Modification of interparticle forces for nanoparticles using atomic layer deposition

Chemical Engineering Science 62 (2007) 6199 – 6211 www.elsevier.com/locate/ces

Modification of interparticle forces for nanoparticles using atomic layer deposition L.F. Hakim a , J.H. Blackson b , A.W. Weimer a,∗ a Department of Chemical and Biological Engineering, University of Colorado, Boulder, CO 80309, USA b The Dow Chemical Company, Midland, MI 48640, USA

Received 20 February 2007; received in revised form 6 July 2007; accepted 13 July 2007 Available online 19 July 2007

Abstract Silica and titania nanoparticles were individually coated with ultrathin alumina films using atomic layer deposition (ALD) in a fluidized bed reactor. The effect of the coating on interparticle forces was studied. Coated particles showed increased interactions which impacted their flowability. This behavior was attributed to modifications of the Hamaker coefficient and the size of nanoparticles. Stronger interparticle forces translated into a larger mean aggregate size during fluidization, which increased the minimum fluidization velocity. A lower bed expansion was observed for coated particles due to enhanced interparticle forces that increased the cohesive strength of the bed. Increased cohesiveness of coated powders was also determined through angle of repose and Hausner index measurements. The dispersability of nanopowders was studied through sedimentation and z-potential analysis. The optimum dispersion conditions and isoelectric point of nanoparticle suspensions changed due to the surface modification. A novel atomic force microscope (AFM) technique was used to directly measure interactions between nanoparticles dispersed on a flat substrate and the tip of an AFM cantilever. Both Van der Waals and electrostatic interactions were detected during these measurements. Long and short range interactions were modified by the surface coating. 䉷 2007 Elsevier Ltd. All rights reserved. Keywords: Interparticle forces; Fluidization; Atomic layer deposition; Nanostructure; Films; Imaging

1. Introduction The flow of particles is significantly influenced by the interactions existing between them. These interactions are dependent upon several factors such as particle size and roughness, surface chemistry, density, surface charge and more (Israelachvili, 2002). Owing to their small dimensions, nanoparticles commonly aggregate, forming structures that are several times bigger than the primary particles (Seville et al., 1997). The aggregation of particles complicates the control of manufacturing operations. When aggregates are present, transport mechanisms through the aggregates become important, making it more difficult to predict and control the quality of the final product. Therefore, there is a fundamental need to understand the forces existing between ultrafine particles. If interactions

∗ Corresponding author. Tel.: +1 303 492 3759; fax: +1 303 492 4341.

E-mail address: [email protected] (A.W. Weimer). 0009-2509/$ - see front matter 䉷 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2007.07.013

between particles could be controlled without affecting their size distribution, the performance of several applications could be improved. Depending upon the application, it is desirable to have either decreased or increased interparticle forces. In some instances, such as the formulation of paints, it is imperative that pigments like sub-micron sized titania particles be homogeneously dispersed (Tiarks et al., 2003). Similarly, superior dispersion of nanosized titania particles is required in sunscreen materials to optimally block ultraviolet radiation (Allen et al., 2004). Ultrafine boron nitride particles are used as fillers in epoxy resins to improve the thermal conductivity of microelectronics packages. For this application, it is critical to appropriately disperse the boron nitride particles in order to reduce the viscosity of the slip and to increase the loading of particles for improved heat transfer (Bian et al., 2005). On the other hand, the sintering of ultrafine tungsten carbide or silicon nitride powders, for the production of circuit board micro-drill bits or turbochargers/valves for internal combustion engines, respectively, requires the existence of strong

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interparticle forces to achieve high-density sintered parts (Azcona et al., 2002; Zhu et al., 2006). During the tabletting of various powders, a granulation step is required in order to yield tablets with a high mechanical strength at low compression loads. This operation involves increasing the interactions between powders to enhance their compaction behavior (Murakami et al., 2001). Surface modification is a common step performed in particle technology in order to give special characteristics to particle substrates (Misev and Van der Linde, 1998). The forces controlling the interactions between fine particles are dependent, among other factors, upon the surface chemistry. Therefore, when surface modification is carried out, a change in the interparticle forces is expected. However, this modification may also have undesirable effects on other particle properties. Surfactants used to disperse pigment titania powders in paints may react with the photo-catalytically active titania substrate resulting in a detrimental change of color of the paint. Likewise, additives to reduce the viscosity of boron nitride particles in epoxies may have a negative impact on the interfacial adhesion between the particles and the resin in the cured package. Delamination of the embedded filler particles with the polymer would result in significantly reduced thermal conductivity of the microelectronics package. Hence, there is a need to control the interparticle forces between ultrafine primary particles via surface modification while maintaining the bulk particle properties. Preferably, this surface tuning is achieved by a chemical reaction in which the surface properties are permanently modified and not the result of a physical modification of the surface that can degrade over time. Since many ultrafine powders are produced or processed in gas phase aerosol flow and fluidized bed reactors, there is also a need to understand the effect of surface modification on the behavior of ultrafine powders in these unit operations. Atomic layer deposition (ALD) is the only known process to allow the chemical modification of primary dry nanoparticle surfaces with highly conformal ultrathin films without significantly affecting the particle size distribution. In this work, the effects of alumina ALD coatings on the interparticle forces of ceramic nanoparticles are studied.

