Negatively-Doped Single-Walled Carbon Nanotubes Decorated with Carbon Dots for Highly Selective NO₂ Detection

In this study, we demonstrated a highly selective chemiresistive-type NO₂ gas sensor using facilely prepared carbon dot (CD)-decorated single-walled carbon nanotubes (SWCNTs). The CD-decorated SWCNT suspension was characterized using transmission electron microscopy (TEM), X-ray diffraction (XRD), and UV-visible spectroscopy, and then spread onto an SiO₂/Si substrate by a simple and cost-effective spray-printing method. Interestingly, the resistance of our sensor increased upon exposure to NO₂ gas, which was contrary to findings previously reported for SWCNT-based NO₂ gas sensors. This is because SWCNTs are strongly doped by the electron-rich CDs to change the polarity from p-type to n-type. In addition, the CDs to SWCNTs ratio in the active suspension was critical in determining the response values of gas sensors; here, the 2:1 device showed the highest value of 42.0% in a sensing test using 4.5 ppm NO₂ gas. Furthermore, the sensor selectively responded to NO₂ gas (response ~15%), and to other gases very faintly (NO, response ~1%) or not at all (CO, C₆H₆, and C₇H₈). We propose a reasonable mechanism of the CD-decorated SWCNT-based sensor for NO₂ sensing, and expect that our results can be combined with those of other researches to improve various device performances, as well as for NO₂ sensor applications.