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3 Analysis and Imaging of Small Particles Although the research community has studied nano- possible. Doing so is a daunting task given the size and mass particles for several decades and has made many advances of an individual aerosol particle and their low concentrations with imaging and analyzing the chemical composition of in the atmosphere. Particle concentrations range from a few to a few thousand particles/cm3. However, said Zelenyuk, individual and mixtures of nanoparticles, it still struggles with understanding how nanoparticles interact and undergo instruments are now available that are capable of looking at changes in different environments. In particular, investiga- the properties of individual particles, with high sensitivity tors are just now developing methods for determining the and resolution, and of determining many properties for the three-dimensional structure and chemical composition of same particle. nanoparticles in the atmosphere and in nanocomposites. One of the main instruments in use is the single par- They also are designing new techniques for studying and ticle laser ablation time-of-flight mass spectrometer called modeling how particles form in the atmosphere and how SPLAT II. This instrument, shown in Figure 3-2, is used in those processes ultimate determine the nanoparticles’ prop- the laboratory and in the field (Zelenyuk and Imre, 2009, erties and their impact on the environment. Nanoparticle Zelenyuk et al., 2009). SPLAT II was flown for a month in an structure and composition also is critically important for the airplane to determine which particles form in ice clouds and materials and catalyst industries, both for understanding how how they affect climate to better understand sources of air existing materials and catalysts behave and for improving pollution over Alaska. Zelenyuk and her collaborators also their design and function. Speakers in this session, as well have developed software capable of examining millions of as the subsequent sessions, discussed the specific challenges particles to establish correlations between different proper- of imaging and analyzing nanoparticles and the wide-ranging ties and different sources. benefits that will come from solving those challenges. Zelenyuk and her colleagues are attempting to use SPLAT II and software to quantitatively determine many particle properties, including particle number and concentra- MULTIDIMENSIONAL CHARACTERIZATION OF tion, size, composition, and density. They also are examining INDIVIDUAL AEROSOL PARTICLES different aspects of particle shape and dynamic shape factor, Alla Zelenyuk of the Pacific Northwest National Labo- that is, whether a particle is spherical or symmetric, and ratory (PNNL) reiterated the important point that aerosols particle morphology in terms of what is on the outside of the are everywhere, with impacts on the climate and health and particle. Measuring the content of the very thin outer layer of potential for misuse as agents of terror (Figure 3-1). She also a particle is important because layers of secondary organic noted that aerosols arise from a variety of sources, each of aerosols on top of a hydroscopic layer can change water which produces particles of unique structure and composi- retention and completely stop the water content of these tion. What makes nanoparticle characterization even more particles from evaporating. After describing how SPLAT II challenging is the fact that aerosols are most often mixtures works (Figure 3-2, right panel), Zelenyuk discussed some of particles. While it is important to understand the mixture, of the results from the Alaska study. Flying through clouds, Zelenyuk and her coworkers are first trying to look at one for example, the instrument showed that very few particles particle at a time to determine as many relevant properties as do not activate and form droplets. By repeatedly sampling 21
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22 CHALLENGES IN CHARACTERIZING SMALL PARTICLES FIGURE 3-1 The importance of aerosols to society are many and varied. SOURCE: Zelenyuk, 2010. R02144 Figure 3-1 uneditable raster bitmap FIGURE 3-2 SPLAT II, an ultrasensitive high-precision instrument for multidimensional single particle characterization. SOURCE: Zelenyuk, 2010; modified from Zelenyuk et al., 2009. R02144 the free particles, SPLAT II provided information that may 2005). They found, for example, that the shape of sodium reveal what is special about these particular particles in terms chloride particles could vary from spherical to the more Figure 3-2 of their size, composition, and morphology. Modelers will uneditable raster bitmap and rectangular, and that the particles could typical cubic then be able to use this information to improve their pre- agglomerate into structures with much larger shape factors. dictions about cloud formation. Zelenyuk noted that any par- In addition, the researchers demonstrated that they could ticle more than 100 nanometers in diameter can be detected measure the symmetry of different types of particles and at levels as low as 1 particle/cm3 in 1 second of sampling even separate particles in real time based on their shapes. and can be sized with an accuracy of close to 100 percent. For each shape, they could then measure density, size, and Zelenyuk and her colleagues recently demonstrated that composition (Zelenyuk et al., 2006). Turning to the issue of they can determine the density of a particle, which is very particle morphology, Zelenyuk showed that it is possible to important for determining the mass of particles, the value of drill down into a particle to study particle composition as which is regulated by the Environmental Protection Agency. a function of depth (Figure 3-3) (Zelenyuk et al., 2008). In her initial experiments, she worked with the sodium chloride model system and coated the particles with liquid organics Characterizing Particle Morphology and solid organics. The experiments revealed that different When SPLAT II was first developed, Zelenyuk and her organic layers deposited over time on the particles do not colleagues used it to study ultrapure molten salts that form mix with one another, contrary to predictions of modeling metastable phases in far-from-equilibrium states. These studies. Instead, the organics develop a layered structure. studies enabled them to report the first density measure- The researchers were able to create some of these structures ments for hygroscopic particles found in the atmosphere and to show that they can be stable for many hours, which that exist in highly metastable phases (Zelenyuk et al., Zelenyuk said was surprising.
