Sol-Gel Derived Eu3+-Doped Gd2Ti2O7 Pyrochlore Nanopowders

Ćulubrk, Sanja; Antić, Željka; Lojpur, Vesna; Marinović-Cincović, Milena; Dramićanin, Miroslav D.

doi:https://doi.org/10.1155/2015/514173

Journal of Nanomaterials

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Abstract Introduction Materials and Methods Results and Discussion Conclusion Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2015 | Article ID 514173 | https://doi.org/10.1155/2015/514173

Sol-Gel Derived Eu³⁺-Doped Gd₂Ti₂O₇ Pyrochlore Nanopowders

Sanja Ćulubrk,¹Željka Antić,¹Vesna Lojpur,¹Milena Marinović-Cincović,¹and Miroslav D. Dramićanin¹

Academic Editor: Alan K. T. Lau

Received04 Dec 2014

Revised10 Feb 2015

Accepted11 Feb 2015

Published02 Mar 2015

Abstract

Herein we presented hydrolytic sol-gel synthesis and photoluminescent properties of Eu³⁺-doped Gd₂Ti₂O₇ pyrochlore nanopowders. According to Gd₂Ti₂O₇ precursor gel thermal analysis a temperature of 840°C is identified for the formation of the crystalline pyrochlore phase. Obtained samples were systematically characterized by powder X-ray diffraction, scanning and transmission electron microscopy, and photoluminescence spectroscopy. The powders consist of well-crystalline cubic nanocrystallites of approximately 20 nm in size as evidenced from X-ray diffraction. The scanning and transmission electron microscopy shows that investigated Eu³⁺-doped Gd₂Ti₂O₇ nanopowders consist of compact, dense aggregates composed entirely of nanoparticles with variable both shape and dimension. The influence of Eu³⁺ ions concentration on the optical properties, namely, photoluminescence emission and decay time, is measured and discussed. Emission intensity as a function of Eu³⁺ ions concentration shows that Gd₂Ti₂O₇ host can accept Eu³⁺ ions in concentrations up to 10 at.%. On the other hand, lifetime values are similar up to 3 at.% (~2.7 ms) and experience decrease at higher concentrations (2.4 ms for 10 at.% Eu³⁺). Moreover, photoluminescent spectra and lifetime values clearly revealed presence of structural defects in sol-gel derived materials proposing photoluminescent spectroscopy as a sensitive tool for monitoring structural changes in both steady state and lifetime domains.

1. Introduction

The insulating rare-earth titanate pyrochlore oxides described by the general formula RE₂Ti₂O₇ (RE = Ln³⁺, Sc³⁺, or Y³⁺) have the face-centered cubic crystal structure with the space group Fdm. In the structure RE³⁺ and Ti⁴⁺ ions occupy 16d and 16c sites, respectively, with the symmetry. Due to the ability to accept rare-earth elements in high concentrations, chemical stability, and the fact that they do not show electronic conductivity at ordinary temperatures, rare-earth titanate pyrochlores could be proposed as a potential phosphor and laser host materials. However, due to their unusual structural, magnetic, and electrical conductivity properties most of the research in this field is done on the structural trends [1], thermodynamic and dynamic magnetic properties [2–4], disorders [5, 6], and nonstoichiometry [7].

Many synthesis approaches for rare-earth pyrochlores such as solid state [8–10], sol-gel [11, 12], molten salt [13], hydrothermal [14], stearic acid method [15], and Pechini process [16] have been developed. Among them sol-gel method has been considered as a low cost approach for the preparation of novel nanostructured materials. It is a wet-chemical technique widely used in the fields of materials science and engineering for the fabrication of materials starting from a chemical solution—sol which acts as the precursor for an integrated network of discrete particles—gel. It refers broadly to the room temperature solution routes and enables synthesis of materials with desirable hardness, optical transparency, chemical durability, tailored porosity, and thermal resistance.

