In this work the peculiarities of oxygen vacancies formation in cerium oxide nanoparticles for different external influences have been investigated by spectroscopic methods. The features of oxygen vacancies and therefore non-stoichiometric cerium oxide formation in CeO2 nanocrystals depending on the atmosphere of high temperature treatment were investigated. Stimulation of oxygen vacancies formation in reducing and neutral atmospheres was revealed. Occurrence of two different luminescence centers (viz. the charge-transfer complexes formed by Ce4 + and O2 – ions, and Ce3 + ions stabilized by vacancies) after cerium oxide nanoparticles annealing in a neutral atmosphere has been observed.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3094

Maksimchuk, P.O.

Malyukin, Yu.V.

Masalov, A.A.

Electronic Sumy State University Institutional Repository

JOURNAL OF NANO- AND ELECTRONIC PHYSICS ЖУРНАЛ НАНО- ТА ЕЛЕКТРОННОЇ ФІЗИКИ Vol. 5 No 1, 01004(4pp) (2013) Том 5 № 1, 01004(4cc) (2013)   2077-6772/2013/5(1)01004(4) 01004-1  2013 Sumy State University Spectroscopically Detected Formation of Oxygen Vacancies in Nano-Crystalline CeO2 – x  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 15 November 2012; revised manuscript received 05 March 2013; published online 28 March 2013)  In this work the peculiarities of oxygen vacancies formation in cerium oxide nanoparticles for different external influences have been investigated by spectroscopic methods. The features of oxygen vacancies and therefore non-stoichiometric cerium oxide formation in CeO2 nanocrystals depending on the atmosphere of high temperature treatment were investigated. Stimulation of oxygen vacancies formation in reducing and neutral atmospheres was revealed. Occurrence of two different luminescence centers (viz. the charge-transfer complexes formed by Ce4 + and O2 – ions, and Ce3 + ions stabilized by vacancies) after cerium oxide nanoparticles annealing in a neutral atmosphere has been observed.  Keywords: Cerium oxide nanoparticles, Oxygen vacancies, High temperature treatment.   PACS number: 78.67.Bf                                                                * pmaksimchuk@isma.kharkov.ua The article was reported at the 2nd International Conference «Nanomaterials: Applications  Properties-2012» 1. INTRODUCTION  One of the most important functional rare earth ox-ides, cerium dioxide (CeO2), has attracted much atten-tion for promising applications in catalysts [1], fuel cells [2, 3], oxygen sensors [4], mechanical polishing [5], ultraviolet blocks [6], and luminescent materials [7, 8.] due to its oxygen storage capacity via facile Ce4 + / Ce3 + redox cycles, high mechanical strength, and optical properties.  Cerium dioxide is a wide bandgap semiconductor that has been synthesized and studied in both thin-film [9, 10] and nanoparticle form [11-13]. As a thin-film, cerium oxide has unique properties such as a high re-fractive index, a high dc dielectric constant and a lat-tice constant similar to Si, making it suitable as an insulating material in Si device technology [14]. These properties make cerium oxide useful for applications in microelectronics and optics. Recently, ceria nanoparticles have attracted atten-tion within the biomedical research community as a potential agent to inhibit cellular aging [15, 16]. Mixed brain cell cultures have been shown to have an in-creased lifespan when a solution containing ceria na-noparticles is introduced into their environment. The likely mechanism for the longevity increase is the scav-enging of free radical species in the cells that would normally damage the cell, causing the cell to age [17]. The scavenging effect is attributed to the presence of Ce3 + ions that reduce the free radical species as the Ce3 + ions are oxidized to Ce4 +. Literature data on the structure and properties of nanosized CeO2 – x often contradict each other. Moreo-ver, most of the properties of nanocrystalline CeO2 – x, remain almost unexplored.  