DOI: 10.1021/cg900809b
Growth Mechanism and Surface Chemical Characteristics of Dicarboxylic Acid-Modified CeO2 Nanocrystals Produced in Supercritical Water: Tailor-Made Water-Soluble CeO2 Nanocrystals
2009, Vol. 9 5297–5303
Minori Taguchi,*,† Seiichi Takami,‡ Takashi Naka,† and Tadafumi Adschiri‡ †
National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan and ‡Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan Received July 15, 2009; Revised Manuscript Received October 10, 2009
ABSTRACT: Tailor-made surface-modified metal oxide nanocrystals enable various applications including medical, electronic, magnetic, and photovoltaic devices. Both the synthesis and application of surface-modified metal oxide nanocrystals rely on the interaction between organic molecules and the surface of metal oxides. From this viewpoint, we have focused on the synthesis of metal oxide nanocrystals using supercritical water in the presence of organic molecules as a surface modifier. Here, we describe the use of dicarboxylic acids with various chain lengths as the modifiers of CeO2 nanocrystals. The morphology and displayed crystallite plane of CeO2 nanocrystals could be controlled by the length of dicarboxylic acids. Long dicarboxylic acids produced cuboctahedral or cubic CeO2 nanocrystals, possibly because of the decreased growth rate of the {200} plane. The growth mechanism of the CeO2 nanocrystals is discussed in detail. Furthermore, dicarboxylic acids on the surface of the CeO2 nanocrystals changed the isoelectric point of the nanocrystals by displaying carboxyl groups. As a result, we have succeeded in synthesizing water-soluble CeO2 nanocrystals with various morphologies using dicarboxylic acids.
Introduction The quantum size effects and high surface-to-volume ratio of nanoscaled materials impart distinctive size-tunable magnetic,1-3 electronic,4,5 optical,6-9 and catalytic10-12 properties. In particular, rare-earth metal oxides in various nanostructures are of great importance because their 4f electrons realize unique optical, magnetic, and chemical properties,13-29 which results in high-performance luminescent devices, biological labeling, magnets, and catalysts. Among rear-earth metal oxides, cerium oxide (CeO2) nanoparticles have been investigated most intensively because they can be used as three-way catalysts for exhaust gas from automobiles, oxygen ion conductors in solid oxide fuel cells, polishing agents for chemical mechanical planarization, gate oxides in metal oxide semiconductor devices, and ultraviolet (UV)-shielding materials.20-29 In these applications, controlling the morphology of the nanocrystals, that is, the crystallographic orientation of the faces of the nanocrystals, is important because they determine various properties, including catalysis and glass polishing ability.22-26 However, there are few reports on the control of the morphology of CeO2 nanocrystals.20-22,29 On the other hand, organic-inorganic hybrid nanomaterials have attracted great interest because they exhibit improved mechanical, thermal, catalytic, medical, or optical properties.30-40 The incorporation of nanocrystals into organic materials32-35 or polymers36-40 enables the realization of hybrid nanomaterials with the merits of organic materials, such as wide tunability, ease of processing, and structural flexibility, together with the physical properties of inorganic nanocrystals including magnetic, electronic, and optical properties. To prepare the organic-inorganic hybrid nanomaterials, we have to control the surface chemical characteristics of *E-mail:
[email protected]. r 2009 American Chemical Society
