Hydrothermally processed SnO₂ nanocrystals for ultrasensitive NO sensors

Abstract

SnO₂ nanocrystals have been successfully synthesized by a hydrothermal method assisted by biuret. When tested as the gas-sensing material, these hydrothermally processed SnO₂ nanocrystals displayed an ultrasensitive response towards NO and ultrahigh selectivity due to their physicochemical nature.

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Gas sensors have attracted great interest due to their importance and wide-ranging applications in the environmental, medical and production fields.^1,2 Generally, their gas-sensing properties are strongly correlated with the size, shape and structure of the sensing materials, which are determined by synthetic methods. So far, gas-sensing materials have been intensively studied, and great progress has been achieved.^3–15

Tin oxide (SnO₂) is of important scientific interest in gas sensors due to its excellent response towards gases.^16–20 Extensive experiments have demonstrated that the nanostructures with smaller size facilitate the response of materials towards target gases.^21–24 In addition, the as-employed synthetic methods also have important influences on the gas-sensing properties of as-synthesized materials.^21–24 Over the past years, tremendous work has been focused on the control of the size of SnO₂ nanomaterials.^21–30 However, the facile routes to synthesize the ultrasmall SnO₂ nanocrystals are still open for challenge.

Herein, we propose a facile hydrothermal route to preparing ultrasmall SnO₂ nanocrystals with the assistance of biuret. The effect of biuret on the formation of ultrasmall SnO₂ nanocrystals is also discussed. Moreover, the ultrasmall SnO₂ nanocrystals are demonstrated to be of excellent gas-sensing materials with fast response, high sensitivity and excellent selectivity towards NO.

During this experiment, we have successfully synthesized the ultrasmall SnO₂ nanocrystals by biuret-assisted hydrothermal method. The phase and structural information of the as-synthesized ultrasmall SnO₂ nanocrystals were characterized by X-ray diffraction (XRD) technique. The XRD pattern in Fig. 1 confirms that the as-synthesized SnO₂ nanocrystals can be attributed to pure tetragonal SnO₂ (PDF: 41-1445). Just as most methods of synthesizing SnO₂, the present method also gives SnO₂ with a pure phase. The XRD pattern of SnO₂ sample synthesized without any additive (Fig. S1†) is similar to that of SnO₂ nanocrystals synthesized with biuret.

Fig. 1 XRD pattern of the as-synthesized ultrasmall SnO₂ nanocrystals with the assistance of 1.03 g biuret.

In order to study the effect of biuret, a series of experiments were conducted in our work. The morphology and shape of the SnO₂ samples synthesized with different amount of biuret are shown in Fig. 2 and S2–3.† As shown in Fig. 2a–d, one can find that all SnO₂ samples are composed of the nanocrystals with ultrasmall sizes. However, one can also find that their sizes seem to be different from each other. As shown in Fig. 2a and b, one can find that the average size of the SnO₂ nanocrystals synthesized with 10 mmol of biuret is about 4.5 nm. High-resolution transmission (HRTEM) images of as-synthesized SnO₂ with biuret (Fig. 2b) and as-synthesized SnO₂ without any additive (Fig. 2d) clearly show a single crystalline structure with a lattice fringe of 0.334 nm corresponding to a crystal plane of (110). Here, it should be noted that there are also some nanoplates (Fig. S1†) for the SnO₂ sample synthesized without any additive, except ultrasmall nanocrystals (Fig. 2c). When the amount of biuret is changed to 5 mmol, the nanoplates can still be observed in Fig. S3.† However, the nanoplates can disappear from the SnO₂ sample while increasing the amount of biuret to 10 mmol, even to 20 mmol (Fig. S4†). The difference in nanocrystals size may be attributed to the growth environment with different amount of biuret, which not only provide coordinating groups for the Sn⁴⁺ ions to change crystal growth rate, but also give rise to different amount of OH⁻ due to its hydrolysis. This is also proofed by the addition of urea. When 15 mmol urea replaced the 10 mmol biuret with other conditions kept constant, the similar uniform SnO₂ nanocrystals could be obtained, as shown in Fig. S5a and b.†

Fig. 2 TEM images of the different SnO₂ samples synthesized with: (a and b) 10 mmol of biuret; (c and d) without any additive.

The porous structure of SnO₂ samples synthesized with biuret and without any additive were analyzed by N₂ adsorption–desorption isotherms, as shown in Fig. 3a and b respectively. The BET surface areas of the two samples are 128 m² g⁻¹ and 119 m² g⁻¹, as shown in Fig. 3a and b. According to our previous report,³¹ the BET surface area of commercial SnO₂ is as low as 5.9 m² g⁻¹. The pore-size distribution calculated using the Barrett–Joyner–Halenda (BJH) method was shown in the inset of Fig. 3. As revealed from the BJH pore size distribution, the average mesopore size of SnO₂ synthesized with biuret and SnO₂ synthesized without any additive is about 7.69 nm and 5.48 nm, respectively.

