Formation of Ca2Gd8(SiO4)6O2 Nanorod Bundles Based on Crystal

Article pubs.acs.org/crystal

Formation of Ca2Gd8(SiO4)6O2 Nanorod Bundles Based on Crystal Splitting by Mixed Solvothermal and Hydrothermal Reaction Methods G. Seeta Rama Raju, Yeong Hwan Ko, E. Pavitra, and Jae Su Yu* Department of Electronics and Radio Engineering, Kyung Hee University, 1 Seocheon-dong, Giheung-gu, Yongin-si, Gyeonggi-do 446-701 Republic of Korea

Jin Young Park, Hong Chae Jung, and Byung Kee Moon* Department of Physics, Pukyong National University, Daeyon 3 dong, Busan-608-737, Republic of Korea S Supporting Information *

ABSTRACT: Oxyapatite Ca2Gd8(SiO4)6O2 (CGS) nanostructures with nanorod bundle-like morphology are prepared by mixed solvothermal and hydrothermal reaction methods. Detailed structural and morphological studies are performed using X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and transmission electron microscopy measurements. CGS nanorod bundles are formed by crystal splitting, and the growth mechanism as a function of reaction time is discussed. The size and crystal splitting of the nanorod bundles are controlled by varying the concentration of 2-propanol. The annealing temperature does not have any effect on the morphology of CGS nanorod bundles, and the bundles can sustain high temperatures, which confirms the crystal splitting of nanorod bundles. Photoluminescence and cathodoluminescent studies are carried out by activating the Eu3+ ions in the CGS host lattice as a function of annealing temperature. The corresponding CIE chromaticity coordinates are in close proximity to the commercial red emitting phosphor chromaticity coordinates.

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INTRODUCTION In recent years, the synthesis of novel inorganic nanomaterials with tunable size and morphology with different dimensions (zero, one, two, and three) have received great attention owing to their superior physical and chemical properties along with promising applications in various fields, such as nanoscale electronics and optoelectronics.1−5 It also offers an opportunity to study and imitate the biological functions at the nanoscale.3 Among the different dimensions, nowadays, one-dimensional (1D) metal oxide nanowires have attracted intensive research interest due to their diverse applications in nanoscale devices with designed functions. However, the weak transport properties of the individual 1D nanostructures have hampered optical applications. A novel solution is to synthesize the nanorod bundles or to align and arrange a number of parallel 1D nanostructures in close vicinity, which could make the most unique structural and optical properties and boost device performance.6 The kinetic control of the nanoparticle growth gives the possibility for greater control of complex shapes, such as rods, tetrapods, arrows, sheaf-like or bundle shapes, and hyperbranched structures.7−9 Currently, a large number of soft © 2011 American Chemical Society

chemistry routes have been successfully developed for the synthesis of simple binary metal oxide nanostructures such as TiO2, ZrO2, SiO2, and ZnO.10−13 However, techniques leading to the formation of doped or undoped polynary complex compounds by soft chemistry routes are difficult and rarely reported due to the difficulty to match the reactivity of the different metal ions in the solution during the fabrication. Therefore, the development of a novel method is imperative for the synthesis of nanostructured polynary complex compounds with a controllable size and shape. It has been recently established that efficient and excitation induced tunable luminescent properties were observed from the trivalent rare-earth ions (RE3+) activated Ca2Gd8(SiO4)6O2(CGS) host lattice.14−17 The ternary CGS host lattice belongs to the family of oxyapatite M4I M6II(XO4)6O2 hexagonal structures with space group P63/m. MI and MII positions correspond to the two distinct low symmetry (C3 and Cs) crystallographic sites, which accommodate metal ions such Received: October 28, 2011 Revised: December 5, 2011 Published: December 7, 2011 960

