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Optimization of dispersed LaPO4:Tb nanosol and their photoluminescence properties
Optical Materials 97 (2019) 109366
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Optimization of dispersed LaPO4:Tb nanosol and their photoluminescence properties
T
Mahboob Ullah, Se-Min Ban, Dae-Sung Kim∗ Energy and Environment Division, Korea Institute of Ceramic Engineering and Technology (KICET), 101, Soho-ro, Jinju-si, Gyeongsangnam-do, 52851, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: LaPO4:Tb nanophosphor Photoluminescence Nanosol Bead milling
LaPO4:Tb nanophosphor with different concentrations of Tb3+ 0.05–0.15 mol % has been synthesized via sol-gel process. The synthesized nanophosphor was dried under 80 °C for 24 hrs. Dependent on the different concentration of Tb3+, the optimum concentration for the enhanced luminescence intensity was determined to be 0.15 mol %. The high luminescent nanophosphor with Tb3+ = 0.15 mol % was heat treated at 200–800 °C for 4 hrs. The nanophosphor heat-treated at 600–800 °C showed high photoluminescence (PL) property by monitoring at 254 nm excitation wavelength. The nanophosphor heat-treated at 200–400 °C is well crystalline in the hexagonal phase, but the crystal phase nature is shifted to monoclinic in case of the sample that heat treated above 400 °C. On the basis of weakly agglomerated nanoparticles, the nanophosphor was dispersed and converted into nanosol under controlled bead mill wet process. Based on particle size analyzer (PSA) the obtained nanosol median particle size (D50) was below 200 nm. Finally, the low PL properties of LaPO4:Tb nanosol with their characteristics green color emission was recovered after calcination at 650 °C under N2 atmosphere for 2 hrs. The crystallinity, morphology and particle size of the nanophosphors were investigated using X-ray diffraction (XRD), FE-SEM, and PSA. The nanosol with green color emission prepared in this work potentially meets their applications for security coatings.
1. Introduction As the industries grow rapidly, the counterfeiting of important products and documents raises economic and social problems which considered as a critical issue in the market. With the rapid increase of internet and smart technology, the public can easily counterfeit the mods and technology that violates the property rights of companies, society, also making confusion in the market economy which is very difficult for even experts to identify them [1,2]. To overcome these issues, various researchers are focusing to develop anti-counterfeiting materials, in the form of security pigments, unique markers, plasmonic labels and so forth. To introduce security materials, it should have various characteristics against counterfeiting to maintain the quality and function of the products. Thus, fluorescent nanophosphors are excellent and effective materials against counterfeiting [3,4]. Lanthanide-based nanophosphor (LaPO4) has been an excellent host material for activating Ln3+ doped ions to generate different color emission for various applications [5–7]. These nanophosphors have played an immense role as security materials due to their easy synthesis, high thermal and chemical stability, and excellent emission properties when exposing to ultraviolet (UV) light [6,7]. The
∗
luminescent nanophosphors, based on LaPO4 have been a great interest of their applications in various fields such as lasers, X-ray detectors, cathode ray tubes, fluorescent lamps, optical data storage, and so forth [7–10]. Based on previous investigations, our experimental results have shown that LaPO4 is an excellent host lattice for activating Tb3+ ion [7,11]. Therefore, our group has been focusing on the synthesis of various color emitting nanophosphors using the sol-gel method, their homogeneous dispersed nanosol in an aqueous medium and coating on the surface of product materials [9,10]. For commercial purposes, inorganic phosphors are applied in powder form to spread on a substrate with the help of binders or in the form of nanosol followed by wet coating techniques. In the current investigation, we hypothesized to convert this product into highly dispersed nanosol in an aqueous system under a controlled wet milling process for security coating applications. However, LaPO4:Tb nanophosphors are seemed to agglomerate due to their larger surface area or high surface energy. To prevent the agglomerated nanoparticles formation, their surface must be modified by adding dispersing agents. Based on LaPO4:Tb, there is no report on preparing their nanosol using sol-gel and wet milling process.
Corresponding author. E-mail address: [email protected] (D.-S. Kim).
https://doi.org/10.1016/j.optmat.2019.109366 Received 18 June 2019; Received in revised form 14 August 2019; Accepted 4 September 2019 0925-3467/ © 2019 Elsevier B.V. All rights reserved.
Optical Materials 97 (2019) 109366
M. Ullah, et al.
2. Experimental section
Table 1 Wet milling conditions for dispersing LaPO4:Tb (0.15 mol %) nanophosphor into nanosol.
2.1. Materials
Milling conditions
In this work all precursors’ material for the synthesis of LaPO4:Tb nanophosphor, La2O3 and Tb2O3 (Aldrich, 99.99%), HNO3 (Daejung Chem, 60%), KH2PO4 (99%) and KOH (50%) were of analytical grade and used without further purification. Double distilled water was used throughout the experiment. Acrylate copolymer (BYK Co.) was used as a dispersible agent in an aqueous medium.
In the experimental, 10 wt % La(NO3)3 or Tb(NO3)3 solutions were prepared by the dropwise addition of 6 M nitric acid (HNO3) into La2O3 or Tb2O3 slurry. The resultant solutions were continuously stirred at 75 °C for 24 hrs to completely react the suspended particles. The solutions pH was adjusted between 6 and 7. General chemical reaction for the synthesis of La(NO3)3, Tb(NO3)3 or LaPO4 at ambient temperature is obtained from Eqs. (1)–(3) respectively. (1)
Tb2 O3 + 6HNO3 → 2Tb (NO3)3 + 3H2 O
(2)
La (NO3)3 + KH2 PO4 → LaPO4 + KNO3 + 2HNO3
(3)
Solvent
LaPO4:Tb/BYK wt %
ZrO2 beads wt (kg)
Milling time (min)
LaPO4:Tb (0.15 mol %)
DI water
2/20
1
0–30
dispersing agent and 1 kg ZrO2 beads with size 0.1 Φ was added into 250 ml DI water and milled at 2100 rpm for 0–30 min respectively. The results for wet milling condition can be seen in Table 1.
2.2. Synthesis of LaPO4:Tb nanophosphor
La2 O3 + 6HNO3 → 2La (NO3)3 + 3H2 O
Sample
2.3. Characterization The crystal phase identification was analyzed by Powder X-ray Diffraction (PXRD, DMAX 2500, Rigaku), with an internal standard CuKα as radiation source (1.5405 Å) with a scanning speed of 5°/2θ per minute. The median particle size (D50) was analyzed by particle size analyzer (PSA, ELS-Z2, Otsuka Electronics). Field Emission- Scanning Electron Microscopy (FE-SEM, JSM 6700F, JEOL Ltd.) was used to analyze the morphology of the nanophosphors. The emission and excitation properties of the synthesized nanophosphors were analyzed by spectrophotometer (PerkinElmer LS 50) or commercially available UV hand lamp 254 nm.
