外文资料翻译---具有高灵敏度的甲醛气体传感器的制备及其气敏特性

1 外文资料翻译译文

具有高灵敏度的甲醛气体传感器的制备及其气敏特性相对甲醛混有氧化铬的氧化铟气体传感器特性已经研究过了。间接加热式气体传感器是用敏感材料进行制备的。最终的材料的状态和传感层的形态通过x射线衍射和扫描电子显微镜分别在焙烧前后观察到其特点。操作温度对传感器响应的影响氧化铬和氧化铟传感器的气体浓度特性的对比已经研究过了。结果表明，在低操作温度该传感器对于甲醛具有良好的反应性能，使他们成为甲醛气体检测最有希望的候选材料。

1介绍

作为一个重要的工业化学品，甲醛被应用于制造业，建筑板，胶合板和漆这样的材料。此外，它还是消费产品中一个中间添加物，如洗涤剂和肥皂。由于其杀菌性能也可用于药理学和药物中。然而，调查结果表明，因为它是挥发性有害化合物，所以甲醛会对人体造成许多损害。因此，需要一种有效的方法来监测甲醛进而进行气体环境测量与控制。制造气体传感器被认为是一个理想的监测气体的手段。我们目前的调查主要涉及与甲醛的检测。

虽然半导体金属氧化物气体传感器提供了对有毒气体或可燃性气体的安全检测，但是他们仍然有一定的局限性，如灵敏度，选择性，长期稳定性等等。为了克服半导体金属氧化物气体传感器的缺点，半导体金属氧化物的制备与掺杂的研究已经做过了。氧化铟是一个有希望的具有宽禁带的半导体材料（3.70电子伏特），其电子浓度主要取决于计量缺陷的浓度（如氧空位）就像其他金属氧化物半导体。就传感机制来说，颗粒的大小，缺陷，表面与界面的性能和化学计量学直接影响了传感器表面的氧化物种类的状态和数量，最后影响了金属氧化物传感器的性能。因此，为了提高并改善气体传感性能（敏感性，选择性，较好的热稳定性和较低的操作温度），氧化铟通常用于纳米结构形式或掺杂合适的贵金属和金属氧化物。作为一个单组分氧化物，由于其良好的灵敏度，氧化铟是一种很有前途的氧化性气体检测的候选者。因此，当其他金属氧化物掺杂氧化铟，对于不同的气体可调谐的气体灵敏度也不同。他们已经很好的研究了检测大部分重要气体的传感器材料，如乙醇，一氧化碳，二氧化氮，和氢气。然而，研究很少集中在甲醛传感器的材料特性。在本次调查中，用固态合成技术制备氧化镉和氧化铟的混合物，通过X射线衍射和扫描电镜图像来观察其特点。基于化镉和氧化铟的混合物的间接加热的气体传感器就被制备好了。甲醛传感器中混合物的特性也就确定了。

2实验

所有的来源于商业用于实验的化学试剂需要保证没有进一步提纯。根据我们的初步实验，氧化铟或氧化镉对于甲醛来说不具有良好的传感特性。氧化铟或氧化镉粉末是由碳酸盐和氧化铟制备的。不同阶段碳酸盐–氧化铟样品的成分已经研究过了。氧化镉：氧化铟=1 : 2.5重量比被认为是最有希望的甲醛气体的传感特性。混合蒸馏去离子水的碳

酸盐–氧化铟样品仔细研磨至约50-500纳米大小的颗粒。然后对样品分别以500，650，750和850摄氏度在空气中煅烧1个小时。

X射线衍射（X射线衍射, Rigaku D/MAX-3B粉末衍射仪）有着用于鉴定阶段目标的铜质物和K辐射（λ=1.54056a°），其中衍射X射线强度被记录为一个2θ。该样本是从10度到70°（2θ）以0.02°为一个单位进行扫描。根据Scherrer的公式：Rx = 0.9λ/(B cos θ),平均晶粒尺寸（接收）测量从X射线衍射峰以每分钟2°的速度进行扫描，λ是X射线的波长，θ是衍射角，B是真正的半个波峰宽度。扫描电子显微镜（扫描电镜）的照片是由荷兰的xesem-tmp获得的。

该间接加热传感器可以根据文献[13]进行制备。混合材料作为一个敏感的物质由带有金属电极和铂电线氧化铝管制备成的。镍–铬合金线缠绕在氧化铝管上被用作电阻。这个电阻需要确保有基本的加热和温度控制。这些元素在650摄氏度的空气中烧结1小时。焙烧后，基于氧化铟的敏感物质的厚度大约为0.6 毫米。为了提高其稳定性和重复性，气体传感器适宜在150摄氏度的空气中烧1小时。该传感器电阻通过在电压为5伏下的串联电阻相连接的传统电路进行测量。气体反应β被定义为在真空和气体中的电阻比。

