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Fabrication of Y2Ti2O7:Yb3+,Ho3+ nanoparticles by a gel-combustion approach and upconverting luminescent properties
Journal of Alloys and Compounds 608 (2014) 165–169
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Fabrication of Y2Ti2O7:Yb3+,Ho3+ nanoparticles by a gel-combustion approach and upconverting luminescent properties Zhongsheng Chen a,⇑, Min Wang a, Haiqing Wang a, Zhanggao Le a, Guolin Huang a, Lixia Zou a, Zhirong Liu a, Dianyuan Wang b, Qingkai Wang b, Weiping Gong c a b c
State Key Laboratory Breeding Base of Nuclear Resources and Environment, East China Institute of Technology, Nanchang, Jiangxi 330013, China College of Science, Jiujiang University, Jiujiang, Jiangxi 332005, China Electronic Science Department, Huizhou University, Huizhou, Guangdong 516001, China
a r t i c l e
i n f o
Article history: Received 15 February 2014 Received in revised form 15 April 2014 Accepted 16 April 2014 Available online 24 April 2014 Keywords: Yttrium titanate Gel-combustion Upconverting luminescence Nanophosphors
a b s t r a c t Yb3+, Ho3+ co-doped pyrochlore-structured (Y0.99xHo0.01Ybx)2Ti2O7 (x = 0, 2.5, 5.0, 7.5, 10.0 and 12.5 mol%) nanoparticles (NPs) were successfully fabricated via a gel-combustion approach. The products as-obtained were characterized by various techniques, i.e. X-ray diffraction, transmission electron microscope, Fourier transformed infrared spectra and upconverting spectra. The results indicate that the bright green (540 nm) and red (660 nm) emissions are observed in Y2Ti2O7:Ho3+,Yb3+ NPs under the 980 nm excitation, which is ascribed to the radiative transitions (5F4,5S2) ? 5I8 and 5F5 ? 5I8 of Ho3+ ions, respectively. It is also found that the calcining temperature and Yb3+ ion doping level have a great influence on the upconverting spectra of (Y0.99xHo0.01Ybx)2Ti2O7 NPs. The emission intensities increase initially and then fall down from 800 to 1000 °C. The optimum doping level of Yb3+ ions is 7.5 mol%, and the intensity of upconverting emissions for (Y0.915Ho0.01Yb0.075)2Ti2O7 NPs is enhanced by the fold of 32 compared to the Yb3+-free samples. The dependence of upconverting intensity on the excitation power reveals the contribution of two photons to both the green and red upconverting process under lower excitation power, and the possible upconverting mechanisms have been proposed accordingly. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction In more recent years, research on rare-earth ions doped upconverting nano-materials has received considerable attention owing to their unique ability to convert the near-infrared (NIR) excitation to visible (Vis) or even ultraviolet emissions, which has opened up vast opportunities for creating novel biomedical imaging, cancer therapy, temperature sensing, detection of heavy metals, spectra solar converters for solar cells, and so on [1–5]. To investigate the upconverting properties of host matrices with low phonon energy, such as NaReF4, ReF3 and Re2O3 (Re = rare earth), is of special interest [6–9] as the lower phonon energy can help reduce the possibility of nonradiative relaxation, thus resulting in the strong NIR-to-VIS upconverting emissions. It is noteworthy that pyrochlore-structured Re2Ti2O7 recently has been proposed as one of potential host material for excellent upconverting luminescence [10–14] due to their unique structural characteristics, such as the relatively low phonon energy
⇑ Corresponding author. Tel.: +86 79183897510; fax: +86 79183879031. E-mail address: [email protected] (Z. Chen). http://dx.doi.org/10.1016/j.jallcom.2014.04.101 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
(<712 cm1), high refractive index value (2.2 at 550 nm), high optical band gap (3.7 eV), the ability to accept rare earth elements in solid solutions, as well as excellent thermal and chemical stability. In these previous research, Er3+ ions are used as the activator, and much efficient upconverting process can be achieved when co-doped with Yb3+ as sensitizer for Yb3+ ion has a larger absorption cross section (11.7 1021 cm2) under the 980 nm excitation [15] and there occurs the large spectral overlap between Er3+ absorption (4I15/2 ? 4I11/2) and Yb3+ emission (4F5/2 ? 4F7/2). Ho3+ ion, another most popular emission center, can give rise to the strong green and red emissions because of energy levels capable of infrared pumping that closely match the most attractive 980 nm. To the best of our knowledge, however, Yb3+, Ho3+ co-doped Y2Ti2O7 nanophosphors and their upconverting luminescence have not yet been reported systematically so far. Therefore, it is urgent to make a deeper research in the upconverting properties of Ho3+, Yb3+ co-doped Y2Ti2O7 NPs. Herein, in the present work, a series of Yb3+, Ho3+ co-doped Y2Ti2O7 nanophosphors were fabricated via gel-combustion approach using glycine as the fuel. The influence of calcining temperature and Yb3+ ion doping concentration on the phase evolution as well as upconverting properties have been discussed
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in detail. And the possible upconverting mechanisms have been also proposed through the power dependence of upconverting intensities. 2. Experimental section Ho3+, Yb3+ co-doped Y2Ti2O7 nanocrystals were synthesized via a glycine– nitrate gel-combustion method. The appropriate conditions (the pH value of the precursor is 2.0, the fuel-to-oxidant ratio is 2.0) are chosen, and the details of the procedure have been given in our previous article [16]. Tetrabutyl titanate, Y2O3, Ho2O3 and Yb2O3 were employed as basic starting materials. Firstly, a given amount of Y2O3, Yb2O3 and Ho2O3 was dissolved with concentrated HNO3 to convert into Y(NO3)3, Yb(NO3)3 and Ho(NO3)3 solutions, respectively. By fixing the molar ratio of n(Y+Ho+Yb)/nTi 1:1 and varying Yb3+ ion doping level, various Ho3+,Yb3+ co-doped (Y0.99xHo0.01Ybx)2Ti2O7 (x = 0–12.5 mol%) NPs were fabricated. The crystalline structure of as-prepared samples was determined by X-ray diffraction (XRD, D/ruax2550PC, Tokyo, Japan) using Cu Ka radiation (k = 0.154056 nm) over a 2h range from 10° to 80° at a scanning rate of 4°/min. And the average crystallite size D (nm) of nanocrystals can be estimated from the XRD spectra according to the Scherrer’s formula [17]:
D¼
crystallizes from the amorphous structure between 700 and 800 °C. The series of characteristic planes (2 2 2), (4 0 0), (3 3 1), (4 4 0), (6 2 2), etc., is indexed based on face-centered cubic lattice titanate (JCPDS No. 42-0413) as reported previously [18]. Additionally, as the temperature increases, no other possible impure phases are detected. And the increase in the diffraction intensity with the rise of the calcining temperature indicates the crystallite growth of the Y2Ti2O7 powders. According to the Scherrer’s formula [17], the mean crystallite sizes of products calcined at 800, 900 and 1000 °C are estimated at 24.8, 30.1, and 45.6 nm, respectively. On the other hand, other as-prepared co-doped samples (Y0.99xHo0.01Ybx)2Ti2O7 (x = 0, 2.5, 5.0, 10.0 and 12.5 mol%) also exhibit the same effect of the calcining temperature on the phase evolution (not shown here). Fig. 2 gives the typical TEM images of Ho3+, Yb3+ co-doped Y2Ti2O7 NPs calcined at 800 °C and 1000 °C for 1.0 h, respectively. Both the samples exhibit the same morphology characteristics:
Kk b cos h
where D (nm) is the crystallite size; K is a Scherrer constant and equals 0.9, k is the X-ray wavelength 0.1540 nm, b (rad) is the full-width-at-half-maximum (FWHM) of a characteristic diffraction peak and h (°) is the diffraction angle corresponding to the XRD peak being considered, and here the most intensive XRD peaks (2 2 2) are used. The morphologies and average size of samples were observed by transmission electron microscopy (TEM, JEOL-2010F, Tokyo, Japan) operated at 200 kV. TEM samples were prepared by dispersing the powder in absolute alcohol by ultrasonic treatment, dropping onto a porous carbon film supported on a copper grid, and then dried in air. The adsorption of impurities on the host surface was conducted by Fourier transform infrared spectra (FT-IR, Avatar 380, Thermo Nicolet Corporation, USA) in the scan range of 4000–400 cm1 with KBr pellet technique. The measurement of upconverting emission was carried out using a power-controllable 980 nm diode laser (Newport Corporation, California, USA) as the excitation source. The laser was focused on the sample (spot size of 1.0 mm) by use of a 10 cm focal length lens and the fluorescence emission was collected in a reflection mode, in an angular direction of 45° to normal incidence. The maximum pump power at the sample surface was 2500 mW. The upconverting emission spectra were collected by a spectrophotometer (Hitachi F-4500, Tokyo, Japan).
