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Facile surfactant-free synthesis and luminescent properties of hierarchical europium-doped lutetium oxide phosphors
Journal of Colloid and Interface Science 394 (2013) 216–222
Contents lists available at SciVerse ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Facile surfactant-free synthesis and luminescent properties of hierarchical europium-doped lutetium oxide phosphors Qi Zhao a,b, Ning Guo a,b, Yongchao Jia a,b, Wenzhen Lv a,b, Baiqi Shao a,b, Mengmeng Jiao a,b, Hongpeng You a,⇑ a b
State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China Graduate University of the Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e
i n f o
Article history: Received 21 September 2012 Accepted 7 November 2012 Available online 29 November 2012 Keywords: Lu2O3 Hydrothermal method Hierarchical structures Luminescence
a b s t r a c t Novel three-dimensional (3D) hierarchical architectures of Lu2O3:Eu3+ have been successfully prepared through a simple hydrothermal process followed by a subsequent calcination process without using any surfactant, catalyst, or template. According to the X-ray diffraction, thermogravimetric analysis and differential scanning calorimetry, and Fourier transform infrared spectroscopy results, the precursors were determined to have the structure formula of Lu4O(OH)9NO3. The morphologies of the precursors could be modulated by altering pH value of the reaction system. On the basis of detailed time-dependent experiments, the growth process of architectures was discussed. The as-formed precursors could transform into Lu2O3:Eu3+ with their original hierarchical structures maintained. The Lu2O3:Eu3+ submicroarchitectures exhibit strong red emission corresponding to the 5D0 ? 7F2 transition of the Eu3+ ions under ultraviolet (UV) excitation, which have potential applications in novel optoelectronic devices. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction Recently, the interest in the fabrication of inorganic micro/ nanomaterials with controllable size, dimensionality, and structure has soared because it provides alternative means for tailoring the chemical and physical properties of functional materials [1–5]. Among the various micro/nanostructures, the hierarchical assembly of nanoscale building blocks into ordered superstructures or complex architectures has attracted considerable attention owing to the novel and collective properties, which are not found on the level of individual nanoparticles [6–9]. Therefore, much attention has been paid to the fabrication of hierarchical micro/nanomaterials by various synthesis techniques. Among them, hydrothermal synthesis has been widely used for the preparation of the micro/nanomaterials [10–15], due to its advantages in reducing processing temperature and yielding fine crystalline powders. In this synthesis route, catalysts, templates, or surfactants are frequently required as capping agents, stabilizing agents, directing agents, or soft templates to form the hierarchical frameworks [16,17]. However, the introduction of these additives into synthetic process will bring heterogeneous impurities, increase the production cost, and present difficulty for scale-up production [18,19]. Moreover, the surfactants are usually organic molecules which are difficult to be eliminated completely and will produce serious drawbacks to luminescence intensity for phosphors [20]. ⇑ Corresponding author. Fax: +86 431 85698041. E-mail address: [email protected] (H. You). 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.11.029
This situation entails efforts to develop more effective and economical methods for preparing micro/nanostructures. Rare earth compounds have been widely used in highperformance luminescent devices, magnets, catalysts, and other functional materials because of the novel electronic, optical, and chemical characteristics resulting from the 4f electrons of rare earth ions [21,22]. Specifically, lutetium oxide (Lu2O3) is an excellent phosphor host materials due to its favorable physical properties, such as high melting point, phase stability, and low thermal expansion [23]. Eu3+-doped lutetium oxide have long been an important red phosphor in color television picture tubes, white LEDs, and field emission displays [24,25]. Conventionally, Lu2O3 and Ln3+-doped Lu2O3 materials are prepared via methods such as combustion (propellant) process using urea (glycine/citric acid) as fuel [25,26], coprecipitation method [27,28], and the Pechini sol–gel procedure [29]. Unfortunately, the morphology of products is often irregular and uncontrollable. Recently, Lu2O3 nanorods [30,31], nanowires [32], and nanosheets [33] have been synthesized via the hydrothermal or solvothermal method followed by a subsequent heat-treatment process. However, the reports on the synthesis of complex hierarchical architectures of rare-earth oxides were relatively rare. So, how to fabricate 3D hierarchical Lu2O3 controllably through a facile and large-scale method remains a tremendous challenge for material chemists. Herein, we report the preparation of 3D hierarchical Lu2O3:Eu3+ through a facile hydrothermal process by a subsequent calcination process in the absence of any surfactants, template supporting, and structure-directing reagents. By simply regulating the pH values of
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the initial solution, various hierarchical architectures of Lu2O3:Eu3+ were achieved. The possible formation mechanisms of the novel architectures were proposed. Finally, we investigated the photoluminescence properties of the as-obtained Lu2O3:Eu3+ and the relationship between luminescence intensity and morphologies. 2. Experimental section 2.1. Reagents The rare-earth oxides RE2O3 (RE = Lu, Eu) (99.99%) were purchased from Shanghai Yuelong Non-Ferrous Metals Limited. The other analytical chemicals were purchased from Beijing Chemical Co. and used as received without further purification. Rare-earth nitrate stock solutions were prepared by dissolving the corresponding metal oxide in nitric acid under heating with agitation.
Fig. 1. XRD patterns of lutetium oxide precursor prepared at pH values of (a) 8 (S1), (b) 9 (S2), (c) 10 (S3), and the standard data for Y4O(OH)9NO3 (JCPDS No. 79-1352) as a reference.
