Growth of Highly Crystalline CaMoO4:Tb3+ Phosphor Layers on

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Growth of Highly Crystalline CaMoO4:Tb3+ Phosphor Layers on Spherical SiO2 Particles via Sol-Gel Process: Structural Characterization and Luminescent Properties Guangzhi Li,†,‡ Zhenling Wang,† Zewei Quan,† Chunxia Li,† and Jun Lin*,†

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 9 1797-1802

Key laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China and College of Chemistry and Medicine, Jiamusi UniVersity, Jiamusi 154007, P. R. China ReceiVed February 28, 2007; ReVised Manuscript ReceiVed May 29, 2007

ABSTRACT: Highly crystalline CaMoO4:Tb3+ phosphor layers were grown on monodisperse SiO2 particles through a simple sol-gel method, resulting in formation of core-shell structured SiO2@CaMoO4:Tb3+ submicrospheres. The resulting SiO2@CaMoO4: Tb3+ core-shell particles were fully characterized by powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectra (EDS), transmission electron microscopy (TEM), photoluminescence (PL), low-voltage cathodoluminescence (CL), and kinetic decays. The XRD results demonstrate that the CaMoO4:Tb3+ layers begin to crystallize on the SiO2 spheres after annealing at 400 °C and the crystallinity increases with raising the annealing temperature. SEM and TEM analysis indicates that the obtained submicrospheres have a uniform size distribution and obvious core-shell structure. SiO2@CaMoO4:Tb3+ submicrospheres show strong green emission under short ultraviolet (260 nm) and low-voltage electron beam (1-3 kV) excitation, and the emission spectra are dominated by a 5D4-7F5 transition of Tb3+ (544 nm, green) from the CaMoO4:Tb3+ shells. Moreover, the PL intensities of Tb3+ increase with increasing annealing temperature and number of coating cycles. The optimum concentration for Tb3+ was determined to be 5 mol % of Ca2+ in CaMoO4 host shells. 1. Introduction Nanometer- and micrometer-sized core-shell structured particles with special physical and chemical properties have attracted great attention due to their potential applications in photonic crystals, catalysis, diagnostics, and pharmacology.1-5 Core-shell particles have been synthesized using metals, semiconductors, metal oxides, alloys, dyes, biomolecules, etc., encapsulated inside silica or polymer shells (lattices) or vice versa.6,7 Usually silica or a polymeric shell protects the liquid or solid core material by isolation from the media (liquid or gas) in which the core-shell particles reside. However, core material is still available for application, for example, by photoexcitation,8 diffusion through the pores of the shell,9 or magnetic fields.10 Thin or thick shells can be synthesized depending upon the applications. A variety of techniques including the inverse micelle method, pretreatment steps in electroless plating, double-jet precipitation, layer-by-layer technique, template directed self-assembly, and encapsulation of silica nanoparticles by in situ polymerization were used to synthesize core-shell materials.11-14 The current demand for high resolution, high brightness, and high efficiency in phosphors for cathode ray tubes, field emissive displays, and plasma display panels has promoted development of phosphors. In particular, phosphors with nonagglomerated, monodisperse, spherical ( 0.099 nm, M ) Ca, Ba, Pb, Sr) exist in the socalled scheelite structure form (scheelite ) CaWO4), where the molybdenum atom adopts tetrahedral coordination.17 Scheelitetype molybdates of the same divalent metal are reciprocally soluble over the entire compositional range, which results in a rich family of solid-state solution compounds. CaMoO4 is important among metal molybdate families that have potential applications in various fields, such as in photoluminescence,18 microwave applications,19 white light emitting diodes,20 and laser materials.21 Silica submicrospheres prepared by the Sto¨ber method are the ideal core materials because of their inexpensiveness, easiness to get spherical particles with narrow size distribution, chemical inertness, and optical transparency. Tb3+ is an important lanthanide ion with green light emitting. In this paper, we report the preparation of monodisperse and coreshell structured SiO2@CaMoO4:Tb3+ submicrospheres through the sol-gel method, which was easy to scale up. By controlling the silica cores the obtained particles had spherical, monodisperse morphology and uniform shell thickness. The core-shell structure and morphology, photoluminescence, and cathodoluminescence properties of obtained products were investigated. 2. Experimental Section Synthesis of Silica Cores. Amorphous submicrometer spheres of silica in the size range of 500-600 nm were synthesized by basecatalyzed hydrolysis of tetraethoxysilane (TEOS) via the well-known Sto¨ber process, i.e., hydrolysis of TEOS in an ethanol solution containing water and ammonia.22 This method yielded the colloidal solution of silica particles with a narrow size distribution in the submicrometer range, and the particle size of silica depended on the relative concentration of the reactants. In a typical experiment the mixture containing 0.1 mol/L TEOS (99 wt %, analytical reagent, A. R., Beijing Beihua Chemicals Co., Ltd), 1 mol/L H2O, and 7 mol/L NH4OH (25 wt %, A. R.) was stirred at room temperature for 4 h, resulting in formation of a white silica colloidal suspension. The silica particles were centrifugally separated from the suspension and washed with ethanol four times.

