- Email: [email protected]
HF-free synthesis of K2SiF6 and BaSiF6 nanoparticles by thermal decomposition
Journal Pre-proofs HF-free synthesis of K2SiF6 and BaSiF6 nanoparticles by thermal decomposition Vinod Kumar, Audrey Potdevin, Philippe Boutinaud, Damien Boyer PII: DOI: Reference:
S0167-577X(19)31755-0 https://doi.org/10.1016/j.matlet.2019.127123 MLBLUE 127123
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
24 October 2019 19 November 2019 4 December 2019
Please cite this article as: V. Kumar, A. Potdevin, P. Boutinaud, D. Boyer, HF-free synthesis of K2SiF6 and BaSiF6 nanoparticles by thermal decomposition, Materials Letters (2019), doi: https://doi.org/10.1016/j.matlet. 2019.127123
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 B.V.
HF-free synthesis of K2SiF6 and BaSiF6 nanoparticles by thermal decomposition Vinod KUMARa, Audrey POTDEVINa, Philippe BOUTINAUDa and Damien BOYERa* a Université
Clermont Auvergne, CNRS, SIGMA Clermont, ICCF, F-63000 Clermont–
Ferrand, France. *Corresponding author. Tel. : + 33 4 73 40 76 47 E-mail address: [email protected]
ABSTRACT K2SiF6 and BaSiF6 nanoparticles are synthesized for the first time by thermal decomposition of a mixture consisting of oleic acid and 1-octadecene using ammonium fluoride as fluorine source instead of highly toxic hydrofluoric acid. The structural and morphological properties of the asobtained nanocrystals are investigated.
Keywords: Thermal decomposition; KSF; Nanoparticles; HF-free INTRODUCTION During the two last decades, phosphor-converted white LEDs (pc-WLEDs) have attracted considerable attention due to their low power consumption, very good luminous efficiency, long lifetime, robustness and low environmental impact. Most of the pc-LEDs contain phosphors doped with rare-earth (RE) ions such as divalent europium and trivalent cerium [1, 2] . These ions exhibit broad emission bands and can be easily excited by a blue light with high quantum
efficiency. To improve the color rendering index of the white LEDs, red phosphors are generally added to the blue LED chip and yellow phosphor (YAG:Ce) combination [2]. These red phosphors must fulfill several requirements such as a good stability upon photonic and thermal stresses in addition to a high emission quantum yield. Recently, a new family of efficient narrow-band red-emitting phosphors has emerged. They consist of a fluoride host lattice, either X2MF6 (X = K, Na, and Cs; M = Si, Ge, Zr, Sn, and Ti) or ZSiF6 (Z = Ba or Zn), activated with Mn4+ transition metal ions. These phosphors exhibit sharp and intense emission lines below 650 nm and show good thermal stability under the operating temperatures of LEDs [3-5]. Besides, in the race for miniaturization of optical systems like lighting or display devices, the demand for nanophosphors is dramatically increasing. Since then, many synthesis methods have been experienced already for the preparation of potassium silicon fluoride (K2SiF6 or KSF) and barium fluorosilicate (BaSiF6 or BSF) nanoparticles [4, 6-8]. Most of them, however, use hazardous and non-eco-friendly aqueous HF as fluorine source. This highly corrosive contactpoison can cause severe skin burns and eye damage, is fatal if swallowed, fatal in contact with skin and fatal if inhaled. Substitution of aqueous HF to another source of fluorine ions is therefore highly recommended and has entailed a lot of interest in recent years [9, 10]. In this Letter, we introduce a new HF-free process based on thermal decomposition (or thermolysis) method for preparing undoped KSF and BSF nanoparticles. This method has already been employed for synthesizing NaLnF4 (Ln = Y3+, Gd3+) nanoparticles in a mixture of octadecene (OD) and oleic acid (OA) at high temperature[11-13] but never applied to fluorides like KSF and BSF.
