Nanoparticles


Sheryl Ehrman

Sometimes called the building blocks of nanotechnology, nanoparticles (particles with diameters less than 100nm) constitute a commercially important sector of the nanotechnology market. Unlike many speculative applications of nanotechnology, nanoparticles have been with us for a while. Nanoscale gold clusters have been used to color ancient glass as far back as Roman civilization.[1] More recently, in the twentieth century, carbon blacka material composed of nanoparticles of high-grade carbon "soot"was incorporated into tires, resulting in greatly improved durability. By the year 2000, carbon black for tires was a 6-million-tons-per-year global market.[2] Nanoparticles are useful when their properties at the nanoscale (mechanical, optical, magnetic, and so on) are different from those at the bulk in some beneficial way and also when their size enables interactions with biological systems.

An example nanoparticle is shown in Figure 13-1.

Figure 13-1. TiO2 particles (micron scale formed from nanoscale particles, used for clarity). (Courtesy of Altair Nanotechnologies, Inc.)


Applications of Nanoparticles

Remember white, pasty sunscreen? It is now transparent because the key ingredient, zinc oxide, which absorbs ultraviolet light, is transparent if it is made of nanoparticles but white when made of larger, micron-sized particles. The same holds for titanium dioxidewhich is a white paint pigment and an additive to improve opacity in colored paintsif the particles are hundreds of nanometers in diameter or greater. Now, these transparent nanoparticles, which still absorb in the UV range, are finding their way into modern, transparent, and highly effective sunscreens.[3]

Nanoparticles of titania are also a key ingredient in dye-sensitized solar cells. The efficiency of these cells is boosted by incorporating nanoparticles that increase the total surface area for harvesting light by a factor of a thousand as compared with a single crystal of titania.[4] Compared with silicon-based photovoltaic materials, dye-sensitized solar cells are much less expensive to produce, and in 2001 a manufacturing plant opened in Australia.[5]

Another significant market for nanoparticles is in the semiconductor industry, in a process known as chemical mechanical planarization (CMP). In the manufacturing of computer chips, the component materials must be coated onto wafers, and at several points in the process, the coatings must be rendered nearly atomically smooth across the entire wafer. For 300-mm wafers, this is no small challenge! In CMP, slurries of nanoparticles are used. In this process, a combination of chemical removal and mechanical abrasion acts to accomplish this atomic-level polishing task. Silica, alumina, and ceria nanoparticles, the most common materials, are very effective. As a result, the global market for CMP has grown rapidly, from $250 million in 1996 to more than $1 billion in 2000, with the market for CMP consumables alone (slurries and polishing pads) expected to hit $800 million by 2005.[6] With ever-increasing miniaturization in device features, resulting in a need for increasing precision in manufacturing, the CMP market will surely continue to grow.

In addition to these existing large-scale applications for nanoparticles, many more products have been recently commercialized or are in development. Professors Paul Alivisatos of the University of California, Berkeley, and Moungi Bawendi of MIT developed processes for making semiconducting nanoparticles out of materials including cadmium selenide (CdS) and cadmium telluride (CdTe). These particles, coated with zinc sulfide, absorb light in the UV range, and then because of size-dependent quantum confinement effects, they emit light in the visible range, at a wavelength dependent on the particle size. Their stability and brightness are much better than those of conventional fluorescent chemical dyes, and when they are functionalized with proteins, oligonucleotides, or smaller molecules of interest, they can be used for fluorescent labeling applications in biotechnology.[7] Silicon nanocrystals less than 4nm in diameter also emit light in the visible range, with the wavelength depending on particle size. These nanoparticles are of interest for solid-state lighting applications and promise much higher efficiency as well as longer lifetimes compared with current incandescent and fluorescent lighting technology.[8]

As well as these optical properties of nanoparticles, chemical propertiesin particular, catalytic activitycan be greatly improved at the nanoscale. One important example is gold. In bulk form, gold is relatively inert. However, when deposited at very low concentrations (0.2 to 0.9 atomic %) in a nonmetallic form onto nanoparticles of cerium dioxide, it becomes very active for the watergas shift reaction.[9] In this reaction, carbon monoxide and water are converted to carbon dioxide and hydrogen. For fuel cells that are powered by hydrocarbon fuels, converted first to hydrogen and carbon-containing by-products, the watergas shift reaction is key for maximizing the amount of hydrogen while minimizing the amount of carbon monoxide, a molecule that poisons the electrocatalysts in the fuel cell itself. Because of the small amount of precious metal used in this material, the economics are favorable compared with catalysts that contain precious metals at up to 10% atomic percent concentration.

