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From Al4B2O9 Nanowires to Al18B4O33:Eu Nanowires Elawad Elssfah* and Chengcun Tang Department of physics, Central China Normal UniVersity, Wuhan, 430079, People’s Republic of China ReceiVed: February 4, 2007; In Final Form: March 29, 2007
In this paper we report on the synthesis of single-crystal Al4B2O9 nanowires by calcining the precursor powder made of disodium tetraborate decahydrate and aluminum nitride in air conditions. The as-received nanowires possess smooth surface with diameters varying between 20 and 50 nm. We can also obtain Al18B4O33:Eu nanowires with diameters varying between 50 and 70 nm by recalcining a mixture powder of the above obtained Al4B2O9 nanowires, K2SO4, and Eu2O3 in air. The structural and compositional characteristics of the as-synthesized products have been investigated by XRD, SEM, TEM, EDX, and SAED techniques. Photoluminescence investigations reveal that Al18B4O33:Eu nanowires display emission peaks at 590.0, 613.4, and 628.8 nm.
1. Introduction It is well-known that the reduction of microstructure size of crystalline materials into nanometer size can dramatically result in modification of their optical and electronic properties due to their high surface ratio to volume and quantum size effect. Onedimensional (1D) nanostructures such as nanowires/nanorods and nanotubes are critical and important work directed toward understanding the fundamental physical concepts and their potential applications.1-3 The rare earth elements (RE)-doped nanosize materials are found to be interesting for photoluminescence properties. The europium (Eu) among rare earth elements has been used as an activator in the red phosphors because of its strong red emission. Also, Eu can display two valence states: trivalent (Eu3+) and divalent (Eu2+). Eu3+ ion which displays a trivalent state is supersensitive to the evolution of its surroundings.4-6 In addition, Eu3+ ion is also observed as phonon sideband on highenergy side of 7F0 f 5D2 transition, due to the local structures coordinating Eu3+ ion.7 Therefore, on the basis of aforementioned futures, Eu3+ ion ought to be a good probe and favorable for the investigation of the local structure. Moreover, most of host materials selected for red phosphors can be doped by remarkably small amount of europium due to concentration quenching. Recently, the researches on the synthesis and optical properties of rare-earth ion-doped one-dimensional nanomaterials have been investigated, which exhibiting significant luminescent properties.8,9 Borate systems have proved to be potential candidates for aforementioned applications.10-12 Aluminum borate (Al18B4O33) material, besides its excellent features based on their excellent mechanical properties, chemical inertness, low thermal expansion coefficient, high-temperature stability, and low-cost production,13-16 is a suitable host material for high efficiently optical materials. Refractory compound Al18B4O33 has the high melting point approaching 1440 °C and is considerably stable even in an oxidizing environment.12,13 In previous work the luminescent properties of Al18B4O33:Eu3+ material has been investigated.7,21,25 Recently, aluminum borate nanowires have * Corresponding author. Fax: 86-27-67861185. Tel: 86-27-67861185. E-mail:
[email protected].
Figure 1. X-ray diffraction pattern of as-synthesized (a) Al4B2O9 and (b) Al18B4O33:Eu.
been synthesized successfully.17-25 It is well-known that aluminum borate Al4B2O9, which has a melting point of about 1030 °C, can be easily converted to Al18B4O33 phase at reaction temperature higher than 1030 °C. This paper reports on the conversion of the as-synthesis Al4B2O9 nanowires to Al18B4O33 one-dimensional nanostructures doped with Eu at 1150 °C in order to examine the photoluminescence properties.
10.1021/jp070977r CCC: $37.00 © 2007 American Chemical Society Published on Web 05/19/2007
From Al4B2O9 Nanowires to Al18B4O33
J. Phys. Chem. C, Vol. 111, No. 23, 2007 8177
Figure 2. (a) Low-magnification SEM image showing a large amount of Al4B2O9 nanowires; (b) the corresponding EDX spectrum; (C) TEM image establishing smooth morphology of Al4B2O9 nanowires; (d) an individual nanowire of Al4B2O9 and (inset) the corresponding SAED pattern.
