A nanocrystal or nanoparticle (not fully crystalline) is defined as a particle with size in range of 1 to 100 nm (102 to 107 atoms) from zero (0D) to three dimensions (3D), which exhibits the unique physiochemical properties due to the quantum size effect that cannot be anticipated from bulk counterparts. Strictly speaking, the name of “nanocrystal” is only used for crystalline nanoparticle, and is however a more general term which can refer to both crystalline and non-crystalline nanoparticles. Accordingly, their particle size is intermediate between the size of molecule and bulk solid (Rao, Müller and Cheetham 2005, Sorensen 2009). Nanocrystals can be formed in a variety of shapes including dot, sphere, cube, rod, triangle, hexagon and many others. In this size range, they possess an immense surface area per unit volume, a very large percentage of atoms in the surface. As a result, their unexpected properties can be obtained as compared to those of both individual atoms/molecules and bulk counterpart of the same chemical composition.
Size- and shape-dependent properties of the nanocrystals can be tuned by changing the dimension and designing the shape (Rao et al. 2005). Due to the materials at the nanoscale, low coordination number, surfaced edge and corner atoms are usually chemically reactive, catalytically active and polarisable surface, contributing to their high chemical potential. For example, the high surface area is of particular importance regarding heterogeneous catalytic reactions, because of the increase of interaction of reactive molecules and active sites on the catalyst surface (Abbet and Heiz 2005). Furthermore, the particle size not only affects their surface area, but also arise new properties, due to the quantum-size effect (e.g., electron confinement and surface effect) (Kroes et al. 2002, Kamat et al. 2010).
Considerable efforts have recently been devoted to the preparation of metal oxide and mixed oxide nanomaterials due to both their unique properties and their technological applications (Seshadri 2005, Burda et al. 2005, Mao et al. 2007, Yin and Alivisatos 2005). Metal oxides including the transition metals and rare earths, display a wide variety of complex structures and interesting electronic and magnetic properties associated with the changes in electronic structure and bonding and in the presence of ordered defect complexes or extended defects. The nanostructured mixed oxides can greatly generate new synergetic
properties and improve the overall application performance, that is not available from single metal oxide species, due to the appropriate combination of individual oxide
components (Redl et al. 2003). Furthermore, the size- and shape-dependent properties of nanomaterials raising expectations for a better performance generally are a consequence of quantum confinement within the particle (Alivisatos 1996). The precise controlled synthesis of the size, shape, chemical composition, crystal structure, and surface chemistry of nanomaterials allows to obtain their unique properties, which have become one of the most challenging issues faced by nanomaterial researchers. The properties of nanocrystals are drastically altered in the shape and size change, making nanocrystals as ideal candidates for many applications, such as in catalysis, energy storages, optoelectronics, sensing, magnetic resonance imaging, biomedicine. (Jun, Choi and Cheon 2006b, Hao et al. 2010, Kinge, Crego-Calama and Reinhoudt 2008, Ying 2000, Na, Song and Hyeon 2009, Jun, Choi and Cheon 2006a). A general picture in Figure 1 is schematically illustrated for the features and new phenomena of the nanoscale materials.
The size- and shape-dependent properties of colloidal metal oxide and mixed oxide structures at the nanoscale makes great demands to the synthetic methodology. Therefore, it is a great challenge to develop a “synthetic chemistry” of nanocrystals that is as precise as that used to make building units. This allows scientists to study the effect of these synthesis parameters which impart to the unique collective properties of monodisperse nanocrystals. There are two different approaches to synthesize nanocrystals: the“top-down” approach, which utilizes physical methods, and the “bottom-up” approach, which employs solution-phase colloidal chemistry. Using the top-down approach, the production of a large quantity of nanocrystals can be achieved, however, uniform-sized nanocrystals and their size control is very difficult to obtain. In contrast, using the “bottom-up” approach, the colloidal chemical methods can be used to synthesize uniform nanocrystals with controlled particle size through chemical nucleation and growth process in bulk solution, although generally only subgram quantities are produced (Schmid 2005). The use of atoms or molecules as building blocks has the advantage, that provides control over crystallite size and shape with a precision well beyond that of top-down lithography (Park et al. 2007). Indeed, bottom-up assembly of well-defined nanoscale building blocks into nanocrystals with controlled size and shape represents a powerful tool to fabricate novel multi-component materials and devices (Nagarajan 2008). In order to control the crystal growth, the capping agents are often used to decrease the surface energies of crystals. The surfactants bind selectively to the different crystallographic faces, so that shape of nanocrytals can be controlled by the nonselective or selective surfactants (2008, Cushing, Kolesnichenko and O'Connor 2004). The use of surfactant molecules, and consequently, result in oxide nanocrystals comprising an inorganic core coated with a layer of organic surfactant molecules. This organic capping provides electronic and chemical passivation of the surface dangling bonds, prevents uncontrolled growth and agglomeration of the nanoparticles, and permits chemical manipulations of the nanoparticles similarly to large molecules having their solubility and reactivity determined by the nature of the surface ligands. The most commonly used ones in colloidal syntheses include alkyl- thiols, long chain amines, carboxylic and phosphonic acids, phosphine oxides, phosphine, phosphates, phosphonates, and various coordinating (e.g., ethers, THF, DMF) or non-coordinating solvents (e.g., alkanes, alkenes).
