Effects Of Doping And Surface Structures On Photocatalytic Activities Of Hematite ??-Fe₂O₃? Via First Principles Investigation

The technology of photolysis water means that the water molecule is split by solar energy into energy carrier H2 and O2 using semiconductor as catalyst.Currently,this technology is considered to be a feasible method for solving energy crisis in future.Ideally,if photons in solar spectrum with wavelength smaller than 800 nm can be used by semiconductor,the solar energy conversion efficiency can reach 32%.Therefore,in order to meet the energy conversion efficiency of the industrial production requirement?>10%?,searching for semiconductor photocatalyst with low cost,which can split water efficiently using visible light,has been an important objective ever since the discovery of Honda-Fujishima phenomenon.The bandgap of Fe₂O₃ is about 2.2 eV,which can cover 40%of the solar spectrum,and the energy conversion efficiency of solar energy-hydrogen is theoretically predicted to be 12.9%.However,the conversion efficiency of Fe₂O₃ is lower than theoretical value due to suffering from low conductivity,high recombination rate,the low absorption coefficient in the visible light region,and slow surface reaction rate.In order to improve the energy conversion efficiency,we study the basic properties of intrinsic Fe₂O₃,the doping effect on photoelectrchemical activity of Fe₂O₃,as well as the dependence of hydrolysis kinetics on surface microstructure based on the first-principles calculations.The main content and conclusion can be summurized as follows:1.The energy band structure,density of electronic states,complex dielectric function,and optical absorption spectra of bulk Fe₂O₃ are studied adopting GGA+U method.The electronic structure indicates that Fe₂O₃ is an indirect-gap semiconductor with bandgap of 2.25 eV,and the valence band maximum?VBM?and the conduction band minimum?CBM?are composed by O 2p and Fe 3d,respectively.The simulated optical properties point out that photons absorbed by Fe₂O₃ mainly locate in the ultraviolet region of solar spectrum.The simulated results and experiments are consistent with each other.2.The electronic structures of 4d and 5d transition metal doped Fe₂O₃ are systematically calculated based on density functional theory,and Ru doping effect is experimentally verified.The calculated results show that factors affecting photocatalytic efficiency of Fe₂O₃ mainly derive from d electrons of doped transition metal,which can be divided into three categories:?1?d electrons of Y,Rh,and Ir can elevate the position of valence band maximum,thus reducing bandgap;?2?In addition to narrowed bandgap,d electrons of Zr,Mo,Tc,Ru,Hf,Ta,W,Os,and Pt will form impurity levels in the bandgap,converting photons lower than the bandgap in energy into electron-hole pairs;?3?the CBM composed by Fe 3d states can be modified by Ru 4d electrons,which will reduce the effective electron mass of Fe₂O₃ and enhance the mobility of electrons.These results reveal the origin of enhancement of photocatalytic efficiency of Fe₂O₃ by doping with transition metals,and provide a basic guide in designing iron oxide based photocatalyst by doping.In the experiment of Ru doped Fe₂O₃,the absorbtion of visible light and photoelectrochemical activity?photocurrent?are modified,which verify the prediction of electronic structure of Ru doped Fe₂O₃.3.Density functional theory is adopted to predict the electronic structure of?Ti/Zr,N?compensated codoped Fe₂O₃.?Ti/Zr,N?compensated codoping method is effective to elevate VBM of Fe₂O₃ to reduce the band gap.The optical absorption coefficient points out that codoping method can expand the spectral absorption range,and enhance the absorption ability of photons in the visible region.After codoping,the spatial separation of the charge density of VBM and CBM is helpful to avoid recombination between excited electrons and holes.In addition,the inflated charge density isosurface indicates the delocalized property of the electrons,which indicates a better carrier transport property in the bulk system.4.The surface chemical activity is a critical factor affecting the photocatalytic efficiency of hematite.In this study,we systematically investigate the thermodynamic adsorption and heterolytic dissociation of H2O molecule and the reactive behavior of hydrogen generation by water splitting on four kinds of hematite?0001?surfaces,namely perfect and defective O-and Fe-terminated surfaces based on first-principles calculations.The simulation results illustrate that the chemical reaction of H2O molecule is sensitive to the morphology of the hematite?0001?surface.The adsorption energies demonstrate that H2O molecule is more easily adsorbed on the surface containing Fe vacancies or atoms 0 in the initial stage of water molecule contacting with surface.For water heterolytic dissociation,the hydrogen atom is apt to drop from water molecule on O-terminated?0001?surface with an energy barrier smaller than 0.35 eV,and this value is larger than 0.69 eV on Fe-terminated?0001?surface.Compared with other surfaces,the Fe-terminated?0001?perfect surface is a preferable candidate for hydrogen generation,on which the whole process needs to overcome a rate determined barrier of 2.77 eV.