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Study on the surface decorating of nanoTb2O3 in the ethanol sol and its fluorescence characteristics
ARTICLE IN PRESS
Journal of Luminescence 126 (2007) 309–313 www.elsevier.com/locate/jlumin
Study on the surface decorating of nanoTb2O3 in the ethanol sol and its fluorescence characteristics Qian-huo Chen, Wen-gong Zhang, Xiu-xiu Huang College of Chemistry and Materials Science, Fujian Normal University, 350007 Fuzhou, PR China Received 5 March 2006; received in revised form 11 July 2006; accepted 24 July 2006 Available online 1 September 2006
Abstract A sort of surface fine-decorated in situ Tb2O3 nanoparticles in ethanol sols were successively prepared by focused pulsed laser ablation at the interface of Tb2O3 target submerged in flowing liquid containing modifiers, and it is found that the nanoTb2O3 sol decorated by acac and 2,20 -bipy can emit very intense characteristic fluorescence at 549 nm of Tb3+ ions, which is different from nondecorated nanoTb2O3 ethanol sols with only low fluorescence at 415 nm. r 2006 Elsevier B.V. All rights reserved. Keywords: NanoTb2O3; Pulsed laser ablation; Flowing liquid; Decoration in situ; Fluorescence; Modifier
1. Introduction As we all know, all nanoparticles, including rare earth oxides, have strange size-dependent luminescent properties owing to their size effect and quantum effect [1–5], but the poor luminescence is still at quite a distance from the demands of actual application. For some nano-rare-earth oxides such as Tb2O3 with characteristic luminescence properties of long lifetime, extremely sharp emission bands and high luminescence efficiency, their luminescent intensities are not high enough because there does not exist energy transferring with high efficiency to centered terbium atoms such as those in its corresponding coordinate complex [6–8]. It is well known that nanoparticles with size of 1 nm have large surface areas and 99% of total atoms are located at the surface of nanoparticles. If modifiers with energy level matching with that of centered terbium ions are used to decorate the surface Tb3+ ions of Tb2O3 nanoparticles, can the excited energy absorbed by modifier be efficiently transferred to centered terbium ions? Can this decoration greatly enhance the luminescence intensities of Tb2O3 Corresponding author. Tel.: +86 591 83465297; fax: +86 591 83465376. E-mail addresses: [email protected] (Q.-h. Chen), [email protected] (W.-g. Zhang).
0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.07.010
nanoparticles? And can the decorated Tb2O3 nanoparticles emit intense fluorescence as same as the corresponding terbium complexes? And what is the fluorescent mechanism? All the above questions interest us very much. In the present study, a sort of surface fine-decorated in situ Tb2O3 nanoparticles in ethanol sols that can emit very intense characteristic fluorescence at 549 nm of Tb3+ ions are successively prepared by the method of focused pulsed laser ablation at the interface of Tb2O3 target submerged in flowing liquid containing modifiers. The fluorescence characteristics and luminescent mechanism of the sols are investigated via transmission electron microscope, fluorescence spectrum and fluorescence lifetime. 2. Experiment The ethanol solutions that contain different combinational modifiers were used as flowing liquid, which submerged the Tb2O3 target (499.99%). The Tb2O3 target was irradiated by the focused output of 532 nm of DCR3G Nd:YAG laser (Spectra Physics Inc.) operating at 10 Hz at the fluence of 200 mJ pulse1 with a pulse width of 8 ns. The spot size of the laser beam on the surface of target was less than 1 mm, and the flowing liquid was flowing over the target at the speed of approximately 0.15 mL s1 and in the submerging depth of 12 nm. The whole preparation
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process was in high pure nitrogen atmosphere at room temperature. The fluorescence spectra of the obtained Tb2O3 ethanol sol were measured by an Edinburgh FL/FS 920 fluorophotometer at room temperature with 450 W xenon lamp as excited source (slits width of a ¼ b ¼ 0.3 mm, c ¼ d ¼ 0.8 mm). The fluorescence lifetime was obtained by monitoring 549 nm emission under excitation at 309 nm by using an Edinburgh FL/FS 920 fluorophotometer with pulsed xenon microsecond flashlamp. The TEM photographs were obtained on a transmission electron microscopy (H-600, Hitachi). Electron micrographs were carried out through using a drop sol onto a copper mesh coated with an amorphous carbon film. Phen (1,10-phenanthroline), TTA (trifluorothenoyl-acetone), 2,20 -bipy (2,20 -dipyridyl) and acac (acetylacetone) were used as modifiers in experiments, where the concentrations of TTA and acac were 4.2 104 M and the concentrations of Phen and 2,20 -bipy were 1.4 104 M, respectively.
