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Langmuir 2006, 22, 8850-8859
Structural Effects Recorded for AFM Tips Interacting with Individual Nanoparticles and Their Clusters Deposited on Substrates Jaroslaw Drelich,*,† Zhenghe Xu,‡ and Jacob Masliyah‡ Department of Materials Science and Engineering, Michigan Technological UniVersity, Houghton, Michigan 49931, and Department of Chemical and Materials Engineering, UniVersity of Alberta, Edmonton, AB T6G 2G6 Canada ReceiVed May 15, 2006. In Final Form: August 12, 2006 Mica and alumina were coated with nanoparticles using aqueous suspensions while managing attractive substrateparticle electrostatic forces. Using nanoparticle-coated substrates, structural forces were measured for 10 nm silica particles deposited on the alumina substrate and 5-80 nm alumina particles on mica using an atomic force microscopy technique. For nanoparticles forming clusters, oscillation of structural forces was recorded with a periodicity that is close to the size of nanoparticles used. Positioning the AFM tip over the single particles allowed, on the other hand, the study of probe-nanoparticle colloidal forces.
Introduction Systems of nanoparticles dispersed in liquids such as nanofluids for cooling systems, dispersions for the formulation of nanocomposites, or nano-slurries for ultra-polishing and ultra-grinding of electronic materials have only recently been developed and utilized. The mechanisms by which nanoparticles interact with surrounding fluids, neighbor nanoparticles, and macroscopic surfaces are not understood at this stage of development, and these mechanisms are expected to be more complex than for larger particles. This is mainly caused by a large interfacial area of such finely dispersed systems and the expectation that the surface properties of nanoparticles are different than the properties of macroscopic surfaces, mainly due to high curvature of the nanoparticle surface. Although the size of the nanoparticles dispersed in a liquid is an important factor for the control of sedimentation and agglomeration, solid-liquid interactions and resulting liquid ordering in a vicinity of the solid surface as well as interparticle forces also must be understood and manipulated if suspensions of nanoparticles with sufficient stability and properties for practical applications are to be developed. For example, because of the small dimensions of nanoparticles and the correspondingly large solid-liquid surface area, a significant fraction of the liquid phase in a suspension of nanoparticles resides within a few nanometers of the nanoparticle surfaces. Consider, for example, 10 nm spherical particles dispersed in a liquid at a concentration of 5 vol %. If the solid-liquid interactions cause ordering of liquid layers to a depth of 1 nm (this seems a realistic dimension for many systems1-3) as much as 3.8 vol % of liquid in nanofluid will exist in an ordered state. This volume will increase to 9.2 and 17.3 vol % if the diameter of the nanoparticles is reduced to 5 and 3 nm, respectively. Undoubtedly, there is a critical concentration of nanoparticles in suspensions which when exceeded promotes a suspension to have properties that are entirely * Corresponding author. E-mail:
[email protected]. † Michigan Technological University. ‡ University of Alberta. (1) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: New York, 1992. (2) Rabinovich, Y. I.; Derjaguin, B. V.; Churaev, N. V. AdV. Colloid Interface Sci. 1982, 16, 63. (3) Peschel, G.; Belouschek, P.; Muller, M. M.; Muller, M. R.; Konig, R. Colloid Polym. Sci. 1982, 260, 444.
