Journal of Alloys and Compounds 453 (2008) 253–260
Luminescence properties of SiO2:Eu3+ nanopowders:Multi-step nano-designing
V. Jokanovic a,∗, M.D. Dramicanin a, Z. Andric a, B. Jokanovic b,Z. Nedic c, A.M. Spasic d
a Institute of Nuclear Sciences “Vinca”, PO Box 522, 11001 Belgrade, Serbiab Institute fur Metallurgie, Department fur Termochemie und Makrokinetic, Claustahal, Germany
c Faculty of Physical Chemistry, University of Belgrade, Studentski trg, 11001 Belgrade, Serbiad Institute for Technology of Nuclear and Other Mineral Raw Materials, Belgrade, Serbia
Received 6 July 2006; received in revised form 5 November 2006; accepted 14 November 2006Available online 4 January 2007
bstract
The structure and optical properties, the luminescence, of the spherical SiO2:Eu3+ particles have been discussed. These particles were formedsing the combination of precursor hydro-thermal synthesis and its ultrasound spray pyrolysis. By the synthesis itself a morphology and/or structuralesign (particle design) and sub-structural design (sub-particles design/sub-structural elements as the sols, the silicon chains and the tetrahedrons)ere defined. The particle mean size and size distribution spectrum of the SiO2:Eu3+ systems were determined by the SEM analysis. The obtained
esults were compared with those derived from the theoretical model. Distribution uniformity and the size and morphology of the sol particles that3+
onstitute the SiO2:Eu systems were determined by the TEM microscopy and by the DLS. The length and composition of silicon chain inside the
ilicon sol particles was determined using the TGA and the IRS. Optical properties were investigated using luminescence excitation and emissionpectroscopy. The assembled results indicate the possibility of a particle structure designing of the SiO2:Eu3+ powder over a various hierarchicalevels.
Nanopowder systems based on the SiO2:Eu3+ cover wideange of applications: integrated opto-couplers, light coloronverters, fluorescence tubes, X-ray detectors, xerographsnd various display devices/equipment, photo-luminescent,athode-luminescent and electro-luminescent materials, lasersnd electronic devices [1–14].
In this paper we present a new approach to the multi-stepesigning or self-assembling of the SiO2:Eu3+ nanopowder,rom the silicon tetrahedrons toward a complex, assembled par-icle, which starts with the hydrothermal synthesis and continues
ith the ultrasound spray pyrolysis. The basis of this approach
s the precise powder particle structural designing on successivetructural levels during the synthesis process. These structural
evels are: structural units or the silica tetrahedrons (molecu-ar level), silica chains and rings, sols, sub-particles, and finallyiO2:Eu3+ particles.
In brief, the spray-pyrolytical self-assembling process inacroblocs of particles and sub-particles may be understood as
n interaction between the two physical fields, internal (charac-eristics of material) and external (applied ultrasound) [15]. Pro-essing parameters are chosen in such way to make this interac-ion constrained to preserve all inherent materials physical prop-rties. Theoretical and experimental background, obtained onhe structural and sub-structural designing of a various ceramicystems (alumina, mulite, cordierite, calciumhydroxyapatite,itanium di-oxide, wolfram bronzes doped with a phosphorus,rgentum iodate) [15–25], is used in presented work to designnd investigate SiO2:Eu3+ powder self-assembling process. The
esulting SiO2:Eu3+ powder system is described by experimen-ally determined distributions of particle and sub-particle sizes,hich are further compared to the theoretical calculations. Lumi-escence of trivalent europium ion incorporated in this specific
dcal variations. Fig. 1 shows the TEM micrograph of SiO2:Eu3+
particles obtained after hydrothermal process.The particle size distribution from DLS measurements is
presented in Table 1.
Table 1Particle size distribution of the SiO2 sol determined by the dynamic lightscattering
d (nm) Participation (%)
<4.5 05.0 05.8 06.6 07.7 988.9 0
10.3 011.8 013.7 015.8 018.3 021.1 0
54 V. Jokanovic et al. / Journal of Allo
orm of silica is measured to investigate local environment influ-nce on characteristic europium optical transitions.