Fig. 1. TEM images of the nanoTb2O3 ethanol sols decorated by acac and 2,20 -bipy, where aging time was 2 min (a) and 3 d (b).
3. Results and discussion During the preparation process, the Tb2O3 target that was immersed in flow liquid was ablated by focused pulsed laser beam under pure nitrogen atmosphere. Because the power density of the pulsed laser beam in the focus spot is as high as 108 W/cm2, it produces a kind of micro-region with high temperature and high pressure on the interface of the solid target and flowing liquid, which causes Tb2O3 in the micro-region to gasify, ionize and become a plasma mass within a very short moment. Subsequently, it condenses to produce Tb2O3 nanoparticles with many dangling bonds on its surface in the course of sharp cooling and decompression. At the same time, its modifiers, such as acac and 2,20 -bipy, in the liquid decorated in situ the surface Tb3+ ions of Tb2O3 nanoparticles with many dangling bonds. Different from other pulsed laser ablation at static liquid [6–14] and at vacuum or inert atmosphere, [15] production of Tb2O3 nanoparticles, decoration in situ, and distributing in the liquid occur at the same time in our preparation procession. Transmission electron micrographs of the obtained nanoTb2O3 ethanol sols which was decorated in situ by acac and 2,20 -bipy are shown in Fig. 1. As can be seen from the images, 2 min after preparation most of the nanoparticles diameter is less than 20 nm in an anomalous shape distributing uniformly; after 3 days most of the decorated Tb2O3 nanoparticles aggregate into floccules, and some of them aggregate into ball particles with diameter of 30 nm. It could be speculated that the diameter of embryonic Tb2O3 nanoparticles produced immediately at first under our experimental conditions is far less than 20 nm. And in situ decorations of acac and 2,20 -bipy cannot completely prevent the agglomerating of embryonic Tb2O3 nanoparticles, but it seems that the decoration of acac and 2,20 -bipy for the Tb2O3 nanoparticles promotes the self-fabrication of Tb2O3 nanoparticles (see Fig. 1b).
Fig. 2. The emission spectra of nanoTb2O3 ethanol sols decorated by different combinational modifiers.
Fig. 2 shows the emission spectra of the nanoTb2O3 ethanol sols decorated by different combinational modifiers excited by the wavelength of 309 nm. As can be seen from Fig. 2, the fluorescence intensity of the sols changes significantly with different combinational modifiers. It is found that the Tb2O3 ethanol sols decorated by the combinations of acac and Phen or acac and 2,20 -bipy exhibit three major peaks in the range of 480–600 nm corresponding to the transitions from the 5D4 states: 5 D4-7F6 (490 nm), 5D4-7F5 (549 nm) and 5D4-7F4 (583 nm), [16] where the strongest emission peak locates at 549 nm. However, the ethanol sols of nanoTb2O3 decorated by the combinational modifiers of TTA and Phen or TTA and 2,20 -bipy have very poor luminescence. It is noteworthy that on comparing the nanoTb2O3 ethanol sols decorated by acac and 2,20 -bipy to the nondecorated nanoTb2O3 ethanol sol (A) (see Figs. 2. 1and 5), the later
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Fig. 3. The excitation spectra of nanoTb2O3 ethanol sols decorated by different combinational modifiers.
