In this work, the effect of humidity and water intercalation on the friction and wear behavior of few-layers of graphene and graphene oxide (GO) was studied using friction force microscopy. Thickness ...measurements demonstrated significant water intercalation within GO affecting its surface topography (roughness and protrusions), whereas negligible water intercalation of graphene was observed. It was found that water intercalation in GO contributed to wearing of layers at a relative humidity as low as ∼30%. The influence of surface wettability and water adsorption was also studied by comparing the sliding behavior of SiO2/GO, SiO2/Graphene, and SiO2/SiO2 interfaces. Friction for the SiO2/GO interface increased with relative humidity due to water intercalation and condensation of water. In contrast, it was observed that adsorption of water molecules lubricated the SiO2/SiO2 interface due to easy shearing of water on the hydrophobic surface, particularly once the adsorbed water layers had transitioned from “ice-like water” to “liquid-like water” structures. Lastly, an opposite friction trend was observed for the graphene/SiO2 interface with water molecules failing to lubricate the interface as compared to the dry graphene/SiO2 contact.
Abstract
Many of the best-performing perovskite photovoltaic devices make use of 2D/3D interfaces, which improve efficiency and stability – but it remains unclear how the conversion of 3D-to-2D ...perovskite occurs and how these interfaces are assembled. Here, we use in situ Grazing-Incidence Wide-Angle X-Ray Scattering to resolve 2D/3D interface formation during spin-coating. We observe progressive dimensional reduction from 3D to
n
= 3 → 2 → 1 when we expose (MAPbBr
3
)
0.05
(FAPbI
3
)
0.95
perovskites to vinylbenzylammonium ligand cations. Density functional theory simulations suggest ligands incorporate sequentially into the 3D lattice, driven by phenyl ring stacking, progressively bisecting the 3D perovskite into lower-dimensional fragments to form stable interfaces. Slowing the 2D/3D transformation with higher concentrations of antisolvent yields thinner 2D layers formed conformally onto 3D grains, improving carrier extraction and device efficiency (20% 3D-only, 22% 2D/3D). Controlling this progressive dimensional reduction has potential to further improve the performance of 2D/3D perovskite photovoltaics.
The energy landscape of reduced-dimensional perovskites (RDPs) can be tailored by adjusting their layer width (n). Recently, two/three-dimensional (2D/3D) heterostructures containing n = 1 and 2 RDPs ...have produced perovskite solar cells (PSCs) with >25% power conversion efficiency (PCE). Unfortunately, this method does not translate to inverted PSCs due to electron blocking at the 2D/3D interface. Here we report a method to increase the layer width of RDPs in 2D/3D heterostructures to address this problem. We discover that bulkier organics form 2D heterostructures more slowly, resulting in wider RDPs; and that small modifications to ligand design induce preferential growth of n ≥ 3 RDPs. Leveraging these insights, we developed efficient inverted PSCs (with a certified quasi-steady-state PCE of 23.91%). Unencapsulated devices operate at room temperature and around 50% relative humidity for over 1,000 h without loss of PCE; and, when subjected to ISOS-L3 accelerated ageing, encapsulated devices retain 92% of initial PCE after 500 h.A scheme to control the confinement within 2D/3D perovskite heterostructures results in stable, efficient inverted perovskite solar cells.
Lightweight high-density polyethylene (HDPE)–graphene nanoplatelet (GnP) composite foams were fabricated via a supercritical-fluid (SCF) treatment and physical foaming in an injection-molding ...process. We demonstrated that the introduction of a microcellular structure can substantially increase the electrical conductivity and can decrease the percolation threshold of the polymer–GnP composites. The nanocomposite foams had a significantly higher electrical conductivity, a higher dielectric constant, a higher electromagnetic interference (EMI) shielding effectiveness (SE), and a lower percolation threshold compared to their regular injection-molded counterparts. The SCF treatment and foaming exfoliated the GnPs in situ during the fabrication process. This process also changed the GnP’s flow-induced arrangement by reducing the melt viscosity and cellular growth. Moreover, the generation of a cellular structure rearranged the GnPs to be mainly perpendicular to the radial direction of the bubble growth. This enhanced the GnP’s interconnectivity and produced a unique GnP arrangement around the cells. Therefore, the through-plane conductivity increased up to a maximum of 9 orders of magnitude and the percolation threshold decreased by up to 62%. The lightweight injection-molded nanocomposite foams of 9.8 vol % GnP exhibited a real permittivity of ε′ = 106.4, which was superior to that of their regular injection-molded (ε′ = 6.2). A maximum K-band EMI SE of 31.6 dB was achieved in HDPE–19 vol % GnP composite foams, which was 45% higher than that of the solid counterpart. In addition, the physical foaming reduced the density of the HDPE–GnP foams by up to 26%. Therefore, the fabricated polymer–GnP nanocomposite foams in this study pointed toward the further development of lightweight and conductive polymer–GnP composites with tailored properties.
Fatigue of graphene Cui, Teng; Mukherjee, Sankha; Sudeep, Parambath M ...