在650摄氏度煅烧的氧化铬与氧化铟材料的传感性能优于在500，750，和850摄氏度进行煅烧的传感性能。在本文中，我们主要讨论在650摄氏度焙烧的材料。

X射线粉末衍射模式的准备和煅烧材料如图1所示。氧化铟和氧化铬峰值可以通过在650摄氏度情况下煅烧1小时的样品的外形进行观察。氧化铟和氧化铬的外形展现了一个很高的结晶度。400摄氏度碳酸盐分解，氧化铬形成。相氧化铟的状态没有改变，和其他的状态（例如，CdIn2O 4）在650度燃烧后并不能看出来。.另一方面，氧化铟的X射线衍射峰的宽度在煅烧前后并不改变，从中我们可以看出氧化铬能有效地抑制晶粒生长。根据雪莱的方程计算的晶粒平均尺寸28纳米，煅烧后氧化铟30纳米，氧化铬31纳米。

比较的结果是，扫描电镜图片显示了制备和煅烧样品中各种大小的粒子。大颗粒由小微晶组成。图2（a）和（b）分别显示了制备和焙烧过程中的扫描电镜图像。大多数粒子有不规则的形态，颗粒大小的范围是100–500纳米。

传感器的导电率取决于气体种类，同时也取决于暴露在测试性气体中的传感材料的操作温度，这些问题以经解决了。图3描述传感器的反应和操作温度之间的关系。操作温度对反应有重大影响。有趣的是，反应首先逐渐增加，然后随着操作温度的提高减少。可以看出，对于甲醛气体在低温范围内，基于氧化铬和氧化铟的传感器具有优异的气敏特性。在95摄氏度它展出了对甲醛气体最高的响应。较低的工作温度在应用中是一个优点。

如图4所示，响应的抵押–氧化铟基于传感器的氧化铬和氧化铟在95度操作时的

响应展示了对气体浓度的良好依赖性。该传感器对酒精和汽油有着非常小的反应，但对于甲醛气体有着较大响应。百万分之十的甲醛气体的反应超过了百万之八十的甲醛气体的响应。本反应是大大高于最近报道氧化锌和氧化铅，三氧化钨和氧化铅，镍，和基于甲醛气体的la0.68pb0.32feo3。这种气体传感器展现了对甲醛气体的较大反应和对酒精与汽油的较高选择性。这一结果表明，氧化铬和氧化铟是一个良好的检测甲醛气体的气敏材料，可用于监测和控制甲醛气体。

一个良好的反应和快速响应、恢复时间可以用这种传感器在最佳工作温度95摄氏度下进行观察。针对不同甲醛气体浓度(10–100 ppm)的器皿传感器如图5所示。作为一个高灵敏度的传感器，它可以测量非常低浓度，甚至百万分之一。随着甲醛气体浓度的增加输出电压的增加呈线性关系并且有较短的响应时间。响应时间和恢复时间（定义为达到最终平衡值90%）为2分钟，恢复时间为4分钟。

气敏机理是基于氧化铬和氧化铟材料的电导的变化。材料的表面对氧的吸收影响了氧化铬和氧化铟传感器的导电性。氧的吸附取决于颗粒大小，较大的材料面积，和合适的传感器操作温度。随着空气中温度的增加，氧的状态被吸附在氧化铬和氧化铟材料的表面的氧的状态在下面的反应中发生。氧从材料中捕获电子，导致了空穴浓度的增加和电子浓度的减少。当传感器接触甲醛气体时，被捕获的电子以吸附状态被释放，导致传感器电阻减小。因此，氧化铬和氧化铟传感器甲醛气体的减少是敏感的。

该传感器具有良好的稳定性（没有显示的数据）。稳定性机制更为复杂和进一步的工作是得到了一一个明确的认识。

4总结

通过固态合成技术氧化铬和氧化铟样本的制备甲醛探测的传感材料已被证明是可行的。制作好的传感器显示了很大程度的反应，高选择性，快速反应，和在低操作温度时良好的恢复性。实验结果表明了混有氧化铬的氧化铟气体传感器的材料潜力。

鸣谢

这项工作得到了中国国家自然科学基金会和中国云南省自然科学基金支持。

2外文译文

The fabrication and gas-sensing characteristics of the

formaldehyde gas sensors with high sensitivity

Abstract

Gas-sensing characteristics of CdO-mixed In2O3 to formaldehyde were investigated. Gas sensors of indirect heating type were fabricated by the sensitive materials. The phases in the resulting materials and the morphologies of the sensing layers were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively, before and after calcination. The effects of operating temperature on the sensor response and the response versus gas concentration properties of the CdO–In2O3 sensors were investigated. It was shown that the sensors exhibited good response properties to formaldehyde gas at low operating temperature, making them to be promising candidates for practical detectors to formaldehyde gas.

