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Sustainable Preparation of Metal Nanoparticles

Methods and Applications

The Royal Society of Chemistry

Introduction

RAFAEL LUQUE AND RAJENDER S. VARMA

In recent years, we have experienced a "nano" revolution in which science was directly impacted with nanotechnologies forming the basis of the so-called nanoscience that are just starting nowadays to be realized as a major step forward towards future technological progress. The possibility of manipulating matter at such an ultrasmall scale (i.e. within the nanometer range) has paved the way to the development of numerous nanoentities and nanosystems which currrently start to be part of our daily lives and consumer products in optics, electronic devices, sensors and even in the textile industry. The ability to directly work and control systems at the same scale as nature (e.g. DNA, cells) can potentially provide a very efficient approach to the production of chemicals, energy and materials (Figure 1.1).

Another important asset of nanomaterials is its inherent multidisciplinarity with a wide range of possibilities in terms of synthesis and applications that these nanoentities hold. Several subfields have been investigating nanoscale effects, properties, and applications from its infancy; every different subdiscipline is involved in modern nanoscience and technology. Inputs from physicists, biologists, chemists and engineers have been a hallmark from the very early developments including the advances in nanoscience to achieve a better understanding of the preparation, application and impact of these new nanotechnologies.

A nanoparticle can be generally defined as a particle that has a structure in which at least one of its phases has one or more dimensions in the nanometer size range (1 to 100 nm, Figure 1.1). Nanoparticles (NPs) have remarkably different properties as compared to their bulk equivalents that mainly include a degenerated density of energy states (as compared to bulk metals) and a large surface to volume ratio together with the sizes in the nanometer scale. These nanoparticles have associated remarkable properties including a relatively high chemical activity and specificity of interaction as compared to bulk metals (e.g. Au). With all the aforementioned advantages and outstanding features of NPs, it is not surprising that the interest in NPs has experienced a staggering exponential increase over past years, with over 10 000 publications referring to NPs in 2010. The amplitude of research efforts is expected to continue increasing as beneficial application of the chemical properties achieved at the nanolevel become increasingly apparent.

One of the key driving forces for the rapidly developing field of nanoparticle synthesis is the contrasting physicochemical properties of nanoparticles compared to their bulk counterparts. Nanoparticles typically provide highly active centers but they are very small and not thermodynamically stable. Structures in this size regime are generally unstable due to their high surface energies and large surfaces. To achieve stable NPs, the particle growth reaction has to be carefully controlled and minimized. This has been rendered feasible by a number of methods including the addition of organic ligands, inorganic capping materials or metal salts, colloids or soluble polymers creating core shell type particle morphologies. These materials can be grouped in the so-called "unsupported" MNPs.

In parallel, a significant volume of research has been devoted to protocols to achieve homogeneous size dispersed nanoparticles on different supports including porous materials. These nanoentities can consequently be grouped in the so-called "supported" nanoparticles (SNPs).

Recent advances in the design and preparation of nanomaterials have shown that a wide variety of them can be synthesized through different preparation routes and tailored to a desired size and distribution, overcoming the limitations of traditional synthetic methodologies.

In conjuction with the nanorevolution, environmental issues, growing demand for energy, political concerns and medium-term depletion of petroleum-derived products have created the need to develop sustainable technologies and low environmental impact processes not only for the production of chemicals, fuels and materials but also for the generation of nanomaterials, nanoparticles and related nanoentities. The state-of-the-art preparation techniques of many NPs attempt to follow more efficient and sustainable routes, taking special considerations to the safety and toxicity of the prepared nanoparticles. These routes include the use of alternative energy input methodologies, such as ultrasound-, microwave irradiation, and ball milling, the use of natural products and biomass (e.g. vitamins, fruits, agricultural residues, etc.) for NP preparation, and the controllable deposition and stabilization of NP using a related technology, that of nanoporous materials.

This monograph is intended to be a contribution towards the aforementioned selected methodologies for the environmentally friendly preparation of nanoparticles and their applications in various fields including energy storage, environmental remediation, biomedical applications, production of fine chemicals, and biofuels from biomass, with two additional contributions on the toxicology of designer nanoparticles and an introduction to nanosafety. Due to the rapidly expanding nature of this field, this book is hoped to provide a useful introduction to readers to this exciting research area.

