Journal of Materials Research and Technology Journal of Materials Research and Technology
J Mater Res Technol 2017;6:297-301 DOI: 10.1016/j.jmrt.2017.03.001
Short Communication
Preparation and luminescence of a new violet blue phosphor derived from proton-type zirconium phosphate
Susumu Nakayamaa,, , Masatomi Sakamotob
a Department of Applied Chemistry and Biotechnology, National Institute of Technology, Niihama College, Niihama-shi, Japan
b Material and Biological Chemistry, Graduate School of Science and Engineering, Yamagata University, Yamagata-shi, Japan
Received 15 September 2016, Accepted 29 March 2017
Abstract

A violet blue phosphor was prepared by thermal treatment of HZr2(PO4)3 at 700°C in a reducing atmosphere and its PL was investigated. The phosphor shows violet blue emissions at around 392nm when excited by 254nm UV light. Its absorptivity, internal and external quantum efficiency are found to be 43, 41, and 18%, respectively, at room temperature when excited at 254nm. Analyses using non-dispersive infrared absorption, X-ray fluorescence, scanning electron microscopy, X-ray photoelectron spectroscopy, and X-ray diffraction suggest that such an emission originates from the formation of hydrogen and oxygen vacancies in the phosphor during the thermal treatment of HZr2(PO4)3.

Keywords
Ceramics, Chemical synthesis, Heat treatment, Photoluminescence, Defects
1Introduction

Hydrogen gas has been widely used as one of the deoxidizing agents to develop various metal oxides with oxygen vacancies and of particular interest is its application for the fabrication of a green phosphor, ZnO [1–5]. Such phosphors, which do not contain light-emitting metal elements (extrinsic activators: Eu, Tb, Tm, etc.), have attracted great attention in respect different from Zn2SiO4:Eu3+, Ca12Al14O32Cl2:Tb3+, and Gd4O3F6:RE3+ (RE=Yb, Er, Ho and Tm) phosphors [6–8]. These studies have inspired us to investigate whether zirconium phosphates including HZr2(PO4)3 will form oxygen and hydrogen vacancies. The proton-type zirconium phosphate HZr2(PO4)3, which features a Nasicon-type (NZP, NaZr2(PO4)3) crystal structure with rhombohedral symmetry consisting of a three-dimensional rigid framework with corner linked ZrO6 octahedra and PO4 tetrahedra, has been known to be an effective ion exchanger and to possess high resistance to heat, chemicals, radiation, corrosion, and mechanical shock [9,10]. Fig. 1(a) shows the crystal structure proposed for HZr2(PO4)3. Preliminary experiments revealed that HZr2(PO4)3 powders contain such vacancies and can act as a bright phosphor as expected. The fluorescence was a violet emission at around 392nm under 254nm excitation. So far, SiO2GeO2 amorphous films, which contain oxygen vacancies, are known to show a bright PL of violet color [11,12]. In addition, ZrO2:Ti,P blue phosphor, which also consists of oxygen vacancies, has been reported [13]. On the other hand, Ce3+-doped Gd2SiO5[14] and Ce3+-doped BaZrSi2O9[15] are well-known as very bright violet or blue phosphors, which contain extrinsic activators. The proton-type zirconium phosphate presented here is a new type of violet blue phosphor, which has not been reported yet. This kind of phosphor is worth studying for the following reasons: (1) it does not contain extrinsic activators, (2) it is easy to synthesize, (3) it has the potential to outperform existing materials, and (4) it is expected to possess interesting luminescence properties. In this work, more detailed PL properties of this new type of violet blue phosphor are investigated.

Fig. 1.
(0.19MB).

Crystal structure proposed for HZP (a); SEM micrographs of HZP (b) and HZP-r (c).

2Experimental

Crystalline powders of proton-type zirconium phosphate, HZr2(PO4)3 (hereafter abbreviated as HZP), were prepared by thermal decomposition of NH4Zr2(PO4)3 at 700°C for 2h in air. The as-prepared HZP powder was thermally treated at 700°C for 2h in a 300mLmin−1 3%-H2 (N2 balance) gas flow to obtain a violet phosphor (here after abbreviated as HZP-r). NH4Zr2(PO4)3 was synthesized as follows. The precipitate was prepared by a hydrothermal reaction at 95°C for 72h from a solution containing 200cm3 of 0.41moldm−3-ZrOCl2·8H2O, 300cm3 of 0.47moldm−3-H2C2O4·2H2O, and 100cm3 of 1.09moldm−3-NH4H2PO4 adjusted to pH 4.5 by 3moldm−3 NH3 aqueous ammonia. The precipitate obtained was filtered and dried at 250°C for 24h.