3. Results and discussion The XRD patterns of the products (Y0.915Ho0.01Yb0.075)2Ti2O7 calcined in air at various temperatures for 1.0 h are presented in Fig. 1. It is apparent that the powders calcined at 600 and 700 °C are primarily amorphous, as evidenced by the broad continuum diffraction peak centered at around 30° (2h) in Fig. 1b and c. Nevertheless, all of the XRD patterns exhibit the single phase with pyrochlore structure when calcined at 800 °C or higher temperatures. Therefore, it can be safely concluded that the products
Fig. 1. XRD patterns of (Y0.915Ho0.01Yb0.075)2Ti2O7 NPs calcined at various temperatures for 1.0 h.
Fig. 2. Typical TEM images of Ho3+, Yb3+ co-doped Y2Ti2O7 NPs calcined at 800 °C and 1000 °C for 1.0 h, and the inserts show the particle size distribution.
Z. Chen et al. / Journal of Alloys and Compounds 608 (2014) 165–169
most of the NPs are either spherical or spheroidal, and of good dispersity. On the basis of the size distribution, the average particle size of the samples prepared at 800 and 1000 °C is estimated to be 25.2 and 45.20 nm, respectively, which is fully consistent with the aforementioned XRD results determined by the Scherrer’s formula. In order to demonstrate the adsorption of impurities on the host calcined at various temperatures, the FT-IR spectra were examined, as shown in Fig. 3. From Fig. 3(a), the two absorption bands at 1522 and 1080 cm1 can be observed, which are ascribed to the carbonate. Generally, the carbonates are the unavoidable residues after the thermal decomposition of precursors containing organic [19,20]. The stretching vibration bands (3450 cm1) and bending vibration bands (1636 cm1) of hydroxyl groups (OAH), and the characteristic absorption bands (1384 cm1) of nitrate ions (ANO 3) still exists after combustion, implying H2O molecules in the air and NO 3 are adsorbed on the products. When the powders were calcined at 600 °C for 1.0 h, the adsorption of carbonate is reduced dramatically, as the characteristic absorption bands (1522 and 1080 cm1) become much weaker, and almost disappeared when calcining at 800 °C. The strength of the characteristic absorption bands of H2O and CO2 molecules are decreased gradually with the increase of calcining temperatures, and then keep almost unchanged at 800 °C or higher temperatures, which showed that the adsorption amount of H2O and CO2 molecules is reduced. When the powders were calcined at 800 or high temperatures, the FT-IR spectra of as-obtained products share the similar absorption bands except for the difference in the absorption strength, which can be accounted for by the fact that the FT-IR spectra are not very sensitive to the particle sizes of nanomaterials [21]. In Fig. 3, the low wavenumber absorption bands appearing at 568, 468 and 410 cm1 are assigned to TiAO, YAO1 and YAO2 stretching vibrations in the Y2Ti2(O1)6O(2) polyhedron of Y2Ti2O7, respectively, which are the main features of the titanate pyrochlore [13]. Fig. 4 shows the effect of calcining temperatures on upconverting spectra of (Y0.915Ho0.01Yb0.075)2Ti2O7 NPs under the 980 nm excitation. In terms of all the samples excited by the weak pump power of 610 mW, bright upconverting emissions could be observed, suggesting that (Y0.915Ho0.01Yb0.075)2Ti2O7 is a high efficient upconverting material. As shown in Fig. 4, the upconverting spectra exhibit the green1 and red emission bands in the visible region of 500–700 nm, which is similar to the upconverting systems, Lu2O3:Ho3+,Yb3+ and Y3Al5O12:Ho3+,Yb3+ [22–24], and the emission bands centered at 540 and 660 nm are originated from (5F4, 5S2) ? 5I8, and 5F5 ? 5I8 electronic transitions of Ho3+ ions, respectively. And also, it is evidently seen from Fig. 4 that the upconverting intensity is increased greatly with the temperature rise from 600 to 800 °C, approaches to the maximum, and then decreases from 800 to 1000 °C, this trend is not identical to our previous findings [10], in which upconverting intensity (Y0.91Er0.015Yb0.075)2Ti2O7 NPs is increased gradually with the temperature rise from 600 to 1000 °C. The reason for this may be accounted for by the different preparation method of phosphors. As a matter of fact, the influence of the calcining temperature on the upconverting intensity seems to be a little more complicated, which may be explained by the three contradictory factors: the higher energy impurities adsorbed, the surface defects and the local environment of activator ions. Firstly, the higher energy contaminants, i.e. OH and CO2 3 , could be readily absorbed onto the sample surface, consequently reducing the upconverting intensity [25]. As demonstrated in the FT-IR spectra above, the quenching centers, i.e. hydroxyl and
1 For interpretation of color in Figs. 4–6, the reader is referred to the web version of this article.
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Fig. 3. FT-IR spectra of the products after combustion and calcined at various temperatures, (a) after combustion, (b) 600 °C, (c) 700 °C, (d) 800 °C, (e) 900 °C and (f) 1000 °C.
Fig. 4. The effect of the calcining temperatures on upconverting spectra of (Y0.915Ho0.01Yb0.075)2Ti2O7 NPs (under the 980 nm excitation with the pump power of 610 mW).
carbonate ion, decrease with calcining temperatures from 600 to 800 °C, then keep constant essentially. Secondly, the surface defects act also as nonradiative transition centers, and the upconverting intensity may be influenced by the grain size (due to the different surface-to-volume ratios) [26,27]. As aforementioned in XRD and TEM results, the increase of the average particle size of (Y0.915Ho0.01Yb0.075)2Ti2O7 nanophosphors prepared at 800, 900, and 1000 °C implies that the surface defects are reduced with the rise of calcining temperature, thereby leading to the increase of luminescence intensity [26]. Thirdly but very importantly, however, the local symmetry of Ho3+ lattice sites can also be promoted with the increase of calcining temperature, therein resulting in the reduction of luminescence intensity [28]. Taking these factors above into consideration, the first reason may be predominant to account for the effect of calcining temperatures on the upconverting intensity. It is proposed that, therefore, to reduce the hydroxyl and carbonate groups absorbed on the host and to modify the local symmetry of crystalline field of rare earth ions may improve the upconverting efficiency of nanophosphors greatly. Fig. 5 shows the effect of Yb3+ ion doping concentrations on the upconverting properties of (Y0.99xHo0.01Ybx)2Ti2O7 NPs calcined at 800 °C for 1.0 h. Apparently, the Yb3+ ion doping level has a great
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Fig. 5. The effect of the Yb3+ ion doping concentrations on upconverting spectra of (Y0.99xHo0.01Ybx)2Ti2O7 NPs calcined at 800 °C for 1.0 h (under the 980 nm excitation with the pump power of 610 mW).