EXPERIMENTAL SECTION The methodology is as follows: for KSF, 4 mmol of potassium fluoride is dissolved in absolute methanol and stirred for 5 min. After dissolution, 8 mmol of ammonium fluoride is poured into the solution and stirred 15 min at room temperature. Then 6 mL of OA and 15 mL of OD are added and the solution is successively heated at 120°C for 30 min in air to remove the 2
methanol/water content and then at 120°C for 30 min under vacuum. Methanol started to evaporate at 65°C. After cooling down to room temperature under vacuum, 2 mmol of tetraethyl orthosilicate (TEOS) are added dropwise into the solution under stirring. The temperature of the solution is then raised to 320°C for 30 min under argon (Ar) atmosphere and maintained at this temperature for 1 h under Ar flow before being cooled down to 80°C in Ar and then to room temperature (RT) in air. Finally, 10 mL of absolute ethanol is added to precipitate the nanoparticles. For BSF, 6 mmol of ammonium fluoride is first dissolved with absolute methanol under stirring. After complete dissolution, 1 mmol of TEOS is injected dropwise and stirred 30 min at RT. OA and OD, respectively 6 mL and 15 mL, are then added to this solution, which was heated at 120°C for 30 min in air and then under vacuum to remove the methanol/water content. The Ba solution is prepared separately by dissolving Ba(Ac)2 (1 mmol) into 3 ml of acetic acid under stirring for 30 min at RT. This Ba solution is then incorporated to the fluoride solution. This final mixture is heated at 120°C for 30 min under air and 30 min under vacuum before being cooled down to RT. It is heated again at 320°C for 1h under Ar atmosphere before the addition 10 ml of ethanol. In both cases, a suspension of nanoparticles is obtained. The particles are collected by centrifugation at 10000 rpm for 20 min, washed twice in a mixture of 20 ml of ethanol and cyclohexane in the volume ratio of 1:1 and dried overnight at 80°C in air oven. The X-ray diffraction (XRD) patterns of the undoped KSF/BSF powders were recorded with a Philips Xpert Pro diffractometer operating with the Cu-Kα1 radiation (λ= 1.5406 Å). Transmission Electronic Microscopy (TEM) images were recorded on a Hitachi H-7650 microscope. Scanning electron (SEM) images of the samples were collected on a ZEISS Supra 55 FEG-VP microscope operated at 3kV. FTIR spectra were obtained on polycrystalline samples with a Thermo Electron (Nicolet 5700-FTIR model) spectrometer in the ATR mode. TGA measurements were carried out using a Setaram SetSys Evolution instrument, in air in two
3
steps, first up to 100 °C at a rate of 10°C/min for 30 min to remove traces of organics and moistures and then up to 800°C at a rate of 10°C/min.
RESULTS AND DISCUSSION The XRD patterns of the as-obtained KSF and BSF nanoparticles (Figure 1) match perfectly the JCPDS standard patterns 85-1382 (KSF) and 84-0628 (BSF). No extra phase is detected. The diffraction peaks are sharp and intense, indicating good level of crystallinity.
(b)
Intensity (a.u.)
Intensity (a.u.)
(a)
85-1382
10
20
30
40
50
60
70
84-0628
80
10
2Theta (°)
20
30
50 40 2 Theta (°)
60
70
80
Figure 1: XRD patterns of as-synthesized (a) K2SiF6 and (b) BaSiF6 and corresponding JCPDS standards.
The SEM image of KSF (Figure 2(a)) consists of quasi-spherical nanoparticles with an average diameter around 100 nm. The presence of agglomerated nanoparticles having a diameter less than 20 nm is also noticed. BSF crystallizes as nanorods of ≈100 nm in diameter and lengths varying from ≈100 to several hundred of nm (Figure 2(b)). This is consistent with the hexagonal crystal structure of this compound that facilitates the growth along the c-axis. This specific morphology is further confirmed by TEM, as shown in the inset of Figure 2(b). The poly-disperse character of the microstructure of KSF and BSF nanocrystals suggests the occurrence of an Ostwald ripening process [12, 14].
