TiO2 and non-metal doped TiO2 nanoparticles: Synthesis and applications for green energy production

TiO₂ nanoparticles (NPs), due to their outstanding electrochemical properties, stability, low-cost and nontoxicity, have potential applications in multiple fields including green energy production. However, the practical applications of TiO₂ NPs as photocatalysts are limited due to their wide bandgap (i.e., 3.0–3.2 eV). Thanks to the non-metal doping strategy-a potential and a bandgap engineering approach to reduce the bandgap of semiconductors-the activation of TiO₂ NPs can be achieved by visible light via doping TiO₂ with non-metal elements. In this review, the promising synthesis methods of TiO₂ and non-metal doped TiO₂ (NM-doped TiO₂) NPs, including hydrothermal, sol-gel, ionic liquid assisted, electrospinning, and microbial-mediated and plant-mediated methods, have been summarized. In addition, their applications in green energy production (i.e., generation of H₂, CH₄ etc.) were discussed in detail. Moreover, the mechanism and kinetics of photocatalytic water splitting, photocatalytic reforming, and CO₂ and N₂ reduction by TiO₂ and NM-doped TiO₂ NPs were explained. Due to their higher efficiency, the applications of single-atom catalysts (SACs) anchored on TiO₂ for H₂ production have also been discussed. Finally, the major challenges and issues in practical applications of TiO₂-based nanomaterials for green energy production have been underlined. This review serves as a roadmap for the researchers working in the field of environmental sustainability and green energy production to twist the TiO₂-based nanomaterials to overcome their inherent shortcomings which prevent them from large-scale practical applications.