Researchs On Photocatalytic Degradation Properties And Mechanism Of Tungsten-based Plasmonic Nanomaterials

Photocatalysis has been extensively investigated for nearly 50 years and tremendous progress has been achieved in fields of the photocatalytic hydrogen evolution and environment remedies et al.As an ideal route to convert solar energy into chemical energy,efficient photocatalysis with suitable photocatalysts has becoming a promising technology to provide clean renewable energy for the sustainable development of human society.Due to mutilpe valance states and localized surface plasmonic resonance(LSPR),tungsten-based materials present unique photocatalytic activity from ultraviolet,visible light to near-infrared(NIR)wavelengths.When LSPR effect was excited by incident light,it could greatly enhance the local electricmagnetic filed,facitilize the generation of photocarriers and promote photocatalytic reaction of photocatalyst.Therefore,it is worth to systematically study the impact of LSPR on tunning the light absorption?superation of photocarricers and photocatalytic mechanism of tungsten-based photocatalyst,which can lay a theoretical foundation for its application in industrial wastewater treatment,photocatalytic degradation of organic pollutants,photocatalytic sterilization and other fields.Despite great efforts and achievements of numerous researchers in pursuing high and applicable photocatalysts,several scientific and technical issues remain need to be clarified.Firstly,in-situ growth of noble metal nanoparticles on the semiconductor generally demands extreme experimental environment and expensive experimental facilities.Low economic cost synthesis method remains difficult to be realized.Secondly,the mechanism of photoabsorption,band structure,plasmonic behavior and photocatalytic performance tuned by oxygen vacancies remains unclear.Thirdly,most of reported near-infrared driven photocatalyst suffer from the low photocatalytic efficiency,and few photocatalyst presents near-infrared plasmonic response.In addition,researches on NIR photocatalytic mechanism are still insufficient,especially for surface chemical reaction dynamic.Therefore,this dissertation is aimed to find ideal solutions for the above issues by taking the tungsten-based materials into consideration during the innovation process.The main contents of this dissertation are following:Firstly,to simplify the synthesis process of in-suit grow nanoparticle and disclosing the mechanism of the noble metal nanoparticles enhanced photocatalysis,a facial Na BH₄chemical reduction method was proposed to synthesize Ag nanoparticles with uniform size on Ag₂WO₄ nanowires.On the basic of density functional theory calculation,the impacts of surface silver nanoparticles on band structure and charge transfer are analyzed.The localized surface plasmonic resonance(LSPR)behaviors of surface Ag nanoparticles were also simulated by finite elementary analysis(FEA)and proved by absoption spectra,the results indicat that the plasmonic light absorption peak of Ag NPs was located at visible light region.The photocatalytic degradation of methylene blue(MB)was adopted to evaluate the outcome performance of the Ag NPs decorated Ag₂WO₄ nanowires.The results show that the photocatalytic rate and degradation rate of Ag/Ag₂WO₄ nanowires was increased significantly by 1.5 and 0.5 times,indicating the LSPR effect of Ag NPs has great contribution in the photocatalysis enhancement.For the lack of mechanism studies about the oxygen vacancy tuned light absorption,band structure,LSPR behaviors and photocatalytic performance on heterojunction,the WO_3-x/TiO_2-x with different oxygen vacancy circumstance was systematically investigated in chapter 3.The TiO₂ nanotube arrays were first synthesized via electrical reduction.Then tungsten oxide was deposited on the TiO₂ nanotube arrays by radio frequency sputtering to obtain WO_3-x/TiO₂.By air annealing and hydrothermal treatment,WO₃/TiO₂ and WO_3-x/TiO_2-x were further obtained.The impact of oxygen vacancies on the photocatalysis ability of WO₃/TiO₂ and WO_3-x/TiO_2-x was evaluated according to their performance on the photo absorbtion,photoelectron response and degradation of MB.On the basic of experimental results,a series of WO_3-x/TiO_2-x models with different oxygen vacancies were considered in the DFT calculation.The theoretical results are in great agreement with the experimental results,both indicating that the oxygen vacancies can reduce the band gaps of TiO₂ and WO₃,and enhance the LSPR effects of WO_3-x.Therefore,the synergetic effect of oxygen vacancy introduced Ti³⁺self-doping and WO_3-x LSPR enhancement were proposed to explain the improvement of PEC performance.Because most of reported near-infrared(NIR)driven photocatalyst present low reactivity,few available plasmonic NIR photocatalyst and insufficient NIR photocatalytic mechanism studies,a metallic tungsten carbide with plasmonic NIR absorption was proposed to realize high performance NIR photocatalysis.According to comprehensively electronic structure characterization,physicals properties measurement and NIR photocatalytic degradation of MB test,tungsten carbide nanoparticles show metallic nature,normal NIR plasmonic absorption and photocatalytic degradation of MB.Furthermore,the plasmonic NIR behaviors of tungsten carbide NPs also proved by DFT calculations and finite elementary analysis.Therefore,a plausible NIR photocatalytic mechanism based on the plasmonic NIR absorption of metallic tungsten carbide was proposed.To emphasize the concerns about insufficient theoretical research on the mechanism of near-infrared photocatalysis and its surface dynamics,a low work function and NIR plasmonic WN photocatalyst was proposed to realize high reactive oxygen species(ROS)activation and NIR driven photocatalysis.Firstly,the DFT calculations indicate that cubic phase WN present more desired lower work function and stronger plasmonic NIR light absorption.Then,cubic phase WN nanocubes were synthesized via hydrothermal-ammonia heat treatment,and the corresponding NIR photocatalytic performance and mechanism were also studied.The results show that WN nanocubes present a broad band NIR plasmonic absorption and normal photocatalytic degradation of MB.The FEA smunilations and absorption spectrum test also prove the plasmonic NIR light absorption behavior.The DFT calculations for H₂O and O₂on(100),(110),(111)crystal facets were further applied to simulate the surface chemical reaction dynamic.The results show that the(110)facet of WN presents the best catalytic performance on ROS activation owing to its lowest adsorption energy and largest charge transfer between the sliced surface and molecules.Therefore,by combining all experimental and theoretical calculations,the synergetic mechanism of low work function enhancing high ROS activation and LSPR NIR light absorption were proposed to explain the good NIR-driven photocatalytic performance of the metallic WN.