Subsequent to this introductory chapter, the first part of the book commences with a chapter by Varma et al. that includes a description of sustainable, novel and innovative methodologies for the development of biosynthetic methods for NP preparation including the use of fungi, bacteria, algae, plants, carbohydrates and vitamins. A range of nanoparticles with different nanoparticles sizes and shapes can be achieved using these interesting methods. Chapter 3 by Özkar et al. then continues along the lines of sustainable ways to synthesize nanoparticles stabilized in the framework of porous materials (supported metal nanoparticles, SMNPs). This chapter reviews protocols and preparation routes of SMNPs, including physico-chemical methods, the aforementioned alternative methodologies, and detailed case studies on the utilization of various supports such as zeolites, clays, porous silica's, carbonaceous materials, MOFs and some others. This chapter also delineates some interesting catalytic applications of these materials in an array of catalytic processes including coupling and redox chemistries.

After these introductory chapters pertaining to the nanoparticle preparation and associated applications in catalysis, the second part of the book focuses on applications of nanoparticles in various research areas. Chapter 4 from Wang et al. deals with an interesting topic of energy conversion and storage through nano-particles where the authors discuss the possibilities of quantum confinements in nanoparticles, preparation of quantum dots and applications in solar cells and lithium ion batteries. Following this chapter, Dionysiou et al. disclose the greener preparation of an assortment of nanomaterials including metal and metal oxide NPs using various methodologies for their utilization in photocatalytic applications for environmental remediation in Chapter five. The chapter includes interesting sections on the immobilization of nanoparticles and the subsequent applications to sustainable environmental systems.

Chapter 6 from Katti et al. deals with the uses of nanoparticles (particularly gold NPs synthesized from natural sources) for biomedical applications and treatment of tumors.

The last chapter of the applications section by Obare et al. comprises an overview of selected nanomaterials and nanosystems for the production of high-value added chemicals and biofuels from biomass valorization practises. This encompasses some synthetic protocols for the preparation of metallic and biometallic nanoparticles all the way to various applications in chemical processes including conversion of sugars, production of hydrocarbons, synthesis of biodiesel and the design of fuel cells, with some future perspectives in the field.

The final part of the book consists of two chapters devoted to the toxicology of designer/engineered nanoparticles by Ming et al. (Chapter 8) and a brief introduction to nanosafety in the lab (Chapter 9) by Balas. The later provides a novel and unique approach to issues associated to the use of nanoparticles, often missing in most nanoparticle-related books to date. Chapter 8 contains some critical information of biophysicochemical interactions at the nano/bio interface, with some important aspects on nanotoxicity. Chapter 9 wraps up the book with some fresh concepts on nanosafety, a relatively novel concept and approach. This Chapter aims to provide some discussion on the introductory issues of risks in handling nanoparticles and strategies for risk reduction together with some general guidelines on safety and prevention in a nanotechnology laboratory from control banding to techniques related to the assessment of nanoparticle emissions.

With the 21st century heralding the dawn of a new age in materials science (where scientists no longer observe the behavior of matter but with the advent of nanoparticles, materials and technology but is able to predict and manipulate matter for specific applications, with sensitivity and efficiency far surpassing previous systems), we hope this book can provide a starting point to readers in the fascinating nanoworld as well as some useful points in terms of nanosafety and nanotoxicity/environmental impact associated with nanoparticles.

Acknowledgments

The authors are grateful to Departamento de Química Orgánica, Universidad de Córdoba and the Environmental Protection Agency (EPA) in Cincinnati, respectively, for their support during the assembly and organization as well as preparation of this monograph. Rafael Luque would also like to thank Ministerio de Ciencia e Innovación, Gobierno de España, for the provision of a Ramon y Cajal (RyC) contract (ref. RYC-2009-04199) and funding under projects P10-FQM-6711 (Consejeria de Ciencia e Innovacion, Junta de Andalucia) and CTQ2011 28954-C02-02 (MICINN) as well as project IAC-2010-II granted to Rafael Luque as a "Estancia de Excelencia" at the EPA in Cincinnati from July to September 2011.