has no obvious emission peak at 549 nm and only a weak wide emission peak centered at 415 nm which can be attributed to the emission of the Tb2O3 nanoparticles. Similar result was observed in the literature [17] that the nano europium oxide ethanol sol has a wide emission peak centered at 408 nm. The excitation spectra of the nanoTb2O3 ethanol sols decorated by different combinational modifiers by monitoring the Tb3+ emission at 549 nm are shown in Fig. 3. The four excitation spectra of nanoTb2O3 ethanol sol decorated by different modifiers show a maximum symmetric peak at 309 nm, which can be attributed to the absorption of modifiers, [18] and the Tb2O3 ethanol sols decorated by acac and 2,20 -bipy have strongest absorption peak at 309 nm. There is no obvious absorption of f–f transitions of Tb3+ in the excitation spectra. The spectrum of Tb2O3 ethanol sol without modifier has only two asymmetric weak peaks at 271 and 308 nm, respectively, which can be attributed to the absorption of terbium ions. The decay curves of Tb3+ ions 5D4-7F5 emission of Tb2O3 ethanol sols decorated by acac and 2,20 -bipy are showed in Fig. 4. The decay curves can be fitted by the biexponentialfunction: y ¼ y0 þ A1 expðx=t1 Þ þ A2 expðx=t2 Þ. By fitting, the luminescence lifetime of Tb3+ (5D4) in the decorated Tb2O3 ethanol sol are determined to be 2.59 and 240.81 us with relative weights of 0.4 and 99.6%, respectively. This result indicates that the energy transfers between Tb3+ ions of the decorated Tb2O3 nanoparticles is closely related to the short lifetime. Moreover, the integrated signal from the slow decay (long lifetime) is much more intense than that from the faster decay (short lifetime). Similar biexpenential fluorescence decay was found for terbium complex in LB film [19]. From the above findings, it could be inferred that the selection and combination of best modifiers are very important for obtaining the decorated nanoTb2O3 ethanol sols with high luminescence. The results show that acac and 2,20 -bipy are best combinational modifiers in the decorated
311
Fig. 4. The decay durves of the Tb3+ 5D4-7F5 emission in nanoTb2O3 ethanol sols decorated by acac and 2,20 -bipy (lex ¼ 309 nm, lem ¼ 549 nm).
Fig. 5. Fluorescence spectra of nanoTb2O3 ethanol sols with adding modifiers (acac and 2,20 -bipy) to flowing liquid before preparation (—) and adding them to flowing liquid after preparation (- - - - -).
nanoTb2O3 ethanol sols due to the match of energy levers between the center Tb3+ ions and the modifiers. Moreover, there is a same or similar luminescence mechanism between the decorated Tb2O3 ethanol sols and terbium complexes that the ligands or modifiers absorb ultraviolet light and produce emission with good efficiency via intramolecular energy transfer from ligands or modifier to terbium ions, and then the terbium ions emit characteristic fluorescence [20,21]. Because the fluorescence intensity of decorated nanoTb2O3 ethanol sol is wonderfully stronger at 549 nm, it could be believed that the grainsize of the embryonic Tb2O3 nanoparticles produced in our experiments should be very small, and most of Tb3+ ions in the surface of nanoparticles can be fully decorated by the combinational modifiers. As can be seen from Fig. 5, when the modifiers of acac and 2,20 -bipy have not been pre-added to the flowing
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Fig. 6. The fluorescence intensity in 549 nm of decorated Tb2O3 ethanol sol as a function of aging time.
