Reverse microemulsions/micelles method
The reverse micelle approach is one of the recent promising routes to nanocrystalline materials. Several recent studies have shown that this approach is a potential candidate to synthesize nanocrystalline metal oxide powders with well-defined and controlled properties.[83-90] By carefully controlling reaction parameters, this technique affords a great deal of control over the particle size and shape.
Surfactants dissolved in organic solvents form spheroidal aggregates called reverse micelles. In the presence of water, the polar head groups of the surfactant molecules organize themselves around small water droplets, small water pools 100 A), leading to dispersion of the aqueous phase in the continuous oil phase as shown in Fig. 4.[91-93]
Reverse micelles are used to prepare nanoparticles by using a water solution of reactive precursors that can be converted to insoluble nanoparticles. Nanoparticle synthesis inside the micelles can be achieved by different methods including hydrolysis of reactive precursors, such as alkoxides, and precipitation reactions of metal salts.[84,85] Solvent removal and subsequent calcination leads to the final product. A variety of surfactants can be used in these processes such as, pentadecaoxyethylene nonylphenylether (TNP-35),[85] decaoxyethylene nonyl-phenyl ether (TNT-10),[85] poly(oxyethylene)5 nonyl phenol ether (NP5),[90] and many others that are commercially available. Several parameters, such as the concentration of the reactive precursor in the micelle and the weight percentage of the aqueous phase in the microemulsion, affect the properties, including particle size, particle size distribution, agglomerate size, and phases of the final oxide powders. There are several advantages to using this method—the preparation of very small particles and the ability to control the particle size. Disadvantages include low production yields and the need to use large amount of liquids.
Fig. 4 Schematic representation of (a) micelle and (b) inverse micelle.
Low-temperature wet-chemical synthesis; precipitation from solutions
One of the conventional methods to prepare nanoparticles of metal oxide ceramics is the precipitation method.[94-96] This process involves dissolving a salt precursor, usually chloride, oxychloride, or nitrate, such as AlCl3 to make Al2O3, Y(NO3)3 to make Y2O3, and ZrCl2 to make ZrO2. The corresponding metal hydroxides usually form and precipitate in water by adding a base solution such as sodium hydroxide or ammonium hydroxide solution. The resulting chloride salts, i.e., NaCl or NH4Cl, are then washed away and the hydroxide is calcined after filtration and washing to obtain the final oxide powder. This method is useful in preparing composites of different oxides by coprecipitation of the corresponding hydroxides in the same solution. One of the disadvantages of this method is the difficulty to control the particle size and size distribution. Very often, fast (uncontrolled) precipitation takes place resulting in large particles.
Colloidal methods
Some nanostructured metal oxides can also be prepared through the oxidation of metal colloids. Nanosized (i.e., 3-5 nm) colloidal Fe(0), Co(0), and Ni(0) particles are very oxophilic both in solution and in powder form, and cannot be redispersed after exposition to air. However, the precisely controlled, stoichiometric addition of argon-diluted air to an organic solution of a 3-nm Fe(0)-sol stabilized by N(octyl)4+Br" leads to a rusty-brown solution of colloidal Fe3+ oxide, which can be isolated and redissolved, e.g., in THF.[97] Colloidal CoO nanoparticles have also been prepared by air oxidation of N(octyl)4+Br" stabilized Co(0) particles[98] (Fig. 5).
Particles prepared via the colloidal approach are also easily supported to form heterogeneous catalysts. It was shown that air oxidation at room temperature leads to surface passivation. Consequently, the resulting particles show a composite structure with a metallic core surrounded by an oxide surface layer.[99] Recently, a new process for the manufacture of a water-soluble PtO2 colloid has been developed, which is significant because of its use as a water-soluble ”Adams catalyst.”[100,101] Colloidal PtO2 stabilized by carbo- or sulfobetaines, respectively, were prepared by simple hydrolysis/condensation of metal salts under basic aqueous conditions in the presence of the surfactants. This method was further exploited to give bi-and trimetallic colloidal metal oxides used as precursors for fuel cell catalysts, e.g., colloidal Pt/RuOx and Pt/Ru/ WOx.[102]
Fig. 5 Schematic representation of the oxidation of tetraalkylammonium stabilized colloids.
