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ScienceWeek
2004 25 June B3 3. MATERIALS SCIENCE: ON HOLLOW NANOCRYSTALS
The following points are made by Y. Yin et al (Science 2004 304:711):
1) Porous solid materials are important in many areas of modern science and technology, including ion exchange, molecular separation, catalysis, chromatography, microelectronics, and energy storage (1-3). Notable examples are microporous ( less than 2 nm) zeolites and mesoporous (2 to 50 nm) silicate and carbonaceous materials. The ability to manipulate the structure and morphology of porous solids on a nanometer scale would enable greater control of the local chemical environment (4,5).
2) It has been known for more than half a century that porosity may result from differential solid-state diffusion rates of the reactants in an alloying or oxidation reaction. In 1947, Smigelkas and Kirkendall reported the movement of the interface between a diffusion couple, i.e., copper and zinc in brass, as the result of the different diffusion rates of these two species at an elevated temperature. This phenomenon, now called the "Kirkendall effect", was the first experimental proof that atomic diffusion occurs through vacancy exchange and not by the direct interchange of atoms. The net directional flow of matter is balanced by an opposite flow of vacancies, which can condense into pores or annihilate at dislocations.
3) Directional material flows also result from coupled reaction-diffusion phenomena at solid/gas or solid/liquid interfaces, leading to deformation, void formation, or both during the growth of metal oxide or sulfide films. These voids are usually explained by outward transport of fast-moving cations through the oxide layer and a balancing inward flow of vacancies to the vicinity of the metal-oxide interface. Interface motion and the formation of pores have been studied because of their impact on the reproducibility and reliability of solders, passivation layers, diffusion barriers, etc., but not generally as a method of preparing porous materials.
4) The pores produced at a metal-metal diffusion couple or near the metal-oxide interface of a growing oxide do not yield monodisperse, ordered arrays but instead form a very heterogeneous ensemble. The observed volume fraction for pores is also commonly much smaller than would be expected for the known material flows. These observations are a direct result of the large volume of material that vacancies can diffuse into and the large number of defects with which they can react.
5) In summary: Hollow nanocrystals can be synthesized through a mechanism analogous to the Kirkendall Effect, in which pores form because of the difference in diffusion rates between two components in a diffusion couple. Starting with cobalt nanocrystals, the authors demonstrate that their reaction in solution with oxygen and either sulfur or selenium leads to the formation of hollow nanocrystals of the resulting oxide and chalcogenides. The authors suggest this process provides a general route to the synthesis of hollow nanostructures of a large number of compounds. A simple extension of the process yielded platinum-cobalt oxide yolk-shell nanostructures, which may serve as nanoscale reactors in catalytic applications.
References (abridged):
1. D. Zhao, P. Yang, Q. Huo, B. F. Chmelka, G. D. Stucky, Curr. Opin. Solid State Mater. Sci. 3, 111 (1998)
2. S. A. Johnson, P. J. Ollivier, T. E. Mallouk, Science 283, 963 (1999)
3. A.-P. Li, F. Müller, A. Birner, K. Nielsch, U. Gösele, Adv. Mater. 11, 483 (1999)
4. D. Trong On, D. Desplantier-Giscard, C. Danumah, S. Kaliaguine, Appl. Catal. 222, 299 (2001)
5. M. E. Davis, Nature 417, 813 (2002)
Science http://www.sciencemag.org
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Related Material:
CREATING NANOCAVITIES OF TUNABLE SIZES: HOLLOW HELICES
The following points are made by B. Gong et al (Proc. Nat. Acad. Sci. 2002 99:11583):
1) Based on the folding of biopolymers, nature has developed astonishingly efficient and sophisticated strategies for generating various nanostructures. Of particular interests is the availability of a wide variety of nanosized cavities and holes that are responsible for numerous biological processes and functions. In recent years there has been intense interest in developing folding oligomers and polymers (foldamers) with unnatural backbones that adopt well-defined structures (1-3), which may eventually lead to protein-like molecular objects with sizes in the nanometer range. Many foldamer systems have been described (4,5).
