Detection of Novel Gaseous States at the Highly

at room temperature. In addition to the spherical-cap-shaped nanobubbles reported by many researchers, flat (quasi- two-dimensional, pancake-like) gas...
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Langmuir 2007, 23, 1778-1783

Detection of Novel Gaseous States at the Highly Oriented Pyrolytic Graphite-Water Interface Xue Hua Zhang,*,†,‡ Xiaodong Zhang,† Jielin Sun,† Zhixiang Zhang,§ Gang Li,† Haiping Fang,§ Xudong Xiao,*,| Xiaocheng Zeng,*,⊥ and Jun Hu*,†,§ Nanobiology Laboratory, Bio-X Life Science Research Center, College of Life Science and Biotechnology, Shanghai Jiaotong UniVersity, Shanghai 200030, China, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, P.O. Box 800-204, Shanghai 201800, China, Department of Physics, Hong Kong UniVersity of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China, and Department of Chemistry and Centre for Materials and Nanoscience, UniVersity of NebraskasLincoln, Lincoln, Nebraska 68588 ReceiVed August 2, 2006. In Final Form: October 25, 2006 We report a novel form of the gaseous state at the interface of water and highly oriented pyrolytic graphite (HOPG) that is induced by local supersaturation of gas. Such local supersaturation of gas next to the HOPG substrate can be achieved by (1) displacing an organic liquid with a gentle flow of water, (2) displacing cold water (∼0 °C) with a gentle flow of warm water (∼40 °C), or (3) preheating the HOPG substrate to ∼80 °C before exposing it to water at room temperature. In addition to the spherical-cap-shaped nanobubbles reported by many researchers, flat (quasitwo-dimensional, pancake-like) gas layers and nanobubble-flat gas layer composites (spherical-cap-shaped nanobubbles sitting on top of the quasi-two-dimensional gas layers) were detected. These entities disappeared after the system was subjected to a moderate level of degassing (∼0.1 atm for 1.5 h), and they did not form when the liquids involved in the aforementioned displacing procedure (to induce local supersaturation of gas) had been predegassed (to ∼0.1 atm). The stability and some physical properties of these newly found gaseous states are examined.

Introduction The properties of interfacial water have long been recognized to hold the key to the understanding of phenomena as diverse as protein folding, lipid aggregation, and chemical self-assembly of macroscopic objects.1 Recently, observations of accumulated gas at water-hydrophobic solid interfaces have raised new puzzles regarding the behaviors of water and attracted much attention from a wide audience of scientists.2-8 Gas at solid-water interfaces, in the form of nanobubbles, was initially proposed as a possible explanation for the mysterious long-range hydrophobic attractive force.9 Later the atomic force microscopy (AFM) force measurement between a hydrophobic surface and a hydrophobic colloid showed stepwise behavior in the force curves,10 which were attributed to the gas nanobubbles formed between the colloid probe and the substrate. Since then, many studies have provided various pieces of experimental evidence in support of the existence of nanobubbles on a * To whom correspondence should be addressed. E-mail: xuehuaz@ unimelb.edu.au (X.H.Z.); [email protected] (J.H.); [email protected] (X.X.); [email protected] (X.Z.). † Shanghai Jiaotong University. ‡ Current address: Department of Chemical and Biomolecular Engineering, The University of Melbourne, Melbourne, Victoria 3010, Australia. § Chinese Academy of Sciences. | Hong Kong University of Science and Technology. ⊥ University of NebraskasLincoln. (1) Ball, P. Nature 2003, 423, 25. (2) Attard, P. AdV. Colloid Interface Sci. 2003, 104, 75. (3) Jensen, T. R.; Jensen, M. O.; Reitzel, N.; Balashev, K.; Peters, G. H.; Kjaer, K.; Bjornholm, T. Phys. ReV. Lett. 2003, 90, 086101. (4) Tyrrell, J. W. G.; Attard, P. Phys. ReV. Lett. 2001, 8717, 176104. (5) Ishida, N.; Inoue, T.; Miyahara, M.; Higashitani, K. Langmuir 2000, 16, 6377. (6) Yang, J. W.; Duan, J. M.; Fornasiero, D.; Ralston, J. J. Phys. Chem. B 2003, 107, 6139. (7) Steitz, R.; Gutberlet, T.; Hauss, T.; Klosgen, B.; Krastev, R.; Schemmel, S.; Simonsen, A. C.; Findenegg, G. H. Langmuir 2003, 19, 2409. (8) Switkes, M.; Ruberti, J. Appl. Phys. Lett. 2004, 84, 4759. (9) Parker, J. L.; Claesson, P. M.; Attard, P. J. Phys. Chem. 1994, 98, 8468.

