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One dimensional (1D) nanostructure like nanorods, nanowires, nanotubes and nanofibers have aroused great attention owing to their exceptional properties like high surface-to-volume ratio, excellent electron and thermal transport and also plays as interconnects during fabricating nanoscale devices. In this work, one dimensionally aligned Carbon nanofibers(CFs)@AuNPs-PtNPs/NiNPs-PtNPs have been developed via a single step co-axial electrospinning integrated with in-situ photo-reduction. The carbon nanofibers with in-situ functionalized bimetallic nanoparticles possessed well-defined core-shell structure with carbon nanofibers (CFs) as the core and AuNPs-PtNPs/NiNPs-PtNPs as the surface anchored heterojunctions. Homogeneously distributed AuNPs-PtNPs/Ni NPs-Pt NPs, which are identified as chemical sensitizers, triggers the dissociation of H2 through the spill-over effect, by successive diffusion of these H+ on carbon nanofiber surface, thereby altering the entire surface of the CFs into reaction sites for H2. Aligned core-shell CFs@Ni NPs-PtNPs possessed high sensitivity of 124% (v/v) at room temperature as compared with aligned core-shell CFs@PtNPs NPs (52%) and aligned core-shell CFs@AuNPs-PtNPs (65%), which may be due to the high catalytic adsorption and desorption properties of Ni and Pt towards hydrogen gas. Moreover, co-axially aligned CFs@NiNPs-PtNPs based sensor also demonstrates outstanding selectivity towards hydrogen compared to other gases like ammonia, ethanol, hydrogen sulphide, and acetone with excellent long-term stability. In addition, hydrogen adsorption kinetics of co-axially aligned CFs@NiNPs-PtNPs based sensor was verified theoretically the adsorption rate constant of (0.42) which showed a comparable experimental value (0.38) of adsorption rate constant. The potential suitability of co-axially aligned CFs@NiNPs-PtNPs based sensor towards H2 at room temperature leading towards real time applications also have been demonstrated

Flexible electronic gas sensors working at room temperature have acquired enormous attention in recent years due to their suitability to be integrated into various wearable electronic products. In ...

Production and alignment of heterojunction metal oxide semiconductor nanomaterial-based sensing elements for microsensor devices have always posed fabrication challenges since they involve multi-step synthesis processes. Herein, we demonstrate a coaxial electrospinning with an in situ photoreduction process for the fabrication of MEMS-compatible, 1D aligned gold nanoisland (GNI) anchored ZnO-based Heterojunction Nanofibers (HNFs) and their complex plasmon-mediated NO2 gas sensing mechanism has been investigated using operando photoluminescence studies. Evaluation of the gas sensing properties of aligned ZnO-GNI HNFs has exhibited excellent sensor response (196%) with rapid response time towards 500 ppb NO2 gas at reduced operating temperature (200 °C). Operando studies using photoluminescence and electrical measurements ascertained the existence of supplementary active sites in the multicrystalline nanofibers owing to the existence of Au nanograins over aligned ZnO nanofibers (NFs), thereby inducing a spill-over zone and aiding the charge transfer phenomenon. The operando PL studies also revealed the plasmonic effect of metallic Au and the role of zinc interstitial (Zni) defects and oxygen vacancies (V+o), which influence the charge transfer between ZnO and surface anchored Au nanoislands. This study revealed atomistic insights into the structural defects and charge transport properties of metal oxide semiconductor heterojunction materials, and the coaxial integration method of 1D aligned ZnO-GNI HNFs paves the way for unique strategies and the development of suitable MEMS based gas sensors for real-time applications.

Herein, we report the formation of well-aligned ultrathin films of polyaniline-functionalized multiwalled carbon nanotubes (PANI@MWCNTs) with a high orientational order over a macroscopic area by L...

A comparative analysis of NO2 gas sensing performances of geometry-controlled Au decorated ZnO heterojunction nanostructures (nanospheres, nanorods, ultra-long nanorods and nanofibers) has been dem...

Interaction of water on heterogeneous nonwetting interfaces has fascinated researchers' attention for wider applications. Herein, we report the evolution of hierarchical micro-/nanostructures on superhydrophobic pseudoboehmite surfaces created from amorphous Al

Electrospun carbon nanofibers (CNF) with surface-anchored bimetallic gold–platinum nanoislands (CNFs@Au–Pt NIs) have been effectively developed by electrospinning and chemical reduction methods, an...

