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Microemulsion-derived ZrO2:Ce3 + nanoparticles: Phase transformation and photoluminescence characterization
Materials Characterization 106 (2015) 20–26
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Microemulsion-derived ZrO2:Ce3 + nanoparticles: Phase transformation and photoluminescence characterization Subrata Das, Chih-Cheng Chang, Che-Yuan Yang, Sudipta Som, Chung-Hsin Lu ⁎ Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 11 February 2015 Received in revised form 23 April 2015 Accepted 2 May 2015 Available online 5 May 2015 Keywords: Nanoparticles Microemulsion ZrO2 Phosphors Photoluminescence
a b s t r a c t Bluish green-emitting ZrO2:Ce3+ nanoparticles were synthesized via a microemulsion route. The effects of the annealing temperature on the phase evolution and luminescence properties were investigated in detail. As the aqueous micelles confined the constituent cations during crystallization, nanosized particles were formed. The formed ZrO2 nanoparticles were observed to undergo a phase transition from tetragonal to monoclinic structure with the increase in annealing temperatures. The reduction of structural defects with annealing temperatures promoted the phase transformation. When the annealing temperature exceeded 900 °C, only the monoclinic phase was present. The photoluminescence emission appeared in the bluish green region around 487 nm under ultraviolet excitation. The emission intensity increased with the annealing temperatures. The optimal Ce3+-dopant concentration in ZrO2 nanoparticles was determined, and the concentration quenching mechanism was discussed. The present work suggests that microemulsion-derived ZrO2:Ce3+ nanoparticles are suitable for optoelectronic applications. © 2015 Published by Elsevier Inc.
1. Introduction Zirconia is one of the most important ceramic materials owing to its excellent mechanical, electrical, thermal and optical properties. Zirconia ceramics are known for the superior hardness, good chemical and photochemical stability, large dielectric constant and optical transparency [1–4]. Recently, increasing attention has been paid to the luminescence properties of various rare earth-doped ZrO2 nanoparticles because of their low phonon energy and high optical output. Nanotechnology based on ceramic materials that involve zirconia has attracted considerable attention because of the demand for not only sophisticated optoelectronic and photonic devices but also a broad range of biomedical applications [5–7]. Various routes for preparing ZrO2 nanoparticles with spherical morphology and uniform size distribution need to be developed for optical and luminescence applications. Therefore, the synthesis of rare earth ion-doped zirconia nanoparticles with controlled sizes, structures and surface properties is of technological and fundamental importance. It is worth to mention that the phosphors with a spherical morphology have appreciable packing density, effective slurry properties, and low light scattering properties which improve luminescent intensity. Numerous synthetic routes, such as the hydrothermal method [8], the sol–gel [9] precipitation method [10] and the pulsed plasma in liquid method [7] have been utilized for the preparation of zirconia nanoparticles. However, the conventional processes are not capable to produce ⁎ Corresponding author. E-mail address: [email protected] (C.-H. Lu).
http://dx.doi.org/10.1016/j.matchar.2015.05.004 1044-5803/© 2015 Published by Elsevier Inc.
ZrO2 nanoparticles with spherical morphology, since these processes require high heating temperatures and long reaction duration. On the contrary, the microemulsion method is useful for preparing ceramic nanopowders with spherical morphology. The microemulsion method is helpful for preparing ceramic powders with nanosized particles and spherical morphology [11–14]. A microemulsion system is a thermodynamically stable solution that contains aqueous micelles, which are uniformly dispersed throughout a continuous oil phase. The micelles act as nanoreactors and restrict the growth of particles [15,16]. Accordingly, the morphology and the size of the particles in the produced powders can be easily controlled using the microemulsion technique. Microemulsion-derived particles are expected to have a uniform nanoscale size distribution with spherical morphology, owing to the confinement of the constituent cations by the aqueous micelles. Trivalent cerium (Ce3+) ions are well-known rare earth activators in various inorganic lattices that exhibit intense blue luminescence [11, 17]. Ce3 + has one 4f electron and the lowest excited configuration is 5d1 level. Since the crystal field strongly influences the unshielded 5d excited state, the symmetry of the local crystal field may induce splitting of the d-orbitals, thereby affecting the emission wavelength and intensity of the material. Hence, the luminescence properties of Ce3+ ions in different host matrices have been extensively investigated. Materials doped with Ce3 + ions have attracted substantial interest owing to their applications as phosphors, scintillators for elementary particles, detectors of ionizing radiation, UV absorbing filters and UV emitters. A wide variety of host matrices of Ce3+, such as borates, niobates, sulfates and phosphates, have been investigated [11,17].
S. Das et al. / Materials Characterization 106 (2015) 20–26
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The present research work aims at utilizing a modified microemulsion route to synthesize the spherical Ce3+-doped ZrO2 nanoparticles for the first time. Detailed experimental investigations of the structural and luminescence properties of ZrO2:Ce3 + nanoparticles were performed using various annealing temperatures. The growth mechanism and phase transformation of Ce3 +-doped ZrO2 were determined in detail. The effects of Ce3+-concentration on the crystal structure and luminescence properties of monoclinic ZrO2:Ce3+ nanoparticles were demonstrated. The average lifetimes of ZrO2:Ce3+ nanoparticles were also analyzed. The present results demonstrate that the microemulsion route is effective for preparing ZrO2:Ce3+ nanoparticles with spherical morphology for various optoelectronic applications. 2. Experimental The starting materials namely ZrO(NO3)2.xH2O (99.9%, Sigma Aldrich), Ce(NO3)3.6H2O (99%, Sigma Aldrich), cyclohexane (99%, Sigma Aldrich), OP-10 (95%, Sigma Aldrich), and 1-hexanol (95%, Nacalai Tesque) were used without further purification. Initially the desired amount of ZrO(NO3)2.xH2O and Ce(NO3)3.6H2O was taken and dissolved in 99% pure ethyl alcohol. The concentration of Ce3+ with respect to Zr4+ was varied from 1 mol% to 5 mol%. Cyclohexane was used as oil, while olyoxyethylene-10-octylphenyl ether (OP-10) and 1-hexanol were used as the surfactant and co-surfactant, respectively. A mixture of cyclohexane, OP-10, and 1-hexanol in a volume ratio of 10:2:3 was used for getting the oil phase. Under sonication, the mixed aqueous phase was dispersed into the oil phase to form the water in-oil (W/O) microemulsion. Moreover, the volume ratio of water phase to oil phase was adjusted at 0.05. The as prepared microemulsion solution was slowly dropped into hot kerosene (180 °C) and heated at around 180 °C for water evaporation to form gels. Further the calcination of the gels was performed at 600 °C for 3 h in air to form the precursor powders. Finally, the precursor powders were heated at various temperatures ranging from 700 °C to 1100 °C for 4 h in a reducing atmosphere (vol. 5% of H2–vol. 95% of N2). The crystal structures of the prepared powders were identified via X-ray diffraction (XRD; Philips X'pert/MPD, Amsterdam, the Netherlands) using CuKα radiation at room temperature. The morphologies and particle sizes were studied using scanning electron microscopy (SEM, Hitachi S-800) and transmission electron microscopy (TEM; JEOL TEM-3010, Tokyo, Japan). The photoluminescence spectra of the synthesized samples were recorded by using a fluorescence spectrophotometer (Hitachi, F-4500, Tokyo, Japan) with xenon lamp operated at 150 W as an excitation source. 3. Results and discussion 3.1. Phase characterization of ZrO2:Ce3+ nanoparticles Fig. 1 shows the XRD patterns of microemulsion-derived ZrO2:1 mol%Ce3 + precursors that were annealed at various temperatures. The XRD patterns of the as-synthesized precursors were entirely consistent with the tetragonal ZrO2 phase [JCPDS 81-1544], as presented in Fig. 1(a). Upon heating at 700 °C, the tetragonal ZrO2 phase was found to contain a small amount of monoclinic ZrO2 phase, as displayed in Fig. 1(b). As the heating temperature increased to 900 °C, the amount of tetragonal phase formed declined while the amount of monoclinic phase increased, as shown in Fig. 1(c) and (d). Upon heating at 1000 °C and above, pure monoclinic phase [JCPDS 88-2390] was formed, and no tetragonal phase was detected, as observed in Fig. 1(e). Further increasing the annealing temperature to 1100 °C increased the XRD intensity owing to the enhancement of crystallinity (Fig. 1(f)). Based on the above results, microemulsion-derived tetragonal ZrO2 nanoparticles are suggested to be metastable and easily transform to the monoclinic phase upon heating. The average crystallite sizes were
Fig. 1. XRD patterns of microemulsion-derived ZrO2:1 mol%Ce3+ precursors (a) preheated at 600 °C in air, and annealed at (b) 700 °C, (c) 800 °C, (d) 900 °C, (e) 1000 °C, and (f) 1100 °C in H2–N2 atmosphere.
