Electrostatically enabled dye reduction using laser synthesized gold nanoparticles

Abstract

The size and shape-dependent catalytic activities of noble metal nanoparticles (NPs) have been well studied. However, the effect of the surface coating on their catalytic performance is relatively less explored. Herein, we demonstrate that the catalytic activity of NPs has a marked dependence on the type of their surfactant coating. Gold NPs were synthesized by pulsed laser ablation and subsequently coated with different surfactants such as cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and polyvinylpyrrolidone (PVP). This enabled us to keep the size, size distribution, and concentration of Au NP the same for the comparative catalysis study. The bare and coated NPs were employed as a catalyst to carry out the reduction of methylene blue (MB) and methyl orange (MO). The results showed remarkable dependence of the reaction rates on the type of surfactant coating of the catalyst NPs. Bare NPs were found to be far superior to the coated NPs for both reactions. In the case of bare NPs both the dyes were completely reduced in less than 5 mins. Whereas, for coated NPs, the zeta potential values determined their effectiveness for a certain reaction. For instance, the reduction of the cationic dye MB was relatively more effective with NPs with a negative zeta potential value, whereas the reduction of anionic dye MO was better for NPs possessing a positive zeta potential. The apparent rate constant of SDS-coated NPs was 40 times higher than that of the CTAB-coated NPs for the reduction of MB. Conversely, the rate constant is 27-fold higher for CTAB-coated NPs compared to SDS-coated NPs for the reduction of MO. These results imply that there is a strong dependence of the catalytic reaction rates on the relative charge of reactants and NPs. Moreover, the better catalytic performance of bare NPs in both cases highlights the significance of the availability of the unhindered surface for catalysis. These results signify the importance of engineering the right surface coating for NPs for their best catalytic performance.