Rhombohedrally stacked layered transition metal dichalcogenides and their electrocatalytic applications.

Layered transition metal dichalcogenides (TMDCs) are extensively investigated as catalyst materials for a wide range of electrochemical applications due to their high surface area and versatile electronic and chemical properties. Bulk TMDCs are van der Waals solids that possess strong in-plane bonding and weak inter-layer interactions. In the few-layer 2D TMDCs, several polymorphic structures have been reported as each individual layer can either retain octahedral or trigonal prismatic coordination. Among them, 1T (tetragonal), 2H (hexagonal) and 3R (rhombohedral) phases are very common. These polymorphs can display discrepancies in their catalytic activity as their electronic structure diverges due to different d orbital filling states. The broken inversion symmetry and large exposed edge sites of some of the 3R-phase TMDCS such as MoS2, NbS2 and TaS2 appear to be advantageous for electrocatalytic water reduction and battery applications. We describe recent studies in phase engineering of 2D TMDCs, particularly aiming at the 3R polytype and their electrocatalytic properties. Redox ability primarily depends on a distinct polymorphic phase in which TMDCs are isolated, and hence, with rich polymorphic structures being reported, numerous new catalytic applications can be realized. Phase conversion from 2H to 3R phase in some TMDCs enhances structural integrity and establishes robustness under harsh chemical conditions.

3R-NbS2 as a highly stable anode for sodium-ion batteries.

Anode materials for advanced sodium-ion batteries (SIBs) require major improvements with regard to their cycling stability, which is a crucial parameter for long-term battery operation. Herein, we report 3R-NbS2, synthesised by a simple solid-state annealing route, as an anode for SIBs with remarkable cycling stability for 2500 cycles at 0.5 A g-1. The stable nature of the NbS2 anode was attributed to its dominant capacitive behaviour.

Neuromorphic devices realised using self-forming hierarchical Al and Ag nanostructures: towards energy-efficient and wide ranging synaptic plasticity.

Closely mimicking the hierarchical structural topology with emerging behavioral functionalities of biological neural networks in neuromorphic devices is considered of prime importance for the realization of energy-efficient intelligent systems. In this article, we report an artificial synaptic network (ASN) comprising of hierarchical structures of isolated Al and Ag micro-nano structures developed via the utilization of a desiccated crack pattern, anisotropic dewetting, and self-formation. The strategically designed ASN, despite having multiple synaptic junctions between electrodes, exhibits a threshold switching (Vth ∼ 1-2 V) with an ultra-low energy requirement of ∼1.3 fJ per synaptic event. Several configurations of the order of hierarchy in the device architecture are studied comprehensively to identify the importance of the individual metallic components in contributing to the threshold switching and energy-minimization. The emerging potentiation behavior of the conductance (G) profile under electrical stimulation and its permanence beyond are realized over a wide current compliance range of 0.25 to 300 µA, broadly classifying the short- and long-term potentiation grounded on the characteristics of filamentary structures. The scale-free correlation of potentiation in the device hosting metallic filaments of diverse shapes and strengths could provide an ideal platform for understanding and replicating the complex behavior of the brain for neuromorphic computing.

Metavalent Bonding in 2D Chalcogenides: Structural Origin and Chemical Mechanisms.

An unusual set of anomalous functional properties of rocksalt crystals of Group IV chalcogenides were recently linked to a kind of bonding termed as metavalent bonding (MVB) which involves violation of the 8-N rule. Precise mechanisms of MVB and the relevance of lone pair of Group IV cations are still debated. With restrictions of low dimensionality on the possible atomic coordination, 2D materials provide a rich platform for exploration of MVB. Here, we present first-principles theoretical analysis of the nature of bonding in five distinct 2D lattices of Group IV chalcogenides MX (M: Sn, Pb, Ge and X: S, Se, Te), in which the natural out-of-plane expression of the lone pair versus in-plane bonding can be systematically explored. While their honeycomb lattices respecting the 8-N rule are shown to exhibit covalent bonding, their square and orthorhombic structures exhibit MVB only in-plane, with cationic lone pair activating the out-of-plane structural puckering that controls their relative stability. Anomalies in Born-effective charges, dielectric constants, Grüneisen parameters occur only in their in-plane behaviour, confirming MVB is confined strictly to 2D and originates from p-p orbital interactions. Our work opens up directions for chemical design of MVB based 2D materials and their heterostructures.

