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Cubic or monoclinic Y2O3:Eu3+ nanoparticles by one step flame spray pyrolysis
Chemical Physics Letters 415 (2005) 193–197 www.elsevier.com/locate/cplett
Cubic or monoclinic Y2O3:Eu3+ nanoparticles by one step flame spray pyrolysis Adrian Camenzind, Reto Strobel, Sotiris E. Pratsinis
*
Particle Technology Laboratory, Institute of Process Engineering, Department of Mechanical and Process Engineering, Sonneggstrasse 5, ETH Zurich, CH-8092 Zurich, Switzerland Received 30 June 2005; in final form 27 August 2005 Available online 28 September 2005
Abstract Continuous, single-step synthesis of monocrystalline Y2O3:Eu3+ nanophosphor particles (10–25 nm in diameter and 5 wt% Eu) was achieved by flame spray pyrolysis (FSP). The effect of FSP process parameters on materials properties was investigated by X-ray diffraction (XRD), nitrogen adsorption (BET) and transmission electron microscopy (TEM). Photoluminescence (PL) emission were measured as well as the time-resolved PL-intensity decay. Controlled synthesis of monoclinic or cubic Y2O3:Eu3+ nanoparticles was achieved without post-treatment by controlling the high temperature residence time of these particles. The cubic nanoparticles exhibited longer decay times but lower maximum PL intensity than commercial micron-sized bulk Y2O3:Eu3+ phosphor powder. Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction Europium-doped yttria is one of the most often used red emitting phosphors and applied in fluorescent lamps and plasma display panels [1,2]. New ideas regarding high resolution displays have increased the interest into using smaller Y2O3:Eu3+ particles. In the last years several studies investigated nanosized Y2O3:Eu3+ particles towards their physical, optical and electronic properties [3]. Different preparation techniques for nanosized europium-doped yttria have been reported, i.e., combustion synthesis [4], chemical deposition [5], laser ablation [3,6], spray pyrolysis in hot-wall [7–9] or flame [10,11] reactors. With the latter technique aqueous yttrium/europium solutions are nebulized and brought into a methane–oxygen diffusion flame where the phosphor is formed. Kang et al. [10] used a spray flame to produce monoclinic Y2O3:Eu3+ nanoparticles, which had to be post-treated to form the desired cubic crystal structure. Using a similar process, Chang et al. [11] synthesized predominantly hollow parti-
*
Corresponding author. Fax: +41 44 632 15 95. E-mail address: [email protected] (S.E. Pratsinis).
0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.09.002
cles 0.2–0.7 lm (TEM) consisting of small cubic crystals (XRD: 40–50 nm). In both studies it was proposed that flame temperature is critical for synthesis of cubic instead of monoclinic Y2O3:Eu3+. Most successful approaches leading directly to such cubic crystals had long residence times of particles at high temperatures. This was reported by Shimomura and Kijima [8], where the residence time at 1500–1800 °C was about 10–40 s resulting in micron-sized, cubic, solid phosphors. Schmechel et al. [9] reported a residence time of 20 s at 1200 °C resulting in cubic, agglomerated nanoparticles. Lenggoro et al. [7] reported residence times of less than 0.1 s (1500–1700 °C) for synthesis of cubic Y2O3:Eu3+. A residence time in the order of milliseconds led to solid and hollow, monoclinic micron-sized particles [10] or hollow/shell-like cubic particles in the range of 260–740 nm [11]. Here, synthesis of solid Y2O3:Eu3+ nanoparticles of controlled size and crystallinity by flame spray pyrolysis (FSP) of appropriate precursors is presented. A combustible precursor solution containing both yttrium and europium is dispersed by a nozzle forming a fine spray which is then ignited [12]. The FSP setup allows to adjust production rate, flame temperature, residence time of particles in the
