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Sr3Al2O6 mixed phase nanophosphor
Journal Pre-proof Investigative study of Mn2+ concentration on the structure, morphology and photoluminescence of sol-gel ZnAl2O4/ZnO/ SrAl2O4/Sr3Al2O6 mixed phase nanophosphor
M.R. Mhlongo, L.F. Koao, R.E. Kroon, S.V. Motloung PII:
S0921-4526(19)30648-9
DOI:
https://doi.org/10.1016/j.physb.2019.411746
Reference:
PHYSB 411746
To appear in:
Physica B: Physics of Condensed Matter
Received Date:
31 May 2019
Accepted Date:
03 October 2019
Please cite this article as: M.R. Mhlongo, L.F. Koao, R.E. Kroon, S.V. Motloung, Investigative study of Mn2+ concentration on the structure, morphology and photoluminescence of sol-gel ZnAl2O4 /ZnO/ SrAl2O4/Sr3Al2O6 mixed phase nanophosphor, Physica B: Physics of Condensed Matter (2019), https://doi.org/10.1016/j.physb.2019.411746
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier.
Journal Pre-proof Investigative study of Mn2+ concentration on the structure, morphology and photoluminescence of sol-gel ZnAl2O4/ZnO/ SrAl2O4/Sr3Al2O6 mixed phase nanophosphor
M.R. Mhlongoa,*, L.F. Koaob, R.E. Kroonc, S.V. Motlounga,d*
aDepartment
of Physics, Sefako Makgatho Health Sciences University, P.O. Box 94, Medunsa, 0204, South Africa
bDepartment
of Physics, University of the Free State (Qwaqwa Campus), Private Bag X 13, Phuthaditjhaba, 9866, South Africa
cDepartment
of Physics, University of the Free State, P. O. Box 339, Bloemfontein, 9300, South Africa
dDepartment
of Physics, Nelson Mandela University, P. O. Box 77000, Port Elizabeth 6031, South Africa
*Corresponding authors: [email protected] and [email protected] (Setumo. V. Motloung) Telephone numbers: +2712 521 3966 and +2741 504 4540. ABSTRACT The sol-gel technique was used to synthesize mixed phases of ZnAl2O4/ZnO/SrAl2O4/Sr3Al2O6 (ZZSS) nanophosphors. The study investigated the effect of varying the Mn2+ doping concentration on the structural, morphological and optical properties of the prepared phosphor materials. X-ray diffraction (XRD) patterns revealed that the structure of the un-doped ZZSS powder was not influenced by varying the Mn2+ concentration. Scanning electron microscopy (SEM) images showed that the morphological features of the prepared nanophosphors were influenced by the Mn2+ concentration. Transmission electron microscopy (TEM) confirmed that the prepared materials are on the nanoscale. Ultraviolet-visible (UV-vis) diffuse reflection spectroscopy showed that the band gap energy can be tuned in the range 4.78 - 4.92 eV. The photoluminescence (PL) results showed that the un-doped ZZSS exhibited an emission peak at 580 nm when excited at 374 nm, which is attributed to the defect centres within the ZnAl2O4 and ZnO. The doped samples showed broad emission bands at 600 nm, which were attributed to the 4T1g(G) → 6A1g(S) transitions of Mn2+ ions. The Commission
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Journal Pre-proof International de I’Eclairage (CIE) chromaticity colour coordinates showed that the emission colour of the prepared phosphors can be tuned from yellow to orange by varying the Mn2+ concentration.
Keywords: Mixed phases, Nanophosphor, Mn2+, sol-gel, Luminescence, CIE
1. Introduction Recently, research and development has mainly been concentrated on new types of materials, especially nanomaterials, because of their numerous potential applications. Nanomaterials may be attractive because of improved physical and chemical properties [1]. Zinc aluminate (ZnAl2O4), zinc oxide (ZnO) and strontium aluminates (SrAl2O4, Sr3Al2O6) nanomaterial systems have recently been investigated by many researchers around the globe. ZnAl2O4 has drawn a lot of attention because of its high mechanical strength, high thermal and chemical stability, low sintering temperature, low surface acidity, wide band gap (Eg) ⁓3.8 eV and excellent optical properties [2-4]. ZnAl2O4 is suitable for thermal control coatings for spacecraft and also for ultraviolet optoelectronic device applications [5,6]. ZnO nanomaterials have attracted tremendous interest for their advantages such as high quantum efficiency, non-toxicity and low cost [7]. They are regarded as one of the very significant and adaptable semiconductors, with a direct band gap of ⁓3.37 eV and a large exciton binding energy of 60 meV at room temperature. ZnO is also a promising candidate for functional components of devices and materials in photonic crystals, gas sensors, light-emitting diodes, solar cells and photo electrochemical cells [8]. Other nanomaterials of interest to researchers are strontium aluminates: SrAl2O4 and Sr3Al2O6. SrAl2O4 exhibits excellent properties such as suitable emission colour, high quantum efficiency, long afterglow, and good chemical stability [9,10] which result in an unexpectedly large field of applications such as luminous paints in highways, airports, buildings and ceramic products [11]. SrAl2O4 with a band gap of ~6.5 eV has a pure monoclinic crystal structure with space group P21 and offers the possibility of generating broad band emission [12], whereas Sr3Al2O6 with a band gap of ~6.3 eV has a cubic structure with space group Pa-3 (No. 205) [13,14].
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Journal Pre-proof Sr3Al2O6 is also applicable in transparent ceramics, mechanoluminescence and long-lasting luminescence [15].