dx.doi.org/10.1021/cg2014286 | Cryst. Growth Des. 2012, 12, 960−969

Crystal Growth & Design

Article

as divalent alkaline-earth or RE3+ elements. The X site is occupied by P-block elements such as P, As, Si, or Ge. In the CGS host lattice, Ca2+ and Gd3+ ions are randomly distributed in the C3 point symmetry of the 9-fold coordinated 4f site, and Gd3+ ions are completely occupied in the Cs point symmetry of 7-fold coordinated 6h sites.15,17,18 Both sites are very suitable for the luminescence of RE3+ ions owing to their low symmetry features. Among the RE3+ ions, europium (Eu3+) shows good luminescent properties.19 The red (5D0 → 7F2) emission of Eu3+ belongs to the hypersensitive (forced electric dipole) transition with the selection rule, ΔJ = 0, ± 2, which is strongly affected by the outside surrounding environment, which helps the stress-free analysis for the suitability and kind of site symmetry and the number of available emission sites in the host lattice. So, Eu3+ ion activated CGS host lattice has been selected for the present work. Recently, Peng et al. synthesized oxyapatite CGS nanobelts and microfibers by the electrospinning method,20 and no other reports have been found on the synthesis of 1D CGS nanostructures thus far. In the present work, we report the synthesis of 1D CGS nanostructures by mixed solvothermal and hydrothermal synthesis. For the first time, we obtained 1D nanorod bundles by the crystal splitting of ternary complex compound. The size and crystal splitting of CGS nanorod bundles were controlled by adding the 2-propanol, and the shapes were tuned by the reaction time. The growth mechanism of oxyapatite CGS nanostructures was explained as a function of reaction time. The photoluminescence and cathodoluminescence properties of Eu3+ activated CGS nanorod bundles were studied as a function of annealing temperature.

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ions. The characterization techniques are presented in the Supporting Information.

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RESULTS AND DISCUSSION Effect of Reaction Temperature. One of the important experimental factors is the reaction temperature, which was considered to investigate the growth process of CGS nanorods. Initially, we prepared the 2-propanol (2-PrOH) mediated CGS amorphous powder at the reaction temperature of 230 °C. Using the amorphous CGS powder, 1D nanostructures were prepared by hydrothermal and solvothermal methods. Figure 1

Figure 1. (a, b) XRD patterns of CGS nanostructures at different reaction temperatures.

shows the XRD patterns of the as-prepared CGS at the reaction temperatures of 250 and 300 °C. The hydrothermally obtained products had an amorphous nature at the reaction temperature of 250 °C (Figure 1a) and became pure hexagonal crystal form of oxyapatite based on the space group P63/m (Figure 1b), which is in good agreement with the standard JCPDS card [PDF (28-0212)]. This means that the high reaction temperature is required for the growth of CGS nanorod bundles because it increases the transfer rates of ions and the frequency of collisions between the ions, thus allowing complete crystal growth.21 On the other hand, the solubility of various compounds increases in water at a higher reaction temperature. As a result, the pH value of the solution decreases through hydrolysis during the hydrothermal process by dissolving all the compounds.22 Figure 2a,b shows the field emission scanning electron microscopy (FE-SEM) images of the DI water mediated asprepared CGS samples at the different reaction temperatures (250 and 300 °C). From the FE-SEM results, as-prepared CGS shows the nanorod bundle almost surrounded by amorphous natured particles at the reaction temperature of 250 °C (Figure 2a). When the reaction temperature increases to 300 °C, 90% of the as-prepared product exhibits exclusively 1D nanorod bundles with uniform diameters of 1.3 μm and lengths ranging from 4 to 5 μm as shown in Figure 2b. Generally, from Figure 2a,b, we can understand that a precursor exists by dissolving from the initial amorphous particles to the final formation of crystalline nanorod bundles. The effect of the reaction temperature on the formation of CGS nanorod bundles is basically assumed through the mechanism of dissolution− precipitation. For further clarification, we performed TEM, as can be seen in Figure 2c,d. Interestingly, the TEM image of

EXPERIMENTAL SECTION

Synthesis. The 1D hexagonal CGS nanorod bundles were fabricated by mixed solvothermal and hydrothermal synthesis. At first, the CGS precursor powder was prepared by taking stoichiometric amounts of high purity grade calcium nitrate tetrahydrate (Ca(NO3)2·4H2O), gadolinium nitrate hexahydrate (Gd(NO3)3·6H2O), and tetraethyl orthosilicate (Si(OC2H5)4). All the reagents were taken without any further purification and dissolved in 650 mL of 2-propanol with vigorous stirring using a magnetic stirrer. The solution was transferred into a stainless steel autoclave (total inner volume of 1356 cm3) with a Teflon liner (1290 cm3 volume and 50% filling capacity). It was then heated to 230 °C at a rate of 2 °C/min and maintained for 5 h with magnetic stirring to make stable networks between the reactants. After gradually cooling down to room temperature, the precipitate was separated by a centrifugation with 5000 rpm for 5 min and dried at 60 °C for a day in ambient atmosphere. Second, 0.5 g of dried powder was dissolved in 60 mL of triple distilled water and stirred for 4 h to make the homogeneous solution. The solution was transferred into a stainless steel autoclave (total inner volume of 250 cm3) with a Teflon liner (120 cm3 volume with 50% filling). The solution was heated to 250 °C at a rate of 2 °C/min and held at this temperature for 30 min with magnetic stirring. The reaction temperature was then raised from 250 to 300 °C at a rate of 1 °C/ min and held at this temperature without magnetic stirring. The precipitate of the CGS nanorod bundles was formed at the bottom of the Teflon liner. After gradually cooling down to room temperature, the precipitate was washed separately with ethanol and water. The obtained precipitate was dried at 60 °C for a day in the ambient atmosphere. The experiment was repeated for optimizing the synthesis conditions such as the water and 2-propanol ratio, reaction temperature, reaction time, and annealing temperature. CGS/Eu3+ nanorod bundles were prepared with the same procedure except by doping 5 mol % of Eu(NO3)3·5H2O as the Eu3+ source instead of Gd3+ 961



