The Activity For Hydrogen Production By Photocatalytic Splitting Water And Band Structure Of TiO₂ Doped With Co

Heterogeneous semiconductor photocatalysis is a hot topic in Photocatalytic water splitting for hydrogen (H₂) evolution by utilizing the solar energy. Among the most of semiconductor photocatalysts, TiO₂ photocatalyst has received much attention and become the core of the theoretical study of photocatalysis due to its nontoxicity, lowcost, high stability and catalytic activity. However, the highly efficient use of TiO₂ is sometimes prevented by its wide band gap that caused the utilization ratio of the sunlight and the quantum yield is low. In this work, TiO₂ was doped with ion to improve the photoefficiency.In this paper, cobalt doped TiO₂ photocatalysts are prepared by sol-gel method using Ti(OC₄H₉)₄ for raw materials and Co(NO₃)₂·6H₂O for donor dopant ions. The prepared samples are characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and UV-vis absorption spectroscopy (UV-vis) to investigate the effects of preparation conditions on structure and performance of photocatalysts. The photocatalytic activity of cobalt doped TiO₂ prepared was researched by the experiment of water splitting for the production of hydrogen, investigating the proportion of cobalt doping and calcination temperature influence on photocatalytic activity. In addition, based on the density functional theory (DFT) of the first-principles the mechanism of Co-doped anatase TiO₂ enhancing photocatalytic activity has been discussed by calculating the crystal structure, band gap, density of states and optical absorption coefficient of pure anatase TiO₂ and anatase TiO₂ doped with cobalt adopting the CASTEP module of Materials Studio 4.0.The results show that Co doped in TiO₂ can inhibit phase transition of anatase to rutile and the growth of crystallite. The doping of cobalt made the absorption threshold values TiO₂ extending up to the visible light range but not reducing the absorption of ultraviolet. Co existing as Co²⁺, acted as hole traps to prohibit the recombination between active electrons and holes,improving the photocatalytic activity. The results demonstrate that Co doped TiO₂ photocatalyst has good optical stability, and its photocatalytic activity is significant higher than that of pure TiO₂. The rate of hydrogen production rate is up to 103.04μmol·h^-1 under the conditions of the doping content of Co for 2.0%(molar ratio), and calcination temperature of photocatalyst preparation at 500℃.The calculation results got can explain the experimental phenomenon reasonably. Compared with pure TiO₂, the steady state energy and the volume of Cobalt doped TiO₂ cell increase. Thus it is that some energy accumulated in the crystal and consumed by Volume expansion caused the phase transition from anatase to rutile temperature improving. The Co²⁺ doping changes the band structure of TiO₂ and reduces its band gap energy successfully. The 3d orbital of cobalt involves in the formation of the valence band and conduction band by forming hybrid orbital. And new impurity level appears between valence band and conduction band, they are propitious to the separation of electron and hole, reducing the recombination between active electrons and hole. At the some time, the excess impurity level may provide new hole and electron recombination centers, and we got an optimum Cobalt doping ratio in the experiment. Consistent with the UV-vis result, Co doping TiO₂ presents some absorption in the visible light region and its optical absorption threshold shifts towards the long wavelength (>400 nm).