Self-Purification-Dependent Unique Photoluminescence Properties of

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J. Phys. Chem. C 2010, 114, 9245–9250

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Self-Purification-Dependent Unique Photoluminescence Properties of YBO3:Eu3+ Nanophosphors under VUV Excitation Qizheng Dong, Yuhua Wang,* Zhaofeng Wang, Xue Yu, and Bitao Liu Department of Materials Science, School of Physical Science and Technology, Lanzhou UniVersity, Lanzhou, 730000, People’s Republic of China ReceiVed: January 10, 2010; ReVised Manuscript ReceiVed: March 14, 2010

A series of different concentrations of Eu3+ doping in YBO3 nanophosphors were successfully prepared by a surfactant-assisted solvothermal and heat-treatment process. Their photoluminescence (PL) properties were investigated under vacuum ultraviolet (VUV) and ultraviolet (UV) excitation. In comparison with the change trend of emission intensities in UV spectra, we found a unique phenomenon in the VUV spectra of YBO3: Eu3+ nanophosphors: the PL intensities are not sensitive to the activator doping concentrations. Although the doping concentrations were changed in a large range (2.5-25%), the PL intensities were maintained in a certain value. This phenomenon was also not observed in the bulk YBO3 phosphor. We gave a detailed explanation to this phenomenon by using the “self-purification” effect in nanosystems. It indicated that the activator in the nanophosphor is not a homogeneous distribution but tends to gather on the surface, which will also cause effective activator amounts for VUV luminescence to be very low. 1. Introduction Recently, much interest has focused on nanoscale phosphors due to their characteristics compared to those of their bulk counterparts. It is expected that the nanosized phosphors can improve not only the luminescence quantum yield but also the resolution of display.1-4 Extensive studies on quantum dots or nanocrystal phosphors have shown that nanocrystals doped with optically active luminescent centers may create new opportunities in the study and application of nanoscale materials.5,6 YBO3:Eu3+, one of the best red phosphors to be widely used in plasma display panels (PDPs), has become a focus of research owing to its strong photoluminescence (PL) intensity, high vacuum ultraviolet (VUV) transparency, and exceptional optical damage threshold.7,8 Moreover, nanosized YBO3:Eu3+ was considered to have better color purity than its bulk counterpart.9 Various synthesis techniques have been developed to prepare nanosized YBO3:Eu3+ phosphors, such as sol-gel pyrolysis,10 hydrothermal,11,12 solvothermal,13 and solution precursor methods,14 but their research on the PL properties were all limited in ultraviolet (UV) spectra. For using in PDPs, it is more meaningful to study the luminescence properties of nanophosphors under VUV excitation. A phosphor under VUV excitation has different PL mechanisms. The energy of VUV light cannot excite the activator directly but must first be absorbed by host lattice and then energy transfer to the activator,15 which will be possible to generate some novel phenomena in nanophosphors. Although the excitation spectra in the VUV region and the quenching concentration of YBO3:Eu3+ nanophosphors under VUV excitation have been studied,16 the difference of PL properties under UV and VUV excitation has not been reported, and the doping efficiency under VUV excitation has also not been discussed. In this work, YBO3 nanophosphors with different Eu3+ doping concentrations were obtained by a surfactant-assisted solvothermal-treatment process. The PL intensities of YBO3 nano* To whom correspondence should be addressed. E-mail: [email protected]. Tel: +086-931-8912772. Fax: +086-931-8913554.

phosphors under UV/VUV excitation changed with the Eu3+ doping concentration and are compared with that of bulk samples. On the basis of this, the effective Eu3+ doping concentration for VUV luminescence is also discussed. 2. Experimental Section 2.1. Preparation. The starting materials were Y2O3 (99.99%), Eu2O3 (99.99%), and H3BO3 (99.5%). Stoichiometric amounts of Y2O3 (65-100 wt %) and Eu2O3 (0-35 wt %) were dissolved in diluted nitric acid by heating and then dried to get a mixture of Y(NO3)3 and Eu(NO3)3. A stoichiometric amount of H3BO3 and 0.125 g of CTAB (CTAB/Y ) 1:20) were added in the mixture. The mixture was dissolved in 50 mL of mixed water and n-propanol hybrid solution (water/n-propanol ) 1:1) under ultrasonic mixing for 1 h. The initial pH value was adjusted to 8 with ammonia because the reaction was a process of producing H+, and weak alkaline conditions were propitious to the reaction production.17 The solution was transferred into a 100 mL Teflonlined, stainless steel autoclave. The solution was heated at 230 °C, and the reaction time was 3 h. White precipitates were collected, filtered, washed with distilled water and ethanol several times, and dried in an oven at 80 °C. At last, the products were heated in 900 °C for 2 h in air. For comparison, the corresponding doped bulk YBO3:Eu3+ was also synthesized by a solid-state reaction at 1100 °C for 5 h. 2.2. Characterization. All the samples were characterized by powder X-ray diffraction (XRD, Rigaku D/max-2000) using Cu KR (λ ) 1.5418 Å) radiation. Some of the samples were characterized by scanning electron microscopy (SEM, S-4800) and transmission electron microscopy (TEM, JSM-1200ex). The UV emission spectra were measured using an Edinburgh Instruments FLS920T. The VUV emission spectra were collected on a vacuum monochromator (VM504, Acton Research Corporation). 3. Results and Discussion 3.1. X-ray Diffraction Analysis. Figure 1 shows XRD patterns of YBO3:Eu3+ samples synthesized by a solvothermal

