Carbone Monoxide (CO) Detection Device Based on the Nickel Antimonate Oxide and a DC Electronic Circuit

applied sciences

Article Carbone Monoxide (CO) Detection Device Based on the Nickel Antimonate Oxide and a DC Electronic Circuit

José Trinidad Guillen Bonilla 1,2,* ,Héctor Guillén Bonilla 3, Verónica María Rodríguez Betancourtt 4, Antonio Casillas Zamora 3, Jorge Alberto Ramírez Ortega 3, Lorenzo Gildo Ortiz 5, María Eugenia Sánchez Morales 6 , Oscar Blanco Alonso 5 and Alex Guillén Bonilla 7,* 1 Departamento de Electrónica, Centro Universitario de Ciencias Exactas e Ingenierías (C.U.C.E.I.), Universidad de Guadalajara, Blvd. M. García Barragán 1421, Guadalajara 44410, Jalisco, Mexico 2 Departamento de Matemáticas, Centro Universitario de Ciencias Exactas e Ingenierías (C.U.C.E.I.), Universidad de Guadalajara, Blvd. M. García Barragán 1421, Guadalajara 44410, Jalisco, Mexico 3 Departamento de Ingeniería de Proyectos, Centro Universitario de Ciencias Exactas e Ingenierías (C.U.C.E.I.), Universidad de Guadalajara, Blvd. M. García Barragán 1421, Guadalajara 44410, Jalisco, Mexico 4 Departamento de Química, Centro Universitario de Ciencias Exactas e Ingenierías (C.U.C.E.I.), Universidad de Guadalajara, Blvd. M. García Barragán 1421, Guadalajara 44410, Jalisco, Mexico 5 Departamento de Física, Centro Universitario de Ciencias Exactas e Ingenierías (C.U.C.E.I.), Universidad de Guadalajara, Blvd. M. García Barragán 1421, Guadalajara 44410, Jalisco, Mexico 6 Departamento de Ciencias Tecnológicas, Centro Universitario de la Ciénega (CUCienéga), Universidad de Guadalajara, Av. Universidad No. 1115, LindaVista, Ocotlán C.P. 47810, Jalisco, Mexico 7 Departamento de Ciencias Computacionales e Ingenierías, Centro Universitario de los Valles (CUValles), Universidad de Guadalajara, Carretera Guadalajara-Ameca Km. 45.5, Ameca 46600, Jalisco, Mexico * Correspondence: [email protected] (J.T.G.B.); [email protected] (A.G.B.); Tel.: +52-(375)-7580-500 (ext. 47417) (A.G.B.) Received: 17 August 2019; Accepted: 9 September 2019; Published: 11 September 2019

Featured Application: Our CO detection device finds practical safety applications where there is possible leakage of carbon monoxide by combustion and it is desirable to detect it. For example: cracked heat exchangers, locked chimneys, boiler safety systems, inadequate installation of boilers, heating devices without exhaust gas ducts and inverted fireplace effect.

Abstract: Carbon monoxide (CO) is very toxic to health. CO gas can cause intoxication and even death when the concentration is high or there are long exposure times. To detect atmospheres with CO gas concentration detectors are placed. In this work, a novel CO detection device was proposed and applied for CO detection. For its implementation, four stages were developed: Synthesis of nickel antimonite (NiSb2O6) oxide powders, physical characterization of NiSb2O6 powders, Pellet fabrication and sensitivity test in CO atmospheres and electronic circuit implementation where signal adaptation and signal ampliﬁcation were considered. Experimentally, a chemical sensor was built and characterized, its signal adaptation circuit was implemented and also it was proved using CO concentrations from 1 to 300 ppm with the operating temperatures of 100, 200, and 300 ◦C. Its optimal operation was at 300 ◦C. From the experimental results, the CO detection device had excellent functionality because the chemical sensor based on the nickel antimonite oxide had high sensitivity and good electrical response, whereas the DC electronic circuit had good performance.

Keywords: a novel CO detection device; CO detection; nickel antimonite oxide; chemical sensor; electronic circuit implementation; high sensitivity

Appl. Sci. 2019, 9, 3799; doi:10.3390/app9183799 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 3799 2 of 17

1. Introduction

Air pollution produced by the emission of gases such as CO, CO2, CH4,C3H8, SO2, and H2S, among others [1–3], has caused a substantial decrease in the quality of life of humans and other species. This emission of toxic gases into the atmosphere of large cities by industries and internal combustion engines is considered to be the main cause of diseases due to respiratory and cardiovascular nature, not to mention the greenhouse effect and the ecological and environmental imbalance they also provoke [4–9]. To mitigate the damage, it is necessary to have better control of the concentrations of harmful gases. For this purpose, different binary semiconductor oxides, such as SnO2, Mn2O3, GeO2, MoO3, and Nb2O5, among others, have been investigated in the past [10–12]. It has been recently suggested that ternary semiconductor oxides are strong candidates to be applied as gas sensors [13–16]. Among these materials, we can find the perovskite (e.g., LaFeO3)[17] and the spinel (e.g., CuFe2O4) type structures [18]. On the other hand, other studies report that semiconductor oxides with trirutile-type structure possess interesting detection properties since they are chemically stable and can operate at different temperature ranges (mainly between 200 and 350 ◦C) [19]. The trirutile-type oxides most commonly employed for this application are CuSb2O6 [20], NiSb2O6 [21], MgSb2O6 [22], ZnSb2O6 [23], and more recently CoSb2O6 [24]. It has been established that the high efficiency and good response of semiconductor materials for detecting toxic gases are mainly due to their morphology and porosity, and the nanometric size of the particles [19,22]. To obtain this type of microstructural features, different synthesis routes have been used in the past, like the solid-state reaction, hydrothermal, coprecipitation, and sol-gel processes [25–27]. On the other hand, the colloidal synthesis method has been increasingly employed because through this process it is possible to obtain porous surfaces, diverse morphologies (nanoparticles, nanowires, nanorods, and nanobelts), and particle sizes less than 100 nm [22–24]. The NiSb2O6 oxide was applied for the Liquefied Petroleum Gas (LPG) and Carbon dioxide (CO2) detection. The oxide was synthetized using a via sol-gel spin coating method. In the experimental work, the electrical resistance changed when the NiSb2O6 oxide was exposed to LPG and CO2. The measurement concentrations were into the interval of 1000 until 5000 ppm [21]. In reference [28], the authors proposed a chemical detector based on NiSb2O6 oxide. The NiSb2O6 powders were synthetized using the microwave-assisted colloidal method. In the experiments, propane (C3H8) and Carbon monoxide (CO) were detected, the concentrations were between 0 and 300 ppm, the operation temperatures were from 100 to 300 ◦C and the maximum sensitivity was approximately 2.14 (to 300 ◦C). On the other hand, using the microwave-assisted wet chemical method, the chemical sensor based on the NiSb2O6 oxide was better. The sensitivity was increased from 2.1 to 14.1 (to 300 ◦C) [29]. Again, in the experimental work, C3H8 and CO were used and the concentrations were between 0 and 300 ppm. However, to our knowledge, the signal adaptation was not proposed for the chemical sensor based on the NiSb2O6 oxide. In this work, the nickel antimonite (NiSb2O6) oxide was synthetized, characterized and applied as a chemical sensor for the carbon monoxide detection. Experimentally, its signal adaptation was implemented using a DC electronic circuit which was based on a Wheatstone bridge and operational amplifiers. The prototype device can detect CO concentration into the interval of 1 until 50 ppm and its optimal operating temperature was 300 ◦C. Our CO detection device has easy implementation, low cost, high sensitivity, good functionality, and also many practical safety applications.

2. CO Device Implementation

To build the CO detection device four stages were required: Synthesis of NiSb2O6 oxide powders, physical characterization of NiSb2O6 powders, Pellet fabrication and sensitivity test in CO atmospheres, Signal adaptation and signal ampliﬁcation where the DC electronic circuit was supplied with a DC supply voltage. Figure1 shows the four stages. Each stage is described in the next sections. Appl. Sci. 2019, 9, 3799 3 of 17 Appl. Sci. 2019, 9, x FOR PEER REVIEW 3 of 16

FigureFigure 1. 1. StagesStages required required for for th thee CO CO detection detection device. device.

