Enhanced gas-sensing performance of SnO₂/Nb₂O₅ hybrid nanowires

Abstract

Semiconductor tin oxide (SnO₂) is acknowledged to be one of the semiconductor oxides with most potential for utilization as nitrogen dioxide (NO₂)-sensor materials. However, the challenge to improve the gas sensitivity still remains, and this limits the full realization of the SnO₂ sensor potential. In this paper, enhanced gas sensitivity obtained by the formation of a SnO₂/niobium pentoxide (Nb₂O₅) hybrid structure is reported. As the NO₂ concentration is lower to 3 ppm, the measured resistance in the SnO₂/Nb₂O₅-based sensor is still 2.5 times larger than that in a pristine SnO₂-based sensor. With an increase in the NO₂ concentration, the response time of the SnO₂/Nb₂O₅-based sensor decreases sharply whereas the pristine SnO₂-based sensor response time increases. The enhanced performance is attributed to the presence of more oxygen vacancies and a higher specific surface area in the hybrid structure induced by the introduction of Nb₂O₅. These results provide some general guidelines for the selection of compositions to enhance the sensor performance.

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Introduction

More and more environmental concerns such as acid rain, ozone depletion and the greenhouse effect result from gaseous pollutants, which severely influence health and society. Therefore, the gas sensor utilized for monitoring these pollutants becomes a part of environmental protection. Recently, the semiconducting metal oxides [such as tin oxide (SnO₂), indium oxide (In₂O₃) and tungsten trioxide] become more preferable for use on account of their easily measurable resistance changes during monitoring of pollutants.^1–5 When exposed to the reducing gases such as hydrogen sulfide, ammonia and so on, the surface of the metal oxide forms a charge depletion layer because of the adsorbed oxygen trapping electrons from the conduction band of the metal oxide. Subsequently, the oxygen species formed react with the adsorbed reducing gas molecules.^6–9 As a result, the reducing gas can be detected by monitoring the change of the resistance in the metal oxide. In the detection of the oxidizing gas, the oxygen vacancies play a crucial role as chemisorption sites on the surface of the metal oxide.^10,11 Generally, the oxygen vacancies carry a positive charge, which can trap the electrons. During the reactions with adsorbed gas molecules, the electron transfer occurring in the conduction band of the metal oxide induces a resistance change. Therefore, the oxidizing gas can be detected by exploring the resistance change in the metal oxide. Compared to other semiconductors, SnO₂ has less barrier potential to adsorb nitrogen dioxide (NO₂) molecules onto its surface.^12–14 In addition, the natural formation of oxygen vacancies in SnO₂ facilitates the electron transfer during the process of adsorption and desorption.^15–17 Therefore, SnO₂ has become one of the most popular materials for commercial NO₂ gas sensor manufacture. Considering the sensor performance, the material surface structure plays a key role in the sensitivity, selectivity, and stability.^18,19 Generally increase of the active sites on the surface is a prevalent strategy to enhance its sensitivity performance as a gas sensor.^20,21 It has been reported that the microstructure of SnO₂ as well as its specific surface area can be tuned by mixing it with other different elements.^22–24 Keying Shi et al. have demonstrated an enhanced sensitivity performance of SnO₂/In₂O₃ nanorods by increasing the specific surface area.²⁴ With the presence of the strong oxidizing element cerium, an increased amount of oxygen vacancies were observed in SnO₂/cerium oxide spheres, leading to an enhanced gas sensitivity performance.²⁵ However, a challenge to enhance both the specific surface area and oxygen vacancies in SnO₂ still remains. In this paper, an enhanced gas sensitivity performance in a SnO₂/niobium pentoxide (Nb₂O₅) (SN) hybrid nanowire structure with both enhanced oxygen vacancies and specific surface area induced by the introduction of Nb₂O₅ is demonstrated. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and nitrogen (N₂) adsorption–desorption isotherms (Brunauer–Emmett–Teller; BET) were utilized for characterizing the structure and chemical composition of the hybrid nanowires. The sensitivity performance of the hybrid structure based gas sensor was examined in both NO₂ and an ambient atmosphere environments.

Materials and methods

Chemicals and materials

Poly(vinyl pyrrolidone) (PVP, molecular weight (M_w) = 1 300 000) and niobium chloride (NbCl₅) were purchased from Alfa Aesar (China) Chemical Co. Ltd. Stannous chloride (SnCl₂), ethanol and dimethyl formamide (DMF) were purchased from Sinopharm Chemical Reagent Co. Ltd.

