Sol–gel preparation and photoluminescence properties of tetragonal ZrO2:Y3+, Eu3+ nanophosphors

Optical Materials 35 (2012) 274–279

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Sol–gel preparation and photoluminescence properties of tetragonal ZrO2:Y3+, Eu3+ nanophosphors Jinsheng Liao, Dan Zhou, Bin Yang ⇑, Ruiqing Liu, Qian Zhang School of Metallurgy and Chemistry Engineering, Jiangxi University of Science and Technology, Ganzhou, Jiangxi 341000, China

a r t i c l e

i n f o

Article history: Received 7 July 2012 Received in revised form 27 August 2012 Accepted 29 August 2012 Available online 10 October 2012 Keywords: ZrO2 Phosphor Sol–gel Luminescence Tetragonal

a b s t r a c t By controlling Eu3+ doping concentration, ZrO2:Y3+, Eu3+ (YSZ:Eu) nanophosphors with tetragonal structure have been prepared by sol–gel method. The properties of the resulting phosphors are characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), photoluminescence (PL) spectra and decay curve. The excitation spectra of YSZ:Eu phosphors are mainly attributed to O2- ? Eu3+ chargetransfer (CT) band at about 254 nm and some sharp lines of Eu3+ f–f transitions in near-UV region with one strong peak at 395 nm. The emission spectra were used to probe the local environments of Eu3+ ion in ZrO2 crystal. Based on the high-resolution emission and excitation spectra at 10 K, it is identified that the lattice site of Eu3+ in tetragonal ZrO2 nanocrystals descending from D4h to approximate C2V as a result of the lattice distortion. Under the 395 nm excitation, intense red emission peaked at 607 nm corresponding to 5D0 ? 7F2 transition of Eu3+ is observed for 6 at.% YSZ:Eu phosphors as the optimal doping concentration. The luminescence properties suggest that YSZ:Eu phosphor may be regarded as a potential red phosphor candidate for near-UV light emitting diodes (LEDs). Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction At present, solid-state white light-emitting diodes (LEDs) have received much attention because they offer some advantages over conventional fluorescent and incandescent lamps, such as low energy consumption, more compact size, higher light output yield, longer service lifetime [1,2]. As a conventional white LED, white light is generated by combining GaN-based blue chip with yellow YAG:Ce3+ phosphor [3], however, this type of white light has a less satisfactory color rendering index (Ra  70) for the deficiency of color in red region [4]. One of the most promising approaches to solve the problem is the utilization of a near-UV-LED chip (360– 410 nm) in combination with blue, green and red phosphors, which provides a more balanced white emission spectrum and a higher color rendering. Presently, the main commercial red phosphor for near-UV-LED is Y2O2S:Eu3+ phosphor. However the type red phosphors have certain drawbacks such as low luminescence efficiency, chemical instability and short lifetime. Therefore, more attention has been paid to the investigation of the red emitting materials using near-UV-LED as the excitation source in the past few years [5–7], and the development of a new stable and high luminescence red phosphor with high absorption in the near-UV spectral region is an attractive and challenging research task. Zirconia (ZrO2) has received much attention due to a wide application in high-performance ceramics, catalysts, gas sensors and ⇑ Corresponding author. Tel.: +86 797 8312015; fax: +86 797 83122015. E-mail address: [email protected] (B. Yang). 0925-3467/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2012.08.016

optical devices [8–14]. The wide applicability of ZrO2 is owed to its excellent physical and chemical properties, which consist of high coefficient of thermal expansion, low thermal conductivity, high thermal shock resistance, high fracture toughness, optical transparency, polymorphic nature and high corrosion resistance [15,16]. ZrO2 possesses three different crystalline structures: monoclinic (m-), tetragonal (t-) and cubic (c-) polymorphs. The monoclinic phase is stable at room temperature. Tetragonal and cubic phase are stable at the temperatures higher than 1170 °C and 2370 °C, respectively [17,18]. The mechanical, electrical, catalytic and optical properties of ZrO2 are dependent on the crystal structures [19–21]. Particularly, stabilization of the high-temperature t- and c-phases at room temperature has been of significant interest for engineering applications and has generally been achieved by substituting Zr4+ with divalent or trivalent elements, such as Mg2+, Ca2+, and Y3+ [22–24]. Rare earth (RE) ions incorporated in host matrices have demonstrated to be optimum optical activators for use in luminescent devices [25]. Compared with bulk RE-doped ZrO2, nanostructured ZrO2 luminescent materials can be exploited for fluorescent biolabels [26] and nanophosphor devices [27]. Such materials with nanostructure bring about novel optical properties that influence luminescence lifetime, luminescence quantum efficiency and concentration quenching [28,29]. Despite their potential use in the fabrication of photonic crystals, relatively few reports on the synthesis of RE-doped ZrO2 nanoparticles have been published thus far [21,26,30–32]. But to the best of our knowledge, studies on sol–gel method and the luminescence properties of tetragonal YSZ:Eu nanocrystals have not yet been systemically

