pubs.acs.org/Langmuir © 2009 American Chemical Society
Tunable Luminescence in Monodisperse Zirconia Spheres Cuimiao Zhang, Chunxia Li, Jun Yang, Ziyong Cheng, Zhiyao Hou, Yong Fan, and Jun Lin* State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China, and Graduate University of the Chinese Academy of Sciences, Beijing 100049, People’s Republic of China. Received January 13, 2009. Revised Manuscript Received February 3, 2009 In this article, monodisperse spherical zirconia (ZrO2) particles with a narrow size distribution were prepared by the controlled hydrolysis of zirconium butoxide in ethanol, followed by heat treatment in air at low temperature from 300 to 500 °C. X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, thermogravimetric and differential thermal analysis (TG/DTA), scanning electron microscopy (SEM), transmission electron microscopy (TEM), photoluminescence (PL) spectra, kinetic decay, and electron paramagnetic resonance were used to characterize the samples. The experimental results indicate that the annealed ZrO2 samples exhibit broad, intense visible photoluminescence. The annealing temperature is indispensable for the luminescence of the obtained ZrO2 particles. The emission colors of the ZrO2 samples can be tuned from blue to nearly white to dark orange by varying the annealing temperature. According to the spectral analysis, luminescence lifetimes, and EPR results, the luminescent centers might be attributed to the carbon impurities in the ZrO2 samples. The possible luminescence mechanism for ZrO2 samples has been presented in detail.
1.
Introduction
The use of phosphors represents a fast-growing industry, owing to their applications in white-light-emitting diodes (LEDs), field emission displays (FEDs), and plasma display panels (PDPs). White LEDs have drawn much attention in recent years.1,2 Most of the commercially available phosphors generally require short-wavelength ultraviolet (UV) as the excitation source and the rare earth ions as activators.3-5 The use of mercury (for the production of the UV excitation source) will give rise to environmental contamination in disposing of junk.4 In addition, rare earth ions such as Eu3+ and Tb3+ that are used in the phosphors are often very expensive.5 For these reasons, much effort has been devoted *Corresponding author. E-mail:
[email protected]. (1) Bredol, M.; Kynast, U.; Ronda, C. Adv. Mater. 1991, 3, 361. (2) Yen, W. M.; Shinonoya, S.; Yamamoto, H. Phosphor Handbook; CRC Press: New York2007. (3) Ropp, R. C. Luminescence and the Solid State; Elsevier: Amsterdam, 1991; Vol. 12, p 238. (4) Hayakawa, T.; Hiramitsu, A.; Nogami, M. Appl. Phys. Lett. 2003, 82, 2975. :: (5) Feldmann, C.; Justel, T.; Ronda, C. R.; Schmidt, P. J. Adv. Funct. Mater. 2003, 13, 511. (6) Green, W. H.; Le, K. P.; Grey, J.; Au, T. T.; Sailor, M. J. Science 1997, 276, 1826. (7) (a) Brankova, T.; Bekiari, V.; lianos, P Chem. Mater. 2003, 15, 1855. (b) Bekiari, V.; Lianos, P. Langmuir 1998, 13, 33459. (c) Bekiari, V.; Lianos, P. Chem. Mater. 1998, 10, 3777. (8) Yold, B. E. J. Non-Cryst. Solids 1992, 147, 614. (9) (a) Carlos, L. D.; Sa Ferreira, R. A.; Pereira, R. N.; Assunc~ ao, M.; Bermudez, V.; de, Z. J. Phys. Chem. B 2004, 108, 14924. (b) Fu, L.; S a Ferreira, R. A.; Silva, N. J. O.; Carlos, L. D.; Bermudez, V.; de Rocha, Z. J. Chem. Mater. 2004, 16, 1507. (c) Carlos, L. D.; Bermudez, V.; de, Z.; S a Ferreira, R. A.; Marques, L.; Assunc- ~ao, M. Chem. Mater. 1999, 11, 581. (10) (a) Cordoncillo, E.; Guaita, F. J.; Escribano, P.; Philippe, C.; Viana, B.; Sanchez, C. Opt. Mater. 2001, 18, 309. (b) Lin, J.; Baerner, K. Mater. Lett. 2000, 46, 86. (11) Ogi, T.; Kaihatsu, Y.; Iskandar, F.; Wang, W.; Okuyama, K. Adv. Mater. 2008, 20, 3235. (12) (a) Lin, C. K.; Luo, Y.; Quan, Z.; Zhang, J.; Fang, J.; Lin, J. Chem. Mater. 2006, 18, 458. (b) Lin, C. K.; Zhang, C. M.; Lin, J. J. Phys. Chem. C 2007, 111, 3300. (c) Lin, C. K.; Yu, M.; Cheng, Z. Y.; Zhang, C. M.; Meng, Q. G.; Lin, J. Inorg. Chem. 2008, 47, 49. (d) Zhang, C. M.; Lin, C. K.; Li, C. X.; Quan, Z. W.; Liu, X. M.; Lin, J. J. Phys. Chem. C 2008, 112, 2183.
