CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 10 2193-2196
Communications Growth and Characterization of Single-Crystal Y2O3:Eu Nanobelts Prepared with a Simple Technique Xia Li,† Qiang Li,*,† Zhiguo Xia,† Lin Wang,† Wenxun Yan,† Jiyang Wang,‡ and R. I. Boughton§ Department of Chemistry, Tsinghua UniVersity, Beijing 100084, P. R. China, The State Key Laboratory on Crystal Materials, Shandong UniVersity, Jinan 250100, China, and Department of Physics and Astronomy, Bowling Green State UniVersity, Bowling Green, Ohio 43403 ReceiVed January 21, 2006; ReVised Manuscript ReceiVed July 31, 2006
ABSTRACT: In this paper, we report on the synthesis of single-crystalline Y2O3:Eu nanobelts via a simple, rapid, and efficient one-step technique without using any templates or catalysts. The prepared products were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), high-resolution transmission microscopy (HRTEM), and selected area electron diffraction (SAED). The effects of pH, reaction temperature, and process time on the phase structure and morphology of the product are studied. The possible growth mechanism of Y2O3 nanobelts is discussed on the basis of the crystal structure of the materials. Photoluminescence results indicate that the Y2O3:Eu nanobelts have a strong red 5D0f7F2 transition. One-dimensional nanomaterials of different shapes, such as wire, rodlike, and tubular forms, have attracted much attention since the discovery of carbon nanotubes in 1991.1 The reason is that they may be suitable for various applications such as electronic devices, sensors, and energy-storage media.2 One-dimensional rare earth oxides have been widely used as high-performance magnets, luminescence devices, catalysts, and other functional materials because of their electronic, optical, and chemical properties resulting from the 4f shell of the ions.3-7 These properties depend strongly on the materials’ composition and structure. If rare earth oxides were obtainable in a nanostructure form, they could hold promise as highly functionalized materials as a result of both shape-specific and quantum-size effects. Indeed, Wakefield 8 has reported on some 1D nanomaterials in which the luminescence properties of the rare earth ions are modified in comparison to traditional micrometersized powders. Many methods have been used for the preparation of onedimensional rare earth nanostructures, including solution combustion (propellant) synthesis,9,10 the homogeneous precipitation method,11-13 and the hydrothermal method,14-16 among others.17,18 Among these methods, catalyst or template-based techniques have been widely used to prepare 1D nanobelts. In recent work,12 Yada’s group described the synthesis of rare earth oxide nanotubes templated by dodecyl sulfate assemblies, realized by homogeneous precipitation with urea. Conversion into a hollow nanotube with an inner diameter of 3 nm was accomplished after anionic exchange of a surfactant with the acetate ions. However, the selection of suitable catalysts or templates for the reaction system can be a complicated process, and their addition may result in the presence of impurities in the final product. Developing the synthesis of nanorods and nanowires * Corresponding author. Tel: 86-10-62797871. mail.tsinghua.edu.cn. † Tsinghua University. ‡ Shandong University. § Bowling Green State University.
E-mail:
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that involves a template-less and noncatalyst process presents a tremendous challenge. Recently, Wang et al.14 prepared a series of rare earth compound nanotubes, including hydroxides, oxides, oxysulfides, and hydroxyfluorides. These materials have been successfully synthesized on the basis of a simple hydrothermal method followed by subsequent dehydration, sulfuration, or fluoridation processes (700 °C for 2h). Eu-doped Y2O3 phosphor is a well-known red phosphor that is used widely in fluorescent lamps and cathode-ray tubes (CRT). For this reason, studies on the luminescence properties of nanocrystalline Y2O3:Eu have attracted extensive interest during the past few years. Wu et al. also reported the preparation of 1DY2O3:Eu NTs by the surfactant assembly method.19 In this paper, we present a simple, rapid, and efficient one-step technique to synthesize very thin, single-crystalline Y2O3 nanobelts and nanorods, using yttrium nitrate in a mixed solvent system at about 200 °C for 3 h. The nanobelts typically have an average thickness of ca. 10 nm, width of 40-100 nm, width-to-thickness ratio of 4-10, and a length of up to several micrometers. The present work suggests that it is possible to grow rare earth nanobelts using an aqueous, solution-based chemical technique under controlled conditions without any catalysts or templates. The growth mechanism of the nanobelts is also discussed on the basis of the crystal structure of the materials. (Y0.95Eu0.05)2O3 nanobelts were prepared by a mixed solvothermal process. In the preparation, stoichiometric amounts of yttrium oxide (Y2O3 99.99%) and europium oxide (Eu2O3 99.99%) were dissolved in diluted nitric acid (A. R.) under vigorous stirring. Then, 15% NaOH (or KOH) solution was added to adjust the system to a pH of ∼8-14. Next, 100 mL of precursor precipitate was placed in an autoclave container with a volume of 500 mL. A quantity of 300 mL of ethanol was added to it, and the resulting suspension was finally heated to the desired temperature for 2 h and allowed to cool to room temperature naturally. The gray precipitate was
10.1021/cg0600400 CCC: $33.50 © 2006 American Chemical Society Published on Web 08/29/2006
2194 Crystal Growth & Design, Vol. 6, No. 10, 2006
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Figure 3. (a) TEM image of the as-synthesized Y2O3 product. (b) TEM images and magnification of a single Y2O3 nanobelt. (c) Tip of a single nanobelt. Figure 1. XRD patterns of the samples prepared by the mixed solvothermal method at different temperatures for 2 h: (a) 200, (b) 220 °C.
