Combustion synthesis and photoluminescence of ZnS:Mn+2 particles

ARTICLE IN PRESS Journal of Luminescence 130 (2010) 1026–1031

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Combustion synthesis and photoluminescence of ZnS:Mn + 2 particles C.W. Won a, H.H. Nersisyan a,n, H.I. Won a, D.Y. Jeon b, J.Y. Han b a b

RASOM, Chungnam National University, Yuseong, Daejeon 305-764, South Korea Department of Nano Materials Engineering, Chungnam National University, Yuseong, Daejeon 305-764, South Korea

a r t i c l e in f o

a b s t r a c t

Article history: Received 1 September 2009 Received in revised form 31 December 2009 Accepted 15 January 2010 Available online 21 January 2010

An efficient process based on a solid-state combustion technique has been developed to produce high crystalline and micrometer sized particles of ZnS:Mn + 2 phosphor with sphalerite structure. The precursor mixture of 0.915Zn+S +0.05Mn+0.035ZnCl2 +kNaCl composition (where k is the mole number of NaCl) was combusted under the argon atmosphere followed by post-heat treatment procedure at 700 1C. It was shown that photoluminescence (PL) intensity of ZnS sample can be easily controlled through adjusting NaCl concentration. In the optimized reaction conditions ZnS samples have showed PL intensity almost comparable to that of a commercial one, despite the relatively low purity of precursor materials used. Many interesting phenomena such as high luminescent efficiency, pure cubic ZnS formation after the post-heat treatment and strong influence of Cl ion on PL intensity have been observed and discussed. & 2010 Elsevier B.V. All rights reserved.

Keywords: Zinc sulfide Phosphor Sodium chloride Combustion synthesis Photoluminescence

1. Introduction Zinc sulfide based phosphor materials have attracted a great deal of interest because of a wide variety of applications ranging from conventional fluorescent lighting to cathode ray tubes, field emission displays, plasma displays, scintillation, etc. Among ZnS based phosphors Mn2 + -doped ZnS (ZnS:Mn2 + ), which emits yellowish orange under 350–360 nm UV excitation, is one of the most important phosphors used in displays and is conventionally produced by a two-stage solid-state process at high temperatures (900–1200 1C) [1]. Conventional process consists of a series of calcining, grinding and annealing steps and requires controlled heating at high temperature and long processing time, which has negative effects on process efficiency and often results in final products with broad particle size distribution. Recently, a number of techniques such as precipitation, microemulsion, sol–gel, chemical vapor deposition, molecular beam epitaxy and spray pyrolysis have been employed in the synthesis of ZnS:Mn2 + nanocrystals [2–8]. However, these methods have low production efficiency and it is still a challenge to produce ZnS:Mn + 2 powders with improved luminescence properties. Combustion synthesis (also known as self-propagating high temperature synthesis (SHS)) is one of the simplest and costeffective techniques that can be applied for a large scale production of ZnS powders. The synthesis of ZnS powder from zinc–sulfur precursor mixture via combustion technique has been

n

Corresponding author. Tel.: + 82 42 821 6587; fax: 82 42 822 9401. E-mail address: [email protected] (H.H. Nersisyan).

