Synthesis of SnO 2 nanowires forCO,CH 4 and CH 3 OH gases sensing

Abstract

Synthesis of one-dimensional nanostructures, such as nanowires, is of vigorous significance for achieving the desired properties and fabricating functional devices. In this work, we report the synthesis of tin oxide (SnO₂) nanowires on gold-catalyzed silicon substrate by carbothermal reduction process. SnO₂ nanowires were synthesized with SnO₂ and graphite powders as the source materials at atmospheric pressure and temperature of 900°C in the ambience of nitrogen (N₂) gas. First, the effect of source material ratio SnO₂:C on growth of SnO₂ nanowires was studied. The structural, morphological and compositional properties of the samples were investigated by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. The scanning electron microscopy investigation reveals that uniform dense nanowires of SnO₂ (diameter ~127 nm and length ~40 µm) were synthesized with vapour–liquid–solid mechanism. Ultraviolet–visible spectra estimate that the optical band gap of the synthesized SnO₂ nanowires was 3.72 eV. Second, the gas sensing performance of synthesized SnO₂ nanowires was investigated by testing with carbon monoxide (CO), Methane (CH₄) and methanol (CH₃OH) gases at different operating temperatures and concentrations. Results indicate that the synthesized SnO₂ nanowires are highly promising for gas sensing applications.

Keywords

Nanowires synthesis sensing applications optical band gap

Introduction

Synthesis of one-dimensional (1D) nanostructures has attracted increasing courtesy in recent years due to research and practical applications of these novel structures in next-generation devices and sensors.^1–5 Tin oxide (SnO₂) is a typical wide-band-gap semiconductor (E_g = 3.6 eV, at 300 K for bulk SnO₂). It exhibits n-type conduction due to excess of oxygen vacancies. It is well known for its potential applications in solid-state gas sensors,⁶ transparent conducting electrode,⁷ dye-based solar cells,⁸ catalyst supports⁹ and optoelectronic devices. SnO₂ 1D nanostructures have been shown to exhibit superior chemical and physical properties than their bulk crystal due to size confinement effect and surface effect. That is why SnO₂ 1D nanostructures have attracted considerable attention followed by extensive researches.^10–13 1D SnO₂ nanostructures (e.g. nanowires, nanobelts and nanorods) have been successfully synthesized by various methods such as laser ablation,¹⁴ electro-deposition,¹⁵ hydrothermal,¹⁶ chemical vapour deposition¹⁷ and micro-emulsion technique.¹⁸ At the same time, different complex 1D SnO₂ nanostructures including nanorings,¹⁹ falcon like nanosheets, dendritic nanorods,²⁰ V-shaped nanorods²¹ and zigzag nanostructures^22,23 have also been fabricated.

Gas sensing using nanostructures is a versatile and attractive application for environmental and security purposes. The advantages of using SnO₂ nanowires for chemical sensing are very diverse. Nanowires with high surface-to-volume ratio and Debye length matching with small diameter are greatly affected by the surface processes and leads to higher gas sensing performances such as sensitivity and response time.²⁴ The review article by Korotcenkov,¹⁰ which includes analysis of various parameters of metal oxides and the search of criteria, which could be used during material selection for solid-state gas sensor applications, demonstrated that SnO₂ is one of the best materials for gas sensing applications. Thong et al.¹ synthesized SnO₂ nanowires and hierarchical SnO₂ nanostructures by two different thermal evaporation processes and a combination of the two processes without reseeding of Au catalyst, respectively. Furthermore, comparative study of gas sensor performance of SnO₂ nanowires and their hierarchical nanostructures showed that hierarchical SnO₂ nanostructures have enhanced gas sensing performance in comparison with SnO₂ nanowires materials.¹ Fabrication, characterization and tests of the practical gradient microarray electronic nose with SnO₂ nanowire gas sensing elements were reported by Sysoev et al. This novel device demonstrated an excellent performance as a gas sensor and e-nose system capable of promptly detecting and reliably discriminating between several reducing gases in air at a ppb level of concentration.³ The transport properties and gas sensing performance of chemiresistors based on quasi-1D, single-crystal, SnO₂ nanostructures with deliberately synthesized (encoded) segmented morphology were explored by Dmitriev et al.⁵ As an important group of wide-band-gap semiconductors, 1D SnO₂ nanostructures have been extensively used to fabricate nanoscale gas sensors.

