Study On Synthesis Of G-C₃N₄/TiO₂and G-C₃N₄/Bi₂O₃/TiO₂Nanotubes For Photoelectrocatalytic Degradation Of Pollutants And Mechanism

Semiconductor photocatalysis has been extensively explored for a variety of environmental control applications and solar energy conversion in terms of photocatalytic water-splitting. However, only UV light (4%of the solar spectrum) at λ≤385nm is required to activate anatase TiO2. In this study, we employed a thermal poly-condensation and dip-coating method to modify TiO2-NTs with g-C3N4and Bi2O3for achieving high visible light photocatalytic activity.The g-C3N4/TiO2-NTs electrodes were synthesized by a dip-coating procedure on TiO2-NTs surface and then followed by high-temperature annealing. The photocurrent of g-C3N4/TiO2-NTs electrode prepared by melamine is ca.2times as high as that of g-C3N4/TiO2-NTs electrode prepared by urea at0.35V (vs Ag/AgCl). g-C3N4/TiO2-NTs electrode accelerates the separation of photo-generated electron-hole pairs in photoelectrochemical process under UV-visible light irradiation.A simultaneous manner for rapid reduction of Cr(VI) and degradation of phenol was observed in a photoelectrochemical system with the g-C3N4/TiO2-NTs electrode under UV-visible light irradiation compared to photocatalytic or electrocatalytic system. The effects of different systems, initial concentration of Cr(VI) or phenol in mixture system, applied potential, pH value and other organic pollutants species on pollutant degradation were investigated systematically. And with adding different scavengers, it indicated that the superoxide radicals were the most major active species for the reduction of Cr(VI) and removal of phenol in Cr(VI)/phenol system. In addition, we also found the synergistic effect in Cr(VI)/benzyl alcohol system and Cr(VI)/MB system. Based on the above results, a suggested mechanism for the reduction of Cr(VI) and degradation of phenol is illustrated schematically.g-C3N4and Bi2O3were successfully incorporated into TiO2nanotubes (TiO2-NTs) to produce a composite nanotube material designated as2. The UV-vis absorption spectrum of composite material Ts was red-shifted toward a narrower band gap energy (Eg). The valence band (VB) of g-C3N4/Bi2O3/TiO2-NTs was shifted to a more positive potential resulting in an increased driving force for water oxidation. The photocurrent generated by g-C3N4/Bi2O3/TiO2-NTs was approximately15times higher and the incident photon-to-current efficiency (IPCE at λ=400nm) was higher than that of the naked TiO2-NTs. This response is attributed to an increase in the lifetimes of the photo-generated electron-hole pairs.The composite g-C3N4/Bi2O3/TiO2-NTs material gave a significantly higher photoelectrochemical response in terms of the bleaching of methylene blue (MB). The effects of applied potential, pH value, initial concentration on MB bleach were studied systematically. The optimal photoelectrochemical process parameters for g-C3N4/Bi2O3/TiO2-NTs were6.0for pH,3V for applied potential. It was also found that the hydroxyl radical and superoxide radicals were the most major active species for the bleaching of MB. In addition, key steps of the sequence and mechanism of photoelectrochemical reactions about MB were summarized.