Electrospun V2O5 composite fibers: Synthesis, characterization and ammonia sensing properties

TSF-31924; No of Pages 6 Thin Solid Films xxx (2013) xxx–xxx

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Electrospun V2O5 composite fibers: Synthesis, characterization and ammonia sensing properties V. Modafferi a, S. Trocino a,⁎, A. Donato a, G. Panzera a, G. Neri b a b

Dept. of Mechanics and Materials, University of Reggio Calabria,Via Graziella, Feo di Vito, 89124, Reggio Calabria,Italy Dept. of Electronic Engineering, Chemistry and Industrial Engineering, University of Messina, Contrada Di Dio (S.Agata), 98166, Messina, Italy

a r t i c l e Available online xxxx Keywords: Ammonia sensor Electrospinning V2O5 Composite materials

i n f o

a b s t r a c t In the present work, vanadium oxide (V2O5) fibers have been investigated for monitoring ammonia (NH3) at ppb levels in air. A simple sol gel-based electrospinning process has been applied for the synthesis of vanadium oxide/polyvinyl acetate (PVAc) and vanadium oxide/polyvinylpyrrolidone (PVP) composite fibers. Composite fibers doped with platinum (Pt) have been also prepared. The pure and Pt-doped metal oxide phase has been subsequently obtained by removing the polymer binder at high temperature in air. The samples have been widely studied to characterize their morphological and microstructural properties by X-Ray Diffraction, Fourier Transform InfraRed spectroscopy, X-ray Photoelectron Spectroscopy, and Scanning Electron Microscopy investigations. The application of the produced fibers in highly sensitive ammonia resistive sensors has been demonstrated. The influence of the nature of polymer binder and platinum addition on the sensing performances of the V2O5 fibers has been investigated and discussed.V2O5fibers produced by using PVP as a polymer binder have shown higher sensitivity toward ammonia at ppb concentrations than fibers obtained with PVAc. Pt-doped samples have shown a lower response compared to un-doped samples. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Semiconductor transition metal oxides with d 0 electron configuration, such as vanadium oxide (V2O5), containing active sites able to adsorbing gaseous molecules and catalyze reactions on their surface, have attracted many interest in the past years due to their chemical, electronic and catalytic properties and have found application in many technological fields [1]. From the electrical point of view, V2O5 is a n-type semiconducting oxide whose electric conductivity increases when the V 5+ species are reduced to V 4+ during the interaction with reducing gases. Therefore, due to these promising sensing properties, V2O5-based materials have been exploited for designing resistive gas sensors. Recently, the synthesis of V2O5 nanostructures has been the goal of numerous researches. Nanoparticles, nanotubes, nanowires, and nano-rods of V2O5 have been synthesized by different methods such as sol–gel, reverse micelle, chemical vapor deposition and so on [2–7]. Nanostructured materials show high surface area and, consequently,

⁎ Corresponding author at: Via Graziella, Feo di Vito, 89124, Reggio Calabria, Italy. Tel.: + 39 0965/875462; fax: + 39 0965/875248. E-mail addresses: [email protected] (V. Modafferi), [email protected] (S. Trocino), [email protected] (A. Donato), [email protected] (G. Panzera), [email protected] (G. Neri).