In this paper, Eu³⁺-doped Gd₂Ti₂O₇ nanopowders with different dopant concentrations (0.5; 1; 3; 5; 7; 10; and 15 at.% with respect to Gd) were fabricated using sol-gel technique. There are two reasons for using europium. Firstly, it provides an intense red emission and therefore Gd₂Ti₂O₇ powders are new red nanophosphors. Secondly, due to nondegenerated ground (⁷F₀) and excited (⁵D₀) states together with nonoverlapping ^2S+1L_J multiplets that give emission spectra with clear dependence on the host material structure and site symmetry, Eu³⁺ is well-known structural probe ion [17, 18]. In this way photoluminescent spectroscopy could be used as a structural probe technique. The thermal decomposition behavior and proper sintering temperature of the Gd₂Ti₂O₇ samples are determined using thermogravimetric/differential thermal analysis (TG-DTA). Fourier transform infrared spectroscopy (FTIR) is used to identify the changes taking place within the precursor gel during drying and calcination. Detailed structural and morphological characterization of the nanocrystalline powders was performed using X-ray diffraction (XRD), scanning (SEM), and transmission electron microscopy (TEM). Optical properties were determined by photoluminescence (PL) excitation, emission, and lifetime measurements. The doping concentrations were systematically investigated in order to obtain the pure phase pyrochlore structure, strongest photoluminescent intensity, and the longest lifetime.

So far there is a lack in reports on photoluminescent properties of rare-earth doped Gd₂Ti₂O₇ powders. To date, photoluminescent properties of Eu³⁺-doped Gd₂Ti₂O₇ have been investigated by others in thin film [19], core-shell [20], and nanocomposite [21] form or codoped with V⁴⁺ ions in powder form [11, 22, 23]. Our previous article reported photoluminescent properties of Sm³⁺ and Eu³⁺-doped Gd₂Ti₂O₇ [16] obtained by mixed metal-citric acid synthesis and this study is the continuation of our research on the rare-earth doped Gd₂Ti₂O₇ powders. Moreover, aim of this report is to introduce luminescent spectroscopy as a sensitive tool for monitoring structural changes.

2. Materials and Methods

2.1. Synthesis of Gd₂Ti₂O₇:Eu³⁺ Nanoparticles

To produce undoped and Eu³⁺-doped pyrochlore Gd₂Ti₂O₇ in the form of nanopowders the hydrolytic sol-gel route has been adopted, starting from rare-earth nitrates and titanium(IV)-isopropoxide. For the synthesis titanium(IV)-isopropoxide (Alfa Aesar, 97%), water, ethanol, and nitric acid were mixed in molar ratio of 1 : 3 : 20 : 0.08 [24]. In the first step, stoichiometric quantities of (Alfa Aesar, 99.9%) and Eu₂O₃ (Alfa Aesar, 99.9%) were dissolved in appropriate amount of water and HNO₃. Then titanium(IV)-isopropoxide was dissolved in ethanol under constant magnetic stirring and added to the rare-earth nitrates mixture. For undoped sample only appropriate amount of Gd(NO₃)₃ was used. Transparent gels were obtained within few minutes and dried at 70°C for 5 h under atmospheric pressure. In order to obtain pure phase Gd₂Ti₂O₇ nanoparticles the undoped and Eu⁺³-doped dried gels were calcinated at 840°C for 4 hours followed by natural furnace cooling to room temperature. The calcination temperature is chosen according to TG/DTA analysis, the results of which are presented in Section 3.1. Finally, undoped Gd₂Ti₂O₇ and 7 Gd₂Ti₂O₇ samples doped with at.% Eu³⁺ ( = 0.5; 1; 3; 5; 7; 10; and 15 with respect to Gd³⁺) were synthesized.