10.1021/cg0701978 CCC: $37.00 © 2007 American Chemical Society Published on Web 08/03/2007

1798 Crystal Growth & Design, Vol. 7, No. 9, 2007 Scheme 1.

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Formation Process of SiO2@CaMoO4:Tb3+ Core-Shell Particles

Figure 1. X-ray diffraction patterns for the as-formed SiO2 (without annealing) (a), 900 °C-annealed [email protected] core-shell particles (b), and pure Ca0.95Tb0.05MoO4powders (c) as well as the JCPDS card 29-0351 for CaMoO4 (d). technique. The sample morphologies were inspected using a field emission scanning electron microscope (FESEM, XL30, Philips) equipped with an energy-dispersive X-ray spectrum (EDS, JEOL JXA840) and transmission electron microscope (TEM) and high-resolution transmission electron microscopy (HRTEM) (FEI Tecnai G2 S-Twin transmission electron microscope) with a field emission gun operating at 200 kV. The photoluminescence (PL) and cathodoluminescence (CL) spectra were taken on a Hitachi F-4500 spectrofluorimeter equipped with a 150 W xenon lamp and 1-3 kV electron beam (self-made electron gun, 10-6 Pa vacuum, filament current 102.5 mA) as the excitation source. Luminescence decay curves were obtained from a Lecroy Wave Runner 6100 Digital Oscilloscope (1GHz) using a 260 nm laser (pulse width ) 4 ns, gate ) 50 ns) as the excitation source (Continuum Sunlite OPO). All measurements were performed at room temperature (RT).

3. Results and Discussion

Coating of SiO2 Cores with CaMoO4:Tb3+ Shells. SiO2 coreCaMoO4:Tb3+ shell particles (SiO2@CaMoO4:Tb3+) were prepared by a sol-gel process.23 The doping concentrations of Tb3+ were 1-11 mol % that of Ca2+ in CaMoO4:Tb3+ host. Stoichiometric amounts of CaCO3 (99.99%, Beijing Beihua Chemicals Co., Ltd.) and Tb4O7 (99.99%, Shanghai Yuelong Nonferrous Metals Limited) were dissolved in dilute HNO3 (A. R., Beijing Beihua Chemicals Co., Ltd.), under vigorous stirring, and the pH of the solution was kept between 2 and 3. Then a suitable amount of water-ethanol (v/v ) 3:1) solution and a stoichiometric amount of ammonium molybdate [(NH4)6Mo7O24‚ 4H2O, A. R., Beijing Beihua Chemicals Co., Ltd.] were added to the solution. Citric acid (A. R., Beijing Beihua Chemicals Co., Ltd.) was added to the above solution as a chelating agent for the metal ions. The molar ratio of metal ions to citric acid was 1:2. A certain amount of polyethylene glycol (PEG, molecular weight ) 10 000, A. R., Beijing Beihua Chemicals Co., Ltd) was added as a cross-linking agent. Highly transparent sols were obtained after stirring for a few hours, and then the as-prepared fresh silica particles (without high-temperature annealing) were added under stirring. The suspension was further stirred for another 3 h, and then the silica particles were separated by centrifugation. The samples were dried at 100 °C for 1 h and then annealed to the desired temperature (400-900 °C) with a heating rate of 1 °C/min and held there for 2 h in air. The above process was repeated several times (i.e., to form several phosphor layers on SiO2 cores) to increase the thickness of the CaMoO4:Tb3+ shells. In this way, the core-shell structured SiO2@CaMoO4:Tb3+materials were obtained, and the whole process is shown in Scheme 1. For the purpose of comparison, the coating sol was evaporated to form powders, which were annealed in a similar process to produce the pure CaMoO4:Tb3+ powder phosphors. Characterization. The powder X-ray diffraction (XRD) of the samples was examined on a Rigaku-Dmax 2500 diffractometer using Cu KR radiation (λ ) 0.15405 nm). FT-IR spectra were measured with Perkin-Elmer 580 B infrared spectrophotometer with the KBr pellet

