WO₃ Heterojunction Photocatalysis: Design,Photocatalytic Performance,and Mechanism

Solar energy has received widespread attention,due to the advantages,such as huge storage,environment friendly,no need to storage and transportation.In recent years,energy shortages and environmental pollution have become the focus of world attention.As one of the most promising solutions for these problems,semiconductor photocatalysis has attracted much attention,since it is a green technology for decomposing water into hydrogen and oxygen.To efficiently utilize the visible region,which covers the largest proportion of the solar spectrum,the development of visible-light-driven photocatalysts is the current trend.Researches on WO₃ have received extensive attention because of its attractive properties such as narrow band-gap,visible light consumption,non-toxicity,stable physicochemical properties and resistance to photocorrosion.However,photocatalysis activity of WO₃ is still an unavoidable drawback hindering their practical application.There are three mainly processes of photocatalysis,contain the light-absorption of the materials,the separation and migrated of electron-hole pairs,and the catalytic reaction on the materials surface.Among these processes of photocatalysis,the separation and migrated of electron-hole pairs is quite important,which closely affect the photocatalytic performance.Therefore,promoting the separation efficiency of the photo induced electron-hole pairs is the important research direction for increasing photocatalytic performance.In order to inhibit the recombine of the charge carriers in WO₃,a novel way by fabricating of“junctions”,including phase junction and heterojunction,on semiconductor surfaces has been demonstrated to be an effective strategy for promoting charge separation in photocatalysis.Our recent results indicates fabricating of hexagonal/monoclinic phase junctions in WO₃ by a simple annealing process has led to a 2 times higher photodegradation rate for RhB than the pure monoclinic WO₃?m-WO₃?sample.Furthermore,we fabricate heterojunction on WO₃ surfaces to improve the photocatalytic efficiencies of photocatalysts,in this work.A series of MWO₄/WO₃?M=Cu?Zn?Ni?heterostructure were obtained by in-situ synthesis reaction on the surface of WO₃ base,since the process of the WO₃ base dipping in various concentrations of nitrate?M=Cu?Zn?Ni?solutions and then calcination.The crystalline phases of the obtained MWO₄/WO₃ samples were investigated using X-ray diffraction?XRD?.The morphology of the samples were characterized by scan electron microscopy?SEM?,transmission electron microscopy?TEM?and high resolution transmission electron microscopy?HR-TEM?.The specific surface area of the samples were calculated from N₂adsorption-desorption?BET?,and the optical properties of the obtained samples were characterized by UV-visible diffuse reflectance spectra?UV-Vis DRS?and photoluminescence spectroscopy?PL?.The photocatalytic activity was studied by calculating the degradation efficiency of the RhB solution under visibile light irradiation.The results indicate that MWO₄/WO₃photocatalyst exhibit a higher photocatalytic activity than that of pure WO₃.The reactive species in the photocatalytic reactions such as superoxide radical anions?·O₂^-?,hydroxyl radical?·OH?,and holes?h⁺?,were investigated by adding the radical scavengers and terephthalic acid photoluminescence probing technique?TA-PL?.The results reveal that the efficiency of CuWO₄/WO₃ increases in the order of·OH<h⁺<·O₂^-,furthermore,only h⁺and·O₂^-groups react in ZnWO₄/WO₃ and Ni WO₄/WO₃,in which the efficiency increases in the order of h⁺<·O₂^-.The second,CuS-WO₃ composite photocatalysts were synthesized by a facile one-step chemical bath method at low temperature.The structure,morphology and optical property of CuS-WO₃ heterogenous samples were characterized by XRD,SEM,XPS?X-ray photoelectron spectroscopy?,UV-Vis and PL,respectively.The degradation results of the RhB solution indicate that the highest degradation efficiency?96%?was obtained since the compound percentage of CuS-WO₃ was 1 wt%,which was higher than its individual compositions.The reactive species in the photocatalytic reactions were investigated by adding the radical scavengers and Nitrotetrazolium Blue chloride?NBT?transformation technology,and proposed a direct Z-scheme photocatalytic mechanism.Compared with CuS-WO₃ heterostructure using liquid phase method,in this part the CuS was fabricated by an element-direct-reaction route using copper and sulfur powder,in order to in situ load CuS on WO₃ surface at low temperature.The structure,morphology and optical property of CuS-WO₃heterogenous samples were characterized by XRD,SEM,XPS,UV-Vis DRS and PL,respectively.As a result,the structure,size and specific surface area of WO₃ have not significant effectted by adding CuS,except increasing the vis-light absorption of WO₃.The degradation results of the RhB solution indicate that the CuS-WO₃ heterostructure photocatalysts exhibit higher photocatalytic activity than that of single CuS or WO₃ sample.Based on the uv-vis analysis,PL,and the active species trapping testing results of band CuS and WO₃,a“direct Z-scheme”mechanism for the enhanced photocatalytic activity of CuS-WO₃ was proposed.