We have measured forces between mica surfaces across two hydrophobic ionic liquids with a surface forces apparatus. Both surfaceadsorbed water and alkyl-chain length on the imidazolium cation influence the structure of the nanoconfined film and the dynamics of film-thickness transitions. Friction shows accumulative microslips as precursors to collective "avalanches" that abruptly reduce friction momentarily. This behavior is interpreted as a consequence of interlayer ion correlations within the 1 to 2 nm thick film; we identify this to be analogous to the friction response of crackling noise systems over a broad range of sizes.

Single-atom catalysts with maximum metal utilization efficiency show great potential for sustainable catalytic applications and fundamental mechanistic studies. We here provide a convenient molecular tailoring strategy based on graphitic carbon nitride as support for the rational design of single-site and dual-site single-atom catalysts. Catalysts with single Fe sites exhibit impressive oxygen reduction reaction activity with a half-wave potential of 0.89 V vs. RHE. We find that the single Ni sites are favorable to promote the key structural reconstruction into bridging Ni-O-Fe bonds in dual-site NiFe SAC. Meanwhile, the newly formed Ni-O-Fe bonds create spin channels for electron transfer, resulting in a significant improvement of the oxygen evolution reaction activity with an overpotential of 270 mV at 10 mA cm−2. We further reveal that the water oxidation reaction follows a dual-site pathway through the deprotonation of *OH at both Ni and Fe sites, leading to the formation of bridging O2 atop the Ni-O-Fe sites.

We have investigated the influence of ambient humidity on the nanoconfined structure and response to shear of ionic liquids. Three ionic liquids (ILs) were selected, namely, 1-ethyl-3-methyl imidazolium ethylsulfate ([EMIM][EtSO4]), 1-ethyl-3-methyl imidazolium tris(pentafluoroethyl)trifluorophosphate ([EMIM] [FAP]), and 1-hexyl-3-methyl imidazolium tris(pentafluoroethyl)trifluorophosphate ([HMIM][FAP]), to investigate the influence of hygroscopic and hydrophobic anions, as well as different alkyl chain lengths. We employed an extended surface forces apparatus (eSFA) to ascertain the structure of the confined films, whereas colloidal-probe lateral force microscopy (CPM) was used to measure shear forces in the nanosized contact between mica and a silica sphere. The presence of water, the anion, and the alkyl chain length of the imidazolium cation were found to influence the equilibrium structure of the nanoconfined film, as well as its dynamic properties. Adsorbed water appears to change both the ion-pair orientation and the slip condition for film-thickness transitions, that is, the resistance of the IL layers to being squeezed out from the contact. Three lubrication regimes have been identified: a boundary-film lubrication regime with the lowest friction, an intermediate lubrication regime that is highly dependent on the IL anion, and an isoviscous rigid hydrodynamic lubrication regime (with Newtonian fluid-film behavior). It is shown how IL composition and water both influence speed and load dependence of shear forces at the nanoscale. Understanding the response to shear provides further insight into the properties of nanoconfined IL films

Studies of 1-hexyl-3-methyl-imidazolium ethylsulfate ([HMIM] EtSO4) using an extended surface forces apparatus show, for the first time, an ordered structure within the nanoconfined ionic liquid (IL) between mica surfaces that extends up to ∼60 nm from the surface. Our measurements show the growth of this ordered IL-film upon successive nanoconfinements-the structural changes being irreversible upon removal of the confinement-and the response of the structure to shear. The compressibility of this system is lower than that typically measured for ILs, while creep takes place during shear, both findings supporting a long-range liquid-to-solid transition. AFM (sharp-tip) studies of [HMIM] EtSO4 on mica only reveal ∼2 surface IL-layers, with order extending only ∼3 nm from the surface, indicating that confinement is required for the long-range IL-solidification to occur. WAXS studies of the bulk IL show a more pronounced ordered structure than is the case for [HMIM] with bis(trifluoromethylsulfonyl)imide as anion, but no long-range order is detected, consistent with the results obtained with the sharp AFM tip. These are the first force measurements of nanoconfinement-induced long-range solidification of an IL.

