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Aqueous Synthesis of (21̅0) Oxygen-Terminated Defect-Free Hierarchical ZnO Particles and Their Heat Treatment for Enhanced Reactivity

Pourrahimi, Amir Masoud, Liu, Dongming, Andersson, Richard L., Ström, Valter, Gedde, Ulf W., Olsson, Richard T.
Langmuir 2016 v.32 no.42 pp. 11002-11013
annealing, electron microscopy, heat treatment, nanoparticles, nanosheets, nitrates, oxygen, photocatalysis, photolysis, porosity, surface area, temperature, zinc oxide
Controlled aqueous growth of 1 μm flower-shaped ZnO particles with a hierarchical subset of exposed nanosheets represented by {21̅0} crystal faces, followed by annealing at temperatures up to 1000 °C, is presented. The flower-shaped particles showed superior photocatalytic performance compared to the crystal faces of 20 nm ZnO nanoparticles. The photocatalytic reaction rate of the flower-shaped particles before annealing was 2.4 times higher per m² compared with that of the nanoparticles with double specific surface area. Crystal surface defects and nanosized pores within the flower-shaped particles were revealed by porosity measurements and electron microscopy. A heat treatment at 400 °C was found to be optimal for removal of nanoporosity/surface defects and impurities while retaining the hierarchical superstructure. The heat treatment resulted in a photodegradation efficiency that increased by an additional 43%, although the specific surface area decreased from 16.7 to 13.0 m²g–¹. The enhanced photocatalytic effect remained intact under both acidic and alkaline environments owing to the {21̅0} crystal surfaces, which were less prone to dissolution than the nanoparticles. The photocatalytic performance relied on primarily three factors: the removal of surface impurities, the oxygen termination of the {21̅0} crystal faces, and the promotion of charge carrier lifetime by removal of lattice defects acting as recombination centers. The synthesis presented is an entirely hydrocarbon- and surfactant-free (“green”) preparation scheme, and the formation of the flower-shaped particles was favored solely by optimization of the reaction temperature after the correct nitrate salt precursor concentrations had been established.