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Introduction: Hydrogen is one of the most promising sources of energy for next generation power and transportation sectors.Owing to the inflammable properties of H2 gas, it is highly essential to develop technologies for its rapid detection to ensure safety and leakage detection in various application domains. In this regard, researchers have focused their attention on development of new chemical sensors based on solid electrolytes, metal oxides, polymers and carbon-based gas sensors [1-2]. Among these materials, carbon-based gas sensors have received immense attention owing to their exceptional structural properties, including high electrical conductivity, chemical and thermal stability and high surface area. Especially, carbon nanofibers (CNFs) have earned significant attention due to their notable electrical conductivity and stability with tailor-made one-dimensional structures. But, the sensors made of CNFs are less explored due to selectivity issues. Integration of catalytically active metal nanoparticles onto 1-D carbon nanostructures have been attempted by many groups [3-4]. For sensing hydrogen, expensive Pt catalysts have shown excellent performance. In order to improve the sensitivity at affordable cost, it is proposed that Pt can be used on mesoporous carbon nanofibers (pCNF). Herein, we report development of a novel room-temperature hydrogen sensor based on Pt@pCNF nanohybrids fabricated by electrospinning. Gas sensing properties were systematically investigated towards trace level detection of hydrogen gas which showed enhanced hydrogen sensing properties at room temperature with fast response/recovery time. Synthesis of Pt@pCNF nanohybrids: Carbon nanofibers were fabricated using electrospinning technique. The electrospun PAN fibers collected on copper sheet were subjected to a two-step thermal process to convert into carbon nanofibers by a stabilization (280°C) step followed by carbonization (800°C) in air and N2 atmosphere with a ramping rate of 2°C/min and 3°C/min by holding time of 1hr. Platinum nanoparticles were chemically deposited on functionalized carbon nanofibers via urea assisted ethylene glycol synthesis method. For fabrication of mesoporous carbon nanofibers (pCNF), NaHCO3 nanoparticles were used as pore forming agent during electrospinning process which was thermally decomposed for introduction of mesopores in carbon nanofibers. Results and Discussion: XRD patterns of Pt@CNF and Pt@pCNF confirms the fcc structure, which corresponds to (111), (200), (220) and (222) planes of Platinum (JCPDS card 04-0802). A broad diffraction peak was appeared at 24.7° in CNF@Pt nanohybrids, which is ascribed to the amorphous phase of carbon ((002) plane) in the carbon nanofibers assigned to hexagonal graphite (JCPDS card 41-1487). The Pt formed over the surface of CNFs exhibited evident diffraction peaks, signifying the formation and favorable crystallization of Pt@CNF and Pt@pCNF nanohybrids. Further structural characterization of the monometallic nanoislands was performed using HRTEM analysis. The lattice d-spacing of 0.227 nm corresponds to the (111) crystalline plane of face centered cubic crystal structure of Pt on the surface of Pt@CNF. Porous structure of carbon nanofibers where studied using HRTEM and SEM analysis. These results indicate that homogenous distribution of Pt nanoparticles over CNF and pCNF. Evaluation of Hydrogen gas sensor properties: H2 gas sensing properties of Pt@CNF and Pt@pCNF nanohybrids were evaluated using in-house gas sensor test station at room temperature. The dynamic gas sensing response of Pt@pCNF nanohybrids towards trace level detection of hydrogen from 0.1% to 4% exhibited sensitivity in the range of 6% to 47% at room temperature. Gas sensing response of CNFs with surface anchored platinum nanoparticles is due to high adsorption property of H2 on monometallic active sites. Upon exposure to H2 gas, H2 molecules initially physisorbed on the sensor and later chemisorbed. Due to this process, work function of the platinum is lowered and hence electron transfer occurs from platinum to supporting materials. Hence, the resistance decreases due to the accumulation of electrons which is evident from the gas sensing response of Pt@pCNF upon exposure to reducing gas, like H2. References: Korotcenkov, S. D. Han, and J. R. Stetter, Review of Electrochemical Hydrogen Sensors, Chem. Rev. (2009) 109, 1402–1433,doi: 10.1021/cr800339k. Hübert, L. Boon-Brett, G. Black, U. Banach, Hydrogen sensors – A review, Sensors and Actuators B 157 (2011) 329– 352, doi: 10.1016/j.snb.2011.04.070. Wang, S. Rathi, B. Singh, I. Lee, H. Joh and G. Kim, Alternating Current Dielectrophoresis Optimization of Pt-Decorated Graphene Oxide Nanostructures for Proficient Hydrogen Gas Sensor, ACS Appl. Mater. Interfaces 2015, 7, 13768-13775, doi: 10.1021/acsami.5b01329. Jung, M. Han, and G. S. Lee , Fast-response room temperature hydrogen gas sensors using Pt-coated spin-capable carbon nanotubes, ACS Appl. Mater. Interfaces,2015, doi: 10.1021/am506578j.

