TiO2纳米片_巢状分级结构纳米阵列薄膜的制备及其在染料敏化太阳能电池中的应用.pdf

TiO 2 纳米片 /巢状分级结构纳米阵列薄膜的制备及其在染料敏化太阳能电池中的应用常萌蕾 1,2 李新军 1, *(1中国科学院可再生能源与天然气水合物重点实验室 , 中国科学院广州能源研究所 , 广州 510640;2 中国科学院研究生院 , 北京 100049)摘要 : 采用水热合成法在氟掺杂二氧化锡 (FTO) 导电玻璃基底上得到 TiO 2 纳米阵列薄膜 , 并进一步通过NaOH 溶液水热处理制备了由巢状纳米阵列及纳米片覆盖层构成的 TiO2 纳米阵列分级结构一体化薄膜 . 采用场发射扫描电镜 (FE-SEM), X 射线衍射 (XRD), 紫外 -可见 (UV-Vis) 漫反射光谱和吸收光谱技术对 TiO 2 薄膜的结构和性质进行表征 . FE-SEM 结果表明 : 分级结构 TiO 2 薄膜膜厚为 1.5 μ m, 薄膜由一层纳米片覆盖层 (约 0.2 μ m高 )和一层巢状纳米阵列层 (约 1.3 μ m 高 )组成 . XRD 谱图表明 TiO 2 薄膜为锐钛矿相 . UV-Vis 光谱显示分级结构TiO2 薄膜具有较强的光捕获能力和染料吸附能力 . TiO2 纳米片 /巢状分级结构纳米阵列薄膜作为光阳极 , 可有效地提高染料敏化太阳能电池的光电转换效率 , 其短路电流 (J sc)为 7.79 mA· cm- 2 , 开路电压 (Voc)为 0.80 V, 填充因子 (FF) 为 0.40, 光电转换效率 (η )为 2.48%, 其光电转换效率较 TiO 2纳米阵列薄膜提高了近 10 倍 .关键词 : TiO2 ; 纳米片 /巢状纳米阵列 ; 水热合成 ; 染料敏化太阳能电池 ; 光电转换中图分类号 : O649Fabrication of Nanosheet/Nestlike Nanoarray Hierarchical TiO 2 Film forDye-Sensitized Solar CellCHANG Meng-Lei 1,2 LI Xin-Jun 1, *(1Key Laboratory of Renewable Energy and Gas Hydrate, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences,Guangzhou 510640, P. R. China; 2Graduate University of ChineseAcademy of Sciences,Beijing 100049, P. R. China)Abstract: A hierarchical configuration of TiO 2 nanoarray film, comprising a nestlike TiO 2 nanoarray layerintegrated with a nanosheet network overlayer, was constructed. The hierarchical TiO 2 film was obtained bythe post-hydrothermal treatment with NaOH solution on hydrothermally synthesized TiO 2-derivednanostructured arrays grown on fluorine-doped tin oxide substrate (FTO). The TiO2 films were characterizedby field-emission scanning electron microscope (FE-SEM), X-ray diffraction (XRD), ultraviolet-visible (UV-Vis)diffuse reflectance spectroscopy, and absorbance spectroscopy. FE-SEM shows that the hierarchical TiO 2film with a thickness of 1.5 μ m is composed of a nanosheet overlayer (~0.2 μ m height) and the nestlikenanoarray layer (~1.3 μ m height). XRD patterns display that the TiO 2 films have pure anatase phasestructure. UV-Vis spectra reveal enhanced light scattering and dye adsorption ability of the hierarchical TiO 2film. For the dye-sensitized solar cell (DSSC) based on the nanosheet/nestlike nanoarray hierarchical TiO 2film, a short-circuit current (J sc) of 7.79 mA· cm - 2, open-circuit voltage (Voc) of 0.80 V, fill factor (FF) of 0.40, andphotoelectric conversion efficiency (η ) of 2.48% are achieved. Within the dye-sensitized solar cell, thephotoelectric conversion efficiency of the hierarchical TiO2 film was nearly ten times higher than that of[Article] doi: 10.3866/PKU.WHXB201203161 www.whxb.pku.edu.cn物理化学学报 (Wuli Huaxue Xuebao)Acta Phys. -Chim. Sin. 2012, 28 (6), 1368- 1372 JuneReceived: February 17, 2012; Revised: March 14, 2012; Published on Web: March 16, 2012.? Corresponding author. Email: lixj@ms.giec.ac.cn; Tel: +86-20-87057781.The project was supported by the National Natural Science Foundation of China (51172233) and National Key Basic Research Program of China(973) (2009CB220002).国家自然科学基金 (51172233) 与国家重点基础研究发展计划项目 (973) (2009CB220002) 资助? Editorial office of Acta Physico-Chimica Sinica1368CHANG Meng-Lei et al .: Fabrication of Nanosheet/Nestlike Nanoarray Hierarchical TiO2 Film for DSSCNo.6nanostructured array film.Key Words: TiO 2; Nanosheet/nestlike nanoarray; Hydrothermal synthesis; Dye-sensitized solar cell;Photoelectric conversion1 IntroductionDye-sensitized nanocrystalline solar cells (DSSCs) based onTiO2-based nanomaterial have been studied extensively sinceGr?tzel and co-workers first reported a low cost and high effi-ciency DSSC in 1991. 1,2 A typical DSSC consists of a dye-sen-sitized photoanode and a Pt counter electrode filled with a liq-uid redox electrolyte. 