Electrochemical Roughening and Annealing of Au(111) Surfaces in Perchloric and Sulfuric Acid Electrolytes Studied by STM
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ELECTROCHEMICAL ROUGHENING AND ANNEALING OF Au(111) SURFACES IN PERCHLORIC AND SULFURIC ACID ELECTROLYTES STUDIED BY STM Karrie J. Hanson and Michael P. Green AT&T Bell Laboratories, 600 Mountain Ave., Murray Hill, NJ 07974
ABSTRACT A technique commonly used as the final stage of cleaning gold electrode surfaces is to electrochemically form and reduce a monolayer of oxide on the surface. We have investigated the changes on the Au(111) surface during this procedure using in situ electrochemical scanning tunneling microscopy. INTRODUCTION The ability of the scanning tunneling microscope (STM) to resolve in situ images of electrode surfaces on a nanometer length scale offers the promise of correlating structural changes on a surface with voltammetric data. In this paper we examine the differences in response of a Au(111) electrode during a voltammetric sweep through oxide formation and reduction for two electrolytes: perchloric acid and sulfuric acid. The Au oxidation and reduction reactions have been previously studied with atomic force microscopy (AFM)[1I and STM[21 in dilute perchloric acid electrolytes. Gewirth et al.[1], using AFM, noted a disordered oxide film formed in the hydroxide potential region, which smoothed over time at potentials positive of the reduction wave (1.1V vs. AgQRE). Stripping the oxide revealed a surface similar to the initial one, again showing the characteristic Au(111) corrugation. In an STM study, Trevor et al.[21 demonstrated that Au(111) surfaces roughen during the formation and reduction of more than a monolayer of oxide, and that they anneal in minutes at moderate potentials. EXPERIMENTAL A NanoScope II STM was used to take images in situ with the electrode under potential control. Electrochemical sweep rates, electrolyte compositions, and STM parameters are noted in the figure captions. Reference electrodes were a charged Pd wire for the perchloric acid experiments and a Pt wire in case of sulfuric acid. These were chosen in order to-reduce the possibility of contamination with Cl- or with other metals. In all cases the coated STM tip was maintained at a constant potential with respect to the reference electrode to reduce the electrochemical currents at the tip. Quoted potentials have been converted to the NHE scale. Gold substrates were made by evaporation onto mica at 300*C, a process documented to produce large (111) terraces[31. RESULTS/DISCUSSION The current vs. potential plots for the anodic sweeps in perchloric acid (10mV/s) and sulfuric acid (2mV/s) electrolytes are shown in Fig. If and Fig. 2g, Mat. Res. Soc. Symp. Proc. Vol. 237. 01992 Materials Research Society
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respectively. These curves were taken in the STM cell during the scans shown in the figures. This was the first oxidation sweep for the Au electrode in both electrolytes. The two peaks at 1200 and 1450mV in Fig. if are attributed to the formation of gold hydroxide and oxide, respectively[4]. In sulfuric acid (Fig. 2g), we do not see the peak associated with hydroxide, presumably because an adsorbed sulfate
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