The Effect of the Incident Collision Energy on the Porosity of Vapor

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J. Phys. Chem. B 2009, 113, 4000–4007

The Effect of the Incident Collision Energy on the Porosity of Vapor-Deposited Amorphous Solid Water Films R. Scott Smith, Tykhon Zubkov, Zdenek Dohna´lek, and Bruce D. Kay* Fundamental and Computational Sciences Directorate, Pacific Northwest National Laboratory, P.O. Box 999, Mail Stop K8-88, Richland Washington 99352 ReceiVed: June 3, 2008; ReVised Manuscript ReceiVed: August 4, 2008

Molecular beam techniques are used to grow water films on Pt(111) with various incident angles and collision energies from 5 to 205 kJ/mol. The effect of the incident angle and collision energy on the porosity and surface area of the vapor-deposited water films was studied using nitrogen physisorption and infrared spectroscopy. At low incident energy (5 kJ/mol), the infrared spectra, which provide a direct measure of the surface area, show that the surface area increases with incident angle and levels off at angles > 65°. This is in contrast to the nitrogen uptake data, which display a maximum near 65° because of the decrease in nitrogen condensation in the larger pores that develop at high incident angles. Both techniques show that the morphology of vapor-deposited water films depends strongly on the incident kinetic energy. These observations are consistent with a ballistic deposition shadowing model used to describe the growth of highly porous materials at glancing angle. The dependence of film morphology on incident energy may have important implications for the growth of porous materials via glancing angle deposition and for the structure of interstellar ices. 1. Introduction It has been shown that the porosity and morphology of vapor-deposited materials can be controlled by varying the incident deposition angle. In particular, vapor deposition at large incident angles from normal can result in high surface area, highly porous materials.1-32 This approach has been used to grow highly porous films for a wide range of materials including amorphous solid water (ASW),9,13,15,16,20,31,32 Si,17,21-23,25,30 metals,1,2,4-6,8,10,11,14,19,24,27 andmetaloxides.17-19,27,29 The success of the technique for such a wide variety of materials is because the fundamental mechanism is based on a simple physical principle that can be described using a ballistic deposition model.3 The term “ballistic deposition” refers to a statistical growth model where incident particles (atoms or molecules) “stick” where they “hit”. If the incident particles “hit and stick”, then a simple shadowing picture can be used to understand the dependence of morphology on the growth angle. At normal incidence, an incoming particle sees a nearly unobstructed path to the surface creating a relatively dense, nonporous film. At glancing angles, however, random height differences that arise during the initial film growth can block incoming flux. This essentially creates shadows that result in void regions in the shadowed region. The length of the shadowed region is proportional to the tangent of the incident angle, and thus the film morphology is strongly dependent on the incident deposition angle. Once a height disparity is created, further deposition results in preferential growth in the direction of the incoming flux. In this way, porous films with filamentous columnar morphologies tilted in the incoming flux direction are created. Films with structures from nonporous to highly porous can be grown by simply increasing the incident deposition angle. Inherent in the above description is that incident particles truly do hit and stick or at least have limited mobility. If this were not the case, particle mobility/diffusion could act to fill * Author to whom correspondence should be addressed. Phone: (509) 371-6143; fax: (509) 371-6139; e-mail: [email protected].

the void regions thereby reducing the film porosity. We have previously shown that at a fixed incident deposition angle the porosity of a film decreases with increasing substrate temperature confirming that particle mobility can act to fill in shadowed void regions.15,16,18,20,24,29 In the present paper, we study the effect of the incident beam collision energy on the porosity of vapor-deposited amorphous solid water (ASW) films. Amorphous solid water is a metastable phase of water that can be formed by water vapor deposition on cold surfaces (30°, which is in agreement with our previous nitrogen uptake studies.9,15,16 Above this onset, the dangling bond intensity in Figure 4 continues to increase up to the maximum dose angle of 85°. As we show below in section 3C (and have shown previously9,15,16), these results are similar to our nitrogen uptake results up to 70° but differ at higher angles from the nitrogen uptake experiments, which have a maximum in the nitrogen uptake near 70°. The explanation for this difference is that the infrared and nitrogen uptake techniques are measuring different physical quantities: surface area in the case of IRAS and surface area and a contribution from pore condensation in the case of nitrogen physisorption. The reason for the decrease in the nitrogen uptake experiments relates to the ability of nitrogen to condense in pores and will be discussed later in this paper.