The luminescent properties of nanomaterials have been reported to depend strongly on the particle size, crystal structure, distribution of activators in the host lattice, morphology, doping concentration and preparation methods [16,17]. Doping is an effective way to manipulate physical and optical properties of wide band gap semiconductor families such as ZnO, ZnAl2O4, SrAl2O4 and Sr3Al2O6. Incorporating selected foreign ions, like Mn2+, into semiconductors serves as the primary method for engineering and controlling the optical, electrical and conductivity properties. The Mn2+ ion has attracted much attention in recent years as an excellent luminescent centre which gives emission at wavelengths in the region from 490 to 750 nm. However, the emission wavelength or colour highly depends on the host matrix crystal field, because the d–d transition in Mn2+ is quite sensitive to the crystal field [18-20]. Luo et al. [21] investigated the crystal structure and magnetic properties of Mn2+ doped ZnO nanoparticles prepared by the combustion method. The X-ray diffraction (XRD) results revealed that the lattice parameters a and c of Zn1-xMnxO increased linearly with the Mn2+ content, indicating that Mn2+ ions substituted Zn2+ ions in the crystal lattice. Furthermore it was found that the average crystallite size decreased with increasing Mn2+ concentration. Ponkumar et al. [22] prepared ZnAl2O4:Mn2+ via the combustion technique. The photoluminescence (PL) results showed two green broad emission bands located at 485 and 512 nm under 377 nm excitation. These emissions were attributed to 4T2 (G)→6A1(S) and 4T1 (G)→6A1(S) transitions of Mn2+, respectively. The intensity of these peaks increased with the increase in Mn2+ concentration until it reached its optimum at 0.06%. SrAl2O4:Mn2+, co-doped with Nd3+, was synthesized by a traditional solid-state reaction method [23]. The PL results showed a green emission around 550 nm due to the 4T1(G)→6A1(S) Mn2+ transition. The Mn2+ ions substituted Al3+ ions on the Al tetrahedral cation sites not occupying the mirror plane. Therefore, the Mn2+ ions had a tetrahedral coordination because each Mn2+ ion had four bonds. Tetrahedrally coordinated Mn2+ ions are subjected to a weak crystal field. Non-radiative energy transfer from Mn2+ to Nd3+ was observed [23]. Sr3Al2O6:Mn2+ has not yet been reported in the literature. Essentially, it can be seen that Mn2+ has been previously doped in bulk ZnO, ZnAl2O4 and 3
Journal Pre-proof SrAl2O4 hosts materials [21-23], although Mn2+ doping of Sr3Al2O6 has not been reported. However, doping Mn2+ in the mixed nanoscale phases of these hosts, i.e. ZnAl2O4/ZnO/SrAl2O4/Sr3Al2O6 (ZZSS), has not been attempted. Hence, we have explored the material properties of ZZSS:x% Mn2+ (0 ≤ x ≤ 2). In particular, this study investigated the effect of varying the Mn2+ concentration on the structural and optical properties of the ZZSS with the aim of developing alternative phosphor materials based on the mixed phases which might have the combined properties of the bulk counterparts. The principal aim is targeted in practical applications such as for liquid crystal displays (LCD). The physics behind the emission channels in this system ZZSS:x% Mn2+ (0 ≤ x ≤ 2) is discussed.
2. Experimental 2.1 Synthesis The citrate sol-gel technique was used to synthesize the ZZSS and ZZSS:x% Mn2+ (0 ≤ x ≤ 2) nanophosphor. 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. The nitrate salts were dissolved in 60 ml of deionized water to form a precursor solution. Mn2+ doped samples were prepared by adding appropriate amounts of Mn(NO3)2·4H2O (99%) to the ZZSS precursor solutions. The solutions were stirred with magnetic stirrers while maintained at a constant temperature of ⁓ 80 oC until a gel formed. This was left to dry for 24 h at room temperature and then annealed in a furnace (in air) at 1000 oC for 2 h. The resulting products were ground using a mortar and pestle in order to fabricate the powder samples, which were then ready for characterization using different analysis techniques.
2.2 Characterization The crystal structure of the samples was characterized by powder X-ray diffraction (XRD) (Bruker AXS Discover diffractometer) with Cu Kα (1.5418 Å) radiation. The morphology of the prepared phosphors was investigated using Zeiss Supra 55 scanning electron microscope (SEM). A JEOL JEM 4
Journal Pre-proof 1010 transmission electron microscopy (TEM) was used to study the crystallite size of the prepared nanopowders. The absorption characteristics were studied using ultraviolet-visible (UV-vis) diffuse reflection spectroscopy with a PerkinElmer Lambda 7505 system. The emission and excitation spectra were measured with an Edinburgh Instruments FLS980 fluorescence spectrometer having double monochromators, using a steady state xenon lamp as excitation source and a R928P photomultiplier tube as detector. All characterizations were carried out at room temperature.
3. Results and Discussion 3.1 X-ray diffraction Fig. 1 shows the XRD patterns of ZZSS:x% Mn2+ (0 ≤ x ≤ 2) samples. The patterns show diffraction peaks indicating the crystalline nature of the prepared samples. Fig. 1 (a) shows the pattern of ZZSS phase which resembles the mixture of ZZ and SS phases. The pattern in Fig. 1 (b) shows that the ZZSS phase reflects the combination of the mixed phases of the monoclinic SrAl2O4 (ICSD:91361, 29%), cubic Sr3Al2O6 (ICSD:71860, 12%) and ZnAl2O4 (ICSD:94160, 37%) and hexagonal ZnO (ICSD:65119, 22%) crystal structures. Doped samples are presented in Fig. 1 (c), which showed similar diffraction patterns to the un-doped ZZSS material. Hence, it can be concluded that the dopant Mn2+ is well incorporated within the ZZSS crystal structure and doping does not change the crystal structure of the prepared samples, as also observed in our previous study [24].
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Fig. 1. The XRD patterns for (a) SS, ZZ and ZZSS samples (b) the un-doped ZZSS sample and ICSD standards cards, (c) the ZZSS:x% Mn2+ (0.4 ≤ x ≤ 2) series. 6
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The most intense diffraction peaks corresponding to the ZnAl2O4, ZnO, SrAl2O4 and Sr3Al2O6 phases are 131, 101, 112 and 044, respectively. The analysis of these most intense diffraction peaks of individual phases in the ZZSS:x% Mn2+ (0 ≤ x ≤ 2) series is illustrated in Fig. 2. Analysis of all the peaks in Fig. 2 showed that all shifted towards lower angles compared to the un-doped ZZSS sample, suggesting the expansion of lattice parameters [25]. This behaviour can be attributed to the substitution of smaller ions by those of larger ionic radii. Considering the ionic radii of Mn2+ (0.83 Å), Sr2+ (1.21 Å), Zn2+ (0.74 Å) and Al3+ (0.39 Å) [26], it is likely that the Mn2+ substituted the smaller Zn2+ and Al3+ in all Mn doped ZZSS phases which then caused the shift to lower diffraction angles. Similar results were reported by Maphiri et al. [27] in Eu3+ doped in Mg1.5Al2O4.
