Ethanol and acetone sensing properties of plasma sprayed copper oxide coating
Introduction
Air quality monitoring in domestic, industrial and commercial applications include not only toxic gases but also volatile organic compounds (VOCs) [1]. VOCs possess low boiling point, tend to vaporize at ambient temperature and release large amount of molecules to the surrounding atmosphere. Potential sources for VOC emission include oil refineries, paints and varnishes, carpets and adhesives, solvents, cleaning and dis-infecting chemicals [1]. Amongst different VOCs, ethanol (C2H5OH) and acetone (CH3COCH3) traces are widely found in different applications. C2H5OH can be extracted from different grains and crops (corn, sorghum, barley, sugar cane, etc.) having higher starch and sugar content [1]. It is widely used in numerous applications, namely, food, biomedical, transportation sector. Since long term exposure to C2H5OH vapor leads to different health hazards namely, headache, drowsiness, eye irritation and breathing difficulty [1]; C2H5OH detection is also important for drink and drive applications in order to ensure social security [2]. CH3COCH3 is another volatile organic compound that is used in nail polish remover, production of bath and fragrance products, hair and skin care products [1]. Exposure to 300–500 ppm acetone for around 5 min causes irritation to nose, eyes and lungs [1]. High concentrations of acetone (>300 ppm) may lead to headache, dizziness, nausea, fatigue, dryness in the mouth, muscle pain, loss of coordinated speech [1]. In a nut shell, detection of C2H5OH and CH3COCH3 is important to prevent potential hazards associated with exposure to these VOCs as discussed earlier.
Different type of gas detectors based on electrochemical, catalytic, optical, thermo conductive and chemi-resistive are available in the market [3]. In comparison with other sensors, chemi-resistive sensors offer good stability of base resistance, excellent sensitivity, quick response time, good accuracy, good durability, easier maintenance, excellent portability and safety during the operation, cost effectiveness aspects and preferred for detection of toxic, harmful gas and volatile organic compounds as well [1]. Chemi-resistive gas sensing elements are available using polymer and metal oxide candidates [3]. Polymeric sensors could be used for room temperature detection. However, sensitivity to ultra violet radiation and elevated temperatures in harsh environments may degrade the sensory function and affect the sensor performance in terms of stability of baseline resistance [3]. Metal oxide sensors usually require elevated temperatures for efficient operation in order to produce measurable gas response, quick response time, and recovery time [3]. In contrast with polymer type sensors, metal oxide sensors could be regenerated by heating at elevated temperatures [3]. Therefore, metal oxide gas sensor outperforms in terms of life span as well as stability in contrast with polymeric gas sensor [3]. Copper oxide (CuO) semiconductor based gas sensors in different configurations, namely, different nanostructures [4,5] and thin film [6] have been explored for the detection of harmful gases and VOCs have been reported in the recent literature [7].
Thermal spray technique is widely used in industries to develop functional surfaces to resist friction [8], wear [9], abrasion [10,11] and other tribological applications [12]. Atmospheric plasma spray (APS) is viable deposition route to fabricate functional coatings owing to its high throughput manufacturing capability and ease of integration with three dimensional robotics [13]. In APS, plasma jet functions as a heat source to melt and accelerate the powder particles towards the substrate at high velocity [14,15]. Powder particles interact with plasma plume, gain momentum from expanding plasma gases, undergo melting and subsequently travel towards the substrate to be coated [16,17]. Molten particles undergo flattening upon impacting over the substrate to form a spat [18]. A single layer is typically composed of series of several splats [19]. The next layer is immediately produced over the previously produced layer and this way, coating formation happens layer by layer [20].
