Carbon-doped titania (C-TiO2) nanoparticles were synthesized by the sol–gel method at different calcination temperatures (300–600
C) employing titanium tetraisopropoxide (TTIP) as the titanium source and polyoxyethylene sorbitan monooleate (Tween 80) as the carbon source. The physical properties of C-TiO2 samples were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The photocatalytic activities were checked through the photodegradation of phenolphthalein (PHP) under ultraviolet irradiation. The UV spectrum showed that the carbon doping extends the absorption range of TiO2 to the visible region. However, the photocatalytic activity is affected by the electron–hole recombination phenomenon, as revealed by the photoluminescence (PL) study. According to the PL spectra, carbon doping reduces the edge-to-edge electron–hole recombination. Nevertheless, the number of defect sites is greatly influenced by the calcination temperature of C-TiO2. C-TiO2 that was calcined at 400
C showed the highest photodegradation percentage of PHP, which was mainly attributed to the synergic effect of the low direct edge-to-edge electron–hole recombination, high content of defect sites, and retention of active electrons on the surface hydroxyl group.