The electrochemical fabrication of porous bimetallic structures and their applications in catalysis and sensing

摘要

The electrochemical fabrication of porous bimetallic structures was investigated via a hydrogen bubble templating method. The templating method involves the evolution of hydrogen from the substrate surface, while simultaneously depositing the metal structure. As the hydrogen gas escapes the metal deposit, it leaves behind porous pathways that are maintained post fabrication. Initially, porous Cu deposition was investigated on a variety of substrates (Cu, Au, Pd and glassy carbon (GC)). Cu was capable of displaying the highest porosity and multi-layered stacking due to its relatively medium hydrogen exchange potential (compared to Au and Pd). The deposition of Cu onto GC formed unreliable samples as the adherence of the Cu deposit to the GC surface was poor, usually resulting in dislodged samples. Active sites were most prominent on Pd and Au substrates, however, Cu was selected as the optimal substrate for the deposition of bimetallic structures owing to the well-defined morphology, strong sample adherence, availability and cost. The metal combinations investigated were Cu/Pd, Cu/Au and Cu/Ag, with varying metal concentrations. Two systems were formed for each metal combination; system 1 having a constant Cu concentration with varying secondary metal (Pd, Au or Ag) and system 2 maintaining a constant Pd, Au or Ag concentration with varying Cu concentration. The ideal deposition time was determined to be 15 s, as this formed rigid, well defined and porous structures. The electrodeposited samples were characterised by SEM, XRD, XPS and AAS and applied to various (electro)-catalytic and sensing applications: - reduction of ferricyanide (FCN) by sodium thiosulphate (STS) - reduction of 4-nitrophenol (NP) by sodium borohydride (SBH) - hydrogen evolution reaction (HER) 5 - determination of rhodamine B by SERS The reduction of both FCN and NP is reliant on an electron transfer from STS to FCN or SBH to NP. These reactions are extremely slow and require the addition of a catalyst which acts as an electron proxy for this transfer. In both reactions, the morphology of the samples played a large role towards the activity of the samples, with an increase in dendritic structures exceeding the activity of globular-like morphology. Alongside the morphology, the samples composition present played an influential role which was attributed to the electronegativity’s of the metals. Cu/Ag resulted in the formation of highly dendritic structures and showed the highest activity towards the reduction of both NP and FCN. The HER was dominated by the Cu/Pd systems, as would be expected due to the high activity of Pd towards this reaction. Additions of Au had a slight initial positive influence towards the HER due to the high electronegativity of Au which is known to promote the HER. However, once a certain concentration of Au is exceeded, the promotional effects are reduced Sensing of rhodamine B was performed by SERS which is reliant on the electromagnetic and chemical enhancement effects. Palladium samples displayed the lowest SERS activity followed by bimetallic samples containing gold. Copper and silver bimetallic structures displayed the highest SERS activity due to the surface plasmon resonance of silver in particular, coupled with the dendritic morphology that resulted in hot spots on the surface. In conclusion, the successful combination of certain coinage metals via a simple, quick and clean electrochemical templating method has been shown. The combination of Cu and Ag was seen to be the most promising material towards the reduction of NP and FCN and also as a SERS sensing material.