J Nanopart Res (2014) 16:2326 DOI 10.1007/s11051-014-2326-1
RESEARCH PAPER
Synthesis, spectroscopic and luminescent properties of nanosized powders of yttrium phosphates doped with Er3+ ions Polina A. Ryabochkina • Svetlana A. Antoshkina • Alexander S. Vanetsev • Ilmo Sildos • Olga M. Gaitko • Vladimir M. Kyashkin • Sergey N. Ushakov Andrey A. Panov • Natalie Yu. Tabachkova • Konstantin N. Nishchev
•
Received: 28 November 2013 / Accepted: 6 February 2014 / Published online: 22 February 2014 Ó Springer Science+Business Media Dordrecht 2014
Abstract Here, we report synthesis of pure crystalline nanopowders of yttrium orthophosphate Y0.95 Er0.05PO4 with xenotime-type structure and hydrate of yttrium orthophosphate Y0.95Er0.05PO40.8H2O with rhabdophane-type structure using microwave-hydrothermal treatment of freshly precipitated gels. To establish phase composition of the synthesized nanopowders, we have carried out XRD analysis. Also, we have investigated their morphology by means of TEM and SAXS. We have registered scattered reflection spectra for synthesized Y0.95Er0.05PO4 and Y0.95Er0.05
PO40.8H2O powders. Also, we have obtained luminescent spectra of these nanocrystalline samples for 4 I13/2 ? 4I15/2 transition of Er3? ion with excitation of 4 I11/2 level. We have proposed the mode of application of nanocrystalline powders Y0.95Er0.05PO4 and Y0.95Er0.05PO40.8H2O for the development of composite materials based on polymer capsules that are being used in targeted drug delivery. Keywords Nanocrystalline powders Ortophosphate Luminescence spectra
Introduction P. A. Ryabochkina (&) S. A. Antoshkina V. M. Kyashkin A. A. Panov K. N. Nishchev Ogarev Mordovia State University, Bolshevistskaya Street 68, Saransk, Russia e-mail:
[email protected] A. S. Vanetsev I. Sildos Institute of Physics, University of Tartu, Riia 142, Tartu, Estonia O. M. Gaitko Kurnakov Institute of General and Inorganic Chemistry RAS, Lenin Avenue 31, Moscow, Russia S. N. Ushakov Prokhorov Institute of General Physics RAS, Vavilov Street 38, Moscow, Russia N. Yu. Tabachkova National University of Science and Technology MISiS, Lenin Avenue 4, Moscow, Russia
Substantial efforts were applied recently to develop new methods of synthesis of RE-activated nanosized crystalline materials and study their physical properties (Lucas et al. 2004, 2006; Di et al. 2007; Heesun et al. 2007; Lai et al. 2008; Li et al. 2009; Lin et al. 2010; Vanetsev et al. 2011). Many authors reported the synthesis, structure, morphology, spectral, and luminescent properties of rare-earth-doped crystalline nanoparticles. This raising interest is due to emerging industrial applications of these materials as phosphors for fluorescent lamps, plasma panels, FED, etc. Nanosized crystalline powders YPO4 doped by rare-earth ions satisfy for practical applications described above, and moreover, they can be used in biology and medicine. One from probable modes of using in medical applications would be described below at this paper.
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Literature data (Assaaoudi et al. 2001; Boatner 2002; Lin et al. 2010; Balakrishnaiah et al. 2010; Li et al. 2009; Parchur et al. 2010; Moine et al. 2011; Vanetsev et al. 2011; Mezentseva et al. 2012; Rodriguez-Liviano et al. 2012; Seminko et al. 2012) indicate that the most common methods for obtaining nanosized crystalline powder orthophosphates are: hydrothermal synthesis (Li et al. 2009; Lin et al. 2010; Vanetsev et al. 2011), sol–gel (Mezentseva et al. 2012), synthesis by a homogeneous precipitation [14, 16] (Balakrishnaiah et al. 2010; Rodriguez-Liviano et al. 2012), solid-state reaction (Assaaoudi et al. 2001), and colloidal synthesis (Seminko et al. 2012). Previously, we have reported (Vanetsev et al. 2011) synthesis of nanodispersed powders of YV1-xPxO4:Eu using microwave-hydrothermal treatment and studied their morphology and luminescent properties. We have shown that this approach is promising for synthesis of non-aggregated and well-crystalline nanoparticles with uniform shape and narrow size distribution. Most of the existing publications are devoted to the study of nanocrystalline powders of YPO4 doped with Eu3? ions (Assaaoudi et al. 2001; Boatner 2002), while the nanopowders doped with other rare-earth ions, for example, YPO4:Er3?, were studied to a much lesser extent. Synthesis, structure, morphology, and vibrational spectra of nanoparticles of hydrates of rare-earth phosphates RePO4nH2O (Re = La, Ce or Y) were described by authors (Hikichi et al. 1989; Assaaoudi et al. 2001; Lucas et al. 2004, 2006); also, structural and morphological changes of these hydrates during thermal treatment and the domains of thermal stability of different phases were presented. It should be noted that until now, the investigations of spectroscopic and luminescent characteristics of rare-earth ions in hydrates phosphate matrix were seldom and unsystematic, despite their importance for practical applications. Therefore, aims of present work include microwave-hydrothermal synthesis of nanocrystalline powders of tetragonal Y0.95Er0.05PO4 phase and hexagonal Y0.95Er0.05PO40.8H2O phase and study of their phase composition, morphology, and spectral-luminescent properties and suggestion of the method to use these nanoparticles for destruction of polymeric capsules by laser excitation of rare-earth ions for targeted drug delivery.