hydrophobic surface in water, including direct imaging of nanobubbles by tapping mode AFM and rapid cryofixation scanning electron microscopy (SEM).4-6,8 More recently, water layers with a reduced density of ∼15-20% were reported from reflectivity studies with neutrons and X-rays at the interface between bulk water and a hydrophobic surface.3,7,11,12 The thickness of the region with a reduced water density is around 1-5 nm, which can be explained by the presence of a thin gas layer.7,11,13 Apart from surface forces,14 the gas accumulated at the interface has evoked reconsiderations in many processes including hydrodynamics,15,16 the adsorption of biomolecules at surfaces,17 the formation of complicated crystal nanostructures,18 microboiling,19 and the design of microdevices.20 However, the spontaneous formation of nanobubbles on a surface immersed in water has always been doubted from thermodynamics.21 Moreover, the experimental observations of nanobubbles preexisting at the interface are often thought to be irreproducible or to be artifacts due to contamination.22-26 (10) Carambassis, A.; Jonker, L.; Attard, P.; Rutland, M. Phys. ReV. Lett. 1998, 80, 5357. (11) Schwendel, D.; Hayashi, T.; Dahint, R.; Pertsin, A.; Grunze, M.; Steitz, R.; Schreiber, F. Langmuir 2003, 19, 2284. (12) Koishi, T.; Yoo, S.; Yasuoka, K.; Zeng, X. C.; Narumi, T.; Susukita, R.; Kawai, A.; Furusawa, H.; Suenaga, A.; Okimoto, N.; Futatsugi, N.; Ebisuzaki, T. Phys. ReV. Lett. 2004, 93,185701. (13) Simonsen, A.; Hansen, P.; Klosgen, B. J. Colloid Interface Sci. 2004, 273, 291. (14) Christenson, H. K.; Claesson, P. M. AdV. Colloid Interface Sci. 2001, 91, 391. (15) de Gennes, P. G. Langmuir 2002, 18, 3413. (16) Dammer, S. M.; Lohse, D. Phys. ReV. Lett. 2006, 96, 206101. (17) Wu, Z. H.; Zhang, X. H.; Zhang, X. D.; Gang, L.; Sun, J. L.; Zhang, Y.; Li, M. Q.; Hu, J. Surf. Interface Anal. 2005, 37, 797. (18) Fan, Y. W.; Wang, R. Z. AdV. Mater. 2005, 17, 2384. (19) Thomas, O. C.; Cavicchi, R. E.; Tarlov, M. J. Langmuir 2003, 19, 6168. (20) Paxton, W. F.; Kistler, K. C.; Olmeda, C. C.; Sen, A.; St Angelo, S. K.; Cao, Y. Y.; Mallouk, T. E.; Lammert, P. E.; Crespi, V. H. J. Am. Chem. Soc. 2004, 126, 13424. (21) Eriksson, J. C.; Ljunggren, S. Colloids Surf., A 1999, 159, 159. (22) Evans, D. R.; Craig, V. S. J.; Senden, T. J. Physica A 2004, 339, 101.

10.1021/la062278w CCC: $37.00 © 2007 American Chemical Society Published on Web 12/14/2006

