Synthesis of ZrO2-Shelled SnO2 Nanowires for NO2 Gas Sensing
Authors: Bang Jae Hoon , Han Seungmin , Lee Ha Young , Shin Ka Yoon , Kim Hyoun Woo 
Published: July 2021

Introduction

NO2 gas, with its highly toxic nature, is mainly produced from automobile exhaust or the burning of fossil fuels [1]. This gas could lead to damage in the respiratory system when inhaled [2]. Moreover, when exposed to high concentration, could lead to cancer or related disease.
One unique type of nanowires is core-shell (C-S) nanowires, where nanowires are covered by a thin shell. In this study, SnO2 nanowires covered by ZrO2 shell has been explored.
The oxygen ions in ZrO2 are actively transported after it reaches 600°C limiting the application field. SnO2 has often been applied for gas sensing due to its physiochemical stability and high mobility of charge carriers. SnO2-ZrO2 C-S NWs are expected to show a prominent result in respect of sensor temperature and sensor response. This study has been published on Sensors and Actuators B: Chemical 319 (2020) 128309.

Method

SnO2 NWs were fabricated using the vapor-liquid-solid (VLS) method. Metallic Sn powders in alumina tube were placed into a furnace and were thermally evaporated at 900°C for 15 min. Evaporated Sn reacts with oxygen from the air, finally, SnO2 NWs are synthesized.
ZrO2 was coated on SnO2 NWs by a thermal ALD system (Lucida D100, NCD Technology, Korea). The precursor and reactant for the ALD process were tris(dimethylamino) cyclopentadienyl zirconium (Cp-Zr) and ozone (O3), respectively. Cp-Zr was heated to 90°C in a canister, and Ar gas was used as a bubbler. The pressure and temperature of the ALD process were kept at 0.5 Torr and 300°C, respectively [3,4].
A bi-layer electrode (200-nm thick Au and 50-nm thick Ti) was sputter-deposited on the samples for making of the gas sensors. The sensors were placed inside a gas chamber in a tubular furnace. The gas concentrations were controlled by mass flow controllers with air as a background gas. The total flow rate was fixed to 500 sccm during all measurements.

Result and Conclusions

Fig. 1(A,B), (C,D), (E,F) and (G,H) show TEM and high resolution TEM (HR-TEM) images of SnO2-ZrO2 C-S NWs with 50, 100, 150, 200 ALD cycles respectively. Owing to the very thin nature of the shell, the lattice fringes from the underlying SnO2 core were able to be observed.
Fig. 2A and B show EDAX data of SnO2-ZrO2 C-S NWs after 50 cycles for core and shell parts, respectively. For evaluation of the presence of Zr in the core and shell regions, the ratio of Zr/(Zr+Sn) was used. The ratio of Zr/(Zr+Sn) in the core region was 0.08 and that of the shell region was 0.98. This demonstrates the successful deposition of Zr in the shell region. The same trend can be seen for other C-S NWs. Therefore, it can be concluded that ZrO2 is mainly concentrated in the outer regions, whereas the SnO2 is concentrated in the core region, regardless of ALD cycle time. The value of Zr/(Zr+Sn) in the core region becomes higher at higher ALD cycles, as the ZrO2 shell gets thicker, whereas the SnO2 core is constant.
Fig. 3 depicts the response to 10 ppm NO2 gas versus temperature for pristine and C-S gas sensors. At low temperatures, the response is low and after reaching a peak at a particular temperature, the response decreases. The highest response for pristine SnO2 gas sensor occurred at 250°C, which was 20.9. For C-S NW gas sensors with a ZrO2 shell deposited at 50, 100, 150, and 200 cycles, the maximum responses were 14.9, 16.2, 24.7, and 14.2, respectively. The sensor with ZrO2 shell being deposited with 150 ALD cycles exhibited the highest gas response. All C-S gas sensors except that deposited with 100 ALD cycles showed their optimal sensing temperature at 150°C. However, for the sensor with ZrO2 shell deposited with 100 ALD cycles, the optimal sensing temperature was 100°C.
The selectivity of SnO2-ZrO2 (150 cycles) C-S NW to 10 ppm of various gases at 150°C is presented in Fig. 4. The response to NO2 (24.7) was much higher than that of the other gases.
To conclude, the optimal sensing temperature was varied from 100°C for the sensor with a ZrO2 shell deposited with 100 cycles to 250°C for pristine SnO2 gas sensor. Also, NO2 gas sensing results demonstrated a dependence of response to the thickness of the ZrO2 shell. The optimal sensor with a ZrO2 shell thickness of 24.1 nm (150 ALD cycles) revealed a high response to NO2 gas and also a good selectivity to NO2 gas.

References

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Figure 1