Study of Water Droplets and Films on Graphite by Noncontact

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Study of Water Droplets and Films on Graphite by Noncontact Scanning Force Microscopy M. Luna, J. Colchero,* and A. M. Baro´ Lab. NueVas Microscopı´as, Depto. Fı´sica de la Materia Condensada, C-III, UniVersidad Auto´ noma de Madrid, 28049 Madrid, Spain ReceiVed: April 19, 1999; In Final Form: August 23, 1999

The scope of the noncontact scanning force microscopy technique concerning the analysis of fragile and weakly attached samples is shown by proving its ability to investigate the water-graphite interface. After a macroscopic quantity of purified water has been shacked out from the graphite surface, a noncontact image taken in air at a relative humidity value of 60% reveals nanodroplets attached to the steps. In a high relative humidity atmosphere (>90%), water adsorbs on the surface forming flat rounded islands of 5 nm in height that transform to 2 nm high islands when the relative humidity stabilizes to 90%. This process is induced by the presence of the scanning tip. Desorption of the water present on the surface is achieved after the exposure of the sample to a dry atmosphere for several hours. The adsorption-desorption cycle is reversible.

Introduction One of the most important features of scanning probe microscopy is its ability to work at air atmospheric pressure,1 and other environments, which in particular include several liquids.2 An interesting application of this property is to study the liquid itself in order to obtain information about the solidliquid-vapor interface. A particularly interesting case is that of water. Different reasons make the study of the water layer attractive: (i) the polar character of water; (ii) its tetrahedral structural arrangement associated with the importance of hydrogen bonding; (iii) its crucial role in our day life; (iv) its importance as a medium in biological material. Several studies of water adsorption at low temperatures and ultrahigh vacuum (UHV) have been performed in the past, such as, for example, the excellent review that Thiel et al.3 report; this includes surface structure studies by scanning tunneling microscopy.4 Adsorption of water on graphite has been studied at low temperature and UHV by vibrational spectroscopy (HREELS) and thermal desorption.5 At low temperature (85 K), H2O is adsorbed associatively on graphite via hydrogen bonding; for low coverage, the vibrational modes show the presence of H-bonded networks. The possibility of formation of 3D clusters is also suggested. All these studies, however, treat typically the adsorption properties of H2O in the monolayer regime, and/or the growth of a limited number of multilayers. In the present paper our aim is to deal with a mesoscopic liquid film in an environmental ambient vapor pressure. Such studies have been previously performed by using optical techniques such as interferometry6 and elipsometry7 and by the surface force apparatus.8 All these techniques are limited, however, by their poor lateral resolution. Thus, to increase our knowledge of water adsorption and mesoscopic liquid formation, we propose to use microscopic techniques, which are able to give a good resolution both laterally and vertically. From that respect, scanning force microscopy (SFM) is an excellent candidate, provided that the interaction force between tip and sample is carefully controlled * Corresponding author. Fax: 00-34-91-397-3961. E-mail: [email protected]

and understood. Because of the soft character of the liquidvapor interface, the force needs to be kept to a minimum value. For that purpose, the first reported study of liquid films by SFM techniques was performed with the so-called scanning polarization force microscopy (SPFM).9 It consists of the application of an electric voltage to the tip that induces an electrostatic force between tip and sample. Because of the long-range character of the force, the imaging of the liquid layer is taken at a distance of the order of 10-30 nm, avoiding the problem of damage of the liquid, but at the expense of loosing resolution. This technique, however, is not free of other problems: the analysis of the water layer thickness depends on the knowledge of the dielectric constant of water, which might not necessarily be the same as the macroscopic value; in addition, the electrostatic interaction of the particular tip-sample geometry is not easy to model. Finally, it is not clear how the existence of an electric field could interact with the strong dipole moment of H2O and with ionic species commonly present in both liquid and solid phases which might modify the measured data. To avoid these problems, we have performed a SFM study of liquid water without applying any electrical voltage. As a consequence, tip-sample distance is shorter, but tip-sample interaction has to be kept to an absolute minimum. To be more precise, the interaction of the liquid with the tip has to be much lower than its interaction with the substrate. Therefore, if liquids are to be imaged by SPM, the interaction between tip and sample used for the imaging process should be some weak van der Waals force (that is, a van der Waals force at a rather large tip-sample distance) or some other even weaker interaction. Consequently, any imaging mode based on mechanical contact between tip and sample is not suited for imaging liquid structures, as we will show in this paper. In principle, also the so-called “intermittent contact mode scanning force microscopy” (IC SFM),10 which has been used successfully on many “soft” but solid samples, should not work for the application discussed here, since, as the name indicates, the tip seemingly touches the surface during a short period of time.11 However, quite recently, imaging of liquid drops by IC SFM has been reported.12 Moreover, in a different work,13 we have shown that for a certain

