Microwave-Assisted Sol−Gel Synthesis and Photoluminescence

J. Phys. Chem. C 2008, 112, 11679–11684

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Microwave-Assisted Sol-Gel Synthesis and Photoluminescence Characterization of LaPO4:Eu3+,Li+ Nanophosphors Wei Li† and Joonho Lee*,‡ State Key Laboratory for Powder Metallurgy, Central South UniVersity, Changsha, Hunan 410083, P.R. China and Department of Materials Science and Engineering, Korea UniVersity, 5-1 Anam-dong, Seongbuk-gu, Seoul, 136-713, Republic of Korea ReceiVed: January 6, 2008; ReVised Manuscript ReceiVed: April 18, 2008

Here, we describe the fast and mass fabrication of monazite lanthanum orthophosphate (LaPO4) nanoparticles via a simple sol-gel method under the assistance of microwave irradiation. The procedure involves formation of homogeneous, transparent, metal-citrate-EDTA gel precursors using both citric acid (CA) and ethylenediamine tetraacetic acid (EDTA) as the complexing agent followed by microwave irradiation, which promotes prompt thermal decomposition of the metal-citrate-EDTA gel precursors to yield the final nanoparticles. Thermogravimetric/differential scanning calorimetry (TG-DSC), X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM) were used to characterize the as-synthesized nanoparticles. About 23 g of single monoclinic phase, approximately 100 nm diameter, LaPO4 spherical nanoparticles were readily obtained at 800 °C within 0.5 h, and the nanospheres were themselves composed of Ultrafine nanocrystals of a few nanometers in diameter. Furthermore, photoluminescence (PL) characterization of the Li+- and Eu3+codoped LaPO4 nanocrystals was carried out. The effects of microwave irradiation temperature and Eu3+ active center concentration, especially the doping concentration of Li+ on the PL properties, were elaborated in detail. Room-temperature photoluminescence (PL) characterization revealed that the optical brightness as well as the intensity ratio of 5D0-7F1 to 5D0-7F2 is highly dependent on the Li+ ions concentration. Introduction of 5 mol % Li+ into the crystal structure enhanced the PL emission brightness more than 2-fold, and the Li0.05Eu0.05La0.9PO4 nanophosphor showed the relatively most promising PL performance with the most intense emission. 1. Introduction Monazite-type, lanthanum orthophosphate (LaPO4) crystallizes in a monoclinic structure, which has very low solubility in water (on the level of 10-25 to 10-27),1 high thermal stability (∼2300 °C),2 and a high index of refraction resulting from the specific 4f shell atomic configuration.3 These properties provide the basis for interest in a wide range of applications such as phosphors,4 sensors,5 proton conductors,6 catalysts,7 timeresolved fluorescence labels for biological detection,8 and heatresistant materials.9 Very recently, LaPO4 has been reported to act as an excellent host for lanthanide ions, such as cerium, terbium, and europium, to produce phosphors emitting a variety of colors for development of photoluminescent materials with applications in optoelectronic devices, solid-state lasers, displays, and phosphors.10 Small and spherical particles are desirable in phosphors due to their ease of processing into devices with high resolution, high screen coverage, intense emission, as well as long service life. The conventional solid-state reaction synthesis for LaPO4 normally results in irregular particles shape and size, irrespective of the high temperature and cumbersome grinding and firing steps.11 Therefore, over the last few decades various solutionphase routes, including combustion,5,6,12 sol-gel,13,14 precipitation,15 water-oil microemulsion,16 polyol-mediated process,17 * To whom correspondence should be addressed. Phone: 0082-2-32903287. Fax: 0082-2-928-3584. E-mail: [email protected]. † Central South University. ‡ Korea University.