Understanding the behavior of ionic liquids (ILs) either confined between rough surfaces or in rough nanoscale pores is of great relevance to extend studies performed on ideally flat surfaces to real applications. In this work we have performed an extensive investigation of the structural forces between two surfaces with well-defined roughness (<9 nm RMS) in 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide by atomic force microscopy. Statistical studies of the measured layer thicknesses, layering force, and layering frequency reveal the ordered structure of the rough IL-solid interface. Our work shows that the equilibrium structure of the interfacial IL strongly depends on the topography of the contact.

With reference to our previous surface-force study on 1-hexyl-3-methylimidazolium ethylsulfate ([HMIM] EtSO4) using an extended surface forces apparatus, which showed an ordered structure within the nanoconfined dry ionic liquid (IL) between mica surfaces that extended up to ∼60 nm from the surface, this work focuses on the influence of the environmental humidity on the bulk, interfacial and nanoconfined structure of [HMIM] EtSO4. Infrared spectroscopy and rheometry reflect the changes in chemical and physical properties of the bulk IL due to the uptake of water when exposed to ambient humidity, while wide-angle X-ray scattering shows a mild swelling of the bulk nanostructure, and the AFM sharp tip reveals an additional surface layer at the mica-IL interface. When the water-containing [HMIM] EtSO4 is nanoconfined between two mica surfaces, no long-range order is detected, in contrast to the results obtained for the dry IL, which demonstrates that the presence of water can prevent the liquid-to-solid transformation of this IL. A combination of techniques and the calculated Bjerrum length indicate that water molecules weaken interionic electrostatic and hydrogen-bonding interactions, which lessens ion-ion correlations. Our work shows that the solid-like behavior of the nanoconfined IL strongly depends on the presence of absorbed water and hence, it has implications with regard to the correct interpretation of laboratory studies and their extension to real applications in lubrication.

In this study, 1-ethyl-3-methyl imidazolium trifluoro tris-(pentafluoroethyl) tris(perfluoroalkyl)trifluorophosphate [EMIM] FAP and 1-hexyl-3-methyl imidazolium tris(pentafluoroethyl) tris(perfluoroalkyl)trifluorophosphate [HMIM] FAP were selected as lubricants for silica/silicon surfaces. Pin-on-disk tribometry was used to test the performance of these lubricants under two different environmental conditions (humid air and a nitrogen atmosphere). The surface reactivity of the ionic liquids under mechanical stress was investigated ex situ by X-ray photoelectron spectroscopy. Environmental conditions appear to affect the mechanism of boundary lubrication in different ways, depending on the contact pressure. Tests carried out at 0.5 N applied load showed low friction and no detectable wear in a nitrogen atmosphere, and a substantial increase in both wear and friction in humid air. It is proposed that the presence of water in the IL induces a change in the structure of the confined lubricant film, leading to contact between the sliding surfaces. At higher load (4.5 N), the observation of wear, both under nitrogen and in humid air, reveals that the film is no longer able to prevent contact between asperities, which now dominates the observed tribological behavior. XP-spectra acquired on samples tribostressed at high load, under the two environmental conditions, reveal evidence for the formation of a reaction layer that is hydrolyzed or oxidized in the presence of water and oxygen, suggesting that the variation of wear with the environment is related to changes in the tribochemical reactions involving the silicon surface.

Studying the frictional properties of interfaces with dynamic chemical bonds advances understanding of the mechanism underlying rate and state laws, and offers new pathways for the rational control of frictional response. In this work, we revisit the load dependence of interfacial chemical-bond-induced (ICBI) friction experimentally and find that the velocity dependence of friction can be reversed by changing the normal load. We propose a theoretical model, whose analytical solution allows us to interpret the experimental data on timescales and length scales that are relevant to experimental conditions. Our work provides a promising avenue for exploring the dynamics of ICBI friction.