Ion Implantation Modification Of Metal Oxide Semiconductor Photoelectrode For Water Splitting

Hydrogen energy is a clean energy resource.The solar energy is regarded as the most abundant renewable energy resource.It can be converted into hydrogen energy by water splitting and considered to be the ultimate solution to address the global energy problem.However,compared to fossil energy,the development of high-efficiency and low-cost solar-hydrogen system to meet the demand on a global scale is still a huge challenge.The best system is the photoelectrochemical(PEC)tandem system for unassisted water splitting which is consist of a p-type photocathode and n-type photoanode.The photovoltaic generated from them provides the energy supply.There are many photoanode semiconductor materials including metal oxides,metal nitrides,metal sulfides,et al were studied.Among them,metal oxides are extensively studied as photoanode materials due to their high photostablity and low-cost.However,the band edge positions of the oxide semiconductors do not match the redox potential of water;the band gap is too large,which limits its utilization of sunlight;its poor electrical conductivity which increases the electron-hole recombination rate.These shortcomings severely limit the photoelectric conversion efficiency.The principle of semiconductor photoelectrochemical water splitting is based on the theory of band gaps in solid state physics.Therefore,many efforts foucs on the modification of the band structure and defects of metal oxide semiconductors,which can affect the optical and electrical properties to enhance the PEC performance and further understand the intrinsic mechanism.Ion implantation technology is commonly used to modify the semiconductor and other materials through doping or forming nanostructures.Its application in Si industry has promoted the development of Integrated Circuit.Compared with other doping method,such as,hydrothermal method and sol-gel method,for ion implantation,all the atoms can be implanted into materials and the implanted atoms is not limited by the solid solubility of target material,and it is also not affected by the diffusion coefficient and the chemical binding energy.The ion implantation process could ensure the uniformity and purity of the introduced elements in materials.It can also control the concentration and distribution of ions in material through controlling ion fluence and energy.Meanwhile,the collision cascade process can produce a lot of vacancies inside the materials.In this thesis,we focus on how to modify the band structure,controlling the defects and forming nanostructure in metal oxides semiconductor by ion implantation technology to improve the PEC performance of photoanodes.We choose the Zr+ ions to dope the WO3 aiming to study the influence of band structure on the PEC performance;and use the Ar+ions to irradiate the TiO2 aiming to study the influence of the different types of defects on the PEC performance,also use the He+ ions to implant into the α-Fe2O3 nanorods aiming to study the influence of the nanovoids on the PEC performance.The detailed contents include the following:(1)We successfully obtain the Zr+ ions doped WO3 nanofilms and modify its band structure.Through changing the band structure,the onset potential of WO3 photoanode is reduced and the photocurrent density is also increased.The WO3 films synthesized by reacting magnetron sputtering deposition were implanted by Zr+ ions to different fluences at 200 keV and subsequently thermal annealled in O2 atmosphere.The results indicate that the implanted Zr+ ions form the Zr4+ substitutional positions in the lattices.Two combined factors lead to the upward shift of the conduction band.One is due to the higher energy level of the Zr 4d orbital than that of the W 5d orbital.Another is the strain induced due to the larger ionic radii of Zr4+ than that of W6+.Meanwhile,the oxygen vacancies introduced during the ion implantation can cause an upward shift of the valence band maximum.The results indicate that the upward shifts of the conduction band minimum and valence band maximum can enhance the band bending between the semiconductor/liquid interfaces which benefits to the separation of the electron-hole pairs.The holes are more easily transferred to the electrolyte solution to oxidize the water.Therefore,the upward shifts of the conduction band and valence band lead to the cathodic shifts of the onset potential.(2)Fabrication of defect rich TiO2 with Ti vacancies and O vacancies by Ar+ ions irradiation.In this work,we demonstrated that the ion irradiation and post-annealing is a simple,effective process for preparing oxygen-deficient TiO2-x thin films.The prepared TiO2-x thin film showed an enhanced PEC property for solar water splitting.The highest photoconversion efficiency η of the TiO2-x photoanode was 0.28%,which is 0.5-fold higher than that of the pristine TiO2 photoanode.Combing the experimental characterization with the theoretical calculation,the oxygen vacancies induced by ion irradiation introduces vacancy energy level in TiO2 which improves the concentration of charge carriers and enhances their separation,while the formed titanium vacancies act as trapping centers for charge carriers and decrease the performance of the samples.Therefore,the reasonable vacancy defects are beneficial to improve the photoelectrode performance.(3)α-Fe2O3 nanorod arrays with nanocavity structure were fabricated by He+ ion implantation and thermal annealing treatment.The formed nanocavities contribute to the increase of the photocurrent density to 1.27 mA/cm2 at 0.6 V vs.Ag/AgCl,which is 1-fold higher than that of the pristine sample.We attribute the enhanced PEC performance to the formed nanocavities which reduced the carrier diffusion path and improved the separation efficiency of the photogenerated electron-hole pairs.The results indicate that the ion beam can not only make the planer structure to nanostructure but also can further form the nanocavities within the nanostructure which can enhance the separation of the charge carriers.This work provides a new strategy for improving the photoelectrochemical performance of semiconductor materials.