Article pubs.acs.org/JPCA
Effects of Postannealing on the Photoluminescence Properties of Coprecipitated Nanocrystalline BaFCl:Sm3+ Zhiqiang Liu,† Marion A. Stevens-Kalceff,‡ and Hans Riesen*,† †
School of Physical, Environmental and Mathematical Sciences, The University of New South Wales, UNSW Canberra (ADFA), Canberra, ACT 2600, Australia ‡ School of Physics and Electron Microscope Unit, Mark Wainwright Analytical Centre, The University of New South Wales, Sydney, NSW 2052, Australia ABSTRACT: Nanocrystalline BaFCl:Sm3+, as prepared by coprecipitation from aqueous solutions, is an efficient photoluminescent X-ray storage phosphor. In the present study, we report effects on its photoluminescence properties resulting from postannealing treatment in air in the temperature range between 100 to 900 °C. Interestingly, upon annealing at temperatures from 200 to 600 °C in air, a small fraction of the Sm3+ ions in nanocrystalline BaFCl can be reduced to Sm2+ ions. In addition to the creation of Sm2+ ions, two different sites of Sm3+ ions, denoted as sites A and B, are observed when the nanocrystalline BaFCl:Sm3+ is annealed between 500 to 900 °C. The temperature dependence of photoluminescence properties of the two different sites in the 500 °C annealed sample reveals that the Sm3+ ions at site A are possibly located at or near the crystallite surface, whereas site B is situated in a very ordered environment. To the best of our knowledge, this is the first report on the reduction of Sm3+ ions doped in alkaline-earth fluorohalides to Sm2+ ions by annealing in air.
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photoluminescence properties of nanocrystalline BaFCl:Sm3+ X-ray storage phosphor, prepared by coprecipitation. In particular, this is the first report on the observation of the reduction of Sm3+ ions doped in alkaline-earth fluorohalides to Sm2+ ions by annealing in air. These results are of significance for a better understanding of the storage mechanism, which, in turn, may lead to the design of other samarium(III) activated materials with even higher X-ray storage efficiencies.
INTRODUCTION Considerable attention has been paid to the spectroscopic properties of samarium doped alkaline-earth fluorohalides MeFX (Me = Ca, Sr, and Ba; X = Cl, Br, and I) since the first observation of photon-gated spectral hole-burning in BaFCl:Sm2+ by Winnacker et al.1 We have recently reported that nanocrystalline BaFCl:Sm3+ can also serve as a very efficient photoluminescent storage phosphor for ionizing radiation, based on the reduction of Sm3+ to Sm2+ upon exposure to ionizing radiation.2−4 Samarium ions can be stable in the trivalent (Sm3+) and divalent oxidation states (Sm2+). The reduction of Sm3+ to Sm2+ ions in the solid state has been realized by several methods such as annealing in H2/N2 and X-ray and γ-ray irradiation in a variety of crystal and glass hosts such as alkali earth fluorohalides,5 alkaline earth metal borophosphates,6 silicate glasses,7 and sodium aluminoborate glasses.8 Interestingly, it was reported that the reduction of Sm3+ to Sm2+ in some hosts can be realized by annealing in air, in particular in many alkaline earth borate hosts, such as in BaB8O13,9 SrB4O7,10−12 SrB6O10,13,14 CaSO4,15 and Li2O-SrO-B2O3 glass-ceramics.16 In these hosts, it is believed that the rigid three-dimensional network of BO4 tetrahedra plays an important role in the reduction of Sm3+ ions.12 Mikhail et al.11 proposed that the reduction of Sm3+ to Sm2+ ions in SrB4O7 occurs via the crystallization of the host matrix during the annealing process. In the present article, we report the effects of postannealing in air at temperatures from 100 to 900 °C on the © 2013 American Chemical Society
