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Aqueous Solution Structure over r-Al2O3(011j2) Probed by Frequency-Modulation Atomic Force Microscopy Takumi Hiasa,*,† Kenjiro Kimura,† Hiroshi Onishi,† Masahiro Ohta,‡ Kazuyuki Watanabe,‡ Ryohei Kokawa,†,‡,§ Noriaki Oyabu,§,| Kei Kobayashi,⊥ and Hirofumi Yamada| Department of Chemistry, Kobe UniVersity, Kobe 657-8501, Japan, Shimadzu Corporation, Kyoto 604-8511, Japan, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan, Department of Electronic Science and Engineering, Kyoto UniVersity, Kyoto 615-8530, Japan, and InnoVatiVe Collaboration Center, Kyoto UniVersity, Kyoto 606-8501, Japan ReceiVed: June 22, 2010; ReVised Manuscript ReceiVed: October 17, 2010
An R-Al2O3(011j2) wafer was immersed in an aqueous KCl solution of 1 mol L-1 and observed with a frequency-modulation atomic force microscope. The tip-surface force was precisely determined as a function of the tip-surface distance. The force-distance relationship contained oscillations accompanied with an exponentially decayed, electric double layer force. The force oscillations were ascribed to liquid water layers confined over the Al2O3 surface. 1. Introduction Aluminum oxides have a variety of applications in aqueous solutions including catalysts, abrasives, and biocompatible materials. Research has successfully been performed on singlecrystalline Al2O3 wafers and films exposed to water vapor as well-defined models of the solution-Al2O3 interfaces.1-8 A future issue to be examined is how liquid water molecules behave at real interfaces in solutions. It is thought that at a solution-solid interface, the solution structure is modified in the proximity of the solid. The freedom of translation is confined along the perpendicular direction to the surface. The confined water molecules present layered arrangements called hydration layers. Surface force9 and friction10 measurements are a good method for identifying water layers confined between two macroscopic solid walls. The surface force was measured with sapphire-solution-sapphire11,12 and silica-solution-sapphire13 interfaces. On the other hand, it is not easy to observe hydration layers open to a solution. Over solution-alumina interfaces, the adsorbed structure of metal cations was determined using X-ray absorption fine structure14-18 and X-ray reflectivity19 analysis. The potential of zero charge was estimated with sumfrequency spectroscopy18,20,21 or surface force measurement.13 A direct method to determine the structure of hydration layers is X-ray crystal truncation rod (XCTR) analysis.22-24 The electron density distribution is quantitatively determined in the proximity of the surface. These methods provide properties of interest laterally averaged over the interface. In the present study, we applied frequency-modulation atomic force microscopy (FMAFM) for observing hydration layers at different lateral positions over an R-Al2O3 surface. When a solid is immersed in a liquid and scanned by an AFM tip, liquid molecules are confined between the solid and tip, as also happens in a miniaturized surface force apparatus.25-28 Layered structures of confined * Corresponding author. Phone/Fax: +81-78-803-5674. E-mail: hiasa@ stu.kobe-u.ac.jp. † Kobe University. ‡ Shimadzu. § Japan Science and Technology Agency. | Department of Electronic Science and Engineering, Kyoto University. ⊥ Innovative Collaboration Center, Kyoto University.
liquids have been found in AFM studies on inorganic and organic materials including CaCO3,29 graphite,30,31 and mica32 immersed in water or organic solvents. Further sensitive force detection has been achieved in liquids by FM-AFM equipped with a low-noise optical deflection sensor.33 Using improved microscopes, hydration-induced modifications of force-distance curves have been observed on polydiacetylene,34 mica,35 TiO2,36 graphite,37 and a lipid bilayer.38 The latest study39,40 was performed on a mica and bacteriorhodopsin layer, where crosssectional force distributions were visualized along planes perpendicular to the surface. The observed force distribution was quantitatively compared with a theoretically simulated density distribution gradient of open water over mica. The authors proposed that the intrinsic water structure at the interfaces is projected on the force distribution probed by the AFM tip. In the present study, we follow their proposal and compare our force-distance curves with the density distribution gradient on Al2O3. The water density distribution was experimentally determined on a (011j2) surface of this oxide using XCTR.23 2. Experimental Methods An R-Al2O3(011j2) wafer (10 × 10 × 0.5 mm3, Shinko-Sha) was washed with concentrated nitric acid, ultrasonically rinsed with pure water, dried in a N2 flow, and calcined in air at 1273 K for 12 h. The wafer was cooled, immediately placed in a homemade liquid AFM cell, and immersed in a KCl aqueous solution of 1 mol L-1, which was prepared with KCl (99.5%, Nakarai) and Millipore water. Minimizing the electric double layer force was important to observe force modulations caused by hydration layers. This was achieved with the strong screening in the electrolyte solution of this concentration. Imaging scans and force spectroscopy measurements were performed with a microscope (Shimadzu, SPM9600) modified with the low-noise optical deflection scheme after Fukuma et al.33 The deflection noise was less than 20 fm Hz-1/2. Gold-coated silicon cantilevers with a nominal spring constant of 42 N m-1 (Nanosensors, PointProbe(R) Plus NCH-AuD) were used. The resonance frequency and Q factor of the cantilever oscillation were typically 140 kHz and 10 in the solution. The oscillation amplitude was regulated to be constant with a feedback loop.
