The use of colloidal inorganic nanoparticles (NPs) goes back centuries. Examples of applications of gold colloids, for example in medicinal practices in the Middle Ages, are well known, not to mention older practices such as the manufacture of glasses containing metal particles, as with the famous fourth century Lycurgus cup whose dichroism is due to the presence of Au (ca. 40 ppm) and Ag (ca. 300 ppm) NPs.^1,2  On the other hand, Nature has been prodigal in producing inorganic NPs, so these systems have always surrounded humans as part of natural processes and thereby an integral part of eco-systems.³  And yet we associate the term “nanoparticle” with a certain modernity, framed by relatively recent discoveries, especially after the 1980s, and preceded by a broader technological context that was conventionally called “nanotechnology”. This type of technology integrates a vast set of procedures whose main aim is the manufacture of new devices at the nanometric or molecular scale, taking advantage of phenomena characteristic of matter at this scale. This is perceived as a technological evolution towards the miniaturization of devices aiming at a more efficient performance, seeking to respond to more complex operational situations. The aspect that we would like to emphasize here is that despite the ubiquity of NPs, the development of deep scientific knowledge about these systems has been relatively recent, namely by taking in consideration their technological potential.^4–6  This knowledge relies heavily on new techniques of chemical synthesis of materials, on the existence of more powerful nanoscale characterization instruments, with special emphasis on microscopy techniques, and on the storage and processing capacity of information currently available at the computational level.⁷  Since the later decades of the twentieth century, these wide avenues for scientific development have acquired a gradually increasing impact, although they cannot be considered revolutionary in the sense of newly established scientific paradigms. Indeed, there has been a consolidation of knowledge that simultaneously has been inducing new discoveries, through the crossroads of several branches of science. Not surprisingly, nanotechnology is, in its genesis and procedures, a multi- and interdisciplinary area. For its relevance in the context of this book, it is appropriate to start with a brief chronological survey of some facts and scientific discoveries more related to the synthesis and characterization of colloidal inorganic NPs. Although at the time of most of these discoveries the term “nanoparticle” was not used, their impact on subsequent developments is still evident today.

It is unimaginable today to consider any understanding of the world without the vast scientific contributions of Michael Faraday (Figure 1.1) in different areas of knowledge. This also applies to the use of colloidal NPs, because Faraday described one of the first scientific approaches in the interpretation of the optical properties of gold colloids, having described for that purpose processes of colloidal synthesis that in some of their aspects are reproduced today or at least adapted.⁸  In the 1857 publication Experimental Relations of Gold (and Other Metals) to Light, Faraday describes various procedures for obtaining finely divided gold dispersed in a liquid medium.⁹  This publication included a qualitative interpretation of the observation of the colour of Au colloids obtained by reducing the respective salt as an optical effect resulting from the interaction of light with the suspended particles of Au. It is interesting that today, Au NPs have been the most widely investigated colloidal inorganic system in the broad context of nanotechnology and nanosciences, with an increasing number of publications in recent years. One of the most discussed aspects is the control of the optical properties of the Au NPs during their chemical synthesis and often the reduction of an Au salt solution is used, recalling Faraday's experiments. Indeed, the observations reported by Faraday in terms of the preparation and characterization of gold colloids constitute an important landmark for the later development of colloid science, as an amalgam of knowledge in chemistry and physics, in terms that already in 1927 had been proposed by a few scientists, especially Wolfgang Ostwald.^10, 11 

Ostwald's book The World of Neglected Dimensions (1914) is a landmark reference in colloid science (Figure 1.2). Among other reasons for this, it assumes that different specialties intersect in this branch of scientific knowledge and therefore should not be seen as a branch of physical chemistry. It is interesting that nanotechnologies are also currently associated with cross-disciplinary knowledge of different specialties, in which colloidal NPs are undoubtedly relevant systems for studies and applications. It was also at the beginning of the twentieth century that very profound changes occurred in science, with the foundation of quantum mechanics, allowing us to understand matter at the atomic scale, and Einstein's theory of relativity. Several discoveries associated to the Periodic Table of the chemical elements (M. Curie, W. Ramsay, T. Richards), the laws of chemical equilibrium (J. H. van't Hoff), electrolytic theory (S. Arrhenius), organic synthesis (E. Fischer, A. von Baeyer), coordination chemistry (A. Werner) and thermochemistry (W. Nernst) are among many examples that have established the pillars of modern chemistry. It is in this context of revolutionary knowledge and true scientific novelty that important contributions to the science of colloids have also emerged. The interpretation of the optical properties of metal colloids was published by Gustav Mie, based on Maxwell's formalism applied to electromagnetic radiation.¹²  This work is still fundamental in understanding the optical behaviour of NPs of metals with an impact in nanotechnology applications, such as in biosensors of Au and other plasmonic-based applications.¹³  The 1925 Nobel Prize in Chemistry was awarded to Richard Zsigmondy for his decisive contributions to understanding the heterogeneous nature of colloids, having established microscopy as a fundamental technique in the study of these materials and other systems with comparable dimensions. The Nobel lecture by Zsigmondy describing his experiments with colloids, and expressing his appreciation of scientists and scientific contributions that preceded him, is still today essential reading for anyone beginning a study of colloidal NPs.¹⁴ 

The main focus of this book is to present fundamental aspects of the surfaces of colloidal inorganic nanocrystals (NCs), mainly those prepared using wet chemical methods, with impact in several applications. It is now appropriate to clarify the terminology that will be valid throughout this book. Rigorously, the boundaries delimiting the solid particulate and the liquid dispersing medium should be termed “interfaces”. However, we felt that the term “surfaces” is better suited to represent a number of processes that occur at such interfaces, namely by considering that NCs are intended to be used as functional units in nanotechnological devices, not necessarily in contact with a liquid dispersing medium. Hence, we assume that there is some sacrifice in the accuracy of the terminology employed in certain situations to the detriment of conceptual generalization, but that nevertheless the use of the terms “surface” and “interface” will be clear in the respective contexts. We also use the term “nanoparticle” in latus sensus, i.e. to describe solid particulates whose dimensions are typically between 1 and 100 nm and show size- and surface-dependent properties. The term “nanocrystal” will be used to describe NPs of crystalline inorganic materials, such as metals, metal chalcogenides and metal oxides. Figure 1.3 is reproduced from a publication that addresses current terminology applied to nanomaterials science, not only by taking into consideration the morphological specifications but also by assessing their applicability and safety concerns, which ultimately put also in perspective the agreement of regulatory definitions.¹⁵ 

Studies on the synthesis and bulk characterization of colloidal inorganic particles, mainly in aqueous dispersing media, have increased since the first quarter of the last century. However, phenomena related to interfaces in these systems have been less developed, a trend that still applies today in a number of situations. The important role of surfaces in materials composed of small particles was identified earlier, and here we can mention the work of Paul Sabatier (awarded the 1912 Nobel Prize in Chemistry) on finely divided metal particles (e.g. nickel) as important catalysts in the hydrogenation of organic compounds. The recognition that physicochemical processes in colloidal systems are markedly dependent on surface chemistry appeared later, namely with the work of Irving Langmuir (awarded the 1932 Nobel Prize in Chemistry) and Katharine Blodgett on thin films (monolayers) and adsorption phenomena. Obviously, knowledge applied to surfaces in general existed earlier, at least since the nineteenth century with the work of Thomas Young, but the main point here is the lack of understanding of surface chemistry as a consequence of chemical interactions occurring at the boundaries of a solid particulate and its surroundings.

In the middle of the twentieth century, several papers were published that were aimed at understanding the mechanisms of formation of colloidal inorganic particles in liquid media. These represent some of the contributions that later proved to be fundamental in understanding the theory and mechanisms of the formation of morphologically uniform colloidal NPs, such as the work of Victor LaMer and Robert Dinegar (1950) in monodispersed hydrosols.¹⁶  Several of these papers describe methods of synthesis that in their most distinctive aspects are still mimicked in the synthesis of colloidal NPs. An illustrative example is a paper co-authored by John Turkevich, Peter Stevenson and James Hillier (1951),¹⁷  which addressed the mechanistic aspects involved in the synthesis of colloidal Au, including a method using sodium citrate as a reducing agent, and, although subject to later adaptations, it still essentially maintains its relevance.¹⁸  In addition to the experimental simplicity and practical utility of the synthesis of Au NCs, this paper¹⁷  made a fundamental contribution to addressing the nucleation and particle growth mechanisms and questioning the effect of the addition of certain chemical agents (e.g. citrate) in the synthesis of colloids. A number of current methods of chemical synthesis of colloidal NPs with controlled size and shape are based on the understanding of these processes.^19–23  The preparation of metal NCs, in particular of noble metals, typically involves the reduction of a metal salt in the presence of stabilizing agents that prevent aggregation of the formed NPs, due to interactions that are established with their surfaces.