2.2. Preparation In a typical procedure, 1.9 mL of Lu(NO3)3 (1 M) and 2 mL of Eu(NO3)3 (0.05 M) were added into 35 mL of ethanol. Subsequently, 25 wt.% of ammonia solution was introduced dropwise to adjust the pH value. After additional agitation for 30 min, the feedstock was transferred to a 50 mL Teflon-lined stainless autoclave and heated at 200 °C for 24 h. When the autoclave was cooled to room temperature naturally, the precursors were separated by centrifugation, washed with ethanol and deionized water several times, and dried at 60 °C in air. The precursors prepared at pH = 8, 9, and 10 were labeled as S1, S2, and S3, respectively. The final products were prepared by calcinating precursors at 800 °C for 2 h with a heating rate of 2 °C min1. The calcined samples for S1, S2, and S3 were denoted as S4, S5, and S6, respectively. 2.3. Characterization The samples were characterized by powder X-ray diffraction (XRD) performed on a D8 Focus diffractometer (Bruker). Fourier transform infrared spectroscopy (FT-IR) spectra were measured by a Perkin–Elmer 580B infrared spectrophotometer with the KBr pellet technique. The size and morphology of the samples were inspected using a field emission scanning electron microscope equipped with an energy-dispersive spectrometer (EDS) (FE-SEM, S-4800, Hitachi, Japan). The transmission electron microscopy (TEM) and selected area electron diffraction (SAED) patterns were obtained by a JEOL-2010 transmission electron microscope at the accelerating voltage of 200 kV. Thermogravimetric analyses (TGAs) were performed with a thermal analysis instrument (SDT2960, TA Instruments, New Castle, DE) at 10 °C min1 in an air flow of 100 mL min1 from room temperature to 1000 °C. Photoluminescence (PL) excitation and emission spectra were recorded with a Hitachi F-4500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source at room temperature. The luminescence decay curves were obtained from a Lecroy Wave Runner 6100 Digital Oscilloscope (1 GHz) using a tunable laser (pulse width = 4 ns, gate = 50 ns) as the excitation source (Continuum Sunlite OPO). All the measurements were performed at room temperature.
process. According to the XRD results, the hydrothermal process at 200 °C did not yield Lu2O3 directly. On the basis of the JCPDS reference database, the patterns could not be indexed to any known lutetium compounds. However, the diffraction peaks for all samples can be finely indexed to monoclinic phase of yttrium oxide hydroxide nitrate Y4O(OH)9NO3 (JCPDS No. 79-1352) except for a spectral shift toward the larger angle side, suggesting that the asobtained precursors are isostructural with the Y4O(OH)9NO3 structure [34,35]. So, the as-obtained precursors are presumed to have a structural formula of Lu4O(OH)9NO3. The spectral shift of the diffraction peaks is ascribed to the variation of ionic radii. When Lu3+ (85 pm) with a smaller radius substitutes for the Y3+ (88 pm), the crystal lattice constants as well as d-spacing decreased and thus the diffraction angles increased following the Bragg equation
2d sin h ¼ k; where d is the distance between two crystal planes, h is diffraction angle of an observed peak, and k is the X-ray wavelength (0.15405 nm). Fig. 2 shows the FT-IR spectra of the lutetium oxide precursors synthesized at pH = 8 (S1), 9 (S2), and 10 (S3), respectively. The similarity of the spectra indicates that the precursors prepared at different pH values share the identical structure, which is in accordance with the result of the XRD (Fig. 1). The sharp bands in the range of 3450–3700 cm1 are assigned to the stretching vibration of O–H groups, confirming the existence of OH in the precursors. The peaks located at bout 723, 832, 1064, and 1368 cm1 can be
3. Results and discussion 3.1. Phase identification, morphology, and growth mechanism of lutetium oxide precursor Fig. 1 presents the X-ray diffraction patterns of the samples obtained at pH = 8 (S1), 9 (S2), and 10 (S3) through the hydrothermal
Fig. 2. FT-IR spectra of samples (a) S1, (b) S2, and (c) S3.
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Fig. 3. TG–DSC curves of the as-obtained precursor prepared at pH = 9 (S2).
