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

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

The effect of electrode material on the electrochemical formation of porous copper surfaces using hydrogen bubble templating

The electrodeposition of copper onto copper, gold, palladium and glassy carbon (GC) electrodes via a hydrogen bubble templating method is reported. It is found that the composition of the underlying electrode material significantly influences the morphology of the copper electrodeposit. Highly ordered porous structures are achieved with Cu and Au electrodes, however on Pd this order is disrupted and a rough randomly oriented surface is formed whereas on GC a bubble templating effect is not observed. Chronopotentiograms recorded during the electrodeposition process allows bubble formation and detachment from the surface to be monitored where distinctly different potential versus time profiles are observed at the different electrodes. The porous Cu surfaces are characterised with scanning electron microscopy, X-ray diffraction and cyclic voltammetric measurements recorded under alkaline conditions. The latter demonstrates that there are active sites present on electrodeposited copper whose coverage and reactivity depend on the underlying electrode material. The most active Cu surface is achieved at a Pd substrate for both the hydrogen evolution reaction and the catalytic reduction of ferricyanide ions with thiosulphate ions. This demonstrates that the highly ordered porous structure on the micron scale which typifies the morphology that can be achieved with the hydrogen bubbling template method is not required in producing the most effective material

Electrochemical properties of galvanically replaced iron nanocubes with gold and palladium

The galvanic replacement reaction has received considerable interest due to the creation of novel bimetallic nanomaterials that minimise the use of expensive metals while maintaining enhanced electrocatalytic properties for certain reactions. In this work we investigate the galvanic replacement of electrochemically synthesised iron nanocubes on glassy carbon, with gold and palladium. The resultant nanomaterials demonstrate quite a difference in morphology; the original cuboid like template is maintained in the case of gold but destroyed when palladium is used. The electrochemical and electrocatalytic behaviours of these materials are reported for reactions such as methanol oxidation, hydrogen evolution and oxygen reduction

Cathodic corrosion of Cu substrates as a route to nanostructured Cu/M (M=Ag, Au, Pd) surfaces

The electrochemical formation of nanostructured materials is generally achieved by reduction of a metal salt onto a substrate that does not influence the composition of the deposit. In this work, we report that Ag, Au and Pd electrodeposited onto Cu under conditions where galvanic replacement is not viable and hydrogen gas is evolved results in the formation of nanostructured surfaces that unexpectedly incorporate a high concentration of Cu in the final material. Under cathodic polarisation conditions, the electrodissolution/corrosion of Cu occurs, which provides a source of ionic copper that is reduced at the surface-electrolyte interface. The nanostructured Cu/M (M=Ag, Au and Pd) surfaces are investigated for their catalytic activity for the reduction of 4-nitrophenol by NaBH4, where Cu/Ag was found to be extremely active. This work indicates that a substrate electrode can be utilised in an interesting manner to make bimetallic nanostructures with enhanced catalytic activity

Decoration of active sites to create bimetallic surfaces and its implication for electrochemical processes

The creation of electrocatalysts based on noble metals has received a significant amount of research interest due to their extensive use as fuel cell catalysts and electrochemical sensors. There have been many attempts to improve the activity of these metals through creating nanostructures, as well as post-synthesis treatments based on chemical, electrochemical, sonochemical and thermal approaches. In many instances these methods result in a material with active surface states, which can be considered to be adatoms or clusters of atoms on the surface that have a low lattice co-ordination number making them more prone to electrochemical oxidation at a wide range of potentials that are significantly less positive than those of their bulk metal counterparts. This phenomenon has been termed premonolayer oxidation and has been reported to occur on a range of metallic surfaces. In this work we present findings on the presence of active sites on Pd that has been: evaporated as a thin film; electrodeposited as nanostructures; as well as commercially available Pd nanoparticles supported on carbon. Significantly, advantage is taken of the low oxidation potential of these active sites whereby bimetallic surfaces are created by the spontaneous deposition of Ag from AgNO3 to generate Pd/Ag surfaces. Interestingly this approach does not increase the surface area of the original metal but has significant implications for its further use as an electrode material. It results in the inhibition or promotion of electrocatalytic activity which is highly dependent on the reaction of interest. As a general approach the decoration of active catalytic materials with less active metals for a particular reaction also opens up the possibility of investigating the role of the initially present active sites on the surface and identifying the degree to which they are responsible for electrocatalytic activity