Introduction: Harmful and toxic gases such as nitrogen dioxide (NO2), carbon-monoxide (CO) and VOCs are largely released into environment due to increased industrial revolution. NO2 is considered as one of the most dangerous air pollutants, which plays a vital role in the formation of ozone (O3) to produce acid rains and therefore, it is essential to monitor trace level NO2 gas in environment for human safety. One-dimensional (1D) nanostructures have become an attractive candidate for sensors owing to their superior spatial resolution and rapid response due to the high surface-to-volume ratio compared to thin film gas sensors [1–4]. Metal oxide based tungsten oxide (WO3) is an important n-type semiconductor material with a bandgap of 2.7 eV, suitable for gas sensor applications [5]. Though, WO3 is widely used gas sensor material, poor selectivity, high operating temperature and reliability hinders their practical application. Similarly, indium oxide (In2O3) has demonstrated as another promising gas sensor material specifically to detect NO2 gas at room-temperature. In this investigation, development of n-n type WO3@ In2O3 heterojunction nanorods has been implemented to form core-shell architecture which exhibited distinguished sensing properties at reduced temperature towards trace level NO2 gas with excellent sensitivity, high selectivity and fast response/recovery characteristics and a plausible mechanism is deduced. Synthesis of WO3@In2O3 core-shell heterojunction nanorods: 1D WO3 nanorods were synthesized using hydrothermal method as per previous report [6]. In this work, WO3@In2O3 core-shell heterojunction nanorods were subsequently prepared by solvothermal method. Briefly, In(CH3COO)3. xH2O (0.5 mmol) was dissolved in a binary solvent mixture (1:2) with ethylene glycol (17 mL) and ethanol (34 mL), followed by vigorous stirring for 40 min. Meanwhile, the surface treatment of 200 mg of WO3 nanorods were carried out under UV-irradiation (254 nm) for 2 min. The above mixture was subjected to ultrasonication for 30 min. The solution was further transferred into a Teflon-lined stainless-steel autoclave (100 mL capacity) and maintained at 160oC for 5 h. The product was separated, washed and centrifuged several times with ultrapure water followed by ethanol in order to eliminate the organic and redundant In2O3 impurities. The precipitate was dried in a vacuum oven at 80oC for 24 h. Finally, the resultant product was annealed at 500oC in air for 2 h at a heating rate of 2oC/min. The color of the precipitate was changed to pale yellow, implying the functionalization of In2O3 nanoparticles on the surface of WO3 nanorods. Results and Discussion: XRD patterns and the Raman spectral analysis of WO3 nanorods and WO3@In2O3 core-shell heterojunction nanorods confirmed the structural purity and formation of heterojunction materials. The peaks in XRD pattern of the WO3 nanorods could be well-indexed to the hexagonal phase of WO3 (JCPDS 85-2460). No additional peaks were observed, which confirmed the phase purity of synthesized WO3 nanorods. Further, core shell structure of the WO3@In2O3 nanorods was analyzed using HRTEM and SEM. The results indicated the homogenous distribution of In2O3 nanoparticles over WO3 nanorods. Evaluation of NO2 gas sensor properties: NO2 gas sensing properties of WO3 nanorods and WO3@In2O3 core-shell heterojunction nanorods were evaluated using in-house gas sensor test station at reduced temperature. The dynamic gas sensing response of WO3@In2O3 core-shell heterojunction nanorods towards trace level detection of NO2 in the range of 500 ppb to 3 ppm exhibited sensitivity upto to S = 280% at 150oC. The enhanced gas sensing response of WO3 nanorods with surface anchored In2O3 nanoparticles is attributed to high adsorption property of NO2 on active sites and also due to directed electron transport mechanism. Upon exposure to NO2, the gas molecules initially physisorbed at the heterojunctions and trap electrons from the heterojunction interfaces of WO3@In2O3 nanorods. Since electron transport between WO3 and In2O3 is based on work function difference, the electrons can accumulate at the heterojunction interfaces leading to increased space-charge depletion region. Hence, the resistance increases due to the depletion of electrons which is evident from the enhanced gas sensing behaviour of WO3@In2O3 heterojunction nanorods upon exposure to NO2. References: Kolmakov, Y. Zhang, G. Cheng, M. Moskovits, “Detection of CO and O2 Using Tin Oxide Nanowire Sensors”Adv. Mater.15, 997 (2003) doi:10.1002/adma.200304889 Liu, E. Koep, M. Liu, “A Highly Sensitive and Fast-Responding SnO2 Sensor Fabricated by Combustion Chemical Vapor Deposition”, Chem. Mater.17, 3997 (2005)doi:10.1021/cm050451o H. Lin, M.W. Huang, C.K. Liu, J.R. Chen, J.M. Wu, H.C. Shih, “The Preparation and High Photon-Sensing Properties of Fluorinated Tin Dioxide Nanowires”J. Electrochem. Soc.156, K196 (2009)doi:10.1149/1.3223984 S. Ramgir, I.S. Mulla, K.P. Vijayamohanan, “A room temperature nitric oxide sensor actualized from Ru-doped SnO2 nanowires”,Sens. Actuators B 107, 708 (2005) doi:10.1016/j.snb.2004.12.073 T. Sun, C. Cantalini, “Microstructural effect on NO2 sensitivity of WO3 thin film gas sensors Part 1. Thin film devices, sensors and actuators”, Thin Solid Films 287, 258 (1996).doi:10.1016/S0040-6090(96)08745-7 Jinmin Wang et, al., “Controlled Synthesis of WO3 Nanorods and Their Electrochromic Properties in H2SO4 Electrolyte”, J. Phys. Chem. C 2009,113,9655–9658.doi:10.1021/jp901650v

