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Y2O3:Tb Nanocrystals Self-Assembly into Nanorods by Oriented Attachment Mechanism Yunxia Zhang,† Jun Guo,‡ Tim White,‡ Timothy Thatt Yang Tan,*,† and Rong Xu*,† School of Chemical and Biomedical Engineering, and School of Materials Science and Engineering, Nanyang Technological UniVersity, Singapore ReceiVed: March 7, 2007; In Final Form: March 25, 2007
Y2O3:Tb nanorods were obtained by self-assembly of nanocrystals using long-chain alkyl amines as the stabilizing agent. Spherical nanoparticles and nanorods can be selectively prepared by varying the synthesis parameters, including the reaction temperature and time or the ratio of yttrium precursors to amine. X-ray diffraction (XRD), transmission electron microscopy (TEM), and high-resolution TEM (HRTEM) have been employed to characterize the as-prepared Y2O3 nanostructures. Nanobeads like structures provide powerful evidence for the formation of Y2O3 nanorods via an oriented attachment mechanism. Room-temperature photoluminescence (PL) spectra were used to investigate the optical properties of the Y2O3 nanostructures.
1. Introduction The controlled synthesis of nanocrystals has drawn much attention in recent years due to the shape and size dependence of optical electronic and magnetic properties, as well as promising applications in nanodevices.1-4 Earlier studies of anisotropic nanocrystals (e.g., quantum rods and quantum wires) have already offered new possibilities for tailoring material properties.2,5,6 To realize anisotropic growth, many techniques have been developed, including solution-liquid-solid (SLS) methods, vapor-liquid-solid (VLS) procedures, and colloid chemistry approaches. For example, VLS method can be used to grow nanowires out of catalytic particles on solid substrates,7-10 while metal nanoparticles can seed quantum rods or wires in SLS routes, where each metal particle promotes unidirectional growth of nanocrystals directly in solution.11-13 The colloidal chemical method adjusts the ligand molecules to control the growth and shape of nanocrystals.14-17 In the past few years, the synthesis of rare earth compound nanocrystals has attracted extensive attention due to their unique properties and potential applications in luminescent displays, biological chemical probes and medical diagnostics. Rare earth compound nanocrystals as biolabels have many advantages over semiconductor quantum dots and organic phosphors: their large Stokes shift, sharp emission spectra, long lifetime, multiphoton and up-conversion excitation, reduced photobleaching, and low toxicity.18-20 Therefore, they offer tremendous potential for future medical diagnosis and therapy. For these applications, it is highly desirable to develop highly dispersed phosphors with narrow size distribution and controlled morphologies. Yttrium oxide-based films and particles are being extensively studied to replace conventional red-green-blue phosphors used for a new type of field emission display (FED) technology.21 A variety of methods have been developed to synthesize Y2O3 nanostructures, such as spray pyrolysis,22 homogeneous precipitation technique,23 microwave hydrothermal method,24 solvothermal method,25 and combustion synthesis.26 Recently, Wang et al.27 synthesized well-dispersed Y2O3:Eu nanocrystals and nanodisks using a non-hydrolytic route, noting that yttrium precursors have * Correspondence authors. E-mails:
[email protected];
[email protected]. † School of Chemical and Biomedical Engineering. ‡ School of Materials Science and Engineering.
