TiO2–Biochar-Based Photocatalysts for Organic Pollutants Removal: From Synthesis Parameters to Degradation Performance

TiO2–biochar-based photocatalysts are one of the materials exhibiting adsorption-photocatalytic synergy. They have been widely used in the remediation of water systems. Current reviews in this field predominantly focus on the following aspects. These include the preparation methods for composite materials, the pollutant removal performance, the adsorption–photocatalytic synergy, and environmental applications. However, there are still gaps in understanding the intrinsic relationships among photocatalyst morphology, surface functional groups, reactive oxygen species (ROS) generation, pollutant removal, and interfacial charge-transfer mechanisms. This restricts the potential for further enhancement of photocatalytic performance. To fill this gap, this review provides a comprehensive summary of the impact of various parameters on the morphology of TiO2–biochar-based photocatalysts during in situ synthesis. These factors include titanium sources, carbon sources, preparation methods, solvents, pyrolysis conditions, and doping modifications. Further analysis is conducted to investigate the effects of morphological structure on the distribution characteristics of surface functional groups (e.g., oxygen- and nitrogen- containing groups), the generation of ROS, and the removal behavior of organic pollutants. Furthermore, this review focuses on the effects of three typical morphologies. The three typical morphologies include surface-adhered, pore-embedded, and interlayer-distributed. The role of morphology in charge transport behavior at interfaces is also examined. We systematically elucidate the mechanisms of coupled interactions among material morphology, surface functional groups, ROS, interfacial charge transport, and photocatalytic performance. An analytical framework is established to explore the relationships among morphology control, structural characteristics, and photocatalytic performance. Lastly, the limitations of TiO2–biochar-based photocatalysts in environmental remediation processes are summarized. It also points the way forward for future development. Overall, this review provides a new theoretical perspective on the rational design and environmental applications of high-performance TiO2–biochar-based photocatalysts.