10.1021/jp1002287  2010 American Chemical Society Published on Web 05/04/2010

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Figure 2. TEM photographs of YBO3:Eu3+ with different concentrations of Eu3+ doping: (a) 5%, (b) 25%, (c) 30%.

Figure 1. XRD patterns of the nanosized YBO3 samples with different concentrations of Eu3+ doping and bulk samples (5% Eu3+ doping).

TABLE 1: Mean Grain Size of the Nano-YBO3 Samples with Different Concentrations of Eu3+ Doping Eu3+ doping concentration

0% 2.5% 5% 10% 15% 20% 25% 30% 35%

mean grain size (nm) 22.6 21.5 25.2 22.3 22.6 23.4 24.0 20.7 25.2

reaction with different concentrations of Eu3+ doping. All samples were well-crystallized, and they could all be recognized as single phase in terms with JCPDF (16-0277). No second phase was observed. The EuBO3 has a similar crystal structure (PDF 13-0485) with that of YBO3 (PDF 16-0277), and it has a similar diffraction pattern but a little shift to the lower angle. In Figure 1, the diffraction peak position of the undoped sample is in accord with the PDF 16-0277, but the diffraction peaks are gradually shifted to the lower angle with the doping concentration increased. The peak positions of doped samples are all between the positions in PDF 16-0277 and PDF 13-0485. However, the peaks in all patterns are not splitting even in the high angle of the pattern. It indicates that the Eu3+ is doped in the YBO3, but EuBO3 is not formed individually. As the doping concentration increased, the intensities of diffraction peaks slightly gradually decreased. This shows that the crystallinity was deteriorated with doping involved. A similar phenomenon has been also observed in the other systems, such as CdS:Eu18 and ZnO:Al.19 Compared with the bulk pattern (5% Eu3+ doping sample), the diffraction peaks of the nanosamples were broader. It indicated that the grain size of the samples became small. By using the Scherrer formula to calculate and average the grain size from each diffraction peak of the samples (Table 1), it can be found that all of these samples reached nanoscale, and the grain sizes did not change significantly (20.7-25.2 nm) with increased activator doping. 3.2. Electron Microscopy Analysis. Some representative TEM photographs of the 5, 25, and 30% doping samples are shown in Figure 2, and the corresponding SEM photographs are shown in Figure 3. As can be seen in Figure 2a,b, although the doping concentrations are greatly different, the particles of the two samples before being quenched are similar. Most of them are spindle-like or rodlike with a size of 50 nm in diameter and 100 nm in length. The size is bigger than the calculated

Figure 3. SEM photographs of YBO3:Eu3+ with different concentrations of Eu3+ doping: (a, b) 5%, (c) 25%, (d) 30%. (e) EDS spectrum of YBO3:5%Eu3+.

results from XRD, indicating that a particle in this sample is polycrystalline and consists of several monocrystalline particles, and smaller nanograins will contribute more to the broadening of the XRD diffraction peaks.20 When the doping concentration reaches 30%, the morphology of the nanophosphor changed greatly (Figure 2c). The particle is polygon-like nanosheet approximately 200 nm long and much thinner. The SEM results correspond to them and are shown in Figure 3. The particles in the 5% and 25% samples both aggregate into spherules (Figure 3a,c), and the diameter of each spherule is about 6-8 µm. In an enlarged figure (Figure 3b), it could be found that every spherule consists of a mass of nanoparticles. However, in the 30% samples, the spherule is aggregated bigger and the diameter of each spherule is about 16 µm. These results indicate that the morphology of nanophosphors will change greatly when the doping concentration exceeds a certain value. The doping concentration will influence the morphology of nanomaterials, which has also been observed in other nanosystems.21,22 The possible reason that causes this phenomenon here will be discussed later. The EDS was used to further characterize the composition of the as-prepared product. The EDS spectrum (Figure 3e) of the YBO3:5%Eu3+ sample shows the presence of Y, O, B, and Eu. Quantitative testing of the atom concentrations of the series of samples was performed, and the results are listed in Table 2.

