Microwave-assisted synthesis of SnO₂ nanorods for oxygen gas sensing at room temperature

Abstract

High-quality single-crystalline SnO₂ nanorods were synthesized using a microwave-assisted solution method. The nanorods were characterized using X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), ultraviolet-visible and Raman spectroscopy, Brunauer–Emmett–Teller (BET), and electrical resistance measurements. The XRD pattern indicated the formation of single-phase SnO₂ nanorods with rutile structure. FE-SEM and TEM images revealed tetragonal nanorods of about 450–500 nm in length and 60–80 nm in diameter. The nanorods showed a higher BET surface area of 288 m²/g, much higher than that of previously reported work. The Raman scattering spectra indicated a typical rutile phase of the SnO₂. The absorption spectrum showed an absorption peak centered at 340 nm, and the band-gap value was found to be 3.64 eV. The gas-sensing properties of the SnO₂ nanorods for oxygen gas with different concentrations were measured at room temperature. It was found that the value of resistance increased with the increase in oxygen gas concentration in the test chamber. The SnO₂ nanorods exhibited high sensitivity and rapid response-recovery characteristics to oxygen gas, and could detect oxygen concentration as low as 1, 3, 5, and 10 ppm.

Introduction

During the past decade, metal oxide semiconductor material-based gas sensors have attracted extensive attention.¹ Among them, as an n-type wide band-gap semiconductor, SnO₂ is regarded as one of the promising materials for gas sensing because of its excellent characteristics, such as low cost, high sensitivity, rapid response, and fast recovery.^2,3 In addition, it has potential applications in detecting polluted or toxic gases and other species,^4–6 as well as successful use in optoelectronic devices.^7–13 Oxygen gas monitoring is an important issue in various fields, such as the environment, transportation, medicine, and agriculture.^14,15 Metal oxide semiconductor gas sensors offer an inexpensive and simple method for monitoring gases, due to low cost, small size, and real-time detection.^16–18 Oxygen gas sensors have been widely used in industrial heating furnaces, to monitor such environments as underground mines and oil fields, and to prevent gas poisoning. However, most sensors made from semiconducting oxide materials have a high operating temperature, which is becoming an obstacle to their application in different areas. Additionally, 1-D nanostructures, including nanorods, nanowires, nanotubes, and nanobelts, are excellent candidates for chemical sensing purposes,^19–22 because of their ultrahigh surface-to-volume ratio, high sensitivity, and short response time to chemical surroundings. Due to high surface-to-volume ratios of these 1-D nanostructures, their first major application was in chemical/gas sensors, as was demonstrated with SnO₂-coated carbon nanotubes,²³ functionalized silicon nanowires,²⁴ and SnO₂ nanowires/nanowhiskers.^25,26 It has been reported that a gas sensor is a chemical sensor that converts chemical information, which is determined by different concentrations of gaseous chemical species, into an electrical signal.²⁷ Thus, a chemical gas sensor gives an electrical signal that can be related to the chemical environment if it is exposed to gas in some way. The sensor response in most cases is defined as difference or ratio between the steady-state sensor response when exposed to the sample gas and the sensor response when exposed to a reference atmosphere (not sample gas). The concentration-versus-response relationship for most gas sensors approximately exhibits either saturated linear behavior (ie, linear for low concentrations and saturated for higher concentrations) or logarithmic behavior. However, most of the drawbacks of commonly used gas-sensing technologies come from their lack of stability. There are other demands to be met while producing gas sensors, such as short response time, good reversibility, low cost, small size, and low power consumption. In general, oxygen-related gas-sensing mechanisms involve chemisorptions of oxygen on the oxide surface followed by charge transfer during the reaction between chemisorbed oxygen-reducing and target-gas molecules. To reveal the mechanism of gas sensors, many models, such as the point-contact model,²⁸ face-contact model,²⁹ and surface-depletion model,^2,3,30 have been established. These models emphasize the importance of charge transfer in the sensing mechanism. They can illustrate this adsorption process qualitatively, but it is difficult to predict sensing results of gas sensors.