the inorganic nanocrystals to hybridize them with organic materials. To date, various strategies have been developed for years.30-40 However, these studies focused on the technique for the preparation of the surface-modified nanoparticles, with little discussion on how the organic molecules bound on to the surface of nanocrystals and the effect of surface modification on the growth of the nanocrystals. The interaction between the organic molecules and the surface of the nanocrystals enables not only the design of various functional hybrid nanomaterials but also the discovery of the novel properties at the interface between organic molecules and inorganic compounds. In this paper, we discuss the growth mechanism and surface chemical characteristics of dicarboxylic acid-modified CeO2 nanocrystals produced in supercritical water (SCW). To synthesize surface-modified metal oxide nanoparticles, our group has proposed the in situ surface modification method in SCW.41,42 SCW is chemically stable and environmentally benign.43 This has led to its increased use in materials chemistry,43-46 more specifically, nanocrystal synthesis in SCW.47-51 Performing the hydrothermal synthesis at supercritical conditions has advantages for the control of hydrolysis and nucleation, because the properties of supercritical water, such as density and dielectric constant, can be widely changed by pressure and temperature.43,44 Increased hydrolysis rate at high temperatures and reduced solubility due to the change in dielectric constant resulted in accelerated nucleation rate and smaller size with narrower distribution. Moreover, organic molecules can be dissolved, owing to the decreased dielectric constant under supercritical conditions. That is, SCW acts to aid nucleation and crystallization of metal oxide, and organic ligand molecules are miscible in SCW. So, we can prepare the surface-modified metal oxide nanoparticles in SCW. To date, monocarboxylic acids, such as decanoic acid, have been used as the capping reagent in our synthetic approaches to Published on Web 11/09/2009
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synthesizing surface-modified metal oxide nanocrystals.41,42 Here, we studied the synthesis of surface-modified CeO2 nanocrystals in the presence of dicarboxylic acids with various chain lengths in SCW. In particular, we have investigated the relationship between the alkyl chain length of dicarboxylic acids and the morphologies of the CeO2 nanocrystals. In addition, we quantitatively evaluated the morphology of the dicarboxylic acids-modified CeO2 nanocrystals. The obtained results showed that the morphology and displayed crystallographic planes of the CeO2 nanocrystals can be controlled by the length of the dicarboxylic acids. Furthermore, the use of dicarboxylic acid, which has two carboxyl groups in its linear structure, controlled the surface chemical characteristics of the synthesized CeO2 nanocrystals by displaying one carboxyl group on the surface of the nanocrystals. On the basis of experimental results, we discuss the interaction between the crystal surface and modifiers, and the effect of the chain length of dicarboxylic acids. Our approach provides a synthetic procedure for tailor-made CeO2 nanocrystals with various shapes and surface characteristics. As an example, we also showed water-dispersed surface-modified CeO2 nanocrystals. Experimental Section Synthesis of Surface-Modified CeO2 Nanocrystals. Ce(OH)4 was purchased from Aldrich. Adipic acid (AA), pimelic acid (PA), sebacic acid (SA), and dodecanedioic acid (DA) were purchased from Wako Chemicals.
Taguchi et al.
Figure 1. XRD patterns of prepared CeO2 nanocrystals: (a) adipic acid-modified; (b) pimelic acid-modified; (c) sebacic acid-modified; (d) dodecanedioic acid-modified; (e) unmodified; (f) JCPDS Card No. 43-1002. Table 1. Lattice Constant a Calculated from XRD Patterns, and the Mean Particle Sizes from XRD Line Broadening (DXRD) and TEM Images (DTEM) of Prepared CeO2 Nanocrystals particle sizes (nm) CeO2 nanocrystals
lattice constant (a/nm)
DXRD
DTEM
adipic acid pimelic acid sebacic acid dodecanedioic acid unmodified
0.5414 0.5421 0.5436 0.5436 0.5414
18.2 ( 1.54 18.5 ( 1.23 13.6 ( 0.66 15.1 ( 1.66 17.8 ( 1.54
12.3 ( 3.91 11.3 ( 2.61 9.80 ( 2.51 11.0 ( 1.81 15.3 ( 3.91
DTG-60H (Rigaku). Zeta potential analysis was used to measure the particle surface charge as a function of pH and to determine optimum dispersion conditions. Zeta potentials were measured with a Zetasizer system (MPT-2, Malvern Instruments). The pH of suspensions was adjusted to values ranging from 3 to 11 using appropriate amounts of HCl and KOH.