2.2. Instruments and Measurements

TG/DTA analysis was performed on the SETARAM SETSYS Evolution-1750 instrument. The Gd₂Ti₂O₇ dried gel (~10–15 mg) was heated at 10°C min⁻¹ heating rate, in air atmosphere (air flow 16 mL min⁻¹), from 30 to 1000°C. Fourier transmission infrared (FTIR) measurements were carried out on the Thermo Nicolet 380 FT-IR instrument, in a reflection mode with a resolution of 4 cm⁻¹. X-ray diffraction (XRD) measurements were performed using Rigaku SmartLab diffractometer. Diffraction data were recorded in a 2θ range from 10° to 90°, counting 0.7°/minute in 0.02° steps. Relevant results of structural analysis (unit cell parameter, crystal coherence size, microstrain values, and data fit parameters) were obtained using built-in package software. Microstructural characterization was done using a JEOL JSM–6610LV scanning electron microscope (SEM) with an acceleration voltage of 20 kV. Microstructure at a local level was analyzed by transmission electron microscopy (TEM) using JEOL-JEM 2100 LaB₆ operated at 200 kV.

Photoluminescence (PL) measurements were performed under room temperature conditions. Measurements were done on Fluorolog-3 Model FL3-221 spectrofluorometer system (Horiba Jobin-Yvon), utilizing 450W Xenon lamp as excitation source for emission measurements and Xenon-Mercury pulsed lamp for lifetime measurements. TBX-04-D PMT detector is used for both lifetime and steady state acquisitions. The line intensities and positions of the measured spectra were calibrated with a standard mercury-argon lamp. All photoluminescence measurements were performed on pellets prepared from the powders under a load of 5 tons and without any additives.

3. Results and Discussion

3.1. Thermal Analysis (TG/DTA)

The Gd₂Ti₂O₇ gels prepared with hydrolytic sol-gel route were of amorphous nature and under appropriate sintering they transformed to crystalline Gd₂Ti₂O₇. Thermal analysis of dried gel was performed in order to determine proper sintering temperature for the transformation to pyrochlore phase. Results of TG/DTA analysis clarified the existence of three temperature regions (see Figure 1). Between room temperature and 275°C strong endothermic peak attributed to the vaporization of adsorbed organic molecules occurs in the DTA curve. In the same temperature range the TG curve shows a marked weight loss (~30%). In the second temperature range between 275°C and 500°C two endothermic peaks are indicated by DTA. At the same time weight loss (~20%) indicated by TG could be related to the elimination of the residual organic compounds. In the third region a DTA exothermic peak at ~815°C could be attributed to the amorphous to crystalline transformation of Gd₂Ti₂O₇, while after 500°C the mass remains constant. According to these results calcination temperature of 840°C was chosen. Comparing with our previous results on rare-earth doped Gd₂Ti₂O₇ obtained with mixed metal-citric acid synthesis (appropriate sintering temperature was ~880°C, [16]) this temperature is somewhat lower. Additionally, more structured shape of TG/DTA curves shows that thermal behavior of sol-gel derived materials is more complex compared to mixed metal-citric acid ones.

According to our previous study on the crystallization of phosphor materials prepared by sol-gel method [25], one should expect that materials crystallize in a two-step process. In the primary crystallization stage nonconstant radial growth rate is present, while in the second stage two-dimensional crystal growth occurs.

3.2. Fourier Transform Infrared Spectroscopy (FTIR)

Figure 2 presents the FTIR spectra of undoped Gd₂Ti₂O₇ gel, dried gel, and powder obtained after calcination at 840°C for 4 hours. Broad band with a maximum at ~3310 cm⁻¹ arises from the stretching vibrations of OH⁻ groups [26] and is especially visible in the Gd₂Ti₂O₇ gel. Many peaks in the gel’s and precursor’s spectra could be attributed to the organic and nitrate presence (see Table 1, [26]). One can notice that after heat treatment peaks from H₂O, groups and organic disappear completely. Due to formation of new lattice, the main feature of the titanate pyrochlores, the strong band in the range 600–400 cm⁻¹ [27], is only visible after thermal treatment at 840°C.

3.3. X-Ray Diffraction (XRD)

Gd₂Ti₂O₇ pyrochlore crystallizes in the face-centered cubic lattice (space group Fdm, number 227) with the unit cell containing eight molecules () and four crystallographically nonequivalent sites (Ti at 16c, Gd at 16d, O at 48f, and O′ at 8b). In the studied structure, Ti⁴⁺ and Gd³⁺ cations (and consequently rare-earth dopants) are situated in a centrosymmetric site of symmetry.

