Synthesis, characterization, and magnetic properties of monodisperse CeO2 nanospheres prepared by PVP-assisted hydrothermal method

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.


Background
Oxide-dilute magnetic semiconductors (O-DMSs) such as ZnO, TiO 2 , SnO 2 , and In 2 O 3 doped with transition metal (TM) ions have recently attracted much attention due to their potential use in magneto-optoelectronic applications [1][2][3]. These O-DMSs are optically transparent and exhibit ferromagnetism (FM) at room temperature (RT) and even well above RT. Recently, TM-doped CeO 2 have been also reported to exhibit ferromagnetism at and above room temperature [4][5][6][7][8][9][10]. Unlike other O-DMSs, CeO 2 has a cubic structure with a lattice parameter a = 0.54113 nm [11] that will facilitate the integration of spintronic devices with advanced silicon microelectronic devices.
Early work on CeO 2 -based O-DMSs was focused on thin films [4][5][6] and only a few works have been carried out on powders, bulk, or nanocrystalline form [9][10][11][12]. Tiwari et al. [4] firstly discovered room temperature ferromagnetism (RT-FM) in Ce 1−x Co x O 2−δ (x ≤ 0.05) films deposited on a LaAlO 3 (001) substrate by pulsed laser deposition (PLD) technique. These films are transparent in a visible regime and exhibit a very high Curie temperature (T C ) 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. [5] reported successful fabrication ofCe 1−x Co x O 2−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 CeO 2 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. [6] also reported FM in Co-doped CeO 2 thin films grown by PLD on SrTiO 3 and Si substrate. The films were ferromagnetic with a T C 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. [9] reported the ferromagnetism observed in pure and Co-doped CeO 2 powders. The RT-FM in pure CeO 2 originated from oxygen vacancies while a slight Co doping in CeO 2 caused a nearly two-order enhancement of saturation magnetization (M s ) to 0.47 emu/g as compared with the pure sample. The authors suggested that the large RT-FM observed in Co-doped CeO 2 powder originated from a combination effect of oxygen vacancies and Co doping. Similarly, Ou et al. [10] reported RT-FM for Ce 1−x Co x O 2 (0 < x < 0.10) nanorods prepared by electrodeposition route. The nanorods were ferromagnetic with a high T C of about 870 K and the largest M s of 0.015 emu/g. They suggested that the RT-FM observed in Co-doped CeO 2 nanorods was adjusted by the structural defects including oxygen vacancies. The same behavior was found in nanoparticles of Fe-doped CeO 2 [11] with an M s value of 0.0062 emu/g in 3 at % Fe prepared by a sol-gel method and Fe-doped CeO 2 [12] with an M s 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 HfO 2 [13], TiO 2 and In 2 O 3 [14], and nanoparticles of CeO 2 , Al 2 O 3 , ZnO, In 2 O 3 , and SnO 2 [15], while the corresponding bulk samples are diamagnetic. Most recently, there are some studies reporting ferromagnetism observed in pure CeO 2 on powders, nanocrystalline, or cubes [16][17][18]. Liu et al. [16] studied the size-dependent ferromagnetism in CeO 2 powders synthesized by precipitation route. They found that ferromagnetism was observed only in sub-20-nm powders with an M s value of 0.08 emu/g. Similarly, Chen et al. [17] reported RT-FM in CeO 2 nanoparticles prepared by thermal decomposition method with an M s 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 [4]. Recently, Ge et al. [18] observed ferromagnetism in CeO 2 nanocubes with an M s 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 CeO 2 nanocubes.
However, magnetic properties of monodisperse nanospheres of pure CeO 2 have not yet been reported. In this work, we report the ferromagnetism observed in monodisperse CeO 2 nanospheres with a particle size of approximately 200 nm synthesized by hydrothermal treatment of Ce(NO 3 ) 3 Á6H 2 O 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), highresolution 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 CeO 2 is also discussed.  3 .8H 2 O (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.

Methods
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 CeO 2 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 L 3 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).