ultrasonification,18 hydrothermal,19–21 and mechanochemical method,22 have been tried to reduce the reaction temperature and obtain high-quality LaPO4-based nanoparticles. However, the simple and mass fabrication of LaPO4 nanocrystals with narrow grain size distribution and uniform morphology still remains a challenge. Spray pyrolysis is one of the accessible processes to induce spherical morphology, fine size, and narrow size distribution.23,24 However, high temperature is required for such pyrolysis, and the resulting particles are frequently hollow due to the fast evaporation of solvent from the droplet surface. The sol-gel process is superior to other preparation methods since the intimate mixing of components ensures homogeneity of the final product. Doping, which forms the basis of most photonic applications of rare earth phosphates, is best achieved through a sol-gel process.25 In view of the high cost and toxicity of the alkoxide precursors used in the conventional sol-gel process, in this paper we prepared the nanocrystalline LaPO4: Eu3+,Li+ phosphors by a microwave-assisted, sol-gel process using general inorganic salts instead of alkoxides as the main precursors. Most importantly, the sol-gel process offers a unique, stable, and low-cost sol precursor for the future fabrication of high-quality, luminescent thin films via dip coating or spin coating. The significant ionic radius difference between Li+ (0.68 Å), La3+ (1.06 Å), and Eu3+ (0.95 Å) makes the extensive substitution of Li+ for La3+ difficult. Thus, addition of lithium ion into the monoclinic structure of LaPO4 should facilitate formation of vacancy and crystallization and thereby cause wide-ranging effects

10.1021/jp800101d CCC: $40.75  2008 American Chemical Society Published on Web 07/12/2008

11680 J. Phys. Chem. C, Vol. 112, No. 31, 2008 on the PL properties of the LaPO4:Eu3+. For instance, lithium ion has been introduced into different phosphor host lattices, such as Y2O3:Eu3+,26 Gd2O3:Eu3+,27 SnO2:Eu3+,28 ZnS:Tm3+,29 MgO: Eu3+,30 and ZnO:Dy3+,31 to act as a coactivator and charge compensator, and the resulting photoluminescence (PL) intensity enhancement has been observed. Moreover, in some reports, the effect of Li+ substitution on the luminescence properties has been investigated in various host lattices such as SrTiO3:Pr3+ 32 and Gd2xYxO3:Eu3+,33 with results revealing that Li+ addition remarkably affects the particle morphology and photo- and cathodoluminescent efficiency of the phosphors. In a microwave system, heating arises from either dipole rotation or ion migration induced by the microwave field, which subsequently generates fast homogeneous nucleation and easy gel dissolution.34 Consequently, compared with the usual methods, microwave irradiation offers several advantages, including shortened reaction time, small particle size, narrow particle size distribution, better selectivity, and higher reaction yield.35 Microwave irradiation as a heating source has been successfully developed for a number of chemical approaches to produce binary and ternary solid-state compounds.36 Different types of nanoparticles such as metal,37 ferrites,38 II-VI group semiconductors,39 and oxides40 have been synthesized in the presence of microwave irradiation. Here, we described a novel, microwave-assisted, sol-gel approach for the fast and mass preparation of uniform, spherical, Li+ and Eu3+ codoped LaPO4 nanoparticles that combines the merits of sol-gel and microwave irradiation. The use of ethylenediamine tetraacetic acid (EDTA) and citric acid (CA) as double chelating agents ensures formation of homogeneous, transparent, metal-citrate-EDTA gels, while that of microwave irradiation as a heating source promotes the prompt thermal decomposition of the gels into uniform spherical LaPO4-based nanocrystallines. PL performances are examined, and special attention is paid to the effects of Li+ ion doping on the PL properties of the obtained LaPO4:Li+,Eu3+ nanophosphors. 2. Experimental Section 2.1. Sample Preparation. All chemicals used in the experiments were of analytical purity, bought from Sigma-Aldrich, and used without further purification. The precursor solutions were prepared and standardized via the inductively coupled plasma atomic emission spectrometer (Perkin-Elmer ICP-AES Plasma 1000). In a typical procedure for synthesizing 0.1 mol of LaPO4 nanoparticles, 0.2 mol of EDTA, and 0.2 mol of CA were dissolved altogether in 200 mL of 1 M ammonia solution to produce transparent, mixed solution. Then 50 mL of 2 M La(NO3)3 aqueous solution was added dropwise to the above mixed solution under vigorous stirring at 80 °C to produce a clear La-citrate-EDTA mixed solution. After that, 100 mL of 1 M NH4H2PO4 aqueous solution was added dropwise into the La-citrate-EDTA mixed solution under vigorous stirring, and a transparent mixed sol was consequently obtained. During this mixing procedure the temperature was controlled at around 80 °C using a water bath. The obtained sol was then transferred into a microwave oven (UM-03, 2 KW, 220 V, 1600 °C) with reflux exchanger equipment, which was then kept at a temperature of 80 °C for 0.5 h to produce a transparent and viscous gel. The obtained gel was subsequently irradiated at 800 °C for 0.5 h to produce the final white LaPO4 nanoparticles. Eu3+ and Li+ codoped LaPO4 nanoparticles were synthesized via the same route by substituting La3+ with a desired amount of Eu3+ and Li+ in the raw materials preparation. In an attempt to investigate the irradiation temperature, Li+ ion doping concentration, and Eu3+ active center concentration dependence

