Local Structure and Spectroscopic Properties of Eu3+-Doped BaZrO3

Article pubs.acs.org/IC

Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Local Structure and Spectroscopic Properties of Eu3+-Doped BaZrO3 Arup K. Kunti,*,†,‡ Nirmalendu Patra,§ Richard A. Harris,‡ Shailendra K. Sharma,† Dibyendu Bhattacharyya,§ Sambhu N. Jha,§ and Hendrik C. Swart*,‡ †

Department of Applied Physics, Indian Institute of Technology (Indian School of Mines), Dhanbad 826004, India Atomic & Molecular Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India ‡ Department of Physics, University of the Free State, P.O. Box 339, Bloemfontein, 9300, South Africa

Inorg. Chem. Downloaded from pubs.acs.org by WEBSTER UNIV on 02/21/19. For personal use only.

§

ABSTRACT: Pristine and Eu3+-doped BaZrO3 were synthesized via a solid-state reaction method, and the synthesized samples were systematically characterized. X-ray diffraction confirmed the formation of single and pure phases of cubic-structured BaZrO3. Extended X-ray absorption fine structure (EXAFS) spectroscopy revealed the site occupancy of Eu3+ and coordination environment around the different atomic sites. Photoluminescence (PL) excitation and emission spectra revealed the dominant absorption at 275 nm and a broad emission centered at 400 nm due to oxygen vacancies below the conduction band (CB). The PL emission intensity at 597 nm increased with increasing Eu3+ doping concentration; simultaneously, emission from the defect level decreased. This confirmed the efficient energy transfer from oxygen vacancies to Eu3+. Density functional theory was employed to calculate the density of states (DOS) to explain the mechanisms of the PL phenomenon. DOS also showed the presence of impurity states due to Eu3+ doping within the band-gap region. The coincidence of the oxygen vacancy state with Eu f state at the bottom of the CB confirmed the PL energy-transfer mechanisms from the oxygen vacancy to europium. The excited-state lifetime values of the 5D0 state decreased with increasing doping concentration due to the increase of the nonradiative transition rate. The internal quantum efficiency, small excited-state lifetime, and photometric parameters indicated that 3 mol % Eu3+-doped BaZrO3 can be a suitable candidate for the red-light-emitting device applications.

■

INTRODUCTION Rare-earth (RE)-doped materials have attracted extensive attention for their potential application in different domains of research, including phosphors, lighting, display systems, imaging, scintillators, lasers, and fiber-optic communications, sensors, and photovoltaics, because of their unique emissive properties.1−4 Recently, different RE complexes were developed in a wide range of optoelectronic systems because of efficient energy transfer (ET) from the triplet state of the ligands to the RE ions.5 Implementation to some practical applications such as solid-state lasers and phosphor devices is lacking because of their poor thermal and chemical stability.6 Oxide materials are beneficial because of their low preparation cost, nontoxicity with good thermal stability, chemical stability, and nongyroscopic properties.7 The Eu3+ ion shows red emission for the f−f transition, which can be used for modern developments in the displays and light-emitting diodes (LEDs). A blue-emitting InGaN LED chip with yellowemitting Y3Al5O12:Ce3+ phosphors is commonly used in © XXXX American Chemical Society

commercially available white LEDs. However, these phosphors produce a lower color rendering index and a high colorcorrelated temperature (CCT) for the lack of the red component in the emission spectrum.8,9 Besides these, commercially used Y2O2S:Eu3+ red phosphor exhibits a lower efficiency by excitation of near-UV or blue light for white-light generation.10 Therefore, the selection of barium zirconate (BaZrO3) as a host for the efficient red-emitting phosphor excited by an UV−LED is highly acceptable because of its wide band gap and efficient energy-transferring properties to the RE ions. Many research works have been carried out on the luminescence properties of undoped and RE-doped BaZrO3 phosphors for different applications. Cavalcante et al. reported UV-excited blue and green emissions from an undoped BaZrO3 phosphor powder.11,12 Nishi et al. and Ma et al. successfully developed a titanium- and magnesium-doped Received: November 1, 2018

A

DOI: 10.1021/acs.inorgchem.8b03088 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Two ionization chambers with a length of 30 cm were used for data collection in the transmission mode. The first ionization chamber was used to collect the incident flux (I0), and the second ionization chamber was used to collect the transmitted flux (It). The X-ray absorption coef ficient at each energy was determined by the following relationship: μ = ln(I0/It). An appropriate mixture of gases with an optimum pressure was fixed to achieve 10−20% absorption in the first ionization chamber and 70−90% absorption in the second ionization chamber to obtain a better signal-to-noise ratio. In order to get a proper edge jump, a 15-mm-diameter pallet was prepared by mixing cellulose with a proper ratio of the samples and placed on Teflon tape. The samples were placed at 45° to the incident beam for the EXAFS measurements in the fluorescence mode. The fluorescence signal (If) was detected by a Si drift detector (90° to the incident beam) within a properly selected region of interest. In this case, the Xray absorption coeff icient was determined by μ = ln(If/It), and the spectra were obtained as a function of the energy by scanning the monochromator over the specified range. However, it should be noted that, because of the low doping concentration in the samples, the Eu L3-edge data were very noisy in the higher-energy range.

BaZrO3 phosphor using a solid-state reaction for long-lasting emission and photocatalytic applications.13,14 The luminescence properties of RE (Yb3+, Er3+, Tm3+, Sm3+, and Tb3+)doped BaZrO3 phosphors have been investigated for various optical applications.15−17 Limited work has been performed on the UV-excited luminescence properties of Eu3+-doped BaZrO3 for LED applications. Drag-Jarzabek et al. studied Eu3+-doped BaZrO3 for display applications.18 Additionally, the luminescence properties of Eu3+-doped BaZrO3 have been investigated for LED applications.15,19−23 However, the details of the ET mechanism and spectroscopic properties with density functional theory (DFT) were not widely explored. In this Article, we have systematically doped Eu3+ in the BaZrO3 lattice. The site occupancy of Eu3+ and the local structure around Eu3+ were discussed using extended X-ray absorption fine structure (EXAFS) spectroscopy. The ET mechanisms were developed using UV-excited photoluminescence (PL) measurements. DFT was employed to calculate the density of states (DOS) to give evidence of an established mechanism explaining the PL emission.

■

■

RESULTS AND DISCUSSION X-ray Diffraction (XRD): Phase Purity and Structural Parameter. Figure 1 shows the diffraction patterns of the

EXPERIMENTS

Development of Eu3+-Doped BaZrO3 Phosphors. Material Preparation. A series of undoped and Eu3+-doped (1, 2, 3, 4, and 5 mol %) BaZrO3 phosphors were synthesized by the solid-state reaction method. Phosphors were prepared using high-purity reagents of barium carbonate (BaCO3), zirconium oxide (Zr2O3), and europium oxide (Eu2O3). The reagents were taken in a stoichiometric ratio and ground using an agate mortar for a few hours for uniform mixing. The ground powder was calcined at 800 °C for 4 h in an alumina crucible at a heating rate of 10 °C/min. The calcined powder was ground again and then annealed at 1400 °C for 4 h at a heating rate of 10 °C/min. Characterization Details. The crystal phases of the synthesized phosphors were characterized by using a Rigaku TTRAX III diffractometer with a Cu Kα source (λ = 0.15418 nm) in the 2θ range of 10−90°, and the scan rate was maintained at 1°/min with a 0.02° step size. Field-emission scanning electron microscopy (FESEM) images were captured using a Supra 55 (Carl Zeiss, Germany) microscope. The diffuse-reflectance spectrometry (DRS) spectra were recorded by an Agilent Cary-5000 UV−vis−near-IR spectrophotometer in the wavelength region of 200−800 nm. PL excitation and emission curves were obtained by a Hitachi F-2500 spectrophotometer within the wavelength range of 220−800 nm. The room temperature excited-state lifetime of the 5D0 state of Eu3+ was determined using an Agilent Cary Eclipse spectrometer. X-ray Absorption Techniques. In the present experiment, the synchrotron-radiation-based X-ray absorption near-edge structure (XANES) and EXAFS studies were carried out on undoped and Eu3+-doped BaZrO3 samples with different doping concentrations at the Zr K-edge and Ba and Eu L3-edges. The XANES and EXAFS measurements of the prepared phosphors were carried out at the Energy Scanning EXAFS beamline (BL-9) at an INDUS-2 synchrotron source (2.5 GeV, 100 mA) at RRCAT, Indore, India. A double-crystal monochromator (DCM) was used in the beamline that works in the photon energy range of 4−25 keV having a resolution of 104 at 10 keV. The beam was collimated using a 1.5 m horizontal premirror with a meridonial cylindrical curvature prior to the DCM, and higher harmonic generation was stopped. The horizontal focusing of the beam was made through a sharp-edge cylindrical second crystal of the DCM having a radius of curvature in the range 1.28−12.91 m, and vertical focusing of the beam to the sample was made by a Rh/Pt-coated bendable postmirror. EXAFS measurements were performed in the transmission mode for the Zr Kedge (17998 eV) and Ba L3-edge (5247 eV), while the fluorescence mode was used at the Eu L3-edge (6977 eV).