the sensing efficiency is strongly enhanced. In particular, V2O5 processed in the form of one-dimensional structures such as nanowires and nano-belts has been used to detect ammonia at low concentrations in air [3,4]. Electrospinning (ES) has been proposed as a technique for the low-cost preparation of such one-dimensional transition metal oxides based nanostructures [8–12]. In a previous paper we have used the electrospinning technique, combined with conventional sol–gel processing, to synthesize vanadium oxide/polyvinyl acetate (V2O5/PVAc) composites fibers [13]. Combining sol–gel processing with electrospinning has some advantages, allowing to obtain uniform fibers with very reduced diameter, and not requiring any further expensive purification. In this work, using polyvinylpyrrolidone (PVP) as polymeric binder, we extended the study to the preparation of vanadium oxide/ polyvinylpyrrolidone (V2O5/PVP) composite fibers. Moreover, platinum (Pt)-doped V2O5/PVP composite fibers have been also prepared. The changes in microstructure and composition of the as-spun fibers after annealing at high temperatures (400–500 °C) have been investigated by the aid of several characterization techniques, i.e. ThermoGravimetric Analysis-Differential Scanning Calorimetry (TGA-DSC), X-Ray Diffraction (XRD), Fourier Transform InfraRed spectroscopy (FTIR), X-ray Photoelectron Spectroscopy (XPS), and Scanning Electron Microscope (SEM).Resistive sensors were fabricated printing the annealed samples on an alumina substrate provided with interdigitated Pt electrodes, and tested for the monitoring of sub-ppm levels of ammonia in air.

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2. Experimental details 2.1. Synthesis of composite fibers A sol–gel route combined with electro-spinning was used for the preparation of composite fibers. A detailed description of the procedure, developed on the basis of that reported by Kim et al. [10], can be found elsewhere [8–11]. The synthesis proceeded through the following steps: i) Preparation of sol solution by the addition of vanadium oxide precursor, vanadium oxytriisopropoxide VOTIP (Aldrich) to dry ethanol (Aldrich). Such a solution is hydrolyzed with distilled water upon vigorous agitation and subsequently continuously stirred for 12 h. The so-obtained solution is mixed with PVAc (Aldrich, molecular weight ~100,000) or PVP (Aldrich, molecular weight ~1,300,000) and kept under stirring for 5 h. Weight ratios of the reagents are: PVAc:VOTIP1.5:1; PVP: VOTIP1.5:1.1 wt.% Pt-doped samples have been also prepared. In this latter case the sol solution has been prepared by using an ethanol-dimethylformamide 70:30 (v/v) mixture. ii) Each sol-solution obtained at the end of step one is introduced into the glass syringe of the ES apparatus (CH-01 Electrospinner 2.0 — Linari Engineering s. r. l.). The glass syringe, shielded by a plexiglass panel, is equipped with a 0.8 mm gauge stainless steel needle. The distance between in the tip of the syringe needle and the collector of the plate was fixed at 13 cm. A voltage in the range 16–18 kV was applied to the needle, while the collector is grounded and continuously translated along the x-direction during the spinning process. The feeding rate of solution was adjusted at a constant rate of 1.41 ml/h using a syringe pump. iii) After drying at room temperature (RT) for one day, the as-spun fibers were kept at 80 °C in air for 4 h and subsequently annealed in air at high temperature (400–500 °C).

allows to operate at controlled temperature and perform resistance measurements while varying the ammonia concentration in the carrier stream, from 0.1 to 0.8 ppm, making use of a permeation tube (FINE permeation tubes, Messina - Italy).Measurements were performed under a dry air total stream of 50sccm, registering the sensors resistance data in the four point mode by means of an Agilent 34970A multimeter, checking and adjusting the sensor temperature by the Agilent E3631A. The gas response is defined as S = [(R0 − R)/R] × 100, where R0 is the electrical resistance of the sensor in dry air and R its electrical resistance at different ammonia concentrations.

3. Results and discussion 3.1. Samples characterization The behavior to the temperature of the as-spun fiber composites was studied by TGA-DSC. Fig. 1 shows the results obtained for some samples. The TG profile of the V2O5/PVAc reveals an initial mass loss due to the evaporation of humidity adsorbed and/or residual ethanol retained in the fiber structure. Two additional and larger weight losses are observed at higher temperatures. Some aids to interpret this behavior comes from DSC analysis. Two negative peaks, centered approximately at 240 °C and 400 °C, indicate the occurrence of exothermic reactions due to the combustion of the organic phase. No further mass loss is observed above 450 °C, suggesting that the polymer binder is completely eliminated and residual matter at the end of TG measurements is only related to vanadium oxide phase. A similar pattern has been registered for the V2O5/PVP system. However, in this case exothermic peaks in the DSC curve are shifted at higher temperature, approximately at 330 °C and 460 °C, as a consequence of the higher resistance to oxidation of PVP compared to PVAc. Pt-doped samples show TGA-DSC patterns similar to those obtained with the parent un-doped samples.

