Liquid-Phase Growth Of Titanium Dioxide Nanorod Arrays And Branched Structure For Photoelectrochemical Solar Energy Conversion

Recently, TiO₂ nanorod arrays have presented a wide application prospect in the field of photoelectrochemical solar energy conversion benefiting from the favorable features of good one-dimensional electronic transmission performance, excellent chemical stability and suitable energy band position. It is well known that the controllable growth of the one-dimensional TiO₂ nanorod arrays is the basis to achieve photoelectrochemical solar energy conversion. In order to further improve the conversion efficiency, it is a promising approach to effectively inhibit the charge recombination at the nanorod arrays/electrolyte interface. Taking these above into account, this thesis focuses on the following aspects: regulating the morphology and structure of the TiO₂ nanorod arrays through the solution-based controlled growth as well as optimizing surface properties of Ti O₂ nanorods to inhibit interface electron recombination, thereby achieving the improved performance of photoelectric conversion efficiency for the dye-sensitised solar cells and photoelectrochemical water splitting. Some important results and conclusions are listed as follows:（1） Rutile TiO₂ film with solution-based deposition as the seed layer was used to regulate the growth of TiO₂ nanorods. A dense rutile TiO₂ film could be conveniently prepared on the substrate surface of Sn O₂: F（FTO） conductive glass by TiCl4 precursor solution hydrolysis method, which serving as seed layer can achieve the controllable growth of TiO₂ nanorods as well as can effectively eliminate the influence of the FTO substrate. The diameter, density and orientation of the TiO₂ nanorods were well controlled by adjusting solution concentration of TiCl4 precursor. More importantly, the dense rutile films serving as the blocking layers can effectively restrain charge recombination at the interface between FTO substrate and electrolyte in the process of photoelectrochemical. As a result, the rutile TiO₂ thin film has significantly increased the lifetime of photo-generated electron in TiO₂ nanorods photoanode sensitized by the N719 dye, which also obviously increased the open-circuit voltage of dye-sensitized solar cells from 680 mV to 790 mV, improving the photoelectric conversion efficiency.（2） Type II anatase/rutile TiO₂ branched homogenous junction was constructed to restrain the interface electron recombination. Type II homogenous TiO₂ nanorods branch structure could be fabricated by decorating rutile TiO₂ nanorods with anatase TiO₂ nanostructures through the hydrothermal method, which sensitized by the N719 dye could be used as photoanode for the dye-sensitized solar cells. Compared to the TiO₂ nanorod arrays, the branched structure can increase the specific surface area with enhanced adsorbing capacity of photosensitive dye so as to improve the ability of the light capture in the dye-sensitized solar cells, presenting 2 times improvement for the short current. More importantly, the homogeneous junctions constructed by rutile and anatase TiO₂ constitute as developed barriers of electron recombination would effectively restrain charge recombination for the electrons in the rutile TiO₂ nanorods and holes in the electrolyte. Moreover, the electron lifetime has been also increased about 2 times, thus, improving the photoelectric conversion efficiency in the dye-sensitized solar cells.（3） Enhances the photoelectrochemical water splitting performance of one-dimensional TiO₂ nanostructures by high temperature rapid thermal annealing（RTA） in the air. The rutile TiO₂ nanorod arrays treated by RTA process at 700 oC for 30 seconds can not only can keep the conductive substrate understroved, but also effectively passivate surface defect states of TiO₂ nanorods, which would inhibit the interface charge recombination and accelerate the separation of the photogenerated electron, resulting in the significantly improved performance of photoelectrochemical water splitting for the TiO₂ nanorods. The samples treated by RTA demonstrated increased photocurrent by 40% under the same potential compared to the untreated samples.Compared with TiO₂ nanorod photoanodes, the water splitting performance of branching structure photoanode was not significantly enhanced by the RTA process. This can be ascribed to the polycrystalline nature of anatase branches, whose large numbers of grain boundries, dominating the transport and recombination of photogenerated charges, are less affected by the RTA process.