Facile Preparation of Mn-doped CeO 2 Submicrorods by Composite-Hydroxide-Salt-Mediated Approach and Their Magnetic Property

Mn-CeO 2 submicrorods have been obtained from anomalous CeO 2 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 °C and normal atmosphere without using an organic dispersant or capping agent. The magnetic measurement of the Mn-CeO 2 submicrorods exhibits an enhanced ferromagnetic property at room temperature with a remanence magnetization (M r ) of 1.4 × 10 emu.g and coercivity (H c ) of 75 Oe. The UV-visible spectra reveal that the absorption peak of the CeO 2 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-CeO 2 submicrorods would have wide applications in spintroics and photocatalysis field.


Introduction
Ceria (CeO 2 ) is an ionic and insulating oxide with the cubic fluorite structure (CaF 2 ).It has been widely used in fuel cells 1,2 , treatment of industrial wastewater [3][4][5] .In addition, they have been used as polishing media in optics 6 , as components for H 2 production 7 and as efficient catalysts 8 .More recently, ceria doped with metal ions has exhibited novel properties in many aspects.Xia et al. 9 have prepared Mn-doped CeO 2 nanorods by facile composite-hydroxide-mediated (CHM) approach.They suggested that Mn-doped CeO 2 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 Cu 2+ in diluted solid solutions of Cu in CeO 2 .The results suggest that the solid solution of Cu in CeO 2 exhibits clustering effects.Copper-doped CeO 2 is used as a catalyst, especially for the reduction of SO 2 by CO [11][12][13] .G. Qi et al. [14][15] have reported the application of Mn-CeO 2 in the low-temperature selective catalytic reduction (SCR) of NO x .Corma et al. 16 found that CeO 2 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-CeO 2 in a frequency range of 0.2-1.8THz was less than 0.35 cm -1 .The result indicates that Fe-CeO 2 may be a potential candidate as THz optical materials.Chen et al. 18 have investigated the synthesis and room-temperature ferromagnetism of CeO 2 nanocrystals with nonmagnetic Ca 2+ doping (Ca-CeO 2 ).Zhang et al. 19 have synthesized Ba-doped CeO 2 nanowires and found that their humidity sensitivity was greatly enhanced in comparison with pure CeO 2 particles.Additionally, Gd-, Sm-, Y-, Tb-and Fe-doped CeO 2 have been studied extensively [20][21][22][23][24] .
In the current research, we report a novel approach for the synthesis of Mn-CeO 2 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 °C and normal atmosphere without using an organic dispersant or capping agent.Although the melting points (T m ) of both pure potassium hydroxide and potassium nitrate are over 300 °C, T m = 404 °C for KOH and T m = 337 °C for KNO 3 , the eutectic point at KOH/KNO 3 = 63.8:36.2 is only about 225 °C.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-CeO 2 submicrorods were investigated.Photoabsorption of Mn-CeO 2 submicrorods and pure CeO 2 particles were comparatively studied via UV-visible diffuse reflectance spectra.

Experimental
Mn-CeO 2 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 KNO 3 was mixed at a ratio of 63.8:36.2 and placed in a 25 mL Teflon vessel.(2) A mixture of 1 mmol CeO 2 and 1 mmol Mn(NO 3 ) 2 •6H 2 O 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 °C.(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 = 1.5418Å) in the 2θ range from 20° to 70° 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 BaSO 4 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.

Results and Discussion
Figure 1 shows the XRD patterns of the source material CeO 2 and the Mn-CeO 2 sample synthesized at 235 °C 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 CeO 2 with that of the Mn-CeO 2 submicrorods, there are no additional diffraction peaks, indicating that Mn ions might have entered into the CeO 2 lattices and there are no secondary phases or precipitates in the Mn-CeO 2 sample.Additionally, we also find that diffraction peaks of the Mn-CeO 2 submicrorods slightly shift to larger Bragg angles.One possible explanation might be that the incorporation of Mn 2+ or Mn 3+ into the lattice results in the decrease of the lattice parameters as the ionic radius of Mn 2+ (0.066 nm) and Mn 3+ (0.062 nm) are smaller than that of Ce 4+ (0.097 nm) 9 .
Figure 2a, c shows the SEM images of the source material CeO 2 and the Mn-CeO 2 submicrorods, respectively.The anomalous particles morphology of the source material CeO 2 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-CeO 2 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 CeO 2 from anomalous particles into single-crystal submicrorods might be explained by a dissolution-recrystallization mechanism 25 .The crystal growth experiences first the CeO 2 and Mn(NO 3 ) 2 dissolving in the molten composite alkali and salt of the same metal solvent, and then CeO 2 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 CeO 2 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 CeO 2 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 Ce 4+ species by comparison with data reported in the literature 26 .The two peaks of the Mn region at 640.8 eV and 652.7 eV are assigned to Mn 2p 3/2 and Mn 2p 1/2 , demonstrating the existence of Mn 2+ [27] .The O 1s peak centered at 529.3 eV belongs to the O 2-in Ce-O bond 9 .
Figure 4 shows the UV-visible diffuse reflectance spectra of the raw material CeO 2 (Figure 4a) and the as-synthesized Mn-CeO 2 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 CeO 2 and Mn-CeO 2 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 CeO 2 .They found that the band gap changed from 3.80 to 3.23 eV when the CeO 2 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 band 29,30 .In addition, we have also observed that the absorption range of the Mn-CeO 2 submicrorods is wider than that of the pure CeO 2 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-CeO 2 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 cell 16 .
The magnetization hysteresis (M-H) loops of the Mn-CeO 2 submicrorods at 293 K have been measured, as shown in Figure 5. Room temperature ferromagnetism (weak magnetism) of the pure CeO 2 particles has been observed 31 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-CeO 2 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 (M r ) of 1.4 × 10 -3 emu.g -1 and coercivity (H c ) of 75 Oe was observed in Figure 4b.The magnetic properties of CeO 2 doped with metal ions, such as Co, Ni, Ca ions 25,32,33 have been suggested to originate from a combination of oxygen vacancies and metal ions doping.

Conclusions
Mn-CeO 2 submicrorods are synthesized from the reaction of CeO 2 with Mn(NO 3 ) 2 •6H 2 O 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 CeO 2 shows that the peak shifts from the ultraviolet region to the visible light region after being doped with Mn ions.The Mn-CeO 2 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-CeO 2 submicrorods suggest potential applications in spintroics and photocatalysis field.

Figure 2 .
Figure 2. SEM images of the source material CeO 2 (a) and TEM images, electron diffraction pattern (inset of b) of the Mn-CeO 2 submicrorods sample synthesized at 235 °C for 24 hours (b).

Figure 1 .
Figure 1.XRD patterns of the source material CeO 2 and the Mn-CeO 2 submicrorods obtained at 235 °C for 24 hours.

Figure 5 .
Figure 5. Magnetization hysteresis loop for the obtained Mn-CeO 2 submicrorods at temperature of 293 K.