Synthesis and photoluminescence of nano-Y2O3:Eu3+ phosphors

Synthesis and photoluminescence of nano-Y2O3:Eu3+ phosphors

Materials Research Bulletin 38 (2003) 973–979 Synthesis and photoluminescence of nano-Y2O3:Eu3þ phosphors Chuan Hea, Yongfeng Guana, Lianzeng Yaoa, W...

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Materials Research Bulletin 38 (2003) 973–979

Synthesis and photoluminescence of nano-Y2O3:Eu3þ phosphors Chuan Hea, Yongfeng Guana, Lianzeng Yaoa, Weili Caia,*, Xiaoguang Lia, Zhen Yaob a

Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, PR China b Department of Physics, University of Texas at Austin, Austin, TX 78712, USA Received 4 November 2002; received in revised form 23 February 2003; accepted 7 March 2003

Abstract We report nano-Y2O3:Eu3þ phosphors with particle size of about 50 nm and relatively high photoluminescence (PL) intensity which is close to the standard for application. The influences of the dope amount, the surfactant and the precipitation pH on the PL intensity, the particle size and the dispersion have been studied. It has been found that 4% is the best Eu3þ molar concentration to get the highest PL intensity for both nano- and micro-Y2O3:Eu3þ. The addition of butanol as a surfactant inhibits the grain growth and the agglomeration of particles efficiently by reducing the oxygen bridge bonds. As the pH rises, the PL intensity and the particle size increase due to the formation of oxygen bridge bonds. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: A. Nanostructures; A. Optical materials; B. Chemical synthesis; D. Luminescence

1. Introduction Y2O3:Eu3þ and related materials are common phosphors in optical displays and lighting applications. The resolution of images on a cathode-ray tube display is closely related to the particle size of phosphors. In general, smaller particles are favored for higher resolution. Conventional phosphors, which lie in the micrometer size regime (1–10 mm), are not suitable for high definition televisions. Thus, application of nanophosphors for displays has gathered great interest. There have been many reports on nano-Y2O3:Eu3þ in the literature [1–4]. Homogenous precipitation [5] and the sol–gel method [6] are used extensively to prepare nanophosphors. Apart from these methods, Xie used the combustion method to prepare nano-Y2O3:Eu3þ phosphors with a series of particle sizes from 10 to 80 nm [7]. Williams prepared nanocrystalline Y2O3:Eu3þ by a gas-phase-condensation technique using CO2-laser vaporization of pressed and sintered pallets of 0.1% Y2O3:Eu3þ [4]. The *

Corresponding author. Tel.: þ86-551-360-1702; fax: þ86-551-360-1592. E-mail address: [email protected] (W. Cai).

0025-5408/03/$ – see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0025-5408(03)00089-8

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optical properties of nano-Y2O3:Eu3þ have been studied intensively, too. But, reports on nano-Y2O3:Eu3þ with photoluminescence (PL) intensity high enough for application are scarce [5]. In this paper, we report on nano-Y2O3:Eu3þ phosphors with a particle size of about 50 nm that were synthesized using the coprecipitation method with a surfactant. The PL intensity of the nanophosphors can reach as high as 70% of the corresponding micro-Y2O3:Eu3þ phosphors’. In addition, the influences of the Eu3þ doping amount, the surfactant and the precipitation surroundings on the PL intensity, the particle size and the dispersion are also discussed.