Much effort has been focused on the fabrication of nanostructured SnO₂ by employing various techniques, such as the molten-salt method, hydrothermal method, laser-ablation deposition, and direct oxidation growth.^31–41 However, it still remains a great challenge to find reliable techniques with low cost for the fabrication of low-dimensional SnO₂ nanostructures. In comparison to other approaches, the solution method has its own advantages, such as low temperature, high production yield, and high quality. Nowadays, a new method – microwave-assisted synthesis – is being used for the synthesis of nanomaterials. Due to this method’s unique features, such as short reaction time, enhanced reaction selectivity, energy saving, and high reaction rate,^42,43 its use has been rapidly increasing.^43–47 Microwave heating not only reduces the chemical reaction times by several orders of magnitude but also suppresses side reactions, and thus improves the yield and reproducibility of a specific synthesis protocol. All these features represent important parameters that have to be considered during the development of “greener” synthesis methodologies, and therefore it can be expected that the microwave technique will also play a fundamental role in a future environmentally friendlier “synthetic nanotechnology.”

In this paper, we report the low-concentration detection of oxygen gas at room temperature using SnO₂ nanorods prepared by the microwave-assisted solution method. Synthesized SnO₂ nanorods showed excellent gas sensitivity, and the reported technique is fast and cost-effective, which is important for large-scale applications in gas sensors and can be applicable at the industrial level.

Materials and methods

All of the reagents involved in the experiments were of analytical grade and utilized as received without further purification. The synthesis was carried out in a domestic microwave oven. In a typical synthesis, an equal (0.05 M) molar ratio of SnCl₂:2H₂O (99.99%, Sigma-Aldrich, St Louis, MO, USA) and NaOH (99.99%, Sigma-Aldrich) was dissolved in 50 mL deionized water in a round-bottom flask, and the solution was put into a microwave oven and treated at a power of 300 W for 20 minutes. After microwave processing, the solution was cooled down to room temperature. The resulting precipitate was separated by centrifugation, then washed with deionized water and absolute ethanol several times, and finally dried in an oven at 80°C for 24 hours. Prior to the characterization, the samples were annealed at different temperatures (400°C and 600°C) for 2 hours.

X-ray diffraction (XRD) measurements were carried out using an X-ray diffractometer with Cu Kα radiation (λ =1.5406 Å) operated at a voltage of 40 kV and current of 30 mA. The morphologies were investigated on a field-emission scanning electron microscope (FE-SEM). The elemental composition of the SnO₂ was determined by energy-dispersive X-ray spectroscopy (EDX) attached to the FE-SEM. Transmission electron microscopy (TEM) images, selected-area electron diffraction (SAED) patterns, and high-resolution TEM (HR-TEM) images were obtained by FE-TEM operated at 200 kV. For the TEM observation, the samples were collected on a carbon holey grid. The room-temperature optical absorption spectrum was recorded in the range of 200–800 nm using an ultraviolet-visible (UV-vis) spectrophotometer. In order to get the phonon vibrational study of the SnO₂ nanorods, a micro-Raman spectrometer was used with a 532 nm solid-state primary laser as an excitation source in the backscattering configuration at room temperature. The Brunauer–Emmett–Teller (BET) specific surface-area measurements were carried out by nitrogen adsorption using an Autosorb®-1 (Quantachrome Instruments, Boynton Beach, FL, USA).

Gas-sensing properties of the SnO₂ nanorods were examined for the O₂ gas at room temperature. For gas-sensing measurements, the sensor was put in a test chamber. High-purity O₂ gas was passed through the test chamber at different flow rates controlled by a mass flow controller. The sample resistance of the SnO₂ nanorods sensor was measured using an HP (Palo Alto, CA, USA) 34401 multimeter as a function of time, which was interfaced with a personal computer through a general-purpose interface bus card. From these measurements, gas-sensor parameters, including sensitivity, response and recovery times, were determined.

Results and discussion

Figure 1 shows the XRD patterns of the as-prepared and annealed samples of SnO₂ at different temperatures. In this experiment, SnCl₂ was used as the precursor, which eventually underwent oxidation to form SnO₂ under the microwave condition. Before oxidation, Sn was found to be in the form of hydroxide intermediate Sn₆O₄(OH)₄, which was explicitly revealed by XRD (see Figure 1). XRD patterns indicated that for the as-prepared sample, the precursor SnCl₂ mainly transformed into poorly crystallized Sn₆O₄(OH)₄ (International Centre for Diffraction Data [ICDD] 46–1486). It can be seen that as the annealing temperature increases to 400°C, the diffraction peaks of Sn₆O₄(OH)₄ gradually vanish, and at 600°C, rutile SnO₂ was formed. All the diffraction peaks of the sample annealed at 600°C are indexed to tetragonal SnO₂ with a rutile structure with lattice parameters of a =4.7390 Å and c =3.1861 Å. These results are very well matched with the ICDD card 41–1445. The sharpness of the peaks shows the high crystallinity of the prepared samples.

