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Sr3Al2O6:0.025% Tb3+ nanophosphor synthesized by sol-gel technique
Journal of Molecular Structure 1184 (2019) 92e101
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Effects of annealing period on the structure and photoluminescence of the mixed phases ZnAl2O4 /ZnO/SrAl2O4/Sr3Al2O6:0.025% Tb3þ nanophosphor synthesized by sol-gel technique M.R. Mhlongo a, *, L.F. Koao b, R.E. Kroon c, T.E. Motaung d, S.V. Motloung a, e, ** a
Department of Physics, Sefako Makgatho Health Sciences University, P.O. Box 94, Medunsa, 0204, South Africa Department of Physics, University of the Free State (Qwaqwa Campus), Private Bag X 13, Phuthaditjhaba, 9866, South Africa Department of Physics, University of the Free State, P. O. Box 339, Bloemfontein, 9300, South Africa d Department of Chemistry, University of Zululand, KwaDlangezwa, 3886, South Africa e Department of Physics, Nelson Mandela University, P. O. Box 77000, Port Elizabeth, 6031, South Africa b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 December 2018 Received in revised form 5 February 2019 Accepted 6 February 2019 Available online 8 February 2019
Nanophosphor powders of the mixed phases of ZnAl2O4/ZnO/SrAl2O4/Sr3Al2O6 (ZZSS) doped with Tb3þ (ZZSS:0.025%Tb3þ) were successfully prepared by sol-gel technique. The effect of the annealing period at a fixed annealing temperature (1000 C) and dopant concentration (0.025% Tb3þ) on the structure and photoluminescence properties was investigated. X-ray diffraction results revealed that the crystallite size was influenced by the annealing period. Scanning electron microscopy showed that varying the annealing period influenced the particle morphology of the prepared nanophosphor material. High resolution transmission electron microscopy confirmed that the prepared material is on the nanoscale. The photoluminescence results showed that the ZZSS emits at 585 nm when excited at 374 nm, which is attributed to the defects centres within the ZnO phase. The Tb3þ doped samples showed emissions peaks at 545, 590 and 623 nm which were attributed to the 4f transitions of Tb3þ, specifically 5D4 / 7FJ (J ¼ 5, 4, 3). Increasing the annealing period up to 5.4 h led to luminescence enhancements, while a further increase led to quenching. The Commision Internationale de l’Eclairage coordinates showed that the greenish emission colour could be tuned by varying the annealing period. © 2019 Elsevier B.V. All rights reserved.
Keywords: Mixed phases Annealing period Tb3þ Sol-gel Luminescence
1. Introduction In recent years, interests have been focused on the development of new luminescent materials (nanophosphors). In particular, oxide matrices are attractive host materials for the development of advanced phosphors due to their ease of synthesis and chemical stability [1]. As one of the most promising phosphor materials, zinc aluminate (ZnAl2O4) has been extensively studied for application in thin film electroluminescence displays, aerospace, paint and stress imaging devices [2]. In addition, ZnAl2O4 is widely used in many catalytic reactions, cracking, dehydration, hydrogenation and dehydrogenation, ceramic and electro-conductive materials
* Corresponding author. ** Corresponding author. Department of Physics, Sefako Makgatho Health Sciences University, P.O. Box 94, Medunsa, 0204, South Africa. E-mail addresses: [email protected] (M.R. Mhlongo), [email protected] (S.V. Motloung). https://doi.org/10.1016/j.molstruc.2019.02.021 0022-2860/© 2019 Elsevier B.V. All rights reserved.
because of its high thermal stability, high mechanical resistance, low surface acidity and excellent optical properties [3]. ZnAl2O4 is one of the well-known wide-bandgap (~3.8 eV) semiconductors with a spinel structure [4,5] belonging to the Fd3m space group. On the other hand, zinc oxide (ZnO), an n-type semiconductor, has drawn tremendous attention from researchers around the globe. It displays a hexagonal crystalline wurtzite-type structure [6] with space group P6mc and lattice parameters of a ¼ b ¼ 0.3250 nm and c ¼ 0.5207 nm [7]. The importance of ZnO is due to its unusual physical properties such as high conductance, chemical stability, harmless to the environment and inexpensive [8]. Moreover, it has good radiation resistance [9], a wide and direct band gap (~3.37 eV) and a high exciton binding energy of 60 meV [6]. Another oxide material being investigated is strontium aluminate (SrAl2O4), which offers excellent properties such as high quantum efficiency, long persistence of phosphorescence and good stability [10]. SrAl2O4 has a wide band gap (~6.5 eV) which offers the possibility of generating broad band emission [11] and much attention has been
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paid to it owing to its higher radiation-resistance [12]. The synthesis and optical properties of SrAl2O4 in a bulk form or as films or nanoparticles have been extensively explored during the past decades [1]. On the other hand, the oxygen deficient structure of strontium aluminate with Sr3Al2O6 formula is less reported as an individual group of oxide materials. Chakoumakos et al. [13] reported that Sr3Al2O6 has a cubic crystal unit containing puckered six-membered AlO4 tetrahedral rings. The luminescent properties of phosphor materials have been reported to depend strongly on the particle size, crystal structure, distribution of activators in the host lattice, morphologies and preparation methods [5,14]. Doping with rare earth ions (RE3þ) influence the morphology, the particle size and the band structure of the nanocrystals [15,16]. Doping also plays key roles in luminescence efficiency and the position of emission bands, thus influencing their practical applications. Among the RE3þ ions, Tb3þ is a popular efficient luminescent dopant which is being considered in many studies [16]. For an example, Koao et al. [17] prepared ZnO nanoparticles doped with Tb3þ using the chemical bath deposition method. The doped samples showed emission peaks centred at 431, 489, 545, 585 and 621 nm for all Tb3þ doping concentrations. These emission peaks were attributed to the intra-4f transitions of Tb3þ in particular, 5 D3/7F4, 5D4/7F6, 5D4/7F5, 5D4/7F4 and 5D4/ 7F3 transitions, respectively. Omkaram et al. [18] have shown that Tb3þ can be used to form a green emitting phosphor due to 5D4/7F5 transitions in the Tb3þ ions. Rare earth doped Sr3Al2O6 prepared via microwave or sol-gel synthesis methods have been reported [19e23]. Different studies have been attempted on the mixed phases of ZnAl2O4/ZnO [24e27]. Motloung et al. [24] reported the effect of annealing period on the structure and optical properties of the mixed phases ZnAl2O4/ZnO and found that the optimum luminescence is obtained for an annealing period of 2 h. Yuan et al. [27] showed that the mixed oxide ZnO/ZnAl2O4 has excellent stability and much higher photocatalytic activity than their bulk oxide counterparts. With this in mind, and to the best of our knowledge, the effect of annealing period on the mixed phases ZnAl2O4/ZnO/SrAl2O4/ Sr3Al2O6:0.025% Tb3þ (ZZSS:0.025% Tb3þ) has not been investigated. The primary aim of this study was to synthesize a phosphor material based on mixed oxides for practical applications such as in light emitting diodes. This study investigates the effect of annealing period on the structure and photoluminescence (PL) of the mixed phase ZZSS:0.025% Tb3þ nanophosphor. The emission channels associated with the observed PL emissions are also proposed.