The main goal of the work was to find a way to de-termine the oxygen vacancies concentration in the ce-rium dioxide nanoparticles using optical spectroscopy. The optical spectroscopy techniques used were spectro-photometry, to measure the excitation spectrum of the samples, and fluorometry, to measure the luminescence spectrum of the cerium dioxide nanoparticles. These techniques are widely used to characterize semicon-ducting materials [18], but have not been extensively used on ceria nanoparticles. One advantage of these techniques is that the optical spectroscopy systems are inexpensive and have fast data acquisition.   2. MATERIALS AND METHODS  By means of Pechini-type sol-gel process cerium di-oxide samples with an average diameter of 40 nm and narrow size distribution were synthesized (Fig. 1).    Fig. 1 – TEM image of synthesized CeO2 nanocrystals  The structural characteristics of nanocrystalline samples were examined from X-ray diffraction (XRD) (Fig. 2). All the diffraction peaks in these patterns can be indexed to a fluorite cubic phase of CeO2 (JCPDS 34-0394) (Fig. 2). There are diffraction angles of 28, 33, 47,  P.O. MAKSIMCHUK, A.A. MASALOV, YU.V. MALYUKIN J. NANO- ELECTRON. PHYS. 5, 01004 (2013)   01004-2 56, 59, 69 and 76, which are corresponding to ceria (111), (200), (220), (311), (222), (400) and (331) respec-tively. No impurity is observed, indicating that pure CeO2 is synthesized by a Pechini-type sol-gel process.    Fig. 2 – XRD of the synthesized CeO2 nanocrystals  Luminescence spectra of CeO2 nanocrystals have been obtained by means of the spectrofluorimeter based on grid monochromator. Reciprocal linear dispersion of monochromator was equal to 1.6 nm / mm. Registration of spectra was carried out by photomultiplier tube  operating in photon counting mode. Luminescence was excited by helium-cadmium laser at 325 nm. Possibility of temperature variation was obtained by placing of the sample in the cryostat.  3. RESULTS AND DISCUSSION  Cerium dioxide has the ﬂuorite structure, which consists of a simple cubic oxygen sub-lattice with the cerium ions occupying alternate cube centres (Fig. 3). In the perfect ceria the oxygen ions have a formal charge of 2 –; the anions are tetrahedrally coordinated to the cerium ions and octahedrally coordinated to the surrounding oxygen atoms. The cerium ions have a formal charge of 4 + (the electronic structure is [Ne]5d10) and are coordinated to eight oxygen ions.   Fig. 3 – Structure of cerium dioxide  Cerium with a 4f25d0f6s2 electron configuration can exhibit both the + 3 and + 4 oxidation states, and in-termediate oxides with composition in the range be-tween Ce2O3 and CeO2  can be formed. Thermodynamic data indicate that cerium is unstable in the presence of oxygen and that Ce2O3 and CeO2 are easily formed [19]. Due to the low value of oxidation-reduction poten-tial of the Се4 + / Се3 + pair (1.61 eV) cerium dioxide is able to attach and release oxygen rather easily (due to Ce3 +-Ce4 + transition). At high temperatures and low oxygen pressure cerium dioxide tends to reduce Ce4 + ions to Ce3 + ones and forms a continuous series of oxy-gen-deficient, nonstoichiometric oxides (CeO2 – x). These oxides are characterized by disordered arrangement of oxygen vacancies. At the same time at low tempera-tures these oxides have a highly organized fluorite structure CeO2 [20, 21]. Oxygen nonstoichiometry de-fines basic biomedical properties of cerium oxide – the ability to participate in redox processes taking place in the organism. CeO2 belongs to a class of materials whose optical properties are determined by the charge-transfer tran-sitions. The formation of the charge transfer complexes is  a common phenomenon for all rare-earth ions. Such transitions can be identified by broad bands in the ab-sorption or excitation spectra. However, most of the rare-earth ions do not exhibit CT-luminescence due to nonradioactive relaxation from the CT-state. Another typical feature of the CT luminescence is a very low temperature