Iron Oxides: An Example of Structural Versatility

Iron is Earth’s fourth most widespread element (6.2% in mass), behind oxygen, silicon, and aluminum. It exists mostly as ferric oxide and oxyhydroxide (Fig. 7.1a) and to a lesser extent as sulfide (pyrite), carbonate (siderite), and silicate (fayalite). Iron oxides are largely used in technological areas such as metallurgy, colored pigments, magnetic materials, and catalysts. They also play an important role in the environment because the dissolution of ferric oxides in natural waters, promoted by acid–base, redox, photochemical phenomena, and also microbial mediation, allows iron to be involved in many biogeochemical processes. Iron is present in many living organisms such as plants, bacteria, mollusks, animals, and humans in various forms: . . . Porphyrinic complexes of iron, which are active centers of hemoglobin and several ferredoxins involved in biological functions, especially respiration mechanism and photosynthesis. Nanoparticles of amorphous ferric oxyhydroxides in animal and human organisms as ferritin, which allows regulation and storage of iron and in various nanophases present in plants as phytoferritin. Crystalline iron oxy(hydroxi)des produced by biomineralization processes. Goethite, lepidocrocite, and magnetite are the main constituents of radulas and the teeth of mollusks (limpets, chitons). Magnetite nanoparticles produced by magnetotactic bacteria (Fig. 7.1b), as well as by bees and pigeons, are used for purposes of orientation and guiding along the lines of force of the Earth’s magnetic field. Green rusts are also ferric- ferrous compounds belonging to the biogeochemical cycle of iron. . . . The crystal chemistry of iron oxy(hydroxi)des is very rich. The ferric, ferrous, and mixed ferric- ferrous oxygenated compounds correspond to around a dozen crystal structural types (Fig. 7.2). Most of these crystal phases can be synthesized from solutions in the laboratory, giving rise to a most diversified chemistry. They are also formed in nature because of the large variability of physicochemical conditions: an acidity range from around pH 0 to 13; redox conditions from oxic to totally anoxic media; bacterial activity that can be extremely intense; salinity largely varying from almost pure waters to real brines; presence of many organic and inorganic ligands; and various photochemical processes.

Condensation in Solution: Polycations and Polyanions

Condensation of metal complexes in solution forms entities in which the cations are linked by hydroxo (HO−) or oxo (O2−) bridges. The reaction is initiated by the addition of a base to an aquocomplex: . . . 2[Cr(OH2)6]3++ 2HO- → [Cr2(OH)2(OH2)8]4+ + 2 H2O . . . or by the addition of an acid to an anionic complex: . . . 2 [CrO4]2- + 2H+ → [Cr2O7]2- + H2O . . . Thus, purely aquo- and purely oxocomplexes are stable in solution, and the condensation of cations is initiated by hydroxylation. With regard to electrically charged hydroxylated complexes, the reaction forms discrete and soluble entities—polycations and polyanions with a molecular complexity which depends on acidity conditions. This chapter presents a detailed study of their formation and structure. With regard to noncharged hydroxylated complexes, the condensation reaction is no longer limited and leads to the formation of a solid (a subject that is examined in the following chapters). The hydroxylation reaction is the key stage to initiate the condensation of cations in solution. It is thus important to precise the mechanism of the successive steps of the process, in order to understand why the behavior of a cation is closely related to its oxidation state, and why the reaction product may be a discrete molecular species or a solid. As a cation generally exhibits its maximum coordination number in the initial monomeric complex and in condensed species, the condensation reaction is a substitution that proceeds according to one of three basic mechanisms: dissocia­tion, association, and interchange or direct displacement [1, 2]. Dissociative substitution is a two-step process involving the formation of a reduced-coordination intermediate: In the first step, a labile ligand, the leaving group, breaks its bond in the starting complex before a nucleophilic entering group completes, in the second step, the cation coordination (Fig. 3.1 a). Associative substitution is also a two-step process in which the intermediate temporarily has increased coordination. The bond with the nucleophilic entering group (first step) occurs prior to the release of the leaving group (second step) (Fig. 3.1 b).

