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Improved luminescent properties of novel nanostructured Eu3+ doped yttrium borate synthesized with carbon nanotube templates
Journal of Alloys and Compounds 584 (2014) 471–476
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Improved luminescent properties of novel nanostructured Eu3+ doped yttrium borate synthesized with carbon nanotube templates D. Zou, Y.Q. Ma ⇑, S.B. Qian, B.T. Huang, G.H. Zheng, Z.X. Dai Anhui Key Laboratory of Information Materials and Devices, School of Physics and Materials Science, Anhui University, Hefei 230039, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 11 July 2013 Received in revised form 7 September 2013 Accepted 11 September 2013 Available online 19 September 2013 Keywords: Luminescence Carbon nanotube template Color purity
a b s t r a c t Eu3+-doped yttrium borate samples were prepared via a hydrothermal method using borate or tributyl borate as B3+ sources, with and without carbon nanotubes as templates. The morphology of the samples is easily altered by varying the carbon nanotube template, B3+ source and post-annealing treatment. Photoluminescence measurements indicate that the color purity of samples is clearly improved by templateassisted synthesis. Furthermore, with tributyl borate as the B3+ source, CIE coordinates of (0.67, 0.33), very similar to the (0.665, 0.334) coordinates for the commercial red phosphor Y2O2S:Eu3+, can be obtained. Finally, an extra Eu3+O2 excitation band is observed for samples made by template-assisted synthesis. The underlying mechanism is discussed in detail. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Recently, luminescent materials containing rare earth ions have attracted great interest since they can be potential candidates for many applications in white LED, fluorescent lamps, display devices, solid-state laser, biological labeling and so on [1–5]. As one of the best red phosphors to be widely used in plasma display panels (PDPs), YBO3:Eu3+ has become a focus of research because of its low toxicity, strong luminescence intensity, high chemical stability, vacuum ultraviolet (VUV) transparency, and exceptional optical damage threshold [6–14]. In the last decades, much attention has been paid to nanostructured luminescent materials, because on the one hand they exhibit novel properties that depend on their size and morphology [15– 18], and on the other hand they generate technological interest because nanosized phosphors are expected to improve display resolution and enable other applications [19,20]. For example, it was reported that the Eu3+O2 charge-transfer excitation bands of YBO3:Eu3+ in nanowires and nanotubes are blue-shifted in contrast to those in the bulk; this effect was attributed to variation in the coordination environments [21,22]. The fluorescence intensity and color purity were also better in nanosized YBO3:Eu3+ [23,24]. These results indicate that nanosized YBO3:Eu3+ is promising for applications in PDPs and optical devices. In YBO3:Eu3+, the luminescence originating from transitions between 4f levels of Eu3+ is predominantly because of electric dipole (5D07F2) and/or magnetic dipole (5D07F1) transitions [25–28].
⇑ Corresponding author. Tel./fax: +86 551 63861820. E-mail address: [email protected] (Y.Q. Ma). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.09.065
The intensity of electric dipole transitions depends strongly on the site symmetry in the host crystal. Magnetic dipole f–f transitions are not greatly affected by the site symmetry, because they are parity-allowed. The intensities of different 5D07FJ transitions and the splittings of these emission peaks depend on the local symmetry of the crystal field of the Eu3+ ion. If the Eu3+ ion occupies a centrosymmetric site in the crystal lattice, the magnetic dipole transition 5D07F1 (orange) is the dominant transition; otherwise, the electric dipole transition 5D07F2 (red) becomes dominant. YBO3:Eu3+ possesses a hexagonal vaterite-type structure and the Eu3+ ions occupy the Y3+ site which has point symmetry S6. Generally, the orange emission at 592 nm from the transition 5D07F1 is dominant [29], which results in a lower value of intensity ratio, R/ O, between red and orange emission. Good color purity requires a high R/O value, and thus many studies have attempted to improve R/O. For instance, R/O values of 0.81, 0.78, 0.74 and 0.69 were found for YBO3:Eu3+ hydrothermally synthesized at temperatures of 200, 220, 240, and 260 °C, respectively [30]. For YBO3:Eu3+ synthesized by a more facile sol–gel pyrolysis process, the R/O value changed from 0.577 to 2.761 with decreasing particle size [31]. Based on the previous reports, the R/O value increases as the particle size is reduced; this results from a surface effect, as is frequently found for nanoparticles [29,31]. In this work, Eu3+-doped yttrium borate samples were prepared by a hydrothermal method. Borate and tributyl borate were used as B3+ sources. COOH-modified multiwalled carbon nanotubes (hereafter referred to as CNT) were used as templates during synthesis to increase the surface area of the samples. The CNT templates greatly improve the color purity and cause other new phenomena because of surface effects.
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2. Experimental The starting materials included yttrium nitrate hexahydrate (Y(NO3)36H2O), Eu2O3 (99.99%), nitric acid (HNO3), aqueous ammonia (NH3H2O), ethanol (C2H5OH), sodium dodecyl benzene sulfonate (SDBS) and deionized water. Tributyl borate (C12H27BO3) and boric acid (H3BO3) were used as B3+ sources for preparing YBO3:Eu3+. Eu2O3 was dissolved in HNO3. Y(NO3)36H2O and H3BO3 were dissolved in deionized water with stirring for 10 min to form separate solutions. Stoichiometric amounts of Eu(NO3)3, Y(NO3)3 and H3BO3 solutions were mixed with continuous stirring. The obtained solution was divided into two parts (solutions I and II). Appropriate amounts of CNTs and SDBS (as a dispersing agent) were dispersed in deionized water and sonicated until a homogeneous suspension was obtained; this suspension was added to solution II. Next, an appropriate amount of C2H5OH was added into solutions I and II, and NH3H2O was added to adjust their pH values to 8. Finally, the two precursor solutions were placed in a 50 mL capacity, Teflon-lined stainless-steel autoclave and heated at 180 °C for 12 h. After the autoclave was cooled to room temperature naturally, the products were separated by centrifugation, washed with ethanol and deionized water several times, and dried at 80 °C to obtain precursor powders. The precursor powders were subsequently sintered at different temperatures to obtain the final samples. The samples obtained from precursor solutions I and II are discussed in Sections 3.1 and 3.2, respectively. For comparison, a sample was also prepared using C12H27BO3 (instead of H3BO3) as the B3+ source (solution III) with the other processing conditions the same as those used for solution II. The samples obtained from this solution are discussed in Section 3.3. The crystal structure of the products was characterized by X-ray diffraction using an X-ray diffractometer (XRD; DX-2000 SSC) with Cu Ka radiation (k = 1.5418 Å) over a scanning range of 15–85° with a step of 0.02°. Scanning electron microscopy (SEM, S-4800, Hitachi) and (high resolution) transmission electron microscopy ((HR) TEM, JEOL JEM-2100) were used to observe the morphologies and microstructures. The excitation and emission spectra were measured on a FL fluorescence spectrophotometer (F-4500, Hitachi). All the measurements were carried out at room temperature.
