- Email: [email protected]
Controlled synthesis and magnetic properties of bowknot-like CeO2 microstructures by a CTAB-assisted hydrothermal method
Materials Letters 119 (2014) 1–3
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Controlled synthesis and magnetic properties of bowknot-like CeO2 microstructures by a CTAB-assisted hydrothermal method Cheng Zhang a, Fanming Meng a,b,n, Leini Wang a a b
School of Physics and Materials Science, Anhui University, 111 Jiulong Road, Hefei 230601, PR China Key laboratory of Materials Modification by laser, Ion and Electron Beams (Dalian University of Technology), Ministry of Education, Dalian 116024, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 15 November 2013 Accepted 21 December 2013 Available online 27 December 2013
Bowknot-like CeO2 microstructures have been successfully synthesized by a CTAB (cetyltrimethyl ammonium bromide)-assisted hydrothermal method using Ce(NO3)3 6H2O as cerium source and urea as mineralizer. X-ray diffraction (XRD) inferred that the synthesized CeO2 microstructures exhibited a fluorite cubic structure. A scanning electron microscope (SEM) revealed that the CeO2 microstructures have a bowknot-like microstructure. X-ray photoelectron spectroscopy (XPS) and the Raman spectroscopy reflected the existence of the oxygen vacancies and Ce3 þ ions in the bowknot-like CeO2 microstructures. Magnetization measurements exhibited room temperature ferromagnetism (RTFM) with a saturation magnetization (Ms) of 3.97 10 3 emu/g and coercivity (Hc) of 200 Oe, which is likely associated with Ce3 þ ions and oxygen vacancies in the bowknot-like CeO2 samples. & 2013 Elsevier B.V. All rights reserved.
Keywords: Magnetic materials CeO2 microstructure Hydrothermal method X-ray techniques
1. Introduction Recently, oxide-based diluted magnetic semiconductor systems have attracted considerable attention because of the reports of room temperature ferromagnetism (RTFM) in several systems and their projected potential in a spintronic device [1–3]. It is generally accepted that magnetic order in a semiconductor requires a few percent of transition metallic dopants that have partially filled shells of d or f electrons to mediate ferromagnetism (FM) [4,5]. However, some scholars have reported RTFM in various pure oxides such as CeO2, TiO2, Al2O3 and ZnO [6]. Ceria (CeO2) and CeO2 based system have a numerous applications including oxygen storage capacity [7,8], structural chemistry [9,10], optical [11,12], electrical [13,14], catalytic [15,16] and magnetic properties [17,18]. As we all know, the functionality of nano-CeO2 strongly depends on the morphology and size of nanostructures, therefore, the control of morphology and size is of great interest. So far, the preparation of CeO2 nanostructures is by no means a new subject, however, much more efforts is still needed to synthesize nanoCeO2 with controllable morphology and size because of the inherited excellent physical and chemical properties which are hard to achieve in their bulk counterparts. Among numerous growth methods, a low temperature hydrothermal process has been proven to be a simple and cost-effective approach for the
n Corresponding author at: School of Physics and Materials Science, Anhui University, 111 Jiulong Road, Hefei 230601, PR China. Tel.: þ 86 551 63861257; fax: þ86 551 63861992. E-mail address: [email protected] (F. Meng).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.12.087
preparation of nano-CeO2. In this work, we report the synthesis and RTFM of bowknot-like CeO2 microstructures by a CTABassisted hydrothermal method using Ce(NO3)3 6H2O as cerium source and urea as mineralizer. 2. Experimental All the reagents were of analytical grade purity and used as received without further purification. The detailed synthetic process was as following: 4 mmol cetyltrimethyl ammonium bromide (CTAB) and 2 mmol Ce(NO3)3 6H2O were dissolved in 20 mL distilled water under vigorous stirring for 30 min, and 30 mmol urea was dissolved in 20 mL distilled water under vigorous stirring for 30 min. After that, 20 mL urea aqueous solution was added to 20 mL CTAB and Ce(NO3)3 6H2O aqueous solution under continuous stirring 30 min, forming a homogeneous solution. The mixed solution was transferred into a 50 mL Teflon-lined autoclave and heated at 180 1C for 12 h. After the autoclave was cooled to room temperature naturally, fresh precipitates were washed with distilled water and ethanol for three times, and then dried at 70 1C overnight. CeO2 microstructures were obtained by calcining at 500 1C for 5 h, accompanied by a color change from white to slight yellow. The crystal phases of the products were analyzed by an X-ray diffractometer (XRD) with Cu Kα radiation (λ ¼0.1506 nm). The morphologies were characterized by a scanning electron microscope (SEM, S-4800). The chemical state was analyzed by X-ray photo-electron spectroscopy (XPS, ESCALAB 250 US Thermo Electron Co.). The Raman spectrometer system (inVia-Reflex) using
2
C. Zhang et al. / Materials Letters 119 (2014) 1–3
Fig. 1. SEM images of (a) bowknot-like CeOHCO3 obtained at 180 1C for 12 h and (b) CeO2 obtained by calcining as-prepared bowknot-like CeOHCO3 at 500 1C for 4 h; insets in (a) and (b) are the corresponding XRD patterns.