NbO2 a Highly Stable, Ultrafast Anode Material for Li- and Na-Ion Batteries.

Anode materials with fast charging capabilities and stability are critical for realizing next-generation Li-ion batteries (LIBs) and Na-ion batteries (SIBs). The present work employs a simple synthetic strategy to obtain NbO2 and studies its applications as an anode for LIB and SIB. In the case of the LIB, it exhibited a specific capacity of 344 mAh g-1 at 100 mA g-1. It also demonstrated remarkable stability over 1000 cycles, with 92% capacity retention. Additionally, it showed a unique fast charging capability, which takes 30 s to reach a specific capacity of 83 mAh g-1. For the SIB, NbO2 exhibited a specific capacity of 244 mAh g-1 at 50 mA g-1 and showed 70% capacity retention after 500 cycles. Furthermore, detailed density functional theory reveals that various factors like bulk and surface charging processes, lower ion diffusion energy barriers, and superior electronic conductivity of NbO2 are responsible for the observed battery performances.

High-Flux lamellar MoSe2 membranes for efficient dye/salt separation.

Membrane-based technology is emerging as an efficient technique for wastewater treatment in recent years. Membranes made up of two-dimensional materials provide high selectivity and water flux compared to conventional polymeric membranes. Herein, we report the synthesis and use of MoSe2 membrane for dye and drug separation in wastewater, mainly from textile and pharmaceutical industries. The as-prepared MoSe2 membrane shows âˆ¼ 100% rejection for organic dyes and ciprofloxacin drug with a water flux reaching up to âˆ¼ 900 Lm-2h-1bar-1. Further, the MoSe2 membrane shows lower NaCl rejection of âˆ¼ 1.9% for the dye/salt mixture. The interlayer spacing in the MoSe2 membrane allows the water molecules and ions from the salt to pass through freely but restricts the movement of large contaminants. The membrane is stable against the bovine albumin serum fouling with a flux recovery rate of 96%. It also shows good performance even in harsh environments (pH 3-10). To the best of our knowledge, the MoSe2 membranes were fabricated for the first time for wastewater treatment application. The dye/salt separation performance of the MoSe2 membrane is significantly better than several other membranes. This work highlights the promising potential for using two-dimensional materials for textile and pharmaceutical wastewater treatment.

A new precursor route for the growth of NbO2thin films by chemical vapor deposition.

Niobium dioxide (NbO2) exhibits metal-insulator transition (Mott transition) and shows the potential for application in memristors and neuromorphic devices. Presently growth of NbO2thin films requires high-temperature reduction of Nb2O5films using H2or sophisticated techniques such as molecular beam epitaxy and pulsed laser deposition. The present study demonstrates a simple chemical route of the direct growth of crystalline NbO2films by chemical vapor deposition using a freshly prepared Nb-hexadecylamine (Nb-HDA) complex. X-ray diffraction studies confirm the NbO2phase with a distorted rutile body-centered-tetragonal structure and the film grown with a highly preferred orientation onc-sapphire. X-ray photoelectron spectroscopy confirms the +4 oxidation state. The present method offers facile growth of NbO2films without post-reduction steps which will be assumed to be a cost-effective process for NbO2based devices.

Metavalent Bonding Origins of Unusual Properties of Group IV Chalcogenides.

A distinct type of metavalent bonding (MVB) is recently proposed to explain an unusual combination of anomalous functional properties of group IV chalcogenide crystals, whose electronic mechanisms and origin remain controversial. Through theoretical analysis of evolution of bonding along continuous paths in structural and chemical composition space, emergence of MVB in rocksalt chalcogenides is demonstrated as a consequence of weakly broken symmetry of parent simple-cubic crystals of Group V metalloids. High electronic degeneracy at the nested Fermi surface of parent metal drives spontaneous breaking of its translational symmetry with structural and chemical fields, which open up a small energy gap and mediate strong coupling between conduction and valence bands making metavalent crystals highly polarizable, conductive, and sensitive to bond-lengths. Stronger symmetry-breaking structural and chemical fields, however, transform them discontinuously to covalent and ionic semiconducting states. MVB involves bonding-antibonding pairwise interactions alternating along linear chains of at least five atoms, which facilitate long-range electron transfer in response to polar fields causing unusual properties. The precise picture of MVB predicts anomalous second-order Raman scattering as an addition to set off their unusual properties, and will guide in design of new metavalent materials with improved thermoelectric, ferroelectric and nontrivial electronic topological properties.

