- Email: [email protected]
Synthesis, photoluminescence, cathodoluminescence, and thermal properties of novel Tb3+-doped BiOCl green-emitting phosphors
Accepted Manuscript Synthesis, photoluminescence, cathodoluminescence, and thermal properties of 3+ novel Tb -doped BiOCl green-emitting phosphors Xiaoyong Huang, Bin Li, Heng Guo PII:
S0925-8388(16)33704-5
DOI:
10.1016/j.jallcom.2016.11.224
Reference:
JALCOM 39721
To appear in:
Journal of Alloys and Compounds
Received Date: 26 September 2016 Revised Date:
11 November 2016
Accepted Date: 16 November 2016
Please cite this article as: X. Huang, B. Li, H. Guo, Synthesis, photoluminescence, 3+ cathodoluminescence, and thermal properties of novel Tb -doped BiOCl green-emitting phosphors, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.11.224. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Synthesis, photoluminescence, cathodoluminescence, and thermal properties of novel Tb3+-doped BiOCl green-emitting phosphors Xiaoyong Huang*, Bin Li, Heng Guo Key Lab of Advanced Transducers and Intelligent Control System, Ministry of Education and
*Corresponding author, E-mail: [email protected]
Abstract
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Taiyuan 030024, P. R. China
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Shanxi Province, College of Physics and Optoelectronics, Taiyuan University of Technology,
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Tb3+-doped BiOCl phosphors were synthesized by a facile solid-state reaction method. The obtained samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence (PL) and cathodoluminescence (CL) spectra, and PL thermal stability. Under 377 nm near-ultraviolet light excitation, all the samples showed the characteristic green emissions of Tb3+ ions owing to the 5D4→7FJ (J = 6, 5, 4, 3) transitions.
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The dependence of PL emission intensity on the Tb3+ doping concentration was investigated and the optimal doping concentration was found to be 9 mol%. With the help of theoretical calculation, the critical distance was determined to be about 10.58 Å and the concentration quenching was dominated by multipole-multipole interaction. Furthermore, thermal stability
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of these phosphors was investigated by measuring the temperature-dependent PL spectra. In addition, these Tb3+-doped BiOCl phosphors also exhibited strong CL green emissions, and
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the CL emission intensity did not show saturation with the increase of accelerating voltage and filament current. These results indicated that the Tb3+-doped BiOCl phosphors may have potential applications in white light-emitting diodes (LEDs) and field emission displays (FEDs) as green-emitting phosphors.
Keywords: Lanthanide ions; Phosphors; Photoluminescence; Cathodoluminescence; Tb3+; BiOCl.
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ACCEPTED MANUSCRIPT 1. Introduction In the past decades, inorganic luminescent materials doped with trivalent lanthanide (Ln3+) ions have attracted considerable research attention, since they show great promise in many
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applications, such as white light-emitting diodes (LEDs), remote temperature monitoring, biological imaging, field emission displays (FEDs), and solar cells [1-9]. For the above applications, the desirable inorganic phosphors are required to exhibit highly efficient
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luminescent performance. In fact, the luminescent properties of Ln3+-doped phosphors are
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strongly dependent on the crystal field around the Ln3+ ions in the host lattices, and therefore selecting a suitable host material is of great importance. Until now, some inorganic materials, such as molybdates, oxides, fluorides, and silicate, were extensively studied as the luminescent host [10-21]. The fluorides possess low cut-off phonon energy, which is benefit
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for decreasing the possibility of non-radiative transition, and thus leading to superior luminescent properties. However, the fluorides generally have low thermal and physical stability, and consequently, they cannot be used for some particular applications. In contrast,
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other luminescent hosts also suffer from some drawbacks, such as relatively high cut-off
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phonon energy and high fabrication temperature. Therefore, searching for a novel luminescent host, which possesses low cut-off photon energy, low fabrication temperature, high thermal and physical stability, is required. Recently, bismuth oxychloride (BiOCl) was selected as suitable luminescent host owing
to its low fabrication temperature, high chemical stability and unique spatial structure [22,23]. BiOCl has a tetragonal matlockite PbFCl structure with [Cl–Bi–O–Bi–Cl] layers stacked together by nonbonding van der Waals interaction through the halogen atoms along c-axis [24, 2
ACCEPTED MANUSCRIPT 25], as demonstrated in Fig. 2(b). Meanwhile, the Bi3+ ions are surrounded by four oxygen atoms and four chlorine atoms. Furthermore, the BiOCl also has a cut-off photon energy of about 400 cm-1 [26]. Thanks to these merits, efficient luminescence could be achieved in
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Ln3+-doped BiOCl phosphors. It was recently reported that the Eu3+-doped BiOCl phosphors can give rise to intense red emissions under the excitation of near-ultraviolet (NUV) light [27]. In addition, the luminescent properties of BiOCl phosphors doped with Dy3+, Er3+/Yb3+ or
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Ho3+/Yb3+ ions were also studied in recent years [28-30]. In spite of these achievements, more
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research efforts are still needed to systematically investigate the luminescent properties and thermal stability of Ln3+-doped BiOCl phosphors. On the other hand, among the Ln3+ ions, terbium (Tb3+) ion has been proven to be an efficient green-emitting activator due to its dominant green emission originating from the 5D4→7F5 transition [31-34]. Recently, Ju et al.
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reported the bright green emissions with high color purity in Tb3+-doped ZnMoO4 phosphors [35]. Yu et al. also pointed out that the CaGd2ZnO5:Tb3+ nanophosphors were suitable for white LEDs applications as green-emitting phosphors [36]. However, to the best of our the
investigation
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knowledge,
on
the
luminescent
properties,
especially
the
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cathodoluminescence (CL) and thermal stability, of the Tb3+-doped BiOCl phosphors is still lacking. In this present paper, we reported the luminescent properties of Tb3+-doped BiOCl phosphors prepared by a facile solid-state reaction method. The phase structure, morphology, photoluminescence (PL), and thermal stability of the obtained BiOCl:Tb3+ phosphors were studied in detail. Meanwhile, the CL properties of the as-synthesized phosphors were also systematically analyzed with the aim of exploring their potential application in FEDs. 2. Experimental 3
ACCEPTED MANUSCRIPT The BiOCl:x%Tb3+ phosphors doped with different Tb3+ concentrations (x = 1, 3, 5, 7, 9, and 11) were prepared by means of a facile solid-state reaction method. Appropriate amounts of Bi2O3 (99.9%), Tb(NO3)3·6H2O (99.9%), and NH4Cl (99.5%) were weighted and ground
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together thoroughly in an agate mortar. Excess NH4Cl (20 mol%) was used in order to compensate the loss of volatilization. Subsequently, these mixtures were transferred to the alumina crucibles followed by calcination in a furnace at 500 ºC for 4 h in air. Finally, these
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mixtures were then cooled down naturally to room temperature and ground for further
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characterization.