XRD analysis
The XRD patterns of the samples prepared by hydrothermal reaction at 160°C for 12 h are shown in Figure 1a. The sample obtained with CeCl 3 Á7H 2 O as the starting agent shows no XRD peaks, indicating that it is amorphous, whereas the sample from Ce(NO 3 ) 3 Á6H 2 O exhibits XRD peaks that correspond to the (111), (200), (220), (311), (222), and (400) planes, which are consistent with the face-centered cubic fluorite structure of CeO 2 in the standard data from the Joint Committee on Powder Diffraction Standards (JCPDS) 34-0394, indicating that pure CeO 2 was successfully synthesized via these procedures.
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(NO 3 ) 3 Á6H 2 O shows that the sample consisted of sphere-like particles with diameters of 100 to 250 nm (Figure 1b), whereas other cerium sources such as CeCl 3 Á7H 2 O, Ce(CH 3 CO 2 ) 3 ÁxH 2 O, and Ce 2 (SO 4 ) 3 Á8H 2 O 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 CeO 2 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 [19]. However, the real reason for the morphology variation of the cerium source and surfactants has yet to be fully understood. Figure 2 shows the XRD patterns of the pure CeO 2 from cerium nitrate at various hydrothermal treatment durations and temperatures. All the samples exhibited six typical peaks corresponding to the (111), (200), (220), (311), (222), and (400) planes, which are consistent with the face-centered cubic fluorite structure of CeO 2 in the standard data from JCPDS 34-0394, and this is in agreement with the selected area electron diffraction (SAED) patterns shown in Figure 3d. The values of the lattice constant calculated from the XRD spectra are shown in Table 1. The average crystallite size of all the samples was calculated from X-ray line broadening of the peaks at the (111), (200), (220), and (311) planes using Scherrer's equation, (as listed in Table 1). We observed that the lattice parameter decreases with increasing crystallite size. This decrease is possibly due to the introduction of Ce 3+ ions into the crystal lattice. Ce 3+ ions have a higher ionic radius (1.034 Å) compared with the Ce 4+ ions (0.92 Å) and introduce oxygen vacancies. Therefore, the concentration of Ce 3+ ions increases, and there is also an increase in the number of oxygen vacancies. It is observed that the pure CeO 2 nanoparticles experience considerable lattice distortion, which is in good agreement with earlier reports on CeO 2 nanoparticles [9,20], which indicated that this causes a change in the Ce-O bond length (lattice distortion) and the overall lattice parameter.

Raman analysis
The formation of a cubic structure in the CeO 2 nanospheres was further supported by the Raman spectra. Figure 4 shows typical spectra of CeO 2 . The Raman active modes are shifted from 458 to 461 cm −1 for the CeO 2 samples heated at 160°C to 200°C. These Raman active modes are attributed to a symmetrical stretching mode of the Ce-80 vibrational unit, and therefore, they are very sensitive to any disorder in the oxygen sublattice that resulted from thermal, doping, or grain size [21][22][23][24]. The effect of the microstructure of CeO 2 on the shape of the Raman spectra was observed by the broadening of the line and by increases in its asymmetry, which are attributed to the reduction of the phonon lifetime in the nanocrystalline regime [25,26]. The particle size of the CeO 2 sample can be also estimated from the Raman line broadening using the following Equation 1 [23,24,27]: where Γðcm À1 Þ is the full width at half maximum of the Raman active mode peak and D R is the particle size of a CeO 2 sample. This relation can be used to determine the crystal size of the CeO 2 samples as shown in Table 1. This calculated crystal size is near that obtained from X-ray line broadening.

FE-SEM, TEM, and HRTEM analyses
The morphology and structure of CeO 2 nanospheres were investigated by FE-SEM and TEM as shown in Figure 3. TEM bright field images show that the samples contain monodisperse nanospheres with a narrow size distribution. The high-magnification TEM image of a single particle (Figure 3b) indicates that the sphere has a diameter of about 218 nm. This result is similar to the work reported by Zhou et al. [28], in which spherical CeO 2 crystallites assembled by nanoparticles were synthesized by hydrothermal treatment because small nanoparticles of CeO 2 aggregated and gradually evolved into a spherical assembly, achieving a low surface energy. The corresponding SAED patterns (inset in Figure 3d of the products show spotty ring patterns indicative of a face-centered cubic structure of CeO 2 (JCPDS 34-0394), which is in agreement with the XRD results. The HRTEM images of the CeO 2 sample prepared at 200°C for 12 h and the CeO 2 sample prepared at 200°C for 12 h followed by annealing in Ar at 400°C for 2 h, are shown in Figure 5a,b, respectively. The d spacings of the lattice fringes of approximately 0.30 and 0.31 nm for the CeO 2 sample prepared at 200°C for 12 h (Figure 5a) calculated from the HRTEM images correspond to

