Bandgap-Engineered Graphene Quantum Dot Photosensitizers for Tunable Light Spectrum-Activated NO₂ Sensors

Visible-light activation is highly desirable for gas sensors due to its energy-efficient operation and broad accessibility. Photocatalysis offers a promising strategy for visible-light activation; however, a limited understanding of the band engineering-mediated activation process restricts the rational design of photocatalysts for gas sensors. In this work, we systematically investigate the impact of band tuning in photocatalysts on the nitrogen dioxide (NO2) sensing performance of In2O3-based sensors, employing graphene quantum dots (GQDs) as photosensitizers. By controlling the sp2 carbon core size in GQDs, the bandgaps are tuned from 3.3 to 1.9 eV, enabling precise band engineering. It modulates the carrier transfer dynamics between GQDs and In2O3 layers, while surface functional groups of GQDs facilitate gas adsorption through their catalytic effects. By integrating sensitization effects, 7 nm GQDs optimize the photocarrier efficiency under visible light (blue light), leading to enhanced NO2 sensing performance in the GQD-decorated In2O3 system (Rg/Ra = 97.1 toward 1 ppm) with a fast response/recovery time (T90/T10 = 136/100 s). The bandgap tuning of GQDs highlights the critical role of band engineering in light-assisted gas sensing, enabling the photocatalyst-based sensor system construction for visible-light activation.