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flame and hence particle growth and morphology which finally forms tailor-made nanophosphors. In general, FSP is a versatile, single-step process for synthesis of nanoparticles for applications in ceramics, electronics, catalysis and optics [13]. 2. Experimental 2.1. Particle synthesis The yttrium precursor was prepared by converting Y(III)-nitrate-hexahydrate (Aldrich, 99.9%) into yttrium hydroxide with an aqueous ammonia solution and subsequent washing with distilled water [14]. The resulting Y(OH)3 was then converted into yttrium(III)2-ethylhexanoate solution (0.8 M) by refluxing in 2-ethylhexanoic acid (Riedel-de Hae¨n, 99%) and acetic anhydride (Riedelde Hae¨n, 99%) at 65 °C for 4 h. The solution was diluted with toluene (Aldrich, 99.8%) and the appropriate amount of Eu(III)2-ethylhexanoate (Strem Chemicals, 99.9%) was added resulting in a total metal concentration of 0.4 M. The as-prepared precursor mixture was fed into a nozzle at a constant feed rate of 5–8 ml/min using a syringe pump (Inotech) as described in detail elsewhere [15]. At the end of the nozzle the precursor solution was dispersed by 3–5 l/ min oxygen forming a spray with a pressure drop of 1.5 bar at the nozzle tip. The spray was ignited by supporting flamelets fed with oxygen (2.4 l/min) and methane (1.13 l/min) which are positioned in a ring around the nozzle outlet [15]. After precursor droplet evaporation and combustion, nanosized Y2O3:Eu3+ particles were formed and collected on a glass microfibre filter (GF/A Whatman, 257 mm in diameter) by the aid of a vacuum pump (Busch, Seco SV 1040C). The as-prepared materials were designated as YEu-x/y, where x stands for the precursor feed rate in ml/min and y for the oxygen dispersion gas flow rate in l/min. Some powders were compacted by pressing tablets (with 5 tons) and subsequent grinding for 5 min using a mortar. A commercial micron-sized bulk Y2O3:Eu3+ phosphor was used for comparison regarding PL emission spectra and PLintensity decay. The europium content for all YEu-x/y par-
ticles was kept constant at 5 wt% which is standard for Y2O3:Eu3+ phosphors. 2.2. Characterization The specific surface area (SSA) was determined according to Brunauer–Emmett–Teller (BET) at 77 K (Micromeritics Tristar). The samples were outgassed at 150 °C for 1 h prior to analysis. The BET particle diameters (Table 1) were calculated accounting for the mass fraction of each Y2O3 crystal phase as well as the corresponding densities (Y2O3-monoclinic: 5.5 g/cm3, Y2O3-cubic: 5.01 g/cm3, Eu2O3: 7.42 g/ cm3). X-ray diffraction (XRD) patterns were recorded with a Bruker D8 advance diffractometer (40 kV, 40 mA, Cu Ka) at 2h = 20–80° with a step size of 0.06° and a scan speed of 0.72°/min. With the fundamental parameter approach and Rietveld method [16] the phase mass fraction and the corresponding monoclinic or cubic sizes were calculated using the software TOPAS. The structural parameters of cubic yttria (Inorganic Crystal Structure Database [ICSD] Coll. Code: 26190 [17]) and monoclinic yttria (modified structural parameters of ICSD Coll. Code.: 84125 [18]) were applied. TEM observations were taken on a Tecnai 30F microscope (Philips; Field emission cathode, operated at 300 kV). TEM images were recorded on a slow-scan CCD camera. The emission spectra were recorded at room temperature using a Fluorescence Spectrophotometer (Varian Cary Eclipse) with a spectral resolution of 0.25 nm. Samples of 100 mg each were filled into a cylindrical powder holder (10 mm in diameter) and exposed to UV photons supplied by a flash xenon lamp (80 flashes/s). The emission spectra (ex: 254 ± 2.5 nm) were measured in the range of 570– 640 nm at a scan speed of 167 nm/min. The decay time of the PL emission was determined by time-resolved measurements of the PL intensity (em: 611 nm, ex: 254 nm) at a resolution of 0.11 ms. A software supplied by Varian fitted (one- or two-) exponential decay curves into the measured data points. 3. Results Fig. 1 shows HR-TEM images of Y2O3:Eu3+ particles made at different flame conditions. A relatively low enthalpy
Table 1 BET- and XRD-derived sizes of as-prepared and compacted Y2O3:Eu3+ particles containing 5 wt% Eu made at different flame conditions Sample
Crystal sizea Monoclinic (nm)
Cubic (nm)
YEu-5/5 YEu-5/5 (compacted) YEu-6/3 YEu-7/3 YEu-8/3 YEu-8/3 (compacted)
13 13 18 20 20 22
– – 27 32 31 29
a b c d e
Mass fraction b mon./cubic (%)
Average densityc (g/cm3)
BETd diameter (nm)
Specific enthalpye (kJ/ldisp)
100/0
5.50
33.4
54/46 30/70 16/84
5.27 5.16 5.09
11 19 16 19 23 33
XRD particle diameter of cubic and monoclinic species using the Rietveld refining method. Mass fraction of crystal phase determined by Rietveld refining method. Averaged density q considering mass fraction and qmon. = 5.5 g/cm and qcub. = 5.01 g/cm. BET equivalent particle diameter d = 6/q Æ SSA. Enthalpy of liquid feed rate (kJ/min) divided by the dispersion gas flow rate (ldisp/min).
66.8 77.9 89.1
Fig. 1. HR-TEM images of flame-made Y2O3:Eu3+ nanoparticles made in: (a) low (YEu-5/5) and (b) high (YEu-8/3) enthalpy flames.
