Do Stable Nanobubbles Exist in Mixtures of Organic Solvents and

May 3, 2010 - Nanoscale gas bubbles at solid-water interfaces have been observed previously by many ... University of Melbourne. ‡ Virginia Tech. J...
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Do Stable Nanobubbles Exist in Mixtures of Organic Solvents and Water? Annette Ha¨bich,† William Ducker,‡ Dave E. Dunstan,† and Xuehua Zhang*,† Department of Chemical and Biomolecular Engineering, UniVersity of Melbourne, Victoria 3010, Australia, and Department of Chemical Engineering, Virginia Tech, Blacksburg, Virginia 24061 ReceiVed: December 15, 2009; ReVised Manuscript ReceiVed: April 14, 2010

Several recent papers have described the existence of stable nanobubbles in bulk, which is surprising given that the high curvature of these bubbles is expected to place such bubbles under a high pressure and therefore lead to rapid dissolution. Here, we investigate the possible existence of nanobubbles in mixtures of water plus an organic solvent using both static and dynamic light scattering and infrared spectroscopy. The mixing of solvents was designed to introduce nanobubbles into bulk solution via supersaturation of the solution. The solutions scatter light for a long period (days) after mixing, which is consistent with the formation of nanoscale objects, but we show that these scattering objects originate from water-insoluble impurities in the organic solvents. Our results are inconsistent with the presence of gas nanobubbles in bulk solution: Degassing the solutions, either before or after mixing, has a minimal effect on the scattering, and purification of the organic solvent before mixing reduces the scattering after mixing. Therefore, previous reports of nanobubbles based on scattering experiments should be reconsidered with the hypothesis that the scattering objects are not actually gaseous. Introduction Nanoscale gas bubbles at solid-water interfaces have been observed previously by many research groups.1-11 The typical morphology of these nanobubbles is a spherical cap with a height of ∼20 nm and a lateral diameter of ∼1 µm.1,2,12 These nanobubbles are usually formed immediately after a disturbance from equilibrium, for example, by gas supersaturation, mixing of solvents, a change in temperature, or chemical reaction.2,8,11,13,14 Once formed, the interfacial nanobubbles can stay at a hydrophobic surface for periods of several days.12,15 There are many reports of the formation of micro- and nanobubbles in bulk solution.16-20 The transfer of gas from a bulk phase to a nanobubble requires an input of energy to account for the increased area of the gas-liquid interface. The energy required to form the bubbles has been provided by electrochemical reaction,17,21 flow through a porous membrane,22 or ultrasound.23,24 The continued existence of a free bubble relies on the existence of an activation barrier to the efflux of the gas, so the stability of these bubbles may be enhanced through the adsorption of surfactants,25,26 polymers,18 or lipids24 at the interface. There are also claims for the spontaneous formation of bubbles and the stabilization of bubbles through the adsorption of ions (“bubstons”)27 or impurities.28 The formation and stability of such bubbles are interesting from an application viewpoint, where the nanobubbles could be used as highly efficient ultrasound contrast agents.23,24,26 Nanobubbles are also of interest from a scientific viewpoint, as their presence challenges our ideas of water structure and stability. Theory based on solubilization kinetics suggests that nanobubbles with a diameter 100 nm would disappear in less than 100 µs.29 If nanobubbles are present, they should affect the properties of water, such as the viscosity and density,30 and the behavior of processes in the liquid, such as the interaction between colloidal * To whom correspondence should be addressed. E-mail: xuehuaz@ unimelb.edu.au. † University of Melbourne. ‡ Virginia Tech.

