Two-dimensional layered materials (like MoS 2 and WS 2 ) those are being used as sensing layers in chemoresistive gas sensors suffer from poor sensitivity and selectivity. Mere surface functionalization (decorating of material surface) with metal nanoparticles (NPs) might not improve the sensor performance significantly. In this respect, doping of the layered material can play a significant role. Here, we report a simple yet effective substitutional doping technique to dope MoS 2 with noble metals. Through various material characterization techniques like X-ray diffraction, scanning tunneling spectroscopy images, and selected area electron diffraction pattern, we were able to put forward the difference between surface decoration and substitutional doping by Au at S-vacancy sites of MoS 2 . Lattice strain was found to exist in the Au-doped MoS 2 samples, while being absent in the Au NPdecorated samples. Surface chemistry studies performed using X-ray photoelectron spectroscopy showed a shift of Mo 3d peaks to lower binding energies, thus realizing p-type doping due to Au. The blue shift of the peaks as observed in Raman spectroscopy further confirmed the p-type doping. We found that gold-doped MoS 2 was more sensitive and selective toward ammonia (with a response of 150% for 500 ppm of ammonia at 90 °C) as compared to gold NP-decorated MoS 2 . The advantages of substitutional doping and the gas-sensing mechanism were also explained by the density functional theory study. From the first principles study, it was found that the adsorption of Au atoms on the S-vacancy site of a monolayer of the MoS 2 sheet was thermodynamically favorable with the adsorption energy of 2.39 eV. We also successfully doped MoS 2 with Pt using the same technique. It was found that Ptdoped MoS 2 gives huge response toward humidity (60,000% at 80% relative humidity). Thus, various noble metal doping of MoS 2 selectively improved the sensing response toward specific analytes. From this work, we believe that this method could also be useful to dope other layered nanomaterials to design gas sensors with improved selectivity.

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A forest like 3D carbon structure formed by reduced graphene oxide (RGO) was prepared to use as an electrode material for a highly power efficient supercapacitor. To improve the specific energy of the electrode, pore like defects were incorporated on the RGO forests by atomic oxygen etching, during the UV-ozone treatment. The modified surface helps to increase the net capacitance by permitting the electrolyte to the inner core of the active material and improving the minimal quantum capacitance. Density functional theory based first principle studies were carried out to find DOS at the Fermi level of defect induced RGO sheet and hence to validate the effect of quantum capacitance on net capacitance. Specific capacitance of RGO forest was increased by almost 150% after introduction of the defects. The best performing material exhibits 18.87 mF cm−2 areal capacitance at 2 mA cm−2 current density which is equivalent to 70 F cm−3 at 3.7 A cm−3 current density, and it was used to fabricate the supercapacitor. Two supercapacitors were fabricated, (i) on graphite sheet (non-flexible) and (ii) on scotch tape (flexible). Here PVA-KOH gel soaked filter paper was used as electrolyte-separator. Both the prepared supercapacitors on graphite sheet and scotch tape are able to transfer electrical energy with ultra high specific power (656.25 mW cm−3 and 164.06 mW cm−3 respectively) while maintaining moderate energy densities. The first device can withstand its primary capacitance by 90% even after 10 K charge–discharge cycles and the flexible device was able to hold 96% of its capacitance after 1 K bending cycles.

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This work reports an in-situ, one-step hydrothermal preparation procedure of a binder-free electrode growth of Ni6Se5 on nickel foam (Ni6Se5/NF) with a rod-like structure. Ni6Se5 is an enveloped transition metal chalcogenides of formula M(n+1)Xn (where 2≤n≤8, M is a transition metal and X is chalcogen) of the nickel selenide family. The Ni6Se5/NF electrode described here demonstrates an exceptional lifetime of 81% capacitance retention over 20000 cycles and a high specific capacitance of 473.5 Fg-1 at a current density of 4 Ag-1. The Ni6Se5/NF/activated carbon asymmetric supercapacitor exhibits a remarkable 97.3 Whkg-1 energy density and a 2325 Wkg-1 power density. Ni6Se5 served as an active electrode material in supercapacitor applications and offered exceptional power density and long cycle life. Ni6Se5/NF, used as an anode for Li-ion batteries (LIB), has a lithium storage capacity of 939.7 mAhg-1 at 100 mAg-1 current density. Ni6Se5 active electrode material's excellent energy storage capability, which was previously unreported, is particularly beneficial for electrochemical energy storage device applications.