Low temperature microwave solid-state synthesis of red-emitting CaMoO₄:Eu³⁺, Li⁺ phosphors with controlled morphology

Abstract

CaMoO₄:Eu³⁺, Li⁺ red phosphors have been successfully achieved by a microwave-assisted solid-state reaction, and their morphologies and luminescence properties have been studied in this paper. The as-prepared Ca_0.9MoO₄:0.05Eu³⁺, 0.05Li⁺ powders occur as uniform spherical particles with a size distribution of 1–2 μm via the optimized reaction temperature at 600 °C and reaction time of 1 h. The phosphors showed an intense red emission with maxima at 615 nm, which was ascribed to the Eu³⁺ electric dipole transition of ⁵D₀ → ⁷F₂. The comparison on the morphology and luminescence property between the as-prepared samples and the samples obtained by the conventional method was discussed in detail. A possible microwave-assisted solid-state reaction mechanism was proposed, and the effect of the microwave sintering temperature and time was also discussed.

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1. Introduction

Rare earth ion (RE³⁺)-doped metal molybdates, MMoO₄:RE³⁺ (M = Ca, Sr, Ba, Pb, Cd, etc.), with a scheelite structure are widely known as multifunctional materials which possess unique physical and chemical properties.¹ These compounds generally have a significant potential for applications in devices such as lasers, scintillators, catalysts, and phosphors for white light-emitting diodes (WLEDs). Eu³⁺-doped CaMoO₄, in particular, has been investigated extensively as a red-emitting phosphor and considered as an ideal substitute for commercial red phosphors for WLEDs,^2,3 because CaMoO₄:Eu³⁺ red phosphors have various optimal performances in stability, rich 4f–4f energy level and efficient emission of the electric dipole transition ⁵D₀ → ⁷F₂. CaMoO₄:Eu³⁺ red phosphors are commercially prepared by conventional solid-state reactions; however, the as-prepared particles of phosphors are large-sized irregular aggregates, which seriously affect their luminescence properties.⁴ Therefore, it is still one of the most challenging issues to prepare CaMoO₄:Eu³⁺ red phosphors with small-sized, uniformly dispersed spherical particles.

Many studies on micro/uniform structures with nano sizes provide an excellent chance to review the excellent properties of CaMoO₄ compared with the traditional bulk counterparts.^5–8 CaMoO₄ micro/nanomaterials, including nanorods,⁹ nanofibers,¹⁰ nanoflakes,^5,11 persimmon-like,¹² and flower-like three-dimensional (3D) superstructures,¹³ have been successfully fabricated by different methods such as the co-precipitation method,¹⁴ microwave radiation,¹⁵ hydro/solvothermal synthesis,^11,16 solution-phase rapid-injection based route,¹² and chemical deposition.¹⁷ Among the various synthesizing techniques, microwave-assisted solid-state sintering is fundamentally different from conventional sintering. Over the last decade, microwave heating as a process has developed into a highly valuable technique, offering an efficient alternative energy source for numerous chemical reactions and processes. Compared to conventional methods, microwave-assisted synthesis shows many advantages such as rapid heating, non-contact heating, energy transfer instead of heat transfer, rapid start-up and stopping of heating, higher yields in shorter reaction times, low reaction temperature and homogeneous thermal transmissions.^18–20 Moreover, the key point is that the sample absorbs microwave energy and is heated by volumetric heating during microwave-assisted sintering.²¹ It is accepted that the microwave-assisted method has become one of the promising routes for the preparation of phosphors with extremely fine particles and different morphologies.

Generally, the realization of target samples with various microstructures require complicated manipulation or rigid conditions, and it is apparent that the complicated process restricts the popularization of synthetic strategies or technique applications in practical productions. Therefore, the rapid and large-amount preparation of the CaMoO₄:Eu³⁺, Li⁺ red phosphor is still a challenge. It is strongly desirable to develop an easy and energy-efficient route to obtain the targeted materials with a uniform and controlled morphology. In this work, Ca_0.9MoO₄:0.05Eu³⁺, 0.05Li⁺ phosphors with uniform spherical particles have been obtained by a microwave-assisted solid-state process, and the samples were evaluated from the crystallization process, particle morphology evolution and photoluminescence property.