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to developing a series of stable, efficient, inexpensive, and environmentally friendly luminescent materials that do not contain expensive or toxic elements and can be excited by long-wavelength (340-400 nm) UV light.6-13 Zirconia (ZrO2) particles are widely used as high-performance ceramics, catalysts, or in cosmetics.14,15 New applications for zirconia powders are high-temperature fuel cells and bioceramics such as dental prostheses.14-16 Recently, some attention has been paid to the luminescence properties of ZrO2.17 For example, the tetragonal ZrO2 nanoparticles synthesized by irradiation show a sharp emission peak at 254 nm excitation and a broad-band emission at 608 nm under 412 nm excitation;14 whereas a two-phase process used to synthesize colloidal ZrO2 nanocrystals produces a broad-band emission centered at 365 nm under 250 nm UV excitation.18 In both cases, the luminescence intensity of ZrO2 nanocrystals seemed to be very weak. In addition, our group has also reported the photoluminescence properties of Pechini-type sol-gel-derived nanocrystalline ZrO2 powders that exhibit white-blue emission (425 nm) and blue-green emission (470 nm). In this case, the high annealing temperature caused the aggregation.12b Therefore, it would be of great interest and significance to prepare monodisperse ZrO2 particles by other methods and investigate their luminescence properties together with their mechanisms to determine if (13) Wegh, R. T.; Donker, H.; Oskam, K. D.; Meijerink, A. Science 1999, 283, 663. (14) Liang, J.; Deng, Z.; Jiang, X.; Li, F.; Li, Y. Inorg. Chem. 2002, 41, 3602. (15) Konishi, J.; Fujita, K.; Oiwa, S.; Nakanishi, K.; Hirao, K. Chem. Mater. 2008, 20, 2165. (16) Widoniak, J.; Eiden-Assmann, S.; Maret, G. Eur. J. Inorg. Chem. 2005, 2005, 3149. (17) (a) Chernov, V.; Belykh, A.; Melendrez, R.; Barboza-Flores, M. J. Non-Cryst. Solids 2006, 352, 2543. (b) Liu, L.; Jiang, X.; Liang, J. H.; Li, Y. D.; Li, F. L. Spectrosc. Spectral Anal. 2005, 25, 1026.cKiisk, V.; Sildos, I.; Lange, S.; Reedo, V.; Tatte, T.; Kirm, M.; Aarik,, J. Appl. Surf. Sci. 2005, 247, 412. (d) Lai, L. J.; Lu, H. C.; Chen, H. K.; Cheng, B. M.; Lin, M. I.; Chu, T. C. J. Electron Spectrosc. Relat. Phenom. 2005, 144, 865. (18) Zhao, N.; Pan, D.; Nie, W.; Ji, X. J. Am. Chem. Soc. 2006, 128, 10118.