Figure 1. XRD patterns of BaZrO3,Eu3+-doped BaZrO3 and the JCPDS pattern of BaZrO3 with red peaks on the top of the image.

undoped and Eu3+-doped BaZrO3 (BZO) phosphors. All of the diffraction patterns matched very well with the cubic space group Pm3m (No. 221; PDF 00-06-0399) and point-group symmetry (Oh). Single and sharp diffraction peaks confirmed the formation of a pure and well-formed crystalline phase. There were no traces of any other secondary crystalline phases. The Ba atom occupies the cube corner positions (0, 0, 0), the Zr atom occupies the body-centered position (1/2, 1/2, 1/2), and the O atoms occupy the face-centered positions (1/2, 1/2, 0) in the cubic unit cell of BaZrO3. Moreover, the Zr atoms are coordinated with six O atoms, which forms an octahedral [ZrO6] cluster, while the Ba atoms are coordinated with 12 O atoms in a cuboctahedral configuration of [BaO12] clusters.24 In order to confirm the cubic structure and coordination of the atoms, Rietveld refinement was performed by using the Fullprof software. The refinement was continued until the smallest values for RBragg and χ2 were achieved to get the best quality of refinement. All of the refined parameters are tabulated in Table 1. The refined results are in good agreement B


Article

Inorganic Chemistry Table 1. Rietveld Refinement Results of Undoped and Eu3+Doped BaZrO3 Phosphors doping % of Eu3+

a=b=c (Å)

V (Å3)

Ba−O (Å)

Zr−O (Å)

RBragg

χ2

0 1 2 3 4 5

4.1927 4.1932 4.193 4.1919 4.1926 4.1928

73.7008 73.7287 73.7197 73.6575 73.696 73.7097

2.9647 2.965 2.9649 2.9641 2.9646 2.9648

2.0963 2.0966 2.0965 2.0959 2.0963 2.0964

1.68 1.54 2.41 1.45 2.05 2.2

1.15 1.24 1.49 1.36 1.67 1.69

shapes and sizes. Agglomeration is a common problem in the solid-state reaction method because of the high synthesis temperature. The average particle size of the phosphors was estimated to be about 200 nm. UV−Vis Spectroscopy. In order to obtain the effect of doping on the energy band gap with respect to the undoped sample, only the most intense red-light-emitting sample and the undoped samples were chosen for energy-band-gap calculation. DRS spectra of the undoped and 3 mol % Eu3+doped BaZrO3 phosphors are shown in Figure 4a. The energy band gap of the prepared nanophosphors was determined by the Kubelka−Munk (K−M) function25 and Tauc relationship. The [F(R∞)hν]2 versus hν graph is plotted for a direct band gap, and the linear portion was extrapolated up to zero. The intersection point on hν axis was taken as the energy band gap of the phosphors, as shown in Figure 4b. It was found that the energy band gaps of the undoped and doped BaZrO3 were about 5.35 and 5.20 eV, respectively. The energy band gaps of the phosphors decreased after doping. Eu3+ doping introduced intermediate energy levels below the conduction band (CB) or above the valence band (VB) and narrowed the energy band gap with increasing doping concentration, which is discussed in more detail later.26 X-ray Absorption Fine Structure Spectroscopy. XANES. The maximum doping percentage was 5 mol % in this system. It is very difficult to get large changes in the EXAFS spectra with such a small variation in the doping percentage. That is why only 1%, 3%, and 5% Eu3+-doped samples were considered for XANES analysis to get a larger change with the doping concentration. Parts a−c of Figure 5 show the normalized XANES spectra of the Eu3+-doped BaZrO3 samples along with their standards at the Zr K-edge and Ba and Eu L3-edges, respectively. Upon a comparison of the XANES features of the samples with that of the standards, qualitative information regarding the samples can be obtained. The XANES spectrum of a standard ZrO2 sample is also shown in Figure 5a along with that of the BaZrO3 samples. It can be seen from Figure 5a that the absorption edges of the samples coincide with that of ZrO2, manifesting that the oxidation state of Zr is 4+ in the above samples. Two peaks denoted by A and B at 18021 and 18033 eV, respectively, obtained in the BaZrO3 samples just above the Zr absorption edge were found to be similar to those obtained by Fassbender et al.27 and Giannici et al.28 and are due to the octahedral oxygen coordination of Zr4+

with the theoretical values. Rietveld refinement revealed that Eu3+ doping distorts its ideal structure. Doping exhibited very little change in the lattice structure of BaZrO3 because of Eu3+ doping at the Zr4+ site. There is little deviation of the lattice parameter along with the bond length, which suggests structural distortions or defects present within the system. Distortion of its ideal structure may influence the oxygen vacancies within the system. The fitted graphs for undoped and 3 mol % Eu3+-doped BaZrO3 are shown in Figure 2. FESEM. The solid-state reaction method has been applied for the synthesis of materials. During the process, at high reaction temperature, the particles were agglomerated, and there was no significant effect on the particle size with the small doping percentages due to a large number of agglomerations. So, to get an idea about the change in the average particle size of the prepared phosphors, the undoped and most intense red-emitting samples were chosen for FESEM measurements. FESEM images were collected for the undoped BaZrO3 and 3 mol % Eu3+-doped BaZrO3 phosphors to obtain the surface morphology, size distribution, and shape of the prepared phosphors. Parts a and b of Figure 3 show the FESEM images of undoped and doped samples. This shows that the particles were grown in a very densely agglomerated structure. The FESEM images show that the particles were a spherical shape, which consisted of either some single particles or clusters of particles. The image reveals a well-defined particle-like morphology, having an abundance of polygon-shaped particles with agglomeration. Parts c and d of Figure 3 show the frequency plot of the size distribution obtained by measuring the size of a large number of particles of the samples. The size distribution of the particles obeyed the normal distribution function. FESEM images of the phosphors indicate an agglomeration of particles with different

Figure 2. Rietveld refinement fittings of (a) undoped and (b) 3 mol % Eu3+-doped BaZrO3. C


Article

Inorganic Chemistry

Figure 3. FESEM images of (a) undoped and (b) 3 mol % Eu3+-doped BaZrO3. Particle-size distributions of (c) undoped and (d) 3 mol % Eu3+doped BaZrO3.

Figure 4. (a) DRS spectra and (b) K−M function plot for undoped and doped phosphors.

a Hf-doped BaZrO3 system.27 Additionally, a small shifting of the absorption edge was observed at the Zr K-edge with increasing Eu3+ doping concentration. The shifting of the absorption edge of Zr was observed due to the effect of Eu3+ doping at the Zr site. The normalized XANES spectra at the Ba L3-edge shown in Figure 5b are characterized by a sharp white line, which is the main absorption peak due to the transition 2p3/2 → 5d. It has been observed that there is no difference in the absorption edge features among the doped and undoped samples, manifesting that Eu3+ ions were not in the Ba sites of the lattice. Eu L3-edge XANES spectra of the samples, as shown in Figure 5c, show that the absorption edges coincide

in the samples. A wide shoulder (C) at 18063 eV, which was completely absent in the ZrO2 sample having tetrahedral coordination, further supports the octadedral oxygen coordination of Zr in the BaZrO3 samples. The existence of shoulder C is reported as a strong correlation between the Zr−Zr and Zr−Ba atomic pairs and the multiple scattering of the [ZrO6] octahedron.27 The overall shape of the XANES spectra of the samples remains unchanged upon doping, which suggests that the octahedral symmetry did not break after doping. However, the intensity ratio at the white line of peaks B/A decreased gradually after doping with Eu3+, manifesting the presence of Eu at Zr sites and the distortion from a perfect cubic structure. A similar result has also been observed by Fassbender et al. for D


Article

Inorganic Chemistry

Figure 5. Normalized XANES spectra of the Eu3+-doped BaZrO3 samples at the (a) Zr K-edge, (b) Ba L3-edge, and (c) Eu L3-edge.

[BaO12]) shape in the first coordination shells, as shown in Figure 8a,b. Theoretical EXAFS spectra have been generated using the above structure of the samples and fitted with the experimental data. During the fitting process, the oxygen coordination number (CN), bond distance (R) between the respective atomic pairs, and disorder factor (Debye−Waller factor σ2), which gives the mean-square fluctuation in the distances and thermal disorder in the system, have been used as fitting parameters. The goodness of fit in the above process is generally expressed by Rfactor, which is defined as

with that of a standard Eu2O3 sample, suggesting that Eu3+ remained in the Eu3+ oxidation state in the samples. EXAFS. Parts a−c of Figure 6 present the normalized EXAFS spectra [μ(E) vs E], and parts a−c of Figure 7 present the k2weighted χ(k) spectra of the Eu3+-doped BaZrO3 samples at the Zr K-edge, Ba L3-edge, and Eu L3-edge, respectively. The energy-dependent absorption coefficient μ(E) has been converted to the energy-dependent absorption function χ(E) and then to the wavenumber-dependent absorption coefficient χ(k) in order to take care of the oscillations in the absorption spectra. To obtain the real distance from the center of the absorbing atoms, k2-weighted χ(k) spectra were transformed into R space to generate the χ(R) versus R (or FT-EXAFS) plots using a Fourier transformation method. Analysis of the EXAFS data has been performed following the standard procedure29,30 using the IFEFFIT software package.31 BaZrO3 is known to have a cubic perovskite structure (ABO3) with the space group Pm3m (No. 221), with the value of the lattice parameter of ∼4.193 Å obtained from XRD. A fragment of this cubic structure has been shown in Figure 8 using the VESTA code.32 As pointed out above from the XRD analysis in this cubic structure, the Ba, Zr, Eu, and O atoms occupy the lattice positions 1a (0, 0, 0), 1b (1/2, 1/2, 1/2), lb (1/2, 1/2, 1/2), and 3d (1/2, 0, 0), respectively. In this arrangement, the Zr atoms are coordinated with six O atoms in a regular octahedral (6-fold [ZrO6]) shape, while the Ba atoms are coordinated with 12 O atoms in a cuboctahedral (12-fold