quenching. As was already mentioned, cerium can exhibit both the + 3 and + 4 oxidation states, and oxygen deficient non-stoichiometric oxides with composition in the range between Ce2O3 and CeO2 can be formed. In this case there is a significant dependence of the stoichiom-etry of the sample from the atmosphere in which high-temperature treatment is carried out. Reducing atmos-phere stimulates the formation of oxygen vacancies and, consequently, the formation of nonstoichiometric oxides. In the oxidizing atmosphere number of oxygen vacancies in the crystal is much lower thus leading to formation of CeO2 crystalline phase. So the processes of oxygen vacancies formation in CeO2 crystals can be controlled by changing the atmosphere of high-temperature treatment. In addition, increased role of the surface for CeO2 nanocrystals compared to the bulk should stimulate the formation of oxygen vacancies that may lead to the nonstoichiometric structure even in the absence of a reducing atmosphere.  Cerium compounds in which the valence of cerium is 4+ have no luminescence in contrast to trivalent ce-rium compounds, which has an intense 4f-5d transition in the blue region of the luminescence spectrum. Thus, by the intensity of the 4f-5d luminescence it is possible to make conclusions about the presence of the Ce3 + ions, and therefore the degree of nonstoichiometric of the structure of the nanocrystals. In this work the peculiarities of oxygen vacancies formation in cerium oxide nanoparticles for different external influences have been investigated by spectro-scopic methods. In order to explore the features of the formation of oxygen vacancies in the nanocrystals CeO2, we have carried out studies of the luminescence spectra after high temperature treatment in different atmospheres.  SPECTROSCOPICALLY DETECTED FORMATION OF OXYGEN VACANCIES… J. NANO- ELECTRON. PHYS. 5, 01004 (2013)   01004-3 High-temperature annealing was carried out in an oxi-dizing (air), inert (Ar) and reducing (hydrogen) envi-ronments at 1000 C for 2 hours. The measurement of luminescence spectra was carried out at room tempera-ture (293 K) and liquid nitrogen (77 K) temperatures. All three spectra are characterized by an intense luminescence band with maximum at 625 nm. This luminescence band can be attributed to the charge-transfer (CT) transition, which occurs due to electron transfer from the ligand to the cerium ion. CT-transitions in the absorption spectra have been ob-served for almost all rare-earth ions [22, 23]. In [24, 25] it was shown that the luminescence of cerium in the crystal Sr2CeO4 is also caused by charge transfer tran-sitions. Such transitions can be identified by broad bands in the excitation spectra. The broad band in the excitation spectrum of CeO2 nanocrystals heat-treated in an oxidizing environment comes from the charge transfer (CT) transition from O2 – to Ce4 + (Fig. 4). The fact that the luminescence in CeO2 nanocrystal belongs to CT-transition is also confirmed by the study of low-temperature spectra.    Fig. 4 – Excitation spectrum of CeO2 nanocrystals heat-treated in an oxidizing environment  In the luminescence spectrum of nanocrystals heat-treated in an oxidizing environment (Fig. 5) only lumines-cence band at 625 nm related to charge-transfer transition is present. The absence of luminescence of trivalent ceri-um indicates that the number of oxygen vacancies in the nanocrystal is small, so that the structure of the nano-crystal is close to the stoichiometric CeO2. In a reducing environment (Fig. 6) in addition to the charge transfer band an intense luminescence band with maximum at 390 nm is observed. Occurrence of this band confirms the statement that the reducing environment stimulates the for-mation of a large number of vacancies in the crystal structure of CeO2, which leads to a high degree of stoi-chiometry and formation of Ce3 + ions. The transition of some of the cerium ions to the trivalent state in a re-ducing atmosphere has been shown previously for the bulk crystal. However, such a transition was observed only at sufficiently high pressures. Thus, the transition from bulk to nanoscale crystals stimulates the for-mation of oxygen vacancies in the crystal lattice.    