The voltammetry of metallic nanoparticle arrays
Keywords: electro-chemistry
voltammetry
nanoparticles

The experimental work discussed in this thesis examines the effect of voltammetry of nanoparticle arrays with a specific focus on the effect of nanoparticle size and surface coverage on the substrate electrode. These effects are investigated in relation to the reduction of hydrogen peroxide, 4-nitrophenol and the hydrogen evolution reaction. In each case, the experimental data obtained is subsequently fitted using numerical simulations to extract quantitative kinetic data.
Distinct differences are noted between macro – and nano – scale. The reduction of H₂O₂ reveals the absence of autocatalysis at the nano – scale. Furthermore, peak potential shifts positively with surface coverage under Case 3 diffusion conditions. On the other hand, at essentially isolated particles (Case 1) Ep varies logarithmically with nanoparticle radius. By decreasing nanoparticle size, we promote convergent diffusion and enhance irreversibility of the process. The shift in Ep with surface coverage can be accounted for as the diffusion layers begin to overlap heavily, tending towards more macro – disk type behaviour.
We also study the hydrogen evolution reaction at an array of AgNPs. By fitting of the experimental data with numerical simulations we demonstrate altered kinetics between the macro – and nano – scale. Voltammetry at AgNP – arrays also display dependence between surface coverage and current. We attribute this to increased electro – active surface area and decrease in irreversibility of the process, with increasing surface coverage.
Numerical simulations are also used to fit experimental data obtained for the reduction of 4 – nitrophenol in acidic media. The AgNP – arrays exhibit significantly different electrode kinetics compared to a macro – disk. Examination of the data obtained for AgNP – arrays at two different acid concentrations implies the rate - determining step is likely the electron transfer process. We therefore infer a change in α the mechanism of the rate – determining step between macro – and nano – scale. An unusually low value of α at the NP – array may indicate that adsorption plays some role in the process.
Furthermore, we discuss the size – dependent adsorption behaviour exhibited by silver nanoparticle arrays, such that small particles with diameters below ~ 50 nm do not display the typical underpotential deposition characteristics of corresponding bulk materials or larger nanoparticles. This phenomenon is reported for the deposition of heavy metals (thallium, lead and cadmium) at silver nanoparticle arrays.
The stripping voltammetry of arrays of silver nanoparticles has also been investigated. The stripping peak potential is dependent on the degree of surface coverage. Modelling of this system has shown that a one – dimensional diffusion model is appropriate for high surface coverage; essentially it is experiencing planar diffusion. However, for an array of widely dispersed particles, the individual nanoparticle size and convergent diffusion begin to dominate behaviour.
A detailed overview of the literature is first discussed, relating to the synthesis of a wide variety of metallic nanoparticles and their practical applications, such as the determination of pH using platinum nanoparticles as covered in Chapter 7. We also discuss the fundamental principles governing the voltammetric behaviour of nanoparticles.