2.1. Synthesis of NiSb2O6 Oxide Powders 2.1. Synthesis of NiSb2O6 Oxide Powders The production of the trirutile oxide powders was done through a microwave-assisted The production of the trirutile oxide powders was done through a microwave-assisted wet- wet-chemistry process [28–30] using 1.4560 g of Ni(NO3)2 6H2O (Aldrich, 99%), 2.2802 g of SbCl3 chemistry process [28–30] using 1.4560 g of Ni(NO3)2·6H2O (Aldrich,· 99%), 2.2802 g of SbCl3 (Sigma- (Sigma-Aldrich 99%), and 0.5 mL of ethylenediamine (Sigma-Aldrich 99%). The reagents were Aldrich ≥ 99%), and≥ 0.5 mL of ethylenediamine (Sigma-Aldrich ≥ 99%). The≥ reagents were diluted in diluted in 5 mL of absolute ethyl alcohol (Jalmek) and then left stirring for 20 min at room temperature. 5 mL of absolute ethyl alcohol (Jalmek) and then left stirring for 20 min at room temperature. The The solutions of nickel nitrate and antimony chloride were added dropwise to the ethylenediamine solutions of nickel nitrate and antimony chloride were added dropwise to the ethylenediamine solution. The resulting mixture produced a green precipitate, which was kept under stirring at solution. The resulting mixture produced a green precipitate, which was kept under stirring at room room temperature for 24 h at a speed of 300 rpm. Evaporation of the solvent was done by applying temperature for 24 h at a speed of 300 rpm. Evaporation of the solvent was done by applying seventeen 90-s exposures to a low power (140 W) microwave radiation using a domestic microwave seventeen 90-s exposures to a low power (140 W) microwave radiation using a domestic microwave oven (General Electric, model JES796WK). The energy absorbed by the solution was estimated at oven (General Electric, model JES796WK). The energy absorbed by the solution was estimated at ~214.2 kJ. Subsequently, using a programmable Novatech muﬄe, two thermal treatments at a rate of ~214.2 kJ. Subsequently, using a programmable Novatech muffle, two thermal treatments at a rate of 100 C/h in the air were applied to the precursor material: A drying at 200 C for 8 h and calcination at 100 °C/h◦ in the air were applied to the precursor material: A drying at 200◦ °C for 8 h and calcination 800 C for 5 h. at 800◦ °C for 5 h.

2.2. Physical Characterization of NiSb2O6 Powders 2.2. Physical Characterization of NiSb2O6 Powders The characterization of the crystalline phase of the trirutile oxide NiSb2O6 was carried out using powderThe X-raycharacterization diffraction of (XRD). the crystall The analysisine phase was of done the trirutile using Panalytical oxide NiSb Empyrean2O6 was carried equipment out using with powder X-ray diffraction (XRD). The analysis was done using Panalytical Empyrean equipment with CuKα radiation and a wavelength of 1.546 Å. The diffractograms were obtained at a 2θ scan from 10◦ CuKα radiation and a wavelength of 1.546 Å. The diffractograms were obtained at a 2θ scan from 10° to 60◦, applying 0.026◦-steps at a rate of 30 s per step. The porosity, morphology, and particle size of the to 60°, applying 0.026°-steps at a rate of 30 s per step. The porosity, morphology, and particle size of material dried at 800 ◦C were analyzed using a scanning electron-microscope (SEM, JEOL JSM-6390LV) thein modalitymaterial dried of high at vacuum 800 °C were and secondaryanalyzed usin electrong a scanning emission. electron-microscope (SEM, JEOL JSM- 6390LV) in modality of high vacuum and secondary electron emission. 2.3. Pellet Fabrication and Sensitivity Test in CO Atmospheres 2.3. Pellet Fabrication and Sensitivity Test in CO Atmospheres The sensitivity tests consisted of measuring the changes in the NiSb2O6 pellets’ electrical resistance at diThefferent sensitivity operating tests temperatures consisted (100, of measuring 200, and 300 the◦C) changes and diff erentin the concentrations NiSb2O6 pellets’ of CO electrical (0, 5, 50, resistance100, 200, and at different 300 ppm). operating The pellets temperatures of the material (100, were 200, elaboratedand 300 °C) using and 0.3different g of the concentrations powder dried of at CO800 (0,◦C 5, and 50, compacting 100, 200, and it by 300 means ppm). of The a hydraulic pellets of press the (Simplexmaterial Italwere Equip-25 elaborated Tons) using applying 0.3 g 10of tonsthe powderfor 60 s. dried The dimensionsat 800 °C and of compacting the pellets wereit by means of 0.5 mmof a inhydraulic thickness press and (Simplex 12 mm in Ital diameter. Equip-25 Prior Tons) to applyingthe gas detection 10 tons fo measurements,r 60 s. The dimensions two ohmic of contactsthe pellets of colloidalwere of 0.5 silver mm paint in thickness (Alfa Aesar, and 99%)12 mm were in diameter.placed on Prior the surface to the gas of thedetection pellets. measurements, Afterward, the tw NiSbo ohmic2O6 pellets contacts were of colloidal placed inside silver a paint measuring (Alfa 3 2 6 Aesar,chamber 99%) (Chambers) were placed with on a vacuumthe surfac capacitye of the of pellets. 10− torr. Afterward, The partial the pressure NiSb O of pellets the gases were inside placed the inside a measuring chamber (Chambers) with a vacuum capacity of 10-3 torr. The partial pressure of the gases inside the chamber was controlled using a TM20 Leybold detector. The electrical resistance Appl. Sci. 2019, 9, 3799 4 of 17

Appl. Sci. 2019, 9, x FOR PEER REVIEW 4 of 16 chamber was controlled using a TM20 Leybold detector. The electrical resistance measurements were mademeasurements using a Keithley were made 2001 using multimeter. a Keithley Finally, 2001 the mu responseltimeter. (sensitivity)Finally, the re wassponse estimated (sensitivity) according was toestimated the formula according [20,22,24 to]: the S = (GformulaG )[20,22,24]:/G , where, S G= (GyG G−GOare)/G theO, where, pellets’ G conductancesG y GO are the (1/ electricpellets’ G− O O G O resistance)conductances in the (1/electric test gas (CO)resistance) and air, in respectively.the test gas (CO) and air, respectively.

2.4.2.4. ElectronicElectronic CircuitCircuit ForFor thethe signalsignal adaptationadaptation ofof ourour chemicalchemical sensor,sensor, anan economiceconomic electronicelectronic circuitcircuit waswas proposed.proposed. TheThe electronicelectronic circuitcircuit consistsconsists ofof aa WheatstoneWheatstone bridge,bridge, aa didifferentialfferential amplifieramplifier andand ananoperational operational amplifieramplifier (Op-amp) (Op-amp) comparator comparator have have been been applied. applied. TheThe electronicelectronic circuitcircuit hashas beenbeen suppliedsupplied byby Vcc1Vcc1 and Vcc sources. The Wheatstone bridge consists of four resistors: R = R are precision resistors, R is and Vcc sources. The Wheatstone bridge consists of four resistors:1 𝑅 2=𝑅 are precision resistors,s the resistance of the chemical sensor and R is a variable resistance for calibration. The differential 𝑅 is the resistance of the chemical sensorx and 𝑅 is a variable resistance for calibration. The amplifierdifferential consists amplifier of an operationalconsists of amplifier an operational and four precision amplifier resistors, and Rfour3, R4 , precisionR5, and R 6.resistors, Finally, the Op-am comparator will be an operational amplifier. Basically, the signal adaptation circuit requires 𝑅,𝑅,𝑅,and 𝑅. Finally, the Op-am comparator will be an operational amplifier. Basically, the signal twoadaptation stages: calibrationcircuit requires and detection.two stages: calibration and detection.

2.4.1. Signal Adaptation Circuit (Wheatstone Bridge Calibration) 2.4.1. Signal Adaptation Circuit (Wheatstone Bridge Calibration) In the calibration stage, the chemical sensor is installed in atmospheres with no presence of CO, In the calibration stage, the chemical sensor is installed in atmospheres with no presence of CO, the sensor terminals are connected to the Wheatstone bridge according to Figure2 and the Wheatstone the sensor terminals are connected to the Wheatstone bridge according to Figure 2 and the bridge is calibrated by varying Rx until the following condition is met: Wheatstone bridge is calibrated by varying 𝑅 until the following condition is met:

V 𝑉 ==𝑉V −𝑉VB ==00 (1) (1) AB A− and then the next condition 𝑉 =𝑉 is true. and then the next condition VA = VB is true.

