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SrF2 hierarchical flowerlike structures: Solvothermal synthesis, formation mechanism, and optical properties
Materials Research Bulletin 44 (2009) 1009–1016
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SrF2 hierarchical flowerlike structures: Solvothermal synthesis, formation mechanism, and optical properties Zewei Quan a,b, Dongmei Yang a,b, Chunxia Li a, Piaoping Yang a, Ziyong Cheng a, Jun Yang a,b, Deyan Kong a,b, Jun Lin 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
A B S T R A C T
Article history: Received 21 August 2008 Received in revised form 12 October 2008 Accepted 8 November 2008 Available online 21 November 2008
We present a solvothermal route to the synthesis of SrF2 hierarchical flowerlike structures based on thermal decomposition of single source precursor (SSP) of strontium trifluoroacetate in benzylamine solvent. These flowerlike superstructures are actually composed of numerous aggregated nanoplates, and the growth process involves the initial formation of spherical nanoparticles and subsequent transformation into nanoplates, which aggregated together to form microdisks and finally flowerlike superstructures. The results demonstrate the important role of benzylamine in the formation of welldefined SrF2 superstructures, not only providing size and shape control to form nanoplates but also contributing to the self-assembly behavior of nanoplates to build into flower-like superstructures. Additionally, the photoluminescence properties of the obtained SrF2 superstructures are studied. ß 2008 Elsevier Ltd. All rights reserved.
Keywords: A. Fluorides A. Nanostructures B. Crystal growth D. Luminescence
1. Introduction The synthesis of inorganic nano- and micromaterials with welldefined and controllable morphologies has stimulated considerable attention, because it is well known that the properties of the materials closely interrelate with geometrical factors such as morphology, dimensionality, and size [1–5]. Recently, there are increasing effort devoted to the preparation of organized extended structures (superstructures) based on the assembly of nanostructured building blocks due to their potential applications in catalysis, medicine, electronics, ceramics, pigments and cosmetics [6,7]. Such a hierarchical superstructure with the cooperation of microstructure and nanostructure provides a novel approach to bring forth new properties [8]. Because of the distinct size, shape, and chemical functionality, these hierarchical superstructures possess the advantages from microstructure and nanostructure and can be promising candidates for many applications [9]. Therefore, a number of strategies have been developed to spatially pattern and control higher-order organization [10–12]. Among them, solvothermal method has been proven as an effective and convenient process in preparing various inorganic materials with diverse controllable morphologies and hierarchical architectures in terms of cost and potential for large-scale production [13–15].
* Corresponding author. Tel.: +86 431 85262031; fax: +86 431 85698041. E-mail address: [email protected] (J. Lin). 0025-5408/$ – see front matter ß 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2008.11.011
Solid inorganic fluorides have attracted vast attention due to their uncommon properties, such as electron-acceptor behavior, high resistivity, and anionic conductivity [16]. In particular, due to their low energy phonons and high ionicity, inorganic fluorides have a wide range of potential optical applications such as windows, lenses, scintillation crystals and as host crystals for rare earth ions exhibiting interesting properties in optoelectronics such as lasing, light amplification and up conversion [17]. As an important kind of alkaline earth metal fluorides, strontium fluoride (SrF2) are dielectric and thus has great applications in microelectric and optoelectric devices [18]. However, to the best of our knowledge, there are few reports on the controlled synthesis of well-defined SrF2 nano/microstructures [19]. The major reason rests with the rapid precipitation reaction between soluble strontium salts and NaF/NH4F in aqueous solutions, making it difficult to achieve controlled nucleation and growth process that is prerequisite to obtain uniform and well-defined nano/microcrystals. Recently, Yan and co-workers has reported the synthesis of a series of high-quality rare earth fluoride nanocrystals based on the thermal decomposition of corresponding rare earth trifluoroacetates in high-boiling point solvents [20–22]. In these complex trifluoroacetates, both metal and fluorine elements are integrated in the same compound, which may provide a much better controlled-synthesis of fluoride nanocrystals than the direct liquid precipitation methods. Based on these results, it is natural to come up with such imagination: the adoption of Sr(CF3COO)2 as single source precursor (SSP) could be applied to obtain well-defined SrF2 nano/microstructures.
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In this paper, we for the first time prepare well-defined SrF2 hierarchical structures through a simple one-pot solvothermal process with Sr(CF3COO)2 as precursor. A phenomenological growth process for these flowerlike superstructures has been proposed, that is, from spherical nanoparticles to microdisks as intermediate products, and finally to flowerlike superstructures composed of numerous nanoplates. The adoption of benzylamine as solvent plays important roles in the formation of the subunits, nanoplates, and the subsequent self-assembly into flower-like superstructures. The resultant SrF2 superstructures can emit purple light under UV irradiation. 2. Experimental Fig. 1. XRD pattern (a) of SrF2 superstructures, as well as the standard data (b) for SrF2 crystal (ICDD 06-0262).
2.1. Materials Strontium carbonates (SrCO3), trifluoroacetic acid (CF3COOH), benzylamine, absolute ethanol and cyclohexane were all analytical grade (A.R.) and purchased from Beijing Beihua Chemicals Co., Ltd. All of the materials were used without further purification.
Fig. 2. SEM images of SrF2 superstructures at various magnifications.