In this work, we report the synthesis of 1D SnO₂ nanowires by chemical vapour deposition technique. It is the most commonly used technique due to its process simplicity, easy-to-handle equipment, low-cost infrastructure and high throughput. Using the mixture of SnO₂ and graphite powders as the source material, we investigated the effect of ratio of the source materials SnO₂:C (1:4, 1:1, 4:1) on density of synthesized 1D SnO₂ nanostructures. Then, the gas sensing performance of these synthesized SnO₂ nanowires is evaluated at different temperatures and concentrations for different chemicals such as CO, CH₄ and CH₃OH.

Experimental details

1D SnO₂ nanostructures were synthesized on a silicon wafer (n-type). Dust and other organic substances were removed from the silicon substrates using isopropyl alcohol (IPA) in ultrasonic bath. Then, these Si substrates were coated with 10 nm of Au catalyst in radiofrequency sputtering chamber at the pressure of 10⁻³ torr and 20 mA electric current. In order to prepare source material, SnO₂ was thoroughly mixed with graphite with 1:4 (sample A), 1:1 (sample B) and 4:1 (sample C) ratios.

The schematic illustration of the experimental setup is shown in the Figure 1. The source material was placed in a ceramic boat and the gold-coated Si wafer was placed on the boat. A quartz tube (3.5 cm in diameter and 100 cm in length) was mounted in a horizontal tube furnace and was served as growth chamber for samples. The ceramic boat along with source material and substrates was then inserted into quartz tube in such a way that it must be in the uniform heating zone. One end of the quartz tube was connected to gas supply of high-purity nitrogen (N₂) gas. During experiment, N₂ gas was used as a carrier gas and was kept flowing throughout the experiment with a flow rate of 60 sccm. The furnace temperature was raised from room temperature to growth temperature 900°C at a rate of 30°C min⁻¹. This growth temperature was maintained for 45 min. Then, the furnace temperature was decreased from 900°C to room temperature. After that, the samples were taken out from the horizontal tube furnace and white layer of wool like products were observed on the Si substrates.

Figure 1.

(a) The schematic illustration of experimental setup; (b, c) SEM images of SnO₂ nanostructures synthesized with source material ratios: SnO₂:C 1:4 (sample A) and 1:1 (sample B).

The morphology and chemical composition of the synthesized SnO₂ nanostructures were characterized and analysed by scanning electron microscopy (JEOL JSM 5910), equipped with electron energy dispersive X-ray (EDX) spectrometer. The crystal structure was investigated by X-ray diffraction patterns using PAN analytical X-pert-Pro diffractometer (Cu–Kα radiations of wavelength 1.5405 Å). Ultraviolet–visible spectroscopy (UV/VIS/NIR) spectrometer was employed to measure the ultraviolet–visible (UV/VIS) absorption spectrum.

In order to analyse current–voltage (I-V) characteristics as well as CO, CH₄ and CH₃OH sensing properties of synthesized SnO₂ nanowires, the synthesized SnO₂ nanowire samples were dispersed in IPA solution by ultrasonic process and deposited on a pre-fabricated gold electrodes. These sensors were heat treated in air for 3 h at 400°C.

The sensing characteristics of the sensors were investigated with the help of a home-made setup by recording the electrical response of the sensors to periodical changes between air and chemicals environment. Sensing response (S) of sensors can be defined as R_a/R_g, where R_a and R_g are the resistances in air and chemical environment, respectively. Keithley digital multimeter (model 2100) was used to measure the electrical changes, and display data are acquired in a computer. The gas sensing properties were studied at different temperatures and gas concentrations.

Results and discussion

SEM and EDX analysis

Figure 1(b) and (c) depicts the SEM images of sample A and sample B, respectively. From these figures, it can be seen that no wire-like structures were found at all with source material ratio SnO₂:C of 1:4. However, with source material ratio SnO₂:C of 1:1, only few nanowires were synthesized on the substrate and the density of these nanostructures was very low. This may be attributed to the fact that the excess carbon atoms surrounded the tin oxide particles and prevented them from reaction to form SnO₂ nanowires.²⁵

Figure 2(a) and (b) depicts the SEM images of sample C, in which SnO₂ nanostructures were synthesized with source material ratio SnO₂:C of 4:1, and very attractive results were found. These images showed that synthesized nanowires have cylindrical shape and are uniformly distributed. Hence, the ratio of SnO₂:C of 4:1 was selected for further study. From high-resolution image, we can see that there is always a nanoparticle on the tips of nanowires. The presence of these nanoparticles on the tips of nanowires is considered to be the evidence of the vapour–liquid–solid (VLS) mechanism.²⁶ The average length, diameter and aspect ratio (average length/average diameter) of synthesized SnO₂ nanowires are 40 µm, 127 nm and 317, respectively.