1.1. Main interparticle forces Several types of interactions promote aggregation between ultrafine particles. Liquid bridging, electrostatic and adhesive forces are some of the most important. Liquid bridging originates when water or another liquid vapor adsorbs onto the surface of individual particles forming links with other particles and promoting aggregation (McLaughlin and Rhodes, 2001). Particles are more or less likely to adsorb water depending on their hydrophilic/hydrophobic behavior. This characteristic is generally controlled by chemical surface groups on the surface of particles. This means that only a chemical surface modification of individual particles would allow for controlling the tendency of particles to adsorb liquids. This could then limit the extent of liquid bridging and aggregation of particles. In

order to achieve a noticeable effect, this surface modification would need to occur on the surface of all primary particles. If only the surface of aggregates is modified, liquid bridging between particles forming the aggregates would still remain and particle aggregation would not be reduced significantly. Electrostatic interactions also occur due to specific functional groups that exist on the surface of particles. Such chemical species commonly originate as a residue from the particle manufacturing process. This electrostatic interaction between particles is not necessarily detrimental. When packed very closely together, particles with the same surface charge will tend to repel each other, thus reducing aggregation. However, interactions between charged particles and the walls of pipes and vessels are undesirable. When electrostatic interactions exist between ultrafine particles, they are commonly larger in magnitude than the inertial (due to their weight) forces of the particles (Seville et al., 2000). Consequently, particles will tend to adhere to the walls of all processing equipment. This again would diminish the quality of the flow of particles and thus worsen the performance of important applications. The charge on the surface of particles can be altered by modifying the particle surface chemistry. Most of the methods available to reduce electrostatic interactions rely on the use of liquid surfactants, which limits the application of such techniques in gas–solid operations. An alternative surface modification approach is the dry encapsulation of particles using materials with a more convenient surface charge. However, conventional techniques for encapsulation yield coatings that are very thick, relative to the size of ultrafine particles, which would adversely modify the bulk particle properties. Therefore, there is an opportunity for the surface modification of very fine particles using ultrathin coatings that alter the surface properties of particles while maintaining desired bulk characteristics. The last, and probably most intriguing, type of forces found affecting ultrafine particles is the dispersion London–Van der Waals interactions, generally known as cohesive forces (Hamaker, 1937). The importance of such interactions rests on the fact that they always exist, as they are related to the chemical nature of materials. London–Van der Waals forces derive from a temporary fluctuating dipole character that molecules show due to a distortional electron density (Visser, 1989). When surfaces are close to each other, this phenomenon results in a generalized attractive interaction. Due to the small mass of nanoparticles, this force becomes more dominant than inertial forces and aggregation is commonly observed. The Van der Waals force between two macroscopic spheres of the same diameter can be approximated by (Israelachvili, 2002) Fvw =

rA , 12a 2

(1)

where r is the particle radius; A is the Hamaker coefficient; and a is the interparticle separation distance. The Hamaker coefficient is a measure of the cohesive interaction between different substrates (Avishai et al., 2005; Podgornik et al., 2006). Controlling the attraction of particles due to London–Van der Waals forces is difficult. In order to control these forces, entire particles would need to be encapsulated using a material

