CO and C3H8 Sensitivity Behavior of Zinc Antimonate Prepared by a Microwave-Assisted Solution Method

Abstract

ZnSb2O6 has been synthesized by a microwave-assisted solution method in order to test its possible application as a gas sensor. Zinc nitrate, antimony trichloride, and ethylenediamine were used as precursors and deionized water as solvent. Microwave radiation, with a power of ~350 W, was applied for solvent evaporation. The thermal decomposition of the precursors leads to the formation of ZnSb2O6 at 600°C. This oxide crystallized in a tetragonal structure with cell parameters a = 4.66 Å, c = 9.26 Å and space group P42/mnm. Microwires and microrods formed by nanocrystals were observed by means of scanning and transmission electron microscopies (SEM and TEM, resp.). Pellets of the oxide were tested as gas sensors in flowing atmospheres of carbon monoxide (CO) and propane (C3H8). Sensitivity increased with the gas concentration (0-300 ppm) and working temperatures (ambient, 150 and 250°C) increase. The results indicate high sensitivity of ZnSb2O6 in both gases at different concentrations and operating temperatures.

Hector Guillen-Bonilla 1 and Veronica-M. Rodriguez-Betancourtt 1 and Jose-Trinidad Guillen-Bonilla 2 and Juan Reyes-Gomez 3 and Lorenzo Gildo-Ortiz 3 and Martin Flores-Martinez 1 and M. de la L. Olvera-Amador 4 and Jaime Santoyo-Salazar 5

Academic Editor:Hassan Karimi-Maleh

1, Departamento de Ingenieria de Proyectos, CUCEI, Universidad de Guadalajara, 44410 Guadalajara, JAL, Mexico
2, Departamento de Electronica y Computacion, CUCEI, Universidad de Guadalajara, 44410 Guadalajara, JAL, Mexico
3, Facultad de Ciencias, Universidad de Colima, 28045 Colima, COL, Mexico
4, Departamento de Ingenieria Electrica-SEES, Centro de Investigacion y de Estudios Avanzados del Instituto Politecnico Nacional, 07360 Mexico, DF, Mexico
5, Departamento de Fisica, Centro de Investigacion y de Estudios Avanzados del Instituto Politecnico Nacional, 07360 Mexico, DF, Mexico

Received 21 September 2015; Accepted 1 December 2015

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. 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 [1-5]. 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 SnO₂ is regarded as one of the most promising materials for such purpose [8-11], because it possesses some of the above desired features, like high sensitivity and chemical stability [12-14]. However, some other semiconductor types, with more complex crystal structures, are currently investigated, like the ones with perovskite structure: [figure omitted; refer to PDF] , SmFeO₃ , and SrFeO₃ [15-17]; spinel structure: MgAl₂ O₄ , CdCr₂ O₄ , and ZnFe₂ O₄ [18-20]; and oxides with trirutile-type structure: CoTa₂ O₆ , NiTa₂ O₆ , and CoSb₂ O₆ [21-23]. Among the latter, the type-n semiconductor zinc antimonate ZnSb₂ O₆ 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 ZnSb₂ O₆ through a colloidal method and probed its sensing capabilities in O₂ , CO, and CO₂ 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 ZnSb₂ O₆ through a dip-coating method, obtaining good selectivity and stability in H₂ S atmospheres, attributing this to the film's porous structure.

Generally speaking, the zinc antimonate adopts a formula ASb₂ O₆ , 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 P 4₂ /mnm [28]. In general, the trirutile-type materials are synthesized by a ceramic-method [29] but ZnSb₂ O₆ has been better prepared by a colloidal-route [25]. In our work, we synthesized the oxide through a microwave-assisted solution-method and prepared pellets with it in order to test its sensing capabilities in CO and C₃ H₈ atmospheres at relative low temperatures.

2. Experimental Procedures

2.1. Synthesis of ZnSb₂ O₆

We synthesized ZnSb₂ O₆ preparing three solutions: 1.48 g (0.005 mol) of Zn(NO₃ )₂ ·6H₂ O (J. T. Baker), 2.28 g (0.01 mol) of SbCl₃ (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).

2.2. Physical Characterization of ZnSb₂ O₆ Powders

The synthesized zinc antimonate (ZnSb₂ O₆ ) powders were analyzed by means of X-ray diffraction (Siemens D 500 XRD-system, CuKα radiation, 2θ scanning-angle from 10 to 70° and 1-second 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 μ 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).

2.3. 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 μ m.

The pellets of ZnSb₂ O₆ , with a thickness of 500 μ m, were exposed to carbon monoxide (CO) and propane (C₃ H₈ ) 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 ( [figure omitted; refer to PDF] ) were evaluated using [30-32] [figure omitted; refer to PDF] where [figure omitted; refer to PDF] and [figure omitted; refer to PDF] are electrical conductance of the ZnSb₂ O₆ pellets measured in gas (CO or C₃ H₈ ) 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.

Figure 1: Experimental array for the sensitivity testing of ZnSb₂ O₆ .