particles.31,32 Meanwhile, the existence of nanobubbles in bulk could provide a mechanism for the formation of nanobubbles at interfaces through collisions with the solid. Of particular interest are recent reports that nanobubbles can be formed in aqueous solutions of small organic molecules simply by mixing the solvents with water.19,33,34 Jin et al. report that aqueous solutions of tetrahydrofuran, ethanol, urea, or R-cyclodextrin cause scattering of light. They conclude that the scattering is due to the presence of stable nanobubbles and that “the formation of such nanobubbles in small organic molecule aqueous solution is a universal phenomenon”. The scattering objects (“bubbles”) have a diameter of about 100 nm, and the proposed mechanism for stabilization is via the amphiphilic nature of the organic molecules. Recently, a series of works have reported strong scattering from a wide range of solutions.35-37 Given the implications of the widespread formation of stable nanobubbles simply by mixing organic molecules with water, further investigation to understand these important effects is worthwhile. In this work, the existence of colloidal-sized bubbles is investigated by both static and dynamic light scattering (DLS) and infrared spectroscopy. Experiments are performed on both solutions that are equilibrated with gas at one atmosphere and on degassed solutions. We attempt to generate colloidal bubbles through changes in solubility of the gas in the liquid and then observe the presence and time dependence of colloidal bubbles. Changes in gas solubility are generated by mixing together two gas-saturated solvents so that the mixture is then supersaturated and has the opportunity to form nanobubbles dispersed throughout the mixture. We use a standard mixture of 20% v/v ethanol because the solubility of atmospheric gases (N2 and CO2) reaches a minimum at this concentration,38 which means that a maximum in supersaturation of gas in an ethanol-water solution is obtained.7,12 Strong light scattering from the solutions is observed, as has been reported previously.19 We conclude that the scattering is not caused by nanobubbles but instead arises

10.1021/jp911868j  2010 American Chemical Society Published on Web 05/03/2010

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from trace water-insoluble impurities in the solvents or contamination from the plastic labware. Experimental Section Materials. Water (18.2 MΩ) was received from a Millipore Simplicity purification system (Millipore, Watertown, MA), and 99.5% ethanol (AR grade, Chem-Supply, Gillman, SA, Australia) was used as received (we refer it as “AR ethanol”) or distilled twice (“distilled ethanol”). Other chemicals used were 99.8% methanol (pro analysi, Merck, Darmstadt, Germany), 99.5% propan-1-ol (AnalaR, VWR International, Poole, England), 99.7% propan-2-ol (AnalaR, Merck), 99.8% acetone (pro analysi, Merck), 99.7% acetic acid (ACS reagent, SigmaAldrich, St. Louis, MO), and hydrochloric acid (AnalaR, VWR International). Solvents were handled in a laminar flow cabinet to prevent contamination. Glassware was soaked in warm 10% w/w sodium hydroxide solution and then rinsed copiously with water. Careful cleaning and avoidance of all plastic products such as polyethylene syringes or polystyrene cuvettes were crucial to the experiments. Solution Preparation. For the experiments with carbon dioxide-saturated solutions, high-purity carbon dioxide (BOC gases, Chatswood, NSW, Australia) was used to purge the solvents for 30 min at an overpressure of 5-10 kPa. Carbon dioxide gas was led through a gas-washing bottle filled with Milli-Q water before bubbling through the solvents to remove dust and other impurities from the gas. Solvents were degassed in separate desiccators during about 14 h under a vacuum of ∼20 kPa. For a more thorough removal of gas from the solutions of organic solvent and water, three cycles of freeze-pump-thaw (F-P-T) were applied. In each cycle, the solution was frozen in a Schlenk tube using liquid nitrogen, then the pressure inside the tube was pumped down to 0.01 kPa, and the solution was thawed under vacuum. Solutions (20% v/v) were prepared by slowly adding water to the organic solvent using a glass syringe. The water and solvents were equilibrated with air at 1 atm, saturated with CO2 at 1 atm, or degassed. The way of mixing the two solvents was important. The water had to be slowly added to the ethanol. Adding the water quickly or adding ethanol to the water did not lead to the formation of scattering objects. This indicates that the nucleation process for the formation of the objects is related to the rate of mixing of the solvents or the order of mixing of the solvents. Laser Light Scattering. Static light scattering (SLS) was measured using a Malvern 4700 instrument (Malvern Instruments Ltd., Malvern, Worcestershire, United Kingdom) equipped with a 10mWAR+ ion laser at 488 nm. The scattered light was measured at a constant angle of 90° and a temperature of 25 °C. The aperture was adjusted to the largest setting with an opening of 500 µm in diameter. The intensities of the scattered light are used as a measure of the presence and time dependence of the nanobubbles. The scattered intensities from the samples were obtained using this method. We used SLS only to detect the scattering intensities from the solutions at a fixed angle. We did not conduct a typical measurement of SLS, which usually involves measurements at a number of angles. After we measured the scattering intensity at a constant angle, we then used DLS to obtain the hydrodynamic diameter of the particles. The same conditions and the same instrument were used for both static and DLS. For a strongly scattering sample, the aperture of the laser was adjusted so that the count rate of the scattering signal was between 100

