Built-in-field carbon quantum dot-nanotube heterointerfaces enable concurrent solar H2 evolution and pollutant degradation.

A scalable route is presented to upcycle biomass-derived hydrochars into functional carbon quantum dots (CQDs) and apply them as interfacial modifiers for dual-functional solar photocatalysis. Hydrochars obtained by hydrothermal carbonization of bamboo, flax shives, eucalyptus, barley straw, and maple leaves are converted into monodisperse CQDs (2.18-3.46 nm) through mild alkaline peroxide oxidation, yielding high quantum yields and strong visible emission. Integration of these CQDs onto titanium dioxide (TiO2) nanotubes (NTs) forms electronically coupled CQD/TiO2 heterostructures that simultaneously drive hydrogen (H2) evolution and degradation of multiple organic pollutants under solar irradiation. The optimized composite (CTN3) delivers an H2 evolution rate of 108.7 ± 9.8 μmolg-1h-1 together with an imidacloprid destruction rate of 18.5 ± 2.3 μmolg-1h-1, and maintains performance over five cycles (H2 and degradation retention ratios of 0.94 and 0.92, respectively). Photoelectrochemical and spectroscopic analyses indicate that CQD coupling reduces interfacial charge-transfer resistance, enhances photocurrent, and suppresses radiative recombination, supporting improved carrier separation and utilization. Density functional theory (DFT) calculations further reveal Ti 3 d-C 2p electronic hybridization, favorable adsorption energetics for key surface intermediates, and interfacial charge redistribution, suggesting the presence of an interfacial electric field that facilitates directional carrier migration. Normalized benchmarking metrics, including quantum yield, space-time yield, and figure-of-merit, position these CQD-TiO2 NT composites among competitive dual-functional photocatalysts for energy-environment coupling.