Fig. 5 – Luminescence spectrum of CeO2 nanocrystals heat-treated in an oxidizing environment    Fig. 6 – Luminescence spectrum of CeO2 nanocrystals heat-treated in an reducing environment    Fig. 7 – Excitation spectrum of CeO2 nanocrystals heat-treated in a reducing environment  The excitation spectrum of the band at 390 nm is significantly different from the charge transfer complex excitation band (Fig. 7).  P.O. MAKSIMCHUK, A.A. MASALOV, YU.V. MALYUKIN J. NANO- ELECTRON. PHYS. 5, 01004 (2013)   01004-4 The excitation spectrum contains characteristic bands corresponding to 4f-5d transitions of the Ce3 + ion. This band can be clearly attributed to 4f-5d transi-tion of trivalent cerium. Even more significant evidence of this statement was the presence of additional band in the lumines-cence spectra of CeO2 nanocrystals after high tempera-ture treatment in an inert atmosphere (Fig. 8).    Fig. 8 – Luminescence spectrum of CeO2 nanocrystals heat-treated in an inert environment  As in the case of a reducing atmosphere this band can be attributed to 4f-5d transition of trivalent ceri-um. The formation of a significant number of oxygen vacancies in the absence of a reducing atmosphere is not typical for bulk CeO2 crystals and, thus, this effect is a manifestation of the increased role of the surface in nanocrystals.   4. CONCLUSION  Thus, peculiarities of oxygen vacancies formation in cerium oxide nanoparticles for different external influ-ences have been investigated by spectroscopic methods. Features of oxygen vacancies and therefore non-stoichiometric cerium oxide formation in CeO2 nano-crystals depend on the atmosphere of high temperature treatment. Reducing and neutral atmospheres stimu-lates oxygen vacancies formation. Occurrence of two different luminescence centers (viz. the charge-transfer complexes formed by Ce4 + and O2 – ions, and Ce3 + ions stabilized by vacancies) after  cerium oxide nanoparti-cles annealing in a neutral atmosphere has been ob-served.       REFERENCES  1. Z. Zhan, S.A. Barnett, Science 308, 844 (2005). 2. D.R. Ou, T. Mori, H. Togasaki, M. Takahashi, F. Ye, J. Drennan, Langmuir 27, 3859 (2011). 3. Z.C. Kang, Z.L. Wang, Adv. Mater. 15, 521 (2003). 4. H. Imagawa, A. Suda, K. Yamamura, S. Sun, J. Phys. Chem. C 115, 1740 (2011).  5. Z. Zhang, L. Yu, W. Liu, Appl. Surf. Sci. 256, 3856 (2010). 6. D.M. Kempaiah, S. Yin, T. Sato, Cryst. Eng. Comm 13, 741 (2011). 7. Y. Yoshida, S. Fujihara, Eur. J. Inorg. Chem. 2011, 1577 (2011) 8. X.J. Yu, P.B. Xie, Q.D. Su, Phys. Chem. Chem. Phys. 3, 5266 (2001). 9. P. Patsalas, S. Logothetidis, L. Sygellou, S. Kennou, Phys. Rev. B 68, 035104 (2003). 10. T. Suzuki, I. Kosacki, H. Anderson, P. Colomban, J. Am. Ceram. Soc. 84, 2007 (2001). 11. S. Tsunekawa, J.T. Wang, Y. Kawazoe, J. Alloy. Compd. 408, 1145 (2006). 12. F. Zhang, S.W. Chan, J.E. Spanier, E. Apak, Appl. Phys. Lett. 80, 127 (2002). 13. T. Sakata, H. Mori, Chem. Mater. 9, 2197 (1997). 14. L. Tye, N.A. El-Masry, Appl. Phys. Lett. 65, 3081 (1994). 15. B. Rzigalinski, Technol. Cancer Res. T. 4, 651 (2005). 16. D. Bailey, L. Chow, S. Merchant, S.C. Kuiry, Nat. Biotech-nol. 14, 112 (2003). 17. B.A. Rzigalinski, K. Meehan, R.M. Davis, Y. Xu, W.C. Miles, C.A. Cohen, Nanomedicine 4, 399 (2006). 18. M.D. Sturge, Phys. Rev. 127, 768 (1962). 19. L.R. Morss, Handbook on the Physics and Chemistry of Rare Earths (New York: Elsevier Science: 1994). 20. G. Adachi, N. Imanaka, Chem. Rev. 98, 1479 (1998).  21. D.J.M. Bevan, J. Kordis, J. Inorg. Nucl. Chem. 26, 1509 (1964). 22. P. Dorenbos, J. Phys.: Condens. Mat. 15, 8417 (2003).  23. E. Nakazawa, J. Lumin. 100, 89 (2002). 24. A.A. Masalov, O.G. Viagin, I.I. Ganina, Functional mate-rials 16, 442 (2009). 25. O. Viagin, A. Masalov, I. Ganina, Opt. Mater. 31, 1808 (2009).   