. Synthesis and characterization methods
The silica sol is prepared by mixing the water glass solutiona2O·3SiO2 (the ratio SiO2/Na2O is equal to 3.75 and its viscos-
ty is equal to 2.2 Pa s), with the 0.1N HCl solution. The soakings accomplished by dripping the 0.1N HCl into the water glassolution, vigorously mixed with a magnetic stirrer. The solutions held at 80 ◦C during mixing. Total amount of the added HClorresponds to the stoichiometric ratio needed to complete theeutralization reaction. In the starting mixture the ratio of theolid phase to the liquid phase was 1:4. After the addition of.1N HCl the mixture was transferred into an autoclave and theoaking is continued for 5 h at the temperature of 120 ◦C andhe pressure of 3 bar. Then the silicon acid precipitate is sepa-ated from the solution using an ultracentrifuge at 10,000 rpm,ollowed by an extra addition of the de-ionized water in order toet the liquid/solid phase ratio to 4:1. In addition, a small amountf HCl (a fifth of the previously dissolved amount) is drippednto the solution. The mixture is transferred into an autoclave forurther treatment according to the previously given procedure.inally, the silicon acid precipitate is separated from the liquidhase using the ultra centrifuging and the decanting operations.
In the second step of preparation, the silicon acid precipitates transformed into the sol. This was achieved by taking 20 g ofhe silicon acid obtained by soaking, according to the describedrocedure, and by adding 200 ml of de-ionized water, with pHet to 10 (using 0.1N NaOH solution). The system is intensivelyixed, again at 80 ◦C for a further 2 h, and its pH is set again on
0 by dripping 0.1N NaOH solution. The content is transferrednto an autoclave and treated further at the 120 ◦C and 3 bar forh. After hydro-thermal treatment, the silicon acid sol had pH 9.oncentrations of obtained sol solutions are set to values of 2, 1nd 0.1 M of silica. Then europium nitrate is added in an amounthat corresponds to 3 at.% in final solution. Precursors, obtainedn this way, are subjected to the ultrasonic spray pyrolysis. Theonditions during the pyrolysis process were: the frequency ofhe ultrasonic atomizer was f = 1.7 MHz, the working tempera-ure in the tubular furnace was T = 1000 ◦C, and the flow velocityf the carrier gas/air was v = 0.011 m/s.
Material phase analysis is performed using the transmis-ion electron microscopy—TEM (JOEL JEM 2000 FX) and thenfrared IR spectroscopy (PERKIN ELMER 983G). The thermoravimetric and differential thermal analysis—TGA/DTAAMNICO) of SiO2 powder are used to determine structuralotives of the SiO2 sol particles, based on the determination of
ounded water molecules in the system. Heating rates from roomo the working temperature of 800 ◦C were 5 and 50 ◦C/min.
Light scattering LSC is measured in the dynamical modedynamic light scattering-DLS) on the “Light Scattering Sys-em BI-200SM, Brookhaven Instruments” device equipped with
he BI-200SM goniometer, the BI-9000AT correlator, tempera-ure controller, and the Coherent INOVA 70C argon-ion laser.LS measurements are performed using 135 mW intense laser
xcitation at 514.5 nm and at detection angle of 90◦. Particle>
Fig. 1. TEM micrograph of the SiO2:Eu3+ particle.
ize distribution is calculated using the Brookhaven Instrumentsarticle sizing software.
Morphology, size distributions, and average size of sili-on dioxide particles and sub-structures are determined usinghe scanning electron microscopy—SEM (JEOL: JKSM-5300).amples for SEM measurements are prepared by coating ofowder with gold, using the PVD method. The particle sizesre found with the line intersection method using the scanningicrophotographs (accounting over 200 particles).
. Results and discussion
.1. Self-assembling during hydrothermal synthesis
TEM and DLS techniques are used for the particle sizeetermination and for the evaluation of particle morphologi-
24.4 028.2 132.6 133 0
V. Jokanovic et al. / Journal of Alloys and
Fr
7oo
usIc
3ccsrlceu
toa
n
wmo
a
n
F
ccte
F3ttcrbccamsin[
cowa
ig. 2. TGA curves for the silica sol samples depending on the various heatingates.
One can see from Fig. 1 that the mean particle size is aroundnm, and that the particles are mainly spherical. The resultsbtained by DLS well correspond to TEM observations, i.e. 98%f particles have 7.7 nm diameter.
Internal structure of the SiO2:Eu3+ particle is determinedsing the combination of the TG analysis (the mean length ofilica chains as particle constitutive elements, Fig. 2), and theR spectroscopy (the structure of the silica tetrahedrons as silicahain constitutive elements, Fig. 3).