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were investigated using a Zeiss Supra 55 scanning electron microscope (SEM) with an energy dispersive X-ray spectroscopy (EDS). Information on the crystallite sizes were obtained using a JEOL 1010 high resolution transmission electron microscope (HRTEM). The emission and excitation spectra measurements were made with an Edinburgh Instruments FLS980 fluorescence spectrometer having double monochromators, using a steady state xenon lamp as excitation source and R928P photomultiplier tube detector. Lifetime measurements were acquired using a xenon flashlamp pulsing at 100 Hz. All characterizations were carried out at room temperature. 3. Results and discussion 3.1. XRD Fig. 1 shows the XRD patterns of the prepared powder samples. Fig. 1 (a) shows the ZZSS pattern which could be indexed to the mixed phases of the cubic ZnAl2O4 (ICSD:4160), Sr3Al2O6 (ICSD:1860), hexagonal ZnO (ICSD:5119) and Sr2Al2O4 (ICSD:31e1336) structures. Fig. 1 (b) presents the ZZSS:0.025%Tb3þ for various annealing periods, which showed similar diffraction patterns to that of the un-doped ZZSS sample. In addition, the results suggest that doping does not change the crystal structure of the prepared samples, especially when comparing the un-doped and doped samples for annealing period of 2 h. These results are consistent with the results obtained by Melato et al. [28] where both In3þ doping and annealing period did not alter the crystal
2. Experimental ZnAl2O4/ZnO (ZZ), SrAl2O4/Sr3Al2O6 (SS) and ZnAl2O4/ZnO/ SrAl2O4/Sr3Al2O6 (ZZSS) nanophosphors were successfully prepared using the citrate sol-gel method. Zn(NO3)2$6H2O (98%), Sr(NO3)2 (98%), Al(NO3)3$9H2O (98%) and citric acid C8H8O7$H2O (99%) precursors purchased from Sigma Aldrich were used as received without further purification. For ZZSS the starting amounts of these precursors were 4.373, 3.111, 10.808 and 2.333 g, respectively. They were dissolved in 60 ml of deionized water. The doped sample (ZZSS:0.025%Tb3þ) was prepared by adding 0.002 g of Tb(NO3)3$5H2O (98%) to the solution. The solution was stirred with a magnetic stirrer at a temperature of ~80 C until a gel was formed, which was left to dry for 2 h at room temperature and subsequently annealed in a furnace at 1000 C for different annealing periods varying from 1.5 to 8 h. The products from the furnace were ground into ultrafine powders using a mortar and pestle and analysed using different techniques. The crystal structures of the samples were characterized by powder X-ray diffraction (XRD) (Bruker AXS Discover diffractometer) with Cu Ka (1.5418 Å) radiation. The morphologies of the prepared phosphors
Fig. 1. The XRD patterns for the (a) ZZSS sample and JCDPS standards sample (b) doped samples for various annealing periods.
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structure of the MgAl2O4. Thus, the results suggest that Tb3þ is incorporated successfully into the crystal structure of each phase of the ZZSS. To further motivate this, consider the analysis of the most intense diffraction peaks for the un-doped and doped samples at various annealing periods shown in Fig. 2 (a) e (c). The most intense diffraction peaks are 131, 101, 102 and 044 corresponding to the ZnAl2O4, ZnO, SrAl2O4 and Sr3Al2O6 phases, respectively. The peak intensities change as the annealing period increases, which suggest that the annealing period influenced the crystallinity of the prepared nanophosphors [29]. In all ZZSS phases, the results clearly show that there is a diffraction peak shift to the lower angle when doping with Tb3þ, which could be attributed to the increase of the lattice parameters [30]. The increase in lattice parameters is attributed to the replacement of the smaller atoms with the bigger atom in the crystal lattices of the individual phases. The ionic radii of Zn2þ, Sr2þ and Al3þ are 0.74, 1.13 and 0.50 Å, respectively [31e33]. When the smaller ions (Zn2þ or Al3þ) are replaced by the larger Tb3þ ion (1.00 Å) [34], this will result in an increase in lattice parameter [15]. Thus, we propose that the Tb3þ ions substitute the Zn2þ or Al3þ ions in the crystal lattice of the ZZSS phases. The lattice parameters for the cubic (ZnAl2O4, Sr3Al2O6) and hexagonal (ZnO, SrAl2O4) phases were calculated from equations (1) and (2), respectively.
a dhkl ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi h2 þ k2 þ l2
(1)
1 dhkl ¼ rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi 2 4 h2 þ k2 þ hk 3a2 cl 2
(2)
where a and c are the lattice constants, dhkl is the interplanar distance and hkl are the Miller indices [35]. The average lattice constant for the cubic ZnAl2O4 and Sr3Al2O6 were calculated to be a ¼ b ¼ c ¼ 8.028 and 15.737 Å respectively, which are similar to reported values [35,36]. The lattice constants of the hexagonal ZnO were calculated to be a ¼ b ¼ 3.240 Å and c ¼ 5.174 Å, which are close to the values reported in the literature [24,37], while those of the hexagonal SrAl2O4 were calculated to be a ¼ b ¼ 4.911 Å and c ¼ 8.462 Å, which is comparable to the previous values reported [30,33]. The crystallite size (D) for each of the phases in the ZZSS nanophosphor was estimated from the full width at half maximum (FWHM) of the most intense diffraction peaks of each phases using Scherrer's equation [38].
D¼
0:9l bSinq
(3)
l stands for the X-ray wavelength, b is the full width at half maximum (radians) and q is the angle of diffraction (radians). The calculated crystallite sizes for various annealing periods are presented in Table 1 and show that annealing period influences the crystallite size of the prepared nanophosphors.
Fig. 2. The most intense diffraction peaks of the different phases (a) 131 of ZnAl2O4 and 101 of ZnO (b) 044 of Sr3Al2O6 (c) 102 of SrAl2O4.
M.R. Mhlongo et al. / Journal of Molecular Structure 1184 (2019) 92e101 Table 1 Crystallite sizes (nm) of the different phases. All samples were doped except as indicated. Annealing period (h)
131 of ZnAl2O4
101 of ZnO
102 of SrAl2O4
044 of Sr3Al2O6
1.5 2 (undoped) 2 3 4 5 6 7 8
29 28 29 27 31 29 23 26 20
35 35 33 30 37 35 26 28 23
30 31 31 28 35 32 27 24 22
31 35 32 32 34 35 28 31 34
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3.2. SEM and EDS Fig. 3 shows the EDS spectrum of the un-doped sample annealed for 2 h. It shows that the sample is composed of Zn, Sr, Al and O. The carbon peak is due to the conductive carbon films coated on the sample holders during the course of EDS measurement. Apart from the anticipated elements, there were no other peaks detected. The morphological aspect of the selected ZZSS nanophosphor samples were analysed using SEM, as shown in Fig. 4. The micrograph in Fig. 4 (a) reveals that the morphology for the un-doped ZZSS consisted of the agglomeration of irregular particles with grain boundaries and voids. Rod-like structures were also observed on the surface. The micrograph of the doped ZZSS sample annealed for 2 h is displayed in Fig. 4 (b) and the morphology is similar to that of the un-doped ZZSS. The agglomeration of the rod-like structures appears less. The sample annealed for 5 h is shown in Fig. 4 (c) and the micrograph reveals that the sizes of the particles and rod-like structures had increased. Fig. 4 (d) shows the after 7 h of annealing the rod-like structures have been destroyed. Based on the XRD results, the observed hexagonal shape particle circled in red is either ZnO or SrAl2O4 rather than the cubic ZnAl2O4 or Sr3Al2O6. The results show clearly that while doping had little influence on the morphology, the annealing period has a strong influence on the morphology of the prepared phosphors. 3.3. HR-TEM
Fig. 3. EDS spectrum of the ZZSS nanophosphor annealed for 2 h.
Further analysis was done by HR-TEM to confirm the crystallite size of the prepared nanophosphor. Fig. 5 (a) and (b) present the images of the un-doped and magnified ZZSS sample, respectively. The ZZSS image confirms the agglomeration of the particles as
Fig. 4. SEM micrographs of samples having annealing periods of (a) 2 h (un-doped ZZSS) (b) 2 h (c) 5 h and (d) 7 h.
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Fig. 5. HR-TEM images for (a) un-doped ZZSS (b) magnified un-doped ZZSS and (c) doped sample annealed for 5 h.
observed in SEM results. Fig. 5 (b) indicates the existence of hexagonal-like structures, which is agreeing very well with the XRD and SEM results. The results also shows the presence of lattice fringes which confirms that our nanophosphor has a crystallite structure. Fig. 5 (c) shows the doped sample annealed for 5 h. The results shows the lattice fringes, it can also be observed that the sample have different planes which can be confirmed by different miller indices from our XRD results. 3.4. Photoluminescence PL excitation and emission spectra of the ZZ and SS nanopowders annealed for 2 h are shown in Fig. 6 (a). In both cases the excitation spectra were recorded when monitoring the emission at 452 nm and there is an excitation peak located at 374 nm. Considering the band gaps of ZnAl2O4(~3.8 eV) [4] and ZnO (~3.37 eV) [6], it is clear that for the ZZ material the 374 nm (~3.32 eV) excitation energy is slightly lower than the band gap energy of both phases and this will then suggest the existence of the intermediary energy level for both phases just below their conduction bands [39]. For the ZZ phases, the emission spectra excited at 374 nm showed
emission peaks located at 430, 452 and 585, 677e730 nm. Previously, similar emissions from both ZnAl2O4 and ZnO were observed [39e42]. ZnO is composed of extrinsic and intrinsic deep level defects that emit different colours in the visible region [40]. The various defects such as oxygen vacancies (Vo), zinc vacancies (Vzn), oxygen interstitial (Oi), zinc interstitial (Zni) and anitisite oxygen (Ozn) are the reason behind the deep level emission at visible range. In this study the 585 nm emission is from ZnO and is attributed to the Oi and believed to be due to band transition from Zni to Oi defect levels in ZnO [42], whereas 430 and 452 nm can be assumed to be from the aluminate site such as ZnAl2O4. Mikenda et al. [43] reported the red luminescence (around 700 nm, here at 677, 690, 700, 713, 725 and 730 nm) to be due to the spin forbidden 2Eg - 4A2g, N-zero phonon lines of Cr3þ at the octahedral site of Al3þ ions. The presence of Cr3þ in aluminium compounds is observed due to trace impurities in the precursor materials, of which it can be a possibility in our case. In the SS phase the 374 nm excitation is also lower than the band gap energies of SrAl2O4 (~6.5 eV) [11] and Sr3Al2O6 (~6.2 eV) [44] which suggests that there are defect levels below the conduction band. This excitation produced emissions at 430, 452 and 490 nm.
