Particuology 8 (2010) 572–577
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Particuology journal homepage: www.elsevier.com/locate/partic
Scalable gas-phase processes to create nanostructured particles J. Ruud van Ommen a,∗ , Caner U. Yurteri a , Naoko Ellis b , Erik M. Kelder a a b
Delft University of Technology, Department of Chemical Engineering, Julianalaan 136, 2628 BL Delft, The Netherlands University of British Columbia, Department of Chemical and Biological Engineering, 2360 East Mall, Vancouver, B.C., Canada V6T 1Z3
a r t i c l e
i n f o
Article history: Received 25 May 2010 Accepted 25 July 2010 Keywords: Nanoparticles Nanocomposite materials Coating Films Particle coating Atomic layer deposition Core–shell particles Electrospraying Electrohydrodynamic atomization Electrostatic forces Fluidization
a b s t r a c t The properties of nanoparticles are often different from those of larger grains of the same solid material because of their very large specific surface area. This enables many novel applications, but properties such as agglomeration can also hinder their potential use. By creating nanostructured particles one can take optimum benefit from the desired properties while minimizing the adverse effects. We aim at developing high-precision routes for scalable production of nanostructured particles. Two gas-phase synthesis routes are explored. The first one – covering nanoparticles with a continuous layer – is carried out using atomic layer deposition in a fluidized bed. Through fluidization, the full surface area of the nanoparticles becomes available. With this process, particles can be coated with an ultra-thin film of constant and well-tunable thickness. For the second route – attaching nanoparticles to larger particles – a novel approach using electrostatic forces is demonstrated. The micron-sized particles are charged with one polarity using tribocharging. Using electrospraying, a spray of charged nanoparticles with opposite polarity is generated. Their charge prevents agglomeration, while it enhances efficient deposition at the surface of the host particle. While the proposed processes offer good potential for scale-up, further work is needed to realize large-scale processes. © 2010 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
1. Introduction In the past decade, interest in nanoparticles has been strongly increasing. As the size of particles becomes smaller, i.e., down to the nano-scale, the surface area per unit volume substantially increases, thus dramatically changing the particle properties. This may result in unique added value to the particulate materials because of their specific chemical, electro-magnetic, optical or other physical properties. However, certain properties that are typical for nanoparticles can make it challenging during production and application (Liu & Bell, 2006). For example, the high surface energy of nanoparticles leads to quick agglomeration, which makes it difficult to disperse. By smart design of the nanoparticles and the processes to manufacture them, these challenges could be overcome. For example, nanoparticles of active pharmaceutical ingredients are in principle very effective, but hard to transport into the lungs for treatment of respiratory diseases. Depositing them on micron-sized carrier particles can facilitate the dispersion and delivery of nanoparticles to targeted areas (Islam & Gladki, 2008). On the other hand, nanoparticles of LiMn2 O4 are a promis-
∗ Corresponding author. Fax: +31 15 2785006. E-mail address:
[email protected] (J.R. van Ommen).
