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CO gas sensing performance of electrospun Co3O4 nanostructures at low operating temperature
Journal Pre-proof CO gas sensing performance of electrospun Co3 O4 nanostructures at low operating temperature C. Busacca, A. Donato, M. Lo Faro, A. Malara, G. Neri, S. Trocino
PII:
S0925-4005(19)31392-9
DOI:
https://doi.org/10.1016/j.snb.2019.127193
Reference:
SNB 127193
To appear in:
Sensors and Actuators: B. Chemical
Received Date:
4 April 2019
Revised Date:
18 September 2019
Accepted Date:
24 September 2019
Please cite this article as: Busacca C, Donato A, Lo Faro M, Malara A, Neri G, Trocino S, CO gas sensing performance of electrospun Co3 O4 nanostructures at low operating temperature, Sensors and Actuators: B. Chemical (2019), doi: https://doi.org/10.1016/j.snb.2019.127193
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CO gas sensing performance of electrospun Co3O4 nanostructures at low operating temperature C. Busaccaa, A. Donatob, M. Lo Faroa, A. Malarab, G. Neric, S. Trocinoa* a
National Council of Research - Institute for Advanced Energy Technologies (CNR-ITAE), 98126 Messina, Italy b Department of Civil, Energy, Environment and Material Engineering, Mediterranean University of Reggio Calabria, 89124 Reggio Calabria, Italy c Department of Electronic Engineering, Chemistry and Industrial Engineering, University of Messina, 98166 Messina, Italy
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Cobalt oxide nanostructures synthesized by the electrospinning method; Physico-chemical characterizations show a well-defined spinel structure; Performances as CO gas resistive sensor; Ability to detect up to 5 ppm CO at a temperature value of 100 °C; Good sensitivity, selectivity, reproducibility and stability.
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Highlights
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*Corresponding author: [email protected]
ABSTRACT
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Cobalt oxide (Co3O4) nanostructures are synthesized by the electrospinning method and their performances as CO gas sensor are investigated. Ethanol (EtOH) and N,N-Dimethylformamide
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(DMF) are selected as solvents to be separately mixed with cobalt(II) nitrate hexahydrate
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(Co(NO3)2·6H2O) and polyvinylpyrrolidone (PVP) in order to obtain two different solutions for the electrospinning and consequently two different morphologies of the electrospun materials after thermal treatment at 600°C in air. The material obtained using ethanol as a solvent was denoted as CoEt while the one obtained using N,N-Dimethylformamide was indicated as CoDMF. Physicochemical characterizations such as X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) show a well-defined spinel structure for both synthesized samples while scanning electron microscopy (SEM) confirms the presence of two different morphologies. In particular, EtOH 1
promotes the formation of a Co3O4 nanofiber morphology while DMF leads to the formation of Co3O4 in nanosheets. The influence of Co3O4 morphology on the ability to detect up to 5 ppm CO at a temperature value of 100°C is evaluated for both the synthesized samples. The CoEt shows a significant response value (R/R0) of about 2.4 as well as fast response and recovery times of 14 s and 36 s, respectively. The CoDMF exhibits a poor response value and a dynamic response slower than CoEt in the same operating conditions. Moreover, good selectivity, reproducibility and stability data are obtained for the CoEt. The enhanced sensing performances of the CoEt are attributable to the
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nanofiber morphology fibrous. Keywords: Gas sensor; Carbon monoxide, Electrospinning; Cobalt oxide; Semiconductors; Pollutants.
1. Introduction
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The need to develop more sensitive, selective and inexpensive gas sensors has directed the scientific research towards an investigation on the gas sensing properties of semiconductor metal oxides [1,2].
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These latter ones have a high sensitivity toward some relevant gases (i.e. CO, H2, and NOx), long-
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term stability and lower cost respect to other gas sensing technologies [3,4]. For these reasons, they were widely used for gas leak detection indoor and monitoring atmospheric pollutants (CO, NOx) in
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the industrial and automotive sectors [5,6]. Semiconductor metal oxides are divided into two main classes depending on the conduction mechanism of electrical charges: the n-type semiconductors
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where most of the charge carriers are the electrons and the p-type where most of the charge carriers are the holes. When an oxidizing gas (i.e. NO2 or O2) adsorbs on the surface of semiconductor metal
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oxide, an increase or a decrease in its conductivity occurs depending on whether it is a p-type or ntype semiconductor respectively. The interaction of reducing gas (i.e. CO, H2, and CH4) with the surface of semiconductor metal oxide produces a variation of its conductivity opposite with respect to the oxidizing gas. The n-type semiconductors (i.e. SnO2, ZnO, TiO2, Fe2O3, and In2O3) were extensively studied as materials for gas sensing applications [7–9]. Despite their high sensitivity, these gas sensors are not suitable for quantitative analysis due to their poor selectivity in the presence 2
of gas mixtures and low stability in humidity conditions. To overcome these limitations, gas sensors based on p-type semiconductor oxides (i.e. Cr2O3, Co3O4, NiO, CuO) are under consideration [10– 12]. Among these, cobalt oxide with spinel structure (Co3O4) is an interesting material due to its low cost, easy availability and its electrical and chemical properties which make it suitable for utilization as a resistive gas sensor. As widely reported in literature, spinel cobalt oxide (Co3O4) with different morphologies (i.e. nanowires, nanorods, nanospheres and nanocubes) exhibits good properties in the detection of some gases such as CO, NO2, H2, CH4, NH3, in a temperature range between room
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temperature and 400°C [13–16]. In particular, several studies were carried out on Co3O4-based materials as CO gas sensors. The relevant interest towards this kind of sensors is due to the high toxicity and dangerousness of the carbon monoxide, produced by an incomplete combustion process
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both in industrial and residential sectors. For example, Patil et al. [17] investigated the CO gas detection properties of Co3O4 nanorods prepared by the co-precipitation method. This material
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showed excellent ability in detecting CO up to 5 ppm at an operating temperature of 250°C. Dou et
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al. [18] achieved high performance in the detection of 50 ppm CO gas at 100°C using porous Co3O4 nanowires synthesized by hydrothermal method. Recently, Vladimirova et al. [19] synthesized Co3O4 nanocrystallines by precipitation method. Their properties in the detection of CO gas at a
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concentration between 6.7 - 20 ppm and in a temperature range of 80 - 120°C were investigated. A response value (R/R0) of about 3 as well as a response/recovery time of about 120 - 270 sec was
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obtained at 6.7 ppm and at a temperature of 100°C. In our work, spinel cobalt oxide (Co3O4) was
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synthesized by electrospinning method in order to obtain a nanofiber structure able to improve the material sensing performance. As well known, the electrospinning is a simple, versatile and inexpensive technique that allows obtaining a nanofiber structure with suitable control of the morphology and large-scale material production. The nanofiber structure, with respect to other morphologies, allows a more appropriate diffusion of the target gas thanks to the higher specific surface area [20,21]. This means that a higher sensitivity of the gas sensor occurs. To our best
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knowledge, the results showed in the present work represent improvement respect to the state of the art in terms of gas response, response - recovery time, selectivity to CO gas and long-term stability.
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2. Material and methods 2.1 Synthesis of Co3O4 nanostructures
Cobalt oxide (Co3O4) was synthesized by electrospinning method starting from a solution obtained
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by mixing 16 gr of ethanol (EtOH, Sigma-Aldrich, 99%) with 2 gr of cobalt nitrate hexahydrate (Co(NO3)2.6H2O, Sigma-Aldrich, 99%) and 2 gr of polyvinylpyrrolidone (PVP, Sigma-Aldrich, Mw
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= 1.300.000). The relative amount of polymer and metal precursor were chosen in order to obtain a
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clear solution without any precipitate suitable for electrospinning process [22]. The obtained mixture was stirred for three hours at room temperature and then was loaded into a 20 ml glass syringe for the electrospinning. The electrospinning apparatus has three basic elements: an electrical generator (high
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voltage supply), a metallic capillary connected to the generator (source electrode) and a grounded collector (target). High voltage (20 kV) was applied to create an electrically charged jet (0.707 ml/h
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injection rate) of the solution containing Co-nitrate and polymer; then the jet solidified on the
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aluminum target placed at 10 cm from the tip of the needle due to the evaporation of the solvent. During the electrospinning procedure, the temperature was maintained constant at 21°C as well as the relative humidity (RH) < 40%. The resulting deposited layer appeared as an interconnected web of small fibers. The electrospun Co(NO3)2/PVP composite material was dried for 12 h at room temperature, and then calcined in air at 600°C for 2 h to remove the polymer matrix and allow the formation of cobalt oxide powder (successively utilized to realize the sensitive films). As a comparison, Co(NO3)2/PVP composite material was prepared by the same technique under the same 4
operating conditions and using N,N-Dimethylformamide (DMF, Sigma-Aldrich, 99.8%) instead of ethanol as a solvent. The collected Co(NO3)2/PVP nanofibers was dried for 12 h at room temperature, and then calcined in air at 600°C for 2 h. The obtained samples were named CoEt and CoDMF as a function of the utilized solvent, EtOH and DMF respectively.