controlled by the solids. The scenario for suspensions with electrolytes is even more complicated due to competitive solidion and solid-liquid interactions causing the concentration of ions to increase significantly near the solid surface as compared to the ion concentration in the bulk liquid. An electric double layer formed by ions around the dispersed charged particles has a thickness up to a few tens of nanometers in aqueous phase and up to a few microns in organic phase with a composition and thus properties different than the bulk solution.1 The thickness of the electric double layer is often larger than the separation distance between nanoparticles in nanofluids. As a result, the electric double layers of neighbor nanoparticles overlap, and nanofluids lack what is called “bulk liquid” in suspensions with microparticles. None of the current theories predict the distribution and concentration of ions in suspensions where practically the entire liquid phase can be considered as an interfacial liquid. The distance between nanoparticles in suspensions can be very small. For example, a distance of about 30 nm separates particles having a diameter of 15 nm if their concentration is 1 vol %. This distance drops to about 10 nm for particles 5 nm in diameter. Attractive van der Waals forces operate at less than 5-10 nm distances and will cause nanoparticles to agglomerate in the absence of any repulsive short-range forces such as solvation or steric forces (colloidal forces are reviewed by Israelachvili1). The agglomeration of nanoparticles caused by van der Waals forces, mainly imposed by a physical factor of geometry in concentrated suspensions, significantly reduces the chances for formulation of homogeneous suspensions with well-dispersed individual particles. Additionally, agglomeration of nanoparticles can, perhaps, be initiated locally; for example, during thinning of a thin macroscopic film of suspension when the concentration of particles might increase as the result of particles’ entrapment in the thin liquid film. The nature of the particle-liquid, particleparticle, and particle-surface interactions in systems with nanoparticles is, therefore, of fundamental importance to the eventual understanding and control of these systems. In our previous papers, we reported structural effects measured with atomic force microscopy (AFM) for highly concentrated suspensions of nanoparticles.4,5 Previous results suggest that (4) Drelich, J.; Nalaskowski, J.; Long, J.; Xu, Z.; Masliyah, J. In Functional Fillers and Nanoscale Minerals: New Markets/New Horizons; Kellar, J. J., Ed.; SME: Littleton, CO, 2006; pp 217-229.
10.1021/la061375m CCC: $33.50 © 2006 American Chemical Society Published on Web 09/16/2006
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Table 1. Particle Size Distribution for Alumina Powder Used in This Study as Determined from High-Resolution Scanning Electron Microscopy Images for a Population of 1234 Particles5 particle diameter [nm] population [%]
5-10 18.7
10-20 27.8
20-40 33.7
40-60 14.2
60-80 5.6
sufficiently high concentrations of particles have a tendency to either form ordered layers or such organized structures are induced by the AFM probe in a thin liquid film separating two solid surfaces. The AFM force-distance curves with a stepwise character were recorded4,5 suggesting that ordered nanoparticles are responsible for additional stabilizing forces called “structural forces”.1,6,7 The distance between step transitions appeared to be directly proportional to the effective size of the nanoparticles dispersed in the suspension. It has been unclear, however, whether the recorded steps on the force-distance curve result from stratification of nanoparticles in thin film induced by the AFM probe approaching the substrate surface or they are caused by clustering of nanoparticles on the substrate and/or probe surfaces. Our previous studies also showed that nanoparticles can easily coat solid surfaces from suspensions that promote attractive substrate-nanoparticle electrostatic interactions.4,5 The attachment of fine particles to surfaces is not an unusual phenomenon as “slime coating” of mineral particles and oil droplets has been observed in mineral processing and oil recovery operations for several decades.8-14 In this research, we study the structural effects for the AFM tips that interact with substrates coated with nanoparticles. Such systems mimic what might happen if nanoparticles are attracted to larger solids and “precipitate” on their surfaces as individual particles, or more often form clusters/aggregates. There are two identified practical benefits from the approach used in this study. We demonstrate that clusters of nanoparticles can be deposited on a substrate from suspension, for which structural effects can be studied with atomic force microscopy. Such systems are, for example, relevant to mineral processing separation technologies as discussed in a separate study.15 We also show that surface forces can be measured between AFM tips and deposited individual nanoparticles, which may lead to