ing cathode material for Li-ion batteries, but unstable because of the dissolution of transition metal ions in an electrolyte. This can be alleviated by forming an Al2 O3 layer of a few atoms thick on the surface of LiMn2 O4 (Beetstra, Lafont, Nijenhuis, Kelder, & van Ommen, 2009). For these examples to be applied in practice, quantities of coated nanostructured particles in the kg to tonne scale will be needed. Currently, there are few examples of nanoparticles produced on a large scale, e.g., titania and carbon black. More sophisticated nanostructured particles are still typically produced in very small quantities for research purposes. Most research in this field is devoted to the specific properties of nanostructured particles produced in small scale, and not to scalable production processes for nanostructured particles. Furthermore, many synthesis approaches for nanostructured particles are carried out in the liquid phase, partly because of historical reasons: many more chemists are researching nanoparticles, and they tend to work in the liquid phase. Prof. Kwauk (2004) clearly pointed out how the ‘conventional’ chemical industry has benefited from the introduction of engineering concepts such as unit operations and transport phenomena. We believe that chemical engineers can contribute in a similar manner to the development of novel processes for production of nanostructured particles. In several cases, it is advantageous to look beyond the commonly used liquid-phase approach for
1674-2001/$ – see front matter © 2010 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.partic.2010.07.010
J.R. van Ommen et al. / Particuology 8 (2010) 572–577
nanoparticle production. Gas phase methods offer inherent advantages such as the absence of solvent waste, the feasibility of continuous processing as opposed to batch processing in the liquid phase, a better potential for scaling-up, and the versatility of these processes with respect to particle material and size and structure (Samyel-Shall & Schmidt-Ott, 2006). In this paper, we will give two examples of gas phase production routes for nanostructured particles that can be scaled-up to produce large volumes without loosing control of particle specifications. 2. Atomic layer deposition in a fluidized-bed reactor A common technique for gas-phase coating objects with a closed layer is chemical vapour deposition (CVD). In a typical CVD process the substrate is exposed to one or more gaseous precursors, which react on a surface to produce the desired film. CVD is commonly used in the semiconductor industry, but can also be used to produce coated particles, e.g., noble metal catalyst particles and layered luminescent pigments (Czok & Werther, 2006). Since different chemical reactants coexist in the gas phase during the CVD reaction, homogeneous reactions can take place that form nanoparticles contaminating the product. Moreover, truly uniform and conformal films on individual nanoparticles have not been achieved (Hakim, Blackson, George, & Weimer, 2005). Instead of CVD, atomic layer deposition (ALD) can provide particles with an ultra-thin, uniform layer. This technique is different from CVD in that the chemistry is split into two half-reactions: the different reactant gases are fed to the sample consecutively rather than simultaneously. For example, for an alumina coating process, a precursor such as tri-methyl-aluminium (TMA) binding to the surface by chemisorption in step (A) reacts with an oxidiser such as water in step (B) (see Fig. 1). A simplified version of the reaction scheme is (Puurunen, 2005): (A) ||Al-OH + Al(CH3 )3 (g) → ||Al-O-Al(CH3 )2 + CH4 (g) (B) ||Al-CH3 Al-CH3 + H2 O (g) → ||Al-OH + CH4 (g)
(1)
where || denotes the solid surface. The number of times the (A)-(B) cycle is repeated determines the thickness of the coating, resulting in full control over the layer thickness at the atomic level. Atomic layer deposition can be applied to a wide range of particles sizes (∼10 nm–500 m) and materials. Weimer and coworkers (e.g., Ferguson, Weimer, & George, 2000; Hakim et al., 2005) showed that applying atomic layer deposition (ALD) to particles is best carried out when these particles are fluidized, i.e., suspended in an upward gas stream, to ensure good gas-solids contacting. While fluidization is widely applied to disperse and process particles on the micron-scale, it seems counterintuitive to apply it to nanoparticles. It is, however, possible, since they are not fluidized
Fig. 1. Schematic representation of ALD half reactions: (1) chemisorption of precursor (e.g., tri-methyl-aluminium) at substrate surface and (2) reaction of adsorbed species with second reactant (e.g., water).