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2.2. Phisico-chemical characterization The thermal stability of the electrospun composite materials was analyzed by thermogravimetricdifferential scanning calorimetry (TG-DSC, Netzsch STA 409). The analyses were performed in
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static air at a heating rate of 10°C min-1 from 30°C up to 1000°C. The structural identification of the calcined samples was carried out by X-ray diffraction (XRD) using a Philips X-pert 3710 X-ray
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diffractometer and Cu-Kα radiation with = 1.540 Å, operating at 40 kV and 20 mA. The data were
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collected over the 2θ range of 10 - 90 degrees, with a step size of 0.05 degrees at a speed of 0.05 degrees per second. Diffraction peaks identification was performed on the basis of the JCPDS database of the reference compounds. The average crystallite size of CoET and CoDMF was
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determined from the Scherrer's equation. The surface composition of the calcined samples was investigated by X-ray photoelectron spectroscopy (XPS) using a Physical Electronics (PHI) 5800-01
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spectrometer equipped with a monochromatic Al Kα X-ray source at 300 W. The XPS spectra were
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obtained by using a pass energy of 58.7 eV for elemental analysis and a pressure in the analysis chamber of 10-9 Torr. The XPS instrument is equipped with a PHI Multipack library which was utilized for the identification of surface species. The peaks deconvolution analysis was carried out by using the PHI Multipak 6.1 software. Quantitative analyses were carried out by dividing the integrated peak area by atomic sensitivity factors, which were calculated from the ionization cross-section, the mean free electron escape depth and the measured transmission functions of the spectrometer. The morphology of the calcined samples was investigated by emission scanning electron microscopy 5
(SEM) using a Philips XL30 S FEG scanning electron microscope operating to an accelerating voltage of 20 kV. Transmission electron microscopy (TEM) was carried out using a FEI CM12 instrument equipped with LaB6 filament. Surface area (SABET) values were measured from the nitrogen adsorption isotherm at -196°C using an ASAP 2020 (Micromeritics Instruments) adopting a preliminary degassing procedure for the samples at 200°C until reaching a residual pressure lower than 0.01 atm. The data were elaborated in accordance with the BET method. Temperatureprogrammed reduction (TPR) measurements were performed using a conventional TPR apparatus.
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The calcined samples (10 mg) were heated at a linear rate of 10°C min-1 from 25 to 1000°C in 5%vol. H2/Ar mixture at a flow rate of 20 cc min-1. H2 consumption was monitored by a thermal conductivity detector (TCD). A molecular sieve cold trap (maintained at -80°C) and a tube filled with KOH, placed
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by injecting into the carrier a known amount of H2.
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before the TCD, were used to block water and CO2, respectively. The calibration of signals was made
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2.3. Sensing tests
The obtained powder Co3O4 by electrospinning process was dispersed in water and the sensor devices were realized by drop coating particles dispersion to deposit films (about 30 μm thick with a
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geometrical area of 3 × 3 mm2) on customized commercial devices. These latter are made of an alumina substrate (6 × 3 × 0.5 mm3), with gold (Au) parallel interdigitated screen-printed electrodes
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and a gold (Au) heater located in the backside (Fig. 1a). Moreover, the sample holder is able to realize
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a hermetic environment when inserted in the stainless-steel test chamber for the electrical characterization procedure. The operating temperature is set by biasing the gold heater; the same resistance is employed as heater and as temperature sensor in a range up to 400°C. The experimental setup for the sensor’s characterization, reported in Fig. 1b, allows carrying out measurements in controlled atmosphere. Sensing tests were carried out by varying the carbon monoxide (CO) concentration in the carrier stream making use of Bronkhorst mass flow controllers. Measurements were carried out in the temperature range from 50°C to 400°C, with steps of 50°C, under a dry air 6
total stream of 100 cm3 min−1, collecting the sensors resistance data in the four-point mode. The volume of the chamber is about 10 cm3, and the small dimensions allow a rapid set/purge of the gas concentration value in the chamber. A multimeter data acquisition unit Agilent 34970 A equipped with an Agilent 34901 A 20-channel (2 - 4 wires) multiplexer board was used for this purpose, while a dual-channel power supplier instrument Agilent E3632A was employed to bias the built-in heater of the sensor to perform measurements at super-ambient temperatures. The gas response is defined as Response = R/R0, where R is the resistance of the sensor at different concentrations of carbon
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monoxide (CO) and R0 the electrical resistance in dry air. In the case of the selectivity tests shown in Figure 14, the sensor response to oxidizing gas such as NO2 is defined as Response = R0/R. The response time is defined as the time required for the sensor to reach 90% of the saturation signal and
3. Results and discussion
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3.1 Physico-chemical characterization
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the recovery one the time needed to bring the signal back to 90% of the baseline signal.
TG-DSC analyses were carried out in order to individuate the appropriate calcination temperature to obtain the Co3O4 pure phase from the Co(NO3)2/PVP composite samples. The TG-DSC curves of
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both Co(NO3)2/PVP composites were reported in Fig. 2. The TG-DSC profiles of the Co(NO3)2/PVP obtained using ethanol (solid line in Fig. 2) showed a weight loss at about 125°C which could be
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ascribed to the removal of humidity and water molecules. A second weight loss, in the temperature
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range between 250°C - 300°C, was attributed to the decomposition of nitrate and the formation of cobalt oxide [23]. The weight loss observed at temperatures higher than 300°C was mainly ascribed to the polymer degradation [24]. A similar TG-DSC profile was observed for the sample prepared using DMF as solvent with a significant shift toward a lower temperature value for each occurred event. No further weight losses were recorded for temperature values above 600°C, this is indicative of a complete formation of pure Co3O4. Therefore, the temperature value of 600°C was considered appropriate for the calcination treatment of electrospun samples in order to obtain the Co3O4 pure 7
phase. The crystal structure of the samples calcined at 600°C was determined by XRD analysis. The XRD spectra of CoEt and CoDMF (Fig. 3) showed the typical diffraction peaks of spinel cobalt oxide (Co3O4). The crystallite size calculated by Scherrer's equation according to the (311) diffraction peak of Co3O4 was 15 nm for CoEt and 25 nm for CoDMF. No other peaks were revealed confirming the absence of phase impurities. The morphology of the synthesized samples was investigated by scanning electron microscopy (SEM). The electrospun Co(NO3)2/PVP composite material prepared using ethanol as solvent showed (Fig. 4a) a fibrous morphology consisting of smooth fibers with a
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diameter in the range of 200 - 400 nm. The morphology (Fig. 4c) of the electrospun Co(NO3)2/PVP composite material prepared using DMF as a solvent was characterized by the presence of nanofibers and polymer agglomerates incorporating cobalt nitrate. The inhomogeneity of this latter material can
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be attributed to the bead formation during the electrospinning process as well as to the slow solvent evaporation due to the high surface tension of DMF [25]. A different morphology (Fig. 4b-4d) was
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obtained for both electrospun samples after the calcination treatment at 600°C. The CoEt preserved
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a fibrous morphology in which the nanofibers are not smooth but consisting of Co3O4 nanoparticles uniformly distributed. The particles diameter was in the range of 10 - 20 nm confirming the crystallite size calculated by the Scherrer's equation. On the contrary, the CoDMF sample showed a sheet-like
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structure consisting of connected primary nanoparticles having a diameter in the range of 20 - 30 nm. This particular morphology of CoDMF could be attributed to a sintering process occurring at the high
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calcination temperature which involves, in particular, the metal nanoparticles incorporated within the
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polymer agglomerates formed during the electrospinning process [26]. TEM images reported in Fig. 5 and Fig. 6 confirmed the formation of nanofiber structure consisting of interconnected nanoparticles for the CoEt sample and sheet-like morphology for CoDMF sample respectively. The lattice fringes, showed in the higher magnification image of both synthesized samples (Fig. 5c and Fig. 6c), can be indexed to the (311) crystal plane of Co3O4 as also revealed by XRD. The particle size distribution was obtained by calculating the particle average diameter of one hundred selected particles (Fig. 5d and Fig. 6d). The peak value of the distribution curve was taken as an average particle size of about 8
