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

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 finalmorphologies of Gd2O3 are directly transferred from the shapes of hydrolyzed precursors. Further ring size control could be accomplishedusing 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 convenientlyprepared by thermal decomposition of metal precursors withoxygen-containing ligands1 or oxidant-assisted decomposition ofmetal precursors.2 Although anisotropic nanocrystals have beenobtained by these methods in several cases,2c,d the most frequentlyencountered morphology for the resulting nanocrystals has beenthe spherical form. The interaction between the metal oxidenanoparticle surface and the employed surfactant usually is notstrong enough to guide the anisotropic crystal growth at hightemperatures, and the sometimes obtained anisotropic productmorphology is usually the result of the intrinsic crystal growthbehavior of metal oxides. Metal hydroxides, on the other hand, canbe prepared from metal precursors or metal oxides at relativelylow temperatures, and thus, the overall shape of metal hydroxidesmight be greatly influenced by the interaction with the surfactantsand, more importantly, by the interaction between surfactants onseparate nano-objects. Furthermore, it was recently shown thatcertain metal hydroxides can directly transfer their structural featuresor growth behaviors to the corresponding metal oxide systemsduring thermal dehydration.3 We have thus examined the possibilityof preparing new metal oxide morphologies via their hydroxideswith surfactant-surfactant interaction guided shapes. Herein, wereport the synthesis of novel colloidal nanorings and nanoplates ofseveral rare earth metal oxides from thermal dehydration ofhydrolyzed 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 tubethat was connected to a bubbler was heated at 90°C with a vigorousmagnetic stirring for 24 h.4 The resulting slurry was subsequentlyheated at 320°C in a preheated oil bath for 1 h under a N2 flowwithout magnetic stirring,5 and the resulting reaction mixture wascooled to room temperature to give a yellowish solution. (Caution!The abrupt pressure build-up by the hydrazine-waterVapor at hightemperatures can be dangerous. Therefore, the experiment shouldbe carried out under an efficient fume hood with a proper shielding.The N2 flow is intended to remoVe the hydrazine-waterVapor fromthe reaction system.) Addition of toluene (10 mL) and precipitationby added methanol (20 mL) produced a tan powder, which can beeasily redispersed in organic solvents such as hexane, toluene, anddichloromethane.

The X-ray powder diffraction (XRD) pattern as shown in Figure1 indicates that the product has the cubic Gd2O3 phase (JCPDScard 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

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 openstructures, though very few, are also observed (Figure 2c), indicatingthat the ring structure is another variation of one-dimensionalstructures such as nanorods and nanowires. It has been previouslyshown that linear nanostructured Gd(OH)3 can be prepared byhydrolysis of bulk Gd2O3 in the absence of surfactants.6 In addition,some rings contain a very small nanoparticle, which might haveinitiated the ring formation and caused the observed variation inring diameters (vide infra) in the center, as shown in Figure 2b.The TEM analysis for the intermediate prepared after initial heattreatment at 90°C for 24 h reveals aggregated ring structures (seethe Supporting Information). Evidently, the final ringlike morphol-ogy for the Gd2O3 has been directly transferred from the hydrolyzedGd precursor-surfactant aggregates. The less commonly observednanocoils might have originated from breakage of some ringstructures of the hydrolyzed Gd precursor-surfactant aggregatesbecause of rapid dehydration and accompanying shrinkage at hightemperatures. The high-resolution TEM (HRTEM) analysis clearlyshows the crystalline nature of the Gd2O3 nanorings (Figure 2d).Although the ring structure is not single-crystalline because of thesevere curvature, it is interesting to note the manifest (222) growth

* 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 aring with a trapped nanoparticle, (c) magnified view of a short nanocoil,(d) high-resolution TEM image of a ring. Unlabeled scale bars correspondto 2.5 nm.

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direction along the rim. Nano- and microring structures with muchlarger diameters have been recently fabricated by edge-spreadinglithography,7 sacrificial template synthesis,8 and molecular beamepitaxy.9 However, colloidal-chemistry-based one-pot synthesis ofnanorings with small diameters (<10 nm) has never been docu-mented to the best of our knowledge.