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YSZ:Eu phosphors with different Eu3+ doping concentration were prepared by sol–gel method. The starting materials for the sample preparations included ZrOCl28H2O (AR), Eu2O3 (99.99%), Y2O3 (99.99%), HNO3 and citric acid. A procedure for the sample synthesis is typically described as follows: 0.1320 g Eu2O3 and 0.0508 g Y2O3 were first dissolved in dilute nitric acid under heating. After the Eu2O3 (acted as both activating agent and stable agent of tetragonal ZrO2) and Y2O3 (acted as stable agent of tetragonal ZrO2) were completely dissolved, the excess nitrite acid was removed at high temperature. Then de-ionized water was added to obtain Eu(NO3)3 and Y(NO3)3 solution. Meanwhile citric acid was added to the above solution as chelating agent for the metal ions, and the molar ratio of total chelated metal cations to citric acid was 1:2. Subsequently, 4.4471 g ZrOCl28H2O were dissolved in a suitable volume of de-ionized water while stirring. After this procedure, 1.0 g of polyethylene glycol (molecular weight 20,000) as cross-linking agent was also added. At the end, the mixed solution was slowly dropped into rare earth solution with magnetic stirring, and the highly transparent solution was heated at 80 °C in a water bath to produce a light yellow transparent gel, and the gel was further dried at 120 °C in oven for 15 h to obtain yellow and dried gel. In the last step, the dried gel was annealed at 800 °C for 8 h in air to obtain the white phosphor sample. To investigate the effect of Eu3+ content on the structure of ZrO2 and the luminescence intensity, the other phosphors with different Eu3+doped concentration were synthesized by the same procedure with the corresponding starting materials. The thermogravimetric (TG) and differential thermal analysis (DTA) data of the complex precursors were recorded with thermal analysis instrument (Diamond TG), using a sample weight of about 10 mg and a heating rate 10 °C/min in air atmosphere. Powder X-ray diffraction patterns were obtained on a Panalytical X’Pert diffractometer using CuKa1 radiation (k = 0.154187 nm). The morphology of the samples was characterized by a JEOL-2010 transmission electron microscope (TEM) equipped with the energy dispersive X-ray spectrum (EDS). The photoluminescence spectra and the luminescence decay were recorded on an Edinburgh Instruments FLS920 spectrofluorimeter equipped with both continuous (450 W) xenon and pulsed xenon (microsecond) lamps. For low temperature measurements, samples were mounted on a closed cycle cryostat (10–350 K, DE202, Advanced Research Systems). The line intensities and positions of the measured spectra were calibrated according to the FLS920 correction curve and standard mercury lamp.

3.2. XRD and TEM characterization Fig. 2 shows XRD pattern of the YSZ:Eu samples with different Eu3+ content prepared by the sol–gel method (900 °C for 10 h) in comparison with the Joint Committee for Power Diffractions Standards (JCPDS) card (tetragonal JCPDS No. 79-1769 and monoclinic JCPDS No. 78-0047). When the Eu3+ concentrations vary from 0.5 to 4 at.%, the diffraction peaks of the tetragonal phase become stronger while those of monoclinic phase become weaker. When the Eu3+ concentration is 5 at.%, no traces of additional peaks from monoclinic phase are observed. With the increase of the Eu3+ concentration, it is clearly seen that the samples are in the pure tetragonal phase. The above-stated results indicate that the Eu3+ concentration is very important for preparing the pure tetragonal phase ZrO2. In addition, it is clearly seen that with the increase of the Eu3+ concentration the diffraction peak (1 0 1) of YSZ:Eu samples slightly shift to the high-angle. Because the radius of Eu3+ (0.107 nm) is quite close to that of Zr4+ (0.089 nm), Eu3+ can easily substitute Zr4+ ions in the host lattice. We conclude that Eu3+ has been efficiently incorporated into the host lattice of ZrO2 crystal. The crystallite sizes of the nanoparticles were calculated using the Scherrer equation:

D ¼ Kk=b cos h where K = 0.9; k is the wavelength of CuKa radiation (k = 0.15418 nm); b is the corrected half width of the diffraction peak, D represents the size of particle. By the equation, the size of 6 at.% YSZ:Eu nanocrystal is about 16 nm.