4004 J. Phys. Chem. B, Vol. 113, No. 13, 2009

Figure 5. The relative dangling bond intensity as a function of incident kinetic energy for 100 ML ASW films deposited at 23 K and at a deposition angle of 65°. The beam energy was varied from 5 to 83 kJ/mol. The relative dangling bond intensity decreases with increasing incident kinetic energy. This indicates that the incident energy does affect the ASW film morphology. The straight line is shown to guide the eye and is not based on any structural model. (Inset) The ratio of 2C to 3C dangling bonds as a function of incident energy. The ratio is not significantly affected by the kinetic energy of impinging H2O molecules.

The higher incident energy experiments in Figure 4 show a similar trend as the effusive beam data except that there is a shift in the onset of porosity to larger incident angles with increasing energy. In particular, the onset of porosity is shifted to 40-45° for the 25 kJ/mol, to 55-60° for the 83 kJ/mol, and to 70-75° for the 205 kJ/mol experiments. Also apparent is that at any given angle the porosity of the ASW films dramatically decreases with incident energy. This effect is best demonstrated in Figure 5 where the relative intensity of the integrated dangling OH stretch (2C and 3C) is plotted versus incident energy for films deposited at 65°. The data show that the relative dangling bond intensity decreases with incident kinetic energy. The inset in Figure 5 is a plot of the ratio of the 2C and 3C dangling bond intensities versus incident energy. These data show that the 2C/3C ratio remains relatively unchanged with incident energy despite the large change in film porosity. This suggests that while the overall morphology of the film is affected by the incident energy, the local surface structure remains unaffected. The infrared results presented in Figure 4 and Figure 5 clearly demonstrate that the incident kinetic energy has a dramatic effect on the morphology of vapor-deposited porous ASW films. C. Effects of Incident Kinetic Energy on ASW Film Morphology: Nitrogen Uptake Results. In this section, we use nitrogen uptake experiments to probe the effects of incident kinetic energy on ASW film morphology. The analogous set of experiments to those shown in Figure 4 is displayed in Figure 6. As was the case above, 100 ML thick films were grown at 23 K with incident kinetic energies of 5, 25, 83, and 205 kJ/ mol. In Figure 6, the saturation uptake of nitrogen is plotted versus deposition angle for incident angles from 0° to 85°. The effusive H2O beam (5 kJ/mol) results show the onset of nitrogen uptake (porosity) near 30°, which then increases with incident angle up to a maximum uptake near 65°. At this maximum, the uptake is 45 ML of N2 for the 100 ML film, which is in

Smith et al.

Figure 6. Nitrogen uptake as a function of growth angle for films deposited with incident kinetic energies of 5, 25, 83, and 205 kJ/mol. The 100 ML thick ASW films were grown at 23 K and at incident angles from 0° to 85°. At incident angles close to normal, the films grow dense with very little nitrogen uptake. At off-normal incident angles above ∼35°, the nitrogen uptake for the effusive beam data (5 kJ/mol) increases up to a maximum uptake near 65°. At higher kinetic energy, the onset of porosity is shifted to 40-45° at 25 kJ/mol, to 55-60° at 83 kJ/mol, and to 70-75° at 205 kJ/mol. Despite the shift, a maximum is still apparent in the higher energy data sets. At all incident angles, the porosity decreases with incident kinetic energy.

