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

Author:
Pourrahimi, Amir Masoud, Liu, Dongming, Andersson, Richard L., Ström, Valter, Gedde, Ulf W., Olsson, Richard T.
Source:
Langmuir 2016 v.32 no.42 pp. 11002-11013
ISSN:
1520-5827
Subject:
annealing, electron microscopy, heat treatment, nanoparticles, nanosheets, nitrates, oxygen, photocatalysis, photolysis, porosity, surface area, temperature, zinc oxide
Abstract:
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.