CdS量子点敏化TiO_2薄膜电极的制备和光电化学性能_英文_

任耀宇等：高温电解水蒸汽制氢关键材料研究进展· 1075 ·第39卷第7期CdS量子点敏化TiO2薄膜电极的制备和光电化学性能周婧1，赵高凌1，臧金鑫1，宋斌2，李红3，韩高荣1(1. 硅材料国家重点实验室，浙江大学材料科学与工程系，杭州 310027；2. 浙江大学理学院物理系，杭州 310027；3. 浙江大学图书馆，杭州 310027)摘要：用水热微乳法制备了结晶性良好的CdS量子点，并通过连接分子巯基乙酸将CdS量子点自组装到多孔TiO2薄膜电极上。

结果表明：CdS纳米颗粒的平均直径约为5nm，小于其量子尺寸；微乳体系为化学反应提供了良好的微型反应器，抑制了CdS晶粒的长大，而水热过程增强了CdS纳米颗粒的结晶性；由于CdS的可见光响应作用和CdS量子点的量子效应，CdS量子点敏化TiO2薄膜电极样品具有比纯TiO2薄膜电极样品更高的可见光响应和光电流。

讨论了CdS量子点通过连接分子组装到TiO2薄膜上的机理。

关键词：量子点；水热微乳法；连接分子；光电化学性能中图分类号：TQ132 文献标志码：A 文章编号：0454–5648(2011)07–1075–05Fabrication and Photoelectrochemical Performance of CdS Quantum DotsSensitized TiO2 Film ElectrodesZHOU Jing1，ZHAO Gaoling1，ZANG Jinxin1，SONG Bin2，LI Hong3，HAN Gaorong1(1. State Key Lab of Silicon Material, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027;2. Department of Physics, Zhejiang University, Hangzhou 310027;3. Zhejiang University Libraries, Hangzhou 310027, China) Abstract: The CdS quantum dots (QDs) material was prepared by a microemulsion-hydrothermal method. This material was self-assembled onto the porous TiO2 film electrode with the molecular linker mercaptoacetic acid. The results show that the average diameter of CdS nanoparticles is approximately 5nm, which is below the quantum size of CdS. The current microemulsion system provided a micro-reactor for the reaction, inhibiting the growth of CdS crystallite. The hydrothermal process enhanced the crystalliza-tion of CdS nanoparticles. The porous TiO2 film sensitized by the self-assembled microemulsion-hydrothermal method with the CdS QDs material gave a higher photoresponse in the visible region and the corresponding film electrode exhibited a higher photocurrent than that of a pure TiO2 film electrode. This was ascribed to both the visible light response of CdS and the quantum effect of CdS QDs. The mechanism of CdS QDs self-assembling onto the porous TiO2 film electrode with the intermediate mercaptoacetic acid was also discussed. Key words: cadmium sulfide quantum dot; microemulsion-hydrothermal method; link molecule; photoelectrochemical performanceDye-sensitized nanocrystalline TiO2 solar cells (DSCs) have attracted attentions since a pioneer work by Grätzel.[1–3] The energy conversion efficiency of DSCs made from nano-structured TiO2 films achieved is 11%.[4] However, the development and application of the DSCs are restricted due to the corresponding cost and low sta-bility of organic dye. One effective way to solve the problems is to replace the organic dye sensitizer by inor-ganic semiconductor quantum dots (QDs).[5–6] The band gap of the film photoelectrode can be tailored in a wide range by customizing the size of QDs when QDs are used as the sensitizers in solar cell devices. Furthermore, a greater production of quantum could be obtained due to the impact ionization,[7] which may bring a unique poten-tial capability of the QD-sensitized solar cells.The CdS with a direct band gap of 2.4eV is a typical semiconductor to sensitize TiO2 film electrodes. In order to obtain a CdS QDs sensitized TiO2 film electrode with a收稿日期：2011–01–11。