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EXPERIMENTAL SECTION ACS reagent grade chemicals were used without further purification. The nanocrystalline BaFCl:Sm3+ was prepared by a coprecipitation method17 and then annealed in an open ceramic crucible in a muffle furnace. The atmosphere in the muffle furnace was not controlled. The photoluminescence spectra of the annealed samples were measured by a Horiba Jobin-Yvon Spex Fluoromax-3 fluorometer at 293 K. The X-ray irradiation of the samples was conducted by exposure in a powder X-ray diffractometer (40 kV, 25 mA, Cu Kα), and the radiation dose was cross-calibrated against a dental X-ray unit (Belmont Searcher model DX-068). For the temperature dependence of the photoluminescence spectra, the annealed nanocrystalline BaFCl:Sm3+ samples were mounted on the coldfinger of a closed-cycle cryostat (CCR, Janis/Sumitomo SHI-4.5) and excited with a focused 405 nm Received: November 30, 2012 Revised: February 3, 2013 Published: February 6, 2013 1930
dx.doi.org/10.1021/jp3118105 | J. Phys. Chem. A 2013, 117, 1930−1934
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Figure 1. Room-temperature photoluminescence spectra of coprecipitated nanocrystalline BaFCl:Sm3+ after annealing at different temperatures from 100 to 900 °C for 1 h in air. Major 4GJ−6HJ (Sm3+) and 5DJ−7FJ (Sm2+) transitions are denoted. The spectra were excited at 401 nm. The emission lines originating from the Sm3+ and Sm2+ transitions are labeled as 3 and 2, respectively, in part b. The spectra are offset on the vertical axis for clarity.
defects in the host lattice. In the as-prepared nanocrystalline BaFCl:Sm3+, the Sm3+ ions enter the host lattice by replacing Ba2+ ions. Because of the differences in the ionic radii of the Ba and Sm ions and the requirement of charge compensation, the presence of various defects around the Sm3+ ions, such as oxygen defects and F or Cl vacancies in the BaFCl host lattice, can be expected. Oxygen is a common contaminant in alkaline earth fluorohalides and is often introduced into the host lattice during the preparation procedure as the oxygen defects (O2−) can substitute for halide anions.20 Optical spectroscopy of oxygen defects in BaFCl crystals has established that there are two different kinds of O2− centers (types I and II).21−23 Type I oxygen defects occupy the F− sites and neighboring Cl− vacancies (OF2−−Cl−), and type II centers completely substitute for the Cl sublattice (OCl2−−Cl−). We have conducted preliminary electron spin resonance experiments on nanocrystalline BaFCl:Sm3+, and they are in accord with the presence of oxygen defects in the BaFCl host lattice. It is possible that during the annealing process electrons from the oxygen defects that are already built into the host lattice are becoming available to reduce the Sm3+ ions, leading to the creation of Sm2+ ions. In addition, the oxygen defects could also be incorporated into the BaFCl host lattice during the annealing process considering that oxygen contamination is very hard to avoid during the heat treatment in air. In addition to the reduction of Sm3+ to Sm2+ ions, another significant effect of the postannealing treatment on photoluminescence properties of nanocrystalline BaFCl:Sm3+ is observed for the spectrum of the Sm3+ ions. As is seen in Figure 1 in the photoluminescence spectrum of the sample annealed at 500 °C, in addition to the four broad emission lines at 559, 594, 639, and 701 nm originating from Sm3+ ions and the narrow lines of the 5DJ−7FJ transitions of Sm2+ ions, a new group of emission lines, at 568, 575, 605, 612, 626, 652, 659, and 712 nm, originating from the 4GJ−6HJ transitions of Sm3+ ions can be observed. These new emission lines of Sm3+ become the dominant feature in the photoluminescence spectra of the samples annealed at 600 and 700 °C. For the discussion below, the emission lines at 559, 594, 639, and 701 nm are denoted as group A and those at 568, 575, 605, 612, 626, 652, 659, and 712 nm as group B. As is illustrated in Figure 1,
blue−violet laser diode. The laser emission was collimated and then focused onto the entry slit of a Spex 1404 monochromator (1200 grooves/mm holographic grating) using 75 and 200 mm lenses. The light was modulated with a chopper and detected by a photomultiplier (Hamamatsu R928). The signal was processed by a current-to-voltage preamplifier (Femto DLPCA200) and a lock-in amplifier (Stanford Research System SR810 DSP) before being collected on a PC.