3. Results and discussion 3.1. Structure, morphology and luminescence of samples obtained from solution I Fig. 1. shows the XRD patterns of samples (a) as-prepared at 180 °C and (b) annealed at 1000 °C. The diffraction peak positions and the relative intensities are well matched with those of the standard JCPDS card (No. 16-0277) for YBO3, which has a hexagonal vaterite-type structure with space group P63/m (176). Consistent with previous reports, no secondary phases were detected [13]. Thus single phase YBO3:Eu3+ can be obtained at 180 °C by the hydrothermal method. The crystallite sizes, calculated by MDI Jade 5.0 software, are about 19 and 40 nm for the as-prepared and annealed samples, respectively. Furthermore, the lattice parameters of the hexagonal structure YBO3:Eu3+ can be calculated 2 2 2 2 Þ h from: 4 sin ¼ 4ðh þhkþk þ cl 2 , where a and c are the lattice parame3a2 k2 ters, h the diffraction angle, k the wavelength of the Cu Ka
Fig. 1. XRD patterns of samples from solution I: (a) as-prepared at 180 °C and (b) after annealing at 1000 °C.
radiation, and (h k l) the crystal plane index of the particular peak. The obtained lattice parameters are a = 0.3779 nm, c = 0.8865 nm for the as-prepared sample and a = 0.3783 nm, c = 0.8819 nm for the annealed sample. Fig. 2 shows low and high magnification SEM images of the asprepared sample (a and b) and the sample annealed at 1000 °C (c and d). The as-prepared sample is composed of nearly spherical particles with diameters of 5–10 lm, as shown in Fig. 2a. The high magnification SEM image (Fig. 2b) shows that each nearly-spherical particle is made up of loosely agglomerated nano-flakes, which extend outward from the center of the microstructure to form a flower-like structure. This structure has also been observed in YBO3:Tb3+, and is believed to arise from van der Waals attractions [32]. The growth mechanism has been suggested by Tang et al. [33]. After annealing at 1000 °C, some residual inorganic and organic components decompose during the calcination process, and the nano-flakes appear thicker; large cracks also appear at the surfaces of the nearly-spherical particles. Fig. 3 shows the excitation spectra measured by monitoring the 627 nm (kem = 627 nm) emission of Eu3+ (a and c) and emission spectra measured under kex = 247 nm (b and d) for the as-prepared and annealed samples. The excitation spectrum of the as-prepared sample (Fig. 3a) shows a peak at 208 nm, assigned to the charge transfer transition between Y3+ and O2. In the longer wavelength range, there is a broad excitation band at 247 nm, resulting from the charge transfer transition of Eu3+, i.e., the electronic transition from the 2p orbital of O2 to the 4f orbital of Eu3+ [21]. The excitation intensity of Eu3+O2 is much weaker than that of Y3+O2, which can be understood as follows: in YBO3, the trivalent yttrium ion is 8-fold coordinated by oxygen, i.e., the structure comprises YO8 polyhedra, which are more or less distorted from the ideal S6 point symmetry [34]. For Eu3+ doped YBO3, the Eu3+ substituted for Y3+ should occupy the interstitial site of the oxygen octahedra. For the as-prepared sample at 180 °C, we suggest that not all of Eu3+ ions could enter the interstitial site of the oxygen octahedra because the ionic radius of Eu3+ (0.95 Å) is larger than that of Y3+ (0.89 Å). Therefore, the excitation intensity of Eu3+O2 is weaker than that of Y3+O2. The emission spectrum of the as-prepared sample (Fig. 3b) has sharp lines at 594 (5D07F1), 615 and 627 (5D07F2), 650 and 675 (5D07F3), and 707 (5D07F4) nm, resulting from the emission of Eu3+ due to the 5D07FJ (J = 1, 2, 3 and 4)
Fig. 2. SEM images of samples from solution I: (a and b) as-prepared at 180 °C and (c and d) after annealing at 1000 °C.
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Fig. 3. (a and c) Excitation spectra measured by monitoring the 627 nm (kem = 627 nm) emission of Eu3+ and (b and d) emission spectra measured under kex = 247 nm for samples from solution I. (a and b) as-prepared at 180 °C and (c and d) after annealing at 1000 °C.
transitions. The most intense red emission (R) at 627 nm results from the electric dipole transition 5D07F2, while the orange emission (O) at 594 nm results from a typical magnetic-dipole transition 5D07F1. The emission intensity ratio between red (627 nm) and orange (594 nm), R/O, is 1.40. The emission color purity can also be expressed by CIE coordinates (x, y). The calculated CIE chromaticity coordinates (x, y) are (0.65, 0.35). For the sample annealed at 1000 °C, the excitation band of Eu3+O2 is much stronger than that of Y3+O2, as shown in Fig. 3c. We suggest that more Eu3+ ions enter the interstitial sites of the oxygen octahedra to form more Eu3+O2 bonds, leading to a stronger Eu3+O2 excitation band. The emission spectrum (in Fig. 3d) of the annealed sample has similar features to that of the as-prepared sample (Fig. 3b), and the origins of the features are the same; however the emission intensity is increased approximately fivefold upon annealing. Additionally, the R/O ratio is 1.1, smaller than that of the as-prepared sample (1.4), and the CIE coordinates become (0.655, 0.346). The change in luminescence upon annealing can be understood as follows: (1) The enhanced emission can be attributed to the better crystallization after annealing; this is supported by the XRD results, which show increased crystallite size. (2) As mentioned above, annealing at 1000 °C enables more Eu3+ ions to enter the Y3+ lattice site which possesses inversion symmetry. Therefore, the orange emission should be enhanced, leading to the lower R/O value.
Fig. 4. (a) XRD patterns of samples from solution II: as-prepared at 180 °C and after annealing at 500, 600, 700, 800 or 1000 °C; (b) TEM and (c) SEM images for samples as-prepared at 180 °C and after annealing at 1000 °C; inset in (c) shows the hollow structure.
a SEM image of the sample after annealing at 1000 °C. The dispersed nanowires of Fig. 4b came together to form dendritic structures because some residual organic components and CNTs were decomposed or burned out, and the branches making up the dendrites are 50 nm in diameter. Fig. 5 shows the excitation spectra measured by monitoring the 627 nm (kem = 627 nm) emission of Eu3+ (a) and emission spectra measured under kex = 247 nm (b) for samples as-prepared from solution II. The wavelengths of excitation and emission peaks (and their corresponding transitions), R/O ratios and CIE coordinates of the samples are listed in Table 1. As shown in Fig. 5a, the as-prepared sample does not exhibit any clear excitation peak. After the sample is annealed at 500 °C, a distinct peak appears at 214 nm, which is assigned to the Y3+O2 excitation. After annealing above 600 °C, the spectra also show a kink around 247 nm, assigned to the Eu3+O2 excitation. As well as this, another excitation band occurs in the longer wavelength range of 270–290 nm; as far as we know, this band has not previously been reported. The underlying mechanism for such an excitation band can be understood as follows: Compared with the sample obtained from solution I (Fig. 2), the sample prepared from solution II contains
3.2. Structure, morphology and luminescence of samples obtained from solution II Fig. 4a shows the XRD patterns of the samples as-prepared at 180 °C and annealed at 500, 600, 700, 800 or 1000 °C. We can see that single phase YBO3:Eu3+ was not obtained at 180 °C in the presence of CNT templates. An amorphous phase is detected in the samples annealed at 500 and 600 °C, with only two broad bands observed around 2h = 30° and 50°. Treatment at 700 °C is clearly required to initiate crystallization — diffraction peaks of YBO3:Eu3+ began to appear at 700 °C, as indexed in Fig. 4a. Besides the YBO3:Eu3+, some other peaks correspond to Y3BO6:Eu3+ phase, which has a monoclinic structure with space group C2/m(12), according to JCPDS card 34-0291. Fig. 4b shows a TEM image of the sample as-prepared at 180 °C, which exhibits long, uniform and well-dispersed nanowires, about 50 nm in diameter, in contrast to the microspheres observed in Fig. 2. This indicates that the CNT templates essentially determined the shape produced. Fig. 4c shows
Fig. 5. (a) Excitation spectra measured by monitoring the 627 nm (kem = 627 nm) emission of Eu3+ and (b) emission spectra measured under kex = 247 nm for samples from solution II: as-prepared at 180 °C and after annealing at 500, 600, 700, 800 or 1000 °C.