large influence on the morphology of the products. Very recently, Yan et al.28 synthesized high quality rare earth oxide nanocrystals using various rare earth complexes as the precursors. In the present work, we demonstrate the shape evolution of rare earth-doped Y2O3 nanostructures from spherical nanocrystals to anisotropic nanorods where long-chain alkylamines serve as stabilizing agents with different yttrium precursor. It is noteworthy that anisotropic nanostructures can be formed in high symmetrical cubic structures. 2. Experimental Section Chemicals. All chemicals were used directly without further purification. Yttrium oxide (99.99%), HNO3 (analytical reagents, 69-70%), and oleic acid (tech. 90%) were purchased from Alfa Aesar. NaOH (reagent grade, 97%, beads), terbium (III) chloride hexahydrate (99.9%), oleylamine (tech., 70%), and 1-dodecylamine (98%) were purchased from Aldrich. Hexadecylamine (puriss, g99%) was purchased from Fluka. Ethanol and hexane were of analytical reagent grade. Synthesis of Yttrium (10 mol % Terbium)-Oleate Complex. Yttrium (terbium)-oleate complex was prepared using Y2O3 powder, terbium chloride, oleic acid, and sodium hydroxide. In a typical preparation process, 4 mmol of Y2O3 was dissolved in 1.6 mL of HNO3 (70%), and 0.88 mmol of TbCl3‚6H2O added. To this precursor, 8.4 mL of oleic acid, 30 mL of hexane, 18 mL of ethanol, and 12 mL of H2O was added with stirring for 2 h. Subsequently, 1.06 g of NaOH dissolved in 12 mL of H2O was slowly added dropwise and aged in a silicon oil bath at 70 °C for 6 h. The solution was then separated from two different layers using a separatory funnel. The above organic layer containing Y (Tb)-oleate complex was collected, washed with water four times and heated in an oven overnight at 80 °C to evaporate off the water and hexane. Synthesis of Tb-Doped Y2O3 Nanorods and Nanocrystals. The synthesis process described by Wang et al.27 was used with modification. Y2O3:Tb nanocystals or nanorods were synthesized by the thermal decomposition of Y(Tb)-oleate complex in the presence of long-chain organic amines. In a typical synthesis process for nanorods, 0.5 mmol of yttrium (10 mol % terbium)oleate complex was dissolved in 5 mL of oleylamine in a three-
10.1021/jp071877o CCC: $37.00 © 2007 American Chemical Society Published on Web 05/09/2007
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TABLE 1: Relationship between Morphology and Experimental Conditions no.
stablizing agent
Y:amine ratio
reaction temperature (°C)
growth time (h)
morphology
1 2 3 4 5 6
oleylamine oleylamine oleylamine oleylamine dodecylamine hexadecylamine
1:30 1:30 1:30 1:10 1:30 1:30
280 280 230 280 265 280
2 0.5 2 2 2 2
nanorods nanocrystals + a small amount of nanorods Nanocrystals + a small amount of nanorods nanocrystals nanocrystals nanorods
neck flask. The solution mixture was purged with N2 for 30 min at room temperature, then heated to the crystallization temperature at 3.3 °C/ min and maintained for a certain time at that temperature in order to induce the growth of nanorods. Afterward, the solution was cooled to room temperature and precipitated by excess ethanol. The precipitate was collected by centrifugation and the supernatant decanted. The isolated solid was dispersed in hexane with the above centrifugation and the precipitation-redispersion process was repeated several times to purify the as-prepared Y2O3 nanocrystals. For the synthesis of nanocrystals, 1.5 mmol of yttrium (10 mol % terbium)-oleate complex was dissolved in 5 mL of oleylamine using the process described above. For experiments using other alkyl amines as the stabilizing agents, 0.5 mmol of yttrium (10 mol % terbium)-oleate complex was dissolved in 3.62 g of hexadecylamine and 2.78 g of dodecylamine, respectively. Characterization. Powder X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and energy-dispersive X-ray (EDX) spectroscopy were used to characterize the shape, size, and composition of the as-synthesized Y2O3:Tb nanorods. The XRD patterns were recorded using a D8 Advance X-ray diffractometer with Cu KR radiation source (λ ) 0.15406 nm). TEM and HRTEM images were obtained on a JEOL JEM-2100F electron microscopy operating on 200 kV. Photoluminescence spectra were collected on a Shimadzu RF-5301 PC Spectrofluorophotometer using 150 W xenon lamp as an excitation source. Dispersions of Y2O3: Tb nanocrystals in hexane were measured in standard quartz cuvettes at room temperature. 