ABSTRACT
More than thirty years ago, it was theoretically predicted that friction for incommensurate contacts between atomically smooth, infinite, crystalline materials (e.g., graphite, MoS_{2}) is vanishing in the low speed limit, and this corresponding state was called structural superlubricity (SSL). However, experimental validation of this prediction has met challenges, since real contacts always have a finite size, and the overall friction arises not only from the atoms located within the contact area, but also from those at the contact edges which can contribute a finite amount of friction even when the incommensurate area does not. Here, we report, using a novel method, the decoupling of these contributions for the first time. The results obtained from nanoscale to microscale incommensurate contacts of graphite under ambient conditions verify that the average frictional contribution of an inner atom is no more than 10^{-4} that of an atom at the edge. Correspondingly, the total friction force is dominated by friction between the contact edges for contacts up to 10 µm in lateral size. We discuss the physical mechanisms of friction observed in SSL contacts, and provide guidelines for the rational design of large-scale SSL contacts.
ABSTRACT
Direct characterizations of the two component surfaces of a solid-solid interface are essential for understanding its various interfacial mechanical, physical, and electrical behaviors. Particularly, the fascinating phenomenon termed structural superlubricity, a state of nearly zero friction and wear, is sensitively dependent on the interface structure. Here we report a controllable pick-and-flip technique to separate a microscale contact pair for the characterization of its two component surfaces for van der Waals layered materials. With this technique, the interface of a graphite superlubric contact is characterized with resolution from microscale down to the atomic level. Imaging of the graphite lattice provides direct proof that this superlubric interface consists of two monocrystalline surfaces incommensurate with each other. More importantly, the structure-property relationship for this contact is investigated. Friction measurements combined with fully atomistic molecular dynamics reveal that internal structures [internals steps, pits, and bulges buried underneath the topmost graphene sheet(s)] have negligible contribution to the total friction; in contrast, external defects lead to a high friction. These results help us to better understand the structure of highly oriented pyrolytic graphite and the fundamental mechanisms of structural superlubricity, as well as to guide the design of superlubricity-based devices.
ABSTRACT
The rapid development of both wearable and implantable biofuel cells has triggered more and more attention on the lactate biofuel cell. The novel lactate/oxygen biofuel cell (L/O-BFC) with the direct electron transfer (DET)-type lactate oxidase (LOx) anode and the platinum group metal (PGM)-free Fe-N-C cathode is designed and constructed in this paper. In such a reasonable design, the surface-controlled direct two-electron electrochemical reaction of the lactate oxidase was determined by cyclic voltammetry (CV) on the carbon nanotube (CNT) modified electrode with favorable high electrochemical active surface area and electronic conductivity. Additionally, the biosensor based on DET-type LOx modified electrode impressively presented linear response to lactate with different concentrations from 0.000 mM to 12.300 mM. In particular, the apparent Michealis-constant (KMapp) calculated as 0.140 mM clearly indicates that LOx on CNT has strong affinity to the substrate lactate. Meanwhile, 4e- transfer oxygen reduction reaction (ORR) was proven to take place on the Fe-N-C catalysts inthe 0.1 M PBS system, indicating the advantage by using the Fe-N-C catalysts at the cathode of L/O-BFC. Last but not least, the L/O-BFC with the direct electron transfer (DET)-type lactate oxidase(LOx) anode and the Fe-N-C cathode produced an superior open circuit potential (OCP) of 0.264 V and a maximum output power density (OPD) of 24.430 µW cm-2 in O2 saturated 95.020 mM lactate solution. The above results will not only bring about significant interest in developing a DET-type biofuel cell, but also offer guiding direction to explore novel catalyst materials for the biofuel cell. This work enriches the research content and may push developments of the implantable and wearable biofuel cell forward.
ABSTRACT
The sliding friction of a graphene flake atop strained graphene substrates is studied using molecular dynamics simulation. We demonstrate that in this superlubric system, friction can be reduced nonmonotonically by applying strain, which differs from previously reported results on various 2D materials. The critical strain needed for significant reduction in friction decreases drastically when the flake size increases. For a 250 nm flake, a 0.1% biaxial strain could lead to a more than 2-order-of-magnitude reduction. The underlying mechanism is revealed to be the evolution of Moiré patterns. The area of the Moiré pattern relative to the flake size plays a central role in determining friction in strain engineering and other scenarios of superlubricity as well. This result suggests that strain engineering could be particularly efficient for friction modification with large contacts.
ABSTRACT
Structural superlubricity, a nearly frictionless state between two contact solid surfaces, has attracted rapidly increasing attention during the past few years. Yet a key problem that limits its promising applications is the high anisotropy of friction which always leads to its failure. Here we study the friction of a graphene flake sliding on top of a graphene substrate using molecular dynamics simulation. The results show that by applying strain on the substrate, biaxial stretching is better than uniaxial stretching in terms of reducing interlayer friction. Importantly, we find that robust superlubricity can be achieved via both biaxial and uniaxial stretching, namely for stretching above a critical strain which has been achieved experimentally, the friction is no longer dependent on the relative orientation mainly due to the complete lattice mismatch. The underlying mechanism is revealed to be the Moiré pattern formed. These findings provide a viable approach for the realization of robust superlubricity through strain engineering.
ABSTRACT
Electrical energy generated directly from sunlight and biomass solution with a Photoelectrochemical Biofuel Cell (PEBFC) was investigated. The PEBFC consisted of a meso-tetrakis(4-carboxyphenyl)porphyrin (TCPP)-sensitized nanocrystalline titanium dioxide (TiO(2)) mesoporous film (NTDMF) as the photoanode and platinum black as the cathode. The interaction between TCPP sensitizer and NTDMF was evaluated by X-ray photoelectron spectra and FT-IR absorption spectra, indicating that the TCPP sensitizer was adsorbed on the NTDMF by bridging or bidentate coordinate bonds. The spectroscopic properties of pure TCPP ethanol solution and TCPP-sensitized NTDMF were obtained by UV-vis absorption spectra, demonstrating that the characteristic absorption peaks of TCPP on NTDMF displayed slight red shift compared with pure TCPP ethanol solution. The performances of the PEBFC were obtained by photocurrent-photovoltage characteristic curves. The open-circuit photovoltage (V(oc)), the short-circuit photocurrent (I(sc)) and the maximum power density (P(max)) was 0.74 V, 69.96 µA and 33.94 µWcm(-2) at 0.45 V, respectively. The fill factor (FF) was 0.19 and the incident photo-to-current efficiency (IPCE) was 36.0% at 436 nm. The results demonstrated that the TCPP was an appropriate photosensitizer for PEBFC.
