Gadolinium Oxide Nanoring and Nanoplate: Anisotropic Shape Control Jungsun Paek,† Chang Hoon Lee,† Jiyoung Choi,† Sung-Yool Choi,‡ Ansoon Kim,‡ Ju Wook Lee,‡ and Kwangyeol Lee*,† Department of Chemistry and Center for Electro- and Photo-ResponsiVe Molecules, Korea UniVersity, Seoul, Korea 136-701, and Electronics and Telecommunications Research Institute (ETRI), Daejeon, Korea 305-350
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 8 1378-1380
ReceiVed March 9, 2007; ReVised Manuscript ReceiVed July 3, 2007
ABSTRACT: Colloidal cubic Gd2O3 nanorings and nanoplates are selectively prepared from low-temperature (90 °C) hydrolysis of Gd(acac)3 and subsequent in situ thermal dehydration of the hydrolyzed precursor-surfactant aggregates at 320 °C. Notably, the final morphologies of Gd2O3 are directly transferred from the shapes of hydrolyzed precursors. Further ring size control could be accomplished using surfactants with different lengths, and importantly, the reported method could be extended to other metals such as Er and Yb. Various nanosized colloidal metal oxides have been conveniently prepared by thermal decomposition of metal precursors with oxygen-containing ligands1 or oxidant-assisted decomposition of metal precursors.2 Although anisotropic nanocrystals have been obtained by these methods in several cases,2c,d the most frequently encountered morphology for the resulting nanocrystals has been the spherical form. The interaction between the metal oxide nanoparticle surface and the employed surfactant usually is not strong enough to guide the anisotropic crystal growth at high temperatures, and the sometimes obtained anisotropic product morphology is usually the result of the intrinsic crystal growth behavior of metal oxides. Metal hydroxides, on the other hand, can be prepared from metal precursors or metal oxides at relatively low temperatures, and thus, the overall shape of metal hydroxides might be greatly influenced by the interaction with the surfactants and, more importantly, by the interaction between surfactants on separate nano-objects. Furthermore, it was recently shown that certain metal hydroxides can directly transfer their structural features or growth behaviors to the corresponding metal oxide systems during thermal dehydration.3 We have thus examined the possibility of preparing new metal oxide morphologies via their hydroxides with surfactant-surfactant interaction guided shapes. Herein, we report the synthesis of novel colloidal nanorings and nanoplates of several rare earth metal oxides from thermal dehydration of hydrolyzed metal precursor-surfactant nanoaggregates. A slurry of Gd(acac)3 (1 mmol), palmitic acid/hexadecylamine (HA/PA, 0.5 mmol/0.5 mmol), hydrazine monohydrate (4 mmol), and trioctylamine (TOA, 7 mL) prepared in a 100 mL Schlenk tube that was connected to a bubbler was heated at 90 °C with a vigorous magnetic stirring for 24 h.4 The resulting slurry was subsequently heated at 320 °C in a preheated oil bath for 1 h under a N2 flow without magnetic stirring,5 and the resulting reaction mixture was cooled to room temperature to give a yellowish solution. (Caution! The abrupt pressure build-up by the hydrazine-water Vapor at high temperatures can be dangerous. Therefore, the experiment should be carried out under an efficient fume hood with a proper shielding. The N2 flow is intended to remoVe the hydrazine-water Vapor from the reaction system.) Addition of toluene (10 mL) and precipitation by added methanol (20 mL) produced a tan powder, which can be easily redispersed in organic solvents such as hexane, toluene, and dichloromethane. The X-ray powder diffraction (XRD) pattern as shown in Figure 1 indicates that the product has the cubic Gd2O3 phase (JCPDS card 12-0797). The analysis by transmission electron microscopy (TEM) reveals the ringlike morphology for the Gd2O3 product (Figure 2a). The rings are rather monodisperse, with an average * To whom correspondence should be addressed. E-mail: kylee1@ korea.ac.kr. Fax: (82)-2-3290-3121. Phone: (82)-2-3290-3139. † Korea University. ‡ Electronics and Telecommunications Research Institute.
Figure 1. XRD pattern of Gd2O3 nanorings.
Figure 2. (a) TEM image of Gd2O3 nanorings, (b) magnified view of a ring with a trapped nanoparticle, (c) magnified view of a short nanocoil, (d) high-resolution TEM image of a ring. Unlabeled scale bars correspond to 2.5 nm.
inner diameter of 5.5 ( 0.5 nm and an average rim thickness of ∼1 nm, and the rim thickness is uniform within a ring structure. Although most rings have a seamless rim (Figure 2b), coil-like open structures, though very few, are also observed (Figure 2c), indicating that the ring structure is another variation of one-dimensional structures such as nanorods and nanowires. It has been previously shown that linear nanostructured Gd(OH)3 can be prepared by hydrolysis of bulk Gd2O3 in the absence of surfactants.6 In addition, some rings contain a very small nanoparticle, which might have initiated the ring formation and caused the observed variation in ring diameters (vide infra) in the center, as shown in Figure 2b. The TEM analysis for the intermediate prepared after initial heat treatment at 90 °C for 24 h reveals aggregated ring structures (see the Supporting Information). Evidently, the final ringlike morphology for the Gd2O3 has been directly transferred from the hydrolyzed Gd precursor-surfactant aggregates. The less commonly observed nanocoils might have originated from breakage of some ring structures of the hydrolyzed Gd precursor-surfactant aggregates because of rapid dehydration and accompanying shrinkage at high temperatures. The high-resolution TEM (HRTEM) analysis clearly shows the crystalline nature of the Gd2O3 nanorings (Figure 2d). Although the ring structure is not single-crystalline because of the severe curvature, it is interesting to note the manifest (222) growth
10.1021/cg070229r CCC: $37.00 © 2007 American Chemical Society Published on Web 07/18/2007
Communications
Crystal Growth & Design, Vol. 7, No. 8, 2007 1379 Scheme 1
Figure 3. (a-c) TEM images of Gd2O3 nanoplate “fallen dominoes” superstructures at various concentrations. Inset in (a) shows SAED. (d) HRTEM image of stacked Gd2O3 nanoplates. (e) Simulated image of the selected area of a “fallen domino” nanoplate in (d). (f) image profile along the direction.
direction along the rim. Nano- and microring structures with much larger diameters have been recently fabricated by edge-spreading lithography,7 sacrificial template synthesis,8 and molecular beam epitaxy.9 However, colloidal-chemistry-based one-pot synthesis of nanorings with small diameters (
