Abstract

Background: Titanium dioxide (TiO₂) powders with visible-light photoactivities are in great demand for application in solar cells and solar photocatalysis. Although pure TiO₂ is active only under ultraviolet light, its band gap can be narrowed down to the visible-light by impurity doping and creation of surface defects. Visible-light active anatse TiO₂ powder attracts a great research interest because of its higher efficiency observed in water splitting, hydrogen generation and solar photocatalysis.

Methods: Nitrogen doped TiO₂-C composite nanoparticles were synthesized in large scale by a simple and efficient chemical reaction using oleic acid and titanium tetra-isopropoxide as the precursors. The composition of the powder was analyzed by X-ray photoelectron spectroscopy, the structure was studied with X-ray diffraction and transmission electron microscopy and the optical property was studied by uv-vis absorption. The visible photocatalytic activity of black TiO₂ powder was investigated by studying the photobleaching of methylene blue under solar irradiation. The effect of surface concentration of the carbon and nitrogen on the visible photocatalytic efficiency of the black-TiO₂ was studied and reported. White TiO₂ powder without surface carbon was also synthesized and all the properties of the above was compare with that of the black TiO₂ powder.

Results: Black TiO₂ was purely anatase whereas white TiO₂ had mixed phase character with 68% anatase and 32% rutile. Compared to white TiO₂, black TiO₂ could absorb more visible light. Unlike white TiO₂, Black TiO₂ is an efficient solar photocatalyst responsible for the photodegradation of 52 µM aqueous solution of MB with a first order rate constant, k = 0.02 min-1. The photocatalytic activity of black-TiO₂ strongly depended on the surface carbon concentration. The photocatalytic activity of black-TiO₂ increases with increased carbon content. When the surface concentration exceeded 28%, the catalytic activity decreased presumably due to slower rate of charge transfer onto the TiO₂ nanoparticle surface. No such systematic change in the photocatalytic activity of black-TiO₂ was observed with change in its N content.

Conclusion: The visible-light active TiO₂-C nanocomposite powder is very important for environmental applications. Environmental remedies such as water purification, prevention of indoor air pollution and outdoor self-cleaning process can be taken up with a low cost but highly efficient black TiO₂-C photocatalyst.

Keywords: Black titanium dioxide, raman spectroscopy, transmission electron microscopy, visible photocatalysis.

Graphical Abstract