Published on Web 03/02/2009
Langmuir 2009, 25(12), 7078–7083
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some novel, strong, useful luminescence can be obtained from the monodisperse ZrO2 particles. In this article, we have prepared monodisperse ZrO2 particles by the controlled hydrolysis of zirconium butoxide [Zr(OBu)4] in alcohol solution and have investigated their luminescence properties as a function of annealing temperature. It is interesting to find that, after being annealed at 300 °C, the monodisperse ZrO2 sample exhibits an intense blue emission centered at 407 nm under a wide range of UV light excitation, making it suitably excited by different light sources. With the increase in annealing temperature from 300 to 500 °C, the emission from ZrO2 particles shows an obvious red shift (blue to nearly white to dark orange), and the center of the emission spectra also shifts from 407 to 550 nm. XRD, FTIR, TG/DTA, SEM, TEM, and EPR were employed to characterize the samples, and possible mechanisms have been proposed to explain the observed luminescence phenomena.
2.
Figure 1. XRD patterns of ZrO2 samples annealed at different temperatures: (a) as-synthesized (uncalinced) and (b) 300, (c) 400, and (d) 500 °C, respectively. The standard JCPDS card data for tetragonal ZrO2 is provided as reference.
Experimental Section
All reagents and solvents were used as received without further purification. Zirconium butoxide (80% solution in butanol, Zr[O(CH2)3CH3]4) was purchased from Aldrich. Other chemicals were analytical-grade reagents and were purchased from Beijing Chemical Corporation. 2.1. Preparation. A series of zirconia spheres were synthesized by the procedures reported previously.16 In a typical synthesis, 100 mL of ethanol mixed with 0.4 mL of sodium chloride aqueous solution (0.1 M) and a certain amount hexadecyltrimethylammonium bromide (CTAB) were added to a 250 mL three-necked flask as solvents under vigorous stirring. Then, 3.4 mL of zirconium butoxide was added to the above solution at 60 °C under an inert N2 atmosphere and vigorous stirring. After the solution was stirred for 5 h, the reaction was finished, and the precipitates were separated by centrifugation, washed with ethanol, and then dried in air at 80 °C for 12 h to obtain the zirconia spheres. Finally, the obtained ZrO2 particles were calcined at 300-500 °C for 2 h to obtain the final ZrO2 phosphor samples. In addition, 2 mmol of ZrOCl2 3 8H2O was directly annealed at 400 °C for 2 h in air (solid-state process, SS) to synthesize ZrO2 (without carbon impurities) for comparison. 2.2. Characterization. X-ray powder diffraction (XRD) measurements were performed on a Rigaku-Dmax 2500 diffractometer at a scanning rate of 12°/min in the 2θ range from 10 to 80°, with graphite-monochromatized Cu KR radiation (λ = 0.15405 nm). The morphology and composition of the samples were inspected using a scanning electron microscope (SEM, XL30, Philips). A Fourier transform infrared (FTIR) spectrum was recorded with a Perkin-Elmer 580B infrared spectrophotometer using the KBr pellet technique. Thermogravimetric and differential thermal analysis (TG/DTA) data were recorded with a Thermal Analysis instrument (SDT 2960, TA Instruments, New Castle, DE) with the heating rate of 10 °C min-1 in an air flow of 100 mL min-1. Transmission electron microscopy (TEM) was performed using an FEI Tecnai G2 S-Twin with a field-emission gun operating at 200 kV. Images were acquired digitally on a Gatan multipole CCD camera. The photoluminescence (PL) excitation and emission spectra were recorded with a Hitachi F-4500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. Luminescence lifetimes were measured with a Lecroy Wave Runner 6100 digital oscilloscope (1 GHz) using a tunable laser (pulse width = 4 ns, gate = 50 ns) as the excitation (Contimuum Sunlite OPO). Electron paramagnetic resonance (EPR) spectra were taken on the JESFE3AX electronic spin resonance spectrophotometer. All of the measurements were performed at room temperature. Langmuir 2009, 25(12), 7078–7083
Figure 2. FTIR spectra for the (a) as-synthesized ZrO2, (b) ZrO2 at 300 °C, (c) ZrO2 at 400 °C, and (d) ZrO2 at 500 °C samples.
3.
