Co-catalytic and electro-kinetic properties of Au nanostructures dispersed in solvents of varying dipole moments

This study demonstrates the influence of electrokinetic parameters; surface charge and zeta potential on the optical absorption band and co-catalytic activity of Au nanospheres (AuNS) dispersed in polar solvents, i.e., methanol, propanol, dimethylformamide, and dimethyl-sulphoxide. The narrow intense surface plasmon (SP) absorption band of AuNS (520 nm) becomes broadened and a SP band at >970 nm is appeared, whose absorption intensity varies from 0.05 to 0.13 a.u. with the extent of aggregation in solvents of varying dipole moment from 1.66 D to 3.96 D while no such alteration in SP band occurs for AuNS dispersion in non-polar CCl4 as confirmed by DLS and TEM analysis. It found that the zeta potential (+50.32 mV) and conductance (1673 μS) of AuNS in water has been decreased to +24.06 mV and 1480 μS in methanol, respectively. The effective electronic charge ca. +4306 μequiv./l present over AuNS surface in water suspension is highly reduced to +512 μequiv./l in photoreaction (TiO2 + salicylic acid + AuNS) mixture and the zeta potential (+50.32 mV) and conductance (1673 μS) of AuNS in water has been decreased to +24.06 mV and 1480 μS in methanol, respectively. The co-catalytic activity of aggregated AuNS (formed in polar solvents) varied in a diverse extent depending on its degree of agglomeration, where the non-aggregated aqueous AuNS + TiO2 mixture displayed higher degradation efficiency (r = 9.990 × 10−3 mM/min) of salicylic acid than that (r = 9.172 × 10−3 mM/min) of aggregated AuNS in MeOH + TiO2 mixture under UV light irradiation.