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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
The major topical and societal issues of energy transition and environmental conservation have benefited from the contribution of nanotechnologies and nanomaterials. Nanomaterials, including carbon-based newcomers, have helped to improve in particular the performance of energy storage and conversion devices. Some of these nanomaterials, including fullerenes, carbon nanotubes, nanodiamonds and carbon dots, were discovered well before the 2000s. Others are more recent, including graphene (the leading material of the 21st century) as well as many mineral materials developed at the nano scale: atomic clusters, metal or semiconductor nanoparticles, two-dimensional inorganic materials, metal-organic frameworks (MOF) and luminescent quantum dots. All of these are involved in the realization of devices for energy purposes. Nanotechnology and Nanomaterials for Energy provides a critical analysis of the latest work in the fields of batteries, photovoltaics, fuel cells and catalysis as well as lighting, with the advent of light-emitting diodes.
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Table of Contents
Cover
Title Page
Copyright
Introduction
I.1. History of nanotechnology
I.2. Outline of this book
I.3. Dedication
PART 1 Nanomaterials and Nanotechnologies
1 Carbon-based Nanomaterials
1.1. Fullerenes
1.2. Carbon nanodiamonds
1.3. Carbon dots or carbon quantum dots
1.4. Carbon nanotubes
1.5. Graphene
1.6. Graphene quantum dots
1.7. Conclusions and perspectives of carbon-based nanomaterials
2 Inorganic Nanomaterials
2.1. Metallic nanoparticles
2.2. Metal nanoclusters
2.3. Semiconductor quantum dots
2.4. Two-dimensional inorganic lamellar nanosheets
2.5. Hybrid metal-organic frameworks
2.6. Conclusions on inorganic nanomaterials
PART 2 Nanotechnology and Nanomaterials for Energy
3 Energy Storage
3.1. Worldwide energy use
3.2. Energy storage systems
3.3. Conclusions on energy storage
4 Energy Conversion
4.1. Photovoltaics
4.2. Electroluminescence, lighting and display
4.3. Conclusions on energy conversion
5 Electro- and Photocatalysis
5.1. Water splitting
5.2. Electrolysis techniques
5.3. HER and OER processes in water splitting
5.4. Photoelectrochemical water splitting
5.5. Fuel cells
Conclusion
References
Index
End User License Agreement
Guide
Cover
Table of Contents
Title Page
Copyright
Introduction
Begin Reading
Conclusion
References
Index
End User License Agreement
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Series EditorPierre-Noël Favennec
Nanotechnology and Nanomaterials for Energy
Pierre Camille Lacaze
Jean-Christophe Lacroix
First published 2021 in Great Britain and the United States by ISTE Ltd and John Wiley & Sons, Inc.
Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms and licenses issued by the CLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at the undermentioned address:
ISTE Ltd27-37 St George’s RoadLondon SW19 4EUUK
www.iste.co.uk
John Wiley & Sons, Inc.111 River StreetHoboken, NJ 07030USA
www.wiley.com
© ISTE Ltd 2021The rights of Pierre Camille Lacaze and Jean-Christophe Lacroix to be identified as the authors of this work have been asserted by them in accordance with the Copyright, Designs and Patents Act 1988.
Library of Congress Control Number: 2021941647
British Library Cataloguing-in-Publication DataA CIP record for this book is available from the British LibraryISBN 978-1-78630-497-1
Introduction
The term “nanomaterials”, by general consensus, is used to designate both nano-objects and nanostructured materials. To qualify as a nanomaterial, the dimensions of an object or of a structural element of a macroscopic material must be less than about one hundred nanometers. In addition to this first, generic definition, a distinction is made between zero-dimensional nanoparticles (0D-NP); nanowires (NW) and nanotubes (NT), which are one-dimensional (1D-NP); nanosheets and multi-sheets, which are two-dimensional (2D-NP); and, finally, three-dimensional (3D) nanostructured materials. Further distinctions are made between these forms based on their material composition: carbonaceous, inorganic or hybrid.
I.1. History of nanotechnology
Nanotechnologies first came into public view in the year 2000. This was the year the National Nanotechnology Initiative1, or NNI, was launched in the United States, propelled by the NSF (National Science Foundation), a large number of academic societies and multiple federal departments. The new and innovative properties of nanomaterials discovered over the course of the 1980s and 1990s opened a vast range of perspectives in terms of applications, attracting the attention of researchers and industrialists, who began to devote considerable time and attention to exploiting these new opportunities. These initiatives attracted almost unprecedented support from public authorities, and President Clinton, visiting Caltech in January 2000, specifically designated this initiative as a national developmental priority. Different fields of application were defined in collaboration with several government agencies, who also provided funding; this new area of research was seen as a likely source of significant scientific progress with the potential to benefit the whole of society.
Why, then, did nanotechnology attract such levels of attention from the US government in the early 2000s? Without going into too much detail, it is important to note that the 1980s and 1990s were marked by a number of important scientific discoveries relating to extremely small elements; this went hand-in-hand with technological developments which greatly increased the capacity to observe the state and performances of materials.