10.1021/jp991256y CCC: $18.00 © 1999 American Chemical Society Published on Web 10/21/1999

Water Droplets and Films on Graphite

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range of experimental parameters the tip does not touch the surface in the so-called “intermittent contact mode”. In the present work, we extend this preliminary study to show that an intermittent “noncontact” mode can be used to image liquid structures adsorbed on solid surfaces under ambient conditions. The crucial point is the development of a dissipative interaction at very short distances, which allows controlling the tip-sample gap without mechanical contact. Tip-Sample Interaction To accurately adjust imaging conditions and for a correct interpretation of experimental data, it is fundamental to determine precisely the tip-sample interaction, since the corresponding signal is used for feedback and is thus basic for the imaging process. This is true for any SPM-based technique and even more important on delicate samples such as the ones discussed in this work. Therefore, in this section we will discuss in some detail aspects of tip-sample interaction which we consider relevant for the imaging mode we have used in our experiments. As discussed above, it has been recently shown that for a certain range of experimental parameters it is possible to avoid mechanical contact between tip and sample during tip oscillation in high-amplitude vibration SFM when working under ambient conditions.13 In another work, we found that, as the tip approaches the sample, a liquid meniscus forms spontaneously even before tip and sample are in mechanical contact.14 With soft cantilevers (90% RH), an image of the surface in noncontact mode presents initially flat rounded islands of several sizes in diameter (300-600 nm) and a height of 5 nm (Figure 3a). If we take a sequence of consecutive images in these conditions, we observe a movement of the islands and a tendency to coalesce and form larger islands. The easy movement of the 5 nm islands, even if enhanced by the scanning motion of the tip, indicates a low activation for diffusion, which could be interpreted as being due to a fluidlike character of the layer. Moreover, the area of the image covered by the islands increases with the scanning time, probably due to the presence of the scanning tip. Another interesting feature observed in our experiments is the formation of a second layer of 2 nm in height (Figure 3b), which grows at the expense of the previous one of larger height. The RH at this stage was kept constant at a value of about 90%. In fact, the evolution of the layers is to form large islands of 2 nm in height, which eventually have little patches of 5 nm in height. We believe that the tendency of water islands under these conditions is to transform to 2 nm high islands, since this value remains constant with time once it is produced. Unlike the 5 nm high layer, that of 2 nm presents a high activation barrier for diffusion, and its perimeter is preferentially oriented along

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Figure 3. (a) SFM noncontact image showing initial flat rounded islands of several sizes of 300-600 nm in diameter with a height of 5 nm at a RH of more than 90%. (b) Image of the same area as before but taken a few minutes later, when the RH has stabilized at 90%. An island of a lower height (2 nm) grows at the expense of the previous one of larger height. (c) Phase contrast image taken simultaneously to part b. The phase contrast is higher on the 5 nm in height island.

the high-symmetry directions of the graphite crystal. We tentatively interpret that as an indication of a solidlike character of the layer.

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Figure 4. SFM noncontact image (∼90% of RH) taken after 2 h of nonstop scanning on an area close to the area of Figure 3b. It can be clearly observed that the 2 nm high islands are oriented with respect to the directions of the graphite surface. This suggests that the 2 nm high layer grows epitaxially on top of graphite.

Both the 5 nm and the 2 nm high islands completely disappear from the surface after some hours when the RH decreases to very low values (2%). This process is reversible, since after an increase of the relative humidity to the previous value (>90%), the 5 nm thick islands appear again. As reported in the previous section, we verified that a contact image taken at the same place where islands were present (parts a and b of Figure 3) showed only a bare graphite surface, and the same result was obtained in a subsequent noncontact image. Since our microscope is able to show two simultaneous images, we could also measure the image of the phase output of the lock-in amplifier (Figure 3c), together with the topography. The phase output is higher on the water island than on the graphite substrate, and it is dependent on the height of the water layer, being larger on the 5 nm island. It is also worth noticing that, while the height contrast of the thicker island compared to the thinner one is 2.7, the phase contrast is 5 times higher. The time evolution of the water layer is certainly influenced by the tip interaction during scanning. This is seen in the image shown in Figure 4, where after 2 h of continuous scanning on a region close to the area of Figure 3b, at 90% of RH, the 2 nm high islands are perfectly oriented along the high-symmetry directions of the graphite surface. Moreover, a tendency to form narrow elongated islands along these directions is clearly seen. Such an effect does not occur without scanning the surface. One might wonder why water could be ever observed on a highly hydrophobic surface like graphite. The evidence comes from the following considerations. First, there is a definite correlation between the observation of water films and the value of the relative humidity, i.e., the formation of liquid water occurs at a high RH value, and once it is formed it disappears when RH decreases; moreover, this is a reversible process. Second, water films and droplets are only observed in noncontact imaging, and they disappear completely in contact imaging. Nevertheless, we believe that the presence of the scanning tip produces the condensation of water on the surface, because the water islands preferentially appear on the scanned area. In addition, after carefully decreasing the tip-sample distance though keeping the tip-sample system in the noncontact regime15 we observe more water on the scanned surface.

