Ammonia selective sensors based on cobalt spinel prepared by combustion synthesis

Solid State Ionics 337 (2019) 91–100

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Ammonia selective sensors based on cobalt spinel prepared by combustion synthesis

T

Daniele Zieglera,b, Andrea Marchisioa,b, Giuliana Ercolinoa, Stefania Specchiaa, ⁎ Jean-Marc Tulliania,b, a b

Politecnico di Torino, Department of Applied Science and Technology, Corso Duca degli Abruzzi, 24, 10129 Torino, Italy INSTM R.U PoliTO-LINCE Laboratory, Italy

ARTICLE INFO

ABSTRACT

Keywords: Spinel cobalt oxide combustion synthesis p-type semiconductor gas sensor ammonia detection

Nano-crystalline cobalt spinel was prepared by combustion synthesis and used as ammonia sensing material. After synthesis, the powder was calcined at 600 °C for 4 h and characterized by thermal analysis, X-ray diffraction, Raman spectroscopy, X-ray Photoelectron Spectroscopy, nitrogen adsorption (B.E.T, Brunauer, Emmet, Teller and B.J.H., Barrett, Joyner, Halenda techniques), H2 temperature programmed reduction, H2O adsorption and field emission scanning electron microscopy. Sensors were screen-printed onto α-alumina substrates with platinum interdigitated electrodes and fired at 700 °C for 1 h in air, after drying overnight. The sensor response was measured in the range 150 °C–250 °C under 1–50 ppm of NH3. Best results were obtained at 225 °C, with R (the ratio between the impedance of the film under gas exposure at the equilibrium and the impedance under dry air) equal to 1.83 under 50 ppm NH3. Response time and recovery time (e.g., the times taken by the sensor to attain 90% of total impedance change from its initial impedance value) were determined, together with cross-sensitivity tests towards CH4, CO, N2O, humidity, O3, CO2 and NO2 at the best operating temperature.

1. Introduction Chemo-resistive gas sensors based on metal oxide semiconductors (MOSs) have been extensively studied for the detection of toxic and explosive gases, due to their ease of operation, low cost and high sensitivity [1,2]. Among them, p-type MOSs with strong catalytic ability and high solubility of oxygen can be used effectively to produce highperformance gas sensors [3]. Co3O4, spinel cobalt oxide, is a mixed valence oxide of CoO and Co2O3 with a strong catalytic activity, above 200 °C, for the oxidation of reducing gases because of its multiple oxidation states [4]. Cobalt can accommodate Co2+, Co3+, and Co4+ oxidation states in the oxide structure, with Co4+ being the strongest electron acceptor, depending on the thermodynamic reaction conditions [5]. Co3O4 is also a typical p-type gas sensing material with an indirect band gap of 1.6–2.2 eV [4]. Usually, p-type conductivity in MOSs is due to an excess of oxygen ions caused by metal vacancies in the crystal lattice and/or interstitial oxygen ions. Thus, intrinsic acceptor states exist which, by trapping electrons, create electron holes that act as charge carriers. In air, other oxygen species will chemically adsorb on the surface of Co3O4,

⁎

increasing holes' concentration in the surface-near region and forming a holes' accumulation layer. Oxidizing gases will enhance this effect, while reducing gases will decrease the surface-trapped negative charges and then, the electrical conductivity [4]. Therefore, Co3O4 with various morphologies and doping has been extensively studied as gas sensor material [4]. For instance, Co3O4 has been proposed for hydrocarbons detection of: ethanol [6–15], acetone [6,16–18], benzene [6], toluene [19–22], xylene [7,19–21,23–25], ethyl ether [26], formaldehyde [8,27] and triethylamine [28], as well as of reducing (CO [9,29–34], H2 [30,35], NO [36], H2S [10], isobutane [37], NH3 [34,38–43]) and of oxidizing gases (NO2 [44–46]) and humidity [47]. Ammonia is generated by the production of nitrogenous fertilizers and other chemical substances and it is also used as a refrigerant [48]. Moreover, handling of manure on farms results in ammonia volatilization that has a negative impact on the environment because these emissions contribute to the acidification and eutrophication of ecosystems [49]. They also play a crucial role in the formation of secondary particulate matter (PM) in the atmosphere as NH3-derived compounds like ammonium sulphate ((NH4)2SO4) and ammonium nitrate (NH4NO3) increase PM mass concentrations [50]. In the European

Corresponding author at: Politecnico di Torino, Department of Applied Science and Technology, Corso Duca degli Abruzzi, 24, 10129 Torino, Italy. E-mail address: [email protected] (J.-M. Tulliani).

https://doi.org/10.1016/j.ssi.2019.03.026 Received 3 January 2019; Received in revised form 22 February 2019; Accepted 26 March 2019 Available online 25 April 2019 0167-2738/ © 2019 Elsevier B.V. All rights reserved.

