CO and C 3 H 8 Sensitivity Behavior of Zinc Antimonate Prepared by a Microwave-Assisted Solution Method

1Departamento de Ingenieŕıa de Proyectos, CUCEI, Universidad de Guadalajara, 44410 Guadalajara, JAL, Mexico 2Departamento de Electrónica y Computación, CUCEI, Universidad de Guadalajara, 44410 Guadalajara, JAL, Mexico 3Facultad de Ciencias, Universidad de Colima, 28045 Colima, COL, Mexico 4Departamento de Ingenieŕıa Eléctrica-SEES, Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, 07360 México, DF, Mexico 5Departamento de Fı́sica, Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, 07360 México, DF, Mexico


Introduction
The need of reliable detection devices for dangerous atmospheres has promoted a huge research since past decades on the development of semiconductor materials suitable as gas sensors.In general, the gas sensors are used in domestic applications and industrial processes like monitoring of automobile exhaust gases, flue-gases in incinerators, the possible toxicity in the air, and so forth [1][2][3][4][5].Important criteria for the fabrication of these sensors are low price, chemical stability, selectivity, and very good sensitivity [6,7].Among all semiconductors that can be used as sensors, the binary oxide SnO 2 is regarded as one of the most promising materials for such purpose [8][9][10][11], because it possesses some of the above desired features, like high sensitivity and chemical stability [12][13][14].However, some other semiconductor types, with more complex crystal structures, are currently investigated, like the ones with perovskite structure: Ca  Pb 1− TiO 3 , SmFeO 3 , and SrFeO 3 [15][16][17]; spinel structure: MgAl 2 O 4 , CdCr 2 O 4 , and ZnFe 2 O 4 [18][19][20]; and oxides with trirutile-type structure: CoTa 2 O 6 , NiTa 2 O 6 , and CoSb 2 O 6 [21][22][23].Among the latter, the type-n semiconductor zinc antimonate ZnSb 2 O 6 has been extensively studied for gas sensing applications due to its high response to several toxic gases [24].Our work presented here is devoted to such material.
Michel et al. [25] synthesized ZnSb 2 O 6 through a colloidal method and probed its sensing capabilities in O 2 , CO, and CO 2 atmospheres by means of dynamic tests using frequencies from 0.1 to 100 kHz at a temperature of 400 ∘ C. Their results showed good selectivity, reproducibility, and chemical stability.Tamaki et al. [26] prepared thick films of ZnSb 2 O 6 through a dip-coating method, obtaining good selectivity and stability in H 2 S atmospheres, attributing this to the film's porous structure.
Generally speaking, the zinc antimonate adopts a formula ASb 2 O 6 , where A can be substituted by the divalent Zn ion or the ions Ni, Co, Mg, and Cu, among others [27].This oxide belongs to the family of trirutile-type materials and crystallizes in a tetragonal structure with a space group P4 2 /mnm [28].In general, the trirutile-type materials are synthesized by a ceramic-method [29] but ZnSb 2 O 6 has been better prepared by a colloidal-route [25].In our work, we synthesized the oxide through a microwave-assisted solutionmethod and prepared pellets with it in order to test its sensing capabilities in CO and C 3 H 8 atmospheres at relative low temperatures.

Experimental Procedures
2.1.Synthesis of ZnSb 2 O 6 .We synthesized ZnSb 2 O 6 preparing three solutions: 1.48 g (0.005 mol) of Zn(NO 3 ) 2 ⋅6H 2 O (J. T. Baker), 2.28 g (0.01 mol) of SbCl 3 (Sigma-Aldrich), and 0.5 mL of ethylenediamine (Sigma), dissolving each separately in 5 mL of distilled water.The solutions were magnetically stirred at 350 rpm during 20 min.After that, the antimony chloride and ethylenediamine solutions were mixed under stirring.The zinc nitrate solution was next slowly poured to the mixed solution under stirring at 350 rpm during 24 h.After this, the solvent was evaporated by means of fourteen 180 s-microwave-exposures at ∼350 W (General Electric microwave oven, model JES769WK; the absorbed energy was estimated to 863 kJ), recording a temperature of ∼92 ∘ C after each exposure.Temperature and exposure time were controlled in order to avoid material loss by splashing.After the evaporation, the product was heated at 200 ∘ C during 8 h and afterward calcined in air at 600 ∘ C during 5 h with a heat rate of 100 ∘ C/h using a temperature-controlled oven (Vulcan 3-550).