Understanding growth behavior and morphology evolution is crucial for an efficient synthesis and a good control of inorganic nanocrystals. In the bottom-up syntheses, for the growth process of nuclei, the behavior was described by the classical Ostwald ripening mechanism, in which the growth of larger particles at the expense of smaller ones driven by surface energy reduction. This phenomenon was extensively used to explain the formation of thermodynamically stable nanocrystals with nearly spherical morphologies. For the controlled self-assembly of nanoparticles into well-defined anisotropic nanostructures, organic capping reagents usually play critical roles in reducing the activity of the nanocrystal surface to promote or tune the ordered self-assembly (Kinge et al. 2008, Sellinger et al. 1998, Malenfant Patrick et al. 2007, Yin and Alivisatos 2005). An oriented attachment mechanism could offer as an additional tool to design advanced materials with anisotropic properties and could be used for the synthesis of more complex crystalline one-dimensional structures. In addition, the sterically diffusive kinetics and selective binding or nonbinding of surfactant molecules to different faces of the growing nanocrystal can also control the product’s morphology due to the possibility of breaking the limitations of crystal growth dynamically. In some cases, the formation of the intrinsic anisotropic nanocrystals is found to be a highly kinetics-driven process, which occurs far away from the thermodynamic equilibrium, and must be overdriven by high precursor monomer concentrations.
The reaction medium is crucial in the solution-based approaches. To date, noble metal nanocrystals obtained from solution-based methods that mainly used the organic reagent as solvent medium such as toluene, diphenyl ether, oleic acid/oleylamine, etc. In organic solvent systems, expensive organometallic precursors, toxic and environmentally unfriendly organic solvents are often not compatible with biomedical applications. Water, as an environmentally friendly solvent with the most abundant resource, and most metal nitrates and chloride salts were used as starting materials, can overcome these barriers. Further, due to the high solubility of metal salt precursors in aqueous media, the aqueous-based routes can be used for the synthesis of pure products in high yield. Therefore, the development of general synthetic strategies to produce the size- and shape-controlled metal oxide, mixed oxide nanocrystals in terms of low cost, environmentally benign reagents, mild synthesis conditions, and potential for large-scale production are needed among the important research topics of the advanced materials chemistry.
In the present chapter, we provide a brief account of our own recent results to synthesize different types of monodisperse colloidal metal oxide and mixed oxide nanocrystals, focusing on one-phase and two-phase solvo-hydrothermal surfactant-assisted approaches. Based on our approaches, a variety of metal oxide and mixed oxide nanocrystals with different sizes, shapes, and phases are obtained using simple chemical reactions (e.g., solvo-hydrothermal reactions), choosing appropriate reaction systems (precursor, surfactant.), and controlling reaction parameters (monomer concentration, temperature and time). The chapter is organized as follows: Introduction in Section 1; Essential concepts in the nucleation and crystal growth process of size- and shape-controlled nanocrystals in Section 2; subsequently, we present the main results in our laboratory along with the results from other research groups related to metal oxide nanocrystals in Section 3, followed by mixed metal oxide nanocrystals in Section 4. The synthetic procedures, the formation mechanisms, and the controlled growth of nuclei based on kinetic and thermodynamic conditions as well as the selection of capping agents will also be discussed to control the size and shape, and conclusion in Section 5.
2. General strategies for surfactant-assisted synthesis of colloidal metal oxide and mixed oxide nanocrystals
In a typical synthesis of inorganic nanocrystals, the precursor compound in bulk solution is decomposed to generate atoms followed by the precipitation starting from dissolved atoms as building blocks to form the nanocrystals. A understanding of the process and parameters controlling the precipitation helps to improve the engineering of the growth of nanocrystals to the desired size and shape. The precipitation process then basically consists of a nucleation step followed by crystal growth stages. Generally, there are three kinds of nucleation processes: homogeneous nucleation, heterogeneous nucleation, and secondary nucleation. For the chemical colloidal nanocrystal synthesis, homogeneous nucleation occurs in the absence of a solid interface by combining solute molecules to produce nuclei. Homogeneous nucleation occurs due to the driving force of the thermodynamics because the supersaturated solution is not stable in energy. Seed formation proceeds according to the LaMer model are shown in Figure 2. This mechanism reported in the early 50’s on the basis of the crystallization study of the solution-phase synthesis of monodisperse sulfur colloids in ethanol (LaMer and Dinegar 1950). According to LaMer plot for the crystal nucleation process, in which the concentration of atoms steadily increases with time as the precursor is decomposed by heating, colloidal nanocrystal formation comprises the following three steps: (i) The atoms start to aggregate into nuclei via self-nucleation as increasing the monomer concentration in the solution to supersaturation levels; (ii) Then monomers continuously aggregate on the pre-existing nuclei or seed which leads to gradual decrease in the monomer concentration. As long as the concentration of reactants is kept below the critical level, further nucleation is discouraged; (iii) With a continuous supply of atoms via ongoing precursor decomposition, the nuclei will grow into nanocrystals of increasingly larger size until an equilibrium state is reached between the atoms on the surface of the nanocrystal and the atoms in the solution (Watzky and Finke 1997).