Figure 1. Scattering intensity from ethanol, water, and mixtures of ethanol and water. The aqueous solution of CO2-treated or airequilibrated AR ethanol scattered the light significantly more than the single solvents did. The scattering was measured under the same conditions (aperture and temperature) so that the scattering could be compared for different solution compositions.

and 200 kcps (kilocounts per second). For a weakly scattering sample, the aperture was fully opened (500). At least three measurements were performed for each sample. The time autocorrelation functions were analyzed by the inverse Laplace transform algorithm, CONTIN. Only glass or quartz cells were used. Particle size distributions were obtained using this method and are reported as number distributions (% in number). Infrared Spectroscopy. Transmission IR spectra were recorded using a Nicolet 5700 Fourier Transform infrared spectrometer from Thermo Electron Corporation. All spectra were recorded at 24 ( 1 °C using a MCT/A detector, KBr beam splitters, a mirror velocity of 1.8988 cm/s, an aperture of 4, and a gain setting of 1. For each spectrum, 100 scans were added with a resolution of 0.125 cm-1. A solution of degassed solvents was used as a background. An EZ-Flow cell from Thermo Spectra-Tech with ZnSe windows and a 200 µm Teflon spacer was used. The solutions could be injected via tubes from the outside of the IR sample compartment. The IR sample compartment was purged with dry and carbon dioxide-free air delivered from a Parker Balston FTIR Purge Gas Generator model 75-52 for about 1 h prior to recording the spectra, so that the CO2 background was stable before the background spectra were collected. Atomic Force Microscopy (AFM). Tapping mode AFM was used to image a highly oriented pyrolytic graphite surface in ethanol solution within a closed fluid cell by using a MFP-3D Atomic Force Microscope (Asylum Research, Santa Barbara, United States). All the parts of the fluid cell were cleaned with ethanol and water and dried with high-purity nitrogen before use. The cantilevers (0.32 N/m, NP probe, Veeco, United States) were treated by UV for ∼15 min before use. Results and Discussion Light Scattering from Air-Equilibrated Solutions. SLS measurements of air-equilibrated solvents are shown in Figure 1. The scattering of all single solvents was less than 20 kcps. The scattering from the ethanol-water mixtures at 20% v/v ethanol, 200 kcps, was much greater than the pure solvents. If water and ethanol were treated by CO2, which has a higher solubility in the solvents, the observed scattering intensity was even higher at approximately 350 kcps. The strong scattering was also observed when any one of acetic acid, propan-1-ol, propan-2-ol, methanol, or acetone was mixed with water (Figure

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Figure 2. Scattering intensity of mixtures of 20% v/v organic solvents and water. All solvents were CO2-treated prior to mixing.

2). Thus, the increase in scattering intensity occurs for a range of water-miscible organic solvents. It should be noted that the observed intensities are stable with time. The highest scattering was observed in mixtures of either ethanol, acetic acid, or propan-1-ol mixtures with water. In this paper, we focus on ethanol-water mixtures. Infrared Spectra of CO2-Saturated Solutions. Light scattering measurements cannot determine the chemistry of scattering objects, so we used infrared spectroscopy to determine the chemical identity of the scattering objects in the ethanol aqueous solution. We used CO2 to treat the solvents prior to the mixing, because gaseous CO2 produces a distinctive infrared spectrum that contains a fine structure due to the rotation of gaseous molecules.12,15,39,40 The scattering intensity of the solution of CO2-treated ethanol and water was around 350 kcps (Figure 1). The hydrodynamic diameter of the particles was determined by DLS and was in the range of 130-180 nm with a very narrow particle size distribution (Figure 3). Twenty-four hours after the sample preparation, the scattering intensity was still very high (around 340 kcps), and the change of the hydrodynamic diameter was not significant (Figure 3). If the observed light scattering was due to CO2 nanobubbles, then we expected to detect the CO2 gas-phase spectrum at the same time as the scattering. The transmission IR spectrum of a mixture of CO2-treated ethanol and water was measured directly after mixing. A solution of degassed ethanol and water (20% v/v) was used as a background. Bands for CO2(gas), displaying a rotational fine structure, superimposed on the broad band for CO2(aqueous), were visible immediately after the solution was introduced into the cell (Figure 4A). The bands for CO2(gas) diminished quickly and disappeared after 15 min, as shown in Figure 4B. On closer visual examination of the IR cell, macroscopic bubbles (diameter ∼1 mm) were observed in the cell immediately after mixing, and these bubbles rose out of the beam during the first 15 min. The gaseous adsorption shown in the spectra may be due to these bubbles. After the gaseous bands disappeared, the solution was collected from the transmission cell, and the SLS was measured. The solution scattered the light intensely (342 kcps). This showed that the scattering objects were not responsible for the gas-phase CO2 infrared bands. This lack of a gaseous IR response from scattering objects could be due to one of two reasons: (1) The objects in solution were not gas bubbles; therefore, no gaseous spectra could be detected. (2) These objects were gas bubbles, but the concentration of gas in the nanobubbles was too low to be determined by transmission IR spectroscopy. We have evaluated the sensitivity of the infrared equipments (see the Supporting Information). The minimum number density of nanobubbles that