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Fig. 2. The most intense diffraction peaks of the different phases (a) 131 and 101 (b) 044 and (c) 112
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Journal Pre-proof The crystallite sizes (𝐷) relevant for each of the phases were estimated from the full width at half maximum (FWHM) of the most intense diffraction peak of each phase using Scherrer’s equation [28] 0.9𝜆
𝐷 = 𝛽 sin𝜃
(1)
where λ stands for the X-ray wavelength, β is the FWHM (radians) and θ is the angle of diffraction (radians). The calculated crystallite sizes for each component phase in the prepared samples are presented in Table 1 and shown as a function of the Mn2+ concentration in Fig. 3. It is clear that the Mn2+ concentration influenced the crystallites sizes. Despite fluctuations, it is quite interesting to note that the trend is almost the same for each phase present in the samples.
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Crystallite sizes (nm)
35 30 25 ZnAl2O4 ZnO SrAl2O4 Sr3Al2O6
20 15 10 0.0
0.5
1.0
1.5
2.0
2+
Mn concentration
Fig. 3. Crystallite sizes as a function of the Mn2+ concentration.
Table 1. Crystallite sizes (nm) of the different phases. Sample ID
131 of ZnAl2O4
101 of ZnO
112 of SrAl2O4
044 of Sr3Al2O6
ZZSS
28
30
31
35
0.4% Mn2+
26
30
26
30
0.8% Mn2+
29
34
30
32
1% Mn2+
30
38
32
41
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Journal Pre-proof 1.2% Mn2+
31
38
32
37
1.4% Mn2+
30
37
30
33
1.8% Mn2+
27
31
13
27
2% Mn2+
30
36
29
35
3.2 Scanning electron microscopy The SEM images of the prepared ZZSS:x% Mn2+ (0 ≤ x ≤ 2) series is shown in Fig. 4. Fig. 4 (a) shows the un-doped ZZSS sample and it can be observed that morphology consists of the agglomeration of irregular particles with hexagonal rod-like structures distributed over the surface. The hexagonal rod-like structures may be attributed to the hexagonal ZnO, as confirmed by the XRD results in Fig. 1 (b). Fig. 4 (b) shows the powder sample containing 0.4% Mn2+, which shows similar morphology as the un-doped sample. The rod-like structures diminished for the 1.2% Mn2+ doped sample as shown in Fig. 4 (c). However, the cubic or monoclinic-like structure, which are attributed to the Sr3Al2O6, ZnAl2O4 or Sr3Al2O6 (based on the XRD results in Fig. 1 (b)) are also observed. Further increasing the Mn2+ concentration to 2% in Fig 4 (d) resulted in a similar morphology as observed in Fig. 4 (c). Thus, varying the doping concentration of Mn2+ in ZZSS influenced the morphology of the prepared nanophosphors.
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Fig. 4. SEM micrographs of ZZSS: (a) un-doped, (b) 0.4% Mn2+ (c) 1.2% Mn2+ and (d) 2% Mn2+.
Fig. 5 shows the Energy dispersive X-ray spectroscopy (EDS) data for the un-doped ZZSS and ZZSS:2% Mn2+ sample. Fig. 5 (a) for the un-doped ZZSS confirmed that the powder sample was composed of Zn, Sr, Al and O elements, while the ZZSS:2% Mn2+ doped sample in Fig. 5 (b) confirmed the presence of the Mn dopant. The additional peak of carbon (C) is attributed to the conductive carbon film coated on the samples in preparation for SEM. Apart from the anticipated elements, there were no other extra peaks, which agrees with the XRD results in Fig. 1.
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Fig. 5. The EDS measurements of the (a) ZZSS and (b) ZZSS:2% Mn2+. 3.3 TEM TEM images for the ZZSS and ZZSS:2% Mn2+ samples presented in Fig. 6 in order to confirm the particle sizes. The results show agglomeration of particles of different shapes, with an average particle size of around 30 nm. These results means that the particle sizes is the same as the crystallite sizes obtained from XRD results presented in Table 1 and Fig. 3.
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Fig. 6. TEM images of (a) un-doped ZZSS and (b) ZZSS:2% Mn2+.
3.4 Ultraviolet-visible diffuse reflectance The absorption characteristics of the mixed phases of un-doped ZZ, SS and ZZSS samples are shown in Fig. 7 (a). The results show that there are two absorption bands at around 251 and 374 nm in the SS phase. These bands may be attributed to structural defects in the SrAl2O4 sample [29]. The absorption band at 374 nm occurs in both the ZZ and SS phases. In the ZZ phase this band is attributed to the electronic transition from the valence band to conduction band of ZnO [30,31]. Fig. 7 (b) shows the reflectance spectra of the ZZSS:x% Mn2+ (0 ≤ x ≤ 2) series. In addition to the bands shown in Fig. 7 (a), there is an additional absorption around 500 nm for the Mn2+ doped samples. This absorption is attributed to the spin-allowed 6A1g(S)→4T1g(G) transition of Mn2+ ions [32]. Generally, the results also show that the reflectance decreased monotonically as the Mn2+ concentration was increased, indicating stronger absorbance as the Mn2+ concentration increased.
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Fig. 7. Diffuse reflectance spectra of (a) ZZ, SS, ZZSS and (b) the ZZSS:x% Mn2+ (0 ≤ x ≤ 2) series.
The effective band gap of the composite phase materials was estimated using the Kubelka-Munk function [33] as shown in Fig. 8. Taking into account that the band gap of the ZnO, ZnAl2O4, Sr3Al2O6 and SrAl2O4 are respectively 3.37, 3.8, 6.3 and 6.5 eV, it is clear that the effective band gap values of the prepared composite samples given in Fig. 8 are therefore within this range. Varying the Mn2+ concentration influenced the effective band gap of the prepared materials.