3. Results and Discussion Temperature, reaction time, the amount of surfactant, and the type of amine used have a great influence on the morphology and size of Tb-doped Y2O3 nanocrystals (Table 1). When the growth temperature was fixed at 280 °C, nanocrystal morphologies are obviously dependent on reaction time. TEM images in Figure S1 (see Supporting Information) show the evolution of morphologies from small nanoparticles to nanorods. A high proportion of spherical particles are obtained after 5 min reaction (as shown in Figure S1a). Nanocrystals obtained are highly monodispersed with the size of around 2 nm. And some nanoparticles begin to coalesce in a certain direction into rod-shaped nanoparticles. After 30 min, many pearl-like aggregates are formed (as shown in Figure S1b), and extending the heating time to 2 h promotes self-assembly into 2 nm × 15-20 nm nanorods (as shown in Figure 1a). Longer heating durations induce the oriented attachment of the nanoparticles into necklace-like structures, and then the bottlenecks between adjacent particles are presumably filled up by the conventional mechanism of dissolution and growth of monomers, finally resulting in the formation of single-crystalline nanorods.29 Interestingly, the diameter of nanorods remains the same as the nanoparticles and does not increase with reaction time. Figure 1b is a large area HRTEM image of several parallel Y2O3 nanorods. It can be seen that some nanorods are not
straight. Figure 1c shows HRTEM image of a somewhat bent nanorods. The clear lattice stripes in Figure 1d show that it is of single crystalline with the interplanar distances of 0.19 nm, which corresponds to that of the {440} plane of cubic Y2O3. The corresponding FFT pattern in Figure 1e characterizes further the cubic structure. The crystal structure of the as-synthesized nanocrystals and nanorods were also analyzed by powder X-ray diffraction (XRD). Figure 2a shows typical powder XRD patterns of as-prepared Tb-doped Y2O3 nanorods and nanoparticles. All the diffraction peaks can be assigned to cubic Y2O3 (JCPDS file #89-5592), which confirms the crystal structure of as-prepared Y2O3 samples. It can be easily seen that the relative intensity of (440) diffraction peak for nanorods is higher than that for nanoparticles, which indicates their anisotropic growth and is in agreement with HRTEM results. EDX spectrum (Figure 2b) shows that the samples are composed of C, Y, O, and Tb. Besides the reaction time, reaction temperature is another factor which affects the morphology of Tb-doped Y2O3 nanocrystals. Under fixed growth time (2 h), varying the temperature leads to different morphologies of nanocrystals while keeping the other experimental conditions unchanged. Very few nanorods are observed at 230 °C with more formed at 250 °C and a maximum yield at 280 °C. The above results mean that the growth kinetics can manipulate the shape control of the nanocrystals. At higher temperatures, the growth rate is increased significantly and the anisotropic nanorods are favored.5 The ratio of Y complex to oleylamine has an obvious influence on the morphology of Y2O3:Tb nanocrystals. When the mole ratio of Y complex to oleylamine is 1:10, only nanocrystals are obtained (Figure 3a). HRTEM image (the inset in Figure 3a) of the Y2O3:Tb nanocrystals shows a nearly spherical crystalline particle with lattice plane distances identified to Y2O3. And if the ratio increases to 1:30, most of nanocrystals aggregate into nanorods (as shown Figure 3b). From the above results, a low oleylamine to Y complex seems to favor the formation of Tb-doped Y2O3 nanocrystals. And a higher ratio easily induces the self-assembly of the nanocrystals and leads to the formation of nanorods. As the concentration of the surfactant is increased relative to the particle concentration, the formation of ordered structures appears to be more favorable than the formation of isolated particles. Long-chain amines are standard reagents both for the control of the self-assembly of different metal and semiconductor nanoparticles and for the fabrication of nanorods. The alkylamine surfactant plays three important roles: (1) it is responsible for uniform anisotropic growth in nanorods or nanowires; (2) it stabilizes the nanoparticles; (3) it defines the lateral spacing between the nanoparticles.4 The chain length of the amines used also affects the morphology and self-assembly of the Tb-doped Y2O3 nanocrystals. Panels a and b of Figure 4 are two TEM images of as-prepared samples using dodecylamine and hexadecylamine as stabilizing agents, respectively, with the 1:30 ratio of yttrium to precursor amine. As shown in Figure 4a, only nanocrystals were obtained when using dodecylamine as stabilizing agent. When hexadecylamine is used as the stabilizing
Y2O3:Tb Nanocrystal Self-Assembly
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Figure 3. TEM images of different ratio of to Y complex to amine: (a) 1:10; (b) 1:30.
Figure 1. (a) A typical TEM image of Y2O3:Tb nanorods obtained at 280 °C for 2 h; (b) large-area HRTEM image of several nanorods; (c) HRTEM image of bent nanorods; (d) HRTEM image of a straight nanorod; (e) the corresponding FFT pattern of nanorod in panel d.