R1 R2

I1 I2

Vcc1 A B

Terminals Is Ix

Rs Rx Chemical sensor based on the NiSb2O6 oxide

D Figure 2. The conditions required for the calibration of a Wheatstone bridge. Figure 2. The conditions required for the calibration of a Wheatstone bridge. Based on the Equation (1) and Figure2, we will have Based on the Equation (1) and Figure 2, we will have VCA VCB VA = VB 𝑉 = 𝑉 (2) 𝑉 =𝑉→ →VAD = VBD (2) 𝑉 𝑉 Observing Figure2, VCA, VAD, VCB, and VBD are deﬁned as Observing Figure 2, 𝑉, 𝑉, 𝑉, and 𝑉 are defined as

VCA𝑉 I𝐼1R𝑅1 V𝑉CB 𝐼I2𝑅R2 == and 𝑎𝑛𝑑 == (3)(3) VAD𝑉 I𝐼sR𝑅s V𝑉BD 𝐼Ix𝑅Rx

Since the Wheatstone bridge is calibrated, then 𝐼 =𝐼 and 𝐼 =𝐼 are true and as a consequence, Equation (2) can express through Appl. Sci. 2019, 9, 3799 5 of 17

Appl. Sci. 2019, 9, x FOR PEER REVIEW 5 of 16 Since the Wheatstone bridge is calibrated, then I1 = Is and I2 = Ix are true and as a consequence,

Equation (2) can express through 𝑅 𝑅 𝑉 =𝑉 → R1 = R2 (4) VA = VB 𝑅 =𝑅 (4) → Rs Rx From Equation (4), the resistance 𝑅 can be determined by From Equation (4), the resistance Rx can be determined by 𝑅 𝑅 =𝑅 (5) R𝑅2 Rx = Rs (5) R1 When Equation (5) is satisfied, then the Whetstone bridge is completely calibrated. When Equation (5) is satisfied, then the Whetstone bridge is completely calibrated. 2.4.2. Detection Circuit (Signal Adaptation + Signal Amplification) 2.4.2. Detection Circuit (Signal Adaptation + Signal Amplification) In the detection stage, the following five steps are carried out: (a) The sensor terminals are connectedIn the to detection the Wheatstone stage, the bridge following according five to steps Figu arere 3, carried (b) The out: chemical (a) The sensor sensor is exposed terminals to arethe connectedcarbon monoxide to the Wheatstone atmosphere bridge placing according the electronic to Figure circuit3, (b) in The a secure chemical area, sensor (c) When is exposed the chemical to the carbonsensor monoxidedetects the atmosphere presence of placing CO, the the Wheatstone electronic circuit bridge in has a secure an imbalance area, (c) Whenand the the following chemical sensorconditions detects are the met: presence of CO, the Wheatstone bridge has an imbalance and the following conditions are met: 𝑉 ≠𝑉 𝑎𝑛𝑑 𝑉 <𝑉 (6) VA, VB and VA < VB (6)

(d) Both voltages 𝑉 and 𝑉 have been compared applying a differential amplifier based on the (d) Both voltages VA and VB have been compared applying a differential amplifier based on the operational amplifier,amplifier, seesee FigureFigure3 .3.

Figure 3. Electronic circuit proposed for the CO detection. Figure 3. Electronic circuit proposed for the CO detection. From Figure3, the di fferential amplifier satisfies From Figure 3, the differential amplifier satisfies ! ! R𝑅6 R𝑅4 R𝑅4 V𝑉o==𝑉VB 1+1 + −𝑉VA (7)(7) R𝑅6++𝑅R5 R𝑅3 − R𝑅3

If 𝑅 =𝑅 and 𝑅 =𝑅, Equation (7) takes the form

𝑉 = 𝐴𝑉 −𝑉 (8) Appl. Sci. 2019, 9, 3799 6 of 17

If R3 = R5 and R4 = R6, Equation (7) takes the form

Vo = A(VB V ) (8) Appl. Sci. 2019, 9, x FOR PEER REVIEW − A 6 of 16 R4 R6 where the amplification, A = = is a factor and Vo is an output voltage of our differential amplifier. where the amplification, 𝐴=R3 =R5 is a factor and 𝑉 is an output voltage of our differential Immediately, we impose the conditions R3 = R4 and R5 = R6. Then, the output voltage is the difference amplifier. Immediately, we impose the conditions 𝑅 =𝑅 and 𝑅 =𝑅. Then, the output voltage is the difference Vo = VB V (9) − A 𝑉 =𝑉 −𝑉 (9) (e) The voltage Vo is applied as an input signal to the comparator circuit verifying that: (e) The voltage 𝑉 is applied as an input signal to the comparator circuit verifying that:

VAlarm = AolVo (10) 𝑉 = 𝐴𝑉 (10) wherewhereV 𝑉Alarmis is the the alarm alarm signal signal produced produced by the by detection the detection device anddeviceAol isand the 𝐴 gain is of the the gain operational of the amplifieroperational in anamplifier open loop. in an Thus, open theloop. signal Thus, is amplifiedthe signal byis theamplified operational by the amplifier. operational According amplifier. to EquationAccording (10), to theEquation device (10), will generatethe device an will alarm genera signalte whenan alarm the chemicalsignal when sensor the is chemical exposed tosensor carbon is monoxide,exposed to CO.carbon monoxide, CO.

3.3. Experimental Results

3.1.3.1. XRD Analysis FigureFigure4 4shows shows the the di diffractogramﬀractogram of of the the material material obtained obtained from from the the chemical chemical reaction reaction between between the nickelthe nickel nitrate nitrate and and the antimonythe antimony chloride, chloride, plus plus ethylenediamine, ethylenediamine, and calcinedand calcined at 800 at◦ C.800 In °C. the In XRD the pattern, the characteristic peaks of NiSb O located at points 2θ = 19.23 , 21.41 , 27.15 , 33.49 , 34.99 , XRD pattern, the characteristic peaks of2 NiSb6 2O6 located at points 2θ ◦= 19.23°,◦ 21.41°,◦ 27.15°,◦ 33.49°,◦ 38.7734.99°,◦, 40.1638.77°,◦, 43.6340.16°,◦, 44.7143.63°,◦, 44.71°, 53.21◦, 53.21°, and 56.20 and◦ can56.20° be clearlycan be clearly observed. observed.

FigureFigure 4.4. DiDiffractogramﬀractogram ofof thethe oxideoxide NiSbNiSb22OO66 obtained by calcination at 800 ◦°CC in air.

TheThe mainmain crystallinecrystalline phasephase ofof thethe oxideoxide waswas identifiedidentified byby meansmeans ofof thethe PDFPDF filefile No.No. 86-0110. AccordingAccording toto this,this, thethe NiSbNiSb22OO66 crystallizedcrystallized intointo aa tetragonaltetragonal structurestructure withwith cellcell parametersparameters aa= = bb == 4.6414.641 ÅÅ andand cc == 9.2199.219 Å,Å, withwith spatialspatial groupgroup P4P422//mnmmnm (136).(136). It has been previouslypreviously reportedreported thatthat thethe peakspeaks ofof thethe oxide’soxide’s mainmain phasephase (see(see FigureFigure4 4)) belong belong to to the the trirutile-type trirutile-type family family of of structures structures or or the the trirutile-typetrirutile-type antimonatesantimonates [[21,28].21,28]. ThisThis characteristiccharacteristic isis obtainedobtained byby modifyingmodifying thethe axis’axis’ c-directionc-direction ofof the rutile cell [31]. On the other hand, a small portion of inorganic material outside the main phase can be seen in the diffractogram. This reflection was located at point 2θ = 37.1° and corresponds to a secondary phase of NiO according to the PDF file No. 65-2901. The results shown in Figure 4 are consistent with literature reports [32,33] where different synthesis methods of the NiSb2O6’s crystalline phase were employed, like the solid-state reaction [31] and the colloidal method [34]. In this work, an economical and simple to implement alternative, a Appl. Sci. 2019, 9, 3799 7 of 17 the rutile cell [31]. On the other hand, a small portion of inorganic material outside the main phase can be seen in the diffractogram. This reflection was located at point 2θ = 37.1◦ and corresponds to a secondary phase of NiO according to the PDF file No. 65-2901. Appl.The Sci. results 2019, 9, shownx FOR PEER in Figure REVIEW4 are consistent with literature reports [ 32,33] where different synthesis7 of 16 methods of the NiSb2O6’s crystalline phase were employed, like the solid-state reaction [31] and the colloidalwet-chemistry method process, [34]. In thiswas work, used successfully. an economical It was and possible simple to to implement obtain with alternative, its particle sizes a wet-chemistry less than process,100 nm. was used successfully. It was possible to obtain with its particle sizes less than 100 nm.

