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Hydrothermal synthesis of hexagonal CeO2 nanosheets and their room temperature ferromagnetism
Journal of Alloys and Compounds 647 (2015) 1013e1021
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Hydrothermal synthesis of hexagonal CeO2 nanosheets and their room temperature ferromagnetism Fanming Meng a, b, *, Cheng Zhang a, Zhenghua Fan a, Jinfeng Gong a, Aixia Li a, Zongling Ding a, Huaibao Tang a, Miao Zhang a, Guifang Wu a a
School of Physics and Materials Science, Anhui University, 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 b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 April 2015 Received in revised form 5 June 2015 Accepted 21 June 2015 Available online 24 June 2015
Hexagonal CeO2 nanosheets of 40e50 nm in thickness and 300e400 nm in side-length have been successfully synthesized via controlling the morphology of CeCO3OH precursors by a facile hydrothermal technique using CeCl3$7H2O as cerium source, ammonium hydrogen carbonate as precipitants, and ethylenediamine as complexant. The reaction time and the amount of CeCl3$7H2O and ethylenediamine were systematically investigated. The as-synthesized hexagonal CeO2 nanosheets were examined by XRD, SEM, TEM, XPS, Raman scattering and magnetization measurements. It is found that the amount of CeCl3$7H2O and ethylenediamine are key parameters for controlling the final morphology. The hexagonal CeO2 nanosheets have a fluorite cubic structure and there are Ce3þ ions and oxygen vacancies in surface of samples. The synthesized CeO2 shows excellent room temperature optical properties. MeH curve exhibits excellent room-temperature ferromagnetism (RTFM) with saturation magnetization (Ms) of 3.02 102 emu/g, residual magnetization (Mr) of 0.68 102 emu/g and coercivity (Hc) of 210 Oe, which is likely attributed to the effects of the Ce3þ ions and oxygen vacancies. © 2015 Elsevier B.V. All rights reserved.
Keywords: Nanostructures Electron microscopy X-ray photo-emission spectroscopy Magnetic properties
1. Introduction Morphology-controlled synthesis of inorganic nanomaterials has been a focus of many researches due to their unique morphology dependent properties and their capability of selfassembly as building blocks [1]. Ceria (CeO2, Eg z 3.2 eV), as a well-known and extensively studied rare earth oxide, has attracted more attention for its promising application in catalysts, fuel cells, oxygen sensors, mechanical polishing, ultraviolet blocks, and luminescent materials [2e6]. Furthermore, nanocrystalline CeO2 shows improved and size-dependent properties [7]. Ceria with various morphologies such as nanocubes, nanotubes, nanowires, nanopoles, mesoporous and flower-like structures have been synthesized in recent years [8e10]. Nevertheless, the synthesis of twodimensional (2D) CeO2 structures is rarely reported. Moreover, a number of precipitants including sodium hydroxide, ammonium hydroxide, ethanediamine and urea have been used to synthesis
* Corresponding author. School of Physics and Materials Science, Anhui University, 111 Jiulong Road, Hefei 230601, PR China. E-mail address: [email protected] (F. Meng). http://dx.doi.org/10.1016/j.jallcom.2015.06.186 0925-8388/© 2015 Elsevier B.V. All rights reserved.
the shape-controlled CeO2 structures. Zhou et al. [11] obtained nanotubes using Ce2(SO4)3$9H2O as cerium resource, NaOH as mineralizer. Sun et al. [12] synthesized flower-like CeO2 microspheres using Ce(NO3)3,6H2O as cerium resource and NH3$H2O as mineralizer. Meng et al. [13] synthesized CeO2 nanopoles using CeCl3$7H2O as cerium resource, NaOH as mineralizer, and ethylenediamine as complexant. But, ammonium hydrogen carbonate could be hydrolyzed to produce ammonium and bicarbonate ions, but its involvement as a precipitant in preparing nano-CeO2 was seldom noted. Therefore, it is a commendable precipitant to fabricate CeO2 nanosheets via annealing CeCO3OH nanosheets. As we all know, magnetic order in a semiconductor requires doping a few percent of transition metal that have partially filled shells of d and f electrons to mediate ferromagnetic (FM) [14]. However, there have been several reports on pure CeO2 exhibiting FM at room temperature [13,15,16]. As a result, oxygen vacancy is a most important factor to RTFM of CeO2 microstructures. Ge et al. [8] reported FM in CeO2 nanoparticles (NPs), and their calculations from first principles revealed that oxygen vacancies, especially at the surface, can induce magnetic moments in CeO2 NPs. And Bernardi et al. [17] also suggest that the presence of oxygen vacancies is
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not a sufficient condition to mediate ferromagnetism in the CeO2 system, and only oxygen vacancies in the surface of nanostructures would lead to such a long range magnetic order. In contrast, some other researchers reported that FM is related not to the surface oxygen vacancies but to the Ce3þ/Ce4þ pairs. Li et al. [18] reported that Ce3þ/Ce4þ pairs in the surface is a most important factor to RTFM of CeO2 microstructures. On the basis of these experimental results, defect-induced mechanism and charger transfer have been used to describe the formation of FM in nanomaterials. Therefore, the mechanism of FM in pure CeO2 nanomaterials is still without a unified statement. Herein, we report a simple hydrothermal method to obtain hexagonal CeO2 nanosheets by thermal decomposition of assynthesized CeCO3OH nanosheets. The mechanism for the transition of phase and morphology are discussed. And the possible mechanism is discussed in terms of the influence of Ce3þ ions. 2. Experimental procedure 2.1. Material preparation All the reagents were of analytical grade purity and used as received without further purification. The detailed synthetic process was as following: 4 mmol CeCl3$7H2O and 40 mmol NH4HCO3 were dissolved in 10 and 15 mL distilled water under vigorous stirring for 0.5 h, respectively. And then, 15 mL NH4HCO3 aqueous solution were gradually added to 10 mL CeCl3$7H2O aqueous solution to form colorless precipitation. After continuous stirring for 0.5 h, 10 mL of ethylenediamine was added dropwise to the CeCl3 solution, the solution was further stirred for 0.5 h. The mixed solution was transferred into a 50 ml Teflon-lined autoclave and heated at 180 C for 1e48 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 C overnight. CeO2 microstructures were obtained by calcining at 400 C for 5 h, accompanied by a color change from white to slight yellow. 2.2. Characterization The crystal phases of the products were analyzed by X-ray diffractometer (XRD) with Cu Ka radiation (l ¼ 0.1506 nm) and the selected area electron diffraction (SAED). The morphology was characterized by scanning electron microscope (SEM, S-4800) and transmission electron microscope (TEM, JEM-2100). The chemical state was analyzed by X-ray photo-electron spectroscopy (XPS, ESCALAB 250 US Thermo Electron Co). Raman spectra were recorded by a Raman spectrometer system (inVia-Reflex) using a laser with 532 nm excitation at room temperature. Photoluminescence (PL) spectra were obtained by a fluorescence spectrophotometer (HORIBA FluoroMax-4P, HORIBA Jobin Yvon) using excitation light of 340 nm. The MeH curve was measured at room temperature by vibrating sample magnetometer (BHV-55). 3. Results and discussion 3.1. Structure and morphology analysis X-ray powder diffraction (XRD) analysis was carried out to investigate the phases of the products as-synthesized and calcined at 400 C for 5 h. A typical XRD pattern of the as-synthesized product is shown in Fig. 1(a). All of the diffraction peaks in Fig. 1(a) can be exactly indexed to the hexagonal CeCO3OH with lattice constants a ¼ 12.530 Å, b ¼ 12.530 Å, c ¼ 10.000 Å, which are in good agreement with the literature values (JCPDS no. 52-0352). It
Fig. 1. XRD patterns of (a) the as-synthesized CeCO3OH samples at 180 C for 48 h and (b) CeO2 obtained by calcining the CeCO3OH samples at 400 C for 5 h.
