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adma.20101039.pdf

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www.MaterialsViews.com

Applications of Ultrasound to the Synthesis of Nanostructured Materials

By Jin Ho Bang and Kenneth S. Suslick*

clusters that small have electronic structures that have a high density of states, but not yet continuous bands. Nanostructured materials have been prepared by a variety of synthetic methods, including gas phase techniques (e.g., molten metal evaporation, flash vacuum thermal and laser pyrolysis decom- position of volatile organometallics), liquid phase methods (e.g., reduction of metal halides with various strong reductants, colloidal techniques with controlled nuclea- tion), and mixed phase approaches (e.g., synthesis of conventional heterogeneous catalysts on oxide supports, metal atom vapor deposition into cryogenic liquids, explosive shock synthesis). One could claim that selecting an appropriate synthetic route ultimately determines the success or failure of nanostructured materials synthesis, because physical properties and applications of nanostructured materials are heavily dependent upon how they are prepared. The importance of choosing a proper synthetic route in designing nanostructured materials has been a driving force for the development of new methodologies for several decades. Indeed, this has led scien- tists’ interest to the development of versatile and generalized synthetic methods readily adaptable for the preparation of a variety of nanostructured materials. Among a variety of approaches, the utilization of ultrasound for materials synthesis has been extensively

examined over many years, and is now positioned as one of the most powerful tools in nanostructured materials synthesis. In this review, the two most successful ultrasound-assisted synthetic methods (sonochemistry and ultrasonic spray pyrolysis) will be discussed to provide a fundamental understanding of their basic principles and to demonstrate the powerful and unique aspects of ultrasound in nanostructured materials synthesis.

2. Sonochemistry

2.1. Acoustic Cavitation

www.advmat.de

Recent advances in nanostructured materials have been led by the develop- ment of new synthetic methods that provide control over size, morphology, and nano/microstructure. The utilization of high intensity ultrasound offers a facile, versatile synthetic tool for nanostructured materials that are often unavailable by conventional methods. The primary physical phenomena associated with ultrasound that are relevant to materials synthesis are cavitation and nebulization. Acoustic cavitation (the formation, growth, and implosive collapse of bubbles in a liquid) creates extreme conditions inside the collapsing bubble and serves as the origin of most sonochemical phenomena in liquids or liquid-solid slurries. Nebulization (the creation of mist from ultrasound passing through a liquid and impinging on a liquid-gas interface) is the basis for ultrasonic spray pyrolysis (USP) with subsequent reactions occurring in the heated droplets of the mist. In both cases, we have examples of phase- separated attoliter microreactors: for sonochemistry, it is a hot gas inside bubbles isolated from one another in a liquid, while for USP it is hot droplets isolated from one another in a gas. Cavitation-induced sonochemistry provides a unique interaction between energy and matter, with hot spots inside the bubbles of 5000 K, pressures of 1000 bar, heating and cooling rates of >1010 K s 1; these extraordinary conditions permit access to a range of chemical reaction space normally not accessible, which allows for the synthesis of a wide variety of unusual nanostructured materials. Complementary to cavitational chemistry, the microdroplet reactors created by USP facilitate the formation of a wide range of nanocomposites. In this review, we summarize the fundamental principles of both synthetic methods and recent development in the applications of ultrasound in nanostructured materials synthesis.

1. Introduction

Nanoscience and nanotechnology have grown at an enormous rate for the last three decades, and recent advances in nanostructured materials have opened up new opportunities for diverse applica- tions in electronics, catalysis, energy, materials chemistry and even biology. Materials in the nanometer-size regime often exhibit properties distinct from their bulk counterparts, in part because

[*] Prof. K. S. Suslick, Dr. J. H. Bang

School of Chemical Sciences

University of Illinois at Urbana-Champaign

600 South Mathews Avenue, Urbana, Illinois 61801 (USA) E-mail: ksuslick@illinois.edu

DOI: 10.1002/adma.200904093

Adv. Mater. 2010, 22, 1039–1059 ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1039

Chemistry deals with the interaction between energy and matter, and chemical reactions require some form of energy (e.g., heat,

REVIEW

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