Ascorbic acid-mediated solid-state synthesis of cavity-engineered CuO-Cu₂O-Cu heterostructures: structural...

The discharge of toxic Cr(vi) into water presents environmental health hazards, requiring efficient, sustainable photocatalysts for its reduction to Cr(iii). In this work, a simple, environmentally friendly solvent-free solid-state route was developed to synthesize CuO-Cu₂O-Cu heterostructures, where the novel use of solid ascorbic acid (AA) as a green reductant enables both the solid-state formation of Cu species and controlled reduction of CuO to Cu₂O and Cu by adjusting the AA amount. Structural analyses confirmed the coexistence of Cu, Cu₂O, and CuO phases, while FESEM and HRTEM revealed cavity-like core-shell architectures with dense heterointerfaces facilitating charge transfer. The optimized CuO-A1.1 showed a narrower band gap (1.42 eV vs. 1.67 eV for CuO) and a higher Cu⁺/Cu²⁺ ratio with oxygen vacancies, boosting its photocatalytic performance. The photocatalytic detoxification of Cr(vi) under visible light showed that all heterostructures were active. The CuO-Cu₂O-Cu sample prepared by reducing 1 g of CuO with 1.1 g of ascorbic acid (CuO-A1.1) was the most efficient. It achieved complete Cr(vi) reduction within 30 min. Its pseudo-first-order rate constant was 0.1036 min^-1, which is nearly 50 times higher than that of pristine CuO (0.0021 min^-1). OFAT screening identified pH, Cr(vi) concentration, and catalyst dosage as key factors. One-way ANOVA with Tukey's test showed that pH (F = 762.06, p < 0.0001, R ² = 0.9965) and concentration (F = 92.46, p < 0.001, R ² = 0.97) significantly affected reduction efficiency, while catalyst dosage showed no significant effect (p > 0.05). Levene's test confirmed variance homogeneity, validating the results. Radical scavenging confirmed electrons and ˙O₂ ^- as key reactive species. Post-reaction XPS analyses and EPR measurements verified the proposed mechanism in which interfacial charge transfer among Cu, Cu₂O, and CuO promotes electron migration and suppresses recombination. These results establish CuO-Cu₂O-Cu heterostructure photocatalysts for Cr(vi) reduction and provide mechanistic insights into phase-function relationships in copper oxide.