Fig. 6. Enhancement factors of the green, red, total upconverting emissions and Ired/Igreen of (Y0.99xHo0.01Ybx)2Ti2O7 NPs as a function of Yb3+ concentrations (the integrated intensity of Yb3+-free sample is scaled as unit, under the 980 nm excitation). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
influence on the intensities of upconverting spectra. As for Yb3+-free samples, the upconverting emissions are very weak. This is due to the fact of the spectra mismatch between the energy level of Ho3+ ions and the 980 nm excitation although Ho3+ ions can be excited by the 980 nm diode laser directly. The upconverting efficiency of uni-doped Y2Ti2O7:Ho3+ nanophosphors is rather low. By co-doping Yb3+ ions, however, bright upconverting emissions could be observed even by naked eyes, which confirms that there exists the efficient energy transfer from Yb3+ to Ho3+. According to Fig. 5, the integral intensity of upconverting green, red and overall emissions is normalized, respectively, and the effect of the green, red, total upconverting emission and the Ired/Igreen as a function of Yb3+ ion doping concentrations can be obtained, as shown in Fig. 6. Obviously, the green, red, total UC emission share the same trend with the Yb3+ ion doping level: the emission intensity increases drastically with increasing Yb3+ ions, up to a critical value of 7.5 mol%, and then decreases for more Yb3+ ions, i.e. the so-called concentration quenching effect takes place. This suggests that there exists an optimal Yb3+ ion concentration that gives a maximum emission intensity in (Y0.99xHo0.01Ybx)2Ti2O7 nanophosphors.
As it well known, Yb3+ ion has a larger absorption cross section (11.7 1021 cm2) at 980 nm excitation [15]. Within a certain range of Yb3+ ion doping concentration, therefore, the increase of Yb3+ ions favors the absorption of pumping photons, and then facilitates the efficient resonant energy transfer from Yb3+ to Ho3+ ions, thus improving the efficiency of upconverting emissions. Too higher Yb3+ ions, however, may increase the odds of the back energy transfer Ho3+ ? Yb3+ [29,30], and on the other hand, it is possible that the pairing or aggregation of Yb3+ ions could come into being, which resultantly may turn these ions into new quenching centers [31]. As a consequence, the Yb3+ ions at increased doping level may act as trapping centers and induce nonradiative relaxation, herein reducing the upconverting efficiency. From Fig. 6, it is also found that at Yb3+ ions of 7.5 mol%, the intensities of green, red, total upconverting emission is enhanced by 32 folds compared to Yb3+-free samples, which evidences the important role of the sensitizer Yb3+ ions in the upconverting process. In addition, the intensity ratios (Ired/Igreen) of the red to green emission keep almost constant as the Yb3+ ion concentration increases from 0 to 12.5 mol%, suggesting that Yb3+ ions have a simultaneous influence on the (5F4, 5S2) ? 5I8 and 5F5 ? 5I8 radiative transitions of Ho3+ ions. To get a better understanding of the possible upconverting mechanism involved in Y2Ti2O7:Ho3+,Yb3+ NPs, the pump power dependence of the integral intensity of upconverting emissions is plotted in Fig. 7. It has been widely recognized that, for nearly any unsaturated upconverting process, the slope of the doublelogarithmic plots of the integral intensity versus pump power is the number of NIR photons absorbed per visible photon emitted [32]. At relatively low pump powers, the slopes determined through the linear fitting are 2.15 ± 0.02 and 2.18 ± 0.02 for the green ((5F4, 5S2) ? 5I8) and red (5F5 ? 5I8) transitions, respectively, indicating the involvement of two photons excited to (5F4, 5S2) and 5 F5 levels. This is consistent with other upconverting system, such as Y3Al5O12:Ho3+,Yb3+ and NaYF4:Ho3+,Yb3+ NPs [23,24,33]. When the pump power increases to a relatively high region, however, the so-called ‘‘saturation effect’’ exists for both the red and green emissions, i.e. the intensity decreases dramatically instead with the increase of the pump power. This phenomena can be attributed to thermal quenching caused by the absorption of the 980 nm light when excited under too high pump power [34–36]. Fig. 8 shows the energy level diagrams of Ho3+ and Yb3+ ions, as well as the proposed upconverting processes under the laser excitation of 980 nm. Firstly, the electrons of Yb3+ ions absorbs one pump photon, and are excited from the ground state 4F7/2 to the level 4F5/2 due to its lager cross sections, and then the energy
Fig. 7. The pump power dependence of the red and green emissions of (Y0.915Ho0.01Yb0.075)2Ti2O7 NPs calcined at 800 °C for 1.0 h (under the 980 nm excitation). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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optimum doping level of Yb3+ ions is 7.5 mol%, and the intensity of upconverting emissions for (Y0.915Ho0.01Yb0.075)2Ti2O7 NPs is increased by 32 times compared to the Yb3+-free samples. The quadratic pump power dependence of the green and red intensities indicates two photons are responsible for the upconverting process under low pump power, and the potential upconverting mechanisms have been proposed. Acknowledgments
Fig. 8. Energy level diagrams of Ho3+ and Yb3+ ion as well as the proposed upconverting mechanisms of Y2Ti2O7:Ho3+,Yb3+ NPs.
