Gas Sensing Properties of NiSb2O6 Micro- and Nanoparticles in Propane and Carbon Monoxide Atmospheres

1Departamento de Quı́mica, CUCEI, Universidad de Guadalajara, 44410 Guadalajara, JAL, Mexico 2Departamento de Ingenieŕıa de Proyectos, CUCEI, Universidad de Guadalajara, 44410 Guadalajara, JAL, Mexico 3Departamento de Ciencias Computacionales e Ingenieŕıas, CUVALLES, Universidad de Guadalajara, Carretera Guadalajara-Ameca Km 45.5, 46600 Ameca, JAL, Mexico 4Departamento de Electrónica y Computación, CUCEI, Universidad de Guadalajara, 44410 Guadalajara, JAL, Mexico 5CONACYT, University of Guadalajara, CUCEI, Blvd. Marcelino Garćıa Barragán 1421, Ciudad Universitaria, 44430 Guadalajara, Jal, Mexico 6Departamento de Ingenieŕıa Eléctrica-SEES, Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, 07360 México City, Mexico

Recently, some authors have placed great emphasis on the study of materials capable of detecting propane (C 3 H 8 ) and carbon monoxide (CO) at different concentrations and operating temperatures [5].ZnO and SnO 2 are the most commonly used oxides to detect C 3 H 8 and CO because they possess good thermal stability in these gases [11,12].The semiconductor oxides LaCoO 3 and CoSb 2 O 6 are currently investigated as detectors of C 3 H 8 and CO, showing good stability and a high response at moderate temperatures (between 250 and 350 ∘ C) [13,14].It is believed that the good response of these materials is largely due to the morphology and the nanometric size of the obtained particles.In fact, the response of any material used as gas detector is highly related to the morphology, the porosity, and the particle size; such microstructural features determine the efficiency of the chemical interaction between the gas molecules and the surface of the detecting material [15].It is therefore of great importance (especially from an economic point of view) to achieve synthesis methods of materials with optimal morphologies and high efficiency.In this work, the microwave-assisted colloidal method was successfully used to synthesize micro-and nanoparticles of NiSb 2 O 6 with great ability to detect different concentrations of carbon monoxide (CO) and propane (C 3 H 8 ) at moderate temperatures.

Results and Discussion
3.1.XRD Analysis.The X-ray diffraction pattern of the calcined material at 600 ∘ C is shown in Figure 2. The presence of the main phase corresponding to the NiSb 2 O 6 is verified.The oxide was identified through the JCPDF card-file 38-1083.According to this, the oxide belongs to the materials with a trirutile-like structure [16], crystallizing with a space group P4 2 /mnm (136) [17][18][19][20][21] and cell parameters  = 4.641 Å and  = 9.223 Å.The width of the peaks in the diffractogram is indicative of a small particle size with a low noise level, which means a sample with high crystallinity [22].Small portions of other materials outside the main phase can be also identified: SbO 2 (card-file 65-2446) located at the points 28.0 ∘ , 30.3 ∘ , and 47.3 ∘ and NiO located at 37.0 ∘ (card-file 65-2901).The crystal size () was estimated using Scherrer's equation [13] and the XRD-peaks (Figure 2): where  is the radiation-wavelength (1518 nm),  is the Bragg angle, and  is the full width at half maximum (FWHM) of the diffraction peak.All peaks of the main phase of the trirutile-like structure were considered, finding an average crystal size of about 31.19 nm.
According to the results shown in Figure 2, the synthesis of the oxide NiSb 2 O 6 using the microwave-assisted colloidal method was successful.Several other authors have synthesized trirutile-type oxides following different synthesis processes [23].Larcher  Raman shift (cm −1 ) ] 14 ] 15 ] 16 ] 17 ] 18 the NiSb 2 O 6 at a significantly lower temperature following an alternative route of synthesis.