2. Experimental procedure

2.1 Samples preparation

Ca(NO₃)₂·4H₂O, (NH₄)₆Mo₇O₂₄·4H₂O, Li₂CO₃ and Eu₂O₃ with a purity of 99.99% were used as starting materials. The powder samples were weighed as per the designed formula of Ca_0.9MoO₄:0.05Eu³⁺, 0.05Li⁺ and mixed in an agate mortar for 1 h. After drying at 95 °C for 12 h, the Ca_0.9MoO₄:0.05Eu³⁺, 0.05Li⁺ phosphor powders were finally synthesized via a microwave assisted sintering in a microwave sintering furnace (Qingdao MKX-M1, 2.45 GHz) with a controllable power up to 800 W and tunable temperature up to 1000 °C. Fig. 1 shows the schematic flow chart for the preparation of the Ca_0.9MoO₄:0.05Eu³⁺, 0.05Li⁺ phosphor. The mixed powders were sintered at selected temperatures such as 150, 200, 400, 600, 700 and 800 °C for different times, such as 1 h with a microwave power of 750 W. In order to compare the effect of different sintering techniques, the Ca_0.9MoO₄:0.05Eu³⁺, 0.05Li⁺ phosphor was also prepared at 800 °C for 2 h using a conventional sintering furnace.

Fig. 1 Flow chart for the preparation of the Ca_0.9MoO₄:0.05Eu³⁺, 0.05Li⁺ phosphors prepared by microwave-assisted sintering and conventional sintering.

2.2 Characterization

The crystalline phases of the as-prepared samples were identified by an X-ray diffractometer (Bruker D8 advance) operating at 40 kV and 40 mA with Cu Kα radiation (λ = 1.5406 Å). The microstructure and morphology were analyzed by a scanning electron microscope (SEM) (Model CSM950, OPTON, Germany) and transmission electron microscope (TEM, JEM-2010) with an accelerated voltage of 200 kV. The photoluminescence (PL) spectra were characterized on an F-4600 fluorescence spectrophotometer with a photomultiplier tube operating at 400 V, and a 150 W Xe lamp as the excitation source. The particle size distribution was measured by a laser particle size analyzer (Mastersizer 2000, Malvern Instruments Ltd.).

3. Results and discussion

Fig. 2 shows the XRD patterns of the Ca_0.9MoO₄:0.05Eu³⁺, 0.05Li⁺ phosphors prepared by microwave-assisted sintering at 150–800 °C for 1 h. Evidently, the sample obtained at 150 °C is poorly crystalline; thus, such a temperature is not enough for the phase formation, though several weak peaks assigned to CaMoO₄ can be also identified. It can be seen that there were still some impurities of MoO₃ for the sample obtained at 200 °C, but prominent peaks corresponding to the crystalline CaMoO₄ phase appeared, which could be identified as the tetragonal structure CaMoO₄ (JCPDS card no. 29-0351). Above 400 °C, the characteristic diffraction peaks of the CaMoO₄ phase were further enhanced, and additional impurities were not detected. In order to make a comparison between the microwave and conventional solid-state methods, the XRD patterns of Ca_0.9MoO₄:0.05Eu³⁺, 0.05Li⁺ synthesized by the conventional method at 200, 300, 600 and 800 °C for 1 h are given in Fig. 3. It can be seen from Fig. 3 that the samples prepared at 200 and 300 °C are poorly crystalline, and an impurities phase can also be observed even when it was prepared at 600 °C. Obviously, the microwave reaction method is more efficient to prepare the crystalline CaMoO₄ with low temperatures.

Fig. 2 XRD patterns of microwave synthesized sample at different temperatures for 1 h: S₁-150 °C, S₂-200 °C, S₃-400 °C, S₄-600 °C, S₅-700 °C, S₆-800 °C.