In addition to colloidal Au, colloidal Ag has also been used for many years, particularly because of its antimicrobial properties.^24,25  Silver colloids do not exhibit the chemical robustness of the Au analogues, as they are more prone to oxidation in a normal atmosphere. However, the preparative methods for Ag NPs are typically based on the reduction of an Ag(i) salt in aqueous solution, using reducing agents such as citrate, sodium borohydride or H₂.²⁶  The morphology observed for these NPs depends very much on the chemical nature of the reagents used as precursors, although there is still an incipient knowledge about the mechanisms involved, especially with regard to the chemistry of surfaces. Currently there is a wide range of strategies for the synthesis of metallic NCs that are not limited to aqueous solution syntheses. An important example is those resulting from adaptations of the polyol method presented by Fiévet and co-workers in the late 1980s, using poly(ethylene glycol) as the solvent and as the reducing agent of a metal salt.^27,28  The method was initially developed for the preparation of micro- and sub-micrometric particles of metals, such as Co, Ni and Cu, but subsequently underwent several adaptations in order to obtain metal NPs of various metals with diverse morphologies. In essence, the method involves dissolving the precursor salt in a liquid polyol, then promoting the reduction of the cation by raising the reaction temperature, resulting in a homogeneous nucleation process and growth of the metal particles.

At the end of the 1960s, but with greater intensity from the 1970s, there was marked interest in controlling the morphology of colloidal inorganic particles during the synthesis processes. This is a topic that, although always relevant to colloid science, may have been by this time more closely connected with technological areas that seek to explore the properties of materials dependent on shape and size. A paradigmatic example is the synthesis and use of colloidal particles in the processing of glass and ceramic materials using the sol–gel method. The method was described in 1968 by Stöber, Fink and Bohn in the Journal of Colloid and Interface Science,²⁹  and was applied to amorphous SiO₂ particles, but was subsequently extended to metal oxides, such as TiO₂ and ZrO₂,³⁰  and other forms of non-particulate materials, such as thin films and fibres. Typically, Si alkoxides are used as precursors undergoing a hydrolysis reaction, forming SiO₂ oligomers that give rise to a sol, which in turn acts as a precursor of a gel composed of discrete particles or SiO₂-based polymers. This process currently has unique importance in the development of chemically functionalized nanomaterials, for example in the modification of inorganic NC surfaces by the growth of SiO₂ shells, aimed at diverse applications.^31–34  It is in this context that the sol–gel method will often be referred to in this book and less from its use in obtaining materials based solely on SiO₂ or sol–gel-derived metal oxides.

As noted above, the control of the size and shape of colloidal particles has always been a central aspect of the science of colloids. The behaviour of such colloids was often assessed by a number of instrumental techniques and explained on the basis of theoretical models that took into account practical applications dependent on those morphological properties.³⁵  With respect to inorganic particulates, the achievement of an impressive range of morphologically uniform systems by colloidal processes has developed remarkably since the work of Egon Matijević at Clarkson University in Potsdam, NY, USA (Figure 1.4).^36–38 

In addition to the purely academic interest in controlling the morphology of colloidal particles of a wide range of inorganic solids, namely through strict control of the chemical speciation of the reaction mixture and the experimental parameters involved, a central idea emerged at the technological level that the properties of the final materials can be tailored by the morphological properties of the constituent particles. It is not an overstatement to say that since then, a new paradigm has been established, in particular in the field of materials science and engineering, by considering that the optimal conditions of processing and the final functionality of materials can, to a large extent, be determined in the initial steps of the chemical synthesis of precursor solids. In this regard, it should be recalled that it was during the transition from the 1980s to the 1990s that we witnessed the emergence of new journals in the field of materials chemistry, such as Advanced Materials (Wiley, 1988), Chemistry of Materials (American Chemical Society, 1989) and the Journal of Materials Chemistry (Royal Society of Chemistry, 1991). This is in line with the increasing number of research laboratories that shared a widespread perception of the importance of chemical methods in materials design with target functionalities.³⁹  Thus, a considerable number of publications by this time concerned the morphological control of inorganic powders obtained from colloidal synthesis, owing to its relevance for those endeavours. These syntheses involved aqueous solutions, such as the controlled hydrolysis of precursor metal salts, subsequently adding other chemical agents, such as chelating organic ligands. Although these methods were based on previously established mechanisms of homogeneous nucleation and growth of inorganic particles, such as the aforementioned work of LaMer and Dinegar,¹⁶  there were attempts to extend the field of application to many other solids with diverse chemical compositions and particle shapes, besides the quasi-spherical morphology. The strategies of synthesis adopted at that time varied widely, from methods of controlled hydrolysis to syntheses in organic media, exploring the effect of structuring agents, such as micelles, or the application of polymers as stabilizers. In addition to the chemical composition and rheological behaviour of the colloidal particles, the control of the morphological characteristics of the particles, namely their homogeneity, is itself an important goal in these procedures. The term “monodispersed” had acquired a renewed semantic value in the scientific lexicon, trying to describe systems that have high chemical and morphological homogeneity, previously known for a limited number of colloids, such as gold, sulfur and latexes.^17,40,41  Although true “monodispersed” systems can be realistic for pure molecular-like particles (e.g. fullerene C₆₀ and Au₂₅ nanoclusters), this is an ideal concept when applied to inorganic colloids but that nevertheless has been employed to denote colloidal particles with a very narrow particle size distribution (typically less than 5%).

The above brief chronological summary of important findings in inorganic colloid synthesis is only apparently sequential. It is well known that the scientific development of a particular area has repercussions and intersections at various levels, establishing a complex web of knowledge that is not limited to restricted periods of time. The synthesis of morphologically uniform inorganic NPs was already known for several systems. As mentioned above, for the morphological control processes of colloidal particles, typically with micro- or sub-micrometric dimensions, a number of synthetic protocols have been established. In parallel, several publications appeared that were focused on the study of colloids comprising inorganic particles of nanometric dimensions, typically less than 100 nm. Among the several systems investigated, semiconductor colloidal NPs started to attract increasing attention because they constituted clear evidence of changes in the electronic structure of a material on decreasing its dimensions. Although there was previous experimental evidence that the optical properties of NPs of semiconductors differ from conventional properties,⁴²  it was mainly from the mid-1980s that this area of research acquired the importance that is recognized today, in the more general frame of nanotechnology.

The systematic observation of particle size effects in the optical spectra of semiconductor colloidal NPs was the experimental anchor for subsequent interpretations based on the alteration of the electronic structure of semiconducting materials. The interpretation of the optical spectra of colloids of semiconductor compounds based on quantum-size effects underwent remarkable developments in the early 1980s and mainly during the 1990s. Henglein⁴³  and Brus⁴⁴  performed pioneering studies on the synthesis and optical properties of colloidal quantum-sized semiconductors, while Ekimov et al. published seminal studies on NCs of semiconductors dispersed in glass matrices.⁴⁵  The unique optical properties observed in such nanosized metal chalcogenides and silver halides catapulted this research area into many laboratories, giving rise to competitive research for new methods of synthesis, including alternatives to those normally used in conventional colloid chemistry and that allowed reproducibility of observations of size effects in nearly monodispersed samples. It is important to note that these earlier observations were not due to particle size effects that could be interpreted on the basis of the classical theory of the interaction between particulate matter and electromagnetic radiation. These effects, until then little explored, arise by changes in the band structure of the semiconductor when the constituent NCs reach very small dimensions, typically smaller than the Bohr exciton radius of the semiconductor under analysis.^46–50  It is a quantum size effect that causes changes in the electronic structure of the solid and as such it depends on the particle's dimensions below a certain size threshold. These quantum size effects involve 3D quantum confinement of the charge carriers into a 0D crystalline structure (dot), hence these NPs are often called quantum dots (QDs). There are other quantum-sized structures, such as 1D quantum wires and 2D quantum wells, but these will not be addressed in this book.⁵¹ 

In the 1990s, it became clear that for colloidal semiconductor NCs, with dimensions in a certain size range, the electronic structure of such a semiconductor depends on particle size, hence their properties can be tailored by controlling this parameter during the synthesis of the colloid. In addition, it was also evident from the pioneering studies mentioned above that the surface nature of the NCs had a marked effect on their properties, particularly their luminescence behaviour. Although surface-dependent properties are intuitively expected to be more relevant in the case of smaller particles, owing to the increase in their surface area per unit volume of material, the chemical tools that allow some degree of control are still a hot topic of investigation today and to a large extent were a strong reason to write this book. A major challenge relates to the need to integrate concepts and practices usually explored in different domains. In this respect, it is interesting that it was from a fortunate merger between colloid chemistry and inorganic chemistry that early efficient synthesis methods were developed to obtain monodispersed QDs, which today are used in various devices. As will be resumed later, the research of Bawendi and co-workers (1993) is now considered a landmark in colloidal NCs synthesis, in describing the synthesis of Cd chalcogenide NCs by the injection of organometallic precursors into a surfactant [tri-n-octylphosphine oxide (TOPO)] at elevated temperatures (>120 °C). By applying this hot-injection method, a chemical route was demonstrated that provided samples with a narrow particle size distribution and surfaces passivated with TOPO molecules, whose optical spectra reflected their morphological characteristics in a size range from about 1.4 to 11 nm. In the following years, this work promoted the development of new methods for the synthesis of inorganic NCs using high-temperature chemical reactions in solvents with the ability for surface coordination to the surfaces of the particles. Thus, Trindade and O'Brien described the synthesis of semiconductor NCs in TOPO by using single-molecule precursors⁵²  and, later, Peng and Peng reported QDs obtained in this type of solvent but using readily available reagents, making the method easier and accessible to most laboratories.⁵³  Chaudret and co-workers explored the use of organometallic compounds for the preparation of metal NCs, thus applying new organometallic methods to the synthesis of metals of catalytic interest and chemisorption phenomena.^54,55  In their various modalities, these hot-injection methods still constitute powerful routes to obtaining high-quality colloidal NCs of metals, semiconductors and metal oxides, but other strategies are now available. The understanding of the surface phenomena is relevant not only to tailoring the optical properties of the NCs but also to developing, for example, particles with other shapes, such as demonstrated by Alivisatos and co-workers for several systems.^56,57  The application of these methods in the synthesis of NCs will often be referred to in this book, because the materials obtained therefrom can be exploited in different applications by making use of their properties, which are to a large extent determined by the surface chemistry, the central aspect of this book.