4
(a)
(b)
100 nm
200 nm
200nm nm 200
Figure 2: SEM images of (a) KSF and (b) BSF powders; inset: TEM image of BSF suspension
The FTIR spectra reproduced in Figure 3 indicate that KSF and BSF particles are covered with OA molecules. The characteristic modes corresponding to antisymmetric and symmetric stretching of the C-H bonds of alkyl chains and the C=O bond from the carboxyl group of OA are observed at 2923, 2854 and 1710 cm-1, respectively [13, 15]. The C=O stretch band of the carboxyl group (1710 cm−1) is noteworthy weak in comparison to the pure OA [15, 16] whereas a band at 1546 cm-1, characteristic of the asymmetric νas(COO–) stretch [16] is observed. Moreover, one can notice two other modes observed at 1070 and 1190 cm-1 assigned to C-O bonds. These signals have already been observed on solvothermal treated OA and KSF-OA particles but not specifically interpreted [15]. If we consider the work of Zhang et al. [16] on OA coated Fe3O4 nanoparticles, who evidenced the same kind of behavior (absence of signal at 1710 cm-1 and presence of bands at 1541 and 1050 cm-1), FTIR spectra of KSF and BSF clearly evidence the bonding between the metal and the carboxylate groups of OA. The remaining vibration modes at 725 and 480 cm-1 in KSF (Figure 4(a)) and at 705, 500 and 460 cm-1 in BSF (Figure 3(b)) are ascribed to Si-F vibrations of SiF62- groups [6, 8, 15].
5
(a)
CH 2923
(b)
Si-F bonds in KSF
CH
log(1/R) (a.u.)
K2SiF6
log(1/R) (a.u.)
Si-F bonds in BSF
2
BaSiF6 2
2854
K2SiF6 BaSiF6
C-O 1070
C=O 1710
2500
3000
3500
-1
Wavenumber (cm )
2000
COO
-
1190
1546
1500
1000
500
-1
Wavenumber (cm )
Figure 3: FTIR spectra of KSF and BSF powders at (a) high and (b) low wavenumber
The TGA analyses (not shown) indicate that the amount of OA attached onto the surface of
NPs represents 5 wt. % in KSF and 35 wt. % in BSF. This is particularly desired to enhance the stability of the fluoride particles towards moisture, as demonstrated earlier in Mn4+-doped KSF [15].
CONCLUSION KSF and BSF nanocrystals were successfully synthesized by an HF free thermal decomposition method. The as-obtained particles are coated with OA molecules, which is an asset for moisture resistance. Further work will be dedicated to use this HF-free synthetic method to prepare luminescent KSF and BSF nanophosphors doped with Mn4+ in order to achieve red phosphors for LED applications.
Acknowledgements The authors acknowledge financial support from Université Clermont Auvergne for the post-doctoral fellowship of Dr Vinod Kumar.
References [1] G. Li, Y. Tian, Y. Zhao, J. Lin, Recent progress in luminescence tuning of Ce3+ and Eu2+-activated phosphors for pc-WLEDs, Chem. Soc. Rev. 44(23) (2015) 8688-8713. [2] Z. Xia, Z. Xu, M. Chen, Q. Liu, Recent developments in the new inorganic solidstate LED phosphors, Dalton Trans. 45(28) (2016) 11214-11232.
