Article pubs.acs.org/JPCC
Systematic Studies on RE2Hf2O7:5%Eu3+ (RE = Y, La, Pr, Gd, Er, and Lu) Nanoparticles: Effects of the A‑Site RE3+ Cation and Calcination on Structure and Photoluminescence Madhab Pokhrel, Kareem Wahid, and Yuanbing Mao* Department of Chemistry, University of Texas Rio Grande Valley, 1201 West University Drive, Edinburg, Texas 78539, United States S Supporting Information *
ABSTRACT: A series of 5 mol % Eu3+-doped rare earth (RE) hafnium oxide RE2Hf2O7 (RE = Y, La, Pr, Gd, Er, and Lu) nanoparticles (NPs) have been synthesized, calcinated, and systematically investigated using X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, Fourier transform infrared spectroscopy, and optically excited luminescence. Effects of the A-site RE3+ cation and calcination on the crystal structure of the RE2Hf2O7:5% Eu NPs were distinguished using XRD and Raman studies. Spectroscopic analysis showed that the La2Hf2 O7 :5%Eu 3+ and Pr2Hf2O7:5%Eu3+ possessed ordered pyrochlore structures while the RE2Hf2O7:5%Eu3+ compositions (RE = Y, Er, and Lu) possessed disordered fluorite structure and were thermodynamically stable up to the highest calcination temperature employed in this study (1500 °C); however, a disordered−ordered transition observed in the Gd2Hf2O7:5%Eu3+ composition indicated that it was not thermodynamically stable. Detailed photoluminescence (PL) studies, including quantum yield and decay properties of each sample before and after calcination, were performed and correlated with their compositions and crystal structures. These results suggest that the A site RE3+ cations and calcination of these RE2Hf2O7:5% Eu NPs play important roles in their PL properties.
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INTRODUCTION Rare earth (RE)-doped inorganic compounds have been widely used as phosphors for a wide variety of applications.1−4 Recently there has been a great amount of interest in the derivative structures, such as A2B2O7, of dioxide BO2 (e.g., B = Zr, Ce, and Hf) type compounds as RE host materials due to their binary metal cation sites, where the larger A site cation can be replaced with trivalent RE ions and B is a smaller transitionmetal cation.5−9 Compounds with the chemical composition A2B2O7 can exist in two closely related structures known as ordered pyrochlore (Fd3̅m) and disordered fluorite (Fm3̅m) depending on the trivalent RE3+ ion in the A site and tetravalent ion in the B site. If the size of ions on A and B sites is comparable (radius ratio rA/rB < 1.46), the ordered pyrochlore structure is not stable and the disordered fluorite structure becomes favorable.10 In addition, if the size difference between the A-site and B-site cations is not so distinct, the two cations have a chance to swap their positions in the lattice, which yields cation antisite defects.11 In the ordered pyrochlore crystal structure A3+ and B4+ ions reside on the A and B sites, respectively, and the two anion sites (8b and 48f) are also fully occupied (Scheme 1a). The A-site cations are 8-fold coordinated with oxygen, and the B-site cations are 6-fold coordinated with oxygen. In the fluorite structure, oxygen vacancies are randomly distributed on the anion site (Scheme 1b), so it is also known as defect or disordered fluorite structure. © 2016 American Chemical Society
Scheme 1. Crystal Structure of (a) Ordered Pyrochlore, in Which All of the A3+ Ions Occupy at the A-site with 8-Fold Coordination with Oxygen and the Smaller B4+ Cations Occupy at the B-Site and Have 6-Fold Coordination with Oxygen, and (b) Disordered Fluorite, in Which All of the A3+ and B4+ Cations Occupy the Same Cation Site with 7-Fold Coordination
The order−disorder transition in A2B2O7 oxides can be realized by various factors, such as chemical doping, treatment by high temperature, and radiation exposure.10 The structural transformation through heating is normally achieved by Received: May 11, 2016 Revised: June 16, 2016 Published: June 22, 2016 14828
DOI: 10.1021/acs.jpcc.6b04798 J. Phys. Chem. C 2016, 120, 14828−14839