3.1. Formation, Structure, and Morphology. (a) XRD. The XRD results demonstrate that the core-shell particles begin to crystallize after being annealed at 400 °C. Typical XRD patterns for the as-formed fresh SiO2 (without annealing) (a), 900 °C-annealed [email protected] (b), and pure Ca0.95Tb0.05MoO4 (c) powder samples as well as the JCPDS card (No. 29-0351) for CaMoO4 (d) are shown in Figure 1. For SiO2 particles (Figure 1a) no diffraction peak is observed except for a broad band centered at 2θ ) 22.00°, which is the characteristic peak for amorphous SiO2 (JCPDS 29-0085). For the [email protected] core-shell sample (Figure 1b) besides the broad band at 2θ ) 22.00° from amorphous SiO2 all the diffraction peaks belonging to crystalline CaMoO4 are present, suggesting that the coatings of Ca0.95Tb0.05MoO4 have crystallized well on the surfaces of amorphous silica particles. This is in good agreement with the situation for the pure Ca0.95Tb0.05MoO4 powder sample (Figure 1c in which well crystalline CaMoO4 is observed). No additional phase was detected, indicating that no reaction occurred between the core and shell components during the annealing process. (b) FT-IR. The FT-IR spectra of the as-formed fresh SiO2 (without annealing), 900 °C-annealed [email protected] core-shell sample, and the pure Ca0.95Tb0.05MoO4 powders are shown in Figure 2a, b, and c, respectively. In Figure 2a for the as-formed SiO2 particles the absorption bands due to OH (3435 cm-1), H2O (1637 cm-1), Si-O-Si (νas, 1100 cm-1; νs, 803 cm-1), Si-OH (νs, 949 cm-1), and Si-O (δ, 469 cm-1) bonds (where νas ) asymmetric stretching, νs ) symmetric stretching, δ ) bending) are observed.24 This indicates that the as-formed SiO2 particles contain a large amount of OH groups and H2O on their surfaces.25 The surface Si-OH groups play an important role for bonding the metal ions (Ca2+, Tb3+) from the coating

Highly Crystalline CaMoO4:Tb3+ Phosphor Layers

Figure 2. FT-IR spectra of the as-formed SiO2 (without annealing) (a), 900 °C- annealed [email protected] core-shell samples (b), and Ca0.95Tb0.05MoO4 powder (c).