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TG curve up to 900 °C with the increase of temperature. There occurred three processes of weight loss in the TG curve from room temperature to 900 °C. The first process of weight loss between room temperature and 190 °C, the weight loss of 4.1% in TG curve is attributed to the removal of residual water molecules or the surface absorbed water. The second weight loss in the range of 190– 300 °C is related to the combustion of citric acid and the corresponding weight loss of 28.7% in TG curve. The third step from 300 to 600 °C may be related to the decomposition of the residual nitrates. The above processes lead to the total weight loss (59.3%). No further obvious weight loss was registered above 600 °C, which indicates that all compounds in the precursors are decomposed completely below 600 °C. The above three processes of weight loss along with several endothermic and exothermic peaks from room temperature to 900 °C are also shown in the DTA curve of Fig. 1. The strongest exothermic peak at about 500 °C represents the formation of the crystallized ZrO2, and when the temperature reaches 900 °C, it has been crystallized very well.

TG (%)

reported till now. A comprehensive investigation of these aspects may obtain more insights into the understanding of optical behaviors of Eu3+ ions in tetragonal ZrO2 nanocrystals, which is especially important to optimize their optical performance for further technological applications. In this paper, we report a facile method to produce Eu3+, Y3+codoped tetragonal ZrO2 nanophosphors by controlling Eu3+-doping concentration. Eu3+ ions site in the tetragonal YSZ nanocrystals has been identified by the high-resolution emission and excitation spectra at 10 k. In addition, the effect of Eu3+-doped concentration on the luminescence intensity is also systematically investigated.

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3. Results and discussion

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3.1. Thermal analysis

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Temperature (ºC) TG–DTA curves of the as-prepared yellow precursor of the sample YSZ:Eu are shown in Fig. 1, where the weight loss occurs in the

Fig. 1. TG–DTA curves of the phosphor YSZ:Eu precursor in air.

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2θ (degree) Fig. 2. XRD patterns forYSZ:Eu samples with different Eu3+ content prepared by sol–gel method. The standard data for two types ZrO2 (tetragonal JCPDS No.79-1769 and monoclinic JCPDS No.78-0047) are also presented in the figure.

TEM was employed to obtain direct information about the size and structure of the produced YSZ:Eu nanocrystals. Morphological observations by TEM (Fig. 3a) indicate that the 6 at.% YSZ:Eu sample consisted of aggregated nanoparticles. The average size of YSZ:Eu nanocrystal is about 15 nm, which is in good agreement with the size estimated by the Scherrer equation from the XRD pattern. Fig. 3b shows the selected area electron diffraction (SAED) pattern from YSZ:Eu nanocrystals. The diffraction rings are ob-

served. It should be noted that the diffraction rings are discontinuous and consist of rather spots, which indicates that the YSZ:Eu nanocrystals are polycrystalline. Combining with the high resolution TEM image (HRTEM) (Fig. 3c), it can be clearly seen that the lattice fringes with the interplanar spacing of 0.294 nm corresponds to the distance of the (1 0 1) plane of the tetragonal ZrO2 structure, which suggests that high quality tetragonal ZrO2 nanocrystals are formed. The EDS was used to further determinate the chemical composition of the as-obtained crystals. The EDS spectrum (Fig. 3d) of the 6 at.% YSZ:Eu sample shows the presence of Y, Zr, O and Eu elements. Combining with the XRD analysis above, these results further indicate that Eu3+ has been effectively built into the ZrO2 host lattice. 3.3. Photoluminescence spectra analysis Fig. 4 presents the excitation spectra of 6 at.% YSZ:Eu phosphors by monitoring the emission at 607 nm at various temperatures (10–298 K). The intense broad band centered at 254 nm is mainly attributed to charge transfer transition from O2 to Eu3+ [14]. In addition, the sharp 4f–4f transitions of Eu3+ (7F0 ? 5D0. 1. 2. 3. 4, 5 L6. 7, 5H3, 3P0) are identified and labelled in Fig. 4. Among these excitation transitions, 7F0 ? 5L6 (395 nm) is the most intense peak, which matches well with the commercially available near-UV GaN-based LED chips. From Fig. 4, it is found that 7F0 ? 5D0 (579 nm) is the most weak single peak at 10 K excitation spectra, which indicated Eu3+ occupied one lattice site. It is also noted (inset of Fig. 4) that there exists the only one peak (526.5 nm 7 F0 ? 5D1) around 530 nm at 10 K excitation spectra, while there

Fig. 3. TEM graph and EDS of the 6 at.% YSZ:Eu samples prepared by sol–gel method: (a) TEM image of the nanocrystals aggregate; (b) SAED spectrum of a nanocrystal; (c) HRTEM image of a nanocrystal aggregate; (d) EDS data taken from a naocrystal aggregate.