In this section, we address some fundamentals related to the surface science of inorganic particles that are considered particularly relevant for understanding the peculiarities of the surface chemistry of NCs. Inorganic NCs suspended in a liquid are treated as lyophobic colloids, which means that at the nanoscale the particulates do not form a homogeneous phase with the solvent in which they are dispersed.

For a given volume of a material, a decrease in the average size of its constituent particles causes an increase in the surface area per unit volume of that solid sample. That is, the specific surface area of this solid material increases with decreasing particle size. Figure 1.5 illustrates in a pictorial manner the increase in surface area for the same volume of a material when it is finely divided into smaller particles. A corresponding illustrative image for this purpose is that a sugar cube of edge about 1 cm, when divided into 1 nm edge nanoscopic cubes, gives a surface area comparable to one football pitch! In terms of powder-based technological applications, it is well known that the increase in the specific surface area observed on decreasing the particle size has been used with advantage in various domains, for example in decreasing the temperature for solid-state chemical reactions to occur extensively, heterogeneous catalysis and adsorption processes, among many others. Intuitively, we expect that also in the case of colloidal NCs, which are systems characterized by high specific surface areas, surface effects will be major players in the nanotechnology arena. This is due not only to the huge area available, hence with a number of interfaces that can interact chemically with the surrounding environment, but also because there are physical properties intrinsically dependent on the nature of the surface, such as the optical properties of metal and semiconductor NCs.

Creating new surfaces in a crystalline solid material is a thermodynamically unfavourable process with respect to the formation of the crystal lattice of the same material but within the solid interior. The associated energy cost is the reversible work to create a surface of unit area (A) as given by the surface free energy (σ), which for the same solid is surface orientation dependent:

In an ideal crystalline solid, the constituent atoms, ions or molecules are arranged in regularly ordered arrays, forming a three-dimensional structure. For most of the situations addressed in this book, those solids are particulates composed of either atoms or ions, hence for convenience we will use the term “atom” unless this generalization raises confusion in a certain context. There is an excess of free energy, for a given material at a certain temperature T and pressure P, for the atoms at the surface of the particle relative to those located inside the crystalline lattice. The surface energy is understood as the difference in energy between a particle and an entity with the same number of atoms but in an infinitely extended solid that mimics a specific surface. Compared with the atoms located in the interior, the surface atoms do not have saturated coordinating environments, hence due to the presence of dangling chemical bonds this results in an inward net force, which translates into changes in the chemical bond lengths between the surface atoms and the underlying neighbours (Figure 1.6). Because in a crystalline solid the different facets have atoms with different bondings and spatial configurations, the surface energy and generated surface stresses depend on the orientation. Surface stress is present because the surface atoms tend to rearrange in order to minimize the energy of the crystal as a whole, thus leading to atomic positions different from those observed in the bulk solid, which causes surface strain. This type of structural change can be small but sufficiently relevant to decrease the lattice constant in nanocrystalline materials and eventually even changing the type of structure relative to the macrocrystal, by keeping constant the temperature and pressure.

Figure 1.7 shows how the particle size influences the percentage of surface atoms and bulk atoms in gold NCs. Surprisingly, in general, the effect of particle size on the surface energy of NPs is still not very well understood, hence this is an important aspect that deserves further attention.⁵⁸  For borderline situations, where particle sizes approach the typical dimensions of molecules, whereas the chemical composition and structure are well defined, these entities are often called nanoclusters.^59,60  In these cases, the most stable structures do not necessarily correspond to the crystalline structures found in the larger NCs of the same type of material. As such, these quasi-molecular entities cannot be understood as nanometric fragments of the crystalline lattice of the corresponding bulk material and will not be a major topic in this book.

A nanocrystalline material dispersed in a liquid has a high number of interfaces per total volume of crystalline solid. Moreover, crystals have defects that become more relevant in determining properties as the particle size decreases, because a large percentage of those defects are located at the surfaces. Nanocrystals dispersed in a liquid are in fact metastable systems because the tendency is for the system to evolve to decrease the surface free energy in order to achieve the minimum total free energy, for example by particle agglomeration. We will discuss here important mechanisms by which the decrease in surface free energy can occur, for the case of a system composed of colloidal particles. For the sake of clarity, we will distinguish between mechanisms that occur at the surface as structurally and chemically individualized entities from those mechanisms that are associated with the collective behaviour of particles when suspended in a liquid. It is important to make this distinction because the behaviour of a single NC depends on its morphological characteristics, which, strictly, are not uniform within an ensemble of NCs.

In the absence of external action, in particles of a metal or semiconductor, the surfaces change in order to achieve a free energy minimum in equilibrium conditions, thereby decreasing the surface free energy. Energy reduction in a given surface of a colloidal NC occurs by important mechanisms such as surface restructuring, adsorption and surface relaxation.⁶¹  In the first case, atoms close to the surface, characterized by lower coordination numbers than in analogous species located within the interior of the NC's lattice, establish new chemical bonds with each other. The presence of neighbouring atoms with dangling bonds leads to local rearrangement of these atoms, establishing new chemical bonds with each other, which although subject to strain, leads to a decrease in the number of non-compensated chemical bonds at the surface. NC atoms can also interact with surface-adsorbed chemical species, which frequently are different from those that make up the solid's crystalline lattice. The interaction of adsorbed chemical species may occur via chemical bonds established between surface sites and adsorbate atoms, which is described as chemisorption, or via weaker electrostatic or van der Waals interactions, in this case adsorption by physisorption. Post-synthesis chemical modification and functionalization of the surfaces aimed at passivating defects can be regarded here as a distinct case of chemisorption, because it is usually treated as an additional step after the fabrication of NCs.

The mechanism corresponding to surface relaxation involves the displacement of surface atoms within the NC due to the net force to which these atoms are subjected, because of uncompensated chemical bonds at the surfaces. As noted above, this mechanism leads to changes in the length of the chemical bond between surface atoms and their direct neighbours, while maintaining the typical crystalline lattice of the solid. Although this effect might be negligible at the macrocrystal level, it is a mechanism whose contribution to the decrease in surface free energy of a crystalline system increases as the percentage of atoms at the surface also increases, i.e. as the particle size is reduced to nanoscale dimensions. A colloidal system consisting of NCs dispersed in a liquid tends to decrease the total surface free energy also due to the collective activity of the constituent particles. One possible mechanism involves decreasing the total surface area of the NCs in contact with the dispersing medium. Assuming for now that NCs are isotropic, i.e. they can be treated as quasi-nanospheres and therefore have on average the same surface energy over the surfaces, the aggregation of colloidal NCs into larger structures is a process that will occur spontaneously over time. Although thermodynamically favourable, this process can be kinetically controlled by creating barriers to the aggregation of the suspended particles, for example through molecular stabilizers adsorbed on their surfaces. Even in these cases, the system will evolve towards decreasing the total surface area, with those NCs clustering for given extended periods of time depending on the colloidal system and ambient conditions. Although the reference here is mainly to NCs dispersed in a liquid medium, sometimes these particulates are isolated for later use as nanodispersed powders, for example for subsequent manufacture of devices. Also, in such situations, it is crucial to keep the surfaces chemically protected so that the morphological integrity of the NCs is maintained for a reasonable period of time. Failure to do so will result in the coalescence of NCs into polydispersed particulates of larger dimensions, in a process called sintering, the kinetics of which may be increased with increase in temperature. Powder sintering is a well-known processing step in the ceramics and metallurgical industries in which there is a decrease in the number of interfaces (grain boundaries) by volume to produce dense pieces of a given material. Sintering occurs at elevated temperature as this process involves mass transport through the diffusion of atoms or ions in a crystalline lattice, which can be interpreted by applying Fick's laws of diffusion.⁶² 

