Showing papers by "S. B. Majumder published in 2015"

Properties of indium doped nanocrystalline ZnO thin films and their enhanced gas sensing performance

Abstract: Zinc oxide (ZnO) is one of the most promising semiconducting metal oxides, particularly for gas sensing applications. Impurity doped ZnO has offered much improved sensing performance, as compared to its undoped counterpart. In this work, undoped as well as indium doped nanocrystalline ZnO thin films are synthesized by a low cost chemical solution deposition route. X-ray diffraction patterns of the synthesized films reveal preferential orientation along the (002) plane. The surface and cross section morphology has clearly changed with the variation of indium content. Optical transmittance values increase with the increase in indium concentration and the band gap energy is found in the range 3.210 eV to 3.221 eV. PL spectra reveal three characteristic peaks, owing to band to band transition and defect level emissions. The effect of indium doping on the electrical parameters of ZnO is analyzed through Hall effect measurements at room temperature. The gas sensing characteristics of these sensors offer good reproducibility and stability towards various reducing gases, with an enhancement of response% and lower detection limit as compared to undoped ZnO. A doping level of 3 wt% of indium in ZnO is found to give optimum response and the lowest detection limit of hydrogen of 1 ppm or even lower. However, further increase in the doping concentration resulted in reduced sensing performance. This is attributed to the gas sensing mechanism related to the substitution of In3+ ions at Zn2+ ion sites enhancing the number of free charge carriers at the optimum level of indium. Through exploring this gas sensing mechanism, it is argued that the sensor performance can be dramatically improved by tailoring the indium concentration in ZnO for its practical application in various sectors as an effective gas sensor.

Electrospun hollow glassy carbon–reduced graphene oxide nanofibers with encapsulated ZnO nanoparticles: a free standing anode for Li-ion batteries

Abstract: Although zinc oxide (ZnO) has a high theoretical capacity for lithium (Li) storage, it has poor cyclability because of the huge volume changes during charge–discharge cycles resulting in particle pulverization and detachment from the current collector. In this paper, a novel anode architecture for Li-ion batteries fabricated by encapsulation of ZnO nanoparticles in the hollow core of glassy carbon–reduced graphene oxide (C–rGO) electrospun composite nanofibers, is described. A one-step, co-axial electrospinning method is used to synthesize a mat of core–shell structured composite nanofibers composed of rGO embedded in poly(acrylonitrile) (shell) and a ZnO nanoparticle precursor with a carrier polymer (core). Subsequent calcination and carbonization produce a mechanically stable anode material, which is used directly as a free-standing anode (∼60 μm thick) without any binder and current collector, which are inactive materials that only add to the battery mass and volume. The ZnO–C–rGO nanofiber composite was characterized by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy and X-ray diffraction. The electrochemical performance of the composite was studied by galvanostatic charge–discharge measurements at different current densities, slow scan cyclic voltammetry (CV) and impedance measurements. Incorporation of an rGO network in the glassy nanofiber shell enhances both the capacity and electrical conductivity of the mat electrode resulting in faster electron kinetics, and thus, an improved rate capability. The interior void spaces combined with the mechanical strength and flexibility of the C–rGO shell act as a structural buffer effectively relieving the volumetric stresses generated during charge–discharge cycles. The synergistic effect of the metal oxide, rGO and the core–shell design results in a high capacity of 815 mA h g−1 at a current density of 50 mA g−1 with capacity retention of almost 80% after 100 cycles, thus demonstrating significant potential as an anode substitute for next generation Li-ion batteries.

A comparative study on the structural, optical and magnetic properties of Fe3O4 and Fe3O4@SiO2 core-shell microspheres along with an assessment of their potentiality as electrochemical double layer capacitors.

Abstract: Herein, we report a comprehensive and comparative study on the crystal structure, and microstructural, optical, magnetic, hyperfine and electrochemical properties of Fe3O4 microspheres (S1) of diameter ∼418 nm and Fe3O4@SiO2 core–shell microspheres (S2) of diameter ∼570 nm. Each asymmetric unit of the crystalline Fe3O4 has one cation vacancy at the octahedral [B] site. At 300 K the saturation magnetization and coercivity of ferrimagnetically ordered S1 and S2 are 63.5, 38.5 emu g−1 and 200 and 120 Oe, respectively. We have shown that the synthesis procedure, morphology, surface properties, interparticle interaction manifesting the collective properties of the nanoparticle assembly and the average size of individual Fe3O4 nanoparticles forming the spherical ensemble play a crucial role in determining the magnetic properties of Fe3O4 and Fe3O4@SiO2 microspheres while the diameter of the microsphere does not have significant influence on magnetic properties of such a system. Further, the photoluminescence intensity of Fe3O4 microspheres gets significantly enhanced upon SiO2 coating. A cyclic voltammetric study suggests that S1 can act as a good electrical double layer capacitor (EDLC) above a scan rate of 0.04 V s−1 while S2 exhibits excellent performance as EDLC in a scan range from 0.01 to 0.06 V s−1. Thus, S2 is a potential candidate for fabrication of EDLCs.