Raman Spectroscopy Analysis.
The Raman spectroscopy results of the calcined powders are shown in Figure 3, where the bands ] 14 to ] 18 , in the range 500-800 cm −1 , correspond to the vibration of the Sb 2 O 6 units of the oxide's crystals, and the bands ] 1 to ] 9 , which are below 400 cm −1 , are due to the influence of Ni 3+ .In detail, in the range 600-800 cm −1 , the stretching modes of the Sb-O p bonds predominate, while, in the range 400-500 cm −1 , the deformation modes of the Sb-O p bonds, coupled to the vibrations of the Ni-O bonds, are dominant.The 500-600 cm −1 bands are due to the elongation modes of the Sb-O cyc bonds [19,20].This supports the results obtained by XRD.The microstructure of the NiSb 2 O 6 oxide calcined at 600 ∘ C is shown in Figure 4(a); it is observed that the material's surface is composed by the agglomeration of particles and the growth of microrods.The microrods appear on the entire material's surface, individually agglomerating at a single origin point and growing in different directions.The topography of this morphology is very fine (smooth) and appears to be solid; however, some microrods are hollow (see insert in Figure 4(a)).The microrods' dimensions were estimated at ∼3.32 m long and ∼2.71 m wide, while the length was measured in the range of 5 to 37 m, with an average of ∼10.56 m and a standard deviation of ∼5.4 m.We reported in previous works that it is possible to obtain microrods of CoSb   In the same image, it can be seen that the spheres are the result of the agglutination and agglomeration of particles, which take the surface of the sphere as a substrate.The size of the particles that comprise the spheres was calculated in the range of 0.22 to 1.8 m, with an average of ∼0.760 m and a standard deviation of ∼0.28 m (see inserted histogram in Figure 4(b)).It has been reported in the literature that through the aerosolroute and by the Stöber method, it is possible to obtain different sized microspheres [27].In our case, as already mentioned, the microspheres were obtained following an alternative method of synthesis: the colloidal method in presence of ethylenediamine.The microspheres formation is attributable not only to the effect of the temperature (when it was calcined at 600 ∘ C) but also to the coalescence of very fine particles present on the material's surface.
An additional point on the surface was analyzed.The growth of this type of microstructure is due to the fact that ethylenediamine produces coordinated compounds, such as the NiSb 2 O 6 , that are capable of modifying the microstructure and the particle size in transition metals [28][29][30].In this case, the ethylenediamine is incorporated into the inorganic solid forming complexes, which join together after both the reaction and the heat treatment took place, giving rise to the formation of the microstructure shown in Figure 4.
The preparation of inorganic compounds using the colloidal method in presence of ethylenediamine has been discussed extensively in previous works [31,32].Some authors obtained similar morphologies to those presented here, reporting that by such synthesis method it is possible to have a better control during the particle nucleation and growth processes, producing materials with desirable morphologies [31][32][33].Therefore, the microstructures obtained are in accordance with the crystallization criteria described by LaMer and Dinegar [14,15,25,26,34], which establish that the crystallization of stable nuclei provokes a strong chemical reaction (the colloidal dispersion), which makes it possible to obtain inorganic materials with different morphologies.Different sized nanoparticles and a very homogeneous morphology can be observed in Figure 5(a).In addition, some nanoparticles were found on a microbase, taking it as a substrate for nucleation and growth.The high-resolution analysis (HRTEM) on a selected area of the nanoparticles (Figure 5(b)) revealed the formation of the material's crystalline planes, confirming the crystalline nature of the nanoparticles (see inserted figures).The distance between planes () was found in the range of ∼3,289 to 4,615 Å, corresponding to the planes (110) and (002), respectively.These results support the X-ray diffraction analysis (Figure 2).Broadly speaking, the size of the nanoparticles was found in the range of 2 to 20 nm, with an average of ∼10.7 nm and a standard deviation of ∼3.4 nm (Figure 5(c)).

Sensing Properties Analysis. Figures 6(a) and 6(b) show
the results of the response tests in gas propane, which reveals that the NiSb 2 O 6 pellets are very sensitive to the test gas concentrations and the operating temperature: the material's response increases as the temperature and the C 3 H 8 concentration rise.Increased response is caused by the increase in the number of gas molecules that react with oxygen on the material's surface.In addition, the oxide's response is associated with the effect of temperature, which increases during the tests [14].The highest response recorded was ∼1.125 corresponding to 500 ppm of propane, at a temperature of 300 ∘ C. At such temperature, the response values were 0, 0.0105, 0.0436, 0.178, 0.360, 0.6123, 0.881, and 1.125 at concentrations 1, 5, 50, 100, 200, 300, 400, and 500 ppm, respectively.However, at temperatures below 200 ∘ C there were no changes in the material's electrical resistance because the terminal energy is not enough to produce the oxygen-desorption reaction [35], regardless of the propane concentration [13].These results are consistent with similar oxides that have been tested in propane atmospheres [11-14, 25, 26].
The gas sensing mechanism is based on the change of the electrical resistance (or conductance) due to the adsorption and desorption of oxygen on the NiSb 2 O 6 pellets' surface.When the oxygen species adsorb on the surface, the material undergoes changes in its conductance, provoking the formation of the electron depletion layer, thus resulting in changes in the response of the oxide [36].The chemical reaction between the surface of a semiconductor material and propane has been discussed in previous works [37,38].In our case, the reaction may be the one reported in [36,39]: The results of tests in CO atmospheres at different concentrations and temperatures are presented in Figures 7(a) and 7(b).As in the propane experiments, the NiSb 2 O 6 pellets showed a good response by raising the temperature and the CO concentration.However, at temperatures below 200 ∘ C, there were no changes in the electrical resistance regardless of the CO concentration.In contrast, when the temperature increased from 200 to 300 ∘ C, the response increased significantly.Apparently, the response of the material is associated with the increase of the oxygen adsorbed on the material's surface at high temperatures [13,35].Chang [35] and Balamurugan et al. [40] report that the adsorption of different oxygen species on the sensor's surface depends on the operating temperature [41,42].Therefore, when the tests were performed at temperatures below 200 ∘ C, the oxygen species were mainly O 2 − ; on the other hand, by raising the temperature to 300 ∘ C, the oxygen species that are chemically adsorbed are O − and O 2− , which signifies changes in the electrical resistance and, consequently, an increase in the response of the NiSb 2 O 6 .The maximum response obtained was of ∼2.14 at 300 ∘ C for 300 ppm of CO.Other values at the same temperature were 0.013, 0.109, 0.321, 0.482, 1.25, and 2.14.
In order to discern the best conditions for a maximum performance of the NiSb 2 O 6 nanoparticles in C 3 H 8 and CO atmospheres, Figure 8 shows a graph of response versus concentration of both gases at a temperature of 300 ∘ C. According to these results, the response of the nanoparticles is clearly dependent on the temperature and the concentration of the test gases [42,43].The optimum operating temperature of the oxide was of 300 ∘ C for both gases, and the best response was recorded in the CO atmosphere at a concentration of 300 ppm.This trend is associated with the fact that during the tests in CO atmospheres, the surface of the pellets was enriched with oxygen species, giving rise to a greater response in this atmosphere.Therefore, the NiSb 2 O 6 oxide possesses a high selectivity in CO concentrations.
These results were compared with those for similar oxides tested also in C 3 H 8 and CO atmospheres.For example, SnO 2 showed in [11] maximum responses of ∼0.4 and ∼0.6 at concentrations of 100 and 500 ppm of propane at 300 ∘ C, respectively.In our case, the highest response was of ∼1.125 at a C 3 H 8 concentration of 500 ppm and of ∼2.14 at a CO concentration of 300 ppm, both at 300 ∘ C. In addition, we have previously reported [14,15,25,27] that the trirutile-type