Spectroscopically Detected Formation of Oxygen Vacancies in Nano-Crystalline CeO2 – x

SocioEconomic Challenges Journal (Sumy State University)

JOURNAL OF NANO- AND ELECTRONIC PHYSICS ЖУРНАЛ НАНО- ТА ЕЛЕКТРОННОЇ ФІЗИКИ 
Vol. 5 No 1, 01004(4pp) (2013) Том 5 № 1, 01004(4cc) (2013) 
 
 
2077-6772/2013/5(1)01004(4) 01004-1  2013 Sumy State University 
Spectroscopically Detected Formation of Oxygen Vacancies in Nano-Crystalline CeO2 – x 
 
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 15 November 2012; revised manuscript received 05 March 2013; published online 28 March 2013) 
 
In this work the peculiarities of oxygen vacancies formation in cerium oxide nanoparticles for different 
external influences have been investigated by spectroscopic methods. The features of oxygen vacancies and 
therefore non-stoichiometric cerium oxide formation in CeO2 nanocrystals depending on the atmosphere of 
high temperature treatment were investigated. Stimulation of oxygen vacancies formation in reducing and 
neutral atmospheres was revealed. Occurrence of two different luminescence centers (viz. the charge-
transfer complexes formed by Ce4 + and O2 – ions, and Ce3 + ions stabilized by vacancies) after cerium oxide 
nanoparticles annealing in a neutral atmosphere has been observed. 
 
Keywords: Cerium oxide nanoparticles, Oxygen vacancies, High temperature treatment. 
 
 PACS number: 78.67.Bf 
 
 
                                                             
*
 pmaksimchuk@isma.kharkov.ua 
The article was reported at the 2nd International Conference «Nanomaterials: Applications  Properties-2012» 
1. INTRODUCTION 
 
One of the most important functional rare earth ox-
ides, cerium dioxide (CeO2), has attracted much atten-
tion for promising applications in catalysts [1], fuel 
cells [2, 3], oxygen sensors [4], mechanical polishing [5], 
ultraviolet blocks [6], and luminescent materials [7, 8.] 
due to its oxygen storage capacity via facile Ce4 + / Ce3 + 
redox cycles, high mechanical strength, and optical 
properties.  
Cerium dioxide is a wide bandgap semiconductor 
that has been synthesized and studied in both thin-film 
[9, 10] and nanoparticle form [11-13]. As a thin-film, 
cerium oxide has unique properties such as a high re-
fractive index, a high dc dielectric constant and a lat-
tice constant similar to Si, making it suitable as an 
insulating material in Si device technology [14]. These 
properties make cerium oxide useful for applications in 
microelectronics and optics. 
Recently, ceria nanoparticles have attracted atten-
tion within the biomedical research community as a 
potential agent to inhibit cellular aging [15, 16]. Mixed 
brain cell cultures have been shown to have an in-
creased lifespan when a solution containing ceria na-
noparticles is introduced into their environment. The 
likely mechanism for the longevity increase is the scav-
enging of free radical species in the cells that would 
normally damage the cell, causing the cell to age [17]. 
The scavenging effect is attributed to the presence of 
Ce3 + ions that reduce the free radical species as the 
Ce3 + ions are oxidized to Ce4 +. 
Literature data on the structure and properties of 
nanosized CeO2 – x often contradict each other. Moreo-
ver, most of the properties of nanocrystalline CeO2 – x, 
remain almost unexplored.  
The main goal of the work was to find a way to de-
termine the oxygen vacancies concentration in the ce-
rium dioxide nanoparticles using optical spectroscopy. 
The optical spectroscopy techniques used were spectro-
photometry, to measure the excitation spectrum of the 
samples, and fluorometry, to measure the luminescence 
spectrum of the cerium dioxide nanoparticles. These 
techniques are widely used to characterize semicon-
ducting materials [18], but have not been extensively 
used on ceria nanoparticles. One advantage of these 
techniques is that the optical spectroscopy systems are 
inexpensive and have fast data acquisition.  
 
2. MATERIALS AND METHODS 
 
By means of Pechini-type sol-gel process cerium di-
oxide samples with an average diameter of 40 nm and 
narrow size distribution were synthesized (Fig. 1). 
 
 
 
Fig. 1 – TEM image of synthesized CeO2 nanocrystals 
 
The structural characteristics of nanocrystalline 
samples were examined from X-ray diffraction (XRD) 
(Fig. 2). All the diffraction peaks in these patterns can 
be indexed to a fluorite cubic phase of CeO2 (JCPDS 34-
0394) (Fig. 2). There are diffraction angles of 28, 33, 47, 
 P.O. MAKSIMCHUK, A.A. MASALOV, YU.V. MALYUKIN J. NANO- ELECTRON. PHYS. 5, 01004 (2013) 
 
 
01004-2 
56, 59, 69 and 76, which are corresponding to ceria 
(111), (200), (220), (311), (222), (400) and (331) respec-
tively. No impurity is observed, indicating that pure 
CeO2 is synthesized by a Pechini-type sol-gel process. 
 
