Langmuir 1999, 15, 3015-3017
Aluminum Oxide Probes for AFM Force Measurements: Preparation, Characterization, and Measurements Henrik Guldberg Pedersen* Department of Chemistry, Technical University of Denmark, DK-2800 Lyngby, Denmark Received May 27, 1998. In Final Form: February 16, 1999
Introduction The state and the resulting properties of a colloidal suspension are to a large extent determined by the magnitude, range, and sign of the interactions between the individual particles.1-3 This interaction can be investigated indirectly by macroscopic methods (e.g., rheology and sedimentation), but direct measurement of the interactions provides the actual force-distance profiles.1-6 The atomic force microscope has become an important tool for investigation of surface chemistry through such measurements.6 One advantage of the AFM methodsalso called the colloidal probe techniquesis that any materials in principle can be employed as probe and sample. Hence, the probe material can be that of commercial AFM cantilevers, that is, Si3N4,7 Si and its oxides, a surface coating,8 or a particle glued onto the end of the AFM cantilever.9-15 Particles for this purpose typically have sizes in the upper range of the colloidal regime, that is, a few micrometers. Some requirements for appropriate particles should be fulfilled: well-defined geometry, high rigidity, and smooth contact area. A simple geometry, such as spherical, simplifies comparison of the measured surface forces with theory. A number of inorganic materials have been used; for example, the interactions between SiO2,9 TiO2,10 Al2O3,11 ZrO2,12 and ZnS13 surfaces and combinations14,15 have been measured by AFM. R-Al2O3 is an important raw material for preparing advanced technical ceramics, and these materials are usually * Current Affiliation: Haldor Topsøe A/S, DK-2800 Lyngby, Denmark (1) Russel, W. B.; Saville, D. A.; Schowalter, W. R. Colloidal Dispersions; Cambridge University Press: Cambridge, U.K., 1995. (2) Israelachvili, J. Intermolecular and Surface Forces, 2nd ed.; Academic Press: San Diego, CA, 1991. (3) Pugh, R. J., Bergstro¨m, L., Eds. Surface and Colloid Chemistry in Advanced Ceramics Processing; Marcel Dekker: New York, 1994. (4) Horn, R. G.; Clarke, D. R.; Clarkson, M. T. J. Mater. Res. 1988, 3, 413-416. (5) Ducker, W. A.; Xu, Z.; Clarke, D. R.; Israelachvili, J. N. J. Am. Ceram. Soc. 1994, 77, 437-443. (6) Butt, H.-J.; Jaschke, M.; Ducker, W. Bioelectrochem. Bioenerg. 1995, 38, 191-201. (7) Senden, T. J.; Drummond, C. J.; Ke´kicheff, P. Langmuir 1994, 10, 358-362. (8) Ito, T.; Namba, M.; Bu¨hlmann; P.; Umezawa, Y. Langmuir 1997, 13, 4323-4332. (9) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Langmuir 1992, 8, 1831-1836. (10) Larson, I.; Drummond, C. J.; Chan, D. Y. C.; Grieser, F. J. Am. Chem. Soc. 1993, 115, 11885-11890. (11) Pedersen, H. G.; Høj, J. W.; Engell, J. In Fourth Euro Ceramics; Galassi, C., Ed.; Gruppo Editoriale Faenza Editrice S.p.A., Faenza, 1995; Vol. 2, pp 31-38. (12) Prica, M.; Biggs, S.; Grieser, F.; Healy, T. W. Colloids Surf., A 1996, 119, 205-213. (13) Toikka, G.; Hayes, R. A.; Ralston, J. Langmuir 1996, 12, 37833788. (14) Larson, I.; Drummond, C. J.; Chan, D. Y. C.; Grieser, F. J. Phys. Chem. 1995, 99, 2114-2118. (15) Larson, I.; Drummond, C. J.; Chan, D. Y. C.; Grieser, F. Langmuir 1997, 13, 2109-2112.
