Paper AS+NS+SA-WeM5
Microstructural Effects on Surface Potential of Amorphous Solid Water

Wednesday, October 24, 2018, 9:20 am, Room 204

Amorphous solid water (ASW) formed by vapor deposition on substrates <~130 K is of interest for its abundance in Earth’s upper atmosphere, icy planetary bodies, and throughout the interstellar medium, as well as its use as model material in many disciplines. Two crucial characteristics of ASW are a self-induced negative surface potential and formation of nanopores [1]. Here, we examine the role of microstructure, including nanopores, on the spontaneous surface potential of ASW, and describe the complementary experimental techniques used, which have application for other microporous solids.

ASW films were deposited by directed vapor beams onto a He-cooled quartz-crystal microbalance (QCM) under ultra-high vacuum. The integrated pore volume (porosity) was calculated by combining the areal mass measured via QCM and thickness measured by UV-visible interferometry. The integrated surface area was indicative by the abundance of incompletely coordinated surface water molecules (H₂O) on the pores, using the O-H dangling bonds (DBs) measured by FT-IR spectroscopy. An in-situ Kelvin probe measured film surface potential. A long-distance optical microscope monitored film morphology in vacuo. Annealing effects were investigated by heating the film at 1.8 K/min.

The magnitude of the negative surface potential (|V_s|) increased linearly with film thickness at rates (|ΔV_s/ΔL|) that decreased with increasing growth temperature (T_g = 10–110 K), keeping deposition angle at θ = 0° (angle between vapor beam and QCM normal); at T_g = 30 K, the |ΔV_s/ΔL| decreased with increasing θ (= 10−75°). ASW porosity showed no dependence on T_g at θ = 0°, but increased significantly with increasing θ. The H₂O DBs decreased/increased with increasing T_g/θ, showing similar trends as the |ΔV_s/ΔL|. Upon heating, the most striking result was that the DB at ~3720 cm⁻¹ (from two-coordinated H₂O) and the |V_s| had similar temperature-dependent evolutions. By correlating all measurements, we propose that the observed intrinsic ASW surface potential results from aligned incompletely-coordinated H₂O on the pore surfaces [2].

The |V_s| decreased abruptly when ASW thickness exceeded a critical value (L_c), and cracks appeared in the optical images of the films. The L_c, ~1−5 µm (T_g = 10–50 K; θ = 0–55°), increased with T_g and θ, suggesting dependences on the microporous structure. We explain such dependences of L_c in the context of Griffith theory and estimate the tensile strength of ASW to be ~25–40 MPa [3].

We acknowledge support from the NASA LASER Program.

[1]Raut et al., J. Chem. Phys.127, 204713 (2007); [2]Bu et al., J. Chem. Phys.143, 074702 (2015); [3]Bu et al., Appl. Phys. Lett.109, 201902 (2016).