A low-temperature n-propanol gas sensor based on TeO₂ nanowires as the sensing layer

Abstract

TeO₂ nanowires were synthesized on Au-coated glass substrates using a thermal evaporation method. Structural characterization showed that TeO₂ nanowires with high surface-to-volume ratio and large mass production had a single-crystal tetragonal structure. A gas sensor based on TeO₂ nanowires showed a quick response–recovery speed and a reversible response to n-propanol gas at low operating temperatures.

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Recently, one dimensional (1D) metal oxide semiconductor nanomaterials such as nanowires, nanobelts, nanorods, and nanotubes have attracted significant attention due to their large surface-to-volume ratio, high size effect, unique physical and chemical properties, which makes them promising candidates for novel potential applications.^1–4 Among these 1D nanomaterials, semiconducting nanowires have shown exciting potential and relevance for their special technological applications due to not only the superior optical, electrical, thermal and mechanical properties, but also their high efficiency and activity caused by their large surface area and high nano-scale size.^5–7

As a wide band gap semiconductor (E_g = 3.8 eV at 300 K), TeO₂ is regarded as a key functional material used for optical devices such as deflectors, modulators, laser devices, tunable filters, and optical data storage due to its remarkable acousto- and electro-optical properties.^8–12 The conventional processing of TeO₂ materials has just been used for preparing bulk materials and thin films. Kokh et al. prepared large TeO₂ single crystals by a low temperature gradient Czochralski method with nonuniform heating in a modified furnace.¹³ Li et al. reported that TeO₂ single crystals with large size of 52 × 52 × 80 mm³ and high quality were obtained along 〈110〉 direction by the modified Bridgman method.¹⁴ Siciliano et al. deposited TeO₂ thin films on quartz substrates by sputtering a Te metal target in an Ar/O₂ gas mixture.¹⁵ Lin et al. prepared TeO₂ thin films on glass substrates by nonhydrolytic sol–gel process.¹⁶

Although TeO₂ materials have been widely investigated in optical applications, only few works are related to the synthesis and n-propanol gas sensing properties of TeO₂ nanostructures up to now.^17–19 Insufficiency of surface-to-volume ratio and mass production also further limited the development of TeO₂ nanostructures in the research field of gas sensors. Therefore, many efforts are currently in progress to optimize the growth conditions and clarify the sensing mechanism.

In this study, high-quality single crystalline TeO₂ nanowires with large mass production were synthesized on Au-coated glass substrate by thermal evaporation method. In a typical procedure, 0.5 g of Te powders with a high purity of 99.999% was placed in an Al₂O₃ boat and positioned in the horizontal quartz tube in a tubular electric furnace. The glass substrates coated with an Au film with a thickness of about 10 nm by ion beam sputtering method were covered on the top of Te powders with a vertical distance of 5 mm. The furnace was then heated to 450 °C and maintained at this temperature for 2 h in air. After the furnace was cooled down to room temperature naturally, a layer of white wire-shaped products was obtained on the substrates. The structural characterization of the products was determined by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and Fourier transform infrared spectrum (FTIR). Further experimental details on the synthesis of TeO₂ nanowires and analysis of structure characterization are provided in the ESI.†

XRD pattern of TeO₂ nanowires is represented in Fig. 1. All the diffraction peaks mainly centered at 21.8°, 26.2°, 28.7°, 29.9°, 37.3°, 47.7°, 48.6°, 53.9°, and 55.3° well correspond to (101), (110), (111), (102), (200), (004), (212), (220), and (114) lattice planes, which are well indexed to the tetragonal TeO₂ structure with lattice constants of a = b = 4.810 Å and c = 7.613 Å according to JCPDS card no. 11-0693, indicating that the obtained TeO₂ nanowires are of high quality and purity. The strongest diffraction peak due to the reflection from the (102) lattice plane reveals that TeO₂ nanowires grow with a strong preferred orientation.

Fig. 1 XRD pattern of TeO₂ nanowires.

The morphology of TeO₂ nanowires was first observed by FESEM, and the FESEM images are shown in Fig. 2. Fig. 2a illustrates the overall wire-like nanostructures of the obtained TeO₂ nanowires with a diameter of 70–200 nm and a length of several hundreds of micrometers to 2 mm. In addition, it can be clearly seen that a high-yield TeO₂ nanowires are distributed on the substrate. The high magnification FESEM image in Fig. 2b provides a clear view of TeO₂ nanowires, indicating that of TeO₂ nanowires are structurally uniform with a smooth surface. The maximal surface-to-volume ratio of TeO₂ nanowires is estimated to be approximately 3 × 10⁴, revealing that these TeO₂ nanowires show a high surface-to-volume ratio and great surface activity.

Fig. 2 Typical FESEM images of TeO₂ nanowires. (a) Low magnification FESEM image. (b) High magnification FESEM image.

