh,A.R.Mohamed/Journal of Alloys and Compounds509 (2011) 1648–1660Table2Summary of studies on photocatalytic degradation of dyes in wastewater in the presence of TiO2as photocatalyst.Compound degraded Photocatalyst used Parameter studied Comments ReferencesMethyl Orange(MO)Natural zeolitessupported TiO2Initial concentration of MO,catalyst concentration,pHand species of natural zeolite•Supported TiO2showed higher reaction rate thanpure TiO2regardless of the initial MO initialconcentration.Li et al.[84]•Photocatalytic degradation rate constant,k1increased sharply from0.025min−1to0.060min−1while increasing the catalyst dosage from0.006to0.04g/10mL.•Both decolorization efﬁciency and reaction rateconstant(k1)decreased with increasing initial pHfrom acidic to basic.•Natural zeolites did not directly participate inphotocatalytic degradation,however their species andthe presence of impurities played minor role inenhancing the photodegradation of MO.C.I.Basic Violet10Y zeolite supported TiO2TiO2content,calcinationstemperature,pH,initial dyeconcentration and catalystloading •The photocatalytic degradation of C.I.Basic Violet10by Y zeolite supported TiO2was aﬁrst-order kineticsreaction.Wang et al.[85]•Optimum photocatalytic performance was achievedwith high TiO2content(20%),high calcinationstemperature(600◦C),alkaline pH(9–10)catalystconcentration of5333ppm and small initial dyeconcentration(10ppm).Acid Orange7(AO7)Silica gel supported TiO2Calcinations temperature,catalyst loading •The photocatalytic activity of silica gel supportedTiO2(31%TiO2/SiO2)was reduced when the catalystwas calcined at higher temperature,which attributedto the decrease in BET surface area available for theadsorption of AO7.Chen et al.[86]•31%TiO2/SiO2showed2.3and12.3times fasterphotodegradation rate than P-25and TiO2(Shanghai),respectively.This could be explained by the strongadsorption of AO7on31%TiO2/SiO2as well as thehigher surface area of31%TiO2/SiO2compared toP-25and TiO2(Shanghai).Rhodamine B TiO2loaded onmesoporous graphiticcarbon(TiO2/GC-950)N/A•TiO2/GC-950showed better photoactivity than pureTiO2with reaction constant,k1of0.012min−1(0.006min−1for pure TiO2).This enhancement inphotoactivity was due to the conﬁnement in themesopores of graphitic carbon(GC-950),high degreeof anatase crystallization as well as the graphiticproperty of GC-950.Xiao et al.[87]Malachite Green(MG)TiO2Degussa P-25TiO2quantity and pH•99.9%of MG was degraded with the addition of0.5g/L TiO2.Chen et al.[88]•Photodegradation rate of MG increased along withincreasing pH as pH higher than pH ZPC(ZPC=zeropoint charge).Malachite Green TiO2prepared byhydrothermalcrystallization in organicmedia Physical properties of TiO2•TiO2prepared from this method exhibited a higherlevel of activity than commercial P-25TiO2.Kominami et al.[89]•Adsorptivity and desorptivity of MG on the catalystas well as the crystallinity of TiO2are decisive factorsin the bleaching of MG.•Decomposition of MG increases with higher degreeof crystallinity and adsorptivity and desorptivity.Triphenylmethane dye (Gentian Violet)TiO2Degussa P-25pH,catalyst concentration,substrate concentration,types of TiO2•Degussa P-25showed better performance than otherTiO2,i.e.,UV100and PC500.Muneer and Saquib[90]•pH value of3.5and11and higher substrateconcentration(no more than0.25mM)are the criteriafor better photocatalytic performance.Methyl Orange(MO)Nanocrystalline TiO2prepared by stearic acidgel method Loading of TiO2•Photocatalytic degradation of MO increases withincreasing loading of TiO2because the probability ofabsorbing photons increases with increasing loadingof TiO2.Ruan et al.[91]Reactive Red195(RR 195)TiO2nanoparticles Initial pH,concentration ofdye,and concentration ofadsorbent•Optimum pH of sorption=3.Acidic pH promotes theelectrostatic attractions between the positivelycharged surface of the adsorbent and anionic dye.Belessi et al.[92]•Percentage of color removal decreased withincreasing initial dye concentration,while the amountof dye adsorbed per unit of adsorbent mass wasenhanced.•Dye adsorption yield increased with increasingadsorbent dose.C.I.Reactive Red TiO2Degussa P-25pH,dye concentration,electron acceptor(hydrogenperoxide,persulfate,andcopper ions),and hydroxylradical trap(ethanol)•Increasing quantity of persulfate ions added wouldincrease the decolorization rate because persulfate(aselectron acceptor)suppressed the recombination of photogenerated electron–hole pairs.Wu[93]•Addition of ethanol inhibited decolorization as thepredominant decolorization pathway involveshydroxyl radicals.。

METHOD FOR DETECTION OF THE PHOTOCATALYTIC DEGRADA

专利名称：METHOD FOR DETECTION OF THEPHOTOCATALYTIC DEGRADATION OFORGANIC DYES BY MEANS OFFLUORESCENCE ANALYSIS发明人：NEUMANN, FRANK,CEREZUELA BARRETO, MERCEDES申请号：EP05749114.4申请日：20050602公开号：EP1751524A1公开日：20070214专利内容由知识产权出版社提供摘要：The invention relates to a method for the quantitative determination of the photocatalytic degradation of organic dyes on photocatalytically active surfaces by means of fluorescence analysis. The photocatalysts for investigation and photocatalytically inactive reference substrates are coated with organic dyes. The samples are then exposed to UV or visible light of known intensity and spectral distribution and the intensity of fluorescence measured by means of a fluorescence scanner, chip-reader or fluorescence microscope before and after exposure and, depending on the device configuration, during exposure as well. The subsequent reduction on fluorescence of the dye-coated photocatalysts by comparison to the also coated but photocatalytically inactive reference (for example quartz glass) is used as a measure for the photocatalytic activity of the sample under investigation.申请人：FRAUNHOFER-GESELLSCHAFT ZUR FOERDERUNG DER ANGEWANDTEN FORSCHUNG E.V.地址：Hansastrasse 27c 80686 München DE 国籍：DE代理机构：Pfenning, Meinig & Partner GbR 更多信息请下载全文后查看。

