Synthesis and Characterization of the CaTiO3:Eu3+ Red Phosphor by an Optimized Microwave-Assisted Sintering Process

The synthesis process has a significant influence on the properties of Ca1-xTiO3:Eu3+x phosphors; thus, an optimized process will lead to a better performance of the Ca1-xTiO3:Eu3+x phosphors. In this work, the feasibility of synthesizing the Ca1-xTiO3:Eu3+x phosphor with a good luminescent performance by combining the chemical co-precipitation method and microwave-assisted sintering was studied. The precursor of Ca1-xTiO3:Eu3+x phosphors were prepared by the chemical co-precipitation method. To find an optimized process, we applied both of the traditional (furnace) sintering and the microwave-assisted sintering to synthesize the Ca1-xTiO3:Eu3+x phosphors. We found out that a sintering power of 528 W for 50 min (temperature around 950 °C) by a microwave oven resulted in similar emission intensity results compared to traditional furnace sintering at 900 °C for 2.5 h. The synthesized Ca1-xTiO3:Eu3+x phosphors has an emission peak at 617 nm (5D0→7F2), which corresponds to the red light band. This new synthesized method is an energy efficient, time saving, and environmentally friendly means for the preparation of Ca1-xTiO3:Eu3+x red phosphor with good luminescent performance.


Introduction
Calcium titanate (CaTiO 3 ) represents one of the most important classes of mixed oxides owing to its good chemical and thermal stability as well as great mechanical resistance. Particularly, rare earth-activated CaTiO 3 phosphors [1][2][3][4] exhibit high-efficiency luminescent properties and, therefore, are promising for using in various optoelectronic devices, e.g., field-emission displays and long persistent phosphorescent phosphors. Such phosphors are crucial for applications in white light-emitting diodes to adjust their chromogenic performance, color and temperature [5].
The chemical co-precipitation method [9,10] to prepare Ca 1-x TiO 3 :Eu 3+ x phosphors is processed by dissolving the calcium carbonate (CaCO 3 ) in the acetic acid and then mixing the solution with europium acetate and the titanium dioxide by stirring for the precipitation. The precipitation is washed several times, which is followed by a traditional sintering (in a muffle furnace) to obtain the Ca1-xTiO3:Eu 3+ x phosphors powder.
In recent years, the microwave-assisted sintering method [2,[11][12][13][14] has been widely applied in the synthesis of phosphor and ceramic materials due to its advantages of rapid heating rate and energy efficiency [15]. Different from the conventional sintering method, with radiation or conduction from the surface to the core as the main heat transfer mechanism, the microwave-assisted sintering procedure heats the whole sample by volumetric heating from microwaves [12]. Thus, to combine advantages from the chemical co-precipitation method and the microwave-assisted sintering method will be an optimized process (e.g., time and energy saving) to prepare Ca1-xTiO3:Eu 3+ x phosphors with better performance.
In this work, the precursor of Ca1-xTiO3:Eu 3+ x phosphor was synthesized by the chemical coprecipitation method. Then, we applied both the traditional sintering and the microwave-assisted sintering to prepare the Ca1-xTiO3:Eu 3+ x phosphor. The luminescence properties from the optimized process were studied and compared to the traditional method.
The sample synthesizing process is presented in Figure 1. Firstly, we applied the chemical coprecipitation method to synthesize the precursor. The C2H2O4·2H2O and C16H36O4Ti were dissolved in the ethanol to get solutions A and B. The (NH4)2C2O4 was dissolved in the distilled water to obtain a solution C. The Ca(NO3)2·4H2O and Eu(NO3)3·6H2O were dissolved in the distilled water to get solution D. Solution A and solution C were dropwise added to solution B and magnetic stirred to obtain a clear solution E, followed by dropwise adding solution D to solution E and magnetic stirring for 2 h. The precipitation was centrifuged washing and dried to get the precursor. Then, the precursors were transferred into alumina crucibles and heated in a muffle furnace at 600-900 °C for 2.5 h in air as well as heated in a microwave oven (Galanz, G80F 2.45G Hz) at 528-800 W for 10-60 min to prepare the Ca1-xTiO3:Eu 3+ x phosphors. During the microwave assisted sintering, a small crucible with dry Ca0.97TiO3: Eu 3+ 0.03 phosphor powder was placed in a large crucible filled with 3/4 volume of activated carbon. The precursor was heated by heat transfer from the activated carbon to prepare the Ca0.97TiO3: Eu 3+ 0.03 phosphor.