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23 ANALYSIS AND IMAGING OF SMALL PARTICLES FIGURE 3-3 Characterizing particle composition as a function of depth. SOURCE: Reprinted (adapted) with permission from Zelenyuk et al., 2008. R02144 Zelenyuk also noted that small amounts of organic vapors, Figure 3-3 analysis tools, it is possible to tease out information that data such as those emitted in auto exhaust, can have a profound would be otherwise invisible.” impact on particle morphology and behavior. uneditable raster bitmap One series of measurements, for example, showed that particle chemistry MATERIALS DESIGN AND SYNTHESIS changed as the aircraft travelled through a cloud, implying In his presentation, Ralph Nuzzo from the University that the chemistry in a cloud is heterogeneous and chang- ing with location and over time. Another set of experiments of Illinois in Urbana-Champaign discussed his work on found that a thin layer of organics can reduce particle evapo- nanoscale characterization from the perspective of under- ration by 96 percent over 24 hours. Those data are now being standing structural dynamics in the context of catalysis. used by modelers to attempt to predict the properties and “First and foremost,” he said, “there’s a gigantic toolbox life cycles of different types of particles in the atmosphere. that can be applied to this area.” Among the examples he As a final example of the type of studies that SPLAT II can cited, which were developed through large investments by enable, Zelenyuk briefly discussed work being done on the Department of Energy, include neutron- and x-ray-based engine exhaust. One finding from those studies is that par- approaches and emerging technologies such as analytic ticles emitted by new-generation fuel-neutral1 spark-ignition electron microscopy. The development of methods that cor- direct injection engines are fractal in structure, and that they rect for the complications that come from both chromatic incorporate polyaromatic hydrocarbons and nitropolyaro- and spherical aberrations were the key factors that enable matic hydrocarbons at levels as high as 40 percent on their electron microscopy to reach atomistic resolution. surfaces, which she said was also surprising. Using these new techniques, it is possible to obtain atomic resolution images that clearly delineate the atoms in polymer-capped platinum and palladium nanoparticles, Discussion for example, as shown in Figure 3-4. A comparison of these Doug Ray of PNNL commented that there is a similarity two images shows that while the platinum particles have a between the data acquisition presented by Zelenyuk and the very well-defined order, the palladium nanoparticles have work presented in Chapter 2 by Gerry McDermott, in that a significant amount of disorder that is independent of the high-throughput analysis of large numbers of items allows orientation. It is possible from these images to count atoms critically new conclusions to be extracted from these data. He and to correlate particle size with atom count in various said, “That is a theme. If a tool can be built with the capability types of particle morphologies. This study can be extended to perform high-throughput measurements with the proper to more complex structures, including core-shell platinum- palladium and palladium-platinum nanoparticles that are relevant to catalysis (Sanchez et al., 2009a). These types of experiments have yielded important insights into particle 1Not requiring a specific transportation fuel.
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24 CHALLENGES IN CHARACTERIZING SMALL PARTICLES FIGURE 3-4 Atomic resolution electron micrographs of platinum and palladium nanoparticles. SOURCE: Reprinted (adapted) with permission from Sanchez et al., 2009a. R02144 nucleation and growth and have provided input for modeling Figure 3-4 a detailed modeling study, combined with nano-area work, studies that have advanced our theoretical understanding of coherent electron diffraction data of gold nanoparticles, uneditable raster bitmap the vertices of gold clusters are deformed more these important processes. revealed that Complementary to these imaging studies are those that than expected in an “ideal” structure. This finding makes involve spectroscopy, which is an averaging technique. sense, Nuzzo noted, because atoms at the vertices have the Taking a population of clusters, it is possible to use x-ray lowest coordination numbers, and, as a result, their structural absorption spectroscopy to measure properties such as the relaxations are most profound (Huang et al., 2008). This type average coordination number for an absorbing atom and of structural behavior has been very hard to characterize in average bond distances and bond disorder. Combining the past. Nuzzo also discussed work done on the impregna- microscopy and spectroscopy data can provide information tion and reduction of an iridium and platinum bimetallic catalyst supported on γ-alumina. He showed images detailing on mesoscopic phenomena such as how bond distances change with temperature. Nuzzo discussed one set of mea- the atomic-level structure of this system. The support lattice surements made on platinum γ-alumina, a quintessential was observable and identifiable in these images to be near heterogeneous catalyst that is used to make gasoline. These the metal clusters and aligned with the zone axes. These experiments showed that bond distances contract as tempera- images, he said, illustrate that it is now possible to directly ture rises when particle diameter reaches sizes as small as correlate specific lattice planes in face-centered cubic struc- 1 nanometer. This phenomenon, known as negative thermal tures, which are essentially single crystals, and to map them onto specific orientations of the γ-alumina structure. Another expansion, was correlated with changes in electronic struc- ture (Sanchez et al., 2009b). Molecular dynamics simulations technique that researchers are putting to use in atomic-level of a 10-atom platinum cluster supported on γ-alumina deter- studies is electron energy loss spectroscopy (EELS), which mined that the bonding between the cluster and the support can be used to characterize the electronic structure of a mate- was dynamic in nature. rial at the atomic level. EELS can elucidate the patterns of charge transfers using an aberration-corrected microscope and can identify regions in a catalyst support that are not Understanding Defects homogenous. Nuzzo showed images of a gold cluster on a γ-Alumina is an interesting support at an atomistic level titanium dioxide support that clearly identify areas in the lat - because it has a great many defined defects created by tice that contain a disproportionate number of titanium (III) oxygen atom vacancies that cause electronic perturbations in centers located under the gold clusters (Sivaramakrishnan et the support. These perturbations occur on a scale that is of the al., 2009). He noted that these types of studies are providing same order as the size of the platinum clusters and therefore information about the nature of catalyst-support interactions perturb the static disorder of the clusters on the support. The and their structural and electronic consequences, which have level of disorder in these structures is also highly sensitive to been the “dark matter of catalysis.” nanoparticle size and the presence of reactive gas. In related