attributed to the bending vibrations and the stretching vibration of NO3groups in the precursors. The band located around 571 cm1 is assigned to the Lu–O stretching vibrations [36,37]. As shown in Fig. 3, the TG–DSC curve demonstrates the thermal behavior of typical lutetium oxide precursor. There are two major stages of weight loss in the TG curve at 373 and 492 °C, accompanying their corresponding endothermic peaks. The total weight loss was measured to be 15.79%, which agrees with the theoretical value of assumed structure Lu4O(OH)9NO3 (14.5%), calculated from the reaction of complete decomposition into Lu2O3. The results of IR spectra and TG–DSC measurement further support the assumption of structure formula [Lu4O(OH)9NO3]. Fig. 4 demonstrates the morphologies of the lutetium oxide precursors obtained at different reaction conditions. Fig. 4a is a wide-field FESEM image of the hydrothermal products (S2) when pH value of the colloidal solution is 9. Interestingly, highly uniform and well dispersed submicrocolumns were obtained despite that no surfactants and templates were employed in hydrothermal
process. These columns have a length in the range of 600– 750 nm and a diameter of about 400–500 nm. A close observation reveals that the column-like submicrostructures are actually composed of several nanoplates, with 600–750, 100–200, and 50– 60 nm in length, width, and thickness, which array orderly around the central axes of columns (Fig. 4b). As the pH value of the colloids decreased to 8, similar column-like hierarchical structures (S1) were yielded with 0.8–1.3 lm and 0.6–1.1 lm in length and diameter. But, as shown in Fig. 4c, large quantities of nanosheets are densely arranged in a well ordered way to form column-like structures. These constituent nanosheets are 10–20 nm thick, much thinner than the counterparts at the pH of 9. In contrast, the morphology of the as-formed precursors S3 varied from submicrocolumns to bundle-like structures composed of many side-by-side nanowires at the pH of 10 (Fig. 4d). Each bunch has average widths of 150 nm and lengths of about 550–700 nm. Furthermore, it can be observed that the mean aspect ratios are about 1.2, 1.5, and 4.5 for the samples S1, S2, and S3, respectively, revealing that the aspect ratio increases as the pH value rises. To shed light on the formation process of these interesting hierarchical architectures, time-dependent experiments were performed with the sample at pH of 9 as representation. Before hydrothermal process, the mixture was composed of numerous aggregated nanoparticles (Fig. 5a). The XRD pattern (Fig. 5f) shows that the sample remained amorphous by this time. After a short reaction time of 1 h (Fig. 5b), several columns came into being, which were constructed by plenty of nanosheets. A careful observation found that a lot of grains adhered to the cylindrical surfaces, indicating that the columns were evolved from the nanoparticles. Here, legible diffraction peaks appeared in the XRD pattern and were accord with those of the final precursor in spite of low crystallinity. After reacting for 2 h, nanoparticles disappeared and completely converted to column-like structures (Fig. 5c). A single constituent nanosheet had a thickness of 10–15 nm. With the increase in hydrothermal time to 4 h, the densely assembled nanosheets became loose and the thickness increased to 20–25 nm (Fig. 5d). Upon further prolonging the reaction time to 36 h, the
Fig. 4. SEM images of samples S2 (a and b), S1 (c), and S3 (d).
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Fig. 5. SEM images of lutetium oxide precursors obtained at pH = 9 after the reaction proceeded for (a) 0 h, (b) 1 h, (c) 2 h, (d) 4 h, and (e) 36 h and (f) their corresponding XRD patterns.
building blocks were thickened and smoothened obviously (Fig. 5e). According to the XRD results, the diffraction intensity became stronger as reaction time extended, indicating that crystallinity increased (Fig. 5f). On the basis of the experimental observations, the formation process of Lu4O(OH)9NO3 hierarchical column-like structures can be divided into two steps: dissolution–recrystallization and Ostwald-ripening. Before hydrothermal treatment, a rapid adjustment of the pH value of the solution led to the formation of colloid particles. In the initial stages, the amorphous nanoparticles dissolved and generated Lu3+ ions under hydrothermal conditions.
Scheme 1. Schematic illustration of the growth mechanism for the hierarchical structures.
The OH and NO3 in the solution could react with Lu3+ ions to form sheet-like Lu4O(OH)9NO3 which spontaneously aggregated to large columns to minimize their surface area. Considering the fact that there were no catalyst or template to guide the formation of the nanosheets, the driving force for the anisotropic growth of the precursors may derive from the inherent highly anisotropic
Fig. 6. XRD patterns of samples (a) S4, (b) S5, and (c) S6 after calcining samples S1, S2, and S3 at 800 °C for 2 h. The standard data for Lu2O3 (JCPDS card 86-2475) are also presented in the figure for comparison.
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monoclinic structure of Lu4O(OH)9NO3 and the chemical potential in solution. As Li proposed, a higher pH value implies a higher OH
ion concentration and a higher chemical potential in solution, which is preferable for the anisotropic growth [38]. What is more,
Fig. 7. (a) SEM and (b) TEM images of sample S5. (c) SEM and (d) TEM images of sample S4. (e) SEM, (f) low-and (g) high-magnification TEM images of sample S6. (h) HRTEM images of the single nanorod from a submicrobundle. (i) The electron diffraction pattern taken on an individual bundle.