SPECIFIC PROPERTIES AND APPLICATIONS
As previously mentioned, the properties of nanoparticles are usually size-dependent. When prepared in nanometer size particles, materials exhibit unique chemical and physical properties that are remarkably different than those of the corresponding bulk materials. The study of physical and chemical properties of nanoparticles is of great interest as a way to explore the gradual transition from atomic or molecular to condensed matter systems.
As the size of a particle decreases, the percentage of atoms residing on the surface increases. As an example, a study on different samples of MgO nanoparticles has revealed that for particles ~ 4 nm in diameter, ~ 30% of the atoms are surface atoms.[18] Naturally, surface atoms/ ions are expected to be more reactive than their bulk counterparts as a result of coordinative unsaturation. Because of this and the fact that the surface-to-volume ratio is large, it is not unusual to see unique behavior and characteristics for nanoparticles. This particle size effect is a characteristic of different nanomaterials including metal oxides.
In this section, we will briefly discuss some selected properties of nanophase metal oxides showing significant size dependence.
Chemical Properties: Acid/Base Behavior of Metal Oxide Surfaces
Metal oxides are often hard acids or bases (e.g., MgO, Al2O3), so they possess sites capable of catalyzing acid/ base chemistry. Several insulating oxides and oxide composites were found to be potential catalysts for a variety of important reactions as a result of their surface basicity or acidity.[103-107] Some selected reactions typical to metal oxides include dehydration of alcohols, cracking of hydrocarbons, isomerization of olefins and parrafins, dehydrohalogenations, alkylations, and esterifications.
Acidity and basicity vary from one metal oxide to another. Several metal oxides exhibit surface basic behavior, such as MgO, CaO, and SrO, while others are considered to be acidic solids that possess more and stronger acidic sites on their surfaces, such as Al2O3. Acid/base behavior and the presence of several types of deficiencies in the lattice and on the surface are two major driving forces for surface reactivity of metal oxides. When metal oxides are prepared in nanostructures, the percentage of coordinatively unsaturated ions, especially on edges and corners, increases significantly. Consequently, surface chemistry effects, which are barely noticeable in large particle systems, become overwhelming in nanopar-ticle systems. These effects are demonstrated by enhanced surface reactivities and catalytic potentials possessed by many nanoparticle systems of metal oxides.[108-112]
Two of the most intensively studied nanoparticulate systems of the metal oxides are MgO and CaO. Two types of nanocrystalline oxides have been prepared and thoroughly studied; a ”conventional preparation” (CP), and an ”aerogel preparation ” (AP).[1,17,18,81] Nanocrystalline MgO prepared by a modified aerogel procedure (AP), yields a fine, white powder of 400-500 m2/g and 4 nm average crystallite size. High-resolution transmission electron microscope (TEM) imaging of a single crystallite indicated a polyhedral structure suggesting the presence of high surface concentrations of edge/corner sites, and various exposed crystal planes (such as 002, 001, 111).[113] Conversely, the conventional preparation (CP) yields particles with surface areas of 150-200 m 2/g and 8 nm average crystallites.
If intrinsic surface chemistry differences due to size are to be uncovered, consider that in bulk MgO the effective ionic charges are close to + 2, whereas the MgO molecule is much more covalent with effective charges close to + 1.[1] Lower coordination surface ions such as Mg3c2+, Mg4c2+, O3c2 ", and O4c2" are expected to have effective charges between +1 and +2. Surface sites on crystalline and powdered MgO have been probed by theoretical as well as experimental efforts. Ab Initio calculations with H2 have been used to probe perfect crystal surfaces and various defect sites. On the perfect (100) MgO surface, H2 has a small adsorption energy and does not dissociate. However, temperature programmed desorption methods have shown that polycrystalline samples do dissociate H2, probably on O3c-Mg3c sites. These sites are apparently very active for heterolytic H2 dissociation. The micro-faceted (111) surface of MgO is particularly reactive, and steps, kinks, and point defects (ion vacancies and substitutions) are also important. Indeed, the unique catalytic properties of defective MgO surfaces also depend on a plethora of unusual coordination sites.