 
 
Fig. 2 – XRD of the synthesized CeO2 nanocrystals 
 
Luminescence spectra of CeO2 nanocrystals have 
been obtained by means of the spectrofluorimeter based 
on grid monochromator. Reciprocal linear dispersion of 
monochromator was equal to 1.6 nm / mm. Registration 
of spectra was carried out by photomultiplier tube  
operating in photon counting mode. Luminescence was 
excited by helium-cadmium laser at 325 nm. Possibility 
of temperature variation was obtained by placing of the 
sample in the cryostat. 
 
3. RESULTS AND DISCUSSION 
 
Cerium dioxide has the fluorite structure, which 
consists of a simple cubic oxygen sub-lattice with the 
cerium ions occupying alternate cube centres (Fig. 3). 
In the perfect ceria the oxygen ions have a formal 
charge of 2 –; the anions are tetrahedrally coordinated 
to the cerium ions and octahedrally coordinated to the 
surrounding oxygen atoms. The cerium ions have a 
formal charge of 4 + (the electronic structure is 
[Ne]5d10) and are coordinated to eight oxygen ions. 
 
 
Fig. 3 – Structure of cerium dioxide 
 
Cerium with a 4f25d0f6s2 electron configuration can 
exhibit both the + 3 and + 4 oxidation states, and in-
termediate oxides with composition in the range be-
tween Ce2O3 and CeO2  can be formed. Thermodynamic 
data indicate that cerium is unstable in the presence of 
oxygen and that Ce2O3 and CeO2 are easily formed [19]. 
Due to the low value of oxidation-reduction poten-
tial of the Се4 + / Се3 + pair (1.61 eV) cerium dioxide is 
able to attach and release oxygen rather easily (due to 
Ce3 +-Ce4 + transition). At high temperatures and low 
oxygen pressure cerium dioxide tends to reduce Ce4 + 
ions to Ce3 + ones and forms a continuous series of oxy-
gen-deficient, nonstoichiometric oxides (CeO2 – x). These 
oxides are characterized by disordered arrangement of 
oxygen vacancies. At the same time at low tempera-
tures these oxides have a highly organized fluorite 
structure CeO2 [20, 21]. Oxygen nonstoichiometry de-
fines basic biomedical properties of cerium oxide – the 
ability to participate in redox processes taking place in 
the organism. 
CeO2 belongs to a class of materials whose optical 
properties are determined by the charge-transfer tran-
sitions. The formation of the charge transfer complexes 
is  a common phenomenon for all rare-earth ions. Such 
transitions can be identified by broad bands in the ab-
sorption or excitation spectra. However, most of the 
rare-earth ions do not exhibit CT-luminescence due to 
nonradioactive relaxation from the CT-state. Another 
typical feature of the CT luminescence is a very low 
temperature quenching. 
As was already mentioned, cerium can exhibit both 
the + 3 and + 4 oxidation states, and oxygen deficient 
non-stoichiometric oxides with composition in the 
range between Ce2O3 and CeO2 can be formed. In this 
case there is a significant dependence of the stoichiom-
etry of the sample from the atmosphere in which high-
temperature treatment is carried out. Reducing atmos-
phere stimulates the formation of oxygen vacancies 
and, consequently, the formation of nonstoichiometric 
oxides. In the oxidizing atmosphere number of oxygen 
vacancies in the crystal is much lower thus leading to 
formation of CeO2 crystalline phase. So the processes of 
oxygen vacancies formation in CeO2 crystals can be 
controlled by changing the atmosphere of high-
temperature treatment. 
In addition, increased role of the surface for CeO2 
nanocrystals compared to the bulk should stimulate 
the formation of oxygen vacancies that may lead to the 
nonstoichiometric structure even in the absence of a 
reducing atmosphere.  
Cerium compounds in which the valence of cerium 
is 4+ have no luminescence in contrast to trivalent ce-
rium compounds, which has an intense 4f-5d transition 
in the blue region of the luminescence spectrum. Thus, 
by the intensity of the 4f-5d luminescence it is possible 
to make conclusions about the presence of the Ce3 + 
ions, and therefore the degree of nonstoichiometric of 
the structure of the nanocrystals. 
In this work the peculiarities of oxygen vacancies 
formation in cerium oxide nanoparticles for different 
external influences have been investigated by spectro-
scopic methods. 
In order to explore the features of the formation of 
oxygen vacancies in the nanocrystals CeO2, we have 
carried out studies of the luminescence spectra after 
high temperature treatment in different atmospheres. 
 SPECTROSCOPICALLY DETECTED FORMATION OF OXYGEN VACANCIES… J. NANO- ELECTRON. PHYS. 5, 01004 (2013) 
 