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with different dielectric properties than those of the particle substrate. If this encapsulation is applied to ultrafine particles, even extremely thin films will represent an important fraction of the coated particle volume and a change in cohesive forces could be achieved. Ultrathin films would also assure that the bulk properties of the particles would not be affected. Moreover, if this coating process could be controllable at the atomic scale so that the thickness of the thin film could be precisely adjusted, the cohesive interaction between particles could be tuned depending on the required application. 2. Experimental 2.1. Nanoparticle coating Silica (Aerosil䉸 OX-50, Degussa, 40 nm) and titania (Aeroxide䉸 P-25, Degussa Corp., 21 nm) nanoparticles were coated using an ALD Fluidized Bed Reactor. The system was operated at reduced pressures and under mechanical vibration. Deionized water and trimethylaluminum (TMA) (97%, Sigma-Aldrich Co.) were used as reactants and the reaction temperature was 450 K. A detailed description of the experimental procedure for ALD coating of nanoparticles is included in previous publications by the authors (Hakim et al., 2005a,b, 2006). 2.2. Flowability tests Titania nanoparticles were used to investigate the effect of surface modification on the flowability of powders. The fluidization behavior of particles before and after 50 ALD coating cycles was studied using the fluidization apparatus described in a previous work (Hakim et al., 2005c). Before analysis, powders were sieved using a 100 mesh (150 m) tray to eliminate large agglomerates that may have formed in packing conditions. Minimum fluidization velocity was determined by fluidizing particles at low pressure ( ∼ 15 Pa) and under mechanical vibration. Two pressure transducers were used to record the change in pressure drop through the bed versus superficial gas velocity. The change in bed expansion during fluidization was measured at atmospheric pressure and without mechanical agitation. The bed height at various gas flowrates was recorded using a scale placed on the outside of the glass fluidization column. The size and sphericity of nanoparticle aggregates during atmospheric fluidization were analyzed using a high-speed laser imaging system. The Hausner index (HI) and angle of repose of nanoparticles were determined before and after ALD coating. HI was obtained by calculating the ratio of the tapped density to the bulk density of powders. Both densities were determined experimentally using a graduated cylinder. Tapped density measurements were carried out after the powder sample was subjected to vibration at constant force using a device with a frequency of 20 Hz and an amplitude of 3 mm. Angle of repose was obtained by calculating the inverse tangent of the ratio of the height to the radius of a heap formed when the powder

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was allowed to flow freely through a 1.5-cm aperture from a fixed height of approximately 15 cm. 2.3. Dispersion experiments Dispersability of titania nanoparticles before and after coating was analyzed using a UV/Vis spectrometer (Lambda 35, PerkinElmer) and a zeta potential analyzer (Nicomp 380/ZLS, Particle Sizing Systems). Experiments also investigated the effect of pH on dispersability. UV/Vis spectrometry allowed for tracking the absorbance of visible light (directly related to concentration) as a function of time in order to study the dispersion and sedimentation rate of particle suspensions. This test was carried out using a single wavelength of 420 nm. Zeta potential analysis was used to measure the particle surface charge as a function of pH and determine the optimum dispersion conditions. Suspensions were prepared by adding the nanopowders to 20 ml of deionized water. The mass of nanoparticles used for these suspensions was 3 ± 0.4 and 6 ± 0.3 mg for uncoated and coated samples, respectively. Larger amounts were used for coated particles, considering their higher density, in order to approximately maintain the same volume of particles. The pH of suspensions was then adjusted to values ranging from 3 to 11 using appropriate amounts of HCl and NaOH. While the powder suspensions were being agitated using a vortex mixer, 1 ml samples were removed and added to transparent cuvettes that were used for analysis. UV/Vis spectrometry and zeta potential measurements were started immediately after vortexing of suspensions at a frequency of 50 Hz. Analyses were performed directly after preparing the suspensions, in order to eliminate any possible effects of the liquid on the alumina film. 2.4. Atomic force microscopy studies Direct measurement of interactions between single nanoparticles and the tip of an atomic force microscope (AFM) cantilever was performed. For this objective, it was crucial to assure the appropriate dispersion of particles on a flat substrate. First, silica nanoparticles before and after 50 ALD cycles were dispersed in aqueous solutions at pH values of 3 and 11, respectively. These optimum dispersion conditions were found using UV/Vis spectrometry analysis, as described in the previous section. Immediately after vortexing, the nanoparticle suspension was poured onto a small piece of polished silicon wafer while it was rotated at high speeds using a spinning plate. This technique allowed for maintaining the dispersed state of nanoparticles while achieving a rapid removal of the liquid carrier. This step was critical as it prevented re-aggregation of particles during drying. An AFM (PicoSPM II, Molecular Imaging) was used to evaluate the dispersion of powders on the flat substrate and to perform force-displacement analysis between single particles and the cantilever tip. In order to reduce the effect of electrostatic interactions between the particles and the tip, the AFM analyses were performed inside a liquid cell. This is justified because

