Silica Nanofiber Mats containing Vanadium Pentoxide for Gas Sensing
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Silica Nanofiber Mats containing Vanadium Pentoxide for Gas Sensing Jeanne E. Panels and Yong L. Joo School of Chemical & Biomolecular Engineering, Cornell University Ithaca, NY 14853, U.S.A ABSTRACT For the first time, silica nanofiber mats containing vanadium pentoxide crystals have been made via a sol-gel/electrospinning technique without using any polymer binder. XRD analysis after calcination of the fibers indicates the presence of vanadium pentoxide crystals. The average diameter of 20mol%V fibers after calcination at 800°C is 420nm. SEM and TEM images demonstrate crystal growth on the surface of the fibers after calcination at 800°C. XPS analysis also indicates the growth of V2O5 crystals at high enough temperatures. Physisorption of 20mol%V fibers after calcination at 800°C gave a BET surface area of 1.9 m2/g and an average pore diameter of 6 nm. INTRODUCTION Electrospinning is a fiber formation process which relies on electrical rather than mechanical forces to form fibers with diameters ranging from 100 nm to 10 microns. A strong electric field is used to draw a solution from the tip of a capillary to the collector. The electrostatic field causes a pendant droplet of the solution at the capillary tip to deform into a conical shape or a Taylor cone. When the electrical force at the surface of the solution overcomes the surface tension a fine, charged jet is ejected. The jet moves towards a ground plate acting as a counter electrode. The solvent begins to evaporate immediately after the jet is formed causing the deposit of a thin fiber on the collector plate [1, 2]. Silica nanofiber mats can be made through a sol-gel synthesis/electrospinning technique without using any polymer binder [2]. One dimensional, inorganic, nanosized mats are of interest for their high thermal stability and large surface to mass ratios. Silica nanofiber mats provide an ideal platform for the incorporation of transition metals [3]. The metals may be included by adding them to the sol-gel precursor [4]. When the viscosity of the solution reaches a certain range (10 to 1000 Poise), it may be electrospun [5]. The current work focuses on silica mats containing vanadium pentoxide for its gas sensing capabilities. Currently, thin films containing V2O5 are used for sensing toxic or flammable gases, but this method only utilizes the surface layer of the film because the gas flows over the film [6, 7]. Nanofiber mats provide a system where the gas can flow through it and thereby effectively utilize the entire active sites on the large surface areas of the fibers. We anticipate that this simple and versatile electrospinning/sol-gel directed approach to conducting layers that combine a large surface area as well as electrical interactions will lead to more sensitive detection, e.g., in sensing gases. Since the electrically conducting nanomats will be deposited directly on surface oxidized silicon substrates, the presence of gas molecules can be obtained by measuring the resistance variation due to change of the surface barrier of the
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