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a higher OH ion concentration could accelerate the nucleation of Lu4O(OH)9NO3 and result in small crystal size [39]. Thus, samples with higher aspect ratio and smaller size were obtained as the pH value increased, as presented in Fig. 4. These nanosheets rapidly self-assembled into columns face-to-face probably because the crystal facets along the surfaces of the sheets matched well, resulting in a low surface energy. In the second step, Ostwald-ripening was mainly going on to make smoother and larger building blocks [40]. The schematic mechanisms for the precursors are summarized in Scheme 1. 3.2. Phase identification and morphology of the Lu2O3:Eu3+ phosphors According to the TGA–DSC data, the lutetium oxide precursors were calcined at 800 °C in air for 2 h to ensure their complete decomposition. As identified in XRD patterns of samples after calcinations in Fig. 6, all diffraction peaks of the as-obtained products (S4, S5, and S6) can be readily indexed to pure cubic phase of Lu2O3 (JCPDS No. 86-2475). This implies that precursors synthesized at different pH values have all successfully converted into lutetium oxide with high crystallinity. The EDS spectrum of sample S5 shows the presence of Re (Lu, Eu) and O with an atomic ratio of 2:3.08, which matches well with that of lutetium oxide Lu2O3 within the experimental error of EDS experiments (Fig. S1). The EDS result suggests that the precursor has decomposed into crystalline Lu2O3 during the calcining process, further supporting the XRD analysis above. In Fig. 7, panels a–g show typical SEM and TEM images of Lu2O3:Eu3+ samples S4, S5, and S6. It can be seen that the Lu2O3:Eu3+ hierarchical architectures inherited the parents’ shapes. On one hand, high activation energies are needed for the collapse of these submicron structures. On the other hand, the low heating rate and appropriate calcination temperature and time contribute to the maintenance of the hierarchical structures. However, the calcination gave shrunken sizes and smoother boundaries compared with the precursors. This is partly ascribed to the release of gaseous carbon oxides and H2O during the calcination process. Another reason for the morphologic change lies in the sintering and crystallization processes at 800 °C. The relatively large size of the samples S4 and S5 has prevented the TEM analysis on their detailed structures, so only sample S6 is measured as representative. Panel g shows the detailed view of the as-obtained bundles, which presents the constituent nanorods evidently. Panel i shows the selected area electron diffraction (SAED) pattern taken on an individual bundle, which contains partial ring and dot patterns, indicating that the Lu2O3:Eu3+ hierarchical architectures are of polycrystalline nature. And the patterns can be indexed as the (2 1 1), (2 2 2), (4 0 0), and (4 4 0) reflections of the cubic Lu2O3, in accordance with the XRD results. The high resolution transmission electron microscopy (HR-TEM) image (Fig. 7h) of a single nanorod from a submicrobundle clearly shows a lattice fringe with interplanar spacing of 0.300 nm that corresponds to the (2 2 2) plane of Lu2O3. The obvious lattice fringes further confirm the high crystallinity of the sample.
Fig. 8. PL emission spectra of the samples (a) S5, (b) S4, and (c) S6 and the typical excitation spectrum (inset).
599, 610, 631, 649, and 706 nm. They come from the 5D1 ? 7F1 and 5D0 ? 7FJ (J = 0, 1, 1, 1, 2, 2, 3, 4) transitions of the Eu3+ ions, respectively. The strongest red emission at 610 arises from the forced electric-dipole 5D0 ? 7F2 transitions of the Eu3+ ions. It is important to note that the morphologies of obtained samples have an important effect on their luminescent intensity. As Fig. 8 presents, the PL intensity of the submicrocolumns (S5) is highest, while the intensity of the bundle-like structures (S6) is the lowest. As is well known, large surface area usually introduces a large number of defects into the phosphor crystal, which provide non-radiative recombination routes for electrons and holes and lead to luminescence quenching [41]. According to the SEM and TEM results demonstrated above (Fig. 7), the size of sample S6 is much smaller that sample S4 and S5, which is responsible for its largest surface area. Although the overall size of sample S4 is larger than that of sample S5, its components are much thinner, increasing the surface area greatly. In contrast, the building blocks of the sample S5 are larger, which gives rise to small surface area and
3.3. Luminescence properties Fig. 8 shows the emission and excitation (inset) spectra of the Lu2O3:Eu3+ samples of different morphologies (S4, S5, and S6), all of which have similar photoluminescent (PL) properties. The excitation spectrum monitored at 610 nm emission of Eu3+ ions consists of a broad band centered at 245 nm and weak sharp lines, which are due to the charge-transfer band (CTB) between the O2 and Eu3+ ions and the f–f transition of the Eu3+ ions, respectively. Upon excitation into the CTB of the Eu3+ ions at 245 nm, the emission spectrum is composed of a group of lines peaking at about 533, 580, 586, 592,
Fig. 9. Decay curves for the 5D0 ? 7F2 (610 nm) emission of Eu3+ of the samples (a)S4, (b) S5, and (c) S6.
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high PL intensity accordingly. As a result, the PL intensity could be arranged as a function of morphologies: S5 > S4 > S6. Fig. 9 shows the fluorescent decay curves of the Eu3+ ions in Lu2O3 host lattices with different morphologies, respectively. All curves can be well fitted to a single exponential function as I = I0 exp (t/s), where s is the decay lifetime. The lifetimes are determined to be 1.23, 1.49, and 1.19 ms for sample S4, S5 and S6, respectively. These results are basically in agreement with the previously reported Lu2O3:Eu3+ phosphors [32]. The longest lifetime of the sample S5 may be mainly attributed to the decrease in nonradiative transition rate caused by surface defects in nanocrystals, which is accorded with the PL intensity.
4. Conclusions In summary, column-like and bundle-like Lu2O3:Eu3+ submicrostructures have been successfully prepared via a facile hydrothermal method combining with a post-calcining process. Highly uniform and well dispersed Lu4O(OH)9(NO3) hierarchical architectures were yielded without the assistance of any surfactant, catalyst, or template. The formation process of the hierarchical precursors includes two stages: dissolution–recrystallization and Ostwald-ripening. The investigation of synthesis parameters reveals that pH value has a significant impact upon the size and aspect ratio of the precursors. The final products of Lu2O3:Eu3+ inherit their parents’ morphologies after calcining and exhibit a strong red emission under the UV excitation. Such hierarchical submicrostructures with unique morphologies and excellent luminescence properties are promising candidates for technical applications in the display systems, optoelectronic device, and biosensor.
Acknowledgments This work is financially supported by the National Natural Science Foundation of China (Grant No. 20771098) and the Fund for Creative Research Groups (Grant No. 20921002), and the National Basic Research Program of China (973 Program, Grant No. 2007CB935502).
