Photocatalytic Properties Of SnO₂ And ZnO With Optimized Energy Band

Environmental pollution and energy shortage compromise human sustainable development and social stability.Photocatalytic technology continues to be an exceedingly active approach to efficient environmental remediation and renewable production of clean energy because of its environmentally friendly nature and high-eff iciency properties.There are two mainly limitations of photocatalytic efficiency:?1?light absorption;?2?charge transfer and separation.Electronic structure in principle determines the light absorbance,charge transfer and separation,and consequently,photocatalytic property of a photocatalyst.Bsed on the energy band engineering theory,we can optimize the electronic structure by the introduction of homogeneous oxygen vacancies.And the optimized electronic structure could show a narrowed bandgap and an increased valance band width.The narrowed bandgap further contributes to extended light absorption range and the increased valance band width leads to enhanced mobility of holes for efficient charge transfer and separation,hence facilitating the photoreactivity.This is a hot research topic in recent years.We chose typical SnO₂ and ZnO semiconductor photocatalysts as the research objects.We introduced oxygen vacancies into SnO₂ and ZnO to optimize their electronic structures,and the mechanisms of enhanced photocatalytic properties have been studied.We synthesized SnO₂ nanoparticles with homogeneous oxygen vacancies via a one-pot hydrothermal process.All the XPS,Raman,ESR and PL spectra demonstrate the homogeneous oxygen vacancies confined in SnO₂ nanoparticles.Moreover,the first principle calculations theoretically reveal the desirable electronic structure.The narrowed bandgap further contributes to extended light absorption range,so the defected SnO₂ with oxygen vacancies shows a broad absorption range?UV-VIS-NIR?,i.e.,the defected SnO₂ can use more sunlight.And the increased valance band width leads to enhanced mobility of holes for efficient charge transfer and separation,thus,defected SnO₂ exhibits a high photocurrent density.The extended light absorption range and efficient charge transfer and separation jointly facilitate the visible light photoreactivity.As a result,the defected SnO₂ exhibits a superior visible light photocatalytic activity.More strikingly,the photodegration of methyl orange?MO?is completely accomplished within only 20 min under ? ? 420 nm.Briefly,this work both experimentally and theoretically indicates homogeneous oxygen vacancies confined in SnO₂ nanoparticles lead to the optimized electronic structure and,consequently,the remarkable visible light photocatalytic activity.We synthesized SnO₂ quantum dots with homogeneous oxygen vacancies via a one-pot solvothermal method.TEM shows that the grain size of the defected SnO₂ quantum dots is about 2 nm around.And the SnO₂ quantum dots have uniform distribution and good dispersion.The incorporation of homogeneous oxygen vacancies into the SnO₂ quantum dots induces the narrowing of band gap and broadening of VB width,which further contribute to improved visible light harvesting and eff icient charge transfer and separation,hence promoting visible light photocatalytic performance.Moreover,SnO₂ quantum dots have a large specific surface area and a large number of reaction sites,and the path of the photogenerated carriers diffusion from the bulk phase to the surface is short.Thus,SnO₂ quantum dots with homogeneous oxygen vacancies exhibits a superior visible light photocatalytic activity.The ZnO nanospheres doped with oxygen vacancies were prepared by a simple two-step method.We adjusted the oxygen vacancy concentration of ZnO samples by annealing ZnO₂ at different temperatures.And the band structure was tuned by the introduction of different oxygen vacancies concentration.The XPS and PL spectra confirm the oxygen vacancies confined in ZnO nanospheres.The oxygen vacancies concentration of ZnO sample annealed at 450 ? is high.This ZnO sample shows a wide range of light absorption and high transfer and separation efficiency of photogenerated carriers.Consequently,it showed the best photocatalytic activity in the degradation of methylene blue.