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RESULTS AND DISCUSSION Effect of Annealing Temperature on the Photoluminescence Properties of Nanocrystalline BaFCl:Sm3+. Figure 1 displays the room temperature photoluminescence spectra of nanocrystalline BaFCl:Sm3+ annealed at temperatures between 100 and 900 °C in air. It appears that the photoluminescence spectrum for samples annealed at 100 °C presents mainly four broad emission lines, at 559, 594, 639, and 701 nm, that can be assigned to the 4GJ−6HJ transitions of Sm3+ ions.18 These lines are identical to those observed for the as-prepared nanocrystalline BaFCl:Sm3+ without annealing at elevated temperatures.5 After annealing at 200 °C, narrow peaks at 687 and 727 nm due to the 5D0−7F0 and 5D0−7F2 transitions of Sm2+ ions,19 respectively, can be observed. This clearly implies the reduction of Sm3+ to Sm2+ ions in nanocrystalline BaFCl by annealing in air. With increasing annealing temperature above 200 °C, the photoluminescence intensities of the Sm2+ emission lines at 687 and 727 nm increase and reach a maximum when the sample is annealed at 500 °C. The photoluminescence intensities of the two peaks of Sm2+ ions decrease when the annealing temperature is raised to 600 °C, and finally, for the samples annealed at temperatures at 700 °C or above, no emission lines associated with Sm2+ ions can be detected. It is important to note that the photoluminescence intensities of the Sm2+ and Sm3+ ions cannot be used directly to compare the concentrations of the two oxidation states since the Sm2+ ions are excited via a parityallowed f−d transition, whereas the Sm3+ ions are excited by a parity-forbidden f−f transition at the excitation wavelength of 401 nm. The reduction of Sm3+ ions in nanocrystalline BaFCl:Sm3+ by annealing in air can be explained by the presence of various 1931
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Figure 2. (a) Temperature dependence of the nonselectively excited photoluminescence spectra in the region of the 4G5/2−6H7/2 transition of Sm3+ ions in nanocrystalline BaFCl:Sm3+, annealed at 500 °C. The emission lines at 594 and 605 nm that are due to sites A and B, respectively, are denoted. The spectra are offset on the vertical axis for clarity. The spectra were excited with a 405 nm blue−violet laser diode. (b) The temperature dependence of the line width of the 4G5/2−6H7/2 transition of Sm3+ ions at 595 nm (site A) and 605 nm (site B) as evaluated from the spectra shown in part a. The data were fitted with a power function with exponents of 1.2 and 2.3 for the 594 and 605 nm lines, respectively.
Figure 3. (a) Room-temperature photoluminescence spectra of nanocrystalline BaFCl:Sm3+ as a function of annealing time at 500 °C. The arrows indicate the photoluminescence intensity changes with the increasing annealing time from 600 to 15 000 s. The spectra were excited at 401 nm. Major 4GJ−6HJ (Sm3+) and 5DJ−7FJ (Sm2+) transitions are denoted. The emission lines originating from the Sm3+ and Sm2+ transitions are labeled as 3 and 2, respectively. (b) Photoluminescence intensities of Sm3+ ions at 594 nm (site A) and 605 nm (site B) and Sm2+ ions at 686 nm as a function of annealing time. The data were normalized to the intensities observed for annealing at 500 °C for 600 s.