NoVel Gaseous States at the HOPG-Water Interface

We took a different approach to study this controversial problem in our previous work.27-32 A reproducible method was established to artificially induce nanobubbles on solid-water interfaces. In essence, nanobubbles can be formed when a solid surface is exposed to an aqueous medium that is supersaturated with gases. Supersaturation of a gas can be achieved by displacing a liquid with another that has a lower gas solubility, such as when ethanol is displaced with water. This method was found to work on various substrates including highly oriented pyrolytic graphite (HOPG), octadecyltrimethylsilane (OTS), and mica. In this paper, for the first time we report that, in addition to the ordinary spherical-cap-shaped nanobubbles, we can also produce other morphologies of the gaseous state at the HOPG-water interface: flat (quasi-two-dimensional, pancake-like) gas layers and nanobubble-flat gas layer composites (spherical-cap-shaped nanobubbles sitting on top of the quasi-two-dimensional gas layers). Experimental Section Materials and Equipment. All water used was purified using a Milli-Q system (Millipore Corp., Boston). Acetone (99.9%), acetic acid (99.9%), ethanol (99.9%), methanol (99.9%), propanol (99.9%), and tert-butyl alcohol (99.0%) were all obtained from the Chinese Chemical Reagent Co. (Shanghai, China). The MultiMode Nanoscope IIIa SPM equipped with a liquid cell (Digital Instruments Veeco Metrology Group, New York) was used for imaging in tapping mode (TM). The cantilever used was silicon nitride with a nominal spring constant of 0.32 N/m (NP-S, Digital Instruments Veeco Metrology Group). The AFM tips were cleaned by acetone, ethanol, and water in this order before use. The fluid cell and O-ring were cleaned by ethanol and water and dried with an air stream. A temperature-controller accessory (Digital Instruments Veeco Metrology Group) was utilized to control the temperature in the liquid cell, and a single-probe thermocouple was used to measure the temperature of liquids in the cell. The environmental temperature was 22-25 °C. Tapping Mode AFM Imaging of the Interfacial Gas on the HOPG Surface in Water. To obtain stable tapping mode AFM images of the interfacial gas on HOPG surfaces in water, we followed the following procedure: (1) Before any liquid was injected into the fluid cell, the AFM tip was brought close to the surface, and then the O-ring was mounted. In this way, the deformation of the O-ring during the approach of the tip to the surface could be reduced. (2) In water, the drive frequency for the cantilever was set at 6-12 kHz, and the drive amplitude was 180-220 µV. Under these settings of drive frequency and drive amplitude, the cantilever could yield at least 0. 5 V of free peak amplitude. (3) During imaging, the set point ratio rsp ) A/A0 was usually above 0.9, where A0 is the free amplitude of the cantilever and A is the set point amplitude used during imaging. (4) It was noted that sometimes it was more difficult to obtain better images at slow scan rates, presumably due to the interaction between the tip and gas-water interface. (5) Plasma treatment of AFM tips (the water vapor plasma treatment was carried out in a custom-built plasma reactor to render the cantilever hydrophilic, using a typical (23) Tsionsky, V.; Kaverin, A.; Daikhin, L.; Katz, G.; Gileadi, E. Phys. Chem. Chem. Phys. 2005, 7, 1830. (24) Mao, M.; Zhang, J. H.; Yoon, R. H.; Ducker, W. A. Langmuir 2004, 20, 1843. (25) McKee, C. T.; Ducker, W. A. Langmuir 2005, 21, 12153. (26) Seo, Y. S.; Satija, S. Langmuir 2006, 22, 7113. (27) Lou, S. T.; Ouyang, Z. Q.; Zhang, Y.; Li, X. J.; Hu, J.; Li, M. Q.; Yang, F. J. J. Vac. Sci. Technol., B. 2000, 18, 2573. (28) Zhang, X. H.; Zhang, X. D.; Lou, S. T.; Zhang, Z. X.; Sun, J. L.; Hu, J. Langmuir 2004, 20, 3813. (29) Zhang, X. H.; Li, G.; Wu, Z. H.; Zhang, X. D.; Hu, J. Chin. Phys. 2005, 14, 1774. (30) Zhang, X. H.; Wu, Z. H.; Zhang, X. D.; Li, G.; Hu, J. Int. J. Nanosci. 2005, 4, 399. (31) Zhang, X. H.; Maeda, N.; Craig, V. S. J. Langmuir 2006, 22, 5025. (32) Zhang, X. H.; Li, G.; Maeda, N.; Hu, J. Langmuir 2006, 22, 9238