IMPROVEMENT OF ADHESION RESISTANCE OF PHOTOGRAPHIC

专利名称：IMPROVEMENT OF ADHESION RESISTANCE OF PHOTOGRAPHIC SENSITIVE SILVERHALIDE MATERIAL FOR PRINTING发明人：KAMEOKA KIMITAKA,ISHIGAMI MAKOTO申请号：JP7411180申请日：19800602公开号：JPS57642A公开日：19820105专利内容由知识产权出版社提供摘要：PURPOSE:To improve the adhesion resistance of a photographic sensitive material for printing and the residual color after development without deteriorating the dimensional stability by adding acid-treated gelatin and a gelatin hardening agent having a vinylsulfonic acid group to the backing layer. CONSTITUTION:A backing layer is formed on one side of a support of polyester or the like by applying and drying a coating material prepared by adding acid-treated gelatin, a compound having a vinylsulfonic acid group and represented by formula I as a gelatin hardening agent, fine particles of SiO2, polymethyl methacrylate or the like as a matting agent, a polymer latex and a dye necessary for gelatin. On the other side of the support a silver halide emulsion layer and a protective layer are formed in order. Thus, the backing layer is prevented from adhering to other substance owing to an increase in the adhesion and tackiness at high temp. and humidity during contact with the substance, a dimensional change of the photographic sensitive material due to expansion and contraction of the hydropilic colloid layer is inhibited, and no dye is left in the backing layer after developmhent.申请人：FUJI PHOTO FILM CO LTD更多信息请下载全文后查看。

激光近视的利弊英语作文

激光近视的利弊英语作文题目，The Pros and Cons of Laser Vision Correction。

In recent years, laser vision correction, particularly for myopia (nearsightedness), has gained significant popularity as a solution for those seeking freedom from glasses or contact lenses. While this procedure has its advantages, it also comes with its own set of disadvantages. In this essay, we will explore both the pros and cons of laser vision correction.Pros:1. Improved Vision: One of the most significantbenefits of laser vision correction is the improvement in vision. Many individuals experience clearer vision afterthe procedure, reducing or eliminating their dependence on glasses or contact lenses.2. Convenience: Laser vision correction offersconvenience to those who lead active lifestyles or engage in sports activities. No longer having to worry about glasses or contacts provides a sense of freedom and ease.3. Quick Procedure: The procedure itself is relatively quick, usually taking only a few minutes per eye. Patients can typically resume their normal activities within a day or two after the surgery.4. Long-Term Cost Savings: While laser visioncorrection may seem expensive initially, it can result in long-term cost savings by eliminating the need for regular purchases of glasses, contact lenses, and associated accessories.5. High Success Rate: With advancements in technology and surgical techniques, the success rate of laser vision correction has significantly increased. Many patients achieve the desired outcome of improved vision with minimal complications.Cons:1. Potential Risks: Like any surgical procedure, laser vision correction carries inherent risks, including infection, overcorrection, undercorrection, and vision disturbances such as glare or halos, particularly at night.2. Cost: The cost of laser vision correction can be prohibitive for some individuals, especially if it is not covered by insurance. Additionally, there may be additional costs for follow-up visits or enhancements.3. Not Suitable for Everyone: Laser vision correction may not be suitable for individuals with certain eye conditions or health issues. Factors such as age, corneal thickness, and stability of vision can impact candidacy for the procedure.4. Temporary Discomfort: While the procedure itself is relatively painless due to numbing eye drops, some patients may experience temporary discomfort or irritation in the days following surgery as the eyes heal.5. Potential Regression: While many patients experience long-lasting results from laser vision correction, some may experience regression of vision over time, necessitating the need for retreatment or the continued use of glasses or contacts.In conclusion, laser vision correction offers significant benefits in terms of improved vision, convenience, and long-term cost savings for many individuals. However, it is essential to weigh these benefits against the potential risks and drawbacks associated with the procedure. Ultimately, the decision to undergo laser vision correction should be made in consultation with a qualified eye care professional, taking into account individual factors and preferences.。

提高你的摄影技术英语作文

As a high school student with a passion for photography, Ive always been on the quest to improve my skills. The journey to becoming a better photographer is a continuous one, filled with trials, errors, and countless moments of learning. Heres my story on how Ive been striving to enhance my photography techniques.It all started when I received my first DSLR camera as a birthday gift. The excitement was palpable as I unboxed it, but the reality of not knowing how to use it effectively hit me like a ton of bricks. The camera manual was as thick as a novel, and the technical jargon was overwhelming. But my love for capturing moments was stronger than the intimidation of the new gadget.The first step in my journey was to familiarize myself with the cameras settings. I spent hours poring over the manual, taking notes, and practicing with different modes. I remember the frustration of not getting the right exposure or focus, but each failure was a lesson. I learned the importance of understanding ISO, aperture, and shutter speed the three pillars of photography.To deepen my understanding, I enrolled in a photography class at a local community center. The instructor, a seasoned professional, shared invaluable insights into composition, lighting, and storytelling through images. The class was a mix of theory and practical sessions, which allowed me to apply what I learned in reallife scenarios. I vividly recall the first time I captured a sunset, the oranges and reds blending seamlessly into the horizon. It was a moment of triumph, a testament to my growing skills.As I progressed, I realized that photography is not just about technical proficiency but also about creativity and perspective. I started experimenting with different styles, from portraits to landscapes, and even macro photography. Each genre presented its unique challenges and rewards. For instance, capturing the perfect portrait required understanding the subjects emotions and using lighting to accentuate their features.One of the most significant turning points in my photography journey was when I joined a local photography club. It was a treasure trove of knowledge and inspiration. The club members, ranging from beginners to professionals, shared their experiences and tips. We organized regular photo walks and contests, which pushed me out of my comfort zone. I learned to see the world from different angles, to notice the details that others might overlook.Moreover, I made it a habit to review my photos critically. I would analyze the composition, lighting, and the emotions conveyed in each image. This selfcritique helped me identify areas of improvement and avoid repeating mistakes. I also sought feedback from others, which was sometimes hardto swallow but ultimately beneficial for my growth as a photographer.In addition to handson practice, I immersed myself in the works of renowned photographers, studying their techniques and the stories behind their iconic images. I was particularly inspired by the works of Ansel Adams, known for his black and white landscapes, and Steve McCurry, famous forhis powerful portraits. Their dedication to their craft and the impact of their images on society fueled my passion for photography.Another aspect that contributed to my improvement was staying updated with the latest trends and technologies in photography. With the advent of digital photography and editing software, the possibilities are endless. I learned to use tools like Adobe Lightroom and Photoshop to enhance my images, adding a new dimension to my work.Lastly, I believe that patience and persistence are key to improving any skill, including photography. There were days when I felt like I wasnt making progress, but I reminded myself that every great photographer started somewhere. I kept pushing forward, taking more photos, learning from my mistakes, and refining my craft.In conclusion, my journey to improve my photography skills has been a rewarding one. It has taught me to see the world through a different lens, to appreciate the beauty in everyday moments, and to express myself creatively. While I have come a long way, I know that there is still much to learn. Photography is an art form that evolves with time, and I am excited to continue this journey, capturing memories and telling stories through my lens.。

