Design, Preparation, and Characterization of a Novel Red Long

Nov 11, 2015 - ABSTRACT: Currently, the development of efficient red- emitting persistent phosphor is still an ongoing challenge. Herein, a novel red-...
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Design, Preparation, and Characterization of a Novel Red LongPersistent Perovskite Phosphor: Ca3Ti2O7:Pr3+ Bo Wang,† Hang Lin,*,†,‡ Ju Xu,†,‡ Hui Chen,† Zebin Lin,† Feng Huang,†,‡ and Yuansheng Wang*,†,‡ †

Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, University of Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China ‡ Fujian Provincial Key Laboratory of Nanomaterials, Fuzhou, Fujian 350002, P. R. China S Supporting Information *

ABSTRACT: Currently, the development of efficient redemitting persistent phosphor is still an ongoing challenge. Herein, a novel red-emitting LPL phosphor Ca3Ti2O7:Pr3+ is successfully prepared by a high-temperature solid-state method. XRD Rietveld refinement analyses demonstrate the high phase purity of the sample which crystallizes in an orthorhombic Ccm21 space group with lattice parameters of a = 5.7702(5) Å, b = 19.4829(7) Å, and c = 5.1214(2) Å. Electronic structure of the host matrix is analyzed by the firstprinciple calculation using CASTEP code. The calculation results show that Ca3Ti2O7 has a direct band gap with CB and VB mainly composed of the Ti-3d and O-2p states, respectively. On the basis of the DR spectrum, the band gap is determined to be 3.6 eV. It is demonstrated that the 612 nm red-emitting persistent luminescence of Ca3Ti2O7:Pr3+ can be either activated by Ti4+−O2− → Ti3+−O− host absorption and Pr3+−O−Ti4+ → Pr4+−O−Ti3+ IVCT in the UV region, or Pr3+:3H4 → 3PJ transition in the blue region. The red afterglow can last for ∼5 min observed by the naked eyes in the dark after ceasing the irradiation source. On the basis of the TL analyses, the trap is found exponentially distributed in the host with the depth of 0.69−0.92 eV. Finally, a possible LPL mechanism for Ca3Ti2O7:Pr3+ is proposed.

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INTRODUCTION Long-persistent luminescence (LPL) is an interesting phenomenon whereby the light can last for a long duration at room temperature after removal of the UV−vis irradiation source.1−4 Such phosphorescence decay is generated by the thermally stimulated recombination of charge carriers (i.e., electrons and/ or holes) trapped at intrinsic defect sites of the host. In the past decades, great efforts have been employed to the design of novel LPL materials for their wide application in the fields of emergency lighting, interior decoration, road signs, AC-LEDs, and vivo bioimaging, etc.5−10 In principle, we can readily get any color emitting LPL by mixing three primary colors (RGB) emitting persistent components at some appropriate ratio.11,12 However, fullcolor emitting LPL has not yet been realized up to now, which is attributed to the development of a red persistent phosphor being far behind the commercialized blue and green counterparts.13 Among the state-of-the-art red persistent phosphors, europium activated sulfides (e.g., CaS: Eu2+,Tm3+) and oxysulfides (e.g., Y2O2S: Eu3+, Ti4+, Mg2+) exhibit superior LPL performance, but they are chemically unstable and sensitive to moisture, and the S as a sulfurization agent is harmful to the environment. 14,15 The nitrides (e.g., Ca2Si5N8:Eu2+) and oxynitrides (SrSi2O2N2:Eu2+) seem to be promising candidates; unfortunately, the high cost caused by © 2015 American Chemical Society

