JOURNAL OFSC'IKNCi: & rKChm>i.um * no.»/ - mn
PKEPARATION AND CHARACTERIZATION
OF MANGANESE - DOPED ZINC ORTHOSILICATE PHOSPHOR POWDERS BY A CO-PRECIPITATION METHOD
T 6 N G HOP VA DAC TINH CUA BOT PHAT QUANG KEM SILICAT KICH HOAT BOI MANGAN THi:() PHU'ONG PHAP DONG K^TTUA Bui Thi Van Anh, Nguyen Anh Dung, Le Xuan Thanh
Hanoi University of Science and Technology Received March 21, 2012; accepted April 26. 2012
ABSTRACT
Manganese doped zinc silicate phosphors (Zn2Si04:Mn) were synthesized by a co-preeipitaHai method combined with a furnace firing. This paper includes the synthesis and investigation of how ^ annealing temperature and concentration affect the photolumineseenee properties of the Mn'* ions m ZnjSiOA crystals At various finng temperatures, two different erystallographic modifications ofZnj^, can be realized. The XRD pattern shows that ^-ZnjSiOi is metastable. since it completely converts to a-ZniSiOt at 90(fC. The synthesized phosphor that consists of a-Zn2Si04 Mn emits a greep luminescence band at 525 nm. the synthesized phosphor that consists of ^-Zn^SiOd Mn emits a ye/to*
luminescence band at 576 nm when excited by 325nm wavelength. The luminescence mten&tyis strongly dependent on the manganese doping concentrations; the optimal doping concentration is 1 mol%. The result further confirms that Zn^' ions can be replaced by Mn'' ions within zinc silicak lattice. The micro-structure examined by TEM shows that the particles were spherical and higt^
dispersed with particle size of 40 nm
TOM lAT
Chat phat quang kem silieat kieh hoat b&i mangan Zn2Si04:Mn dd dupe tdng hgp thdnh cflng theo phuong phdp dong kit tua. Bdi bdo ndy trinh bay mdt s6 kdt qud nghidn cuu dnh hu&ng cua nhidt do nung va hdm luxjng chdt kieh hoat ddn tinh chdt phdt quang cua ZniSi04:Mn. Si/ t$o thM tinh the p- ZnsSiOd.Mn bdt diu & 75(fC. tdng tidp nhidt dd nung. dang fi- Z/J. S;0, Mn chuydn ddn ve dang a- Zn2Si04:Mn. 0 90(fC, sdn phdm thu tfupc chi chira mdt dang tinh Ihd duy nhdt id a- ZHSSIO^ Mn. Dang p- ZozSiOt phdt quang mdu vdng & bu&c sdng A = 576 nm. a- ZnsSiOd phdt quang mau lyc & bu&c sdng Amax = 525 nm khi bf kieh thieh b&i tia UV ed bu&c sdng A = 325 nm. Tir ng/iBn ciru anh hu&ng cOa hdm lugng chdt kieh boat mangan ddn cir&ng d& phdt quang cua sdn phdm (H dua ra ket lu$n v&i ty 1$ s6 moi mangan bdng 1% so vdi s6 moi cua k&m cho cu&ng dd phdt quang cao nhdt Kdt qud cung khing dinh rdng cdc ion Mnj+ dS thay thd mdt phdn cdc ion Zn^+ trong mm tmh the ZniSiO.,. Dt/a vdo dnh TEM cua sdn phim. ta thdy sdn phim thu dugc gdm nhidu hat hinh cdu neng bidt cd kich thu&c khodng 40 nm.
.. INTRODUCTION
/ii-SiOj Mn is an eftlcienl green phosphor widely used in plasma display panels (PDPs), cathode ray lube (CRT), in-color lamps, color television and ihin film eleclrolumincscenl devices because of iis high luminescence cfricicncN, color punly. cliemica!
stability and Uirgc color gaiiuil. Zn-,Si(),-Mn have been mainly fabricated with the solid-slate reaclion mclhod ll(u\c\cr. the soiid-slaie reaction mclhod has some disadvanlages such as high firing lemperature and diftlcullics in controlling the parliclc sizes and shapes despite
of simple processing. Many new methods for preparation of the Zn.-SiO^.Mn have been proposed. including sol-gel method.
h\drothennal method, spray pyrolysis method, chemical vapor synthesis, and solution combustion method. In this paper, we have studied the optimal preparation conditions for Zn;SiOj:Mn phosphors prepared by a co- precipitation mclhod combined with a furnace tiring. This paper includes the s\ nihesis anil investigation of how the annealing temperature and concentration affect the photo luminescence properties of the M n ' ions in Zn:Si04 crystals
JOURNAL OF SCIKNCE & TECHNOLOGY * No. 87 - 2012 The optical and structural characteristics of
obtained Zn2Si04:Mn have been investigated.