视力下降的原因并且提出建议英语作文

视力下降的原因并且提出建议英语作文全文共3篇示例，供读者参考篇1Causes and Recommendations for Vision ImpairmentIntroductionVision impairment is a common health issue that affects people of all ages. It can be caused by a variety of factors, both genetic and environmental. In this article, we will discuss the primary reasons why vision can deteriorate and offer some practical recommendations on how to prevent and manage vision impairment.Causes of Vision Impairment1. AgeOne of the most common reasons for vision impairment is aging. As we grow older, the lenses in our eyes become less flexible, making it harder to focus on close objects. This condition, known as presbyopia, affects nearly everyone over the age of 40. Additionally, older adults are more prone todeveloping age-related eye diseases such as cataracts and glaucoma, which can cause further vision loss.2. GeneticsGenetics play a significant role in determining our eye health. Some people may inherit genes that make them more susceptible to conditions like myopia (nearsightedness) or astigmatism. While we cannot change our genetic makeup, understanding our family history can help us take proactive steps to maintain our eye health.3. Lifestyle FactorsSeveral lifestyle factors can contribute to vision impairment. Excessive screen time, poor lighting, and lack of eye protection can strain our eyes and lead to conditions like computer vision syndrome or dry eye. Smoking, poor nutrition, and high alcohol consumption can also increase the risk of developing eye diseases such as macular degeneration and diabetic retinopathy.4. Environmental FactorsEnvironmental factors like UV radiation and air pollution can damage the delicate structures of the eye. Prolonged exposure to sunlight without proper protection can increase the risk of developing cataracts and age-related macular degeneration.Indoor air pollution from smoke, dust, and chemicals can irritate the eyes and cause inflammation or infection.Recommendations for Preventing Vision Impairment1. Get Regular Eye ExamsRegular eye exams are essential for detecting vision problems early and preventing further damage. Adults should have a comprehensive eye exam every 1-2 years, while children and seniors may need more frequent screenings. An eye care professional can assess your eye health, prescribe corrective lenses if needed, and recommend preventive measures to maintain good vision.2. Follow a Healthy DietA balanced diet rich in fruits and vegetables can provide essential nutrients that support eye health. Foods high in antioxidants, vitamins A, C, and E, and omega-3 fatty acids can help protect against age-related eye diseases. Some eye-friendly foods include leafy greens, citrus fruits, nuts, fish, and eggs. Drinking plenty of water and limiting sugary and processed foods can also benefit your eye health.3. Protect Your EyesWearing sunglasses with UV protection can shield your eyes from harmful sun rays and reduce the risk of developing cataracts or macular degeneration. Safety glasses or goggles are recommended when working in hazardous environments or participating in sports activities. Remember to take breaks from digital devices, adjust the lighting in your workspace, and blink regularly to prevent eye strain.4. Quit Smoking and Limit Alcohol IntakeSmoking is a major risk factor for several eye diseases, including cataracts, macular degeneration, and diabetic retinopathy. If you smoke, consider quitting or seeking help to kick the habit. Limiting alcohol consumption can also benefit your eye health, as excessive drinking can impair your vision and increase the risk of developing eye conditions.5. Practice Good HygieneMaintaining good hygiene habits can prevent eye infections and reduce the risk of inflammation or irritation. Wash your hands frequently, avoid touching your eyes with dirty hands, and clean your contact lenses properly to prevent contamination. If you wear makeup, replace old products regularly, and avoid sharing cosmetics with others to prevent bacterial or viral transmission.ConclusionVision impairment can have a significant impact on our daily lives, affecting our ability to work, learn, and enjoy activities. By understanding the causes of vision deterioration and following preventive measures, we can protect our eyes and maintain good vision throughout our lives. Remember to prioritize your eye health, schedule regular check-ups, and make healthy lifestyle choices to safeguard your precious gift of sight.篇2Causes of Decreased Vision and SuggestionsIntroductionWith the increasing prevalence of digital devices and the aging population, more and more people are suffering from decreased vision. There are various factors that can contribute to vision loss, including age, genetics, lifestyle choices, and underlying medical conditions. In this article, we will explore some of the common causes of decreased vision and provide suggestions on how to prevent or slow down its progression.Causes of Decreased Vision1. Age-related Macular Degeneration (AMD): AMD is a common eye condition that affects the macula, the central part of the retina responsible for sharp central vision. As people age, the macula gradually deteriorates, leading to central vision loss. Factors such as genetics, smoking, and poor diet can increase the risk of developing AMD.2. Cataracts: Cataracts are a clouding of the lens in the eye, which can cause blurry vision and difficulty seeing in low light. Age, diabetes, smoking, and excessive sunlight exposure are common risk factors for cataracts.3. Glaucoma: Glaucoma is a group of eye conditions that damage the optic nerve, leading to peripheral vision loss. Increased intraocular pressure is often a key factor in the development of glaucoma. Genetics, age, and certain medical conditions can also contribute to the risk of glaucoma.4. Diabetic Retinopathy: Diabetic retinopathy is a complication of diabetes that affects the blood vessels in the retina. High blood sugar levels can damage the blood vessels, leading to vision loss. Poorly controlled diabetes and high blood pressure are significant risk factors for diabetic retinopathy.5. Refractive Errors: Refractive errors such as myopia (nearsightedness), hyperopia (farsightedness), and astigmatismcan cause blurry vision and difficulty focusing. These errors can be corrected with glasses, contact lenses, or refractive surgery.Suggestions for Preventing Decreased Vision1. Get regular eye exams: Routine eye exams are essential for maintaining healthy vision and detecting eye conditions early. Adults should have an eye exam at least every two years, and more frequently if they have risk factors for eye diseases.2. Eat a healthy diet: A diet rich in fruits, vegetables, and omega-3 fatty acids can help protect the eyes from age-related macular degeneration and other eye conditions. Foods such as leafy greens, fish, nuts, and citrus fruits are beneficial for eye health.3. Protect your eyes from sunlight: UV radiation can damage the eyes and increase the risk of cataracts and macular degeneration. Wearing sunglasses with UV protection and a wide-brimmed hat can help protect the eyes from harmful sun exposure.4. Quit smoking: Smoking is a major risk factor for many eye diseases, including cataracts, macular degeneration, and diabetic retinopathy. Quitting smoking can significantly reduce the risk of vision loss and improve overall eye health.5. Manage underlying medical conditions: Conditions such as diabetes, high blood pressure, and high cholesterol can affect the eyes and lead to vision loss. It is essential to manage these conditions through medication, lifestyle changes, and regular medical check-ups.ConclusionDecreased vision can have a significant impact on a person's quality of life, making it essential to take proactive steps to protect the eyes and maintain healthy vision. By following the suggestions outlined in this article, individuals can reduce their risk of developing common eye conditions and preserve their eyesight for years to come. Remember, it is never too late to start taking care of your eyes – your future self will thank you for it.篇3Reasons for Decreased Eyesight and Suggestions for ImprovementIntroductionEyesight is one of the most important senses, allowing us to see and interact with the world around us. However, many people experience a decline in their vision as they age or due to various factors. In this article, we will discuss the reasons fordecreased eyesight and provide suggestions for improving and maintaining good vision.Reasons for Decreased EyesightThere are several factors that can contribute to a decline in eyesight, including:1. Age-related changes: As we age, the lens of the eye becomes less flexible, leading to difficulty focusing on close objects. This condition, known as presbyopia, is a common reason for decreased eyesight in older adults.2. Refractive errors: Refractive errors such as nearsightedness, farsightedness, and astigmatism can cause blurry vision and difficulty seeing objects at a distance. These conditions can usually be corrected with glasses, contact lenses, or surgery.3. Eye diseases: Conditions such as glaucoma, cataracts, and macular degeneration can cause permanent damage to the eyes if left untreated. Regular eye exams are important for early detection and treatment of these conditions.4. Lifestyle factors: Poor diet, lack of exercise, and excessive screen time can all contribute to decreased eyesight. A healthylifestyle that includes a balanced diet, regular exercise, and breaks from screen time can help prevent vision problems.Suggestions for ImprovementTo improve and maintain good eyesight, consider the following suggestions:1. Get regular eye exams: Annual eye exams are essential for early detection of vision problems and eye diseases. A comprehensive eye exam can help identify issues that may be affecting your vision and allow for timely treatment.2. Eat a healthy diet: A diet rich in fruits and vegetables, particularly those high in vitamins A, C, and E, can help support eye health. Foods such as carrots, spinach, and berries are especially beneficial for maintaining good vision.3. Take breaks from screen time: Staring at a screen for extended periods can strain the eyes and contribute to decreased eyesight. Take regular breaks to rest your eyes and reduce the risk of eye strain.4. Wear protective eyewear: When engaging in activities that can pose a risk to your eyes, such as sports or DIY projects, be sure to wear protective eyewear. This can help prevent injuries and maintain good vision.5. Maintain a healthy lifestyle: Regular exercise, adequate sleep, and avoiding smoking can all contribute to good eye health. These lifestyle factors can help reduce the risk of eye diseases and maintain optimal vision.ConclusionWhile decreased eyesight is a common issue that many people face, there are steps that can be taken to improve and maintain good vision. By getting regular eye exams, eating a healthy diet, taking breaks from screen time, wearing protective eyewear, and maintaining a healthy lifestyle, you can support your eye health and enjoy clear vision for years to come.。