agreement with our previous nitrogen uptake studies.9,15,16 The maximum arises because the nitrogen uptake consists of two components: nitrogen adsorbed directly on the ASW surface and nitrogen condensed into pores. Condensation occurs, even though the adsorption temperature was set at 27 K to avoid multilayer formation (determined on a flat surface), because the concave curvature of the pores lowers the local vapor pressure as described by the Kelvin equation.15,16,20 At large incident deposition angles, the pore radii become larger and the lowering of the vapor pressure due to pore curvature is less resulting in the observed maximum in Figure 6. As mentioned above in section 3B, the analogous infrared results in Figure 4 do not have this maximum. The results in Figure 6 clearly show the effects of increased incident energy. In the higher incident energy data, there is a shift to larger incident angle for the onset of porosity. The observed shifts are the same as those observed for the corresponding energy experiments in Figure 4. Despite the shifts in the onset angle, the higher energy data sets also display maxima in the nitrogen uptake versus angle curves albeit these maxima are also shifted to higher angles. As was observed in the infrared experiments, the nitrogen uptake data show that the increased incident energy lowers the porosity of the ASW film. The effect of the incident energy on porosity is more transparent in Figure 7, which displays the normalized nitrogen uptake versus incident energy for various incident growth angles from 40° to 80°. In Figure 7, the nitrogen uptake data (minus 1 ML to account for the amount that would adsorb on a dense film) are normalized to the uptake of the effusive beam uptake at the same angle. At all incident angles, the porosity decreases nonlinearly with incident energy. The rate of decrease depends on the incident angle with the most rapid decrease observed for the 40° data and the slowest for the data at 85°. For incident angles of 65° and below, the normalized nitrogen uptake eventually approaches zero. Above 65°, however, the films maintain a residual amount of porosity even at the highest incident energy of 205 kJ/mol.

Effect of Collision Energy on Solid Water Films

J. Phys. Chem. B, Vol. 113, No. 13, 2009 4005 parameter is inversely related to the rate of decrease in porosity with incident energy. The E* parameter (solid circles) increases with incident angle up to ∼70° and then levels off for higher angles. The line through the data is intended as a guide for the eye to illustrate the saturation at higher angles. The E* dependence with incident energy essentially captures the rate of porosity decrease dependence on incident angle observed in Figure 7. Its overall angular dependence is similar to that of the surface area in Figure 4. The nitrogen uptake results presented in Figure 6 and Figure 7, in concurrence with the infrared data above, clearly show that the morphology of ASW films is strongly affected by the incident collision energy. 4. Discussion

Figure 7. Normalized nitrogen uptake as a function of incident kinetic energy for 100 ML ASW films deposited at 23 K and for deposition angles from 40° to 80°. The nitrogen uptake data (minus 1 ML to account for the amount that would adsorb on a dense film) are normalized to the uptake of the effusive beam results at the same angle. The beam energy was varied from 5 to 205 kJ/mol. At all incident angles, the porosity decreases nonlinearly with incident energy with the rate of decrease dependent on the incident angle. The lines are fits to the empirical equation, R + β exp(-E/E*), where E is the incident kinetic energy and R, β, and E* are fit parameters. The R parameter accounts for the relative porosity that persists at high incident energies, and the E* parameter is related to the rate of decrease in porosity with incident energy. The β parameter is a normalization parameter.

Figure 8. A double-Y plot of the parameters E* (circles, left-hand side) and R (squares, right-hand side) obtained from a fit of the data in Figure 7 to the empirical equation, R + β exp(-E/E*). The normalized nitrogen uptake versus incident energy was fit for each incident angle data set. The R parameter, which represents the extent of porosity that persists at high incident kinetic energies, increases exponentially with deposition angle. The E* parameter, which is inversely related to the decrease in porosity with incident energy, increases with deposition angle but levels off above 75°. The solid lines are fits intended to guide the eye.