2. EXPERIMENTAL
The two solutions having Zn:Si molar ratio of 2:1 included solution A containing Zn(CH3COO)2 and MnSOa with Mn:Zn molar ratio of 0-1.5 mol% and solution B containing Na^SiOj and NH4OH with Si:OH molar ratio of 1:2. Both solutions A and B were poured in the reactor, and then stirred for 15 minutes at room lemperature. Then obtained precursors were filtered, washed Wid dried at SST for 8 hours.
Obtained precursors TvBWf 'calcinated to synthesize Zn2Si04:Mn at various temperatures fi-om 700 to IIOO'C, with heating rate of IO''C/minute and heat maintenance time of 30 minutes. After the calcination, the product was naturally cold in the furnace.
The crystalline phase was characterized by the X-Ray Diffraction (XRD) using a D8- Advance Bucker equipment with graphite monoehromatized Cu Ka radiation (X = 1.5418 A"). Photolumineseenee (PL) spectra were measured at room temperature by the spectrophotometer using an excitation wavelength of 325 nm. The size and Ihe shape of the particles were observed by a transmission electron microscopy (TEM) using a JEMIOIO- JEOL equipment.
3. RESULTS AND DISCUSSION 3.1. Study on the eH^ect of firing temperature
For the purpose of research, sample preparation process was conducted as described in section 2.1; Zn:Mn molar ratio was fixed to an identical value, i.e., 100:1 (1 mol% Mn- doped Zn2Si04 powders). Obtained precursors were calcinated to synthesize Zn2Si04:Mn at various temperatures, with heating rate of lO^C/minute and heat maintenance time of 30 minutes.
Fig. 1 shows the XRD patterns of 1 mol% Mn-doped Zn2Si04 powders after heat treatment at various temperatures for 30 minutes. The sample remained amorphous after calcination up to 725''C. For samples calcined at 750°C, the XRD results only saw a single crystalline phase that was P-Zn3Si04 [JCPDS No. 14-0653], At SOOT, p-ZnjSiOj appeared as
a crystalline phase; there also existed a-Zn2Si04 [JCPDS No. 79-2005]. At the higher temperatures than 9 0 0 ^ , a-Zn2Si04 existed as the only crystalline phase, which was a rhombohedra structure consisting of comer- joined lelrahedral group of [Zn04] and [Si04].
It is also confirmed in XRD results that the size of P-Zn2Si04:Mn crystals was smaller Ihan a- Zn2Si04:Mn crystals. Based on Debye- Scherrer's formula [1], Ihe average crystallite size of P-Zn2Si04:Mn is 8 nm and that of a- Zn2Si04:Mn is 42 nm.
(220)
j
(MO) 1
.1 11-.
'"f'T
1 1 || >
J1 t
0)
T "}"
T
. ) i , ....Ji
Ktrc
_..."?
,«
. — , , - ,——, 1 . 1 , 2-Theta-Scale (degree)
Fig. I. X-ray diffraction patterns of 1 mol%
Mn-doped Zn2Si04 powders after heal treatment al various temperatures
The presence of P-Zn3Si04'Mn, after the precursors were annealed at 750°C, was described in many previous studies. Marit Mai, Claus Feldmann obtained P-Zn2Si0d;Mn after annealing precursors at 750''C for 15 minutes.
In similar conditions, when precursors were annealed at 1OOOT for 15 minutes, only crystalline a-ZoiSiOa was obtained [2]. When annealing hemimorphite Zn4(OH);Si07, N.
