Sol–gel pyrolysis and photoluminescent characteristics of
europium-ion doped yttrium aluminum garnet nanophosphors
Chung-Hsin Lu
∗, Wei-Tse Hsu, J. Dhanaraj
1, R. Jagannathan
1Electronic and Electro-Optical Ceramics Laboratory, Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan, ROC Received 16 March 2003; received in revised form 5 December 2003; accepted 13 December 2003
Available online 21 April 2004
Abstract
Europium-ion doped Y3Al5O12garnet nanophosphors (YAG:Eu3+) with wide ranging size tunability (40–150 nm) have been prepared via
a sol–gel pyrolysis method employing a fuel system that combined urea and polyvinyl alcohol (PVA). Well dispersed nanoparticles were prepared at 1000◦C. This temperature is lower than that required for synthesizing YAG:Eu3+via the solid-state reaction route. The particle
size and morphology of the synthesized powders are found to have critical dependence on the oxidizer (metallic nitrates) to fuel ratio. The importance of using an organic polymeric dispersion matrix to obtain well dispersed YAG:Eu3+nanoparticles has been demonstrated. The
photoluminescene properties of the prepared YAG:Eu3+phosphors are profoundly dependent on the preparation conditions. The emission
intensity of well dispersed YAG:Eu3+nanoparticles is found to be much stronger than that of the bulk sample. The excitation spectrum of well dispersed nanoparticles shows an extension of the excitation peak in the high-energy region. These unique properties of YAG:Eu3+
nanoparticles can be rationalized by considering numerous surface states due to the large surface area to volume ratio of the nanoparticles. In addition, using the hypersensitive5D0 →7F2transition of Eu3+as a local probe, the role of surface states that modify the optical properties
of YAG:Eu3+nanophosphors has also been illustrated. © 2004 Elsevier Ltd. All rights reserved.
Keywords: Sol–gel process; Y3Al5O12; Nanoparticles; Phosphors
1. Introduction
Yttrium aluminum garnet Y3Al5O12 (YAG) has wide
applications from conventional structural ceramics to photonics.1,2 YAG doped with lanthanides such as Nd3+ and Ce3+is already used in the construction of dye lasers and new generation lighting devices.3,4 Eu3+ doped YAG phosphors and thin films also have the potential for applica-tion in field emission devices.5,6 Phosphors for field
emis-sion and vacuum fluorescent display devices have critical dependence on their particle sizes. Optimum performance in these devices can be achieved by employing ultrafine phosphor particles.7,8
In view of the tremendous scope for application, a num-ber of physicochemical methods have been employed for the
∗Corresponding author. Tel.:+886-2-2363-5230;
fax:+886-2-2362-3040.
E-mail address: [email protected] (C.-H. Lu). 1Present address: Luminescence Group, CECRI (CSIR), Karaikudi-630006, Tamil Nadu, India.
preparation of YAG powders. In the typical solid-state syn-thesis method, Y2O3and Al2O3, used as the starting
mate-rials, are mixed and heated at high temperature (∼1600◦C) for several hours. Two intermediate phases, YAM (Y4Al2O9,
yttrium aluminium monoclinic) and YAP (YAlO3, yttrium
aluminium perovskite) are easily formed as by-products with YAG, even if the synthesis is performed with stoichiometric mixtures of Y2O3and Al2O3.9For the formation of the pure
YAG phase, repeated calcination processes and prolonged heating are required.10 The prolonged heating at elevated temperatures results in inevitable coarsening of the grains. Therefore, the formed particles tend to exhibit a large grain size, a wide size distribution, and irregular morphology.
For correcting these drawbacks of the solid-state reaction process, different kinds of wet-chemical processes have been applied to synthesize YAG phosphors. In these chem-ical routes, precise control of particle size and chemchem-ical homogeneity is possible since the chemical reaction in the liquid phase takes place at the molecular level. Among various chemical routes such as co-precipitation,11,12spray drying,13,14 and sol–gel pyrolysis,15–17 the last one has an
0955-2219/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2003.12.009
edge over the other chemical routes. It has the advantages of both wet-chemical and solid-state synthesis methods, such as low temperature synthesis (1000◦C), well dispersed nanoparticles, inexpensive precursors, ease of preparation, and large upscalability. Thus, these foregoing facts jus-tify the sol–gel polymer pyrolysis employing urea and polyvinyl alcohol (PVA) combined fuel system as a more facile method for the preparation of YAG powders.
