The paper presents an analysis of the luminescent properties of Ce3+-doped calcium hydroxyapatite 
(Ca10(PO4)6(OH)2, HAp) nanopowders prepared by the chemical precipitation and sol-gel method. It was 
shown that in nanoscale HAp Ce3+ ions are strongly perturbed by surface defects and their local symmetry 
differs from that of Ce3+ ions in the bulk crystals. Possible origin of defects in nanosized HAp is also discussed.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3489

Berezovskaya, I.V.

Dotsenko, V.P.

Efryushina, N.P.

Zubar, E.V.

SocioEconomic Challenges Journal (Sumy State University)

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE NANOMATERIALS: APPLICATIONS AND PROPERTIES 
Vol. 1 No 1, 01PCN24(4pp) (2012) 
 
 
2304-1862/2012/1(1)01PCN24(4) 01PCN24-1  2012 Sumy State University 
Distribution and Luminescent Properties of Ce3+ Ions in Nanosized Calcium Hydroxyapatite 
 
I.V. Berezovskaya, N.P. Efryushina, E.V. Zubar, V.P. Dotsenko 
 
A.V. Bogatsky Physico-Chemical Institute, National Academy of Sciences of Ukraine, Lustdorfskaya doroga, 86, 
65080 Odessa, Ukraine 
 
(Received 19 June 2012; published online 24 August 2012) 
 
The paper presents an analysis of the luminescent properties of Ce3+-doped calcium hydroxyapatite 
(Ca10(PO4)6(OH)2, HAp) nanopowders prepared by the chemical precipitation and sol-gel method. It was 
shown that in nanoscale HAp Ce3+ ions are strongly perturbed by surface defects and their local symmetry 
differs from that of Ce3+ ions in the bulk crystals. Possible origin of defects in nanosized HAp is also dis-
cussed. 
 
Keywords: Calcium hydroxyapatite, Synthesis, Nanopowders, Cerium, Luminescence. 
 
 PACS numbers: 78.47.jd; 81.07.Wx 
 
 
1. INTRODUCTION 
Compounds and solid solutions with the apatite-
type structure are of great interest since they are used 
as phosphors, biomaterials and ionic exchangers. Be-
sides, it has been recently shown that lanthanide-doped 
calcium hydroxyapatite (HAр) nanoparticles can be 
used as bioactive, fluorescent carriers of biological mol-
ecules and drugs [1,2]. HAp belongs to the hexagonal 
system with the P63/m space group. In this lattice, two 
unequivalent calcium sites are present: Ca(I) with C3 
symmetry is surrounded by nine oxygen atoms, and 
Ca(II) with CS symmetry is coordinated to six oxygen 
atoms and one OHˉ group [3]. It is accepted that the 
disordering of OHˉ ions in c-axis columns causes a 
structural complexity of HAp. It is well known that 
apatites can accommodate a large number of different 
ions in their lattice. The distribution of trivalent lan-
thanides (Ln3+) between the two alkaline earth sites in 
different apatites has been the subject of numerous 
studies [4-7]. It has been found that trivalent lantha-
nides in Ca(II) position are characterized by abnormal-
ly high values of Stark splitting. This provided a dis-
tinction of the spectral features of these ions from those 
of Ln3+ in Ca(I) position. However, the results obtained 
are contradictory. For example, the luminescence prop-
erties of Eu3+ in calcium apatites under UV excitation 
have been studied by several authors [4,6]. 
It was concluded that Eu3+ ions occupy mainly the 
Ca(II) positions. In contrast to this conclusion, Karbow-
iak and Hubert [7] have reported that the nature of site 
occupancy by Eu3+ in calcium fluoroapatite varies de-
pending on the material preparation method. Some of 
us have studied the luminescence properties of Ce3+ 
ions in Ca10(PO4)6(OH)2 prepared by solid state reaction 
method at 1100°C [8]. It has been found that at rela-
tively low concentrations (≤ 0.5 at.%) Ce3+ ions tend to 
occupy preferably Ca(I) sites, and these Ce3+ centers 
cause an intense emission with maxima at 335 and 362 
nm. The aim of this work is to study the influence of 
the preparation route on the distribution and lumines-
cent properties of Ce3+ ions in HAp. 
 