The lines in Figure 7 are fits to the empirical equation, R + β exp(-E/E*), where E is the incident kinetic energy, and R, β, and E* are fit parameters. The obtained fit values for R and E* for each incident angle data set in Figure 7 are displayed in a double-Y plot in Figure 8. The R parameter (solid squares) is a measure of the relative porosity that persists at high incident energies. The value of R is near zero at incident angles e65° but increases for angles of 70° and above. The line through the data is a fit to an exponential, which confirms the rapid increase in R above 65°. This clearly shows that the degree of porosity that remains in the film, despite the high incident energy, increases roughly exponentially with incident angle. The E*

The infrared experiments in this paper provide important data to characterize the morphology of ASW films. These data complement the previous results and provide new insights into ASW film morphology that are not directly obtainable from other techniques (e.g., nitrogen uptake and optical interference). For comparison with the results of other techniques, we need to focus on the low incident energy (5 kJ/mol) experiments since the previous work was performed at that energy. For example, the ASW morphology as a function of thickness was studied using both infrared and nitrogen uptake experiments. The thickness dependence data displayed in Figure 3 show that, after some nominal dose (∼10 ML), there is a linear increase in the dangling bond intensity. The same thickness dependence was observed in the analogous nitrogen uptake experiments.9,16 In this case, the combined infrared and nitrogen results lead to the same conclusion that the overall ASW morphology is independent of thickness. The significance is that the structure is likely homogeneous throughout the film. The low incident energy infrared and nitrogen uptake results of the ASW morphology as a function of incident angle, however, yield slightly different results. The infrared data displayed in Figure 4 show that the dangling bond intensity increases with angle and eventually plateaus at very high angles. The nitrogen uptake data in Figure 6, and elsewhere,9,15,16,20 show an increase in uptake up to ∼65° and then a decrease at higher angles. This maximum has been explained as being due to the ability of nitrogen to condense into pores thus increasing the uptake capacity of the film.15,16,20 In a recent study, we observed that ASW films saturated with nitrogen have a significant fraction of the molecules that are not in a direct contact with the ASW surface, which indicates multilayer pore condensation.31 The extent of pore condensation depends on pore size (radius) as a result of vapor pressure lowering described by the Kelvin equation.15,16,20 For concave surfaces, the vapor pressure of an adsorbate decreases as the pore radius decreases. This is in contrast to the vapor pressure of a convex spherical drop whose vapor pressure increases with decreasing radius (Kelvin effect). For small pore radii, the adsorbate vapor pressure is lowered and condensation can occur. At large incident angles, the pores become larger with larger radii and thus there is less pore condensation at very high incident angles. This explanation is supported by ballistic deposition simulations15,16,20 and by the experimental observation that the angle where the maximum occurs depends on the adsorbate gas (e.g., methane, argon, etc.).15,16 Consequently, the maximum is not due to a decrease in film porosity at high incident angles but is due to the inability of an adsorbate to completely fill the pores above a certain growth angle. Confirmation for this interpretation comes from optical interference experiments where the film porosity (density) was found to increase monotonically with

4006 J. Phys. Chem. B, Vol. 113, No. 13, 2009 incident angle showing no maximum.20 The differences in the infrared and nitrogen uptake experiments arise because the two techniques are not measuring the same physical property. The uptake experiments measure both surface area and pore volume whereas the infrared data measure only the surface area. The infrared experiments in Figure 4 provide a measure of the “true” film surface area. The data show that the surface area eventually plateaus at very high incident angle. This appears to be at odds with the optical interference results, which show a monotonic increase in porosity with incident angle.20 The plateau in surface area was first observed in model simulations where it was found that at angles >65° there was no significant increase in film surface area.20 The explanation obtained from the simulations was that spacing between individual filaments becomes larger as the incident angle is increased. This results in larger void volumes and hence more porosity. However, because the interfilament spacing is larger, there are fewer filaments. Since the total water dose is the same, there is a tradeoff between having fewer but longer filaments at higher angle and having more but shorter filaments at a lower angle. In this way, the surface area of the filaments can remain nearly constant as the porosity continues to increase. In a recent paper, we used the nitrogen condensation coefficient behavior to estimate the surface area of ASW films as a function of growth angle.31 In that work, it was observed that from 35° to 60° there was a rapid increase in surface area but that above ∼65° the surface area remained nearly constant. The infrared results presented here show the same behavior and provide a direct measure of the surface area and its dependence on growth angle. The combined infrared and nitrogen uptake results show that the morphology of ASW films depends on the incident kinetic energy. More specifically, the infrared data (Figure 4) show that the internal surface area decreases while the nitrogen uptake data (Figure 6) show that the porosity decreases. These results are consistent with the fundamental shadowing mechanism proposed for the growth of porous materials at glancing deposition angles. This mechanism is based on a hit and stick scenario. The increased kinetic energy may allow particles to avoid sticking on the first collision and instead to move to fill in void regions. These results lend credence to the shadowing model description. While it is clear that the incident kinetic energy affects the morphology of ASW films, our prior study showed that it had no effect on its crystallinity or crystallization kinetics.62 In that paper, the lack of an effect was attributed to the collective and cooperative motion that may be required for crystallization. In the present work, such cooperative motions are not required for a single incoming particle to translate or diffuse once it has collided with the surface. Despite the dramatic effect the incident kinetic energy has on the film morphology, the data in the inset of Figure 5 show that the local bonding structure remains unaffected. These data showed that the ratio of 2C to 3C dangling bonds remained relatively unaffected by the incident energy. This suggests that while a particle with higher kinetic energy can translate further along the surface, the final bonding configuration is dominated by local particle surface interactions. These experimental results are consistent with a theoretical study by Batista and Jo´nsson.81 They found that admolecules on a crystalline ice surface prefer to sit in noncrystallographic positions, that is, those positions not in registry with the crystalline ice structure. This is because an isolated absorbed water molecule can maximize its hydrogen bonding to the underlying crystalline layers by adopting a noncrystalline configuration. They argue that water molecules impinging at

