Semiconducting Ratio Modulation of Single-Walled Carbon Nanotubes for Efficient NO₂ Detection at Low-Temperature

This study presents a nitrogen dioxide (NO₂) gas sensor based on single-walled carbon nanotubes (SWCNTs) with varying proportions of semiconducting SWCNTs (s-SWCNTs). The SWCNTs were synthesised using a floating catalyst chemical vapor deposition (FCCVD) method. The purity of the s-SWCNTs was controlled by adjusting the carbon source, reactor temperature, and gas flow rates. The FCCVD-synthesised CNT thin films can be transferred via a simple one-step process without the need for solvents, a significant advantage over conventional CNTs. As reported in our previous publication [S. Kim,<i> et al</i>., Adv. Mater., (2024) 2313830.], sensors utilising FCCVD CNTs demonstrate superior performance in sensitivity, response time, and detection limit among CNT-based NO₂ sensors. Additionally, the optimal operating temperature for these sensors was found to be 150 °C, significantly lower than that of metal oxide-based NO₂ sensors, which typically operate around 400 °C. In this work, we further improved the sensing performance and reduced the optimal operating temperature by controlling the ratio of s-SWCNTs. A higher proportion of semiconducting content increased the binding energy between the gas molecules and the CNT surface, significantly enhancing the sensor performance, even at lower temperatures of 120 °C. A sensor with 94% s-SWCNT exhibited an exceptionally high response (83.2% at 500 ppb) and a fast response time (~8.6 s) compared to previously reported studies [M. Jeon, et al., Appl. Phys. Lett., (2017), 022102.; L. Sacco, et al., Carbon, (2020), 631.]. Conversely, a sample with 87% s-SWCNT demonstrated an extremely rapid recovery (72.8 s). Additionally, the proposed sensor showed superior selectivity compared to other gases and maintained stable properties for up to 6 months. These experimental and theoretical findings suggest that efficient control of s-SWCNT content can pave the way for innovative approaches in various sensor applications in the near future.