提高我的摄像水平英语作文

Improving your photography skills is an exciting journey that can lead to a deeper appreciation of the world around you. Here are some tips to enhance your photography abilities:1. Understand Your Camera: Familiarize yourself with the settings and features of your camera. Read the manual and practice using different modes to understand how they affect your photos.2. Learn the Basics of Composition: Composition is key in photography. Learn about the rule of thirds, leading lines, and framing to create more engaging images.3. Play with Lighting: Light is the most important element in photography. Experiment with natural light, golden hour, and different types of artificial lighting to see how they change the mood and quality of your photos.4. Experiment with Perspectives: Try shooting from different angles and heights to create unique perspectives. Get low to the ground, or shoot from above to see how it changes the composition.5. Master the Exposure Triangle: Learn how aperture, shutter speed, and ISO work together to control the exposure in your photos. Understanding this will give you more control over the final look of your images.6. Practice Regularly: The more you shoot, the better youll get. Take your camera with you everywhere and practice in different situations and lighting conditions.7. Study the Work of Others: Look at the work of professional photographers and try to understand what makes their photos successful. This can give you ideas and inspiration for your own work.8. Edit Your Photos: Learn how to use photo editing software to enhance your images. Adjusting the contrast, saturation, and sharpness can make a big difference in the final product.9. Take a Class or Workshop: There are many resources available, from online courses to inperson workshops, that can help you learn new techniques and get feedback on your work.10. Seek Feedback: Share your photos with others and ask for constructive criticism. This can help you identify areas for improvement and give you new ideas to explore.11. Challenge Yourself: Set personal projects or participate in photography challenges to push your boundaries and try new techniques.12. Stay Updated with Trends: Photography is an evolving art form. Keep up with the latest trends and technologies to stay inspired and relevant.13. Invest in Good Equipment: While the best camera is the one you have with you, investing in a good quality camera and lenses can significantly improve your photography.14. Patience: Great shots often require patience. Wait for the right moment, whether its for the perfect light or the right subject to enter your frame.15. Enjoy the Process: Most importantly, enjoy the process of learning and improving. Photography is a creative outlet, so have fun with it and let your passion show in your work.By following these tips and dedicating time to practice and learn, you can significantly improve your photography skills and create images that you are proud of.。

Photo Catalytic Degradation of Imidachloprid Under Solar Light

L. Gomathi Devi (&) Á B. Narasimha Murhty Á S. Girish Kumar Department of Post Graduate Studies in Chemistry, Bangalore University, Central College City Campus, Bangalore 560001, India e-mail: gomatidevi_naik@yahoo.co.in
Abstract Anatase TiO2 was doped with metal ions like Th4?, V5? and Mo6? and tested for the degradation of imidachloprid under solar light. X-ray diffraction results inferred that all the dopants stabilized the anatase phase irrespective of their nature, oxidation state and ionic size. The undoped and transition metal ion doped TiO2 were completely transformed to rutile phase at 700 °C while rare earth Th4? doped sample completely transformed to rutile phase at 1,000 °C. The rare earth dopant stabilized the anatase phase by hindering the growth of crystallite size. Among the photo catalysts used, Th4? (0.06%)-TiO2 showed highest activity and its efﬁciency was 2.8 times higher than that of Degussa P-25. The Th4? ion lowered the band gap of TiO2 to 2.6 and 2.5 eV facilitating solar light absorption. Detrapping of the trapped charge carriers depends on electronic conﬁguration and the oxidation state of the dopants. Keywords Stabilization of anatase phase in TiO2 Á Th4? doping Á V5? and Mo6? doping Á Imidachloprid Á Photodegradation under solar light

