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Self-assembled light lanthanide oxalate architecture with controlled morphology, characterization, growing mechanism and optical property
Materials Research Bulletin 46 (2011) 1546–1552
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Self-assembled light lanthanide oxalate architecture with controlled morphology, characterization, growing mechanism and optical property Hongmei He, Youjin Zhang *, Wei Zhu, Ao Zheng Department of Chemistry, University of Science and Technology of China, 96 Jinzhai Road, Hefei, 230026, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 27 April 2011 Received in revised form 2 June 2011 Accepted 16 June 2011 Available online 23 June 2011
Flower-like Sm2(C2O4)310H2O had been synthesized by a facile complex agent assisted precipitation method. The flower-like Sm2(C2O4)310H2O was characterized by X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, field-emission scanning electron microscopy, thermogravimetry–differential thermal analysis and photoluminescence. The possible growth mechanism of the flower-like Sm2(C2O4)310H2O was proposed. To extend this method, other Ln2(C2O4)3nH2O (Ln = Gd, Dy, Lu, Y) with different morphologies also had been prepared by adjusting different rare earth precursors. Further studies revealed that besides the reaction conditions and the additive amount of complex agents, the morphologies of the as-synthesised lanthanide oxalates were also determined by the rare earth ions. The Sm2(C2O4)310H2O and Sm2O3 samples exhibited different photoluminescence spectra, which was relevant to Sm3+ energy level structure of 4f electrons. The method may be applied in the synthesis of other lanthanide compounds, and the work could explore the potential optical materials. ß 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Inorganic compounds B. Chemical synthesis C. Electron microscopy C. X-ray diffraction D. Optical properties
1. Introduction In current years, the precise design and synthesis of lanthanide compound micro/nanostructure materials with well-defined morphologies and tunable sizes remain the research hot, because the chemical and physical properties of functional lanthanide compounds are fundamentally related to their size, morphology, and dimensionality [1–5]. Particularly, as an important lanthanide compounds, lanthanide oxalates were widely used in rare earth purification industry [6], or as the precursor to prepare lanthanide oxides [7]. There has been an increasing number of reports on the precise design and synthesis of rare earth oxalates including dysprosium gadolinium oxalate [8], neodymium praseodymium oxalate [9], lanthanide calcium oxalate [10] and lanthanide metalorganic frameworks [11] and Ln2(C2O4)310H2O (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Y and Yb) by various synthesis methods, such as hydrothermal synthesis without complex agent, sol–gel method, emulsion liquid membrane (ELM, water-in-oil- in-water emulsion) system, etc. [12–18]. Herein, a simple and mild complex agent assisted precipitation method at room temperature was proposed for the synthesis of Ln2(C2O4)3nH2O (Ln = Sm, Gd, Dy, Lu, Y) micro-particles, in which trisodium citrate (Na3Cit) was chosen as the complex agent. A series of experiments for the reaction temperature, the reaction time and
* Corresponding author. Tel.: +86 551 3492145; fax: +86 551 3492083. E-mail address: [email protected] (Y. Zhang). 0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2011.06.029
the molar ratio of Na3Cit to Sm3+ had been carried out to investigate their influence on the flower-like Sm2(C2O4)310H2O micro-particles. In addition, more and more attentions have been focused on the study of Sm3+ activated inorganic materials for application in luminescent materials because of its electron transitions within the 4f shell. However, to the best of our knowledge, no studies have been conducted on the luminescence performance of pure Sm2(C2O4)310H2O crystal. Thus the photoluminescence properties of the pure Sm2(C2O4)310H2O and Sm2O3 obtained by calcining Sm2(C2O4)310H2O crystal, were investigated in the paper. 2. Experimental 2.1. Materials All reagents (samarium oxide, gadolinium oxide, dysprosium oxide, lutetium oxide, yttrium oxide, oxalic acid, trisodium citrate (Na3Cit), ethylene diamine tetraacetic acid (EDTA)) used were of analytical purity, obtained from Shanghai Chemical Reagent Ltd. Co. of China and used without further purification. 2.2. Synthesis of Sm2(C2O4)310H2O LnCl3 (Ln = Sm, Gd, Dy, Lu and Y) aqueous solution was obtained by dissolving Ln2O3 (99.99%) (Ln = Sm, Gd, Dy, Lu and Y) in 1 mol/L HCl solution and dried by heating to produce LnCl3 crystal. In a typical procedure, 0.5 mmol Sm2O3 was dissolved in 5.0 mL of 1 mol/L HCl and dried by heating. The reactant was dissolved in
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30 mL of deionized water and then 1.2 mmol Na3Cit was added to the solution with stirring. The mixed aqueous solution was stirred at room temperature for 15 min to form samarium citrate complexes, then 1.5 mmol H2C2O42H2O was introduced to the samarium citrate complexes. After agitation for 10 min, the asobtained mixture was transferred into a 50-mL Teflon liner and some distilled water was added up to 80% of the total volume. The autoclave was sealed for 24 h at room temperature. The obtained precipitate was collected after being washed several times with deionized water and dried at 60 8C in air. The product was finally obtained. A similar process was applied to prepare Ln(C2O4)3nH2O (Ln = Gd, Dy, Lu and Y) samples by using a proper amount of Gd(Cl)3, Dy(Cl)3, Lu(Cl)3, and Y(Cl)3 solutions instead of Sm(Cl)3 at the initial stage as described above. Samples with other parameters were prepared in a similar way. 2.3. Synthesis of Sm2O3 Sm2O3 was calcined from Sm2(C2O4)310H2O at 850 8C for 6 h.
