Phase-selective fabrication of Cu-based films by magnetic field-assisted sparking process for improved photocatalytic dye degradation and CO2 reduction

Copper-based films (Cu₂(OH)₃NO₃, Cu₂O, and CuO) were successfully synthesized using a sparking process under an external magnetic field, without requiring an annealing process. We thoroughly investigate the impact of magnetic field orientation on the structural, morphological, and optical properties of the films, along with their performance in photocatalytic degradation of methylene blue (MB) and carbon dioxide (CO₂) reduction. Field-emission scanning electron microscopy (FE-SEM) images reveal that the CuEB(S) sample displays secondary particles forming spherical, dense clusters, whereas the CuEB(N) sample exhibits larger, loosely packed clusters. In contrast, the cluster-like structures disperse in the CuEB(P) sample, resulting in a smoother film surface aligned with the parallel magnetic flux direction. Upon close observation, rice-shaped particles of varying sizes (26.3–60.3 nm) were found stacked together. This study presents a facile method to selectively fabricate films comprising single-phase Cu₂(OH)₃NO₃, a CuO/Cu₂O composite, or a combination of all phases by manipulating the external magnetic field orientation during the sparking process. All synthesized materials exhibit photocatalytic activity, with the heterojunction composed of CuO, Cu₂O, and Cu₂(OH)₃NO₃ demonstrating superior performance. This heterostructure achieved up to a 180 % enhancement in MB degradation and approximately doubled the CO₂-to-CO conversion rate to 16.09 μmol g^–¹ h^–¹ compared to the sample prepared without a magnetic field.