Taghavinia [3] also staled that obtained phases of crystalline Zn;Si04 were differeni when Ihe synthesis conditions were different. Masafiimi Takesue [4] showed that p-Zn2Si04 was obtained at 725-760''C and the crystal system was identified to be orthorhombic. The orthorhombic P-Zn;Si04 was transformed into a-Zn;Si04 by heating to 960T. Thus, p- Zn2Si04 is a melastable phase, which exists al 750-850"C and at higher temperatures it transforms into a-Zn2Si04. As seen in XRD patterns, a-Zn:Si04:Mn was obtained at 900"C,
JOURNAL OF SCIENCE A Te;CHNOLOUV * No. 87-2012 as well as at higher temperatures. Therefore, it
can be concluded that a-Zn2Si04:Mn can be obtained when precursors are annealed at 900''C. This preparation temperature is lower than that in other methods such as the solid- state reaction method (1200-I400"C) and sol- gel method (10QO-I200*'C). When calcined at 700-ilOOT, all of the powders were white, which indicates that the manganese ions were in the divalent state.
The photolumineseenee intensity of the samples annealed at 750''C. ROOT, 900"C and HOOT was measured at room temperature using excitation wavelength of 325 nm. Figure 2 shows that the sample annealed at 750"C and 800T emitted yellow light at the wavelength of 576 nm and had a weak PL intensity. The sample annealed at 800"C showed a weaker PL intensity. It can be explained thai this sample was not of unique phase and p-Zn>Si04 was obtained as the dominant phase besides Ihe presence of a-Zn:Si04 Mn. Similarly, the sample annealed at 900T had a high PL intensity, twice as high as that of the sample annealed at 800''C due to ihe presence of a- Zn;SiOj:Mn as the only phase. According to the results, the samples annealed at 900T, I lOOT lo prepare a-Zn:Si04:Mn that had similar XRD patterns showed similar PL iniensii\. It is stated that the annealing process to prepare a- Zn;Si04:Mn should be carried out at 900T.
3.2. Study on the effect of manganese dopttg concentration
The concentration of doping substances is the most important factor aficciing HK intensity of the phosphors. In many studies tn synthesis of manganese doped zinc orthosilicab phosphors, the effects of manganese dopii^
content has been mentioned. In each method and condition of synthesis, the optima) manganese concentration to dope the desired phoioluminescencc intensity can be found. To investigalc (he relationship between manganese doping concentration and the emission inlensi^
from Zn.-SiOiMn phosphors, samples with different Mn:Zn molar ratios (mol%) were
synlhcM/cd ^ Fig. 3 shows Ihe emission intensiqiof
powders heated al 9 0 0 T depended on manganese doping concentrations when excited by 325 nm wavelength . It can be seen that the luminescence iniensiiv increased wboi manganese doping concentration increased However, the emission intensity did not increase monotonously with the increase of MR doping concenlraiion. To increase the luminescence intensity, the host crystals woe doped with Ions in high concentration. When the doping concentration was greater dian a critical \alue. impurity clusters were formed, then could reduce or quench luminesced*
[51,[6j. The maximum brightness increased Mn
Fig 2 Phatolumiiuwccnce \f)CL!rii of I moi"., Mn- doped Zn>Si04 powdeiw anneadcd at various lemperaiures when excited hy 325nm wavelength
l-lg.3- Pholohiininescence spectra nj InSiOt-Ui' powders healed at 900"C for various inangoitM doping eoiuenlralions when e.xciicd bv i 2 j | wavelength
M
.lOURNAL OF SCIENCE & TECHNOLOGY * No. 87 - 2012
Table. I. Composition and lattice parameters of a- Zn2SiOi:Mn with differeni Mn:Zn molar ratios Mn.Zn molar ratio
(mol%}
a,A°
c.A' V. (A-f
Zn,SIO.C) 13.948
9.315 1569413
0 13.947
9.313 1568.851
0.50 13.959
9.329 1574.252
1.00 13.962
9.333 1575.604
1.25 13.926
9.309 1563.459 Zn2Si04(*): standard sample (registered by JCPDS No, 79-2005)
concentration up to 1 mol%, but declined significantly as doping increased above 1.25 mol%. The optima! Mn doped concentration of green emitting samples was about 1 mol%. It is noted that 1 moI% of optimal Mn concentration for our as-synthesized samples is significantly lower than the reported methods [6]-[l 1]. When the Mn doping concentration was 1.5 mol%, obtained Zn2Si04:Mn powder that was not luminescent was slightly yellowish. This is probably due to presence of some Mn"*^. Thus, by co-precipitation method, synthesized Zn2Si04:Mn with a manganese doping concentration of 1 mol% has the highest luminescence intensity.