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Wavelength (nm) Fig. 4. The excitation spectra (10–298 K) of 6 at.% YSZ:Eu samples prepared by sol– gel method, monitoring the Eu3+ emission at 607 nm.

are three main peaks (526.5, 535.5, and 547 nm) at 298 K. It is clearly seen from the inset figure that the other two peaks (535.5, 547 nm) gradually appear in the excitation spectra with increase of temperature. The energy levels of 7F1 (251, 337, 354 cm1) and the lowest energy level of 7F2 (726 cm1) are identified according to 10 K emission spectra (see Fig. 5). These values (7F1:337 cm1; 7F2:726 cm1) are close to the energy differences of the 526.5 nm and 535.5 nm peaks (319 cm1), as well as 526.5 nm and 547 nm peaks (712 cm1), respectively. In addition, there exist thermal populations of 7F1 and 7F2 energy levels with at room temperature (298 K), while there are no thermal populations of 7F1 and 7 F2 energy levels at low temperature (10 K). Therefore, we deduce that the peaks (535.5, and 547 nm) in the 298 K excitation spectra should be the transitions 7F1 ? 5D1 and 7F2 ? 5D1, respectively. Fig. 5 shows the emission spectra of 6 at.% YSZ:Eu phosphor (excited at 395 nm) at 10 K and at room temperature. The emission spectra of YSZ:Eu phosphor consist of narrow peaks mainly located in the wavelength range from 575 to 760 nm. These narrow peaks correspond to transitions from the 5D0 state to the 7FJ (J = 0, 1, 2, 3, 4, 5) state of the 4f6 configuration of Eu3+, as clearly marked in Fig. 5. It is well known that the 5D0 ? 7F2 lines of Eu3+ are of electric-dipole (ED) nature and very sensitive to site symmetry, while the 5D0 ? 7F1 lines are primarily of magnetic-dipole (MD) nature and insensitive to site symmetry. The intensity ratio of the above two transitions may provide structural information such as distortion of ligand environment and site symmetry. Radiative

transitions from 5D0 to levels with J = 0 or odd J (J = 3, 5) are both ED and MD forbidden, and only weak transitions from 5D0 to these levels are observed due to the crystal field induced J-mixing effect [33]. In Fig. 5, the ED transition of Eu3+ ions in the YSZ:Eu nanocrystals is stronger than the MD transition, suggesting that Eu3+ ions occupy low-symmetry sites without an inversion center. Zr4+ ions sit at a D4h site in the tetragonal lattice. The substitution of Zr4+ with a little larger Eu3+ leads to a descent of the intrinsic D4h to lower site symmetry (S4, C2V, C4 or D2), according to the ranching rules of the 32 point groups [34]. Theoretically, if Eu3+ ions situate at S4, D4h or C4 lattice sites, only two lines for J = 0 to J = 1 transition and three lines (S4), zero line (D4h) or two lines (C4) for the J = 0 to J = 2 crystal filed transition are allowed. However, three lines (589.6, 592.6, 593.2 nm) for 5D0 ? 7F1 transition and four lines (607.2, 613.6, 626.4, 633.4 nm) for the 5D0 ? 7F2 transition of Eu3+ ions in the inset of Fig. 5 can be clearly identified. Moreover, according to the ED selection rule, the 5D0 ? 7F0 (0–0) transitions only allowed in the following 10 site symmetries: CS, C1, C2, C3, C4, C6, C2V, C3V, C4V, and C6V [33,35]. The appearance of the 0–0 line (581.0 nm) suggests that Eu3+ ions may occupy a C2V symmetry. It is noted that the most intensive emission at 607 nm (the room temperature emission spectra) is very narrow with a full width at half-maximum (fwhm) less than 2.5 nm. The most intense peak in YSZ:Eu phosphor obtained by sol–gel method becomes much narrower. It indicates that the phosphor nanoparticles have excellent crystallinity and fewer defects. The dependences of red emission (5D0 ? 7F2 transition) on the 3+ Eu doping concentration in the YSZ:Eu (0.5, 1, 2, 4. 5, 6, 8, 10 at.%) phosphors are shown in Fig. 6. In this study, the emission intensities of YSZ:Eu phosphors excited under 395 nm were compared. It is obviously seen that the maximum emission intensity of the YSZ:Eu phosphors appears the Eu3+ doping concentration of 6 at.%. When the Eu3+ doping concentration is higher above 6 at.%, the luminescence intensity reduces contrarily owing to the concentration quenching effect. It is well known that the dopant concentration that determines the average distance between the two neighboring activator ions has a great impact on the photoluminescence efficiency in RE3+ ions doped nanocrystals [36]. High doping concentrations of Eu3+ ions in YSZ nanocrystals may bring about deleterious cross relaxations between the adjacent Eu3+ ions, resulting in the quenching of excitation energy and thereby weak luminescence of Eu3+ [37]. Therefore, the optimum dopant concentration of Eu3+ in YSZ:Eu phosphors is about 6 at.%. Presently, the main commercial red phosphor for near-UV-LEDs is Y2O2S:Eu3+.

