Enhancement of adsorption and photocatalytic activity of TiO2 by using carbon nanotubes for the treatment of azo dye

The effect of carbon nanotubes (CNTs) on the adsorption and the photocatalytic properties of TiO2 (P25) for the treatment of azo dyes, including one monoazo dye Procion Red MX-5B, and two diazo dyes Procion Yellow HE4R and Procion Red HE3B, are investigated by Brunauer-Emmett-Teller (BET) measurement, spectrophotometer, total organic carbon (TOC), high proficiency liquid chromatography (HPLC), Raman spectroscopy (Raman), photoluminescence (PL), electron paramagnetic resonance (EPR) and transmission electron microscopy (TEM) analyses. The results show that compared with activated carbon (AC), CNTs can comparatively better improve the adsorption of the dyes onto P25 due to the strong interaction between P25 and CNTs. Residual TOC in the solutions and the amount of cyanuric acid evolved after degradation both confirm that the adsorption ability of P25 is enhanced by CNTs. CNTs also facilitate the photocatalytic activity of P25 in the degradation of the three azo dyes more efficiently than AC. With PL and EPR analysis, the mechanism of the enhancement of the photocatalytic activity of P25 by CNTs is proposed. The excited e− in conduction band of TiO2 may migrate into CNTs, of which have special structure and the ability for e− transport. Thus, the possibility of the recombination of e−/h+ pairs decreases. Meanwhile, O2 adsorbed on the surface of CNTs may accept e− and form O2−, which also leads to the formation of OH in the system. Therefore, there are more radicals in the system, resulting in the quicker degradation of the dyes.