Inorganic Nanoparticles: Synthesis, Applications, and Perspectives

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Inorganic Nanoparticles: Synthesis, Applications, and Perspectives

Nanomaterials and Their Applications Series Editor: M. Meyyappan Carbon Nanotubes: Reinforced Metal Matrix Composites A

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Nanomaterials and Their Applications Series Editor: M. Meyyappan

Carbon Nanotubes: Reinforced Metal Matrix Composites Arvind Agarwal, Srinivasa Rao Bakshi, Debrupa Lahiri

Inorganic Nanoparticles: Synthesis, Applications, and Perspectives Edited by Claudia Altavilla, Enrico Ciliberto

Nanorobotics: An Introduction Lixin Dong, Bradley J. Nelson

Graphene: Synthesis and Applications Wonbong Choi, Jo-won Lee

Edited by

Claudia Altavilla Enrico Ciliberto

Boca Raton London New York

CRC Press is an imprint of the Taylor & Francis Group, an informa business

CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 © 2011 by Taylor and Francis Group, LLC CRC Press is an imprint of Taylor & Francis Group, an Informa business No claim to original U.S. Government works Printed in the United States of America on acid-free paper 10 9 8 7 6 5 4 3 2 1 International Standard Book Number-13: 978-1-4398-1762-9 (Ebook-PDF) This book contains information obtained from authentic and highly regarded sources. Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, please access www.copyright.com (http:// www.copyright.com/) or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com

This book is dedicated to my parents Ida and Nicolò “the guiding lights,” to my husband Giuseppe, “the true love,” and to my daughter Marida “the best reason to become a better person” Claudia Altavilla To my Family and to my Mentors Enrico Ciliberto

Contents Foreword..........................................................................................................................................ix Acknowledgments..........................................................................................................................xi Editors............................................................................................................................................ xiii Contributors....................................................................................................................................xv 1 Inorganic Nanoparticles: Synthesis, Applications, and Perspectives—An Overview................................................................................................................................... 1 Claudia Altavilla and Enrico Ciliberto 2 Inorganic Nanoparticles for the Conservation of Works of Art.................................. 17 Piero Baglioni and Rodorico Giorgi 3 Magnetic Nanoparticle for Information Storage Applications.................................... 33 Natalie A. Frey and Shouheng Sun 4 Inorganic Nanoparticles Gas Sensors............................................................................... 69 B.R. Mehta, V.N. Singh, and Manika Khanuja 5 Light-Emitting Devices Based on Direct Band Gap Semiconductor Nanoparticles........................................................................................................................ 109 Ekaterina Neshataeva, Tilmar Kümmell, and Gerd Bacher 6 Formation of Nanosized Aluminum and Its Applications in Condensed Phase Reactions.................................................................................................................... 133 Jan A. Puszynski and Lori J. Groven 7 Nanoparticles for Fuel Cell Applications....................................................................... 159 Jin Luo, Bin Fang, Bridgid N. Wanjala, Peter N. Njoki, Rameshwori Loukrakpam, Jun Yin, Derrick Mott, Stephanie Lim, and Chuan-Jian Zhong 8 Inorganic Nanoparticles for Photovoltaic Applications.............................................. 185 Elif Arici 9 Inorganic Nanoparticles and Rechargeable Batteries.................................................. 213 Doron Aurbach and Ortal Haik 10 Quantum Dots Designed for Biomedical Applications.............................................. 257 Andrea Ragusa, Antonella Zacheo, Alessandra Aloisi, and Teresa Pellegrino 11 Magnetic Nanoparticles for Drug Delivery................................................................... 313 Claudia Altavilla 12 Nanoparticle Thermotherapy: A New Approach in Cancer Therapy.......................343 Joerg Lehmann and Brita Lehmann vii

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Contents

13 Inorganic Particles against Reactive Oxygen Species for Sun Protective Products................................................................................................................................. 355 Wilson A. Lee and Miriam Raifailovich 14 Innovative Inorganic Nanoparticles with Antibacterial Properties Attached to Textiles by Sonochemistry............................................................................................ 367 Nina Perkas, Aharon Gedanken, Eva Wehrschuetz-Sigl, Georg M. Guebitz, Ilana Perelshtein, and Guy Applerot 15 Inorganic Nanoparticles for Environmental Remediation......................................... 393 Thomas B. Scott 16 Inorganic Nanotubes and Fullerene-Like Structures—From Synthesis to Applications.......................................................................................................................... 441 Maya Bar-Sadan and Reshef Tenne 17 Inorganic Nanoparticles for Catalysis............................................................................. 475 Naoki Toshima 18 Nanocatalysts: A New “Dimension” for Nanoparticles?............................................ 511 Paolo Ciambelli, Diana Sannino, and Maria Sarno Index.............................................................................................................................................. 547

Foreword Development, characterization, and exploitation of nanophase materials are all fundamental to the anticipated nanotechnology revolution. In the last decade, research activities on carbon nanotubes, inorganic nanowires, quantum dots, and nanoparticles have increased exponentially, as evidenced by the large number of papers in peer-reviewed journals and conference presentations across the world. Among the various nanomaterials, inorganic nanoparticles assume special importance because they are easier and cheaper to synthesize in the laboratory and to mass produce than some other nanomaterials like carbon nanotubes, for example. It is for this reason also that they can be more readily integrated into applications. As synthesis, characterization, and application development using nanoparticles continues strongly, there is a need to capture the fundamentals and the current advances in a textbook for the benefit of researchers, graduate students, and colleagues in various industries. This book by Drs. Claudia Altavilla and Enrico Ciliberto meets the above need admirably. An excellent group of experts have been assembled to discuss the diverse applications of inorganic nanoparticles, which would otherwise have been impossible to cover by just one or two people. After an overview on material synthesis and general perspectives in Chapter 1, the book delves into myriad applications of nanoparticles. Chapter 2 covers a very interesting and unique application in the conservation of art. Magnetic materials have found their way into magnetic storage media long ago, and Chapter 3 covers the use of nanoparticles in this domain. Oxide thin films, especially tin oxide, have been the conducting media in commercial gas and vapor sensors, and Chapter 4 provides a discussion as to how their performance can be improved using metal and oxide nanoparticles. Solid-state lighting has attracted attention worldwide due to its higher efficiency compared to conventional lighting, but the costs remain very high. Advances in materials, device fabrication, and large-scale production are urgently required to reduce global energy demands. Chapter 5 discusses the advances in semiconductor nanoparticles for light-emitting devices. Besides lighting, other areas related to the energy sector, such as solar energy and energy storage devices (fuel cells, rechargeable batteries, etc.), can also benefit from the properties of nanomaterials. These are covered in Chapters 7 through 9. Another industrial sector that is likely to feel the impact of nanotechnology is the biomedical field. Several chapters are devoted to quantum dots for bioimaging, nanoparticle-based cancer therapy, drug delivery, antibacterial agents, and others. Last but not the least is the long-standing application in catalysis and the role of nanosized particles in this established field. I hope the readers find this treatise useful as a textbook and research resource. Nora Konopka of CRC Press deserves praise for initiating the book series on nanomaterials. Meya Meyyappan Moffett Field, California

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Acknowledgments We thank all the contributors to this book for their extra effort in presenting state-of-the-art developments in their areas of expertise. This book would not have been possible without them. Additionally, we would like to acknowledge Dr. Meya Meyyappan for his trust and support in the realization of this project, and Nora Konopka and Kari Budyk of CRC Press for their constant technical support during all the stages of production. Tom Schott, who designed the cover of the book, is also heartily acknowledged. Finally, a special thanks to our families for their endless patience, which allowed us to spend time on the preparation of this book.

