Triboelectrification and Unique Frictional Characteristics of Germanium-Based Nanofilms.

In this study, germanium arsenide (GeAs) is investigated as a promising nanomaterial for application in triboelectric nanogenerators and green energy harvesting. The mechanical and electrical properties of mechanically exfoliated GeAs on silica substrates are evaluated through friction force microscopy and Kelvin probe force microscopy, respectively. First, it is observed that the surface potential/work function of GeAs varied with thickness. Second, thickness-dependent friction on GeAs films is found. However, the variation of friction with GeAs thickness followed an inverse trend typically observed for most other 2D material systems: larger friction is measured on thicker GeAs films. The higher friction is attributed to the higher surface potential of thicker GeAs, resulting from the accumulation of electrons on the GeAs surface that also resulted in higher adhesion between GeAs surface and the tip. Finally, history-dependent friction is observed and resulted from a continual increase in the friction force as the surface is scanned and originated from the triboelectrification of the surface. The dynamic triboelectrification behavior of thick GeAs during the scanning process is further verified and visualized by a serial experiment, where the GeAs is tribo-electrified through scanning and gradually de-electrified/discharged upon ceasing the scan.

Blowing-inspired ex situ preparation of ultrathin hydrogel coatings for visibly monitoring humidity and alkaline gas.

Compared with the in situ preparation of ultrathin hydrogel coatings through successive yet tedious steps, ex situ strategies decouple the steps and greatly enhance the maneuverability and convenience of preparing hydrogel coatings. However, the difficulty in preparing sub-micron-thick coatings limits the applicability of ex situ methods in nanotechnology. Herein, we report the ex situ preparation of centimeter-scale ultrathin hydrogel coatings by applying omnidirectional stretching toward pre-gelated hydrogels with necking behaviors. This process involves blowing a bubble directly from a pre-gelated hydrogel and subsequently transferring the resulting hydrogel bubble to different substrates. The as-fabricated coatings exhibit peak-shaped thickness variations, with the thinnest part as low as ∼5 nm and the thickest part controllable from ∼200 nm to several microns. This method can be universally applied to hydrogels with necking behavior triggered by internal particles with partial hydrophobicity. Due to the overall near- or sub-micron thickness and unique thickness distribution, the coatings present concentric rings of different interference colors. With such an observable optical characteristic, the as-prepared hydrogel coatings are applied as sensors to visibly monitor humidity changes or alkaline gas through the visibly observable expansion or contraction of concentric interferometry rings, which is triggered by adsorbing/desorbing the surrounding water or alkaline molecules and the resultant swelling/deswelling of the coatings, respectively. With the universality of the method, we believe that the ex situ strategy can be used as a simple yet efficient environmental nanotechnology to fabricate various types of nanometer-thick hydrogel coatings as detectors to sensitively and visibly monitor surrounding stimuli on demand.

Electrode Materials for Enhancing the Performance and Cycling Stability of Zinc Iodide Flow Batteries at High Current Densities.

Aqueous redox flow battery systems that use a zinc negative electrode have a relatively high energy density. However, high current densities can lead to zinc dendrite growth and electrode polarization, which limit the battery's high power density and cyclability. In this study, a perforated copper foil with a high electrical conductivity was used on the negative side, combined with an electrocatalyst on the positive electrode in a zinc iodide flow battery. A significant improvement in the energy efficiency (ca. 10% vs using graphite felt on both sides) and cycling stability at a high current density of 40 mA cm-2 was observed. A long cycling stability with a high areal capacity of 222 mA h cm-2 is obtained in this study, which is the highest reported areal capacity for zinc-iodide aqueous flow batteries operating at high current density, in comparison to previous studies. Additionally, the use of a perforated copper foil anode in combination with a novel flow mode was discovered to achieve consistent cycling at exceedingly high current densities of >100 mA cm-2. In situ and ex situ characterization techniques, including in situ atomic force microscopy coupled with in situ optical microscopy and X-ray diffraction, are applied to clarify the relationship between zinc deposition morphology on the perforated copper foil and battery performance in two different flow field conditions. With a portion of the flow going through the perforations, a significantly more uniform and compact zinc deposition was observed compared to the case where all of the flow passed over the surface of the electrode. Results from modeling and simulation support the conclusion that the flow of a fraction of electrolyte through the electrode enhances mass transport, enabling a more compact deposit.

Revisiting Frictional Characteristics of Graphene: Effect of In-Plane Straining.

Supreme mechanical performance and tribological properties render graphene a promising candidate as a surface friction modifier. Recently, it has been demonstrated that applying in-plane strain can effectively tune friction of suspended graphene in a reversible manner. However, since graphene is deposited on solid surfaces in most tribological applications, whether such operation will result in a similar modulation effect becomes a critical question to be answered. Herein, by depositing graphene onto a stretchable substrate, the frictional characteristics of supported graphene under a wide range of strain are examined with an in situ tensile loading platform. The experimental results show that friction of supported graphene decreases with increasing graphene strain, similar to the suspended system. However, depending on the adherence state of the graphene/substrate interface, the system exhibits two distinct friction regimes with significantly different strain dependences. Assisted by detailed atomic force microscopy imaging, we attribute the unique behavior to the transition between two friction modulation modes, i.e., contact-quality-dominated friction and puckering-dominated friction. This work provides a more comprehensive view of the influence of strain on surface friction of graphene, which is beneficial for active modulation of graphene friction through strain engineering.