XRD patterns of undoped and seven Eu⁺³-doped Gd₂Ti₂O₇ powders together with ICDD card number 01-074-9640 are presented in Figures 3(a) and 3(b). No other phase peaks or traces of impurities were detected up to 10 at.% Eu³⁺ indicating that, up to this concentration, dopant Eu³⁺ ions have been effectively incorporated into the Gd₂Ti₂O₇ host lattice. However, at 15 at.% Eu³⁺, barely visible traces of impurities are present. In order to investigate presence of trace impurities in more detail diffractogram of 15 at.% Eu³⁺-doped sample is recorded under longer acquisition time and presented in Figure 4. Additional diffractions could be assigned to the traces of Ti₅O₉ (ICDD card number 01-075-1815) and Ti₆O₁₁ (ICDD card number 01-085-1058). According to the Ti–O phase diagram [28] large number of oxides and suboxides, TiO₂, Ti₆O, Ti₃O, Ti₂O, TiO, Ti₂O₃, and Ti₃O₅, as well as Magnelli phases (), could be present at 298 K. However, for given stoichiometry, besides TiO₂, most probable is formation of Magnelli phase impurities. Even though amounts of all constituting ions are precisely calculated and weighted, presence of bigger in size dopant Eu³⁺ in high concentration leads to structural misbalance and formation of stoichiometrically close Ti₅O₉ and Ti₆O₁₁ impurity phases traces.

(a)

(b)

Compared to our previous results [16], one can conclude that sol-gel route allows incorporation of Eu⁺³ ions into Gd₂Ti₂O₇ host lattice in lower concentrations (up to 10 at.%) compared with mixed metal-citric acid synthesis (up to 15 at.%). However, more detailed investigation (between the range of 10 and 15 at.% Eu³⁺) is necessary to precisely determine the extent to which the Eu³⁺ ion can be accommodated into sol-gel obtained Gd₂Ti₂O₇ pyrochlore matrix.

Relevant results of structural analysis (unit cell parameter, crystal coherence size, microstrain values, and data fit parameters) are presented in Table 2. The starting parameters for the structural analysis were taken according to [9]. Unit cell parameter, , increases with rare-earth doping due to replacement of smaller Gd³⁺ ( = 0.938 Å) with bigger in size Eu³⁺ ( = 0.947 Å) ion. The largest unit cell parameter was found for the sample with the highest Eu³⁺ dopant concentration (10 at.%). Similar crystallite size of around 20 nm found for all samples shows that dopant concentration does not affect crystallite size. Data fit parameters, the regression sum of relative errors (Rp), the regression sum of weighted squared errors (Rwp), and the goodness of fit (GOF) are small indicating a highly satisfactory reliability. Microstrain values are low suggesting good ion ordering in the nanocrystals.

3.4. Scanning and Transmission Electron Microscopy (SEM/TEM)

A SEM study was conducted to investigate the surface morphology of the powders and representative sample images are given in Figures 5(a) and 5(b). The micrographs show that representative 3 at.% Eu³⁺-doped Gd₂Ti₂O₇ consists of compact, dense aggregates composed entirely of nanoparticles. Microstructure at a local level investigated by TEM (Figures 5(c) and 5(d)) revealed that sol-gel synthesis produces powders with irregular both particles size and shape. The nanoparticles are organized in agglomerates, some of which formed the bigger crystals. This was visible for all Eu³⁺-dopant concentrations as well. The local crystal structure was investigated with the selected area electron diffraction technique (SAED) and one representative ring electron diffraction pattern is presented as Figure 5(c) inset. The presence of rings is the evidence of Gd₂Ti₂O₇ polycrystalline nature, while their grainy appearance is connected to the fact that the constituent crystallites have a size of ~20 nm.