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Improvement of Photocatalytic Degradation Activity of Visible-Light-Responsive TiO2by Aid of Ultraviolet-Light PretreatmentTaizo Sano,*,†Eric Puzenat,‡Chantal Guillard,‡Christaphe Geantet,‡Sadao Matsuzawa,†andNobuaki Negishi†Research Institute for En V ironmental Management Technology,National Institute of Ad V anced Industrial Scienceand Technology(AIST),16-1Onogawa,Tsukuba,Ibaraki305-8569,Japan,and Institut de recherches sur lacatalyse et l’en V ironnement de Lyon(IRCELYON),2A V enue Albert Einstein,Villeurbanne Cedex69626,FranceRecei V ed:September10,2008;Re V ised Manuscript Recei V ed:January30,2009The photocatalytic degradation activity of visible-light-responsive TiO2,which was developed as oxygen-deﬁcient TiO2,was improved by pretreatment with UV-light irradiation in the presence of oxygen and adsorbedwater.The C2H2degradation rate under visible light was increased up to twice by the UV pretreatment.During the UV pretreatment,peroxo species was produced on the surface,and the visible-light absorbance ofthe photocatalyst between400and500nm was increased.The modiﬁcation of the surface accompanied bythe formation of peroxo species seems to enhance the photocatalytic oxidation of C2H2.The increased activitywas gradually diminished within24h under dark condition,suggesting that the modiﬁed surface is metastableand is reverted to the original surface.IntroductionTitanium dioxide(TiO2)has been getting attention as a photocatalyst that can be utilized for the oxidative removal of environmental pollutants,such as NOx and VOC,under ultraviolet(UV)irradiation(λ<380nm).1-3However,the activity of TiO2in a closed environment is often low since UV light,which is essential for activating pure TiO2,is insufﬁcient. Therefore,many researchers are making efforts to develop visible-light-responsive photocatalysts.The major part of indoor light is visible light,and therefore an efﬁcient utilization of visible light will improve the degradation rate of the pollutants. Because TiO2has many advantages against other photocatalysts, such as high photostability,high resistance for acid and base, and low price,the modiﬁcation of TiO2is an attractive method to develop a visible-light-responsive photocatalyst. Recently,the replacement of the oxide ion of the TiO2lattice with a nitrogen atom or an oxygen defect has been studied to improve the visible-light response.4-9Nitrogen-doped TiO2(N-TiO2)is prepared by reduction with gaseous NH3,spattering, or hydrolysis of titanium salt by ammonia solution.4,5We also succeeded in the preparation of different types of N-doped TiO2 from the titanium-bipyridine complex(TBC).6The newly prepared TiO2oxidized NO under visible light with longer wavelengths(λ<650nm)and the oxidation rate at wavelengths above450nm was more than10times that by conventional N-TiO2.However,the photocatalyst derived from TBC did not degrade acetaldehyde,while conventional N-TiO2decomposed acetaldehyde to CO2under visible light.The presence of a carbon atom in the anatase lattice is proposed to explain the different response to visible light from the conventional N-TiO2. The H2-plasma treatment on an Ultraﬁne particle of TiO2 caused the creation of oxygen defects in the anatase lattice,and formed oxygen-deﬁcient TiO2(TiO2-δ)oxidized NO under visible light up to ca.600nm.7Ihara et al.8reported the synthesis of oxygen-deﬁcient TiO2in the aqueous phase.They concluded that oxygen-deﬁcient sites formed in grain boundaries are important to develop the visible-light responsibility and that the nitrogen atom doped in a portion of the oxygen defects worked as a blocker for reoxidation.BA-PW25(Ecodevice Co.)is an anatase-type photocatalyst commercialized as an oxygen-deﬁcient TiO2.BA-PW25is prepared by the calcinations of titanium hydroxide precursor made from titanium(IV)sulfate with ammonia solution.9This preparation method is quite similar to that described in the report by Ihara et.al.8BA-PW25degrades2-propanol and acetone into CO2under blue LED light,and the visible-light response was attributed to the oxygen deﬁciency of TiO2.We have studied the photocatalytic degradation of C2H2,10,11 since the oxidation of C2H2is much more difﬁcult than that of acetaldehyde and therefore it reveals the oxidative power of each photocatalyst.Under visible light(λ>400nm),BA-PW25 decomposed C2H2into CO2while N-TiO2and conventional TiO2 (P25,Degssa)did not degrade C2H2.10The formation of formic acid on the surface of BA-PW25was transiently observed,and the formic acid was decomposed into CO2by further visible-light irradiation(eq1).It is inferred that the active species effective for C2H2degrada-tion is formed on the midgap level of BA-PW25between the top of the valence band of conventional TiO2and the N-induced midgap level.The UV-vis absorption edge of BA-PW25 located at a slightly longer wavelength(400nm)than that of N-TiO2prepared with gaseous NH3treatment of TiO2,although XRD,XPS,FT-IR,and TG did not show any clear evidence for the presence of oxygen deﬁciency or any special structures in the bulk.Therefore,it is inferred that the novel degradation activity of BA-PW25is due to the anomalous surface morphol-ogy as mentioned in the previous report.8*To whom correspondence should be addressed.E-mail:sano-t@ aist.go.jp.†National Institute of Advanced Industrial Science and Technology (AIST).‡Institut de recherches sur la catalyse et l’environnement de Lyon (IRCELYON).C2H2f(HCOOH)f CO2+H2O(1)J.Phys.Chem.C2009,113,5535–5540553510.1021/jp808032y CCC:$40.75 2009American Chemical SocietyPublished on Web03/18/2009BA-PW25showed a novel phenomenon in our recent experiments.It was found that the photocatalytic degradation rate of C 2H 2or acetaldehyde with visible light was improved by UV irradiation (UV pretreatment)to the BA-PW25before the degradation experiment.If the conventional TiO 2was used,this phenomenon can be hardly observed since the conventional TiO 2does not degrade C 2H 2or acetaldehyde under visible light in the ﬁrst place.Thus,BA-PW25showed the novel photo-catalytic activities;however,the mechanisms of visible-light responsibility of BA-PW25have not been well-understood.In this research,we studied the effect of UV pretreatment on the C 2H 2degradation activity of visible-light-responsive TiO 2(BA-PW25),and developed an understanding of its novel characteristics.We also analyzed the roles of the adsorbed water on the photocatalyst,the atmosphere,and peroxo species formed on the surface during the UV pretreatment.Experimental SectionThe degradation of C 2H 2was analyzed with a batch reactor.10The typical experimental procedure with UV pretreatment was simply shown in Figure 1.A 40mg portion of visible-light-responsive