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 (R_g/R_a = 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.