Fig. 8. Kubelka-Munk function band gap estimation of the ZZSS:x% Mn2+ (0 ≤ x ≤ 2) series.
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Journal Pre-proof Table 2. CIE chromacity colour coordinates of the ZZSS and ZZSS:x% Mn2+ (0 ≤ x ≤ 2) samples. Sample ID
CIE coordinates (x ; y)
ZZSS
(0.400 ; 0.460)
0.4 % Mn2+
(0.500 ; 0.401)
0.8% Mn2+
(0.510 ; 0.400)
1% Mn2+
(0.550 ; 0.401)
1.2% Mn2+
(0.550 ; 0.390)
1.4% Mn2+
(0.580 ; 0.410)
1.8% Mn2+
(0.510 ; 0.410)
2% Mn2+
(0.560 ; 0.430)
3.5 Photoluminescence analysis Fig. 9 (a) shows the excitation and emission spectra of the un-doped ZZ and SS samples. The results show that the excitation peak is located at 374 nm when monitoring the emission at 580 nm. Excitation at 374 nm produced violet, blue, yellow and sharp red emissions located at 430, 452, 580 and 677-732 nm, respectively, particularly for the ZZ phase. The emission peaks at 430 and 452 nm are similar to those from Al2O3 [34] and are attributed the ZnAl2O4 or the SS aluminate phases. In the SS sample the emissions at 430 nm is due to the F–centres like those in α–Al2O3, present in SrAl2O4 [35-37]. The broad band centred at 580 nm can either be the deep level defect emission from the ZnO ascribed to the presence of oxygen interstitials (Oi) [38] or an emission from ZnAl2O4 intrinsic defects [39]. The emissions (677 – 732 nm) cannot be attributed to transition from zinc interstitial (Zni) to Oi defect levels in ZnO [40] due to the sharpness of the peaks, rather they can be associated with the Mn4+ contamination [41]. Fig. 9 (b) shows the excitation and emission spectra of the un-doped ZZSS sample. The maximum emission peak is observed at around 580 nm. The peak is attributed to both the ZnO and ZnAl2O4 as discussed for Fig. 9 (a). Fig. 9 (c) shows the excitation and emission spectra of the ZZSS:x% Mn2+ (0 ≤ x ≤ 2) when excited at 374 nm. The results revealed that there are two emission peaks located at 580 and 600 nm with some shoulders at 414 and 434 nm. We reported 15
Journal Pre-proof similar peaks around 412 and 432 nm in our previous study [24]. The emission peak at 580 nm is attributed to originate from the same notion as explained for Fig. 9 (a). The normalized emission spectra are shown in Fig. 9 (d), which clearly confirms two peaks at 580 and 600 nm. The emission at 600 nm originates from Mn2+, specifically from the 4T1g(G) → 6A1g(S) transition [42]. The emission intensity as a function of the Mn2+ concentration is shown in Fig. 9 (e). This shows a decrease in luminescence intensity as the Mn2+ doping concentration increases, which is attributed to the wellknown luminescence quenching normally observed at the higher dopant concentration. The Mn2+ concentration window at the range (0 ≤ x ≤ 0.4) must be investigated in order to explore the possibilities of enhancing the emission intensity.
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Fig. 9. The excitation and emission spectra of the (a) ZZ and SS (b) ZZSS (c) ZZSS:x% Mn2+ (0 ≤ x ≤ 2) series (d) normalized emission of the ZZSS:x%Mn2+ (0 ≤ x ≤ 2) series and (e) emission intensity as a function of the Mn2+ concentration.
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Journal Pre-proof 3.6 CIE The Commission International de I’Eclairage (CIE) chromaticity diagram of the ZZSS:x% Mn2+ (0 ≤ x ≤ 2) series is shown in Fig 10. The CIE chromaticity coordinates were calculated from the emission spectra excited at 374 nm and the coordinates are presented in Table 2. The results show that there is a shift from yellow towards orange when the Mn2+ concentration is increased. This is in agreement with the PL results in Fig 9(d), showing that when Mn2+ was added a shift from 580 – 600 nm was observed.
Fig. 10. CIE chromaticity diagram for the ZZSS:x% Mn2+ (0 ≤ x ≤ 2) series.
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Conclusion
ZZSS:x% Mn2+ nanophosphors were successfully prepared via the citrate sol-gel method. XRD showed that the prepared samples consisted of mixed phases of monoclinic SrAl2O4, cubic Sr3Al2O6 cubic ZnAl2O4 and hexagonal ZnO. Variation of the Mn2+ doping concentration influenced the morphology of the prepared samples. EDS confirmed the expected elemental composition. UV-vis difuse reflection results revealed that the effective band gap of the ZZSS:x% Mn2+ composites can be tuned by varying the Mn2+ concentration. PL results revealed the emissions originating from the various phases and the 4T1g(G) → 6A1g(S) Mn2+ transition. CIE data showed that the emission colour
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Acknowledgements This work is supported by the South African National Research Foundation (NRF) Thuthuka programme (fund numbers: UID 99266 and 113947) and 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 authors would like to acknowledge Dr James Wesley- Smith and the team from the Electron Microscope Unit at Sefako Makgatho Health Science University for the SEM and TEM images.