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electrostatic interactions are long ranged since their energy decays slowly, as 1/r, and remains strong at large distances. However, in a medium of high dielectric constant such as water, the strength and range of such interaction is much lower due to electroneutrality and ionic screening effects (Israelachvili, 2002). AFM scans were carried out using the contact mode of the microscope. Typical scan speeds were of the order of 500 nm/s. In order to minimize possible perturbations from other particles, a high aspect ratio AFM probe (NP Series, Silicon Nitride, Veeco Instruments) with a nominal tip radius of less than 10 nm was used. After the coordinates of single nanoparticles were found on a high-resolution scan, the AFM tip was moved to one of these locations using the microscope control software. At this position, an automatic routine varied the distance between the AFM tip and the substrate, while recording the change in force. This process was repeated several times for different nanoparticles in order to obtain a statistically significant value of the interaction force of particles before and after coating. 3. Results and discussion 3.1. Nanoparticle coating An example of alumina-coated silica nanoparticles is shown in Fig. 1. The deposited alumina films are extremely conformal and uniform around individual nanoparticles. Additionally, films deposited on different particles are clearly differentiated because no sintering between particles occurs during the ALD surface reaction. The growth rate is calculated to be approximately 0.2 nm per coating cycle. Similar growth rates have been observed for other nanoparticle systems (Ferguson et al., 2000a,b). Film growth rates observed on nanoparticles are

Fig. 2. Fluidized aggregates of (a) uncoated and (b) alumina-coated titania nanoparticles.

typically higher than those measured on flat substrates. For high aspect ratio surfaces, less steric effects occur and gas precursors can reach the available active sites more easily. 3.2. Flowability analysis

Fig. 1. Silica nanoparticles individually coated with alumina ultrathin films.

The role of interparticle forces on the flowability of titania nanoparticles was investigated through the fluidization of powders before and after surface modification. The size of aggregates of nanoparticles was measured using a high-speed laser imaging system. Figs. 2(a) and (b) show representative images of fluidizing aggregates of nanoparticles before and after coating, respectively. It has been demonstrated that nanoparticles fluidize in the form of aggregates of several microns to hundreds of microns in size. These aggregates show a dynamic aggregation behavior where they break apart and reform during fluidization (Hakim et al., 2005c). Owing to the aggregation behavior of nanoparticles, their fluidization properties are dictated by the aggregate properties rather than those of primary nanoparticles. Modifying forces in the fluidized bed such as electrostatic interactions and liquid bridging has an important effect on

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properties like the aggregate size and sphericity. Similarly, changes in the particle surface chemistry are expected to modify interparticle forces. The results for the aggregate size and sphericity of nanoparticles as measured by the Visizer TM system are shown in Table 1. Results show that the aggregate size of coated particles was almost twice that of uncoated particles. Stronger interparticle forces would promote the formation of larger structures in the fluidized bed. Since nanoparticles are individually coated by ALD, the material on the surface of particles changed from titania to alumina during the coating process. It has also been demonstrated that ALD coating does not promote particle sintering, which minimizes the possibility of undesirable particle size effects. Additionally, a small decrease in the sphericity of aggregates of coated particles with respect to those of uncoated particles was observed. It is possible that the increased size observed for aggregates of coated particles motivated this relative decrease in sphericity. As aggregates become larger, the equilibrium between cohesive and inertial forces also changes. More massive particle aggregates would tend to show lower sphericities because they would collide with other aggregates with a larger momentum. The analysis performed using the laser imaging system is relevant because it allows for investigating the mesostructure of agglomerates while they are fluidizing. This intermediate structure of aggregates is of special importance in the case of nanoparticle systems as it defines the interactions between different levels of agglomeration and describes the

Table 1 Aggregate size and sphericity analysis for titania nanoparticles before and after coating Aggregate property

Uncoated

Coated (50 cycles)

Mean size (m) Sphericity (a.u.)

39.3 ± 8.2 0.64 ± 0.04

69.8 ± 27.9 0.59 ± 0.09

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progression of a system dominated by cohesive forces to one dominated by inertial forces (Chaouki et al., 1985; Arastoopour et al., 1988; Wang et al., 1998). Methods that study the formation of aggregates after the bed has defluidized have inherent limitations for capturing this intermediate structure. The bed expansion of nanoparticles during fluidization at atmospheric pressure was measured before and after surface modification. The results for this analysis are shown in Fig. 3. The increase in bed expansion was very similar for both samples at low gas velocities. However, the coated nanoparticle sample reached a maximum expansion of about 47%, whereas the bed of uncoated particles continued to expand until a value of 66% was reached. Once a superficial gas velocity of approximately 4.5 cm/s had been reached the bed expansion did not change significantly with increasing gas flowrates. This suggests that full fluidization was achieved at this gas velocity. This result has a direct relationship with the interparticle forces in a fluidized bed. During the expansion of a particle bed, the average distance between particles increases. If interparticle forces are increased, the separation between particles during fluidization is expected to be smaller. Larger interparticle forces also increase the tensile strength of the bed, which limits changes in its structure, such as expansion. The fluidization of nanoparticles at low pressure and under mechanical vibration was also studied. Bed pressure drop versus superficial gas velocity for titania nanoparticles before and after coating is shown in Fig. 4. The minimum fluidization velocity is the point at which the bed pressure drop remains constant, independent of gas velocity. The minimum fluidization velocity of titania nanoparticles increased from 0.2 to 0.25 cm/s after coating. This result is directly related to the increase in aggregate size presented in Table 1. As the interparticle forces are increased, larger structures are formed in the fluidized bed. More massive aggregates require higher gas flowrates to fluidize and thus higher minimum fluidization velocities are expected. Further analysis of the effect of surface modification on the flowability of powders was performed by measuring the Hausner index (HI) of nanopowders. This property is a measure

