Tailoring oxygen vacancies in HfO2-x and constructing In2S3/HfO2-x/SnS2 dual direct Z-scheme heterojunctions for efficient visible-light photocatalytic degradation of tetracycline

Abstract

Hafnium dioxide (HfO₂) is known for its high thermal and chemical stability. However, despite these advantages, it remains inactive under visible light due to its wide bandgap. In this study, HfO<inf>2-x</inf> with high level of Ov concentration was prepared using a scalable solution-based synthesis confirmed by EPR analysis. By employing this method, the HfO<inf>2-x</inf> average particle sizes can be minimized to 10.6 nm validated by HRTEM. While the incorporation of Ov enhances visible light absorption, it also shifts the band edges unfavorably, reducing redox potential and promoting charge recombination. To overcome this challenge, a dual direct Z-scheme heterojunction was constructed by coupling HfO<inf>2-x</inf> with In₂S₃ and SnS₂, forming the In₂S₃/HfO₂₋ₓ/SnS₂ (IS3) composite. The photocatalytic activity of IS3 achieved 66.4 % degradation of 20 ppm tetracycline under visible light irradiation in 30 min. This primary advantage of IS3 in photocatalytic activity can be attributed to the effective charge carrier separation and reducing recombination compared to other catalysts. The multi-heterojunction, driven by differences in Fermi levels and their disparity, promoted efficient charge separation which was confirmed through TRPL. Moreover, the XPS analysis has shown the formation of new bonds resulting in multi-channel for charge transfer reducing recombination. Characterization (XRD, SEM-EDX, FTIR, and UV–Vis spectroscopy) confirmed structural and electronic modifications. Mechanistic insights were provided through Mott-Schottky, KPFM, and degradation pathway studies (TOC, COD, and LC-MS/MS). These findings underscore the development of the inducing Ov method in semiconductors and heterojunction engineering for the rational design of next-generation photocatalysts environmental remediation.