sol and forming the Ca0.95Tb0.05MoO4 layers on the SiO2 surfaces in the following annealing process, as shown in Scheme 1. In Figure 2b for the [email protected] core-shell sample the characteristic absorption peak of the Mo-O bond (νs, 818 cm-1),26 Si-O-Si bond (1103 cm-1), and Si-O bond (469 cm-1) for amorphous SiO2 (Figure 2a) have been observed clearly. For pure Ca0.95Tb0.05MoO4 powders (Figure 2c) the Mo-O bond (νs, 818 cm-1) is observed also. This suggests that the crystalline phase of CaMoO4 has formed after annealing at 900 °C, agreeing well with the results of XRD. The signal of OH groups from the as-formed silica particles have almost disappeared for [email protected] core-shell particles annealed at 900 °C, further demonstrating formation of crystalline Ca0.95Tb0.05MoO4 coatings on the silica surfaces via the sol-gel deposition and annealing process. (c) FESEM and TEM. Figure 3 shows the FESEM micrographs of the as-formed SiO2 particles (a), SiO2 particles coated by four layers of Ca0.95Tb0.05MoO4 (b), and pure Ca0.95Tb0.05MoO4 powders (c). From the FESEM micrograph of Figure 3a we can observe that the as-formed SiO2 consists of spherical particles with an average size of 500 nm, and these particles are nonaggregated with narrow size distribution. After

Crystal Growth & Design, Vol. 7, No. 9, 2007 1799

functionalizing the silica particles by Ca0.95Tb0.05MoO4 coatings the resulting [email protected] particles still keep the morphological properties of the silica particles, i.e., these particles are still spherical and nonaggregated, but are slightly larger than the pure silica particles due to the additional layers of Ca0.95Tb0.05MoO4 on them, as shown in Figure 3b. This indicates that all of the Ca0.95Tb0.05MoO4 materials have been coated on the surfaces of silica particles by our experimental process. In contrast, the pure Ca0.95Tb0.05MoO4 powders contain aggregated particles, and their size is about 80-110 nm, as shown in Figure 3c. However, it should be mentioned that the FESEM micrographs can only provide basic information on the morphology of [email protected] particles on a large scale (namely, all of the SiO2 particles remain spherical and nonaggregated after coating with Ca0.95Tb0.05MoO4 layers on them) and the core-shell structure of [email protected] particles cannot be resolved from the FESEM micrographs due to the low magnification. The result of EDS (Figure 3d) for [email protected] shows that the composites are composed of Si, O, Ca, Tb, and Mo elements, which is consistent with the formula composition. To see the core-shell structure of [email protected] particles TEM was performed. Representative TEM micrographs for the SiO2 particles coated four times (layers) by Ca0.95Tb0.05MoO4 shells as well as a high-resolution TEM (HRTEM) image of Ca0.95Tb0.05MoO4 shells are shown in Figure 4a and b, respectively. In Figure 4a the core-shell structure for the [email protected] particles can be clearly seen due to the different electron penetrability for the cores and shells. The cores are black spheres with an average size of 500 nm, and the shells have gray color with an average thickness of 65 nm. The HRTEM image of [email protected] core/shell (Figure 4b) sample indicates that the shell of Ca0.95Tb0.05MoO4 is highly crystalline, and the distances between the adjacent lattice fringes, measured as 0.32 nm, are the interplanar distances of the CaMoO4 (112) plane, agreeing well with the (112) d spacing in the literature, 0.31 nm (JCPDS No. 29-0351). 3.2. Photoluminescence and Cathodoluminescence Properties. (a) Photoluminescence Properties. The SiO2@CaMoO4: Tb3+ submicrospheres show strong green emission under short

Figure 3. SEM micrographs of the as-formed SiO2 (a), the SiO2 particles coated with four layers of Ca0.95Tb0.05MoO4 (b), pure Ca0.95Tb0.05MoO4 powders (c), and the energy-dispersive X-ray analysis of [email protected] sample in b.

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Figure 5. Excitation (a) and emission [email protected] core-shell particles.

(b)

spectra

of

Figure 6. Decay curve for the 5D4-7F5 (544 nm) emission of Tb3+ in [email protected] sample annealed at 900 °C (λex ) 260 nm laser).

