| A solar cell is a device that converts the energy of sunlight directly into electricity based on the photovoltaic effect. The development of a low-cost and high-efficiency solar cell is highly significant for utilization of solar energy, which will help solve the serious energy crisis and environmental problem faced by the whole world. Among solar cells, silicon-based solar cell and thin-film solar cell have been commercially available. In recent years, dye sensitized solar cells (DSSCs) are receiving increasing attention as a potential cost-effective alternative due to its low cost and simplicity. Usually, the energy conversion efficiency of the DSSCs is limited by the photoanode performance. As a key material in photoanode, fast electron transfer kinetics of TiO2 is necessary to avoid photoelectron recombination, which usually depends strongly on the micromorphology and crystallographic structure of TiO2. Therefore, it is very valuable to synthesize controllably titania nanomaterials with specific structures and optimize the electron transfer kinetics based on nanotechnology.In this work, protonated titanate nanocrystallites, nanorods, and nanotubes were used as precursors to fabricate titania photoelectrode materials. We focused mainly on the conversion of the phase structure and micromorphology after calcination at different temperature. In particular, the effect of such conversion on photoelectrochemical properties of the products obtained subsequently was investigated in detail. In the case of protonated titanate nanoparticles, it is found that the transformation of the crystallographic structure occurs from titanate to TiO2-B and anatase with increasing calcination temperature, respectively. Meantime, the nanocrystalline size grows gradually. Among all products, nanocrystallites obtained after calcination at 500℃show the best photoelectrochemical properties. Similarly, the strcture transformation can be also observed from protonated titanate nanorods to TiO2-B nanorods and anatase nanorods with increasing calcination temperature. However, such a phase transformation temperature is obviously higher. Moreover, the rod-like morphology could be destroyed under high calcination temperature. It is noted that nanorods calcined at 700℃show the optimized photoelectrochemical properties. Finally, it is found that titanate nanotubes can convert to TiO2-B nanotubes (300-400℃), anatase nanorods (500-600℃), and anatase nanoparticles (700℃) with increasing calcination temperature. The obtained anatase nanorods after calcination at 600℃present the exellent photoelectrochemical properties.Intensity-modulated photocurrent and photovoltage spectroscopies (IMPS and IMVS, respectively) were used to investigate the electron transport and recombination processes of the various titania nanomaterials. The results indicate that titania nanocrystallites provide a slow electron transport and a short electron lifetime, which leads to a high electron recombination rate due to the configuration confinement and the existence of abundant surface defects. In the case of titania nanorods with a relatively large size, the one-dimensional structure and single crystallites allow a fast electron transport, a long electron lifetime, and a low electron recombination rate. In respect of nanotubes, the electron transfer kinetics is varied obviously with the change in morphology and crystallographic structure. The limited electron transport and serious electron recombination can be found for TiO2-B nanotubes obtained after calcination at 300℃and 400℃due to the existence of abundant surface defects. The anatase nanorods with a relatively small size obtained at 500℃and 600℃allow a fast electron transfer, and electron lifetime of nanorods obtained at 600℃is long up to 106 ms. It is indicated from above results that the rod-like morphology can provide the optimized electron transfer kinetics, which is beneficial to provide important information for further fabricating one-dimensional titania nanomaterials with enhanced photoelectrochemical properties.To optimize electron transfer kinetics of titania materials, one-dimensional titania nanocomposites, in which titania nanocrystallites are dispersedly supported on the surface of nanorods, were fabricated based on the above results. It is demonstrated that one-dimensional titania nanocomposites can combine the high surface area of nanocrystallites and the electron transport advantage of nanorods, which may present a long electron lifetime and a low electron recombination rate. As anticipated, the energy conversion efficiency of one-dimensional titania nanocomposites is increased by 5 times as compared to that of nanorods. To further increase the surface area of titania nanocomposites, nanorods with a smaller size, obtained by calcining titanate nanotubes at 500℃, were used to support titania nanocrytallites to fabricate one-dimensional nanocomposites. It is shown that the charge transfer resistance is reduced and the adsorption of dye is increased for such one-dimensional nanocomposites with the large surface area. Besides, the fast electron transport is also obtained due to the contribution of nanorods. Therefore, the photovoltaic conversion efficiency of the nanocomposites is greatly improved to 7.87 %, the highest value obtained under our experimental conditions in this work.In summary, titania nanocomposites with desired structure and morphology are used to prepare photoanode for DSSCs. The one-dimensional nanocomposites prepared here have optimized electron transfer kinetics due to the combination of the advantages of nanocrystallites and nanorods, resulting in an obviously improved energy conversion efficiency of DSSCs. The results obtained in this work can provide a potential approach to fabricate new photoanode materials and improve energy conversion efficiency of DSSCs. |
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