Sensitivity, selectivity, and stability of a palladium-loaded single-walled carbon nanotube methane gas sensor

Abstract

There exists a need to detect methane (CH₄) gas, and optical sensors provide many advantages over electronic sensors. Single-walled carbon nanotubes (SWNT) can be used as optical sensors, as SWNTs emit fluorescence upon photoexcitation. The photoluminescent (PL) intensity upon excitation depends on the electronic structure of the SWNT. Here, SWNTs are loaded with palladium (Pd) nanoparticles using sodium dodecyl sulfate (SDS). The Pd-loaded SWNTs offer a detection mechanism for methane (CH 4) by comparing the PL intensity before and after exposure to CH₄.A significant PL change (relative to the experiment) indicates a "turn-on" response to CH₄ Preliminary experiments showed no response to CH4 at zero and high (>10mM) Pd loading in both aqueous solution and film, indicating that there must be an optimal Pd concentration in the middle. This optimal loading for methane response was found to be in the range 2.61mM - 5.21mM Pd²+. The Pd-SWNTs in aqueous solution showed selectivity to CH₄ over nitrogen gas (N₂ and ambient air (20-22% O₂). SDS-SWNT (the control) showed a significant response to N₂ and air while Pd-SWNTs in aqueous solution showed the greatest response to methane. In film, the optimal Pd-loading (5.21mM) for sensitivity was found to exhibit an inexplicable high response to N₂ but still displayed selectivity to methane over ambient air. Pd-SWNTs in aqueous solution and film showed stability of response over time. Pd- SWNTs in aqueous solution maintained the PL intensity with a minor decrease (3- 19% decrease) in Day 14 as compared to Day 1. Pd-SWNT in aqueous solution showed turn-on response under the methane gas in Day 14. In film, the optimal Pd-loading (5.21mM) and a lower Pd-loaded SWNT (2.61mM) displayed a turn-on response to methane after 14 days, showing stability in response over time. This work has thus demonstrated an optimally Pd-loaded SWNT that is sensitive, selective, and stable over time to methane gas.