CHAPTER 5 Silica and Titania Nanodispersions J. Nestor, J. Esquena Institute of Advanced Chemistry of Catalonia, Department of Chemical and Biomolecular Nanotechnology, Spanish National Research Council (IQAC-CSIC) and CIBER on Bioengineering, Biomaterials and Nanomedicine (CIBER-BBN), Barcelona, Spain 5.1 Introduction Tremendous effort devoted to nanomaterials has resulted in a rich database for their synthesis, properties, modification, and application. Continuous breakthroughs in the synthesis and modification of nanomaterials have brought new properties and new applications with improved performance. The synthesis and processing of fine ceramic particles at the nanometer scale have recently received considerable attention owing to their interesting optical, electronic, and catalytic properties. Significant progress has been made in the preparation of a large number of inorganic colloids consisting of particles with different chemical compositions, sizes, and morphologies. However, a number of questions still have to be answered satisfactorily, including chemical and physical mechanisms of their formation and growth, as well as the control of particle morphology. This chapter provides insight into silica (SiO2) and titania (TiO2) nanoparticles (NPs). Silicon dioxide, universally known as silica, can be natural or synthetic, crystalline or amorphous, and it has been used in glass manufacturing since ancient times. Silica is highly ubiquitous and abundant, being the major constituent of sand. It is most commonly found in nature as quartz and is present in various living organisms such as diatoms.1 However, silica is also a highly complex material, existing in many different mineral forms in nature and also being synthetically produced. Notable examples of synthetic silica include fused quartz, fumed silica NPs, and silica hydrogels, xerogels, and aerogels.2 Applications range from semiconductor devices to ceramics and metallurgy, to name just a few. This chapter deals mostly with synthetic amorphous silica in colloidal dispersions. All forms of silica contain the SieO bond, which is the most stable of all SieX covalent bonds. The SieO siloxane bond is just 0.162 nm long, which is considerably shorter than the sum of covalent radii of silicon and oxygen atoms (0.191 nm).3 The short length of the siloxane bond largely accounts for its partial ionic character, and it is responsible for its Nanocolloids. http://dx.doi.org/10.1016/B978-0-12-801578-0.00005-9 Copyright © 2016 Elsevier Inc. All rights reserved. 159 160 Chapter 5 high chemical stability. Silica NPs can be prepared by many different techniques. This chapter focuses on methods used to prepare colloidal silica, which refers to stable dispersions of discrete amorphous silica particles. Special attention is devoted to solegel chemistry and processing. Another inorganic oxide of great interest is titanium dioxide (TiO2), also known as titania. It is being commercially manufactured at the scale of millions of tons because of its chemical stability, low cost, availability, and its remarkably high refractive index (w2.4). TiO2 is widely used as a white pigment in paints, as an ultraviolet (UV) absorber in sunscreen creams, and as an abrasive in toothpastes. Since 1972, when Fujishima and Honda4 discovered the photocatalytic splitting of water on TiO2 electrodes, TiO2 constitutes the archetypal photocatalyst in heterogeneous photocatalysis because of its relatively high efficiency and chemical stability.5 The discovery of this phenomenon generated a tremendous amount of research related to TiO2 photocatalysis, including solar fuels and environmental remediation. TiO2 has three common crystalline polymorphic phases: rutile (tetragonal), anatase (tetragonal), and brookite (orthorhombic). Rutile and anatase are the two most common crystal structures. Although all of them are found in nature, rutile is the most abundant since it is the only thermodynamically stable mesomorphic phase.6 Rutile is usually commercially used as a white pigment, whereas anatase is used because of its photocatalytic performance. Anatase is considered the most important phase of TiO2 in terms of trade, but it rarely exists in nature in pure form. This phase is only stable at temperatures lower than 500 C and becomes rutile at temperatures higher than 600 C. Anatase is used as a photocatalyst for many purposes, especially in water and air treatment, removing pollutants, decomposing microorganisms such as bacteria and viruses, and removing organic contaminants, such as phenol and its derivatives, as well as textile paint formulations. TiO2 NPs possess interesting optical, dielectric, and photocatalytic properties. Therefore many efforts have sought to produce TiO2 NPs with controlled size, morphology, and porosity for thin films, ceramics, composites, and catalysts. 5.2 Synthesis Methods Based on SoleGel Processing in Solution Solegel is a rather old term that has been used since the 1930s to describe the synthesis and processing of glass and ceramics.7 Initially, solegel referred to the formation of solid suspensions in a liquid phase (denoted as “sols”) and the formation of a covalently cross-linked network with a very high viscosity (denoted as “gels” or, preferably, “hydrogels”), forming a soft material that can be casted into molds. Nowadays, the term solegel refers to synthesis based on two consecutive reaction steps: (1) formation of solutions of solvated metal precursors, generally by hydrolyzation reactions; and (2) polycondensation or polyesterification of the hydrolyzed species. Solegel processing Silica and Titania Nanodispersions 161 allows advanced materials to be fabricated in a wide variety of forms: ultrafine or spherical powders, thin-film coatings, fibers, porous or dense materials, and extremely porous aerogel materials.2 An overview of various solegel processes is illustrated in Fig. 5.1. Stabilized under proper conditions, particle dispersions (sols) do not become a gel or settle even after several years of storage, and they may contain up to about 50% silica, with particle sizes up to 300 nm, although particles larger than about 70 nm slowly settle. Pioneering work in silica chemistry by Iler1 led to the industrial development of colloidal silica dispersions, commercially known as Du Pont’s Ludox silica. Stöber et al.8 extended Iler’s findings to show that using ammonia as a catalyst for the tetraethyl orthosilicate (TEOS) hydrolysis reaction, one could control precisely the size of microparticles, yielding so-called Stöber spherical silica microparticles,8 Figure 5.1 Schematic representation of a solegel processes for synthesis of nanomaterials. Adapted with permission from Brinker, C. J.; Scherer, G. W. SoleGel Science: The Physics and Chemistry of SoleGel Processing; Academic Press: London, 1990. 162 Chapter 5 which are nonporous particles with a narrow size distribution (practically monodisperse) and a smooth surface. The most important advantage of solegel processing is that it allows extremely pure oxide materials to be formed, which is a factor that is not only an advantage, but a necessity for applications in electronics and high-energy optics. Other potential advantages of solegel particles over particles obtained by other methods are controlled size and morphology, molecular scale homogeneity,9e11 and enhanced reactivity because of lower processing temperatures. For successful commercial applications, these advantages must outweigh inherent disadvantages such as higher costs, lengthy processing times, and low yields. Mackenzie12e14 summarizes a number of potential advantages and disadvantages and the relative economics of solegel methods. Hench and colleagues15,16 compare quantitatively the merits of solegel-derived silica materials for optics over alternative high-temperature processing methods. The most widely used precursors in solegel processing are alkoxides (M(OR)n), because they are stable at standard conditions they are easily hydrolyzed with water, and they do not generate toxic by-products. This family of precursors has been widely used to obtain a large variety of inorganic oxide NPs, including TiO2,17,18 Al2O3,19 Fe2O3,20 and YBa2Cu3O7 superconductor.21 5.2.1 Synthesis of Silica NPs by Hydrolysis of Alkoxides Solegel processing In the case of silica, the most-used alkoxide precursor is TEOS, since it is stable at room temperature in normal laboratory conditions. The solegel reaction of alkoxides consists of two steps: hydrolysis and polycondensation. Hydrolysis follows the following reaction2,22: SiðORÞ4 þ n H2 O/SiðORÞ4 nðOHÞn þ 4 n ROH [5.1] The fully hydrolyzed product is silicic acid (Si(OH)4), which is rather unstable and polycondensates, producing species with high molecular mass. The condensation of the hydrolyzed species is described by the reactions 2 SiðORÞ3 OH/ðORÞ3 Si O SiðORÞ3 þ H2 O ðORÞ3 Si OR þ HO SiðORÞ3 /ðORÞ3 Si O SiðORÞ3 þ ROH [5.2] [5.3] These three reactions normally take place simultaneously, leading to the formation of siloxane (SieOeSi) bridges and finally producing highly pure silica. The overall reaction is SiðORÞ4 þ 4 H2 O/SiO2 þ 4 ROH [5.4] Silica and Titania Nanodispersions 163 In this reaction a solvent with intermediate hydrophilicity, such as an alcohol, is required since water and alkoxysilanes are mutually immiscible, as shown in the TEOS/ethanol/ water ternary phase diagram in Fig. 5.2. However, gels can be prepared in silicon alkoxideewater mixtures without added a solvent, since the alcohol released as a by-product of the hydrolysis is sufficient to homogenize the initially phase-separated system. The formation of silica hydrogels was described long ago by Carman23 and studied in detail by Iler.1 According to this model, silica primary particles are formed by hydrolysis of a precursor and successive polymerization. Nucleation of primary particles is produced, and particle nuclei evolve, forming a stable suspension (sol) at a pH between 7 and 10 and in the absence of salt. Otherwise, at pH <7 and/or in the presence of electrolytes, the primary particles aggregate, producing a three-dimensional (3D) network (gel). The gel structure can be controlled depending on the size of the primary particles when the solegel transition occurs. Figure 5.2 Ternary phase diagram of tetraethyl orthosilicate/distilled alcohol (95% ethyl alcohol, 5% water)/ water at 25 C. For pure ethanol, the miscibility line is shifted slightly to the right. Reproduced with permission from Brinker, C. J.; Scherer, G. W. SoleGel Science: The Physics and Chemistry of SoleGel Processing; Academic Press: London, 1990. 164 Chapter 5 Nuclear magnetic resonance (NMR) results provides further information about the mechanism, demonstrating that condensation takes place in such a way that maximizes the number of SieOeSi bonds and minimizes the number of terminal hydroxyl groups through internal condensation. Thus siloxane rings are quickly formed, to which monomers are added, creating 3D particles. These particles condense to the most compact state, leaving silanol OH groups on the outside. According to Iler,1 the 3D species serve as nuclei. Further growth occurs by an Ostwald ripening mechanism whereby particles increase in size and decrease in number, as highly soluble small particles dissolve and reprecipitate on larger, less soluble nuclei. The growth stops when the difference in solubility between the smallest and largest particles becomes only a few parts per million. As a consequence, this mechanism makes it possible to obtain monodisperse silica particles. Summarizing the Iler mechanism, silica gels are formed as a result of three steps that can occur simultaneously: 1. polymerization of monomers to form NPs 2. growth of NPs 3. Linking of NPs into chains, thus forming three-dimensional networks that extend throughout all the liquid medium and thicken into a gel Solegel processes are modulated by different variables such as pH and the dielectric constant of the medium. It has been shown that at a pH between 7 and 10, and in the absence of electrolytes, silica particle growth follows a “monomer cluster” type, leading to stable colloidal dispersions without particle aggregation. However, at a pH <7 (or a higher pH in the presence of salts) the growth can be defined as “clusterecluster,” leading to the aggregation of particles and forming a hydrogel tridimensional network, instead of stable discrete particles. Catalysis and control of solegel reactions Hydrolysis and condensation can be catalyzed in an acidic or basic medium. Furthermore, strongly nucleophile anions such as fluoride ions (F) can accelerate these