Functionalization of antimonene and bismuthene with Lewis acids.

Elemental 2D pnictogens (group 15) are an interesting class of materials with tunable band structures and high carrier mobilities. Heavier pnictogens (Sb and Bi) are stable under ambient conditions compared to lighter members (P and As) and are emerging as interesting candidates for various electronic and optoelectronic applications. The reactivity of these materials is due to the presence of a lone pair which can be effectively utilized to tune material properties via different functionalization strategies. In this work, we have synthesized antimonene and bismuthene nanosheets by liquid exfoliation which are emissive in the visible range and functionalized these nanosheets with group 12 and 13 Lewis acids (ZnCl2, CdCl2, BCl3, GaCl3, AlCl3, and InCl3). Interaction of these Lewis acids with the lone pairs on Sb/Bi leads to the formation of Lewis acid-base adducts with the corresponding changes in the bonding environment along with lattice distortion and rehybridization of the band structure. Interestingly, the changes in band structure upon functionalization were realized as a blue shift in the emission of few-layered Sb and Bi. This is the first report on the functionalization of heavier pnictogens by the formation of Lewis acid-base adducts and opens a path for tuning their properties for integration in electronic and optoelectronic devices.

Atomic Layer Deposited Ti2 O3 Thin Films.

Ti2 O3 thin films have been prepared through atomic layer deposition and subjected to electrical resistivity measurements as a function of temperature. The as-prepared films were stable for up to three weeks. In Ti2 O3 thin films, the insulator-metal transition is observed at ∼80 K, with nearly 3-4 orders of magnitude change in resistivity. The anomalous increase in electrical resistivity in the films is in accordance with the two-band model. However, the energy interval between the bands depending on the crystallographic c/a ratio leads to a change in electrical resistivity as a function of temperature.

Designing electrode materials for the electrochemical reduction of carbon dioxide.

Electrochemical reduction of carbon dioxide is a viable alternative for reducing fossil fuel consumption and reducing atmospheric CO2 levels. Although, a wide variety of materials have been studied for electrochemical reduction of CO2, the selective and efficient reduction of CO2 is still not accomplished. Complex reaction mechanisms and the competing hydrogen evolution reaction further complicates the efficiency of materials. An extensive understanding of reaction mechanism is hence essential in designing an ideal electrocatalyst material. Therefore, in this review article we discuss the materials explored in the last decade with focus on their catalytic mechanism and methods to enhance their catalytic activity.

Anderson localization of IR light in 1D nanosystems.

This contribution reports on the observation of a strong light localization of Anderson type in 1D systems consisting of ship-shaped carbon nanotubes. Such a localization of infrared (IR) light was observed using Fourier transform infrared spectroscopy under attenuated total reflection geometry within the spectral range of 2-20 µm. Such an IR light localization manifests itself in the form of a significant interference profile of the optical transmission over the full wavenumber range of 400-4000cm-1.

Metals and non-metals in the periodic table.

The demarcation of the chemical elements into metals and non-metals dates back to the dawn of Dmitri Mendeleev's construction of the periodic table; it still represents the cornerstone of our view of modern chemistry. In this contribution, a particular emphasis will be attached to the question 'Why do the chemical elements of the periodic table exist either as metals or non-metals under ambient conditions?' This is perhaps most apparent in the p-block of the periodic table where one sees an almost-diagonal line separating metals and non-metals. The first searching, quantum-mechanical considerations of this question were put forward by Hund in 1934. Interestingly, the very first discussion of the problem-in fact, a pre-quantum-mechanical approach-was made earlier, by Goldhammer in 1913 and Herzfeld in 1927. Their simple rationalization, in terms of atomic properties which confer metallic or non-metallic status to elements across the periodic table, leads to what is commonly called the Goldhammer-Herzfeld criterion for metallization. For a variety of undoubtedly complex reasons, the Goldhammer-Herzfeld theory lay dormant for close to half a century. However, since that time the criterion has been repeatedly applied, with great success, to many systems and materials exhibiting non-metal to metal transitions in order to predict, and understand, the precise conditions for metallization. Here, we review the application of Goldhammer-Herzfeld theory to the question of the metallic versus non-metallic status of chemical elements within the periodic system. A link between that theory and the work of Sir Nevill Mott on the metal-non-metal transition is also highlighted. The application of the 'simple', but highly effective Goldhammer-Herzfeld and Mott criteria, reveal when a chemical element of the periodic table will behave as a metal, and when it will behave as a non-metal. The success of these different, but converging approaches, lends weight to the idea of a simple, universal criterion for rationalizing the instantly-recognizable structure of the periodic table where …the metals are here, the non-metals are there … The challenge of the metallic and non-metallic states of oxides is also briefly introduced. This article is part of the theme issue 'Mendeleev and the periodic table'.