must be pointed that two tiny visible peaks at around 28 and 33 can be indexed to CeO2. After annealing the precursors, all of the diffraction peaks in Fig. 1(b) can be exactly indexed to CeO2 with face-centered cubic (FCC) structure (JCPDS no. 34-0394). Considering the sensibility of XRD, in the best of the cases, that cannot detect fraction of secondary phases or segregates with concentration lower than 2%. The sharp peaks demonstrate that both the asobtained and calcined materials show good crystallinity. The morphologies of the CeCO3OH and CeO2 are shown in Fig. 2. Fig. 2(a) and (b)show that the as-synthesized CeCO3OH nanosheets are uniform and the size of the hexagonal CeCO3OH nanosheets are 40e50 nm in thickness and 300e400 nm in side-length. From Fig. 2(c) and (d), it can be seen that the post-heat-treatment process does not ruin the morphology of the products, and morphology of CeO2 nanosheets is almost the same as its counterpart. The morphologies of the hexagonal CeO2 nanosheets are further examined by (HR)TEM. Fig. 3(a) and (b) show TEM images of hexagonal CeO2 nanosheets with different magnification, which indicate that the hexagonal CeO2 nanosheets are loose and porous. Fig. 3(c) and (d) show the high-resolution TEM (HRTEM) images of the marked area in Fig. 3(a). From Fig. 3(d), it can be seen that the distance between neighboring planes is about 0.177 nm, representing the crystallographic plane of CeO2. The corresponding
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Fig. 2. SEM images of (a, b) the as-synthesized CeCO3OH samples at 180 C for 48 h and (c, d) CeO2 obtained by calcining the CeCO3OH samples at 400 C for 5 h.
select area electron diffraction (SAED) pattern, as shown Fig. 3(e), further indicates that the hexagonal CeO2 nanosheets have a monocrystalline structure and expose its (111) planar on its both sides. 3.2. The formation mechanism of nanosheets To reveal the formation mechanism of hexagonal CeCO3OH nanosheets, the detailed time-dependent morphology and crystal phase evolution process are investigated. Fig. 4 shows SEM images of products prepared at 180 C for different reaction time. In the early stage of hydrothermal treatment, nanowires with the diameter of 10 nm are obtained and intended to aggregate together (for example Fig. 4(a)). As the time elongated, nanowires dissolve and reunite to nanorods of 20 nm in diameter and 100e300 nm in length (as shown in Fig. 4(b)). With the increase of reaction time, the growth of nanorods is restrained along a same direction and the small nanosheets are obtained (Fig. 4(c, d)). When the reaction time reaches up to 48 h, the nanorods completely disappear and the bigger hexagonal nanosheets are obtained. Fig. 5 shows the XRD patterns of the products synthesized at 180 C for different reaction time. The products prepared at 1 h show pure orthorhombic crystal phase (JCPDS no. 41-0013), and the weak peaks demonstrate that the as-obtained materials show poor crystallinity. With the increase of reaction time to 6 and 9 h, the products possess a mixture crystal phase of orthorhombic (JCPDS no. 41-0013) and hexagonal (JCPDS no. 52-0352). As shown in Fig. 1(a), when the reaction time is 48 h, no orthorhombic is detected but hexagonal crystal phase, and the as-obtained materials show good crystallinity. Thus, on the basis of above discussion, it can be reasonably concluded that, with the increase of reaction time, the nanowires with orthorhombic phase are gradually transformed into nanosheets with hexagonal phase. Ethylenediamine is a common complexant to control the release
of isolated Ce3þ ions via the coordination interaction between Ce3þ ions and ethylenediamine. The reaction can be summarized as below:
3þ Ce3þ þ 2NH2 CH2 CH2 NH2 / CeðNH2 CH2 CH2 NH2 Þ2
(1)
In addition, ethylenediamine can increase the viscosity of the solution and influence the diffusion coefficient of the building blocks [19], which should help the formation of hexagonal CeCO3OH nanosheets. The formation of CeO2 involves several complicated reactions. At the very beginning, ammonium hydrogen carbonate (NH4HCO3) provides ammonium (NHþ 4 ), hydroxyl (OH ) and carbonate anions 2 (CO3 ), and the CeCO3OH nanocrystals with irregular shapes are formed. The main reactions in the system can be expressed as follows [Eqs. (2)e(6)]:
NH4 HCO3 þ H2 O/NH3 $H2 O þ H2 CO3
(2)
NH3 $H2 O#NHþ 4 þ OH
(3)
H2 CO3 # 2Hþ þ CO2 3
(4)
i h Ce3þ þ yH2 O/ CeðOHÞ$ðH2 O m1 2þ þ H3 Oþ
(5)
h i CeðOHÞ$ðH2 O m1 2þ þ CO3 2 /CeOHCO3 ðCeCO3 OHÞ þðn 1ÞH2 O
(6)
As is known to all, many factors, such as reaction temperature and time, solvent, concentration in the reaction system, may also influence the crystal growth in the hydrothermal condition. The Xray diffraction patterns of the nanowires show a poor crystallinity,
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processes from nanowires to nanosheets. At the very beginning, a large amount of CeCO3OH particles with orthorhombic structure formed and then quickly self-assembled into nanowires to reduce their surface free energy. Afterward, because of its poor crystallinity, the CeCO3OH nanowires were dissolved. The Ce3þ will reunite with OHand CO2þ 3 to form CeCO3OH with hexagonal or orthorhombic and the ethylenediamine will also control the release of isolated Ce3þ ions to ensure CeCO3OH selectively absorbed on the (001) face of hexagonal CeCO3OH, which could reduce the surface energy and restrain the growth along [001] direction [24]. As a result, the hexagonal CeCO3OH grow along [100] and [010] direction and transform into a nanosheet structure ultimately. 3.3. Tunable factors on the synthesis of hexagonal CeCO3OH nanosheets
Fig. 3. (a, b) TEM images and (c,d) HRTEM images of hexagonal CeO2 nanosheets, and (e) SEAD of (a).