Herein, n–n type one dimensional ZnO@In2O3 heterojunction nanowires have been developed and their local electron transport properties during trace-level NO2 gas sensing process have been probed at room-temperature using conducting probe atomic microscopy. Solvothermally synthesized 1D ZnO@In2O3 heterojunction nanowires have been characterized by various spectroscopic and microscopic techniques, which revealed the mesoporous structure indicating their enhanced sensing properties. The dangling bonds and fraction of metal ions to oxygen ions existing on the exposed crystal facets of the heterojunction nanowires have been visualized by employing crystallographic simulations with TEM analysis, which aided in forecasting the nature of surface adsorption of NO2 gas species. In situ electrical characteristics and Scanning Spreading Resistance Microscopic (SSRM) imaging of single ZnO@In2O3 heterojunction nanowires revealed the local charge transport properties in n–n type ZnO@In2O3 heterojunction nanowires. Moreover, the ZnO@In2O3 heterojunction nanowires based sensor exhibited excellent sensitivity (S = 274%), a fast response (4–6 s) and high selectivity towards trace-level concentration (500 ppb) of NO2 gas under ambient conditions with low power consumption. Spatially resolved surface potential (SP) variations in ZnO@In2O3 heterojunction nanowires have been visualized using in situ Scanning Kelvin Probe Force Microscopy (SKPM) under NO2 gas environment at room temperature, which was further correlated with its energy band structure. The work functions of the material evaluated by SKPM reveal considerable changes in the energy band structure owing to the local electron transport between ZnO and In2O3 at the heterojunctions upon exposure to NO2 gas indicating the charge carrier recombination. A plausible mechanism has been proposed based on the experimental evidences. The results suggest that new insights into complex sensing mechanisms deduced from the present investigation on n–n type MOS based heterojunction nanowires under ambient conditions can pave the way for the novel design and development of affordable and superior real-time gas sensors.

Herein, porous 1D n–p type ultra-long ZnO@Bi2O3 heterojunction nanorods have been synthesized by a solvothermal method and their complex charge transport characteristics pertaining to NO2 gas sensing properties have been investigated. The porous structure of the ZnO@Bi2O3 heterojunction nanorods assisted in achieving superior sensing properties compared to pristine ZnO nanorods. Temperature-dependent in situ electrical studies of the porous heterojunction nanorods explored the unique electron transport properties under different environments, which revealed the accumulation/depletion of electrons and charge carrier recombination leading to band bending at the metal oxide heterojunctions. The formation of electron depletion layers at n-ZnO/p-Bi2O3 interfaces is believed to increase the adsorption of oxidizing gas, resulting in a fast response time (10–12 s) and 10 times higher sensitivity than that of the ZnO nanorod-based sensor towards 500 ppb NO2. To study the structure–property correlation of the ultra-long ZnO@Bi2O3 heterojunction nanorods-based sensor, a crystallographic model supported by transmission electron microscopy analysis was adopted to understand the NO2 gas adsorption properties on the surface. The crystallographic model helps to visualize the dangling bonds and the ratio of metal to oxygen ions present at the exposed crystal planes. The results suggest that porous, ultralong n–p type ZnO@Bi2O3 heterojunction nanorods could be a promising candidate for a high performance NO2 sensor for real time applications.

Unique phosphorous doped onion-like carbon nanostructures (P-OLCs) with superior surface properties have been synthesized through a temperature and pressure controlled thermal decomposition method. The substitutional doping of phosphorous in OLCs nanostructures could facilitate the charge transfer during ammonia gas sensing. Presence of polar metaphosphates/phosphonate species in the active sites of highly crystalline P-OLCs due to interstitial doping of oxygen incorporated with substitutionally doped phosphorous atoms significantly enhanced the sensitivity with fast response and recovery characteristics (2.8 s and 4.5 s) towards NH3 at room temperature. Further, ammonia gas adsorption kinetics in OLCs and P-OLCs have been meticulously investigated which revealed the faster adsorption rate in P-OLCs (2.17 x 10-7 Ω-1s-1) compared to pristine OLCs (8.599 x 10-8 Ω-1s-1). This method can widely open up the feasibility to induce enhanced surface activity (phosphorus-containing groups allows the access of oxyge...

