The Preparation And Modification Of Fe-based Complex Oxide Thin Films For Photoelectrochemical Water Splitting

Solar engery is the most sufficient,clean and sustainable energy on the earth.Hydrogen production by artificial photosynthesis is one of the important methods to solve the world’s energy problem.Photoelectrochemical（PEC）water splitting is one of the most promising artificial photosynthetic method for solar hydrogen production.Photoelectrochemical water splitting can produce chemical fuel with high energy,and it can be stored as H₂ which is convenient for transport and use.The mass energy density of hydrogen is 3-4 times higher than gasoline,and it can be burned directly or used as a hydrogen fuel cell.Every year,there is about 10⁵ TW of solar energy reaching on the Earth,of which about 36,000 TW is on land.This means that the PEC batteries with efficiency of only 1%covered 10%on the land can generate 36 TW of energy per year,which is enough to meet the estimated global energy consumption in 2050.So far,it remains a challenge to build an efficient and stable solar-to-hydrogen（STH）energy conversion device.First,the semiconductor electrode material is particularly critical because it is an important component of photoelectrochemical water splitting devices,which plays the role of light absorption and catalyst.The ideal photoelectrode material should have suitable band gap to absorb visible light and suitable band position.The component of electrode material should be earth-abandant,cheap and non-toxic,moreover it should have high catalytic activity and stability.However,no single material has been found to meet all of the above requirements.Therefore,it is essential to search for new electrode materials that meet all of the above requirements.In this paper,nanostructured novel ZnFe₂O₄ and Fe₂TiO₅ thin films are prepared by electrospray technique as photoanode materials,and photocatalytic performance of Fe₂TiO₅ is further improved through surface modifications:1.Nanostructured ZnFe₂O₄ thin films were prepared by electrospray technique as photoanode material for solar water splitting.The effect of experimental parameters on the morphology and catalytic performance was investigated.With the temperature increasing,the morphology of the changes from tense to nanostructured,which is composed of nanoparticles.The film consisted of small sized nanoparticles possesses higher surface which shorten the carrier fusion distance and exposes more interfacial areas between electrode and the electrolyte,which increases its oxygen evolution activity.Additionally,the film thickness also has a great effect on its oxygen evolution activity.The film with suitable thickness is more benefit for improving water splitting performance.At last,the effect of discharge potential on water splitting efficiency is also studied.Higher potential makes the droplet carry more electron and the droplet will explode to smaller droplets and finally generate smaller nanoparticles.2.Nanostructured Fe₂TiO₅ thin films were prepared by electrospray technique.By adjusting the substrate temperature,nanostructured Fe₂TiO₅ film was obtained at 350℃,which was composed of nanoparticles,which had a large specific surface area and sufficient contact with the electrolyte,which reduced the transmission distance of photogenerated carriers.Therefore,the catalytic performance is superior to that of the Fe₂TiO₅ film of other structures.According to the XRD results,the films prepared at different substrate temperatures were all pure phases of bismuth brookite Fe₂TiO₅ after annealing at 550℃.In addition,we have carried out surface F treatment on the nanostructured Fe₂TiO₅ film,which greatly improved its catalytic activity and more than doubled the photocurrent density.The sturctures,electronic states and morphologies of the samples before and after treatment were characterized to investigate its structure-activity relationship and the improvement mechanism.3.Nanostructured Fe₂TiO₅ thin film were prepared by electrospray technique,and the sample was subjected to surface Al and Fe co-modification to improve its photocatalytic performance.Through the sequential use of these two surface treatment methods,the photocurrent has been significantly improved.At 1.23 V_RHE,the photocurrent density increased from 0.18 mA cm^-2 of the untreated sample to 0.31 mA cm^-2,which in turn increased to 0.58 mA cm^-2,approximately 3.6 times than that of the original sample.At the same time,after two surface-modified electrodes,the onset potential was also cathodicly shifted by nearly 300mV.Through electrochemical impedance spectroscopy,we found that the surface modification not only improves the transport and separation efficiency of carriers inside the electrode,but also promotes the migration of carriers at the interface between the electrode and the electrolyte,and effectively participates in the catalytic reaction.In this paper,nanostructured ZnFe₂O₄ and Fe₂TiO₅ films were prepared by electrospray technique and surface modification was applied to significantly improve the catalytic performance of Fe₂TiO₅.Our research focuses on the development and improvement of novel photoanode materials through different strategies.This research provides a new method and idea for the preparation of high efficiency photoanode materials.