After the formation of nuclei, the subsequent growth stages also strongly govern the final morphology of the nanocrystals. Generally, the nanocrystal growth can occur under two different regimes, either in a thermodynamically controlled or kinetically controlled growth regime. The manipulation between thermodynamic and kinetic growth regimes is thus a critical factor in determining nanoparticle shape (Kwon and Hyeon 2009) The final nanoparticle morphology can be controlled by dictating the shape of nuclei and directing the growth of the nuclei and/or nanocrystals. Nuclei can take on a variety of shapes determined by the chemical potentials of the different crystallographic faces, which are in turn highly dependent on the reaction environment such as temperature and solute concentration. The nuclei shape can have a strong effect on the final nanocrystal shape, for example, through selected growth of high-energy crystal faces of the nuclei (Skrabalak and Xia 2009, Pileni 2007, Searcy 1983). In the present of surfactant in bulk solution, the products are capped by surfactant molecules, resulting in the restriction of the particle growth as well as the good dispersibility of the product in reaction solvent. This is particularly important for shape control: to obtain a highly shape-monodisperse yield of nanocrystals, nucleation must occur rapidly and instantaneously.
The surfactant-assisted synthetic methods provide convenient and powerful pathway for the reproducible controlled synthesis of nanocrystals because these methods allow for the metal oxide and mixed oxide nanocrystals to be precisely adjusted in terms of their size, shape, composition, and phase structure on the nanometer scale. Nanocrystals obtained by the surfactant-assisted route, in general, exhibit excellent crystallinity and monodispersity. In the following section, we develop the simple one-phase and two-phase surfactant-assisted routes for the shape-and size-controlled synthesis of colloidal monodisperse metal oxide and mixed metal oxide nanocrystals.
Critical reaction parameters that have strong effect on the growth of the metal oxide and mixed oxide nanocrystals including precursor and surfactant concentration, the molar ratio of precursor to capping ligand, and reaction temperature and time, are precisely adjusted to control over their sizes and shapes in the crystal nucleation-growth stages. In addition, to understand the formation process of nanocrystals in the bulk solution, we discuss the possible mechanism for the shape and size control of the nanocrystals obtained from our surfactant-assisted approaches. In all cases, there are some advantages for these routes because of the use of inorganic salt precursors, instead of expensive metal alkoxides, and quite mild synthetic conditions. Particularly, these synthesis methods are scalable to multigram in a single run using the same synthetic conditions.
3. Metal oxide nanocrystals
Within the broad family of functional materials, metal oxides play a very important role in many scientific and technological areas (Lu, Chang and Fan 2006) For decades they have been extensively investigated their physiochemical properties and useful applications by solid-state chemists. Metal oxides including the transition metals and rare earths are able to form a large diversity of oxide compounds, giving the inspiration for designing new materials. The crystal structures ranging from simple rock salt to complex oxide are often built by the metal-oxygen bonds varying nearly ionic to covalent or metallic. The oxidic materials exhibit fascinating electronic and magnetic properties associating with the changes in electronic structure and bonding (Gariglio, Gabay and Triscone 2010). Additionally, metal oxides having multivalent oxidation states have attracted much attention among specialists because they often exhibit superior catalytic reaction performance (Antonini and et al. 1987) Many progresses, such as hot injection, co-precipitation, microemulsion, nonhydrolytic sol-gel process, and so on, have been devoted to fabricate metal oxide nanocrystals.
Vanadium oxides (V2O5-
The monodisperse vanadium oxide NCs with different sizes and shapes including nanospheres, nanocubes, nanorices, and nanorods can be achieved by the control of various reaction parameters, such as types of V(V) diperoxo alkylammonium complexes, and alkyl chain length of capping agents in synthesis mixture. Remarkably, a significant effect of water content on the size and shape of vanadium oxide NCs has been observed. Figure 3B shows representative TEM images of these samples synthesized at different water/toluene volume ratio (W/T) varing from 0:40 to 2:40, 8:40, and 20:40. Only uniform quasispherical NCs with an average size of 4 nm were obtained in the absence of water in the synthesis medium. While the W/T value was as low as 2:40, aggregated nanoparticles beside some un-uniformed small nanorods were formed. When the W/T value increased to 8:40, mostly short vanadium oxide nanorods were generated. However, when the W/T value increased to 20:40, the vanadium product is composed of uniformly sized and shaped rods with 20 nm in width and 150-300 nm in length. The SAED pattern taken from a single vanadium oxide nanorod reveals the single crystal nature of the nanorod, and it further confirmed that the nanorods’ elongation axis was along the  direction. The shape elongation of vanadium oxide nanocrystals in the increase of water could be explained by a lateral aggregation of individual nanorods along the longitudinal axis and further their fusion to form aligned nanorods at the high water content in the synthesis mixture.
Some research groups reported the preparation of size-turnable VOx nanotubes via the aging and hydrothermal process of various vanadium sources including bulk V2O5 powders, vanadium(V) peroxo gels, and vanadium(V) triisopropoxides using aliphatic amines as structure-directing templates (Tenne 2004, Spahr et al. 1998, Corr et al. 2008) Schlogl et al. (Pinna et al. 2003) presented a reverse micelle technique to prepare V2O5 nanorods and nanowires from a colloidal self-assembly made of sodium bis(ethyl-2-hexyl)sulfosuccinate Na(AOT)/isooctane/H2O. Park et al. (Guiton et al. 2004) synthesized single-crystalline VO2 nanowires with rectangular cross sections using a vapor transport method. Zhang et al. (Li et al. 2007) showed that the belt-, olive-, petal-shaped VO2 NCs could be synthesized with high concentrations of the reducing oxalic acid agent through the hydrothermal route. Baughman et al. (Gu et al. 2003) reported the synthesis of V2O5 nanofibers at room temperature from ammonium metavanadate and acidic ion-exchange resin in water. The resulting V2O5 nanofibers could deliver dramatically higher specific discharge capacities than micrometer-sized V2O5 fibers. The Whittingham’s group has also developed a method to produce vanadium oxide nanofibers with dimensions of less than 140 nm by coating oxides on polylactide fibers (Lutta et al. 2005).