Fig. 1. XRD patterns of the Ln2(C2O4)3nH2O obtained at room temperature for 24 h with the molar ratio of Na3Cit to Ln3+ of 1.2:1: (a) Lu; (b) Dy; (c) Y; (d) Gd; (e) Sm.
2.4. Characterization Powder X-ray diffraction (XRD) was carried out with a Japan Rigaku D/max rA X-ray diffractometer equipped with graphitemonochromatized high-intensity Cu Ka radiation (l = 0.15478 nm). The scanning rate was 0.058 s1 in the 2u range from 108 to 708. The X-ray photoelectron spectroscopy (XPS) analysis was performed on an ESCALAB MK II X-ray photoelectron spectrometer, using Mg Kr radiation as the excitation source. The Fourier transform infrared (FTIR) spectroscopic study was performed on a MAGNA-IR 750 FTIR spectrometer. The field-emission scanning electron microscopy (FESEM) images were obtained on a JEOL-6300F field-emission scanning electron microscope with an accelerating voltage of 15 kV. The thermogravimetry–differential thermal analysis (TG–DTA) was conducted on a Rigaku Standard Model thermal analyzer (in air atmosphere, flow rate: 90 mL min1; heat rate: 10 8C min1). The photoluminescence (PL) was carried out using a Jobin Yvon Fluorolog-3-TAU steadystate/lifetime spectrofluorometer. 3. Results and discussion 3.1. Characterization, morphology and the growth mechanism of Sm2(C2O4)310H2O crystal The overall crystallinity and purity of the Ln2(C2O4)3nH2O samples obtained at room temperature for 24 h with the molar ratio of Na3Cit to Ln3+ of 1.2:1 were characterized by X-ray powder diffraction as shown in Fig. 1. All the XRD patterns of the asprepared products can be indexed to anorthic Lu2(C2O4)36H2O (space group:P1, JCPDS card 49-1244), monoclinic Dy2(C2O4)310H2O (space group:P21/c, JCPDS card 21-0315), monoclinic Y2(C2O4)310H2O (space group:P21/c, JCPDS card 331460), monoclinic Gd2(C2O4)310H2O (space group:P21/c, JCPDS card 20-0411) and monoclinic Sm2(C2O4)310H2O (space group:P21/c, JCPDS card 20-1021) from Fig. 1a–e, respectively. No XRD peaks for single metal oxides Ln2O3 in the lanthanide oxalates were detected, which indicated the pure lanthanide oxalate crystals can be achieved by this method. The Sm2(C2O4)310H2O crystal obtained at room temperature for 24 h with the molar ratio of Na3Cit to Sm3+ of 1.2:1 was also characterized by XPS for the evaluation of its composition on the particle surface (Fig. 2). The binding energies obtained in the XPS analysis are corrected for specimen charging by referencing the C 1s orbital to 284.17 eV. The Sm 4d5/2, 4p3/2 and 3d5/2 peaks centered at 132.54 eV, 249.44 eV and 1083.21 eV, respectively
(Fig. 2b and c) [19,20]. No characteristic peaks of other metals are detected. The O 1s spectrum of the sample demonstrates a O2 peak at around 530.31 eV [21] (Fig. 2d). To confirm the formation of the Sm2(C2O4)310H2O crystal obtained at room temperature for 24 h with the molar ratio of Na3Cit to Sm3+ of 1.2:1, FTIR spectroscopy was performed in the wave number range from 200 to 4000 cm1, as illustrated in Fig. 3. A strong absorption band at around 805 cm1 can be attributed to the absorption of Sm–O [22] and a weak band at 486 cm1 represents the CO2 wagging mode [9], respectively. There is water of crystallization in the crystal, which is evidenced by the intense band at around 3375 cm1 [23]. The absorption bands located at about 3375 and 1321 cm1 can be corresponded to the bending mode of water and asymmetric stretch of CO2 [12,24]. The morphology of Sm2(C2O4)310H2O crystal was investigated by FESEM. The higher magnification FESEM images illustrated the flower-like Sm2(C2O4)310H2O, with the diameter about 5 mm, was obtained through the simple precipitation process at room temperature (Fig. 4a and b). It could be clearly seen the flowerlike Sm2(C2O4)310H2O was self-assembled by many thick and smooth flakes from Fig. 4a, which should be beneficial to reduce the surface area and surface energy of the product. It is well-known that the crystal morphology has important effect on its physical and chemical properties. The morphological uniformity of the product is found to be highly correlative with the reaction temperature, reaction time and the additive amount of the complex agent [25]. To control the morphology of Sm2(C2O4)310H2O architectures, the influence of reaction temperature, reaction time and the molar ratio of Na3Cit to Sm3+ on the morphologies of the products was investigated by FESEM. The FESEM images indicated the reaction time and reaction temperature had a great role on the flower-like Sm2(C2O4)310H2O (Fig. 5). The reaction for too short or too long a time was unfavorable for the formation of the flower-like Sm2(C2O4)310H2O. The flower-like Sm2(C2O4)310H2O was not well developed when the reaction time was 12 h, indicating that the self-assembling process was incomplete (Fig. 5a). The regular flower-like Sm2(C2O4)310H2O architecture was obtained when the reaction time was prolonged to 24 h (Fig. 5b). As the reaction time further increased to 30 h (Fig. 5c) or the temperature increased to 60 8C (Fig. 5d), the flower-like architecture fell to pieces. The morphologies of the products obtained with different dosages of Na3Cit at room temperature for 24 h illuminated the
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Fig. 2. XPS spectra of the as-synthesized Sm2(C2O4)310H2O: (a) survey spectrum; (b) Sm 4d and Sm 4p region; (c) Sm 3d region; (d) O 1s region.
