Chemically stabilized epitaxial wurtzite-BN thin film

We report on the chemically stabilized epitaxial w-BN thin film grown on c-plane sapphire by pulsed laser deposition under slow kinetic condition. Traces of no other allotropes such as cubic (c) or hexagonal (h) BN phases are present. Sapphire substrate plays a significant role in stabilizing the metastable w-BN from h-BN target under unusual PLD growth condition involving low temperature and pressure and is explained based on density functional theory calculation. The hardness and the elastic modulus of the w-BN film are 37 & 339 GPa, respectively measured by indentation along direction. The results are extremely promising in advancing the microelectronic and mechanical tooling industry

Operando Insights on the Degradation Mechanisms of Rhenium doped Molybdenum Disulfide Nanocatalysts for Electrolyzer Applications

MoS2 nanostructures are promising catalysts for proton-exchange-membrane (PEM) electrolyzers to replace expensive noble metals. Their broadscale application demands high activity for the hydrogen evolution reaction (HER) as well as good durability. Doping in MoS2 is commonly applied to enhance the HER activity of MoS2-based nanocatalysts, but the effect of dopants in the electrochemical and structural stability is yet to be discussed. Herein, we correlate operando electrochemical measurements to the structural evolution of the materials down to the nanometric scale by identical location electron microscopy and spectroscopy. Different degradation mechanisms at first electrolyte contact, open circuit stabilization and HER conditions are identified for MoS2 nanocatalysts with and without Rhenium doping. Our results demonstrate that doping in MoS2 nanocatalysts can not only improve their HER activity, but also their stability. Doping of MoS2-based nanocatalysts is validated as a promising strategy to follow for the continuous improvement of high performance and durable PEM electrolyzers

A Low-Temperature Efficient Approach for the Fabrication of ZnO-rGO heterostructures for Applications in Optoelectronic Applications

In recent years, graphene oxides (GO)/reduced graphene oxide (rGO) and its derivatives have garnered/gained the attention of the scientific and research community due to their superior candidature in various electronic and optoelectronic devices due to their exceptional solution processability, easy fabrication, and tunable electron transport properties. However, the requirement of high-temperature processing steps and complicated processes motivates the scientific community to find simple, efficient, and low-temperature methods. Here, we report the synthesis of GO/rGOs and ZnO-rGO nanocomposite at a relatively low temperature of 150 °C using a simple and efficient solution-processed methodology. The SEM/EDX, XRD, Raman spectroscopy, FTIR, and UV-vis spectroscopy performed to investigate the morphological, structural, and optical properties confirmed the successful synthesis of GO, rGO, and ZnO-rGO with an enhanced carbon-carbon (sp2 and sp $^{3}$ ) component and reduced oxygen-containing functional group and the restoration of the graphitic domain in the hybrid nanocomposite, attributed to the possible chemical interaction between the rGO and ZnO through oxygen-containing functional groups. The bandgap of ZnO-rGO is modulated from 3.27 eV to 2.72 eV in comparison to pure ZnO. Using Hall measurement the carrier concentration was found to be $3.077\times 10^{17}$ cm $^{-3}$ , $4.518\times 10^{20}$ cm $^{-3}$ , and $2.973\times 10^{19}$ cm $^{-3}$ for ZnO, rGO, and ZnO-rGO, respectively, and the mobility was calculated as 16.787 cm2/V.s, 46.112 cm2/V.s and 25.953 cm2/V.s, respectively. The fabricated cell exhibited a power conversion efficiency of 6.17 % ( $\text{V}_{\mathrm {oc}}$ = 0.551 V and $\text{J}_{\mathrm {sc}}$ = 24.33 mA/cm2. After 8 weeks, 90 % of the initial efficiency could be achieved, suggesting excellent stability of the fabricated devices. The prepared samples have potential applications in different electronics and optoelectronics devices for enhanced performance

Mechanistic insights on Bi-potentiodynamic control towards atomistic synthesis of electrocatalysts for hydrogen evolution reaction

Abstract Herein, electrochemically assisted dissolution-deposition (EADD) is utilized over a three-electrode assembly to prepare an electrocatalyst for hydrogen evolution reaction (HER). Cyclic voltammetry is performed to yield atomistic loading of platinum (Pt) over SnS2 nanostructures via Pt dissolution from the counter electrode (CE). Astonishingly, the working electrode (WE) swept at 50 mV/s is found to compel Pt CE to experience 1000–3000 mV/s. The effect of different potential scan rates at the WE have provided insight into the change in Pt dissolution and its deposition behaviour over SnS2 in three electrode assembly. However, uncontrolled overpotentials at CE in a three-electrode assembly made Pt dissolution-deposition behavior complex. Here, for the first time, we have demonstrated bi-potentiodynamic control for dissolution deposition of Pt in four-electrode assembly over Nickel (Ni) foam. The dual cyclic voltammetry is applied to achieve better control and efficiency of the EADD process, engendering it as a pragmatically versatile and scalable synthesis technique