According to the formula of La 1-x PO4:Tbx, the Tb content (x) was varied from 0.05 to 0.15 mol %. The total precursor concentration was 0.2 M. The volume for each mixture was adjusted to 50 ml by adding distilled water. The mixtures were stirred for 20 min at room temperature added 0.4 M KH2PO4, 0.6 M KOH 50 ml to the above mixtures and stirred again for 15 min to complete the reaction. After reaction completion, the resultant nanophosphors were filtered under vacuum, washed one time with distilled water, ethanol and dried in an electric oven under 80 °C for 24 hrs. Heat treatment of the nanophosphor were made at 200–800 °C in an electric furnace (5 °C/min) for 4 hrs, then followed by natural cooling to room temperature. The experimental scheme is shown in Fig. 1. For nanodispersion, the agglomerated nanophosphor was converted into nanosol by controlled bead mill wet process. 5 g LaPO4:Tb (0.15 mol %) nanophosphor with 20 wt % acrylate copolymer as a
3. Results and discussion 3.1. X-ray diffraction study of LaPO4:Tb nanophosphor Fig. 2 shows x-ray diffraction (XRD) patterns of the La1-xPO4:Tbx nanophosphors for x = 0.05, 0.10, and 0.15. XRD patterns of each sample exhibit single phase of LaPO4.nH2O with hexagonal structure. The high peak intensity of each sample confirms their good crystallinity and matched well with the characteristics peaks of LaPO4.nH2O (JCPDS No. 46–1439). No additional phases, such as TbPO4 and La3PO7 were detected in the studied range. Based on the aforementioned observation, it can be suggested that Tb3+ have been successfully incorporated into the LaPO4 structure. In the above synthesized sample with x = 0.15 showed excellent photoluminescence properties, which would be discussed later have chosen for the subsequent investigations. Fig. 3 displays the XRD patterns for La1-xPO4:Tbx (x = 0.15) nanophosphors heat treated at 200–600 °C for 4 hrs. No structural changes
Fig. 2. XRD patterns of the synthesized La 1-x PO4:Tbx nanophosphors using (a) 0.05, (b) 0.10, and (c) 0.15 mol % of Tb3+ contents (x value) dried under 80 °C for 24 hrs.
Fig. 1. Experimental process for the synthesis of La 1-x PO4:Tbx nanophosphors to obtain dispersed nanosol. 2
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Fig. 3. XRD patterns of the synthesized LaPO4:Tb (0.15 mol %) nanophosphors heat treated at (a) 200, (b) 400, and (c) 600 °C for 4 hrs.
have been observed for samples treated at 200 °C and 400 °C and the corresponding peaks matched well with the hexagonal phase of LaPO4.nH2O (JCPDS No. 46–1439). However, as the temperature increased to 600 °C, the corresponding peaks transform from hexagonal to monoclinic phase and the corresponding peaks matched with LaPO4:Tb (JCPDS No. 32–0439). The suggested transformation could be due to the dehydration of compound. As in case of low temperature calcination, this phase has sufficient water molecules which surrounded by Ln3+ in the form of Ln (H2O)3+ [12–14]. However, in the case of high temperature calcination i.e., 600 °C, the water molecules is removed that might surround on Ln3+ in [Ln(H2O)]3+, or could be due to Coulombic interactions among Ln3+ and PO43− charges occur to form LnPO4 [15–17].
3.2. Morphological study of LaPO4:Tb nanophosphor 3.2.1. Before milling The surface morphology of the synthesized La1-xPO4:Tbx nanophosphors for x = 0.05, 0.10, and 0.15 dried under 80 °C for 24 hrs displayed in Fig. 4. A needle-like morphology is shown by using 0.05 mol % Tb3+ sample as depicted in Fig. 4a, while an agglomerated like nanoparticles has been obtained by the samples using 0.1 or 0.15 mol % Tb3+ as shown in Fig. 4b-c. Fig. 5 displays the surface morphology of LaPO4:Tb (0.15 mol %) nanophosphors heat-treated at 200-800 °C for 4 hrs. As can be seen in Fig. 5a-c, weakly agglomerated like nanoparticles has been obtained for the samples heat-treated at 200–800 °C for 4 hrs. However, for sample heat-treated at 800 °C for 4 hrs shows bigger and cubic aggregated nanoparticles. For particle size measurement, at least 20 particles were averaged using Mac-View Ver. 4 (MOUNTECH Co. Ltd.) software. It shows that samples heat treated at 200–600 °C have particle size about 76–80 nm, while the sample treated at 800 °C treated sample have an average particle size of 150 nm. This suggest that temperature plays an important role to influence the morphology, structure, and size of the nanoparticles. When the particles are formed by the heating process, they coalesce with each other either form agglomerate or larger particles. The process depends upon the available energy or temperature. That's why the particle size and morphology have a direct relationship with temperature or available energy.
Fig. 4. FE-SEM images of the synthesized La 1-x PO4:Tbx nanophosphors using (a) 0.05, (b) 0.10, and (c) 0.15 mol % of Tb3+ content (x value) dried under 80 °C for 24 hrs (scale bar100 nm).
3.2.2. After milling Fig. 6 displays FE-SEM images for unmilled and 20–30 min milled LaPO4:Tb (0.15 mol %) sample that heat-treated at 200–600 °C for 4 hrs. The results for 800 °C treated milled sample is not included here due to low yield of nanosol, the reason may be due to the hardly aggregated nanoparticles is difficult to disperse. Fig. 6a shows the representative FE-SEM images of unmilled and 20–30 min milled samples that treated at 600 °C for 4 hrs. It can be seen that the unmilled sample consists of agglomerated nanoparticles. However, as the milling time is increased to 20 min, the larger agglomerates are disappeared and uniformly dispersed into nanosol. At 30 min milling, the smaller particles are converted into bigger agglomerates. Fig. 6b shows FE-SEM images for unmilled and after 20–30 min milled sample that treated at 600 °C for 4 hrs. Bigger agglomerated nanoparticles can be seen for the unmilled sample. However, as the milling time is increased to 20 min the larger agglomerate is converted into dispersed nanosol. Furthermore, when the milling time is increased to 30 min the smaller particles re-aggregates and converted into bigger 3
Optical Materials 97 (2019) 109366
M. Ullah, et al.
Fig. 5. FE-SEM images of LaPO4:Tb (0.15 mol %) nanophosphor heat treated at (a) 200, (b) 400, (c) 600, and (d) 800 °C for 4 hrs (scale bar 100 nm).
3.3. Photoluminescence study of LaPO4:Tb nanophosphor
agglomerated like nanoparticles. These results indicate that 20 min milling with a 20 wt % dispersing agent is a better condition to achieve LaPO4:Tb (0.15 mol %) dispersed nanosol. It also indicates that 20 wt% dispersant in LaPO4:Tb nanosol for 20 min milled samples prevents the re-aggregation of nanoparticles in an aqueous medium. The reason of dispersant in the nanosol contributes to the repulsion of electrostatic or steric force together with the help of wetting and dispersing agents chemically. The re-agglomeration of smaller particles into bigger one may be due to the unstable charge on Tb3+ or the dissolution of LaPO4 surfaces in aqueous solution. The phenomenon could also be due to the amorphous or low crystallinity of particles which can easily be attacked by water molecules and settled down due to gravity [9,10].
