An infrared spectroscopic study of water thin films on NaCl (100)

Surface Science 427–428 (1999) 102–106

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An infrared spectroscopic study of water thin films on NaCl (100) Michelle Foster, George E. Ewing * Department of Chemistry, Indiana University, Bloomington, IN 47405, USA

Abstract Using infrared spectroscopy, we were able to determine that water adsorbs onto the surface of NaCl (100) in a liquid-like thin film over the temperature range of −27°C to 40°C. Photometric methods allow coverages to be monitored from near 0.1 monolayer to over three monolayers. The shifting band center of the OH stretching vibration absorption indicates changing hydrogen-bonding environments with coverage. Below a monolayer the band center is blue-shifted relative to liquid water absorption, suggesting some sort of unique hydrogen bonding arrangement. In the region between one and two monolayers, the coverage and change of band center with relative humidity appears discontinuous. Coverages near two monolayers produce spectra essentially indistinguishable from that of a saturated salt solution, suggesting a liquid-like hydrogen-bonded network, even at temperatures as low as −27°C. A model for thin film water growth on NaCl (100) is proposed. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Adsorption isotherms; Alkali halides; Amorphous thin films; Infrared adsorption spectroscopy; Photon adsorption spectroscopy; Physical adsorption; Single crystal surfaces; Water

1. Introduction Thin films of water coat many surfaces under ambient conditions. These films are important to both chemical and physical processes, yet very little is understood about their properties on a molecular level. We use transmission Fourier transform infrared (FTIR) spectroscopy to study the properties of these thin water films on a model ionic insulator surface, NaCl (100). The thin films are prepared in equilibrium with water vapor at carefully defined pressures and temperatures. Frequency shifts, bandwidths and absorbance values of the water features are all used to interpret the structure and phase of the interfacial region, * Corresponding author. Fax: +1 812-855-8300. E-mail address: [email protected] (G.E. Ewing)

giving us both a qualitative and a quantitative understanding of water adlayers. Previous work with this system includes the electrical conductivity measurements of Hu¨cher et al. [1] and a variety of atomic force microscopy (AFM ) step mobility studies [2–4], both collected as a function of relative humidity (RH ) at room temperature. The Hu¨cher et al. study [1] found that for RH up to 40% there is an exponential growth in the conductance. Between 40 and 50% RH there is a more gentle increase and above 50% RH there is a jump in conductance. The AFM step mobility measurements also find three distinct regions of surface change with water vapor pressure. Below 40% RH there is no step mobility, yet above 40% RH the step velocity increases with increasing humidity until approximately 75% RH when the step structure disappears with the onset

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of dissolution. Our FTIR measurements and the structural information they contain fill in the picture of adlayer changes with RH, suggesting a possible mechanism for water thin film growth on the NaCl (100) surface.

2. Experimental The FTIR spectrometer and the vacuum line have been described elsewhere [5,6 ]. For these spectroscopic measurements as a function of temperature and pressure, 11 crystals, cleaved along the (100) faces of NaCl (Harshaw Bicron), were placed in a cylindrical stainless steel cell. Spacers made of 0.1 mm diameter Ta wire were sandwiched between the crystals so that the NaCl faces did not touch. Wedged silicon windows (Infrared Optical Products) were attached to both ends of the cell, and sealed with Viton O-rings. Copper tubing coiled around the exterior of the cell was attached to a re-circulating bath from a variabletemperature unit (Julabo, FP50), which was used to alter the temperature of the crystals within the range −27 to 40°C and maintain their temperature within ±1°C. Temperature was monitored via a thermocouple within the cell. Evacuated chambers were attached to either side of the main cell with CaF windows ( Infrared Optical Products) to ease 2 any temperature differential between the cell and room temperature. The water vapor was obtained from a liquid sample (Sigma, HPLC grade). Before experiments, the crystal temperature was allowed to equilibrate overnight. Water vapor was then introduced at the desired pressure, and the adlayer of water on the NaCl (100) face was monitored by transmission spectroscopy.

3. Results and discussion A typical absorption spectrum of adlayer water on NaCl (100) is shown in Fig. 1. This spectrum was taken with a water pressure of 0.2 mbar (35% RH ) and a temperature of −27°C. The broad feature at 3400 cm−1 corresponds to the OH stretching vibration of the adsorbed water. Even at this low temperature, which is significantly

Fig. 1. Infrared absorption of water adsorbed to NaCl (100) at −27°C and 0.2 mbar of water. The coverage is calculated to be H=1.5.

below the −20°C freezing point of a saturated NaCl solution [7], this feature is remarkably similar to that of bulk liquid water, centered at 3390 cm−1 [8]. Had this been a solid-like layer we would expect the peak frequency to be nearer to the ice peak frequency of 3250 cm−1 [9]; had it been a lattice gas we would expect the peak frequency to be nearer the gas-phase frequencies of 3756 and 3657 cm−1 [10]. For convenient comparison, the OH stretching frequencies of water in its various phases are shown at the top of Fig. 1. Since the absorption closely resembles that of bulk water we shall use its optical properties to estimate adlayer coverage. We make use of a modified Beer–Lambert relationship [11,12] for integrated absorbance of surface species given by nsS ˜ = : H2O . A 2.303

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˜ (cm−1) of the The integrated absorbance A adsorbed water was determined by numerical integration over the frequency range 3800– 2800 cm−1, S is the surface density of adsorbed HO water, and n is2the number of exposed NaCl (100) faces. The integrated cross-section, s: (cm/molecule), was determined from the measured frequency-dependent imaginary component k of the index of refraction of bulk water [8], using [13] s: =

