Contributions of morphological and structural parameters at different hierarchical morphology levels to photocatalytic activity of mesoporous nanostructured ZnO

Abstract

Hierarchical mesoporous zinc oxide (ZnO) polyhedral micro-lumps were prepared by annealing zinc oxalate dihydrate (ZnC2O4 ⋅ 2H2O) precursor in an air atmosphere at various temperatures. The development of the mesoporous nanoarchitecture of the ZnO micro-lumps was studied by XRD analysis corroborated with the analysis of their specific surface area utilizing nitrogen sorption via BET method. Investigations by high-resolution SEM revealed a morphology resembling calcined mesocrystals. A description of the sintering process and the crystallite growth is given together with an analysis of the activation energy of the grain and crystallite growth, respectively. The self-diffusion coefficient, diffusion mechanisms, and its activation energy involved in the growth of crystallites were identified. The crystallite size and the specific surface area can be tuned by varying the annealing temperature, while the UV–vis and PL spectra remain nearly unchanged. The morphological and structural parameters are correlated with photocatalytic activity evaluated by the UV discolouration of the Methyl violet 2B. An interpretational framework, including three hierarchical morphology levels of the micro-lumps, was developed. The interplay between the contributions of the different promoting and impeding effects emerging on various levels can explain the non-monotonous dependence of the photocatalytic activity on the catalyst annealing temperature. © 2020