Erbium-compound nanomaterials consisting of hexagonal Er(OH)3, monoclinic ErOOH, or cubic Er2O3 are particularly attractive among the rare earth oxides due to their remarkably electrical and optical properties. These unique properties orginate from the intra Er3+ 4
Li et al. (Wang et al. 2005b) reported a general hydrothermal method for the synthesis of a variety of nanocrystals by a liquid-solid-solution reaction. The system consists of metal salt, sodium linoleate (solid), ethanol-linoleic acid liquid phase and water-ethanol solution at different reaction temperatures under hydrothermal conditions. TiO2 nanoparticles and nanorods can be also obtained by solvothermal reaction of titanium butoxide, linoleic acid, triethylamine, and cyclohexane (Li et al. 2006). The decomposition of NH4HCO3 which provide H2O for the hydrolyzation reaction is found to be an important factor to shape evolution of particles. In the presence of NH4HCO3, the fast hydrolyzation of precursors with the water leads to the formation of nanoparticles. In the absence of NH4HCO3, in contrast, the slow nonhydrolytic condensation of precursors produces titania nanorods with uniform diameters of 3.3 nm, and a length of up to 25 nm. TiO2 nanowires could be also produced by solvothermal treatment of a mixture containing titanium tetra-isopropoxide, ethylenediamine, and ethylene glycol (Xie and Shang 2007). The diameter of nanowires was controlled by changing the amount of ethylenediamine.
Our group recently also synthesized the TiO2 nanocrystals with well-controlled shapes on the basis of solvothermal technique using both acid oleic acid (OA) and oleyamine (OM) as two capping surfactants, and water vapor as hydrolysis agent (Dinh et al. 2009). It is demonstrated that, the presence of water vapor along with the desired OA:OM molar ratio plays crucial roles in controlling size and shape of TiO2 nanocrystals. In particular, the shape of TiO2 changed from rhombic to truncated rhombic and to sphere as the OA:OM ratio increased from 4:6 to 5:5 and to 6:4, respectively. Increasing the amount of titanium butoxide (TB) led to the formation of elongated particles. For example, when the TB:OA:OM molar ratio changed from 1:6:4 to 2:6:4, the shape of TiO2 evolved from spheres to dog bone-like particles with uniform size. The solvothermal reaction of Mn(NO3)2/oleylamine/dodecanol recently flourished by Li et al. (Li et al. 2010b) was a successful way for shape control of highly monodisperse Mn3O4 nanocrystals with dot, rod, wire shapes. Moreover, the as-prepared hydrophobic spherical or elongated nanoparticles were used as building blocks to be rationally assembled into three-dimensional (3D) Mn3O4 colloidal spheres with a facile ultrasonication strategy. The as-prepared colloidal spheres were chemically converted to LiMn2O4 nanomaterials in a simple solid-state reaction. Such materials showed distinct electrochemical performance, mainly depending on their crystallinity and particle size.
The Niederberger’s group developed a nonaqueous sol-gel approach for the synthesis a variety of metal oxide nanocrystals involving the solvothermal treatment of metallic alkoxide precursors in benzyl alcohol solvent (Garnweitner and Niederberger 2008). As the presence of halide impurities in the final oxides obtained by these routes may be a drawback, alternative halide-free methods have been developed as well.
The shape of SmVO4 and CeVO4 NCs can be controlled by the synthesis temperature. TEM images of the samples synthesized solvothermally at 150º C and 180º C for 16 h are shown in Figures 8a,b (Nguyen et al. 2009a). Nearly cubic-shaped SmVO4 and round-shaped CeVO4 nanocrystals with an average diameter of 15 nm were found at 150º C. When the synthesis temperature increased to 180º C for 16 h, both uniform SmVO4 and CeVO4 nanospheres were observed, however, the diameter is unchanged. The transformation of both cubic-shaped SmVO4 and round-shaped CeVO4 NCs into uniform nanospheres, while preserving the particle size by increasing synthesis temperature from 150º C to 180º C can be explained by Wuff facets theory. The XPS results for characterization of these nanomaterials exhibite that only one oxidation state of samarium, cerium, and vanadium for each metal (e.g., Sm3+, Ce3+, V5+) was observed on the particle surface at the nanoscale, even after calcination, while the existence of two oxidation states of these metals was found (e.g., Sm3+/Sm2+, Ce4+/Ce3+, V5+/V4+) in the corresponding single metal oxide nanocrystals (Nguyen et al. 2009a).
For this approach, SmVO4 was selected as a typical example for discussion on the experimental results. It was found that their size and shape were controlled by the nature
and amount of capping surfactant as well as the metal complex precursor concentration (Nguyen et al. 2009b) Figure 6c-f displays TEM images of the SmVO4 nanocrystals obtained using two different surfactants (e.g., oleylamine and oleic acid), and the corresponding SAED patterns. When only oleylamine was used, nanospheres with ~17 nm in diameter were obtained (Figure 6c,d). However, using oleic acid as capping surfactant instead of oleylamine under the same synthesis conditions, hexagonal-like SmVO4 nanocrystals with no significant change in particle size (~17 nm in diameter) were formed (Figures 6e,f). The SAED results revealed that both the samples are indexed to a tetragonal SmVO4 single crystal. The selective-shape formation of the SmVO4 nanocrystals nanospheres and nanohexagonons can be resulted in the nonselective and selective absorption of oleylamine or oleic acid, respectively.