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manufactured by a process having at least one aqueous, colloidal step. Therefore, optimal processing requires information on the interactions between R-Al2O3 particles.3,16 The forces between two R-Al2O3 surfaces (i.e. symmetric system) have been studied with the surface forces apparatus (SFA) using single-crystal sapphire platelets4,5 and in a preliminary atomic force microscope (AFM) study with a tabular shaped R-Al2O3 particle as probe.11 At surface separations greater than a few nanometers, the measured forces agreed well with the DLVO (Derjaguin-Landau-Verwey-Overbeek) theory.4,5,12 In this theory the total interaction is calculated as the sum of attractive van der Waals forces and repulsive doublelayer forces.1,2 AFM force measurements with asymmetric systems (R-Al2O3 as sample) have been carried out with a silica sphere15 and a standard Si3N4 AFM tip17 as probe. For preparation of inorganic spheres with diameters in the micrometer range, a number of methods have been used, for example, nozzle-reactor and emulsion techniques.18,19 Most of these procedures require specialized equipment, while the amount needed for force measurements is smallsa few grams suffices. To increase the versatility of the AFM as a forcemeasuring device, a simple laboratory-scale method for fabricating micron-sized spheres has been developed. The method can be characterized as a sol-gel-assisted reversed emulsion (water-in-oil) technique, and as an example R-Al2O3 spheres have been prepared. The suitability as AFM probes was investigated through force measurements with consolidated spheres in aqueous solutions and by comparison of the obtained data to DLVO theory. Experimental Section A surfactant solution was prepared by mixing 16 g of SPAN 80 (sorbitan monooleate, Fluka) and 400 mL of mineral oil (technical grade) in a 1 L conical flask during magnetic stirring. An aqueous boehmite sol (25 wt % γ-AlOOH, AL20, Nyacol Products) was peptized by addition of 1 M HNO3 until pH 4, and subsequent treatment in an ultrasound bath for 20 min. One hundred twenty milliliters of this sol was emulsified by slow addition to the surfactant solution during vigorous stirring (1000 rpm). The formed spherical boehmite sol droplets were then gelled by passing pressurized air through a bubbling flask containing 30% NH3 and into the water-in-oil emulsion. During this addition the pH in the water phase was raised to approximately 8. Water was removed from the emulsion droplets by heating to 110 °C during magnetic stirring. The dried spheres were separated, dried overnight in a heating cabinet, and heated in an R-Al2O3 crucible to the desired temperature and annealed for 30 min. After this consolidating calcination the spheres were characterized by single-point BET (Micromeritics Flowsorb II 2300). X-ray diffraction (Philips PW 1705 powder diffractometer) was used to identify crystalline phases, and from line broadening the average crystallite size was determined. One emulsion sample was treated in an ultrasound bath for 15 min prior to gelation by ammonia addition. This resulted in much smaller spheres, ≈0.5-5 µm, than those prepared by magnetic stirring alone (5-50 µm). Aqueous suspensions (0.1 vol %) of the smaller spheres were used for electrophoresis experiments (Zetamaster, Malvern Instruments, U.K.). AFM probes were prepared by gluing 10-30 µm spheres onto the end of standard triangular Si3N4 cantilevers (Digital Instru(16) Lange, F. F. J. Am. Ceram. Soc. 1989, 72, 3-15. (17) Arai, T.; Daisuke, A.; Yoh, F.; Fijuhara, M. Thin Solid Films 1996, 273, 322-326. (18) Wilcox, D. L., Sr.; Berg, M. Mater. Res. Soc. Symp. Proc. 1995, 372, 3-13. (19) Deptula, A.; Rebandel, J.; Drozda, W.; Lada, W.; Olczak, T. Mater. Res. Soc. Symp. Proc. 1992, 271, 277-283.
10.1021/la980621u CCC: $18.00 © 1999 American Chemical Society Published on Web 03/25/1999
3016 Langmuir, Vol. 15, No. 8, 1999
Notes
Figure 2. Crystallite size (closed circles) and specific surface area (open squares) of gelled boehmite (AlOOH) spheres calcined for 30 min at different temperatures.
Figure 1. SEM micrograph of an R-Al2O3 probe after use in AFM force measurements. ments) using a method adapted from Ducker et al.9 with a twocomponent epoxy resin (Araldit, Casco Nobel) as adhesive. Sphere diameters with an accuracy of (0.7 µm were determined by image analysis of optical micrographs. Cantilever spring constants were determined to 0.13 ( 0.02 N/m by the method described by Cleveland et al.20 A cantilever with a mounted R-Al2O3 sphere is shown in Figure 1. Forces between the probe and a sintered, polished polycrystalline R-Al2O3 sample plate were measured with a Burleigh ARIS 3600 AFM. After polishing, the plate was boiled in a HCl/ H2O2/H2O solution (3:2:50 by volume) for 2 min, rinsed in water, boiled in a NH3/H2O2/H2O solution (3:2:50 by volume) for 5 min, and rinsed. The root mean square roughness of the polished, rinsed surface was determined to 3 nm for a 4 × 4 µm2 area by AFM examination. A small poly(ethylene) plate glued on top of the cantilever holder served as a simple liquid cell. Filtered water (18.2 MΩ‚cm, Millipore, MilliQ grade) and analytical grade NaCl, NaOH, and HCl were used to prepare solutions of desired pH and ionic strength. Just prior to force measurements, probe and sample were rinsed in ethanol and water and irradiated with UV light for 20 min. The liquid cell was flushed with ≈5 times the cell volume of sample solution, and the system was allowed to equilibrate for 15 min before force measurements. To prevent effects from evaporation, the solution was exchanged every 1015 min. Five different probes and two different sample plates prepared as described above were used to test the reproducibility. Raw data were converted to force-distance curves by the method of Ducker et al.,9 where zero surface separation is defined at the region of constant compliance and zero force is defined at large separation with no detectable surface forces.