NCs dispersed in solution may be subject to morphological changes by dissolution of material and crystal regrowth, depending on the solubility of the solid under the experimental conditions employed. This is a well-known mechanism used for ageing poorly soluble salts obtained by precipitation in solution. This mechanism, known as (Wilhelm) Ostwald ripening or ageing or sol coarsening, occurs when suspended polydispersed particles increase the average size at the expense of reprecipitation of ions coming from dissolution of the smaller particles. Hence, once the equilibrium state has been achieved, the number of particles will be smaller for the same amount of material in a closed system.^63,64  This mechanism is important even in colloidal NCs kept at room temperature, but can be accelerated by increasing the temperature, which usually leads to the dissolution of smaller particles into the respective ionic species. This mechanism also occurs in several post-synthesis treatments of colloidal NCs, in order to narrow the particle size distribution of the as-prepared colloid. In these cases, the average particle size increases as the system evolves towards quasi-monodispersity. The Ostwald ripening mechanism has been used to explain the growth dynamics of colloidal particles, but other mechanisms have also been proposed. The mechanism of coarsening due to island diffusion and dynamic coalescence dates back to the pioneering work of Marian Smoluchowski (1872–1917), whose ideas established the foundations of the theory of stochastic processes, among other fundamental scientific contributions.⁶⁵  The Smoluchowski coarsening mechanism can be particularly relevant for NCs supported on solid substrates.^66,67  Figure 1.8 highlights diagrammatically the difference between the Ostwald ripening and Smoluchowski coarsening mechanisms.

Although we assumed above that NCs are isotropic in terms of the distribution of free energy on their surfaces, this is not strictly true for real crystalline systems. NCs of metals and semiconductors are nanosized fragments of extended three-dimensional crystalline lattices and, as such, these NPs have exposed faces corresponding to distinct crystallographic planes. These facets have different surface energies so a decrease in overall surface free energy can be achieved by the growth of certain faces with lower surface energy relative to the more energetic faces. Effectively, in the case of macrocrystals generated in homogeneous solution, just by taking into account thermodynamic criteria, the particles reach their minimum energy in the equilibrium situation, giving rise to polyhedral structures according to their crystallographic habit.^67,68  Ideally, the final shape of the crystals can be deduced through the so-called Wulff constructions, which take into account purely thermodynamic criteria for predicting the shape of crystals, under vacuum conditions and at 0 K.⁶⁸  In reality, there are other factors that affect the final morphology of the particles generated in the liquid phase. These include, among others, the synthesis reaction kinetics, the existence of twin defects, the presence of impurities at the growing surfaces and the selective adsorption of certain chemical species. This means that the final shapes of the NCs are not necessarily those predicted by the Wulff plots. For example, in metals with a face-centred cubic structure, a single NC would appear as a truncated octahedron because this is the Wulff polyhedron corresponding to the equilibrium shape. Although these structures might appear in the initial stages of NC formation (seeds), a number of parameters will determine their final shape, which might happen to be far from the predicted equilibrium structure (Figure 1.9). As will be apparent later, for a great number of practical situations we can treat NCs as quasi-spherical particles, but this is an approximation to the real faceted nanostructures.

Adsorption is a key aspect in the general study of solid surfaces.^69–71  Colloidal particles are characterized by a high surface area per unit volume of material, hence adsorption phenomena are of great importance, which is easily perceived given some aspects already mentioned above. Adsorption of chemical species in NCs is a possible mechanism for decreasing the total surface free energy, altering the colloidal stability, which, depending on the adsorbate, either promotes their stability for longer periods of time or causes particle aggregation. Many of the methods for the surface modification of colloidal NCs envisaging interaction with target species involve adsorption phenomena.^72–77  Furthermore, the surface characterization of these finely divided systems can be performed using techniques that are based on adsorption phenomena.^35,69–71  Therefore, this section will present some of the main concepts related to adsorption in solid particulates in general, but bearing in mind their application to NCs dispersed in the liquid phase. Although this is a very broad subject for which an in-depth approach requires interaction with many other fundamental areas, we have selected here important concepts for understanding the subjects discussed later in this book, namely those concerned with surface chemical modification (Chapters 3 and 4) and technological applications of NCs (Chapter 6).

Here, the term adsorption is used in situations where molecules or other chemical species interact with the surface of the particle, forming a new interface with the surrounding medium. Desorption is the reverse process, that is, the release of adsorbed molecules from the surface of the solid. For the sake of explanation, let us assume first that the adsorbates are molecules of a given gaseous substance. The extent to which the surface of the solid is covered due to adsorption of gas molecules is given by

The extent of solid surface coverage will depend on the temperature and pressure conditions, and also on the initial number of adsorbate molecules. Interaction of the adsorbate molecules with the surface of the solid can be accomplished through the formation of strong chemical bonds, yielding locally adsorbate–surface site conjugates, which are chemically distinct from the starting components. This irreversible adsorption phenomenon is called chemisorption, involving a relatively high heat of adsorption, and it is actually a surface chemical reaction. Hence there is an energy activation barrier that has to be overcome for establishing chemical bonds between the adsorbate and the surface of the solid. On the other hand, for a reversible adsorption process, the energy activation barrier is generally negligible because the adsorbed molecules are attached at the solid surface through weak physical interactions, such as van der Waals interactions. Figure 1.10 typifies both adsorption mechanisms in terms of the potential energy as a function of the molecular adsorbate–surface distance.⁷⁸  Note that in the case of physisorption, the chemical identity of the adsorbed molecules is maintained and the heat of adsorption involved in this type of interaction with the surface of the solid is low. This causes desorption without a great energetic input to the system because weak van der Waals interactions are present. The chemisorption phenomenon occurs at the level of the first monolayer, when the adsorbed molecules establish chemical bonds with solid surface sites, in this case forming self-assembled monolayers. Physisorption can occur in multilayers, notably by gas condensation in the first monolayer and later layers, for higher partial pressure regimes (gas) or high concentrations of adsorbate species (solutes).

The study of adsorption behaviour allows us to collect important data about the surface of solids, such as the specific surface area and the associated type of porosity. The specific surface area (m² g⁻¹) is the overall sum of the surface areas exposed by the particles per unit mass of sample. Despite the limitations in terms of its theoretical foundations, the methodology proposed by Brunauer, Emmett and Teller (BET) in 1938 remains widely used to describe the surface areas of crystalline solids.⁷⁹  These measurements can be performed using samples of powders after outgassing, i.e. with particle surfaces previously cleaned, following heat treatment under vacuum. The sample is then treated with an inert gas stream, typically N₂ at a temperature near the boiling point, at a constant temperature and by varying the amount of gas (relative pressure) in contact with the powder. The experimental result is an isothermal curve that represents the amount of gas adsorbed as a function of the relative pressure of that gas. The BET adsorption isotherm follows the equation

where n_a is the total amount of adsorbed gas per unit mass of sample (mol g⁻¹), p/p₀ is the relative pressure of the gas, C is the BET constant and n_m is the amount (mol g⁻¹) of adsorbed gas in the first layer. The representation of the amount of gas adsorbed as a function of the relative pressure of the gas follows a linear relationship for a relative pressure range typically from 0.05 to 0.3, where the intercept at the origin is given by 1/n_mC and the curve slope is given by (C – 1)/n_mC. It can be easily demonstrated that the amount of adsorbed gas in the first monolayer (n_m) is equal to the inverse of the sum of the values obtained for the slope and the intercept at the origin. The n_m value corresponds to the number of gas molecules (mol g⁻¹), typically N₂, that form the first adsorbed layer over the solid. This value, once known, allows us to calculate the specific surface area from the expression

where N_A is the Avogadro constant and A_CS is the adsorbate cross-sectional area, which is assumed to be 16.2 Ų in the case of nitrogen gas.

Although the BET method is widely used as an experimental technique for determining surface areas based on the adsorption of a gas on a solid surface, a brief mention should be made of other methodologies. The concept of a monolayer originated from pioneering research on oil films performed by Irving Langmuir, the first non-academic chemist to be awarded the Nobel Prize for Chemistry (1932), for outstanding achievements in surface chemistry research.⁸⁰  Langmuir considered surfaces to be divided into tiny squares, where the area of each of these squares could be occupied by only one molecule (or atom) in the adsorption process. The set of molecules that occupy the whole surface forms a monolayer the thickness of which corresponds to the size of one of the equally adsorbed species. The monolayer concept, which was also used in the formulation of the BET model, has had huge implications in many branches of chemistry and is still the basis for interpreting many experimental facts today.⁷¹  In heterogeneous catalysis, for example, it was possible to explain higher chemical rates by the proximity of the reactant molecules to neighbouring sites where they were adsorbed. The formation of self-assembled monolayers (SAMs) of thiol compounds over Au surfaces had a huge impact on nanotechnology, such as in self-assembly-mediated nanofabrication processes. Also, in determining the specific surface area of certain microporous solids, the monolayer concept is used to explain the adsorption of a gas in these porous materials. In these cases, the adsorption is limited to the formation of a monolayer at the solid/gas interface, which, at a constant temperature, has an extent that is dependent on the gas pressure, which in an equilibrium situation follows the equation

where K is the equilibrium constant, which is equal to the ratio of the kinetic constants of adsorption and desorption of gas molecules on the solid surface, which are processes having equal rates under equilibrium conditions. The Langmuir isotherm assumes that the adsorption sites have the same affinity for the adsorbate molecules and that the adsorption is limited to a monolayer, where no interactions occur between adsorbed adjacent species. This is indeed a possible interpretation for adsorption phenomena, when for example the gas molecules are relatively few (low pressure). The BET model [eqn (1.3)] extends the adsorption process to other common situations, such as the case of multilayer adsorption where gas condensation occurs on the top of the underlying layers.⁷⁹  On the other hand, it still considers that the surface sites where adsorption occurs are equivalent and do not take into account interactions between adjacent adsorbed molecules.