molar ratio of Na3Cit to Sm3+ had crucial effect on the formation of the flower-like Sm2(C2O4)310H2O. For a lack of Na3Cit (with the molar ratio of Na3Cit to Sm3+ 0:1 or 0.6:1), the samples were composed of some assembled micro-particles but not regular flower-like structure (Fig. 6a and b). However, when the molar ratio was increased to 1.8:1, all the as-prepared flower-like Sm2(C2O4)310H2O crystal was inclined to fragmentation and the flake became larger (Fig. 6d). Only when the molar ratio was 1.2:1, could the uniform flower-like Sm2(C2O4)310H2O crystal be observed clearly (Fig. 6c). On the basis of the above experiment results, it can be generalized a conclusion that the uniform flower-like Sm2(C2O4)310H2O could
Fig. 3. The FTIR spectrum of Sm2(C2O4)310H2O sample.
be fabricated at room temperature for 24 h with the molar ratio of Na3Cit to Sm3+ of 1.2:1. A possible process for the self-assembled flower-like Sm2(C2O4)310H2O is shown in Fig. 7. In the paper, it is found that the Cit3 plays a key role in the formation of flower-like Sm2(C2O4)310H2O. At first, the Cit3 is a strong chelating agent with three carboxylate groups for metal ions [26]. In the solution, Sm3+ ions are protected by forming Sm (Cit)23 units (Eq. (1)), greatly decreasing the free Sm3+ ion concentration. When oxalic acid solution is added, Sm3+ would be released gradually from the Sm(Cit)23 units and the free Sm3+ ions can react with the vicinal C2O42 ions to form numerous tiny Sm2(C2O4)3 crystalline nuclei (Eq. (2)). During the subsequent crystal growth stage, these tiny nuclei grow to form particles. According to LaMer’s model, the formation of such complexes could control the concentration of free Sm3+ ions in the solution, and thus help to control the nucleation and growth rate of the Sm2(C2O4)3 particles. The relatively slow generation rate of particles is favorable for the subsequent growth of hierarchical structures [27]. The kinetic control leads to the formation of the flakes. When the reaction time was increased, these initial flakes continued to grow in a self-assembled growth style, and finally the flower-like structure was formed. Sm3þ þ 2Cit3 Ð SmðCitÞ2 3
(1)
Sm3þ þ C2 O4 2 þ 10H2 O Ð Sm2 ðC2 O4 Þ3 10H2 O
(2)
To further investigate the influence of complex agents on the Sm2(C2O4)310H2O morphology, the products with different complex agents were obtained. The FESEM image showed many large flakes appeared in the product when EDTA is used as the complex agent (Fig. 8b).
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Fig. 4. FESEM images of the as-synthesized product obtained at room temperature for 24 h with the molar ratio of Na3Cit to Sm3+ of 1.2:1.
The different impacts of Na3Cit and EDTA on the product morphology have some connection with their different chelating constants and coordination modes with Sm3+. On the one hand, the chelating constant for Cit3 is smaller than that for EDTA [26,28]. The smaller the chelating constant, the faster the nucleation rate, which would influence the growth rate of the Sm2(C2O4)310H2O particles. On the other hand, the coordination modes between the complex agents with Sm3+ are clearly different [29]. A citrate molecule (COOHCH2–COHCOOH–CH2COOH) has four binding sites, including three carboxylic groups and one hydroxyl group, whereas an EDTA molecule ((HOOCCH2)2–NCH2CH2N– (CH2COOH)2) has six binding sites, including four carboxylic groups and two single pairs of electrons on the nitrogen atom. The difference might lead to the different morphology because of its influence on growth orientation of the crystals. The thermal behavior of the flower-like Sm2(C2O4)310H2O was investigated with TG–DTA (Fig. 9a). TG curve shows that the total weight loss is 53.115 wt% and can divide into two processes. The first process from 40 to 403 8C belongs to the evaporation of the crystal water. The weight loss of 24.39 wt% is in agreement with
the theoretical value of 24.17% according to Eq. (3). The second weight loss (28.725 wt%) is consistent with the theoretical value of 29.00% according to Eq. (4). The process is from 403 to 850 8C and has a strong endothermic peak at 421 8C in the DTA curve due to the decomposition of Sm2(C2O4)3 (Eq. (4)), which can also be proved by the XRD in Fig. 9b. The XRD pattern of Fig. 9b can be indexed to the cubic Sm2O3 (space group:I231, JCPDS card 74-1989). Sm2 ðC2 O4 Þ3 10H2 O ! Sm2 ðC2 O4 Þ3 þ 10H2 O
(3)
Sm2 ðC2 O4 Þ3 ! Sm2 O3 þ 3CO þ 3CO2
(4)
3.2. Synthesis of Gd2(C2O4)310H2O, Dy2(C2O4)310H2O, Lu2(C2O4)36H2O and Y2(C2O4)310H2O Monoclinic Gd2(C2O4)310H2O, Dy2(C2O4)310H2O, Y2(C2O4)310H2O and anorthic Lu2(C2O4)36H2O also had been successfully synthesized by the above method (Fig. 10). It is well known that rare earth elements are frequently divided into two groups based on the atomic weight and chemical properties. The
Fig. 5. FESEM images of the samples obtained (a–c) at room temperature with the molar ratio of Na3Cit to Sm3+ of 1.2:1 for different times: (a) 12 h; (b) 24 h; (c) 36 h; (d) at 60 8C for 24 h with the molar ratio of Na3Cit to Sm3+ of 1.2:1.
