Sensitivity of Mesoporous CoSb2O6 Nanoparticles to Gaseous CO and C3H8 at Low Temperatures

Abstract

Mesoporous CoSb2O6 nanoparticles, synthesized through a nonaqueous method (using cobalt nitrate, antimony trichloride, ethylenediamine, and ethanol as a solvent), were tested to establish their sensitivity to CO and C3H8 atmospheres at relatively low temperatures. The precursor material was dried at 200°C and calcined at 600°C. X-ray diffraction and scanning electron microscopy were employed to verify the existence of crystal phases ([subscript]P42[/subscript] /mnm ) and the morphology of this trirutile-type CoSb2O6 oxide. Pyramidal and cubic shaped crystals (average size: 41.1 nm), embedded in the material's surface, were identified. Mesopores (average size: 6.5 nm) on the nanoparticles' surface were observed by means of transmission electron microscopy. The best sensitivity of the CoSb2O6 in a CO atmosphere was at the relatively low temperatures of 250 and 350°C, whereas, in a C3H8 atmosphere, the sensitivity increased uniformly with temperature. These results encourage using the CoSb2O6 nanoparticles as gas sensors.

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

Academic Editor:Rakesh K. Joshi

1, Facultad de Ciencias Quimicas, Universidad de Colima, 28400 Coquimatlan, COL, Mexico
2, Facultad de Ciencias, Universidad de Colima, 28045 Colima, COL, Mexico
3, Departamento de Ingenieria Electrica-SEES, Centro de Investigacion y de Estudios Avanzados del Instituto Politecnico Nacional, 07360 Mexico, DF, Mexico
4, Departamento de Fisica, Centro de Investigacion y de Estudios Avanzados del Instituto Politecnico Nacional, 07360 Mexico, DF, Mexico
5, Centro Universitario de Ciencias Exactas e Ingenierias, Universidad de Guadalajara, 44410 Guadalajara, JAL, Mexico

Received 14 November 2014; Accepted 7 January 2015; 29 January 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

In recent years, nanoscience and nanotechnology have had a huge impact on many fields of scientific research. New materials with nanometric sized particles (1 nm = 10^-9 m) show interesting physical and chemical properties for different applications, including gas sensors [1-10].

Several synthesis methods are currently employed to obtain materials with nanometric particles and desired morphological features, like colloid, sol-gel, coprecipitation, ultrasonic spray, polymerization, and so forth [11-16]. For gas sensor applications, the morphology and a reduced particle size play an important role due to the increase of the specific surface area and, therefore, of the sensitivity, enhancing at the same time the chemical adsorption and the physical adsorption of the gases [17-20].

SnO₂ , LaFeO₃ , TiO₂ , ZnO, In₂ O₃ , and WO₃ , among other oxides, have been extensively used in gas sensor applications [21-25]. However, in recent years, trirutile-type oxides are of interest as gas sensors, because they show good electric response and chemical stability, as well as gas selectivity [26-28]. CoSb₂ O₆ has shown, for example, very good performance as CO₂ and O₂ sensor, due to its morphology and particle size [29], and ZnSb₂ O₆ has been found to be very suitable for detecting H₂ S, due to its porosity [30].

In order to obtain the trirutile-type CoSb₂ O₆ with specific size, porosity, and morphological features for gas sensing, a nonaqueous chemical method was used in this work, which presented the additional advantage of not being expensive [31]. The material's characterization was made by scanning and transmission electron microscopies (SEM and TEM, resp.). The good sensitivity results of the obtained mesoporous CoSb₂ O₆ nanoparticles in CO and C₃ H₈ atmospheres support the use of these oxides as gas sensors.

2. Experimental Procedures

CoSb₂ O₆ nanoparticles with trirutile-type structure were prepared employing a colloidal method [29]; however, in order to modify the microstructure, cobalt nitrate (Jalmek), antimony trichloride (Sigma-Aldrich), 4 mL of ethylenediamine (Sigma), and analytic grade ethanol (CTR) were employed in this work. The solvent was evaporated by means of a microwave oven (General Electric, model JES769WK) at a power of 70 W and applying pulses at 20-30 s intervals. After this, the precursor material was dried at 200°C during 8 h and calcined at 600°C at a heating rate of 100°C/h using a programmable muffle (Vulcan 3-550).

The crystallinity characterization was carried out at room temperature by means of Siemens D500 X-ray powder diffraction (XRD) system using a nickel filter and CuKα radiation. A 2θ continuous diffraction scanning was performed from 20° to 70° and 1 s-steps of 0.02°.