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Table 1 Wavelengths of excitation and emission peaks (and the corresponding transitions), R/O ratios and CIE coordinates for samples from solution II. Processing temperatures Excitation bands(nm) Emission bands(nm) (Transitions) R/Oa CIE (x, y)
180 °C
500 °C 214
597(5D0–7F1) 623(5D0–7F2) 704(5D0–7F4) 1.36 (0.46, 0.51)
5
7
628( D0– F2) 706(5D0–7F4) 2.90 (0.57, 0.42)
600 °C
700 °C
800 °C
214, 247, 272 598(5D0–7F1) 621(5D0–7F2) 705(5D0–7F4) 3.56 (0.61, 0.38)
214, 247, 272 594(5D0–7F1) 625(5D0–7F2) 706(5D0–7F4) 2.48 (0.64, 0.36)
214, 247, 282 5
1000 °C
7
630( D0– F2) 709(5D0–7F4) 4.2 (0.66, 0.34)
214, 247, 287 629(5D0–7F2) 708(5D0–7F4) 4.73 (0.66, 0.34)
a For samples annealed at 500, 800 and 1000 °C, the 5D0–7F1 transitions did not generate any peaks; instead, shoulders around 597 nm were seen – we used the intensities at 597 nm to calculate the R/O ratios for these samples.
much smaller particles, as shown in Fig. 4b and c. Furthermore, after annealing at higher temperature, the nanoparticles show a hollow tube-like structure, as shown in the inset of Fig. 4c; such a structure has both an inside and outside surface. In other words, these samples have larger surface-to-volume ratios than those of the samples in section 3.1. This means that more Eu3+ ions reside at the surface of the nanoparticles, leading to the Eu3+O2 excitation band seen above 270 nm. As shown in Fig. 5b, the emission intensity first increases with increasing temperature, and the sample annealed at 800 °C exhibits the most intense emission. Annealing at 1000 °C weakens the emission, as previously reported [26,35]. This trend can be explained as follows: with increasing annealing temperature, the crystallization of the samples becomes better, resulting in a decrease in the number of fluorescence quench centers (i.e., crystal defects) and leading to a decrease in non-radiative recombination, consequently enhancing the emission intensity. However, with further increase in annealing temperature, small particles will come together and form larger particles, meaning that the available cross-section for receiving excitation light becomes smaller, and consequently resulting in a decrease in luminescence intensity. Furthermore, increasing annealing temperature results in the diffusion of dopants in the lattice, and this also affects the emission intensity. Compared with the results in Fig. 3, novel luminescent properties are observed for the samples obtained from solution II. They are summarized as follows: (1) the two separate red emissions from 5D07F2 transitions in Fig. 3. merge into one peak, located at 620–630 nm in Fig. 5; (2) the R/O ratio becomes much larger, with a maximum of 4.73 for the sample annealed at 1000 °C; and (3) the CIE coordinates change slightly from (0.655, 0.346) to (0.66, 0.34) for the samples annealed at 1000 °C, which is closer to the (0.665, 0.334) for commercial red phosphor Y2O2S:Eu3+ [36], thus satisfying the requirement for red phosphors. As mentioned above, the 5D07F2 transition is a hypersensitive (forced dielectric dipole) transition with the selection rule DJ = 2, and therefore is strongly influenced by the surrounding environment. When Eu3+ is located at a low-symmetry local site (without an inversion center), the emission from the 5D07F2 transition often dominates the emission spectra; this has been observed in many other host materials [37–40]. However, the reason for the low-symmetry environment surrounding Eu3+ in the present samples is somewhat different from that in previously reported samples (where the low-symmetry environment is because of the symmetry of the crystal lattice itself). In both pure YBO3:Eu3+ and Y3BO6:Eu3+, the emissions owing to the 5D07F2 transitions are usually split into several peaks [24–28]. The present sample contains both Y3BO6:Eu3+ and YBO3:Eu3+, but no splittings are observed for the 5D07F2 transition. Therefore we suggest that the dominant red emission from the 5D07F2 transition is not governed by the symmetry of the crystal lattice itself. Similar phenomena were also observed in nanosized YBO3:Eu3+ synthesized by co-precipitation [24] and solvothermal [29] methods. The abnormal luminescent behavior was assigned to a surface effect [29]. As
shown in Fig. 4c and its inset, the addition of CNTs reduces the particle size to the nanometer scale, and the nanoparticles show a hollow tube-like structure after annealing at higher temperature. Their small size and hollow structure increase their effective surface area. Many atoms at the nanoparticle surface cannot be completely bound, leading to numerous defects on the surface. These defects may increase the degree of disorder in the crystal field symmetry, and lower the local symmetry of the Eu3+ ions. This then increases the probability of the 5D07F2 red-emission transition, meaning that this transition dominates the emission spectra, resulting in a higher R/O value, as listed in Table 1. 3.3. Structure, morphology and luminescence of samples obtained from solution III Fig. 6 shows the XRD patterns of samples prepared from solution III, both as-prepared at 180 °C and annealed at 500, 600, 700, 800 or 1000 °C. The crystallization of the samples is similar to that observed in Fig. 4a, except for the difference that the sample prepared from solution III and annealed at 700 °C was almost single phase YBO3:Eu3+, indicating that using tributyl borate as the B3+ source groups on the outside of the carbon nanotube templates can be esterified by tributyl borate, as reported before [41], producing surface-esterified carbon nanotube templates. The precipitate containing Y3+, Eu3+ and B3+ may more easily and uniformly form on such surface-esterified templates, making it easier for YBO3:Eu3+ to crystallize. As shown in Fig. 7a, the as-prepared sample exhibits similar morphology to that shown in Fig. 4b. Fig. 7b shows that the CNTs are coated uniformly to give an outer diameter of about 50 nm. Fig. 7c shows a SEM image of the sample annealed at 1000 °C. This sample is made up of large honeycomb-like particles with many pores within their surfaces. The magnified SEM image, shown in Fig. 7d, reveals that the honeycomb-like particles consist of numerous fine particles, each several tens to hundreds of nanometers
Fig. 6. XRD patterns of samples from solution III: as-prepared at 180 °C and annealed at 500, 600, 700, 800 or 1000 °C.
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Fig. 7. (a and b) TEM images of sample hydrothermally prepared from solution III at 180 °C and (c and d). SEM images for sample annealed at 1000 °C.
across. This sample has a distinctly different morphology to that of the sample prepared using borate as the B3+ source. As suggested above, the carboxyl-modified carbon nanotube template becomes surface-esterified when tributyl borate is used as the B3+ source. Compared with the bare carboxyl-modified template, the surface-esterified template will release more CO2 and H2O during decomposition, leading to the honeycomb-like particles. Fig. 8 shows the excitation spectra measured by monitoring the 627 nm (kem = 627 nm) emission of Eu3+ (a) and the emission spectra measured under kex = 247 nm (b) for samples as-prepared at 180 °C and annealed at 500, 600, 700, 800 or 1000 °C. The wavelengths of the main excitation and emission peaks (and their corresponding transitions), values of R/O ratio and CIE coordinates are listed in Table 2.