Luna et al. We can speculate whether the islands of 2 nm in height are closely connected with the structural properties of water adsorbed on graphite. For that it is worth introducing here the structure of hexagonal Ih ice. Water molecules form bilayers of 0.324 nm thickness, growing above the substrate surface. To obtain 2 nm in height we would need six bilayers. Another point to analyze is the fit of the bilayer to the substrate, in this case, the graphite surface structure. For that purpose we need to study the lattice structures of both substances (water/ice and graphite) and to compare their lattice parameters. If both parameters approximately match, the adsorption of water molecules in a crystalline form could take place. In the ice-like structure, the nearest oxygen-oxygen distance is 0.274 nm. However, they are raised (or lowered) with respect to the vertical direction by 0.048 nm. Thus, the closest oxygen neighbors within the same horizontal plane are the next-closest neighbors, which are at a distance of 0.45 nm. For adsorption on a substrate, this is the relevant distance because the oxygen atoms that are adsorbed to the surface are the ones within the same horizontal plane. On the other hand, the graphite has an elongated hexagonal unit with a horizontal lattice parameter of 0.246 nm. The corresponding (x3 × x3)R30° superstructure has a lattice constant of 0.426 nm. Therefore, water molecules arranging in a (x3 × x3)R30° superlattice would have a mismatch of about 5% with respect to the normal ice lattice. This lies in the lower limit where a similar ordered structure has been observed in UHV studies.3 The fact that we observe two distinct heights of water layers at high humidity, one of 5 nm, the other of 2 nm, is also significant. The two heights could be attributed to the existence of two different water phases on graphite. An effective surface potential having two minima could explain this.6,21 We expect such a situation to happen if the water layer is subject to short range and long-range interactions. Then, each phase would produce a different thickness of the water layer: one corresponding to the long range, a thick layer, whereas the other one corresponding to the shortest range, a thin layer. We can even be more explicit, in the sense that the short-range interaction could be due to the binding to the graphite substrate. This is confirmed by the 2 nm high layer ordered along the high-symmetry directions of graphite, which we suggest to correspond to a solidlike behavior. This hypothesis is further supported by the high value of surface diffusion of the 5 nm islands (fluidlike behavior), in contrast with the 2 nm islands, which become attached to specific directions of the substrate (solidlike behavior). Finally, the phase value measured on the two layers of different thickness gives additional evidence for the existence of the two different phases, even if we do not know the exact physical meaning of this quantity. The SPFM experiments performed by Xu et al.22 show that water adsorbs on mica forming flat layers. The data are interpreted in terms of two water phases, I and II. Also, the frontier of the two different phases follows the high-symmetry directions of the mica crystal underneath. The height of these phases is, however, the same, i.e., one monolayer, though we believe that the measurement of the height by SPFM might be questionable. In our case, the vertical distance measurements are closely related to the real topography. We have verified that the height of a biatomic graphite step gives the correct value of about 0.7 nm. Other authors have already reported high water layer thickness values. Because of the importance of hydrogen bonding, water presents 3D clusters even at low coverage. This is due to the highly associative character of the water molecules, which is well-known and also observed in our experiments.

