Design, Preparation And Interface Modification Of Photoelectrochemical Electrodes Based On Nanostructured Silicon Heterojunction For Water Splitting

Among all chemical fuels,hydrogen is considered to be an ideal,green and clean one.Since Fujishima and Honda utilized a semiconductor for solar water splitting to generate hydrogen in 1972,photoelectrochemical(PEC)water splitting has intrigued widespread attention.In semiconductor materials that applied in PEC electrodes,silicon(Si)is a low-coat,earth abundant material with wide light absorption range and long carrier diffusion length,which is widely investigated in PEC electrodes.However,there still exist some drawbacks in Si-based PEC electrodes.First,the high reflection of crystal Si result in a significant lost of incident light.Second,Si is unstable in most electrolytes,in which oxidation or corrosion would happen.Third,the valence band edge of Si is too shallow to inject photogenerated holes into oxygen evolution reaction.Aiming to solve these issues in Si-based heterojunction PEC electrodes,it is investigated in this thesis how morphologies and modifications of Si/catalyst or Si/electrolyte interfaces affect the performance of PEC electrodes.And issues of interface effect in Si-based PEC electrodes are discussed significantly.Furthermore,textured Si is applied to improve the absorption of incident light.Functional layers are introduced for passivation and protection.The heterojunction is modified in PEC electrodes to improve their performance.The works in the thesis including:1.Si NW arrays are prepared by low-temperature solution etching methods.The lengths of NWs are controlled by varying etching time and the density is controlled by second time etching in PCl5 solution.PEC cathodes based on Si NW arrays with different NW lengths and densities are constructed for hydrogen evolution reaction(HER).And their performances are measured and compared.Furthermore,polymer with low workfunction and well catalytic capacity is incorporated in the PEC cathodes.As a result,the photocathode performance is enhanced,due to the decrease of the overportential and the increment of barrier height in Si depletion layer.2.Textured Si/organic heterojunction is prepared via spin-coating of organic solutions,in order to passivate Si.The contact at Si/organic interface is modified by controlling the length and density of Si NWs.And then with a cross-linking method,the interface contact is enhanced.Additionally,this method also passivates textured Si,which decreases surface recombination rate significantly and improves the minority carrier lifetime for over 1.5 times.3.Cross-linking method is utilized to prepare PEC anodes with Si/organic heterojunction,in which a record photovoltage of 657 mV is achieve with a PEC conversion efficiency of over 11 %.Variation of the workfunctions of the organic films prepared by cross-linking method and traditional solution treatment,are investigated by UV-vis absorption,X-ray photoelectron spectroscopy(XPS),ultraviolet photoelectron spectroscopy(UPS).And their electrode performances are also compared.4.Si-based PEC anodes for oxygen evolution reaction(OER)with metal-insulator-semiconductor(MIS)structure are fabricated.How the thickness of each layer in MIS structure influence the performance of the PEC electrode is investigated.Molybdenum oxide(Mo O3)and nickel oxide(NiOx)are introduced in the PEC anodes,respectively,and the performance of the electrodes is improved.5.Ferroelectric(FE)films are incorporated in Si-based PEC anodes as a tunable interface dipole.The morphologies of the FE films are investigated by scanning electron microscopy(SEM)and atomic force microscopy(AFM).In the PEC anodes with FE film,photovoltages are controllable via poling with an external electric field.After a forward poling process,the photovoltage of the photoanode is improved for 17 %.In this thesis,Si-based PEC electrodes with relative high performance were fabricated.Their performance was improved and costs were reduced via the application of surface morphologies and inter layers on Si.To some extent,these works will play guiding roles to fabrication and industrialization of high performance PEC electrodes.