Li and Lee

Figure 1. TG-DSC curves of the as-synthesized metal-citrate-EDTA precursors.

of PL performances, several experiments were conducted by varying the temperatures and molar ratio of Eu3+ and Li+ to La3+, as indicated in the text. 2.2. Sample Characterization. The crystal phase of the prepared products was identified by powder X-ray diffraction (XRD, Bruker D8) using Cu KR radiation (λ ) 1.5418Å) at a scan rate of 4 °C/min. The microstructural morphology of the final products was characterized with a Hitachi H-800 transmission electron microscope (TEM) operated at 200 kV, highresolution transmission electron microscope (HRTEM, JEOL3010), and a scanning electron microscope (SEM) equipped with a Strata DB235 focusing ion beam (FIB). The thermal decomposition behavior of the citrate gel precursors was examined by a thermogravimetric analyzer (TGA, Perkin-Elmer, TAC 7/DX) using air as the working gas. For DSC measurements, the samples were heated from room temperature to 1000 °C at a heating rate of 15 K/min. R-Al2O3 was used as the reference material, and the samples were run in open platinum pans. The PL measurements were carried out using a Hitachi F-4500 fluorescence spectrophotometer. The excitation spectra were corrected for the beam intensity variation in the Xe light source used. For comparison, all excitation and emission spectra were measured at room temperature with the same instrumental parameters. 3. Results and Discussions 3.1. Thermal Analysis. The thermal decomposition procedure of the obtained La-P-Citrate-EDTA gel precursors was studied by thermogravimetric/differential scanning calorimetry (TG-DSC) as shown in Figure 1. From the TG curve, throughout the temperature range from ambient to 950 °C, three weightloss regions occurred at 25-200 °C (about 4.8%), 200-400 °C (about 29.5%), and 400-800 °C (about 47.6%). The change of weight loss was minimized at temperatures higher than 800 °C. Correspondingly, three discrete, phase transformation regions can be observed in the DSC curve centered at around 100, 287, and 691 °C. The first weight loss corresponds to elimination of water. The second weight-loss region can be ascribed to the pyrolysis of NO3- and organic phases (CA and EDTA) to give an amorphous inorganic phase, which will be further discussed in detail based on XRD analysis in the following section. Subsequent elimination of the remaining organic materials (carbon and organic compounds) occurs in the temperature range from 400 to 500 °C, and crystallized LaPO4 inorganic phase is simultaneously formed.19

Characterization of LaPO4:Eu3+,Li+ Nanophosphors

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Figure 2. Evolution of XRD patterns of the Li0.1La0.9PO4 nanoparticles as a function of microwave irradiation temperature. The bottom lines indicate the diffractions of monoclinic LaPO4 according to JCPDS840600.