Figure 4. TEM images of dodecylamie (a) and hexadecylamine (b) as stabilizing agents.
Figure 2. (a) XRD patterns of the as-prepared Y2O3:Tb nanorods and nanoparticles; (b) EDX spectrum showing that the as-prepared nanorods are composed of C, Y, O, and Tb.
agent, nanorods with similar size are formed but with random arrangement. More ordered nanorods can be obtained only when oleylamine is used as the stabilizing agents (as shown in Figure 1a). The room-temperature photoluminescence (PL) spectra of Tbdoped Y2O3 nanocrystals and nanorods dispersed in hexane are shown in Figure 5. Figure 5a shows photoluminescence excitation and emission spectra of Y2O3:Tb nanorods. The emission spectrum of Y2O3:Tb nanorods under 230 nm excitation displays four narrow emission bands between 450 and 650 nm assigned to 4f f 4f transitions within Tb3+ ions. The most intense peak at 545 nm corresponds to the 5D4 f 7F5 transition, while the peaks at 489, 585, and 622 nm correspond to transitions from 5D to 7F , 7F , and 7F , respectively.30 No emission in the blue 4 6 4 3
spectral region from the high energy 5D3 is observed, and is typical of luminescent material with a high concentration of Tb3+ ions as cross-relaxation produces an increase in the population of the 5D4 states at the expense of 5D3 state.31,32 Similar PL spectra can be obtained under the excitation of 245 nm. It is obvious that the strongest excitation is located at 245 nm. Figure 5(b) shows room-temperature PL spectra of the asprepared Y2O3:Tb nanocrystals, nanorods and pure Y2O3 nanocrystals synthesized in oleylamine. Obviously, pure Y2O3 nanocrystals have no emission at room temperature. The spectra for nanoparticles and nanorods are very similar, suggesting that the diameter is the major controlling factor for the confinement effect.4 The emissions of nanocrystals with the same doping level in different alkylamines display very interesting findings (Figure 5c). Although the position of the emission peaks in these samples are identical, the emission intensities of Y2O3:Tb nanocrystals in oleylamine and hexadecylamine are more than five times as high as those in dodecylamine. Although the transition intensity of 5D4 f 7F5 for hexadecylamine and oleylamine as capping agents is very similar, the intensity ratio of 5D4 f 7F5 to 5D4 f 7F4 transitions in the former is almost two times of that in the latter. That means that the sensitivity of various transitions is different under the different environment. Usually, the difference in luminescent properties can be ascribed to the quantum confinement effects, various dimensions, morphologies, and crystal structures. It is plausible that it is originated from the lower symmetry in the former two as it is well-known that the lower the symmetry is around the emitting ion, the stronger is the emission in lattices.33 However, it does not seem to apply to the present situation because the emission intensity for the nanocrystals and nanorods is very similar in Figure 5b. Yan et al.34 reported that the PL spectra of YBO3: Eu nanocrystals showed a size dependent characteristic. Different sizes and shapes result in different combinative abilities between the surface and the absorbed species so as to produce the different quenching abilities when the PL spectra are measured in suspension.35 Earlier studies indicated that the
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Figure 5. Photoluminescence spectra of as-prepared Y2O3:Tb samples.