ethanol but added to the nondecorated nanoTb2O3 ethanol sol (C) after preparation, another decorated Tb2O3 ethanol sol (B) can obtained. It is found that although the fluorescence spectrum of sol (B) is the same as that of sol (A), the fluorescence intensity of (B) decreases sharply and is only about 1/3 of that of sol (A). The reason for this is that during the preparation of sol (A), the Tb2O3 nanoparticles in the sol are decorated in situ by acac and 2,20 -bipy in the flowing liquid as soon as they were produced by pulsed laser ablation. However, during the preparation of (B), when adding acac and 2,20 -bipy to nondecorated Tb2O3 ethanol sol (C) after preparation, the Tb2O3 nanoparticles in the sol have self-aggregated into larger particles to some degree before decoration, so their surface area can be decorated by modifiers decreased greatly. Certainly, the fluorescence intensity decreased vastly also. In order to understand the stability of the decorated Tb2O3 ethanol (A), the influence of the aging time (the laid time after preparation) on the fluorescence intensity of sol (A) at 549 nm is shown in Fig. 6. It is found that the fluorescence intensity decreases rapidly in the first three days, keeps stable in the next 6 days and subsequently decreases quickly. This finding can be explained by the fact that in the first 3 days the part of bigger decorated nanoparticles deposits from the sol because of selfaggregation of nanoparticles, which results in rapid decreasing of the fluorescence intensity; the smaller decorated nanoparticles have higher stability, so the fluorescence intensity can keep stable in the next 6 days. However, the decoration of modifiers for the Tb2O3 nanoparticles cannot completely prevent the aggregation, but can promote the self-fabrication of decorated Tb2O3 nanoparticles and subsequently results in deposit from the sol, so the relative emission intensity decreases rapidly in the final 3 days. The results indicate that in order to obtain decorated nanoTb2O3 materials with high fluorescence, we must design a way to make the decorated Tb2O3
nanoparticles disperse evenly and be fixed in some solid medium at the most proper aging time of the sol. Research regarding this has been in progress, and its results will be reported in another paper. According to above discussions, the production and decorating mechanism of the decorated Tb2O3 nanoparticles could be described as follows: when pulsed laser beam ablates at the interface of Tb2O3 target which is submerged in flowing liquid containing the modifiers of acac and 2,20 -bipy, Tb2O3 in the ablate spot is gasified, decomposed, ionized and becomes a plasma mass, and condenses within a very short moment of 8 ns, so the Tb2O3 nanoparticles produced in the special surroundings of high temperature, high pressure, high density and sharp cooling have a very vast special surface with varying dangling bonds, which can easily combine with the modifiers of acac and 2,20 -bipy in the flowing liquid, and produce the decorated Tb2O3 nanoparticles. Obviously, during the preparation process, there is a competition between the self-agglomerating and the decorating of embryonic Tb2O3 nanoparticles. When the decorating is superior to the self-agglomerating, the decorated Tb2O3 nanoparticles with smaller embryonic size and high luminescence can be obtained easily. Otherwise, the decorated Tb2O3 nanoparticles with larger embryonic size and low luminescence can be obtained.
4. Conclusion In brief, a sort of cluster Tb2O3 ethanol sols which are deorated in situ were successively prepared by focused pulsed laser ablation of Tb2O3 target in the interface of solid and flowing liquid containing some suitable modifiers. The decoration for surface terbium ion of Tb2O3 nanoparticles can change the luminescent wavelength from 415 to 549 nm. Compared to nondecorated nanoTb2O3 ethanol sol with very low fluorescence of nanoparticles at 415 nm, the nanoTb2O3 ethanol sols decorated in situ by acac and 2,20 -bipy can emit very intense characteristic fluorescence at 549 nm of terbium ions. It seems that the decorations of the modifiers for the embryonic Tb2O3 nanoparticles promotes the self-fabrication of the Tb2O3 nanoparticles. It is also considered that the luminescence mechanism of the decorated nanoTb2O3 ethanol sols may be the same as that of the corresponding terbium complex. We also believe that our study presents a convenient method to obtained other decorated metal oxide nanoparticles with novel properties.
Acknowledgments The authors are indebted to the financial support of the National Natural Scientific Foundation of China (Grant 50272014), the Key Nano Special Item of Fujian Province (Grant 2005HZ01-5) and the key item of Natural Scientific Foundation of Fujian Province (Grant 2001F005).
