Introduction

Water is unique in that it exists in several different identifiable amorphous forms, low-density amorphous (LDA), high-density amorphous (HDA) and very-high-density amorphous (VHDA), with a density difference of 33% between LDA and VHDA1,2,3,4. Several metastable amorphous forms have also been observed at various pressures and temperatures5,6. It has been proposed that LDA and HDA are glassy forms of low-density liquid (LDL) and high-density liquid (HDL), respectively, in a liquid-liquid transition scenario1,7,8,9,10, if these liquids would exist as distinct phases. There are many crystalline phases that resemble some of the amorphous forms, but much higher density ices can also be formed under pressure; e.g., ice VII has a density 74% higher than hexagonal ice Ih11. All of these forms of crystalline ice phases formed at modestly high pressures, as well as amorphous or glassy water, typically have structures in which there is an inward collapse of the 2nd coordination shell. Similarly, water under pressure at room temperature responds through a continuous collapse of the 2nd shell with increasing pressure12 where the extrapolated pair-correlation function (PCF) of HDL water has a broad 2nd-shell distribution in between those of HDA and VHDA2; a similar structural rearrangement has been observed in simulations of water in silica nano-confinement13. Water thus seems to have the ability to exhibit rather dramatic structural differences with a large range of densities and only minor differences in enthalpy1. Here we demonstrate that it is possible to generate an extremely compressed HDL-like form of water at ambient conditions at the BaF2(111)/water interface, which acts as the pressurizing medium.

The influence of the substrate on interfacial water has often been discussed in terms of ice-like and liquid-like structural signatures14,15,16,17. In this respect, BaF2(111) terminated by F−-Ba2+-F− trilayers, with a surface lattice parameter of 4.39±0.1 Å, is very close to that of the basal plane of ice Ih (4.52 Å). Thus, an epitaxial layer of ice growth is expected to form on BaF2(111) at ambient conditions. Although earlier optical absorption experiments suggested the formation of ice-like overlayers under low temperature18 and ambient conditions19, theoretical estimates of the stability of the epitaxial ice layers are not consistent with these experimental observations20. Also in nano-confinement Raman scattering experiments on water have found a non-freezing water layer at the water/solid interface down to extremely low temperatures21. Arguments about the stability of epitaxial wetting layers based on molecular dynamics simulations22 have not been corroborated because of the effects of surface defects23,24,25. Theoretical calculations20 using the ASP-W2K potential find an adsorption energy of −39.78 kJ/mol for an isolated water molecule on BaF2. Higher coverages resulted in a bilayer hexagonal ice film with an increase of the adsorption energy to −51.4 kJ/mol, which, albeit higher than the enthalpy of condensation of bulk water (−44 kJ/mol), demonstrates only a weak trend towards wetting.

In the present study, the molecular structure of thin-film water under ambient conditions (up to 2.4 monolayers (ML)) as well as ice under low-temperature conditions in ultra-high vacuum (UHV) were elucidated on BaF2(111) using x-ray absorption spectroscopy (XAS) and x-ray photoelectron spectroscopy (XPS). O K-edge XAS is a powerful local probe for identifying the structures of water in solid and liquid phases; the spectral features are directly related to the hydrogen-bonding environments26. The experimental data are complemented with molecular dynamics (MD) simulations, where the first interfacial layer indicates a very high local density (1.23 g/cm3) and the O-O pair-correlation function shows a near-complete collapse of the 2nd shell, well beyond what is seen in HDL and close to merging with the first shell to form a two-dimensional hexagonal structure.