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25 ANALYSIS AND IMAGING OF SMALL PARTICLES Turning Low-Resolution into High-Resolution Images refined using diffractive imaging (Figure 3-6). In the latter image, the resolution was sufficient to see the separation As a final example, Nuzzo described the use of coherent between cadmium and selenium atoms (Huang et al., 2009). diffractive imaging to provide atomic-resolution structural Nuzzo explained that there are still some important limi- determinations even when an atomic-resolution imaging lens tations to current analytical techniques that point to future is not available. Using synergistic information from electron directions for research. Current methods can reveal atomic diffraction and low-resolution images, Nuzzo’s team was structure, speciation of elements at the nanoscale, and able to reconstruct an image showing the chirality and regis- electronic structure at the atomic scale. However, structural tration of the two concentric walls of a double-walled carbon dynamics is still needed, because current methods provide nanotube (Figure 3-5; Zuo et al., 2003). He also showed a limited or no temporal resolution to monitor ongoing pro- low-resolution image of a cadmium selenide quantum dot cesses. Also needed is the ability to characterize adsorbate- and the subsequent, far more detailed, image that had been interface bonding at atomic resolution, particularly in terms FIGURE 3-5 Coherent diffractive imaging reveals the chirality and registration of the two walls of a double-walled carbon nanotube. SOURCE: Zuo et al., 2003. R02144 Figure 3-5 uneditable raster bitmap FIGURE 3-6 Coherent diffractive imaging was used to refine a low-resolution image of a cadmium selenide quantum dot (left) and to en - able visualization of the separation between atoms in the crystal. SOURCE: Huang et al., 2009. R02144 Figure 3-6 uneditable raster bitmap
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26 CHALLENGES IN CHARACTERIZING SMALL PARTICLES of the dynamics of that bonding given that clusters are superior barriers. Particle dimensionality plays a large role vibrating and moving, not merely sitting, on the support in defining the bulk physical properties that a nanomaterial structure. Finally, there is a need to better merge theory and can add to the material being designed, and Silverman briefly experimentation to sort out the many atoms and many excited described the design rules that come from that relationship. states that actually exist on the catalyst’s surface. “If you want transparency or photonic properties, you use spheres,” he explained. “If you want electrical conductivity or thermomechanical behavior, you use rods. If you want PARTICLE CHARACTERIZATION barrier properties or flame retardant properties, you turn to NEEDS FOR NANOCOMPOSITES plates.” Once the nanomaterial is chosen, the polymer chem- Lee Silverman of DuPont’s Central Research and Devel- ist selects the polymer that will serve as the matrix based opment Laboratory provided an industrial perspective on the on other physical properties such as temperature capability kinds of tools needed to analyze nanomaterials. Nanotech- or tribilogical properties. The real work, said Silverman, nology, he said, is a huge field that includes nanostructured comes in developing the nanocomposite so that it has the materials, nanotextured surfaces, nanoscale-thick surface desired properties and is manufacturable. “Anyone who’s films, nanoscale devices, and nanoparticles. “DuPont’s done polymer processing understands that it’s really easy interest is in adding nanoparticles to polymers to try to aug- to take an extruder that’s full of polyester, throw clay in it, ment the properties of already existing polymer platforms and make something that comes out with the mechanical and extend the material applications,” said Silverman. “We properties of chalk and not useful to anybody.” In the end, believe that manipulation of materials on a very fine scale nanocomposite systems require compatible particles, poly- is broadly applicable across all sorts of material platforms, mers, and processes. and nanotechnology enables you to combine different prop- erty sets into specific materials.” As examples, Silverman Probing Complex Materials said that nanoparticles added to a polymer can improve its rheological properties in the molten state and its mechanical Although it is interesting scientifically to examine single properties once the material has cooled. Nanomaterials can particles, polymer chemists are more interested in materials add barrier properties to a film while enabling it to remain with high loadings of the nanoscale filler, and analysis at transparent. For single property materials, turning to nano - this level is very difficult (Figure 3-7). It would be useful to materials is not necessary. Aluminum, for example, makes know the spacing of the particles in a matrix, and researchers a great conductive film, and metal films in general make have tried to use small-angle x-ray scattering (SAX) to get FIGURE 3-7 Silica nanoparticles in polystyrene. SOURCE: Silverman, 2010. R02144 Figure 3-7 uneditable raster bitmap
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27 ANALYSIS AND IMAGING OF SMALL PARTICLES at the microstructure of a heavily loaded polymer. However, composites. The next level adds in multimodal distributions SAX only provides limited detail when an 80-nanometer- of different types of particles or differently shaped particles. thick film is loaded with 20-nanometer-diameter particles Particles can also be bent and have kinks, which makes them and there is little information about how the particles are very interesting from the perspective of creating a nano- spatially organized. Silverman also noted that transmission composite but introduces still another level of complexity electron tomography is useless in this type of system because that cannot yet be analyzed at any satisfactory level. Cer- the particles are too concentrated. tainly, Silverman noted, this field is hampered by a lack of In his research, Silverman is most interested in plates and the kind of physical characterization data that would enable rods, because he is concerned with creating nanocomposites a polymer chemist to predict how any given composite will with useful mechanical characteristics such as barrier or behave. Silverman summarized the situation by stating that permeability properties for gases and liquids. Studies on particle shape, size, and size distribution are critical deter- permeability conducted in the late 1960s showed that rela- minants not only for creating useful materials but also for tive permeability falls substantially as the aspect ratio of the understanding how they will behave from an environment, nanorod increases (Nielsen, 1967). Because aspect ratio is health, safety, and stewardship perspective. “We know how to the key feature, it would be desirable