3.3.1. Before milling Fig. 8 shows photoluminescence (PL) properties of La1-xPO4:Tbx nanophosphor dried under 80 °C for 24 hrs with different concentration of Tb3+ content x = 0.05–0.15 mol%. The excitation spectrum was monitored at the emission wavelength of 545 nm while, the emission spectrum was monitored at 254 nm as an excitation wavelength. In Fig. 8a the strong emission peak from 225 to 240 nm is due to the charge transfer band of oxygen to Tb3+. It can be expected that the electronegativity of La3+ (1.10) is smaller than Tb3+ (2.20), which may cause an easier charge transfer and improve the emission intensity of nanophosphors. The peak observed between 255 and 300 nm could be the transition of La3+ to Tb3+. The weak peak range from 338 to 392 nm is due to the transition of f-f electrons among Tb3+ ions [18]. In Fig. 8b, when the graph is viewed, the emission starts at 490 nm, then falls off near at 518 nm, and then significantly enhances at 545 nm, again goes down at 568 nm, then increases at 586 nm, then decreases at 604 nm and again enhances at 620 nm. The phenomenon is discussed as the weak peak at 490 nm (blue) is due to the transition of electrons from 5D4-7F6 in the Tb3+. The strong green emission band in the range of 545 nm is due to the transition of electrons from 5D4-7F5. The appearance of weak emission band for yellow and red in the range of 586 and 620 nm is attributed to the transition of electrons from 5D4-7f4, 5 D4-7f3 respectively [18]. Generally, the luminescence behavior of nanophosphors significantly decreases by using high contents of activator ions as Tb3+. Referring to previous researchers, the low PL phenomenon is due to the concentration quenching effect and cross relaxation arising among activator ions such as Tb3+-Tb3+ [11,18]. Based on our results, high concentration of Tb3+ contents above their critical level may be due to the concentration quenching effect showed low luminescence property which is not included in this paper. However, the luminescence properties of green emitting nanophosphor are switched on or off by performing low and high Tb3+ contents [19].
3.2.3. Particle size analysis of dispersed LaPO4:Tb nanosol Fig. 7 displays the median particle size (D50) analysis of LaPO4:Tb (0.15 mol%) slurries or nanosols of the unmilled and 20 min milled samples that heat treated at 200, 600, and 800 °C for 4 hrs. The D50 values for these three unmilled samples were found to be 642, 256 and 645 nm respectively. After 20 min milling, the nanosols particle size was reduced and dispersed uniformly in an aqueous medium. The D50 values for the above three samples were reduced to 185, 150, and 175 nm respectively. From these results, all these samples dispersed well but different yield was obtained. The yield for sample heat-treated at 800 °C decreased from over 80% to below 30%. As shown in Fig. 5d that high heat treated particles became bigger and strongly aggregated, this could be due to the easy precipitation and re-aggregation of bigger particles in aqueous solution. Similarly, highly dispersed nanosol without sedimentation was obtained for the 600 °C treated sample. It postulates that the dispersant is easily adsorbed on the surfaces of LaPO4:Tb in aqueous solution to prevent the re-aggregation between particles. It also postulates that good dispersion is due to the weakly agglomerated nanoparticles as can be seen in the FE-SEM image of Fig. 5c.
4
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M. Ullah, et al.
Fig. 6. FE-SEM images of unmilled and milled LaPO4:Tb (0.15 mol %) samples heat treated at (a) 200 and (b) 600 °C for 4 hrs (scale bar 1 μm).
In addition, a dominant peak intensity was found at 0.15 mol % of Tb3+ concentration. Thus for bright green emission, the optimum concentration for Tb3+ was chosen to be 0.15 mol %. Fig. 9 shows the PL properties of the synthesized LaPO4:Tb (0.15 mol %) nanophosphor heat-treated at 200–800 °C for 4 hrs. The excitation wavelengths are monitored at 254 nm. As shown in Fig. 9a, the emission intensity of the nanophosphor heat-treated at 600–800 °C is higher than that treated below this temperature. The inset photographs in Fig. 9a clearly shows their temperature dependent characteristics green color emission from lower to higher after exposure to UV 254 nm hand lamp. The high emission phenomenon is due to the increase of temperature decreases the number of contaminations like, OH, NO3 in the host LaPO4, which definitely enhance the optical property and vice versa [13]. The high or low emission intensity may be also due to the phase transition. As indicated in Fig. 9b, low emission intensity is achieved by the samples that heat treated below 600 °C, this property may be due to the phase transition from hexagonal to monoclinic as indicated in the XRD data. In hexagonal phase there are sufficient amount of water molecules or other contaminants which are
responsible for low optical properties compared to monoclinic phase [13,14]. 3.3.2. After milling Fig. 10 indicates the PL properties of LaPO4:Tb (0.15 mol%) nanophosphor (pristine) that treated at 600 °C, nanosol that calcined in N2 atmosphere for 2 hrs, and nanosol before calcination. Fig. 10a shows the excitation spectra for the above 3 samples. The excitation spectra (λem = 545 nm) in the range of 254–300 nm exhibit a strong absorption band for nanophosphor (pristine), a slightly weak band for nanosol calcined at 650 °C, and a very weak band for nanosol before calcination. Fig. 10b shows emission spectra (λex = 254 nm) for nanophosphor (pristine) exhibit an intense emission band in the range of 545–550 nm. A slightly low emission band is exhibited by the after calcined sample, and a very weak peak is obtained for the nanosol without calcination. Generally, milling of phosphors into smaller particles reduces PL properties due to their reduced crystallinity and the formation of defects on the particle surface. The main reason is due to the finer particles of nanoparticles which causes detriment to the PL properties or 5
Optical Materials 97 (2019) 109366
M. Ullah, et al.
Color purity =
(x − x i )2 + (y − yi )2 (x d − x i )2 + (yd − yi )2
× 100%
Where (x, y) means CIE chromaticity coordinates of the material, (xi, yi) represents white illumination, and (xd, yd) are the dominated wavelengths respectively. The CIE color coordinates values for nanophosphor (pristine), nanosol before and after calcination were investigated to be x, y = (0.31, 0.59), (0.30, 0.58), and (0.31, 0.59) respectively. The color purity for nanophosphor, nanosol before and after calcination were calculated to be 73.9%, 69.3%, and 72.4%. Notably, the 72.4% color purity for nanosol after calcination at 650 °C in N2 atmosphere have an obvious increase which can also be proved from their corresponding CIE values and high emission intensity band.