research areas that have not been explored yet. Experimental Section Alumina particles with a size range of 5-80 nm were purchased from Nanophase Technologies Co. and they were suspended in water or 10-4M KCl solution at a concentration of 1-5 wt %. The isoelectric point for the alumina particles was determined in our previous study to be at about pH 8.7-8.9.5 The particle size distribution determined from high-resolution scanning electron microscopy (SEM) images for a population of over 1200 particles is shown in Table 1.5 The SEM images of alumina powder also revealed that these nanoparticles are spherical. (5) Drelich, J.; Long, J.; Xu, Z.; Masliyah, J.; Nalaskowski, J.; Beauchamp, R.; Liu, Y. J. Colloid Interface Sci. 2006, 301, 511. (6) Sethumadhavan, G. N.; Nikolov, A.; Wasan, D. Langmuir 2001, 17, 2059. (7) Sethumadhavan, G. N.; Nikolov, A.; Wasan, D. J. Colloid Interface Sci. 2001, 240, 105. (8) Fuerstenau, D. W.; Gaudin, A. M.; Miaw, H. L. Am. Inst. Min. Eng. Trans. 1958, 211, 792. (9) Gaudin, A. M.; Fuerstenau, D. W.; Miaw, H. L. Trans. Can. Inst. Min. Metall. 1960, 56, 960. (10) Attia, Y. A.; Deason, D. M. Colloids Surf. 1989, 39, 227. (11) Huynh, L.; Feiler, A.; Michelmore, A.; Ralston, J.; Jenkins, P. Miner. Eng. 2000, 13, 1059. (12) Xu, Z.; Liu, J.; Choung, J. W.; Zhou, Z. Int. J. Miner. Process. 2003, 68, 183. (13) Zhou, Z. A.; Hussein, H.; Xu, Z.; Czarnecki, J.; Masliyah, J. H. J. Colloid Interface Sci. 1998, 204, 342. (14) Liu, J.; Xu, Z.; Masliyah, J. Can. J. Chem. Eng. 2004, 82, 655. (15) Drelich, J.; Xu, Z.; Masliyah, J. In Proceedings of the Conference of Metallurgists, COM 2006 Montreal, in press.
Suspension of 10 nm silica particles at a particle concentration of 30 wt %, pH ∼10.5, density 1200 kg/m3, and viscosity 8 mPas was received from Eka Chemicals Inc. The commercial name for this product containing 0.55 wt % Na2O is Nyacol 830. This commercial product was diluted 10-20× and the pH was reduced to pH 9-9.2 before use. The zeta potential values presented in our previous paper show that these particles are negatively charged in water having pH > 5.4 The imaging of silica nanoparticles with the SEM was unsuccessful. The AFM images of these particles suggest that they are spherical in shape. Freshly cleaved muscovite mica (Ward’s Natural Science) and a re-polished optical-grade sapphire (R-alumina) disk (Harrick Scientific Co.) were used as substrates, but the substrates were coated with nanoparticles of alumina or silica before surface force measurements. Coating of the substrates with nanoparticles was usually done before installation of the substrate in the AFM fluid cell. Mica was immersed in 1-5 wt % alumina suspension with pH 8.5-8.7 and alumina disk in 1.5-3 wt % silica suspension with pH 9.0-9.2 for 2-5 min. The coated substrates were washed with deionized water, blew with a stream of dry nitrogen to remove excessive water, and immediately installed on the AFM piezoelectric transducer and covered with the fluid cell to avoid overdrying. In selected experiments, a suspension of alumina particles was injected for 1-2 min to the AFM fluid cell equipped with a mica substrate and cantilever prior to the surface force measurements. The fluid cell was washed with deionized water after removal of suspension for each test. The surface force measurements were performed using a Nanoscope E AFM (Digital Instruments Inc.) in a fluid cell. The measurements of surface forces in AFM operational contact mode between probes and substrates were carried out in aqueous suspensions of nanoparticles using the colloidal probe technique.16 This commonly used surface force measurement technique and the interpretation of recorded results are well described in the literature17-21 and will not be repeated here. Triangular-shaped contact-mode cantilevers (NP, Veeco) with pyramidal silicon nitride tips having a spring constant of 0.12 N/m and the tip curvature radius of 20-60 nm were used in all experiments. Probes were cleaned by UV irradiation for at least 30 min prior to the experiments. The measurements of surface forces between AFM tips and substrates were carried out in deionized water or 10-4 M KCl (pH ) 6.0-6.5). The force curves were analyzed with the SPIP software (Image Metrology, Lyngby, Denmark), which translates the cantilever deflection-piezo extension/retraction data to force-separation profiles. The processing of the force curves include baseline and hysteresis corrections. Sensitivity values were taken from the recorded cantilever deflection-piezo extension curves and varied from about 36 to 38 mV/nm. The force graphs in this paper present individual tipsubstrate approaching curves and no averaging of the data points was performed. Topographical images were obtained for nanoparticle-coated substrates using the intermittent contact mode operation on a Digital Instruments Nanoscope E AFM. Images with 512 × 512 resolution were captured using RTESP10 cantilevers from Veeco with the resonance frequency of 262-316 kHz, spring constant of 20-80 N/m, and radius of tip curvature of about 10 nm at a scan rate of 1 Hz.
