Silver Electrodeposition on Nanocrystalline Graphite: Galvanostatic Control As a Tool in the Study Electrodes with High Resistance

Abstract

Carbon based materials, such as glassy carbon or highly oriented pyrolytic carbon (HOPG), have been used extensively as working electrodes for electrodeposition. Due to their chemical inertness and electrochemical stability they are commonly used as substrates for electrodeposition studies. HOPG in particular has attracted great deal of attention due to its close relation to carbon nanotubes and graphene [1]. Miranda-Hernández et al. [2] investigated silver electrodeposition on HOPG, mechanically polished glassy carbon and fractured glassy carbon. Their results showed that the surface conditions, including morphology of the carbon substrate, have an effect on the overpotential required to start a 3D silver electrocrystallization. Additionally, their results led to the conclusion that the active sites do not depend on physical features on the electrode's surface and thus their nature must be electronic. In carbon materials, the shape and magnitude of the density of electronic states (DOS) distribution vary according to their structure [3]. Since the rate of heterogeneous electron transfer depends on the electrode’s DOS [4], it is expected that different carbon materials would exhibit different electrodeposition behaviors. Furthermore, simple electrodeposition systems could be used to characterize the properties of novel carbon materials. Recently, Cholula-Díaz et al. [5] used aerosol assisted chemical vapor deposition to synthesize nanocrystalline graphite (NCG) thin films. The low nanoscale roughness of this material may lead to some insight into the electrodeposition process. Additionally, their complex electronic behavior may lead to the observation of new phenomena. These samples were used as working electrodes for silver electrodeposition. A single compartment electrochemical cell was used, with Hg|HgSO4 and a Pt wire as reference and counter electrodes, respectively. The composition of the platting bath was: 10 mM AgNO3, 1 M KNO3 and 1.6 M NH4OH [2]. Cyclic voltammetry experiments resulted in tilted voltammograms, which are typical of a high ohmic resistance. Since the platting bath had a high ionic concentration, this behavior is adscribed to the resistance of the NCG sample. The cell voltage U is equal to the sum of the double-layer potential φDL and the ohmnic drop. U= φDL + iR Hence, under potentistatic control, current changes result in variations of the ohmic term, to which the system responds by adjusting the double-layer potential to comply with the applied voltage value. This feedback loop between the double-layer potential and the current makes it difficult to establish a clear relationship between the experimental conditions and the characteristics of the deposited material. In contrast, galvanostatic control offers a far simpler scenario; since the current is fixed in the experiment, it is easier to determine the double-layer potential. Figure 1a shows the time evolution of the double-layer potential during controlled current experiments. The time scale on which the potential stabilizes is larger for the one observed for similar experiments on glassy carbon. This shows the importance of galvanostatic control as means to study electrodeposition samples with high resistivity. Figure 1b shows an SEM image in which submicrometric silver particles can be identified. We explore the dependence between the current pulses applied and the resulting particle size distributions. References R. Unwin, Faraday Discuss., 172, 521 (2014)Miranda-Hernández, I. González, and N. Batina, J. Phys. Chem. B, 105, 4214 (2001).Richard L. McCreery, Chem. Rev., 108, 2646 (2008)J. Royea, T. W. Hamann, B. S. Brunschwig, N. S. J. Lewis, Phys. Chem. B, 110, 19433 (2006).L. Cholula-Díaz, J. Barzola-Quiquia, H. Krautscheid, U. Teschner, P. Esquinazi, Carbon, 67, 10 (2014). Figure 1. a) Time dependence of nanocrystalline graphite electrode potential after applying cathodic current pulses from −50 to −325 µA in 10 mM AgNO3 + 1 M KNO3 + 1.6 M NH4OH electrolyte. b) SEM image (BS detector) of the surface of the nanocrystalline graphite showing particles of silver deposit. Figure 1