Aluminum Oxides: Alumina and Aluminosilicates

Aluminum is the third most abundant element in Earth’s crust (8.3% in mass), behind oxygen (45.5%) and silicon (27.2%). It forms in nature various oxygenated mineral phases: hydroxides Al(OH)3, oxyhydroxides AlOOH, of which bauxite is the main ore, and oxides, Al2O3, alumina. Corundum, α- Al2O3, is the component of many gems: sapphire (pure Al2O3, perfectly colorless), ruby (red colored due to the presence of Cr3+ ions), and blue sapphire (blue colored by the presence of Ti4+ and Fe2+ ions), among many others. The content of foreign elements substituted for Al3+ ions in these phases accounts for only a small percentage of the total. Aluminum also forms many natural phases in combination with various elements, especially silicon in aluminosilicates, such as feldspars, clays, zeolites, allophanes, and imogolites. The biochemical cycling of the elements involves many soluble complexes of aluminum in natural waters [1, 2]. Aluminum oxides and oxy(hydroxi)des are important materials and nanomaterials used in many fields: for instance, as active phase for adsorption in water treatment; as inert support and active phase in catalysis; as active phase in flame-retardant polymers; as refractory material for laboratory tools and in the ceramics industry; and as abrasives [3, 4]. Alumina Al2O3 is produced in various forms (tubes, balls, fibers, and powders) for numerous industrial uses (laboratory tools, filtration membranes, ball bearings, fine powders as catalysis supports, etc.). The structural chemistry of aluminum oxy(hydroxi)des is rich. There are various hydroxides, Al(OH)3 (gibbsite, also named hydrargillite, bayerite, and some other polytypes such as nordstrandite and doyleite), oxyhydroxides, AlOOH (boehmite and diaspore), and a series of oxides, Al2O3, so-called transition aluminas. These last phases have different degrees of hydration and different degrees of order of the Al3+ cations within the cubic close packing of oxygen atoms according to the temperature at which they have been submitted. They belong to various structural types (γ, δ, θ, η, κ, etc.). These aluminas of huge specific surface areas are usually used in catalysis, especially γ-alumina of spinel crystal structure.

Conclusion

Metal oxide nanostructures are of major interest in technology. It is therefore es­sential to have a full understanding of the phenomena involved in the aqueous synthesis of nanoparticles, so that their properties can be adjusted to a desired application. Understanding these phenomena is also important in other fields, for instance, in geology and environmental sciences, enabling us to explain the presence and formation of a given mineral. The precipitation of metal oxy(hydroxi)des is a complex phenomenon initiated by hydroxylation of the cations in solution and resulting from condensation of the hydroxylated species. Therefore, the acidity of the cations is the main charac­teristic of their reactivity. Three main parameters are essential in predicting and rationalizing the behavior of metal cations in water: the formal charge (the oxida­tion degree), size, and electronegativity, which determine the degree of polariza­tion of the oxygenated ligands. One may thus define five classes of cations: . . . The too weakly polarizing cations that form only aquocomplexes unable to condense and to precipitate; for instance, the alkaline cations M+, The cations that condense by olation and form polycations and hydroxides, typically, the divalent cations and also Al3+. The cations that condense by olation and oxolation and form oxyhydroxides and oxides (such as Cr3+, Fe3+, and Mn3+). The cations that condense essentially by oxolation and form oxides that are more or less hydrated (Ti4+, Mn4+, V5+). The cations that form anionic oxocomplexes and exhibit no trend toward condensation, typically, MnVII. . . . This series thus includes cations of increasing polarizing power, that is cations of increasing oxidation degree and electronegativity. Precipitation usually generates nanosized particles. In a system that is not con­tinuously fed, in which a limited amount of matter is available in the reactor, the nucleation step is always sudden and easy enough, allowing lower supersaturation and creating nuclei that have stopped growing because of the too low concentra­tion in soluble precursor. That does not, however, exclude an intense dynamics of dissolution–crystallization because of the evolution of the criticality of the particle size during the decrease in supersaturation.