the harsh preparation condition limits their practical application.16 Thereupon, recently more attention has been paid to one kind of praseodymium activated ABO3 (A = Ca, Sr, Ba; B = Zr, Ti, Sn) perovskite, which shows good persistent characteristics with chromaticity coordinate close to the “ideal red” as well as the technological benefits of low price, excellent chemical/physical stability, and eco-friendliness.17−20 As a perovskite-related compound, Ca3Ti2O7 draws our attention. This compound belongs to the Ruddlesden−Popper phase with composition Can+1TinO3n+1 [n(CaTiO3)·CaO, n = ∞] and crystallizes with excess Ca cations structurally accommodated by regular insertion of distorted NaCl-type layers between perovskite blocks.21 It is usually regarded as an excellent ferroelectric material,22 but seldom reported yet as the luminescent host. There has been only one study in the literature proposing Ca3Ti2O7:Eu3+ potentially applicable in UV excited LEDs.23 As a matter of fact, Ca3Ti2O7 should be an ideal luminescent host considering the CaTiO3 blocks in the crystal structure. Upon doping of Pr3+, it is reasonable to assume its luminescent characteristics similar to those of the well-known CaTiO3:Pr3+.18,19 Moreover, it has been demonstrated that huge amounts of the excess Ca2+ induced defects, Received: August 18, 2015 Published: November 11, 2015 11299

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such as oxygen and/or titanium vacancies,24,25 and Ruddlesden−Popper like planar faults,26 are generated during the phase transformation from CaTiO3 to Ca3Ti2O7. These defects are very likely to reside in the Ca3Ti2O7 host owing to the local structure imperfections, which may benefit the LPL performance. Herein, we report the synthesis, microstructure, electronic structure, and luminescent properties of a novel Ca3Ti2O7:Pr3+ persistent phosphor. As we anticipated, the sample yields sound red-emitting phosphorescence which can be activated by both the UV and blue light. We further probe the trap status in the host and discuss the electron charging and detrapping processes.

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RESULTS AND DISCUSSION Microstructure. Figure 1 displays XRD patterns of the Ca3‑xTi2O7:xPr3+ (x = 0, 0.002, 0.005, 0.01, and 0.02) samples

EXPERIMENTAL SECTION

Sample Preparation. A series of Ca3Ti2O7 powder phosphors by varying Pr3+ doping concentration were synthesized via a conventional high-temperature solid-state reaction method. The raw materials CaCO3 (99.9%), TiO2 (99.99%), and Pr6O11 (99.99%) were weighed according to the nominal compositions of Ca3‑xTi2O7:xPr3+ (x = 0, 0.002, 0.005, 0.01, and 0.02), and 3 wt % H3BO3 was added as a flux. For each batch, the powders were ground in an agate mortar for 30 min. Then, the mixture was transferred into an alumina crucible, and calcined at 1350 °C for 6 h in a furnace. In order to protect Pr3+ from oxidizing to Pr4+, a small amount of carbon granules was placed in the furnace to produce a weak reducing atmosphere. Finally, the asobtained samples were cooled down to room temperature naturally, and ground to fine powders for subsequent usage. Characterization. XRD measurements of all the samples were executed on a powder diffractometer (Rigaku, Miniflex600, Cu Kα, λ = 1.5418 Å), with a continuous scanning rate of 5°/min for phase identification and a step scanning rate of 8 s per step (step size: 0.02°) for Rietveld analysis. The crystal structure of Ca3Ti2O7:Pr3+ was refined using the Rietveld method with a General Structure Analysis System (GSAS) software suite. The diffuse refection (DR) spectra of the samples were measured by an UV−vis−NIR spectrophotometer (Lambda 950, PerkinElmer), using BaSO4 as a standard reference. The photoluminescence (PL), photoluminescence excitation (PLE), temperature-dependent PL spectra, and persistent decay curves were recorded by a spectrophotometer (Edinburgh Instruments, FLS920) equipped with 450 W xenon lamps as lighting source. For the thermoluminescence (TL) measurements, the samples were mounted on a thermal stage (77−873 K, THMSE6300, Linkam Scientific Instruments), exposed to UV irradiation for 5 min at 300 K, and then heated to 450 K at a heating rate of 1 K/s. Meanwhile, the luminescent intensity variation of Pr3+ by monitoring at 612 nm was recorded by employing the kinetic mode of FLS920. The final TL curves were obtained by transforming the measured time-dependent luminescent curves to the temperature-dependent ones. Electronic Structure Calculations. To investigate the electronic structures of pure Ca3Ti2O7, calculations were carried out with density functional theory (DFT) framework using the Cambridge Serial Total Energy Package (CASTEP) code.27 The generalized gradient approximations (GGAs) with the Perdew−Burke−Ernzerhof functional were chosen for the theoretical basis of density function. Two steps were necessary to calculate the electronic band structure of Ca3Ti2O7. The first step was to optimize the crystal structure using the single crystal data reported in the literature. The second step was to calculate the band structure and the density of states (DOS) of Ca3Ti2O7 for the optimized structure. The lattice parameter and the atomic coordinates were fixed at the values obtained by the crystal structure optimization process in the first step. For the two steps, the basic parameter were chosen as follows in setting up the CASTEP run: The kinetic energy cutoff = 300 eV, k-point spacing = 0.05 Å−1, sets of k points = 4 × 4 × 4, self-consistent field tolerance thresholds = 2.0 × 10−6 eV/atom, and space representation = reciprocal.