Overall loss of mass is 3.02% at heating rate of 5 ◦C/min and.47% at heating rate of 50 ◦C/min, as can be seen in Fig. 2,urves 1 and 2, respectively. The difference between results isaused by the trapped water molecules in nano-pores of the driedilica sol, which cannot be completely unconfined for heatingate of 50 ◦C/min. Therefore, it seems that the value of massoss at heating rate 5 ◦C/min is more acceptable for further cal-ulations. Assuming that observed mass loss originates just fromvaporation of bonded water we find the fraction of OH− groups,OH, in the average silica chain equal to that value.
In order to determine the length of silica chains it is assumedhat OH− groups are present on both sides of the chain. Then the
verall SiO2 average silica chain mass, MSiO2 , can be expresseds
ASi + (2n − 1)AO + 2MOH = MSiO2 , (1)
batp
Fig. 3. IR spectra of the SiO2 (a) and SiO2:Eu3+ (b) p
Compounds 453 (2008) 253–260 255
here ASi, AO, MOH and MSiO2 are the atomic or molecularasses of the Si, O, and OH, respectively, and n is the number
f Si atoms/tetrahedrons.Further on
2MOH
nASi + (2n − 1)AO + 2MOH= uOH (2)
nd finally we find:
= 2MOH(1 − uOH) − uOHAO
uOH(ASi + 2AO). (3)
rom Eqs. (1)–(3) we calculated value for n to be 18.2.The assumption that the basic structural motive of the chain
onsists of two connected molecules of the dehydroxylated sili-on acid (the two interconnected silica tetrahedrons) implies thathe total number of such elements in the average silica chain isqual to 9.
Regarding the IR spectrum of the SiO2:Eu3+ presented inig. 3 the following absroptions may be observed: bands at382 and 3382 cm−1 correspond to the extended vibration ofhe silanol group (Si–OH), band at 1628 cm−1 corresponds tohe twisting vibration of OH− group of molecular water/in thisase shifted to 1577 cm−1, bands at 1057 and 1030 cm−1 cor-espond to the transversal asymmetric Si–O–Si vibrations. Theands at 750 and 416 cm−1 shifted toward smaller wave numbersorrespond to the rocking vibration deformations in Si–O–Sihains due to the coupling of transversal symmetric O atomslong the bisection line of the Si–O–Si angle with simultaneousovement of Si cations. Finally, the band at 1325 cm−1 corre-
ponds to the transversal and longitudinal asymmetric vibrationsn the rigid siloxan rings–silica tetrahedrons, where a domi-ant participation of the longitudinal vibrations is disappeared26–31].
Considering the IR spectrum it is obvious that the SiO2:Eu3+
hains are similar to those of SiO2, i.e. that the chains consistsf the inter-connected tetrahedrons: (1) the tetrahedron groundsere the neighboring Si atoms that are bonded with three O
toms, so forming the ground-ground type of the bond; (2) the
onds where the neighboring Si atoms are bonded with one Otom (O atom at the top of the tetrahedron) thus forming theop–top type of the bond. For the Eu3+ ions more convenientositions are at the top of the tetrahedrons, close to the O neigh-
owders obtained by the spray pyrolysis process.
2 ys and
btbttriet
3
3
ahlptom
wtloticu[
d
wsu
o(if[
wcn
o
d
Td
d
wMtm
[
I
waedwcbrna
I
3
gtwdt
wd
t
wi
r
wdifferent solutions (values of the numerical constant) as a set
56 V. Jokanovic et al. / Journal of Allo
orhood. At these positions the Eu3+ ions are bonded both, athe surface of SiO2 sols and inside the chain (infiltration in theulk). Such positioning of the Eu3+ ions influence the deforma-ion of the Si–O–Si bonds; that is the reason for decrease ofhe dominant participation of the longitudinal vibration in theigid siloxan rings–silica tetrahedrons. These structural changesnside the SiO2 tetrahedrons are, probably, caused by the influ-nce of the Eu3+ ions on the deformation of Si–O–Si bonds athe top of tetrahedrons.