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Journal of Luminescence 126 (2007) 309–313 www.elsevier.com/locate/jlumin
Study on the surface decorating of nanoTb2O3 in the ethanol sol and its fluorescence characteristics Qian-huo Chen, Wen-gong Zhang, Xiu-xiu Huang College of Chemistry and Materials Science, Fujian Normal University, 350007 Fuzhou, PR China Received 5 March 2006; received in revised form 11 July 2006; accepted 24 July 2006 Available online 1 September 2006
Abstract A sort of surface fine-decorated in situ Tb2O3 nanoparticles in ethanol sols were successively prepared by focused pulsed laser ablation at the interface of Tb2O3 target submerged in flowing liquid containing modifiers, and it is found that the nanoTb2O3 sol decorated by acac and 2,20 -bipy can emit very intense characteristic fluorescence at 549 nm of Tb3+ ions, which is different from nondecorated nanoTb2O3 ethanol sols with only low fluorescence at 415 nm. r 2006 Elsevier B.V. All rights reserved. Keywords: NanoTb2O3; Pulsed laser ablation; Flowing liquid; Decoration in situ; Fluorescence; Modifier
1. Introduction As we all know, all nanoparticles, including rare earth oxides, have strange size-dependent luminescent properties owing to their size effect and quantum effect [1–5], but the poor luminescence is still at quite a distance from the demands of actual application. For some nano-rare-earth oxides such as Tb2O3 with characteristic luminescence properties of long lifetime, extremely sharp emission bands and high luminescence efficiency, their luminescent intensities are not high enough because there does not exist energy transferring with high efficiency to centered terbium atoms such as those in its corresponding coordinate complex [6–8]. It is well known that nanoparticles with size of 1 nm have large surface areas and 99% of total atoms are located at the surface of nanoparticles. If modifiers with energy level matching with that of centered terbium ions are used to decorate the surface Tb3+ ions of Tb2O3 nanoparticles, can the excited energy absorbed by modifier be efficiently transferred to centered terbium ions? Can this decoration greatly enhance the luminescence intensities of Tb2O3 Corresponding author. Tel.: +86 591 83465297; fax: +86 591 83465376. E-mail addresses: [email protected] (Q.-h. Chen), [email protected] (W.-g. Zhang).
0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.07.010
nanoparticles? And can the decorated Tb2O3 nanoparticles emit intense fluorescence as same as the corresponding terbium complexes? And what is the fluorescent mechanism? All the above questions interest us very much. In the present study, a sort of surface fine-decorated in situ Tb2O3 nanoparticles in ethanol sols that can emit very intense characteristic fluorescence at 549 nm of Tb3+ ions are successively prepared by the method of focused pulsed laser ablation at the interface of Tb2O3 target submerged in flowing liquid containing modifiers. The fluorescence characteristics and luminescent mechanism of the sols are investigated via transmission electron microscope, fluorescence spectrum and fluorescence lifetime. 2. Experiment The ethanol solutions that contain different combinational modifiers were used as flowing liquid, which submerged the Tb2O3 target (499.99%). The Tb2O3 target was irradiated by the focused output of 532 nm of DCR3G Nd:YAG laser (Spectra Physics Inc.) operating at 10 Hz at the fluence of 200 mJ pulse1 with a pulse width of 8 ns. The spot size of the laser beam on the surface of target was less than 1 mm, and the flowing liquid was flowing over the target at the speed of approximately 0.15 mL s1 and in the submerging depth of 12 nm. The whole preparation
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process was in high pure nitrogen atmosphere at room temperature. The fluorescence spectra of the obtained Tb2O3 ethanol sol were measured by an Edinburgh FL/FS 920 fluorophotometer at room temperature with 450 W xenon lamp as excited source (slits width of a ¼ b ¼ 0.3 mm, c ¼ d ¼ 0.8 mm). The fluorescence lifetime was obtained by monitoring 549 nm emission under excitation at 309 nm by using an Edinburgh FL/FS 920 fluorophotometer with pulsed xenon microsecond flashlamp. The TEM photographs were obtained on a transmission electron microscopy (H-600, Hitachi). Electron micrographs were carried out through using a drop sol onto a copper mesh coated with an amorphous carbon film. Phen (1,10-phenanthroline), TTA (trifluorothenoyl-acetone), 2,20 -bipy (2,20 -dipyridyl) and acac (acetylacetone) were used as modifiers in experiments, where the concentrations of TTA and acac were 4.2 104 M and the concentrations of Phen and 2,20 -bipy were 1.4 104 M, respectively.