Figure 4. TEM micrograph of SiO2 coated with four layers of Ca0.95Tb0.05MoO4 (a), and HRTEM image of the marked area (b).

ultraviolet (260 nm) excitation. Figure 5 shows the excitation (a) and emission (b) spectra of [email protected] coreshell phosphors. The excitation spectrum was obtained by monitoring the emission of the Tb3+ 5D4-7F5 transition at 544 nm. It can be seen clearly that the excitation spectrum consists of a strong and broad band from 200 to 350 nm with a maximum at about 260 nm, which corresponds to the charge-transfer transitions within the MoO42- groups.27 The f-f transitions within the Tb3+ 4f8 configuration can hardly be detected because of their weaker intensity. Upon excitation into the MoO42- at 260 nm the obtained emission spectrum (Figure 5b) contains exclusively characteristic emission of Tb3+ with 5D4-7F5 green emission (544 nm) as the most prominent group (other transitions are labeled in the figure). The blue emission from MoO42is completely quenched, suggesting that an efficient energy transfer from MoO42- to Tb3+ has occurred in the [email protected] sample. Furthermore, emission from

the 5D3 level of Tb3+ is much weaker than that from the 5D4 level due to the cross-relaxation effect of Tb3+.28 The representative decay curve for the luminescence of Tb3+ in the [email protected] core-shell phosphors annealed at 900 °C is shown in Figure 6. The decay curve for 5D4-7F5 (544 nm) of Tb3+ can be well fitted into a double-exponential function as I ) A1exp(-t/τ1) + A2 exp(-t/τ2) (τ1 and τ2 are the fast and slow components of the luminescence lifetimes and A1 and A2 are the fitting parameters), and the fitting results are shown in Figure 6. The average lifetime for 5D4-7F5 (544 nm) of Tb3+ is about 8.11 ms, as determined by the formula as τ ) (A1τ12 + A2τ22)/(A1τ1 + A2τ2).29 The double-exponential decay behavior of the activator is frequently observed when the excitation energy is transferred from the donor to acceptor.29,30 (b) Cathodoluminescence Properties. Similar to the emission under UV light illumination, the [email protected] core-shell particles also exhibit strong green luminescence under excitation of an electron beam. Typical emission spectra under excitation of an electron beam (1-3 kV) are shown in Figure 7, which are basically consistent with the PL emission spectrum (Figure 5b). In the CL spectra for Tb3+ in [email protected] only emissions from 5D4-7F3,4,5,6 are observed due to an efficient energy transfer from MoO42- to

Highly Crystalline CaMoO4:Tb3+ Phosphor Layers

Figure 7. CL spectra of [email protected] core-shell particles as a function of accelerating voltage.

Tb3+ as well as direct excitation of Tb3+ by the plasmas produced by the incident electrons. In Figure 7 it can also be seen clearly that the CL intensity increases with the increase of accelerating voltage from 1 to 3 kV. It is known that the penetration depth of electrons into a particular specimen is determined by the energy of the electron beams. The higher the energy, the greater the penetration depth. The electron penetration depth (L) can be estimated by the empirical formula L [Å] ) 250(A/F)(E/Z1/2)n, where n ) 1.2/(1-0.29 log10Z), A is the atomic or molecular weight of the material, F is the density, Z is the atomic number or the number of electrons per molecule in the case compounds, and E is the anode voltage (kV).31 For CaMoO4 phosphors the electron penetration depths at the anode voltage of 1, 1.5, 2, 2.5, and 3 kV are estimated to be 2, 6, 14, 26, and 44 nm, respectively. These values are within the shell thickness (65 nm) of CaMoO4:Tb3+ in the [email protected] core-shell particles. With the increase of accelerating voltage, more plasmas will be produced by the incident electrons, resulting in more MoO42- and Tb3+ ions being excited and thus higher CL intensity resulting. 3.3. Tuning the PL Emission Intensity. (a) Temperature Effect. The PL emission intensity (defined as the integrated area intensity of 5D4-7F5 emission peak, and this holds for all the following sections) of the Tb3+ in [email protected] core-shell phosphors was affected by the annealing temperature. Figure 8shows the effect of annealing temperature on the PL intensity. Clearly, the PL intensity increases with increasing annealing temperature. This is because with the increase of annealing temperature the content of impurities in the [email protected] core-shell phosphors such as _OH, NO3-, _CH2, and others decreases and the crystallinity of the Ca0.95Tb0.05MoO4 shell increases. Quenching of the luminescence of the rare-earth ions by vibrations of these impurities decreases, resulting in the increase of the emission intensity. (b) Concentration Effect. By varying the content of the Tb3+ in CaMoO4 host we determined the compositions with the highest PL emission intensity. Figure 9 shows the dependence of the PL emission intensity of Tb3+ on its doping concentration (x) in SiO2@Ca(1-x)TbxMoO4core-shell phosphors. It can be found that the PL emission intensity of Tb3+ increases with the increase of its concentration (x) first, reaching a maximum value at x ) 5 mol %, and then decreases with increasing content (x) due to the concentration quenching effect. Thus, the optimum concentration for Tb3+ is 5 mol % of Ca2+ in CaMoO4 host. (c) Number of Coatings (N) Effect. The number of the coatings (N) is also an important factor influencing PL intensity.

