Luminescent Characteristics and Microstructures of Sr 2 CeO 4 Phosphors Prepared via Sol-gel and Solid-state Reaction Routes

O R I G I N A L P A P E R

Luminescent characteristics and microstructures of Sr

CeO

phosphors prepared via sol–gel and solid-state reaction routes

Received: 27 October 2006 / Accepted: 28 February 2007 / Published online: 6 April 2007

Springer Science+Business Media, LLC 2007

Abstract Blue-light-emitting Sr2CeO4 phosphors were synthesized via a sol–gel process and the conventional solid-state method in this study. The developed sol–gel process lowered the synthesis temperature of monophasic Sr2CeO4to as low as 900 C. In comparison with the solid-state derived powders, the sol–gel derived powders had more uniform morphology and smaller particle sizes. In addition, sol–gel derived Sr2CeO4displayed higher lumi-nescent intensity than that prepared via the solid-state route under the same heating conditions. This is attributed to the improved compositional homogeneity and crystallinity in the sol–gel process. During the heating processes, Sr2CeO4 tended to thermally decompose at elevated temperatures. This decomposition reaction resulted in the formation of an impurity phase- SrCeO3 and thereby a decrease in the luminescent intensity. For obtaining Sr2CeO4 phosphors with high luminescent intensity, the heating conditions in both processes need to be well modulated.

Keywords Luminescence
Sr2CeO4
Sol–gel
Phosphor

1 Introduction

Much attention has been paid to oxide-based luminescent materials for exploring their luminescence properties. Oxide-based materials appear to be the focus of extensive

research interest in the past few decades because of their good stability upon excitation by electron beam [1]. The popularity of oxide-based luminescent materials is due to the fact that they exhibit superior photoluminescence (PL) and cathodoluminescence (CL) properties which make them useful as important components of color emission in field emission displays (FEDs), plasma display panel de-vices, and lamps [2–5]. Among these luminescent materi-als, Sr2CeO4is a promising one for FEDs and lamps due to its efficient luminescence under ultraviolet, cathode ray, and X-ray excitation [6–9]. Sr2CeO4has an orthorhombic structure, which consists of one-dimensional chains of edge-sharing CeO6octahedra linked by strontium ions [6]. This structure of Sr2CeO4can absorb energy by itself and acts as a sensitizer to transfer the absorbed energy to the luminescence centers—CeO6octahedra responsible for the occurrence of blue emission [10]. The luminescence of Sr2CeO4 is considered to originate from charge-transfer (CT) transition [11].

Sr2CeO4 powders are usually prepared via a conven-tional solid-state route which generally requires prolonged heating at elevated temperatures and thus results in coars-ening of the obtained powders [7–11]. Other methods, such as the co-precipitation process and sol–gel method using PEG polymer, have been used to synthesize Sr2CeO4[12,

13]. However, high-temperature heating is still required in these processes for obtaining pure Sr2CeO4. Yu et al. uti-lized the citrate-gel method to prepare Sr2CeO4 [14] and detected the presence of an impurity phase-Ce4SrO7phase in Sr2CeO4 powders because of insufficient homogeneity of the starting materials. In order to improve the drawbacks of the previous processes, a sol–gel method employing citric acid (CA) and ethylene glycol (EG) was developed in this study for synthesizing Sr2CeO4 powders. CA forms stable metal-ion complexes, and their polyesterification

C.-H. Lu (&)
C.-T. Chen

Department of Chemical Engineering, National Taiwan University, No.1, Sec. 4, Roosevelt Road, Taipei 106, Taiwan, R.O.C.

e-mail: [email protected] DOI 10.1007/s10971-007-1565-3

with EG produces polymeric resins [15,16]. Immobiliza-tion of metal complexes in rigid organic polymer networks is considered to decrease segregation of metal ions, thereby improving the compositional homogeneity in the starting materials of Sr2CeO4. Therefore, the formed metal com-plex will enhance the formation of Sr2CeO4and suppress the formation of impurity phases. Increasing the CA/metal ion molar ratio was reported to produce more carboxylic group and polymer resin and enhance the formation of pure phases [16]. The CA/metal ion and CA/EG molar ratio were also found to significantly vary the microstructures of the prepared materials [17, 18]. On the other hand, the combustion method has also been applied to synthesize Sr2CeO4 [19, 20]. This method is an alternative wet-chemical method. It is an exothermic reaction and occurs with the evolution of heat. This process is different from the sol–gel process because fuels and oxidizers are required during combustion reactions and no polymerization reac-tions occur. The effects of the oxidant-to-fuel ratio was found to greatly affect the phase formation [20]. Recently the above sol–gel process has been applied to a spray pyrolysis process for preparing particles with fine size and regular morphology [21].