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individually, but fluidized as large, very porous, fractal structured agglomerates due to the large cohesive forces between them (see, for example, Chaouki, Chavarie, Klvana, & Pajonk, 1985; Jung & Gidaspow, 2002; Wang, Gu, Wei., & Wu, 2002; Wang, Kwauk, & Li, 1998). During fluidization of nanopowders, problems such as bubbling, channelling, clustering, and elutriation of particles are often encountered, which disrupt good dispersion of the powder in the gas phase and lead to appreciable gas bypassing. Various assisting remedial methods have been developed to overcome these problems and enhance the fluidization of nanopowders (Quevedo, Omosebi, & Pfeffer, 2010; van Ommen & Pfeffer, 2010). In this study, we have used vibration to enhance fluidization. In the semi-conductor industry, ALD is typically carried out under vacuum to enhance the removal of non-reacted precursors and gaseous by-products. Typically Weimer and co-workers (e.g., Ferguson et al., 2000; Hakim et al., 2005) applied ALD to particles at low pressure, ∼100 Pa. However, we recently showed that ALD of fluidized particles can also be carried out at atmospheric pressure (Beetstra, Nijenhuis, Kelder, & van Ommen, 2007a, 2007b) which simplifies the fluidization of the particles and facilitates process scale up. While continuous coating of nanoparticles can be used for a wide range of applications (e.g., catalysis or imaging), we first aimed at the production of nanoparticulate cathode material for Liion batteries. Use of nanoparticles increases the effective capacity of batteries and reduces the charging time through reduction of the diffusion length. We have used LiMnO2 nanoparticles (∼100 nm) supplied by Erachem Comilog (St-Ghislain, Belgium). The reactor consisted of a 26 mm internal diameter, 500 mm long glass tube that was filled with 100–120 g of particles. The reactor was placed on a shaker driven by two vibromotors that produced a low amplitude vibration at adjustable frequency to assist fluidization. For the experimental runs reported here, the vibration frequency was set to 45 Hz. The nitrogen flow, controlled through the reactant bubblers, was set to 0.5 L/min; in some experiments this was diluted with a secondary flow of nitrogen to a total flow of 1.0 L/min. The gas entering the reactor was preheated in an inline heater to around 80 ◦ C; the reactor temperature was kept at 160 ◦ C using an infrared lamp. Effluent gases from the reactor were led through a double set of gas washers filled with mineral oil. The streams containing TMA and streams containing water were led through separate gas washers. Any TMA absorbed in the gas washers was neutralised after the experiment. The effluent from the gas washers was filtered using Pall Kleenpak pharmaceutical-grade sterilizing filters to capture elutriated nanoparticles. Pressure at the outlet was atmospheric. Presently, the drawback of using materials such as LiMn2 O4 for Li-ion batteries is that they suffer from degradation in the chemical environment of the battery as Mn-ions dissolve in the electrolyte. Since this is a surface related process, the problem increases when nanoparticles are used, as well with the presence of high voltages. Therefore, the surface of LiMn2 O4 was coated with an inert layer (in this case Al2 O3 ) that prevents dissolution of Mn-ions but does not influence the electrochemical behaviour of the battery, that is, the Li-ions should be able to diffuse through the layer. Fig. 2 shows transmission electron microscopy (TEM) images of the coated and uncoated material. The crystalline surfaces in these images are the LiMn2 O4 base particles. Coatings formed by ALD are generally amorphous, and are therefore more diffuse in the images. A coating cannot always be distinguished in the images, e.g., the coating in the lower part of Fig. 2(d) seems to disappear towards the top of the image, indicating that the coating process was not fully homogeneous, possibly due to the lattice orientation of the crystal. It may be harder for TMA to adsorb on certain lattice faces of the spinel material. Alternatively, the treatment of the particles after the process may also break up some agglomer-
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Fig. 2. TEM photos of coated and uncoated LiMn2 O4 particles: (a) uncoated material – overview; (b) uncoated material; (c) coated after 5 ALD cycles and (d) coated after 28 ALD cycles.
ates and therefore expose non-coated particle surfaces (Beetstra et al., 2009). Although a high surface coverage is attractive, to obtain 100% coverage is not necessary to lead to a much better battery performance in terms of degradation. To determine whether or not primary particles are coated rather than the agglomerates as a whole, we measured the surface area of the particles before and after coating, using the BET adsorption method, as shown in Table 1. The surface area decreases slightly, and the effective particle diameter increases from 7.3 × 102 to 1.1 × 103 nm. If the agglomerates are coated as a whole, the effective particle diameter of the coated material would be close to the agglomerate size, around 30 m. The decrease of the total surface area is due to some of the smaller gaps between grains getting filled with alumina. As can be seen in Fig. 2(a), some of the primary particles are in close proximity. With several ALD-cycles, they can be ‘glued’ together, resulting in the observed decrease of surface area. However, the majority of the particles are coated as individual particles by the process.