Fig. 3 XRD patterns of the samples synthesized by the conventional method at 200, 300, 600 and 800 °C for 1 h.

The photoluminescence excitation (PLE) and emission (PL) spectra of the Ca_0.9MoO₄:0.05Eu³⁺, 0.05Li⁺ phosphor obtained at 600 °C for 1 h are illustrated in Fig. 4. The excitation spectrum on the left was monitored at 615 nm and is composed of an intense, broad band ranging from 200 to 340 nm and a sequence of narrow weak peaks ranging from 350 to 480 nm. In fact, the intense and broad band can be decomposed into two Gaussian components as mentioned in the ref. 22, in which one peak around 265 nm is assigned to the charge transfer (CT) band of Eu³⁺ → O²⁻, and the other one around 310 nm corresponds to the CT band of Mo⁶⁺ → O²⁻. Noticeably, only one intense and broad peak centered at 285 nm can be found in Fig. 4, which cannot be distinguished due to the spectral overlap as also mentioned in other ref. 23 and 24. Moreover, the intense, broad band in the UV region may consist of the charge transfer excitation of the Eu³⁺ ions and energy-transfer transition from the molybdate groups to the Eu³⁺ ions. The CT band can be attributed to the electronic transition from the 2p orbital of O²⁻ to the 4f orbital of Eu³⁺, which is closely related to the covalency between O²⁻ and Eu³⁺ and the coordination environment around Eu³⁺. The decrease in energy for the electron transfer in O²⁻ to Eu³⁺ represents the increase in the covalency and the decrease in the ionicity of oxygen and Eu³⁺. Above 350 nm, the narrow weak peaks are attributed to the 4f–4f transitions of the Eu³⁺ ions within its 4f⁶ configuration.²⁵ Among these characteristic excitation transitions, the ⁷F₀ → ⁵L₆ line at around 393 nm is the most intense one. For the emission spectrum, upon the excitation of 285 nm, we can find groups of several narrow sharp lines, which correspond to the ⁵D₀ → ⁷F_J (J = 0, 1, 2, 3, 4) transition belonging to the characteristic emission of the Eu³⁺ ions.²⁶ Among these peaks, the weak emissions located at 536 and 571 nm are ascribed to the Eu³⁺ magnetic dipole transition of ⁵D₀ → ⁷F₀ and ⁵D₀ → ⁷F₁, respectively, which are insensitive to the site symmetry. The stronger red emission around 615 nm is ascribed to the Eu³⁺ electric dipole transition of ⁵D₀ → ⁷F₂ and is much more intense than that of ⁵D₀ → ⁷F₀ or ⁵D₀ → ⁷F₁, which suggests that the Eu³⁺ ions occupy the lattice sites without inversion symmetry.

Fig. 4 Photoluminescence excitation and emission spectra of the CaMoO₄:Eu³⁺, Li⁺ phosphor obtained at 600 °C for 1 h.

As is well known, charge compensation is adopted to enhance the luminescent intensities of the Eu³⁺-doped CaMoO₄ phosphor, since the Eu³⁺ ions replaces the Ca²⁺ sites with a non-equivalence substitution. Fig. 5 shows the emission spectra excited at 285 nm of two types of samples for Ca_0.9MoO₄:0.05Eu³⁺, 0.05Li⁺ and Ca_0.95MoO₄:0.05Eu³⁺ obtained at various temperatures. As a result, the emission intensity of Ca_0.9MoO₄:0.05Eu³⁺, 0.05Li⁺ in Fig. 5(a) is much stronger than that of the sample without charge compensation in Fig. 5(b) at the same temperature. It can also be observed that the effect of promotion on the emission intensity from the Li⁺ charge compensation is demonstrated more distinctly as the temperature increases. It is believed that these changes are ascribed to the smaller ionic radius of Li⁺ (92 pm), which is similar to that of Ca²⁺ (112 pm), and Li⁺ is expected to enter into the matrix lattice smoothly and make the lattice distorted along with reducing the symmetry. Also, the reduction in symmetry enhances the Eu³⁺ ions to deviate from centrosymmetric sites in the lattice. Accordingly, the more Eu³⁺ ions deviate from the centrosymmetric sites, the more beneficial ⁵D₀ → ⁷F₂ transition of Eu³⁺ around 615 nm occurs.²⁷ In addition, though the Li⁺ as a charge compensator can dramatically increase the intensity of the emission peak, the shape and position of the present peak seldom changes by the impact of charge compensation. Concurrently, as can be demonstrated in any one of the emission spectra in Fig. 5(a) or (b), the luminescent intensity of the characteristic emission under excitation at 285 nm initially increased and then decreased slightly with the rise in temperature. Also, the optimum experimental condition for the sample is at 600 °C for 1 h.

