From Mo ⁴⁺ -Induced Heterojunction Construction to Mo ⁶⁺ -Boosted Charge Transfer: A Stepwise Strategy for High-Performance MoO ₂ /Cu ₂ O Visible-Light Catalysts

Efficient charge separation remains a critical challenge in semiconductor photocatalysis. This study addresses this by designing a MoO 2 /Cu 2 O heterojunction through a valence-modulation strategy, where Mo 4+ in MoO 2 acts as an in situ reductant for Cu 2 O formation. The concomitantly generated Mo 6+ species further enhance the interfacial charge dynamics. The synergistic interplay between the constructed heterojunction interface and the built-in electric field thus significantly promotes charge transfer and separation. As a result, the composite exhibits markedly improved optoelectronic properties: an electrochemical impedance as low as 1140 Ω, a photocurrent density of 0.19 μA·cm –2, and an open-circuit voltage of 1.23 mV, alongside an extended charge carrier lifetime. The photocatalyst demonstrates high performance in tetracycline degradation, achieving 96.3% removal within 60 min under visible light. The reaction mechanism, investigated via density functional theory calculations, HPLC–MS/MS, and toxicity assessment, aligns with a type-II heterojunction model. Intermediates analysis reveals effective detoxification pathways. This work provides a rational design strategy for high-performance transition metal oxide photocatalysts by harnessing valence-state chemistry.