Esra Öztürk*
Photoluminescence Properties of Eu
3+
-activated
Silicate Phosphors
DOI 10.1515/htmp-2015-0263
Received November 20, 2015; accepted April 20, 2016
Abstract: The silicate-type and Eu3+-activated Sr3SiO5
and Mg3SiO5were prepared through the high
tempera-ture solid state reaction method under an open atmo-sphere. DTA/TG analysis was conducted to obtain information about the thermal behaviors of the mixed reactants. Using the DTA/TG results, the sintering process was achieved and the phase properties were characterized by X-ray diffraction (XRD). The effects of the same activator (Eu3+) and co-dopant (Dy3+) on the photoluminescence (PL) properties of the host lat-tices were investigated by using a photoluminescence spectrometer.
Keywords: silicate structure, high temperature solid state reaction method, photoluminescence, rare earth ions
Introduction
Photoluminescent materials have attracted much interest due to their possible applications in many different areas, such as emergency and general illumination in a dark environment, LEDs, display devices, optical sto-rage, etc. [1]. Among the inorganic phosphor systems, silicate-based phosphors are known for being more che-mically and thermally stable, and are less costly com-pared with sulfide or even aluminate-based phosphors. Until now, intense, different types of color emitting oxide phosphors have been commercially and scientifically researched which have better properties than other phos-phor systems. Therefore, much research on phosphos-phors with silicate type hosts has been conducted owing to the growing interest in silicate-based photoluminescent materials [2, 3]. Also, the family of phosphors activated with Eu2 + or Eu3 + rare-earth ions is commonly used as components for conventional lighting to obtain blue to
red region emissions due to their high efficiencies and color purities [1]. Among the silicate systems, for exam-ple the SrO–SiO2binary system, there are three transition
compounds, namely SrSiO3, Sr2SiO4 and Sr3SiO5 [4]. In
this research, the photoluminescent properties of the silicate-based Sr3-x-ySiO5: xEu3+, yDy3+, Mg3SiO5: xEu3+,
yDy3+ and Sr
3-x-y-zSiO5: xEu3+, yDy3+, zMg2+ (x: 0.50,
y: 0.25 and z: 0. 1) phosphor systems, in particular were investigated by means of the high temperature solid state reaction method.
Material and methods
The silicate-based Sr1.25Eu0.5Dy0.25SiO5, Mg1.25Eu0.5
Dy0.25SiO5 and Sr1.15Mg0.1Eu0.5Dy0.25SiO5 phosphor
sys-tems were synthesized via the high temperature ceramic method. The compositions were stoichiometrically calcu-lated and appropriate amounts of the high purity starting reagents, namely 4MgCO3·Mg(OH)2·5H2O (A.R.), SrCO3
(99.9 %), SiO2 (99.9 %), Eu2O3 (99.99 %) and Dy2O3
(99.99 %), were thoroughly mixed and ground in a agate mortar to ensure fine and homogenous particle mixing. Subsequently, the heat treatments of the sam-ples were done in pure alumina crucibles in a muffle furnace (Protherm PTF 16/50/450); they were then cooled down slowly to room temperature. The sintered samples were again ground to powder form prior to the characterizations.
Simultaneous differential thermal analysis (DTA) and thermogravimetric (TG) analysis (Seiko Instruments Inc./Exstar TG/DTA 6200) at a heating rate of 10 °C/min from room temperature to 1,300 °C were performed to determine the decomposition and the oxidation process of the reactants. Then the pre-sintering (calcination) and sintering processes were applied according to the DTA/ TG results, and a BRUKER AXS D8 ADVANCE model X-ray diffractometer, which was run at 40 kV and 30 mA (Cu-Kα radiation) in a step-scan mode (0.02°/2θ), was used to obtain the phases after sintering. Finally, the photoluminescent spectra which showed the excitations and emissions of the phosphors were analyzed by a spectrophotometer (Photon Technology International (PTI), QuantaMasterTM30).
*Corresponding author: Esra Öztürk, Department of Materials Science and Engineering, Engineering Faculty, Karamanoğlu Mehmetbey University, Karaman, Turkey,
Results and discussion
Thermal analysis
The thermal behaviors of the phosphor systems, which were basically composed of 4MgCO3·Mg(OH)2·5H2O
and SrCO3, were obtained between 50 °C and 1,300 °C
(Figures 1–3).