2.2. Chemical and physical characterization

2.3. Electrical characterization and sensing test Sensor devices were fabricated mixing made fibers with water to obtain a paste and then printing it on alumina substrates (3 mm × 6 mm) supplied with interdigitated Pt electrodes and a Pt resistance as heating element. Sensing tests were carried in a system which

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The texture and morphology of as-spun and annealed fibers were investigated by means of SEM. For this purpose a Philips XL-30-FEG scanning electron microscope operating at an accelerating voltage of 5 kV has been used. The thermal stability of as-spun composite fibers was analyzed by TGA-DSC using a TG/DSC (Netzsch STA 409) instrument. The analyses were performed at a heating rate of 10 °C/min in static air up to 650 °C. XRD was performed using a Philips X-Pert diffractometer equipped with a Ni β-filtered Cu- Kα radiation at 40 kV and 30 mA. XRD patterns were recorded over the 10–50° 2θ-range at a scan speed of 0.05°/s, with a scan step of 0.05°. Diffraction-peak identification was performed on the basis of the JCPDS database of reference compounds. XPS measurements were performed by using a Physical Electronics (PHI 5800-01) spectrometer. A monochromatic Al Ka X-ray source was used at a power of 350 W. XPS data have been interpreted by using the on-line library of oxidation states implemented in the PHI MULTIPAK 6.1 software and the PHI Handbook of X-ray photoelectron spectroscopy [14]. FTIR spectra of the pressed powder samples into thin, self-supporting wafers were registered on a Bruker FTIR Equinox 55 spectrophotometer equipped with a Mercury Cadmium Telluride detector in the 7200–450 cm −1 range and with a resolution of 2 cm −1.

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On the basis of TGA-DSC, as-spun PVAc-based composite fibers were annealed at 400 °C, whereas as-spun PVP-based composites were annealed at 500 °C. At these temperatures, maintained for 2 h in air, the polymeric binder is completely eliminated, as confirmed by the characterization techniques below reported. The surface morphology of the as-prepared and annealed samples was studied by using SEM. A collection of images showing the morphology of PVAc- and PVP-based composites are reported in Fig. 2. A SEM micrograph of the as-spun V2O5/PVAc sample is shown in Fig. 2a. It can be clearly observed the presence of randomly oriented fibers having high aspect ratio and with a smooth and uniform surface. The fiber diameter varies from 1 to 4 μm, and no bead formation could be seen. The fiber diameter is almost regular along the whole length of each fiber. After annealing at the temperature of 400 °C, the fibers of V2O5/PVAc (Fig. 2b) and Pt/V2O5/PVAc samples are characterized by a reduced length, rough surface and a hollow structure and show some crystal growth on the fiber surface. Fig. 2c–d shows SEM micrographs of the as-spun Pt/V2O5/PVP composite fibers. The fibers produced present a different morphology and had much smaller

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diameters than those observed on PVAc-based composite. Indeed, the diameter of PVP-based composites fibers is in the range from 100 to 500 nm. Moreover, beaded structures are present diffusely. It is well

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Fig. 2. SEM micrograph showing the morphology of the as-spun and annealed samples: a) as spun V2O5/PVAc; b) annealed V2O5/PVAc; c, d) as spun Pt-V2O5/PVP; e, f) annealed Pt-V2O5/PVP.