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The Journal of Physical Chemistry C calcinating samples above 1000 °C if first prepared in the cubic disordered fluorite structure at lower temperatures (below 1000 °C).12 Oxygen vacancies in the disordered fluorite structure and cation sites become ordered during the transformation process.11 Interestingly, A2B2O7 type pyrochlore oxides can exist as insulators.13 There are a few pyrochlores exhibiting dielectric, piezoelectric, and ferroelectric properties if A and B ions remain in the highest oxidation state.14−17 Some radiation tolerant pyrochlore compositions have been used to immobilize plutonium, americium, and other minor actinides.11 Although they find a wide range of applications, we confine our interests on these compositions as hosts of RE ions. As host materials, the photoluminescence (PL) properties of disordered fluorite and ordered pyrochlore structures with the same compositions have been rarely investigated. Moreover, there is minimal information available on the differences in PL properties between fluorite and pyrochlore structures of the same compositions along with their QY and the effect of phase transformation on PL properties. We hypothesize that RE2Hf2O7 nanophosphors exhibit PL properties dependent on the trivalent rare-earth ions used as dopants. According to theoretical studies, Eu 3+ -doped RE2Hf2O7 has a valence band largely consisting of bonding O2− 2p and RE3+ 6s orbitals and a conduction band largely consisting of Hf4+ 5d, antibonding O2− 2p, and RE3+ 6p orbitals.13,18 Eu3+ emits red under UV excitation through a charge-transfer band (CTB), followed by energy transfer to Eu3+. In addition, there are already sufficient literature reports about the local structure of the Eu3+ ion site that can be easily obtained from the f−f transition spectra.19−24 The ground state (7F0) and excited state (5D0) of the Eu3+ ion are nondegenerate and give information about the local symmetry and inhomogeneity of the surrounding ligands.25 Therefore, Eu3+ dopants can be used as a probe to estimate the local structure of host ions. There are previous reports on PL properties of disordered fluorite and ordered pyrochlore oxide particles; however, most of the reported synthesis techniques utilize the solid-state synthesis method.4,7,26,27 The solid-state synthesis method usually requires high temperatures above 1000 °C, where it is difficult to control the morphology and crystal phase of the particles. In comparison, the synthesis of binary oxides in the molten salt method is more attractive than many other solidstate methods due to the homogeneous nanosized morphology of products and lower synthesis temperatures.28 Molten salt synthesis involves the use of a molten salt as the medium for preparing complex oxides from their constituent materials (oxides and carbonates).29 Eutectic mixtures of salts are used to lower the liquid formation temperature.29 Moreover, the disordered fluorite phase could be obtained due to the lower synthesis temperature of molten salt synthesis; then, the relevant ordered pyrochlore phase could be prepared through phase transition endowed by calcination of the as-synthesized fluorite particles. Owing to the refractory nature of RE2Hf2O7 synthesis, PL studies at different temperatures and for RE hafnium oxides are limited. Although PL studies on limited compositions of hafnates have been carried out individually,4,26,27 RE-site-dependent PL analysis on a series of RE hafnium oxides RE2Hf2O7:5%Eu (RE = Y, La, Pr, Gd, Er, and Lu) has not been presented before. To the best of our knowledge, no systematic comparison studies on the effect of A site RE dependence and calcination on the quantum yield (QY)
and decay kinetics of RE2Hf2O7:5%Eu (RE = Y, La, Pr, Gd, Er, and Lu) NPs have been reported before under optical excitation. To fill this knowledge gap, we first synthesized a series of RE2Hf2O7:5%Eu NPs (RE = Y, La, Pr, Gd, Er, and Lu) and calcinated them at different temperatures. We then investigated their crystal structure and morphology by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), and their downconversion (Stokes) emission properties in the range of 500 to 750 nm with ultraviolet (258 nm) excitation. Detailed studies on the effects of A site RE3+ ions and calcination temperature on the crystal phase, PL, QY and decay mechanisms of these RE2Hf2O7:5%Eu NPs were performed.