15 nm for CoEt and 27 nm for CoDMF according to the values calculated by Scherrer's equation. The BET surface area measured for both electrospun samples was equal to 120 m2 g-1 and 80 m2 g-1 for CoEt and CoDMF, respectively. The obtained results are consistent with the XRD characterization data and the observed morphology of both synthesized materials. Furthermore, a pore volume of about 0.125 cm3 g-1 and 0.045 cm3 g-1 was calculated for CoEt and CoDMF respectively, indicative of a high porosity structure. H2-TPR measurements were carried out in order to study the reducibility of the electrospun samples. The H2-TPR profiles (Fig. 7) of both samples were characterized by a
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broad and asymmetric peak with a shoulder at lower reduction temperature that is indicative of two reduction reactions. The first reaction is due to the reduction of Co3O4 to CoO at lower temperatures and the second one is the subsequent reduction reaction from CoO to metallic Co at higher
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temperatures [27]. The CoEt profile showed a lower onset of reduction temperatures respect to CoDMF indicating that the CoEt was characterized by a higher surface area and easily reducible sites
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thanks to a weaker Co-O bond strength [28]. Moreover, the shift to lower reduction temperatures
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observed for the CoEt could be attributed to the presence of a larger number of oxygen vacancies respect to CoDMF. As is known, this means higher oxygen mobility in the oxide which promotes the Co3O4 catalytic activity for CO oxidation [29]. A further confirmation of the greater amount of oxygen
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vacancies in CoEt than CoDMF was provided by XPS spectra in the O1s binding energy region reported in Fig. 8b and Fig. 8d respectively. The deconvolution of O1s spectra revealed for both
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electrospun samples the presence of three peaks centered at 529.8, 531.4 and 532.6 eV that can be
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ascribed to the lattice O2- species (Co-O bonds), to the O2- ions in oxygen-deficient regions within the matrix of Co3O4 (oxygen defects or vacancies) and to chemisorbed and dissociated oxygen species (O2−, O2−, or O−), respectively [30–32]. The relative amount of oxygen species was reported in Table 1 for both the investigated samples. The percentage of oxygen vacancies was calculated by the curve fitting of O1s XPS spectrum and it corresponds to the underlying area of the relative deconvolution peak at 531.4 eV. A larger amount of oxygen vacancies was present on the CoEt than the CoDMF highlighting a more density of lattice defects and confirming characterization data 9
previously reported. In Fig. 8a and Fig. 8c were shown the XPS spectra of CoEt and CoDMF in the Co2p binding energy region for the CoEt and CoDMF respectively. Two main peaks were present at binding energy value of about 780 eV (Co2p3/2) and 795 eV (Co2p1/2) and the respective satellites at about 789 eV and 805 eV in the spectra of both electrospun samples. The deconvolution of Co2p spectra revealed the coexistence of both Co2+ and Co3+ species due to the peaks at 779.7 and 794.5 eV, 781.1 and 797.2 eV, respectively. The binding energy difference between the Co2p1/2 and Co2p3/2 peaks is about 15 eV in agreement with the value reported in the literature for Co3O4. Moreover, the
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shake-up satellites were at a binding energy of 9 eV from the main spin-orbit components indicating only the presence of Co3O4 and not CoO [33].
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3.2 Sensing tests
In order to evaluate the properties of both electrospun Co3O4 samples in the detection of CO, sensing
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tests were conducted in the temperature range between 100 and 400°C by using a CO gas
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concentration of 5 ppm (Fig. 9). The CoEt showed a CO gas response (R/R0) of about 2.4 at 100°C that decreases with the temperature up to be irrelevant at 300°C due to a gas saturation. Moreover, a single measurement (Fig. 10a) on CoEt sensor, was carried out at 50°C showing a very long recovery
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time due to an incomplete desorption of CO. On the contrary, the CoDMF sample didn’t provide any response towards CO gas in the range temperature investigated, in fact, the resistance in carbon
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monoxide environment is comparable to the resistance in dry air. Moreover, the CoEt showed a
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response time of about 14 s and a recovery time of about 36 s at 100°C (Fig. 10b) while the CoDMF was characterized by a slow dynamic response. On the other hand, the resistive response at 150°C was slightly lower (R/R0 = 1.8) than 100°C while the dynamic response was similar. The CO detection limit at 100°C was evaluated for both CoEt and CoDMF by exposing them to CO pulses in the concentration range of 5 - 40 ppm (Fig. 11). The CoDMF gas response was characterized by an irregular trend whereas the CoEt was more reliable and showed an increasing and reversible gas response in the concentration range investigated. A large difference in resistance between CoEt and 10
CoDMF is also evident in Fig. 11, this can be due to a larger amount of O2- species adsorbed on the CoEt surface (reported as oxygen vacancies by XPS data) that take electrons from the oxide surface with a consequent increase of charge carriers (holes) and conductivity. Nevertheless, a saturation effect is evident for the CoEt at concentration values higher than 20 ppm (Fig. 12) probably due to the formation of carbonates species at a low temperature which produces a deactivation of Co3O4 catalysts in the CO oxidation [15]. The CO gas sensing performances of electrospun CoEt sample can be attributed to the fibrous structure, composed by nanoparticles uniformly distributed, producing of
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a large accessible surface area as well as a high porosity favorable the diffusion of the target gas [16,20,21]. As known, the sensitivity of metal-oxide gas sensors to gases depends not only on the porosity of materials but also on the grain size [34]. Rothschild et al. [35] stated that the sensor’s
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response signal increases with decreasing the grain size of sensor material when the grain size is below about 20 nm. As reported by physico-chemical characterization data, the CoEt crystallite size
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is about 15 nm therefore it is possible to attribute the CO gas performances of the CoEt also to the
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crystallites size. Furthermore, a large number of oxygen defects or vacancies on the CoEt surface, as reported by XPS characterization data, is responsible for more active site availability and enhancement of the Co3O4 catalytic activity towards CO oxidation according to the Mars-van-
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Krevelen type mechanism [15]. A further investigation of the CO gas sensing properties of CoEt was the reproducibility and stability. These important parameters were evaluated by repeating the test at
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100°C with a CO concentration of 5 ppm for 10 times. As shown in Fig. 13, the CoEt gas response
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in different tests was approximately constant (R/R0 = 2.4) indicating the repeatability of the sensor. The stability test (Fig. 14) was carried out at 100°C with a 5 ppm of CO, after 30 days the sensor response was comparable with the initial one (zero day). This test was achieved with an averaging of CO response in about 2 hours per day. Finally, the selectivity to CO was measured by exposing the CoEt sample to the same concentration (5 ppm) of different gas as NO2, C2H6O and H2 at 100°C. The obtained results (Fig. 15) indicated that the response of CoEt to these gases was more than one order of magnitude lower than the CO gas response. In comparison with the state of the art (Table 2), in 11
this work a good gas response and a very fast dynamic response were obtained for the detection of very low concentrations of CO at low temperature. Therefore, the CoEt nanofibers can be used to develop solid-state sensors for monitoring the CO presence at ppm levels in the air.
4. Conclusions Spinel cobalt oxide (Co3O4) was synthesized by electrospinning method in order to obtain a nanofiber structure able to improve the material sensing performance. As a comparison, the composite material
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was prepared by the same technique under the same operating conditions and using a different solvent. The CO gas sensing performances of both electrospun samples were investigated at different CO concentrations and temperatures. CoEt showed, at low concentrations of CO and at low
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temperature, good resistive response and a very short response - recovery time in comparison of the state of the art. In the same operating conditions, the CoDMF didn’t provide any appreciable response.
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The better CO sensing performances of CoEt can be attributed to the nanofiber morphology
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characterized by high surface area as well as a large number of oxygen defects or vacancies on the CoEt surface responsible of more active site availability. Furthermore, the results concerning the repeatability, stability and selectivity to the CO gas in different tests showed the reliability of CoEt
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nanofibers for the detection of CO at very low concentrations (5 ppm) at low temperature (100°C) with a very fast dynamic response (very short response and recovery time). For these reasons, this
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approach can be a fundamental step to developing at a pre-commercial level a CO resistive gas sensor.