The phase composition and crystal structure of as-synthesized samples were characterized by X-ray diffraction (XRD) using a Bruker D8 Advance diffractometer with Cu Kα radiation. The morphology of the samples were examined by a field-emission scanning electron
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microscope (FE-SEM; Philips XL-30). The PL spectra were measured by an Edinburgh FS5 spectrometer. The temperature ranging from 303 to 443 K was controlled by a homemade temperature stage. The decay curves of the studied samples were collected by a fluorescence
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spectrophotometer (Photon Technology International, USA) attached with a Xe flash lamp of
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25 W power. The CL properties were investigated by utilizing a Gatan (UK) MonoCL3 system attached with a Hitachi S-4300 SEM at room temperature.
3. Results and discussion To identify the crystal structure and phase purity of Tb3+-doped BiOCl phosphors, the XRD measurement was carried out. Fig. 1 shows the representative XRD patterns of the as-synthesized samples. All the diffraction peaks of the obtained products agree well with the 4
ACCEPTED MANUSCRIPT tetragonal phase of BiOCl (JCPDS 06-0249) without detecting any other impurity peaks from the secondary phase. This result clearly demonstrates that the as-synthesized samples possessed pure tetragonal phase and the Tb3+ ions were completely doped into the BiOCl host
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lattices by replacing the Bi3+ ions. According to the previous reports [37,38], to form a new solid solution, the radius percentage difference (Dr) between the dopants and the possible
Dr =
R1 (CN) − R2 (CN) × 100 , R1 (CN)
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substituted ions should be no more than 15% and it can be expressed as:
(1)
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where R1(CN) is the ionic radius of the host cation, R2(CN) is the ionic radius of the dopant. Herein, the values of R1(CN) and R2(CN) were 1.17 and 1.04 Å, respectively, when the coordinate number was 8. Therefore, by means of Eq. (1), the Dr value of Bi3+/Tb3+ was calculated to be around 11.11%, which was smaller than 15%, further confirming that the
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Tb3+ ion can occupy the site of Bi3+ ions in the BiOCl host. Furthermore, it can be seen from Fig. 1 that the diffraction peak positions were slightly shifted to smaller angle with the
ions.
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increase of Tb3+ doping concentration due to the different ionic radii between Bi3+ and Tb3+
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For the sake of investigating the crystal structure of obtained samples as well as proving
the Tb3+ ions occupied the Bi3+ ions’ sites in BiOCl host, the Rietveld XRD refinement of the typical BiOCl:9%Tb3+sample was performed via a General Structure Analysis System software. Fig. 2(a) describes the refinement diffraction pattern of the BiOCl:9%Tb3+ sample and the corresponding refined lattice parameters were displayed in Table 1. Obviously, the obtained sample exhibited pure tetragonal phase with a space group of P4/nmm(129). As presented in Table 1, the refinement data values were Rp = 6.38%, Rwp = 8.78% and χ2 = 2.13, 5
ACCEPTED MANUSCRIPT while the lattice parameters for BiOCl:9%Tb3+ sample were determined to be approximately a = b = 3.88222 Å, c = 7.34426 Å, V = 110.69Å3. It was found that the calculated lattice parameters for the as-synthesized sample were a little smaller than those of BiOCl host, which
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can be attributed to the fact that Bi3+ ions were replaced by the smaller sized Tb3+ ions. These results further proved that the Tb3+ ions were preferable to substitute the Bi3+ ions and did not induce any significant changes to the host structure.
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The representative FE-SEM images of BiOCl:Tb3+ phosphors with the doping
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concentration of 1, 5, 9 and 11 mol% are illustrated in Fig. 3. As can be seen, the obtained samples were made up of plate-like agglomerated particles with the size ranging from around 1 to 4 µm. Furthermore, the shape and size of the particles were changed little with the addition of Tb3+ ions, suggesting that the dopant concentration did not have any effect on the
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morphological properties of the as-synthesized samples.
Fig. 4 shows the PL excitation (PLE) and PL spectra of the BiOCl:9%Tb3+sample. The PLE spectrum, which was monitored at the green emission wavelength of 543 nm, consisted
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of a broad band and two sharp peaks. In particular, the broad band from 220 to 335 nm with a
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center at around 300 nm was originated from the inter-configurational 4f8→4f75d1 transition of Tb3+ ions [35,36], while these narrow bands located at around 354 and 377 nm were assigned to the intra-4f transitions of Tb3+ ions (namely, 7F6→5L9 and 7F6→5G6 transitions, respectively) [39,40]. Among these excitation bands, the intensity of the excitation band at 377 nm, which is matched well with the wavelength of commercial NUV LED chip, was the highest, suggesting that the as-synthesized phosphors can be effectively pumped by NUV light. Upon excitation at 377 nm, the PL spectrum of BiOCl:9%Tb3+ sample was measured 6
ACCEPTED MANUSCRIPT and displayed in Fig. 4. Obviously, the PL spectrum consisted of a series of sharp emission peaks at about 489, 543, 585 and 621 nm, which can be assigned to the 5D4→7F6, 5D4→7F5, 5
D4→7F4 and 5D4→7F3 transitions of Tb3+ ions, respectively [41,42]. To comprehend the
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involved PL processes, the simplified energy level diagram of Tb3+ ions as well as the proposed luminescent mechanism was shown in Fig. 5. Under 377 nm excitation, the electrons were first excited from the ground state of 7F6 to the excited state of 5G6. Then, the
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non-radiative transition took place, resulting in the population of metastable 5D3 level.
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Subsequently, the second non-radiative transition occurred and the electrons was decayed to the 5D4 level from 5D3 level. Finally, the radiative transitions (namely, 5D4→7F6 at 489 nm, 5
D4→7F5 at 543 nm, 5D4→7F4 at 585 nm, and 5D4→7F3 at 621 nm) happened, giving rise to the
characteristic emissions of Tb3+ ions.
