Pt Nanoislands Functionalized Mesoporous Carbon Nanofibers Based Chemical Sensor for Trace-Level Detection of Hydrogen Gas

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. 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