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Figure 2. (a, b) SEM images of CGS nanorod bundles prepared at 250 and 300 °C, and (c) low magnification (d) high magnification TEM image of CGS nanorod bundles prepared at 300 °C.

Figure 3. (a) TEM image of individual CGS nanorod, (b) SAED pattern, and (c) HRTEM of CGS single nanorod.

phase of CaCO3. From the earlier reports, the formation of a CGS nanorod bundle looks like the assessment of the sheaf-like structured Bi2S37 and hyperbranched Co2P.23 For strong support of the crystal splitting, the splitting from the solid core and the presence of a solid core inside the bundle are also shown in the SEM image of Figure S1a,b (Supporting Information). The tips of the nanorods and different resolutions of TEM images corresponding to the as-prepared

Figure 3b shows a high density region inside the nanorod bundle and indicates the presence of a solid core, which further confirmed that the CGS bundle is not an aggregate of many individual nanorods but a single crystal growing from the original particle with the splitting of nanorods, hence the name “bundle or sheaf or hyperbranched structures”. These particular structures are unusual in nature and are only found in some minerals such as stilbite [NaCa4(Si27Al9)O72·nH2O], a mineral of zeolite group and an aragonite, which is the orthorhombic 962



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CGS sample are presented in Figure S2 (Supporting Information). To obtain further insight into the crystallographic structure and chemical composition of as-prepared CGS nanorod bundles were investigated by the selected area electron diffraction (SAED) and high-resolution TEM (HRTEM) of individual nanorods (Figure 3). Figure 3b shows the corresponding SAED pattern taken from the single nanorod and displays the regularly arranged diffraction spots indicating their single crystalline nature. The pattern can be indexed to the reflection of an oxyapatite hexagonal CGS structure, consistent with the XRD results presented above. The twist angle (ϕ = 23.1°) was similar to that in the reports on the Sr2RE8(SiO4)6O2 host lattice taken from the [001] zone by Lieu et al. and Masubuchi et al.24,25 The HRTEM image (Figure 3c) of the CGS individual nanorod clearly shows lattice fringes with interplanar spacing of 2.77 Å corresponding to the (112) plane, which is the preferential growth direction of CGS nanorods. The growth direction can also be supported by relative diffraction peak intensity at 32.172° in the XRD patterns. We predicted that the HRTEM study on the core part of the CGS bundle may explain the formation of such structures by providing the structural information. However, the thickness of the CGS bundle core part has prevented such measurements due to strong electron scattering. According to the literature, the perfect reason for crystal splitting is unidentified and varies depending on the system. Generally, the crystal splitting is related to both kinetic and thermodynamic factors: fast crystal growth and extra molecules appearing in the parent solution. The crystal growth is subject to the solution supersaturation with the other conditions being constant. Subject to the level of supersaturation or impurity concentration, crystals can adopt the different degrees of splitting, resulting in a number of subforms of split crystal. Punin et al.26 suggested that the splitting is only possible if the supersaturation exceeds a certain critical level, which is specific for each crystal (mineral) under the given conditions. In our present work, the synthetic parameters such as temperature, reaction time, and liquid−liquid interface play a key role in the formation of crystal splitting. Effect of Reaction Time. To investigate the formation mechanism of the CGS nanorod bundles, their growth process has been followed by extracting the product after different hydrothermal treatment times of 13, 15, 25, 35, 45, and 55 h at 300 °C and the other conditions being constant. Figure 4 shows a series of SEM images of the products by changing the reaction time. After 13 h of hydrothermal reaction, the tiny equiaxial nanoparticles were collected (Figure 4a).When the reaction time was extended to 15 h, absolutely well-aligned 1D nanorod bundles appeared due to crystal splitting during their growth with an average diameter of 42.5 nm for each individual nanorod (Figure 4b). From Figure 5a,b, we believed that the Ostwald’s ripening process27,28 is dominated for the growth of CGS nanorod bundles because in our process at the initial stage tiny crystalline nuclei that act as the centers of crystallization were formed in a supersaturated solution. Then the crystal growth follows, and bigger particles grow with crystal splitting at the cost of smaller crystals, as described by the Gibbs− Thomson’s equation.29 It is also interesting to note that the reaction time is increased to 25 h, the nanorods split into nanowires, and the length of the bundle also decreases from 4 to 5 to 2−3 μm. The average diameter of the bundle increases from 1.3 to 1.8 μm and the diameter of individual wire is about

