Preparation And Photocatalytic H₂ Production Activity Of One-dimensional ZnO Or ZnS-based Composite Photocatalysts

With the progress of the society and the development of science and technology,energy shortage and environmental pollution has become serious problems facing the world today. As a clean energy, hydrogen has received extensive attention.There are a lot of methods to produce hydrogen, among them the photocatalytic water splitting is the most environmental friendly method, since it just needs light, water, and photocatalyst.ZnO and ZnS can quickly generate electron-hole pairs upon light irradiation, and the positions of the conduction band are more negative than the reduction potential of H⁺/H₂, thus H+ will be reduced to H₂. However, due to the wide bandgap, ZnO and ZnS only absorb ultraviolet light, which greatly limits their practical applications. Therefore, it is significantly important to make ZnO and ZnS responsive to visible light. In this thesis, composites of ZnO nanorods and ZnS nanotubes with other semiconductors with narrow bandgap（CuS, CdS and In₂S₃） are synthesized, and their photocatalytic property for hydrogen production from Na₂S and Na₂SO₃ aqueous solution in the absence of any cocatalysts under visible light irradiation are investigated. Our study includes the following three parts:1. A series of ZnO/ZnS-CuS core/shell nanorods with different Cu2+/Zn2+ molar ratios are synthesized by low-temperature growth, sulfuration reaction and subsequent cation exchange technique. The ZnO/ZnS-CuS nanocomposites are characterized by SEM, TEM, XRD, XPS, PL, UV-vis DRS. And their photocatalytic performance for H2 generation under 300 W Xe-lamp（λ > 400 nm） irradiation is investigated. The as-prepared ZnO/ZnS-CuS core-shell nanorods have a wide and strong photo absorption in the visible region and exhibit a high photocatalytic activity for H₂ generation from 0.35 M Na₂ S and 0.25 M Na₂SO₃ mixed aqueous solution. The rate of H₂ generation obviously depends on the content of CuS in ZnO/ZnS-CuS composite. The highest rate of H₂ generation reaches 52.75 μmol·h-1, which is more than 50 times higher than that of ZnO/ZnS nanorod. The excellent photocatalytic performance of ZnO/ZnS–CuS core/shell nanorods may be due to the heterostructure and efficient charge separation.2. ZnS/CuS nanotubes with a length of hundreds of nanometers to several micrometers and a thickness of about 40 nm in tube wall are synthesized by sulfuration reaction, followed by treatment with KOH and subsequent cation exchange reaction. Their photocatalytic performance for H₂ generation under 300 W Xe-lamp（λ > 420 nm） irradiation is studied. The as-prepared ZnS/CuS nanotubes with 2.27 mol% of CuS exhibit the highest rate of H₂ production with 56.47 μmol·h-1, which is more than 85 times higher compared with pure ZnS nanotubes. The high visible light photocatalytic H₂-production activity may be due to the higher specific surface area, the heterostructure and interfacial charge transfer（IFCT） from the valence band of ZnS to CuS, which causes the reduction of partial CuS to Cu₂S and thus enhances the photocatalytic activity.3. ZnO/CdS-In₂S₃ heterostructure nanorods are synthesized through surface functionalization by using citric acid as a functionalizing agent, followed by a cation exchange reaction. The photocatalytic activity and stability for hydrogen production from 0.35 M Na₂S and 0.25 M Na₂SO₃ aqueous solution under visible light（λ > 420 nm） irradiation have been significantly improved after introducing a small amount of In₂S₃. The highest rate of H₂ production is 78.41 μmol·h-1, which is two times of that of ZnO/CdS nanorod. The excellent photocatalytic activity may be originated from the formation of heterostructure and efficient inhibition of photocorrosion by In₂S₃.