Transmission electron microscopy (TEM) was first commercialized by Siemens and Ernst Ruska2 and continued to develop over the following decades. By the 1990s, TEM performance had reached a level where it was possible to observe objects on a nanometric scale [RUS 86]. Scanning electron microscopy (SEM) developed in parallel, although commercial applications appeared at a later stage; this technique is based on the analysis of reflected secondary electrons. The first apparatus was launched onto the market by Cambridge Scientific Instruments in the 1960s. SEM makes it possible to observe the texture of material surfaces at nanometric level, constituting a breakthrough in terms of observing nano-objects in 3D.
The decisive breakthrough into the “nanoworld”, however, was not made until the mid-1980s. Rohrer and Binnig’s invention of near-field microscopy [ROH 86], including scanning tunneling microscopy (STM) and atomic force microscopy (AFM), marked a further step in the observation of sub-nanometric objects: the long-pursued goal of atomic-level observation had finally been attained. These new microscopy techniques, which now play a central role in the study of nanomaterials, were increasingly used to produce high-resolution images. Compared to classic TEM and SEM techniques, they also present a practical advantage in that they do not need to be used in a vacuum, and can indeed be used in “everyday” atmospheric conditions.
These technological innovations in the field of observation went hand-in-hand with the discovery of new materials, on a nanometric scale, which displayed entirely new physical and chemical properties due to their very smallness. Thus, in 1985, Kroto et al. (Nobel Prize winners in 1996) discovered fullerenes: new carbon nanomaterials with exceptional properties [KRO 85]. The first fullerenes were obtained in the form of traces; they are now produced commercially on a large scale. Fullerenes consist of a spherical assembly of carbon atoms, assembled in hexagon and pentagon shapes. Often considered as “electron wells”, they have particularly interesting chemical properties and are widely used in various domains, notably in plastic photovoltaic cells; their application in this field in the 1990s resulted in significant gains in energy yield [YU 95].
A few years later, in Japan, Lijima [LIJ 91] discovered the tubular structure of carbon nanotubes (CNTs), then of single wall carbon nanotubes (SWCNTs) [BET 93], which also consist of a hexagonal assembly of carbon atoms. These nanotubes vary from one to several nanometers in diameter, and have a length of several hundred nanometers. As we shall see later, the mechanical properties of these nanotubes largely surpass those of steel; furthermore, they also possess exceptional electronic properties.
These discoveries rapidly attracted the attention of scientific agencies in the United States. An ambitious program of fundamental and applied research into nanotechnologies was launched in the year 2000, with both scientific and industrial support. A series of directives and recommendations issued by the NNI attracted attention on the world stage, and a number of other nations launched their own initiatives, highlighting the strategic importance of the domain.
The scale of technological research increased considerably from the early 2000s on, and the number of publications in connection with nanomaterials or nanotechnologies continues to grow. While the creation and use of nanometric elements in the field of electronics are well established, other fields, including chemistry, the energy sector, biology and medicine, stand to make considerable gains from the progression of nanotechnologies. New fields of investigation are opened up on a regular basis, and their results look highly likely to revolutionize both theoretical knowledge and practical applications in these domains.
I.2. Outline of this book
Our main aim in this book is to highlight new breakthroughs and areas of research in nanotechnologies which have appeared in recent years, notably since 2010; however, we shall begin by presenting a brief overview of earlier discoveries, essential to understanding later work. Many of the new findings presented here have yet to be used commercially, but their interest in terms of research and potential future applications is immense.
We have chosen to focus on problems and solutions relating to the energy sector. Our discussion is split into two parts: first, a description of nanomaterials and their properties, and second, a discussion of the ways these materials are, or may be, used in the energy sector for storage and conversion, electrocatalysis and photocatalysis.
Chapter 1 is devoted to the subject of carbon nanomaterials, and includes a description of recent preparation methods, properties and major applications. Some of these materials (fullerenes, carbon nanotubes, nanodiamonds, etc.) largely predate the year 2000. However, they are yet to reveal all of their secrets, notably in terms of the remarkable properties they possess and in new contexts of application. Twodimensional families of nanomaterials, derived from graphene, are a far more recent discovery; graphene itself looks set to become the star material of the 21st century. More recently still, the family of 2D carbon composites has expanded to include materials such as graphdiyne, with a huge range of potential applications.
Chapter 2 concerns the family of inorganic nanomaterials. This field, too, has undergone considerable development in recent years, and new properties have emerged with the discovery of materials at an increasingly small level. Atom clusters, which contain from ten to a few hundred atoms and are smaller than 1.5 nm in size, possess physico-chemical properties which set them apart from larger NPs; this is a result of their spatial confinement. Quantum dots (QD), which are generally made up of binary alloys of semiconductors (SC), constitute another important family of nanomaterials and are particularly notable for their luminescent properties. The latest set of 2D materials include dichalcogenides of transition metals, along with a significant number of inorganic lamellar materials; the interest of these materials is already apparent, particularly in terms of their applications in the energy sector.
The second part of this book constitutes a critical study of new problems in the domain of energy, notably in terms of energy transition and preserving the environment. The fight to reduce greenhouse gas emissions raises two major problems: those of storing and converting energy. Nanotechnologies and nanomaterials stand to make a decisive contribution to solving these issues.
