Unveiling the Electrocatalytic Activity of 1T'-MoSe2 on Lithium-Polysulfide Conversion Reactions.

The dissolution of intermediate lithium polysulfides (LiPS) into an electrolyte and their shuttling between the electrodes have been the primary bottlenecks for the commercialization of high-energy density lithium-sulfur (Li-S) batteries. While several two-dimensional (2D) materials have been deployed in recent years to mitigate these issues, their activity is strictly restricted to their edge-plane-based active sites. Herein, for the first time, we have explored a phase transformation phenomenon in a 2D material to enhance the number of active sites and electrocatalytic activity toward LiPS redox reactions. Detailed theoretical calculations demonstrate that phase transformation from the 2H to 1T' phase in a MoSe2 material activates the basal planes that allow for LiPS adsorption. The corresponding transformation mechanism and LiPS adsorption capabilities of the as-formed 1T'-MoSe2 were elucidated experimentally using microscopic and spectroscopic techniques. Further, the electrochemical evaluation of phase-transformed MoSe2 revealed its strong electrocatalytic activity toward LiPS reduction and their oxidation reactions. The 1T'-MoSe2-based cathode hosts for sulfur later provide a superior cycling performance of over 250 cycles with a capacity loss of only 0.15% per cycle along with an excellent Coulombic efficiency of 99.6%.

Reliable and highly sensitive biosensor from suspended MoS2 atomic layer on nano-gap electrodes.

The uneven morphology and the trapped charges at the surface of the traditionally used supporting substrate-based 2D biosensors produces a scattering effect, which leads to a irregular signals from individually fabricated devices. Though suspended 2D channel material has the potential to overcome scattering effects from the substrates but achieving reliability and selectivity, have been limiting the using of this biosensor technology. Here, we have demonstrated nanogap electrodes fabrication by using the self-assembly technique, which provides suspension to the 2D-MoS2. These nano-spacing electrodes not only give suspension but also provide robustness strength to the atomic layer, which remains freestanding after coating of the Hafnium oxide (HfO2) as well as linkers and antibodies. For evaluating the electrical characteristics of suspended MoS2 FET, gating potential was applied through an electrolyte on the suspended MoS2 transistor. This helped in achieved a lower subthreshold swing 70 mV/dec and ON/OFF ratio 107. Later, pH detection was conducted at room temperature, which showed an impressive sensitivity of ~880 by changing 1 unit of pH. We have also successfully shown Escherichia coli (E. coli) bacteria sensing from the suspended MoS2 transistor by functionalizing dielectric layer with E. coli antibodies. The reported biosensor has shown the ~9% of conductance changes with a lower concentration of E. coli (10 CFU/mL; colony-forming unit per mL) as well as maintain the constant sensitivity in three fabricated devices. The obtained enhancement in the sensitivity of devices and its effect on biomolecules detection can be extened to other biomolecules and this type of architecture has the potential to detect COVID-19 viruses based biomolecules.

Experimental Studies and Numerical Simulation of Polypyrrole Trilayer Actuators.

Conducting polymer actuators have shown wide application prospects in the field of biomedical sensors and micro-/nanorobotics. In order to explore more applications in biomedical sensing and robotics, it is essential to understand the actuator static behavior from an engineering perspective, before incorporating them into a design. In this article, we have established the mathematical model of a trilayer polypyrrole (PPy) cantilever actuator and validated it experimentally. The model helps in enhancing the efficiency and in improving the performance, predictability, and control of the actuator. The thermal expansion analogy, which is similar to volume change of the multilayer PPy actuator due to ion migration, has been considered to develop a mathematical model in COMSOL Multiphysics. To further validate the actuator deformation predicted by the mathematical modeling, a multilayer PPy actuator was fabricated by electrochemical synthesis and the experimentally determined deflection of the actuator was compared to simulation data. Both the theoretical and experimental results depict that the model is effective for predicting the bending behavior of multilayer PPy actuators at different input voltages.

CVD-Grown MoSe2 Nanoflowers with Dual Active Sites for Efficient Electrochemical Hydrogen Evolution Reaction.

Due to its unique electronic band characteristics (presence of d-orbital in both Mo and Se atoms), MoSe2 has potential to exhibit high electrical conductivity and superior hydrogen evolution reaction (HER) kinetics when compared to other transition-metal dichalcogenides. Though various strategies were employed earlier to obtain MoSe2 structure with different shapes and morphologies, precise control on achieving both Mo- and Se-edge sites and understanding their interaction with reactants in HER remains to be challenging. Here, we successfully demonstrate the vapor diffusion method to grow highly crystalline MoSe2 nanoflowers on carbon cloth in a vertical orientation. Uniformly dispersed nanoflowers with Mo- and Se-edge sites exhibited remarkable electrocatalytic activity on hydrogen reduction in terms of low Tafel slope and high exchange current density. The existence of a strong interaction between MoSe2 and carbon cloth assists in long-term hydrogen production and confirms the exceptional durability of the catalyst. A comprehensive evidence for hydrogen adsorption on dual active sites, viz., Mo- and Se-edges of MoSe2, is provided using X-ray photoelectron spectroscopy and in situ Raman spectroscopy containing a specially designed liquid immersion objective lens.

Transition Metal Dichalcogenide Atomic Layers for Lithium Polysulfides Electrocatalysis.

Lithium-sulfur (Li-S) chemistry is projected to be one of the most promising for next-generation battery technology, and controlling the inherent "polysulfide shuttle" process has become a key research topic in the field. Regulating intermediary polysulfide dissolution by understanding the metamorphosis is essential for realizing stable and high-energy-density Li-S batteries. As of yet, a clear consensus on the basic surface/interfacial properties of the sulfur electrode has not been achieved, although the catalytic phenomenon has been shown to result in enhanced cell stability. Herein, we present evidence that the polysulfide shuttle in a Li-S battery can be stabilized by using electrocatalytic transition metal dichalcogenides (TMDs). Physicochemical transformations at the electrode/electrolyte interface of atomically thin monolayer/few-layer TMDs were elucidated using a combination of spectroscopic and microscopic analysis techniques. Preferential adsorption of higher order liquid polysulfides and subsequent conversion to lower order solid species in the form of dendrite-like structures on the edge sites of TMDs have been demonstrated. Further, detailed electrochemical properties such as activation energy, exchange current density, rate capabilities, cycle life, etc. have been investigated by synthesizing catalytically active nanostructured TMDs in bulk quantity using a liquid-based shear-exfoliation method. Unveiling a specific capacity of 590 mAh g-1 at 0.5 C rate and stability over 350 cycles clearly indicates yet another promising application of two-dimensional TMDs.

A flexible and implantable microelectrode arrays using high-temperature grown vertical carbon nanotubes and a biocompatible polymer substrate.

This paper presents a novel microelectrode arrays using high-temperature grown vertically aligned carbon nanotubes (CNTs) integrated on a flexible and biocompatible parylene substrate. A simple microfabrication process is proposed to unite the high quality vertical CNTs grown at high temperature with the heat sensitive parylene substrate in a highly controllable manner. Briefly, the CNTs electrode is encapsulated by two layers of parylene and the device is released using xenon difluoride (XeF2). The process is compatible with wafer-scale post complementary metal oxide semiconductor integration. Lower impedance and larger interfacial capacitance have been demonstrated using CNTs compared to a Pt electrode. The flexible CNT electrodes have been utilized for extracellular neuronal recording and stimulation in rats. The signal-to-noise ratio of the device is about 12.5. The threshold voltage for initiating action potential is about 0.5 V.