Mechanistic Study of Chemical Deposition of ZnS Thin Films from Aqueous Solutions Containing Zinc Acetate and Thioacetamide by Comparison with Homogeneous Precipitation

Chemical bath deposition of ZnS thin films (CBD) in zinc acetate-thioacetamide (TAA) aqueous solutions has been studied in the temperature range between 50 and 90 °C. The rate of film growth is compared to that of homogeneous precipitation of ZnS. The film growth was found to proceed by accumulation of ZnS nanocrystallites (4 nm) formed in the solution according to a cluster-by-cluster mechanism. The rate of the precipitation reaction in the present system has been phenomenologically well described by the rate equation, dP_t/dt = k[Zn(II)]ⁿ[TAA]ⁿ, where P_t is the concentration of ZnS precipitate at time t, k is the temperature-activated reaction rate constant, and n is the apparent reaction order, which was determined as 0.64. The same rate equation was successfully applied to the film growth based on the assumption that the rate of the film growth is proportional to that of the precipitation, i.e., F_t = mP_t, where F_t is the film thickness at time t and m is a proportionality factor. This result shows that the rate of the film growth is under the control of ZnS precipitation in the solution. However, linear increase of m has been found when the ratio of initial concentrations of the two components, [Zn(II)]₀/[TAA]₀, is larger than unity, indicating the increase in the rate of the film growth, while m remains constant when [Zn(II)]₀/[TAA]₀ is less than unity. The variation of m and the coincident morphological change of both the film and the precipitate indicate enhanced aggregation of ZnS nanocrystallites in the presence of excess zinc ion, which is most likely explained by a decrease of the repulsive electrostatic force among ZnS nanocrystallites by charge neutralization. The rate-limiting step for the formation of ZnS clusters is attributed to the initial decomposition of a complex between zinc acetate and thioacetamide species.