1. Introduction

As an important industrial chemical, formaldehyde is utilized in the manufacturing of building boards, plywood, and lacquer materials [1,2]. Moreover, it is an intermediate in consumer products, such as detergents and soaps, and also used in pharmacology and medicine because of its sterilization property. However, the investigated results showed that formaldehyde could cause many damages to the human body because it is a volatile and deleterious compound [3,4]. Therefore, effective methods to monitor formaldehyde have been demanded for atmospheric environmental measurement and control. The fabrication of gas sensors is thought to be a desirable means for monitoring the gases. Our present investigation mainly deals with the detection of formaldehyde.

Although gas sensors based on semiconductor metal oxides provide the safe detection of toxic or flammable gases, they still have some limitations and challenges such as sensitivity, selectivity, long-term stability, and so on. To overcome the disadvantages of semiconductor metal oxide gas sensors, the research on preparation and doping of semiconductor metal oxides had been done. Indium oxide is a promising semiconductor material with a wide band gap (3.70 eV), whose electron concentration is determined mainly by the concentration of stoichiometric defects (such as oxygen vacancy) like other metal oxide semiconductors.

In view of the sensing mechanism, the particle size, defects, the properties of surface and interface, and stoichiometry directly affect the state and amount of oxygen species on the surface of sensors, and consequently the performance of the metal oxide-based sensors.

Therefore, in order to enhance and improve the gas sensing performances (sensitivity, selectivity, good thermal stability, and lower operating temperature), In2O3 is usually prepared in a nanostructured form and/or doped with suitable noble metals and/or metal oxides [5–11].Asa single-component oxide, In2O3 is a promising candidate for the detection of oxidizing gases because of its good sensitivity [12]. Thus, when other metal oxides were doped into In2O3, the resulting materials have the potential for tunable sensitivity for different gases [6]. They have been well studied as the sensor material to detect most of the key gases, such as ethanol [5],CO [6,7],NO2 [7,8], and H2 [9]. Nevertheless, research has seldom been focused on the formaldehyde sensing properties of the material. In this investigation, the CdO-mixed In2O3 was prepared with solid-state synthesis technology and characterized by X-ray diffraction and SEM images. Gas sensors for indirect-heating based on CdO-In2O3 sensing materials were fabricated. The formaldehyde sensing properties of the mixed oxides were determined.

2. Experimental

All the chemical reagents used in the experiments were obtained from commercial sources as guaranteed-grade reagents and used without further purification. Based on our preliminary experiments, In2O3 or CdO does not have good sensing properties to formaldehyde. CdO-In2O3 powders were prepared from CdCO3 and In2O3. CdCO3–In2O3 samples with various phase compositions were studied. The Cd:In = 1:2.5 weight ratio was found to be the most promising for the sensing properties to formaldehyde gas. CdCO3–In2O3 samples mixed with distilled deionized water were round carefully to about 50–500 nm size, and then the samples were calcined at 500, 650, 750, and 850 .C,respectively, for 1 h in air. X-ray diffraction (XRD, Rigaku D/MAX-3B powder diffractometer) with a copper target and K. radiation (λ = 1.54056 °A) was used for the phase identification, where the diffracted X-ray intensities were recorded as a function of 2θ. The sample was scanned from 10. to 70. (2θ) in steps of 0.02.. The mean crystallite sizes (Rx) were measured from XRD peaks at a scan rate of 2./min based on the Scherrer’s equation: Rx = 0.9λ/(Bcos θ), where λis the wavelength of X-ray, θ is the diffraction angle, and B is the true half-peak width. Scanning electron microscopy (SEM) photographs were obtained by XL30ESEM-TMP, Holland.

The sensors of indirect heating were fabricated according to the literature [13]. The mixed material used as a sensitive body was fabricated on an alumina tube with Au electrodes and platinum wires. A Ni–Cr alloy wire crossing the alumina tube was used as a resistor. This resistor ensured both substrate heating and temperature control. The elements were sintered at

650 .C for 1 h in air. Thickness of the sensitive body based on In2O3 was approximately 0.6 mm after calcination. In order to improve their stability and repeatability, the gas sensors were aged at an operating temperature of 150 .C for 150 h in air. The sensor resistance was measured by using a conventional circuit in which the element was connected with an external resistor in series at a circuit voltage of 5 V. The gas response β was defined as the ratio of the electrical resistance in air (Ra)to that in gas(Rg).