Magnetic properties of nanoparticles can also be exploitedfor example, the tendency of small magnetic nanoparticles to be superparamagnetic. Superparamagnetic nanoparticles, in the absence of a magnetic field, have random magnetic moments at temperatures above their Curie temperature, but in the presence of an external magnetic field, they align with the field, producing a high magnetic moment. One application of this phenomenon is in magnetic resonance imaging (MRI). MRI contrast is aided in the body by the presence of naturally occurring materials such as deoxyhemoglobin but can be greatly enhanced by introduction of superparamagnetic iron oxide nanoparticles (SPIONs). These particles have cores consisting of magnetite (Fe3O4), maghemite (gamma Fe2O3), or some combination of both, and are coated to enhance colloidal stability and biocompatibility. The type of contrast enhancement enhanced by SPION is useful in the imaging of features that contain large amounts of fluid, such as internal injuries or cancerous lesions. These nanoparticles are commercially available from several sources.[10] The surface of the SPION can be further functionalized to engineer interactions between the contrast agents and specific tissue and cell types, and this is an active area of research.[11]

Production of Nanoparticles

Production methods for nanoparticles can be loosely classified into three general categories: wet synthesis, dry synthesis, and milling. In both wet and dry synthesis, nanoparticles are generally produced in a bottom-up way from atomic precursors, whereas in the milling approach, nanoparticles are produced from the top down by mechanically breaking down larger particles. Wet approaches include sol-gel and precipitation methods, whereas dry approaches encompass combustion, furnace, and plasma synthesis of nanoparticles.

In all cases, there are concerns about the narrowness of the size distribution of the nanoparticles, and concern about the degree of agglomeration. All processes for making nanoparticles lead to some spread in the particle size. The size distribution can be modified somewhat by adjusting the process parameters, or the size distribution can be tailored by removing the tails of the distribution through additional separation steps. This typically leads to lower process yield. With respect to agglomeration, nanoparticles have a high ratio of surface area to volume, and it is much more energetically favorable for them to reduce their surface area by coalescing together. Thus, materials that melt at high temperatures if they are in bulk form may fuse together at much lower temperatures if they are nanoparticles.

Some applications, such as the fluorescing quantum dots mentioned earlier, have very tight requirements for particle size and aggregation. Other applications, such as CMP slurries, may not have such narrow constraints. There is no one perfect process. Milling is very energy-intensive, and it may not work at all for some materials, such as pure metals, that are malleable. Precipitation methods may require the addition of capping ligands to the nanoparticle suspension, to stop particle growth and to prevent the particles from agglomerating together. These ligands bind to the surface of the particles, and if they do not impart the desired functionality to the particles, they must be displaced in a separate processing step.

In high-temperature synthesis of nanoparticles, agglomeration can be avoided for some materials by simultaneously quenching and diluting, but this presents a challenge when the process is scaled up. If the nanoparticles suspended in the gas are more dilute, more energy is required to recover them. For some materials that are viscous glasses at high temperature, such as silica, diluting and quenching may not help. Some processes, such as electro-spray or plasma-based syntheses, produce particles that are highly charged, and this aids in reducing agglomeration.

Outlook for Nanoparticles

Before a process can be considered commercially viable, there are additional economic concerns. Many processes for nanoparticles have been developed at the laboratory scale, but they are not yet commercialized because of constraints, including scalability considerations and precursor costs. Aerosol pyrolysisthe process used to make nanoparticles of silica, alumina, and titania by companies such as Cabot, Degussa, and DuPonthas long been in existence. Plasma-based processes, because of their potential for high-throughput reduced agglomeration, have been more recently scaled up in industry.[12] Many other processes could perhaps be scaled up, but there is a catch-22 involved: Without a sufficient commercial market for the nanoparticles, there are insufficient resources available for process development. Without sufficient quantities of nanoparticles available, it is difficult to conduct product development research.

In the past several years, there has been increasing public concern about the occupational and environmental risks associated with nanoparticles.[13] The correlation between particle phase air pollution and adverse health effects has been noted for some time.[14] There is now concern that nanoparticles may also threaten human health and ecosystems. It is important to consider that materials that may not pose health risks in bulk may become toxic in nanoparticle form because of their small size. Safety precautions that are appropriate for materials in bulk form may prove insufficient for nanoparticles. As a result, research into potential health risks associated with these new materials, supported by U.S. government agencies such as the Environmental Protection Agency and the National Science Foundation, is actively under way.




Nanotechnology. Science, Innovation, and Opportunity
Nanotechnology: Science, Innovation, and Opportunity
ISBN: 0131927566
EAN: 2147483647
Year: 2003
Pages: 204

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