There have also been studies of the Lewis acid and base sites of metal oxide nanoparticles using a variety of techniques. For example, through the use of probe molecules, electron spin resonance (ESR) was used to quantify the Lewis acid and base site on AP-MgO.[114] Surprisingly, AP MgO was found to possess both types of Lewis sites, which is very interesting because MgO is not typically associated with acid catalysis, and it is believed that this is the first observation of this type of Lewis acid activity on MgO.[115]
Catalytic properties of transition metal oxides
The development of nanoscale transition metal oxides has been of importance to several applications, especially in catalysis. Transition metal oxides that are electrically conductive are of fundamental importance to catalysis and, in particular, to fuel cells. Ion and electron transfer reactions required for these applications require high surface area materials with defective or charged sur-faces.[116] Water-soluble PtO2 (i.e., a colloidal ”Adams catalyst”) has been applied in the immobilized form for the reductive amination of benzaldehyde by n-propyl-amine.[100,101] The selectivity in favor of the desired monobenzylated product was found to be >99% and the immobilized PtO2 was found to be 4-5 times more active than the commercial Adams catalysts. The PtO2 colloid was also effective in the hydrogenation of carbonyl compounds, or of olefins in solution or in immobilized form.
The most active and CO-tolerant fuel cell catalysts [direct methanol fuel cell (DMFC) and proton exchange membrane fuel cell (PEMFC)] have been shown to contain oxides and hydrous oxides of Pt and Ru. Recent studies have revealed that practical Pt-Ru blacks are not single phase materials, but are instead bulk mixtures of Pt metal, Pt hydrous oxides, and hydrous and dehydrated RuO. A proposed mechanism for the increased CO tolerance is that Pt-adsorbed CO is removed via an oxygen transfer step from electrogenerated Ru-OH because Ru(0) transfers oxygen more effectively than Pt(0). Additionally, recent studies have suggested that the presence of metal oxides (in particular, Ru, Sn, and Mo) in electrocatalysts working with a carbon containing feed show improved CO tolerance.[117,118] It is generally accepted that this is a result of the oxide interacting with the CO-poisoned metal (usually Pt) and oxidizing the CO to CO2. Because these metal oxides are composed of metals with high oxidation potentials, the hydrous oxides are readily regenerated by water in the feed.
Furthermore, by combining Pt/Ru alloy catalysts with transmission metal oxides (WOx, MoOx, VOx) improved DMFC catalysts have been produced. Electrochemical results demonstrated that the introduction of the oxides leads to an improvement of the catalytic activity toward methanol oxidation.[119] The addition of a transition metal oxide to the PtRu catalyst led to a decrease in the methanol oxidation and surface oxide formation with the most effective being VOx.
Ruthenium oxide, in particular, has been the subject of numerous investigations because of the numerous chemical and electrical applications it can be used for. Ruthenium oxide also catalyzes the Fischer Tropsch methanation of CO2, and selectively hydrogenates benzene and its derivatives to cyclohexane and relevant cycloalkenes.[120] Solid state nuclear magnetic resonance (NMR) investigations of hydrous ruthenium oxide prepared using LiOH have demonstrated that the mobility of water molecules and their interaction with ruthenium oxides play an important role in proton charge density.[121]
RuO2-TiO2 aerogels have been prepared and the redistribution of electrical properties on the nanoscale have been studied. It was found that the electrical (electronic and protonic) transport properties of the bulk RuO2-TiO2 are redistributed when synthesized as an aerogel. Electron transport dominates the characteristics of the dense form, while protonic transport of the hydrous oxide surfaces governs the electrical properties of the aerogel.[122] Anhydrous RuO2 is also used as a thick film resistor but the hydrous oxide is preferred in electrocata-lysis.[123] RuO2 electrodes are generally prepared by the thermal decomposition of RuCl3yH2O,[124] which produces hydrous materials that are more correctly described as RuOxyH20 or RuOxHy.
