Pulsed laser deposition of CoFe2O4/CoO hierarchical-type nanostructured heterojuction forming a Z-scheme for efficient spatial separation of photoinduced electron-hole pairs and highly active surface area

The ablation of a target, composed of a homogeneous mixture of Fe & Co metal with H3BO3 powder, produces core-shell type particles on the as-deposited coating surface. Annealing at 600 °C in air transforms the core-shell into urchin-like particles having nanowires of 10–30 nm in diameter and 0.5–1 μm in length, grown radially from the particle surface as a result of the stress created between core and shell. Microstructural and analytical characterization confirms that nanowires are composed of CoFe2O4 and CoO phases forming a heterojunction, which showed outstanding performance as a photocatalyst, requiring half the time for an organic water pollutant degradation reaction in comparison to similarly nanostructured single-metal oxide counterparts (Co3O4 and Fe2O3). Degradation reactions conducted in controlled conditions demonstrate that the urchin-like mixed oxide coatings are highly efficient both in direct photocatalysis and in photo-Fenton reaction. This overall enhancement in activity is due to a synergistic effect created at the heterojunction of CoFe2O4/CoO that allows the formation of a Z-scheme mechanism to facilitate the spatial separation of photoinduced electron-hole pairs. Moreover, the hierarchical urchin-like nanostructures provide a large number of active sites for photocatalytic reactions, while the lower band gap of CoFe2O4 permits better visible light absorption to effectively degrade organic pollutants even under visible light. Based on all the results, a plausible reaction mechanism is proposed related to Z-scheme heterojunction formed with CoFe2O4/CoO.