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known that beads are more easily formed as the fiber diameter decreases. Therefore, it is very difficult to make beads-less fibers with the very thin fiber diameter. Fig. 2e-f remark the fibers morphology of the Pt/V2O5/PVP composite after annealing at T = 500 °C. The fiber structure is preserved, forming an extensive network with the individual fibers which appear melted among them at the contact points. FTIR spectra of the as-prepared and annealed samples are shown in Fig. 3. The profile of as-spun PVAc-based fibers reveals the major absorption peaks associated with the bending and stretching frequencies of poly(vinylacetate). In particular it can be seen the typical signal of C_O stretching of unconjugated ester [νs(CO)] at approximately 1736 cm−1, the methyl group peak of ester displayed at around 1375 cm−1 and the C\O stretching peak at about 1238 cm−1. C\H

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broad alkyl stretching band is evident between 2850 and 3000 cm−1. In the lower wavenumber region, the detection of the stretching of the V_O bonds at 947 cm−1 and 1022 cm−1 proves the presence of vanadium with an oxidation state 5 + [13,15,16]. Moreover, no peak is observed at 920 cm−1, indicating that no V+4 species is present [17]. The profile of as-spun PVP-based fibers reveals the major absorption peaks associated with bending and stretching frequencies of poly(vinylpyrrolidone). The ring C\C stretching vibrations occur in the region 1500–1600 cm−1. The bands around 3295 cm−1 can be assigned to vibrations of C\H ring. A number of C\H in plane deformation bands occur in the region of 1290–1210 cm−1. The bands in the region 1190–1210 cm−1 are assigned to C\N stretching. The signal corresponding to C_O vibrations is situated to 1657 cm−1. After annealing, signals referred to PVAc and PVP binders, disappear completely, hinting the complete polymer degradation, in agreement with the indications of the TG-DSC analysis. Correspondingly, the signals due to V2O5 phase increase in intensity. In particular, the peak at around 1020 cm −1 is due to the ν(VO) mode of V2O5, correspondent to the terminal oxygen strongly bonded to only one vanadium atom, while the vibration located at about 850 cm −1 is due to the bridging oxygen with the stretching modes of the VOV bonds [13]. XRD analysis has shown that as-spun composites samples have an amorphous nature. Annealed samples (Fig. 4) show the crystalline peaks of the orthorhombic V2O5 phase [PDF-card n. 9-387]. These observations are in full agreements with previous results obtained on the V2O5/PVAc sample [13]. No diffraction peak can be instead attributed to the presence of platinum species. This could be determined by the small amount of platinum (1 wt.%) present on the samples. XPS measurements were performed mainly to clarify the nature of the fibers constituents the after annealing, i.e. in the conditions in which they are used as sensing element. As an example, here are

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reported results regarding the Pt/V2O5/PVP sample, annealed at 500 °C. XPS spectra of this sample are shown in Fig. 5. In the C 1s spectrum is clearly evidenced only the presence of the adventitious carbon (COx) chemisorbed on the surface. This confirms that the polymer binder has been eliminated with the thermal treatment. Then, the O1s spectrum clearly point to the formation of oxides belong to V and/or Pt ions as proved by the O1s peak placed at 530.4 eV that monitors the O 2− ions in the lattice of metal oxides. The large presence of vanadium ions in the sample is proved by the characteristic V2p3/2 and V2p1/2 spin-orbit transitions located at binding energies of517.5 eV and 525.0 eV, respectively, which are in agreement with the formation of V2O5 phase as suggested by the other characterization techniques reported. Finally, XPS give also helpful indications about the presence of platinum. The characteristic spin-orbit transitions Pt4f7/2 and Pt4f5/2, partially convoluted with the 3s transition of vanadium ions, belong to platinum ions (Pt4f7/2 at 73.2 eV and Pt4f5/2 at 76.6 eV).

