Effect of Heterojunction Interface (SnO2\ZnO) on Gas Sensing Properties of Core-Shell Nanostructures
Authors: Karnati Priyanka , Akbar Sheikh Ali , Morris Patricia 
Published: May 2020

Introduction

Metal oxide-based core-shell (C-S) nanostructures have been studied for gas sensing applications to eliminate poor selectivity [1]. When designing core-shell nanostructures with n-n, p-n and n-p junctions, an equalization of the Fermi level induces the formation of accumulation and depletion layers at the interface through charge redistribution, modifying the conduction channel. The selection of the core and the shell material is important as it would affect the properties of the heterojunction interface. Parameters such as the thickness of the shell layer, type of sensing material and working temperature have been studied for C-S nanostructure-based gas sensors. However, less attention has been given to factors affecting the conduction mechanisms in C-S nanostructures.

Gas Sensing Mechanism

In C-S nanostructures optimizing the shell layer thickness to the Debye length range of the material has proven to enhance the sensor performance [2]. Mechanisms such as potential barrier carrier transport and surface depletion layer formation play important roles in enhancing the gas sensing performance of C-S nanostructures [3]. Understanding the gas sensing mechanism is not straightforward due to the presence of the heterojunction interface. Therefore, there is a need to understand the sensing mechanism in C-S nanostructures to determine the role of heterojunction. One way to determine the role of the heterojunction is by exchanging the core and shell layers in C-S nanostructures and analyzing the gas sensing properties.

Method

Metal oxides such as Tin dioxide (SnO2) and Zinc oxide (ZnO) are chosen to synthesize C-S nanostructures. SnO2 nanowires were synthesized by using the VLS (Vapor-liquid- Solid) process with Au as the catalyst. The ZnO layer was deposited by using a sputtering technique. The thickness of the ZnO shell layer can be varied by changing the deposition time. To study the effect of exchanging the core and shell layer, ZnO nanorods were synthesized on a Si substrate by using the hydrothermal process. The Sn layer was deposited on ZnO nanorods by using a sputtering technique. Then the samples were annealed in air to form the SnO2 layer. X-ray diffraction (XRD), Scanning electron microscopy (SEM) and Transmission electron microscopy (TEM) characterizations are used to confirm the phase, morphology, and dimensions of the synthesized C-S nanostructures. Kelvin probe force microscopy is used to calculate the work function of C-S nanostructures to determine the formation of the heterojunction. Gas sensors are fabricated by pipetting the C-S nanostructures solution on to the interdigitated Pt electrodes deposited on Si substrate. Response curves are measured in a custom built probe station towards 1000 ppm of H2 gas. The effect of shell layer thickness is analyzed towards maximum gas sensing response. Impedance spectroscopy (AC) measurements are performed to determine the contribution from different parts of C-S nanostructures towards the gas sensing response. This study that is ongoing will help in the understanding of the gas sensing mechanism and the role of heterojunction in C-S nanostructures.

Results and Conclusions

ZnO was sputtered on SnO2 nanowires grown through the VLS process for 30 seconds. The sputtering resulted in synthesizing ZnO decorated SnO2 nanowires which can be confirmed from the SEM image in Figure 1a. To study the effect of the heterojunction on gas sensing, SnO2 was grown on ZnO nanorods by sputtering Sn for 30 seconds and annealing in air at 400°C which is confirmed in Figure 1b. Figure 1c shows the response curve towards 1000 ppm hydrogen gas at 300°C for ZnO decorated SnO2 nanowires. The inset figure in Figure 1c shows the response of pristine SnO2 nanowires towards hydrogen gas. Response (Ra/Rg) is calculated as the ratio between resistance in air (Ra) and resistance in hydrogen gas (Rg). The response of ZnO decorated SnO2 nanowires (11) is enhanced when compared to the pristine SnO2 nanowires (7.39). The enhanced response proves the formation of a heterojunction between sputtered ZnO and SnO2 nanowires. Figure 1d shows the impedance curves for ZnO decorated SnO2 nanowires in nitrogen gas at different temperatures. It is evident from the plots that as the temperature is increased the resistance of the decorated nanostructures is decreasing which is the expected behavior for semiconductors. The ability to measure impedance curves also proves the stability of the hetero-nanostructures. Future work involves measuring response and impedance curves for SnO2 decorated ZnO nanorods towards 1000 ppm hydrogen gas. The effect on heterojunction (SnO2/ZnO) will be analyzed by fitting the impedance curves.

References

[1] P. Karnati, S.A. Akbar, P.A. Morris, Conduction mechanisms in one dimensional core-shell nanostructures for gas sensing: A review, Sensors Actuators, B Chem. 295 (2019) 127–143. doi:10.1016/j.snb.2019.05.049.
[2] S.W. Choi, A. Katoch, G.J. Sun, J.H. Kim, S.H. Kim, S.S. Kim, Dual functional sensing mechanism in SnO2-ZnO core-shell nanowires, ACS Appl. Mater. Interfaces. 6 (2014) 8281–8287. doi:10.1021/am501107c.
[3] D.R. Miller, S.A. Akbar, P.A. Morris, Nanoscale metal oxide-based heterojunctions for gas sensing: A review, Sensors Actuators, B Chem. 204 (2014) 250–272. doi:10.1016/j.snb.2014.07.074.

Figure 1