3. Electrospray deposition of nanoparticles on host particles The second route to fabricate nanostructured particles we will discuss is the deposition of nanoparticles (guest particles) on larger particles (host particles), e.g., for transportation, catalytic activity, or as biological contact points. This is referred to as discrete coating (Pfeffer, Dave, Wei, & Ramlakhan, 2001; see Fig. 3). Most processes currently used in the upcoming field of discrete coating use mechanical forces in order to bring the nanoparticles in contact with the micron-sized particles (Pearnchob & Bodmeier, 2003; Pfeffer et al., 2001). In order to achieve better control over discrete coating, we have developed a method using electrostatic forces to bring guest and host particles into contact. This has the advantage of the coating being less dependent on the stochastic mixing of the particles, thus easier to optimize.
Table 1 Surface area of coated and uncoated particles, determined by BET adsorption. # cycles
Surface area (m2 /g)
Effective particle diameter (nm)
0 5 11 28
1.9 1.7 1.5 1.3
7.3 × 102 8.2 × 102 9.3 × 102 1.1 × 103
Fig. 3. Schematic of discrete coating Adapted from Pfeffer et al. (2001).
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To charge and disperse the nanoparticles, we have used electrohydrodynamic atomization (EHDA), also called electrospraying. Along with applications in mass spectrometry, the EHDA is receiving much attention as a potential source for production of structured micro- and nano-materials that have a variety of applications in medicine, biology, chemistry and electronics. Furthermore, the electrospraying of nanoparticle laden liquids resolves the apparent problem of effectively dispersing the nanoparticles. Electrospraying of such suspension generates a spray of charged droplets that are seeded with nanoparticles. Thus EHDA offers a solution for dispersing and depositing nanoparticles on microparticles. EHDA is a process in which a liquid jet breaks up into droplets under the influence of electrical forces (Cloupeau & Prunet-Foch, 1994; Grace & Marijnissen, 1994). A liquid is pumped through a nozzle at a low flow rate (L/h to mL/h). An electric field is applied over the liquid by applying a potential difference between the nozzle and a counter electrode. When the electric stresses overcome the surface tension stresses, the emerging liquid meniscus from the tip of the nozzle is transformed into a conical shape. From the cone apex, a jet emerges which breaks up into quasi-monodisperse droplets. Unipolarity of the droplets prevents their coagulation and dispersion is enhanced. Recently, the application of electrospraying of suspensions of nanoparticles has been demonstrated (Suh, Han, Okuyama, & Choi, 2005), generating a spray of charged nanoparticle laden droplets. The utilization of a volatile liquid leads to its fast evaporation. Thus the droplets shrink, and at some critical diameter, break up into smaller droplets. This occurs at the Rayleigh charge limit, which is reached when the mutual repulsion of electric charges at the surface exceeds the confining force of surface tension. This process repeats itself until droplets are formed which contain one or a few nanoparticles, depending on their initial concentration in the suspension. Eventually, when the liquid phase is completely evaporated, a spray of charged nanoparticles is obtained. EHDA leads to the formation of unipolar charged suspensions of guest nanoparticles, while host particles can be charged with opposite polarity by means of tribocharging, corona, or inductive charging. When these particles are brought into contact in an appropriate way, the mutual electrostatic attraction force between the negative and positive charges will cause a coating to be deposited on the surface. Interaction can be realized in two ways: host particles can be encapsulated with a polymer and nanoparticles; or nanoparticles can be discretely deposited on the surface of the host particles. We will focus on the latter case, in which fast evaporating nanoparticle suspensions are sprayed to deposit a uniform coating. Depending on the initial nanoparticle concentration and size of electrospray formed droplets, single nanoparticles or nanoparticle agglomerates will be deposited to the surface. For most applications, the deposition of single nanoparticles is preferred. We apply tribocharging (or electrostatic charging) to give host particles the opposite polarity. For example, using a Teflon container, alumina particles can be provided with a relatively high net positive charge. While the charge obtained by tribocharging is much lower than that by electrospraying, some preliminary experiments and simulations we performed showed an unexpected strong effect of tribocharging (Dabkowski, van Ommen, Yurteri, Hochhaus, & Marijnissen, 2007). By subsequent contacting of guest and host particles in an appropriate manner, the electric attraction forces will bring the host and guest particles into contact and van der Waals forces will effectuate the coating. We have studied two possibilities for mutually interacting oppositely charged particles in order to deposit nanoparticles on micro ones. These processes can be named as the grounded moving target (GMT) method (Dabkowski et al., 2007), as depicted in
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Fig. 4. Schematic of the grounded moving target set-up for controlled depositing of nanoparticles on host particles. The particle feeder provides charged host particles. Note that the host particles are not drawn to scale.