Fig. 3. Bed expansion during fluidization at atmospheric pressure of uncoated and alumina-coated titania nanoparticles.

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Fig. 4. Bed pressure drop versus superficial gas velocity at low pressure for titania nanoparticles before and after coating. Minimum fluidization velocities are marked with vertical lines.

Table 2 Powder classification based on Hausner index Hausner index (HI)

Geldart’s classification

HI > 1.4 HI < 1.2 1.2 < HI < 1.4

Type C powder Type A powder May show the behavior of both Type A and Type C powders

of the compressibility of a granular material and it is defined as the ratio of the tapped density to the bulk density of the powder (Rambali et al., 2001; Thalberg et al., 2004). The HI is also a good indicator of interparticle forces, since increased forces between particles generate stronger structures in a bed of powder and make a sample more likely to compact under the repeated application of a constant force. Powders can be classified relating the HI to the Geldart’s powder classification (Geldart, 1973) as shown in Table 2 (Alavi and Caussat, 2005). The tapped and bulk densities as well as the calculated HI for titania nanoparticles before and after ALD coating are shown in Fig. 5. Both bulk and tapped densities are higher for the coated sample. This is explained because the effective particle density increases during the coating process, due to the higher density of the alumina films with respect to the titania substrate. The independent analysis of tapped and bulk densities does not provide information on the interparticle forces. Only the ratio of these two properties is relevant for this purpose. The values of the HI for uncoated and coated particles are 1.38 ± 0.04 and 1.47 ± 0.01, respectively. This means that stronger interactions occur for coated particles, which makes them more cohesive and increases their tendency to compact under applied vibration. This result further demonstrates that the change in surface chemistry provided by the ALD coating indeed modifies the flowability of nanopowders. Additional flowability tests were performed by measuring the angle of repose of nanoparticles before and after coating. This measurement is the angle () formed between a horizontal plane

and the side of a heap of powder formed in a non-contained surface (Carr, 1976). This quantity can also be calculated by taking the inverse tangent of the ratio of the height to the radius of the powder heap. The value of the angle of repose is also used as a measure of the cohesiveness of powders since stronger interparticle forces provide larger strength to the heap and allow the formation of steeper structures. Conversely, powders with weak interactions will tend to flow easily and form very shallow structures. The flowability of powders can be qualitatively predicted based on their angle of repose as indicated in Table 3 (Alavi and Caussat, 2005). Two examples of the structures obtained during the angle of repose measurements are shown in Figs. 6(a) and (b). The angles measured for uncoated and coated nanoparticles were 46.2◦ ± 1.9◦ and 52.8◦ ± 2.9◦ , respectively. Based on the classification shown in Table 3, coated nanoparticles showed a more cohesive behavior than their uncoated counterparts. Another observation is that due to their reduced cohesiveness, uncoated powders showed increased flowability during testing and formed more uniform heaps. This is another confirmation that coated nanoparticles had stronger interparticle forces caused by the ALD surface modification and that such increased interactions affected the flowability of the powders studied. It is important to highlight that multiple effects may be involved in the stronger interactions observed for coated nanoparticles. On one hand, the Hamaker coefficient of titania (A ∼ 43) is larger than that of alumina (A ∼ 6) (Israelachvili, 2002), which would suggest stronger forces for uncoated particles. On the other hand, cohesive interactions may also be affected by changes in the particle size. Even though the ALD coating process does not promote particle sintering, the particle diameter is increased a few nanometers due to the deposited film. For the case of nanoparticles, this change in diameter represents an important fraction of the starting particle size. For example, a 3 nm film deposited on a 21 nm particle represents almost 53% of the volume of the coated particle. From

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Fig. 5. Density measurements and Hausner index calculation for uncoated and alumina-coated titania nanoparticles.