photocatalyst (BA-PW25(Ecodevice Co.))was suspended in 1cm 3of distilled water and the suspension was dispersed on a glass plate (9.0cm 2),followed by drying in air at 50or 110°C for 24h.The samples dried at 50and 110°C were denoted as S50and S110,respectively.The glass plate with photocatalyst was placed in a reactor (320cm 3)with a cover window of Pyrex glass.The temperature of the reactor was maintained at 15°C by cooling water.A light source (high-pressure Hg lamp (Philips HPK-125W))was placed on the upper side of the reactor.A water cell was placed between the reactor and the light source to absorb infrared light.When only visible light is used for irradiation,a UV-cut ﬁlter (Sumitomo kagaku,LF-41)was placed under the water cell.The transmit-tance of LF-41at 380,400,and >430nm was 0.05%,1%,and 90%,respectively.In the UV preatment of photocatalyst,UV light was irradiated to the photocatalyst sample for 30min to 3h while synthetic air was passed through the reactor.The UV intensity was 3.4mW/cm 2(2.1×10-8mol/cm 2/s (300<λ<380nm)).When the activity without UV pretreatment was evaluated,synthetic air was passed for 30min in darkness.After the pretreatment,the reactor was closed and then the required amount of C 2H 2gas (Air Liquide)was introduced.The reaction atmosphere was vigorously stirred by a magnetic stirrer to make the atmosphere homogeneous.Any decreases in the C 2H 2concentration by adsorption were not observed in all the experiments in this study.The degradation of C 2H 2was performed by irradiating visible light.The photon ﬂux at the sample surface was 3.1×10-8mol/cm 2/s (380<λ<600nm).The UV intensity was less than the detection limit (1µW/cm 2)of the UV analyzer (Vilber Lourmat,VLX-3W with CX365).The concentrations of C 2H 2and CO 2were analyzed by gas chromatographs with a ﬂameionization detector (Intersmat:IGC120FL)and a thermal conductivity detector.The solid phases were mainly analyzed with UV -vis diffuse reﬂectance spectroscopy (Perkin-Elmer,Lambda 45).H 2O 2was analyzed with a colorimetric method,using N ,N -diethyl-p -phenylenediamine (DPD)and horseradish peroxidaze (POD).12,13A 3cm 3sample solution was fed into the quartz cell,and solutions of DPD (10g/dm 3,0.030cm 3)and POD (1g/dm 3,0.030cm 3)were added in sequence with vigorous mixing.The absorbance at 551nm was measured with a UV -vis -NIR spectrometer (Shimadzu UV-3600)after 30s of addition of POD.Results and DiscussionFigure 2shows the time courses of C 2H 2concentration in the reactor,in which the visible-light-responsible photocatalyst (S50)was placed.The C 2H 2concentration decreased while the photocatalyst was irradiated with visible light.When the C 2H 2concentration became lower than ca.30ppm,the reaction atmosphere was replaced with synthetic air,and then the degradation of C 2H 2was repeatedly performed.Before the ﬁrst,second,and sixth runs,the photocatalyst was irradiated with UV light for 1h (UV pretreatment),and the degradation with visible light was observed.In the third -ﬁfth runs,the C 2H 2degradation was performed without UV pretreatment.It is obvious that the C 2H 2concentration decreased rapidly after UV pretreatment.This suggests that the characteristics of the sample were changed by absorbing UV light and that the sample became more active for C 2H 2degradation.Meanwhile,the UV pretreat-ment was not essential for the degradation of C 2H 2since the sample repeatedly degraded C 2H 2at a constant rate without UV pretreatment.The effect of UV pretreatment was further analyzed.Figure 3a shows the decrease in the C 2H 2concentration under visible light with the photocatalyst dried at 50°C (S50).S50was irradiated with UV light or visible light before C 2H 2degradation,and the degradation rates were compared.The degradation rate by the samples after UV pretreatment for 1h was signiﬁcantly larger than that by the sample without light pretreatment.Furthermore,the UV pretreatment for 17h provided a higher degradation rate than the treatment for 1h.On the other hand,the pretreatment with visible light did not change the degradation rate.Thus,the C 2H 2degradation activity of S50was improved by the UV pretreatment.Next,the effect of UV pretreatment on the photocatalyst dried at 110°C (S110)wasanalyzedFigure 1.Experimental procedure for C 2H 2degradation with visiblelight.Figure 2.Degradation of C 2H 2by visible-light-responsive photocatalyst (BA-PW25)under visible light (λ>400nm).Before the ﬁrst,second,and sixth run,UV light was irradiated to the photocatalyst for 1h.5536J.Phys.Chem.C,Vol.113,No.14,2009Sano et al.(Figure 3b).The degradation rate by S110was higher than that by S50without UV pretreatment.This suggests that the dried surface is favorable for the degradation of C 2H 2.In the case of S110,the rates for decrease in C 2H 2concentration were almost the same regardless of the pretreatments,and the activity was not improved by the UV pretreatment.The difference between the effects of UV pretreatment on the S50and S110suggests that adsorbed water has an important role for improving photocatalytic activity by UV pretreatment.Further discussions are given later.Figure 4shows the relation between the initial rate of C 2H 2degradation by S50and UV pretreatment time.The degradation rate increased signiﬁcantly with the UV pretreatment time between 0and 3h,and the effect of UV pretreatment was saturated.The maximum degradation rate was approximatelytwice that without UV pretreatment.Also,the samples that were UV irradiated for more than 1h exhibited higher activities than S110.The improved activity of S50by UV pretreatment was gradually diminished by keeping the sample without light in dry air (Figure 5).The degradation rate of the sample after 24h from the UV pretreatment was equal to that before UV pretreatment.This suggests that the condition of photocatalyst was reverted to the original condition and that the improved activity is due to a reversible reaction.The lifetime of the improved activity was much longer than those of radicals formed by TiO 2(e.g.,O 2-•∼200s,OH •∼1s),13and the time scale is relatively close to the superhydrophilicity.14The photocatalyst (S50)after 3min from UV pretreatment for 2h was contacted with C 2H 2in a dark condition.The concentration of C 2H 2did not change.This indicates that an active species that can directly oxidize C 2H 2was not present on the photocatalyst after 3min from the UV pretreatment.It is inferred that the surface of photocatalyst was modiﬁed by UV-light absorption and that a metastable surface structure,whose lifetime is several hours,was formed.The metastable structure possibly enhances the production of active species for C 2H 2degradation with visible light.In the UV -vis diffuse reﬂectance spectra of S50and