Herein, we demonstrate a novel approach for development of TiO2 grafted 2D-TiC nanosheets (TiO2@2D-TiC) based room temperature operable, flexible ethanol gas sensor. The homogeneous distribution, unique composition, and crystalline microstructure of TiO2 nanoparticles grafted 2D-TiC nanosheets have been found to enhance the surface reactivity and efficiency of its transducer–receptor functions. The electron–hole recombination at the TiO2/2D-TiC interfaces offered superior sensor performance with fast response and recovery times. Moreover, TiO2@2D-TiC nanosheets based flexible sensor exhibited high selectivity toward trace-level ethanol gas (10 ppb–60 ppm) with extremely low noise-to-signal ratio and excellent stability. The results suggest that the development of low-cost flexible sensors based on TiO2@2D-TiC nanosheets could be applied for potential applications, such as printed/wearable electronics, biomedical sector and environmental monitoring.

A 3D porous nitrogen-doped graphene foam with a uniformly grafted platinum electrocatalyst (2–3 nm) (3D-Pt@N-graphene foam) has been developed using a simple hydrothermal synthesis for high performance proton exchange membrane (PEM) fuel cells. Excellent electrocatalytic activity towards the ORR in acidic medium has been exhibited by Pt@N-graphene foam with a favorable 4-electron transfer pathway resulting in the augmentation of active triple phase boundary sites. The positive shift of onset potential (989 mV) and half wave potential (895 mV) with a high limiting current density of 4.675 mA cm−2 exhibited by the 3D-Pt@N-graphene foam was found to be comparatively greater than that of the Pt/C catalyst with high durability. The presence of pyridinic-N and pyrrolic-N species in the highly compressible 3D-Pt@N-graphene foam contributed to the enhancement of the electrocatalytic activity, while graphitic-N improved the electrical conductivity of the material facilitating the electron transport during the ORR process. Compared to the commercial Pt/C electrocatalyst, the 3D-Pt@N-graphene foam based on a membrane electrode assembly exhibited four times higher power density (394 mW cm−2) and current density (1036 mA cm−2) owing to the favorable mass transport due to the hierarchical porous network, high specific surface area, nitrogen doping, uniform distribution of Pt nanoparticles and abundant active sites on the support material. The material demonstrates a new rational design for the fabrication of a highly efficient electrocatalyst assembly for flexible PEMFCs.

Herein, we have successfully developed an integrated strategy to develop antireflective coatings with self-cleaning capabilities based on periodic double-sided photonic γ-AlOOH nanostructures to transmit maximum incident light photons. Interfacial reflections are instinctive and one of the fundamental phenomena occurring at interfaces owing to refractive index mismatch. The eradication of such undesirable light reflection is of significant consideration in many optical devices. A systematic approach was carried out to eradicate surface reflection and enhance optical transmission by tailored γ-AlOOH nanostructures. The γ-AlOOH photonic nanostructures with subwavelength features exhibited a gradient index, which almost eliminated the refractive index mismatch at the interface. Optical transmittance of 97% was achieved in the range of 350-800 nm at normal incidence compared to uncoated glass (89%). A multilayer model approach was adopted to extract the effective refractive index of the coating, which showed a graded index from the top to the bottom surface. Further, to fully comprehend the optics of these nanostructures, the omnidirectional (20-70°) antireflective property has been explored using variable-angle specular reflectance spectroscopy. The hierarchical γ-AlOOH nanostructures exhibited only ∼1.3% reflectance at the lower incident angle in the visible spectra and an average reflectance of ∼7.6% up to an incident angle of 70°. Moreover, the hierarchical nanostructures manifested contact angle (CA) >172° and roll-off angle (RA)

Herein, we report a facile approach for the synthesis of TiO2 nanoparticles tethered on 2D mixed valent vanadium oxide (VOx/TiO2) nanoflakelets using a thermal decomposition assisted hydrothermal method and investigation of its temperature-independent performance enhancement in oxygen-sensing properties. The material was structurally characterized using XRD, TEM, Raman, DSC, and XPS analysis. The presence of mixed valent states, such as V2O5 and VO2 in VOx, and the metastable properties of VO2 have been found to play crucial roles in the temperature-independent electrical conductivity of VOx/TiO2 nanoflakelets. Though pristine VOx exhibited characteristic semiconductor-to-metal transition of monoclinic VO2, pure VOx nanoflakelets exhibited poor sensitivity toward sensing oxygen. VOx/TiO2 nanoflakelets showed a very low temperature coefficient of resistance above 150 °C with improved sensitivity (35 times higher than VOx for 100 ppm) toward oxygen gas. VOx/TiO2 nanoflakelets exhibited much higher response,...