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2.2. Preparation of trifluoroacetate precursors Precursors were prepared according to the literature method [23]. In a typical synthesis, SrCO3 was added in slight excess to trifluoroacetic acid with the evolution of heat and continuous stirring to make the reaction occur rapidly. Then the mixture was allowed to stand overnight, and then the excess carbonate or oxide was filtered. The resultant transparent trifluoroacetate solution was dried in oven at 100 8C for 12 h to obtain Sr(CF3COO)2 powders. 2.3. Synthesis of SrF2 flowerlike superstructures In a typical synthesis, 1 mmol Sr(CF3COO)2 and 40 mL benzylamine were directly added into 45 mL Teflon-lined stainless autoclave, sealed, and heated at 240 8C for 24 h. After the autoclave was cooled to room temperature naturally, the products were separated by filtration, washed three times with ethanol, and dried in an oven at 100 8C for 2 h. The final products were dispersed in cyclohexane solution for further characterizations.
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and differential thermal analysis (TG–DTA) data were recorded with Thermal Analysis Instrument (SDT 2960, TA Instruments, New Castle, DE) with the heating rate of 10 8C/min in an air flow of 100 mL/min. Fourier transform infrared (FT-IR) spectra were measured with a PerkinElmer 580B infrared spectrophotometer with the KBr pellet technique. Transmission electron microscopy (TEM) was performed using FEI Tecnai G2 S-Twin with a field emission gun operating at 200 kV and images were acquired digitally on a Gatan multiple CCD camera. The excitation and emission photoluminescence (PL) spectra were taken on an F-4500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The luminescence decay curves were obtained from a Lecroy Wave Runner 6100 Digital Osilloscope (1 GHz) using a tunable laser (pulse width = 4 ns, gate = 50 ns) as the excitation (Contimuum Sunlite OPO). The decay curves and spectra were obtained from samples dispersed in cyclohexane solutions. All the measurements were performed at room temperature. 3. Results and discussion 3.1. Phase identification
2.4. Characterizations Powder X-ray diffraction (XRD) measurements were performed on a Rigaku-Dmax 2500 diffractometer with graphite monochromatized Cu Ka radiation (l = 0.15405 nm). Thermogravimetric
The composition and phase purity of the product were first examined by XRD, as shown in Fig. 1a. Vertical bars (Fig. 1b) indicate a standard cubic bulk SrF2 peak position from International Center for Diffraction Data (ICDD No. 06-0262). It is obvious
Fig. 3. TEM (a and b) and HRTEM images (c) of SrF2 superstructures, as well as HRTEM image (d) of two separated nanoplates.
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that all the XRD peaks of the resulted SrF2 superstructures can be indexed as a pure face centered cubic phase (fcc, space group: Fm3m), agreeing well with the values in the standard card. The calculated lattice constant of pure SrF2 superstructures, a = 0.580 nm, is the same with that from the standard card (a = 0.580 nm). It can be seen that the diffraction peaks of XRD patterns are obviously broadened, revealing the small size nature of components. The peak broadening can be utilized to calculate the average crystallite size by Scherer’s formula, Dh k l = 0.89l/ (b cos u), where l is the X-ray wavelength (0.15405 nm), b and u are the full width at half-maximum and the diffraction angle, respectively. The average crystallite size was finally estimated to be about 12 nm, which was close to the edge length of subunits, nanoplates (see the next part). 3.2. Morphology SEM images provide direct information about the size and typical shapes of the as-synthesized SrF2 sample, as shown in Fig. 2. The low-magnification SEM image (Fig. 2a) reveals that the product consists of exclusively round flowers with a relatively uniform size, which indicates the high yield of the product obtained by this simple solvothermal method. The analysis of a number of SrF2 flowers shows that these structures have average diameter of 250 nm and thickness of 150 nm. Further close investigations under higher magnification (Fig. 2b) and different incidence angles (Fig. 2c and d) reveal that these flowers are actually composed of many layered nanoparticles and slightly concaved in the center region. TEM investigations can provide further insight into the microstructure details of the flowers. Fig. 3a shows the TEM image of the product, clearly revealing the round shape with concaved centers due to the brighter transmission region of the center compared to the peripheric parts of particles. Furthermore, the layered structure of the flowers can be demonstrated from the particle standing on the side face perpendicular to the substrate. From the magnified TEM image of an individual particle shown in Fig. 3b, we can observe that the round flower is actually composed of numerous square nanoplates. This point can be proven from the magnified HRTEM image of the edge of the flower (Fig. 3c). In this HRTEM image, taken with the electron beam perpendicular to the c-axis of the round flower, the interplanar distances between adjacent lattice fringes are determined as 0.291 nm, agreeing well with the dspacing value (0.290 nm) of the (1 0 0) planes of cubic SrF2 crystals. Among them, there is a group of parallel lattice fringes with identical interplanar spacing (0.290 nm) marked with an arrow in Fig. 3c, which may be ascribed to the side surface of square nanoplates. Therefore, we can conclude that the square nanoplates are actually composed of six (1 0 0) planes. Further HRTEM investigation of two separated square nanoplates (Fig. 3d) can reveal that the good crystallinity and single crystal nature of subunits, nanoplates.