Fig. 8. (a) Excitation spectra measured by monitoring the 627 nm emission of Eu3+ and (b) emission spectra measured under kex = 247 nm of the samples from solution III, as-prepared at 180 °C and annealed at 500, 600, 700, 800 or 1000 °C.
The spectral features in Fig. 8. and the underlying mechanism are the same as those discussed in Fig. 5. When we consider these results along with those of Section 3.2, we note that: (1) The position of the excitation bands around 247 nm hardly changes while the band in the region 270–290 nm clearly varies with annealing temperature, as shown in Tables 1 and 2. This is consistent with the excitation bands around 247 nm resulting from the Eu3+O2 excitation inside the lattice and those around 270–290 nm resulting from the Eu3+O2 excitation at the surfaces of the nanoparticles. The position of the excitation band from the Eu3+O2 excitation at the surfaces of the nanoparticles would be expected to change with annealing temperature, because annealing temperature may alter the distribution and coordination number of the surface Eu3+. However, the Eu3+ inside the lattice stably occupies the Y3+ sites, and therefore the position of the excitation band around 247 nm does not alter significantly with annealing. (2) The intensities of both excitation and emission in Figs. 5 and 8 show an abrupt increase at an annealing temperature of 700 °C; this is because the sample starts to crystallize at this temperature, as evidenced by the XRD patterns. (3) The sample annealed at 700 °C is a single phase YBO3:Eu3+, which should have a lower R/ O value according to the discussion in Section 3.1 and in other previous reports [24,26,27]. However, it has the maximum R/O value of 4.06. This phenomenon further confirms that the disorder in crystal field symmetry at the surfaces of the nanoparticles lowers the local symmetry of the Eu3+ ions, and consequently enhances the 5D07F2 red-emission, leading to the higher R/O value. (4) The CIE coordinates of samples annealed at 800 and 1000 °C are both (0.67, 0.33), almost the same as that (0.665, 0.334) for the commercial red phosphor Y2O2S:Eu3+, indicating that the sample made using tributyl borate as the B3+ source has better color purity. 4. Conclusions Three groups of samples were prepared by hydrothermal methods and subsequently post-annealed at different temperatures. Using borate as the B3+ source, the first and second groups were synthesized without and with carbon nanotube templates, respectively. The third group was prepared using tributyl borate as the B3+ source and carbon nanotube templates. The samples obtained from the first group after initial synthesis at 180 °C contained single phase YBO3:Eu3+ and were microspheres of diameter 5–10 lm. The intensity ratios between red and orange emission were 1.4 and 1.1 and the CIE coordinates were (0.65, 0.35) and (0.665, 0.346), for the samples as-prepared at 180 °C and after annealing at 1000 °C, respectively. The samples from the second group began to crystallize after annealing at 700 °C and contained YBO3:Eu3+ and Y3BO6:Eu3+ phases. The sample as-prepared at 180 °C exhibited long, uniform and well-dispersed nanowires with diameters of about 50 nm; these transformed into a dendritic structure after annealing at 1000 °C. The maximum value of intensity ratio between red and orange emission was obtained, 4.73, indicating that color purity was improved by addition of carbon nanotubes to the precursor solution.
Table 2 Wavelengths of excitation and emission peaks (and the corresponding transitions), R/O ratios and CIE coordinates for samples from solution III. Processing temperatures
180 °C
500 °C
600 °C
700 °C
800 °C
1000 °C
Excitation bands(nm) Emission bands(nm)
214 597(5D0–7F1) 619(5D0–7F2) 700(5D0–7F4) 1.55 (0.61, 0.39)
214, 272 599(5D0–7F1) 621(5D0–7F2) 704(5D0–7F4) 3.35 (0.65, 0.35)
214, 272 599(5D0–7F1) 621(5D0–7F2) 706(5D0–7F4) 3.46 (0.65, 0.35)
214, 247 595(5D0–7F1) 625(5D0–7F2) 708(5D0–7F4) 4.06 (0.66, 0.34)
214, 247, 275 594(5D0–7F1) 613(5D0–7F2) 707(5D0–7F4) 3.07 (0.67, 0.33)
214, 247, 281 595(5D0–7F1) 626(5D0–7F2) 707(5D0–7F4) 3.66 (0.67, 0.33)
(Transitions) R/O CIE (x, y)
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The third group of samples also crystallized at 700 °C, but formed nearly single-phase YBO3:Eu3+. Annealed at temperatures over 700 °C produced samples containing both YBO3:Eu3+ and Y3BO6:Eu3 phases. The as-prepared sample again consisted of well-dispersed nanowires, while the sample annealed at 1000 °C exhibited a different, honeycomb-like morphology. The single phase YBO3:Eu3+ annealed at 700 °C had a much larger intensity ratio between red and orange emission, 4.06, than previously reported for YBO3:Eu3+. Furthermore, the optimal CIE coordinates were obtained, (0.67, 0.33), almost the same as the (0.665, 0.334) of commercial red phosphor Y2O2S:Eu3+, indicating that the sample using tributyl borate as the B3+ source has better color purity. Additionally, we observed the Eu3+O2 excitation band from the nanoparticles’ surface; and excitation band position changed in the region of 270–290 nm with the annealing temperature, because annealing at different temperature resulted in the different distribution and coordination number of Eu3+. Acknowledgment This work was supported by the National Natural Science Foundation of China (Grant Nos. 11174004 and 50901074). References [1] Q.L. Xiao, J.X. Meng, J.R. Qiu, J. Alloys Comp. 574 (2013) 600–603. [2] L. Chen, A.Q. Luo, Y. Zhang, X.H. Chen, H. Liu, Y. Jiang, S.F. Chen, K.J. Chen, H.C. Kuo, Y. Tao, G.B. Zhang, J. Solid State Chem. 201 (2013) 229–236. [3] Y.H. Fu, H.F. Jiu, L.X. Zhang, Y.X. Sun, Y.Z. Wang, Mater. Lett. 91 (2013) 265– 267. [4] X.L. Wu, J.G. Li, D.H. Ping, J.K. Li, Q. Zhu, X.D. Li, X.D. Sun, Y. Sakka, J. Alloys Comp. 559 (2013) 188–195. [5] J. Huang, Y.H. Song, G.W. Wang, Y. Sheng, K.Y. Zheng, H.B. Li, H.G. Zhang, Q.S. Huo, X.C. Xu, H.F. Zou, J. Alloys Comp. 574 (2013) 310–315. [6] Y. Zhang, Y.D. Li, J. Alloys Comp. 384 (2004) 88–92. [7] Y. Zhang, Y.D. Li, Y.S. Yin, J. Alloys Comp. 400 (2005) 222–226. [8] L. He, Y.H. Wang, J. Alloys Comp. 454 (2008) 250–254. [9] G. Bertrand-Chadeyron, M. El-Ghozzi, D. Boyer, R. Mahiou, J.C. Cousseins, J. Alloys Comp. 317–318 (2001) 183–185. [10] R. Balakrishnaiah, S.S. Yi, K. Jang, H.S. Lee, B.K. Moon, J.H. Jeong, Mater. Res. Bull. 46 (2011) 621–626. [11] B. Moine, J. Mugnier, D. Boyer, R. Mahiou, S. Schamm, G. Zanchi, J. Alloys Comp. 323–324 (2001) 816–819.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Improved luminescent properties of novel nanostructured Eu3+ doped yttrium borate synthesized with carbon nanotube templates D. Zou, Y.Q. Ma ⇑, S.B. Qian, B.T. Huang, G.H. Zheng, Z.X. Dai Anhui Key Laboratory of Information Materials and Devices, School of Physics and Materials Science, Anhui University, Hefei 230039, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 11 July 2013 Received in revised form 7 September 2013 Accepted 11 September 2013 Available online 19 September 2013 Keywords: Luminescence Carbon nanotube template Color purity