Fig. 4, and the falling curtain method. In the GMT method, a continuous deposition method, the coating level is controlled by changing the residence time of the host particles in the spraying zone via conveyor speed and changing the concentration of the suspension. In the preliminary experiments, three types of nanoparticle deposits were identified: single, in groups, and in agglomerates (see Fig. 5). The latter type is presumably explained by the deposition of droplets with a high concentration of nanoparticles, or by agglomerates already present in the suspension used. The efficiency and homogeneity of the obtained coating have been investigated with techniques such as BET surface analysis, fluorescence microscopy, and scanning electron microscopy. Tribocharging of host particles can be improved by constructing the particle feeder/charger out of a material, which is far away removed from the host particle in the tribo-series, e.g., for glass host particle, the particle feeder is made out of Teflon. However, charging the host particles too high causes particles to stick on the feeder making it difficult to handle to the conveyor for coating. The falling curtain set up, on the other hand, omits the contact of particles with the conveyor surface, while reducing the particle residence time. Applying multiple electrosprays as well as applying a counter air flow increases the residence time of the particles in the spray zone and thus enhances targeting of the nanoparticles on the host particles. This can lead to higher surface coverage, similar to those obtained with stationary host particles (see Fig. 6).
Fig. 5. SEM picture of 65 nm polystyrene particles deposited on a 165 m alumina particle. Three different deposition types are distinguished: singles, groups and agglomerates.
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Fig. 6. SEM image of 500 nm polystyrene particles deposited with electrospraying on a falling flow of 200 m glass beads.
4. Outlook Due to their unique properties, nanoparticles are finding increasing applications in daily life and industrial processes. We have successfully demonstrated two techniques for producing such nanostructured particles: atomic layer deposition (ALD) in a fluidized bed to provide nanoparticles with a continuous film; and deposition of nanoparticles on larger host particles by electrostatic forces, electrohdrodynamic atomization (EHDA). Both techniques have the advantage that they can be applied to a wide range of materials, both concerning the core particle and the coating material. ALD can be used to deposit a wide range of metal oxides, pure metals, and other inorganic materials (Puurunen, 2005). Also with electrospray deposition, a wide range of materials can be deposited. We have recently demonstrated that not only nanoparticles but also proteins can be deposited on carrier particles in this way (van Ommen, Tavares Cardoso, Talebi, & Yurteri, 2010). Both ALD and electrospray deposition are in principle fit to be scaled up. However, there are some hurdles to overcome before large quantities can be processed. ALD has already been demonstrated to be able to produce particles on the kg scale. Recently Spencer, King, van Ommen, and Weimer (2009) showed that particle ALD can be scaled up to 150 mm; there is no clear limitation to further increase of this diameter. However, in order to obtain smooth fluidization behaviour for a wide range of materials, assistance methods are needed. Recently, Quevedo et al. (2010) demonstrated that adding a secondary flow in the form of a high-velocity jet produced by one or more micronozzles pointing vertically downward toward the distributor is an attractive assistance method. They showed that such a micro-jet is effective in improving fluidization, simple to use, does not require expensive equipment or adding foreign materials to the bed, and can be used to mix and blend different species of nano-particles on the nanoscale to form nano-composites. Quevedo