6
[3] C.C. Lin, A. Meijerink, R.-S. Liu, Critical Red Components for Next-Generation White LEDs, J. Phys. Chem. Lett. 7(3) (2016) 495-503. [4] Q. Zhou, L. Dolgov, A.M. Srivastava, L. Zhou, Z. Wang, J. Shi, M.D. Dramićanin, M.G. Brik, M. Wu, Mn2+ and Mn4+ red phosphors: synthesis, luminescence and applications in WLEDs. A review, J. Mater. Chem. C 6(11) (2018) 2652-2671. [5] T.-D. Tran, D.-H. Nguyen, T.-H. Pham, D.-C. Nguyen, T.-T. Duong, Achieving High Luminescent Performance K2SiF6:Mn4+ Phosphor by Co-precipitation Process with Controlling the Reaction Temperature, J. Electron. Mater. 47(8) (2018) 4634-4641. [6] G. George, S.L. Jackson, Z.R. Mobley, B.R. Gautam, D. Fang, J. Peng, D. Luo, J. Wen, J.E. Davis, D. Ila, Z. Luo, Fast luminescence from rare-earth-codoped BaSiF6 nanowires with high aspect ratios, J. Mater. Chem. C 6(27) (2018) 7285-7294. [7] H.F. Sijbom, R. Verstraete, J.J. Joos, D. Poelman, P.F. Smet, K2SiF6:Mn4+ as a red phosphor for displays and warm-white LEDs: a review of properties and perspectives, Opt. Mater. Express 7(9) (2017) 3332-3365. [8] Y. Pan, Z. Chen, X. Jiang, S. Huang, M. Wu, A Facile Route to BaSiF6:Mn4+ Phosphor with Intense Red Emission and Its Humidity Stability, J. Am. Ceram. Soc. 99(9) (2016) 3008-3014. [9] Z. Hou, X. Tang, X. Luo, T. Zhou, L. Zhang, R.-J. Xie, A green synthetic route to the highly efficient K2SiF6:Mn4+ narrow-band red phosphor for warm white light-emitting diodes, J. Mater. Chem. C 6(11) (2018) 2741-2746. [10] L. Yang, S. Wang, Y. Wang, J. Wang, Narrow-Band Red-Emitting Phosphor K2SiF6:Mn4+: HF-Free Synthesis, Surface Modification, and Application for Warm White LEDs, ChemistrySelect 4(13) (2019) 3891-3897. [11] P. Rahman, M. Green, The synthesis of rare earthfluoride based nanoparticles, Nanoscale 1(2) (2009) 214-224. [12] H.-X. Mai, Y.-W. Zhang, R. Si, Z.-G. Yan, L.-d. Sun, L.-P. You, C.-H. Yan, HighQuality Sodium Rare-Earth Fluoride Nanocrystals: Controlled Synthesis and Optical Properties, J. Am. Chem. Soc. 128(19) (2006) 6426-6436. [13] N. Francolon, D. Boyer, F. Leccia, E. Jouberton, A. Walter, C. Bordeianu, A. Parat, D. Felder-Flesch, S. Begin-Colin, E. Miot-Noirault, J.-M. Chezal, R. Mahiou, Preparation of core/shell NaYF4:Yb,Tm@dendrons nanoparticles with enhanced upconversion luminescence for in vivo imaging, Nanomedicine: Nanotechnology, Biology and Medicine 12(7) (2016) 2107-2113.
7
[14] A. Naduviledathu Raj, T. Rinkel, M. Haase, Ostwald Ripening, Particle Size Focusing, and Decomposition of Sub-10 nm NaREF4 (RE = La, Ce, Pr, Nd) Nanocrystals, Chem. Mater. 26(19) (2014) 5689-5694. [15] P. Arunkumar, Y.H. Kim, H.J. Kim, S. Unithrattil, W.B. Im, Hydrophobic Organic Skin as a Protective Shield for Moisture-Sensitive Phosphor-Based Optoelectronic Devices, ACS Appl. Mater. Interfaces 9(8) (2017) 7232-7240. [16] L. Zhang, R. He, H.-C. Gu, Oleic acid coating on the monodisperse magnetite nanoparticles, Appl. Surf. Sci. 253(5) (2006) 2611-2617.