Figure 3. Number distributions from DLS for particles in mixtures of CO2-treated AR ethanol and water. (A) The distribution curve of three consecutive measurements of a solution of CO2-treated solvents at intervals of 10 min. It shows the variance in the measurements. (B) The distribution curves of the solution of CO2-treated solvents in an interval of 24 h.

Figure 4. Transmission IR spectra of the solution of CO2-treated ethanol and water. (A) Spectrum immediately after preparation. The rotational fine structure indicating gaseous CO2 is visible. The broad band is due to absorption by dissolved CO2. (B) Spectrum collected 15 min after that in panel A. The bands showing gaseous CO2 are now absent, demonstrating the absence of gas-phase CO2.

can be detected by infrared is about order of 108/mL. We combined degassing and light scattering to identify the source of the scattering as discussed below. Effect of Degassing on Light Scattering. In this section, we investigated the effect of the gas pressure on the formation and stability of the scattering objects. If the scattering was due

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Figure 5. Number distributions from DLS for particles in mixtures of CO2-treated and degassed AR ethanol and water. The size distribution of the scattering objects from these two solutions is similar.

to nanobubbles that were formed as a result of supersaturation of solvents during mixing, then removing the gas before mixing should prevent formation of the bubbles. Figure 5 shows the number distributions of ethanol and water mixtures prepared from (1) a mixture of CO2-treated ethanol and CO2-treated water and (2) a mixture of degassed ethanol and degassed water. The size distribution (diameter ∼100 nm) from the solution was very similar to CO2-treated solutions. The scattering intensity from SLS was around 200 kcps for degassed solvents, which is very similar to the scattering from air-equilibrated solutions (Figure 1). The removal of gas from the solvents before mixing did not affect the formation of the scattering objects, which is inconsistent with the formation of bubbles during supersaturation. We also investigated the influence of gas pressure after the formation of the scattering objects. The stability of bubbles with a radius of curvature of approximately 70 nm is problematic because such a highly curved object would be under a high pressure (the Laplace pressure). For example, if a bubble of radius 70 nm has a surface tension of about 30 mJ m-2, then the pressure in the bubble is approximately 8.5 atm greater inside the bubble than outside the bubble. Over time, this high pressure would be expected to cause an efflux of gas out of the bubble, which makes the bubble unstable. If nanobubbles were to exist, then possible hypotheses are (1) that the bubbles are closed or (2) that there is sufficient adsorption of material around the bubbles that the surface tension is decreased to levels where the difference in pressure between the inside and the outside of the bubble is negligible. For example, the surface tension would need to be less than 7 mJ m-2, so that the pressure across the interface would be less than about 1 atm. If the surface tension were to fall that low by adsorption of contaminants, it is likely that those contaminants would hinder diffusion of gas from the bubble, so that the bubble would act more like a closed bubble. Two types of experiments were performed on the scattering objects. In the first, we attempted to remove as much gas as possible by the F-P-T method. This technique allows the removal of 99.999% of gases from liquids.41,42 We mixed a solution of CO2-treated ethanol and CO2-treated water and measured the scattering. Then, we degassed the solution by repeating F-P-T three times. The scattering from the degassed solutions was unchanged after thoroughly degassing by F-P-T and was similar to that from nondegassed solution shown in Figure 6. This shows that the scattering objects are not influenced by degassing. This shows that the bubbles, if present, are closed; there was no loss of volume when the gas was removed from the liquid phase. In the second experiment, we monitored the scattering intensity after the solution pressure was reduced from 1 atm to

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Figure 6. Number distributions from DLS for scattering objects in mixtures of AR ethanol and water under different conditions. The scattering from solution after three cycles F-P-T measured under a vacuum of ∼0.01 kPa or under 100 kPa (ambient pressure) is similar to the scattering from solution before F-P-T.