In the sol–gel polymer pyrolysis process, the preparation of inorganic powder is based on pyrolyzing the mixtures of the corresponding metallic nitrates acting as reactants and a polymeric gel base acting as dispersion medium in the pres-ence of suitable fuels by a self-sustained combustion pro-cess. The effective total valency ratio,Φ, between the ox-idizer (metallic nitrates) and fuels (urea and PVA) is criti-cal in determining the sustainability of the combustion reac-tion and, hence, the acquirement of the desired monophasic products.15 The effective total valency ratio (Φ) is defined as8,15 Φ = − xiMini xjFjnj (1)
where Miand Fjare the number of ions present in oxidizers and fuels, respectively, xiand xjare the corresponding num-ber of units in oxidizers and fuels, respectively, and n is the corresponding valence for each species. This ratio is criti-cal for the growth of the ceramic particles prepared using the sol–gel polymer pyrolysis method. However, the infor-mation regarding the growth of phosphors prepared withΦ values different from unity is limited. In the present investi-gation, an attempt has been made to understand the growth and luminance of YAG:Eu3+ phosphors prepared with dif-ferent values of Φ. Two different fuel systems (PVA and urea combined system, and urea only system) were applied to prepare YAG:Eu3+ phosphors to elucidate the effects of fuel systems on the growth as well as the luminance charac-teristics. The influence of the particle sizes on the photolu-minescene properties of the prepared YAG:Eu3+phosphors was also examined.
2. Experimental
The sol–gel pyrolysis method employed in this investi-gation for the preparation of YAG:Eu3+ nanophosphors is described inFig. 1. In all the above preparations, the Eu3+ concentration was fixed at an optimum value of 5 mol% with respect to Y3+(Y2.85Eu0.15Al5O12). Samples were prepared
by combining PVA (with degree of polymerization around 1500) with urea together as fuel. Two sets of samples were prepared with different fuel systems, where A (A1, A2, and
A3) denotes the mixture of equi-molar PVA and urea used
as fuel; B (B1, B2, and B3) represents the system using only
urea as the fuel, and the subscripts (1, 2, and 3) indicate different oxidizer to fuel ratios (Φ, equal to 0.5, 1, and 2.0, respectively). The fuel system of each sample is
summa-Fig. 1. Flow chart showing various stages in the preparation of YAG:Eu3+ phosphors via sol–gel polymer pyrolysis.
rized inTable 1. The corresponding nitrates of each element were dissolved in deionized water, followed by adding the designed amount of PVA and urea solution. The resulting solution was stirred and heated at 150◦C on a hot plate for 2 h. The clear solution was turned into yellowish gel by heat treatment at 250◦C for 2 h. The dry gel was then calcined at 1000◦C for 2 h, and uniform white powder was obtained. On the other hand, YAG:Eu3+powders were also prepared via the solid-state reaction process using Al2O3, Y2O3, and
Table 1
Preparation conditions and characteristics of YAG:Eu3+phosphors using
sol–gel pyrolysis
Fuel system Sample Φa Tf (K)b C (%)c Crystallite size (nm) PVA and urea A1 0.5 2204 35.8 41.1
A2 1.0 2146 67.9 45.2
A3 2.0 700 59.3 43.1
Urea B1 0.5 2036 10.6 25.8
B2 1.0 1607 23 43.1
B3 2.0 390 17.7 36.2
a Φ: the ratio between the sum of oxidizing valencies and the sum of reducing valencies.
b Tf: flame temperature.
Eu2O3 (all analytical grade) as starting materials, through
heating at 1500◦C for 6 h. The solid-state reaction derived sample was named sample S.