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Biographies Concetta Busacca received the Master Degree in Industrial Chemistry in 2004 and Ph.D. Degree (in Materials Chemistry) in 2009 from University of Catania and University of Reggio Calabria respectively. Her research interests are focused on materials development for redox batteries and gas sensors. Actually, she is working in National Council of Research - Institute for Advanced Energy Technologies (CNR-ITAE) in Messina. She is author of 21 papers (h-index 9) on international journal (Scopus and Web of Science).
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Andrea Donato was graduated in Industrial Chemistry in 1981. Since 2001 he is Full Professor of Chemistry at the Department of Mechanical and Materials of the University of Reggio Calabria. From 2008 he is member of the Doctorate in “Geotechnical engineering and Chemistry of the Materials” at the Faculty of Engineering of the “Mediterranea” University of Reggio Calabria. Prof. Donato is author of more than 50 scientific publications in international journals and around 100 communication to national and international congresses. His research interests are devoted to synthesis and application of nanostructured materials for gas sensors applications.
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Massimiliano Lo Faro, in 2013, was awarded a degree in Industrial Chemistry at the University of Catania. After being awarded his BA degree, he attended a Master on “Design and Testing of fuel cells electrical propulsion equipment for sustainable mobility” at the CNR – Advanced Energy Technology Institute “Nicola Giordano” (ITAE). In March 2008, he was awarded a PhD in Environmental and Energy Materials at the University of Roma TorVergata. Currently, he is a permanent researcher at the National Research Council (CNR) and carries out the research activity on the development of materials and components for solid oxide fuel cells (SOFC) operating at intermediate temperatures (600-800 °C). He took part in various national and international projects, as well as private partnership contracts, and he also is the referee for many science magazines and sector science projects.
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Angela Malara received the Master Degree in Materials Engineering and Ph.D. Degree (in Materials Chemistry) from University of Messina and University of Reggio Calabria respectively. Her research interests are focused on materials development for gas sensors. Actually, she is working in “Mediterranea” University of Reggio Calabria. She is author of 17 papers (h-index 5) on international journal (Scopus and Web of Science).
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Giovanni Neri was born in Reggio Calabria, Italy, in 1956. He received the M.S. degree in chemistry from the University of Messina, Messina, Italy, in 1980. From 2004 to 2007, he was the Head of the Department of Industrial Chemistry and Materials Engineering, University of Messina. He was a Visiting Scientist/Professor with the University of Michigan, Ann Arbor, MI, USA, and also with the University of Alagappa, Karaikudi, India. Since 2002, he has been a Full Professor of chemistry with the University of Messina. He has authored over 200 papers on international journals, cited over 6600 times and h-index = 46 (source: ISI Web of Science). His current research interests include the synthesis and characterization of nanostructured materials for chemical sensors, applied in a wide range of sectors from medical diagnostics to automotive, industrial processes, and environmental control. Stefano Trocino received the Master Degree in Electronic Engineering in 2010 and Ph.D. Degree (in Materials Chemistry) in 2014 from University of Messina and University of Reggio Calabria respectively. His research interests are focused on electrochemical characterization of Solid Oxide Fuel Cell, photo-electrolyzers, electrolyzers, high temperature batteries (metal-air), development of hardware and software for electrochemical systems, semiconductor gas sensor characterization and modelling, development of measurement system for sensors, characterization of electronic devices up to microwave range. He is a developer in LabView environment. Actually, he is working in 17
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National Council of Research - Institute for Advanced Energy Technologies (CNR-ITAE) in Messina. He is author of 25 papers (h-index 9) on international journal (Scopus and Web of Science).
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Figure captions Table 1. Functional groups resulting from curve fitting of O1s spectra (%). Table 2. A comparison between the CO gas sensing properties of home-made electrospun Co3O4 nanofibers in this work and different Co3O4-based materials reported in literature at different temperatures and CO concentrations. Fig. 1. Schematic representation of the holder and the sensor (a); scheme of the experimental setup
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(b). Fig. 2. TG-DSC profiles of electrospun Co(NO3)2/PVP composite samples obtained using EtOH as solvent (solid line) and DMF as solvent (dotted line).
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Fig. 3. XRD diffraction patterns comparison among a) CoEt; b) CoDMF.
Fig. 4. SEM images of a) Co(NO3)2/PVP composite obtained using EtOH as solvent; b) CoEt; c)
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Co(NO3)2/PVP composite obtained using DMF as solvent; d) CoDMF.
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Fig. 5. TEM images ((a), (b), (c)) and particle size distribution (d) of CoEt. Fig. 6. TEM images ((a), (b), (c)) and particle size distribution (d) of CoDMF. Fig. 7. H2-TPR profile of a) Co(NO3)2-PVP composite obtained using EtOH as solvent; b) Co(NO3)2-
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PVP composite obtained using DMF as solvent.
Fig. 8. XPS spectra of (a), (c) Co2p and (b), (d) O1s for CoEt and CoDMF sample.
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Fig. 9. Response of the two sensors to 5 ppm of CO as a function of temperature.
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Fig. 10. Response and recovery time of CoET to 5 ppm of CO at 50°C (a) and 100°C (b). Fig. 11. Response of the different gas sensors to different concentrations of CO at 100°C. Fig. 12. Calibration curves of the different CO gas sensors at 100°C. Fig. 13. Repetition response curves of the CoEt to 5 ppm of CO at 100°C. Fig. 14. Long-term stability of CoEt to 5 ppm of CO at 100°C. Fig. 15. Response of the CoEt to 5 ppm of different gases at 100°C.
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Fig. 2.
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* Co3O4 [JCPDS No:01-080-1533] Intensity (a.u.)
* (a)
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10
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40
50 60 70 2 (degree)
80
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Fig. 4.
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size / nm Particle (nm) diameter Particle
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Fig. 6.
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H2 uptake (a.u)
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Temperature (°C)
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(b)
Co2p3/2
Intensity (a.u.)
Co2p1/2
800 795 790 785 Binding Energy (eV)
Intensity (a.u.)
(c)
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(d)
Co2p3/2 Co2p1/2
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536 535 534 533 532 531 530 529 528 527 526 Binding Energy (eV)
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800 795 790 785 Binding Energy (eV)
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Intensity (a.u.)
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536 535 534 533 532 531 530 529 528 527 526 Binding Energy (eV)
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Intensity (a.u.)
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Fig. 8.
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3.0 5 ppm CO
CoDMF CoEt
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1.0 100
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Response (R/R0)
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Working Temperature (°C)
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Fig. 10.
30
20k
100°C C T == 100
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CoDMF CoEt
16k
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20 ppm CO
5 ppm CO 10 ppm CO
10k
40 ppm CO
20 ppm CO
5 ppm CO
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Resistance (k)
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40 ppm CO
10 ppm CO
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Fig. 12.
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Fig. 13.
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5 T = 100°C 5 ppm CO
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Response (R/R0)
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T = 100 °C
CoEt
3 2 1
-1 -2 -3 -4 CO
C2H6O C2H6O
NO22 NO
Gas
H2 H2
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Fig. 15.
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Response (R0/R)
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Samples
Table 1.
CoEt CoDMF
Chemisorbed oxygen species (%) (532.6 eV) 7 6
Co-O (%) Ovac. (%) (529.8 eV) (531.4 eV) 53 62
40 32
Table 2. Gas respons e (R/R0)
Respons e time (s)
Recover y Time (s)
Operating temperatu re (°C)
CO Concentra tion (ppm)
Electrospinning
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100
5
6.5
3
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250
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13
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3.5
120
270
100
6.7
Coprecipitation Hydrothermal Coprecipitation
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Co3O4 nanofibers (this work) Porous Co3O4 nanorods [15] Co3O4 nanowires [20] Co3O4 nanocrystalline [21]
Preparation method
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CO gas sensing materials
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PII:
S0925-4005(19)31392-9
DOI:
https://doi.org/10.1016/j.snb.2019.127193
Reference:
SNB 127193
To appear in:
Sensors and Actuators: B. Chemical
Received Date:
4 April 2019
Revised Date:
18 September 2019
Accepted Date:
24 September 2019
Please cite this article as: Busacca C, Donato A, Lo Faro M, Malara A, Neri G, Trocino S, CO gas sensing performance of electrospun Co3 O4 nanostructures at low operating temperature, Sensors and Actuators: B. Chemical (2019), doi: https://doi.org/10.1016/j.snb.2019.127193
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CO gas sensing performance of electrospun Co3O4 nanostructures at low operating temperature C. Busaccaa, A. Donatob, M. Lo Faroa, A. Malarab, G. Neric, S. Trocinoa* a
National Council of Research - Institute for Advanced Energy Technologies (CNR-ITAE), 98126 Messina, Italy b Department of Civil, Energy, Environment and Material Engineering, Mediterranean University of Reggio Calabria, 89124 Reggio Calabria, Italy c Department of Electronic Engineering, Chemistry and Industrial Engineering, University of Messina, 98166 Messina, Italy
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Cobalt oxide nanostructures synthesized by the electrospinning method; Physico-chemical characterizations show a well-defined spinel structure; Performances as CO gas resistive sensor; Ability to detect up to 5 ppm CO at a temperature value of 100 °C; Good sensitivity, selectivity, reproducibility and stability.