Conclusions
Micro-and nanoparticles of trirutile-type NiSb 2 O 6 were synthesized using the microwave-assisted colloidal method.This synthesis route is economically very suitable and allows good fabrication efficiency because it makes it possible to have an excellent control of the morphology and the particle size.It was found that NiSb 2 O 6 nanoparticles are highly sensitive to various concentrations of C 3 H 8 and CO, operating at relatively low temperatures.The oxide's response increases directly proportional to the temperature and the concentrations of the test gases, obtaining a very good response in both gases.Due to the above, NiSb 2 O 6 with trirutile-type structure is a great candidate to be used as a gas sensor.

NiSb 2 O 6 Figure 1 :
Figure 1: Schematic representation of the experimental steps used in the preparation of NiSb 2 O 6 powders.

3. 3 .
SEM Analysis.Three SEM photomicrographs of the NiSb 2 O 6 oxide's morphology are depicted in Figures4(a)-4(c).The images were obtained at magnifications of 836x, 3.48kx, and 4.98kx, respectively.The insert in Figure4(b)shows the size distribution histogram of the particles that comprise the microspheres.
Figure 4(c) shows a surface composed of different particles, which grew agglomerated by the effect of temperature.Generally speaking, Figures 4(a)-4(c) show that these morphologies appeared commonly on the oxide's surface.

3. 4 .
TEM Analysis.Two TEM images of the oxide are shown in Figures5(a) and 5(b).Nanoparticles appearing on the NiSb 2 O 6 surface are depicted in Figure 5(a); Figure 5(b) shows a high-resolution image (HRTEM) of the nanoparticles' surface; Figure 5(c) depicts a histogram of the nanoparticle-size distribution.

Figure 6 :
Figure 6: Response of NiSb 2 O 6 nanoparticles as a function of: (a) C 3 H 8 concentration and (b) operating temperature.

Figure 7 :C 3 H 8 Figure 8 :
Figure 7: Response of the NiSb 2 O 6 nanoparticles to (a) carbon monoxide concentration and (b) operating temperatures.
−   )/  , where   and   are the pellets' electrical conductance in the gases (C 3 H 8 or CO) and in the air, respectively.The conductance was recorded by means of a Keithley digital multimeter (model 2001) as a function of the operating temperature and the concentration of the tested gas.
was then evaporated by microwave radiation at low power using an LG domestic device (model MS1147X) in periods of 30 to 40 s, reaching a temperature of 70 ∘ C. The precursor material was dried at 200 ∘ C for 8 h and then calcined at 600 ∘ C in air for 5 h, at a heating rate of 100 ∘ C/h.The calcination was carried out in a programmable muffle with temperature control (Vulcan 3-550).Figure1shows the steps followed to obtain NiSb 2 O 6 powders.
et al. used precursor oxides MO (M = Cu, Ni, or Co) and Sb 2 O 5 to synthesize the materials MSb 2 O 6 at 800 ∘ C by means of the solid-state reaction method [24].Ehrenberg et al. prepared the same oxide as Larcher et al. but at a temperature of 1450 ∘ C [16].Comparing our results with those aforementioned, it was possible for us to obtain 2 O 6 and ZnSb 2 O 6 [15, 25], mesoporous nanoparticles of CoSb 2 O 6 [14], nanorods of MgSb 2 O 6 [26],