8
Declaration of interests
☒ 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.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
9
K2SiF6 and BaSiF6 nanoparticles were synthesized for the first time by thermal decomposition method. This synthesis process does not require the use of highly toxic hydrofluoric acid as fluorine source. The as-obtained particles are coated with OA molecules, which is an asset for moisture resistance.
10
S0167-577X(19)31755-0 https://doi.org/10.1016/j.matlet.2019.127123 MLBLUE 127123
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
24 October 2019 19 November 2019 4 December 2019
Please cite this article as: V. Kumar, A. Potdevin, P. Boutinaud, D. Boyer, HF-free synthesis of K2SiF6 and BaSiF6 nanoparticles by thermal decomposition, Materials Letters (2019), doi: https://doi.org/10.1016/j.matlet. 2019.127123
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 B.V.
HF-free synthesis of K2SiF6 and BaSiF6 nanoparticles by thermal decomposition Vinod KUMARa, Audrey POTDEVINa, Philippe BOUTINAUDa and Damien BOYERa* a Université
Clermont Auvergne, CNRS, SIGMA Clermont, ICCF, F-63000 Clermont–
Ferrand, France. *Corresponding author. Tel. : + 33 4 73 40 76 47 E-mail address: [email protected]
ABSTRACT K2SiF6 and BaSiF6 nanoparticles are synthesized for the first time by thermal decomposition of a mixture consisting of oleic acid and 1-octadecene using ammonium fluoride as fluorine source instead of highly toxic hydrofluoric acid. The structural and morphological properties of the asobtained nanocrystals are investigated.
Keywords: Thermal decomposition; KSF; Nanoparticles; HF-free INTRODUCTION During the two last decades, phosphor-converted white LEDs (pc-WLEDs) have attracted considerable attention due to their low power consumption, very good luminous efficiency, long lifetime, robustness and low environmental impact. Most of the pc-LEDs contain phosphors doped with rare-earth (RE) ions such as divalent europium and trivalent cerium [1, 2] . These ions exhibit broad emission bands and can be easily excited by a blue light with high quantum
efficiency. To improve the color rendering index of the white LEDs, red phosphors are generally added to the blue LED chip and yellow phosphor (YAG:Ce) combination [2]. These red phosphors must fulfill several requirements such as a good stability upon photonic and thermal stresses in addition to a high emission quantum yield. Recently, a new family of efficient narrow-band red-emitting phosphors has emerged. They consist of a fluoride host lattice, either X2MF6 (X = K, Na, and Cs; M = Si, Ge, Zr, Sn, and Ti) or ZSiF6 (Z = Ba or Zn), activated with Mn4+ transition metal ions. These phosphors exhibit sharp and intense emission lines below 650 nm and show good thermal stability under the operating temperatures of LEDs [3-5]. Besides, in the race for miniaturization of optical systems like lighting or display devices, the demand for nanophosphors is dramatically increasing. Since then, many synthesis methods have been experienced already for the preparation of potassium silicon fluoride (K2SiF6 or KSF) and barium fluorosilicate (BaSiF6 or BSF) nanoparticles [4, 6-8]. Most of them, however, use hazardous and non-eco-friendly aqueous HF as fluorine source. This highly corrosive contactpoison can cause severe skin burns and eye damage, is fatal if swallowed, fatal in contact with skin and fatal if inhaled. Substitution of aqueous HF to another source of fluorine ions is therefore highly recommended and has entailed a lot of interest in recent years [9, 10]. In this Letter, we introduce a new HF-free process based on thermal decomposition (or thermolysis) method for preparing undoped KSF and BSF nanoparticles. This method has already been employed for synthesizing NaLnF4 (Ln = Y3+, Gd3+) nanoparticles in a mixture of octadecene (OD) and oleic acid (OA) at high temperature[11-13] but never applied to fluorides like KSF and BSF.