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EXPERIMENTAL SECTION Chemicals. The starting materials including RE nitrate hexahydrate (RE(NO3)3·6H2O, 99.0%, RE = Y, La, Pr, Gd, Er, and Lu), hafnium dichloride oxide (HfOCl2·xH2O, 99.9%), europium(III) nitrate hexahydrate (Eu(NO3)3·6H2O, 99.9%), potassium nitrate (KNO3, 99.9%), sodium nitrate (NaNO3, 98%) and ammonium hydroxide (NH4OH, 28.0−30.0%) were purchased from Sigma-Aldrich. All reagents are of analytical grade and used directly without further purification. Synthesis of RE2Hf2O7:5%Eu3+ NPs (RE = Y, La, Pr, Gd, Er, and Lu). RE2Hf2O7:5%Eu3+ NPs were prepared by a twostep process following our previous reports.24,30,31 First, a single-source complex precursor of RE(OH)3·5%Eu(OH)3· HfO(OH)2·nH2O) was synthesized via a coprecipitation route. In the second step, size-controlled RE2Hf2O7:5%Eu3+ NPs were synthesized through a facile molten salt synthetic process using the single-source complex precursors of RE(OH)3·5%Eu( O H ) 3 · H f O ( O H ) 2 · n H 2 O ) a n d a n i t r a t e m i xt u r e (NaNO3:KNO3 = 1:1, molar ratio) at 650 °C for 6 h. In order to further investigate the effect of heat treatment and possible crystal phase transformation, the as-prepared RE2Hf2O7:5%Eu3+ NPs were further calcinated at 1000 and 1500 °C in air for 6 h (see details in S1 in the Supporting Information (SI)). Characterization. Powder XRD patterns of the RE2Hf2O7:5%Eu3+ powders (RE = Y, La, Pr, Gd, Er, and Lu) were measured by a Bruker D8 ADVANCE, X-ray diffractometer with CuKα1 radiation (λ = 0.15406 nm). The XRD data were collected by utilizing a scanning mode of 2θ ranging from 20° to 70° with a scanning step size of 0.04° and a scanning rate of 1.0° min−1. The morphologies of the powders were observed by means of a field emission scanning electron microscope (Carl Zeiss Sigma VP FESEM) equipped with a field emission gun operated at 5 kV. Energy Dispersive X-ray (EDX) spectral mapping analysis was performed under 25 kV. Raman spectra were collected using a Bruker Senterra-system using a 785 nm helium−neon laser with a spatial resolution of 2 μm. FT-IR spectra were recorded on a Bruker Alpha modular Platinum-ATR FT-IR spectrometer with OPUS software, using the samples directly (neat) without making pallets. XPS spectra were obtained with a 180° double focusing hemispherical analyzer with a 128-channel detector using nonmonochromatic Al Kα radiation (1486.86 eV) with a power of 240 W (Thermo Scientific K-Alpha+ X-ray Photoelectron Spectrometer System). This X-ray source was chosen to minimize the effects of superposition of photoelectron and Auger lines of constituent elements. The diameter of the X-ray beam was ∼0.4 mm. The 14829
DOI: 10.1021/acs.jpcc.6b04798 J. Phys. Chem. C 2016, 120, 14828−14839
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The Journal of Physical Chemistry C
Figure 1. XRD patterns of the RE2Hf2O7:5%Eu3+ samples (RE = Y, La, Pr, Gd, Er, and Lu): (a) as-synthesized and (b) the enlarged view of the diffraction peak corresponding to the (222) plane, (c) calcinated at 1000 °C and (d) the enlarged view of the diffraction peak corresponding to the (222) plane, and (e) calcinated at 1500 °C and (f) the enlarged view of the diffraction peak corresponding to the (222) plane.
hydroxyl and NO3− species on the particle surface (which could be responsible for quenching the PL emission through nonradiative routes as discussed later), and calcination significantly reduced the adsorbed hydroxyl and NO3− species from the particle surface. The trace amount of hydroxyl and NO3− species on the surface of the as-synthesized NPs could not be completely removed after being washed with deionized water four times and drying at 120 °C for 12 h. Small amounts of OH− contained on the surface of the as-synthesized NPs could be due to the exposure of the samples under ambient conditions before FT-IR and XPS measurements. The XRD patterns with four prominent diffraction peaks of the as-synthesized RE2Hf2O7:5%Eu3+ NPs (RE = Y, La, Pr, Gd, Er, and Lu) are shown in Figure 1a. The intense and sharp diffraction peaks suggest well-crystallized forms of the NP samples. Within the detection limit of the X-ray spectrometer, no additional peaks were observed. The absence of typical superlattice peaks (at 2θ = 27, 36, 50, etc.) indicated that none of the compositions were found to have ordered pyrochlore type lattice.26 These peaks are the only allowed reflections in the angular range between 20 and 60° for an oxide with the fluorite crystal structure, especially for those oxides close to the parent fluorite (MO2) structures.26 They correspond to disordered fluorite structure with space group Fm3̅m (JCPDF
energy resolution of the instrument was chosen to be 0.73 eV, so as to have sufficiently small broadening of natural core level lines together with a reasonable signal-to-noise ratio. The BE scale was calibrated with reference to Cu 3p3/2 (75.1 eV) and Cu 2p3/2 (932.7 eV) lines, giving an accuracy of 0.1 eV in any peak energy position determination. The excitation, decay lifetime, emission spectra and QY of the RE2Hf2O7:5%Eu3+ powders were measured using an Edinburgh Instruments FLS980 fluorometer system (See S2 in the SI for details).