Results and Discussion Coatings of Substrates by Nanoparticles. Attractive electrostatic forces between oppositely charged substrates and nanoparticles were used to fabricate coatings of alumina nanoparticles on mica (Figure 1) as well as coatings of silica (16) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Langmuir 1992, 8, 1831. (17) Cappella, B.; Dietler, G. Surf. Sci. Rep. 1999, 34, 1. (18) Butt, H.-J.; Cappella, B.; Kappl, M. Surf. Sci. Rep. 2005, 59, 1. (19) Veeramasuneni, S.; Yalamanchili, M. R.; Miller, J. D. J. Colloid Interface Sci. 1996, 184, 594. (20) Liu, J.; Xu, Z.; Masliyah, J. Colloids Surf. A: Physicochem. Eng. Aspects 2005, 260, 217. (21) Liu, J.; Zhang, L.; Xu, Z.; Masliyah, J. Langmuir 2006, 22, 1485.
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Figure 1. AFM tapping mode topographic image of a mica substrate covered with 5-50 nm alumina particles. Arrow points into individual alumina particle (∼40 nm in diameter), circle marks location of a cluster made of three alumina nanoparticles, and square frames a large aggregate made of at least 14 alumina nanoparticles. Note that some nanoparticles and their clusters can have distorted shapes and dimensions due to the tip convolution effects. For example stretching of the image in the section between y ) ∼0.27 and ∼0.35 µm is the result of temporary attachment of an alumina nanoparticle to the AFM tip.
Drelich et al.
Figure 3. AFM tapping mode topographic image of an alumina substrate covered with a multilayer of ∼10 nm silica particles. Note that the size of silica nanoparticles appear larger than 10 nm due to the tip convolution effects.
Figure 4. Schematic drawing of structures of nanoparticles that are deposited from aqueous suspensions on the surface of a charged substrate. In this drawing, the particles are shown to be positively charged and substrate negatively charged. Clusters are considered in this study as small aggregates composed of a few nanoparticles and aggregates are made of a large number of nanoparticles, often forming multilayer construction.
nanoparticles on alumina disk (Figures 2 and 3). Similar approach was used by Atkins et al.22 They made use of the attractive electrostatic forces to deposit 4-5 nm cerium oxide nanoparticles on mica from suspensions, as well as prepare coatings of 9-10 nm silica on top of the ceria-coated mica. Atkins et al. used these coated materials to measure colloidal forces in water of varying pH using a surface force apparatus. Nanoparticle-based coatings deposited on the substrates from suspensions in this study were not uniform and comprised of a mixture of single nanoparticles, clusters, and aggregates, all randomly distributed over the substrate surface. Clusters and aggregates of alumina nanoparticles were already present in preformulated suspensions and were difficult to avoid due to
strong particle-particle interactions. Clusters/aggregates of silica nanoparticles most likely formed directly on the alumina disk of rough and heterogeneous characteristics. Figures 1 and 2 show the AFM topographic images for two of the nanoparticle-coated substrates used in this study. Note that the shape and size of nanoparticles and their clusters are often distorted on such images due to the convolution effects caused by the AFM tip whose curvature dimension was close to or larger than the size of some of the nanoparticles deposited on the substrate.23 The artifacts in the images were sometimes worsen by the nanoparticles transferring, permanently or temporarily, from the substrate to the AFM tip during scanning, enlarging the size of the scanning tip. As a result the imaged nanoparticles appear larger than they actually are. Schematic drawing of common structures observed during scanning of nanoparticle-coated alumina and mica substrates is shown in Figure 4. In Figures 1 and 2, as examples, the arrows point to the location of individual particles, circles surround clusters made of a few nanoparticles, and squares mark larger aggregates made of at least several nanoparticles (Figure 4). Similar structures of random density were identified at several coated substrates and at different locations. Our experiments indicate that the number of nanoparticles deposited on the substrate surface could be controlled by, at least, the concentration of particles in the suspension, pH
(22) Atkins, D.; Kekicheff, P.; Spalla, O. J. Colloid Interface Sci. 1997, 188, 234.