EXPERIMENTAL SECTION The methodology is as follows: for KSF, 4 mmol of potassium fluoride is dissolved in absolute methanol and stirred for 5 min. After dissolution, 8 mmol of ammonium fluoride is poured into the solution and stirred 15 min at room temperature. Then 6 mL of OA and 15 mL of OD are added and the solution is successively heated at 120°C for 30 min in air to remove the 2
methanol/water content and then at 120°C for 30 min under vacuum. Methanol started to evaporate at 65°C. After cooling down to room temperature under vacuum, 2 mmol of tetraethyl orthosilicate (TEOS) are added dropwise into the solution under stirring. The temperature of the solution is then raised to 320°C for 30 min under argon (Ar) atmosphere and maintained at this temperature for 1 h under Ar flow before being cooled down to 80°C in Ar and then to room temperature (RT) in air. Finally, 10 mL of absolute ethanol is added to precipitate the nanoparticles. For BSF, 6 mmol of ammonium fluoride is first dissolved with absolute methanol under stirring. After complete dissolution, 1 mmol of TEOS is injected dropwise and stirred 30 min at RT. OA and OD, respectively 6 mL and 15 mL, are then added to this solution, which was heated at 120°C for 30 min in air and then under vacuum to remove the methanol/water content. The Ba solution is prepared separately by dissolving Ba(Ac)2 (1 mmol) into 3 ml of acetic acid under stirring for 30 min at RT. This Ba solution is then incorporated to the fluoride solution. This final mixture is heated at 120°C for 30 min under air and 30 min under vacuum before being cooled down to RT. It is heated again at 320°C for 1h under Ar atmosphere before the addition 10 ml of ethanol. In both cases, a suspension of nanoparticles is obtained. The particles are collected by centrifugation at 10000 rpm for 20 min, washed twice in a mixture of 20 ml of ethanol and cyclohexane in the volume ratio of 1:1 and dried overnight at 80°C in air oven. The X-ray diffraction (XRD) patterns of the undoped KSF/BSF powders were recorded with a Philips Xpert Pro diffractometer operating with the Cu-Kα1 radiation (λ= 1.5406 Å). Transmission Electronic Microscopy (TEM) images were recorded on a Hitachi H-7650 microscope. Scanning electron (SEM) images of the samples were collected on a ZEISS Supra 55 FEG-VP microscope operated at 3kV. FTIR spectra were obtained on polycrystalline samples with a Thermo Electron (Nicolet 5700-FTIR model) spectrometer in the ATR mode. TGA measurements were carried out using a Setaram SetSys Evolution instrument, in air in two
3
steps, first up to 100 °C at a rate of 10°C/min for 30 min to remove traces of organics and moistures and then up to 800°C at a rate of 10°C/min.
RESULTS AND DISCUSSION The XRD patterns of the as-obtained KSF and BSF nanoparticles (Figure 1) match perfectly the JCPDS standard patterns 85-1382 (KSF) and 84-0628 (BSF). No extra phase is detected. The diffraction peaks are sharp and intense, indicating good level of crystallinity.
(b)
Intensity (a.u.)
Intensity (a.u.)
(a)
85-1382
10
20
30
40
50
60
70
84-0628
80
10
2Theta (°)
20
30
50 40 2 Theta (°)
60
70
80
Figure 1: XRD patterns of as-synthesized (a) K2SiF6 and (b) BaSiF6 and corresponding JCPDS standards.
The SEM image of KSF (Figure 2(a)) consists of quasi-spherical nanoparticles with an average diameter around 100 nm. The presence of agglomerated nanoparticles having a diameter less than 20 nm is also noticed. BSF crystallizes as nanorods of ≈100 nm in diameter and lengths varying from ≈100 to several hundred of nm (Figure 2(b)). This is consistent with the hexagonal crystal structure of this compound that facilitates the growth along the c-axis. This specific morphology is further confirmed by TEM, as shown in the inset of Figure 2(b). The poly-disperse character of the microstructure of KSF and BSF nanocrystals suggests the occurrence of an Ostwald ripening process [12, 14].