4. Conclusion LaPO4:Tb nanophosphors with different concentration of Tb3+ (0.05–0.15 mol %) were synthesized via sol-gel method using nontoxic and cheap inorganic precursors. The optimum concentration for emitting green color was chosen to be 0.15 mol %. The excitation wavelength was monitored at 254 nm. The resultant nanophosphors were heat-treated at 200–800 °C for 4 hrs. The nanophosphors showed high photoluminescence (PL) property with a strong green color that treated under high temperature. The homogeneous and dispersed nanosol of weakly agglomerated nanophosphors were successfully achieved under controlled bead mill wet process using 20 wt % acrylate copolymer as a dispersing agent. The median particle size (D50) of nanosol was about 150–200 nm at 20 min milling time. The low PL property of nanosol for 600 °C treated sample with bright green color emission was recovered after calcination at 650 °C for 2 hrs under an N2 atmosphere. The nanosol with green color emission prepared in this work potentially meets their applications for security coatings.
Declaration of competing interest
Fig. 7. Median particle size (D50) of LaPO4:Tb (0.15 mol %) slurries or nanosols for (a) 200, (b) 600 and (c) 800 °C treated samples.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
due to the formation of defects and dislocation on grain surfaces [9,10]. More simply, decrement in PL properties is related to a decrease in particle size. Interestingly, the low PL property of the obtained nanosol was recovered after calcination at 650 °C for 2 hrs under N2 atmosphere. The photographs inside Fig. 10b also shows the recovery of green color from light green to bright green. The color purity of the samples was calculated using the following formula.
Acknowledgments This work was financially supported by the Research Fund of Advanced Technology Center (ATC) Project (Project No. 10052088) under the Ministry of Trade, Industry and Energy (MOTIE) of the Republic of Korea.
Fig. 8. Photoluminescence spectra of the synthesized La1-x PO4:Tbx nanophosphors as a function of Tb3+ content (x value) dried under 80 °C for 24 hrs (a) excitation, (b) emission. 6
Optical Materials 97 (2019) 109366
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Fig. 9. Photoluminescence properties of LaPO4:Tb (0.15 mol %) nanophosphor heattreated at 200–800 °C for 4 hrs (a) emission (λex = 254 nm), the photographs show their characteristic green color after exposure to UV 254 nm hand lamp, (b) dependence of the intensity variation, and their phase change from hexagonal to monoclinic as a function of temperature. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 10. Photoluminescence spectra and photographs of 600 °C treated LaPO4:Tb (0.15 mol %) sample that milled for 20 min and their calcination at 650 °C for 2 hrs in N2 atmosphere.
References
[10] S.M. Ban, J.M. Park, K.Y. Jung, B.K. Choi, K.J. Kang, M.C. Kang, D.S. Kim, Preparation of nanosized Gd2O3:Eu3+ red phosphor coated on mica flake and its luminescent property, J. Korean Powder Metall. Inst. 24 (2017) 457–463. [11] M. Yu, J. Lin, J. Fu, H.J. Zhang, Y.C. Han, Sol-gel synthesis and photoluminescent properties of LaPO4:A (A = Eu3+, Ce3+, Tb3+) nanocrystalline thin films, J. Mater. Chem. 13 (2003) 1413–1419. [12] R.C.L. Mooney, X-ray diffraction study of cerous phosphate and related crystals. I, hexagonal modification, Acta Crystallogr. 3 (1950) 337–340. [13] C. Fu, G. Li, M. Zhao, L. Yang, J. Zheng, L. Li, Solvent-driven room temperature synthesis of nanoparticles BiPO4:Eu3+, Inorg. Chem. 51 (2012) 5869–5880. [14] M. Ferhi, K. Horchani-Naifer, M. Ferid, Hydrothermal synthesis and photoluminescence of the monophosphate LaPO4:Eu (5%), J. Lumin. 128 (2008) 1777–1782. [15] Y.P. Fang, A.W. Xu, R.Q. Song, H.X. Zhang, L.P. You, J.C. Yu, H.Q. Liu, Systematic synthesis and characterization of single-crystal lanthanide orthophosphate nanowires, J. Am. Chem. Soc. 125 (2003) 16025–16034. [16] X. Xiao, B. Yan, Reproducible solvent-thermal synthesis, controlled microstructure, and photoluminescence of REPO4:Eu3+, Tb3+ (RE=Y, La, and Gd) nanophosphor, J. Am. Ceram. Soc. 93 (2010) 2195–2201. [17] C. Zollfrank, H. Scheel, S. Brungs, P. Greil, Europium (III) orthophosphate: synthesis, characterization, and optical properties, Cryst. Growth Des. 8 (2008) 766–770. [18] G. Phaomei, R.S. Ningthoujam, W.R. Singh, R.S. Loitongbam, N.S. Singh, A. Rath, R.R. Juluri, R.K. Vatsa, Luminescence switching behavior through redox reaction in Ce3+ co-doped LaPO4:Tb3+ nanorods: re-dispersible and polymer film, Dalton Trans. 40 (2011) 11571–11580. [19] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer-Verlag Berlin, Heidelberg, 1994, pp. 10–32.
[1] Y. Cui, R.S. Hegde, I.Y. Phang, H.K. Lee, X.Y. Ling, Encoding molecular information in plasmonic nanostructures for anti-counterfeiting applications, Nanoscale 6 (2014) 282–288. [2] K. Jiang, L. Zhang, J. Lu, C. Xu, C. Cai, H. Lin, Triple-mode emission of carbon dots: applications for advanced anti-counterfeiting, Angew. Chem. Int. 55 (2016) 7231–7235. [3] K.Y. Jung, B.H. Min, J.C. Lee, D.S. Kim, B.K. Choi, W.J. Kang, An aerosol synthesis of CeO2:Eu3+/Na+ red nanophosphor with enhanced photoluminescence, RSC Adv. 6 (2016) 81203–81210. [4] L. Li, Technology designed to combat fakes in the global supply chain, Bus. Horiz. 56 (2013) 167–177. [5] Z. Wang, J.G. Li, Qi Zhu, B.N. Kim, X. Sun, Tartrate promoted hydrothermal growth of highly [001] oriented (La0.95-xBixEu0.05)PO4 (x=0-0.01) nanowires with enhanced photoluminescence, Mater. Des. 126 (2017) 115–122. [6] K.Y. Jung, J.H. Han, D.S. Kim, B.K. Choi, W.J. Kang, Aerosol synthesis of Gd2O3:Eu/ Bi nanophosphor for preparation of photofunctional pearl pigment as security material, J. Korean Chem. Soc. 55 (2018) 461–472. [7] U. Rambabu, N.R. Munirathnam, T.L. Prakash, S. Buddhudu, Emission spectra of LnPO4:RE3+ (Ln= La, Gd; RE= Eu, Tb and Ce) powder phosphors, Mater. Chem. Phys. 78 (2003) 160–169. [8] A.P.D. Silva, V.A. Fassel, X-ray excited optical fluorescence of trace rare earths in yttrium phosphate and yttrium vanadate hosts, part per giga level determination of rare earth impurities in yttrium oxide, Anal. Chem. 45 (1973) 542–547. [9] J.M. Park, S.M. Ban, K.Y. Jung, B.K. Choi, K.J. Kang, D.S. Kim, Dispersion and shape control on nanoparticles of Gd2O3:Eu3+ red phosphor prepared by template method, Korean J. Mater. Res. 27 (2017) 534–543.