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TABLE 2: Atom Concentrations and Doping Concentrations Measured by EDS for the Series of YBO3:Eu3+ Nanophosphors atom concentration % initial doping concentration

B

O

Y

Eu

EDS measured doping concentration

2.5% 5% 10% 15% 20% 25% 30%

66.58 ( 5.73 61.43 ( 6.02 63.04 ( 5.36 72.25 ( 7.88 66.65 ( 9.58 63.41 ( 6.22 66.49 ( 5.70

28.63 ( 1.14 34.05 ( 0.74 32.68 ( 0.66 26.13 ( 1.36 30.60 ( 1.02 30.87 ( 1.21 29.14 ( 0.95

4.68 ( 0.05 4.22 ( 0.06 3.71 ( 0.06 1.37 ( 0.04 2.09 ( 0.08 4.35 ( 0.06 3.00 ( 0.06

0.12 ( 0.03 0.30 ( 0.05 0.58 ( 0.05 0.25 ( 0.04 0.65 ( 0.09 1.38 ( 0.09 1.37 ( 0.07

2.5% 6.6% 13.5% 15.4% 23.7% 24.1% 31.4%

The Eu doping concentrations are calculated by the Eu atomic concentration divided by the sum of Y and Eu atomic concentrations, which are also shown in Table 2. It can be seen that some measured results have some deviation from the initial values, which is due to the random selection of the measuring point. Nevertheless, we could still find the increasing trend of the doping concentration, which indicates that Eu ions are successfully involved in the nanophosphor particles. 3.3. Spectra Analysis. The emission spectra excited at 254 and 147 nm are displayed in Figures 4 and 5, respectively. All spectra consist of sharp lines ranging from 580 to 700 nm, which are associated with the transitions from the excited 5D0 level to

the 7FJ (J ) 1, 2, 3, 4) levels of Eu3+ activators.23,24 The major emissions of YBO3:Eu3+ is at 592 nm (5D0 f 7F1) and 611 and 627 nm (5D0 f 7F2). On comparison with bulk YBO3:Eu3+, the 5 D0 f 7F2 transition (611 nm) of nanosized YBO3:Eu3+ is stronger than the 5D0 f 7F1 transition (592 nm) (in Figures 4 and 5), which indicates a better color purity of nanosized YBO3: Eu3+ than bulk counterparts. The color purity of Eu3+ doping phosphors can be characterized as the intensity ratio between the red emission (5D0 f 7F2) and the orange emission (5D0 f 7 F1) (R/O ratio). The R/O ratios and chromaticity coordinates are shown in Table 3. It can be found that the R/O ratio of the nanosample was significantly higher than that of the bulk

Figure 4. Emission spectra of a series of YBO3:Eu3+ (λex ) 254 nm). The inset shows the integrated emission intensity of Y1-xEuxBO3 as a function of Eu3+ concentration (2.5-35%). (a) Nanosized samples. (b) Bulk samples.

Figure 5. Emission spectra of a series of YBO3:Eu3+ (λex ) 147 nm). The inset shows the integrated emission intensity of Y1-xEuxBO3 as a function of Eu3+ concentration (2.5-35%). (a) Nanosized samples. (b) Bulk samples.

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TABLE 3: R/O Ratios and Chromaticity Coordinates of the Nanosample and Bulk Sample λex ) 254 nm