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Journal Pre-proof Conflict of Interest form
I hereby declare that there is no conflict of interest. I do not know or have any relationship with the possible reviewers, editor or any person who will review this manuscript. I have acknowledged all the sponsorships contributed in this manuscript
Journal Pre-proof H Dixit, CMT-group and EMAT, Departement Fysica, Universiteit Antwerpen Groenenborgerlaan 171, B-2020 Antwerpen, Belgium E-mail: [email protected] Prof A Meijerink Chemistry (condensed matter and interfaces) CMI, Debye Institute for Nanomaterials Science, Utrecht University, P.O. Box 80 000, 3508 TA Utrecht, The Netherlands. E-mail: [email protected]
Prof. Ulrich Kynast Department of Chemical Engineering, Mu¨nster University of Applied Sciences Stegerwaldstraße 39, 48565 Steinfurt, Room: C 252 Tel: +49 2551 9-62119 Fax: +49 2551 9-62187 [email protected]
M.R. Mhlongo, L.F. Koao, R.E. Kroon, S.V. Motloung PII:
S0921-4526(19)30648-9
DOI:
https://doi.org/10.1016/j.physb.2019.411746
Reference:
PHYSB 411746
To appear in:
Physica B: Physics of Condensed Matter
Received Date:
31 May 2019
Accepted Date:
03 October 2019
Please cite this article as: M.R. Mhlongo, L.F. Koao, R.E. Kroon, S.V. Motloung, Investigative study of Mn2+ concentration on the structure, morphology and photoluminescence of sol-gel ZnAl2O4 /ZnO/ SrAl2O4/Sr3Al2O6 mixed phase nanophosphor, Physica B: Physics of Condensed Matter (2019), https://doi.org/10.1016/j.physb.2019.411746
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Journal Pre-proof Investigative study of Mn2+ concentration on the structure, morphology and photoluminescence of sol-gel ZnAl2O4/ZnO/ SrAl2O4/Sr3Al2O6 mixed phase nanophosphor
M.R. Mhlongoa,*, L.F. Koaob, R.E. Kroonc, S.V. Motlounga,d*
aDepartment
of Physics, Sefako Makgatho Health Sciences University, P.O. Box 94, Medunsa, 0204, South Africa
bDepartment
of Physics, University of the Free State (Qwaqwa Campus), Private Bag X 13, Phuthaditjhaba, 9866, South Africa
cDepartment
of Physics, University of the Free State, P. O. Box 339, Bloemfontein, 9300, South Africa
dDepartment
of Physics, Nelson Mandela University, P. O. Box 77000, Port Elizabeth 6031, South Africa
*Corresponding authors: [email protected] and [email protected] (Setumo. V. Motloung) Telephone numbers: +2712 521 3966 and +2741 504 4540. ABSTRACT The sol-gel technique was used to synthesize mixed phases of ZnAl2O4/ZnO/SrAl2O4/Sr3Al2O6 (ZZSS) nanophosphors. The study investigated the effect of varying the Mn2+ doping concentration on the structural, morphological and optical properties of the prepared phosphor materials. X-ray diffraction (XRD) patterns revealed that the structure of the un-doped ZZSS powder was not influenced by varying the Mn2+ concentration. Scanning electron microscopy (SEM) images showed that the morphological features of the prepared nanophosphors were influenced by the Mn2+ concentration. Transmission electron microscopy (TEM) confirmed that the prepared materials are on the nanoscale. Ultraviolet-visible (UV-vis) diffuse reflection spectroscopy showed that the band gap energy can be tuned in the range 4.78 - 4.92 eV. The photoluminescence (PL) results showed that the un-doped ZZSS exhibited an emission peak at 580 nm when excited at 374 nm, which is attributed to the defect centres within the ZnAl2O4 and ZnO. The doped samples showed broad emission bands at 600 nm, which were attributed to the 4T1g(G) → 6A1g(S) transitions of Mn2+ ions. The Commission
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Journal Pre-proof International de I’Eclairage (CIE) chromaticity colour coordinates showed that the emission colour of the prepared phosphors can be tuned from yellow to orange by varying the Mn2+ concentration.
Keywords: Mixed phases, Nanophosphor, Mn2+, sol-gel, Luminescence, CIE
1. Introduction Recently, research and development has mainly been concentrated on new types of materials, especially nanomaterials, because of their numerous potential applications. Nanomaterials may be attractive because of improved physical and chemical properties [1]. Zinc aluminate (ZnAl2O4), zinc oxide (ZnO) and strontium aluminates (SrAl2O4, Sr3Al2O6) nanomaterial systems have recently been investigated by many researchers around the globe. ZnAl2O4 has drawn a lot of attention because of its high mechanical strength, high thermal and chemical stability, low sintering temperature, low surface acidity, wide band gap (Eg) ⁓3.8 eV and excellent optical properties [2-4]. ZnAl2O4 is suitable for thermal control coatings for spacecraft and also for ultraviolet optoelectronic device applications [5,6]. ZnO nanomaterials have attracted tremendous interest for their advantages such as high quantum efficiency, non-toxicity and low cost [7]. They are regarded as one of the very significant and adaptable semiconductors, with a direct band gap of ⁓3.37 eV and a large exciton binding energy of 60 meV at room temperature. ZnO is also a promising candidate for functional components of devices and materials in photonic crystals, gas sensors, light-emitting diodes, solar cells and photo electrochemical cells [8]. Other nanomaterials of interest to researchers are strontium aluminates: SrAl2O4 and Sr3Al2O6. SrAl2O4 exhibits excellent properties such as suitable emission colour, high quantum efficiency, long afterglow, and good chemical stability [9,10] which result in an unexpectedly large field of applications such as luminous paints in highways, airports, buildings and ceramic products [11]. SrAl2O4 with a band gap of ~6.5 eV has a pure monoclinic crystal structure with space group P21 and offers the possibility of generating broad band emission [12], whereas Sr3Al2O6 with a band gap of ~6.3 eV has a cubic structure with space group Pa-3 (No. 205) [13,14].
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Journal Pre-proof Sr3Al2O6 is also applicable in transparent ceramics, mechanoluminescence and long-lasting luminescence [15].