Fig. 5 The emission spectra of Ca_0.9MoO₄:0.05Eu³⁺, 0.05Li⁺ (a) and Ca_0.95MoO₄:0.05Eu³⁺ (b) under the excitation of 285 nm at different temperatures for 1 h.

It is known that the luminescence characteristics of phosphor particles also strongly depend on the morphology of the particles, such as size, shape, and size distribution. Fig. 6 displays the SEM images of the Ca_0.9MoO₄:0.05Eu³⁺, 0.05Li⁺ phosphors sintered at various temperatures in the microwave furnace. It is found that the morphologies of the samples obtained at different temperatures are much different from each other with the increasing reaction temperature. When reaction temperature is 150 °C, some irregular amorphous particles can be found, and nearly-spherical microstructures with good crystallinity can be observed only when temperature reaches to 400 °C, which corresponds to the pure phase formation obtained by the XRD analysis. Obviously, the as-prepared Ca_0.9MoO₄:0.05Eu³⁺, 0.05Li⁺ powders obtained at 600 °C possess the most uniform spherical particles with a narrow particle size distribution of 1–2 μm, and these spherical particles connect with each other to form a layer of reticular structure, which is also verified by the strongest photoluminescence intensity, as discussed in Fig. 5. When the reaction temperature is further increased, some agglomerates can be formed corresponding to the excessive nucleation behavior, which in turn will induce some defect and decrease the emission intensity.^28,29 The SEM images for the samples obtained by different sintering techniques, the microwave heating process and conventional heating process, are comparatively given in Fig. 7. It is observed that the samples sintered by microwave-assisted sintering have a smaller grain size than that of the samples produced by the conventional process. As shown in Fig. 7(b), compared with the sample obtained by the microwave process, the surface of the samples by the conventional method is rather rough, and the size distribution is located in a wide range. In order to clearly show the results, Table 1 summarizes the information on the experimental conditions and morphology characteristics of the as-prepared samples. From the data in Table 1, we can find that the average size of the particles tends to be smaller or homogeneous, and the surface of the micro-spheres becomes much smoother as the temperature rises up to 600 °C. Furthermore, the morphology for the selected Ca_0.9MoO₄:0.05Eu³⁺, 0.05Li⁺ sample obtained at 600 °C was further characterized by TEM (Fig. 8). Moreover, a high-resolution TEM (HRTEM) image is shown in Fig. 8(b). The distance between the adjacent lattice fringes is 2.608 Å, corresponding to the spacing of the (200) planes of CaMoO₄. The chemical composition of the CaMoO₄:Eu³⁺, Li⁺ particle was also checked by EDS, which demonstrates that it consisted of Ca, Mo and O.

Fig. 6 SEM images of CaMoO₄:Eu³⁺, Li⁺ prepared at different temperatures: (a) 150 °C, (b) 200 °C, (c) 400 °C, (d) 600 °C, (e) 700 °C, (f) 800 °C.

Fig. 7 SEM images of CaMoO₄:Eu³⁺, Li⁺ prepared at 800 °C by different preparation processes: S₆-(a) microwave process, S₇-(b) conventional process.

