Preparation and Photoluminescence of Single-Crystalline GdVO4:Eu3+ Nanorods by Hydrothermal Conversion of Gd(OH)3 nanorods Meng Gu, Qun Liu, Shengping Mao, Dali Mao, and Chengkang Chang*
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 4 1422–1425
School of Materials Science and Engineering, Shanghai Jiaotong UniVersity, 1954 Huashan Road, Shanghai 200030, P. R. China ReceiVed February 7, 2007; ReVised Manuscript ReceiVed December 12, 2007
ABSTRACT: This paper reports a hydrothermal conversion process of a rare earth nanostructure, Gd(OH)3 nanorods, into photoluminescent GdVO4 nanorods, together with the investigation on the morphological changes and related photoluminescent properties. Gd(OH)3 precursor with nanorod shape, uniform diameter, and good crystallinity was prepared by a colloidal hydrothermal method. High-resolution transmission electron microscopy (HRTEM) revealed the single-crystal nature of the precursor and preferred growth along the [001] direction. After hydrothermal conversion, well-crystallized GdVO4 nanorods were obtained. The growth direction of the newly formed nanorods was also confirmed as [001] from HRTEM investigation. TEM observations of the nanorod samples hydrothermally treated for 4 and 8 h indicated that the nanorods were formed by a surface deposition with a subsequent crystal growth procedure, during which the VO43- deposited onto the surface of the Gd(OH)3 precursor and then reacted gradually with the inner core to generate the GdVO4 nanorods. Such GdVO4 nanorods showed strong red emission upon UV illumination when 5 at % Eu3+ was doped into the lattice, due to the intrinsic Eu3+ transition between 5D0 and 7Fj configuration, showing potential application of the nanorods in the photoluminescence field. Introduction Nanostructures have received wide recognition for their novel properties generated by size effects. Current challenges in the synthesis of one-dimensional nanomaterials essentially focus on control over a single-crystalline nanostructure, such as the control of morphology, size, and growth direction. Recently, research attention has been directed to the field of rare earth materials, since they have many potential applications, such as high-performance luminescent devices, highly efficient catalysts, and other functional materials based on their novel electronic and optical properties resulting from their 4f electrons.1–5 GdVO4 is reported as an excellent matrix for photoluminescence application, and Eu3+ serves as a highly efficient red-emitting luminescent center in many hosts. In a crystalline GdVO4:Eu3+ lattice, Eu3+ replaces Gd3+with a D2d symmetry and occupies a 4a (000) lattice position of space group I4/amd.6 Therefore, well-crystallized GdVO4 nanorods are of great significance in improving the phosphor application. Various methods, such as the floating zone method 7,8 and the Czochralski method,9 were employed to prepare the material in macrosize, and the combustion process10 was used to prepare the phosphor at nanoscale. But morphologically controlled GdVO4 single crystals at nanoscale have not been reported yet. Recently, another vanadate, tetragonal phase LaVO4:Eu, was synthesized via an ethylenediamine tetra-acetic acid (EDTA) assisted hydrothermal method, and the transformation from monazite to the metastable zircon structure for LaVO4:Eu, which resulted in a remarkable improvement of the luminescent properties, was reported.11–13 In another report, Fan et al.14 synthesized tetragonal LaVO4 single-crystalline nanorods via a conventional hydrothermal reaction. But the crystal size and the aspect ratio of the obtained nanorods by the two routines are quite different, which indicates the importance of the anisotropic growth environment that the EDTA provides. LnVO4 * Corresponding author. Tel: 00-86-21-34202748. Fax: 00-96-21-34202748. E-mail:
[email protected].
