Ostwald Ripening during Electrophoretic Deposition: Control of Dispersity and Size of Gold Nanoparticles by KBr

This work explores size transformations during the electrophoretic deposition (EPD) of citrate-coated Au nanoparticles (NPs). The EPD method involves a transparent ITO-glass electrode subjected to 1.0 V (vs Ag/AgCl) immersed in a solution of as-synthesized Au NPs with hydroquinone (HQ) serving for the deposition of NPs. We observed that as-synthesized monodisperse 6.0 nm average diameter Au NPs became polydisperse within the first 300 s of EPD, forming two well-defined sizes (∼5.0 and 10 nm), as determined by AFM and anodic stripping voltammetry (ASV). Size transformation is attributed to electrochemical Ostwald ripening occurring during EPD. Control experiments demonstrated that HQ serves a redox regulator, allowing size transformations to a certain extent. Surprisingly, however, incorporating 20 mM KBr into the EPD solution transformed as-synthesized polydisperse Au NPs into a monodisperse film. It also reduced the size of Au NPs from ∼9.0 to 5.1 nm within 900 s of EPD. The addition of KBr to the EPD solution causes the stripping of Au to dominate over the reduction since Au NPs are continuously exposed to anodic potentials (1.0 V) in the presence of bromide. This leads to overall small ∼5.0 nm Au NPs in the film accompanied by an irreversible loss of Au mass as indicated by the decrease in the area under the ASV peak (representing moles of oxidized Au). The predominance of small Au NPs exhibits superior electrocatalytic activity after electrochemically removing bromide with NaOH. The ability to rapidly control the monodispersity, size, and coverage during film deposition is crucial for both fundamental studies and applications in electronics, catalysis, and sensors.