The DTA/TG/DTG analysis results of Sr3SiO5: Eu3 +,
Dy3 + are shown in Figure 1. Figure 1 shows that there is a
significant weight loss between 800 and 1,150 °C which is due to the decomposition of SrCO3and removal of CO2in
the system. The decomposition of SrCO3 under heating
can be given as follows: SrCO3Δ
!SrO + CO2 (1)
The endothermic peaks at 944 and 1,040 °C are attributed to an orthorhombic to rhombohedral transition and then the decomposition of SrCO3 to SrO [5]. The TG curve
exhibits a mass loss equal to 25.0 %, which is almost similar to the calculated mass loss (~25.0 %).
Figure 2 shows that the decomposition and dehydra-tion of 4MgCO3·Mg(OH)2·5H2O and SrCO3for the Sr3SiO5:
Eu3+, Dy3+, Mg2+system are similar to the DTA/TG/DTG curves of Sr3SiO5: Eu3+, Dy3+, as expected.
The Sr3SiO5: Eu3+, Dy3+, Mg2+ system has almost
similar DTA/TG/DTG results. Thus, this system exhibits the decomposition of SrCO3and also the decomposition
of a small amount of 4MgCO3·Mg(OH)2·5H2O as the MgO
source in the system. The decomposition of SrCO3under
heating is given for the previous system.
Therefore, the endothermic peaks at 944 and 1,040 °C were attributed to an orthorhombic to rhombohedral
transition followed by the decomposition of SrCO3 to
SrO [5], and that of 4MgCO3·Mg(OH)2·5H2O to MgO. The
TG curve exhibits a mass loss equal to 25.3 %, which is almost similar to the calculated mass loss (~25.0 %).
Figure 3, which is shown below, shows the thermal analysis results of Mg3SiO5: Eu3 +, Dy3 + phosphor which
is mainly composed of 4MgCO3·Mg(OH)2·5H2O.
When subjected to thermal analysis, hydrated magne-sium carbonates are decomposed by endothermic reac-tions and result in the departure of H2O and CO2from the
compound due to decomposition of 4MgCO3·Mg(OH)2·5H2O
to MgO. Much research has been conducted and it was found that the thermal decomposition of hydromagnesite proceeds via dehydration at 100–300 °C and decarbona-tion at 350–650 °C toward the end product, namely MgO [6]. Therefore, in our analysis, the weight losses starting from 150 to 600 °C are related to the decomposition of
Figure 1: DTA/TG/DTG curves of Sr3SiO5: Eu 3 +
, Dy3 +.
Figure 3: DTA/TG/DTG curves of Mg3SiO5: Eu3+, Dy3+.
4MgCO3·Mg(OH)2·5H2O in the system (Figure 3). The
decomposition of 4MgCO3·Mg(OH)2·5H2O under heating
can be summarize as follows:
4MgCO3 Mg OHð Þ2 5H2O Δ
!4MgCO3 Mg OHð Þ2+ 5H2O
(2)
4MgCO3 Mg OHð Þ2!Δ 4MgCO3+ MgO + H2O (3)
4MgCO3Δ
!4 MgO + 4CO2 (4)
The first endothermic peak (at 267 °C) is attributed to the partition of the hydroxyl group from Mg(OH)2.The second
endothermic peak to (at 450 °C) belongs the decomposi-tion of MgCO3to MgO. The TG curve exhibits a total mass
loss equal to 47.2 %, which is similar to calculated mass loss (~48.8 %).
X-ray diffraction (XRD) analysis
The heat treatment temperatures of each phosphor sys-tem were determined according to thermal analysis results. The sintering processes of the phosphor systems were applied as a pre-sintering stage at 800 °C for 2 h followed by main sintering processes. The base sintering temperatures are described below:
After high temperature sintering processes, the XRD analysis was applied. Figure 4 shows the XRD pattern of Sr3SiO5: Eu3 +, Dy3 +.