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As we know, the PL emission intensity of the Ln3+-doped phosphors is strongly dependent on the activator’s concentration. To figure out the optimal doping concentration of Tb3+ ions in BiOCl host material, a series of Tb3+-doped BiOCl phosphors were prepared and
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their luminescent properties were also studied. Fig. 6 depicts the PL spectra of the
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BiOCl:x%Tb3+ phosphors as a function of Tb3+ ion concentration (x = 1, 3, 5, 7, 9, and 11). As can be seen, the doping concentration had little influence on the PL spectrum profile, whereas the PL emission intensity changed a lot with raising the Tb3+ doping concentration. It is clear that the PL emission intensity of the BiOCl:x%Tb3+ phosphors first exhibited an increasing tendency with the increase of Tb3+ doping concentration and reached maximum at 9 mol% Tb3+. However, the PL emission intensity began to decrease with further raising the Tb3+ ion concentration owing to concentration quenching effect mainly caused by the non-radiative 7
ACCEPTED MANUSCRIPT energy transfer between the adjacent Tb3+ ions [38]. To explore the aforementioned concentration quenching mechanism, the critical distance (Rc) between Tb3+ ions in BiOCl:x%Tb3+ phosphors was evaluated by the following formula [43]: 1/3
3V Rc = 2 , 4π Zxc
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(2)
where V is the volume of the unit cell, Z is the number of cations per unit cell, and xc is the
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critical doping concentration. In terms of the BiOCl host, V = 111.57 Å3, Z = 2, and the critical doping concentration for Tb3+ ions in BiOCl host was 9 mol%. Hence, by using the
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Eq.(2), the critical distance was determined to be around 10.58 Å. To realize the non-radiative energy transfer between the activators, there are mainly two mechanisms: exchange interaction and multipole-multipole interaction [38]. General speaking, the exchange interaction dominates in the non-radiative energy transfer between the activators when the
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critical distance is less than 5 Å, whereas the multipole-multipole prevails if the critical distance is larger than 5 Å. In our case, the critical distance for the Tb3+ ions in the
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BiOCl:Tb3+ phosphors was much larger than 5 Å, and therefore, the multipole-multipole interaction is the dominant mechanism underlying the concentration quenching of the
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Tb3+-activated BiOCl phosphors.
The Commission International De I’Eclairage (CIE) chromaticity coordinates of the
Tb3+-doped BiOCl phosphors with optimal doping concentration were calculated from the PL spectrum and corresponding results were depicted in Fig. 7. The color coordinates of the BiOCl:9%Tb3+ phosphors were found to be (x = 0.353, y = 0.572), and thus located in the green region. Meanwhile, the calculated CIE chromaticity coordinates were also close to that of the commercial green-emitting Y2O3:Tb3+ phosphors (x = 0.319, y = 0.597) (see Fig. 7) 8
ACCEPTED MANUSCRIPT [36]. Furthermore, bright green emission was achieved in BiOCl:9%Tb3+ sample when excited at 377 nm, as shown in the inset of Fig. 7. These results indicated that the Tb3+-doped BiOCl phosphors may have potential application in white LEDs as green-emitting phosphors.
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The decay curves of the Tb3+-doped BiOCl phosphors with different Tb3+ ion concentration were recorded under the excitation wavelength of 377 nm and emission
double exponential expression:
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I = I 0 + A1 exp ( −t τ 1 ) + A2 exp ( −t τ 2 ) ,
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wavelength of 543 nm. As presented in Fig. 8, all the decay curves can be well fitted by the
(3)
In this equation, I0 and I are luminescent intensities when the time is t = 0 and t, respectively. A1 and A2 are constants, τ1 and τ2 are rapid and slow decay time for the exponential components, respectively. Meanwhile, the average lifetime τav can be defined as below:
A1τ 12 + A2τ 22 , A1τ 1 + A2τ 2
(4)
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τ av =
According to the fitting results, the lifetimes of Tb3+ emission at 543 nm were found to be
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around 82, 70, 49, 47, 12 and 10 µs for the samples with Tb3+ doping concentration of 1, 3, 5, 7, 9 and 11%, respectively (see Fig. 8). The short decay times make these BiOCl:Tb3+
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phosphors promising for application in white LEDs [44]. For examining the practicability of the obtained phosphors for white LEDs applications, it
is very necessary to investigate their thermal stability since it can affect the light output. Herein, the temperature-dependent PL spectra of BiOCl:9%Tb3+ phosphors with the temperature heating from 303 to 443 K and cooled from 443 to 303 K were recorded and illustrated in the inset of Fig. 9. It can be seen that the PL emission intensity monotonously decreased with increasing the temperature, while for the cooling process, the PL emission 9
ACCEPTED MANUSCRIPT intensity showed an upward tendency with the decrement of temperature. Comparing the heating and cooling processes, no distinct changes were observed for their PL profiles and the temperature-dependent PL spectra can be exactly repeated, demonstrating that the Tb3+-doped
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BiOCl phosphors possessed reversible thermal behavior and good thermal stability. Furthermore, as demonstrated in Fig. 9, the PL emission at 423 K retained around 24.6% of the initial value at room temperature (303 K). The repaid decrement of PL emission intensity
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with the elevated temperature would be attributed to the non-radiative process. In general, the
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non-radiative transition possibility is proportion to the absolute temperature, and consequently, higher temperature will result in higher probability of the non-radiative transition and thus the decreased PL emission intensity [45]. To further comprehend the thermal quenching of the studied samples, the activation energy is needed to evaluate and it can be defined as [37]:
I0 , 1 + exp ( − E kT )
(5)
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I=
where I0 and I are the PL emission intensity at initial and testing temperature, respectively. E
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is the activation energy, k is the Boltzmann constant and T is the absolute temperature. The plot of ln(I0/I-1) versus 1/kT is displayed in the inset of Fig. 10. Obviously, the experimental
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data can be linear fitted with a slope of −0.31, as a result, the involved activation energy in this work was determined to be 0.31 eV. Apart from the PL measurement, the CL performance of the Tb3+-doped BiOCl phosphors
was also necessary to analyze for the purpose of exploring their suitability for FEDs applications as green-emitting phosphors. Figure 11(a) shows the CL spectrum of the BiOCl:9%Tb3+ phosphors at the accelerating voltage of 10 kV and the filament current of 55 µA. The CL spectrum was composed of four sharp emission bands at around 489 (5D4→7F6 10
ACCEPTED MANUSCRIPT transition), 543 (5D4→7F5 transition), 585 (5D4→7F4 transition) and 621 nm (5D4→7F3 transition), corresponding to the characteristic emissions of Tb3+ ions. It is noted that the CL emission intensity was strongly dependent on the accelerating voltage and filament current.