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Union of 28 States (EU-28), 94% of ammonia emissions derived from agriculture in 2015 [50]. However, ammonia is a primary irritant to eye and upper respiratory tract, thus, the Time-Weighted Average (TWA) over 8 h should not exceed 25 ppm and the short-term exposure over 15 min is limited to 25–35 ppm [48]. Therefore, there is a need for reliable and effective ammonia gas sensors. In addition, environmental monitoring and clinical diagnostics require NH3 gas sensors to display a sensitivity at ppm levels and to be able to work at room temperature. In this frame, it appears crucial to develop ammonia sensors with fast response times, good selectivity, low detection limit, low power consumption and low cost for practical applications. Thus, in this work, nano-crystalline cobalt spinel was synthesized by combustion synthesis and used as ammonia sensing material. Combustion synthesis is an intriguing, smart, and appealing technique to synthesize a wide range of highly pure solid inorganic nanomaterials, and it is easily adaptable to various shapes of supports for the fabrication of structured catalysts or sensors [51,52]. The synthesis process is based on the propellant chemistry. Basically, a self-propagating exothermic reaction starts in a aqueous or sol-gel medium after a short heating, resulting in highly pure nanocrystalline compounds [53,54]. The synthesis process is easily controllable, highly efficient, and versatile, allowing a fine-tuning of the properties of the desired products. Combustion synthesis can be considered as a sustainable, environmentally friendly, and easily scalable preparation technique, according to the principles of circular economy [55]. The manufactured cobalt spinel powder was characterized by thermal analysis (Differential Scanning Calorimetry-Thermo Gravimetric Analysis, DSC-TGA), X-ray diffraction (XRD), Raman spectroscopy, X-ray Photoelectron Spectroscopy (XPS), nitrogen adsorption (B.E.T, Brunauer, Emmet, Teller, and B.J.H., Barrett, Joyner, Halenda, techniques), H2 temperature programmed reduction (H2TPR), H2O adsorption, and Field Emission-Scanning Electron Microscopy. The sensor response was measured in the range 150 °C–250 °C under 1–50 ppm NH3. Finally, cross-sensitivity tests towards CH4, CO, N2O, humidity, O3, CO2 and NO2 were also performed in order to evaluate the selectivity of the cobalt oxide sensor for ammonia detection.

GC 200 flow controller and a Huber cooling system, Columbus, USA) was performed on Co3O4 (25 mg) in the temperature range 25–1000 °C with a heating ramp of 10 °C/min under an air flow of 50 mL min−1. XRD was also carried out on the annealed powder. Spectra were recorded on a Pan'Analytical X'Pert Pro instrument (Pan'Analytical, Almelo, The Netherlands) with Cu K radiation (0.154056 nm) in the range between 20 and 70° 2θ, working with a step size of 0.026° of 2θ and an acquisition time per step equal to 46 s. Diffraction patterns were indexed by means of the Powder Data File database (P.D.F. 2000, International Centre of Diffraction Data, Newtown Square, PA, USA). The average crystallite size was estimated according to the Scherrer equation, for a qualitative comparison with the available data in literature (Eq. (2)):

