High-response H₂S sensor based on ZnO/SnO₂ heterogeneous nanospheres

Abstract

Designing nanostructured materials to enhance gas-sensing performance is a key objective for sensor technology. In this paper, ZnO/SnO₂ heterogeneous nanospheres have been successfully synthesized through annealing as-synthesized SnO₂ nanospheres immersed in aqueous Zn(NO₃)₂ solution. Compared to the untreated pure SnO₂ nanospheres and commercial ZnO gas sensors, the ZnO/SnO₂ heterogeneous sensors showed exceptional electrical responses to H₂S gas at 300 °C. The response of the as-obtained ZnO/SnO₂ sensors towards 10 ppm H₂S can reach up to 99.6. Meanwhile, the sensors exhibited obvious sensitivity and fast response/recovery behavior towards trace H₂S gas, and achieved a detection limit of less than 1 ppm. Moreover, the gas test results revealed that the design of the ZnO/SnO₂ heterogeneous nanostructures enhanced the H₂S gas response and the selectivity in response to interfering gases such as NO, SO₂, CO, CH₄, and C₂H₅OH. The high sensitivity and dynamic repeatability observed in these sensors reveal that these heterogeneous nanostructures are promising as sensitive and reliable chemical sensors.

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1. Introduction

Real-time monitoring of pollutant, toxic, and harmful gases is important for the health and safety of industrial workers and the general population.^1,2 Hydrogen sulfide (H₂S) is a naturally occurring gas found in oil deposits, biogas, and natural gas fields. It is toxic and harmful at concentrations as low as hundreds of parts per million. Currently, there are many methods for detecting gases, but most available sensors are not suitable for detecting trace gas concentrations.

Nanostructured materials have received great attention as gas-sensing materials due to their high sensitivity, good transduction properties, fast response time, and excellent selectivity.³ In the past decade, many semiconductor compounds, such as Fe₂O₃,^4,5 SnO₂,^6–9 ZnO,^10,11 WO₃,^12,13 NiO, and CuO,^14,15 have been intensively studied as gas-sensing materials. Although some progress has been achieved, it is still a big challenge to synthesize ultra-sensitive materials for detecting a trace gas. Thus, exploring ultra-sensing materials has become an extremely important task for developing high-performance gas sensors.

In recent years, inorganic semiconductor composites have been of great interest due to their improved ability to support charge transfer on their interfaces.^16–18 As mentioned above, ZnO and SnO₂, as wide direct-band-gap (E_g = 3.37 and 3.6 eV at 300 K, respectively) semiconductors, have attracted much attention for gas sensor applications.¹⁹ Nevertheless, both sensing materials react with several gases simultaneously. Therefore, poor selectivity and reliability are the main drawbacks of these gas-sensing materials. Various methods have been applied to enhance gas sensing performance. ZnO/SnO₂ heterogeneous composites have also been used as gas-sensing materials, and exhibited good sensing properties with potential application in gas sensors. Many strategies have been applied to fabricate ZnO/SnO₂ sensing materials with various structures such as core–shell nanowires/nanospheres,^20–24 hollow structures,^25–28 nanofibers,^29–31 hierarchical structures,^32,33 etc. Khoang et al. prepared a ZnO–SnO₂ heterostructure with ZnO nanorod branches that were uniformly grown on SnO₂ nanowires, and it showed high sensitivity to ethanol at 400 °C.³³ In addition, nanocomposites of ZnO and SnO₂ showed much better performance compared with pure SnO₂ or ZnO. Choi et al. combined electrospinning with atomic layer deposition to obtain ZnO-covered SnO₂ fibers for detecting O₂ and NO₂.²² Thus, designing ZnO/SnO₂ nanostructures is expected to result in excellent gas-sensing performance.

Our group recently found that it was feasible to prepare inorganic composites through annealing porous materials immersed in another aqueous salt, and successfully prepared Fe₂O₃/NiO nanoplates with a large response to H₂S.³⁴ In this context, we have applied this method to synthesize ZnO/SnO₂ heterogeneous nanospheres, which demonstrated their performance as an excellent H₂S-sensing material with a fast, sensitive, and good selective response towards H₂S gas.