Water and Metal Cations in Solution

Water has an exceptional ability to dissolve minerals. It is safe and chemically stable, and it remains liquid over a wide temperature range. Thus, it is the best solvent and reaction medium for both laboratory and industrial purposes. Water is able to dissolve ionic and ionocovalent solids because of the high polarity of the molecule (dipole moment μ = 1.84 Debye) as well as the high dielectric constant of the liquid (ε = 78.5 at 25°C). This high polarity allows water to exhibit a strong solvating power: that is, the ability to fix onto ions as a result of electrical dipolar interactions. Water is also an ionizing liquid able to polarize an ionocovalent molecule. For example, the solvolysis phenomenon increases the polarization of the HCl molecule in aqueous solution. Finally, owing to the high dielectric constant of the liquid, water is a dissociating solvent that can decrease the electrostatic forces between solvated cations and anions, allowing their dispersion as H+solvated and Cl−solvated through the liquid. (The attractive force F between charges q and q′ separated by the distance r is given by Coulomb’s law, F = qq′/εr2.) These characteristics are rarely found together in common liquids. The dipole moment of the ethanol molecule (μ = 1.69 Debye) is close to that of water, but the dielectric constant of ethanol is much lower (ε = 24.3). Ethanol is a good solvating liquid, but a poor dissociating one; consequently, it is considered a bad solvent of ionic compounds. Dissolution in water of an ionic solid such as sodium chloride is limited to dipolar interactions with Na+ and Cl− ions and their dispersion in the liquid as solvated ions, regardless of the pH of the solution. Cations with higher charge, especially cations of transition metals, retain a fixed number of water molecules, thereby forming a true coordination complex [M(OH2)N]z+ with a well-defined geometry. In addition to the dipolar interactions, water molecules behave as true ligands because they are Lewis bases exerting an electron σ-donor effect on the empty orbitals of the cation.

Nanomaterials: Specificities of Properties and Synthesis

The concept of material concerns matter in solid state that is endowed with usable properties for practical applications. It is indeed in the solid state that matter exhibits the highest mechanical strength and chemical inertness, providing solidity and sustainability because the solid is based on an extended stiff crystalline framework. It is also in the solid state that many properties exist, including optical, electrical, and magnetic properties, providing great technological progress. A typical example is electronics which owes its enormous development to doped silicon. A material may therefore be defined as a useful solid. The properties of a solid depend directly on its chemical composition, crystalline and electronic structures, texture, as well as morphology and casting. This last point, which is often neglected, is illustrated by amorphous silica glass, which is used largely for its properties such as chemical inertness, mechanical strength, optical transparency, and low thermal and electrical conductivities. These various properties are highlighted through the many possibilities of casting and shaping: flat glass (optical transparency for glazing); hollow glass (chemical inertness and mechanical strength for bottling); short fibers (glass wool for heat insulation) and long fibers (optical fibers); massive pieces (insulators for electric power lines); and thin films (insulating layers for miniaturized electronics). Metal oxides exhibit a wide range of exploitable properties useful for innumerable applications. Silica, SiO2, as flat glass, has excellent optical properties, but other oxides such as LiNbO3 and KTiOPO4 exhibit interesting nonlinear optical properties, allowing changes in the wavelength of the transmitted light. Certain oxides are good electrical insulators (SiO2), but others are true elec­tronic conductors (VO2, NaxWO3), ionic conductors (β-alumina NaAl11O17, NaSiCON Na3Zr2PSi2O12, yttria-stabilized cubic zirconia Zr1–xYxO2–x/ 2), and also superconductors (cuprates such as YBa2Cu3O7–x and Bi4Sr3Ca3Cu4O16+x). Compounds such as BaTiO3, PbZr1–xTixO3, and PbMg1/3Nb2/3O3 are ferroelectric solids used largely as miniaturized electronic components, whereas spinel ferrite γ-Fe2O3, barium hexaferrite BaFe12O19, and garnet Y3Fe5O12 are more or less coercive ferrimagnetic solids used in magnetic recording or as permanent magnets.

Titanium, Manganese, and Zirconium Dioxides

The dioxides of titanium (TiO2), manganese (MnO2), and zirconium (ZrO2) are important materials because of their technological uses. TiO2 is used mainly as white pigment. Because of its semiconducting properties, TiO2, in its nanomaterial form, is also used as an active component of photocells and photocatalysis for self-cleaning glasses and cements . MnO2 is used primarily in electrode materials. ZrO2 is used in refractory ceramics, abrasive materials, and stabilized zirconia as ionic conductive materials stable at high temperature. Many of these properties are, of course, dependent on particle size and shape (§ Chap. 1). Dioxides of other tetravalent elements with interesting properties have been studied elsewhere in this book, especially VO2, which exhibits a metal–isolator transition at 68°C, used, for instance, in optoelectronics (§ 4.1.5), and silica, SiO2 (§ 4.1.4), which is likely the most ubiquitous solid for many applications and uses. Aqueous chemistry is of major interest in synthesizing these oxides in the form of nanoparticles from inorganic salts and under simple, cheap, and envi­ronmental friendly conditions. However, as the tetravalent elements have re­stricted solubility in water (§ 2.2), metal–organic compounds such as titanium and zirconium alkoxides are frequently used in alcoholic solution as precursors for the synthesis of TiO2 and ZrO2 nanoparticles. An overview of the conversion of alkoxides into oxides is indicated about silica formation (§ 4.1.4), and since well-documented works have already been published, these compounds are not considered here. The crystal structures of most MO2 dioxides are of TiO2 rutile type for hexacoordinated cations (e.g., Ti, V, Cr, Mn, Mo, W, Sn, Pb) and CaF2 fluorite type for octacoordinated, larger cations (e.g., Zr, Ce), but polymorphism is common. Some dioxides of elements such as chromium and tin form only one crystal­line phase. So, hydrolysis of SnCl4 or acidification of stannate [Sn(OH)6]2− leads both to the same rutile-type phase, cassiterite, SnO2. Many other dioxides are polymorphic, especially TiO2, which exists in three main crystal phases: anatase, brookite, and rutile; and MnO2, which gives rise to a largely diversified crystal chemistry.