of the excited electrons is transferred to the neighboring electrons of Ho3+ ions at the ground state 5I8 (ETU1 in Fig. 8), thereby bringing about the population of the intermediate excited state 5I6 with the excess energy consumed by the host lattice vibration. Then, most Ho3+ ions at the state 5I6 are pumped to the levels (5F4, 5S2) via excited state absorption (ESA) and energy transfer (ETU2). Lastly, the green luminescence is observed via the radiative transitions from (5F4, 5S2) to 5I8, which is also depicted in detail as follows: 3þ
3þ
4
F 7=2 ðYb Þ þ hm980 ! 4 F 5=2 ðYb Þ ðGSAÞ
5
3þ
I8 ðHo3þ Þ þ 4 F 5=2 ðYb Þ ! 5 I6 ðHo3þ Þ þ 4 F 7=2 ðYb Þ ðETU1Þ
5
I6 ðHo3þ Þ þ 4 F 5=2 ðYb Þ ! ð5 F 4 ; 5 S2 ÞðHo3þ Þ þ 4 F 7=2 ðYb Þ ðETU2Þ
5
3þ
3þ
3þ
I6 ðHo3þ Þ þ hm980 ! ð5 F 4 ; 5 S2 ÞðHo3þ Þ ðESAÞ
ð F 4 ; 5 S2 ÞðHo3þ Þ ! 5 I8 ðHo3þ Þ þ hmgreen 5
ðRTÞ
In the case of the red emission, the two possible approaches are responsible for the population of Ho3+ ions at the 5F5 level. On one hand, the excited electrons at the states (5F4, 5S2) decay nonradiatively to the level of 5F5 with the help of multi-phonon relaxation (MPR). On the other hand, a portion of electrons of Ho3+ ions at the intermediate excited state 5I6 decay to the level 5I7 via nonradiative transition (NRT), and then giving rise to the population of the state 5F5 through the energy transfer (ETU3) or excited-state absorption (ESA). As a result, the red emission around 660 nm is observed by the transition from the level 5F5 to 5I8, which can be described as follows. 4
3þ
3þ
F 7=2 ðYb Þ þ hm980 ! 4 F 5=2 ðYb Þ ðGSAÞ 5
3þ
3þ
3þ
I8 ðHo Þ þ 4 F 5=2 ðYb Þ ! 5 I6 ðHo3þ Þ þ 4 F 7=2 ðYb Þ ðETU1Þ
ð5 F 4 ; 5 S2 ÞðHo3þ Þ ! 5 F 5 ðHo3þ Þ ðMPRÞ 5
I6 ðHo3þ Þ ! 5 I7 ðHo3þ Þ ðMPRÞ
5
I7 ðHo3þ Þ þ 4 F 5=2 ðYb Þ ! 5 F 5 ðHo3þ Þ þ 4 F 7=2 ðYb Þ ðETU3Þ
3þ
3þ
3þ
3þ
5
I7 ðHo Þ þ hm980 ! 5 F 5 ðHo Þ ðESAÞ
5
F 5 ðHo3þ Þ ! 5 I8 ðHo3þ Þ þ hmred
ðRTÞ
4. Conclusions In summary, a variety of Yb3+, Ho3+ co-doped Y2Ti2O7 NPs were prepared via the glycine-combustion approach. Under the 980 nm excitation, the bright (540 nm) and red (660 nm) emissions are observed, which is attributed to the (5F4,5S2) ? 5I8 and 5F5 ? 5I8 transitions of Ho3+ ions, respectively. The calcining temperatures and Yb3+ ion doping level have a great influence on the upconverting spectra of (Y0.99xHo0.01Ybx)2Ti2O7 NPs. The intensities of emissions increase initially and then decrease after 800 °C. The
This work is supported by Natural Science Foundation of China (Nos. 51362002, 11375043, 51171069 and 51062008), Science & Technology Pillar Program from Jiangxi Provincial Department of Science and Technology(No. 20122BBG70084-2), and the financial supports from Key Laboratory of Radioactive Geology and Exploration Technology Fundamental Science for National Defence (No. 2010RGET15) and State Key Laboratory Breeding Base of Nuclear Resources and Environment (No. 060614 and NRE1319) are also greatly appreciated. References [1] L. Cheng, C. Wang, Z. Liu, Nanoscale 5 (2013) 23–37. [2] K.E. Sapsford, W.R. Algar, L. Berti, K.B. Gemmill, B.J. Casey, E. Oh, M.H. Stewart, I.L. Medintz, Chem. Rev. 113 (2013) 1904–2074. [3] S. Zhou, S. Jiang, X. Wei, Y. Chen, C. Duan, M. Yin, J. Alloys Comp. 588 (2014) 654–657. [4] X. Huang, S. Han, W. Huang, X. Liu, Chem. Soc. Rev. 42 (2013) 173–201. [5] Q. Liu, W. Feng, F. Li, Coord. Chem. Rev. (2014), http://dx.doi.org/10.1016/ j.ccr.2014.01.004. [6] D. Gao, X. Zhang, H. Zheng, W. Gao, E. He, J. Alloys Comp. 554 (2013) 395–399. [7] C. Li, Y. Mei, J. Xie, W. Dai, G. Du, Z. Li, J. Alloys Comp. 560 (2013) 10–14. [8] Y. Li, S. Xu, J. Alloys Comp. 601 (2014) 195–200. [9] L. Li, H. Lin, X. Zhao, Y. Wang, X. Zhou, C. Ma, X. Wei, J. Alloys Comp. 586 (2014) 555–560. [10] Z.S. Chen, W.P. Gong, T.F. Chen, S.L. Li, D.Y. Wang, Q.K. Wang, Mater. Lett. 68 (2012) 137–139. [11] X. Li, H. Cai, L. Ding, X. Dou, W. Zhang, J. Alloys Comp. 541 (2012) 36–40. [12] C.C. Ting, Y.C. Chien, W.F. Sung, ECS J. Solid State Sci. Technol. 2 (2013) R105– R110. [13] B.P. Singh, A.K. Parchur, R.K. Singh, A.A. Ansari, P. Singh, S.B. Rai, Phys. Chem. Chem. Phys. 15 (2013) 3480–3489. [14] B.S. Cao, J.L. Wu, N.S. Yu, Z.Q. Feng, B. Dong, Thin Solid Films 550 (2014) 495– 498. [15] F. Vetrone, J.C. Boyer, J.A. Capobianco, A. Speghini, M. Bettinelli, J. Phys. Chem. B 107 (2003) 1107–1112. [16] Y. Zhang, M. Wang, Z. Le, G. Huang, L. Zou, Z. Chen, Ceram. Int. 40 (2014) 5223– 5230. [17] H.P. Klug, L.E. Alexander, X-Ray Powder Diffraction Procedures, John Wiley & Sons, New York, 1959. [18] L. Brixner, Inorg. Chem. 3 (1964) 1065–1067. [19] P.A. Tanner, L.S. Fu, B.M. Cheng, J. Phys. Chem. C 113 (2009) 10773–10779. [20] H. Palneedi, V. Mangam, S. Das, K. Das, J. Alloys Comp. 509 (2011) 9912–9918. [21] K. Koteswara Rao, T. Banu, M. Vithal, G.Y.S.K. Swamy, K. Ravi Kumar, Mater. Lett. 54 (2002) 205–210. [22] L.Q. An, J. Zhang, M. Liu, S.W. Wang, J. Am. Ceram. Soc. 88 (2005) 1010–1012. [23] M. Liu, S.W. Wang, D.Y. Tang, L.D. Chen, J. Ma, J. Rare Earth 27 (2009) 66–70. [24] X.P. Chen, W.J. Zhang, Q.Y. Zhang, Physica B 406 (2011) 1248–1252. [25] G.J. De, W.P. Qin, J.S. Zhang, Y. Wang, C.Y. Cao, Y. Cui, Solid State Commun. 137 (2006) 483–487. [26] M.L. Bailly, G. Costentin, H. Lauron-Pernot, J.M. Krafft, M. Che, J. Phys. Chem. B 109 (2005) 2404–2413. [27] T. Moon, S.T. Hwang, D.R. Jung, D. Son, C. Kim, J. Kim, M. Kang, B. Park, J. Phys. Chem. C 111 (2007) 4164–4167. [28] C.C. Ting, Y.S. Chiu, C.W. Chang, L.C. Chuang, J. Solid State Chem. 184 (2011) 563–571. [29] L. Zhang, H. Hu, C. Qi, F. Lin, Opt. Mater. 17 (2001) 371–377. [30] F. Auzel, G. Baldacchini, L. Laversenne, G. Boulon, Opt. Mater. 24 (2003) 103– 109. [31] J.L. Wagener, P.F. Wysocki, M.J.F. Digonnet, H.J. Shaw, D.J. Digiovanni, Opt. Lett. 18 (1993) 2014–2016. [32] M. Pollnau, D.R. Gamelin, S.R. Luthi, H.U. Gudel, M.P. Hehlen, Phys. Rev. B 61 (2000) 3337–3346. [33] Y. Bai, Y. Wang, G. Peng, K. Yang, X. Zhang, Y. Song, J. Alloys Comp. 478 (2009) 676–678. [34] Y.Q. Lei, H.W. Song, L.M. Yang, L.X. Yu, Z.X. Liu, G.H. Pan, X. Bai, L.B. Fan, J. Chem. Phys. 123 (2005) 174710. [35] A.H. Li, Q. Lü, EPL 96 (2011) 18001. [36] J. Petit, B. Viana, P. Goldner, Opt. Express 19 (2011) 1138–1146.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Fabrication of Y2Ti2O7:Yb3+,Ho3+ nanoparticles by a gel-combustion approach and upconverting luminescent properties Zhongsheng Chen a,⇑, Min Wang a, Haiqing Wang a, Zhanggao Le a, Guolin Huang a, Lixia Zou a, Zhirong Liu a, Dianyuan Wang b, Qingkai Wang b, Weiping Gong c a b c
State Key Laboratory Breeding Base of Nuclear Resources and Environment, East China Institute of Technology, Nanchang, Jiangxi 330013, China College of Science, Jiujiang University, Jiujiang, Jiangxi 332005, China Electronic Science Department, Huizhou University, Huizhou, Guangdong 516001, China
a r t i c l e
i n f o
Article history: Received 15 February 2014 Received in revised form 15 April 2014 Accepted 16 April 2014 Available online 24 April 2014 Keywords: Yttrium titanate Gel-combustion Upconverting luminescence Nanophosphors
