Demonstrating the Critical Role of Band-Edge Potential Alignment in Photocatalytic Cu(II) Reduction by ZnO and TiO2

Abstract

Photocatalytic reduction of Cu(II) offers a sustainable approach for recovery of metallic copper from aqueous solution under mild reaction conditions. However, the factors governing the thermodynamics feasibility of Cu(II) reduction remain insufficiently understood, and photocatalytic performance is frequently interpreted primarily in terms of semiconductor bandgap energy. Although zinc oxide (ZnO) and titanium dioxide (TiO₂) possess similar optical bandgaps (∼3.2 eV), they exhibit markedly different Cu(II) reduction behaviors. This discrepancy arises not from their optical bandgap, but from differences in their band-edge potentials, which control the thermodynamic feasibility of electron transfer between the photocatalyst and the media. For the Cu²⁺/Cu⁰ couple (+0.34 V vs. normal hydrogen electrode (NHE)), a photocatalyst must exhibit a conduction band minimum (CBM) more negative than the redox potential to achieve reduction. This manuscript theoretically and experimentally demonstrates that ZnO, with a CBM (−4.84 eV), effectively reduces Cu(II) to Cu(0), whereas TiO₂ (−4.48 eV), despite its comparable bandgap, showed predominantly adsorption with only limited evidence of Cu reduction under the investigated conditions. The results confirm that band-edge alignment, rather than bandgap energy alone, is the key predictive parameter for selecting suitable photocatalysts for redox reactions.