Heterostructural Nano-Scale Conductometric Sensing Devices to Improve Selective Gas Detection
Authors: Walker Janine , Akbar Sheikh Ali , Morris Patricia 
Published: May 2020

Introduction

Semiconducting metal oxides have been investigated since the 1960’s for highly sensitive conductometric gas sensing applications [1]. These sensors operate based on modulating bulk conduction via the exchange of electrons between a gas analyte and the sensing material. Size reduction of these materials has shown to increase sensitivity by reaching complete depletion conditions due to electronic exchange between the sensing material and gas analyte. For this reason, nanotechnology has been utilized to further improve the performance of these sensors leading to sensors exhibiting high gas sensitivity [2]. Increased understanding of selectivity observed for this class of sensor technology has been difficult to achieve due to the wide range of parameters utilized in fabricating these materials including materials selection, synthesis methods, and measurement techniques.
To improve selective sensing, heterostructural sensor devices have been investigated to improve both electronic properties as well as gas-surface interactions [3]. Tin dioxide nanowires functionalized with copper oxide have been fabricated and their sensing capabilities have been tested. The response towards hydrogen and carbon monoxide have been compared between pristine nanowire sensors and the same sensors after deposition of copper oxide. This study demonstrates different response towards each gas between the unfunctionalized and functionalized sensing material. These experimental results have been compared to a database developed by this research group to find trends in sensing behavior in order to elucidate underlying sensing mechanisms.

Synthesis and Methods

SnO2 nanowires were synthesized via a vapor-liquid-solid (VLS) method. A 1 cm2 (100) p-type silicon substrate was deposited with a 1.4 nm layer of gold using a Leica ACE 600 nm sputter coater. Approximately 0.025 grams of SnO powder was measured on an alumina half cylinder. The gold coated substrate was placed 3 cm downstream of the alumina boat containing the SnO powder in a tube furnace. The tube furnace was heated to 950°C and held at this temperature for 4.5 hours. A 5% H2 gas balanced with nitrogen was flowed in the tube furnace for 90 minutes with a flowrate of 200 SCCM. This was followed by flowing 245 SCCM of 4.8 grade argon gas for 180 minutes. The obtained substrate covered with SnO2 nanowires was then added to 10 mL of DI water and sonicated for 1 minute to separate the nanowires from the substrate. A 5 µL portion of this solution was deposited onto an interdigitated Pt electrode. Tests were performed for this device then a Cu(NO3)2 precursor solution was deposited onto the sensor and repeated testing was performed. The sensor was annealed at 400°C for 2 hours before each round of gas testing.
The fabricated sensors were inserted into a custom-built probe station and heated to 300°C. Two gold coated tungsten probes were placed on each electrode contact. A 0.1 V bias was applied across the sensing material and the corresponding resistance of each sensing material was measured. Air was flowed at a rate of 1000 standard cubic centimeters per minute (SCCM) until resistance stabilized. Gas testing was performed by alternating between flowing 1000 SCCM of dry air, 1000 SCCM of 1000 ppm of either hydrogen or carbon monoxide, and 1000 SCCM of dry air. The Gas tests were performed for unfunctionalized SnO2 nanowires and SnO2 nanowires functionalized with CuO for hydrogen and carbon monoxide gas. Response was calculated by taking the ratio of sensor resistance in air to the resistance in the gas analyte.

Results and Conclusions

It was observed that the response towards hydrogen and carbon monoxide for the unfunctionalized SnO2 sensor was approximately 2.24 and 1.13, respectively. After functionalization, the response was approximately 1.68 and 4.50 for hydrogen and carbon monoxide, respectively. The decreased response towards hydrogen and increased response towards carbon monoxide showed that CuO functionalization resulted in improved selectivity for carbon monoxide over hydrogen. Furthermore, the increased baseline resistance in air of the functionalized SnO2 sensor indicated charge carrier depletion in the SnO2 nanowires due to the formation of a heterojunction between the SnO2 and CuO. A search was performed in the database for response towards CO for SnO2-CuO heterostructural sensors. Here literature compiled in the database supported the trend of improved CO response of SnO2 functionalized by CuO over pristine SnO2. This study proposed that the mechanism dictating enhanced sensing behavior was the formation of a p-n junction between the p-type CuO and n-type SnO2. However, it is expected that charge carrier depletion due to the formation of a p-n junction would result in an overall improved response towards either gas tested. Therefore, a surface chemistry effect between the gas analytes and the CuO is expected to be the cause for the observed changes in response. Studies to characterize these gas-surface interactions should be carried out in the future and compared to literature. Comparison between experimental results and literature results compiled by the database currently under development should help streamline future resistive-type sensing research.

References

[1] T. Seiyama, A. Kato, K. Fujiishi, and M. Nagatani, “A New Detector for Gaseous Components Using Semiconductive Thin Films,” Anal. Chem., vol. 34, no. 11, pp. 1502–1503, 1962.
[2] M. E. Franke, T. J. Koplin, and U. Simon, “Metal and Metal Oxide Nanoparticles in Chemiresistors: Does the Nanoscale Matter?,” Small, vol. 2, no. 1, pp. 36–50, Jan. 2006.
[3] J. Walker, S. Akbar, and P. Morris, “Synergistic Effects in Gas Sensing Semiconducting Oxide Nano-Heterostructures: a Review,” Sensors Actuators B Chem., vol. 286, pp. 624–640, May 2019.