Table 3 Expected flowability of powders based on angle of repose Angle of repose ()

Expected flowability

55 <  < 70 45 <  < 55 38 <  < 45 30 <  < 38 25 <  < 30

Very cohesive Cohesive Low flowability Medium flowability High flowability

Eq. (1), changes in particle size will directly impact the Van der Waals forces. It is then likely that the contribution from the increase in particle size was more significant than that from the reduction in the Hamaker coefficient. 3.3. Dispersion of nanoparticles It is widely known that individual nanoparticles do not disperse naturally in dry systems. In a liquid environment, the equilibrium of forces changes and good dispersion of particles is feasible. Nonetheless, even in a colloidal state, interactions between nanoparticles are present. The general form of interaction potential energy of Van der Waals attraction between two colloidal particles of the same size is given by (Hiemenz and Rajagopalan, 1997)    A 2r 2 2r 2 f1 Wvw = − , (2) + + ln 6 f1 f2 f2 (3) f1 = a 2 + 4ar, 2 2 f2 = a + 4ar + 4r , (4) where A is the Hamaker coefficient; r is the particle radius; and a is the interparticle separation distance. The Hamaker coefficient of a specific material interacting with itself across a vacuum (A11 ) can be determined by using the Tabor–Winterson approximation which is based on the Lifshitz theory (Bergstrom, 1997): A11 =

3h (n21 − 1)2 , √ 16 2 (n21 + 1)3/2

(5)

Fig. 6. Images of (a) uncoated and (b) alumina-coated titania nanoparticles utilized for angle of repose measurements.

where n1 is the refractive index of the material in the visible light range; h is the Planck’s constant; and  is the plasma frequency of ∼ 3 × 1015 Hz. The quantity A11 can be used to calculate the Hamaker coefficient of the material interacting with itself in a water medium (A131 ) using the following (Sun et al., 2004):  2  A11 − A33 , (6) A131 = where A33 is the Hamaker coefficient of water interacting with itself across a vacuum. Using a similar approach, the Hamaker

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Fig. 7. Visible light absorbance of nanoparticle suspension at t = 0 and 15 h and different pH conditions for (a) uncoated and (b) alumina-coated titania particles.

coefficient of any given pair of materials interacting across any given media can be numerically approximated. Since cohesive Van der Waals forces are still present in a colloidal environment, additional methods are required in order to overcome such forces and improve the dispersability of nanoparticles. One method to achieve this goal is to modify the pH of the liquid media (Ramachandra Rao et al., 1999; Zhang et al., 2004; Addai-Mensah and Ralston, 2005; Mandzy et al., 2005; Singh et al., 2005). Altering the pH of a particulate suspension generates like charges of sufficient magnitude on the surface of the suspended particles. This causes strong electrostatic repulsions between the suspended particles and thereby dispersion of the suspension. When the magnitude of the surface charge density is higher, relative to the interparticle Van der Waals forces, a low-viscosity stable suspension is achieved. In order to determine the effect of pH and surface modification on the dispersability of TiO2 nanoparticles, aqueous particle suspensions with different pH values were prepared as described in the experimental section. The concentration of nanoparticles in dispersion was tracked over time using a UV/Vis spectrometer. The transmittance of the samples

is related to the absorbance of light (Al ) which is directly proportional to the dispersion concentration following the Beer–Lambert law   I Al = −log10 = cl b, (7) I0 where c is the concentration of the absorbing species; l is a constant known as the molar absorptivity or extinction coefficient; and b is the pathlength through the sample. The results of dispersion experiments for uncoated and coated titania nanoparticles are shown in Figs. 7(a) and (b), respectively. These plots show the light absorption of nanoparticle dispersions at the beginning of the test and after 15 h of sedimentation. Higher absorbance directly indicated a higher concentration. Since the mass of nanoparticles was maintained constant throughout samples with the same surface, higher concentration was an indication of improved dispersability. Analyses were performed at pH values in the range of 3–11. Several observations can be made from these results. First, for all pH values, the light absorbance at the beginning of the test was higher for uncoated particles than for coated samples.

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Fig. 8. Zeta-potential analysis for titania nanoparticles before and after ALD coating.