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Fig. 6. The photoluminescence emission intensity (integrated intensity of 5 D0 ? 7F2) of Eu3+ ions as a function of its doping concentration in ZrO2 host prepared by sol–gel method (excitation wavelength 395 nm).

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Thus the sample of Y2O2S:Eu3+ is selected to compare with our phosphors. The luminescence spectra of Y2O2S:Eu3+ and YSZ:Eu under the excitation wavelength of 395 nm are also shown in Fig. 5. The red emission intensity of YSZ:Eu is comparable to that of Y2O2S:Eu3+. In order to investigate the luminescence dynamic of Eu3+ in tetragonal YSZ nanocrystals, the luminescent decay curves of 5 D0 ? 7F2 were measured at various temperatures (10–318 K). Fig. 7 shows that the decay curves from the 5D0 state deviate from a single exponential (Four typical curves at 10, 200, 298, and 318 K are shown). Therefore, the average luminescence lifetime sav is adopted the following equation [38]:

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sav ¼ R01 0

R max tIðtÞdt tIðtÞdt ffi R0max ; IðtÞdt IðtÞdt 0

Table 1 The average luminescence lifetimes (sav) of the 5D0 state of 6 at.% YSZ:Eu at various temperature. Temperature (K)

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6 at.% YSZ:Eu (2.33 ms). The results further confirm that there happen concentration quenching when the Eu3+doping concentration is higher above 6 at.%, agreeing with the luminescence intensity dependence of concentration analysis above. 4. Conclusions

where I(t) stands for the luminescence intensity at time t corrected for the background and the integrals are evaluated on a range 0 < t < tmax where tmax  sav. The obtained sav values for the nanocrystalline samples for various temperatures are listed in Table 1. It is seen from Table 1 that the average luminescence lifetimes decrease with the increase of temperature. The result indicates that the luminescence quench becomes strong with the increase of temperature. The non-exponential shape of luminescence decay curves could be due to energy transfer processes between the Eu3+ ions [38]. To further investigate concentration quenching effect, the decay curves for higher doping concentration (8, 10 at.%) samples were measured. The sav values for YSZ:Eu samples (8 and 10 at.%) are 2.05 and 1.86 ms, respectively, which are less than that of

The tetragonal YSZ:Eu nanophosphors can be synthesized by sol–gel process. The excitation spectra show that the most intense peak at 395 nm is in good agreement with the emission wavelength of applied UV-LED chips. The emission spectra show the strong red emission at 607 nm corresponding to the 5D0 ? 7F2 transition of YSZ:Eu phosphors, which indicated that the local environments of Eu3+ ions in ZrO2 crystal had no inversion symmetry. We conclude from the experimental data that the Eu3+ ions should occupy C2V Site. The optimum concentration for Eu3+ was determined to be about 6 at.% of Eu3+ ions in YSZ:Eu phosphors. Therefore, the high emission intensity and thermal stability of this system make them potential red phosphor for application in white LEDs. Acknowledgements

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This work was financially supported by the National 863 Plan Project (2010AA03A408) and National Natural Science Foundation of China (No. 51162012).

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Time (ms) Fig. 7. Luminescence decays of the 5D0 state of 6 at.% YSZ:Eu at various temperature (excitation wavelength 395 nm, monitoring the Eu3+ emission at 607 nm). Four typical curves at 10, 200, 298, and 318 K are shown.

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