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From Fig. 11(b), one can see that the CL emission intensity exhibited an elevating tendency with the increased accelerating voltage from 5 to 10 kV when the filament current was 55 µA. Furthermore, under a fixed accelerating voltage of 10 kV, the CL emission intensity was also
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greatly enhanced by raising the filament current from 34 to 55 µA (see Fig. 11(c)). For CL,
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the Tb3+ ions were excited by the incident electrons induced plasmas. Generally, with the increase of accelerating voltage and filament current, more plasma would be produced, leading to more excited Tb3+ ions [46]. As a consequence, the CL emission intensity was enhanced with elevating the accelerating voltage and filament current. In addition, the
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improved CL emission intensities could be also attributed to the deeper penetration of the electrons into the host body and larger electron beam current density. On the basis of previous studies [47,48], the electron penetration depth for the BiOCl:9%Tb3+ phosphors can be
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roughly estimated by the following empirical expression: n
(6)
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A E 1.2 L = 250 1/2 , and n = 1 − 0.29 log Z ρ Z
In the above expression, L stands for electron penetration depth, A is the molecular weight of the studied product, ρ represents the bulk density (for BiOCl, the density is 7.717 g/cm3), E is the accelerating voltage and Z is the atomic number per molecule. In this present work, the values of A, ρ and Z for BiOCl were constants. From Eq. (6), it is clear that the electron penetration depth for the BiOCl:9%Tb3+ phosphors was proportion to the accelerating voltage. Thus, the larger accelerating voltage results in deeper electron penetration depth and the 11
ACCEPTED MANUSCRIPT enhanced CL emission intensity. Notably, the CL emission intensity did not exhibit the saturate with raising the accelerating voltage and filament current, as described in Fig. 11. These results suggested that the Tb3+-doped BiOCl phosphors possessed superior CL
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properties, which make them promising candidates for FEDs as green-emitting phosphors.
4. Conclusion
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In summary, a series of Tb3+-doped BiOCl green-emitting phosphors were synthesized
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through a facile solid-state reaction method. The XRD results confirmed that all the samples possessed pure tetragonal phase and the Tb3+ ions occupied the Bi3+ ions’ sites in BiOCl host lattices. Upon excitation at 377 nm, the characteristic emissions of Tb3+ ions were observed. Meanwhile, the PL emission intensity was strongly dependent on the Tb3+ doping
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concentration and the optimal doping concentration was determined to be 9 mol%. The multipole-multipole interaction was the dominant mechanism for the concentration quenching effect. With the increase of the temperature, the PL emission intensity exhibited a declining
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trend and the activation energy was estimated to be around 0.29 eV. Furthermore, the
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BiOCl:Tb3+ phosphors possessed excellent CL properties and the CL emission intensity could be significantly enhanced with elevating the accelerating voltage and filament current. These characteristics make the Tb3+-doped BiOCl phosphors are promising candidates for white LEDs and FEDs as green-emitting phosphors.
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ACCEPTED MANUSCRIPT Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51502190), the Program for the Outstanding Innovative Teams of Higher Learning
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Institutions of Shanxi, the Start-up Research Grant of Taiyuan University of Technology (No.Tyutrc201489a), the Excellent Young Scholars Research Grant of Taiyuan University of Technology (No. 2014YQ009 and No. 2015YQ006), and the Open Fund of the State Key
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Laboratory of Luminescent Materials and Devices (South China University of Technology,
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No. 2015-skllmd-10).
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[45] H. Yi, F. Li, L. Wu, L. Wu, H. Wang, B. Wang, Y. Zhang, Y. Kong, J. Xu, RSC Adv., 4 (2014) 64244-64251.
[46] N. Zhang, C. Guo, J. Zheng, X. Su, J. Zhao, J. Mater. Chem. C, 2 (2014) 3988-3994. [47] P. Du, J.S. Yu, RSC Adv., 5 (2015) 60121-60127.
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[48] K. Li, Y. Zhang, X. Li, M. Shang, H. Lian, J. Lin, Phys. Chem. Chem. Phys., 17 (2015)
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ACCEPTED MANUSCRIPT Figure captions Fig. 1. Representative XRD patterns of BiOCl:x%Tb3+ (x = 1, 5, 9, and 11) phosphors sintered at 500 °C. Fig. 2. (a) Retiveld refinement for XRD pattern of BiOCl:9%Tb3+ phosphors. (b) The crystal
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structure of BiOCl 3 × 3 × 3 super cell. Fig. 3. Typical FE-SEM image of BiOCl:x%Tb3+ phosphors (a) x = 1, (b) x = 5, (c) x = 9 and (d) x = 11.
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Fig. 4. Room temperature PLE (λem = 543 nm) and PL (λex = 377 nm) spectra of
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BiOCl:9%Tb3+ phosphors.
Fig. 5. Energy level diagram of Tb3+ ions along with the proposed PL processes. Fig. 6. PL spectra of Tb3+-doped BiOCl phosphors with different Tb3+ doping concentration excited at 377 nm.
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Fig. 7. CIE chromaticity coordinates of BiOCl:9%Tb3+ and commercial green-emitting Y2O3:Tb3+ phosphors. Inset shows the luminescent image of BiOCl:9%Tb3+ excited at 377 nm.
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Fig. 8. PL decay curves of the BiOCl:x%Tb3+ (x = 1, 3, 5, 7, 9 and 11) phosphors monitored at 543 nm and with an excitation wavelength of 377 nm.
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Fig. 9. Normalized PL emission intensity of BiOCl:9%Tb3+ phosphors. Inset shows the temperature-dependent PL spectra of BiOCl:9%Tb3+ phosphors (a) heat from 303 to 443 K and (b) cooled from 443 to 303 K. Fig. 10. Plots of ln(I0/I-1) vs. 1/kT for BiOCl:9%Tb3+ phosphors. Fig. 11. (a) CL spectrum of BiOCl:9%Tb3+ phosphors recorded at 10 kV of accelerating voltage and 55 µA of filament current. CL emission intensity as a function of (b) accelerating voltage at 55 µA of filament current and (c) filament current at 10 kV of accelerating voltage.