Crystal Growth & Design, Vol. 7, No. 9, 2007 1801

Figure 8. PL emission intensities of Tb3+ in [email protected] sample as a function of annealing temperature.

Figure 9. PL emission intensities of Tb3+ as function of its concentration (x) in SiO2@Ca(1-x) Tb x MoO4 samples.

Figure 10. PL emission intensities of Tb3+ as a function of the number of layers (coating times, N) of Ca0.95Tb0.05MoO4on SiO2 particles. The PL intensity of pure Ca0.95Tb0.05MoO4 powder measured under the same experimental conditions is also given for comparison.

Figure 10 shows the effect of coating number (i.e., the number of layers) on the PL intensity of [email protected] core-

1802 Crystal Growth & Design, Vol. 7, No. 9, 2007

shell phosphors annealed at the same temperature. The PL intensity increases with increasing coating number, which is due to the increase of the thickness of Ca0.95Tb0.05MoO4 shells on the SiO2 spheres. When the coating number is 9, the PL intensity of core-shell phosphors is about 88% of that of pure Ca0.95Tb0.05MoO4 powders excited at a wavelength of 260 nm. Here it should be mentioned that the luminescent properties (e.g., PLE and PL spectra, decay lifetime) between a bulk Ca0.95Tb0.05MoO4 and core-shell structured [email protected] are similar except for the PL intensity discussed above. 4. Conclusions Submicrometer SiO2@CaMoO4:Tb3+ core-shell phosphors were successfully prepared by the sol-gel process. The obtained SiO2@CaMoO4:Tb3+ core-shell phosphors have spherical morphology, submicrometer size, and narrow size distribution. The PL intensities of the core-shell phosphors can be tuned by the annealing temperature and number of coatings. With the increase of annealing temperature and number of coatings the PL intensities increase. The optimum concentration for Tb3+ was determined to be 5 mol % of Ca2+ in CaMoO4 host. Acknowledgment. This project was financially supported by the foundation of “Bairen Jihua” of the Chinese Academy of Sciences, the MOST of China (2003CB314707), and the National Natural Science Foundation of China (50572103, 20431030, 00610227). References (1) Schartl, W. AdV. Mater. 2000, 12, 1899. (2) Caruso, F. AdV. Mater. 2001, 13, 11. (3) Suryanarayanan, V.; Nair, A. S.; Tom, R. T. J. Mater. Chem. 2004, 14, 2661. (4) Oldenberg, S. J.; Averitt, R.D.; Westcott, S. L.; Halas, N. J. Chem. Phys. Lett. 1998, 288, 243. (5) LizMarzan, L. M.; Giersig, M.; Mulvaney, P. Langmuir 1996, 12, 4329. (6) Sertchook, H.; Avnir, D. Chem. Mater. 2003, 15, 1690. (7) (a) Liz-Marz´an, L. M.; Correa-Durate, M. A.; Pastorza-Santos, I.; Mulvaney, P.; Ung, T.; Giersig, M.; Kotov, N. A. In Handbook of Surfaces and Interfaces of Materials: Nanostructured Material, Micelles and Colloids; Nalwa, H. S., Ed.; 2001; Academic Press:

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