修改稿收到日期：2011–03–22。

基金项目：浙江省科研厅项目(Y200909120)资助。

第一作者：周婧(1985—)，女，博士研究生。

通信作者：赵高凌(1965—)，女，教授。

Received date:2011–01–11. Approved date: 2011–03–22.First author: ZHOU Jing (1985–), female, postgraduate student for doctor degree.E-mail: zhoujing0205@Correspondent author: ZHAO Gaoling (1965–), female, professor.E-mail: glzhao@第39卷第7期2011年7月硅酸盐学报JOURNAL OF THE CHINESE CERAMIC SOCIETYVol. 39，No. 7J u l y，2011硅酸盐学报· 1076 ·2011年high conversion efficiency, one of the important aspect is to fabricate the highly crystallized CdS QDs. Quantum effect dominates when the size of QDs approaches the excitonic Bohr radius. Another aspect is to achieve the well combined CdS QDs with TiO2 film. It was reported that the use of linker molecules between synthesized CdSe QDs and TiO2 film could increase the cell per-formance.[8]In this work, the following approaches were proposed to prepare CdS QDs sensitized TiO2 film electrodes, namely, (a) A hydrothermal method was used to synthe-size highly crystallized CdS QDs in a microemulsion system; and (b) In order to ensure the charge transfer between CdS QDs and TiO2 film, mercaptoacetic acid, which can combine CdS QDs with porous TiO2 film by chemical adsorption, was used as an intermediate to as-semble CdS QDs onto the TiO2 film electrode. The pho-toelectrochemical (PEC) properties of thus prepared CdS QDs sensitized TiO2 film electrode were investigated. A model of the CdS QDs self-assembling onto the porous TiO2 film electrode was proposed.1 Experimental procedure1.1 Preparation of the samplesAll the chemicals (Sinopharm Chemical Reagent Co. Ltd., China) were analytical pure. The CdS QDs were prepared by a microemulsion-hydrothermal method. A polyoxyethylenelaurylether of 3.1g was heated to melt in a water bath. Then, n-butyl alcohol of 10mL, cyclohex-ane of 25mL, Cd(NO3)2·4H2O of 0.62g, thiourea of 0.45 g and H2O of 2.7g were added under vigorously stirring, obtaining a microemulsion system. The solution was transferred to the autoclave and maintained at 180℃ for 12h. The products were then washed with distilled water and ethanol for three times and dried in a vacuum at 60℃ for 24h. TiO2 porous films were prepared.[9] In order to prepare the CdS QDs sensitized TiO2 film electrode, the TiO2 film was dipped into the ethanol solu-tion with mercaptoacetic acid of 7mol/L for 24h. Then, TiO2 film electrode connected with mercaptoacetic acid was washed with distilled water and ethanol. It was dipped into the CdS ethanol solution of 5mol/L for 24h. TiO2 film electrode sensitized by in-situ produced CdS QDs was also prepared for the comparison. The TiO2 film was dipped in the Cd(NO3)2·4H2O aqueous solution of 20 mmol/L, thiourea of 140mmol/L and ammonia of 230 mmol/L for 12h, and rinsed with ethanol. The samples of pure TiO2 film electrode, and CdS QDs sensitized TiO2 film electrode prepared in-situ and a microemulsion- hydrothermal method were named as Samples A, B and C, respectively.1.2 CharacterizationThe microstructure of samples were characterized by X-ray powder diffraction (XRD) using a D/max-RA model X-ray diffractometer (XRD) with a Cu Kα radiation. The morphology of the CdS QDs was observed by a JEM-100CX transmission electron microscope (TEM). The morphologies of the TiO2 porous film were observed by a FEI- SIRION field emission scanning electron mi-croscope (FESEM). The optical absorption spectra of the samples were measured by a Perkin-Lamba-20 ultravio-let–visible (UV–Vis) spectrophotometer.1.3 PEC measurementThe curves of current vs potential (I–V) were measured by a three-electrode cell consisting of the film electrode (Samples A, B and C, respectively), a Platonized Pt elec-trode and a saturated calomel electrode (SCE) as the working, counter and reference electrodes, respectively.A buffer solution with Na2B4O7 of 0.14mol/L, Na2SO4 of0.2mol/L and H2SO4 of 0.3mol/L was used as the sup-porting electrolyte. A 450W xenon lamp (Thermo Orial, model: 66921) was used as a light source. The LAND series battery test system was used to record the I–V curves.2 Results and discussion2.1 Formation of CdS QDs sensitized TiO2 filmelectrodes2.1.1 Highly crystallized CdS QDs Figure 1(a) shows the XRD pattern of the microemulsion-hydrothermally synthesized sample. All the diffraction peaks can be in-dexed as a hexagonal wurtzite CdS phase (JCPDS No. 41–1049). The average crystallite size of CdS nanocrys-tals was determined to be 5nm according to the Scherrer formula.Figure 1(b) shows the typical TEM image of the mi-croemulsion-hydrothermally synthesized CdS nanoparti-cles. Clearly, the average diameter of CdS nanoparticles is approximately 5nm. It was indicated that the well crystallized CdS QDs could be obtained by the microe- mulsion-hydrothermal method. This superior crystallinity can be ascribed to the high temperature and high pressure in hydrothermal process. The small size can be attributed to the microemulsion system.