A Peltier stage cooled sample holder tip for the in situ study of water vapor / liquid equilibrium and the hygroscopic behaviour of aerosols in an ETEM
Résumé
The constant development of atmospheric close cells since a few decades permits today a wide range of in situ studies of nanomaterials in liquids, or liquids themselves in a Transmission Electron Microscope (TEM). However, Liquid Cell TEM fails to properly study liquid-vapor reactions of outermost importance in many societal subjects especially concerning water interactions with nanosystems, such as in the field of atmospheric chemistry (see e.g. [1]) or in pharmaceutical sciences [2]. Such domains can fortunately be investigated in a dedicated Environmental TEM (FEI Titan ETEM), where the absence of sealing membranes around the sample allows direct exchanges between the vapor (gas) introduced into the microscope and a solid. Several studies exist of the hygroscopic behaviour of model or atmospheric aerosols in such ETEMs, see for example [3]. A cryo-holder is used to cool down the specimen (typically in the range 0 to 18 °C) in order to reach the vapor-liquid equilibrium in the pressure range eligible in an ETEM (below 20 mbar).
We present here a new approach, where the sample is cooled by means of a micro-Peltier stage mounted on the type of a commercial MEMS-based ‘heating holder’ from DENSsolutionsTM, see Fig. 1a). This montage [4] offers a bunch of advantages as compared to the above ETEM approach. One is its rapidity to reach a given relative humidity (linked to the temperature-pressure conditions), which allows to study very accurately the first stages of deliquescence (dissolution) or efflorescence (recrystallisation) of soluble solids, such as NaCl nanocubes as illustrated in Fig. b-d) [5].
Figure 1. A micro-Peltier stage cooled sample holder type to study the hygroscopic behaviour of aerosols in a Titan ETEM. a): montage of the type (to be detailed during the conference). b)-d): illustration of a deliquescence sequence of NaCl nanocubes (STEM imaging, 300 kV). Note the appearance of a water layer (arrows) around crystals in c). The vapor pressure, temperature (not measured but estimated, Relative Humidity (RH, estimated but partly dictated by the behaviour of the NaCl-water system – deliquescence of NaCl is known to occur at RH = 75 % –). The labels Peltier ON / OFF indicate whether the stage is powered or not; time is also reported in seconds.
References
[1] A.G. Tereshchenko. J. of Pharmaceutical Sciences, 2015, 104, 3639-3652.
[2] Y-L. Pan et al., J. of Aerosol Science, 2021, 155, 105767.
[3] M.E. Wise et al., Aerosol Science and Technology, 2008, 42, 281-294.
[4] J.V. Vas et al., Microsc. Microanal. 2022, 28 (Suppl 1), 818-819.
[5] Thanks are due to CLYM (www.clym.fr) for the access to the microscope. This study is supported by the French National Research Agency (ANR) under the ‘WATEM’ project n° ANR-20-CE42-0008.