(23) Butler Velegol, S. Dekker Encyclopedia of Nanoscience and Nanotechnology; Marcel Dekker: New York 2004, pp 143-153.
Figure 2. AFM tapping mode topographic image of an alumina substrate covered with ∼10 nm silica particles. Arrow points into location of individual silica nanparticle, circle marks location of a cluster made of 6-7 nanoparticles, and square frames a large aggregate made of a few tenths of nanoparticles. Note that some nanoparticles and their clusters can have distorted shapes and dimensions due to the tip convolution effects.
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Figure 5. Force versus separation curves for a silicon nitride tip interacting with mica surface (triangle) and with a single alumina particle (∼30 nm in diameter) deposited on mica (squares) in ∼10-4 M KCl solution (pH 6.5-6.7). Zero separation position corresponds to tip in contact with the mica surface. Theoretical curve for the tip-mica geometry was plotted based on equations presented in the Appendix and with the following parameters: R ) 48 nm, 2γ ) 70°, κ-1 ) 35 nm, σ1 ) -3.8 mC/m2 for AFM tip, σ2 ) -6 mC/m2 for mica, and A ) 2.7 × 10-20 J. Table 2. Zeta Potential Values for Materials Used material
zeta potential [ref] (1 mM KCl, pH 6.0-6.5)
silica alumina mica silicon nitride
-45 to -50 mV [27]; -80 to -85 mV [28] +30 to +40 mV [5] -70 to -75 mV [28] -2 to -10 mV (in 0.01M KCl) [25]
of suspension, and time of immersion. However, no systematic study was undertaken in this direction. A few observations from this study suggest that the controlled deposition of nanoparticles on a substrate, regarding their number and distribution, is difficult in this coating process. Only when deposition of nanoparticles on the substrate surface took place from highly concentrated suspensions, more dense and uniform coatings were observed (Figure 3). Such coatings however, with excessive number of particles, were of a little interest to us in this study. After substrate coating, the AFM tip, made of silicon nitride, was moved stepwise to various locations of the substrate and then along one of the axes in 3-5 nm steps by an operatorcontrolled offset distance, and surface forces were measured between the AFM tip and nanoparticles or substrate. Because of the random distribution of nanoparticles on the substrate, sitting as individual nanoparticles or forming either smaller clusters or larger aggregates, the surface forces measured reflected the interaction of the AFM tip with individual particles, their clusters/ aggregates, or uncoated substrate. Examples of the recorded surface forces and their interpretations are presented in the following sections. Surface Forces for the AFM Tip-Single Nanoparticle Systems. Alumina Nanoparticles on Mica. Figure 5 shows an example of recorded surface force versus separation curves for the AFM tip interacting with mica as well as with an individual alumina nanoparticle deposited on the mica surface. Mica is negatively charged in KCl aqueous solutions at pH 6.0-6.524 (Table 2), and therefore, repulsive forces were recorded between the AFM tip and mica in the absence of deposited nanoparticles, (24) Lyons, J. S.; Furlong, D. N.; Healy, T. W. Aust. J. Chem. 1981, 34, 1177.