The morphology of the calcined CoSb₂ O₆ was characterized by means of a scanning electron microscopy (SEM) system (Jeol, model JSM-6390LV) in high vacuum and using the secondary electron emission. The particle size and morphology were analyzed with a transmission electron microscopy (TEM) system (Jeol, model JEM-2010) at a 200 kV acceleration voltage. Prior to this, the CoSb₂ O₆ powder was dispersed in isopropyl alcohol by means of a sonicator during 5 min.

For the sensitivity tests, 0.5 g of CoSb₂ O₆ powder was pressed at 15 ton during 90 min with a manual pressing machine (Simplex Ital Equip-25 tons). The obtained pellet, with a 12 mm diameter and a 0.5 mm thickness, was heated at 200°C during 1 h and set after that in a vacuum chamber at 10^-3 Torr (see Figure 1).

Figure 1: Arrangement employed for the sensitivity testing of CoSb₂ O₆ .

[figure omitted; refer to PDF]

The electrical resistance tests of the CoSb₂ O₆ pellets were carried out in carbon monoxide (CO) and propane (C₃ H₈ ) atmospheres to the concentrations 0, 5, 50, 100, 200, and 300 ppm at the temperatures 23, 150, 250, and 350°C. The gas detection sensitivity ( [figure omitted; refer to PDF] ) was calculated according to the following formula [32-35]: [figure omitted; refer to PDF] where [figure omitted; refer to PDF] and [figure omitted; refer to PDF] are the electrical conductance of the CoSb₂ O₆ pellets measured in gas (CO or C₃ H₈ ) and air, respectively. The conductance was measured by means of a digital multimeter (Keithley, model 2001). Electrical contact with the pellet was made using silver electrodes, as shown in Figure 1.

3. Results and Discussion

3.1. X-Ray Diffraction Analysis

Figure 2 shows the diffractogram of the CoSb₂ O₆ powders obtained at 600°C. The characteristic peaks of the oxide are clearly discernible and were identified by the JCPDF index-card 18-0403. According to this, CoSb₂ O₆ is a trirutile-type oxide with tetragonal structure (cell parameters: [figure omitted; refer to PDF] = 4.65 Å, [figure omitted; refer to PDF] = 9.28 Å) and P 4₂ /mnm space group [28]. The height and width of the peaks are indicative of a small particle size [36]; the presence of slight fluorescence shows high crystallinity. Further, a secondary phase was identified: Co_2.33 Sb_0.67 O₄ (JCPDF, 15-0517), located at 2θ = 36.4°, 42.0°, and 61.1°. These results are consistent with other studies on the same oxide and similar ones (despite being synthesized by different methods and at higher temperatures) [26, 27, 29, 37]. We emphasize again the relative low temperature (600°C) at which the oxide was synthesized for this study.

Figure 2: X-ray diffractogram of the calcined powder at 600°C, identifying the main and secondary phases: CoSb₂ O₆ and Co_2.33 Sb_0.67 O₄ , respectively.

[figure omitted; refer to PDF]

3.2. Scanning Electron Microscopy Analysis

Three scanning electron microscopy (SEM) photomicrographs of the oxide's surface are depicted in Figure 3. Figure 3(a) shows the microstructure of CoSb₂ O₆ , where irregular particles of tetragonal morphology agglomerate on the whole porous surface. In Figure 3(b), tetragonal and cubic microcrystals are clearly discernible, showing also a multidirectional growth. The plane length of the cubic crystals is approximately 0.2-1.2 μ m, ~0.735 μ m on average, and has a standard deviation of ±0.2 μ m. In the more magnified photomicrograph, Figure 3(c), the multidirectional growth of the tetrahedral crystals is more evident. It can be observed that every single face of the monocrystals is taken as a substrate for growing in different directions. The effect of ethylenediamine on the microstructure of materials has been reported in previous works [38]. The ethylenediamine acts as a template which is incorporated into the inorganic framework first and then escapes from it during the thermal treatment to form particles of desired morphologies [39]. Studies made with colloidal dispersions have obtained morphologies similar to those obtained in this study. In such cases, the morphologies have originated from the growth process of stable nuclei of the colloidal systems [40, 41], agreeing with the crystallization principles proposed by Lamer and Dinegar [42], which were described in three stages: the first one states that the concentration of the reagents in colloidal dispersions gradually increases; the second stage states that the concentration of the reagents reaches an oversaturation limit and the nucleation happens rapidly forming the nuclei of the crystals; and, finally, the third stage states that the growth of the crystals is originated by diffusion of the dissolved species to nuclei and thus their morphology is clear. Based on these principles, compounds with different morphologies can be obtained using colloidal methods.

SEM images of CoSb₂ O₆ at several magnifications: (a) 1000x, (b) 4500x, and (c) 6000x.