Fig. 1. TEM images of the nanoTb2O3 ethanol sols decorated by acac and 2,20 -bipy, where aging time was 2 min (a) and 3 d (b).
3. Results and discussion During the preparation process, the Tb2O3 target that was immersed in flow liquid was ablated by focused pulsed laser beam under pure nitrogen atmosphere. Because the power density of the pulsed laser beam in the focus spot is as high as 108 W/cm2, it produces a kind of micro-region with high temperature and high pressure on the interface of the solid target and flowing liquid, which causes Tb2O3 in the micro-region to gasify, ionize and become a plasma mass within a very short moment. Subsequently, it condenses to produce Tb2O3 nanoparticles with many dangling bonds on its surface in the course of sharp cooling and decompression. At the same time, its modifiers, such as acac and 2,20 -bipy, in the liquid decorated in situ the surface Tb3+ ions of Tb2O3 nanoparticles with many dangling bonds. Different from other pulsed laser ablation at static liquid [6–14] and at vacuum or inert atmosphere, [15] production of Tb2O3 nanoparticles, decoration in situ, and distributing in the liquid occur at the same time in our preparation procession. Transmission electron micrographs of the obtained nanoTb2O3 ethanol sols which was decorated in situ by acac and 2,20 -bipy are shown in Fig. 1. As can be seen from the images, 2 min after preparation most of the nanoparticles diameter is less than 20 nm in an anomalous shape distributing uniformly; after 3 days most of the decorated Tb2O3 nanoparticles aggregate into floccules, and some of them aggregate into ball particles with diameter of 30 nm. It could be speculated that the diameter of embryonic Tb2O3 nanoparticles produced immediately at first under our experimental conditions is far less than 20 nm. And in situ decorations of acac and 2,20 -bipy cannot completely prevent the agglomerating of embryonic Tb2O3 nanoparticles, but it seems that the decoration of acac and 2,20 -bipy for the Tb2O3 nanoparticles promotes the self-fabrication of Tb2O3 nanoparticles (see Fig. 1b).
Fig. 2. The emission spectra of nanoTb2O3 ethanol sols decorated by different combinational modifiers.
Fig. 2 shows the emission spectra of the nanoTb2O3 ethanol sols decorated by different combinational modifiers excited by the wavelength of 309 nm. As can be seen from Fig. 2, the fluorescence intensity of the sols changes significantly with different combinational modifiers. It is found that the Tb2O3 ethanol sols decorated by the combinations of acac and Phen or acac and 2,20 -bipy exhibit three major peaks in the range of 480–600 nm corresponding to the transitions from the 5D4 states: 5 D4-7F6 (490 nm), 5D4-7F5 (549 nm) and 5D4-7F4 (583 nm), [16] where the strongest emission peak locates at 549 nm. However, the ethanol sols of nanoTb2O3 decorated by the combinational modifiers of TTA and Phen or TTA and 2,20 -bipy have very poor luminescence. It is noteworthy that on comparing the nanoTb2O3 ethanol sols decorated by acac and 2,20 -bipy to the nondecorated nanoTb2O3 ethanol sol (A) (see Figs. 2. 1and 5), the later
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Fig. 3. The excitation spectra of nanoTb2O3 ethanol sols decorated by different combinational modifiers.
