Modification And Photochemical Properties Of Hematite Photoanodes For Solar Water Splitting

With the increasing environmental problems and demand for energy,clean energy has attracted global attentions.Hydrogen is customarily viewed as a clean and efficient energy substitute of fossil oil and gas.Since water oxidation was found on TiO₂ electrode,photoelectrochemical?PEC?water splitting has been considered as one of potential techniques for hydrogen production.Many semiconductors have been applied in solar water splitting.Hematite??-Fe₂O₃?has been widely used as photoanode because of its low cost,narrow band gap,good chemical stability and ready availability.Due to inherent shortcomings,such as short hole diffusion length,low charge-carrier concentration,high suface states density and poor water oxidation reaction kinetics,the PEC performance of hematite is far below the theoretical predicted value.The main contents can be summarized as follows:?1?The effect of oxygen vacancy on photoelectrochemical performance of hematite.Sn Doping and creation of oxygen vacancies have been adopted universally to overcome the poor electric conductivity and unfavorable hole diffusion length of?-Fe₂O₃photoanodes.Generally,Sn doping is realized via longitudinal migration of tin element from FTO?fluorine-doped tin oxide?substrates into?-Fe₂O₃ at high temperature.To introduce oxygen vacancies along with Sn into hematite for further promoting its electric conductivity,we have created a local reducing atmosphere via partial oxidation of graphite while doping hematite with Sn.The donor density of the resultant Fe₂O₃ photoanode annealed on graphite?G-Fe₂O₃?at 700°C for 20 min is increased to¹.7 times that of the counterpart annealed on SiO₂ powders?S-Fe₂O₃?,indicating that the electric conductivity of hematite is improved after introduction of oxygen vacancies.Moreover,oxygen vacancies have been demonstrated to significantly reduce the charge transfer resistance of Sn doped hematite.Consequently,the photocurrent density of G-Fe₂O₃ is enhanced remarkably?⁷0%?compared with S-Fe₂O₃.However,the improvement in photocurrent density due to oxygen vacancies becomes less significant when more Sn is doped into hematite.The strategy for creation of oxygen vacancies reported here can be extended to other photoanodes for better understanding the effect of oxygen vacancies on PEC performance.?2?The PEC water splitting performance of NiCo-Pi/?-Fe₂O_3.High surface states density of hematite photoanodes results in their low water oxidation kinetics and high surface electrons-holes recombination.To overcome these inherent drawbacks,various methods have been adopted,especially loading oxygen evolutioncatalysts and depositing oxide passivation layers.We report here an efficient way to promote the photocurrent of Fe₂O₃ photoanodes via depositing a thin layer of Co_0.84Ni_0.16-Pi.With the Co_0.84Ni_0.16-Pi deposit on its surface,the photocurrent density of Fe₂O₃ increases by ca.42%at 0.23 V vs.Ag/AgCl,and the onset potential shifts 200 mV cathodically.In contrast,Co-Pi@Fe₂O₃ photoanode shows only 20%enhancement in photocurrent density under otherwise identical condition.The dark current densities of the photoanodes give an evidence that both Co_0.84Ni_0.16-Pi and Co-Pi are good oxygen evolution catalysts.Moreover,different from sparsely distributed Co-Pi nanoparticles,a 2³ nm amorphous Ni doped cobalt phosphate layer can be also an effective passivation layer for surface states of hematite,which has been demonstrated by the analyses of Mott-Schottky plots and electrochemical impedance spectroscopy.This work demonstrates the dual roles of anamorphous oxygen evolution co-catalyst on hematite photoanodes and provides a simple method for designing highly efficient photoanodes.?3?The PEC water splitting performance of Ni-Pi/?-Fe₂O₃.Photoinduced charging process often occurs in nickel based O₂ evolving catalyst?OEC?decorated photoanodes and hinders the charge transfer across the OECs layer to the electrolyte,resulting in relatively lower activity of the photoanodes for solar water oxidation.In this work,we oxidize nickel phosphate/Ti doped hematite?Ni-Pi/Ti-Fe₂O₃?photoelectrode with a concentrated NaClO solution to suppress the photocharging of Ni²⁺/Ni³⁺.The oxidized nickel phosphate significantly enhances the photoelectrochemical performance of Ti-Fe₂O₃ electrode with a³50 mV cathodic shift in onset potential and 100%enhancement in photocurrent at 1.23 V vs.RHE.X-ray photoelectron spectroscopy,Mott-Schottky analysis and electrochemical impedance spectroscopy have been adopted to investigate the origin of the considerable activity enhancement that is attendant to oxidation.After pre-oxidation,some nickel?II?phosphate species are transformed into active NiOOH species and thereby photoinduced charging of Ni²⁺/Ni³⁺can be greatly suppressed.Electrochemical experiments demonstrate that improved oxygen evolution reaction?OER?activity and reduced photo-charging effect enable accelerated charge transfer across the OECs layer to the electrolyte,resulting in markedly enhanced solar water oxidation performance of oxidized Ni-Pi/Ti-Fe₂O₃ photoelectrode.These results indicate pre-oxidation is an efficient way to activate nickel based OECs on semiconductor photoelectrodes for highly active photoanodes.