0022-2313/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2010.01.018

reported by several authors [9–14]. The unique properties of zinc– sulfur mixture such as high combustion temperature ( 2000 1C), short reaction time (10 s) and low ignition temperature (  450 1C) make this synthesis process attractive when compared with traditional techniques for zinc sulfide synthesis, for example the heating of reactant precursor powders in a resistive furnace (conventional process) or the precipitation with hydrogen sulfide from an aqueous solution of the corresponding salts. To permit the combustion process to proceed entirely at such high temperature, the process was conducted under 3–5 MPa of argon atmosphere. Synthesized in the stated conditions the reaction samples were completely melted (Tmelt ZnS =1700 1C) and identified as a high temperature wurtzite phase of ZnS [11]. Tanaka et al. [15] investigated the combustion process of (1 x)Zn + xMn+ yS (x r0.05, y=1.1) system to produce luminescent materials. It was shown that during the synthesis Mn2 + ions penetrated uniformly into the vacancies of Zn2 + ions in ZnS matrix shifting the peaks of photoluminescent (PL) spectra of ZnS from 480 nm (x= 0) to 580 nm (x =0.05). It was supposed that the characteristic emission at 480 nm was related to a self-activation of ZnS caused from some vacancies of Zn2 + ions in the ZnS matrix, and that at 580 nm was from 3d5 orbital transition of Mn2 + ions. However, the PL intensity of the ZnS samples synthesized from Zn+ S reaction mixture was sufficiently low for the practical application. To the best of our knowledge no further attempts have been made to improve the morphological and structural characteristics of ZnS powder prepared by the combustion process. In the present paper an efficient and cost-effective combustion route based on the precursor mixture of 0.915Zn+ S+0.05Mn

ARTICLE IN PRESS C.W. Won et al. / Journal of Luminescence 130 (2010) 1026–1031

2.5 TmeltZnS = 1830 ± 20°C

2000

2

1.5

Tad CZn(sol)

CZnS(liq)

1000

500

0.5

CNaCl(sol) CZn(gas)

0 0

0

1

2

3

kNaCl , mole Fig. 1. . Thermodynamic analysis of Zn + S+ kNaCl system.

1600

0.2

1200

0.15 Tc

800

Uc

3.1. Thermodynamics and synthesis

400 To estimate the influence of NaCl concentration on the adiabatic combustion temperature (Tad) and equilibrium composition of reaction products, thermodynamic analysis of Zn+ S+ kNaCl+ 0.035ZnCl2 (Mn was not included in this calculation) system was performed using software package ‘‘Thermo’’ developed in Ref. [16]. The equilibrium characteristics of Zn+ S+ kNaCl+ 0.035ZnCl2 composition were calculated for the 0 rkr3 interval at given conditions (P= 0.5 MPa, T= 300 K). The

1

C, mole

CNaCl(liq)

1500

0.1

Uc , cm/s

3. Results and discussion

2500

Combustion limit !

Zinc powder (99.0% purity and particle size r20 mm, Daejung Chemical and Metals Co., Ltd., South Korea), sulfur powder (99.0% purity and particle size o5.0 mm, Daejung Chemical and Metals Co., Ltd., South Korea), manganese powder (99.0% purity and particle size r20 mm, Aldrich, USA), sodium chloride (99.5% purity and particle size 50–150 mm, Daejung Chemical and Metals Co., Ltd., South Korea), zinc chloride (99.0% purity and particle size 50–200 mm, Samchun Pure Chemicals Co., Ltd., South Korea) and cubic zinc sulfide (98.0% purity and particle size r 5 mm, Alfa Aesar, USA) were used in the experiments. Solid state combustion process followed by post-heat treatment operation is suggested for the ZnS:Mn + 2 phosphor preparation. For the combustion experiment a controlled amount of reactant powders was weighed and thoroughly mixed by ballmilling for several hours. Then, a reaction pellet (5.0 cm in diameter) prepared from 150 to 250 g of powder mixture was placed into a reaction vessel and the combustion process was ignited under 0.5 MPa of argon atmosphere by hot nickel– chromium filament system. The combustion temperature (Tc) and the reaction velocity (Uc) were calculated from temperature– time profiles measured by WR-20/WR-5 thermocouples inserted into the reaction pellets. After the combustion process was completed, the sample was cooled down to room temperature and was driven out. Then the combusted sample was put into a covered 300 ml alumina crucible, heated in a laboratory box furnace at 700 1C for 2 h, and cooled down to room temperature by itself in air. The heated product in the crucible was transferred to a beaker of 1 l, and washed several times by DI water to remove the remaining NaCl, and dried at 120 1C. The crystal structure and morphology of ZnS powders were characterized by an X-ray diffractometer with Cu Ka radiation (Siemens D5000, Germany) and a scanning electron microscope (SEM; JSM 5410, JEOL, Japan). Photoluminescence (PL) spectrum was recorded on a fluorescence spectrophotometer (F-7000, Hitachi, Japan) using a Xe lamp with an excitation wavelength of 355 nm at room temperature. Specific surface area analysis was performed on a Coulter SA3100 instrument using nitrogen gas sorption. Weight content (wt%) of elements was measured by SEA 5120 Fluorescence X-ray Element Monitor (Seiko Instruments Inc.).