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Highlights
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*Corresponding author: [email protected]
ABSTRACT
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Cobalt oxide (Co3O4) nanostructures are synthesized by the electrospinning method and their performances as CO gas sensor are investigated. Ethanol (EtOH) and N,N-Dimethylformamide
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(DMF) are selected as solvents to be separately mixed with cobalt(II) nitrate hexahydrate
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(Co(NO3)2·6H2O) and polyvinylpyrrolidone (PVP) in order to obtain two different solutions for the electrospinning and consequently two different morphologies of the electrospun materials after thermal treatment at 600°C in air. The material obtained using ethanol as a solvent was denoted as CoEt while the one obtained using N,N-Dimethylformamide was indicated as CoDMF. Physicochemical characterizations such as X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) show a well-defined spinel structure for both synthesized samples while scanning electron microscopy (SEM) confirms the presence of two different morphologies. In particular, EtOH 1
promotes the formation of a Co3O4 nanofiber morphology while DMF leads to the formation of Co3O4 in nanosheets. The influence of Co3O4 morphology on the ability to detect up to 5 ppm CO at a temperature value of 100°C is evaluated for both the synthesized samples. The CoEt shows a significant response value (R/R0) of about 2.4 as well as fast response and recovery times of 14 s and 36 s, respectively. The CoDMF exhibits a poor response value and a dynamic response slower than CoEt in the same operating conditions. Moreover, good selectivity, reproducibility and stability data are obtained for the CoEt. The enhanced sensing performances of the CoEt are attributable to the
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nanofiber morphology fibrous. Keywords: Gas sensor; Carbon monoxide, Electrospinning; Cobalt oxide; Semiconductors; Pollutants.
1. Introduction
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The need to develop more sensitive, selective and inexpensive gas sensors has directed the scientific research towards an investigation on the gas sensing properties of semiconductor metal oxides [1,2].
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These latter ones have a high sensitivity toward some relevant gases (i.e. CO, H2, and NOx), long-
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term stability and lower cost respect to other gas sensing technologies [3,4]. For these reasons, they were widely used for gas leak detection indoor and monitoring atmospheric pollutants (CO, NOx) in
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the industrial and automotive sectors [5,6]. Semiconductor metal oxides are divided into two main classes depending on the conduction mechanism of electrical charges: the n-type semiconductors
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where most of the charge carriers are the electrons and the p-type where most of the charge carriers are the holes. When an oxidizing gas (i.e. NO2 or O2) adsorbs on the surface of semiconductor metal
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oxide, an increase or a decrease in its conductivity occurs depending on whether it is a p-type or ntype semiconductor respectively. The interaction of reducing gas (i.e. CO, H2, and CH4) with the surface of semiconductor metal oxide produces a variation of its conductivity opposite with respect to the oxidizing gas. The n-type semiconductors (i.e. SnO2, ZnO, TiO2, Fe2O3, and In2O3) were extensively studied as materials for gas sensing applications [7–9]. Despite their high sensitivity, these gas sensors are not suitable for quantitative analysis due to their poor selectivity in the presence 2
of gas mixtures and low stability in humidity conditions. To overcome these limitations, gas sensors based on p-type semiconductor oxides (i.e. Cr2O3, Co3O4, NiO, CuO) are under consideration [10– 12]. Among these, cobalt oxide with spinel structure (Co3O4) is an interesting material due to its low cost, easy availability and its electrical and chemical properties which make it suitable for utilization as a resistive gas sensor. As widely reported in literature, spinel cobalt oxide (Co3O4) with different morphologies (i.e. nanowires, nanorods, nanospheres and nanocubes) exhibits good properties in the detection of some gases such as CO, NO2, H2, CH4, NH3, in a temperature range between room
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temperature and 400°C [13–16]. In particular, several studies were carried out on Co3O4-based materials as CO gas sensors. The relevant interest towards this kind of sensors is due to the high toxicity and dangerousness of the carbon monoxide, produced by an incomplete combustion process
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both in industrial and residential sectors. For example, Patil et al. [17] investigated the CO gas detection properties of Co3O4 nanorods prepared by the co-precipitation method. This material
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showed excellent ability in detecting CO up to 5 ppm at an operating temperature of 250°C. Dou et
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al. [18] achieved high performance in the detection of 50 ppm CO gas at 100°C using porous Co3O4 nanowires synthesized by hydrothermal method. Recently, Vladimirova et al. [19] synthesized Co3O4 nanocrystallines by precipitation method. Their properties in the detection of CO gas at a
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concentration between 6.7 - 20 ppm and in a temperature range of 80 - 120°C were investigated. A response value (R/R0) of about 3 as well as a response/recovery time of about 120 - 270 sec was
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obtained at 6.7 ppm and at a temperature of 100°C. In our work, spinel cobalt oxide (Co3O4) was
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synthesized by electrospinning method in order to obtain a nanofiber structure able to improve the material sensing performance. As well known, the electrospinning is a simple, versatile and inexpensive technique that allows obtaining a nanofiber structure with suitable control of the morphology and large-scale material production. The nanofiber structure, with respect to other morphologies, allows a more appropriate diffusion of the target gas thanks to the higher specific surface area [20,21]. This means that a higher sensitivity of the gas sensor occurs. To our best
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knowledge, the results showed in the present work represent improvement respect to the state of the art in terms of gas response, response - recovery time, selectivity to CO gas and long-term stability.
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2. Material and methods 2.1 Synthesis of Co3O4 nanostructures
Cobalt oxide (Co3O4) was synthesized by electrospinning method starting from a solution obtained
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by mixing 16 gr of ethanol (EtOH, Sigma-Aldrich, 99%) with 2 gr of cobalt nitrate hexahydrate (Co(NO3)2.6H2O, Sigma-Aldrich, 99%) and 2 gr of polyvinylpyrrolidone (PVP, Sigma-Aldrich, Mw
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= 1.300.000). The relative amount of polymer and metal precursor were chosen in order to obtain a
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clear solution without any precipitate suitable for electrospinning process [22]. The obtained mixture was stirred for three hours at room temperature and then was loaded into a 20 ml glass syringe for the electrospinning. The electrospinning apparatus has three basic elements: an electrical generator (high
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voltage supply), a metallic capillary connected to the generator (source electrode) and a grounded collector (target). High voltage (20 kV) was applied to create an electrically charged jet (0.707 ml/h
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injection rate) of the solution containing Co-nitrate and polymer; then the jet solidified on the
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aluminum target placed at 10 cm from the tip of the needle due to the evaporation of the solvent. During the electrospinning procedure, the temperature was maintained constant at 21°C as well as the relative humidity (RH) < 40%. The resulting deposited layer appeared as an interconnected web of small fibers. The electrospun Co(NO3)2/PVP composite material was dried for 12 h at room temperature, and then calcined in air at 600°C for 2 h to remove the polymer matrix and allow the formation of cobalt oxide powder (successively utilized to realize the sensitive films). As a comparison, Co(NO3)2/PVP composite material was prepared by the same technique under the same 4
operating conditions and using N,N-Dimethylformamide (DMF, Sigma-Aldrich, 99.8%) instead of ethanol as a solvent. The collected Co(NO3)2/PVP nanofibers was dried for 12 h at room temperature, and then calcined in air at 600°C for 2 h. The obtained samples were named CoEt and CoDMF as a function of the utilized solvent, EtOH and DMF respectively.
