Studies On Fabrication, Modification Of TiO₂Nanotube Arrays And Their Properties Of Photoelectrochemical H₂Generation

Hydrogen energy is regarded as an ideal green energy. Photoelectrochemical water splitting is one of the most promising ways to produce hydrogen by using the renewable solar energy. TiO2is the most extensively used semiconductor photocatalyst and there are different forms. TiO2nanotube arrays is one of the specific forms which is highly ordered, precisely oriented and possessing larger specific area allowing the vertical transfer of electrical charges across the length of nanotubes. So it can be used as photoanode to photoelectrochemical split water effectively. The amount of water on earth is extremely rich but most of them are seawater, up to93%, while fresh water is limited. So it is significative to make use of seawater to obtain hydrogen through photoelectrochemical splitting technology. In this work we mainly focus on the preparation and modification of TiO2nanotube arrys for photoelectrochemical water splitting. The thesis composed of three parts:1. TiO2nanotube arrays (TiO2-NTs) perpendicular to the substrate were fabricated by anodic oxidation of a pure titanium in three different electrolytes: fluoride-free inorganic system (HCI+H2O2), fluoride-inorganic system (HF) and fluoride-organic system (EG). The influences of the electrolytes on the formation of the nanotubes were investigated.The morphology of the as-fabricated samples were studied with FESEM. The TiO2-NTs were used as photoanodes to photoelectrochemically split artificial seawater into hydrogen. The results showed that TiO2-NTs obtained in EG electrolytes have a higher activity for hydrogen evolution (156.13μmol/h) because of relatively better surface smoothness and tube orderliness, which are beneficial to transporting the photoinduced electrons and reducing recombination of the electron-hole pairs.2. We successfully prepared N-modified TiO2nanotube arrays (N-TiO2-NT) through urea pyrolysis in semienclosed system. When the as-modified N-TiO2-NT was undergone further heat treatment under N2atomsphere, its performance of phtoelectrochemical response increased markedly. XPS analysis shows that the surface hydroxyls of TiO2-NTs act as catalysts during the process of urea pyrolysis and polycondensation can occure and finally generate carbonitride similar to C3N4which has visible light activity. UV-vis absorption spectra displays that the absorption edge of the N-TiO2-NT moves toward long wavelength (redshift)489nm. The photocurrent density is431μA/cm2at natural pH at-0.1V bias, while it reaches as high as1.06mA/cm2after adjusting the pH of electrolyte to7.0,1.5times of the former. TG-DSC and XPS analyses suggest that during the third heat treatment some small polymer molecules might sublimate from the N-TiO2-NT and at the same time some molecules condensed further. As a result, its photocurrent is higher after the heat treatment.3. Zr-doped TiO2nanotube arrays (Zr-TiO2-NT) were prepared in situ via anodic oxidation of titanium foils. It does not affect markedly the morphology when Zr at low concentration. However, the tube mouth becomes very irregular and the tube length is only7.4μm as the concentration up to2.5mmol L-1. Mott-Schottky analysis shows that the flat band potential of Zr(1.0)-TiO2-NT shifts negatively about0.025eV. Zr doping can not only enhance the polymerization degree of surface-modified carbon nitride polymers but also promote the N doping. As a result, Zr doping improves markedly visible-light photocurrent response and the photoelectrochemical H2production of the modified Zr-TiO2-NT.