Results

In order to characterize the structure of the thin water films we used O K-edge XAS to obtain spectral signatures indicative of specific hydrogen-bonding configurations. The obtained spectra of thin layers of liquid water, however, do not exhibit the expected templating effect resulting in hexagonal ice, but instead show strong similarity to high-pressure ices, such as ice VII with a density of 1.6 g/cm3. XAS spectra measured at 1.5 Torr water at 30% (288 K), 60% (270 K) and 90% (259 K) relative humidity (RH), with the polarization vector of the incident x-ray out-of-plane and in-plane, are shown in Figs. 1(a and 1(b)), respectively. These films correspond to 1.2, 1.8 and 2.4 ML coverage of water (see Supplementary Information for details). At all RHs studied, the XAS spectral features are rather similar: strong pre-edge and weak post-edge features are observed, indicating that, irrespective of the polarization vector orientation and the temperature (~300–259 K), the structure of thin-film water on BaF2(111) does not depend on coverage. At 30% RH, a monolayer of water forms, as seen in the uptake measurements.

Figure 1
figure 1

XAS spectra of water on BaF2(111) at 1.5 Torr water pressure with the polarization vector (a) perpendicular and (b) parallel to the surface plane.

RH was varied from 30% to 90%. The incident angle (α) of in-plane polarized light and the take-off angle (θ) are ~10° and 35°, respectively.

Full size image

The pre-edge feature at 535 eV has been proposed to be mainly related to weakened H-bonds26,27 whereas the post-edge feature appearing at 540–542 eV is due to H-bonds and is particularly strong for tetrahedrally H-bonded water as in hexagonal ice (ref. 26,28 and discussions therein regarding various interpretations). The main-edge feature, at ~537 eV is enhanced upon formation of HDA29 and various crystalline high-pressure ice phases such as III, IVI, IVII and IVIII30. This enhancement is related to the fact that the second shell collapses from the tetrahedral distance of 4.52 Å in ice Ih to much shorter distances, leading to an overall higher density; note that this also slightly increases the pre-edge intensity.

In Fig. 2a, we compare the XAS spectrum of thin-film water on BaF2(111) at 90% RH with the spectrum of crystalline ice formed at low pressures (Ih or Ic ) on BaF2(111) and with the relative change between ice Ih and IVII30. The latter corresponds to a high-pressure crystalline ice phase with a density of 1.60 g/cm3. First, we note that the thin water film XAS spectrum lacks the post-edge feature which is seen in crystalline ice. Furthermore no low pressure ice-like spectral features were observed at any coverage as RH was increased from 30% to 90%, even at temperatures as low as 259 K. Considering the arguments of surface-templated ice formation, the templating effect of BaF2(111) on the structure of the water film is expected to be strongest under this condition. Secondly, we instead note that the water thin-film spectrum has a close resemblance with the high-pressure spectrum of ice IVII.

Figure 2
figure 2

(a) Comparison of XAS spectra of (i) bulk ice Ih (dashed), ice VII (IVII) (solid)30 with the spectra of (ii) 2 ML crystalline ice and (iii) supercooled water on BaF2(111) at 1.5 Torr at 259 K (90% RH). Crystalline ice on BaF2(111) was prepared by annealing 10 ML LDA to 175 K. XAS spectra of crystalline ice on BaF2(111) were recorded via exciting with in-plane polarized light. (b) Comparison of XAS spectra of (i) supercooled water on BaF2(111) at 1.5 Torr at 259 K (90% RH) with (ii, solid) liquid water and (ii, dashed) 6 M NaCl solution measured at room temperature31. The supercooled water spectrum is an average of two spectra obtained using in-plane and out-of-plane polarizations, shown in Fig. 1. The shaded area represents the difference between bulk ambient liquid and thin-film water. XAS spectra of liquid water and the 6 M NaCl solution were recorded in transmission mode31. The sample was embedded between two Si3N4 windows separated by a 300 nm thick spacer. All spectra are normalized by area.