Figure 2.

(a, b) SEM images of SnO₂ nanowires synthesized with source material ratio SnO₂:C 4:1 (sample C); (c) EDX spectrums of SnO₂ nanowires (sample C); and (d) XRD pattern of synthesized SnO₂ nanowires (sample C).

Quantitative compositional analyses of the synthesized SnO₂ nanostructures were carried out using EDX analysis. Figure 2(c) confirms the presence of Sn and O elements in the EDX spectrum. The strong peaks of Tin and oxygen in the spectra show that the concentrations of these elements are higher as compared to other elements. The presence of extra peak of Si in the EDX spectra is due to the Si substrate. No other peaks of any element were present in the EDX spectra, which show that the synthesized SnO₂ nanowires were quite pure and do not have any other impurity.²⁷

Crystal structure analysis

The X-ray diffraction (XRD) patterns of synthesized SnO₂ nanowires are shown in Figures 2(d). Very sharp peaks were observed, showing it as a perfect crystal.²⁴ The XRD pattern showed that the synthesized SnO₂ nanowires have tetragonal structure, so that the lattice parameters for tetragonal structure were calculated using the equation (1)

\frac{1}{d^{2}} = \frac{h^{2} + k^{2}}{a^{2}} + \frac{l^{2}}{c^{2}}

(1)

The XRD peaks of sample are matched with standard JCDPS of reference code number 01-071-5323. The average values of the crystallographic parameters of synthesized SnO₂ nanowires are a = b = 4.74 Å, and c = 3.20 Å.

UV/visible absorbance and photoluminescence spectroscopy

For nanocrystalline materials, a high energy shift of an absorption edge is expected. UV/VIS/NIR spectrophotometer was used to characterize the optical absorption properties of synthesized SnO₂ nanowires. Figure 3(a) shows the absorbance spectra of the synthesized SnO₂ nanowires. The optical transition of tin oxide is characterized to be a direct type. The band gap energy of direct band gap materials can be obtained from intercepts of the tangent to the plot.²⁸ Furthermore, quantum size effect may affect the optical band gap of nanostructures thereby increasing its value as compared to that for bulk materials. In our study, the optical band gap of synthesized nanowires was found to be 3.72 eV, and this value is larger than the value of 3.6 eV for bulk SnO₂, which is in agreement with previous studies.^28–31 Moreover, the larger band gap of the nanowires agrees well with their lower sizes as observed from SEM examinations. The energy band gap of synthesized SnO₂ nanowires was calculated using equation (2)²⁸

E_{g} = \frac{h c}{λ}

(2)

Figure 3.

UV/visible is absorbance and photoluminescence spectra of synthesized SnO₂ nanowires: (a) shows the absorbance spectra of the synthesized SnO₂ nanowire and (b) shows photoluminescence spectrum of the synthesized SnO₂ nanowires.

Figure 3(b) shows the room-temperature photoluminescence (PL) spectrum of the synthesized SnO₂ nanowires with He-Cd laser excitation at 325 nm. Peak at 411 nm is due to trapped states within the band gap. Broad emission peak at 572 nm (2.19 eV) was observed for nanowires sample suggesting defect-related (oxygen vacancies and Sn interstitials) electronic states in the band gap.^32,33 These defects are quite useful for gas sensing properties; the greater the defects, the more will be the sensing response for gases.