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ACCEPTED MANUSCRIPT Table 1. Refined lattice parameters of Tb3+-doped BiOCl phosphors as well as the structural parameters of BiOCl host. Parameters
Compounds BiOCl (JCPDS 06-0249) BiOCl:%9Tb3+ Tetragonal Tetragonal P4/nmm(129) P4/nmm(129) 3.891 3.88222 7.369 7.34426 90° 90° 111.57 110.69 2 2 6.38% 8.78% 2.13
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Phase structure Space group a = b (Å) c (Å) α=β=γ V (Å3) Z Rp Rwp χ2
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Fig. 11
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ACCEPTED MANUSCRIPT Highlights: (1) BiOCl:Tb3+ phosphors were prepared by a facile solid-state reaction method. (2) Photoluminescence and cathodoluminescence spectra were investigated.
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(3) Thermal stability was studied according to the temperature-dependent emission spectra.
S0925-8388(16)33704-5
DOI:
10.1016/j.jallcom.2016.11.224
Reference:
JALCOM 39721
To appear in:
Journal of Alloys and Compounds
Received Date: 26 September 2016 Revised Date:
11 November 2016
Accepted Date: 16 November 2016
Please cite this article as: X. Huang, B. Li, H. Guo, Synthesis, photoluminescence, 3+ cathodoluminescence, and thermal properties of novel Tb -doped BiOCl green-emitting phosphors, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.11.224. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Synthesis, photoluminescence, cathodoluminescence, and thermal properties of novel Tb3+-doped BiOCl green-emitting phosphors Xiaoyong Huang*, Bin Li, Heng Guo Key Lab of Advanced Transducers and Intelligent Control System, Ministry of Education and
*Corresponding author, E-mail: [email protected]
Abstract
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Taiyuan 030024, P. R. China
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Shanxi Province, College of Physics and Optoelectronics, Taiyuan University of Technology,
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Tb3+-doped BiOCl phosphors were synthesized by a facile solid-state reaction method. The obtained samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence (PL) and cathodoluminescence (CL) spectra, and PL thermal stability. Under 377 nm near-ultraviolet light excitation, all the samples showed the characteristic green emissions of Tb3+ ions owing to the 5D4→7FJ (J = 6, 5, 4, 3) transitions.
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The dependence of PL emission intensity on the Tb3+ doping concentration was investigated and the optimal doping concentration was found to be 9 mol%. With the help of theoretical calculation, the critical distance was determined to be about 10.58 Å and the concentration quenching was dominated by multipole-multipole interaction. Furthermore, thermal stability
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of these phosphors was investigated by measuring the temperature-dependent PL spectra. In addition, these Tb3+-doped BiOCl phosphors also exhibited strong CL green emissions, and
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the CL emission intensity did not show saturation with the increase of accelerating voltage and filament current. These results indicated that the Tb3+-doped BiOCl phosphors may have potential applications in white light-emitting diodes (LEDs) and field emission displays (FEDs) as green-emitting phosphors.
Keywords: Lanthanide ions; Phosphors; Photoluminescence; Cathodoluminescence; Tb3+; BiOCl.
1
ACCEPTED MANUSCRIPT 1. Introduction In the past decades, inorganic luminescent materials doped with trivalent lanthanide (Ln3+) ions have attracted considerable research attention, since they show great promise in many
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applications, such as white light-emitting diodes (LEDs), remote temperature monitoring, biological imaging, field emission displays (FEDs), and solar cells [1-9]. For the above applications, the desirable inorganic phosphors are required to exhibit highly efficient
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luminescent performance. In fact, the luminescent properties of Ln3+-doped phosphors are
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strongly dependent on the crystal field around the Ln3+ ions in the host lattices, and therefore selecting a suitable host material is of great importance. Until now, some inorganic materials, such as molybdates, oxides, fluorides, and silicate, were extensively studied as the luminescent host [10-21]. The fluorides possess low cut-off phonon energy, which is benefit
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for decreasing the possibility of non-radiative transition, and thus leading to superior luminescent properties. However, the fluorides generally have low thermal and physical stability, and consequently, they cannot be used for some particular applications. In contrast,
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other luminescent hosts also suffer from some drawbacks, such as relatively high cut-off
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phonon energy and high fabrication temperature. Therefore, searching for a novel luminescent host, which possesses low cut-off photon energy, low fabrication temperature, high thermal and physical stability, is required. Recently, bismuth oxychloride (BiOCl) was selected as suitable luminescent host owing
to its low fabrication temperature, high chemical stability and unique spatial structure [22,23]. BiOCl has a tetragonal matlockite PbFCl structure with [Cl–Bi–O–Bi–Cl] layers stacked together by nonbonding van der Waals interaction through the halogen atoms along c-axis [24, 2
ACCEPTED MANUSCRIPT 25], as demonstrated in Fig. 2(b). Meanwhile, the Bi3+ ions are surrounded by four oxygen atoms and four chlorine atoms. Furthermore, the BiOCl also has a cut-off photon energy of about 400 cm-1 [26]. Thanks to these merits, efficient luminescence could be achieved in
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Ln3+-doped BiOCl phosphors. It was recently reported that the Eu3+-doped BiOCl phosphors can give rise to intense red emissions under the excitation of near-ultraviolet (NUV) light [27]. In addition, the luminescent properties of BiOCl phosphors doped with Dy3+, Er3+/Yb3+ or
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Ho3+/Yb3+ ions were also studied in recent years [28-30]. In spite of these achievements, more
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research efforts are still needed to systematically investigate the luminescent properties and thermal stability of Ln3+-doped BiOCl phosphors. On the other hand, among the Ln3+ ions, terbium (Tb3+) ion has been proven to be an efficient green-emitting activator due to its dominant green emission originating from the 5D4→7F5 transition [31-34]. Recently, Ju et al.
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reported the bright green emissions with high color purity in Tb3+-doped ZnMoO4 phosphors [35]. Yu et al. also pointed out that the CaGd2ZnO5:Tb3+ nanophosphors were suitable for white LEDs applications as green-emitting phosphors [36]. However, to the best of our the
investigation
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knowledge,
on
the
luminescent
properties,
especially
the
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cathodoluminescence (CL) and thermal stability, of the Tb3+-doped BiOCl phosphors is still lacking. In this present paper, we reported the luminescent properties of Tb3+-doped BiOCl phosphors prepared by a facile solid-state reaction method. The phase structure, morphology, photoluminescence (PL), and thermal stability of the obtained BiOCl:Tb3+ phosphors were studied in detail. Meanwhile, the CL properties of the as-synthesized phosphors were also systematically analyzed with the aim of exploring their potential application in FEDs. 2. Experimental 3
ACCEPTED MANUSCRIPT The BiOCl:x%Tb3+ phosphors doped with different Tb3+ concentrations (x = 1, 3, 5, 7, 9, and 11) were prepared by means of a facile solid-state reaction method. Appropriate amounts of Bi2O3 (99.9%), Tb(NO3)3·6H2O (99.9%), and NH4Cl (99.5%) were weighted and ground
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together thoroughly in an agate mortar. Excess NH4Cl (20 mol%) was used in order to compensate the loss of volatilization. Subsequently, these mixtures were transferred to the alumina crucibles followed by calcination in a furnace at 500 ºC for 4 h in air. Finally, these
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mixtures were then cooled down naturally to room temperature and ground for further
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characterization.
