Photocatalytic Material Design And Mechanism Study On Pure Water Splitting Under Visible Light

Photocatalytic water splitting,known as the"holy grail"in chemistry,can convert solar energy directly into green storable chemical energy.Up to now,photocatalytic water splitting is still in the experimental research stage.Many photocatalysts have been found for overall water splitting.It has been reported that a solar-to-hydrogen energy conversion efficiency of particulate photocatalyst sheets exceeds 1%.However,it is still far from the practical application requirement of at least 10%solar energy conversion efficiency.Thus,discovering new materials and improving solar energy conversion efficiency are still two major themes in research.In this paper,the new visible-light photocatalysts Bi_xY_1-xVO₄ solid solution and g-C₃N₄ for photocatalytic water splitting will be studied.The specific research results are listed as follows:Firstly,we successfully prepared Bi_xY_1-xVO₄ solid solution by low temperature hydrothermal method,which has a new dodecahedron shape with two facets{101}and{100}exposed.With the doping of Y,The crystal system of BiVO₄ changes from monoclinic to tetragonal.The improved symmetry of crystal system makes the?100?and?010?of{100}facets entirely identical.As a result,the decahedron BiVO₄ increases two faces and changes to dodecahedron Bi_xY_1-xVO₄.By experiment and calculation,redox properties of the two facets are different.The photo-generated electrons and holes can be separated between the different facets.Through the studying of Bi_xY_1-xVO₄ with only Pt as co-catalyst,Bi_xY_1-xVO₄ can split pure water with stoichiometric ratio.This novel dodecahedron Bi_xY_1-xVO₄ may be another semiconductor photocatalyst with only two facets exposed for overall water splitting after the finding of TiO₂.On the other hand,Bi_xY_1-xVO₄ prepared by solid-state reaction always shows low photocatalytic activity and bad repeatability.In this work,diluted acid was introduced to modify the Bi_xY_1-xVO₄prepared by solid-state reaction.The photocatalytic activity of Bi_xY_1-xVO₄ can be increased nearly four times after diluted acid treatment.The key factor for enhanced photocatalytic water splitting activity is the disappearance of BiO_x clusters on the surface which can trap photo-generated electrons with lower conduction band to H₂/H⁺.It is speculated that a novel surface self-heterojunction built between BiO_x clusters and Bi_xY_1-xVO₄ induces photo-generated electrons transferring to BiO_x clusters.The light absorption edge of Bi_xY_1-xVO₄ solid solution is 410 nm.In order to expand the spectral absorption,g-C₃N₄ is studied to achieve overall water splitting.g-C₃N₄ with the light absorption edge about 430 nm was prepared by calcining of urea.The Pt particles were used as cocatalyst loaded by in situ photodeposition of H₂PtCl₆ aqueous solution.Through photocatalytic water splitting test,H₂ and O₂ can be produced simultaneously with the stoichiometric ratio.However,the activity is instability.In particular,the stoichiometric ratio for H₂ and O₂ is more sensitive for light intensity and spectrum.On the other hand,the photocatalytic activity of bulk g-C₃N₄ is rather low.Many strategies have been adopted to improve the photocatalytic performance of g-C₃N₄.Here,bulk g-C₃N₄ was modified by molten salt method.Alkali metal K⁺was introduced into bulk g-C₃N₄,which can reduce the bandgap and promote the separation of the photogenic carrier.Moreover,Na and O co-doped g-C₃N₄ was prepared by a feasible solvothermal method.The band structure is modulated mainly by introducing O into lattice of g-C₃N₄,resulting in simultaneously enhanced absorption and red shift.The separation and migration of photogenerated carriers under visible light is enhanced by Na doping.In addition,well dispersion of?Na,O?-g-C₃N₄ in pure water can be observed due to its high surface electronegativity.The photocatalytic H₂ production activity of?Na,O?-g-C₃N₄ is improved tremendously.Finally,absorption from near-field dielectric scattering was observed in case of Pt nanoparticles directly supported on P25.This absorbed scattered light could drive visible-light photocatalytic pure water splitting??>420 nm?.It is interesting that the roles of Pt and P25 in photocatalytic hydrogen generation were switchable under different irradiation conditions.Under UV-vis irradiation,P25 represents a photocatalyst while Pt is cocatalyst.Under visible light irradiation??>420 nm?,Pt would prefer to act as a photocatalyst while P25 is analogous to cocatalyst.Moreover,defects(Ti³⁺or V_O)on the surface of Pt/P25 would be generated under UV irradiation.These defects are similar with a sacrificial agent to consume photogenerated holes,which are necessary for visible-light-drive photocatalytic pure water splitting.