The effect of the amount of CeCl3$7H2O and ethylenediamine on the formation of nanosheets are investigated. When the amount of CeCl3$7H2O is less 4 mmol, the nanosheets are also formed, but most of morphologies are just some irregular nano-fragments, as shown in Fig. 7 (a, b). However, when the amount of CeCl3$7H2O is more than 6 mmol, a large number of nanosheets are formed, but there are also a few of nanorods presented simultaneously (Fig. 7(c)), and this phenomenon is more prominent with increasing the amount of CeCl3$7H2O continuously, as shown in Fig. 8(d). Based on the formation mechanism of nanosheets, we can conclude that when the amount of CeCl3$7H2O is more than 4 mmol, the reaction time should be prolonged to obtain that morphology and uniform nanosheets. When 5 ml ethylenediamine or less is employed, no nanosheets form, but only some irregular nanoparticles, as shown in Fig. 8(a). Some fragments can be obtained with 7.5 mL ethylenediamine (Fig. 8(b)). While the amount of ethylenediamine increased to 10 mL or more, hexagonal CeCO3OH nanosheets are presented as Fig. 2(a). 3.4. Composition and chemical state
which implies that the nanowires with orthorhombic phase contain large number of defects, which lead to nanowires with high free energy and unstable. On the basis of lowest energy principle, the high free energy will promote the transformation from nanowires with orthorhombic phase to the stable phase with lower free energy. At relatively low reaction temperature, the CeCO3OH may have two phases, i.e. orthorhombic and hexagonal. According to the degree of symmetry (higher symmetry, lower degree of ordering and higher free energy) and literature [20,21], the orthorhombic phase of CeCO3OH is more stable than its hexagonal phase. In general, in terms of the free energy and Ostwald ripening rule [22], the most stable crystal structure phase or intermediate state phase with lower nucleation barrier is preferred to emerge first and then transforms to the most stable phase. Here, the situation is rather complicated due to the sophistical composition and medium temperature. Nevertheless, the defects in the initial orthorhombic phase may play an important role in this phase and morphology transition. The large number of defects result initial orthorhombic phase extremely unstable and dissolve easily and then recrystallize a higher quality crystal phase. We have succeed in converting that stable phase to unstable one and the same phenomenon also was detected by others [23]. The hexagonal phase is rather stable even prolonged the reaction time to a week. Based on the time-dependent morphology evolution evidences, a defect driven dissolutionerecrystallization mechanism has been designed to explain the transformation from nanowires to nanosheets. Fig. 6 schematically illustrates the possible growth
To investigate oxidation state of Ce in the obtained CeO2 nanostructures, XPS analyses was carried out. Fig. 9 typically depicts the Ce 3d and O 1s X-ray photo-electron spectra (XPS) of the hexagonal CeO2 nanosheets. From Fig. 9(a), it can be seen that six Ce 3d BE peaks, at U000 (916.37 eV), U00 (906.95 eV), U (900.49 eV), V000 (897.96 eV), V00 (888.45 eV), and V (881.98 eV), respectively, were assigned to Ce 3d5/2 for Ce4þ state [25,26], indicating the main valences of cerium in the sample was þ4. However, two weak peaks at U0 (902.83 eV) and V0 (884.06 eV) should be assigned to Ce 3d3/2 for Ce3þ state [27], indicating a small amount of Ce3þ ions existed in the samples. It is clear that cerium exists mainly as Ce(IV) in the sample, but a small quantity of Ce(III) is also detected. This result implies existing defect of sample and reflects the concentration of oxygen vacancies. The ratio between fitted peak areas of Ce4þ and Ce3þ for ceria can be used to estimate the concentrations by use of valence states and the presence of Ce3þ. A semiquantitative analysis of the integrated peaks area can provide the concentration of Ce3þ ions in the CeO2 sample. It can be calculated as [28,29]:
½Ce3þ ¼
Au0 þ Av0 Au000 þ Au00 þ Au0 þ Au þ Av000 þ Av00 þ Av0 þ Av
(7)
½Ce4þ ¼
Au000 þ Au00 þ Au þ Av000 þ Av00 þ Av Au000 þ Au00 þ Au0 þ Au þ Av000 þ Av00 þ Av0 þ Av
(8)
where Ai is the integrated area of peak “i”.
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Fig. 4. SEM image of products synthesized at 180 C for (a) 1 h, (b) 3 h, (c) 6 h, and (d) 9 h.
Table 1 shows the integrated area of Ce3d and O1s. According to Eqs (6) and (7), we can calculate that the concentration of Ce3þ for sample is 27.83%. The trivalent Ce3þ ions can be distributed either in region of sesquioxide Ce2O3 or around oxygen vacancies in CeO2. In order to determine whether the trivalent Ce3þ ions are associated with Ce2O3 or oxygen vacancies, we can calculate the oxygen content, which is the sum of the required oxygen to fully oxidize Ce4þ and Ce3þ and to form CeO2 and Ce2O3, respectively. Then taking into account the difference in stoichiometry x ¼ [O]/[Ce] in CeO2 (x ¼ 2) and in Ce2O3 (x ¼ 1.5), the ratio of O to the total Ce ions (Ce4þþCe3þ) is determined from the concentration of [Ce4þ] and
[Ce3þ] according to following equations；
c¼
i h i ½O 3 h ¼ Ce3þ þ 2 Ce4þ ½Ce 2
As shown in Fig. 9(b), the O1s spectra consist of two peaks at binding energy 531.56 and 529.33 eV, respectively. The peak at binding energy 531.56 eV can be attributed to Ce3þ-carbonates and/ or Ce4þ-hydroxides formed at the surfaces of the samples [30,31]; the peak at 529.33 eV originates from lattice oxygen ions in CeO2 [32], and O relates to the Ce3þ ions (Ce3þ-hydroxide and Ce3þ-oxide) [33]. The actual stoichiometry x0 ¼ [O]/[Ce] can be calculated directly from the XPS integrated areas of the O1s and Ce3d peaks according to the following equation:
c0 ¼
Fig. 5. XRD patterns of products synthesized at 180 C for 1, 3, 6, and 9 h; the symbol # ascribes to hexagonal CeCO3OH and * to orthorhombic CeCO3OH.
(9)
Ols Ao S ¼ Ce Ce3d ACe So
(10)
where AO and ACe are the XPS integrated areas of the O1s and Ce3d peaks, and SCe (¼7.399) and SO (¼0.711) are sensitivity factors of Ce and O atoms, respectively. Table 2 shows the stoichiometry variations with the concentration of Ce3þ (requiring O to fully oxidize Ce3þand Ce4þ and direct comparison of the O1s and Ce3d XPS peak intensities). From Table 2, we can see that the stoichiometric ratio oxygen and cerium (x0 ¼ [O1s]/[Ce3d]) is 0.08 smaller than stoichiometric ratio Eq. (8), which indicated that the part of Ce3þ in the sample is consumed in forming Ce2O3, and part of Ce3þ is forming oxygen vacancies, suggesting that Ce2O3 and oxygen vacancies coexist in the hexagonal CeO2 nanosheets. To better understand the defects in the hexagonal CeO2 nanosheets, the Raman scattering was carried out and is shown in Fig. 10. One strong Raman peak centered at about 460 cm1 dominates the spectrum. This peak originates from the F2g Ramanactive mode of CeO2 cube structure [34], i.e. a symmetrical
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Fig. 6. Schematic depicting possible growth processes from nanowires to nanosheets.
stretching mode of the Cee8O vibrational unit [35], which is very sensitive to any disorder in the oxygen sub-lattices. As we know that the main peak of bulk CeO2 is at 466 cm1, 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 464 cm1 peak [36]. It is likely that Ce3þ ions and oxygen vacancies in the samples are responsible for the changes in the Raman scattering as supported by the XPS data. The peak near 267 cm1 can be contributed to disorder in the system [37], and the peak at 577 cm1 can be attributed to the presence of Ce3þ and oxygen vacancies [35]. Fig. 11 shows the PL spectra of CeO2 samples measured at room temperature using excitation light of 340 nm. Numerous emission peaks located at ~373, 384, 401, 420, 440, 450, 463, 468, 473, 482, 494, and 564 nm are observed. Among them, the emission peak at 468 nm is the strongest peak. These emission peaks between 410
and 510 nm form a broad emission band. The emission peaks located at ~374, 383 and 401 nm can be attributed to excitonic recombination corresponding to the near-band-edge emission of CeO2 [38]. They are due to the 5d-4f transitions of Ce3þ between the 2 D (5d1) ground state and the 2F5/2 (4f1) state [39]. The other emission peaks between 410 and 510 nm may be attributed to the transitions from different defect levels to O2p band. As we know that defects have an important influence on the chemical and physical properties of CeO2 nanocrystals, including oxygen transportation, catalysts, fuel cells, and so on. Interestingly, the emission peaks at 620 and 630 nm as shown in Fig. 11 have not been reported in the literature. The nature behind this phenomenon is not clear so far. It may be induced by the oxygen vacancies in the crystal with electronic energy levels below the Ce 4f band or by the transition from some localized states within the band gap to the valence band. More investigations need to be carried out in order to clarify the origin of these new emission peaks.
