Ceria nanocrystal

Deshpande, A. S. Pinna, N. Smarsly, B. Antonietti, M. Niederberger, M. 2005. Controlled assembly of preformed ceria nanocrystals into highly ordered 3D nanostructures. Small 1 313-316. 

A. Hartridge, A. Bhattacharya, Preparation and analysis of zirconia doped ceria nanocrystal dispersions, J. Nanoparticle Res. 1 (2001) 75-80. 

Nonaqueous solution methods In addition to the aqueous methods, ceria nanocrystals could also be obtained through a number of nonaqueous solution mefhods, such as the solvothermal methods, ionic liquids routes, polyol routes, as well as reactions in coordinating high-boiling oil solvents. 

The ceria nanocrystals could be synthesized through solvothermal routes, where a nonaqueous reaction media, such as ethanol, CCI4, or mixed solvents, are used (Li et al., 2001 Verdon et al., 1995 ... 

Wang et al., 1996, 2002). Zhang et al. reported the solvothermal s)mthesis of ceria nanoparticles in ethanol (Zhang et al., 2003a). Ce(N03)3 6H2O and (NH4)2Ce(N03)g are used as cerium source respectively with KOH or NaOH powders. The alcohothermal treatment was performed at 180 °C to produce ceria nanocrystals. To attain full crysfallization via a dissolution and recrystallization mechanism, small amount of water is required in the reactions. 

Gu and Soucek reported the synthesis of ceria nanoparticle in hydrocarbon solvents. Cerium-oleate complex was refluxed in high boiling point organic solvents such as octyl ether, 1-tetradecene, decalin, dipropylene glycol monomethyl ether, dipropylene glycol n-butyl ether, and 2,2,4-trimethyl-l,3-pentanediol monoisobutyrate, to decompose and form ceria nanocrystals. The sizes of the nanocrystals are uniform and could be... 

Ceria NWs (1.2 nm in width) and tadpole-shaped nanocrystals (3.7 nm in width) are obtained by a nonhydrolytic sol-gel process in presence of diphenyl ether in a mixed high boiling solvent of oleic acid and oleyla-mine at 320 °C (Yu et al., 2005c Figure 7). The NWs reported by Yu et al. are among the thinnest ones known with a reasonable yield, showing a growth direction of [100] of the fluorite structure. Spherical ceria nanocrystals of 3.5 nm could also be obtained if oleic acid is absent from the system. 

FIGURE 7 TEM images of the tadpole-shaped ceria nanocrystals. The inset is the HRTEM image of a tadpole shaped nanocrystal. Reprinted with permission from Yu et al. (2005c). Copyright 2005 Wiley-VCH. 

Zhang et al. reported a nanocomposite membrane of shuttle shaped ceria nanocrystals (Guo et al., 2008), SWNTs, and ILsl-butyl-3-methyli-midazolium hexafluorophosphate (BMIMPFg), which was incorporated on the glassy carbon electrode for electrochemical sensing of the immobilization and hybridization of DNA (Zhang et al., 2009). The electron transfer resistance (Pgt) of the electrode surface increased after the immobilization of probe ssDNA on the Ce02-SWNTs-BMIMPF6 membrane and rose further after the hybridization of the probe ssDNA with its complementary sequence. 

Yu, T.Y., Joo,)., Park, Y.I. and Hyeon, T. (2005) Large-scale nonhydrolytic sol-gel synthesis of uniform-sized ceria nanocrystals with spherical, wire, and tadpole shapes. Angewandte Chemie-Intemational Edition, 44, 7411-14. 

Ceria has a cubic phase with a fluorite structure. Therefore, common as-prepared ceria nanocrystals are isotropic in shape. On the other hand, the fluorite structure has a different atomic arrangement in different facets, which leads to distinct catalytic activity. Controlling the shape of nanocrystals can be used to tune the catalydc properties of such materials. The size of nanocrystals is another important factor in determining catalytic properties because of the surface effect. Various methods have been developed to control the size and shape of ceria nanomaterials. In this section, we will describe recent advances in the synthesis of this kind of nanomaterial and present some typical examples to introduce the characteristics of the various methods. 

The common synthesis techniques of nanocrystals can be easily applied to the synthesis of ceria nanocrystals. Because of the ultra-low solubility of ceria, basic co-precipitation can be applied directly and commercial ceria nanocrystals are obtained by this simple method. An alkali solution is added to a Ce(III) or Ce(IV) solution to initiate the hydrolysis of Ce(III) and Ce(IV) ions. Ce(OH)3 can be easily oxidized to Ce(OH)4. The as-produced Ce(OH)4 quickly loses HgO and forms ceria. Because of its ultra-low solubility, ceria nanocrystals are usually small and hard to grow, which is different from the usual recrystallization in the synthesis of other kinds of nanocrystals. 