Characterization Techniques
The chemical structure of the precursor was measured by applying Fourier transform infrared spectroscopy (FT-IR, Nexus-670, Waltham, MA, USA) measurements, where the precursor samples were mounted in the KBr pelletized disks and measured in the 400-3600 cm −1 region with a resolution of 2 cm −1 . The thermal properties of the precursor were tested using a simultaneous thermal analysis (TG-DSC, STA409PC, Selb, Germany) in air atmosphere. The crystal information was analyzed by an X-ray diffractometer (XRD, D/max-2500PC, Rigaku, Japan) in air atmosphere. The micromorphology of the samples was studied by using field emission scanning electron microscope (SEM, S-3000N, Tokyo, Japan). The luminescent properties of the phosphors were tested by the fluorescence spectroscopy (FP-6600, Tokyo, Japan).

Precursor Analysis
In order to understand the structure of precursor (from the chemical co-precipitation method [9,10]) and carry out subsequent processing, the precursor was tested by FT-IR, and the results are shown in the The chemical structure of the precursor was measured by applying Fourier transform infrared spectroscopy (FT-IR, Nexus-670, Waltham, MA, USA) measurements, where the precursor samples were mounted in the KBr pelletized disks and measured in the 400-3600 cm −1 region with a resolution of 2 cm −1 . The thermal properties of the precursor were tested using a simultaneous thermal analysis (TG-DSC, STA409PC, Selb, Germany) in air atmosphere. The crystal information was analyzed by an X-ray diffractometer (XRD, D/max-2500PC, Rigaku, Japan) in air atmosphere. The micromorphology of the samples was studied by using field emission scanning electron microscope (SEM, S-3000N, Tokyo, Japan). The luminescent properties of the phosphors were tested by the fluorescence spectroscopy (FP-6600, Tokyo, Japan).

Precursor Analysis
In order to understand the structure of precursor (from the chemical co-precipitation method [9,10]) and carry out subsequent processing, the precursor was tested by FT-IR, and the results are shown in the Figure 2. From the FT-IR curve, the peak at around 3434 cm −1 is the anti-symmetry and telescopic symmetrical vibration of OH, which corresponds to the H-O-H group of water bound in the precursor. The peak around 1600 cm −1 is the vibration of C=O, which corresponds to the carbonoxygen double bond inside the MTiO(C2O4)·4H2O. The metal cations and complexes of TiO(C2O4) 2− are uniformly represented by MTiO(C2O4) [16]. To optimize the sintering process, the understanding of the thermal properties of the precursor is crucial. Figure 3 shows the thermal decomposition of the precursor. The TG curve has a clear threestage decline during the heating process which is corresponding to the three endothermic peaks in the DSC curve [17][18][19]. At the temperature before 200 °C (stage one), the precursor first lost the free water and then lost the bound water. The endothermic peak around 200 °C in the DSC curve indicates the loss of bound water. The reaction [17] can be represented by Equation (1). MTiO(C2O4)·4H2O→MTiO(C2O4)2 + 4H2O (1) The second stage at the temperature range of 275−457 °C, where the precursor decomposed to MCO3 and TiO2 by releasing the CO and CO2. The process companies by an obvious weight loss [17] which can be represented by Equation (2). To optimize the sintering process, the understanding of the thermal properties of the precursor is crucial. Figure 3 shows the thermal decomposition of the precursor. The TG curve has a clear three-stage decline during the heating process which is corresponding to the three endothermic peaks in the DSC curve [17][18][19]. At the temperature before 200 • C (stage one), the precursor first lost the free water and then lost the bound water. The endothermic peak around 200 • C in the DSC curve indicates the loss of bound water. The reaction [17] can be represented by Equation (1).
