An <i>operando</i> photoconductivity platform reveals defect–adsorbate–carrier coupling in ultrathin MoS ₂ for solar-driven CO ₂ reduction

Understanding the interfacial charge dynamics between a catalyst and adsorbate is vital for advancing solar-driven CO₂ reduction. However, the mechanistic coupling between defects, surface adsorbates, and photogenerated carriers remains poorly defined under <i>operando</i> conditions, limiting rational catalyst design. Here, we introduce a high-sensitivity <i>in situ</i> photoconductivity (PC) platform to probe carrier dynamics in a 3 nm-thick MoS₂ film under simulated gas-phase CO₂ photoreduction conditions (AM1.5G illumination, CO₂/H₂O atmosphere). Prominent persistent PC (PPC) was observed under vacuum/UV irradiation, attributed to approximately 5% sulfur vacancies acting as deep-level traps (bandgap: 1.76 eV; electron capture barrier: 0.47 eV). Upon exposure to humid CO₂, the photocurrent dropped by 82%, with significantly shortened rise and decay time constants, indicating accelerated recombination <i>via</i> enhanced defect-adsorbate interactions. To explain these findings, we propose a conceptual defect-adsorbate energy alignment model describing three distinct electronic coupling scenarios, including (i) energetic alignment of defect-adsorbate states, (ii) modulation of adsorbate LUMO levels to higher energies, and (iii) lowering of adsorbate LUMO levels under specific coupling conditions. This model represents the first mechanistic framework linking adsorption, defect energetics, and charge retention in defective ultrathin 2D photocatalysts. It establishes a foundation for rational catalyst design, with future theoretical and spectroscopic validation expected to further strengthen this framework. Our <i>operando</i> PC platform offers mechanistic insights into defect-adsorbate-carrier coupling and provides a promising diagnostic tool for guiding the development of high-efficiency ultrathin 2D CO₂ catalysts.