Tad, °C

2. Experimental

results of the calculation are shown in Fig. 1. The adiabatic combustion temperature of Zn +S binary mixture at k= 0 is higher (Tad =1970 1C) than ZnS melting point (Tmelt = 1830720 1C); therefore the ZnS sample is expected to be in molten form. However, an effective decrease of Tad from k is also predicted by thermodynamic analysis: Tad drops from 1970 to 800 1C, when k increases from 0 to 3 mole. Adiabatic combustion temperature below NaCl melting point can be reached with or above 3 mole of NaCl. In the investigated diapason of k the solid reaction products consisted of ZnS and water soluble NaCl were predicted. The combustion parameters of 0.915Zn+ S+ 0.05Mn +0.035ZnCl2 +kNaCl system versus k are shown in Fig. 2. Almost linear decreases for the combustion temperature (Tc) and wave propagation velocity (Uc) were revealed upon NaCl concentration. Temperature change from 1400 to 800 1C is recorded for 1rkr2.75. Below this temperature an attenuation of combustion process occurred. Experimentally measured values of combustion temperature are in good agreement with thermodynamically calculated ones. We also observed that combustion process was in steady-state mode and measured values of combustion velocity were quite moderate:

Tc ,°C

+ 0.035ZnCl2 + kNaCl composition was developed for synthesizing well-crystalline cubic phase ZnS:Mn + 2 microparticles with high PL characteristics. The precursor mixture used in our experiments contains two different salts (NaCl and ZnCl2) for regulation of combustion temperature and PL intensity of ZnS phosphor samples. The concentrations of ZnCl2 and Mn powder were initially chosen to be 0.035 and 0.05 mole, respectively, and this choice was made based upon preliminary experiments.

1027

0.05

0

0 0.5

1.5

2.5

3.5

kNaCl, mole Fig. 2. . Combustion parameters (Tc, Uc) as a function of NaCl concentration.

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3.2. XRD and SEM studies The photographs of ZnS:Mn + 2 samples after the combustion and the post-heat treatment processes are shown in Fig. 3. These photographs were made from the samples excited by UV-lamp (350 nm), in the dark. Combustion derived sample shows yellow colored surface with many dark spots (Fig. 3 (a)), whereas after the post-heat treatment at 700 1C, color purity was gradually improved and the dark spots almost disappeared from the surface (Fig. 3(b)). It is obvious that low color purity of the sample shown in Fig. 3(a) was conditioned by a poor doping of ZnS with Mn + 2 ions because of a relatively short period of the combustion process. Fig. 4 shows the X-ray diffraction patterns of ZnS:Mn + 2 samples synthesized from the 0.915Zn+ S+ 0.05Mn+ 0.035ZnCl2 + kNaCl mixture. It reveals that both sphalerite (cubic) and wurtzite (hexagonal) phases are presented in the final combustion product, where the portion of hexagonal phase has been decreasing with increase in k. However, single cubic phase ZnS was not obtained after the combustion process. Therefore the combusted samples were subjected to a secondary heat treatment at 700 1C in air. The results of X-ray investigation are shown in Fig. 5. As seen, after the heat treatment almost a single phase cubic ZnS was obtained at kZ1.75 (Fig. 5 (a, b). For the lower concentrations of k (e.g. k= 1) a noticeable amount of hexagonal phase still remains in the final product (Fig. 5 (c)). XRD results clearly indicate that hexagonal–cubic phase transformation during the secondary heat treatment was subjected to the synthesis conditions; i.e. the reaction product prepared at the temperature less than 1000 1C can be converted to the single phase cubic ZnS after the secondary heat treatment at 700 1C; meanwhile the reaction sample obtained at higher synthesis temperature (more than 1000 1C) failed to produce cubic phase ZnS (Fig. 5(c)). Fig. 6 shows SEM micrographs of ZnS powder before and after the post-heat treatment procedure. ZnS particles of various shapes (round, flake type, undefined, etc.) and very broad distribution were obtained after the combustion process (Fig. 6(a)), meanwhile after the post-heat treatment procedure the morphology of particles was substantially improved (Fig. 6(b))