Materials 2020, 13, 874 4 of 10 The second stage at the temperature range of 275−457 • C, where the precursor decomposed to MCO 3 and TiO 2 by releasing the CO and CO 2 . The process companies by an obvious weight loss [17] which can be represented by Equation (2).
The third stage mainly occurred in solid-phase reaction at 500-900 • C. The reaction process [17] is shown in Equation (3).
The TG curve is getting flat when the temperature is higher than 715 • C, which indicates that the reaction was completed. MTiO(C2O4)2→MCO3 + TiO2 + CO + CO2 (2) The third stage mainly occurred in solid-phase reaction at 500-900 °C. The reaction process [17] is shown in Equation (3).
The TG curve is getting flat when the temperature is higher than 715 °C, which indicates that the reaction was completed.

Ca1-xTiO3:Eu 3+ x Phosphor Synthesized by the Traditional Sintering Processes
Based on the TG-DSC curve (Figure 3), the MTiO3 will be generated at a temperature higher than 600 °C. Thus, the effect of sintering temperature on crystal formation and luminescence properties are firstly explored. Figure 4 shows the XRD patterns of the Ca1-xTiO3:Eu 3+ x phosphor samples at different sintering temperatures and the same isothermal holding for 2.5 h. It can be seen from Figure 4 that the Ca1-xTiO3:Eu 3+ x has been synthesized at 600 °C while contains a certain amount of TiO2 [20,21]. When the sintering temperature is higher than 650 °C, the pure Ca1-xTiO3:Eu 3+ x phosphor can be produced (without TiO2 crystal peak). The diffraction peak (diffraction angle: 33.24°, 47.74°, 59.54°) corresponds to the standard card JCPDS NO.42-0423, which is the orthorhombic CaTiO3 structure. Moreover, the intensity of the standard diffraction peak is getting stronger with increasing the sintering temperature. The intensity of the standard diffraction peak is greatly enhanced at sintering temperature higher than 800 °C, which indicates good crystallization.

Ca 1-x TiO 3 :Eu 3+ x Phosphor Synthesized by the Traditional Sintering Processes
Based on the TG-DSC curve (Figure 3), the MTiO 3 will be generated at a temperature higher than 600 • C. Thus, the effect of sintering temperature on crystal formation and luminescence properties are firstly explored. Figure 4 shows the XRD patterns of the Ca 1-x TiO 3 :Eu 3+ x phosphor samples at different sintering temperatures and the same isothermal holding for 2.5 h. It can be seen from Figure 4 that the Ca 1-x TiO 3 :Eu 3+ x has been synthesized at 600 • C while contains a certain amount of TiO 2 [20,21]. When the sintering temperature is higher than 650 • C, the pure Ca 1-x TiO 3 :Eu 3+ x phosphor can be produced (without TiO 2 crystal peak). The diffraction peak (diffraction angle: 33.24 • , 47.74 • , 59.54 • ) corresponds to the standard card JCPDS NO.42-0423, which is the orthorhombic CaTiO 3 structure. Moreover, the intensity of the standard diffraction peak is getting stronger with increasing the sintering temperature. The intensity of the standard diffraction peak is greatly enhanced at sintering temperature higher than 800 • C, which indicates good crystallization. Figure 5 shows the emission spectra under an excitation wavelength of 398 nm of Ca 0.97 TiO 3 :Eu 3+ 0.03 prepared at different traditional sintering temperature. It can be seen from Figure 5 that the luminous performance of the Ca 0.97 TiO 3 :Eu 3+ 0.03 powder is continuously increasing with increasing of sintering temperature. The overall trend of luminescent properties is consistent with the trend of the crystal structure presented by XRD. In other words, the crystallization is not complete at relatively lower temperatures under 800 • C. The incomplete crystallization will result in weak light intensity.   Figure 5 shows the emission spectra under an excitation wavelength of 398 nm of Ca0.97TiO3:Eu 3+ 0.03 prepared at different traditional sintering temperature. It can be seen from Figure 5 that the luminous performance of the Ca0.97TiO3:Eu 3+ 0.03 powder is continuously increasing with increasing of sintering temperature. The overall trend of luminescent properties is consistent with the trend of the crystal structure presented by XRD. In other words, the crystallization is not complete at relatively lower temperatures under 800 °C. The incomplete crystallization will result in weak light intensity. Based on the results from the XRD and the luminescence properties, a traditional sintering temperature of 900 °C is the best process for preparing the Ca0.97TiO3:Eu 3+ 0.03 phosphor.