Chapter 3 is devoted to the key question of energy storage, a major challenge linked to the development of renewable energies. This field encompasses large-capacity static storage, the production of batteries and supercapacitors for electric vehicles, and also microbatteries, designed to provide power for small portable electronic devices. All of these different systems are subject to different constraints, leading us to focus on different design choices.
Chapter 4 concerns energy conversion, notably in terms of photovoltaic sources and lighting. Solar power is nothing new, but huge progress has been made in recent years, notably with the development of organic photovoltaics and that of perovskites; these solutions provide a genuine alternative to inorganic silicon-based systems, and, importantly, are more cost-effective. Lighting and display devices constitute another important subject in the energy field, notably in connection with the general phenomenon of electroluminescence. The overarching goal is to reduce energy consumption while providing a better quality of lighting than that obtained using incandescent bulbs or neon tubes. Considerable advances have been made over the last few years; as before, parallel developments in inorganic and organic nanomaterials have found practical applications in this area. The development of new forms of memory electronics (not covered in this book) is another important area of research, where once again the reduction of energy consumption is a major goal.
Chapter 5 concerns electrocatalysis and photocatalysis. Electrocatalysis relates primarily to the development of electrolysis cells (water splitting) and fuel cells, a main focus for future advances in converting chemical energy into electrical energy. Hydrogen is a particularly promising source of fuel: it is non-polluting, and constitutes a highly efficient means of transporting and storing energy. One especially promising area of research relates to the production of hydrogen by electrolyzing water, powered by a photovoltaic cell. This development is connected to the broader aim of developing systems which use non-precious metals to catalyze oxygen evolution reactions (OER) and oxygen reduction reactions (ORR). This area of research is essential for the optimization of water splitting and H2/O2 fuel cells. Extensive research is also underway on the subject of photocatalysis: the aim in this case is to optimize the use of solar energy for carrying out chemical transformations. Photocatalysis can also be combined with electrolysis reactions (photo-electrocatalysis) in order to transform ordinary compounds into high-added-value compounds, or into energy products which may be used either for storage or as a fuel source.
I.3. Dedication
We dedicate this book in memory of Jacques-Emile Dubois, eminent member of the Science Faculty of the University of Paris and founder of the ITODYS laboratory, who would have celebrated his 100th birthday this year, were he still among us. Professor Dubois was a pioneer and outstanding practitioner of the multidisciplinary approach to science in the realm of Physical Organic Chemistry. He blazed a trail in his revolutionary application of Computer Science to Chemistry, as well as in launching the study of phenomena at the solid-liquid interface. His drive and enthusiasm for uncovering and delving into virgin areas of research were and remain a source of inspiration to many.
1
The term nanotechnology was first used in the 1970s by the Japanese scientist Noria Tanegouchi to denote the body of research concerning materials with dimensions in the tens of nanometers at most.
2
Ernst Ruska was the first to suggest using electron beams to observe microscopic objects via a transmission mechanism. Ruska produced the first commercial electron microscope, in collaboration with Siemens, in 1933. He went on to devote his whole scientific career to improve this technique, which earned him a Nobel Prize in 1986.
PART 1Nanomaterials and Nanotechnologies
1Carbon-based Nanomaterials
Fullerenes, carbon nanotubes (CNTs), nanodiamonds (NDs) and carbon quantum dots (CQDs) are nothing new, and were discovered long before the year 2000; these discoveries were followed, in 2005, by that of graphene. Due to their remarkable physico-chemical properties, fullerenes, CNTs and graphene have given rise to innumerable applications, and their use has progressed continually; nevertheless, the use of graphene has increased much faster than that of fullerenes and CNTs (Figure 1.1).
Figure 1.1.Number of publications on the subject of fullerenes, CNTs and graphene from 2005 to 2014. Based on [LI 15]
The number of publications concerning these three families of compounds between 2005 and 2014 clearly shows an “explosion” in the number of articles on the subject of graphene; over the course of 7 years, from 2007 to 2014, this went from around 50 to over 16000, exceeding scientific production on the subject of CNT, discovered a whole decade earlier than graphene.
Moreover, the implication of Asian countries (notably China, South Korea and Japan) in this research is notable, representing more than 50% of total publications between 2005 and 2014 (China alone accounts for 36% of production); the United States accounts for just over 22%, and the rest of the world for 27%.
1.1. Fullerenes
Since Kroto et al.’s [KRO 85] discovery of buckminsterfullerene C60 (henceforth C60 fullerene) in 1985, where they demonstrated that it had a balloon-like structure made up of an assembly of sp2 carbon atoms arranged in the form of pentagons (12) and hexagons (20), a whole area of chemistry has developed in connection with fullerenes; a whole host of applications have also emerged in biology, medicine, electronics and energy.
Following on from C60 and C70, the two most stable and widespread forms (Figure 1.2), new fullerenes with higher carbon numbers have subsequently been synthesized.
