Application of Black Titanium Dioxide and Its Composite Materials in Photocatalytic Degradation of Water Pollutants

Abstract

Photocatalysis, prized for its effectiveness, environmental soundness, and affordability, is finding promise across an ever-widening range of uses. Titanium dioxide (TiO₂) remains the benchmark photocatalyst, yet its inherently broad bandgap restricts activation to ultraviolet wavelengths, while rapid electron–hole recombination additionally saps its efficiency. Black titanium dioxide (TiO₂₋ₓ), formed by embedding surface defects — oxygen vacancies above all — overcomes these limitations: the shrunken bandgap together with the proliferation of active sites strengthens its response and quantum yield throughout the visible and near-infrared regions, elevating overall catalytic performance. Consequently, how TiO₂₋ₓ is synthesized, modified, and incorporated into composites has become a subject of intense investigation. Herein, we delineate the principal defect architectures of TiO₂₋ₓ and the analytical methods suited to probing them, recapitulate its major fabrication routes, and review available modification strategies. Particular attention is devoted to the behavior of TiO₂₋ₓ and its hybrids in degrading aqueous pollutants, with the relevant decomposition mechanisms dissected and consolidated. We close by appraising the obstacles and opportunities that will shape the deployment of TiO₂₋ₓ-based materials in future water-remediation efforts.