We report a facile, one pot solvothermal route to prepare ultrathin molybdenum disulfide supported on nanocarbon hybrid (MoS2@NCHy) network with varied mass loading of MoS2 (20 wt%, 40 wt% and 60 wt%) for catalyzing hydrogen evolution reaction (HER). A large current density of −451 mAcm−2 at an overpotential of −480 mV vs. RHE, proves 40 wt% MoS2@NCHy/GCE nanocomposite as a highly HER active catalytic material. This value was found to be 50 times, 6 times and 2.5 times higher compared to MoS2/GCE (−8 mAcm−2), 20 wt% MoS2@NCHy/GCE (−75 mAcm−2) and 60 wt% MoS2@NCHy/GCE (−180 mAcm−2) respectively. Meanwhile, η10 (overpotential required to attain a current density of −10 mAcm−2) is −186 mV for 40 wt% MoS2@NCHy/GCE, which is much lower to MoS2/GCE (−498 mV), NCHy/GCE (−334 mV), 20 wt% MoS2@NCHy/GCE (−264 mV) and 60 wt% MoS2@NCHy/GCE (−204 mV). The overall activity of the catalyst was manifested by the least overpotential, η10 = −186 mV vs. RHE, Tafel slope of 53 mVdec−1 and exchange current density (j0) of 6.3 × 10−2 mAcm−2. The superior performance of 40 wt% MoS2@NCHy should be attributed to the highly conductive and porous support of three dimensional NCHy architecture as well as the enlarged interlayer spacing of MoS2. This study paves a new paradigm to exploit the synergistic effect of MoS2 and NCHy resulting in strong electronic coupling for enhancement in the HER activity.

Highly monodispersed, mesoporous ZnO@Au heterojunction micro-spheres have been successfully synthesized using a two-stage facile chemical method and their surface bound NO2 sensing properties were explored. Room temperature Photoluminescence (RTPL) reveal the presence of high degree of oxygen defects and zinc interstitials for the pristine ZnO mesoporous-spheres and a drastic reduction in PL intensity of the ZnO@Au heterojunction mesoporous-spheres pointing towards the utilization of surface defects for the Au cluster growth, facilitating electron transfer process between ZnO and Au. NO2 gas sensor property analysis of the ZnO@Au mesoporous-spheres showed an extraordinary sensitivity and selectivity at a lower operating temperature of 250 °C than pristine ZnO mesoporous-spheres (450 °C). The enhanced sensing behavior of the ZnO@Au heterojunction mesoporous-spheres can be ascribed to the synergetic effect of Au nanoclusters at the heterojunctions which acts as spill-over zone for the physisorption mediated sensing process and the inherent high surface area and surface defects.

Pt-grafted, hierarchical mesoporous carbon nanofibers (Pt/MPCNFs) electrocatalysts have been developed using electrospinning for high-performance PEM fuel cells. The morphological analysis of Pt/MPCNFs revealed uniformly dispersed Pt nanoparticles (2–3 nm) strongly grafted onto the hierarchical nanochannels of mesoporous carbon nanofibers (MPCNFs). Pyridinic and pyrrolic nitrogen species formed in the sp2 graphitic structure during the carbonization process of the MPCNFs have been found to play a major role in augmentation of the triple phase boundaries in the catalyst support materials. Pt/MPCNFs exhibited outstanding electrocatalytic performance towards the oxygen reduction reaction (ORR) with a positively shifted onset potential (54 mV), half-wave potential (78 mV) and high limiting current density (4.75 mA cm−2 at 0.4 V) compared to state-of-the-art Pt/C electrocatalysts in acidic medium and they exhibited superior long-term stability (89.8% retention after 30 000 s and less change in activity after 10 000 potential sweeps). Pt/MPCNFs as a cathode catalyst yielded a maximum power density of 428.6 mW cm−2 during single cell testing, which is 2.08 times higher than a commercial Pt/C electrocatalyst. The electrochemical performance evaluation clearly implied that the unique combination of ultrathin nanofibers with a three-dimensional mesoporous structure, high electrical conductivity, enhanced specific surface area, homogeneous dispersion of Pt nanocatalysts and the presence of optimal nitrogen doping offers superior electrocatalytic activity via a favorable four-electron pathway and long-term operating stability during repeated cycles. Performance analysis in a single PEM fuel cell shows twice the power density (428 mW cm−2) with Pt/MPCNFs compared to commercial electrocatalyst membranes due to the effective enhancement in the triple phase boundaries, indicating that Pt/MPCNFs are potential candidates for high-performance, durable PEMFCs.