2. Experimental

2.1. Synthesis of ZnO/SnO₂ heterogeneous nanospheres

The synthesis of ZnO/SnO₂ nanospheres involves two steps: (i) the synthesis of SnO₂ nanospheres. The synthesis of SnO₂ nanospheres was based on our previous work.⁷ Typically, 0.27 g Na₂SnO₃·3H₂O and 0.2 g tris(hydroxymethyl)aminomethane (THAM) were first dissolved in 35 ml distilled H₂O, and then heated at 120 °C for 10 h under hydrothermal conditions. The as-obtained white sample was rinsed with deionized water and pure ethanol, and finally dried under ambient conditions. (ii) For the synthesis of ZnO/SnO₂ heterogeneous nanospheres, 0.1 g of the as-synthesized SnO₂ nanospheres were immersed in a 3 ml solution containing 1 mmol zinc nitrate for 30 min and then dried at 120 °C in air. The product was heated at 300 °C in air for 1 h with a heating rate of 4 °C min⁻¹ in a muffle furnace, and then cooled to room temperature naturally.

2.2. Characterization

The morphology and crystal structure were characterized using X-ray diffraction (XRD, Rigaku D/max 2500 diffractometer), scanning electron microscopy (SEM, Hitachi S4800), transmission electron microscopy (TEM; JEOL 2010 with an accelerating voltage of 200 kV) and high-angle annular dark field scanning TEM (HAADF-STEM) images (JEOL JEM-2100F electron microscope).

2.3. Gas sensing measurements

The fabrication and testing principles for the gas sensor are similar to those described in our previous reports.^8,9 Firstly, the gas-sensing samples were mixed with terpineol to form a paste and then coated onto the outside surface of an alumina tube with a diameter of 1 mm and a length of 5 mm. A platinum coil through the tube was employed as a heater to control the operating temperature. To improve their stability and repeatability, the gas sensors were aged at 300 °C for 10 h in air. Here, the sensing properties of the sensors were measured by a NS-4003 series gas-sensing measurement system (China Zhong-Ke Micro-nano IOT, Internet of Things, Ltd.). The relative humidity (RH) was about 45%. The response and recovery times were defined as the time required for a change of the resistance to reach 90% of the equilibrium value after injecting the air/gas mixture and that for removing the detected gas, respectively. When the air and ppm-level target gas were blown through the sensor element, the corresponding steady-state resistances of the sensor in air (R_air) and in the air–gas mixture (R_gas) were recorded, respectively. The as-fabricated ZnO/SnO₂, ZnO, or SnO₂ sensor gas response (S) for reducing gases (H₂S, CH₄, SO₂, CO, or ethanol) is defined as the ratio of R_air/R_gas, while the sensor gas response for oxidizing gas (NO) is defined as the ratio of R_gas/R_air.

3. Results and discussion

The ZnO/SnO₂ heterogeneous nanospheres were synthesized through two steps: (i) preparing the SnO₂ nanospheres by the bio-inspired hydrothermal method according to our previous work;⁷ (ii) annealing the as-synthesized SnO₂ nanospheres with adsorbed Zn(NO₃)₂, which extends our previous method for the preparation of Fe₂O₃-loaded NiO nanoplates.³⁴ The as-synthesized sample was found to be a mixed phase of ZnO (JCPDS no. 36-1451) and SnO₂ (JCPDS no. 41-1445), which can be observed in the X-ray diffraction (XRD) pattern in Fig. 1. No characteristic peaks for any impurity, such as ZnSnO₃ or Zn₂SnO₄, were observed.

Fig. 1 XRD patterns of as-synthesized ZnO/SnO₂ heterogeneous nanospheres and bare SnO₂ nanospheres.

The morphology, composition, and elemental distribution of the ZnO/SnO₂ heterogeneous nanospheres were characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM), and further mapped through TEM and energy-dispersive X-ray spectroscopy (EDS) by displaying the integrated intensity of the oxygen, tin, and zinc signals as a function of the beam position when operating the TEM in scanning mode (STEM). In the SEM images of Fig. 2a and b, one can find that the as-synthesized ZnO/SnO₂ heterogeneous nanospheres are well dispersed in a similar way to the untreated SnO₂ nanospheres.⁷ The TEM images in Fig. 2c and d show that there are some nanoplates on the structure of the as-synthesized ZnO/SnO₂ heterogeneous nanospheres, which originate from the ZnO incorporated into the SnO₂ nanospheres. These clear high-magnification TEM images are obviously different from that of the surface of the untreated SnO₂ nanospheres in Fig. 3b. Both the SEM and the TEM results indicate that our synthetic method is effective for preparing well-defined ZnO/SnO₂ heterogeneous nanospheres. Fig. 4 shows a high-angle annular dark field scanning TEM (HAADF-STEM) image of the as-synthesized ZnO/SnO₂ heterogeneous nanospheres. The results shown in Fig. 4a–d reveal that the three elements O, Sn, and Zn are distributed very homogeneously in the heterogeneous nanospheres. Moreover, the average Sn/Zn ratio measured from the heterogeneous nanospheres via EDS analysis is 0.664.