[10] The current microemul-sion system is composed of poly oxyethylene lauryl ether/ water/cyclohexane/butanol, in which the micro-reactors form. The micro-reactors could affect the growth of CdS crystallite.2.1.2 Porous TiO2 film Figure 2 shows the XRD pattern of the porous TiO2 film before sensitization. It is seen that the prominent peaks are indexed to an anatase phase (JCPDS No.21–1272). Meanwhile, a peak at about 30.4°was found to correspond to a brookite phase.The morphologies of the porous TiO2 film were ob-served by FESEM. Figure 3(a) shows the surface image of the porous TiO2 film. The film is porous in the pore size range from 10 to 25nm. The pores provide spaces enough for anchoring CdS QDs, which are approximately周婧等：CdS量子点敏化TiO2薄膜电极的制备和光电化学性能· 1077 ·第39卷第7期Fig.1 XRD pattern and typical TEM image of the CdS sample synthesized by microemulsion-hydrothermal methodFig.2 XRD pattern of the porous TiO2 film5nm in diameter. Figure 3(b) shows the cross-section of the porous TiO2 film, indicating that the typical film thickness is 2.5μm.2.1.3 CdS QDs sensitized TiO2 film electrodes Figure 4 shows the optical absorption spectra of samples A, B and C, respectively. sample A has the absorption edge at 380nm, and no absorption is detected in the visible re-gion. For both sample B and sample C, the optical ab-sorptions extend to the long wavelength region (aboutFig.3 FESEM images of surface and cross-section of the porous TiO2 filmFig.4 UV-Vis spectra of samples A, B, and C520nm), which is due to the visible light response of CdS QDs. Compared to sample B, sample C exhibits a stronger optical absorption in the visible region. This implies that an abundance of highly crystallized CdS QDs is assembled to the TiO2 film electrode by current self-assembly method.Figure 5 shows the schematic diagram of CdS QDs self-assembling onto the porous TiO2 film electrode. The硅酸盐学报· 1078 ·2011年Fig.5 Schematic diagram of the self-assembly process for CdS QDs anchoring onto the TiO2 film electrodeCdS QDs self-assembling onto the TiO2 film electrode can be ascribed to the effect of the molecular linker, i.e., mercaptoacetic acid, which is used as a stabilizer in the synthesis of CdS. When the TiO2 film is dipped into the mercaptoacetic acid solution, the carboxylate groups in mercaptoacetic acid react to the hydroxyl groups in TiO2. Therefore, the mercapto groups in mercaptoacetic acid anchor to the CdS QDs.[11] As a bridge, mercaptoacetic acid could favor the combination of the CdS QDs with the TiO2 film. This adsorption is a chemical process, so that the connection between the CdS QDs and the TiO2 film is stable. The pores in porous TiO2 film are larger than 5nm, which offer spaces enough for the adsorption of CdS QDs. The CdS QDs are anchored to the TiO2 film on the surface and in the pores. A higher optical absorp-tion in the visible region for sample C is due to the large amount of the adsorbed highly crystallized CdS QDs.2.2 PEC properties of CdS QDs sensitized TiO2film electrodesFigure 6 shows the curves of current vs potential for the film electrodes. The dark current of the samples is similar. The density of anodic photocurrent is in a in-creasing order. Sample C, i.e., the self-assembled CdS QDs sensitized TiO2 film electrode, exhibits the highest photocurrent, which is 3.5 times higher than that of pure TiO2 film electrode (sample A). Such an enhancement of the photocurrent response can be explained by two fac-tors. Firstly, it is due to the charge injection process in the TiO2-CdS system, as shown in Fig.7(a). Upon optical excitation of CdS, the photogenerated electrons are transferred to TiO2, and the holes accumulate in the CdS QDs. If the holes reach the surface and react to species in the electrolyte, the excited electrons left behind will dif-fuse to the back-contact where they are withdrawn as a photocurrent. The quick transport characteristics of elec-trons of CdS will decrease the recombination of photo-generated carriers and suppress the electron transfer to the electrolyte.