3.2. XRD Analysis. Figure 2 shows the changing of XRD patterns of the as-obtained Li0.1La0.9PO4 nanoparticles as a function of microwave irradiation temperature. It can be observed that the materials calcined at below 500 °C are amorphous, while the main planes of [111], [120], [-112]/[012], and [103]/[-311], referring to the monoclinic structured LaPO4 (JCPDS-840600), appear, which indicates decomposition of organic compounds in the precursors and formation of inorganic phase Li-La-P-O at a temperature ranging from 300 to 400 °C, which is consistent with the results presented in the above thermal analysis section. For the sample calcined at 500 °C, all diffraction peaks can be readily indexed to phase-pure LaPO4 (JCPDS-840600). However, the wide and weak diffraction peaks are indicative of the poor crystallinity as well as fine grain size of the as-synthesized sample at 500 °C. When the irradiation temperature was further raised from 600 to 1000 °C, the intensity of all the diffraction peaks gradually increased and the peak width gradually narrowed, but no secondary phase diffractions were found in the XRD patterns, revealing that the particle size increases and crystallization improves with increasing temperature while remaining phase pure of monoclinic LaPO4. Figure 3 shows the XRD patterns of the as-synthesized (a) LixEu0.05La0.95-xPO4 and (b) Li0.05EuyLa0.95-yPO4 nanoparticles with varying Li+ and Eu3+ doping concentration. It is clear that all the diffractions can be readily indexed to phase-pure LaPO4, indicating that the obtained lanthanum phosphate nanoparticles remain a single phase after doping La3+ with Li+ or Eu3+ at concentrations up to 15%, which indicates that the Li+ and Eu3+ ions are effectively doped into the crystal lattices of the monoclinic LaPO4 phase. 3.3. SEM and TEM Analysis. Figure 4a shows an SEM image of the Li0.05Eu0.05La0.9PO4 sample after microwave irradiation treatment at 800 °C for 20 min. The sample is composed of uniform, spherical, 100 nm sized nanoparticles. TEM images of this sample shown in Figure 4b and 4c clearly demonstrate that the obtained Li0.05Eu0.05La0.9PO4 particles were spherical, 100 nm diameter aggregates, which were further constructed of primary Ultrafine nanoparticles of a few nanometers in diameter. Figure 4d is an HRTEM image of this sample, which indicates that the obtained Li0.05Eu0.05La0.9PO4 primary nanoparticles are well crystallized and ordered in crystallography. 3.4. Photoluminescence Characterization. Room-temperature excitation and emission spectra of the Li0.05Eu0.05La0.9PO4

Figure 3. XRD patterns of the LaPO4-based nanoparticles after substituting La3+ by various content of (a) Li+ and (b) Eu3+ concentrations. The bottom lines indicate diffractions of monoclinic LaPO4 according to JCPDS-840600.

nanophosphors obtained at 800 °C are presented in Figure 5. In the excitation spectrum (Figure 5, left), the broad band centered at 260 nm was attributed to the charge-transfer band (CTB) between Eu3+ and the surrounding oxygen anions.41–45 Successful doping with europium was evident from the splitting and intensity pattern of the emission lines.21c The emission spectrum of the Li0.05Eu0.05La0.9PO4 nanophosphors consisted of lines mainly located in the orange-red spectral area (from 550 to 720 nm) (Figure 5, right). These lines correspond to transitions from the excited 5D levels to the 7F (J ) 0, 1, 2, 3, 4) levels of the f configuration 0 J of Eu3+ as marked in Figure 5.41–45 The orange-red emission lines at around 590 nm originating from the magnetic dipole transition 5D -7F 0 1 were the dominant bands for the as-synthesized Li0.05Eu0.05La0.9PO4 nanocrystals. Figure 6a presents the evolution of the emission spectra of the obtained LixEu0.05La0.95-xPO4 nanophosphors with the doped Li+ concentration. All spectra showed the typical features specific to LaPO4:Eu3+, enabling two straightforward conclusions to be drawn from Figure 6a: emission brightness was greatly enhanced after doping of Li+ ions, and the Li0.05Eu0.05La0.90PO4 sample showed the most intense emission among the tested samples. The Li+ concentration dependence of the emission intensity of the four main characteristic peaks centered at 588, 594, 612, and 622 nm is summarized in Figure 6a, and the results are displayed in Figure 6b and Table 1. This summary revealed that the emission intensity resulting from the 5D0-7F1 transition (Peak 1 + Peak 2) was

11682 J. Phys. Chem. C, Vol. 112, No. 31, 2008

Li and Lee

Figure 4. (a) SEM and TEM images of the as-obtained Li0.05Eu0.05La0.9PO4 nanoparticles under (b) low and (c) high magnification. (d) HRTEM pattern of this sample.

