Simultaneous growth of spherical, bipyramidal and wire-like gold nanostructures in solid and solution phases: SERS and electrocatalytic applications

Anisotropic growth of gold nanostructures (AuNS) on an indium tin oxide (ITO) substrate and in growth solution was achieved by in situ electrochemical reduction of Au⁺ ions from the growth solution. Initially, the growth solution was prepared by adding the shape-directing agent Cu²⁺ into a solution of cetyltrimethylammonium bromide (CTAB), HAuCl₄ and ascorbic acid (AA). The Au⁺-CTAB complex from the solution was electrochemically reduced to deposit AuNPs on ITO, which act as nucleation centers. The HR-TEM and SEM images exhibit spherical, bipyramidal and wire-like AuNS grown both in solution and on ITO after 1, 3 and 6 h, respectively. The crystallite size and lattice strain were calculated and it was found that the lattice strain decreases with an increase in nucleation time. The ITO/Au-nanowires showed a higher surface coverage (10.1%) and electrochemically active surface area (0.69 cm²) than the other AuNS. Further, the ITO/AuNS exhibited shape-dependent surface enhanced Raman scattering enhancement (SERS) towards 4-aminothiophenol adsorbed on the grown AuNS surface. The ITO/Au-nanowires showed a higher enhancement factor of 9.7 × 10⁵ when compared to the other AuNS. Further, the electrocatalytic activity of the different shaped AuNS was examined towards the reduction of hydrogen peroxide (HP). The Au-nanowire modified ITO electrode exhibited higher electrocatalytic activity towards HP reduction by not only shifting its reduction potential towards less positive potential but also by enhancing its reduction current.