Figure 2. IR spectrum of the as-prepared Y2O3 nanobelt.
Figure 4. (a) TEM image of a single nanobelt (b) SAED pattern reveals the [001] growth direction of the Y2O3 nanobelt. (c) HRTEM image of the belt’s top surface, exhibiting marked interplanar d-spacing (1.03 nm) corresponding to the {001} lattice planes of cubic Y2O3.
filtered using suction filtration and washed several times each with distilled water and absolute ethanol. Results on the purity and phase structure of the products were obtained by X-ray diffraction in a D max-γ A model (Japan Rigaku) X-ray with Ni-filtered CuKR radiation (λ ) 0.1541784 nm). The morphology of the samples was observed by transmission electron microscopy (TEM) measurements, which were performed on a Hitachi Model H-800 transmission electron microscope using an accelerating voltage of 200 kV with a tungsten filament. The microstructure of the Y2O3:Eu nanorods was analyzed by highresolution transmission electron microscopy (HRTEM), which was performed with a Philips Tecnai 20v-Twin transmission electron microscope using an accelerating voltage of 200 kV. Samples for the TEM and HRTEM were prepared by ultrasonically dispersing the as-synthesized products into absolute ethanol, placing a drop of this suspension onto a copper grid coated with an amorphous carbon film, and drying it in air. The room-temperature emission spectra were obtained on a spectrophotometer (model Edinburgh FLS920). Figure 1 shows a typical XRD pattern of the Y2O3:Eu samples at different temperatures. All of the peaks can be indexed to the pure cubic phase of Y2O3 with a measured lattice constant of a0 ) 1.0602 nm (JCPDS 88-1040). No purity assessment can be determined from the XRD analysis in view of the technique’s detection limit. According to X-ray diffraction data, there are no differences in the peak positions observed between the two samples except for the latter having higher intensity. This indicates that the crystallite size of the Y2O3 increases at higher reaction temperatures. Yttrium oxide, as well as the entire rare earth sesquioxide series, presents polymorphic forms, denoted as A, B, and C, and classified as being hexagonal, monoclinic, and cubic, respectively. The C-form structure is of the cubic bixbyite type, related to a doubled-edge fluorite in a regular way. For Y2O3, it is the low-temperature form at ordinary pressures.20 Therefore, a pure Y2O3 phase was apparently obtained using this technique. Figure 2 is the IR spectrum of the prepared product. The absorption band centered around 3443 cm-1
can be attributed to moisture absorbed on the surface of the sample. The peaks centered about 1539,1450, and 1396 cm-1 can be attributed to CO32- in the bond-stretching mode. The strong metaloxygen vibrations centered at 460 and 560 cm-1, in accord with earlier reports, are characteristic of the Y-O stretching frequencies. This result is in agreement with the above XRD result. The morphology of the Y2O3 nanobelt was characterized by transmission electron microscopy (TEM). As can be seen from Figure 3a, the product consists of a uniform nanobelt (over 90 vol %), in addition to a small number of nanorods. The TEM images can provide more geometric detail on the Y2O3. As shown in panels b and c of Figure 3, most of the nanobelts are very thin, and each nanobelt has a uniform width and thickness along its entire length. The nanobelts have thicknesses of 10-20 nm and typical lengths of several micrometers. The width-to-thickness ratio of the nanobelts is estimated to lie in the range of 4-10. It can be seen from the enlarged selected area image that most of the nanobelts have rectangularly shaped tips that are easily viewed at the open end. The tips of the nanobelts are a regular rectangular shape, different from the faceted tip morphology of other oxide nanobelts and nitride nanobelts.21,22 Detailed microstructure information of individual Y2O3 nanobelts was further examined by HRTEM and nanobeam ED analysis. Figure 4a is a TEM image of a segment of such an individual nanobelt. As shown in Figure 4b, the ED pattern recorded with the incident electron beam perpendicular to the wide surface of an individual nanobelt can be indexed to the diffraction pattern of the [110] zone axis of cubic single-crystal yttrium. The selected-area electron diffraction (SAED) pattern (Figure 4b,c) reveals that the as-synthesized Y2O3 nanobelts are structurally uniform, single crystalline, free of detectable defects and dislocations, and have a growth direction of [001]. Diffraction patterns taken from different regions along the nanobelt axis show the same features, indicating the same crystal orientation along the entire length of the nanobelts. The high temperature and high-pressure field produced during this method provides a favorable environment for the anisotropic