.2. The particle self-assembling during the spray pyrolysis
.2.1. Particle levelThe waves formed by transferring ultrasound oscillations on
given precursor, depending on the superposition conditions,ave a complex spatial form that primarily depend on oscil-ation damping factors in different directions (in the directionarallel or perpendicular to the direction of deformation) and onhe forced frequency of the ultrasonic generator [15–25]. Basedn the 3D model of capillary standing waves, formed at theeniscus surface, the following expression is obtained:
∂ϕ
∂t+ gξ − σ
ρ
[∂2ξ
∂x2 + ∂2ξ
∂z2
]= 0, (4)
here ϕ is the potential rate of a capillary standing waves, σ
he solution surface tension, g the inertial force acting on theiquid in contact with the ultrasonic oscillator, ξ the amplitudef the standing wave, x and z are space coordinates and t ishe time coordinate. Under the assumption that the differencen the damping factors of transversal and longitudinal wavesan be neglected (in the case of very small thickness of liq-id column), the solution of Eq. (4) can be written in the form15–25]:
=(
πσ
ρf 2
)1/3
, (5)
here d is the mean radius of the formed aerosol droplet, ρ theolution density and f is the excitation frequency of the givenltrasonic oscillator.
In the case when the difference in the damping factorsf transversal and longitudinal waves can not be neglected,because the thickness of the liquid column is significant),t is possible to present the standing capillary wave in theorm of the Laplace equation expressed in polar coordinates15–25]:
ρ∂2ϕ
∂t2
∣∣∣∣r=R
− σ
R2
⎧⎨⎩2
∂ϕ
∂r+ ∂
∂r
⎡⎣ 1
sin θ
∂(
sin θ∂ϕ∂θ
)∂θ
+ 1
sin2 θ
∂2ϕ
∂ε2
⎤⎦
⎫⎬⎭ = 0,
(6)
here r is the radius of aerosol droplet, ε and θ are the anglesorresponding to the equation transformed into the polar coordi-ates. The solution of this equation gives a set of discrete values
o2ea
Compounds 453 (2008) 253–260
f aerosol droplet diameters:
= 1
π
(2σπ
ρf 2
)1/3
[l(l − 1)(l + 2)]1/3, (7)
he values of particle diameters formed from the aerosolroplets, through its solidification are given by
p = dd
(cprMp
ρpMpr
)1/3
(8)
here dp is the powder particle diameter, ρp the powder density,p the molecular mass of the powder, cpr the precursor concen-
ration (the solution used for spraying) and Mpr is the molecularass of the precursor.The particle size distribution can be estimated by equation
15–25]:
1 : I2 : · · · : IN = 1
f1:
1
f2: · · · :
1
fn
(9)
here I1, I2, IN are the intensities (the participation) of theppearance of corresponding discrete values of the particle diam-ters in a given spectra, or the corresponding numerical valuesefining the appearance frequencies of a given discrete values,hile f1, f2, . . ., fn are displacements of the real frequen-
ies from the forced frequency of the ultrasonic oscillator causedy damping factor of the precursor liquid column. The relativeatios of intensities (the frequency of a particular diameter) whenormalized gives the basic relationship for calculation of thebsolute value of the intensity [15–25]:
1 + I2 + · · · + IN = 1 (10)
.2.2. Sub-particle levelThe radius of nano-elements; nano-droplets that form the
iven aerosol droplet sub-structure and nano-particles formed byheir solidification, can be determined using the solution of theave equation of the centrally symmetric standing wave (formeduring an transferred excitation from the ultrasonic generator tohe droplet itself) [18,21]:
∂2ϕ
∂t2 = c2 1
r2
∂
∂r
(r2 ∂ϕ
∂r
), (11)
here c is the rate of disturbance spreading and r is the aerosolroplet radius. The solution is derived and given by [18,21]:
g kr = kr, (12)
here k is the wave number (k = 2πf/c) whose numerical solutions given by the set [18,21]:
= Nc
f(13)
here N is a numerical constant that has different values for
f solutions of Eq. (12). These values are: 0.175; 1.21; 1.72;.23; 2.74; 3.82; they depend on the form of the sub-structurallements comprising the particle formed in the droplet (aerosol)nd the sub-droplet (solidification process stage).
V. Jokanovic et al. / Journal of Alloys and Compounds 453 (2008) 253–260 257
pyro
3o
dt[
ttssa
a1eS
TE
d
0000111111
(is
t00o(
tswD
Fig. 4. Typical distribution of the SiO2:Eu3+ particles obtained by the spray
.3. Assessment of experimentally and theoreticallybtained size distributions
Based on the break-up model of standing capillary waves,escribed in previous section, one can predict and describehe complete particle and sub-particle distribution spectrum16,18,21].
