A study of structural, optical and ferromagnetic properties of sol-gel derived Cr and Fe co-doped CeO2 nanoparticles


 The Cr and Fe co-doped CeO2 nanoparticles (Ce0.98−xCrxFe0.02O2: where x = 0, 0.01, 0.02, 0.03) were prepared by sol-gel method. Effects of additional Cr dopant on structural, optical and magnetic properties of Fe doped CeO2 nanoparticles have been investigated by X-ray diffraction (XRD), optical absorption spectra, Raman spectroscopy (Raman) and physical property measurement system (PPMS). XRD and Raman studied showed that all samples are single-phase of CeO2 original cubic fluorite crystal structure, Cr can readily be incorporated into the lattice of Fe doped CeO2 and no ferromagnetic secondary phase was found. With the increase of Cr doping concentration, the grain size and crystal quality decreases. The values of optical bandgap energy extracted from the absorption coefficient increase with the increase of Cr doping concentration. The PPMS studied show hysteresis phenomenon, which indicates that the samples have ferromagnetic properties at 300 K. With the increase of Cr content, the saturation magnetization increase obviously. Based on the results of XRD and Raman, it can be concluded that the ferromagnetism is the intrinsic property of the sample.


Introduction
Diluted magnetic oxide semiconductors (DMOS) have great potential applications in spintronic devices because of their optical, electrical and magnetic properties [1][2][3]. Transition metal doped CeO 2 is one of the most promising and widely studied DMOS systems [2][3][4][5]. However, there is a heated debate on the origin of ferromagnetic properties of these DMOS systems. CeO 2 belongs to uorite structure in cubic system. The coordination number of oxygen is four and that of cerium is eight. CeO 2 can be converted into nonstoichiometric CeO 2−δ at high temperature, and the deviation δ is 0 < δ < 0.5 [6]. Nonstoichiometric CeO 2−δ can also maintain a uorite crystal structure well under the condition of Vo formed by anoxia. Therefore, CeO 2 has a good ability to store and release oxygen and is easy to manipulate Vo defects. The unique physical and chemical properties of CeO 2 make it widely used in the elds of luminescence, polishing, UV absorption, automobile exhaust puri cation, solid oxide fuel cell, optical coating and oxygen sensor. CeO 2 based DMOS has also attracted great attention. The main reasons for these are its structural properties that are similar to silicon [7]. Therefore, CeO 2 is considered as a potential substitute for silicon semiconductor. In addition, CeO 2 has better visible light transmittance, which makes it possible to develop new transparent spintronic devices. A large number of studies have shown that CeO 2 doped with metal or transition metal exhibits good room temperature ferromagnetism [2][3][4][5][6][7][8][9][10]. However, the origin of ferromagnetism in CeO 2 based DMOS system is still controversial. Some studies have found that doping 3d transition metals can provide local magnetism, and the defect in CeO 2 matrix can provide charge, ferromagnetic properties are obtained by s-p-d electron exchange [9,10]. Other studies suggest that ferromagnetism originates from defects in CeO 2 matrix, because ferromagnetism is also observed in undoped CeO 2 and rare earth (such as Nd, Sm or Pr) doped CeO 2 [11][12][13]. In most studies, V O is considered to be an important reason for room temperature ferromagnetism in CeO 2 based DMOS systems. A large amount of V O can be formed in the anoxic CeO 2 matrix. The magnetic exchange coupling induced by V O leads to room temperature ferromagnetism. In view of the current reported in the literature, how to control the various properties of CeO 2 based DMOS system and clarify the related magnetic and transport properties due to the local structure of doped ions and spin are the di culties in the current research. Therefore, it is necessary to further study the preparation and properties of different elements doped CeO 2 based DMOS.