The phases in the prepared samples were examined us-ing a MAC Science MXP3 X-ray powder diffractometer employing CuK␣1 radiation at room temperature. The par-ticle sizes of various samples were calculated from XRD line-width data according to the Debye–Scherrer formula. The morphology and particle size of the prepared samples were analyzed using a Hitachi H-7100 transmission electron microscope (TEM) operated at 75 kV. Photoluminescence spectra were recorded using a Hitachi F-4500 fluorescence spectrophotometer employing Hamamatsu 3788 photomul-tiplier as the light detector, 50 W Xe arc discharge lamp as the light source and gratings with a groove density of 1200 lines per mm. The obtained excitation spectra were corrected for the beam intensity variation of the Xe-light source used.
3. Results and discussion
3.1. Preparation of YAG:Eu3+phosphors via the sol–gel
pyrolysis process
Three different values ofΦ (0.5, 1, and 2.0) were used to prepare YAG:Eu3+phosphors in this study. When the value of Φ equals unity, maximum energy will be released for the combustion reaction to achieve the target materials.18It should be noted that the samples prepared with Φ = 2.0 andΦ = 0.5 correspond to fuel-deficient and fuel-rich sys-tems, respectively. The adiabatic flame temperature during the combustion process can be estimated with the following equation:
Tf= T0+
Hr− Hp
Cp
(2) where Tf is the flame temperature, T0 is the starting
tem-perature of the reaction; Hrand Hp, respectively, are the
formation enthalpies of the reactants and the products, and
Cpis the heat capacity under constant pressure of the
prod-ucts. The flame temperatures for various cases have been calculated and are listed inTable 1. From the XRD patterns, it is found that all the samples prepared in system A and B are well indexable with ICDD file #33-0040 corresponding to the YAG system (Fig. 2 and 3). Both powders in system A and B were calcined at 1000◦C for 2 h. The percentage of crystallinity (C) for all samples was calculated based on the relative intensity of the (420) diffraction line of the YAG system in various cases with respect to that of the solid-state prepared sample (sample S shown inFig. 2), and the derived values are summarized inTable 1. It is obvious that the C value is the maximum for the case ofΦ = 1, regardless of the type of fuel system used. For differentΦ values, the C values of system A are much higher than those of system B. These results are in agreement with the calculated flame temperature.
Fig. 2. X-ray diffraction patterns of YAG:Eu3+ phosphors prepared via
sol–gel pyrolysis using PVA and urea combined fuel system (system A). For sample A1, A2, and A3, the oxidizer to fuel ratioΦ is equal to 0.5, 1.0, and 2.0, respectively. Sample S is prepared via the solid-state reaction.
3.2. Microstructures of YAG:Eu3+phosphors prepared via
the sol–gel pyrolysis process
Fig. 4shows the TEM bright field images of the samples prepared atΦ = 0.5, 1, and 2 in system A (PVA–urea sys-tem) prepared at 1000◦C. It is evident that well dispersed YAG particles (size∼40 nm) with a characteristic polyhe-dral morphology are obtained for the case of Φ = 1 in
Fig. 4(b). For the sample derived from rich fuel given as de-picted inFig. 4(a), two kinds of particles can be observed, i.e. small particles (size∼35 nm) with polyhedral morphol-ogy and large particles (size∼150 nm) with cuboidal mor-phology. It is considered that in the case of Φ = 0.5, the presence of excess fuel might accelerate the particle growth
Fig. 3. X-ray diffraction patterns of YAG:Eu3+ phosphors prepared via sol–gel pyrolysis using urea only as the fuel (system B). For sample B1, B2, and B3, the oxidizer to fuel ratioΦ is equal to 0.5, 1.0, and 2.0, respectively.