(Ln ) Nd, Sm, Eu, Dy) nanorods were also obtained via the same method, but single-crystalline nanorods of GdVO4 were not mentioned in the report. We tried several times according to above two methods but failed to obtain GdVO4 nanorods; only nanosized particles that lack anisotropic growth were obtained after the hydrothermal procedure. Heretofore, the fabrication of the GdVO4 nanorods with well-controlled dimensionality, shape, phase purity, chemical composition, and desired properties remains as one challenging issue faced by chemists. In this paper, a modified hydrothermal routine was designed to synthesize GdVO4 nanostructures with well-controlled morphology. Gd(OH)3 nanorods were used as a template, which provided favorable conditions for the anisotropic growth of tetragonal GdVO4 and finally resulted in the formation of the GdVO4 nanorod. The photoluminescence spectra indicated that such a zircon-type GdVO4:Eu single-crystalline nanorod may be a potential red phosphor for future applications. Experimental Procedure A gadolinium hydroxide template, Gd(OH)3, was prepared15,16 through a hydrothermal method. All the chemicals used in the experiment were of analytic grade and were not further purified. In a typical procedure, 0.01 mol of Gd(NO3)3 · 5H2O was dissolved in 20 mL of distilled water, and 40 mL of 1 mol/L NaOH solution was added dropwise with continuous stirring to the solution to generate a colloidal solution with a pH around 13. This colloidal solution was transferred to an 80 mL autoclave and maintained at 180 °C for 18 h. When the autoclave was cooled to room temperature, a white Gd(OH)3 precipitate was collected. The precipitate was washed with distilled water several times, centrifuged, and redispersed in 100 mL of distilled water. Nanorod-like GdVO4 was prepared by the hydrothermal conversion of the Gd(OH)3 precursor. Na3VO4 · 12H2O (0.002 mol) was first dissolved in 30 mL of distilled water, and then 10 mL of Gd(OH)3 suspension was added with continuous stirring. The pH value of the suspension was adjusted to ca. 13 using 1 M NaOH solution before it was transferred to a 50 mL autoclave and maintained at 180 °C for 18 h. When the autoclave was cooled to room temperature, a white precipitate was obtained. The precipitate was collected, washed with distilled water, and finally centrifuged and dried at 60 °C overnight. In order to realize
10.1021/cg070140l CCC: $40.75 2008 American Chemical Society Published on Web 02/22/2008
GdVO4:Eu3+ Nanorods
Figure 1. Characterization of Gd(OH)3 nanorods: (a) SEM morphology of the Gd(OH)3 template, revealing a rod shape; (b) XRD pattern of the template, showing that a pure phase was obtained; (c)TEM micrograph of the Gd(OH)3 nanorods; (d) HRTEM image indicating the single-crystalline nature. the photoluminescence of the GdVO4 nanorods, 5 mol % Gd3+ was replaced by Eu3+ solution in the starting Gd3+ solution before the formation of the Gd(OH)3 precursor. Various means were employed to characterize the nanorods and investigate potential applications. Both the Gd(OH)3 precursor and the final GdVO4 nanorods were examined by means of scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM) to confirm the morphology, phase composition, and growth direction. The morphologies were observed with a JEOL-JSM6700F field emission scanning electron microscope (FESEM). XRD patterns were obtained on a JEOL-JDX3500 X-ray diffractometer employing Cu KR radiation; TEM observation was carried out with a field emission TEM, JEM2010. Photoluminescent properties of GdVO4:Eu3+ nanorods were measured on a Hitachi F-4500 fluorescence spectrophotometer. The excitation spectrum was recorded from 220 to 450 nm monitored at 618 nm, while the emission spectrum was collected from 400 to 750 nm under the illumination of UV light with a wavelength of 307 nm.