Figure 1. XRD patterns of the Ca3‑xTi2O7:xPr3+ (x = 0−0.02) and the standard data (JCPDS 89-1384) of Ca3Ti2O7 as a reference.

by varying Pr3+ doping concentrations. Obviously, all the diffraction peaks can be well-indexed to the standard data (JCPDS 89-1384) of Ca3Ti2O7 with no detectable impurity phase signal, indicating no phase transformation or structural variation occurs in the host upon Pr3+ doping. Pr3+ ions are expected to substitute Ca2+ sites for their similar ionic radius (rPr = 0.99 Å vs rCa = 1.00 Å) via 2Pr3+ + 3Ca2+ → 2Pr3+• Ca2+ + VCa ″ 2+, producing positive defect Pr3+• ″ 2+ Ca2+ and calcium vacancy VCa to fulfill the charge compensation.28 To get knowledge on the detailed crystal structure of the prepared sample, XRD Rietveld refinement analysis is performed on a representative Ca2.995Ti2O7:0.005Pr3+ which exhibits an optimized persistent performance, using the GSAS program. The starting model for the refinement is built with the crystallographic data taken from pure Ca3Ti2O7 (ICSD-86241). Figure 2a shows the observed, calculated, and difference results for the Rietveld refined XRD patterns. The final refined lattice parameters are a = 5.7702(5) Å, b = 19.4829(7) Å, c = 5.1214(2) Å, as summarized in Table 1. The reliability factors converge to χ2 = 6.92, Rwp = 9.22%, Rp = 8.9%, indicating that the atom positions, fraction factors, and temperature factors of the samples satisfy well the reflection conditions.29 Figure 2b depicts the unit cell structure of Ca3Ti2O7 viewing along the adirection and the coordination environment of cation sites. There are two Ca2+ crystallographical sites (Wyckoff position of 4a and 8b), which possess identical coordination numbers (CN = 6) and similar average Ca−O bond distances of 2.4246 and 2.4009 Å, suggesting the occupancy of Pr3+ ions should be equally probable in the two different Ca sites. Ti4+ ions are coordinated by six oxygen atoms in a regular octahedron with an average Ti−O bond distance being 1.9811 Å. In comparison with the crystal structure of CaTiO3, the excess CaO in Ca3Ti2O7 can be accommodated in an ordered fashion between CaTiO3 perovskite so as to form double CaTiO3 perovskite blocks interleaved with a CaO layer.21 Nevertheless, the excess CaO has been previously demonstrated inducing plenty of lattice defects, including negatively charged calcium vancancies (V″Ca), positively charged oxygen vacancies (VÖ ), Ti vacancies (VTi ⁗), and antisite defects (CaTi ″ ), when fired at high temperature.24,25,30,31 These defects can be retained in the 11300

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Figure 3. (a) Calculated energy band structure of Ca3Ti2O7. (b) Total and partial (Ca, Ti, O atoms) density of states for Ca3Ti2O7. Figure 2. (a) Rietveld refinement of powder XRD profiles of sample Ca2.995Pr0.005Ti2O7. (b) The double perovskite crystal structure of Ca3Ti2O7 viewed along the a axis, and the coordination environment of the [TiO6] and [CaO6] octahedrons.

