In this work effect of the size on the oxygen vacancies concentration in cerium oxide nanocrystals have been investigated by means of luminescence spectroscopy techniques. For determination of changes of oxygen stoichiometry intensity of 5d-4f luminescence of Ce3+ ions were used.  It was shown that for CeO2 nanocrystals decrease of the size from 50 nm to 2 nm manifests itself in 8 times increase of the band intensity associated with vacancy-stabilized Ce3+ ions. The same effects have been observed at atmosphere variation from oxidizing to reducing and are connected with significant increase of concentration of oxygen vacancies. Obtained results allow to determine that decrease of the size stimulate formation of oxygen vacancies in cerium oxide nanocrystals

Maksimchuk, P.O.

Malyukin, Yu.V.

Masalov, A.A.

English

SocioEconomic Challenges Journal (Sumy State University)

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE  
NANOMATERIALS: APPLICATIONS AND PROPERTIES 
Vol. 3 No 2, 02NAESF05(3pp) (2014) 
 
 
2304-1862/2014/3(2)02NAESF05(3) 02NAESF05-1  2014 Sumy State University 
Optically Detected Effect of Size on the Oxygen Vacancies Concentration in Cerium Oxide 
Nanocrystals 
 
P.O. Maksimchuk*, A.A. Masalov, Yu.V. Malyukin 
 
Institute for Scintillation Materials National Academy of Sciences of Ukraine,  
60, Lenin Ave., 61001 Kharkov Ukraine 
 
(Received 24 April 2014; published online 29 August 2014) 
 
In this work effect of the size on the oxygen vacancies concentration in cerium oxide nanocrystals have 
been investigated by means of luminescence spectroscopy techniques. For determination of changes of oxy-
gen stoichiometry intensity of 5d-4f luminescence of Ce3+ ions were used. It was shown that for CeO2 nano-
crystals decrease of the size from 50 nm to 2 nm manifests itself in 8 times increase of the band intensity 
associated with vacancy-stabilized Ce3+ ions. The same effects have been observed at atmosphere variation 
from oxidizing to reducing and are connected with significant increase of concentration of oxygen vacan-
cies. Obtained results allow to determine that decrease of the size stimulate formation of oxygen vacancies 
in cerium oxide nanocrystals. 
 
Keywords: Luminescence spectroscopy, Cerium oxide nanoparticles, Oxygen vacancies. 
 
 PACS number: 78.67.Bf 
 
 
                                                 
* pmaksimchuk@isma.kharkov.ua 
1. INTRODUCTION 
 
In recent years, several authors have demonstrated 
that the size of the nanocrystal can greatly affect the 
oxygen non-stoichiometry of oxide material. 
Dependence of oxygen non-stoichiometry on the particle 
size most clearly manifested for nanosized cerium 
dioxide [1-6]. Cerium dioxide CeO2 has found 
application in different technology fields. Ceria is 
widely used for catalysts [7], fuel cells [8, 9], oxygen 
sensors [10], ultraviolet blocks [11], and luminescent 
materials [12, 13]. Recently, ceria nanoparticles have 
attracted attention within the biomedical research 
community as a potential agent to inhibit cellular aging 
[14, 15]. 
As was shown in [5], 1.9 nm is the critical size of 
cerium dioxide nanoparticles, at which all of cerium 
ions in the crystal lattice of CeO2 – x are triple-charged. 
According to existing ideas, oxygen non-stoichiometry 
defines basic biomedical properties of cerium oxide - the 
ability to participate in redox processes taking place in 
the organism [16, 17]. Extremely high concentrations of 
oxygen vacancies (and correspondingly, Ce3+ ions) in 
cerium oxide nanocrystals were recently determined by 
XPS investigations (17 % of  Ce3+ ions for 30 nm and up 
to 44 % for 3 nm nanoparticles) [18] and EMR (about 
18 % of Ce3+ ions for 3 nm nanoparticles) [19]. 
Literature data on the structure and properties of 
nanosized CeO2 – x often contradict each other. 
Moreover, most of the properties of nanocrystalline 
CeO2 – x, remain almost unexplored.  
The main goal of the present work was to find a 
way to determine the oxygen vacancies concentration in 
the CeO2 – x nanocrystals of different size using optical 
spectroscopy. The optical spectroscopy techniques used 
were fluorometry, to measure the luminescence spec-
trum of the CeO2-x nanocrystals.  These techniques are 
widely used to characterize semiconducting materials 
[20], but have not been extensively used for CeO2 – x 
nanocrystals. One advantage of these techniques is 
that the optical spectroscopy systems are inexpensive 
and have fast data acquisition. 
 