Method

The SN nanowires were prepared using an electrospinning method. The precursor sources of SnO₂ and Nb₂O₅ were SnCl₂ and NbCl₅, respectively. In a typical experiment, 0.5 g of SnCl₂·H₂O was added into a mixture of 2.5 ml ethanol and 2.5 ml DMF which was then stirred for ∼2 h to ensure that the precursors were dissolved. Then 0.15 g of NbCl₅ was added into the solution to dissolve. When the solution became transparent, 0.5 g of PVP was added and the mixture was stirred for about 12 h. The solution was then transferred into a syringe. A high voltage power supply was connected with a 22G sized needle. The flow rate was 0.75 ml l⁻¹ and a voltage of 16 kV was applied. The distance between the needle and a collector (a piece of aluminum foil) was fixed at a value of 16 cm. Then the prepared, electrospun SnCl₂/NbCl₅/PVP nanowires were dried in an oven at a temperature of 80 °C for 12 h in ambient air. To remove the polymer matrix and oxidize two kinds of chloride, the nanowires were annealed in the air at 500 °C for 1 h and the SN hybrid nanowire was obtained. For comparison, the pristine SnO₂ nanowires were prepared using the same method without the addition of NbCl₅.

Characterization

Field-emission scanning electron microscopy (JSM-6700F, Jeol) was used to explore the morphology of the as-prepared SN nanowires. TEM (Talos F200X, FEI) and XRD (MXPAHF, Mac Science Co. Ltd., Japan) were utilized for the microstructure investigations. In addition, these nanowires were also examined using XPS (ESCALAB 250Xi, Thermo Fisher Scientific) with Al Kα radiation (1486.6 eV).

Gas sensing experiments

The samples were ground with deionized water at a volume ratio of 4 : 1 and were painted uniformly on the interdigital electrode. After drying, the device was kept at 400 °C for one day and then it was used for gas sensor investigation.

Results and discussion

The morphologies and microstructures of the as-synthesized nanowires were examined using SEM and TEM as shown in Fig. 1. The rough surface indicates that the nanowires are composed of polycrystalline nanoparticles. During the annealing process, the polymer was oxidized and overflowed from the surface of the nanowires. In the meanwhile, SnCl₂ and NbCl₅ were also oxidized and the SN nanowires were formed internally. The energy dispersive spectroscopy (EDS) characterization confirms the chemical composition of the SN nanowires (Fig. 1c–f). This reveals that niobium (Nb) atoms are mainly located in the nanowire core whereas more tin (Sn) atoms are distributed on the surface of the SN nanowires. Because the ionic radius of Sn⁴⁺ is smaller than Nb⁵⁺, the Sn⁴⁺ ionic becomes easier to research the surface of the nanowires during an annealing process.^19,24 Therefore, a core/shell-like structure is observed in the SN nanowires. Two types of lattice fringes (Fig. 1g) were also observed at the surface and core of the hybrid structure. They are indexed to be (110) plane of SnO₂ shell and (100) plane of the Nb₂O₅ core as shown in the Fig. 1h and i. The XRD pattern of the hybrid structure (Fig. 1j) shows the SnO₂ representative planes of (110), (101) and (211). At 22.3°, the Nb₂O₅ representative peak of (100) was also observed. Because the molar ratio of the Sn and Nb in the synthesis precursors is 5 : 1, the peak intensity of SnO₂ is much stronger than that of Nb₂O₅.

Fig. 1 (a and b) The SEM images of SnO₂/Nb₂O₅ and pure SnO₂ nanowires. (c) The transmission electron microscope image. (d–f) The corresponding energy-filtered transmission electron microscopy (EFTEM) elemental mapping of Sn, Nb and O. (g) The high-resolution TEM (HRTEM image of SnO₂/Nb₂O₅). (h and i) Partially enlarged area of the HRTEM image shown in (g). (j) The XRD patterns of SnO₂/Nb₂O₅ (red line) and SnO₂ (black line) nanowires. The peak of the Nb₂O₅ is labeled with red stars and the others are the peaks of the SnO₂ phase.

To further confirm the structure and the chemical composition of the hybrid structure, XPS was used. The high-resolution spectra (Fig. 2a–c) shows the binding energy of SnO₂ and Nb₂O₅ in the hybrid structure. The O 1s spectra (Fig. 2a) can be divided into two contributions from 529.5 eV and 530.5 eV, which are from the O 1s levels of tin(II) oxide (SnO) and SnO₂, respectively. Compared to the pure SnO₂, the peak intensity of SnO in the hybrid structure is much higher. In general, the existence of the SnO phase in the SnO₂ implies that the formation of oxygen vacancies occurs.^3,26,27 Therefore, the amount of oxygen vacancies in the hybrid structure is much higher than that of pure SnO₂. This is attributed to the strong oxidizing property of Nb, which causes more of the SnO phase to be formed in the hybrid structure.²⁸ As shown in Fig. 2b, there are four different states in the Sn 3d spectra. The peaks labeled by the red line and green lines represent Sn⁴⁺ 3d_3/2, and the peaks labeled by the blue and pink lines represent Sn²⁺ 3d_3/2. This further confirms the existence of the SnO phase in both the SN and SnO₂ nanowires. The XPS spectra of the Nb binding energy (Fig. 2c) also prove that there is Nb₂O₅ phase existing in the hybrid structure, which can be utilized to tune the surface state of the SnO₂.