Further microstructure and chemical components of TeO₂ nanowires were examined using TEM and high-resolution TEM (HRTEM). Fig. 3a shows a typical low magnification TEM image of a single TeO₂ nanowire with a diameter of nearly 90 nm. The corresponding selected area electron diffraction (SAED) pattern shown in Fig. 3b also supports the formation of single crystal tetragonal TeO₂ nanowires, which is consistent with the result of XRD pattern shown in Fig. 1. A HRTEM image in the edge region of this nanowire is shown in Fig. 3c, in which the lattice planes can be clearly seen, indicating an excellent single crystal structure. The interplanar spacings of 0.47 and 0.29 nm correspond to the (010) and (102) lattice planes in a tetragonal TeO₂ structure. To demonstrate the chemical composition of the nanowire, EDX analysis was carried out, and the EDX spectrum is illustrated in Fig. 3d. It is found that, except for the Cu element from the copper grid in TEM measurement, only peaks of Te and O elements are observed, indicating that the single crystalline TeO₂ nanowires do not include any impurity.

Fig. 3 (a) TEM image of a single TeO₂ nanowire with a diameter of 90 nm. (b) SAED pattern of this nanowire. (c) HRTEM image of this nanowire. (d) EDX spectrum of this nanowire.

Usually, two models have been proposed to describe the growth mechanism of TeO₂ nanowires: the catalyst-assisted vapour–liquid–solid (VLS) and the vapour–solid (VS) growth mechanisms.²⁰ The important feature of the VLS growth mechanism is the existence of metal nanoparticles which serve as catalysts between the vapour feed and the solid product. Such metal nanoparticles are located at the ends of the produced nanowires.²¹ In the present study, a layer of Au film with a thickness of 10 nm was coated on the surface of glass substrate. However, no metal nanoparticles are observed at any ends of TeO₂ nanowires as shown in Fig. 2 and 3. Therefore, the growth process of TeO₂ nanowires might be dominated by the VS growth mechanism. At a high temperature of 450 °C, Te powders are vaporized to form the gaseous Te molecules, which subsequently react with oxygen in air to form the gaseous TeO₂ molecules. Then, these gaseous TeO₂ molecules directly condense on the substrate to become the seed crystals serving as the nucleation sites. To minimize the surface energy, the gaseous TeO₂ molecules facilitate directional growth. Therefore, TeO₂ nanowires tend to be produced by continuous aggregation of more molecular TeO₂ on the growth front of the initial TeO₂ nuclei via the VS growth mechanism.²²

The FTIR spectrum of TeO₂ nanowires is shown in Fig. 4. Based on the previous literatures, the peak located at 463 cm⁻¹ is assigned to bending vibrations of Te–O–Te or O–Te–O linkages.²³ The peak at 781 cm⁻¹ and the shoulder peak at about 716 cm⁻¹ match well with the characteristic stretching bands of v₁(A₁) and v₈(B₁), respectively, which are due to the equatorial Te–O bonds.^24,25 The peaks at 636 and 681 cm⁻¹ are attributed to axial symmetric and asymmetric stretching bands of v₂(A₁) and v₆(B₂), respectively.²⁶ The observed FTIR spectrum clearly confirms the formation of TeO₂. In addition, the band gap of TeO₂ nanowires could be calculated based on Tauc/Davis–Mott Model.²⁷ The estimated band gap is 3.68 eV, which is slightly lower than the conventional 3.8 eV. This is maybe due to the chemical defects or vacancies in the present TeO₂ nanowires, forming a new energy level to reduce the band gap energy.

Fig. 4 FTIR spectrum of TeO₂ nanowires.

Gas sensor was fabricated by dispersing TeO₂ nanowires on an Al₂O₃ substrate with a pair of interdigitated Au electrodes and a heating resistance. The gas sensing measurements were performed by a volt-amperometric method at the operating temperatures of 24–100 °C in a gas sensing test system in a fume hood. The experimental details on the fabrication of gas sensor and gas sensing measurement are provided in the ESI.†

The dynamic response–recovery curves of TeO₂ nanowire gas sensor upon exposure to 500 ppm n-propanol gas at different operating temperatures are shown in Fig. 5. It can be clearly seen that the resistance decreases rapidly after introduction of reducing n-propanol gas, and then it recovers to its initial value after removing n-propanol gas, indicating that TeO₂ nanowires are n-type semiconductor which shows a good reversibility. In addition, the response and recovery times are important parameters to evaluate the sensing performance of a sensor. Generally, the response time and recovery time are defined as the time required for the resistance to reach 90% of the equilibrium value after detected gas is introduced, and the time necessary for the sensor to recover 90% of its initial resistance, respectively. As shown in these figures, it is also found that TeO₂ nanowire gas sensor shows quick response and recovery characteristics to n-propanol gas with the response and recovery times of less than 30 s depending on the operating temperatures.