Elemental analyses of H and O were carried out by means of the inert gas melt/non-dispersive infrared (NDIR) method using a simultaneous oxygen/nitrogen/hydrogen analyzer EMGA-930 (HORIBA Ltd.). Contents of Zr and P were determined with an X-ray fluorescence (XRF) spectrometer Simultix 14 (Rigaku Co.).

The obtained violet phosphor was characterized by scanning electron microscopy (SEM) observation, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and fluorescence spectroscopy. The crystal morphology was observed using a field emission SEM with a semi-in-lens JSM-7500F (JEOL Ltd.) at 5kV. XPS analyses for the electronic states of P2p were carried out with X-ray photoelectron spectroscopy on a Quantera SXM (Physical Electronics, Inc.). An Al-Kα monochromatized radiation (50W) was employed as X-ray source. The X-ray beam size was a 100μmϕ. XRD data were obtained using an X-ray diffractometer MiniFlex II (Rigaku Co.) with CuKα1 radiation (λ=0.154nm) and a graphite monochromator. The X-ray generator worked at 30kV and 15mA. The scans were performed for 15min within the 2θ range of 10–40° using a scan step of (2θ)=0.005°. Excitation and emission spectra were measured on a spectrofluorometer FP-6500 (JASCO Co.) with a scan rate of 500nmmin−1 and a step size of 0.1nm using a 150W ozone-free Xenon lamp. Quantum efficiency was determined with a Quantaurus-QY absolute PL quantum yield spectrometer C11347-01 (Hamamatsu Photonics K.K.) at an excitation wavelength of 254nm. The absorptivity (A) was calculated according to the following expression.

The internal quantum efficiency was measured using an integrating sphere attached to a multi-channel CCD detector and a Xenon lamp as excitation source. The external quantum efficiency was calculated by multiplying absorptivity and internal quantum efficiency. The PL lifetime was measured using a Quantaurus-τ fluorescence lifetime spectrometer C11367-24 (Hamamatsu Photonics K.K.) at an excitation wavelength of 280nm and an emission wavelength of 392nm. Measurements except for the determination of the internal quantum efficiency were conducted at room temperature (RT).

3Results and discussion

Element analysis using NDIR and XRF of HZP (starting material of HZP-r) reveals an elemental distribution of 0.31, 38.07, 20.97, and 41.59wt%, whereas HZP-r (sample prepared by the thermal treatment of HZP in 3%-H2 atmosphere) contains 0.17, 38.72, 21.17, and 40.37wt% of H, Zr, P, and O, respectively. Assuming that the number of Zr atoms is 2, the chemical composition of HZP can be estimated to be H1.47Zr2P3.24O12.47 for HZP, suggesting HZr2(PO4)3 and (H3O)Zr2(PO4)3 exist in a molar ratio of 0.76:0.24 as major components along with a small amount of P2O5. In relation to this, we previously reported that HZr2(PO4)3 absorbs moisture from ambient air to form (H3O)Zr2(PO4)3, which reverts to HZr2(PO4)3 at 205°C [16]. As for HZP-r, its chemical composition can be calculated to be H0.80Zr2P3.22O11.90, suggesting the formation of hydrogen and oxygen vacancies during thermal treatment of HZP at 700°C in 3% H2.

The SEM micrographs of HZP and HZP-r are shown in Fig. 1(b) and (c), respectively. The crystal morphology of HZP and HZP-r is cubic with a side length of approximately 1–3μm. Furthermore, no change in crystal morphology induced by H2 reduction can be observed.

Fig. 2(a) shows the XPS wide spectra of HZP and HZP-r. In these two spectra, peaks attributable to C, O, Zr, and P are observed. Among these detected elements, the P2p XPS spectra were further evaluated in order to investigate the valence state of P (Fig. 2(b)). The P2p peaks for HZP and HZP-r are observed at almost identical binding energies of around 133.4eV and their peak shapes are also similar, suggesting no significant difference in the electronic state of P between HZP and HZP-r. It is presumed that the valence of the P atoms in HZP and HZP-r is +5.

Fig. 2.
(0.13MB).

XPS wide spectra (a) and P2p XPS spectra (b) of HZP and HZP-r.

Fig. 3 shows the X-ray powder diffraction patterns of HZP-r, HZP, and a sample prepared by thermal treatment of HZP-r at 700°C for 2h in air. All three XRD patterns are in good agreement with those found in the corresponding ICDD files ((38-4, HZr2(PO4)3, hexagonal, R-3c) and (38–67, (H3O)Zr2(PO4)3, hexagonal, R-3c)). The lattice parameters and unit cell volume of HZP-r are a=0.876nm, c=2.249nm, and 1.495nm3, respectively. These values are smaller than those of HZP (a=0.879nm, c=2.319nm, cell volume=1.552nm3), probably due to the formation of hydrogen and oxygen vacancies in HZP-r induced by H2 reduction of HZP. Lattice parameters and unit cell volume for the sample prepared by thermal treatment of HZP-r at 700°C for 2h in air are estimated to be a=0.879nm, c=2.261nm, and 1.514nm3, respectively, suggesting that the oxygen vacancies were, to some extent, removed during thermal treatment in air as partial oxidation of HZP-r occurred.