Adsorptive Properties
Compared to their conventionally prepared and commercial counterparts, nanoparticles of several metal oxides exhibit a significantly enhanced ability to chemically adsorb and dissociate a variety of organic molecules on their surfaces. One of the great promises that nanoparticles of metal oxides hold in chemical applications is their remarkable ability to chemically adsorb a wide variety of molecules, especially organic molecules that are of concern as environmental hazards.
Several oxides have shown promise in this field including MgO, CaO, Al2O3, SiO2, and ZnO. A wide range of molecules including chlorinated hydrocarbons, phosphorous compounds, alcohols, aldehydes, ketones, and amines were found to strongly adsorb and chemically decompose on the surfaces of these oxides.[125-133] Details and examples on this subject are discussed in the literature.[1,134]
It has been proposed that as particles become smaller in size, they may take on different morphologies, which may alter their surface chemistry and adsorption properties in addition to increasing the surface area and porosities.[135] One of the most intriguing observations was that nano-crystals prepared by the altered aerogel approach have exhibited higher surface chemical reactivities than more conventionally prepared samples (precipitation of hydroxides followed by vacuum dehydration, herein referred to as CP samples).[135] For example, in the reaction of 2CaO+CCl4! 2CaCl2+CO2, AP (aerogel prepared) samples demonstrated reaction efficiencies twice those of CP samples and 30 times higher than commercial sam-ples.[1,136] For the adsorption of SO2, AP MgO adsorbed three times as much as CP MgO/nm2.[137,138] For the destructive adsorption of CH3(CH3O)2PO, the reaction efficiency was four times higher for AP MgO than CP MgO, and 50 times higher than for CM MgO.[139] This high reactivity observed at both room temperature and high temperatures observed for numerous reactions demonstrates that this is not an effect of higher surface area alone. Nanoparticles (especially the AP samples) have been shown to possess a much greater number of defect sites per unit surface area, which are believed to be responsible for the observed chemistry.
Physical/Mechanical Properties
Many physical properties of nanoscale metal oxides are also size-dependent. Most of the physical properties are dominated by those of the surface, which differ from the bulk because of the different bonding geometries present in nanoscale materials. Several systems of nano-phase oxides have exhibited quite interesting and potentially useful mechanical properties, which creates the necessity for much more work on exploring their physical properties.
Improved sintering and hardness properties
Unique consolidation and compaction properties have been observed in ceramics produced from nanophase powders. Ceramic is processed from nanophase powders by first compacting a powder composed of individual ceramic particles (usually less than 50 nm in size) into a raw shape (often called a green body), then it is heated at elevated temperatures. Densification occurs as a result of diffusion of vacancies out of pores (to grain boundaries) leading to sample shrinkage, which is referred to as pressureless sintering. Fortunately, nanophase powders were found to compact as easily as their analogous submicron particles. Samples have to be sintered at the lowest temperature possible for a time sufficient to remove the residual porosity and establish coherent grain boundaries to avoid particle size growth. Successful sintering enhances the hardness of materials. However, if, hardness decreases with sintering, only grain growth is occurring.[54]
Experimental evidence has demonstrated that nano-phase powders densify at faster rates than commercial (submicron) particles. The slow densification of commercial samples is a result of their larger grain and pore sizes. It has also been found that faster densification rates allow achieving a given density at smaller grain sizes, before serious growth takes place. As a result of their small particle and pore sizes, nanocrystalline powders sinter to much greater densities than their conventional analogs at the same temperature. This also demonstrates that nanocrystalline powders, as compared to conventional powders, reach the same density at much lower temperatures, which eliminates the need for very high temperatures.[54,133,140-142]
Nonuniform heating where the outside layers of the particles densifies into a hard impervious shell, which constrains the inside of the sample from normal shrinking leading to some cracking as a result of strain incompatibility, is one disadvantage that can occur with fast densification. This problem can be avoided by several ways. The most efficient way is to heat the samples slowly to reduce the shrinkage in the outer shell while heat is transported to the inner regions.[143] Additionally, high-density nanostructured oxide systems including Y2O3, TiO2 and ZrO2 have been achieved via pressure-assisted sintering, and it has been shown that applying some pressure during sintering can increase the densification rate and suppress the particle growth.[144,145]
