The Effect of Spin Center Parameters on the Photoactivity of Nanocrystalline Titanium Dioxide in the Visible Spectral Range

Abstract

Nanocrystalline TiO₂ samples, obtained by sol–gel and supercritical fluid (SCF) methods, have been studied by optical spectroscopy and electron spin resonance. It is established that the main type of spin centers in SCF samples are Ti³⁺/oxygen vacancy centers and conduction electrons. Radicals (${{{\text{N}}}^{ \bullet }}$, ${\text{N}}{\kern 1pt} {\text{O}}{{{\kern 1pt} }^{ \bullet }}$) and (${{{\text{C}}}^{ \bullet }}$) are found in the samples doped with nitrogen and carbon, respectively. The energy levels of defects in the band gap of the structures studied are determined. It is explained why samples with close values of nanocrystal sizes, specific surface area, and defect concentration have a significantly different photocatalytic activity in the visible spectral range. It is found that impurity centers with energy levels near the middle of the semiconductor band gap must be incorporated into TiO₂ to obtain samples with the highest photocatalysis rate. New possibilities for developing energy-efficient (without UV irradiation) photocatalysts are discovered.