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2012.11.029.
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Contents lists available at SciVerse ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Facile surfactant-free synthesis and luminescent properties of hierarchical europium-doped lutetium oxide phosphors Qi Zhao a,b, Ning Guo a,b, Yongchao Jia a,b, Wenzhen Lv a,b, Baiqi Shao a,b, Mengmeng Jiao a,b, Hongpeng You a,⇑ a b
State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR China Graduate University of the Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e
i n f o
Article history: Received 21 September 2012 Accepted 7 November 2012 Available online 29 November 2012 Keywords: Lu2O3 Hydrothermal method Hierarchical structures Luminescence
a b s t r a c t Novel three-dimensional (3D) hierarchical architectures of Lu2O3:Eu3+ have been successfully prepared through a simple hydrothermal process followed by a subsequent calcination process without using any surfactant, catalyst, or template. According to the X-ray diffraction, thermogravimetric analysis and differential scanning calorimetry, and Fourier transform infrared spectroscopy results, the precursors were determined to have the structure formula of Lu4O(OH)9NO3. The morphologies of the precursors could be modulated by altering pH value of the reaction system. On the basis of detailed time-dependent experiments, the growth process of architectures was discussed. The as-formed precursors could transform into Lu2O3:Eu3+ with their original hierarchical structures maintained. The Lu2O3:Eu3+ submicroarchitectures exhibit strong red emission corresponding to the 5D0 ? 7F2 transition of the Eu3+ ions under ultraviolet (UV) excitation, which have potential applications in novel optoelectronic devices. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction Recently, the interest in the fabrication of inorganic micro/ nanomaterials with controllable size, dimensionality, and structure has soared because it provides alternative means for tailoring the chemical and physical properties of functional materials [1–5]. Among the various micro/nanostructures, the hierarchical assembly of nanoscale building blocks into ordered superstructures or complex architectures has attracted considerable attention owing to the novel and collective properties, which are not found on the level of individual nanoparticles [6–9]. Therefore, much attention has been paid to the fabrication of hierarchical micro/nanomaterials by various synthesis techniques. Among them, hydrothermal synthesis has been widely used for the preparation of the micro/nanomaterials [10–15], due to its advantages in reducing processing temperature and yielding fine crystalline powders. In this synthesis route, catalysts, templates, or surfactants are frequently required as capping agents, stabilizing agents, directing agents, or soft templates to form the hierarchical frameworks [16,17]. However, the introduction of these additives into synthetic process will bring heterogeneous impurities, increase the production cost, and present difficulty for scale-up production [18,19]. Moreover, the surfactants are usually organic molecules which are difficult to be eliminated completely and will produce serious drawbacks to luminescence intensity for phosphors [20]. ⇑ Corresponding author. Fax: +86 431 85698041. E-mail address: [email protected] (H. You). 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.11.029
This situation entails efforts to develop more effective and economical methods for preparing micro/nanostructures. Rare earth compounds have been widely used in highperformance luminescent devices, magnets, catalysts, and other functional materials because of the novel electronic, optical, and chemical characteristics resulting from the 4f electrons of rare earth ions [21,22]. Specifically, lutetium oxide (Lu2O3) is an excellent phosphor host materials due to its favorable physical properties, such as high melting point, phase stability, and low thermal expansion [23]. Eu3+-doped lutetium oxide have long been an important red phosphor in color television picture tubes, white LEDs, and field emission displays [24,25]. Conventionally, Lu2O3 and Ln3+-doped Lu2O3 materials are prepared via methods such as combustion (propellant) process using urea (glycine/citric acid) as fuel [25,26], coprecipitation method [27,28], and the Pechini sol–gel procedure [29]. Unfortunately, the morphology of products is often irregular and uncontrollable. Recently, Lu2O3 nanorods [30,31], nanowires [32], and nanosheets [33] have been synthesized via the hydrothermal or solvothermal method followed by a subsequent heat-treatment process. However, the reports on the synthesis of complex hierarchical architectures of rare-earth oxides were relatively rare. So, how to fabricate 3D hierarchical Lu2O3 controllably through a facile and large-scale method remains a tremendous challenge for material chemists. Herein, we report the preparation of 3D hierarchical Lu2O3:Eu3+ through a facile hydrothermal process by a subsequent calcination process in the absence of any surfactants, template supporting, and structure-directing reagents. By simply regulating the pH values of
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the initial solution, various hierarchical architectures of Lu2O3:Eu3+ were achieved. The possible formation mechanisms of the novel architectures were proposed. Finally, we investigated the photoluminescence properties of the as-obtained Lu2O3:Eu3+ and the relationship between luminescence intensity and morphologies. 2. Experimental section 2.1. Reagents The rare-earth oxides RE2O3 (RE = Lu, Eu) (99.99%) were purchased from Shanghai Yuelong Non-Ferrous Metals Limited. The other analytical chemicals were purchased from Beijing Chemical Co. and used as received without further purification. Rare-earth nitrate stock solutions were prepared by dissolving the corresponding metal oxide in nitric acid under heating with agitation.
Fig. 1. XRD patterns of lutetium oxide precursor prepared at pH values of (a) 8 (S1), (b) 9 (S2), (c) 10 (S3), and the standard data for Y4O(OH)9NO3 (JCPDS No. 79-1352) as a reference.