D = k / cos

where k is a constant assumed to be equal to 0.9, λ is the Cu Kα wavelength (0.154056 nm), θ is the half of Bragg (in rad) and β is the full width at half maximum of the X-ray diffraction peaks, that are commonly associated with the crystallite size, presence of defects, and peak broadening caused by the instrument [59]. Raman spectroscopy (Renishaw InVia spectrometer with confocal microscope Leica DMLM, Gloucestershire, UK) was realized on the powder at room temperature recording the spectra in the range 100–1000 cm−1, using a 50× magnification, with CCD detector and an excitation wavelength λ = 514 nm. The signal-to-noise ratio was optimized by accumulating ten scans for each measurement. The minimal spot diameter of the laser on the sample for the used spectrometer was 836 nm (1.22 λ/NA, where numerical aperture NA = 0.75 for Leica 50× lens [60]). The applied laser power was 0.25 mW (0.5% of the 50 mW maximum laser out-put power). XPS analysis were carried out with a PHI 5800 Versaprobe (Physical Electronics, Chanhassen, MN, USA) scanning X-ray photoelectron spectrometer (monochromatic Al Kα X-ray source with 1486.6 eV energy, 15 kV voltage, and 1 mA anode current), with 100 μm X-rays spots, to investigate the surface chemical composition of the semiconductor together with possible shifts in high resolution for Co and O. A dual beam charge neutralization method was applied, combining both low energy ions and electrons, to reduce any possible charging effects of X-rays. Prior to the measurement, the samples were outgassed in an ultrahigh vacuum chamber at 2.5 × 10−6 Pa for 12 h. Survey scans were recorded in the range 0-1200 eV. The high-resolution O1s spectrum was collected from 523 to 543 eV, and Co2p from 765 to 815 eV. For spectra collection, the instrument was calibrated against a value of the C1s binding energy of 284.5 eV, with a standard deviation equal to 0.3 eV. Nitrogen physisorption was performed at −196 °C on a Micromeritics ASAP 2020 (Micromeritics, Norcross, USA). Before the measurement, about 100 mg of annealed cobalt spinel was outgassed overnight at 150 °C in low vacuum (7 Pa). The specific surface area was evaluated by the BET method between 0.05 and 0.30 p/p0. The pore diameter distribution was evaluated by the Barrett–Joyner–Halenda (BJH) method, calibrated for cylindrical pores according to the improved Kruk–Jaroniec–Sayari (KJS) method, with the corrected form of the Kelvin equation, from the desorption branches of the isotherms. H2-TPR tests were also carried out using a Quantachrome ChemBET Pulsar TPR-TPD with a TCD detector (Boynton Beach, USA), using 5% H2/Ar with flow rate of 15 mL·min−1 and heating rate of 5 °C/min. The experiment was performed on 30 mg cobalt spinel inserted in a quartz U-tube, in the temperature range from 25 to 700 °C, with a heating rate 15 °C∙min−1. A cold trap between the cell and the detector was used to remove water vapor from the Co3O4 reduction. Water adsorption capacity was measured in the same TG-DSC apparatus, to assess the capability of the material in adsorbing humidity. The experiments were conducted under an Ar atmosphere (flow rate 50 mL·min−1) at the constant temperature of 30 and 250 °C. The experiment was designed to test water adsorption in the H2O flow at or

2. Experimental 2.1. Powder synthesis Co3O4 was synthetized by combustion synthesis mixing in aqueous solution glycine (NH2CH2COOH, ≥99% purity, Sigma-Aldrich) as the organic fuel with cobalt nitrate (Co(NO3)2∙6H2O, ≥98% purity, SigmaAldrich). 1/4 of the stoichiometric glycine was added [58], according to the following reaction (Eq. (1)):

3Co(NO3 )2 +

28 NH2 CH2 COOH 9

Co3O4 +

(2)

56 70 41 CO2 + H2 O + N2 9 9 9 (1)

The organic fuel favors a suitable homogeneity of the mixture while preventing the preferential precipitation of ionic species in the first stages of the reaction. Then, during the combustion process, it reacts with nitrates, according to the chemistry of propellants [51,52,56]. First, reagents were dissolved in aqueous solution by stirring at 120 °C up to the formation of a gel. The solution was subsequently placed in an oven pre-heated at 250 °C to initiate the reaction. 20 min were enough to obtain a fine powder [57,58]. The collected powder was then ground in a mortar. Finally, for the physical-chemical characterization of the powder and the preparation of the sensor films, cobalt spinel was annealed at 600 °C for 4 h in static air. 2.2. Powder and film characterization TG-DSC (Mettler Toledo TGA/DSC1 STARe System equipped with a 92

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close to the equilibrium conditions. The saturation of Ar gas with water was done by bubbling the gas through the water containing gas washing bottle, which resulted in ~1 vol% H2O/Ar. The sample was equilibrated in the gas flow, and then the water-saturated Ar was introduced to the sample at the same gas flow rate. When the steady signal was attained, the dry Ar flow was restored. An empty crucible was tested with the same experimental procedure, to evaluate the buoyancy effect. Finally, morphologies of powders and films were examined by means of a FE-SEM (Zeiss Supra-40, Oberkochen, Germany) equipped with an Oxford Energy Dispersive X-ray detector. All observations were performed after chromium film sputtering. Image analysis (Image-Pro Plus 4.5 by Media Cybernetics Inc., Rockville, MD, USA) was performed on FE-SEM micrographs of the powders heat treated at 600 °C for 4 h and at 600 °C for 4 h + 700 °C for 1 h. The aim of these measurements was to evidence any grain growth due to additional thermal treatment necessary on screen-printed sensors to promote the adhesion of the films onto the alumina substrates. Grains detection was done image by image, as a function of grey values thresholds. Once the grains were highlighted, the system measured the average diameter, i.e. the average value of diameters measured at 2degree intervals and passing through object's centroid. Finally, all the detected objects were manually checked to remove artifacts (grains or parts of grains that were not correctly detected).

