Study On Photocatalytic Performance Of Direct Z-Scheme Heterojunction Based On 2D WO₃ Nanosheets

As an environment-friendly and sustainable technology,semiconductor photocatalysis has attracted extensive attention in the fields of water decomposition,carbon dioxide reduction and pollutant degradation.However,the efficiency of photocatalytic reaction is still very low due to the rapid recombination of electron-hole pairs and low utilization of light.To solve these problems,many heterojunction photocatalyst systems have been designed,which usually occur traditional type-?heterojunction photocatalytic mechanism.The traditional heterojunction system can effectively promote the spatial separation and separation efficiency of the carriers,but these advantages are at the cost of the redox capacity of the carriers.Therefore,the direct Z-scheme heterojunction system which can reduce carrier recombination and maximize photocatalyst redox potential is considered to be a better solution strategy.It is worth noting that the work function of the photocatalyst used for reduction should be smaller than that of the photocatalyst used for oxidation in order to successfully construct Z-scheme heterojunction photocatalyst.WO₃ has been used as a visible light driven photocatalyst with a band gap of about2.4?2.8 eV and a valence band(VB)potential of 2.7?3.4 eV.The advantages of high VB potential and visible light drive properties make WO₃ has great potential in the construction of high performance heterojunction photocatalysts.Looking for a semiconductor with low conduction band(CB)potential to construct a direct Z-scheme system photocatalyst with WO₃ is an effective way to solve the problem of carrier recombination and improve the photocatalytic efficiency.As a medium band gap(2.7eV)semiconductor,g-C₃N₄ has good visible light response,high stability and low CB position(?-1.2 eV),which makes it an excellent for a coupling reduction semiconductor.In addition,SiC with appropriate band gap(2.3?3.3 eV)and low CB position(?-1.4 eV)is also a research hotspot,and the study of direct Z-scheme WO₃/SiC photocatalyst has not been reported.In this paper,two-dimensional(2D)WO₃were prepared by simple water oxidation precipitation method,WO₃/g-C₃N₄ direct Z-scheme photocatalyst and WO₃/SiC direct Z-scheme photocatalyst were constructed,and their photocatalytic performance and photodegradation mechanism were explored.The main research contents and results of this paper are as follows:(1)The standard monoclinic WO₃ crystals with different morphology scales was prepared by a simple water oxidation precipitation method.2D nanosheets with the size of 100?200 nm were prepared by adding nitric acid at 80?(WO₃-HNO₃),and the flake structure with the size of more than 1?m was prepared by adding hydrochloric acid at room temperature(WO₃-HCl).The photodegradation efficiency of Rh B(5 mg/L)by WO₃-HNO₃ was 1.99 times that of WO₃-HCl.Photoluminescence(PL)spectroscopy and transient photocurrent response analysis showed that WO₃-HNO₃ has higher separation efficiency of photogenerated electron-hole pairs due to the structure of the2D nanosheet,thus enhancing the photocatalytic degradation activity.(2)The direct Z-scheme heterojunction WO₃/g-C₃N₄ was constructed by the same preparation method to maximize the redox potential of photocatalyst WO₃/g-C₃N₄.The samples were characterized by XRD,SEM,XPS,UV-Vis diffuse reflectance spectroscopy and active species trapping experiments.The results indicated that the 2D WO₃ were tightly covered on the surface of g-C₃N₄ and formed Z-scheme heterojunction.The degradation efficiency and rate constant of Rh B(8 mg/L)photodegraded by WO₃/g-C₃N₄-1:2 were 7.1 times and 33.9 times higher than that of WO₃,respectively.In addition,WO₃/g-C₃N₄ composites have good photostability and reusability.The photocatalytic mechanism of the composites and the main active species in the process of photodegradation were investigated by free radical trapping experiment and energy band structure.(3)The direct Z-scheme photocatalyst WO₃/SiC(WS)was prepared for the first time.The composition and interfacial band structure of direct Z-scheme heterojunction WO₃/SiC(WS)were characterized by XPS,KPFM and Mott-Schottky methods.The degradation efficiency and rate constant of WS-1 photodegradation of Rh B(8 mg/L)were 2.5 times and 5.3 times higher than that of WO₃,respectively.ESR tests and free radical trapping experiments confirmed that the WS-1 photocatalyst produced·OH and·O₂^-active species,further confirming that the photogenerated carriers are transmitted through the Z-scheme mode.The band structure studies showed that the direct Z-scheme structure composed of WO₃ with high VB and SiC with low CB could maintain the high photocatalytic activity of the active species.The research in this paper will provide a feasible strategy for the design and construction of novel direct Z-scheme photocatalyst.