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Figure 7. TEM images of Y2O3 nanobelts obtained after processing for (a) 1, (b) 2, and (b) 3 h. Figure 5. TEM images of the as-prepared product under various pH conditions: (a) pH 8, (b) pH 10.
Figure 6. TEM of microsized rods.
growth of nanocrystals. For a better understanding of the synthesis of Y2O3 nanobelts, we carried out a series of experiments employing different reaction conditions. We found that the pH of the starting reaction system plays a crucial role in the formation of Y2O3 nanobelts and related nanostructures under ambient solvothermal conditions. Other research groups23 have found that the same phenomenon, i.e, pH of the mixture solution, affects the morphology of the synthesized product. The uniform Y2O3 nanobelts with a large aspect ratio can be obtained only when the reaction system is adjusted so that the pH lies in the range 12-13. With decreasing pH, the tendency to grow along a certain direction is weakened to some extent, and the product obtained exhibits a lower aspect ratio and less uniform morphology, as shown in Figure 5. A possible explanation of this result is provided as follows. In the case for which the pH was around 8-10 in the precursor solution, the Y source was primarily in the form of [Y(C2H5O)6]3-, whereas the remaining Y source existed in the form of Y(OH)3 precipitates. During the reaction process, large quantities of Y2O3 nuclei were first formed because of the decomposition of part of the Y(OH)3 precipitates. On the other hand, certain Y(OH)3 precipitates transformed into the growth units of [Y(OH)6]3- under alkaline conditions. It is well-known that the polar growth of Y2O3 crystal along the [001] direction proceeds through the adsorption of growth units of [Y(OH)6]3- onto (001) plane. However, in the present case, the negatively charged [Y(C2H5O)6]3- complexes preferably adsorbed on the (001) plane, thus repelling the growth units of [Y(OH)6]3-. Accordingly, the intrinsically anisotropic growth of Y2O3 along the [001] direction was substantially suppressed. As a consequence, the flakelike Y2O3 crystals were formed. In addition, the reaction temperature, time, and pressure are important factors for the synthesis of yttrium nanobelts. When the reaction temperature is higher than 220 °C with all other conditions remaining fixed, only microsized rods were obtained (as shown in Figure 6). Moreover, we found that the morphology of the as-prepared product depends strongly on the reaction time. After a
hydrothermal treatment for 1 h, a typical TEM image (Figure 7a) reveals that the sample is composed of Y2O3 nanobelts with lengths of several nm. The XRD results prove that this precipitate is a crystalline cubic structure. When the reaction time is increased to 2 h, the belts grow to a diameter of 20-30 nm and to lengths of up to about 2-3 µm. After 3 h of treatment, the widths of the nanobelts are invariable, but the lengths are increased to 5-6 µm. All of the above studies show that growth kinetics affects the morphology of the yttrium product. We speculated that proper control of the reaction conditions may lead to the formation of yttrium nanobelts. Until now, template-directed approaches based on the use of polymers, surfactants, or strong chelating ligands24,25 have proven to be particularly versatile in making confined structures and obtaining a controlled size and shape. However, there are also much simpler systems, where control over the dimensionality of the nanoparticles is achieved by the solvent.26 In our previous section, it was proved that the ratio of Y(OH)3 to [Y(C2H5O)6]3- substantially determines the morphology of Y2O3 crystals. Concretely speaking, under the condition that the precursor solution contained a large quantity of Y(OH)3 and a small quantity of [Y(C2H5O)6]3-, the anisotropic growth of Y2O3 was favorable when the pH was greater than 10, because certain Y(OH)3 precipitates transformed into the growth units of [Y(OH)6]3- under strong alkaline conditions. According to the “oriented attachment” view of crystal growth