Water Droplets and Films on Graphite Another important difference between our experiments and those performed by Xu et al. is the substrate that has been used, HOPG versus mica. It is known that water adsorbs uniformly on the mica surface due to its highly hydrophilic character. On the other hand, adsorption on graphite only happens at very high RH, induced by the scanning with the tip, forming isolated water islands. A recent study has been reported6 about the patterns formed by evaporation of thick water films on mica substrates. The films are observed optically by interference microscopy, and a thick layer is found to thin uniformly until it reaches a few tens of nanometers, where an instability, described as dewetting, occurs. Two different phases appear at this point: one thick layer with a fluid nature and a thin layer with a nonfluid character that the authors suggest to be of 2 nm in height. These data together with the present work could indicate that this scenario concerning two phases is quite general, so that our results on graphite by SPM, which have a good resolution both laterally and vertically, could probably be extended to other substrates, including particularly the case of mica. That is surprising, since the properties of mica and graphite are quite different, but could correspond to some kind of universality. This idea has been advanced in the literature and it is related to the existence of a correlation length of the surface layer. When this length increases, molecular interactions lose their local character and specificity. Summary and Conclusions In summary, we have described a noncontact SFM method suitable to image weakly attached, soft samples or liquids such as water on graphite. This method offers real topographic values for samples of homogeneous composition and does not require either metallic tips or samples. From the purified water adsorption study on a graphite surface, several results are obtained. After a macroscopic purified water droplet has been removed from a graphite surface, a noncontact image taken at 60% RH reveals nanosize water droplets attached to the steps. The contact angle is smaller than the one expected for macroscopic droplets. On the other hand, at high relative humidity (g90%), water condenses on the graphite surface under the influence of the scanning tip. The water adsorbs, forming flat round islands of 5 nm in height that transform to flat islands

J. Phys. Chem. B, Vol. 103, No. 44, 1999 9581 of 2 nm at about 90% RH. The higher diffusion coefficient together with their rounded contour and a higher value of the phase signal lead to the interpretation of a more “liquidlike” nature for the higher aggregates. On the other hand, the lower ones would present a more “solidlike” structure. Desorption of the condensed water is completed by drying the ambient atmosphere for several hours, and this is a reversible process. Acknowledgment. This work was supported by the Spanish DGES through the Project PB95-0169. We thank J. Go´mezHerrero, J. J. Sa´enz, L. Mederos, C. Rasco´n, and P. J. de Pablo for fruitful discussions. M. Luna acknowledges financial support from the Ministerio de Educacio´n y Cultura, and J. Colchero a fellowship from the Comunidad de Madrid. References and Notes (1) Baro´, A. M.; Miranda, R.; Alama´n, J.; Garcı´a, N.; Binnig, G.; Rohrer, H.; Gerber, Ch.; Carrascosa, J. L. Nature 1985, 315, 253. (2) Sonnenfeld, R.; Hansma P. K. Science 1986, 232, 211. (3) Thiel, P.; Madey, E. Surf. Sci. Rep. 1987, 7, 211. (4) Morgenstern, M.; Michely, T.; Comsa, G. Phys. ReV. Lett. 1996, 77, 703. (5) Chakarov, D. V.; O ¨ sterlund, L.; Kasemo, B. Langmuir 1995, 11, 1201. (6) Lipson, S. G.; Samid-Merzel, N.; Tannhauser, D. S.; Europhys. News 1998, May/June, 116. (7) Heslot, F.; Fraysse, N.; Cazabat, A. M. Nature 1989, 338, 640. (8) Israelachvili, J. N. Acc. Chem. Res. 1987, 20, 415. (9) Hu, J.; Xiao, X.-D.; Ogletree, D. F.; Salmero´n, M. Science 1995, 268, 267. (10) Zong, Q.; Immiss, D.; Kjoller, K.; Elings, V. B. Surf. Sci. 1993, 290, L688. (11) Tamayo, J.; Garcı´a, R. Langmuir 1996, 12, 4430. (12) Herminhaus, S.; Fery, A.; Reim, D. Ultramicroscopy 1997, 69, 211. (13) Luna, M.; Colchero, J.; Baro´, A. M. Appl. Phys. Lett. 1998, 72, 3461. (14) Colchero, J.; Storch, A.; Luna, M.; Go´mez-Herrero, J.; Baro´, A. M. Langmuir 1998, 14, 2230. (15) Luna, M.; Colchero, J.; Baro´, A. M. To be published. (16) NanoTec Electro´nica S. L.; C/ Padilla, 3-1° Dcha. Madrid. (17) Testo 610 ((2% of error). Instrumentos Testo S. A. (18) Olympus Opt. Co. LDT, 2-3 kuboyama-cho Hachioji-shi, 192 Tokyo. (19) Hu, J.; Xiao, X-d.; Salmeron, M. Appl. Phys. Lett. 1995, 67, 476. (20) Rieutord, F.; Salmero´n, M. J. Phys. Chem. 1998, 102, 3941. (21) de Gennes, P. G. ReV. Mod. Phys. 1985, 57, 827. (22) Xu, L.; Lio, A.; Hu, J.; Ogletree, D. F.; Salmero´n, M. J. Phys. Chem. , 1998, 102, 540