3.2.3.2. SEM SEM Analysis Analysis

Figure 5 shows three typical photomicrographs of the calcined NiSb2O6’s surface at different Figure5 shows three typical photomicrographs of the calcined NiSb 2O6’s surface at diﬀerent magniﬁcations:magnifications: 500 500×,, 7000 7000×,, andand 1500015000×. . × × ×

Figure 5. SEM images of the calcined (at 800 ◦C) NiSb2O6’s microstructure at magnifications: (a) 500 , Figure 5. SEM images of the calcined (at 800 °C) NiSb2O6’s microstructure at magnifications: (a) 500×, × (b) 7000 ,(c) 15000 , and (d) Particle size distribution. (b) 7000×,× (c) 15000×,× and (d) Particle size distribution. According to the results shown in Figure5a,b, a large agglomeration of di fferently sized particles According to the results shown in Figure 5a,b, a large agglomeration of differently sized particles can be identified. The morphology of the particles is irregular, showing a rough surface generated can be identified. The morphology of the particles is irregular, showing a rough surface generated by by the elimination of organic matter when the material is calcined at 800 C (see Figure5c). Growth the elimination of organic matter when the material is calcined at 800 °C (see◦ Figure 5c). Growth and andnucleation nucleation of this of this type type of morphology of morphology can be can attrib beuted attributed to the residence to the residence time of timethe material of the materialinside inside the muffle and to the effect of the ethylenediamine used for the preparation of the NiSb O . the muffle and to the effect of the ethylenediamine used for the preparation of the NiSb2O6. We have2 6 Wereported have reported in previous in previous works that, works by that,using by ethyle usingnediamine ethylenediamine during the during synthesis the synthesisprocess, it process, was it waspossible possible to obtain to obtain different different types types of ofmorphologi morphologies,es, like like microrods, microrods, octahedrons, octahedrons, hexagonal, hexagonal, microplates,microplates, and and mesoporous-nanoparticles mesoporous-nanoparticles suitablesuitable for being being used used as as gas gas sens sensorsors [19,20,22–24,28,29]. [19,20,22–24,28,29 ]. TheThe size size of of the the particles particles was was estimated estimated inin thethe range of 0.1–0.9 0.1–0.9 μµm,m, with with an an average average of of ~0.457 ~0.457 μmµ m andand a standarda standard deviation deviation of of ±0.1250.125 µμm.m. These results are are shown shown in in the the form form of of a ahistogram histogram of ofthe the ± particleparticle size size distribution distribution in Figurein Figure5d. The5d. The porosity porosi onty the on material’sthe material’s surface surface is largely is largely due todue the to release the of gases,release suchof gases, as water such as vapor, water NO vapor,x, and NO COx, and2 during CO2 during the heat the treatment heat treatment applied applied to the to material the material [30]. [30].In general, the ethylenediamine’s effect on the production of different morphology types has been thoroughlyIn general, discussed the inethylenediamine’s previous works [ 32effect,33]. on The the ethylenediamine production of different is incorporated morphology into the types inorganic has frameworkbeen thoroughly and then discussed lost by the in heatprevious treatment, works thus [32,33 emerging]. The ethylenediamine different types ofis incorporated microstructures into [32 the,33 ], asinorganic occurred inframework this work. and LaMer then and lost Dinegar by the [35 heat] proposed treatment, a mechanism thus emerging that involves different the types formation of of nanoparticlesmicrostructures with [32,33], diff erentas occurred types ofin morphologiesthis work. LaMer (mainly and Dinegar colloidal [35] dispersions). proposed aThese mechanism authors that involves the formation of nanoparticles with different types of morphologies (mainly colloidal dispersions). These authors refer that the growth and nucleation of inorganic monodisperse colloidal particles are based on three fundamental stages: The first one states that the concentration of the reactants in the colloidal dispersions increases gradually, the second one establishes that the Appl. Sci. 2019, 9, 3799 8 of 17 refer that the growth and nucleation of inorganic monodisperse colloidal particles are based on three fundamental stages: The first one states that the concentration of the reactants in the colloidal dispersions increases gradually, the second one establishes that the concentration of the reactants reaches a limit of supersaturation and thus, nucleation occurs rapidly forming the nuclei of the crystals,

finally,Appl. the Sci. third 2019, one9, x FOR refers PEER thatREVIEW the growth of the particles occurs, originating their morphology.8 of 16 Agreeing with these authors, the microstructure depicted in Figure5 can be attributed to the production of stableconcentration nuclei formed of the during reactants the reaches reaction a limit of theof supersaturation reactants when and the thus, ethylenediamine nucleation occurs is rapidly added to the process,forming giving the nuclei rise toof athe colloidal crystals, dispersionfinally, the third [19] and,one refers as a consequence,that the growth the of the morphology particles occurs, and the particleoriginating sizes shown their in morphology. this work. Agreeing with these authors, the microstructure depicted in Figure 5 can be attributed to the production of stable nuclei formed during the reaction of the reactants when 3.3. Sensingthe ethylenediamine Properties Analysis is added to the process, giving rise to a colloidal dispersion [19] and, as a consequence, the morphology and the particle sizes shown in this work. Figure6 shows the response of the NiSb 2O6 pellets as a function of the CO concentration at the operating3.3. Sensing temperatures Properties 100, Analysis 200, and 300 ◦C. It can be clearly verified in the graph that the pellets are sensitiveFigure to the 6 shows concentrations the response of of the the gas. NiSb The2O6 highpellets response as a function of the of materialthe CO concentration is closely related at the to the chemicaloperating adsorption temperatures of the 100, gas 200, on and the 300 pellets’ °C. It can surface, be clearly as wellverified as toin the graph increase that ofthe the pellets test are gases’ concentrationsensitive and to the of theconcentrations temperature. of the This gas. is corroboratedThe high response in Figure of the6 materialsince as is the closely concentration related to ofthe the gas andchemical the operating adsorption temperature of the gas increase, on the pellets’ the effi surface,ciency of as the well pellets as to tothe detect increase the of CO the concentrations test gases’ improvesconcentration substantially. and of The the results temperature. obtained This in is thecorrob gasorated are attributed in Figure 6 to since the as fact the that concentration the increase of in the operatingthe gas temperatureand the operating favors temperature the reaction increase, kinetics the between efficiency the surfaceof the pellets of the to pellets detect and the theCO test gas [20],concentrations contributing improves to a higher substantially. oxygen desorption The results during obtained the in adsorption the gas are ofattributed the test to gas the [20 fact,22 that,24,30 ], the increase in the operating temperature favors the reaction kinetics between the surface of the which leads to changes in the material’s electrical resistance and, consequently, on the high response pellets and the test gas [20], contributing to a higher oxygen desorption during the adsorption of the obtainedtest during gas [20,22,24,30], the detection which tests.leads to The changes adsorption in the material’s of different electrical types resistance of oxygen and, consequently, species at high temperatureson the high has response been reported obtained in during the literature the detection [36 ].tests. It can The beadsorption seen in Figureof different6 that types the of maximum oxygen sensitivityspecies magnitudes, at high temperatures ~0.05 and has ~0.35, been werereported respectively in the literature obtained [36]. atIt can 200 be and seen 300 in◦ FigureC for a6 carbonthat monoxidethe maximum concentration sensitivity of 300 magnitudes, ppm. ~0.05 and ~0.35, were respectively obtained at 200 and 300 °C Thefor response a carbon monoxide of the NiSb concentration2O6 pellets of is associated300 ppm. to the mechanism of oxygen desorption at high temperaturesThe [ 36response], which of impliesthe NiSb2 thatO6 pellets at a temperature is associated to lower the mechanism than 150 ◦ofC, oxygen the thermal desorption energy at high isnot enoughtemperatures to provoke the[36], oxygen which implies desorption that at reactions a temperature so that lower no electrical than 150 °C, response the thermal occurs energy regardless is not of enough to provoke the oxygen desorption reactions so that no electrical response occurs regardless the gas concentration. In general, the most abundant oxygen species at temperatures below 150 ◦C of the gas concentration. In general, the most abundant oxygen species at temperatures below 150 °C are the O2− ions. Above 150 ◦C (in our case 300 ◦C) the formation of oxygen species such as theO− 2are the 𝑂 ions. Above 150 °C (in our case 300 °C) the formation of oxygen species such as the 𝑂 and O and− ions 𝑂 takes ions place,takes place, which which are moreare more reactive reactive [22 [22,24,28,30,36].,24,28,30,36]. Furthermore,Furthermore, the the increase increase of the of the temperaturetemperature causes causes a rise a in rise the in solid-gasthe solid-gas interactions interactions [19 [19,22,30],,22,30], leadingleading to to a a good good material’s material’s response response in CO atmospheres.in CO atmospheres. Sensitivity

Figure 6. Response of NiSb2O6 pellets as a function of (a) CO and concentrations. Figure 6. Response of NiSb2O6 pellets as a function of (a) CO and concentrations. In order to know the variations of the electrical response (resistivity) in the presence of different carbon monoxide (CO) concentrations, the changes in the resistivity were estimated as a function of Appl. Sci. 2019, 9, 3799 9 of 17

Appl.In Sci. order 2019, 9 to, x knowFOR PEER the REVIEW variations of the electrical response (resistivity) in the presence of diﬀerent9 of 16 carbon monoxide (CO) concentrations, the changes in the resistivity were estimated as a function of the gasthe concentration gas concentration and the and operating the operating temperature. temperatur Thee. calculation The calculation was made was with made the with formula the formula [20,37]: ρ[20,37]:= RA/t, ρ where = RA/t, R where is theelectrical R is the electrical resistance resistance in the test in gases, the test A gases, is the areaA is ofthe the area cross-section, of the cross-section, and t is theand thickness t is the thickness of the pellets of the (= pellets0.5 mm, (=0.5 diameter mm, diameter= 12 mm). = 12 The mm). results The are results shown are in shown Figure in7. Figure 7.

