Understanding the potential band position and e–/h⁺ separation lifetime for Z-scheme and type-II heterojunction mechanisms for effective micropollutant mineralization: Comparative experimental and DFT studies

A new approach to determine the importance of band potential by comparing two different electron charge transfer mechanism, via Z-scheme and type-II heterojunction. Through microwave hydrothermal (MWH) treatment and subsequent thermal polycondensation, the released ammonia gas from the formation of oxidized GCN simultaneously reducing the surface of TiO₂ (designated as mwh-CNTO), hence creating a sub-gap state between the interface of these two catalysts. Compared to pristine photocatalysts, mwh-CNTO-0.1 (0.1 g TiO₂ with 6 g melamine) has shown superior photocatalytic activities (between 6 to 34-folds) under monochromatic LED (400 nm) and natural sunlight. Since TiO₂ in the composite cannot be activated under LED, the bands alignment from type-II heterojunction decreases the overall band potential, resulting in mainly ·O₂⁻ (anionic) generated. Consequently, non-charged BPA was effectively degraded with a kinetic rate constant of 0.0310 min^–1, while negatively charged ATZ had much lower rate constant (0.0043 min^–1) due to their repulsive properties. In contrast, natural sunlight (full spectrum) could not only activate both TiO₂ and GCN of mwh-CNTO-0.1, but also induce Z-scheme mechanism via driving the photogenerated electrons (TiO₂) through the created sub-gap state and ultimately recombining at valence band (VB) of GCN. As proven by detection of DMPO-·OH, scavenging tests and DFT modeling, this scheme effectively degraded both BPA (0.0379 min^–1) and ATZ (0.0474 min^–1) owing to the VB position of TiO₂ being maintained to generate non-selective ·OH. Overall, in comparison to other studies, the proposed Z-scheme on mwh-CNTO-0.1 had much higher energy efficiencies for BPA (8.2 × 10^–3 min^–1 W^–1) and ATZ removal (1.0 × 10^–2 min^–1 W^–1) under natural sunlight.