Fig. 7. SEM images of products synthesized at 180 C for 48 h doped CeCl3$7H2O of (a) 1 mmol, (b) 2 mmol, (c) 6 mmol, and (d) 8 mmol.
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Fig. 8. SEM images of products synthesized at 180 C for 48 h doped ethylenediamine of (a) 5 mL and (b) 7.5 mL. Fig. 9. XPS cores level spectra of Ce3d (a) and O1s (b) of hexagonal CeO2 nanosheets.
3.5. Ferromagnetism analysis Fig. 12 shows MeH curve of the hexagonal CeO2 nanosheets and the inset of Fig. 12 is the magnetization of the central part. It can be seen that saturation magnetization (Ms) is 3.02 102 emu/g, residual magnetization (Mr) is 0.68 102 emu/g and coercivity (Hc) is 210 Oe. It is notable that the value of Ms is larger than those previous reports [40] and comparable to those previous reports in which CeO2 nanoparticles were doped with another elements, such as Co, Cu and Zn [41e44]. From those reports, we can conclude that magnetic not only from doped elements, but a part of CeO2 itself becomes magnetic. It is well known that the crystal structure of CeO2 is face-centered cubic, which determined Ce4þ and O2 ions are interwoven (Ce4þ-O2-Ce4þ) (as shown in Fig. 13), whose electron orbits are symmetrical and the spins are also coupled, so bulk CeO2 is paramagnetic. Based on XPS and Raman analysis, it can be concluded that Ce3þ and oxygen vacancy existed in hexagonal CeO2 nanosheets. When the valence of Ce changes from þ4 to þ3, the electron orbits (Ce4þ-O2-Ce3þ) are no longer symmetrical and an uncoupled spins in the Ce f orbit are generated. Furthermore, the Ce3þ ions could exchange to Ce4þ ions and enhance the transfer between Ce 4f and O 2p. The room temperature ferromagnetic (RTFM) might consequently arise from a nearesteneighbor interaction: double exchange (Ce4þeO2eCe3þ) [40]. Otherwise, the
existence of Ce3þ in the lattice may cause local charge compensations. The formation of one oxygen vacancy (Vo) is complex between two Ce3þ ions as indicated in Fig. 13. It is well known that oxygen vacancies were assumed to give rise to the RTFM by experimental and theoretical studies. M. I. B. Bernardi et al. [17] has confirmed that the presence of substantial oxygen vacancies would lead to magnetic order. So the RTFM of the hexagonal CeO2 nanosheets can be attributed to the effects of the Ce3þ ions and oxygen vacancies.
4. Conclusions Hexagonal CeO2 nanosheets of 40e50 nm in thickness and 300e400 nm in side-length have been successfully synthesized via controlling the morphology of CeCO3OH precursor by a facile hydrothermal technique using CeCl3$7H2O as cerium source, ammonium hydrogen carbonate as precipitant and ethylenediamine as complexant. The dependences of morphologies and phase transitions of CeCO3OH precursors on reaction time, chemical compositions are investigated. Results show that the precursor exhibits changes both in morphology and phase structure as reaction time
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Table 1 Integrated areas of individual XPS peaks of the Ce3d and O1s from Gear-like CeO2 sample prepared with 4 mmol Ce(NO3)3$6H2O. Ce (3d3/2)
Ce (3d5/2)
U000 49391.83
U00 19757.96
U0 30474.33
U 57303.20
V000 80149.97
O1s V00 61001.08
V0 100853.50
V 72896.72
Ce4þeO 60263.86
Ce3þeO 20349.76
Table 2 Concentrations of Ce3þ and Ce4þ ions and stoichiometry x ¼ [O]/[Ce] of Gear-like CeO2 sample prepared with 4 mmol Ce(NO3)3$6H2O. [Ce3þ]
[Ce4þ]
x ¼ [O]/[Ce]a
x’ ¼ [O1s]/[Ce3d]b
27.83%
72.17%
1.86
1.78
a b
Using Eq. (8). Using Eq. (9).
Fig. 12. M-H curve of hexagonal CeO2 nanosheets.
Fig. 10. Raman spectrum of hexagonal CeO2 nanosheets.
elongated. One-dimensional nanowire morphology with an orthorhombic phase structure is obtained when the reaction time is less than 1 h, while two-dimensional nanosheet with a hexagonal
Fig. 11. PL spectra of CeO2 samples.
phase structure is formed when the reaction time is more than 6 h. And a defect driven dissolutionerecrystallization mechanism is suggested to explain the transformation from the initial orthorhombic nanowires to hexagonal nanosheets. Experimental results reveal that the amount of CeCl3$7H2O and ethylenediamine are key parameters for the nucleation and crystal growth of CeCO3OH nanosheets. Moreover, the hexagonal CeO2 nanosheets have a fluorite cubic structure and there are Ce3þ ions and oxygen vacancies in surface of samples. The synthesized CeO2 shows excellent room temperature optical properties. MeH curve exhibits excellent room-temperature ferromagnetism (RTFM) with saturation magnetization (Ms) of 3.02 102 emu/g, residual magnetization (Mr) of 0.68 102 emu/g and coercivity (Hc) of 210 Oe, which is likely attributed to the effects of the Ce3þ ions and oxygen vacancies. The controllable morphology and RTFM should make the hexagonal CeO2 nanosheets as an excellent candidate for applications in related areas.
Fig. 13. Schematic representation of ion interactions in bulk and nanoscale ceria, in the latter case: the formation of oxygen vacancy.