Physical Characterization of ZnSb 2 O 6
Powders.The synthesized zinc antimonate (ZnSb 2 O 6 ) powders were analyzed by means of X-ray diffraction (Siemens D 500 XRD-system, CuK radiation, 2 scanning-angle from 10 to 70 ∘ and 1second steps of 0.02 ∘ ).The powders were analyzed by Raman spectroscopy using a 1000B microRaman Renishaw system, calibrated with a silicon semiconductor with its characteristic Raman peak at 520 cm −1 .The laser (excitation wavelength of 830 nm) was focused on the surface of the powders (spot size of approximately 20 m) by means of a Leica optical microscope (DMLM) integrated to the microRaman system.The radiation energy on the sample was 4.5 mW during 60 s.The microstructure was analyzed by means of scanning electron microscopy (JEOL JSM-6390LV SEM-system, in high-vacuum mode, using secondary-electron-emission) and a dispersion in isopropyl alcohol of the powders was put in a copper grid and analyzed by means of transmission electron microscopy (JEOL JEM-2010 TEM-system, at a 200 kV accelerating voltage).

Pellets Preparation for Gas Sensitivity Analysis.
For the sensitivity studies, pellets from the calcined powders were prepared using 0.353 g of them and compressed by means of a 25-ton manual-pressing-equipment (SIMPLEX ITAL EQUIP).The best pressing parameters were 20 tons during 120 min, obtaining pellets with a diameter of 12 mm, and a thickness of 500 m.
The pellets of ZnSb 2 O 6 , with a thickness of 500 m, were exposed to carbon monoxide (CO) and propane (C 3 H 8 ) flows at concentrations 5, 50, 100, 200, and 300 ppm of both gases.Working temperatures were 23 ∘ C (ambient), 150 ∘ C, and 250 ∘ C. The sensitivity changes () were evaluated using [30][31][32] where  G and  O are electrical conductance of the ZnSb 2 O 6 pellets measured in gas (CO or C 3 H 8 ) and air, respectively.For this analysis, a measuring vacuum chamber with a vacuum capacity of 10 −3 Torr was used.Gas concentration and partial pressure were controlled using a TM20 Leybold detector.Electric resistance measures were carried out by means of a digital multimeter (model Keithley 2001).See Figure 1.

Results and Discussion
3.1.XRD Analysis.Figure 2 shows a diffractogram depicting the zinc antimonate (ZnSb 2 O 6 ) oxide's peaks corresponding to the different crystal planes, which were identified by means of the JCPDF file 38-0453.It was found that the oxide exhibits a tetragonal crystal structure with cell parameters  = 4.66 Å,  = 9.26 Å and a P4 2 /mnm spatial group.That means the oxide is a trirutile phase type [24,26,33,34], which agrees with the results in [35,36].Furthermore, the small peaks at 2 = 29.62 ∘ , 42.54 ∘ , and 60.61 ∘ belong to the secondary phase ZnSb 2 O 4 [25], which were identified through the JCPDF file 15-0802.
In previous works, the ZnSb 2 O 6 oxide has been prepared through different synthesis routes, obtaining the oxide generally above a temperature of 600 ∘ C. For example, Wu et al. [37] prepared the oxide by means of a solid-state-reaction, mixing ZnO and Sb 2 O 3 at 700-1000 ∘ C, and Zhu et al. [38] used the vapor-phase-oxidation method in a Ar + O 2 atmosphere at a temperature between 500 and 900 ∘ C. In the present work, the synthesis of the oxide by means of a microwave-assisted solution method was done at a temperature of 600 ∘ C, which brings some advantages against the previously cited works.

Raman Spectroscopy Analysis.
The Raman spectrum of the oxide is depicted in Figure 3, where the main bands ] 7 to ] 10 , in the range 500-800 cm −1 , correspond to the vibration of the Sb 2 O 10 units of the oxide's crystals, and the weaker bands ] 1 to ] 6 , which are below 400 cm −1 , are due to the influence of Zn 2+ .In more 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 Zn-O bonds, are dominant.The 500-600 cm −1 bands are due to the elongation modes of the Sb-O cyc bonds [39,40].This analysis supports the results obtained by XRD, shown above.A dendrite-like formation of microwires and microrods is depicted in Figure 4(a), at 100x (see the inserted zoom), which can be due to the rise of the temperature during the calcinations, the lasted time by the material in the muffle, and the ethylenediamine concentration [41].About the latter, the ethylenediamine incorporates into the organic frame and then escapes as a result of the thermal treatment, giving rise to the desired morphologies, like those obtained in this work.