Fig. 5 Dynamic response–recovery curves of TeO₂ nanowire gas sensor upon exposure to 500 ppm n-propanol gas at different operating temperatures: (a) 24 °C; (b) 50 °C; (c) 75 °C; (d) 100 °C.

Fig. 6 presents the corresponding response of TeO₂ nanowire gas sensor upon exposure to 500 ppm n-propanol gas at different operating temperatures. In this study, the sensor response was defined as R_a/R_g, where R_a and R_g were the electrical resistances before and after the introduction of n-propanol gas, respectively. It is found that the response is highly dependent on the operating temperature. At the operating temperature lower than 50 °C, the response significantly increases and obtains the maximum value of 3.15 at 50 °C. However, the response decreases as the operating temperature further increases. Therefore, 50 °C is the optimum operating temperature for the following gas sensing measurements.

Fig. 6 Response of TeO₂ nanowire gas sensor upon exposure to 500 ppm n-propanol gas at different operating temperatures.

In order to further investigate the reversibility and stability, the dynamic response–recovery curves of TeO₂ nanowire gas sensor upon exposure to n-propanol gases with different concentrations in the sequence of 100, 200, 500, 800, and 1000 ppm at an operating temperature of 50 °C are presented in Fig. 7. For each n-propanol gas concentration, the resistance decreases abruptly after introducing n-propanol gas and then recovers to the initial value after n-propanol gas is released. In addition, the change in the resistance increases with the increase of n-propanol gas concentration, indicating that the response greatly enhances as n-propanol gas concentration increases. The relationship between the response of TeO₂ nanowire gas sensor and n-propanol gas concentration at 50 °C is illustrated in Fig. 8. As shown in this figure, there is an almost linear relationship between the response and n-propanol gas concentration, which indicates that it is possible to determine a wide range of n-propanol gas concentration from the response signal. The responses upon exposure to 100, 200, 500, 800, and 1000 ppm n-propanol gases are 1.88, 2.34, 3.15, 3.72, and 4.45, respectively. In particular, the response and recovery times are less than 20 s at a low operating temperature of 50 °C, which can satisfy the demand of practical application for a gas sensor, and are convenient for the continuous detection.

Fig. 7 Dynamic response–recovery curves of TeO₂ nanowire gas sensor upon exposure to n-propanol gases with different concentrations at an operating temperature of 50 °C.

Fig. 8 Relationship between the response of TeO₂ nanowire gas sensor and n-propanol gas concentration at 50 °C.

The electron depletion theory is widely used for explaining the sensing mechanism of gas sensor based on n-type oxide semiconductor.^28,29 For TeO₂ nanowires, the oxygen molecules in air adsorb on the surface of TeO₂ nanowires and trap electrons from the conduction band of TeO₂ nanowires to form adsorbed oxygen species such as O₂⁻, O⁻, and O²⁻ at a proper operating temperature. Consequently, an electron depletion layer is formed in the surface region of TeO₂ nanowires, resulting in a decrease of conductance. When TeO₂ nanowire gas sensor is exposed to the reducing n-propanol gas, these detected gas molecules react with adsorbed oxygen species to form CO₂ and H₂O, releasing the trapped electron back to the conduction band of TeO₂ nanowires and subsequently leading to an increase of conductance. As n-propanol gas concentration increases, the electron release aggravates to reduce the width of the depletion layer and consequently decreases the resistance of TeO₂ nanowires gradually at a fixed operating temperature.

Compared with the previous TeO₂ nanomaterials, the present TeO₂ nanowires can provide a higher surface-to-volume ratio and better electron mobility, which leads to more surface active sites available for sensing reaction, thus further improving gas sensing performance at low operating temperatures. On the other hand, at a low operating temperature, the adsorbed oxygen species show small number and low activity, resulting in weak reaction and low response. At a high operating temperature, the reaction occurred on TeO₂ nanowires is very fast, which limits the effective diffusion of n-propanol gas and decreases the gas concentration on the surface of TeO₂ nanowires, leading to the decrease of the response. Therefore, an optimum operating temperature of 50 °C is obtained to show the highest response.

In conclusion, high-yield single crystal TeO₂ nanowires were synthesized by thermal evaporation of high purity Te powders in air. TeO₂ nanowires have a tetragonal phase structure and show a high surface-to-volume ratio. Gas sensor based on TeO₂ nanowires showed a reversible response and quick response–recovery speed to n-propanol gas at low operating temperatures, demonstrating the possibility of fabricating low power consumption gas sensors using TeO₂ nanowires.

Acknowledgements

The project was supported by the National Natural Science Foundation of China (51422402), Fundamental Research Funds for the Central Universities (N140105002), Program for Liaoning Excellent Talents in University (LJQ2013025), Specialized Research Fund for the Doctoral Program of Higher Education of China (20130042120033), and Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (47-3). The authors also wish to gratefully thank the teachers worked at Research Institute of Northeastern University Analysis and Testing Center for their collaboration in SEM and TEM measurements.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra00867k

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