To determine the best concentration of Eu 3+ in the Ca1-xTiO3:Eu 3+ x phosphor, the Ca1-xTiO3:Eu 3+ x phosphors doped with various concentrations of Eu 3+ are synthesized. The XRD patterns of the Ca1-   Figure 5 shows the emission spectra under an excitation wavelength of 398 nm of Ca0.97TiO3:Eu 3+ 0.03 prepared at different traditional sintering temperature. It can be seen from Figure 5 that the luminous performance of the Ca0.97TiO3:Eu 3+ 0.03 powder is continuously increasing with increasing of sintering temperature. The overall trend of luminescent properties is consistent with the trend of the crystal structure presented by XRD. In other words, the crystallization is not complete at relatively lower temperatures under 800 °C. The incomplete crystallization will result in weak light intensity. Based on the results from the XRD and the luminescence properties, a traditional sintering temperature of 900 °C is the best process for preparing the Ca0.97TiO3:Eu 3+ 0.03 phosphor.
To determine the best concentration of Eu 3+ in the Ca1-xTiO3:Eu 3+ x phosphor, the Ca1-xTiO3:Eu 3+ x phosphors doped with various concentrations of Eu 3+ are synthesized. The XRD patterns of the Ca1- Based on the results from the XRD and the luminescence properties, a traditional sintering temperature of 900 • C is the best process for preparing the Ca 0.97 TiO 3 :Eu 3+ 0.03 phosphor. To determine the best concentration of Eu 3+ in the Ca 1-x TiO 3 :Eu 3+ x phosphor, the Ca 1-x TiO 3 :Eu 3+ x phosphors doped with various concentrations of Eu 3+ are synthesized. The XRD patterns of the Ca 1-x TiO 3 :Eu 3+ x phosphors doped with various concentrations of Eu 3+ are shown in Figure 6. The main diffraction peak (diffraction angle: 33.24 • , 47.74 • , 59.54 • ) is consistent with the standard card JCPDS NO.42-0423, which is the orthorhombic CaTiO 3 structure. The TiO 2 peaks are also detected in the XRD pattern, which is due to the increase of Eu 3+ accompanied by an excessive adding of Ti to ensure the complete chemical reaction of the metal cation (Ca 2+ , Eu 3+ ) and [TiO(C 2 O 4 ) 2 ] 2− . Table 1 shows the lattice constant of the Ca 1-x TiO 3 :Eu 3+ x phosphors calculated by JADE software (6.5). It can be seen in Table 1 that the lattice constant decreases slightly with the addition of Eu 3+ content, which is as a reason for the distance decrease of the lattice and the surrounding O 2− ion. Indeed, the ion radius (94.7 pm) of Eu 3+ is slightly smaller than the ion radius (100 pm) of Ca 2+ . Moreover, the change of lattice constant also proved that the Eu 3+ ions are successfully doped into the crystal structure.