Precipitation: Structures and Mechanisms

The formation of a solid, and especially an oxide, from soluble metal complexes is usually called “precipitation.” This term is a generic name that includes a set of complex and intricate phenomena. The process is governed by thermodynamic, structural, and kinetic contingencies, which should be examined in detail to understand the role of synthesis conditions and their influence on the solid obtained. The chemistry of the process involves a condensation reaction, olation or oxolation, between uncharged hydroxylated complexes. It forms particles of widely variable size over the nano- to micrometric range. These particles are portions of a solid identifiable in using techniques such as X-ray diffraction, absorption, and diffusion, electron microscopy, light-scattering, and various spectroscopies. Of course, these particles have the properties typical of the corresponding bulky solid, but they may be modulated because of the size effect, especially in the nanometric range (Chap. 1). Because of their small size, these objects have a large surface area highlighting their surface physicochemistry, such as ability to disperse in aqueous or nonaqueous medium, to aggregate, and to fix various species from solution, that allows the surface energy to be controlled to adjust the shape and size of these objects (Chap. 5). The crystal structure of polymorphic solids can also be controlled by the choice of the pathway of their formation. Thus, knowledge of the processes involved allows us to exploit the large versatility of the nanostructures synthesized in solution. This chapter has two main objectives. The first is to show that the crystalline structure of the solid may in many cases be anticipated from the characteristics of the precursor in solution, such as functionality, geometry, reactivity, and elec­tron configuration. This point concerns the structural aspect of the formation of the solid. The second objective is to understand why precipitation forms small particles, generally of nano- or micrometric size, and how the crystallization mechanism influences their morphology. These questions concern the kinetics and dynamics of the precipitation phenomenon.

Surface Chemistry and Physicochemistry of Oxides

Oxide particles resulting from precipitation have at least one dimension less than a few nanometers. Therefore, as their specific area (surface-to-mass ratio) may reach several hundred square meters per gram, the behavior of these particles is closely related to their surface physical-chemical characteristics. Thus, the dispersion state of particles in solution is dependent on attractive and repulsive forces between surfaces. The balance control of these forces limits the aggregation of particles and promotes the formation of sols or gels, or, contrariwise, flocculates the particles and separates them from a suspension. The divison state of solids resulting from precipitation is ruled by forces that exert themselves onto the surface (interfacial—or surface—tension). They determine the extent of the surface area and, therefore, the particle size. Adsorption of ions or molecules within the dispersion depends on forces exerting between soluble species and the surface. These forces may be due to electrostatic charges on the surface. They may also be due to the ability of the surface cations to be coordinated by soluble species and/or the ability of surface oxygenated groups to coordinate cations from solution. The attachment of various species on the surface of oxide particles plays a major role in various fields—for instance, the transport of matter in natural or industrial waters, catalysis and corrosion phenomena, formation of stable and homogeneous dispersions. It is somewhat difficult to characterize the surface of nanometer-sized objects from structural as well as chemical standpoints. The geometry of such small particles is not easily defined with precision, and the surface often includes defects such as steps, truncations, and stacking faults. These sites are difficult to recognize but exhibit largely variable chemical reactivities. In addition, the study of the oxide– solution interface is complicated because few of its physical quantities are experimentally accessible. These quantities are treated as fitting parameters in more or less complex modelings. The current state of the art, however, allows suit­able interpretation of experimental data.