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Figure 1. AFM images of the HOPG surface in air and in water: (a) height image of freshly cleaved HOPG in air (height scale 15 nm); (b) height image of HOPG in water; (C) phase image of HOPG in water. power level of 30 W for 20 s) improved the acquisition of stable images because the tip could be more hydrophilic after the treatment. Formation of Interfacial Gases. HOPG (ZYH, NT-MDT, Moscow, Russia) was used as the substrate. The freshly cleaved HOPG surface is chemically homogeneous and provides a wellcharacterized substrate. As noted above, nanobubbles can form when a hydrophobic surface is exposed to an aqueous medium that is supersaturated with gas. Supersaturation of a gas can be achieved by displacing a liquid with another that has a lower solubility of the gas. Since our interest is to have water as the final medium, we need to use a liquid that has a higher gas solubility than water. For example, an organic liquid, such as ethanol, methanol, propanol, tert-butyl alcohol, acetone, or acetic acid, has a higher gas solubility than water and, importantly, also wets HOPG better (has a smaller contact angle on HOPG) than water. Thus, it is expected that nanobubbles can be induced when water displaces one of these organic liquids. We refer to this method as method 1. Also, cold water (near 0 °C) has a higher solubility of gas than warm water (∼40 °C). Thus, it is expected that nanobubbles can be induced when warm water displaces cold water, too. We refer to this method as method 2. Alternatively, supersaturation of gases can be achieved by introducing appropriate temperature gradients in the system. For example, preheating of the HOPG substrate (to ∼80 °C) prior to the introduction of water at room temperature is expected to raise the temperature of the water layer next to the HOPG substrate and cause supersaturation of gases in that layer. We refer to this method as method 3. Degassing Process. When predegassing of liquids for the exchange process was necessary, the liquids to be predegassed were put inside a desiccator and the pressure inside the desiccator was pumped down to ∼0.1 atm using a vacuum pump. The liquids were kept under the low pressure for at least 2 h. The liquids were then used for the exchange process (method 1) within 1 min after being taken out of the desiccator. The details as to how to degas the system after nanobubbles and/ or the other novel forms of gaseous states were induced and detected by the TM-AFM imaging can be found in our previous work.32 Here we only outline the procedure. The HOPG surface has a layered structure, and the steps on the surface can be used as references in AFM images to identify the location of the imaged areas. The features on the HOPG substrate, such as distinct patterns of the cleavage steps, and the relative position of the AFM cantilever were recorded using a CCD attached to the optical microscope (magnification 250×). The (hydrophilic) fluid cell was taken carefully away from the AFM head so that the water remains on the hydrophobic HOPG substrate as a droplet. The HOPG and the water droplet were then put into a desiccator for degassing (∼0.1 atm). After a desired degassing time (typically 1 h), the HOPG with the water droplet was placed back in the AFM head for imaging. The areas that had been imaged prior to the partial degassing were identified from the features of the cleavage steps recorded by the CCD.

Results and Discussion Formation of Novel Forms of the Gaseous State at the HOPG-Water Interface. The high-resolution AFM images of the HOPG substrate (Figure 1a) show that, regardless of whether the water at room temperature has been degassed or not,

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Figure 2. AFM image and section of spherical-cap nanobubbles on HOPG formed after displacement of ethanol with water.

nanobubbles or other forms of the gaseous state were not observed when the freshly cleaved HOPG substrate was simply immersed into water, i.e., without one of the procedures to induce supersaturation of gases mentioned above. This result is consistent with the reports that there are few nanobubbles that can be detected simply by immersing a hydrophobic substrate into water.24,25,31,33 We observed spherical-cap nanobubbles (Figure 2) using method 1. The morphology of the nanobubbles is similar to those previously reported by many researchers.5,8,20,27,34,35 The morphological and physical properties of spherical-cap nanobubbles at the HOPG-water interface have been studied extensively in our previous work.31 In addition to these spherical-cap nanobubbles, we also observed a state of vicinal gas that has not been reported so farsvery flat (quasi-two-dimensional, pancakelike) gas layers (Figure 3a-c). Here we call this newly discovered form of gas a “micropancake”, while the spherical-cap nanobubble we simply call a “nanobubble”. We could at the same time detect nanobubble-micropancake composites (spherical-cap nanobubbles on top of flat gas layers; see Figure 3d-f). The micropancakes could also be produced by methods 2 and 3, as shown in Figure 4. We note that these novel forms of gaseous states were detected on HOPG substrates but not on OTS substrates. We also note that only special-cap nanobubbles could be formed on HOPG when a low concentration (2-5 vol %) of ethanol in aqueous solution was used instead of Milli-Q water to displace the (100%) ethanol employed in method 1. Degassing Effects on the Formation and the Stability of the Gaseous States. It has been known that dissolved gases in an aqueous phase play a major role in the stability of hydrophobic entities, and therefore, removing these gases results in a marked difference in the stability of these entities.36-38 We used two different ways of degassing in this study. One is to partially predegas all the liquids involved in the exchange processes to ∼0.1 atm and use these liquids in method 1 or 2. The other is (33) Takata, Y.; Cho, J. H. J.; Law, B. M.; Aratono, M. Langmuir 2006, 22, 1715. (34) Holmberg, M.; Kuhle, A.; Garnaes, J.; Morch, K.; Boisen, A. Langmuir 2003, 19, 10510. (35) Ishida, N.; Higashitani, K. Miner. Eng. 2006, 19, 719. (36) Alfridsson, M.; Ninham, B.; Wall, S. Langmuir 2000, 16, 10087. (37) Maeda, N.; Rosenberg, K.; Israelachvili, J.; Pashley, R. Langmuir 2004, 20, 3129. (38) Pashley, R. M. J. Phys. Chem. B 2003, 107, 1714.

