Superfine Sr2CeO4 powder with blue-emission prepared by microemulsion method

Materials Letters 59 (2005) 948 – 952 www.elsevier.com/locate/matlet

Superfine Sr2CeO4 powder with blue-emission prepared by microemulsion method De-Song Xing, Jian-Xin Shi*, Meng-Lian Gong School of Chemistry and Chemical Engineering, State Key Laboratory of Optoelectronic Materials and Technologies, Sun Yat-sen University, Guangzhou 510275, P. R. China Received 1 July 2004; accepted 4 October 2004 Available online 18 December 2004

Abstract Powder samples of Sr2CeO4 were prepared by microemulsion-heating method. Field emission scanning electron microscopy (FE-SEM) images showed that the sample fired at 850 8C for 4 h was spherical with an average diameter of 70~80 nm while the samples which sintered at 900 8C for 4 h and 1000 8C for 4 h were shuttle-like shape and spherical respectively both with sizes less than 1 Am. X-ray diffraction (XRD) patterns disclosed that the superfine Sr2CeO4 exhibited an orthorhombic crystal structure. Room-temperature photoluminescence (PL) analysis indicated that there were three excitation peaks located at around 260, 280, and 350 nm, and all the Sr2CeO4 samples showed intensely blue emission at 470 nm. Compared with Sr2CeO4 samples prepared with other methods, the Sr2CeO4 phosphor fabricated with this method had a controllable shape with smaller size, lower calcination temperature, and shorter calcinations time. D 2004 Elsevier B.V. All rights reserved. Keywords: Sr2CeO4; Superfine; Microemulsion; Blue-emission

1. Introduction As new phosphor candidates for field emission displays, materials with good luminescent properties have recently attracted considerable attention. Sulfide-based phosphors [1–3] and organic materials [4–6] have been widely studied due to their high luminescence characteristics. However, these materials are unstable at high temperature and are unamiable to environment. Since oxide-based phosphors can hurdle these disadvantages successfully, metal oxides with PL, especially rare earth based composite materials, have been of interest with respect to possible applications. In fact, lanthanide ions offer almost unlimited possibilities for the design of new luminescence materials. When excited, these materials convert absorbed energy into electromagnetic radiation in ultraviolet, visible, and/or infrared regions, and the luminescence of rare earth based phosphors also permits the development of trichromatic * Corresponding author. Tel.: +86 20 8411 2830; fax: +86 20 8411 2245. E-mail address: [email protected] (J.-X. Shi). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.10.071

luminescence lighting. For instance, white color can be produced by the emission in the blue, green, and red respectively at 450, 550 and 610 nm. In 1998, a blue phosphor compound, Sr2CeO4, possessing one-dimensional chain of edge-sharing CeO6 octahedra, was identified by Danielson and his co-workers with combinatorial chemistry [7]. It exhibited a blue–white emission band that peaks at 485 nm, with a quantum yield of 0.48F0.02 under 254 nm excitation. The luminescence was suggested to originate from a ligand-to-metal Ce4+ charge transfer. Recently, some groups began using all kinds of methods to fabricate this promising material and research its luminescent properties. Jiang et al. [8] prepared Sr2CeO4 by chemical coprecipitation technique and showed very promising results for phosphor in field emission display applications. The phosphor has an emission peak at 470 nm with a luminescence efficiency of 5.4 lm/W at 4 kV and 29.0 lm/W at 10 kV. Serra et al. [9] synthesized Sr2CeO4 by Pechini’s method. It was reported that the excitation spectra present two broad bands with maxima at 294 nm and 344 nm and the emission spectrum has a broad band centered at

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475 nm. Y.B. Khollam et al. prepared Sr2CeO4 by microwave-accelerated hydrothermal method and got a broad emission peak at 504 nm [10]. Tang Y.X. et al. [11] fabricated Sr2CeO4 by microwave calcination and its thin films by using 355-nm pulsed laser deposition technique. The time-resolved PL spectroscopy showed that the PL decay lifetime of the phosphor was about 18 As. Pieterson et al. [12] explained the long excited state lifetime and the temperature dependence of the emission intensity of Sr2CeO4 phosphor. Sankar and Subba Rao [13] reported a PL study on undoped and Eu3+-doped Sr2CeO4 synthesized by the conventional high-temperature solid-state reaction. Hinattsu et al. [14] doped Pr4+ ions into Sr2CeO4 phosphor and showed that the crystal field of Pr4+ is effective to some extent on the behavior of a 4f electron in Sr2CeO4. To our knowledge, there was no report on the preparing of Sr2CeO4 phosphor with microemulsion method. Here, we report the preparation of superfine Sr2CeO4 phosphor via microemulsion-heating method. The experimental results showed that the size of the phosphor is easily controlled to less than 1 Am with this method, and the calcined temperature was lower and the calcined time became shorter too.