There are many methods for preparing different elements doped CeO 2 based DMOS nanoparticles, such as sol-gel, thermal decomposition, hydrothermal, microemulsion and ball milling methods [2][3][4][5][6][7][8][9][10][11][12][13]. In these methods, sol-gel method is more attractive because of its simple equipment, convenient operation, low cost, low sintering temperature and easy control of chemical components. was used as solvent and acetylacetone (CH 3 COCH 2 COCH 3 ) was used as stabilizer. These solutes were completely dissolved in 2-methoxyethanol with stirring for 1 h at 65 o C, then the stabilizer acetylacetone was added, and nally stirred for 3 h at room temperature. In order to study the effect of Cr doping on the structural, optical and magnetic properties of 2% Fe doped CeO 2 based DMOS, here we kept Fe doping concentration as a constant of 2 at.% with the variation of Cr doping content from 0 to 3 at.%. Cr doping was achieved by the introduction of appropriate amount of Cr(NO 3 ) 3 ·9H 2 O. These prepared precursor solutions were put in a furnace for 24 h at 100°C to eliminate excess water and form the xerogel. Then, the xerogel was calcined at 800°C for 2 h to eliminate organic materials, and nally Ce 0.98−x Cr x Fe 0.02 O 2 nanoparticles were prepared.
The crystal structure of Ce 0.98−x Cr x Fe 0.02 O 2 nanoparticles were characterized by using RIGAKU D-MAX 2200 VPC X-ray diffractometer (XRD) equipped with a Cu-Kα (λ = 1.54 Å) source. The room temperature Raman spectroscopy of these nanoparticles was measured by using Confocal Micro-Raman Spectrometer (inVia Re ex, Renishaw) with 514 nm excitation source under air ambient condition. Optical absorption spectra were measured in a band ranging from the ultraviolet to visible regions (UV-Vis) by using a UV-3150 SHIMADZU. Magnetic measurements as a function of temperature ( M ~ T ) at H = 1000 Oe in the temperature range from 10 to 300 K and magnetization curves as a function of magnetic eld ranging − 100 < H < 100 kOe (M ~ H) measured at T = 300 K were done by commercial physical property measurement system (PPMS, quantum design, model 6000). In Table 1, some properties of Ce 0.98−x Cr x Fe 0.02 O 2 (where x = 0, 0.01, 0.02 and 0.03) nanoparticles, such as particle size (D), optical band gap energy (E g ), saturation moment (M s ), and coercivity (H C ) are given.  Fig. 1, it is also seen that the intensity of diffraction peak decreases as the increase of Cr doping concentration. This indicates that the quality of crystallization is deteriorated. All diffraction peaks are broadened as the Cr content increase, which indicated the ne nature of the small particles. By using Debye Scherrer formalism , where β is the fullwidth at half-maximum in radians, is the Bragg's angle in degrees, and λ is the wavelength of X-rays  Table 1). The radii of Cr and Fe ions are about 0.73 Å and 0.61 Å, respectively. The radii of Ce ions in CeO 2 are 0.97 Å. The radii of Cr or Fe ions are smaller than those of Ce ions. If substitution doping occurs, the grain size will decrease, which is consistent with the results of XRD. On the other hand, as the Cr content increases, the diffraction peak shifts to a large angle. This indicates the lattice constant of the sample decrease gradually. The reason why additional Cr dopant affects the structure of Fe doped CeO 2 nanoparticles is that Cr will inhibit the crystallization of the samples. When Cr or Fe ions replace Ce ions, V O will be produced due to different valence states, and different ion radii will lead to lattice distortion or other defects. V O and lattice distortion or other defects will increase with the increase of Cr doping concentration.
The structure of Ce 0.98-x Cr x Fe 0.02 O 2 nanoparticles was further studied by Raman spectroscopy. Fig. 2 shows the room temperature Raman spectra of Ce 0. It related to a rst-order symmetrical stretching mode of the Ce-O 8 vibrational unit. This Raman peak (at 462.5 cm -1 ) changes with the crystal structure (such as defects, Vo, grain size, etc.). Generally, the strength of the peak is related to the grain size. The larger the grain size, the stronger the peak strength.