PL emission intensity / arb. units
A. Camenzind et al. / Chemical Physics Letters 415 (2005) 193–197
195
compacted powder
YEu-8/3 YEu-7/3 YEu-6/3
content and short flame (YEu-5/5) results in the formation of spherical particles with an approximate size of 11 nm (Fig. 1a), whereas a hotter and longer flame (YEu-8/3) gives larger and rather rhombohedrally shaped particles of 15–25 nm (Fig. 1b). The former are pure monoclinic (YEu-5/5) while the latter are mostly cubic (YEu-8/3). Table 1 shows the BET and crystal sizes and phase composition of Y2O3:Eu3+ particles made at four flame conditions with or without compaction. The BET particle diameter ranged from 11 nm (YEu-5/5) to 23 nm (YEu-8/3) depending on flame conditions. Fig. 2 depicts the evolution of the as-prepared BET particle diameter (circles) and crystallinity (boxes) in the FSP process diagram: the flame height (as measured from the nozzle to the end of the visual flame is related to high temperature particle residence time) versus the specific combustion enthalpy (feed rate [kJ/min] divided by
Fig. 2. Diagram of the employed FSP conditions in terms of specific 1 combustion enthalpy ½kJ l1 disp =ldisp min and resulting visual flame height [cm]. The size of the circle corresponds to the Y2O3:Eu3+ grain diameter (BET) while its shade corresponds to the flame condition in the legend (YEu-x/y), where x is the liquid precursor flow rate [ml/min] and y is the oxygen dispersion [ldisp/min]. The dotted line is the best fit between the centers of the circles. The region of the monoclinic or cubic crystallinity in the as-prepared Y2O3:Eu3+ are depicted also.
YEu-5/5 570
580
590 600 610 620 emission wave length / nm
630
640
Fig. 3. Photoluminescence emission pattern (excitation at 254 nm) with characteristic peaks of the cubic (612 nm) and monoclinic (625 nm) crystals shown on top for four flame-made Y2O3:Eu3+. Short and cold flames (YEu-5/5) produced pure monoclinic Y2O3:Eu3+ particles while hot and long flames (YEu-8/3) produced mostly cubic. Compacted (doted lines) particles showed slightly increased PL emission intensity.
the dispersion gas flow rate [ldisp/min]). Smaller flames (lower specific combustion enthalpies) led to smaller, monoclinic particles, whereas longer flames (higher specific combustion enthalpies) resulted in the formation of larger particles with pronounced cubic crystal structure. Fig. 3 shows the photoluminescence (PL) emission spectra of all flame-made Y2O3:Eu3+ particles. The characteristic PL emission spectrum of the cubic phase (strongest peak at 612 nm) differs strongly from the monoclinic one (strongest peak at 625 nm). The doted lines depict compacted YEu-8/3 and YEu-5/5 particles which showed slightly higher PL emission intensities. The characteristic PL emission spectra caused by the monoclinic phase are consistent with literature [6]. Fig. 4 shows the PL emission decay time of the best flame-made Y2O3:Eu3+ and a commercial phosphor. The micron-sized commercial phosphor was fitted with an one-exponential decay (s1 = 1.05 ms). The YEu-8/3 showed a two-exponential decay (s1 = 0.12 and s2 = 2.67 ms) that was slower than that of the commercial phosphor. The compacted YEu-8/3 particle showed a oneexponential decay behaviour similar to the bulk phosphor (s1 = 1.58 ms). The initial fast decay vanished but a lower intensity decay was still observed. 4. Discussion Selecting an appropriate organometallic precursor and controlling its combustion [14] allowed synthesis of monoclinic as well as cubic Y2O3:Eu3+ nanoparticles with homogeneous composition and morphology (Fig. 1). Whereas most previous flame studies used precursors based on metal
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PL intensity / arb. units (norm.)
YEu-8/3 YEu-8/3 (compacted) commercial powder
0
1
2
3
4
5
radiative decay time / ms Fig. 4. PL emission intensity decay curves (normalized) of as-prepared (solid line) and compacted (broken line) YEu-8/3 and commercial (dotted line) powder (measured at 612 nm and excited at 254 nm).