(a) [figure omitted; refer to PDF]

(b) [figure omitted; refer to PDF]

(c) [figure omitted; refer to PDF]

3.3. Transmission Electron Microscopy Analysis

Three bright field transmission electron microscopy (TEM) photomicrographs of the oxide's surface are shown in Figure 4. The formation of the tetrahedral nanometric monocrystals is depicted in Figure 4(a). The particle size is 70-150 nm. The agglomeration of nanoparticles, joined by necks originated from particle coalescence, is depicted in Figures 4(b) and 4(c). Uniformly distributed mesopores on the nanoparticles' surface are discernible.

Bright field TEM photomicrographs of CoSb₂ O₆ showing (a) cubes and tetrahedral pyramids and (b-c) mesoporous nanoparticles.

(a) [figure omitted; refer to PDF]

(b) [figure omitted; refer to PDF]

(c) [figure omitted; refer to PDF]

Size distribution of the nanoparticles and mesopores is shown in Figure 5. The estimated nanoparticle size is in the range 20-70 nm, ~41.1 nm on average, and has a standard deviation of ±12 nm. From TEM images, the mesopores diameter was estimated in the range 5-8.5 nm, ~6.5 nm on average, and has a standard deviation of ±0.81 nm. The diameter agrees with those obtained in other studies, despite using different synthesis methods [43-47].

Size distribution of CoSb₂ O₆ : (a) nanoparticles and (b) mesopores.

(a) [figure omitted; refer to PDF]

(b) [figure omitted; refer to PDF]

3.4. Gas Sensing Properties

Figure 6 depicts the sensitivity of the mesoporous CoSb₂ O₆ nanoparticles in terms of the CO concentrations and operating temperatures. At temperatures 23°C (ambient) and 150°C, no significant change on the sensitivity to CO is discernible (Figure 6(a)), but, at 250 and 350°C, a change was detected (Figure 6(b)). In particular, at 250°C, the gas sensitivity values calculated were 0, 0.10, 2.26, 3.80, and 5.10 in presence of 0, 5, 50, 100, and 200 ppm of CO, respectively, while, at 350°C, the gas sensitivity magnitude increased to 0, 0.61, 2.63, 4.70, and 7, under the same gas concentrations, respectively. Table 1 summarizes the obtained values. It can be seen that an increase in concentration and temperature meant an increase in sensitivity [48]. To explain this, it has been reported that the ionization states of the chemically adsorbed oxygen highly depend on temperature; at temperatures lower than 150°C, [figure omitted; refer to PDF] species are present. However, at temperatures greater than that, the more reactive O^- and O^2- species are predominant [49, 50]. The formation at high temperature of the latter species means a rise in the gas-solid interaction in the presence of CO [51], causing an increase in the sensitivity.

Table 1: Sensitivity values of CoSb₂ O₆ in CO atmospheres.

Temperature (°C)	Concentration CO (ppm)	Sensitivity(S)	Temperature (°C)	Concentration CO (ppm)	Sensitivity (S)
250	0	0	350	0	0
5	0.10	5	0.61
50	2.26	50	2.63
100	3.80	100	4.70
200	5.10	200	7.00

Gas sensitivity of CoSb₂ O₆ oxide as a function of (a) CO concentration and (b) CO operation temperature.

(a) [figure omitted; refer to PDF]

(b) [figure omitted; refer to PDF]

When the CO molecules make contact with the surface of the CoSb₂ O₆ pellets at moderate temperatures (250°C and 350°C), the adsorbed CO reacts with the oxygen anions chemisorbed on the surface, yielding CO₂ and a release of electrons back into the conduction band [48, 52, 53]. The mechanism of CO adsorption greatly depends on the temperature, and at least one of the following reactions may occur at different operating temperatures [17, 52, 53]: [figure omitted; refer to PDF] where the suffix (ads) denotes the adsorbed species [17, 52]. In this case, the electrons present in the reactions combine with charge carriers in the oxide's valence band. According to these chemical paths, the gas-sensing mechanism in semiconductor materials is based on the electric resistance change (i.e., conductance) produced by the electron transfer that takes place during the chemical adsorption [18, 29]. In the presence of a reducing electron-donator species, like CO, the whole concentration diminishes and the resistance increases, causing changes in the material's response.