It has been demonstrated before that the ALD coating does not significantly affect the particle size distribution of particles. Therefore, a reduction in the initial dispersion of particles is mainly due to stronger interparticle forces that promote higher levels of aggregation. Second, optimum dispersion for uncoated particles was obtained at a pH of 3, whereas for the coated sample it was found at a pH of 11. The charging of a surface in a liquid can occur by two possible mechanisms: (i) the ionization of surface groups and (ii) the adsorption of ions from solution (Israelachvili, 2002). These phenomena will determine the characteristics of different ionic layers (Stern layer and electric double layer) that may interact with the substrate. It is likely that when surface modification is applied these charging mechanisms will change. After particle coating, the type and density of functional surface groups may change. Additionally, the tendency for ions to adsorb on a substrate may vary for different materials. At optimum dispersion conditions, the electric repulsion between nanoparticles is highest. If such repulsion is controlled by charging mechanisms, the optimal dispersion conditions for nanoparticles are expected to change with surface modification. Finally, it was also observed that the sedimentation velocity for coated particles was much higher than for uncoated particles. At optimum dispersion conditions, the absorbance of the uncoated sample decreased from 1.76 to 1.1 (or a change of 37.5%) after 15 h. On the other hand, coated nanoparticle suspensions showed a decrease in absorbance from 0.36 to 0.06, or a change of 83%. This result cannot be solely attributed to modified interparticle forces. After the ALD coating process, the effective particle density increased, due to the higher density of alumina versus that of titania. This has a direct impact on the sedimentation velocity of particles. Nonetheless, from the measurements at the beginning of the test, it was observed that coated particles showed a higher level of agglomeration due to stronger interactions. These larger structures will settle faster than smaller aggregates of uncoated particles, which would partially explain the higher sedimentation velocities observed.

Results from sedimentation experiments were confirmed by zeta-potential measurements. This property is the electrical potential that exists on the neighboring area of a colloidal particle. Zeta potential can be derived from measuring the mobility of particles in suspension when they are subjected to an electric field. Light scattering techniques are typically used to track particles with respect to time and measure their mobility. The mobility is defined as the velocity of particles per electric field unit. Results for zeta-potential analysis are shown in Fig. 8. An important quantity obtained from zeta-potential analysis is known as the isoelectric point. This is defined as the pH at which the substrate has a net surface charge equal to zero. This means that the isoelectric point is the condition of poorest dispersion, since there are not electrostatic interactions competing against Van der Waals forces. In a plot of zeta potential versus pH, the isoelectric point is the value of pH at which the zeta potential changes from positive to negative, or vice versa. The isoelectric point is also determined by charging mechanisms and therefore is expected to change with surface modification. The isoelectric point of titania particles changed from a pH of approximately 9.8 to a pH of about 7.9 after coating as shown in Fig. 8. This result further demonstrates that the ALD coating indeed modifies the interactions between particles in suspension. As an additional observation, the optimum dispersion conditions observed during sedimentation experiments (pH of 3 for uncoated and pH of 11 for coated particles) were also detected during zeta-potential analysis. This suggests that both techniques are suitable for studying the dispersability of nanoparticles in suspension. 3.4. Direct measurement of interaction forces The use of AFM has been suggested as a method to determine the interactions between particles and a reference substrate (Ducker et al., 1992; Lee, 1993; Witte, 1993; Pollock et al., 1995; Milling et al., 1996; Gady et al., 1997; Veeramasuneni et al., 1998; Yalamanchili et al., 1998;

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Fig. 9. Atomic force microscopy analysis for (a) uncoated and (b) alumina-coated silica nanoparticles.

Ramirez-Aguilar et al., 1999; Portigliatti et al., 2000; Eve et al., 2002; Forsyth et al., 2002; Andersson and Bergstrom, 2005). In the reported studies, micron-sized particles were physically attached to the tip of an AFM cantilever using techniques such as micro-manipulators and diverse types of adhesive materials. Force–displacement measurements were performed with these modified tips and a reference substrate. The interactions measured were between the micron-sized particle and flat surfaces. Due to the small size of nanoparticles, their individual manipulation is a major challenge. However, if nanoparticles are appropriately dispersed on a flat surface, interactions between the tip of a cantilever and individual particles can be potentially measured. Using the techniques as the one described before, the optimum dispersion conditions of 40 nm silica nanoparticles were determined. Following the procedure described in the experimental section, imaging of dispersed nanoparticles was performed by AFM. AFM scans of dispersed nanoparticles before and after coating are shown in Figs. 9(a) and (b). Despite the large cohesive interactions between nanoparticles, this technique allowed for dispersing particles fairly well on a flat substrate. This condition is necessary for directly measuring interactions between a single