 
01004-3 
High-temperature annealing was carried out in an oxi-
dizing (air), inert (Ar) and reducing (hydrogen) envi-
ronments at 1000 C for 2 hours. The measurement of 
luminescence spectra was carried out at room tempera-
ture (293 K) and liquid nitrogen (77 K) temperatures. 
All three spectra are characterized by an intense 
luminescence band with maximum at 625 nm. This 
luminescence band can be attributed to the charge-
transfer (CT) transition, which occurs due to electron 
transfer from the ligand to the cerium ion. CT-
transitions in the absorption spectra have been ob-
served for almost all rare-earth ions [22, 23]. In [24, 25] 
it was shown that the luminescence of cerium in the 
crystal Sr2CeO4 is also caused by charge transfer tran-
sitions. Such transitions can be identified by broad 
bands in the excitation spectra. The broad band in the 
excitation spectrum of CeO2 nanocrystals heat-treated 
in an oxidizing environment comes from the charge 
transfer (CT) transition from O2 – to Ce4 + (Fig. 4). The 
fact that the luminescence in CeO2 nanocrystal belongs 
to CT-transition is also confirmed by the study of low-
temperature spectra. 
 
 
 
Fig. 4 – Excitation spectrum of CeO2 nanocrystals heat-
treated in an oxidizing environment 
 
In the luminescence spectrum of nanocrystals heat-
treated in an oxidizing environment (Fig. 5) only lumines-
cence band at 625 nm related to charge-transfer transition 
is present. The absence of luminescence of trivalent ceri-
um indicates that the number of oxygen vacancies in the 
nanocrystal is small, so that the structure of the nano-
crystal is close to the stoichiometric CeO2. 
In a reducing environment (Fig. 6) in addition to the 
charge transfer band an intense luminescence band 
with maximum at 390 nm is observed. 
Occurrence of this band confirms the statement 
that the reducing environment stimulates the for-
mation of a large number of vacancies in the crystal 
structure of CeO2, which leads to a high degree of stoi-
chiometry and formation of Ce3 + ions. The transition of 
some of the cerium ions to the trivalent state in a re-
ducing atmosphere has been shown previously for the 
bulk crystal. However, such a transition was observed 
only at sufficiently high pressures. Thus, the transition 
from bulk to nanoscale crystals stimulates the for-
mation of oxygen vacancies in the crystal lattice. 
 
 
 
Fig. 5 – Luminescence spectrum of CeO2 nanocrystals heat-
treated in an oxidizing environment 
 
 
 
Fig. 6 – Luminescence spectrum of CeO2 nanocrystals heat-
treated in an reducing environment 
 
 
 
Fig. 7 – Excitation spectrum of CeO2 nanocrystals heat-
treated in a reducing environment 
 
The excitation spectrum of the band at 390 nm is 
significantly different from the charge transfer complex 
excitation band (Fig. 7). 
 P.O. MAKSIMCHUK, A.A. MASALOV, YU.V. MALYUKIN J. NANO- ELECTRON. PHYS. 5, 01004 (2013) 
 
 
01004-4 
The excitation spectrum contains characteristic 
bands corresponding to 4f-5d transitions of the Ce3 + 
ion. This band can be clearly attributed to 4f-5d transi-
tion of trivalent cerium. 
Even more significant evidence of this statement 
was the presence of additional band in the lumines-
cence spectra of CeO2 nanocrystals after high tempera-
ture treatment in an inert atmosphere (Fig. 8). 
 
 
 
Fig. 8 – Luminescence spectrum of CeO2 nanocrystals heat-
treated in an inert environment 
 
As in the case of a reducing atmosphere this band 
can be attributed to 4f-5d transition of trivalent ceri-
um. The formation of a significant number of oxygen 
vacancies in the absence of a reducing atmosphere is 
not typical for bulk CeO2 crystals and, thus, this effect 
is a manifestation of the increased role of the surface in 
nanocrystals.  
 