1220)
(300)
J '' (1131 (*10)
1 )
il ii
(223)
_J.-^-..
(333)
Fig. 4. XRD pattern of samples with various manganese doping concentrations
Fig. 4 shows the XRD pattern of samples with various manganese doping concentrations annealed at 900T for 30 minutes. Table. 1 lists the composition and lattice parameters of a- Zn2Si04:Mn with different Mn:Zn molar ratios evaluated from the XRD patterns. Zn:Si04 sample without manganese and the cell parameters a and c show the results that are similar to standard ZmSi04 sample; there is no
considerable deviation. XRD examination reveals that single-phase a- Zn:Si04:Mn was formed by the co-precipitation method with proper doping concentrations, suggesting that Ihe Mn'* ion were well dispersed as substitutes for Zn"' in Ihe zinc silicate host lattice after heating. Oxidation stale of manganese in the Zn;SiOd crystal is the most important factor to decide Ihe luminescence of Zn3Si04:Mn. It is confirmed in many documents that manganese that exists as Mn'* oxidation would luminance [11],[12]. Moreover, the Zn:Si04:Mn powders can only be luminescent when they are white, indicating that the manganese ions were in the divalent stale. It can be stated that manganese in the Zn2Si04:Mn crystal exists in the +2 oxidation state. When manganese was added, the lattice parameters a, c were changed. It is concluded that Ihe small amount of added manganese affected Ihe unit cell size, which suggests that Mn'* ion entered the lattice of the host Zn^SiOd crystal. It shows that the cell volume of zinc silicate increased gradually with the increases of Mn doping concentration. The result further confirms that Zn'* ions can be replaced by Mn'* ions within zinc silicate lattice in large scale of molar ratio due to their similar ionic radii (ionic radius Zn"* is 0.074 nm; ionic radius of Mn"* is 0.080 nm) [6],[13].
According to the obtained results above, when concentrations of manganese activated increased, obtained Zn2Si04;Mn crystal had the increasing lattice parameter This is quite consistent with a number of published document [6], However, after replaced Mn"*
ions reached the highest number (1 moi %), manganese doping concentration continued to be increased; there was a considerable decrease in the lattice parameters and the luminescence intensity.
.JOURNAL OF SCIF,NeK & TECHNOLOGY * No. 87 - 2012 Fig. 5 shows TEM images of I moi%
Mn-doped Zn2Si04 powders after healing at 9 0 0 T for 30 minutes. As can be seen, the nanocrysiallites aggregated to form uniform spherical particles, the particle size of Zn2Si04- :Mn synthesized by a co-precipitation method combined with furnace firing was about 40 nm.
•;i.
Fig. 5. TEM image of I mol% Mn-doped Zn2Si04 powders after healing at 900'C for 30 minutes
REFKRENCES 1
4. CONCLUSION •'- In summary, Zn2Si04:Mn phosphor powders have successfully been synthesized by a co-precipitation method combined with furnace firing. At various firing temperatures, two dilTerent erystallographic modifications of Zn2Si04 can be realized. It is confirmed in situ XRD measurement results that p-ZniSi04:Mn is metasiabic, since it completely converts to a- Zn:Si04:Mn al 9 0 0 T . The synthesized phosphor thai consists of a-Zn;Si04:Mn emits a green luminescence at the 525 nm, the synthesized phosphor that consists of P- Zn2Si04:Mn emits a yellow luminescence at the 576 nm when excited by 325nm wavelength.
The luminescence intensity is strongly dependent on the manganese doping concentrations; the optimum doping concentration Is I moI%. The result fiirthw confirms that Zn"" Ions can be replaced by Mn^
ions within zinc silicate lattice. The micro- structure examined by TEM shows that the particles were spherical and highly dispersed of panicle size as 40 nm.
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Atiihor's address: Bui Thi Van Anh - Tel. (+84)912 993.598, Email: anhbtv-fct@mail hut.edu.vn School of Chemical Engineering
Hanoi Uni\crsit) of Science and Technology No.l Dai Co Viel Sir.. Ha Noi, Viet Nam
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