In this study, Sr2CeO4powders were prepared via both the sol–gel route and solid-state reaction method. The luminescent properties and microstructures of the powders derived from the above two methods were compared. The relation between the luminescent intensity and calcination conditions was also investigated. The sol–gel method was found to improve the microstructures and luminescent properties of the obtained powders.

2 Experimental

Sr2CeO4was prepared via the sol–gel method using CA as the chelating agent and EG as the polymerizing agent. Stoichiometric amounts of strontium nitrate, cerium nitrate hexahydrate, and citric acid monohydrate were dissolved in deionized water with constant stirring. The solution was stirred for 1.5 h, followed by adding EG. The molar ratio of EG: CA: metal ions was set to be 4:2:1. The prepared solution was heated at 130C on a hot plate for 1.5 h and later heated at 300C until the dried gels were formed. The resulting gels were calcined at 500C in air for 2 h at a heating rate of 10C/min, and the obtained powders were ground thoroughly. Subsequently, the samples were cal-cined at various temperatures in air for 4 h at a heating rate of 10C/min. For the synthesis of Sr2CeO4via the solid-state method, stoichiometric amounts of strontium car-bonate and cerium oxide were ball-milled for 12 h in ethyl alcohol using zirconia balls. The mixed powders were then heated in air at various temperatures.

The obtained Sr2CeO4powders from both methods were characterized via X-ray diffraction (XRD) analysis using Cu-K radiation (k = 1.54180 A˚ ) (MAC Science, MXP3). The microstructures of Sr2CeO4 prepared via the sol–gel and solid state methods were examined via scanning electron microscopy (SEM) (Hitachi S-800 Field Emission Scanning Electron Microscope). A differential scanning calorimeter (Netzsch, DSC 404) was used to analyze the sol–gel derived precursors. The voltage used in SEM analysis was 20 kV. The PL spectra were recorded at room temperature via a fluorescence spectrophotometer (Hitachi F-4500) using a Xenon lamp as an excitation source.

3 Results and discussion

3.1 Formation and decomposition of Sr2CeO4

Sr2CeO4powders prepared via the solid-state route were heated at temperatures between 700 and 1,400 C in air for 4 h, and the products were characterized via XRD analysis (Fig.1). The relations between the intensity of the main diffraction peaks for each phase and the heating tempera-ture are illustrated in Fig.2. It is shown in Fig.1that only starting materials were observed at 700C, indicating no occurrence of reactions. When the mixed powders were

Fig. 1 X-ray diffraction patterns of the starting materials of Sr2CeO4

calcined at various temperatures for 4 h in the solid-state reaction process

heated at 850C, the diffraction peaks of an intermediate phase-SrCeO3 were found. Further raising the heating temperature led to an increase in the amount of SrCeO3. When the heating temperature reached 1,000C, SrCeO3 was transformed into Sr2CeO4 completely and the target compound was obtained. However, as the heating tem-perature was raised to 1,100C, SrCeO3 reappeared as seen in Fig.1. This indicates that increasing heating tem-perature resulted in decomposition of Sr2CeO4 to form SrCeO3. Further raising the heating temperature led to greater decomposition of Sr2CeO4.

The precursors of Sr2CeO4 derived from the sol–gel method were calcined at various temperatures for 4 h. The XRD patterns of the heated powders are illustrated in Fig.3. The relations between the diffraction intensity of the major peak for each phase and the calcination temperature are illustrated in Fig.4. SrCO3 and CeO2 were observed at 700C. As the heating temperature was increased to 800C, Sr2CeO4began to appear. It is noted that no SrCeO3 was formed prior to the appearance Sr2CeO4. After heating at 900C, pure Sr2CeO4phase was formed and the recorded diffraction patterns matched well with the standard ICDD pattern (No. 89-5546) [22]. The DSC analysis for the sol–gel derived precursors was performed, and the curve is illustrated in Fig.5. It was found that an endothermal peak started from 800C, and its peak temperature occurred at around 900C. The temperature range of this peak matched with the temperature range for the formation of Sr2CeO4.