Results and Discussion Characterization of the Gd(OH)3 Nanorod Precursor. Nanorod-like Gd(OH)3 template was prepared by the hydrothermal method. The SEM micrograph shown in Figure 1a revealed a typical morphology of the prepared precursor. Rodlike structures were found with uniform diameter and variable rod length. The average diameter was estimated as 40–50 nm from the SEM micrograph. The XRD pattern shown in Figure 1b indicated a pure single-phased material. All the peaks in the pattern can be indexed to a hexagonal structure, which is in good agreement with the PDF 83-2037. No other phases were identified from the pattern. A TEM micrograph of the template is shown in Figure 1c, from which uniform rod-like structures were identified, showing good consistency with the result from the SEM micrograph. A HRTEM image of an individual nanorod is shown in Figure 1d. The fine fringes on the HRTEM indicated the single-crystalline nature and good crystallinity of the nanorod. The spacing between the two sets of fringes was
Crystal Growth & Design, Vol. 8, No. 4, 2008 1423
Figure 2. Characterization of GdVO4 nanorods after hydrothermal conversion: (a) XRD pattern of the nanorods, showing that a pure phase was obtained; (b) SEM morphology of the GdVO4, showing that the rod-like shape of the precursor was kept; (c) TEM micrograph; (d) HRTEM image, from which a preferred growth along the c axis was determined.
0.36 and 0.63 nm, which is equal to the d value of the lattice plane (001) and (100), respectively, and the angle between the fringes is nearly 90°. The above results strongly suggest that the HRTEM was taken from the [010] axis zone. The rod direction was also marked on the micrograph, which was found to be parallel to [001] direction, suggesting that the nanorod has a preferred growth along the c axis direction. Characterization of GdVO4 Nanorods. After hydrothermal conversion, pure phased GdVO4 with a rod-like shape was obtained. The XRD pattern shown in Figure 2a revealed a pure phase, and all the diffraction peaks are well-consistent with reported XRD profile of tetragonal GdVO4 with a space group I4/amd. The morphology and size of the products were evaluated by SEM and TEM observation. Representative images shown in Figure 2b,c presented a rod-like morphology, implying that the newly formed nanorods inherited the morphology from their mother crystals. A big difference in the size between the two kinds of nanorods was also observed. The diameter of the GdVO4 nanorods is about 70–80 nm, which is obviously bigger than that of the precursor. The HRTEM image shown in Figure 2d presented two sets of clear fringes from which the growth direction of the GdVO4 nanorod was determined. The spacing between the two fringes was measured as 0.63 and 0.71 nm, corresponding to lattice plane (100) and (001). The angle between the two fringes was very close to 90°, showing that the HRTEM was taken from the [010] zone axis. Therefore, it can be concluded that the single-crystalline GdVO4 nanorods have a preferred growth direction along the c axis direction, the same as their mother crystals. Mechanism of Formation of the GdVO4 Nanorods. Up to now, we demonstrated herein a hydrothermal routine to prepare GdVO4 nanorods with uniform size. One question remaining is how the nanorods were formed. Generally, there are two possible ways to produce the GdVO4 product during the hydrothermal
1424 Crystal Growth & Design, Vol. 8, No. 4, 2008
Gu et al.
Figure 3. TEM micrograph and EDS results of the nanorod sample prepared with 4 h hydrothermal reaction: (a) TEM micrograph, revealing a nanorod with inner core and outer layer; (b) HRTEM for the tip of the nanorod, indicating a poor crystallinity of the newly formed layer; (c) EDS of the inner core, showing the presence of a Gd-rich core; (d) EDS of the outer layer, implying that the outer layer is rich in V.