FigureFigure 7. 7. ElectricalElectrical responseresponse (resistivity) of NiSb NiSb22OO6 6pelletspellets as as a afunction function of of(a) ( aCO) CO concentration, concentration, (b) (bCO) CO operating operating temperature. temperature.

AccordingAccording to to the the results, results, the the electrical electrical resistivity resistivit ofy theof the material material in CO in CO atmospheres atmospheres decays decays as the as concentrationthe concentration and theand operating the operating temperature temperature increase. increa Thisse. This means means that that the electricalthe electrical conductivity conductivity of theof the material material increases increases proportionally proportionally as theas the resistivity resistivity decreases decreases (see (see Figure Figure7a,b). 7a,b). This This behavior behavior is is mainlymainly attributed attributed to to the the fact fact that, that, due due to to the the increase increase of of temperature, temperature, the the charge charge carriers carriers show show more more mobilitymobility on on the the surface surface of of the the pellets, pellets, leading leading to to an an increase increase in thein the conductivity conductivity [20 ,[20,24].24]. In addition,In addition, it is it evidentis evident that that increasing increasing the concentrationthe concentration of the of testthe test gases gases and and the operatingthe operating temperature, temperature, it favors it favors the decreasethe decrease in resistivity. in resistivity. This This phenomenon phenomenon occurs occurs because because thekinetic the kinetic activity activity of the of gasthe moleculesgas molecules on theon pellets’the pellets’ surface surface is increased is increased by the by rise the in rise operating in operating temperature temperature [24] (in our[24] case(in our at 200 case and at 300200◦ andC). It300 has °C). been It reportedhas been in repo therted literature in the that literature the temperature that the temp anderature the geometric and the shape geometric of the shape sensor of (i.e., the pelletssensor or (i.e., films) pellets plays or a films) quite importantplays a quite role importan in the chemicalt role in interaction the chemical between interaction the test between gases and the thetest chemisorbedgases and the oxygen chemisorbed on the material’soxygen on surface the material’s [20,24,38 surface,39], contributing [20,24,38,39], to producingcontributing changes to producing in the electricalchanges resistivity,in the electrical as shown resistivity, in Figure as7 showna,b. This in trendFigure is 7a,b. typical This of trend a semiconductor is typical of material a semiconductor like the onematerial studied like in the this one work studied [20,24 in,37 this,38, 40work]. [20,24,37,38, 40]. PerformingPerforming the the test test on on a a particle particle at at 300 300◦ C,°C, the the resistivity resistivity of of the the material material was was high, high, with with an an almost almost linearlinear trend trend regardless regardless of of the the CO CO concentrations. concentrations. However, However, by by increasing increasing the the operating operating temperature temperature toto 200 200◦ C,°C, the the conductivity conductivity increased, increased, and and the the resistivity resistivity decreased. decreased. InIn thethe casecase ofofthe the oxide oxide pellets, pellets, theythey showed showed inflection inflection points points at at 200 200◦ C°C for for both both the the CO CO atmospheres atmospheres (see (see Figure Figure7b). 7b). The The resistivity resistivity inflectioninflection points points shown shown by by the the pellets pellets at at this this temperature temperature are are clear clear indications indications of of the the transition transition that that thethe material material undergoes undergoes due due to to the the increase increase in in the the operating operating temperature temperature of of the the sensor. sensor. On On the the other other hand,hand, at at 300 300◦ C,°C, the the resistivity resistivity in in the the atmospheres atmospheres decreased decreased significantly, significantly, where where the the minimum minimum value value ofof resistivity resistivity inin carbon monoxide monoxide was was of of 94.0 94.0 ΩmΩ mat 300 at 300 ppm. ppm. Considering Considering the results the results shown shown in Figure in Figure7a,b, 7thea,b, high the highresponse response is attributed is attributed mainly mainly to tothe the nanometric nanometric size size obtained duringduring thethe synthesis synthesis processprocess of of the the oxide. oxide. Recent Recent reports reports have have shown shown that that by by reducing reducing the the particle particle size size of of semiconductors,semiconductors, thethe area-to-volume area-to-volume ratio ratio considerably considerably increases increases thus thus favoring favoring high high gas gas adsorption adsorption on the on surfacethe surface of the of materialthe material [30,40 [30,40,,41], leading 41], leading to a high to a responsehigh response in diff inerent different types types of gases. of gases.

3.4.3.4. Resistance Resistance vs. vs. COCO ConcentrationConcentration FromFrom Figure Figure7 a,7a, the the electrical electrical resistance resistance has has been been calculated calculated for for our our chemical chemical sensor sensor through through EquationEquation R R= =ρ ρt /t/A,A, and and then, then, the the behavior behavior of of resistance resistance vs. vs. CO CO concentration concentration can can be be estimated estimated and and graphedgraphed as as Figure Figure8 illustrates.8 illustrates. Appl.Appl. Sci. Sci.2019 2019, 9, ,9 3799, x FOR PEER REVIEW 1010 ofof 17 16 ) Ω Resistance (

FigureFigure 8. 8.Response Response of of NiSb NiSb2O2O6 6pellets: pellets: Resistance Resistance vs. vs.CO COconcentrations. concentrations.

ObservingObserving Figure Figure8, 8, when when the the chemical chemical sensor sensor and and CO CO gas gas make make a a reaction, reaction, the the resistance resistance decays decays forfor three three temperatures, temperatures, 100, 100, 200 200 and and 300 300◦ C.°C. The The sensor sensor has has the the highest highest resistance resistance if if both both temperatures temperatures areare 100 100 and and 200 200◦ C.°C. However, However, when when the the temperature temperature is is 300 300◦ C,°C, sensor’s sensor’s resistance resistance is is lower, lower, its its decline decline isis major major but but the the sensor sensor has has more more sensitivity. sensitivity. As As a consequence, a consequence, the the operating operating sensor sensor is better is better at 300 at ◦300C when°C when the sensor´sthe sensor´s resistance resistance is into is theinto interval the interval of 578 ofΩ 578and Ω 431andΩ 431. Ω. 3.5. CO Detection Circuit 3.5. CO Detection Circuit Based on Figures3,7a and8, a new CO detection system would be implemented using the Based on Figures 3, 7a and 8, a new CO detection system would be implemented using the NiSb2O6 pellets and a DC electronic circuit. Its characteristics are: its operating concentration is into the NiSb2O6 pellets and a DC electronic circuit. Its characteristics are: its operating concentration is into interval of 5 and 300 ppm, its operating temperatures are 100 C, 200 C, and 300 C, high sensitivity at the interval of 5 and 300 ppm, its operating temperatures◦ are 100◦ °C, 200 ◦°C, and 300 °C, high 300 C, economic fabrication and easy implementation. To operate the CO detection system at 300 C sensitivity◦ at 300 °C, economic fabrication and easy implementation. To operate the CO detection◦ and 50 ppm (or major), we propose the electronic circuit as Figure9 illustrates. system at 300 °C and 50 ppm (or major), we propose the electronic circuit as Figure 9 illustrates. Observing Figure9, R1, R2, R3, R4, R5, and R6 have values of 1 KΩ (1000 Ω), the sensor´s resistance Observing Figure 9, 𝑅 , 𝑅 , 𝑅 , 𝑅 , 𝑅, and 𝑅 have values of 1 KΩ (1000 Ω), the sensor´s Rs was measured to ~560 Ω (Figure8) and consequently, resistance Rx was calibrated to ~560 Ω resistance 𝑅 was measured to ~560 Ω (Figure 8) and consequently, resistance 𝑅 was calibrated to (Equation (5)). The Wheatstone bridge is supplied by Vcc1 5 V. Both operational amplifiers are ~560 Ω (Equation (5)). The Wheatstone bridge is supplied by≈ 𝑉 ≈5 V. Both operational amplifiers supplied by Vcc = 12 V. The differential amplifier has unitary amplification. Finally, the comparator are supplied± by ±𝑉± = ±12 V. The differential amplifier has unitary amplification. Finally, the has its gain in the operational amplifier in an open loop, Aol = 10,000 (see Equation (10)). comparator has its gain in the operational amplifier in an open loop, 𝐴 = 10,000 (see Equation On the other hand, Figure 10 shows the electronic diagram of our DC supply voltage. The voltage (10)). source has an error of 0.2 V, high repeatability, low cost, and good precision. In the power supply, On the other hand,± Figure 10 shows the electronic diagram of our DC supply voltage. The the transformer reduces the alternate current (AC) from 120 V to 18 V, the bridge diode transforms voltage source has an error of ±0.2 V, high repeatability, low cost, and good precision. In the power the alternate current (AC) to direct current (DC), the capacitors filter the electrical signal and devices, supply, the transformer reduces the alternate current (AC) from 120 V to 18 V, the bridge diode LM317 T, LM7812, and LM7912 regulate the current voltages, V 5 V and Vcc = 12 V. Notice transforms the alternate current (AC) to direct current (DC), thecc 1capacitors≈ filter± the ±electrical signal that, Vcc1 has a fine-tuning through the resistor RAF. and devices, LM317 T, LM7812, and LM7912 regulate the current voltages, 𝑉 ≈5 V and ±𝑉 = ± 12 V. Notice that, 𝑉 has a fine-tuning through the resistor 𝑅. Appl. Sci. 2019, 9, 3799 11 of 17 Appl. Sci. 2019, 9, x FOR PEER REVIEW 11 of 16