3,4 The morphology and structure of photo-anode play important roles in determining the charge transport,dye-loading, and light-harvesting properties, which have signif-icant effects on the final cell performance.5It is expected that a direct conduction pathway from thepoint of photogenerated charge carriers to the collecting elec-trode would improve the conversion efficiency of DSSC.6- 8 Inparticular, highly ordered TiO 2 nanoarrays have been widelyused to enhance the electron transportation within the photo-anode by providing a direct conduction pathway for the rapidcollection of photogenerated electrons.9- 13Another way to increase the efficiency of DSSC is to in-creasethe light-harvesting capability by introducing light scat-terers into the photoelectrode.14- 16 Many previous studies havedemonstrated that optical absorption of TiO 2 nanocrystallinefilms can be promoted due to the light scattering with the addi-tion of large-sized TiO2 particles.17- 19The highly ordered TiO2 nanoarrays covered with a light-scattering overlayer could improve the photochemical conver-sion efficiency due to the enhanced electron transportation andlight-harvesting capability. Herein, we reported a novel inte-grated hierarchical structure, nanosheet/nestlike nanoarrayTiO2 film. By integrating the TiO 2 films into the DSSC struc-ture, the photoelectric conversion performances were investi-gated.2 Experimental2.1 MaterialsAll reagents were of analytical grade and used without anyfurther purification. N-719 dye was obtained from Solaronix,Switzerland. Chloroplatinic acid hexahydrate (H2PtCl6· 6H 2O,37.5% Pt basis), lithium iodide (LiI), iodide (I 2), and 4-tert-bu-tylpyridine were supplied by Westingarea Corporation. Titani-um butoxide was purchased from Aladdin Chemistry Co. Ltd.Sulphuric acid (H2SO4, 95%- 98%), sodium hydroxide (NaOH),and anhydrous acetonitrile were obtained from GuangzhouChemical Reagent Factory.2.2 Preparation of TiO 2 nanostructured array filmThe TiO 2 nanostructured array film was prepared by the hy-drothermal synthesis method. Firstly, 10 mL of concentratedH2SO4 solution was added to 50 mL of water, and then 1 mL oftitanium butoxide was added under stirring for 10 min. The so-lution was transferred to a 100 mL Teflon-lined stainless steelautoclave. Subsequently, the cleaned fluorine-doped tin oxide(FTO) glass was placed at an angle against the wall of the Tef-lon-liner with the conducting side facing down, and then hydro-thermally treated at 150 ° C for 16 h. Finally, the FTO glasswas taken out, washed thoroughly with deionized water, driedin air at room temperature, and heated in air at 500 °C for 1 h.2.3 Preparation of a nanosheet/nestlike nanoarrayhierarchical TiO 2 filmThe above prepared TiO 2 nanostructured array film on FTOsubstrate was placed in an autoclave. The NaOH solution (60mL, 10 mol· L - 1) was added to the above autoclave. The hydro-thermal reaction was kept at 150 °C for 4 h. The resultant TiO 2film was thoroughly rinsed with deionized water and then treat-ed with 0.1 mol· L - 1 HNO 3 solution for 30 min. It was againthoroughly rinsed with deionized water and then dried at roomtemperature for subsequentcharacterization and measurement.2.4 CharacterizationThe morphology of the films was inspected using a field-emission scanning electron microscope (FE-SEM, HitachiS-4800). The crystalline structure of the films was examinedby powder X-ray diffraction (XRD, Philip X ?pert Pro MPD)with Cu Kα radiation from 10° to 80°. The diffuse reflectancespectra of TiO 2 films and the absorption spectra of dye-loadedTiO 2 films were recorded by an ultraviolet-visible (UV-Vis)spectrophotometer (U-3010, HITACHI) equipped with an inte-grating sphere in the range of 350- 700 nm. The width of slitwas 2.0 nm and the step was 0.5 nm. Photocurrent- voltagecurves of DSSCs were