Figure 4. SEM images of CGS nanorod bundles at different reaction times of (a) 13 h, (b) 15 h, (c) 25 h, (d) 35 h, (e) 45 h, (f) 55 h, and (g) schematic diagram of CGS nanostructure growth mechanism.

14.7 nm, which has been shown in Figure 4c. The conversion of nanorods into nanowires is clearly observed in the magnified SEM image (Figure S3a, Supporting Information). When the reaction time was further extended to 35 h, the nanowires converted to nanofilaments (Figure 4d and Figure S3b in Supporting Information) and the length of bundles again decreased without any aggregation, which indicates that the rate of nucleation increases due to an increase in the thermodynamic driving force.30,31 It can be clearly observed that the reaction time further increased to 45 h as shown in Figure 4e, and the nanofilament bundles continued to grow and coalesced with each other, which are actually constructed from even smaller bundles of nanofilaments with a high aspect ratio.32 Figure 4f shows the sample prepared at 55 h of reaction time, and the sample almost turned into spherical particles made of numerous nanofilaments (Figure S4, Supporting Information) by self-assembling. On the basis of the above observations, we are apt to describe the crystal growth mechanism of the CGS nanostructures as involving a sequence of nucleation, crystal splitting, dissolution, renucleation/recrystallization, and selfassembling of nanofilaments into spherical-like morphology. The schematic diagram of the CGS crystal growth is displayed in Figure 4g. 963



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Figure 5. SEM images of nanorod bundles at difference liquid−liquid (H2O−2PrOH) interface effect: (a) 100:0, (b) 90:10, (c) 80:20, (d) 60:40, (e) 40:60, and (f) 80:20 (all the scale bars are 1 μm) (inset figures show the corresponding XRD patterns).

300 °C. Further, with reducing the water content by adding 2PrOH, the polarity of the solvent decreases, and thus the solubility of the matter also decreases, which indicates that the number of carbon (C) (because increasing the C atoms decreases the polarity) atoms and the polarity of the solvent show a dynamic role on variation of crystal splitting. The corresponding XRD patterns are shown in the insets of Figure 5, which confirms their hexagonal crystalline nature. However, it is also clear that by the XRD pattern (inset of Figure 5f) and SEM image (Figure 5f) that the particles show an amorphous nature for the sample prepared with 100% 2-PrOH. This suggests that the polarity should be high enough for crystal splitting and growth of the CGS nanorod bundles. On the other hand, since the solubility decreases by increasing the 2-PrOH content, the supersaturation degree under the hydrothermal condition becomes higher, and then the nucleation rate increases due to decreased the kinetic barrier.30 Thus, more particles are growing at the same time and so the crystal splitting nature decreases. Hence, it could be concluded that the rate of nucleation and the polarity of the solvent are

Solvent Interface Effect. The solvent is also an important factor to control the morphology evaluation of the CGS. In order to investigate the DI water/2-PrOH interface effect on the growth of the nanorods, the DI water content was reduced by adding 2-PrOH. Figure 5 shows the SEM images of the different percentages of water and 2-PrOH interface (100:0, 90:10, 80:20, 60:40, 40:60, and 20:80). It was found that in the presence of 2-PrOH content from 0 to 80%, the crystal splitting gradually decreases, and as a result the length of a bundle decreases. However, the nanorod bundles were not broken and maintained almost uniform diameter. We can also observe that the yield amount was increased by increasing the 2-PrOH content. The high magnification SEM image with inside core of the (80% 2-PrOH/20% H2O) CGS bundles is shown in Figure S5 (Supporting Information). The above observations indicate that the splitting extent decreases with increasing the 2-PrOH content. This inhibiting effect of 2-PrOH can be speculated as follows: on the one hand, water is a much more polar molecule than 2-PrOH, and from the above observations, the CGS powder completely dissolved and crystallized in the water at 964