Figure 1.

X-ray diffraction patterns of as-prepared and annealed SnO₂ samples.

Figure 2A shows the FE-SEM image of the SnO₂ nanorods annealed at 600°C for 2 hours. The nanorods have a uniform length of about 450–500 nm and a diameter of about 60–80 nm. TEM and HR-TEM investigations give further insight into the morphologies and the structural features of SnO₂ nanorods. The TEM image in Figure 2B shows that the nanorods are needle-like, with a diameter of 50 nm at the tip and 75 nm in the middle, and a length of about 500 nm. The clear lattice fringes in the HR-TEM images (Figure 2C) show the single crystal nature of the SnO₂ nanorods. The spacing between two adjacent lattice planes is 0.33 nm, corresponding to the (110) planes of SnO₂, which indicates that the nanorods are bound by (110) facets. Therefore, the [001] direction is the favored growth direction for the nanorods, which is further confirmed by the SAED pattern taken from individual nanorods (inset of Figure 2C).

Figure 2.

(A) Field emission scanning electron microscopy image, (B) transmission electron microscopy (TEM) image, (C) high-resolution TEM (HRTEM) image, and (D) energy-dispersive X-ray spectroscopy spectrum of SnO₂ nanorods annealed at 600°C for 2 hours. Inset of (C) shows corresponding selected-area electron diffraction pattern.

The elemental composition of the SnO₂ nanorods was determined by EDX. Figure 2D shows the corresponding EDX spectrum of the annealed SnO₂ nanorods. The EDX analysis of these nanorods demonstrates that the nanorods are only composed of elemental Sn and O.

The formation of a tetragonal rutile structure of SnO₂ nanorods was further supported by Raman spectra employed at room temperature. The rutile structure SnO₂ belongs to the space group P4₂/mnm,^48–50 with Sn and O atoms in 2a and 4f positions, respectively. Figure 3 shows the Raman spectrum of the SnO₂ nanorods in the wave-number range 300–850 cm⁻¹. The Raman spectrum of the SnO₂ nanorods shows conventional vibration modes^49–52 of E_g, (A_2u)V_(TO), A_1g, (A_2u)V_(LO), and B_2g centered at 473 cm⁻¹, 498 cm⁻¹, 629 cm⁻¹, 689 cm⁻¹, and 770 cm⁻¹, respectively. This is in agreement with the results of group-theory analysis.^53,48

Figure 3.

Room-temperature Raman spectrum of SnO₂ nanorods annealed at 600°C for 2 hours.

In order to study the optical properties of SnO₂ nanorods, UV-vis spectroscopy was used. The UV-vis absorption spectrum of SnO₂ nanorods annealed at 600 for 2 hours is presented in Figure 4. It is clear from the absorption spectrum that there is a strong absorption peak positioned at 340 nm. It is well known that the theory of optical absorption gives the relationship between the absorption coefficients α and the photon energy hv for direct allowed transition as⁵⁴:

Figure 4.

Room-temperature absorption spectrum of SnO₂ nanorods annealed at 600°C for 2 hours. Inset shows corresponding optical band-gap plot.

$${(\alpha hv)}^2=\text{A} (hv-E_g)$$ (1)

where A is a constant characteristic of the semiconductor, hν is the photon energy, E_g is the apparent optical band gap, and α is the absorption coefficient. The direct band gap is determined using this equation when the straight portion of the (αhν)² against the hν plot is extrapolated to intersect the energy axis at α =0. The inset of Figure 4 shows the plot of (αhν)² versus hν for SnO₂ nanorods. The E_g value of the SnO₂ nanorods was determined from the intercept of (αhν)² versus (hν) curves and found to be 3.64 eV. It is clear that the optical band gap is larger than the value of 3.62 eV for bulk SnO₂.

Figure 5 gives the nitrogen adsorption–desorption isotherm and the surface-area plot of the annealed SnO₂ nanorods. The isotherm shows that nitrogen-adsorption volume gradually increased as relative pressure increased and then decreased as relative pressure decreased (Figure 5A). The BET surface area of the annealed SnO₂ nanorods was found to be 288 m²/g. The SnO₂ nanorods (prepared by this method) showed higher surface area than other reported SnO₂ nanostructures (mesoporous, nanotubes, nanosheet, nanowires).^55,56 It was also much higher than commercially produced SnO₂ nanoparticles, such as SnO₂ (BET 47.2 m²/g, 18.3 nm in diameter, 549657-25 G; Sigma-Aldrich), SnO₂ (BET 25.9 m²/g, 34 nm in diameter; Yamanaka, Osaka, Japan), SnO₂ (BET 23 m²/g, 26 nm in diameter, 37314-13, NanoTek®; CI Kasei, Tokyo, Japan) or In₂O₃–SnO₂ (BET 3–6 m²/g, 100–300 nm in diameter; Sumitomo Chemical, Tokyo, Japan).