Figure 1.2.Fullerenes: C60 (a) and C70 (b)
COMMENT ON FIGURE 1.2.– a) The C60 fullerene consists of an assembly of 10 pentagonal and 20 hexagonal carbon rings. The red and white symbols represent the C atoms in the foreground and background of the projection plane. b) The structure of the C70 fullerene is equivalent to that of two C60 hemispheres joined by 10 C atoms (six shown in dark blue, four in light blue, two of which cannot be seen) at the midplane of the molecule. Adapted from [BAL 98a].
The stability of these fullerenes results from the arrangement of hexagons and pentagons. The IPR (Isolated Pentagon Rule) notably states that a fullerene, with a given number n of carbon atoms, will be stable if each pentagon is surrounded by hexagons and has no contact with another pentagon. In the case of the C60 fullerene, there are 1812 geometric isomers of which only one (with symmetry Ih) verifies the IPR. The energy difference between the most stable (noted 1) and the least stable (noted 1812) isomers in the series is of the order of 540 kcal/mol [SUR 17]. This energy largely results from the tension exerted by the curvature of the bonds.
1.1.1. Properties of fullerenes
Given their cage-like carbon structure, featuring only covalent bonds between sp2 carbons, fullerenes are naturally hydrophobic and therefore insoluble in aqueous media. In order to use fullerenes for biological purposes, transformations must be carried out to make them more hydrophilic; this is achieved by grafting hydroxylated groups (COOH or OH), corresponding to exo-functionalization reactions, notably onto C60 and a number of other fullerenes (C70, C80, etc.) which are produced on a commercial scale. However, these latter fullerenes are expensive due to the difficulties inherent in large-scale production.
The presence of a cavity suggests that it might be possible to encapsulate chemical species. This is of interest with regard to the design of various markers, such as those used in medical imaging. The encapsulation of metals would result in a whole new category of nanomaterials, corresponding to the formation of endometallofullerenes (EMF), whose stability is a function of the fullerene and the small number of metal atoms encapsulated within the cage. The electrophilic character of fullerenes and their strong electronic affinity also mean that they are strong electron receivers, and are sometimes considered as “electron sinks”.
1.1.1.1. Electrophilic and antioxidant properties of fullerenes
The electrophilic character of fullerenes was established as early as the 1990s. This discovery suggested the possibility of forming anions with six negative charges, corresponding to the acceptance of six electrons by the LUMOs (lowest unoccupied molecular orbitals) of the fullerene. This hypothesis was confirmed by the existence of potassium salts (K6C60) in crystalline form.
More evidently still, the progressive six-step electrochemical reduction of C60 and C70 demonstrates the possibility of obtaining six stable reduction states of fullerene [XIE 92] (Figure 1.3).
Figure 1.3.Voltametric curves showing redox of C60 in a low temperature acetonitrile/toluene solution (−10°C)
COMMENT ON FIGURE 1.3.– The reduction curve obtained by a potential sweep from 0 to –3.5 V (blue arrow) shows six distinct reduction peaks, each corresponding to the gain of an electron. The reverse oxidation curve (red arrow) is clearly defined and also shows six oxidation peaks, corresponding to the six successive anionic states; this proves the stability and reversibility of these six redox states. Sweep speed: 100 mV/s. Potentials given against the ferrocene/ferricenium+ (Fc/Fc+) redox couple. Adapted from [XIE 92].
The electronic affinity of fullerenes means that they can be used as electron carriers in photovoltaic cells, achieving energy yields of the order of 10% (see section 4.1). Fullerenes also have numerous applications in the biomedical field. C60 and its derivatives, in combination with various hydrophilic groups with relevant biological properties, have been used in cancer and AIDS therapies (anti-retroviral therapy against HIV-1). Functionalized by carbohydrate chains, fullerenes have also proven to be good antibacterial agents. Their high affinity for radicals, which results from radical addition on the numerous double bonds of the fullerene, effectively makes them act as “sponges” for these radicals. As powerful antioxidants, fullerenes are used in biology to neutralize radical oxygen species (ROS) such as the O2•– superoxide ion, HO• hydroxyls, and hydrogen peroxide, H2O2, which are particularly damaging to DNA and certain proteins. They are also used in “niche” cosmetic products, including creams based on liposoluble C60 fullerenes, marketed as antioxidants, with a power comparable to, or better than, that of vitamin E-based products [ACQ 17]. All these properties, used in a medical context, have been described in well-documented papers [BAK 07, LAL 13, CAS 17].
1.1.1.2. Chemical reactivity and exo-functionalization
As fullerenes are considered to be electrophilic reagents, a whole chemistry of functionalization, corresponding to nucleophilic or radical additions or even cycloaddition reactions of the Diels-Alder and Prato type1, can be envisaged. This has been described in detail by Taylor and Walton [TAY 93], and more recently by Georgakylas et al. [GEO 15] in the case of C60 and C70.
Among fullerene derivatives, the fullerenols and carboxyfullerenes obtained by grafting hydroxyl or carboxyl groups onto the surface of fullerenes present an advantage in comparison with simple fullerenes in that they are soluble in aqueous and biological media. This is the case of hexacarboxylated fullerenes (carboxyfullerenes), which are fullerenes carrying three pairs of carboxylic acids, comprising a mixture of two stereoisomers, C3-C60 and D3-C60 (Figure 1.4)2.