Inks for screen printing were prepared by mixing the Co3O4 powder with an organic vehicle (ethylene glycol monobutyral ether, Emflow, Emca Remex, USA), to reach the correct viscosity. Polyvinyl butyral (PVB, Sigma Aldrich, Milan, Italy) was also added as a temporary binder for the film before firing. After screen-printing deposition with a 325-mesh steel mask, sensors were dried at 80 °C overnight and fired at 700 °C in air for 1 h (2 °C/ min heating and cooling ramps). This step is necessary to remove all the organic residues from the solvent and PVB as well as, to ensure the correct adhesion of the ceramic material to the substrate. In Fig. 1 a scheme of the sensor is reproduced. Sensors were soon after tested in a home-made system where ammonia in helium (450 ppm) was diluted with synthetic air by means of flow meters (Teledyne Hastings Instruments HFM 300 controller and flow meters HFC 302, Teledyne Hastings, Hampton, VA, USA) in a constant air flow of 1000 sccm (standard cubic centimeters per minute). The investigated range of ammonia concentration was 1–50 ppm. All the sensors were heated by a Ni-Cr wire, located underneath the sensor, alimented with a DC power supply (Peak Tech, Nanjing, Jiangsu, China). A PT1000 resistance temperature detector (RS Pro, London, UK) was used for sensors temperature determination. Two different sensors were then tested between 150 °C and 250 °C in a chamber with a volume of 0.1 L. The films' impedance was measured by means of an LCR meter (Hioki 3533-01, Nagano, Japan) by alimenting the sensors with an AC tension of 1 V at 1 kHz. Finally, cross sensitivity measurements were carried out towards CH4 (50.0 ppm in air), CO (10.0 ppm in air), N2O (15.0 ppm in air), CO2 (500.0 ppm in air), O3 (0.5 ppm in air), NO2 (0.1 ppm in air), and humidity (50% of relative humidity at room temperature) under the same flow of 1000 sccm. The response of the sensor (R) was calculated according to Eq. (3), for reducing gases:

2.3. Fabrication and measurement of gas sensors Co3O4 spinel sensors were prepared by screen-printing technique onto α-alumina substrates (Coors Tek, Golden, CO, USA, ADS-96, 96% alumina, 8 mm × 16 mm, Fig. 1) with platinum interdigitated electrodes. These electrodes were realized by screen printing a Pt ink (5545LS, ESL, King of Prussia, PA, USA) over the ceramic substrate and then firing at 980 °C (2 °C/min heating and cooling ramps) for 18 min to optimize their electrical conductivity and to ensure a correct adhesion onto the ceramic substrate, according to the manufacturer's recommendations. Electrodes have a thickness of 400 μm and are spaced 450 μm one from the other.

(3)

R = Zg /Z 0 and according to Eq. (4) for oxidizing gases:

Fig. 1. Scheme of the sensor prepared by screen printing technique: (a) front view, (b) side view. The proportions between the parts are exaggerated for explanatory purpose, while the quotes are referred to the real dimensions. In the picture, solid black lines represent Pt interdigitated electrodes, dark grey layer is the active sensing material (Co3O4), and white area is the α-alumina substrate.

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Fig. 2. TG-DSC curve of cobalt oxide powder pre-treated at 600 °C for 4 h in static air.

Fig. 3. XRD pattern of Co3O4 powder pretreated at 250 °C for 20 min (a), heat treated at 600 °C for 4 h in static air (b) and then subsequently calcined at 700 °C for 1 h (c).

(4)

R = Z 0 /Zg

where Zg and Z0 are respectively the impedance values under target gas and under dry air flow of the oxide layer at the equilibrium. The response times (the time needed by a sensor to achieve 90% of the total impedance change in the case of gas adsorption) together with the recovery times (the time necessary to reach 90% of the total impedance variation in the case of gas desorption) were also determined in this work. 3. Results and discussion 3.1. Powder and film characterization Fig. 2 illustrates TG-DSC curve related to pre-calcined cobalt oxide powder at 600 °C for 4 h in static air. TG analysis displays a first weight loss of about 2% between 300 °C and 650 °C, probably due to combustion of precursors residues and a second one at high temperature in the range 900°–930 °C, generated by the decomposition of spinel into CoO according to Eq. (5):

Co3 O4

3CoO +

1 O2 2

Fig. 4. Raman spectrum of cobalt oxide powder pretreated at 600 °C for 4 h in static air.