lattice constant of the Ca1-xTiO3:Eu 3+ x phosphors calculated by JADE software (6.5). It can be seen in Table 1 that the lattice constant decreases slightly with the addition of Eu 3+ content, which is as a reason for the distance decrease of the lattice and the surrounding O 2− ion. Indeed, the ion radius (94.7 pm) of Eu 3+ is slightly smaller than the ion radius (100 pm) of Ca 2+ . Moreover, the change of lattice constant also proved that the Eu 3+ ions are successfully doped into the crystal structure.   Figure 7 shows the emission spectra under an excitation wavelength of 398 nm of Ca1-xTiO3:Eu 3+ x phosphors doped with various concentrations of Eu 3+ . The emission spectrum mainly presents four emission peaks, located at 597 nm, 617 nm, 658 nm, and 700 nm, respectively, under excitation wavelength of 398 nm, which correspond to excited state 5 D0→ 7 FJ (J = 1~4) transition of the Eu 3+ ion. The main emission peak is at 617 nm ( 5 D0→ 7 F2), which corresponds to the red light band. The 5 D0→ 7 F2 transition is the electric dipole transition. When the Eu 3+ ion occupies the asymmetric position, the electric dipole transition will be the main transition, which also proves the replacement of Eu 3+ ions in the crystal structure to Ca 2+ ions.  Figure 7 shows the emission spectra under an excitation wavelength of 398 nm of Ca 1-x TiO 3 :Eu 3+ x phosphors doped with various concentrations of Eu 3+ . The emission spectrum mainly presents four emission peaks, located at 597 nm, 617 nm, 658 nm, and 700 nm, respectively, under excitation wavelength of 398 nm, which correspond to excited state 5 D 0 → 7 F J (J = 1~4) transition of the Eu 3+ ion. The main emission peak is at 617 nm ( 5 D 0 → 7 F 2 ), which corresponds to the red light band. The 5 D 0 → 7 F 2 transition is the electric dipole transition. When the Eu 3+ ion occupies the asymmetric position, the electric dipole transition will be the main transition, which also proves the replacement of Eu 3+ ions in the crystal structure to Ca 2+ ions.  With the addition of Eu 3+ ion doping amount, more luminescent centers are formed in the matrix, which increases the luminescence intensity. The emission intensity appears to reduce after it reaches the maximum at x = 0.03 (3 mol%). This is due to the interaction between the luminescent centers by increasing the Eu 3+ concentration, the continuous migration of the excitation energy between the centers will not generate effective radiation. Therefore, 3 mol% is the best doping concentration of Eu 3+ .  With the addition of Eu 3+ ion doping amount, more luminescent centers are formed in the matrix, which increases the luminescence intensity. The emission intensity appears to reduce after it reaches the maximum at x = 0.03 (3 mol%). This is due to the interaction between the luminescent centers by increasing the Eu 3+ concentration, the continuous migration of the excitation energy between the centers will not generate effective radiation. Therefore, 3 mol% is the best doping concentration of Eu 3+ . Microwave-assisted sintering has many advantages, including rapid heating and energy saving. However, the Ca 0.97 TiO 3 : Eu 3+ 0.03 phosphor cannot absorb the microwaves; thus, a small crucible with dry Ca 0.97 TiO 3 : Eu 3+ 0.03 phosphor powder was placed in a large crucible filled with 3/4 volume of activated carbon. The activated carbon was heated using 2.45 GHz microwave radiation with a maximum power of 800 W. The precursor was heated by heat transfer from the activated carbon to prepare the Ca 0.97 TiO 3 : Eu 3+ 0.03 phosphor. We note that the sintering atmosphere in the microwave oven might also slightly change due to the existence of activated carbon underneath the sample crucible. To record the temperature in the microwave oven, we placed a thermocouple into the crucible. The insulation inside the microwave oven is relatively poor, thus, we used an electric potential difference meter to calibrate the instantaneous temperature. Figure 8 shows the XRD patterns of the Ca 0.97 TiO 3 :Eu 3+ 0.03 phosphor samples sintered at different power. It can be seen in Figure 8a Figure 9 shows the emission spectra under an excitation wavelength of 398 nm of Ca0.97TiO3:Eu 3+ 0.03 prepared at different microwave sintering power (800 W, 680 W, and 528 W). It can be seen from Figure 9 that, under an excitation wavelength of 398 nm, the emission spectrum mainly includes four emission peaks (597 