Modified co-precipitation methods are also applied in the synthesis of ceria nanocrystals. NaOH, ammonia, urea, and Na2C03 can all be used as precipitation agents. Cetyl trimethylam-monium bromide (CTAB), carboxymethylcellulose sodium," and polyvinylpyrrolidone (PVP) are used as surfactants to control the size of the nanocrystals. 

Oh et al investigated the influence of the dielectric properties of solvents used in the hydrothermal reaction on the morphology and particle size of ceria nanocrystals. Cerium nitrate and KOH were used as starting materials, and methanol, ethanol, ethylene glycol, and 1,4-butylene glycol were chosen as different dielectric solvents. They found that the size of the final products increased along with an increase of the dielectric constant of the solvent and reaction temperature, and a decrease in the pff values of the reaction medium. 

Li and co-workers investigated the evolution of the morphology in the solvothermal synthesis of ceria nanoparticles. Rod-like and prism-like Ce(0H)C03 and polygon-like CeOg can be obtained with urea as precipitator. Using the organic base triethanolamine or inorganic NaOH would result in cube-like ceria nanocrystals. 

Adachi and co-workers synthesized ultra-fine ceria nanocrystals within reverse micelles. The particle size was less than 5 nm, which can be tuned by applying different water/oil ratios. 

Because of their different surface structures and catalytic properties, the exposed facets of the ceria nanoparticles also need to be tuned. The most common exposed facets of ceria nanocrystals are 200, 111, and 110. Both theoretical and experimental results... 

Chen, Yang, and co-workers investigated the doping effect to control the size and shape of ceria nanocrystals. Doped by trivalent rare-earth ions, the surfaces of the nanocrystals adsorb more OH , which accelerates recrystallization to give regular-shaped nanocubes. 

Generally, ID ceria nanocrystals can be obtained through two methods. The first is the template method, which uses a hard or soft template to restrict the growth of the ceria nanocrystals to one specific direction. The other method adjusts surface energies in which some facets are protected by specific capping ligands, which restrict growth. 

Chen et al. employed ID attapulgite with a uniform diameter ranging from 20 to 40 nm as a template to grow ceria nanotubes. Attapulgite rods were first treated with acid, followed by the precipitation of ceria nanocrystals on the surface. After etching by an NaOH solution, the template was removed from the ceria nanotubes. 

Yuan, Su, and co-workers used CTAB as a surfactant to modify the morphology of nanocrystals. The authors realized that hydrated Ce(IV) species can incorporate with CTA at some pH value and together form a micelle structure to direct the growth of ceria nanocrystals. 

Jin and co-workers obtained 1.5-nm ultra-thin ceria nanorods from a cerium stearate precursor. The reaction was carried up at 270°C in 1-octadecene with the addition of 1-octadecanol. The authors ascribed the formation of nanorods to the oriented attachment of ultra-small ceria nanocrystals grown in the initial stage of the reaction. 

Two-dimensional ceria nanocrystals are usually obtained through the template method because it is difficult to tune the growth rate of different facets to form this type of morphology. 

Soft templates have also been used for the synthesis of ceria nanoplates. Murray and co-workers used a high-boiling solvent system for the synthesis of ceria nanocrystals. Oleic acid and oleylamine were chosen as the solvent and capping agents and sodium diphosphate or sodium oleate were used as mineralizers in the experiment. Square-shaped ceria nanoplates were prepared and assembled into wire-like structures. 

Another method to obtain 2D ceria nanocrystals is the electrochemical route. Li, Tong, and co-workers developed the method, which involves the electrochemical deposition of Ce(III) in a NH4NO3 solution." Ceria nanosheets and nanobelts were obtained by applying lower and higher current densities, respectively. The CeOg and Ce(OH)3 nuclei were proposed as two important intermediates in the formation of these two kinds of material. 

Zhang, Shi, and co-workers also described a method to prepare a new type of hollow structure assisted by CNTs as templates. " Ceria nanocrystals were grown on the CNTs and formed necklace-like hybrid structures. After calcination, the CNTs were removed to give ceria nanomaterials with a ring structure. 

Hollow ceria nanostructures can be also prepared with the help of other hard templates. Jian, Du, and co-worker used a silica sphere as a hard template." Ceria nanocrystals were precipitated on the silica sphere to form a polycrystalline shell. After etching with NaOH solution to remove the silica, hollow spheres were obtained. Strandwitz and Stucky also used this method to obtain uniform hollow ceria spheres." ... 

Litzelman et al attempted to modify the space-charge potential of ceria nanocrystals by controlled in-diffusion of cations. They observed a decrease in electrical conductivity by more than one order of magnitude, while the modified films remained predominantiy... 

Dai QG, Huang H, Zhu Y, et al Catalysis oxidation of 1,2-dichloroethane and ethyl acetate over ceria nanocrystals with weU-defmed crystal planes, Appl Catal B Environ 117-118 360-368, 2012. 

Xu J, Harmer J, Li G, et al Size dependent oxygen buffering capacity of ceria nanocrystals, Chem Commun 46(11) 1887—1889, 2010. 

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