Figure 1.4.Structure of two carboxyfullerene stereoisomers, C3-C60 and D3-C60. Based on [DUG 97]
Like simple fullerenes – but with the advantage of aqueous solubility – these derivatives have strong antioxidant properties; they notably have the capacity to destroy the peroxide ion O2•–, a toxic subproduct of cell metabolism. This property makes them excellent neuroprotectors, and research in this area is still ongoing [DUG 97, ALI 04, GHA 05, YE 15b].
1.1.1.3. Endometallofullerenes
The idea that the interior of the fullerene “cage” to trap atoms came hot on the heels of the discovery of fullerenes themselves. As early as 1985, Heath et al. [HEA 85] identified the first fullerene made up of 60 carbon atoms and containing a lanthanum atom3 using mass spectrometry. A few years later, the same authors isolated several metallofullerenes with 60, 70, 74 and 82 carbon atoms, the last of which, La@C824, is the only one which is stable in contact with air [CHA 91].
One remarkable feature of these new compounds is that the metal is not released when the compound is placed in a biological medium; this constitutes a considerable advance in comparison with classic metal chelates. This stability probably results from an electron transfer between the La atom and the fullerene, creating a La3+@C823- ion pair, and presumably also to the fact that the mesh of the fullerene is small enough to prevent the La3+ ion from diffusing outward.
This discovery, which opened promising perspectives in the medical field (notably for diagnostic and therapeutic applications) resulted in an almost immediate upsurge in research into synthesizing new EMF, some of which are useful in the field of oncology. For instance, the gadolinium fullerenol Gd@C82(OH)22, initially used as a contrast agent in nuclear magnetic resonance imaging (MRI), has also been shown to have strong anti-cancer properties, different from those of simple fullerenes; furthermore, it retains low cytotoxicity [KAN 14]5.
1.1.1.4. Endocluster fullerenes
This new family of metallofullerenes was isolated for the first time in 1999 by Dorn and coworkers [STE 99]; as before, these molecules are obtained by producing an electrical discharge between graphite electrodes, containing a metal oxide6, in a reactor. The difference here lies in the inclusion of a small quantity of dinitrogen in the reactor (Figure 1.5).
The remarkable result of this reaction is that it produces a new type of fullerene in which a cluster corresponding to a trimetallic nitride is embedded. The most abundant endocluster fullerenes (ECF) of this family is obtained using scandium. It has the formula Sc3N@C80 and consists of an 80-atom cage, into which scandium nitride with a symmetrical plane structure is inserted. A detailed description of the various functionalization reactions carried out on the surface of the cage of metallocluster fullerenes, which sometimes present higher levels of reactivity compared to simple fullerenes, can be found in a number of recent articles [GOO 17, YAN 17, BAO 18].
Figure 1.5.Structure of the C80 fullerene and of the ECF Sc3N@C80. a) C80 fullerene (symmetry Ih, symmetry axis of order 5) showing a 5 C ring (light blue) surrounded by five rings of 6 C atoms (dark blue). b) Structure of the Sc3N@C80 endocluster fullerene. Based on [STE 99]
An additional family of EMF has been developed by replacing nitride clusters with clusters of metal dicarbides of the form M2C2n, which can exist in the form M2C2@C2n-2. Depending on the atomic volume of the metal, the size of the fullerene cages can vary from 72 to 88 carbon atoms (in the case of scandium) and from 90 to 104 atoms (lanthanum). More recently still, a series of “giant” fullerenes has been discovered involving yttrium carbide clusters: the size of the cages in this case can be up to 130 carbon atoms. The formula for these clusters is Y2C2n where 2n is from 92 to 130 inclusive [PAN 18].
All these compounds have immense potential applications, but the fact that they are currently obtained in only very small quantities (a few tens of mg) and in the form of mixtures, requiring painstaking separation using preparative chromatography, obviously limits their practical use. In contrast with simple fullerenes (C60 and C70) and their exo-functionalized derivatives, which have already been subject to extensive research in connection with biomedical applications [CAS 17], ECF and EMF are generally studied in the context of fundamental research, aimed at better understanding the behavior of clusters of a small number of atoms, confined in a constrained space and interacting with carbon atoms.
1.1.1.5. Onion-like fullerenes
Onion-like fullerenes (OLFs) are a family of multi-shell fullerenes consisting of a concentric, nested assembly of multiple fullerenes of no more than around 10 nm. Various methods are used to create OLFs; the most common approach involves heat annealing, a powder made up of carbon nanodiamonds (NDs; see section 1.2) to produce large quantities of these OLFs [ZEI 16] (Figure 1.6).