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Editors Dr. Claudia Altavilla graduated in chemistry (cum laude) in 2001 from the University of Catania, Italy. She received her PhD in chemistry in 2006 from the same university with a dissertation on the synthesis and characterization of nanostructured materials assembled on inorganic substrates. She worked as a visiting scientist at Ludwig Maximillians Universitat in Munich, Germany, with Professor Wolfgang Parak, and at the University of Florence, Italy, with Professor Dante Gatteschi, where she was involved in the magnetic characterization of nanoparticle monolayers on silicon substrates. Since 2005, she has been a professor of inorganic protective and consolidant methods in cultural heritage at the University of Catania. Dr. Altavilla’s current research includes the chemical synthesis of inorganic nanoparticles of ferrite, chalcogenite, and metals functionalized by different organic coatings for application in magnetic storage media, lubricants, magnetorheological fluids, and biomedicine; and self-assembled monolayers of inorganic and organic nanostructures on different substrates and CVD synthesis of carbon nanotubes on silicon substrates using transition metal oxide nanoparticles as catalyst. She has published several papers and monographs. She is a referee for international journals on material science and nanotechnology such as ACS Nano, Chemistry of Materials, and the Journal of Material Chemistry. Currently she is a research fellow in the Department of Chemical and Food Engineering, University of Salerno, Italy. Dr. Enrico Ciliberto is a full professor of inorganic chemistry at the University of Catania and the president of the Cultural Heritage Technologies Faculty at the University of Syracuse, Italy. His research focuses on the chemistry of materials, including surface science and cultural heritage materials, both from an archaeometric and conservative point of view, and covers Minoan mortars in Crete, Michelangelo’s David in Florence, and Saint Mark’s Basilica in Venice. His current scientific interest includes the application of nanotechnologies for the conservation of works of art. He has also published over 100 scientific papers.

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Contributors Alessandra Aloisi National Nanotechnology Laboratory of CNR-INFM Italian Institute of Technology Research Unit Lecce, Italy

Piero Baglioni Department of Chemistry and Consorzio Interuniversitario per lo Sviluppo dei Sistemi a Grande Interfase University of Florence Sesto Fiorentino, Italy

Claudia Altavilla Department of Chemical and Food Engineering University of Salerno Fisciano, Italy

Maya Bar-Sadan Institute of Solid State Research Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons Research Centre Juelich Juelich, Germany

Guy Applerot Department of Chemistry and Kanbar Laboratory for Nanomaterials Bar-Ilan University Center for Advanced Materials and Nanotechnology Bar-Ilan University Ramat-Gan, Israel Elif Arici Linz Institute for Organic Solar Cells Institute of Physical Chemistry Johannes Kepler University Linz, Austria Doron Aurbach Department of Chemistry Bar-Ilan University Ramat Gan, Israel Gerd Bacher Werkstoffe der Elektrotechnik and Center for Nanointegration Duisburg-Essen University Duisburg-Essen Duisburg, Germany

Paolo Ciambelli Department of Chemical and Food Engineering and Centre NANO_MATES University of Salerno Fisciano, Italy Enrico Ciliberto Dipartimento di Scienze Chimiche Università di of Catania Catania, Italy Bin Fang Department of Chemistry State University of New York, Binghamton Binghamton, New York Natalie A. Frey Department of Chemistry Brown University Providence, Rhode Island

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xvi

Aharon Gedanken Department of Chemistry and

Contributors

Wilson A. Lee Estee Lauder Companies, Inc. Melville, New York

Kanbar Laboratory for Nanomaterials Bar-Ilan University Center for Advanced Materials and Nanotechnology Bar-Ilan University Ramat-Gan, Israel

Brita Lehmann Department of Radiology School of Medicine University of California Davis Sacramento, California

Rodorico Giorgi Department of Chemistry and Consorzio Interuniversitario per lo Sviluppo dei Sistemi a Grande Interfase University of Florence Sesto Fiorentino, Italy

Joerg Lehmann Department of Radiation Oncology School of Medicine University of California Davis Sacramento, California

Lori J. Groven Chemical and Biological Engineering Department South Dakota School of Mines and Technology Rapid City, South Dakota Georg M. Guebitz Institute of Environmental Biotechnology Graz University of Technology Graz, Austria Ortal Haik Department of Chemistry Bar-Ilan University Ramat Gan, Israel Manika Khanuja Thin Film Laboratory Department of Physics Indian Institute of Technology New Delhi, India Tilmar Kümmell Werkstoffe der Elektrotechnik and Center for Nanointegration Duisburg-Essen University Duisburg-Essen Duisburg, Germany

Stephanie Lim Department of Chemistry State University of New York, Binghamton Binghamton, New York Rameshwori Loukrakpam Department of Chemistry State University of New York, Binghamton Binghamton, New York Jin Luo Department of Chemistry State University of New York, Binghamton Binghamton, New York B.R. Mehta Thin Film Laboratory Department of Physics Indian Institute of Technology New Delhi, India Derrick Mott Department of Chemistry State University of New York, Binghamton Binghamton, New York Ekaterina Neshataeva Werkstoffe der Elektrotechnik and Center for Nanointegration Duisburg-Essen University Duisburg-Essen Duisburg, Germany

xvii

Contributors

Peter N. Njoki Department of Chemistry State University of New York Binghamton Binghamton, New York Teresa Pellegrino National Nanotechnology Laboratory of CNR-INFM Italian Institute of Technology Research Unit Lecce, Italy and Istituto Italiano di Tecnologia Genova, Italy Ilana Perelshtein Department of Chemistry and Kanbar Laboratory for Nanomaterials Bar-Ilan University Center for Advanced Materials and Nanotechnology Bar-Ilan University Ramat-Gan, Israel Nina Perkas Department of Chemistry and Kanbar Laboratory for Nanomaterials Bar-Ilan University Center for Advanced Materials and Nanotechnology Bar-Ilan University Ramat-Gan, Israel Jan A. Puszynski Chemical and Biological Engineering Department South Dakota School of Mines and Technology Rapid City, South Dakota Miriam Raifailovich Material Science and Engineering Department Stony Brook University Stony Brook, New York

Andrea Ragusa National Nanotechnology Laboratory of CNR-INFM Italian Institute of Technology Research Unit Lecce, Italy Diana Sannino Department of Chemical and Food Engineering and Centre for NANOMAterials and NanoTEchnology University of Salerno Fisciano, Italy Maria Sarno Department of Chemical and Food Engineering and Centre for NANOMAterials and NanoTEchnology University of Salerno Fisciano, Italy Thomas B. Scott Interface Analysis Centre University of Bristol Bristol, United Kingdom V.N. Singh Thin Film Laboratory Department of Physics Indian Institute of Technology New Delhi, India Shouheng Sun Department of Chemistry Brown University Providence, Rhode Island Reshef Tenne Materials and Interfaces Department Weizmann Institue of Science Rehovot, Israel

xviii

Naoki Toshima Department of Applied Chemistry Tokyo University of Science, Yamaguchi Sanyo-Onoda, Japan

Contributors

Jun Yin Department of Chemistry State University of New York, Binghamton Binghamton, New York

and Core Research for Evolutional Science and Technology (CREST) Japan Science and Technology Agency Kawaguchi, Japan Bridgid N. Wanjala Department of Chemistry State University of New York, Binghamton Binghamton, New York Eva Wehrschuetz-Sigl Institute of Environmental Biotechnology Graz University of Technology Graz, Austria

Antonella Zacheo National Nanotechnology Laboratory of CNR-INFM Italian Institute of Technology Research Unit Lecce, Italy

Chuan-Jian Zhong Department of Chemistry State University of New York, Binghamton Binghamton, New York

1 Inorganic Nanoparticles: Synthesis, Applications, and Perspectives—An Overview Claudia Altavilla and Enrico Ciliberto Contents 1.1 Introduction.............................................................................................................................1 1.2 Properties of Nanoparticles................................................................................................... 5 1.3 Synthesis Strategies................................................................................................................ 6 1.4 Applications............................................................................................................................. 8 1.5 Conclusion............................................................................................................................. 13 References........................................................................................................................................ 14