Herein we report excellent gas sensor properties of Polyaniline functionalized multiwalled carbon nanotubes (PANI/MWCNTs) based nanocomposite for trace level detection of ammonia (NH3) gas. PANI/MWCNTs nanocomposite was synthesized by in-situ chemical oxidative polymerization of aniline monomer with carboxylated multiwalled carbon nanotubes (C-MWCNTs). The material was structurally characterized by UV–vis Spectroscopy, FT-IR Spectroscopy, Raman Spectroscopy, X-ray Photoelectron Spectroscopy (XPS) and High Resolution Transmission Electron Microscopy (HRTEM). Uniform PANI layer with ∼7 nm thickness was formed on the external walls of the MWCNTs. The gas sensor properties of C-MWCNT and PANI/MWCNTs nanocomposite towards NH3 gas exposure at trace level concentrations (2–10 ppm) under ambient conditions were analyzed and their performances were compared. PANI/MWCNT nanocomposite based sensor exhibited excellent enhancement in sensor response and response/recovery characteristics with good reproducibility towards NH3 gas in comparison with C-MWCNT. The response and recovery time of the PANI/MWCNTs nanocomposite based sensor were found to be significantly improved in the order of a few seconds (6 s) towards NH3 gas. The results reveal the potential application of this sensor in monitoring trace level NH3 gas for varied applications.

A rapid synthesis route for hybrid ZnO@Au core–shell nanorods has been realized for ultrasensitive, trace-level NO2 gas sensor applications. ZnO nanorods and hybrid ZnO@Au core–shell nanorods are structurally analyzed using X-ray diffraction (XRD), high resolution transmission electron microscopy (HR-TEM) and X-ray photoelectron spectroscopy (XPS). Optical characterization using UV–visible (UV–vis), photoluminescence (PL) and Raman spectroscopies elucidate alteration in the percentage of defect and charge transport properties of ZnO@Au core–shell nanorods. The study reveals the accumulation of electrons at metal–semiconductor junctions leading to upward band bending for ZnO and thus favors direct electron transfer from ZnO to Au nanoclusters, which mitigates charge carrier recombination process. The operating temperature of ZnO@Au core–shell nanorods based sensor significantly decreased to 150 °C compared to alternate NO2 sensors (300 °C). Moreover, a linear sensor response in the range of 0.5–5 ppm of NO2 concentration was observed with a lowest detection limit of 500 ppb using conventional electrodes. The defects with deep level, observed in ZnO nanorods and hybrid ZnO@Au core–shell nanorods influences local electron density, which in-turn indirectly influence the gas sensing properties. The ZnO@Au core–shell nanorods based sensor exhibited good selectivity toward NO2 and was found to be very stable.

The present investigation is related to synthesis of high surface area lanthanum phosphate by an aqueous sol–gel process involving post gelation treatment by solvent exchange resulting in a soft agglomerated gel. Precursor sol, prepared from lanthanum nitrate through precipitation and peptization had an average particle size of 34 nm. The sol to gel transformation was carried out in presence of ammonia and resulted in a porous gel structure. Washing of the gel with different alcohols led to solvent exchange and subsequent drying resulted in the high surface area lanthanum phosphate. The surface area increased with the increase in boiling point of the alcohol used for washing. A surface area of 140 m 2 /g was obtained for the octanol washed lanthanum phosphate calcined at 400 °C.

In the present study, highly monodispersed polyaniline (PANI) and gold nanoparticles grafted PANI nanospheres (Au@PANI) have been fabricated through a facile wet-chemical synthesis process by in-situ polymerization of aniline. The structural characterization of the PANI and Au@PANI nanospheres were carried out by UV-Vis spectroscopy, FT-IR spectroscopy, and High Resolution Transmission Electron Microscopy (HR-TEM). Stable uniform sized nanospheres of PANI and Au@PANI obtained were found to exhibit superior gas sensor performance towards trace-level concentration of ammonia (NH3) gas in the range of 1-5 ppm. A comparative analysis of the sensor properties of PANI and Au@PANI nanospheres were performed under ambient conditions. Au@PANI nanospheres exhibited enhanced gas sensing performance with high sensitivity (58.2%), rapid response (48 s) and recovery (58 s) characteristics (1 ppm of NH3). The results indicate the potential use of Au@PANI nanospheres for fabrication of efficient NH3 sensors for varied applications.