F. Meng et al. / Journal of Alloys and Compounds 647 (2015) 1013e1021
Acknowledgments This work was supported by the Anhui Provincial Natural Science Foundation of China (1508085SME219) and the Scientific Research Foundation of Education Ministry of Anhui Province of China (KJ2012A028 and KJ2011A010). References [1] S.W. Yang, L. Gao, J. Am. Chem. Soc. 128 (2006) 9330. [2] F. Esch, S. Fabris, L. Zhou, T. Montini, C. Africh, P. Fornasiero, G. Comelli, R. Rosei, Science 309 (2005) 752. [3] Z. Zhan, S.A. Bernett, Science 308 (2005) 844. [4] S. Kaur, G. Pal Singh, P. Kaur, D.P. Singh, J. Lumin 143 (2013) 31. [5] T. Feng, X. Wang, G. Feng, Mater. Lett. 100 (2013) 36. [6] B. Choudhury, A. Choudhury, Curr. Appl. Phys. 13 (2013) 217. [7] S. Deshpande, S. Patil, S. Kuchibhatla, S. Seal, Appl. Phys. Lett. 87 (2005) 133113. [8] M.Y. Ge, H. Wang, E.Z. Liu, J.F. Liu, J.Z. Jiang, Y.K. Li, Z.A. Xu, H.Y. Li, Appl. Phys. Lett. 93 (2008) 062505. [9] X. Lu, X. Li, J. Qian, Z. Chen, Powder. Technol. 239 (2013) 415. [10] D.M. Kempaiah, S. Yin, T. Sato, Cryst. Eng. Comm. 13 (2011) 741. [11] K. Zhou, Z. Yang, S. Yang, Chem. Mater. 19 (2007) 1215. [12] C.W. Sun, J. Sun, G.L. Xiao, H.R. Zhang, X.P. Qiu, H. Li, L.Q. Chen, J. Phys. Chem. B 110 (2006) 13445. [13] F. Meng, L. Wang, Mater. Lett. 100 (2013) 86. [14] H. Wei, T. Yao, Z. Pan, C. Mai, Z. Sun, Z. Wu, F. Hu, Y. Jiang, W. Yan, J. App. Phys. 105 (2009) 043903. [15] R.K. Singhal, S. Kumar, A. Samariya, M. Dhawan, S.C. Sharma, Y.T. Xing, Mater. Chem. Phys. 132 (2012) 534. [16] S.Y. Chen, Y.H. Lu, T.W. Huang, D.C. Yan, C.L. Dong, J. Phys. Chem. C 114 (2010) 19576. [17] M.I.B. Bernardi, A. Mesquita, F. Beron, K.R. Pirota, A.O.d. Zevallos, A.C. Doriguetto, H.B.d. Carvalho, Phys. Chem. Chem. Phys. 17 (2015) 3072. [18] M.J. Li, S.H. Ge, W. Qiao, L. Zhang, Y.L. Zuo, S.M. Yan, Appl. Phys. Lett. 94 (2009) 152511. [19] C. Sun, H. Li, H. Zhang, Z. Wang, L. Chen, Nanotechnology 16 (2005) 1454.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Hydrothermal synthesis of hexagonal CeO2 nanosheets and their room temperature ferromagnetism Fanming Meng a, b, *, Cheng Zhang a, Zhenghua Fan a, Jinfeng Gong a, Aixia Li a, Zongling Ding a, Huaibao Tang a, Miao Zhang a, Guifang Wu a a
School of Physics and Materials Science, Anhui University, 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 b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 April 2015 Received in revised form 5 June 2015 Accepted 21 June 2015 Available online 24 June 2015
Hexagonal CeO2 nanosheets of 40e50 nm in thickness and 300e400 nm in side-length have been successfully synthesized via controlling the morphology of CeCO3OH precursors by a facile hydrothermal technique using CeCl3$7H2O as cerium source, ammonium hydrogen carbonate as precipitants, and ethylenediamine as complexant. The reaction time and the amount of CeCl3$7H2O and ethylenediamine were systematically investigated. The as-synthesized hexagonal CeO2 nanosheets were examined by XRD, SEM, TEM, XPS, Raman scattering and magnetization measurements. It is found that the amount of CeCl3$7H2O and ethylenediamine are key parameters for controlling the final morphology. The hexagonal CeO2 nanosheets have a fluorite cubic structure and there are Ce3þ ions and oxygen vacancies in surface of samples. The synthesized CeO2 shows excellent room temperature optical properties. MeH curve exhibits excellent room-temperature ferromagnetism (RTFM) with saturation magnetization (Ms) of 3.02 102 emu/g, residual magnetization (Mr) of 0.68 102 emu/g and coercivity (Hc) of 210 Oe, which is likely attributed to the effects of the Ce3þ ions and oxygen vacancies. © 2015 Elsevier B.V. All rights reserved.
Keywords: Nanostructures Electron microscopy X-ray photo-emission spectroscopy Magnetic properties
1. Introduction Morphology-controlled synthesis of inorganic nanomaterials has been a focus of many researches due to their unique morphology dependent properties and their capability of selfassembly as building blocks [1]. Ceria (CeO2, Eg z 3.2 eV), as a well-known and extensively studied rare earth oxide, has attracted more attention for its promising application in catalysts, fuel cells, oxygen sensors, mechanical polishing, ultraviolet blocks, and luminescent materials [2e6]. Furthermore, nanocrystalline CeO2 shows improved and size-dependent properties [7]. Ceria with various morphologies such as nanocubes, nanotubes, nanowires, nanopoles, mesoporous and flower-like structures have been synthesized in recent years [8e10]. Nevertheless, the synthesis of twodimensional (2D) CeO2 structures is rarely reported. Moreover, a number of precipitants including sodium hydroxide, ammonium hydroxide, ethanediamine and urea have been used to synthesis
* Corresponding author. School of Physics and Materials Science, Anhui University, 111 Jiulong Road, Hefei 230601, PR China. E-mail address: [email protected] (F. Meng). http://dx.doi.org/10.1016/j.jallcom.2015.06.186 0925-8388/© 2015 Elsevier B.V. All rights reserved.
the shape-controlled CeO2 structures. Zhou et al. [11] obtained nanotubes using Ce2(SO4)3$9H2O as cerium resource, NaOH as mineralizer. Sun et al. [12] synthesized flower-like CeO2 microspheres using Ce(NO3)3,6H2O as cerium resource and NH3$H2O as mineralizer. Meng et al. [13] synthesized CeO2 nanopoles using CeCl3$7H2O as cerium resource, NaOH as mineralizer, and ethylenediamine as complexant. But, ammonium hydrogen carbonate could be hydrolyzed to produce ammonium and bicarbonate ions, but its involvement as a precipitant in preparing nano-CeO2 was seldom noted. Therefore, it is a commendable precipitant to fabricate CeO2 nanosheets via annealing CeCO3OH nanosheets. As we all know, magnetic order in a semiconductor requires doping a few percent of transition metal that have partially filled shells of d and f electrons to mediate ferromagnetic (FM) [14]. However, there have been several reports on pure CeO2 exhibiting FM at room temperature [13,15,16]. As a result, oxygen vacancy is a most important factor to RTFM of CeO2 microstructures. Ge et al. [8] reported FM in CeO2 nanoparticles (NPs), and their calculations from first principles revealed that oxygen vacancies, especially at the surface, can induce magnetic moments in CeO2 NPs. And Bernardi et al. [17] also suggest that the presence of oxygen vacancies is
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not a sufficient condition to mediate ferromagnetism in the CeO2 system, and only oxygen vacancies in the surface of nanostructures would lead to such a long range magnetic order. In contrast, some other researchers reported that FM is related not to the surface oxygen vacancies but to the Ce3þ/Ce4þ pairs. Li et al. [18] reported that Ce3þ/Ce4þ pairs in the surface is a most important factor to RTFM of CeO2 microstructures. On the basis of these experimental results, defect-induced mechanism and charger transfer have been used to describe the formation of FM in nanomaterials. Therefore, the mechanism of FM in pure CeO2 nanomaterials is still without a unified statement. Herein, we report a simple hydrothermal method to obtain hexagonal CeO2 nanosheets by thermal decomposition of assynthesized CeCO3OH nanosheets. The mechanism for the transition of phase and morphology are discussed. And the possible mechanism is discussed in terms of the influence of Ce3þ ions. 2. Experimental procedure 2.1. Material preparation All the reagents were of analytical grade purity and used as received without further purification. The detailed synthetic process was as following: 4 mmol CeCl3$7H2O and 40 mmol NH4HCO3 were dissolved in 10 and 15 mL distilled water under vigorous stirring for 0.5 h, respectively. And then, 15 mL NH4HCO3 aqueous solution were gradually added to 10 mL CeCl3$7H2O aqueous solution to form colorless precipitation. After continuous stirring for 0.5 h, 10 mL of ethylenediamine was added dropwise to the CeCl3 solution, the solution was further stirred for 0.5 h. The mixed solution was transferred into a 50 ml Teflon-lined autoclave and heated at 180 C for 1e48 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 C overnight. CeO2 microstructures were obtained by calcining at 400 C for 5 h, accompanied by a color change from white to slight yellow. 2.2. Characterization The crystal phases of the products were analyzed by X-ray diffractometer (XRD) with Cu Ka radiation (l ¼ 0.1506 nm) and the selected area electron diffraction (SAED). The morphology was characterized by scanning electron microscope (SEM, S-4800) and transmission electron microscope (TEM, JEM-2100). The chemical state was analyzed by X-ray photo-electron spectroscopy (XPS, ESCALAB 250 US Thermo Electron Co). Raman spectra were recorded by a Raman spectrometer system (inVia-Reflex) using a laser with 532 nm excitation at room temperature. Photoluminescence (PL) spectra were obtained by a fluorescence spectrophotometer (HORIBA FluoroMax-4P, HORIBA Jobin Yvon) using excitation light of 340 nm. The MeH curve was measured at room temperature by vibrating sample magnetometer (BHV-55). 3. Results and discussion 3.1. Structure and morphology analysis X-ray powder diffraction (XRD) analysis was carried out to investigate the phases of the products as-synthesized and calcined at 400 C for 5 h. A typical XRD pattern of the as-synthesized product is shown in Fig. 1(a). All of the diffraction peaks in Fig. 1(a) can be exactly indexed to the hexagonal CeCO3OH with lattice constants a ¼ 12.530 Å, b ¼ 12.530 Å, c ¼ 10.000 Å, which are in good agreement with the literature values (JCPDS no. 52-0352). It
Fig. 1. XRD patterns of (a) the as-synthesized CeCO3OH samples at 180 C for 48 h and (b) CeO2 obtained by calcining the CeCO3OH samples at 400 C for 5 h.