The effect of oleylamine (OM) and metal complex precursor concentrations on the growth of SmVO4 nanoparticles was also studied, as shown in Figure 9 (Nguyen et al. 2009b). In the absence of OM, only irregular nanocrysqtals with aggregated pearl-chain-like structures often formed. When the OM concentration increased from 0.025 to 0.060, 0.129, 0.230, 0.034, and 0.43 M, the particle size of SmVO4 nanoparticles decreased from ~30 to 3 nm. The reason for this behaviour may be due to the high degree of surfactant protection and stabilization of nanocrystals with increasing the OM concentrations in the bulk reaction solution. The larger and irregular sizes of the nanocrystals at low OM concentrations as compared to those obtained at high OM concentrations may result in insufficient coverage to the nanocrystal surface and induce their aggregation. Furthermore, in all the cases, the spherical nanocrystals were produced. The formation of spherical NPs could be due to the nonselective surfactant character of oleylamine (OM). On the other hands, it was found that the shape of SmVO4 nanocrystals elongated from ~3 nm cores into ~3 nm x 200 nm wires as increasing the metal complex precursor concentrations from 0.065 to 0.130, 0.195, 0.260 M. These results reveal that the shape evolution from nanocores to nanowires can be controlled by increasing the precursor monomer concentration, which is strongly associated with the increase of chemical potential in the bulk solution as well as the dominant oriented attachment for the formation of nanowires (Nguyen et al. 2009b)
Due to their remarkable luminescence properties, Li’s group(Liu and Li 2007) has also developed a general oleic acid-assisted hydrothermal method for the synthesis of a series of colloidal rare earth orthovanadate nanocrystals through the hydrothermal reaction of metal nitrate, NaOH, NH4VO3, oleic acid, ethanol/water mixture. The morphologies of all products were mainly square sheetlike shape with average diameters of ~20-40 nm. The products were formed by capping of oleic acid to Ln3+ first, then oleic acid attached rare earth ion (Ln3+) reacted with VO43- to form LnVO4 nuclei at the water-oleic acid interface. This was followed by crystal growth until the nanocrystals were large enough to fall to the bottom of the vessel. Further, Eu3+-doped LnVO4 nanocrystals emitted intense red light. The author also used this route to synthesize colloidal uniform rare earth floride nanocrystals (Li, Peng and Li 2009, Wang et al. 2006). Lin et al. (Xu et al. 2010) synthesized and studied the luminescence properties of Ln3+ (Ln = Eu, Dy, Sm, Er)-doped YVO4 nanocrystals via the trisodium citrate-assisted hydrothermal process. Haase et al (Sun et al. 2006c) also demonstrated an increase of visible emission intensity of Er3+-doped YVO4 nanocrystals due to photoadsorption and energy transfer of Er3+ ions to the host YVO4. An another general ultrasonic irradiation route for the lanthanide orthovanadate LnVO4 (Ln = La-Lu) nanocrystals from the aqueous solution of Ln(NO3)3 and NH4VO3 without any surfactant were also reported by Lin et al. (Yu et al. 2008). The resulting LnVO4 nanocrystals had
spindle-like shape with the equatorial diameter of 30-70 nm and the length of 100-200 nm, which were the aggregates of small particles of 10-20 nm. Furthermore, the Eu3+ and Dy3+-doped LnVO4 (Ln = La, Gd, Lu) samples showed the characteristic dominant emissions of Eu3+ at 613 nm and Dy3+ at 572 nm, respectively, as a result of an energy transfer from VO43- to Eu3+ or Dy3+.
Manganese tungstate (MnWO4) is one of the most promising mixed metal oxide nanomaterials, which exhibits high sensitivity to humidity change and unique magnetic property (Qu, Wlodarski and Meyer 2000, Arkenbout et al. 2006, Heyer and et al. 2006). Hence, it has attracted considerable research interest for potential applications such as photocatalysts, humidity sensors, optical fibers, photoluminescence and scintillator materials (Xing et al. 2008, Qu et al. 2000, Bharati, Singh and Wanklyn 1982). Several efforts have been devoted to the synthesis of MnWO4 nanoparticles and especially focused on the shape and dimensional control (Zhou et al. 2008b, Zhang et al. 2008b). We recently developed a new approach for the aqueous-phase “one-step” synthesis of uniform single-crystalline MnWO4 nanoparticles with controlled shape and the self-assembled mesocrystal microspheres/microapples with high yield using Mn(NO3)2 and Na2WO4 as precursors and bifunctional amino acid biomolecules as capping agent. MnWO4 nuclei were early formed by the combination of Mn2+ cations and WO42- anions and then grew into nanocrystals (Nguyen et al. 2011b). The nanoparticle products were capped by the amino head groups of 6-aminohexanoic acid (AHA) biomolecules and their surface become hydrophilic owing to other end of the uncoordinated carboxylic groups. Because of the hydrophilic surface character, the final products can be suspended in water medium.