Results and Discussion
3.1. Structure and Morphology. 3.1.1. XRD. Figure 1 shows the XRD patterns of ZrO2 powders annealed at different temperatures: (a) as-synthesized (uncalcined), (b) 300 °C, (c) 400 °C, and (d) 500 °C. The X-ray diffraction patterns show that the as-synthesized ZrO2 powders (obtained via hydrolysis of zirconium butoxide) and those annealed at 300 °C are both amorphous (Figure 1a,b). For ZrO2 annealed at 400 °C, besides a weak, broad peak at 20-30° (amorphism), some peaks at 2θ = 30.2, 50.3°, and 60.2° are present (Figure 1c) and are assigned to the (011), (112), and (121) reflections of tetragonal ZrO2, respectively. This indicates that the phase transition from the amorphous to hexagonal phase occurs at 400 °C. With the increase in annealing temperature to 500 °C, the diffraction peaks of the as-prepared sample (Figure 1d) can be indexed as a pure tetragonal phase with a space group of P42/nmc (137), which coincides well with the standard data for tetragonal ZrO2 (JCPDS no. 50-1089). 3.1.2. FTIR. The FTIR spectra for the derived ZrO2 samples annealed from 300 to 500 °C are shown in Figure 2. In Figure 2, the broad absorption band below 1000 cm-1 corresponds to Zr-O vibrations. This indicates the formation of -Zr-O-Zr- networks. The bands at 1627 and 3421 cm-1 are ascribed to the O-H vibration of Zr-OH or absorbed H2O in the samples.19 With the increase in annealing temperature, the FTIR signals for the O-H vibration decrease. In addition, the small contributions at 1300-1550 cm-1 can be ascribed to the carbon-related
(19) Mizuno, M.; Sasaki, Y.; Lee, S.; Katakura, H. Langmuir 2006, 22, 7137.
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impurities, which exist in the ZrO2 samples annealed at low temperature (300-500 °C).12b 3.1.3. Thermal Analysis. The simultaneous TG/DTA traces for ZrO2 particles synthesized by the hydrolysis of zirconium butoxide are shown in Figure S1 (Supporting Information). The first DTA peak is a broad endothermic peak below 200 °C with a minimum at 95 °C that can be attributed to the combined loss of water and residual organics from the preparation. This peak occurs concurrently with a TG weight loss of 19.2%. The second peak, a sharp exotherm ranging from 255 to 404 °C with a maximum at 329 °C, may be due to the oxidation of the residual organic compounds (alkoxy groups) and corresponds to weight loss of about 3.2%. In addition, the very sharp exothermic peak from 418 to 496 °C occurs with a maximum at 445 °C whereas the corresponding weight loss is not obvious in the TG curve, revealing the phase transition from amorphous zirconia to the tetragonal phase. Therefore, the crystallization of ZrO2 was expected to occur at around 400 °C and to be complete at 500 °C. The TG/DTA results are in good agreement with the XRD patterns (Figure 1). 3.1.4. Morphologies. Zirconia particles with a narrow size distribution were obtained in a hydrolysis reaction from zirconium alkoxide and water in an alcohol solution. Figure 3 shows the morphologies of the as-synthesized ZrO2 sample and the representative calcined ZrO2 powders: (a) lowmagnification SEM, (b) enlarged SEM, and (c) TEM images of as-synthesized ZrO2 and SEM images of (d) ZrO2 at 300 °C, (e) ZrO2 at 400 °C, and (f) ZrO2 at 500 °C. As can be seen from a low-magnification SEM image (Figure 3a), the as-synthesized ZrO2 sample consists of a large quantity of monodisperse spheres with perfect uniformity. The analysis of a number of the spheres shows that they have