Figure 9. AA CO CO detection detection system system proposed proposed at 300 ◦°CC andand 5050 ppmppm (or(or higher).higher). Appl. Sci. 2019, 9, 3799 12 of 17 Appl. Sci. 2019, 9, x FOR PEER REVIEW 12 of 16

FigureFigure 10. 10.Electronic Electronic diagram diagram of of our our DC DC supply supply voltage. voltage.

3.6.3.6. CO CO Detection Detection Device Device FigureFigure 11 11 shows shows our our CO CO detection detection device. device. For For its it construction,s construction, the the electronic electronic circuit circuit which which was was shownshown in in Figure Figure9 9and and thethe DC supply supply source source shown shown in in Figure Figure 10 10were were considered. considered. The Printed The Printed Circuit CircuitBoard Board(PBC) was (PBC) designed was designed using the using CAD the Altium CAD desi Altiumgn program, design program,version 17. version Finally, 17. the Finally,required thematerials required were: materials a transformer were: a transformer 120:24 120:24Volts, Volts,a diode a diode bridge bridge KBL610 KBL610 (3A), (3A), sevenseven capacitorscapacitors (C1 = C2, = C3 = C4 = 2200 µF (50 V), C5 = C6 = 0.1 µF, C7 = 1 nF), seven precision resistors 𝐶 =𝐶,=𝐶 =𝐶 = 2200 μF 50 V,𝐶 =𝐶 =0.1 μF,𝐶 =1 nF , seven precision resistors 𝑅 = (R1 = R2 = R3 = R4 = R5 = R6 = I KΩ, R7 = 2 KΩ), a potentiometer (Rv = 10 KΩ), four rectangular 𝑅 =𝑅 =𝑅 =𝑅 =𝑅 =𝐼 𝐾Ω,𝑅 =2 𝐾Ω, a potentiometer 𝑅 =10 KΩ, four rectangular cermet cermet trimmer potentiometers (Rx = 1 KΩ, RAF = 1 KΩ, RG1 = 10 KΩ, RG2 = 10 KΩ), a TL084 (3 V trimmer potentiometers 𝑅 =1 KΩ,𝑅 =1 KΩ,𝑅 =10 KΩ,𝑅 = 10 KΩ, a TL084 (3 V to 32 V, to 32 V, DIP, 14 Pines), a phenolic one side plate (15 cm 15 cm), three screw connectors, an LM317T DIP, 14 Pines), a phenolic one side plate (15 cm × ×15 cm), three screw connectors, an LM317T (regulator to 5 V), an LM7812 (regulator to 12 V), and an LM7912 (Regulator to 12 V). (regulator to 5 V), an LM7812 (regulator to 12 V), and an LM7912 (Regulator to− −12 V). In the CO detection device, the sensitivity was increased collocating two resistors, RG1 and RG2. In the CO detection device, the sensitivity was increased collocating two resistors, 𝑅 and 𝑅. That is possible because R4 and RG1 were connected using a serial array whereas R6 and RG2 were That is possible because 𝑅 and 𝑅 were connected using a serial array whereas 𝑅 and 𝑅 were also connected to applying a serial array. Therefore, Equation (8) is satisfied if and only if RG1 = RG2. also connected to applying a serial array. Therefore, Equation (8) is satisfied if and only if 𝑅 =𝑅. The output voltage Vo will be The output voltage 𝑉 will be Vo = Am(VB V ) (11) − A 𝑉 = 𝐴𝑉 −𝑉 (11) R4+RG1 R6+RG2 where the modified amplification is given by Am = = and its value can only be between R3 R5 where the modified amplification is given by 𝐴 = = and its value can only be between 1 and 11. 1 andOn 11. the other hand, the device operates as Figure 12 illustrates. If CO concentration is equal or greaterOn the than other 50 ppm, hand, the the sensor device´s operates resistance as diminishesFigure 12 illustrates. and as a consequence,If CO concentration the Wheatstone is equal or bridgegreater is unbalanced,than 50 ppm, provoking the sensor´s the resistance signal alarm diminiV shes and11.2 asV a(Alarm consequence, state “On”). the Wheatstone However, ifbridge CO Alarm ≈ concentrationis unbalanced, is lowerprovoking than 50ppm, the signal the Equationalarm 𝑉 (6) is≈11.2 not satisfied V (Alarm and thenstate the“On”). signal However, alarm is closeif CO toconcentration zero, V is 0lowerV (Alarm than 50ppm, state “O theff”). Equation For our (6) new is not toxic satisfied gas detector and then device, the signal the thresholding alarm is close Alarm ≈ valueto zero, can 𝑉 be selected≈0 V through (Alarm Rstatex: if R“Off”).x > 560 ForΩ thenour new the device toxic gas will detector detect CO device, concentrations the thresholding lower thanvalue 50 can ppm be (or selected higher) through but if R x𝑅<: 560if 𝑅Ω>then 560 theΩ then device the will device detect will CO detect concentrations CO concentrations greater thanlower 50than ppm. 50 ppm (or higher) but if 𝑅 < 560 Ω then the device will detect CO concentrations greater than 50 ppm. Appl. Sci. 2019, 9, x FOR PEER REVIEW 13 of 16 Appl. Sci. 2019, 9, 3799 13 of 17 Appl. Sci. 2019, 9, x FOR PEER REVIEW 13 of 16

Figure 11. CO detection device built and its electronic elements.

FigureFigure 12. 12.Signals Signals measuredmeasured withwith our CO detection devi devicece (operating (operating temperature temperature to to 50 50 ppm ppm and and T = T300= 300 °C).◦C). Figure 12. Signals measured with our CO detection device (operating temperature to 50 ppm and T = 4. Discussion300 °C). 4. Discussion Based on the experimental results, a new prototype was proposed, developed, and applied for CO4. Discussion gasBased detection. on the Theexperimental prototype results, device consisteda new prot ofotype a chemical was proposed, sensor and deve aloped, DC electronic and applied circuit. for CO gas detection. The prototype device consisted of a chemical sensor and a DC electronic circuit. The chemicalBased on sensorthe experimental was made results, using the a new nickel prot antimonateotype was oxide.proposed, The deve oxideloped, was and synthetized applied for by The chemical sensor was made using the nickel antimonate oxide. The oxide was synthetized by meansCO gas of detection. an alternative The prototype microwave-assisted device consisted wet-chemistry of a chemical process, sensor using and nickel a DC nitrate,electronic antimony circuit. means of an alternative microwave-assisted wet-chemistry process, using nickel nitrate, antimony chloride,The chemical ethylenediamine, sensor was made and ethylusing alcohol. the nickel The antimonate precursor materialoxide. The was oxide dried was at 200 synthetized◦C and later by chloride, ethylenediamine, and ethyl alcohol. The precursor material was dried at 200 °C and later calcinedmeans of at an 800 alternative◦C in static microwave-assisted air. The crystalline wet-chemistry phase of the calcined process, material using nickel was analyzednitrate, antimony by X-ray calcined at 800 °C in static air. The crystalline phase of the calcined material was analyzed by X-ray dichloride,ﬀraction, ethylenediamine, ﬁnding the cell parametersand ethyl alcohol. a = b = The4.641 precursor Å and c =material9.219 Å, was and dried spatial at 200 group °C P4and2/mnm later diffraction, finding the cell parameters a = b = 4.641 Å and c = 9.219 Å, and spatial group P42/mnm calcined at 800 °C in static air. The crystalline phase of the calcined material was analyzed by X-ray diffraction, finding the cell parameters a = b = 4.641 Å and c = 9.219 Å, and spatial group P42/mnm Appl. Sci. 2019, 9, 3799 14 of 17