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Information, Figure S6). The merging might be attributed to the melting process. Lai et al.35 reported that the surface to volume ratio of nanosize particles is high, and at high temperatures the surface energy substantially affects the interior bulk properties of the materials. These annealing temperature results are also in good agreement with the report of Chen et al.36 The assessment of crystallite size from the XRD patterns was performed by Scherrer’s equation37,38 (see the Supporting Information), and the estimated crystallite size along with an average diameter of the individual rod and an average length and diameter of a bundle are summarized in Table 1. Clearly, the crystallite size increases with raising the annealing temperature except at 900 °C, and the same trend was observed for the individual nanorod and nanorod bundles. Luminescence Properties. Photoluminescence Properties. The model diagram for the possible mechanism of excitation and emission from the nanorod bundles is shown in Figure 8. In order to investigate the luminescence properties, 6 mol % of Eu3+ was doped for Gd3+ in CGS host lattice, which has been optimized previously.39 Figure 9a shows the photoluminescence excitation (PLE) spectra of the CGS/Eu3+ nanorod bundles at different annealing temperatures by monitoring the emission wavelength at 616 nm. The PLE spectrum of precursor CGS/Eu3+ nanorod bundle reveals the broad excitation band overlapped with the f−f transitions of Gd3+ ion in the shorter wavelength region, and it also consists of sharp excitation bands in the longer wavelength region due to the f−f transitions of Eu3+ ion. The broad excitation band also called the charge transfer band (CTB) is due to the charge transfer between the completely filled 2p orbital of O2− ion and the partially filled 4f orbital of the Eu3+ ion, and the position of this band depends strongly on the host lattice. The other sharp excitation peaks due to intraconfigurational f−f transitions of Gd3+ and Eu3+ ions are assigned to the electronic transitions of (8S7/2 → 6I11/2) at 275 nm, (8S7/2 → 6I9/2) at 277 nm for Gd3+ and (7F0 → 5D4) at 362 nm, (7F0 → 5G3) at 376 nm, (7F0 → 5 G4) at 382 nm, (7F0 → 5L6) at 395 nm, (7F0 → 5D3) at 412 nm, (7F0 → 5D2) at 464 nm for Eu3+. These Eu3+ sharp excitation peaks indicate that violet and blue laser diodes/LEDs are efficient pumping sources in obtaining Eu3+ emissions. Figure 9a shows that by increasing the annealing temperature from 300 (S3) to 700 °C (S7), the broadness (or strength) of the CTB increases and (7F0 → 5L6) transition in the longer wavelength region also increases. Additionally, the CTB maxima shift toward the lower energy side, which indicates that the quantum efficiency of Eu3+ decreases with increasing the annealing temperature.40 The CTB shift to a lower energy side is a good sign because the forced electric dipole transition needs its CTB at lower energies, and thus the parity-forbidden of the Eu3+ ions borrow the energy from the lowest strong absorption band.41 As shown in Figure 9a, it can also be clear that by increasing annealing temperature, the f−f transitions of Gd3+ ions almost disappeared and the strength of the CTB increased. Furthermore, when the annealing temperature increases to 900 °C (S9), the band maxima shift toward the higher energy side. The CTB is related to the stability of the electron of the surrounding O2− ion; that is, the CT transition is sensitive to ligand environment (the bonding energy between the central ion and the ligand ions). The deviation of CTB described in different reports is somewhat incompatible. For example, in Y2O3/Eu3+ nanocrystals, some authors observed a blue shift, whereas the others observed a red shift.42,43 The red shift can be explained as (i) in nanoscale samples, especially

responsible for the variation of crystal splitting and size of the particle. The obtained results are unlike the assessment of Bi2S3,7 Fe2P,8 and Co2P.23 The XRD is a powerful tool for analyzing the crystalline phase, but FTIR measurements were also carried out for further confirmation of phase evaluation at different liquid interface effects of CGS nanorod bundles, as shown in Figure 6. All the

Figure 6. FTIR spectra of CGS nanorod bundles at different ratios of H2O−2PrOH.