R factor =

∑

{Im[χdat (ri) − χth (ri)]}2 + {Re[χdat (ri) − χth (ri)]}2 {Im[χdat (ri)]}2 + {Re[χdat (ri)]}2 (1)

where χdat and χth refer to the experimental and theoretical χ(R) values, respectively, and Im and Re refer to the imaginary and real parts of the respective quantities. Zr K-Edge. In order to study the local surroundings around the Zr atoms, the normalized absorption spectra have been properly Fourier-transformed using the Hanning window function within the k range of 2−9 Å−1 and in the R range of 1−3.6 Å. An oscillation below 1.15 Å has been subtracted by the background subtraction routine available in the ATHENA E


Article

Inorganic Chemistry

Figure 6. Normalized EXAFS spectra of the undoped and Eu3+-doped BaZrO3 samples at the (a) Zr K-edge, (b) Ba L3-edge, and (c) Eu L3-edge.

subroutine of FEFF 6.0. The experimentally obtained phaseuncorrected data fitted with the theoretically generated model at the Zr K-edge are shown in Figure 5a. In Figure 9a, the first peak in the R range of 1−2 Å appears because of backscattering of the photoelectrons from six O atoms situated in the first coordination shell with Oh symmetry at a bond distance of 2.09 Å. This result corroborates with the XANES result presented above. The next peak with a lesser amplitude between 3 and 3.5 Å appears because of the second coordination shell coordinated by eight Ba atoms at 3.63 Å. In the fitting process, only a multiple-scattering path (Zr−O−O−Zr), which appears in the previously mentioned single-scattering paths, has been neglected because of its smaller amplitude and noncolinearity of the constituent paths. The best-fit values of the EXAFS fitting are summarized in Table 2. The value of the Zr−O bond length in the first coordination shell is similar to the value obtained by Giannici et al.28 From Table 2, it has been observed that the Zr−O bond length decreased with Eu doping. The decrease in the bond length at higher doping concentration clearly signifies the doping of Eu atoms at the Zr site. However, this observation is an indication that Eu atoms are occupying the Zr sites. In octahedral coordination, Eu3+ ions have ionic radii of 0.95 Å, which are higher than those of Zr4+ ions (0.72 Å). Thus, when Eu atoms go to the Zr sites, the Zr−O bond lengths get contracted to accommodate the Eu−O bonds. The effect of Eu doping at the Zr site was also confirmed from the changes observed in the Zr K-edge XANES spectra of the samples upon Eu doping. The values of the Debye−Waller factors of the first shell remained the same throughout the whole series, which

manifests lesser structural disorder of the anions around the Zr atoms. However, the coordination spheres involving the cations (Zr−Ba) were found to be more disordered than the anions (Zr−O), which is reflected in the higher values of the Debye−Waller factors, which leads to the amplitude of the higher peaks getting reduced. Ba L3-Edge. The normalized absorption spectra measured at the Ba L3-edge have been properly Fourier-transformed using the Hanning window function in the k range of 2.5−8 Å−1. The experimentally obtained phase-uncorrected data at the Ba L3edge and the best-fit theoretical curve in the R range of 1−3.6 Å, generated using the structure described above, are shown in Figure 9b. In the Fourier transform spectra shown as a function of the radial distribution function, the first intense peak within 1.8−2.8 Å appears because of backscattering of a photoelectron from the first shell coordinated by 12 O atoms at a bond distance of 2.96 Å. The contribution of the next shell appears within 2.8−3.8 Å because of 8 Zr atoms situated at a distance 3.63 Å form the central Ba atom. No multiplescattering paths appear within these single-scattering paths, and the best-fit results of the EXAFS parameters are shown in Table 3. From the above table, it can be seen that variation in the Ba−O distance over the samples is negligible, and it remained the same almost around the value of the undoped sample with a much higher value of the Debye−Waller factor. The negligible variation in the Ba−O bond length can also indicate the nonoccupancy of the Eu atoms at the Ba site. It can be seen from Table 3 that the average Ba−O distances obtained from EXAFS analysis of the samples are significantly F


Article

Inorganic Chemistry

Figure 7. k2-weighted χ(k) spectra of the undoped and Eu3+-doped BaZrO3 samples at the (a) Zr K-edge, (b) Ba L3-edge, and (c) Eu L3-edge.

Figure 8. (a) Octahedron around the Zr atoms and (b) cuboctahedron around the Ba atom of the BaZrO3 structure.

in the BaZrO3 sample due to intrinsic structural disorder have been observed by Giannici et al. also.28 PL Studies. Figure 10a depicts the room temperature PL excitation and emission spectra of the pure BaZrO3 and 3 mol % Eu3+-doped BaZrO3 phosphors. The PL excitation spectrum of BaZrO3 was monitored, keeping the emission wavelength fixed at 400 nm. The excitation spectrum of BaZrO3 revealed a broad peak centered at 275 nm accompanied by a smaller peak centered at 236 nm. The peak centered at 236 nm belongs to the band-gap absorption of BaZrO3 of 5.20 eV (238 nm).18,34 The broad intense excitation peak centered at 275 nm is firmly believed to be due to absorption of the (Vo) defect centers below the CB. When the excitation spectrum of Eu3+-doped

smaller compared to the values obtained from XRD analysis or structural inputs. A similar discrepancy in the values of the bond lengths obtained from EXAFS and XRD has also been observed by Lebedev and Sluchinskaya33 for BaZrO3 samples and has been attributed to the structural instability in the sample. The thermal motion of the center of the octahedral relative to the Ba atom is characterized by the σ2 value. It can be seen from Table 3 that the σ2 value of the Ba−O shell is higher than that of the Ba−Zr shell, which has also been observed by the above authors and has been attributed to the rotation of ZrO6 octahedra of BaZrO3, leading to its structural instability. High values of σ2 for the Ba−O coordination shell G


Article

Inorganic Chemistry

Figure 9. (a) Zr K-edge and (b) Ba L3-edge Fourier-transformed χ(R) versus R plot of the experimentally obtained data fitted with the theoretically generated plot.

Table 2. Values of the CN, Bond Length, and Disorder Factor Obtained from EXAFS Analysis of Eu3+-Doped BaZrO3 Samples at the Zr K-Edge scattering path

parameter

BZO

1% Eu:BZO

3% Eu:BZO

5% Eu:BZO

Zr−O (×6)

CN R (Å) σ2 CN R (Å) σ2 Rfactor

5.30 ± 0.30 2.10 ± 0.01 0.003 ± 0.001 8 3.72 ± 0.018 0.011 ± 0.003 0.003

5.44 ± 0.25 2.08 ± 0.01 0.003 ± 0.001 8 3.72 ± 0.016 0.013 ± 0.002 0.003

5.76 ± 0.45 2.07 ± 0.01 0.003 ± 0.001 8 3.72 ± 0.013 0.014 ± 0.002 0.002

6.00 ± 0.22 2.05 ± 0.01 0.003 ± 0.001 8 3.70 ± 0.014 0.011 ± 0.002 0.006

Zr−Ba (×8)

Table 3. Values of the CN, Bond Length, and Disorder Factor Obtained from EXAFS Analysis of Eu3+-Doped BaZrO3 Samples at the Ba L3-Edge scattering path

parameter

BZO

1% Eu:BZO

3% Eu:BZO

5% Eu:BZO

Ba−O (×12)

CN R (Å) σ2 CN R (Å) σ2 Rfactor

12 2.85 ± 0.02 0.014 ± 0.002 8 3.58 ± 0.01 0.002 ± 0.001 0.002

11.50 ± 0.45 2.85 ± 0.01 0.012 ± 0.001 8 3.58 ± 0.01 0.008 ± 0.001 0.004

11.79 ± 0.11 2.86 ± 0.01 0.014 ± 0.002 8 3.64 ± 0.01 0.003 ± 0.001 0.004

11.65 ± 0.20 2.87 ± 0.01 0.012 ± 0.003 8 3.66 ± 0.01 0.004 ± 0.002 0.004

Ba−Zr (×8)

BaZrO3 was recorded, keeping the emission wavelength fixed at 597 nm, the host absorption band was absent. The defect center band shows red shifting after Eu3+ doping in the lattice, as shown in Figure 10b. Apart from this, in the range of 350− 550 nm, feeble excitation peaks are observed that are ascribed to the f−f transitions of Eu3+. This implies that a high quantum efficiency of PL excitation originates from the trap level.18

The shallow and deep trap levels originate from structural disorder and the simultaneous presence of the h•/e′ pair in the BaZrO3 lattice. In the ordered BaZrO3 structure, Zr atoms in two [ZrO6] octahedra are connected by corner-sharing O atom, as shown in Figure 11a. According to the disordered model proposed by Cavalcante et al., the Zr−O bond breaks, which displaces the Zr atom in the [001] direction.35 This H


Article

Inorganic Chemistry

Figure 10. (a) Excitation and emission spectra of BaZrO3 and 3 mol % Eu3+-doped BaZrO3. (b) Enlarged excitation spectra.