(a)

(b)

(c)

(d)

3.5. Photoluminescence (PL)

In Figure 6(a) excitation spectrum of representative Gd₂Ti₂O₇:10 at.% Eu³⁺ sample with marked transitions, measured in the spectral region between 325 and 560 nm at room temperature, is presented. Photoluminescence emission spectra of samples with different Eu³⁺ concentration are presented in Figure 6(b). Emission intensity as a function of Eu³⁺ ions concentration (see inset in Figure 6(b)) shows that emission increases with Eu³⁺ doping and that Gd₂Ti₂O₇ host can accept Eu³⁺ ions in concentrations up to 10 at.% without concentration quenching. Figures 6(c)-6(d) show more clearly emission spectra of Gd₂Ti₂O₇:10 at.% Eu³⁺ and 15 at.% Eu³⁺ samples recorded at room temperature. Excitation into the ⁵L₆ level ( nm) produces distinct luminescence bands in the visible spectral region. Both spectra show five characteristic bands placed around 579 nm, 589 nm, 612 nm, 652 nm, and 712 nm associated with ⁵D₀ → ⁷F_J () spin forbidden f-f transitions, respectively. However, in the 10 at.% Eu³⁺ spectrum the most intense sharp orange-reddish line placed around 589 nm (magnetic-dipole ⁵D₀ → ⁷F₁ transition) is followed by quite broad red emission placed around 612 nm (electric-dipole ⁵D₀ → ⁷F₂ transition). On the other hand, in the 15 at.% Eu³⁺ sample spectrum this ratio is inverted. It is generally acknowledged that when a rare-earth ion is situated in a centrosymmetric site then, according to the Laporte rule, only magnetic-dipole transitions are possible. According to Tanner [29] when Eu³⁺ ion is placed in crystallographic site of point symmetry (with a strict center of symmetry) only doublet corresponding to ⁵D₀ → ⁷F₁ transition should be visible. Therefore, presence of dominant sharp orange-reddish ⁵D₀ → ⁷F₁ doublet in 10 at.% sample suggests a crystalline environment of Eu³⁺ ions and their incorporation into nanoparticles. On the other hand, observation of broad forbidden ⁵D₀ → ⁷F_J () electric-dipole transitions could be related to the distorted sites near nanoparticle’s surface or other defects. Observed most intense ⁵D₀ → ⁷F₂ transition in 15 at.% sample clearly indicates presence of additional defects in the structure. Even though XRD spectra showed hardly noticeable difference, photoluminescence spectroscopy clearly revealed presence of additional impurities. This observation indicates that luminescence spectroscopy is an effective method for investigating the presence of defects and could be used as extremely sensitive structural probe technique.

(a)

(b)

(c)

(d)

Figure 6

(a) Excitation spectrum ( = 589 nm) of representative Gd₂Ti₂O₇:10 at.% Eu³⁺ sample with marked transitions recorded at room temperature and (b) emission spectra of Gd₂Ti₂O₇: at.% Eu³⁺ ( = 0.5; 1; 3; 5; 7; 10) samples ( = 393 nm) with dependence of the emission intensity versus Eu³⁺ concentration as inset. All spectra were recorded at room temperature and under the same experimental conditions. Photoluminescent emission spectra recorded in the spectral range 525–725 nm at room temperature of (c) Gd₂Ti₂O₇:10 at.% Eu³⁺ and (d) Gd₂Ti₂O₇:15 at.% Eu³⁺ sample.

The emission decay curves of the ⁵D₀ emitting level are obtained under excitation at 393 nm ( nm) (see Figure 7(a)) and indicate a complex luminescence decay process. As can be seen, all curves could be fitted with at least two exponential functions. For this reason we found it more appropriate to calculate an average lifetime using the following equation [30]: where represents the luminescence intensity at time , corrected for the background, and the integrals are evaluated on the range , where .

(a)

(b)

Figure 7(b) shows how the dopant concentration influences lifetime values. Up to 3 at.% Eu³⁺ values are similar experiencing decrease at higher concentrations. Generally, these lifetime values are almost two times shorter than our previous results on the same material obtained by mixed metal-citric acid method [16]. This clearly indicates that sol-gel method produces materials with more defects in the structure and confirms that photoluminescent spectroscopy could be used as a sensitive tool for monitoring structural changes in both steady state and lifetime domains.