to have a technique for characterize monodispersed spherical systems, but character- measuring the aspect ratio in a clay nanocomposite, but such ization of plate and rod-like materials is onerous at best, and a technique does not exist. Silverman cited another example, is almost impossible in real nanocomposites,” he stated. He this one from the mid-1990s, of a model that relates the added that surface chemistry of nanocomposites is another percolation threshold of a composite to the ellipsoid aspect area that must be better understood, and one that also suffers ratio of the filler particles as they progress from plates to from a lack of analytical techniques applicable to real-world spheres to rods (Garboczi et al., 1995). The most complete materials. An audience member from the National Institute of picture of such composites covers spheres, but they are very Standards and Technology (NIST) commented that NIST has uninteresting when it comes to them serving as barriers. developed some special techniques for making subsurface The problem arises when trying to measure the aspect ratio measurements in some types of composites using scanning of plates or rods when they are buried inside a composite. electron microscopy and scanning probe microscopy. Other properties, such as conductivity, thermal conductivity, and elastic modulus also require particles with larger aspect QUANTIFYING THE CHEMICAL COMPOSITION OF ratios, not spheres. ATMOSPHERIC NANOPARTICLES James Smith, of the U.S. National Center for Atmo- Environment, Health, and Safety spheric Research (NCAR) and a visiting professor, addressed The other issue that DuPont worries about, said Silverman, the phenomenon of new particle formation in the atmosphere is environmental, health, and safety and product stewardship. and the recent progress that has been made in quantifying “We believe that we are going to need to understand these the composition of these spontaneously formed nanoparticles materials very well before we start putting them in consumer in our atmosphere. Nanoparticles form in the atmosphere products. We just cannot risk having another ‘asbestos’ by condensation to stable clusters formed by nucleation or another kind of incident like that,” he explained. For (Figure 3-8), or they can be emitted directly from sources materials that may shed fibers, that understanding must such as diesel engines. include a complete characterization of fiber dimensions and According to the theory of classical nucleation, particle biopersistence, which are key factors in determining the formation is an endothermic process that creates a stable pathogenicity of a fiber. Silverman believes that no usable cluster in the atmosphere. As an endothermic process, mol- techniques exist today that can provide that data for materials ecules that collide and stick to one another tend to fall apart, that are densely packed with nanomaterials. Many techniques and so the key is to cause enough collisions to occur that are available for studying dry or dispersed spherical particles, a pair is formed, then a triplex, and so on, until a “critical but spheres are not very useful in making high-performance cluster” is formed. This cluster can contain any number of nanocomposites. For rods and plates, scanning electron different compounds. Smith explained that sulfuric acid is microscopy (SEM) can provide some information, but only if a particularly sticky molecule in the atmosphere and that it the particles are in specific orientations and in dilute solution. plays a key role in the formation of critical clusters. Once a SEM is not useful for composites. Transmission electron critical cluster has formed, any additional collisions involv- microscopy and SAX are more useful, but the information ing the cluster will actually cause the particle to grow, which they generate is only helpful if the plates or rods are aligned may be a rapid process, although it depends on other factors in the sample. including constraints in kinetics, concentration and chemical Particle size distribution represents just one level of com- nature of gaseous species, and particle surface properties. A plexity to the analytical challenges found in dealing with post-doctoral fellow in Smith’s group at NCAR has developed
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28 CHALLENGES IN CHARACTERIZING SMALL PARTICLES random collisions and intermolecular cluster grows indefinitely by condensation forces cause molecules to form and and coagulation until it becomes a particle break apart (a particle) energy a stable "critical cluster" forms! minimum detectable diameter by particle instrumentation ~3nm cluster or particle size cluster or particle size FIGURE 3-8 Formation of atmospheric nanoparticles by classical nucleation. SOURCE: Smith, 2010. a unique instrument that for the first time provides direct the growth rates are between a factor of 2 and a factor of 50 chemical measurements of neutral clusters in the atmosphere. higher than is predicted by the only species we really know Smith asked, “Why should we care about new particle forma- with 100 percent certainty contributes to growth, and that’s tion?” To answer that question, he presented data from the Po sulfuric acid,” said Smith. “So the question is, what species Valley in Italy, which is a much polluted region. At one time, are contributing to this?” researchers thought that particle formation would not occur in such heavily polluted areas, because existing aerosols in Uncovering the Role of Amines the local environment would capture all of the small clusters before they could grow large enough to act as nucleation The challenge in searching for these mystery agents is centers. That idea was proven wrong when it was observed that the quantities of material that need to be analyzed fall that there can be sudden bursts of particle formation when the in the picogram range. Ideally, Smith will collect about atmospheric boundary layer (part of troposphere closest to 15 picograms of 5-nanometer particles, but at best, he will the Earth’s surface) lifts in the afternoon. Particle formation collect 800 picograms of a 20-nanometer particle. To ana- can produce as many as 100,000 particles/cm3, and growth lyze these samples, his team has developed an instrument can be as rapid as 20 nanometers/hour. At 100 nanometers in they call the thermal desorption chemical ionization mass size these particles can then act as nuclei for cloud droplet spectrometer (TD-CIMS) for characterizing the composi- formation. Researchers have been modeling this event. They tion of 8- to 50-nanometer particles. After describing how estimate that new particle formation can contribute up to the instrument works, Smith presented data produced by the 40 percent of the cloud condensation nuclei in the boundary instrument from 20-nanometer particles sampled in Atlanta, layer and up to 90 percent in the remote troposphere. Given a strongly sulfur-dominated environment. The instrument these numbers, said Smith, “It is imperative to understand revealed large amounts of sulfate compounds, as expected, this growth event and be able to predict it in models in but also dimethylamine. Smith