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Hydroxyl-Quenching Effects on the Photoluminescence Properties of SnO2:Eu3+ Nanoparticles Taeho Moon, Sun-Tae Hwang, Dae-Ryong Jung, Dongyeon Son, Chunjoong Kim, Jongmin Kim, Myunggoo Kang, and Byungwoo Park* Department of Materials Science and Engineering, and Research Center for Energy ConVersion and Storage, Seoul National UniVersity, Seoul 151-744, Korea ReceiVed: NoVember 1, 2006; In Final Form: January 20, 2007
The effects of hydroxyl quenching were examined on the photoluminescence properties of SnO2:Eu3+ nanoparticles. High-quality SnO2:Eu3+ nanoparticles were simply synthesized from SnCl4, EuCl3, and ethylene glycol. The photoluminescence spectra showed a reddish orange emission, which gradually increased with the calcination temperature in the range from 700 to 1000 °C. As the calcination temperature varied, the change of the OH-/O2- integrated-intensity ratios from X-ray photoelectron spectroscopy (XPS) was qualitatively consistent with that of the photoluminescence intensities. The samples obtained after the hydrothermal treatment and after reheating, respectively, exhibited a decline and recovery of their emission intensities, and this behavior with XPS confirmed the hydroxyl-quenching effect.
Introduction For display devices, such as plasma display panel (PDP), field emission display (FED), etc., nanophosphors have potential advantages over traditional micron-sized phosphors. Nanophosphors offer higher packing densities, lower scattering of light, and a larger fraction of cathodoluminescent active material at low excitation voltages, thereby resulting in a higher luminescent efficiency and resolution.1-3 However, nanoparticles have generally showed poor luminescence efficiencies, and one possible reason for this is hydroxyl quenching due to the presence of some adsorbates during the synthesis of the nanoparticles.1 Hydroxyl-quenching effects were reported for rare-earth ions and nanoparticles suspended in water.4,5 However, reports on the characterizations of phosphor itself have been rare, and it is easily expected that hydroxyl quenching will largely depend on the thermal histories of the nanoparticles. Therefore, it is necessary to systematically characterize the luminescence properties to develop high-efficiency nanophosphors. The Eu3+ ion as an activator has been investigated most frequently because of its unique fluorescent properties, which are attributed to its stability and high emission-quantum yield. Also, the luminescence properties of rare-earth ions strongly depend on the crystal nanostructures of the host matrix. Recently, it was reported that among the various combinations of micron-sized SnO2 hosts and rare-earth activators, the Eu3+ ion showed the best cathodoluminescence properties.6 Herein, we report the hydroxyl-quenching-related photoluminescence (PL) properties of SnO2:Eu3+ nanoparticles prepared using a simple solvothermal method. This hydroxyl-quenching effect may yield useful information that can be applied to many other nanoparticles as well as SnO2. Experimental Section The SnO2:Eu3+ nanoparticles were synthesized from SnCl4, EuCl3, and ethylene glycol (C2H6O2) by a solvothermal method. * Corresponding author. E-mail:
[email protected].
Ethylene glycol functions as a complexion agent to form a polymeric network and also as a spacer to modulate the distance between the metal ions, thus preventing the metal oxides from aggregating during the earlier stage of organic removal.7,8 The SnCl4 (1 mL, 8.55 mM) and EuCl3 (0.25 g, 0.95 mM) dissolved in ethylene glycol (40 mL) were under vigorous stirring. The mixtures were maintained in a Teflon-sealed autoclave at 180 °C for 12 h. After cooling the colloids to room temperature, the precipitates were centrifuged/washed several times in water and ethanol. To remove the organic groups, the precipitates were calcined in a furnace at various calcination temperatures ranging from 700 to 1000 °C for 3 h under air. To investigate the effect of the hydroxyl group on the PL properties, the SnO2:Eu3+ nanoparticles (calcined at 1000 °C) were hydrothermally treated at 140 °C for 12 h.9 After the treatment, the precipitates were centrifuged and oven-dried. For a systematic comparison, half of the samples were reheated at 700 °C for 3 h under air. The PL spectra were measured using a spectrofluorometer (FP-6500, JASCO) with a Xe lamp, and the absorption spectra were recorded with the colloid solutions in ethanol by means of a UV/vis spectrophotometer (DU-65, Beckman). The particle sizes and local strains were analyzed by X-ray diffraction (XRD: M18XHF-SRA, MAC Science). The Fourier transform infrared (FT-IR: Nicolet Magna 550, Midac) spectra were recorded using pellets with KBr. Inductively coupled plasma-atomic emission spectroscopy (ICP: Optima 4300DU, Perkin-Elmer) was used to determine the actual doping concentrations of Eu. The chemical-bond states were analyzed by X-ray photoelectron spectroscopy (XPS: AXIS, Kratos) with Mg KR radiation. Thermogravimetric analysis (TGA: SDT Q600, TA instrument) was conducted under air with a heating rate of 10 °C/min. The actual doping concentration of Eu was measured to be only ∼0.1 at. % by ICP, which was much lower than that on synthesis. This low doping concentration is due to the low solubility and/or the doping kinetics during the colloidal growth.10,11
10.1021/jp067217l CCC: $37.00 © 2007 American Chemical Society Published on Web 03/01/2007
PL Properties of SnO2:Eu3+ Nanoparticles
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Figure 1. PL spectra (excited at 325 nm) of the SnO2:Eu3+ nanoparticles with various calcination temperatures ranging from 700 to 1000 °C. Figure 3. (a) TGA profile of the as-synthesized SnO2:Eu3+ nanoparticles (before calcination) under air with a heating rate of 10 °C/min. (b) The first derivative of weight for the same nanoparticles.