Figure 5.

(A) Nitrogen adsorption–desorption isotherm of SnO₂ nanorods annealed at 600°C for 2 hours; (B) corresponding Brunauer–Emmett–Teller surface-area plot.

Figure 6 shows the response-recovery characteristics for the SnO₂ nanorod sensors operated at room temperature under different oxygen gas concentrations. Figure 6 depicts the change in resistance as a function of time with different oxygen concentrations ranging from 1 to 10 ppm. The sensor clearly shares a common feature at all concentrations, wherein resistance increases sharply when the oxygen gas is on, yet returns to its original state when the oxygen gas is off. SnO₂ is known to be of an n-type semiconducting nature, and its sensing properties are controlled exclusively by a change in surface resistance. Therefore, the species and amount of the chemisorbed oxygen species on the oxide surface play an important role in influencing the functionality.^57–59 In general, sensing materials with low-dimensional structures should have a large surface area, which could not only provide more adsorption sites for the oxygen species and the tested gases but also facilitate the interaction between the oxide surfaces and gas molecules.⁶⁰ This induces a more significant degree of electron transfer, and hence more pronounced output of electric signal, which is detected by the electric circuit.

Figure 6.

Plot of change in resistance of SnO₂ nanorod gas sensors for different oxygen concentrations.

The mechanism of sensing of SnO₂ nanorod sensors can be explained by the modulation model of the depletion layer.⁶¹ The electrical conductance of nanorods is determined by the amount of electrons in a conduction band of nanorods. The more electrons are present inside the rod, the higher the conductivity the nanorods possess. If the surrounding analyte gas molecules are not absorbed at all by the nanorods, there is no electron exchange between the nanorods and the gas molecules. Hence, the conductivity of the nanorods remains at the same level as without the gas surroundings. On the other hand, when an SnO₂ nanorod sensor is exposed to oxygen gas, oxygen molecules are captured by the surface electrons and become adsorbed oxygen. The adsorbed oxygen creates a depletion layer on the surface of the nanorods and increases the energy barrier, thereby increasing the electrical resistance of the nanorods. For gas-sensor applications, the larger the variation in the resistance, the higher the sensitivity that can be achieved. The gas sensitivity (S) was estimated using the relationship S = (R_g – R₀)/R₀, where R₀ is the initial resistance in the absence of oxygen gas, and R_g is the resistance measured in the presence of oxygen gas. Figure 7 shows the sensitivities calculated from Figure 6 for different oxygen concentrations. It is clear from Figure 7 that the sensitivity increases with increasing oxygen gas concentration, hence a linear relationship is obtained between sensitivity and oxygen gas concentration. The enhanced sensitivity at room temperature as operating temperature is attributed to the higher surface-to-volume ratio of the nanorods. Larger surface-to-volume ratio of nanorods creates more oxygen adsorption and increases the resistance of the material, as shown in Figure 7. Hence, the increase in resistance generates a corresponding increase in the sensor’s sensitivity.

Figure 7.

Plot of sensitivity as a function of oxygen concentration.

Conclusion

In summary, we have successfully fabricated a sensor made of SnO₂ nanorods by a rapid and cost-effective microwave-assisted solution method and investigated its gas-sensing properties. XRD, SAED, HR-TEM, and Raman analyses revealed that the SnO₂ nanorods annealed at 600°C for 2 hours have a single-crystalline nature with rutile structure. FE-SEM and TEM images show that the SnO₂ nanorods have length ~500 nm and diameter of ~80 nm, respectively. BET analysis showed that SnO₂ nanorods had surface area of 288 m²/g, which was much higher than that of the SnO₂ reported earlier or commercially produced nanoparticles. The UV-vis spectrum showed the maximum absorption band at 340 nm and optical band gap of 3.64 eV. The SnO₂ nanorod-based oxygen gas sensor operated at room temperature showed good sensitivity for various concentrations of oxygen gas. The sensitivity of the gas sensor is linearly proportional to oxygen concentrations ranging from 1 to 10 ppm. The present SnO₂ nanorod gas sensor represents an important step forward in exploring gas sensors that can be operated at room temperature for future practical applications.