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

Leila Pashazanusi,Noshir S Pesika,Kai Kristiansen,Shaowei Li,Yu Tian

doi:10.3389/fmech.2019.00039

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.

Highlights

The study of molecular interactions between surfaces and colloids has been of great interest (Hugel and Seitz, 2001; Israelachvili, 2011; Balzer et al, 2013; Pashazanusi et al, 2017a) for several decades
Based on prior literature presented in the introduction and our past work (Pashazanusi et al, 2017b), we hypothesize that the underlying and dominant mechanism that modulates the friction force is hydrogen bonding between oriented interfacial water layer (IWL) and between the IWLs and the atomic force microscopy (AFM) tip
Our assumption is further supported by recent work which reproduced our results and further demonstrated that the ability of the tip to experience hydrogen bonding affected the friction forces (Li et al, 2018)

Summary

Introduction

The study of molecular interactions between surfaces and colloids has been of great interest (Hugel and Seitz, 2001; Israelachvili, 2011; Balzer et al, 2013; Pashazanusi et al, 2017a) for several decades. We presented an exploratory study on the effect of an applied potential on the friction force between an AFM tip and a gold electrode surface over a wider range of applied potentials (i.e., −0.6 to +0.6 V vs Ag) (Pashazanusi et al, 2017b).

Results

Conclusion