2. EXPERIMENTAL 
 
Samples of nominal formula Ca10(1-x)Ce10x(PO4)6(OH)2 
(x  0.005) used in the present work were prepared by 
two different methods. The samples were checked by X-
ray diffraction (XRD) using Cu Kα radiation. No impurity 
phases were detected in the XRD patterns. Morphologi-
cal investigations were carried out by scanning electron 
microscopy (SEM) on JEOL JSM 6390LV electron micro-
scope. The emission and excitation spectra in UV-visible 
region were recorded at room temperature using a 
Fluorolog Fl-3 (Horiba Jobin Yvon) spectrofluorometer 
equipped with a xenon lamp. The measurements of 
emission spectra were also performed at 8 and 293 К 
using synchrotron radiation and the equipment of the 
SUPERLUMI experimental station [9] of HASYLAB 
(Hamburg, Germany). The Fourier-transformed infrared 
absorption spectra were recorded using a Bruker Vertex 
70 spectrometer. 
 
3. RESULTS AND DISCUSSION 
 
In this work, Ce3+-doped Ca10(PO4)6(OH)2 with dif-
ferent particle sizes have been synthesized by chemical 
precipitation and sol-gel method. To prepare calcium 
phosphate nanoparticles by a precipitation process, an 
aqueous solution of (NH4)2HPO4 was slowly added to an 
aqueous solution of calcium nitrate tetrahydrate 
Ca(NO3)2·4H2O upon continuous stirring. The pH of the 
mixture was adjusted to 10 by adding NH4OH and the 
reaction temperature was maintained at 60°C. The 
resulting precipitate was aged for 0.5 h under stirring, 
and then filtered, washed with distilled water and 
dried at 105°C. The preparation of HAp powders by the 
sol-gel method [10] involved the use of aqueous solution 
of calcium nitrate tetrahydrate Ca(NO3)2·4H2O and 
alcohol solution of trimethyl phosphate (CH3O)3PO as 
the precursors. Some amount of citric acid (as a chelat-
ing agent) was added into aqueous solution of 
Ca(NO3)2·4H2O. The solutions were mixed at room 
temperature and kept at pH 7.5 for 48 h. The obtained 
gel was dried at 190°C for 2 h, and then fired at 600°C 
for 1 h. In the both cases cerium nitrate was added to 
the calcium nitrate solutions to obtain molar ratio of 
Ce/(Ca+Ce)  0.005. The luminescent properties of the 
samples were analyzed in the as prepared-state and 
after annealing at 800°C for 2-3 h in air or in a reduc-
ing medium of CO. The average sizes of crystallites (d) 
of the as-prepared polycrystalline materials were calcu-
lated from XRD-patterns by the Scherrer equation [11]. 
 I.V. BEREZOVSKAYA, N.P. EFRYUSHINA, E.V. ZUBAR, V.P. DOTSENKO PROC. NAP 1, 01PCN24 (2012) 
 
 
01PCN24-2 
The reflections from the (300) and (002) planes were 
used. The obtained values varied from 34 to 70 nm, in 
agreement with the results reported previously for HAp 
nanopowders prepared either by chemical precipitation 
method [1,12] or by the sol-gel process [10]. A typical 
SEM image of HAp prepared by the sol-gel method is 
presented in Fig. 1. It is seen that the material is com-
posed of crystallites with an irregular form, and which 
exhibit a high degree of aggregation.  
 
 
 
Fig. 1 – SEM photograph of Ca10(1-x)Ce10x(PO4)6(OH)2 
(x  0.005) synthesized by the sol-gel method 
 
The emission spectra of the obtained materials de-
pend on the preparation conditions. Fig. 2 shows the 
emission spectra of Ca10(1-x)Ce10x(PO4)6(OH)2 (x  0.005) 
prepared by the chemical precipitation method. Under 
excitation in the 240-300 nm region, the spectrum of 
the as-prepared sample extends from 310 to 430 nm. It 
has a maximum around 370 nm and a shoulder at 
about 340 nm. As can be seen from Fig. 2, the anneal-
ing of this sample at 800°C significantly increases the 
emission intensity, and two maxima at ~ 338 nm and 
368 nm are clearly observed. These changes can be at-
tributed to the increase in the solubility of CeO2 in 
HAp, and the suppression of the luminescence-
quenching action produced by surface defects. One can 
expect that in nanoscale HAp Ce3+ ions in the surface 
region are strongly perturbed by the surface states and 
their point symmetry somewhat differs from that of 
Ce3+ ions in the bulk crystals. 
Fig. 3 shows the low temperature time-resolved 
emission spectra of the Ca10(1-x)Ce10x(PO4)6(OH)2 
(x  0.005) sample annealed at 800°C. Both spectra can 
be reasonably decomposed into three Gaussian-type 
bands peaked at 335, 362 and 388 nm. Their origin has 
been discussed previously by some of us [8]. The bands 
at 335 and 362 nm must be attributed to the transi-
tions from the lowest Ce3+ 5d excited state to the 4f 
ground state levels 2F5/2 and 2F7/2. The energy gap be-
tween the maxima (1960 cm-1) coincides with the spin-
orbit splitting of Ce3+ ground state. 
We will denote all the centers related to the 335 and 
362 nm emission bands as CeI. It is clear that the spec-
tra in Figs. 2,3 are mainly due to the CeI emission. The 
centers related to the 388 nm emission band will be de-
noted as CeII. Note that the CeII emission band does not 
show its characteristic doublet structure, indicating that 
there are several Ce3+ centers with overlapping spectra.  
 