reactions. Hydrolysis reactions Hydrolysis described in Eq. [5.1] corresponds to a nucleophilic substitution of alkoxy groups (OR) by hydroxyl group (OHe) following an SN2-type mechanism. Although hydrolysis can occur without the addition of an external catalyst, this reaction is more rapid and efficient when a catalyst is used. Various authors24,25 described hydrolysis of alkoxy silanes as a function of pH (Fig. 5.3). Hydrochloric acid and ammonia are generally used instead of other catalysts such as acetic acid, potassium hydroxide, amines, potassium fluoride, and hydrofluoric acid.2 Silica and Titania Nanodispersions 165 Figure 5.3 pH rate profile for hydrolysis in an aqueous solution, showing the hydrolysis kinetic constant k. Reproduced with permission from Pohl, E. R.; Osterholtz, F. D. In Molecular Characterisation of Composites Interfaces; Kruna, G., Ishida, H., Eds.; Plenum: New York, 1985. Furthermore, it has been shown that the rate and extent of the hydrolysis reaction is influenced mostly by the strength and concentration of the acid or base catalyst.26 Acid-catalyzed hydrolysis mechanism Aelion et al.26 found that all strong acids behave similarly, whereas weaker acids require longer reaction times to achieve the same extent of alkoxide hydrolysis. From a plot of the logarithm of the hydrolysis rate constant versus acid concentration, a slope of 1 was obtained. They concluded that the reaction was of the first order in acid concentration. Under acidic conditions, the alkoxide group is protonated in a rapid first step. Electron density is depleted from the silicon atom, making it more electrophilic and thus more susceptible to attack from water. This results in the formation of a penta-coordinate transition state with a significant SN2-type character. The transition state decays by displacement of an alcohol and inversion of the silicon tetrahedron, as seen in Fig. 5.4. Base-catalyzed hydrolysis mechanism Under basic conditions, the hydrolysis reaction was found to be of the first order in [OH] concentration. However, if TEOS concentration is increased, the reaction deviated from a simple first-order to a more complex second-order reaction. If weaker bases are used, such as ammonium hydroxide and pyridine, measurable speeds of reaction were obtained only if large concentrations were present. Therefore, compared with acidic conditions, hydrolysis kinetics at a basic pH are more strongly affected by the nature of the solvent. In any case, base-catalyzed hydrolysis of silicon alkoxides take places more slowly than acid-catalyzed hydrolysis at an equivalent catalyst concentration.26 Basic alkoxide oxygens 166 Chapter 5 Figure 5.4 Acid-catalyzed hydrolysis. Reproduced with permission from Brinker, C. J.; Scherer, G. W. SoleGel Science: The Physics and Chemistry of SoleGel Processing; Academic Press: London, 1990. Copyright 1990 Elsevier Inc. tend to repel the nucleophile OH. Once an initial hydrolysis has already occurred, however, the following condensation reactions progress steadily, with each subsequent alkoxide group eliminated from the monomer more easily than the previous one.27 Therefore, partially hydrolyzed alkoxides are more prone to attack. In addition, hydrolysis of the forming polymer is more sterically hindered than the hydrolysis of a monomer. Although hydrolysis in alkaline environments is slow, it still tends to be complete and irreversible.28 Thus, under basic conditions, hydroxyl anions attack silicon atoms. Again, a SN2-type mechanism has been proposed in which the eOH displaces eOR, with inversion of the silicon tetrahedron. This is seen in Fig. 5.5. Influence of the water-to-silicon molar ratio (r) on hydrolysis and condensation of silicon alkoxides Hydrolysis is usually performed with r values ranging from less than 1 to over 50, depending on the desired polysilicate product. From Eq. [5.2] it is inferred that increasing r is expected to promote the hydrolysis reaction. Aelion et al.26 described that acid-catalyzed hydrolysis of TEOS is of the first order in water concentration. However, they also observed an apparent zero-order dependence of the water concentration under Figure 5.5 Base-catalyzed hydrolysis. Reproduced with permission from Brinker, C. J.; Scherer, G. W. SoleGel Science: The Physics and Chemistry of SoleGel Processing; Academic Press: London, 1990. Silica and Titania Nanodispersions 167 basic conditions. This is probably a result of the production of monomers by siloxane bond hydrolysis and redistribution reactions (ie, the reverse reactions of those described in Figs. 5.4 and 5.5. Nonetheless, the most obvious effect of the increased value of r is the acceleration of the hydrolysis reaction. In addition, higher values of r cause the rather complete hydrolysis of monomers, before significant condensation occurs. In any case, the overall process is complex, since the different extents of monomer hydrolysis influence the relative rates of the alcohol- or water-producing condensation reactions. In general, with under-stoichiometric additions of water (r 2), the alcohol-producing condensation mechanism (Eq. [5.3]) is favored, whereas the water-forming condensation reaction (Eq. [5.2]) is favored when r > 2.29 Although increased values of r generally promote hydrolysis, when r is increased while maintaining a constant solvent-to-silicate ratio, the silicate concentration is reduced. This in turn reduces the hydrolysis and condensation rates, resulting in longer gel times. Therefore, two opposing factors define gelification kinetics. These were described by Klein,30 who studied gel times for acid-catalyzed TEOS systems as a function of r (H2O-to-Si molar ratio) and the initial ethyl alcoholetoeTEOS ratio. He found that gelification time is minimal at H2O/TEOS ¼ 4, in the case of ethanol/TEOS ¼ 1. Finally, since water is the by-product of the condensation reaction (Eq. [5.2]), large values of r promote siloxane bond hydrolysis (the reverse of Eq. [5.1]). Condensation reactions for the formation of silica As mentioned before, polymerization to form siloxane bonds occurs by either an alcohol-producing or a water-producing condensation reaction. Engelhardt et al.31 showed that a typical sequence of condensation products is monomer, dimer, linear trimer, cyclic trimer, cyclic tetramer, and higher-order rings (Fig. 5.3). This sequence of condensation requires both depolymerization (ring opening) and the availability of monomers that are in solution equilibrium with the oligomeric species and/or are generated by depolymerization. Influence of pH The rate of these ring-opening polymerizations and monomer-addition reactions is dependent on pH. In polymerizations below a pH of 2, condensation rates are proportional to the [Hþ] concentration. Because the solubility of silica is quite low below a pH of 2 (see Fig. 5.8), formation and aggregation of primary silica particles occur simultaneously, and ripening (growth of a network) contributes little to growth after particles exceed 2 nm in diameter. Thus, 3D gel networks comprising small primary particles are developed. It is generally agreed that between pH 2 and pH 6, condensation rates are proportional to [OH] concentrations. Condensation preferentially occurs in highly condensed species, reacting with less highly condensed silica species, which are somewhat neutral. 168 Chapter 5 This suggests that the rate of dimerization is low. Once dimers form, however, they react preferentially with monomers to form trimers, which in turn react with other monomers to form tetrameters. Cyclization occurs because of the proximity of the chain ends and the substantial depletion of the monomer population. Further growth occurs by adding lower-molecular-weight species to more highly condensed species and by aggregation of the condensed species to form chains and networks. The solubility of silica in this pH range is again low, and particle growth stops when the particles reach 2e4 nm in diameter.28 Polymerization of silica species occurs rapidly at pH values above 2. However, at higher pH values (mainly above pH 7), condensed species are strongly ionized and therefore mutually repulsive. Thus growth occurs primarily through the addition of monomers to the more highly condensed particles rather than by particle aggregation. However, because of the greater solubility of silica species at basic pH, particles increase in size and decrease in number as slightly more soluble small particles dissolve and re-precipitate on larger, less soluble particles. Growth stops when the difference in solubility between the smallest and largest particles becomes indistinguishable. This process is denoted as Ostwald ripening of solid particles. As a consequence, particle size is strongly dependent on temperature: higher temperatures produce larger particles. In addition, in the pH 2e6 range, the growth rate also depends on the particle size distribution. Nature and concentration of catalyst As well as hydrolysis, condensation can proceed without a catalyst. However, the use of catalysts for the condensation of organosiloxanes is very helpful. Furthermore, the same catalysts are used: generally those compounds that exhibit acidic or basic characteristics. It has been shown that condensation reactions are acid and base specific.24 In addition, Iler1 showed that under more basic conditions, gel times increase. Condensation reactions continue to proceed; however, gelation does not occur. Again, catalysts that dictate a specific pH can and do drive the type of silica particle produced, as noted in the previous discussion of pH. Acid-catalyzed mechanism It is generally believed that the acid-catalyzed condensation mechanism involves a protonated silanol species. Protonation of silanol makes the silicon more electrophilic and thus susceptible to nucleophilic attack. The most basic silanol species (silanols contained in monomers or weakly branched oligomers) are the most likely to be protonated. Therefore condensation reactions may occur preferentially between neutral species and protonated silanols situated on monomers, end groups of chains, and so on. Base-catalyzed mechanism The most widely accepted mechanism for the base-catalyzed condensation reaction involves the attack of a nucleophilic deprotonated silanol on a neutral silicic acid (Fig. 5.6). Silica and Titania Nanodispersions 169 Figure 5.6 Nucleophilic attack to form a siloxane bond. Furthermore, it is generally believed that the base-catalyzed condensation mechanism involves penta- or hexa-coordinated silicon intermediates or transition states that are similar to those of an SN2-type mechanism. Preparation of monodisperse silica by precipitation in solution As described earlier, the classic method for preparing silica microparticles from alkoxides is well known as the Stöber method.8 Tetraethoxysilane or tetraethyl TEOS is a silane monomer produced from tetrachlorosilane, which in turn is derived from metallurgical-grade silicon. The silicon itself is obtained by the reduction of naturally occurring silica at temperatures over 1900 C in the presence of carbon. Using the Stöber method, different sizes of silica NPs can be prepared, ranging from 50 to 1 mm, with a very narrow size distribution. Typical Stöber particles are dense and nonporous, have a spherical shape, and possess very smooth surfaces. The size of these particles depends on the alkoxide chain type and the alcohol mixture used as a solvent.8 Particles prepared in methanol solutions are the smallest, whereas the particle size increases with an increase in the alcohol chain length. The particle size distribution also becomes broader when longer-chain alcohols are used as solvents. Many studies have been devoted to unraveling the mechanisms of Stöber silica formation.32e37 An example of a typical colloidal silica with a quite uniform distribution made by the Stöber process is shown in Fig. 5.7. Formation of monodisperse silica by the Stöber-Fink-Bohn method can be explained by two mechanisms: homogeneous nucleation and aggregative growth.33e35 Homogeneous nucleation is believed to take place via the LaMer model (monomer addition),39,40 in which nucleation is a fast process, followed by a particle growth process without further nucleation.41e43 By contrast, the aggregative growth (aggregation) model considers that nucleation and aggregation occur simultaneously throughout the reactions, and the nuclei (primary particles) aggregate together to form dimers, trimers, and larger particles (secondary particles). These models lead to the formation of either spherical particles (LaMer model) or gel network (aggregation model), depending