(5)

The measured weight loss of this decomposition is equal to 6.3 wt%, close to the theoretical value of 6.6 wt%. The total weight loss is around 8%. DSC curve also reveals one small exothermic peak at 310 °C that could be ascribed to the combustion of carbon and a sharp endothermic one at 918 °C that is associated to spinel decomposition into CoO, that is a thermal reduction of Co3+ to Co2+ [61]. The formation and crystallization of Co3O4 phase after pre-treatment at 250 °C for 20 min, firing at 600 °C for 4 h and further calcination at 700 °C (following the same thermal cycle of the thick-films) are confirmed by XRD analysis as shown in Fig. 3. The XRD pattern of the cobalt oxide nano-powder confirms the formation of the cubic Co3O4 phase indexed in the Fd3m space group (JCPDF card n°01-080-1533). By applying the Scherrer equation the average crystallite sizes are equal to 50 nm for the sample pre-treated at 250 °C for 20 min, 76 nm for the powder annealed at 600 °C for 4 h and 77 nm for the sample annealed at 600 °C for 4 h and then at 700 °C for 1 h. The last thermal treatment does not increase significantly the crystallite size of the cubic Co3O4 nanoparticles. Moreover, the Raman spectrum of the cobalt oxide nano-powder confirms the formation of Co3O4, revealing five main Raman peaks (Fig. 4) located near 196, 483, 525, 623, and 692 cm−1, corresponding

to the F2g, Eg, 2 × F2g, and A1g modes of crystalline Co3O4, respectively [60]. The chemical states of Co and O atoms in the cobalt oxide powder were investigated by XPS: Fig. 5 shows the high resolution spectra. For oxygen, the high resolution XPS analysis shows asymmetric O1s spectrum, which can be deconvoluted into a main peak followed by a shoulder at a higher binding energy value, as shown in Fig. 5a, centered at 529.7 and 531.6 eV, respectively. The first peak at the low binding energy side of 529.7 eV is attributed to O2– ions in the Co3O4, which are surrounded by Co atoms (lattice oxygen) [62]. Whereas the shoulder located at the higher binding energy value can be assigned to O2– ions in oxygen-deficient regions within the matrix of Co3O4 (oxygen vacancies) and to oxygen species in adsorbed Co-OH and H2O molecules, respectively [34,42,43]. The high-resolution spectrum of Co2p peak can be deconvoluted into two major peaks centered at around 780 and 795 eV, corresponding to the binding energies of the Co 2p3/2 and Co 2p1/2, respectively (Fig. 5b) [42,43]. This spectrum is typical of the cobalt spinel structure without CoO phase on the surface [61]. From the decomposition of the Co 2p3/2 band into the components originating 94

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Fig. 5. High resolution XPS spectra of cobalt oxide powder pre-treated at 600 °C for 4 h in static air: O1S (a) and Co2p (b).

from Co2+ (band at 780.6 eV) and Co3+ (band at 779.6 eV), the Co2+/ (Co2+ + Co3+) ratio resulted equal to 0.21, well below to the theoretical value of 0.67. Low Co2+ amounts may be due to the sub-stoichiometric amount of glycine used during the combustion synthesis [63]. B.E.T measurements on Co3O4 powder evidenced a low specific surface area of 4.5 m2·g−1, with modest porosity (total B.J.H. pore volume of 0.003 cm3 g−1, with average pore width of 2 nm). The H2-TPR profile shown in Fig. 6 is typical of cobalt spinel, with an asymmetric reduction signal maximum centered at 437 °C [64]. In this specific case, the absence of a double maximum, which usually can be observed for Co3O4 [65,66], can be a consequence of the morphology and size of the constituting particles, which can affect the reduction profile [67]. Fig. 7 shows the results of TGA water adsorption experiments. The system response to the introduction of 1 vol% H2O saturated Ar was evaluated by performing a blank test with an empty crucible. The apparent change in the mass for the blank test was 0.007 mg (Fig. 7a). This value was subtracted from the mass of adsorbed water determined for Co3O4 (Fig. 7b, c). Considering the low specific surface area of the cobalt spinel, a surface concentration of reversibly adsorbed water equal to 1 and 0.3 H2O molecule per nm2 was estimated at 30 and 250 °C, respectively [68]. Such an amount of water is very small. Thus, we can conclude that water interferences have a minimal influence on sensing process.