has no obvious emission peak at 549 nm and only a weak wide emission peak centered at 415 nm which can be attributed to the emission of the Tb2O3 nanoparticles. Similar result was observed in the literature [17] that the nano europium oxide ethanol sol has a wide emission peak centered at 408 nm. The excitation spectra of the nanoTb2O3 ethanol sols decorated by different combinational modifiers by monitoring the Tb3+ emission at 549 nm are shown in Fig. 3. The four excitation spectra of nanoTb2O3 ethanol sol decorated by different modifiers show a maximum symmetric peak at 309 nm, which can be attributed to the absorption of modifiers, [18] and the Tb2O3 ethanol sols decorated by acac and 2,20 -bipy have strongest absorption peak at 309 nm. There is no obvious absorption of f–f transitions of Tb3+ in the excitation spectra. The spectrum of Tb2O3 ethanol sol without modifier has only two asymmetric weak peaks at 271 and 308 nm, respectively, which can be attributed to the absorption of terbium ions. The decay curves of Tb3+ ions 5D4-7F5 emission of Tb2O3 ethanol sols decorated by acac and 2,20 -bipy are showed in Fig. 4. The decay curves can be fitted by the biexponentialfunction: y ¼ y0 þ A1 expðx=t1 Þ þ A2 expðx=t2 Þ. By fitting, the luminescence lifetime of Tb3+ (5D4) in the decorated Tb2O3 ethanol sol are determined to be 2.59 and 240.81 us with relative weights of 0.4 and 99.6%, respectively. This result indicates that the energy transfers between Tb3+ ions of the decorated Tb2O3 nanoparticles is closely related to the short lifetime. Moreover, the integrated signal from the slow decay (long lifetime) is much more intense than that from the faster decay (short lifetime). Similar biexpenential fluorescence decay was found for terbium complex in LB film [19]. From the above findings, it could be inferred that the selection and combination of best modifiers are very important for obtaining the decorated nanoTb2O3 ethanol sols with high luminescence. The results show that acac and 2,20 -bipy are best combinational modifiers in the decorated
311
Fig. 4. The decay durves of the Tb3+ 5D4-7F5 emission in nanoTb2O3 ethanol sols decorated by acac and 2,20 -bipy (lex ¼ 309 nm, lem ¼ 549 nm).
Fig. 5. Fluorescence spectra of nanoTb2O3 ethanol sols with adding modifiers (acac and 2,20 -bipy) to flowing liquid before preparation (—) and adding them to flowing liquid after preparation (- - - - -).
nanoTb2O3 ethanol sols due to the match of energy levers between the center Tb3+ ions and the modifiers. Moreover, there is a same or similar luminescence mechanism between the decorated Tb2O3 ethanol sols and terbium complexes that the ligands or modifiers absorb ultraviolet light and produce emission with good efficiency via intramolecular energy transfer from ligands or modifier to terbium ions, and then the terbium ions emit characteristic fluorescence [20,21]. Because the fluorescence intensity of decorated nanoTb2O3 ethanol sol is wonderfully stronger at 549 nm, it could be believed that the grainsize of the embryonic Tb2O3 nanoparticles produced in our experiments should be very small, and most of Tb3+ ions in the surface of nanoparticles can be fully decorated by the combinational modifiers. As can be seen from Fig. 5, when the modifiers of acac and 2,20 -bipy have not been pre-added to the flowing
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Fig. 6. The fluorescence intensity in 549 nm of decorated Tb2O3 ethanol sol as a function of aging time.