4
(a)
(b)
100 nm
200 nm
200nm nm 200
Figure 2: SEM images of (a) KSF and (b) BSF powders; inset: TEM image of BSF suspension
The FTIR spectra reproduced in Figure 3 indicate that KSF and BSF particles are covered with OA molecules. The characteristic modes corresponding to antisymmetric and symmetric stretching of the C-H bonds of alkyl chains and the C=O bond from the carboxyl group of OA are observed at 2923, 2854 and 1710 cm-1, respectively [13, 15]. The C=O stretch band of the carboxyl group (1710 cm−1) is noteworthy weak in comparison to the pure OA [15, 16] whereas a band at 1546 cm-1, characteristic of the asymmetric νas(COO–) stretch [16] is observed. Moreover, one can notice two other modes observed at 1070 and 1190 cm-1 assigned to C-O bonds. These signals have already been observed on solvothermal treated OA and KSF-OA particles but not specifically interpreted [15]. If we consider the work of Zhang et al. [16] on OA coated Fe3O4 nanoparticles, who evidenced the same kind of behavior (absence of signal at 1710 cm-1 and presence of bands at 1541 and 1050 cm-1), FTIR spectra of KSF and BSF clearly evidence the bonding between the metal and the carboxylate groups of OA. The remaining vibration modes at 725 and 480 cm-1 in KSF (Figure 4(a)) and at 705, 500 and 460 cm-1 in BSF (Figure 3(b)) are ascribed to Si-F vibrations of SiF62- groups [6, 8, 15].
5
(a)
CH 2923
(b)
Si-F bonds in KSF
CH
log(1/R) (a.u.)
K2SiF6
log(1/R) (a.u.)
Si-F bonds in BSF
2
BaSiF6 2
2854
K2SiF6 BaSiF6
C-O 1070
C=O 1710
2500
3000
3500
-1
Wavenumber (cm )
2000
COO
-
1190
1546
1500
1000
500
-1
Wavenumber (cm )
Figure 3: FTIR spectra of KSF and BSF powders at (a) high and (b) low wavenumber
The TGA analyses (not shown) indicate that the amount of OA attached onto the surface of
NPs represents 5 wt. % in KSF and 35 wt. % in BSF. This is particularly desired to enhance the stability of the fluoride particles towards moisture, as demonstrated earlier in Mn4+-doped KSF [15].
CONCLUSION KSF and BSF nanocrystals were successfully synthesized by an HF free thermal decomposition method. The as-obtained particles are coated with OA molecules, which is an asset for moisture resistance. Further work will be dedicated to use this HF-free synthetic method to prepare luminescent KSF and BSF nanophosphors doped with Mn4+ in order to achieve red phosphors for LED applications.
Acknowledgements The authors acknowledge financial support from Université Clermont Auvergne for the post-doctoral fellowship of Dr Vinod Kumar.
References [1] G. Li, Y. Tian, Y. Zhao, J. Lin, Recent progress in luminescence tuning of Ce3+ and Eu2+-activated phosphors for pc-WLEDs, Chem. Soc. Rev. 44(23) (2015) 8688-8713. [2] Z. Xia, Z. Xu, M. Chen, Q. Liu, Recent developments in the new inorganic solidstate LED phosphors, Dalton Trans. 45(28) (2016) 11214-11232.
