Surface Modified Hematite Nanomaterials For Efficient Photoelectrochemical Water Splitting

With the gradual advancement of the global industrialization process,the importance of energy issues has become more pronounced.By now it is urgent to find new energy sources to replace fossil fuels.Hydrogen energy as a clean and effective energy source has attracted wide attention in recent years.Although the commercial hydrogen production process is mature,it is not clean and new methods to produce hydrogen are highly needed.Based on the use of solar energy,photoelectrochemical water splitting using semiconductor materials as a clean and effective way to produce hydrogen has been a hot topic and attracted much attention.a-Fe2O3 is a promising photoanode due to its low cost,non-toxic,abundance and chemical stability.It can absorb most of the sun light with a suitable band gap(2.0 e V-2.2 e V).However,it also has several drawbacks in practical applications,such as unfavorable band position,poor conductivity,short hole diffusion length(2-4 nm)and poor oxygen evolution reaction kinetics,which highly limit its practical performance.Therefore,it is useful to modify hematite and then improve its photoelectrochemical properties.In this thesis,we have improved the hematite photoelectrode by the surface modification and the substrate/photoelectrode interface modification.The contents are listed as bellow:(1)We have prepared surface B-passivated hematite(B-Fe2O3)and Ti-incorporated hematite(B-Ti-Fe2O3)photoelectrodes using a simple surface treatment method.Actually,a large cathodic shift(up to 100 m V)of the onset potential is observed in the B-treated hematite when compared to the pristine sample.The B-passivation is also effective for Ti-treated hematite to significantly lower the onset potential and improve the photocurrent to 2.61 m A/cm2 at 1.23 V vs.RHE(with Co-Pi co-catalyst).Fe(Ⅳ)in the surface region is highly oxidative and can modify B(Ⅲ)to a higher chemical state,which can be observed in the XAS spectra.The B-terminated Fe(Ⅳ)is reduced to Fe(Ⅲ),which confirmed the effective passivation of Fe(Ⅳ)defects by B.(2)C,Co-Ti-Fe2O3 photoelectrodes have been prepared by the surface hydrothermal method.This treatment could effectively improve the performance and reduce onset potential.In particular,the photocurrent density of C,Co-Ti-Fe2O3 photoelectrode at 1.23 V vs.RHE is 2.24 m A/cm2,and the onset potential is reduced by nearly 300 m V.TEM image reveals that there is a carbon layer on the sample surface,which could promote the surface charge separation.XPS spectra show that there is Co2+ in hematite as an active site for the water oxidation.(3)We also synthesize W,Zr-Fe2O3 by a two-step hydrothermal method.WO3 deposited on the FTO substrate can be clearly confirmed by XRD patterns and be observed from the SEM image.TEM images showed W and Zr have diffused to the bulk phase of hematite after high temperature annealing.The performance of W,Zr-Fe2O3(2.0 m A/cm2)is better than that of WO3-Fe2O3(1.55 m A/cm2).In addition,the XPS data confrims the existence of W6+ and Zr4+.These results show that the co-modification of W and Zr plays a synergistic effect in improving the performance of hematite.