2. Experimental section 2.1. Preparation of Sr2CeO4 phosphor Powder samples of Sr2CeO4 were prepared with microemulsion-heating method. Strontium and cerium oxalate precursors were prepared from appropriate mixtures of nitrates with a Sr2+/Ce4+ ratio of 2:1 in mixed solvents of poctyl polyethylene glycol penylher, 1-pentanol, and cyclohexane. The reaction mixture was stirred for 50 min, and then excessive oxalic acid was dropped in. A white precipitate was formed and was separated in a high-speed centrifugal machine and rinsed with deionized water and anhydrous ethanol for several times, then was dried in an oven at 120 8C for 36 h. The precursor was calcined on a high-temperature oven at different temperature. Before and after calcination, the powder was triturated enough.

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min) in a range of 23~930 8C with a calefactive rate of 10.0 8C/min. The weight of the sample was 6.93 mg. DTA curve was measured on a Differential Scanning Catorimeter (DSC-1700, Perkin-Elmer of America) in a range of 0~1200 8C with a scan rate of 10 8C/min in air (45.00 ml/min). The weight of the sample was 11.2 mg. Excitation and emission spectra were taken with a Hitachi F-4500 fluorescence spectrophotometer (Japan) with both excitation and emission slits of 5 nm, a scan rate of 240 nm/min, and a PMT at 700 V at room temperature. Decay curve was measured on the FLS920 (England) fluorescence spectrophotometer at the wavelength of 470 nm at room temperature.

3. Results and discussion Fig. 1 is the TG-DTA curves for the sample prepared with microemulsion. It was found that the sample lost its weight in three stages. The first stage was between 30~220 8C with exothermic peak at 206 8C. In this stage, the sample lost its adsorbed moisture and the weight-loss was 4.83 %. The second step is between 220~530 8C, the main radiative peaks are at 350 8C and 479 8C. In this stage, the organisms began to burn and the lost weight was 2.16 %. The third stage was decomposition of SrC2O4 and Ce2(C2O4)3, release of CO2, and formation of Sr2CeO4 between 530~1100 8C. In this stage, the weight-loss was smaller, only 0.5%. In fact, the TG curve became almost a straight line from 865 8C. However, in the DSC curve, two exothermic peaks at 963 8C and 1056 8C appeared, which may be arisen by the transformation of crystal modality. In order to verify the above deduction, XRD analysis was performed for the samples calcined at 850, 900, 1000, and 1100 8C, respectively (Fig. 2). The XRD patterns showed that at 850 8C only a little Sr2CeO4 was created, and most were mixture of SrO and CeO2. At 900 8C, Sr2CeO4 and 100

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2.2. Apparatuses and measurements TG/%

X-ray powder diffraction (XRD) analysis was carried out on a D/max-IIIA diffractometer (RIGAKU of Japan) with divergence slit of 1 degree, scattering slit of 1 degree, voltage of 35 kV and current of 25 mA. The radiation was Cu target, Ka1 k=1.54056 2. FE-SEM images were taken on a field emission scanning electron microscopy (JSM-6330F, JEOL of Japan). Platinum power was sprayed onto the sample surface before FE-SEM observation. TG curve was measured on a Thermogravimetric Analyzer (NETZSCH TG 209, German) in air (20.00 ml/

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T/˚C Fig. 1. TG–DTA curves for SrC 2 O4 +Ce 2 (C2 O4 ) 3 prepared with microemulsion.