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Fig. 6. The excitation and emission spectra of the (a) ZZ and SS samples (b) ZZSS samples.
These emissions can be associated with defects within the aluminate in both SrAl2O4 and Sr3Al2O6, as they are also observed in ZnAl2O4. Similar emissions were observed by Tamrakar et al. [45] in SrAl2O4 system. Fig. 6 (b) shows the ZZSS mixed phase excitation and emission spectra. It can be seen that there is one emission peak located at 585 nm with some shoulders at 412 and 432 nm when the sample was excited at 374 nm. The emission peak at 585 nm is therefore considered to originate from the ZZ phases (in particular
from ZnO as discussed for Fig. 6 (a)). Generally, the PL results confirmed the presence of the mixed ZZSS phases predicted by the XRD results. Fig. 7 (a) shows a comparison between the ZZSS (un-doped) and ZZSS:0.025 Tb3þ samples annealed for 2 h. An excitation wavelength of 374 nm only produced the emission at 585 nm (with small shoulders at 412 and 432 nm) which is from the ZZ material. Since the emissions peaks for both samples are located at the same
Fig. 7. The excitation and emission spectra of (a) ZZSS and ZZSS:0.025%Tb3þ annealed for 2 h (b) ZZSS:0.025%Tb3þ annealed for different periods and (c) intensity as a function of annealing period.
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positions, the results suggest that there is no emission from the Tb3þ ions for this excitation wavelength but all of the emissions should be attributed to the ZZSS phases (as discussed for Fig. 6). Fig. 7 (b) shows the ZZSS:0.025 Tb3þ samples for various annealing periods when the samples were excited at 374 nm. The results show the change in emission intensities as the annealing period changes, as presented in Fig. 7 (c). The sample annealed for 6 h had the highest emission intensity, even though there are fluctuations. We propose that the fluctuations on the emission intensity might possibly be due to the existence of various phases in ZZSS. It is noted from Fig. 7 (a) and (b) that the 374 nm wavelength only excited the ZZSS phases and not the Tb3þ ions. Tb3þ ions can generally be effectively excited at very short wavelengths corresponding to their strong f-d transition and the emission curve of the ZZSS:0.025%Tb3þ sample annealed for 8 h, when excited at 230 nm, showed a clear Tb3þ emission line at 542 nm superimposed on the broad emission band. To minimize the broad emission and focus on the Tb3þ emission, the sample was further excited with a wavelength of 485 nm which is longer than the excitation range of the broad emission and corresponds to an f-f excitation line of Tb3þ. The weak data from Fig. 8 (a) is zoomed in Fig. 8 (b) and shows the characteristic emissions of Tb3þ at 545, 590 and 623 nm. Fig. 8 (c) shows all the doped samples for various annealing periods when excited at 485 nm. The broad band centred at 585 nm is emission from ZnO and can be ascribed to the presence of oxygen interstitials (Oi) [46]. The relatively sharp peak near
590 nm (with Stark split sublevels at 585 and 594 nm) is due to the 5 D4/7F4 transition of Tb3þ [47]. The 545 nm (with 542 and 548 nm Stark components) and 623 nm emissions are attributed to the intra-4f transitions of Tb3þ specifically, 5D4/7F5 and 5D4/ 7F3 transitions [47]. To examine the effect of the annealing period, all the doped samples were excited with 485 nm as shown in Fig. 8 (c). Fig. 8 (d) shows the emission intensity as a function of annealing period, which can be fit by a Gaussian curve, suggesting that 5.4 h is the optimum annealing period for the ZZSS:0.025% Tb3þ system to obtain maximum Tb3þ emission. The result clearly indicate that when the annealing period is increased, the luminescence intensity increases and reach a maximum value for 5.4 h and when further increased the PL intensity decreases. Melato et al. [28] have observed similar behaviour in a MgAl2O4:0.3%In3þ system. Fig. 9 illustrates the proposed schematic band diagram for the observed PL emission mechanisms or channels for the (a) ZnAl2O4 (b) ZnO (c) SrAl2O4 (d) Sr3Al2O6 (e) Tb3þ and (f) Cr3þ [48] ions, respectively. 3.5. Lifetime measurements The PL lifetime decay curves of the 542 nm emission from Tb3þ (excited at 485 nm) are presented in Fig. 10. The results show that all the prepared nano-phosphor samples have the same phosphorescence mechanism irrespective of the annealing period. All the decay curves of intensity ðIÞ against time ðtÞ after a pulsed excitation were fitted using the second order decay shown in equation 4
Fig. 8. (a) Excitation and emission spectra of ZZSS:0.025%Tb3þ (b) zoomed image of (a). (c) excitation and emission spectra of ZZSS:0.025 Tb3þ samples for various annealing periods (d) maximum intensity as a function of annealing period.
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Fig. 9. The proposed pathway mechanism for the (a) ZnAl2O4 (b) ZnO (c) SrAl2O4 (d) Sr3Al2O6 (e) Tb3þ (f) Cr3þ.
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3.6. Chromaticity The chromaticity coordinates of the ZZSS:0.025% Tb3þ samples excited at 374 and 485 nm for various annealing periods were determined using the Commision Internationale de l’Eclairage (CIE) coordinate calculator software and are represented in Fig. 11 (a) and (b), respectively, as well as Table 2. As anticipated from the PL results in Fig. 7(b), it can be observed that the colour cannot be tuned when the samples are excited at 374 nm. The results also show that at 485 nm excitation the intensity of the greenish colour of our nanophosphor can be enhanced as expected from Fig. 8 (c).
4. Conclusion
Fig. 10. The decay curves of ZZSS:0.025%Tb3þ at various AP.
IðtÞ ¼ I0 þ A1 et=t1 þ A2 et=t2
(4)
where I0 is the background, A1 , A2 are constants and t1, t2 are the different times coming from Tb3þ incorporated in different phases, which are presented in Table 2.
ZZSS:0.025% Tb3þ nanophosphors were successfully prepared using sol-gel technique. The XRD patterns showed that the prepared material is composed of the mixed phases of ZnAl2O4, ZnO, SrAl2O4 and Sr3Al2O6. SEM images revealed that morphology of the nanophosphor changes as the annealing period was varied. Therefore doping and varying annealing period influenced the crystallite size and morphology of the material. HR-TEM confirmed that the prepared samples are on the nanoscale. The PL results illustrated that exciting the samples at 485 nm showed emissions at 545, 590 and 623 nm from Tb3þ. The decay curves showed that Tb3þ ions occupied at least two different sites because fitting required a double exponential function. CIE coordinates showed that the green emission can be enhanced by varying the annealing period.
Table 2 Summary of the sample identification, lifetime measurements and CIE coordinates. Annealing period (h)
t1 (ms)
t2 (ms)
CIE Coordinates (x; y) 374 nm 452 nm
1.5 2 3 4 5 6 7 8
1.518 ± 0.069 1.429 ± 0.059 1.409 ± 0.063 1.410 ± 0.062 1.458 ± 0.062 1.424 ± 0.069 1.519 ± 0.066 1.493 ± 0.065
3.906 ± 1.074 3.923 ± 1.125 3.718 ± 0.952 3.789 ± 0.820 3.918 ± 1.123 3.640 ± 0.923 3.961 ± 1.106 3.964 ± 1.108
(0.416; (0.409; (0.417; (0.415; (0.415; (0.416; (0.418; (0.420;
0.407) 0.410) 0.410) 0.409) 0.406) 0409) 0.411) 0.410)
Fig. 11. CIE diagram for the ZZSS:0.025% Tb3þ for various annealing periods when excited at (a) 374 nm (b) 485 nm.