An ultrasensitive and selective chemiresistive sensor interface based on Ag nanocluster integrated polyaniline functionalized multi-walled carbon nanotubes (AgNC@PANI/MWCNTs) has been developed for trace-level detection of NH3 gas at room temperature. AgNC@PANI/MWCNTs was synthesized using surfactant-free, one-pot wet-chemical process, by controlled integration of active Ag sites onto MWCNTs. The structure and morphology of AgNC@PANI/MWCNTs nanocomposite have been extensively studied by various characterization techniques. The gas sensing properties of AgNC@PANI/MWCNTs nanocomposite towards trace-level concentrations NH3 (2-10 ppm), an important biomarker in exhaled human breath, was systematically evaluated. The sensor exhibited dramatic enhancement in the sensor response (26 %), fast response (∼5 s) and recovery (∼4 s) characteristics with good reproducibility and selectivity upon exposure to NH3 gas. The excellent performance of the sensor towards NH3 could be attributed to the rapid electronic sensitization of surface engineered active AgNC sites in the composite in which oxidized AgNC were found to play a critical role. Effect of humidity and the kinetics of the NH3 gas adsorption on nanocomposite were analyzed. The possible interactions between NH3 and AgNC@PANI/MWCNTs nanocomposite were discussed. This investigation can pave the way to novel strategies for designing and fabricating low-cost, high performance NH3 gas sensors for clinical breath analyzer application.

A controlled synthetic strategy is established for the development of TiO2@Au heterojunction nanorods using a facile wet-chemical method for the detection of NO2 gas under atmospheric pressure conditions. Structural studies reveal the existence of metastable anatase phase along with thermodynamically stable rutile phase with high degree of crystallinity. The structural analysis divulges the uniform surface anchoring of Au nanoclusters onto mono-dispersed TiO2 nanorods introducing interfacial metal–semiconductor heterojunctions. TiO2@Au heterojunction nanorods exhibited excellent sensor performance towards trace level exposure of NO2 gas. Owing to the interfacial electron transfer process at the heterojunction the optimum operating temperature of TiO2@Au heterojunction nanorods determined to be 250 °C, which is much less as compared to pristine TiO2 gas sensors (400 °C). Sensor response was found to be linear for the trace level concentration range of 0.5–5 ppm with lowest detection limit as 500 ppb. The TiO2@Au heterojunction nanorods exhibited higher sensitivity at atmospheric pressure conditions compared to vacuum conditions because of the changes in surface O2 adsorption properties of the heterojunction material at different oxygen partial pressure and existence of mixed phases in TiO2 nanorods. The superior gas sensor performance of the material under atmospheric pressure conditions point towards their potential for real-time applications.

This work demonstrates the development of Ag@polyaniline/multi-walled carbon nanotube nanocomposite-based sensor strips and a suitable integrated electronic read-out system for the measurement of trace-level concentrations of ammonia (NH3). The sensor is optimized under various operating conditions and the resulting sensor exhibited an enhanced response (32% for 2 ppm) with excellent selectivity. Stable performance was observed towards NH3 in the presence of high concentrations of CO2 (>40 000 ppm), simulated and real breath samples. A suitable electronic sensor read-out system has also been designed and developed based on multi-scale resistance-to-voltage conversion architecture, processed by a 32-bit microcontroller which is operatable over a wide range of sensor resistance (1 kΩ to 200 MΩ). As a proof of concept, integration of gas-sensing strips with the electronic read-out system was tested with various levels of NH3 ( 2 ppm as critical and 2 ppm as threshold), which is important for clinical breath analyzer applications. The developed prototype device can be readily incorporated into a portable, low-cost and non-invasive point-of-care breath NH3 detection unit for portable pre-diagnostic breath analyzer applications.