Figure 6. (a) Evolution of the emission spectra of the LixEu0.05La0.95nanophosphors with varying Li+ doping concentration. (b) The total emission intensity (Peak 1 + Peak 2) resulting from the 5D0-7F1 transition as a function of Li+ concentration.

xPO4

Figure 5. Room-temperature excitation and emission spectra of the Li0.05Eu0.05La0.9PO4 nanophosphors obtained at 800 °C.

enhanced more than 2-fold after substituting 1% La3+ with Li+ and that the emission intensity further increased with increasing Li+ concentration. The luminescence peaked at 5% Li+ and slightly decreased after that. As is well known, the 5D0 f 7F2 transition is only possible when Eu3+ is embedded at a site of noninversion symmetry, while the 5D0 f 7F1 transition is possible at a site with centrosymmetry. Therefore, the fluorescence intensity ratio of 5D f 7F to 5D f 7F , known as the symmetry ratio, gives a 0 1 0 2 measure of the degree of distortion from the inversion symmetry of the local environment surrounding the Eu3+ ions in the host matrix.41–45 Here, we employ two methods to calculate the symmetry ratio: the ratio of intensity of the most prominent emissions from 5D0 f 7F1 (Peak 2 at 594 nm) to that of 5D0 f 7F (Peak 3 at 612 nm) and the ratio of the total intensity of 2 (Peak 1 + Peak 2) to that of (Peak 3 + Peak 4). The symmetry ratio of the LixEu0.05La0.95-xPO4 nanophosphors with varying Li+ concentration is calculated and listed in Table 1. It reveals that after doping 1% Li+ ions into the crystal lattice of LaPO4:Eu3+, the symmetry ratio was increased considerably and the

Li0.05Eu0.05La0.90PO4 sample had the most symmetric lattice conditions for the Eu3+ active center. This result confirmed that introduction of Li+ ions substantially improved the emission intensity. Table 1 also shows the effect of Li+ doping on the CIE 1931 chromaticity points of the LixEu0.05La0.95-xPO4 nanophosphors. When 1% Li+ ions was introduced into the crystal lattice of Eu0.05La0.95PO4, the CIE coordinates (x, y) were varied from (0.609, 0.391) to (0.606, 0.394). However, the chromaticity remains almost stable when further increasing the doping concentration of Li+ ions. This confirms the blue shift of the emission band after introducing the Li+ ions into the crystal structure of Eu0.05La0.95PO4. The emission bands of the assynthesized LixEu0.05La0.95-xPO4 nanophosphors are located between the orange and red regions. The mechanism of the effect of Li+ on the optical improvement has not yet been well established. It is speculated that the improved PL brightness resulting from the low fraction Li+ substitution (0-5%) in the lattice was due not only to the fast energy transfer from the host to the Eu3+ ions but also to a decrease in interstitial oxygen and hence to an increase in the hole concentration, leading to a decrease in competitive absorption and thus a higher quantum yield.26–33 Moreover, the enhancement can also be attributed to the reduced internal reflection that occurs for the spherical morphology of the obtained LixEuyLa1-x-yPO4 particles. However, doping of Li+ might also give rise to formation of a defective structure. Once

Characterization of LaPO4:Eu3+,Li+ Nanophosphors

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TABLE 1: Characteristic PL Featuresa As Summarized in Figure 7 Li+ concentration 0 0.01 0.05 0.10 0.15