samples exhibit similar spectral profiles, and there appears lack of absorption bands that are characteristics of OH groups with stretching (3572 cm−1) and libration (630 cm−1) modes in the apatite structure, indicative of the successful formation of the oxyapatite phase in our samples. The IR absorption due to SiO4 units can be assigned based on four kinds of modes consisting of symmetric stretching (ν1), symmetric bending (ν2), antisymmetric stretching (ν3), and antisymmetric bending (ν4). As seen from Figure 6, the stretching vibrations ν1 and ν3 give fairly intense and broad absorption bands with the band maxima at the wavenumbers of 870 and 924 cm−1, respectively. The bending vibrations of ν2 and ν4 are observed at the wavenumbers of 415 and 495 cm−1, respectively. This result is consistent with that reported in the literature for IR absorption of the SiO4 unit in the apatite structure.33 The Gd−O absorption band was observed at 550 cm−1, 34 and the IR spectrum exhibits very weak bands between 1800 and 1200 cm−1 constituted by the absorption of C−H bending (1372 cm−1) and CO stretching (1737 cm−1). Effect of Annealing Temperature. Figure 7 shows the SEM images of the CGS nanorod bundles at different annealing temperatures and also shows the magnified SEM images of the corresponding structures. The annealing temperature did not have any effect on the morphology of CGS nanorod bundles even at a very high temperature (900 °C). The average diameter of individual nanorods increased from 42.5 to 77.5 nm when the precursor was annealed at different temperatures. Moreover, the diameter and length of the bundle increase with increasing the annealing temperature since the transformation of smaller nanorods into larger ones occurs during an extended annealing process. In Figure 7d, we observed that the annealing temperature increases to 900 °C, the nanorods came into contact and formed as a number of nanorod pairs (marked by circles) due to merging (clear figure shown in the Supporting 965



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Figure 7. SEM images of CGS nanorod bundles (right side high magnification SEM) at different annealing temperatures of (a) 300 °C, (b) 500 °C, (c) 700 °C, and (d) 900 °C (all the scale bars for low magnification are 1 μm, and high magnification scale bars are 100 nm).

Table 1. Crystallite Size, Average Diameter, and Length of the CGS Nanorod Bundles As a Function of Annealing Temperature average diameter (μm) annealing temperature (°C)

crystallite size (nm)

individual nanorod

bundle

average length of the bundle (μm)

as-prepared 300 500 700 900

44 46 58.3 74.5 72.3

42.5 47.5 59.5 77.5 77.5

1.3 1.57 1.86 2.0 2.2

4.5 5.5 6.1 6.4 6.4

Figure 8. Schematic diagram for the possibility of excitation and emission mechanism of CGS nanorod bundles.

ions are less stable for the nanoscale particles, and the surface to volume ratio is high as compared to bulk sample, causing the degree of disorder of the nanostructured system to increase. As a result, it requires less energy to remove an electron from an O2− ion; therefore, the CTB is shifted toward the lower energy side. It is noted that, at a higher annealing temperature of about 900 °C the CTB shifts toward the higher energy side due to the increased potential field, and the energy requirement also

very tiny ones, the host lattice is imperfect and incompact and the Eu−O distance is long, which indicates that the Eu−O bonds become weaker and less covalent and with high ionicity, weakening the binding energy of an electron to O2−. Therefore, the electron needs less energy to transfer from O2− to Eu3+, resulting in the CTB shift to lower energy. (ii) Electrons in O2− 966



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Figure 9. (a) PLE spectra and (b) PL spectra with their corresponding color coordinates of CGS nanorod bundles at different annealing temperatures.

increases to transfer an electron from the O2− ion to the cation (Eu3+). Therefore, the charge transfer band moves to the higher energy side. These results are unlike our earlier report on the CGS nanophosphors39 in that the CGS phosphors showed the blue shift by increasing the annealing temperature. However, the results are consistent with Hoefdraad’s study on the CTB of Eu3+ in oxides, where he concluded that the CTB band position of Eu-doped oxide depends on the bond length of Eu−O and the coordination environment around Eu3+.44 The PL spectra at different annealing temperatures of Eu3+ activated CGS nanorod bundles by exciting at 395 nm are shown in Figure 9b, and the Supporting Information (Figure S7) shows the PL spectra at different excitation wavelengths. The CGS nanorod bundles exhibit an intense emission band with a peak maxima at 616 nm, which can be ascribed to the forced electric-dipole or hypersensitive 5D0 → 7F2 transition of the Eu3+ ions. The intense emission peak confirmed that the Eu3+ ions are located at the sites without inversion symmetry. The moderate emission bands between 590 and 603 nm correspond to the magnetic-dipole 5D0 → 7F1 transition, and the weak emission band at 650 nm is due to the 5D0 → 7F3 transition, and also other two emission bands at 579 nm, 586 nm are caused by 5D0 → 7F0 transitions of Eu3+ ions. The position of the hypersensitive transition is slightly shifted to Isaac’s reported result, in which the strongest Ca2Gd8Si6O26/ Eu3+ emission is observed at around 614.7 nm.45 The assynthesized CGS nanorod bundles exhibited an unresolved broader emission band with the fwhm of 8.1 nm and after increasing the annealing temperature, the broadness and the fwhm decreased. The calculated fwhm values are presented in Table 2. The reason is similar to the above discussion in the excitation part for the nanoscale samples, and it is also probably because only a small portion of Eu3+ ions entered into the nanocrystalline lattice and remaining Eu3+ ions might be located at the surface or close to the surface of the low symmetry sites of CGS. Accordingly, the PL spectrum of as-