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2.2. Phisico-chemical characterization The thermal stability of the electrospun composite materials was analyzed by thermogravimetricdifferential scanning calorimetry (TG-DSC, Netzsch STA 409). The analyses were performed in
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static air at a heating rate of 10°C min-1 from 30°C up to 1000°C. The structural identification of the calcined samples was carried out by X-ray diffraction (XRD) using a Philips X-pert 3710 X-ray
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diffractometer and Cu-Kα radiation with = 1.540 Å, operating at 40 kV and 20 mA. The data were
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collected over the 2θ range of 10 - 90 degrees, with a step size of 0.05 degrees at a speed of 0.05 degrees per second. Diffraction peaks identification was performed on the basis of the JCPDS database of the reference compounds. The average crystallite size of CoET and CoDMF was
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determined from the Scherrer's equation. The surface composition of the calcined samples was investigated by X-ray photoelectron spectroscopy (XPS) using a Physical Electronics (PHI) 5800-01
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spectrometer equipped with a monochromatic Al Kα X-ray source at 300 W. The XPS spectra were
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obtained by using a pass energy of 58.7 eV for elemental analysis and a pressure in the analysis chamber of 10-9 Torr. The XPS instrument is equipped with a PHI Multipack library which was utilized for the identification of surface species. The peaks deconvolution analysis was carried out by using the PHI Multipak 6.1 software. Quantitative analyses were carried out by dividing the integrated peak area by atomic sensitivity factors, which were calculated from the ionization cross-section, the mean free electron escape depth and the measured transmission functions of the spectrometer. The morphology of the calcined samples was investigated by emission scanning electron microscopy 5
(SEM) using a Philips XL30 S FEG scanning electron microscope operating to an accelerating voltage of 20 kV. Transmission electron microscopy (TEM) was carried out using a FEI CM12 instrument equipped with LaB6 filament. Surface area (SABET) values were measured from the nitrogen adsorption isotherm at -196°C using an ASAP 2020 (Micromeritics Instruments) adopting a preliminary degassing procedure for the samples at 200°C until reaching a residual pressure lower than 0.01 atm. The data were elaborated in accordance with the BET method. Temperatureprogrammed reduction (TPR) measurements were performed using a conventional TPR apparatus.
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The calcined samples (10 mg) were heated at a linear rate of 10°C min-1 from 25 to 1000°C in 5%vol. H2/Ar mixture at a flow rate of 20 cc min-1. H2 consumption was monitored by a thermal conductivity detector (TCD). A molecular sieve cold trap (maintained at -80°C) and a tube filled with KOH, placed
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by injecting into the carrier a known amount of H2.
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before the TCD, were used to block water and CO2, respectively. The calibration of signals was made
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2.3. Sensing tests
The obtained powder Co3O4 by electrospinning process was dispersed in water and the sensor devices were realized by drop coating particles dispersion to deposit films (about 30 μm thick with a
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geometrical area of 3 × 3 mm2) on customized commercial devices. These latter are made of an alumina substrate (6 × 3 × 0.5 mm3), with gold (Au) parallel interdigitated screen-printed electrodes
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and a gold (Au) heater located in the backside (Fig. 1a). Moreover, the sample holder is able to realize
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a hermetic environment when inserted in the stainless-steel test chamber for the electrical characterization procedure. The operating temperature is set by biasing the gold heater; the same resistance is employed as heater and as temperature sensor in a range up to 400°C. The experimental setup for the sensor’s characterization, reported in Fig. 1b, allows carrying out measurements in controlled atmosphere. Sensing tests were carried out by varying the carbon monoxide (CO) concentration in the carrier stream making use of Bronkhorst mass flow controllers. Measurements were carried out in the temperature range from 50°C to 400°C, with steps of 50°C, under a dry air 6
total stream of 100 cm3 min−1, collecting the sensors resistance data in the four-point mode. The volume of the chamber is about 10 cm3, and the small dimensions allow a rapid set/purge of the gas concentration value in the chamber. A multimeter data acquisition unit Agilent 34970 A equipped with an Agilent 34901 A 20-channel (2 - 4 wires) multiplexer board was used for this purpose, while a dual-channel power supplier instrument Agilent E3632A was employed to bias the built-in heater of the sensor to perform measurements at super-ambient temperatures. The gas response is defined as Response = R/R0, where R is the resistance of the sensor at different concentrations of carbon
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monoxide (CO) and R0 the electrical resistance in dry air. In the case of the selectivity tests shown in Figure 14, the sensor response to oxidizing gas such as NO2 is defined as Response = R0/R. The response time is defined as the time required for the sensor to reach 90% of the saturation signal and
3. Results and discussion
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3.1 Physico-chemical characterization
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the recovery one the time needed to bring the signal back to 90% of the baseline signal.
TG-DSC analyses were carried out in order to individuate the appropriate calcination temperature to obtain the Co3O4 pure phase from the Co(NO3)2/PVP composite samples. The TG-DSC curves of
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both Co(NO3)2/PVP composites were reported in Fig. 2. The TG-DSC profiles of the Co(NO3)2/PVP obtained using ethanol (solid line in Fig. 2) showed a weight loss at about 125°C which could be
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ascribed to the removal of humidity and water molecules. A second weight loss, in the temperature
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range between 250°C - 300°C, was attributed to the decomposition of nitrate and the formation of cobalt oxide [23]. The weight loss observed at temperatures higher than 300°C was mainly ascribed to the polymer degradation [24]. A similar TG-DSC profile was observed for the sample prepared using DMF as solvent with a significant shift toward a lower temperature value for each occurred event. No further weight losses were recorded for temperature values above 600°C, this is indicative of a complete formation of pure Co3O4. Therefore, the temperature value of 600°C was considered appropriate for the calcination treatment of electrospun samples in order to obtain the Co3O4 pure 7
phase. The crystal structure of the samples calcined at 600°C was determined by XRD analysis. The XRD spectra of CoEt and CoDMF (Fig. 3) showed the typical diffraction peaks of spinel cobalt oxide (Co3O4). The crystallite size calculated by Scherrer's equation according to the (311) diffraction peak of Co3O4 was 15 nm for CoEt and 25 nm for CoDMF. No other peaks were revealed confirming the absence of phase impurities. The morphology of the synthesized samples was investigated by scanning electron microscopy (SEM). The electrospun Co(NO3)2/PVP composite material prepared using ethanol as solvent showed (Fig. 4a) a fibrous morphology consisting of smooth fibers with a
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diameter in the range of 200 - 400 nm. The morphology (Fig. 4c) of the electrospun Co(NO3)2/PVP composite material prepared using DMF as a solvent was characterized by the presence of nanofibers and polymer agglomerates incorporating cobalt nitrate. The inhomogeneity of this latter material can
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be attributed to the bead formation during the electrospinning process as well as to the slow solvent evaporation due to the high surface tension of DMF [25]. A different morphology (Fig. 4b-4d) was
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obtained for both electrospun samples after the calcination treatment at 600°C. The CoEt preserved
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a fibrous morphology in which the nanofibers are not smooth but consisting of Co3O4 nanoparticles uniformly distributed. The particles diameter was in the range of 10 - 20 nm confirming the crystallite size calculated by the Scherrer's equation. On the contrary, the CoDMF sample showed a sheet-like
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structure consisting of connected primary nanoparticles having a diameter in the range of 20 - 30 nm. This particular morphology of CoDMF could be attributed to a sintering process occurring at the high
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calcination temperature which involves, in particular, the metal nanoparticles incorporated within the
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polymer agglomerates formed during the electrospinning process [26]. TEM images reported in Fig. 5 and Fig. 6 confirmed the formation of nanofiber structure consisting of interconnected nanoparticles for the CoEt sample and sheet-like morphology for CoDMF sample respectively. The lattice fringes, showed in the higher magnification image of both synthesized samples (Fig. 5c and Fig. 6c), can be indexed to the (311) crystal plane of Co3O4 as also revealed by XRD. The particle size distribution was obtained by calculating the particle average diameter of one hundred selected particles (Fig. 5d and Fig. 6d). The peak value of the distribution curve was taken as an average particle size of about 8
15 nm for CoEt and 27 nm for CoDMF according to the values calculated by Scherrer's equation. The BET surface area measured for both electrospun samples was equal to 120 m2 g-1 and 80 m2 g-1 for CoEt and CoDMF, respectively. The obtained results are consistent with the XRD characterization data and the observed morphology of both synthesized materials. Furthermore, a pore volume of about 0.125 cm3 g-1 and 0.045 cm3 g-1 was calculated for CoEt and CoDMF respectively, indicative of a high porosity structure. H2-TPR measurements were carried out in order to study the reducibility of the electrospun samples. The H2-TPR profiles (Fig. 7) of both samples were characterized by a
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broad and asymmetric peak with a shoulder at lower reduction temperature that is indicative of two reduction reactions. The first reaction is due to the reduction of Co3O4 to CoO at lower temperatures and the second one is the subsequent reduction reaction from CoO to metallic Co at higher
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temperatures [27]. The CoEt profile showed a lower onset of reduction temperatures respect to CoDMF indicating that the CoEt was characterized by a higher surface area and easily reducible sites
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thanks to a weaker Co-O bond strength [28]. Moreover, the shift to lower reduction temperatures
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observed for the CoEt could be attributed to the presence of a larger number of oxygen vacancies respect to CoDMF. As is known, this means higher oxygen mobility in the oxide which promotes the Co3O4 catalytic activity for CO oxidation [29]. A further confirmation of the greater amount of oxygen
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vacancies in CoEt than CoDMF was provided by XPS spectra in the O1s binding energy region reported in Fig. 8b and Fig. 8d respectively. The deconvolution of O1s spectra revealed for both
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electrospun samples the presence of three peaks centered at 529.8, 531.4 and 532.6 eV that can be
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ascribed to the lattice O2- species (Co-O bonds), to the O2- ions in oxygen-deficient regions within the matrix of Co3O4 (oxygen defects or vacancies) and to chemisorbed and dissociated oxygen species (O2−, O2−, or O−), respectively [30–32]. The relative amount of oxygen species was reported in Table 1 for both the investigated samples. The percentage of oxygen vacancies was calculated by the curve fitting of O1s XPS spectrum and it corresponds to the underlying area of the relative deconvolution peak at 531.4 eV. A larger amount of oxygen vacancies was present on the CoEt than the CoDMF highlighting a more density of lattice defects and confirming characterization data 9
previously reported. In Fig. 8a and Fig. 8c were shown the XPS spectra of CoEt and CoDMF in the Co2p binding energy region for the CoEt and CoDMF respectively. Two main peaks were present at binding energy value of about 780 eV (Co2p3/2) and 795 eV (Co2p1/2) and the respective satellites at about 789 eV and 805 eV in the spectra of both electrospun samples. The deconvolution of Co2p spectra revealed the coexistence of both Co2+ and Co3+ species due to the peaks at 779.7 and 794.5 eV, 781.1 and 797.2 eV, respectively. The binding energy difference between the Co2p1/2 and Co2p3/2 peaks is about 15 eV in agreement with the value reported in the literature for Co3O4. Moreover, the
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shake-up satellites were at a binding energy of 9 eV from the main spin-orbit components indicating only the presence of Co3O4 and not CoO [33].