The experimental results revealed that the shape elongation of MnWO4 nanoparticles is affected by capping 6-aminohexanoic acid concentration in the aqueous solution. Figure 10 shows TEM/HRTEM images of the MnWO4 nanoparticles synthesized using AHA concentration ranging from o.031 to 0.305, 0.610, 0.915 M, corresponding to the AHA/(Mn+W) molar ratio ranging from 2:1 to 10:1, 20:1, 30:1 (Nguyen et al. 2011b). The elongation of nanoparticles from 25 nm x 50 nm-sized nanobar to 25 nm x 150 nm-sized nanorods with increasing 6-aminohexanoic acid (AHA) concentration from 0.031 to 0.610 M was clearly observed. The high-resolution TEM results (Figure 8b) of a MnWO4 nanobar suggest that the nanobar is single crystal with an interplanar spacing of 0.30 nm, which corresponds to the separation between the (200) lattice planes of monoclinic MnWO4. The side surfaces of the nanobar were bounded by (100) plane and the ends of the nanobar were enclosed by the (021) plane. The (100) planes are oriented parallel to the nanobars’ growth axis, suggesting that the growth direction of the single-crystalline nanobar occurs preferentially along the  direction (
In this synthesis approach, water is adopted as the continuous solution phase and inorganic salts were used as starting materials. Due to the high solubility of the salts in aqueous solution, it is applicable to synthesize the nanoparticles in scale up by using the high precursor monomer concentrations. This aqueous-based method is thus a promising way in the academic laboratory as well as can be expanded to the industrial scale in a simple way. In fact, we obtained as much as 16 grams of ~25 nm x 50 nm MnWO4 nanobars per single run in a 700 mL-sized autoclave when the high precursor concentration of 0.122 M was used (Figure 8g) (Nguyen et al. 2011b).
The self-assembly of tailored nanobuilding units into three-dimensional (3D) mixed metal oxide microarchitectures has recently received considerable interest. Many novel and fascinating properties of these materials and useful applications are predicted depending not only on the complex morphology but also on the order degree of single-crystalline nanoparticles in microarchitectures. Sacrificial organic surfactants can act as structure-directing agents or soft templates and are widely used to design nano/microstructures with peculiar morphologies. The amino acid molecules are considered as fine assemblied agents. In this work, we also found that the formation of self-assemblied MnWO4 3D hierarchical microspheres from nanobars is favorable at relatively low precursor concentration (0.0076 M) of Mn2+ cation and WO42- anion precursors in the initial synthesis solution in the presence of 6-aminohexanoic acid surfactant (Nguyen et al. 2011b) SEM/TEM images in Figure 11A shows that self-assemblied products is quite monodisperse microspheres with two size populations, 3-5
The FTIR results clarified that the assembly mechanism of the formed microspheres caused by generating the peptide chains in bulk solution during synthesis. Only the amino (-NH2) group of AHA molecules capped on the surface of MnWO4 nanoparticles and the free carboxylic (-COOH) terminus was oriented outward. The polypeptide chains were formed
through the interaction between this uncoordinated carboxylic group on the nanobar surface and the amino group of residual 6-aminohexanoic acid in aqueous solution. The self-assembly of the peptide structure led to produce the assembled MnWO4 microspheres. Further, the microsphere morphology can thus be controlled by adjusting the concentration of the peptide, that is, the concentration of 6-aminohexanoic acid (Nguyen et al. 2011b).
This suggests that with a low concentration of Mn2+ and WO42- precursors (0.0076 M, (Mn+W)/AHA molar ratio of 0.25:1) and a low nuclei of MnWO4 was formed and a small amount of MnWO4 nanoparticles was produced. Due to the relatively high surfactant concentration (0.031 M), an excess amount of free 6-aminohexanoic acid exists in the aqueous synthesis solution, the dipeptide/polypeptide process of both -NH2 and -COOH groups of free 6-aminohexanoic acid occurred on the basis of nucleophile mechanism, resulting in the formation of polypeptide chains. Because only small amount of MnWO4 nanobars was yielded in bulk solution and because of the high AHA concentration, the peptide reaction continued and the reaction between the amino groups of the formed amino acid sequence of protein and the uncoordinated carboxylic groups on the nanoparticle surface to generate the polypeptide-stabilized MnWO4 nanobars. Subsequently, highly oriented backbone-backbone intermolecular hydrogen-bonding interactions of numerous amphiphilic polypeptide chains via either antiparallel or parallel arrangements were proceeded to spontaneous-assemble into polypeptide-stabilized MnWO4 microspheres. These interactions are thermodynamically favorable due to the reduction of the particle surface energy when the interface is eliminated. On the contrary, a large amount of monodisperse nanobars was yielded using a higher precursor monomer concentration (0.015 M, (Mn+W)/AHA molar ratio of 0.48:1) with a faster nucleation of MnWO4. The excess amount of free 6-aminohexanoic acid in synthesis solution decreased strongly because they were consumed more for the capping on nanobar surface. A possible proposed mechanism of the “one-step” formation of self-assemblied MnWO4 hierarchical microspheres is illustrated in Figure 11B. The photoluminescence results indicated that the PL emission intensity of the MnWO4 nanobars is higher than that of the MnWO4 microspheres indicating the decrease in the luminescence efficiency of the microspheres due to nanobars inside of microspheres (Nguyen et al. 2011b).