a b s t r a c t Yb3+, Ho3+ co-doped pyrochlore-structured (Y0.99xHo0.01Ybx)2Ti2O7 (x = 0, 2.5, 5.0, 7.5, 10.0 and 12.5 mol%) nanoparticles (NPs) were successfully fabricated via a gel-combustion approach. The products as-obtained were characterized by various techniques, i.e. X-ray diffraction, transmission electron microscope, Fourier transformed infrared spectra and upconverting spectra. The results indicate that the bright green (540 nm) and red (660 nm) emissions are observed in Y2Ti2O7:Ho3+,Yb3+ NPs under the 980 nm excitation, which is ascribed to the radiative transitions (5F4,5S2) ? 5I8 and 5F5 ? 5I8 of Ho3+ ions, respectively. It is also found that the calcining temperature and Yb3+ ion doping level have a great influence on the upconverting spectra of (Y0.99xHo0.01Ybx)2Ti2O7 NPs. The emission intensities increase initially and then fall down from 800 to 1000 °C. The optimum doping level of Yb3+ ions is 7.5 mol%, and the intensity of upconverting emissions for (Y0.915Ho0.01Yb0.075)2Ti2O7 NPs is enhanced by the fold of 32 compared to the Yb3+-free samples. The dependence of upconverting intensity on the excitation power reveals the contribution of two photons to both the green and red upconverting process under lower excitation power, and the possible upconverting mechanisms have been proposed accordingly. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction In more recent years, research on rare-earth ions doped upconverting nano-materials has received considerable attention owing to their unique ability to convert the near-infrared (NIR) excitation to visible (Vis) or even ultraviolet emissions, which has opened up vast opportunities for creating novel biomedical imaging, cancer therapy, temperature sensing, detection of heavy metals, spectra solar converters for solar cells, and so on [1–5]. To investigate the upconverting properties of host matrices with low phonon energy, such as NaReF4, ReF3 and Re2O3 (Re = rare earth), is of special interest [6–9] as the lower phonon energy can help reduce the possibility of nonradiative relaxation, thus resulting in the strong NIR-to-VIS upconverting emissions. It is noteworthy that pyrochlore-structured Re2Ti2O7 recently has been proposed as one of potential host material for excellent upconverting luminescence [10–14] due to their unique structural characteristics, such as the relatively low phonon energy
⇑ Corresponding author. Tel.: +86 79183897510; fax: +86 79183879031. E-mail address: [email protected] (Z. Chen). http://dx.doi.org/10.1016/j.jallcom.2014.04.101 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
(<712 cm1), high refractive index value (2.2 at 550 nm), high optical band gap (3.7 eV), the ability to accept rare earth elements in solid solutions, as well as excellent thermal and chemical stability. In these previous research, Er3+ ions are used as the activator, and much efficient upconverting process can be achieved when co-doped with Yb3+ as sensitizer for Yb3+ ion has a larger absorption cross section (11.7 1021 cm2) under the 980 nm excitation [15] and there occurs the large spectral overlap between Er3+ absorption (4I15/2 ? 4I11/2) and Yb3+ emission (4F5/2 ? 4F7/2). Ho3+ ion, another most popular emission center, can give rise to the strong green and red emissions because of energy levels capable of infrared pumping that closely match the most attractive 980 nm. To the best of our knowledge, however, Yb3+, Ho3+ co-doped Y2Ti2O7 nanophosphors and their upconverting luminescence have not yet been reported systematically so far. Therefore, it is urgent to make a deeper research in the upconverting properties of Ho3+, Yb3+ co-doped Y2Ti2O7 NPs. Herein, in the present work, a series of Yb3+, Ho3+ co-doped Y2Ti2O7 nanophosphors were fabricated via gel-combustion approach using glycine as the fuel. The influence of calcining temperature and Yb3+ ion doping concentration on the phase evolution as well as upconverting properties have been discussed
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in detail. And the possible upconverting mechanisms have been also proposed through the power dependence of upconverting intensities. 2. Experimental section Ho3+, Yb3+ co-doped Y2Ti2O7 nanocrystals were synthesized via a glycine– nitrate gel-combustion method. The appropriate conditions (the pH value of the precursor is 2.0, the fuel-to-oxidant ratio is 2.0) are chosen, and the details of the procedure have been given in our previous article [16]. Tetrabutyl titanate, Y2O3, Ho2O3 and Yb2O3 were employed as basic starting materials. Firstly, a given amount of Y2O3, Yb2O3 and Ho2O3 was dissolved with concentrated HNO3 to convert into Y(NO3)3, Yb(NO3)3 and Ho(NO3)3 solutions, respectively. By fixing the molar ratio of n(Y+Ho+Yb)/nTi 1:1 and varying Yb3+ ion doping level, various Ho3+,Yb3+ co-doped (Y0.99xHo0.01Ybx)2Ti2O7 (x = 0–12.5 mol%) NPs were fabricated. The crystalline structure of as-prepared samples was determined by X-ray diffraction (XRD, D/ruax2550PC, Tokyo, Japan) using Cu Ka radiation (k = 0.154056 nm) over a 2h range from 10° to 80° at a scanning rate of 4°/min. And the average crystallite size D (nm) of nanocrystals can be estimated from the XRD spectra according to the Scherrer’s formula [17]:
D¼
crystallizes from the amorphous structure between 700 and 800 °C. The series of characteristic planes (2 2 2), (4 0 0), (3 3 1), (4 4 0), (6 2 2), etc., is indexed based on face-centered cubic lattice titanate (JCPDS No. 42-0413) as reported previously [18]. Additionally, as the temperature increases, no other possible impure phases are detected. And the increase in the diffraction intensity with the rise of the calcining temperature indicates the crystallite growth of the Y2Ti2O7 powders. According to the Scherrer’s formula [17], the mean crystallite sizes of products calcined at 800, 900 and 1000 °C are estimated at 24.8, 30.1, and 45.6 nm, respectively. On the other hand, other as-prepared co-doped samples (Y0.99xHo0.01Ybx)2Ti2O7 (x = 0, 2.5, 5.0, 10.0 and 12.5 mol%) also exhibit the same effect of the calcining temperature on the phase evolution (not shown here). Fig. 2 gives the typical TEM images of Ho3+, Yb3+ co-doped Y2Ti2O7 NPs calcined at 800 °C and 1000 °C for 1.0 h, respectively. Both the samples exhibit the same morphology characteristics:
Kk b cos h
where D (nm) is the crystallite size; K is a Scherrer constant and equals 0.9, k is the X-ray wavelength 0.1540 nm, b (rad) is the full-width-at-half-maximum (FWHM) of a characteristic diffraction peak and h (°) is the diffraction angle corresponding to the XRD peak being considered, and here the most intensive XRD peaks (2 2 2) are used. The morphologies and average size of samples were observed by transmission electron microscopy (TEM, JEOL-2010F, Tokyo, Japan) operated at 200 kV. TEM samples were prepared by dispersing the powder in absolute alcohol by ultrasonic treatment, dropping onto a porous carbon film supported on a copper grid, and then dried in air. The adsorption of impurities on the host surface was conducted by Fourier transform infrared spectra (FT-IR, Avatar 380, Thermo Nicolet Corporation, USA) in the scan range of 4000–400 cm1 with KBr pellet technique. The measurement of upconverting emission was carried out using a power-controllable 980 nm diode laser (Newport Corporation, California, USA) as the excitation source. The laser was focused on the sample (spot size of 1.0 mm) by use of a 10 cm focal length lens and the fluorescence emission was collected in a reflection mode, in an angular direction of 45° to normal incidence. The maximum pump power at the sample surface was 2500 mW. The upconverting emission spectra were collected by a spectrophotometer (Hitachi F-4500, Tokyo, Japan).
