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The aim of this article was an explanation of the gold nanoparticle (AuNP) formation mechanisms which take place in a redesigned Ultrasonic Spray Pyrolysis (USP). Depending on the synthesis parameters of gold concentration in the precursor solution, gas flow and reaction temperature, we have previously obtained a combination of spherical, irregular and cylindrical AuNPs. Two parameters (gold concentration and gas flow) were determined to be the most influential on the shapes and sizes of the produced AuNPs. The effects of these two influential synthesis parameters were evaluated on controlling the formation mechanisms: Gold concentration (2.5 and 0.5 g/l Au) in the precursor solution of HAuCl 4 dissolved in water and gas flows of (i) aerosol carrier gas N 2 (1.5, 3.0 and 4.5 l/min) and (ii) reduction gas H 2 (1.0, 1.5 and 2.0 l/min). Depending on the parameter conditions, the AuNPs are formed from a combination of the liquid/solid phase (Droplet-to-Particle mechanism, DTP), the gas phase (Gas-to-Particle mechanism, GTP) and from intermediate secondary droplets, formed from primary droplet explosions. Increasing the gas flow affected the evaporation of the solvent (water) and diffusion of the solute ([AuCl 4 ] − ) in the aerosol droplets, which resulted in the formation of more uniformly shaped AuNPs and with narrower size distributions than before. With favorable parameter conditions increased control over AuNP synthesis with USP has been achieved.

Nanostructured zinc oxide (ZnO) particles were synthesized by the one step Ultrasonic Spray Pyrolysis (USP) process from nitrate salt solution (Zn(NO3)2&middot, 6H2O). Various influential parameters, from Zn(NO3)2&middot, 6H2O concentrations (0.01875&ndash, 0.0375 M) in the initial solution, carrier gas (N2) flow rates (0.5&ndash, 0.75 L/min) to reaction temperature (400&ndash, 800 &deg, C), were tested to investigate their role on the final ZnO particles&rsquo, morphology. For this purpose, Scanning Electron Microscopy (SEM), High Resolution Transmission Electron Microscopy (HRTEM) and (Selected Area Electron Diffraction) SAED techniques were used to gain insight into how the ZnO morphology is dependent on the USP process. It was revealed that, by certain parameter selection, different ZnO morphology could be achieved, from spherical to sphere-like structures assembled by interwoven nanoplate and nanoplate ZnO particles. Further, a more detailed crystallographic investigation was performed by XRD and Williamson-Hall (W-H) analysis on the ZnO with unique and non-typical planar morphology that was not reported before by USP synthesis. Moreover, for the first time, a flexible USP formation model was proposed, ending up in various ZnO morphologies rather than only ideal spheres, which is highly promising to target a wide application area.

This article explains the mechanism of the metal/oxide core-shell Ag/TiO2 and Au/TiO2 nanoparticle formation via one-step ultrasonic spray pyrolysis (USP) by establishing a new model. The general knowledge on the standard “droplet-to-particle” (DTP) mechanism, nucleation, and growth processes of noble metals, as well as physical and chemical properties of core and shell materials and experimental knowledge, were utilized with the purpose of the construction of this new model. This hypothesis was assessed on silver (Ag)/titanium oxide (TiO2) and gold (Au) TiO2 binary complex nanoparticles’ experimental findings revealed by scanning electron microscopy (SEM), focused ion beam (FIB), high-resolution transmission electron microscopy (HRTEM), and simulation of crystal lattices. It was seen that two mechanisms run as proposed in the new model. However, there were some variations in size, morphology, and distribution of Ag and Au through the TiO2 core particle and these variations could be explained by the inherent physical and chemical property differences of Ag and Au.

Metals : open access journal 8(4), 278 (2018). doi:10.3390/met8040278 special issue: "Special Issue "Powder Synthesis and Processing" / Guest Editor Prof. Dr. Claude Estournès", Published by MDPI, Basel