Plasmonic photodegradation of textile dye Reactive Black 5 under visible light: a vibrational and electronic study

Heterogeneous photocatalysis using semiconductor oxides as a photocatalyst is an intensely employed technique in photodegradation processes under UV irradiation. More recently, plasmonic photocatalysis came in as a technique that promotes intensification of photodegradation processes, in addition to allowing moving the excitation toward the visible range of wavelengths. Such improvements are achieved by hybrid photocatalysts formed between coinage metal nanoparticles, which present the LSPR (Localized Surface Plasmon Resonance) phenomenon, and semiconductor oxides, resulting in plasmonic photocatalysts. In this paper, Ag/ZnO hybrid nanostructured photocatalysts were obtained by associating silver (AgNP) and zinc oxide nanoparticles suspensions (mZnO/mAg ratio of 114.5). The plasmonic photocatalyst Ag∕ZnO was characterized by UV-Vis and powder XRD and used in the degradation of the Reactive Black 5 (RB5) textile dye under visible radiation. The photodegradation process of RB5 was monitored by UV-Vis, resonance Raman (RR), and surface enhanced Raman spectroscopies (SERS) and showed a gain of 54% in the rate constant for the Ag/ZnO plasmonic photocatalyst as compared to the process using only ZnO as the catalyst. Resonance Raman and SERS results provided valuable mechanistic information of degradation pathway under visible light and showed that the initial steps in the mineralization of the dye involve the cleavage of at least one of the azo groups in RB5 and consequent rupture of the chromophoric conjugated π system.