The formation temperature of pure Sr2CeO4in the sol– gel process is lower than that in the solid-state reaction

process. Pure Sr2CeO4was present when heating temper-ature was raised from 900 to 1,200C. As the heating temperature reached above 1,200C, the diffraction intensity of Sr2CeO4 gradually decreased along with an

Fig. 2 Diffraction intensity of the resultant phases versus calcination temperature for the starting materials of Sr2CeO4heated at elevated

temperatures in the solid-state reaction process

Fig. 3 X-ray diffraction patterns of the sol–gel derived precursors of Sr2CeO4calcined at various temperatures for 4 h

Fig. 4 Diffraction intensity of the resultant phases versus calcination temperature for the precursors of Sr2CeO4 heated at elevated

increase in the diffraction intensity of SrCeO3. This phe-nomenon indicates thermal decomposition of Sr2CeO4 at elevated temperatures.

These results clearly demonstrate that the sol–gel method used in this study can successfully synthesize the pure phase of Sr2CeO4 at temperature as low as 900C, which is much lower than that required in the chemical coprecipitation method reported by Masui et al. [7]. The CA used can form stable metal-ion complexes, and the polyesterification between the formed complexes and EG results in the formation of polymeric resins. As a result, immobilization of metal-ion complexes in such rigid or-ganic polymeric networks reduces segregation of particular metal ions and thereby ensures compositional homogeneity [23]. Therefore, pure Sr2CeO4 oxides can be synthesized from the sol–gel derived powders at low temperatures in this study.

It is also found that the solid-state derived Sr2CeO4 was readily decomposed to SrCeO3 at 1,100 C. On the other hand, the sol–gel derived Sr2CeO4 had enhanced thermal stability as compared to the solid-state derived samples. The exact decomposition temperature of Sr2CeO4has not been reported in literature. According to the XRD patterns shown in Fig.1, pure Sr2CeO4seemed to be formed in the 1,000C-heated powders in the solid-state processes. However, it is believed that a small amount of the impurity phase-SrCeO3 coexisted in this sample. This tiny amount of SrCeO3 was beyond the detection of XRD analysis, so that no SrCeO3 could be found. The product of the thermal decomposition of Sr2CeO4 is SrCeO3. This impurity phase-SrCeO3 in the solid-state derived powders serves as nuclei, and these nuclei can trigger the decomposition reactions at low temperatures. As a result, the thermal stability of the solid-state derived Sr2CeO4is diminished.

3.2 Microstructures of Sr2CeO4powders

The microstructures of the solid-state derived Sr2CeO4are shown in Fig. 6a, b. As shown in Fig.6a, the grain size of 1,000C-heated Sr2CeO4was around 0.8 lm. With a rise in the heating temperature, the grain size of the powders gradually increased. When the heating temperature reached 1,300C, significant coarsening of grains was observed with the grain size increased to around 2 lm. As shown in Fig.6b, the grain shape became irregular and a large number of small particles were formed. The microstruc-tures of sol–gel derived Sr2CeO4are shown in Fig.6c, d. The 1,000C-heated sample had uniform morphology and the grain size was around 0.4 lm which was smaller than of that of the solid-state derived powders. As the temper-ature was raised to 1,300 C, the agglomeration of grains occurred with the grain size increased to around 1.5 lm. From the above results, it is noted that the sol–gel process can produce Sr2CeO4with smaller grain size and uniform morphology.

3.3 Luminescent properties of Sr2CeO4powders

The emission spectra of Sr2CeO4powders prepared via the solid-state method are illustrated in Fig.7. The emission spectra were recorded with an excitation wavelength of 282 nm. When the sample was heated 900C, a very weak peak was observed. After heating at 1,000C, a broad emission peak was found at 471 nm which is resulted from Ce4+(4f0) to O2+(2p6) charge transfer transition [6]. This emission is attributed to the transition from metal-to-ligand charge-transfer excited state to the ground state [24]. When the heating temperature was raised above 1,000C, the emission intensity reversely decreased due to the decom-position of Sr2CeO4 at elevated temperatures. Once the decomposition of Sr2CeO4occurred, SrCeO3was formed. According to the study reported by Goubin et al. [25], SrCeO3does not have luminescence properties. Therefore, the decomposition reaction resulted in a reduction in the emission intensity of Sr2CeO4.

The excitation spectra of solid-state derived Sr2CeO4 powders are illustrated in Fig.8. The excitation spectra were measured at an emission wavelength of 471 nm. When the sample was heated 900 C, the excitation peak was very weak. When the samples were heated at elevated tempera-tures, two excitation peaks were observed at 282 and 340 nm. These peaks were due to the interaction of the central Ce4+ion with neighboring oxygen ligands in CeO6 octahedra leading to ligand-to-metal charge-transfer tran-sition. These two excitation peaks are attributed to two excitation states arising from ligand molecular orbitals t1g fi f and t1u fi f charge transfer states [24]. The highest excitation intensity was found in the 1,000

C-he-Fig. 5 DSC analysis of the precursors of Sr2CeO4prepared by the

Fig. 7 Emission spectra of Sr2CeO4prepared via the solid-state route

Fig. 8 Excitation spectra of Sr2CeO4 prepared via the solid-state

route Fig. 6 Scanning electron

micrographs of the solid-state derived Sr2CeO4heated at (a)

1,000C and (b) 1,300 C for 4 h, and the sol–gel derived Sr2CeO4heated at (c) 1,000C

ated sample. Further increasing temperature led to a de-crease in the excitation intensity.