energy state in the valence band are both located at the same crystal momentum (k-vector) G in the Brillouin zone, indicating the orthorhombic Ca3Ti2O7 is a direct band gap semiconductor. The computed energy gap is approximately 2.619 eV, implying Ca3Ti2O7 is a good luminescent host for accommodating both the ground and excited states of luminescent ions within the wide band gap.32 Since the adopted GGA exchange-correlation function usually underestimates the size of energy band gap,33 the calculated value is much lower than the experimental one of ∼3.62 eV that is determined by the diffuse reflectance spectrum in Figure 4. Composition of the calculated energy bands can be further resolved with the help of partial density of states (PDOS) and total density of states (TDOS) diagram. Figure 3b depicts the PDOS for Ca, Ti, O atoms, as well as the TDOS for the Ca3Ti2O7 host. It can be seen that the valence band is mainly composed of the valence electrons of O-2p, Ti-3d, and Ca-3d states, ranging from −5 eV to the Fermi level (Ef, set as 0 eV); therein, the O-2p state is almost fully occupied within the valence band. The band between −20 and −15 eV is created by O-2s states and Ca-3p states. The lower part of the valence band consists of three peaks at −32, −38, and −56 eV, which are due to the 3p orbital of Ti, 2s orbital of Ca, and 2s orbital of Ti, respectively. As for the conduction band in the range from 2.5 to 5.0 eV, it is composed mostly of the Ti-3d states, and there is only a slight contribution from Ca-3d and O-2p states.

Table 1. Refined Crystallographic Data and Reliability Factors for Ca2.995Pr0.005Ti2O7 formula symmetry space group a/Å b/Å c/Å Z Rp (%) Rwp (%) χ2 (%)

Ca2.995Pr0.005Ti2O7 orthorhombic Ccm21 5.7702 19.4829 5.1214 4 9.22 8.9 6.92

crystal structure and served as electron/hole trap centers to endow the material with persistent luminescent properties. Electronic Structure. The electronic structure of Ca3Ti2O7 was then calculated using DFT methods based on the lattice parameters and atomic coordinates derived from the single crystal data, as presented in Figure 3a. It is revealed that the minimal energy state in the conduction band and the maximal 11301

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Figure 4. Diffuse reflection spectrum of the undoped Ca3Ti2O7. The inset shows the relationship of [F(R∞)hν]2 versus photon energy hν.

Therefore, it is concluded that the host absorption for Ca3Ti2O7 primarily originates from the antibonding π interaction between Ti-3d and O-2p orbitals. UV−vis diffuse reflectance (DR) spectrum of the undoped Ca3Ti2O7 is exhibited in Figure 4. The reflectance shows a remarkable drop around 320 nm which is attributed to the electron transition between the valence and conduction bands of the host. The optical band gap (Eg) of Ca3Ti2O7 can be estimated according to eq 134 [F(R ∞)hν]n = A(hν − Eg )

(1)

where hν represents the photon energy, A is a proportional constant, n equals 2 since the TiO68− host absorption is a direct allowed transition. F(R∞) denotes the Kubelka−Munk function expressed as [F(R ∞)] = (1 − R )2 /2R

(2)

where R is the reflection coefficient. Then, the plot of [F(R∞) × hν]2 versus hν was made according to the Taus method, as shown in the inset of Figure 4. The optical band gap energy of Ca3Ti2O7 is determined to be 3.62 eV by extrapolating the linear portion to the photon energy axis. Photoluminescence Properties. Figure 5 shows the normalized PLE, PL, together with DR spectra for the Ca3Ti2O7:0.005Pr3+ sample at room temperature. Upon 330 nm excitation, the PL spectrum is found comprising one dominant red emission band at 612 nm designated as Pr3+:1D2 → 3H4 and one weak near-infrared emission band at 700 nm attributed to Pr3+:1D2 → 3H5. The absence of greenish-blue emission from higher excited levels of Pr3+:3PJ (J = 0−2) is probably caused by their complete depopulation via a thermal activated crossover to a low-lying Pr3+−Ti4+ intervalence charge transfer (IVCT) state.35 As displayed in the inset of Figure 5a, the sample yields intense red emission when exposed to 365 nm UV lamp irradiation. On the basis of the PL spectrum, Commission Internationale de l’Eclairage (CIE) chromaticity coordinate of the sample is calculated to be (0.670, 0.329), which is close to the “ideal red” light (0.670, 0.330).18 It is also found that PLE and DR spectra coincide precisely with each other, showing two distinguishable broad bands centered around 330 and 375 nm, and a series of characteristic 4f−4f narrow bands of Pr3+ around 450 nm originating from Pr3+:3H4 → 3PJ (J = 0−2). These spectral features are in agreement with those of a previously reported Pr3+-doped perovskite,17−19 demonstrating the successful incorporation of Pr3+ into the