Figure 2. Absorption spectrum of the SnO2:Eu3+ nanoparticles calcined at 1000 °C. The inset shows the corresponding plot of (RhV)2 vs photon energy, and the linear fit over Eg.
Results and Discussion Figure 1 shows the PL spectra (excited at 325 nm) of the SnO2:Eu3+ nanoparticles with various calcination temperatures ranging from 700 to 1000 °C. The PL spectra show a reddish orange emission, and gradually increased with the calcination temperature. The three strong lines at ∼590, ∼594, and ∼600 nm, and the weak line at ∼610 nm are attributed, respectively, to the 5D0-7F1 magnetic-dipolar transition and 5D0-7F2 electricdipolar transition of the Eu3+ ions.12 (No other peaks were found in the 520-580 nm range, where 5D1-7F0, 5D1-7F1, and 5D17F transitions may be observed.) If Eu3+ is centrosymmetric 2 in the lattice, only magnetic-dipolar transitions can occur. Without this inversion symmetry, the electric-dipolar transitions are no longer strictly forbidden and appear in the luminescence spectra.13 Therefore, the 5D0-7F2/5D0-7F1 emission intensity ratio indicates how far the local environment of the Eu3+ ions is centrosymmetric, and the results show that the symmetry of the substitutional octahedral site is not significantly distorted at various calcination temperatures. The absorption spectrum shows an intense UV line (SnO2 interband transition) and a small peak at ∼380 nm (for the Eu3+ direct excitation, 7F0-5L6), as shown in Figure 2.12 The PL analysis excited at 380 nm (for the Eu3+ direct excitation) shows no observable peak (not shown). Therefore, it appears that a reasonable energy transfer from the SnO2 host to the Eu3+ ions occurs. The corresponding plot of (RhV)2 versus photon energy (hV) gives a band gap (Eg)
Figure 4. XRD patterns of the SnO2:Eu3+ nanoparticles with various calcination temperatures ranging from 700 to 1000 °C. The ideal peak positions and intensities for rutile SnO2 (JCPDS #41-1445) are marked at the bottom.
of ∼3.9 eV. (The fitting results with various calcination temperatures did not show any significant changes.) The TGA profile of the as-synthesized SnO2:Eu3+ nanoparticles (before calcination) shows the weight losses that may be assigned to the removal of ethylene glycol organics (200-450 °C) and the removal of chemically bonded OH groups (above 450 °C), as shown in Figure 3.7,14 (However, the origin of many peaks in the temperature range of 200-450 °C from the residual ethylene glycol needs to be identified.) The XRD patterns of the SnO2:Eu3+ nanoparticles with various calcination temperatures show rutile structures without any secondary phases, as shown in Figure 4. To confirm the narrowing of the diffraction peaks, the widths (fwhm) for the six (hkl) peaks were fitted using a double peak Lorentzian function, considering the effect of KR1 and KR2. To correct the instrumental broadening effect, a resolution function estimated from a silicon powder (Aldrich) was subtracted after fitting for each peak. As the calcination temperature increases from 700 to 1000 °C, the grain size (calculated by a Scherrer equation) gradually increases to 21.4 ( 1.7 nm, 34.4 ( 3.3 nm, 33.0 ( 1.7 nm, and 45.9 ( 6.0 nm, respectively. Because the surface defects act as nonradiative transition centers, PL intensity may be influenced by the grain
4166 J. Phys. Chem. C, Vol. 111, No. 11, 2007
Figure 5. Plots of ∆k vs k from the X-ray diffraction of the SnO2: Eu3+ nanoparticles with various calcination temperatures ranging from 700 to 1000 °C. The slope of the linear-fitted lines describes a nonuniform distribution of local strain.