(0.350; (0.330; (0.313; (0.330; (0.320; (0.332; (0.326; (0.289;
0.496) 0.545) 0.563) 0.563) 0.612) 0.580) 0.585) 0.588)
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Acknowledgements This work is supported by the South African National Research Foundation (NRF) Thuthuka programme (fund number: UID 99266 and 113947), Sefako Makgatho Health Science University (SMU) Research Development Grant (RDG). This work is based on the research supported in part by the National Research Foundation of South Africa (R.E. Kroon, Grant Number 93214). The author would like to thank Dr James Wesley-Smith and SMU Electron Microscope Unit for all SEM and TEM images. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.molstruc.2019.02.021. References [1] B.Y. Geng, J.Z. Ma, F.M. Zhan, J. Alloy. Comp. 473 (2009) 530e533. [2] N. Pathak, S.K. Gupta, K. Sanyal, M. Kumar, R.M. Kadam, V. Natarajan, Dalton Trans. 43 (2014) 9313e9323. [3] D. Xiulan, Y. Duorong, W. Xinqiang, X.U. Hongyan, J. Sol. Gel Sci. Technol. 35 (2005) 221. ~es, M.M. Bassaco, L.S.F. Pereira, E.M. de [4] E.L. Foletto, S. Battiston, J.M. Simo Moraes Flores, E.I. Müller, Microporous Mesoporous Mater. 163 (2012) 29e33. [5] S.F. Wang, G.Z. Sun, L.M. Fang, L. Lei, X. Xiang, X.T. Zu, Sci. Rep. 5 (2015) 12849. [6] L.F. Koao, F.B. Dejene, H.C. Swart, S.V. Motloung, T.E. Motaung, Opt. Mater. 60 (2016) 294e304. [7] R. Liu, A.A. Vertegel, E.W. Bohannan, T.A. Sorenson, J.A. Switzer 13 (2001) 508e512. [8] C.V. Jagtap, V.S. Kadam, T.T. Ghogare, Y.A. Inamdar, A.A. Shaikh, R.S. Mane, A.V. Shaikh, J. Mater. Sci. Mater. Electron. 27 (2016) 12335e12339. [9] C. Coskun, D.C. Look, G.C. Farlow, J.R. Sizelove, Semicond. Sci. Technol. 19 (2004) 752e754. [10] D.S. Kshatri, Ayush khare, piyush JHA, Chalcogenide Lett. 10 (3) (2013) 121e129. [11] S.K. Sharma, Strontium Aluminates-From Synthesis to Applications, Lampert academic publishing, 2014. [12] Y. Zhang, L.I. Lan, X. Zhang, D. Wang, S. Zhang, J. Rare Earths 26 (2008) 656. [13] B.C. Chakoumakos, G.A. Lager, J.A. Fernandez-Baca, Acta Crystallogr. Sect. C Cryst. Struct. Commun. 48 (3) (1993) 414e419. [14] M.T. Tsai, Y.X. Chen, P.J. Tsai, Y.K. Wang, Thin Solid Films 518 (2010) e9ee11. [15] L.F. Koao, B.F. Dejene, R.E. Kroon, H.C. Swart, J. Lumin. 147 (2014) 85e89. [16] V. Kumar, S. Som, V. Kumar, V. Kumar, O.M. Ntwaeaborwa, E. Coetsee, H.C. Swart, J. Chem. Eng. 255 (2014) 541e552. [17] L.F. Koao, F.B. Dejene, H.C. Swart, S.V. Motloung, T.E. Motaung, S.P. Hangothi,
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Adv. Mater. Lett. 7 (2016) 529e535. [18] I. Omkaram, G. Seeta Rama Raju, S. Buddhudu, J. Phys. Chem. Solid. 69 (8) (2008) 2066e2069. [19] P. Zhang, L. Lingxia, T. Yuming, Phys. B Condens. Matter 404 (21) (2009) 4286e4289. [20] P. Zhang, M. Xu, Z. Zheng, B. Sun, Y. Zhang, Trans of Non-ferrous Met Soc of China 16 (2006) 423e425. [21] P. Zhang, M. Xu, Z. Zheng, L. Liu, L. Li, J. Sol. Gel Sci. Technol. 43 (1) (2007) 59e64. [22] L. Chen, Z. Zhang, Y. Tian, M. Fei, L. He, P. Zhang, J. Rare Earths 35 (2) (2017) 127e134. [23] P. Huang, Q. Zhang, C. Cui, J. Li, Opt. Mater. 33 (8) (2017) 1252e1257. [24] S.V. Motloung, P. Kumari, L.F. Koao, T.E. Motaung, T.T. Hlatshwayo, M.J. Mochane, Materials Today Communications 14 (2018) 294e301. [25] L. Zhang, J. Yan, M. Zhou, Y. Yang, Y.N. Liu, Appl. Surf. Sci. 268 (2013) 237e245. [26] X. Zhao, L. Wang, X. Xu, X. Lei, S. Xu, F. Zhang, AIChE J. 58 (2011) 573e582. [27] X. Yuan, X. Cheng, Q. Jing, J. Niu, D. Peng, Z. Feng, X. Wu, Materials 11 (2018) 1624. [28] L.T. Melato, T.E. Motaung, O.M. Ntwaeaborwa, S.V. Motloung, Opt. Mater. (2017) 319e326. [29] V. Ghafouri, A. Ebrahimzad, M. Shariatti, Sci. Iran. 20 (3) (2013) 1039e1048. [30] V.P. Singh, S.B. Rai, H. Mishra, Chandana Rath, Dalton Trans. 43 (2014) 5309e5316. [31] G. Rani, Powder Technol. 312 (2017) 354e359. [32] G. Li, Y. Lai, T. Cui, H. Yu, D. Liu, S. Gan, Mater. Chem. Phys. 124 (2010) 1094e1099. [33] Q. Hou, F. Meng, J. Sun, Nano Res. Lett. 8 (2013) 144. [34] K.G. Tshabalala, S.H. Cho, J.K. Park, S. Shrreyas, I.M. Pilate, R.E. Kroon, H.C. Swart, O.M. Ntwaeaborwa, J. Alloy. Comp. 509 (2011), 10115e10120. [35] S. Iaiche, D. Djelloul, J. Spectrosc. 2015 (2015) 836859. [36] J.A. Alonso, I. Rasines, J.L. Soubeyroux, Inorg. Chem. 29 (1990) 4768e4771. [37] L.F. Koao, F.B. Dejene, M. Tsega, H.C. Swart, Physica B 480 (2016) 53e57. [38] B.D. Cullity, Elements of X-Ray Diffraction, second ed., Addison Wesley, 1978, p. 285, 1956. [39] L. Cornu, M. Gaudon, V. Jubera, J. Mat. Chem. C, Royal. Soc. Chem. 1 (34) (2013) 5419e5428. [40] R. Amiruddin, M.C. Santosh Kumar, J. Lumin. 155 (2014) 149e155. [41] H.A. Ahn, Y.Y. Kim, D.C. Kim, S.K. Mohanta, H.K. Cho, J. Appl. Phys. 105 (2009), 013502. [42] N.H. Alvi, K.U. Hasan, O. Nur, M. Willander, Nanoscale Res Lett 6 (1) (2011) 130. [43] W. Mikenda, A. Preisinger, J. Lumin. 26 (1981) 53e83. [44] D. Dutczak, T. Justel, C. Rondac, A. Meijerink, Phys. Chem. Chem. Phys. 17 (2015) 15236. [45] R.K. Tamrakar, K. Upadhyay, Res. J. Recent Sci. 6 (4) (2017) 28e31. [46] P.S. Xu, Y.M. Sun, C.S. Shi, F.Q. Xu, H.B. Pan, Nucl. Instrum. Methods Phys. Res. B 199 (2003) 286e290. [47] S.V. Motloung, K.G. Tshabalala, R.E. Kroon, T.T. Hlatshwayo, M. Mlambo, S. Mpelane, J. Mol. Struct. (2019) 241e252. [48] R. Ma, C. Ma, J. Zhang, J. Long, Z. Wen, X. Yuan, Y. Cao, Opt. Mater. Express 7 (2) (2017) 454.