NO2 sensing and selectivity characteristics of tungsten oxide thin films

Abstract: Using sol–gel synthesis, we have synthesized phase pure, porous, WO 3 thin film (thickness ∼70 nm) on optically flat quartz substrates. The deposited films were characterized in terms of their structure, microstructure, and gas sensing characteristics. We have investigated the NO 2 (0.2–50 ppm), CO (10–500 ppm), methane, and butane (50–500 ppm) sensing characteristics of the synthesized films. The low ppm NO 2 sensing characteristics of these films are found to be superior than most of the nano-structured WO 3 sensing elements reported in recent literatures. For higher test gas concentrations, we found these sensing elements are sensitive to carbon monoxide (>10 ppm), methane, and butane gases (>50 ppm). We demonstrated that exponential moving average feature extraction algorithm of the resistance transients in conjunction with pattern recognition analyses is one of the most viable approach to address the cross-sensitivity of WO 3 thin film NO 2 sensors towards other reducing gases.

Improvement of the Electrochemical Characteristics of Lithium and Manganese Rich Layered Cathode Materials: Effect of Surface Coating

Abstract: Surface coating with electrochemically inert materials are found to be fruitful to improve the cycleability and rate capability characteristics of lithium and manganese rich composite cathode materials. In order to understand the structure-property relation between the nature of coating and the electrochemical performance, surface modification of composite cathodes was carried out either by a thin layer of carbon or zirconia particles. Zirconia coating helps to sustain 86% capacity retention after 50 cycles as compared to bare composite which exhibits 68% capacity retention when cycled at 10 mAg(-1). Among 1 wt%, 2.5 wt% and 5 wt% zirconia coated cathode materials, 2.5 wt% zirconia coating exhibits best rate capability. We have demonstrated that the porous particulate ZrO2 coating improved the capacity retention of the composite cathodes by suppressing the impedance growth at the electrodes-electrolyte interface. (C) 2015 The Electrochemical Society.

Correlation of carbon monoxide sensing and catalytic activity of pure and cation doped lanthanum iron oxide nano-crystals

Abstract: For nano-crystalline cobalt (Co) and cobalt/lead (Pb) co-doped lanthanum ferrite (LaFeO 3 ) ceramics, we have investigated the carbon monoxide (CO) sensing characteristics. The catalytic activities toward CO oxidation were investigated for LaFeO 3 and LaFe 0.8 Co 0.2 O 3 powders. At an operating temperature as low as 175 °C, Co doped LaFeO 3 sensors exhibit excellent low concentration ( 0.8 Pb 0.2 Fe 0.8 Co 0.2 O 3 (LPFC) ceramics selectively senses CO at 150 °C. For LaFe 0.8 Co 0.2 O 3 , 0.056 mmol of CO conversion per gram of catalyst was measured at 200 °C. On the other hand, for LaFeO 3 ceramics, CO conversions per gram of catalyst were recorded to be 0.004 mmol. The superior low temperature CO sensing characteristics of nano-crystalline LaFe 0.8 Co 0.2 O 3 ceramics correlates well with its excellent catalytic activity toward CO oxidation. For perovskite LaFe 0.8 Co 0.2 O 3 sensors it was demonstrated that lower metal-oxygen binding energy, favorable d-orbital electron configuration of Co cation, and nature of surface composition are the three dominant factors control its superior catalytic activity and the CO sensing characteristics.

The role of catalytic cobalt-modified lanthanum ferrite nano-crystals in selective sensing of carbon monoxide

Abstract: In the present work we have investigated the carbon monoxide (CO) sensing characteristics of cobalt-modified lanthanum iron oxide perovskite sensors. Nano-particles of lanthanum ferrite and cobalt-modified lanthanum ferrites have been synthesized by auto-combustion route. We have reported that Co-modified LaFeO3 sensor is capable to sense low-concentration (<100 ppm) CO selectively at operating temperature as low as 100 °C. The selective CO sensing characteristics at lower operating temperature are correlated to the superior catalytic activities of these perovskite toward CO oxidation. For these perovskite sensors it was demonstrated that the lower metal–oxygen binding energy, favorable d orbital electron configuration of transition metal cation/(s), and the nature of surface composition are the three dominant factors that control the catalytic activity and thereby the CO sensing characteristics.