estimated from the full width at half maxima (FWHM) and strain (ε) of the XRD peak, using the Hall and Williamson relation [18]: βcosθ=λ ¼ 1=D þ ε sinθ=λ
ð1Þ
where D represents the crystallite size; β is the FWHM of the XRD peak; λ is the incident wavelength, and θ is the diffraction angle. For precursor powders that were annealed from 700 °C to 1100 °C, the average crystallite sizes were increased from 45 nm to 121 nm, respectively, whereas that of microemulsion-derived precursor powders was calculated to be 18 nm. To analyze the phase transformation clearly, the magnified XRD patterns of microemulsion-derived ZrO2:1 mol%Ce3+ precursors annealed at various temperatures are displayed in Fig. 2. As the heating temperature was increased from 700 °C to 1000 °C, the XRD reflection (101) of the tetragonal phase suppressed gradually with the simultaneous evolution of two reflections 111 and (111) of the monoclinic phase, as presented in Fig. 2(a) to (e). Heating to 1100 °C the XRD intensity of the monoclinic phase increased (Fig. 2(f)). The volume fraction of the monoclinic-ZrO2 was estimated using the relationship [19], Vm ¼ ½1:311Xm =ð1 þ 0:311Xm Þ
ð2Þ
where Xm represents the XRD intensity ratio and is defined as h i h i Xm ¼ Im ð111Þ þ Im 111 = Im ð111Þ þ Im 111 þ It ð101Þ
ð3Þ
where Vm is the volume fraction of the monoclinic phase, and Im(111) and Im 111 denoted the line intensities of the (111) and 111 peaks for monoclinic-ZrO2, respectively. It(101) is the intensity of the (101) peak for tetragonal-ZrO2. The volume fraction (Vm) of the monoclinic-ZrO2 was estimated to be 0.29, 0.43 and 0.84 for the samples that were heated to 700 °C, 800 °C and 900 °C, respectively. In monoclinic ZrO2, which is stable at room temperature, the coordination number of the Zr4 + cations is seven, whereas that in both
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tetragonal and cubic-ZrO2 is eight. The strongly covalent Zr–O bond favors a coordination number of seven. Therefore, monoclinic ZrO2 is observed to be thermodynamically stable at room temperature. However, the tetragonal phase of ZrO2 is normally stable above 1170 °C [4,20]. The stability of the tetragonal phase of bulk ZrO2 is attributable to the presence of oxygen ion vacancies that are formed by doping tri-, tetra, and pentavalent impurities into the ZrO2 lattice [21]. In nanosized ZrO2, the tetragonal phase can be stabilized by the aggregation of ZrO2 nanocrystallites [21,22]. Based on the above discussion, the small crystallite size (~18 nm) and the presence of oxygen vacancies stabilized the tetragonal phase of ZrO2 precursors at room temperature [21,22]. Hydrolysis during the microemulsion process introduced hydroxyl groups into the amorphous ZrO2 structure. During heat treatment in air, the hydroxyl groups were driven out from ZrO2 precursors in the form of water, and the subsequent amount of Zr–OH in the lattice was thereby decreased. Such dehydroxylation reduced the valance of zirconium ions to low-valence numbers resulting in the generation of charge-compensated oxygen vacancies, according to following equation [3,23] Zr4þ –OH− þ Zr4þ –OH− →Zr3þ –Vo þ Zr4þ –O− þ H2 O:
ð4Þ
The oxygen vacancies in ZrO2 are considered to be responsible for the formation of the tetragonal phase upon crystallization, instead of the thermodynamically stable monoclinic phase [3,4]. The autoreduction of zirconium ion species with the generation of oxygen vacancies may explain the formation of the tetragonal phase of the microemulsion-derived ZrO2 precursors at room temperature [3,24]. However, oxygen vacancies may also be introduced by the incorporation of trivalent Ce3+ ions [4,25]. Further annealing of ZrO2 precursors in the reduction furnace accelerated the tetragonal-to-monoclinic phase transformation from 700 °C to 900 °C. During heat treatment, the penetrating OH groups became O2− and subsequently reduced the oxygen vacancies that were responsible for the formation of the tetragonal phase [3,24,25].
Fig. 2. Magnified XRD patterns of microemulsion-derived ZrO2:1 mol%Ce3+ precursors (a) preheated at 600 °C in air, and annealed at (b) 700 °C, (c) 800 °C, (d) 900 °C, (e) 1000 °C, and (f) 1100 °C in H2–N2 atmosphere.
Fig. 3. XRD patterns of undoped and xmol%Ce3+-content doped ZrO2 precursors preheated at 600 °C and annealed at 1100 °C in H2–N2 atmosphere. (a) undoped, (b) 1 mol% Ce3+, (c) 2 mol% Ce3+, (d) 3 mol% Ce3+, (e) 4 mol% Ce3+, and (f) 5 mol% Ce3+.