[12] Moreover, it can be ascribed to the quantum effect of CdS QDs. Figure 7(b) shows the sche-matical process of impact ionization. Energy distributes discontinuously for semiconductor QDs. The quantum effect dominates when the size of nanoparticles ap-proaches the exitonic Bohr radius. When the size of the carriers in the semiconductor QDs confined by potential barriers is smaller than or equals to the excitonic Bohr radius, the hot carrier cooling rates could be reduced, and the rate of impact ionization could become competitive with the rate of carrier cooling.[7] So the energetic hot carriers can produce a second (or more) electron-hole pair through impact ionization, leading to an enhanced photocurrent. As a result, the higher photocurrent response is expected for both the visible light response of CdS and the quantum effect of CdS QDs.Furthermore, it is found that the photocurrent of sample C is 1.5 times higher than that of sample B (see Fig.6). The lower photocurrent response of sample B can be as-cribed to a lower crystallinity and a smaller amount of in-situ synthesized CdS QDs.Fig.6 Current-potential curves for the film electrodes of Samples A, B, and CFig.7 Process of charge injection and impact ionization for CdS QDs sensitized TiO2 film solar cell3 ConclusionsThe well crystallized CdS QDs were prepared by a microemulsion-hydrothermal method. The synthesized CdS QDs were self-assembled onto the porous TiO2 film via a linker molecule, i.e., mercaptoacetic acid. The re-sults showed that the porous TiO2 film electrode sensi-周婧等：CdS量子点敏化TiO2薄膜电极的制备和光电化学性能· 1079 ·第39卷第7期tized by self-assembled CdS QDs exhibited an enhanced photoresponse in the visible region and a higher photo-current, compared to the pure TiO2 film electrode. It was contributed to the visible light response of CdS and the quantum effect of the CdS QDs. Compared to the TiO2 film electrode sensitized by in-situ produced CdS QDs, the self-assembled CdS QDs sensitized TiO2 film electrode exhibited a higher photocurrent due to the better crystal-linity and the larger amount of CdS QDs. This approach to fabricate CdS QDs sensitized TiO2 film electrode could be promising in the development and application of the QD-sensitized solar cells.References:[1] O'REGAN B, GRÄTZEL M. A low-cost, high-efficiency solar cellbased on dye-sensitized colloidal TiO2 films [J]. Nature, 1991, 353: 737–740.[2] GRÄTZEL M. Photoelectrochemical cells [J]. Nature, 2001, 414: 338–344.[3] GRÄTZEL M. Solar energy conversion by dye-sensitized photovoltaiccells [J]. Inorg Chem, 2005, 44(20): 6841–6851.[4] NAZEERUDDIN K M, de ANGELIS F, FANTACCI S, et al. Com-bined experimental and DFT-TDDFT computational study of photo-electrochemical cell ruthenium sensitizers [J]. J Am Chem Soc, 2005, 127(48): 16835–16847.[5] VOGEL R, HOYER P, WELLER H. Quantum-sized PbS, CdS, Ag2S,Sb2S3, and Bi2S3 particles as sensitizers for various nanoporous wide- bandgap semiconductors [J]. J Phys Chem, 1994, 98(12): 3183–3188.[6] ROBEL I, SUBRAMANIAN V, KUNO M, et al. Quantum dot solarcells. Harvesting light energy with CdSe nanocrystals molecularly linked to mesoscopic TiO2 films [J]. J Am Chem Soc, 2006, 128(7): 2385–2393[7] NOZIK J. Quantum dot solar cells [J]. Physica E, 2002, 14(1–2):115–120.[8] BANG H J, KAMAT V P. Quantum dot sensitized solar cells: A tale oftwo semiconductor nanocrystals: CdSe and CdTe [J]. ACS Nano, 2009, 3(6): 1467–1476.[9] BARBE J, ARENDSE F, COMTE P, et al. Nanocrystalline titaniumoxide electrodes for photovoltaic applications [J]. J Am Ceram Soc, 1997, 80(12): 3157–3171.[10] ZANG Jinxin, ZHAO Gaoling, HAN Gaorong. Preparation of Cdsnanoparticles by hydrothermal method in microemulsion [J]. Chin J Inorg Chem, 2006, 22(5): 917–920.[11] GUIJARRO N, LANA VILLARREAL T, MORA SERO I, et al. CdSequantum dot-sensitized TiO2 electrodes: effect of quantum dot cover-age and mode of attachment [J]. J Phys Chem C, 2009, 113(10): 4208–4214.。