In Situ Live Observation of Nucleation and Dissolution of Sodium Chlorate Nanoparticles by Transmission Electron Microscopy Yuki Kimura,* ,† Hiromasa Niinomi, †,§ Katsuo Tsukamoto, † and Juan M. García-Ruiz* ,‡ † Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, Aramakiaza-Aoba 6-3, Aoba-ku, Sendai 980-8578, Japan ‡ Laboratorio de Estudios Cristalogra ́ ficos, Instituto Andaluz de Ciencias de la Tierra, Consejo Superior de Investigaciones Científicas-Universidad de Granada, Av. de las Palmeras 4, 18100 Granada, Spain * S Supporting Information ABSTRACT: The formation of crystals from solution requires the initial self-assembly of units of matter into stable periodic structures reaching a critical size. The early stages of this process , called nucleation, are very difficult to visualize. Here we describe a novel method that allows real time observation of the dynamics of nucleation and dissolution of sodium chlorate clusters in an ionic liquid solution using in situ transmission electron microscopy. Using ionic liquids as solvent circumvents the problem of evaporation and charging, while the nucleation frequency was reduced by using saturated solutions. We observe simultaneous formation and dissolution of prenucleation clusters, suggesting that high-density fluctuations leading to solid cluster formation exist even under equilibrium conditions. In situ electron diffraction patterns reveal the simultaneous formation of crystalline nuclei of two polymorphic structures, the stable cubic phase and the metastable monoclinic phase, during the earliest stages of nucleation. These results demonstrate that molecules in solution can form clusters of different polymorphic phases independently of their respective solubility. T he birth of a crystal from its mother solution is one of the most intriguing problems in solid-state physics. 1 The current view of the process was formalized during the first half of the 20th century and is based on the competition between the free energy created when molecules in the solution cluster to form a solid phase and the free energy consumed as a result of the surface creation. 2 Accordingly, the energy required to form a crystalline cluster is maximum for a particular size, called the size of the critical nucleus. 3 This view of crystal formation has recently been challenged by a “two-step” approach in which the pathway for overcoming the energy barrier can be lowered by prior formation of either dense liquid fluid or amorphous but stable clusters that might later form the crystalline nucleus. 4-8 Testing the actual nucleation pathway experimentally at the nanoscale presents formidable challenges. Atomic force microscopy and novel techniques of transmission electron microscopy (TEM) using fluid cells have provided exquisite information on the size and structure of clusters in frozen stages at the nanoscale, 9-11 as well as a dynamic view of the growth and coalescence of nuclei. 12-14 However, a live observation of the dynamics during the earliest stages of nucleation, those taking place before the formation of a stable crystal, has never been achieved. The reason is that in situ observation of nucleation at the nanoscale using TEM faces serious difficulties, particularly those related to solvent evaporation, charge dissipation and image acquisition speed. To circumvent the problem of evaporation and charging, we have used an ionic solvent having negligible vapor pressure and relatively high electrical conductivity. 15 To overcome the problem of visual- ization, we have used saturated solutions, where crystalline clusters are expected either not to form or to do it at a slow rate, and will never reach a critical size, thus making the observation of the dynamics easier, cleaner and more informative. Ionic liquids have previously been used for direct observation of organic materials, such as seaweed by scanning electron microscopy (SEM) 16 and dispersed metallic nanoparticles by TEM. 17 There are several hundreds of ionic liquids that have been categorized into several systems, such as aliphatic, imidazolium, or pyridium. We selected the following five ionic liquids (Kanto Chemical Co., Inc., Tokyo, Japan) as candidate solvents, taking into account their melting point, dissociation temperature, price, and availability. The ionic liquid must be liquid at room temperature for nucleation and must be stable, showing no dissociation during the heating experiments; 1,3-diallylimidazolium bromide, 1-allyl-3-butylimidazolium bro- mide, 1-ethyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3- methylimidazolium tetrafluoroborate, and 1-allyl-3-ethylimida- zolium bromide. The first of these ionic liquids was found the best solvent to study NaClO 3 nucleation. Its chemical formula weight is 229.12 and its decomposition temperature is 271 °C. Its chemical formula is C 9 H 13 BrN 2 . Saturated solutions of NaClO 3 (analytical grade, Wako Pure Chemical) in ionic solution at 80 °C were prepared as follows. NaClO 3 powder was poured into the ionic liquid (500 μL) and stirred using ultrasound. The solution was stored for 2 days at 85 °C, then cooled down to 80 °C and stored for 1 day. A residue of undissolved NaClO 3 was observed at the bottom of the ionic solution confirming that the supernatant ionic liquid solution was saturated with NaClO 3 at 80 °C. Saturated ionic solution of 5-10 μL was removed from the middle of the solution using a pipet previously warmed to 80 °C. The Received: November 27, 2013 Communication pubs.acs.org/JACS © XXXX American Chemical Society A dx.doi.org/10.1021/ja412111f | J. Am. Chem. Soc. XXXX, XXX, XXX-XXX