Table 2. Full Width at Half Maximum (fwhm), Asymmetry (R) Ratio, and Corresponding CIE Chromaticity Coordinates As a Function of Annealing Temperature annealing temperature (°C)

fwhm (nm)

as-prepared 300 500 700

8.1 6.2 5.7 5.3

R = (5D0 → F2)/(5D0 → 7 F1 )

CIE (x, y) chromaticity coordinates

7

2.53 3.57 4.03 4.84 commercial Y2O3

900

4.75

4.76

(0.619, (0.641, (0.644, (0.645, (0.650,

0.379) 0.354) 0.354) 0.350) 0.346)

(0.644, 0.355)

synthesized CGS/Eu3+ shows an inhomogeneously broadened emission spectrum. While increasing the sintering temperature, the broadness with fwhm was decreased, suggesting that the incorporation of Eu3+ ions into the low symmetry sites of CGS host lattice increases.19 No emissions were observed from the higher 5D1,2 levels in the shorter wavelength range of 400−550 nm. The reason is because the smaller energy gaps between 5D2 and 5D1 or 5D1 and 5D0 of Eu3+ can be bridged by the vibration energies of the silicate groups present in the oxyapatite. The higher 5D2 and 5D1 excited levels are then relaxed once to the lowest 5D0 excited level, producing an efficient multiphonon relaxation from the upper excited states to 5D0. It is worthy to note that as-prepared CGS sample shows a single emission band at 579 nm due to the 5D0 → 7F0 transition. While increasing the annealing temperature, unusual emissions of Eu3+ were observed at 579 nm and 586 nm due to the 5D0 → 7F0 transition because Stark splitting of either 5D0 or 7 F0 is not possible and the presence of two peaks must be due to separate emissions from the two different sites occupied by the Eu3+ ion.46 Evidently, with increasing the annealing temperature, the intensity of 5D0 → 7F1 decreases compared to the 5D0 → 7F0 transition. The reason for the appearance of 967



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nanorod bundles (annealing at 700 °C) under the low electron beam excitation (accelerating voltage: 5 kV and filament current: 55 μA). The shape of the CL spectrum is similar to that of PL spectrum; however, the asymmetric ratio increases greatly to 5.49 as compared to PL (4.84) and the calculated CIE chromaticity coordinates (0.650, 0.348) are very close to those of the commercial Y2O3 phosphor (0.650, 0.346), suggesting that the CGS nanorod bundles are suitable for the development of efficient red region in FED systems. Figure 10b,c shows the CL intensities of both as-prepared and S7 samples of CGS/Eu3+ nanorod bundles as a function of accelerating voltage and filament current. From Figure 10b, it is evident that, when the filament current is fixed at 55 μA, the CL intensity of both as-prepared and S7 nanorod bundles increases with an increase in the accelerating voltage from 1 to 5 kV and no saturation was observed up to 5 kV. Figure 10c shows that the CL intensity increases for the both samples by increasing the filament current from 35 to 55 μA under a fixed accelerating voltage of 5 kV. The behavior of increased CL intensity with increasing accelerating voltage and filament current is a good indication that these nanorod bundles may be used in FED systems. At the initial level (1−3 kV), the CL intensity of both samples increases linearly with an increase in the accelerating voltage, and with a further increase of accelerating voltage above 3 kV, the CL intensity increases rapidly for both samples. The reason is that the CL emission of activator from the boundaries or surface of the nanoparticles is weaker than that of the inside of the particles at below 3 kV due to surface states that may lead to nonradiative decay of the excited electrons. The rapid increase of CL intensity above 3 kV can be explained by the fact that more plasma will be produced due to the heavier electron penetration depth by the recombination of more electron−hole (excitons) pairs, resulting in more Eu3+ ions from the boundary or surface including inside of nanoparticles being excited.48