3. Results and discussion The XRD patterns of the products (Y0.915Ho0.01Yb0.075)2Ti2O7 calcined in air at various temperatures for 1.0 h are presented in Fig. 1. It is apparent that the powders calcined at 600 and 700 °C are primarily amorphous, as evidenced by the broad continuum diffraction peak centered at around 30° (2h) in Fig. 1b and c. Nevertheless, all of the XRD patterns exhibit the single phase with pyrochlore structure when calcined at 800 °C or higher temperatures. Therefore, it can be safely concluded that the products
Fig. 1. XRD patterns of (Y0.915Ho0.01Yb0.075)2Ti2O7 NPs calcined at various temperatures for 1.0 h.
Fig. 2. Typical TEM images of Ho3+, Yb3+ co-doped Y2Ti2O7 NPs calcined at 800 °C and 1000 °C for 1.0 h, and the inserts show the particle size distribution.
Z. Chen et al. / Journal of Alloys and Compounds 608 (2014) 165–169
most of the NPs are either spherical or spheroidal, and of good dispersity. On the basis of the size distribution, the average particle size of the samples prepared at 800 and 1000 °C is estimated to be 25.2 and 45.20 nm, respectively, which is fully consistent with the aforementioned XRD results determined by the Scherrer’s formula. In order to demonstrate the adsorption of impurities on the host calcined at various temperatures, the FT-IR spectra were examined, as shown in Fig. 3. From Fig. 3(a), the two absorption bands at 1522 and 1080 cm1 can be observed, which are ascribed to the carbonate. Generally, the carbonates are the unavoidable residues after the thermal decomposition of precursors containing organic [19,20]. The stretching vibration bands (3450 cm1) and bending vibration bands (1636 cm1) of hydroxyl groups (OAH), and the characteristic absorption bands (1384 cm1) of nitrate ions (ANO 3) still exists after combustion, implying H2O molecules in the air and NO 3 are adsorbed on the products. When the powders were calcined at 600 °C for 1.0 h, the adsorption of carbonate is reduced dramatically, as the characteristic absorption bands (1522 and 1080 cm1) become much weaker, and almost disappeared when calcining at 800 °C. The strength of the characteristic absorption bands of H2O and CO2 molecules are decreased gradually with the increase of calcining temperatures, and then keep almost unchanged at 800 °C or higher temperatures, which showed that the adsorption amount of H2O and CO2 molecules is reduced. When the powders were calcined at 800 or high temperatures, the FT-IR spectra of as-obtained products share the similar absorption bands except for the difference in the absorption strength, which can be accounted for by the fact that the FT-IR spectra are not very sensitive to the particle sizes of nanomaterials [21]. In Fig. 3, the low wavenumber absorption bands appearing at 568, 468 and 410 cm1 are assigned to TiAO, YAO1 and YAO2 stretching vibrations in the Y2Ti2(O1)6O(2) polyhedron of Y2Ti2O7, respectively, which are the main features of the titanate pyrochlore [13]. Fig. 4 shows the effect of calcining temperatures on upconverting spectra of (Y0.915Ho0.01Yb0.075)2Ti2O7 NPs under the 980 nm excitation. In terms of all the samples excited by the weak pump power of 610 mW, bright upconverting emissions could be observed, suggesting that (Y0.915Ho0.01Yb0.075)2Ti2O7 is a high efficient upconverting material. As shown in Fig. 4, the upconverting spectra exhibit the green1 and red emission bands in the visible region of 500–700 nm, which is similar to the upconverting systems, Lu2O3:Ho3+,Yb3+ and Y3Al5O12:Ho3+,Yb3+ [22–24], and the emission bands centered at 540 and 660 nm are originated from (5F4, 5S2) ? 5I8, and 5F5 ? 5I8 electronic transitions of Ho3+ ions, respectively. And also, it is evidently seen from Fig. 4 that the upconverting intensity is increased greatly with the temperature rise from 600 to 800 °C, approaches to the maximum, and then decreases from 800 to 1000 °C, this trend is not identical to our previous findings [10], in which upconverting intensity (Y0.91Er0.015Yb0.075)2Ti2O7 NPs is increased gradually with the temperature rise from 600 to 1000 °C. The reason for this may be accounted for by the different preparation method of phosphors. As a matter of fact, the influence of the calcining temperature on the upconverting intensity seems to be a little more complicated, which may be explained by the three contradictory factors: the higher energy impurities adsorbed, the surface defects and the local environment of activator ions. Firstly, the higher energy contaminants, i.e. OH and CO2 3 , could be readily absorbed onto the sample surface, consequently reducing the upconverting intensity [25]. As demonstrated in the FT-IR spectra above, the quenching centers, i.e. hydroxyl and
1 For interpretation of color in Figs. 4–6, the reader is referred to the web version of this article.
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Fig. 3. FT-IR spectra of the products after combustion and calcined at various temperatures, (a) after combustion, (b) 600 °C, (c) 700 °C, (d) 800 °C, (e) 900 °C and (f) 1000 °C.
Fig. 4. The effect of the calcining temperatures on upconverting spectra of (Y0.915Ho0.01Yb0.075)2Ti2O7 NPs (under the 980 nm excitation with the pump power of 610 mW).
carbonate ion, decrease with calcining temperatures from 600 to 800 °C, then keep constant essentially. Secondly, the surface defects act also as nonradiative transition centers, and the upconverting intensity may be influenced by the grain size (due to the different surface-to-volume ratios) [26,27]. As aforementioned in XRD and TEM results, the increase of the average particle size of (Y0.915Ho0.01Yb0.075)2Ti2O7 nanophosphors prepared at 800, 900, and 1000 °C implies that the surface defects are reduced with the rise of calcining temperature, thereby leading to the increase of luminescence intensity [26]. Thirdly but very importantly, however, the local symmetry of Ho3+ lattice sites can also be promoted with the increase of calcining temperature, therein resulting in the reduction of luminescence intensity [28]. Taking these factors above into consideration, the first reason may be predominant to account for the effect of calcining temperatures on the upconverting intensity. It is proposed that, therefore, to reduce the hydroxyl and carbonate groups absorbed on the host and to modify the local symmetry of crystalline field of rare earth ions may improve the upconverting efficiency of nanophosphors greatly. Fig. 5 shows the effect of Yb3+ ion doping concentrations on the upconverting properties of (Y0.99xHo0.01Ybx)2Ti2O7 NPs calcined at 800 °C for 1.0 h. Apparently, the Yb3+ ion doping level has a great
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Fig. 5. The effect of the Yb3+ ion doping concentrations on upconverting spectra of (Y0.99xHo0.01Ybx)2Ti2O7 NPs calcined at 800 °C for 1.0 h (under the 980 nm excitation with the pump power of 610 mW).