a b s t r a c t Eu3+-doped yttrium borate samples were prepared via a hydrothermal method using borate or tributyl borate as B3+ sources, with and without carbon nanotubes as templates. The morphology of the samples is easily altered by varying the carbon nanotube template, B3+ source and post-annealing treatment. Photoluminescence measurements indicate that the color purity of samples is clearly improved by templateassisted synthesis. Furthermore, with tributyl borate as the B3+ source, CIE coordinates of (0.67, 0.33), very similar to the (0.665, 0.334) coordinates for the commercial red phosphor Y2O2S:Eu3+, can be obtained. Finally, an extra Eu3+O2 excitation band is observed for samples made by template-assisted synthesis. The underlying mechanism is discussed in detail. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Recently, luminescent materials containing rare earth ions have attracted great interest since they can be potential candidates for many applications in white LED, fluorescent lamps, display devices, solid-state laser, biological labeling and so on [1–5]. As one of the best red phosphors to be widely used in plasma display panels (PDPs), YBO3:Eu3+ has become a focus of research because of its low toxicity, strong luminescence intensity, high chemical stability, vacuum ultraviolet (VUV) transparency, and exceptional optical damage threshold [6–14]. In the last decades, much attention has been paid to nanostructured luminescent materials, because on the one hand they exhibit novel properties that depend on their size and morphology [15– 18], and on the other hand they generate technological interest because nanosized phosphors are expected to improve display resolution and enable other applications [19,20]. For example, it was reported that the Eu3+O2 charge-transfer excitation bands of YBO3:Eu3+ in nanowires and nanotubes are blue-shifted in contrast to those in the bulk; this effect was attributed to variation in the coordination environments [21,22]. The fluorescence intensity and color purity were also better in nanosized YBO3:Eu3+ [23,24]. These results indicate that nanosized YBO3:Eu3+ is promising for applications in PDPs and optical devices. In YBO3:Eu3+, the luminescence originating from transitions between 4f levels of Eu3+ is predominantly because of electric dipole (5D07F2) and/or magnetic dipole (5D07F1) transitions [25–28].
⇑ Corresponding author. Tel./fax: +86 551 63861820. E-mail address: [email protected] (Y.Q. Ma). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.09.065
The intensity of electric dipole transitions depends strongly on the site symmetry in the host crystal. Magnetic dipole f–f transitions are not greatly affected by the site symmetry, because they are parity-allowed. The intensities of different 5D07FJ transitions and the splittings of these emission peaks depend on the local symmetry of the crystal field of the Eu3+ ion. If the Eu3+ ion occupies a centrosymmetric site in the crystal lattice, the magnetic dipole transition 5D07F1 (orange) is the dominant transition; otherwise, the electric dipole transition 5D07F2 (red) becomes dominant. YBO3:Eu3+ possesses a hexagonal vaterite-type structure and the Eu3+ ions occupy the Y3+ site which has point symmetry S6. Generally, the orange emission at 592 nm from the transition 5D07F1 is dominant [29], which results in a lower value of intensity ratio, R/ O, between red and orange emission. Good color purity requires a high R/O value, and thus many studies have attempted to improve R/O. For instance, R/O values of 0.81, 0.78, 0.74 and 0.69 were found for YBO3:Eu3+ hydrothermally synthesized at temperatures of 200, 220, 240, and 260 °C, respectively [30]. For YBO3:Eu3+ synthesized by a more facile sol–gel pyrolysis process, the R/O value changed from 0.577 to 2.761 with decreasing particle size [31]. Based on the previous reports, the R/O value increases as the particle size is reduced; this results from a surface effect, as is frequently found for nanoparticles [29,31]. In this work, Eu3+-doped yttrium borate samples were prepared by a hydrothermal method. Borate and tributyl borate were used as B3+ sources. COOH-modified multiwalled carbon nanotubes (hereafter referred to as CNT) were used as templates during synthesis to increase the surface area of the samples. The CNT templates greatly improve the color purity and cause other new phenomena because of surface effects.
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2. Experimental The starting materials included yttrium nitrate hexahydrate (Y(NO3)36H2O), Eu2O3 (99.99%), nitric acid (HNO3), aqueous ammonia (NH3H2O), ethanol (C2H5OH), sodium dodecyl benzene sulfonate (SDBS) and deionized water. Tributyl borate (C12H27BO3) and boric acid (H3BO3) were used as B3+ sources for preparing YBO3:Eu3+. Eu2O3 was dissolved in HNO3. Y(NO3)36H2O and H3BO3 were dissolved in deionized water with stirring for 10 min to form separate solutions. Stoichiometric amounts of Eu(NO3)3, Y(NO3)3 and H3BO3 solutions were mixed with continuous stirring. The obtained solution was divided into two parts (solutions I and II). Appropriate amounts of CNTs and SDBS (as a dispersing agent) were dispersed in deionized water and sonicated until a homogeneous suspension was obtained; this suspension was added to solution II. Next, an appropriate amount of C2H5OH was added into solutions I and II, and NH3H2O was added to adjust their pH values to 8. Finally, the two precursor solutions were placed in a 50 mL capacity, Teflon-lined stainless-steel autoclave and heated at 180 °C for 12 h. After the autoclave was cooled to room temperature naturally, the products were separated by centrifugation, washed with ethanol and deionized water several times, and dried at 80 °C to obtain precursor powders. The precursor powders were subsequently sintered at different temperatures to obtain the final samples. The samples obtained from precursor solutions I and II are discussed in Sections 3.1 and 3.2, respectively. For comparison, a sample was also prepared using C12H27BO3 (instead of H3BO3) as the B3+ source (solution III) with the other processing conditions the same as those used for solution II. The samples obtained from this solution are discussed in Section 3.3. The crystal structure of the products was characterized by X-ray diffraction using an X-ray diffractometer (XRD; DX-2000 SSC) with Cu Ka radiation (k = 1.5418 Å) over a scanning range of 15–85° with a step of 0.02°. Scanning electron microscopy (SEM, S-4800, Hitachi) and (high resolution) transmission electron microscopy ((HR) TEM, JEOL JEM-2100) were used to observe the morphologies and microstructures. The excitation and emission spectra were measured on a FL fluorescence spectrophotometer (F-4500, Hitachi). All the measurements were carried out at room temperature.