Fig. 2 The high-resolution XPS spectra showing the binding energy of (a) O 1s and (b) Sn 3d and (c) Nb 3d in SnO₂/Nb₂O₅ (lower spectra) and SnO₂ (upper spectra) nanowires, respectively.

To explore the specific surface area of the samples, N₂ adsorption–desorption isotherms were measured and characterized using the BET pore size distribution. These measurements revealed that the BET surface area of the SN hybrid nanowire is 31.7476 m² g⁻¹, which is twice that of the pristine SnO₂ (14.6543 m² g⁻¹) as shown in Fig. 3. The pore size of the SN hybrid nanowires are mainly distributed around 110 nm whereas the pristine SnO₂ has a four times larger value of the pore size as shown in the insets of Fig. 3.

Fig. 3 BET results showing the specific surface area and pore size distributions of (a) SnO₂/Nb₂O₅ and (b) SnO₂ nanowires.

In order to explore the gas sensing properties of the SN hybrid nanowires, a gas sensor device was fabricated by painting the nanowires onto an interdigital electrode. The resistance change of the device was monitored as it was exposed to NO₂ of different concentration ranges from 0.5 ppm to 50 ppm. These experiments were carried out under a dry air diluted NO₂ environment. The parabolic shaped curve shown in Fig. 4a shows the response of the SN nanowires as a function of temperature, indicating that the best working temperature for the device is 200 °C. The dynamic sensing response versus time curves is shown in Fig. 4b. The sensor sensitivity was defined as R = R_g/R_a. R_a and R_g represent the resistance in air and NO₂, respectively. At low concentrations, the sensitivity of the SN nanowires is much better than that of pure SnO₂, and there is 3.5 times larger resistance change at a NO₂ concentration of 3 ppm. As demonstrated, in the range of 0.5 ppm to 50 ppm, the SN nanowire-based devices maintain a strong sensitivity performance compared to that of the pristine SnO₂. The response time and recovery time are another two important parameters used to evaluate the performance of a sensor. The response time of the hybrid structure based device decreases (Fig. 4c) whereas the response time of the SnO₂-based device exhibits certain fluctuations as the NO₂ concentration increases. Compared to the recovery time of SnO₂, the hybrid structure based device under different NO₂ concentrations presents a more stable behavior as shown in Fig. 4d.

Fig. 4 (a) Resistance change response of SnO₂/Nb₂O₅ at different temperatures; (b) the resistance change response of SnO₂/Nb₂O₅ (red line) and SnO₂ (black line) under an NO₂ environment; (c and d) the response time and recovery time of SnO₂/Nb₂O₅ (red line) and SnO₂ (black line) nanowires.

In the hybrid structure, the SnO₂ phase is mixed with the Nb₂O₅ phase as discussed previously. Therefore, the strong oxidizing property of Nb traps more oxygen surrounding the Nb atoms and causes less oxygen to be surrounding the Sn atoms in the hybrid nanowires during the synthesis process.²⁸ As a result, there are more oxygen vacancies formed in the hybrid structure compared to the pristine SnO₂. This is consistent with XPS results obtained in this research, which indicate more of the SnO phase is observed in the hybrid structure. As shown in Fig. 1, the core of the hybrid nanowire is mostly occupied by the aggregated Nb₂O₅ nanoparticles and therefore the surface of the hybrid structure was mainly coated with SnO₂ nanoparticles. Compared to the pristine SnO₂, there are more active reaction sites exposed to the NO₂ gas in the hybrid structure. As a result, the SN nanowires exhibit a better sensitivity performance as a NO₂ gas sensor with the introduction of Nb. Selectivity and stability are two of the important parameters used for evaluating the sensing performance. These results reveal that the SnO₂/Nb₂O₅ exhibits an apparently higher sensing performance in NO₂ compared to other different gas environments. In addition, the SN nanowires exhibit an excellent stable sensing performance (see the ESI†). To explore the effect of gas on the morphology, structure, vacancy concentration, SEM, XPS characterizations were used on the sample of SN nanowires after the sensing measurement (see the ESI†). The results show that there is rarely any effect on the SN nanowires during the sensor performance. Although the response and recovery time of the sample was slightly longer, the best working temperature was lower than that of the reported SnO₂ and In₂O₃ sensor devices (Table S1, ESI†).

Conclusion

In this paper, a hybrid structure of SN nanowires is reported which can be applied in a NO₂ gas sensor. The strong oxidizing property of Nb introduced into the hybrid structure caused a large amount of oxygen vacancies. This indicated that the aggregation behavior of Nb₂O₅ nanoparticles led to more SnO₂ being exposed to the surface of the hybrid structure compared to with pristine SnO₂. Therefore, the hybrid structure based sensor device exhibited an enhanced sensitivity performance. These results provide an effective method to tune the material surface state with a selection of compositions.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21373196, 11434009, 11474249), the National Program for Thousand Young Talents of China and the Fundamental Research Funds for the Central Universities (WK2340000050, WK2060140014).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra21672b

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