COO disappear, and a set of characteristic absorption peaks of benzylamine molecules (C6H5CH2NH2) are present. It shows two bands at 2963 and 3033 cm1 that are characteristic of C–H stretching vibration of benzene ring and three other bands (1667, 1572, and 1464 cm1) due to the C–C skeletal modes of benzene ring, implying the presence of benzene ring in the final product [24]. Besides, the sharp peaks at 2855 and 2921 cm1 are assigned to the symmetric and antisymmetric stretching vibration of the – CH2 group. It also shows a broad absorption peak centered at 3425 cm1 due to the asymmetric stretching vibration of the primary amino group, which to some extent overlaps with the H2O frequency [25]. Through above analysis, we can conclude that the as-prepared SrF2 flowerlike superstructures have been successfully coated with benzylamine molecules. This fact can be further confirmed from the presence of C and O peaks in the EDS pattern for SrF2 superstructures (Fig. 4c). Note that the Au peak in the figure comes from the coating from the measurement. It is the presence of benzylamine ligand on the crystal surface that makes the obtained SrF2 flowerlike superstructures stable for a period of time in cyclohexane solutions. 3.4. Formation process To uncover the formation process of SrF2 flowerlike superstructures, time-dependent experiments were carried out. Generally, the intermediates obtained at different reaction intervals are be used to shed light on the growth mechanism of the crystals. So far, this method is widely used to study the morphological formation of various kinds of crystals under different reaction
3.3. Surface structure Fourier transform infrared measurement was carried out to prove the presence of benzylamine molecules in the obtained flowerlike superstructures. Fig. 4 displays the FT-IR spectrum of the Sr(CF3COO)2 precursor (a) and as-synthesized SrF2 flowerlike superstructures (b). The absorption peaks located at 1678 and 1460 cm1 in the FT-IR spectrum of Sr(CF3COO)2 (Fig. 4a) can be ascribed to the stretching vibrations of carboxylate COO [24]. The broad absorption band peaking at 3670 and 3430 cm1 is due to the stretching vibration of the hydroxyl group (–OH) of absorbed water molecules [20]. As for the FT-IR spectrum of the resultant SrF2 flowerlike superstructures (Fig. 4b), the absorption peaks from
Fig. 4. FT-IR spectra of Sr(CF3COO)2 precursor (a) and SrF2 superstructures (b), as well as the EDS pattern (c) of SrF2 superstructures.
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Fig. 5. TEM (a) and HRTEM images (b) of spherical nanoparticles, as well as SEM (c) and HRTEM images (d) of microdisks. Inset of (c) is the TEM image of an individual microdisk. (e) Schematic illustration of the formation of microdisks.
Fig. 6. TG–DTA curves of Sr(CF3COO)2.
conditions [26,27]. Fig. 5a shows the TEM image of intermediate product obtained after 2 h reaction, revealing their exclusively uniform pseudo-spherical morphology with average size of 20 nm. Further HRTEM image of an individual nanoparticle (Fig. 5b) reveals many groups of lattice fringes with adjacent distance of 0.349 nm, which matches well with that of (1 1 1) planes of bulk SrF2. When the reaction proceeds up to 8 h, the obtained sample (Fig. 5c) is mainly composed of microdisks with edge length of 250 nm and thickness of 20 nm. Besides, there are still certain amounts of spherical nanoparticles present in the sample, which can be obviously observed from the TEM image (inset in Fig. 5c). Additionally, it also demonstrates that the obtained microdisks are actually composed of many interconnected square nanoplates bounded by six (1 0 0) planes, which can clearly seen from the HRTEM image shown in the inset of Fig. 5d. From this image, we can observe that two groups of lattice fringes with identical spacing (0.291 nm) coming from the top and side surfaces of
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nanoplates, respectively, intersect at the edge of each nanoplate. This spacing agrees well with the d-spacing value (0.290 nm) of the (1 0 0) planes of cubic SrF2 crystals. HRTEM image of the edge of an individual microdisk (Fig. 5d) reveals the co-presence of lattice fringes of (1 0 0) planes of nanoplates and (1 1 1) planes of nanoparticles highlighted with cycles. Above results indicate that as the reaction proceeds, the small spherical nanoparticles undergo a structure transformation to form 2D nanoplates bounded by six (1 0 0) planes, and subsequently aggregate together to build into larger microdisks. The schematic illustration of the formation of the microdisks is shown in Fig. 5e. During the subsequent aging period, the residual spherical nanoparticles tend to locate on both the circumference and surface of microdisks and subsequently transform into plates to increase the lateral and longitudal dimensions of the microdisks through the well-established Oswald ripening process. It means that the aggregation seems to be not strictly restricted within two dimensions, and can be extended to a few layers along the disk thickness direction, resulting in the formation of final architecture, flowerlike superstructures, as revealed by Fig. 2. 3.5. Growth mechanism It is known that metal trifluoroacetates can thermally decompose to produce the corresponding metal fluorides and various fluorinated and oxyfluorinated carbon species [28,29]. Based on
this reaction, we adopted Sr(CF3COO)2 as SSP to prepare SrF2 crystals in this system, and the thermal decomposition reaction should proceed as follows: SrðCF3 COOÞ2 ! SrF2 þ C2 F4 þ 2CO2 To obtain the quantitative information of the decomposition process, the TG–DTA analysis of Sr(CF3COO)2 powder precursor was performed, as shown in Fig. 6. The TG curve of Sr(CF3COO)2 shows two stages of weight loss. The first weight loss step (about 3.5%) is observed between 54 and 110 8C due to the evaporation of absorbed water that is accompanied with an endothermic peak at 105 8C in the DTA curve. Based on this weigh loss value, the amount of water molecule in the precursor can be determined to be one. The second weight loss step is observed in the temperature range from 265 to 350 8C, accompanied by a strong exothermic peak at 347 8C in DTA curve due to the transformation of Sr(CF3COO)2 into SrF2, and the obtained weight loss value (56.7%) is in good agreement with the theoretical value (56.6%). However, this decomposition reaction in our system can occur at a relatively lower reaction temperature (240 8C), possibly due to the presence of Sr(CF3COO)2 in the form of molecular species in benzylamine solvent. It is well accepted that formation of crystals is mainly achieved through two stages: nucleation and growth. In our case, seed formation proceeds according to LaMer model for homogeneous nucleation, in which thermal decomposition of Sr(CF3COO)2 occurs to generate SrF2 in solution. Above the critical concentration,
Fig. 7. TEM and HRTEM images of SrF2 crystals prepared under equal amount of ethylamine and benzylamine (a and b) and pure ethylamine (c and d), respectively.