The emission spectra of sol–gel derived Sr2CeO4 pow-ders are illustrated in Fig.9. All emission bands showed a peak at 471 nm due to the transition from metal-to-ligand charge-transfer excited state to the ground state. The highest emission intensity was found in the 1,200 C-he-ated sample. As the heating temperature reached above 1,200C, thermal decomposition of Sr2CeO4occurred to lead to a decrease in the emission intensity. It is worth noting that the sol–gel method resulted in higher emission intensity as compared with the solid-state method under the same heating conditions. The increase in luminescence intensity is attributable to the improved compositional homogeneity and enhanced crystallinity in the sol–gel process [4, 26, 27]. The excitation spectra of sol–gel de-rived Sr2CeO4are shown in Fig.10. The highest excitation intensity was found in the 1,200C-heated sample. It was also found that there were red shifts in the excitation spectra when the heating temperatures were increased. This phenomenon is considered to be related to the decompo-sition Sr2CeO4 at elevated temperatures. The thermal decomposition will result in the compositional deviation, thereby affecting the excitation spectra. The effects of the composition on the luminescence properties of Sr2CeO4 will be investigated in details.

The effects of thermal decomposition of Sr2CeO4on the luminescence intensity are shown in Fig.11. The decom-position ratio of Sr2CeO4 is defined as ISrCeO3= IðSr2CeO4þ ISrCeO₃Þ, where ISr2CeO4 and ISrCeO3 represent the diffraction intensities of the major peaks of Sr2CeO4 and SrCeO3,

respectively. The figure indicates that as the decomposition ratio of Sr2CeO4increases, the luminescence intensity of Sr2CeO4correspondingly decreases in both processes. It is realized that the thermal decomposition of Sr2CeO4 at elevated temperatures directly causes the luminescence

Fig. 9 Emission spectra of Sr2CeO4prepared via the sol–gel route

Fig. 10 Excitation spectra of Sr2CeO4prepared via the sol–gel route

Fig. 11 Decomposition ratio of Sr2CeO4 and emission intensity at

471 nm of the samples calcined at various temperatures. (a) The solid-state derived samples and (b) the sol–gel derived samples

intensity to reduce. As demonstrated in Figs.7and9, raising the temperature up to the decomposition temperature can increase the luminescent intensity because of the enhanced reaction and improved crystallinity of Sr2CeO4. However, increasing temperature causes more Sr2CeO4to decompose once the heating temperature is above the decomposition temperature, thereby resulting in a reduction in the lumi-nescent intensity. Based on the above results, it is known that the heating conditions in both processes need to be well modulated for obtaining Sr2CeO4 phosphors with high luminescent intensity.

4 Conclusions

Sr2CeO4phosphors were successfully prepared via the sol– gel method using CA and ethyl glycol. The developed sol–gel process resulted in the formation of pure Sr2CeO4 at 900C. The formation temperature of pure Sr2CeO4in the sol–gel process was lower than that in the solid-state reaction process. The sol–gel derived powders had more uniform morphology and smaller particle sizes than the solid-state derived powders. The sol–gel route resulted in higher luminescent intensity of Sr2CeO4 than the solid-state route when the precursors were treated under the same heating conditions. This is ascribed to the improved com-positional homogeneity and enhanced crystallinity in the sol–gel process. Sr2CeO4tended to thermally decompose at elevated temperatures and formed an impurity phase-SrCeO3. This decomposition reaction caused the lumines-cent intensity of Sr2CeO4 to decrease. The synthesis and luminescent properties of Sr2CeO4 were explored in this study, and these results can serve to provide better under-standing for preparing Sr2CeO4 with high luminescent intensity and for its future application in luminescence devices.

Acknowledgments The authors would like to thank Dr. K. Krish-nan and Dr. B. Bhattacahrjee for helpful discussion. We would also like to thank Mrs. C. Y. Lin for her assistance on SEM. The authors would also like to thank National Science Council of the Republic of

China, Taiwan for financially supporting this research under contract No. NSC 95–2221-E002-345-MY3.

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