Chemical sensors based on nanostructured metal oxide semiconductors are widely used in detecting and monitoring toxic and explosive gases. Due to the rapid increase in popularity and demand for gas sensors, there is a high need to improve their performance efficiency. In a recently published literature, titanium oxide has been identified as a good material for enhancing the sensor performance owing to its high sensitivity, thermal stability, nontoxicity, and self-cleaning properties. In particular, titanium oxide can be presented in the form of nanotubes which are specifically suitable for use in sensors.
Presently, several methods for the preparation of titanium oxide nanotubes including the sol-gel and template-assisted methods are available. Consequently, the use of perovskite materials in either filling, modifying or decorating titanium oxide nanotubes has been explored in various applications such as solar cells and photocatalysis processes.
In a recent paper published in Journal of Alloys and Compounds, researchers from the National Kaohsiung University of Science and Technology: Dr. Kuo-Chin Hsu, Professor Te-Hua Fang and Miss Pei-Che Wu in collaboration with Professor Yu-Jen Hsiao at Southern Taiwan University of Science and Technology fabricated and explored the structural surfaces and properties of carbon-dioxide thin-film sensors with heterojunction structure. Their objective was to significantly improve the design and utilization of titanium dioxide nanotubes in different practical applications.
Briefly, the research team started their experimental studies by preparing titanium dioxide nanotubes using the electrochemical method. On the other hand, the LSCNO perovskite film was prepared using the sol-gel process. The two were combined to form a PN heterojunction structure. This structure was eventually tested in a carbon monoxide sensor. Furthermore, the structural surface properties of the nanotubes were tested at different annealing conditions.
For different annealing atmospheres on the nanotubes, no significant change in the length or size of the pipe was observed. However, significant variance in the contact angles was noted. Nitrogen produced the maximum and hydrophobic surface contact angle while argon produced the minimum and hydrophilic contact area. The sensing characteristics of the heterojunction films operating at different temperatures were investigated. The film exhibited good recovery, reproducibility and stability thus making it suitable for use as a gas sensor. A small change in the resistance value and response for different carbon monoxide concentration was noted.
When the PN heterojunction film was tested, a moderate response (38.41%) was recorded at an operation temperature of 200°C and carbon monoxide concentration of 400 ppm. Therefore, the authors concluded that the PN junction makes a good carbon dioxide sensor at 200°C. Even though a single film exhibited the ability to sense carbon dioxide gas, it was outperformed by the developed heterojunction film which was more sensitive.
In summary, the research team successfully demonstrated the efficiency and applicability of the heterogeneous structure gas sensors that currently form the focus of future research. Based on the properties of the heterojunction structures, the study provides essential information that will pave way for developed of various sensors based on heterojunction structures.
Hsu, K., Fang, T., Hsiao, Y., & Wu, P. (2019). Response and characteristics of TiO2/perovskite heterojunctions for CO gas sensors. Journal of Alloys and Compounds, 794, 576-584.