Figure 11. (a) Ordered structure of BaZrO3. (b) Schematic representation of ET from the oxygen vacancy to Eu3+ in a disordered BaZrO3:Eu3+ structure. (c) Schematic diagram of the PL mechanism of BaZrO3:Eu3+ phosphors.

displacement forms two environments of Zr; one is [ZrO5· VOz], where VOz = VOx, VO•, and VO•• are designed as squarebase pyramids and [ZrO6] is designed as an octahedron, as shown in Figure 11b. Thus, the disordered model can be represented as [ZrO5·VOz] − [ZrO6]. The disordered structure produces a charge gradient due to breaking of the Zr−O bond. This charge gradient and the presence of the localized states offer good conditions for the trapping of electrons and holes.35 Before excitation, the disordered structure creates a hole and electron that behave as the acceptor and donor according to the following equation:36 [ZrO6 ]o x + [ZrO6 ]d x → [ZrO6 ]o ′ + [ZrO6 ]d•

(2)

[ZrO6 ]x + [ZrO5ΔVo x]→[ZrO6 ]′+[ZrO5·ΔVo•]

(3)

[ZrO6 ]x + [ZrO5 ·Vo•]→[ZrO6 ]′+[ZrO5 ·Vo••]

(4)

deep defect levels because of the creation of holes and electrons in the band-gap region. According to Longo et al., the violet and blue emissions are ascribed to shallow-level defects, and the blue-green emission is attributed to the deep-level defect within the band gap.11,35 Apart from the trap-level emission, the PL emission spectrum of 3 mol % Eu3+-doped BaZrO3 exhibited an intense sharp emission at 597 nm that corresponds to the 5D0 → 7F1 magnetic-dipole (MD) transition and is much stronger than that of the 5D0 → 7F2 electric-dipole (ED) transition.37 It is well-known that ED and MD transitions are strong, depending on the symmetry and environment around the Eu3+ ion. The MD transition is dominant when the Eu3+ ions occupy a site with inversion symmetry. On the other hand, the ED transition is dominant when Eu3+ ions occupy without inversion symmetry.38 Because the MD transition is stronger, it is confirmed that Eu3+ ions must occupy the inversion symmetry site in the BaZrO3 matrix. Because of the difference in the ionic sizes of Ba2+ (135 Å), Zr4+ (72 Å), and Eu3+ (95 Å), it could be considered that Eu3+ ions must occupy centrosymmetric Zr4+ sites in the BaZrO3 lattice.18,37 The emission spectrum of Eu3+-

where [ZrO6]ox is the ordered cluster, [ZrO6]dx is the disordered cluster, [ZrO6]′ is the electron donor, [ZrO5·ΔVo•] is donor or acceptor capable of capturing or emitting both electrons and holes, and [ZrO5·ΔVo••] is the acceptor. This suggests that the disorder structure introduces shallow and I


Article

Inorganic Chemistry

Figure 12. (a) Excitation spectra. (b) Emission spectra. (c) Intensity versus Eu3+ concentration. (d) Logarithmic plot of the intensity for different Eu3+ concentrations.

ground-state electrons, and the 5D0 level is populated.26,44−46 Thus, Eu3+ exhibits its characteristic emission from 5D0 to 7 F1−2, and the mechanism is depicted in Figure 11c. Figure 12b displays a plot of the Eu3+ emission at 597 nm and trap-level emission intensity against the Eu3+ concentration. From Figure 12c, it has been observed that the defect level emission decreased gradually with an increase of Eu3+; conversely, Eu3+ emission increased to a maximum at 3 mol % doping. It is well-known that shallow defects can create localized energy states below the CB. When light of 275-nm wavelength was used to excite the electrons, the excited electrons emitted blue emission via defect states. The blue emission intensity decreased with increasing doping concentration. The decreasing nature of blue emission with increasing doping concentration denotes the efficient ET from the defect state to Eu3+ ions. This confirms that the ET efficiency (η) from the trap level to Eu3+ ion increased (shown in Figure 12c) with the doping concentration. Figure 12c reveals that the emission intensity of 5D0 → 7F1 (at 597 nm) reached a maximum at 3 mol % Eu3+ doping due to concentration quenching. Concentration quenching can be attributed to nonradiative ET between the neighboring Eu3+ ions. Nonradiative ET can be ascribed by exchange or multipolar interaction.47 Exchange interaction plays an important role if the distance between two neighboring RE ions is sufficiently low. The nature of the interaction mechanism can be well understood by the critical distance

doped samples shows some weak peak above 700 nm due to the f−f transition of the Eu3+ ion. Effect of the Eu3+ Doping Concentration. Parts a and b of Figure 12 show the excitation and emission spectra of Eu3+doped BaZrO3 phosphors under 275 nm excitation. Different kinds of possible models have been established to explain the ET mechanism between the RE ions and host via defect states, such as the defect-related auger transition model, resonant ET model, bound exciton model, shallow center model, etc.39−42 In the present case, different photoexcitation channels were involved in the emission associated with the Eu3+ ions. In the first case, when Eu3+-doped BaZrO3 phosphors are excited, the excitation energy promoted the electrons to the defect levels below the CB. The excited electrons are trapped in different trap states and recombine with the holes, resulting in a broad emission centered at 400 nm. Some of the excited electrons from the defect states hop to the excited states of Eu3+ such as the 5DJ (J = 4−1) directly or via several oxygen vacancy levels below the CB. The excited electrons nonradiatively relax to the 5 D0 state of Eu3+, as shown in Figure 11c. Thus, the 5D0 state is populated nonradiatively and gives radiative emission.16,26,43 Another possible mechanism may be the excited electron absorbed in the defect states, and broad-band emission generates after recombination with the hole. At the same time, a part of the absorbed excited electrons in the defect levels, nonradiatively recombine with holes above the VB, and part of the energy resonantly or by phonon-mediated processes is transferred to the surrounding Eu3+ ions, which excites the J


Article

Inorganic Chemistry Table 4. Spectral Parameters of the Eu3+-Doped BaZrO3 Phosphors concn of Eu3+ (mol %)

Ω2 (pm2)

Ω4 (pm2)

R (I2/ I1)

A1 (s−1)

A2 (s−1)

A4 (s−1)

τexp (ms)

Arad (s−1)

Anr (s−1)

β1 (%)

β2 (%)

β4 (%)

IQE (%)

1 2 3 4 5

0.669 0.63 0.534 0.692 0.841

0.024 0.033 0.034 0.045 0.046

0.43 0.41 0.34 0.45 0.54

117.2 117.2 117.2 117.2 117.2

52.34 49.25 41.74 54.07 65.77

0.85 1.27 1.28 1.74 1.75

3.44 3.24 3.12 2.74 2.54

170.39 167.73 160.23 173.01 184.72

120.3 140.91 160.28 191.94 208.97

68.78 69.87 73.14 67.74 63.44

30.71 29.36 26.05 31.25 35.6

0.49 0.76 0.8 1 0.94

58.61 54.34 49.99 47.4 46.92

The other two parameters Ω4 and Ω6 are less sensitive to the symmetry environment and reflect the information about longrange effects. J−O parameters were determined from the radiative transition probability of the 5D0 level of Eu3+ following the described procedure.55 In this method, the radiative transition probability and the integrated emission intensities between the two manifolds 5D0 and 7FJ (J = 2, 4, 6) are related to each other by the relationship

(Rc), which can be calculated using the Blasse equation48,49 expressed as ÄÅ É Å 3V ÑÑÑ1/3 ÑÑ R c = 2ÅÅÅÅ ÅÅÇ 4πCN ÑÑÑÖ

(5)

where C is the critical concentration of the Eu3+ ions and N the number of cations in the unit cell of volume V. For the BaZrO3 host, C = 0.03, N = 8, and V = 73.6575 Å3 are used, and Rc is obtained at about 8.37 Å. Here, exchange interaction is not possible between the Eu3+ ions because a Rc value greater than 5 Å is indicated. The larger value of Rc indicates that multipolar interaction was responsible for the concentration quenching. Different kinds of multipolar interaction may be involved in the process of ET, such as dipole−dipole (d−d), dipole− quadrupole (d−q), and quadrupole−quadrupole (q−q). According to the Dexter theory,50 the emission intensity (I) per activator ion is given by I K = C β(C)s/3

A 0 − 2,4 A 0−1

=

I0 − 2,4 hν0 − 1 I0 − 1 hν0 − 2,4

(7)

where I0−J denotes the integrated emission intensity and hν0−J is the energy barycenter corresponding to the transition 5D0 → 7 FJ (J = 1, 2, 4). The MD radiative transition probability, A0−1, can be calculated by the expression56 A0−1 = 0.31 × 10−11n3(hν0−1)3, and its value can be determine to be approximately 117.20 s−1. The radiative transition probabilities A0−2,4 for ED transitions can be represented as a function of the J−O intensity parameter:

(6)

A 0−J =

where C is the activator concentration, K and β are constants for a particular interaction, and s determines the nature of the interaction. s values of 6, 8, and 10 indicate the d−d, d−q, and q−q exchange interactions, respectively. The electric multipolar character was determined by the slope (−s/3) of plot log(I/C) versus log C. The slope value of −1.78 was determined from the linear curve of log(I/C) versus log C, as presented in Figure 12d. Hence, the s value was determined as 5.34, which is quite close to the theoretical value of 6 for the d−d interaction. It can be concluded that the d−d-type exchange interaction is the main mechanism for the concentration quenching. It is confirmed that the MD transition was the dominant emission. Therefore, the asymmetry ratio R = I(5D0→7F2)/ I(5D0→7F1) calculation is required to represent the degree of covalency and symmetric nature around the Eu3+ ion in the host lattice.51 The asymmetric ratio helps to predict the nature of the covalence bonding, polarization environment around the Eu3+ ions by the short-range effects, and centrosymmetry distortion of the Eu3+ ion site. The asymmetry ratio decreased by up to 3 mol % Eu3+ doping and then increased with further doping. The descending nature of the R value indicates that the symmetry increased around the Eu3+ ions and higher Eu−O covalence and vice versa. Spectroscopic Parameters. The nature of the PL behavior of Eu3+ in the host BaZrO3 can be investigated by Judd−Ofelt (J−O) intensity parameters Ωt (t = 2, 4, 6).52,53 The site symmetry and internal quantum efficiency (IQE) were examined by J−O analysis. The Ω2 parameter is sensitive to the symmetry and reflects the information about the local environment around Eu3+ and the polarizability of Eu3+ and O bonding.54

64π 4(ν0 − 2,4)3 e 2 3hc 3

1 χ 4πε0

∑ J = 2,4

ΩJ 5D0 |U (J )|7F2,4 2

(8)

where χ represents the Lorentz local-field correction factor, n(n2 + 2)2

given as χ = , and n is the refractive index of the host. 9 Thus, Ω2,4 can be determined using eqs 7 and 8. In this case, Ω6 cannot be determined because the 5D0 → 7F4 transition is absent. The calculated J−O intensity parameters are tabulated in Table 4. Because Ω2 is especially sensitive to the local environment of Eu3+, it can be properly understood by the prominent change of its value with the systematic variation of the Eu3+ doping concentration. Table 4 depicts that Ω2 does not vary in a greater value with the RE concentration. It indicates that Eu3+ doping in BaZrO3 does not have much affect on the symmetry around the doping site. Similar results are reflected by the asymmetry ratio (R). Ω2 indicates the strengthening of the covalency of the Eu−O bond. First, Ω2 decreased up to 3 mol % doping, and upon further doping, the Ω2 value as well as the value of R increased; it tends to increase with higher doping concentration. This result implies that the covalency of the Eu−O bonds and distortion of the symmetry of the Eu3+ sites increased with the doping concentration up to 3 mol %.53 The changes of the R and Ω2 values can be described by the displacements of the Zr atom due to the substitution of Eu3+ at the Zr4+ site in the BaZrO3 lattice. The value of Ω4 is not directly related to the symmetry around the Eu3+ ions but associated with the electron density in the surrounding O2− anions, and its value decreased when the electron density on the ligands increased. The total transition probability (AT) can be determined using the following relationship: K


Article

Inorganic Chemistry A T (ψ J ) =

∑ AJ− J′

The calculated IQE is tabulated in Table 4. Table 4 shows that IQE is also decreased with increasing doping concentration due to concentration quenching. DFT Calculations. Methodology. The calculations presented here employ DFT with the generalized gradient approximation (GGA-DFT), implemented in the DMOL3 code. The exchange and correlation energies are described using the parametrization of Perdew and Wang, PW91. A plane-wave basis set with three-dimensional periodic boundary conditions were applied, and the double numeric plus polarization basis set was employed with an self-consistentfield tolerance of 1.0 × 10−5 Ha. Spins remained unrestricted, with the formal spin set as the initial spin, and symmetry was also employed. Multiple simulations were run where substitutional Eu3+ ions were systematically added at appropriate lattice positions to the BaZrO3 crystal lattice with a lattice constant of 4.1927 Å. Likewise, oxygen vacancies were added, the DOS for these different simulations were monitored, and the results are presented in Figure 14. DOS. The PL emission curve suggested that oxygen-related defects influence the PL emission and ET rate. Therefore, it is necessary to investigate the electronic DOS of ideal, oxygen vacancy, and Eu3+-incorporated BaZrO3. To create the defect structure, first O atoms were removed from the structure one by one (up to four O atoms within the BaZrO3 lattice of the ideal BaZrO3 structure), and the DOS was calculated. Similarly, Eu3+ ions were substituted at the Zr sites one by one (up to four Eu3+ ions within the 3 × 3 × 3 unit cell of the ideal BaZrO3). Furthermore, oxygen vacancies were also taken into account with the Eu3+-doped BaZrO3 structure to investigate the DOS. Both the oxygen vacancy and Eu3+ incorporation were considered one by one, with up to two oxygen vacancies introduced and four Eu3+ ions incorporated within the lattice of the ideal BaZrO3. In BaZrO3, the top of the VB is mainly composed of 2p orbitals of O and a slight contribution from 4d orbitals of Zr. The bottom of the CB is mainly composed of 4d orbitals of Zr with a small contribution from 2p orbitals of O. Figure 14 depicts the DOS of BaZrO3, where the Fermi energy (EF) level is set to 0 eV on the energy scale. Figure 14a represents the DOS of pristine BaZrO3. It is observed that the Fermi energy level coincides with the top of the VB. The estimated energy band gap is found to be 2.69 eV. The electronic band gap is underestimated with respect to the experimentally calculated band gap from the DRS spectrum. Such an underestimation of the band-gap values is common for the different exchange-correlation functionals of the DFT calculations. A pseudo band gap is present in the VB that signifies the strong covalent bonding between Zr and O. The Ba is weakly hybridized with O in the BaZrO3 structure. The DOS of BaZrO3 in the presence of the oxygen vacancy (Ov) is shown in Figure 14a. The DOS shows that the EF level shifts to near the bottom of the CB and an appreciable amount of impurity states are present at the EF level and in the vicinity of the Fermi level. When an O ion is removed, a defective BaZrO3 model with an oxygen vacancy is created. The total energy is then lowered by the attraction of the surrounding Ba and Zr ions, and these ionic site distortions leave the defect levels shifted with respect to the band’s original position to the middle of the band gap. The overall nature of the VB remain same, but the shape of the CB is changed. The CB is composed of the two states mentioned as L (lower part of the CB) and U (upper part of the CB). The relative number of states of the L

(9)

J′

5

The excited-state radiative lifetime of D0 can be determined by the following relationship: 1 τrad(ψJ ) = A T (ψ J ) (10) The corresponding branching ratio of the emission wavelength from the excited state can be determined by the following relationship: β(ΨJ ) = (AJ − J /A T) × 100% (11) ′ Excited-State Lifetime and IQE. The total transition probability is related to the radiative and nonradiative transition probabilities by the following relationship: 1 1 1 = A nrad + A rad = + AT = τexp τnrad τrad (12)

where τexp denotes the experimentally measured excited-state lifetime from the decay curve. Figure 13 shows the decay curves for PL emission of Eu3+ at 597 nm (5D0 → 7F1) in the BaZrO3 phosphor. The decay

Figure 13. PL decay curve for 1−5 mol % Eu3+-doped BaZrO3 phosphors.

curve was fitted by employing a biexponential decay function:57 I = A1e−τ / τ1 + A 2 e−τ / τ2

(13)

where A1 and A2 are the decay constant, τ denotes time, and τ1 and τ 2 are the decay times for corresponding exponential components. The average excited-state lifetime was determined using the following relationship: τexp =

A1τ12 + A 2 τ2 2 A1τ1 + A 2 τ2

(14)

It can be observed that the lifetime values of the 5D0 states decrease with an increase of the Eu3+ doping concentration. The decreasing nature of the lifetime can be ascribed to the increase of the nonradiative transition rate caused by concentration quenching.58 The IQE of the 5D0 level is determined by A rad IQE (%) = × 100 A rad + A nrad (15) L


Article

Inorganic Chemistry

Figure 14. (a) DOS of pristine, oxygen vacancy, and Eu3+-incorporated BaZrO3 phosphors. (b) DOS of BaZrO3:Eu3+ with the presence of oxygen vacancy.