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RESULTS AND DISCUSSION Summary of SEM, FT-IR, and XRD Analysis. SEM images were first taken to evaluate the size and morphology of the RE2Hf2O7:5%Eu3+ (RE = Y, La, Pr, Gd, Er, and Lu) particles. The average particle size of the as-synthesized RE2Hf2O7:5% Eu3+ NPs was estimated to be 32 ± 5 nm from the SEM images (S3 and S4 in the SI). After calcination at 1500 °C, the particles grew tremendously in particle size (S5 in the SI). From EDX spectra (S6 in the SI), the calculated Eu3+ concentration was found to be ∼5 mol %, consistent with the anticipated value based on the added amount of the precursors. To understand the associated structural modification with calcination, FT-IR spectra of the powders (S7 in the SI) demonstrated that the assynthesized RE2 Hf2O 7:5%Eu3+ NPs possessed adsorbed 14830
DOI: 10.1021/acs.jpcc.6b04798 J. Phys. Chem. C 2016, 120, 14828−14839
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measured lattice parameters a0 were 10.74 and 10.30 Å for the La2Hf2O7:5%Eu3+ and Lu2Hf2O7:5%Eu3+ NPs, respectively. After calcinating the as-synthesized RE2Hf2O7:5%Eu3+ NPs (RE = Y, La, Pr, Gd, Er, and Lu) at 1000 and 1500 °C for 6 h, no extra diffraction peaks corresponding to the ordered pyrochlore phase were observed for any of the samples (Figure 1c−f), even though the lattice parameters of the La2Hf2O7:5% Eu3+ and Pr2Hf2O7:5%Eu3+ NPs increased, those of the Y2Hf2O7:5%Eu3+ and Gd2Hf2O7:5%Eu3+ NPs decreased, and those of the rest did not show any change after calcination, even at 1500 °C (Table 1). This suggests that RE2Hf2O7:5% Eu3+ NPs (RE = Y, La, Pr, Gd,) were not resistant to heatinduced disordered to ordered transitions. These results match with the conclusion made in the Raman study later, which shows that the structural transformations are associated with the compounds, where cation ionic radius ratio (rA/rB) is the largest. The absence of pyrochlore phase in this series based on XRD data could be due to the inability of XRD analysis to resolve those super lattice peaks corresponding to pyrochlore structure: Disordered fluorite and ordered pyrochlore structures have the same parent pattern, and the emergence of minor reflections is the only indication of the pyrochlore structure. Therefore, the accurate identification of disordered fluorite and ordered pyrochlore structure through regular XRD power diffraction analysis is difficult; Raman spectroscopy was used to further investigate the structure of all of these samples. Raman Analysis. Raman spectroscopy has been widely utilized as a tool to distinguish between the disordered fluorite and the ordered pyrochlore compositions.26,34 On the basis of the group theory, the cubic fluorite A2B2O7 with space group Fm3̅m has only one Raman active mode T2g, where pyrochlore with Fd3m ̅ symmetry possesses six modes within the range of 200−1000 cm−1.32,34 A detailed Raman spectroscopic characterization was carried out on the as-synthesized RE2Hf2O7:5% Eu3+ NPs (RE = Y, La, Pr, Gd, Er, and Lu) in the range of 170− 1000 cm−1 (Figure 2a and S8 in the SI). To avoid the Eu luminescence interference with the Raman features of the RE2Hf2O7 (RE = Y, La, Pr, Gd, Er, and Lu) host, we used an excitation wavelength of 785 nm. The Raman spectra of the RE2Hf2O7:5%Eu3+ NPs (RE = Y, Gd, and Lu) consist of broad
card no. 78-1292) and could be indexed as (222), (662), (440), and (400) reflections with a parent fluorite unit cell; however, variation on the broadening of the XRD peaks was found to depend on the identity of the RE ion in the RE2Hf2O7:5%Eu3+ NPs (RE = Y, La, Pr, Gd, Er, and Lu), especially based on the expanded view of the peak corresponding to the (222) plane (Figure 1b). The noticeably sharper peaks for La2Hf2O7:5% Eu3+ indicated larger particle size compared with the other compositions (S1 in the SI). A peak shift from the reflection peak corresponding to the (222) plane of the La2Hf2O7:5%Eu3+ NPs at 28.76° toward higher angle for other compositions was observed (Figure 1b). The cell parameters, in conformity with the fluorite phase, for all compositions were calculated using diffraction peaks corresponding to the plane (222) of the RE2Hf2O7:5%Eu3+ NPs (Table 1). The variation of lattice Table 1. Lattice Parameters (Å) of the RE2Hf2O7:5%Eu3+ Samples (RE = Y, La, Pr, Gd, Er, and Lu) along with the Ionic Radius Ratios of RE3+/Hf4+
