Application Of Partially Reduced Titanium Dioxide In Visible Light Catalytic Water Splitting

With the development of society,the demand for energy in human society is increasing day by day.At the same time,the problem of environmental pollution caused by using a large amount of fossil energy has increasingly become an important factor restricting mankind development.It is important to develop new high-efficiency alternative fuels.The hydrogen energy is green and environmentally friendly with high burning value,which is an ideal fossil energy alternate.The use of solar energy to split water in a photocatalytic system is one of the best solutions to resolve energy crisis.TiO₂ is a widely used photocatalyst because of its stable chemical properties,low cost,easy availability and low toxicity to organisms.However,its poor response for solar energy is a key factor limiting its application.In order to expand the absorption of TiO₂,the scientists have made a lot of exploration.The method of introducing oxygen vacancies and Ti³⁺into titanium dioxide enables the TiO₂ to achieve visible light response.As a photocatalyst,semiconductor materials generally face the problem of poor photo-generated charge separation and slow surface reaction kinetics.Loading other semiconductor to build heterojunction and supporting cocatalysts can significantly enhance photo-generated charge separation while improving surface reaction kinetics.Among the water-splitting catalysts,polyoxometalates?POMs?have been reported to efficiently catalyze water oxidation and proton reduction.In recent years,the study of POMs molecular catalysts has progressed from noble metals Ru,Ir to transition metals?Mn,Fe,Co,Ni,Cu?substituted POMs.In order to expand the application of visible light responsive TiO₂ in photocatalytic water splitting system and explore the research of POMs in semiconductor photocatalytic systems,we conducted the following research:1.The visible light-responsive Ti³⁺/TiO₂ was synthesized under hydrothermal conditions using titanium powder as the titanium source and applied in the photocatalyticwateroxidationprocess.AnironcontainingPOMs Na₂₇[Fe₁₁?H₂O?₁₄?OH?₂(W₃O₁₀)₂(?-SbW₉O₃₃)₆]?Fe11?was used as cocatalyst in Ti³⁺/TiO₂ photocatalytic water oxidation system.It was found that Fe11 POM can promote the oxygen evolution of Ti³⁺/TiO₂ photocatalytst.After several characterizations,we found that Fe11 cocatalyst is unstable during the photocatalytic water oxidation process.A layer of amorphous FeO_x species decomposed from Fe11was deposited on the surface of Ti³⁺/TiO₂ nanorod.This amorphous FeO_x species improve the oxygen production of Ti³⁺/TiO₂ by reducing the charge transfer impedance and promoting electron-hole separation.The amorphous film produced by in-situ deposition from Fe11 POM is a better co-catalyst than the material produced by the deposition of the simple inorganic salt FeCl₃.2.Using low-valence TiH₂ as the titanium source and H₂O₂ as the oxidant,anatase TiO_2-x was synthesized by the method of room temperature stirring with high-temperature calcination under inert gas protection.The material can be excited by visible light due to the presence of oxygen vacancies and trivalent titanium Ti³⁺.When applied to a hydrogen production system in which Na₂S and Na₂SO₃ as sacrificial agents,TiO_2-x shows poor activity.The ternary sulfide ZnIn₂S₄ has a small band gap and can absorb visible light,which is beneficial to the separation of photogenerated electrons and holes.When ZnIn₂S₄ and TiO_2-x were combined together to construct a heterojunction,the hydrogen production activity of TiO_2-x was greatly improved.SEM and TEM results show that TiO_2-x is embedded in ZnIn₂S₄nanosheets.The close contact between them is beneficial for electron transport.The energy band arrangement result based on the UV-vis spectra and Mott-Schottky results shows that when the composite ZnIn₂S₄/TiO_2-x are excited by light,the electrons will transfer to TiO_2-x,and holes will transmit to ZnIn₂S₄.Such directed charge transfer is benefic to efficient photocatalytic process.The increase in hydrogen production performance of TiO_2-x after loading ZnIn₂S₄ can be summarized as extended absorbance range and more efficient electron transfer.This work will provide new concept and route for constructing an efficient photocatalytic system.