531.32
915.11
896.86
880.82
Intensity (a.u.)
900.78 905.96
882.84 887.35
Intensity (a.u.)
899.30
527.83 528.64
920
910
900
890
880
Binding Energy (eV)
534
532
530
528
526
Binding Energy (eV)
Fig. 2. XPS core level spectra of Ce3d (a) and O1s (b) of bowknot-like CeO2 samples.
a laser with 532 nm excitation at room temperature was also used. The UV–vis absorption spectroscopy was measured by an ultraviolet–visible–near infrared spectrophotometer (U-4100). Photoluminescence (PL) spectra were obtained by a fluorescence spectrophotometer (HORIBA FluoroMax-4P, HORIBA Jobin Yvon) using excitation light of 340 nm. The M–H curves were measured at room temperature by a vibrating sample magnetometer (BHV-55).
3. Results and discussion The insert patterns in Fig. 1a and b are the XRD patterns of the samples as-obtained and calcined at 500 1C for 4 h, respectively. All peaks of the pattern in Fig. 1a can be indexed to hexagonal CeOHCO3 (JCPDS no. 32-0189). And after annealing the precursors, the bowknot-like CeO2 with pure face-centered cubic (JCPDS no. 43-1002) structure is obtained, as shown in Fig. 1b. The sharp peaks demonstrate that both the as-obtained and calcined materials show good crystallinity. The morphologies of the CeOHCO3 and CeO2 are shown in Fig. 1a and b, respectively. Fig. 1a shows the bowknot-like CeOHCO3 microstructures of 3 mm in diameter and 10 mm in length, and this microstructure is made up of nanorods of 100 nm in diameter and 1 mm in length. After being annealed, the bowknot-like structure did not change on the overall appearance but nanorods became more slender and compact. The variation can be attributed to the decomposition of the precursors which is shown by the following equation: 4CeOHCO3 þO2-4CeO2 þ 4CO2 þ2H2O
It is well know that there are two different oxidation states for elemental Ce, namely, Ce(III) and Ce(IV). However, Ce(IV) is more stable than Ce(III) in the presence of air [19]. Fig. 2a shows Ce3d XPS spectra of bowknot-like CeO2 microstructures. The corresponding fitted deconvolutions of the Ce3d levels are also included in Fig. 2a. The peaks located in 915.11, 905.96, 899.30, 896.86, 887.35 and 880.82 eV are attributed to Ce4 þ ions, while those at 900.78 and 882.84 eV are the characteristic peaks of Ce3 þ [20]. This result indicates that both Ce4 þ and Ce3 þ ions coexist, and there are surface Ce4 þ /Ce3 þ pairs in the bowknot-like CeO2 microstructures. Fig. 3b shows O1s core levels spectra for the bowknot-like CeO2 sample. From Fig. 3b, it can be seen that the O1s spectra consist of three peaks at binding energy 527.83 eV, 528.64 eV, and 531.32 eV, which can be attributed to lattice oxygen ions in CeO2, absorbed oxygen and lattice oxygen ions in Ce2O3 [17,21,22], respectively. To better understand the defects in the bowknot-like CeO2 microstructures, the Raman scattering was carried out and is shown in Fig. 3a. One strong Raman peak centered at about 462 cm 1 dominates the spectra. This peak originates from the F2g Raman-active mode of CeO2 cube structure [23], which is very sensitive to any disorder in the oxygen sublattice [24]. As we know that the main peak of bulk CeO2 is at 466 cm 1, but in this case, the main peak shifts toward lower wavenumber and becomes more asymmetric. Several factors, such as phonon confinement, defects and variation in phonon relaxation with particle size, may contribute to the changes in the position and asymmetry of the 462 cm 1 peak [18]. Based on the XPS analysis, it can be reasonably concluded that Ce3 þ and oxygen vacancies have the main contribution to the changes in the position and asymmetry of the
C. Zhang et al. / Materials Letters 119 (2014) 1–3
14
462 cm-1
4
Magnetization (×10-3emu/g)
Intensity (×103 a.u.)
