Humins-Derived Carbon Quantum Dots for Enhanced Visible Light Photocatalytic Activity of TiO2

This work introduces a green and scalable hydrothermal route for converting humins—an abundant waste stream from terrestrial and marine biomass—into small-sized, highly homogeneous and bright blue-emitting carbon quantum dots (CQDs). The obtained CQDs were subsequently integrated with commercial Degussa titanium dioxide (P-TiO2) and NaOH-modified TiO2 (M-TiO2) to form CQD@TiO2 heterostructures. TEM, SEM, BET, and XRD analyses show that M-TiO2 evolves into a nanoparticle–nanotube hybrid architecture with a dramatically increased surface area (up to 203 m2/g), hierarchical mesoporosity, and an enhanced anatase–rutile heterojunction density. These features create an efficient scaffold for CQD anchoring and play a decisive role in improving charge separation and photocatalytic performance. The intrinsic bandgaps of P-TiO2 (3.10 eV) and M-TiO2 (2.91 eV) remained essentially unchanged upon CQD incorporation, indicating that the enhanced visible-light response arises from CQD photosensitization and interfacial charge-transfer processes rather than bandgap narrowing. The CQD@TiO2 nanocomposites exhibited markedly improved photocatalytic activity under visible light. In comparison with our previously reported humins-derived CQD/ZnO photocatalysts, the CQD@TiO2 heterostructures display significantly faster kinetics, achieving 93–98% methylene blue degradation within only 60 min. Preliminary diclofenac degradation tests further demonstrates their applicability toward persistent pharmaceutical pollutants.