Figure 6. XPS peaks corresponding to (a) Sn 3d and (b) O 1s for the SnO2:Eu3+ nanoparticles with various calcination temperatures. The marked peak positions are from the SnO2 standard sample. (c) The OH-/O2- integrated-intensity ratios from the O 1s peaks for the SnO2: Eu3+ nanoparticles with various calcination temperatures. (d) The typical fitting results of the O 1s peak (900 °C) with two Gaussian functions.
size (due to the different surface-to-volume ratios). The slopes from the ∆k vs k plots describe a nonuniform distribution of local strain (Figure 5).15,16 The variation of nonuniform distribution of local strains with the calcination temperature is negligible, such as -0.021 ( 0.057% (700 °C), -0.093 ( 0.024% (800 °C), -0.054 ( 0.019% (900 °C), and 0.010 ( 0.018% (1000 °C). (However, the meaning of the negative slope is not yet clear.17) The XPS peaks corresponding to Sn 3d and O 1s for the SnO2:Eu3+ nanoparticles with various calcination temperatures are shown in Figure 6(a),(b). The Eu peaks (∼136 eV for 4d18) were not clearly observed due to the low concentration of Eu and the overlapping with the Sn peaks (∼137 eV for 4s19). The Sn 3d5/2 (∼486 eV) and 3d3/2 (∼495 eV) peaks are not significantly changed with the calcination temperatures.20 In the case of the O 1s peaks, a shoulder at ∼531 eV is observed with the main peak at ∼530 eV. The main peak is assigned to the lattice oxygen, and the shoulder is due to the oxygen of the metal-OH bonds.20,21 Various hydroxyl bonds, such as Sn-OH, Eu-OH, and hydroxide bonds (OxHy), may have slightly
Moon et al.
Figure 7. FT-IR spectrum of the SnO2:Eu3+ nanoparticles calcined at 1000 °C.
Figure 8. (a) PL spectra of the SnO2:Eu3+ (1000 °C) nanoparticles, before and after the hydrothermal treatment at 140 °C, and reheated at 700 °C. The corresponding variations of the (b) Sn 3d and (c) O 1s peaks from the XPS spectra.
different binding energies.22 The O 1s peaks were therefore fitted with two Gaussian functions without fixing the width or position due to the difficulty in defining each hydroxide, as shown in Figure 6(d). Figure 6(c) shows an overall decrease of the OH-/O2- integrated-intensity ratios with the calcination temperature, and this result is comparable to the TGA profile, as shown in Figure 3. As the calcination temperature varies, the OH-/O2- integrated-intensity ratios are qualitatively consistent with the PL intensities, as shown in Figure 1. The FT-IR spectrum confirms that some hydroxyl groups still remain, even in the samples calcined at 1000 °C, as shown in Figure 7.23,24 (Before the measurement, the samples and KBr were oven-dried at 100 °C to minimize the effect of the water molecules.) To examine only the influence of the OH groups without the nanoparticle-size effect, the SnO2:Eu3+ (1000 °C) nanoparticles were hydrothermally treated. Figure 8(a) shows the PL spectra of the SnO2:Eu3+ nanoparticles, before and after the hydrothermal treatment, and reheated. The samples after the hydrothermal treatment show an ∼20% decrease of the 5D0-7F1 integrated intensity. Afterward, their subsequent reheating (at 700 °C) gives rise to the full recovery of the PL intensity. The
PL Properties of SnO2:Eu3+ Nanoparticles corresponding variations of the Sn 3d and O 1s peaks from XPS are shown in Figure 8. For the Sn peaks after the hydrothermal treatment, the binding-energy positions become slightly higher, and the shoulders appear at a high binding energy, due to the presence of Sn-OH-like species.25 The shift of the O peak may indicate that most of the nanoparticle surfaces changes into a hydroxide phase. (Even though the peak position looks different between the hydrothermal-treatment experiment at 140 °C (Figure 8) and the calcination-temperature experiment (Figure 6), the peak shift can be attributed to the hydroxyl bonds.) The reheated samples show the XPS and PL spectra, similar to those of the samples before the hydrothermal treatment. The qualitative correlation between the PL intensities and the XPS results suggests that the reversible change of the PL intensities is caused by the hydroxyl groups. The strong drop of the PL intensity was reported for Y2O3:Eu3+ nanoparticles suspended in water, which is explained by a nonradiative energy transfer through the coupling of the 5D0 states of the Eu3+ ions to the O-H vibration states.4,5 The Eu ions in a few layers near the surface can be influenced by hydroxyl groups due to the energy-transfer distance. Therefore, some of the adsorbates during the synthesis of the nanoparticles and the large surfaceto-volume ratio can give rise to many nonradiative pathways, and the variation of the PL intensities with the calcination temperature can be explained by the removal of the hydroxyl groups and by the decrease of the surface area due to the growth of the nanoparticles. Conclusions The SnO2:Eu3+ nanoparticles were synthesized by a simple solvothermal method. The PL spectra showed a reddish orange emission, which gradually increased with the calcination temperature. As the calcination temperature increased, the change of the OH-/O2- integrated-intensity ratios from XPS was qualitatively consistent with that of the PL intensities. The samples obtained after the hydrothermal treatment and after subsequent reheating, respectively, exhibited a decline and recovery of their emission intensities, and this behavior with XPS confirmed the hydroxyl-quenching effect. The increase of the PL intensities with the calcination temperature was attributed to the removal of the hydroxyl groups and to the decrease of the surface area caused by the growth of the nanoparticles. Detailed studies are needed to clarify the dopant distribution and defect chemistry, and thereby correlate those with the PL properties.
