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.

The 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 as PowerPoint slide