Nitrogen Doped Titanium Dioxide (N-TiO2): Synopsis of Synthesis Methodologies, Doping Mechanisms, Property Evaluation and Visible Light Photocatalytic Applications

Titanium dioxide (TiO2) is one of the stable and potential metal oxide semiconductor nanomaterials with flexible properties which allows them to be used in a variety of applications (i.e., environmental remediation, energy storage and production, and also as a pigment in personal care products, etc.). However, its low surface area, poor adsorption capacity and high bandgap energy (~3.2 eV) prevents its full potency. Especially, TiO2 with high bandgap (~3.2 eV) reduces its visible light absorption capacity and catalytic efficiency. Various modification processes (i.e., metal and non-metal doping, composite materials (mixed metal oxide, high surface area adsorbents), and dye sensitization etc.) have been accomplished for stimulating the characteristics of TiO2 and the associated catalytic efficiency. Among the modifications, the non-metal doping process in TiO2, specifically nitrogen doping, is one of the efficient dopants for enhancing the photocatalytic efficiency of TiO2 in the presence of visible light irradiation. However, the morphology of TiO2, structural changes in TiO2 during N-doping, properties (e.g., morphology and electronic) of N-doped TiO2 and also reaction operational parameters (e.g., doping concentration) hold a greater impact for enhancing the photocatalytic properties of TiO2 either positively or negatively. Furthermore, the synthesis methodologies have a major influence on the synthesis of stable N-TiO2 with pronounced photocatalytic efficiencies. Nevertheless, the methodologies for highly stable N-TiO2 synthesis, properties evaluation and their correlation with photocatalytic efficiencies are still not appropriately stabilized to accomplish the commercial utilization of N-TiO2. Therefore, this review article focuses on the synopsis of various synthesis methodologies and either their efficiencies or inefficiencies, the mechanism involved in the doping processes, changes in the structural, electronic and morphological properties observed due to the N-doping along with the photocatalytic capacity. Furthermore, the opportunities, challenges and future requirements linked to the development of durable N-doped TiO2-based semiconductor nanomaterials for efficient catalytic performance is also represented.