1.1  Introduction Over the last few years, a variety of inorganic nanomaterials such as nanoparticles, nanowires, and nanotubes have been created or modified in order to obtain superior properties with greater functional versatility. The advent of nanoscale science and technology has stimulated a big effort to develop new strategies for the synthesis of nanomaterials of a controlled size and shape. In particular, nanoparticles due to their size, in the range of 1–100 nm, have been examined for their uses as tools for a new generation of technological devices. Moreover, due to their dimensions and shapes being similar to several biological structures (e.g., membrane cell genes, proteins, and viruses), they have been proposed for investigating biological processes as well as for sensing and treating diseases. Nowadays, the volume of studies dealing with these topics represents one of the most impressive phenomenon in all of scientific history. Even so only one Nobel prize, shared by three scientists, has been awarded for the development of the studies in this field in the last 20 years, in 1996, Robert F. Curl Jr., Sir Harold W. Kroto, and Richard E. Smalley were awarded for their discovery of fullerenes. In Figure 1.1, the number of scientific articles and papers with reference to the themes of nanoparticles from 1996 until 2009 is reported: the exponential trend clearly indicates that the scientific and technological interest is continuing to increase. Compared with the notable amount of scientific and technological studies in this field, only one Nobel prize could sound quite inadequate. One reason can probably be attributed to the fact that “nanotechnologies” are very old, even though several of the relationships between dimension and properties have only been clarified in the nineteenth century. In fact, very few people know that even in the sixth century BC, nanotechnology was commonly used in the Attic region (Greece). During the Archaic and Classical periods, roughly 620–300 BC, in the region of Attica, dominated by the city of Athens, the production of 1

2

3.41% 2002

2009

2.73% 2001

11.46%

9.08%

18.28% 2008

2.45% 2000

2005

1.76% 1999

6.49%

1.41% 1998

2004

1.02%

2003

0.35%

0

1997

5,000

1996

10,000

4.78%

15,000

2006

Record count

20,000

14.33%

>100,000 records

2007

25,000

21.78%

Inorganic Nanoparticles: Synthesis, Applications, and Perspectives

Publication year FIGURE 1.1 Temporal evolution in the number of scientific papers on nanoparticles published from 1996 to 2009. More than 100,000 records were found and more than 54% of the articles have been published in the last 3 years. (Data from ISI Web of Knowledge.)

decorated vases reached an extraordinary artistic level due to the development of a highly original firing technique that obtained a magnificent black/red dichromatism, the secret of Greek vases (Figure 1.2) (Boardman 1991). The reason why a deep black color formed on the vase surface was discovered only a few years ago. During the firing process, spinel-like nanoparticles formed inside a glassy layer, which is a few microns thick (Maniatis et al. 1992). In Figure 1.3, a secondary electron microscope (SEM) image of submicron particles inside a glossy layer of a sixth century BC Greek vase is reported. The magnetite particles, looking whitish in the backscattered mode, show different sizes (100–300 nm) and different shapes. A skillful alternation of the

FIGURE 1.2 Attic black figure krater, sixth century BC. (Courtesy of Prof. Enrico Ciliberto.)

Inorganic Nanoparticles: Synthesis, Applications, and Perspectives—An Overview

1 µm*

3

Mag = 26.76 K X Signal A = QBSD Date : 20 Jun 2008 WD = 8 mm EHT = 15.00 kV Time : 12:58:08

FIGURE 1.3 Back scattered electron image of submicron magnetite particles inside the black glossy layer of the vase reported in Figure 1.2. (Courtesy of Prof. Enrico Ciliberto.)

oxidizing and reduction processes, induced by either opening or closing the oven vents, stimulated the formation of black magnetite nanoparticles (Hemelrijk 1991). In addition, “luster” ceramic decorations have been revealed by transmission electron microscopy (TEM) to have been ancient nanostructured metallic thin films made by man. Considering this type of decoration in the context of cultural heritage, it is a remarkable discovery in the history of technology, because nanocrystal films have been produced empirically since medieval times (Borgia et al. 2002; Padovani et al. 2003). Luster is a type of ceramic decoration, which results in a beautiful metallic shine and colored iridescence on the surface of the ceramic object. The earliest luster was probably made in Iraq in the early ninth century AD on tin-glazed ceramics. However, luster technology spread from the Middle East to Persia, Egypt, Spain, and Italy, and its splendid production continued in the centuries that followed through to the present day. In TEM, luster layers appear with a homogeneous surface microstructure formed by small quasispherical clusters, embedded in an amorphous glassy matrix (Figure 1.4). The total thickness of this structure is 200–500 nm. An initial outer layer is formed by the biggest clusters, which have a diameter of about 50 nm. The diameter of the next layer, with smaller inner clusters, is 5–20 nm. With respect to the composition of these clusters, transmission electron microscopy (TEM) fitted with energy dispersive x-ray spectroscopy (EDS) analyses indicate that the nanoclusters are particles of pure copper and silver (PerezArantegui and Larrea 2003). In addition, red glasses that are very ancient are colored due to the presence of nanoparticles. In fact, excavations at Qantir, on the Nile Delta, have given insight into the organization and development of an industrial estate in Ramesside, Egypt. In founding the new capital of Egypt, Piramesses, during the nineteenth dynasty, a huge bronze-casting factory was built, accompanied by a range of other, nonmetallic high-temperature industries.

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Inorganic Nanoparticles: Synthesis, Applications, and Perspectives

30 nm

FIGURE 1.4 TEM image of a smalt from an Italian Renaissance Luster Majolica. Copper nanoparticles show diameters ranging from 7 to 10 nm. (Courtesy of Prof. Bruno Brunetti.)

Besides, an abundant production of faience implements, coated with copper-colored glazes, and the manufacturing of Egyptian blue, the coloration of large quantities of red glass also played a major role. The production of glass is attested by numerous crucibles, mostly with adhering traces of red glass. While evidence of glass working by artisans is absent, there are indications that the production of both raw glass and glass coloring took place. The nature and complexity of high-temperature industrial debris found at Qantir suggest a highly specialized organization of labor within a framework of shared technologies and skills of closely controlled temperatures and redox conditions. This cross-craft workshop pattern further reveals a significant level of intracraft specialization as well as the spatial separation of glass making, coloring, and finally working in the Late Bronze Age Egypt (Rehren et al. 1998). We now know that the red color is due to metal nanoparticles contained in the glass network. The use of metal nanoparticles dispersed in an optically clear matrix by potters and glassmakers from the Bronze Age up to the present time has been reviewed by Colomban from a solid-state chemistry and material science point of view. The nature of metal (gold, silver, or copper) and the importance of some other elements (Fe, Sn, Sb, and Bi) added to control metal reduction in the glass in relation to the firing atmosphere (combined reducing oxidizing sequences and role of hydrogen and water) are considered in the light of ancient treatises and recent analyses using advanced techniques (TEM, extended x-ray absorption fine structure (EXAFS), etc.) as well as classical methods (optical microscopy, UV–visible absorption). The different types of color production, by absorption/reflection (red and yellow) or diffraction (iridescence), as well as the relationship between nanostructures (metal particle dispersion and layer stacking) and luster color have been also discussed. It has also been shown that Raman scattering is a very useful technique in order to study the local glass structure around the metal particles as well as detect incomplete metal reduction or residues tracing the preparation route; therefore, making it possible to differentiate between genuine artifacts and fakes (Colomban 2009). In all the aforementioned cases, old technology surpassed the scientific interpretation of the related phenomena and, today, experimental experience remains the basis of modern

Inorganic Nanoparticles: Synthesis, Applications, and Perspectives—An Overview

5

progress, even if thermodynamic and quantum mechanics have already explained many of the properties of nanoscale materials (Lafait 2006; Cavalcante et al. 2009).

