Nanomaterials for Advanced Applications

Nanomaterials Yesterday, Today and Tomorrow

Thomas Juehne

The union of distinct scientific disciplines is revealing the leading edge of Nanotechnology. Fifteen to twenty years ago, the interdisciplinary activity of geneticists, biologists, immunologists and organic chemists created a more diverse toolbox now known as life science. Bioconjugates were created to help us move from outside the cell to the inside. Enabling technologies brought about the ability to create, identify and specifically manipulate genetic maps to engineer designer proteins.¹ In parallel, physicists, chemists, polymer chemists and engineers were creating the foundation for the small world of nano materials science. Fullerenes,² carbon nanotubes³ and atomic force microscopy⁴ were in their infancy. In less than a decade, materials science and life science together are unraveling the mysteries of controlling, on a molecular level, the structure of matter.

Particles, complexes, tubes, coatings, active surfaces and devices are being explored on the nanoscale. Assembly of nature’s building blocks (e.g. carbon, nucleic acids, lipids and peptides) along with the combination of different materials (e.g. CdSe/ZnS, Au, Ag, Si(n)OH(n), light harvesting dendrimers and thin films) are leading to insightful understanding and the creation of new scientific tools. Chemists and physicists have been manipulating matter on the molecular level for centuries. Some say this is nothing new. When one looks at the absolute elegance of the nanometer scale biological system, however, one is compelled to create, understand, manipulate and control systems with equal elegance.

This high level of activity and promise has attracted private and government funding with considerable economic impact and growth. Since the launch of the National Nanotechnology Initiative (www.nano.gov) in 2000, there have been hundreds of start-up companies emerging in the market. At universities, there are increased investments in nanotechnology programs and facilities. Northwestern University’s Institute for Nanotechnology (www.nanotechnology.northwestern.edu) is a direct result of the high level of activity and pioneering work of Prof. Chad Mirkin. As a result of massive grant support and continued focus from the university, the Center for Nanofabrication and Molecular Self Assembly building has been constructed. Similarly, The Molecular Foundry (www.foundry.lbl.gov), a Department of Energy Nanoscale Science Research Center, is under construction at Berkeley in California. The research center is focused on the dissemination of nanoscale techniques and methods to enable scientists to delve into nano research. The Molecular Foundry is the direct result of Prof. Paul Alivasatos’ discoveries and benchmark work. We see an increase in programs, facilities, career opportunities and educational outreach. It is a massive explosion of activity in study on a very small scale. Science is beginning to set free nanotechnology. Soon the only limit will be the imagination.

Nanoparticle-Catalyzed Growth of Semiconductor Nanowires

Answers from the Past

William E. Buhro

Over the past twenty years, homogeneous nucleation in solution has proven to be an effective method for the synthesis of both metallic⁵ and semiconductor nanocrystals.⁶ Nearly isotropic, pseudospherical morphologies are typically produced. Other nanocrystalline morphologies are often desirable, and homogeneous nucleation has been adapted for the synthesis of rod-shaped morphologies.⁷ However, the growth of nanowires by homogeneous nucleation is apparently limited to materials having particularly favorable, highly anisotropic crystal structures.⁸ How then might nanowire morphologies be generally prepared?

The answer was uncovered by Wagner and Ellis in 1964,⁹ who discovered that micrometer-scale silicon whiskers (wires) could be grown from gold-droplet catalysts under Chemical Vapor Deposition (CVD) conditions at about 1000 °C. The process was named the “Vapor-Liquid-Solid” (VLS) mechanism after the three phases involved. VLS growth was extended to micrometer-scale whiskers of many inorganic materials and intensively studied for over a decade, before fading into relative obscurity.