0.01 kPa in a closed Schlenk tube. No significant change in scattering was observed, as shown in Figure 6. This again shows that the bubbles are closed; the bubbles did not degas and disappear. If the putative gas bubbles are closed, it shows that the bubbles must be under a high Laplace pressure. If they were under a low Laplace pressure, then the decrease in solution pressure would have a large effect on the absolute pressure in the bubble. For example, if the Laplace pressure were only 1 atm, the decrease in the surrounding pressure by 1 atm should cause a doubling of volume for a closed bubble. This was not observed. The only way that the bubble can remain at a constant volume at reduced solution pressure is for it to be either under a high Laplace pressure or inside a rigid container. Rather than postulate the existence of a closed high pressure or rigid container, it is more likely that the scattering objects are simply not bubbles. In the next section, we examine what else could be scattering the light. What Is Scattering the Light? The results from infrared spectroscopy, predegassed solvents, and thoroughly degassing the solution are all inconsistent with the presence of gas bubbles. The question then arises, what is scattering the light? We repeated the experiments with double-distilled ethanol. Figure 7A shows that the scattering intensity from the aqueous solution of distilled ethanol was much smaller (8 kcps) than that from an aqueous solution of AR ethanol (202 kcps, see Figure 1) and was in fact similar to the scattering intensity from distilled ethanol itself (see Figure 7A). This shows that impurities in ethanol are critical for the formation of scattering objects in aqueous solution. Our hypothesis is that any solute including trace contaminants in the solvent could precipitate once the solvency is reduced by mixing with water and that it is the trace impurities that are removed by distillation that cause the scattering. Major impurities in AR grade ethanol are methanol (0.1% v/v), propan-2-ol (0.015% v/v), and acetone (0.1% v/v). Aqueous solutions of distilled ethanol containing a small amount of methanol or acetone did not cause greater light scattering than the single solvent. When 5-10 ppm of decane was added to distilled ethanol, and then, the decane-ethanol solution was mixed with water, the scattering of the solution was six times higher than that from AR ethanol aqueous solution. The size of the scattering objects was smaller (Figure 7B), and the lifetime was shorter (about 1 day). We also found that using plastic materials, such as polypropylene syringes or polystyrene cuvettes, to transport distilled ethanol could also cause very strong scattering intensity (about 4 times higher than AR ethanol aqueous solution) with similar size distribution as shown in

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Figure 7. Effect of impurities on the light scattering. (A) Scattering intensity from distilled ethanol and distilled ethanol aqueous solution. The scattering from the aqueous solution of CO2-treated or airequilibrated distilled ethanol is similar to single solvents. For comparison, the scattering from the aqueous solutions of CO2-treated and air-equilibrated AR ethanol under identical conditions shown in Figure 1 is 345 and 202 kcps, respectively. (B) Number distribution of scattering objects generated by various “impurities” in air-equilibrated ethanol and water solutions. The impurities were introduced by doping decane in distilled ethanol or by using a plastic syringe to transport distilled ethanol.

Figure 7B. All of these results show that the scattering could be produced by traces of water-insoluble impurities. Why did the scattering increase with CO2 treatment? Treatment with carbon dioxide not only increases the concentration of gas in the solution38,43 but also lowers the pH by forming carbonic acid.44 Water saturated with CO2 has a pH of 3.8 ( 0.2.12 When an aqueous hydrochloric acid solution at pH 3.5 was mixed with ethanol, the scattering intensity and particle size were similar to what was observed for CO2-treated solutions. This indicates that the higher scattering came from the change in pH, which presumably affected the stability or solubility of the scattering particles. Implication for Nanobubbles at Interfaces. Nanobubbles have been shown to be stable at interfaces. Whereas the results here suggest that nanobubbles have not yet been found to occur in bulk solution, it is possible that the small scattering objects can affect the formation of bubbles at interfaces that are formed by ethanol/water exchange.12,15 A hydrophobic surface (freshly cleaved HOPG) was exposed to the following solutions: (1) a solution of CO2-treated ethanol and water, which had intense light scattering; and (2) a solution of CO2-treated distilled ethanol and water, which only had background light scattering. The surface was imaged by tapping mode AFM in the above solution. In neither case were any nanobubbles or particles observed on the surface (Figure 8). If the highly scattering solution was composed of nanobubbles, then some of these