4. Conclusion

Eu³⁺-doped Gd₂Ti₂O₇ samples produced via sol-gel synthesis and annealed at 840°C for 4 h were studied in order to observe the dopant’s influence on the structural and optical properties. XRD measurements revealed that up to 10 at.% dopant Eu³⁺ ions have been effectively incorporated into the Gd₂Ti₂O₇ host lattice while at higher dopant concentrations barely visible traces of impurities are present. Microstructure at a local level revealed that sol-gel synthesis produces particles with well-defined crystallinity and irregular size and shape, organized in agglomerates, some of which formed the big crystals. The dependence of luminescence emission intensity on Eu³⁺ concentration shows that the strongest emission is found for sample containing 10 at.%. On the other hand, with an increase in Eu³⁺ concentration the lifetime of the ⁵D₀ level decreases from ~2.8 ms (for 1 at.% Eu³⁺) to ~2.4 ms (for 10 at.% Eu³⁺). Correlation between structural and photoluminescent properties reveals that photoluminescent spectroscopy of rare-earths (e.g., Eu³⁺ ions) could be proposed as an effective method for investigating the presence of defects and structural changes in optically active materials.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

This work is supported by the Ministry of Education, Science and Technological Development of the Republic of Serbia (Grants nos. 172056 and 45020). Authors would like to thank Dr. Phillip S. Ahrenkiel, South Dakota School of Mines & Technology, Rapid City, South Dakota, USA, for transmission electron microscopy images.