From VLS to SLS

In 1995, my group reported that III-V semiconductor nanowires could be grown in solution from indium nanoparticle catalysts by a process analogous to the VLS mechanism, and by analogy we named it the “Solution- Liquid-Solid” (SLS) mechanism.¹⁰ Shortly thereafter, Lieber and coworkers described laser-ablation adaptations of the VLS mechanism that afforded silicon and a wide range of other semiconductor nanowires (Laser- Assisted Catalyzed Growth, LCG).^11-12 Many others made seminal contributions to nanowire growth by VLS adaptations, most notably Korgel (Supercritical Fluid-Liquid- Solid, SFLS, mechanism)¹³ and Yang.¹⁴ We now routinely prepare soluble, III-V and II-VI semiconductor nanowires with controlled diameters in the strong-confinement regime of about 3–20 nm by bismuth-nanoparticle-catalyzed SLS growth (Figure 1).^15-16 The nanoparticle-catalyzed VLS mechanism and its solution-phase variants have emerged as the popular, widely practiced, general methods for the synthesis of semiconductor nanowires.¹⁷ Gold nanoparticles are presently by far the most commonly employed catalysts.

/US/en/product/aldrich/546461Quantum Wires to Nanophotonics

The advantages of nanoparticle-catalyzed nanowire growth include its general applicability to a wide variety of materials, the diameter control afforded, the uniformity of the wires (lack of significant diameter fluctuations) and their oriented, near-single crystallinity. The surface passivation, solubility and length of the wires may also be systematically varied. Small-diameter “quantum” wires are ideal specimens for fundamental studies of two-dimensional (2D) quantum-confinement phenomena and for property comparisons to 3D-confined quantum dots, 1D-confined quantum wells and anisotropically 3D-confined quantum rods. Potential applications of semiconductor nanowires in nanophotonics and lasing, nanoelectronics, solar-energy conversion and chemical detection are under active development. Exciting progress and advances in the semiconductor-nanowire field, enabled by the emergence of nanoparticle-catalyzed growth, are anticipated in the immediate future.

Nanomaterials in Organic Photovoltaic Devices

Dr. Sean Dingman

The ability to create high-efficiency solar cells is a key strategy to meeting growing world energy needs. Nanotechnology is currently enabling the production of high-efficiency organic photovoltaics (OPVs) to help meet this challenge.¹⁸ Organic photovoltaics are nanostructured thin films composed of layers of semiconducting organic materials (polymers or oligomers)¹⁹ that absorb photons from the solar spectrum. These devices will revolutionize solar energy harvesting, because they can be manufactured via solution-based methods, such as ink-jet or screen printing, enabling rapid mass-production and driving down cost.

OPVs currently lag behind their “inorganic” counterparts because of low solar energy conversion efficiencies (approximately 1-3%). Several research groups are addressing conversion efficiency by employing a combination of nanomaterials and unique nanoscale architectures. These hybrid organic-inorganic photovoltaics consist of light-absorbing polymers in contact with semiconductor nanocrystals, fullerenes or nanostructured metals. The nanomaterials affect electro-optical properties of the conducting polymer, which include assisting in absorption of red and near-IR photons, a significant portion of the solar spectrum. Examples of OPVs designs employing nanomaterials include:^20-23

Polymer-Fullerene Heterojunctions: Cells where a chemically modified fullerene (C60) layer, acting as electron acceptors, is in close physical contact with a polymeric organic electron donor (MDMO-PPV catalog number 546461, or poly-(3-hexylthiophene), P3HT, catalog number 445703 and 510823). This contact improves efficiency by allowing charge transfer to take place at the sub 10-nanometer scale, on the order of the diffusion length of an exciton generated from organic semiconductors. The most recent cells exhibit conversion efficiencies of ~5%.

Organic-Nanocrytsal Solar Cells: Blends of semiconducting polymers and semiconducting quantum dots or nanorods (CdSe or CdTe) are mixed in a manner similar to the polymer-fullerene blends. The polymers are modified to give rise to chemical bonding between the nanocrystal and polymer. The nanocrystals can be tailored to a wide variety of optical band gaps, which depend on the size of the nanocrystal (or the diameter of the nanorod).

Dye-Sensitized Cells: These cells employ complex dye molecules attached to the surface of nanostructured oxides like titanium(IV) oxide (TiO2) or niobium(V) oxide (Nb2O5). The dyes exhibit broad lightabsorption profiles and rapid photoinduced charge transfer of electrons to the nanocrystals. These cells show solar conversion efficiencies of ~4%.