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postdegassing of the substrate and the water that already contains nanobubbles and/or pancakes (confirmed by AFM imaging) to ∼0.1 atm. In the first method, initially nondegassed ethanol was displaced with nondegassed water, which resulted in the formation of nanobubbles and micropancakes, as shown in Figure 5a). Then degassed ethanol was used to displace the water, which was found to remove all nanobubbles and micropancakes as reported30 and discussed below. Finally, the degassed ethanol was displaced with degassed water. This resulted in formation of a few nanobubbles but no micropancakes or nanobubble-micropancake composites, as shown in the in situ AFM image in Figure 5b). When these exchange processes were repeated several times, the in situ AFM images showed that micro-pancakes could be produced by the exchange of nondegassed ethanol and water, but not the exchange of degassed ethanol and water. The statistical coverage ratio of the micropancake, which is the area of micropancakes (µm2) per image area (µm2), is shown in Figure 5c. The difference between the exchange of nondegassed and the exchange of degassed liquids is dramatic. A few nanobubbles still formed after the exchange of degassed ethanol and water, presumably because of the moderate level of degassing used in this study (∼0.1 atm). These results showed that the formation of gaseous states at the HOPG surface is closely related to the amount of dissolved gas. For the second method, we degassed the water and the substrate (to ∼0.1 atm for 1.5 h) after we had confirmed the formation of micropancakes from AFM imaging. The in situ AFM images after the partial degassing show that the micropancakes had disappeared (Figure 6). This result indicates that the presence of nanobubbles, micropancakes, or nanobubble-micropancake composites requires a sufficient amount of gas (air) dissolved in water to be stable. The details about the degassing effect on nanobubbles have been discussed in our recent work.32 Morphology of Different Gaseous States at the HOPGWater Interface. The morphological characteristics of the micropancakes are quite different from those of spherical-cap nanobubbles.4-7,28 The micropancakes have a comparatively flat top surface and a sharp (apparently high curvature) boundary. Their lateral length scale spans from a few tens of nanometers to several micrometers,while their apparent height measured from the AFM images is generally less than 3 nm, that is, about 2-3 orders of magnitude less than their lateral length scale. Their area coverage is also much higher than that of the spherical-cap nanobubbles when they were formed at the same time. These morphological features of the micropancakes are quite distinct from those of spherical-cap nanobubbles, and we believe that the newly detected micropancake state is indeed an alternative form of gas that can emerge at the HOPG-water interface. We believe that the observed spherical-cap nanobubbles and micropancakes are both made of gas. Methods 2 and 3, which do not use any organic solvents, generate essentially the same features as those generated by method 1. This result proves quite conclusively that if the observed spherical-cap nanobubbles and micropancakes were some sort of contaminants they could not be originated from the organic solvent used in method 1. Contaminants in the Milli-Q water or on the HOPG are also unlikely to be responsible for the formation of spherical-cap nanobubbles or micropancakes, because we observed neither of these entities prior to one of the three generation methods. We also note that we had analyzed the chemical composition of the Milli-Q water droplet by mass spectrometry and found no evidence of contamination. On the contrary, the spherical-cap nanobubbles induced on HOPG or OTS substrates were found to behave as

NoVel Gaseous States at the HOPG-Water Interface

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Figure 3. New states of interfacial gas, micropancakes and nanobubble-pancake composites formed after displacement of ethanol with water: (a) height image, (b) section of the height image, and (c) phase image of micropancakes; (d) height image, (e) section of the height image, and (f) phase image of nanobubble-pancake composites.