Abnormal Raman Characteristics of Graphene Originating from Contact Interface Inhomogeneity.

The Raman peak position shift rate per strain (RSS) coefficient of graphene is crucial for quantitative strain measurement by Raman spectroscopy. Despite its essential role, the experimentally measured RSS values are found to be highly scattered and many times significantly lower than the theoretical prediction. Here, using in situ Raman spectroscopy with a tensile test system, we resolve this controversy by examining the Raman characteristics of graphene derived from chemical vapor deposition (CVD) transferred on polymer substrates. Our experiments show that the Raman 2D-peak position of CVD graphene can shift nonlinearly with applied strain, in contrast to its intrinsically linear trait. More importantly, the resultant RSS coefficient at the steady state is much lower than the theoretical prediction. By analyzing atomic force microscopy (AFM) phase images and full width at half-maximum (FWHM) of Raman spectra, we attribute the abnormal behavior to nanometer-scale inhomogeneity of the graphene/substrate contact interface. Assisted by a simplified discrete interface slip model, we correlate the evolution of nanometer-scale inhomogeneity with that of the apparent Raman response. The theoretical model provides a useful tool for understanding and optimizing the contact interface behavior of various two-dimensional materials on substrates; the revealed mechanism is critical for correct interpretation of data obtained by Raman or any other spectroscopies based on homogenized laser signals.

Graphene-encapsulated nickel-copper bimetallic nanoparticle catalysts for electrochemical reduction of CO2 to CO.

Highly selective CO2 electroreduction to CO (∼90% faradaic efficiency) was achieved on NiCu0.25 bimetallic nanoparticle catalysts. By combining Synchrotron based X-ray absorption and in situ Raman spectroscopy studies, we found that there is a negative correlation between the Cu content in NiCux and CO selectivity due to redistribution of the 3d electrons.

Anomalous hydrogen evolution behavior in high-pH environment induced by locally generated hydronium ions.

Most fundamental studies of electrocatalysis are based on the experimental and simulation results obtained for bulk model materials. Some of these mechanistic understandings are inapplicable for more active nanostructured electrocatalysts. Herein, considering the simplest and most typical electrocatalytic process, the hydrogen evolution reaction, an alternative reaction mechanism is proposed for nanomaterials based on the identification of a new intermediate, which differs from those commonly known for the bulk counterparts. In-situ Raman spectroscopy and electrochemical thermal/kinetic measurements were conducted on a series of nanomaterials under different conditions. In high-pH electrolytes with negligible hydronium (H3O+) concentration in bulk phase, massive H3O+ intermediates are found generating on the catalytic surface during water dissociation and hydrogen adsorption processes. These H3O+ intermediates create a unique acid-like local reaction environment on nanostructured catalytic surfaces and cut the energy barrier of the overall reaction. Such phenomena on nanostructured electrocatalysts explain their widely observed anomalously high activity under high-pH conditions.

Interfacial Mechanical Properties of Double-Layer Graphene with Consideration of the Effect of Stacking Mode.

The mechanical performance and the effect of the stacking mode of the double-layer graphene interface are studied. Three kinds of double-layer graphene-PET composite structure specimens with different stacking methods are designed. By combining micro-Raman spectroscopy with a microtensile loading device, in situ and real-time measurements are carried out for the specimens during the uniaxial loading process. Based on mechanical analysis, a method for peak splitting of the Raman spectra of double-layer polycrystalline graphene is developed to extract the strain information for each layer of graphene. The strain distribution and shear stress distribution of graphene in each layer during the loading process are determined experimentally. The strain transfer between the two interfaces is analyzed, and the mechanical parameters of interfaces are given quantitatively, the interlayer shear stress of graphene is 0.084 MPa. Finally, double-layer graphene with different stacking modes is studied. The results show that the different lengths of the upper and lower layers of graphene lead to a stress concentration of 0.7-1 GPa at the boundary of the short layer of graphene when stacked. The stress concentration problem of double-layer graphene should be considered for the practical application in microelectrical components.

Bronze alloys with tin surface sites for selective electrochemical reduction of CO2.

Low concentration tin bronze alloys show high selectivity for CO2 electroreduction to CO, while high concentration tin bronze alloys show high selectivity for formate. The tin surface sites appear to control selectivity by influencing the binding characteristics toward the first reaction intermediate on copper-based alloys.

Size Effect of the Interfacial Mechanical Behavior of Graphene on a Stretchable Substrate.

The size effect and deformation transfer of the interface between graphene and a polymer substrate are experimentally investigated. Eight composite specimens, containing polyethylene terephthalate substrate (PET) and eight different sizes of graphene, are designed. The specimens are studied by a series of experiments to explore how the mechanical properties of the tangential interface between graphene and the substrates can be influenced by the size of graphene. Micro-Raman spectroscopy is employed to measure the full-field strain of graphene subjected to a uniaxial tensile loading process, based on which the evolution of the bonding states of the interface is obtained. The existence of a size effect in the interfacial strain transfer process at the graphene/PET interface is observed, and this phenomenon is characterized by a size threshold and an innovatively defined parameter called the critical relative transfer length. Combined with previous experimental results on the tangential interface of graphene, we observe that the size effect of the interfacial shear stress of graphene is the main cause for the inconformity of experimental data published in previous reports.