conversion. A possible explanation is the ordinary templateaided nucleation and subsequent crystal growth procedure, during which an outlayer rich in VO43- deposits on the surface of the precursor and then gradually reacts with the inner Gd(OH)3 core to form GdVO4. Another possible explanation could be the dissolution-precipitation process, during which the Gd(OH)3 precursor dissolves gradually in the water and then reacts with the VO43- to form new GdVO4 crystals. To verify which mechanism dominated during the GdVO4 formation, two intermediate samples prepared under the same routine but different reaction time (4 and 8 h) were investigated under a transmission electron microscope. Figure 3a shows a typical TEM micrograph of the sample powder after 4 h hydrothermal conversion, from which a single crystal with inner core and outer layer was observed. HRTEM of the tip of the crystal is shown in Figure 3b. The vague fringes observed on the micrograph indicated the crystalline nature of the core and the surface layer but with low crystallinity. EDS investigations for both the inner core and the outer layer are presented in Figure 3c,d. It is clear from the spectra that the inner core has a chemical composition quite different from that of outer layer: the core is rich in Gd, while the outer layer is rich in V. Such a result showed us that a VO43--containing layer was deposited onto the Gd(OH)3 surface after 4 h hydrothermal treatment. A TEM micrograph for the sample hydrothermally treated for 8 h is shown in Figure 4a, from which a rod-like crystal with a surface layer was observed. HRTEM observation of the nanorod shown in Figure 4b clearly showed that the nanorod is composed of an inner core and an outer layer. Compared with the results obtained from Figure 3, it seemed clear that the outer layer became thinner and the inner core turned more crystalline. Two sets of fringes were obtained from the HRTEM micrograph, and the spacings between the two sets of fringes were measured
Figure 4. TEM micrograph of the nanorod obtained after 8 h hydrothermal conversion: (a) a TEM micrograph, revealing good crystallinity of the inner core; (b) HRTEM for the nanorod, showing that a GdVO4 crystal with preferred growth along the c axis was formed after 8 h hydrothermal treatment.
as 0.63 and 0.72 nm, which is very close to the d values of (100) and (001) lattice plane of tetragonal GdVO4. The above results strongly indicated that tetragonal GdVO4, with a preferred growth along the c axis direction, was formed after 8 h hydrothermal conversion. Therefore, the mechanism of formation of the GdVO4 nanorods was determined as the first one, the surface deposition mechanism. Such a growth manner provided us a hint that tube-like GdVO4 single crystals could be obtained by etching the Gd(OH)3 inner core when the hydrothermal routine was carefully controlled and ideal nanorod crystals containing a GdVO4 outer layer and Gd(OH)3 inner core were obtained. Related work is continuing and will be reported later. Photoluminescent Properties of GdVO4 Nanorods. The GdVO4 nanorods showed strong photoluminescent behavior when 5 at % Eu3+ was introduced into the crystal lattice. Comparison
GdVO4:Eu3+ Nanorods
Crystal Growth & Design, Vol. 8, No. 4, 2008 1425
rods as the precursor. HRTEM investigation of the nanorod revealed a single-crystal nature of the GdVO4 nanorods and a preferred growth along the c axis direction. Such rod-like nanocrystals were formed by a surface nucleation and subsequent crystal growth mechanism, in which VO43-deposited on the surface of the Gd(OH)3 precursor and then reacted with the inner core to form the final GdVO4 product. The Eu3+-doped GdVO4 nanorod showed strong red emission centering at 618 nm upon UV excitation, and this implied potential application in the photoluminescent field. Acknowledgment. The research was financially supported from Shanghai Pujiang Project with Project Number 06PJ14048 and PRP project from Shanghai Jiaotong University with Project Number T05010005.