Results and Discussion
Ce(OH)4 (0.25 mmol), the dicarboxylic acid (0.25 mmol), and distilled water (2.5 mL) were transferred to a pressure-resistant Hastelloy C vessel (inner volume: 5.0 mL). A hydrothermal reaction was carried out using an electric furnace at 400 °C and 38 MPa for 10 min. The reaction was terminated by submerging the vessel in a water bath at room temperature. After the reaction, unreacted organic ligands were removed by a combination of repeated centrifugation and decantation, alternately with water and methanol. Finally, the products were dried in air. Hereafter, these products are designated as 1 (AA-modified CeO2), 2 (PA-modified CeO2), 3 (SAmodified CeO2), and 4 (DA-modified CeO2). Unmodified nanocrystals were also prepared under the same conditions (at 400 °C and 38 MPa for 10 min) and with the same procedure. Physical Methods. UV-visible absorption spectra were recorded using a V-570 spectrophotometer (JASCO), and Fourier transform infrared (FT-IR) spectra were recorded using a FT/IR-680 Plus (JASCO) with the products in KBr pellets. A transmission electron microscope (TEM, JEM-1200EX, JEOL) was used to image the products. X-ray diffraction (XRD) patterns were recorded using a RINT-2000 spectrometer (Rigaku) with Cu KR (λ = 1.5405 A˚) radiation. Thermogravimetric analysis (TGA) was carried out at a ramp rate of 10 °C/min under an argon atmosphere using a
Characterization of Dicarboxylic Acid-Modified CeO2 Nanocrystals. The crystallite phases of the obtained products were examined by XRD. Figure 1 shows the XRD patterns of products and the standard data for bulk CeO2. The XRD results indicate that the products are well crystallized and have a cubic structure (space group Fm3m) known for bulk CeO2 (JCPDS Card No. 43-1002). According to the Scherrer equation, the average crystallite size was between 13 and 18 nm. The calculated lattice constants (a) ranged from 0.5414 to 0.5436 nm. The crystallite sizes and lattice constants are summarized in Table 1. The detailed morphology and structure of the products were investigated by TEM (Figure 2). Products 1 and 2 seemed to be a hexagonal shape. Hence, they are described as cuboctahedral.20-22,52 Products 3 and 4 seemed to be an almost squarelike shape. Hence, they are described as cubic.20-22,29,52 In addition, the unmodified nanocrystals are considered truncated octahedral. However, there was the variability of morphology in each product. The observed particle sizes are similar to those determined from the XRD patterns (see summary in Table 1 and Figure S1 of the Supporting Information). FT-IR spectroscopy was used to characterize the carboxyl groups present on the surface of CeO2 nanocrystals
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Figure 3. (A) FT-IR spectra of prepared CeO2 nanocrystals: (a) adipic acid-modified; (b) pimelic acid-modified; (c) sebacic acidmodified; (d) dodecanedioic acid-modified; (e) unmodified. (B) Schematic illustration of the carboxyl groups coordinated to the metal atom. (C) Schematic illustration of the structure of each dicarboxylic acid-modified CeO2 nanocrystal. Figure 2. TEM images of prepared CeO2 nanocrystals: (a) adipic acid-modified; (b) pimelic acid-modified; (c) sebacic acid-modified; (d) dodecanedioic acid-modified; (e) unmodified, scale bar = 20 nm. (f) Structural models of the cubic, cuboctahedral, and truncated octahedra.
(Figure 3A), and the spectra of the original dicarboxylic acids are shown in Figure S2. In product 1, a strong peak was observed at 1535 cm-1, which is assigned to the asymmetric (υas) stretching vibration, and peaks at 1456, 1433, and 1403 cm-1 were observed, which are assigned to the symmetric (υs) stretching vibrations of the -COO- group.53-56 In product 2, these peaks were also observed, but they appeared at 1525 (υas), 1455 (υs), 1433 (υs), and 1404 cm-1 (υs), respectively. On the other hand, in products 3 and 4, the weak stretching mode of the -COOH group was observed at 1707 cm-1, respectively, suggesting the presence of free carboxyl groups on the surface of CeO2 nanocrystals.53-56 In product 3, broadening peaks were observed at 1539 and 1435 cm-1, which are ascribed to the asymmetric (υas) and symmetric (υs) stretching vibrations of the -COO- group, respectively. In product 4, these peaks were observed at 1545 (υas) and 1443 cm-1 (υs), respectively. In the previous studies on carboxylic acid attached to the surface of metal oxides, the binding state between carboxylate groups and metal atoms was categorized as unidentate, bridging bidentate, or chelating bidentate (Figure 3B).53,55,56 These binding states can be identified using the wavenumber separation Δ between the asymmetric (υas) and symmetric (υs) IR bands. Unidentate complexes exhibit Δ values (ranging from 200 to 320 cm-1) that are much greater than those of bidentate complexes (ranging from 140 to