nm, 617 nm, 658 nm, 700 nm), which correspond to the 5 D0→ 7 FJ (J = 1~4) transition of Eu 3+ ion. Moreover, with the sintering time prolonged, the emission intensity firstly increased to a peak at 50 min and then decreased. A sintering time of 60 min will not significantly increase the emission intensity, which is due to the completion of the crystallization  mainly includes four emission peaks (597 nm, 617 nm, 658 nm, 700 nm), which correspond to the 5 D 0 → 7 F J (J = 1~4) transition of Eu 3+ ion. Moreover, with the sintering time prolonged, the emission intensity firstly increased to a peak at 50 min and then decreased. A sintering time of 60 min will not significantly increase the emission intensity, which is due to the completion of the crystallization process.  Figure 9 shows the emission spectra under an excitation wavelength of 398 nm of Ca0.97TiO3:Eu 3+ 0.03 prepared at different microwave sintering power (800 W, 680 W, and 528 W). It can be seen from Figure 9 that, under an excitation wavelength of 398 nm, the emission spectrum mainly includes four emission peaks (597 nm, 617 nm, 658 nm, 700 nm), which correspond to the 5 D0→ 7 FJ (J = 1~4) transition of Eu 3+ ion. Moreover, with the sintering time prolonged, the emission intensity firstly increased to a peak at 50 min and then decreased. A sintering time of 60 min will not significantly increase the emission intensity, which is due to the completion of the crystallization process.  Figure 10 shows the emission spectra under an excitation wavelength of 398 nm of Ca0.97TiO3:Eu 3+ 0.03 synthesized by microwave-assisted methods and traditional furnace sintering method. A sintering power of 528 W for 50 min by microwave oven gets the similar emission intensity results as the traditional furnace sintering at 900 °C for 2.5 h. The emission intensity will significantly decrease when we further increase the power. Indeed, the microwave-assisted sintering will greatly save the energy and time for the preparation of Ca0.97TiO3:Eu 3+ 0.03 red phosphor with good luminescent performance.  Figure 10 shows the emission spectra under an excitation wavelength of 398 nm of Ca 0.97 TiO 3 :Eu 3+ 0.03 synthesized by microwave-assisted methods and traditional furnace sintering method. A sintering power of 528 W for 50 min by microwave oven gets the similar emission intensity results as the traditional furnace sintering at 900 • C for 2.5 h. The emission intensity will significantly decrease when we further increase the power. Indeed, the microwave-assisted sintering will greatly save the energy and time for the preparation of Ca 0.97 TiO 3 :Eu 3+ 0.03 red phosphor with good luminescent performance.
Materials 2020, 13, x FOR PEER REVIEW 9 of 11 Figure 10. Emission spectra under an excitation wavelength of 398 nm of the Ca0.97TiO3:Eu 3+ 0.03 synthesized by the traditional sintering process (furnace) and microwave-assisted sintering process. Figure 11 shows the morphology of Ca0.97TiO3:Eu 3+ 0.03 phosphors (best doping) synthesized by the different synthesize methods and scanned by SEM. It can be seen from Figure 11a,b that both of the CaTiO3:Eu 3+ 0.03 phosphors have a rod shape, which agrees with the orthorhombic perovskite structure from the XRD results. However, the CaTiO3:Eu 3+ 0.03 phosphors synthesized by the microwave-assisted sintering process are slightly bigger while containing numerous defects on the surface compared to the ones prepared by the traditional sintering processes. The imperfection of the shape of the CaTiO3:Eu 3+ 0.03 phosphors is attributed to the rapid and shorter sintering process in a microwave oven. a b synthesized by the traditional sintering process (furnace) and microwave-assisted sintering process. Figure 11 shows the morphology of Ca 0.97 TiO 3 :Eu 3+ 0.03 phosphors (best doping) synthesized by the different synthesize methods and scanned by SEM. It can be seen from Figure 11a,b that both of the CaTiO 3 :Eu 3+ 0.03 phosphors have a rod shape, which agrees with the orthorhombic perovskite structure from the XRD results. However, the CaTiO 3 :Eu 3+ 0.03 phosphors synthesized by the microwave-assisted sintering process are slightly bigger while containing numerous defects on the surface compared to the ones prepared by the traditional sintering processes. The imperfection of the shape of the CaTiO 3 :Eu 3+ 0.03 phosphors is attributed to the rapid and shorter sintering process in a microwave oven.