Fig. 2 (a and b) SEM images, and (c) TEM image of ZnO/SnO₂ heterogeneous nanospheres; (d) TEM image of bare SnO₂ nanospheres.

Fig. 3 HR-TEM images of (a) ZnO/SnO₂ heterogeneous nanospheres, and (b) bare SnO₂ nanospheres.

Fig. 4 (a) HAADF-STEM image of ZnO/SnO₂ heterogeneous nanospheres; and (b–d) corresponding O, Sn, and Zn elemental maps.

It is well known that ZnO and SnO₂ are two important fundamental active materials in gas sensors. Semiconducting metal oxides can exhibit a conductivity change due to the adsorption and reactions of molecules from the gas phase with the surface.⁶ Moreover, combining two materials will form n–n heterojunctions at the borders, which speeds up the electronic transport.²⁵ Hence, it is necessary to study the sensing performance of the ZnO/SnO₂ gas sensor.

In order to confirm the optimum operating temperature of the ZnO/SnO₂ sensors, the sensors were exposed to 10 ppm H₂S at different temperatures. Commercial ZnO and untreated SnO₂ nanospheres were also tested for a comparison. In this work, H₂S gas was chosen as the test gas due to its high toxicity. As shown in Fig. 5a, the optimum working temperature for the ZnO/SnO₂ heterogeneous nanospheres and commercial ZnO was 300 °C, at which both of them achieve the highest response (99.6 and 33.1) towards 10 ppm H₂S gas. Then, the responses of both the ZnO/SnO₂ and the commercial ZnO obviously decrease with increasing temperature. This behavior can be explained by the kinetics and mechanics of the gas reaction on the surface of the active materials. Meanwhile, at higher operating temperature, the enhancement of desorption reduces the quantity of the adsorbed molecules and thus leads to the low response when the sensor is operated at high temperature. It is also clearly found that the response of SnO₂ nanospheres towards 10 ppm H₂S gas is enhanced dramatically after incorporating ZnO into their structure. The sensitivities of ZnO/SnO₂ and commercial ZnO dramatically increased when the operating temperature was changed from ambient temperature to 300 °C, and then decreased on further increasing the operating temperature. The response of the untreated SnO₂ nanospheres, however, showed only a slight change with the rising temperature. What is more, the sensitivity of the ZnO/SnO₂ heterogeneous nanospheres was always higher than that of either the commercial ZnO or the untreated SnO₂ nanospheres.

Fig. 5 (a) Response, and (b) response time of three different materials towards 10 ppm H₂S at different operating temperatures.

The response times of these three gas-sensing materials were also studied, as shown in Fig. 5b. The response times of all the three sensors dropped dramatically as the temperature rose from ambient temperature to 350 °C. Moreover, the response time of the ZnO/SnO₂ heterogeneous nanospheres was much shorter than those of commercial ZnO and SnO₂ when the working temperature was above 100 °C.

To understand the sensing characteristics of ZnO/SnO₂ sensors, the dynamic response curves towards H₂S gas with increasing concentrations, ranging from 0.5 to 100 ppm at an operating temperature of 300 °C, were investigated. From the curves in Fig. 6a, it can be seen that the conductance of the sensor undergoes a rapid rise upon injection of the H₂S gas and drops back to its initial state after the sensor is exposed to air. Moreover, the responses obviously increase with increasing concentration of H₂S gas, and the ZnO/SnO₂ sensors exhibit a higher response than the commercial ZnO or the untreated SnO₂ nanosphere sensors for the same concentrations. The better response could be attributed to the stronger interaction between the gas and the surface-adsorbed oxygen species (O₂⁻, O⁻, or O²⁻), and higher release of trapped electrons in ZnO/SnO₂ heterogeneous nanospheres. In air, the oxygen molecules could be adsorbed onto the grains of the nanospheres due to the special heterogeneous nanostructure which has more active sites for adsorption and dissociation of the gas molecules. But with the rise of operation temperature, more thermal energy was provided to overcome the activation energy barrier of the reaction between O²⁻, O⁻, or O₂⁻ ions and gas molecules, and the reaction rate would be improved resulting in the increase of feedback electrons.^35,36 Upon exposure of these materials to the H₂S gas, the chemisorbed oxygen reacts with the H₂S gas, and electrons are subsequently reintroduced into the conduction band, leading to the increase in conductivity.³⁷ Fig. 6b shows the sensitivity of the ZnO/SnO₂ heterogeneous nanospheres to different concentrations of H₂S at the optimum temperature. We find that the sensitivity to the 0.5 ppm H₂S is around 3.94 and that to 100 ppm H₂S gas is around 350.6, which are both higher than the responses of commercial ZnO and untreated SnO₂ nanospheres.