Fig. 6. Enhancement factors of the green, red, total upconverting emissions and Ired/Igreen of (Y0.99xHo0.01Ybx)2Ti2O7 NPs as a function of Yb3+ concentrations (the integrated intensity of Yb3+-free sample is scaled as unit, under the 980 nm excitation). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
influence on the intensities of upconverting spectra. As for Yb3+-free samples, the upconverting emissions are very weak. This is due to the fact of the spectra mismatch between the energy level of Ho3+ ions and the 980 nm excitation although Ho3+ ions can be excited by the 980 nm diode laser directly. The upconverting efficiency of uni-doped Y2Ti2O7:Ho3+ nanophosphors is rather low. By co-doping Yb3+ ions, however, bright upconverting emissions could be observed even by naked eyes, which confirms that there exists the efficient energy transfer from Yb3+ to Ho3+. According to Fig. 5, the integral intensity of upconverting green, red and overall emissions is normalized, respectively, and the effect of the green, red, total upconverting emission and the Ired/Igreen as a function of Yb3+ ion doping concentrations can be obtained, as shown in Fig. 6. Obviously, the green, red, total UC emission share the same trend with the Yb3+ ion doping level: the emission intensity increases drastically with increasing Yb3+ ions, up to a critical value of 7.5 mol%, and then decreases for more Yb3+ ions, i.e. the so-called concentration quenching effect takes place. This suggests that there exists an optimal Yb3+ ion concentration that gives a maximum emission intensity in (Y0.99xHo0.01Ybx)2Ti2O7 nanophosphors.
As it well known, Yb3+ ion has a larger absorption cross section (11.7 1021 cm2) at 980 nm excitation [15]. Within a certain range of Yb3+ ion doping concentration, therefore, the increase of Yb3+ ions favors the absorption of pumping photons, and then facilitates the efficient resonant energy transfer from Yb3+ to Ho3+ ions, thus improving the efficiency of upconverting emissions. Too higher Yb3+ ions, however, may increase the odds of the back energy transfer Ho3+ ? Yb3+ [29,30], and on the other hand, it is possible that the pairing or aggregation of Yb3+ ions could come into being, which resultantly may turn these ions into new quenching centers [31]. As a consequence, the Yb3+ ions at increased doping level may act as trapping centers and induce nonradiative relaxation, herein reducing the upconverting efficiency. From Fig. 6, it is also found that at Yb3+ ions of 7.5 mol%, the intensities of green, red, total upconverting emission is enhanced by 32 folds compared to Yb3+-free samples, which evidences the important role of the sensitizer Yb3+ ions in the upconverting process. In addition, the intensity ratios (Ired/Igreen) of the red to green emission keep almost constant as the Yb3+ ion concentration increases from 0 to 12.5 mol%, suggesting that Yb3+ ions have a simultaneous influence on the (5F4, 5S2) ? 5I8 and 5F5 ? 5I8 radiative transitions of Ho3+ ions. To get a better understanding of the possible upconverting mechanism involved in Y2Ti2O7:Ho3+,Yb3+ NPs, the pump power dependence of the integral intensity of upconverting emissions is plotted in Fig. 7. It has been widely recognized that, for nearly any unsaturated upconverting process, the slope of the doublelogarithmic plots of the integral intensity versus pump power is the number of NIR photons absorbed per visible photon emitted [32]. At relatively low pump powers, the slopes determined through the linear fitting are 2.15 ± 0.02 and 2.18 ± 0.02 for the green ((5F4, 5S2) ? 5I8) and red (5F5 ? 5I8) transitions, respectively, indicating the involvement of two photons excited to (5F4, 5S2) and 5 F5 levels. This is consistent with other upconverting system, such as Y3Al5O12:Ho3+,Yb3+ and NaYF4:Ho3+,Yb3+ NPs [23,24,33]. When the pump power increases to a relatively high region, however, the so-called ‘‘saturation effect’’ exists for both the red and green emissions, i.e. the intensity decreases dramatically instead with the increase of the pump power. This phenomena can be attributed to thermal quenching caused by the absorption of the 980 nm light when excited under too high pump power [34–36]. Fig. 8 shows the energy level diagrams of Ho3+ and Yb3+ ions, as well as the proposed upconverting processes under the laser excitation of 980 nm. Firstly, the electrons of Yb3+ ions absorbs one pump photon, and are excited from the ground state 4F7/2 to the level 4F5/2 due to its lager cross sections, and then the energy
Fig. 7. The pump power dependence of the red and green emissions of (Y0.915Ho0.01Yb0.075)2Ti2O7 NPs calcined at 800 °C for 1.0 h (under the 980 nm excitation). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Z. Chen et al. / Journal of Alloys and Compounds 608 (2014) 165–169
169
optimum doping level of Yb3+ ions is 7.5 mol%, and the intensity of upconverting emissions for (Y0.915Ho0.01Yb0.075)2Ti2O7 NPs is increased by 32 times compared to the Yb3+-free samples. The quadratic pump power dependence of the green and red intensities indicates two photons are responsible for the upconverting process under low pump power, and the potential upconverting mechanisms have been proposed. Acknowledgments
Fig. 8. Energy level diagrams of Ho3+ and Yb3+ ion as well as the proposed upconverting mechanisms of Y2Ti2O7:Ho3+,Yb3+ NPs.