Full size image

In Fig. 2b, we note that the XAS spectrum of thin-film water on BaF2(111) at ambient conditions more closely resembles the spectrum of liquid water than that of low-density crystalline ice on BaF2, as seen from the weak post-edge and strong main-edge and pre-edge features. Because the post-edge is enhanced for tetrahedral H-bonding configurations, we infer that low-density crystalline ice-like local arrangements are very sparse in the water layer on BaF2(111). This inference is quite surprising because the substrate temperature of 259 K is deep in the supercooled state of water and the driving force for tetrahedral structuring should be rather strong according to expectations from thermodynamics arguments that enthalpy and entropy of water films converge to values typical of ice19. Contrary to these enthalpy and entropy estimates from quantification of IR spectral features, the tetrahedral structure of ice for water films on BaF2(111) has not been found to be the most favorable based on theoretical calculations20 and polarizability measurements25.

Even more surprising is the observation that, due to the loss of intensity in the post-edge region, the spectral width spanned by the main-edge and post-edge peaks of supercooled water on BaF2(111) is somewhat smaller than that for liquid water. In Fig. 2b we also compare the thin-film water spectrum with the spectrum of 6 M NaCl solution at room temperature31 and note in the small post-edge a stronger similarity for thin-film liquid on BaF2(111) with the NaCl solution than with bulk water. In alkaline aqueous solutions the interaction of water with the alkali cations is weak26,31, while neutron diffraction measurements32 indicate that the resulting structure is still similar to that of water under pressure, which is also supported by MD simulations33. The close spectral similarity of the thin-film water spectrum and the spectra of the NaCl solution and IVII indicates that the water structure should resemble more a HDL-related structure as typically seen upon applying pressure on water with an inward collapse of the 2nd shell.

From the MD simulations, we find a strong layering of water molecules parallel to the surface. The inset of Fig. 3 shows a dramatic localization of the first water layer; the distribution of the molecules within subsequent layers is broader, but the influence of the 1st layer on the 2nd and 3rd layers is evident. To further determine the structure within these layers, we calculated oxygen-oxygen PCFs within layers of defined thickness perpendicular to the surface and compared the calculated PCFs with the experimental PCFs of HDL water34, NaCl solution31 and the PCF of a slab of bulk TIP4P/2005 water of the same thickness as for water on BaF2 (Fig. 3). The first three peaks in the TIP4P/2005 bulk water PCF represent the 1st, 2nd and 3rd coordination shells located at 2.75, 4.4 and 6.7 Å, respectively, in this model. The 1st layer of 1 Å thickness of thin-film TIP4P/2005 water on BaF2(111), on the other hand, shows a significantly altered structure compared to the 2nd and 3rd layers of the same thickness. The 2nd coordination shell of the 1st layer is collapsed as in the case of HDL34 and the 3rd peak shifted inwards to 6 Å, indicating that the 3rd coordination shell adapts to the high-density profile. The 2nd and 3rd layers of the thin film, on the other hand, show a structure similar to that of the bulk. For T = 260 K, the structure is slightly more pronounced than for T = 290 K, but does not change qualitatively. These findings are universal and do not change with the different force fields commonly utilized to simulate the properties of water (see Supplementary Information for details).

Figure 3
figure 3

Oxygen-oxygen pair-correlation functions (a) calculated within the first 1 Å layer of H2O on the BaF2(111) surface from MD simulations, full line T = 260 K, dotted line T = 290 K; (b) of HDL34; (c) of a NaCl solution of concentration 1:10 salt to water mole ratio44; (d) calculated within a 1 Å layer of bulk H2O from MD simulation at T = 260 K; (e) calculated within the second 1 Å layer of H2O on the BaF2(111) surface from MD simulations, full line T = 260 K, dotted line T = 290 K.

Note that the pair-correlation functions calculated within a 1 Å layer of water (a, d, e) are normalized differently than in (b, c), cf. Supplementary Information. Inset: Histogram of distances between the oxygen atoms of H2O and the surface atoms.