I-V characteristics

Figure 4 shows current–voltage (I-V) characteristics of the synthesized SnO₂ nanowires gas sensor at different temperatures. This plot shows a linear relationship between current and voltage, which indicates that fabricated sensor, represents good ohmic behaviour. This ohmic behaviour is quite important to the gas sensing properties, because the sensitivity of the gas sensor is affected by contact resistance. During fabrication of gas sensors, ohmic contacts with low contact resistance are essential since they are the means of signal transfer between the semiconductor and the external circuitry. An ohmic contact should have a linear and symmetric current–voltage characteristic and the resistance of the contact negligible as compared to the bulk of the device. The resistance is characterized by the contact resistance (Ω) and the specific contact resistivity, ρ_c (Ω cm²), also called contact resistivity or specific contact resistance. The specific contact resistance includes the interface as well as the regions immediately above and below the interface, and is independent of the geometry of the contact. This is useful when comparing contacts with different areas. High-temperature anneal is typically necessary to achieve ohmic behaviour of a deposited metal contact. For high-temperature sensor operation in corrosive environments, further development of the ohmic contact material is necessary to achieve contacts that are stable for extended periods of time.

Figure 4.

I-V characteristics of fabricated sensor at 25°C and 50°C temperatures.

I-V curve of as fabricated device also reveals that the synthesized SnO₂ nanowires show semiconducting behaviour because as the temperature of I-V measurement is increased, a significant decrease is measured in resistance of SnO₂ nanowires. According to our calculation, the resistance of synthesized SnO₂ nanowires was decreased from 479.6×10⁶ Ω to 291×10³ Ω as temperature was increased from 25°C to 50°C. In addition, the measured I-V characteristics at 25°C and 50°C temperatures showed that there is a linear relationship between I and V at both temperatures. If the temperature is further increased, then there will also be a linear relationship between I and V as reported previously by Johari et al.²⁹

Gas sensing properties of SnO₂ nanowires

The gas sensing properties of synthesized SnO₂ nanowires were checked using the electrical resistance versus time variation for different chemicals like CO, CH₄ and CH₃OH at different temperatures. The schematic illustration of gas sensing setup and fabricated sensor is shown in Figure 5. It has been demonstrated by Thong et al. that the response (S) of oxide semiconductor sensors is usually calculated using the following equation (1)

S = \frac{R_{a}}{R_{g}}

(3)

where R_a and R_g are the resistances in air and chemical environment, respectively.

Figure 5.

(a) The schematic illustration of gas sensing setup and (b, c) schematic illustration of fabricated sensor.

The sensor was tested at different temperatures ranging from 100°C to 500°C with an interval of 100°C for all three gases and the sensitivity of synthesized SnO₂ nanowires is found to be increased with increase in temperature, and it is optimized in temperature range 390°C–410°C, 290°C–310°C and 390°C–410°C for CO, CH₄ and CH₃OH, respectively. Plots between operating temperatures and sensing responses for CO, CH₄ and CH₃OH are shown in Figures 6(a), 7(a) and 8(a), respectively. From these figures, it is clear that temperature has an obvious influence on the response of sensor to all of these gases. The maximum response values of CO, CH₄ and CH₃OH gas sensors are 3.20, 1.20 and 6.65, respectively. As we increase the operating temperature from optimal temperature, the sensitivity of the sensor decreases for all three gases. The exact reason for this behaviour is unknown.³³ It may be attributed to the fact that at high temperature, the mobility of oxygen vacancies becomes appreciable in metal oxide semiconductors such as SnO₂ and the mechanism of conduction turns into a mixed ionic/ electronic conduction.

Figure 6.

Gas sensing response of synthesized SnO₂ nanostructures (a) for 20 ppm CO against operating temperature, (b) for 20 ppm CO at 400°C, (c) CO gas concentration and (d) response and recovery time of sensor for 20 ppm CO gas at 400°C.

Figure 7.

Gas sensing response of synthesized SnO₂ nanostructures (a) for 400 ppm of CH₄ against operating temperature, (b) for 400 ppm of CH₄ at 300°C, (c) CH₄ gas concentration and (d) response and recovery time of sensor for 400 ppm CH₄ gas at 300°C.

Figure 8.

Gas sensing response of synthesized SnO₂ nanostructures (a) for 50 ppm of CH₃OH against operating temperature, (b) for 50 ppm of CH₄ at 400°C, (c) CH₄ gas concentration and (d) response and recovery time of sensor for 50 ppm CH₃OH gas at 400°C.