MnWO4 nanomaterials have been synthesized and revealed several morphologies such as flower-like clusters, nanowires, nanoplates, and nanorods (Zhang et al. 2007b, Zhou et al. 2008b, Thongtem, Wannapop and Thongtem 2009, Chen et al. 2003, Zhou et al. 2008a). Additionally, the kinetic control of MnWO4 nanoparticles grown for tailored structural properties were studied by Li et al. (Tong et al. 2010). To extend for these tungstates, Wong et al. (Zhang et al. 2008a) developed a room-temperature template-directed method for the synthesis of single-crystalline alkaline-earth-metal tungstate AWO4 (A = Ca, Sr, Ba) nanorods with unique optoelectronic properties. The 2D self-organization of BaWO4 nanorods at the water-air interface was performed on the basis of a Langmuir-Blodgett technique (Kim et al. 2001). By using the catanionic reversed micelle templating method, single-crystalline BaWO4 nanorods with high-aspect-ratio were produced (Shi et al. 2003b). The penniform BaWO4 nanostructures could be generated in reverse micellar system containing double-hydrophilic block copolymers (Shi et al. 2003a). Also used a microemulsion with solvothermal association, the SrWO4 nanorods of 100 nm in diameter and 500-1500 nm in length were yielded (Sun et al. 2006b). In the solution-based self-assembly process, through properly employing organic additives to the reaction system, the growth of inorganic nanocrystals can be rationally directed to obtain products with desirable morphologies and/or hierarchical structures. For example, Cascales et al. (Esteban-Betegón, Zaldo and Cascales 2010) have produced the tetragonal scheelite-phase Yb3+-doped NaGd(WO4)2 micro/nanostructures with preserved photoluminescence properties from hydrothermal preparations using metal nitrate and chloride reagents. Shen et al. (Liu et al. 2004) synthesized the single crystalline ZnWO4 nanorods via the direct aggregation of the amorphous nanoparticles, and promote the crystallization process of the amorphous ZnWO4 particles derived from the precipitation reaction in a nanorod/amorphous nanoparticle coexisting system. Yu et al. (Zhang et al. 2007c) have used the refluxing method to fabricate the uniform core-shell heterostructured ZnWO4MWO4 (M = Mn, Fe) nanorods with optical and antiferromagnetic characters. The direct crystallization of the MWO4 nuclei occurred on the backbone of ZnWO4 nanorods without surfactant. The formation of the hybrid ZnWO4MWO4 (M = Mn, Fe) nanorods based on the oriented aggregation. The shell thickness of MWO4 could be tuned by changing the molar ratio of these raw materials.
Many other research groups have also used the solvo-hydrothemal method to synthesize the mixed metal oxide nanocrystals. For example, Yu et al. (Zhou et al. 2009) reported an effective ethylene glycol (EG)-assisted solvothermal method to synthesize hierarchical FeWO4 microcrystals using FeCl3 6H2O and Na2WO4 2H2O as precursors. It was found that the organic solvent EG played a critical role as both a reducing agent and a structure-directing agent in driving such architectures assembled by oriented attachment of primary nanoparticles. Moreover, a certain amount of CH3COONa was necessary for the formation of such unique platelike FeWO4 microcrystals. The photocatalytic property of as-synthesized hierarchical FeWO4 microcrystals was also studied, which shows excellent photocatalytic activity for the degradation of rhodamine B. Cascales et al. (Esteban-Betegón et al. 2010) has produced the tetragonal scheelite-phase Yb3+-doped NaGd(WO4)2 micro/nanostructures with preserved photoluminescence properties from hydrothermal preparations using metal nitrate and chloride reagents. The Li’s group has successfully prepared a novel hollow CeO2-ZrO2 nanocages with controlled shapes, sizes, and compositions by adding zirconium(IV) into the glycol solution containing CeO2 nanospheres under solvothermal treatment (Liang et al. 2008). The formation of cage-structured CeO2-ZrO2 nanomaterials was explained by the Kirkendall effect. When Zr4+ ions were added into the system, they readily doped into ceria to form a solid solution of the type Ce1-
Related progress using this two-phase approach was also applied for the synthesis of a variety of mixed inorganic nanomaterials under solvo/hydrothermal treatment. A modified two-phase approach for the synthesis of the CdS nanocrystals at the water/toluene interface was carried out by mixing cadimium myristate toluene solution and thioure aqueous solution (Wang et al. 2005a). Additionally, these authors used a seeding-growth technique to change the CdS nanocrystal sizes. They also synthesized the CdSe and CdSe/CdS core/shell nanocrystals using selenourea as a selenium precursor by this two-phase approach. The CdSe/CdS core/shell nanocrystals exhibited quantum yields up to 60-80%, the photoluminescence of these nanocrystals can be clearly observed even without UV lamp irradiation (Pan et al. 2005a). By an alternant growth technique, the authors also synthesized two multi-shell nanocrystals of CdS/CdSe/CdS/CdSe/CdS and CdSe/CdS/CdSe/CdS/CdSe/CdS nanocrystals (Pan et al. 2006). In all cases, the obtained products had the narrow size distributions due to the use of low reaction precursors. Xu et al. (Wang et al. 2009) have recently achieved rare-earth doped NaYF4 up conversion fluorescent nanocrystals using water/ethanol/rare-earth stearate/oleic acid system. Kaskel et al. (Du et al. 2007) also used this two phase solvothermal method to synthesize of BaTiO3 nanocrystals using mixed titanium(IV)-
Our group recently reported a modified two-phase method for the synthesis of alkyl chain-capped metal particles (e.g., Cu and Au) and metal oxide nanoparticles (TiO2 and ZrO2), followed by their cooperative assemblage into unusual hybrid metal/metal oxide nanocrystal mesostructured materials (Mrabet, Zahedi-Niaki and Do 2008). The catalytic activity of these hybrid products for the CO oxidation was also employed. Indeed, these hybrid metal/metal oxide nanocatalysts exhibited high surface areas, narrow pore size distributions, and exceptional catalytic properties in the oxidation of CO, even surpassing the performance of commercial noble metal catalysts.