10.1021/jp1057447 2010 American Chemical Society Published on Web 11/12/2010
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Figure 2. Frequency shift-distance curves over the Al2O3(01-12) surface. Panel a shows a two-dimensional distribution of the frequency shifts. Four curves observed at positions i-iv were shown in panel b. Forward i curve and backward i curve were determined with the tip approaching the surface and leaving from the surface, respectively. Cantilever oscillation amplitude ) 0.2 nm.
Figure 1. Constant frequency-shift topography of the Al2O3(01-12) surface immersed in the KCl solution. Frequency shift (∆f) ) +800 Hz, cantilever oscillation amplitude ) 0.8 nm.
The absolute amplitude was estimated by comparing the theoretical amplitude of the cantilever Brownian motion to the cantilever deflection sensor output recorded with a spectrum analyzer (Agilent, 4395A). In our microscope, the divided photodiodes had a differential readout of 1 V when the cantilever was deflected by 5.1 nm. The frequency shift of the cantilever oscillation was detected with a phase locked loop. The vertical drift of the tip was less than 0.1 nm min-1 relative to the surface. The typical acquisition time of one ∆f-distance curve was 0.25 s. 3. Results and Discussion 3.1. Topography of Al2O3. Figure 1 shows the constant frequency-shift topography of the calcined R-Al2O3(011j2) wafer observed in the KCl solution. In this imaging scan, the tip-surface distance was regulated to keep the frequency shift of cantilever oscillation (∆f) constant, +800 Hz. A positive ∆f represents repulsive tip-surface force. Steps appear from the top right to bottom left of the image. The height of the steps was 0.3 nm reproducing the minimum repetition length of the Al2O3 crystal along the (011j2) direction. The steps and terraces were stable under repeated scans with no sign of corrosion. Atom-scale features were not resolved on the terraces. A periodic, hexagonal lattice arrangement was observed on an R-Al2O3(0001) wafer in water using a contact-mode AFM.41 There were bumps over the terraces. The topographic height of the bumps was less than 0.1 nm. This number is too small to assign the bumps into some chemical species at the surface. Instead, the authors assume that the bumps appeared from some errors in the constant frequency-shift feedback. 3.2. ∆f-Distance Curve over Al2O3. Over the surface shown in Figure 1, ∆f was precisely determined as a function of the tip-surface distance. The amplitude of cantilever oscillation was reduced at 0.2 nm to identify hydration layers. This amplitude is comparable to the size of a water molecule. One hundred ∆f-distance curves were observed at different lateral positions over the Al2O3 surface. Panel a of Figure 2 presents a twodimensional ∆f distribution constructed with the observed curves. Four of individual curves are shown as i-iv in panel b. The four curves were observed with lateral distances less than 1 nm, as evidenced in panel a, and presented different ∆f
features as a function of the vertical distance. The forward i curve was determined with the tip approaching the surface, whereas the backward i curve was determined with the tip leaving the surface at the same lateral position. The identical features of the two curves suggest that the ∆f-distance measurements were reproducible at a fixed lateral position. In addition, the tip-surface near-contact does not alter the solution structure. The different shapes of curves i-iv indicate that the tipsurface force is heterogeneous over the surface. Positive (or negative) shifts of frequency represent repulsive (or attractive) tip-surface forces. Sader and Jarvis42 established a quantitative relationship to covert the frequency shifts to the force. The 100 ∆f-distance curves were thereby converted to 100 force-distance curves. Definition of the zero tip-surface distance is not a trivial issue. Our current definition in Figure 2b was the distance where the converted repulsive force exceeds 350 pN. 3.3. Electric Double Layer Force. The AFM tip and Al2O3 surface are accompanied by their own electric double layers when immersed in the electrolyte solution. The gradient of the ion concentration is modified in the overlapped portion of the double layers. The energy consumption or gain to modify the gradients causes the tip-surface force. In our KCl solution of 1 mol L-1, this electric double layer (EDL) force was short ranged. Butt et al.43,44 described an analytical form of EDL force with a spherical tip of radius R and a plane surface as a function of tip-surface distance d.