The luminescent properties of nanomaterials have been reported to depend strongly on the particle size, crystal structure, distribution of activators in the host lattice, morphology, doping concentration and preparation methods [16,17]. Doping is an effective way to manipulate physical and optical properties of wide band gap semiconductor families such as ZnO, ZnAl2O4, SrAl2O4 and Sr3Al2O6. Incorporating selected foreign ions, like Mn2+, into semiconductors serves as the primary method for engineering and controlling the optical, electrical and conductivity properties. The Mn2+ ion has attracted much attention in recent years as an excellent luminescent centre which gives emission at wavelengths in the region from 490 to 750 nm. However, the emission wavelength or colour highly depends on the host matrix crystal field, because the d–d transition in Mn2+ is quite sensitive to the crystal field [18-20]. Luo et al. [21] investigated the crystal structure and magnetic properties of Mn2+ doped ZnO nanoparticles prepared by the combustion method. The X-ray diffraction (XRD) results revealed that the lattice parameters a and c of Zn1-xMnxO increased linearly with the Mn2+ content, indicating that Mn2+ ions substituted Zn2+ ions in the crystal lattice. Furthermore it was found that the average crystallite size decreased with increasing Mn2+ concentration. Ponkumar et al. [22] prepared ZnAl2O4:Mn2+ via the combustion technique. The photoluminescence (PL) results showed two green broad emission bands located at 485 and 512 nm under 377 nm excitation. These emissions were attributed to 4T2 (G)→6A1(S) and 4T1 (G)→6A1(S) transitions of Mn2+, respectively. The intensity of these peaks increased with the increase in Mn2+ concentration until it reached its optimum at 0.06%. SrAl2O4:Mn2+, co-doped with Nd3+, was synthesized by a traditional solid-state reaction method [23]. The PL results showed a green emission around 550 nm due to the 4T1(G)→6A1(S) Mn2+ transition. The Mn2+ ions substituted Al3+ ions on the Al tetrahedral cation sites not occupying the mirror plane. Therefore, the Mn2+ ions had a tetrahedral coordination because each Mn2+ ion had four bonds. Tetrahedrally coordinated Mn2+ ions are subjected to a weak crystal field. Non-radiative energy transfer from Mn2+ to Nd3+ was observed [23]. Sr3Al2O6:Mn2+ has not yet been reported in the literature. Essentially, it can be seen that Mn2+ has been previously doped in bulk ZnO, ZnAl2O4 and 3
Journal Pre-proof SrAl2O4 hosts materials [21-23], although Mn2+ doping of Sr3Al2O6 has not been reported. However, doping Mn2+ in the mixed nanoscale phases of these hosts, i.e. ZnAl2O4/ZnO/SrAl2O4/Sr3Al2O6 (ZZSS), has not been attempted. Hence, we have explored the material properties of ZZSS:x% Mn2+ (0 ≤ x ≤ 2). In particular, this study investigated the effect of varying the Mn2+ concentration on the structural and optical properties of the ZZSS with the aim of developing alternative phosphor materials based on the mixed phases which might have the combined properties of the bulk counterparts. The principal aim is targeted in practical applications such as for liquid crystal displays (LCD). The physics behind the emission channels in this system ZZSS:x% Mn2+ (0 ≤ x ≤ 2) is discussed.
2. Experimental 2.1 Synthesis The citrate sol-gel technique was used to synthesize the ZZSS and ZZSS:x% Mn2+ (0 ≤ x ≤ 2) nanophosphor. 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. The nitrate salts were dissolved in 60 ml of deionized water to form a precursor solution. Mn2+ doped samples were prepared by adding appropriate amounts of Mn(NO3)2·4H2O (99%) to the ZZSS precursor solutions. The solutions were stirred with magnetic stirrers while maintained at a constant temperature of ⁓ 80 oC until a gel formed. This was left to dry for 24 h at room temperature and then annealed in a furnace (in air) at 1000 oC for 2 h. The resulting products were ground using a mortar and pestle in order to fabricate the powder samples, which were then ready for characterization using different analysis techniques.
2.2 Characterization The crystal structure of the samples was characterized by powder X-ray diffraction (XRD) (Bruker AXS Discover diffractometer) with Cu Kα (1.5418 Å) radiation. The morphology of the prepared phosphors was investigated using Zeiss Supra 55 scanning electron microscope (SEM). A JEOL JEM 4
Journal Pre-proof 1010 transmission electron microscopy (TEM) was used to study the crystallite size of the prepared nanopowders. The absorption characteristics were studied using ultraviolet-visible (UV-vis) diffuse reflection spectroscopy with a PerkinElmer Lambda 7505 system. The emission and excitation spectra were measured with an Edinburgh Instruments FLS980 fluorescence spectrometer having double monochromators, using a steady state xenon lamp as excitation source and a R928P photomultiplier tube as detector. All characterizations were carried out at room temperature.
3. Results and Discussion 3.1 X-ray diffraction Fig. 1 shows the XRD patterns of ZZSS:x% Mn2+ (0 ≤ x ≤ 2) samples. The patterns show diffraction peaks indicating the crystalline nature of the prepared samples. Fig. 1 (a) shows the pattern of ZZSS phase which resembles the mixture of ZZ and SS phases. The pattern in Fig. 1 (b) shows that the ZZSS phase reflects the combination of the mixed phases of the monoclinic SrAl2O4 (ICSD:91361, 29%), cubic Sr3Al2O6 (ICSD:71860, 12%) and ZnAl2O4 (ICSD:94160, 37%) and hexagonal ZnO (ICSD:65119, 22%) crystal structures. Doped samples are presented in Fig. 1 (c), which showed similar diffraction patterns to the un-doped ZZSS material. Hence, it can be concluded that the dopant Mn2+ is well incorporated within the ZZSS crystal structure and doping does not change the crystal structure of the prepared samples, as also observed in our previous study [24].
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Fig. 1. The XRD patterns for (a) SS, ZZ and ZZSS samples (b) the un-doped ZZSS sample and ICSD standards cards, (c) the ZZSS:x% Mn2+ (0.4 ≤ x ≤ 2) series. 6
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The most intense diffraction peaks corresponding to the ZnAl2O4, ZnO, SrAl2O4 and Sr3Al2O6 phases are 131, 101, 112 and 044, respectively. The analysis of these most intense diffraction peaks of individual phases in the ZZSS:x% Mn2+ (0 ≤ x ≤ 2) series is illustrated in Fig. 2. Analysis of all the peaks in Fig. 2 showed that all shifted towards lower angles compared to the un-doped ZZSS sample, suggesting the expansion of lattice parameters [25]. This behaviour can be attributed to the substitution of smaller ions by those of larger ionic radii. Considering the ionic radii of Mn2+ (0.83 Å), Sr2+ (1.21 Å), Zn2+ (0.74 Å) and Al3+ (0.39 Å) [26], it is likely that the Mn2+ substituted the smaller Zn2+ and Al3+ in all Mn doped ZZSS phases which then caused the shift to lower diffraction angles. Similar results were reported by Maphiri et al. [27] in Eu3+ doped in Mg1.5Al2O4.