Figure 1.6.Progressive transformation of nanodiamonds (ND) into OLF. a) Beginnings of the formation of a fullerene structure at 700–800°C on the surface of the ND. b) Formation of multiple concentric layers of fullerenes at 900–1100°C. c) Terminal phase in obtaining an OLF at T > 1700°C and the corresponding TEM image, showing multiple concentric carbon layers. Adapted from [ZEI 16]
The transformation of NDs (which are commercially available) into OLFs essentially consists of a gradual change from sp3 to sp2 carbons. This transformation, which takes place in vacuum or in the presence of inert gases, starts to take place at temperatures around 1300°C and is completed at temperatures of up to 1800–2000°C. Under these conditions, we obtain OLFs which are homogeneous in size, about 5 nm in diameter, but in agglomerated form. Stable solutions can be obtained after disaggregation and an acid treatment, and these solutions are used in manufacturing conductive inks [VAN 17].
These compounds have electrical conduction properties far superior to those of activated carbon7 and are thus highly valuable for creating supercapacitors with extremely fast response times [PEC 10]. They are eminently suited for use as power devices for energy storage, notably in the realization of pseudo-supercapacitors (see Chapter 3), combining a high charge/discharge speed with larger amounts of stored energy than those found in simple supercapacitors [VAN 17]. A very large number of alternative preparation techniques, which do not use NDs, may be used; these involve degrading various low-cost carbon products, and this makes them more economically competitive in relation to activated carbon [ZEI 16].
1.2. Carbon nanodiamonds
Although NDs were first discovered in the 1960s8, they were not widely used until the late 1990s, once a detonation-based production technique had been firmly established [GRE 88]. These nanomaterials are now produced on a commercial scale, and are used both for improving the mechanical properties of plastics and for diagnosis and treatment in the field of medicine.
1.2.1. Principal techniques used in creating nanodiamonds
Owing to growing awareness of their importance, many different techniques have been developed for producing NDs. The most widespread approaches currently include detonation in a closed chamber, laser ablation and grinding using diamond microcrystal microbeads at high temperature and high pressure [MOC 12]. Their size is between 4 and 5 nm. Nanodiamonds can also be produced using CVD (chemical vapor deposition) techniques, in the form of films of varying thicknesses, between 20 nm and 5 μm [BUT 08]. The detonation method consists of introducing a mixture of two explosive compounds, such as the trinitrate derivatives trinitrotoluene (TNT-C7N3O6H5), trinitrobenzene (TNB-C6N3O6H3) or hexogen (C3N6O6H6), which do not release oxygen on explosion, into a closed metal chamber. The reaction takes place in a humid atmosphere of nitrogen and carbon dioxide (Figure 1.7).
Figure 1.7.Simplified diagram showing the detonation technique for nanodiamond production
COMMENT ON FIGURE 1.7.– I) Front of the shockwave resulting from the explosion. II) Products resulting from the explosion of a combination of TNT/TNB. III) Region in which the temperature (from 2200 to 4500 K) and pressure (from 13.5 to 16.5 GPa, i.e., approximately 150000 atmospheres) correspond to the formation of carbon clusters. IV) Coagulation of clusters in the form of nanodroplets. V) Crystallization, growth and agglomeration of NDs. Adapted from [MOC 12].
As soon as the explosion occurs, the temperature and pressure immediately increase, establishing within a confined space the conditions needed for carbon nanoclusters to form. These nanoclusters become ND (in soot form) on reaching the coldest parts of the reactor [DAN 05]. The soot formed after the explosion of the charge may contain up to 75% ND, ranging from 4 to 5 nm in size; the remainder is made up of graphitic carbon (between 25% and 75% by weight, depending on the conditions), metals and oxides (between 1% and 8 % by weight), and nitrogenous derivatives. The yield in terms of carbon-based products is around 4%–10% of the mass of the explosive charge9.
Nanoparticles (NPs) are essentially composed of sp3 carbons, but they may also be partially covered by layers of graphitic carbon (sp2) on the surface, or functionalized by various chemical groups resulting from reactions with dangling bonds on the surface.
1.2.2. Key properties of nanodiamonds
From a mechanical perspective, the properties of NDs are very similar to those of pure diamonds. They are characterized by extreme hardness and a very high Young elasticity modulus, making them ideal for use in polishing hard surfaces such as ceramics. Their high level of chemical stability means that they can also be used in very hostile environments.
Fluorescence and biocompatibility are two further interesting properties, opening the way for biomedical applications in both diagnostics and treatment, due to the ease of surface functionalization.
1.2.2.1. Fluorescent nanodiamonds
Fluorescent nanodiamonds (FNDs) are a new family of nanomaterials, ranging in size from 35 to 100 nm [HSI 16] and characterized by the presence of a structural defect inside the crystal (Figure 1.8a) [CHA 08].
This defect, named NV, corresponds to a coupling between a vacancy V (absence of a carbon atom) and a nitrogen atom N, adjacent to the vacancy. It can be created easily by irradiating ND crystallites with a beam of helium (He+) ions [CHA 08] or protons (H+)10. Nanodiamonds treated in this way (equivalent to n-doped NDs) emit stable red fluorescence when stimulated by a laser. The intensity of the fluorescence is a function of the concentration of NV, which increases as the energy of the He+ ion beam used to create the defect is strong (NV concentration may vary between 10 and 30 ppm in relation to the number of carbon atoms; in the absence of treatment, their concentration is lower than 1 ppm)11. The biocompatibility and high stability of FNDs, along with their non-toxic nature and high-intensity red fluorescence, make them ideal for use in the biomedical sphere; they are a helpful replacement for inorganic quantum dots, which are usually made up of toxic elements (Cd, Se, Pb, etc.).