The phase composition and crystal structure of as-synthesized samples were characterized by X-ray diffraction (XRD) using a Bruker D8 Advance diffractometer with Cu Kα radiation. The morphology of the samples were examined by a field-emission scanning electron
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microscope (FE-SEM; Philips XL-30). The PL spectra were measured by an Edinburgh FS5 spectrometer. The temperature ranging from 303 to 443 K was controlled by a homemade temperature stage. The decay curves of the studied samples were collected by a fluorescence
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spectrophotometer (Photon Technology International, USA) attached with a Xe flash lamp of
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25 W power. The CL properties were investigated by utilizing a Gatan (UK) MonoCL3 system attached with a Hitachi S-4300 SEM at room temperature.
3. Results and discussion To identify the crystal structure and phase purity of Tb3+-doped BiOCl phosphors, the XRD measurement was carried out. Fig. 1 shows the representative XRD patterns of the as-synthesized samples. All the diffraction peaks of the obtained products agree well with the 4
ACCEPTED MANUSCRIPT tetragonal phase of BiOCl (JCPDS 06-0249) without detecting any other impurity peaks from the secondary phase. This result clearly demonstrates that the as-synthesized samples possessed pure tetragonal phase and the Tb3+ ions were completely doped into the BiOCl host
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lattices by replacing the Bi3+ ions. According to the previous reports [37,38], to form a new solid solution, the radius percentage difference (Dr) between the dopants and the possible
Dr =
R1 (CN) − R2 (CN) × 100 , R1 (CN)
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substituted ions should be no more than 15% and it can be expressed as:
(1)
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where R1(CN) is the ionic radius of the host cation, R2(CN) is the ionic radius of the dopant. Herein, the values of R1(CN) and R2(CN) were 1.17 and 1.04 Å, respectively, when the coordinate number was 8. Therefore, by means of Eq. (1), the Dr value of Bi3+/Tb3+ was calculated to be around 11.11%, which was smaller than 15%, further confirming that the
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Tb3+ ion can occupy the site of Bi3+ ions in the BiOCl host. Furthermore, it can be seen from Fig. 1 that the diffraction peak positions were slightly shifted to smaller angle with the
ions.
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increase of Tb3+ doping concentration due to the different ionic radii between Bi3+ and Tb3+
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For the sake of investigating the crystal structure of obtained samples as well as proving
the Tb3+ ions occupied the Bi3+ ions’ sites in BiOCl host, the Rietveld XRD refinement of the typical BiOCl:9%Tb3+sample was performed via a General Structure Analysis System software. Fig. 2(a) describes the refinement diffraction pattern of the BiOCl:9%Tb3+ sample and the corresponding refined lattice parameters were displayed in Table 1. Obviously, the obtained sample exhibited pure tetragonal phase with a space group of P4/nmm(129). As presented in Table 1, the refinement data values were Rp = 6.38%, Rwp = 8.78% and χ2 = 2.13, 5
ACCEPTED MANUSCRIPT while the lattice parameters for BiOCl:9%Tb3+ sample were determined to be approximately a = b = 3.88222 Å, c = 7.34426 Å, V = 110.69Å3. It was found that the calculated lattice parameters for the as-synthesized sample were a little smaller than those of BiOCl host, which
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can be attributed to the fact that Bi3+ ions were replaced by the smaller sized Tb3+ ions. These results further proved that the Tb3+ ions were preferable to substitute the Bi3+ ions and did not induce any significant changes to the host structure.
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The representative FE-SEM images of BiOCl:Tb3+ phosphors with the doping
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concentration of 1, 5, 9 and 11 mol% are illustrated in Fig. 3. As can be seen, the obtained samples were made up of plate-like agglomerated particles with the size ranging from around 1 to 4 µm. Furthermore, the shape and size of the particles were changed little with the addition of Tb3+ ions, suggesting that the dopant concentration did not have any effect on the
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morphological properties of the as-synthesized samples.
Fig. 4 shows the PL excitation (PLE) and PL spectra of the BiOCl:9%Tb3+sample. The PLE spectrum, which was monitored at the green emission wavelength of 543 nm, consisted
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of a broad band and two sharp peaks. In particular, the broad band from 220 to 335 nm with a
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center at around 300 nm was originated from the inter-configurational 4f8→4f75d1 transition of Tb3+ ions [35,36], while these narrow bands located at around 354 and 377 nm were assigned to the intra-4f transitions of Tb3+ ions (namely, 7F6→5L9 and 7F6→5G6 transitions, respectively) [39,40]. Among these excitation bands, the intensity of the excitation band at 377 nm, which is matched well with the wavelength of commercial NUV LED chip, was the highest, suggesting that the as-synthesized phosphors can be effectively pumped by NUV light. Upon excitation at 377 nm, the PL spectrum of BiOCl:9%Tb3+ sample was measured 6
ACCEPTED MANUSCRIPT and displayed in Fig. 4. Obviously, the PL spectrum consisted of a series of sharp emission peaks at about 489, 543, 585 and 621 nm, which can be assigned to the 5D4→7F6, 5D4→7F5, 5
D4→7F4 and 5D4→7F3 transitions of Tb3+ ions, respectively [41,42]. To comprehend the
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involved PL processes, the simplified energy level diagram of Tb3+ ions as well as the proposed luminescent mechanism was shown in Fig. 5. Under 377 nm excitation, the electrons were first excited from the ground state of 7F6 to the excited state of 5G6. Then, the
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non-radiative transition took place, resulting in the population of metastable 5D3 level.
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Subsequently, the second non-radiative transition occurred and the electrons was decayed to the 5D4 level from 5D3 level. Finally, the radiative transitions (namely, 5D4→7F6 at 489 nm, 5
D4→7F5 at 543 nm, 5D4→7F4 at 585 nm, and 5D4→7F3 at 621 nm) happened, giving rise to the
characteristic emissions of Tb3+ ions.