particle and the cantilever tip while minimizing perturbative effects from other particles. If effective dispersion was not achieved, interactions with aggregates rather than with individual particles would be measured. When the material on the particle surface is modified, interactions with other particles are expected to change. As shown in Eq. (1), the attractive Van der Waals force between particles increases with an increasing value of the Hamaker coefficient. According to Lifshitz theory, cohesive forces scale as the dielectric properties of the particle volume. It is important to recognize that since the dielectric properties of silica and alumina are in the same order of magnitude, the expected change in cohesive forces might be small when thin films are applied to micron-sized particles. However, when encapsulation is applied to nano-sized particles, even extremely thin films will represent an important fraction of the coated particle volume and a change in cohesive forces may be achieved. Results for the force–displacement analysis of nanoparticles before and after coating are shown in Fig. 10. As mentioned before, these curves were obtained when an automatic routine in the AFM software varied the distance between the cantilever tip and the substrate, while recording the change in voltage. The interaction force was calculated by using the measured voltage, the deflection sensitivity (18.9 nm/V) and the spring constant of the cantilever (0.58 N/m). For clarity purposes, only the region near the tip-particle contact is shown in Fig. 10. The curves show a relative separation distance which does not correspond to the exact separation between the tip and the particles. This occurs because calibration of the tip position was done with respect to a flat substrate, which explains why the particle–tip contact occurred several nanometers above the substrate. Nonetheless, the displayed curves give equivalent information regarding the interactions between particles and the tip. For the two force–displacement curves shown, the value of zero force corresponded to the point where the cantilever was completely retracted from the sample. At this point, the separation distance between the tip and the particles was more than 1m. Considering the short range character of Van der Waals forces, it is reasonable to think that significant cohesive interactions did not occur between the particles and the tip at this point. However, the force–displacement curves show that the values of the interaction force before contact (indicated by relatively flat regions at higher separation distances) were not equal to zero. One possible explanation is that electrostatic interactions also existed between the tip and the particle. Even though these interactions were being considerably reduced by using the liquid cell, they could not be totally eliminated. Moreover, these forces have a long range character, which explains why they were present even when the relative separation distance was increased. One important observation is that the value of these electrostatic forces was different for uncoated and coated samples, which may be explained by the modification of the particle charging mechanism promoted by the alumina coating. As the separation distance between the tip and the substrate was reduced, a point was reached where Van der Waals attractions became important and a sharp decrease (indicative of stronger attractive interactions) in the value of the force was

L.F. Hakim et al. / Chemical Engineering Science 62 (2007) 6199 – 6211

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Fig. 10. Force–displacement analysis for uncoated and alumina-coated silica nanoparticles.

observed. This value continued to decrease until the tip and the particle were so close that they touched and started to undergo mutual deformation of their structures. At this point, the interaction force increased, rapidly achieving positive values. The difference (indicated in Fig. 10 by vertical arrows) between the force at larger separations and the maximum negative force observed is a measure of the short range forces that occur immediately before contact. For the case of uncoated nanoparticles this value was 13.2±2.6 nN after several measurements on different particles in the sample. For coated particles, this value was 6.2 ± 3.1 nN. Similar to previous results, variations in the interparticle forces may be explained by concurrent changes in both the Hamaker coefficient and the size of the nanoparticles. Since both Van der Waals and electrostatic forces were present during these measurements, it is not possible at this point to determine the individual contribution of each force. Nonetheless, this result demonstrates that the alumina ALD coating modified the interactions between the particles and the tip. The behavior of the interaction force observed during AFM studies cannot be directly related to the results of flowability or dispersability tests. For AFM measurements, interactions between the tip and the particles were detected, whereas in the other experiments, interactions between like particles were studied. Nonetheless, the combination of experiments described in this work demonstrates that ALD surface modification has an impact on forces of individual nanoparticles. Further improvements to the AFM technique presented here would allow for directly measuring the forces between individual nanoparticles and compare such results with theoretical calculations. The direct measurement of interparticle forces and the tailoring of such interactions using ALD represent an important scientific advancement in particle technology.

interparticle forces were attributed to changes in the Hamaker coefficient and the size of nanoparticles. Due to increased interactions, larger aggregate sizes and higher fluidization velocities were observed. Coated particles showed a lower bed expansion due to an increased strength of the particle bed. Higher cohesiveness of coated powders was also determined through angle of repose and HI measurements. Z-potential and sedimentation analyses allowed for determining the optimum dispersion conditions of nanoparticles. The isoelectric point of nanoparticle suspensions changed due to the deposited alumina film. Interactions between dispersed nanoparticles and the tip of an AFM cantilever were directly measured. Long (electrostatic) and short (Van der Waals) range interactions were altered by the surface modification.

Notation a A Al Anmn Ann

4. Conclusions

b c Fvw h HI I I0 ni

The effect of deposited ALD films on interparticle forces was investigated. The flowability of alumina-coated nanoparticles was impacted by the coating. Variations in the

r  Wvw

interparticle separation distance Hamaker coefficient absorption of light Hamaker coefficient of material n interacting with itself across a medium m Hamaker coefficient of material n interacting with itself across a vacuum pathlength through the sample concentration of absorbing species Van der Waals force Planck’s constant Hausner index intensity of sample beam intensity of reference beam refractive index of material i in the visible light range particle radius plasma frequency Van der Waals interaction potential energy

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Greek letters l 

molar absorptivity angle of repose

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