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Optimization of dispersed LaPO4:Tb nanosol and their photoluminescence properties
T
Mahboob Ullah, Se-Min Ban, Dae-Sung Kim∗ Energy and Environment Division, Korea Institute of Ceramic Engineering and Technology (KICET), 101, Soho-ro, Jinju-si, Gyeongsangnam-do, 52851, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: LaPO4:Tb nanophosphor Photoluminescence Nanosol Bead milling
LaPO4:Tb nanophosphor with different concentrations of Tb3+ 0.05–0.15 mol % has been synthesized via sol-gel process. The synthesized nanophosphor was dried under 80 °C for 24 hrs. Dependent on the different concentration of Tb3+, the optimum concentration for the enhanced luminescence intensity was determined to be 0.15 mol %. The high luminescent nanophosphor with Tb3+ = 0.15 mol % was heat treated at 200–800 °C for 4 hrs. The nanophosphor heat-treated at 600–800 °C showed high photoluminescence (PL) property by monitoring at 254 nm excitation wavelength. The nanophosphor heat-treated at 200–400 °C is well crystalline in the hexagonal phase, but the crystal phase nature is shifted to monoclinic in case of the sample that heat treated above 400 °C. On the basis of weakly agglomerated nanoparticles, the nanophosphor was dispersed and converted into nanosol under controlled bead mill wet process. Based on particle size analyzer (PSA) the obtained nanosol median particle size (D50) was below 200 nm. Finally, the low PL properties of LaPO4:Tb nanosol with their characteristics green color emission was recovered after calcination at 650 °C under N2 atmosphere for 2 hrs. The crystallinity, morphology and particle size of the nanophosphors were investigated using X-ray diffraction (XRD), FE-SEM, and PSA. The nanosol with green color emission prepared in this work potentially meets their applications for security coatings.
1. Introduction As the industries grow rapidly, the counterfeiting of important products and documents raises economic and social problems which considered as a critical issue in the market. With the rapid increase of internet and smart technology, the public can easily counterfeit the mods and technology that violates the property rights of companies, society, also making confusion in the market economy which is very difficult for even experts to identify them [1,2]. To overcome these issues, various researchers are focusing to develop anti-counterfeiting materials, in the form of security pigments, unique markers, plasmonic labels and so forth. To introduce security materials, it should have various characteristics against counterfeiting to maintain the quality and function of the products. Thus, fluorescent nanophosphors are excellent and effective materials against counterfeiting [3,4]. Lanthanide-based nanophosphor (LaPO4) has been an excellent host material for activating Ln3+ doped ions to generate different color emission for various applications [5–7]. These nanophosphors have played an immense role as security materials due to their easy synthesis, high thermal and chemical stability, and excellent emission properties when exposing to ultraviolet (UV) light [6,7]. The
∗
luminescent nanophosphors, based on LaPO4 have been a great interest of their applications in various fields such as lasers, X-ray detectors, cathode ray tubes, fluorescent lamps, optical data storage, and so forth [7–10]. Based on previous investigations, our experimental results have shown that LaPO4 is an excellent host lattice for activating Tb3+ ion [7,11]. Therefore, our group has been focusing on the synthesis of various color emitting nanophosphors using the sol-gel method, their homogeneous dispersed nanosol in an aqueous medium and coating on the surface of product materials [9,10]. For commercial purposes, inorganic phosphors are applied in powder form to spread on a substrate with the help of binders or in the form of nanosol followed by wet coating techniques. In the current investigation, we hypothesized to convert this product into highly dispersed nanosol in an aqueous system under a controlled wet milling process for security coating applications. However, LaPO4:Tb nanophosphors are seemed to agglomerate due to their larger surface area or high surface energy. To prevent the agglomerated nanoparticles formation, their surface must be modified by adding dispersing agents. Based on LaPO4:Tb, there is no report on preparing their nanosol using sol-gel and wet milling process.
Corresponding author. E-mail address: [email protected] (D.-S. Kim).
https://doi.org/10.1016/j.optmat.2019.109366 Received 18 June 2019; Received in revised form 14 August 2019; Accepted 4 September 2019 0925-3467/ © 2019 Elsevier B.V. All rights reserved.
Optical Materials 97 (2019) 109366
M. Ullah, et al.
2. Experimental section
Table 1 Wet milling conditions for dispersing LaPO4:Tb (0.15 mol %) nanophosphor into nanosol.
2.1. Materials
Milling conditions
In this work all precursors’ material for the synthesis of LaPO4:Tb nanophosphor, La2O3 and Tb2O3 (Aldrich, 99.99%), HNO3 (Daejung Chem, 60%), KH2PO4 (99%) and KOH (50%) were of analytical grade and used without further purification. Double distilled water was used throughout the experiment. Acrylate copolymer (BYK Co.) was used as a dispersible agent in an aqueous medium.
In the experimental, 10 wt % La(NO3)3 or Tb(NO3)3 solutions were prepared by the dropwise addition of 6 M nitric acid (HNO3) into La2O3 or Tb2O3 slurry. The resultant solutions were continuously stirred at 75 °C for 24 hrs to completely react the suspended particles. The solutions pH was adjusted between 6 and 7. General chemical reaction for the synthesis of La(NO3)3, Tb(NO3)3 or LaPO4 at ambient temperature is obtained from Eqs. (1)–(3) respectively. (1)
Tb2 O3 + 6HNO3 → 2Tb (NO3)3 + 3H2 O
(2)
La (NO3)3 + KH2 PO4 → LaPO4 + KNO3 + 2HNO3
(3)
Solvent
LaPO4:Tb/BYK wt %
ZrO2 beads wt (kg)
Milling time (min)
LaPO4:Tb (0.15 mol %)
DI water
2/20
1
0–30
dispersing agent and 1 kg ZrO2 beads with size 0.1 Φ was added into 250 ml DI water and milled at 2100 rpm for 0–30 min respectively. The results for wet milling condition can be seen in Table 1.
2.2. Synthesis of LaPO4:Tb nanophosphor
La2 O3 + 6HNO3 → 2La (NO3)3 + 3H2 O
Sample
2.3. Characterization The crystal phase identification was analyzed by Powder X-ray Diffraction (PXRD, DMAX 2500, Rigaku), with an internal standard CuKα as radiation source (1.5405 Å) with a scanning speed of 5°/2θ per minute. The median particle size (D50) was analyzed by particle size analyzer (PSA, ELS-Z2, Otsuka Electronics). Field Emission- Scanning Electron Microscopy (FE-SEM, JSM 6700F, JEOL Ltd.) was used to analyze the morphology of the nanophosphors. The emission and excitation properties of the synthesized nanophosphors were analyzed by spectrophotometer (PerkinElmer LS 50) or commercially available UV hand lamp 254 nm.