1.2  Properties of Nanoparticles On the nanoscale, materials behave very differently compared to larger scales. In fact, nanoparticles often have unique physical and chemical properties. For example, the electronic, optical, and chemical properties of nanoparticles may be very different from those of each component in the bulk. By increasing the surface area with respect to the volume of a particle, a corresponding increasing of importance of the behavior of the surface atoms can be observed, and a modification of the properties of the particle itself as well as of its interaction with the surrounding environment take place. Moreover, in order to become small enough, a transition from classical physics behavior to a quantum mechanic, one describes the particle that can now be viewed as an artificial atom, an object that possesses discrete electronic states, similar to naturally occurring atoms. An electron in an artificial atom that can be described by a quantum wave-function that is similar to the one used for an electron in a single atom, even though its energy is spread coherently over the lattice of atomic nuclei. As the size of a crystal decreases to the nanometer regime, the size of the particle begins to modify the properties of the crystal. The electronic structure is altered by the continuous electronic bands to discrete or quantized electronic levels. As a result, the continuous optical transitions between the electronic bands become discrete and the properties of the nanomaterial become sizedependent. Therefore, optical, thermal, and electrical properties of the particles become dependent on their sizes and shapes. These properties have been recently reviewed by Burda et al. (2005). However, some of the properties of the nanoparticles might not be predicted by understanding the increasing influence of surface atoms or quantum effect. For instance, it has been shown that silicon nanoparticles in the range of 20–100 nm are superhard in the 30–50 GPa range after work hardening (Gerberich et al. 2003). The nanosphere hardness falls between the conventional hardness of sapphires and diamonds, which are among the hardest known materials. The extremely small dimensions of nanobuilding blocks have created difficult challenges to many existing instruments, methodologies, and even theories. The methods that have been developed and used for measuring the mechanical properties of isolated individual nanobuilding blocks include uniaxial tensile loading using a nanomanipulation stage, in-situ compression of nanoparticles and nanopillars, mechanical/electric-field-induced resonance, atomic force microscopy (AFM) bending, and nanoindentation (Uchic et al. 2004). These methods certainly represent important instruments that help scientists in designing low-cost superhard materials from nanoscale building blocks. While nanoparticles display properties that differ from those of bulk samples of the same material, groups of nanoparticles can have collective properties that are different to those displayed by individual nanoparticles and bulk samples. For realizing versatile functions, an assembly of nanoparticles in regular patterns on surfaces and at interfaces is required (Altavilla 2007). Assembling nanoparticles generates new nanostructures, which have unforeseen collective, intrinsic physical properties. These properties can be exploited for multipurpose applications in nanoelectronics, spintronics, sensors, etc. (Nie et al. 2010).

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Inorganic Nanoparticles: Synthesis, Applications, and Perspectives

1.3  Synthesis Strategies There are a wide variety of techniques for producing nanoparticles. These essentially fall into three categories: physical methods, chemical syntheses, and mechanical processes such as milling. Among the physical methods, pulsed laser ablation has been demonstrated to be a powerful and versatile technique for preparing high-purity nanoparticles or nanofilms (Longstreth-Spoor et al. 2008). In general, the targets used for the preparation of nanoparticles or films by laser ablation are bulk sizes, and the lasers are either excimer, pulsed yttrium aluminium garnet (YAG), or femtosecond lasers. The quality and sizes of the nanoparticles prepared by these systems are controlled by optimizing either the laser parameters or ambient-gas pressure. The main advantage of laser ablation is the congruent (stoichiometric) material transport above the threshold fluence, which is used for depositing complex compounds such as high-Tc superconductors (Kang et al. 2006). In addition, high-melting-point materials (e.g., C, W, and refractory ceramics) are easily deposited (Ullmann et al. 2002; Chen et al. 2004). Passivated α-Fe nanoparticles can also be prepared at atmospheric pressure by pulsed laser ablation of an Fe wire and a bulk Fe target (Wang et al. 2009). Other physical methods used in preparing nanoparticles belong to the category of vapor condensation. This approach is used to prepare metallic and metal oxide ceramic nanoparticles. It involves the evaporation of a solid metal followed by rapid condensation to form the final nanostructured material. Different methods can be adopted to produce metal vapors. An inert gas is also used to inhibit oxidizing phenomena but in some cases, oxygen atmosphere is used to make metal oxide nanoparticles. The main advantage of this approach is low contamination levels. Final particle size is controlled by the variation of temperature, flux parameters, and gas environment (Swihart 2003). The most widely used chemical synthesis essentially consists of growing nanoparticles in a liquid medium made up of various reactants. The chemical growth of bulk or nanometer-sized materials inevitably involves the process of precipitation of a solid phase from a solution. For a particular solvent, there is a certain solubility for a solute, whereby addition of any excess solute will result in the precipitation and formation of nanocrystals. Thus, in the case of nanoparticle formation, for nucleation to occur, the solution must be supersaturated either by directly dissolving the solute at higher temperatures and then cooling to low temperatures or by adding the necessary reactants to produce a supersaturated solution during the reaction. The precipitation process then basically consists of a nucleation step followed by particle growth stages (Peng et al. 1998). For a homogeneous nucleation that occurs in the absence of a solid interface, the phenomenon can be described by the overall free energy change (ΔG) because the supersaturated solutions are not stable from a thermodynamic point of view. It has been demonstrated that ΔG depends on the saturation ratio of the solution as well as the radius of nuclei formed (Burda et al. 2005). ΔG shows a maximum critical value of the radius (rc) that corresponds to a critical size of the particle (see Figure 1.5). This maximum free energy is the activation energy for nucleation. Nuclei larger than the critical size will further decrease their free energy for growth and form stable nuclei that grow to form particles. The growth process of nanocrystals can occur in two different ways, “focusing” and “defocusing,” depending on the concentration of the solution. A critical size exists at any given concentration. At a high concentration, the critical size is small so that all the particles grow. In this situation, smaller particles, slightly larger than the critical size, have a high free energy driving force and grow faster than the larger ones. As a result, the size

Inorganic Nanoparticles: Synthesis, Applications, and Perspectives—An Overview

7

∆G

rc

r

FIGURE 1.5 Free energy ΔG as a function of the radius of particle; rc, critical radius size.

distribution can be focused down to one that is nearly monodisperse. If the monomer concentration is below a critical threshold, small nanocrystals are depleted as larger ones grow and the size distribution broadens, or defocuses (Yin and Alivisatos 2005). The preparation of nearly monodisperse spherical particles can be achieved by stopping the reaction while it is still in the focusing regime, with a large concentration of monomer still present (Peng et al. 1998). In general, it is desirable for nucleation to be separated in time from the growth step in order to obtain relatively monodisperse samples. This means that nucleation must occur on a short timescale. This may be achieved by rapidly injecting suitable precursors into the solvent at high temperatures to generate transient supersaturation in solutions and induce a nucleation burst. In addition to this kind of growth, where soluble species deposit on the solid surface, particles can grow by aggregation with other particles, and this is called secondary growth. The rate of particle growth by aggregation is much larger than by molecular addition. Finally, the control over size, size distribution, and secondary growth becomes a more challenging problem in such dimensional regimes. In the synthesis of colloidal nanoparticles, the key strategy stands within the use of specific molecules, which act as terminating or stabilizing agents, ensuring a slow growth rate, preventing interparticle agglomeration, and conferring stability as well as further processability to the resulting nanoparticles. These molecules are often chosen among various classes of surfactants. Surfactants are molecules composed of a polar head group and one or more hydrocarbon chains with a hydrophobic nature. The most commonly used in colloidal syntheses include alkyl thiols, amines, carboxylic and phosphonic acids, phosphines, phosphine oxides, phosphates, phosphonates, as well as various coordinating solvents (Cozzoli et al. 2006). An important step in the generation of colloidal inorganic nanoparticles is the identification of suitable precursor molecules such as metal complexes and organometallic compounds. The precursors need to rapidly decompose or react at the required growth temperature to yield reactive atomic or molecular species (often called monomers), which then cause nanocrystal nucleation and growth (Stuczynski et al. 1989; Steigerwald 1994). In this sense, these chemical methods operating in solutions can be related to the metal organic chemical vapor deposition where volatile precursors in vapor phase react and/or decompose on the substrate surface to produce a desired deposit at much higher growth