(136). Their sensitivity tests were developed applying static tests (Direct current, DC), an excellent electrical response was obtained. For its signal adaptation a DC electronic circuit was built which was based on a Wheatstone bridge, a DC source, a differential amplifier, and an operational amplifier comparator. That electronic circuit was implemented on a PBC of size 15 cm 15 cm. × The CO detection device has good characteristics: low cost, high sensitivity, good performance, fast response, selective sensitivity (through trimmer potentiometers), adaptability, CO concentration detected into the interval of 1 to 300 ppm, operating temperatures 100, 200, and 300 ◦C, optimal operating temperature at 300 C, its dimension (15 cm 15 cm), AC supply voltage (120 V) and DC ◦ × output voltage (11.2 V). However, such characteristics can be improved using better technology for its implementation. In reference [42], the authors proposed a CO2 detection system based on CoSb2O6 oxide. For its signal adaptation, the impedance response was analyzed. From the analysis, an electronic circuit was proposed. Its implementation had high complexity because the signal analysis was done on the s-plane, and the electronic circuit was based on the impedance. In this work, another chemical sensor was applied for the CO gas detection but its signal adaptation was made based on the direct current, simplifying the analysis and its implementation. Therefore, comparing both proposals, our proposal reduced complexity, analysis, cost, materials, electronic components and optimized the performance. The CO detection device finds practical safety applications where CO detection is desirable. Some examples are boiler safety system, locked chimneys, inverted fireplace effect, corroded ventilation pipes and more. Therefore, our future work is in the following direction: CO detector applications, the detector device would be optimized through digital electronics and the nickel antimonate (NiSb2O6) oxide would be applied for the propane (C3H8) gas detection.

5. Conclusions We have employed a wet-chemistry synthesis process for producing particles of nanometric size (~23.24 nm on average) for their application in gas detection. This preparation method is economical and very efficient to obtain different morphologies. Furthermore, it is possible to obtain the crystalline phase at relatively low temperatures compared to traditional methods such as the solid-state reaction with this process. The NiSb2O6 nanoparticles showed high sensitivity in carbon monoxide (CO) atmospheres at different operating temperatures (100, 200, and 300 ◦C). The optimum performance of the oxide was at concentrations of 300 ppm of CO at 300 ◦C. The maximum value of the sensitivity was of ~0.35 in CO. Based on these results, a new prototype device was implemented for CO gas detection. Its operating concentration was into the interval of 1 to 300 ppm. Its operating temperatures were 100, 200 and 300 ◦C. However, its optimal operation was at 300 ◦C. The CO detection device was built using analogic electronics: a Wheatstone bridge and electronic circuits based on operational amplifiers. The device had good performance, fast response, selective sensitivity, adaptability, and low cost. Our prototype device finds practical application where it is desirable to detect carbon monoxide leaks.

Author Contributions: A.G.B. and J.A.R.O. synthetized the NiSb2O6 Oxide Powders; H.G.B., V.M.R.B., L.G.O., O.B.A. developed the Physical characterization of NiSb2O6 Powders; J.T.G.B., A.C.Z., M.E.S.M. developed the signal analysis and proposed the electronic circuit. All authors wrote the paper. Funding: This research received no external funding. Acknowledgments: The authors thank the Mexico’s National Council of Science and Technology (CONACyT) for the support granted. Antonio Casillas Zamora expresses his gratitude to CONACyT for his scholarship. This investigation was carried out following the line of research “Nanostructured Semiconductor Oxides” of the academic group UDG-CA-895 “Nanostructured Semiconductors” of CUCEI, University of Guadalajara. Likewise, we thank Sergio Oliva-León, Miguel-Ángel Luna-Arias, M. L. Olvera-Amador, for their technical assistance. Conﬂicts of Interest: The authors declare no conﬂict of interest. Appl. Sci. 2019, 9, 3799 15 of 17

References

1. Wetchakun, K.; Samerjai, T.; Tamaekong, N.; Liewhiran, C.; Siriwong, C.; Kruefu, V.; Wisitsoraat, A.; Tuantranont, A.; Phanichphant, S. Semiconducting metal oxides as sensors for environmentally hazardous gases. Sens. Actuators B 2011, 160, 580–591. [CrossRef]

2. Niu, X.; Du, W.; Du, W. Preparation and gas sensing properties of ZnM2O4 (M. = Fe, Co, Cr). Sens. Actuators B 2004, 99, 405–409. [CrossRef] 3. Navarro-Botella, P.; García-Aguilar, J.; Berenguer-Murcia, A.; Cazorla-Amorós, D. Pd and Cu-Pd nanoparticles

supported on multiwall carbon nanotubes for H2 detection. Mater. Res. Bull. 2017, 93, 102–111. [CrossRef] 4. Fleischer, M.; Meixner, H. Fast gas sensors based on metal oxides which are stable at high temperatures. Sens. Actuators B 1997, 43, 1–10. [CrossRef] 5. Fiels, L.L.; Zheng, J.P.; Cheng, Y.; Xiong, P. Room-temperature low-power hydrogen sensor based on a single tin dioxide nanobelt. Appl. Phys. Lett. 2006, 88, 263102. [CrossRef]

6. Yoon, J.W.; Grilli, M.L.; Di Bartolomeo, E.; Polini, R.; Traversa, E. The NO2 response of solid electrolyte sensors made using nano-sized LaFeO3 electrodes. Sens. Actuators B 2001, 76, 483–488. [CrossRef] 7. Makri, A.; Stilianakis, N.I. Vulnerability to Air Pollution Health Eﬀects. Int. J. Hyg. Environ. Health 2008, 211, 326–336. [CrossRef] 8. Zuidema, T.; Nentjes, A. Health Damage of Air Pollution: An Estimate of a Dose Response Relationship for the Netherlands. Environ. Resour. Econ. 2007, 9, 291–308. [CrossRef] 9. Brunekreef, B.; Holgate, S.T. Air pollution and health. Lancet 2002, 360, 1233–1242. [CrossRef]

10. Sukunta, J.; Wisitsoraat, A.; Tuantranont, A.; Phanichphant, S.; Liewhiran, C. Highly-sensitive H2S sensors based on flame-made V-substituted SnO2 sensing films. Sens. Actuators B 2017, 242, 1095–1107. [CrossRef] 11. Afzal, A.; Cioffi, N.; Sabbatini, L.; Torsi, L. NOx sensors based on semiconducting metal oxide nanostructures: Progress and perspectives. Sens. Actuators B 2012, 171–172, 25–42. [CrossRef] 12. Kanazawa, E.; Sakai, G.; Shimanoe, K.; Kanmura, Y.; Teraoka, Y.; Miura, N.; Yamazoe, N. Metal oxide

semiconductor N2O sensor for medical use. Sens. Actuators B 2001, 77, 72–77. [CrossRef] 13. Addabbo, T.; Bertocci, F.; Fort, A.; Gregorkiewitz, M.; Mugnaini, M.; Spinicci, R.; Vignoli, V. Gas sensing

properties and modeling of YCoO3 based perovskite materials. Sens. Actuators B 2015, 221, 1137–1155. [CrossRef]

14. Yuan, Y.; Wang, B.; Wang, C.; Li, X.; Huang, J.; Zhang, H.; Xia, F.; Xiao, J. Eﬀects of CoFe2O4 electrode microstructure on the sensing properties for mixed potential NH3. Sens. Actuators B 2017, 239, 462–466. [CrossRef] 15. Zhao, J.; Liu, Y.; Li, X.; Lu, G.; You, L.; Liang, X.; Liu, F.; Zhang, T.; Du, Y. Highly sensitive humidity sensor

based on high surface area mesoporous LaFeO3 prepared by a nanocasting route. Sens. Actuators B 2013, 181, 802–809. [CrossRef] 16. Moseley, P.T.; Williams, D.E.; Norris, J.O.W.; Toﬁeld, B.C. Electrical conductivity and gas sensitivity of some transition metal tantalates. Sens. Actuators 1988, 14, 79–91. [CrossRef] 17. Wang, B.; Yu, Q.; Zhang, S.; Wang, T.; Sun, P.; Chuai, X.; Lu, G. Gas sensing with yolk-shell LaFeO3 microspheres prepared by facile hydrothermal synthesis. Sens. Actuators B 2018, 258, 1215–1222. [CrossRef] 18. Yang, X.; Zhang, S.; Yu, Q.; Sun, P.; Liu, F.; Lu, H.; Yan, X.; Zhou, X.; Liang, X.; Gao, Y.; et al. Solvothermal

synthesis of porous CuFe2O4 nanospheres for high performance acetone sensor. Sens. Actuators B 2018, 270, 538–544. [CrossRef] 19. Guillén-Bonilla, H.; Gildo-Ortiz, L.; Olvera-Amador, M.D.L.L.; Santoyo-Salazar, J.; Rodríguez-Betancourtt, V.M.;