12 10 8 6 4 2
3
T=300K 2
0
-2
-4
0 250 300 350 400 450 500 550 600 650 700 750 800
-4000
Raman shift (cm-1)
-2000
0
2000
4000
Magnetic field (Oe)
Fig. 3. (a) The Raman spectra and (b) the M–H curve of bowknot-like CeO2 samples.
462 cm 1. The shoulder peak from 560 to 600 cm 1 can be attributed to the presence of Ce3 þ and oxygen vacancies [17]. The peak near 254 cm 1 can be attributed to disorder in the system [25]. Fig. 3b shows the M–H curve of bowknot-like CeO2 samples and the inset of Fig. 3b is the magnetization of the central part. It can be seen that saturation magnetization (Ms) is 3.97 10 3 emu/g, residual magnetization (Mr) is 0.75 10 3 emu/g and coercivity (Hc) is 200 Oe. It is noted that the value of Hc is larger than those in previous reports [26] and Ms and Mr are smaller than those in previous reports [27]. It is well known that the stoichiometrical CeO2 is paramagnetic. When the valence of Ce changes from þ4 to þ3, an unpaired spin in the Ce f orbit is generated. Based on XPS and the Raman analysis, it can be concluded that Ce3 þ and oxygen vacancy existed in CeO2 deposits. The Ce3þ ions change to Ce4þ ions and enhance the transfer between Ce 4f and O 2p and result in the charge transfer transition between O 2p and Ce 4f bands. The RTFM might consequently arise from a nearest-neighbor interaction: double exchange (Ce3þ –O–Ce4þ ) or superexchange (Ce3 þ –O–Ce3 þ ) [28]. So the RTFM of the bowknotlike CeO2 samples can be attributed to the effects of the Ce3 þ and oxygen vacancies. 4. Conclusion In summary, the bowknot-like CeO2 has been successfully synthesized by a CTAB-assisted hydrothermal method. The XRD pattern showed that the synthesized CeO2 have the pure facecentered cubic structure. XPS and the Raman analysis indicated that the obtained CeO2 contains a small number of Ce3 þ and oxygen vacancies at CeO2 surface. The synthesized CeO2 shows excellent RTFM with Ms of 3.97 10 3 emu/g, Hc of 200 Oe and Mr of 0.75 10 3 emu/g, which may be attributed to the Ce3 þ and oxygen vacancies. The controllable morphology and RTFM should make the bowknot-like CeO2 an excellent candidate for applications in related areas. Acknowledgments This work was supported by the Anhui Provincial Natural Science Foundation of China (1208085ME81) and the Scientific
Research Foundation of Education Ministry of Anhui Province of China (KJ2011A010 and KJ2012A029).