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Fig. 6. FESEM images of the samples obtained at room temperature for 24 h with different molar ratio of Na3Cit to Sm3+: (a) 0:1; (b) 0.6:1; (c) 1.2:1; (d) 1.8:1.
‘‘light’’ rare earths consist of the elements with atomic number 57– 63, such as Sm. The ‘‘heavy’’ rare earths consist of Sc, Y and the elements 64–71, such as Dy and Lu. Meanwhile, the element 64 Gd possesses the similar chemical behavior with the ‘‘light’’ rare earths because they are very close. It is interesting to find the selfassembled architecture of the Gd2(C2O4)310H2O crystal is similar to that of the Sm2(C2O4)310H2O (Fig. 10a) and only large irregular nubby-like particles for Dy2(C2O4)310H2O, Lu2(C2O4)36H2O and Y2(C2O4)310H2O are obtained (Fig. 10b–d), which might result from the differences of their ionic radius. These results show that besides the reaction conditions and the additive amount of complex agent, the morphology evolution of lanthanide oxalates is also determined by the rare earth ions. 3.3. Photoluminescence property
Fig. 7. Schematic illustration for the possible formation mechanism of flower-like Sm2(C2O4)310H2O crystal.
Many studies have been focused on the luminescent application of Sm3+ activated inorganic materials such as YVO4:Sm3+ and LuVO4:Sm3+ [26,30], but the papers on the intrinsic photoluminescence property of pure Sm2(C2O4)310H2O material are not reported up to now. In this work, the photoluminescence property of the as-prepared Sm2(C2O4)310H2O and Sm2O3 which was obtained by calcining Sm2(C2O4)310H2O at 850 8C for 6 h was
Fig. 8. The FESEM images of the samples obtained room temperature for 24 h with the molar ratio of different complex agents to Sm3+ of 1.2:1: (a) Na3Cit; (b) EDTA.
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Fig. 9. (a) TG–DTA curve of the flower-like Sm2(C2O4)310H2O; (b) XRD pattern of the Sm2O3.
Fig. 10. The FESEM images of the samples with the molar ratio of Na3Cit to Ln3+ (Ln = Gd, Dy, Lu and Y) of 1.2:1 for 24 h at room temperature: (a) Gd2(C2O4)310H2O; (b) Dy2(C2O4)310H2O; (c) Lu2(C2O4)36H2O; (d) Y2(C2O4)310H2O.
Fig. 11. (a) Excitation spectrum for Sm2(C2O4)310H2O, monitoring the emission at 645 nm; (b) emission spectra for Sm2(C2O4)310H2O and Sm2O3 under direct excitation of 402 nm; (C) positions of the Sm2(C2O4)310H2O (A), Sm2O3 (B) in the chromaticity diagram.
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studied. The excitation spectrum of the flower-like Sm2(C2O4)310H2O sample consists of a strong absorption band with a maximum at 402 nm and lots of weak bands monitoring with 642 nm emission of Sm3+ (Fig. 11a). The room-temperature emission spectrum of the flower-like Sm2(C2O4)310H2O exhibits the typical red-orange emissions at 561, 596, 640 and 703 nm under 402 nm excitation, which are ascribed to the transitions 4G5/ 6 4 6 4 6 4 6 2 ! H5/2, G5/2 ! H7/2, G5/2 ! H9/2 and G5/2 ! H11/2, respectively (black line of Fig. 11b), while the Sm2O3 powder shows obviously different PL spectral emission (red line of Fig. 11b). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.) The emission spectrum of Sm2O3 displays a broad emission band in the wavelength range from 550 to 705 nm. It is well known that the energy level structure of 4f electrons is the key factor controlling the PL spectrum of rare earth ions, while the energy level structure depends on the crystal field [31]. The different PL spectral emission may be firstly explained by the different crystal field of the monoclinic Sm2(C2O4)310H2O and cube Sm2O3. Secondly, the crystal field, namely, the energy level structure of 4f electrons in microcrystal specimen is also influenced by the defects and the reconstruction of atomic arrangement on the surface. Besides them, reabsorption effect has a certain influence on the emission, which occurs as dips in the broad emission feature and makes PL spectral emission of Sm2O3 more unconspicuous [32,33]. As illustrated in Fig. 11c, the emission colour of the above samples can be expressed by the CIE (Commission International de I’Eclairage 1931 chromaticity) coordinates. The flower-like Sm2(C2O4)310H2O and Sm2O3 emit red-orange and violet light and their chromaticity coordinates are x = 0.4905, y = 0.3792 and x = 0.3943, y = 0.2609, respectively. 4. Conclusions In summary, a convenient and facile complex agent assisted precipitation method has been utilized for the preparation of Ln(C2O4)3nH2O (Ln = Sm, Gd, Dy, Lu, Yb). The condition experiment results show room temperature, 24 h and the molar ratio of Na3Cit to Sm3+ of 1.2:1 are the preferable experiment parameters to synthesize the flower-like Sm2(C2O4)310H2O. Further researches show that besides the reaction conditions and the additive amount of the complex agent, the morphology of the lanthanide oxalates is also determined by the rare earth ions. The optical properties of Sm2(C2O4)310H2O and Sm2O3 samples are distinct, which is relevant to Sm3+ energy level structure of 4f
electrons. This work may present a way for the morphologycontrolled synthesis of other inorganic materials. Acknowledgments This research is supported by the Structure Research Laboratory of CAS. We thank S.Q. Fu (University of Science and Technology of China) for the helpful discussion. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]