Fig. 4. Micro Raman analysis of microemulsion-derived ZrO2:1 mol%Ce3+ precursors (a) preheated at 600 °C in air, and annealed at (b) 800 °C, and (c) 1000 °C in H2–N2 atmosphere.
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XRD measurements were made to elucidate the effects of Ce3+ concentration on the structure and crystallinity of ZrO2. Fig. 3 displays the XRD patterns of undoped and Ce3+-doped ZrO2 nanoparticles at various concentrations that were calcined at 1100 °C for 4 h. The XRD pattern of the undoped ZrO2 was highly consistent with pure monoclinic ZrO2 with a space group of P21/c (14) [JCPDS 88-2390], as shown in Fig. 3(a). The incorporation of Ce3+ ions up to 4 mol%Ce3+ showed no significant effect on the host structure, as shown in Fig. 3(b) to (e). When the doping concentration reached 5 mol%, a small amount of tetragonal Zr0.5Ce0.5O2 phase [JCPDS 38-1436] was formed, as displayed in Fig. 3(f). 3.2. Micro-Raman analysis of ZrO2:Ce3+ nanoparticles The systematic phase transformation of ZrO2:Ce3 + owing to the increase in the heating temperature was elucidated from the room-
23
temperature Raman spectra, since Raman spectroscopy is more sensitive to the presence of secondary phases than is XRD. Fig. 4(a) displays the Raman spectra of microemulsion-derived ZrO2:1 mol%Ce3+ precursors. The intense Raman peaks from 100 cm−1 to 650 cm−1 could be easily attributed to the pure tetragonal zirconia, as indicated in Fig. 4(a) [26]. Upon heating the precursors, an increasing amount of the monoclinic phase was formed, as shown in Fig. 4(b). From the relevant spectrum, the major phase of ZrO2:Ce3+ was found to be monoclinic, coexisting with a small amount of the tetragonal phase. The signature bands of the tetragonal phase clearly appeared at around 608 and 640 cm−1 along with other bands, indicating the phase transformation from tetragonal to monoclinic. These results are in good agreement with the XRD results, as shown in Fig. 1. The Raman spectra revealed a complete transformation from the tetragonal to the monoclinic phase, as presented in Fig. 4(c). In this figure, the sharp Raman peaks from 140 cm−1 to 690 cm−1 are associated with pure monoclinic
Fig. 5. TEM images and the corresponding SAED patterns (insets) of microemulsion-derived ZrO2:1 mol%Ce3+ precursors (a) preheated at 600 °C in air, and annealed at and calcined at (b) 700 °C, (c) 800 °C, (d) 900 °C, (e) 1000 °C, and (f) 1100 °C in H2–N2 atmosphere.
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precursors that were annealed at various temperatures. From the SAED patterns of microemulsion derived precursors, lattice planes (101), (102) and (112) of tetragonal phase could be identified, as shown in Fig. 5(a). The SAED patterns in Fig. 5(e) and (f) are consistent with lattice planes 111 , (110) and (022) of the monoclinic phase. The coexistence of tetragonal and monoclinic phases in ZrO2:Ce3+ was also verified from the SAED patterns of the samples that were prepared at annealing temperatures from 700 °C–900 °C, as seen in the insets in Fig. 5(b)–(d). Moreover, the discontinuity in the diffraction rings and the increase in spots with the annealing temperature were observed. The above results demonstrate that the polycrystallinity in ZrO2:Ce3+ nanoparticles increased with processing temperature. A tetragonal-tomonoclinic phase transformation is clearly observed. 3.4. Photoluminescence characterization of ZrO2:Ce3+ nanoparticles
Fig. 6. (a) Photoluminescence emission spectra of microemulsion-derived ZrO2:1%Ce3+ precursors preheated at 600 °C in air and calcined at various temperatures in H2–N2 atmosphere. (b) Photoluminescence emission spectra of various Ce3+-content doped ZrO2 precursors preheated at 600 °C and annealed at 1100 °C in H2–N2 atmosphere. Inset: deconvoluted emission spectrum of ZrO2:4 mol%Ce3+.
Fig. 6(a) shows the photoluminescence emission spectra of microemulsion-derived ZrO2:Ce3+ nanoparticles that were calcined at various temperatures (λEX = 300 nm). The photoluminescence intensity increased with the annealing temperatures. The increase in annealing temperature caused constituent Ce3+ ions to have a high diffusion rate. It helps to increase the energy of Ce3+ ion over the activation energy of Zr4+ in the lattice during reactions. Therefore, at high annealing temperatures, sufficient Ce3+ ions were easily incorporated into Zr4+ lattice sites, and the ZrO2 host tended to have a low concentration of defect centers. As a result, the luminescent intensity of ZrO2:1 mol%Ce3+ increased with the annealing temperature. Fig. 6(b) shows the emission spectra of various Ce3 + ion-doped ZrO2 nanoparticles that were synthesized at an annealing temperature of 1100 °C. Although the emissive nature of the Ce 3 +-doped ZrO2 did not vary with temperatures but increased with the concentration of Ce3 + ions up to 4 mol%. Above 4 mol% of Ce3 + doping, the photoluminescence started to quench because of the substantial increase in the amount of the impurity in Zr0.95Ce0.05O2 phase, as observed in the XRD analysis in Fig. 4. The emission spectrum was well fitted by the sum of two Gaussian bands A and B (484 and 543 nm), corresponding to the transitions 5d (2A1g) → 2Fj and 5d (2B1g) → 2Fj, respectively, of the Ce3+ ions (inset in Fig. 6(b)) [29,30]. The photoluminescence excitation spectra of ZrO2:1 mol%Ce3 + nanoparticles demonstrated an increase in intensity with annealing temperature. Fig. 7 exhibits the excitation spectrum (λem = 484 nm)
zirconia, as indicated [26–28]. Hence, as the annealing temperature was increased above 900 °C, the crystallites turned into a pure monoclinic phase, and the Raman resonances became significantly more intense. 3.3. TEM analysis of ZrO2:Ce3+ nanoparticles In order to obtain insight information about the surface morphology and particle size of the samples, TEM studies had been performed. Fig. 5 presents the TEM images of the prepared samples. The TEM image of microemulsion-derived ZrO2:1 mol%Ce3+ precursors revealed that the sample precipitated into agglomerated nanosized particles with an average particle size of 20 nm (Fig. 5(a)). Heating to 700 °C reduced the agglomeration and increased the particle size, as shown in Fig. 5(b). As the annealing temperature increases from 700 °C to 1100 °C, the average nanoparticle size increased from 48 nm to 135 nm, as estimated from the TEM images in Fig. 5(b) to (e). The size distribution of ZrO2:Ce3+ nanoparticles was almost uniform with little aggregation. The insets in Fig. 5 present the selected area electron diffraction (SAED) patterns of microemulsion-derived ZrO2:1 mol%Ce3 +
Fig. 7. Photoluminescence excitation spectra of microemulsion-derived ZrO2:1 mol%Ce3+ precursors preheated at 600 °C in air and calcined at 1100 °C in H2–N2 atmosphere.
