2D-3D metal oxide heterojunction nanostructures for catalytic applications

A simple template-free method was used to fabricate unique undoped, Cr-doped, and S-doped mesoporous 2D-3D ceria (CeO_2-x) nanostructures by using triethanolamine (TEA) to exfoliate electrodeposited cerium coordination polymer (Ce-CP), followed by heat treatment at 450 °C. For undoped CeO_2-x fabrication, varying the amount of TEA impacted the exfoliation extent and the resultant microstructural and defect characteristics, with the optimal Ce-CP to TEA ratio determined to be 30 mg:0.35 mL. For doped CeO_2-x, the introduction of dopants (substitutionally and/or interstitially) and heterojunction formation resulted in the creation of defects from charge compensation (ionic, electronic, and redox) which influenced charge carrier concentrations for reactions. The precipitation of excess dopants on the surface to create heterojunctions allowed for reduced crystallite sizes, lowering of the optical band gaps, and increased charge carrier separation, which in turn minimised charge carrier diffusion distance, ultimately increasing photocatalytic efficiency. Dye degradation tests showed that 10 mol% Cr-doped CeO_2-x revealed superior photocatalytic performance with a reaction rate of 15.88 × 10⁻³ min⁻¹ (87% degradation after 160 min) compared to both 2.5 mol% S-doped CeO_2-x and undoped CeO_2-x (68% and 57% after 160 min, respectively). With sulphur addition, although the defect concentration increased and the crystallite sizes and band gaps were lowered, the degradation performance declined with increasing dopant amount owing to the reduced porosity from liquid-phase microstructural densification. Thus, the positive changes to the electronic and microstructural features from dopant-related heterojunction formations were observed to be the most critical factors impacting the photocatalytic performance of these unique nanostructures.