Synthesis, characterization, and magnetic properties of monodisperse CeO2 nanospheres prepared by PVP-assisted hydrothermal method
© Phokha et al.; licensee Springer. 2012
Received: 7 May 2012
Accepted: 14 July 2012
Published: 31 July 2012
Ferromagnetism was observed at room temperature in monodisperse CeO2 nanospheres synthesized by hydrothermal treatment of Ce(NO3)3·6H2O using polyvinylpyrrolidone as a surfactant. The structure and morphology of the products were characterized by X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy, high-resolution transmission electron microscopy, and field-emission scanning electron microscopy (FE-SEM). The optical properties of the nanospheres were determined using UV and visible spectroscopy and photoluminescence (PL). The valence states of Ce ions were also determined using X-ray absorption near edge spectroscopy. The XRD results indicated that the synthesized samples had a cubic structure with a crystallite size in the range of approximately 9 to 19 nm. FE-SEM micrographs showed that the samples had a spherical morphology with a particle size in the range of approximately 100 to 250 nm. The samples also showed a strong UV absorption and room temperature PL. The emission might be due to charge transfer transitions from the 4f band to the valence band of the oxide. The magnetic properties of the samples were studied using a vibrating sample magnetometer. The samples exhibited room temperature ferromagnetism with a small magnetization of approximately 0.0026 to 0.016 emu/g at 10 kOe. Our results indicate that oxygen vacancies could be involved in the ferromagnetic exchange, and the possible mechanism of formation was discussed based on the experimental results.
KeywordsCeO2 Nanospheres Dilute magnetic oxide Ferromagnetism Oxygen vacancies Valence states
Oxide-dilute magnetic semiconductors (O-DMSs) such as ZnO, TiO2, SnO2, and In2O3 doped with transition metal (TM) ions have recently attracted much attention due to their potential use in magneto-optoelectronic applications [1–3]. These O-DMSs are optically transparent and exhibit ferromagnetism (FM) at room temperature (RT) and even well above RT. Recently, TM-doped CeO2 have been also reported to exhibit ferromagnetism at and above room temperature [4–10]. Unlike other O-DMSs, CeO2 has a cubic structure with a lattice parameter a = 0.54113 nm  that will facilitate the integration of spintronic devices with advanced silicon microelectronic devices.
Early work on CeO2-based O-DMSs was focused on thin films [4–6] and only a few works have been carried out on powders, bulk, or nanocrystalline form [9–12]. Tiwari et al.  firstly discovered room temperature ferromagnetism (RT-FM) in Ce1−xCo x O2−δ (x ≤ 0.05) films deposited on a LaAlO3 (001) substrate by pulsed laser deposition (PLD) technique. These films are transparent in a visible regime and exhibit a very high Curie temperature (TC) at approximately 740 to 875 K with large magnetic moments of 6.1 ± 0.2 to 8.2 ± 0.2 μ B /Co. Following the work by Tiwari et al., Song et al.  reported successful fabrication ofCe1−xCo x O2−δ (x = 0.03) thin films with (111) preferential orientation deposited on a Si (111) substrate by a PLD technique. Their deposited films show RT-FM with large magnetic moment of 5.8 μ B /Co and coercivity of 560 Oe. The authors also showed that the films could be deposited on glass but with smaller magnetic moment and coercivity. These results suggested that the FM in Co-doped CeO2 depend not only on the doping concentration of transition element, but also on the microstructure of film, including its crystallization, defects, vacancies, etc. Vodungbo et al.  also reported FM in Co-doped CeO2 thin films grown by PLD on SrTiO3 and Si substrate. The films were ferromagnetic with a TC above 400 K. These authors found that the amount of structural defects had a little effect on FM, but the presence of oxygen during the growth or annealing reduced drastically the FM, suggesting that oxygen vacancies played an important role in the magnetic coupling between Co ions, while Wen et al.  reported the ferromagnetism observed in pure and Co-doped CeO2 powders. The RT-FM in pure CeO2 originated from oxygen vacancies while a slight Co doping in CeO2 caused a nearly two-order enhancement of saturation magnetization (Ms) to 0.47 emu/g as compared with the pure sample. The authors suggested that the large RT-FM observed in Co-doped CeO2 powder originated from a combination effect of oxygen vacancies and Co doping. Similarly, Ou et al.  reported RT-FM for Ce1−xCo x O2 (0 < x < 0.10) nanorods prepared by electrodeposition route. The nanorods were ferromagnetic with a high TC of about 870 K and the largest Ms of 0.015 emu/g. They suggested that the RT-FM observed in Co-doped CeO2 nanorods was adjusted by the structural defects including oxygen vacancies. The same behavior was found in nanoparticles of Fe-doped CeO2 with an Ms value of 0.0062 emu/g in 3 at % Fe prepared by a sol–gel method and Fe-doped CeO2 with an Ms value of 0.10 emu/g in 1 at % Fe prepared by the proteic sol–gel process. The authors suggested that the RT-FM originated from an exchange of F-center, which involved a combination of oxygen vacancies and TM doping.