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peak III at around 290 nm (~ 4.27 eV) is attributable to the f–d transitions of Ce3+ ions in the host lattice. In addition, the consequent splitting of the ground state (2Fj of the 4f1 electron configuration) for the Ce3+ ions is ascribed to two sublevels 2F7/2 and 2F7/2 under the spin– orbit interaction. The photoluminescence decay profiles of the bluish green-emitting ZrO2:xmol%Ce3 + nanoparticles were also recorded as functions of Ce3+ concentration. The resulting decay curves were found to be double exponential. According to the literature [32], the main cause for the multiexponential decay is the transfer of excitation energy from donor to lanthanide activators. As seen in the inset of Fig. 6(b), the dopant Ce3 + has two emission centers namely Ce(A) and Ce(B). The biexponential decay behavior indicates the predominance of inter-ionic interactions and energy transfer between Ce(A) and Ce(B). Such interionic interactions resulted in fast and slow photoluminescence decay components. Therefore, the decay curves were observed to be double exponential in nature. The as-recorded non linear decay curves were well fitted by the double exponential function, as follows [33,34]: Iðt Þ ¼ I 1 e−t=τ1 þ I2 e−t=τ2
ð5Þ
where I(t) represents the intensity of luminescence at a particular time, and τ1 and τ2 are the short and long lifetimes that correspond to the intensity coefficients I1 and I2, respectively. The average lifetime is defined as follows [33,34]: τav ¼ I1 τ 1 2 þ I 2 τ2 2 =ðI1 τ 1 þ I2 τ 2 Þ: Fig. 8. The emission decay curves of the spectra for the (a) ZrO2:3 mol%Ce3+, (b) ZrO2:4 mol%Ce3+ and, (c) ZrO2:5 mol%Ce3+ nanoparticles preheated at 600 °C in air and calcined at 1100 °C in H2–N2 atmosphere.
and the Gaussian components of the ZrO2:1 mol%Ce3+ sample that was calcined at 1100 °C, which shows three excitation peaks centered at around 234, 244 and 290 nm. Excitation peak I, located at 234 nm (~5.3 eV), arose from electron transfer from the valence band (VB) to the conduction band (CB) of the ZrO2 host [31]. Peak II at 244 nm (~5.1 eV) may have arisen from a charge (electron) transfer transition associated with cerium–oxygen interactions. The broad absorption
ð6Þ
The average decay time was observed to increase from 0.013 ms to 0.079 ms, as the concentration of Ce3+-dopant increased from 1 mol% to 4 mol%. On the other hand, the average decay time of ZrO2:5 mol%Ce3+ nanoparticles was estimated to be 0.039 ms. Sudden quenching at 5 mol%Ce 3 + concentration was observed owing to the formation of the impurity phase. Fig. 8 presents the emission decay curves of the ZrO2:xmol% Ce3 + (x = 3, 4, and 5). The above results demonstrate that the optimized microemulsion synthesis scheme and Ce 3 + -concentrations effectively controlled the formation of the ZrO2:Ce3+ phase and improved the photoluminescence intensity. The as-presented photoluminescence suggests that microemulsion-derived ZrO2:Ce3+ nanoparticles have the potential to be utilized in optoelectronic devices. Fig. 9 shows the variation of emitted color of ZrO2:x%Ce3 + nanocrystals with the Ce3+ concentration. According to Fig. 9, the bright bluish green emission was achieved for the present phosphors [25]. The inset of Fig. 9 shows the CIE chromaticity diagram for ZrO2:x%Ce3 + nanocrystals synthesized at various annealing temperatures ranging from 700 °C to 1100 °C for 4 h. The effects of the annealing temperatures on the emission color, shown in the inset of Fig. 9, elaborated that the emission color remained about unchanged with the variation of annealing temperatures. 4. Conclusions
Fig. 9. CIE diagram of the various Ce3+-content doped ZrO2 precursors preheated at 600 °C in air and calcined at 1100 °C in H2–N2 atmosphere. Inset: CIE diagram of the microemulsion-derived ZrO2:1%Ce3+ precursors preheated at 600 °C in air and calcined at various temperatures in H2–N2 atmosphere. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Tetragonal ZrO2:Ce3 + nanoparticles were successfully synthesized via a microemulsion method, followed by annealing at elevated temperatures. The XRD pattern of as-formed precursor was consistent with the tetragonal phase. The reduction of structural defects promoted the tetragonal to monoclinic phase transformation. The amount of the monoclinic phase was increased by heat treatment at high temperatures owing to the suppression of structural defects. Above 900 °C, the crystal structure was completely converted to single monoclinic phase. Ce3 +-doped ZrO 2 exhibited strong bluish green photoluminescence under excitation by UV at 300 nm. The luminescence efficiency increased with the annealing temperature. ZrO 2 nanoparticles that were doped with 4 mol% Ce 3 + exhibited the highest photoluminescence intensity with an average lifetime
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of 0.079 ms. Additionally, emission measurements suggested that the emissive properties of ZrO 2:Ce3 + nanoparticles involved two site symmetries. The results of the present work elucidate the optimization and suitability of the microemulsion route for synthesizing ZrO2:Ce3 + nanoparticles. References [1] Z. Shu, X. Jiao, D. Chen, Hydrothermal synthesis and selective photocatalytic properties of tetragonal star-like ZrO2 nanostructures, CrystEngComm 15 (2013) 4288–4294. [2] R.D. Monte, J. Kaspar, Nanostructured CeO2–ZrO2 mixed oxides, J. Mater. Chem. 15 (2005) 633–648. [3] S. Chang, R. Doong, Chemical-composition-dependent metastability of tetragonal ZrO2 in sol–gel-derived films under different calcination conditions, Chem. Mater. 17 (2005) 4837–4844. [4] S. Xie, E. Iglesia, A.T. Bell, Water-assisted tetragonal-to-monoclinic phase transformation of ZrO2 at low temperatures, Chem. Mater. 12 (2000) 2442–2447. [5] B.M. Tissue, Synthesis and luminescence of lanthanide ions in nanoscale insulating hosts, Chem. Mater. 10 (1998) 2837–2845. [6] V.K. Vidhu, D. Philip, Phytosynthesis and applications of bioactive SnO2 nanoparticles, Mater. Charact. 101 (2015) 97–105. [7] L. Chen, T. Mashimo, E. Omurzak, H. Okudera, C. Iwamoto, A. Yoshias, Pure tetragonal ZrO2 nanoparticles synthesized by pulsed plasma in liquid, J. Phys. Chem. C 115 (2011) 9370–9375. [8] A. Behbahani, S. Rowshanzamir, A. Esmaeilifar, Hydrothermal synthesis of zirconia nanoparticles from commercial zirconia, Procedia Eng. 42 (2012) 908–917. [9] C. Viazzi, A. Deboni, J.Z. Ferreira, J.P. Bonino, F. Ansart, Synthesis of yttria stabilized zirconia by sol–gel route: influence of experimental parameters and large scale production, Solid State Sci. 8 (2006) 1023–1028. [10] N.L. Wu, T.F. Wu, Enhanced phase stability for tetragonal zirconia in precipitation synthesis, J. Am. Ceram. Soc. 83 (2000) 3225–3227. [11] J.R. Syu, S. Kumar, S. Das, C.H. Lu, Microemulsion-mediated synthesis and characterization of YBO3:Ce3+ phosphors, J. Am. Ceram. Soc. 95 (2012) 1814–1817. [12] C.Y. Chung, C.H. Hsu, C.H. Lu, Preparation and mechanism of nest-like YBO3:Tb3+ phosphors synthesized via the microemulsion-mediated hydrothermal Process, J. Am. Ceram. Soc. 94 (2011) 2884–2889. [13] C.Y. Chung, C.H. Lu, Reverse-microemulsion preparation of visible-light-driven nano-sized BiVO4, J. Alloys Compd. 502 (2010) L1–L5. [14] C.H. Lu, C.H. Lee, C.H. Wu, Microemulsion-mediated solvothermal synthesis of copper indium diselenide powders, Sol. Energy Mater. Sol. Cells 94 (2010) 1622–1626. [15] C.H. Lu, Y.K. Lin, Microemulsion preparation and electrochemical characteristics of LiNi1/3Co1/3Mn1/3O2 powders, J. Power Sources 189 (2009) 40–44. [16] C.H. Lu, H.C. Wang, Microemulsion-mediated synthesis and electrochemical characterization of nanosized LiNi0.25Co0.5Mn0.25O2 powders, J. Electrochem. Soc. 152 (2005) C341–C347.
