Decoupling Thermodynamic and Kinetic Processes in Sub-ppm H2S Sensing at Room Temperature using Sulfur Vacancy-Rich N-Doped Bi2S3-x Nanotubes as a Model System.

Understanding the sensing mechanisms of reducing gases remains a significant challenge, particularly at low concentrations and room temperature. Herein, sulfur vacancy-rich N-doped Bi2S3-x (VsN-Bi2S3-x) nanotubes are employed as a model system to decouple the thermodynamic and kinetic processes in H2S sensing. It is demonstrated that activated oxygen species (O2-) play an important role in governing both processes. From a thermodynamic perspective, density functional theory calculations reveal that nitrogen doping and sulfur vacancies synergistically promote the adsorption of O2 and H2S, where the higher O2- and H2S surface concentrations jointly contribute to an enhanced sensing response. This mechanism fundamentally differs from the direct charge transfer behavior of the well-documented oxidizing gases NO2. From a kinetic standpoint, an apparent reaction-rate model incorporating surface reaction and diffusion is established, showing that the O2- concentration also controls the electron generation rate and accelerates the response kinetics under sub-ppm conditions, where conventional adsorption-based models fail to apply. Benefiting from these effects, the optimized sensor exhibits a response of 24.3 and a response time of 75 s toward 1 ppm H2S at room temperature, outperforming pristine Bi2S3 by factors of 3.9 and 4.4, respectively. To validate these mechanistic insights, a wireless sensing platform is further developed as a proof of concept for live-tracking meat freshness. Overall, this work establishes a unified thermodynamic-kinetic framework based on the O2- surface concentration, providing new insights into reducing gas sensing beyond empirical material optimization.