The development of hybrid materials with high pseudocapacitance and electrical double layer capacitance is an urge for the fabrication of efficient supercapacitor electrodes. Herein, a highly effective, easily performable, high-yielding, and economical method is demonstrated for the preparation of α -Fe2O3 nanorods incorporated defect-free graphene nanosheets that showed excellent supercapacitor performance. α -Fe2O3/graphene nanocomposite has been synthesized by an interlayer catalytic exfoliation of α -Fe2O3 intercalated graphite which showed high conductivity of 3.1 × 106 S/cm. α -Fe2O3/graphene nanocomposite modified carbon paste electrode exhibited ultrahigh specific capacitance values of 1135 mF/cm2 at a scan rate of 5 mV/s and 815 mF/cm2 at a current density of 0.5 mA/cm2. The fast ionic diffusion with no charge transfer resistance of the electrode material was revealed from the electrochemical impedance spectroscopic analysis. α -Fe2O3/graphene nanocomposite showed 100 folds of increase in the specific capacitance compared to α -Fe2O3 and graphene, suggesting their synergistic effect in the nanocomposite leading to a drastic increase in the specific capacitance. The composite displayed good cyclic stability of 79.9% retention of the initial capacitance after 1500 cycles and 54.9% after 10,000 repeated cycles.

Aquatic contamination from the accumulation of pharmaceuticals has induced severe toxicological impact to the ecological environment, especially from non-steroidal anti-inflammatory drugs (NSAIDs). Real-time monitoring of flutamide, which is a class of NSAIDs, is very significant in environmental protection. In this work, we have synthesized the hexagonal-h boron nitride decorated on bismuth oxide (Bi2O3/h-BN) based nanocomposite for the effective electrochemical detection of flutamide (FTM). The structural and morphological information of the heterostructured Bi2O3/h-BN nanocomposite was analyzed by using a sequence of characterization methods. Voltammetric techniques were used to evaluate the analytical performance of the Bi2O3/h-BN modified screen-printed carbon electrode (SPCE) for the FTM detection. The Bi2O3/h-BN modified SPCE displays a synergetic catalytic effect for the reduction of FTM due to large surface area, numerous active sites, fast charge transfer and abundant defects. The proposed electrochemical sensing platform demonstrates high selectivity, low detection limit (9.0 nM), good linear ranges (0.04–87 μM) and short response time for the detection of FTM. The feasibility of the electrochemical sensor has been proved by the successful application to determine FTM in environmental samples.

Novel copper nanoparticles supported on starch micro particles (CuNPs@MS) were prepared successfully and characterized by SEM, EDX, TEM, SAED, TGA, XPS, and XRD analyses. The easily synthesized and thermally stable CuNPs@MS have been applied as a green, degradable, and eco-friendly catalyst in a three component reaction of amines, aldehydes and alkynes for the preparation of diverse propargylamines. The reactions were performed under heterogeneous conditions and the catalyst was successfully recycled for five consecutive runs with a small decrease in activity.

Understanding the origin of magnetic ordering in an undoped semiconductor with native defects is an open question, which is being explored actively in research. In this investigation, the interplay between magnetic ordering and excess induced native defects in undoped anatase TiO2 nanoparticles is explained using an experimental and theoretical approach. It is demonstrated that structurally disordered TiO2 nanoparticles with a high concentration of native defects such as titanium interstitials and oxygen vacancies are synthesized using controlled atmospheric rapid cooling (i.e. quenching) process. The structural disorders in the lattice have been examined using various spectroscopic and microscopic analyses revealed the existence of Ti deficiency in both pristine and quenched TiO2 nanoparticles. A possible origin of magnetic ordering in titanium deficient anatase TiO2 system is elucidated based on first-principle calculations. It was found that the overall magnetic moment of Ti deficient TiO2 system is determined by the distance between Ti interstitials and its neighboring vacancies (i.e. either V Ti or V Os). However, quenched TiO2 nanoparticles possess excess Ti interstitials, Ti and O vacancies and therefore the net magnetic moment of the system is reduced due to anti-ferromagnetically coupled neighboring Tilattice ions.

α-Fe2O3 functionalized, less-defective graphene sheets have been synthesized by a cost effective green solvent-assisted ultrasonication induced graphite exfoliation followed by hydrothermal method and its chemiresistive ammonia gas sensing properties have been investigated. The less-defective nature of graphene and the strong interaction of aromatic graphene network with α-Fe2O3 have been revealed from Raman spectroscopic and XPS and Raman spectroscopic analyses. The α-Fe2O3/graphene nanocomposite exhibited outstanding chemiresistive sensing properties towards NH3 gas, which is a toxic industrial pollutant. High sensor response, selectivity and repeatability are offered by the α-Fe2O3/graphene nanohybrid towards NH3 gas at 250 °C. The sensor response was linear in the range of 10–50 ppm with a regression coefficient of 0.95. A plausible sensing mechanism is suggested, indicating the role of both α-Fe2O3 and graphene leading to a synergistic response towards NH3.

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.

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.