## 3 Results and Discussion
### Synthesis by Direct Ultrasonic Aerosolisation and Comparison with Non-Aerosolised Synthesis
However, nanosynthesis processes in which sprays or aerosolised phases participate can be divided into two categories: those in which nanosynthesis takes place by mutual coalescence between droplets [48] or by mixing of droplets with other phases [49], and those in which it takes place by chemical/physical processes occurring within the droplets themselves and are independent of the above phenomena, as is generally the case for spray/aerosol pyrolysis [50] and for some variants of chemical vapour deposition (CVD) [51]. The present technique belongs to the former category, and hence it is consistent with the related beneficial effects in terms of polydispersity when compared to standard batch processes. The only basic difference with respect to the synthesis method of [PERSON] et al. [48] lies in the absence of an external energy contribution, as reaction (1) occurs spontaneously at room temperature, being favoured by thermodynamic and kinetic conditions. Figure 6: Plot of particle diameter distribution function obtained by DLS. (**a**) Direct aerosolisation with CaCl\({}_{2}\) and Na\({}_{2}\)CO\({}_{3}\) at concentration C\({}_{0}\) = 0.472 [M]. (**b**) Direct aerosolisation, but with reagent concentrations C\({}_{0}\)/4. (**c**) Reagent mixing in stirred tank reactor, with impulsive injection of reagents at concentrations adopted in (**a**). (**d**) Reagent mixing in stirred tank reactor, with impulsive injection of reagents at concentrations adopted in (**b**). Curves are interpolations from frequency histograms derived from averages over three replicate samples, with a maximum experimental error not exceeding 10%. This fact makes the present method rather uncommon and potentially attractive in terms of inherently safe operation and energy saving, as will be discussed in the next subsection. In Figure 7, FESEM images of the NP aggregates obtained by direct aerosolisation at two different values of precursor concentration, namely C\({}_{0}\) and C\({}_{0}\)/4, are reported. By simple inspection, the dimension of primary particles is qualitatively consistent with the DLS results. In Figure 8, the energy-dispersive X-ray spectroscopy has been carried out for the solid phase obtained by the aerosol-assisted method, with reagent concentrations equal to those adopted in Figure 7a. Although the analysis has been carried out up to 20 kV, the horizontal axis has been intentionally cut at a maximum of 10 kV in order to improve the resolution of visible peaks, especially since no other significant peaks were detected for values greater than 10 kV. The presence of Cu is due to the FESEM metal substrate on which the nanostructured phase was deposited, while Na and Cl ions are accessory ions present in both precursors and forming NaCl according to reaction (1). The measured atomic ratio Ca/C = 0.815 is different from the expected value Ca/C = 1, corresponding to a solid phase made of pure CaCO\({}_{3}\). This result can be ascribed to an excess of C related to the presence of a thin graphitisation layer deposited on the solid phase to make it conductive, according  
Figure 7: FESEM images of CaCO\({}_{3}\) NPs obtained by direct aerosolisation. (**a**) Case with CaCl\({}_{2}\) and Na\({}_{2}\)CO\({}_{3}\) concentrations C = C\({}_{0}\). (**b**) The same as (**a**), but with C = C\({}_{0}\)/4. to the preparatory technique required by FESEM analysis. The composition of the NPs obtained in other experimental configurations proposed here does not differ significantly from those of Figure 8, and hence it has not been reported.