Cerium doped lanthanum(III) carbonate hydroxide (Ce
We recently reported the fabrication of undoped and cerium doped LaCO3OH annular-shaped nanoarchitectures with high specific surface area via the thermolysis of the binary source precursor, Ce
The XRD and SEM/TEM results (Figure 10B) revealed that the formed product without cerium doping exhibited the pure hexagonal-phase LaCO3OH structure with annular shape (Nguyen et al. 2011a). The monodisperse LaCO3OH annular-shaped nanoarchitectures revealed rough surface, narrow size distribution, average particle diameter of <400 nm, high specific surface area (~100 m2.g-1), and are composed of numerous small 3-5 nm particle assemblies. The phase structure and morphology of Ce
The photoluminescence (PL) emission spectra (Figure 11) of the colloidal Ce
Xie et al. (Xie et al. 2009) fabricated the 1D layer-by-layer hexagonal-phased La(OH)CO3 hierarchical nanomaterials with a diameter of ~700 nm and lengths in the range of 6-8 μm from the self-assembly of the single La(OH)CO3 nanoplates via the hydrothermal reaction of bulk La2O3 powders with NH2CH2COOH. Under the thermal procedure, the obtained 1D La(OH)CO3 nanostructures could be converted to porous La(OH)3 and La2O3 nanorods. The La(OH)CO3 was first transformed into La2O3 through calcination at 900º C and then further change into hexagonal-phased La(OH)3 by a hydrolysis process. The doping ion obviously plays a crucial role in modifying the interesting optical properties of the produced mixed oxides. The orthorhombic-phase Eu3+/Tb3+ doped La2OCO3/La2O3 nano/microcrystals with multiform morphologies were synthesized via a homogeneous precipitation (Li et al. 2010a). The multiform LaCO3OH products such as flake, flower, rhombuse, two-double hexagram, sandwichlike spindle, peach-nucleus-shaped nanocrystals obtained by changing the carbon sources [CO(NH2)2, Na2CO3, NaHCO3, (NH4)2CO3, and NH4HCO3], NH4+, Na+ ions, and pH values of the initial solution. LaCO3OH was easily converted to La2O2CO3 and La2O3 under annealing at suitable temperatures. The excitation and selective emission spectroscopy were applied to the microstructures and difference in 5D0 → 7F2 transition of Eu3+ ions in La2O2CO3 and La2O3 host lattices. In addition, the optical luminescence properties of 1-5 mol% Eu3+/Tb3+-doped La2OCO3/La2O3 phosphors were strongly dependent on their morphologies and sizes. Reddy et al. (Reddy, Katta and Thrimurthulu 2009) synthesized the novel nanocrystalline Ce1-
A variety of nanostructured metal oxide and mixed oxide materials have been synthesized by various methods, covering a wide range of compositions and tunable size/shape, especially over the past decade. In comparison, microemulsions can also be used to synthesize monodisperse mixed oxide nanocrystals with various morphologies; however, this method requires a large amount of solvent and small-scale production. In terms of size and shape control of the magnetic nanocrystals, thermal decomposition seems an improved method has been developed to date; however, this techniques often require the use of expensive organometallic or metal alkoxide precursors and performed at high temperature under argon or nitrogen atmosphere. In terms of simplicity of the synthesis, one-phase and two-phase solvo-hydrothermal surfactant-assisted methods are almost the preferred pathways for the large-scale synthesis of high-quality inorganic nanocrystals with controlled size and shape at relatively mild reaction temperature.
Eventually, the obtained metal oxide and mixed oxide nanomaterials are of great vitality and offers immense opportunities for chemistry, physics, biology, materials and engineering. Interaction among scientists with different backgrounds will undoubtedly create new science, and in particular new materials, with unforeseen technological possibilities. What is noteworthy is that such nanomaterials are likely to benefit not only the catalytic industry, but also to contribute to most aspects of the electronic and biotechnological industries.
5. Summary and outlook
In summary, this chapter provides an overview of some recent progresses related to solution-based syntheses of monodisperse metal oxide and mixed oxide colloidal nanocrystals. Compared to the traditional methods such as sonochemistry, solid-phase reaction, gas-phase reaction, coprecipitation, the one-phase and two-phase solvo-hydrothermal surfactant-assisted pathways are much more easily controlled in the size, shape, composition, and phase structure. Several current approaches developed in our group have some conveniences including nontoxic and inexpensive reagents (e.g., inorganic metal salts as starting precursors), water as environmentally benign medium, high-yield, and large-scale products.
It is pointed out that the major interest of such metal oxide and mixed oxide nanocrystals is systematically studied their catalytic properties in our laboratory for the next time. The ability to manipulate precisely the size/shape and the surface of nanocrystals has opened up a number of potential applications for the new further materials: nanoelectronics, optics, solar cells, magnetic resonance imaging (MRI), biomedicine, etc. Considering the astonishing rate at which progress is being made on several areas, we may expect that the impact of this field of nanotechnology on our daily lives will grow markedly in the near future. Finally, we expect that this chapter would be useful to readerships and could also form the basis of a course on the nanoscale shape-controlled subject.
This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) through a strategic grant.
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