must be pointed that two tiny visible peaks at around 28 and 33 can be indexed to CeO2. After annealing the precursors, all of the diffraction peaks in Fig. 1(b) can be exactly indexed to CeO2 with face-centered cubic (FCC) structure (JCPDS no. 34-0394). Considering the sensibility of XRD, in the best of the cases, that cannot detect fraction of secondary phases or segregates with concentration lower than 2%. The sharp peaks demonstrate that both the asobtained and calcined materials show good crystallinity. The morphologies of the CeCO3OH and CeO2 are shown in Fig. 2. Fig. 2(a) and (b)show that the as-synthesized CeCO3OH nanosheets are uniform and the size of the hexagonal CeCO3OH nanosheets are 40e50 nm in thickness and 300e400 nm in side-length. From Fig. 2(c) and (d), it can be seen that the post-heat-treatment process does not ruin the morphology of the products, and morphology of CeO2 nanosheets is almost the same as its counterpart. The morphologies of the hexagonal CeO2 nanosheets are further examined by (HR)TEM. Fig. 3(a) and (b) show TEM images of hexagonal CeO2 nanosheets with different magnification, which indicate that the hexagonal CeO2 nanosheets are loose and porous. Fig. 3(c) and (d) show the high-resolution TEM (HRTEM) images of the marked area in Fig. 3(a). From Fig. 3(d), it can be seen that the distance between neighboring planes is about 0.177 nm, representing the crystallographic plane of CeO2. The corresponding
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Fig. 2. SEM images of (a, b) the as-synthesized CeCO3OH samples at 180 C for 48 h and (c, d) CeO2 obtained by calcining the CeCO3OH samples at 400 C for 5 h.
select area electron diffraction (SAED) pattern, as shown Fig. 3(e), further indicates that the hexagonal CeO2 nanosheets have a monocrystalline structure and expose its (111) planar on its both sides. 3.2. The formation mechanism of nanosheets To reveal the formation mechanism of hexagonal CeCO3OH nanosheets, the detailed time-dependent morphology and crystal phase evolution process are investigated. Fig. 4 shows SEM images of products prepared at 180 C for different reaction time. In the early stage of hydrothermal treatment, nanowires with the diameter of 10 nm are obtained and intended to aggregate together (for example Fig. 4(a)). As the time elongated, nanowires dissolve and reunite to nanorods of 20 nm in diameter and 100e300 nm in length (as shown in Fig. 4(b)). With the increase of reaction time, the growth of nanorods is restrained along a same direction and the small nanosheets are obtained (Fig. 4(c, d)). When the reaction time reaches up to 48 h, the nanorods completely disappear and the bigger hexagonal nanosheets are obtained. Fig. 5 shows the XRD patterns of the products synthesized at 180 C for different reaction time. The products prepared at 1 h show pure orthorhombic crystal phase (JCPDS no. 41-0013), and the weak peaks demonstrate that the as-obtained materials show poor crystallinity. With the increase of reaction time to 6 and 9 h, the products possess a mixture crystal phase of orthorhombic (JCPDS no. 41-0013) and hexagonal (JCPDS no. 52-0352). As shown in Fig. 1(a), when the reaction time is 48 h, no orthorhombic is detected but hexagonal crystal phase, and the as-obtained materials show good crystallinity. Thus, on the basis of above discussion, it can be reasonably concluded that, with the increase of reaction time, the nanowires with orthorhombic phase are gradually transformed into nanosheets with hexagonal phase. Ethylenediamine is a common complexant to control the release
of isolated Ce3þ ions via the coordination interaction between Ce3þ ions and ethylenediamine. The reaction can be summarized as below:
3þ Ce3þ þ 2NH2 CH2 CH2 NH2 / CeðNH2 CH2 CH2 NH2 Þ2
(1)
In addition, ethylenediamine can increase the viscosity of the solution and influence the diffusion coefficient of the building blocks [19], which should help the formation of hexagonal CeCO3OH nanosheets. The formation of CeO2 involves several complicated reactions. At the very beginning, ammonium hydrogen carbonate (NH4HCO3) provides ammonium (NHþ 4 ), hydroxyl (OH ) and carbonate anions 2 (CO3 ), and the CeCO3OH nanocrystals with irregular shapes are formed. The main reactions in the system can be expressed as follows [Eqs. (2)e(6)]:
NH4 HCO3 þ H2 O/NH3 $H2 O þ H2 CO3
(2)
NH3 $H2 O#NHþ 4 þ OH
(3)
H2 CO3 # 2Hþ þ CO2 3
(4)
i h Ce3þ þ yH2 O/ CeðOHÞ$ðH2 O m1 2þ þ H3 Oþ
(5)
h i CeðOHÞ$ðH2 O m1 2þ þ CO3 2 /CeOHCO3 ðCeCO3 OHÞ þðn 1ÞH2 O
(6)
As is known to all, many factors, such as reaction temperature and time, solvent, concentration in the reaction system, may also influence the crystal growth in the hydrothermal condition. The Xray diffraction patterns of the nanowires show a poor crystallinity,
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processes from nanowires to nanosheets. At the very beginning, a large amount of CeCO3OH particles with orthorhombic structure formed and then quickly self-assembled into nanowires to reduce their surface free energy. Afterward, because of its poor crystallinity, the CeCO3OH nanowires were dissolved. The Ce3þ will reunite with OHand CO2þ 3 to form CeCO3OH with hexagonal or orthorhombic and the ethylenediamine will also control the release of isolated Ce3þ ions to ensure CeCO3OH selectively absorbed on the (001) face of hexagonal CeCO3OH, which could reduce the surface energy and restrain the growth along [001] direction [24]. As a result, the hexagonal CeCO3OH grow along [100] and [010] direction and transform into a nanosheet structure ultimately. 3.3. Tunable factors on the synthesis of hexagonal CeCO3OH nanosheets
Fig. 3. (a, b) TEM images and (c,d) HRTEM images of hexagonal CeO2 nanosheets, and (e) SEAD of (a).