S110,the absorption shoulder was observed in the visible light region between 400and 550nm neighboring the UV absorption (Figure 6).The absorbance (∝Kubelka -Munk function,if the scattering coefﬁcient is identical)of S50between 400and 500nm was increased signiﬁcantly by the UV irradiation in air for 1h.The visible-light irradiation on S50also increased the absorbance in the similar wavelength range.The irradiations increased the absorbance in the visible-light region by approximately 1.5times.Since the increment did not coincide with the increase in the degradation rate (ca.2times for UV pretreatment and 0times for visible pretreatment),the increase in the degradation rate did not directly depend on the increase in the number of photons absorbed.Remarkably,the spectrum of S110was not changed by the UV irradiation.This is similar to the result that the activity of S110was not changed by UV pretreatment while that of S50was improved.Since the effect of UV pretreatment on the samples dried at 50°C (S50)and 110°C (S110)was different,the characteristics of the BA-PW25photocatalyst should be changed during heating the sample from 50to 110°C.Figure 7shows the TG-DTA curves of S50.The rapid weight loss with the endothermic process was observed between 50and 100°C.Then,the weight of sample decreased slowly above 100°C.NeithersigniﬁcantFigure 3.Degradation of C 2H 2under visible light by the samples dried at 50(a)and 110°C(b).Figure 4.Increase in degradation rate of C 2H 2with UV-pretreatmenttime.Figure 5.Decay curve of C 2H 2degradation rate with keeping time in darkness.The degradation rate was obtained under visible light.Improvement of Photocatalytic Degradation Activity J.Phys.Chem.C,Vol.113,No.14,20095537increase in the weight nor exothermic process was observed.The weight loss between 50and 100°C is due to the release of adsorbed water in layer II,which was proposed by Nosaka et al.15The adsorbed water in layer II is less mobile and harder to volatilize than the water in the outermost layer (layer III),and is weakly stabilized compared with the water in the closest layer to the solid surface (layer I).Water in layers III and II is released below 50and 100°C,respectively,and that in layer I is not released at around 100°C.Therefore,S50contained water in layers I and II,and S110contained only water in layer I.Asdescribed above,the activity and the visible-light absorbance of S50are changed with UV pretreatment while those of S110were not changed.Therefore,the adsorbed water in layer II seems to be responsible for the improvement of the C 2H 2degradation activity and the visible-light absorbance with UV pretreatment.The effect of adsorbed water was further conﬁrmed by the rehydration experiment.A small amount of distilled water was added to S110powder at room temperature and then the powder was dried at 50°C.The C 2H 2degradation rate by the rehydrated powder was almost the same as that by S50.Additionally,the increments in the degradation rate by UV pretreatment on the rehydrated powder and S50were similar (Figure 8).These results support the speculation that the water in layer II is essential for the improvement of the activity with UV pretreat-ment.Furthermore,it is inferred that the drying process between 50and 110°C is not an irreversible process like a change in the bulk structure but a reversible process around the neighbor-hood of the surface.Oxygen was essential for the improvement of C 2H 2degrada-tion activity by UV pretreatment.When the UV pretreatment on S50was performed in a nitrogen gas stream instead of an air stream,the degradation rate was almost the same as that without UV pretreatment (Figure 9).Also,the absorption spectrum of S50was not changed by UV irradiation in nitrogen (Figure 10).Generally,an oxygen molecule enhances photo-catalytic processes by accepting a conduction bandelectronFigure 6.UV -vis diffuse reﬂectance spectra of photocatalyst powder dried at 50°C (S50)or 110°C (S110)before and after UV (or visible light)irradiation for 1h.The Kubelka -Munk function is proportional to the absorption coefﬁcient if the scattering coefﬁcient isidentical.Figure 7.Thermogravimetry with differential thermal analysis (TG-DTA)on BA-PW25(S50)recorded in an air stream at a heating rate of 2deg/min.Figure 8.Effect of rehydration to S110on C 2H 2degradation.The photocatalyst dried at 110°C (S110)was rehydrated with a small amount of water,followed by drying at 50°C and UV irradiation for 3h.Degradation was performed under visiblelight.Figure 9.Effect of atmosphere during UV pretreatment on degradation of C 2H 2.The degradation of C 2H 2was observed under visible light in air.5538J.Phys.Chem.C,Vol.113,No.14,2009Sano et al.formed by UV-light absorption.It is considered that the lack of oxygen prohibited the photocatalytic process that improves the C 2H 2degradation activity and that increases the visible-light absorbance.The improvement of degradation activity is probably related to the increase in the visible light absorbance by UV pretreat-ment,since the factors necessary for improving the degradation activity were very similar to the factors for increasing visible light absorbance:oxygen,adsorbed water in layer II,and UV light were necessary.And these results indicate that the observed behaviors are due to the photoinduced reaction with adsorbed water and oxygen.However,there is one pretreatment condition that increased the visible-light absorbance but did not change the activity.The visible-light pretreatment increased the visible-light absorbance,whereas it did not change the degradation activity.Therefore,it is considered that the change in the visible-light absorbance is partially related to the improvement of the degradation activity.The participation of peroxo species was analyzed,since the production of hydrogen peroxide species is one of the important reactions with photocatalyst,water,and oxygen.2,16-18It is proposed that H 2O 2is produced photocatalytically as shown by the following equations.where adsorbed species and free species were not distinguished.Also,the formation of surface peroxo species,Ti -O -O -Tiand Ti -O -OH,is reported.18The formation mechanisms were estimated as follows:Since the adsorbed water and oxygen were essential for the improvement of the activity by UV pretreatment,there is a high possibility that the peroxo species are involved in the improve-ment of activity.On the basis of the above consideration,the production of peroxo species by BA-PW25was analyzed.A 50mg sample of photocatalyst was dispersed into Millipore water and coated on the Petri dish (26cm 2).The dish was dried at 50°C for 24h.The dried photocatalyst was irradiated with UV light (3.0mW/cm 2)or visible light (6×103Lx,ca.5mW/cm 2)passed through