Study On The Synthesis, Modification And Photoelectrochemical Properties Of Anodic TiO₂ Nanotube Arrays

Using semiconductor nanocatalysts to convert the solar energy to chemical energy is an effective way to solve the current energy crisis. Among all of semiconductors, Ti O2 has been extensively studied and used in the fields of photocatalysis for hydrogen and organic pullutants degradation, dye-sensitized solar cells, sensors, electrochemical capacitors and biomedicine due to its excellent photocatalytic activity, thermal and chemical stability, non-toxocity and low-cost. Compare to the bulk particulate Ti O2, the one-dimensional Ti O2 nanotube arrays(TNTs) prepared by anodization method possess high aspect ratio, large surface area, and provide a perpendicular pathway for photogenerated electrons separation and transfer, which lead to superb photoelectrocatalytic performance. However, the wide bandgap of Ti O2 which only responses to UV light and relatively fast recombination of photogenerated electron-holes on the surface of catalyst siganificantly limit the photoelectrocatalytic applications of TNTs. Hence, improving the preparation methods of TNTs and reasonably modifying their properties are important in enhancing the photoelectrocatalytic activity of TNTs. In this thesis, a new and gentle BF4- contained electrolyte is employed to fabricate TNTs and the influences of their morphologies and structures on photoelectrocatalytic activity are also investigated. In addition, Ti3+ self-doped, nano-gold and graphene quantum dots loading and Z-scheme hybrid photocatalyst constructed by Cd S nanocrystallites and reduced graphene oxide are used to extend the sprectrum responsive range and boost the separation and transfer of photogenerated electron-holes, thus greatly improve photoelectrocatalytic efficiency. The details are presented as below:The influences of different potentials on morphologies and structures of naontubes and photoelectrocatalytic performance in a new BF4- contained organic electrolyte are studied. The results show that the BF4-, similar to the conventional F-, can also act as the etching ions to form the highly ordered Ti O2 nanotube arrays in the anodization process. The anion BF4- are easily decomposed to release the fluorion ions into the electrolyte under high electric field and thus single-walled Ti O2 nanotube arrays(SW-TNTs) are formed at high potentials but double-walled Ti O2 nanotube arrays(DW-TNTs) are formed at relatively low potentials. The DW-TNTs show more excellent photoelectrocatalytic performance than the SW-TNTs due to larger specific area and better light absorption of double-walled structure.Extremely smooth and boron-fluorine co-doped Ti O2 nanoube arrays(BF-TNTs) are prepared by two-steps anodization method in a new BF4- contained electrolyte. The results show that the “smooth” walls nanotubes are obtained in the contained electrolyte while the “rippled” walls nanotubes(F-TNTs) are formed in the conventional under the same condition of relatively high water content. The non-metal boron and fluorine elements which may come from the partial decomposition of BF4- are doped into the Ti O2 nanotubes arrays during the anodization process. As a result, the BF-TNTs exhibit better photoelectrocatalytic activity than the F-TNTs.A microwave-assisted chemical reduction method with Na BH4 is employed to prepare bulk abundant Ti3+ self-doped Ti O2 nanotube arrays(MR-TNTs). The optimized saturation photocurrent density and photoconversion efficiency of the MR-TNTs under simulated solar illumination are measured to be 3.05 m A/cm2 and 1.66%, respectively, which are the highest values ever reported for doped TNTs photoelectrodes. Moreover, the MR-TNTs exhibit much more stable PEC performance than the Na BH4 treated TNTs(R-TNTs) without microwave assistance, which is attributed to the self-doped Ti3+ mainly exist in the bulk rather than on the surface.A coaxial heterogeneous graphene quantum dots sensitized Ti O2 nanotube arrays(EPD-GQDs/TNTs) is prepared by a coupling technique of linker molecules binding and electrophoretic deposition(EPD). The silane linker molecules act as a superb medium for integrating GQDs and TNTs by covalent amide linkage, thus preventing GQDs clogging on the tube entrances and forming an uniform GQDs layer tightly attached to the inside tube walls during the following EPD process. By adjusting the time of EPD, appropriate thickness of the deposited GQDs in the internal tube walls of TNTs can be controlled. Compared to the pristine TNTs and GQDs/TNTs prepared by conventional impregnation-precipitation method(IP-GQDs/TNTs), the EPD-GQDs/TNTs exhibit significantly enhanced photoelectrochemical water splitting activity and photocatalytic organic dye decomposition performance for their broad photo-absorption range, fast separation of photogenerated charge and stability.Ternary nanocomposite photoelectrodes composed of Cd S nanocrystallites, reduced graphene oxide(RGO) and Ti O2 nanotube arrays(TNTs) are prepared by a coupling technique of electrophoretic deposition(EPD) and successive ionic layer adsorption and reaction(SILAR). Compare to pure TNTs, RGO/TNTs, and Cd S/TNTs, the ternary Cd S/RGO/TNTs hybrids show much higher visible-light-driven photoelectrochemical and photocatalytic activity due to that the outer layer of Cd S acts as sensitizer for trapping substantial photons from the visible light, the middle layer of RGO not only serves as electrons mediator and transporter for suppressing the recombination of photogenerated carriers, but also plays as a green sensitizer for enhancing visible light absorption, and the inner TNTs with narrowed band gap collect the hot electrons form the Cd S and RGO to participate subsequent redox reaction for hydrogen production and organic pollutants degradation.