J. Phys. Chem. C, Vol. 111, No. 11, 2007 4167 Acknowledgment. The authors are grateful to Jiwoo Ahn for the materials characterizations. This work was supported by the ERC program of MOST/KOSEF (R11-2002-102-000000) and by the Basic Research Program (R01-2004-000-101730) of KOSEF. References and Notes (1) Nelson, J. A.; Brant, E. L.; Wagner, M. J. Chem. Mater. 2003, 15, 688. (2) Yu, M.; Lin, J.; Fang, J. Chem. Mater. 2005, 17, 1783. (3) Kang, Y. C.; Roh, H. S.; Kim, E. J.; Park, H. D. J. Electrochem. Soc. 2003, 150, H93. (4) Horrocks, W. D., Jr.; Sudnick, D. R. Acc. Chem. Res. 1981, 14, 384. (5) Schmechel, R.; Kennedy, M.; von Seggern, H.; Winkler, H.; Kolbe, M.; Fischer, R. A.; Xaomao, L.; Benker, A.; Winterer, M.; Hahn, H. J. Appl. Phys. 2001, 89, 1679. (6) Kang, J. H.; Kim, J. Y.; Jeon, D. Y. J. Electrochem. Soc. 2005, 152, H33. (7) Zhang G.; Liu, M. J. Mater. Sci. 1999, 34, 3213. (8) Moon, T.; Kim, C.; Hwang, S.-T.; Park, B. Electrochem. SolidState Lett. 2006, 9, A408. (9) Yoshimura, M.; Noma, T.; Kawabata, K.; Somiya, S. J. Mater. Sci. Lett. 1987, 6, 465. (10) Matsuoka, T.; Kasahara, Y.; Tsuchiya, M.; Nitta, T.; Hayakawa, S. J. Electrochem. Soc. 1978, 125, 102. (11) Erwin, S. C.; Zu, L.; Haftel, M. I.; Efros, A. L.; Kennedy, T. A.; Norris, D. J. Nature 2005, 436, 91. (12) Yanes, A. C.; Castillo, J. D.; Torres, M.; Peraza, J.; Rodrı´guez, V. D.; Me´ndez-Ramos, J. Appl. Phys. Lett. 2004, 85, 2343. (13) Blasse, G.; Grabmaier, B. C. Luminescent Materials; SpringerVerlag: New York, 1994; pp 41-44. (14) Peng, Z. S.; Wan, C. R.; Jiang, C. Y. J. Power Sources 1998, 72, 215. (15) Kim, T.; Oh, J.; Park, B.; Hong, K. S. Appl. Phys. Lett. 2000, 76, 3043. (16) Kim, Y.; Oh, J.; Kim, T.-G.; Park, B. Appl. Phys. Lett. 2001, 78, 2363. (17) Gu, F.; Wang, S. F.; Lu¨, M. K.; Zhou, G. J.; Xu, D.; Yuan, D. R. J. Phys. Chem. B 2004, 108, 8119. (18) Schneider, W.-D.; Laubschat, C.; Nowik, I.; Kaindl, G. Phys. ReV. B 1981, 24, 5422. (19) Moulder, J. F.; Chastrain, J.; King, R. C. Handbook of X-Ray Photoelectron Spectroscopy; Physical Electronics Inc.: Eden Prairie, MN, 1995. (20) Ramgir, N. S.; Mulla, I. S.; Vijayamohanan, K. P. J. Phys. Chem. B 2005, 109, 12297. (21) Epling, W. S.; Mount, C. K.; Hoflund, G. B. Appl. Surf. Sci. 1998, 134, 187. (22) Suporhina S.; Guire, M. R. Thin Solid Films 2000, 371, 1. (23) Srivastava, D. N.; Chappel, S.; Palchik, O.; Zaban, A.; Gedanken, A. Langmuir 2002, 18, 4160. (24) Kersen, U ¨ .; Sundberg, M. R. J. Electrochem. Soc. 2003, 150, H129. (25) Donley, C.; Dunphy, D.; Paine, D.; Carter, C.; Nebesny, K.; Lee, P.; Alloway, D.; Armstrong, N. R. Langmuir 2002, 18, 450.