Fig. 3.
(0.1MB).

X-ray powder diffraction patterns of HZP-r (a), starting material HZP (b), and a sample prepared by thermal treatment of HZP-r (c). Closed circles represent peak data of HZr2(PO4)3 reported in ICDD No. 38-4.

Excitation and emission spectra of HZP-r are shown in Fig. 4. HZP-r can be efficiently excited by UV light of 254nm, yielding a violet blue emission at around 392nm. The CIE (x, y) was calculated as (x, y)=(0.188, 0.140). Thus, the HZP-r emission is exactly on the violet blue light emitting area under 254nm excitation, indicating that the HZP-r is a violet blue phosphor. Absorptivity, internal and external quantum efficiency of the phosphor, HZP-r, at the excitation wavelength of 254nm are 43, 41, and 18%, respectively. As a comparison, the internal quantum efficiency of Ce3+-doped BaZrSi2O9 phosphor, which has a very bright PL at around 432–451nm, was 92.3% under 340nm excitation [15], about double the value of HZP-r. In contrast to HZP-r, HZP exhibited no violet blue emission, and no significant emission was observed for the sample prepared by thermal treatment of HZP-r at 700°C in air. These different observations suggest that there is a threshold concentration of hydrogen and/or oxygen vacancies above which the emission can be clearly observed. It is plausible that the strong violet blue emission of HZP-r is caused by its hydrogen and oxygen vacancies; elucidation of the emission mechanisms is in progress. In Fig. 4, the PL decay profile of the phosphor, HZP-r is inserted. It is assumed that the decay is non-exponential and the average PL lifetime was estimated to be 6.4ns. We believe that the non-exponential decay originates from a continuous distribution of lifetimes due to an inhomogeneity of the PL centers or some exotic recombination processes, such as tunneling recombination.

Fig. 4.
(0.1MB).

Excitation (left, emission: 392nm) and emission (right, excitation: 254nm) spectra of HZP-r at room temperature. The inset is the PL decay profile at 392nm of HZP-r at room temperature (excitation wavelength: 280nm).

The temperature dependence of the internal quantum efficiency of the phosphor, HZP-r, was investigated in the range between 25 and 163°C. Temperature quenching is generally observed in phosphors; however, a temperature rise of a device using the phosphors as a constituent material is inevitable due to heat radiation from the high power light emitting diode (LED) and the backlight of the liquid crystal display (LCD). Therefore, phosphors featuring suppressed temperature quenching are required. As shown in Fig. 5, the efficiency decreases linearly with a rise in temperature and is found to be 20% at 163°C, which is about half of that obtained at 25°C. For comparison, the measured PL intensity of Ce3+-doped BaZrSi2O9 phosphor at 123°C retained 44.6% of the value measured at room temperature [15], which is nearly equal to that of HZP-r.

Fig. 5.
(0.07MB).

Temperature dependence of the internal quantum efficiency of HZP-r. The excitation wavelength is 254nm. The inset shows a digital photograph of HZP-r irradiated under 254nm UV light at room temperature.

4Conclusions

A new type of violet blue phosphor, H0.80Zr2P3.22O11.90, was prepared by thermal treatment of proton-type zirconium phosphate, HZr2(PO4)3, at 700°C in 3%-H2 atmosphere and its PL was investigated. No change in crystal morphology of proton-type zirconium phosphate induced by H2 thermal treatment was observed. The internal quantum efficiency of the violet blue phosphor at the excitation wavelength of 254nm was 41%. The type of PL decay of H0.80Zr2P3.22O11.90 was found to be non-exponential and the average PL lifetime was about 6.4ns. The PL intensity decreased linearly with increasing temperature (163°C) retaining about 50% of the value measured at 25°C. It is plausible that the emission is due to the hydrogen and oxygen vacancies in the phosphor. Research on H2 treatment conditions and hydrogen/oxygen defects observations are in progress in order to further investigate the PL mechanisms.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

We would like to thank Hamamatsu Photonics K.K. for the measurements of quantum efficiency and PL lifetime, HORIBA Ltd. for the elemental analyses of H, and Editage (www.editage.jp) for help with English language editing.