SCHEME 1 : Proposed Formation Mechanism of the Oriented Aggregation of Nanocrystals into Nanorods
surface structure is a key factor for the occurrence of band gap states that quench the exciton luminescence.36,37 Wuister et al.38 investigated the influence of the length of alkyl chain on the luminescence properties of CdSe quantum dots and found that the antiquenching temperature shifts to higher temperatures with the increase of the length of alkyl chains. This demonstrates the crucial role of the capping molecules (e.g., alkyl amines) on the luminescence of nanocrystals to some extent. Due to the quantum size confinement, the larger surface-to-volume ratio in any nanometric materials implies that the state of their surface will play an important role in determining their optical properties. The capping molecules not only prevent the agglomeration of nanocrystals but also passivate their surfaces, thereby limiting the recombination of the photogenerated electron and hole. The surface modification of the nanocrystals effectively removes the surface defects and accordingly enhances the intensity of luminescence. It is because the longer the length of alkyl amines, the larger the separation distance between the donor and acceptor in the samples, and the stronger the emission intensity. It is well-known that the driving force for the growth of anisotropic nanostructures is the anisotropy of crystal structure or crystal surface reactivity. However, Y2O3 is of cubic structure, and therefore, difficult to form the anisotropic nanostructures based on the classical Ostwald ripening mechanism. Recently,
the oriented attachment mechanism has been found to be applicable in the growth of anisotropic nanostructures in many oxides, such as TiO239, 40, ZnO,29 MnO,41 and even cubic CdTe,42 PbSe,43 in which the nanoparticles can be selfassembled by the dipole-dipole interactions. Zeng et al. have realized the nonlinear 1 D arrangement of nanocrystals through the “fusion” of nanocrystals.44 Dipolar interaction is the most probable candidate for the driving force inducing nanoparticles to assemble into chains. The ligands adsorb to the nanocrystal surface and control growth by providing steric stabilization and preventing aggregation. The adsorbed ligands can change the growth kinetics and surface energies of different crystal faces, which can ultimately lead to anisotropic growth of low dimensional nanostructures, such as nanorods, nanodisks, and nanowires.45 According to morphologies evolution of nanocrystals under different growth times (as shown in Figure S1), a simple scheme to visualize the growth mechanism is given in Scheme 1. To investigate the growth of nanorods, we intercept the intermediate steps in the early stage of the formation of nanorods. As shown in Figure 6a, “pearl-neck” aggregates are observed and each “nanorods” is composed of several nanoparticles. The arrows show the interstice between nanoparticles. Figure 6b shows the oriented attachment of two nanocrystals. It seems that the cubic
Y2O3:Tb Nanocrystal Self-Assembly
Figure 6. (a) TEM and (b) HRTEM images of the intermediates showing the oriented attachment of Y2O3:Tb nanocrystals.
Y2O3 nanocrystals first form chainlike aggregates due to dipoledipole interactions and then fuse gradually, recrystallize into single crystalline nanorods. The similarity of the diameters of nanoparticles and nanorods substantiates this oriented attachment mechanism. Tang et al. suggested that the force producing chains of nanocrystals is dipole-dipole attraction, because it has a long range and is surprisingly strong.42 Shim et al.46 reported large permanent dipole moments and charges in colloidal semiconductor quantum dots in detail. These dipole moments may be intrinsic attributes to interplay between the surface localized charges and surface strain at the semiconductor/ organic cap interface, and large permanent dipole moments may not be a specific property of the II-VI semiconductor nanocrystals but a generic attribute to all dielectric materials of nanometer scale. When two nanoparticles approach each other closely, they are attracted by van der Waals forces. However, due to their thermal energy they can still rearrange to find the low-energy configuration represented by a coherent particle-particle interface.47 In the oriented attachment, particles appear fused almost endto-end along the longitudinal axis and form linear chains. The attachment leads to a lowering of the surface energy after the elimination of highly curved surfaces of individual spheroids and is an enthalpy favorable process.48,40 4. Conclusion The spherical nanoparticles and anisotropic nanorods have been successfully synthesized in a controlled fashion via different capping agents by adjusting the experimental factors. The nanobeads like structures formed in the intermediate step provide strong evidence for the oriented attachment mechanism of the nanorods formation, which is highly noteworthy because anisotropic nanostructures can be formed for Y2O3 with a symmetrical cubic structure. Intense green-light emission for the obtained materials can be observed at room temperature using an excitation source of 230 nm. Different surface alkyl amines have a large influence on the PL properties and the longer the length of alkyl amines, the stronger the emission intensity. These Y2O3:Tb nanocrystals will find many potential applications in future medical diagnosis and therapy. Acknowledgment. Financial support by Centre of Advanced Bionanosystems under Grant M61120003, Nanyang Technological University, is gratefully acknowledged. Supporting Information Available: TEM images of Y2O3: Tb nanocrystals prepared at 280°C for 5 and 30 min using oleylamine as stablizaing agents. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Coe, S.; Woo, W. K.; Bawendi, M.; Bulovic, V. Nature 2002, 420, 800. (2) Huynh, W.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 225.
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