Structure-photocatalytic activity relationship of carbon doped titanium dioxide analyzed by density functional theory and fuzzy logic graph

Abstract

Carbon doping is a promising way to modify the properties of TiO2 for enhancing its photocatalytic performance. Although there are many publications about the enhancement of photocatalytic activity of TiO2, the relationship between the structural and physical properties with the photocatalytic activity of TiO2 is still not clearly understood. A new approach has been proposed to evaluate the structurephotocatalytic activity relationship with the aim to better understand the dominant properties that determine the photocatalytic activities of C-doped TiO2. Fuzzy logic graph has been used as a new approach in determining the dominant factor for the structure-photocatalytic activity relationship of C-doped TiO2. Characterization results from experimental study were used in the fuzzy logic graph. For the experimental study, two types of C-doped TiO2 were successfully synthesized by the sol-gel method with addition of cetyltrimethylammonium bromide (CTAB) surfactant and without the addition of CTAB, at different calcination temperatures, to compare with commercial TiO2. The synthesized photocatalysts were characterized using several characterization techniques. Photooxidation of styrene with aqueous hydrogen peroxide has been used as the model reaction for organic pollutants to study the structure-photocatalytic activity relationship under UV and visible light irradiation. Xray photoelectron spectroscopy (XPS) spectra show that C was doped into TiO2’s lattice with the amount of C of about 2.5 at% for CTAB-C/TiO2-500 samples and about 10.5 at% for C/TiO2-500 samples at interstitial and substitutional positions of anatase TiO2. Energy dispersive X-ray spectroscopy (EDX) and XPS results for CTAB-C/TiO2 samples show a lower amount of C incorporated into TiO2 as compared to C/TiO2 without the addition of CTAB, which may be caused by the removal of C impurity by the CTAB surfactant. Furthermore, the effects of calcination temperature from 300 to 700°C on the physicochemical properties of the C-doped TiO2 were also studied. Calcination temperature affected the phase, morphology, surface area, porosity, crystallite size and amount of C. The surface area of CTAB-C/TiO2 and C/TiO2 samples is shown to decrease as the calcination temperature increased. Additionally, the confirmation on the effect of C on the band gap energy of the anatase TiO2 was investigated using density functional theory (DFT). Total density of states (TDOS) shows that the C affects the band gap energy of TiO2 by introducing the mid gap states between the band gap. Based on DFT analysis and photocatalytic experiment, six physical properties have been chosen to be used for fuzzy logic graph, i.e. surface area, phase, amount of electron-hole recombine, band gap energy, existence of sub-band gap and amount of C. Fuzzy logic graph analysis shows that surface area is a dominant factor for the photooxidation of styrene under UV and visible light irradiations, followed by phase , amount of C and amount of electron-hole recombine. This study demonstrated that the combination of photocatalytic experiment, DFT and fuzzy logic graph analysis can be used to clarify the structure-photocatalytic activity relationship in TiO2 photocatalytic systems.