Anand P. Tiwari

Highly Stretchable and Conductive Silver Nanoparticle Embedded Graphene Flake Electrode Prepared by In situ Dual Reduction Reaction

The emergence of stretchable devices that combine with conductive properties offers new exciting opportunities for wearable applications. Here, a novel, convenient and inexpensive solution process was demonstrated to prepare in situ silver (Ag) or platinum (Pt) nanoparticles (NPs)-embedded rGO hybrid materials using formic acid duality in the presence of AgNO3 or H2PtCl6 at low temperature. The reduction duality of the formic acid can convert graphene oxide (GO) to rGO and simultaneously deposit the positively charged metal ion to metal NP on rGO while the formic acid itself is converted to a CO2 evolving gas that is eco-friendly. The AgNP-embedded rGO hybrid electrode on an elastomeric substrate exhibited superior stretchable properties including a maximum conductivity of 3012 S cm-1 (at 0 % strain) and 322.8 S cm-1 (at 35 % strain). Its fabrication process using a printing method is scalable. Surprisingly, the electrode can survive even in continuous stretching cycles.

Superconductivity at 7.4 K in Few Layer Graphene by Li-intercalation

Superconductivity in graphene has been highly sought after for its promise in various device applications and for general scientific interest. Ironically, the simple electronic structure of graphene, which is responsible for novel quantum phenomena, hinders the emergence of superconductivity. Theory predicts that doping the surface of the graphene effectively alters the electronic structure, thus promoting propensity towards Cooper pair instability (Profeta et al (2012) Nat. Phys. 8 131-4; Nandkishore et al (2012) Nat. Phys. 8 158-63) [1, 2]. Here we report the emergence of superconductivity at 7.4 K in Li-intercalated few-layer-graphene (FLG). The absence of superconductivity in 3D Li-doped graphite underlines that superconductivity in Li-FLG arises from the novel electronic properties of the 2D graphene layer. These results are expected to guide future research on graphene-based superconductivity, both in theory and experiments. In addition, easy control of the Li-doping process holds promise for various device applications.

Two-Dimensional WO3 Nanosheets Chemically Converted from Layered WS2 for High-Performance Electrochromic Devices

Two-dimensional (2D) transitional metal oxides (TMOs) are an attractive class of materials due to the combined advantages of high active surface area, enhanced electrochemical properties, and stability. Among the 2D TMOs, 2D tungsten oxide (WO3) nanosheets possess great potential in electrochemical applications, particularly in electrochromic (EC) devices. However, feasible production of 2D WO3 nanosheets is challenging due to the innate 3D crystallographic structure of WO3. Here we report a novel solution-phase synthesis of 2D WO3 nanosheets through simple oxidation from 2D tungsten disulfide (WS2) nanosheets exfoliated from bulk WS2 powder. The complete conversion from WS2 into WO3 was confirmed through crystallographic and elemental analyses, followed by validation of the 2D WO3 nanosheets applied in the EC device. The EC device showed color modulation of 62.57% at 700 nm wavelength, which is 3.43 times higher than the value of the conventional device using bulk WO3 powder, while also showing enhancement of ∼46.62% and ∼62.71% in switching response-time (coloration and bleaching). The mechanism of enhancement was rationalized through comparative analysis based on the thickness of the WO3 components. In the future, 2D WO3 nanosheets could also be used for other promising applications such as sensors, catalysis, thermoelectric, and energy conversion.

Chemical strain formation through anion substitution in Cu2WS4 for efficient electrocatalysis of water dissociation

Researchers have revealed that the electrocatalytic activity can be improved by creation of defects in the crystal lattice of 2D layered transition metal dichalcogenides (TMDCs) or ternary metal chalcogenides (TMCs) such as MoS2 or Cu2MoS4, respectively. However, the role of anion substitution in the enhancement of overall electrocatalytic activity in TMCs remains unproven. Here, we show the substitution of anion atom sulfur (S) with selenium (Se) in a new electrocatalyst Cu2WS4 for efficient hydrogen evolution reaction (HER) activity. The higher electrocatalytic activity of Cu2WS4 after anion atom substitution can be attributed to the creation of chemical strain in the lattice, which causes an increase of active sites for hydrogen adsorption and desorption. Experimentally, the anion substituted Cu2W(SySe1 − y)4 samples show superior electrocatalytic activities with a low onset potential of −0.320 V at 10 mA cm−2 for the HER, which is two-fold lower than that of the pristine Cu2WS4 (−0.650 V at 10 mA cm−2) sample. In addition, after 1000 cycles with continuous electrolysis in an acid electrolyte for 12 h, the anion substituted samples Cu2W(SySe1 − y)4 preserve their structure and robust catalytic activity perfectly. As a result, our work demonstrates a new approach for developments of real applications of TMCs in energy conversion.

Enhanced Electrocatalytic Activity by Chemical Nitridation of Two-Dimensional Titanium Carbide MXene for Hydrogen Evolution

Developing active and stable electrocatalysts from Earth-abundant elements is the key to water splitting for hydrogen production through electrolysis. Here, we report a strategy to turn non-electrocatalytic Ti2CTx into an active electrocatalyst by the nitridation of two-dimensional (2D) titanium carbide MXene (Ti2CTx) nanosheets using sodium amide (NaNH2). The addition of NaNH2 results in the chemical bonding of Ti–Nx at 500 °C on the surface of Ti2CTx, which was designed as an efficient electrocatalytic material for the hydrogen evolution reaction (HER). When used as an electrocatalytic material for the HER, the nitrided-Ti2CTx (N-Ti2CTx) exhibited high activity with an overpotential of −215 mV vs. NHE for the hydrogen evolution reaction (HER) at 10 mA cm−2. These values are over three times smaller than those for pristine-Ti2CTx (−645 mV vs. NHE for the HER). The as-synthesized sample showed excellent durability under acidic (0.5 M H2SO4) conditions, indicating its robust catalytic activity towards the HER. The nitridation strategy implemented here could be extended to other 2D transition metal carbide electrocatalysts to improve their catalytic performance.

FeIn2S4 Nanocrystals: A Ternary Metal Chalcogenide Material for Ambipolar Field‐Effect Transistors

Abstract An ambipolar channel layer material is required to realize the potential benefits of ambipolar complementary metal–oxide–semiconductor field‐effect transistors, namely their compact and efficient nature, reduced reverse power dissipation, and possible applicability to highly integrated circuits. Here, a ternary metal chalcogenide nanocrystal material, FeIn2S4, is introduced as a solution‐processable ambipolar channel material for field‐effect transistors (FETs). The highest occupied molecular orbital and the lowest unoccupied molecular orbital of the FeIn2S4 nanocrystals are determined to be −5.2 and −3.75 eV, respectively, based upon cyclic voltammetry, X‐ray photoelectron spectroscopy, and diffraction reflectance spectroscopy analyses. An ambipolar FeIn2S4 FET is successfully fabricated with Au electrodes (E F = −5.1 eV), showing both electron mobility (14.96 cm2 V−1 s−1) and hole mobility (9.15 cm2 V−1 s−1) in a single channel layer, with an on/off current ratio of 105. This suggests that FeIn2S4 nanocrystals may be a promising alternative semiconducting material for next‐generation integrated circuit development.