The isotherms discussed above aim to interpret the adsorption phenomenon under specified conditions that are framed by the assumptions of the respective theoretical models. These theoretical frameworks do not necessarily approximate many practical situations. The adsorption isotherm proposed by Herbert Freundlich (1907)^69,81  is an empirical relationship that shows how the extent of adsorption varies as a function of gas equilibrium pressure p:

The constants c₁ and c₂ are characteristic of the system under analysis at constant temperature. These constants adjust eqn (1.6) to experimental data for a given pressure range. Despite being empirical, Freundlich's isotherm may nevertheless be a useful tool in estimating the extent of adsorption on a heterogeneous surface of a solid, namely when the Langmuir and BET isotherms fail to match the experimental conditions to the theoretical assumptions of the respective models.

The above adsorption models have been presented for the adsorption of gas molecules on the surface of a solid. These adsorption models can be adapted to the cases of solute adsorption on the surfaces of solid particulates that are dispersed in a solvent. Figure 1.11 illustrates schematically the profiles of the three adsorption isotherms for the cases mentioned above, but with the equilibrium concentration (q_e) of a solute adsorbed on a solid surface plotted as a function of the concentration of solute (C_e) in the solution with which the adsorbent is in contact. However, the reader should keep in mind that there are other types of adsorption isotherms that might be useful for many other situations. In addition, the adsorption of solutes is a competitive process with solvent molecules that are also adsorbed on the surface of the dispersed particles. The system in this case evolves into an equilibrium situation, which results in an overall decrease in its free energy, thus corresponding to an adsorption extension that also depends on the nature of surface sites available for the interaction with the adsorbate molecules. For a given system, the rate at which the adsorption equilibrium is reached depends on a number of factors, such as temperature, the energy activation barrier required to exchange solvent molecules by those of adsorbate and the diffusion coefficient of adsorbate molecules in the solution.

The synthesis of colloidal inorganic NCs will be explored in Chapters 2 (water-compatible systems) and 3 (organic-coated surface), but it is now instructive to address here the main characteristics of chemical methods in order to stabilize inorganic colloidal particles. First, attention is drawn to a consideration that requires clarification throughout the text. Although there are numerous chemical methods for the synthesis and stabilization of inorganic NCs, even by restricting these methods to typical colloid chemistry strategies, those selected here are intended to illustrate relevant surface chemistry aspects envisaging selected properties and target applications. The emphasis on the relationship between synthesis strategy and surface-mediated properties is not fortuitous. It should be recalled that in past decades, the synthesis of NCs was developed in order to control properties that derive from the size and shape of the particles, in addition to the interactions that those particles establish with the surrounding environment through their surfaces.^82–85  For example, the optical and magnetic properties that depend on particle size effects can be conveniently understood and explored only if methods are available not only to control their morphology but also to produce stable colloids that retain a high number of interfaces. These endeavours invariably require the understanding of the physicochemical processes that occur at the surfaces of colloidal NCs, namely aiming at their colloidal stabilization. Furthermore, the particles' morphology itself has an influence on the chemistry associated with such nanoscale materials. In fact, all of these surface-related aspects lie in the realm of nanochemistry.^86,87 

In general terms, the methods discussed below are based on chemical reactions that give rise to stable colloidal particles via electrostatic repulsions or to steric hindrance (entropic stabilization), the latter due to (macro)molecules attached to the surface of the particles. There is often a combined effect of these two factors, for example on the use of polymers (e.g. bulky polyelectrolytes) as surface modifiers for nanocomposite formulations. The preparation of nanocomposites containing inorganic NCs will be treated separately in Chapter 4. This section focuses on particles without adsorbed foreign species, namely organic moieties, which for the sake of simplicity are termed non-coated particles.

Although not restricted to aqueous methods, the synthesis of inorganic NCs stabilized by electrostatic repulsions has been developed to a large extent using water as a solvent with a high dielectric constant. The synthesis and stabilization of colloidal particles in aqueous solution have been known for a long time and usually require relatively simple technical procedures. The apparent simplicity of these laboratory methods does not preclude rigorous laboratory skills. In the case of metals, the NCs are usually obtained from the reduction of water-soluble salts using reducing agents, which are present in solution or added in a later step, or due to an externally controlled effect such as UV irradiation of the reaction system. In the case of binary NCs, typically metal oxides and chalcogenides, the particles are generated in homogeneous aqueous solution by judicious adjustment of experimental parameters such as the relative quantities of reagents and chemical additives and stirring speed, temperature and reaction time, among others. Knowledge of the surface charge distribution in the as-prepared colloidal particles is crucial because the stabilization in these situations is achieved due to inter-particle electrostatic repulsions. Experimentally, measurements of zeta potential values in a given pH range are in these cases widely used to assess the stability of the suspended particles with respect to their aggregation.³⁵  Relatively long-term stable colloids are observed for high (negative or positive) zeta potential values, typically greater than 30 mV in modulus. The zeta potential can be determined by electrophoretic measurements, in which the colloidal particles migrate under the influence of an electric potential difference. These mobile entities are understood as being composed of the crystalline core (NCs) and the ionic species chemisorbed/physisorbed at the surface. Other experimental methods can be found in the literature, however, and these have been used in different contexts.⁶⁹ 

The spontaneous aggregation of aqueous colloidal particles can be prevented by strong repulsive electrostatic forces between the suspended particles. This is due to the existence of electric charge separation at the solid/liquid interface, which, in the vicinity of a solid particle, can be described by the electrical double layer (EDL) model.^35,69  The nature of the EDL at the solid/liquid interface in particles dispersed in water varies with the colloidal system and here we treat the subject in its general aspects. In Chapter 2, we will present different synthesis methods that allow the stabilization of inorganic NCs using the stabilization of the EDL. For the moment, let us consider first that the surface potential of the solid particulate is determined solely by the concentration of ions of the same type as those pertaining to the respective crystalline lattice. The change in electric potential, in this case the surface potential (ψ₀) due to chemical change, is given by the Nernst equation:

where µ$iL$ and µ$iS$ are the standard chemical potentials of the ion in bulk solution and in the solid, respectively, z_i is the ion valence, C_i is the ion concentration in solution, F is the Faraday constant and R is the universal gas constant.

The EDL model was originally proposed by Helmoltz in 1879,⁸⁸  and later developed into improved versions in order to agree with an increasing number of experimental facts. The EDL corresponds to a structure formed at the solid/liquid interface, where neutralization of the surface charge in the solid originates from the electrostatic attraction of dissolved ionic species, hence forming an adjacent layer in solution containing an excess of charge ions (counterions) in relation to surface ions (co-ions). In its simplest form, this structure can be visualized as a capacitor consisting of two adjacent layers of opposite charge at a distance d from one another, where the electric potential decreases abruptly with the distance d (Figure 1.12A). The Gouy–Chapman model presented about 30 years later considers the layer containing the counterions as a diffuse structure.^89–91  Hence, by taking into account also the diffusion effect due to thermal energy, there is a higher concentration of counterions near the surface, which decreases as the distance from the surface to the bulk solution increases (Figure 1.12B). As a result, the electric potential decreases exponentially from its surface value to zero, for a point in the bulk solution sufficiently far from the surface of the solid. Although this model can reasonably be used in the interpretation of the EDL structure formed at the solid/liquid colloidal particle interface, in some situations it disagrees with some experimental data and is based on approximate concepts. Right from the start, the model assumes the ions to be point charges and does not distinguish the distinct behaviour of ions with equal charge adsorbed on the solid surface. An improvement to the Gouy–Chapman model of the EDL was presented by Stern in 1924, in order to circumvent some conceptual limitations encountered in the previous models (Figure 1.12C). As such, the structure of the layer was understood as being formed by two regions, an inner layer (Stern layer) containing ions adsorbed at the surface of the solid and a diffuse outer layer (Gouy–Chapman layer). The Stern layer is composed of ions charged oppositely to the surface and follows the Langmuir isotherm. The distance to the surface of the solid is determined by the dimensions of the ions that are distributed along a plane at a distance d from the surface of the solid (Stern plane). The charge of the Stern layer is distributed across the Stern plane and, in this case, the electric potential varies linearly from its value at the solid surface to the electric potential value that characterized the Stern plane (ψ_d). After that, the potential along the Gouy–Chapman layer varies exponentially as mentioned above, because it is associated with a diffuse ion layer that also contributes to charge neutralization that was not completed just by the formation of the Stern layer. It should be noted that, unlike the Gouy–Chapman model, this EDL model takes into account the adsorption behaviour observed at the surface of the solid. Thus, the electric potential in the Stern layer may present values corresponding to reversal of charge depending on the nature of the ionic species that constitute this layer. It turns out that the Stern layer was described later as being composed of two regions: a layer close to the surface comprising specifically adsorbed ions, delimited by the inner Helmoltz plane, and an outer layer of hydrated and non-specifically adsorbed ions, delimited by the outer Helmoltz plane. Other formalisms have been developed that aim not only to improve the EDL model but also to explain certain processes that are not taken into account in previous models, namely when studying colloids of nanoscale materials.^35,92  Nevertheless, the Gouy–Chapman–Stern model of the EDL is adequate for interpreting a large number of experimental observations. Despite its limitations, it is also useful as a pedagogical tool for describing important features of solid/liquid interfaces, such as those formed in dispersing colloidal NCs in aqueous solutions.

