In Situ Monitoring of Particle Formation with Spectroscopic and Analytical Techniques Under Solvothermal Conditions The formation of multifunctional materials involves various simultaneous and closely entangled physicochemical processes including nucleation, growth, aggre- gation, and in some cases also growth by oriented aggregation. These elementary particle formation processes occur in broad size ranges from the molecular to micrometer level and time scales ranging from seconds to hours. This contribu- tion demonstrates that the in situ monitoring of reaction mixtures and dispersions with several simultaneous analytical and spectroscopic techniques is an effective and widely applicable strategy to achieve an in-depth understanding of the forma- tion of complex particulate structures and to move towards the rational design of materials. Keywords: Carbon nanodots, Mesocrystals, Metal-organic frameworks, Oriented attachment, Particle formation Received: September 23, 2019; revised: March 21, 2020; accepted: March 23, 2020 DOI: 10.1002/ceat.201900520 1 Introduction Understanding the formation mechanism of complex particu- late structures is a fundamental challenge and a prerequisite for the rational design of multifunctional materials with tunable properties. During particle formation, various closely entangled physicochemical processes involving nucleation, growth, aggre- gation, and in some cases, also growth by oriented aggregation occur simultaneously. Processes taking place in broad size ranges from the molecular to micrometer level and time scales ranging from seconds to hours further complicate the develop- ment of mechanistic understanding [1]. One possible strategy to tackle the challenge of understanding the formation mecha- nisms of solid phases is the implementation of in situ tech- niques to monitor the evolution of synthesis solutions in real time. One of the most robust methods for the production of inor- ganic functional materials is synthesis under hydrothermal, or more general, solvothermal conditions in sealed vessels, usually stainless-steel autoclaves, under autogeneous pressures gener- ated at temperatures above 100 °C or above the boiling point of the solvent, respectively [2, 3]. Under hydro- and solvothermal conditions, in principle it is possible to achieve highly crystal- line materials with narrow particle size distributions, well-de- fined shapes, and properties. Studies of the formation mecha- nisms under such experimental conditions have traditionally been carried out by ex situ characterization of quenched sam- ples. These time-resolved studies are performed under the assumption that the solids recovered from the reaction mixture do not undergo modification upon transfer to ambient temper- ature and pressure [3]. To overcome this limitation and fully exploit the advantages of synthesis under hydro- and solvothermal conditions, several groups have developed various experimental setups making use of in situ techniques. The first reactors of this kind were designed to perform in situ X-ray diffraction measurements [4–9]. Over the years, the formation of several metal oxide materials such as WO 3 [10, 11], ZrO 2 [12], iron oxides [13, 14], ZnO [15], SnO 2 [16], CeO 2 [17], yttria-stabilized zirconia par- ticles [18], and of several multicomponent oxides such as LiCoO 2 [19], Li 2 TiO 3 [20], has been monitored under solvo- thermal and hydrothermal conditions using in situ total-scat- tering study and X-ray diffraction techniques, both at laborato- ry scale and at X-ray/neutron synchrotron sources [21]. However, in situ diffraction techniques are able to detect only the phase evolution of crystalline materials, whereas the detection of initial or intermediate amorphous solids remains inaccessible. Therefore, the implementation of simultaneous Chem. Eng. Technol. 2020, 43, No. 5, 879–886 ª 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA. www.cet-journal.com Heidemarie Embrechts 1,2 Martin Hartmann 2,3 Wolfgang Peukert 1,2 Monica Distaso 1,2, * This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distri- bution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes. – 1 Heidemarie Embrechts, Prof. Dr.-Ing. Wolfgang Peukert, Dr. Monica Distaso monica.distaso@fau.de Institute of Particle Technology, FAU Erlangen-Nuremberg, Cauer- strasse 4, 91058 Erlangen, Germany. 2 Heidemarie Embrechts, Prof. Dr.-Ing. Wolfgang Peukert, Dr. Monica Distaso Interdisciplinary Center for Functional Particle Systems, FAU Erlan- gen-Nuremberg, Haberstrasse 9a, 91058 Erlangen, Germany. 3 Prof. Dr. Martin Hartmann Erlangen Center for Interface Research and Catalysis (ECRC), FAU Erlangen-Nuremberg, Egerlandstrasse 3, 91058 Erlangen, Germany. Research Article 879