According to the formula of La 1-x PO4:Tbx, the Tb content (x) was varied from 0.05 to 0.15 mol %. The total precursor concentration was 0.2 M. The volume for each mixture was adjusted to 50 ml by adding distilled water. The mixtures were stirred for 20 min at room temperature added 0.4 M KH2PO4, 0.6 M KOH 50 ml to the above mixtures and stirred again for 15 min to complete the reaction. After reaction completion, the resultant nanophosphors were filtered under vacuum, washed one time with distilled water, ethanol and dried in an electric oven under 80 °C for 24 hrs. Heat treatment of the nanophosphor were made at 200–800 °C in an electric furnace (5 °C/min) for 4 hrs, then followed by natural cooling to room temperature. The experimental scheme is shown in Fig. 1. For nanodispersion, the agglomerated nanophosphor was converted into nanosol by controlled bead mill wet process. 5 g LaPO4:Tb (0.15 mol %) nanophosphor with 20 wt % acrylate copolymer as a
3. Results and discussion 3.1. X-ray diffraction study of LaPO4:Tb nanophosphor Fig. 2 shows x-ray diffraction (XRD) patterns of the La1-xPO4:Tbx nanophosphors for x = 0.05, 0.10, and 0.15. XRD patterns of each sample exhibit single phase of LaPO4.nH2O with hexagonal structure. The high peak intensity of each sample confirms their good crystallinity and matched well with the characteristics peaks of LaPO4.nH2O (JCPDS No. 46–1439). No additional phases, such as TbPO4 and La3PO7 were detected in the studied range. Based on the aforementioned observation, it can be suggested that Tb3+ have been successfully incorporated into the LaPO4 structure. In the above synthesized sample with x = 0.15 showed excellent photoluminescence properties, which would be discussed later have chosen for the subsequent investigations. Fig. 3 displays the XRD patterns for La1-xPO4:Tbx (x = 0.15) nanophosphors heat treated at 200–600 °C for 4 hrs. No structural changes
Fig. 2. XRD patterns of the synthesized La 1-x PO4:Tbx nanophosphors using (a) 0.05, (b) 0.10, and (c) 0.15 mol % of Tb3+ contents (x value) dried under 80 °C for 24 hrs.
Fig. 1. Experimental process for the synthesis of La 1-x PO4:Tbx nanophosphors to obtain dispersed nanosol. 2
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Fig. 3. XRD patterns of the synthesized LaPO4:Tb (0.15 mol %) nanophosphors heat treated at (a) 200, (b) 400, and (c) 600 °C for 4 hrs.
have been observed for samples treated at 200 °C and 400 °C and the corresponding peaks matched well with the hexagonal phase of LaPO4.nH2O (JCPDS No. 46–1439). However, as the temperature increased to 600 °C, the corresponding peaks transform from hexagonal to monoclinic phase and the corresponding peaks matched with LaPO4:Tb (JCPDS No. 32–0439). The suggested transformation could be due to the dehydration of compound. As in case of low temperature calcination, this phase has sufficient water molecules which surrounded by Ln3+ in the form of Ln (H2O)3+ [12–14]. However, in the case of high temperature calcination i.e., 600 °C, the water molecules is removed that might surround on Ln3+ in [Ln(H2O)]3+, or could be due to Coulombic interactions among Ln3+ and PO43− charges occur to form LnPO4 [15–17].
3.2. Morphological study of LaPO4:Tb nanophosphor 3.2.1. Before milling The surface morphology of the synthesized La1-xPO4:Tbx nanophosphors for x = 0.05, 0.10, and 0.15 dried under 80 °C for 24 hrs displayed in Fig. 4. A needle-like morphology is shown by using 0.05 mol % Tb3+ sample as depicted in Fig. 4a, while an agglomerated like nanoparticles has been obtained by the samples using 0.1 or 0.15 mol % Tb3+ as shown in Fig. 4b-c. Fig. 5 displays the surface morphology of LaPO4:Tb (0.15 mol %) nanophosphors heat-treated at 200-800 °C for 4 hrs. As can be seen in Fig. 5a-c, weakly agglomerated like nanoparticles has been obtained for the samples heat-treated at 200–800 °C for 4 hrs. However, for sample heat-treated at 800 °C for 4 hrs shows bigger and cubic aggregated nanoparticles. For particle size measurement, at least 20 particles were averaged using Mac-View Ver. 4 (MOUNTECH Co. Ltd.) software. It shows that samples heat treated at 200–600 °C have particle size about 76–80 nm, while the sample treated at 800 °C treated sample have an average particle size of 150 nm. This suggest that temperature plays an important role to influence the morphology, structure, and size of the nanoparticles. When the particles are formed by the heating process, they coalesce with each other either form agglomerate or larger particles. The process depends upon the available energy or temperature. That's why the particle size and morphology have a direct relationship with temperature or available energy.
Fig. 4. FE-SEM images of the synthesized La 1-x PO4:Tbx nanophosphors using (a) 0.05, (b) 0.10, and (c) 0.15 mol % of Tb3+ content (x value) dried under 80 °C for 24 hrs (scale bar100 nm).
3.2.2. After milling Fig. 6 displays FE-SEM images for unmilled and 20–30 min milled LaPO4:Tb (0.15 mol %) sample that heat-treated at 200–600 °C for 4 hrs. The results for 800 °C treated milled sample is not included here due to low yield of nanosol, the reason may be due to the hardly aggregated nanoparticles is difficult to disperse. Fig. 6a shows the representative FE-SEM images of unmilled and 20–30 min milled samples that treated at 600 °C for 4 hrs. It can be seen that the unmilled sample consists of agglomerated nanoparticles. However, as the milling time is increased to 20 min, the larger agglomerates are disappeared and uniformly dispersed into nanosol. At 30 min milling, the smaller particles are converted into bigger agglomerates. Fig. 6b shows FE-SEM images for unmilled and after 20–30 min milled sample that treated at 600 °C for 4 hrs. Bigger agglomerated nanoparticles can be seen for the unmilled sample. However, as the milling time is increased to 20 min the larger agglomerate is converted into dispersed nanosol. Furthermore, when the milling time is increased to 30 min the smaller particles re-aggregates and converted into bigger 3
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Fig. 5. FE-SEM images of LaPO4:Tb (0.15 mol %) nanophosphor heat treated at (a) 200, (b) 400, (c) 600, and (d) 800 °C for 4 hrs (scale bar 100 nm).
3.3. Photoluminescence study of LaPO4:Tb nanophosphor
agglomerated like nanoparticles. These results indicate that 20 min milling with a 20 wt % dispersing agent is a better condition to achieve LaPO4:Tb (0.15 mol %) dispersed nanosol. It also indicates that 20 wt% dispersant in LaPO4:Tb nanosol for 20 min milled samples prevents the re-aggregation of nanoparticles in an aqueous medium. The reason of dispersant in the nanosol contributes to the repulsion of electrostatic or steric force together with the help of wetting and dispersing agents chemically. The re-agglomeration of smaller particles into bigger one may be due to the unstable charge on Tb3+ or the dissolution of LaPO4 surfaces in aqueous solution. The phenomenon could also be due to the amorphous or low crystallinity of particles which can easily be attacked by water molecules and settled down due to gravity [9,10].