[figure omitted; refer to PDF]

3. Results and Discussion

3.1. XRD Analysis

Figure 2 shows a diffractogram depicting the zinc antimonate (ZnSb₂ O₆ ) 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 [figure omitted; refer to PDF] Å, [figure omitted; refer to PDF] Å and a P 4₂ /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 2θ = 29.62°, 42.54°, and 60.61° belong to the secondary phase ZnSb₂ O₄ [25], which were identified through the JCPDF file 15-0802.

Figure 2: Diffractogram of the zinc antimonate (ZnSb₂ O₆ ) powders calcined at 600°C in air.

[figure omitted; refer to PDF]

In previous works, the ZnSb₂ O₆ 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 Sb₂ O₃ at 700-1000°C, and Zhu et al. [38] used the vapor-phase-oxidation method in a Ar + O₂ 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.

3.2. Raman Spectroscopy Analysis

The Raman spectrum of the oxide is depicted in Figure 3, where the main bands [figure omitted; refer to PDF] to [figure omitted; refer to PDF] , in the range 500-800 cm^-1 , correspond to the vibration of the Sb₂ O₁₀ units of the oxide's crystals, and the weaker bands [figure omitted; refer to PDF] to [figure omitted; refer to PDF] , which are below 400 cm^-1 , are due to the influence of Zn²⁺ . In more detail, in the range 600-800 cm^-1 , the stretching modes of the Sb- [figure omitted; refer to PDF] bonds predominate, while in the range 400-500 cm^-1 , the deformation modes of the Sb- [figure omitted; refer to PDF] 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- [figure omitted; refer to PDF] bonds [39, 40]. This analysis supports the results obtained by XRD, shown above.

Figure 3: Raman spectrum of the ZnSb₂ O₆ powders calcined at 600°C in air.

[figure omitted; refer to PDF]

3.3. Scanning Electron Microscopy Analysis

Figure 4 shows three typical SEM photomicrographs of trirutile-type zinc antimonate (ZnSb₂ O₆ ) powders after calcination at 600°C. These images present three different magnifications: 100x, 500x, and 1500x.

Figure 4: SEM images of trirutile-type zinc antimonate (ZnSb₂ O₆ ) powders calcined at 600°C at different magnifications: (a) 100x, (b) 500x, and (c) 1500x.

(a) [figure omitted; refer to PDF]

(b) [figure omitted; refer to PDF]

(c) [figure omitted; refer to PDF]

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. The importance of ethylenediamine in the formation of nano- and microstructures has been discussed in previous works [42, 43]. In our case, the microrod's length was estimated in the range 10-100 μ m with a standard deviation of 21.3 μ m and an average of 59.5 μ m. The averaged diameter was of 1.5 μ m (see Figure 4(b)). As can be observed, tiny crystals (average size of 1.1 μ 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, [figure omitted; refer to PDF] , and CO₂ [31, 44].

3.4. 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 μ m and a diameter of 0.857 μ 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.

Figure 5: TEM analysis of trirutile-type zinc antimonate (ZnSb₂ O₆ ) powders (600°C): ((a), (b)) microrods formed by crystals, (c) HRTEM image showing the lattice fringes.

[figure omitted; refer to PDF]

3.5. Sensing Properties

The variations of the sensitivity's magnitude are shown in Figures 6(a)-6(c) in carbon monoxide (CO) atmospheres.

Figure 6: Sensitivity of ZnSb₂ O₆ pellets as a function of (a) CO concentration and (b) CO operating temperature; (c) maximum values of [figure omitted; refer to PDF] at 250°C for every CO concentration.

(a) [figure omitted; refer to PDF]

(b) [figure omitted; refer to PDF]

(c) [figure omitted; refer to PDF]

These results show that ZnSb₂ O₆ 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 [figure omitted; refer to PDF] , which is not produced or is detected because the thermal energy is not enough for that) at temperatures above 150°C [46]. ZnSb₂ O₆ strongly reacts above such temperature with CO [31, 32] of the atmosphere yielding CO₂ 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)-7(c)). 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 [figure omitted; refer to PDF] 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 SnO₂ 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].

Figure 7: Sensitivity of ZnSb₂ O₆ pellets as a function of (a) propane (C₃ H₈ ) concentration and (b) operating temperature; (c) maximum values of [figure omitted; refer to PDF] at 250°C at different propane concentrations.

(a) [figure omitted; refer to PDF]

(b) [figure omitted; refer to PDF]

(c) [figure omitted; refer to PDF]

4. Conclusions

Zinc antimonate (ZnSb₂ O₆ ), 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 ZnSb₂ O₆ were clearly sensitive and uniform in response to the operating temperatures and gas concentrations in carbon monoxide (CO) and propane (C₃ H₈ ) atmospheres. A good sensitivity was obtained at 250°C, with a maximum [figure omitted; refer to PDF] (CO) and [figure omitted; refer to PDF] (C₃ H₈ ) at 300 ppm. It has been found that ZnSb₂ O₆ is a strong candidate for being applied as an environmental gas sensor.

Acknowledgments

Hector Guillen-Bonilla acknowledges Mexico's Consejo Nacional de Ciencia y Tecnologia (CONACyT) for the received financial support. The technical support received from Dario Pozas Zepeda and Miguel Ángel Luna Arias is also appreciated. This work was partially supported by Project no. 263656 from CONACyT, Project no. 254790 from CONACyT, and Project no. 784-12 FRABA.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

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