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nucleation results in a rapid depletion of the reactants such that all subsequent growth occurs on the pre-existing nuclei [30]. As dictated by thermodynamics (i.e. Wulff construction), the SrF2 with fcc structure are expected to nucleate and grow into tetradecahedron seeds enclosed by a mix of (1 1 1) and (1 0 0) planes to minimize the total surface energy [31]. During the subsequent crystal growth process, it has been illustrated that the final shape of an fcc nanocrystal is mainly determined by the ratio (R) between the growth rates along the h100i and h111i directions. Cubes bounded by six equivalent (1 0 0) planes will be formed when R = 0.58, while octahedrons bounded by the eight equivalent (1 1 1) planes will result if R is increased to 1.73 [32]. The growth rates at different facets are dominated by their surface energies, that is, the facets with higher surface energy will grow faster. Generally speaking, the surface energies on (1 1 1), (1 0 0), and possibly (1 1 0) crystallographic planes in a fcc structure could be distinctive because of the different atomic densities, electronic structures, bindings, and possibly chemical reactivities on them [31,32]. In the case of SrF2, the intrinsic surface energy of (1 0 0) planes is higher than that of (1 1 1) planes [33], and therefore relatively fast growth along six equivalent h100i directions from the tetradecahedron seeds are expected, finally leading to the formation of octahedrons. However, as stated above, the final product is actually composed of nanoplates (Fig. 6a and b). We therefore reasoned that coordinating ability of benzylamine solvent plays an important role in this formation of nanoplates. In a solution-phase process, impurities or capping agents are usually adopted to alter the surface free energies via adsorption or chemical interaction and thus induce new shapes. As for our system, benzylamine molecules can preferably and efficiently interact with active (1 0 0) planes to significantly reduce their surface energy and block the growth on these planes, and therefore
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facilitate the growth on (1 1 1) planes [33], finally resulting in the formation of nanoplates exposing six (1 0 0) planes. Similar situation has also been observed for the synthesis of anatase nanoplates using benzylamine as solvent [34]. Another important role of benzylamine in the formation of well-defined superstructures is that it can efficiently direct the assembly of nanoplates. This point can be proven from two control experiments in which the benzylamine was partly or completely replaced by ethylamine under otherwise same experimental conditions. Fig. 7 shows the TEM and HRTEM images of samples obtained under conditions of equal amount of benzylamine and ethylamine (Fig. 7a and b) and pure ethylamine (Fig. 7c and d). The former sample (Fig. 7a) is composed of irregular plate-like structures and loosely aggregated structures, while the latter sample (Fig. 7c) contains almost irregular plate-like structures. HRTEM images taken from the plate-like structures of these two samples (Fig. 7b and d) show the (1 0 0) lattice fringes of SrF2, which are the same as that of SrF2 superstructures. From this tendency, we can conclude that the adoption of benzylamine as solvent is essential to the formation of well-defined hierarchical SrF2 flowerlike superstructures. This is because the benzylamine species bound to the (1 0 0) surfaces interact with each other, through p–p interactions, driving the stacking of the nanoplates into aggregated superstructures [34]. It should be noted that the obtained SrF2 superstructures are remarkably stable, and it is impossible to separate the aggregated nanoplates even by applying extensive ultrasonication and washing treatments in various solvents. This special function of benzylamine molecules has been widely utilized for the synthesis of highly ordered organic– inorganic nanocomposites [35–37]. Finally, it has turned out that the role of benzlylamine in the system can be clearly identified, providing not only size and shape control to form nanoplates but
Fig. 8. (a) PL excitation and emission spectra and the corresponding digital image under 254 nm irradiation of SrF2 superstructures dispersed in cyclohexane solution. (b) Decay curve of SrF2 superstructures dispersed in cyclohexane solution.
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also contributing to the self-assembly behavior of nanoplates to build into flower-like superstructures. 3.6. Luminescent properties The PL properties of the SrF2 superstructures were investigated here, as shown in Fig. 8. Under 254 nm irradiation, it can show purple emission, which is displayed on the right part of Fig. 8a. Monitored with the emission wavelength of 388 nm, the excitation spectrum (Fig. 8a) consists of a broad band from 250 to 375 nm with two wellresolved peaks at 289 and 322 nm. Upon excitation at 289 nm, the obtained emission spectrum (Fig. 8a) exhibits broad emission band ranging from 350 to 450 nm centered at 388 nm. The photoluminescence decay curve of SrF2 superstructures is shown in Fig. 8b. It can be fitted to a single-exponential function as I = I0 exp(t/t) (t is lifetime), from which the lifetime is determined to be 8.04 ns. 4. Conclusions SrF2 hierarchical flowerlike superstructures were first prepared via the thermal decomposition of single source precursor, Sr(CF3COO)2, under solvothermal conditions. The adoption of Sr(CF3COO)2 and benzylamine as precursor and solvent, respectively, was both crucial to the formation of the well-defined flowerlike superstructures. Time-dependent experiments were conducted to reveal the formation process of these flowerlike superstructures. This versatile nonaqueous route with benzylamine as solvent can be potentially extended to the synthesis of other alkaline earth metal nano/microstructures. Acknowledgments This work is financially supported by the ‘‘Bairen Jihua’’ of Chinese Academy of Sciences, the National Natural Science Foundation of China (50702057, 50872131) and the MOST of China (No. 2007CB935502). References
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SrF2 hierarchical flowerlike structures: Solvothermal synthesis, formation mechanism, and optical properties Zewei Quan a,b, Dongmei Yang a,b, Chunxia Li a, Piaoping Yang a, Ziyong Cheng a, Jun Yang a,b, Deyan Kong a,b, Jun Lin 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
A B S T R A C T
Article history: Received 21 August 2008 Received in revised form 12 October 2008 Accepted 8 November 2008 Available online 21 November 2008
We present a solvothermal route to the synthesis of SrF2 hierarchical flowerlike structures based on thermal decomposition of single source precursor (SSP) of strontium trifluoroacetate in benzylamine solvent. These flowerlike superstructures are actually composed of numerous aggregated nanoplates, and the growth process involves the initial formation of spherical nanoparticles and subsequent transformation into nanoplates, which aggregated together to form microdisks and finally flowerlike superstructures. The results demonstrate the important role of benzylamine in the formation of welldefined SrF2 superstructures, not only providing size and shape control to form nanoplates but also contributing to the self-assembly behavior of nanoplates to build into flower-like superstructures. Additionally, the photoluminescence properties of the obtained SrF2 superstructures are studied. ß 2008 Elsevier Ltd. All rights reserved.
