Nanoscale 2D semi-conductors – Impact of structural properties on light propagation depth and photocatalytic performance

The performance of photocatalytic materials are largely dictated by the crystalline and optical properties of the semi-conductors. The density of photo-generated electron and hole pairs is greatly influenced by the light penetration in photocatalytic material, leading to specific degradation kinetics. In this work, the relationship between the metal oxide film thickness and the overall materials optical and photocatalytic performances are systematically established for the first time. Thin films of semiconductor metal oxides such as TiO₂ and ZnO were prepared by atomic layer deposition (ALD) on stainless steel sputtered silicon wafers. The thickness of the metal oxide thin films was controlled by varying the number of deposition cycles (50–1000 cycles). The fabricated films were fully characterized to examine the change in morphology, roughness, crystallinity, optical and structural properties with varying thickness by several techniques such as Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), X-Ray Diffraction (XRD), Diffuse Reflectance Spectroscopy (DRS) and X-Ray Photo-electro Spectroscopy (XPS). The films were generated to yield very consistent crystallinity, roughness and light absorption properties. Critical thicknesses were observed when a plateau in photocatalytic efficiency was reached at the thickness of 31 nm in the case of the TiO₂ and 89 nm thickness for ZnO films. The dependency of the thickness of nanometric ALD films on their photocatalytic efficiency results from the light diffusion and penetration within the material which was investigated through fundamentals and modelling of light-matter interaction in photocatalytic processes. This work establishes a new fundamental understanding of the operation and performance of photocatalysts for further development of advanced reactors and their scale-up.