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3.2 Sensing tests
In order to evaluate the properties of both electrospun Co3O4 samples in the detection of CO, sensing
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tests were conducted in the temperature range between 100 and 400°C by using a CO gas
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concentration of 5 ppm (Fig. 9). The CoEt showed a CO gas response (R/R0) of about 2.4 at 100°C that decreases with the temperature up to be irrelevant at 300°C due to a gas saturation. Moreover, a single measurement (Fig. 10a) on CoEt sensor, was carried out at 50°C showing a very long recovery
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time due to an incomplete desorption of CO. On the contrary, the CoDMF sample didn’t provide any response towards CO gas in the range temperature investigated, in fact, the resistance in carbon
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monoxide environment is comparable to the resistance in dry air. Moreover, the CoEt showed a
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response time of about 14 s and a recovery time of about 36 s at 100°C (Fig. 10b) while the CoDMF was characterized by a slow dynamic response. On the other hand, the resistive response at 150°C was slightly lower (R/R0 = 1.8) than 100°C while the dynamic response was similar. The CO detection limit at 100°C was evaluated for both CoEt and CoDMF by exposing them to CO pulses in the concentration range of 5 - 40 ppm (Fig. 11). The CoDMF gas response was characterized by an irregular trend whereas the CoEt was more reliable and showed an increasing and reversible gas response in the concentration range investigated. A large difference in resistance between CoEt and 10
CoDMF is also evident in Fig. 11, this can be due to a larger amount of O2- species adsorbed on the CoEt surface (reported as oxygen vacancies by XPS data) that take electrons from the oxide surface with a consequent increase of charge carriers (holes) and conductivity. Nevertheless, a saturation effect is evident for the CoEt at concentration values higher than 20 ppm (Fig. 12) probably due to the formation of carbonates species at a low temperature which produces a deactivation of Co3O4 catalysts in the CO oxidation [15]. The CO gas sensing performances of electrospun CoEt sample can be attributed to the fibrous structure, composed by nanoparticles uniformly distributed, producing of
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a large accessible surface area as well as a high porosity favorable the diffusion of the target gas [16,20,21]. As known, the sensitivity of metal-oxide gas sensors to gases depends not only on the porosity of materials but also on the grain size [34]. Rothschild et al. [35] stated that the sensor’s
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response signal increases with decreasing the grain size of sensor material when the grain size is below about 20 nm. As reported by physico-chemical characterization data, the CoEt crystallite size
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is about 15 nm therefore it is possible to attribute the CO gas performances of the CoEt also to the
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crystallites size. Furthermore, a large number of oxygen defects or vacancies on the CoEt surface, as reported by XPS characterization data, is responsible for more active site availability and enhancement of the Co3O4 catalytic activity towards CO oxidation according to the Mars-van-
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Krevelen type mechanism [15]. A further investigation of the CO gas sensing properties of CoEt was the reproducibility and stability. These important parameters were evaluated by repeating the test at
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100°C with a CO concentration of 5 ppm for 10 times. As shown in Fig. 13, the CoEt gas response
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in different tests was approximately constant (R/R0 = 2.4) indicating the repeatability of the sensor. The stability test (Fig. 14) was carried out at 100°C with a 5 ppm of CO, after 30 days the sensor response was comparable with the initial one (zero day). This test was achieved with an averaging of CO response in about 2 hours per day. Finally, the selectivity to CO was measured by exposing the CoEt sample to the same concentration (5 ppm) of different gas as NO2, C2H6O and H2 at 100°C. The obtained results (Fig. 15) indicated that the response of CoEt to these gases was more than one order of magnitude lower than the CO gas response. In comparison with the state of the art (Table 2), in 11
this work a good gas response and a very fast dynamic response were obtained for the detection of very low concentrations of CO at low temperature. Therefore, the CoEt nanofibers can be used to develop solid-state sensors for monitoring the CO presence at ppm levels in the air.
4. Conclusions Spinel cobalt oxide (Co3O4) was synthesized by electrospinning method in order to obtain a nanofiber structure able to improve the material sensing performance. As a comparison, the composite material
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was prepared by the same technique under the same operating conditions and using a different solvent. The CO gas sensing performances of both electrospun samples were investigated at different CO concentrations and temperatures. CoEt showed, at low concentrations of CO and at low
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temperature, good resistive response and a very short response - recovery time in comparison of the state of the art. In the same operating conditions, the CoDMF didn’t provide any appreciable response.
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The better CO sensing performances of CoEt can be attributed to the nanofiber morphology
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characterized by high surface area as well as a large number of oxygen defects or vacancies on the CoEt surface responsible of more active site availability. Furthermore, the results concerning the repeatability, stability and selectivity to the CO gas in different tests showed the reliability of CoEt
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nanofibers for the detection of CO at very low concentrations (5 ppm) at low temperature (100°C) with a very fast dynamic response (very short response and recovery time). For these reasons, this
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approach can be a fundamental step to developing at a pre-commercial level a CO resistive gas sensor.
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Biographies Concetta Busacca received the Master Degree in Industrial Chemistry in 2004 and Ph.D. Degree (in Materials Chemistry) in 2009 from University of Catania and University of Reggio Calabria respectively. Her research interests are focused on materials development for redox batteries and gas sensors. Actually, she is working in National Council of Research - Institute for Advanced Energy Technologies (CNR-ITAE) in Messina. She is author of 21 papers (h-index 9) on international journal (Scopus and Web of Science).
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Andrea Donato was graduated in Industrial Chemistry in 1981. Since 2001 he is Full Professor of Chemistry at the Department of Mechanical and Materials of the University of Reggio Calabria. From 2008 he is member of the Doctorate in “Geotechnical engineering and Chemistry of the Materials” at the Faculty of Engineering of the “Mediterranea” University of Reggio Calabria. Prof. Donato is author of more than 50 scientific publications in international journals and around 100 communication to national and international congresses. His research interests are devoted to synthesis and application of nanostructured materials for gas sensors applications.