3. Results and discussion 3.1. Structure, morphology and luminescence of samples obtained from solution I Fig. 1. shows the XRD patterns of samples (a) as-prepared at 180 °C and (b) annealed at 1000 °C. The diffraction peak positions and the relative intensities are well matched with those of the standard JCPDS card (No. 16-0277) for YBO3, which has a hexagonal vaterite-type structure with space group P63/m (176). Consistent with previous reports, no secondary phases were detected [13]. Thus single phase YBO3:Eu3+ can be obtained at 180 °C by the hydrothermal method. The crystallite sizes, calculated by MDI Jade 5.0 software, are about 19 and 40 nm for the as-prepared and annealed samples, respectively. Furthermore, the lattice parameters of the hexagonal structure YBO3:Eu3+ can be calculated 2 2 2 2 Þ h from: 4 sin ¼ 4ðh þhkþk þ cl 2 , where a and c are the lattice parame3a2 k2 ters, h the diffraction angle, k the wavelength of the Cu Ka
Fig. 1. XRD patterns of samples from solution I: (a) as-prepared at 180 °C and (b) after annealing at 1000 °C.
radiation, and (h k l) the crystal plane index of the particular peak. The obtained lattice parameters are a = 0.3779 nm, c = 0.8865 nm for the as-prepared sample and a = 0.3783 nm, c = 0.8819 nm for the annealed sample. Fig. 2 shows low and high magnification SEM images of the asprepared sample (a and b) and the sample annealed at 1000 °C (c and d). The as-prepared sample is composed of nearly spherical particles with diameters of 5–10 lm, as shown in Fig. 2a. The high magnification SEM image (Fig. 2b) shows that each nearly-spherical particle is made up of loosely agglomerated nano-flakes, which extend outward from the center of the microstructure to form a flower-like structure. This structure has also been observed in YBO3:Tb3+, and is believed to arise from van der Waals attractions [32]. The growth mechanism has been suggested by Tang et al. [33]. After annealing at 1000 °C, some residual inorganic and organic components decompose during the calcination process, and the nano-flakes appear thicker; large cracks also appear at the surfaces of the nearly-spherical particles. Fig. 3 shows the excitation spectra measured by monitoring the 627 nm (kem = 627 nm) emission of Eu3+ (a and c) and emission spectra measured under kex = 247 nm (b and d) for the as-prepared and annealed samples. The excitation spectrum of the as-prepared sample (Fig. 3a) shows a peak at 208 nm, assigned to the charge transfer transition between Y3+ and O2. In the longer wavelength range, there is a broad excitation band at 247 nm, resulting from the charge transfer transition of Eu3+, i.e., the electronic transition from the 2p orbital of O2 to the 4f orbital of Eu3+ [21]. The excitation intensity of Eu3+O2 is much weaker than that of Y3+O2, which can be understood as follows: in YBO3, the trivalent yttrium ion is 8-fold coordinated by oxygen, i.e., the structure comprises YO8 polyhedra, which are more or less distorted from the ideal S6 point symmetry [34]. For Eu3+ doped YBO3, the Eu3+ substituted for Y3+ should occupy the interstitial site of the oxygen octahedra. For the as-prepared sample at 180 °C, we suggest that not all of Eu3+ ions could enter the interstitial site of the oxygen octahedra because the ionic radius of Eu3+ (0.95 Å) is larger than that of Y3+ (0.89 Å). Therefore, the excitation intensity of Eu3+O2 is weaker than that of Y3+O2. The emission spectrum of the as-prepared sample (Fig. 3b) has sharp lines at 594 (5D07F1), 615 and 627 (5D07F2), 650 and 675 (5D07F3), and 707 (5D07F4) nm, resulting from the emission of Eu3+ due to the 5D07FJ (J = 1, 2, 3 and 4)
Fig. 2. SEM images of samples from solution I: (a and b) as-prepared at 180 °C and (c and d) after annealing at 1000 °C.
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Fig. 3. (a and c) Excitation spectra measured by monitoring the 627 nm (kem = 627 nm) emission of Eu3+ and (b and d) emission spectra measured under kex = 247 nm for samples from solution I. (a and b) as-prepared at 180 °C and (c and d) after annealing at 1000 °C.
transitions. The most intense red emission (R) at 627 nm results from the electric dipole transition 5D07F2, while the orange emission (O) at 594 nm results from a typical magnetic-dipole transition 5D07F1. The emission intensity ratio between red (627 nm) and orange (594 nm), R/O, is 1.40. The emission color purity can also be expressed by CIE coordinates (x, y). The calculated CIE chromaticity coordinates (x, y) are (0.65, 0.35). For the sample annealed at 1000 °C, the excitation band of Eu3+O2 is much stronger than that of Y3+O2, as shown in Fig. 3c. We suggest that more Eu3+ ions enter the interstitial sites of the oxygen octahedra to form more Eu3+O2 bonds, leading to a stronger Eu3+O2 excitation band. The emission spectrum (in Fig. 3d) of the annealed sample has similar features to that of the as-prepared sample (Fig. 3b), and the origins of the features are the same; however the emission intensity is increased approximately fivefold upon annealing. Additionally, the R/O ratio is 1.1, smaller than that of the as-prepared sample (1.4), and the CIE coordinates become (0.655, 0.346). The change in luminescence upon annealing can be understood as follows: (1) The enhanced emission can be attributed to the better crystallization after annealing; this is supported by the XRD results, which show increased crystallite size. (2) As mentioned above, annealing at 1000 °C enables more Eu3+ ions to enter the Y3+ lattice site which possesses inversion symmetry. Therefore, the orange emission should be enhanced, leading to the lower R/O value.
Fig. 4. (a) XRD patterns of samples from solution II: as-prepared at 180 °C and after annealing at 500, 600, 700, 800 or 1000 °C; (b) TEM and (c) SEM images for samples as-prepared at 180 °C and after annealing at 1000 °C; inset in (c) shows the hollow structure.
a SEM image of the sample after annealing at 1000 °C. The dispersed nanowires of Fig. 4b came together to form dendritic structures because some residual organic components and CNTs were decomposed or burned out, and the branches making up the dendrites are 50 nm in diameter. Fig. 5 shows the excitation spectra measured by monitoring the 627 nm (kem = 627 nm) emission of Eu3+ (a) and emission spectra measured under kex = 247 nm (b) for samples as-prepared from solution II. The wavelengths of excitation and emission peaks (and their corresponding transitions), R/O ratios and CIE coordinates of the samples are listed in Table 1. As shown in Fig. 5a, the as-prepared sample does not exhibit any clear excitation peak. After the sample is annealed at 500 °C, a distinct peak appears at 214 nm, which is assigned to the Y3+O2 excitation. After annealing above 600 °C, the spectra also show a kink around 247 nm, assigned to the Eu3+O2 excitation. As well as this, another excitation band occurs in the longer wavelength range of 270–290 nm; as far as we know, this band has not previously been reported. The underlying mechanism for such an excitation band can be understood as follows: Compared with the sample obtained from solution I (Fig. 2), the sample prepared from solution II contains
3.2. Structure, morphology and luminescence of samples obtained from solution II Fig. 4a shows the XRD patterns of the samples as-prepared at 180 °C and annealed at 500, 600, 700, 800 or 1000 °C. We can see that single phase YBO3:Eu3+ was not obtained at 180 °C in the presence of CNT templates. An amorphous phase is detected in the samples annealed at 500 and 600 °C, with only two broad bands observed around 2h = 30° and 50°. Treatment at 700 °C is clearly required to initiate crystallization — diffraction peaks of YBO3:Eu3+ began to appear at 700 °C, as indexed in Fig. 4a. Besides the YBO3:Eu3+, some other peaks correspond to Y3BO6:Eu3+ phase, which has a monoclinic structure with space group C2/m(12), according to JCPDS card 34-0291. Fig. 4b shows a TEM image of the sample as-prepared at 180 °C, which exhibits long, uniform and well-dispersed nanowires, about 50 nm in diameter, in contrast to the microspheres observed in Fig. 2. This indicates that the CNT templates essentially determined the shape produced. Fig. 4c shows
Fig. 5. (a) Excitation spectra measured by monitoring the 627 nm (kem = 627 nm) emission of Eu3+ and (b) emission spectra measured under kex = 247 nm for samples from solution II: as-prepared at 180 °C and after annealing at 500, 600, 700, 800 or 1000 °C.