The EDL at the solid/liquid interface stabilizes colloidal NCs in aqueous solution due to the presence of inter-particle repulsive electrostatic interactions. In the absence of such repulsive interactions, ubiquitous van der Waals attractive forces between the dispersed particles would lead to particle aggregation in order to decrease the surface energy of the system. In fact, the EDL structure can be altered in order to overlap and eventually collapse, thereby inducing the aggregation of the colloid. This aggregation step can be deliberately promoted, for example, by adding an electrolyte to the colloid that causes the ionic strength of the dispersing medium to change. The ionic strength I depends on the ion concentration (C_i) and charge (z_i) of the respective ions as follows:

The thickness of the EDL tends to decrease with increase in the concentration of counterions in the diffuse layer, a process that may otherwise be described as a decrease in the value of the Debye length (κ⁻¹), i.e. the inverse of the Debye parameter that models the exponential decrease in counterion concentration in the diffuse layer. The Debye parameter characterizes the electrostatic screening power of the electrolyte solution and is a function of the ionic strength of the medium.

The balance between van der Waals attractive forces towards particle aggregation and the repulsive electrostatic interactions for colloidal stabilization can be appreciated on the basis of the DLVO (Derjaguin–Landau–Verwey–Overbeek) theory.^93–95  Considering an aqueous colloid, it is assumed that only long-range interactions occur between the dispersed particles, whereas the resulting interaction potential is a function of the potential that result from the electrostatic repulsive forces and the van der Waals attractive forces [eqn (1.9)]. The net potential is thus a function of the inter-particle distance (d), which can be expressed by the sum of an attractive and a repulsive term:

The DLVO curve (Figure 1.13) shows the variation of the net potential V_net as a function of the distance between two spherical colloidal particles. Particulates suspended in a liquid, at a given temperature T, are in permanent movement (Brownian motion) due to collisions with the dispersing medium molecules that compose the solvent. These molecules are animated by a translational intrinsic kinetic energy equal to (3/2)kT, which for these corpuscular entities is not negligible. Note that as two solid particles approach as a result of the Brownian motion, a potential energy barrier V_pe higher than the thermal energy kT must be overcome for the formation of the two-particle aggregate. Also, the interaction potential curve shows two minima corresponding to distinct particle aggregation processes (Figure 1.13). The first minimum corresponds to an irreversible aggregation process, commonly referred to as coagulation, which differs from flocculation, which is a reversible process. In the latter case, corresponding to the secondary minimum in the potential curve, the aggregated particles can be separated again by applying an external action such as mechanical stirring. According to the DLVO theory, it is possible to demonstrate that for homogeneous colloidal particles of radius r dispersed in a liquid medium, which are influenced only by electrostatic repulsive interactions and attractive van der Waals forces, the resulting interaction potential is given by³⁵ 

Eqn (1.10) allows us to interpret to some extent the stability of particles dispersed in a liquid medium with respect to aggregation at a given temperature T. The first term corresponds to the attraction of particles due to van der Waals interactions, which are integrated into the value of A, denoted the Hamaker parameter, and is a constant for a given material.⁹⁶  The second term corresponds to the potential due to repulsive interactions involving the EDLs at the particle/liquid interface. As mentioned above, this term depends strongly on the Stern potential, which can be experimentally adjusted using electrolytes introduced into the system. The second term also depends on the Debye constant (κ), which is a function of ion concentrations and their respective valence. The equation is only valid for colloidal particles subjected to van der Waals and electrostatic interactions, the so-called electrocratic colloids, which nevertheless include a large number of colloidal NCs dispersed in aqueous solution. Furthermore, the DLVO formalism is a well-established theoretical framework that acts as a good starting point for understanding the stability of colloids relative to the aggregation process.³⁵ 

It is useful to know laboratory techniques that achieve sufficiently stable colloidal NCs. In this way, we can take advantage of the high number of interfaces that such functional nanomaterials create with distinct environments, for a wide range of applications. The approach presented here is obviously common to other colloidal systems that are not necessarily made up of nanosized particles. However, in the case of colloidal NCs, namely metals and semiconductors, not only does the colloid stability maintain a high number of solid/liquid interfaces, but also there are intrinsic properties of these nanomaterials that depend on, and scale with, particle size. These properties can be conveniently adjusted and exploited provided that there are methods for controlling the morphological properties of the suspended particles, including the surface modification with chemical agents of nature distinct from that of the respective inorganic cores. Hence this section mentions common methodologies for promoting the colloidal stability of NCs due to the presence of chemical species distinct from the inorganic lattice. For the sake of clarity, NCs stabilized in this way will be referred to as surface-coated NCs, although in a strict sense the surface of a colloidal NC always has a chemical coating even if it is made up of ions common to the crystal lattice.

As already mentioned, the stability of colloidal particles towards electrostatic repulsion can be achieved because there is an EDL at the solid/liquid interface, namely due to the adsorption of ions of the same type as those that form the crystalline lattice of the solid. The inter-particle repulsion due to electrostatic interactions can be promoted by the deliberate addition of ions common to the crystalline network. In practice, the most frequent situation for stabilization of ionic adsorption involves other chemical species that will guarantee greater colloidal stability, by promoting inter-particle repulsive electrostatic interactions or/and avoiding particle aggregation by steric hindrance. The chemical species that act as colloidal stabilizers may integrate the reaction system during the colloid synthesis or may be added after the synthesis stage. A common strategy for stabilizing colloidal NCs, especially in aqueous systems, is the adsorption of ionized chemical species, such as ions of organic compounds containing carboxylate, thiolate and selenate anionic groups or ammonium or phosphonium cationic groups.

The surfaces of metal and semiconductor NCs are often coated with (macro)molecules that prevent their aggregation due to steric hindrance. As will be described later, the surface modification of NCs with surfactant molecules is one of the most common procedures for obtaining stable colloids for extended periods that eventually will be incorporated into devices. A high performance of such devices requires that not only the chemical identity but also the morphological integrity should be preserved. Furthermore, this passivation process allows the adjustment and optimization of size-dependent physical properties of NCs, such as the plasmonic and photoluminescence properties.^97–100  Typically, in these cases, the NCs are coated with surfactant molecules in which the polar component, with Lewis acid characteristics, coordinates to surface sites of the NC, leaving the non-polar component directed outwards. In such surface-coated NCs, colloidal stabilization arises because the non-polar component of the surface-attached molecules has a strong affinity for the solvent, thus forming a hydrophobic surface layer that acts as a barrier to particle aggregation. Obviously, these situations relate to stable colloidal NCs in non-polar solvents, characterized by a low dielectric constant, but whose further use in aqueous solution is possible after replacing the surfactant molecules with molecules or polymers with hydrophilic properties.¹⁰¹ 

The use of surfactants in NC stabilization can involve a non-polar solvent as the dispersing medium, by which the hydrophilic component is adsorbed on the surface of the solid, but there are also other situations where this is not the case. For example, the widely used surfactant sodium dodecyl sulfate (SDS) may interact with the surface of NPs through the alkyl chain, leaving the polar sulfate group pointing outwards, thus conferring hydrophilic characteristics to the colloidal particles.¹⁰²  However, this not the most frequent situation, at least in the methods for the synthesis of metal and semiconductor NCs that were developed from the 1980s onwards, in order to produce (quasi-)monodispersed NCs.^46–50  A strategy in which colloidal stabilization using surfactants has been widely used involves the synthesis of NCs in self-assembled amphiphilic molecules that form inverse micelles (W/O: water-in-oil).^103–107  These amphiphilic structures can be regarded as permeable nanoreactors, in the interior of which can occur the nucleation and growth of NCs, their dimensions being confined by the micellar three-dimensional structure itself. The system obtained by using such nanoreactors consists of a hydrophobic colloid containing inorganic cores whose surfaces are stabilized by the surfactant molecules.