1 - ZnS Sphalerite 2 - ZnS Wurtzite-2H

1

Intensity (a.u.)

Uc = 0.05–0.12 cm/s for 1rkr2.75. It should be noted that the pressure change during the combustion process was negligible, and no condensed mass through evaporation was found in the reaction chamber after the synthesis. After the combustion process white-cream color samples with low shrinkage were obtained and post-heat treatment at 700 1C (2 h) was carried out to increase PL emission.

2

2

1 2

2

(a) 2

2

(b) 2 (c)

20

2

2

1 2 1

1

30

2

2

2

2

40

2

1

2

2 2

1 1

1

1

1

1

60

70

1

50 2θ (°)

80

Fig. 4. . XRD patterns of as-combusted ZnS samples: (a) k=1.0; (b) k= 1.75; (c) k= 2.5.

1 - ZnS Sphalerite 2 - ZnS Wurtzite-2H

1

Intensity (a.u.)

1028

(c)

2

2

(b)

1

(a)

20

2

1

30

40

2

1

2

22

1 2 2 2

1

1 1

1

1

1

1 1

1

1

50 2θ (°)

60

70

80

Fig. 5. . XRD patterns of ZnS samples after post-heat treatment: (a) k =1.0; (b) k= 1.75; (c) k= 2.5.

Fig. 3. . ZnS:Mn + 2 samples derived from the Zn+ S+ 2.5NaCl mixture: (a) as-combusted; (b) post-heat treated.

ARTICLE IN PRESS C.W. Won et al. / Journal of Luminescence 130 (2010) 1026–1031

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Fig. 6. . SEM microsrtucture of ZnS:Mn + 2 phosphor (k= 2.5): (a) as-combusted; (b) post-heat treated.

and 10 mm size polycrystalline particles of ZnS:Mn + 2 phosphor became dominant.

472 nm

2 1

400

450

500

550

600

Wavelength, nm

4000

340 nm

Excitation spectra

ZnS:CI3000 Intensity (a.u.)

