Continuous Crystallization of ZnO Nanoparticles by Spray Flash Evaporation versus Batch Synthesis Regarding the scaled up and accelerated processing of ZnO particles, an advanced continuous pilot plant device is used for ZnO nanoparticle production from a zinc acetate dihydrate solution. A qualitative and mechanistic study is presented to show the feasibility of the process with respect to the formation of semiconducting ZnO nanoparticles. In order to evaluate the continuous approach based on the particle characteristics, a laboratory batch synthesis approach is additionally con- sulted for conventional preparation. In both cases, the precursor solutions are identical, on the basis of ethanol; no alkaline precipitant is added. The main find- ing in this comparison is the differing surface chemistry of the particles derived from these two approaches which triggers a whole cascade of differences in par- ticle size distribution and spectroscopic and morphological properties of the par- ticles. Keywords: Agglomeration, Continuous preparation, Nanoparticles, Spray flash evaporation, ZnO Received: January 26, 2015; revised: May 6, 2015; accepted: May 27, 2015 DOI: 10.1002/ceat.201500053 1 Introduction Nanostructured semiconducting ZnO has attained increasingly more attention from the scientific and industrial community since it is a multidisciplinary and nearly all-purpose com- pound. It has achieved high prominence from its use in (opto)- electronics (because of its wide band gap of 3.37 eV and large exciton binding energy of 60meV, both at room temperature), as a catalyst or sensor, or even in medicine and cosmetics etc. [1–8]. Since the synthesis strategies determine the final applica- tion, a simple laboratory batch process is demonstrated and compared with an advanced continuous synthesis on the basis of the spray flash evaporation (SFE) process. Particular empha- sis lies on accelerated processing of ZnO nanopowders with common wurtzite structure, also for industrial scale-up. Apparently, more batch syntheses than continuous processes have been presented in the scientific literature. It seems likely that this is due to the fact that the specific setup of a simple batch approach is usually less complex and can be more easily implemented in the laboratory field than a continuous-flow apparatus which needs the continuous addition of precursor and the continuous removal of product. Within industrial pro- cessing, the dominance of these setups is reversed and batch processing is rather rare – having the stigma of not being ‘‘engi- neered’’ and therefore not sophisticated [9]. Nevertheless, some continuous approaches for processing ZnO nanoparticles have also been published in scholarly areas ranging from liquid phase to gas phase synthesis. Here, liquid phase processes are mostly based on microreactors (microfluidic/hydrothermal reaction systems) combining two precursor solutions (zinc source and base) within a mixer (usually a T-mixer). The latter necessitates efficient mixing of the two reactants in order to form a homogeneous product in diverse solvents including water, supercritical water, alcohols, dimethylsulfoxide (DMSO), etc. [10–13]. ZnO products show different morphologies rang- ing from quasi-spheres, rods, nanowires and prisms to complex geometric structures like double cone-shaped nanoparticles [12, 14, 15]. In the end, the synthesis leads to nanoparticulate ZnO suspensions. Furthermore, there are state-of-the-art con- tinuous gas phase syntheses described as aerosol-based tech- niques. The flame reactor is used for the continuous prepara- tion of ZnO nanorods and the flowing gas phase reaction method was developed to fabricate ZnO; in the latter case, ZnO nanotetrapods have been described [16, 17]. Finally, the above- mentioned aerosol-based techniques lead to dry nanopowders for further processing. In this context, the direct synthesis of ZnO nanoparticles via spray drying has not been reported up to now. In this work, a completely new strategy is used to show the manufacturing of ZnO nanoparticles. Based on the flash evapo- ration effect, continuous synthesis of nanostructured powders Chem. Eng. Technol. 2015, 38, No. 8, 1477–1484 ª 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cet-journal.com Martin Klaumu ¨ nzer Laurent Schlur Fabien Schnell Denis Spitzer NS3E, ISL-CNRS-UNISTRA (Nanomate ´ riaux pour les Syste ` mes Sous Sollicitations Extre ˆ mes) UMR 3208, French- German Research Institute of Saint-Louis, Saint-Louis, France. Supporting Information available online – Correspondence: Martin Klaumu ¨nzer (martin.klaumuenzer@isl.eu, martin.klaumuenzer@fau.de), NS3E, ISL-CNRS-UNISTRA (Nano- mate ´riaux pour les Syste `mes Sous Sollicitations Extre ˆmes) UMR 3208, French-German Research Institute of Saint-Louis, 5, rue du Ge ´ne ´ral Cassagnou, B.P. 70034, 68301 Saint-Louis, France. Research Article 1477