Ambient humidity control during dip coating: a critical parameter for tailoring microstructure and photocatalytic activity of CdO + CdTiO₃ sol–gel thin films

This study investigates the impact of absolute humidity (AH) at the substrate extraction point during the dip coating process on the microstructure and photocatalytic performance of multicomponent CdO + CdTiO₃ thin films produced by sol–gel. Unlike prior research that focused solely on the water content in the sol–gel solution, this study demonstrates the critical importance of controlling AH during film formation. The precursor composition (water content and Ti/Cd ratio) and withdrawal speed were kept constant to isolate the influence of AH during film formation. This study reveals that varying AH from 3.47 to 29.28 g/m³ significantly influences the film uniformity, the crystalline grain distribution, and the photocatalytic degradation of methylene blue (MB). Notably, increasing AH enhances photocatalytic activity, doubling the reaction rate constant (k) to around 0.6 h⁻¹. This improvement is due to a more uniform distribution and closer crystalline grain sizes of CdO and CdTiO₃ phases_, which leads to improved coupling. The size of the crystalline grains varies linearly with the increase in AH from 11.5 g/m³. The crystalline size of CdTiO₃ increases as CdO decreases, with both phases approaching 20 nm. The findings show that proper management of ambient humidity during extraction improves structural homogeneity and photocatalytic activity.

Homogeneous Cd and Ti atomic distribution from EDS images in S7 film deposited at the highest AH during its coating, concerning S1 with the lowest AH. The highest MB photodegradation of S7 is attributed to the best CdO and CdTiO₃ grains coupling.