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Materials Research

versão impressa ISSN 1516-1439

Mat. Res. vol.16 no.4 São Carlos jul./ago. 2013  Epub 22-Mar-2013 

Facile preparation of Mn-doped CeO2 Submicrorods by composite-hydroxide-salt-mediated approach and their magnetic property



Jie Tan*; Wei Zhang; Yao-Hui Lv; Ai-Lin Xia

Anhui Key Laboratory of Metal Materials and Processing, School of Materials Science and Engineering, Anhui University of Technology, Anhui, Maanshan 243002, P.R. China




Mn-CeO2 submicrorods have been obtained from anomalous CeO2 particles though a novel composite-hydroxide-salt-mediated (CHSM) approach. This method is based on a reaction between a metallic salt and a metallic oxide in a solution of composite-hydroxide-salt eutectic at ~225 ºC and normal atmosphere without using an organic dispersant or capping agent. The magnetic measurement of the Mn-CeO2 submicrorods exhibits an enhanced ferromagnetic property at room temperature with a remanence magnetization (Mr) of 1.4 × 10-3 emu.g-1 and coercivity (Hc) of 75 Oe. The UV-visible spectra reveal that the absorption peak of the CeO2 shifts from ultraviolet region to visible light region after being doped with Mn ions. The room temperature ferromagnetic properties and light absorption of the Mn-CeO2 submicrorods would have wide applications in spintroics and photocatalysis field.

Keywords: Mn-CeO2 submicrorods, crystal growth, morphology, magnetic properties



1. Introduction

Ceria (CeO2) is an ionic and insulating oxide with the cubic fluorite structure (CaF2). It has been widely used in fuel cells1,2, treatment of industrial wastewater3-5. In addition, they have been used as polishing media in optics6, as components for H2 production7 and as efficient catalysts8. More recently, ceria doped with metal ions has exhibited novel properties in many aspects. Xia et al.9 have prepared Mn-doped CeO2 nanorods by facile composite-hydroxide-mediated (CHM) approach. They suggested that Mn-doped CeO2 nanorods would have potential applications in photocatalysis and building of photovoltaic devices. R.S. de Biashi et al.10 have investigated electron spin resonance spectra of Cu2+ in diluted solid solutions of Cu in CeO2. The results suggest that the solid solution of Cu in CeO2 exhibits clustering effects. Copper-doped CeO2 is used as a catalyst, especially for the reduction of SO2 by CO11-13. G. Qi et al.14-15 have reported the application of Mn-CeO2 in the low-temperature selective catalytic reduction (SCR) of NOx. Corma et al.16 found that CeO2 nanoparticles as well as rare-earth-doped ceria did not need photosensitization to have photovoltaic activity in the visible region. Wen et al.17 have found that the absorption coefficient of Fe-CeO2 in a frequency range of 0.2-1.8 THz was less than 0.35 cm-1. The result indicates that Fe-CeO2 may be a potential candidate as THz optical materials. Chen et al.18 have investigated the synthesis and room-temperature ferromagnetism of CeO2 nanocrystals with nonmagnetic Ca2+ doping (Ca-CeO2). Zhang et al.19 have synthesized Ba-doped CeO2 nanowires and found that their humidity sensitivity was greatly enhanced in comparison with pure CeO2 particles. Additionally, Gd-, Sm-, Y-, Tb- and Fe-doped CeO2 have been studied extensively20-24.

In the current research, we report a novel approach for the synthesis of Mn-CeO2 submicrorods. The method is based on a reaction between a metallic salt and a metallic oxide in a solution of molten mixed potassium nitrate and potassium hydroxide eutectic at ~225 ºC and normal atmosphere without using an organic dispersant or capping agent. Although the melting points (Tm) of both pure potassium hydroxide and potassium nitrate are over 300 ºC, Tm = 404 ºC for KOH and Tm = 337 ºC for KNO3, the eutectic point at KOH/KNO3 = 63.8:36.2 is only about 225 ºC. During the reaction process, hydroxides work not only as the solvent but also as the reactant for reducing the reaction temperature. The advantage of this methodology is the easy recycle of by-products, owing to applying the salt nitrate and hydroxide of the same metal. Additionally, this methodology provides a one-step, convenient, low-cost, nontoxic, and mass-production route for the synthesis of nanostructures of functional oxide materials.