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Effects of annealing period on the structure and photoluminescence of the mixed phases ZnAl2O4 /ZnO/SrAl2O4/Sr3Al2O6:0.025% Tb3þ nanophosphor synthesized by sol-gel technique M.R. Mhlongo a, *, L.F. Koao b, R.E. Kroon c, T.E. Motaung d, S.V. Motloung a, e, ** a
Department of Physics, Sefako Makgatho Health Sciences University, P.O. Box 94, Medunsa, 0204, South Africa Department of Physics, University of the Free State (Qwaqwa Campus), Private Bag X 13, Phuthaditjhaba, 9866, South Africa Department of Physics, University of the Free State, P. O. Box 339, Bloemfontein, 9300, South Africa d Department of Chemistry, University of Zululand, KwaDlangezwa, 3886, South Africa e Department of Physics, Nelson Mandela University, P. O. Box 77000, Port Elizabeth, 6031, South Africa b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 December 2018 Received in revised form 5 February 2019 Accepted 6 February 2019 Available online 8 February 2019
Nanophosphor powders of the mixed phases of ZnAl2O4/ZnO/SrAl2O4/Sr3Al2O6 (ZZSS) doped with Tb3þ (ZZSS:0.025%Tb3þ) were successfully prepared by sol-gel technique. The effect of the annealing period at a fixed annealing temperature (1000 C) and dopant concentration (0.025% Tb3þ) on the structure and photoluminescence properties was investigated. X-ray diffraction results revealed that the crystallite size was influenced by the annealing period. Scanning electron microscopy showed that varying the annealing period influenced the particle morphology of the prepared nanophosphor material. High resolution transmission electron microscopy confirmed that the prepared material is on the nanoscale. The photoluminescence results showed that the ZZSS emits at 585 nm when excited at 374 nm, which is attributed to the defects centres within the ZnO phase. The Tb3þ doped samples showed emissions peaks at 545, 590 and 623 nm which were attributed to the 4f transitions of Tb3þ, specifically 5D4 / 7FJ (J ¼ 5, 4, 3). Increasing the annealing period up to 5.4 h led to luminescence enhancements, while a further increase led to quenching. The Commision Internationale de l’Eclairage coordinates showed that the greenish emission colour could be tuned by varying the annealing period. © 2019 Elsevier B.V. All rights reserved.
Keywords: Mixed phases Annealing period Tb3þ Sol-gel Luminescence
1. Introduction In recent years, interests have been focused on the development of new luminescent materials (nanophosphors). In particular, oxide matrices are attractive host materials for the development of advanced phosphors due to their ease of synthesis and chemical stability [1]. As one of the most promising phosphor materials, zinc aluminate (ZnAl2O4) has been extensively studied for application in thin film electroluminescence displays, aerospace, paint and stress imaging devices [2]. In addition, ZnAl2O4 is widely used in many catalytic reactions, cracking, dehydration, hydrogenation and dehydrogenation, ceramic and electro-conductive materials
* Corresponding author. ** Corresponding author. Department of Physics, Sefako Makgatho Health Sciences University, P.O. Box 94, Medunsa, 0204, South Africa. E-mail addresses: [email protected] (M.R. Mhlongo), [email protected] (S.V. Motloung). https://doi.org/10.1016/j.molstruc.2019.02.021 0022-2860/© 2019 Elsevier B.V. All rights reserved.
because of its high thermal stability, high mechanical resistance, low surface acidity and excellent optical properties [3]. ZnAl2O4 is one of the well-known wide-bandgap (~3.8 eV) semiconductors with a spinel structure [4,5] belonging to the Fd3m space group. On the other hand, zinc oxide (ZnO), an n-type semiconductor, has drawn tremendous attention from researchers around the globe. It displays a hexagonal crystalline wurtzite-type structure [6] with space group P6mc and lattice parameters of a ¼ b ¼ 0.3250 nm and c ¼ 0.5207 nm [7]. The importance of ZnO is due to its unusual physical properties such as high conductance, chemical stability, harmless to the environment and inexpensive [8]. Moreover, it has good radiation resistance [9], a wide and direct band gap (~3.37 eV) and a high exciton binding energy of 60 meV [6]. Another oxide material being investigated is strontium aluminate (SrAl2O4), which offers excellent properties such as high quantum efficiency, long persistence of phosphorescence and good stability [10]. SrAl2O4 has a wide band gap (~6.5 eV) which offers the possibility of generating broad band emission [11] and much attention has been
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paid to it owing to its higher radiation-resistance [12]. The synthesis and optical properties of SrAl2O4 in a bulk form or as films or nanoparticles have been extensively explored during the past decades [1]. On the other hand, the oxygen deficient structure of strontium aluminate with Sr3Al2O6 formula is less reported as an individual group of oxide materials. Chakoumakos et al. [13] reported that Sr3Al2O6 has a cubic crystal unit containing puckered six-membered AlO4 tetrahedral rings. The luminescent properties of phosphor materials have been reported to depend strongly on the particle size, crystal structure, distribution of activators in the host lattice, morphologies and preparation methods [5,14]. Doping with rare earth ions (RE3þ) influence the morphology, the particle size and the band structure of the nanocrystals [15,16]. Doping also plays key roles in luminescence efficiency and the position of emission bands, thus influencing their practical applications. Among the RE3þ ions, Tb3þ is a popular efficient luminescent dopant which is being considered in many studies [16]. For an example, Koao et al. [17] prepared ZnO nanoparticles doped with Tb3þ using the chemical bath deposition method. The doped samples showed emission peaks centred at 431, 489, 545, 585 and 621 nm for all Tb3þ doping concentrations. These emission peaks were attributed to the intra-4f transitions of Tb3þ in particular, 5 D3/7F4, 5D4/7F6, 5D4/7F5, 5D4/7F4 and 5D4/ 7F3 transitions, respectively. Omkaram et al. [18] have shown that Tb3þ can be used to form a green emitting phosphor due to 5D4/7F5 transitions in the Tb3þ ions. Rare earth doped Sr3Al2O6 prepared via microwave or sol-gel synthesis methods have been reported [19e23]. Different studies have been attempted on the mixed phases of ZnAl2O4/ZnO [24e27]. Motloung et al. [24] reported the effect of annealing period on the structure and optical properties of the mixed phases ZnAl2O4/ZnO and found that the optimum luminescence is obtained for an annealing period of 2 h. Yuan et al. [27] showed that the mixed oxide ZnO/ZnAl2O4 has excellent stability and much higher photocatalytic activity than their bulk oxide counterparts. With this in mind, and to the best of our knowledge, the effect of annealing period on the mixed phases ZnAl2O4/ZnO/SrAl2O4/ Sr3Al2O6:0.025% Tb3þ (ZZSS:0.025% Tb3þ) has not been investigated. The primary aim of this study was to synthesize a phosphor material based on mixed oxides for practical applications such as in light emitting diodes. This study investigates the effect of annealing period on the structure and photoluminescence (PL) of the mixed phase ZZSS:0.025% Tb3þ nanophosphor. The emission channels associated with the observed PL emissions are also proposed.