F)
4πRσtipσsur exp(-κd) εκ
(1)
σtip and σsur are the surface charge density on the tip and surface. ε is the electric permittivity of water. The Debye length in the solution, 1/κ, is given by,
1 ) κ
εkBT e2z2cbulk
(2)
cbulk represents the ion concentration of the bulk solution. z and e are the valence of the electrolyte and the elementary charge. The 100 ∆f-distance curves were converted to 100 forcedistance curves. The 100 force-distance curves were averaged and fitted to eq 1, as shown in Figure 3. The best fit was achieved with,
Aqueous Solution Structure over R-Al2O3(01-12)
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Figure 3. The force-distance curve averaged over the solution-Al2O3 interface. The exponential function fitted to the observed curve is shown with the gray line.
F ) 3.2 × 10-10N exp(-6.8 × 109d/m)
(3)
The Debye length deduced from the fitted function was 0.15 nm. This length agreed with the length from eq 2, 0.20 nm. A negative σtip is assumed on a silicon tip after ref 45. Thus the positive sign of the preexponential factor of the fitted eq 3 suggests a negative σsur of the Al2O3 wafer. The suggested negative charge density is consistent with that reported in the literature. The isoelectronic point was reported to be pH 5 in a sum-frequency study20 and pH 5-6 in a surface force study.13 Negative charge densities were estimated on solution-alumina interfaces at pH 7.46,47 The authors recognize that the propriety of fitting to eq 1 is limited in the electrolyte solution of 1 mol L-1 and avoid quantitative discussion about the surface charge densities provided from the fit. A short-ranged, monotonic, repulsive surface force was found in sapphire-solution-sapphire interfaces.12 The observed surface force was interpreted with energy cost to collapse the hydration shell of solute cations. A number of hydrated cations should have been pinched and collapsed in the two sapphire plates of a macroscopic size. On the other hand, pinching a K cation should be rare in the small tip apex and the sapphire surface. The contribution of the shell-collapse force is thus less probable with the tip-surface force depicted in Figure 3. 3.4. Hydration Structure. The EDL force represented by eq 3 was subtracted from the force curves. Figure 4 shows the four subtracted curves deduced from curves i-iv in Figure 2. Each subtracted force-distance curve should contain two force terms: the force induced by water molecules pushing the tip, and the van der Waals (VDW) force pulling the tip to the solid. The repulsive contribution of the VDW force is apparent at distances smaller than 0.1 nm. The attractive contribution of the VDW force is less evident and perhaps present at distances of 0.2-0.5 nm. The water-induced force provides force oscillations. The force curves present different features as a function of the distance with local maxima at distances of 0.2, 0.4, 0.6, and 0.8 nm. We ascribe the heterogeneity of force-distance curves to the uneven distribution of hydration layers over the surface. When a minitip is attached to the tip apex, as frequently assumed in AFM studies, different manners of hydration can be probed at different lateral positions. The layered structure of water distribution is present in a limited region from the surface, whose thickness should be less than 1 nm. The minitip penetrates into the structured region. Structured water pushes the minitip, and the resonant oscillation of the whole cantilever is affected by the force applied on the minitip. The tip body protrudes out of the structured region and remains in the bulk solution. The number of water molecules in contact with the tip body, and
Figure 4. The subtracted force-distance curves over the solution-Al2O3 interface. The ∆f-curves i-iv in Figure 2 were converted to force-distance curves. The averaged EDL force in Figure 3 was subtracted from each converted force curve. Curves I-IV were deduced from converted curves i-iv in Figure 2. Local maxima of the repulsive tip-surface force are indicated with circles. The electron density distribution found in an XCTR study23 was differentiated with respect to the vertical coordinate, inverted in the sign, and inserted on the top of the figure. Local minima of the electron density gradient are marked with arrows.