the UV cut ﬁlter for 1h in air (relative humidity 50%).After the irradiation was ﬁnished,10cm 3of sodium phosphate buffer (pH 6.0)was added to the Petri dish and the product was extracted with a shaker for 20min.After ﬁltration,the concentration of H 2O 2was analyzed by the colorimetric method with DPD and POD.In this procedure,chemical species that react with water to form H 2O 2are counted as H 2O 2.In other words,surface peroxo species,Ti -O -O -Ti and Ti -O -OH,could be detected as H 2O 2.For the photocata-lyst samples irradiated with UV and visible light,7.3×10-4and 5.0×10-4µmol/cm 2of H 2O 2were detected,respectively.Without light irradiation,H 2O 2was not produced.Thus,it was conﬁrmed that BA-PW25produced peroxo species on the surface by absorbing UV or visible light.Since the production was much smaller than the decomposed C 2H 2(ca.9×10-2µmol/cm 2in 2h),it seems that the peroxo species produced are not major reactants for the oxidation of C 2H 2.Alternatively it is possible that peroxo species modify the surface structure (or the band structure)of TiO 2to increase the photocatalytic efﬁciency with visible light.The following changes can be expected to increase the efﬁciency:increase in surface defect site,enhancement of charge separation,formation of midgap level,increase in the number of active sites or adsorption sites,etc.As shown in Figure 5,the improved activity diminished within 24h.This suggests that the modiﬁed surface is metastable and is gradually reverted to the original surface.At present,the time course of peroxo species has not been analyzed,and moreover,the extraction efﬁciency of peroxo species is uncer-tain.Further analyses of peroxo species are required to conclude the exact role of the peroxo species for the C 2H 2degradation.Next,the effect of supplementation of H 2O 2to the photo-catalyst was analyzed to study inﬂuences of peroxo species on the photocatalyst.A 4cm 3sample of H 2O 2solution (5mmol/dm 3)was added to 1.0g of S50powder,and then the sample was dried at 50°C for 24h.Figure 11shows the diffuse reﬂectance spectra of S50before and after H 2O 2treatment.The visible light absorbance between 400and 500nm was increased signiﬁcantly by the H 2O 2supplementation.The change was consistent with the increase in the visible light absorbance with UV pretreatment and visible pretreatment on S50(Figure 6).This result supports an assumption that the lightpretreatmentFigure 10.UV -vis diffuse reﬂectance spectra after UV treatment in air and nitrogen gas.TiO 2+h νf e -+h +(2)e -+O 2f O 2-·(3)h ++H 2O f OH ·+H +(4)O 2-·+H ++e -f HO 2-(5)2O 2-·+H +f HO 2-+O 2(6)HO 2-+H +f H 2O 2(7)OH ·+OH ·f H 2O 2(8)Ti s O s Ti ·+H 2O +h +f Ti s O ·+Ti s OH +H +(9)2[Ti s O ·+Ti s OH]f Ti s O s O s Ti +Ti s O s Ti +H 2O (10)Ti s O s O s Ti +H 2O f Ti s O s OH +Ti s OH (11)Improvement of Photocatalytic Degradation Activity J.Phys.Chem.C,Vol.113,No.14,20095539produced H 2O 2or other peroxo species which increase the visible-light absorbance as shown in Figure 12.However,the C 2H 2degradation rate was not improved by the visible-light pretreatment,although peroxo species were detected and the visible-light absorbance was increased.Therefore,the production of peroxo species is not a sufﬁcient factor for the improvement of C 2H 2degradation activity.We speculate that the kind of surface peroxo species formed with visible light is different from that with UV light and that only the latter is effective for the improvement of activity.Additionally,we infer that the peroxo species formed with visible-light pretreatment were also produced during the deg-radation of C 2H 2under visible light even without the pretreat-ment and therefore the formation of peroxo species with visible-light pretreatment was not beneﬁcial to improve the activity.In this stage,the peroxo species formed with UV and visible light have not been characterized yet except for their activities.Also,the speciﬁc surface structure of BA-PW25has not been well analyzed yet although it is strongly related to the formation of effective peroxo species since the degradation activity and the visible-light absorbance of conventional TiO 2(P25)were not changed by UV pretreatment.To characterize the effective peroxo species and the speciﬁc surface structure of BA-PW25,further analyses of the surface modiﬁed by UV pretreatment,such as in situ infrared or near-infrared spectroscopic,and electric microscopic techniques,are necessary.It is known that H 2O 2production is enhanced by organic substances as electron donors.2In this study,organic substances were not fed into the system during UV pretreatment.However,commercialized photocatalysts often contain organic impurities on the surface.It is possible that the formation of peroxo speciesin the UV pretreatment is affected by the organic impurities.This will be discussed elsewhere.ConclusionThe new aspects about UV pretreatment on the visible-light-responsive TiO 2(BA-PW25)were conﬁrmed.By UV pretreat-ment,in which UV light was irradiated to the photocatalyst in the presence of oxygen and the adsorbed water in layer II,the C 2H 2degradation activity under visible light was improved,and simultaneously,the visible-light absorbance between 400and 500nm was increased.These phenomena seem to be due to the metastable surface structure accompanied by the formation of peroxo species induced by UV-light absorption.It is considered that the peroxo species were not the major reactants for C 2H 2degradation but the formation of peroxo species improved the photocatalytic efﬁciency.Unfortunately,the mechanism of C 2H 2degradation by using the modiﬁed surface has not been clariﬁed yet.Further analyses of the modiﬁed surface using spectroscopic and microscopic techniques seem interesting future works to distinguish the effective peroxo species for improving the degradation activity.If a photocatalyst containing a lot of the effective peroxo species can be developed,visible light will be utilized more efﬁciently for the photoinduced degradation of organic pollutants.Ad-ditionally,the UV pretreatment would be an attractive method to improve the degradation activity of visible-light-responsive photocatalyst without chemical reagent.Acknowledgment.This work was partially supported by the ECSAW program (Environmental Catalysis for Sustaining Clean Air and Water)from 2005to 2006.We greatly appreciate Dr.J.M.Herrmann and all the friends of Institut de recherches sur la catalyse et l’environnement de Lyon (IRCELYON).References and Notes(1)Ibusuki,T.;Takeuchi,K.J.Mol.Catal.1994,88,93–102.(2)Hoffmann,M.R.;Martin,S.T.;Choi,W.Y.;Bahnemann,D.W.Chem.Re V .1995,95,69–96.(3)Sano,T.;Negishi,N.;Takeuchi,K.;Matsuzawa,S.Solar Energy 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