Fig. 4. Transmission electron microscope images of YAG:Eu3+phosphors prepared via sol–gel pyrolysis using PVA and urea combined fuels (system A) with (a)Φ = 0.5, (b) Φ = 1.0, and (c) Φ = 2.0.
process locally and result in particles with different mor-phologies. For the sample provided with fuel-deficient (Φ = 2.0) system as shown in Fig.4(c), agglomerates of smaller particles (size∼20 nm) having nearly polyhedral morphol-ogy were observed. It is not surprising that in the case of
Fig. 5. Crystallite size of YAG:Eu3+phosphors prepared via the sol–gel pyrolysis and solid-state reaction.
fuel-deficient system, the particle growth could not get com-pleted owing to the lack of sustainability of the combustion reaction, therefore, accounting for smaller particles. In addi-tion, the scope of agglomeration might be considerably re-duced so that individual crystallites showing single-crystal features were observed.
The values of crystallite size calculated from the (420) XRD peak in conjunction with the Scherrer formula are in agreement with the estimated size derived from the electron microscopy results. The results for the crystallite size as calculated from XRD peak-width data for phosphors of both system A and B prepared at 1000◦C for 2 h are given in
Fig. 5. This again confirms that nanoparticles with the largest size are obtained only for the case of Φ = 1. Also for system A, it is found that the pyrolysis reaction starts at around 800◦C and produces a highly agglomerated sample with individual crystallites having an average size of 5 nm. Well dispersed nanoparticles are obtained only for samples heated at 1000◦C and above. The foregoing observations clearly verify that the optimum ratio of oxidizer to fuel for the preparation of well crystallized nanoparticles via the pyrolysis reaction isΦ = 1.
For system B that only involved urea as fuel, the corre-sponding electron microscopy images of samples heated at 1000◦C for 2 h with differentΦ values are shown inFig. 6. This figure clearly suggests a highly agglomerated nature of the YAG particles. In all cases of Φ = 0.5, 1, and 2, no distinct crystallite features can be observed regardless of the temperature of synthesis (800–1000◦C). These agglom-erates might be the result of absence of any organic cages serving as the dispersion agent during the course of pyrol-ysis reaction. As shown inTable 1, it is found that the per-centage of crystallinity (C) in the samples of system B is approximately 1/3 of that of corresponding samples in sys-tem A. Moreover, the theoretical flame sys-temperatures (Tf) of
iden-Fig. 6. Transmission electron microscope images of YAG:Eu3+phosphors prepared via sol–gel pyrolyis using urea as the only fuel (system B) with (a)Φ = 0.5, (b) Φ = 1.0, and (c) Φ = 2.0.
tical Φ values. The decrease in percentage of crystallinity is attributed to the low temperature of sustained pyrolysis reaction. In system A, where the fuel is a mixture of PVA and urea, sufficient heat is generated when the combustion of all the vinyl groups happens. The results in Fig. 4
evi-Fig. 7. Photoluminescence emission spectra of YAG:Eu3+phosphors pre-pared via sol–gel pyrolysis employing (a) PVA and urea, (b) urea only along with YAG:Eu3+samples prepared by the solid-state reaction (under
excitation of 220 nm radiation).
dently demonstrate the necessity to employ an organic poly-meric network such as PVA in addition to the fuel for pro-viding organic cages for the preparation of well dispersed nanoparticles. Furthermore, the particle size and morphol-ogy of YAG:Eu3+ are found to depend on both the fuels employed and the ratio of oxidizer to fuel.
3.3. Luminescence transition in YAG:Eu3+phosphors
Photoluminescence emission spectra of Eu3+doped YAG prepared by the sol–gel pyrolysis method employing urea and PVA as fuels at different oxidizer to fuel ratios are il-lustrated inFig. 7. From this figure, it is found that the red emission around 608 nm due to 5D0 → 7F2 transition is
weak in this system. This is because it is reduced in the YAG system due to the centrosymmetric surroundings.19 On the other hand, the5D0→7F1transition involving a magnetic
dipole mechanism is structure independent. Therefore, this can serve as an internal standard to assess the strength of the various f–f transitions of Eu3+. The intensity ratio of5D0→ 7F
2to5D0→7F1can be viewed as a clue concerning the
nature of the chemical surroundings of the luminescent cen-ter and its symmetry. The values of the relative strength of
5D
0→7F2to5D0→7F1for sample A2and S are 0.5 and
0.24, respectively. It indicates that in the case of YAG:Eu3+ nanoparticles (A2), there is a doubling in the intensity of the 5D
0 →7F2transition. This suggests that slightly modified
chemical surroundings can enhance the transition strength of the hypersensitive transition.