 
 
Fig. 2 – The emission spectra of Ca10(1-x)Ce10x(PO4)6(OH)2 
(x  0.005) prepared by the chemical precipitation method: 1- 
as-prepared sample; 2- after firing at 800°C for 2 h in air. The 
spectra were recorded at 293 K upon excitation at 266 nm 
 
 
 
Fig. 3 – The time-resolved emission spectra of the 
 Ca10(1-x)Ce10x(PO4)6(OH)2 (x  0.005) sample at 8 K (λexc  266 
nm). The spectra were recorded for two different time inter-
vals (∆t) after picosecond pulse excitation: (1) ∆t  1-190 ns; (2) 
∆t  1-8 ns 
 
It was shown that the dominant center (CeI) with 
emission maxima at 335 and 362 nm consists of a Ce3+ 
ion in the Ca(I) position [8]. In this case, the charge 
compensation for Ce3+ ions is provided by calcium va-
cancies, and in samples with relatively high concentra-
tions of Ce3+, a single calcium vacancy will compensate 
one Ce3+ ion distantly and one Ce3+ locally. 
The other type of Ce3+ centers (CeII) can be attribut-
ed to a Ce3+ ion on the Ca(II) position. The charge com-
pensation of Ce3+ ions occupying the Ca(II) positions is 
provided by substitution of OHˉ ions by O2-. This mech-
anism implies the formation of the neutral (CeCa•OOH)× 
associates. This causes the lower-energy position of the 
CeII emission band (~390 nm) as compared to the CeI 
emission band due to a larger crystal field splitting of 
the Ce3+ 5d configuration and to strengthening of the 
covalent bond between Ce3+ and its environment. 
 
 DISTRIBUTION AND LUMINESCENT PROPERTIES OF CE3+ IONS… PROC. NAP 1, 01PCN24 (2012) 
 
 
01PCN24-3 
 
 
Fig. 4 – The emission spectra of Ca10(1-x)Ce10x(PO4)6(OH)2 
(x  0.005) prepared by the sol-gel method: 1 - as-prepared 
sample; 2- after firing at 800°C for 3 h in a reducing medium 
of CO; 3 - after firing at 800°C for 3 h in air. The spectra were 
recorded at 293 K upon excitation at 288 nm 
 
The emission spectra of Ca10(1-x)Ce10x(PO4)6(OH)2 
(x  0.005) prepared by the sol-gel method are present-
ed in Fig. 4. The emission spectrum of the as-prepared 
sample extends from 300 to 500 nm and has a local 
maximum around 360 nm, which can be attributed to 
the Ce3+ 5d → 4f transitions. In addition to the 
Ce3+ 5d → 4f emission band the spectrum contains rela-
tively intense features at 414 and 434 nm. It is seen 
that the annealing of the sample at 800°C suppresses 
completely the emission in the 400-460 nm region, 
whereas the high energy band shows clearly the dou-
blet structure and its intensity increases significantly. 
The positions of the maxima (335 and 362 nm) are in 
agreement with those found for the CeI emission in 
HAp prepared either by the chemical precipitation 
method (see Fig. 2) or by solid state reactions [8].  
The results presented in Fig. 4 indicate that the 
emission in the 410-500 nm range is not an intrinsic 
property of HAp, but is due to unknown defects or impu-
rities. The probable candidates are CO2ˉ radicals situat-
ed in numerous pores of the nanosized HAp.  
Indeed, Zhang et al. [13] have revealed that strontium 
hydroxyapatite microspheres prepared by facile sol-
vothermal process exhibit an intense broadband emis-
sion with a maximum at 427 nm. This emission was ten-
tatively assigned to the CO2ˉ radicals formed during the 
synthesis process. Earlier, Angelov et al. [14] have re-
ported that the CO2ˉ radicals in interstitial positions of 
the aragonite lattice are probably responsible for self-
activated luminescence around 490 nm in SrCO3. This 
interpretation is indirectly supported by the following 
experimental facts: a) defects responsible for the emis-
sion at 414 and 434 nm are not stable at 800°C (see Fig. 
4), b) Ca10(1-x)Ce10x(PO4)6(OH)2 (x  0.005) prepared by 
the sol-gel method contains significant quantity of car-
bonate ions (CO32-). Fig. 5 shows the typical FTIR ab-
sorption spectrum of HAp prepared by the sol-gel meth-
od. It contains the stretching and bending vibration 
bands of OHˉ groups at 3572 and 631 cm-1, asymmeric 
(ν3) and symmetric (ν1) stretching vibration bands of the 
isolated PO43- groups at 1090, 1039 and 960 cm-1. 
 
