Chris Salter

A Joint Experimental and Computational Search for Authentic Nano‐electrocatalytic Effects: Electrooxidation of Nitrite and L‐Ascorbate on Gold Nanoparticle‐Modified Glassy Carbon Electrodes

The investigation of electrocatalytic nanoeffects is tackled via joint electrochemical measurements and computational simulations. The cyclic voltammetry of electrodes modified with metal nanoparticles is modeled considering the kinetics of the electrochemical process on the bulk materials of the different regions of the electrode, that is, the substrate (glassy carbon) and the nanoparticles (gold). Comparison of experimental and theoretical results enables the detection of changes in the electrode kinetics at the nanoscale due to structural and/or electronic effects. This approach is applied to the experimental assessment of electrocatalytic effects by gold nanoparticles (Au NPs) in the electrooxidation of nitrite and L-ascorbate. Glassy carbon electrode is modified with Au NPs via seed-mediated growth method. Divergence between the kinetics of these processes on gold macroelectrodes and gold nanoparticles is examined. Whereas claimed catalytic effects are not observed in the electrooxidation of nitrite, electrocatalytic nanoeffects are verified in the case of L-ascorbate. This is probably due to that the electron transfer process follows an adsorptive mechanism. The combination of simulation with experiments is commended as a general strategy of authentification, or not, of nanoelectrocatalytic effects.

Anodic Stripping Voltammetry of Silver Nanoparticles: Aggregation Leads to Incomplete Stripping

The influence of nanoparticle aggregation on anodic stripping voltammetry is reported. Dopamine-capped silver nanoparticles were chosen as a model system, and melamine was used to induce aggregation in the nanoparticles. Through the anodic stripping of the silver nanoparticles that were aggregated to different extents, it was found that the peak area of the oxidative signal corresponding to the stripping of silver to silver(I) ions decreases with increasing aggregation. Aggregation causes incomplete stripping of the silver nanoparticles. Two possible mechanisms of ‘partial oxidation’ and ‘inactivation’ of the nanoparticles are proposed to account for this finding. Aggregation effects must be considered when anodic stripping voltammetry is used for nanoparticle detection and quantification. Hence, drop casting, which is known to lead to aggregation, is not encouraged for preparing electrodes for analytical purposes.

The expansion/contraction of gold microparticles during voltammetrically induced amalgamation leads to mechanical instability

The mechanical stability of gold microparticles during anodic stripping voltammetric (ASV) detection over a large range of mercury concentrations was investigated. Mercury was detected at gold microparticles chemically deposited onto glassy carbon microspheres using ASV. Oxidation was observed at 0.5 and 0.8 V vs. SCE. Which peak was observed was dependent on the concentration of mercury and the deposition potential. The formation of the amalgam was of interest. As mercury was deposited for longer time intervals, scanning electron microscopy (SEM) analysis showed the microparticles increasing in size from 0.76 ± 0.03 μm (initial) to 1.51 ± 0.14 μm (Hg2+ deposited for 1980 s at 0.35 V) in diameter. In order to ascertain if multiple expansion and contraction cycles damaged the gold microparticles, cyclic voltammetry was used to monitor the amount of gold on the electrode as mercury was deposited and stripped repeatedly. It was seen that the area under the cathodic gold peak decreased with repetitive scans. SEM analysis revealed that the mechanical stress of repetitive deposition and stripping cycles of mercury caused the gold microparticles to fracture, appearing as irregular cuboid crystals rather than as the orderly polycrystallite formations seen initially. Energy dispersive X-ray (EDX) analysis indicated that the composition of the microparticles changed over the course of repetitive deposition and stripping cycles from gold to an Au–Hg amalgam, which may not be in electrical contact with the carbon support.

Electron induced modification of the surface electrochemical properties of diamond electrodes

A novel method has been developed to modify the surface properties of boron-doped CVD diamond electrodes by electron stimulated desorption (ESD) of hydrogen and oxygen from as grown diamond surfaces allowing the selective electrochemical silver deposition on the areas not irradiated with electrons.

Achieving High Spatial Resolution in Elemental Mapping of Metal Samples from Archaeological Contexts

Improving the characterization of archaeological artifacts brings a need to understand better the relationships between composition, structure and properties. With archaeological material there is also a requirement to consider the effects of ageing and environmental interactions in altering the original structure and composition, both in the bulk and at the surface. However, curatorial constraints and, frequently, the condition of the objects preclude the sampling methods required for the most powerful means of structural analysis of materials, the high resolution transmission electron microscope. The samples normally available are small bulk samples and we must find other means of maximizing spatial resolution in microchemical and microstructural analysis of both bulk and surface regions of the samples. This paper describes ways in which this is being achieved using the scanning proton microprobe (SPM) with both particle induced X-ray emission (PIXE) and Rutherford back scattered proton (RBS) spectra at resolutions down to ca. 1?m, electron probe microanalysis (EPMA) at 250-300nm, and scanning Auger microscopy (SAM) at resolutions of 10-20nm, but only from the surface layers of atoms in a sample. Examples will be given which demonstrate the contribution that each instrument can make, and that new and useful information is obtained each time resolution is increased. They will also show that structural features can be identified which are invisible to other microscopies. It will also be shown how modern PC-based software has greatly enhanced the mapping capability of all instruments.

Metalworking at Hengistbury Head, Dorset and the Durotrigan Coinage: A Reinterpretation of an Iron Age and Roman Industrial Site

The excavation of the metalworking areas of the late Iron Age and early Roman port of Hengistbury Head in southern England revealed evidence of a wide range of processes. These involved gold, silver and copper alloys and their connections with the local Iron Age coinage as well as the casting of bronze artefacts and ironworking. Since publication, in 1987 re-analysis of some material coupled with an extensive analysis of the associated coinage has led to a re-interpretation of the material. This paper presents the new perspective on the site as a metallurgical centre that has emerged as a result of our analyses.