Keywords: A. Fluorides A. Nanostructures B. Crystal growth D. Luminescence
1. Introduction The synthesis of inorganic nano- and micromaterials with welldefined and controllable morphologies has stimulated considerable attention, because it is well known that the properties of the materials closely interrelate with geometrical factors such as morphology, dimensionality, and size [1–5]. Recently, there are increasing effort devoted to the preparation of organized extended structures (superstructures) based on the assembly of nanostructured building blocks due to their potential applications in catalysis, medicine, electronics, ceramics, pigments and cosmetics [6,7]. Such a hierarchical superstructure with the cooperation of microstructure and nanostructure provides a novel approach to bring forth new properties [8]. Because of the distinct size, shape, and chemical functionality, these hierarchical superstructures possess the advantages from microstructure and nanostructure and can be promising candidates for many applications [9]. Therefore, a number of strategies have been developed to spatially pattern and control higher-order organization [10–12]. Among them, solvothermal method has been proven as an effective and convenient process in preparing various inorganic materials with diverse controllable morphologies and hierarchical architectures in terms of cost and potential for large-scale production [13–15].
* Corresponding author. Tel.: +86 431 85262031; fax: +86 431 85698041. E-mail address: [email protected] (J. Lin). 0025-5408/$ – see front matter ß 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2008.11.011
Solid inorganic fluorides have attracted vast attention due to their uncommon properties, such as electron-acceptor behavior, high resistivity, and anionic conductivity [16]. In particular, due to their low energy phonons and high ionicity, inorganic fluorides have a wide range of potential optical applications such as windows, lenses, scintillation crystals and as host crystals for rare earth ions exhibiting interesting properties in optoelectronics such as lasing, light amplification and up conversion [17]. As an important kind of alkaline earth metal fluorides, strontium fluoride (SrF2) are dielectric and thus has great applications in microelectric and optoelectric devices [18]. However, to the best of our knowledge, there are few reports on the controlled synthesis of well-defined SrF2 nano/microstructures [19]. The major reason rests with the rapid precipitation reaction between soluble strontium salts and NaF/NH4F in aqueous solutions, making it difficult to achieve controlled nucleation and growth process that is prerequisite to obtain uniform and well-defined nano/microcrystals. Recently, Yan and co-workers has reported the synthesis of a series of high-quality rare earth fluoride nanocrystals based on the thermal decomposition of corresponding rare earth trifluoroacetates in high-boiling point solvents [20–22]. In these complex trifluoroacetates, both metal and fluorine elements are integrated in the same compound, which may provide a much better controlled-synthesis of fluoride nanocrystals than the direct liquid precipitation methods. Based on these results, it is natural to come up with such imagination: the adoption of Sr(CF3COO)2 as single source precursor (SSP) could be applied to obtain well-defined SrF2 nano/microstructures.
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In this paper, we for the first time prepare well-defined SrF2 hierarchical structures through a simple one-pot solvothermal process with Sr(CF3COO)2 as precursor. A phenomenological growth process for these flowerlike superstructures has been proposed, that is, from spherical nanoparticles to microdisks as intermediate products, and finally to flowerlike superstructures composed of numerous nanoplates. The adoption of benzylamine as solvent plays important roles in the formation of the subunits, nanoplates, and the subsequent self-assembly into flower-like superstructures. The resultant SrF2 superstructures can emit purple light under UV irradiation. 2. Experimental Fig. 1. XRD pattern (a) of SrF2 superstructures, as well as the standard data (b) for SrF2 crystal (ICDD 06-0262).
2.1. Materials Strontium carbonates (SrCO3), trifluoroacetic acid (CF3COOH), benzylamine, absolute ethanol and cyclohexane were all analytical grade (A.R.) and purchased from Beijing Beihua Chemicals Co., Ltd. All of the materials were used without further purification.
Fig. 2. SEM images of SrF2 superstructures at various magnifications.
