Ligand-capped pristine and doped ZnO2 nanoparticles for enhanced photocatalytic methylene blue degradation: A DFT-supported study

In the present work, ligand-assisted zinc peroxide (ZnO₂) nanoparticles (NPs) and doped ZnO₂ NPs (manganese (Mn) and cobalt (Co)) were synthesized via the co-precipitation method and characterized through complementary techniques. Organic ligands, including citrate (cit), 1,5-diphenyl-1,3,5-pentanetrione (pent), and dimethyl-L-tartrate (dmlt), were used to stabilize the NPs and tune their bandgap through Mn and Co doping, thereby enhancing the photocatalytic degradation of methylene blue (MB) dye. The XRD data refinement revealed cuboctahedral-shape crystallites/NPs with diameters ranging from 3 to 12.7 nm. Tauc's relation supported an optical bandgap of 2.96 eV upon 3 % Mn incorporation, which is lower than 3.07 eV for pristine ZnO₂ (cit-capped) NPs. In 150 min, cit-capped pristine ZnO₂ NPs degraded 63.34 % of MB dye; this rate increased to 77.94 %, 93 % upon 3 %, 5 % Mn-doping, and 85.13 %, 81.24 % upon 3 %, 5 % Co doping, respectively. A similar trend was observed for dmlt-capped NPs; pristine ZnO₂ NPs degraded 63.72 % of dye and reached to 72.26 %, 74.25 % upon 3 %, 5 % Mn-doping, and 78.56 %, 79.71 % upon 3 %, 5 % Co-doping in 150 min, respectively. Notably, pent-capped ZnO₂ NPs degraded 98.89 % in 90 min. However, the performance was adversely affected by Mn and Co-doping (83.20 %, 97.35 % for 3 %, 5 % Mn-doped and 78.65, 73.68 % for 3 %, 5 % Co-doped). DFT calculations suggest that Mn and Co-incorporation into ZnO₂ lattice improved thermodynamic and mechanical stability as well as chemical reactivity. The kinetics analysis showed that all reactions follow the pseudo-first-order kinetic model with R-squared values ranging from 84.0 to 98.5. This work demonstrates how doping can improve the performance of ligand-assisted ZnO₂ photocatalysts for enhanced dye degradation.