The morphology of Co3O4 powder and sensor was observed by FESEM (Fig. 8a, b). The powder, after thermal treatment at 600 °C for 4 h, consists of typical clusters of nanocrystals with the characteristic rombicuboctahedral shapes grains (prevailing of (100) and (111) planes) [68]. The crystals present a typical spongy structure, with edges not so clearly defined, due to the gases released during the combustion synthesis process [51,52]. FE-SEM analysis repeated on prepared sensors fired at 700 °C for 1 h (Fig. 8c, d) revealed that Co3O4 grains maintained their typical rombicuboctahedral morphology, without suffering during the screen-printing process of preparation and following thermal treatment. Finally, FE-SEM observations were also performed perpendicularly to screen-printed sensors (not shown here) and allowed estimating film thickness, which was found to be equal to 20.6 ± 2.7 μm (average value on 10 measurements). Image analysis on FE-SEM micrographs of the powders did not evidence any significant increase of grain size due to additional thermal treatment after screen-printing step: the average diameters determined on about 250 measured objects were 0.39 μm ± 0.20 μm and 0.51 μm ± 0.20 μm, respectively for the powders heat treated at 600 °C for 4 h and at 600 °C for 4 h + 700 °C for 1 h. These results confirm the conclusion drawn from crystallite size estimation of both powders by means of Scherrer's equation. 3.2. Gas sensing properties The sensors were first tested towards ammonia (50.0 ppm) at different temperatures (in the range from 150 to 250 °C), evaluating the optimum working temperature. Different ammonia concentrations were then investigated at the optimum temperature, between 1.0 and 50.0 ppm, comparing the obtained results with those of analogous works. Fig. 9 displays the sensor response at different temperatures. Co3O4 sensors behave as p-type semiconductors and their impedance increased from about 1 to 13 kΩ under dry air, up to 1.2–17 kΩ when exposed to ammonia 50.0 ppm in function of the temperature (Table 1). Considering the sensor's response as well as, the response and recovery times, the optimum working temperature was equal to 225 °C as reported in Fig. 9 and Table 1. The response and recovery times at 225 °C under 50.0 ppm of ammonia were not the fastest in literature (around 1 min and 6 min 36 s, respectively; Fig. 10 and Table 1), but for some practical applications these features can be tolerated. Table 2 reports the Co3O4 sensor response towards ammonia 1.0, 2.5, 5.0, 10.0, 25.0 and 50.0 ppm at 225 °C as well as the corresponding response and recovery times. These results were also used to determine the sensitivity of the sensor. According to IUPAC definition, the

Fig. 6. H2-TPR profile of cobalt oxide powder pre-treated at 600 °C for 4 h in static air. 95

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impedance under pure air over the same amount of time (Table 3). In addition, a polar plot of the cross-sensitivity test performed at 225 °C is depicted in Fig. 13 confirming a noticeable selectivity for ammonia of Co3O4 film even under humid conditions (50% and 90% of RH). Surprisingly for common metal oxide semiconductors, Co3O4 exhibits an excellent selectivity for ammonia detection at ppm level, not in total agreement with some previous works [9,29–34,41–43]. In agreement with TGA water adsorption experiments performed at 30 °C, only a modest interference is constituted by water molecules, acting as reducing species on the surface of spinel oxide at 225 °C. More in detail, when exposed to 25 ppm of ammonia, sensor response is equal to 1.55 with response time of 83 s, while under 25 ppm of ammonia in humid condition (50 RH%) the sensor response remains almost unchanged, but the response time increases sharply to 784 s: this is a proof of the competition for the same adsorption sites between ammonia and water molecule as reducing species. The effect of the sensor response towards 25 and 1 ppm of ammonia in a baseline of 90% of relative humidity was studied and impedance's variations are illustrated in the Fig. 14. The spinel Co3O4 sensors was exposed in a 90% humid atmosphere and the sensor response R was equal to 1.14 (while R:1.55 in dry conditions was measured) confirming the competition between water and ammonia for the same adsorption sites and still the sensors showed a variation of impedance equal to 3% when exposed to NH3 1 ppm under 90% of relative humidity. Finally, in Table 4, results of this work are compared to previous results on Co3O4 based sensor for ammonia detection. Comparing our results with those of other studies based on cobalt oxide films, maximum response is ordinarily achieved in the 150°–200 °C temperature range, though in refs. [34,39], the Authors obtained the maximum response at room temperature. The results available in the literature can be due to a synergistic effect of adsorption oxygen and electron transport in the Co3O4/polyethyleneimine‑carbon nanotubes composites, or because the tested sensor was a pellet. Zhou et al. [43] also obtained interesting results working at room temperature. Considering Table 4, it seems that manufacturing sensing materials via the hydrothermal synthesis achieves higher sensing responses than other preparation techniques. This may be due to the higher specific surface area achieved by these materials. However, one drawback of hydrothermal synthesis is the limited amount of powder that can be obtained by each synthesis batch. Therefore, we think that the proposed combustion synthesis is an extremely interesting process because it allows overcoming limitations in the amount of powder prepared in one batch compared to the hydrothermal process. This possibility can be useful in the case of industrial fabrication of sensors, where the requested amount of spinel could be large. Grain size and film thickness are certainly the main parameters responsible for sensor's performance. According to Gardon et al. [69], at lower temperature the gas penetration profile is higher for thick-devices, enabling a sharp accessibility to inner grains for gas molecules. So, activation energy is lower for thick film-based metal oxide sensors because of heating of gas molecules upon diffusion through the metal oxide. For this reason, thick film-based gas sensors often exhibit an increased response at lower temperature in comparison with thin film technology. The working principle of gas sensing materials depends on gas adsorption which causes a change in the conductivity/resistance of the surface exposed to the gas. At low temperatures, O2− is chemisorbed (Eq. (6)), while above 150 °C, O2− and O− are chemisorbed (Eqs. (7) and (8):