In-depth experimental and theoretical investigations on Co-SAC catalyzed transfer hydrogenation of azo compounds using methanol and ethanol

Transfer hydrogenation of non-polar bond using methanol or ethanol such hydrogen source is of great challenge, and development of efficient catalyst for performing such challenging reactions always an exciting area to explore. Herein, using Co-SAC various azo bonds were very efficiently hydrogenated to the corresponding amines, including commercially used dyes. A number of kinetics study and Hammett studies were to investigate the plausible mechanism and electronic effects. Further, a detailed DFT-calculation was performed to get a deeper insight about the mechanism

Substrate induced tuning of compressive strain and phonon modes in large area MoS2 and WS2 van der Waals epitaxial thin films

Large area MoS2 and WS2 van der Waals epitaxial thin films with control over number of layers including monolayer is grown by pulsed laser deposition utilizing slower growth kinetics. The films grown on c-plane sapphire show stiffening of A(1g) and E-2g(1) phonon modes with decreasing number of layers for both MoS2 and WS2. The observed stiffening translate into the compressive strain of 0.52% & 0.53% with accompanying increase in fundamental direct band gap to 1.74 and 1.68 eV for monolayer MoS2 and WS2, respectively. The strain decays with the number of layers. HRTEM imaging directly reveals the nature of atomic registry of van der Waals layers with the substrate and the associated compressive strain. The results demonstrate a practical route to stabilize and engineer strain for this class of material over large area device fabrication. (C) 2017 Elsevier B.V. All rights reserved

Compositional defects in a MoAlB MAB phase thin film grown by high-power pulsed magnetron sputtering

Various compositional defects such as Mo3Al2B4, Mo4Al3B4, Mo6Al5B6 and Al3Mo, together with MoB MBene, are observed to be coexisting in a MoAlB MAB phase thin film grown at 800 °C by high-power pulsed magnetron sputtering. An overall film composition of Mo0.29Al0.33B0.38 is measured by time-of-flight elastic recoil detection analysis. The concurrent formation of these compositional defects in the MoAlB matrix occurs during the synthesis without using any chemical reagent, and their coexistence with the MAB phase is thermodynamically possible, as elucidated by density functional theory simulations. These defect phases are imaged at the atomic scale by aberration-corrected scanning transmission electron microscopy. A rough estimation of defect populations of 0.073, 0.037, 0.042 and 0.039 nm−1 for Mo3Al2B4, Mo4Al3B4, Mo6Al5B6 and Al3Mo compositional defects, respectively, is performed within the MoAlB matrix. The calculated energies of formation reveal that the Mo4Al3B4 and Mo6Al5B6 defect phases form spontaneously in the MoAlB host matrix, while the energy barrier towards the formation of the metastable Mo3Al2B4 phase is approx. 20 meV per atom. The small magnitude of this barrier is easily overcome during vapor phase condensation, and the surface diffusion of adatoms during deposition leads to local compositional variations and the coexistence of the defect phases in the host matrix. Additionally, at grain boundaries, the presence of MoB MBene is observed, with an interlayer spacing between two Mo2B2 units increasing up to ∼50% compared to the pristine MoAlB phase

Direct MoB MBene domain formation in magnetron sputtered MoAlB thin films

Two-dimensional (2D) inorganic transition metal boride nanosheets are emerging as promising post-graphene materials in energy research due to their unique properties. State-of-the-art processing strategies are based on chemical etching of bulk material synthesized via solid-state reaction at temperatures above 1000 degrees C. Here, we report the direct formation of MoB MBene domains in a MoAlB thin film by Al deintercalation from MoAlB in the vicinity of AlOx regions. Hence, based on these results a straightforward processing pathway for the direct formation of MoB MBene-AlOx heterostructures without employing chemical etching is proposed here

Synthesis and Properties of Orthorhombic MoAlB Coatings

MoAlB is a potential candidate for high-temperature application since a dense, adherent alumina scale is formed. While, based on X-ray diffraction investigations, the formation of phase pure orthorhombic MoAlB coatings is observed, energy dispersive X-ray spectroscopy carried out in a scanning transmission electron microscope reveals the presence of Al-rich and O-rich regions within the MoAlB matrix. The oxidation kinetics of coatings and bulk is similar to the scale thickness formed on the MoAlB coating after oxidation at 1200 degrees C for 30 min is similar to the one extrapolated for bulk MoAlB. Furthermore, the oxidation kinetics of MoAlB coatings is significantly lower than the one reported for bulk Ti2AlC. Finally, the elastic properties measured for the as-deposited coatings are consistent ab initio predictions