As well as all previously discussed systems, here presentedhe SiO2:Eu3+ system, (Fig. 4) has been discussed consideringhe particle size distribution specter, i.e. the average particleize, the value of each particular diameter inside the distributionpecter, and the relative frequency of the particular diameterppearance among all possible values [15–25].
The mean particle diameters for the samples of SiO2:Eu3+,
nd corresponding concentrations 2, 1 and 0.1 M of SiO2, are.1, 0.87 and 0.71 �m, respectively (Table 2). The particle diam-ter distributions were limited to the ranges 0.5–1.66 �m (2 MiO2:Eu3+), 0.5–2.86 �m (1 M SiO2:Eu3+) and 0.36–1.71 �m
able 2xperimentally determined particle size distribution
p(2 M) (�m) Ip(2 M) dp(1 M) (�m) Ip(1 M)
.66 0.11 0.71 0.14
.83 0.07 0.86 0.11
.94 0.12 0.93 0.23
.99 0.1 1.0 0.06
.05 0.06 1.07 0.13
.11 0.14 1.14 0.05
.16 0.09 1.21 0.05
.38 0.06 1.43 0.09
.22 0.11 1.57 0.05
.55 0.14 2 0.09
(Sefif
tpt
TEa
C
210
lysis process: (a) 2 M SiO2:Eu3+; (b) 1 M SiO2:Eu3+; (c) 0.1 M SiO2:Eu3+.
0.1 M SiO2:Eu3+). According to the morphological character-stics, these particles have, usually, spherical and/or ellipsoidalhape.
The sphericity or elipsoidicity of these particles is given ashe ratio of the biggest over the smallest diameter, and its value is.75–0.86 for 2 M SiO2:Eu3+, 0.61–0.75 for 1 M SiO2:Eu3+, and.78–0.81 for 0.1 M SiO2:Eu3+. It may be remarked that somef the particles have extreme values of eccentricity (0.53–0.66)Table 3).
Also, in some systems, e.g. 0.1 M SiO2: Eu3+ few of the par-icles have well defined fiber structure at the surface, and theirhape is polygonal. It is possible to explain by low concentration,hen preferably dominant mechanism is surface precipitation.ue to this precipitation mechanism the thin wall hollow spheres
calculated thickness 7 nm, that correspond to the diameter ofiO2:Eu3+ sol particle) are obtained. These hollow spheres mayasily be transformed under the action of some temperatureeld in a more thermodynamically stable polyhedral particleorms.
In parallel with these experimental results the estimation ofhe correspondent theoretical values is performed. The meanarticle diameter, and the complete spectrum of discrete par-icle diameters and their frequencies of appearance in overall
able 3xperimentally determined average particle diameter, the minimal, the maximand the average eccentricity
(mol) da (�m) emax emin eaverage
M 1.10 0.75 0.86 0.82M 0.87 0.56 0.75 0.6.1 M 0.71 0.53 0.82 0.78
258 V. Jokanovic et al. / Journal of Alloys and
Table 4Predicted particle size distribution
dd (�m) Id dp(2 M) (�m) dp(1 M) (�m) dp(0.1 M) (�m)
Table 5Other characteristic parameters of the predicted particle size distribution
f (MHz) f (MHz) dp (�m)
2.37 0.67 1.2210
na
0Tv1wi
d
s4c(eMSti
Fp
osm
pttud
3
pEl
tegaps5s[
seaol(I
.23 0.47 1.89
.79 0.91 2.55
umber of particles are calculated (Eqs. (5), (7)–(9) and (10),nd Tables 4 and 5).