The shift of the peak position is also related to the grain size. The optical absorption measurement can investigate the behavior of semiconductor nanostructure. The optical bandgap energy (E g ) can be modi ed by particle size due to the effect of quantum con nement.
The UV-Vis absorption spectra of Ce 0.98-x Cr x Fe 0.02 O 2 (where x =0, 0.01, 0.02 and 0.03) nanoparticles are presented in Fig.3. All samples show strong absorption below 400 nm (3.10 eV). The abrupt reduction in the absorption curves is equivalent to the electronic excitation of electrons passing through the energy gap from the valence band. In order to extract the E g values, rst the Tauc plots which are the plots of (ahu) 2 vs photon energy hu were achieved. These are shown on Fig. 4. Then, according to the Tauc equation (ahu) 2 =A(hu-E g ), here, a is the optical absorption coe cient, hu is the photon energy. A is a constant, and E g is the optical bandgap energy, the E g values can be extracted by the linear extrapolation of the linear portion of the Tauc plots [15]. As shown in Fig. 4 Table 1). With increasing of Cr doping content, the E g value increases slightly. Accordingly, the analysis of the UV-Vis spectra approves the reduction of the particle size detected from XRD investigations and proposes the increasing of E g value with increasing the Cr content into CeO 2 semiconducting nanoparticles. Hence, the observed increase of the E g value of additional Cr doping Fe-doped CeO 2 nanoparticles is due to the decrease in the particle size of the samples with Cr content that attributed to a strong quantum con nement. Similar result was reported for Cr doped ZnO nanoparticles [16]. The surface and interface effects are the other reason for the increasing E g value with increasing of Cr doping content. Larger particles show enlarged scattering which produces broaden and shift for the absorption peak towards longer wavelengths due to larger optical cross sections. However, for small particles, the absorption peak is damped due to the reduced mean free path  [19][20][21]. In addition, Paula et. al. reported that the E g value of CeO 2 nanoparticles was 3.26 eV [22]. This value is very close to the E g value (3.2 eV) of ntype semiconductor CeO 2 , and is also close to the value obtained in this experiment.   Table 1). The coercive eld (H C ) values calculated from hysteresis loops were in the range from 36 to 62 Oe for different Cr concentration (also shown in Table 1). The hysteresis loops have small coercive eld and low remanence featured a soft ferromagnetism. This observation is in agreement with the reported data [2][3][4][5][6][7][8][9][10][11][12][13]. Meanwhile, the lattice distortion or defect will be produced due to the difference of ionic radius. Defects or V O may be responsible for the ferromagnetism of these samples [23]. On the basis of F-center mediated ferromagnetic coupling mechanism (FCE) used in insulated DMO [24][25][26][27], more Vo will provide more coupling centers thus induce larger Ms. The increase in Cr doping concentration favored FM interactions because most of the V O are formed. Therefore, the Ms is enhanced with additional Cr dopant.
Defects have also been reported as one of the possible reason for the FM origination [28]

Conclusions
In summary, Ce 0.98−x Cr x Fe 0.02 O 2 (where x = 0, 0.01, 0.02 and 0.03) nanoparticles were successfully prepared by sol-gel method. The results of XRD show a cubic structure (Fm-3m space group) and particle average sizes varying from 24.8 to 10.9 nm. Raman results are in good accordance with XRD data. Within the XRD and Raman detection limit, no secondary phase related to Cr and Fe is found, and Ce 0.98−x Cr x Fe 0.02 O 2 nanoparticles have the same structure of pure CeO 2 . The optical absorption spectra of the nanoparticles show strong absorption below 400 nm (3.10 eV). The bandgap energy was found to increase slightly from 3.21 eV (x = 0) to 3.33 eV (x = 0.03) with increasing the additional Cr doping level which is related to the quantum con nement effect. Declarations