References [1] Chen SY, Lu YH, Huang TW, Yan DC, Dong CL. J Phys Chem C 2010;114: 19576–81. [2] Meng F, Zhang C, Bo Q, Zhang Q. Mater Lett 2013;99:5–7. [3] Bouaine A, Green RJ, Colis S, Bazylewski P, Chang GS, Moewes A, et al. J Phys Chem C 2011;115:1556–60. [4] Sun Z, Yan W, Zhang G, Oyanagi H, Wu Z, Liu Q, et al. Phys Rev B 2008;77:245208–13. [5] Singhal A, Achary SN, Manjanna J, Jayakumar OD, Kadam RM, Tyagi AK. J Phys Chem C 2009;113:3600–6. [6] Sundaresan A, Bhargavi R, Rangarajan N, Siddesh U, Rao CNR. Phys Rev B 2006;74:161304–7. [7] Ozawa M, Yuzuriha H, Haneda M. Catal Commun 2013;30:32–5. [8] Newton MA, Michiel MD, Kubacka A, Iglesias-Juez A, Fernandsez-Garcia M. Angew Chem Int Ed 2012;51:2363–7. [9] Achary SN, Sali SK, Kulkarni NK, Krishna PSR, Shinde AB, Tyagi AK. Chem Mater 2009;21:5848–59. [10] Mandal BP, Roy M, Grover V, Tyagi AK. J Appl Phy 2008;103:033506. [11] Sujana MG, Chattopadyay KK, Anand S. Appl Surf Sci 2008;254:7405–9. [12] Zeng S, Tang K, Li T, Liang Z. J Colloid Interface Sci 2007;316:921–9. [13] Pfau A, Schierbaum KD. Surf Sci 1994;321:71–80. [14] Pfau A, Schierbaum KD. Surf Sci 1995;331:1476–85. [15] Esch F, Fabris S, Zhou L, Montini T, Africh C, Fornasiero P, et al. Science 2005;309:752–5. [16] Campbell CT, Peden CH. Science 2005;309:713–4. [17] Wang L, Meng F. Mater Res Bull 2013;48:3492–8. [18] Singhal RK, Kumar S, Samariya A, Dhawan M, Sharma SC, Xing YT. Mater Chem Phys 2012;132:534–9. [19] Lu XH, Zhai T, Cui HN, Shi JY, Xie SL, Huang YY, et al. J Mater Chem 2011;21:5569–72. [20] Deshpande S, Patil S, Kuchibhatla S, Seal S. Appl Phys Lett 2005;87:133113-13. [21] Yu R, Yan L, Zheng P, Chen J, Xing X. J Phys Chem C 2008;112:19896–900. [22] Lasachi F, Pierre J, Adnot A, Bernis A. Appl Surf Sci 2002;195:236–50. [23] Phoka S, Laokul P, Swatsitang E, Promarak V, Seraphin S, Maensiri S. Mater Chem Phys. 2009;115:423–8. [24] Lu XH, Huang X, Xie SL, Zheng DZ, Liu ZQ, Liang CL, et al. Langmuir 2010;26:7569–73. [25] Kosacki I, Petrovsky V, Anderson HU, Colomban PJ. J Am Ceram Soc 2002;85:2646–50. [26] Meng F, Wang L. Mater Lett 2013;100:86–8. [27] Liu Y, Lockman Z, Aziz A, MacManus-Driscoll J. J Phys: Condens Matter 2008;20:165201–5. [28] Chen S, Tsai C, Huang M, Yan D, Huang T, Gloter A, et al. J Phys Chem C 2011;116:8707–13.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Controlled synthesis and magnetic properties of bowknot-like CeO2 microstructures by a CTAB-assisted hydrothermal method Cheng Zhang a, Fanming Meng a,b,n, Leini Wang a a b
School of Physics and Materials Science, Anhui University, 111 Jiulong Road, Hefei 230601, PR China Key laboratory of Materials Modification by laser, Ion and Electron Beams (Dalian University of Technology), Ministry of Education, Dalian 116024, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 15 November 2013 Accepted 21 December 2013 Available online 27 December 2013
Bowknot-like CeO2 microstructures have been successfully synthesized by a CTAB (cetyltrimethyl ammonium bromide)-assisted hydrothermal method using Ce(NO3)3 6H2O as cerium source and urea as mineralizer. X-ray diffraction (XRD) inferred that the synthesized CeO2 microstructures exhibited a fluorite cubic structure. A scanning electron microscope (SEM) revealed that the CeO2 microstructures have a bowknot-like microstructure. X-ray photoelectron spectroscopy (XPS) and the Raman spectroscopy reflected the existence of the oxygen vacancies and Ce3 þ ions in the bowknot-like CeO2 microstructures. Magnetization measurements exhibited room temperature ferromagnetism (RTFM) with a saturation magnetization (Ms) of 3.97 10 3 emu/g and coercivity (Hc) of 200 Oe, which is likely associated with Ce3 þ ions and oxygen vacancies in the bowknot-like CeO2 samples. & 2013 Elsevier B.V. All rights reserved.
