Ultrasound-assisted synthesis of ZnO photocatalysts for gas phase pollutant remediation: Role of the synthetic parameters and of promotion with WO 3 .

The synthesis of ZnO photocatalysts by ultrasound-assisted technique was here investigated. Several experimental parameters including the zinc precursor (acetate, chloride, nitrate), sonication conditions (amplitude, pulse) and post-synthetic thermal treatment (up to 500 °C) were studied. Crystalline ZnO samples were obtained without thermal treatments due to the adopted reactant ratios and synthesis temperature. Sonication plays a major role on the morphological oxide features in terms of particle size and surface area, the latter showing a 20-fold increase with respect to conventional synthesis. Interestingly, 1 and 3 s sonication pulses led to morphological properties similar to continuous sonication. A thermal treatment at moderate temperatures (400-450 °C) promoted the loss of surface hydroxylation and the formation of lattice defects, while higher temperatures were detrimental for the sample morphology. The prepared ZnO was decorated with WO ₃ particles comparing an ultrasound-assisted technique using 1 s pulses with a conventional approach, giving rise to composites with promoted visible light absorption. Samples were tested towards the photocatalytic degradation of nitrogen oxides (500-1000 ppb) in humidified air under both UV and visible light. By carefully controlling the synthetic procedure, better performance were observed with respect to the commercial benchmark. Samples from ultrasound-assisted syntheses, also in the case of pulsed sonication, showed consistently better results than conventional references, in particular for ZnO-WO ₃ composites. The composite by ultrasound-assisted synthesis showed > 95% degradation in 180 min and doubled NO _x degradation under visible light with respect to the conventional composite.
Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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