Thermal desorption of H2O ice: from nanoscale films to the bulk

Abstract

ABSTRACT The desorption properties of H2O films are investigated across a wide range of film thicknesses from 53 nanometres (nm) to 101 micrometres (μm) using a quartz-crystal microbalance (QCM) and temperature-programmed desorption. Three desorption stages are observed belonging to amorphous solid water (ASW), stacking disordered ice I (ice Isd), and hexagonal ice I (ice Ih). The desorption of ASW is only detectable for the ≥10 μm films and is separated from the ice I desorption by 10–15 K with an associated desorption energy of ∼64 kJ mol−1. The desorption energy of the 53-nm film was found to be near 50 kJ mol−1 as also noted in the literature, but with increasing film thickness, the desorption energy of ice I rises, reaching a plateau around 65–70 kJ mol−1. The reason for the increased desorption energy is suggested to be due to molecules unable to desorb due to the thick covering layer of H2O and possibly re-adsorption events. Before complete desorption of ice I at around 220 K for the 101 μm film, a two-stage ice I desorption is observed with the QCM for the ≥10 μm films near 200 K. This event corresponds to the desorption of ice Isd as corroborated by X-ray diffraction patterns collected upon heating from 92 to 260 K at ambient pressure. Cubic ice is not observed as is commonly stated in the literature as resulting from the crystallization of ASW. Therefore, ice Isd is the correct terminology for the initial crystallization product of ASW.