Figure 1.8.Fluorescence of ND nanocrystallites containing NV defects
COMMENT ON FIGURE 1.8.– a) Structure of an NV defect resulting from the proximity of a nitrogen atom (impurity due to ND preparation) to a vacancy corresponding to the absence of a carbon atom. The lobes shown in purple correspond to the dangling bonds of the carbon atoms neighboring the vacancy V. b) Fluorescence spectrum of FND (size: 35 nm) obtained by laser irradiation at 532 nm. The pink rectangle represents the window for which the autofluorescence of biological tissue is negligible. c) Time difference between the fluorescence lifetimes of an FND (pink) and that of a biological cell due to the presence of endogenous fluorophores (green). The use of a 10 ns shift between excitation and detection neutralizes the effect of the underlying fluorescence of the biological medium. Adapted from [HSI 16].13
1.2.2.2. Boron-doped diamonds
Nanodiamonds made up of a core of sp3 carbons are insulators, with resistivity reaching values of the order of 1012 Ω cm-1; the band gap, typically that of an insulating material, is equal to 5.47 eV [HE 19].
These characteristics are completely reversed when NDs are doped with boron. Unlike nitrogen doping, which does not exceed concentrations of 30 ppm, boron doping can be very high: it is possible to obtain concentrations of up to 30000 ppm, transforming the boron-doped nanodiamonds (BDD) into conductive materials suitable for use in electrochemistry. In cases of high boron doping (1020–1021 atoms/cm3), the resistivity of BDD is very low, corresponding to that of a semi-metal (10-3 Ω cm), meaning that this new material is suitable for use as electrodes. Lower levels of doping, around 108 atoms/cm-3, give much higher resistivities of around 104 Ω cm [PAN 05].
BDDs containing very low levels of graphitic carbon constitute an exceptional material for electrodes [GAR 15a, HE 19], giving an electroactivity window, for both oxidation and reduction, which is much higher than that of platinum or glassy carbon. The reaction corresponding to the reduction of H+ ions in an acidic medium with the release of dihydrogen occurs at –1.2 V compared to an SHE (standard H2/H+ reference electrode), and the reaction corresponding to the oxidation of water with the release of dioxygen occurs at 2.4 V/SHE, that is, in both cases, there is considerable overvoltage compared to thermodynamic equilibrium voltages12. This property, which is a priori counterproductive from a catalytic point of view, has been used for the oxidation of organic pollutants in water [PAN 05].
Several research groups currently consider that electrodes made of BDD films are the best available option for the destruction of organic materials [HE 19]. Their high resistance to corrosion, non-fouling of their surface due to their hydrophobic character and the very high overvoltage required for oxygen production mean that large quantities of •OH radicals are produced and adsorbed on the electrode surface in the form of BDD-OH•. These highly reactive radical species have the effect of significantly increasing the oxidizing power of the electrodes with respect to organic compounds. Given their very high oxidizing power, their efficiency in wastewater treatment is much better than that obtained with platinum, graphite or even DSA (a “dimensionally stable anode” by DeNora) electrodes, which are usually used for this kind of application [GAR 15b].
BDD electrodes can also be produced in the form of porous films, increasing the active surface of the electrodes and thus improving electrochemical performance. However, this raises a number of difficulties, principally in terms of the adhesion of the BDD film to the substrate (usually Ti or Si); this must be improved before the film can be used for long-term electrolysis operations [GAR 15b].
1.3. Carbon dots or carbon quantum dots
Carbon dots (CDs) or carbon quantum dots (CQD) are smaller than 10 nm in size, and consist of a core of graphitic-type sp2 carbons, corresponding to fragments of graphene or graphene oxide (GO), linked together by sp3 carbons. Their surface contains significant quantities of oxygen, essentially in the form of COOH, carbonyls and hydroxyl groups [LIM 15] (Figure 1.9). Discovered by chance during the preparation and purification of CNTs [XU 04], CDs rapidly attracted attention for their fluorescent properties, which mean they have the potential to be used as reagents for biomedical imaging. As in the case of NDs, their non-toxic nature presents an additional advantage, notably making them preferable to inorganic, heavy metal semiconductor-based quantum dots (QD) in many cases.
Figure 1.9.Structure of carbon quantum dots (CQD) showing the association of graphene oxide and graphene fragments. Adapted from [DEM 13] and [LIM 15]
1.3.1. CQD production methods
CQDs were first discovered in carbon soot. Produced by an electric arc discharge between graphite electrodes, CQDs were identified and isolated during electrophoresis separation of the various carbon NPs contained in the soot, thanks to their fluorescence [XU 04]. Interest in these new fluorescent nanomaterials soon resulted in the development of other methods, such as laser ablation of carbon-based precursors [SUN 06, HU 09].
Many different production methods have been described in the literature [WAN 14b, FER 15, LIM 15]. As fluorescence is a key property of CQDs, the main objective is generally to obtain the best possible fluorescence properties at the lowest cost; different approaches result in different specific properties. The most common production methods involve pyrolysis, applied to hydroxylated or easily oxidizable chemical compounds (amino products), carried out by microwave irradiation of the precursor product to create carbonization. This is the simplest and least expensive method.