of the excited electrons is transferred to the neighboring electrons of Ho3+ ions at the ground state 5I8 (ETU1 in Fig. 8), thereby bringing about the population of the intermediate excited state 5I6 with the excess energy consumed by the host lattice vibration. Then, most Ho3+ ions at the state 5I6 are pumped to the levels (5F4, 5S2) via excited state absorption (ESA) and energy transfer (ETU2). Lastly, the green luminescence is observed via the radiative transitions from (5F4, 5S2) to 5I8, which is also depicted in detail as follows: 3þ
3þ
4
F 7=2 ðYb Þ þ hm980 ! 4 F 5=2 ðYb Þ ðGSAÞ
5
3þ
I8 ðHo3þ Þ þ 4 F 5=2 ðYb Þ ! 5 I6 ðHo3þ Þ þ 4 F 7=2 ðYb Þ ðETU1Þ
5
I6 ðHo3þ Þ þ 4 F 5=2 ðYb Þ ! ð5 F 4 ; 5 S2 ÞðHo3þ Þ þ 4 F 7=2 ðYb Þ ðETU2Þ
5
3þ
3þ
3þ
I6 ðHo3þ Þ þ hm980 ! ð5 F 4 ; 5 S2 ÞðHo3þ Þ ðESAÞ
ð F 4 ; 5 S2 ÞðHo3þ Þ ! 5 I8 ðHo3þ Þ þ hmgreen 5
ðRTÞ
In the case of the red emission, the two possible approaches are responsible for the population of Ho3+ ions at the 5F5 level. On one hand, the excited electrons at the states (5F4, 5S2) decay nonradiatively to the level of 5F5 with the help of multi-phonon relaxation (MPR). On the other hand, a portion of electrons of Ho3+ ions at the intermediate excited state 5I6 decay to the level 5I7 via nonradiative transition (NRT), and then giving rise to the population of the state 5F5 through the energy transfer (ETU3) or excited-state absorption (ESA). As a result, the red emission around 660 nm is observed by the transition from the level 5F5 to 5I8, which can be described as follows. 4
3þ
3þ
F 7=2 ðYb Þ þ hm980 ! 4 F 5=2 ðYb Þ ðGSAÞ 5
3þ
3þ
3þ
I8 ðHo Þ þ 4 F 5=2 ðYb Þ ! 5 I6 ðHo3þ Þ þ 4 F 7=2 ðYb Þ ðETU1Þ
ð5 F 4 ; 5 S2 ÞðHo3þ Þ ! 5 F 5 ðHo3þ Þ ðMPRÞ 5
I6 ðHo3þ Þ ! 5 I7 ðHo3þ Þ ðMPRÞ
5
I7 ðHo3þ Þ þ 4 F 5=2 ðYb Þ ! 5 F 5 ðHo3þ Þ þ 4 F 7=2 ðYb Þ ðETU3Þ
3þ
3þ
3þ
3þ
5
I7 ðHo Þ þ hm980 ! 5 F 5 ðHo Þ ðESAÞ
5
F 5 ðHo3þ Þ ! 5 I8 ðHo3þ Þ þ hmred
ðRTÞ
4. Conclusions In summary, a variety of Yb3+, Ho3+ co-doped Y2Ti2O7 NPs were prepared via the glycine-combustion approach. Under the 980 nm excitation, the bright (540 nm) and red (660 nm) emissions are observed, which is attributed to the (5F4,5S2) ? 5I8 and 5F5 ? 5I8 transitions of Ho3+ ions, respectively. The calcining temperatures and Yb3+ ion doping level have a great influence on the upconverting spectra of (Y0.99xHo0.01Ybx)2Ti2O7 NPs. The intensities of emissions increase initially and then decrease after 800 °C. The
This work is supported by Natural Science Foundation of China (Nos. 51362002, 11375043, 51171069 and 51062008), Science & Technology Pillar Program from Jiangxi Provincial Department of Science and Technology(No. 20122BBG70084-2), and the financial supports from Key Laboratory of Radioactive Geology and Exploration Technology Fundamental Science for National Defence (No. 2010RGET15) and State Key Laboratory Breeding Base of Nuclear Resources and Environment (No. 060614 and NRE1319) are also greatly appreciated. References [1] L. Cheng, C. Wang, Z. Liu, Nanoscale 5 (2013) 23–37. [2] K.E. Sapsford, W.R. Algar, L. Berti, K.B. Gemmill, B.J. Casey, E. Oh, M.H. Stewart, I.L. Medintz, Chem. Rev. 113 (2013) 1904–2074. [3] S. Zhou, S. Jiang, X. Wei, Y. Chen, C. Duan, M. Yin, J. Alloys Comp. 588 (2014) 654–657. [4] X. Huang, S. Han, W. Huang, X. Liu, Chem. Soc. Rev. 42 (2013) 173–201. [5] Q. Liu, W. Feng, F. Li, Coord. Chem. Rev. (2014), http://dx.doi.org/10.1016/ j.ccr.2014.01.004. [6] D. Gao, X. Zhang, H. Zheng, W. Gao, E. He, J. Alloys Comp. 554 (2013) 395–399. [7] C. Li, Y. Mei, J. Xie, W. Dai, G. Du, Z. Li, J. Alloys Comp. 560 (2013) 10–14. [8] Y. Li, S. Xu, J. Alloys Comp. 601 (2014) 195–200. [9] L. Li, H. Lin, X. Zhao, Y. Wang, X. Zhou, C. Ma, X. Wei, J. Alloys Comp. 586 (2014) 555–560. [10] Z.S. Chen, W.P. Gong, T.F. Chen, S.L. Li, D.Y. Wang, Q.K. Wang, Mater. Lett. 68 (2012) 137–139. [11] X. Li, H. Cai, L. Ding, X. Dou, W. Zhang, J. Alloys Comp. 541 (2012) 36–40. [12] C.C. Ting, Y.C. Chien, W.F. Sung, ECS J. Solid State Sci. Technol. 2 (2013) R105– R110. [13] B.P. Singh, A.K. Parchur, R.K. Singh, A.A. Ansari, P. Singh, S.B. Rai, Phys. Chem. Chem. Phys. 15 (2013) 3480–3489. [14] B.S. Cao, J.L. Wu, N.S. Yu, Z.Q. Feng, B. Dong, Thin Solid Films 550 (2014) 495– 498. [15] F. Vetrone, J.C. Boyer, J.A. Capobianco, A. Speghini, M. Bettinelli, J. Phys. Chem. B 107 (2003) 1107–1112. [16] Y. Zhang, M. Wang, Z. Le, G. Huang, L. Zou, Z. Chen, Ceram. Int. 40 (2014) 5223– 5230. [17] H.P. Klug, L.E. Alexander, X-Ray Powder Diffraction Procedures, John Wiley & Sons, New York, 1959. [18] L. Brixner, Inorg. Chem. 3 (1964) 1065–1067. [19] P.A. Tanner, L.S. Fu, B.M. Cheng, J. Phys. Chem. C 113 (2009) 10773–10779. [20] H. Palneedi, V. Mangam, S. Das, K. Das, J. Alloys Comp. 509 (2011) 9912–9918. [21] K. Koteswara Rao, T. Banu, M. Vithal, G.Y.S.K. Swamy, K. Ravi Kumar, Mater. Lett. 54 (2002) 205–210. [22] L.Q. An, J. Zhang, M. Liu, S.W. Wang, J. Am. Ceram. Soc. 88 (2005) 1010–1012. [23] M. Liu, S.W. Wang, D.Y. Tang, L.D. Chen, J. Ma, J. Rare Earth 27 (2009) 66–70. [24] X.P. Chen, W.J. Zhang, Q.Y. Zhang, Physica B 406 (2011) 1248–1252. [25] G.J. De, W.P. Qin, J.S. Zhang, Y. Wang, C.Y. Cao, Y. Cui, Solid State Commun. 137 (2006) 483–487. [26] M.L. Bailly, G. Costentin, H. Lauron-Pernot, J.M. Krafft, M. Che, J. Phys. Chem. B 109 (2005) 2404–2413. [27] T. Moon, S.T. Hwang, D.R. Jung, D. Son, C. Kim, J. Kim, M. Kang, B. Park, J. Phys. Chem. C 111 (2007) 4164–4167. [28] C.C. Ting, Y.S. Chiu, C.W. Chang, L.C. Chuang, J. Solid State Chem. 184 (2011) 563–571. [29] L. Zhang, H. Hu, C. Qi, F. Lin, Opt. Mater. 17 (2001) 371–377. [30] F. Auzel, G. Baldacchini, L. Laversenne, G. Boulon, Opt. Mater. 24 (2003) 103– 109. [31] J.L. Wagener, P.F. Wysocki, M.J.F. Digonnet, H.J. Shaw, D.J. Digiovanni, Opt. Lett. 18 (1993) 2014–2016. [32] M. Pollnau, D.R. Gamelin, S.R. Luthi, H.U. Gudel, M.P. Hehlen, Phys. Rev. B 61 (2000) 3337–3346. [33] Y. Bai, Y. Wang, G. Peng, K. Yang, X. Zhang, Y. Song, J. Alloys Comp. 478 (2009) 676–678. [34] Y.Q. Lei, H.W. Song, L.M. Yang, L.X. Yu, Z.X. Liu, G.H. Pan, X. Bai, L.B. Fan, J. Chem. Phys. 123 (2005) 174710. [35] A.H. Li, Q. Lü, EPL 96 (2011) 18001. [36] J. Petit, B. Viana, P. Goldner, Opt. Express 19 (2011) 1138–1146.