nitrates dissolved in water that resulted in rather inhomogeneous particles [10,11]. Feeding aqueous metal nitrate solutions to FSP often leads to inhomogeneous particles with a bimodal particle size distribution (CeO2) and/or hollow particles (Bi2O3) [19]. The monoclinic, YEu-5/5 particles changed to mostly cubic YEu-8/3 particles by increasing the enthalpy density of the process (Table 1 and Fig. 3). Higher feed rates of precursor combined with lower dispersion gas flow rates led to elongated flames (Fig. 2): the 5/5 flame is shorter than the hotter 8/3 flame (Fig. 2). As a result, particles formed in the latter experience longer residence time at high temperature allowing them to grow and transform into the cubic phase. Fig. 2 is consistent with literature of monoclinic particles made at short residence times [10] and cubic ones at longer such times [7–9,11]. Here, the particle residence time for synthesis of cubic nanoparticles was less than 100 ms [20] and shorter than past studies [8,9] or at least in the same order [7]. Furthermore, higher precursor feed rates increased the flame enthalpy density (resulting in higher temperatures) and the metal concentration in the flame resulting in higher nucleation and coagulation rates [21] as directly by quantitative detailed models of nanoparticles formation in flame sprays. Lower dispersion gas flow rate results in a denser aerosol and less entrainment of surrounding air. As a result, increased precursor flow rate and decreased dispersion gas flow rate contribute to synthesis of bigger particles in agreement with BET and XRD as well as TEM (Table 1, Figs. 1 and 2). Additionally, changes in particle shape and the transformation from compact monoclinic to less dense cubic contribute to the increased BET-particle sizes. TEM analysis showed, that some monoclinic, round particles (YEu-5/5) have become larger than some rhombohedrally shaped,
cubic YEu-8/3 particles (marked particles in Fig. 1). The altered shape of the cubic particles indicates that a phase transformation from monoclinic to cubic has taken place. Evidently, aside from particle size and corresponding surface energies [22], the high temperature particle residence time determines the crystal phase of Y2O3:Eu3+ as in flame synthesis of TiO2 [21] and ZrO2 [14]. Compacting the powders had no influence on crystal phase composition and little on crystal sizes (Table 1). However, the compacted materials showed lower specific surface areas (SSA) caused by stronger agglomeration of the particles. Large particles (commercial product) showed higher brightness but a faster PL-intensity decay compared to YEu-8/3 ones. Crystal defects alter the lattice constants at the particle surface [9]. The surface-to-volume ratio is large for small particles while the concentration of defects at their surface is higher than that of the commercial powder. Hence, altered host lattice properties can reduce the PL emission intensity [9]. The smooth transition from the pure monoclinic (YEu-5/5) to the nearly complete cubic PL spectra (YEu-8/3) corroborate the XRD results (Table 1). The excited YEu-8/3 particles showed slower intensity decay consistent with Schmechel et al. [9] who reported longer lifetimes for smaller particles. The significant influence of the surrounding medium (here air) was investigated by Meltzer et al. [23]. Compacting flame-made materials increased the filling factor (fraction of space occupied by Y2O3) and decreased radiative decay time [23]. 5. Conclusions Cubic Y2O3:Eu3+ nanoparticles (<30 nm) were prepared directly without post-processing by flame spray pyrolysis (FSP) of appropriate organometallic precursors. The crystal size and composition could be closely controlled from monoclinic to cubic by selecting the FSP-process parameters that determine the high temperature particle residence time. Size effects for both the photoluminescent (PL) emission intensity and for the radiative decay time were found: These nanosized flame-made particles showed lower PL intensities than a commercial phosphor but prolonged radiative decay. Compacted powders showed higher PL emission intensities and bulk-like emission intensity decay. Acknowledgement We thank Dr. Frank Krumeich (ETH) for the TEM measurements and financial support by ETH Zurich (TH 2/03-2) is kindly acknowledged. References [1] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer, Berlin, 1994. [2] T. Justel, H. Nikol, C. Ronda, Angew. Chem. Int. Edit. 37 (1998) 3085. [3] B.M. Tissue, Chem. Mater. 10 (1998) 2837.
A. Camenzind et al. / Chemical Physics Letters 415 (2005) 193–197 [4] J.W. Wang, Y.M. Chang, H.C. Chang, S.H. Lin, L.C.L. Huang, X.L. Kong, M.W. Kang, Chem. Phys. Lett. 405 (2005) 314. [5] A. Patra, E. Sominska, S. Ramesh, Y. Koltypin, Z. Zhong, H. Minti, R. Reisfeld, A. Gedanken, J. Phys. Chem. B 103 (1999) 3361. [6] H. Eilers, B.M. Tissue, Chem. Phys. Lett. 251 (1996) 74. [7] I.W. Lenggoro, Y. Itoh, K. Okuyama, T.O. Kim, J. Mater. Res. 19 (2004) 3534. [8] Y. Shimomura, N. Kijima, Electrochem. Solid State 7 (2004) H1. [9] R. Schmechel, M. Kennedy, H. von Seggern, H. Winkler, M. Kolbe, R.A. Fischer, X.M. Li, A. Benker, M. Winterer, H. Hahn, J. Appl. Phys. 89 (2001) 1679. [10] Y.C. Kang, D.J. Seo, S.B. Park, H.D. Park, Jpn. J. Appl. Phys. 40 (2001) 4083. [11] H. Chang, I.W. Lenggoro, K. Okuyama, T.O. Kim, Jpn. J. Appl. Phys. 43 (2004) 3535. [12] L. Ma¨dler, H.K. Kammler, R. Mueller, S.E. Pratsinis, J. Aerosol Sci. 33 (2002) 369.