on the reaction conditions.33,44e46 170 Chapter 5 Figure 5.7 Left: Scanning electron micrograph of Stöber spherical silica powders. Right: Size distribution of typical Stöber silica particles. Reprinted with permission from Khadikar, C. The Effect of Adsorbed Poly(Vin 1 Alcohol) on the Properties of Model Silica Suspensions (Ph.D. dissertation); University of Florida: Gainesville, FL, 1988. The stoichiometric amount of water for hydrolysis is 4 mol for 1 mol of silicon alkoxide, or only 2 mol water if condensation is completed. In the preparation of particles, however, the typical ratio of water to TEOS is higher than 20:1, and the pH is very high, since both factors promote condensation. This leads to the formation of compact structures (Stöber dense particles) rather than the extended polymeric networks generally found in hydrogels. Formation and stability of sols As predicted by Iler1 and confirmed by Harris52 in most sols that consist of discrete spherical particles of amorphous silica, the interior of the particles is made of anhydrous SiO2 with a density of 2.2 g/cm3. The silicon atoms located at the surface hold OH groups that are not lost when the silica is dried to remove free water. Calculations of the silanol number of the silica surface by purely geometric considerations and the density of amorphous silica indicated that there should be 7.8 silicon atoms/nm2 around the surface. Silica sols do not conform to the DerjaguineLandaueVerweyeOverbeek theory because they are apparently stabilized by a layer of adsorbed water that prevents coagulation even at the isoelectric point. This stabilization by repulsive hydration forces is possible because of the unusually small Hamaker constant of silica. To destabilize an aqueous silica sol, the degree of hydration must be reduced. Allen and Matijevı́c47,48 showed that the addition of salt produces surface ion exchange, and stability is reduced near the isoelectric point. Several researchers33,44e46 made many attempts to determine the size of the primary particles using various techniques. Through small-angle X-ray scattering, Green et al.46 Silica and Titania Nanodispersions 171 reported that the size of primary particles was 10.3 nm (in methanol) or 20.7 nm (in ethanol). Later, Rahman et al.49 produced homogeneous and stable silica NPs with mean particle size of 7.1 1.9 nm (in ethanol). In all cases these particles were nonporous. Iler1 attributes the dense structure to the solubility of the oxide in the aqueous medium and speculates that this might not be the case in a nonaqueous (eg, alcoholic) solution. This hypothesis was confirmed later by Ramsay and Booth.50 Although the cores of the primary particles in aqueous sols are nonporous, the presence of hydroxyl groups reduces their density. The stability of sols is explained by the fact that the repulsive barrier increases with the size of the particle; for this reason the nuclei may be unstable against aggregation until they reach a certain size. Mechanisms of particle growth Harris and coworkers46 investigated the growth of particles produced by the Stöber method, assuming that the first hydrolysis of an alkoxide group is the rate-limiting step in the formation of nuclei. The remaining groups hydrolyze rapidly, and small nuclei are created from fully hydrolyzed species. In the early stages of the reaction these small nuclei aggregate, forming colloidally stable seed particles, and in the later stages of the reaction particle growth occurs mainly by the addition of a monomer to the surface, not by aggregation of the small nuclei particles, as claimed by Bogush et al.33e35 Using the results of cryoetransmission electron microscopy experiments, Bailey and Mecartney51 postulated that hydrolyzed monomer polymerizes to form microgel clusters as a result of polysilicic acid cross-linking; these clusters collapse upon reaching a certain size and cross-linking density. Collapsed particles made more dense by condensation are colloidally stable with respect to each other. The denser seed particles grow by adding a hydrolyzed monomer or polymer to their surfaces. The rate of growth of the polymers must be slow enough so that after a sufficient number of seeds has been formed, the polymers attach to a particle’s surface before they grow to a large enough size to collapse and form a seed particle themselves. Boukari et al.52 proposed that the initial particles that also have a polymeric structure are better described as mass fractals, in which the nucleating backbones or seeds are used to build the compact and stable particles observed later during growth. Before equilibrium is reached, particles of various sizes and fractality are distributed. In addition, under conditions in which condensation is rapid compared with hydrolysis, polysiloxane chains or rings are produced, whereas in the reverse case more extensively cross-linked polymeric clusters are formed. Therefore polymers formed in base-catalyzed reactions generate highly branched clusters.53 The effect of electrolytes on the size of silica NPs was described by Bogush et al.33e35 and in their studies, they reported that when the electrolyte (NaCl) concentration was increased from 0 to 104 M, the particle size increased from 340 to 710 nm. 172 Chapter 5 Surface structure of silica The surface of hydrated silica is covered with hydroxyl groups (silanols) that are attached in various ways to silicon atoms. Using an infrared spectroscopy method, Yaroslavsky and colleagues,54e56 proved for the first time the existence of hydroxyl groups on a silica surface (porous glass). This fact was soon confirmed by Kurbatov and Neuymin.57 Now, numerous spectral and chemical data unambiguously confirm the presence of OH groups on such SiO2 surfaces. The hydroxyl groups at the surface are involved in temperature-dependent dehydroxylation-hydroxylation, producing siloxane bridges. The reaction is written as: ^SieOH%SieOeSi^ þ H2 O [5.5] Dehydroxylation usually starts at temperatures above 500 K, forming siloxane bridges on the surface that are less polar than hydroxyl groups. On the other hand, it must be emphasized that a hydrated silica surface, upon being exposed to a sufficiently high pressure of water vapor, is able to take up water by means of physical adsorption and capillary condensation. The surface properties of amorphous silica, which is considered to be an oxide adsorbent, in many cases depend on the presence of silanol groups. At a sufficient concentration these groups make such a surface hydrophilic. The OH groups act as the centers of molecular adsorption during their specific interaction with adsorbates that are capable of forming a hydrogen bond with the OH groups. The removal of the hydroxyl groups from the surface of silica leads to a gradual increase in hydrophobic properties.1,58 It is commonly accepted that several types of hydroxyl groups with different reactivity coexist on the surface of silica. Surface OH groups are subdivided as (1) isolated free (single silanols; SiOH); (2) geminal free (geminal silanols or silanediols); ]Si(OH)2; (3) vicinal, or bridged, OH groups bound through the hydrogen bond (H-bonded single silanols, H-bonded geminals, and their H-bonded combinations) (Fig. 5.8). On the SiO2 surface there also exist surface siloxane groups or SieOeSi bridges with oxygen atoms on the surface. Moreover, internal silanol groups are often present inside the silica skeleton, located in very fine ultramicropores (<1 nm), which may also contain hydration water molecules. Chemical modification of the silica surface The chemical modification of the silica surface with organo-functional groups is an important step toward the preparation of silicae polymer nanocomposites. More precisely, surface modifications have been reported to enhance the affinity between the organic and inorganic phases and at the same time improve the dispersion of silica NPs within the polymer matrix.60 Modification of the silica surface with silane coupling agents is one of the most effective techniques available. Silane coupling agents (Si(OR)3R0 ) have the ability to bond inorganic materials such as silica NPs Silica and Titania Nanodispersions 173 Figure 5.8 Types of silanol groups and siloxane bridges on the surface of amorphous silica, and internal OH groups (see text). Qn terminology is used in nuclear magnetic resonance, where n indicates the number of bridging bonds (OeSi) tied to the central silicon atom: Q4, surface siloxanes; Q3, single silanols; Q2, geminal silanols (silanediols). Reproduced with permission from Zhuravlev, L. T. The Surface Chemistry of Amorphous Silica. Zhuravlev Model. Colloids Surf. A 2000, 173 (1e3), 1e38. Copyright 2000, Elsevier Inc. to organic resins. In general, the Si(OR)3 portion of the silane-coupling agents reacts with the inorganic reinforcement, while the organofunctional group (R0 ) reacts with the resin. Table 5.1 shows some common silane-coupling agents used for modification of a silica surface. In general, the chemical modification of a silica surface using silane-coupling agents can be conducted via an aqueous or nonaqueous system that is also known as post modification. The nonaqueous system is usually used for grafting 3-aminopropyltriethoxysilane (APTES) molecules onto the silica surface. The main reason for using a nonaqueous system is to prevent hydrolysis. Silanes such as APTES, which carries amine groups (base), can undergo uncontrollable hydrolysis and polycondensation reactions in an aqueous system. Therefore the use of an organic solvent provides better control of reaction parameters and is preferred for coupling reaction using APTES. For a nonaqueous system, the silane molecules are attached to the silica surface via a direct condensation reaction, and the reaction is usually conducted at reflux conditions. On the other hand, an aqueous system is favorable for large-scale production. In this system the silanes undergo hydrolysis and condensation before being deposited on the surface. 174 Chapter 5 Table 5.1: Commonly used silane-coupling agents. Name (Acronym) Formula Vinyltriethoxysilane (VTS) Methacryloxypropyltriethoxysilane (MPTS) 3-Glycidyloxypropyltrimethoxysilane (GPTS) 3-Aminopropyltriethoxysilane (APTES) 3-Mercaptopropyltriethoxysilane (McPTS) Chloropropyltriethoxysilane (CPTS) The alkoxy molecules are hydrolyzed in contact with water. This is followed by self-condensation reactions between the hydrolyzed silanes. Then the silane molecules are deposited on the silica surface through the formation of siloxane bonds between the silanol groups and hydrolyzed silanes with the release of water molecules.61 A slight increase in the particle size (z25%) after surface modification was observed.62 Silica and Titania Nanodispersions 175 5.2.2 Synthesis of TiO2 NPs by Hydrolysis of Alkoxides TiO2 powders possess interesting optical, dielectric, and catalytic properties, which lead to many industrial applications such as pigments, fillers, catalyst supports, and photocatalysts. Since commercial production started in the early 20th century, TiO2 has been widely used as a pigment,63 in sunscreen formulations,64,65 paints,66 ointments, and toothpaste,67 among others. Synthesis of TiO2 NPs by precipitation methods, using reactants such as titanium tetrachloride and titanium tetrabutoxide, is common. Titanium tetrachloride and titanium tetrabutoxide are dissolved in a solvent (usually water), and precipitation is induced by adding a basic solution. Various factors affect this precipitation method: 1. Use of an inhibitor, accelerator, and surfactant: Absorption of the inhibitor on the crystal surface prevents the growth of NPs, and it can be used to produce fine crystals. Adding inhibitors in other methods (eg, microemulsion) has the same effect. A surfactant covers the crystal surface and, by changing the crystal’s interfacial energy, prevents the growth particles; thus it can be used to produce fine NPs.157 2. Reaction temperature: An increase in temperature, and the consequent increase in diffusion coefficient, reduces the time needed for formation of stable nuclei and thus decreases the induction time for nucleation. In most cases high temperatures also increase solubility and decrease supersaturation; consequently, low supersaturation growth favors homogeneous particle growth, creating larger particles with lower polydispersity. The final properties of TiO2 nanopowders depend on size, morphology, and crystalline phase. To prepare a TiO2 nanostructured material with controlled properties, several processes have been developed in the past decade and can be classified as liquid processes (solegel,68e70 solvothermal,71 hydrothermal72,73), solid-state processing routes (mechanical alloying/milling,74 mechanochemical,75,76 radiofrequency thermal plasma77), and other routes such as laser ablation.78 Nowadays, the method most used to industrially produce nanopowders of TiO2 is based on plasma spray synthesis. This technique allows high productivity but leads to the production of NPs containing impurities, which reduce the quality grade of the product. Moreover, control of crystallite size distribution and shape of the NPs is not straightforward and depends strongly on the spray feedstock.79 On the contrary, production of NPs by a chemical solegel method is the best method to ensure the high purity of the final product. Furthermore, the solegel process gives the ability to go all the way from the molecular precursor to the product, making it possible to synthesize tailor-made materials with controlled particle size distribution. 