The as-synthesized submicrorods were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM). The magnetic properties of the Mn-CeO2 submicrorods were investigated. Photoabsorption of Mn-CeO2 submicrorods and pure CeO2 particles were comparatively studied via UV-visible diffuse reflectance spectra.


2. Experimental

Mn-CeO2 submicrorods were synthesized by the composite-hydroxide-salt-mediated (CHSM) method without using any capping agent. All reactants were of analytical grade. The synthesis steps are as follows: (1) a total of 20 g of KOH and KNO3 was mixed at a ratio of 63.8:36.2 and placed in a 25 mL Teflon vessel. (2) A mixture of 1 mmol CeO2 and 1 mmol Mn(NO3)2·6H2O each was used as the raw material for reaction, and was placed on the top of the hydroxide and salt nitrate in the vessel. (3) The Teflon vessel was put into a furnace preheated to 235 ºC. (4) After the hydroxide and salt nitrate were totally molten (30 minutes later), the molten reactants were mixed uniformly by shaking the covered vessel. (5) 24 hours later, the vessel was taken out and cooled to room temperature naturally. The product was collected by centrifugation and through washing with deionized water and ethanol.

X-ray diffraction measurement (XRD, D8-Advance, Germany) with the use of Cu Kα radiation (λ = 1.5418 Å) in the 2θ range from 20º to 70º were used to investigate the crystal phase. The morphology and the size of the synthesized samples were characterized by a field emission scanning electron microscopy (SEM, S-4800, Japan) and at 120 kV by a transmission electron microscopy (TEM, JEM-100CXII, Japan). Energy-dispersive spectroscopy (EDS) was employed to determine the final actual Mn concentration in the composites. The selected-area diffraction (SAED) pattern was taken on the TEM. Ultraviolet-visible (UV-vis) diffuse reflectance spectra were obtained for the dry-pressed disk samples by using a Shimadzu UV-2550 recording spectrophotometer, which was equipped with an integrating sphere, and BaSO4 was used as a reference. The magnetic measurement was obtained on a Micromag Model 2900 Alternating Gradient Magnetometer (AGM). The magnetic hysteresis loop was observed in the range of -10 kOe < H < 10 kOe at temperature of 293 K.


3. Results and Discussion

Figure 1 shows the XRD patterns of the source material CeO2 and the Mn-CeO2 sample synthesized at 235 ºC for 24 hours. All the peaks are assigned by using the JCPDS file (no. 34-0394) which is a pure cubic phase (Fm3m). Comparing the curves of the pure CeO2 with that of the Mn-CeO2 submicrorods, there are no additional diffraction peaks, indicating that Mn ions might have entered into the CeO2 lattices and there are no secondary phases or precipitates in the Mn-CeO2 sample. Additionally, we also find that diffraction peaks of the Mn-CeO2 submicrorods slightly shift to larger Bragg angles. One possible explanation might be that the incorporation of Mn2+ or Mn3+ into the lattice results in the decrease of the lattice parameters as the ionic radius of Mn2+ (0.066 nm) and Mn3+ (0.062 nm) are smaller than that of Ce4+ (0.097 nm)9.



Figure 2a, c shows the SEM images of the source material CeO2 and the Mn-CeO2 submicrorods, respectively. The anomalous particles morphology of the source material CeO2 can be seen with the diameter ranging from 2-8 µm. When doped with Mn ions, the particles convert gradually into submicrorods with lengths of 2-3 µm, as shown in Figure 2b. For Mn-CeO2 submicrorods, the crystal face is clean and sharp, and no amorphous layer is present, because no organic reagent of capping material was introduced during the synthesis process. The interesting morphological conversion of the CeO2 from anomalous particles into single-crystal submicrorods might be explained by a dissolution-recrystallization mechanism25. The crystal growth experiences first the CeO2 and Mn(NO3)2 dissolving in the molten composite alkali and salt of the same metal solvent, and then CeO2 recrystallizing while some Mn ions dope in, and finally crystal growth into the submicrorod morphology. The selected area electron diffraction (SAED) pattern of the corresponding edge of the submicrorod in Figure 2b demonstrates its single-crystalline structure. The growth direction of the submicrorods is [110]. The EDS indicates the existence of Ce, Mn and O in the submicrorods (Figure 2d). In addition, EDS measurement shows that the ratio of the elements in the product is Mn/Ce/O = 1:1:4, demonstrating the controllability in chemical composition.

