Surface modification of TiO2 with polydopamine and its effect on photocatalytic degradation mechanism

Abstract

TiO2 presents a broad application prospect in the field of degradation of environmental pollutants. However, the development of TiO2 photocatalyst has been limited by two factors, namely, narrow spectral response range and low quantum efficiency. Surface coating can selectively change the physical and chemical properties of core particles, whereas the synergistic action between core and shell can optimize material performance. In this paper, surface modification of titanium dioxide (TiO2) by dopamine polymerization was used to improve its photocatalytic performance. Among polydopamine (PDA)-modified TiO2, TiO2@PDA-0.4 mg/mL-2 h showed the best degradation rate of Rhodamine B (RhB): the degradation rate reached 99.28% and 77.40% after 1.5 h of ultraviolet–visible (UV–vis) and visible (Vis) light irradiation, respectively. Compared with the uncoated TiO2, the degradation rate has shown improvement, especially under Vis light irradiation, with the photocatalytic performance improving by 35.04%. Absorption, emission, and impedance spectra showed that surface modification of TiO2 with suitable thickness of PDA effectively increased light absorption and promoted separation of photogenerated carriers. High-performance liquid chromatography–mass spectrometry revealed the differences in photocatalytic degradation mechanism of TiO2@PDA to RhB under UV–vis and Vis light irradiation. Compared with those under UV–vis light irradiation, the intermediate products under Vis light irradiation are relatively simple. TiO2@PDA generated more electrons, which interacted with H2O to produce large amounts of ·OH and holes under UV–vis light irradiation in comparison with that under Vis light irradiation. In addition to deacetylation, deamination, and decarboxylation by hydroxylation, excess ·OH in the photocatalytic process would attack benzene rings directly to produce N,N-diethylacetamide.