state increased, and the relative number of states of the U state decreased. This indicates that most of the electrons are localized at the L state. A new small energy state formed near the EF level, 1.71 eV above the VB maxima (VBM), and another impurity state formed between the L and U states. These impurity states appeared because of the presence of Ov in the BaZrO3 lattice. The CB is majorly influenced by Ov in the lattice. For this case, the difference between the VBM and CB minima (CBM) is 2.174 eV. The energy band gap of the material was reduced because of the presence of the oxygen vacancy. In the previous section, the excitation spectrum of the pristine material showed that the maximum absorption at 275 nm (4.5 eV) was due to Ov, which is 0.50 eV (Eg = 5.0 eV) below the CBM. Thus, the theoretically calculated position of the Ov level matches very well with our experimental results. Figure 14a also depicts the DOS of BaZrO3 with the Eu3+ ion incorporated at the Zr site. The DOS illustrates that the overall shapes of the VB and CB remain mainly unaltered when the Zr site is replaced with one Eu3+ ion. However, the nature of the CB changed significantly, increasing the value of the L state more than that of the U state in the CB. The EF level shifts to 0.44 eV above the VBM. An impurity state arises at the EF level because of Eu3+ ion incorporation in the ideal lattice. In this case, the difference between the VBM and CBM is now 2.224 eV. The band gap of the material decreased as a result of Eu3+ incorporation. The overall nature of the VB and CB altered systematically with Eu3+ incorporation one by one, up to four ions, respectively. Two impurity states appeared within the band gap above the VBM as a result of more than one Eu3+ incorporation. One is very close to the EF level, and another one is approximately 1.45 eV above the VBM. The state near the EF level slightly shifts toward the CB for two Eu3+

incorporations. However, very close observation shows that the DOS remains the same for three Eu3+ incorporations, with only a small shift observed for both of these states toward the VB. The localization states increase with Eu3+ incorporation at the bottom of the CB, and the value of the L state increases relatively high compared to the U state for two Eu3+ incorporations. All of the states are localized at the bottom of the CB, and the U state disappeared from the CB when more than two Eu ions are incorporated in the lattice. The EF level shifts toward the VB accordingly, with increasing Eu doping due to increasing hole creation with Zr4+ replacement by Eu3+. The EF level also shifts to the top of the VB for four Eu3+ incorporations. Figure 14a shows that the nature of the VB also changed, especially the state’s majorly localized upper part of the VB than the lower one with Eu3+ incorporation, respectively. Also, a weak new impurity state appears at the top of the upper part of the VB, which is 0.95 eV below the VBM. This state becomes dominant, and the lower state of the VB decreases when three Eu3+ ions are incorporated. This state is also significantly increased when four Eu 3+ ions are incorporated, and the lower state of the VB becomes feeble. It is quite interesting that the impurity states above the VBM are shifted 0.45 eV from the EF level and the difference between the two states is reduced to 0.9 from 1 eV for four Eu3+ incorporations. The whole DOS shifted toward higher energy. All of these impurity states are composed of Eu f states and form a band that is responsible for emission from the Eu3+ ion due to the f−f transition. It can be assumed that the band 0.95 eV below the VBM is composed of ground states (7FJ, J = 6−0) of the Eu3+ ion and the band 1.30 eV above the VBM is composed of excited states (5DJ, J = 2−0) of the Eu3+ ion. The energy difference between these two bands is 2.25 eV (550 nm), which corresponds to the 7F2 → 5D1 transition, as shown M


Article

Inorganic Chemistry

it is noted that the Ov states are always strongly localized below the CB and above the Eu f states. The impurity states of Ov and Eu f coincide with each other at the bottom of the CB; this also confirms that Ov interacts with Eu; hence, energy is transferred from Ov to Eu. The photoexcited electrons in the VB may migrate to the impurity states associated with Ov and can be transferred nonradiatively to the excited states of Eu f. Thus, the oxygen vacancy favors ET to the dopant. This significantly supports the PL mechanisms explained in the previous section. Photometric Characterization. The color coordinates for all of the phosphors are tabulated in Table 5. Figure 15 depicts

in Figure 12a. The excitation peak at 550 nm confirms our assumptions. Figure 14b depicts the DOS of BaZrO 3 with Eu 3+ incorporation with the presence of oxygen vacancy. The overall nature of the VB remains unaltered with one Eu3+ incorporation and one oxygen vacancy, but the nature of the CB changes, reducing the value of the L state. Eu 3+ incorporation and Ov influence the DOS at the upper part of the CB. In this case, the energy difference between the VBM and CBM is 2.42 eV, which is a smaller value compared to the pristine material (2.69 eV). This energy band gap is greater with respect to the Ov-associated system. The EF level lies above the VBM, and the position is between the EF level position of the Eu3+-incorporated and Ov-associated system. The impurity states arise 0.25 eV above the VBM, the EF level, and 1.76 eV above the VBM. The positions of these states match very closely those of the states of Ov and Eu. The first two states above the VB are due to the Eu3+ ions. The impurity state above the EF level or in another sense near the CB is due to Ov. There is little deviation of the energy position of the states from one Eu3+- and one Ov-incorporated states. This deviation occurred because of the overlapping of the states Eu3+ and Ov. The Ov state is pushed toward the EF level due to the presence of Eu, and the gap between the impurity states and EF level is reduced. When two Ov were introduced with only one Eu3+ ion, the nature of the VB and CB returned to the nature of the pristine sample. The EF level also shifted toward the CB by 0.39 eV compared to the system with only one Ov and one Eu3+ ion. The impurity states appear predominantly within the band-gap region. A small impurity state appeared between the L and U states in the CB. The difference in energy between the VBM and CBM is 2.22 eV, and the reduction of the electronic band gap is less in this case compared to the system with one Ov and one Eu3+ ion. The impurity states extend deeply into the VB and CB, which results in a reduction in the band gap by 0.47 eV with respect to the pristine material. Another impurity state appears in the VB because of Ov. When one more extra Eu3+ ion is inserted with this combination, several impurity states are generated within the band-gap region along with the states deeply into the VB and CB, which extends the tails of the VB and CB, as shown in Figure 14b. A slight impurity level arises 0.50 eV below the VBM, and another two impurity levels arise at 0.28 and 1.28 eV above the VBM, respectively. These levels correspond to the energy levels to the Eu f states. Apart from these, another impurity level appears 1.70 eV above the VBM, which is associated with the states of Ov. The value of the impurity state within the L and U states increased and the energy separation between the L and U states increased because of the overlapping of states of Eu and Ov within the CB. Thus, the nature of the VB and CB alter, and the reduction of the electronic band gap is observed by 0.31 eV compared to the pristine system. When four Eu3+ ions are incorporated with one Ov, the nature of the VB and CB is changed. The impurity states generate deeply into the CB and VB, and the EF level shifts into the CB. The states in the CB overlap each other and form a single band. A similar phenomenon happens in the VB: the states in the upper part of the VB increase, and the states in the lower part almost diminish. The VB and CB tails spread into the band-gap region, and the electronic band gap reduces to 1.75 eV because of heavy Eu doping with Ov. From Figure 14b,

Table 5. Photometric Parameters of Undoped and Eu3+Doped BaZrO3 Phosphors Eu3+ concn (mol %) 0 1 2 3 4 5

(x, y) 0.17, 0.32, 0.38, 0.47, 0.40, 0.40,

CCT (K)

0.10 0.20 0.25 0.31 0.26 0.26

>100000 17486 2386 1796 2175 2156

Figure 15. CIE plot for undoped and different Eu3+-doped BaZrO3 phosphors.

that color emission traverses from the blue region to the red region with a change in the Eu3+ concentration. The 3 mol % Eu3+-doped sample showed red emission (0.47, 0.31), which was very close to ideal red light. The color-correlated temperature (CCT) was calculated using the McCamy empirical formula59 CCT = − 437n3 + 3601n2 − 6861n + 5514.31

(16)

where n = (x − xe)/(y − ye), xe= 0.3320, and ye = 0.1858. Usually, a CCT value below 5000 K defines warm light, which can be used for the commercial lighting applications. In this case, CCT < 5000 K was observed from 2 mol % Eu3+ doping. The color purity changes with the doping concentration. A 35.1% color purity was achieved for 3 mol % doping. The photometric parameters, small excited-state lifetime, and IQE N


Inorganic Chemistry

■

ACKNOWLEDGMENTS The authors are highly thankful to the Indian Institute of Technology (Indian School of Mines), Dhanbad, India, for providing financial support to carry out the research work. Financial support from the University of the Free State is also acknowledged. This research is supported by the South African Research Chairs Initiative of the Department of Science and Technology (Grant 84415).

suggest that these materials can be potential candidates for optoelectronic applications.

■

CONCLUSION Undoped and Eu3+-doped BaZrO3 were synthesized by a solidstate reaction method and characterized by XRD, EXAFS, UV−vis, FESEM, and PL techniques. XRD confirmed the phase purity and formation of the cubic phase of BaZrO3. Rietveld refinement was performed to calculate the structural parameters. A EXAFS study confirmed the structural parameters obtained from the Rietveld refinement. EXAFS spectra confirmed 12 and 6 coordination environments around Ba and Zr. EXAFS spectra showed a negligible variation in the Ba−O bond length, which indicated the nonoccupancy of the Eu atoms at the Ba site. It has been observed that, upon doping with Eu, the Zr−O bond distance decreased. Although this decrease was not significant, it may be due to the low doping concentration of Eu atoms. However, this observation confirmed that Eu atoms occupied the Zr sites. The energyband-gap estimation from DRS spectra showed a reduction of the energy band gap due to Eu3+ doping. The PL excitation spectrum showed dominant absorption at 275 nm due to oxygen vacancies below the CB. PL spectra showed broad defect-related emission centered at 400 nm and dominant emission at 597 nm, which is the MD transition because Eu3+ occupied the centrosymmetric Zr4+ site in the cubic BaZrO3 lattice. The PL intensity at 597 nm increased with increasing Eu3+ doping concentration; simultaneously, the emission from the defect level decreased. This confirms that energy was efficiently transferred from the defect states to the Eu3+ ion. DOS calculations indicated the presence of the oxygen vacancy below the CB. DOS also showed the presence of impurity states due to Eu3+ doping within the band-gap region. The coincidence of the oxygen vacancy state with the Eu f states at the bottom of the CB confirmed the PL ET mechanisms from the oxygen vacancy state to the Eu f states. The CIE diagram showed that color emission traverses from the blue to red region with the Eu3+ doping concentration. It could be possible to tune the color emission by varying the doping concentration. It was found that the 3 mol % Eu3+-doped sample emitted red color (0.47, 0.31) that was very near to ideal red light. Excited-state lifetime values of the 5D0 state decreased with an increase of the Eu3+ doping concentration. The decreasing nature of the lifetime can be ascribed to the increase of the nonradiative transition rate caused by concentration quenching. The average excited-state lifetime was estimated to be 3.12 ms, and the IQE was found to be 50% for 3 mol % Eu3+ doping. The IQE, small excited-state lifetime, and photometric parameters indicate that 3 mol % Eu3+-doped BaZrO3 can be used for red-light-emitting device applications.