The calculated diameters are 0.98 �m for 2 M SiO2:Eu3+,.78 �m for 1 M SiO2:Eu3+ and 0.36 �m for 0.1 M SiO2:Eu3+.he difference between calculated and experimentally obtainedalues for two systems of higher concentrations were around1%, but for the third, the lowest, concentration the differenceas 49%; this was due to the surface precipitation mechanism
n this system.In a similar way the sub-particle sizes in the SiO2:Eu3+ pow-
ers are analyzed (Fig. 5).The experimentally obtained values of the sub-particle mean
izes for the systems 2 and 1 M SiO2:Eu3+ are in the range5–55 nm, and the calculated values obtained by theoreti-al model of the sub-designing are 35–40 nm (Eqs. (13) and8)) [18,21]. From these data one can see that the obtainedxperimental and theoretical data are in a fair agreement.
ore important difference appears only for the system 0.1 M
iO2:Eu3+; this is probably due to the specific precipita-ion mechanism. Furthermore, considering the percolor index,n some parts of the bulk entanglement and solidification
ig. 5. Typical sub-structure of the SiO2:Eu3+ particles obtained by the sprayyrolysis process.
dt
s5
Compounds 453 (2008) 253–260
ccurs; at the same time in the center of the sphere “empty”pace appears because of the deficiency of correspondingaterial.Regardless the small difference between these results, it is
ossible to calculate a diameter of the hollow sphere ring ando predict its design. Finally, it is possible, starting from theheoretical model and choosing the adequate frequency of theltrasound generator, to predict a type and complete quantitativeefinition of particles and sub-particles structuring process.
.4. Luminescence properties of the SiO2:Eu3+ powders
The luminescence of powders (2, 1 and 0.1 M SiO2:Eu3+ sam-les) is measured at room temperature and presented in Fig. 6.xcitation spectroscopy is performed using Perkin Elmer LS45
uminescence spectrometer.One can note the six resolved bands that originate from
ransitions within f-electron shell of the europium ions. Thenergy difference between 7F1,2 states of Eu3+ ion and itsround state 7F0 are about 360 and 1000 cm−1, respectively,llowing these states to be effectively populated at room tem-erature. This enables absorption from 7F0,1,2 to the excitedtates followed by characteristic de-excitations from: 5D1 at30 nm, 5D2 at 460 nm, 5D3 at 420 nm, 5L6 at 396 nm—thetrongest one, 5GJ and 5L8 around 380 nm, and 5D4 at 360 nm32–44].
The room temperature luminescence emission spectra of theame samples, excited at 396 nm, are presented in Fig. 7. Themission spectra are recorded after excitation into the 7F0 → 5D2bsorption band. The excitation source was an optical parametricscillator (OPO) pumped by the third harmonic of the Nd:YAGaser. The emission is analysed using HR250 monochromatorJobin–Yvon) and then detected by an ICCD camera (Princetonnstrument). Luminescence spectra are obtained with 1 ms timeelay after the laser pulse in order to limit the contribution of
he 5D1 emission.
Upon excitation europium doped silica powders exhibittrong red luminescence. The five emission bands, at around79, 590, 613, 653 and 700 nm, correspond to the internal con-
Fig. 6. Excitation spectra of the SiO2:Eu3+ powders.
V. Jokanovic et al. / Journal of Alloys and Compounds 453 (2008) 253–260 259
a of th
fi45
itbiElf[
5
ia
R
I2Sepw
tts�stood
4
oatstp
cSScsSscmirlsSfh7
mti
Fig. 7. Emission spectr
guration transitions of the Eu3+ ion 5D0 → 7FJ (J = 0, 1, 2, 3,). A selection rules allows band at 590 nm to be attributed to theD0 → 7F1 magnetic-dipole transition (J = 1), which is ratherndependent of local site symmetry. The 5D0 → 7F2,3,4 transi-ions are electric-dipole allowed transitions. The most intenseand is at 613 nm. It comes from the 5D0 → 7F2 transition, whichs hypersensitive to the symmetry of the crystal field around theu3+ and will be relatively strong if the local site symmetry is
ow. Well resolved emission bands at 660 and 705 nm originaterom the 5D0 → 7F3 and the 5D0 → 7F4 transitions, respectively32–44].
The asymmetry ratio of the integrated intensity of theD0 → 7F2 and 5D0 → 7F1 transitions can be considered asndicative of the asymmetry of the coordination environmentround the Eu3+ ion, and is given by
= I(5D0 → 7F2)
I(5D0 → 7F1). (14)
n this case very high values of asymmetry ratio are: 14.6 forM SiO2:Eu3+, 12.3 for 1 M SiO2:Eu3+, and 9.9 for 0.1 MiO2:Eu3+. This is evidence of strong structural disorder arounduropium ions and well correlates with supposition of Eu ionositioning in investigated structure presented earlier in the texthen discussing IR measurements.Influence of the internal defects on the luminescence proper-
ies of a crystal systems may be considered in a similar way forhe amorphous SiO2:Eu3+ system; the structure of the SiO2:Eu3+
ystem, at short distance, is similar to the structure of the crystal-quartz. As it is well known, the structural unit of amorphousilica is the SiO4 tetrahedron, where the Si atom is bonded to
he four oxygen atoms, and the O–Si–O bonds make an anglef 109.5◦. The lack of a regular distribution is due to the spreadf tetrahedral linkage angle of the Si–O–Si that is statisticallyistributed between 120◦ and 180◦.
