Crystallization Kinetics of Thin Amorphous Water Films on Surfaces

Abstract

The paper presents experimental investigation of the crystallization kinetics of thin (from several and up to 100 monolayers, ML) amorphous ice films grown in a vacuum on clean and adsorbate modified Pt(111) and graphite (0001) surfaces. The crystallization kinetics was followed by the associated decrease of the desorption rate by isothermal desorption mass spectroscopy. The process is strongly substrate dependent for thin films (below 20-30 ML), gradually becoming substrate independent in 30-100 ML range, and approaching the bulk ice characteristics for thicker films. For thin films, the enthalpy of vaporization is 52 kJ/mol for the amorphous and 54 kJ/mol for the crystalline films. The activation energy for crystallization was estimated to be 75 kJ/mol. The crystallization is accompanied by an enhanced mobility in the film as demonstrated experimentally by following O2 release during crystallization of ice films on O2 precovered Pt(111).

Reflection Absorption Infrared Spectroscopy and Temperature-Programmed Desorption Studies of the Adsorption and Desorption of Amorphous and Crystalline Water on a Graphite Surface

“…Previous isothermal TPD studies of the adsorption of water on HOPG and on Pt{111} have shown that the time taken for crystallization becomes independent of film thickness for sufficiently thick films. 15 However, this was not observed for the adsorption of water on HOPG as shown in Figure 5B. Note that for low exposures (e6 langmuirs), region A is followed directly by region C. This implies that, for exposures e6 langmuirs, desorption of multilayers takes place simultaneously with the crystallization of the ASW.…”

“…A temperature of 142 K was chosen in accordance with previous isothermal TPD studies of water on HOPG. 12,14,15 Figure 5A shows an isothermal TPD spectrum recorded following a 40 langmuir exposure of water on HOPG. The spectrum is divided into three regions, labeled A, B, and C. Initially there is an increase in the desorption rate as desorption of the adlayer takes place during both the initial temperature ramp and with the sample held at 142 K. The desorption rate then rapidly decreases.…”

“…In this region, islands of water ice alternate with bare areas of the HOPG surface. 14,15 Sublimation of the ice causes a monotonic decrease in island size and effective island surface area. This leads to the monotonic decrease in desorption rate observed in region C. Note that region B will only be observed if the water ice film is sufficiently thick for multilayers to form.…”

“…12 Isothermal TPD experiments have also been carried out to investigate the crystallization kinetics of water on HOPG. 14,15 The structure of water adsorbed on a surface is highly dependent on the deposition conditions and the surface temperature. If deposition occurs below 135 K, ice grows as amorphous solid water (ASW), [16][17][18] with the exact morphology depending on the dosing conditions.…”

Communication: Distinguishing between bulk and interface-enhanced crystallization in nanoscale films of amorphous solid water

“…The crystallization kinetics will provide information about the properties of supercooled liquid water. A variety of experimental techniques (including desorption, [16][17][18][19][20] physisorption, 11,21-24 infrared spectroscopy, 5,10,[25][26][27][28] and electron microcopy 12,29 ) have been used to study ASW crystallization. The majority of the above mentioned studies report that crystallization occurs via bulk nucleation although a few studies suggest that nucleation may start at the vacuum interface.…”