■

Article

■

REFERENCES

(1) Li, J. G.; Wang, X.; Watanabe, K.; Ishigaki, T. Phase structure and luminescence properties of Eu3+-doped TiO2 nanocrystals synthesized by Ar/O2 radio frequency thermal plasma oxidation of liquid precursor mists. J. Phys. Chem. B 2006, 110 (3), 1121−1127. (2) Wang, H.-Q.; Batentschuk, M.; Osvet, A.; Pinna, L.; Brabec, C. J. Rare-Earth Ion Doped Up-Conversion Materials for Photovoltaic Applications. Adv. Mater. 2011, 23 (22−23), 2675−2680. (3) Sabikoglu, I.; Ayvacıklı, M.; Bergeron, A.; Ege, A.; Can, N. Photoluminescence investigations of Li 2 SiO 3 Ln (Ln = Er3+Eu3+Dy3+Sm3+) phosphors. J. Lumin. 2012, 132 (6), 1597−1602. (4) Sailaja, S.; Dhoble, S. J.; Sudhakar Reddy, B. Synthesis and photoluminescence properties of Sm3+ and Dy3+ ions activated Ca2Gd2W3O14 phosphors. J. Mol. Struct. 2011, 1003 (1−3), 115−120. (5) Greco, C.; Moro, G.; Bertini, L.; Biczysko, M.; Barone, V.; Cosentino, U. Computational Investigation on the Spectroscopic Properties of Thiophene Based Europium beta-Diketonate Complexes. J. Chem. Theory Comput. 2014, 10 (2), 767−777. (6) Gu, H.; Hou, Y.; Xu, F.; Wang, S. Electrospinning preparation, thermal, and luminescence properties of Eu2(BTP)3(Phen)2 complex doped in PMMA. Colloid Polym. Sci. 2015, 293 (8), 2201−2208. (7) Conde-Gallardo, A.; Garcıa-Rocha, M.; Hernández-Calderón, I.; Palomino-Merino, R. Photoluminescence properties of the E3+ activator ion in the TiO2 host matrix. Appl. Phys. Lett. 2001, 78 (22), 3436. (8) Som, S.; Kunti, A. K.; Kumar, V.; Kumar, V.; Dutta, S.; Chowdhury, M.; Sharma, S. K.; Terblans, J. J.; Swart, H. C. Defect correlated fluorescent quenching and electron phonon coupling in the spectral transition of Eu3+ in CaTiO3 for red emission in display application. J. Appl. Phys. 2014, 115 (19), 193101. (9) Xia, Z.; Zhuang, J.; Liao, L. Novel red-emitting Ba2Tb(BO3)2Cl:Eu phosphor with efficient energy transfer for potential application in white light-emitting diodes. Inorg. Chem. 2012, 51 (13), 7202−7209. (10) Ju, Z.; Wei, R.; Gao, X.; Liu, W.; Pang, C. Red phosphor SrWO4Eu3+ for potential application in white LED. Opt. Mater. 2011, 33 (6), 909−913. (11) Cavalcante, L. S.; Sczancoski, J. C.; Espinosa, J. W. M.; Mastelaro, V. R.; Michalowicz, A.; Pizani, P. S.; De Vicente, F. S.; Li, M. S.; Varela, J. A.; Longo, E. Intense blue and green photoluminescence emissions at room temperature in barium zirconate powders. J. Alloys Compd. 2009, 471 (1−2), 253−258. (12) Cavalcante, L. S.; Sczancoski, J. C.; Longo, V. M.; De Vicente, F. S.; Sambrano, J. R.; de Figueiredo, A. T.; Dalmaschio, C. J.; Li, M. S.; Varela, J. A.; Longo, E. Intense violet−blue photoluminescence in BaZrO3 powders: A theoretical and experimental investigation of structural order−disorder. Opt. Commun. 2008, 281 (14), 3715− 3720. (13) Moon, C.; Nishi, M.; Miura, K.; Hirao, K. Blue long-lasting phosphorescence of Ti-doped BaZrO3 perovskites. J. Lumin. 2009, 129 (8), 817−819. (14) Ma, X.; Zhang, J.; Li, H.; Duan, B.; Guo, L.; Que, M.; Wang, Y. Violet blue long-lasting phosphorescence properties of Mg-doped BaZrO3 and its ability to assist photocatalysis. J. Alloys Compd. 2013, 580, 564−569. (15) Gupta, S. K.; Pathak, N.; Kadam, R. M. An efficient gelcombustion synthesis of visible light emitting barium zirconate perovskite nanoceramics: Probing the photoluminescence of Sm3+ and Eu3+ doped BaZrO3. J. Lumin. 2016, 169, 106−114.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Arup K. Kunti: 0000-0002-9340-1107 Richard A. Harris: 0000-0003-0366-9941 Hendrik C. Swart: 0000-0001-5233-0130 Notes

The authors declare no competing financial interest. O


Article

Inorganic Chemistry

intense visible green photoluminescence in BaZrO3 powders. J. Appl. Phys. 2008, 103 (6), 063527. (36) Moreira, M. L.; Buzolin, P. G.; Longo, V. M.; Nicoleti, N. H.; Sambrano, J. R.; Li, M. S.; Varela, J. A.; Longo, E. Joint experimental and theoretical analysis of order-disorder effects in cubic BaZrO3 assembled nanoparticles under decaoctahedral shape. J. Phys. Chem. A 2011, 115 (17), 4482−4490. (37) Kanie, K.; Seino, Y.; Matsubara, M.; Nakaya, M.; Muramatsu, A. Hydrothermal synthesis of BaZrO3 fine particles controlled in size and shape and fluorescence behavior by europium doping. New J. Chem. 2014, 38 (8), 3548. (38) Fu, Z. L.; Yang, H. K.; Moon, B. K.; Choi, B. C.; Jeong, J. H. La2Sn2O7Eu3+ Micronanospheres: Hydrothermal Synthesis and Luminescent Properties. Cryst. Growth Des. 2009, 9 (1), 616−621. (39) Tachikawa, T.; Ishigaki, T.; Li, J. G.; Fujitsuka, M.; Majima, T. Defect-mediated photoluminescence dynamics of Eu3+-doped TiO2 nanocrystals revealed at the single-particle or single-aggregate level. Angew. Chem., Int. Ed. 2008, 47 (29), 5348−5352. (40) Waldorf, A. J.; Dobrowolski, J. A.; Sullivan, B. T.; Plante, L. M. Optical coatings deposited by reactive ion plating. Appl. Opt. 1993, 32 (28), 5583. (41) Zhang, B.; Zou, H.; Guan, H.; Dai, Y.; Song, Y.; Zhou, X.; Sheng, Y. Lu2O2S: Tb3+Eu3+ nanorods:luminescence, energy transfer, and multicolour tuneable emission. CrystEngComm 2016, 18, 7620. (42) Dong, G.; Hou, C.; Yang, Z.; Liu, P.; Wang, C.; Lu, F.; Li, X. Color-tunable, single phased BaLa2−x−yZnO5 xBi3+ yEu3+ phosphors with efficient energy transfer under ultraviolet excitation. Ceram. Int. 2014, 40 (9), 14787−14792. (43) Ghosh, S.; Das, K.; Sinha, G.; Lahtinen, J.; De, S. K. Bright white light emitting Eu and Tb co-doped monodisperse In2O3 nanocrystals. J. Mater. Chem. C 2013, 1 (35), 5557. (44) Planelles-Aragó, J.; Cordoncillo, E.; Ferreira, R. A. S.; Carlos, L. D.; Escribano, P. Synthesis, characterization and optical studies on lanthanide-doped CdS quantum dots: new insights on CdS → lanthanide energy transfer mechanisms. J. Mater. Chem. 2011, 21 (4), 1162−1170. (45) Luo, W.; Li, R.; Liu, G.; Antonio, M. R.; Chen, X. Evidence of Trivalent Europium Incorporated in Anatase TiO2 Nanocrystals with Multiple Sites. J. Phys. Chem. C 2008, 112, 10370. (46) Datta, A.; Patra, A. Bright white light emission from In2S3:Eu3+ nanoparticles. J. Phys. D: Appl. Phys. 2009, 42 (14), 145116. (47) Saha, S.; Das, S.; Ghorai, U. K.; Mazumder, N.; Ganguly, D.; Chattopadhyay, K. K. Controlling Nonradiative Transition Centers in Eu3+ Activated CaSnO3 Nanophosphors through Na+ Co-Doping: Realization of Ultrabright Red Emission along with Higher Thermal Stability. J. Phys. Chem. C 2015, 119 (29), 16824−16835. (48) Nikolić, M. G.; Antić, Ž .; Ć ulubrk, S.; Nedeljković, J. M.; Dramićanin, M. D. Temperature sensing with Eu3+ doped TiO2 nanoparticles. Sens. Actuators, B 2014, 201, 46−50. (49) Blasse, G. Energy transfer between inequivalent Eu2+ ions. J. Solid State Chem. 1986, 62 (2), 207. (50) Min, X.; Huang, Z.; Fang, M.; Liu, Y. G.; Tang, C.; Wu, X. Energy transfer from Sm3+ to Eu3+ in red-emitting phosphor LaMgAl11O19Sm3+Eu3+ for solar cells and near-ultraviolet white lightemitting diodes. Inorg. Chem. 2014, 53 (12), 6060−6065. (51) Boyer, J. C.; Vetrone, F.; Capobianco, J. A.; Speghini, A.; Bettinelli, M. Variation of Fluorescence Lifetimes and Judd-Ofelt Parameters between Eu3+ Doped Bulk and Nanocrystalline Cubic Lu2O3. J. Phys. Chem. B 2004, 108, 20137. (52) Xiao, Q.; Liu, Y.; Liu, L.; Li, R.; Luo, W.; Chen, X. Eu3+-Doped In2O3 Nanophosphors: Electronic Structure and Optical Characterization. J. Phys. Chem. C 2010, 114, 9314. (53) Grzyb, T.; Lis, S. Structural and spectroscopic properties of LaOF:Eu3+ nanocrystals prepared by the sol-gel Pechini method. Inorg. Chem. 2011, 50 (17), 8112−8120. (54) Wiglusz, R. J.; Bednarkiewicz, A.; Strek, W. Role of the sintering temperature and doping level in the structural and spectral properties of Eu-doped nanocrystalline YVO4. Inorg. Chem. 2012, 51 (2), 1180− 1186.

