Stick–slip behaviour on Au(111) with adsorption of copper and sulfate

Abstract

SummarySeveral transitions in the friction coefficient with increasing load are found on Au(111) in sulfuric acid electrolyte containing Cu ions when a monolayer (or submonolayer) of Cu is adsorbed. At the corresponding normal loads, a transition to double or multiple slips in stick–slip friction is observed. The stick length in this case corresponds to multiples of the lattice distance of the adsorbed sulfate, which is adsorbed in a √3 × √7 superstructure on the copper monolayer. Stick–slip behaviour for the copper monolayer as well as for 2/3 coverage can be observed at F N ≥ 15 nN. At this normal load, a change from a small to a large friction coefficient occurs. This leads to the interpretation that the tip penetrates the electrochemical double layer at this point. At the potential (or point) of zero charge (pzc), stick–slip resolution persists at all normal forces investigated.

In situ friction study of Ag Underpotential deposition (UPD) on Au(111) in aqueous electrolyte

“…The COF then increases above a critical load, which depends on potential (regime α). We had observed this effect before for the Cu‐upd system and also for sulfate on Au(111) and tentatively ascribed it to penetration of the tip into the sulfate adlayer, [12,13] where more and more adsorbate species interact with the tip and have to be pushed away. For higher normal loads, as suggested also by the modelling in, [25] the adsorbate is completely squeezed out from the confined region underneath the tip, “and the number of particles in contact with the tip levels off.…”

“…We observed such an increasing high friction on Au(111) in pure sulfuric acid, when with increasing potential the sulfate coverage increases, as long as the adlattice is disordered below the potential of the well‐known spike indicating the phase transition to the ordered structure [12] . Similarly, in 0.4 mM CuSO 4 /0.05 M H 2 SO 4 , friction decreases when sulfate is desorbed upon a potential sweep in negative direction reaching a minimum before Cu sets in [13] . Such a dependence of friction on coverage by a mobile phase is also in agreement with the MD simulation of Ouyang et al.…”

“…The strength of interaction between surface and AFM strongly depends on the species adsorbed on the surface. Thus, adsorbed molecules on substrates play a crucial role in adhesion, friction and wear behaviors [10–22] . Interestingly, molecular dynamics(MD) simulation demonstrated that for given external conditions, such as normal load, temperature and adsorbate surface coverage, the observed regime of friction is determined by the strength of the adsorbate‐substrate interaction [23–25] …”

“…In aqueous electrolytes, the surface of Au(111) can be reversibly modified by anions and cations. Our group also has studied the influence of several adlayers on Au(111) on friction force under electrochemical condition [11–14] . For sulfate ions on Au(111) the friction starts to increase at the potential where the lifting of reconstruction of Au(111) occurs and continuously increases with increasing sulfate coverage until the potential arrives at the spike, which indicates the transition of disordered to ordered sulfate adlayer [12] .…”

Advanced atomic force microscopy techniques III

“…Santiago Solares and Enrique A. López-Guerra presented different approaches to model such viscoelastic properties within AFM simulations [ 23 ]. Sliding contact properties like several transitions in the friction coefficient with increasing load have been found on Au(111) in sulfuric acid electrolyte containing Cu ions by Helmut Baltruschat an co-workers [ 24 ] and the stiffness of micron-sized sphere-plate contacts was studied by Diethelm Johannsmann et al by employing high frequency tangential excitation [ 25 ].…”