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[13] L. Ma¨dler, KONA 22 (2004) 107. [14] R. Jossen, R. Mueller, S.E. Pratsinis, M. Watson, M.K. Akhtar, Nanotechnology 16 (2005) S609. [15] L. Ma¨dler, W.J. Stark, S.E. Pratsinis, J. Mater. Res. 17 (2002) 1356. [16] R.W. Cheary, A.A. Coelho, J. Appl. Crystallogr. 31 (1998) 851. [17] B.H. OConnor, T.M. Valentine, Acta Crystallogr. B 25 (1969) 2140. [18] B. Antic, M. Mitric, D. Rodic, J. Phys. Condens. Mat. 9 (1997) 365. [19] R. Jossen, S.E. Pratsinis, W.J. Stark, L. Madler, J. Am. Ceram. Soc. 88 (2005) 1388. [20] M. Heine, S.E. Pratsinis, Ind. Eng. Chem. Res. 66 (2005) 6222. [21] S.E. Pratsinis, W.H. Zhu, S. Vemury, Powder Technol. 86 (1996) 87. [22] M.R. Ranade, A. Navrotsky, H.Z. Zhang, J.F. Banfield, S.H. Elder, A. Zaban, P.H. Borse, S.K. Kulkarni, G.S. Doran, H.J. Whitfield, Proc. Natl. Acad. Sci. USA 99 (2002) 6476. [23] R.S. Meltzer, S.P. Feofilov, B. Tissue, H.B. Yuan, Phys. Rev. B 60 (1999) R14012.
Cubic or monoclinic Y2O3:Eu3+ nanoparticles by one step flame spray pyrolysis Adrian Camenzind, Reto Strobel, Sotiris E. Pratsinis
*
Particle Technology Laboratory, Institute of Process Engineering, Department of Mechanical and Process Engineering, Sonneggstrasse 5, ETH Zurich, CH-8092 Zurich, Switzerland Received 30 June 2005; in final form 27 August 2005 Available online 28 September 2005
Abstract Continuous, single-step synthesis of monocrystalline Y2O3:Eu3+ nanophosphor particles (10–25 nm in diameter and 5 wt% Eu) was achieved by flame spray pyrolysis (FSP). The effect of FSP process parameters on materials properties was investigated by X-ray diffraction (XRD), nitrogen adsorption (BET) and transmission electron microscopy (TEM). Photoluminescence (PL) emission were measured as well as the time-resolved PL-intensity decay. Controlled synthesis of monoclinic or cubic Y2O3:Eu3+ nanoparticles was achieved without post-treatment by controlling the high temperature residence time of these particles. The cubic nanoparticles exhibited longer decay times but lower maximum PL intensity than commercial micron-sized bulk Y2O3:Eu3+ phosphor powder. Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction Europium-doped yttria is one of the most often used red emitting phosphors and applied in fluorescent lamps and plasma display panels [1,2]. New ideas regarding high resolution displays have increased the interest into using smaller Y2O3:Eu3+ particles. In the last years several studies investigated nanosized Y2O3:Eu3+ particles towards their physical, optical and electronic properties [3]. Different preparation techniques for nanosized europium-doped yttria have been reported, i.e., combustion synthesis [4], chemical deposition [5], laser ablation [3,6], spray pyrolysis in hot-wall [7–9] or flame [10,11] reactors. With the latter technique aqueous yttrium/europium solutions are nebulized and brought into a methane–oxygen diffusion flame where the phosphor is formed. Kang et al. [10] used a spray flame to produce monoclinic Y2O3:Eu3+ nanoparticles, which had to be post-treated to form the desired cubic crystal structure. Using a similar process, Chang et al. [11] synthesized predominantly hollow parti-
*
Corresponding author. Fax: +41 44 632 15 95. E-mail address: [email protected] (S.E. Pratsinis).