λex ) 147 nm

chromaticity coordinates

chromaticity coordinates

sample

R/O

x

y

R/O

x

y

NTSC nano bulk

1.05 0.46

0.67 0.640 0.636

0.33 0.359 0.363

1.16 0.68

0.67 0.645 0.642

0.33 0.354 0.358

samples. Compared with the National Television Systems Committee (NTSC), the chromaticity coordinates of the nanosample correspond more with the standard red light (x ) 0.67, y ) 0.33) than those of the bulk phosphor. It reflects the advantage of nanoscale YBO3:Eu3+ phosphors.9,16 To have a more detailed research on Eu3+ doping, the relationship between the variation of the integrated intensities of spectra (from 550 to 700 nm) with the change of Eu3+ concentration is listed in the inset of Figures 4 and 5. Under UV excitation, the PL intensities became enhanced as the Eu concentration increased (the inset of Figure 4a). The quenching concentration is about 25% for nanosized YBO3:Eu3+ and about 20% for bulk YBO3:Eu3+ (the inset of Figure 4b). Observed in this work, the quenching concentration in VUV spectra is similar with that in UV spectra (25%), which is consistent with Wei′s reports.9,16 However, under VUV excitation, a noticeable change trend of YBO3:Eu3+ nanophosphors could be found (the inset of Figure 5a): the PL intensities are not increased with the Eu3+ doping concentration but reach to the strongest at first (2.5%) and then decreased very slowly. The PL intensities remain almost unchanged until the concentration quenching happened. In other words, the PL intensity under VUV excitation is not sensitive to the change of activator concentration. This phenomenon cannot be observed in that of bulk samples (Figure 5b) and also differs from it in UV spectra (Figure 4a). This phenomenon, which could be expressed as the PL intensity of nanophosphors under VUV excitation is not sensitive to doping concentration, is different with that observed in Wei’s work.16 However, the phenomenon could be observed when we repeated preparing the same samples several times. Furthermore, we chose another Eu3+ doped nanophosphor (YVO4:Eu3+), and an analogous phenomenon also appeared under VUV excitation (see figures in the Supporting Information). This indicates that this phenomenon is not caused by experimental error. The phenomenon has not been reported before, and it also cannot be explained by the traditional luminescence mechanism used in bulk, which shows that there could be a new phenomenon of nanophosphors excited by VUV light. In the bulk phosphor, the quenching concentration in VUV spectra is usually lower than that in UV spectra,25,26 which was also observed in this work (Figures 4b and 5b), and it was considered to be due to different the luminescence mechanism in VUV excitation.15 The quenching concentrations of nanosized YBO3 in UV/VUV spectra are similar. The same phenomenon could also been found in Wei’s work,9,16 but they did not make further discussions about this. In this work, we consider that it must relate to the change of morphology that has been observed in the Electron Microscopy Analysis section. However, whether in UV or VUV, the PL intensities are all sensitive to the activator concentration. Here, we consider this issue from the different penetration depths of these two excitation sources. It is known that UV light has a large penetration depth for phosphors (several micrometers),27 but it is generally considered that the penetration depth of the VUV for phosphors is in nanoscales.

The concrete values reported in different studies vary greatly.28-31 The exact depth in this work is hard to test, so we could only estimate the value from another reference. In Modi’s work,28 the depth of incident radiation are all restricted to 8.0 nm (10-250 eV). According to Terekhin’s work,30 it shows that the penetration depth is determined by many factors, such as photon energy, diffusion to the surface, dielectric permittivity of materials, and incident angle. The depth presented in this reference changed irregularly, but the majority of the values is restricted to 10 nm when the photon energy is below 30 eV. In our test equipment, the incident angle is about 45°. A typical value in Terekhin’s work30 is 10.3 nm when the incidence angle is at 42°. We estimate that the penetration depth in our test condition is not more than 10 nm for the nanoparticles. It will cause that the VUV light could only irradiate on the part of the phosphor particle where it is near the surface. On the basis of these views, we consider that the activator in the nanophosphor is not a homogeneous distribution, but the activator concentration on the nanophosphor surface and in the interior is different, which causes different change trends in their UV and VUV spectra. The main reason is due to the “self-purification” mechanisms, which have been applied in many nanosystems.32-34 It could be expressed as the impurities are hard to be doped in nanocrystals and easy to be expelled and move to the surface. The reference33 point is that the formation energies of impurity defects increase as the size of the nanocrystal decreases. That is, the smaller the particle size, the more difficult to form doping. We can express the relationship between dopant formation energy (Ef) and particle radius (r) as

Ef ∝ r-n

(1)

where n is a fixed value greater than zero. Here, the formula is only used to reflect the relationship between Ef and r. Moreover, the formation energy Ef determines the impurity concentration c in thermal equilibrium (maximum doping concentration), through the expression

c ) Ns exp(-Ef /kBT)