6fSd
e SiO2:Eu3+ powders.
. Conclusions
The synthesis of the self-assembled SiO2:Eu3+ particles,btained by the spray pyrolysis of the mixture silica-sol–Eu3+
s a precursor, has been presented. The structural design of par-icles at different levels, starting with the particle as a whole, viaub-particles participating in the formation of these particles, tohe silica sol particles is investigated on the basis of measuredarticle and sub-particle size distributions and IR spectroscopy.
According to the SEM and TEM analysis the mean parti-le size after the spray pyrolysis process were 1.1 �m for 2 MiO2:Eu3+, 0.87 �m for 1 M SiO2:Eu3+, and 0.71 �m for 0.1 MiO2:Eu3+; the sub-particle size was around 35–40 nm. Theorresponding calculated values, obtained on the basis of the pre-ented model, were 0.98 �m for 2 M SiO2:Eu3+, 0.78 �m for 1 MiO2:Eu3+, and 0.36 �m for 0.1 M SiO2:Eu3+ and sub-particleize 38 nm, respectively. The average diameter of the sol parti-les is found to be 7.7 nm, obtained by dynamic light scatteringeasurements. The calculations based on the presented theoret-
cal model are in a fair agreement with experimentally obtainedesults, both those on the powder particle and the sub-particleevels, suggesting the applicability of the model to the SiO2:Eu3+
ystem and similar systems. The difference noticed for 0.1 MiO2:Eu3+ system is caused by the surface precipitation typicalor all systems with a small concentration of precursors. Theollow spheres obtained in that case had the ring diameter ofnm and preferred polyhedron form of particles.
Although quenched by water, observed in both TG and IReasurements, luminescence of trivalent europium ion in this
ype of silica structure is very strong. All 5D0 → 7FJ character-stic transitions are observed with the strongest one located at
13 nm (5D0 → 7F2). High values of the asymmetry ratio areound in all samples (14.6 for 2 M SiO2:Eu3+, 12.3 for 1 MiO2:Eu3+, and 9.9 for 0.1 M SiO2:Eu3+) suggesting highlyisordered local environment for the europium ion.
This research was supported by Ministry of Science and Envi-onmental Protection of the Republic of Serbia by the grants42066 and 142034G.
eferences
[1] S.W. Kang, Y.S. Kim, J. Korean Inst. Met. Mater. 34 (1996) 1375.[2] B. Jokanovic, T. Damjanovic, R. Weiss, G. Borchardt, Mater. Sci. Forum
453–454 (2004) 329.[3] M. Ohmori, E. Matijevic, J. Coll. Interf. Sci. 150 (1992) 594–598.[4] H. Sertchook, D. Avnir, Chem. Mater. 15 (2003) 1690.[5] D. Hreniak, E. Zych, L. Kepinski, W. Strek, J. Phys. Chem. Solids 64 (2003)
111.[6] A. Lempicki, A.J. Wojtowich, C. Brecher, in: S.R. Rotman (Ed.), Wide-gap
Luminescent Materials, Theory and Applications, Kluwer, MA, 1996.[7] R. Bertoncello, L. Milanese, R. Negro, L. Saragoni, S. Barison, J. Non-
Cryst. Solids 324 (2003) 73.[8] Y. Fukada, P.S. Nicholson, J. Eur. Ceram. Soc. 24 (2004) 17.[9] M. Morita, D. Rau, S. Kajyama, T. Sakurai, M. Baba, M. Iwamura, Mater.
Sci. Poland 22 (2004) 5.10] O.D. Velev, T.A. Jede, R.F. Lobo, A.M. Lenhoff, Nature 389 (1997) 447.11] H. Zhu, Y. Ma, Y. Fan, J. Shen, Thin Sol. Films 397 (2001) 95.12] F. Geotti-Bianchini, M. Preo, M. Guglielmi, C.G. Pantano, J. Non-Cryst.