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Figure 8. AFM images of (A) the freshly cleaved HOPG surface in the solution of CO2-treated AR ethanol and water and (B) the freshly cleaved HOPG surface in the solution of CO2-treated distilled ethanol and water.

bubbles would be expected to collide with the solid and form interfacial nanobubbles. The absence of interfacial bubbles is consistent with the absence of solution bubbles. It is not surprising that nanoscale gas bubbles are less stable in bulk than at a surface, as attachment to a hydrophobic solid can both decrease the surface energy and lead to pinning on the three phases contact line. Conclusions Strong and stable light scattering was observed after organic solvents were mixed with water. This is consistent with earlier observations.19 The scattering objects have the following characteristics: (1) the formation and the stability of the scattering objects are independent of the amount of dissolved gas. Also, the size of these objects does not change at reduced pressure. (2) For CO2-treated solutions, the presence of the scattering was not accompanied by the presence of the characteristic gas-phase CO2 infrared spectrum. (3) When the solvent is purified, the scattering objects do not form. All of these results are inconsistent with the presence of nanobubbles in these solutions. A more likely explanation is that the scattering objects consist of impurities originating from the solvent or the plastic labware that coalesce into nanoparticles when the solvent quality is changed or that solubilize from the surface of the vessels when the solvent is changed.

Nanobubbles in Mixtures of Organic Solvents and Water Acknowledgment. We thank Prof. Jun Hu at Shanghai Jiao Tong University for valuable suggestions. This research was supported under the Australian Research Council’s Discovery Project funding scheme (DP0880152). X.Z. is the recipient of an Australian Postdoctoral Fellowship. Supporting Information Available: Calculations of the number density of nanobubbles to be detected with FTIR. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Ishida, N.; Inoue, T.; Miyahara, M.; Higashitani, K. Langmuir 2000, 16, 6377–6380. (2) Yang, J.; Duan, J.; Fornasiero, D.; Ralston, J. J. Phys. Chem. B 2003, 107, 6139–6147. (3) Holmberg, M.; Kuhle, A.; Garnaes, J.; Morch, K. A.; Boisen, A. Langmuir 2003, 19, 10510–10513. (4) Tyrrell, J. W. G.; Attard, P. Phys. ReV. Lett. 2001, 87. (5) Ball, P. Nature 2003, 423, 25–26. (6) Attard, P. AdV. Colloid Interface Sci. 2003, 104, 75–91. (7) Zhang, X. H.; Maeda, N.; Craig, V. S. J. Langmuir 2006, 22, 5025– 5035. (8) Yang, S. J.; Dammer, S. M.; Bremond, N.; Zandvliet, H. J. W.; Kooij, E. S.; Lohse, D. Langmuir 2007, 23, 7072–7077. (9) Hampton, M. A.; Donose, B. C.; Nguyen, A. V. J. Colloid Interface Sci. 2008, 325, 267–274. (10) Hampton, M. A.; Nguyen, A. V. Miner. Eng. 2009, 22, 786–792. (11) 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, 2573–2575. (12) Zhang, X. H.; Khan, A.; Ducker, W. A. Phys. ReV. Lett. 2007, 98, 1–4. (13) Zhang, L. J.; Zhang, Y.; Zhang, X. H.; Li, Z. X.; Shen, G. X.; Ye, M.; Fan, C. H.; Fang, H. P.; Hu, J. Langmuir 2006, 22, 8109–8113. (14) Zhang, X. H. Phys. Chem. Chem. Phys. 2008, 10, 6842–6848. (15) Zhang, X. H.; Quinn, A.; Ducker, W. A. Langmuir 2008, 24, 4756– 4764. (16) Johnson, B. D.; Cooke, R. C. Science 1981, 213, 209–211. (17) Vogt, H. J. Appl. Electrochem. 1983, 13. (18) Kim, J.-Y.; Song; Myung-Geun; Kim, J.-D. J. Colloid Interface Sci. 2000, 223, 285–291.

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