measured by a CHI660A electrochemi-cal workstation under the illumination of simulated sunlightwith a 300 W xenon lamp (Changtuo, Beijing, AM 1.5, 100mW· cm- 2).The TiO 2 array film electrodes with the active area of 0.6cm× 0.6 cm were soaked in an ethanol solution of N-719 dyefor about 24 h. Then the dye-adsorbed TiO 2 array film elec-trodes were assembled into sandwich type cells with a counterelectrode (platinum-deposited fluorine-doped tin oxide glassslice) by clamps. A drop of electrolyte composed of 0.1 mol·L - 1 LiI, 0.05 mol· L- 1 I2 , and 0.5 mol· L- 1 4-tert -butylpyridinein acetonitrile was introduced into the clamped electrodes bycapillarity.3 Results and discussionFig.1 shows FE-SEM images of the TiO2 nanostructured ar-ray film and the nanosheet/nestlike nanoarray hierarchical TiO 2film. Fig.1(a) shows a cross-sectional SEM image of the TiO 21369Acta Phys. -Chim. Sin. 2012 Vol.28nanostructured array film obtained by the hydrothermal synthe-sis, which shows that the densely packing TiO2 nanostructuredarrays with ~1.5 μ m height are perpendicularly grown on theFTO substrate. After further hydrothermal treatment withNaOH solution, the nanosheet/nestlike nanoarray hierarchicalTiO2 film was obtained, as shown in Fig.1(b - d). It can be seenfrom Fig.1(b, c) that a nestlike TiO 2 nanoarray layer is integrat-ed with nanosheetnetwork overlayer, and the nanosheet over-layer covers on the nestlike nanoarrays?surface. The nestlikenanoarray layer (~1.3 μ m height) grown on FTO substrate is as-sembled by many arranged nanoparticles and tiny nanosheets,and the top layer (~0.2 μ m height) with nanosheet networksstands on the surface of the nestlike nanoarray layer. Thenanosheets alternately connect with each other to formnanosheet networks (see Fig.1(d)). The TiO 2 nanosheets areabout 10 nm in thickness and hundredsof nanometers in planarsize. It could be proposed that the transformation of TiO 2nanostructured array film to spontaneously self-assemble thenanosheet/nestlike nanoarray TiO 2 hierarchical structure is dueto a dissolution-recrystallization processvia NaOH solution un-der the hydrothermal driving force.Fig.2 shows the XRD patterns of the TiO 2 nanostructured ar-ray film (curve a) and the nanosheet/nestlike nanoarray hierar-chical TiO 2 film (curve b). XRD patterns indicate that the dif-fraction peak of both TiO 2 films appears at 25.3 ° which corre-sponds to TiO2 (101) diffraction of the anatasephase (JCPDSNo. 21-1272). In contrast to FTO, the enhanced diffractionpeak of both TiO2 films at 37.9 ° is from the (004) anataseTiO 2peak completely overlapped with (200) FTO peak. The intensi-ty of diffraction peaks of the nanosheet/nestlike nanoarray hier-archical TiO2 film is similar to that of TiO2 nanostructured ar-ray film.An enhancement of light-scattering ability of the TiO 2 filmis an effective way for DSSCs to improve the light-harvestingefficiency of the TiO2 photoanode.20 The difference in light-scat-tering abilities of the different films could be reflected by the dif-fuse reflectance spectra. As can be seen from Fig.3(a), com-pared with the TiO 2 nanostructured array film, the nanosheet/nestlike nanoarray hierarchical TiO2 film has higher light-scat-tering ability in the visible regions. The enhancement inlight-scattering ability of the hierarchical TiO 2 film is likelydue to the inimitable nanosheetswith hundreds of nanometersin size (see Fig.1(d)). The dye-loaded TiO 2 nanostructured ar-ray film and nanosheet/nestlike nanoarray hierarchical TiO 2film were investigated by means of UV-Vis absorption spectra(Fig.3(b)). The absorption wavelengths of the both TiO 2 filmsFig.1 FE-SEM images of TiO 2 nanostructured array film (cross-sectional view (a)) and the nanosheet/nestlike nanoarray