Structural, optical, photocurrent and mechanism-induced photocatalytic properties of surface-modified ZnS thin films by chemical bath deposition

ZnS thin films were prepared by chemical bath codeposition using ZnSO₄–ZnCl₂ or Zn(CH₃COO)₂–ZnCl₂ as zinc ion sources. The presence of SO₄ ²⁻ favors the heterogeneous growth of ZnS thin film. The coexistence of two zinc salts impedes the formation of homogeneous precipitation and improves the growth rate of ZnS film. XRD and HRTEM results show that all the samples exhibit the cubic structure. EDS analysis shows that Zn/S atom ratios from the codeposition are closer to 1:1 than those deposited from a single zinc salt, and ZnS thin films of S3 and S7 are very uniform without stirring. FTIR reveals that –NH₂ group as a surface modifier is adsorbed on the surface of ZnS nanoparticles. Raman spectra further reveal that S3, S4 and S7 form the ZnS films, and ZnO phase is present in short or middle range of the S6 nanocrystal, indicating that different amounts of zinc salts affect the structure of ZnS films significantly after three 2.5 h deposition cycles. The grain sizes determined by FESEM are inversely proportional to RMS determined by AFM. The band gap values of ZnS thin films agree well with the results of HRTEM. The photocurrent responses of different samples are similar, indicating that different amounts of zinc salts have little effect on the photocurrent of ZnS films. The photocatalytic performance of S6 and S8 is much better than that of S1–S5. S6 decomposes 65 % of methyl orange within 3 h, and its K value is 4.78 × 10⁻¹ h⁻¹. The photocatalytic performance is induced by the growth mechanism, which determines the grain size of ZnS thin film. The tendency of grain sizes of ZnS films agrees well with that of photocatalytic performance, especially under the clusters by clusters deposition.