176 Chapter 5 While silica is the most prevalent type of solegel network, there is increasing interest in systems created from titanium and other transition metal alkoxides. These systems are significantly more difficult to study and control because transition metals are more electrophilic than silicon and consequently exhibit greater rates of hydrolysis and condensation. The tendency of titanium alkoxides to rapidly hydrolyze makes their solegel transformation difficult to control. Titanium alkoxides are quite reactivedmuch more than silicon alkoxidesdand therefore must be handled with care, in a dry environment; they are often stabilized via chemical modification,80e82 which is performed mainly with acetic acid, acetylacetone, or hydrogen peroxide. In any case, hydrolysis of titanium alkoxides occurs much faster than hydrolysis of silica counterparts, and consequently, titanium alkoxides have to be protected from contact with water. Therefore, storage in a nitrogen atmosphere is required. In solegel processes TiO2 is usually prepared by acid-catalyzed reactions of hydrolysis and polycondensation of titanium alkoxide ((TiOR)n) to form oxopolymers, which are transformed into an oxide network. These reactions are similar to those used for the synthesis of silica (see Section 5.3.1.1) and can be schematically presented as follows. Hydrolysis follows a nucleophilic substitution mechanism, where R is an alkyl group: TiðORÞn þ H2 O/TiðORÞn1 ðOHÞ þ ROH [5.6] Condensation dehydration (alcoxolation) is a reaction by which a bridging oxo group is formed through the elimination of an alcohol molecule. The mechanism is basically the same as for hydrolysis: TiðORÞn þ TiðORÞn1 /Ti2 OðORÞ2n2 þ ROH [5.7] Dealcoholation (oxolation) follows the same mechanism as alcoxolation, but the R group of the leaving species is a proton: 2TiðORÞn1 ðOHÞ/Ti2 OðORÞ2n2 þ H2 O [5.8] Thus, the overall reaction is TiðORÞ4 þ 2 H2 O/TiO2 þ 4 ROH [5.9] where the relative rates of hydrolysis and polycondensation strongly affect the structure and properties of metal oxides. Factors influencing particle formation The performance of titania in technological applications could be optimized with specific microstructural and macrostructural control over the morphology of the material. Nucleation and growth of titania need to be controlled at the earliest stages of production Silica and Titania Nanodispersions 177 and stopped at the appropriate stage to obtain titania particles with the desired crystal structure and a well-defined size, shape, and surface structure. One of the main advantages of the solegel technique over other methods, such as hydrothermal synthesis and chemical vapor deposition, is that it produces materials with a large surface area.83 Experimental results have shown that powders created by an uncontrolled solegel method generally lack the properties of uniform size, shape, and unagglomerated state. There are several parameters for controlling the solegel process to prepare a TiO2 nanopowder with desired properties. It has been demonstrated that the precursor’s concentration of titanium alkoxide greatly affects the crystallization behavior and characteristics of the final powder.84 Factors that are crucial in the formation of metal oxides include the reactivity of metal alkoxides, the water-to-alkoxide ratio, the pH of the reaction medium, the nature of the solvent and additives, and the reaction temperature. By varying these process parameters, materials with different surface chemistries and microstructures can be obtained. Typically, in the solegel method, the solegel-derived precipitates are amorphous. Therefore further heat treatment is required to induce crystallization. To induce transition from an amorphous to an anatase phase, an annealing temperature higher than 300 C is generally required; this results in the dramatic increase in of particle size. The transformation of anatase to rutile occurs at temperatures from 400 to 1100 C.85e87 This transformation results in a drastic loss of surface area and porosity.88 The alkoxy groups most often used in the synthesis of TiO2 range from two (ethoxide) to four (butoxide) carbon atoms long. In the same way as silicon alkoxide, the reactivity toward hydrolysis decreases as the alkoxy chain length increases, which provides a means of controlling the rate of particle formation.82,89 Even so, hydrolysis in the presence of excess water is rapid and completed within seconds. To moderate the high reactivity, alkoxides are usually diluted in alcohol before mixing with water.90 In addition, the size, stability, and morphology of the sol produced from alkoxides is strongly affected by the water-to-titanium molar ratio (r ¼ [H2O]/[Ti]).91,92 The formation of colloidal TiO2 at a high r ratio is of great interest because small particles are formed under this condition. One can distinguish two regimes: (1) At low r values (r 10) one obtains spherical, relatively monodisperse particles with diameters of 0.5e1 mm.93e95 (2) At higher r values the particles formed are colloidally unstable and precipitate in the form of large aggregates. These aggregates can be chemically peptized to final sizes that are usually smaller than 100 nm in diameter.96e98 Because of the rapid precipitation in these conditions, which makes kinetic studies extremely difficult, the process remains poorly understood. The titania solegel process follows a different pathway than the silicon-based solegel process. It is generally assumed that individual monosilicates serve as building blocks 178 Chapter 5 during the polymerization process.99 In the case of titania, however, titanium clusters (polyoxoalkoxides) have been suggested to serve as the primary building blocks for particles and gels. In particular, Ti11O13(OC3H7)18 has been proposed as a building block in titanium isopropoxide (TTIP)ebased solegel processes based on results from 17O NMR spectroscopy.100,101 A stable solution of these clusters can be obtained by hydrolysis of TTIP at low hydrolysis ratios (h < 1). However, if h > 1.5, hydrolysis of TTIP does not result in a stable solution of polyalkoxides but rather in the precipitation of TiO2 particles. The precipitation takes place after an induction period in which slow particle growth is observed, followed by rapid precipitation. The presence and size evolution of NPs (1.5e6 nm) during the induction period has been studied by dynamic light scattering.102 Simonsen et al.103 showed that the titanium clusters formed using titanium tetraethoxide were smaller than the clusters formed using TTIP and titanium tetrabutoxide under similar conditions. Furthermore, the small 3-nm particles synthesized at pH 3 grow significantly during the drying process as a result of the destabilization of the colloidal solution, leading to the formation of a gel confirmed by high-resolution transmission electron microscopy. Stabilization of titania NPs Hydroxypropyl cellulose can be used to improve the monodispersity of TiO2 NPs. According to Nagpal et al.,104 TiO2 particles less than 100 nm in diameter can be prepared when tetraethylorthotitanate (TEOT) reacts in the presence of hydroxypropylcellulose (HPC) dispersant for r (H2O/TEOT) ¼ 60/120. In addition, they assumed that the amount of HPC adsorbed onto the surface of TiO2 particles increased with an increasing water concentration, leading to greater steric stabilization. To prevent aggregation, Jean and Ring105 also used HPC during the precipitation of TiO2 particles and synthesized monodispersed powders. In general, TiO2 fine particles were synthesized by the controlled hydrolysis and condensation of TEOT in a dilute alcohol solution106,107 using a batch precipitation technique on a small scale, in which hydrolysis was carried out using a beaker and a magnetic mixer in a nitrogen atmosphere. However, TiO2 particles created through a batch process were not enough to obtain nanophase particles with a narrow particle size distribution. Accordingly, the synthesis of TiO2 NPs using a semibatch process was introduced in this work. The semibatch method, in which system TEOT reactants are fed into a reactor at a constant rate, is better than a batch process at controlling the size, shape, and size distribution because of its short nucleation and slow hydrolysis rate. Peptization One of the problems encountered when making titania is the high reactivity in water of the available precursors, such as alkoxides or chlorides. The reaction invariably leads to a mixture of different polymeric species, which assume a variety of structures and sizes, and finally lead to an amorphous solid. Although certain chemical modifications have been Silica and Titania Nanodispersions 179 successful in controlling the reactivity of titanium alkoxides and have permitted the isolation of intermediate polyoxoalkoxides,108e110 none of these species resemble the structure of the bulk dioxide, and any further hydrolysis again leads to gels and amorphous precipitates. In both cases a peptization process is required to convert these polymers into crystalline particles. The size of the particles at higher pH values is smaller, and they tend to agglomerate. According to several studies, the variety of TiO2 surface charge is dependent on pH. TiO2 in sols is electrically charged because of the absorption of Hþ or OH in an aqueous suspension. According to Bahnemann et al.,111 the surface charges of TiO2 can be determined by chemisorptions: of Hþ ; TiO2 þ nHþ 4TiO2 Hn þ for pH < 3:5 of OH ; TiO2 þ nOH 4TiO2 OHn for pH > 3:5 [5.10] [5.11] In acidic and alkaline media the strong repulsive charge among particles reduces the probability of coalescence, and more stable sols can be formed. Studies by Su et al.112 indicated that the isoelectric point of TiO2 powders varies in the pH range of 5e6.8. The hydrolysis of titanium isopropoxide or titanium ethoxide in excess water, followed by peptization with nitric acid at refluxing temperatures, yields a sol comprising TiO2 NPs.97,98 Green et al.46 found that charged particles have a low sticking probability because of the electrostatic repulsive forces between particles that counteract the attractive van der Waals forces, producing smaller aggregates. Fig. 5.9 shows the effect of pH on the surface charge of TiO2 NPs. Figure 5.9 Effect of pH on the surface charge of an oxide. Adapted with permission from Bischoff, B. L.; Anderson, M. A. Peptization Process in the SoleGel Preparation of Porous Anatase (TiO2). Chem. Mater. 1995, 7 (10), 1772e1778. Copyright 1995 American Chemical Society. 180 Chapter 5 A simple method to obtain TiO2 NPs by hydrolysis and peptization was described by Bartlett et al.113 It consists of weighing TTIP precursor in an inert atmosphere, then adding 18.5% hydrochloric acid to the precursor, giving rise to a transparent precipitate. The precipitate is introduced in dilute hydrochloric acid solution under agitation. After some time (which depends on the pH) a stable TiO2 dispersion is obtained. 