Surprisingly, the researchers report RT-FM of undoping in different oxides, such as thin films of HfO2, TiO2 and In2O3, and nanoparticles of CeO2, Al2O3, ZnO, In2O3, and SnO2, while the corresponding bulk samples are diamagnetic. Most recently, there are some studies reporting ferromagnetism observed in pure CeO2 on powders, nanocrystalline, or cubes [16–18]. Liu et al.  studied the size-dependent ferromagnetism in CeO2 powders synthesized by precipitation route. They found that ferromagnetism was observed only in sub-20-nm powders with an Ms value of 0.08 emu/g. Similarly, Chen et al.  reported RT-FM in CeO2 nanoparticles prepared by thermal decomposition method with an Ms value of 0.12 emu/g. The authors showed that its crystallite size in nanometers would be ferromagnetic because of the large value of the surface-to-volume ratio, leading to the exchange interactions between electron spin moments that resulted from oxygen vacancies at the surface . Recently, Ge et al.  observed ferromagnetism in CeO2 nanocubes with an Ms value of 0.0057 emu/g (an average size of 5.3 nm) prepared by a chemical method. They suggest that oxygen vacancy is essential for the formation of FM in CeO2 nanocubes.
However, magnetic properties of monodisperse nanospheres of pure CeO2 have not yet been reported. In this work, we report the ferromagnetism observed in monodisperse CeO2 nanospheres with a particle size of approximately 200 nm synthesized by hydrothermal treatment of Ce(NO3)3·6H2O using polyvinylpyrrolidone (PVP) as a surfactant. The technique of preparation and the effect of the type of cerium source on the crystallinity and morphology were investigated. The prepared samples were characterized by X-ray diffraction (XRD), Raman spectroscopy, field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), UV and visible spectroscopy (UV–vis), and photoluminescence (PL). The valence states of Ce ions were also investigated by using X-ray absorption near edge spectroscopy (XANES), and the magnetic properties of the samples were determined using a vibrating sample magnetometer (VSM). The origin of RT-FM in this pure CeO2 is also discussed.
In this study, cerium (III) nitrate hexahydrate, Ce(NO3)3·6H2O (99.99% purity; Kanto Corporation, Portland, OR, USA); cerium (III) acetate hydrate, Ce(CH3CO2)3·x H2O (99.9% purity; Sigma-Aldrich Corporation, St. Louis, MO, USA); cerium (III) chloride heptahydrate, CeCl3·7H2O (99.9% purity; Sigma-Aldrich Corporation); cerium (III) sulfate octahydrate, Ce2(SO4)3.8H2O (99.999% purity; Sigma-Aldrich Corporation); and PVP (Sigma-Aldrich Corporation) were used as starting materials. In a typical procedure, one gram of PVP was mixed with 40 mL of deionized water under vigorous magnetic stirring at room temperature (27°C) until a homogeneous solution was obtained. Subsequently, 3 mmol of cerium source was slowly added to the PVP solution under vigorous stirring at room temperature for 2 h, in order to obtain a well-dissolved solution. Throughout the whole process described, no pH adjustment was made. The homogeneous solution was transferred into a Teflon-lined stainless steel autoclave of 50-mL capacity and prepared at 160°C and 200°C for 12 h and 160°C and 200°C for 24 h. After the autoclave was cooled naturally to room temperature, the precipitate was collected and washed several times with distilled water. The final product was then dried in a vacuum at 80°C overnight. In addition, the as-prepared samples were also annealed in argon atmosphere at 400°C for 2 h to study the effect of oxygen vacancies on magnetic properties of the annealed samples.
The prepared samples were characterized using XRD, Raman spectroscopy, FE-SEM, TEM, HRTEM, UV–vis, PL, XANES, and VSM. A Philips X-ray diffractometer (Philips Tecnai, Amsterdam, The Netherlands) with CuKα radiation (λ = 0.15406 nm) was used to study the phases of the pure CeO2 samples. The Raman spectra were recorded at room temperature using a triple spectrometer (Jobin Yvon/Atago-Bussan T-64000, HORIBA Jobin Yvon S.A.S., Chilly-Mazarin, France). The morphology of the sample was obtained from TEM (JEM 2010 200 kV, JEOL Ltd., Akishima, Tokyo, Japan). FE-SEM was performed using a JEOL JSM-6335 F (JEOL Ltd.). The optical absorption spectrum was measured in the range of 200 to 800 nm using a UV-3101PC UV–vis-NIR scanning spectrometer (Shimadzu Corporation, Nakagyo-ku, Kyoto, Japan). PL was carried out on a luminescence spectrometer (PerkinElmer LS-55B, PerkinElmer Instrument, Waltham, MA, USA), using a Xenon lamp as the excitation source at room temperature. The Ce L3 XANES spectrum was studied using XANES in transmission mode at the BL4 Station at Siam Photon Laboratory (Synchrotron Light Research Institute (Public Organization), SLRI) in Nakhon Ratchasima, Thailand. The magnetic measurements were performed at room temperature using a vibrating sample magnetometer (VSM 7403, Lakeshore, Westerville, OH. USA).