Keywords: Magnetic materials CeO2 microstructure Hydrothermal method X-ray techniques
1. Introduction Recently, oxide-based diluted magnetic semiconductor systems have attracted considerable attention because of the reports of room temperature ferromagnetism (RTFM) in several systems and their projected potential in a spintronic device [1–3]. It is generally accepted that magnetic order in a semiconductor requires a few percent of transition metallic dopants that have partially filled shells of d or f electrons to mediate ferromagnetism (FM) [4,5]. However, some scholars have reported RTFM in various pure oxides such as CeO2, TiO2, Al2O3 and ZnO [6]. Ceria (CeO2) and CeO2 based system have a numerous applications including oxygen storage capacity [7,8], structural chemistry [9,10], optical [11,12], electrical [13,14], catalytic [15,16] and magnetic properties [17,18]. As we all know, the functionality of nano-CeO2 strongly depends on the morphology and size of nanostructures, therefore, the control of morphology and size is of great interest. So far, the preparation of CeO2 nanostructures is by no means a new subject, however, much more efforts is still needed to synthesize nanoCeO2 with controllable morphology and size because of the inherited excellent physical and chemical properties which are hard to achieve in their bulk counterparts. Among numerous growth methods, a low temperature hydrothermal process has been proven to be a simple and cost-effective approach for the
n Corresponding author at: School of Physics and Materials Science, Anhui University, 111 Jiulong Road, Hefei 230601, PR China. Tel.: þ 86 551 63861257; fax: þ86 551 63861992. E-mail address: [email protected] (F. Meng).
0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.12.087
preparation of nano-CeO2. In this work, we report the synthesis and RTFM of bowknot-like CeO2 microstructures by a CTABassisted hydrothermal method using Ce(NO3)3 6H2O as cerium source and urea as mineralizer. 2. Experimental All the reagents were of analytical grade purity and used as received without further purification. The detailed synthetic process was as following: 4 mmol cetyltrimethyl ammonium bromide (CTAB) and 2 mmol Ce(NO3)3 6H2O were dissolved in 20 mL distilled water under vigorous stirring for 30 min, and 30 mmol urea was dissolved in 20 mL distilled water under vigorous stirring for 30 min. After that, 20 mL urea aqueous solution was added to 20 mL CTAB and Ce(NO3)3 6H2O aqueous solution under continuous stirring 30 min, forming a homogeneous solution. The mixed solution was transferred into a 50 mL Teflon-lined autoclave and heated at 180 1C for 12 h. After the autoclave was cooled to room temperature naturally, fresh precipitates were washed with distilled water and ethanol for three times, and then dried at 70 1C overnight. CeO2 microstructures were obtained by calcining at 500 1C for 5 h, accompanied by a color change from white to slight yellow. The crystal phases of the products were analyzed by an X-ray diffractometer (XRD) with Cu Kα radiation (λ ¼0.1506 nm). The morphologies were characterized by a scanning electron microscope (SEM, S-4800). The chemical state was analyzed by X-ray photo-electron spectroscopy (XPS, ESCALAB 250 US Thermo Electron Co.). The Raman spectrometer system (inVia-Reflex) using
2
C. Zhang et al. / Materials Letters 119 (2014) 1–3
Fig. 1. SEM images of (a) bowknot-like CeOHCO3 obtained at 180 1C for 12 h and (b) CeO2 obtained by calcining as-prepared bowknot-like CeOHCO3 at 500 1C for 4 h; insets in (a) and (b) are the corresponding XRD patterns.
531.32
915.11
896.86
880.82
Intensity (a.u.)
900.78 905.96
882.84 887.35
Intensity (a.u.)
899.30
527.83 528.64
920
910
900
890
880
Binding Energy (eV)
534
532
530
528
526
Binding Energy (eV)
Fig. 2. XPS core level spectra of Ce3d (a) and O1s (b) of bowknot-like CeO2 samples.
a laser with 532 nm excitation at room temperature was also used. The UV–vis absorption spectroscopy was measured by an ultraviolet–visible–near infrared spectrophotometer (U-4100). Photoluminescence (PL) spectra were obtained by a fluorescence spectrophotometer (HORIBA FluoroMax-4P, HORIBA Jobin Yvon) using excitation light of 340 nm. The M–H curves were measured at room temperature by a vibrating sample magnetometer (BHV-55).
