The role of OH− in the formation of highly selective gold nanowires at extreme pH: multi-fold enhancement in the rate of the catalytic reduction reaction by gold nanowires

Abstract

There is a quest to understand the mechanism governing the morphology and geometry control of the particle growth of nanomaterials for their optical and catalytic applications. In the available literature, the role of OH⁻ in dictating the size and shape of Au nanowires is unknown. As one of the first examples, herein, we explore how excess OH⁻ ions in CTAB micelles play a significant role during the highly selective formation of gold nanowires having controlled diameters of ∼20–25 nm and length >1 μm, by reducing Au³⁺ to Au⁰ in a one pot, simple synthesis procedure in the presence of Ag⁺ ions. At pH 4–11, the same procedure does not harvest Au NWs, but Au NPs of diameter 50–70 nm, indicating that excess OH⁻ is needed for nanowire formation. XRD, TGA, DSC, EDX, FT-IR and fluorescence spectroscopic analysis confirm that both CTAB and curcumin act as capping and stabilizing agents for Au NWs as well as Au NPs – there is no remarkable difference in the curcumin/CTAB content between Au NWs and NPs prepared in different pH environments. However, changing the CTAB micellar media to DPPC liposome media inhibits the formation of nanowires at pH ∼13; the growth of the Au NPs diminishes in DPPC liposomes, offering smaller NPs of diameter ∼25 to 30 nm, suggesting that the role of CTAB is necessary in nanowire formation. The rate of NW formation has been found to be 0.13 h⁻¹ and the growth mechanism advocates elongation in the [110] facet of Au [110] as opposed to the [100] or [111] facets. Curcumin capped Au nanowires serve as excellent nano-catalysts for the reduction of nitro-compounds and the rate of reduction of 4-nitrophenol, a model compound, by curcumin capped Au NWs is found to be ∼10 fold higher, compared to Au NPs, which signifies that catalytic activities can be dictated by the size and shape of Au NPs.