Environmentally Friendly Preparation of Metal Nanoparticles

JURATE VIRKUTYTE AND RAJENDER S. VARMA

2.1 Introduction

Commercial and research interest in nanotechnology significantly increased in the past several years translating into more than US$9 billion in investment from public and private sources.

Nanotechnology is the ability to measure, see, manipulate and manufacture things on an atomic or molecular scale, usually between one and 100 nanometers. These tiny products also have a large surface area to volume ratio, which are the most important characteristics responsible for the widespread use of nanomaterials in mechanics, optics, electronics, biotechnology, microbiology, environmental remediation, medicine, numerous engineering fields and material science.

Unfortunately, high surface area often results in various drawbacks that are closely associated with the surface phenomena, e.g. outer layer atoms in the particle may have a different composition and therefore, chemistry from the rest of the particle. Furthermore, nanoparticle surface will be prone to environmental changes such as redox conditions, pH, ionic strength, microorganisms, etc. Also, small size and large surface to volume ratio may lead to both chemical and physical differences in their properties including mechanical, biological and sterical, catalytic activity, thermal and electrical conductivity, optical absorption and melting point, compared to the bulk of the same chemical composition.

Generally, the intended nanoparticle application defines its composition, e.g. if a nanoparticle is going to be used to interact with biological systems, functional groups will be attached to its surface to prevent aggregation and/or agglomeration. Also, coatings and other surface active materials could be introduced that form transient van der Waals interactions with the surface of nanoparticles and exist in equilibrium with the free surfactant molecule. Furthermore, if the use is intended for the electronics industry, nanoparticles can be manufactured in a way that significantly enhances the strength and hardness of materials, exhibits enhanced electrical properties by controlling the arrangements within the nanoclusters, etc. Also, if the use is intended for environmental remediation and catalysis, increased activity could be achieved by attaching various functional side groups, doping with ions and anions and variation in size and structure. And finally, nanoparticles with nonconventional properties including superconductivity and magnetism can also be manufactured utilizing appropriate mechanisms and surface functionalization approaches.

According to Christian et al., nanoparticle consists of three layers: i) the surface that can be functionalized, ii) a shell that may be added according to the application needs and iii) the core that can be synthesized using various methods, reaction conditions and precursors. Thus, the surface of a nanoparticle can be functionalized with various metals and metal oxides, small molecules, surfactants and/or polymers. In addition, target nanoparticle surface can be charged (e.g. base-catalyzed hydrolysis of tetraethyl orthosilicate, SiO- M+) or uncharged (citrate, sodium dodecylsulfate (SDS), polyethylene glycol (PEG), etc.), which highly depends on the application and the subsequent use of nanomaterials. In most cases, the shell is made of inorganic material that has a completely different structure than a core, e.g. iron oxide on iron nanoparticles, quantum dots (zinc sulfide on cadmium selenide) and polystyrene–polyaniline nanoparticles. Importantly, the core is usually referred to as the nanoparticle itself and the physicochemical properties of nano-particles are nearly always governed by the properties of the core. However, the environmental fate and transport most likely will be dominated by the core and shell properties rather than core alone. Also, risks associated with the occurrence of nanoparticles in the environment must be related to the surface, core and the shell.

Currently, there are two main methods to synthesize nanomaterials: "top down" and "bottom up" approaches (Figure 2.1). Briefly, the "top-down" approach suggests nanoparticle preparation by lithographic techniques, etching, grinding in a ball mill, sputtering, etc. However, the most acceptable and effective approach for nanoparticle preparation is the "bottom up" approach, where a nanoparticle is "grown" from simpler molecules – reaction precursors. In this way, it is possible to control the size and shape of the nanoparticle depending on the subsequent application through variation in precursor concentrations, reaction conditions (temperature, pH, etc.), functionalizing the nanoparticle surface, using templates, etc.

Altering of the surface properties, or, in other words, functionalization of the surface, is one of the most important aspects of nanoparticle synthesis for the desired applications. For instance, the high chemical activity of nanoparticles with a large surface is usually the main reason for undesirable and most often irreversible processes such as aggregation. Aggregation significantly diminishes particle reactivity through the reduced specific surface area and the interfacial free energy. Thus, in order to avoid aggregation, nanoparticles have to be coated or functionalized with chemicals and or materials to increase their stability during storage, transportation, application and overall life cycle.

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