3.3.1. ZnS:Cl Preliminarily, the influence of donor type impurity (i.e. Cl ion) on emission intensity of ZnS powder was investigated. Two ZnS samples were selected for analysis of photoluminescence (PL) characteristics. The first sample was synthesized from the reaction mixture of Zn+ S+2.5NaCl composition and the second one from Zn +S +2.5NaCl+ 0.035ZnCl2 reaction mixture. Here, in the first composition the source of Cl ions is NaCl, the second one contains two sources of Cl ions: NaCl and ZnCl2. The concentration of NaCl and ZnCl2 shown above was chosen in accordance with optimum experimental conditions. As follows from Fig. 7 (a) emission profiles with 472 nm emission bands were obtained, i.e. doping of ZnS with Cl ions produces sky-blue emission. It is seen that ZnS sample having NaCl alone as a dopant shows low emission intensity (curve 1); however when ZnCl2 is combined with NaCl the emission intensity increases considerably (curve 2). The excitation spectra recorded for 472 nm emission show intra-band gap excitation level at 340 nm (Fig. 7(b)). These results clearly indicate that origin of 472 nm emission is associated with cation vacancies (VZn) formed in the lattice for charge neutrality. Consequently, the presence of ZnCl2 in mixture increases the Cl concentration in ZnS lattice, resulting in higher cation vacancies (VZn) and emission intensity. The doping activity of these chlorides may be related to standard DHNaCl = 98.27 kcal/mole; enthalpy of formation: DHZnCl2 = 120 kcal/mole [17], i.e. 98.27 kcal/mole energy is required to produce one atom of chlorine from NaCl molecule; meanwhile only 60 kcal/mole energy is necessary to produce one chlorine from ZnCl2. The influence of chloride salts, such as NH4Cl (DHNH4 Cl = 75 kcal/mole) and MnCl2 (DHMnCl2 = 115 kcal/mole) [17], on emission intensity of ZnS was similar to ZnCl2. In contrast, the combination of NaCl with another chloride having high standard enthalpy of formation (e.g. KCl, DHKCl = 115 kcal/mole) did not show an effect on the emission intensity of ZnS samples. These results clearly indicate that high PL intensity ZnS samples can be produced by combustion method, if at least one chloride of low decomposition energy (ZnCl2, MnCl2, NH4Cl, etc.) is used as an initial reactant. X-ray fluorescence (XRF) analysis was also utilized to determine the composition of ZnS phosphor obtained with metal chlorides. The resulting percentages of Zn, S and Cl are shown in Table 1. This result is consistent with empirical formula ZnS0.97. The XRF results also suggest that the percentage of chlorine is 120 ppm when only NaCl was used in the combustion experiments. Adding of ZnCl2 to the reaction mixture has increased the content of Cl ions in ZnS lattice up to 250 ppm.

1. ZnS:Cl (NaCl) 2. ZnS:Cl (NaCl, ZnCl2)

Intensity (a.u.)

3.3. Photoluminescence studies

2000

1000

0 250

300

350

400

Wavelength, nm Fig. 7. . PL emission (a) and excitation spectra (b) of ZnS:Cl heat treatment at 700 1C.

phosphors after post-

Thus, the obtained results imply that emission spectrum of blue luminescence (472 nm) probably is due to the incorporation of Cl ions, as a donor type impurity, into the crystal lattice of ZnS.

3.3.2. ZnS:0.05Mn:Cl As discussed, ZnS:Mn + 2 samples were prepared from the following mixture: 0.915Zn + S+0.05Mn + 0.035ZnCl2 + kNaCl. Fig. 8 shows PL spectra of ZnS:Mn + 2 phosphors for three different

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C.W. Won et al. / Journal of Luminescence 130 (2010) 1026–1031

Table 1 XRF analysis of ZnS phosphor. No

Initial mixture

Zn (wt%)

S (wt%)

Mn (wt%)

Cl (wt%)

1 2 3 4

Zn + S+2.5NaCl Zn + S+2.5NaCl + 0.035ZnCl2 0.915Zn+ S+ 0.05Mn +0.035ZnCl2 + 2.0NaCl 0.915Zn+ S+ 0.05Mn +0.035ZnCl2 + 2.5NaCl

67.12 67.43 67.75 67.86

32.85 32.25 31.14 31.0

0 0 1.01 1.08

0.012 0.025 0.0275 0.029

ZnS:Mn+2 Emission spectra Emission intensity (a.u.)

Ref. (100 %) 2.5 NaCl (94.5 %) 1.5 NaCl (80 %) 1.0 NaCl (69 %)

550

600 650 Wavelength, nm

ZnS:Mn+2 Excitation spectra

700

Ref. ZnS (2.5NaCl) ZnS (2.0NCl) ZnS (1.5NaCl)

4000 Intensity (a.u.)

K = 1.0

K = 1.75

K = 2.5

K = 2.75

λem. = 588 nm

500

5000

K = 2.5

λex.= 356 nm

3000 2000 1000 0 250

300

350

400

450

Wavelength, nm Fig. 8. . PL emission (a) and excitation spectra (b) of ZnS:Mn + 2 phosphors after post-heat treatment at 700 1C.