2. Experimental Varying amounts of 0.016 mol/l EuCl3 along with 5 ml of butanol, as a surfactant, were mixed uniformly with 50 ml of 0.384 mol/l YCl3 solution to achieve the desired Eu3þ doping level. The Eu3þ molar concentration was 2, 3, 4 and 5%, respectively. White precipitates appeared as 0.3 mol/l Na2CO3 solution was slowly added to the solution with stirring until the pH was 7. The precipitates were then filtrated, washed with distilled water several times and dried at 60 8C. The precursors were heated at 800 and 1100 8C in air for 1 h, to obtain nano- and micro-Y2O3:Eu3þ phosphors, respectively. Y2O3:Eu3þ phosphors prepared with the surfactant in different precipitation surroundings were obtained using the same procedure by adjusting the pH to 5, 7, 8 and 9, respectively and heating the precipitates at 900 8C in air for 1 h. Precipitates free of the surfactant were prepared using the conventional method, and were heated at 900 8C for 1 h and at 1400 8C for 3 h, respectively. These samples were used for comparison. X-ray diffraction (XRD) patterns for all samples were obtained using a diffractometer (Rigaku D/MAX-gA) with Cu Ka radiation in the range of 10  2y  70 . The grain sizes were estimated using the Scherrer’s formula. The morphology of the samples was observed on a transmission electron microscope (TEM, Hitachi H-800). The particle sizes were calculated statistically by the algebraic average method. The PL spectra for Y2O3:Eu3þ samples were measured at room temperature using a fluorescence spectrophotometer (Hitachi 850) in the wavelength range from 400 to 600 nm. The excitation wavelength was 255 nm.

3. Results and discussion 3.1. Influence of Eu3þ molar concentration on PL intensities Fig. 1 shows XRD patterns of samples prepared with the surfactant and heated at 800 and 1100 8C, separately. It can be seen from Fig. 1 that broadened peaks appear in the diffraction pattern for the sample heated at 800 8C compared to that heated at 1100 8C, owing to its smaller grain size. Table 1 gives the PL intensity of Y2O3:Eu3þ prepared with the surfactant and heated at 800 or 1100 8C as a function of Eu3þ molar concentration. Obviously, the PL intensity of nano-Y2O3:Eu3þ particles heated at 800 8C reaches its maximum when the Eu3þ molar concentration is 4%. The photoluminescence of Y2O3:Eu3þ is attributed to the electron transition of Eu3þ in the S6 and C2 symmetry sites of Y2O3 from 5 D0 to 7 F1a [8]. The transition probability increases as the Eu3þ concentration rises. But, when the Eu3þ concentration is higher than a certain value, which is 4% in our experiments, the fluorescence quenching will appear,

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Fig. 1. XRD patterns of samples prepared with the surfactant and heated at 800 and 1100 8C, respectively. Table 1 PL intensity of Y2O3:Eu3þ with the surfactant as a function of Eu3þ molar concentration Eu3þ molar concentration (%)

T ¼ 800 8C PL intensity (a.u.)

T ¼ 1100 8C PL intensity (a.u.)

2 3 4 5

495 600 620 500

708 998 1054 816

resulting in the drop of the PL intensity. The micro-Y2O3:Eu3þ sample heated at 1100 8C with Eu3þ concentration of 4% also has the highest PL intensity. Apparently, the PL intensities for nano- and microY2O3:Eu3þ phosphors have the same tendency to change with the Eu3þ concentration. 3.2. Influence of surfactant on particle size and dispersion Fig. 2 shows the TEM photographs of Y2O3:Eu3þ particles heated at 900 8C whose precursors were precipitated with and without the surfactant, respectively. Clearly, the particle size of the sample with the surfactant added is much smaller than that free of it, and the dispersion of the former is better than the later. Table 2 gives the particle size and the grain size of the two samples calculated from the TEM images and XRD patterns, respectively. It can be concluded that the surfactant inhibits the grain growth and the agglomeration of particles efficiently. The hard agglomeration, or sintering, is an important Table 2 Influence of surfactant on grain size and particle size of Y2O3:Eu3þ phosphors prepared at 900 8C

Grain size (nm) Particle size (nm)

With the surfactant

Without the surfactant

37 53

50 188

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Fig. 2. TEM images of Y2O3:Eu3þ heated at 900 8C (a) with the surfactant; (b) without the surfactant.