Fig. 7. Thermogravimetric profiles of equilibrium water adsorption experiments performed for cobalt oxide powder pre-treated at 600 °C for 4 h in static air: blank test (a), Co3O4 @ 30 °C (b), Co3O4 @ 250 °C (c).

sensitivity of the sensor can be determined from the slope of the interpolation line R = f([NH3]), as illustrated in Fig. 11. The response of the sensor follows a calibration curve in the form y = a + bxc in the investigated ammonia concentration range (1.0–50.0 ppm; R2 = 0.998) and the calculated sensitivity is equal to 0.051 ppm−1. The long-term stability and repeatability of Co3O4 sensor were also evaluated, by exposing the film towards 3 pulses of 5 ppm of ammonia, as depicted in Fig. 12, after 9 months from the preparation. The sensor response under 5 ppm of NH3 in dry conditions is comparable to the previous test (R = 1.19 vs R = 1.2) and has a large repeatability among different pulses. Since selectivity of semiconductor gas sensors is of paramount importance, different cross-sensitivity tests were carried out at 225 °C. For this reason, impedance variations of the cobalt oxide spinel film were recorded after exposure for 10 min to different target gas comparing its 96

O2 (ads) + e

O2 (ads)

(6)

O2 (ads) + e

O2(ads)

(7)

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Fig. 8. FESEM micrographs of Co3O4 powder low magnification, 25 kx (a); high magnification, 100 kx (b); and of Co3O4 sensor: low magnification, 25 kx (c); high magnification, 100 kx (d).

Fig. 10. Sensor film responses under different ammonia concentrations (1.0, 2.5, 5.0, 10.0, 25.0 and 50.0 ppm) at 225 °C.

Fig. 9. Sensor film response at different operating temperature (150–250 °C) under 50.0 ppm NH3 in synthetic air.

Table 2 Co3O4 response R (Zg/Zo) towards ammonia 1.0, 2.5, 5.0, 10.0, 25.0 and 50.0 ppm at 225 °C and corresponding response and recovery times.

Table 1 Changes in impedance, response and recovery times at different temperature under dry air and 50.0 ppm of ammonia. Temperature (°C)

Zo (kΩ)

Zg (kΩ)

R (Zg/Zo)

Response time (s)

Recovery time (s)

150 175 200 225 250

13.19 5.36 1.82 1.83 0.96

17.07 9.42 3.16 3.35 1.17

1.29 1.76 1.73 1.83 1.24

280 210 381 61 261

585 850 592 396 83

O2(ads) + e

2O

[NH3] NH3 NH3 NH3 NH3 NH3 NH3

(8)

1 ppm 2.5 ppm 5 ppm 10 ppm 25 ppm 50 ppm

2NH3 + 7O

When the sensor was exposed to NH3, ammonia molecules reacted with the surface adsorbed species O2−and O− to produce NO2 and H2O [40], allowing the trapped electrons to come back to the conduction band (Eq. (9)).

Zo (kΩ)

Zg(kΩ)

1.7 1.73 1.75 1.78 1.81 1.83

1.87 2 2.1 2.29 2.8 3.35

R (Zg/Zo) 1.1 1.16 1.2 1.29 1.55 1.83

2NO2 + 3H2 O + 7e

Response time (s) 183 161 124 112 83 61

Recovery time (s) 249 302 313 367 345 396

(9)

This process narrowed the electron depletion layer of the semiconductor, reduced the hole density, leading to an increase of the impedance [40]. When NH3 molecules are removed, re-adsorption of 97

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Fig. 13. Polar plot of Co3O4 cross sensitivity measurements at 225 °C.

Fig. 11. Sensitivity of the sensor at 225 °C.

Fig. 14. Impedance variations at 225 °C for Co3O4 sensor under 90% of RH and 25 and 1 ppm of NH3.