After determining the optimal temperatures, the gas sensing properties of synthesized SnO₂ nanowires for 20 ppm CO at 400°C, 400 ppm CH₄ at 300°C and 50 ppm CH₃OH at 400°C are shown in Figures 6(b), 7(b) and 8(b), respectively. All of these plots show that the sensor has very quick response to all of these three gases. When gas is purged out, the sensor again returns to the original state which shows that the synthesized SnO₂ nanowires have very short recovery and response times.³⁴ This quick response could be attributed to the full exposure of higher number of SnO₂ molecules to the chemical environment which was full of CO/CH₄/CH₃OH molecules with high surface area. The gas is highly reduced with the interaction of SnO₂ nanostructures, and the resistance decreases during CO/CH₄/CH₃OH gas in gas sensing system because, in case of any of these three gases, the quasi-free electrons increase the carrier concentration on the surface of SnO₂ nanowires.

Figures 6(b), 7(b) and 8(b) showed that the performance of the synthesized SnO₂ nanowires sensor has very good repeatability because we measured the sensor response with relative large number of gas pulse sequence and did not found any change in sensor response. Hence, they are promising candidates for gas sensing applications.

The response and recovery times of a gas sensor are also very important for its practical applications. Figures 6(d), 7(d) and 8(d) show the plot of sensor responses for CO, CH₄ and CH₃OH at their optimal temperatures. From this resistance-time data, we calculated the response and recovery time and found that the values of response times were 57, 70 and 32 s for CO, CH₄ and CH₃OH, respectively, whereas, recovery times were 91, 70 and 108 s for CO, CH₄ and CH₃OH, respectively. These values indicate that the fabricated sensor, as compared to that of previously reported sensors, has relatively short response and recovery times for all of these gases.

It is not easy to compare the achieved results with other reported results due to different measurement setups, such as measurements in vacuum conditions prior to target gas exposures^35,36 or different background gases.³⁷ Our results can be most probably compared to the results reported by Comini et al.,³⁸ Barbi et al.³⁹ and Wang et al.,⁴⁰ who have reported SnO₂ nanobelts, films and nanowires sensor responses to CO concentrations of 250–500, 10–100 and 20 ppm, respectively. Kolmakov et al.^41,42 and Ramirez et al.⁴³ attained a clear sensor response to 5 ppm of CO with SnO₂ nanowires sensor. Our results are also comparable to the results reported by Shouli et al.⁴⁴ and Kuang et al.,⁴⁵ who have reported SnO₂ nanocomposites and nanowire sensors for 850 and 500 ppm of CH₄ as well as CO. In case of CH₃OH, our results are comparable to the results reported by Ding et al.³⁶ and Shouli et al.,⁴⁴ who have reported SnO₂ nanorods and nanowires sensors for 100 and 200 ppm of CH₃OH, respectively.

Conclusion

We have demonstrated that SnO₂ nanowires can be successfully synthesized, by carbothermal reduction process, on gold-catalyzed silicon substrate using mixture of SnO₂ and graphite powders as source materials. The XRD analysis shows that the synthesized SnO₂ nanostructures have polycrystalline nature with tetragonal rutile structure, whereas the SEM investigation reveals that the ratio of 4:1 (SnO₂:C) is an appropriate ratio and uniform dense nanowires of SnO₂ (diameter ∼127 nm and length ∼40 µm) were synthesized with VLS mechanism. Gas sensor fabricated from synthesized SnO₂ nanowires was used to study the gas sensing performance of these synthesized nanostructures. Gas sensing tests of synthesized nanowires exhibit enhanced gas sensing performance, which may be attributed to high porosity and more active sites. The stability of the sensor and the simplicity of technology used are also the advantages of the fabricated sensor.

Footnotes

Handling Editor: Kenneth Loh

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors thank Higher Education Commission (HEC) of Pakistan for the financial support through ‘National Research Program for Universities’ and Institute of Industrial Control Systems, Pakistan, for their technical sport. The authors also thank COMSATS Institute of Information Technology, Islamabad, for providing excellent research facilities through project no. 16-27/CRGP/CIIT/IBD/13/225.

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Synthesis of SnO 2 nanowires forCO,CH 4 and CH 3 OH gases sensing

Abstract

Keywords

Introduction

Experimental details

Results and discussion

SEM and EDX analysis

Crystal structure analysis

UV/visible absorbance and photoluminescence spectroscopy

I-V characteristics

Gas sensing properties of SnO2 nanowires

Conclusion

Footnotes

Declaration of conflicting interests

Funding

References

Gas sensing properties of SnO₂ nanowires