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2.2. Preparation of trifluoroacetate precursors Precursors were prepared according to the literature method [23]. In a typical synthesis, SrCO3 was added in slight excess to trifluoroacetic acid with the evolution of heat and continuous stirring to make the reaction occur rapidly. Then the mixture was allowed to stand overnight, and then the excess carbonate or oxide was filtered. The resultant transparent trifluoroacetate solution was dried in oven at 100 8C for 12 h to obtain Sr(CF3COO)2 powders. 2.3. Synthesis of SrF2 flowerlike superstructures In a typical synthesis, 1 mmol Sr(CF3COO)2 and 40 mL benzylamine were directly added into 45 mL Teflon-lined stainless autoclave, sealed, and heated at 240 8C for 24 h. After the autoclave was cooled to room temperature naturally, the products were separated by filtration, washed three times with ethanol, and dried in an oven at 100 8C for 2 h. The final products were dispersed in cyclohexane solution for further characterizations.
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and differential thermal analysis (TG–DTA) data were recorded with Thermal Analysis Instrument (SDT 2960, TA Instruments, New Castle, DE) with the heating rate of 10 8C/min in an air flow of 100 mL/min. Fourier transform infrared (FT-IR) spectra were measured with a PerkinElmer 580B infrared spectrophotometer with the KBr pellet technique. Transmission electron microscopy (TEM) was performed using FEI Tecnai G2 S-Twin with a field emission gun operating at 200 kV and images were acquired digitally on a Gatan multiple CCD camera. The excitation and emission photoluminescence (PL) spectra were taken on an F-4500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The luminescence decay curves were obtained from a Lecroy Wave Runner 6100 Digital Osilloscope (1 GHz) using a tunable laser (pulse width = 4 ns, gate = 50 ns) as the excitation (Contimuum Sunlite OPO). The decay curves and spectra were obtained from samples dispersed in cyclohexane solutions. All the measurements were performed at room temperature. 3. Results and discussion 3.1. Phase identification
2.4. Characterizations Powder X-ray diffraction (XRD) measurements were performed on a Rigaku-Dmax 2500 diffractometer with graphite monochromatized Cu Ka radiation (l = 0.15405 nm). Thermogravimetric
The composition and phase purity of the product were first examined by XRD, as shown in Fig. 1a. Vertical bars (Fig. 1b) indicate a standard cubic bulk SrF2 peak position from International Center for Diffraction Data (ICDD No. 06-0262). It is obvious
Fig. 3. TEM (a and b) and HRTEM images (c) of SrF2 superstructures, as well as HRTEM image (d) of two separated nanoplates.
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that all the XRD peaks of the resulted SrF2 superstructures can be indexed as a pure face centered cubic phase (fcc, space group: Fm3m), agreeing well with the values in the standard card. The calculated lattice constant of pure SrF2 superstructures, a = 0.580 nm, is the same with that from the standard card (a = 0.580 nm). It can be seen that the diffraction peaks of XRD patterns are obviously broadened, revealing the small size nature of components. The peak broadening can be utilized to calculate the average crystallite size by Scherer’s formula, Dh k l = 0.89l/ (b cos u), where l is the X-ray wavelength (0.15405 nm), b and u are the full width at half-maximum and the diffraction angle, respectively. The average crystallite size was finally estimated to be about 12 nm, which was close to the edge length of subunits, nanoplates (see the next part). 3.2. Morphology SEM images provide direct information about the size and typical shapes of the as-synthesized SrF2 sample, as shown in Fig. 2. The low-magnification SEM image (Fig. 2a) reveals that the product consists of exclusively round flowers with a relatively uniform size, which indicates the high yield of the product obtained by this simple solvothermal method. The analysis of a number of SrF2 flowers shows that these structures have average diameter of 250 nm and thickness of 150 nm. Further close investigations under higher magnification (Fig. 2b) and different incidence angles (Fig. 2c and d) reveal that these flowers are actually composed of many layered nanoparticles and slightly concaved in the center region. TEM investigations can provide further insight into the microstructure details of the flowers. Fig. 3a shows the TEM image of the product, clearly revealing the round shape with concaved centers due to the brighter transmission region of the center compared to the peripheric parts of particles. Furthermore, the layered structure of the flowers can be demonstrated from the particle standing on the side face perpendicular to the substrate. From the magnified TEM image of an individual particle shown in Fig. 3b, we can observe that the round flower is actually composed of numerous square nanoplates. This point can be proven from the magnified HRTEM image of the edge of the flower (Fig. 3c). In this HRTEM image, taken with the electron beam perpendicular to the c-axis of the round flower, the interplanar distances between adjacent lattice fringes are determined as 0.291 nm, agreeing well with the dspacing value (0.290 nm) of the (1 0 0) planes of cubic SrF2 crystals. Among them, there is a group of parallel lattice fringes with identical interplanar spacing (0.290 nm) marked with an arrow in Fig. 3c, which may be ascribed to the side surface of square nanoplates. Therefore, we can conclude that the square nanoplates are actually composed of six (1 0 0) planes. Further HRTEM investigation of two separated square nanoplates (Fig. 3d) can reveal that the good crystallinity and single crystal nature of subunits, nanoplates.