Guillen-Bonilla, A.; Reyes-Gómez, J. Sensitivity of Mesoporous CoSb2O6 Nanoparticles to Gaseous CO and C3H8 at Low Temperatures. J. Nanomater. 2015, 2015, 308465. [CrossRef] 20. Guillén-Bonilla, A.; Rodríguez-Betancourtt, V.M.; Guillén-Bonilla, J.T.; Sánchez-Martínez, A.; Gildo-Ortiz, L.;

Santoyo-Salazar, J.; Morán-Lázaro, J.P.; Guillén-Bonilla, H.; Blanco-Alonso, O. A novel CO and C3H8 sensor made of CuSb2O6 nanoparticles. Ceram. Int. 2017, 43, 13635–13644. [CrossRef] 21. Singh, A.; Singh, A.; Singh, S.; Tandon, P. Nickel antimony oxide (NiSb2O6) A fascinating nanostructured material for gas sensing application. Chem. Phys. Lett. 2016, 646, 41–46. [CrossRef] Appl. Sci. 2019, 9, 3799 16 of 17

22. Guillén-Bonilla, H.; Flores-Martínez, M.; Rodríguez-Betancourtt, V.M.; Guillen-Bonilla, A.; Reyes-Gómez, J.;

Gildo-Ortiz, L.; Olvera-Amador, M.D.L.L.; Santoyo-Salazar, J. A Novel Gas Sensor Based on MgSb2O6 Nanorods to Indicate Variations in Carbon Monoxide and Propane Concentrations. Sensors 2016, 16, 177. [CrossRef][PubMed] 23. Guillen-Bonilla, H.; Rodríguez-Betancourtt, V.M.; Guillén-Bonilla, J.T.; Reyes-Gómez, J.; Gildo-Ortiz, L.;

Flores-Martínez, M.; Olvera-Amador, M.D.L.L.; Santoyo-Salazar, J. CO and C3H8 Sensitivity Behavior of Zinc Antimonate Prepared by a Microwave-Assisted Solution Method. J. Nanomater. 2015, 2015, 979543. [CrossRef] 24. Guillen-Bonilla, A.; Blanco-Alonso, O.; Guillen-Bonilla, J.T.; Olvera-Amador, M.D.L.L.; Rodríguez-Betancourtt, V.M.; Sánchez-Martínez, A.; Moran-Lázaro, J.P.; Martínez-García, M.; Guillen-Bonilla, H. Synthesis and characterization of cobalt antimonate nanostructures and their

study as potential CO and CO2 sensor at low temperatures. J. Mater. Sci. Mater. Electron. 2018, 29, 15632–15642. [CrossRef] 25. Montero, M.; Molina, T.; Szafran, M.; Moreno, R.; Nieto, M.I. Alumina porous nanomaterials obtained by colloidal processing using D-fructose as dispersant and porosity promoter. Ceram. Int. 2012, 38, 2779–2784. [CrossRef] 26. Lee, S.J.; Kriven, W.M. Crystallization and Densiﬁcation of Nano-Size Amorphous Cordierite Powder Prepared by a PVA Solution-Polymerization Route. J. Am. Ceram. Soc. 1998, 81, 2605–2612. [CrossRef] 27. Seung-Man, Y.; Shin-Hyun, K.; Jong-Min, L.; Gi-Ra, Y. Synthesis and assembly of structured colloidal particles. J. Mater. Chem. 2008, 18, 2177–2190. 28. Rodríguez Betancourtt, V.M.; Guillen Bonilla, H.; Flores Martínez, M.; Guillen Bonilla, A.; Moran Lazaro, J.P.;

Guillen Bonilla, J.T.; González, M.A.; Olvera Amador, M.D.L.L. Gas Sensing Properties of NiSb2O6 Micro-and Nanoparticles in Propane and Carbon Monoxide Atmospheres. Hindawi J. Nanomater. 2017, 2017, 8792567. [CrossRef] 29. Guillen Bonilla, H.; Olvera Amador, M.D.L.L.; Casallas Moreno, Y.L.; Guillen Bonilla, J.T.; Guillen Bonilla, A.; Gildo Ortiz, L.; Morán Lázaro, J.P.; Santoyo Salazar, J.; Rodríguez Betancourtt, V.M. Synthesis and characterization of nickel antimonate nanoparticles: Sensing properties in propane and carbon monoxide. J. Mater. Sci. Mater. Electron. 2019, 30, 6166–6177. [CrossRef] 30. Gildo-Ortiz, L.; Guillén-Bonilla, H.; Santoyo-Salazar, J.; Olvera-Amador, M.L.; Karthik, T.V.K.; Campos-González, E.; Reyes-Gómez, J. Low-temperature synthesis and gas sensitivity of perovskite-type

LaCoO3 nanoparticles. J. Nanomater. 2014, 2014, 164380. [CrossRef] 31. Ehrenberg, H.; Wltschek, G.; Rodriguez-Carvajal, J.; Vogt, T. Magnetic structures of the tri-rutiles NiTa2O6 and NiSb2O6. J. Magn. Magn. Mater. 1998, 184, 111–115. [CrossRef] 32. Deng, Z.X.; Wang, C.; Sun, X.M.; Li, Y.D. Structure-directing coordination template eﬀect of ethylenediamine in formations of ZnS and ZnSe nanocrystallites via solvothermal route. Inorg. Chem. 2002, 41, 869–873. [CrossRef][PubMed] 33. Wang, X.; Li, Y. Solution-based synthetic strategies for 1-D nanostructures. Inorg. Chem. 2006, 45, 7522–7534. [CrossRef][PubMed] 34. Han, J.; Xu, M.; Jia, M.; Liu, T. Evaluation of reduced grapheme oxide-supported NiSb2O6 nanocomposites for reversible lithium storage. Ceram. Int. 2016, 42, 14782–14787. [CrossRef] 35. Lamer, V.K.; Dinegar, R.H. Theory, production and mechanism of formation of monodispersed hydrosols. J. Am. Chem. Soc. 1950, 72, 4847–4854. [CrossRef] 36. Shih-Chia, C. Oxygen chemisorption on tin oxide: Correlation between electrical conductivity and EPR measurements. J. Vac. Sci. Technol. 1979, 17, 366–369. 37. Kita, J.; Engelbrecht, A.; Schubert, F.; Gro, A.; Rettigand, F.; Moos, R. Some practical points to consider with respect to thermal conductivity and electrical resistivity of ceramic substrates for high-temperature gas sensors. Sens. Actuators B 2015, 213, 541–546. [CrossRef] 38. McAleer, J.F.; Moseley, P.T.; Norris, J.O.W.; Williams, D.E. Tin dioxide gas sensors: Part 1.-Aspects of the surface chemistry revealed by electrical conductance variations. J. Chem. Soc. Faraday Trans. 1 1987, 183, 1323–1346. [CrossRef] 39. Mirzaei, A.; Leonardi, S.G.; Neri, G. Detection of hazardous volatile organic compounds (VOCs) by metal oxide nanostructures-based gas sensors: A review. Ceram. Int. 2016, 42, 15119–15141. [CrossRef] Appl. Sci. 2019, 9, 3799 17 of 17

40. Song, E.; Choi, J.W. Conducting polyaniline nanowire and its applications in chemiresistive sensing. Nanomaterials 2013, 3, 498–523. [CrossRef] 41. Lin, T.; Xin, L.; Li, S.; Wang, Q. The Morphologies of the Semiconductor Oxides and Their Gas-Sensing Properties. Sensors 2017, 17, 2779. [CrossRef][PubMed] 42. Guillén-Bonilla, A.; Rodríguez-Betancourtt, V.M.; Guillén-Bonilla, H.; Gildo-Ortiz, L.; Blanco-Alonso, O.;

Franco-Rodríguez, N.E.; Reyes-Gómez, J.; Casillas-Zamora, A.; Guillen-Bonilla, J.T. A new CO2 detection system based on the trirutile-type CoSb2O6 oxide. J. Mater. Sci. Mater. Electron. 2018, 29, 15741–15753. [CrossRef]

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