Debasish Sarkar

Hydrogenated NiO nanoblock architecture for high performance pseudocapacitor.

Supercapacitor electrodes are fabricated with the self-organized 3D architecture of NiO and hydrogenated NiO (H-NiO) nano-blocks (NBs) grown by the facile electrodeposition and high temperature annealing of the Ni foil on Cu substrate. The unique architecture of H-NiO NBs electrode exhibits excellent cycling stability (only 5.3% loss of its initial specific capacitance after 3000 cycles at current density of 1.1 A g(-1)) along with the high specific and areal capacitance of ∼1272 F g(-1) and 371.8 mF cm(-2), respectively at scan rate of 5 mV s(-1) compared with the pure NiO NBs electrode (∼ 865 F g(-1) and 208.2 mF cm(-2), respectively at scan rate of 5 mV s(-1)). H-NiO NBs electrode also exhibits excellent rate capability; nearly 61% specific capacity retention has been observed when the current density increases from 1.11 to 111.11 A g(-1). This electrode offers excellent energy density of 13.51 Wh kg(-1) and power density of 19.44 kW kg(-1) even at a high current density of 111.11 A g(-1). The superior pseudocapacitive performance of the H-NiO NBs electrode is because of the high electron and ion conductivity of the active material because of the incorporation of hydroxyl groups on the surface of NiO NBs.

Unique hydrogenated Ni/NiO core/shell 1D nano-heterostructures with superior electrochemical performance as supercapacitors

This study demonstrates a scheme to design and fabricate a novel 1D core/shell Ni/NiO nano-architecture electrode as a pseudocapacitor with significantly improved capacitive performance through hydrogenation. The specific capacitance of the as prepared 1D core/shell Ni/NiO nanoheterostructure (717 F g−1 at a scan rate of 2 mV s−1) is nearly 1635 F g−1 after the hydrogenation. The improved pseudocapacitive properties of hydrogenated Ni/NiO nano-heterostructures are attributed to the incorporation of the hydroxyl groups on the NiO surface due to hydrogenation, where the metallic Ni nanowire core of this unique 1D core/shell heterostructure serves as the efficient channel for the fast electron conduction to the current collector. The H–Ni/NiO nanoheterostructures exhibit good rate capability (retaining nearly 60% of their initial charge) and good long-term cycling stability with an excellent specific energy and power density of 49.35 W h kg−1 and 7.9 kW kg−1, respectively, at a current density of 15.1 A g−1. This study demonstrates that the H–Ni/NiO nano-heterostructure is very promising for next generation high-performance pseudocapacitors.

High-Performance Supercapacitor Electrode Based on Cobalt Oxide–Manganese Dioxide–Nickel Oxide Ternary 1D Hybrid Nanotubes

We report a facile method to design Co3O4-MnO2-NiO ternary hybrid 1D nanotube arrays for their application as active material for high-performance supercapacitor electrodes. This as-prepared novel supercapacitor electrode can store charge as high as ∼2020 C/g (equivalent specific capacitance ∼2525 F/g) for a potential window of 0.8 V and has long cycle stability (nearly 80% specific capacitance retains after successive 5700 charge/discharge cycles), significantly high Coulombic efficiency, and fast response time (∼0.17s). The remarkable electrochemical performance of this unique electrode material is the outcome of its enormous reaction platform provided by its special nanostructure morphology and conglomeration of the electrochemical properties of three highly redox active materials in a single unit.

Design and synthesis of high performance multifunctional ultrathin hematite nanoribbons.

1D porous ultrathin nanoribbons of hematite (α-Fe2O3) were prepared by controlled annealing of different oxides and hydroxides of iron obtained from a solvothermal synthesis method. It is found that calcination at a temperature of 500 °C for 150 min decomposes these iron hydroxides into their most stable oxide form, i.e., α-Fe2O3. Driven by different attractive forces, these porous α-Fe2O3 nanoparticles get aggregated in an ordered fashion to form an ultrathin 1D nanoribbon structure, as observed by detailed time dependent TEM and HRTEM analysis. It has been found that the high aspect ratio and porous surface morphology of these nanoribbons have significantly improved their electronic and spintronic properties as manifested by their photocatalysis, gas sensing, electrochemical, and magnetic behaviors. These hematite nanoribbons exhibit a weak ferromagnetic behavior due to surface spin disorder and shape anisotropy. Lateral confinement of electrons increases the band gap of the nanoribbons, as evident from the UV absorption analysis, which in turn improves their photocatalytic degradation efficiency (rate constant ∼0.95 h(-1)) by delaying the electron-hole recombination process. However, their liquid petroleum gas sensing properties have been found to be mainly governed by the improved (porous) surface of the hematite nanoribbons that provides huge interaction sites for the analyte gas. Most of all, these hematite nanoribbons show a significantly enhanced pseudocapacitive performance exhibited by their high specific capacitance of about 145 F g(-1) at a current density of 1 A g(-1), high rate capability, and also long cycle stability (nearly 96% of capacity retention after 1600 charging/discharging cycles).

