Gold oxide films grown in the confined aqueous layer between gold and organic solvents
Introduction
Gold and gold nanoparticles are employed either as substrate or additive in electrochemical sensors in order to improve the analytical selectivity of membranes. On the other hand, drops of organic solvents immiscible in water work as template for the production of gold nanoparticles [1], [2], [3], [4], [5]. Besides, the contact angle (CA) of captive drops (CD) allows evaluation of the surface energy, as in the case of nanofiltration, and the development of less fouling membrane materials [6], [7]. The CD technique involves placing bubbles or droplets against a solid surface where they are held captive in a cell filled with some immiscible fluid. The effect of the electrochemical potential on the surface properties of electrodes and membranes can be measured “in situ”. Under these conditions the complete hydration of the surface avoids hydrophobic contributions to the surface tension produced by adsorbed air or empty vacuoles [8], [9], [10].
This paper investigates the effect of the applied potential program on the structure of oxide films formed on gold using voltammetry, electrochemical impedance spectroscopy (EIS) and CA measurement. The anodization is carried out in both phosphate solutions at pH 6.7 and in the same electrode in contact with the confined water layer forms when the metal is covered by several organic immiscible solvents.
The comparison of the capacitance obtained in different growth conditions contributes to providing a deep insight into the different processes involved in the catalytic behavior of this complex interface.
Section snippets
Experimental
The experimental set-up has been described in previous works [11], [12], [13]. Polycrystalline gold rods (99.999% purity, area 0.5 cm2) were used as working electrodes. Before each experiment, the electrodes were mechanically polished to a mirror finish with alumina of 0.3 and 0.05 μm. The counter electrode was a platinum wire placed around the working electrode. A Pt/H2 electrode coupled to a Luggin–Haber capillary tip was used as reference electrode for all potential measurements.
The
Voltammetric data
Fig. 1 shows the simultaneous current, i, and the potential, E, detection during repetitive cycling at 0.1 V/s with the formation (anodic scan peak) and reduction (cathodic scan peak) of the Au2O3 monolayer.
When at the end of the anodic scan the potential is held at Ea = 1.7 V, i decreases rapidly in the time τ from 450 μA to less than 20 μA after 20 s (see arrow). During the subsequent cathodic scan, narrower reduction peaks result with only a very small current increase after longer holding times at
Discussion
In the potential region corresponding to the gold oxide monolayer formation, very similar voltammetric plots are obtained in buffer solution and with the electrode totally covered with the drop of the different organic solvents (Fig. 1). This indicates that the gold electrode strongly retains a layer of confined water that can achieve a thickness of several nanometers.
Only in the case of butyl acetate, a reduction of up to 30% in the area of both the anodic and the cathodic oxide peak can be
Conclusions
The cycled gold electrode retains a strong adsorbed aqueous thin layer in contact with immiscible solvents. The polarization at 1.7 V shows the formation of the gold oxide monolayer even for the electrode covered with the solvent. For the metal and the oxide covered electrode similar values of double layer capacity are observed. Significant variations depending on the holding time in the double layer potential region, 0.04 V ⩽ E ⩽ 0.6 V are observed in the dielectric properties and the homogeneity of
Conflict of interest
There is no conflict of interest.
Acknowledgements
This research project was supported by the “Comisión de Investigaciones Científicas de la Provincia de Buenos Aires, CIC”, the “Consejo Nacional de Investigaciones Científicas y Técnicas, CONICET”, the “Universidad Nacional de La Plata” and the “Universidad Nacional de San Luis”. Work was presented in the 9th ECHEMS Meeting “Electrochemistry in Particles, Droplets and Bubbles”, 23–26 June 2013, Lochow, Poland, http://echems9.pl/faq.
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2014, Journal of Applied Electrochemistry