From the early tribological studies of Leonardo da Vinci to Amontons' law, friction has been shown to increase with increasing normal load. This trend continues to hold at the nanoscale, where friction can vary nonlinearly with normal load. Here we present nanoscale friction force microscopy (FFM) experiments for a nanoscale probe tip sliding on a chemically modified graphite surface in an atomic force microscope (AFM). Our results demonstrate that, when adhesion between the AFM tip and surface is enhanced relative to the exfoliation energy of graphite, friction can increase as the load decreases under tip retraction. This leads to the emergence of an effectively negative coefficient of friction in the low-load regime. We show that the magnitude of this coefficient depends on the ratio of tip-sample adhesion to the exfoliation energy of graphite. Through both atomistic- and continuum-based simulations, we attribute this unusual phenomenon to a reversible partial delamination of the topmost atomic layers, which then mimic few- to single-layer graphene. Lifting of these layers with the AFM tip leads to greater deformability of the surface with decreasing applied load. This discovery suggests that the lamellar nature of graphite yields nanoscale tribological properties outside the predictive capacity of existing continuum mechanical models.

Efficient water electrolysis for hydrogen production constitutes a key segment for the upcoming hydrogen economy, but has been impeded by the lack of high-performance and low-cost electrocatalysts for, ideally, simultaneously expediting the kinetics of both hydrogen and oxygen evolution reactions (HER and OER). In this study, the favored binding energetics of OER and HER reaction intermediates on iron-doped nickel phosphides are first predicted by density functional theory (DFT) simulations, and then experimentally verified through the fabrication of Fe-doped Ni2P nanoparticles embedded in carbon nanotubes using metal–organic framework (MOF) arrays on nickel foam as the structural template. Systematic investigations on the effect of phosphorization and Fe doping reveal that while the former endows a larger benefit on OER than on HER, the latter enables not only modulating the electronic structure, but also tuning the micromorphology of the catalyst, synergistically leading to both enhanced HER and OER. As a result, extraordinary performances of constant water electrolysis are demonstrated requiring only a cell voltage of 1.66 V to afford a current density of 500 mA cm–2, far outperforming the benchmark electrode couple composed of Pt/C and RuO2. Postelectrolysis characterizations combined with DFT inspection further reveal that while the Fe-doped Ni2P species are mostly retained after prolonged HER, they are in situ converted to Fe/P-doped γ-NiOOH during OER, serving as the actual OER active sites with high activity.

Nanoframes of cobalt–iron bimetal phosphide enhance both HER and OER activities towards efficient overall water splitting.

Cesium lead halide (CsPbX3) nanocrystals have great potential for photovoltaic and optoelectronic applications, but they are sensitive to oxygen, moisture, and light irradiation. Embedding these CsPbX3 nanocrystals into all‐inorganic amorphous solid matrices such as glass is expected to improve their stability. In this work, CsPbX3 nanocrystals are precipitated in boro‐germanate glasses with tunable composition, absorption, and photoluminescence. Quantum efficiency of CsPbBr3 nanocrystals in glass can be as high as ≈80% and ≈20% for CsPb(Cl/Br)3 and CsPb(Br/I)3 nanocrystals, respectively. Thermo‐ and photostabilities of CsPbX3 nanocrystals in glass are greatly improved due to the inert nature of glasses, and intense laser irradiation‐induced damage to CsPbX3 nanocrystals is recoverable through thermal annealing. With CsPbBr3 nanocrystal‐embedded glass slices, a green light‐emitting device with a luminous efficiency of ≈120 lm W−1 and an external quantum yield of ≈30% is achieved. A white‐light‐emitting device consisting of CsPbBr3 nanocrystals and CsPb(Br/I)3 nanocrystal–embedded glass slices shows luminous efficiency in the range of 50–60 lm W−1 and external quantum yield of 20–25%. The thermo‐ and photostabilities along with the chemical stability of CsPbX3 nanocrystal–embedded glasses are promising materials for photoluminescence related applications.

Reversible oxygen conversion is important for various green energy technologies. Herein we synthesize a series of bimetallic coordination polymers by varying the Ni/Co ratio and using HITP (HITP=2,3,6,7,10,11‐hexaiminotriphenylene) as the ligand, to interrogate the role of metal centres in modulating the activity of the oxygen reduction reaction (ORR). Co3HITP2 and Ni3HITP2 are compared. Unpaired 3d electrons in Co3HITP2 result in less coplanarity but more radical character. Thus, despite of a reduced crystallinity and conductivity, the best ORR activity, comparable to 20 % Pt/C, is obtained for Co3HITP2, showing the 3d orbital configuration of the metal centre promotes ORR. Experimental and DFT studies show a transition of ORR pathway from four‐electron for Co3HITP2 to two‐electron for Ni3HITP2. Rechargeable zinc–air batteries using Co3HITP2 as the air cathode catalyst demonstrate excellent energy efficiency and stability.

High efficiency overall water splitting enabled by a hierarchical Ni–C structure derived from 2D MOF sheets without heteroatom doping.

Scheme 1. Schematic illustration of the fabrication process of h-CoFeNi LDHs through MOF-mediated topotactic transformation.

Using atomic force microscopy (AFM), supported by semicontinuum numerical simulations, we determine the effect of tip-subsurface van der Waals interactions on nanoscale friction and adhesion for suspended and silicon dioxide supported graphene of varying thickness. While pull-off force measurements reveal no layer number dependence for supported graphene, suspended graphene exhibits an increase in pull-off force with thickness. Further, at low applied loads, friction increases with increasing number of layers for suspended graphene, in contrast to reported trends for supported graphene. We attribute these results to a competition between local forces that determine the deformation of the surface layer, the profile of the membrane as a whole, and van der Waals forces between the AFM tip and subsurface layers. We find that friction on supported monolayer graphene can be fit using generalized continuum mechanics models, from which we extract the work of adhesion and interfacial shear strength. In addition, we show that tip-sample adhesive forces depend on interactions with subsurface material and increase in the presence of a supporting substrate or additional graphene layers.