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Inorganic Nanoparticles: Synthesis, Applications, and Perspectives

temperatures (Sun et al. 2004; Apátiga et al. 2007; Creighton et al. 2008). The two approaches share many features including similar basic chemical reactions involved. Along with the mechanical techniques used to prepare nanoparticles, a method that has received a great deal of interest from the industrial world is bead milling. The particle size achieved from a bead mill is a direct function of the size of the beads used for the grinding process. The average particle size that can be quickly achieved in a bead mill is about 1/1000 the size of the grinding media. The smallest bead size regularly used on a commercial basis is 200–300 μm. The applications of this media are primarily in the pigment manufacturing and ink industry for the fine grinding and dispersion of pigments such as phthalocyanine blue and green as well as carbon black. The uses for these inks are in the ink jet market, textile inks, etc. (Czekai 1996). Sonochemistry is characterized by both mechanical and chemical properties. In fact, sonochemical methods refer to chemical reactions that are induced by acoustic cavitations. In organic solvents, the high-temperature conditions generated during acoustic cavitations have been used to synthesize metal and other nanomaterials. In water, a variety of primary and secondary radicals are generated during acoustic cavitations that can be used for a series of redox reactions in aqueous solutions. Moreover, it has been demonstrated how the size, size distribution, and, to some extent, the shape of metal nanoparticles may be controlled by the sonochemical preparation method (Muthupandian 2008).

1.4  Applications The goal of this book is to describe the most important applications of nanoparticles. In Chapter 2, Piero Baglioni and Rodorico Giorgi introduce the use of nanoparticles in the field of cultural heritage conservation. The contribution of science to the conservation of cultural heritage has radically increased over the last years, many thanks to the advancements in the knowledge of the physicochemical composition and properties of the materials constituting the works of art (Ciliberto 2000). Nanoparticles of calcium hydroxide give a consistent improvement over the classical application of a calcium hydroxide solution. In fact, the use of Ca(OH)2 dispersions overcome the limitation due to the low solubility in water, alcohols are less aggressive than water toward fragile mural paintings, and the quick carbonation of hydroxides gives a strong consolidation effect. Calcium and magnesium hydroxide as a nonaqueous dispersion also give excellent results for the treatment of cellulose-based materials. These preferably require waterless solvents and need an alkaline reserve to protect the object from further degradation due to pollution or internal acid production as a consequence of the natural aging of the materials. Humble particles of calcium or magnesium hydroxide give excellent results and ensure high physicochemical compatibility with the substrates that grant the durability of the treatment and long-lasting protection of the works of art. With illustrative examples on the consolidation of wall paintings and deacidification of books and wood, this contribution also reports on some recent case studies, highlighting the improved performances of nanoparticles and nanocontainers (micelles, microemulsion, nanogels, etc.) in respect to traditional conservation methodologies. The use of magnetic nanoparticles for an information storage application is discussed by Natalie A. Frey and Shouheng Sun in Chapter 3. High-quality monodisperse magnetic nanoparticles with high coercivity can be made from various chemical synthesis routes

Inorganic Nanoparticles: Synthesis, Applications, and Perspectives—An Overview

9

and provide a way around the superparamagnetic limit that is currently encroaching upon the granular media that is used in hard disk drives today. Considering that synthesized nanoparticles are usually superparamagnetic, some novel approaches have been used to anneal the particles at high temperature, facilitating the face centered cube (fcc) to face centered tetragonal (fct) phase transition in FePt nanoparticles while keeping the particles from sintering and allowing the particles to be dispersed again in organic solvents. In order to increase packing density to maximize areal density for media, the shapes can be controlled and self-assembly can be employed to control interparticle spacing, which in turn provides control over magnetic interactions. Even higher anisotropic rare earthtransition metal nanoparticles are being synthesized, though the challenges associated with their syntheses are significant. In the case of SmCo5, nanoscale powders with high coercivity have been made after reductively annealing core-shell structures. More needs to be done to protect these particles from sintering during the annealing process and the issue of chemical stability needs to be addressed. The results presented paint a promising picture for the future of magnetic nanoparticles in recording technology. Gas sensor technologies have received a significant boost from nanoparticles. There is a large volume of data on the use of metal oxide nanoparticles and nanoparticle layers for gas sensor applications. Lack of accurate and reliable information about nanoparticle size, size distribution, metal additive, composition, and configuration makes the analysis of this data a challenging task, but B.R. Mehta et al. describe the current state of art of this topic in Chapter 4. Due to the percentage of atoms on the surface increasing with the decrease in particle size as the surface-to-volume ratio is inversely proportional to radius, nanoparticles will offer a large surface area for gas adsorption, which is always the first step in the gas-sensing mechanism. However, for a more detailed and clearer understanding of the dependence of gas sensing properties on nanoparticle size and the nature of the metal additive, it is important to use synthesis methods suitable for yielding well-defined nanoparticle sizes and composite configuration. Some of the current research directions include the use of synthesis methods for well-defined nanoparticle sizes, a reliable and scale electronic characterization of nanoparticles using conducting AFM and scanning tunneling microscopy on gas exposure, as well as the fabrication of nanowire–nanoparticle or decorated nanowire composites. Nowadays, the demand for low-cost light emitters is high, covering a wide range of different applications in the advertising and giveaway industry, low-cost indicators, and displays for consumer electronics, mobile phones, toys, and many more. In Chapter 5, Ekaterina Neshataeva et al. discuss light-emitting devices (LEDs) based on semiconductor nanoparticles. Versatile implementations of nanocrystals in LEDs are expected to combine the robustness and efficiency of conventional semiconductor LEDs with low-cost processing techniques used for large-area organic LEDs. This fascinating research field not only requires the development of innovative fabrication and processing techniques using nanoparticles but also opens a path toward novel applications and devices. In the chapter, an overview of various device concepts and technical approaches is given focusing on the devices, where nanoparticles are used as active materials. Both direct and alternating current-driven light emitters are also discussed, covering the time span from early to recent developments in the field. The formation of nanosize aluminum and its applications in condensed phase reaction has been reviewed by Jan A. Puszynski and Lori J. Groven in Chapter 6. They clearly indicate that the use of nanosize reactants in condensed phase exothermic reactions leads to a significant increase in the energy release rate. Such high-energy release rates, not commonly observed between oxidizer and fuel particles, make these nanoenergetic systems