COO disappear, and a set of characteristic absorption peaks of benzylamine molecules (C6H5CH2NH2) are present. It shows two bands at 2963 and 3033 cm1 that are characteristic of C–H stretching vibration of benzene ring and three other bands (1667, 1572, and 1464 cm1) due to the C–C skeletal modes of benzene ring, implying the presence of benzene ring in the final product [24]. Besides, the sharp peaks at 2855 and 2921 cm1 are assigned to the symmetric and antisymmetric stretching vibration of the – CH2 group. It also shows a broad absorption peak centered at 3425 cm1 due to the asymmetric stretching vibration of the primary amino group, which to some extent overlaps with the H2O frequency [25]. Through above analysis, we can conclude that the as-prepared SrF2 flowerlike superstructures have been successfully coated with benzylamine molecules. This fact can be further confirmed from the presence of C and O peaks in the EDS pattern for SrF2 superstructures (Fig. 4c). Note that the Au peak in the figure comes from the coating from the measurement. It is the presence of benzylamine ligand on the crystal surface that makes the obtained SrF2 flowerlike superstructures stable for a period of time in cyclohexane solutions. 3.4. Formation process To uncover the formation process of SrF2 flowerlike superstructures, time-dependent experiments were carried out. Generally, the intermediates obtained at different reaction intervals are be used to shed light on the growth mechanism of the crystals. So far, this method is widely used to study the morphological formation of various kinds of crystals under different reaction
3.3. Surface structure Fourier transform infrared measurement was carried out to prove the presence of benzylamine molecules in the obtained flowerlike superstructures. Fig. 4 displays the FT-IR spectrum of the Sr(CF3COO)2 precursor (a) and as-synthesized SrF2 flowerlike superstructures (b). The absorption peaks located at 1678 and 1460 cm1 in the FT-IR spectrum of Sr(CF3COO)2 (Fig. 4a) can be ascribed to the stretching vibrations of carboxylate COO [24]. The broad absorption band peaking at 3670 and 3430 cm1 is due to the stretching vibration of the hydroxyl group (–OH) of absorbed water molecules [20]. As for the FT-IR spectrum of the resultant SrF2 flowerlike superstructures (Fig. 4b), the absorption peaks from
Fig. 4. FT-IR spectra of Sr(CF3COO)2 precursor (a) and SrF2 superstructures (b), as well as the EDS pattern (c) of SrF2 superstructures.
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Fig. 5. TEM (a) and HRTEM images (b) of spherical nanoparticles, as well as SEM (c) and HRTEM images (d) of microdisks. Inset of (c) is the TEM image of an individual microdisk. (e) Schematic illustration of the formation of microdisks.
Fig. 6. TG–DTA curves of Sr(CF3COO)2.
conditions [26,27]. Fig. 5a shows the TEM image of intermediate product obtained after 2 h reaction, revealing their exclusively uniform pseudo-spherical morphology with average size of 20 nm. Further HRTEM image of an individual nanoparticle (Fig. 5b) reveals many groups of lattice fringes with adjacent distance of 0.349 nm, which matches well with that of (1 1 1) planes of bulk SrF2. When the reaction proceeds up to 8 h, the obtained sample (Fig. 5c) is mainly composed of microdisks with edge length of 250 nm and thickness of 20 nm. Besides, there are still certain amounts of spherical nanoparticles present in the sample, which can be obviously observed from the TEM image (inset in Fig. 5c). Additionally, it also demonstrates that the obtained microdisks are actually composed of many interconnected square nanoplates bounded by six (1 0 0) planes, which can clearly seen from the HRTEM image shown in the inset of Fig. 5d. From this image, we can observe that two groups of lattice fringes with identical spacing (0.291 nm) coming from the top and side surfaces of
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nanoplates, respectively, intersect at the edge of each nanoplate. This spacing agrees well with the d-spacing value (0.290 nm) of the (1 0 0) planes of cubic SrF2 crystals. HRTEM image of the edge of an individual microdisk (Fig. 5d) reveals the co-presence of lattice fringes of (1 0 0) planes of nanoplates and (1 1 1) planes of nanoparticles highlighted with cycles. Above results indicate that as the reaction proceeds, the small spherical nanoparticles undergo a structure transformation to form 2D nanoplates bounded by six (1 0 0) planes, and subsequently aggregate together to build into larger microdisks. The schematic illustration of the formation of the microdisks is shown in Fig. 5e. During the subsequent aging period, the residual spherical nanoparticles tend to locate on both the circumference and surface of microdisks and subsequently transform into plates to increase the lateral and longitudal dimensions of the microdisks through the well-established Oswald ripening process. It means that the aggregation seems to be not strictly restricted within two dimensions, and can be extended to a few layers along the disk thickness direction, resulting in the formation of final architecture, flowerlike superstructures, as revealed by Fig. 2. 3.5. Growth mechanism It is known that metal trifluoroacetates can thermally decompose to produce the corresponding metal fluorides and various fluorinated and oxyfluorinated carbon species [28,29]. Based on
this reaction, we adopted Sr(CF3COO)2 as SSP to prepare SrF2 crystals in this system, and the thermal decomposition reaction should proceed as follows: SrðCF3 COOÞ2 ! SrF2 þ C2 F4 þ 2CO2 To obtain the quantitative information of the decomposition process, the TG–DTA analysis of Sr(CF3COO)2 powder precursor was performed, as shown in Fig. 6. The TG curve of Sr(CF3COO)2 shows two stages of weight loss. The first weight loss step (about 3.5%) is observed between 54 and 110 8C due to the evaporation of absorbed water that is accompanied with an endothermic peak at 105 8C in the DTA curve. Based on this weigh loss value, the amount of water molecule in the precursor can be determined to be one. The second weight loss step is observed in the temperature range from 265 to 350 8C, accompanied by a strong exothermic peak at 347 8C in DTA curve due to the transformation of Sr(CF3COO)2 into SrF2, and the obtained weight loss value (56.7%) is in good agreement with the theoretical value (56.6%). However, this decomposition reaction in our system can occur at a relatively lower reaction temperature (240 8C), possibly due to the presence of Sr(CF3COO)2 in the form of molecular species in benzylamine solvent. It is well accepted that formation of crystals is mainly achieved through two stages: nucleation and growth. In our case, seed formation proceeds according to LaMer model for homogeneous nucleation, in which thermal decomposition of Sr(CF3COO)2 occurs to generate SrF2 in solution. Above the critical concentration,
Fig. 7. TEM and HRTEM images of SrF2 crystals prepared under equal amount of ethylamine and benzylamine (a and b) and pure ethylamine (c and d), respectively.