2. MATERIALS AND METHODS 
 
СеО2 nanocrystals were obtained by two methods: 
Pechini method [21, 22] and colloidal precipitation 
method [23]. Using of two methods of synthesis allow to 
obtain CeO2 nanocrystals of different sizes (2 nm and 
10 nm for colloidal precipitation method; 50 nm for 
Pechini method). Sizes of nanocrystals were controlled 
by TEM (Fig. 1). 
Stoichiometry of the nanocrystal depends on the 
atmosphere in which high-temperature treatment is 
carried out. Reducing atmosphere stimulates the for-
mation of oxygen vacancies and, consequently, the for-
mation of nonstoichiometric oxides. In the oxidizing 
atmosphere number of oxygen vacancies in the crystal 
is much lower thus leading to formation of CeO2 crys-
talline phase. To obtain a different degree of stoichiom-
etry, nanocrystals synthesized using Pechini method 
were annealed during 2 hours in different atmospheres 
– oxidizing (air), neutral (argon) and reducing (hydro-
gen) at 900 C.  
X-ray diffraction patterns of obtained CeO2 
nanocrystals are shown in the Fig. 2 (curve 1-50 nm 
cerium dioxide nanocrystals, curve 2-10 nm cerium 
dioxide nanocrystals and curve 3-2 nm cerium dioxide 
nanocrystals). The structure of the samples 
corresponds with JCPDS card No.34-0394, so the 
nanocrystals are characterized by FCC fluorite-type 
lattice and formation of any additional phases at these 
conditions can be excluded. 
Luminescence spectra of CeO2 nanocrystals have 
been obtained by means of the spectrofluorimeter based 
on grid monochromator. Registration of spectra was 
carried out by photomultiplier tube operating in photon 
counting mode. Luminescence was excited by helium- 
 P.O. MAKSIMCHUK, A.A. MASALOV, ET AL.  PROC. NAP 3, 02NAESF05 (2014) 
 
 
02NAESF05-2 
 
 
Fig. 1 – TEM images of CeO2 nanocrystals of different sizes:  
a – 50 nm nanocrystals obtained by Pechini method, b – 10 nm 
nanocrystals obtained by colloidal precipitation method, c – 
2 nm nanocrystals obtained by colloidal precipitation method 
 
 
 
Fig. 2 – XRD patterns of CeO2 nanocrystals of different sizes: 
50 nm (curve 1), 10 nm (curve 2) and 2 nm (curve 3). 
 
cadmium laser at 325 nm. Possibility of temperature 
variation was obtained by placing of the sample in the 
cryostat. 
 
3. RESULTS AND DISCUSSION 
 
Ceria is wide band gap (~ 6 eV) material with va-
lence band formed by O2p and conduction band by 
Ce5d states [24, 25].  In the band gap of stoichiometric 
CeO2 narrow sub-band is present formed by empty 4f0 
states of Ce4+ ion. For non-stoichiometric CeO2 – x split-
ting of this band to the empty 4f0 and filled 4f1 sub-
bands occurs [26].  
Spectra of CeO2 nanocrystals (exc  325 nm) treated 
in oxidizing atmosphere (curve 1), neutral atmosphere 
(curve 2) and reducing atmosphere (curve 3) at 77 K 
are shown in the Fig. 3. 
 
 
Fig. 3 – Luminescence spectra of CeO2 nanocrystals treated in 
oxidizing atmosphere (curve 1), neutral atmosphere (curve 2) 
and reducing atmosphere (curve 3). exc  325 nm, T  77 K. 
 
