The photocatalytic degradation represents one of the most promising solutions to address the issue of water pollution. In this context, ZnO has received significant attention, due to its non-toxicity, high exciton binding energy, and competitive photocatalytic activity under UV irradiation. However, the ZnO performance is greatly affected by the synthesis method since the morphology plays a very important role in determining its properties [1]. Among different synthesis approaches, the hydrothermal synthesis under subcritical conditions is frequently used. However, the method requires the use of strong alkaline mineralizers to promote crystallization. Instead, the synthesis in supercritical water (SCW), a green reaction medium, allows crystallization of metal oxide nanoparticles through a mineralizer-free process [2].
In this work, we report the synthesis of ZnO nanoparticles by continuous flow SCW method (ZnO-SCW) and the evaluation of its photocatalytic properties in the degradation of methylene blue (MB). The photocatalytic properties of the ZnO-SCW were compared with those of other two ZnO materials, namely a material obtained by the conventional hydrothermal approach (ZnO-HT) [3] and a commercial sample (ZnO-Co).
For production of the ZnO-SCW nanopowder the continuous flow tubular reactor of NISA (Neutron Imaging Supercritical-water Analysis) equipment [4] was used. The synthesis started from aqueous Zn(NO₃)₂ precursor and was carried out for 1 h at 395 ºC and 250 bar. The same precursor was used for the preparation of ZnO-HT sample, in the presence of NH₄OH as mineralizer at 120 °C for 6 h. All ZnO powders were characterized by XRD and SEM. Photocatalytic activity tests were conducted in a cylindrical quartz photoreactor and the dye concentration was determined from a standard curve using the absorbance values measured by UV–Vis spectroscopy.
The XRD indicated that all samples are crystallized in the wurtzite hexagonal phase of ZnO, with the ZnO-SCW sample showing a preferential orientation on the (100) crystallographic plane. The morphological SEM analysis reveals the formation of 1D particles for both ZnO-SCW and ZnO-HT. However, in the case of ZnO-SCW sample interesting faceted and contrast variation on particle surfaces are noticed, suggesting the formation of the lattice defects. The photocatalytic tests indicated a faster decomposition of MB in the presence of ZnO-SCW. In this case, a MB conversion of 98.9 % was obtained after only 10 min while similar conversions were obtained after 45 min and 120 min in the presence of ZnO-HT and ZnO-Co, respectively. The improvement of the ZnO-SCW photocatalytic properties is most probably due to their peculiar structural and morphological characteristics which lead to an enhanced MB adsorption and a faster transfer of the photogenerated electrons.
 
Acknowledgements: This research is funded by Romanian CNCS/CCCDI—UEFISCDI, grant number PN-III-P4-ID-PCE-2020-1241. The collaboration with Prof. C. Ludwig from Paul Scherrer Institute, Switzerland is greatly acknowledged.

References
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