et al. (2010) also made a first step towards scale-up (from a 76 to a 127 cm column), but more insight into the mechanism of the micro-jet is needed for effective scale-up to much larger diameters (van Ommen, King, Weimer, Pfeffer, & van Wachem, 2010). Moreover, the optimum operating pressure needs further research work. For the ALD chemistry, a lower pressure is more favourable to enhance the removal of non-reacted precursors and gaseous by-products. On the other hand, atmospheric operation is easier concerning the hydrodynamics. Scaling-up particle ALD to the tonne range seems feasible. This is attractive, since the method is pre-eminently suited for producing various materials that can be used in energy conversion and storage, e.g., quantum-dots for third generation photo-voltaic cells (Conibeer et al., 2008), hydrogen storage in coated nanoparticles of light metal alloys (Krishnan, Kooi, Palasantzas, Pivak, & Dam,
2010), advanced cathode material for Li-ion batteries (Beetstra et al., 2009). For these applications, large quantities of coated material will be required. The electrospray deposition method is currently carried out at the gram scale. As a first step, we foresee this method suitable for pharmaceutical applications, i.e., scale-up to the kg range to be sufficient as the first target. This will require both changes to the charging of the host particles and the electrospraying of the guest particles. Recently, Ellis, Yurteri, and van Ommen (2010) showed a novel concept of charging the host particles in a fluidized bed, and subsequently entraining them out of the bed, after which the nanoparticles can be deposited onto the charged host particles. To be able to coat a larger flow of host particles and/or obtain a larger surface coverage, it will be essential to use a configuration with multiple electrosprays. Providing a uniform electric field to each nozzle and a uniform flow rate is essential for getting uniform droplets from an outscaled electrospray assembly. Arnanthigo, Yurteri, Marijnissen, and Schmidt-Ott (2010) recently demonstrated a nozzle assembly in which the nozzles are arranged in a close circle, so that the electric field situations are identical for each nozzle. We have limited ourselves in this brief paper to just two methods we are currently working on. However, many more promising approaches for gas phase production of nanostructured particles are being investigated worldwide. Molecular layer deposition is a technique related to ALD; with this coating technique organic layers instead of inorganic layers are deposited (Liang, King, Li, George, & Weimer, 2009). Several authors have been using plasmaenhanced chemical vapour deposition to provide micron-sized and nanoparticles with a very thin layer (Jung, Park, Park, & Kim, 2004; Sanchez et al., 2001; Spilmann, Sonnenfeld, & von Rohr, 2006; van Ommen, Abadjieva, & Creyghton, 2010), although of the listed papers only Spilmann et al. coated nanoparticles. Another successful technique to make nanostructured particles of various compositions in the gas phase is flame spray pyrolysis (Mädler, Kammler, Mueller, & Pratsinis, 2002; Teleki, Heine, Krumeich, Akhtar, & Pratsinis, 2008). Esmaeili, Chaouki, and Dubois (2009) used a fluidized-bed reactor for encapsulating nanoparticles by few nm of polyethylene using Ziegler-Natta catalysts. Chen, Yang, Dave, and Pfeffer (2009) used dry powder coating carried out in a magnetically assisted impaction coater to improve the flowability of 15 m cornstarch by coating it with 7 nm silica nanoparticles. An alternative approach is to employ plasticizer-electrostaticheat-dry-coating to improve the flowability of various powders (Luo, Zhu, Ma, & Zhang, 2008). These are just a few examples; this brief list is far from complete. Many more interesting developments concerning gas-phase processes to create nanostructured particles are taking place, yielding a wide range of novel products.
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