0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.09.002
cles 0.2–0.7 lm (TEM) consisting of small cubic crystals (XRD: 40–50 nm). In both studies it was proposed that flame temperature is critical for synthesis of cubic instead of monoclinic Y2O3:Eu3+. Most successful approaches leading directly to such cubic crystals had long residence times of particles at high temperatures. This was reported by Shimomura and Kijima [8], where the residence time at 1500–1800 °C was about 10–40 s resulting in micron-sized, cubic, solid phosphors. Schmechel et al. [9] reported a residence time of 20 s at 1200 °C resulting in cubic, agglomerated nanoparticles. Lenggoro et al. [7] reported residence times of less than 0.1 s (1500–1700 °C) for synthesis of cubic Y2O3:Eu3+. A residence time in the order of milliseconds led to solid and hollow, monoclinic micron-sized particles [10] or hollow/shell-like cubic particles in the range of 260–740 nm [11]. Here, synthesis of solid Y2O3:Eu3+ nanoparticles of controlled size and crystallinity by flame spray pyrolysis (FSP) of appropriate precursors is presented. A combustible precursor solution containing both yttrium and europium is dispersed by a nozzle forming a fine spray which is then ignited [12]. The FSP setup allows to adjust production rate, flame temperature, residence time of particles in the
194
A. Camenzind et al. / Chemical Physics Letters 415 (2005) 193–197
flame and hence particle growth and morphology which finally forms tailor-made nanophosphors. In general, FSP is a versatile, single-step process for synthesis of nanoparticles for applications in ceramics, electronics, catalysis and optics [13]. 2. Experimental 2.1. Particle synthesis The yttrium precursor was prepared by converting Y(III)-nitrate-hexahydrate (Aldrich, 99.9%) into yttrium hydroxide with an aqueous ammonia solution and subsequent washing with distilled water [14]. The resulting Y(OH)3 was then converted into yttrium(III)2-ethylhexanoate solution (0.8 M) by refluxing in 2-ethylhexanoic acid (Riedel-de Hae¨n, 99%) and acetic anhydride (Riedelde Hae¨n, 99%) at 65 °C for 4 h. The solution was diluted with toluene (Aldrich, 99.8%) and the appropriate amount of Eu(III)2-ethylhexanoate (Strem Chemicals, 99.9%) was added resulting in a total metal concentration of 0.4 M. The as-prepared precursor mixture was fed into a nozzle at a constant feed rate of 5–8 ml/min using a syringe pump (Inotech) as described in detail elsewhere [15]. At the end of the nozzle the precursor solution was dispersed by 3–5 l/ min oxygen forming a spray with a pressure drop of 1.5 bar at the nozzle tip. The spray was ignited by supporting flamelets fed with oxygen (2.4 l/min) and methane (1.13 l/min) which are positioned in a ring around the nozzle outlet [15]. After precursor droplet evaporation and combustion, nanosized Y2O3:Eu3+ particles were formed and collected on a glass microfibre filter (GF/A Whatman, 257 mm in diameter) by the aid of a vacuum pump (Busch, Seco SV 1040C). The as-prepared materials were designated as YEu-x/y, where x stands for the precursor feed rate in ml/min and y for the oxygen dispersion gas flow rate in l/min. Some powders were compacted by pressing tablets (with 5 tons) and subsequent grinding for 5 min using a mortar. A commercial micron-sized bulk Y2O3:Eu3+ phosphor was used for comparison regarding PL emission spectra and PLintensity decay. The europium content for all YEu-x/y par-
ticles was kept constant at 5 wt% which is standard for Y2O3:Eu3+ phosphors. 2.2. Characterization The specific surface area (SSA) was determined according to Brunauer–Emmett–Teller (BET) at 77 K (Micromeritics Tristar). The samples were outgassed at 150 °C for 1 h prior to analysis. The BET particle diameters (Table 1) were calculated accounting for the mass fraction of each Y2O3 crystal phase as well as the corresponding densities (Y2O3-monoclinic: 5.5 g/cm3, Y2O3-cubic: 5.01 g/cm3, Eu2O3: 7.42 g/ cm3). X-ray diffraction (XRD) patterns were recorded with a Bruker D8 advance diffractometer (40 kV, 40 mA, Cu Ka) at 2h = 20–80° with a step size of 0.06° and a scan speed of 0.72°/min. With the fundamental parameter approach and Rietveld method [16] the phase mass fraction and the corresponding monoclinic or cubic sizes were calculated using the software TOPAS. The structural parameters of cubic yttria (Inorganic Crystal Structure Database [ICSD] Coll. Code: 26190 [17]) and monoclinic yttria (modified structural parameters of ICSD Coll. Code.: 84125 [18]) were applied. TEM observations were taken on a Tecnai 30F microscope (Philips; Field emission cathode, operated at 300 kV). TEM images were recorded on a slow-scan CCD camera. The emission spectra were recorded at room temperature using a Fluorescence Spectrophotometer (Varian Cary Eclipse) with a spectral resolution of 0.25 nm. Samples of 100 mg each were filled into a cylindrical powder holder (10 mm in diameter) and exposed to UV photons supplied by a flash xenon lamp (80 flashes/s). The emission spectra (ex: 254 ± 2.5 nm) were measured in the range of 570– 640 nm at a scan speed of 167 nm/min. The decay time of the PL emission was determined by time-resolved measurements of the PL intensity (em: 611 nm, ex: 254 nm) at a resolution of 0.11 ms. A software supplied by Varian fitted (one- or two-) exponential decay curves into the measured data points. 3. Results Fig. 1 shows HR-TEM images of Y2O3:Eu3+ particles made at different flame conditions. A relatively low enthalpy
Table 1 BET- and XRD-derived sizes of as-prepared and compacted Y2O3:Eu3+ particles containing 5 wt% Eu made at different flame conditions Sample
Crystal sizea Monoclinic (nm)
Cubic (nm)
YEu-5/5 YEu-5/5 (compacted) YEu-6/3 YEu-7/3 YEu-8/3 YEu-8/3 (compacted)
13 13 18 20 20 22
– – 27 32 31 29
a b c d e
Mass fraction b mon./cubic (%)
Average densityc (g/cm3)
BETd diameter (nm)
Specific enthalpye (kJ/ldisp)
100/0
5.50
33.4
54/46 30/70 16/84
5.27 5.16 5.09
11 19 16 19 23 33
XRD particle diameter of cubic and monoclinic species using the Rietveld refining method. Mass fraction of crystal phase determined by Rietveld refining method. Averaged density q considering mass fraction and qmon. = 5.5 g/cm and qcub. = 5.01 g/cm. BET equivalent particle diameter d = 6/q Æ SSA. Enthalpy of liquid feed rate (kJ/min) divided by the dispersion gas flow rate (ldisp/min).