Scanning Electron Microscopy Analysis
The importance of ethylenediamine in the formation of nanoand microstructures has been discussed in previous works [42,43].In our case, the microrod's length was estimated in the range 10-100 m with a standard deviation of 21.3 m and an average of 59.5 m.The averaged diameter was of 1.5 m (see Figure 4(b)).As can be observed, tiny crystals (average size of 1.1 m) agglomerate to give shape to the microrods.At 1500x (Figure 4(c)), more details of the microstructure can be seen, where a porous surface and the strong agglomeration of individual particles, with irregular shape, are clearly discernible.The agglomeration is due to the formation of necks caused by the elimination of organic material after the thermal treatment.The porosity is attributed to the release of gases during the thermal decomposition, mainly water vapor, NO  , and CO 2 [31,44].

Transmission Electron Microscopy Analysis.
Figure 5 shows typical images obtained by transmission electron microscopy.In Figures 5(a) and 5(b) photomicrographs of the microrods formed by the tiny cubic-shaped crystals can be observed.The analyzed microrod in Figure 5(a) had a length of 6.6 m and a diameter of 0.857 m, while the tiny crystal's size was around 50-250 nm (Figure 5(b)).High resolution transmission electron microscopy (HRTEM) was performed in a selected zone of the sample (Figures 5(b) and 5(c)).The HRTEM image reveals that the interplanar distance is around 0.46 nm, which corresponds to (002) planes.In order to verify the local crystallinity of the sample, selected area electron diffraction (SAED) was made on several rods.The inset in Figure 5(b) shows a typical pattern; these results confirmed the crystallinity of the sample previously registered in XRD analysis.These results show that ZnSb 2 O 6 is highly sensitive to the employed CO concentrations and to operation temperatures above 150 ∘ C. No significant variations below such temperature were found.The maximum value of the sensitivity was of ∼6.66 for a concentration of 300 ppm at 250 ∘ C. The increase of the sensitivities with the concentration is due to the raise of the number of gas molecules, which react with the adsorbed oxygen, donating electrons to the material's surface  [31,45].The most accepted mechanism to explain this is a model based on the depletion layer's modulation due to the oxygen adsorption [8,11,14].In this model [14], the adsorbed oxygen on the semiconductor's surface ionizes to O − and O 2− , diminishing consequently the carriers concentration and the electrons' mobility, because these species are more reactive than others (like O 2 − , which is not produced or is detected because the thermal energy is not enough for that) at temperatures above 150 ∘ C [46].ZnSb 2 O 6 strongly reacts above such temperature with CO [31,32] of the atmosphere yielding CO 2 and releasing electrons back into the conduction band [25,47], especially at the temperature of 250 ∘ C, producing a high sensitivity.
A similar trend was found for different propane concentrations and operation temperatures (Figures 7(a As expected, at ambient temperature (23 ∘ C) no variation of the sensitivity was detected, but when the temperature was increased, sensitivity variations were observed.The maximum was  ∼ 1.26 at 250 ∘ C for a propane concentration of 300 ppm.A mechanism to explain the propane detection at relative low temperatures, like 250 ∘ C or 350 ∘ C, has not been yet reported.Other investigations, using similar semiconductor oxides (like SnO 2 and ZnO) to detect propane, report a maximum sensitivity of ∼0.7 and ∼0.6 for a propane concentration of 500 ppm at 300 ∘ C [48,49].Comparatively, we obtained better results here.In addition, we have found in previous works, using a cobalt oxide in the same atmospheres, that a trirutile-type structure, like that of the oxide of this work, is highly responsible for a great performance and excellent stability [32].

Conclusions
Zinc antimonate (ZnSb 2 O 6 ), with a trirutile-type structure, forming microstructures like microrods and microwires and intended to be used for gas sensing applications, was successfully synthesized employing a microwave-assisted solution-method at relative low temperatures.Tested pellets of ZnSb 2 O 6 were clearly sensitive and uniform in response to the operating temperatures and gas concentrations in carbon monoxide (CO) and propane (C 3 H 8 ) atmospheres.A good sensitivity was obtained at 250 ∘ C, with a maximum  = 6.66 (CO) and  = 1.2 (C 3 H 8 ) at 300 ppm.It has been found that ZnSb 2 O 6 is a strong candidate for being applied as an environmental gas sensor.

Figure 3 :
Figure 3: Raman spectrum of the ZnSb 2 O 6 powders calcined at 600 ∘ C in air.

Figure 7 :
Figure 7: Sensitivity of ZnSb 2 O 6 pellets as a function of (a) propane (C 3 H 8 ) concentration and (b) operating temperature; (c) maximum values of  at 250 ∘ C at different propane concentrations.