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SrCeO3 were created and the proportion of SrCeO3 was higher than that of Sr2CeO4. As the calcination temperature was increased from 900 8C to 1000 8C, the proportion of Sr2CeO4 became higher than that of SrCeO3. Finally, at 1100 8C, the products were mainly Sr2CeO4, and the peaks completely fit the Joint Committee on Power Diffraction Standards (JCPDS) card (22–1422). The crystal structure of

Sr2CeO4 was triclinic in the JCPDS card. However, Danielson [7] reported the structure of Sr2CeO4 to be orthorhombic. This is probably due to some diffraction peaks, which were pivotal but low, were neglected. We accounted the locations of diffraction peaks of Sr2CeO4 in orthorhombic and in triclinic lattice by XRD program, and found out that our results can better fit the calculated

Fig. 3. FE-SEM images of samples calcined at different temperatures for 4 h: panels (a), (b), and (c) are samples prepared with microemulsion method calcined at 850, 900, and 1100 8C for 4 h, respectively, and panel (d) is the sample prepared with conventional high-temperature solid-state reaction method calcined at 1100 8C for 20 h.

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shape under 100 nm, and Fig. 3(b) is a shuttle-like shape. When the temperature is above 1000 8C, the shape changed back to spherical-like [see Fig. 3(c)], and as the temperature increased, the size of Sr2CeO4 became bigger. The size of the samples prepared with the microemulsion method was all under 1 Am which was much smaller than that of the sample prepared with conventional high-temperature solidstate reaction method [1~5 Am, as shown in Fig. 3(d)]. The excitation and emission spectra for the above samples were taken (see Figs. 4 and 5) to investigate the photoluminescence (PL) characteristics of the samples. There were three excitation bands centered at 263, 281, and 341 nm, respectively. The emission bands of samples were all at 470 nm with a full width at half maximum (FWHM) of about 90 nm. As the calcination temperature increased, the emission intensity increased (Fig. 5, a, b, c, and d), but integrated PL intensity showed non-linear increment. We deduce that the result was induced by the non-linear increment of the particles size. In particular, the emission intensity of sample calcined at 1100 8C was much higher than those of the other samples. This phenomenon meant, on the other hand, that the sample prepared with microemulsion method has created stabilized Sr2CeO4 at 1100 8C and it is a pure phase of Sr2CeO4. When the calcination temperature was below 1100 8C, a mixture of Sr2CeO4 and SrCeO3 will form. A decay curve for the Sr2CeO4 phosphor was measured at room temperature. The decay time was calculated with a simple exponential model to be 36.09 As, similar to that reported in Ref. [11], 35F5 As.

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λ/nm Fig. 4. Excitation spectrum of Sr2CeO4 prepared with microemulsion method calcined at 1100 8C for 4 h; k em=470 nm.

outcomes based on orthorhombic lattice, especially at 2hN358. So we also regard that our Sr2CeO4 samples are orthorhombic. According to Debye–Scherrer equation, D=Kd k/bcosh, the estimated size for the sample calcined at 850 8C for 4 h was 60 nm, while 620 nm for the sample calcined at 1000 8C for 4 h and 690 nm for the sample at 1100 8C for 4 h. Fig. 3 shows the FE-SEM images of the samples. In fact, these images not only give us the shape and size of the samples but also give the shape change during the formation process of the compounds. Fig. 3(a) showed a spherical-like

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λ/nm Fig. 5. Emission spectra of Sr2CeO4. Panels (a), (b), (c), and (d) are the samples prepared with microemulsion method calcined at 850, 900, 1000, and 1100 8C for 4 h, respectively; k ex=281 nm.

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4. Conclusions

References

Superfine powder of Sr2CeO4 phosphor has been successfully prepared by using microemulsion-heating method. XRD patterns and the calculated results show that the structure of the prepared SrC2O4 is orthorhombic. FESEM images showed that the size of the prepared power was less than 1 Am, and their shapes changed with the calcination temperature. Compared with the samples prepared by other methods, the size of the powder fabricated by this method is much smaller, and the calcined temperature was lower and the calcined time became shorter too. The PL intensity increases evidently with increasing calcination temperature; and pure and stable Sr2CeO4 phase forms at 1100 8C.

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Acknowledgements This work was financially supported by the Natural Science Foundations of Guangdong Province (grants No. 021716 and No. 980342) and the Scientific and Technological Projects of Guangdong Province (B10502).