66.8 77.9 89.1
Fig. 1. HR-TEM images of flame-made Y2O3:Eu3+ nanoparticles made in: (a) low (YEu-5/5) and (b) high (YEu-8/3) enthalpy flames.
PL emission intensity / arb. units
A. Camenzind et al. / Chemical Physics Letters 415 (2005) 193–197
195
compacted powder
YEu-8/3 YEu-7/3 YEu-6/3
content and short flame (YEu-5/5) results in the formation of spherical particles with an approximate size of 11 nm (Fig. 1a), whereas a hotter and longer flame (YEu-8/3) gives larger and rather rhombohedrally shaped particles of 15–25 nm (Fig. 1b). The former are pure monoclinic (YEu-5/5) while the latter are mostly cubic (YEu-8/3). Table 1 shows the BET and crystal sizes and phase composition of Y2O3:Eu3+ particles made at four flame conditions with or without compaction. The BET particle diameter ranged from 11 nm (YEu-5/5) to 23 nm (YEu-8/3) depending on flame conditions. Fig. 2 depicts the evolution of the as-prepared BET particle diameter (circles) and crystallinity (boxes) in the FSP process diagram: the flame height (as measured from the nozzle to the end of the visual flame is related to high temperature particle residence time) versus the specific combustion enthalpy (feed rate [kJ/min] divided by
Fig. 2. Diagram of the employed FSP conditions in terms of specific 1 combustion enthalpy ½kJ l1 disp =ldisp min and resulting visual flame height [cm]. The size of the circle corresponds to the Y2O3:Eu3+ grain diameter (BET) while its shade corresponds to the flame condition in the legend (YEu-x/y), where x is the liquid precursor flow rate [ml/min] and y is the oxygen dispersion [ldisp/min]. The dotted line is the best fit between the centers of the circles. The region of the monoclinic or cubic crystallinity in the as-prepared Y2O3:Eu3+ are depicted also.
YEu-5/5 570
580
590 600 610 620 emission wave length / nm
630
640
Fig. 3. Photoluminescence emission pattern (excitation at 254 nm) with characteristic peaks of the cubic (612 nm) and monoclinic (625 nm) crystals shown on top for four flame-made Y2O3:Eu3+. Short and cold flames (YEu-5/5) produced pure monoclinic Y2O3:Eu3+ particles while hot and long flames (YEu-8/3) produced mostly cubic. Compacted (doted lines) particles showed slightly increased PL emission intensity.
the dispersion gas flow rate [ldisp/min]). Smaller flames (lower specific combustion enthalpies) led to smaller, monoclinic particles, whereas longer flames (higher specific combustion enthalpies) resulted in the formation of larger particles with pronounced cubic crystal structure. Fig. 3 shows the photoluminescence (PL) emission spectra of all flame-made Y2O3:Eu3+ particles. The characteristic PL emission spectrum of the cubic phase (strongest peak at 612 nm) differs strongly from the monoclinic one (strongest peak at 625 nm). The doted lines depict compacted YEu-8/3 and YEu-5/5 particles which showed slightly higher PL emission intensities. The characteristic PL emission spectra caused by the monoclinic phase are consistent with literature [6]. Fig. 4 shows the PL emission decay time of the best flame-made Y2O3:Eu3+ and a commercial phosphor. The micron-sized commercial phosphor was fitted with an one-exponential decay (s1 = 1.05 ms). The YEu-8/3 showed a two-exponential decay (s1 = 0.12 and s2 = 2.67 ms) that was slower than that of the commercial phosphor. The compacted YEu-8/3 particle showed a oneexponential decay behaviour similar to the bulk phosphor (s1 = 1.58 ms). The initial fast decay vanished but a lower intensity decay was still observed. 4. Discussion Selecting an appropriate organometallic precursor and controlling its combustion [14] allowed synthesis of monoclinic as well as cubic Y2O3:Eu3+ nanoparticles with homogeneous composition and morphology (Fig. 1). Whereas most previous flame studies used precursors based on metal
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PL intensity / arb. units (norm.)