5.3 Preparation of Silica and TiO2 NP Hydrolyzing Alkoxides in Nanostructured Liquids 5.3.1 Synthesis in Microemulsions Traditional solegel technology has the disadvantage that many precursors are not initially soluble in water, and thus organic solvents (eg, alcohols) are required as reaction media. One solution to this problem is the use of water-in-oil (W/O) microemulsions,114,115 which can solubilize large amounts of both polar and nonpolar components. The use of microemulsions, both oil-in-water (O/W) and W/O, has received considerable attention because of its technological applications in the preparation of NPs with a controlled size, achieving size distributions narrower than those achieved by conventional methods of precipitation in solution. Microemulsions were first used for producing nano-sized particles by Boutonnet et al.116 in 1982; they reported the preparation of platinum, palladium, rhodium, and iridium NPs by reducing metal salts in W/O microemulsions. These NPs were in the range of 3e5 nm, with very narrow pore size distributions. The synthesis procedure, which is still the most widely used, consists of the preparation of two identical microemulsions, each containing a separate reactant. The reaction is initiated after mixing the two systems. The microemulsion droplets are subject to Brownian motion, and they collide continuously to form aggregates, which last for very short periods of time. The internal content of the microemulsion droplets can be exchanged rapidly as a result of continuous coalescence and droplet disruption; consequently, diffusion of the reactants is generally a fast process, and solid particles can be detected in a time scale of minutes. This microemulsion method is versatile, since hydrophilic reactants can be used in W/O microemulsions and lipophilic ones in O/W microemulsions. Consequently, a wide variety of different nano-sized materials can be prepared by this technique. Some examples include gold and silver particles,117 carbonates,118 superconductors,119 semiconductors,120 magnetic materials,121 copper,122 and silica.123,124 Many scientific reports that focus on the use of microemulsions as nanoreactors for the preparation of solid particles can be found in the literature, and reviews are abundant.20,125e131 Osseo-Assure and Arriaga132,133 demonstrated that the particle size of silica NPs obtained in microemulsions was extremely narrow; this had not been achieved before using the Silica and Titania Nanodispersions 181 conventional technique of silica precipitation in a homogeneous alcohol media.8 Friberg and Sjöblom134 studied, using NMR, the distribution of the intermediate species SinO2nr (OH)2rx(OR)x during the formation of silica particles. They concluded that TEOS is located within the lipophilic domains but the reactions proceed rapidly when the molecules reach the interface of the water pools. Particle size can be significantly increased by the addition of co-surfactants. This effect has been observed when adding benzyl alcohol to O/W microemulsions stabilized with sodium bis(2-ethylhexyl) sulfosuccinate (AOT) surfactant132 and adding alcohols to O/W microemulsions prepared with block copolymer surfactants or conventional ethoxylated surfactants.124 All these results indicate that the rates of intermicellar exchange are increased by the addition of co-surfactants. It is well known that alcohols disrupt molecular organization on films, increasing the film’s flexibility.135e138 This aspect has been studied theoretically using Monte Carlo simulations, which have highlighted the importance of film flexibility and molecular exchange between droplets.128 Control of particle size in microemulsion-based synthesis The control of NP size is a complex and intriguing field. Primitive models were based on the assumption that microemulsion droplets were nanoreactors that determined the size of particles, depending on the H2O-to-surfactant ratio and the reactant concentrations in the aqueous domain of the microemulsion droplets. However, these simple considerations were rapidly outdated by the fact the particles (commonly around 30 nm) are significantly bigger than the size of the microemulsion droplets that were used as “templates.” For example, Yamauchi et al.123, who studied the preparation of silica particles using the anionic surfactant AOT, showed that the size of the NPs (ranging from 10 to 80 nm, depending on composition) was much larger than the size of the corresponding microemulsion droplets (5e10 nm). In the majority of cases there is not a direct correlation between the size of the particles and the size of the microemulsion droplets,127 despite the fact that this dependence has been observed in microemulsion systems based in AOT surfactant.127 In most works reported in the literature, there is an increase in size during the reactions as a result of reactant diffusion by particle exchange. Consequently, the microemulsion droplets cannot be considered as “real templates,” since microemulsion droplets are not molds that restrict particle size. For example, very large silica particles can be obtained by adding alcohols as co-surfactants to microemulsions.124 Few particles are initially nucleated, but they can grow rapidly thanks to fast diffusion. Fig. 5.10 shows an example of silica particles with an approximate diameter of 150 nm. The dynamics of NP formation and growth in microemulsion systems are complex, but recent advances have provided clear understanding.20,127,139e141 Monte Carlo simulations 182 Chapter 5 Figure 5.10 Scanning electron microscopy image of silica particles obtained in a system comprising 10 wt% NH3/C12e14E4.5/n-butanol/isooctane/tetraethyl orthosilicate (TEOS) (15 wt% aqueous solution, C12e14E4.5:isooctane ¼ 8:3, 20 wt% butanol, 10 wt% TEOS). Reproduced with permission from Esquena, J.; Tadros, T. F.; Kostarelos, K.; Solans, C. Preparation of Narrow Size Distribution Silica Particles Using Microemulsions. Langmuir 1997, 13 (24), 6400e6406. Copyright 1997 American Chemical Society. have demonstrated the importance of film flexibility and the interdroplet exchange rate, in addition to the H2O-to-surfactant ratio and the concentration of reactants.139,141 The main conclusions, assuming that the chemical reactions are much faster than the material exchange, were summarized by López-Quintela20: 1. An increase in reactant concentrations increases both particle size and polydispersity. 2. An increase in the concentration of one of the reactants, in excess with respect to the second reactant, tends to decrease the particle size. 3. An increase in film flexibility increases the particle size. It can be achieved by adding alcohols to the microemulsions,124 approaching the microemulsion phase boundaries, changing the chain length of oil and co-surfactant, among others. 4. An increase in microemulsion droplet size, depending on the parameter R ¼ [H2O]/ [surfactant], produces an increase in particle size. However, it should be pointed out that the relationship between R and the particle size is generally nonlinear because R also influences film flexibility. Silica and Titania Nanodispersions 183 In the case of TiO2, uncontrolled aggregation and flocculation was reported for microemulsion-based methods,132 except at very low reactant concentrations.142 By contrast, stable, monodisperse SiO2 NPs can be produced because of the relatively low reactivity of silicon alkoxides, allowing better stability, which increases with the surfactant-to-water ratio.2 Colloidal stability can be explained by a combination of steric repulsion with secondary hydration forces.17,18 TiO2 sols, gels, and NPs were produced by the controlled hydrolysis of tetraisopropyltitanate in AOT reverse micelles. NPs with diameters less than 10 nm could be produced at relatively high Ti(IV) concentrations (up to 0.05 mol/dm3). These NPs aggregated into sols, with diameters of 20e300 nm. Lee et al.143 recently showed that TiO2 NPs can be prepared by the hydrolysis of titanium tetraisopropoxide in W/O microemulsions consisting of water, nonionic Brij series surfactants with hydrophilic groups of different lengths, Tween series surfactants with hydrophobic groups of different lengths, and cyclohexane. Particles have a spherical shape and show a uniform size distribution, but the shape becomes distorted with a decrease of hydrophilic group chain length following rapid hydrolysis of water and titanium alkoxide. TiO2 NPs calcined at 500 C have a stable anatase phase that has no organic surfactants; the product completely transforms into the anatase phase above 300 C, and the rutile phase begins to appear at 600 C, regardless of surfactants. 5.3.2 Synthesis in Nano-emulsions One major disadvantage of microemulsion formation is that it requires larger amounts of surfactant than conventional emulsions, typically over 20 wt%. This drawback can be solved by the use of emulsions with droplets in the nanometer range as reaction media, since formation of small-size emulsions requires lower surfactant concentrations, in comparison to microemulsions. These emulsions, termed nano-emulsions, miniemulsions, or ultrafine emulsions, are transparent or translucent (with droplet sizes between 50 and 200 nm) or milky (sizes up to 500 nm)144e148 and exhibit high kinetic stability. Nanoemulsions are a subject of increasing interest in both theoretical discussions and practical applications because of their singular properties, namely extremely small droplet size, kinetic stability, and transparency. In addition, they present several advantages over conventional emulsions, principally because of their similar characteristics to microemulsions.145 In this sense Porras et al.149 used W/O nano-emulsions as reaction media for the synthesis of SiO2 and TiO2 ceramic particles. They demonstrated that SiO2 and TiO2NPs, with sizes ranging from 30 to 230 nm, can be obtained by alkoxide hydrolysis in W/O nano-emulsions. NP sizes can be tuned by controlling the nano-emulsion droplet sizes. Fig. 5.11 shows examples of TiO2 and SiO2 NPs obtained using this method. More recently, Wu et al.150 demonstrated a nano-emulsion templating approach for the synthesis of silica NPs with a compartmentalized, hollow structure, with a uniform diameter of 12 nm and a cavity diameter of 3.3 nm, via an ultrasound-assisted solgel method. 184 Chapter 5 Figure 5.11 (A) Scanning electron micrograph of silica particles obtained from a nano-emulsion. (B) Atomic force micrograph of titania particles obtained from a nano-emulsion. Reproduced from Porras, M.; Martı´nez, A.; Solans, C.; González, C.; Gutiérrez, J. M. Ceramic Particles Obtained Using W/O Nano-Emulsions as Reaction Media. Colloids Surf. A 2005, 270e271 (1e3), 189e194, with permission from Elsevier. 5.4 Preparation of Silica and Titania by Other Methods In general, because of the price of metal alkoxides, the production of NPs via the solgel process is around 100 times more expensive than production using conventional silica melting and casting. As a consequence, the production of NPs via the solgel process makes economic sense only when addressing high value-added products. The most common raw materials used in the preparation of silica products include sodium silicate ((Na2O) (SiO2)y), chlorosilanes (RxSiCl4x), and silicon alkoxides (Si(OR)4). Among these, sodium silicate has the lowest cost on a per-weight basis and is a commodity chemical available in large quantities. Sodium silicate can be readily acidified to produce silicic acid (Si(OH)4), from which a wide range of silica microstructures, ranging from gels with a large surface area to colloidal particles, can be produced. Silicic acid can be subsequently processed (eg, gelled, precipitated) by changing the temperature, pH, and/or solid content. Numerous techniques have been proposed for manufacturing silica sols from soluble silicate, including dialysis,151 electrodialysis,152 peptization,153 acid neutralization,154 and ion exchange.155 The last one has come to be the most commonly used in the industry. This method is described in the flow chart in Fig. 5.12. There are four typical routes to prepare colloidal silica from natural ore. The first among them is the preparation of colloidal silica from liquid sodium silicate through an ion Silica and Titania Nanodispersions 185 Figure 5.12 Main methods applied in the industry to obtain colloidal silica. Reproduced with permission from Lim, H.; Lee, J.; Jeong, J.; Oh, S.; Lee, S. Comparative Study of Various Preparation Methods of Colloidal Silica. Engineering 2010, 2 (12), 998e1005. Copyright 2010 Scientific Research Publishing Inc. exchange process. The sodium silicate is obtained from naturally available silica by melting the silica ore in the presence of alkali flux; subsequently, it is dissolved by heating under pressure to produce liquid sodium silicate, which is commonly known as water glass. The liquid sodium silicate has a high viscosity, and therefore it is diluted to a concentration of 3e5 wt%. Next, it is passed through an ion-exchange resin, and then fed into the alkali solution to form a silica seed, which is then used to grow silica particles. The product is concentrated to 30 wt% to obtain the commercial product. The second method for preparing colloidal silica is from TEOS, which is well known as the Stöber method8 and was described in detail in Section 5.2. TEOS is a silane monomer prepared from tetrachlorosilane, which in turn is derived from metallurgical-grade silicon. The silicon itself is obtained by the reduction of naturally occurring silica ore at temperatures over 1900 C in the presence of carbon. The third method is that of direct oxidation of silicon, wherein colloidal silica is prepared by the direct oxidation of metallurgical-grade silicon without using TEOS. The silicon is treated with water in the presence of alkali catalysts to produce silica along with the evolution of hydrogen and heat. Finally, the fourth method for preparing colloidal silica consists of the milling and peptization of silica (silica gel or fumed silica), which consists of preferentially coacervated or aggregated primary silica particles. The properties of the colloidal silica prepared by this route largely depend not only on the milling and peptization process but also on the properties of the starting silica source, such as purity, shape, and aggregation. 