Role of Interfacial Water and Applied Potential on Friction at Au(111) Surfaces

Abstract

The tribological properties between an AFM tip and a Au(111) surface in an aqueous environment is influenced by an applied electrical potential. Using lateral force microscopy, we measure the resulting friction force, while simultaneously applying a predetermined electrical potential on the Au surface via a three-electrode setup. Applying a positive potential to the Au surface forms an interfacial water layer at the Au/electrolyte interface, which sharply increases friction. However, when an anodic potential is applied, lower friction forces are measured. The potential dependent friction is observed on ultra-smooth gold surfaces as well as Au surfaces with larger roughness. An increase in the ionic strength of the electrolyte is found to lower friction. The use of an aqueous NaOH solution is found to lower the critical potential at which the friction sharply increases. Normal force curves are also measured as a function of approach velocity. The normal force linearly increases as the approach velocity increases in agreement with a drainage model. These results provide valuable insight into the effect of applied electrical potentials on the properties of water at charged surfaces and can potentially impact a wide range of fields including tribology, micro-electro-mechanical systems (MEMS), energy storage devices, fuel cells, and catalysis.

Towards programmable friction: control of lubrication with ionic liquid mixtures by automated electrical regulation

“…Correspondingly, in situ control of friction 2 , 3 , adaptive friction change 4 and superlubricity 5 – 7 were identified as the greatest challenges in tribology. Switchable or controllable friction is thereby recognized as an increasingly significant and important topic 8 – 11 .…”

Potential-Dependent Interfacial Frictional Behavior between Charged Microspheres and Gold in Aqueous Solutions

“…For the GE surfaces with different electric charges, the ions and water molecules between the interfaces would have different structures. According to previous studies, when a gold surface has the appropriate positive potential, water molecules adsorb on the GE to form an ordered ice-like water layer with viscosities 10 5 –10 7 times that of bulk water (Figure a,b). , The ordered water layer exposes a large number of dangling hydroxyl groups, which are capable of hydrogen bonding with polar hydrophilic probes ,,, (panel 1 of Figure c). However, at positive potentials, the Cl – adsorption is inevitable, which would inhibit the formation of ice-like water. , In this experiment, the maximum potential used, V app = +0.6 V, and the amount of adsorbed Cl – was insufficient to disrupt the ice-like water structure, similar to the findings of previous studies. ,, In contrast, when the GE potential was negative, the ordered water structure was disrupted and ice-like water did not form. ,, Due to electrostatic attraction, the hydrated cations were adsorbed on the interface water layer at the GE (Figure ).…”

“…Gold-coated mica was used as the working electrode (Figure ). A NaCl solution of 0.1 M concentration was used as the electrolyte because the Debye length was less than 1 nm at this concentration, which indicated that the double-layer force could be neglected. ,, In this electrochemical system, the electrochemical potential of the gold surface is maintained between −0.6 and +0.6 V because in this potential range, no Faraday process, such as the oxidation of the GE, occurs. , All electrochemical potentials were recorded relative to the quasi-reference electrode of the Ag wire. The potential point of zero charge ( U pzc ) in the electrochemical system is approximately 100 mV …”

“…As shown in the abovementioned examples, most electrochemical AFM studies of the frictional behavior in GE interfaces use the negatively charged silica colloidal probe as a counter-frictional material. ,, Currently, studies that directly compare colloidal probes with different surface electric properties are lacking; such comparative studies could have a wide-range impact. For example, while GEs are widely used as test substrates in the biomedical field, such as cancer cell recognition and targeted drug transport, − the surfaces of most drugs or cells of interest are either positively or negatively charged …”

QCM Study of Tribotronic Control in Ionic Liquids and Nanoparticle Suspensions

“…7). This scenario is supported by the fact that water layers are known to form ice like structures near metal surfaces and reorient with the strength and directly of applied external fields [50]. 6 Example 2…”