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Materials Characterization journal homepage: www.elsevier.com/locate/matchar
Microemulsion-derived ZrO2:Ce3 + nanoparticles: Phase transformation and photoluminescence characterization Subrata Das, Chih-Cheng Chang, Che-Yuan Yang, Sudipta Som, Chung-Hsin Lu ⁎ Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 11 February 2015 Received in revised form 23 April 2015 Accepted 2 May 2015 Available online 5 May 2015 Keywords: Nanoparticles Microemulsion ZrO2 Phosphors Photoluminescence
a b s t r a c t Bluish green-emitting ZrO2:Ce3+ nanoparticles were synthesized via a microemulsion route. The effects of the annealing temperature on the phase evolution and luminescence properties were investigated in detail. As the aqueous micelles confined the constituent cations during crystallization, nanosized particles were formed. The formed ZrO2 nanoparticles were observed to undergo a phase transition from tetragonal to monoclinic structure with the increase in annealing temperatures. The reduction of structural defects with annealing temperatures promoted the phase transformation. When the annealing temperature exceeded 900 °C, only the monoclinic phase was present. The photoluminescence emission appeared in the bluish green region around 487 nm under ultraviolet excitation. The emission intensity increased with the annealing temperatures. The optimal Ce3+-dopant concentration in ZrO2 nanoparticles was determined, and the concentration quenching mechanism was discussed. The present work suggests that microemulsion-derived ZrO2:Ce3+ nanoparticles are suitable for optoelectronic applications. © 2015 Published by Elsevier Inc.
1. Introduction Zirconia is one of the most important ceramic materials owing to its excellent mechanical, electrical, thermal and optical properties. Zirconia ceramics are known for the superior hardness, good chemical and photochemical stability, large dielectric constant and optical transparency [1–4]. Recently, increasing attention has been paid to the luminescence properties of various rare earth-doped ZrO2 nanoparticles because of their low phonon energy and high optical output. Nanotechnology based on ceramic materials that involve zirconia has attracted considerable attention because of the demand for not only sophisticated optoelectronic and photonic devices but also a broad range of biomedical applications [5–7]. Various routes for preparing ZrO2 nanoparticles with spherical morphology and uniform size distribution need to be developed for optical and luminescence applications. Therefore, the synthesis of rare earth ion-doped zirconia nanoparticles with controlled sizes, structures and surface properties is of technological and fundamental importance. It is worth to mention that the phosphors with a spherical morphology have appreciable packing density, effective slurry properties, and low light scattering properties which improve luminescent intensity. Numerous synthetic routes, such as the hydrothermal method [8], the sol–gel [9] precipitation method [10] and the pulsed plasma in liquid method [7] have been utilized for the preparation of zirconia nanoparticles. However, the conventional processes are not capable to produce ⁎ Corresponding author. E-mail address: [email protected] (C.-H. Lu).
http://dx.doi.org/10.1016/j.matchar.2015.05.004 1044-5803/© 2015 Published by Elsevier Inc.
ZrO2 nanoparticles with spherical morphology, since these processes require high heating temperatures and long reaction duration. On the contrary, the microemulsion method is useful for preparing ceramic nanopowders with spherical morphology. The microemulsion method is helpful for preparing ceramic powders with nanosized particles and spherical morphology [11–14]. A microemulsion system is a thermodynamically stable solution that contains aqueous micelles, which are uniformly dispersed throughout a continuous oil phase. The micelles act as nanoreactors and restrict the growth of particles [15,16]. Accordingly, the morphology and the size of the particles in the produced powders can be easily controlled using the microemulsion technique. Microemulsion-derived particles are expected to have a uniform nanoscale size distribution with spherical morphology, owing to the confinement of the constituent cations by the aqueous micelles. Trivalent cerium (Ce3+) ions are well-known rare earth activators in various inorganic lattices that exhibit intense blue luminescence [11, 17]. Ce3 + has one 4f electron and the lowest excited configuration is 5d1 level. Since the crystal field strongly influences the unshielded 5d excited state, the symmetry of the local crystal field may induce splitting of the d-orbitals, thereby affecting the emission wavelength and intensity of the material. Hence, the luminescence properties of Ce3+ ions in different host matrices have been extensively investigated. Materials doped with Ce3 + ions have attracted substantial interest owing to their applications as phosphors, scintillators for elementary particles, detectors of ionizing radiation, UV absorbing filters and UV emitters. A wide variety of host matrices of Ce3+, such as borates, niobates, sulfates and phosphates, have been investigated [11,17].