Figure 5. (a) PLE (λem = 612 nm), PL (λex = 330 nm), and DR spectra of Ca2.995Ti2O7:0.005Pr3+ sample; inset is the corresponding CIE coordinate and the digital photo under 365 nm UV lamp irradiation. (b) PL spectra of Ca3Ti2O7:xPr3+ (x = 0.002−0.02) samples upon 330 nm excitation.

Ca3Ti2O7 host. The 330 nm excitation band can be assigned to the TiO68− host absorption (Ti4+−O2− → Ti3+−O−), in which electrons from the O-2p valence band are excited into the Ti-3d conduction band. The 375 nm band presumably results from the IVCT (Pr3+−O−Ti4+ → Pr4+−O−Ti3+). An empirical equation was then further applied to roughly determine the IVCT energy position36 3+

−1

IVCT(Pr , cm ) = 58 800−49 800

χopt (Ti4 +) d(Pr 3 +−Ti4 +)

(3)

where χopt(Ti ), representing the optical electronegativity of Ti4+, has been calculated to be 2.05,37 and d(Pr3+−Ti4+), being the shortest Pr3+−Ti4+ bond distance in the host structure, is determined to be 3.2004(13) Å based on the above Rietveld refinement result. The theoretical IVCT band is then estimated to be ∼370 nm, agreeing well with the experimental one in the PLE spectrum. Figure 5b exhibits the PL spectra of Ca3Ti2O7:xPr3+ (x = 0.002−0.02) samples upon 330 nm excitation. With increasing Pr3+ concentration, the emission intensity of the phosphor increases gradually and reaches a maximum at x = 0.005, and then the concentration quenching 4+

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Figure 6. (a) Persistent decay curves of the Ca3‑xTi2O7:xPr3+ (x = 0.002−0.020) samples monitored at 612 nm emission after irradiation at 330 nm for 5 min. (b) Afterglow intensity at 10 s (I10s) after ceasing the excitation as a function of excitation wavelength over the 230−490 nm spectral range for the Ca2.995Ti2O7:0.005Pr3+ (blue sphere). For comparison, the PLE spectrum of the Ca2.995Ti2O7:0.005Pr3+ is also provided (solid line). (c) Persistent luminescence spectrum of Ca2.995Ti2O7:0.005Pr3+ recorded immediately after the removal of the excitation source; the inset shows the digital photographs of Ca2.995Ti2O7:0.005Pr3+ after stoppage of 330 or 460 nm excitation source for different durations.

occurs probably due to energy transfer from Pr3+ to some unknown defects which act as killing centers. Persistent Luminescence Properties. We further investigate the persistent luminescence behaviors of the Ca3‑xTi2O7:xPr3+ samples by varying Pr3+ doping contents (x = 0.002−0.020), as presented in Figure 6. The persistent decay curves (Figure 6a) monitored at 612 nm are recorded after irradiation by 330 nm UV light for 5 min. Obviously, the decay process includes a bright fast component in the first few seconds and a weak slow one persisting for a long duration. The optimized sample is the Ca2.995Ti2O7:0.005Pr3+, which exhibits the highest initial afterglow brightness and the longest persistent lifetime. To validate the effectiveness of different excitation wavelengths for activating persistent luminescence, the persistent excitation (PE) spectrum of Ca2.995Ti2O7:0.005Pr3+ monitored at 612 nm is created by plotting the afterglow intensities at 10 s (as a reference point) after the stoppage of excitation versus the excitation wavelength (200−400 nm), as displayed in Figure S1 and Figure 6b. In comparison with the PLE spectrum, the PE one also shows the identical host absorption, IVCT bands in the UV region, and the Pr3+:4f → 4f absorption bands in the blue region; however, the relative intensities of IVCT and Pr3+ absorptions to that of host absorption in the PE spectrum is much higher, which is