values of k (k=1.0, 1.5 and 2.5). It can be seen from Fig. 8(a) that doping with Mn + 2 ions has resulted in an emission profile with a 58771 nm emission band. The excitation spectra recorded for 58771 nm show intra-band gap excitation at the 356 71 nm level (Fig. 8(a)). The excitation band at 356 nm can arise by formation of new cation vacancies (VMn) or VZn vacancies can be partially replaced by VMn. Fig. 8(a, b) also shows a continuous increase of excitation and emission intensity of the samples depending on NaCl content. As shown in the inset of Fig. 8(a), the relative emission intensity of ZnS phosphor increased as much as 26% (69–95%) after the NaCl concentration was changed from 1 to 2.5 mole. These results imply that maximum intensity obtained at k= 2.5, corresponds to the lowest values of the combustion temperature (800–850 1C). According to X-ray fluorescence analysis data (Table 1), only 1 wt% Mn + 2 could be incorporated

Fig. 9. . ZnS:Cl dark.

(a) and ZnS:Mn + 2 phosphors (b–e) under 350 nm UV excitation in

into ZnS crystal lattice, despite its relatively large content in the reaction mixture (0.05 mole or 2.8 wt%). The remaining part of Mn was converted into MnCl2 and washed out from the sample. The concentration of Cl ions is slightly higher (290 ppm) than that in undoped ZnS sample. Table 1 shows that Mn + 2 concentration for both samples is about 1.0 wt%. Consequently, the intensity change may be related to the characteristics of ZnS particles, i.e. particle size and surface purity. According to the experimental results, ZnS particles prepared with 2.5 mole NaCl have smaller size (r =  10 mm) and higher specific surface area measured by BET analysis (S= 0.3 m2/g) than the particles prepared with 2.0 mole NaCl (r =  15 mm, S =0.14 m2/g). Consequently, the emission centers of ZnS samples having small size and relatively large surface area may be largely situated near the surface and can be efficiently excited to obtain higher luminescence efficiency. Also ZnS particles prepared with large amount of flux (NaCl) usually display smoother surfaces (Fig. 6(b)) and good dispersity, thus increasing the luminescence efficiency. This analysis could provide evidence that concentration of NaCl is critical to control the size, dispersity and luminescence efficiency of ZnS samples prepared by the combustion synthesis technique. Photographic images of ZnS phosphor powders prepared by combustion technique under different reaction conditions are shown in Fig. 9. These images were made under 350 nm UV excitation in dark. Sky-blue emission can be seen for ZnS samples doped with Cl ions (Fig. 9(a)). Other samples doped with Mn + 2 ions show yellowish emission, which depends strongly on the concentration of NaCl Fig. 9(b–e).

4. Conclusions In this study an economical and cost-effective method to produce ZnS:Mn + 2 phosphor has been developed based on the combustion technique. In this process the precursor mixtures consisted of Zn, S, NaCl, ZnCl2 and/or Mn powders were combusted under argon atmosphere, followed by post-heat treatment procedure at 700 1C.

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XRD and SEM investigations revealed that cubic phase ZnS phosphor powder of good dispersity and particle size less than 20 mm was obtained by the developed technique. According to X-ray fluorescence analysis data maximum concentration of Cl and Mn + 2 ions introduced in the ZnS crystal lattice was 280 ppm and 1 wt%, respectively. At that point, PL emission profile of ZnS sample with peak maximum at 472 nm was dominant in Cl doped samples. Meanwhile, after co-doping by Mn + 2 ions, the emission profile with 58771 nm emission band was only obtained. The present study also demonstrates that high PL intensity ZnS:Mn + 2 samples can be produced by the combustion method, if at least one chloride of low decomposition energy (e.g. ZnCl2, MnCl2, NH4Cl, etc.) is used as an initial reactant along with NaCl. References [1] S. Shionoya, W.M. Yen, in: Phosphor Handbook, CRC Press, Boca Raton, FL, 1999. [2] R.N. Bhargava, D. Gallagher, X. Hong, A. Nurmikko, Phys. Rev. Lett. 72 (1994) 416.

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