Nanocube-size-dependent Fermi level positioning in Cu2O/TiO2 S-scheme heterojunctions for optimized VOC photodegradation

Precise tuning of the Cu 2 O Fermi-level position is achieved by tailoring its crystal size (30, 55, and 315 nm). The Fermi-level position of Cu 2 O and the resulting interfacial electronic structure of Cu 2 O/TiO 2 S-scheme heterojunctions (CT-x: x= edge length in nm) have been studied as a function of Cu 2 O nanocube size. With a fixed 1 wt% loading, the heterojunction constructed from the smallest nanocubes (CT-30) achieves superior BTX oxidation efficiencies: 84.3% for benzene, 98.2% for toluene, and 94.1% for xylene, significantly outperforming pure TiO 2 , Cu 2 O, and the large-cube composite (CT-315). For toluene, this performance corresponds to an apparent quantum yield of 3.2% and a reaction rate of 6.04 µmol g − 1 h − 1 . Detailed electronic characterization shows that smaller Cu 2 O nanocubes have a more negative conduction band, enlarging the Fermi-level offset (ΔE F ) with TiO 2 . This strengthens the interfacial electric field, forming an efficient S-scheme photocatalyst that separates and preserves high-energy carriers. The retained Cu 2 O conduction band electrons (-0.79 V vs. NHE) drive •O 2 − generation and subsequent reactive •OH formation, enabling efficient BTX mineralization to CO 2 and H 2 O. This work establishes size-mediated Fermi level engineering as a key design principle for high-efficiency S-scheme photocatalysts.