Perhaps a more important example of the synthesis of semiconductor and metal NCs with impact on industrial production and applications is the synthesis of NCs using high boiling point surfactant solvents.²³  Often this type of synthesis integrates procedures of colloid chemistry and inorganic and organometallic chemistry, using liquid-phase thermolysis of molecular precursors using a surfactant as the solvent and dispersing medium.¹⁰⁸  These are hot-injection methods based on the chemical reaction of molecular precursors in solvent surfactants with high boiling points. The original method was proposed in 1993 by Murray, Norris and Bawendi for the synthesis of CdS, CdSe and CdTe NCs from organometallic precursors of Cd and chalcogenide compounds as the respective source, using TOPO (tri-n-octylphosphine oxide) as solvent.¹⁰⁹  Subsequently, variants of this method have emerged, and also their application in the synthesis of NCs of other semiconductors and metals, currently comprising a set of synthetic routes to colloidal NCs, with high laboratory and industrial applicability. This subject will be considered later, but its mention now illustrates the relevance of the use of surfactants in the stabilization of technologically relevant colloidal NCs.

The surfaces of colloidal NCs can be coated with a single chemical agent that prevents particle aggregation due to electrostatic repulsions and steric hindrance. This is conveniently illustrated by considering colloidal NCs that have been surface protected with ionizable polymers such as polyelectrolytes.¹¹⁰  The dispersed NCs acquire a surface charge distribution which, depending on the type of polyelectrolyte used, can be negative (polyanions) or positive (polycations). This strategy of stabilization of colloidal NCs is very versatile because it allows the fabrication of multilayered structures by the layer-by-layer (LbL) deposition of oppositely charged polyelectrolytes.^111,112  In addition, the polymeric chains that wrap the NCs also act as chemical barriers that oppose the aggregation of particles by steric hindrance, resulting in very stable colloids due to electrostatic repulsions and steric effects. The polyelectrolytes can be selected depending on the type of NCs, envisaging their use in different chemical environments. There is a wide range of commercially available polyelectrolytes that may be used as supplied or, optionally, they can be chemically functionalized. The latter possibility has been successfully explored in the development of inorganic NCs for biological applications such as in medicine.^113,114 

Polymers adsorbed on the surface of NCs may be inorganic in nature.¹¹⁵  A well-known example is the use of inorganic polyphosphates dispersed in water aiming at the colloidal stabilization of NCs. Polyphosphates are polymeric oxyanions formed from orthophosphate structural units linked together by sharing oxygen atoms (Figure 1.14). Structurally, these polyanions can be visualized as linear or cyclic chains of PO₄³⁻ tetrahedral units joined at the corners (O atoms). Polyphosphates have long been used as colloidal stabilizers and their importance in various fields should be noted here, including inorganic polyphosphate NPs for regenerative medicine.¹¹⁶  Finally, let us just mention how Nature resorted to phosphate units in the synthesis of polymeric structures (DNA) in living organisms.

NCs coated with polymers of synthetic or natural origin have been widely explored to produce stable colloids. Although numerous strategies are available, here we mention just a few for the moment as this topic will be developed further in Chapter 4.^117–121  Hence, polymer-coated inorganic NCs can be produced by using in situ polymerization strategies, resulting in inorganic–organic hybrid structures, as constituents of nanocomposite materials. In these cases, the surfaces of the NCs are previously grafted with chemical functionalities in order to be integrated in the polymeric matrix or, alternatively, as a way to proceed to polymerization reactions occurring at the surfaces. The use of polymers associated with the stabilization of colloidal NCs is not limited to post-synthesis surface treatment processes. Although these processes also comprise strategies for surface modification of previously prepared NCs, they should be distinguished from the methods in which the NCs are obtained in situ in the presence of polymers. In these cases, the macromolecules act as structuring agents in the controlled synthesis of NCs and as colloidal stabilizers, typically using water as a solvent.

The last group of chemical and colloid stabilization strategies comprises the formation of inorganic shells of a material distinct from the starting NCs. The most common are the so-called core–shell nanostructures, which comprise metal or semiconductor NCs as nuclei and protective outer layers of another inorganic material. Common core–shell inorganic structures are those involving the formation of amorphous SiO₂ shells around metal or semiconductor NCs.^122–124  Therefore, this type of nanostructures will be specially addressed in this book, given their relevance in the application of colloidal NCs and also for the pedagogical value associated with their study. Moreover, these nanomaterials are currently commercialized for several applications depending on the type of core properties and surface functionalization. However, there are other hybrid nanostructures that are distinguished by either their morphological characteristics or the chemical nature of the shell, or even both. In the first case, we have structures obtained by the growth of an inorganic phase leading to distinct morphologies, such as anisotropic and hollow-type nanostructures.^125–128  On the other hand, NCs can be coated with materials other than amorphous SiO₂, such as the use of carbon shells (carbon onions) to protect metal NCs against oxidation.¹²⁹  Another important example comes from semiconductor nanotechnology based on colloidal NCs. This is the epitaxial growth of monolayers of a wide-bandgap semiconductor (e.g. ZnS) on the NC surface of a narrow-bandgap semiconductor (e.g. CdSe), which is used as a photoluminescent material, in order to passivate surface defects that act as traps in radiative light emission mechanisms.^130–132 

One of the reasons for the ubiquity of SiO₂ as a shell material is the versatility that this strategy offers for subsequent surface chemical functionalization envisaging different applications. Indeed, one of the great advantages of using colloidal NCs is the ability to use these systems as surface chemically functionalized vectors, providing a large number of interfaces to interact, for example, at biological and environmental levels. In this context, the wealth of information available for chemical derivatization of SiO₂ surfaces, for example gathered from the long experience with chromatographic techniques, can be used and applied in new contexts.

Surface-coated colloidal NCs can be isolated as solid samples from the dispersing medium without loss of their morphological integrity. This is only feasible provided the surfaces are capped with a coating, the molecules of which are not loosely bound and still retain the ability to disperse the solid particulates in a good solvent. For example, TOPO-capped NCs prepared as mentioned above form stable colloids in toluene, but they can also be precipitated when methanol is added to the sol.¹⁰⁹  The powders, once isolated, are easily redispersed again in toluene. Otherwise, these powder samples can be characterized using solid-state techniques, which is an interesting perspective that became increasingly common in colloid chemistry associated with materials development. The X-ray powder diffractograms of NCs show broad bands instead of the narrow peaks that are characteristic of the polycrystalline counterparts. The peak broadening of the corresponding Bragg reflections, assuming that no other factors are contributing, such as defects in the crystalline lattice, is due to the nanometric dimensions of the constituent particles. Take as an example the Cu Kα radiation often used for recording X-ray diffractograms, which has a wavelength of 1.5406 Å, and inorganic NCs whose dimensions range from 10 to 100 Å. Unlike the macrocrystalline samples, in this case the radiation does not probe a large number of crystallographic planes oriented within the same crystalline domain, owing to the high number of interfaces per unit volume of material, whose particle dimensions are just one to two orders of magnitude larger than the incident radiation. The broadening effect in the powder XRD patterns of a crystalline material due to the existence of very small crystallites is expressed in the Scherrer equation:^133,134 

where τ is the average size of the crystalline domains, which may or may not match the average particle size, K is a dimensionless factor dependent on the shape of the crystallite and whose value is close to 1, λ is the X-ray wavelength, β is the full width at half-maximum intensity (FWHM), after subtracting the instrumental line broadening, in radians and θ is the Bragg angle. This equation allows the estimation of the crystallites' dimensions from experimental X-ray diffractograms of the sample once the factors inherent to the instrumental setup are known.

The decrease in the crystallinity in the X-ray diffractograms of nanosized particles, relative to the corresponding bulk material, should therefore be understood as a lower structural order by volume due to the small number of coherently oriented crystallographic planes. However, it is important to retain as a fundamental concept that, despite their small size, NCs have a similar crystalline lattice as the macrocrystalline material of the same substance. That is, these inorganic particles can be synthesized as nanometric fragments of crystalline materials with the same crystal lattice and with exposed faceted surfaces.¹³⁵  Colloidal NCs are usually obtained by liquid-phase synthetic methods, where particle growth kinetics influence the final particle morphology. The existence of faceted surfaces is not as evident in the case of colloidal NCs compared with the macrocrystalline counterparts. NCs with aspect ratios close to unity are often represented by spheres, typically in illustrative schemes. This simplification is partly due to the fact that NCs with aspect ratios close to one can be treated as quasi-spheres, such as in the analysis of transmission electron microscopy (TEM) images. The high-resolution TEM images depicted in Figure 1.15 show polyhedral NCs. Thus, just as NCs with aspect ratios close to one are represented by nanospheres, faceted nanorods and nanometric flat platelets are represented by nanocylinders and nanodisks, respectively. However, the multifaceted nature of inorganic NC surfaces is shown in practice in a number of situations, such as in the selective adsorption due to specific recognition on crystalline faces of catalysts¹³⁶  and in the growth of semiconductor NCs with morphologies resulting from preferential orientational growth due to the adsorption of certain species in specific facets.¹³⁷  Although not always evident, the multifaceted nature of NCs plays a major role in several aspects of the chemistry of NCs.