Strikingly different platelike morphology is obtained for the cubicGd2O3 product10 by employing an increased amount of surfactants(1.2 equiv of PA/1.2 equiv of HA) and reduced solvent volume(see the Supporting Information). The TEM analysis reveals thesquare plate morphology (average 10× 10 × 1.1 nm3) for theGd2O3 product (Figure 3a). The square nanoplates form one-dimensional superstructures resembling fallen dominoes as far as∼1 µm from the surfactant-surfactant interactions. The interplateinteraction seems to be extremely robust; the one-dimensionalsuperstructures are always attained from widely varying sampleconcentrations (images b and c in Figure 3). Such strong interplateinteractions are known only for nanoplates of Co,11 Cu2S,12

BaCrO4,13 Ag,14 and LaF3.15 The high-resolution TEM (HRTEM)analysis shows the highly single-crystalline nature of the Gd2O3

nanoplates (Figure 3d-f). The TEM image shown in Figure S3 ofthe Supporting Information for the intermediate prepared after initialheat treatment at 90°C for 24 h exhibits lamellar superstructuressimilar to those of Gd2O3 nanoplates. The spacing between platesis 2.7 nm, corresponding to the thickness of the interdigitatedsurfactant bilayer. Again, the final square plate morphology for theGd2O3 has been directly transferred from the hydrolyzed Gdprecursor-surfactant aggregates. Although similar Gd2O3 plateshave been also prepared from the thermal decomposition ofgadolinium precursors,16 nanoplate shapes of Gd2O3 in our workare predetermined by the very strong interaction between hydrolyzedprecursor and surfactants at low temperature of 90°C.

The reduced amount of surfactants per Gd precursor for the ringstructures might favor the formation of concentric in-plane bilayerof surfactants, which would have been facilitated by the initial

presence of very small nanoparticles (see Scheme 1). The presenceof very small Gd2O3 nanoparticles in the center of some rings(images a and b in Figure 2) seems to corroborate this proposedmechanism. It appears that most Gd2O3 rings do not contain smallnanoparticles at the center; the nanoparticles, which initiated theconcentric bilayer formation, might have been lost during thermaltreatment at 320°C. The inner diameter of ring (average 5.5( 0.5nm) is only roughly twice the interplate spacing (2.7 nm), whichis the head-to-head length of the interdigitated surfactant bilayerin the nanoplate superstructure (see Scheme 1); the initial presenceof a small nanoparticle at the center of the ring would result inring diameters larger than 2× 2.7 nm for the hydrolyzed Gdspecies, but size contraction concomitant with dehydration isexpected for the final Gd2O3 phase with a higher density. Theproposed ring formation mechanism seems to be further substanti-ated by the fact that Gd2O3 rings with larger inner diameters(average 7.0( 0.6 nm) are prepared with longer surfactants ofnonadecanoic acid/octadecylamine (see the Supporting Informationfor the TEM image of Gd2O3 rings). The intrastructural surfactantinteractions responsible for the ring formation might not occur inthe plate-forming condition with higher surfactant concentrations.

We could obtain nanorings and nanoplates for other rare earthmetal systems under similar experimental conditions, indicating theapplicability of the reported synthetic methods for the rare earthoxide systems. It is interesting to note that the ring structures arepreferred over a wide range of metal-to-surfactant ratios for theYb system (0.4 equiv of PA/0.4 equiv of HA to 1.2 equiv of PA/1.2 equiv of HA), and the nanodisk structures are obtained only atvery high surfactant concentration (2 equiv of PA/2 equiv of HA).The stacked plate morphology is much favored in the case of Erover wide concentration range of surfactants (0.6 equiv of PA/0.6equiv of HA to 1.2 equiv of PA/1.2 equiv of HA) and the ringstructures (ca. 30%), severely contaminated by the plate morphology(ca. 70%), are obtained only with 0.5 equiv of PA/0.5 equiv ofHA. Interestingly enough, the plate shape is again the preferredmorphology for Er system at lower surfactant concentrations (0.3equiv of PA/0.3 equiv of HA to 0.4 equiv of PA/0.4 equiv of HA).

In summary, we have demonstrated that the strong surfactant-hydroxide precursors and surfactant-surfactant interactions can beexploited to form novel metal oxide nanostructures, which are notreadily accessible by conventional thermal routes. The syntheticprotocol might be extended to other metal oxide systems, and thuswe are investigating metal-hydroxide-mediated morphologicalcontrol of other functional metal oxides. Furthermore, we arecurrently working on the application of new Gd2O3 nanostructuresas MRI agents.

Acknowledgment. This work was supported by MOST (R01-2005-000-10503-0) and a Korea Research Foundation Grant fundedby the Korean Government (MOEHRD) (KRF-2006-311-C00373).S.Y.C. thanks the basic research program of ETRI and 0.1 Tbitnonvolatile memory program of MOCIE. We thank the staff ofKBSI (Chuncheon) for technical assistance in TEM analyses.

Supporting Information Available: Experimental details and furthercharacterization data (PDF). This material is available free of charge viathe Internet at http://pubs.acs.org.

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 theselected area of a “fallen domino” nanoplate in (d). (f) image profile alongthe <222> direction.

Scheme 1

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