The effect of the amount of CeCl3$7H2O and ethylenediamine on the formation of nanosheets are investigated. When the amount of CeCl3$7H2O is less 4 mmol, the nanosheets are also formed, but most of morphologies are just some irregular nano-fragments, as shown in Fig. 7 (a, b). However, when the amount of CeCl3$7H2O is more than 6 mmol, a large number of nanosheets are formed, but there are also a few of nanorods presented simultaneously (Fig. 7(c)), and this phenomenon is more prominent with increasing the amount of CeCl3$7H2O continuously, as shown in Fig. 8(d). Based on the formation mechanism of nanosheets, we can conclude that when the amount of CeCl3$7H2O is more than 4 mmol, the reaction time should be prolonged to obtain that morphology and uniform nanosheets. When 5 ml ethylenediamine or less is employed, no nanosheets form, but only some irregular nanoparticles, as shown in Fig. 8(a). Some fragments can be obtained with 7.5 mL ethylenediamine (Fig. 8(b)). While the amount of ethylenediamine increased to 10 mL or more, hexagonal CeCO3OH nanosheets are presented as Fig. 2(a). 3.4. Composition and chemical state
which implies that the nanowires with orthorhombic phase contain large number of defects, which lead to nanowires with high free energy and unstable. On the basis of lowest energy principle, the high free energy will promote the transformation from nanowires with orthorhombic phase to the stable phase with lower free energy. At relatively low reaction temperature, the CeCO3OH may have two phases, i.e. orthorhombic and hexagonal. According to the degree of symmetry (higher symmetry, lower degree of ordering and higher free energy) and literature [20,21], the orthorhombic phase of CeCO3OH is more stable than its hexagonal phase. In general, in terms of the free energy and Ostwald ripening rule [22], the most stable crystal structure phase or intermediate state phase with lower nucleation barrier is preferred to emerge first and then transforms to the most stable phase. Here, the situation is rather complicated due to the sophistical composition and medium temperature. Nevertheless, the defects in the initial orthorhombic phase may play an important role in this phase and morphology transition. The large number of defects result initial orthorhombic phase extremely unstable and dissolve easily and then recrystallize a higher quality crystal phase. We have succeed in converting that stable phase to unstable one and the same phenomenon also was detected by others [23]. The hexagonal phase is rather stable even prolonged the reaction time to a week. Based on the time-dependent morphology evolution evidences, a defect driven dissolutionerecrystallization mechanism has been designed to explain the transformation from nanowires to nanosheets. Fig. 6 schematically illustrates the possible growth
To investigate oxidation state of Ce in the obtained CeO2 nanostructures, XPS analyses was carried out. Fig. 9 typically depicts the Ce 3d and O 1s X-ray photo-electron spectra (XPS) of the hexagonal CeO2 nanosheets. From Fig. 9(a), it can be seen that six Ce 3d BE peaks, at U000 (916.37 eV), U00 (906.95 eV), U (900.49 eV), V000 (897.96 eV), V00 (888.45 eV), and V (881.98 eV), respectively, were assigned to Ce 3d5/2 for Ce4þ state [25,26], indicating the main valences of cerium in the sample was þ4. However, two weak peaks at U0 (902.83 eV) and V0 (884.06 eV) should be assigned to Ce 3d3/2 for Ce3þ state [27], indicating a small amount of Ce3þ ions existed in the samples. It is clear that cerium exists mainly as Ce(IV) in the sample, but a small quantity of Ce(III) is also detected. This result implies existing defect of sample and reflects the concentration of oxygen vacancies. The ratio between fitted peak areas of Ce4þ and Ce3þ for ceria can be used to estimate the concentrations by use of valence states and the presence of Ce3þ. A semiquantitative analysis of the integrated peaks area can provide the concentration of Ce3þ ions in the CeO2 sample. It can be calculated as [28,29]:
½Ce3þ ¼
Au0 þ Av0 Au000 þ Au00 þ Au0 þ Au þ Av000 þ Av00 þ Av0 þ Av
(7)
½Ce4þ ¼
Au000 þ Au00 þ Au þ Av000 þ Av00 þ Av Au000 þ Au00 þ Au0 þ Au þ Av000 þ Av00 þ Av0 þ Av
(8)
where Ai is the integrated area of peak “i”.
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Fig. 4. SEM image of products synthesized at 180 C for (a) 1 h, (b) 3 h, (c) 6 h, and (d) 9 h.
Table 1 shows the integrated area of Ce3d and O1s. According to Eqs (6) and (7), we can calculate that the concentration of Ce3þ for sample is 27.83%. The trivalent Ce3þ ions can be distributed either in region of sesquioxide Ce2O3 or around oxygen vacancies in CeO2. In order to determine whether the trivalent Ce3þ ions are associated with Ce2O3 or oxygen vacancies, we can calculate the oxygen content, which is the sum of the required oxygen to fully oxidize Ce4þ and Ce3þ and to form CeO2 and Ce2O3, respectively. Then taking into account the difference in stoichiometry x ¼ [O]/[Ce] in CeO2 (x ¼ 2) and in Ce2O3 (x ¼ 1.5), the ratio of O to the total Ce ions (Ce4þþCe3þ) is determined from the concentration of [Ce4þ] and
[Ce3þ] according to following equations；
c¼
i h i ½O 3 h ¼ Ce3þ þ 2 Ce4þ ½Ce 2
As shown in Fig. 9(b), the O1s spectra consist of two peaks at binding energy 531.56 and 529.33 eV, respectively. The peak at binding energy 531.56 eV can be attributed to Ce3þ-carbonates and/ or Ce4þ-hydroxides formed at the surfaces of the samples [30,31]; the peak at 529.33 eV originates from lattice oxygen ions in CeO2 [32], and O relates to the Ce3þ ions (Ce3þ-hydroxide and Ce3þ-oxide) [33]. The actual stoichiometry x0 ¼ [O]/[Ce] can be calculated directly from the XPS integrated areas of the O1s and Ce3d peaks according to the following equation:
c0 ¼
Fig. 5. XRD patterns of products synthesized at 180 C for 1, 3, 6, and 9 h; the symbol # ascribes to hexagonal CeCO3OH and * to orthorhombic CeCO3OH.
(9)
Ols Ao S ¼ Ce Ce3d ACe So
(10)
where AO and ACe are the XPS integrated areas of the O1s and Ce3d peaks, and SCe (¼7.399) and SO (¼0.711) are sensitivity factors of Ce and O atoms, respectively. Table 2 shows the stoichiometry variations with the concentration of Ce3þ (requiring O to fully oxidize Ce3þand Ce4þ and direct comparison of the O1s and Ce3d XPS peak intensities). From Table 2, we can see that the stoichiometric ratio oxygen and cerium (x0 ¼ [O1s]/[Ce3d]) is 0.08 smaller than stoichiometric ratio Eq. (8), which indicated that the part of Ce3þ in the sample is consumed in forming Ce2O3, and part of Ce3þ is forming oxygen vacancies, suggesting that Ce2O3 and oxygen vacancies coexist in the hexagonal CeO2 nanosheets. To better understand the defects in the hexagonal CeO2 nanosheets, the Raman scattering was carried out and is shown in Fig. 10. One strong Raman peak centered at about 460 cm1 dominates the spectrum. This peak originates from the F2g Ramanactive mode of CeO2 cube structure [34], i.e. a symmetrical
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Fig. 6. Schematic depicting possible growth processes from nanowires to nanosheets.