over the entire range of separations, up to about 100 nm. The recorded experimental force data fitted well to a simplified theoretical model, which combines electric double layer and van der Waals forces. Equations for the model are specified in the Appendix. Because the tip curvature radius and surface charge density for both the mica and silicon nitride tip were not measured in this study, the obtained fitting cannot be used in the model validation. Nevertheless, the surface charge densities used in the fitting appear of reasonable values in view of the experimental results reported in the literature for silicon nitride25,26 and mica.1,24 The force versus separation for the AFM tip interacting with mica changed when an alumina nanoparticle was present under the area of tip-mica interactions (Figure 5). This is mainly due to a positive charge of alumina5 and its attractive interactions with the negatively charged AFM tip (the isoelectric point for Si3N4 is located at pH 6-7, depending on the treatment of the surface25). See Table 2 for selected zeta potential values taken from the literature. Because of the small dimension of the nanoparticle, ∼30 nm as per our estimate (see next paragraph), the repulsive forces between the AFM tip and mica dominate at tip-particle surface distances larger than 10-15 nm whereas the tip-alumina attractive forces dominate at 30 nm. The aggregates of fine alumina particles are more difficult to break in liquid during the preparation of suspension than aggregates made of particles with larger dimensions due to the larger surface area per volume for fine particles interacting at contact through the van der Waals forces. Silica on Alumina. Figure 10 shows two examples of force versus separation curves recorded for the AFM tip interacting with clusters/aggregates of silica nanoparticles deposited on the alumina substrate. The forces recorded in this system are repulsive. Both the silicon nitride tip25 and silica nanoparticles5 are negatively charged at the experimental solution pH 6.0-6.5. The force-separation curve is not smooth again. The force profile indicates a stepwise change in the interactions when the tip penetrated through the structure of deposited nanoparticles and approached a “contact” position with the substrate. Two peaks at about 17-19 and 9-11 nm of the tip-to-substrate separation indicate that the tip, most likely, interacted with a cluster made of two “layers” of silica nanoparticles. Difficulties associated with recording force vs separation curves with more than two steps for this system suggest that either three layer structures were not formed in this system or the repulsive forces between the negatively charged tip and the negatively charged silica nanoparticles were sufficient to remove nanoparticles weakly interacting with the alumina substrate and forming a third layer on top of the alumina substrate. The first scenario is more probable in view of the fact that the force curves could be fitted with the DLVO model as discussed later. There is a significant difference in the shape of the force curves recorded for clusters made of silica nanoparticles (Figure 10) as (36) Wasan, D.; Nikolov, A.; Moudgil, B. Powder Technol. 2005, 153, 135.
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Figure 10. Force versus separation curves for silicon nitride probes approaching and penetrating through clusters of 10 nm silica particles deposited on the alumina substrate in water (pH 6.5-6.7).
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Figure 11. Force versus separation curve for a silicon nitride probe approaching and penetrating a cluster of 10 nm silica particles deposited on the alumina surface in 10-4 M KCl, pH 6.5-6.7. Theoretical fitting was plotted based on equations presented in the Appendix and with the following parameters: R ) 30 nm, 2γ ) 70°, κ-1 ) 30 nm, σ1 ) -4 mC/m2 (silicon nitride), σ2 ) -4 mC/m2 (silica), and A ) 1.4 × 10-20 J.
compared to those composed of alumina nanoparticles (Figure 9). The steps of the force curves are above a “baseline”, which expands up to about 50 nm for silica clusters and suggest on repulsive interactions between the AFM tip and silica cluster before their contact and during the tip’s penetration through the cluster. Attractive interactions between alumina cluster-on-mica and the AFM tip were only noted at large separations, before the tip made the first contact with the cluster. The recorded force curves oscillated around the y ) 0 axis after the first contact with alumina cluster was made. It is possible that the negative charge of the penetrating AFM tip was reduced or eliminated soon after its contact with the alumina cluster, during which one alumina nanoparticle or more jumped into the tip surface. Such scenario is not possible during the experiments with silica clusters due to the same negative charge on both the AFM tip and silica. The force curves, especially their section representing a longdistance interaction between the AFM tip and the silica cluster, could be fitted with the DLVO model.37,38 Figure 11 shows an example of the theoretical fitting for the experimental data taken from Figure 10A. The fitting was done for a conical-flat geometry using the equations presented in the Appendix. As shown in Figure 11, experimental data overlap the theoretical curve well at separations larger than about 20 nm. Deviations at shorter separations,