Results and Discussion To obtain the pure R-Al2O3 phase, heating the boehmite gel spheres to 1200 °C was found to be necessary. The crystalline transformation from transition phases (γ-, θ-, and δ-Al2O3) to the desired R-Al2O3 commences at 1100 °C and is accompanied by a significant crystallite growth, as shown in Figure 2. Furthermore, the heat treatment results in a decrease in surface area and an increase in density, as expected during sintering of the primary particles constituting the spheres. Spheres calcined at (20) Cleveland, J. P.; Manne, S.; Bocek, D.; Hansma, P. K. Rev. Sci. Instrum. 1993, 64, 403-405. (21) Bergstro¨m, L. Adv. Colloid Interface Sci. 1997, 70, 125-169. (22) Grabbe, A. Langmuir 1993, 9, 797-801. A DLVO simulation program was developed on the basis of this reference and used in the present calculations (courtesy of A. Grabbe). (23) Stankovich, J.; Carnie, S. L. Langmuir 1996, 12, 1453-1461.
Figure 3. Normalized forces as a function of surface separation between R-Al2O3 surfaces measured in aqueous solutions at pH 6.2 with different concentrations of NaCl. Solid and dashed lines denote constant charge and constant potential fit, respectively. Fitted curves: (squares) 0.0001 M NaCl, |ψ0| ) 26 mV, 1/κ ) 30.2 nm; (circles) 0.001 M NaCl, |ψ0| ) 22 mV, 1/κ ) 9.6 nm; (triangles) 0.01 M NaCl, |ψ0| ) 24 mV, 1/κ ) 3 nm.
1200 °C were chosen for force measurements. These had a specific surface area of 6 m2/g and about 90% of the theoretical density. Force curves measured at pH 6.2 with different NaCl concentrations are shown in Figure 3. The measured force-distance curves were compared to calculated DLVO force curves using a trial-and-error procedure. With optical data for alumina and water from Bergstro¨m,21 the nonretarded dispersive contribution to the Hamaker constant was estimated to be Aν>0 ) 3.47 × 10-20 J and the much smaller static contribution was estimated to be Aν)0 ) 0.2 × 10-20 J. Retardation of the dispersive part of the attractive van der Waals force was calculated according to the approximation by Russel et al.1 The Poisson-Boltzmann equation was solved numerically with the technique described by Grabbe22 to determine the double-layer repulsion under constant charge (σ0) and constant surface potential (ψ0) boundary conditions. According to Stankovich et al.,23 application of the Derjaguin approximation is valid for the present system with a micron-sized sphere and a flat plate:
F(D)sphere-plate ) π‚W(D)plate-plate R
(1)
Notes
Figure 4. Normalized forces as a function of surface separation between R-Al2O3 surfaces measured in aqueous solutions of 0.001 M NaCl at different pH values. Solid and dashed lines denote constant charge and constant potential fit, respectively. Fitted curves: pH 4.6 (squares), |ψ0| ) 12 mV; pH 5.8 (circles): |ψ0| ) 20 mV; pH 8.5 (triangles): |ψ0| ) 28 mV. For all fits: 1/κ ) 9.6 nm.
This expression relates the force normalized with sphere radius (F/R) to the flat-plate interaction energy (W) used in the calculations. The force curves shown in Figure 3 decay exponentially as expected, the decay length (i.e. the Debye screening length, 1/κ) decreasing with increasing electrolyte concentration, and the curves are between the two limiting conditions. Also, the positions of the “jumpin” points, where the force gradient exceeds the cantilever spring constant, are in good agreement with the fitted curves. Deviations at low separations may be caused by a small yet finite surface roughness but could also be due to a short-ranged non-DLVO force. The adhesive minima on the decompression part of the force-distance curves (not shown) decreased as the ionic strength was increased; from ≈ -1000 µN/m in pure Millipore water to ≈ -100 µN/m in 0.01 M NaCl at pH 6.2. This systematic variation with ionic strength may be due to adsorption of ions on the surfaces, resulting in a thin (