Nature materials,
04/2020, Letnik:
19, Številka:
4
Journal Article
Recenzirano
Materials can suffer mechanical fatigue when subjected to cyclic loading at stress levels much lower than the ultimate tensile strength, and understanding this behaviour is critical to evaluating ...long-term dynamic reliability. The fatigue life and damage mechanisms of two-dimensional (2D) materials, of interest for mechanical and electronic applications, are currently unknown. Here, we present a fatigue study of freestanding 2D materials, specifically graphene and graphene oxide (GO). Using atomic force microscopy, monolayer and few-layer graphene were found to exhibit a fatigue life of more than 10
cycles at a mean stress of 71 GPa and a stress range of 5.6 GPa, higher than any material reported so far. Fatigue failure in monolayer graphene is global and catastrophic without progressive damage, while molecular dynamics simulations reveal this is preceded by stress-mediated bond reconfigurations near defective sites. Conversely, functional groups in GO impart a local and progressive fatigue damage mechanism. This study not only provides fundamental insights into the fatigue enhancement behaviour of graphene-embedded nanocomposites, but also serves as a starting point for the dynamic reliability evaluation of other 2D materials.
Droplet friction is common and significant in any field where liquids interact with solid surfaces. This study explores the molecular capping of surface-tethered, liquid-like polydimethylsiloxane ...(PDMS) brushes and its substantial effect on droplet friction and liquid repellency. By exchanging polymer chain terminal silanol groups for methyls using a single-step vapor phase reaction, the contact line relaxation time is decreased by three orders of magnitude-from seconds to milliseconds. This leads to a substantial reduction in the static and kinetic friction of both high- and low-surface tension fluids. Vertical droplet oscillatory imaging confirms the ultra-fast contact line dynamics of capped PDMS brushes, which is corroborated by live contact angle monitoring during fluid flow. This study proposes that truly omniphobic surfaces should not only have very small contact angle hysteresis, but their contact line relaxation time should be significantly shorter than the timescale of their useful application, i.e., a Deborah number less than unity. Capped PDMS brushes that meet these criteria demonstrate complete suppression of the coffee ring effect, excellent anti-fouling behavior, directional droplet transport, increased water harvesting performance, and transparency retention following the evaporation of non-Newtonian fluids.
Electrochemical reduction of carbon dioxide (CO^sub 2^) to carbon monoxide (CO) is the first step in the synthesis of more complex carbon-based fuels and feedstocks using renewable electricity1-7. ...Unfortunately, the reaction suffers from slow kinetics7,8 owing to the low local concentration of CO^sub 2^ surrounding typical CO^sub 2^ reduction reaction catalysts. Alkali metal cations are known to overcome this limitation through non-covalent interactions with adsorbed reagent species9,10, but the effect is restricted by the solubility of relevant salts. Large applied electrode potentials can also enhance CO^sub 2^ adsorption11, but this comes at the cost of increased hydrogen (H^sub 2^) evolution. Here we report that nanostructured electrodes produce, at low applied overpotentials, local high electric fields that concentrate electrolyte cations, which in turn leads to a high local concentration of CO^sub 2^ close to the active CO^sub 2^ reduction reaction surface. Simulations reveal tenfold higher electric fields associated with metallic nanometre-sized tips compared to quasi-planar electrode regions, and measurements using gold nanoneedles confirm a field-induced reagent concentration that enables the CO^sub 2^ reduction reaction to proceed with a geometric current density for CO of 22 milliamperes per square centimetre at -0.35 volts (overpotential of 0.24 volts). This performance surpasses by an order of magnitude the performance of the best gold nanorods, nanoparticles and oxidederived noble metal catalysts. Similarly designed palladium nanoneedle electrocatalysts produce formate with a Faradaic efficiency of more than 90 per cent and an unprecedented geometric current density for formate of 10 milliamperes per square centimetre at -0.2 volts, demonstrating the wider applicability of the field-induced reagent concentration concept.
The periodic permanent magnet electromagnetic acoustic transducer (PPM EMAT) is a sensor that can generate and receive shear horizontal (SH) waves without direct contact with the inspected medium ...using the Lorentz mechanism. However, the PPM EMAT experiences high signal variance on ferromagnetic steel under specific conditions, such as a change in signal amplitude when the sensor is moved in the direction of SH wave propagation. Magnetostriction effects are hypothesized to be the cause of these anomalous behaviors; the objective of this paper is to determine the relative strengths of the magnetostriction and Lorentz wave generation mechanisms for this type of EMAT on steel. This goal is accomplished through the use of a second EMAT, which induces only magnetostriction (MS-EMAT), to calibrate a novel semi-empirical magnetostriction model. It is found that magnetostriction effects reduce the amplitude of the SH wave generated by this particular PPM EMAT transmitter by an average of 29% over a range of input currents. It is also determined that magnetostriction is significant only in the investigated PPM EMAT transmitter, not the receiver. In terms of practical application, it is shown that the MS-EMAT is less sensitive to changes in the static and dynamic fields than PPM EMATs at specific operating points; this makes the MS-EMAT a viable alternative for nondestructive evaluation despite lower amplitudes.
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