Fig. 12. Long term stability and repeatability of Co3O4 sensor towards 5 ppm of ammonia after 9 months from the preparation.

semiconductors, but they usually exhibit the advantage to work at relatively low temperature. Cobalt oxide film displays distinctive catalytic activities that promote the selective oxidation of ammonia due to the high affinity with oxygen and multivalent characteristics. Under oxidizing atmosphere (such NO2 and O3), the resistance and the impedance of p-type oxide semiconductor is known to drop. This is described by an enhanced hole concentration in the sensitive layer due to the ionosorption of oxidizing gas. However, chemo-resistive variation of p-type oxide semiconductors to oxidizing gas seems to be not so high considering the gas sensing mechanism and this could describe the lack of sensor response towards ozone and nitrogen dioxide gases in the presented spinel cobalt oxide sensors [3]. For those reasons, this synthetized Co3O4 sensor could find an application in soil ammonia monitoring where the concentrations of N2O and CH4 are in the range of some ppm [70] and a selective detection of NH3 is required at ppm level also with respect to CO2 [49,71].

Table 3 Cross-sensitivity measurements for Co3O4 sensor at 225 °C. Gas CH4 100 ppm CO 10 ppm O3 0.5 ppm NO2 0.1 ppm N2O 15 ppm CO2 500 ppm RH 50% NH3 25 ppm NH3 25 ppm + RH 50% RH 90% NH3 25 ppm + RH 90%

Zo (kΩ)

Zg (kΩ)

(R)

1.83 1.84 1.83 1.85 1.85 1.83 1.86 1.81 1.86 1.87 1.87

1.83 1.84 1.78 1.88 1.85 1.76 2.5 2.80 2.94 2.28 2.60

1.00 1.00 1.03 1.02 1.00 1.04 1.34 1.55 1.58 1.22 1.39

Response time (s) n.d. n.d. n.d. 18 n.d. 127 276 83 784 157 321

Recovery time (s) n.d. n.d. n.d. 54 n.d. 102 291 345 456 162 290

NB: n.d. = not determined. RH = relative humidity (at ambient temperature).

oxygen species favored fast recovering of the initial impedance value. In fact, the O– and O2– adsorption on cobalt oxide leads to the formation of hole-accumulation layers (HALs), and conduction occurs mainly near-surface of HAL. Thus, the chemo-resistive variations of undoped p-type oxide semiconductors are lower compared to those induced at the electron-depletion layers of n-type oxide

4. Conclusions In this work, nano-crystalline cobalt oxide (Co3O4) was synthesized by solution combustion synthesis (SCS) and used as ammonia sensing material. Sensors were screen-printed onto α-alumina substrates with platinum interdigitated electrodes and fired at 700 °C for 1 h in air. 98

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Table 4 Comparison of the ammonia sensing performances of cobalt oxide described in the literature. Technological route Co3O4/polyethyleneimine‑carbon nanotubes composites by hydrothermal synthesis Co3O4 sol gel spin coated films technique Co3O4 by precipitation route and annealing Hierarchical Co3O4 by hydrothermal synthesis Hybrid Co3O4/SnO2 core−shell nanospheres by hydrothermal synthesis Nano-sheet arrays of Co3O4 by hydrothermal synthesis Dumbbell-like Co3O4 by hydrothermal synthesis Co3O4 thick film by combustion synthesis

Film thickness

Sensitivity (ppm−1)

Sensor response (R = Rg/Ro)

Conditions of measurements

Reference

Thin film

n.d.

Ca. 1.35

R.T., 50 ppm

[34]

Thin film Pellet Thick film Thick film

n.d. n.d. n.d. n.d.

1.28 Ca. 28.5 Ca. 7 Ca. 7.5

200 °C, 50 ppm R.T., 60.6 ppm 160 °C, 50 ppm 200 °C, 50 ppm

[38] [39] [40] [41]

5.2 μm n.d. 20.6 μm

0.0819 n.d. 0.051

Ca. 5.6 Ca. 1.4 1.83

R.T., 60 ppm R.T., 50 ppm 225 °C, 50 ppm

[42] [43] This work

NB: R.T. = room temperature.

Best results were obtained at 225 °C, with R (Zg/Z0) equal to 1.83 under 50 ppm exposure to NH3 and 10% of impedance variation under 1 ppm of ammonia in dry air. Response and recovery time were reasonably fast (in the order of few minutes), and cobalt oxide films exhibit a noticeable selectivity for NH3 respect to CH4, CO, N2O, humidity, O3, CO2 and NO2. These results are extremely encouraging and support the exploitation of Co3O4 as sensing and selective material able to detect ammonia at ppm level from soil volatilization.

[15] [16] [17] [18]

Acknowledgements [19]

The Authors are grateful to Mr. Mauro Raimondo for FESEM observations and Salvatore Guastella for XPS analysis.

[20]

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