Optical properties
The UV-vis absorption spectra of the pure CeO 2 nanospheres are shown in Figure 6a. All the samples show a strong absorption below 400 nm (3.10 eV) with a welldefined absorbance peak at approximately 302 nm (4.10 eV). The direct bandgap energy (E g ) is determined  by fitting the absorption data to the direct transition as shown in Equation 2: where α is the optical absorption coefficient, hν is the photon energy, E g is the direct bandgap, and A is a constant [29]. The extrapolation of the linear portions of the curves towards absorption equal to zero (y = 0) gives E g for direct transitions (Figure 6b). The estimated direct bandgaps of all the samples are shown in Table 1   respectively, due to quantum confinement effect [32]. This phenomenon has been well explained for particle sizes down to less than a few nanometers, but for our results, the bandgaps increased with increasing crystal size, which exhibit blueshifts in the UV absorption spectra inferred from the bandgap calculated for pure CeO 2 nanospheres. This blueshift has been reported to be an electrostatic potential effect due to a cerium valence change when the particle size is larger than a few nanometers (e.g., ≥8 nm). The Ce 4+ ions coexist with Ce 3+ ions, and these ions can be attributed to oxygen vacancies at the surface [33]. Therefore, in our work, the bandgaps increased with increasing crystal size. The crystallite size is in the range of 9 to 19 nm as indicated by the existence of the blueshift for our CeO 2 nanospheres.  (2.35 eV). Our results are consistent with that reported for CeO 2 in the literature. The strong emission peak at 410 nm [34] and 422 nm [35] observed for CeO 2 nanoparticles as-prepared at λ ex = 290 nm. Maensiri et al. [21] reported a blue band at approximately 443 nm, along with a green band at 529 nm for 400°C to 500°C calcined sample, and a strong UV emission band at 392 nm for 600°C calcined sample. The dependence of the PL blueshift peak on CeO 2 particle concentration has also been observed by Sathyamurthy et al. [34] for CeO 2 nanoparticles synthesized by a reverse micelle route. This phenomenon has been explained by charge transitions from the 4f band to the valence band of the CeO 2 in both nanoparticles and thin films [36]. In addition, it is well known that the emission energy from the 4f band to the valence band energy gap of CeO 2 is about 3.0 to 3.38 eV, as determined from the calculation of the electronic structure of CeO 2 [35,37]. Therefore, the emission in our CeO 2 samples could be assumed to be the transition from the Ce 4f band to the O 2p band (valence band) in CeO 2 . The broad PL band ranging from 300 to 550 nm of all the samples could be the result of defects, including oxygen vacancies in the crystal with electronic energy levels below the 4f band [38]. This is confirmed by the enhanced absorption tail below 3 eV in nonstoichiometric CeO 2 that was previously observed and attributed to the presence of oxygen vacancies [39].

