| The photoelectrochemistry(PEC)technology aims to catalyze water splitting by carriers generated when the semiconductors absorb photon.It causes extensive research due to the advantages of moderate equipment cost and relatively high conversion efficiency.Hematite(α-Fe2O3)is one of the most appealing materials for PEC activity,owing to its favorable bandgap(2.1 eV),good stability,and vast earth-abundance.However,the PEC activity is still much lower than the theoretical threshold of 12.6 mA cm-2.The main factors limiting its performance include low carrier mobility,and sluggish oxygen evolution reaction(OER)kinetics et al.Aliovalent doping is a powerful approach for enhancing the PEC activity,yet the underlying mechanism has been remaining elusive.For example,although some researchers spared great effort in correlating improvement of charge transfer efficiency(ηct)with i-SS evolution upon doping,why i-SS evolves is still unclear.In addition,most of the works focus on the improvement of charge separation efficiency(ηcs)based nanostructured samples,with markedly different microstructures.There is very limited work devoted to fundamental study of electronic and optical properties of Sn-doped α-Fe2O3.Herein,we take Sn/Ti-doped α-Fe2O3 for example to explore the mechanism of i-SS evolving with doping elements,and in second part we grow Sn-doped α-Fe2O3 epitaxial film with well-defined structure by pulsed laser deposition(PLD)to study the effect of doping on opt-electronic properties.The intrinsic relationship between the crystal structure,electronic structure,self-trapped exaction dynamics and PEC performance was investigated by a combination of X-ray photoemission spectroscopy(XPS),X-ray absorption spectroscopy(XAS),DFT calculation and Electrochemical impedance spectroscopy(EIS),et al.We explore the important influence of doping on the charge bulk ηcs and interface ηct from the micro level.The content is as follows:(1)The results show that the charge transfer efficiency mainly causes PEC activity different,which it increases for 0.5%Sn doped sample whereas more Sn decreases it,while always improves as Ti content increases.It suggests that the introduction of dopant can tune potential distribution of i-SS with the assistance of EIS analyses,and further proves that the charge transfer rate constant(kct)is proportional to overlapping between the hole-filled i-SS and water oxidation state.The redox potential improvement of i-SS can be explained with the increase in the Fe-O bond covalence,which is caused by the electronegativity of dopant the Fe-O bond length.(2)We show the α-Fe2O3/Al2O3 heterojunction grew in an epitaxial relationship of[1210]Fe2O3‖[1210]A12O3,A combination of X-ray photoemission,O K-edge X-ray absorption spectroscopy and DFT calculations reveals the Ef gradually shifts toward the conduction band minimum with Sn doping and a localized Fe2+-like gap state is observed at the top of valence band.The conduction for all samples is based on a smallpolaron hoping(SPH)mechanism,but Sn doping reduces the activation energy barrier and increases the n-type conductivity of α-Fe2O3.We found that 0.2%Sn doped α-Fe2O3 significantly improve the PEC activity,whereas more Sn decreases it.The enhanced PEC activity is partially attributed to:(a)an increased band bending potential,which facilitates the charge separation at space charge region;(b)the reduced activation energy barriers for SPH,which may facilitate the transport of photo-excited carriers for the improved PEC. |
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