TiO2/ZnO core/shell nano-heterostructure arrays as photo-electrodes with enhanced visible light photoelectrochemical performance

The present article reports a facile method for preparing the vertically-aligned 1D arrays of a new type of type II n–n TiO2/ZnO core/shell nano-heterostructures by growing the nano-shell of ZnO on the electrochemically fabricated TiO2 nanotubes core for visible light driven photoelectrochemical applications. The strong interfacial interaction at the type II heterojunction leads to an effective interfacial charge separation and charge transport. The presence of various defects such as surface states, interface states and other defects in the nano-heterostructure enable it for improved visible light photoelectrochemical performance. The presence of such defects has also been confirmed by the UV-vis absorption, cathodoluminescence, and crystallographic studies. The TiO2/ZnO core/shell nano-heterostructures exhibit strong green luminescence due to the defect transitions. The TiO2/ZnO core/shell nano-heterostructures photo-electrode show significant enhancement of visible light absorption and it provides a photocurrent density of 0.7 mA cm−2 at 1 V vs. Ag/AgCl, which is almost 2.7 times that of the TiO2/ZnO core/shell nano-heterostructures under dark conditions. The electrochemical impedance spectroscopy results demonstrate that the substantially improved photoelectrochemical and photo-switching performance of the nano-heterostructures photoanode is because of the enhancement of interfacial charge transfer and the increase in the charge carrier density caused by the incorporation of the ZnO nano-shell on TiO2 nanotube core.

Designing one dimensional Co-Ni/Co3O4-NiO core/shell nano-heterostructure electrodes for high-performance pseudocapacitor

A high-performance supercapacitor electrode based on unique 1D Co-Ni/Co3O4-NiO core/shell nano-heterostructures is designed and fabricated. The nano-heterostructures exhibit high specific capacitance (2013 F g−1 at 2.5 A g−1), high energy and power density (23u2009Wh kg−1 and 5.5u2009kW kg−1, at the discharge current density of 20.8 A g−1), good capacitance retention and long cyclicality. The remarkable electrochemical property of the large surface area nano-heterostructures is demonstrated based on the effective nano-architectural design of the electrode with the coexistence of the two highly redox active materials at the surface supported by highly conducting metal alloy channel at the core for faster charge transport.

Domain controlled magnetic and electric properties of variable sized magnetite nano-hollow spheres

Here, we report the synthesis of variable sized magnetite (Fe3O4) nano-hollow spheres in one step template free solvothermal method and their size dependent magnetic and electrical properties. Size of the hollow spheres is varied from 100u2009nm to 725u2009nm by changing the concentration of capping agent. Trace of Verway transition is found for all sets of spheres and the Verway transition temperature (TV) increases with increasing size of the spheres. The domain structure of these spheres changes from pseudo single domain to multi domain state as the size increases from 100u2009nm to 725u2009nm as evident from Day plots. This change in domain structure also changes the magnetic and electric properties of these spheres. Temperature dependent of high field magnetization of the hollow spheres can be well explained by Blochs power law with higher than the bulk value of Bloch constant. The Bloch exponent varies from 1.94 to 1.69 with increasing size of the spheres. Frequency dependence of electrical conductivity (σ) shows ...

Micelles induced high coercivity in single domain cobalt-ferrite nanoparticles

We have prepared CoFe2O4 nanoparticles in micellar medium by wet chemical technique and obtained very high coercivity value of 4.4 kOe at room temperature for particle size ∼16u2002nm. A large coercivity (∼20u2002kOe) is observed on cooling down to 2.5 K. We annealed the sample at different temperatures to check the role of micelles and particles size in the change in coercivity value. Here we observed micelles as capping agent playing an important role to enhance the coercivity, as after removal of micelles for the same particle size the coercivity drops from 4.4 kOe to small value ∼350u2002Oe. But the coercivity again increases due to the increase in particle size with increase in annealing temperature from 873 K and above. To obtain structural information and size of particles, we have taken x-ray diffraction spectra from the samples before and after annealing at different temperatures which confirm the spinel phase only.

Enhanced band gap emission and ferromagnetism of Au nanoparticle decorated α-Fe2O3 nanowires due to surface plasmon and interfacial effects

Au nanoparticle decorated α-Fe2O3 NWs exhibit enhanced band gap luminescence due to coupling between excitons in α-Fe2O3 NWs and the surface plasmons of Au particles. The enhanced magnetization in α-Fe2O3 NWs with Au coating is attributed to the interfacial effects arising from the trapped metallic electrons at the Au/α-Fe2O3 NWs interface.

Substrate-integrated core–shell Co3O4@Au@CuO hybrid nanowires as efficient cathode materials for high-performance asymmetric supercapacitors with excellent cycle life

Here we demonstrate a facile strategy to boost the electrochemical performance of Co3O4 based electrodes through a unique Co3O4@Au@CuO core–shell nanowire (NW) electrode design. Ultra-thin interconnected nano-sheets of Co3O4 have been uniformly coated on Au@CuO NWs, formed via Au sputtering on CuO NWs directly grown on Cu foil, providing an ameliorated surface area for electrochemical reactions together with the Au interlayer serving as the current accumulator. The results suggest that the incorporation of the Au interlayer into Co3O4@Au@CuO core–shell NWs dramatically improves their electrochemical performance in terms of capacitance and rate capability as compared to Co3O4@CuO NWs. Accordingly, the Co3O4@Au@CuO core–shell NW electrode reaches a specific capacitance of about 1141 F g−1, equivalent to an areal capacitance of about 240 mF cm−2, at a current density of 4.8 A g−1. Moreover, the ASC device assembled with a Co3O4@Au@CuO core shell NW cathode and an α-Fe2O3 nanorod anode exhibits an impressive volumetric energy density of 0.23 mW h cm−3 (equivalent to 33.8 W h kg−1) with a high power density of 270 mW cm−3 (equivalent to 40.4 kW kg−1) and a remarkable capacitance retention of ∼100% during 10u2006000 cycles. Indeed, this study would help design advanced high-performance ASCs with core–shell NW electrodes.