compared with the group A (site A) emission lines of Sm3+ ions, the group B (site B) lines are much narrower and redshifted by approximately 10 nm. It thus follows that the two groups of emission lines can be attributed to two types of Sm3+ sites with significantly different local coordination environments. With increasing annealing temperature from 600 to 900 °C, the photoluminescence intensities of the group B emission lines decrease. To gain some understanding of the different local coordination environments of sites A and B of the Sm3+ ions, the photoluminescence spectrum of the 500 °C annealed nanocrystalline BaFCl:Sm3+ was measured in the temperature range from 2 to 280 K using a 405 nm blue−violet laser diode as the excitation source. Figure 2 shows some photoluminescence spectra in the region of the 4G5/2− 6H 7/2 transition of Sm3+ ions from 2 to 280 K. At 2 K, narrow
emission lines at 594 and 605 nm that correspond to the G5/2−6H7/2 transition of Sm3+ ions at sites A and B, respectively, are observed. With increasing temperature, the 4 G5/2−6H7/2 emission line at 594 nm due to site A becomes much broader in comparison with that at 605 nm line due to site B. The temperature dependence of the two linewidths is summarized in Figure 2 (b). Whereas the line width for both sites is dominated by inhomogeneous broadening at 2 K, at 280 K, the homogeneous broadening is dominant for site A, but inhomogeneous broadening is still a major contributor to the line width of site B. It follows that, at higher temperatures, the Sm3+ ions in site A are subject to significantly stronger electron−phonon coupling, which is frozen out at 2 K, than the ions in site B. Our previous photoluminescence spectroscopy and cathodoluminescence microanalysis investigations4,24 in4
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Figure 4. (a) Room-temperature photoluminescence spectra of X-irradiated nanocrystalline BaFCl:Sm3+ after annealing for 1 h at temperatures between 100 and 600 °C. The spectra were excited at 415 nm. The sample was irradiated with 600 Gy of X-rays (40 kV, 25 mA, Cu anode). For clarity, the spectra of the X-irradiated samples annealed from 100 to 500 °C are plotted using solid lines and offset on the left vertical axis, whereas the spectrum of the sample annealed at 600 °C is shown as a dashed line and scaled on the right vertical axis. The emission lines originating from the Sm3+ and Sm2+ transitions are labeled as 3 and 2, respectively. Major 4GJ−6HJ (Sm3+) and 5DJ−7FJ (Sm2+) transitions are denoted. (b) Annealing temperature dependence of the normalized photoluminescence intensity of Sm3+ ions at 594 nm (site A) and 605 nm (site B) and Sm2+ ions at 686 nm in X-irradiated nanocrystalline BaFCl:Sm3+.
dicated that the Sm3+ ions at site A are located at or near the crystallite surface; the electron−phonon coupling is therefore governed by the interactions between the f-electrons and pronounced surface modes. In contrast, the emission line at 605 nm from Sm3+ ions in site B is much less dependent on the temperature, which may indicate that the ions are more embedded within the host lattice; this observation may also indicate that there is a mismatch of lattice modes and the local vibrational modes of the Sm3+ ion, i.e., a different coordination environment that does not allow coupling to lattice or surface modes. Considering that the oxygen defects play an important role in the annealing process, it is possible that samarium oxyhalides (where the oxygen defects lie in close proximity to the Sm 3+ ions) are formed when the nanocrystalline BaFCl:Sm3+ is annealed at the temperature range of 600 to 700 °C, and account for site B of the Sm3+ ions. Effect of Annealing Time on the Photoluminescence Properties of Nanocrystalline BaFCl:Sm3+. To further