ascribed to the difference in the persistent luminescence mechanism that will be discussed in detail hereafter. Figure 6c shows the room-temperature persistent luminescence spectrum of the Ca2.995Ti2O7:0.005Pr3+ sample which is recorded immediately after the 330 or 460 nm excitation sources are switched off, demonstrating that the red persistent luminescence originates from the Pr3+ emissive centers. As illustrated by the digital photographs in the inset of Figure 6c, the most efficient activation of persistent luminescence is achieved via host absorption at 330 nm, and the afterglow lasts for ∼5 min by the naked eyes in the dark. Trap Analyses. It is well-known that the creation of persistent luminescence is due to the thermal-stimulated gradual release of charge carriers which are captured by the trap centers. The persistent performance is therefore highly dependent on the trap properties (i.e., trap depth and concentration). On the basis of the knowledge of this trap information in the host lattice, the persistent luminescence mechanism would be clarified. To this end, the thermoluminescence (TL) technique, which is regarded as the most efficient method to probe the nature of traps, is applied to the Ca3‑xTi2O7:xPr3+ (x = 0.002−0.020) phosphors. Figure 7 shows the corresponding Pr3+ concentration-dependent TL spectra. All the curves appear as a broad band in the temperature range 11303

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conforms to the deep traps, in which the immobilized charge carriers combine with the emissive centers at a slow rate. The TL peak wavelength and TL intensity are found to greatly rely on the Pr3+ concentration, indicating that Pr3+ doping favors the trap creation and thus influences the trap depth and concentration. For all the samples, the peak maxima are located at ∼340 K, falling in the recommended range (323−400 K) for good persistent luminescence performance. The TL intensity first increases to a maximum at x = 0.005 and then decreases with further increasing Pr3+ concentration. Such concentration quenching effect of persistent luminescence has been explained by the enhanced interaction of the neighboring traps.28 Noticeably, the broadband feature of the TL curve implies that there is a continuous distribution of trap depth. The small variations in the spatial configurations of the defect aggregates are responsible for such a broadening of the trap depth distribution.38 In order to determine the trap depth distribution, a series of excitation temperature-dependent TL experiments are performed on the representative Ca2.995Ti2O7:0.005Pr3+ sample, as exhibited in Figure 8a. All the curves are measured following the procedure outlined by Eeckout et al.:39 First, the sample is preheated to a certain temperature (Texc) and exposed to 330 nm light for 5 min; then, it is quickly cooled to room temperature (300 K) and subjected to TL measurements with a heating rate of 1 K/s. Such a method cleans different fractions of the occupied traps depending on the Texc. It is observed that TL intensity weakens with increasing Texc, indicating the trapping process is not thermally activated within the studied

Figure 7. TL glow curves of the Ca3‑xTi2O7:xPr3+ (x = 0.002, 0.005, 0.01, and 0.02) samples (excitation wavelength, 330 nm; heating rate, 1 K/s); the inset is the measured (black dot line) and Gaussian-fitted (red solid line) TL glow curves of the Ca2.995Ti2O7:0.005Pr3+.

300−450 K, which can be further deconvoluted into two components based on a Gaussian function (the inset in Figure 7). The low-temperature component corresponds to the shallow traps which have a smaller binding force to stabilize electrons or holes, leading to the quick release of charge carriers after ceasing the excitation. The high-temperature one

Figure 8. (a) TL glow curves by preheating the Ca2.995Ti2O7:0.005Pr3+ sample at various excitation temperatures (excitation wavelength, 330 nm; heating rate, 1 K/s). (b) Initial rise analysis of the corresponding TL glow curves. (c) Estimated trap depth in Ca2.995Ti2O7:0.005Pr3+ as a function of the excitation temperature. (d) Trap depth distribution in Ca2.995Ti2O7:0.005Pr3+. 11304

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Inorganic Chemistry temperature range.40 The peak-shifting of the TL maxima confirms a wide trap depth distribution. An initial rising method is further applied to the measured TL curves to reveal the shallowest occupied trap depth (E) at each Texc, as presented in Figure 8b. This method assumes that the concentration of trapped electrons on the low-temperature side of a TL glow curve remains relatively constant; thus, the TL intensity (I(T)) can be approximately expressed as39 ⎛ −E ⎞ ⎟ I(T ) = C exp⎜ ⎝ kT ⎠

