Kaiming Hou

Structural and tribological characterization of fluorinated graphene with various fluorine contents prepared by liquid-phase exfoliation

The high strength and noble inertness of fluorinated graphene (FG) indicate very promising properties for its application in tribological applications to reduce friction and save energy, yet few works refer to its tribological performance, mostly due to the lack of an effective synthesis method and limited knowledge of FG. In this work, fluorinated graphene (FG) sheets with various fluorine contents are prepared from fluorinated graphite (FGi) by means of controllable chemical reaction with ethylenediamine (EDA) and liquid-phase exfoliation with N-methyl-2-pyrrolidone (NMP) in a one-pot synthesis. Transmission electron microscopy and atomic force microscopy analyses show that the obtained FG sheets possess large lateral size and ultrathin thickness (1.8–4.0 nm). Chemical characterizations indicate the C/F ratio can be readily tuned by adjusting the reaction temperature with EDA, which leads to defluorination and also substitution of a small amount of fluorine atoms by alkylidene amino groups. The tribological performance of FG samples as novel lubricant additives in base oil of polyalphaolefin-40 with different concentrations (0.1–0.4 mg mL−1) is investigated. The tribological tests suggest that the addition of FG at optimum concentration can greatly improve the anti-wear property of the base oil and there exists a strong proportional relationship between anti-wear ability and fluorine content.

In-situ formation of spherical MoS2 nanoparticles for ultra-low friction

The motion resistance and energy dissipation of rolling friction are much lower than those of sliding friction at the macroscale. But at the microscale, the impact of rolling on friction remains an open question. Here, we show that spherical MoS2 nanoparticles can be formed in situ at a friction interface by scrolling and wrapping MoS2 nanosheets under the induction of a reciprocating shear stress, when an MoS2 coating constructed from loosely stacked nanosheets is tested in a vacuum of 3.5 × 10-3 Pa. An ultra-low friction state can be readily realized with friction coefficients of 0.004-0.006, which are one order of magnitude lower than that of a pulse laser deposited MoS2 coating without nanoparticles formed in a friction process. Accordingly, the spherical nanoparticles are highlighted as the key factor in the ultra-low friction. Classical molecular dynamics simulations further reveal that the motion mode of the MoS2 nanoparticle is stress-dependent. This finding confirms access to ultra-low friction by introducing rolling friction based on the microstructural evolution of the coating.

Graphene-based cellular materials with extremely low density and high pressure sensitivity based on self-assembled graphene oxide liquid crystals

Three-dimensional (3D) graphene materials with high elasticity and low density are a prerequisite for achieving high sensitivity in flexible strain sensors. However, conventional 3D graphene materials with extremely low density often struggle to attain excellent mechanical resilience. Here, a material synthesis strategy, including self-assembly and annealing steps, is developed to fabricate a novel 3D cellular material, graphene oxide liquid crystals–konjac glucomannan, with high elasticity and low density for possible application in highly sensitive flexible strain sensors. The novel introduction of biomass, konjac glucomannan, largely enhances the elasticity of the 3D cellular structure, endowing the resulting material with a high Youngs modulus of 10.3 kPa at a low density of 1.56 mg cm−3, and excellent mechanical durability for >1000 cycles. The assembled flexible strain sensor based on cellular material exhibits a particularly high sensitivity of 0.28 kPa−1, a fast response of 40 ms (rising time) and excellent cycling stability. Importantly, practical application of the sensitive strain sensor has been realized through monitoring a variety of human motion in real time, suggesting the creation of a novel candidate for applications in wearable medical devices and electronic skin.