References

B. J. Kennedy, B. A. Hunter, and C. J. Howard, “Structural and bonding trends in tin pyrochlore oxides,” Journal of Solid State Chemistry, vol. 130, no. 1, pp. 58–65, 1997.
View at: Publisher Site | Google Scholar
H. W. J. Blöte, R. F. Wielinga, and W. J. Huiskamp, “Heat-capacity measurements on rare-earth double oxides R₂M₂O₇,” Physica, vol. 43, no. 4, pp. 549–568, 1969.
View at: Publisher Site | Google Scholar
S. T. Bramwell, M. N. Field, M. J. Harris, and I. P. Parkin, “Bulk magnetization of the heavy rare earth titanate pyrochlore—a series of model frustrated magnets,” Journal of Physics Condensed Matter, vol. 12, no. 4, pp. 483–495, 2000.
View at: Publisher Site | Google Scholar
A. P. Ramirez, “Geometrical frustration,” in Handbook on Magnetic Materials, K. H. J. Buschow, Ed., vol. 13, p. 423, Elsevier, Amsterdam, The Netherlands, 2001.
View at: Google Scholar
C. Heremans, B. J. Wuensch, J. K. Stalick, and E. Prince, “Fast-ion conducting Y₂(Zr_yTi_1-y)₂O₇ pyrochlores: neutron rietveld analysis of disorder induced by Zr substitution,” Journal of Solid State Chemistry, vol. 117, no. 1, pp. 108–121, 1995.
View at: Publisher Site | Google Scholar
L. Minervini, R. W. Grimes, K. E. Sickafus, and C. Randall, “Disorder in pyrochlore oxides,” Journal of the American Ceramic Society, vol. 83, no. 8, pp. 1873–1878, 2000.
View at: Google Scholar
C. R. Stanek, L. Minervini, and R. W. Grimes, “Nonstoichiometry in A₂B₂O₇ pyrochlores,” Journal of the American Ceramic Society, vol. 85, no. 11, pp. 2792–2798, 2002.
View at: Google Scholar
A. R. Heredia, M. Q. García, J. L. P. Mazariego, and R. Escamilla, “X-ray diffraction and Raman spectroscopy on ${Gd}_{2} ({Ti}_{2 - y} {Te}_{y}) O_{7}$ prepared at high pressure and high temperature,” Journal of Alloys and Compounds, vol. 504, no. 2, pp. 446–451, 2010.
View at: Publisher Site | Google Scholar
A. F. Fuentes, K. Boulahya, M. MacZka, J. Hanuza, and U. Amador, “Synthesis of disordered pyrochlores, A₂Ti₂O₇ (A = Y, Gd and Dy), by mechanical milling of constituent oxides,” Solid State Sciences, vol. 7, no. 4, pp. 343–353, 2005.
View at: Publisher Site | Google Scholar
N. P. Raju, M. Dion, M. J. P. Gingras, T. E. Mason, and J. E. Greedan, “Transition to long-range magnetic order in the highly frustrated insulating pyrochlore antiferromagnet Gd₂Ti₂O₇,” Physical Review B: Condensed Matter and Materials Physics, vol. 59, no. 22, pp. 14489–14498, 1999.
View at: Publisher Site | Google Scholar
K.-M. Lin, C.-C. Lin, C.-Y. Hsiao, and Y.-Y. Li, “Synthesis of Gd₂Ti₂O₇:Eu³⁺, V⁴⁺ phosphors by sol-gel process and its luminescent properties,” Journal of Luminescence, vol. 127, no. 2, pp. 561–567, 2007.
View at: Publisher Site | Google Scholar
L. Lin and B. Yan, “Rare earth titanates ceramics Na₂La₂Ti₃O₁₀: Pr³⁺ and RE₂Ti₂O₇: Pr³⁺ (R = Gd, Y): sol-gel synthesis, characterization and luminescence,” Journal of Materials Science: Materials in Electronics, vol. 22, no. 6, pp. 672–678, 2011.
View at: Publisher Site | Google Scholar
N. Dharuman and L. John Berchmans, “Low temperature synthesis of nano-crystalline gadolinium titanate by molten salt route,” Ceramics International, vol. 39, no. 8, pp. 8768–8771, 2013.
View at: Publisher Site | Google Scholar
W. Peng, B. Hu, C. Yan et al., “Hydrothermal synthesis and characterization of pyrochlore titanate R₂Ti₂O₇(R=Gd³⁺, Tb³⁺, Dy³⁺),” Chemical Research in Chinese Universities, vol. 27, no. 2, pp. 161–165, 2011.
View at: Google Scholar
W. Zhang, L. Zhang, H. Zhong, L. Lu, X. Yang, and X. Wang, “Synthesis and characterization of ultrafine Ln₂Ti₂O₇ (Ln = Sm, Gd, Dy, Er) pyrochlore oxides by stearic acid method,” Materials Characterization, vol. 61, no. 2, pp. 154–158, 2010.
View at: Publisher Site | Google Scholar
S. Ćulubrk, Ž. Antić, M. Marinović-Cincović, P. S. Ahrenkiel, and M. D. Dramićanin, “Synthesis and luminescent properties of rare earth (Sm³⁺ and Eu³⁺) doped Gd₂Ti₂O₇ pyrochlore nanopowders,” Optical Materials, vol. 37, pp. 598–606, 2014.