While the XRD patterns of the Sr3SiO5:Eu3 +, Dy3 +
phos-phors for different sintering temperatures were achieved according to the DTA/TG/DTG results, several reactions occurred simultaneously, including the grain growth of
SrO and SiO2, the diffusion of Si4+ into the SrO lattice,
and a phase formation reaction of Sr2SiO4-Sr3SiO5 [7].
Therefore, the crystallinity and base phase of the Sr3SiO5:Eu3+, Dy3+ powders improved and were well
indexed with increasing sintering temperature. The Sr3SiO5:Eu3+, Dy3+ powders sintered at 1,470 °C for 12 h
exhibited improved crystallinity compared to powders sintered at 1,300 °C for 6 h, as proved by the relatively high and sharp peaks in the XRD pattern. The phosphor sample XRD results proved that the expected crystal sys-tem could be indexed by increasing sintering sys- tempera-ture. Therefore the XRD pattern of Sr3SiO5:Eu3+, Dy3+
matched well with the PDF 00-026-0984 Sr3SiO5card [8]
owing to at sintering conditions 1,470 °C for 12 h. Almost all the peaks showed a single phase except thatβ-Sr2SiO4
secondary phases were observed despite the higher sintering temperature and holding time at that tempera-ture. The sample crystallized in the tetragonal structure, the lattice parameters of which were a = 6.9476 Å b = 6.9476 Å, c = 10.753 Å, (alpha) α = 90°, (beta) β = 90° and (gamma) γ = 90°.
The XRD patterns of the Sr3SiO5:Eu3+, Dy3+, Mg2+
powders (Figure 5) were observed to be similar to Sr3SiO5: Eu3+, Dy3+, as expected. The obtained products
were mainly composed of Sr3SiO5in the 1,460 °C sintered
sample except that some β-Sr2SiO4 phase existed in the
system. Furthermore the result clearly suggests that trace amounts of Eu3+, Dy3+or Mg2+were incorporated into the lattice and caused no change in the lattice structure, as can be seen by the unchanged diffraction patterns. At lower sintering conditions, for example 1,300 °C for 6 h, β-Sr2SiO4presented as the major phase, but this situation
was changed by the higher temperatures and holding times, then the β-Sr2SiO4 phase was minor and the
Figure 4: The XRD patterns at different sintering temperatures of Sr3SiO5:Eu
3+
, Dy3+.
Figure 5: The XRD patterns at different sintering temperatures of Sr3SiO5: Eu
3+
expected phase was Sr3SiO5. To obtain a pure Sr3SiO5
structure, it is necessary to increase the sintering tem-perature and holding time. The XRD pattern of Sr3SiO5:
Eu3+, Dy3+, Mg2+matched well with the PDF 00-026-0984
Sr3SiO5 card [8] at sintering conditions of 1,460 °C for
12 h. The sample crystallized in the tetragonal structure similar to Sr3SiO5:Eu3+, Dy3+, the lattice parameters of
which were a = 6.9476 Å, b = 6.9476 Å, c = 10.753 Å, (alpha)α = 90°, (beta) β = 90° and (gamma) γ = 90°.
The XRD pattern of the Mg3SiO5:Eu3+, Dy3+phosphor
that was sintered at different temperatures to obtain a pure structure is shown in Figure 6.
It was determined that the base phase is Mg2SiO4; also
the major first two phases of the peaks indicate those of MgO although the sintering temperature was repeated and increased as 1,250 °C. Despite the fact that a higher thermal treatment was applied for this type of phosphor according to DTA/TG results, the expected crystal system could not be indexed in XRD analysis except for Mg2SiO4
and MgO. Also, it is well known that the formation of the Mg3SiO5phase occurs at very high temperatures, namely
above 1,850 °C, according to the MgO-SiO2 phase
dia-gram [9].
Photoluminescence properties
All of the photoluminescence studies gave effective results with excitation and emission spectra. Although not all of the objective phosphor systems could be synthe-sized as pure structures, they still have fascinating PL properties. Therefore, the impressive excitation and emis-sion bands are thanks to activated Eu3 +, rare-earth ion.