Understanding the anomalous conduction behavior in ‘n’ type tungsten oxide thin film during hydrogen gas sensing: Kinetic analyses of conductance transients

Abstract: Promising hydrogen sensing characteristics are reported for sol–gel synthesized ‘n’ type WO 3 thin films. During hydrogen gas sensing using WO 3 films, irrespective of the test gas concentration and thickness of the sensing films, a temperature dependent anomalous change in surface conduction behavior of the sensor is reported. The nature of such gradual change in conduction behavior was further investigated by analyzing the response and recovery conductance transients. It was identified that the observed anomalous characteristics are superposition of several donor and acceptor type adsorption–desorption processes with their characteristic time constants. From the temperature variations of these characteristic time constants we have estimated the activation energies of the processes involved during sensor response and recovery. By correlating the estimated activation energies with the steps of the surface reactions, we have made an attempt to explain the observed gradual change in conduction behavior of WO 3 thin films during hydrogen gas sensing.

Mixed ionic liquid/organic carbonate electrolytes for LiNi0.8Co0.15Al0.05O2 electrodes at various temperatures

Abstract: Mixtures of N-butyl-N-methyl pyrrolidinium bis(trifluoromethanesulfonyl)imide ionic liquid (IL) and conventional organic carbonate electrolyte are used for high-capacity LiNi0.8Co0.15Al0.05O2 (LNCA) electrodes in Li-ion batteries. Increasing the IL content ratio in the mixtures can increase the electrolyte's thermal stability and retard its flammability. However, the optimal electrolyte composition depends on the operating temperature. At 25 °C, the plain organic electrolyte is preferred due to its highest ionic conductivity among the tested electrolytes. This electrolyte is volatile at 50 °C, and thus the incorporation of 25 wt% IL can improve the cyclic stability of the LNCA electrode. The LNCA dissolution and electrolyte decomposition at 75 °C are clearly suppressed with a high IL ratio in the mixed electrolyte. At such a high temperature, with 75 wt% of IL incorporation a high electrode capacity of 195 mA h g−1 is obtained at 30 mA g−1; 50% of this capacity can be retained when the charge–discharge rate increases to 700 mA g−1. Moreover, less than 20% capacity decay is found after 100 cycles.

Hematite iron oxide nano-particles: facile synthesis and their chemi-resistive response towards hydrogen

Abstract: In the present work, hematite iron oxide nano-particles are synthesized through a facile wet chemical precipitation route. The phase formation behavior and microstructure evolution of the synthesized nano-particles are studied using infrared spectroscopy in conjunction with x-ray diffraction analyses and electron microscopy. Chemi-resistive type hydrogen sensing characteristics (e.g. response %, response time, recovery time) of hematite iron oxide nano-particulate sensing element are evaluated using an automated, dynamic flow gas sensing measurement set-up. The sensing characteristics are measured by varying the operating temperature (275–350 °C) of the sensor and concentration of hydrogen (250–1660 ppm). From the operating temperature dependence of response and recovery times, we have estimated the respective activation energies for response and recovery processes.

Selective hydrogen sensing by cobalt doped ZnO thin films: A study on carrier reversal conductivity

Abstract: The n to p type carrier reversal conductivity is observed in cobalt doped zinc oxide (Co_ZnO) thin films with cobalt content from 0.001 to 0.5 wt% in hydrogen (H2) gas. Maximum response (220.6 %) and p-type conductivity is achieved in H2 gas at lower operating temperature (200oC) using 0.01 wt% Co_ZnO thin film where n-type conductivity is observed at higher operating temperatures which provide selective H2 sensing. The n and p type conductivity are observed at 250oC but the response (%) is less. Below 250oC it shows fully p-type conductivity in presence of H2 gas. By decreasing operating temperature, O2" molecular oxygen ions might be dominant to chemi-adsorb and creates OH" on sensor surface by reacting with H2 gas which increase the sensor resistance and shows p-type conductivity. The effect of interstitial defects and oxygen vacancies are also studied for n to p carrier reversal using photo luminescence spectra of undoped and Co_ZnO thin films.