The XPS measurements provided further information for the evaluation of the purity and surface composition of the Mn doped CeO2 sample. The XPS survey spectrum in Figure 3a demonstrates no peaks of other elements than C, Mn, Ce and O in the sample. Figure 3b-d shows the high-resolution XPS spectra of Ce 3d, Mn 2p and O 1s, respectively. Figure 3b illustrates Ce 3d XPS spectra measured for the Mn doped CeO2 samples. The six components observed in the spectrum (882.7, 889.1, 898.6, 900.9, 907.9 and 916.7 eV) could be assigned unambiguously to Ce4+ species by comparison with data reported in the literature26. The two peaks of the Mn region at 640.8 eV and 652.7 eV are assigned to Mn 2p3/2 and Mn 2p1/2, demonstrating the existence of Mn2+[27]. The O 1s peak centered at 529.3 eV belongs to the O2- in Ce-O bond9.

Figure 4 shows the UV-visible diffuse reflectance spectra of the raw material CeO2 (Figure 4a) and the as-synthesized Mn-CeO2 submicrorods (Figure 4b). From this figure, we observe that the absorption peak is in the visible light region centered at 414.5 nm after being doped with Mn ions. Xia et al.9 have reported that the absorption peak of CeO2 and Mn-CeO2 submicrorods was in the ultraviolet region centered at 349.22 nm and the visible light region centered at 403.96 nm, respectively. Thurber et al.28 have reported the similar phenomenon of the Ni-doped CeO2. They found that the band gap changed from 3.80 to 3.23 eV when the CeO2 was doped with 4% Ni. Thurber et al. attributed this change to the extensive structural changes caused by the incorporation of interstitial Ni. Xia et al. speculated that the peak shift was the result of the defects (such as oxygen vacancies) or the impurities caused from the incorporation of the Mn ions as the defects and impurities could result in the formation of sublevels within the band gap. In conclusion, the red-shift phenomenon has been observed in this experiment which could be attributed to electron-phonon coupling. In certain systems, electron-phonon coupling could be strong enough to overcome the spatial confinement to determine the energy of excitons. It determines or modifies the effective mass of carriers and the style of carrier scattering by the lattice, leading to a red-shift of the emission band29,30. In addition, we have also observed that the absorption range of the Mn-CeO2 submicrorods is wider than that of the pure CeO2 particles. This phenomenon suggests the energy absorption form the charge transition between multisublevels, and indirectly reveals the existence of both defects and impurities. The broad light absorption of the Mn-CeO2 submicrorods shows promise in photocatalytic or photovoltaic applications by harvesting solar energy, as demonstrated by the rare-earth-doped ceria in the dye-free solar cell16.

The magnetization hysteresis (M-H) loops of the Mn-CeO2 submicrorods at 293 K have been measured, as shown in Figure 5. Room temperature ferromagnetism (weak magnetism) of the pure CeO2 particles has been observed31 in which the ferromagnetism was assumed to limited surface defects of the micron-sized particles. However, after the incorporation of Mn ions, the magnetism of the Mn-CeO2 submicrorods is greatly enhanced. The magnetization increases almost linearly under an applied magnetic field up to 1000 Oe. Saturated magnetization is not observed even under an applied magnetic field up to 10 kOe. From the hysteresis loop between -2000 and 2000 Oe, the remanence magnetization (Mr) of 1.4 × 10-3 emu.g-1 and coercivity (Hc) of 75 Oe was observed in Figure 4b. The magnetic properties of CeO2 doped with metal ions, such as Co, Ni, Ca ions25,32,33 have been suggested to originate from a combination of oxygen vacancies and metal ions doping.


4. Conclusions

Mn-CeO2 submicrorods are synthesized from the reaction of CeO2 with Mn(NO3)2·6H2O through the CHSM method, which provides a one-step, convenient, low-cost, nontoxic, and mass-production route. Owing to the defects and impurities, the absorption spectra of the CeO2 shows that the peak shifts from the ultraviolet region to the visible light region after being doped with Mn ions. The Mn-CeO2 submicrorods exhibit an enhanced room temperature ferromagnetism originating from the defects and the doped Mn ions. The room temperature ferromagnetic properties and light absorption of the Mn-CeO2 submicrorods suggest potential applications in spintroics and photocatalysis field.



The work is financially supported by the Youth Foundation of Anhui University of Technology and The work is financially supported by the National Natural Science Foundation of China (Grant No. 11204003).



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Received: June 27, 2012
Revised: October 17, 2012



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