Reduced brittleness and enhanced ductility and superplasticity
The ability of some polycrystalline materials to undergo extensive tensile deformation without necking or fracture is referred to as superplasticity and ductility. Theoretical and experimental results provide evidence for the possibility that, traditionally, brittle materials can be ductilized by reducing their particle and grain sizes. Brittle ceramics can be superplastically deformed at modest temperatures and then heat-treated at higher temperatures for high-temperature strengthening when made from nanocrystal-line precursors. The great interest in this property stems from the fact that brittle fracture is a technical barrier in the use of ceramics in load-bearing applications. This interest in the superplasticity of oxide materials has been growing after it was experimentally demonstrated in 1986 that yttria-stabilized tetragonal zirconia polycrystals could be elongated by over 100% in tension.[145,146] Similar behavior was later demonstrated by other nanophase ceramic systems involving Al2O3, TiO2, and ZnO.[147-155]
One important use of superplasticity in ceramics is diffusion bonding, where two ceramic parts are pressed together at moderate temperatures and pressures to form a seamless bond through diffusion and grain growth across the interface. Diffusion bonds form more easily in nanocrystalline ceramics than in larger-grained ceramics as a result of both the enhanced plastic flow of nanocrys-talline ceramics and the larger number of grain boundaries they provide for diffusional flux across the inter- face.[156,157]
Other properties of ceramics that are size-dependent include electrical and optical properties. An increase in the electrical resistance and dielectric constant was observed for nanophase ceramic materials as a result of their small particle sizes.[158,159] An effect on optical properties of ceramic materials was also found because of their nanometer particle sizes. As an example, nanoparticles of TiO2 were found to become a more efficient ultraviolet (UV) absorber.[160] In conclusion, nanophase ceramic powders and metal oxides hold great promise for better materials with unique desired properties and potential applications as compared to their large-grained counterparts.
Additional selected size dependent properties
Lead zirconate-titanate (PZT), a solution of ferroelectric PbTiO3 (Tc=490°C) and antiferroelectric PbZrO3 (Tc= 230°C), belongs to the ferroelectric family of perovskite structure with a general formula of ABO3 (where A=mono or divalent, and B=tri to hexavalent ions).[158] Nanoscale PZT particles (25 nm) were synthesized by using an in situ method. Powder X-ray diffraction (XRD) studies of these particles found the sample X-ray amorphous and produced single phase PZT after heating at 500°C.
Nanophase powders of YxZr1 _xO2 _x/2 have been prepared from a mixture of commercially available ZrO2 and Y2O3 powders.[161] It was found that, depending on the starting powder mixture composition, the yttrium content in the nanophases can be controlled and the tetragonal or cubic phases can be obtained. Tetragonal or a mixture of tetragonal and cubic were observed for low yttria content (3.5% mol yttria), and cubic for higher yttria contents (19, 54, and 76% mol yttria). These powders were found to have a most probable grain radius of about 10-12 nm and the grains appear as isolated unstrained single crystals with polyhedral shapes. The grain shapes appeared to be polyhedral and not very anisotropic. Lattice fringes were parallel to the surfaces demonstrating that (100) and (111) faces dominate.
CONCLUSION
Nanoscale metal oxides are of considerable importance to both the fundamental understanding of size-dependent properties and numerous applications. While, in many cases, a basic understanding of the bonding and structure present in these systems has been determined, there is still a great deal of work to be carried out. Additionally, the methods for the preparation of oxide systems is a continually evolving area of science. Ultimately, developments in the areas of synthesis, instrumentation, and modeling will aid scientists as we try to gain an understanding of the relationships between physical, electronic, and chemical properties.
Developing a ”tool box” of synthetic methods, which would afford scientists the possibility to put atoms together into nanomaterials with predetermined shapes and sizes, is an ongoing effort among preparative chemists. Another ongoing effort is the search for relationships between these shapes and sizes, and the chemical and electronic properties observed. As rapid advances take place in these areas, we will begin to see the potential of nanoscience begin to realize its potential. Metal oxides should be at the forefront of these advances in nanoscience because of their stability and the amount of information that has been gathered about their bulk counterparts through the years.