3.3.1. Before milling Fig. 8 shows photoluminescence (PL) properties of La1-xPO4:Tbx nanophosphor dried under 80 °C for 24 hrs with different concentration of Tb3+ content x = 0.05–0.15 mol%. The excitation spectrum was monitored at the emission wavelength of 545 nm while, the emission spectrum was monitored at 254 nm as an excitation wavelength. In Fig. 8a the strong emission peak from 225 to 240 nm is due to the charge transfer band of oxygen to Tb3+. It can be expected that the electronegativity of La3+ (1.10) is smaller than Tb3+ (2.20), which may cause an easier charge transfer and improve the emission intensity of nanophosphors. The peak observed between 255 and 300 nm could be the transition of La3+ to Tb3+. The weak peak range from 338 to 392 nm is due to the transition of f-f electrons among Tb3+ ions [18]. In Fig. 8b, when the graph is viewed, the emission starts at 490 nm, then falls off near at 518 nm, and then significantly enhances at 545 nm, again goes down at 568 nm, then increases at 586 nm, then decreases at 604 nm and again enhances at 620 nm. The phenomenon is discussed as the weak peak at 490 nm (blue) is due to the transition of electrons from 5D4-7F6 in the Tb3+. The strong green emission band in the range of 545 nm is due to the transition of electrons from 5D4-7F5. The appearance of weak emission band for yellow and red in the range of 586 and 620 nm is attributed to the transition of electrons from 5D4-7f4, 5 D4-7f3 respectively [18]. Generally, the luminescence behavior of nanophosphors significantly decreases by using high contents of activator ions as Tb3+. Referring to previous researchers, the low PL phenomenon is due to the concentration quenching effect and cross relaxation arising among activator ions such as Tb3+-Tb3+ [11,18]. Based on our results, high concentration of Tb3+ contents above their critical level may be due to the concentration quenching effect showed low luminescence property which is not included in this paper. However, the luminescence properties of green emitting nanophosphor are switched on or off by performing low and high Tb3+ contents [19].
3.2.3. Particle size analysis of dispersed LaPO4:Tb nanosol Fig. 7 displays the median particle size (D50) analysis of LaPO4:Tb (0.15 mol%) slurries or nanosols of the unmilled and 20 min milled samples that heat treated at 200, 600, and 800 °C for 4 hrs. The D50 values for these three unmilled samples were found to be 642, 256 and 645 nm respectively. After 20 min milling, the nanosols particle size was reduced and dispersed uniformly in an aqueous medium. The D50 values for the above three samples were reduced to 185, 150, and 175 nm respectively. From these results, all these samples dispersed well but different yield was obtained. The yield for sample heat-treated at 800 °C decreased from over 80% to below 30%. As shown in Fig. 5d that high heat treated particles became bigger and strongly aggregated, this could be due to the easy precipitation and re-aggregation of bigger particles in aqueous solution. Similarly, highly dispersed nanosol without sedimentation was obtained for the 600 °C treated sample. It postulates that the dispersant is easily adsorbed on the surfaces of LaPO4:Tb in aqueous solution to prevent the re-aggregation between particles. It also postulates that good dispersion is due to the weakly agglomerated nanoparticles as can be seen in the FE-SEM image of Fig. 5c.
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Fig. 6. FE-SEM images of unmilled and milled LaPO4:Tb (0.15 mol %) samples heat treated at (a) 200 and (b) 600 °C for 4 hrs (scale bar 1 μm).
In addition, a dominant peak intensity was found at 0.15 mol % of Tb3+ concentration. Thus for bright green emission, the optimum concentration for Tb3+ was chosen to be 0.15 mol %. Fig. 9 shows the PL properties of the synthesized LaPO4:Tb (0.15 mol %) nanophosphor heat-treated at 200–800 °C for 4 hrs. The excitation wavelengths are monitored at 254 nm. As shown in Fig. 9a, the emission intensity of the nanophosphor heat-treated at 600–800 °C is higher than that treated below this temperature. The inset photographs in Fig. 9a clearly shows their temperature dependent characteristics green color emission from lower to higher after exposure to UV 254 nm hand lamp. The high emission phenomenon is due to the increase of temperature decreases the number of contaminations like, OH, NO3 in the host LaPO4, which definitely enhance the optical property and vice versa [13]. The high or low emission intensity may be also due to the phase transition. As indicated in Fig. 9b, low emission intensity is achieved by the samples that heat treated below 600 °C, this property may be due to the phase transition from hexagonal to monoclinic as indicated in the XRD data. In hexagonal phase there are sufficient amount of water molecules or other contaminants which are
responsible for low optical properties compared to monoclinic phase [13,14]. 3.3.2. After milling Fig. 10 indicates the PL properties of LaPO4:Tb (0.15 mol%) nanophosphor (pristine) that treated at 600 °C, nanosol that calcined in N2 atmosphere for 2 hrs, and nanosol before calcination. Fig. 10a shows the excitation spectra for the above 3 samples. The excitation spectra (λem = 545 nm) in the range of 254–300 nm exhibit a strong absorption band for nanophosphor (pristine), a slightly weak band for nanosol calcined at 650 °C, and a very weak band for nanosol before calcination. Fig. 10b shows emission spectra (λex = 254 nm) for nanophosphor (pristine) exhibit an intense emission band in the range of 545–550 nm. A slightly low emission band is exhibited by the after calcined sample, and a very weak peak is obtained for the nanosol without calcination. Generally, milling of phosphors into smaller particles reduces PL properties due to their reduced crystallinity and the formation of defects on the particle surface. The main reason is due to the finer particles of nanoparticles which causes detriment to the PL properties or 5
Optical Materials 97 (2019) 109366
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Color purity =
(x − x i )2 + (y − yi )2 (x d − x i )2 + (yd − yi )2
× 100%
Where (x, y) means CIE chromaticity coordinates of the material, (xi, yi) represents white illumination, and (xd, yd) are the dominated wavelengths respectively. The CIE color coordinates values for nanophosphor (pristine), nanosol before and after calcination were investigated to be x, y = (0.31, 0.59), (0.30, 0.58), and (0.31, 0.59) respectively. The color purity for nanophosphor, nanosol before and after calcination were calculated to be 73.9%, 69.3%, and 72.4%. Notably, the 72.4% color purity for nanosol after calcination at 650 °C in N2 atmosphere have an obvious increase which can also be proved from their corresponding CIE values and high emission intensity band.
