Dense and uniform growth of TiO2 nanoparticles on the pomelo-peel-derived biochar surface for efficient photocatalytic antibiotic degradation

To clarify the impact of TiO2 growth and deposition on an activated biochar substrate and disclose the deciding mechanistic aspect in photocatalysis, we report TiO2 nanoparticle growth and deposition on the pomelo-peel-derived activated biochar surface by a combined process of liquid-phase deposition in the presence of KOH-activated biochar and then pyrolysis in the air. The TiO2 growth process is investigated by varying the liquid-phase deposition duration. Initially, TiO2 grows over the surface of activated biochar into small microcrystallites, leaving much bare surface of the activated biochar. Prolonging the growth time allows the growth to be saturated and gives rise to dense and uniform TiO2 nanoparticles (with an average size of about 28 nm) over the activated biochar surface, with strengthened interfacial contact. The overgrowth is not favorable for the uniform dispersion of TiO2 nanoparticles while forming large-size aggregates with weakened interfacial contact between TiO2 and activated biochar. Meanwhile, the graphitic domains of the underlying activated biochar substrate are significantly restored, with the Raman D-to-G band intensity estimated to be 0.05, contrasting to 0.95 for the activated biochar. Some surface functional groups of the activated biochar participate in anchoring TiO2 nanoparticles, especially C-OH. The optimal activated biochar-TiO2 composite is stable and exhibits efficient and robust photocatalytic degradation of tetracycline antibiotics, with the photocatalytic reaction rate constant estimated to be 0.021 min−1. More importantly, the determining factor that affects the photocatalytic performance is disclosed to be the photogenerated charge carrier separation capability.