APS is widely used to fabricate functional coatings for traditional applications, namely, corrosion and wear resistance, thermal barrier, decorative, and others [21,22]. In addition to these conventional applications, APS was widely attempted by different researchers to develop catalytic coatings [23], photocatalytic coatings [24], electrode for the lithium ion battery [25], strain gauge [26], thermocouple [27], magneto-resistive sensor [28], humidity sensor [29], and gas sensors [30] in the past few decades. Following APS deposition technique, reports presenting nitrogen dioxide (NO2) sensing using APS derived tungsten oxide (WO3) [30], zinc oxide (ZnO) [31] coatings, ammonia (NH3) sensing using APS derived titanium oxide (TiO2) coating [32], ethanol sensing using APS derived tin oxide (SnO2) coating [33] and tungsten oxide-tin oxide (WO3–SnO2) composite coating [34], hydrogen (H2) sensing using APS derived tin oxide (SnO2) coating [35,36], carbon monoxide (CO) sensing using APS derived copper oxide (CuO) coating [37] were reported. Despite of these successful attempts, ethanol (C2H5OH) and acetone (CH3COCH3) gas sensing potential of atmospheric plasma sprayed CuO coating was hardly found in the literature.
The present work attempts to investigate C2H5OH and CH3COCH3 sensing of APS derived CuO coating in the temperature range of 200–300 °C. Sensing measurements were also performed by altering analyte concentration (300-25 ppm) at a constant temperature of 300 °C. Response transients as a function of analyte concentration were analyzed using Freundlich adsorption isotherm to estimate activation energy of adsorption for each analyte. Attempt has been made to understand sensing behavior of C2H5OH and CH3COCH3 in conjunction with estimated activation energy of adsorption over CuO sensor surface. Secondly, feature extraction of conductance transients using principal component analysis is found useful to identify a particular analyte.
Section snippets
Fabrication of sensing element
Copper oxide layer was coated over aluminium oxide plate (size 19 mm × 9.5 mm × 0.65 mm) having two planar silver electrodes. A 9 MB plasma gun (Sulzer Metco, Westburry, New York) was operated using following set of deposition parameters to produce CuO coating: plasma power of 22.1 kW, stand-off distance of 100 mm, N2 flow rate of 50 slpm, powder flow rate of 50 g/min [37,38]. Plasma gun is equipped with a pair of compressed air jet as cooling media in order to circumvent overheating of alumina
Phase structure
Fig. 2 envisages the X-ray diffraction pattern for CuO powder, CuO coating. Since the CuO powder was sprayed onto an alumina plate having silver electrodes, peaks corresponding to Al2O3 (rhombohedral, R–3C, JCPDS-PDF 01-071-1126) and Ag (cubic, Fm-3m, JCPDS-PDF 01-087-0718) have been noticed in addition to CuO (monoclinic, C2/c, JCPDS-PDF 01-089-5895) peaks in the X-ray diffractogram of coated layer [37].
X-ray photoelectron spectroscopy
In the present work, X-ray photoelectron spectroscopy was carried out to ascertain the
Conductance transient analysis
Conductance transient analysis is done based on Langmuir adsorption theory [50]. In the present work, analytes under consideration, namely, ethanol and acetone are adsorbates whereas copper oxide sensor surface is the adsorbent. Assuming (1) homogeneity of surface, (2) monolayer adsorption, and (3) heat of adsorption is invariant of coverage; adsorption of gaseous species over a solid surface could be expressed as [34]:Where N (t) represents the number of gas species N
Conclusion
Though the plasma sprayed CuO coating was found suitable for CO sensing applications, C2H5OH and CH3COCH3 sensing performance of plasma sprayed CuO layer was hardly attempted. This paper addressed C2H5OH and CH3COCH3 sensing characteristics by altering temperature (200–300 °C) and analyte concentration (300-25 ppm). CuO coating showed useful gas response for C2H5OH (~844%) and CH3COCH3 (~600%) vapors. The superior gas response towards ethanol could be attributed to basic nature of CuO sensor,
Credit author statement
V. Ambardekar: Investigation, Data curation, Formal analysis, Writing – original draft, Writing – review & editing; T. Bhowmick: Formal analysis, Writing – review & editing, P.P.Bandyopadhyay: Conceptualization, Methodology, And Supervision, Writing – review & editing; S.B.Majumder: Conceptualization, Funding acquisition, Methodology, Supervision, Writing – review & editing
Data availability statement
The raw and processed data required to produce these finding could not be shared at this time since this data also forms important part of an ongoing study.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was partially funded by following sources: (1) MeiTY, Govt. of India, 5(1)/2017-Nano dated March 28, 2018 and (2) DST/NM/NNETRA/2018(G)-IITKGP dated March 21, 2018.
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