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Fig. 2. The most intense diffraction peaks of the different phases (a) 131 and 101 (b) 044 and (c) 112
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Journal Pre-proof The crystallite sizes (𝐷) relevant for each of the phases were estimated from the full width at half maximum (FWHM) of the most intense diffraction peak of each phase using Scherrer’s equation [28] 0.9𝜆
𝐷 = 𝛽 sin𝜃
(1)
where λ stands for the X-ray wavelength, β is the FWHM (radians) and θ is the angle of diffraction (radians). The calculated crystallite sizes for each component phase in the prepared samples are presented in Table 1 and shown as a function of the Mn2+ concentration in Fig. 3. It is clear that the Mn2+ concentration influenced the crystallites sizes. Despite fluctuations, it is quite interesting to note that the trend is almost the same for each phase present in the samples.
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Crystallite sizes (nm)
35 30 25 ZnAl2O4 ZnO SrAl2O4 Sr3Al2O6
20 15 10 0.0
0.5
1.0
1.5
2.0
2+
Mn concentration
Fig. 3. Crystallite sizes as a function of the Mn2+ concentration.
Table 1. Crystallite sizes (nm) of the different phases. Sample ID
131 of ZnAl2O4
101 of ZnO
112 of SrAl2O4
044 of Sr3Al2O6
ZZSS
28
30
31
35
0.4% Mn2+
26
30
26
30
0.8% Mn2+
29
34
30
32
1% Mn2+
30
38
32
41
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Journal Pre-proof 1.2% Mn2+
31
38
32
37
1.4% Mn2+
30
37
30
33
1.8% Mn2+
27
31
13
27
2% Mn2+
30
36
29
35
3.2 Scanning electron microscopy The SEM images of the prepared ZZSS:x% Mn2+ (0 ≤ x ≤ 2) series is shown in Fig. 4. Fig. 4 (a) shows the un-doped ZZSS sample and it can be observed that morphology consists of the agglomeration of irregular particles with hexagonal rod-like structures distributed over the surface. The hexagonal rod-like structures may be attributed to the hexagonal ZnO, as confirmed by the XRD results in Fig. 1 (b). Fig. 4 (b) shows the powder sample containing 0.4% Mn2+, which shows similar morphology as the un-doped sample. The rod-like structures diminished for the 1.2% Mn2+ doped sample as shown in Fig. 4 (c). However, the cubic or monoclinic-like structure, which are attributed to the Sr3Al2O6, ZnAl2O4 or Sr3Al2O6 (based on the XRD results in Fig. 1 (b)) are also observed. Further increasing the Mn2+ concentration to 2% in Fig 4 (d) resulted in a similar morphology as observed in Fig. 4 (c). Thus, varying the doping concentration of Mn2+ in ZZSS influenced the morphology of the prepared nanophosphors.
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Fig. 4. SEM micrographs of ZZSS: (a) un-doped, (b) 0.4% Mn2+ (c) 1.2% Mn2+ and (d) 2% Mn2+.
Fig. 5 shows the Energy dispersive X-ray spectroscopy (EDS) data for the un-doped ZZSS and ZZSS:2% Mn2+ sample. Fig. 5 (a) for the un-doped ZZSS confirmed that the powder sample was composed of Zn, Sr, Al and O elements, while the ZZSS:2% Mn2+ doped sample in Fig. 5 (b) confirmed the presence of the Mn dopant. The additional peak of carbon (C) is attributed to the conductive carbon film coated on the samples in preparation for SEM. Apart from the anticipated elements, there were no other extra peaks, which agrees with the XRD results in Fig. 1.
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Fig. 5. The EDS measurements of the (a) ZZSS and (b) ZZSS:2% Mn2+. 3.3 TEM TEM images for the ZZSS and ZZSS:2% Mn2+ samples presented in Fig. 6 in order to confirm the particle sizes. The results show agglomeration of particles of different shapes, with an average particle size of around 30 nm. These results means that the particle sizes is the same as the crystallite sizes obtained from XRD results presented in Table 1 and Fig. 3.
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Fig. 6. TEM images of (a) un-doped ZZSS and (b) ZZSS:2% Mn2+.
3.4 Ultraviolet-visible diffuse reflectance The absorption characteristics of the mixed phases of un-doped ZZ, SS and ZZSS samples are shown in Fig. 7 (a). The results show that there are two absorption bands at around 251 and 374 nm in the SS phase. These bands may be attributed to structural defects in the SrAl2O4 sample [29]. The absorption band at 374 nm occurs in both the ZZ and SS phases. In the ZZ phase this band is attributed to the electronic transition from the valence band to conduction band of ZnO [30,31]. Fig. 7 (b) shows the reflectance spectra of the ZZSS:x% Mn2+ (0 ≤ x ≤ 2) series. In addition to the bands shown in Fig. 7 (a), there is an additional absorption around 500 nm for the Mn2+ doped samples. This absorption is attributed to the spin-allowed 6A1g(S)→4T1g(G) transition of Mn2+ ions [32]. Generally, the results also show that the reflectance decreased monotonically as the Mn2+ concentration was increased, indicating stronger absorbance as the Mn2+ concentration increased.
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Fig. 7. Diffuse reflectance spectra of (a) ZZ, SS, ZZSS and (b) the ZZSS:x% Mn2+ (0 ≤ x ≤ 2) series.
The effective band gap of the composite phase materials was estimated using the Kubelka-Munk function [33] as shown in Fig. 8. Taking into account that the band gap of the ZnO, ZnAl2O4, Sr3Al2O6 and SrAl2O4 are respectively 3.37, 3.8, 6.3 and 6.5 eV, it is clear that the effective band gap values of the prepared composite samples given in Fig. 8 are therefore within this range. Varying the Mn2+ concentration influenced the effective band gap of the prepared materials.
Fig. 8. Kubelka-Munk function band gap estimation of the ZZSS:x% Mn2+ (0 ≤ x ≤ 2) series.
