Reasonable Construction Of Z-scheme Photocatalytic Systems To Achieve High Efficiency Overall Water Splitting Under Light Irradiation

As global energy is being consumed at an alarming rate,the development of clean,green,renewable energy is imminent.Photocatalytic water splitting using sunlight has always attracted intense interest and is generally accepted as the most environmentally friendly way to permanently resolve the current energy crises.The overall water splitting system,which is based on two photoreactions?hydrogen evolution reaction,HER,and oxygen evolution reaction,OER?,can work efficiently in water splitting process because it can suppress backward reactions.^[1-3]However,a constant water splitting process requires the two reactions in matched reaction rate,which is based on the semiconductor photocatalysts they applied.Until now,the rate of HER has reached quite high level while the rate of OER is far behind.Therefore,the development of high-efficient OER is one of the urgent questions to level up the whole efficiency of overall water splitting.The reason for its is generally believed that oxygen evolution process is a four-electron process,the electron transfer process is more complicated.On the other hand,the overpotential of oxygen on the surface of the material is much higher than the hydrogen overpotential.Formation of oxygen atoms on the surface of the material requires a higher activation energy.This led to photocatalytic oxygen evolution has been in a downturn situation.In view of the current problems of oxygen evolution,this paper takes WO₃as the catalyst for oxygen evolution and obtains the highest known oxygen production effect by controlling different conditions and combining the synergistic effect of the catalyst crystallinity and the oxygen deficiency.On the basis of this,Z-scheme system was constructed to achieve complete decomposition of water.First section:we use sodium tungstate as a tungsten source by one-step hydrothermal synthesis of nano-flaky WO₃.By controlling different hydrothermal temperature and calcination temperature,catalysts with different oxygen defects were synthesized,and by adjusting the energy levels ofWO_3,the WO₃ valence band was adjusted to reduce the Gibbs free energy of the reaction.On the other hand,the crystallinity of the material is increased by calcination,the photocatalytic electron transfer rate of the catalyst is effectively increased,and the synergistic effect of the crystallinity and the oxygen deficiency makes WO₃ have a higher oxygen evolution effect.Second section:on the basis of better oxygen evolution,combined with a suitable hydrogen production catalyst Zn_0.5Cd_0.5S multi-walled carbon nanotubes as a carrier to build Z-scheme system to achieve the overall water splitting.multi-walled carbon nanotubes?MWCNTs?have attracted great attention as supports for photocatalysts because of their large surface area,special aperture structure,high chemical stability and good electron conductivity.We loaded two kinds of catalysts on carbon nanotubes,and utilized the excellent physical and chemical properties of carbon nanotubes to effectively improve the generation and separation efficiency of photogenerated electrons and holes,and achieved overall water sptting under full light.Third section:the catalyst wants to have good performance,good contact with the catalyst and water is the basis of it's reaction.However,the solubility of tungsten trioxide in water is relatively small,which becomes another factor that limits its photocatalytic performance.in the experiment,We acidify the trioxide to increase its hydrophilicity,so that the catalyst has better contact with water,the catalyst reaction is more sufficient and the photocatalytic efficiency is higher.On the other hand,we add a trace amount of manganese element in the hydrothermal process,and the manganese element has a variety of valence states.Under the excitation of light,it may generate new electronic transition level to change the electronic transition path and improve the photocatalytic performance.