2) Despite the progress made so far, the foldamer field is still in its infancy. One daunting challenge involves the design of foldamers with cavities and holes of tunable sizes in the nanometer range, the realization of which will have far-reaching significance for not only fundamental understanding but also important applications. While cavities and holes are mostly seen at the tertiary and quaternary structural levels of biopolymers, almost all foldamers reported so far fold into secondary structures.
3) In summary: The authors present a general strategy for creating nanocavities with tunable sizes based on the folding of unnatural oligomers. The backbones of these oligomers are rigidified by localized, three-center intramolecular hydrogen bonds, which lead to well-defined hollow helical conformations. Changing the curvature of the oligomer backbone leads to the adjustment of the interior cavity size. Helices with interior cavities of 10 angstroms to >30 angstroms across, the largest thus far formed by the folding of unnatural foldamers, are generated. Cavities of these sizes are usually seen at the tertiary and quaternary structural levels of proteins. The ability to tune molecular dimensions without altering the underlying topology is seen in few natural and unnatural foldamer systems.
References (abridged):
1. Gellman, S. H. (1998) Acc. Chem. Res. 31, 173-180
2. Rowan, A. E. & Nolte, R. J. M. (1998) Angew. Chem. 37, 63-68
3. Hill, D. J. , Mio, M. J. , Prince, R. B. , Hughes, T. S. & Moore, J. S. (2001) Chem. Rev. 101, 3893-4011
4. Appella, D. H. , Christianson, L. A. , Klein, D. A. , Powell, D. R. , Huang, X. L. , Barchi, J. J. & Gellman, S. H. (1997) Nature (London) 387, 381-384
5. Seebach, D. , Abele, S. , Gademann, S. K. & Jaun, B. (1999) Angew. Chem. 38, 1595-1597
Proc. Nat. Acad. Sci. http://www.pnas.org
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Related Material:
ON NANOSCALE ENCAPSULATION
The following points are made by I.G. Loscertales et al (Science 2002 295:1695):
1) Production and control of droplets and particles of micrometer or even nanometer size with a narrow size distribution are of interest for many applications in science and technology. Usually, these particles are formed as either an aerosol or a hydrosol phase. Aerosols and hydrosols of compound particles, such that each particle is made of a small amount of a certain substance surrounded by another substance, are of particular importance for encapsulation of food additives, targeted drug delivery, and special material processing, among other technological fields. In all of these cases, encapsulation is used to provide compound particles in an appropriate size range.
2) One of the most widely adopted methods to obtain micrometer or nanometer capsules is based on emulsion technology. Two immiscible fluids, one carrying the substance to be encapsulated and the other fluid carrying the polymer for the shell, are stirred to form an emulsion. This emulsion is stabilized by pouring it into a third solution ("double emulsion process"), thereby extracting the polymer solvent and solidifying the polymer as a capsule. Related methods also incorporate phase separation and similar physical or chemical phenomena.
3) Other approaches for encapsulation resort to the formation and control of liquid jets with diameters in the micrometer/nanometer range. In the electrospray technique, a conducting liquid is slowly injected through an electrified capillary tube. When the electric potential between the liquid and its surroundings rises to a few kilovolts, the meniscus at the tube exit develops a conical shape, commonly referred to as the "Taylor cone". A thin microthread of liquid is issued from the tip of the Taylor cone, which eventually fragments to form a spray of highly charged droplets. Its most well-known application has been in mass spectrometry, where it has been successfully exploited as a way to produce multiply-charged gas phase ions of huge biomolecules present in the liquid phase.
4) The authors report a method to generate steady coaxial jets of immiscible liquids with diameters in the range of micrometer/nanometer size. This compound jet is generated by the action of electro-hydrodynamic forces with a diameter that ranges from tens of nanometers to tens of micrometers. The eventual jet breakup results in an aerosol of monodisperse compound droplets with the outer liquid surrounding or encapsulating the inner liquid. The authors report they have produced monodisperse capsules with diameters varying between 0.15 and 10 microns, depending on the running parameters.
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