Controllable Synthesis And Photoelectrochemical Properties Of Titanium Dioxide Photoanode Nanomaterials

Currently, dye-sensitized solar cell (DSSC) technology has been considered as a viable competitor to the well developed but relatively expensive solid-state homo-and hetero-junction solar cell technologies. A key parts in DSSC is the nanoporous TiO2 electrode, which serve both as a high-surface-area support for dye molecules and as an electron-transporting medium. In the thesis, from the point of view of light harvesting, charge transfer and optimizing the cell performance, we designed and prepared a series of anatase TiO2 photoanode materials, such as TiO2 nanosheets, Ta-doped TiO2 nanosheets, mesoporous TiO2 nanospheres, irregular mesoporous TiO2 and hierarchical hexagonal TiO2 hollow nanosheets based on improving dye absorbance, changing band gap structure and adjusting the structure of photoanode films. The formation mechanism and the performcance of solar cell fabricated using these materials were also studied systematically.Anatase TiO2 nanosheets were controllable synthesisd by hydrothermal method, taking TiCl4 and NH4F as the reagents. The growth mechanism was dicussed in detail. The presence of F hindered the transformation of TiO2 from anatase to rutile. It is possible to achieve anatase TiO2 single crystals with a high percentage of anatase{001} facets since their surfaces are surrounded by F atoms. Therefore, the{001} facets were reserved even after the hydrothermal prcess. During cell fabrication, two types of electrode films were developed, one was composed of 30 nm-TiO2 nanosheets and 260 nm-TiO2 nanosheets, and the other was composed of P25 TiO2 and 260 nm-TiO2 nanosheets, respectively, and the 260 nm-TiO2 nanosheets were used as light-scattering layer. The experimental results indicated that more incident light was absorbed. The power conversion efficiency for the cell with the scattering layer was increased greatly,25.6% higher than that without scattering layer. The power conversion efficiency for the cell fabricated with P25 TiO2 and 260 nm-TiO2 nanosheets was 10% higher than its counterpart without scattering layer.Mesoporous TiO2 with irregular morphology and well-defined spherical morphology was systhesised by reaction-confined hydrolysis at the n-butanol/H3BO3 solution interface region. The structural characteristics as well as the formation mechanism were studied systematically. The experimental results showed that the mesoporous TiO2 spheres were constructed by the nanoparticle of 8.3 nm via oriented aggregation. The attached planes are not (001) with high surface energy but (101) with low energy. There exists an interface of water/n-butanol and the TBT hudrolyzed into an amorphous TiO2 sol at the beginning of the reaction quickly at the interface region, which gradually ripens into the well-defined TiO2 nanoparticles with aging. By adjusting the concentration of boric acid used, the pH value of the solution varied, which could influence the hydrolysis rate aswell as themotion of the formed TiO2 nanoparticles. Spherical emulsions formed in this interface region may serve as aggregation centers for the primary colloid particles. In this case, the (001) plane can be expected to adsorb many more groups (e.g., boron specimen) than the{101} plane and hindered the contact between (001) planes, resulting in oriented attachment occurring on the{101} plane. Without n-butanol, because of the absence of the interface, the TBT could hydrolyze rather homogeneously in the solution, and only mesoporous TiO2 with irregular morphologies, not spheres, could be obtained. And the specific surface area was 182.1 m2·g-1. Besides these, the mesoporous structure based on the oriented attachment of TiO2 nanocrystals would benefit for electron transfer through the TiO2 layer. The electrode composed of the irregular mesoporous TiO2 exhibited good light harvesting capability because of the large specific surface area. The short circuit current of DSSC reached to 16 mA·cm-2. Boron doping would affect the position of the conduction band minimum and the difference between the flat band energy of TiO2 and redox potential of the dye will be decreased. Therefore, a smaller Voc was obtained. Herein, we fabricated DSSC with bilayer structure composed of P25 TiO2 as dye-absorbed layer and irregular mesoporous TiO2 as light scattering layer. The performance of this kind of DSSC was:Voc=0.77 V, Jsc=15.58 mA·cm-2, FF=0.57, andη=6.94%. In the case of mesoporous TiO2 spheres, The performance of this kind of DSSC was:Voc=0.75 V, Jsc=17.00 mA·cm-2, FF=0.61, andη=7.79%. The large specific surface area as well as the special spherical morphology and size of the mesoporous TiO2 spheres benefit for dye absorbing and utilization of incident light.Hierarchical hexagonal TiO2 hollow nanosheets were prepared via a simple template method by copying hexagonal Cd(OH)2. We proposed the formation mechanism and studied how to control the number of layer of this hierarchical strucuture. During the reaction, (NH4)2TiF6 hudrolyzed into TiO2 nanoparticles on the out surface of Cd(OH)2 nanosheets. Accompanying by the dissolving of Cd(OH)2, the well-defined hexagonal TiO2 hollow nanosheets can be obtained. The experimental results showed that the layer number of the hexagonal TiO2 hollow nanosheets can be controlled readily by repeating the reaction. The unique porous structure possessed by this kind of structure implied the potential applications in the field of controlled realease, catalysis and solar cells. Taking the prepared hexagonal TiO2 hollow nanosheets as the scattering layer of DSSC, the effect of structural characteristics on the cell performace was studied. The experimental results indicated that more the incident light was absorbed. The power conversion efficiency for the cell with the scattering layer was 24% higher than that without scattering layer. In order to obtain the DSSCs with high open circuit voltage, the band gap of TiO2 was modulated by doping. Here, we prepared Ta-doped TiO2 nanosheets by a hydrothermal method. The experiemental results showed that the{001} facets were reserved even after dopig of Ta into the host lattice while the morphology was not changed. Ta doping can restrict the recombination of carries and improve the short circuit current. The flat-band potential VFB of nanosheets was shifted negatively from-0.383 V to-0.425 V after Ta doping. This negative shift in the flat-band potential and open-circuit potential under illumination in Ta-doped nanosheets suggests that a higher quasi-Fermi level in the doped electrode may be responsible for the Voc enhancement in DSSCs and the DSSC device using the Ta-doped nanosheets electrode gives a high photovoltage of 0.82 V and a power conversion efficiency of 6.27%.