Figure 4. Micropancakes produced at the HOPG-water interface: (a) AFM phase image of micropancakes formed after the displacement of cold water with warm water; (b) AFM phase image of micropancakes after preheating from the bottom of the HOPG substrate prior to the introduction of water at room temperature.

one expects gas bubbles would when different types of surfactants were added.31 Comparison between Spherical-Cap Nanobubbles and Micropancakes. Compared to the spherical-cap nanobubbles, the micropancakes and nanobubble-micropancake composites exhibit a number of distinctive properties. First, as can be seen in the phase images of Figure 3c,f, the micropancakes, nanobubbles, and bare HOPG substrate show quite different contrasts. This indicates that the energy dissipation of AFM cantilevers on HOPG, nanobubbles, and micropancakes are different, which might be due to their different curvatures of the gas-water interface. The retracting traces of AFM withdraw force curves (not shown) on both nanobubbles and micropancakes show identically large adhesion, which excludes the possibility that the micropancakes are due to the trapped gases by the delaminated graphite layer. Second, immediately after the displacement of ethanol with water, we found that the micropancakes are unstable and coalesce rapidly, the time scale of which depended on the experimental temperature, and their coverage is strongly time-dependent. In fact, some smaller micropancakes can coalesce into a bigger one during their growth (Figure 7a,b). The change in the morphology of the micropancakes in water was not observed after 1.5 h at 31 °C (Figure 7c). However, when the temperature was increased from 31 to 36 °C, the micropancakes started to grow again (Figure 7d) and coalesce much faster, presumably because the solubility of major atmospherical gases in water at 31 °C is higher than that at 36

Figure 5. Degassing effects on the formation of micropancakes: (a) height image of the HOPG surface when using nondegassed ethanol and water; (b) height image of the HOPG surface when using predegassed ethanol and degassed water (it shows that the micropancakes are not observable except a few nanobubbles); (c) average coverage ratio of the micropancake when nondegassed and degassed ethanol and water are used (the coverage ratio is the area of micropancakes per image area).

°C.39 The growth saturated in a much shorter time (about 0.5 h) at this temperature. These successive morphological changes of the micropancakes toward the final equilibrium state bear resemblance to two-dimensional Ostwald ripening. It appears that the cleavage steps on the HOPG substrate influence the spreading/growth/coalescence of the micropancakes. In contrast, the morphologic characteristics of the spherical-cap nanobubbles on the HOPG substrate generally do not show much time dependence. These results suggest that the micropancakes are more sensitive to the time and the temperature than the nanobubbles. Third, the stability of nanobubbles and micropancakes also shows marked differences in ethanol solution. (39) Pollack, G. L. Science 1991, 251, 1323.

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Figure 6. Removal of micropancakes by partial degassing. Micropancakes were produced on HOPG by the exchange of ethanol and water as shown in (a). After degassing under ∼0.1 atom for 1.5 h, the same region was imaged, as shown in (b). Both (a) and (b) are height images.

Figure 7. Change of micropancakes and nanobubbles as a function of time and temperature: (a) initial pancakes (P ) micropancake) immediately after the displacement of ethanol with water at 31 ( 0.5 °C; (b) after 1.5 h at 31 ( 0.5 °C, P1 and P2 coalesced into a bigger micropancake, P3 (the nanobubbles on the micropancakes could move with time, for example, the one inside the dotted circle); (c) after another 1.5 h at this temperature, the pancakes are not very different from those in image b; (d) recommencement of growth of P3 if the temperature is increased to 36 ( 0.5 °C and held for 0.5 h.

Figure 8. Effect of ethanol solution on the stability of spherical-cap nanobubbles and the micropancakes. All micropancakes in water in image a have diminished in image b after the displacement of water with 10 vol % ethanol solution.