6
[3] C.C. Lin, A. Meijerink, R.-S. Liu, Critical Red Components for Next-Generation White LEDs, J. Phys. Chem. Lett. 7(3) (2016) 495-503. [4] Q. Zhou, L. Dolgov, A.M. Srivastava, L. Zhou, Z. Wang, J. Shi, M.D. Dramićanin, M.G. Brik, M. Wu, Mn2+ and Mn4+ red phosphors: synthesis, luminescence and applications in WLEDs. A review, J. Mater. Chem. C 6(11) (2018) 2652-2671. [5] T.-D. Tran, D.-H. Nguyen, T.-H. Pham, D.-C. Nguyen, T.-T. Duong, Achieving High Luminescent Performance K2SiF6:Mn4+ Phosphor by Co-precipitation Process with Controlling the Reaction Temperature, J. Electron. Mater. 47(8) (2018) 4634-4641. [6] G. George, S.L. Jackson, Z.R. Mobley, B.R. Gautam, D. Fang, J. Peng, D. Luo, J. Wen, J.E. Davis, D. Ila, Z. Luo, Fast luminescence from rare-earth-codoped BaSiF6 nanowires with high aspect ratios, J. Mater. Chem. C 6(27) (2018) 7285-7294. [7] H.F. Sijbom, R. Verstraete, J.J. Joos, D. Poelman, P.F. Smet, K2SiF6:Mn4+ as a red phosphor for displays and warm-white LEDs: a review of properties and perspectives, Opt. Mater. Express 7(9) (2017) 3332-3365. [8] Y. Pan, Z. Chen, X. Jiang, S. Huang, M. Wu, A Facile Route to BaSiF6:Mn4+ Phosphor with Intense Red Emission and Its Humidity Stability, J. Am. Ceram. Soc. 99(9) (2016) 3008-3014. [9] Z. Hou, X. Tang, X. Luo, T. Zhou, L. Zhang, R.-J. Xie, A green synthetic route to the highly efficient K2SiF6:Mn4+ narrow-band red phosphor for warm white light-emitting diodes, J. Mater. Chem. C 6(11) (2018) 2741-2746. [10] L. Yang, S. Wang, Y. Wang, J. Wang, Narrow-Band Red-Emitting Phosphor K2SiF6:Mn4+: HF-Free Synthesis, Surface Modification, and Application for Warm White LEDs, ChemistrySelect 4(13) (2019) 3891-3897. [11] P. Rahman, M. Green, The synthesis of rare earthfluoride based nanoparticles, Nanoscale 1(2) (2009) 214-224. [12] H.-X. Mai, Y.-W. Zhang, R. Si, Z.-G. Yan, L.-d. Sun, L.-P. You, C.-H. Yan, HighQuality Sodium Rare-Earth Fluoride Nanocrystals: Controlled Synthesis and Optical Properties, J. Am. Chem. Soc. 128(19) (2006) 6426-6436. [13] N. Francolon, D. Boyer, F. Leccia, E. Jouberton, A. Walter, C. Bordeianu, A. Parat, D. Felder-Flesch, S. Begin-Colin, E. Miot-Noirault, J.-M. Chezal, R. Mahiou, Preparation of core/shell NaYF4:Yb,Tm@dendrons nanoparticles with enhanced upconversion luminescence for in vivo imaging, Nanomedicine: Nanotechnology, Biology and Medicine 12(7) (2016) 2107-2113.
7
[14] A. Naduviledathu Raj, T. Rinkel, M. Haase, Ostwald Ripening, Particle Size Focusing, and Decomposition of Sub-10 nm NaREF4 (RE = La, Ce, Pr, Nd) Nanocrystals, Chem. Mater. 26(19) (2014) 5689-5694. [15] P. Arunkumar, Y.H. Kim, H.J. Kim, S. Unithrattil, W.B. Im, Hydrophobic Organic Skin as a Protective Shield for Moisture-Sensitive Phosphor-Based Optoelectronic Devices, ACS Appl. Mater. Interfaces 9(8) (2017) 7232-7240. [16] L. Zhang, R. He, H.-C. Gu, Oleic acid coating on the monodisperse magnetite nanoparticles, Appl. Surf. Sci. 253(5) (2006) 2611-2617.
8
Declaration of interests
☒ 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.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
9
K2SiF6 and BaSiF6 nanoparticles were synthesized for the first time by thermal decomposition method. This synthesis process does not require the use of highly toxic hydrofluoric acid as fluorine source. The as-obtained particles are coated with OA molecules, which is an asset for moisture resistance.
10