A review on production, characterization, and photocatalytic applications of TiO₂ nanoparticles and nanotubes

Background: We present a review on recent progress in the research and development of titanium dioxide (TiO₂) nanomaterials. Based on the studies reported in the literature, titanium oxide nanoparticles and nanotubes (NPs and NTs) can be fabricated by inexpensive methods, such as solgel deposition and anodization process. Typically, TiO₂ NPs are synthesized in a particle size less than 10 nm, and the featured dimensions of TiO₂ NTs are about 100 nm in pore diameter, 10¹⁰ pores/cm² in pore density, 25 nm for wall thickness, and several microns in length perpendicular to surface. Methods: Anodic oxide films are normally amorphous because of the formation of defect clusters and the presence of considerable internal mechanical stresses. However, a careful pre-polishing procedure and a slow increasing rate in anodizing voltage (0.1 mV/s) is beneficial to fabricate crystalline epitaxial thin films. Furthermore, with an ultra-slow anodizing condition up to 1 V (saturated calomel electrode; SCE), crystalline TiO₂ with a thickness of 1.65 nm can be formed. Results: In order to increase the effective surface area of TiO₂, the TiO₂ NPs are deposited on TiO₂ NT surfaces. The effect of the TiCl₄ treatment on TiO₂ NT films has been reported to increase the surface area, and thus to enhance the photocatalytic applications. During the deposition process, the Ti(OH)₄ is formed from TiF₄ or TiCl₄ solution by controlling pH value and temperature. Then, the Ti(OH)₄ can be transformed to TiO₂ after heat treatment. TiO₂ NTs/NPs have a larger surface area than a compact TiO₂ film, more materials can be absorbed on the TiO₂ NT surface. With the anodic electrolyte of a higher pH value, the TiO₂ NT film with a longer length and a higher surface area will be produced. Large-scale manufacturing of high-quality and inexpensive TiO₂ NTs/NPs film can be potentially utilized for academic research and industrial applications. Conclusion: The dense TiO₂ film can be formed on matrix Ti through heat-treatment or interference anodization methods. However, anodizing titanium foil in a F− ions and minimizing water content in non-aqueous organic polar electrolyte are beneficial for the formation of closely packed and vertically aligned nanotubes.