YEu-8/3 YEu-8/3 (compacted) commercial powder
0
1
2
3
4
5
radiative decay time / ms Fig. 4. PL emission intensity decay curves (normalized) of as-prepared (solid line) and compacted (broken line) YEu-8/3 and commercial (dotted line) powder (measured at 612 nm and excited at 254 nm).
nitrates dissolved in water that resulted in rather inhomogeneous particles [10,11]. Feeding aqueous metal nitrate solutions to FSP often leads to inhomogeneous particles with a bimodal particle size distribution (CeO2) and/or hollow particles (Bi2O3) [19]. The monoclinic, YEu-5/5 particles changed to mostly cubic YEu-8/3 particles by increasing the enthalpy density of the process (Table 1 and Fig. 3). Higher feed rates of precursor combined with lower dispersion gas flow rates led to elongated flames (Fig. 2): the 5/5 flame is shorter than the hotter 8/3 flame (Fig. 2). As a result, particles formed in the latter experience longer residence time at high temperature allowing them to grow and transform into the cubic phase. Fig. 2 is consistent with literature of monoclinic particles made at short residence times [10] and cubic ones at longer such times [7–9,11]. Here, the particle residence time for synthesis of cubic nanoparticles was less than 100 ms [20] and shorter than past studies [8,9] or at least in the same order [7]. Furthermore, higher precursor feed rates increased the flame enthalpy density (resulting in higher temperatures) and the metal concentration in the flame resulting in higher nucleation and coagulation rates [21] as directly by quantitative detailed models of nanoparticles formation in flame sprays. Lower dispersion gas flow rate results in a denser aerosol and less entrainment of surrounding air. As a result, increased precursor flow rate and decreased dispersion gas flow rate contribute to synthesis of bigger particles in agreement with BET and XRD as well as TEM (Table 1, Figs. 1 and 2). Additionally, changes in particle shape and the transformation from compact monoclinic to less dense cubic contribute to the increased BET-particle sizes. TEM analysis showed, that some monoclinic, round particles (YEu-5/5) have become larger than some rhombohedrally shaped,
cubic YEu-8/3 particles (marked particles in Fig. 1). The altered shape of the cubic particles indicates that a phase transformation from monoclinic to cubic has taken place. Evidently, aside from particle size and corresponding surface energies [22], the high temperature particle residence time determines the crystal phase of Y2O3:Eu3+ as in flame synthesis of TiO2 [21] and ZrO2 [14]. Compacting the powders had no influence on crystal phase composition and little on crystal sizes (Table 1). However, the compacted materials showed lower specific surface areas (SSA) caused by stronger agglomeration of the particles. Large particles (commercial product) showed higher brightness but a faster PL-intensity decay compared to YEu-8/3 ones. Crystal defects alter the lattice constants at the particle surface [9]. The surface-to-volume ratio is large for small particles while the concentration of defects at their surface is higher than that of the commercial powder. Hence, altered host lattice properties can reduce the PL emission intensity [9]. The smooth transition from the pure monoclinic (YEu-5/5) to the nearly complete cubic PL spectra (YEu-8/3) corroborate the XRD results (Table 1). The excited YEu-8/3 particles showed slower intensity decay consistent with Schmechel et al. [9] who reported longer lifetimes for smaller particles. The significant influence of the surrounding medium (here air) was investigated by Meltzer et al. [23]. Compacting flame-made materials increased the filling factor (fraction of space occupied by Y2O3) and decreased radiative decay time [23]. 5. Conclusions Cubic Y2O3:Eu3+ nanoparticles (<30 nm) were prepared directly without post-processing by flame spray pyrolysis (FSP) of appropriate organometallic precursors. The crystal size and composition could be closely controlled from monoclinic to cubic by selecting the FSP-process parameters that determine the high temperature particle residence time. Size effects for both the photoluminescent (PL) emission intensity and for the radiative decay time were found: These nanosized flame-made particles showed lower PL intensities than a commercial phosphor but prolonged radiative decay. Compacted powders showed higher PL emission intensities and bulk-like emission intensity decay. Acknowledgement We thank Dr. Frank Krumeich (ETH) for the TEM measurements and financial support by ETH Zurich (TH 2/03-2) is kindly acknowledged. References [1] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer, Berlin, 1994. [2] T. Justel, H. Nikol, C. Ronda, Angew. Chem. Int. Edit. 37 (1998) 3085. [3] B.M. Tissue, Chem. Mater. 10 (1998) 2837.
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