Ideal crystalline solids are considered to be free from defects when describing their structure and surface. Although this might be convenient when creating certain pedagogical contexts, it must not be forgotten that according to the third law of thermodynamics, defect-free crystalline materials are abstract concepts and do not actually exist. In fact, it is the existence (and control) of certain defects in the crystal lattice that allows the technological application of solids in various applications, such as in semiconductor technologies and solid fuel cells.¹³⁸  Therefore, defects in the interior and on the surface of a crystalline material may or may not be a limiting aspect for a given application. As occurs in macrocrystals at the microstructure level, also in NCs the surface defects not only impact the physical properties of materials but also are determinant in the genesis and growth of the particles. In the case of NCs, the existence of surface defects becomes especially relevant given the high specific surface area, which results in a high number of interfaces when interacting with the surrounding environment. Certain shapes observed in colloidal NCs are achieved by controlling the type of defects in the crystalline facets of embryonic particulates (seeds) used in the growth of those NCs. Surface defects may be of diverse nature, such as vacancies, adsorbed species (adatoms), disrupted chemical bonds and lattice disorder, relative to the ideal situation of a flat and chemically pure crystalline surface at the atomic level. These defects are especially active surface sites that influence the growth and chemical reactivity of the solid. Similarly to macrocrystalline materials, the properties conferred by surface defects depend on the type of defect and its location on the surface of the NC. Nanocrystalline surfaces tend to evolve to a lower free energy state, involving mechanisms whose kinetics are dependent on mass diffusion phenomena that in turn depend on the type of surface defects. The ideal surface model in an elemental crystal, as originally proposed by Walther Kossel and Ivan Nikolov Stranski, continues to be instructive in this context, as it distinguishes the surface energy of a solid by taking into account the different types of defects (Figure 1.16).¹³⁹  In such a crystal, the surface defects are distinguished as terraces, steps (edges) and kinks. Terraces are planar defects that result from the three-dimensional growth of the crystal, distanced from each other by steps, which are associated with line defects (edges) and point defects (kinks). There are also defects corresponding to adsorbed atoms and surface-created vacancies, which may be isolated or form islands, which are very relevant in situations out of equilibrium.

The earlier sections considered fundamental concepts of surface chemistry in general, but which are regarded as particularly relevant when studying colloidal NCs. So far, these aspects have been presented in terms of their relevance for understanding the intrinsic properties of these nanomaterials, such as those that depend on the size and shape and which will be developed later. The past decades have witnessed the development of a large number of methods for the synthesis of colloidal NCs, which allow the manufacture and commercialization of metals and semiconductors in distinct shapes within several size ranges. Well-developed synthesis methods involve surfactant molecules anchored to the surface of NCs, forming an organic coating. This organic cap can in turn be functionalized or modified by post-synthesis methods. The chemical nature of the interfaces that are created between the NCs and the dispersing medium is crucial in various technological applications. Moreover, studies on the impact of nanomaterials on health (nanotoxicology) and the environment (nanoecotoxicology) are strongly guided by surface phenomena.^140,141  For example, understanding the protein corona surrounding NCs dispersed in physiological media is crucial in medical applications among many other applications involving biological systems.¹⁴²  The protein corona is a biointerface that is formed by adsorption of proteins due to their binding affinity to the surface of NCs. The composition and structure of the protein corona depend on many parameters, including the type of surface, that characterize the dispersed NCs. This is an example that emphasizes the importance of studying NC surfaces taking into account target nanotechnology applications, namely the type of biointerfaces formed, which will be described in more detail in Chapter 5.

Surface-mediated phenomena in nanomaterials are determinant in the processing conditions of materials for the manufacture of new products, such as in the metallurgical and ceramic industries. In these cases, the finely divided powders are processed at elevated temperatures, resulting in dense bodies due to sintering of the solid particles. Also, in the conventional electronics industry, top-down approximations are used in device manufacture, for example in the nanopatterning of semiconductor surfaces. These are two examples of processing of inorganic materials using well-established technologies. On the other hand, in recent decades innovative strategies have been developed that rely mainly on a “supra-chemistry” of nanostructures, such as the one that results from the assembly of colloidal NCs as building blocks.^143–145  A large number of these methods are self-assembly processes that occur at a lower temperature than the high temperatures normally used in processing using conventional methods; spontaneous self-assembly of NCs usually occurs under ambient conditions, involving molecular recognition and geometrically driven mechanisms. The term “recent” should be understood in the context of human ingenuity in developing self-assembly methods, otherwise it would be a presumption to forget that Nature has always used such manufacturing methods.^146–148  Indeed, one of the central ideas in nanotechnology is the fabrication of functional materials and devices mimicking Nature in the self-assembly of building blocks with the right size and shape, via specific chemical interactions that include nanoscopic and molecular structures. These processes occur spontaneously in order to reach the minimum energy corresponding to thermodynamic equilibrium or may require external stimuli, under controlled kinetic conditions, leading to metastable states that lead to defect-containing nanostructures. As also occurs in natural processes, ultimately it is intended to develop nanotechnology into a smart technology, with adaptive processing and the possibility of self-correction, especially if the primary function of the device is compromised by the existence of such defects.

In addition to the chemical composition and morphological characteristics of colloidal NCs, the nature of nanocrystalline surfaces is determinant in such assembly methods. It should be recalled that the anatomy of colloidal NCs comprises an inorganic core and a coating of protective organic molecules. Thus, nanostructured materials that result from this process have diverse structural complexity and dimensionality, in addition to their variable chemical composition. In self-assembly methods, short- or long-range interactions are established between the building blocks and other species, such as the molecules of the solvent. Hence self-assembly is mediated by a set of interactions about which there is knowledge based on what occurs in conventional colloidal systems. However, caution is required in this type of reasoning, because colloidal NCs have dimensions that approximate those of molecules and have capped surfaces. In a first approach, assuming the case of quasi-nanospheres, we can generically typify particle–particle interactions that include van der Waals forces, Coulomb-type electrostatic interactions, dipole–dipole interactions and depletion interactions, including those interactions involving surface-coordinated organic molecules. Thus, alongside the synthesis of NCs with well-defined sizes and shapes, controlling the type of interactions that occur between these building blocks allows us to anticipate a new branch of chemistry, which shows some parallel with supramolecular chemistry but in which the superstructures result from the arrangement of fragments of crystal lattices rather than molecular structural units. This is a fascinating area of research that implies a great deal of complexity in itself and it is nowadays in active development.^143–145  There is now a variety of chemical compositions for colloidal crystals, which are 3D ordered arrays composed of monodispersed colloidal NCs as structural units, made by self-assembly processes under controlled conditions.^149–153  In terms of symmetry and the crystal lattice, we have a situation analogous to that found in a crystalline structure of a closely packed solid but where the constituent units of the crystal lattice are NPs rather than atoms. In this case, the organically coated NCs interact via short-range van der Waals forces, forming closed-packed 3D arrays provided that the NCs are nearly monodispersed. The formation of well-ordered assemblies is not limited to the use of just one type of colloidal NC, and more challenging, composite structures comprising distinct types of materials can be made. An instructive example is the synthesis of colloidal binary supracrystals whose structural lattices are tuneable by modifying the growth conditions employed. Noteworthily, the control of the surface chemistry is crucial to achieving such types of metamaterials, because distinct colloidal NC systems exhibit different surface chemistry and interact differently, under the same experimental conditions, compared with the respective unary systems. Wang et al. described free-standing binary supracrystals composed of organically coated Au (5 nm) and Fe₃O₄ (10 nm) NCs (Figure 1.17).¹⁵⁴  They developed an oil-in-water emulsion process in which the droplets act as nanoreactors by confining the long alkyl chain-coated NCs. The co-assembly of the Au and Fe₃O₄ NCs occurs by slow evaporation of the oil phase. The method described allows the preparation of a variety of stable lattices and stoichiometries, by varying the growth conditions, and can be applied to other types of NCs.

In this chapter, we have presented fundamental concepts concerning colloidal particulates that we feel are relevant to exploring the surface chemistry of colloidal NCs. In particular, we hope that this introduction to the nature of the surfaces of NCs will facilitate the reading of the following chapters, which review specific topics related to the thematic of the book. We have also attempted to highlight some selected scientific landmarks, which is always an incomplete task and debatable to some extent, but helps to put into perspective the development of the surface chemistry of nanosized matter along a timeline. Finally, we selected illustrative examples of how the nature of the surfaces at the nanoscale impact on the intrinsic properties of the NCs and metamaterials obtained therefrom, with a deep understanding of their surface chemistry.