reason for the increase of particle size. Hydrogen bonds between chemically combined OH resulting in the formation of oxygen bridge bonds have been cited as the main reason for such hard agglomeration [9,10]. The addition of the surfactant reduces the oxygen bridge bonds between particles and avoids hard agglomeration efficiently. Since the particle size of Y2O3:Eu3þ with the surfactant after heating at 900 8C remains in the nanoscale range, synthesis of nano-Y2O3:Eu3þ phosphors whose PL intensity is close to that of the corresponding micro-ones becomes possible. 3.3. Influence of precipitation surroundings on particle size and PL intensity Fig. 3 demonstrates the TEM photographs of Y2O3:Eu3þ particles prepared with the surfactant in different precipitation surroundings and heated at 900 8C. Table 3 presents the grain size, the particle size and the PL intensity of Y2O3:Eu3þ samples as a function of the precipitation pH prepared at 900 8C. It can be seen from Fig. 3 and Table 3 that the precipitation pH plays an important rule on the microstructure (including the grain size and the particle size) and the optical properties of Y2O3:Eu3þ samples. The higher the pH, the larger the resulting particle size. Under low pH conditions, particles prefer to grow into linear chains, which are easy to bend, circumrotate and deform during heating at high temperatures. In contrast, under high pH conditions, they tend to form interconnected, polymeric Table 3 PL intensity, grain size and particle size as a function of precipitation pH for Y2O3:Eu3þ with the surfactant prepared at 900 8C Precipitation pH

PL intensity (a.u.)

Grain size (nm)

Particle size (nm)

5 7 8 9

500 745 807 860

28 33 55 66

46 60 315 510

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Fig. 3. TEM images of samples with the surfactant precipitated under different pH conditions and heated at 900 8C: (a) pH 5; (b) pH 7; (c) pH 8; (d) pH 9.

chains. Furthermore, the OH radicals which the precipitate combines with chemically are much more prevalent at high pH. Therefore, a high pH is propitious to the formation of oxygen bridge bonds and agglomeration among particles [9,10]. Clearly, the surfactant almost loses its inhibiting effect when the precipitation pH is over 7. The particle size is much smaller under low pH conditions. Our result about the relationship between pH and particle size is consistent with that of Pope [11]. The PL intensity of Y2O3:Eu3þ drops as the particle size decreases as shown in Table 3. It is known that the light absorption of phosphors is determined by two coefficients: one is the absorption

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Fig. 4. PL spectra of nano- and micro-Y2O3:Eu3þ phosphors.

coefficient independent of the particle size, and the other is the scattering coefficient correlating to it. Butler et al. [12] have pointed out that particles whose size is smaller than 3 mm have larger scattering and smaller absorption coefficients for 255 nm UV light. This leads to the decreasing luminescence efficiency. The scattering coefficient of phosphors, s, as a function of the average particle diameter, g, can be expressed as follows: ln s ¼ ln k  ln g þ 0:5 ln2 q where k is a constant and q is the standard deviation of normal distribution of particle size. The expression indicates that smaller particle size and larger q lead to larger scattering coefficient s and less efficient absorption, causing the decreasing luminescence efficiency. In addition, as the particle size decreases, the specific surface area of particles increases. There are plenty of defects and dangling bonds on the surface. These defects and dangling bonds induce fluorescence quenching easily. Water and impurities, such as Cl and NH4þ, absorbed on the particle surface also affect the absorption and scattering of the UV light. That is why there is relatively low PL intensity for nano-Y2O3:Eu3þ particles. Consequently, by choosing a proper preparation condition (using the surfactant, precipitating at pH 7 and heating at 900 8C for 1 h), nano-Y2O3:Eu3þ particles with size of about 50 nm can be obtained. However, their PL intensity reaches as high as 70% of the corresponding micro-phosphors’, which are obtained by heating the precipitates at 1400 8C for 3 h without the surfactant. The PL emission spectra of these two samples are shown in Fig. 4. Comparing with nano-Y2O3:Eu3þ reported before, these nano-Y2O3:Eu3þ phosphors have relatively high PL intensity and are close to the practical application.

4. Conclusion The PL intensities for both nano- and micro-Y2O3:Eu3þ phosphors reach their maximum when the Eu3þ molar concentration is 4%. The surfactant added inhibits the grain growth and the agglomeration

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of particles efficiently, but its inhibiting effect nearly disappears under basic precipitation conditions. The particle size and the PL intensity increase with the rise of pH. Nano-Y2O3:Eu3þ phosphors with size is about 50 nm and PL intensity as high as 70% of the corresponding micro-ones’ were prepared under a proper preparation condition.

Acknowledgements This work was supported by the National Natural Science Foundation of China (NSFC) under grant No. 50128202.

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