S. Das et al. / Materials Characterization 106 (2015) 20–26
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The present research work aims at utilizing a modified microemulsion route to synthesize the spherical Ce3+-doped ZrO2 nanoparticles for the first time. Detailed experimental investigations of the structural and luminescence properties of ZrO2:Ce3 + nanoparticles were performed using various annealing temperatures. The growth mechanism and phase transformation of Ce3 +-doped ZrO2 were determined in detail. The effects of Ce3+-concentration on the crystal structure and luminescence properties of monoclinic ZrO2:Ce3+ nanoparticles were demonstrated. The average lifetimes of ZrO2:Ce3+ nanoparticles were also analyzed. The present results demonstrate that the microemulsion route is effective for preparing ZrO2:Ce3+ nanoparticles with spherical morphology for various optoelectronic applications. 2. Experimental The starting materials namely ZrO(NO3)2.xH2O (99.9%, Sigma Aldrich), Ce(NO3)3.6H2O (99%, Sigma Aldrich), cyclohexane (99%, Sigma Aldrich), OP-10 (95%, Sigma Aldrich), and 1-hexanol (95%, Nacalai Tesque) were used without further purification. Initially the desired amount of ZrO(NO3)2.xH2O and Ce(NO3)3.6H2O was taken and dissolved in 99% pure ethyl alcohol. The concentration of Ce3+ with respect to Zr4+ was varied from 1 mol% to 5 mol%. Cyclohexane was used as oil, while olyoxyethylene-10-octylphenyl ether (OP-10) and 1-hexanol were used as the surfactant and co-surfactant, respectively. A mixture of cyclohexane, OP-10, and 1-hexanol in a volume ratio of 10:2:3 was used for getting the oil phase. Under sonication, the mixed aqueous phase was dispersed into the oil phase to form the water in-oil (W/O) microemulsion. Moreover, the volume ratio of water phase to oil phase was adjusted at 0.05. The as prepared microemulsion solution was slowly dropped into hot kerosene (180 °C) and heated at around 180 °C for water evaporation to form gels. Further the calcination of the gels was performed at 600 °C for 3 h in air to form the precursor powders. Finally, the precursor powders were heated at various temperatures ranging from 700 °C to 1100 °C for 4 h in a reducing atmosphere (vol. 5% of H2–vol. 95% of N2). The crystal structures of the prepared powders were identified via X-ray diffraction (XRD; Philips X'pert/MPD, Amsterdam, the Netherlands) using CuKα radiation at room temperature. The morphologies and particle sizes were studied using scanning electron microscopy (SEM, Hitachi S-800) and transmission electron microscopy (TEM; JEOL TEM-3010, Tokyo, Japan). The photoluminescence spectra of the synthesized samples were recorded by using a fluorescence spectrophotometer (Hitachi, F-4500, Tokyo, Japan) with xenon lamp operated at 150 W as an excitation source. 3. Results and discussion 3.1. Phase characterization of ZrO2:Ce3+ nanoparticles Fig. 1 shows the XRD patterns of microemulsion-derived ZrO2:1 mol%Ce3 + precursors that were annealed at various temperatures. The XRD patterns of the as-synthesized precursors were entirely consistent with the tetragonal ZrO2 phase [JCPDS 81-1544], as presented in Fig. 1(a). Upon heating at 700 °C, the tetragonal ZrO2 phase was found to contain a small amount of monoclinic ZrO2 phase, as displayed in Fig. 1(b). As the heating temperature increased to 900 °C, the amount of tetragonal phase formed declined while the amount of monoclinic phase increased, as shown in Fig. 1(c) and (d). Upon heating at 1000 °C and above, pure monoclinic phase [JCPDS 88-2390] was formed, and no tetragonal phase was detected, as observed in Fig. 1(e). Further increasing the annealing temperature to 1100 °C increased the XRD intensity owing to the enhancement of crystallinity (Fig. 1(f)). Based on the above results, microemulsion-derived tetragonal ZrO2 nanoparticles are suggested to be metastable and easily transform to the monoclinic phase upon heating. The average crystallite sizes were
Fig. 1. XRD patterns of microemulsion-derived ZrO2:1 mol%Ce3+ precursors (a) preheated at 600 °C in air, and annealed at (b) 700 °C, (c) 800 °C, (d) 900 °C, (e) 1000 °C, and (f) 1100 °C in H2–N2 atmosphere.
estimated from the full width at half maxima (FWHM) and strain (ε) of the XRD peak, using the Hall and Williamson relation [18]: βcosθ=λ ¼ 1=D þ ε sinθ=λ
ð1Þ
where D represents the crystallite size; β is the FWHM of the XRD peak; λ is the incident wavelength, and θ is the diffraction angle. For precursor powders that were annealed from 700 °C to 1100 °C, the average crystallite sizes were increased from 45 nm to 121 nm, respectively, whereas that of microemulsion-derived precursor powders was calculated to be 18 nm. To analyze the phase transformation clearly, the magnified XRD patterns of microemulsion-derived ZrO2:1 mol%Ce3+ precursors annealed at various temperatures are displayed in Fig. 2. As the heating temperature was increased from 700 °C to 1000 °C, the XRD reflection (101) of the tetragonal phase suppressed gradually with the simultaneous evolution of two reflections 111 and (111) of the monoclinic phase, as presented in Fig. 2(a) to (e). Heating to 1100 °C the XRD intensity of the monoclinic phase increased (Fig. 2(f)). The volume fraction of the monoclinic-ZrO2 was estimated using the relationship [19], Vm ¼ ½1:311Xm =ð1 þ 0:311Xm Þ
ð2Þ
where Xm represents the XRD intensity ratio and is defined as h i h i Xm ¼ Im ð111Þ þ Im 111 = Im ð111Þ þ Im 111 þ It ð101Þ
ð3Þ
where Vm is the volume fraction of the monoclinic phase, and Im(111) and Im 111 denoted the line intensities of the (111) and 111 peaks for monoclinic-ZrO2, respectively. It(101) is the intensity of the (101) peak for tetragonal-ZrO2. The volume fraction (Vm) of the monoclinic-ZrO2 was estimated to be 0.29, 0.43 and 0.84 for the samples that were heated to 700 °C, 800 °C and 900 °C, respectively. In monoclinic ZrO2, which is stable at room temperature, the coordination number of the Zr4 + cations is seven, whereas that in both
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tetragonal and cubic-ZrO2 is eight. The strongly covalent Zr–O bond favors a coordination number of seven. Therefore, monoclinic ZrO2 is observed to be thermodynamically stable at room temperature. However, the tetragonal phase of ZrO2 is normally stable above 1170 °C [4,20]. The stability of the tetragonal phase of bulk ZrO2 is attributable to the presence of oxygen ion vacancies that are formed by doping tri-, tetra, and pentavalent impurities into the ZrO2 lattice [21]. In nanosized ZrO2, the tetragonal phase can be stabilized by the aggregation of ZrO2 nanocrystallites [21,22]. Based on the above discussion, the small crystallite size (~18 nm) and the presence of oxygen vacancies stabilized the tetragonal phase of ZrO2 precursors at room temperature [21,22]. Hydrolysis during the microemulsion process introduced hydroxyl groups into the amorphous ZrO2 structure. During heat treatment in air, the hydroxyl groups were driven out from ZrO2 precursors in the form of water, and the subsequent amount of Zr–OH in the lattice was thereby decreased. Such dehydroxylation reduced the valance of zirconium ions to low-valence numbers resulting in the generation of charge-compensated oxygen vacancies, according to following equation [3,23] Zr4þ –OH− þ Zr4þ –OH− →Zr3þ –Vo þ Zr4þ –O− þ H2 O:
ð4Þ
The oxygen vacancies in ZrO2 are considered to be responsible for the formation of the tetragonal phase upon crystallization, instead of the thermodynamically stable monoclinic phase [3,4]. The autoreduction of zirconium ion species with the generation of oxygen vacancies may explain the formation of the tetragonal phase of the microemulsion-derived ZrO2 precursors at room temperature [3,24]. However, oxygen vacancies may also be introduced by the incorporation of trivalent Ce3+ ions [4,25]. Further annealing of ZrO2 precursors in the reduction furnace accelerated the tetragonal-to-monoclinic phase transformation from 700 °C to 900 °C. During heat treatment, the penetrating OH groups became O2− and subsequently reduced the oxygen vacancies that were responsible for the formation of the tetragonal phase [3,24,25].