investigate the effect of postannealing, the photoluminescence spectra of nanocrystalline BaFCl:Sm3+ were measured upon annealing for different time intervals at 500 °C. The results are shown in Figure 3, which shows that with increasing annealing time at 500 °C, the photoluminescence intensity at 605 nm due to the 4G5/2−6H7/2 transition of Sm3+ ions in the ordered environment of site B as well as that at 686 nm from the 5 D0−7F0 transition of the Sm2+ ions increase, while the intensity at 594 nm originating from the 4G5/2-6H7/2 transition of Sm3+ ions in the near surface site A decreases. It is possible that, upon annealing at 500 °C, both the reduction of Sm3+ (at site A) to Sm2+ ions and the oxidation of Sm2+ to Sm3+ ions (at site B) occur in nanocrystalline BaFCl:Sm3+, i.e., the creation of Sm2+ ions by annealing treatment appears to be correlated with the conversion of Sm3+ ions from site A to site B. Thermal Bleaching of the Sm2+ Ions Generated by Xrays in Nanocrystalline BaFCl:Sm3+. The thermal stability of the Sm2+ ions generated by X-rays in nanocrystalline BaFCl:Sm3+ was studied since this is an important property for the application of the storage phosphor in dosimetry. The
as-prepared nanocrystalline BaFCl:Sm3+ was irradiated with 600 Gy of X-rays and subsequently annealed at temperatures between 100 to 600 °C. The results are illustrated in Figure 4. From Figure 4a, it follows that the photoluminescence spectrum of the X-irradiated sample without any annealing treatment presents mainly emission lines at 629, 640, 663, 686, 700, and 727 nm due to the 5DJ−7FJ transitions of Sm2+ ions.19 Similar photoluminescence spectra are also observed for the Xirradiated nanocrystalline BaFCl:Sm3+ after annealing at temperatures from 100 to 500 °C. However, a significantly different photoluminescence spectrum is obtained for the Xirradiated sample that is annealed at 600 °C. For this annealing temperature, in addition to the 5DJ−7FJ emission lines of Sm2+ ions, lines at 568, 575, 605, 612, 626, 652, and 659 nm originating from Sm3+ ions in site B are dominant. Figure 4b summarizes the integrated intensities of emission lines of Sm3+ in sites A and B and Sm2+ as a function of increasing annealing temperature from 100 to 600 °C. As illustrated in Figure 4b, both the photoluminescence intensities at 686 nm from the Xray generated Sm2+ ions and 594 nm due to the Sm3+ ions in site A decrease, whereas the intensity at 605 nm from Sm3+ ions in site B increases. It follows that, by annealing at 600 °C in air, the X-irradiation generated Sm2+ ions in nanocrystalline BaFCl:Sm3+ are oxidized to Sm3+ ions in site B. The oxidation of X-irradiation generated Sm2+ ions by heat treatment in air has also been observed in Sm3+ ion doped SrB6O10,10 BaBPO5:Sm3+,25 LiBaB9O15,26 and Al2O3−SiO2 glasses.7 During X-irradiation, electron−hole pairs are created. It is possible that, upon heat treatment, the holes created by Xirradiation in the host lattice are released and captured by the Sm2+ ions, resulting in the formation of Sm3+ ions.
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CONCLUSIONS The effects of postannealing in air at temperatures between 100 and 900 °C on the photoluminescence properties of nanocrystalline BaFCl:Sm3+ X-ray storage phosphor, prepared by coprecipitation, have been investigated. By annealing at temperatures from 200 to 600 °C in air, some of the Sm3+ 1933