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nucleation results in a rapid depletion of the reactants such that all subsequent growth occurs on the pre-existing nuclei [30]. As dictated by thermodynamics (i.e. Wulff construction), the SrF2 with fcc structure are expected to nucleate and grow into tetradecahedron seeds enclosed by a mix of (1 1 1) and (1 0 0) planes to minimize the total surface energy [31]. During the subsequent crystal growth process, it has been illustrated that the final shape of an fcc nanocrystal is mainly determined by the ratio (R) between the growth rates along the h100i and h111i directions. Cubes bounded by six equivalent (1 0 0) planes will be formed when R = 0.58, while octahedrons bounded by the eight equivalent (1 1 1) planes will result if R is increased to 1.73 [32]. The growth rates at different facets are dominated by their surface energies, that is, the facets with higher surface energy will grow faster. Generally speaking, the surface energies on (1 1 1), (1 0 0), and possibly (1 1 0) crystallographic planes in a fcc structure could be distinctive because of the different atomic densities, electronic structures, bindings, and possibly chemical reactivities on them [31,32]. In the case of SrF2, the intrinsic surface energy of (1 0 0) planes is higher than that of (1 1 1) planes [33], and therefore relatively fast growth along six equivalent h100i directions from the tetradecahedron seeds are expected, finally leading to the formation of octahedrons. However, as stated above, the final product is actually composed of nanoplates (Fig. 6a and b). We therefore reasoned that coordinating ability of benzylamine solvent plays an important role in this formation of nanoplates. In a solution-phase process, impurities or capping agents are usually adopted to alter the surface free energies via adsorption or chemical interaction and thus induce new shapes. As for our system, benzylamine molecules can preferably and efficiently interact with active (1 0 0) planes to significantly reduce their surface energy and block the growth on these planes, and therefore
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facilitate the growth on (1 1 1) planes [33], finally resulting in the formation of nanoplates exposing six (1 0 0) planes. Similar situation has also been observed for the synthesis of anatase nanoplates using benzylamine as solvent [34]. Another important role of benzylamine in the formation of well-defined superstructures is that it can efficiently direct the assembly of nanoplates. This point can be proven from two control experiments in which the benzylamine was partly or completely replaced by ethylamine under otherwise same experimental conditions. Fig. 7 shows the TEM and HRTEM images of samples obtained under conditions of equal amount of benzylamine and ethylamine (Fig. 7a and b) and pure ethylamine (Fig. 7c and d). The former sample (Fig. 7a) is composed of irregular plate-like structures and loosely aggregated structures, while the latter sample (Fig. 7c) contains almost irregular plate-like structures. HRTEM images taken from the plate-like structures of these two samples (Fig. 7b and d) show the (1 0 0) lattice fringes of SrF2, which are the same as that of SrF2 superstructures. From this tendency, we can conclude that the adoption of benzylamine as solvent is essential to the formation of well-defined hierarchical SrF2 flowerlike superstructures. This is because the benzylamine species bound to the (1 0 0) surfaces interact with each other, through p–p interactions, driving the stacking of the nanoplates into aggregated superstructures [34]. It should be noted that the obtained SrF2 superstructures are remarkably stable, and it is impossible to separate the aggregated nanoplates even by applying extensive ultrasonication and washing treatments in various solvents. This special function of benzylamine molecules has been widely utilized for the synthesis of highly ordered organic– inorganic nanocomposites [35–37]. Finally, it has turned out that the role of benzlylamine in the system can be clearly identified, providing not only size and shape control to form nanoplates but
Fig. 8. (a) PL excitation and emission spectra and the corresponding digital image under 254 nm irradiation of SrF2 superstructures dispersed in cyclohexane solution. (b) Decay curve of SrF2 superstructures dispersed in cyclohexane solution.
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also contributing to the self-assembly behavior of nanoplates to build into flower-like superstructures. 3.6. Luminescent properties The PL properties of the SrF2 superstructures were investigated here, as shown in Fig. 8. Under 254 nm irradiation, it can show purple emission, which is displayed on the right part of Fig. 8a. Monitored with the emission wavelength of 388 nm, the excitation spectrum (Fig. 8a) consists of a broad band from 250 to 375 nm with two wellresolved peaks at 289 and 322 nm. Upon excitation at 289 nm, the obtained emission spectrum (Fig. 8a) exhibits broad emission band ranging from 350 to 450 nm centered at 388 nm. The photoluminescence decay curve of SrF2 superstructures is shown in Fig. 8b. It can be fitted to a single-exponential function as I = I0 exp(t/t) (t is lifetime), from which the lifetime is determined to be 8.04 ns. 4. Conclusions SrF2 hierarchical flowerlike superstructures were first prepared via the thermal decomposition of single source precursor, Sr(CF3COO)2, under solvothermal conditions. The adoption of Sr(CF3COO)2 and benzylamine as precursor and solvent, respectively, was both crucial to the formation of the well-defined flowerlike superstructures. Time-dependent experiments were conducted to reveal the formation process of these flowerlike superstructures. This versatile nonaqueous route with benzylamine as solvent can be potentially extended to the synthesis of other alkaline earth metal nano/microstructures. Acknowledgments This work is financially supported by the ‘‘Bairen Jihua’’ of Chinese Academy of Sciences, the National Natural Science Foundation of China (50702057, 50872131) and the MOST of China (No. 2007CB935502). References
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