3. Results and discussion

Sensing properties of CdO–In2O3 material calcined at 650 .C is better than those at 500, 750, and 850 .C. In this paper, we mainly discuss the material calcined at 650 .C. The X-ray powder diffraction patterns for the as-prepared and calcined materials are shown in Fig. 1. The peaks of In2O3 and CdO are observed in the pattern of the sample calcined at 650 .C for 1 h. The pattern is indexed as In2O3 (JCPDS No. 06-0416) and CdO (JCPDS No. 65-2908), and shows a high degree of crystallinity. CdCO3 will be decomposed and CdO be formed at400 .C [14]. The phase of In2O3 is not changed and other new phase (for example, CdIn2O4) is not observed after calcining at 650 .C. On the other hand, the width of XRD peaks of In2O3 Fig. 1. The XRD patterns of as-prepared and calcined materials: (a) CdCO3 (JCPDS No. 42-1342), (b) In2O3 (JCPDS No. 06-0416), and (c) CdO (JCPDS No. 65-2908). does not change before and after calcination, revealing that the CdO can effectively inhibit the crystalline grain growth [15]. The crystallite average sizes calculated according to Scherrer’s equation are about 28 nm for In2O3 before calcination, and about 30 nm for In2O3 and 31 nm for CdO after calcination. Comparing with the XRD results, SEM images revealed that there were various sizes of particles in the as-prepared and calcined samples. The large particles were composed of small crystallites. Fig. 2(a) and (b) shows SEM images of the as-prepared and calcined materials, respectively. Most particles have irregular morphology, and the particle size is in the range of 100–500 nm.

It has been addressed that the electrical conductivity of a sensor depends on the gas atmosphere, but also on the operating temperature of the sensing material exposed to the test gas [16]. Fig. 3 depicts the relation between the response and the operating temperature for the sensor. The operating temperature has a great influence on the response. Interestingly, the response first increases gradually and then decreases with increasing the operating temperature. It can be seen that the CdO–In2O3 based sensor shows excellent gas-sensing characteristics to formaldehyde gas in the low temperature range. It exhibited the highest response to formaldehyde gas at 95 .C.The low operating temperature is an advantage in application. As shown in Fig. 4, the response of the CdO–In2O3 based sensor operated at

95 .C shows good dependence on the gas concentration. The sensor exhibits very small response to alcohol and gasoline, but large response to formaldehyde gas. The response to 10 ppm formaldehyde gas is more than 80. The response is considerably larger than those recently reported for SnO2–Sb2O4,WO3–Sb2O4, NiO, and La0.68Pb0.32FeO3-based formaldehyde gas sensors [17–21]. This gas sensor showed large response to formaldehyde gas and high selectivity against alcohol and gasoline. This result indicates that the CdO–In2O3 specimen is a good gas-sensing material for detecting formaldehyde, which can be applied for monitoring and controlling of the formaldehyde gas.

A good response and quick response/recovery time were observed with this sensor at the optimal operating temperature of 95 .C. The response changes of the gas sensor to different formaldehyde gas concentrations (10–100 ppm) are shown in Fig. 5. As a highly sensitive sensor, it can measure very low concentrations, even 10 ppm. The output voltage increases in a linear relation to the formaldehyde gas concentration with a short response time. Response time and recovery time (defined as the time required to reach 90% of the final equilibrium value) was 2 min and the recovery time was 4 min.

The gas-sensing mechanism is based on the changes in conductance of the CdO–In2O3 material. The oxygen adsorbed on the surface of the material influences the conductance of the CdO–In2O3 sensor. The oxygen adsorption depends on the particle size, large specific area of the material, and the operating temperature of the sensor [22]. With the increasing temperature in air, the state of oxygen adsorbed on the surface of the CdO–In2O3 material undergoes the following Fig. 3. The effects of operating temperature on the sensor response to 100 ppm formaldehyde. increasing concentrations operated at an operating temperature of The oxygen species capture electrons from the material, leading to an increase in hole concentration and a decrease in electron concentration. When the sensor is exposed to formaldehyde gas, the electrons

trapped by the adsorptive states will be released, leading to

a decrease in sensor resistance. So, the CdO–In2O3 sensor is

sensitive to reducing formaldehyde gas.

The sensors have good stabilities (no shown data). The stability mechanism is more complicated and further work is to be done to get a definite understanding.

4. Conclusion

Preparation of the CdO–In2O3 specimen used as a sensing material to formaldehyde has

been shown to be feasible by the solid-state synthesis technologies. The fabricated sensor showed large response magnitude, high selectivity, quick responses, and good recovery to formaldehyde gas at a low operating temperature (95 .C). The experimental results indicate the potential of using CdO-mixed In2O3 material for formaldehyde gas sensing.

Acknowledgements

This work was supported by National Natural Science Foundation of China (No. 50662006), and Natural Science Foundation of Yunnan Province, China (No. 2006E0013M).