Chemical Route to Twisted Graphene, Graphene Oxide and Boron Nitride.

The recently discovered twisted graphene has attracted considerable interest. A simple chemical route was found to prepare twisted graphene by covalently linking layers of exfoliated graphene containing surface carboxyl groups with an amine-containing linker (trans-1,4-diaminocyclohexane). The twisted graphene shows the expected selected area electron diffraction pattern with sets of diffraction spots out with different angular spacings, unlike graphene, which shows a hexagonal pattern. Twisted multilayer graphene oxide could be prepared by the above procedure. Twisted boron nitride, prepared by cross-linking layers of boron nitride (BN) containing surface amino groups with oxalic acid linker, exhibited a diffraction pattern comparable to that of twisted graphene. First-principles DFT calculations threw light on the structures and the nature of interactions associated with twisted graphene/BN obtained by covalent linking of layers.

Effect of magnetic field on the hydrogen evolution activity using non-magnetic Weyl semimetal catalysts.

An external switch to control the kinetics of the reaction by manipulating the participating electrons could be interesting as it can alter the rate of the reaction without affecting the reaction pathway. The magnetic field, like a switch, is non-invasive, tunable, and clean; it can also alter the electrons in a material. We study the effect of an applied magnetic field on the hydrogen evolution activity of the NbP family of Weyl semimetals because of their extremely high mobility and large magnetoresistance at room temperature and good hydrogen evolution properties. We find that by applying a magnetic field of ∼3500 G, the hydrogen evolution activity of NbP increases by up to 95%. The other members of this Weyl semimetal family (viz. TaP, NbAs, and TaAs) also exhibit increased hydrogen evolution activity. Thus, our observations suggest an interplay of electronic property, magnetic field, and catalytic activity in this class of compounds, providing evidence of manipulating the catalytic performance of topological materials through the application of a magnetic field.

Simple synthesis of nanosheets of rGO and nitrogenated rGO.

A green and facile approach has been developed for the large-scale synthesis of nanosheets of reduced graphene oxide (rGO) and nitrogenated reduced graphene oxide (N-rGO). This has been achieved by direct thermal decomposition of sucrose and glycine at 475 °C in ca. 7 minutes, respectively. The present protocols for synthesizing rGO and N-rGO are simple and environmentally friendly as we do not use any harmful reagents, metal catalysts and solvents. Along with that, this method offers an inexpensive route with high yields to prepare rGO with a high nitrogen content (20-25 atom %). To further improve the properties of the synthesized rGO sheets, hydrogen treatment has been carried out to reduce the oxygen functional groups. Cyclic voltammograms and charge-discharge experiments have been carried out to understand the supercapacitor behavior of rGO and hydrogen treated (H-rGO) samples.

Effect of Mn2+ substitution on the structure, properties and HER activity of cadmium phosphochlorides.

Cadmium phosphochlorides, Cd4P2Cl3 and Cd7P4Cl6, possess cadmium atoms differently bonded to chlorine and phosphorus ligands. A combined experimental and theoretical study has been carried out to examine the effect of manganese substitution in place of cadmium in these compounds. Experimentally it is found that manganese prefers the Cd7P4Cl6 phase over Cd4P2Cl3. First-principles calculations reveal, stabilization of Cd7P4Cl6 upon Mn-substitution with a significant reduction in the formation energy when Mn2+ is substituted at Cd-sites coordinated octahedrally by Cl-ligands. Substitution of Mn2+ at two different Cd-sites in these compounds not only alters their formation energy differently but also causes a notable change in the electronic structures. In contrast to n-type conductivity in pristine Cd7P4Cl6, Mn2+ substituted Cd7-y Mn y P4Cl6 analogues exhibit p-type conductivity with a remarkable enhancement in the photochemical HER activity and stability of the system. Photochemical properties of pristine and substituted compounds are explained by studying the nature of charge carriers and their dynamics.