compositions YHfO:5%Eu LaHfO:5% Eu PrHfO:5%Eu GdHfO:5% Eu ErHfO:5% Eu LuHfO:5% Eu Eu3+/Hf4+
lattice parameter (Å) of the assynthesized
lattice parameter (Å) after calcination at 1000 °C
lattice parameter (Å) after calcination at 1500 °C
ionic radius ratio (RE3+/Hf4+)
10.44 10.73
10.44 10.73
10.40 10.75
1.38 1.56
10.61 10.55
10.62 10.51
10.66 10.50
1.50 1.44
10.38
10.38
10.37
1.37
10.30
10.30
10.30
1.33 1.44
parameters as a function of the ionic radii of RE3+ ions follows a trend as expected from the lanthanide contraction, which was in good agreement with previous experimental results and theoretical calculations.8,26,32,33 The largest and smallest
Figure 2. Raman spectra of the RE2Hf2O7:5%Eu3+ samples (RE = Y, La, Pr, Gd, Er, and Lu): (a) as-synthesized, (b) calcinated at 1000 °C, and (c) calcinated at 1500 °C. 14831
DOI: 10.1021/acs.jpcc.6b04798 J. Phys. Chem. C 2016, 120, 14828−14839
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Figure 3. XPS spectra of O 1s from the RE2Hf2O7:5%Eu3+ samples (RE = Y, La, Pr, Gd, Er, and Lu): (a) as-synthesized and (b) calcinated at 1500 °C.
then 1500 °C for 6 h each. Interestingly, no extra Raman peaks corresponding to the ordered pyrochlore phase were observed from these calcinated samples except for Gd2Hf2O7:Eu showing a transition from the disordered fluorite structure to the ordered pyrochlore structures (Figure 2b,c and S8 in the SI). This Raman analysis clearly indicated that the disordered fluorite phase of three compositions (RE2Hf2O7:5%Eu3+ (RE = Y, Er and Lu)) was thermodynamically stable up to the highest calcination temperature (1500 °C) performed in this study (Figure 2b,c and S8 in the SI); however, distinct Raman peaks for the Gd2Hf2O7:5%Eu3+ sample were observed after calcination at 1500 °C, where its spectrum was almost identical to those of the La2Hf2O7:5%Eu3+ and Pr2Hf2O7:5%Eu3+ samples. This phenomenon showed that the Gd2Hf2O7:5% Eu3+ particles are not thermodynamically stable and can be converted into the ordered pyrochlore phase when heated at relatively high temperatures (∼1500 °C), while the disordered fluorite phase is commonly metastable in the low-temperature region below 1000 °C. This further indicates that the absence of observed ordered pyrochlore phase in the RE2Hf2O7:5%Eu3+ (RE = Y, Er, and Lu) compositions could be due to the kinetic barrier. XPS Analysis. To further support the Raman observations, we carried out XPS analysis to understand the changes in the valence chemistry and binding energy of constituent element in all compositions before and after calcination to 1500 °C. The O 1s XPS spectra did not show similar surface chemistry before and after calcination (Figure 3a vs Figure 3b). The change in the O 1s XPS spectra after calcination could be due to the removal of the adsorbed NO3− and OH− species on the surface (S4 in the SI) or structural change of the RE2Hf2O7:5%Eu3+ particles (RE = Y, La, Pr, Gd, Er, and Lu). The O 1s profiles were, in general, complicated due to the overlapping contribution of oxygen from the RE and Hf ions. The spectral fitting of the O 1s binding energy (BE) peaks demonstrated changes depending on the compositions. In general, the O 1s BE peak lies within the range of 528−531 eV when bonded to
bands indicating that oxygen ions in the disordered fluorite structure are randomly distributed over the eight anion sites. On comparing with the spectra of the La2Hf2O7:5%Eu3+ (S9 in the SI) and Pr2Hf2O7:5%Eu3+ NPs having ordered pyrochlore structure, we can conclude that the RE2Hf2O7:5%Eu3+ NPs (RE = Y, Gd, and Lu) possess disordered fluorite structure, consistent with the group theory. Moreover, the Raman spectra of Er2Hf2O7:5%Eu3+ NPs clearly indicated that they were crystallized in the disordered fluorite structure, as reported in the literature after neglecting the PL peaks of Er3+.33 It was reported that if the radius ratio of A3+ and B4+ ions is