Full size image

In Fig. 4 we show a snapshot from the simulation with a strong predominance of chain-like conformations in the layer in contact with the substrate. In comparison to low density crystalline ice, the structure of the 1st layer is more similar to close packing at the expense of breaking H-bonds. The density in the 1st layer is estimated to be 1.23 g/cm3 (see Supplementary Information for details), which is similar to the density of VHDA; this similarity also fits well with the 2nd-shell distance of 3.3 Å1,4. However, there are more severe distortions from tetrahedral coordination in the 1st shell in comparison to VHDA which we attribute to the thermal fluctuations prominent near room temperature.

Figure 4
figure 4

A snapshot of the first water layer on BaF2(111).

F− and Ba2+ ions are color-coded by purple and green, respectively. Red and white sticks represent oxygen and hydrogen atoms of water molecules. Intermolecular hydrogen bonding and water-BaF2(111) bonding are indicated by dashed lines. H-bond definition: O-O distance < 3 Å, O-O-H angle <30 deg.

Full size image

Discussion

Previous surface x-ray scattering and diffraction studies of the water–silica35 and water–Cu(111)36 interfaces have shown that the first water layer has a structure with higher density than that of either ice or liquid water. A high-density form of interfacial water has also been identified for water on NaCl(001) in MD simulations37 as well as for water in nano-confinement13,21. In a recent study of the role of lattice mismatch in ice nucleation Cox et al.38 found that the angular flexibility in the second shell allowed closer packing at their model interface. These findings can now be interpreted in the context of the present results with the formation of a highly compressed, HDL-like phase at the interface. In fact, the HDL-like phase persists well below freezing without any signatures of ice formation in a way similar to the freezing point lowering seen in NaCl solutions39.

We have shown that the BaF2(111) surface does not act as an ice formation template for ambient and supercooled water films, notwithstanding the close lattice match. The XAS spectrum of water on BaF2(111), on the contrary, indicates limited tetrahedral H-bonding at 300–259 K, where the similarity to spectra of high density ices suggests a highly compressed HDL-like water structure even in the supercooled regime. The O-O pair-correlation function of the first water layer from MD simulations fully supports this high-density form of water at the interface.

Methods

Measurement techniques

Relative humidity-dependent water uptake curves were obtained using XPS to monitor the water coverage (see Supplementary Information for details). The BaF2(111) surface was exposed to 1.5 Torr water vapor and the uptake was carefully controlled by reducing the sample temperature in an incremental fashion. 2.4 ML of water was the maximum coverage at 90% RH and uptake curves are in good agreement with the ones measured using IR spectroscopy19.

The low temperature, UHV experiments on ice on BaF2(111) were performed at the soft x-ray Spectroscopy Beamline (BL 13.2) at the Stanford Synchrotron Radiation Lightsource (SSRL). X-ray absorption spectra were acquired by the total electron yield (TEY) method. Ambient pressure XPS and XAS experiments were performed at the Molecular Environmental Science Beamline (BL 11.0.2) at the Advanced Light Source (ALS). The differentially pumped electron energy spectrometer (Specs Phobios 150) enables measurements at pressures up to 5 Torr40. XAS data at 1.5 Torr H2O were acquired by the Auger electron yield (AEY) method using a kinetic energy detection window of 370–400 eV. Details of the sample preparation and data analysis are presented in figure captions and in the Supplementary Information.

MD simulations

MD simulations were performed to model the behavior of H2O on a fluorine-terminated BaF2(111) surface (256 BaF2, 500 H2O). We used the GROMACS package ver. 4.5.341, the OPLS-AA force field42 and the TIP4P/2005 water model43; results using alternative force-fields are discussed in the Supplementary Information. Each simulation was equilibrated for at least 5 ns, generic thermodynamic properties were monitored to ensure equilibration. The production runs were performed in the NPT ensemble for 20 ns at a time step of 1 fs using velocity rescaling to set the temperature; the Berendsen barostat was applied for the pressure. All simulation results shown here were obtained at 1 bar and 290 K or 260 K, respectively.