Results and discussion
In this study, we found that the type of cerium source has a great effect on the morphology of the final product. The cerium source from Ce(NO3)3·6H2O shows that the sample consisted of sphere-like particles with diameters of 100 to 250 nm (Figure 1b), whereas other cerium sources such as CeCl3·7H2O, Ce(CH3CO2)3·x H2O, and Ce2(SO4)3·8H2O resulted in irregular shapes and agglomerated particles as shown in Figure 1c,d,e, respectively. Therefore, it is clearly seen that cerium source from nitrate is most favorable for the formation of uniformly sized CeO2 nanospheres. It is possible that the absorption of PVP molecules on various crystallographic planes of cerium source played a major role in determining the product morphology, due to the fact that the supersaturation degree has a significant influence on the crystal nucleation rate and crystal growth rate . However, the real reason for the morphology variation of the cerium source and surfactants has yet to be fully understood.
Summary of crystallite sizes, lattice constant, bandgap, and magnetization of pure CeO 2 nanospheres
Crystallite size from XRD (nm)
Lattice constant a(nm)
E g (eV)
Crystallite size from Raman spectroscopy (nm)
Msat 10 kOe (emu/g)
Before Ar annealing
After Ar annealing
CeO2 at 160 °C for 12 h
9.43 ± 0.41
0.5430 ± 0.0021
CeO2 at 200 °C for 12 h
19.6 ± 0.53
0.5420 ± 0.0003
CeO2 at 160 °C for 24 h
12.2 ± 0.13
0.5430 ± 0.0003
CeO2 at 200 °C for 24 h
15.6 ± 0.20
0.5428 ± 0.0006
FE-SEM, TEM, and HRTEM analyses
Gaussian fitting for percentage of Ce 3+ of pure CeO 2 nanospheres before and after Ar annealing
Peak position (eV)
Peak area (eV)
Percentage of Ce3+(%)
Before Ar annealing
After Ar annealing
Before Ar annealing
After Ar annealing
Before Ar annealing
After Ar annealing
CeO2 at 200 °C for 12 h
CeO2 at 160 °C for 24 h
CeO2 at 200 °C for 24 h
To explain the origin of the ferromagnetic contribution in the CeO2 nanostructures, the following arguments are proposed. The annealing of samples in an Ar atmosphere at 400°C for 2 h could possibly increase the number of oxygen vacancies and Ce3+ ions in the samples. The high concentration of Ce3+ (approximately 13.3% Ce3+ for the sample prepared at 200 °C for 24 h) suggests that defects could be present in the majority of the samples, which activate more coupling between the Ce ions, leading to an increase in Ms. Wen et al.  reported the variation of RT-FM in oxygen and H2 (10%)/Ar (90%) annealed samples of 1% Co-doped CeO2 powder. They found that the sample showed little hysteresis loop after O2 annealing and that the FM signal decreased significantly, while the H2 (10%)/Ar (90%) annealed sample showed an enhanced FM with Ms of about 0.4 emu/g. However, further work is needed to achieve a thorough understanding, and this will be of great interest to researchers in the field of dilute magnetic oxides.
In summary, spheres of pure CeO2 with Ce(NO3)3·6H2O using PVP as a surfactant have been successfully synthesized by hydrothermal method, and their structures, valence state, and magnetic properties were investigated. The XRD and Raman spectroscopy results suggested the formation of CeO2 cubic fluorite structures in the CeO2 samples, which was in agreement with the SAED patterns. It is observed that there is a decrease in the lattice parameters with increasing crystallite size, possibly due to the formation of structure defects/oxygen vacancies in the CeO2 lattice. The bandgaps of our CeO2 nanospheres increased with increasing crystal size indicated by the existence of a blueshift due to a cerium valence change, and this can be attributed to oxygen vacancies at the surface. The surface defects in the CeO2 nanospheres play an important role in the PL properties of our sample. The XANES results reveal that a fraction of the Ce ions are in the 3+ state, and these cause the samples to show weak RT-FM with an Ms value of 0.0026 to 0.016 emu/g. A ferromagnetic exchange mechanism in the pure CeO2 samples is discussed by FCE, and the Ms of samples was shown to change, as well as the proportion of oxygen vacancies.
The authors would like to thank the Department of Chemistry of Khon Kaen University for providing VSM and UV–vis facilities, Ubon Ratchathani University for providing XRD and PL facilities, and the National Metal and Materials Technology Center (MTEC) for providing TEM facilities. We thank the Synchrotron Light Research Institute (Public Organization), Nakhon Ratchasima, Thailand for the XANES facilities. S. Phokha would like to acknowledge the financial support for her Ph.D. studies from the Thailand Research Fund through the Royal Golden Jubilee Ph.D. program (grant no. PHD/0275/2550) and the Graduate School of Khon Kaen University (grant no. 53142103). This work is supported by the ‘Industry/University Cooperative Research Center (I/UCRC) in HDD Component, the Faculty of Engineering, Khon Kaen University, and National Electronics and Computer Technology Center, National Science and Technology Development Agency.’
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