(16) Romero, V. H.; De la Rosa, E.; Salas, P.; Velázquez-Salazar, J. J. Strong blue and white photoluminescence emission of BaZrO3 undoped and lanthanide doped phosphor for light emitting diodes application. J. Solid State Chem. 2012, 196, 243−248. (17) Singh, V.; Rai, V. K.; Al-Shamery, K.; Haase, M.; Kim, S. H. NIR to visible frequency upconversion in Er3+ and Yb3+ co-doped BaZrO3 phosphor. Spectrochim. Acta, Part A 2013, 108, 141−145. (18) Drag-Jarzabek, A.; John, L.; Petrus, R.; Kosinska-Klahn, M.; Sobota, P. Alkaline Earth Metal Zirconate Perovskites MZrO3 (M = Ba2+Sr2+Ca2+) Derived from Molecular Precursors and Doped with Eu3+ Ions. Chem. - Eur. J. 2016, 22 (14), 4780−4788. (19) Guan, L.; Jin, L.; Guo, S.; Liu, Y.; Li, X.; Guo, Q.; Yang, Z.; Fu, G. Luminescent properties of Eu3+-doped BaZrO3 phosphor for UV white light emitting diode. J. Rare Earths 2010, 28, 292−294. (20) Huang, J.; Zhou, L.; Wang, Z.; Lan, Y.; Tong, Z.; Gong, F.; Sun, J.; Li, L. Photoluminescence properties of SrZrO3Eu3+ and BaZrO3Eu3+ phosphors with perovskite structure. J. Alloys Compd. 2009, 487 (1−2), L5−L7. (21) Liu, X.; Wang, X. Preparation and luminescence properties of BaZrO3Eu phosphor powders. Opt. Mater. 2007, 30 (4), 626−629. (22) Guo, L.; Wang, X.; Zhong, C.; Li, L. Preparation and Photoluminescence Properties of Eu3+Doped BaZrO3 Nanotube Arrays. J. Am. Ceram. Soc. 2011, 94 (10), 3175−3177. (23) Zhang, H.; Fu, X.; Niu, S.; Xin, Q. Synthesis and photoluminescence properties of Eu3+ doped AZrO3 (A = Ca, Sr, Ba) perovskite. J. Alloys Compd. 2008, 459 (1−2), 103−106. (24) Macario, L. R.; Moreira, M. L.; Andrés, J.; Longo, E. An efficient microwave-assisted hydrothermal synthesis of BaZrO3 microcrystals: growth mechanism and photoluminescence emissions. CrystEngComm 2010, 12 (11), 3612. (25) Nobbs, J. H. Kubelka−Munk Theory and the Prediction of Reflectance. Rev. Prog. Color. Relat. Top. 1985, 15, 66−75. (26) Kunti, A. K.; Patra, N.; Sharma, S. K.; Swart, H. C. Radiative transition probability enhancement of white light emitting Dy3+ doped and K+ co-doped BaWO4 phosphors via charge compensation. J. Alloys Compd. 2018, 735, 2410−2422. (27) Uarth Fassbender, R.; Strelow Lilge, T.; Cava, S.; Andres, J.; Fernando da Silva, L.; Roberto Mastelaro, V.; Longo, E.; Lucio Moreira, M. Fingerprints of Short-Range and Long-Range Structure in BaZr1−xHfxO3 Solid Solutions: An Experimental and Theoretical Study. Phys. Chem. Chem. Phys. 2015, 17, 11341. (28) Giannici, F.; Longo, A.; Balerna, A.; Kreuer, K.-D.; Martorana, A. Proton Dynamics in In:BaZrO3Insights on the Atomic and Electronic Structure from X-ray Absorption Spectroscopy. Chem. Mater. 2009, 21, 2641. (29) Kelly, S. D.; Hesterberg, D.; Ravel, B. Analysis of soils and minerals using X-ray absorption spectroscopy, in Methods of Soil Analysis, Part 5−Mineralogical methods; Soil Science Society of America: Madison, WI, 2008. (30) Prins, R.; Koningsberger, D. C. X-Ray Absorption: Principles, Applications, Techniques of EXAFS, SEXAFS and XANES; Wiley: New York, 1988. (31) Newville, M.; Ravel, B.; Haskel, B. R. D.; Rehr, J. J.; Stern, E. A.; Yacoby, Y. Analysis of multiple-scattering XAFS data using theoretical standards. Phys. B 1995, 208-209, 154. (32) Momma, K.; Izumi, F. VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data. J. Appl. Crystallogr. 2011, 44, 1272. (33) Lebedev, A. I.; Sluchinskaya, I. A. Combined first-principles and EXAFS study of structural instability in BaZrO3. J. Adv. Dielectr. 2015, 05, 1550019. (34) Moreira, M. L.; Gurgel, M. F.; Mambrini, G. P.; Leite, E. R.; Pizani, P. S.; Varela, J. A.; Longo, E. Photoluminescence of barium titanate and barium zirconate in multilayer disordered thin films at room temperature. J. Phys. Chem. A 2008, 112 (38), 8938−8942. (35) Cavalcante, L. S.; Longo, V. M.; Zampieri, M.; Espinosa, J. W. M.; Pizani, P. S.; Sambrano, J. R.; Varela, J. A.; Longo, E.; Simões, M. L.; Paskocimas, C. A. Experimental and theoretical correlation of very P


Article

Inorganic Chemistry (55) Kodaira, C. A.; Brito, H. F.; Malta, O. L.; Serra, O. A. Luminescence and energy transfer of the europium (III) tungstate obtained via the Pechini method. J. Lumin. 2003, 101, 11. (56) Shyichuk, A.; Moura, R. T.; Neto, A. N. C.; Runowski, M.; Zarad, M. S.; Szczeszak, A.; Lis, S.; Malta, O. L. Effects of Dopant Addition on Lattice and Luminescence Intensity Parameters of Eu(III)-Doped Lanthanum Orthovanadate. J. Phys. Chem. C 2016, 120 (50), 28497−28508. (57) Zaiats, G.; Kinge, S.; Kamat, P. V. Origin of Dual Photoluminescence States in ZnS−CuInS2 Alloy Nanostructures. J. Phys. Chem. C 2016, 120 (19), 10641−10646. (58) de Mayrinck, C.; Schiavon, M. A.; Nascimento, C. S.; Guimarães, L.; Davolos, M. R.; de Sousa Góes, M.; Gonçalves, R. R.; Pires, A. M.; Ribeiro, S. J. L.; Ferrari, J. L. Spherical-shaped Y2O3Eu3+ nanoparticles with intense photoluminescence emission. Ceram. Int. 2015, 41 (1), 1189−1195. (59) McCamy, C. S. Correlated Color Temperature as an Explicit Function of Chromaticity Coordinates. Color Res. Appl. 1992, 17 (2), 142.

Q


Local Structure and Spectroscopic Properties of Eu3+-Doped BaZrO3

Recommend Documents