The effect of calcination temperature on the structure and photocatalytic activity of carbon-doped titanium dioxide prepared via sol-gel route

Abstract

The outstanding accomplishment by Fujishima and Honda (1972) on the discovery of photocatalytic water splitting by titanium dioxide (TiO2) electrodes results in the extensive study of TiO2 for air and water purification for environmental applications. Despite all the advantages provided from TiO2 compared to other semiconductor photocatalysts, its two main concern issues, which are large band gap energy (Eg) and high recombination rate of photogenerated electrons ) and holes (h+) pairs, restraint its usage in practical applications. Therefore, the development of visible light active TiO2 become the challenge of researchers in the field of semiconductors photocatalysis. This study has been designated to the development of modified TiO2 photocatalyst to enhance its photocatalytic activity into visible range and increase the charge carrier separation for more beneficial applications by carbon doping modification. Carbon-doped TiO2 (C-TiO2) nanoparticles with amorphous, anatase and mixed anatase/rutile phase were successfully synthesized via a simple and low-cost sol-gel route based on the self-assembly technique exploiting polyoxyethylenesorbitan monooleate (Tween 80) as a carbon source and calcined at 300-600 oC. The as-prepared powders were characterized by attenuated total reflectance spectra Fourier transform infrared spectroscopy (ATR-FTIR), diffuse reflectance ultraviolet-visible spectroscopy (DR UV-Vis), X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence (PL), and UV-Visible spectrophotometry (UV-Vis). With prolonged calcination temperature, the crystallite size and Eg were increased and the level of carbon doping decreased. All the nanoparticles showed high absorption in UV and visible light region. This is due to the reduction of Eg and the formation of sub-band observed by UV-Vis and PL spectrum, respectively. The formation of sub-band widens the absorption towards the visible light region. The potential of using the synthesized C-TiO2 nanoparticles for photocatalytic environment remediation was demonstrated by the degradation of phenolphthalein (PHP) under UV and visible light irradiation. These photocatalytic activities were attributed to the level of carbon doping and crystallite structure. Carbon acts as an electron-trapping agent and causes structural defects as observed by PL spectrum. Obviously, it produces a low rate of recombination of e-/h+ as observed by PL spectroscopy which will have high separation of charge carrier and better performance of the photocatalytic activity. It was revealed that the amorphous C-TiO2 is inert and the samples calcined from 400 oC to 600 oC have a decreasing trend of photocatalytic activity with the increased in size of nanoparticles. Among all, C-TiO2 photocatalyst calcined at 400oC was found to be more effective with an ideal amount of carbon, the presence of hydroxyl group, better crystallinity of amorphous-anatase mixture, and the existence of sub-band which reduces the band gap energy. Thus, exhibits high degradation percentage of PHP under UV and visible light irradiations, leading to as much as 3.7 and 11.89 % respectively. As a result, carbon doping modification is an appropriate strategy to enhance the photocatalytic activity by improving visible-light absorption and e–/h+ separation.