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

•Au nanoparticles are coagulated in varied extent depending on solvents dipole moment.•Electrokinetic properties of Au nanostructures are greatly changed in different solvents.•Au nanosphere of size ∼3 nm in water suspension agglomerated to ∼40 nm in DMSO.•Co-catalysis of Au nanospheres is changed depending on the degree of agglomeration.

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.

Graphical abstractThe Au nanospheres in water (AuNS-water, size ≈3–6 nm, zeta potential (ζ) = +49.79 mV, and conductance = 1673 μS) has been found to exhibit agglomeration with lowering in zeta potential (≈−31.88 to 24.06 mV) and conductance (≈23 μS) values in polar solvents of varying dipole moments. The AuNS-water suspension and its aggregated morphology (in various polar solvents) is utilized to study the relative co-catalytic activity imparting to Au–TiO2 photocatalysis for salicylic acid oxidation. The highest rate of photo-oxidation was achieved by non-aggregated AuNS in water suspension and aqueous TiO2 mixture followed by aggregated AuNS (formed in polar solvents) depending on the degree of agglomeration as a function of solvent polarity.Figure optionsDownload full-size imageDownload as PowerPoint slide