2. Experimental Two steps were adapted to synthesis Al18B4O33:Eu nanowires. First, Al4B2O9 nanowires have been synthesis as follow: under vigorous stirring, 4.5 g of disodium tetraborate decahydrate (Na2B4O7‚10H2) (99.5%, Beijing Chemical Co.) was completely dissolved in 150 mL of distilled water in a separate beaker to form a colorless solution. Under the stirring, 2 M aluminum nitride (Al (NO3)3‚9H2O) (99%, Shanghai Chemical Reagents Company) solution was then added dropwise to the previously solution until a white precipitation appeared. The precipitation was filtered and washed with absolute methanol and deionized water for several times to remove the remnant reactants. The precursor powder was finally obtained after drying on a hot plate at 50 °C overnight. The sample was held in air at 900 °C for 2 h in a traditional resistance-heated furnace. The white products were washed with dilute HCl solution and distilled water to separate the nanowirs from remnant Na salt flux and then dried in a hot plate at 60 °C. Finally Al4B2O9 nanowires were obtained. Second, the re-ground of the as-achieved Al4B2O9 nanowires, K2SO4 (flux), and Eu2O3 were used as starting materials to fabricate samples of Al18B4O33:Eu nanowires. The doping concentration of Eu ions was fixed to 2.100 mol % of the Al ion. All starting materials were weight in the proper stoichiometries, and mixed in an agate mortar, then placed in aluminum crucible. The samples were placed into alumina tube and fired at 1150 °C for 4 h in the air conditions. After the reaction, white products could be obtained from the above synthetic route. The white products were washed with dilute HCl solution and hot distilled water to remove the possible remnant K salt flux. The crystal structure and phase purity of the products were identified by means of X-ray diffraction (XRD, D/MAX-rB, Cu K
radiation) analysis. The overview of the samples morphology were checked by scanning electron microscopy (SEM, JEM6700F, JEOL), equipped with the system of energy-dispersive X-ray (EDX) analysis. The powder samples were also ultrasonically dispersed in ethanol solution and then were transferred onto a copper grid with carbon film for the transmission electron microscopy (TEM, JEM-2100F, JEOL) and selected area electron diffraction (SAED) examinations. The excitation and emission spectra of Al18B4O33:Eu nanowires were measured at room temperature with a Hitachi F-4500 fluorescence spectrometer. 3. Results and Discussion Figure 1a shows X-ray diffraction (XRD) pattern of the sample synthesized by calcining the precursor powder made of Na2B4O7‚10H2O and Al (NO3)3‚9H2O. All the diffraction peaks can be indexed to the phase of orthorhombic structure of singlecrystalline aluminum borate Al4B2O9 with the calculated lattice parameters of a ) 1.471, b ) 1.513, and c ) 0.545 nm. The peak positions and relative intensities are matches well with those of the bulk Al4B2O9 (JCPDS 09-0158 and JCPDS 290010). Figure 2a shows the SEM image of the as-synthesized Al4B2O9 nanowires, establishing that the product consist of abundant and immense amount of nanowires. These pure nanowires almost have uniform diameter ranging between 20 and 50 nm and lengths up to several micrometers. A typical EDS spectrum taken from a single nanowire was shown in Figure 2b, suggesting that the nanowire was made of Al, B, and O elements. Transmission electron microscopy (TEM) was also employed to study the structure and the morphology of the as-synthesized Al4B2O9 nanostructures. Figure 2c exhibits
8178 J. Phys. Chem. C, Vol. 111, No. 23, 2007
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Figure 3. (a) SEM image showing a general morphology of Al18B4O33:Eu3+nanowires; (b) the corresponding EDX spectrum; (c) low-magnification TEM image showing the state morphology of nanowires; (d) Individual nanowire and SAED (inset).