repeated these measurements order to actually get at the role of aerosols in climate.” The on particles collected at Hyytiälä Forestry Field Station in real mystery is why these nanoparticle growth rates are so Finland, where they found large amounts of aminium ions. high. Smith asked, “What species, other than sulfuric acid, Indeed, measurements from all of the sites his team visited contributes to this remarkable growth?” He presented a col- revealed the presence of amines. From these observations, lection of observations that detail the growth rates of these he concluded that aminium salt formation is an important particle formation events and make clear that something m echanism that accounts for 10 to 50 percent of new other than sulfuric acid is involved. These events occurred in nanoparticle growth in the atmosphere (Smith et al., 2010). a wide range of environments from around the world, from Smith concluded by stating that acid-based chemistry plays Tecamac, Mexico, near Mexico City, to McCrory Island in an important role in the formation and growth of these new the South Pole. “What these data show, universally, is that particles and that amines appear to be important compounds
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29 ANALYSIS AND IMAGING OF SMALL PARTICLES Probing the Interactions Between Catalyst and Support involved in new particle growth. “Time and time again,” he said, “We’re starting to hear in the atmospheric aerosol field Nanoparticles make good catalysts because they provide about the growing awareness of the impact of amines on large surface areas on which both catalysis and contact atmospheric aerosol formation.” But despite this awareness, between the particles and their support material can occur. very little is known about amines, where they come from, The latter is critical because many catalysts are bifunctional and what their fate is in the atmosphere. Acquiring that and require the participation of both nanoparticle and sup- information is critical to understanding their impact on the port to drive catalysis. Datye noted that recent work with environment and climate, and that, explained Smith, requires gold nanoparticles on a titanium dioxide support showed more and better atmospheric measurements. unexpected activity in catalyzing oxidation of carbon mon- oxide. This activity peaked at a particle size of 3 nanometers, Discussion suggesting that some interesting interactions occur between the particles and the support. Because this reaction took place Doug Tobias of the University of California, Irvine, at room temperature and because gold is less expensive than asked if the sources of these amines are being worked platinum, there is a significant incentive to characterize the out. Smith replied that currently there is no real idea of nanoparticle-support interactions to better understand how where they originate.2 However, he said a new instrument to make use of this discovery. In reviewing the challenges to can measure amines in the gas phase. The data from this catalyst characterization, Datye said, “Of course, we want instrument show that the sum of all the amines is about to know the size, shape, bulk and surface structure, composi- the same as the total concentration of ammonia in the tion, oxidation state, and the location of individual atoms.” atmosphere. Work from his team suggests that when an In particular, catalyst researchers would like to pinpoint ammonium sulfate aerosol is exposed to gaseous amines, the location of promoter atoms that are present at parts per the amines can partition into the aerosol and displace the million levels and have a significant impact on a catalyst’s ammonia, producing an aminium sulfate aerosol. Observa - behavior. Catalyst designers also want a better understand- tions by Smith’s team and others suggest that agriculture ing of the interface between the nanoparticle and its support, may be a significant source of amines in some parts of the as well as the location of nucleation sites and atom trapping United States and the rest of the world. Amine levels also sites, all under reaction conditions. This is a difficult chal- show diurnal variation, which Smith hypothesized might be lenge, although the development of aberration-corrected related to temperature control. transmission electron microscopy (TEM) will help the field tremendously. So, too, will recent advances in performing PARTICLE DESIGN AND SYNTHESIS FOR CATALYSTS in situ TEM at pressures up to 1 bar in closed cells, and in energy dispersive x-ray spectroscopy (EDS) and EELS. Datye Abhaya Datye o f the University of New Mexico presented a few examples of how these techniques have been reiterated the importance of catalytic technologies to the used to study catalytic systems. One example showed how U.S. economy. Catalysts, he said, are engines that oper- aberration-corrected TEM was used to reveal surface features ate at the nanoscale and generate more than $1 trillion in on a 6-nanometer platinum nanoparticle (Gontard et al., economic activity in the United States each year. Although 2007). From such images it is possible to see steps on the par- many people imagine catalytic reactors as being enormous, ticle’s surface and therefore to determine how the facets of the on the scale of a chemical refinery, they come in all sizes, nanoparticle interconnect. These interconnects are important some as small as the battery that fits in a laptop. In fact, features because they are where some of the most active cata- one company has developed a catalytic fuel cell designed to lytic sites may lie. Annular dark-field electron microscopy power a typical laptop for about 20 hours. Nonetheless, most is another useful technique that provides atom-by-atom catalysts, certainly in terms of volume, are used in large- structural and chemical information. Images of the atoms scale chemical production where it might take 6 months on a single sheet of boron nitrite clearly show the location to produce one batch of catalyst needed to turn natural gas of boron, nitrogen, carbon, and oxygen atoms (Figure 3-9) into liquid fuels on a scale of metric tons. Given that scale, (Krivanek et al., 2010). This technique acquires images on a it is critically important to be able to make catalysts, which relatively low-power 60 kilovolt microscope. To really under- are composed of complex nanoparticles, in a highly repro - stand how a catalyst works, it is necessary to obtain structural ducible manner, which requires the ability to characterize information at the single atom level. Aberration-corrected catalysts in great detail. TEM images combined with EELS data can provide such information. Datye described how single lanthanum atoms were imaged inside the bulk structure of a calcium titanium 2After the workshop was held, new information on atmospheric amines oxide support (Varela et al., 2004). The small size of the EELS has become available. For example, see Ge, X., A. S. Wexler, and S. L. probe allowed the material to be scanned column by column Clegg, 2011. Atmospheric amines—Part I. A review. Atmospheric Environ- to determine the chemical signature of each atom and its state. ment 45(3):524–546.
