Enhancing Mechanism For Visible-Light Photocatalytic Properties Of Niobium-Modified SnO₂ And The Application On The Degradation Of Microplastics

Microplastics are widely found in the oceans,which is harmful for the marine ecosystems.Meanwhile,microplastics are toxic to aquatic life and human beings,which may cause neurotoxicity and other hazards.Thus,an effective and environmental-friendly is needed for solving the microplastic pollution problem.The photocatalytic degradation is a sustainable method for microplastic pollution remediation due to it can convert organic contaminants into carbon dioxide and water without toxicity.Tin oxide（SnO₂）is chosen as the photocatalytic material for this thesis due to its stability,low cost and environmental friendliness.However,due to the wide band gap of 3.6 e V for the bulk rutile,SnO₂ can only be excited under ultraviolet light illumination,inhibiting its practical scenarios.The effects of transition metal modification on SnO₂-based photocatalysts are expected to decrease the band gap as well as enhance the photocatalytic properties,and then for degrading microplastics.In this work,niobium-modified SnO₂ quantum dots（Nb-SnO₂ QDs）have been prepared.The photocatalytic performances are evaluated by the degradation of organic contaminants.The photocatalytic mechanism is discussed and the application research of microplastic degradation is carried out.Firstly,pristine SnO₂ QDs and Nb-SnO₂ QDs are prepared via a one-step hydrothermal method.The incorporation of Nb atoms into the SnO₂ matrix is determined by the morphology,microstructural and compositional properties of prepared samples.The increased visible light absorbance is determined by ultraviolet-visible absorption spectra,which is attributed to an impurity level incorporated in the band structure by Nb incorporation.The photocatalytic ability of Nb-SnO₂ QDs is approximately twice higher than that of pristine SnO₂,which is attained by the degradation rate constant of organic contaminants under visible-light irradiation.The Nb-SnO₂QDs are stable.Based on above results,the effect of Nb incorporation concentration on the photocatalytic performance of SnO₂ QDs is further investigated.The photocatalytic ability increases with incremental Nb incorporation concentration until it reaches the maximum of0.08474 min^-1 at incorporation ratio of 6%.With the further increase of incorporation concentration,the photocatalytic ability diminishes.Secondly,the first principle calculation based on the density functional theory（DFT）is carried out to describe the band structure and interpret the photocatalytic mechanism of the Nb-modified SnO₂ QDs.The band structure and density of states of pristine and Nb-modified SnO₂are calculated.The calculated results are investigated.The impurity levels are observed in the band by Nb incorporation,which are sorely contributed by the 4d orbits of the Nb atoms.The photo-generated carriers transfer pathway within Nb-SnO₂ QDs is discussed by the joint analysis of experimental and computational results.The superoxide radical(O₂^●-)is the main active radical in the photocatalytic degradation,which is determined by the trapping experiments.The photocatalytic mechanism is described in three parts:carrier transfer pathways,carrier action in the reaction fluid and organic decomposition.Finally,the photocatalytic degradation of polyethylene microplastic is investigated using the 6%Nb-SnO₂ QDs as the photocatalyst.The polyethylene degradation efficiency of 9.6%is determined by the mass loss.The morphological fragmentation further confirms the decomposition of the polyethylene microplastic.The intermediate of the photocatalytic solution is determined by the gas chromatography characterization.The results show the presence of intermediates such as triacontane,hexadecenoic acid,ethyl ester and octanal.The possible degradation pathway of the polyethylene microplastic is discussed.