View at: Publisher Site | Google Scholar
K. Binnemans and C. Görller-Walrand, “Application of the Eu³⁺ ion for site symmetry determination,” Journal of Rare Earths, vol. 14, no. 3, pp. 173–180, 1996.
View at: Google Scholar
V. Đorđević, Ž. Antić, V. Lojpur, and M. D. Dramićanin, “Europium-doped nanocrystalline Y₂O₃-La₂O₃ solid solutions with bixbyite structure,” Journal of Physics and Chemistry of Solids, vol. 75, no. 10, pp. 1152–1159, 2014.
View at: Publisher Site | Google Scholar
M. L. Pang, J. Lin, J. Fu, and Z. Y. Cheng, “Luminescent properties of Gd₂Ti₂O₇: Eu³⁺ phosphor films prepared by sol-gel process,” Materials Research Bulletin, vol. 39, no. 11, pp. 1607–1614, 2004.
View at: Publisher Site | Google Scholar
K.-M. Lin, C.-C. Lin, and Y.-Y. Li, “Luminescent properties and characterization of Gd₂O₃Eu³⁺@SiO₂ and Gd₂Ti₂O₇Eu³⁺@SiO2 core-shell phosphors prepared by sol-gel process,” Nanotechnology, vol. 17, no. 6, pp. 1745–1751, 2006.
View at: Publisher Site | Google Scholar
M. Saif, “Luminescence based on energy transfer in silica doped with lanthanide titania (Gd₂Ti₂O₇:Ln³⁺) [Ln³⁺ = Eu³⁺ or Dy³⁺],” Journal of Photochemistry and Photobiology A: Chemistry, vol. 205, no. 2-3, pp. 145–150, 2009.
View at: Publisher Site | Google Scholar
Y. Zhang, L. Ding, X. Pang, and W. Zhang, “Influence of annealing temperature on luminescent properties of Eu³⁺/V⁵⁺ co-doped nanocrystalline Gd₂Ti₂O₇ powders,” Journal of Rare Earths, vol. 27, no. 6, pp. 900–904, 2009.
View at: Publisher Site | Google Scholar
Y. Zhang, C. Jia, Z. Su, and W. Zhang, “The enhanced and color-tunable photoluminescence of Eu³⁺/V⁵⁺ co-doped Gd₂Ti₂O₇ nanocrystals,” Journal of Alloys and Compounds, vol. 479, no. 1-2, pp. 381–384, 2009.
View at: Publisher Site | Google Scholar
G. Dagan and M. Tomkiewicz, “Titanium dioxide aerogels for photocatalytic decontamination of aquatic environments,” The Journal of Physical Chemistry, vol. 97, no. 49, pp. 12651–12655, 1993.
View at: Publisher Site | Google Scholar
M. Marinović-Cincović, B. Janković, B. Milićević, Ž. Antić, R. K. Whiffen, and M. D. Dramićanin, “The comparative kinetic analysis of the non-isothermal crystallization process of Eu³⁺ doped Zn₂SiO₄ powders prepared via polymer induced sol-gel method,” Powder Technology, vol. 249, pp. 497–512, 2013.
View at: Publisher Site | Google Scholar
G. Socrates, Infrared and Raman Characteristics Group Frequences Tables and Charts, John Wiley & Sons, West Sussex, UK, 3rd edition, 2001.
M. A. Subramanian, G. Aravamudan, and G. V. Subba Rao, “Oxide pyrochlores—a review,” Progress in Solid State Chemistry, vol. 15, no. 2, pp. 55–143, 1983.
View at: Publisher Site | Google Scholar
H. Okamoto, “O-Ti (Oxygen-Titanium),” Journal of Phase Equilibria and Diffusion, vol. 32, no. 5, pp. 473–474, 2011.
View at: Publisher Site | Google Scholar
P. A. Tanner, “Some misconceptions concerning the electronic spectra of tri-positive europium and cerium,” Chemical Society Reviews, vol. 42, no. 12, pp. 5090–5101, 2013.
View at: Publisher Site | Google Scholar
E. Nakazawa, “Fundamentals of luminescence,” in Phosphor Handbook, S. Shionoya and W. M. Yen, Eds., pp. 11–141, CRC Press, New York, NY, USA, 1999.
View at: Google Scholar

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Copyright © 2015 Sanja Ćulubrk et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Journal of Nanomaterials

Sol-Gel Derived Eu3+-Doped Gd2Ti2O7 Pyrochlore Nanopowders

Abstract

1. Introduction

2. Materials and Methods

2.1. Synthesis of Gd2Ti2O7:Eu3+ Nanoparticles

2.2. Instruments and Measurements

3. Results and Discussion

3.1. Thermal Analysis (TG/DTA)

3.2. Fourier Transform Infrared Spectroscopy (FTIR)

3.3. X-Ray Diffraction (XRD)

3.4. Scanning and Transmission Electron Microscopy (SEM/TEM)

3.5. Photoluminescence (PL)

4. Conclusion

Conflict of Interests

Acknowledgments

References

Copyright

Sol-Gel Derived Eu³⁺-Doped Gd₂Ti₂O₇ Pyrochlore Nanopowders

2.1. Synthesis of Gd₂Ti₂O₇:Eu³⁺ Nanoparticles