References
[1]
K. Vanheusden,W.L. Warren,C.H. Seager,D.R. Tallant,J.A. Voigt
Mechanisms behind green photoluminescence in ZnO phosphor powders
J Appl Phys, 79 (1996), pp. 7983-7990 http://dx.doi.org/10.1063/1.362349
[2]
A.K. Mishra,S.K. Chaudhuri,S. Mukherjee,A. Priyam,A. Saha,D. Dasa
Characterization of defects in ZnO nanocrystals: photoluminescence and positron annihilation spectroscopic studies
J Appl Phys, 102 (2007), pp. 103514 http://dx.doi.org/10.1063/1.2817598
[3]
N. Ohashi,Y.G. Wang,T. Ishigaki,Y. Wada,H. Taguchi,I. Sakaguchi
Lowered stimulated emission threshold of zinc oxide by hydrogen doping with pulsed argon–hydrogen plasma
J Cryst Growth, 306 (2007), pp. 316-320 http://dx.doi.org/10.1016/j.jcrysgro.2007.05.024
[4]
Q. Shi,Z. Wang,Y. Liu,B. Yang,G. Wang,W. Wang
Single-phased emission-tunable Mg-doped ZnO phosphors for white LEDs
J Alloys Compd, 553 (2013), pp. 172-176 http://dx.doi.org/10.1016/j.jallcom.2012.11.135
[5]
C. Li,J. Lv,Z. Liang,S. Yao
Study on the optical absorption and green emission of ZnO phosphors by varying Al doping contents
Opt Mater, 35 (2013), pp. 586-589 http://dx.doi.org/10.1016/j.optmat.2012.10.037
[6]
N.A.S. Omar,Y.W. Fen,K.A. Matori
Europium doped low cost Zn2SiO4 based glass ceramics: a study on fabrication, structural, energy band gap and luminescence properties
Mater Sci Semi Proc, 61 (2016), pp. 27-34 http://dx.doi.org/10.1016/j.mssp.2016.12.040
[7]
Y. Chen,X. Wang,Y. Bao
Study on a new green phosphor Ca12Al14O32Cl2: Tb3+ derived from Tb-doped Ca–Al layered double hydroxide
Cur Appl Phys, 17 (2017), pp. 78-84 http://dx.doi.org/10.1016/j.cap.2016.11.003
[8]
J. Zhang,J. Jia
Morphologies and up-conversion luminescence of Gd4O3F6:RE3+ (RE=Yb, Er, Ho and Tm) phosphors by hydrothermal synthesis
[9]
V. Vesely,V. Pekarek
Synthetic inorganic ion-exchangers – I: Hydrous oxides and acidic salts of multivalent metals
Talanta, 19 (1972), pp. 219-262
[10]
B.E. Scheetz,D.K. Agrawal,E. Breval,R. Roy
Sodium zirconium phosphate (NZP) as a host structure for nuclear waste immobilizaition: a review
Waste Manage, 14 (1994), pp. 489-505
[11]
N. Arai,H. Tsuji,M. Hattori,M. Ohsaki,H. Kotaki,T. Ishibashi
Luminescence properties of Ge implanted SiO2:Ge and GeO2:Ge films
Appl Surf Sci, 256 (2009), pp. 954-957 http://dx.doi.org/10.1016/j.apsusc.2009.05.062
[12]
A.N. Trukhin
Luminescence of polymorph crystalline and glassy SiO2, GeO2
J Non-Cryst Solids, 355 (2009), pp. 1013-1019 http://dx.doi.org/10.1016/j.jnoncrysol.2009.01.040
[13]
S. Nakayama,M. Sakamoto
Fluorescence property of ZrO2:Ti phosphor and its enhancement in fluorescent intensity by adding phosphorus
J Mater Res Technol, 5 (2016), pp. 289-292 http://dx.doi.org/10.1016/j.jmrt.2016.03.005
[14]
Y. Parganiha,J. Kaur,V. Dubey,R. Shrivastava,D. Chandrakar
Violet blue emission and thermoluminescence glow curve analysis of Gd2SiO5:Ce3+ phosphor
Optik, 127 (2016), pp. 6243-6252 http://dx.doi.org/10.1016/j.ijleo.2016.04.064
[15]
S. Liu,Y. Liang,Y. Zhu,X. Wu,R. Xu,M. Tong
Synthesis and luminescence properties of novel Ce3+ doped BaZrSi3O9 phosphors
Opt Laser Technol, 84 (2016), pp. 1-8 http://dx.doi.org/10.1016/j.optlastec.2016.04.010
[16]
C. Hashimoto,S. Nakayama
Effect of treatment temperature on the immobilization of Cs and Sr to HZr2(PO4)3 using an autoclave
J Nucl Mater, 440 (2013), pp. 153-157 http://dx.doi.org/10.1016/j.jnucmat.2013.04.085
Corresponding author. (Susumu Nakayama nakayama@chem.niihama-nct.ac.jp)
Copyright © 2017. Brazilian Metallurgical, Materials and Mining Association
J Mater Res Technol 2017;6:297-301 DOI: 10.1016/j.jmrt.2017.03.001