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Massimiliano Lo Faro, in 2013, was awarded a degree in Industrial Chemistry at the University of Catania. After being awarded his BA degree, he attended a Master on “Design and Testing of fuel cells electrical propulsion equipment for sustainable mobility” at the CNR – Advanced Energy Technology Institute “Nicola Giordano” (ITAE). In March 2008, he was awarded a PhD in Environmental and Energy Materials at the University of Roma TorVergata. Currently, he is a permanent researcher at the National Research Council (CNR) and carries out the research activity on the development of materials and components for solid oxide fuel cells (SOFC) operating at intermediate temperatures (600-800 °C). He took part in various national and international projects, as well as private partnership contracts, and he also is the referee for many science magazines and sector science projects.
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Angela Malara received the Master Degree in Materials Engineering and Ph.D. Degree (in Materials Chemistry) from University of Messina and University of Reggio Calabria respectively. Her research interests are focused on materials development for gas sensors. Actually, she is working in “Mediterranea” University of Reggio Calabria. She is author of 17 papers (h-index 5) on international journal (Scopus and Web of Science).
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Giovanni Neri was born in Reggio Calabria, Italy, in 1956. He received the M.S. degree in chemistry from the University of Messina, Messina, Italy, in 1980. From 2004 to 2007, he was the Head of the Department of Industrial Chemistry and Materials Engineering, University of Messina. He was a Visiting Scientist/Professor with the University of Michigan, Ann Arbor, MI, USA, and also with the University of Alagappa, Karaikudi, India. Since 2002, he has been a Full Professor of chemistry with the University of Messina. He has authored over 200 papers on international journals, cited over 6600 times and h-index = 46 (source: ISI Web of Science). His current research interests include the synthesis and characterization of nanostructured materials for chemical sensors, applied in a wide range of sectors from medical diagnostics to automotive, industrial processes, and environmental control. Stefano Trocino received the Master Degree in Electronic Engineering in 2010 and Ph.D. Degree (in Materials Chemistry) in 2014 from University of Messina and University of Reggio Calabria respectively. His research interests are focused on electrochemical characterization of Solid Oxide Fuel Cell, photo-electrolyzers, electrolyzers, high temperature batteries (metal-air), development of hardware and software for electrochemical systems, semiconductor gas sensor characterization and modelling, development of measurement system for sensors, characterization of electronic devices up to microwave range. He is a developer in LabView environment. Actually, he is working in 17
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National Council of Research - Institute for Advanced Energy Technologies (CNR-ITAE) in Messina. He is author of 25 papers (h-index 9) on international journal (Scopus and Web of Science).
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Figure captions Table 1. Functional groups resulting from curve fitting of O1s spectra (%). Table 2. A comparison between the CO gas sensing properties of home-made electrospun Co3O4 nanofibers in this work and different Co3O4-based materials reported in literature at different temperatures and CO concentrations. Fig. 1. Schematic representation of the holder and the sensor (a); scheme of the experimental setup
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(b). Fig. 2. TG-DSC profiles of electrospun Co(NO3)2/PVP composite samples obtained using EtOH as solvent (solid line) and DMF as solvent (dotted line).
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Fig. 3. XRD diffraction patterns comparison among a) CoEt; b) CoDMF.
Fig. 4. SEM images of a) Co(NO3)2/PVP composite obtained using EtOH as solvent; b) CoEt; c)
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Co(NO3)2/PVP composite obtained using DMF as solvent; d) CoDMF.
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Fig. 5. TEM images ((a), (b), (c)) and particle size distribution (d) of CoEt. Fig. 6. TEM images ((a), (b), (c)) and particle size distribution (d) of CoDMF. Fig. 7. H2-TPR profile of a) Co(NO3)2-PVP composite obtained using EtOH as solvent; b) Co(NO3)2-
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PVP composite obtained using DMF as solvent.
Fig. 8. XPS spectra of (a), (c) Co2p and (b), (d) O1s for CoEt and CoDMF sample.
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Fig. 9. Response of the two sensors to 5 ppm of CO as a function of temperature.
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Fig. 10. Response and recovery time of CoET to 5 ppm of CO at 50°C (a) and 100°C (b). Fig. 11. Response of the different gas sensors to different concentrations of CO at 100°C. Fig. 12. Calibration curves of the different CO gas sensors at 100°C. Fig. 13. Repetition response curves of the CoEt to 5 ppm of CO at 100°C. Fig. 14. Long-term stability of CoEt to 5 ppm of CO at 100°C. Fig. 15. Response of the CoEt to 5 ppm of different gases at 100°C.
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na
ur
Jo
Fig. 1.
ro of
-p
re
lP
na
ur
Jo (a) (b)
21
ro of
-p
re
lP
na
ur
Jo
Fig. 2.
22
* Co3O4 [JCPDS No:01-080-1533] Intensity (a.u.)
* (a)
*
* * *
*
* *
**
10
20
30
40
50 60 70 2 (degree)
80
90
100
Jo
ur
na
lP
re
-p
Fig. 3.
ro of
(b)
23
Fig. 4.
ro of
-p
re
lP
na
ur
Jo a b
c
d
24
(b)
(a)
50
ro of
0.24 nm (311)
(d) 40 30 20 10 0 8
10
-p
Frequency / % (%) Particle fraction
(c)
12
14
16
18
20
22
24
26
size / nm Particle (nm) diameter Particle
Jo
ur
na
lP
re
Fig. 5.
25
(b)
(a)
50
(d)
30 20 10 0 10
15
20
25
ro of
0.24 nm (311)
40
30
35
-p
Frequency / % (%) Particle fraction
(c)
40
45
50
size / nm Particle (nm) diameter Particle
Jo
ur
na
lP
re
Fig. 6.
26
H2 uptake (a.u)
(a)
100
300
200
400
500
600
700
-p
Temperature (°C)
ro of
(b)
Jo
ur
na
lP
re
Fig. 7.
27
(b)
Co2p3/2
Intensity (a.u.)
Co2p1/2
800 795 790 785 Binding Energy (eV)
Intensity (a.u.)
(c)
780
(d)
Co2p3/2 Co2p1/2
810
536 535 534 533 532 531 530 529 528 527 526 Binding Energy (eV)
805
800 795 790 785 Binding Energy (eV)
780
ro of
805
Intensity (a.u.)
810
536 535 534 533 532 531 530 529 528 527 526 Binding Energy (eV)
-p
Intensity (a.u.)
(a)
Jo
ur
na
lP
re
Fig. 8.
28
3.0 5 ppm CO
CoDMF CoEt
2.0
1.5
1.0 100
300
200
ro of
Response (R/R0)
2.5
400
Working Temperature (°C)
Jo
ur
na
lP
re
-p
Fig. 9.
29
ro of
-p
re
lP
na
ur
Jo
Fig. 10.
30
20k
100°C C T == 100
18k
CoDMF CoEt
16k
12k
20 ppm CO
5 ppm CO 10 ppm CO
10k
40 ppm CO
20 ppm CO
5 ppm CO
ro of
Resistance (k)
14k
40 ppm CO
10 ppm CO
1
10
20
30
40
50
-p
0
Time (min)
Jo
ur
na
lP
re
Fig. 11.
31
ro of
-p
re
lP
na
ur
Jo
Fig. 12.
32
ro of
-p
re
lP
na
ur
Jo
Fig. 13.
33
5 T = 100°C 5 ppm CO
CoEt
Response (R/R0)
4
3
2
0
5
10
15
20
Time (day)
25
30
Jo
ur
na
lP
re
-p
Fig. 14.
ro of
1
34
Response (R/R0)
4
T = 100 °C
CoEt
3 2 1
-1 -2 -3 -4 CO
C2H6O C2H6O
NO22 NO
Gas
H2 H2
Jo
ur
na
lP
re
-p
Fig. 15.
ro of
Response (R0/R)
0
35
Samples
Table 1.
CoEt CoDMF
Chemisorbed oxygen species (%) (532.6 eV) 7 6
Co-O (%) Ovac. (%) (529.8 eV) (531.4 eV) 53 62
40 32
Table 2. Gas respons e (R/R0)
Respons e time (s)
Recover y Time (s)
Operating temperatu re (°C)
CO Concentra tion (ppm)
Electrospinning
2.4
14
36
100
5
6.5
3
5
250
50
13
60
100
100
50
3.5
120
270
100
6.7
Coprecipitation Hydrothermal Coprecipitation
Jo
ur
na
lP
re
-p
Co3O4 nanofibers (this work) Porous Co3O4 nanorods [15] Co3O4 nanowires [20] Co3O4 nanocrystalline [21]
Preparation method
ro of
CO gas sensing materials
36