186 Chapter 5 In any case, silica and TiO2 NPs can be obtained by many different methods. The next section briefly describes the most usual methods (ion exchange, vapor phase, mechanochemical, hydrothermal/solvothermal, aerosols, and peptization) that are alternatives to typical solegel processing. Other methods to obtain silica sols, including dialysis and electrodialysis, have also been proposed, but they were never applied on an industrial basis and are not in the scope of this chapter. 5.4.1 Preparation of NPs by Ion Exchange Methods Alkaline silicate is a common and inexpensive source of silica. Silica can be precipitated by acidification of alkaline silicate solutions with a mineral acid. Sulfuric acid and sodium silicate solutions are added simultaneously with agitation to water, and silica is precipitated. The choice of agitation, the duration of precipitation, the addition rate of reactants, their temperature and concentration, and the pH can vary the properties of the silica. The formation of a gel stage is avoided by stirring at high temperatures. The obtained NPs are porous, with a diameter of 5e100 nm. However, this precipitation method has the disadvantage that control is difficult, with bad reproducibility when scaling up. The acidification of alkaline silicate solutions can be improved by ion exchange methods. The general principle is to remove sodium from silicate via cation exchange with Hþ, using exchange columns in diluted solutions. When sodium content is reduced at lower pH values, polymerization takes place in a controlled manner, and silica NPs are nucleated homogeneously and begin to grow. After reaching the desired size, the stable NP dispersion is concentrated to the final content. For the preparation of large particles, smaller particles can be used as seeds for further growth. The resulting suspensions are commercially available as aqueous dispersions of colloidal polysilicic acid (resulting from the condensation of two or more molecules of Si(OH)4), with typical loads up to 50 wt% of solid material. pH and conductivity of the concentrated sol are adjusted to ensure stability for at least 6 months. Table 5.2 lists examples of commercially available silica dispersions. Table 5.2: Selected commercially available silica dispersions.157 Trademark Name Ludox Nyacol/Bindzil Levasil Kostrosol Silicadol Supplier W. R. Grace & Co. Eka Chemicals (Akzo Nobel) Bayer Chemiewerke Bad Köstritz GmbH Nippon Chemical Industrial Co. Silica and Titania Nanodispersions 187 5.4.2 Preparation of SiO2 and TiO2 NPs by Vapor Phase Methods The vapor phase method is the most important for the production of TiO2 pigment and fumed silica. It is based on the oxidation of TiCl4 or SiCl4 in a furnace at very high temperatures or with the use of plasma following Eqs. [5.12] and [5.13]: TiCl4 þ O2 /TiO2 þ 2Cl2 SiCl4 þ O2 /SiO2 þ 2Cl2 [5.12] [5.13] In a study by Park and Park,159 SiCl4 vapor was hydrolyzed with water vapor at 150 C as the first step to form oxychloride particles. The sample was then converted into silica particles through further hydrolysis at 1000 C to produce silica particles in a size range of 250e300 nm. Low-temperature vapor phase hydrolysis was used more recently for the synthesis of nanostructured silica materials.160 Fumed silica has a very strong thickening effect. Primary particle size is 5e50 nm. The particles are nonporous and have a surface area of 50e600 m2/g. The density is 160e190 kg/m3. In the case of TiO2, this method is performed at a rate of hundreds of tons per year. The dry process involves a vapor phase oxidation reaction of TiCl4, which leads to the production of amorphous, nano-sized TiO2. The obtained amorphous, nano-sized TiO2 is then annealed at different temperatures to get desired crystalline phases such as anatase or rutile. The resulting powder is nearly monodisperse, with a primary particle size of w250 nm. 5.4.3 Preparation of SiO2 and TiO2 Particles by Mechanochemical Methods One interesting alternative synthesis method for inorganic oxides is the mechanochemical activation of solid-state reactions. The mechanical treatment of powder solids in high-energy mills significantly increases the number of defects in the solid structure, leading to much lower crystallinity, smaller crystallite size, and a large surface area.161,162 During the high-energy ball-milling reaction, several types of impact between reactants, balls, and walls of the container can occur. A schematic of such possible impacts is depicted in Fig. 5.13. In this way the reactivity of the activated solids is remarkably enhanced. Ball milling is a very old method for accelerating solid-state chemical reactions, and it is beeing used as a tool for synthesizing different materials, such as special alloys, nanocrystalline powders, or metaleceramic composites.161 An attractive variation of this method allows nanometric powders of simple metallic oxides to be obtained by solid-state reaction between a metallic salt (Lewis acid) and a base. The mechanochemically activated reaction produces 188 Chapter 5 Figure 5.13 Three examples of possible manners by which attrition balls inside a mill can collide: head-on impact (A); oblique impact (B); multiball impact (C). Reproduced with permission from Zhang, D. L. Processing of Advanced Materials Using High-Energy Mechanical Milling. Prog. Mater. Sci. 2004, 49 (3e4), 537e560. Copyright 2004 Elsevier Inc. oxides with low crystallinity that are immersed in a water-soluble salt by-product, which is later removed by means of a simple washing procedure.163,164 Different oxides (Gd2O3, CeO2, TiO2, etc.)165 and even metallic nanopowders166 have been obtained using this technique. Compared with the most conventional preparative routes, the main advantages of the mechanochemical method are the possibility of obtaining relatively large amounts of product, the simplicity of the process, the low cost of raw materials, and the total absence of organic solvents. 5.4.4 Preparation of SiO2 and TiO2 by Hydrothermal/Solvothermal Methods The hydrothermal method has been widely used to prepare TiO2 nanomaterials.167e172 Hydrothermal/solvothermal synthesis is normally conducted in steel pressure vessels, called autoclaves, under controlled temperature and/or pressure with the reaction in aqueous/organic solutions. The temperature can be elevated above the boiling point of the aqueous/organic solvents, reaching the pressure of vapor saturation. The temperature and the amount of solution added to the autoclave largely determine the internal pressure produced. The solvothermal method is almost identical to the hydrothermal method except that a nonaqueous solvent is used. However, the temperature can be elevated much higher than that in hydrothermal method because a variety of organic solvents with high boiling points can be chosen. The solvothermal method normally provides better control than the hydrothermal method over the size, shape distributions, and the crystallinity of TiO2 NPs. For example, TiO2 nanorods were prepared by hydrothermal treatment of a titanium trichloride aqueous solution supersaturated with NaCl at 160 C for 2 h.168 Different surfactants or compositions can be used to tune the morphology of the resulting Silica and Titania Nanodispersions 189 nanorods.172 When TiO2 powders are put into a 2.5- to 20-mol/L NaOH aqueous solution and held at 20e110 C for 20 h in an autoclave, TiO2 nanotubes are obtained.171 With or without the aid of surfactants, solvothermal methods have been used to synthesize high-quality TiO2 NPs.158,173 5.4.5 Preparation of SiO2 and TiO2 Particles by Aerosol Methods An aerosol is a colloidal dispersion of liquid droplets in a vapor. Aerosols can be used to make oxide powders in a variety of ways. A widely practiced technique, generally known as evaporative decomposition of solutions, involves spraying salt solutions into a furnace, where the droplets dry and the salts decompose into oxides. Matijevic et al.174 propose a variant method in which a gas stream passes through a vapor of AgCl to condense nuclei, then this aerosol is passed over a film of titanium ethoxide or isopropoxide or chloride. The vapor of the titanium compound condenses on the AgCl nuclei, and the droplets then are hydrolyzed at a temperature lower than 100 C in a chamber containing water vapor. Then the droplets are heated at 150 C to complete the reaction. In this method almost monodisperse spherical particles with a diameter ranging from 60 to 600 nm were obtained, as shown in Fig. 5.14. Figure 5.14 (A) Transmission electron micrograph of TiO2 nanoparticles (NPs) prepared by hydrolysis of an aerosol of titanium ethoxide. (B) Scanning electron micrograph of TiO2 NPs prepared by hydrolysis of an aerosol of titanium isopropoxide. Reproduced from Matijevic , E.; Budnik, M.; Meites, L. Preparation and Mechanism of Formation of Titanium Dioxide Hydrosols of Narrow Size Distribution. J. Colloid Interface Sci. 1977, 61 (2), 302e311, with permission from Elsevier. 190 Chapter 5 5.4.6 Redispersion of NPs by Peptization Peptization is the process of redispersing a colloid that has been weakly coagulated. This can be done by adsorbing charged ions (and/or adjusting pH) that reestablish the repulsive electrostatic double layer.2 An example of silica peptization consists of a two-step pH adjustment. In this example an acid such as sulfuric or hydrochloric acid is added to a dilute aqueous solution of sodium silicate while stirring, or while heating as necessary, to neutralize pH and form a precipitate of silica hydrogel, which contains high electrolyte concentrations. Next, this crude silica hydrogel is washed with water, removing the electrolyte, and afterward pH is adjusted to 8.5e10, which is far from the isoelectric point of silica (pH z 2). The precipitate is then heated for several hours in an autoclave (120e150 C) to allow the hydrogel to peptize and form a stable colloidal suspension. During this process, electrostatic repulsion increases and primary coagulated particles are redispersed, forming a silica slurry. The silica sol prepared using this method can contain up to 30 wt% SiO2. However, it has the drawback of creating irregular particles, despite the fact that they can be relatively small (10e20 nm).153 5.5 Properties and Applications The properties of nanomaterials are usually dependent on size. A nanomaterial often exhibits unique physical and chemical properties compared with its bulk counterparts. As described earlier, not much literature is available describing the size-dependent properties of silica NPs. Some properties such as specific surface area and photoluminescence with respect to the particle size are rarely reported. Therefore in this section some general size-dependent properties of nanoceramics are briefly discussed. 