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Experimental As starting compounds for preparation of samples Y0.95Er0.05PO4 and Y0.95Er0.05PO40.8H2O, we have used Y(NO3)36H2O (Aldrich, 99.99 % purity), Er(NO3)36H2O (Aldrich, 99.98 % purity), and K2HPO43H2O (KhimMed, 99.9 % purity). We have prepared solution of Y(NO3)36H2O (4.95 or 4.75 mmols) and Er(NO3)36H2O (0.05 or 0.25 mmol) in 10 ml of deionized water as well as solutions of 5 (for preparation of xenotime-type orthophosphate, Sample 1) or 50 (for preparation of rhabdophane-type orthophosphate hydrate, Sample 2) mmols of K2HPO43H2O in 30 ml of deionized water. After that, we have added solution of nitrates dropwise to solution of phosphate under vigorous stirring. We have diluted the as-precipitated gels in mother solution with 10 ml of deionized water, transferred them into 100-ml Teflon autoclaves, and exposed to microwave-hydrothermal (MW-HT) treatment (200 °C, 2 h) using Berghof Speedwave-3M? laboratory device (2.45 GHz, 1 kW output power). After treatment, we have centrifuged all samples, washed them several times with deionized water, and air-dried at 50 °C for 10 h. We have performed XRD analysis of synthesized samples using PANalitical B.V. Empyrean diffrac˚ ) with vertical tometer (CuKa radiation, k = 1.5414 A goniometer and PIXcel 3D detector. Phases were identified using JSPDS PDF 2 1911 database. Qualitative elemental analysis of samples was performed using EDX unit of scanning electron microscope QUANTA 200 I 3D. Transmission electron microscopy (TEM) was carried out on a JEOL 2100 microscope working at 200 kV. SAXS analysis of synthesized nanoparticles was performed using Hecus S3-MICRO device (CuKa ˚ ) with Kratki collimator, using radiation, k = 1.54 A point collimation, with vertical width of slit 200 lm. For registration of scattering spectra, we used onedimensional gas-filled (90 % argon, 10 % methane) position sensitive X-ray detector Hecus PSD 50 M. Achievable range of registration of scattering vector q ˚ -1. We corresponded to the range of 0.005–0.65 A used silver behenate powder as a calibration sample. Sample-to-detector distance was 281 mm. A Fourier transform IR spectrometer Infralum covering the wavenumbers 400–4,000 cm-1 was used
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Fig. 1 XRD patterns of Y0.95Er0.05PO4 (sample 1)
Fig. 2 XRD patterns of Y0.95Er0.05PO40.8H2O (sample 2)
to record IR spectra of the powdered samples pressed in pellets with KBr. We performed scattering reflection experiments using special accessory to Perkin Elmer Lambda 950 spectrophotometer—150-mm integrating sphere. We registered luminescence spectra using automated installation based on MDR-23 monochromator and semiconductor laser diode (kem & 970 nm) as a source. As a detector, we used germanium photodiode FD-7G. Luminescence spectra were recorded with synchronous detection of signals using an SR-810 lock-in amplifier.
