Preparation And Photocatalytic Water-splitting Into Hydrogen Of Co²⁺/Co³⁺ Surface Modified Ta₃N₅-based Nanocomposite Materials

To solve the problem of increasingly exhausted fossil energy and serious environmental pollution caused by its combustion in the 21st century,modern society has been seeking for a clean,cheap and renewable green energy that can replace fossil fuel,so as to realize sustainable development of human society.As a new-generation energy carrier,H₂ has the lightest weight,ideal heating value,zero carbon emission and no pollution,and is the clean energy with the greatest development potential in the 21st century.Solar energy is inexhaustible.The total energy supply of solar radiation is huge,but the energy density is low.Currently,large-scale and efficient conversion of solar energy into chemical fuels that can be stored,transported and used remains a challenge.Using solar photocatalytic overall water-splitting into hydrogen to convert solar energy into chemical energy is honoured as the"holy grail"of chemistry.It is a key prerequisite for the realization of solar photocatalytic water-splitting into hydrogen to develop advanced photocatalytic materials with strong sunlight absorption capacity,high carrier separation efficiency,fast carrier interface migration and strong photochemical stability.The band gap of Ta₃N₅ is about 2.1 ev.Its absorption band edge can be extended to600 nm.The valence band potential position is ideal.It across the water redox potential,and its theoretical STH is 15.9%.It is a kind of solar photohydrolysis catalytic material which has attracted much attention in recent years.However,Ta₃N₅ has defects such as poor carrier transport,low separation efficiency,poor photocorrosion stability,and difficult synthesis of materials with few surface defects,which seriously limit its practical application.Surface modification comparative technology of Co²⁺/Co³⁺was carried out by high temperature nitriding reduction technique,using Ta₃N₅@Ta₂O₅ as the precursor.The influences of different valence Co,nitriding temperature,nitriding time,Co²⁺/Co³⁺molar ratio,and other conditions on photocurrent were investigated.The results showed that the photocurrent of 1 wt%Co²⁺surface modified Ta₃N₅-based photocatalyst compared with unmodified Ta₃N₅@Ta₂O₅ was 14 times higher,3 wt%Co³⁺surface modified sample was 29 times higher,and 1:1 molar ratio of Co²⁺/Co³⁺synergistic modified sample was 39 times higher.XRD,TEM,XPS,and electrochemical tests showed that the amount and highly active crystal surfaces of Co_xN and Ta₂N can be effectively controlled by regulating nitridation temperature,nitridation time,amount of modification,and their proportions.It can generate multiple heterogeneous interface structures,enhance visible light absorption.Co²⁺/Co³⁺together-midification promoted to generate Co_5.47N and Ta₂N.Co_5.47N appears metallicity and has good transport electron action.Ta₂N also has good electrical conductivity and could transport electron.Co_xN and Ta₂N together with Ta₃N₅ produced coupling action,and promoted photogenerated carrier separation.Mixed Co nitride exhibited bifunctional hydrogen and oxygen evolution action,and improved the surface catalytic reaction rate.The photocatalytic water-splitting into hydrogen efficiencies of 1 wt%Co²⁺and 3 wt%Co³⁺modified Ta3N5-based photocatalysts by 950°C 2 h nitridation were higher than unmodified Ta₃N₅@Ta₂O₅.Their efficiencies were 2.56 and 3.14 times of that,respectively.Co²⁺/Co³⁺mixed modification has synergistic effect to further improve the separation efficiency of photo-generated carriers and stability of photocurrent density.The sample with 1:1 of Co²⁺/Co³⁺molar ratio exhibited the highest photocurrent density.Its enhanced photocatalytic water-splitting into hydrogen efficiency was 3.48 times of Ta₃N₅@Ta₂O₅.The multiple heterogeneous structures were further optimized by regulating the highly active crystals via 1000°C high temperature nitriding reduction.The photocurrent of 3 wt%Co³⁺surface modified sample by nitridation at 1000°C for 1h was 49 times higher than unmodified Ta3N5@Ta2O5.Its improved photocatalytic water-splitting into hydrogen efficiency was 3.85 times of Ta₃N₅@Ta₂O₅.This paper provides a new strategy for the further development of high performance photocatalytic water-splitting into hydrogen materials.It also lays a theoretical and experimental foundation for the development of high performance and stable Ta₃N₅-based photocatalytic water-splitting into hydrogen materials.