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Inorganic Nanoparticles: Synthesis, Applications, and Perspectives

suitable candidates for environmentally benign macro- and microinitiators as well as energetic components of microthrusters and other applications requiring fast combustion front velocities. The recent developments in the formation of aluminum nanopowders indicate that high-temperature methods seem to be more suitable for scale-up than low-temperature wet chemistry synthesis routes. The mechanical reduction of aluminum particle size seems to be another promising approach for making larger quantities of reactive aluminum nanopowders. Fuel cells using hydrogen represent an important form of tomorrow’s energy due to it not only being an efficient fuel but also environmentally clean. The auto industry, which relies on oil-fuelled cars, is perhaps the biggest driving force behind the massive investment in fuel cell development. Jin Luo et al. have reviewed this interesting area of research in Chapter 7. In particular, they claim that the molecular encapsulation approach to the synthesis and processing of bimetallic/trimetallic nanoparticles is effective in producing alloy nanoparticles in the 2–5 nm regime with controllable composition and carbon-supported catalysts for fuel cell reactions. This approach differs from other traditional preparation approaches of supported catalysts in the abilities to control the nanoscale size, multimetallic composition, phase properties, and surface properties. As demonstrated by the bimetallic AuPt alloy nanoparticle catalysts, synergistic activity is possible in which Au atoms surrounding Pt provide effective sites for the reaction adsorbates in the electrocatalytic reaction. The fact that this bimetallic nanoparticle system displays a unique single-phase property different from the miscibility gap of its bulk-scale counterpart serves as an important indication of the operation of nanoscale phenomena in the catalysts, which can be further exploited for the design and preparation of the nanostructured bimetallic catalysts for fuel cells. Trimetallic nanoparticle catalysts have displayed enhanced electrocatalytic activity. For carbon-supported ternary PtVFe and PtNiFe nanoparticle catalysts, the size, composition, and loading of the nanoparticles on carbon support have been shown to be controllable, as well as processible by controlled thermal treatment and calcination, which can be optimized in order to achieve the effective shell removal and alloying of the ternary catalysts. The measurements of the intrinsic kinetic activities of the catalysts toward an oxygen reduction reaction have shown high electrocatalytic activities, and the trimetallic PtVFe nanoparticle catalysts prepared by the nanoengineered synthesis and processing methods have exhibited a much better performance in proton exchange membrane (PEM) fuel cell cathode than the commercial Pt catalyst. It also becomes clear that the synthesis and processing approach to the preparation of nanoparticle catalysts is promising for delivering much higher catalyst utilization than those of conventional methods, which has important implications on the improved design of fuel cell cathode catalysts. Solar cells, devices that convert the energy of sunlight directly into electricity, based on organic–inorganic hybrid blends are discussed by Elif Arici-Bogner in Chapter 8. He describes the current state of art in organic–inorganic hybrid solar cells that use nanocrystalline inorganic materials in two different functions: as anodes and inorganic dyes in dye-sensitized solar cells as well as in bulk heterojunction solar cells. The basic parameters of photovoltaic devices and their characterization, synthesis aspects of inorganic nanoparticles investigated as active materials in solar cells as well as the material characterization methods, and the new developments for integration of inorganic nanoparticles in photovoltaic devices are discussed in this chapter. The relationship between nanoparticles and rechargeable batteries is described in Chapter 9 by Doron Aurbach and Ortal Haik. This chapter deals with the possible use of nanomaterials in devices for energy storage and conversion, with an emphasis on inorganic species (e.g., alloys transition metal oxides and sulfides and carbon nanotubes). Four types

Inorganic Nanoparticles: Synthesis, Applications, and Perspectives—An Overview

11

of devices are discussed and classified: batteries (primary and secondary), fuel cells, super electric double-layer capacitors, and photovoltaic cells with the main focus on rechargeable batteries. For fuel cells, the main interest in nanomaterials relates to the catalysts. For low temperatures, hydrogen/oxygen, and alcohol/oxygen (direct) fuel cells, the catalysts are metallic particles comprising mostly platinum and its alloys. The authors mention dyesensitized photovoltaic cells in which the anode material is semiconducting titanium oxide where the required high surface area is reached through the use of nanoparticles. For super capacitors, whose energy storage mechanism is based on electrostatic interactions, nanostructured carbonaceous materials may provide the necessary high surface area and hence high capacity. For rechargeable batteries, the use of nanomaterials may enable a high capability rate, because of the short length for solid-state diffusion (which is usually the determining step rate for intercalation materials). However, nanomaterials may have high surface reactivity, which can be detrimental for Li ion battery systems, in which there is no thermodynamic stability between most of the relevant electrode materials and the nonaqueous polar electrolyte solutions. There are some cases in which the use of nanomaterials is crucial: LiMPO4 olivine cathode materials, silicon- and tin-based anode materials, as well as anodes based on conversion reactions (e.g., MO + Li = Li2O + M). Nano-alumina and silica may be a desirable component in polymeric electrolytes because of the existence of ionic conductance mechanisms based on the interactions between Li ions and surface oxygens of the nanoparticles. The various battery components are classified and discussed in connection with the possible use of nanomaterials. Nanobiotechnology, the combination of nanotechnology with biology, allows the use of nanotools and nanodevices to interact with, detect, and alter biological processes at a cellular and molecular level. A. Ragusa et al. in Chapter 10 describe the use of semiconductor quantum dots for biomedical applications. Semiconductor nanocrystals, also known as quantum dots (QDs), represent an emerging class of inorganic fluorescent markers. Due to their inorganic nature, they offer revolutionary fluorescence performance including narrow and symmetrical emission spectra for low interchannel overlap, broad adsorption spectra and extremely bright emitting colors for simple single-excitation multicolor analysis, long-term photostability for live-cell imaging, and dynamics studies. Since the first proof of the concept of the application of QDs as fluorescent probe on living cells in 1998, numerous groups have demonstrated the significant potential of such a tool in biology. In this chapter, the authors provide an overview of the exploitation of QDs in different biological applications ranging from biosensoring to labeling and imaging, both on in vitro models and in vivo animal studies. They also consider their use in photodynamic therapy and multimodal imaging techniques—fields of research that have only recently been created but are already attracting a lot of attention. An interesting strategy, with immense potentiality, that can be used to remotely control the delivery of a drug or gene is the use of magnetic nanoparticles manipulated by an external magnetic field. After a brief description of the physical principles underlying some current biomedical applications of nanoparticles (superparamagnetism, hyperthermia, and manipulation of magnetic nanoparticles inside blood vessel), Claudia Altavilla, in Chapter 11, reviews the most important wet chemistry strategies to design, synthesize, protect, and functionalize magnetic nanoparticles and/or multifunctional systems as drug delivery carrier. Some of the most explicative and significant recent studies on the application of these “smart” drug delivery systems in vivo and in vitro are finally reported. Thermotherapy, elevation of tissue temperature to above 40°C–41°C, has long been described and researched for cancer therapy. The addition of magnetic nanoparticles was introduced in the hope for a more focused and homogenous distribution in the cancer

12

Inorganic Nanoparticles: Synthesis, Applications, and Perspectives

tissue while sparing healthy tissue. The principle of nanoparticle thermotherapy, discussed by Joerg Lehmann and Brita Lehmann in Chapter 12, is the excitement of magnetic nanoparticles, which have been brought in close proximity to cancer cells through the use of an alternating magnetic field. Heat is produced through the transfer of the energy of the alternating magnetic field via magnetic hysteresis losses and Brownian relaxation losses. There is evidence that the technology is capable of providing a serious blow to cancer, possibly even complete remission. However, in relation to this point, only mice have been cured. The methods of nanoparticle delivery to cancer cells and creating the alternating magnetic field are reviewed in this chapter, as well as the properties of the nanoparticles. Reference is made to animal studies and initial clinical trials. This truly interdisciplinary field involving chemists, biologists, physicians, and physicists is very much under development. In Chapter 13, Wilson A. Lee and Miriam Raifailovich describe the use of inorganic particles against reactive oxygen species for sun protection products. The chemical grafting of antioxidant molecules and anionic polymer encapsulated in a hydrophobic polymer directly onto TiO2 particle surface is found to mitigate photocatalytic degradation, enabling highly effective filtering against UV radiation. The coating consists of a densely grafted polymer, an anionic polymer, and a free radical scavenger. The addition of the coated particles prevents scission and even possible hydrolysis of the DNA after exposure to UVA, UVB, and even UVC radiation. Metal oxide nanoparticles can be uniformly deposited onto the surface of different kinds of textiles by a sonochemical method in order to achieve antibacterial properties. The topic is discussed by Nina Perkas et al. in Chapter 14. The coating can be performed by a simple, efficient, one-step procedure using environmentally friendly reagents. The physical and chemical analyses demonstrated that nanocrystals of ∼20–30 nm in size are finely dispersed onto fabric surfaces without any significant damage to the structure of the yarn. The mechanism of nanooxide formation and adhesion to the textile is also discussed. It is based on the local melting of the substrate due to the high rate and temperature of the nanoparticles thrown at the solid surface by sonochemical microjets. The strong adhesion of the metal nanooxides to the substrate has been demonstrated in terms of the absence of the leaching of the nanoparticles into the washing solution. The performance of fabrics coated with a low content of nanooxides (99 >99 >99 >99 80 >99 99.5+ —

— — — — — — 20 — — 99+

85 25 20 23 5 12 25 5 20 80

Nanocatalysts: A New “Dimension” for Nanoparticles?