Tandem Cells: The tandem cell acts as a 2-1 cell, harvesting photons from the complete visible spectrum. These cells employ layers of C60 as a strong blue light absorber and copper-phthalocyanine (CuPc) as a red-yellow absorber. Nanosized silver particles act as a charge conduit between the cells but do not absorb photons traveling through the cell because of their nanosized dimensions. These cells have achieved conversion efficiencies of ~6% (Figure 2).

Significant challenges exist to achieving OPV devices that can be mass-produced. Nanotechnology will assist in meeting the technical challenges of this rapidly evolving field.

Coatings

Dr. Luke Grocholl

Nanopowders and nanoparticle dispersions have seen increasing applications in coatings. Due to their small size, very even coating can be achieved by painting nanoparticle dispersions onto a surface and baking off the residual solvent.

Optically Transparent Conductive Coatings: Indium tin oxide (ITO) and antimony tin oxide (ATO) are well known, optically transparent, electrically conductive materials. Nanoparticles of these materials can be painted on surfaces such as interactive touch screens to create a conductive, transparent screen without relying on expensive sputtering techniques. In addition, ITO and ATO can be used as an antistatic coating, utilizing their inherent conductivity to dissipate static charge.

Optically Transparent Abrasion- Resistant Coatings: Nanoscale aluminum oxide and titanium oxide are optically transparent and greatly increase the abrasion resistance of traditional coatings. Titanium oxide is of particular interest in many optical applications, since it is highly reflective for most ultra-violet radiation. Zinc oxide and rare-earth oxides are also UV-reflective, but optically transparent and are therefore effective in protecting surfaces from degradation brought about by exposure to UV radiation.

Carbon Nanotube-Based Biosensors

Michael Strano, Dr. Seunghyun Baik, Paul Barone

Bio-conjugates

Researchers from the University of Illinois at Urbana-Champaign have developed near-infrared optical biosensors based on single-walled carbon nanotubes, which modulate their fluorescence emission in response to specific biomolecules. The viability of sensor techniques was demonstrated by creating a singlewalled carbon nanotube (SWNT) enzyme bio-conjugate that detects glucose concentrations.²⁵

Carbon nanotubes fluoresce in a region of the near-infrared, where human tissue and biological fluids are particularly transparent to their emission. The sensor could be implanted into tissue, excited with a nearinfrared light source, and provide real-time, continuous sensing of blood glucose level by fluorescence response. Figure 3 shows a schematic mechanism of the nanotube sensor. Hydrogen peroxide is produced when glucose reacts with the enzyme, which quickly transforms ferricyanide to modulate near-infrared fluorescence characteristics of the nanotube.

Future Opportunities

The important aspect of this technology is that the technique can be extended to many other chemical systems. New types of non-covalent functionalization are developed, creating opportunities for nanoparticle sensors that operate in strong absorbing media of relevance to medicine or biology.

Layer-by-Layer Nanoassembly for Drug Delivery

Yuri Lvov

Researchers from Louisiana Tech University were among the pioneers of one of the novel nanotechnology methods: layer-by-layer (LbL) nanoassembly by alternate adsorption of oppositely charged polyelectrolytes, nanoparticles and proteins. With this technique, we can assemble ultrathin multilayers with nanometer precision and pre-determined composition across the film, and make nanocapsules from LbL films. We are using such nanocapsules for targeted drug delivery, biocompatible nanocoating and pulp microfiber processing. For this nanoarchitecture, we use nanoblocks, such as nanoparticles. In the development of our research area, we successfully used nanoparticles, such as different diameter and surface-charged gold nanoparticles, silica, nanoclay-montmorillonite, alumina, titanium dioxide and other nanoparticles. Also, we investigated linear polyelectrolytes of different types and molecular weight (especially natural polyelectrolytes), which we used as electrostatic glue to assemble nanoparticle and protein arrays. As any architect, we nanoarchitects need a wide palette of nanoblocks with new properties and dimensions, ideally, monodispersed, stable in solution, charged nanoparticles of noble metals, metal oxides and ceramics with diameters of 5, 10, 20, 50 and 100 nm.

Antibacterial Applications

Dr. Luke Grocholl

Zinc oxide is an effective antibacterial and anti-odor agent. It has been used in deodorants, dental cleansers and diaper creams. The increased ease in dispersibility, optical transparence and smoothness make zinc oxide nanopowder an attractive antibacterial ingredient in many heath care products. Copper oxide nanopowder has also been proposed as an anti-microbial preservative for wood or food products.

