A First-principles Study On Photocatalytic Water Splitting Of Semiconductor Materials For Hydrogen Production

In recent years,energy and environmental problems have become more and more seriously.It is imminent to search a new form of energy which is clean and renewable for representing a viable alternative to the fossil fuels.Hydrogen is the lightest gas in the world,which is non-toxicand and odourless.Its production of combustion is water,and its calorific value is 2.8 times as much as that of gasoline which is one of the ideal clean energy.Thus,how to produce hydrogen efficient in low energy becomes one of research hot spots.Theoretically,barely the water,solar energy,and photocatalyst are needed for photocatalytic water splitting.While the solar energy is inexhaustible for us and water is one of the most plentiful resources on the earth.Therefore,the key index for efficiently photocatalytic water splitting is finding a suitable photocatalyst.One of the effective ways to improve the photocatalytic efficiency of semiconductors for water splitting is to improve its absorption coefficient in visible light range.Based on the first principles calculations,we have investigated the influence of different dopants on photocatalytic water splitting performance.Our study can provide a theoretical guide for the design and preparation of efficient photocatalysts for hydrogen production from water splitting.The main works are as following:1.Based on density functional theory,the optical properties of La-N co-doped TiO₂have been investigated with Hubbard U on-site correction,including the influence of dopants on lattice parameters,dielectric function,absorption coefficient and photoconductive properties for TiO₂.The results reveal that the absorption coefficients of interstitial La doping and N-substitution of O are larger than that of other mono-doped structures which is in great agreement with the experimental results.Moreover,we found that N-substitution of O and La-substitution Ti has the highest absorption coefficient among all doped structures that have not been reported in the literature,which can provide helpful information for preparation of efficient photocatalysts.2.Electronic and optical properties of the pristine and Se/In mono-doped Tl AsS₂have been investigated with meta-GGA+TPSS potential.The calculated energy levels of the valence band maximum and the conduction band minimum reveal that all of these three structures are satisfy the necessary conditions for water splitting.High absorption coefficient in visible light range for the pristine TlAsS₂ can be observed,indicating that it can use solar energy efficiently.Moreover,the absorption coefficient of TlAsS₂ can significantly enhance with the doping of Se and In,which means that all of these structures are promising candidate catalysts for hydrogen generation from water splitting driven by the visible light.3.Electronic and optical properties of the pristine and group VI A elements doped BaTiO₃ have been investigated with meta-GGA+MBJ potential.Four concentrations of dopants are considered.The results show that the absorption coefficient of BaTiO₃ in the visible light range is increased with the increase of doping concentration.However,the energy positions of conduction and valence bands are no longer satisfy the conditions of water splitting when Ba TiO₃ doping with 5 at%Te.4.Electronic and optical properties of the pristine and group IV A elements doped ZnS and SrIn₂O₄ have been investigated.The results reveal that the energy band gap of ZnS is decreased and its absorption coefficient in the visible light range is increased with the introduction of group IV A elements.However,the energy positions of conduction and valence bands of all these structures still satisfy the conditions of water splitting although its energy band gap is decreased,which implies that these structures are promising candidate catalysts that can be driven by the visible light.While the influence of group IV A elements on electronic and optical properties for SrIn₂O₄ is similar to that of ZnS,but it is unfortunate that Si mono-doped SrIn₂O₄ are no longer suitable for photocatalytic water splitting.5.The influence of the different concentration of Hg dopant on electronic and optical properties of ZnS and ZnSe has been investigated.When ZnS doped with Hg,its absorption coefficient in near ultraviolet light range is increased,indicating that it still can not use solar energy for water splitting efficiently.Different with ZnS,the absorption coefficient of ZnSe increased significantly with the introduction of Hg.Moreover,the carrier mobility of ZnS has decreased apparently by doping with Hg.While the radio between the effective mass of electrons and holes for ZnSe is increased with the introduction of Hg although its carrier mobility is also decreased a little,which implies that the photocatalytic water splitting performance of ZnSe can be improved by doping with Hg.