Synthesis And Photoelectrochemical Properties Of Hematite Based Three-dimensional Ordered Nanostructured Photoanodes

The ability to directly convert solar energy into hydrogen has made photoelectrochemical(PEC)water splitting one of the most effective ways to utilize the unlimited solar energy in an economic and environmentally friendly way.Owing to the the advantages of the proper band gap(2.1 e V),good band energy position,high photochemical stability,nontoxicity,abundance on the earch and low cost,the hematite(?-Fe₂O₃)is the candidate materials for photoanodes among a variety of semiconductor photoelectrode materials.However,like many PEC materials,its poor light absorption,short photogenerated hole diffusion length(2-4 nm),easy recombination of photogenerated electron-hole pair make its actual photocurrent efficiency far lower than the ideal theoretical value.How to improve the PEC performance of the hematite is a frontier subject worthy of further investigation.High light absorption,effective carrier separation,and the ability of fast transportion of photogenerated holes and electrons to the surface to participate in the reaction are necessary to obtain efficient PEC water splitting.For planar electrodes,it is difficult to obtain both high light absorption and efficient carrier separation.The introduction of nanostructured photoelectrodes and optimization of structural parameters can improve the light absorption and the carrier separation efficiency of semiconductor materials as much as possible.Therefore,ordered array nanostructures have attracted extensive attention as novel photoelectrodes due to their unique optical properties in recent years.In this thesis,in orde to improve the PEC performance of the hematite,the ordered array nanostructures were designed and optimized to increase the light absorption capacity,shorten the transportion distance of photogenerated holes,and inhibit photogenerated electron-hole recombination.Three kinds of nano ordered structures including the frustum of a cone(FC)arrays with surface plasmon effect,three-dimensional(3D)porous host/guest structure and inverse opal structure with photonic crystal effect were fabricated by magnetron sputtering,thermal evaporation,hydrothermal method,atomic layer deposition(ALD)and other material growth methods,respectively.The effect and mechanism of the nano-array structure on the catalytic performance of the photoelectrode is systematically studied from the aspects of surface plasmon effect,the optimization of nanopore size,and photonic crystal effect.The main results of this thesis are as following:1.Si O₂/ITO/Au/?-Fe₂O₃(a frustum of a cone,FC)nano-grating structure was prepared by reactive ion etching,magnetron sputtering and thermal evaporation methods.The PEC performance of the?-Fe₂O₃ photoanode can be adjusted by optimizing the grating period,the height of the cone or the thickness of?-Fe₂O₃ film.It is found that the photocurrent density of the Si O₂/ITO/Au/?-Fe₂O₃ nano-grating photoanode is largest when the grating period is 600 nm.The photocurrent density both decreases with the increase of the height of the nano truncated cone and the thickness of?-Fe₂O₃,and reaches the maximum for the 300 nm high cone,and the 50 nm thick hematite,respectively.2.The maximum photocurrent density of PEC oxygen evolution reaction(OER)is 1.33m A/cm² for the Si O₂/ITO/Au/?-Fe₂O₃ samples with the cone grating period 600 nm,the height of the cone 300 nm,and the thickness of the deposited Fe 50 nm under 1.23v versus reversible hydrogen electrode(V_RHE),which is about 10 times higher than that of the planar sample.The photoelectric conversion efficiency(IPCE)is improved by more than two orders of magnitude near band gap of?-Fe₂O₃,and the maximum IPCE enhancement is about 20 times in the broadband wavelength range.The SPP-induced electric field plays a dominant role in improving the PEC efficiency by promoting charge separation.The relatively weak electric field caused by light scattering in the nanograting structure also leads to the enhancement of IPCE in the broadband wavelength range.3.The three-dimensional porous ATO/?-Fe₂O₃ host/guest inverse opal photoanode was prepared by self-assembly using polystyrene colloidal spheres(PS)as template.The ATO skeleton is composed of ATO microcrystals with an average size of about 10 nm,and the size of?-Fe₂O₃ particles is about 40 nm.By changing the size of PS(100nm,500nm,800nm,1000nm,1500nm and 2000 nm),the photoanodes with different pore sizes were obtained.When the pore size is 1000 nm,the maximum photocurrent density is 1.1m A/cm² at 1.23 V_RHE,which is much higher than 0.29 m A/cm² of the planar samples.ATO transparent conductive skeleton with continuous channels can improve the electron transport efficiency.The high specific surface area of the 3D skeleton effectively enhanced the light absorption and provided more active sites for the OER of PEC.The excellent PEC performance was obtained only for the samples with the appropriate pore size,at which the electrical conductivity and light absorption reaches a good balance.4.The ATO/Fe₂O₃ photoanode with photonic crystal structure was prepared on FTO conductive substrate by template forced impregnation method and ALD method.The Fe₂O₃ layer with a thickness of 20 nm was uniformly and flatly covered on the ATO skeleton surface.The photonic crystal structures with different photon gap positions were fabricatied by changing the size of PS(360 nm,400 nm,430 nm and 480 nm).It is found that with the increase of the pore diameters,the photonic crystals appear blue-purple,light blue,yellow-green and red,respectively.The photocurrent density(maximum 0.8m A/cm² under 1.23 V_RHE)of ATO/Fe₂O₃-400 photonic crystals with the pore size of 400nm is as large as about 2-3 times of that of the disordered porous crystal samples.The ATO/Fe₂O₃ inverse opal with excellent structural integrity and surface flatness has obvious photonic crystal effect,and the slow light effect of photonic crystal can selectively enhance light absorption and IPCE in the spectral region.At the same time,ATO/Fe₂O₃ structure can reduce holes transmission distance and promote the separation of photogenerated carrier.By studying?-Fe₂O₃-based photoelectrodes with different ordered nanoarray structures,this thesis explores the influence of unique optical properties of nanoarray structures on the improvement of PEC performance.The research results can be Scalable applied to solar energy conversion systems based on other semiconductor materials.This work provides some guidances for the further design and application of nanoarray structure photoelectrodes.