Windowless in situ Water Condensation on NaCl nanocubes in Environmental TEM. - VIRTUEL
Résumé
Observing liquids in a Transmission Electron Microscope (TEM) has long been impossible owing to basic thermodynamic limitations due to the need for a high vacuum, typically 10-5 mbar or better, within the column of the instrument, making it impossible to maintain a liquid state at room temperature. With the development of dedicated sealed liquid cells mounted on specific specimen holders, Liquid Cell TEM (LCTEM) has become possible about a decade ago, opening a huge range of possible applications in the field of biology, crystal growth or electrochemistry [1]. In parallel, Environmental TEM (ETEM) was also developed [2]; here a partial pressure can be maintained in the pole-pieces gap where the tip of the sample holder, including the sample itself, is inserted, allowing to perform observations under gas without any sealing membranes as needed with the close-cell technology. Such an ‘open-cell’ approach was also developed in Scanning EM (ESEM, e.g. [3]). With these dedicated ETEM or ESEM configurations, observing liquid such as water layers is possible under a partial pressure of a few mbar if the temperature is cooled down close to the dew point in order to insure a thermodynamic equilibrium between the solid, gas and liquid states: for water, the liquid state can effectively been stabilized in a temperature and pressure range of typically 0 to 11°C and 6 to 15 mbar respectively [4], which are conditions easily accessible in ETEM and ESEM. While LCTEM in a close-cell permits to reach atmospheric pressure, thus allowing to observe water at room temperature, it has the drawback of its advantage: the presence of top and bottom sealing membranes makes it very difficult to perform water condensation from a humid atmosphere and to control water vapor states. Such experiments are possible in ‘open-cell’ ESEM [5] and ETEM [6] and enhance our understanding of the hygroscopic behavior of atmospheric aerosol particles that are known to act as cloud condensation nuclei [7]. Hygroscopic growth, deliquescence and efflorescence of model and real) atmospheric nanoparticles can be directly visualized by these techniques.
The present contribution aims at establishing conditions under which aerosols can be adequately observed in a Titan ETEM (FEI/TFS). We use a Gatan liquid-nitrogen (LN2) cryo-holder to cool down the specimen around 0°C. We adjust the temperature by mixing LN2 with a controlled quantity of ethanol. For the purpose of this preliminary investigation, we use NaCl nanoparticles, obtained by vaporizing a salt solution onto a classical holey carbon TEM grid, as a model aerosols. Observations were performed at 300 kV under a humid atmosphere generated by pumping a small sealed water reservoir connected to one of the input lines of the ETEM, the pumping being insured by the molecular turbopumps of its vacuum system. The presence of water (vapor) was controlled by the residual gas analyzer equipping the microscope and by Electron Energy-Loss Spectroscopy (EELS). In a first step, and considering the high voltage at which experiments were performed, a control of the electron flux and dose was realized using different illumination settings in order to define safe imaging conditions avoiding noticeable irradiation damage of the nanocrystals (Fig. 1). Then, both water condensation and evaporation have been performed to follow the evolution of NaCl cubes (Fig. 2). Results will be discussed in terms of relationships between percentage of relative humidity and water uptake of the NaCl particles as a function of T and P [8].
References:
[1] FM Ross (Ed.), Liquid Cell Electron Microscopy (Advances in Microscopy and Microanalysis), Cambridge University Press, Cambridge (2017), 524 p.
[2] TW Hansen, J.B. Wagner (Ed.), Controlled Atmosphere TEM, Springer, New York, (2016), 332 p.
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[6] ME Wise et al. Aerosol Science & Technology, 39:9 (2005) 849; C Cassidy et al. Plos One, 12 11 (2017) e0186899; BDA Levin et al. Microscopy and Microanal. (2020) 1.
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[8] The authors acknowledge the Consortium Lyon – St-Etienne de Microscopie (CLYM, www.clym.fr), the Centre Technologique des Microstructures (http://microscopies.univ-lyon1.fr/) for practical assistance and the French National Research Agency (ANR, www.anr.fr) for supporting project ANR-20-CE42-0008.