XANES analysis
The valence state of Ce in pure CeO 2 nanospheres was determined by XANES spectra measured at Ce L 3 edge. Figure 8a shows the edge energies of the Ce (NO 3 ) 3 Á6H 2 O standard and CeO 2 standard. The standard Ce(NO 3 ) 3 Á6H 2 O has a single peak illustrated by one intense white line at approximately 5,726.8 eV, which can be associated with the final state of 2p4f 1 5de g L, where L denotes an oxygen ligand 2p hole, corresponding to the Ce 3+ valence state [40]. In the standard CeO 2 , there are four peaks comprising high energy peak A, main peak B, low energy peak C, Figure 7 Room temperature photoluminescence spectrum of CeO 2 nanospheres using Ce(NO 3 ) 3 Á6H 2 O as starting materials. The sample was dispersed in methanol and the excitation wavelength used in PL measurement was 290 nm. and pre-edge peak D, which have been reported and assigned previously [41][42][43]. Peaks A and B are shifted to higher energies at approximately 5,737.9 and 5,731.3 eV, respectively, and were assigned as being due to a mixture of the multi-electron with the final state of 2p4f 0 5d and 2p4f 1 5dt g L, respectively, which characterizes the Ce in the Ce 4+ valence state [40]. Peak C, at approximately 5,726.8 eV, is observed at the same energy as the white line of a typical Ce (NO 3 ) 3 Á6H 2 O standard, corresponding to the Ce 3+ valence state. Peak D is assigned to the final states of 2p5d with a delocalized d character at the bottom of the conduction band due to the cubic crystal-field splitting of Ce 5d states [44]. Figure 8b shows the XANES spectra of pure CeO 2 for various hydrothermal treatment durations and temperatures before and after annealing in Ar. The quantitative analysis of the valence state of Ce in each of the three states of the CeO 2 nanospheres was performed using multi-peak Gaussian fitting obtained from the XANES spectra, as shown in Figure 8c. The analysis shows that Ce in the samples is in a mixed valence state of Ce 3+ and Ce 4+ . From these results, we can obtain the valence state of Ce according to the fitting parameters of the peak positions and areas listed in Table 2. It is observed that the percentage of Ce 3+ ranges from 7% to 13.7% in the pure CeO 2 samples. These results provide confirmation of the formation of oxygen vacancies on the surface of the CeO 2 samples. It is possible that the concentration and distribution of oxygen vacancies play an important role in the magnetism of our CeO 2 nanospheres. The highest percentage of Ce 3+ is 13.7% for CeO 2 prepared at 200°C for 24 h, with the increase in the number of oxygen vacancies, leading to the highest M s values. The concentration of Ce 3+ in our pure CeO 2 sample is higher than the values reported in the literature for CeO 2 . Zhang et al. [41] reported the average Ce 3+ concentration of 10-and 6-nm CeO 2 nanoparticles prepared by mixing cerium nitrate and hexamethylenetetramine in aqueous solution at room temperature to be 1% and 6.5%, respectively. Chen et al. [17] reported that the concentration of Ce 3+ was higher than 21% for CeO 2 nanoparticles synthesized by the thermal decomposition method, which is higher than the value for pure CeO 2 nanospheres reported in this study.  Table 1). These values are higher than the M s values reported in the literature for pure CeO 2 nanospheres [15,17]. Sundaresan et al. [15] reported an RT-FM with an M s value of approximately 0.0019 emu/g for CeO 2 nanoparticles with an average size of approximately 15 nm. Ge et al. [18] reported weak ferromagnetic behavior at an ambient temperature with an M s value of approximately 0.0007 emu/g for pure CeO 2 nanoparticles with an average size of approximately 100 nm obtained commercially from Sigma-Aldrich Corporation (purity of 99.9%). However, magnetic behavior (M s of approximately 0.0057 emu/g) was also observed in monodisperse CeO 2 nanocubes with an average size of approximately 5.3 nm prepared by a chemical method. The results obtained here (experimental and theoretical) provide evidence that pure CeO 2 samples can indeed have a magnetic moment due to oxygen vacancies. This direct ferromagnetic coupling is called F-center exchange (FCE) [45], as cerium can have both variable valence states (Ce 4+ /Ce 3+ ) and oxygen vacancies on the surface of the CeO 2 nanoparticles. It is possible that oxygen vacancies can create magnetic moments on neighboring Ce ions [46]. In this work, the effect of Ar annealing at 400°C for 2 h on the magnetic properties was also performed to confirm the effect of oxygen vacancies on magnetic properties of the annealed samples. However, this effect was clearly observed only on the CeO 2 sample prepared at 200°C for 12 h and followed by annealing in argon atmosphere at 400°C for 2 h, as its magnetization increased from 0.0025 emu/g to 0.010 emu/g as (see inset of Figure 9b). It is noted that for the samples prepared at 160°C and 200°C for 24 h, annealing did not affect much their magnetic behavior due to the short time of annealing.

Magnetic properties
To explain the origin of the ferromagnetic contribution in the CeO 2 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 Ce 3+ ions in the samples. The high concentration of Ce 3+ (approximately 13.3% Ce 3+ 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 M s . Wen et al. [9] reported the variation of RT-FM in oxygen and H 2 (10%)/Ar (90%) annealed samples of 1% Co-doped CeO 2 powder. They found that the sample showed little hysteresis loop after O 2 annealing and that the FM signal decreased significantly, while the H 2 (10%)/Ar (90%) annealed sample showed an enhanced FM with M s 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.

Conclusions
In summary, spheres of pure CeO 2 with Ce(NO 3 ) 3 Á6H 2 O 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 CeO 2 cubic fluorite structures in the CeO 2 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 CeO 2 lattice. The bandgaps of our CeO 2 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 CeO 2 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 M s value of 0.0026 to 0.016 emu/g. A ferromagnetic exchange mechanism in the pure CeO 2 samples is discussed by FCE, and the M s of samples was shown to change, as well as the proportion of oxygen vacancies.