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Journal Pre-proof Table 2. CIE chromacity colour coordinates of the ZZSS and ZZSS:x% Mn2+ (0 ≤ x ≤ 2) samples. Sample ID
CIE coordinates (x ; y)
ZZSS
(0.400 ; 0.460)
0.4 % Mn2+
(0.500 ; 0.401)
0.8% Mn2+
(0.510 ; 0.400)
1% Mn2+
(0.550 ; 0.401)
1.2% Mn2+
(0.550 ; 0.390)
1.4% Mn2+
(0.580 ; 0.410)
1.8% Mn2+
(0.510 ; 0.410)
2% Mn2+
(0.560 ; 0.430)
3.5 Photoluminescence analysis Fig. 9 (a) shows the excitation and emission spectra of the un-doped ZZ and SS samples. The results show that the excitation peak is located at 374 nm when monitoring the emission at 580 nm. Excitation at 374 nm produced violet, blue, yellow and sharp red emissions located at 430, 452, 580 and 677-732 nm, respectively, particularly for the ZZ phase. The emission peaks at 430 and 452 nm are similar to those from Al2O3 [34] and are attributed the ZnAl2O4 or the SS aluminate phases. In the SS sample the emissions at 430 nm is due to the F–centres like those in α–Al2O3, present in SrAl2O4 [35-37]. The broad band centred at 580 nm can either be the deep level defect emission from the ZnO ascribed to the presence of oxygen interstitials (Oi) [38] or an emission from ZnAl2O4 intrinsic defects [39]. The emissions (677 – 732 nm) cannot be attributed to transition from zinc interstitial (Zni) to Oi defect levels in ZnO [40] due to the sharpness of the peaks, rather they can be associated with the Mn4+ contamination [41]. Fig. 9 (b) shows the excitation and emission spectra of the un-doped ZZSS sample. The maximum emission peak is observed at around 580 nm. The peak is attributed to both the ZnO and ZnAl2O4 as discussed for Fig. 9 (a). Fig. 9 (c) shows the excitation and emission spectra of the ZZSS:x% Mn2+ (0 ≤ x ≤ 2) when excited at 374 nm. The results revealed that there are two emission peaks located at 580 and 600 nm with some shoulders at 414 and 434 nm. We reported 15
Journal Pre-proof similar peaks around 412 and 432 nm in our previous study [24]. The emission peak at 580 nm is attributed to originate from the same notion as explained for Fig. 9 (a). The normalized emission spectra are shown in Fig. 9 (d), which clearly confirms two peaks at 580 and 600 nm. The emission at 600 nm originates from Mn2+, specifically from the 4T1g(G) → 6A1g(S) transition [42]. The emission intensity as a function of the Mn2+ concentration is shown in Fig. 9 (e). This shows a decrease in luminescence intensity as the Mn2+ doping concentration increases, which is attributed to the wellknown luminescence quenching normally observed at the higher dopant concentration. The Mn2+ concentration window at the range (0 ≤ x ≤ 0.4) must be investigated in order to explore the possibilities of enhancing the emission intensity.
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Fig. 9. The excitation and emission spectra of the (a) ZZ and SS (b) ZZSS (c) ZZSS:x% Mn2+ (0 ≤ x ≤ 2) series (d) normalized emission of the ZZSS:x%Mn2+ (0 ≤ x ≤ 2) series and (e) emission intensity as a function of the Mn2+ concentration.
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Journal Pre-proof 3.6 CIE The Commission International de I’Eclairage (CIE) chromaticity diagram of the ZZSS:x% Mn2+ (0 ≤ x ≤ 2) series is shown in Fig 10. The CIE chromaticity coordinates were calculated from the emission spectra excited at 374 nm and the coordinates are presented in Table 2. The results show that there is a shift from yellow towards orange when the Mn2+ concentration is increased. This is in agreement with the PL results in Fig 9(d), showing that when Mn2+ was added a shift from 580 – 600 nm was observed.
Fig. 10. CIE chromaticity diagram for the ZZSS:x% Mn2+ (0 ≤ x ≤ 2) series.
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Conclusion
ZZSS:x% Mn2+ nanophosphors were successfully prepared via the citrate sol-gel method. XRD showed that the prepared samples consisted of mixed phases of monoclinic SrAl2O4, cubic Sr3Al2O6 cubic ZnAl2O4 and hexagonal ZnO. Variation of the Mn2+ doping concentration influenced the morphology of the prepared samples. EDS confirmed the expected elemental composition. UV-vis difuse reflection results revealed that the effective band gap of the ZZSS:x% Mn2+ composites can be tuned by varying the Mn2+ concentration. PL results revealed the emissions originating from the various phases and the 4T1g(G) → 6A1g(S) Mn2+ transition. CIE data showed that the emission colour
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Journal Pre-proof can be tuned from yellow to orange by increasing the Mn2+ doping concentration, although this is accompanied by a decrease in luminescence intensity.
Acknowledgements This work is supported by the South African National Research Foundation (NRF) Thuthuka programme (fund numbers: UID 99266 and 113947) and 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 authors would like to acknowledge Dr James Wesley- Smith and the team from the Electron Microscope Unit at Sefako Makgatho Health Science University for the SEM and TEM images.
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Journal Pre-proof Conflict of Interest form
I hereby declare that there is no conflict of interest. I do not know or have any relationship with the possible reviewers, editor or any person who will review this manuscript. I have acknowledged all the sponsorships contributed in this manuscript
Journal Pre-proof H Dixit, CMT-group and EMAT, Departement Fysica, Universiteit Antwerpen Groenenborgerlaan 171, B-2020 Antwerpen, Belgium E-mail: [email protected] Prof A Meijerink Chemistry (condensed matter and interfaces) CMI, Debye Institute for Nanomaterials Science, Utrecht University, P.O. Box 80 000, 3508 TA Utrecht, The Netherlands. E-mail: [email protected]
Prof. Ulrich Kynast Department of Chemical Engineering, Mu¨nster University of Applied Sciences Stegerwaldstraße 39, 48565 Steinfurt, Room: C 252 Tel: +49 2551 9-62119 Fax: +49 2551 9-62187 [email protected]