Preparation Of Oxygen Vacancy Modified Titanium Dioxide And Application In Photocatalytic Hydrogen Production

Due to natural and human factors,our environmental conditions have undergone tremendous and catastrophic changes over the past few centuries.Environmental and energy issues have become global problems that need to be solved urgently.Solar-driven photocatalytic hydrogen production technology has immeasurable potential in solving above problems simultaneously.Titanium dioxide,a typical semiconductor material,is widely investigated in the field of photocatalysis for its advantages of non-toxicity,chemical stability,light corrosion resistance,favorable band position and low price.However,the wide band gap of TiO₂ make it nearly no response in the range of visible light,the photogenerated electrons and holes recombine quickly and cannot be efficiently transferred.All these problems limit the photocatalytic activity of titanium dioxide seriously.Herein,measures like designing a unique TiO₂ nano-hollow sphere structure,doping non-metallic elements to shorten the band gap,introducing oxygen vacancies to form intermediate state to promote efficient transfer of photogenerated carriers,and plasma treatment to obtain hydrogenated TiO₂ were taken to effectively improve the quantum efficiency,thereby promoting its photocatalytic performance.The main works of the paper are as follow:Firstly,a novel C and N self-doped TiO₂ hollow spheres?CNTH?with oxygen vacancies and Ti³⁺was synthesized by a one-step hydrothermal method in a mixed solution of H₂O-C₂H₅OH-HF-H₂O₂,using TiCN as precursor.The results showed that the as prepared CNTH photocatalyst has significantly enhanced visible light photocatalytic hydrogen production activity(16614?mol·h^-1·g^-1),which is about 939 times that of pure TiO₂ nanosheets.The enhanced activity of CNTH was due to the introduction of C and N self-doping and the existence of oxygen vacancies and Ti³⁺on the one hand.The introduction of C and N narrowed the band gap to enhance visible light response and caused a hybrid 2p band to promote the transfer of electrons under irradiation,while the oxygen vacancies and Ti³⁺created more active sites and prolong the life time of electrons and holes by their donor states.On the other hand,the nanostructure of hollow spheres assembled by the nanosheet can greatly increase the specific surface area of the material,and the increase of active site promote the photocatalytic reactions.The special structure makes it easy to get separated from the slurry system by filtration or sedimentation for reuse.In addition,estimation of the content of trivalent titanium were proven by the synchrotron radiation results.Secondly,TiO₂ nanosheets with exposed?001?crystal faces were placed in a plasma atmosphere of mixed gases Ar-H₂,Ar-H₂-N₂ and Ar-O₂ to change their surface properties and their application for photocatalytic decomposition of water to produce hydrogen.The results show that treatment using Ar-H₂ plasmas produced highly hydrogenated,surface-disordered TiO₂ nanosheets with oxygen vacancies,whereas exposure to Ar-H₂-N₂ plasmas resulted in N doping.Surprisingly,Ar-O₂ plasma treatment did not change surface properties of TiO₂.The hydrogenated titanium dioxide nanosheets exhibited the highest photocatalytic H₂ production rate of 122.6?mol·h^-1·g^-1 under visible light irradiation,which is about 7 times that of untreated nanosheets.And it exhibits excellent photocatalytic activity at a rate of 22500?mol·h^-1·g^-1 under monochromatic illumination of 365 nm and the quantum efficiency reaches to 6.06%.Specifically,surface disorder caused a significant shift in the valence band position of the hydrogenated PT-TiO₂-1?0.28 eV?,accompanied by an increase in visible light absorption due to the darker color.Relatively low energy is required for electrons to reach the oxygen vacancy state,which effectively extends the lifetime of electrons and holes.In addition,oxygen vacancies also cause redistribution of excess electrons in the nearest neighboring Ti atoms to form a donor state,thereby facilitating the separation and migration of electrons and holes generated under visible radiation.This work demonstrates that plasma treatment can change the surface properties of titanium dioxide nanosheets and improve the photocatalytic hydrogen production activity under visible light irradiation.