hierarchical TiO 2film (cross-sectional view (b and c) and top view (d))Fig.2 XRD patterns for the TiO 2 nanostructured array film (a)and the nanosheet/nestlike nanoarray hierarchical TiO 2 film (b)CHANG Meng-Lei et al .: Fabrication of Nanosheet/Nestlike Nanoarray Hierarchical TiO2 Film for DSSCNo.6present an absorption peak centered around 520 nm, corre-sponding to the metal-to-ligand charge-transfer (MLCT) ab-sorption peaks.21 The MLCT absorption originates from thedye molecules adsorbed on the TiO2 surface and is related tothe film structure. Owing to the improvement of the light-har-vesting efficiency and the increase of dye loading, the MLCTabsorption intensity of nanosheet/nestlike nanoarray hierarchi-cal TiO 2 film is significantly higher than that of TiO2 nanostruc-tured array film.The TiO 2 films were also employed as photoanodes in dye-sensitized solar cells. Fig.4 shows typical current density ver-susvoltage curves of the both TiO2 films and the resultant pho-tovoltaic parameters are summarized in Table 1. Comparedwith TiO 2 nanostructured array film, the nanosheet/nestlikenanoarray hierarchical TiO 2 film exhibits the higher open-cir-cuit voltages (Voc) of 0.80 V and a drastic increase of photocur-rent density (Jsc) from 0.84 to 7.79 mA· cm- 2. The photoelectricconversion efficiencies (η ) of the DSSCs for the nanosheet/nestlike nanoarray hierarchical TiO 2 film and the nanostruc-tured array film are 2.48% and 0.23%, respectively. As a result,the nanosheet/nestlike nanoarray hierarchical TiO 2 film exhib-its nearly 10 times higher conversion efficiency as compared tothe nanostructured array film. The improved conversion effi-ciency can be attributed to the nanosheet/nestlike nanoarray hi-erarchical structure which leads to the increase of dye loadingand the higher light-harvesting capability.In the DSSC, the enhancement of JSC, Voc, and photoelectricconversion efficiency for the nanosheet/nestlikenanoarray hier-archical TiO 2 film may be attributed to the textural and structur-al properties. The JSC is dependent on the light-harvesting effi-ciency, dye loading amount, and charge transport properties.22The hierarchical TiO 2 film is comprised of nanosheet networksand TiO2 nestlike nanoarrays. The nanosheetsare submicrome-ter-sized and, thus, can function as efficient light scatterers toenhance the light harvesting efficiency. And the networks pro-vide the film with the necessaryporous structure and large in-ternal surface area for increasing the number of chemisorbeddye molecules. The increase of light scattering ability wouldenhance the light-harvesting capability of the hierarchical TiO 2film and the increase of dye loading amount leads to more elec-tron-hole pairs generated. On the other hand, the nestlike nano-arrays can provide the direct conduction pathways for efficienttransport of photogenerated electrons and hold back the photo-generated carrier recombination. And thus the open circuit volt-age and the short-circuit current density increase. Owing to thehigher Jsc and Voc, the photoelectric conversion efficiency of thenanosheet/nestlike nanoarray hierarchical TiO 2 film is im-proved.4 ConclusionsA novel integrated hierarchical structure, nanosheet/nestlikenanoarray TiO 2 film, has been obtained by NaOH solution post-hydrothermal treatment on hydrothermally synthesized TiO 2-derived nanostructured arrays. The nanosheet/nestlike nanoar-ray hierarchical TiO2 film shows significant improvement inlight-harvesting and dye adsorption. Despite the very low filmthickness of about 1.5 μ m, when integrating the nanosheet/nest-like nanoarray hierarchical TiO 2 film into the DSSC structure,the photoelectric conversion efficiency of 2.48% is achieved,Fig.3 Reflectance spectra of