ZnO nanostructures based piezo-photocatalytic degradation enhancement of steroid hormones

Testosterone (TST) and β-estradiol (E2) are steroid hormones belonging to the emerging contaminants group, which is considered a huge environmental threat. In this context, wastewater remediation is gaining attention, especially piezo-photocatalytic approaches based on nanostructured semiconductors. In this contribution, ZnO nanosheets have been synthesized, characterized and utilized for the piezo-photodegradation of TST and E2 from aqueous solutions, under solar illumination and simultaneous mechanical stimulation induced by ultrasound waves. The peculiar micrometric flat shape of the ZnO nanostructures appears particularly interesting for piezo-photocatalytic applications for two reasons: (i) the large surface area available for contaminant absorption, (ii) the flat structure is very susceptible to the mechanical stimulus by the ultrasound waves. In detail, the piezoelectric properties of the ZnO nanosheets were measured through piezo-response force microscopy (PFM); it is important to note that only ultrasound stimulation does not induce TST degradation. On the contrary, the piezo-features of the synthesized nanostructures support and enhance the photocatalytic performances of the ZnO. In fact, tests with specific scavengers of the produced reactive species have permitted us to comprehend the physical-chemical mechanism behind the hormone degradation, which is activated by hydroxyl radical species. Similarly, E2 is shown to also be efficiently piezo-photodegraded in ca 45 min, further underlining ZnO nanosheets' potential as very efficient piezo-photocatalysts for wastewater remediation.