93
were investigated using a Zeiss Supra 55 scanning electron microscope (SEM) with an energy dispersive X-ray spectroscopy (EDS). Information on the crystallite sizes were obtained using a JEOL 1010 high resolution transmission electron microscope (HRTEM). The emission and excitation spectra measurements were made with an Edinburgh Instruments FLS980 fluorescence spectrometer having double monochromators, using a steady state xenon lamp as excitation source and R928P photomultiplier tube detector. Lifetime measurements were acquired using a xenon flashlamp pulsing at 100 Hz. All characterizations were carried out at room temperature. 3. Results and discussion 3.1. XRD Fig. 1 shows the XRD patterns of the prepared powder samples. Fig. 1 (a) shows the ZZSS pattern which could be indexed to the mixed phases of the cubic ZnAl2O4 (ICSD:4160), Sr3Al2O6 (ICSD:1860), hexagonal ZnO (ICSD:5119) and Sr2Al2O4 (ICSD:31e1336) structures. Fig. 1 (b) presents the ZZSS:0.025%Tb3þ for various annealing periods, which showed similar diffraction patterns to that of the un-doped ZZSS sample. In addition, the results suggest that doping does not change the crystal structure of the prepared samples, especially when comparing the un-doped and doped samples for annealing period of 2 h. These results are consistent with the results obtained by Melato et al. [28] where both In3þ doping and annealing period did not alter the crystal
2. Experimental ZnAl2O4/ZnO (ZZ), SrAl2O4/Sr3Al2O6 (SS) and ZnAl2O4/ZnO/ SrAl2O4/Sr3Al2O6 (ZZSS) nanophosphors were successfully prepared using the citrate sol-gel method. Zn(NO3)2$6H2O (98%), Sr(NO3)2 (98%), Al(NO3)3$9H2O (98%) and citric acid C8H8O7$H2O (99%) precursors purchased from Sigma Aldrich were used as received without further purification. For ZZSS the starting amounts of these precursors were 4.373, 3.111, 10.808 and 2.333 g, respectively. They were dissolved in 60 ml of deionized water. The doped sample (ZZSS:0.025%Tb3þ) was prepared by adding 0.002 g of Tb(NO3)3$5H2O (98%) to the solution. The solution was stirred with a magnetic stirrer at a temperature of ~80 C until a gel was formed, which was left to dry for 2 h at room temperature and subsequently annealed in a furnace at 1000 C for different annealing periods varying from 1.5 to 8 h. The products from the furnace were ground into ultrafine powders using a mortar and pestle and analysed using different techniques. The crystal structures of the samples were characterized by powder X-ray diffraction (XRD) (Bruker AXS Discover diffractometer) with Cu Ka (1.5418 Å) radiation. The morphologies of the prepared phosphors
Fig. 1. The XRD patterns for the (a) ZZSS sample and JCDPS standards sample (b) doped samples for various annealing periods.
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structure of the MgAl2O4. Thus, the results suggest that Tb3þ is incorporated successfully into the crystal structure of each phase of the ZZSS. To further motivate this, consider the analysis of the most intense diffraction peaks for the un-doped and doped samples at various annealing periods shown in Fig. 2 (a) e (c). The most intense diffraction peaks are 131, 101, 102 and 044 corresponding to the ZnAl2O4, ZnO, SrAl2O4 and Sr3Al2O6 phases, respectively. The peak intensities change as the annealing period increases, which suggest that the annealing period influenced the crystallinity of the prepared nanophosphors [29]. In all ZZSS phases, the results clearly show that there is a diffraction peak shift to the lower angle when doping with Tb3þ, which could be attributed to the increase of the lattice parameters [30]. The increase in lattice parameters is attributed to the replacement of the smaller atoms with the bigger atom in the crystal lattices of the individual phases. The ionic radii of Zn2þ, Sr2þ and Al3þ are 0.74, 1.13 and 0.50 Å, respectively [31e33]. When the smaller ions (Zn2þ or Al3þ) are replaced by the larger Tb3þ ion (1.00 Å) [34], this will result in an increase in lattice parameter [15]. Thus, we propose that the Tb3þ ions substitute the Zn2þ or Al3þ ions in the crystal lattice of the ZZSS phases. The lattice parameters for the cubic (ZnAl2O4, Sr3Al2O6) and hexagonal (ZnO, SrAl2O4) phases were calculated from equations (1) and (2), respectively.
a dhkl ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi h2 þ k2 þ l2
(1)
1 dhkl ¼ rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi 2 4 h2 þ k2 þ hk 3a2 cl 2
(2)
where a and c are the lattice constants, dhkl is the interplanar distance and hkl are the Miller indices [35]. The average lattice constant for the cubic ZnAl2O4 and Sr3Al2O6 were calculated to be a ¼ b ¼ c ¼ 8.028 and 15.737 Å respectively, which are similar to reported values [35,36]. The lattice constants of the hexagonal ZnO were calculated to be a ¼ b ¼ 3.240 Å and c ¼ 5.174 Å, which are close to the values reported in the literature [24,37], while those of the hexagonal SrAl2O4 were calculated to be a ¼ b ¼ 4.911 Å and c ¼ 8.462 Å, which is comparable to the previous values reported [30,33]. The crystallite size (D) for each of the phases in the ZZSS nanophosphor was estimated from the full width at half maximum (FWHM) of the most intense diffraction peaks of each phases using Scherrer's equation [38].
D¼
0:9l bSinq
(3)
l stands for the X-ray wavelength, b is the full width at half maximum (radians) and q is the angle of diffraction (radians). The calculated crystallite sizes for various annealing periods are presented in Table 1 and show that annealing period influences the crystallite size of the prepared nanophosphors.
Fig. 2. The most intense diffraction peaks of the different phases (a) 131 of ZnAl2O4 and 101 of ZnO (b) 044 of Sr3Al2O6 (c) 102 of SrAl2O4.
M.R. Mhlongo et al. / Journal of Molecular Structure 1184 (2019) 92e101 Table 1 Crystallite sizes (nm) of the different phases. All samples were doped except as indicated. Annealing period (h)
131 of ZnAl2O4
101 of ZnO
102 of SrAl2O4
044 of Sr3Al2O6
1.5 2 (undoped) 2 3 4 5 6 7 8
29 28 29 27 31 29 23 26 20
35 35 33 30 37 35 26 28 23
30 31 31 28 35 32 27 24 22
31 35 32 32 34 35 28 31 34
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3.2. SEM and EDS Fig. 3 shows the EDS spectrum of the un-doped sample annealed for 2 h. It shows that the sample is composed of Zn, Sr, Al and O. The carbon peak is due to the conductive carbon films coated on the sample holders during the course of EDS measurement. Apart from the anticipated elements, there were no other peaks detected. The morphological aspect of the selected ZZSS nanophosphor samples were analysed using SEM, as shown in Fig. 4. The micrograph in Fig. 4 (a) reveals that the morphology for the un-doped ZZSS consisted of the agglomeration of irregular particles with grain boundaries and voids. Rod-like structures were also observed on the surface. The micrograph of the doped ZZSS sample annealed for 2 h is displayed in Fig. 4 (b) and the morphology is similar to that of the un-doped ZZSS. The agglomeration of the rod-like structures appears less. The sample annealed for 5 h is shown in Fig. 4 (c) and the micrograph reveals that the sizes of the particles and rod-like structures had increased. Fig. 4 (d) shows the after 7 h of annealing the rod-like structures have been destroyed. Based on the XRD results, the observed hexagonal shape particle circled in red is either ZnO or SrAl2O4 rather than the cubic ZnAl2O4 or Sr3Al2O6. The results show clearly that while doping had little influence on the morphology, the annealing period has a strong influence on the morphology of the prepared phosphors. 3.3. HR-TEM
Fig. 3. EDS spectrum of the ZZSS nanophosphor annealed for 2 h.
Further analysis was done by HR-TEM to confirm the crystallite size of the prepared nanophosphor. Fig. 5 (a) and (b) present the images of the un-doped and magnified ZZSS sample, respectively. The ZZSS image confirms the agglomeration of the particles as
Fig. 4. SEM micrographs of samples having annealing periods of (a) 2 h (un-doped ZZSS) (b) 2 h (c) 5 h and (d) 7 h.
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Fig. 5. HR-TEM images for (a) un-doped ZZSS (b) magnified un-doped ZZSS and (c) doped sample annealed for 5 h.
observed in SEM results. Fig. 5 (b) indicates the existence of hexagonal-like structures, which is agreeing very well with the XRD and SEM results. The results also shows the presence of lattice fringes which confirms that our nanophosphor has a crystallite structure. Fig. 5 (c) shows the doped sample annealed for 5 h. The results shows the lattice fringes, it can also be observed that the sample have different planes which can be confirmed by different miller indices from our XRD results. 3.4. Photoluminescence PL excitation and emission spectra of the ZZ and SS nanopowders annealed for 2 h are shown in Fig. 6 (a). In both cases the excitation spectra were recorded when monitoring the emission at 452 nm and there is an excitation peak located at 374 nm. Considering the band gaps of ZnAl2O4(~3.8 eV) [4] and ZnO (~3.37 eV) [6], it is clear that for the ZZ material the 374 nm (~3.32 eV) excitation energy is slightly lower than the band gap energy of both phases and this will then suggest the existence of the intermediary energy level for both phases just below their conduction bands [39]. For the ZZ phases, the emission spectra excited at 374 nm showed
emission peaks located at 430, 452 and 585, 677e730 nm. Previously, similar emissions from both ZnAl2O4 and ZnO were observed [39e42]. ZnO is composed of extrinsic and intrinsic deep level defects that emit different colours in the visible region [40]. The various defects such as oxygen vacancies (Vo), zinc vacancies (Vzn), oxygen interstitial (Oi), zinc interstitial (Zni) and anitisite oxygen (Ozn) are the reason behind the deep level emission at visible range. In this study the 585 nm emission is from ZnO and is attributed to the Oi and believed to be due to band transition from Zni to Oi defect levels in ZnO [42], whereas 430 and 452 nm can be assumed to be from the aluminate site such as ZnAl2O4. Mikenda et al. [43] reported the red luminescence (around 700 nm, here at 677, 690, 700, 713, 725 and 730 nm) to be due to the spin forbidden 2Eg - 4A2g, N-zero phonon lines of Cr3þ at the octahedral site of Al3þ ions. The presence of Cr3þ in aluminium compounds is observed due to trace impurities in the precursor materials, of which it can be a possibility in our case. In the SS phase the 374 nm excitation is also lower than the band gap energies of SrAl2O4 (~6.5 eV) [11] and Sr3Al2O6 (~6.2 eV) [44] which suggests that there are defect levels below the conduction band. This excitation produced emissions at 430, 452 and 490 nm.