hence the force applied to the tip body, is insensitive to the tip-surface distance. Suppose that an infinitely small tip is in the potential given by the convolution of the density distribution of water molecules F and the tip-water pair potential, u.40
U(rt) )
∫ F(r)u(r - rt)dr
(4)
where U is the convoluted potential and rt is the coordinate of the tip. When the pair potential is simplified to a δ function,
u(r - rt) ≈ δ(r - rt)
(5)
the convoluted potential is proportional to the density distribution.
U(rt) ∝ F(rt)
(6)
The force applied on the tip is given by the gradient of U and therefore proportional to the gradient of the density distribution.
F(rt) ∝ -∇F(rt)
(7)
On the basis of this assumption, we compared the force-distance curves with the gradient of the electron density distribution determined in an XCTR study.23 The sign of the electron density gradient is inverted and inserted on the top of Figure 4. According to eq 7, each force maximum should correspond to a local minimum of the electron density gradient. This was the case with the results shown in Figure 4. The four force curves are not completely identical. This suggests different manners of hydration on different lateral positions. Water is dissociated on Al3+ cations to produce hydroxyl species. The hydroxyl species can create hydrogen
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bonds with water in the solution. The water density distribution may be modified in the presence or absence of Al3+ cations and hydroxyl species. The XCTR results23 reveal the density distribution laterally averaged over the interface. On the other hand, the force-distance curves represent the unaveraged density distribution probed by the AFM tip. In this respect, the XCTR-based density distribution gradient can be related to an average of force-distance curves, rather than individual curves. The assumption presented in this subsection still has much room to be improved. A finite-sized AFM tip should be located in its own solution atmosphere. The real tip-water potential should be more than a two-body potential. A better quantitative relation of the density distribution and force-distance curves is an issue for further studies. 4. Summary An R-Al2O3(011j2) surface immersed in a concentrated aqueous electrolyte solution was probed with an advanced FMAFM. In addition to the topography of the surface, the solution structure at the interface was deduced from force-distance curves. The density distribution of water was not uniform in the proximity of the surface, being consistent with the results of a previous XCTR study. The electrostatic force produced by the overlapped electric double layers of the tip and surface was also identified. The promising ability of FM-AFM to observe the layered water distribution and electric double layers is demonstrated on a practically important metal oxide. Acknowledgment. The authors thank J. C. Catalano for his provision of raw XCTR results of ref 23. This work was supported by a Grant-in-Aid for Scientific Research (KAKENHI) on Priority Areas [477] “Molecular Science for Supra Functional Systems”. T.H. was supported by the Japan Society for the Promotion of Science Fellowship. References and Notes (1) Schildbach, M. A.; Hamza, A. V. Surf. Sci. 1993, 282, 306–322. (2) Liu, P.; Kendelewicz, T.; Brown, G. E., Jr.; Nelson, E. J.; Chambers, S. A. Surf. Sci. 1998, 417, 53–65. (3) Elam, J. W.; Nelson, C. E.; Cameron, M. A.; Tolbert, M. A.; George, S. M. J. Phys. Chem. B 1998, 102, 7008–7015. (4) Eng, P. J.; Trainor, T. P.; Brown, G. E., Jr.; Waychunas, G. A.; Newville, M.; Sutton, S. R.; Rivers, M. L. Science 2000, 288, 1029–1033. (5) Barth, C.; Reichling, M. Nature 2001, 414, 54–57. (6) Thiel, P. A.; Madey, T. E. Surf. Sci. Rep. 1987, 7, 211–385. (7) Henderson, M. A. Surf. Sci. Rep. 2002, 46, 1–308. (8) Kelber, J. A. Surf. Sci. Rep. 2007, 62, 271–303. (9) Israelachivili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1992. (10) Sakuma, H.; Otsuki, K.; Kurihara, K. Phys. ReV. Lett. 2006, 96, 046104. (11) Horn, R. G.; Clarke, D. R.; Clarkson, M. T. J. Mater. Res. 1988, 3, 413–416. (12) Ducker, W. A.; Xu, Z.; Clarke, D. R.; Israelachivili, J. N. J. Am. Ceram. Soc. 1994, 77, 437–443.
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