Fig. 3 Nitrogen gas adsorption–desorption isotherms and pore-size distribution (inset) for the sample: (a) as-synthesized SnO₂ with biuret; (b) as-synthesized SnO₂ without any additive.

It is well-accepted that the grain size is reduced to a scale comparable to the space-charge length, the sensitivity could be exponentially enhanced.³² As reported, the space-charge length for SnO₂ is 6 nm.³³ Thus, it is expected to be promising gas-sensing materials for our as-obtained SnO₂ nanocrystals due to their small size. Fig. 4 shows the response of the three SnO₂ sensors with nanocrystals synthesized under different conditions towards 20 ppm of NO gas as a function of operating temperature. For NO gas detection, the optimum working temperature is about 300 °C, under which the three sensors have the highest sensitivity towards NO gas. The results indicate that the response of the three SnO₂ sensors increase when the working temperature is lower than 300 °C, however, the corresponding response decrease quickly with the temperature further increasing. For SnO₂ nanocrystals sensor in Fig. 4a, its sensitivity towards 20 ppm of NO gas at 300 °C is about 865. From the above analysis, thus, in the following section the temperature of 300 °C was chosen for further sensing analysis. Here, the large difference between two SnO₂ samples synthesized with biuret and without any additive might be attributed to their porous size and nonunformal shape.

Fig. 4 Sensitivity of SnO₂ sensors to 20 ppm NO gas at different working temperature: (a) as-synthesized SnO₂ with biuret; (b) as-synthesized SnO₂ without any additive; (c) the commercial SnO₂.

To understand the gas sensing behavior, the dynamic responses of SnO₂ gas sensors are studied. The dynamic responses of three SnO₂ nanocrystals sensors at the working temperature of 300 °C with various concentrations of NO gas are exhibited in Fig. 5a. In Fig. 5a, one can find that the response amplitude of three SnO₂ nanocrystals sensors is increased with increasing the concentration of NO gas, respectively. In Fig. 5b, the corresponding sensitivity of SnO₂ nanocrystals sensors towards NO gas are found to be increased with the concentration of NO gas increasing. Among three SnO₂ nanocrystals sensors, the sensor with SnO₂ synthesized with biuret are superior to the sensors with SnO₂ synthesized without biuret. In Fig. 5b, the corresponding sensitivity of sensor with SnO₂ synthesized with biuret is 1.2 for 0.1 ppm, 1.8 for 1 ppm, 119.5 for 5 ppm, 864.5 for 20 ppm, 991 for 30 ppm, 1173 for 50 ppm and 1187 for 100 ppm, respectively. The excellent sensing performance of SnO₂ nanocrystals sensors might be attributed to the uniformly dispersed nanoparticles with size less than that of space-charge length as well as their unique synthetic method. Since the particle sizes of the sensing layers are different among the three devices, thus it leads to the difference in the NO gas response.

Fig. 5 (a) The real-time response curve of the ultrasmall SnO₂ sensor to NO with increased concentration at a working temperature of 300 °C; (b) the relationship between the sensitivity and the NO concentration.

The gas selectivity of sensors is an important parameters. In this work, the sensor sensitivity of SnO₂ nanocrystals sensor towards 20 ppm of several other gases is also studied. In Fig. 6 and the inset, one can find that three SnO₂ sensors have a higher sensitivity towards NO than other gases (CH₄, CO, SO₂ and H₂S). In addition, among three sensors, the sensor with SnO₂ nanocrystals synthesized with biuret shows the highest sensitivity towards NO gas. The gas-sensing behaviour of SnO₂ nanocrystals sensor towards NO gas can follow the common laws,^33–35 i.e., when exposed to oxidizing gas NO, the electric resistance of the n-type semiconductor SnO₂ sharply increase.

Fig. 6 Sensor response to various gases with 20 ppm at 300 °C: (a) as-synthesized SnO₂ with biuret; (b) as-synthesized SnO₂ without any additive; (c) the commercial SnO₂.

In summary, we have successfully synthesized the ultrasmall SnO₂ nanocrystals by biuret-assisted hydrothermal method. The mechanism of the ultrasmall SnO₂ nanocrystals have also been discussed, combining other experimental parameters. The gas-sensing results have showed that the ultrasmall SnO₂ nanocrystals have excellent gas-sensing characteristics towards NO. It is expected that the as-synthesized ultrasmall SnO₂ nanocrystals are promising to be a candidate for detecting NO in the atmospheric environment.

This work was supported by the National Natural Science Foundation of China (Grant no. 21103046, 21373081, 51302079 and 61376073).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra03121k

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