(process 3). Owing to the severe thermal activation in the lattice, they would release gradually from the traps (process 4). The electrons transfer to the Pr3+:3PJ level, and the holes transfer to the Pr3+:3H4 one. The electron−hole recombination yields red afterglow luminescence. Upon blue light (400−490 nm) activation, the persistent luminescence mechanism should be different, considering that there is a large energy barrier of ∼0.6 eV between the excited Pr3+:3PJ level and the CB bottom which hinders electrons’ delocalization to CB via photoionization. In this case, the “quantum tunneling” mechanism,44,45 i.e., the electrons do not go through the CB, but directly tunnel a short distance between the traps and the ionized Pr3+ ions, might take control. To confirm this, the 460 nm activated persistent decay curve of the Ca3Ti2O7:Pr3+ phosphor at cryogenic temperatures (77 K) was recorded, as shown in Figure S2a. At such a low temperature, the phononassisted transitions, e.g., 3PJ → CB, should be “frozen”, while the afterglow can still be observed. The afterglow decay curve measured at 77 and 300 K is also plotted as reciprocal luminescence intensity (I−1) versus time (t), both of which can be well-fitted by a straight line (Figure S2b,c). The linear dependence of I−1 ∼ t is characteristic of a quantum tunneling process.5,44−46

(4)

where C is the constant including the frequency factor s, and k the Boltzmann constant. By plotting the TL curves as ln(I) vs 1000/T, the shallowest trap depth E can be determined by the slope of a fitted straight section at the low-temperature side. The estimated trap depth is found continuously distributed from 0.69 to 0.92 eV, as shown in Figure 8c. The trap density between two adjacent depths can be roughly calculated from the difference between the integrated intensities of the corresponding two TL glow curves,41,42 as depicted in Figure 8d. Obviously, the filled electrons are distributed in an exponential way and mostly occupy the shallow traps with the depth 0.69−0.77 eV. As for the trap type, it is still unclear. However, many researchers have proposed that there are abundant traps, such •• 3+ as Pr3+• Ca2+, V″ Ca2+, and VO2−, in the Pr -doped perovskite host for 28 •• the nonequivalent substitution. Pr3+• Ca2+ and VO2− can act as electron traps, whereas V″Ca2+ can serve as hole trap.43 Persistent Luminescence Mechanisms. On the basis of the above analyses, we try to elucidate the phosphorescence mechanism for Ca3Ti2O7:Pr3+, as schematically illustrated in Figure 9. In the energy level diagram, the energy gap between

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CONCLUSIONS In conclusion, a novel red long-persistent perovskite, Ca3Ti2O7:Pr3+, is successfully prepared via a high-temperature solid-state reaction route. The microstructure, electronic structure, and luminescent property of the samples are characterized and analyzed in detail. By conducting a series of excitation temperature-dependent thermoluminescence measurements, the continuous exponential distribution of trap depth is confirmed, which is found to be responsible for the persistent luminescence behaviors. It is also revealed that there are two different electron charging and detrapping ways in the persistent luminescence process, depending on the energy of irradiation source.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b01894. Persistence decay curves (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

Figure 9. Schematic representation of the persistent luminescence mechanism for Ca3Ti2O7:Pr3+ phosphor.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (11204301, 51472242, 51172231, and 11304312) and the Natural Science Foundation of Fujian Province (2015J01032).

the conduction band (CB) and valence band (VB) is ∼3.6 eV. The location of the Pr3+ ground level relative to CB is ∼3.3 eV that accords to the IVCT energy. The trap depth varies from 0.69 to 0.92 eV. Upon UV light (330 or 375 nm) irradiation, electrons are promoted from the VB or Pr3+ ground level to the CB, and holes are left behind in the VB (process 1). These electrons and holes move around freely (process 2) until they are captured by the electron and hole traps, respectively

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REFERENCES

(1) Van den Eeckhout, K.; Poelman, D.; Smet, P. F. Materials 2013, 6, 2789−2818.

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DOI: 10.1021/acs.inorgchem.5b01894 Inorg. Chem. 2015, 54, 11299−11306

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DOI: 10.1021/acs.inorgchem.5b01894 Inorg. Chem. 2015, 54, 11299−11306