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Table 1 Wavelengths of excitation and emission peaks (and the corresponding transitions), R/O ratios and CIE coordinates for samples from solution II. Processing temperatures Excitation bands(nm) Emission bands(nm) (Transitions) R/Oa CIE (x, y)
180 °C
500 °C 214
597(5D0–7F1) 623(5D0–7F2) 704(5D0–7F4) 1.36 (0.46, 0.51)
5
7
628( D0– F2) 706(5D0–7F4) 2.90 (0.57, 0.42)
600 °C
700 °C
800 °C
214, 247, 272 598(5D0–7F1) 621(5D0–7F2) 705(5D0–7F4) 3.56 (0.61, 0.38)
214, 247, 272 594(5D0–7F1) 625(5D0–7F2) 706(5D0–7F4) 2.48 (0.64, 0.36)
214, 247, 282 5
1000 °C
7
630( D0– F2) 709(5D0–7F4) 4.2 (0.66, 0.34)
214, 247, 287 629(5D0–7F2) 708(5D0–7F4) 4.73 (0.66, 0.34)
a For samples annealed at 500, 800 and 1000 °C, the 5D0–7F1 transitions did not generate any peaks; instead, shoulders around 597 nm were seen – we used the intensities at 597 nm to calculate the R/O ratios for these samples.
much smaller particles, as shown in Fig. 4b and c. Furthermore, after annealing at higher temperature, the nanoparticles show a hollow tube-like structure, as shown in the inset of Fig. 4c; such a structure has both an inside and outside surface. In other words, these samples have larger surface-to-volume ratios than those of the samples in section 3.1. This means that more Eu3+ ions reside at the surface of the nanoparticles, leading to the Eu3+O2 excitation band seen above 270 nm. As shown in Fig. 5b, the emission intensity first increases with increasing temperature, and the sample annealed at 800 °C exhibits the most intense emission. Annealing at 1000 °C weakens the emission, as previously reported [26,35]. This trend can be explained as follows: with increasing annealing temperature, the crystallization of the samples becomes better, resulting in a decrease in the number of fluorescence quench centers (i.e., crystal defects) and leading to a decrease in non-radiative recombination, consequently enhancing the emission intensity. However, with further increase in annealing temperature, small particles will come together and form larger particles, meaning that the available cross-section for receiving excitation light becomes smaller, and consequently resulting in a decrease in luminescence intensity. Furthermore, increasing annealing temperature results in the diffusion of dopants in the lattice, and this also affects the emission intensity. Compared with the results in Fig. 3, novel luminescent properties are observed for the samples obtained from solution II. They are summarized as follows: (1) the two separate red emissions from 5D07F2 transitions in Fig. 3. merge into one peak, located at 620–630 nm in Fig. 5; (2) the R/O ratio becomes much larger, with a maximum of 4.73 for the sample annealed at 1000 °C; and (3) the CIE coordinates change slightly from (0.655, 0.346) to (0.66, 0.34) for the samples annealed at 1000 °C, which is closer to the (0.665, 0.334) for commercial red phosphor Y2O2S:Eu3+ [36], thus satisfying the requirement for red phosphors. As mentioned above, the 5D07F2 transition is a hypersensitive (forced dielectric dipole) transition with the selection rule DJ = 2, and therefore is strongly influenced by the surrounding environment. When Eu3+ is located at a low-symmetry local site (without an inversion center), the emission from the 5D07F2 transition often dominates the emission spectra; this has been observed in many other host materials [37–40]. However, the reason for the low-symmetry environment surrounding Eu3+ in the present samples is somewhat different from that in previously reported samples (where the low-symmetry environment is because of the symmetry of the crystal lattice itself). In both pure YBO3:Eu3+ and Y3BO6:Eu3+, the emissions owing to the 5D07F2 transitions are usually split into several peaks [24–28]. The present sample contains both Y3BO6:Eu3+ and YBO3:Eu3+, but no splittings are observed for the 5D07F2 transition. Therefore we suggest that the dominant red emission from the 5D07F2 transition is not governed by the symmetry of the crystal lattice itself. Similar phenomena were also observed in nanosized YBO3:Eu3+ synthesized by co-precipitation [24] and solvothermal [29] methods. The abnormal luminescent behavior was assigned to a surface effect [29]. As
shown in Fig. 4c and its inset, the addition of CNTs reduces the particle size to the nanometer scale, and the nanoparticles show a hollow tube-like structure after annealing at higher temperature. Their small size and hollow structure increase their effective surface area. Many atoms at the nanoparticle surface cannot be completely bound, leading to numerous defects on the surface. These defects may increase the degree of disorder in the crystal field symmetry, and lower the local symmetry of the Eu3+ ions. This then increases the probability of the 5D07F2 red-emission transition, meaning that this transition dominates the emission spectra, resulting in a higher R/O value, as listed in Table 1. 3.3. Structure, morphology and luminescence of samples obtained from solution III Fig. 6 shows the XRD patterns of samples prepared from solution III, both as-prepared at 180 °C and annealed at 500, 600, 700, 800 or 1000 °C. The crystallization of the samples is similar to that observed in Fig. 4a, except for the difference that the sample prepared from solution III and annealed at 700 °C was almost single phase YBO3:Eu3+, indicating that using tributyl borate as the B3+ source groups on the outside of the carbon nanotube templates can be esterified by tributyl borate, as reported before [41], producing surface-esterified carbon nanotube templates. The precipitate containing Y3+, Eu3+ and B3+ may more easily and uniformly form on such surface-esterified templates, making it easier for YBO3:Eu3+ to crystallize. As shown in Fig. 7a, the as-prepared sample exhibits similar morphology to that shown in Fig. 4b. Fig. 7b shows that the CNTs are coated uniformly to give an outer diameter of about 50 nm. Fig. 7c shows a SEM image of the sample annealed at 1000 °C. This sample is made up of large honeycomb-like particles with many pores within their surfaces. The magnified SEM image, shown in Fig. 7d, reveals that the honeycomb-like particles consist of numerous fine particles, each several tens to hundreds of nanometers
Fig. 6. XRD patterns of samples from solution III: as-prepared at 180 °C and annealed at 500, 600, 700, 800 or 1000 °C.
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Fig. 7. (a and b) TEM images of sample hydrothermally prepared from solution III at 180 °C and (c and d). SEM images for sample annealed at 1000 °C.
across. This sample has a distinctly different morphology to that of the sample prepared using borate as the B3+ source. As suggested above, the carboxyl-modified carbon nanotube template becomes surface-esterified when tributyl borate is used as the B3+ source. Compared with the bare carboxyl-modified template, the surface-esterified template will release more CO2 and H2O during decomposition, leading to the honeycomb-like particles. Fig. 8 shows the excitation spectra measured by monitoring the 627 nm (kem = 627 nm) emission of Eu3+ (a) and the emission spectra measured under kex = 247 nm (b) for samples as-prepared at 180 °C and annealed at 500, 600, 700, 800 or 1000 °C. The wavelengths of the main excitation and emission peaks (and their corresponding transitions), values of R/O ratio and CIE coordinates are listed in Table 2.