Solids 321 (2003) 110.13] D.R. Daughton, J. MacDonald, N. Mulders, J. Non-Cryst. Solids 319 (2003)
297.14] P. Innocenzi, J. Non-Cryst. Solids 316 (2003) 309.15] V. Jokanovic, in: A.M. Spasic, J.P. Hsu (Eds.), Finelly Dispersed Particles:
Micro-, Nano- and Ato-engineeering, CRC Press, 2005.16] V. Jokanovic, Dj. Janackovic, A.M. Spasic, D. Uskokovic, Mater. Trans.
JIM 37 (1996) 627.17] V. Jokanovic, Dj. Janackovic, D. Uskokovic, Ultrason. Sonochem. 6 (1999)
157.18] V. Jokanovic, Dj. Janackovic, D. Uskokovic, Nanostruct. Mater. 12 (1999)
349.
[
[
[
Compounds 453 (2008) 253–260
19] V. Jokanovic, A.M. Spasic, D. Uskokovic, J. Coll. Interf. Sci. 278 (2004)342.
20] V. Jokanovic, I. Nikcevic, B. Dacic, D. Uskokovic, J. Ceram. Proc. Res. 5(2004) 157.
21] V. Jokanovic, D. Uskokovic, Mater. Trans. JIM 46 (2005) 228.22] V. Jokanovic, U.B. Mioc, Z. Nedic, S.St. Ionics 176 (2005) 2955.23] R.J. Lang, J. Acoust. Soc. Am. 34 (1962) 6.24] R.L. Peskin, R.J. Raco, J. Acoust. Soc. Am. 35 (1963) 1378.25] G.V. Jayan, S.C. Zhang, G.L. Messing, J. Aerosol. Sci. Technol. 19 (1993)
478.26] E. Matijevic, Chem. Mater. 5 (1993) 412.27] A. van Blaaden, A. Vrij, Langmuir 8 (1992) 2921.28] A. Bose, R.K. Gilpin, M. Jaroniec, J. Coll. Interf. Sci. 240 (2001) 224.29] D.R. Vollet, D.A. Donatti, A. Ibanez Ruiz, H. Maceti, J. Non-Cryst. Sol.
324 (2003) 201.30] R. Bertoncello, L. Milanese, R. Negro, L. Saragoni, S. Barison, J. Non-
Cryst. Sol. 324 (2003) 73.31] Y. Fukada, P.S. Nicholson, J. Eur. Ceram. Soc. 24 (2004) 17.32] A.N. Trukhin, M. Goldberg, J. Jansons, H.J. Fitting, I.A. Tale, J. Non-Cryst.
Sol. 223 (1998) 114.33] C.W. Thiel, Y. Sun, R.L. Cone, J. Modern Opt. 49 (2002) 1–18.34] V. Buissette, A. Huignard, T. Gacoin, J.P. Boileau, Chem. Mater. 14 (2002)
2264.35] P.R. Selvin, J. Jancarik, M. Li, L.W. Hung, Inorg. Chem. 35 (1996) 700.36] H. Nozary, C. Piguet, J.P. Rivera, P. Tissot, G. Bernardinelli, N. Vulliermet,
J. Weber, J.C.G. Bunzli, Inorg. Chem. 39 (2000) 5286.37] Q. Xu, L. Fu, L. Li, H. Zhang, R. Xu, J. Mater. Chem. 10 (2000) 2532.38] H.S. Peng, H.W. Song, B.J. Chen, J.W. Wang, S.Z. Lu, X.G. Kong, J.H.
Zhang, J. Chem. Phys. 118 (2003) 3277.39] J. Wang, S. Tian, G. Li, F. Liao, X. Jing, Mater. Res. Bull. 36 (2001)
1855.40] J. Dexpert-Ghys, M. Faucher, P. Caro, Phys. Rev. B 23 (1981) 607.41] J. Dexpert-Ghys, M. Faucher, Phys. Rev. B 20 (1979) 10.
42] O.L. Malta, E. Antic-Fidancev, M. Lemaˆıtre-Blaise, A. Milicic-Tang, M.
Taibi, J. Alloys Compd. 228 (1995) 41.43] E. Antic-Fidancev, J. Holsa, M. Lastusaari, A. Lupei, Phys. Rev. B 64