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Fig. 6. The excitation and emission spectra of the (a) ZZ and SS samples (b) ZZSS samples.
These emissions can be associated with defects within the aluminate in both SrAl2O4 and Sr3Al2O6, as they are also observed in ZnAl2O4. Similar emissions were observed by Tamrakar et al. [45] in SrAl2O4 system. Fig. 6 (b) shows the ZZSS mixed phase excitation and emission spectra. It can be seen that there is one emission peak located at 585 nm with some shoulders at 412 and 432 nm when the sample was excited at 374 nm. The emission peak at 585 nm is therefore considered to originate from the ZZ phases (in particular
from ZnO as discussed for Fig. 6 (a)). Generally, the PL results confirmed the presence of the mixed ZZSS phases predicted by the XRD results. Fig. 7 (a) shows a comparison between the ZZSS (un-doped) and ZZSS:0.025 Tb3þ samples annealed for 2 h. An excitation wavelength of 374 nm only produced the emission at 585 nm (with small shoulders at 412 and 432 nm) which is from the ZZ material. Since the emissions peaks for both samples are located at the same
Fig. 7. The excitation and emission spectra of (a) ZZSS and ZZSS:0.025%Tb3þ annealed for 2 h (b) ZZSS:0.025%Tb3þ annealed for different periods and (c) intensity as a function of annealing period.
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positions, the results suggest that there is no emission from the Tb3þ ions for this excitation wavelength but all of the emissions should be attributed to the ZZSS phases (as discussed for Fig. 6). Fig. 7 (b) shows the ZZSS:0.025 Tb3þ samples for various annealing periods when the samples were excited at 374 nm. The results show the change in emission intensities as the annealing period changes, as presented in Fig. 7 (c). The sample annealed for 6 h had the highest emission intensity, even though there are fluctuations. We propose that the fluctuations on the emission intensity might possibly be due to the existence of various phases in ZZSS. It is noted from Fig. 7 (a) and (b) that the 374 nm wavelength only excited the ZZSS phases and not the Tb3þ ions. Tb3þ ions can generally be effectively excited at very short wavelengths corresponding to their strong f-d transition and the emission curve of the ZZSS:0.025%Tb3þ sample annealed for 8 h, when excited at 230 nm, showed a clear Tb3þ emission line at 542 nm superimposed on the broad emission band. To minimize the broad emission and focus on the Tb3þ emission, the sample was further excited with a wavelength of 485 nm which is longer than the excitation range of the broad emission and corresponds to an f-f excitation line of Tb3þ. The weak data from Fig. 8 (a) is zoomed in Fig. 8 (b) and shows the characteristic emissions of Tb3þ at 545, 590 and 623 nm. Fig. 8 (c) shows all the doped samples for various annealing periods when excited at 485 nm. The broad band centred at 585 nm is emission from ZnO and can be ascribed to the presence of oxygen interstitials (Oi) [46]. The relatively sharp peak near
590 nm (with Stark split sublevels at 585 and 594 nm) is due to the 5 D4/7F4 transition of Tb3þ [47]. The 545 nm (with 542 and 548 nm Stark components) and 623 nm emissions are attributed to the intra-4f transitions of Tb3þ specifically, 5D4/7F5 and 5D4/ 7F3 transitions [47]. To examine the effect of the annealing period, all the doped samples were excited with 485 nm as shown in Fig. 8 (c). Fig. 8 (d) shows the emission intensity as a function of annealing period, which can be fit by a Gaussian curve, suggesting that 5.4 h is the optimum annealing period for the ZZSS:0.025% Tb3þ system to obtain maximum Tb3þ emission. The result clearly indicate that when the annealing period is increased, the luminescence intensity increases and reach a maximum value for 5.4 h and when further increased the PL intensity decreases. Melato et al. [28] have observed similar behaviour in a MgAl2O4:0.3%In3þ system. Fig. 9 illustrates the proposed schematic band diagram for the observed PL emission mechanisms or channels for the (a) ZnAl2O4 (b) ZnO (c) SrAl2O4 (d) Sr3Al2O6 (e) Tb3þ and (f) Cr3þ [48] ions, respectively. 3.5. Lifetime measurements The PL lifetime decay curves of the 542 nm emission from Tb3þ (excited at 485 nm) are presented in Fig. 10. The results show that all the prepared nano-phosphor samples have the same phosphorescence mechanism irrespective of the annealing period. All the decay curves of intensity ðIÞ against time ðtÞ after a pulsed excitation were fitted using the second order decay shown in equation 4
Fig. 8. (a) Excitation and emission spectra of ZZSS:0.025%Tb3þ (b) zoomed image of (a). (c) excitation and emission spectra of ZZSS:0.025 Tb3þ samples for various annealing periods (d) maximum intensity as a function of annealing period.
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Fig. 9. The proposed pathway mechanism for the (a) ZnAl2O4 (b) ZnO (c) SrAl2O4 (d) Sr3Al2O6 (e) Tb3þ (f) Cr3þ.
99
100
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3.6. Chromaticity The chromaticity coordinates of the ZZSS:0.025% Tb3þ samples excited at 374 and 485 nm for various annealing periods were determined using the Commision Internationale de l’Eclairage (CIE) coordinate calculator software and are represented in Fig. 11 (a) and (b), respectively, as well as Table 2. As anticipated from the PL results in Fig. 7(b), it can be observed that the colour cannot be tuned when the samples are excited at 374 nm. The results also show that at 485 nm excitation the intensity of the greenish colour of our nanophosphor can be enhanced as expected from Fig. 8 (c).
4. Conclusion
Fig. 10. The decay curves of ZZSS:0.025%Tb3þ at various AP.
IðtÞ ¼ I0 þ A1 et=t1 þ A2 et=t2
(4)
where I0 is the background, A1 , A2 are constants and t1, t2 are the different times coming from Tb3þ incorporated in different phases, which are presented in Table 2.
ZZSS:0.025% Tb3þ nanophosphors were successfully prepared using sol-gel technique. The XRD patterns showed that the prepared material is composed of the mixed phases of ZnAl2O4, ZnO, SrAl2O4 and Sr3Al2O6. SEM images revealed that morphology of the nanophosphor changes as the annealing period was varied. Therefore doping and varying annealing period influenced the crystallite size and morphology of the material. HR-TEM confirmed that the prepared samples are on the nanoscale. The PL results illustrated that exciting the samples at 485 nm showed emissions at 545, 590 and 623 nm from Tb3þ. The decay curves showed that Tb3þ ions occupied at least two different sites because fitting required a double exponential function. CIE coordinates showed that the green emission can be enhanced by varying the annealing period.
Table 2 Summary of the sample identification, lifetime measurements and CIE coordinates. Annealing period (h)
t1 (ms)
t2 (ms)
CIE Coordinates (x; y) 374 nm 452 nm
1.5 2 3 4 5 6 7 8
1.518 ± 0.069 1.429 ± 0.059 1.409 ± 0.063 1.410 ± 0.062 1.458 ± 0.062 1.424 ± 0.069 1.519 ± 0.066 1.493 ± 0.065
3.906 ± 1.074 3.923 ± 1.125 3.718 ± 0.952 3.789 ± 0.820 3.918 ± 1.123 3.640 ± 0.923 3.961 ± 1.106 3.964 ± 1.108
(0.416; (0.409; (0.417; (0.415; (0.415; (0.416; (0.418; (0.420;
0.407) 0.410) 0.410) 0.409) 0.406) 0409) 0.411) 0.410)
Fig. 11. CIE diagram for the ZZSS:0.025% Tb3þ for various annealing periods when excited at (a) 374 nm (b) 485 nm.