Fig. 6 (a) Dynamic response curves of the three different sensors towards H₂S with increasing concentration at 300 °C; (b) relationship between the response and the H₂S concentration for the three different gas sensors.

The possible mechanism behind the enhanced sensor properties might be ascribed to two main factors: (i) ZnO/SnO₂ could show a highly activated response to the test gas because the synergetic effects of ZnO and SnO₂ facilitate the dissociation of oxygen molecular ions at lower temperature;^38,39 (ii) the ZnO/SnO₂ heterogeneous structure not only could provide more active sites, but also could widen electron depletion layers, facilitating the electron transfer.^40,41 As we know, there are some factors affecting on the gas sensitivity, such as morphology, grain size, structural formation, surface-to-volume ratio and film thickness. For the ZnO/SnO₂ heterogeneous structure, there are two structural advantages for the sensor. Due to the existence of the heterogeneous structure, the band bending at the material/grain surface and the potential barriers that are appearing due to the grain/grain contact of metal oxide, and the response will be affected due to the sensitive material being dominated by the gas-dependent grain–grain potential barriers. Meantime, in the case of semiconductor metal oxide gas sensors, it is generally a surface controlled process that is responsible for the sensitivity. The existence of ZnO nanostructures on the surface of SnO₂ nanospheres is beneficial to increase the sensitivity to H₂S owing to its higher surface-to-volume ratios and different surface states. This type of surface could trap a higher concentration of oxygen ions, and adsorb more H₂S.

Selective detection of a specific gas remains a challenging issue for the commercial application of metal oxide based gas sensors. Metal oxides in their pure form are sensitive to a wide range of gases. Selectivity can be tuned with the use of composite sensing materials, as discussed previously. In order to investigate the selectivity, we have conducted sensing studies of the annealed ZnO/SnO₂ sensor to other gases such as NO, SO₂, CO, CH₄, and C₂H₅OH. The responses of the sensors to 10 ppm of these gases at 300 °C are shown in Fig. 7. As demonstrated in Fig. 7, the sensors show excellent selectivity to H₂S, whereas they have little response to other typical interference gases at the same temperature. For the ZnO/SnO₂ sensor, the response to 10 ppm H₂S was 99.6, whereas the responses to other tested gases were less than 10. The response of the ZnO/SnO₂ sensor to H₂S was approximately 10 times higher than for the other gases. This indicates that the present sensors have quite excellent selectivity towards H₂S.

Fig. 7 Sensor responses for various gases at 10 ppm at 300 °C.

Moreover, the stabilities of these sensors towards 10 ppm H₂S gas at 300 °C were also investigated in this work. As shown in Fig. 8, the sensors maintain their initial response amplitude without a clear decrease through ten successive sensing tests to 10 ppm H₂S gas at 300 °C. These results demonstrate that the ZnO/SnO₂ heterogeneous nanospheres have a good stability, just as the commercial ZnO and untreated SnO₂ nanospheres towards H₂S gas.

Fig. 8 Reproducibility of the performance of the three different sensors on successive exposure to 10 ppm H₂S gas at 300 °C.

4. Conclusions

In summary, we have successfully prepared ZnO/SnO₂ heterogeneous nanospheres with excellent gas-sensing properties. Such ZnO/SnO₂ heterogeneous nanospheres were synthesized through immersing SnO₂ nanospheres in an aqueous Zn(NO₃)₂ solution, followed by annealing these as-immersed SnO₂ nanospheres. The gas-sensing results have demonstrated that the as-synthesized ZnO/SnO₂ heterogeneous nanospheres feature an enhanced response to H₂S gas with high selectivity, fast response, and fast recovery.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant No. 51302079). We also thank Dr Tania Silver from Institute for Superconducting and Electronic Materials (University of Wollongong) for revising our manuscript.

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