Gold thin-film electrodes: an EQCM study of the influence of chromium and titanium adhesion layers on the response

The effect of the adhesion layer between gold films and quartz on the response of an electrochemical quartz crystal microbalance (EQCM) has been investigated in 0.1 M HClO4 and in 0.1 M phosphate buffer pH 7.4. Cyclic voltarmmetry (CV) was applied in an anodic potential range, with polished and unpolished crystals, provided with a Cr or Ti adhesion layer. Crystals with Cr undercoatings showed-in HClO4 and in bufferoxidation currents and frequency shifts due to dissolution of Cr in a potential range in which gold oxidation also takes place. This led to loss of resonance of the crystal and/or to an increase in surface roughness. The deleterious effects of Cr are most pronounced with unpolished crystals. Crystals with Ti undercoatings neither showed a sign of oxidation or dissolution or adhesion metal, nor a significant increase in surface roughness upon repeated cycling, probably due to passivation of Ti by a thin oxide film, and its resistance to dissolution. The small positive frequency shifts observed upon repeated cycling in stirred solutions of HClO4, and phosphate buffer, with unpolished crystals in particular, are most likely due to dissolution of gold. The oxygen chemisorption on gold in HClO4 and in buffer, and the enhanced dissolution of gold in the presence of small concentrations (0.5 mM) of chloride in HClO4, could be very well studied with Ti-coated crystals. For applications in the anodic potential range (extending into the region of oxygen chemisorption on gold), crystals with Cr adhesion layers should not be used. (C) 2001 Elsevier Science Ltd. All rights reserved.