1

2

3

4

588 nm 977 2198 2738 2520 2455

594 nm 1036 2497 2984 2675 2628

612 nm 422 826 969 899 887

622 nm 339 665 784 738 732

5

D0-7F1/ 5D0-7F2

2/3 2.46 3.08 3.06 2.98 2.96

(1 + 2)/(3 + 4) 2.65 3.15 3.26 3.17 3.14

CIE coordinates (x, y) (0.609, 0.391) (0.606, 0.394) (0.605, 0.395) (0.605, 0.394) (0.606, 0.394)

a5 D0-7F1/5D0-7F2 denotes the ratio of intensity of the 5D0-7F1 transition to the 5D0-7F2 transition. 2/3 means the ratio of intensity of Peak 2 to that of Peak 3. (1 + 2)/(3 + 4) means the ratio of intensity of (Peak 1 + Peak 2) to that of (Peak 3 + Peak 4).

resonant energy transfer between Eu3+ ions and a fraction of energy migration to distant killer or quenchers followed by the appearance of quenching behavior.47 The concentration quenching behavior of the as-produced Li0.05EuxLa0.95-xPO4 nanophosphors is observed in Figure 7a. It is apparent that for this type of phosphors, quenching started at an Eu concentration of 5% (molar ratio). Figure 7b shows the microwave irradiation temperature-dependent emission brightness of the 5D0 f 7F1 transition. The PL intensity increased as the irradiation temperature was increased from 500 to 800 °C and then reached saturation above 800 °C. The crystal structure of the sample obtained at lower temperature was fairly disordered, and quenching centers, such as impure NO3- and OH- ions, were present in relatively larger quantities,48 leading to weakened emission. Therefore, high-temperature irradiation treatment greatly reduced these quenching centers, thereby improving the emission. However, when the temperature was raised above the critical point, this effect became less pronounced and the emission intensity was maximized. 4. Conclusions

Figure 7. Emission intensity ascribed to the 5D0-7F1 transition as a function of (a) Eu3+ concentration and (b) microwave irradiation temperature.

Li+ attains a certain concentration (>0.05 in this case), the defects in the host lattice greatly increase and unavoidably reduce the crystallinity and increase the unactive center concentration, thereby leading to luminescence quenching.26–33 Figure 7 summarizes the emission intensity ascribed to the 5D -7F transition as a function of (a) the doped-Eu3+ active 0 1 center concentration and (b) the microwave irradiation temperature. Although detailed experiments are necessary to finely establish the relationship between the emission intensity and the Eu3+ concentration, the data presented in Figure 7a indicate a favorite Eu3+ concentration of around 5 mol %. The PL intensity was highly dependent on activator (Eu3+ in this case) concentration. Generally speaking, the brightness tended to increase with increasing activator concentration. Consequently, the luminescence began to decrease because pairing or aggregation of activator atoms at high concentration led to efficient

In summary, a simple, microwave-assisted, sol-gel technique was successfully established to synthesize monazite-type, lanthanum orthophosphate (LaPO4) nanoparticles in high yield. The prepared LaPO4 crystallized in a single-phase, monoclinic structure with a spherical, approximately 100 nm diameter, particle shape, which was composed of Ultrafine nanocrystals of a few nanometers in diameter. A high fraction of lithium ions and europium ions could be effectively doped into the crystal lattice of the monoclinic LaPO4 through the same microwave-assisted, sol-gel approach to thereby produce a homogeneous, LixEuyLa1-x-yPO4 solid solution. Room-temperature PL characterization revealed that introduction of 5 mol % Li+ into the crystal structure enhanced the PL emission brightness more than 2-fold and that the Li0.05Eu0.05La0.9PO4 nanophosphor showed the most promising PL performance with the most intense emission and purest color. Therefore, the Li+ and Eu3+ codoped LaPO4 obtained by the present method could be easily processed into various types of lamp and display due to its size suitability and spherical morphology. In this respect, our proposed solid-solution system, Li0.05Eu0.05La0.9PO4, is a very promising phosphor. The sol-gel process appears to be most suitable for obtaining small particles with a narrow size distribution. Furthermore, the sol-gel process offers a unique, low-cost solution, comprising dip or spincoating, to the production of high optical quality, luminescent thin films. Most importantly, the method described here can be readily extended to other rare earth phosphates. Acknowledgment. The authors wish to thank Prof. Y.-M. Sung at Korea University for his kind discussion. One of the authors (W.L.) was supported by the Brain Korea 21 Program: Center for Advanced Device Materials.

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