2.2. Preparation In a typical procedure, 1.9 mL of Lu(NO3)3 (1 M) and 2 mL of Eu(NO3)3 (0.05 M) were added into 35 mL of ethanol. Subsequently, 25 wt.% of ammonia solution was introduced dropwise to adjust the pH value. After additional agitation for 30 min, the feedstock was transferred to a 50 mL Teflon-lined stainless autoclave and heated at 200 °C for 24 h. When the autoclave was cooled to room temperature naturally, the precursors were separated by centrifugation, washed with ethanol and deionized water several times, and dried at 60 °C in air. The precursors prepared at pH = 8, 9, and 10 were labeled as S1, S2, and S3, respectively. The final products were prepared by calcinating precursors at 800 °C for 2 h with a heating rate of 2 °C min1. The calcined samples for S1, S2, and S3 were denoted as S4, S5, and S6, respectively. 2.3. Characterization The samples were characterized by powder X-ray diffraction (XRD) performed on a D8 Focus diffractometer (Bruker). Fourier transform infrared spectroscopy (FT-IR) spectra were measured by a Perkin–Elmer 580B infrared spectrophotometer with the KBr pellet technique. The size and morphology of the samples were inspected using a field emission scanning electron microscope equipped with an energy-dispersive spectrometer (EDS) (FE-SEM, S-4800, Hitachi, Japan). The transmission electron microscopy (TEM) and selected area electron diffraction (SAED) patterns were obtained by a JEOL-2010 transmission electron microscope at the accelerating voltage of 200 kV. Thermogravimetric analyses (TGAs) were performed with a thermal analysis instrument (SDT2960, TA Instruments, New Castle, DE) at 10 °C min1 in an air flow of 100 mL min1 from room temperature to 1000 °C. Photoluminescence (PL) excitation and emission spectra were recorded with a Hitachi F-4500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source at room temperature. The luminescence decay curves were obtained from a Lecroy Wave Runner 6100 Digital Oscilloscope (1 GHz) using a tunable laser (pulse width = 4 ns, gate = 50 ns) as the excitation source (Continuum Sunlite OPO). All the measurements were performed at room temperature.
process. According to the XRD results, the hydrothermal process at 200 °C did not yield Lu2O3 directly. On the basis of the JCPDS reference database, the patterns could not be indexed to any known lutetium compounds. However, the diffraction peaks for all samples can be finely indexed to monoclinic phase of yttrium oxide hydroxide nitrate Y4O(OH)9NO3 (JCPDS No. 79-1352) except for a spectral shift toward the larger angle side, suggesting that the asobtained precursors are isostructural with the Y4O(OH)9NO3 structure [34,35]. So, the as-obtained precursors are presumed to have a structural formula of Lu4O(OH)9NO3. The spectral shift of the diffraction peaks is ascribed to the variation of ionic radii. When Lu3+ (85 pm) with a smaller radius substitutes for the Y3+ (88 pm), the crystal lattice constants as well as d-spacing decreased and thus the diffraction angles increased following the Bragg equation
2d sin h ¼ k; where d is the distance between two crystal planes, h is diffraction angle of an observed peak, and k is the X-ray wavelength (0.15405 nm). Fig. 2 shows the FT-IR spectra of the lutetium oxide precursors synthesized at pH = 8 (S1), 9 (S2), and 10 (S3), respectively. The similarity of the spectra indicates that the precursors prepared at different pH values share the identical structure, which is in accordance with the result of the XRD (Fig. 1). The sharp bands in the range of 3450–3700 cm1 are assigned to the stretching vibration of O–H groups, confirming the existence of OH in the precursors. The peaks located at bout 723, 832, 1064, and 1368 cm1 can be
3. Results and discussion 3.1. Phase identification, morphology, and growth mechanism of lutetium oxide precursor Fig. 1 presents the X-ray diffraction patterns of the samples obtained at pH = 8 (S1), 9 (S2), and 10 (S3) through the hydrothermal
Fig. 2. FT-IR spectra of samples (a) S1, (b) S2, and (c) S3.
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Fig. 3. TG–DSC curves of the as-obtained precursor prepared at pH = 9 (S2).