190 cm-1). The Δ values of bridging bidentate complexes are greater than those of chelating bidentate complexes ( n(100) > n(110), on the basis of the structure model for the (100), (110), and (111) planes. The authors showed that the surface energy of the three lowest index planes follows γ{111} < γ{200} < γ{110}, in view of the similarly between the CeO2 crystal and the fcc structure. This result suggests that the surface modification possibly proceeds more easily on the {200} plane than on the {111} plane, because the surface energy of the {200} plane is larger, that is, more unstable than that of the {111} plane. Therefore, the surface modifier might be preferably bound on to the {200} plane of CeO2 nanocrystals to reduce the growth rate of the {200} plane. We then discuss the effect of the alkyl chain length of dicarboxylic acids on the interaction with the surface of CeO2 nanocrystals. One of the properties that depend on the alkyl chain length is the hydrophobic interaction between molecules with long alkyl chains. In general, the strength of the hydrophobic interaction between amphiphilic molecules including surfactants depends on the length of the carbon chain.65 When the carbon chain is short, the repulsive force between hydrophilic parts in the molecules becomes dominant over intermolecular hydrophobic interaction. In product 1 or 2, the CeO2 nanocrystals were sparsely covered by AA or PA (Figure 3C), respectively, possibly because the repulsion among the carboxyl groups is strong in short dicarboxylic acids. In contrast, long dicarboxylic acids, that is, SA or DA, had strong hydrophobic interactions to form a uniformly and densely ordered organic layer on the surface of CeO2 nanocrystal. This discussion is supported by FT-IR and TGA results, which show that carboxyl groups in SA and DA were coordinated to the surface of CeO2 nanocrystals more densely than those of AA and PA. That is, the increased intermolecular hydrophobic interaction in the long dicarboxylic acids resulted in more dense modifiers on the
We have succeeded in synthesizing morphology-controlled CeO2 nanocrystals with dicarboxylic acids as a modifier in SCW. Furthermore, the correlation between the crystal surface of CeO2 and modifiers was investigated, and we proposed a mechanism of the changed morphology of CeO2 nanocrystals with modifiers in SCW. The correlation between the growth rate of a crystallite plane and surface modification was discussed in depth. As a result, the growth rates of the {111} and {200} planes of CeO2 nanocrystals were controlled by modifiers. In particular, the growth rate of the {200} plane depends on the density of the modifiers on the surface of CeO2 nanocrystals. SA (C8) and DA (C10) with long alkyl chains are more densely coordinated to the surface of CeO2 nanocrystals by hydrophobic intermolecular interactions. Thus, the growth rate of the {200} plane was reduced and cubic CeO2 nanocrystals were obtained with the {200} plane. On the other hand, AA (C4) and PA (C5) were sparsely coordinated to the surface of CeO2 nanocrystals owing to the repulsive force between carboxyl groups. As a result, the growth rate of the {200} plane was reduced to some extent and cuboctahedral CeO2 nanocrystals were obtained. These results suggested that the morphology of CeO2 nanocrystals was controlled by the growth rate of the {200} plane. In addition, the growth rate of the {200} plane depends on the density of modifiers on the plane. Interestingly, the synthesized surface-modified CeO2 nanocrystals can be dissolved in water by virtue of surface functional carboxyl groups (Figure S3). The presence of free carboxyl groups on the surface of CeO2 nanocrystals allows further conjugation with various organic compounds, and the conjugated CeO2 nanocrystals derived from a dicarboxylic acidmodified precursor can be used in important fields, such as catalysis and photonics. Furthermore, our work offers a simple, rapid, and green chemistry approach to preparing surfacemodified CeO2 nanocrystals using SCW as a reaction medium. Because of the convenience of the procedure and the ready availability of the chemicals used in this work, this route is expected to be applicable to various metal oxide nanocrystals. Acknowledgment. This work was supported by a Grant-inAid for Scientific Research (S) (KAKENHI) No. 20226015. Supporting Information Available: Particle size distribution from TEM images, FT-IR spectra for original dicarboxylic acids, and photographs showing the surface-modified CeO2 nanocrystals dissolved in water. This material is available free of charge via the Internet at http://pubs.acs.org.
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