Nanomaterials in Fuel Cells

Shelley D. Minteer

Over the last two decades, general interest and research in fuel cells has increased, because they have the potential to be more energy efficient than conventional power generation methods.²⁶ During this time period, researchers have begun using nanomaterials in the catalyst layer of fuel cell electrodes for a variety of reasons, including: increasing the active surface area of the anode and cathode catalyst, increasing the catalytic rate of oxidation or reduction and minimizing the weight of platinum and other precious metals in the fuel cell. The current generated at an electrode is proportional to the active surface of catalyst on the electrode surface, so higher power density fuel cells can be formed from nanomaterials, because nanomaterials have a higher surface area to volume ratio. Researchers have also shown that the electrocatalytic properties of the materials are sensitive to particle size, so increased catalytic activity can be observed for nanoparticles and nanomaterials.^27-29 However, the most important goal has been to decrease the weight of platinum and other precious metals in the catalyst layer of the fuel cell, so that the fuel cell can be costeffective. This has been the main limitation to the widespread use of fuel cells.²⁶ Researchers have employed carbon nanomaterials as supports for dispersions of platinum nanomaterials. This allows for a decrease in the weight of platinum needed to produce the same surface area of active platinum catalyst. The nanomaterials could be carbon foams containing nanopores,³⁰ different types of nanotubes^31-32 or even singlewalled nanohorns.³³ All of these materials can act as a support and a conductor for platinum nanomaterials, making strides toward cost-effective fuel cell catalysts.

Nanomaterials in Other Energy Applications

Environmentally Friendly Energy Sources

Dr. Luke Grocholl

Devising schemes to meet the world’s growing energy demands while simultaneously reducing green house emissions and other pollutants, has become one of the major challenges facing materials scientists. Nanomaterials promise to help solve many of the problems associated with new and emerging energy technologies.

Fuel Cells: Solid oxide fuel cells (SOFCs) offer the advantage over other fuel cell designs in that they do not require expensive, precious metal catalysts and can operate effectively without extensive purification of fuel sources. The activity of doped rare-earth oxide electrodes such as yttrium stabilized zirconia (YSZ) is directly related to their surface areas. Nanoparticles exhibit the high surface required for developing SOFC technologies.

Cleaner Emissions: Catalytic converters on vehicles around the world have significantly reduced the amount of automotive pollution over the last three decades. These devices require large amounts of expensive metals such as platinum, palladium and rhodium. Doped rare-earth metal oxides offer the promise of increased catalytic activity without the heavy reliance on precious metals. In addition, the increased efficiency of the next-generation catalytic converters will result in cleaner emission of existing internal combustion and diesel engines.

Nanotube-Polymer Composites for Ultra Strong Materials

Exceptional Strength

Nicholas A. Kotov

Exceptional mechanical properties of single-walled carbon nanotubes (SWNT) have prompted intensive studies of SWNT-polymer composites. However, the composites made with nanotubes are still holding a substantial reserve of improvement of mechanical properties. The problem is that pristine SWNTs have very poor solubility in polymers, which leads to phase segregation of composites. Severe structural inhomogeneities result in the premature failure of the hybrid SWNT-polymer materials. The connectivity with the polymer matrix and uniform distribution within the matrix are essential structural requirements for strong SWNT composites. This problem is being solved by several approaches. First, by using coatings from surfactants and polymers, such as sodium dodecyl sulfate or poly(styrenesulfonate). This enables formation of better dispersions in traditional solvents including water. Polymeric dispersion agents are strongly preferred for the composite preparation because of (a) tighter bonding with the graphene surface, (b) miscibility with polymer matrixes of composites and (c) substantially smaller concentration necessary for the preparation of SWNT dispersions. Among polymers, different poly(vinyl alcohols) work best as a host matrix for SWNTs, providing the composites with high tensile strength and excellent Young’s modulus.