Fig. 2. Magnified XRD patterns of microemulsion-derived ZrO2:1 mol%Ce3+ precursors (a) preheated at 600 °C in air, and annealed at (b) 700 °C, (c) 800 °C, (d) 900 °C, (e) 1000 °C, and (f) 1100 °C in H2–N2 atmosphere.
Fig. 3. XRD patterns of undoped and xmol%Ce3+-content doped ZrO2 precursors preheated at 600 °C and annealed at 1100 °C in H2–N2 atmosphere. (a) undoped, (b) 1 mol% Ce3+, (c) 2 mol% Ce3+, (d) 3 mol% Ce3+, (e) 4 mol% Ce3+, and (f) 5 mol% Ce3+.
Fig. 4. Micro Raman analysis of microemulsion-derived ZrO2:1 mol%Ce3+ precursors (a) preheated at 600 °C in air, and annealed at (b) 800 °C, and (c) 1000 °C in H2–N2 atmosphere.
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XRD measurements were made to elucidate the effects of Ce3+ concentration on the structure and crystallinity of ZrO2. Fig. 3 displays the XRD patterns of undoped and Ce3+-doped ZrO2 nanoparticles at various concentrations that were calcined at 1100 °C for 4 h. The XRD pattern of the undoped ZrO2 was highly consistent with pure monoclinic ZrO2 with a space group of P21/c (14) [JCPDS 88-2390], as shown in Fig. 3(a). The incorporation of Ce3+ ions up to 4 mol%Ce3+ showed no significant effect on the host structure, as shown in Fig. 3(b) to (e). When the doping concentration reached 5 mol%, a small amount of tetragonal Zr0.5Ce0.5O2 phase [JCPDS 38-1436] was formed, as displayed in Fig. 3(f). 3.2. Micro-Raman analysis of ZrO2:Ce3+ nanoparticles The systematic phase transformation of ZrO2:Ce3 + owing to the increase in the heating temperature was elucidated from the room-
23
temperature Raman spectra, since Raman spectroscopy is more sensitive to the presence of secondary phases than is XRD. Fig. 4(a) displays the Raman spectra of microemulsion-derived ZrO2:1 mol%Ce3+ precursors. The intense Raman peaks from 100 cm−1 to 650 cm−1 could be easily attributed to the pure tetragonal zirconia, as indicated in Fig. 4(a) [26]. Upon heating the precursors, an increasing amount of the monoclinic phase was formed, as shown in Fig. 4(b). From the relevant spectrum, the major phase of ZrO2:Ce3+ was found to be monoclinic, coexisting with a small amount of the tetragonal phase. The signature bands of the tetragonal phase clearly appeared at around 608 and 640 cm−1 along with other bands, indicating the phase transformation from tetragonal to monoclinic. These results are in good agreement with the XRD results, as shown in Fig. 1. The Raman spectra revealed a complete transformation from the tetragonal to the monoclinic phase, as presented in Fig. 4(c). In this figure, the sharp Raman peaks from 140 cm−1 to 690 cm−1 are associated with pure monoclinic
Fig. 5. TEM images and the corresponding SAED patterns (insets) of microemulsion-derived ZrO2:1 mol%Ce3+ precursors (a) preheated at 600 °C in air, and annealed at and calcined at (b) 700 °C, (c) 800 °C, (d) 900 °C, (e) 1000 °C, and (f) 1100 °C in H2–N2 atmosphere.
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precursors that were annealed at various temperatures. From the SAED patterns of microemulsion derived precursors, lattice planes (101), (102) and (112) of tetragonal phase could be identified, as shown in Fig. 5(a). The SAED patterns in Fig. 5(e) and (f) are consistent with lattice planes 111 , (110) and (022) of the monoclinic phase. The coexistence of tetragonal and monoclinic phases in ZrO2:Ce3+ was also verified from the SAED patterns of the samples that were prepared at annealing temperatures from 700 °C–900 °C, as seen in the insets in Fig. 5(b)–(d). Moreover, the discontinuity in the diffraction rings and the increase in spots with the annealing temperature were observed. The above results demonstrate that the polycrystallinity in ZrO2:Ce3+ nanoparticles increased with processing temperature. A tetragonal-tomonoclinic phase transformation is clearly observed. 3.4. Photoluminescence characterization of ZrO2:Ce3+ nanoparticles
Fig. 6. (a) Photoluminescence emission spectra of microemulsion-derived ZrO2:1%Ce3+ precursors preheated at 600 °C in air and calcined at various temperatures in H2–N2 atmosphere. (b) Photoluminescence emission spectra of various Ce3+-content doped ZrO2 precursors preheated at 600 °C and annealed at 1100 °C in H2–N2 atmosphere. Inset: deconvoluted emission spectrum of ZrO2:4 mol%Ce3+.