3.2. NH3 sensing tests In a previous paper, we pointed out on the decisive role of the microstructural properties of the V2O5 sensing element on the ammonia sensing performance. Morphological modifications occurring when the as-prepared V2O5/PVAc sample was treated in air at different temperatures (300, 400 and 500 °C), have been monitored by scanning electron microscopy. Increasing the treatment temperature shorter hollow fibers with a very porous and rough structure have been obtained [13]. The strong response has been correlated to the formation of this extensiveV2O5 fibers network. These optimal conditions were created after the annealing step of the V2O5/PVAC composite fibers, leading to a better interaction between the sensitive phase and the target gas. In order to develop ammonia sensors with enhanced performance, we try to optimize the formulation of the composite fibers by: i) using a different polymer binder and ii) adding a catalytic promoter such as platinum. The fibers produced using PVP as a polymer binder show a different morphology and much smaller diameters than those observed on PVAc-based composite. It is known that the concentration and nature of the polymer binder is critical for electrospinning. Indeed, maintaining constant the other factors, fiber formation is primarily governed by the viscosity of the polymer solution, which essentially depends on the concentration and structure of the polymer [18,19]. A detailed study is planned in order to investigate the mechanism which are responsible of the different fiber morphologies obtained when using PVAc or PVP as polymer binder. Moreover, after annealing at the temperature of 500 °C, the fiber structure on these samples is preserved. The influence of the nature of polymer used for the electrospinning process and of the platinum addition on the sensing performance has

been then investigated. The dynamic response of the V2O5/PVAc sensor, operating at the temperature of 260 °C, to successive ammonia pulses (lasting 20 s) in the range between 0.1 ppm and 0.8 ppm is reported in Fig. 6. In the experimental conditions adopted the response of V2O5/PVP sample to sub-ppm ammonia concentration pulses is very good. Moreover, the signal baseline is almost completely recovered in a short time after each pulse. The calibration curves of the V2O5/PVP and V2O5/PVAc samples, obtained in the experimental conditions above described, are compared in the plot reported in Fig. 7. It can be clearly observed as the response of V2O5/PVP sensor is higher with respect that registered with V2O5/PVAc one. The sensing data here reported suggests that the morphology of the samples prepared by using a different polymeric binder strongly influence the ammonia sensing behavior. Specifically, it appears that the V2O5 network constituted by fibers with a smaller diameter could be the key factor for the enhanced sensitivity of the V2O5sensing layer formed after annealing of the as-spun V2O5/PVP composite. Finally, we investigated the behavior of the Pt-doped V2O5/PVP sensor. It is well known that the addition of noble metals (platinum, palladium, gold) as catalytic additives is an effective way to enhance the sensing properties of metal oxide-based chemoresistive sensors [20]. However, in this case, we noted a lower response to ammonia pulses of the Pt-doped sensor compared to un-doped sensor. The reasons of this are not clear at this moment, and further experiments are in progress. 4. Conclusions V2O5fibers have been synthesized by the electrospinning process. The application of the produced fibers in high performance ammonia resistive sensors able to detect ammonia at concentrations as low as 100 ppb has been demonstrated. V2O5 nanofibers produced by using PVP as a polymer binder have shown higher sensitivity toward ammonia at sub-ppm concentrations than that obtained with PVAc. Pt-doped samples have shown a lower response compared to un-doped samples. References [1] G.T. Chandrappa, N. Steunou, S. Cassaignon, C. Bauvais, J. Livage, Catal. Today 78 (2003) 85. [2] G. Rizzo, A. Arena, A. Bonavita, N. Donato, G. Neri, S. Saitta, Thin Solid Films 518 (2010) 7124. [3] A. Dhayal Raj, T. Pazhanivel, P. Suresh Kumar, D. Mangalaraj, D. Nataraj, N. Ponpandian, Curr. Appl. Phys. 10 (2010) 531. [4] I. Raible, M. Burghard, U. Sclect, A. Yasuda, T. Vossmeyer, Sens. Actuators B 106 (2005) 730. [5] J. Muster, G.T. Kim, V. Krstic, J.W. Park, S. Roth, M. Burghard, Adv. Mater. 12 (2000) 420. [6] N. Pinna, U. Wild, J. Urban, R. Schlógl, Adv. Mater. 15 (2003) 329.

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