Smith et al. normal incidence are easily directed to these lower energy positions by long-range interactions that are relatively unaffected by the velocity (kinetic energy) of the incoming molecule. This concept is consistent with a combined experiment and theory paper that showed that the sticking of water on ice at normal incidence was independent of incident kinetic energy.65 Such substrate/adsorbate interactions are also likely to direct incoming water molecules to these noncrystalline positions after the molecule has lost some of its kinetic energy by translating along the surface for some distance. These results also support the idea that a concerted multimolecule process is necessary to rearrange the local as-deposited structure into a crystalline configuration. 5. Summary In summary, infrared spectroscopy and nitrogen uptake have been used to study the effects of the incident kinetic energy on the morphology of vapor-deposited water films. At low incident energy (5 kJ/mol), the infrared spectra provide a direct measure of the surface area and show that the surface area increases with incident angle and then levels off at angles >65°. This is in contrast to the nitrogen uptake data, which display a maximum near 65° because of the decrease in nitrogen condensation in the larger pores that develop at high incident angles. Both techniques show that the morphology of vapor-deposited water films depends strongly on the incident kinetic energy. These observations are consistent with a ballistic deposition shadowing model used to describe the growth of highly porous materials at glancing angle. The dependence of film morphology on incident energy may have important implications for the growth of porous materials via glancing angle deposition. Effusive metal evaporators are often used to deposit metal atoms for the growth of metal and metal oxide porous films. The temperature needed to sublimate a metal will vary with the material and, as such, so will the average kinetic energy of the incident metal atoms (Eeffusive ) 2kT).68 For example, in a recent paper, the specific surface area of porous TiO2 films deposited at 70° was found to be only ∼100 m2/g.29 This is much less than the ∼1000 m2/g measured for porous MgO films grown in a similar way.18 It was suggested that this discrepancy might be due to differences in the incident energy as a result of the different temperatures used in the Knudsen evaporation cells (1953 K for Ti and 600 K for Mg).29 The work here supports that hypothesis and shows that the incident deposition energy needs to be considered when synthesizing other porous materials via vapor deposition at offnormal angles. Another area where the incident energy may have important implications is in the porosity of interstellar water ices. It is thought that these water ices have some degree of porosity because of the low temperature of the interstellar environments.51-61 Our results show that, depending on the growth mechanism, the incident kinetic energy will affect the ice morphology. For example, less porosity may result in interstellar ices that are created via a mechanism where water is deposited onto particles moving at high velocities. Acknowledgment. This work was supported by the U.S. Department of Energy (DOE), Office of Basic Energy Sciences, Chemical Science Division. The experiments were performed at the W. R. Wiley Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory, which is operated for DOE by Battelle.

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