Preparation Of Step-scheme Heterojunction TiO₂ Based Thin Film Photocatalysts And Its Investigation On Activity Of Photocatalytic Water Splitting

The two great challenges of modern human society are the ever-increasing energy demand and environmental pollution.Thus,developing clean,pollution-free and recyclable green energy has been one of the most important tasks for mankind.Hydrogen energy,as an alternative green energy source,has attracted widespread attention because of its high combustion calorific value,nontoxicity,pollution-free,convenient storage and transportation.At the same time,solar energy is a renewable energy and widely distributed.Therefore,the utilization of solar energy as the driving force for the water splitting into H₂ has been a research hotspot,and the utilization of semiconductors as photocatalysts for the water splitting into H₂ is one of the most ideal ways.TiO₂,a n-type semiconductor metal oxide with wide band-gap,has been regarded as a fascinating material in photocatalytic hydrogen production fields due to its low cost,good chemical stability,high resistance to photocorrosion and strong redox ability.At present,the research of TiO₂ photocatalysts mainly focuses on two aspects:one is to improve the utilization rate of solar energy,the other is to optimize the carriers migration process.Enormous efforts are employed to enhance photocatalytic efficiency by narrowing the band gap,introducing the mid-gap states,constructing built-in electric fields and increasing the active sites.The specific strategies include elemental doping,forming heterojunctions with other semiconductors and loading co-catalysts.This paper focuses on regulating the band structure of TiO₂ photocatalysts,constructing built-in electric fields and introducing co-catalysts to improve the utilization of solar light and promote the rapid separation of charge carriers,achieving the purpose of enhancing photocatalytic hydrogen production activity.First,TiO₂ nanotubes were prepared with Ti foil as the substrate.For the purpose of optimizing the interface and increasing the heterojunction area,TiO₂ nanocrystalline films were prepared on the surface of TiO₂ nanotubes,followed by the narrow band-gap semiconductor g-C₃N₄ coupled with TiO₂ to form the S-scheme heterojunction.The introduction of the g-C₃N₄ will broaden light absorption range and form the S-scheme heterojunction with TiO₂,which in turn conduces to optimize the carrier migration process and improve the photocatalytic activity.In order to further boost the photocatalytic activity,it is feasible to introduce NiO as a co-catalyst,which not only provides abundant active sites,but also synergistic with S-scheme heterojunction to promote the separation of electron-hole pairs.Specific experimental research contents are as follows:（1）TiO₂ nanotubes were prepared on Ti substrate（denoted as TNT）,and the TiO₂ nanotubes prepared under different anodic oxidation voltages（20V,25V,30V）for 1h were investigated.The results show that the TiO₂ nanotubes prepared at 25 V anodic oxidation voltage for 1h have the best crystallinity.The length of the nanotubes is about 1.0μm,and the diameter is about 60 nm.The tubes grow perpendicular to the substrate,which is conducive to the longitudinal charge transport.In order to avoid the TiO₂ nanotube array directly coupling with g-C₃N₄ films to form smaller heterojunction area,an n-type TiO₂ nanocrystalline films were fabricated by a dipping strategy with sol-gel as a precursor.Whereafter,g-C₃N₄ films were prepared by chemical vapor deposition method,and Ti/TNT/TiO₂/g-C₃N₄ heterojunction thin film photocatalyst was obtained.Different the quality of raw materials for preparing g-C₃N₄ films were investigated.The results show that the surface of g-C₃N₄ thin film prepared from urea is both smooth and dense,and it was tightly bonded with TiO₂,There are almost no pores between the interfaces.When the mass of monomer was 6g,Ti/TNT/TiO₂/g-C₃N₄ thin film photocatalyst had the highest photocatalytic efficiency,and the gas yield was 6.13μmol/h·cm².Electron paramagnetic resonance（EPR）spectroscopy was employed to further monitor the interface charge-transfer route of catalytic materials.It can be demonstrated that the charge transfer route in S-scheme mode can well explain the enhancement of photocatalytic activity.（2）NiO co-catalyst was loaded on the surface of Ti/TNT/TiO₂/g-C₃N₄ thin film to further improve the photocatalytic efficiency.Firstly,the precursor solution with nickel element was prepared by chemical bath method,and then the NiO layer was covered onto the g-C₃N₄ using a spin coating method to form Ti/TNT/TiO₂/g-C₃N₄/NiO photocatalyst.The gas evolution rate of Ti/TNT/TiO₂/g-C₃N₄/NiO is 7.97μmol h^-1 cm^-2 in Na OH solution（PH=13）,which was 1.3,3 and 8.3 times as high as that of the Ti/TNT/TiO₂/g-C₃N₄,Ti/TNT/g-C₃N₄and Ti/TNT,respectively.More importantly,the Ti/TNT/TiO₂/g-C₃N₄/NiO thin film photocatalyst can not only generate hydrogen and oxygen on both sides of the Ti foil respectively,which inhibits the formation of surface reverse reaction,but also realize recycling.