Co-ZnO/Co₃O₄ heterostructures for superior dye degradation: mid-gap energy level and intent contact

Improvement in materials' optical and charge transfer properties through simultaneous doping and interfacial contact has received significant attention in the catalysis field. In this study, cobalt-doped ZnO/Co₃O₄ (CZnO) heterostructures have been synthesized. The XRD pattern peak shift and appearance of the Co₃O₄ new peak on the XRD analysis confirm the cobalt ion insertion and development of an interfacial contact. The obtained average crystallite sizes for ZnO and CZnO are 26.3 and 10.7 nm, respectively. The interfacial contact between ZnO (0.273 nm) and Co₃O₄ (0.241 nm) crystals was also ratified from the HRTEM image lattice fringes analysis. The obtained particle size for CZnO is in the range of 27–50 nm, confirming the synthesized material is in the nanoscale size range. The well-scattered cobalt dispersal on the ZnO surface and anticipated elemental composition without impurities were confirmed from the EDS-mapping analysis. The attained indirect bandgap for ZnO and CZnO is 3.05 and 1.71 eV, respectively. The occurrence of a charge transfer process through the ZnO and Co₃O₄ interfacial contact was understood from the total PL intensity reduction for CZnO, compared to the ZnO. The 4.5 times greater methylene blue dye degradation enhancement for the CZnO ( k = 0.141 min −1 ) than ZnO ( k = 0.0312 min −1 ) is due to the improved charge transfer and optical properties. Thus, this porous CZnO heterostructure synthesized using a simple combustion approach has a novel future outlook for scale-up application. • ZnO has decent band edge potential, except for some drawbacks. • Here, ZnO's drawbacks were reduced through simultaneous doping and heterojunction strategies. • Cobalt doping creates a new energy level (sp-d exchange interaction) and changes ZnO's optical property. • A proper p-n heterojunction between ZnO and Co₃O₄ extended electron-hole relaxation time for charge transfer. • Thus, the Co-ZnO/Co₃O₄ showed enhanced dye degradation ability with a rate constant value of 0.141 min −1 .