4. Conclusion LaPO4:Tb nanophosphors with different concentration of Tb3+ (0.05–0.15 mol %) were synthesized via sol-gel method using nontoxic and cheap inorganic precursors. The optimum concentration for emitting green color was chosen to be 0.15 mol %. The excitation wavelength was monitored at 254 nm. The resultant nanophosphors were heat-treated at 200–800 °C for 4 hrs. The nanophosphors showed high photoluminescence (PL) property with a strong green color that treated under high temperature. The homogeneous and dispersed nanosol of weakly agglomerated nanophosphors were successfully achieved under controlled bead mill wet process using 20 wt % acrylate copolymer as a dispersing agent. The median particle size (D50) of nanosol was about 150–200 nm at 20 min milling time. The low PL property of nanosol for 600 °C treated sample with bright green color emission was recovered after calcination at 650 °C for 2 hrs under an N2 atmosphere. The nanosol with green color emission prepared in this work potentially meets their applications for security coatings.
Declaration of competing interest
Fig. 7. Median particle size (D50) of LaPO4:Tb (0.15 mol %) slurries or nanosols for (a) 200, (b) 600 and (c) 800 °C treated samples.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
due to the formation of defects and dislocation on grain surfaces [9,10]. More simply, decrement in PL properties is related to a decrease in particle size. Interestingly, the low PL property of the obtained nanosol was recovered after calcination at 650 °C for 2 hrs under N2 atmosphere. The photographs inside Fig. 10b also shows the recovery of green color from light green to bright green. The color purity of the samples was calculated using the following formula.
Acknowledgments This work was financially supported by the Research Fund of Advanced Technology Center (ATC) Project (Project No. 10052088) under the Ministry of Trade, Industry and Energy (MOTIE) of the Republic of Korea.
Fig. 8. Photoluminescence spectra of the synthesized La1-x PO4:Tbx nanophosphors as a function of Tb3+ content (x value) dried under 80 °C for 24 hrs (a) excitation, (b) emission. 6
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Fig. 9. Photoluminescence properties of LaPO4:Tb (0.15 mol %) nanophosphor heattreated at 200–800 °C for 4 hrs (a) emission (λex = 254 nm), the photographs show their characteristic green color after exposure to UV 254 nm hand lamp, (b) dependence of the intensity variation, and their phase change from hexagonal to monoclinic as a function of temperature. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 10. Photoluminescence spectra and photographs of 600 °C treated LaPO4:Tb (0.15 mol %) sample that milled for 20 min and their calcination at 650 °C for 2 hrs in N2 atmosphere.
References
[10] S.M. Ban, J.M. Park, K.Y. Jung, B.K. Choi, K.J. Kang, M.C. Kang, D.S. Kim, Preparation of nanosized Gd2O3:Eu3+ red phosphor coated on mica flake and its luminescent property, J. Korean Powder Metall. Inst. 24 (2017) 457–463. [11] M. Yu, J. Lin, J. Fu, H.J. Zhang, Y.C. Han, Sol-gel synthesis and photoluminescent properties of LaPO4:A (A = Eu3+, Ce3+, Tb3+) nanocrystalline thin films, J. Mater. Chem. 13 (2003) 1413–1419. [12] R.C.L. Mooney, X-ray diffraction study of cerous phosphate and related crystals. I, hexagonal modification, Acta Crystallogr. 3 (1950) 337–340. [13] C. Fu, G. Li, M. Zhao, L. Yang, J. Zheng, L. Li, Solvent-driven room temperature synthesis of nanoparticles BiPO4:Eu3+, Inorg. Chem. 51 (2012) 5869–5880. [14] M. Ferhi, K. Horchani-Naifer, M. Ferid, Hydrothermal synthesis and photoluminescence of the monophosphate LaPO4:Eu (5%), J. Lumin. 128 (2008) 1777–1782. [15] Y.P. Fang, A.W. Xu, R.Q. Song, H.X. Zhang, L.P. You, J.C. Yu, H.Q. Liu, Systematic synthesis and characterization of single-crystal lanthanide orthophosphate nanowires, J. Am. Chem. Soc. 125 (2003) 16025–16034. [16] X. Xiao, B. Yan, Reproducible solvent-thermal synthesis, controlled microstructure, and photoluminescence of REPO4:Eu3+, Tb3+ (RE=Y, La, and Gd) nanophosphor, J. Am. Ceram. Soc. 93 (2010) 2195–2201. [17] C. Zollfrank, H. Scheel, S. Brungs, P. Greil, Europium (III) orthophosphate: synthesis, characterization, and optical properties, Cryst. Growth Des. 8 (2008) 766–770. [18] G. Phaomei, R.S. Ningthoujam, W.R. Singh, R.S. Loitongbam, N.S. Singh, A. Rath, R.R. Juluri, R.K. Vatsa, Luminescence switching behavior through redox reaction in Ce3+ co-doped LaPO4:Tb3+ nanorods: re-dispersible and polymer film, Dalton Trans. 40 (2011) 11571–11580. [19] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer-Verlag Berlin, Heidelberg, 1994, pp. 10–32.
[1] Y. Cui, R.S. Hegde, I.Y. Phang, H.K. Lee, X.Y. Ling, Encoding molecular information in plasmonic nanostructures for anti-counterfeiting applications, Nanoscale 6 (2014) 282–288. [2] K. Jiang, L. Zhang, J. Lu, C. Xu, C. Cai, H. Lin, Triple-mode emission of carbon dots: applications for advanced anti-counterfeiting, Angew. Chem. Int. 55 (2016) 7231–7235. [3] K.Y. Jung, B.H. Min, J.C. Lee, D.S. Kim, B.K. Choi, W.J. Kang, An aerosol synthesis of CeO2:Eu3+/Na+ red nanophosphor with enhanced photoluminescence, RSC Adv. 6 (2016) 81203–81210. [4] L. Li, Technology designed to combat fakes in the global supply chain, Bus. Horiz. 56 (2013) 167–177. [5] Z. Wang, J.G. Li, Qi Zhu, B.N. Kim, X. Sun, Tartrate promoted hydrothermal growth of highly [001] oriented (La0.95-xBixEu0.05)PO4 (x=0-0.01) nanowires with enhanced photoluminescence, Mater. Des. 126 (2017) 115–122. [6] K.Y. Jung, J.H. Han, D.S. Kim, B.K. Choi, W.J. Kang, Aerosol synthesis of Gd2O3:Eu/ Bi nanophosphor for preparation of photofunctional pearl pigment as security material, J. Korean Chem. Soc. 55 (2018) 461–472. [7] U. Rambabu, N.R. Munirathnam, T.L. Prakash, S. Buddhudu, Emission spectra of LnPO4:RE3+ (Ln= La, Gd; RE= Eu, Tb and Ce) powder phosphors, Mater. Chem. Phys. 78 (2003) 160–169. [8] A.P.D. Silva, V.A. Fassel, X-ray excited optical fluorescence of trace rare earths in yttrium phosphate and yttrium vanadate hosts, part per giga level determination of rare earth impurities in yttrium oxide, Anal. Chem. 45 (1973) 542–547. [9] J.M. Park, S.M. Ban, K.Y. Jung, B.K. Choi, K.J. Kang, D.S. Kim, Dispersion and shape control on nanoparticles of Gd2O3:Eu3+ red phosphor prepared by template method, Korean J. Mater. Res. 27 (2017) 534–543.
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