Table 1 Morphological characteristics of the particles prepared at different temperatures

Samples	Temperature (°C)	Reaction time (h)	Size (μm)	Shape
S1	150	1	5.0	Massive particle with rough surface
S2	200	1	2.1	Sphere-like with rough surface
S3	400	1	1.6	Sphere-like with uniform surface
S4	600	1	1.0	Sphere-like with smooth surface
S5	700	1	1.5	Agglomeration with grains
S6	800	1	2.0	Agglomeration with more grains
S7	800	2	0.5–6	Massive particle with agglomeration

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Fig. 8 TEM images and EDS spectrum of CaMoO₄:Eu³⁺, Li⁺ at 600 °C for 1 h.

The reaction time dependence of the phase structure, morphology, and luminescence properties is also discussed in this work. Fig. 9 gives the XRD patterns of the samples obtained at 600 °C for 0.25–3 h. It can be seen from Fig. 9 that all diffraction peaks of the samples can be well indexed to the pure tetragonal scheelite structure of CaMoO₄ (JCPDS card no. 29-0351), even when the reaction time is only 0.5 h, while there are several impurity diffraction peaks for the sample obtained at 0.25 h. When the reaction time reached 1 h, highly-crystalline CaMoO₄ phases were formed, and this optimized reaction time is used in our present study. Fig. 10 gives the emission spectra of the Ca_0.9MoO₄:0.05Eu³⁺, 0.05Li⁺ samples obtained at different times, which possess similar spectral profiles except for different emission intensities. Several narrow emission lines corresponding to transitions from the ⁵D₀ → ⁷F₀ (536 nm), ⁵D₀ → ⁷F₁ (592 nm), ⁵D₀ → ⁷F₂ (615 nm), ⁵D₀ → ⁷F₃ (654 nm), ⁵D₀ → ⁷F₄ (702 nm) can be observed.³⁰ We can also find that maximum emission intensities can be obtained at 1 h. However, we also observe that emission intensities cannot be enhanced obviously when the sintering time is above 1 h, which may be ascribed to the fact that the size of the particles, distribution and other morphology properties change when the sintering time is above 1 h. Therefore, the heating time of 1 h is most efficient to prepare the Ca_0.9MoO₄:0.05Eu³⁺, 0.05Li⁺ phosphors with excellent red emissions.

Fig. 9 XRD patterns of the microwave sample at 600 °C for different sintering times and the CaMoO₄ simulated pattern.

Fig. 10 The emission spectra of the CaMoO₄:Eu³⁺, Li⁺ phosphors at 600 °C for different sintering times.

We further studied the impact of sintering time on the microstructure of the samples, and Fig. 11 displays SEM images of the Ca_0.9MoO₄:0.05Eu³⁺, 0.05Li⁺ products obtained at 600 °C for various sintering times. The SEM images clearly show the evolution process of the particles' size and morphology. When the sintering time reached 1 h, the size of the sphere-like particles decreased slightly, and the surface of grains became much smoother and homogeneous. Moreover, Table 2 gives additional specific information on the structural features of the samples obtained at 600 °C for different sintering times. From Fig. 11 and Table 2, we can also find that a few changes in the grain size with a diameter of approximately 1–2 μm can be measured; however, some minor agglomeration of the particles appeared when the sintering time was above 1 h. Therefore, the results revealed that the sintering time has a slight influence on the microstructure, especially when the heating time is above 1 h, which is consistent with the PL study. Furthermore, the particle size distribution of the typical Ca_0.9MoO₄:0.05Eu³⁺, 0.05Li⁺ sample synthesized at 600 °C for 1 h by the microwave-assisted process was analyzed in Fig. 12. The center particle d(0.5) of the Ca_0.9MoO₄:Eu³⁺, Li⁺ phosphor was 1.007 μm, and the center peak corresponding to the main particle size ranged from 0.5 to 2 μm, which sufficiently corresponds to the result on the particle size of 1–2 μm measured from the SEM images.