proposed by Penn and Banfield,27 the impetus for the aggregation of nanoparticles is a group for the growth of ZnO nanorods in a solvothermal colloid system. They also found that the nanoparticles served as “seeds” for the growth of the nanobelt and that the growth is increased by higher monomer concentration, which is coincident with our observation during the growth of the Y2O3 nanobelt. We think that in this case, the growth of the Y2O3 nanobelt is also based on an aggregation mechanism; here, [Y(OH)6]3- is the growth unit. As is well-known, the (111), (100), and (110) surfaces of the face-centered cubic structured particles are different, not only in their surface atom densities but also in their electronic structure, bonding, and possibly chemical reactivities.28 The surface free energy of the crystallographic planes descends in the order γ(100) > γ(110) > γ(111). It may thus be inferred that the activation energy of the (100) facet should be lower than those of the (110) and (111) facets, resulting in the bonding ability and chemical reactivity of the (100) facet being greater than those of the other two. Thus preference growth of Y2O3 crystal along the [001] direction proceeds through the adsorption of growth units of [Y(OH)6]3- onto (001) plane. The higher monomer concentration in the reaction solution leads to different growth rates for different facets.29 At the same time, in the strong alkaline solution (pH 13) contained a large quantity of [Y(OH)6]3- to meet the rapid growth of the (001) direction. The limited amount of monomers maintained by diffusion is consumed by the quick growth of the predominant facet. The velocity of crystal growth should be faster because of the higher monomer concentration, and thus nanobelts with smaller diameter and higher aspect ratio can be formed. Our present understanding of the formation mechanism of yttrium nanobelts is still limited. More in-depth studies are in progress. Figure 8. shows the emission spectrum of a (Y0.95Eu0.05)2O3 nanobelt measured at room temperature. Because of the shielding
2196 Crystal Growth & Design, Vol. 6, No. 10, 2006
Figure 8. Emission spectrum of Y2O3:Eu nanobelts.
effect of the 4f electrons by 5s and 5p electrons in the outer shells of the europium ion, narrow emission peaks are expected, consistent with the sharp peak observed. The intensity is comparable with that of ytttria-based crystalline and lamellar nanostructures synthesized by Pinna et al.26 This spectrum comprises a series of resolved features at 586, 592, 598, and 610 nm, which are assigned to the 5D0f7FJ, J ) 1, 2, 3, transitions, respectively. The emission lines reveal much about the local environment of the Eu3+ ion. The emission band at 598 nm, which corresponds to the 5D0f7F1 transition, is a magnetic dipole transition and hardly varies with crystal field strength around Eu3+. However, the hypersensitive transition 5D0f7F2 at 610 nm is electronic dipole allowed. Consequently, it depends on the local electric field and, hence, local symmetry. It is well-established that there are two Y3+ sites in cubic Y2O3; 75% of these sites are noncentrosymmetric with C2 symmetry, and the remaining 25% are centrosymmetric, having S6 symmetry.30 When the Eu3+ ion is located at a low-symmetry local site without an inversion center, this forced-electric dipole transition is often dominant in the emission spectrum. So the strongest 5D f7F transition (610 nm) and nearly all of other features in the 0 2 spectrum are due to the Eu3+ on a C2 site, except for the three other 5D0f7F1 (586, 592, 598 nm) transition lines, which are expected to arise from both Eu3+ C2 and S6 sites. In conclusion, pure cubic yttrium nanobelts were synthesized by a convenient solvothermal synthetic technique. It was demonstrated that the size and morphology of the synthesized product can be controlled by controlling the pH of the starting solution, and the phase of the nanobelts can be controlled by controlling the process temperature. The as-synthesized nanobelts typically have an average thickness of ca. 10 nm, width of 40-100 nm, widthto-thickness ratio in the range of 4-10, and length of up to several micrometers. The photoluminescence results indicate that the Y2O3: Eu nanobelts have a strong red 5D0f7F2 transition. On the basis of this method, other kinds of rare earth compounds and composite nanosized structures show promise for easy synthesis.
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