stretching mode of the Cee8O vibrational unit [35], which is very sensitive to any disorder in the oxygen sub-lattices. As we know that the main peak of bulk CeO2 is at 466 cm1, 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 464 cm1 peak [36]. It is likely that Ce3þ ions and oxygen vacancies in the samples are responsible for the changes in the Raman scattering as supported by the XPS data. The peak near 267 cm1 can be contributed to disorder in the system [37], and the peak at 577 cm1 can be attributed to the presence of Ce3þ and oxygen vacancies [35]. Fig. 11 shows the PL spectra of CeO2 samples measured at room temperature using excitation light of 340 nm. Numerous emission peaks located at ~373, 384, 401, 420, 440, 450, 463, 468, 473, 482, 494, and 564 nm are observed. Among them, the emission peak at 468 nm is the strongest peak. These emission peaks between 410
and 510 nm form a broad emission band. The emission peaks located at ~374, 383 and 401 nm can be attributed to excitonic recombination corresponding to the near-band-edge emission of CeO2 [38]. They are due to the 5d-4f transitions of Ce3þ between the 2 D (5d1) ground state and the 2F5/2 (4f1) state [39]. The other emission peaks between 410 and 510 nm may be attributed to the transitions from different defect levels to O2p band. As we know that defects have an important influence on the chemical and physical properties of CeO2 nanocrystals, including oxygen transportation, catalysts, fuel cells, and so on. Interestingly, the emission peaks at 620 and 630 nm as shown in Fig. 11 have not been reported in the literature. The nature behind this phenomenon is not clear so far. It may be induced by the oxygen vacancies in the crystal with electronic energy levels below the Ce 4f band or by the transition from some localized states within the band gap to the valence band. More investigations need to be carried out in order to clarify the origin of these new emission peaks.
Fig. 7. SEM images of products synthesized at 180 C for 48 h doped CeCl3$7H2O of (a) 1 mmol, (b) 2 mmol, (c) 6 mmol, and (d) 8 mmol.
F. Meng et al. / Journal of Alloys and Compounds 647 (2015) 1013e1021
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Fig. 8. SEM images of products synthesized at 180 C for 48 h doped ethylenediamine of (a) 5 mL and (b) 7.5 mL. Fig. 9. XPS cores level spectra of Ce3d (a) and O1s (b) of hexagonal CeO2 nanosheets.
3.5. Ferromagnetism analysis Fig. 12 shows MeH curve of the hexagonal CeO2 nanosheets and the inset of Fig. 12 is the magnetization of the central part. It can be seen that saturation magnetization (Ms) is 3.02 102 emu/g, residual magnetization (Mr) is 0.68 102 emu/g and coercivity (Hc) is 210 Oe. It is notable that the value of Ms is larger than those previous reports [40] and comparable to those previous reports in which CeO2 nanoparticles were doped with another elements, such as Co, Cu and Zn [41e44]. From those reports, we can conclude that magnetic not only from doped elements, but a part of CeO2 itself becomes magnetic. It is well known that the crystal structure of CeO2 is face-centered cubic, which determined Ce4þ and O2 ions are interwoven (Ce4þ-O2-Ce4þ) (as shown in Fig. 13), whose electron orbits are symmetrical and the spins are also coupled, so bulk CeO2 is paramagnetic. Based on XPS and Raman analysis, it can be concluded that Ce3þ and oxygen vacancy existed in hexagonal CeO2 nanosheets. When the valence of Ce changes from þ4 to þ3, the electron orbits (Ce4þ-O2-Ce3þ) are no longer symmetrical and an uncoupled spins in the Ce f orbit are generated. Furthermore, the Ce3þ ions could exchange to Ce4þ ions and enhance the transfer between Ce 4f and O 2p. The room temperature ferromagnetic (RTFM) might consequently arise from a nearesteneighbor interaction: double exchange (Ce4þeO2eCe3þ) [40]. Otherwise, the
existence of Ce3þ in the lattice may cause local charge compensations. The formation of one oxygen vacancy (Vo) is complex between two Ce3þ ions as indicated in Fig. 13. It is well known that oxygen vacancies were assumed to give rise to the RTFM by experimental and theoretical studies. M. I. B. Bernardi et al. [17] has confirmed that the presence of substantial oxygen vacancies would lead to magnetic order. So the RTFM of the hexagonal CeO2 nanosheets can be attributed to the effects of the Ce3þ ions and oxygen vacancies.
4. Conclusions Hexagonal CeO2 nanosheets of 40e50 nm in thickness and 300e400 nm in side-length have been successfully synthesized via controlling the morphology of CeCO3OH precursor by a facile hydrothermal technique using CeCl3$7H2O as cerium source, ammonium hydrogen carbonate as precipitant and ethylenediamine as complexant. The dependences of morphologies and phase transitions of CeCO3OH precursors on reaction time, chemical compositions are investigated. Results show that the precursor exhibits changes both in morphology and phase structure as reaction time
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F. Meng et al. / Journal of Alloys and Compounds 647 (2015) 1013e1021
Table 1 Integrated areas of individual XPS peaks of the Ce3d and O1s from Gear-like CeO2 sample prepared with 4 mmol Ce(NO3)3$6H2O. Ce (3d3/2)
Ce (3d5/2)
U000 49391.83
U00 19757.96
U0 30474.33
U 57303.20
V000 80149.97
O1s V00 61001.08
V0 100853.50
V 72896.72
Ce4þeO 60263.86
Ce3þeO 20349.76
Table 2 Concentrations of Ce3þ and Ce4þ ions and stoichiometry x ¼ [O]/[Ce] of Gear-like CeO2 sample prepared with 4 mmol Ce(NO3)3$6H2O. [Ce3þ]
[Ce4þ]
x ¼ [O]/[Ce]a
x’ ¼ [O1s]/[Ce3d]b
27.83%
72.17%
1.86
1.78
a b
Using Eq. (8). Using Eq. (9).
Fig. 12. M-H curve of hexagonal CeO2 nanosheets.
Fig. 10. Raman spectrum of hexagonal CeO2 nanosheets.
elongated. One-dimensional nanowire morphology with an orthorhombic phase structure is obtained when the reaction time is less than 1 h, while two-dimensional nanosheet with a hexagonal
Fig. 11. PL spectra of CeO2 samples.
phase structure is formed when the reaction time is more than 6 h. And a defect driven dissolutionerecrystallization mechanism is suggested to explain the transformation from the initial orthorhombic nanowires to hexagonal nanosheets. Experimental results reveal that the amount of CeCl3$7H2O and ethylenediamine are key parameters for the nucleation and crystal growth of CeCO3OH nanosheets. Moreover, the hexagonal CeO2 nanosheets have a fluorite cubic structure and there are Ce3þ ions and oxygen vacancies in surface of samples. The synthesized CeO2 shows excellent room temperature optical properties. MeH curve exhibits excellent room-temperature ferromagnetism (RTFM) with saturation magnetization (Ms) of 3.02 102 emu/g, residual magnetization (Mr) of 0.68 102 emu/g and coercivity (Hc) of 210 Oe, which is likely attributed to the effects of the Ce3þ ions and oxygen vacancies. The controllable morphology and RTFM should make the hexagonal CeO2 nanosheets as an excellent candidate for applications in related areas.
Fig. 13. Schematic representation of ion interactions in bulk and nanoscale ceria, in the latter case: the formation of oxygen vacancy.
F. Meng et al. / Journal of Alloys and Compounds 647 (2015) 1013e1021
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