Sol–gel derived WO₃–TiO₂ films for photocatalysis

ABSTRACT Semiconductor-based heterogeneous photocatalysis has received considerable attention as a sustainable advanced oxidation process for the degradation of persistent organic pollutants in aqueous environments. In this context, TiO₂–WO₃ composite thin films have been widely proposed as promising photocatalytic materials, with WO₃ incorporation expected to suppress electron–hole recombination and extend light absorption into the visible region through heterojunction formation. However, the relationship between composite composition, film microstructure, and actual photocatalytic performance remains incompletely understood, particularly for films deposited by the sol–gel route. In this study, WO₃–TiO₂ composite thin films were fabricated by the sol–gel method and deposited onto substrates via spin coating. Films with varying WO₃ molar ratios were prepared and subjected to controlled thermal treatment to induce crystallization. The structural and morphological properties of the films were characterized using X-ray diffraction (XRD), which confirmed the formation of the anatase TiO₂ phase and provided evidence for WO₃ incorporation at higher loadings. Scanning electron microscopy (SEM) revealed a uniform, crack-free film morphology with grain sizes in the nanometric range. Ultraviolet–visible (UV–Vis) spectroscopy was employed to determine the optical band gap of the films as a function of WO₃ content, confirming a progressive red-shift in absorption onset with increasing tungsten loading. Photocatalytic performance was evaluated by monitoring the degradation of methylene blue (MB) dye in aqueous solution under UV irradiation. Contrary to expectations reported in several composite studies, the incorporation of WO₃ into the TiO₂ matrix did not yield a measurable improvement in photocatalytic degradation efficiency relative to the unmodified TiO₂ reference films prepared under identical conditions. This finding suggests that, under the synthesis and testing conditions employed, the anticipated charge-transfer enhancement at the WO₃–TiO₂ interface was not realized, possibly due to insufficient interfacial contact, phase segregation at higher WO₃ loadings, or a mismatch between the available UV photon flux and the absorption characteristics of the composite films. These results contribute to a more nuanced understanding of the factors governing photocatalytic activity in sol–gel-derived WO₃–TiO₂ thin film systems. The study highlights the critical importance of synthesis parameters ,including precursor chemistry, annealing temperature, and film deposition conditions, in determining the microstructural quality of the heterojunction interface and, consequently, the photocatalytic response. The findings underscore the need for systematic optimization studies and provide a basis for future work directed at improving interfacial charge-transfer efficiency in composite metal oxide photocatalytic films.