Luminescence spectrum of CeO2 nanocystals treated 
in oxidizing atmosphere consists of one band with 
maximum at 630 nm, while spectrum of non-
stoichiometric CeO2 nanocystals consists of two bands 
with maxima at 390 nm and 630 nm. As was recently 
shown in [22], optical properties of CeO2 nanocrystals 
are determined by presence of two different optical 
centers formed by Ce3+ ions and Ce4+-O2 – charge 
transfer (CT) states. The band at 390 nm is 
temperature-independent but its intensity increases 
strongly at atmosphere variation from oxidizing to 
reducing. This band is determined by Ce3+ ion 
luminescence and corresponds to electronic transition 
from excited 5d1 state to the ground 4f 1 state of Ce3+ 
(2F7/2 and 2F5/2 terms) [22, 27]. In reducing atmosphere 
concentration of oxygen vacancies and, 
correspondingly, of Ce3+ ions (that require oxygen 
vacancies for charge compensation) is much more than 
in oxidizing atmosphere, so the dependence of this band 
intensity on the atmosphere of treatment can be easily 
explained. Thus the ratio of Ce4+ / Ce3+ centers has 
been shown to be strongly depended on the amount of 
oxygen vacancies. Therefore, the increasing non-
stoichiometry of CeO2 nanocrystals leads to the 
enhancement of Ce3+ luminescence.  The band at 620 
nm exhibit strong quenching with temperature 
increase and slightly depends on the atmosphere of 
treatment. As was shown previously, 630 nm band is 
determined by radiative relaxation in Ce4+-O2- charge 
transfer state (CT-luminescence). As CT luminescence 
is independent on the vacancy concentration, relative 
intensity of Ce3+ 5d-4f luminescence can be used as a 
measure of Ce3+ (and oxygen vacancies) concentration 
in CeO2 nanocrystals with different sizes. 
In the Fig. 4 luminescence spectra of CeO2 nanocrys-
tals (exc  325 nm) with sizes of about 50 nm (curve 1), 
10 nm (curve 2) and 2 nm (curve 3) are shown.  
Decrease of the size from 50 nm to 10 nm leads to 
increase in 2.5 times of intensity of 390 nm band asso-
ciated with 5d → 4f luminescence of Ce3+ ion. Decrease 
of the size from 10 nm to 2 nm leads to increase in 
3.2 times of intensity of 390 nm band associated with  
 OPTICALLY DETECTED EFFECT OF SIZE… PROC. NAP 3, 02NAESF05 (2014) 
 
 
02NAESF05-3 
 
 
Fig. 4 – Luminescence spectra of CeO2 nanocrystals of 
different sizes: 50 nm (curve 1), 10 nm (curve 2) and 2 nm 
(curve 3). exc  325 nm, T  77 K. 
 
5d → 4f luminescence of Ce3+ ion. Intensity of this band 
is directly proportional to Ce3+ concentration and, cor-
respondingly, to concentration of oxygen vacancies in 
the nanocrystal. The same effects have been observed 
at atmosphere variation from oxidizing to reducing and 
are connected with significant increase of concentration 
of oxygen vacancies. So, it can be concluded that de-
crease of the size of cerium dioxide nanoparticles from 
50 nm to 10 nm and from 10 nm to 2 nm leads to 2.5 
and 3.2 times increase of oxygen vacancy concentration. 
Obtained results allow to determine that decrease of 
the size stimulate formation of oxygen vacancies in 
cerium oxide nanocrystals. 
 
4. CONCLUSIONS 
 
In the paper influence of size on the processes of 
formation of oxygen vacancies in CeO2 nanocrystals has 
been investigated by means of spectroscopic techniques. 
The ratio of Ce4+ / Ce3+ centers has been shown to be 
strongly depended on the amount of oxygen vacancies. 
Therefore, the increasing non-stoichiometry of ceria 
nanocrystals leads to the enhancement of Ce3+ 
luminescence. Thus, variation of the Ce3+/Ce4+ ratio in 
CeO2 allows to control the oxygen non-stoichiometry by 
luminescence properties of this material. Size 
dependence of oxygen non-stoichiometry for CeO2 
nanocrystals was shown. For CeO2 nanocrystals it was 
shown that decrease of the size from 50 nm to 2 nm 
leads to 8 times increase of oxygen vacancy 
concentration. 
 
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Optically Detected Effect of Size on the Oxygen Vacancies Concentration in Cerium Oxide  Nanocrystals

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Optically Detected Effect of Size on the Oxygen Vacancies Concentration in Cerium Oxide Nanocrystals

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