5.5.1 Silica NPs Unique properties of nano-sized SiO2 have led to a marked increase in its application. The easy preparation methods and versatile properties have led to applications in fields and products such as biomedicine, catalysis, sensors, coatings, electronic and thermal insulators, and thin-film substrates.1e5 However, many of these applications are heavily dependent on the size of the NPs. A complete review dealing with recent uses of silica-based materials was published in 2013 by Ciriminna et al.,157 in which emerging applications in the controlled release of fragrance and aromas, active pharmaceutical ingredients and biocides, inks and coatings, and catalysis for fine chemicals are listed. Many advances have also been achieved in more conventional applications such as catalysis, additives, and construction and separation techniques. Silica and Titania Nanodispersions 191 Catalysis Silica NPs have attracted attention in catalysis because of their improved efficiency under mild and environmentally benign conditions in the context of “green” synthesis. Because of their enormously large and highly reactive surface area, NPs have the potential to exhibit superior catalytic activity in comparison to their bulk counterparts. Commonly used metallic/bimetallic nanocatalysts are expensive and toxic. Reports show remarkable catalytic activity of silica NPs synthesized using the Stöber method in bis-Michael addition175 and anti-Markovnikov addition of thiols to alkenes. Mesoporous silica NPs allow the confinement of reactions into the pores. These nanoreactors provide efficient catalytic materials for producing commodity chemicals and energy. Furthermore, surface functionalization allows the excellent control of many reactions. Additives By adding SiO2 NPs, many product characteristics are improved, including suspension stability, thixotropy, adhesion strength between the substrate and coating, and the degree of finish. For example, in paint formulations SiO2 nanopowder is added to water base formulations at 0.3e1% of the total weight (fully dispersed). The main advantage is a significant reduction in paint drying time, which is shortened. Furthermore, other advantages are that resistance to UV radiation is greatly increased, the scrub resistance is also increased by several orders of magnitude, and the stain resistance of the coating is improved significantly. 253,254 SiO2 NPs can be added to plastic, when fully dispersed in polypropylene, polyvinylchloride (PVC) plastic raw materials can significantly improve strength, toughness, wear resistance and aging resistance of plastics. For example, nano-modified polypropylene can match or exceed the performance of nylon 6 in terms of water absorption, insulation resistance, compressive residual deformation, flexural strength, and other indicators. 255 Strengthening fillers for concrete SiO2 NPs can be produced in large quantities and at low cost, allowing application in concrete. It may replace cement in the mix, which is the most costly and environmentally unfriendly component in concrete. The use of NPs makes concrete financially more attractive and reduces the CO2 footprint of the concrete products produced. The NPs also increase the product properties of the concretedthe workability and the properties in a hardened statedenabling the development of high-performance concretes for extreme constructions. That means that a concrete with better performance, lower costs, and an improved ecological footprint can be designed. 192 Chapter 5 Drug delivery systems Amorphous silica particles are nontoxic and are regularly used as food additives and components of vitamin supplements (as colloidal suspensions). Encapsulation in silica has also been found to prolong the shelf life and preserve the metabolic activities of enzymes,176 bacteria,177 and mammalian cells.178 This confirms the high compatibility of silica with biological systems. The use of a solegel process to prepare nanocapsules allows synthesis at low temperatures and makes silica a good alternative to organic polymers to be used as a stable, nontoxic platform for biomedical applications such as drug delivery. Furthermore, the release rates can be precisely controlled by tailoring the internal structure of the particles for a desired diffusion (release) profile.233,234 Stabilization of pickering emulsions An interesting application of silica NPs is the stabilization of O/W emulsions, preparing so-called Pickering emulsions.179 Giermanska-Kahn et al.180 started with classical surfactant-stabilized O/W emulsions, which are sheared and size fractionated to attain monodispersity. Then the surfactant is removed by a dialysis method and simultaneously replaced by silica particles. In this way stable Pickering emulsions are obtained with the same size distribution as the initial surfactant-stabilized ones. Dulle and Glatter181 and Sadeghpour et al.182 obtained very stable O/W emulsions of about 200 nm stabilized by nano-sized hydrophilic silica after a simple surface treatment method with oleic acid, improving the hydrophobicity of the particles while maintaining their charge and stability. 5.5.2 Titania NPs The properties of TiO2 NPs strongly depend on the crystallographic phase, structure and morphology, and particle size. TiO2 exhibits good photocatalytic properties; hence is used in antiseptic and antibacterial formulations, in degrading organic contaminants and germs, as a UV-resistant material, in the manufacture of printing ink, self-cleaning ceramics and glass, coatings, and so on. TiO2 is also used in the manufacturing of cosmetic products such as sunscreen creams, whitening creams, morning and night cosmetic creams, skin milks, and toothpastes. Uses of TiO2 are also found in the paper industry for improving the opacity of paper. Another application is the decontamination of smart textiles, as described by Senic et al.183,184 The use of TiO2 as photoactive catalyst for solar fuels, photovoltaics, and environmental remediation constitutes the main advanced research areas for TiO2. In all these applications TiO2 performs three important functions: first, as a photochemical energy conversion material because of the electronic structure of photocatalysts; second, as a photosensitizer substrate because of its surface area and surface stability in artificial photosynthesis applications; and third, as an electron transport scaffold in photovoltaic Silica and Titania Nanodispersions 193 applications because of its electrical properties. The research findings derived from these investigations have then been used for other advanced applications such as sensing, biomedicine, and electronics.182e193 The next two sections describe in more detail applications of TiO2 NPs in photocatalysis and photovoltaics. Photocatalytic applications The tuning of photocatalytically active materials such as TiO2 has recently attracted attention because of the enormous significance of these materials in the fields of energy conversion and chemical synthesis. There are numerous ways to improve the performance of catalysts, for example, via modifying the composition by doping194e196 or by using cocatalysts. But optimizing the morphology (surface area and porosity),197 the particle size,198,199 and the crystallinity200,201 also offers various possibilities. Because of the presence of a small number of oxygen vacancies, TiO2 is an n-type semiconductor. The valence band of this oxide is mainly formed by the overlap of the oxygen 2p orbitals, whereas the lower part of the conduction band is mainly constituted by the 3d orbitals of Tiþ 4 cations. The band gaps, that is, the void region that extends from the top of the filled valence band to the bottom of the vacant conduction band, are 3.2 and 3.0 eV for anatase and rutile, respectively. Therefore TIO2 is active under near-UV light (UVA band from w3.0e3.9 eV). These large values avoid the absorption of a significant fraction of visible light, resulting in poor solar-to-hydrogen conversion efficiency. Heterogeneous photocatalysis belongs to the group of advanced oxidation processes, which refers to procedures to remove organic (or also inorganic) pollutants from water through reactions involving hydroxyl radicals. Specifically, the process of photocatalysis (either in a gaseous or an aqueous medium) is initiated by light energy equal to or greater than the band gap of the semiconductor. As a consequence, excitation of an electron from the valence band to the conduction band is produced, thus generating electron(eBC)ehole(hþBV) pairs in the semiconductor material, as in the case of a TiO2 particle, illustrated in Fig. 5.15. Such electronehole pairs possess an extremely high capacity to oxidize and thus are able to completely mineralize a large variety of both organic and inorganic chemicals. The lifetime of such electronehole pairs is in the nanosecond regime.202 This is sufficient for the created pair to undergo a charge transfer to adsorbed species on the semiconductor surface from contact with a solution or gas phase. However, the electronehole can also recombine with the release of heat. This process can occur either at the surface or in the particle bulk, and it actually competes with the charge transfer. Recombination depends on parameters such as the degree of crystallinity.203 At the surface, the semiconductor can donate electrons to reduce the oxygen, which is a common electron acceptor in aerated solutions.204 On the other side, the holes can be combined with electrons given by the 194 Chapter 5 Figure 5.15 Different processes that take place in a semiconductor TiO2 particle under near-ultraviolet light irradiation. Adapted with permission from Linsebigler, A. L.; Lu, G.; Yates, J. T., Jr. Photocatalysis on TiO2 Surfaces: Principles, Mechanisms, and Selected Results. Chem. Rev. 1995, 95 (3), 735e758. Copyright 1995 American Chemical Society. donor species. Two main oxidative paths are recognized today.205 In the first the holes might directly oxidize the adsorbent compounds, whereas in the second they might oxide adsorbed water or the hydroxyl groups to form OH radicals, which eventually are responsible for the mineralization of the adsorbed species. It should be pointed out that heterogeneous processes are efficient only when the pollutant concentration is relatively low (up to 100 ppm). Moreover, the reaction is enhanced if the donor species are preadsorbed onto the catalyst’s surface. The performance of TiO2 photocatalysts strongly depends on the crystal phase, particle size, and surface structure (eg, surface hydroxyl, oxygen vacancy, specific surface area).207 In this way the preparation and processing of the photocatalyst have a great influence on the physicochemical properties, which affect the photocatalytic activity. Heterogeneous photocatalysis shows some advantages with respect to other advanced oxidation processes, such as the efficiency in eliminating toxic halogenated chemicals, that additives are not necessary, and that the photocatalyst remains intact after the reaction. Photocatalytic activity of TiO2 is particularly interesting for the development of self-cleaning materials. These materials have recently received substantial interest208 because of their wide range of applications in various fields ranging from indoor applications of fabrics,209 furnishing materials,210 and window glass211 to exterior construction materials, roof tiles,212 car mirrors,210 and solar panels.213 These materials can easily be cleaned by a stream of natural water such as rainfall, which in turn significantly reduces the cost of routine maintenance. Silica and Titania Nanodispersions 195 Photovoltaic applications Photovoltaic applications of TiO2 have been well investigated.214e216 Because of the large surface-to-volume ratio of NPs, photocatalytic processes are favored in the same way as increasing the loading of photoactive sensitizers in hybrid solar cells, and they benefit from optimization at the nanoscale of the diffusion length of electronehole pairs in hybrid-heterojunction solar devices.217 More specifically, photovoltaic process involving TiO2 nanostructures are the result of a complex interplay between concurring phenomena taking place at the nanoscale, which include (1) the interaction of molecules (from small inorganic to large organic compounds) with the surface of nanocrystals and nanostructures; (2) the absorption of light, which raises such interacting systems to high-energy-excited states; and (3) a series of charge transfer or redox processes, which lead the system to recover its ground state by accomplishing the photovoltaic task. Different inorganic materials, particularly NPs, are currently mixed with organic dyes and polymers to enhance a device’s efficiency. These types of cells are called hybrid solar cells. It is well established that NPs play a significant role in the performance of the photovoltaic properties of a device. The introduction of trace amounts of NPs in polymer/ organic solar cells can significantly enhance and tune their mechanical, electrical, and thermal properties.218 5.6 Current Trends and Novel Tendencies 5.6.1 Recent Tendencies on the Preparation of Silica NPs Research on new alkoxides Solegel chemistry offers the possibility to design molecular precursors via chemical modification of metal alkoxides by nucleophilic reagents.80 Most organic ligands are usually removed during hydrolysis and condensation reactions, while more strongly bound organics or adsorbed