Fig. 6(a) shows the photoluminescence emission spectra of microemulsion-derived ZrO2:Ce3+ nanoparticles that were calcined at various temperatures (λEX = 300 nm). The photoluminescence intensity increased with the annealing temperatures. The increase in annealing temperature caused constituent Ce3+ ions to have a high diffusion rate. It helps to increase the energy of Ce3+ ion over the activation energy of Zr4+ in the lattice during reactions. Therefore, at high annealing temperatures, sufficient Ce3+ ions were easily incorporated into Zr4+ lattice sites, and the ZrO2 host tended to have a low concentration of defect centers. As a result, the luminescent intensity of ZrO2:1 mol%Ce3+ increased with the annealing temperature. Fig. 6(b) shows the emission spectra of various Ce3 + ion-doped ZrO2 nanoparticles that were synthesized at an annealing temperature of 1100 °C. Although the emissive nature of the Ce 3 +-doped ZrO2 did not vary with temperatures but increased with the concentration of Ce3 + ions up to 4 mol%. Above 4 mol% of Ce3 + doping, the photoluminescence started to quench because of the substantial increase in the amount of the impurity in Zr0.95Ce0.05O2 phase, as observed in the XRD analysis in Fig. 4. The emission spectrum was well fitted by the sum of two Gaussian bands A and B (484 and 543 nm), corresponding to the transitions 5d (2A1g) → 2Fj and 5d (2B1g) → 2Fj, respectively, of the Ce3+ ions (inset in Fig. 6(b)) [29,30]. The photoluminescence excitation spectra of ZrO2:1 mol%Ce3 + nanoparticles demonstrated an increase in intensity with annealing temperature. Fig. 7 exhibits the excitation spectrum (λem = 484 nm)
zirconia, as indicated [26–28]. Hence, as the annealing temperature was increased above 900 °C, the crystallites turned into a pure monoclinic phase, and the Raman resonances became significantly more intense. 3.3. TEM analysis of ZrO2:Ce3+ nanoparticles In order to obtain insight information about the surface morphology and particle size of the samples, TEM studies had been performed. Fig. 5 presents the TEM images of the prepared samples. The TEM image of microemulsion-derived ZrO2:1 mol%Ce3+ precursors revealed that the sample precipitated into agglomerated nanosized particles with an average particle size of 20 nm (Fig. 5(a)). Heating to 700 °C reduced the agglomeration and increased the particle size, as shown in Fig. 5(b). As the annealing temperature increases from 700 °C to 1100 °C, the average nanoparticle size increased from 48 nm to 135 nm, as estimated from the TEM images in Fig. 5(b) to (e). The size distribution of ZrO2:Ce3+ nanoparticles was almost uniform with little aggregation. The insets in Fig. 5 present the selected area electron diffraction (SAED) patterns of microemulsion-derived ZrO2:1 mol%Ce3 +
Fig. 7. Photoluminescence excitation spectra of microemulsion-derived ZrO2:1 mol%Ce3+ precursors preheated at 600 °C in air and calcined at 1100 °C in H2–N2 atmosphere.
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peak III at around 290 nm (~ 4.27 eV) is attributable to the f–d transitions of Ce3+ ions in the host lattice. In addition, the consequent splitting of the ground state (2Fj of the 4f1 electron configuration) for the Ce3+ ions is ascribed to two sublevels 2F7/2 and 2F7/2 under the spin– orbit interaction. The photoluminescence decay profiles of the bluish green-emitting ZrO2:xmol%Ce3 + nanoparticles were also recorded as functions of Ce3+ concentration. The resulting decay curves were found to be double exponential. According to the literature [32], the main cause for the multiexponential decay is the transfer of excitation energy from donor to lanthanide activators. As seen in the inset of Fig. 6(b), the dopant Ce3 + has two emission centers namely Ce(A) and Ce(B). The biexponential decay behavior indicates the predominance of inter-ionic interactions and energy transfer between Ce(A) and Ce(B). Such interionic interactions resulted in fast and slow photoluminescence decay components. Therefore, the decay curves were observed to be double exponential in nature. The as-recorded non linear decay curves were well fitted by the double exponential function, as follows [33,34]: Iðt Þ ¼ I 1 e−t=τ1 þ I2 e−t=τ2
ð5Þ
where I(t) represents the intensity of luminescence at a particular time, and τ1 and τ2 are the short and long lifetimes that correspond to the intensity coefficients I1 and I2, respectively. The average lifetime is defined as follows [33,34]: τav ¼ I1 τ 1 2 þ I 2 τ2 2 =ðI1 τ 1 þ I2 τ 2 Þ: Fig. 8. The emission decay curves of the spectra for the (a) ZrO2:3 mol%Ce3+, (b) ZrO2:4 mol%Ce3+ and, (c) ZrO2:5 mol%Ce3+ nanoparticles preheated at 600 °C in air and calcined at 1100 °C in H2–N2 atmosphere.
and the Gaussian components of the ZrO2:1 mol%Ce3+ sample that was calcined at 1100 °C, which shows three excitation peaks centered at around 234, 244 and 290 nm. Excitation peak I, located at 234 nm (~5.3 eV), arose from electron transfer from the valence band (VB) to the conduction band (CB) of the ZrO2 host [31]. Peak II at 244 nm (~5.1 eV) may have arisen from a charge (electron) transfer transition associated with cerium–oxygen interactions. The broad absorption
ð6Þ
The average decay time was observed to increase from 0.013 ms to 0.079 ms, as the concentration of Ce3+-dopant increased from 1 mol% to 4 mol%. On the other hand, the average decay time of ZrO2:5 mol%Ce3+ nanoparticles was estimated to be 0.039 ms. Sudden quenching at 5 mol%Ce 3 + concentration was observed owing to the formation of the impurity phase. Fig. 8 presents the emission decay curves of the ZrO2:xmol% Ce3 + (x = 3, 4, and 5). The above results demonstrate that the optimized microemulsion synthesis scheme and Ce 3 + -concentrations effectively controlled the formation of the ZrO2:Ce3+ phase and improved the photoluminescence intensity. The as-presented photoluminescence suggests that microemulsion-derived ZrO2:Ce3+ nanoparticles have the potential to be utilized in optoelectronic devices. Fig. 9 shows the variation of emitted color of ZrO2:x%Ce3 + nanocrystals with the Ce3+ concentration. According to Fig. 9, the bright bluish green emission was achieved for the present phosphors [25]. The inset of Fig. 9 shows the CIE chromaticity diagram for ZrO2:x%Ce3 + nanocrystals synthesized at various annealing temperatures ranging from 700 °C to 1100 °C for 4 h. The effects of the annealing temperatures on the emission color, shown in the inset of Fig. 9, elaborated that the emission color remained about unchanged with the variation of annealing temperatures. 4. Conclusions
Fig. 9. CIE diagram of the various Ce3+-content doped ZrO2 precursors preheated at 600 °C in air and calcined at 1100 °C in H2–N2 atmosphere. Inset: CIE diagram of the microemulsion-derived ZrO2:1%Ce3+ precursors preheated at 600 °C in air and calcined at various temperatures in H2–N2 atmosphere. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Tetragonal ZrO2:Ce3 + nanoparticles were successfully synthesized via a microemulsion method, followed by annealing at elevated temperatures. The XRD pattern of as-formed precursor was consistent with the tetragonal phase. The reduction of structural defects promoted the tetragonal to monoclinic phase transformation. The amount of the monoclinic phase was increased by heat treatment at high temperatures owing to the suppression of structural defects. Above 900 °C, the crystal structure was completely converted to single monoclinic phase. Ce3 +-doped ZrO 2 exhibited strong bluish green photoluminescence under excitation by UV at 300 nm. The luminescence efficiency increased with the annealing temperature. ZrO 2 nanoparticles that were doped with 4 mol% Ce 3 + exhibited the highest photoluminescence intensity with an average lifetime
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