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Self-assembled light lanthanide oxalate architecture with controlled morphology, characterization, growing mechanism and optical property Hongmei He, Youjin Zhang *, Wei Zhu, Ao Zheng Department of Chemistry, University of Science and Technology of China, 96 Jinzhai Road, Hefei, 230026, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 27 April 2011 Received in revised form 2 June 2011 Accepted 16 June 2011 Available online 23 June 2011
Flower-like Sm2(C2O4)310H2O had been synthesized by a facile complex agent assisted precipitation method. The flower-like Sm2(C2O4)310H2O was characterized by X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, field-emission scanning electron microscopy, thermogravimetry–differential thermal analysis and photoluminescence. The possible growth mechanism of the flower-like Sm2(C2O4)310H2O was proposed. To extend this method, other Ln2(C2O4)3nH2O (Ln = Gd, Dy, Lu, Y) with different morphologies also had been prepared by adjusting different rare earth precursors. Further studies revealed that besides the reaction conditions and the additive amount of complex agents, the morphologies of the as-synthesised lanthanide oxalates were also determined by the rare earth ions. The Sm2(C2O4)310H2O and Sm2O3 samples exhibited different photoluminescence spectra, which was relevant to Sm3+ energy level structure of 4f electrons. The method may be applied in the synthesis of other lanthanide compounds, and the work could explore the potential optical materials. ß 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Inorganic compounds B. Chemical synthesis C. Electron microscopy C. X-ray diffraction D. Optical properties
1. Introduction In current years, the precise design and synthesis of lanthanide compound micro/nanostructure materials with well-defined morphologies and tunable sizes remain the research hot, because the chemical and physical properties of functional lanthanide compounds are fundamentally related to their size, morphology, and dimensionality [1–5]. Particularly, as an important lanthanide compounds, lanthanide oxalates were widely used in rare earth purification industry [6], or as the precursor to prepare lanthanide oxides [7]. There has been an increasing number of reports on the precise design and synthesis of rare earth oxalates including dysprosium gadolinium oxalate [8], neodymium praseodymium oxalate [9], lanthanide calcium oxalate [10] and lanthanide metalorganic frameworks [11] and Ln2(C2O4)310H2O (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Y and Yb) by various synthesis methods, such as hydrothermal synthesis without complex agent, sol–gel method, emulsion liquid membrane (ELM, water-in-oil- in-water emulsion) system, etc. [12–18]. Herein, a simple and mild complex agent assisted precipitation method at room temperature was proposed for the synthesis of Ln2(C2O4)3nH2O (Ln = Sm, Gd, Dy, Lu, Y) micro-particles, in which trisodium citrate (Na3Cit) was chosen as the complex agent. A series of experiments for the reaction temperature, the reaction time and
* Corresponding author. Tel.: +86 551 3492145; fax: +86 551 3492083. E-mail address: [email protected] (Y. Zhang). 0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2011.06.029
the molar ratio of Na3Cit to Sm3+ had been carried out to investigate their influence on the flower-like Sm2(C2O4)310H2O micro-particles. In addition, more and more attentions have been focused on the study of Sm3+ activated inorganic materials for application in luminescent materials because of its electron transitions within the 4f shell. However, to the best of our knowledge, no studies have been conducted on the luminescence performance of pure Sm2(C2O4)310H2O crystal. Thus the photoluminescence properties of the pure Sm2(C2O4)310H2O and Sm2O3 obtained by calcining Sm2(C2O4)310H2O crystal, were investigated in the paper. 2. Experimental 2.1. Materials All reagents (samarium oxide, gadolinium oxide, dysprosium oxide, lutetium oxide, yttrium oxide, oxalic acid, trisodium citrate (Na3Cit), ethylene diamine tetraacetic acid (EDTA)) used were of analytical purity, obtained from Shanghai Chemical Reagent Ltd. Co. of China and used without further purification. 2.2. Synthesis of Sm2(C2O4)310H2O LnCl3 (Ln = Sm, Gd, Dy, Lu and Y) aqueous solution was obtained by dissolving Ln2O3 (99.99%) (Ln = Sm, Gd, Dy, Lu and Y) in 1 mol/L HCl solution and dried by heating to produce LnCl3 crystal. In a typical procedure, 0.5 mmol Sm2O3 was dissolved in 5.0 mL of 1 mol/L HCl and dried by heating. The reactant was dissolved in
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30 mL of deionized water and then 1.2 mmol Na3Cit was added to the solution with stirring. The mixed aqueous solution was stirred at room temperature for 15 min to form samarium citrate complexes, then 1.5 mmol H2C2O42H2O was introduced to the samarium citrate complexes. After agitation for 10 min, the asobtained mixture was transferred into a 50-mL Teflon liner and some distilled water was added up to 80% of the total volume. The autoclave was sealed for 24 h at room temperature. The obtained precipitate was collected after being washed several times with deionized water and dried at 60 8C in air. The product was finally obtained. A similar process was applied to prepare Ln(C2O4)3nH2O (Ln = Gd, Dy, Lu and Y) samples by using a proper amount of Gd(Cl)3, Dy(Cl)3, Lu(Cl)3, and Y(Cl)3 solutions instead of Sm(Cl)3 at the initial stage as described above. Samples with other parameters were prepared in a similar way. 2.3. Synthesis of Sm2O3 Sm2O3 was calcined from Sm2(C2O4)310H2O at 850 8C for 6 h.
