Black titanium oxide: synthesis, modification, characterization, physiochemical properties, and emerging applications for energy conversion and storage, and environmental sustainability

Since its advent in 2011, black titanium oxide (B-TiO_x) has garnered significant attention due to its exceptional optical characteristics, notably its enhanced absorption spectrum ranging from 200 to 2000 nm, in stark contrast to its unmodified counterpart. The escalating urgency to address global climate change has spurred intensified research into this material for sustainable hydrogen production through thermal, photocatalytic, electrocatalytic, or hybrid water-splitting techniques. The rapid advancements in this dynamic field necessitate a comprehensive update. In this review, we endeavor to provide a detailed examination and forward-looking insights into the captivating attributes, synthesis methods, modifications, and characterizations of B-TiO_x, as well as a nuanced understanding of its physicochemical properties. We place particular emphasis on the potential integration of B-TiO_x into solar and electrochemical energy systems, highlighting its applications in green hydrogen generation, CO₂ reduction, and supercapacitor technology, among others. Recent breakthroughs in the structure–property relationship of B-TiO_x and its applications, grounded in both theoretical and empirical studies, are underscored. Additionally, we will address the challenges of scaling up B-TiO_x production, its long-term stability, and economic viability to align with ambitious future objectives.