attributed to the bending vibrations and the stretching vibration of NO3groups in the precursors. The band located around 571 cm1 is assigned to the Lu–O stretching vibrations [36,37]. As shown in Fig. 3, the TG–DSC curve demonstrates the thermal behavior of typical lutetium oxide precursor. There are two major stages of weight loss in the TG curve at 373 and 492 °C, accompanying their corresponding endothermic peaks. The total weight loss was measured to be 15.79%, which agrees with the theoretical value of assumed structure Lu4O(OH)9NO3 (14.5%), calculated from the reaction of complete decomposition into Lu2O3. The results of IR spectra and TG–DSC measurement further support the assumption of structure formula [Lu4O(OH)9NO3]. Fig. 4 demonstrates the morphologies of the lutetium oxide precursors obtained at different reaction conditions. Fig. 4a is a wide-field FESEM image of the hydrothermal products (S2) when pH value of the colloidal solution is 9. Interestingly, highly uniform and well dispersed submicrocolumns were obtained despite that no surfactants and templates were employed in hydrothermal
process. These columns have a length in the range of 600– 750 nm and a diameter of about 400–500 nm. A close observation reveals that the column-like submicrostructures are actually composed of several nanoplates, with 600–750, 100–200, and 50– 60 nm in length, width, and thickness, which array orderly around the central axes of columns (Fig. 4b). As the pH value of the colloids decreased to 8, similar column-like hierarchical structures (S1) were yielded with 0.8–1.3 lm and 0.6–1.1 lm in length and diameter. But, as shown in Fig. 4c, large quantities of nanosheets are densely arranged in a well ordered way to form column-like structures. These constituent nanosheets are 10–20 nm thick, much thinner than the counterparts at the pH of 9. In contrast, the morphology of the as-formed precursors S3 varied from submicrocolumns to bundle-like structures composed of many side-by-side nanowires at the pH of 10 (Fig. 4d). Each bunch has average widths of 150 nm and lengths of about 550–700 nm. Furthermore, it can be observed that the mean aspect ratios are about 1.2, 1.5, and 4.5 for the samples S1, S2, and S3, respectively, revealing that the aspect ratio increases as the pH value rises. To shed light on the formation process of these interesting hierarchical architectures, time-dependent experiments were performed with the sample at pH of 9 as representation. Before hydrothermal process, the mixture was composed of numerous aggregated nanoparticles (Fig. 5a). The XRD pattern (Fig. 5f) shows that the sample remained amorphous by this time. After a short reaction time of 1 h (Fig. 5b), several columns came into being, which were constructed by plenty of nanosheets. A careful observation found that a lot of grains adhered to the cylindrical surfaces, indicating that the columns were evolved from the nanoparticles. Here, legible diffraction peaks appeared in the XRD pattern and were accord with those of the final precursor in spite of low crystallinity. After reacting for 2 h, nanoparticles disappeared and completely converted to column-like structures (Fig. 5c). A single constituent nanosheet had a thickness of 10–15 nm. With the increase in hydrothermal time to 4 h, the densely assembled nanosheets became loose and the thickness increased to 20–25 nm (Fig. 5d). Upon further prolonging the reaction time to 36 h, the
Fig. 4. SEM images of samples S2 (a and b), S1 (c), and S3 (d).
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Fig. 5. SEM images of lutetium oxide precursors obtained at pH = 9 after the reaction proceeded for (a) 0 h, (b) 1 h, (c) 2 h, (d) 4 h, and (e) 36 h and (f) their corresponding XRD patterns.
building blocks were thickened and smoothened obviously (Fig. 5e). According to the XRD results, the diffraction intensity became stronger as reaction time extended, indicating that crystallinity increased (Fig. 5f). On the basis of the experimental observations, the formation process of Lu4O(OH)9NO3 hierarchical column-like structures can be divided into two steps: dissolution–recrystallization and Ostwald-ripening. Before hydrothermal treatment, a rapid adjustment of the pH value of the solution led to the formation of colloid particles. In the initial stages, the amorphous nanoparticles dissolved and generated Lu3+ ions under hydrothermal conditions.
Scheme 1. Schematic illustration of the growth mechanism for the hierarchical structures.
The OH and NO3 in the solution could react with Lu3+ ions to form sheet-like Lu4O(OH)9NO3 which spontaneously aggregated to large columns to minimize their surface area. Considering the fact that there were no catalyst or template to guide the formation of the nanosheets, the driving force for the anisotropic growth of the precursors may derive from the inherent highly anisotropic
Fig. 6. XRD patterns of samples (a) S4, (b) S5, and (c) S6 after calcining samples S1, S2, and S3 at 800 °C for 2 h. The standard data for Lu2O3 (JCPDS card 86-2475) are also presented in the figure for comparison.
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monoclinic structure of Lu4O(OH)9NO3 and the chemical potential in solution. As Li proposed, a higher pH value implies a higher OH
ion concentration and a higher chemical potential in solution, which is preferable for the anisotropic growth [38]. What is more,
Fig. 7. (a) SEM and (b) TEM images of sample S5. (c) SEM and (d) TEM images of sample S4. (e) SEM, (f) low-and (g) high-magnification TEM images of sample S6. (h) HRTEM images of the single nanorod from a submicrobundle. (i) The electron diffraction pattern taken on an individual bundle.