The gas sensitivity as a function of the propane (C₃ H₈ ) concentrations and operating temperatures are shown in Figures 7(a) and 7(b), respectively. CoSb₂ O₆ nanoparticles are sensible to both propane gas concentration and operation temperature, even at 150°C. At this temperature, the gas sensitivity values of CoSb₂ O₆ were 0, 0.12, 0.85, 0.92, 1.00, and 1.04 under 0, 5, 50, 100, 200, and 300 ppm of propane, respectively, while, at 250°C, the pellets exhibited higher sensitivity values around 0, 0.22, 1.8, 2.1, 2.4, and 2.7 under the same gas concentrations, respectively. However, at 350°C, the CoSb₂ O₆ nanoparticles exhibited greater sensitivity values around 0, 0.23, 2, 3, 3.7, and 4.8 in the same propane concentrations. The sensitivity increased, because more desorption reactions take place when the propane concentration rises [48, 54]. In addition, the sensitivity increases more uniformly in propane atmosphere. This can be accounted for from the rise of the oxygen's adsorption at high temperatures [54]. At room temperature, no resistance changes are recorded, because the thermal energy is not enough to produce the desorption reactions. Our findings are in agreement with [50]. In addition, we have found that, when the test gases were removed from the vacuum chamber, the oxide's sensitivity decreased, as in a reversed process. Table 2 summarizes the findings.

Table 2: Sensitivity values of CoSb₂ O₆ in C₃ H₈ atmospheres.

Temperature(°C)	Concentration C3 H8 (ppm)	Sensitivity(S)	Temperature (°C)	Concentration C3 H8 (ppm)	Sensitivity (S)
250	0	0	350	0	0
5	0.22	5	0.23
50	1.8	50	2
100	2.1	100	3
200	2.4	200	3.7
300	2.7	300	4.8

Gas sensitivity of CoSb₂ O₆ oxide as a function of (a) C₃ H₈ concentration and (b) C₃ H₈ operation temperature.

(a) [figure omitted; refer to PDF]

(b) [figure omitted; refer to PDF]

According to these results, the detection mechanism of C₃ H₈ at 250°C and 350°C is not obvious. However, some authors have investigated other materials that detect propane at different gas concetraciones and operating temperatures [32, 33, 54]. Then, comparing our results with such studies, the sensitivity of our oxide is far better. For example, [32, 33] report a maximum sensitivity of 0.7 and 0.4 for a propane concentration of 300 ppm at 300°C, using SnO₂ and ZnO, respectively. The value obtained in this work, [figure omitted; refer to PDF] at 350°C, and the same concentration of C₃ H₈ , speaks of the important role played by the nanometric particle size, which we attribute to the synthesis route. As mentioned, CoSb₂ O₆ nanoparticles have been studied as potential gas sensors, due to their ability to detect variations in the concentration of gases, such as CO₂ and O₂ [29]. Now, in the present work, it has been found that CoSb₂ O₆ nanoparticles exhibit very good gas sensitivity in CO and propane atmospheres, even at relatively low temperatures.

Due to the fact that the microstructure of a material used as a gas detector is considered to be of great importance for satisfactory operation, when the particle size is nanometric, the sensitivity can be substantially improved [20], because the average crystal size is smaller than twice the thickness of the charged outer layer ( [figure omitted; refer to PDF] ), which is defined as in [18, 29]: [figure omitted; refer to PDF] where [figure omitted; refer to PDF] is the Debye length, [figure omitted; refer to PDF] is the charge of the electron, [figure omitted; refer to PDF] is the potential of the surface, [figure omitted; refer to PDF] is the Boltzmann constant, and [figure omitted; refer to PDF] is the temperature. Usually, [figure omitted; refer to PDF] values are between 1 and 100 nm [18].

According to some authors (e.g., [55]), the charged outer layer mainly depends on the gas pressure and concentrations. Owing to this, the conductivity will strongly depend on the crystal size ( [figure omitted; refer to PDF] ), yielding three possible scenarios [18]: (1) if [figure omitted; refer to PDF] , the conductivity is limited by the Schottky barrier at the particle border; thus, the gas detection does not depend on the particle size; (2) if [figure omitted; refer to PDF] , the conductivity and the gas sensing properties depend on the formation of necks built by crystals; (3) if [figure omitted; refer to PDF] , the conductivity depends on the crystal size [18, 20, 55]. Based on these three scenarios, the known physical fact that the smaller the particle size (accomplished in our case during the synthesis process), the greater the surface area obtained can be neatly used in the gas sensors field.

4. Conclusions

The trirutile-type oxide was synthesized by a convenient method which allowed using the material at low temperatures. The mesoporous CoSb₂ O₆ nanoparticles are highly sensitive to CO and C₃ H₈ operating at relative low temperatures. It has been observed that when increasing temperature and concentration, the sensitivity rises. The maximum sensitivity was reached for the concentrations 200 and 300 ppm at 350°C. The success using CoSb₂ O₆ as a gas sensor is attributed to the nanometric particle size.

Acknowledgments

Hector Guillen-Bonilla and Lorenzo Gildo-Ortiz express their gratitude to Consejo Nacional de Ciencia y Tecnologia (CONACyT) for the scholarships received. The technical support received from Dario Pozas Zepeda and Miguel Ángel Luna Arias is also appreciated. This work was partially supported by 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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