solvent molecules are burnt during thermal treatment, leading to the formation of oxide glass or ceramics. However, solegel synthesis at room temperature can keep organic functional groups inside the inorganic gel matrix and lead to hybrid materials. These new materials offer many opportunities for applications in different fields, such as patternable optical devices, photonics, sensors, and catalysis. Ormosil, ormocer, or ceramer219,220 materials can be easily synthesized because SiC bonds are highly covalent and therefore do not break upon hydrolysis. Interest in nanocomposites Silica is a chemically stable material that often requires its combination with other materials to form composites and obtain functionality.221e229 Novel silica-based nanocomposites can offer tailor-made properties that target specific applications. An interesting example is imparting electrical conductivity to porous silica made by solegel 196 Chapter 5 processes.223e226 Three approaches have been developed. The first one is postfunctionalization, which consists of the synthesis of pure silica followed by surface deposition of an electrically conductive material,223 obtaining a significant increase in conductivity. In the second approach, functional species are introduced during the silica solegel process, before condensation of silanols and formation of siloxane bonds. For example, formation of silicaegraphene nanocomposites by single-step methods leads to electrical conductivity values around 0.5 S/cm.225 A third approach, different from conventional methods, was described by Warren et al.226 In this approach, a metaleligand solegel precursor is used, incorporating high loads of metal into silica materials. Uniformly sized and functionalized silica NPs doped with a large amount of metal were obtained, achieving remarkable electrical conductivities.226 For example, silicaepalladium nanocomposites exhibited an electrical conductivity only 80 times less than that of bulk palladium, and exceeded the conductivity of porous carbons by a factor of 100.226 These high conductivity values enable these materials to be implemented in high-current devices. Nanocomposites might also have applications in catalysis, where control of surface functionality is required. Hybrid TiO2e2SiO2 NPs can be obtained with a low degree of TiO2 crystallinity,227 by reaction of 3-(propyltrimethoxysilyl) acetylacetone with Ti(OiPr)4. The UV-visible spectrum of the resulting TiO2e2SiO2 materials showed that both fourand six-coordinate titanium atoms are present. An interesting example of catalysis is the immobilization of amino acids (histidine and glutamic acid) by amide bonds on the surface of mesoporous silica supports. These immobilized amino acids were used as ligands for Fe(II), and the obtained biomimetic iron complexes were tested as catalysts for cyclohexane oxidation at mild reaction conditions with hydrogen peroxide as the oxidant.228 Other applications of nanocomposites can be found in nonlinear optics and light-emitting devices. An example is organic/inorganic nanocomposites with lanthanide complexes, which can possess luminescent properties based on energy transfer from the absorbed coordinated ligands to the chelated lanthanide ions. Silica-based NPs with such lanthanide complexes can be formed in situ by solegel chemistry.229 This example illustrates the importance of solegel processes in the development of new materials for nonlinear optics and light-emitting devices. Consequently, solegel nanocomposites have prompted great academic interest in further investigations, and extensive research is focused on the development of novel materials. Research on encapsulation There has recently been an increased interest in the use of solegel silica particles for encapsulation of active molecules and controlled release.230e234 Silica particles offer an interesting alternative to organic delivery systems. Their intrinsic hydrophilicity and Silica and Titania Nanodispersions 197 biocompatibility, as well as the excellent protection they provide for their internal payload, makes them perfect candidates for controlled drug delivery applications. Silica microspheres (with monodisperse dimensions typically between 10 and 100 mm) or silica matrix particles share the same basic advantages: enhanced mechanical strength, chemical and physical stability, biocompatibility, and an environmentally benign nature. The release rates for a given drug can be designed to range from very slow to very fast, as porosity, and thus diffusion through different porous structures, can be easily controlled in any solegel silicate. Furthermore, the surface of the particles can be functionalized to minimize protein interaction and enhance blood circulation for active targeting. 5.6.2 Recent Tendencies on TiO2 NPs Future advanced uses of TiO2 clearly depend on the development of more complex materials with advanced functionalities. Materials that are purpose-built to perform specific tasks are the targets of such current research. For example, for photocatalysis, TiO2 has the advantage of good photocatalytic activity and photostability and low cost.235 However, one limitation of TiO2 as a photocatalyst is its wide band gap, making TiO2 sensitive to only UV light, which covers less than 5% of the solar spectrum. To overcome this intrinsic disadvantage, two strategies have been pursued. The first is to broaden the active spectrum of TiO2 by a variety of chemical modifications that adjust its electronic structure. The other strategy, aimed at improving TiO2 utility, is to create TiO2 with a large surface area. In photocatalysis the photocatalytic conversion scales with the surface area. This implies a great interest in small NPs. For example, TiO2 NPs with sizes around 2e10 nm can be synthesized using a modified solegel method at around 50e100 C.236e241 As mentioned before, TiO2 nanomaterials, including NPs, nanorods, nanowires, and nanotubes, are widely investigated for various applications in photocatalysis, photovoltaics, batteries, photonic crystals, sensors, UV blockers, “smart” surface coatings, pigments, and paints. Various methods such as solegel, sol, hydrothermal/solvothermal, physical/ chemical vapor deposition, and electrodeposition have been successfully used in making TiO2 nanomaterials. New physical and chemical properties emerge at the nanometer scale, and they vary with the sizes and shapes of the nanomaterials. The movement of electrons and holes in semiconductor nanomaterials is governed by the well-known quantum confinement, the transport properties related to phonons and photons are greatly affected by the size and geometry of the materials, and the specific surface area and surface-tovolume ratio increase dramatically as the size of a material decreases. The large surface area brought about by small particle size is beneficial to most TiO2-based devices because it facilitates reaction/interaction between the devices and interacting media, which mainly occurs on the surface and depends on the surface area. As the size, shape, and crystal structure of TiO2 nanomaterials change, not only does surface stability vary but the 198 Chapter 5 transitions between different phases of TiO2 under pressure or heat become size dependent as well. Research on nanocomposites or doped NPs A serious limitation of TiO2-based photocatalysis is its low efficiency. The most important limitations are probably the low rate of electron transfer to oxygen and the high recombination rate of electronehole pairs, which limits the rate of photo-oxidation of organic compounds on the surface of the catalyst. For this reason, many recent studies have been carried out to produce doped TiO2 NPs or nanocomposites. Addition of noble metals such as gold, silver, and platinum to TiO2, which may enhance the overall photocatalytic efficiency, has been reported by many investigators. TiO2 modified by metal clusters exhibits a reduced electronehole recombination rate as a result of the Schottky barrier at the metaleTiO2 interface.206 Moreover, silver is known to have some antimicrobial properties.242,243 Therefore TiO2 modified by silver NPs is a promising material for antimicrobial applications. Among the many various modification strategies for visible lightedriven titania, TiO2 NPs can be hosts for metal or nonmetal dopants.244e246 The band gap of TiO2 can be modified effectively by doping with transition metal ions (eg, vanadium247 or selenium248 at titanium sites). To enhance the visible light efficiency, much research has been undertaken to modify the electron structure of TiO2 and to narrow its band gap by doping with either anions or cations; this body of work has been reported and reviewed elsewhere.249,250 Research on coreeshell NPs Applications of coreeshell materials can be found in the control of surface wetting.251 A new type of surface with supported Ag@TiO2 (core@shell) particles has been described. When irradiated with UV light, these surfaces become superhydrophilic, reducing the water contact angle to zero.251 Surfaces can be converted back again to the superhydrophobic state, with a water contact angle larger than 150 degrees, when samples are heated or illuminated with visible light for a sufficient period of time. This superhydrophobicesuperhydrophilic transition is fully reversible. Other applications of coreeshell NPs can be found in enhanced photocatalytic processes. Metal NPs can be anchored on the TiO2 surface, as isolated “nanoislands,” obtaining heterointerfaces that are able to trap photogenerated electrons. Therefore, TiO2@metal core@shell particles might have applications in the photodegradation of pollutants. However, a typical problem is that metal particles may be corroded or dissolved via photocatalytic reactions. On account of this problem, heterostructures with metal cores and semiconductor shells (metal@TiO2) have attracted much deeper attention. Nevertheless, Silica and Titania Nanodispersions 199 the semiconductor shells in metal@TiO2 NPs might be unstructured, exposing crystal facets with low reactivity or having small specific surface areas. Consequently, it is important to control the structures and morphologies of shells in metal@TiO2 particles, and this objective has recently become an interesting research field. An example is the development of Au@TiO2 NPs with a truncated wedge-shaped morphology,252 which can be synthesized by a relatively simple hydrothermal method, obtaining interesting photocatalytic properties. This example illustrates that the availability of new chemical routes for the synthesis of heteronanostructures is renewing the interest of coreeshell particles for catalytic applications. 5.7 Concluding Remarks Silica and TiO2 NPs have been a focus of interest for both fundamental research and technological developments. Tremendous effort has been devoted to the synthesis of SiO2 and TiO2 NPs with narrow size distributions, the formation of hybrid and/or nanocomposite structures, and the control of properties. As a consequence, a rich database of scientific information on tailor-made NPs has been established. Many synthetic routes have already been investigated by the scientific community, including precipitation in solution, synthesis in microemulsions and nano-emulsions, nucleation and growth by ion exchange, synthesis in the vapor phase, synthesis in aerosols, and synthesis using mechanochemical and solvothermal methods. Fundamental research has often been focused on precipitation, microemulsion, and nano-emulsion methods because of their precise control of particle properties, whereas industry has centered its attention mainly on ion exchange and vapor phase methods, since these methods allow NPs to be produced on a very large scale and at a low cost. In the scientific literature alkoxides are the most commonly used precursors because of their chemical stability, controlled reactivity, and high purity. Consequently, the literature on some alkoxides (eg, TEOS for the synthesis of silica) is extremely wide and comprehensive. However, alkoxides are not often used in industry because of higher costs; halides and water-soluble salts (such as TiCl4 or sodium silicate) are widely preferred in production plants. Therefore some synthetic strategies and precursors of great industrial interest are underrepresented in the scientific literature. For example, ion exchange methods have been widely used in industry for many years at a very large scale, but nevertheless, nucleation/growth processes, as well as nanoparticle stability, have been less studied from a fundamental point of view. 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