Hematite Photoanodes Modified With Various Elements For Efficient Photoelectrochemical Water Splitting

Nowadays,the world's energy demand still mainly depends on fossil fuels,the corresponding problems of energy scarcity and environmental pollution need to be solved urgently.Therefore,it is important to develop the clean energy sources.Among the numerous clean energy sources,hydrogen energy is considered as an ideal alternative to fossil fuels.The traditional way to produce hydrogen energy also relies on fossil fuels,leading to environmental pollution.Therefore,it is one of the most environmentally and energy-saving ways to produce hydrogen energy by using semiconductor nanomaterials as a photocatalyst for photoelectrochemical water splitting,which can convert renewable solar energy into clean hydrogen energy.There are many semiconductor nanomaterials that can be used for photoelectrochemical water splitting.Among them,the nontoxic,cheap and stable ?-Fe2O3 semiconductor is a promising one.?-Fe2O3 has a suitable band gap(2.1-2.2ev),which can absorb and utilize most of the sunlight.Theoretical calculations show that the solar-to-hydrogen efficiency of hematite can reach to 16.8%,corresponding to a photocurrent of 12.6 mA cm-2 at 1.23 V vs.RHE.However,the reported hematite performance was much lower,due to several factors of hematite such as low conductivity,improper band position and short hole life time.By now,Ti-based treatment has been considered as one of the most effective methods to improve the performance of hematite.Then we have selected the similar elements of Zr and Hf in the same family,and Sn with the same high valence state,to modify ?-Fe2O3 for enhanced PEC performance.The specific research contents are listed as follows:(1)We used Zr-MOFs(UiO-66-(COOH)2))as the Zr precursor to deposit bi-functional Fe2ZrO5 on hematite surface.Interestingly,the photocurrent density of Fe2ZrO5-Fe2O3 photoanode is around two times of that for the pristine Fe2O3 photoanode.Moreover,it can be observed that the onset potential is reduced with a value of 180 mV.Combining with the effects of Ti-treatment and Co-Pi cocatalyst,the photocurrent can finally achieve 2.88 mA cm-2 at 1.23 V vs.RHE.The XAS spectra confirm the existence of Fe2ZrO5,which can be used as both the passivation layer and the source of Zr-doping.So the recombination of electron-hole can be suppressed and the bulk conductivity can be improved to accelerate the OER reaction.(2)Sn-Fe2O3 photoanode was prepared by treating the FeOOH with Na2SnO3 solution before subsequent annealing.Then the photoanode was put in in urea solution through a hydrothermal process to get the N-Sn-Fe2O3 photoanode.To further accelerate the OER kinetics,the Co-Pi cocatalyst was deposited on its surface.The Co-Pi-N-Sn-Fe2O3 photoanode could finally achieve a high photocurrent of 2.87mA cm-2 at 1.23 V vs.RHE.The conductivity of Sn-treated hematite is enhanced with the presence of Fe2+,which can be observed in XPS spectra.N signal can also be clearly observed in the XPS spectra after the urea modification.The N-modified hematite has an enhanced charge separation efficiency.The synergistic effect of Sn-treatment and N-modification significantly improves the the PEC performance of ?-Fe2O3.(3)Hf-Fe2O3 photoanode was prepared by using HfCl4 as Hf source.This treatment can increase the donor density,facilitate the charge transport and enhance the light absorption capacity of ?-Fe2O3.Especially,the photocurrent density of Hf-Fe2O3 photoanode has a significant enhancement(1.91 mA cm-2),which is 2.2 times higher than that of the pristine ?-Fe2O3 photoanode.After depositing cocatalyst FeNiOOH,the photocurrelt density shows a great enhancement to be 2.42 mA cm-2.