Figure 5. Photoluminescence of GdVO4:Eu3+ nanorods, indicating potential application in photoluminescent field. 3+
of XRD patterns between the 5% Eu -doped and nondoped GdVO4 powder showed no difference, implying that the introduced Eu3+ ions occupied the lattice sites of the original Gd3+ ions. The photoluminescence of Eu3+ in GdVO4 lattice was investigated using a photoluminescent spectroscope, and the results are shown in Figure 5. It could be seen from the excitation spectrum that the GdVO4 nanorods has a strong absorption upon UV illumination, with a center around 307 nm, which can be ascribed to the chargetransfer band (CTB) of Eu-O bond within the GdVO4 matrix. The emission spectrum shown in Figure 5 revealed normal photoluminescent behavior of Eu3+ ions in an inorganic matrix. Similar to the other phosphors activated by Eu3+, the GdVO4:Eu3+ showed the intrinsic transition between 5D0 and 7Fj (j ) 0, 1, 3, 4) configurations, as marked on the figure. We have experience preparing a Gd2O2S:Eu3+ phosphor by a hydrothermal routine,17 but the emission spectrum seemed somewhat different: the relative emission intensity is quite different. Such difference in the emission spectrum caused different emission colors, orange for Gd2O2S: Eu3+ and red for GdVO4:Eu3+. According to an earlier report,18 such a difference is generated by the difference in crystal environment: the coordination numbers and the site symmetry. In the present case, GdVO4 belongs to tetragonal structure, and Eu ions occupy D2d symmetry with coordination number of eight. In such a condition, the transition from 5D0 to 7F2 is greatly enhanced, and thus a bright red emission is observed. Such photoluminescent behavior of the nanorods indicated a potential application for the rod-like phosphor to serve as a highly efficient red phosphor in photoluminescent field. Conclusion In summary, we presented in this paper a hydrothermal conversion routine for GdVO4 nanorods using Gd(OH)3 nano-
References (1) Tang, Q.; Liu, Z. P.; Li, S.; Zhang, S. Y.; Liu, X. M.; Qian, Y. T. J. Cryst. Growth 2003, 259, 208. (2) Capobianco, J. A.; Vetrone, F.; Boyer, J. C.; Speghini, A.; Bettinelli, M. Opt. Mater. 2002, 19, 259. (3) Palmer, M. S.; Neurock, M.; Olken, M. M. J. Am. Chem. Soc. 2002, 124, 8452. (4) Wang, X.; Li, Y. D. Chem.sEur. J. 2003, 9, 5627. (5) Hasegawa, Y.; Thongchant, S.; Wada, Y.; Tanaka, H.; Kawai, T.; Sakata, T.; Mori, H.; Yanagida, S. Angew. Chem. 2002, 41, 2073. (6) Zhang, Q. L.; Guo, C. X.; Shi, C. S.; Lü, S. Z. J. Alloys Compd. 2000, 309, 10. (7) Higuchi, M.; Kodaira, K.; Urata, Y.; Wada, S.; Machida, H. J. Cryst. Growth 2004, 265, 487. (8) Onodera, K.; Ogawa, T.; Itagaki, H.; Machida, H.; Wada, S. Opt. Mater. 2004, 26, 343. (9) Zhang, H. J.; Meng, X. L.; Zhu, L.; Yang, Z. H. Mater. Res. Bull. 1999, 34, 1589. (10) Mahalley, B. N.; Pode, R. B.; Gupta, P. K. Phys. Status Solidi a 1999, 177, 293. (11) Jia, C. J.; Sun, L. D.; You, L. P.; Jiang, X. C.; Luo, F.; Pang, Y. C.; Yan, C. H. J. Phys. Chem. B 2005, 109, 3284. (12) Jia, C. J.; Sun, L. D.; Luo, F.; Jiang, X. C.; Wei, L. H.; Yan, C. H. Appl. Phys. Lett. 2004, 84, 5305. (13) Oka, Y.; Yao, T.; Yamamoto, N. J. Solid State Chem. 2000, 152, 486. (14) Fan, W. L.; Zhao, W.; You, L. P.; Song, X. Y.; Zhang, W. M.; Yu, H. Y.; Sun, S. X. J. Solid State Chem. 2004, 177, 4399. (15) Chang, C. K.; Zhang, Q. A.; Mao, D. L. Nanotechnology 2006, 17 (8), 1981. (16) Chang, C. K.; Kimura, F.; Kimura, T.; Wada, H. Mater. Lett. 2000, 59 (8–9), 1037. (17) Hang, T.; Liu, Q.; Mao, D. L.; Chang, C. K. Mater. Chem. Phys. 2008, 107 (1), 142. (18) Ropp, R. C. J. Electrochem. Soc. 1965, 112 (2), 181.
CG070140L