(0.350; (0.330; (0.313; (0.330; (0.320; (0.332; (0.326; (0.289;
0.496) 0.545) 0.563) 0.563) 0.612) 0.580) 0.585) 0.588)
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Acknowledgements This work is supported by the South African National Research Foundation (NRF) Thuthuka programme (fund number: UID 99266 and 113947), Sefako Makgatho Health Science University (SMU) Research Development Grant (RDG). This work is based on the research supported in part by the National Research Foundation of South Africa (R.E. Kroon, Grant Number 93214). The author would like to thank Dr James Wesley-Smith and SMU Electron Microscope Unit for all SEM and TEM images. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.molstruc.2019.02.021. References [1] B.Y. Geng, J.Z. Ma, F.M. Zhan, J. Alloy. Comp. 473 (2009) 530e533. [2] N. Pathak, S.K. Gupta, K. Sanyal, M. Kumar, R.M. Kadam, V. Natarajan, Dalton Trans. 43 (2014) 9313e9323. [3] D. Xiulan, Y. Duorong, W. Xinqiang, X.U. Hongyan, J. Sol. Gel Sci. Technol. 35 (2005) 221. ~es, M.M. Bassaco, L.S.F. Pereira, E.M. de [4] E.L. Foletto, S. Battiston, J.M. Simo Moraes Flores, E.I. Müller, Microporous Mesoporous Mater. 163 (2012) 29e33. [5] S.F. Wang, G.Z. Sun, L.M. Fang, L. Lei, X. Xiang, X.T. Zu, Sci. Rep. 5 (2015) 12849. [6] L.F. Koao, F.B. Dejene, H.C. Swart, S.V. Motloung, T.E. Motaung, Opt. Mater. 60 (2016) 294e304. [7] R. Liu, A.A. Vertegel, E.W. Bohannan, T.A. Sorenson, J.A. Switzer 13 (2001) 508e512. [8] C.V. Jagtap, V.S. Kadam, T.T. Ghogare, Y.A. Inamdar, A.A. Shaikh, R.S. Mane, A.V. Shaikh, J. Mater. Sci. Mater. Electron. 27 (2016) 12335e12339. [9] C. Coskun, D.C. Look, G.C. Farlow, J.R. Sizelove, Semicond. Sci. Technol. 19 (2004) 752e754. [10] D.S. Kshatri, Ayush khare, piyush JHA, Chalcogenide Lett. 10 (3) (2013) 121e129. [11] S.K. Sharma, Strontium Aluminates-From Synthesis to Applications, Lampert academic publishing, 2014. [12] Y. Zhang, L.I. Lan, X. Zhang, D. Wang, S. Zhang, J. Rare Earths 26 (2008) 656. [13] B.C. Chakoumakos, G.A. Lager, J.A. Fernandez-Baca, Acta Crystallogr. Sect. C Cryst. Struct. Commun. 48 (3) (1993) 414e419. [14] M.T. Tsai, Y.X. Chen, P.J. Tsai, Y.K. Wang, Thin Solid Films 518 (2010) e9ee11. [15] L.F. Koao, B.F. Dejene, R.E. Kroon, H.C. Swart, J. Lumin. 147 (2014) 85e89. [16] V. Kumar, S. Som, V. Kumar, V. Kumar, O.M. Ntwaeaborwa, E. Coetsee, H.C. Swart, J. Chem. Eng. 255 (2014) 541e552. [17] L.F. Koao, F.B. Dejene, H.C. Swart, S.V. Motloung, T.E. Motaung, S.P. Hangothi,
101
Adv. Mater. Lett. 7 (2016) 529e535. [18] I. Omkaram, G. Seeta Rama Raju, S. Buddhudu, J. Phys. Chem. Solid. 69 (8) (2008) 2066e2069. [19] P. Zhang, L. Lingxia, T. Yuming, Phys. B Condens. Matter 404 (21) (2009) 4286e4289. [20] P. Zhang, M. Xu, Z. Zheng, B. Sun, Y. Zhang, Trans of Non-ferrous Met Soc of China 16 (2006) 423e425. [21] P. Zhang, M. Xu, Z. Zheng, L. Liu, L. Li, J. Sol. Gel Sci. Technol. 43 (1) (2007) 59e64. [22] L. Chen, Z. Zhang, Y. Tian, M. Fei, L. He, P. Zhang, J. Rare Earths 35 (2) (2017) 127e134. [23] P. Huang, Q. Zhang, C. Cui, J. Li, Opt. Mater. 33 (8) (2017) 1252e1257. [24] S.V. Motloung, P. Kumari, L.F. Koao, T.E. Motaung, T.T. Hlatshwayo, M.J. Mochane, Materials Today Communications 14 (2018) 294e301. [25] L. Zhang, J. Yan, M. Zhou, Y. Yang, Y.N. Liu, Appl. Surf. Sci. 268 (2013) 237e245. [26] X. Zhao, L. Wang, X. Xu, X. Lei, S. Xu, F. Zhang, AIChE J. 58 (2011) 573e582. [27] X. Yuan, X. Cheng, Q. Jing, J. Niu, D. Peng, Z. Feng, X. Wu, Materials 11 (2018) 1624. [28] L.T. Melato, T.E. Motaung, O.M. Ntwaeaborwa, S.V. Motloung, Opt. Mater. (2017) 319e326. [29] V. Ghafouri, A. Ebrahimzad, M. Shariatti, Sci. Iran. 20 (3) (2013) 1039e1048. [30] V.P. Singh, S.B. Rai, H. Mishra, Chandana Rath, Dalton Trans. 43 (2014) 5309e5316. [31] G. Rani, Powder Technol. 312 (2017) 354e359. [32] G. Li, Y. Lai, T. Cui, H. Yu, D. Liu, S. Gan, Mater. Chem. Phys. 124 (2010) 1094e1099. [33] Q. Hou, F. Meng, J. Sun, Nano Res. Lett. 8 (2013) 144. [34] K.G. Tshabalala, S.H. Cho, J.K. Park, S. Shrreyas, I.M. Pilate, R.E. Kroon, H.C. Swart, O.M. Ntwaeaborwa, J. Alloy. Comp. 509 (2011), 10115e10120. [35] S. Iaiche, D. Djelloul, J. Spectrosc. 2015 (2015) 836859. [36] J.A. Alonso, I. Rasines, J.L. Soubeyroux, Inorg. Chem. 29 (1990) 4768e4771. [37] L.F. Koao, F.B. Dejene, M. Tsega, H.C. Swart, Physica B 480 (2016) 53e57. [38] B.D. Cullity, Elements of X-Ray Diffraction, second ed., Addison Wesley, 1978, p. 285, 1956. [39] L. Cornu, M. Gaudon, V. Jubera, J. Mat. Chem. C, Royal. Soc. Chem. 1 (34) (2013) 5419e5428. [40] R. Amiruddin, M.C. Santosh Kumar, J. Lumin. 155 (2014) 149e155. [41] H.A. Ahn, Y.Y. Kim, D.C. Kim, S.K. Mohanta, H.K. Cho, J. Appl. Phys. 105 (2009), 013502. [42] N.H. Alvi, K.U. Hasan, O. Nur, M. Willander, Nanoscale Res Lett 6 (1) (2011) 130. [43] W. Mikenda, A. Preisinger, J. Lumin. 26 (1981) 53e83. [44] D. Dutczak, T. Justel, C. Rondac, A. Meijerink, Phys. Chem. Chem. Phys. 17 (2015) 15236. [45] R.K. Tamrakar, K. Upadhyay, Res. J. Recent Sci. 6 (4) (2017) 28e31. [46] P.S. Xu, Y.M. Sun, C.S. Shi, F.Q. Xu, H.B. Pan, Nucl. Instrum. Methods Phys. Res. B 199 (2003) 286e290. [47] S.V. Motloung, K.G. Tshabalala, R.E. Kroon, T.T. Hlatshwayo, M. Mlambo, S. Mpelane, J. Mol. Struct. (2019) 241e252. [48] R. Ma, C. Ma, J. Zhang, J. Long, Z. Wen, X. Yuan, Y. Cao, Opt. Mater. Express 7 (2) (2017) 454.