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30 CHALLENGES IN CHARACTERIZING SMALL PARTICLES FIGURE 3-9 Annular dark-field scanning-tunneling electron microscope image of monolayer boron nitride (BN). (a) As recorded, and (b) Corrected for distortion, smoothed, and deconvolved. The area circled on the right in (a) indicates a single hexagonal ring of the BN R02144 structure, which consists of three brighter nitrogen atoms and three darker boron atoms. The circle on the left indicates a deviation from the pattern. Inset at top right in (a) shows the Fourier transform of an Figure 3-9 image area away from the thicker regions. Its two arrows point to reflec - tions of the hexagonal BN that correspond to recorded spacings of 1.26 and 1.09 Å. The image was recorded at 60 kV primary voltage, and uneditable raster bitmap the probe size was about 1.2 Å. SOURCE: Reprinted with permission, Krivanek et al., 2010.
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31 ANALYSIS AND IMAGING OF SMALL PARTICLES Catalysts by Design Norit activated carbon the particles were distributed evenly throughout the support. This suggests that activated carbon Given this level of detailed structural information, the next provides more nucleation sites to form smaller clusters of step, Datye explained, is to use the information to control metal. To summarize, Datye said that these developments in the features of a catalyst by design. And, in fact, several electron microscopy are providing unprecedented insights groups have been able to do just that. For example, Greeley into the structure of these catalysts. “As we develop better and colleagues were able to design alloys of platinum and strategies, we should be able to make these catalysts more early transition metals that were superior oxygen reduction stable and more active,” he said. electrocatalysts compared to platinum alone (Greeley et al., 2009). Key to the effort’s success was the careful design of NANOPARTICLE DISPERSIONS the catalyst’s surface. Wang and colleagues created a multi- metallic gold, iron, and platinum nanoparticle that proved In his presentation, Yi Qiao of the 3M Corporate Research to be a highly durable electrocatalyst (Wang et al., 2011). In Process Laboratory discussed some of the challenges facing this case, the deposition of 1.5-nanometer iron and platinum those who need to characterize nanoparticle dispersions particles on gold yielded nanoparticles with five-fold sym- used in industrial applications. He said there is a disconnect metry, a structure not seen in bulk platinum materials and between what academia uses to make such measurements one with many exposed facets at which catalysis can occur. and what industry needs to help its efforts in process control This structure was far more stable under catalytic conditions and quality monitoring. To meet the needs of a manufactur- than one constructed from pure platinum particles on a car- ing environment, a measurement technique must be fast bon support. The final step in intentional catalyst design is enough to provide feedback on a meaningful timeframe, have controlling the site of nucleation; that is, controlling the exact few restrictions for sample preparation in terms of nanopar- placement of catalytic nanoparticles on the support surface. ticle concentration and purity, and be able to distinguish Datye, for example, is working with graphene sheets that “good” from “bad” so a line operator can make necessary have corrugations on the order of an angstrom and is using adjustments to the manufacturing process in real time. To those corrugations as nucleation sites to anchor ruthenium address that disconnect, Qiao and his colleagues at 3M have nanoparticles. He said Farmer and colleagues have capital- developed two techniques that are now used in manufactur- ized on information about the atomic-level energetics of ing plants for process monitoring and feedback control. The cerium to stably anchor small gold nanoparticles (Farmer first technique uses a device called a microfluidic Y-cell and Campbell, 2010). In real-life application, catalysts (Figure 3-10). This device takes advantage of the fact that undergo changes over their lifetime. For example, by the fluid flows through a microfluidic device in laminar mode; end of its lifetime, the platinum in an automobile catalytic that is, two fluid streams flowing next to one another will not converter no longer disperses evenly over the support but mix. When a nanoparticle-loaded fluid is introduced next to agglomerates in clumps. It would be useful to understand the a buffer solution at one end of a microfluidic channel and mechanism by which the platinum no longer takes the form the fluids are allowed to flow through the channel, the only of a nanoparticle. Using in situ TEM to study this process, nanoparticles that enter the buffer stream will be those that Datye has discovered that the clumps appear to form via diffuse into it. The diffusion coefficient, which reflects the Ostwald ripening. He explained that it is actually possible to size of a nanoparticle, can then be measured by passing a see particles disappearing rapidly and that, by observing this laser beam through the microfluidic channel (Figure 3-11). happening many times, he concluded that the particles are not T his very simple technique provides a measure of evaporating, which would happen over a much longer time nanoparticle size in real time. A small amount of a process period. Instead, he believes that the particles emit atoms to stream can be diverted into the Y-cell device, providing a line the surface and then diffuse across the support surface. The operator with the information needed to make adjustments particles then form clusters, a few atoms across, at step edges to a process in real time to ensure particle size falls within on the support surface in much the same way that blowing the desired parameters. The technique, however, works best leaves collect against a curb. As the particles grow, they with particle sizes less than 15 nanometers in diameter. For become pinned against these edges and eventually become larger particles, diffusion occurs too slowly for the device to the clumps seen in an aged catalyst. As a final example of measure size changes within a useful timeframe. Dielectro- the type of structural detail that modern microscopy can pro - phoresis is proving useful for characterizing larger particles, vide, Datye described the use of high angular annular dark even in the micron range, in manufacturing settings. Qiao field scanning transmission electron microscopy to produce and colleagues used microfabrication techniques to cre - tomographic images of nanostructured heterogeneous cata- ate an electrode array that can trap nanoparticles when an lysts. In one study, the investigators compared the dispersion alternating electric field is applied to the array. Once the of platinum and rhenium nanoparticles on two different nanoparticles are trapped, the electric field is turned off and supports. On a typical Vulcan carbon black support used the particles are allowed to diffuse, washing out the density in fuel cells, particles were found in localized areas, but on