Fig. 1. XRD patterns of the Ln2(C2O4)3nH2O obtained at room temperature for 24 h with the molar ratio of Na3Cit to Ln3+ of 1.2:1: (a) Lu; (b) Dy; (c) Y; (d) Gd; (e) Sm.
2.4. Characterization Powder X-ray diffraction (XRD) was carried out with a Japan Rigaku D/max rA X-ray diffractometer equipped with graphitemonochromatized high-intensity Cu Ka radiation (l = 0.15478 nm). The scanning rate was 0.058 s1 in the 2u range from 108 to 708. The X-ray photoelectron spectroscopy (XPS) analysis was performed on an ESCALAB MK II X-ray photoelectron spectrometer, using Mg Kr radiation as the excitation source. The Fourier transform infrared (FTIR) spectroscopic study was performed on a MAGNA-IR 750 FTIR spectrometer. The field-emission scanning electron microscopy (FESEM) images were obtained on a JEOL-6300F field-emission scanning electron microscope with an accelerating voltage of 15 kV. The thermogravimetry–differential thermal analysis (TG–DTA) was conducted on a Rigaku Standard Model thermal analyzer (in air atmosphere, flow rate: 90 mL min1; heat rate: 10 8C min1). The photoluminescence (PL) was carried out using a Jobin Yvon Fluorolog-3-TAU steadystate/lifetime spectrofluorometer. 3. Results and discussion 3.1. Characterization, morphology and the growth mechanism of Sm2(C2O4)310H2O crystal The overall crystallinity and purity of the Ln2(C2O4)3nH2O samples obtained at room temperature for 24 h with the molar ratio of Na3Cit to Ln3+ of 1.2:1 were characterized by X-ray powder diffraction as shown in Fig. 1. All the XRD patterns of the asprepared products can be indexed to anorthic Lu2(C2O4)36H2O (space group:P1, JCPDS card 49-1244), monoclinic Dy2(C2O4)310H2O (space group:P21/c, JCPDS card 21-0315), monoclinic Y2(C2O4)310H2O (space group:P21/c, JCPDS card 331460), monoclinic Gd2(C2O4)310H2O (space group:P21/c, JCPDS card 20-0411) and monoclinic Sm2(C2O4)310H2O (space group:P21/c, JCPDS card 20-1021) from Fig. 1a–e, respectively. No XRD peaks for single metal oxides Ln2O3 in the lanthanide oxalates were detected, which indicated the pure lanthanide oxalate crystals can be achieved by this method. The Sm2(C2O4)310H2O crystal obtained at room temperature for 24 h with the molar ratio of Na3Cit to Sm3+ of 1.2:1 was also characterized by XPS for the evaluation of its composition on the particle surface (Fig. 2). The binding energies obtained in the XPS analysis are corrected for specimen charging by referencing the C 1s orbital to 284.17 eV. The Sm 4d5/2, 4p3/2 and 3d5/2 peaks centered at 132.54 eV, 249.44 eV and 1083.21 eV, respectively
(Fig. 2b and c) [19,20]. No characteristic peaks of other metals are detected. The O 1s spectrum of the sample demonstrates a O2 peak at around 530.31 eV [21] (Fig. 2d). To confirm the formation of the Sm2(C2O4)310H2O crystal obtained at room temperature for 24 h with the molar ratio of Na3Cit to Sm3+ of 1.2:1, FTIR spectroscopy was performed in the wave number range from 200 to 4000 cm1, as illustrated in Fig. 3. A strong absorption band at around 805 cm1 can be attributed to the absorption of Sm–O [22] and a weak band at 486 cm1 represents the CO2 wagging mode [9], respectively. There is water of crystallization in the crystal, which is evidenced by the intense band at around 3375 cm1 [23]. The absorption bands located at about 3375 and 1321 cm1 can be corresponded to the bending mode of water and asymmetric stretch of CO2 [12,24]. The morphology of Sm2(C2O4)310H2O crystal was investigated by FESEM. The higher magnification FESEM images illustrated the flower-like Sm2(C2O4)310H2O, with the diameter about 5 mm, was obtained through the simple precipitation process at room temperature (Fig. 4a and b). It could be clearly seen the flowerlike Sm2(C2O4)310H2O was self-assembled by many thick and smooth flakes from Fig. 4a, which should be beneficial to reduce the surface area and surface energy of the product. It is well-known that the crystal morphology has important effect on its physical and chemical properties. The morphological uniformity of the product is found to be highly correlative with the reaction temperature, reaction time and the additive amount of the complex agent [25]. To control the morphology of Sm2(C2O4)310H2O architectures, the influence of reaction temperature, reaction time and the molar ratio of Na3Cit to Sm3+ on the morphologies of the products was investigated by FESEM. The FESEM images indicated the reaction time and reaction temperature had a great role on the flower-like Sm2(C2O4)310H2O (Fig. 5). The reaction for too short or too long a time was unfavorable for the formation of the flower-like Sm2(C2O4)310H2O. The flower-like Sm2(C2O4)310H2O was not well developed when the reaction time was 12 h, indicating that the self-assembling process was incomplete (Fig. 5a). The regular flower-like Sm2(C2O4)310H2O architecture was obtained when the reaction time was prolonged to 24 h (Fig. 5b). As the reaction time further increased to 30 h (Fig. 5c) or the temperature increased to 60 8C (Fig. 5d), the flower-like architecture fell to pieces. The morphologies of the products obtained with different dosages of Na3Cit at room temperature for 24 h illuminated the
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Fig. 2. XPS spectra of the as-synthesized Sm2(C2O4)310H2O: (a) survey spectrum; (b) Sm 4d and Sm 4p region; (c) Sm 3d region; (d) O 1s region.