related to the amount of bound water. According to the literature data (Kolitsch and Holtstam 2004), content of bound water in this phase may vary in a rather wide range from 0.5 to 1.0 molecule per formula unit. As the reference sample in PDF2 database was synthesized in significantly different conditions (Hikichi et al. 1989), one can expect that the quantity of bound water in this sample is also different. This causes differences in XRD spectra in the large angle region corresponding to small distances in the crystal lattice. According to XRD data, the mean size of average scattering region, which in the case of well-crystalline and non-aggregated powders is close to the mean size of the particles, can be estimated using Scherrer equation (Jones 1938):
Experimental results and discussion XRD analysis for sample 1 (Fig. 1) showed that it consists of pure yttrium orthophosphate with xenotime-type structure (space group I41/amd) (Vegard 1927). Refinement of structure from XRD spectrum showed that cell parameters for synthesized sample ˚ and c = 6.013 A ˚ . On the other hand, are a = 9.735 A sample 2 (Fig. 2) consists of rhabdophane-like hydrate of yttrium orthophosphate YPO40.8H2O (space group P6222) (Hikichi et al. 1989) with unit cell parameters ˚ , b = 6.833 A ˚ , and c = 6.291 A ˚ . For a = 6.833 A both samples, we have observed shifting of diffraction peaks to smaller 2h angles which is probably due to doping of yttrium phosphate with larger Er3? ions. One may also note that for rhabdophane-type hydrate of orthophosphate (sample 2), we have found noticeable differences between reference spectrum and our experimental results in the region of large 2h angles. Most likely, this is due to the non-stoichiometry of this phase
b¼
Ck ; K cos h
ð1Þ
where b is the angular breadth of the line defined below, K the effective crystal size, k the wavelength of X-radiation, and h the Bragg angle. C is a constant equal to 0.94 in our case. Analysis of XRD diffractograms (Figs. 1 and 2) allows one to suggest that we are dealing with wellcrystalline samples as intensity of peaks is rather high and noise/signal ratio is low. Therefore, it is possible to connect the mean size of average scattering region with the mean size of particles. Calculated mean size of average scattering region is equal to 19 ± 2 and 15 ± 2 nm for Samples 1 and 2, respectively. For studying of morphology and more precise determination of the mean size of synthesized nanoparticles, we used TEM analysis (Figs. 3, 4). Sample 1 consists of almost isotropic particles, as sample 2
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Fig. 3 Transmission electron micrographs obtained for the sample 1
consists basically of more elongated, ellipsoidal nanoparticles (Fig. 4), typical for the rhabdophane phase (Lucas et al. 2006). Statistical analysis of obtained micrographs allowed us to calculate size distribution and mean size of particles (Fig. 5). We have found that the mean size for particles of Sample 1 is 37 ± 8 nm and for Sample 2 is 30 ± 11 nm. It is worthy to note that distribution patterns for samples are different. For Sample 1 it is very close to normal, but for Sample 2 it is closer to lognormal. Usually, lognormal distribution indicates significant input of growth and/or recrystallization processes. Summarizing this with lower crystallinity of Sample 2 according to XRD data, we may assume that synthesis of YPO40.8H2O phase in specified reaction conditions is hindered comparing to formation of YPO4 phase. This results in smaller size of particles and their wider size distribution. Comparison of the mean sizes of particles estimated from XRD and TEM data shows that relation between the sizes of particles of Sample 1 and Sample 2 remains unchanged though absolute values are significantly larger in the case of TEM data. Usually, this means that visible particles are polycrystalline. From the difference between values obtained from XRD and TEM data, we may assume that the particles of both samples consist at average of one or two crystallites
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Fig. 4 Transmission electron micrographs obtained for the sample 2
Fig. 5 Distribution of sizes of the particles for Sample 1 (filled bars and solid line) and for Sample 2 (empty bars and dashed line)
with unbroken long-range ordering. Thus, obtained nanopowders exhibit relatively low degree of aggregation and may be considerate as candidates for various biomedical applications. Also, for the synthesized powders of tetragonal Y0.95Er0.05PO4 and hexagonal Y0.95Er0.05PO40.8H2O phases, we have performed SAXS experiments. Figure 6 represents the graphs of small-angle X-ray scattering for our samples.