Zhang et al.

also nanobubbles. Compared to nanobubbles, the micropancakes are less stable when the surface tension of the solution is decreased. Possible Formation Mechanism of the Gaseous States at the HOPG-Water Interface. All the experimental evidence suggests that the local supersaturation of gases next to the HOPGwater interface is the key to the formation of nanobubbles. Displacement of ethanol or another organic liquid (that has a higher gas solubility than water and at the same time is miscible with water and wets HOPG better than water) with water, and displacement of warm water with cold water, preheating of HOPG all would generate local supersaturation of gases next to the HOPG-water interface. For example, in the first method ethanol wets HOPG better than water; thus, a thin layer of ethanol will be left next to the HOPG surface as the ethanol is being displaced with water, and this leftover ethanol subsequently dissolves into the water. Then the heterogeneous nucleation of nanobubbles on the HOPG surface from the thin layer of water that has been supersaturated with gases (because of the reduced gas solubility of water compared to ethanol) should still be preferable to the homogeneous nucleation in the bulk water. We believe that the same mechanism caused the formation of the two other newly found gaseous states at the HOPG-water interface. Remaining Questions. The first question is the following: What determines the morphology of the gaseous state? That is, what controls the formation of nanobubbles, micropancakes, or nanobubble-micropancake composites on the HOPG substrate? We found that if we used low concentrations of ethanol in aqueous solution instead of pure water, we could only form nanobubbles on the surfaces.30 Meanwhile, we found that the electrochemical reaction, which is designed to generate hydrogen gas (working electrode, 2H+ + 2e- f H2v; auxiliary electrode, H2O f 1/2O2v + 2H+ + 2e-), can produce a large amount of nanobubbles but not micropancakes.40 The methods described in the Experimental Section could be used to produce all three gaseous states on an HOPG surface. But we do not know why the same method can sometimes form only one state but not the others, as shown in Figure 3. The results also show that micropancakes form on HOPG but not on other substrates such as OTS or mica. It may be related to the chemical homogeneity and the layered structure of the HOPG surface. It was possible to detect spherical-cap nanobubbles on top of micropancakes. These spherical-cap nanobubbles could be moved around by an AFM tip on the top plateau of a micropancake as well as from the top of one micropancake to the top of another micropancake. We also observed that the spherical-cap nanobubbles on the top plateau of a micropancake could move with time. It appears that there is a thin but stable layer of water separating the spherical-cap nanobubbles from the micropancake, although we do not know how the water layer could get between the nanobubbles and the micropancakes or how such a water layer could be sustainable. A significant amount of electrostatic charge at the interfaces would be required for such a water layer to be stable against the hydrophobic attractive force from both sides of the water layer. The underlying physical reasons for these issues are not clear at this stage. We leave these for future study.

Conclusion When the concentration of ethanol in water is greater than 5 vol %, the micropancake state becomes unstable and eventually vanishes, whereas the spherical-cap nanobubbles can survive at up to 10 vol % ethanol (Figure 8). The ethanol aqueous solution has a lower surface tension than water and wets the HOPG surface better, which may destroy the micropancakes and eventually

We have revealed that, in addition to spherical-cap nanobubbles, both gas micropancakes and nanobubble-pancake composites are two alternative states of gas at the interface of HOPG and water. Micropancakes and nanobubble-pancake composites exhibit a number of distinctive properties from spherical-cap (40) Zhang, L. J.; Zhang, Y.; Zhang, X. H.; Li, Z. X.; Shen, G. X.; Ye, M.; Fan, C. H.; Fang, H. P.; Hu, H. Langmuir 2006, 22, 8109.

NoVel Gaseous States at the HOPG-Water Interface

nanobubbles. Although the formation and stability mechanisms of nanobubbles, pancakes and nanobubble-pancake composites are still mysterious, the supersaturation of gas near the surface is shown to be the key for the formation of all these different gaseous states at the HOPG-water interface. Acknowledgment. This work was supported by the National Natural Science Foundation of China, the Chinese Ministry of

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Science and Technology, the Science and Technology Commission of Shanghai Municipality, the Chinese Academy of Sciences, the U.S. National Science Foundation, the Office of Basic Energy Sciences of the U.S. Department of Energy (Materials Sciences and Engineering Division), the John Simon Guggenheim Foundation, and the Nebraska Research Initiative. LA062278W