 
 
Fig. 5 – FTIR absorption spectrum of HAp prepared by the 
sol-gel method 
 
The bending vibrations (ν4, ν2) of the PO43- groups 
cause the bands at 601, 568 cm-1 and 472 and 435 cm-1, 
respectively. In addition to these features, there are 
bands at 1457, 1416 and 873 cm-1, which must be at-
tributed to the stretching and bending vibrations of the 
CO32- anions occupying both the OHˉ and PO43- positions 
[10,12,15]. Since these bands were absent in FTIR ab-
sorption spectrum of HAp prepared by the chemical pre-
cipitation method, one can expect that the incorporation 
of the CO32- groups into the crystal lattice is a conse-
quence of the use of citric acid as a reagent. It is evident 
that this can be also responsible for the presence of CO2ˉ 
radicals in HAp prepared by the sol-gel method. 
 
4. CONCLUSIONS 
 
Ce3+-doped Ca10(PO4)6(OH)2 with different particle 
sizes have been synthesized by the chemical precipita-
tion and sol-gel method. The site preference of Ce3+ ions 
for the two calcium sites Ca(I), Ca(II) in the apatite 
structure was studied by means of luminescent spec-
troscopy. It was found that the distribution of Ce3+ ions 
on the calcium sites depends on the preparation condi-
tions. Although Ce3+ ions tend to occupy the Ca(I) posi-
tions with C3 symmetry in nanoscale HAp they are 
strongly perturbed by different surface defects, so that 
their local symmetry differs from that of Ln3+ ions in 
the bulk crystals. 
 
 
REFERENCES 
 
1. S.P. Mondejar, A. Kovtun, M. Epple, J. Mater. Chem. 17, 
4153 (2007). 
2. P. Yang, Z. Quan, C. Li, X. Kang, H. Lian, J. Lin, Bio-
materials 29, 4341 (2008). 
3. J.M. Hughes, M. Cameron, A. Mariano, Amer. Mineral. 76, 
1165 (1991). 
4. R. Jahannathan, M. Kottaisamy, J. Phys.: Condens. Mat-
ter. 7, 8453 (1995). 
5. A. Serret, M.V. Cabanas, M. Vallet-Regi, Chem. Mater. 12, 
3836 (2000). 
6. R. Sahoo, S.K. Bhattacharya, R. Debnath, J. Sol. State 
Chem. 175, 218 (2003). 
 I.V. BEREZOVSKAYA, N.P. EFRYUSHINA, E.V. ZUBAR, V.P. DOTSENKO PROC. NAP 1, 01PCN24 (2012) 
 
 
01PCN24-4 
7. M. Karbowiak, S. Hubert, J. Alloys Compd. 302, 87 (2000). 
8. I.V. Berezovskaya, N.P. Efryushina, G.B. Stryganyuk, 
A.S. Voloshinovskii, E.V. Zubar, V.P. Dotsenko, Func. Ma-
ter. 15, 164 (2008). 
9. G. Zimmerer, Radiat. Meas. 42, 859 (2007). 
10. C.L. Chu, P.H. Lin, S. Dong, D.Y. Guo, J. Mater. Sci. Lett. 
21, 1793 (2002). 
11.  H.P. Klug, L.E. Alexander, X-ray diffraction procedures 
for polycrystalline and amorphous materials (New York: 
Willey-Interscience: 1974). 
12.  L.M. Rodriguez-Lorenzo, M. Vallet-Regi, Chem. Mater. 12, 
2460 (2000). 
13. C. Zhang, Z. Cheng, P. Yang, Z. Xu, C. Peng, G. Li, J. Lin, 
Langmuir 25, 13591 (2009). 
14. S. Angelov, R. Stoyanova, R. Dafinova, K. Kabasanov, 
J. Phys. Chem. Solids 47, 409 (1986). 
15. I.R. Gibson, I. Rehman, S.M. Best, W. Bonfield, J. Mater. 
Sci. Mater. Med. 12, 799 (2000). 
 


Distribution and Luminescent Properties of Ce3+ Ions in Nanosized Calcium Hydroxyapatite

https://essuir.sumdu.edu.ua/bitstream/123456789/34896/1/princon_2012_1_1_26.pdf

Distribution and Luminescent Properties of Ce3+ Ions in Nanosized Calcium Hydroxyapatite

Abstract

Similar works

Full text

Available Versions

SocioEconomic Challenges Journal (Sumy State University)