Based on the emission spectra given inFig. 7, it is obvious that sample A2shows the maximum luminescence intensity,
which is three times higher than that of the bulk sample S (with a size of 0.9m) prepared by solid-state reaction. The luminescence enhancement in the case of nanophosphors can be explained from the excitation spectra monitoring the
Fig. 8. Photoluminescence excitation spectra of YAG:Eu3+ phosphors prepared using sol–gel pyrolysis and the bulk sample, corresponding to emission at 608 nm due to5D0→7F2electric dipole transition. The solid arrow shows the presence of the sideband around band edge of the YAG system in particular for the nanoparticles prepared using PVA and urea as fuels (A2).
hypersensitive5D0→7F2emission inFig. 8. The emission
wavelength was fixed at 608 nm. All these samples demon-strate strong excitation peaks at 240 nm, which conforms to the value of the charge transfer band in the YAG:Eu3+ system.20 However, it can also be noted that the nanocrys-talline sample (A2) exhibits an extension of the excitation
spectrum in the short wavelength region (at around 220 nm) while the bulk counterpart (sample S) does not show such an excitation band. The presence of an extension of the excita-tion band corresponding to the charge transfer band can be explained by considering multiple surface states because of the large surface area in the nanocrystalline sample. These surface states in the nanocrystals arise from unsaturated chemical bonds, different chemical coordination, and bro-ken lattice periodicity. Owing to the large surface to volume ratio, these surface states play a prominent role in nanocrys-tals. Under photoexcitation near 220 nm, the tendency to un-dergo charge transfer will be more pronounced. Therefore, stronger charge transfer transitions providing the energy to the excited5D levels can lead to luminescence enhancement in well dispersed nanocrystals. On the other hand, the case of agglomerates comprising nanoparticles such as sample B2
appears to be quite different in that the role of surface states seems to be less pronounced. In the case of agglomerates, the particle-to-particle contact may modify the unsaturated bonds on the surface of the crystallites and provide alterna-tive non-radiaalterna-tive relaxation path in the same manner as the situation in the bulk system.Fig. 9illustrates the energy level diagram of the well dispersed YAG:Eu3+nanoparticles. For the well dispersed nanoparticles, the existence of enormous surface states (SS) results in additional charge transfer tran-sition (CTS). This additional trantran-sition (CTS) besides the charge transfer band (CTB) will provide energy to the5DJ
Fig. 9. Schematic diagram of the energy levels in the well dispersed YAG:Eu3+nanoparticles. The gray blocks denote the positions of surface
states (SS). CTB indicates the Eu3+–O2− charge transfer band in the
general YAG:Eu3+system. CTS represents the charge transfer due to the
existence of enormous surface states.
states of Eu3+, and the absorbed energy in5DJ states will
transfer to 7FJ states. Hence, the charge transfer to Eu3+
will become more probable, leading to the enhancement of luminescence emission in the YAG:Eu3+nanoparticles.
4. Conclusions
Sol–gel polymer pyrolysis method employing urea and PVA combined fuel system appears to be a feasible way for the synthesis of YAG:Eu3+ nanophosphors at low temper-atures. Through this method, well dispersed nanoparticles having an average size of 40 nm can be obtained. Well dis-persed nanoparticles are obtained when the oxidizer to fuel ratio (Φ) is maintained at an optimum value of 1. On the other hand, when urea is used as the only effective fuel (i.e. in the absence of polymeric network in the process), seri-ously agglomerated YAG particles will be obtained. Results based on Eu3+ luminescence serving as a local probe con-firm these microstructural changes in these samples. Further-more, Eu3+luminescence in this nanoceramic system shows a size-dependent luminescence enhancement that can be at-tributed to pronounced surface states having charge trans-fer origin. The luminescence enhancement observed in the nanophosphors is of practical importance for this system to be applied to field emission devices.
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