Photocatalytic Material Design and Mechanism Study on Pure Water Splitting under Visible Light

Author:Fang Wen Jian

Supervisor:shang guan wen feng


Degree Year:2018





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 BixY1-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 BixY1-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 BixY1-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 BixY1-xVO4 with only Pt as co-catalyst,BixY1-xVO4 can split pure water with stoichiometric ratio.This novel dodecahedron BixY1-xVO4 may be another semiconductor photocatalyst with only two facets exposed for overall water splitting after the finding of TiO2.On the other hand,BixY1-xVO4 prepared by solid-state reaction always shows low photocatalytic activity and bad repeatability.In this work,diluted acid was introduced to modify the BixY1-xVO4prepared by solid-state reaction.The photocatalytic activity of BixY1-xVO4 can be increased nearly four times after diluted acid treatment.The key factor for enhanced photocatalytic water splitting activity is the disappearance of BiOx 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 BiOx clusters and BixY1-xVO4 induces photo-generated electrons transferring to BiOx clusters.The light absorption edge of BixY1-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(λ>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(λ>420 nm),Pt would prefer to act as a photocatalyst while P25 is analogous to cocatalyst.Moreover,defects(Ti3+or VO)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.