4. CONCLUSION 
 
Thus, peculiarities of oxygen vacancies formation in 
cerium oxide nanoparticles for different external influ-
ences have been investigated by spectroscopic methods. 
Features of oxygen vacancies and therefore non-
stoichiometric cerium oxide formation in CeO2 nano-
crystals depend on the atmosphere of high temperature 
treatment. Reducing and neutral atmospheres stimu-
lates oxygen vacancies formation. Occurrence of two 
different luminescence centers (viz. the charge-transfer 
complexes formed by Ce4 + and O2 – ions, and Ce3 + ions 
stabilized by vacancies) after  cerium oxide nanoparti-
cles annealing in a neutral atmosphere has been ob-
served. 
 
 
 
 
 
 
REFERENCES 
 
1. Z. Zhan, S.A. Barnett, Science 308, 844 (2005). 
2. D.R. Ou, T. Mori, H. Togasaki, M. Takahashi, F. Ye, 
J. Drennan, Langmuir 27, 3859 (2011). 
3. Z.C. Kang, Z.L. Wang, Adv. Mater. 15, 521 (2003). 
4. H. Imagawa, A. Suda, K. Yamamura, S. Sun, J. Phys. 
Chem. C 115, 1740 (2011).  
5. Z. Zhang, L. Yu, W. Liu, Appl. Surf. Sci. 256, 3856 (2010). 
6. D.M. Kempaiah, S. Yin, T. Sato, Cryst. Eng. Comm 13, 741 
(2011). 
7. Y. Yoshida, S. Fujihara, Eur. J. Inorg. Chem. 2011, 1577 
(2011) 
8. X.J. Yu, P.B. Xie, Q.D. Su, Phys. Chem. Chem. Phys. 3, 
5266 (2001). 
9. P. Patsalas, S. Logothetidis, L. Sygellou, S. Kennou, Phys. 
Rev. B 68, 035104 (2003). 
10. T. Suzuki, I. Kosacki, H. Anderson, P. Colomban, J. Am. 
Ceram. Soc. 84, 2007 (2001). 
11. S. Tsunekawa, J.T. Wang, Y. Kawazoe, J. Alloy. Compd. 
408, 1145 (2006). 
12. F. Zhang, S.W. Chan, J.E. Spanier, E. Apak, Appl. Phys. 
Lett. 80, 127 (2002). 
13. T. Sakata, H. Mori, Chem. Mater. 9, 2197 (1997). 
14. L. Tye, N.A. El-Masry, Appl. Phys. Lett. 65, 3081 (1994). 
15. B. Rzigalinski, Technol. Cancer Res. T. 4, 651 (2005). 
16. D. Bailey, L. Chow, S. Merchant, S.C. Kuiry, Nat. Biotech-
nol. 14, 112 (2003). 
17. B.A. Rzigalinski, K. Meehan, R.M. Davis, Y. Xu, 
W.C. Miles, C.A. Cohen, Nanomedicine 4, 399 (2006). 
18. M.D. Sturge, Phys. Rev. 127, 768 (1962). 
19. L.R. Morss, Handbook on the Physics and Chemistry of 
Rare Earths (New York: Elsevier Science: 1994). 
20. G. Adachi, N. Imanaka, Chem. Rev. 98, 1479 (1998).  
21. D.J.M. Bevan, J. Kordis, J. Inorg. Nucl. Chem. 26, 1509 
(1964). 
22. P. Dorenbos, J. Phys.: Condens. Mat. 15, 8417 (2003).  
23. E. Nakazawa, J. Lumin. 100, 89 (2002). 
24. A.A. Masalov, O.G. Viagin, I.I. Ganina, Functional mate-
rials 16, 442 (2009). 
25. O. Viagin, A. Masalov, I. Ganina, Opt. Mater. 31, 1808 
(2009). 
 
 


English

https://essuir.sumdu.edu.ua/bitstream/123456789/30940/1/Maksimchuk%20P.O._Cerium%20oxide%20nanoparticles.pdf

Spectroscopically Detected Formation of Oxygen Vacancies in Nano-Crystalline CeO2 – x

Abstract

Similar works

Full text

Available Versions

Electronic Sumy State University Institutional Repository

SocioEconomic Challenges Journal (Sumy State University)