3. Results and discussion The insert patterns in Fig. 1a and b are the XRD patterns of the samples as-obtained and calcined at 500 1C for 4 h, respectively. All peaks of the pattern in Fig. 1a can be indexed to hexagonal CeOHCO3 (JCPDS no. 32-0189). And after annealing the precursors, the bowknot-like CeO2 with pure face-centered cubic (JCPDS no. 43-1002) structure is obtained, as shown in Fig. 1b. The sharp peaks demonstrate that both the as-obtained and calcined materials show good crystallinity. The morphologies of the CeOHCO3 and CeO2 are shown in Fig. 1a and b, respectively. Fig. 1a shows the bowknot-like CeOHCO3 microstructures of 3 mm in diameter and 10 mm in length, and this microstructure is made up of nanorods of 100 nm in diameter and 1 mm in length. After being annealed, the bowknot-like structure did not change on the overall appearance but nanorods became more slender and compact. The variation can be attributed to the decomposition of the precursors which is shown by the following equation: 4CeOHCO3 þO2-4CeO2 þ 4CO2 þ2H2O
It is well know that there are two different oxidation states for elemental Ce, namely, Ce(III) and Ce(IV). However, Ce(IV) is more stable than Ce(III) in the presence of air [19]. Fig. 2a shows Ce3d XPS spectra of bowknot-like CeO2 microstructures. The corresponding fitted deconvolutions of the Ce3d levels are also included in Fig. 2a. The peaks located in 915.11, 905.96, 899.30, 896.86, 887.35 and 880.82 eV are attributed to Ce4 þ ions, while those at 900.78 and 882.84 eV are the characteristic peaks of Ce3 þ [20]. This result indicates that both Ce4 þ and Ce3 þ ions coexist, and there are surface Ce4 þ /Ce3 þ pairs in the bowknot-like CeO2 microstructures. Fig. 3b shows O1s core levels spectra for the bowknot-like CeO2 sample. From Fig. 3b, it can be seen that the O1s spectra consist of three peaks at binding energy 527.83 eV, 528.64 eV, and 531.32 eV, which can be attributed to lattice oxygen ions in CeO2, absorbed oxygen and lattice oxygen ions in Ce2O3 [17,21,22], respectively. To better understand the defects in the bowknot-like CeO2 microstructures, the Raman scattering was carried out and is shown in Fig. 3a. One strong Raman peak centered at about 462 cm 1 dominates the spectra. This peak originates from the F2g Raman-active mode of CeO2 cube structure [23], which is very sensitive to any disorder in the oxygen sublattice [24]. As we know that the main peak of bulk CeO2 is at 466 cm 1, but in this case, the main peak shifts toward lower wavenumber and becomes more asymmetric. Several factors, such as phonon confinement, defects and variation in phonon relaxation with particle size, may contribute to the changes in the position and asymmetry of the 462 cm 1 peak [18]. Based on the XPS analysis, it can be reasonably concluded that Ce3 þ and oxygen vacancies have the main contribution to the changes in the position and asymmetry of the
C. Zhang et al. / Materials Letters 119 (2014) 1–3
14
462 cm-1
4
Magnetization (×10-3emu/g)
Intensity (×103 a.u.)
12 10 8 6 4 2
3
T=300K 2
0
-2
-4
0 250 300 350 400 450 500 550 600 650 700 750 800
-4000
Raman shift (cm-1)
-2000
0
2000
4000
Magnetic field (Oe)
Fig. 3. (a) The Raman spectra and (b) the M–H curve of bowknot-like CeO2 samples.
462 cm 1. The shoulder peak from 560 to 600 cm 1 can be attributed to the presence of Ce3 þ and oxygen vacancies [17]. The peak near 254 cm 1 can be attributed to disorder in the system [25]. Fig. 3b shows the M–H curve of bowknot-like CeO2 samples and the inset of Fig. 3b is the magnetization of the central part. It can be seen that saturation magnetization (Ms) is 3.97 10 3 emu/g, residual magnetization (Mr) is 0.75 10 3 emu/g and coercivity (Hc) is 200 Oe. It is noted that the value of Hc is larger than those in previous reports [26] and Ms and Mr are smaller than those in previous reports [27]. It is well known that the stoichiometrical CeO2 is paramagnetic. When the valence of Ce changes from þ4 to þ3, an unpaired spin in the Ce f orbit is generated. Based on XPS and the Raman analysis, it can be concluded that Ce3 þ and oxygen vacancy existed in CeO2 deposits. The Ce3þ ions change to Ce4þ ions and enhance the transfer between Ce 4f and O 2p and result in the charge transfer transition between O 2p and Ce 4f bands. The RTFM might consequently arise from a nearest-neighbor interaction: double exchange (Ce3þ –O–Ce4þ ) or superexchange (Ce3 þ –O–Ce3 þ ) [28]. So the RTFM of the bowknotlike CeO2 samples can be attributed to the effects of the Ce3 þ and oxygen vacancies. 4. Conclusion In summary, the bowknot-like CeO2 has been successfully synthesized by a CTAB-assisted hydrothermal method. The XRD pattern showed that the synthesized CeO2 have the pure facecentered cubic structure. XPS and the Raman analysis indicated that the obtained CeO2 contains a small number of Ce3 þ and oxygen vacancies at CeO2 surface. The synthesized CeO2 shows excellent RTFM with Ms of 3.97 10 3 emu/g, Hc of 200 Oe and Mr of 0.75 10 3 emu/g, which may be attributed to the Ce3 þ and oxygen vacancies. The controllable morphology and RTFM should make the bowknot-like CeO2 an excellent candidate for applications in related areas. Acknowledgments This work was supported by the Anhui Provincial Natural Science Foundation of China (1208085ME81) and the Scientific
Research Foundation of Education Ministry of Anhui Province of China (KJ2011A010 and KJ2012A029).