517

process, yielding nanosized particles that are more uniformly sized. When the water/ TPT molar ratio increases, hydrolysis and polymerization accelerate, and rapid hydrolysis could result in the formation of large inhomogeneous nonspherical particles. A two-step modification of sol–gel method has also been performed. TiO2 sol is prepared by a chemical coprecipitation–peptization method (Tseng et al. 2006). An aqueous NH3 solution is added to a TiCl4/DI water solution at 48°C to ensure complete hydrolysis, producing a white precipitate. Subsequently, a yellow transparent TiO2 sol is obtained after 2 h of peptization with hydrogen peroxide (10%) and 24 h of heating at 95°C. The resulting TiO2 sol contains arrowhead-like crystals less than 30 nm in size, with a degree of crystallization in anatase phase strongly dependent on the drying temperature. Another approach in the sol–gel method is based on microemulsions, in which an aqueous phase, a surfactant, and an oil phase are stably and isotropically dispersed in an oil phase. Dispersed water phase droplets (typical size 10–50 nm) are used for nanoconfined synthesis of particles. The formation of stable and nanosized TiO2 nanoparticles via hydrolysis of titanium isopropoxide in microemulsion has been also reported (Zhang and Gao 2002). A solution of TPT in isopropanol was added to water, Span-Tween80, and toluene microemulsion, yielding TiO2 precipitate. The water/surfactant ratio controlled the diameter of nanosized particles, allowing a narrow distribution of spherical nanoparticles to be obtained. High-temperature techniques to produce stable nanophotocatalysts have also been investigated. In the gas-phase decomposition, titanium alkoxide is evaporated at a chosen temperature, usually below 100°C, and the vapor is carried by a high purity inert gas (typically nitrogen), in the absence of water vapor and oxygen, to a reactor kept at 500°C–900°C to induce the organic precursor pyrolysis. Thimble filters are used to collect the particles produced. In the case of nanosized titania prepared via gas-phase decomposition, it is possible to control the crystallinity by changing the reaction temperature with a relatively low influence on the particle size (Jung et al. 2002). Despite the gas-phase decomposition producing nanosized particles, the method suffers from low productivity and is limited to only volatile precursors as raw materials. In order to overcome this limit, the development of aerosol or liquid-feed (FP) has been proposed (Chiarello et al. 2005, 2008). Ultrasound-assisted spray pyrolysis is well known as a technique to prepare ceramic powders of submicrometer size (Kang et al. 1996). In the spray pyrolysis method, which is similar in principle to gas-phase decomposition where a precursor is decomposed at high temperature, a nebulized solution containing the precursor, usually an aqueous and acidic phase where the alkoxide precursors are hydrolyzed, is fed to the decomposition section. The droplets generated by a nebulizer are air carried into the furnace at a temperature of 500°C–900°C and the formed particles are collected by a thimble filter (Figure 18.2). In this range of temperatures, submicronized particles are obtained. FP is a very high temperature, and hence effective, modification of the spray pyrolysis technique. A specifically designed burner (Chiarello 2005) allows the feeding of a mixture of oxygen and an organic solution of the titanium precursors through a nozzle (Figure 18.3), where the solvent acts as fuel for the flame. The mixture is ignited by a surrounding ring of O2 and CH4 or other fuel. The short residence time in the flame and the high temperature assure the decomposition; moreover, optimizing the main operating parameters such as liquid feeding rate, the flow rate of O2/CH4 mixture, the linear velocity of the dispersingoxidizing oxygen, the required crystal phase of the product, and its structural homogeneity

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Inorganic Nanoparticles: Synthesis, Applications, and Perspectives

Furnace

Heating mantle

Furnace

Carrier gas Air

Vent

Controller

Filter

Vent

(a)

Activated copper

Nebulizer

Silica gel

Zeolite A

Thermocouple

Filter

N2

Carrier gas

(b)

FIGURE 18.2 Sketched lab plants for the preparation of titania particles by (a) spray pyrolysis and (b) gas-phase decomposition of TPT. (Reprinted from Jung, K.Y. et al., Appl. Catal. A: Gen., 22, 229, 2002. With permission of Elsevier.) (+)

Exhaust gas E

C D (–)

B

A

PM

O2

Organic solution

To Flamelets

CH4 O2

FIGURE 18.3 Scheme of the FP apparatus. A, burner; B, Pyrex glass conveyor; C, collector; D, multipin effluviator; E, heating mantle. (Reprinted from Chiarello, G.L. et al., Appl. Catal. B: Environ., 84, 332, 2008. With permission of Elsevier.)

can be achieved. Nanometer-sized particles with high surface area (>100 m2/g) and high purity can be obtained with high productivity, phase purity, and improved thermal resistance. In the synthesis of titania nanoparticles, the phase transformation of metastable anatase to rutile can be retarded and reduced by achieving fast crystal growth with a short residence time at a high reaction temperature.

Nanocatalysts: A New “Dimension” for Nanoparticles?

519

Several organic solvents can be used such as xylene, propionic acid, pyridine, methanol, etc. It was found that the surface area of the FP-synthesized nanocatalysts linearly decrease with an increase in the combustion heat of the organic solvent/fuel, as a result of a higher flame temperature with a consequent increase in the rate of particle growth (see below). Thus, larger particles, possessing lower SSA are obtained. It is worthwhile underlining that both the surface area and the titania crystallite dimensions of FP materials depend on the geometry of the burner employed for the synthesis, which allows a more efficient dispersion of the liquid solution by a narrower nozzle. Other high heat sources have been used, as in the case of laser-pyrolysis with titanium alkoxide. It is worthwhile to note that this technique was developed as early as 1992. The titania nanoparticles obtained by CO2 IR laser pyrolysis at high power were quite monodimensional in size (around 50 nm) and very stable against aggregation (Ciambelli et al. 1992, Musci et al. 1992). Titania nanotubes are produced by a variety of techniques, including hydrothermal hydrolysis, template synthesis, and anodization. The latter method enables the production of well-ordered titania nanotube arrays. An example of hydrothermal synthesis of titania nanotubes is reported by Kasuga et al. (Kasuga et al. 1999). In a typical procedure, 2 g of rutile TiO2 powder in 85 mL of 10 M NaOH aqueous solution was into a Teflon-lined autoclave 130°C for 72 h. After filtration, water washing, and drying, in order to improve the crystallization of titanate nanotubes, calcination at 400°C for 1 h in air was performed, yielding TiO2 nanotubes with high crystalline structure. Template synthesis can shape the form and dimensions of a material through a matrix that constitutes the “negative” of the desired architecture. Template synthesis could be conducted as replicas of a porous membrane, typically with cylindrical pores of uniform diameter, and a nanocylinder or a nanofibril could be tailored in each pore in dependence of the properties of the material and the chemistry of the pore wall. On the other hand, a nanocylinder could be shaped around needle-like crystals, such as aragonite calcium carbonate. The tailoring was carried out (Qian et al. 2010) by using needle-like calcium carbonate and octadecylamine as double templates at room temperature in a nonaqueous system with tetrabutoxytitanium titania nanotubes with regular tubular morphology. Titania nanotubes, up to 15 μm long, with inner diameter of 400 nm, and a wall thickness of 40 nm, resulted in a high SSA (112 m2/g). Vertically oriented nanotube arrays were obtained by electrochemical anodization (Ghicov and Schmuki 2009). By adjusting the anodization parameters (temperature, potential rate, applied potential, electrolyte species, electrolyte pH, viscosity, aqueous or organic electrolyte, etc.), well-defined, self-organized, orthogonal titania nanotubular layers can be obtained. The morphological characteristics can be finely controlled, resulting in uniform titania nanotube arrays of various pore sizes (22–110 nm), length (200 nm–1000 μm), and wall thickness (7–34 nm). 18.4.4  Characterization of Nanoparticles for Photocatalysis Scanning electron microscopy (SEM) allows examination of nanoparticle topographies at very high magnifications (up to 300,000×). SEM inspection is often used for the analysis of pores, cracks, and fractures of surfaces as well as morphology of samples. Transmission electron microscopy (TEM) permits higher magnifications and spatial resolution than SEM, in the range of a few nanometers, and gives evidence also for the crystallographic structure, morphology, and of the composition of a nanoparticle.