Fig. 8. (a) Excitation spectra measured by monitoring the 627 nm emission of Eu3+ and (b) emission spectra measured under kex = 247 nm of the samples from solution III, as-prepared at 180 °C and annealed at 500, 600, 700, 800 or 1000 °C.
The spectral features in Fig. 8. and the underlying mechanism are the same as those discussed in Fig. 5. When we consider these results along with those of Section 3.2, we note that: (1) The position of the excitation bands around 247 nm hardly changes while the band in the region 270–290 nm clearly varies with annealing temperature, as shown in Tables 1 and 2. This is consistent with the excitation bands around 247 nm resulting from the Eu3+O2 excitation inside the lattice and those around 270–290 nm resulting from the Eu3+O2 excitation at the surfaces of the nanoparticles. The position of the excitation band from the Eu3+O2 excitation at the surfaces of the nanoparticles would be expected to change with annealing temperature, because annealing temperature may alter the distribution and coordination number of the surface Eu3+. However, the Eu3+ inside the lattice stably occupies the Y3+ sites, and therefore the position of the excitation band around 247 nm does not alter significantly with annealing. (2) The intensities of both excitation and emission in Figs. 5 and 8 show an abrupt increase at an annealing temperature of 700 °C; this is because the sample starts to crystallize at this temperature, as evidenced by the XRD patterns. (3) The sample annealed at 700 °C is a single phase YBO3:Eu3+, which should have a lower R/ O value according to the discussion in Section 3.1 and in other previous reports [24,26,27]. However, it has the maximum R/O value of 4.06. This phenomenon further confirms that the disorder in crystal field symmetry at the surfaces of the nanoparticles lowers the local symmetry of the Eu3+ ions, and consequently enhances the 5D07F2 red-emission, leading to the higher R/O value. (4) The CIE coordinates of samples annealed at 800 and 1000 °C are both (0.67, 0.33), almost the same as that (0.665, 0.334) for the commercial red phosphor Y2O2S:Eu3+, indicating that the sample made using tributyl borate as the B3+ source has better color purity. 4. Conclusions Three groups of samples were prepared by hydrothermal methods and subsequently post-annealed at different temperatures. Using borate as the B3+ source, the first and second groups were synthesized without and with carbon nanotube templates, respectively. The third group was prepared using tributyl borate as the B3+ source and carbon nanotube templates. The samples obtained from the first group after initial synthesis at 180 °C contained single phase YBO3:Eu3+ and were microspheres of diameter 5–10 lm. The intensity ratios between red and orange emission were 1.4 and 1.1 and the CIE coordinates were (0.65, 0.35) and (0.665, 0.346), for the samples as-prepared at 180 °C and after annealing at 1000 °C, respectively. The samples from the second group began to crystallize after annealing at 700 °C and contained YBO3:Eu3+ and Y3BO6:Eu3+ phases. The sample as-prepared at 180 °C exhibited long, uniform and well-dispersed nanowires with diameters of about 50 nm; these transformed into a dendritic structure after annealing at 1000 °C. The maximum value of intensity ratio between red and orange emission was obtained, 4.73, indicating that color purity was improved by addition of carbon nanotubes to the precursor solution.
Table 2 Wavelengths of excitation and emission peaks (and the corresponding transitions), R/O ratios and CIE coordinates for samples from solution III. Processing temperatures
180 °C
500 °C
600 °C
700 °C
800 °C
1000 °C
Excitation bands(nm) Emission bands(nm)
214 597(5D0–7F1) 619(5D0–7F2) 700(5D0–7F4) 1.55 (0.61, 0.39)
214, 272 599(5D0–7F1) 621(5D0–7F2) 704(5D0–7F4) 3.35 (0.65, 0.35)
214, 272 599(5D0–7F1) 621(5D0–7F2) 706(5D0–7F4) 3.46 (0.65, 0.35)
214, 247 595(5D0–7F1) 625(5D0–7F2) 708(5D0–7F4) 4.06 (0.66, 0.34)
214, 247, 275 594(5D0–7F1) 613(5D0–7F2) 707(5D0–7F4) 3.07 (0.67, 0.33)
214, 247, 281 595(5D0–7F1) 626(5D0–7F2) 707(5D0–7F4) 3.66 (0.67, 0.33)
(Transitions) R/O CIE (x, y)
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The third group of samples also crystallized at 700 °C, but formed nearly single-phase YBO3:Eu3+. Annealed at temperatures over 700 °C produced samples containing both YBO3:Eu3+ and Y3BO6:Eu3 phases. The as-prepared sample again consisted of well-dispersed nanowires, while the sample annealed at 1000 °C exhibited a different, honeycomb-like morphology. The single phase YBO3:Eu3+ annealed at 700 °C had a much larger intensity ratio between red and orange emission, 4.06, than previously reported for YBO3:Eu3+. Furthermore, the optimal CIE coordinates were obtained, (0.67, 0.33), almost the same as the (0.665, 0.334) of commercial red phosphor Y2O2S:Eu3+, indicating that the sample using tributyl borate as the B3+ source has better color purity. Additionally, we observed the Eu3+O2 excitation band from the nanoparticles’ surface; and excitation band position changed in the region of 270–290 nm with the annealing temperature, because annealing at different temperature resulted in the different distribution and coordination number of Eu3+. Acknowledgment This work was supported by the National Natural Science Foundation of China (Grant Nos. 11174004 and 50901074). References [1] Q.L. Xiao, J.X. Meng, J.R. Qiu, J. Alloys Comp. 574 (2013) 600–603. [2] L. Chen, A.Q. Luo, Y. Zhang, X.H. Chen, H. Liu, Y. Jiang, S.F. Chen, K.J. Chen, H.C. Kuo, Y. Tao, G.B. Zhang, J. Solid State Chem. 201 (2013) 229–236. [3] Y.H. Fu, H.F. Jiu, L.X. Zhang, Y.X. Sun, Y.Z. Wang, Mater. Lett. 91 (2013) 265– 267. [4] X.L. Wu, J.G. Li, D.H. Ping, J.K. Li, Q. Zhu, X.D. Li, X.D. Sun, Y. Sakka, J. Alloys Comp. 559 (2013) 188–195. [5] J. Huang, Y.H. Song, G.W. Wang, Y. Sheng, K.Y. Zheng, H.B. Li, H.G. Zhang, Q.S. Huo, X.C. Xu, H.F. Zou, J. Alloys Comp. 574 (2013) 310–315. [6] Y. Zhang, Y.D. Li, J. Alloys Comp. 384 (2004) 88–92. [7] Y. Zhang, Y.D. Li, Y.S. Yin, J. Alloys Comp. 400 (2005) 222–226. [8] L. He, Y.H. Wang, J. Alloys Comp. 454 (2008) 250–254. [9] G. Bertrand-Chadeyron, M. El-Ghozzi, D. Boyer, R. Mahiou, J.C. Cousseins, J. Alloys Comp. 317–318 (2001) 183–185. [10] R. Balakrishnaiah, S.S. Yi, K. Jang, H.S. Lee, B.K. Moon, J.H. Jeong, Mater. Res. Bull. 46 (2011) 621–626. [11] B. Moine, J. Mugnier, D. Boyer, R. Mahiou, S. Schamm, G. Zanchi, J. Alloys Comp. 323–324 (2001) 816–819.
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