Figure 7 shows the PL spectra of the Sr3SiO5:Eu3 +,
Dy3 + phosphor system upon excitation at 311 nm in the UV region and the obtained sharp emission with a max-imum at 618 nm in the red region.
It appeared that the PL result of this system indicates maximum emission bands at 617 nm, 579 nm and 698 nm which are attributed to 5D0→ 7F2,5D0→7F1[10–12] and 5D
0→7F4[12, 13] transitions of the Eu3+ion, respectively.
Furthermore, the sharp excitation peaks generally between 250 nm and 500 nm are associated with typical intra-4f transitions of the Eu3+ion. The minör excitation bands at 396 and 465 nm are attributed to 7F0 → 5L6
[12, 14–16] and 7F0 → 5D2 [12, 15–17] transitions of the
Eu3+ion, respectively.
The PL spectra of the Sr3SiO5: Eu3+, Dy3+, Mg2+
phos-phor powders are shown in Figure 8. The sample was excited at 287 nm in the UV region and presented max-imum emission at 586 nm in the yellow-red region.
It appears that the PL result of this system indicates maximum emission bands at 586 nm, 613 nm and 701 nm, correlatively with the Sr3SiO5:Eu3+, Dy3+phosphor, which
are attributed to the 5D0 → 7F1, 5D0 → 7F2 [10–12], and 5D
0→7F4[12, 13] transitions of the Eu3+ion, respectively.
Moreover, the sharp excitation peak at 287 nm and very little peaks at 394 nm and 463 nm are associated with typical intra-4f, 7F0 → 5L6[12, 14–16] and7F0→ 5D2[12,
15–17] transitions of the Eu3+ion, respectively. Recently, Jee et al. reported that the incorporation of Mg2+ into SrSi2O2Ni2:Eu2+improves the luminescence intensity and
thermal quenching [18]. Therefore, in this study, the aim was to achieve more intense luminescence properties
Figure 6: The XRD patterns at different sintering temperatures of Mg3SiO5: Eu3+, Dy3+.
with the Sr3SiO5:Eu3+, Dy3+phosphor. By combining the
effects of replacing Sr2+with Mg2+, significant enhance-ment of emission intensity was obtained while the spec-tral distribution of the emission was maintained, as can be seen from Figures 7 and 8. In Figure 8, the remarkable almost 75 % improvement in emission intensity is high-lighted, with an excitation maximum in the range of 500 nm to 750 nm, which is quite attractive for different types of phosphor applications.
The last PL analysis results of Mg3SiO5: Eu3+, Dy3+are
also similar to the two previous studies, because the same activator, namely Eu3+, was used (Figure 9).
All excitation (between 250–500 nm) and emission bands (between 550–750 nm) are almost the same as the previous two Eu3+-doped and Dy3+co-doped systems. So the transitions of Eu3+ion appeared.
The important and prominent point here is that, apart from having the same PL results, these the intensities change according to the hosts. The Mg3SiO5: Eu3+, Dy3+ system
phosphor has the most intense PL results among them.
Conclusions
Three silicate-type phosphors, namely Sr1.25Eu0.5Dy0.25SiO5,
Mg1.25Eu0.5Dy0.25SiO5 and Sr1.15Mg0.1Eu0.5Dy0.25SiO5, were
prepared by the high temperature solid-state reaction method under an open atmosphere. The Sr3SiO5structure is rather
difficult to achieve when applying higher sintering tempera-tures. However, its formation could be seen by increasing temperature via the XRD results. Also, the Mg3SiO5phase
could not be formed because it needs temperatures of more than 1,800 °C based on the MgO-SiO2phase diagram. Briefly,
photoluminescence analysis exhibited that all of the different activated hosts exhibit emissions due to Eu3+emission center. Therefore, Eu3+-doped hosts generally created red emissions
because of their f-f transitions. Additionally, the two phos-phor systems with the same hosts have different PL intensi-ties due to Mg2+ions occurring in one of them. Finally, it was proved that the activator ions PL properties in the phosphor systems are independent of the phase forming process except for possible different PL intensities.
Funding: The authors would like thank to Karamanoglu Mehmetbey University’s, Scientific Research Projects Commission (BAP), 06-YL-14 project number in the, Republic of Turkey for their financial support.
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