molar ratio of Na3Cit to Sm3+ had crucial effect on the formation of the flower-like Sm2(C2O4)310H2O. For a lack of Na3Cit (with the molar ratio of Na3Cit to Sm3+ 0:1 or 0.6:1), the samples were composed of some assembled micro-particles but not regular flower-like structure (Fig. 6a and b). However, when the molar ratio was increased to 1.8:1, all the as-prepared flower-like Sm2(C2O4)310H2O crystal was inclined to fragmentation and the flake became larger (Fig. 6d). Only when the molar ratio was 1.2:1, could the uniform flower-like Sm2(C2O4)310H2O crystal be observed clearly (Fig. 6c). On the basis of the above experiment results, it can be generalized a conclusion that the uniform flower-like Sm2(C2O4)310H2O could
Fig. 3. The FTIR spectrum of Sm2(C2O4)310H2O sample.
be fabricated at room temperature for 24 h with the molar ratio of Na3Cit to Sm3+ of 1.2:1. A possible process for the self-assembled flower-like Sm2(C2O4)310H2O is shown in Fig. 7. In the paper, it is found that the Cit3 plays a key role in the formation of flower-like Sm2(C2O4)310H2O. At first, the Cit3 is a strong chelating agent with three carboxylate groups for metal ions [26]. In the solution, Sm3+ ions are protected by forming Sm (Cit)23 units (Eq. (1)), greatly decreasing the free Sm3+ ion concentration. When oxalic acid solution is added, Sm3+ would be released gradually from the Sm(Cit)23 units and the free Sm3+ ions can react with the vicinal C2O42 ions to form numerous tiny Sm2(C2O4)3 crystalline nuclei (Eq. (2)). During the subsequent crystal growth stage, these tiny nuclei grow to form particles. According to LaMer’s model, the formation of such complexes could control the concentration of free Sm3+ ions in the solution, and thus help to control the nucleation and growth rate of the Sm2(C2O4)3 particles. The relatively slow generation rate of particles is favorable for the subsequent growth of hierarchical structures [27]. The kinetic control leads to the formation of the flakes. When the reaction time was increased, these initial flakes continued to grow in a self-assembled growth style, and finally the flower-like structure was formed. Sm3þ þ 2Cit3 Ð SmðCitÞ2 3
(1)
Sm3þ þ C2 O4 2 þ 10H2 O Ð Sm2 ðC2 O4 Þ3 10H2 O
(2)
To further investigate the influence of complex agents on the Sm2(C2O4)310H2O morphology, the products with different complex agents were obtained. The FESEM image showed many large flakes appeared in the product when EDTA is used as the complex agent (Fig. 8b).
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Fig. 4. FESEM images of the as-synthesized product obtained at room temperature for 24 h with the molar ratio of Na3Cit to Sm3+ of 1.2:1.
The different impacts of Na3Cit and EDTA on the product morphology have some connection with their different chelating constants and coordination modes with Sm3+. On the one hand, the chelating constant for Cit3 is smaller than that for EDTA [26,28]. The smaller the chelating constant, the faster the nucleation rate, which would influence the growth rate of the Sm2(C2O4)310H2O particles. On the other hand, the coordination modes between the complex agents with Sm3+ are clearly different [29]. A citrate molecule (COOHCH2–COHCOOH–CH2COOH) has four binding sites, including three carboxylic groups and one hydroxyl group, whereas an EDTA molecule ((HOOCCH2)2–NCH2CH2N– (CH2COOH)2) has six binding sites, including four carboxylic groups and two single pairs of electrons on the nitrogen atom. The difference might lead to the different morphology because of its influence on growth orientation of the crystals. The thermal behavior of the flower-like Sm2(C2O4)310H2O was investigated with TG–DTA (Fig. 9a). TG curve shows that the total weight loss is 53.115 wt% and can divide into two processes. The first process from 40 to 403 8C belongs to the evaporation of the crystal water. The weight loss of 24.39 wt% is in agreement with
the theoretical value of 24.17% according to Eq. (3). The second weight loss (28.725 wt%) is consistent with the theoretical value of 29.00% according to Eq. (4). The process is from 403 to 850 8C and has a strong endothermic peak at 421 8C in the DTA curve due to the decomposition of Sm2(C2O4)3 (Eq. (4)), which can also be proved by the XRD in Fig. 9b. The XRD pattern of Fig. 9b can be indexed to the cubic Sm2O3 (space group:I231, JCPDS card 74-1989). Sm2 ðC2 O4 Þ3 10H2 O ! Sm2 ðC2 O4 Þ3 þ 10H2 O
(3)
Sm2 ðC2 O4 Þ3 ! Sm2 O3 þ 3CO þ 3CO2
(4)
3.2. Synthesis of Gd2(C2O4)310H2O, Dy2(C2O4)310H2O, Lu2(C2O4)36H2O and Y2(C2O4)310H2O Monoclinic Gd2(C2O4)310H2O, Dy2(C2O4)310H2O, Y2(C2O4)310H2O and anorthic Lu2(C2O4)36H2O also had been successfully synthesized by the above method (Fig. 10). It is well known that rare earth elements are frequently divided into two groups based on the atomic weight and chemical properties. The
Fig. 5. FESEM images of the samples obtained (a–c) at room temperature with the molar ratio of Na3Cit to Sm3+ of 1.2:1 for different times: (a) 12 h; (b) 24 h; (c) 36 h; (d) at 60 8C for 24 h with the molar ratio of Na3Cit to Sm3+ of 1.2:1.