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As we know, the PL emission intensity of the Ln3+-doped phosphors is strongly dependent on the activator’s concentration. To figure out the optimal doping concentration of Tb3+ ions in BiOCl host material, a series of Tb3+-doped BiOCl phosphors were prepared and
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their luminescent properties were also studied. Fig. 6 depicts the PL spectra of the
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BiOCl:x%Tb3+ phosphors as a function of Tb3+ ion concentration (x = 1, 3, 5, 7, 9, and 11). As can be seen, the doping concentration had little influence on the PL spectrum profile, whereas the PL emission intensity changed a lot with raising the Tb3+ doping concentration. It is clear that the PL emission intensity of the BiOCl:x%Tb3+ phosphors first exhibited an increasing tendency with the increase of Tb3+ doping concentration and reached maximum at 9 mol% Tb3+. However, the PL emission intensity began to decrease with further raising the Tb3+ ion concentration owing to concentration quenching effect mainly caused by the non-radiative 7
ACCEPTED MANUSCRIPT energy transfer between the adjacent Tb3+ ions [38]. To explore the aforementioned concentration quenching mechanism, the critical distance (Rc) between Tb3+ ions in BiOCl:x%Tb3+ phosphors was evaluated by the following formula [43]: 1/3
3V Rc = 2 , 4π Zxc
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(2)
where V is the volume of the unit cell, Z is the number of cations per unit cell, and xc is the
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critical doping concentration. In terms of the BiOCl host, V = 111.57 Å3, Z = 2, and the critical doping concentration for Tb3+ ions in BiOCl host was 9 mol%. Hence, by using the
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Eq.(2), the critical distance was determined to be around 10.58 Å. To realize the non-radiative energy transfer between the activators, there are mainly two mechanisms: exchange interaction and multipole-multipole interaction [38]. General speaking, the exchange interaction dominates in the non-radiative energy transfer between the activators when the
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critical distance is less than 5 Å, whereas the multipole-multipole prevails if the critical distance is larger than 5 Å. In our case, the critical distance for the Tb3+ ions in the
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BiOCl:Tb3+ phosphors was much larger than 5 Å, and therefore, the multipole-multipole interaction is the dominant mechanism underlying the concentration quenching of the
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Tb3+-activated BiOCl phosphors.
The Commission International De I’Eclairage (CIE) chromaticity coordinates of the
Tb3+-doped BiOCl phosphors with optimal doping concentration were calculated from the PL spectrum and corresponding results were depicted in Fig. 7. The color coordinates of the BiOCl:9%Tb3+ phosphors were found to be (x = 0.353, y = 0.572), and thus located in the green region. Meanwhile, the calculated CIE chromaticity coordinates were also close to that of the commercial green-emitting Y2O3:Tb3+ phosphors (x = 0.319, y = 0.597) (see Fig. 7) 8
ACCEPTED MANUSCRIPT [36]. Furthermore, bright green emission was achieved in BiOCl:9%Tb3+ sample when excited at 377 nm, as shown in the inset of Fig. 7. These results indicated that the Tb3+-doped BiOCl phosphors may have potential application in white LEDs as green-emitting phosphors.
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The decay curves of the Tb3+-doped BiOCl phosphors with different Tb3+ ion concentration were recorded under the excitation wavelength of 377 nm and emission
double exponential expression:
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I = I 0 + A1 exp ( −t τ 1 ) + A2 exp ( −t τ 2 ) ,
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wavelength of 543 nm. As presented in Fig. 8, all the decay curves can be well fitted by the
(3)
In this equation, I0 and I are luminescent intensities when the time is t = 0 and t, respectively. A1 and A2 are constants, τ1 and τ2 are rapid and slow decay time for the exponential components, respectively. Meanwhile, the average lifetime τav can be defined as below:
A1τ 12 + A2τ 22 , A1τ 1 + A2τ 2
(4)
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τ av =
According to the fitting results, the lifetimes of Tb3+ emission at 543 nm were found to be
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around 82, 70, 49, 47, 12 and 10 µs for the samples with Tb3+ doping concentration of 1, 3, 5, 7, 9 and 11%, respectively (see Fig. 8). The short decay times make these BiOCl:Tb3+
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phosphors promising for application in white LEDs [44]. For examining the practicability of the obtained phosphors for white LEDs applications, it
is very necessary to investigate their thermal stability since it can affect the light output. Herein, the temperature-dependent PL spectra of BiOCl:9%Tb3+ phosphors with the temperature heating from 303 to 443 K and cooled from 443 to 303 K were recorded and illustrated in the inset of Fig. 9. It can be seen that the PL emission intensity monotonously decreased with increasing the temperature, while for the cooling process, the PL emission 9
ACCEPTED MANUSCRIPT intensity showed an upward tendency with the decrement of temperature. Comparing the heating and cooling processes, no distinct changes were observed for their PL profiles and the temperature-dependent PL spectra can be exactly repeated, demonstrating that the Tb3+-doped
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BiOCl phosphors possessed reversible thermal behavior and good thermal stability. Furthermore, as demonstrated in Fig. 9, the PL emission at 423 K retained around 24.6% of the initial value at room temperature (303 K). The repaid decrement of PL emission intensity
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with the elevated temperature would be attributed to the non-radiative process. In general, the
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non-radiative transition possibility is proportion to the absolute temperature, and consequently, higher temperature will result in higher probability of the non-radiative transition and thus the decreased PL emission intensity [45]. To further comprehend the thermal quenching of the studied samples, the activation energy is needed to evaluate and it can be defined as [37]:
I0 , 1 + exp ( − E kT )
(5)
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I=
where I0 and I are the PL emission intensity at initial and testing temperature, respectively. E
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is the activation energy, k is the Boltzmann constant and T is the absolute temperature. The plot of ln(I0/I-1) versus 1/kT is displayed in the inset of Fig. 10. Obviously, the experimental
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data can be linear fitted with a slope of −0.31, as a result, the involved activation energy in this work was determined to be 0.31 eV. Apart from the PL measurement, the CL performance of the Tb3+-doped BiOCl phosphors
was also necessary to analyze for the purpose of exploring their suitability for FEDs applications as green-emitting phosphors. Figure 11(a) shows the CL spectrum of the BiOCl:9%Tb3+ phosphors at the accelerating voltage of 10 kV and the filament current of 55 µA. The CL spectrum was composed of four sharp emission bands at around 489 (5D4→7F6 10
ACCEPTED MANUSCRIPT transition), 543 (5D4→7F5 transition), 585 (5D4→7F4 transition) and 621 nm (5D4→7F3 transition), corresponding to the characteristic emissions of Tb3+ ions. It is noted that the CL emission intensity was strongly dependent on the accelerating voltage and filament current.