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32 CHALLENGES IN CHARACTERIZING SMALL PARTICLES FIGURE 3-10 Microfluidic Y-cell for nanoparticle size measurement. SOURCE: Qiao, 2010. R02144 Figure 3-10 uneditable raster bitmap FIGURE 3-11 In a microfluidic Y-cell, a laser beam shows no deflection prior to the admission of nanoparticles into the channel (top). Once nanoparticles are introduced into the channel, the laser beam is deflected with a slope that reflects the size of the nanoparticles. SOURCE: Qiao, 2010. R02144 Figure 3-11 uneditable raster bitmap
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33 ANALYSIS AND IMAGING OF SMALL PARTICLES gradient that was created by the electric field. A laser beam source of iron and possibly sodium; therefore, they may play is then used to measure the speed with which the particles an important role in climate processes. move across the gradient. Key to this device is its ability to Stroud and her collaborators at NASA would like to make an electrode array capable of generating a large electric answer three important questions about cosmic dust using field gradient that can overcome Brownian motions. Because state-of-the-art analytical tools: the strength of the electric field is controllable, it is possible to measure both nanoparticle aggregates and individual • Did the dust form in our solar system or around another nanoparticles in real time. star? • How did the dust form? • Is it pristine or has it been heated, shocked, irradiated, DECODING THE UNIVERSE AT THE NANOSCALE or otherwise altered? Rhonda Stroud o f the Naval Research Laboratory explained that the Navy has long been interested in nanopar- Cosmic nanoparticles have a variety of compositions. ticles, primarily for their application as propellants, in Most are silicates, although nanodiamonds may in fact be photovoltaics, and as fuel cell catalysts. As a result, she has more abundant. Research has identified dust particles made developed a wide range of tools for analyzing the composi- of silicon carbide, magnesium aluminum oxide in spinel tion of nanoparticles. Some of these methods have proven form, graphite, aluminum, calcium-aluminum oxides, and useful for studying the cosmic origins of the 40,000 tons of silica nitrite. “The majority of the materials analyzed so far extraterrestrial dust that enters Earth’s upper atmosphere are refractory-type things, essentially interstellar sandpaper annually. Although this type of analysis may seem far afield, materials,” said Stroud. “This is part of why they’ve survived the challenges to characterizing these types of nanoparticles 4.5 billion years.” are the same as those for environmental and engineered nano- particles. Nanoparticles form in large quantities around dying Snapshots of Single Grains of Dust stars and in interstellar gas clouds. Most of the particles that bombard Earth ablate in the upper atmosphere. In particular, One approach to studying cosmic nanoparticles is to the particles in the 100-micron range, which make up most map the isotopic signature of individual grains of a mete- of the dust’s mass, vaporize completely (Figure 3-12). Some orite. Stroud and colleagues have developed methods for of this vapor recondenses to form individual nanoparticles in using a focused ion beam to slice particles as small as the upper atmosphere. These nanoparticles are an important 200 nanometers and a combination of Z-contrast imag- FIGURE 3-12 The flux of extraterrestrial dust. SOURCE: Stroud, 2010. R02144 Figure 3-12
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34 CHALLENGES IN CHARACTERIZING SMALL PARTICLES ing and EDS to measure the elemental composition of the explained Stroud. “We would really love to be able to go in exposed grains (Stroud et al., 2004). She showed an image and locate the individual xenon atom and say, Aha! That one of a silicon carbide nanoparticle with an isotopic signature is probably from a supernova.” Identifying nanodiamonds as indicating it came from a nova star (Figure 3-13). The image coming from outside of the solar system is also problematic also revealed a number of subgrains that Stroud presumes because spectroscopic studies since 1987 have consistently came from the same star because they were trapped inside demonstrated that there is a soot-like component on the nano- this nanoparticle. However, the subgrains are below the size diamonds. Stroud recounted a variety of microscopy stud- limit at which she can measure individual isotopes to confirm ies showing that nanodiamond particle aggregates contain their origin. multiple phases of poorly ordered carbon sheets, agglomer- Stroud explained that the SEM instruments have a beam ated nanodiamonds, and what appears to be ordered graph- spot size of approximately 100 nanometers, and although it ite. To accurately characterize the nanodiamonds, Stroud is possible to make the spot smaller, it has to be big enough needed a better microscope. The new aberration-corrected to capture sufficient numbers of atoms to make accurate NanoSTEM microscope at Oak Ridge National Laboratory isotopic measurement. “Mostly we’re looking for a few (ORNL) fits the bill. Using this instrument, she and her rare isotopes, and there are just not enough atoms present collaborator at ORNL produced images with subnanome- in a 50-nanometer particle in