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Fig. 6. FESEM images of the samples obtained at room temperature for 24 h with different molar ratio of Na3Cit to Sm3+: (a) 0:1; (b) 0.6:1; (c) 1.2:1; (d) 1.8:1.
‘‘light’’ rare earths consist of the elements with atomic number 57– 63, such as Sm. The ‘‘heavy’’ rare earths consist of Sc, Y and the elements 64–71, such as Dy and Lu. Meanwhile, the element 64 Gd possesses the similar chemical behavior with the ‘‘light’’ rare earths because they are very close. It is interesting to find the selfassembled architecture of the Gd2(C2O4)310H2O crystal is similar to that of the Sm2(C2O4)310H2O (Fig. 10a) and only large irregular nubby-like particles for Dy2(C2O4)310H2O, Lu2(C2O4)36H2O and Y2(C2O4)310H2O are obtained (Fig. 10b–d), which might result from the differences of their ionic radius. These results show that besides the reaction conditions and the additive amount of complex agent, the morphology evolution of lanthanide oxalates is also determined by the rare earth ions. 3.3. Photoluminescence property
Fig. 7. Schematic illustration for the possible formation mechanism of flower-like Sm2(C2O4)310H2O crystal.
Many studies have been focused on the luminescent application of Sm3+ activated inorganic materials such as YVO4:Sm3+ and LuVO4:Sm3+ [26,30], but the papers on the intrinsic photoluminescence property of pure Sm2(C2O4)310H2O material are not reported up to now. In this work, the photoluminescence property of the as-prepared Sm2(C2O4)310H2O and Sm2O3 which was obtained by calcining Sm2(C2O4)310H2O at 850 8C for 6 h was
Fig. 8. The FESEM images of the samples obtained room temperature for 24 h with the molar ratio of different complex agents to Sm3+ of 1.2:1: (a) Na3Cit; (b) EDTA.
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Fig. 9. (a) TG–DTA curve of the flower-like Sm2(C2O4)310H2O; (b) XRD pattern of the Sm2O3.
Fig. 10. The FESEM images of the samples with the molar ratio of Na3Cit to Ln3+ (Ln = Gd, Dy, Lu and Y) of 1.2:1 for 24 h at room temperature: (a) Gd2(C2O4)310H2O; (b) Dy2(C2O4)310H2O; (c) Lu2(C2O4)36H2O; (d) Y2(C2O4)310H2O.
Fig. 11. (a) Excitation spectrum for Sm2(C2O4)310H2O, monitoring the emission at 645 nm; (b) emission spectra for Sm2(C2O4)310H2O and Sm2O3 under direct excitation of 402 nm; (C) positions of the Sm2(C2O4)310H2O (A), Sm2O3 (B) in the chromaticity diagram.
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studied. The excitation spectrum of the flower-like Sm2(C2O4)310H2O sample consists of a strong absorption band with a maximum at 402 nm and lots of weak bands monitoring with 642 nm emission of Sm3+ (Fig. 11a). The room-temperature emission spectrum of the flower-like Sm2(C2O4)310H2O exhibits the typical red-orange emissions at 561, 596, 640 and 703 nm under 402 nm excitation, which are ascribed to the transitions 4G5/ 6 4 6 4 6 4 6 2 ! H5/2, G5/2 ! H7/2, G5/2 ! H9/2 and G5/2 ! H11/2, respectively (black line of Fig. 11b), while the Sm2O3 powder shows obviously different PL spectral emission (red line of Fig. 11b). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.) The emission spectrum of Sm2O3 displays a broad emission band in the wavelength range from 550 to 705 nm. It is well known that the energy level structure of 4f electrons is the key factor controlling the PL spectrum of rare earth ions, while the energy level structure depends on the crystal field [31]. The different PL spectral emission may be firstly explained by the different crystal field of the monoclinic Sm2(C2O4)310H2O and cube Sm2O3. Secondly, the crystal field, namely, the energy level structure of 4f electrons in microcrystal specimen is also influenced by the defects and the reconstruction of atomic arrangement on the surface. Besides them, reabsorption effect has a certain influence on the emission, which occurs as dips in the broad emission feature and makes PL spectral emission of Sm2O3 more unconspicuous [32,33]. As illustrated in Fig. 11c, the emission colour of the above samples can be expressed by the CIE (Commission International de I’Eclairage 1931 chromaticity) coordinates. The flower-like Sm2(C2O4)310H2O and Sm2O3 emit red-orange and violet light and their chromaticity coordinates are x = 0.4905, y = 0.3792 and x = 0.3943, y = 0.2609, respectively. 4. Conclusions In summary, a convenient and facile complex agent assisted precipitation method has been utilized for the preparation of Ln(C2O4)3nH2O (Ln = Sm, Gd, Dy, Lu, Yb). The condition experiment results show room temperature, 24 h and the molar ratio of Na3Cit to Sm3+ of 1.2:1 are the preferable experiment parameters to synthesize the flower-like Sm2(C2O4)310H2O. Further researches show that besides the reaction conditions and the additive amount of the complex agent, the morphology of the lanthanide oxalates is also determined by the rare earth ions. The optical properties of Sm2(C2O4)310H2O and Sm2O3 samples are distinct, which is relevant to Sm3+ energy level structure of 4f
electrons. This work may present a way for the morphologycontrolled synthesis of other inorganic materials. Acknowledgments This research is supported by the Structure Research Laboratory of CAS. We thank S.Q. Fu (University of Science and Technology of China) for the helpful discussion. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]
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