Fig. 11 SEM images of CaMoO₄:Eu³⁺, Li⁺ at 600 °C for different sintering times: (a) 0.25 h, (b) 0.5 h, (c) 1 h, (d) 2 h, (e) 3 h.

Table 2 Morphological features of the particles prepared with different times

Reaction time (h)	Temperature (°C)	Size (μm)	Shape
0.25	600	2.1	Sphere-like with rough surface
0.5	600	1.7	Sphere-like with uniform surface
1	600	1.0	Sphere-like with smooth surface
2	600	1.1	Sphere-like with smooth surface
3	600	1.3	Sphere-like with smooth surface

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Fig. 12 Particle size distribution of the CaMoO₄:Eu³⁺, Li⁺ phosphor at 600 °C.

On the basis of the optimization of the microwave-assisted sintering temperature and time, the phase structure, morphology and luminescence property for the Ca_0.9MoO₄:0.05Eu³⁺, 0.05Li⁺ powders have been demonstrated, and the as-prepared samples possess uniform spherical particles with a narrow particle size distribution and excellent luminescence properties, which also shows excellent properties over those obtained by the conventional heating method. As is known, ions can jump from the convex to the concave side of the grain boundaries in the grain growth process, but microwave-assisted sintering does not seem to facilitate such a process, resulting in the formation of small and homogeneous particle grains.³¹ In order to understand the possible formation mechanism, a schematic illustration of the microwave heating reaction is depicted in Fig. 13. As can be seen, the starting materials are solid powders, including oxides, nitrate and molybdate; moreover, a portion of crystalliferous water in the starting materials has the characteristics of polarity, which absorbed microwave irradiation energy adequately in the microwave-assisting heating. The absorbed microwave energy generates thermal energy, and the heat is generated within the material itself, which is also called as “heating within volumetric”. Then, the massive particles (such as molybdate) disperse uniformly within the material when material interacts with microwave power, which engenders the significant advantages of homogeneous heating and short sintering process time in the reaction.

Fig. 13 Illustration of the possible reaction processes of CaMoO₄:Eu³⁺, Li⁺ via the microwave reaction.

4. Conclusion

In summary, Ca_0.9MoO₄:Eu³⁺, 0.05Li⁺ red phosphors have been prepared by a microwave-assisted solid-state reaction. The crystalline CaMoO₄ phase was identified at as low as 300 °C for 1 h. The optimum synthesis condition to obtain the products with a sphere-like morphology was determined to be at 600 °C for 1 h. The Ca_0.9MoO₄:Eu³⁺, 0.05Li⁺ phosphor shows intense red emission lines corresponding to transitions from the ⁵D₀ → ⁷F₀ (536 nm), ⁵D₀ → ⁷F₁ (592 nm), ⁵D₀ → ⁷F₂ (615 nm), ⁵D₀ → ⁷F₃ (654 nm), ⁵D₀ → ⁷F₄ (702 nm), and the emission peak at 615 nm is the strongest one. The possible mechanism via the microwave-assisted sintering was proposed to demonstrate the absorption of the microwaves by crystalliferous water, the decomposition of the starting materials, and the phase formation of the homogeneous particles. The optimization of the microwave synthesis method, including the addition of the charge compensator, reaction temperature and time, is expected to lead to the formation of better phosphors, which are superior to conventionally prepared phosphors. Such an easy, energy-saving and large scale preparation method will be interesting to obtain new functional materials with controlled morphology.

Acknowledgements

The present work was supported by the National Natural Science Foundations of China (Grant no. 51002146, no. 51272242), Natural Science Foundations of Beijing (2132050), the Program for New Century Excellent Talents in the University of the Ministry of Education of China (NCET-12-0950), Beijing Nova Program (Z131103000413047), Beijing Youth Excellent Talent Program (YETP0635) and the Funds of the State Key Laboratory of New Ceramics and Fine Processing, Tsinghua University (KF201306). Z. G. Xia is also grateful for the financial support from University of Science and Technology Beijing.

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