general to get a good isotopic ter resolution that clearly identified the various phases of measurement,” she explained. Analyzing the origins of indi- carbon present as well as individual impurity atoms (Figure vidual nanodiamonds, which average about 2 nanometers 3-14). These images showed that the nanodiamonds contain in diameter, is therefore challenging. In 1987, researchers impurities ranging from fluorine and neon to vanadium and reported identifying nanodiamonds with an isotopic signa- chromium, but nothing nearly as heavy as xenon. The pres- ture indicating they were formed outside of the solar system ence of individual neon atoms in the secondary phases of (Daulton et al., 1996). However, these measurements were carbon, and not in the nanodiamonds, argues for a supernova done as bulk average measurements, and they were identi- origin for that material. fied on the basis of signatures in krypton and xenon isotopes. Stroud used EELS measurements to further characterize “The problem here is that there is only one xenon atom for the sheet-like or sub-nanometer-thick layer of carbon associ- 105 of these nanodiamonds, so it is not clear which fraction ated with the nanodiamonds. She found that the electronic of those nanodiamonds really formed inside our solar sys- profile of sheet-like carbon was spatially distinct from nano- tem and which came from supernova or somewhere else,” diamond surfaces in the agglomerate that makes up the dust particle. Inside diamond, however, the electronic signature was distinctively that of diamond. These data suggest that the nanodiamonds did not form in the supernova, but, rather, in the interstellar medium. Flash heating of organic matter Pt Oxidized Pt (Ti,V)C would have converted some of that matter to nanodiamond crack Mg,Fe and some to amorphous forms of carbon. In summary, Stroud silicate said that the problem of performing multiple, coordinated, TiC Diamond Impurity atom (Ti,V)C Fe Fe,Ni Disordered carbon vacuum 100 nm 1 nm FIGURE 3-13 Z-contrast imaging and energy dispersive spec- FIGURE 3-14 Dark-field scanning transmission electron micro- troscopy reveal subgrain structure and elemental composition of a R02144 silicon carbide cosmic nanoparticle. graph of cosmic particles from the Murchison meteorite. Figure 3-13 SOURCE: Stroud, 2010. SOURCE: Stroud, 2010. R02144 uneditable raster bitmap photo, with vector labels Figure 3-14 uneditable raster bitmap with vector labels and arrows
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35 ANALYSIS AND IMAGING OF SMALL PARTICLES nanoscale analyses on particles 200 nanometers and larger an engineering polymer—a statistical average over a square has been solved. “We can pick up individual grains, do kilometer or 50 pound sample will do. However, such infor- the isotope measurements, do transmission microscopy, mation is needed to understand how specific properties of do whatever we like,” she said. “It gets a lot harder to do a composite material arise, and perhaps more importantly, multiple analyses when you get below 200 nanometers.” to understand how the material can fail. Agglomeration of Aberration-corrected electron microscopes are effective the nanoparticles in a matrix can be meaningless in some tools for doing atomic-scale characterizations on periodic materials or applications and catastrophic in others; under- or ordered samples with well-constrained impurities. Con- standing which will be the case requires the ability to first ducting such analyses on natural samples, where there may create a perfect dispersion to show that agglomeration does not adversely affect material performance. Mark Barteau be large numbers of different elements present and phase mixtures, some of which are disordered, is more difficult but from the University of Delaware asked if work to character- can be done with the right preparation and patience. ize catalyst structure under reducing conditions has been done to the neglect of work to study catalysts under more challenging oxidizing conditions. Nuzzo replied that entire Discussion industries have been built on conducting catalysis under When asked by Barbara Finlayson-Pitts about how the reducing conditions, including the petrochemical industry, noble gas neon manages to remain in a dust fragment for but he agreed that interesting oxidative reaction conditions billions of years, Stroud said that the noble gas atoms are require the same amount of attention. Doing so is proving only seen when more than one layer of carbon is present. to be a big challenge, however, particularly because most They are likely trapped inside C60 cages that formed at the atomic-level characterizations are performed in a vacuum. same time as the nanodiamond. Datye added that new heating elements that can withstand high-temperature, oxidizing conditions are starting to move that aspect of the field along. Levi Thompson of the Univer- OPEN DISCUSSION sity of Michigan asked how meaningful these techniques are In response to a question from Jim Litster of Purdue given the rapid timescale at which catalysis occurs. Nuzzo U niversity about whether he can measure aggregation responded that the rapid timescale of these reactions means or the degree of dispersion of nanoparticles in a matrix, that, from a dynamic perspective, the catalysts are in fact Silverman explained that the methods he uses cannot yet sitting still most of the time. However, techniques have been make those distinctions. Currently, no tool exists that can developed to study the conformational dynamics in proteins provide adequate information at that level of detail. Nuzzo in real time, which may be useful for studying heterogeneous materials. In response to a question from Vicki Grassian then asked if it was really necessary to make measurements with atomic-level detail for materials such as engineering of the University of Iowa about small particle monitoring, polymers that are used in bulk-scale applications. Silverman Lippmann said that there is a clear need for better monitor- responded that it is not necessary from a process control ing of the ultrafine particles to which people are exposed in point of view to know the exact location of every atom in the environment.
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