For instance, the carbonization of citric acid in the presence of amines, used as “passivation” agents13, produces (after microwave irradiation) CQDs with very high monodispersity and a size distribution between 2.3 and 3 nm. Laser irradiation of a solution of these QDs at 360 nm produces an intense blue fluorescence, centered around 450 nm, with a high quantum yield (QY) of 30% [ZHA 12].
Another way of treating a hydroxylated organic precursor consists of successively using a highly concentrated acid (e.g., H2SO4) for its dehydrating power, followed by an acid with a strong oxidizing power (HNO3), which breaks down bonds, leading to the formation of CQDs. Passivation of the CQDs using an amino compound is then used to increase the intensity of fluorescence of the CQDs [WAN 14b]. The same result can be achieved in a single step by heating polyethyleneimine (PEI), in contact with concentrated nitric acid, to 120°C with reflux [SHE 13a]. This technique has the advantage of being readily available and applicable to many organic precursors, but does not allow for very precise control of CQD sizes [WAN 14b].
A variant of alcohol degradation by electrochemical oxidation in a strongly basic medium has also been proposed, and this also results in fluorescent CQDs. This technique has the advantage of simplicity, insofar as CQDs can be obtained in a single step; it is possible to control CQD size using this approach, but unfortunately only a few compounds can be transformed using this method [LI 10a].
The hydrothermal route has also been widely developed, and allows high-quality CQDs to be obtained at relatively low cost. This method consists of heat treating an organic precursor, placed in a sealed reactor, in the presence of high-temperature steam. The carbonization reaction can be carried out by choosing organic precursors such as glucose, citric acid or orange juice, along with a whole series of easily degradable natural compounds [WAN 14b].
When the organic precursor is pyrolyzed in a confined space, it becomes effectively possible to control the size of the CQDs. The use of nanoporous materials such as silica (in the form of microspheres) means that organic precursors can be inserted into the pores, which then act as nanoreactors. Hydrophilic CQDs are thus obtained by pyrolysis in air of silica microspheres (average diameter 1.3 μm, pore size 3.6 nm), soaked in salts and citric acid, at 300°C for 2 hours. After dissolving the silica using hydrofluoric acid, dialysis is used to recover CQDs, which are almost mono-disperse (sized between 1.5 and 2.5 nm). These are characterized by excellent photostability and high QYs in terms of fluorescence (23%) [ZON 11].
CQDs can be given a variety of different properties by doping or functionalizing the surface; one reason to do this is to increase the intensity of fluorescence. The ultimate goal is to match the QYs obtained using certain organic dyes or inorganic QDs such as CdSe, for which QY can reach 80% [LIM 15].
1.3.2. Fluorescence properties of CQDs
Unlike inorganic QDs, for which absorption is determined by the band gap, CQDs are characterized by a very broad absorption band resulting from plasmonic absorption of the π orbitals14. This absorption band covers a large part of the UV-visible spectrum and extends into the near infrared, thus intercepting a large part of the solar spectrum [FER 15].
Two main emission mechanisms, which are not always easy to identify, are responsible for CQD fluorescence. The first results from transitions between energy levels in domains composed of conjugated sp2 carbons; the second, less evident, relates to surface defects present in CQDs. A third type of mechanism has also been identified, corresponding to an up-conversion of photons, where an emission is produced in the visible spectrum as a result of excitation in the red spectrum.
It is widely accepted that photoexcitation of CQDs results in a separation of charges in the core carbons, and that their recombination with surface defects results in fluorescence. This also explains the high degree of variation in emissions, which is not strictly related to a confinement effect, specific to mineral semiconductor QDs (Figure 1.10).
Figure 1.10.Luminescence of an aqueous solution of CQDs produced by laser ablation of graphite powder
COMMENT ON FIGURE 1.10.– a) Absorption (ABS) and luminescence spectra obtained by excitation at different wavelengths (from 400 to 600 nm in 20 nm increments – from left to right) of poly(N-propionylethyleneimine-co-ethyleneimine) (PPEI-EI)-functionalized CQDs. Intensities are normalized with respect to quantum yields; the inset figure shows normalization with respect to peak intensity. b) Photographs of the luminescence of polyethylene glycol (PEG)-functionalized CQD solutions with a molar mass of 1500 and two amino groups – CH2NH2 (PEG1500N) – at chain ends. Irradiation was applied at 400 nm, and the photographs are obtained after passing the emitted light through color filters of the indicated wavelengths (in nm). c) The same CQD solutions, photographed directly after irradiation at the indicated wavelengths (in nm). Adapted from [SUN 06].
One of the most striking features of the fluorescence emission of CQDs is the way the emission spectrum depends on that of the source of excitation. Different explanations have been put forward for this dependence. The quantum size effect, defects and the surface states present in the CQDs, the presence of fluorophores with different degrees of conjugation and the radiative recombination of electron-holes due to clusters of sp2 carbons enclosed in a matrix of sp3