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From Fig. 11(b), one can see that the CL emission intensity exhibited an elevating tendency with the increased accelerating voltage from 5 to 10 kV when the filament current was 55 µA. Furthermore, under a fixed accelerating voltage of 10 kV, the CL emission intensity was also
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greatly enhanced by raising the filament current from 34 to 55 µA (see Fig. 11(c)). For CL,
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the Tb3+ ions were excited by the incident electrons induced plasmas. Generally, with the increase of accelerating voltage and filament current, more plasma would be produced, leading to more excited Tb3+ ions [46]. As a consequence, the CL emission intensity was enhanced with elevating the accelerating voltage and filament current. In addition, the
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improved CL emission intensities could be also attributed to the deeper penetration of the electrons into the host body and larger electron beam current density. On the basis of previous studies [47,48], the electron penetration depth for the BiOCl:9%Tb3+ phosphors can be
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roughly estimated by the following empirical expression: n
(6)
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A E 1.2 L = 250 1/2 , and n = 1 − 0.29 log Z ρ Z
In the above expression, L stands for electron penetration depth, A is the molecular weight of the studied product, ρ represents the bulk density (for BiOCl, the density is 7.717 g/cm3), E is the accelerating voltage and Z is the atomic number per molecule. In this present work, the values of A, ρ and Z for BiOCl were constants. From Eq. (6), it is clear that the electron penetration depth for the BiOCl:9%Tb3+ phosphors was proportion to the accelerating voltage. Thus, the larger accelerating voltage results in deeper electron penetration depth and the 11
ACCEPTED MANUSCRIPT enhanced CL emission intensity. Notably, the CL emission intensity did not exhibit the saturate with raising the accelerating voltage and filament current, as described in Fig. 11. These results suggested that the Tb3+-doped BiOCl phosphors possessed superior CL
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properties, which make them promising candidates for FEDs as green-emitting phosphors.
4. Conclusion
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In summary, a series of Tb3+-doped BiOCl green-emitting phosphors were synthesized
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through a facile solid-state reaction method. The XRD results confirmed that all the samples possessed pure tetragonal phase and the Tb3+ ions occupied the Bi3+ ions’ sites in BiOCl host lattices. Upon excitation at 377 nm, the characteristic emissions of Tb3+ ions were observed. Meanwhile, the PL emission intensity was strongly dependent on the Tb3+ doping
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concentration and the optimal doping concentration was determined to be 9 mol%. The multipole-multipole interaction was the dominant mechanism for the concentration quenching effect. With the increase of the temperature, the PL emission intensity exhibited a declining
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trend and the activation energy was estimated to be around 0.29 eV. Furthermore, the
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BiOCl:Tb3+ phosphors possessed excellent CL properties and the CL emission intensity could be significantly enhanced with elevating the accelerating voltage and filament current. These characteristics make the Tb3+-doped BiOCl phosphors are promising candidates for white LEDs and FEDs as green-emitting phosphors.
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ACCEPTED MANUSCRIPT Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51502190), the Program for the Outstanding Innovative Teams of Higher Learning
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Institutions of Shanxi, the Start-up Research Grant of Taiyuan University of Technology (No.Tyutrc201489a), the Excellent Young Scholars Research Grant of Taiyuan University of Technology (No. 2014YQ009 and No. 2015YQ006), and the Open Fund of the State Key
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Laboratory of Luminescent Materials and Devices (South China University of Technology,
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No. 2015-skllmd-10).
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ACCEPTED MANUSCRIPT Figure captions Fig. 1. Representative XRD patterns of BiOCl:x%Tb3+ (x = 1, 5, 9, and 11) phosphors sintered at 500 °C. Fig. 2. (a) Retiveld refinement for XRD pattern of BiOCl:9%Tb3+ phosphors. (b) The crystal
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structure of BiOCl 3 × 3 × 3 super cell. Fig. 3. Typical FE-SEM image of BiOCl:x%Tb3+ phosphors (a) x = 1, (b) x = 5, (c) x = 9 and (d) x = 11.
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Fig. 4. Room temperature PLE (λem = 543 nm) and PL (λex = 377 nm) spectra of
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Fig. 5. Energy level diagram of Tb3+ ions along with the proposed PL processes. Fig. 6. PL spectra of Tb3+-doped BiOCl phosphors with different Tb3+ doping concentration excited at 377 nm.
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Fig. 7. CIE chromaticity coordinates of BiOCl:9%Tb3+ and commercial green-emitting Y2O3:Tb3+ phosphors. Inset shows the luminescent image of BiOCl:9%Tb3+ excited at 377 nm.
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Fig. 8. PL decay curves of the BiOCl:x%Tb3+ (x = 1, 3, 5, 7, 9 and 11) phosphors monitored at 543 nm and with an excitation wavelength of 377 nm.
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Fig. 9. Normalized PL emission intensity of BiOCl:9%Tb3+ phosphors. Inset shows the temperature-dependent PL spectra of BiOCl:9%Tb3+ phosphors (a) heat from 303 to 443 K and (b) cooled from 443 to 303 K. Fig. 10. Plots of ln(I0/I-1) vs. 1/kT for BiOCl:9%Tb3+ phosphors. Fig. 11. (a) CL spectrum of BiOCl:9%Tb3+ phosphors recorded at 10 kV of accelerating voltage and 55 µA of filament current. CL emission intensity as a function of (b) accelerating voltage at 55 µA of filament current and (c) filament current at 10 kV of accelerating voltage.
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ACCEPTED MANUSCRIPT Table 1. Refined lattice parameters of Tb3+-doped BiOCl phosphors as well as the structural parameters of BiOCl host. Parameters
Compounds BiOCl (JCPDS 06-0249) BiOCl:%9Tb3+ Tetragonal Tetragonal P4/nmm(129) P4/nmm(129) 3.891 3.88222 7.369 7.34426 90° 90° 111.57 110.69 2 2 6.38% 8.78% 2.13
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Phase structure Space group a = b (Å) c (Å) α=β=γ V (Å3) Z Rp Rwp χ2
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ACCEPTED MANUSCRIPT Highlights: (1) BiOCl:Tb3+ phosphors were prepared by a facile solid-state reaction method. (2) Photoluminescence and cathodoluminescence spectra were investigated.
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(3) Thermal stability was studied according to the temperature-dependent emission spectra.