Looking Ahead

An additional resource of SWNTs for improving the mechanical properties of composites is nanotube orientation. Virtually complete alignment of nanotubes can be achieved in SWNT composite fibers. These composites display substantially better mechanical properties than any other SWNT-polymer hybrid. If one can find a simple and controllable method to produce not only fibers but also bulk materials and coatings with nanotubes oriented in a desirable direction, new technological vistas can be opened for various composites. Breakthroughs in this area can come both from the studies of fundamental properties of SWNTs and from development of methods for composite processing. In the past, magnetic field alignment with exceptionally powerful electromagnets and alignment in the flow was used for this purpose.

Metal Matrix Nanocomposites for Structural Applications

Improved Properties

Pradeep K. Rohatgi; Ben Schultz; J.B. Ferguson

Metal matrix composites (MMCs) such as continuous carbon or boron fiber reinforced aluminum and magnesium, and silicon carbide reinforced aluminum have been used for aerospace applications due to their lightweight and tailorable properties.³⁴ There is much interest in producing metal matrix nanocomposites that incorporate nanoparticles and nanotubes for structural applications, as these materials exhibit even greater improvements in their physical, mechanical and tribological properties as compared to composites with micron-sized reinforcements.^35-39 The incorporation of carbon nanotubes in particular, which have much higher strength, stiffness, and electrical conductivity as compared to metals, can significantly increase these properties of metal matrix composites. Nanocomposites are being explored for structural applications in the defense, aerospace and automotive sectors.

Low Cost Solutions

Concurrent with the interest in producing novel nanocomposite materials is the need to develop low cost means to produce these materials. Most of the prior work in synthesizing nanocomposites involves the use of powder metallurgy techniques, which are not only high cost, but also result in the presence of porosity and contamination.^{26, 40-41} Solidification processing methods, such as stir mixing, squeeze casting and pressure infiltration are advantageous over other processes in rapidly and inexpensively producing large and complex near-net shape components, however, this area remains relatively unexplored in the synthesis of nanocomposites. Stir mixing techniques, widely utilized to mix micron size particles in metallic melts,^42-43 have recently been modified for dispersing small volume percentages of nanosize reinforcement particles in metallic matrices. Although there are some difficulties in mixing nanosize particles in metallic melts resulting from their tendency to agglomerate, a research team in Japan has published research on dispersing nanosize particles in aluminum alloys using a stir mixing technique.⁴⁴ Researchers at the Polish Academy of Science⁴⁵ have recently demonstrated the incorporation of greater than 80 volume percent nanoparticles in metals using high-pressure infiltration with pressures in the GPa range. Composites produced by this method possess the unique properties of nanosize metallic grains.

Recently, metal matrix nanocomposites were synthesized at the University of Wisconsin, Milwaukee using aluminum alloy A206 and nanoparticles of alumina (Al₂O₃).⁴⁶ TEM samples of the cast Al-A206/Al₂O₃ clearly show nanoparticles present within the metal matrix (Figure 5A). SAD patterns show the pattern of the matrix as well as the nanoparticles (Figure 5B). EDX indicates that the grains are composed of aluminum, which contains nanosize alumina particles. The distribution of particles throughout the grains of the matrix with an absence of large concentrations at the grain boundaries suggests wetting of the alumina by the liquid metal. In this case the nanoparticles did not appear to act as nucleation sites for nanosized grains.

Books on Nanomaterials and Nanotechnology

The Chemistry of Nanomaterials: Synthesis, Properties and Applications, 2 volumes (Catalog #Z703850)

C. N. R. Rao, A. Müller, A. K. Cheetham (Eds.), Wiley, 2004

With this handbook, the distinguished team of editors has combined the expertise of leading nanomaterials scientists to provide the latest overview of this field. The authors cover the whole spectrum of nanomaterials, ranging from theory, synthesis, properties, characterization and applications.

Nanoparticles: From Theory to Applications(Catalog #Z551376)

G. Schmid (Ed.), Wiley, 2004

An introduction to the science of nanoparticles, from fundamental principles to their use in novel applications. As a basis for understanding nanoparticle behavior, the book first outlines the principles of quantum size behavior, nanoparticles architecture, formation of semiconductor and metal nanoparticles. It then goes on to describe the chemical syntheses of nanoparticles with defined characteristics, their structural, electrical and magnetic properties, as well as current methods to monitor these properties.