two 5D0 → 7F0 emission bands is that there are two low symmetry sites in oxyapatite, that is, 9-fold coordinated 4f sites with C3 point symmetry and 7-fold coordinated 6h sites with Cs point symmetry. Since the 5D0 → 7F0 transition is forbidden both as a magnetic dipole and an electric dipole, and it is often very weak or altogether absent, but both C3 and Cs sites in this crystal have low symmetry, which relaxes the selection rules. These results suggest a strong contribution to overall luminescence from Eu3+ ions on both the sites. As the annealing temperature increases from 300 °C, the intensity of the 5D0 → 7F2 transition increases greatly and the intensity of 5D0 → 7F0 transitions reaches the intensity of 5D0 → 7F1 transitions. The intensity ratio of 5D0 → 7F2 (red) to 5D0 → 7F1 (orange), also called the asymmetric ratio (R), is close to being related to the local environment of Eu3+. Generally, the larger the intensity ratio of 5D0 → 7F2 to 5D0 → 7F1, lower the local symmetry.47 The asymmetric ratios of CGS/Eu3+ nanorod bundles at various annealing temperatures were also calculated, and the results are shown in Table 2. The obtained result shows that the asymmetric ratio increases greatly with increasing annealing temperature up to 700 °C due to the increase of crystallite size, which confirms the decrease in local symmetry and hence an increase in red emission. The annealing temperature increases to 900 °C, the asymmetry ratio slightly decreases as compared to 700 °C due to the merging of CGS nanorods, leading to an increase in local symmetry. The corresponding Commission International De l'Eclairage (CIE) chromaticity coordinates for CGS/Eu3+ nanorod bundles at different annealing temperatures were calculated and indicated in Figure 9b and also listed in Table 2. The CIE chromaticity coordinates clearly indicate that the red color purity increases from yellowish warm white light with increasing the crystallanity by increasing the annealing temperature, and these results are in good agreement with Wang et al.’s report.47 The CIE chromaticity coordinates of the sample annealing at 700 °C are in close proximity to the commercially available Y2O3/Eu3+ phosphor. The relationship between annealing temperature, crystallite size, and emission intensity is also shown in Figure S8 (Supporting Information). Cathodoluminescence Properties. The cathodoluminescence (CL) properties of individual CGS/Eu3+nanorod bundles have been investigated to explore their potentiality in the development of efficient red region for FED systems. Figure 10a shows the CL spectrum of 6 mol % Eu3+ activated CGS

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SUMMARY

For the first time, oxyapatite CGS nanostructures with nanorod bundle-like morphology were successfully prepared by mixed solvothermal and hydrothermal reaction methods. The XRD patterns exhibited hexagonal structure of CGS nanorod bundles, and the FTIR spectra confirmed their oxyapatite nature. From the SEM and TEM measurements, it was confirmed that the CGS nanorod bundles were formed by crystal splitting. The growth mechanism was discussed as a function of reaction time. The size and crystal splitting of the nanorod bundles can be controlled by varying the concentration of 2-propanol. The annealing temperature effect on the morphology of CGS nanorod bundles was also examined, showing that the bundles were not broken even at a very high temperature. The PL and CL studies were performed by activating the Eu3+ ions in the CGS host lattice as a function of annealing temperature. From the PL and CL studies, the Eu3+ emission coming from both C3 and Cs sites of the CGS host lattice indicated that the Eu3+ ions occupied both sites. The corresponding CIE chromaticity coordinates are very close proximity to the commercial red chromaticity coordinates. From the above studies, we are able to conclude that the CGS nanorod bundles are promising materials for their applications in the development of novel optical systems and biological probes or labels.

Figure 10. (a) CL spectrum of CGS nanorod bundle (annealed at 700 °C) and CL intensity comparison as a function of accelerating voltage (b), as a function of filament current (c). 968



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ASSOCIATED CONTENT

S Supporting Information *

Additional data for clear understanding of the crystal splitting, tips of the nanorod bundles shown at different directions of the TEM image, magnified SEM images, PL spectra at different excitation wavelengths, relationship between crystallite size, emission intensity, and annealing temperature, and characterization techniques used for the prepared samples. This information is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.S.Y.), [email protected] (B.M.).

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ACKNOWLEDGMENTS This work was supported by Basic Science Research Program through the NRF funded by the MEST (No. 2010-0025071). REFERENCES

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Formation of Ca2Gd8(SiO4)6O2 Nanorod Bundles Based on Crystal

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