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Inorganic Nanoparticles: Synthesis, Applications, and Perspectives

50 nm (a)

(b)

FIGURE 18.4 TEM images of (a) pseudospherical (P25) and (b) elongated titania nanocrystals by coprecipitation–peptization method. (Reprinted from Tseng, Y.-H. et al., Micro Nano Lett., online no. 20065035, doi:10.1049/mnl:20065035 ristretto, 2006. With permission of Elsevier.)

Two examples regarding titania nanoparticles are presented in Figure 18.4. Characterization in terms of SSA gives another significant contribution to evaluate and compare the nanocatalyst moieties. The measurement is typically based on the physical adsorption (or van der Waals adsorption) of suitable molecules (adsorbate), which can enrich the interfacial layer of a solid (adsorbent) upon exposure to an adsorbing solid to a gas or vapor, without chemical reaction occurring. Energy of interaction is low, less than 15 kJ/mol, and thus the adsorption is a reversible phenomenon. The adsorption extent (denoted n) on a solid surface can be described at constant temperature and within the limits of vacuum and the saturation vapor pressure at which condensation takes place, i.e.,



 p n = f   T , adsorbent, adsorbate  p0 

where p/p0 is the relative gas/vapor pressure. The amount of gas adsorbed when the mono-layer is saturated is proportional to the entire surface area of the sample. From the N2 equilibrium adsorption isotherm as function of N2 partial pressure, typically performed at the boiling point of pure liquid nitrogen (T = 77.2 K), the number of N2 molecules necessary to have a uniform monolayer coverage of solid surface adsorption can be evaluated; it is multiplied by the area projected for a single molecule (6.2 Å2 for N2), to get the SSA value. The low temperature is necessary to guarantee that no dissociation will occur or transformation of nitrogen, which can change the N2 projected area. The measurement has to be carried out on a pretreated sample at suitable temperature and under vacuum to remove the surface contaminants beforehand. Both static volumetric or dynamic apparatus can be useful for the measurement.

Nanocatalysts: A New “Dimension” for Nanoparticles?

521

According to IUPAC classification, six complete adsorption/desorption isotherm types take place in dependence of the different gas–solid interactions, and their trend represents a standardized way to classify the solid and to select the appropriate model to get the value of SSA and porosity characteristics. The Type I isotherm (Figure 18.5) is characteristic of microporous solids (pores below 2 nm). Type II and IV arise from non-porous solids representing unrestricted monolayer– multilayer adsorption (Figure 18.5). Point B, at the beginning of the almost linear rise of adsorbate amount, is often taken to individuate the completion of monolayer coverage and the beginning of multilayer adsorption. Type III and type V are typical of low interaction between adsorbent and adsorbate, for example, water vapor on hydrophobic materials, while type VI is a quite rare step-like isotherm. The hysteresis loop is a feature of Type VI and V isotherms, generated by the capillary condensation of the adsorbate within the mesopores (pores in the ranges 2–50 nm) of the solid. In particular, for type II and IV, the Brunnamer, Emmett, and Teller (BET) model is appropriate to evaluate the SSA, while Dubinin or Langmuir models are used for the evaluation of micropore volume from types I and III isotherms. The micro- and mesopore volume and size distributions can be obtained by the desorption branches of isotherm by different methods such as Dollimore-Heal (DH), Barrett Joiner Halenda (BJH), S&F or by adsorption branch with Horwath and Kawazoe (H&K) theory. For a deeper description, see the book by Gregg and Sing (1982). The pores of the solid could be formed by the aggregation of primary particles (interparticle porosity) or be present in the primary particles (intraparticle porosity). In the absence Type I

n, molecules N2/g

Type II

Type VI

Point B

p/p0 FIGURE 18.5 Type I, II, and IV standard adsorption isotherms according to IUPAC classification.

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Inorganic Nanoparticles: Synthesis, Applications, and Perspectives

of intraparticle porosity, simple geometrical models could be employed to evaluate the particle size. For dense particles, the surface-to-volume ratio, Sp/Vp, divided by the particle density is the specific surface area, SSA: SSA =

Sp ≡ BET surface area Vp × ρp

For a sphere, the particle diameter is Dp =

6 SSA × ρp

where Vp is the particle volume Sp is the surface area of a single particle ρp is the particle density Dp is the particle size Since typically there is a particle size distribution, this simple calculation yields the average particle size. Specific techniques such as laser diffraction are used to evaluate size distribution, apart from direct electron microscopy observation. This calculated size is in good agreement with that measured by SEM for pseudospherical titania particles (Jung et al. 2002), and the comparison permits to confirm of the absence of intraparticle pores. When the latter are present, they contribute to the total specific surface area, giving higher BET values. For the titania particles prepared by the spray pyrolysis of TEOT and gas-phase decomposition of TPT (Jung et al. 2002), the comparison indicated that dense particles were obtained both in submicrometer and nanometer sizes. At similar crystallite sizes, evaluated by XRD (see Figure 18.8), the surface area of titania particles increased by reducing the particle size in the nanometer range. SSA of FP-synthesized samples vs. combustion enthalpy of the solvent/fuel is shown in Figure 18.6. It was observed that both the surface area and the titania crystallite dimensions of FP materials depend on the geometry of the burner. Indeed, FP titania nanopowders prepared with a different burner could possess a higher surface area (106 m2/g) as a consequence of the more efficient dispersion of the liquid solution by a narrower nozzle. This would suppress the formation of the bigger particles, with the consequent increase of surface area. As it is well known, the identification of the crystalline phase and the relevant size is obtained by the XRD patterns. The crystallite size (Figure 18.7) is evaluated by the Scherrer formula



t=

K×λ B × cos θB

where t is the thickness of crystallite in the direction individuated by Miller indices K is the constant dependent on crystallite shape (0.89 for Cu Kα)

523

Nanocatalysts: A New “Dimension” for Nanoparticles?

70

SSA/m2/g

65 60 55 50

15

20

25 –∆Hc

30

/ kJ/cm3

35

40

Intensity, counts

FIGURE 18.6 SSA of titania particles prepared by spray pyrolysis at several temperatures vs. combustion enthalpy of the fuel. (Reprinted from Chiarello, G.L. et al., Appl. Catal. B: Environ., 84, 332, 2008. With permission of Elsevier.)

Imax B ½ Imax

2θ°

2θmax

2θe

2θ, °

FIGURE 18.7 XRD peak broadening as function of crystallite dimension and relevant terms of Scherrer equation.

λ is the x-ray wavelength B is the FWHM (full width at half max) or integral breadth θB is the Bragg angle The Scherrer formula can be applied when the crystallite size is