Thermally tuneable optical and electrochemical properties of Au-Cu nanomosaic formed over the host titanium dimples

Abstract

Au-Cu nanostructures offer unique optical and catalytic properties unlike the monometallic ones resulting from the specific interaction. Among others, they have the ability to exhibit surface plasmon resonance, electrochemical activity towards the oxygen and hydrogen evolution reaction (OER, HER) as well as improved photoresponse in relation to monometalic but those properties depend highly on the substrate where bimetallic structures are immobilized.In this work, bimetallic gold-copper mosaics over the conducting structured titanium substrate were fabricated via following steps: anodization of Ti foil, chemical etching of as-formed titania resulting in nanodimpled Ti substrate (TiND), sputtering of thin metal layer (Au, Cu) in various sequences, and finally thermal treatment in furnace at 450 °C or 600 °C. The morphology, optical and structural properties were investigated in details and it was shown that both arrangements of metallic films and thermal conditions strongly affect the morphology and optical features. The XPS results confirmed the presence of gold-copper alloys and copper oxide species. Last but not least, the electrochemical activities were verified in 0.1 M NaOH using cyclic voltammetry and linear voltammetry measurements performed in dark and under visible light illumination. Among all investigated materials in both anodic and cathodic regimes bimetallic 5Au/5Cu sample annealed at 450 °C exhibits the highest response towards OER and HER, respectively. This is further boosted by the light with λ > 420 nm. Upon exposure to visible light, the current density for 5Au/5Cu and 5Cu/5Au electrodes reached 1.32 mA cm−2 and 1.26 mA cm−2, respectively, while in the case of monometallic structures the current was below 10 μA cm−2. Both optical and electrochemical behaviour indicate a strong synergistic effect arising within the bimetallic mosaic formed over the TiND.