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On other hand, we have determined maximal particle size from distance distribution function p(r), which we have obtained by Fourier inversion of the scattering curve using program GNOM (Glatter and Kratky 1982; Svergun and Feigin 1986): 1 pð r Þ ¼ 2 2p
Zqmax
Iexp ðqÞ
sinðqr Þ dr: qr
ð5Þ
q¼qmin
We have calculated mean radius of gyration Rg from the equation: Fig. 6 SAXS graphs for tetragonal Y0.95Er0.05PO4 and hexagonal Y0.95Er0.05PO40.8H2O powders. Inset represents scattering graphs in Guinier coordinates
From Fig. 6, one can see that dependences of smallangle X-ray scattering decrease monotonically which is typical of systems with small particle size. Since the size distributions for the particles of both samples are relatively narrow, we can use a monodisperse approximation for the calculations. Linear dimensions R for this approximation for spherical particles can be estimated from the ratio: 3 R2g ¼ R2 ; 5
ð2Þ
where Rg is the radius of gyration. The radius of gyration provides the information concerning the distribution of scattering matter around the center of mass of the particle. On one hand, from SAXS results, we have calculated volumetric radius of gyration Rg using Guinier formula (Glatter and Kratky 1982; Svergun and Feigin 1986): I ðqÞ ¼ I0 exp R2g q2 =3 ; ð3Þ where I0 is the scattering strength at zero angle h (q = 0); q ¼ 4p sin the scattering vector, h the k scattering angle, and k the radiation wavelength. From Guinier formula, one can see that Rg value determines the intensity of scattering near q = 0. Taking the logarithm of both sides of Eq. (3), we have: I ðqÞ ¼ lnðI0 Þ R2g q2 =3:
ð4Þ
Thus, Rg value may be calculated from the slope of rectilinear segment of dependence of lnI(q) on q2 (Guinier diagram).
R2g
¼
ZD 0
2
Pðr Þr dr=
ZD
Pðr Þdr:
ð6Þ
0
On the inset to Fig. 5, one can see segments of SAXS curves for experimental samples represented in Guinier coordinates (ln(I) on q2). Using Eq. (3) from slope of these, we have established values of Rg for tetragonal Y0.95Er0.05PO4 and hexagonal Y0.95Er0.05 PO40.8H2O:Er phases. These values appear to be 8,70 ± 0,01 and 8,30 ± 0,09 nm, respectively. Figure 6 represents graphs of calculated distance distribution function p(r) (function of pair distances) for samples. Product 4pp(r)/q gives the number of line segments with length r joining any two elements of the particle volume (Svergun and Feigin 1986). Difference between values Rg determined from distance distribution function p(r) and values calculated using Eq. (3) is 2 and 5 % for samples 1 and 2, respectively. Based on the form of curve of distance distribution function p(r) (Fig. 7), we can conclude about the shape of the scattering particles. For sample 1, it is close to isotropic. Asymmetric form of the curve of distance distribution function p(r) for sample 2 may be due to anisotropic form of particles or their agglomeration. This fact agrees with TEM results. Estimation of mean sizes of particles for Y0.95Er0.05 PO4 and Y0.95Er0.05PO40.8H2O samples gives us values of 22.8 ± 0.3 and 22.5 ± 1.7 nm, respectively. Results for sample 1 are in good correlation with corresponding mean sizes of average scattering region obtained from XRD analysis. Results of sample 2 differ from XRD, because sample 2 consists at average of one or two crystallites and has smaller size of particles according to TEM. FT-IR spectra of samples are presented on Fig. 8. Results of our investigations in comparison with data
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Fig. 7 Distance distribution function p(r) for tetragonal Y0.95Er0.05PO4 and hexagonal Y0.95Er0.05PO40.8H2O powders Fig. 9 Diffused reflection spectra of Y0.95Er0.05PO4 (gray) and Y0.95Er0.05PO40.8H2O (black)
Fig. 8 FT-IR spectra of Y0.95Er0.05PO4 (gray) and Y0.95Er0.05 PO40.8H2O (black)
reported in Li et al. (2009) indicate that vibration bands at 525, 644 cm-1 (sample 1) and 536, 617 cm-1 (sample 2) and a broadband centered at 1,087 cm-1 represent the characteristic absorption of the phosphate groups. The peaks at 3425 and 1635 cm-1 should correspond to the surface-absorbed water and hydroxyl groups. The band at 1,383 cm-1 in sample 2 can be attributed to vibration of NO3 group. Some amount of potassium found by EDX-analysis for sample 2 may indicate that sample 2 contains trace amounts of KNO3. On Fig. 9, we present diffused reflection spectra of Y0.95Er0.05PO4 and Y0.95Er0.05PO40.8H2O samples, transposed according to Kubelka–Munk function: FðR1 Þ ¼
ð1 R1 Þ2 k ¼ ; S 2R1