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an average size of about 500 nm (Figure 3b,c). After annealing at 300 °C in air, the spherical shape of ZrO2 particles is basically retained (Figure 3d). The ZrO2 particles are slightly agglomerated after annealing at 400 °C in air (Figure 3e). In addition, it can be observed that the ZrO2 particles are strongly agglomerated when annealed at 500 °C (Figure 3f). 3.2. Luminescence Properties. Under UV-light excitation, the as-synthesized ZrO2 sample does not show any luminescence, whereas the ZrO2 samples annealed at 300-500 °C in the absence of extrinsic activators exhibit bright luminescence. Figure 4 shows the excitation and emission spectra of ZrO2 at 400 °C as a representative example at room temperature. The sample shown in Figure 4 was prepared at 400 °C for 2 h with 0.5 g of CTAB in a hydrolysis process. Under excitation at 365 nm, the sample of ZrO2 at 400 °C shows a strong emission band ranging from 350 to 600 nm centered at 462 nm (Figure 4a, black line). The corresponding excitation spectrum includes a broad band from 220 to 450 nm (Figure 4a, red line), indicating that the ZrO2 sample can be excited over a wide range. The luminescence decay curve for the ZrO2 at 400 °C sample shown in Figure 4b can be fitted to a singleexponential function as I = I0 exp(-t/τ) (τ is the lifetime), from which the lifetime is determined to be 5.75 ns.12 Furthermore, the effects of the annealing conditions on the PL intensities have been investigated. Figure 5 shows the emission spectra of ZrO2 samples annealed at different temperatures for 2 h in air [(a) 300, (b) 350, (c) 400, (d) 450, and (e) 500 °C], and Figure 6 shows the corresponding luminescence photographs under the excitation of a 365 nm
Figure 3. Morphologies of an as-prepared ZrO2 sample and the representative calcined ZrO2 powders: (a) low-magnification SEM, (b) enlarged SEM, and (c) TEM images of as-prepared ZrO2 and SEM images of (d) ZrO2 at 300 °C, (e) ZrO2 at 400 °C, and (f) ZrO2 at 500 °C, respectively.
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Figure 4. (a) Excitation (red line) and emission (black line) spectra and (b) corresponding decay curve for the ZrO2 phosphor annealed at 400 °C for 2 h in air. Langmuir 2009, 25(12), 7078–7083
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Figure 5. Emission spectra of ZrO2 samples annealed at different temperatures: (a) ZrO2 at 300, (b) ZrO2 at 350, (c) ZrO2 at 400, (d) ZrO2 at 450, and (e) ZrO2 at 500 °C under 365 nm excitation.
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Figure 7. Emission spectra of ZrO2 samples using 0 g (blue line), 0.5 g (black line), and 1.0 g of CTAB (red line) in the hydrolysis process and the SS--ZrO2 sample. All samples were annealed at 400 °C for 2 h.
Figure 6. Luminescence photographs of (a) ZrO2 at 300, (b) ZrO2 at 350, (c) ZrO2 at 400, (d) ZrO2 at 450, and (e) ZrO2 at 500 °C for samples under the excitation of a 365 nm UV lamp.
UV lamp. From Figure 5, we can see that both the emission intensity and emission color (peak position) vary obviously as a function of the annealing temperature. The PL intensity first increases with increasing of annealing temperature, reaching a maximum at 400 °C, and then decreases quickly with further increasing temperature. The peak center positions for the ZrO2 samples shift from 407 (blue emission) to 550 nm (dark-orange emission) when annealed from 300 to 500 °C. In particular, ZrO2 at 400 °C shows nearly white emission ranging from 370 to 620 nm centered at 462 nm, as shown in Figures 5 and 6. The decay curves for the luminescence of (a) ZrO2 at 300, (b) ZrO2 at 350 °C, (c) ZrO2 at 450, and (d) ZrO2 at 500 °C are shown in Figure S2 in the Supporting Information. The luminescence decay curves can be also fitted to a single-exponential function such as I = I0exp(-t/τ) (τ is the lifetime), from which the lifetimes are determined to be 6.02, 5.81, 5.63, and 5.37 ns, respectively.12 It can be seen that the lifetimes decrease only a little with the increase in annealing temperature, which indicates that the luminescence centers of the ZrO2 samples annealed at different temperature are of the same kind. 3.3. Possible Luminescence Mechanisms. Zr4+ itself is nonluminous and has no extrinsic activators, so the observed luminescence from ZrO2 phosphors must be related to some chemical bond breakage with resulting carbon impurities and/or defects in the systems.4,6,7a,9-14,20 As reported previously, amorphous SiO2 (including SiO2 glass,4 SiO2 gels,6 organic/inorganic hybrid silicones,9 etc.) is well known to show luminescence generally from blue to red emission (emission range of 350-700 nm with lifetimes of