a typical TEM morphology of the synthesized Al4B2O9 nanowires, indicates the nanowires with smooth surface and no nonwire-structured morphology can be detected. Indicated in Figure 2d is the magnified TEM image of an individual Al4B2O9 nanowire and the corresponding selected area electron diffraction (SAED) pattern. The SAED pattern is shown in the upper-right inset of Figure 2c, exhibiting a single crystalline nature of these pure nanowires with the lattice constant consistent with XRD analyses presented above. The growth direction of the nanowires was determined to [001] by defocused techniques. Figure 1b shows XRD pattern of the sample fabricated by calcining the mixture powder prepared from Al4B2O9 nanowires (Figure 2a,c), K2SO4, and Eu2O3 at 1150 °C in air conditions. All the diffraction peaks can be readily indexed as an orthorhombic structure aluminum borate (Al18B4O33) with the lattice constants a ) 0.774 nm, b ) 1.504 nm, and c ) 0.567 nm, in good agreement with the standard parameters Al18B4O33 (JCPDS 32-0003). This result can be account to be a good evident to prove that Al4B2O9 phase can be completely decomposed into a single phase Al18B4O33 when reheated above its melting point. Moreover, no X-ray reflection corresponding to any other crystal phase was observed. Scanning electron microscopy (SEM) image (Figure 3a) shows the general morphology of as synthesized Al18B4O33:Eu nanostructures obtained by recalcined the same as-received Al4B2O9 nanowires shown in Figure 2a,c. As can be clearly seen, the as-received of Al18B4O33:Eu nanowires possess diameters vary between 50 and 70 nm. These nanowires are relatively short compared to Al4B2O9 nanowires (Figure 2a), and this is maybe due to the high temperature used for fabricating such Al18B4O33:Eu nanowires. Therefore, we can conclude that the morphology detail depends on the reaction
temperature. A result of the energy dispersive X-ray (EDX) microanalysis of the chemical composition (Figure 3b) determines the doped Eu concentration of ∼2.1 atom %, with respect to the Al18B4O33. The TEM morphology (Figure 3c) of as-synthesized product Al18B4O33:Eu establishing that the distribution of the nanowires typically agrees with SEM observations shown in Figure 3a. Despite the fact that no particle could be found at the tip of the wires, some dark disparities can be detected on surface of nanowires, and this could ascribe to the effect stimulated by the Eu doping. Indicated in Figure 3d are an individual Al18B4O33 nanowire and the corresponding SAED pattern. The selected area electron diffraction (SAED) pattern indicated that the nanowires possessed a single-crystal structure and can be indexed as the orthorhombic Al18B4O33single-crystal recorded from [-210] zone axis with the same crystalline parameters as the calculated results from XRD measurement. Figure 4a shows the excitation and the emission spectra of Al18B4O33:Eu3+ nanowires. The excitation spectrum consists of two parts. One is a wide band extending from 230 to 260 nm, due to the charge transfer (CT) transition from 2p orbital of O-2 ions to the orbital of Eu3+ ions. The position of CT band usually depends upon the length of the Eu-O bond. The larger the strength of bond, the shorter the location of CT bond position.26,27 The other part is some sharp lines, and the stronger peak is at 263 nm, attributed to 4f-4f transitions between the ground state of Eu3+ ion and its excitation states. Figure 4b shows the emission spectrum of Eu3+-doped Al18B4O33 nanowires, which consist of three emission bands peaking at 591.0, 613.4, and 628.8 nm. This finding is considerably different compared to our previously result reported in ref 21, in which no emission band peaking at 628.8 nm could be detected. The
From Al4B2O9 Nanowires to Al18B4O33
J. Phys. Chem. C, Vol. 111, No. 23, 2007 8179 4. Conclusion In summary, uniform Al4B2O9 nanowires were successfully synthesized by directly calcining the precursor powder of desodium tetraborate and aluminum nitride in air. Also Al18B4O33: Eu has been fabricated by calcining the as-prepared Al4B2O9 and Eu2O3 in air. The characterization of ID nanostructure through SEM and TEM shows that the nanowires made of Al4B2O9 possess a diameter of 20-50 nm, whereas nanowires composed of Al18B4O33:Eu possess a diameter of 50-70 nm. Al18B4O33:Eu exhibits an emission spectrum regions, consisting of three emission bands peaking at 591.0, 613.4, and 628.8 nm. Acknowledgment. This work was supported by the Fok Fing Tong Education Foundation (Grant No. 91050) and the National Natural Science Foundation of China (Grant No. 50202007). References and Notes
Figure 4. (a) Excitation (EX) and (b) emission (EM) spectrum of Al18B4O33:Eu3+ nanowires.
strong red emission line at 613.4 nm is due to hypersensitive electronic dipole transition of 5D0 f 7F2, induced by the lack of inversion symmetry at Eu3+ local sites, while the emission near 591 nm is the magnetic dipole transition owing to the 5D0 f 7F1 states, which obey the selection rule (∆j ) 0, (1), and its intensity hardly varies with evolution of the Eu surroundings. Among these luminescence emission peaks, the emission spectrum is dominated by the hypersensitive red emission transition 5D0 f 7F2. The Mmlti-phonon relaxation from higher exited level (such as 5D1,2 to 5D0) has a high probability; this is maybe due to the relatively high phonon energies in aluminum borate nanostructures. Therefore, only 5D0 f 7Fj transitions are observed in the emission spectra. The almost complete absence of emission from the higher states was also observed in this sample, which is expected. This observation may be due to fast, nonradiative decay from high states to the 5D0 state, which can be attributed to the presence of large phonons in the host lattice.7 Also, due to strongly forbidden character of a j ) 0 to 0 transition, the 5D0 to 7F0 transition was not observed. It seems impossible that Eu3+ ions are incorporated into the crystal lattice of Al18B4O33, because Eu3+ ion has a much larger radius, compared with Al3+ or B3+ ion.
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