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ð7Þ
where k is the absorption coefficient, S the scattering coefficient, and R? the relative diffused reflection of the sample, compared to non-absorbing standard (MgO). Absorption bands corresponding to transitions from Er3? ion 4I15/2 ground level to excited multiplets 4I13/2, 4 I11/2, 4I9/2, 4F9/2, 4S3/2, 2H11/2, 4F7/2, 4F5/2, 4F3/2, 2H9/2, and 4G11/2 are clearly seen on both spectra. One can also notice that band structure is different for samples 1 and 2 and this may be due to the differences in local surrounding of Er3? ions in these samples. It is also worthy to note that ratio of intensity of hypersensitive transition 4I15/2 ? 2H11/2 to intensities of other transitions in Y0.95Er0.05PO40.8H2O sample is higher than in Y0.95Er0.05PO4. This effect needs further investigation which we are planning to perform. Luminescence spectra for Er3? ion in xenotimetype Y0.95Er0.05PO4 and rhabdophane-like Y0.95Er0.05 PO40.8H2O are presented on Figs. 10 and 11, respectively. These spectra contain 4I13/2 ? 4I15/2 transition with excitation of 4I11/2 band. Comparative analysis of this luminescent spectrum in the area of 4 I13/2 ? 4I15/2 transition for Er3? ion in Y0.95Er0.05 PO4 showed that it is identical to the one of YPO4:Er presented in Moine et al. (2011). It should be noted that luminescence from the levels 4S3/2, 4F9/2, and 4I11/2 of Er3? ions with excitation of 4S3/2 (kex = 532 nm) has not been obtained in studied nanoscale crystalline particles. We associate this fact with effective nonradiative energy transfer between Er3? ions and OHgroups, the presence of which is confirmed by IR
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visible range of spectrum. However, the biological tissues transmit infrared radiation (*1 l) better than the visible. Gold particles do not satisfy for this condition. From our point of view, this task can be solved using nanoparticle of orthophosphates with rare-earth ions as substitute for gold nanoparticles. In our opinion, the heat produced by the transformation of laser excitation by energy transfer from Er3? ions to OH groups, followed by non-radiative relaxation, can be used to open the polymer capsules with the nanoparticles of Y0.95Er0.05PO4 and Y0.95Er0.05PO4 0.8H2O on their shells. Fig. 10 Luminescence spectra of tetragonal Y0.95Er0.05PO4
Conclusion
Fig. 11 Luminescence spectra of hexagonal Y0.95Er0.05PO4 0.8H2O
spectra. For the same reason, in our opinion, we were unable to register any luminescence of Er3? ions from the levels 4S3/2, 4F9/2 with excitation of 4I11/2 in nanopowders YPO4:Er, which is observed in bulk crystal with Er3? and due to the interaction of excited Er3? ions. Several publications reported on the possibility of using laser radiation for the destruction of polymers (Skirtach et al. 2008; Anipina and Sukhorukov 2011; Pavlov et al. 2011). Microcapsules made from these polymers with gold nanoparticles are developed for targeted drug delivery at the present time (Skirtach et al. 2008; Anipina and Sukhorukov 2011; Pavlov et al. 2011), because it is known that metal nanoparticles as gold, silver, and platinum can transform light energy to heat well. The opening of capsules can be realized by laser irradiation of gold nanoparticles in
In the present work, we have introduced new method of microwave-hydrothermal synthesis of nanosized crystalline powders of yttrium orthophosphate and hydrate of yttrium orthophosphate doped with Er3? ions. Investigations of phase composition and morphology of Y0.95Er0.05PO4 and Y0.95Er0.05PO40.8H2O nanopowders have been carried out. Diffused reflection spectra of synthesized Y0.95 Er0.05PO4 and Y0.95Er0.05PO40.8H2O nanopowders clearly show absorption bands which correspond to transition from Er3? ion 4I15/2 ground level to excited multiplets. We have also obtained luminescence spectra for 4I13/2 ? 4I15/2 transition with excitation of 4I11/2 band of Er3? ions for both samples. We connect absence of other luminescence of Er3? ions with effective non-radiative energy transfer between Er3? ions and OH- groups and proposed method of applying this phenomenon to open polymeric capsules with these nanopowders on shell by laser excitation of Er3? ions in targeted drug delivery. Acknowledgments This work performed as part of the base portion of the state task in the scientific activities of The Ministry of Education and Science of the Russian Federation and is partially supported by European Union through European Social Fund (Mobilitas grant No. MTT 50).
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