4p r

P

kn˜ dn˜ ,

band where r is the molecular density of water at the given temperature. For water at room temperature, s: was found to be 1.4×10−16 cm/molecule. Once S is calculated, the coverage S is easily H2O H2O determined since S for the NaCl (100) face is NaCl 6.4×1014 ion pairs cm−2 [7]. Using this procedure, we have converted all integrated absorbances to coverages H. We realize that different hydrogenbonding arrangements can affect values. For sub-monolayer coverages the value for s: might be markedly different. Consequently, the coverage values we have reported are approximate. The water adlayer, shown in Fig. 1, using this method of analysis corresponds to a coverage of 1.5 monolayers. The adsorption isotherm for −12°C is shown in Fig. 2. The water coverage is plotted as a function of the water pressure at equilibrium within the cell. The closed circles in Fig. 2 represent adsorption measurements and the open circles are the desorption measurements. The highest pressure used in this series of experiments was 1.4 mbar. This was to preserve the crystals, which, at a temperature of −12°C, will start to dissolve at 1.8 mbar of water – the deliquescence point [7]. There are three distinct growth regions within the pressure range we monitored. Below 0.6 mbar, the growth is gradual and linear; a near vertical rise occurs near 0.8 mbar suggesting a discontinuity of the isotherm. After 0.9 mbar the isotherm rolls over. If we had taken the pressure out further, the growth would presumably continue to rise until the deliquescence point was reached, when water

Fig. 2. Adsorption isotherm of water on NaCl (100) at a temperature of −12°C plotted as a function of water pressure. The closed circles represent the adsorption of water on the surface while the open circles are the desorption.

would condense on the surface and bulk NaCl solution form. Except at very low coverages, where the noise level is most troublesome, the data indicate no significant hysteresis. The shape of this isotherm was similar for each temperature we monitored. In fact, if instead of plotting the isotherm as a function of water vapor pressure we use the RH, we see that the adsorption isotherms track each other almost exactly. This similarity can be seen in Fig. 3a. There is, however, a noticeable difference in the onset of the near vertical rise, which shifts with temperature; at the lower temperatures the rise occurs at a lower RH. Not only do the adsorption isotherms as a function of RH look similar irrespective of the temperature of the system, but so too does the band center, as seen in Fig. 3b. At low RHs the band is centered around 3520 cm−1. At the higher RH the peak frequency converges to that of the

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retical studies suggests that the hydrogen-bonding network involves less than a tetrahedral arrangement for sub-monolayer coverages. We now draw on our spectroscopic observations of thin film water on NaCl (100) at a variety of temperatures, together with the findings of others, to suggest a structure for the adlayer and a mechanism for its growth, as seen in Fig. 4. At submonolayer coverages we have two-dimensional islands or strings [17] forming on the surface involving a hydrogen-bonding network among the individual water molecules that is not tetrahedral. This accounts for the ~3500 cm−1 vibrational frequency. This is the region of the exponential growth of the conductance across the NaCl (100) surface seen in the earlier work [1]. At multi-layer coverages, there is three-dimensional growth on the surface with a low concentration of ions from the substrate being incorporated into the thin film. Here the hydrogenbonded network approaches a tetrahedral arrangement. This accounts for the convergence of the Fig. 3. (a) Adsorption isotherm of water on NaCl (100) plotted as a function of RH for temperatures of −12°C and 30°C. (b) Peak frequency shift of the OH stretch of water adsorbed on NaCl (100) as a function of coverage for temperatures of −12°C and 30°C.

bulk saturated NaCl solutions, i.e. 3420 cm−1 [14]. This shift in band center occurs at a slightly lower RH than the jump in coverage, as the comparison of Fig. 3a and b shows. A similar vibrational frequency shift in the hydrogen-bonded OH stretching vibration of water is seen with water clusters. Theoretical studies have shown that, as the sizes of the water clusters increase, the vibrational frequencies are red shifted towards that of bulk water or ice [15,16 ]. These calculations also produce structures that accompany the OH frequency changes. The hydrogenbonding network shifts from an arrangement with water molecules sharing one, two or three neighbors (small clusters) to tetrahedral structures ( large clusters). Thus, although our spectroscopy alone cannot uncover the structure of adsorbed water below one monolayer, a consideration of the theo-

Fig. 4. Proposed model for the growth of water adlayers on NaCl (100) at near ambient conditions.

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peak frequency to that of a saturated salt solution. These ions, lifted from defects on the surfaces, are suggested to be the carriers responsible for surface conductivity at this adlayer coverage [1], and their movement along the surface is believed to produce migration of steps seen with AFM [2–4]. At mid-coverages there is a transition occurring between two-dimensional and three-dimensional growth, and the jump in the isotherms suggests a coexistence region. The region of this jump, at 30– 45% RH, agrees with the previous work with both the conductance measurements [1] and the step mobility measurements [2–4], which both show changes occurring in this same RH range.

4. Conclusions Water reversibly physisorbs onto the NaCl (100) surface forming liquid-like thin films over the temperature range of −27 to 40°C. Adsorption isotherms show an abrupt jump in film thickness from sub-monolayer to multi-layer coverages within the range 30–45% RH. A similar shift in the band center of the OH stretching frequency suggests a change in the hydrogen-bonding network of the adsorbed water molecules within this same region. We speculate that this is a transition from two- to three-dimensional growth on the surface. Hydrogen bonding between adsorbed water molecules is occurring, even at the lowest coverages.

Acknowledgements This work was funded by the National Science Foundation under grants CHE95-05892 and ATM96-31838.

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