Water/solid interfaces play crucial roles in a wide range of physicochemical and technological processes. However, our microscopic understanding of the interfacial water under ambient temperature is relatively primitive. Herein, we report the direct experimental construction of two-dimensional (2D) ice-like water layer on hydrophilic surface at room temperature by using environment-controlled atomic force microscopy. In contrast to the prevailing view that nanoscale confinement is needed for the formation of 2D ice-like water, we find that 2D ice-like water can form on mica surface at temperatures above the freezing point without confinement. The 2D ice-like water layer shows epitaxial relation with the underlying mica lattice and good thermostability. In addition, the growth of ice-like water layer can be well controlled by the mechanical force from the scanning tip. Furthermore, the friction properties of 2D ice-like water layer are also probed by friction force microscopy. It is found that the ice-like water layer can dramatically reduce the friction. These results provide deep understanding of 2D ice-like water formation on solid surfaces without nanoscale confinement and suggest means of growing 2D ices on surfaces at room temperature.

Hydration lubrication has long been invoked to account for the ultralow sliding friction between charged surfaces in aqueous environments, but still not well understood at molecular-level. Herein, we explored the lubrication effect of hydrated halogen anions on positively charged surface at the atomic scale by using three-dimensional atomic force microscopy and friction force microscopy. Atomically resolved three-dimensional imaging revealed that the anion layer was topped by a few hydration layers. The mechanical properties of the hydration layers were found mainly dependent on the concentration of electrolyte solutions and independent of the species of hydrated anions. Atomic-scale friction experiments showed that the hydration friction coefficient and friction dissipation at low concentrations were orders of magnitude lower than that at high concentrations and in pure water. Superlubricity can be achieved in low concentration electrolyte solution. These results indicated that the changes of electrolyte solution concentrations led to different adsorption state of anions on the positively charged surface which gave rise to the difference of the friction behaviors. The findings in this study reveal the role of hydrated anions in hydration lubrication and provide deep insights into the origins of hydration lubrication.

Load-dependent friction hysteresis is an intriguing phenomenon that occurs in many materials, where the friction measured during unloading is larger than that measured during loading for a given normal load. However, the mechanism underlying this behavior is still not well understood. In this work, temperature- controlled friction force microscopy was utilized to explore the origin of friction hysteresis on exfoliated monolayer graphene. The experimental observations show that environmental adsorbates from ambient air play an important role in the load dependence of friction. Specifically, the existence of environmental adsorbates between the tip and graphene surface gives rise to an enhanced tip-graphene adhesion force, which leads to a positive friction hysteresis where the friction force is larger during unloading than during loading. In contrast to positive friction hysteresis, a negative friction hysteresis where the friction force is smaller during unloading than during loading is observed through the removal of the environmental adsorbates upon in situ annealing. It is proposed that the measured friction hysteresis originates from the hysteresis in the contact area caused by environmental adsorbates between the tip and graphene. These findings provide a revised understanding of the friction hysteresis in monolayer graphene in terms of environmental adsorbates.