References [1] Chen SY, Lu YH, Huang TW, Yan DC, Dong CL. J Phys Chem C 2010;114: 19576–81. [2] Meng F, Zhang C, Bo Q, Zhang Q. Mater Lett 2013;99:5–7. [3] Bouaine A, Green RJ, Colis S, Bazylewski P, Chang GS, Moewes A, et al. J Phys Chem C 2011;115:1556–60. [4] Sun Z, Yan W, Zhang G, Oyanagi H, Wu Z, Liu Q, et al. Phys Rev B 2008;77:245208–13. [5] Singhal A, Achary SN, Manjanna J, Jayakumar OD, Kadam RM, Tyagi AK. J Phys Chem C 2009;113:3600–6. [6] Sundaresan A, Bhargavi R, Rangarajan N, Siddesh U, Rao CNR. Phys Rev B 2006;74:161304–7. [7] Ozawa M, Yuzuriha H, Haneda M. Catal Commun 2013;30:32–5. [8] Newton MA, Michiel MD, Kubacka A, Iglesias-Juez A, Fernandsez-Garcia M. Angew Chem Int Ed 2012;51:2363–7. [9] Achary SN, Sali SK, Kulkarni NK, Krishna PSR, Shinde AB, Tyagi AK. Chem Mater 2009;21:5848–59. [10] Mandal BP, Roy M, Grover V, Tyagi AK. J Appl Phy 2008;103:033506. [11] Sujana MG, Chattopadyay KK, Anand S. Appl Surf Sci 2008;254:7405–9. [12] Zeng S, Tang K, Li T, Liang Z. J Colloid Interface Sci 2007;316:921–9. [13] Pfau A, Schierbaum KD. Surf Sci 1994;321:71–80. [14] Pfau A, Schierbaum KD. Surf Sci 1995;331:1476–85. [15] Esch F, Fabris S, Zhou L, Montini T, Africh C, Fornasiero P, et al. Science 2005;309:752–5. [16] Campbell CT, Peden CH. Science 2005;309:713–4. [17] Wang L, Meng F. Mater Res Bull 2013;48:3492–8. [18] Singhal RK, Kumar S, Samariya A, Dhawan M, Sharma SC, Xing YT. Mater Chem Phys 2012;132:534–9. [19] Lu XH, Zhai T, Cui HN, Shi JY, Xie SL, Huang YY, et al. J Mater Chem 2011;21:5569–72. [20] Deshpande S, Patil S, Kuchibhatla S, Seal S. Appl Phys Lett 2005;87:133113-13. [21] Yu R, Yan L, Zheng P, Chen J, Xing X. J Phys Chem C 2008;112:19896–900. [22] Lasachi F, Pierre J, Adnot A, Bernis A. Appl Surf Sci 2002;195:236–50. [23] Phoka S, Laokul P, Swatsitang E, Promarak V, Seraphin S, Maensiri S. Mater Chem Phys. 2009;115:423–8. [24] Lu XH, Huang X, Xie SL, Zheng DZ, Liu ZQ, Liang CL, et al. Langmuir 2010;26:7569–73. [25] Kosacki I, Petrovsky V, Anderson HU, Colomban PJ. J Am Ceram Soc 2002;85:2646–50. [26] Meng F, Wang L. Mater Lett 2013;100:86–8. [27] Liu Y, Lockman Z, Aziz A, MacManus-Driscoll J. J Phys: Condens Matter 2008;20:165201–5. [28] Chen S, Tsai C, Huang M, Yan D, Huang T, Gloter A, et al. J Phys Chem C 2011;116:8707–13.