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a higher OH ion concentration could accelerate the nucleation of Lu4O(OH)9NO3 and result in small crystal size [39]. Thus, samples with higher aspect ratio and smaller size were obtained as the pH value increased, as presented in Fig. 4. These nanosheets rapidly self-assembled into columns face-to-face probably because the crystal facets along the surfaces of the sheets matched well, resulting in a low surface energy. In the second step, Ostwald-ripening was mainly going on to make smoother and larger building blocks [40]. The schematic mechanisms for the precursors are summarized in Scheme 1. 3.2. Phase identification and morphology of the Lu2O3:Eu3+ phosphors According to the TGA–DSC data, the lutetium oxide precursors were calcined at 800 °C in air for 2 h to ensure their complete decomposition. As identified in XRD patterns of samples after calcinations in Fig. 6, all diffraction peaks of the as-obtained products (S4, S5, and S6) can be readily indexed to pure cubic phase of Lu2O3 (JCPDS No. 86-2475). This implies that precursors synthesized at different pH values have all successfully converted into lutetium oxide with high crystallinity. The EDS spectrum of sample S5 shows the presence of Re (Lu, Eu) and O with an atomic ratio of 2:3.08, which matches well with that of lutetium oxide Lu2O3 within the experimental error of EDS experiments (Fig. S1). The EDS result suggests that the precursor has decomposed into crystalline Lu2O3 during the calcining process, further supporting the XRD analysis above. In Fig. 7, panels a–g show typical SEM and TEM images of Lu2O3:Eu3+ samples S4, S5, and S6. It can be seen that the Lu2O3:Eu3+ hierarchical architectures inherited the parents’ shapes. On one hand, high activation energies are needed for the collapse of these submicron structures. On the other hand, the low heating rate and appropriate calcination temperature and time contribute to the maintenance of the hierarchical structures. However, the calcination gave shrunken sizes and smoother boundaries compared with the precursors. This is partly ascribed to the release of gaseous carbon oxides and H2O during the calcination process. Another reason for the morphologic change lies in the sintering and crystallization processes at 800 °C. The relatively large size of the samples S4 and S5 has prevented the TEM analysis on their detailed structures, so only sample S6 is measured as representative. Panel g shows the detailed view of the as-obtained bundles, which presents the constituent nanorods evidently. Panel i shows the selected area electron diffraction (SAED) pattern taken on an individual bundle, which contains partial ring and dot patterns, indicating that the Lu2O3:Eu3+ hierarchical architectures are of polycrystalline nature. And the patterns can be indexed as the (2 1 1), (2 2 2), (4 0 0), and (4 4 0) reflections of the cubic Lu2O3, in accordance with the XRD results. The high resolution transmission electron microscopy (HR-TEM) image (Fig. 7h) of a single nanorod from a submicrobundle clearly shows a lattice fringe with interplanar spacing of 0.300 nm that corresponds to the (2 2 2) plane of Lu2O3. The obvious lattice fringes further confirm the high crystallinity of the sample.
Fig. 8. PL emission spectra of the samples (a) S5, (b) S4, and (c) S6 and the typical excitation spectrum (inset).
599, 610, 631, 649, and 706 nm. They come from the 5D1 ? 7F1 and 5D0 ? 7FJ (J = 0, 1, 1, 1, 2, 2, 3, 4) transitions of the Eu3+ ions, respectively. The strongest red emission at 610 arises from the forced electric-dipole 5D0 ? 7F2 transitions of the Eu3+ ions. It is important to note that the morphologies of obtained samples have an important effect on their luminescent intensity. As Fig. 8 presents, the PL intensity of the submicrocolumns (S5) is highest, while the intensity of the bundle-like structures (S6) is the lowest. As is well known, large surface area usually introduces a large number of defects into the phosphor crystal, which provide non-radiative recombination routes for electrons and holes and lead to luminescence quenching [41]. According to the SEM and TEM results demonstrated above (Fig. 7), the size of sample S6 is much smaller that sample S4 and S5, which is responsible for its largest surface area. Although the overall size of sample S4 is larger than that of sample S5, its components are much thinner, increasing the surface area greatly. In contrast, the building blocks of the sample S5 are larger, which gives rise to small surface area and
3.3. Luminescence properties Fig. 8 shows the emission and excitation (inset) spectra of the Lu2O3:Eu3+ samples of different morphologies (S4, S5, and S6), all of which have similar photoluminescent (PL) properties. The excitation spectrum monitored at 610 nm emission of Eu3+ ions consists of a broad band centered at 245 nm and weak sharp lines, which are due to the charge-transfer band (CTB) between the O2 and Eu3+ ions and the f–f transition of the Eu3+ ions, respectively. Upon excitation into the CTB of the Eu3+ ions at 245 nm, the emission spectrum is composed of a group of lines peaking at about 533, 580, 586, 592,
Fig. 9. Decay curves for the 5D0 ? 7F2 (610 nm) emission of Eu3+ of the samples (a)S4, (b) S5, and (c) S6.
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high PL intensity accordingly. As a result, the PL intensity could be arranged as a function of morphologies: S5 > S4 > S6. Fig. 9 shows the fluorescent decay curves of the Eu3+ ions in Lu2O3 host lattices with different morphologies, respectively. All curves can be well fitted to a single exponential function as I = I0 exp (t/s), where s is the decay lifetime. The lifetimes are determined to be 1.23, 1.49, and 1.19 ms for sample S4, S5 and S6, respectively. These results are basically in agreement with the previously reported Lu2O3:Eu3+ phosphors [32]. The longest lifetime of the sample S5 may be mainly attributed to the decrease in nonradiative transition rate caused by surface defects in nanocrystals, which is accorded with the PL intensity.
4. Conclusions In summary, column-like and bundle-like Lu2O3:Eu3+ submicrostructures have been successfully prepared via a facile hydrothermal method combining with a post-calcining process. Highly uniform and well dispersed Lu4O(OH)9(NO3) hierarchical architectures were yielded without the assistance of any surfactant, catalyst, or template. The formation process of the hierarchical precursors includes two stages: dissolution–recrystallization and Ostwald-ripening. The investigation of synthesis parameters reveals that pH value has a significant impact upon the size and aspect ratio of the precursors. The final products of Lu2O3:Eu3+ inherit their parents’ morphologies after calcining and exhibit a strong red emission under the UV excitation. Such hierarchical submicrostructures with unique morphologies and excellent luminescence properties are promising candidates for technical applications in the display systems, optoelectronic device, and biosensor.
Acknowledgments This work is financially supported by the National Natural Science Foundation of China (Grant No. 20771098) and the Fund for Creative Research Groups (Grant No. 20921002), and the National Basic Research Program of China (973 Program, Grant No. 2007CB935502).
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2012.11.029.
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