(2)

where Ns is the number of sites in the lattice where the impurity can be incorporated, kB is the Boltzmann constant, and T is temperature. As can be seen in formula 2, in nanocrystals, the Ns is limited by smaller particle size, so the doping concentration mainly depends on dopant formation energy. On the basis of this mechanism, we can describe the distribution of the activator in the nanophosphor. In this work, the doping ions preferably occupy the lattice sites that are near the nanoparticle surface, as the surface has the relatively low formation energy (formula 1). Until the surface layer reaches “saturation” concentration (formula 2), the surplus doping ions will distribute from the surface to the interior. Ultimately, the nanophosphor particle forms a similar form like a “core-shell”. There is little activator in the “core”, and the doping ions mainly exist in the “shell” and form an “activator doping layer” (Figure 6). In Figure 6, the thickness of the activator layer is expressed as d. The UV light could penetrate the whole particle, so the PL intensity under UV excitation is associated with the total amounts of activators doped in the layer. The VUV light could only penetrate part of the particle, and the penetration depth is expressed as d1. When the above formulas 1 and 2 are combined, it can be drawn that the same radius of the

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Figure 6. Schematic diagrams of the nanophosphor particles with different doping concentrations.

shell has the same doping concentration. For nanophosphors with different doping concentrations, the d1 is a constant but the d will increase with increasing doping concentration (Figure 6). The PL intensity of nanophosphors under VUV excitation is not sensitive to doping concentration but that under UV excitation increases with increasing doping concentration. This could well explain the different intensity change trends in UV and VUV spectra. From the Electron Microscopy Analysis section, it has been observed that the morphology changes when the doping concentration exceeds a certain value. Here, we can also use the theory to understand this phenomenon: because the activator concentration is controlled by the particle radius and there must be a critical radius (r0), the doping ions could be doped until the particle radius grows larger than r0.35,36 The activator only could exist in the layer. On the other hand, the total activator amounts in the layer are limited by the particle radius. The surplus doping ions in a high activator concentration sample tend to gather to the place far away from the particle. This trend will cause the morphology changes from spherical to flake. This change causes YBO3: Eu3+ to have the same quenching concentration in UV/VUV spectra. From here, we could infer that the morphology and structure changes are mainly dependent on nanosized YBO3: Eu3+ PL intensity and quenching concentration. From the above analysis, we could find when the d exceeds d1; incorporating more activator is no contribution for nanophosphor luminescence under VUV excitation. To find the “effective” doping concentration for nanophosphors under VUV excitation, a series of nanophosphors with relatively low doping concentrations (1-5%) were synthesized with the same process. The emissions spectra excited at 147 nm are shown in Figure 7a. As a contrast, the emission spectra excited at 254 nm are shown in Figure 7b. From the inset of these figures, we can find that the PL intensities under UV excitation have been increasing from 1% to 5% but the PL intensities under VUV excitation reached maximum with the sample with 3% Eu3+ doping. It indicates that the effective Eu3+ doping concentration in nanosized YBO3 is only 3% for VUV emission. Despite that more activator doping will increase the PL intensity under UV excitation, it does not contribute to the VUV PL intensity. From this work, it can be concluded that the VUV spectra could reflect the characteristics on the phosphor surface layer and it should have great potential for application in nanoluminescent materials. 4. Conclusion A series of different concentrations of Eu3+ doping in YBO3 nanophosphors were successfully prepared by a surfactantassisted solvothermal and heat-treatment process, and their spectra properties were characterized by UV and VUV excitation. The results showed that, in a large range of Eu3+ doping concentrations, the PL intensities of nanosized YBO3:Eu3+ under

Figure 7. Emission spectra of a series of YBO3:Eu3+. The inset shows the integrated emission intensity of Y1-xEuxBO3 as a function of Eu3+ concentration (1-5%). (a) λex ) 147 nm. (b) λex ) 254 nm.

VUV excitation are not sensitive to the activator doping concentrations. This phenomenon had not been observed before, and we attributed this phenomenon to the self-purification effect in YBO3:Eu3+ nanophosphors. Due to the existence of this effect, it can be seen that effective Eu3+ doping concentration in nanosized YBO3 is only 3% for emission under VUV excitation. Acknowledgment. This work is supported by the National Natural Science Foundation of China (No. 10874061), the National Science Foundation for Distinguished Young Scholars (No. 50925206), and The Research Fund for the Doctoral Program of Higher Education (No. 200807300010). Supporting Information Available: TEM photograph of nanosized YVO4:5%Eu3+ phosphors synthesized at 200 °C by hydrothermal reaction and the series of emission spectra (excited at 254 and 147 nm) of nanosized YVO4 with different doping concentrations. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Bhargava, R.; Gallagher, D.; Hong, X.; Nurmikko, A. Phys. ReV. Lett. 1994, 72, 416. (2) Li, J.; Li, X.; Sun, X.; Ishigaki, T. J. Phys. Chem. C 2008, 112, 11707.

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