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(12) Pei, Z.; Su, Q.; Zhang, J. The Valence Change From RE3+ to RE2+ (RE = Eu, Sm, Yb) in SrB4O7: RE Prepared in Air and the Spectral Properties of RE2+. J. Alloys Compd. 1993, 198, 51−53. (13) Zeng, Q.; Pei, Z.; Wang, S.; Su, Q. The Reduction of RE3+ in SrB6O10 Prepared in Air and the Luminescence of SrB6O10:RE2+ (Re = Eu, Sm, Tm). Spectrosc. Lett. 1999, 32, 895−912. (14) Zeng, Q.; Pei, Z.; Wang, S.; Su, Q.; Lu, S. Luminescent Properties of Divalent Samarium-Doped Strontium Hexaborate. Chem. Mater. 1999, 11, 605−611. (15) Lapraz, D.; Prévost, H.; Angellier, G.; Mady, F.; Benabdesselam, M.; Dusseau, L. Photoluminescence and Thermally Stimulated Luminescence Properties of CaSO4:Sm3+ and CaSO4:Ce3+, Sm3+; Sm3+ → Sm2+ Conversion and High Temperature Dosimetry. Phys. Status Solidi A 2007, 204, 4281−4287. (16) Jiang, C.; Huang, Y.; Park, S.; Jang, K.; Seo, H. J. Luminescence and Spectral Hole Burning of Sm2+ Doped in Li2O-SrO-B2O3 GlassCeramics. Spectrochim. Acta, Part A 2009, 72, 412−416. (17) Riesen, H.; Kaczmarek, W. A. Efficient X-Ray Generation of Sm2+ in Nanocrystalline BaFCl:Sm3+: A Photoluminescent X-Ray Storage Phosphor. Inorg. Chem. 2007, 46, 7235−7237. (18) Huang, Y.; Wang, J.; Seo, H. J. The Irradiation Induced Valence Changes and the Luminescence Properties of Samarium Ions in Ba2SiO4. J. Electrochem. Soc. 2010, 157, J429−J434. (19) Gacon, J. C.; Grenet, G.; Souillat, J. C.; Kibler, M. Experimental and Calculated Energy Levels of Sm2+:BaFCl. J. Chem. Phys. 1978, 69, 868−880. (20) Falin, M.; Bill, H.; Lovy, D. EPR of Sm3+ in BaFCl Single Crystals. J. Phys.: Condens. Matter 2004, 16, 1293−1298. (21) Eachus, R. S.; McDugle, W. G.; Nuttall, R. H. D.; Olm, M. T.; Koschnick, F. K.; Hangleiter, T.; Spaeth, J. M. Radiation-Produced Electron and Hole Centres in Oxygen-Containing BaFBr: I. EPR and ODEPR Studies. J. Phys.: Condens. Matter 1991, 3, 9327−9338. (22) Eachus, R. S.; Nuttall, R. H. D.; Olm, M. T.; McDugle, W. G.; Koschnick, F. K.; Hangleiter, T.; Spaeth, J. M. Oxygen Defects in BaFBr and BaFCl. Phys. Rev. B 1995, 52, 3941−3950. (23) Radzhabov, E.; Otroshok, V. Optical Spectra of Oxygen Defects in BaFCl and BaFBr Crystals. J. Phys. Chem. Solids 1995, 56, 1−7. (24) Stevens-Kalceff, M. A.; Liu, Z.; Riesen, H. Cathodoluminescence Microanalysis of Irradiated Microcrystalline and Nanocrystalline Samarium Doped BaFCl. Microsc. Microanal. 2012, 18, 1229−1238. (25) Huang, Y.; Zhao, W.; Shi, L.; Seo, H. Structural Defects and Luminescence Properties of Sm2+ Ions Doped in BaBPO5 Phosphor by X-Ray Irradiation. J. Alloys Compd. 2009, 477, 936−940. (26) Wang, J.; Li, Y.; Huang, Y.; Shi, L.; Seo, H. J. The Luminescence and Stabilities of Sm2+ Ions Doped in LiBaB9O15. Mater. Chem. Phys. 2010, 120, 598−602.
ions are reduced to Sm2+ ions due to the capture of electrons from oxygen defects at Sm3+ ions in the host lattice. The oxygen defects are introduced into the BaFCl host lattice during the coprecipitation and postannealing processes. This is the first report on the reduction of Sm3+ ions doped in alkalineearth fluorohalides to Sm2+ ions by postannealing treatment in air. Furthermore, the reduction of Sm3+ to Sm2+ ions upon annealing in nanocrystalline BaFCl:Sm3+ is associated with a change in the local environment of Sm3+ ions between the two different sites, i.e., sites A and B. On the basis of the different temperature dependence of photoluminescence properties of the two sites, it follows that the Sm3+ ion at site A is located near the crystallite surface, whereas site B appears to be well embedded in the crystal lattice with a significantly different coordination environment.
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AUTHOR INFORMATION
Corresponding Author
*Tel: +61 (0)2 6268 8679. Fax: +61 (0)2 6268 8017. E-mail: h.
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The Australian Research Council (ARC Discovery Project DP0772426; ARC Linkage Project LP110100451) is acknowledged for financial support of this work. Z.L. thanks The University of New South Wales Canberra for a Research Publication Fellowship.
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