Design of TiO₂-Based Semiconductor Materials for Near-Infrared Photocatalytic Water Remediation

Abstract

The development of semiconductor materials capable of utilizing low-energy solar radiation remains a major challenge for sustainable photocatalysis and advanced water remediation. In this work, TiO₂-based mixed oxides modified with Zn and Cu were synthesized and investigated to elucidate the relationship between their structural, surface, optical, and photocatalytic properties under near-infrared (NIR) irradiation. The materials were prepared through a sol–gel route and systematically characterized by X-ray diffraction, electron microscopy coupled with energy-dispersive spectroscopy, surface and vibrational analyses, zeta-potential measurements, and UV–Vis–NIR diffuse reflectance spectroscopy. Optical characterization revealed pronounced modifications in the electronic response induced by transition-metal incorporation, with the estimated optical band gap decreasing from approximately 3.20 eV for pristine TiO₂ to 2.73 and 2.54 eV for Zn- and Cu-modified systems, respectively. These changes were accompanied by measurable alterations in surface charge and Ti–O vibrational environments, indicating that metal incorporation modifies the electronic and interfacial structure of the semiconductor. Photocatalytic experiments were conducted using representative organic dyes and antibiotics as model water contaminants under controlled irradiation conditions. The modified materials exhibited measurable photocatalytic activity under NIR irradiation, whereas pristine TiO₂ showed negligible activity. For methyl orange, TiO₂–Zn and TiO₂–Cu reached residual concentration ratios of approximately 0.80 and 0.78–0.80, respectively, after irradiation, demonstrating that transition-metal modification can activate otherwise inaccessible low-energy excitation pathways. Kinetic analysis using Langmuir–Hinshelwood, Weibull, Avrami, and fractal models further revealed contaminant-dependent degradation behavior. These findings demonstrate that transition-metal engineering provides a viable strategy for tailoring the physicochemical properties of TiO₂ toward solar-driven environmental remediation.

Keywords: TiO₂ modification; Near-infrared photocatalysis; Water remediation