ethanol but added to the nondecorated nanoTb2O3 ethanol sol (C) after preparation, another decorated Tb2O3 ethanol sol (B) can obtained. It is found that although the fluorescence spectrum of sol (B) is the same as that of sol (A), the fluorescence intensity of (B) decreases sharply and is only about 1/3 of that of sol (A). The reason for this is that during the preparation of sol (A), the Tb2O3 nanoparticles in the sol are decorated in situ by acac and 2,20 -bipy in the flowing liquid as soon as they were produced by pulsed laser ablation. However, during the preparation of (B), when adding acac and 2,20 -bipy to nondecorated Tb2O3 ethanol sol (C) after preparation, the Tb2O3 nanoparticles in the sol have self-aggregated into larger particles to some degree before decoration, so their surface area can be decorated by modifiers decreased greatly. Certainly, the fluorescence intensity decreased vastly also. In order to understand the stability of the decorated Tb2O3 ethanol (A), the influence of the aging time (the laid time after preparation) on the fluorescence intensity of sol (A) at 549 nm is shown in Fig. 6. It is found that the fluorescence intensity decreases rapidly in the first three days, keeps stable in the next 6 days and subsequently decreases quickly. This finding can be explained by the fact that in the first 3 days the part of bigger decorated nanoparticles deposits from the sol because of selfaggregation of nanoparticles, which results in rapid decreasing of the fluorescence intensity; the smaller decorated nanoparticles have higher stability, so the fluorescence intensity can keep stable in the next 6 days. However, the decoration of modifiers for the Tb2O3 nanoparticles cannot completely prevent the aggregation, but can promote the self-fabrication of decorated Tb2O3 nanoparticles and subsequently results in deposit from the sol, so the relative emission intensity decreases rapidly in the final 3 days. The results indicate that in order to obtain decorated nanoTb2O3 materials with high fluorescence, we must design a way to make the decorated Tb2O3
nanoparticles disperse evenly and be fixed in some solid medium at the most proper aging time of the sol. Research regarding this has been in progress, and its results will be reported in another paper. According to above discussions, the production and decorating mechanism of the decorated Tb2O3 nanoparticles could be described as follows: when pulsed laser beam ablates at the interface of Tb2O3 target which is submerged in flowing liquid containing the modifiers of acac and 2,20 -bipy, Tb2O3 in the ablate spot is gasified, decomposed, ionized and becomes a plasma mass, and condenses within a very short moment of 8 ns, so the Tb2O3 nanoparticles produced in the special surroundings of high temperature, high pressure, high density and sharp cooling have a very vast special surface with varying dangling bonds, which can easily combine with the modifiers of acac and 2,20 -bipy in the flowing liquid, and produce the decorated Tb2O3 nanoparticles. Obviously, during the preparation process, there is a competition between the self-agglomerating and the decorating of embryonic Tb2O3 nanoparticles. When the decorating is superior to the self-agglomerating, the decorated Tb2O3 nanoparticles with smaller embryonic size and high luminescence can be obtained easily. Otherwise, the decorated Tb2O3 nanoparticles with larger embryonic size and low luminescence can be obtained.
4. Conclusion In brief, a sort of cluster Tb2O3 ethanol sols which are deorated in situ were successively prepared by focused pulsed laser ablation of Tb2O3 target in the interface of solid and flowing liquid containing some suitable modifiers. The decoration for surface terbium ion of Tb2O3 nanoparticles can change the luminescent wavelength from 415 to 549 nm. Compared to nondecorated nanoTb2O3 ethanol sol with very low fluorescence of nanoparticles at 415 nm, the nanoTb2O3 ethanol sols decorated in situ by acac and 2,20 -bipy can emit very intense characteristic fluorescence at 549 nm of terbium ions. It seems that the decorations of the modifiers for the embryonic Tb2O3 nanoparticles promotes the self-fabrication of the Tb2O3 nanoparticles. It is also considered that the luminescence mechanism of the decorated nanoTb2O3 ethanol sols may be the same as that of the corresponding terbium complex. We also believe that our study presents a convenient method to obtained other decorated metal oxide nanoparticles with novel properties.
Acknowledgments The authors are indebted to the financial support of the National Natural Scientific Foundation of China (Grant 50272014), the Key Nano Special Item of Fujian Province (Grant 2005HZ01-5) and the key item of Natural Scientific Foundation of Fujian Province (Grant 2001F005).
ARTICLE IN PRESS Q.-h. Chen et al. / Journal of Luminescence 126 (2007) 309–313
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