Photocatalytic Activity Enhancement of ZnO/CZTS Thin Film Heterostructures in Visible Region: Controlling Phase Pure Kesterite CZTS Thin Films and the Effect of the Number of Seed Layers on Photocatalytic Performance

Abstract

This work focuses on optimizing the structure of CZTS/ZnO thin film heterostructure to increase the photocatalytic property in visible region. The CZTS thin films were fabricated on glass substrates by dip coating method. Then ZnO thin films were grown on the CZTS layer by chemical bath deposition method at low temperature. Secondary phase formation in CZTS thin films with four different molar ratios of Cu : Zn : Sn : S of 1.4 : 1 : 1 : 8, 1.6 :1 : 1 : 8, 1.8 : 1 : 1 : 8 and 2 : 1 : 1 : 8 were investigated. In order to improve the crystallinity, CZTS thin films were annealed at 450°C in N₂ atmosphere without sulfurization. The results showed that the CZTS thin film exhibited phase pure kesterite structure and good crystallinity at the ratio of 1.8 : 1 : 1 : 8 (Cu : Zn : Sn : S), absence of secondary phase formation in CZTS thin film was important to reduce charge recombination. For ZnO/CZTS thin film heterostructure, in order to reduce the lattice mismatch of ZnO and CZTS, some seed layers were coated onto the surface of CZTS thin film to support the growth of ZnO thin film in these heterostructures. Raman spectra, XRD patterns and SEM images data confirm that the ZnO/CZTS thin film heterostructures with two seed layers achieved optimal crystal structure and film morphology. UV-Vis spectra of heterostructures suggested that photocatalytic activity is extended toward visible-light region from 380 to 800 nm. Finally, the photocatalytic performance was examined through degradation of methylene blue (MB) and apparent rate constant (k_app), namely: the highest degradation of MB was obtained with ZC2 labelled heterostructure samples up to 92.8% in 150 min; k_app of this sample is much larger (as high as 6.5 and 29.2 times respectively) than that of pristine CZTS and ZnO films.