Optical and magnetic properties of CeO2 nanoparticles synthesized by a hydrothermal method in acid environment

MENG Fanming, LI Huijie, HE Guangyuan, GAO Xin, LIU Daorui

(School of Physics and Materials Science, Anhui University, Hefei 230601, China)

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Optical and magnetic properties of CeO2nanoparticles synthesized by a hydrothermal method in acid environment

MENG Fanming, LI Huijie, HE Guangyuan, GAO Xin, LIU Daorui

(School of Physics and Materials Science, Anhui University, Hefei 230601, China)

Abstract:CeO2 nanoparticles were successfully synthesized by a hydrothermal method using CeCl3·7H2O as cerium source, NH3·H2O as mineralizer and HCl as acidic regulator. The crystal phase, morphology, optical and magnetic properties of the as-synthesized CeO2 nanoparticles were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), UV-Vis spectrophotometer (UV-Vis), photoluminescence spectrometer (PL), Raman spectrometer (Raman) and vibrating sample magnetometer (VSM). It was found that the as-synthesized CeO2 nanoparticles had a fluorite structure, and there were defects and vacancies. The lattice parameter calculated from XRD spectra was slightly higher than bulk CeO2, and the lattice parameter decreased with the increase of pH value. The morphology of all samples was spherical. The direct band gaps estimated from the UV-Vis absorption spectra were smaller than that of bulk CeO2, and the band gaps increased with the increase of pH value. The sample synthesized at pH=2 exhibited room temperature ferromagnetism (RTFM), which was likely associated with the existence of Ce3+and oxygen vacancy.

Keywords:CeO2; nanoparticles; acid environment; optical properties; magnetic properties

0Introduction

The scale of nanoparticle is generally between 1 and 100 nm, which belongs to the transition area between the atom clusters and macro object. From the macro and micro view, the system is neither typical microscopic system nor a typical macro system. So nanometer materials have a series of peculiar properties, such as quantum size effect, small size effect, surface and interface effect as well as the macroscopic quantum tunneling effect. All of these effects make nanometer materials present many peculiar physical and chemical properties[1]. So nanomaterials are considered an important class of advanced materials. As one of the most reactive rare earth metal oxides[2], cerium oxide nanostructures have wide applications such as high-temperature ceramics[3], catalysts[4], fuel cells[5], silicon-on-insulator structures, barrier layers[6-7]or promoter for automotive exhaust gas conversion reaction[8]due to their special optical properties, high thermal stability, electrical conductivity and diffusivity, and the ability to store and release oxygen[9], which depends on Ce3+-Ce4+redox cycles and the oxygen vacancies that make the more efficient for storing oxygen[10]. As a consequence, much effort had been made in preparing CeO2by different methods and in different conditions during the past decades. But the properties of the samples can be greatly influenced by particle size and synthesis method[11]. Therefore finding a suitable method for productive of high quality nano-CeO2is still the most important task. In this paper, the spherical nano-CeO2was success-fully synthesized by a hydrothermal method without any surfactant in the acidic condition and the effect of the pH value on the magnetic and optical properties of nano-CeO2was investigated by measuring VSM, XRD, SEM, UV-Vis, PL and Raman.

1Experimental procedure

1.1Materials synthesis

All the reagents were of analytical grade purity and used without further purification. Typically, 1.490 32 g of cerium chloride (CeCl3·7H2O) and 6 mL of aqueous ammonia (NH3·H2O) were dissolved in 60 mL of deionized water, respectively. The aqueous ammonia was added gradually to the CeCl3aqueous solution to form lavender colorless precipitation. After continuous stirring for 3 h, hydrochloric acid (HCl) was added dropwise to the CeCl3solution to control its pH value to 2, 4 and 6. After further stirred for 3 minutes, the mixed solution was transferred into a 50 mL Teflon-lined autoclave and heated at 200 ℃ for 12 h. After the autoclave was cooled to room temperature naturally, fresh precipitates were washed with deionized water and ethanol for three times, and then dried at 80 ℃ overnight and the straw yellow powder was obtained.

1.2Characterizations

The crystal phases of the products were analyzed by X-ray diffractometer(XD-3)with CuKαradiation (λ=0.150 46 nm). The morphology and size were characterized by scanning electron microscope(S-4800). The Raman spectra was recorded by a Raman spectrometer system (inVia-Reflex) using a laser with 532 nm excitation at room temperature. The UV-Vis absorption spectroscopy (U-4100) was measured by ultraviolet-visible-near-infrared spectrophotometer. Room temperature photoluminescence spectra (F-4500) were measured by a fluorescence spectrophotometer using excitation light of 340 nm. The M-H curves were measured at room temperature by vibrating sample magnetometer (PPMS, ECII(9T)).

2Results and discussion

2.1Size and morphology analysis

Fig.1 shows the XRD patterns of CeO2samples synthesized at different pH values. It can be seen that all diffraction peaks in these patterns can be perfectly indexed to the face-centered cubic structure of ceria (JCPDS Card #34-1002) with space group Fm3m (225). The obtained CeO2samples are pure phase products, no obvious peaks corresponding to other cerium oxides were observed in the powder patterns, indicating that pure CeO2is synthesized. The lattice parameter calculated from XRD spectra with Jade 5 software is about 0.542 6, 0.542 3, 0.541 6 nm for the samples with pH value is 2, 4 and 6, which indicate that the lattice parameter decreases with the increase of pH value. Compared with the lattice parameter of bulk CeO2(0.541 1 nm)[12], these are slightly higher. The potential reason for the observed lattice expansion is the presence of oxygen vacancies in the crystal lattice of nanocrystalline CeO2, which generally possesses high redox capabilities could create larger Ce3+ions for charge compensation (Ce3+and Ce4+have their respective ionic radii of 0.103 4 and 0.092 nm)[13].

Fig.1　XRD patterns for CeO2 nanoparticles synthesized at pH=2, 4, 6

More details about the size and morphology of the as-obtained CeO2nanocrystals synthesized at different pH value were investigated by SEM. As represented in Fig.2, the morphology of samples are all spherical nanocrystals, this should mean that the pH value have little effect on the morphology of ceria nanoparticles under acid condition. It can be seen that CeO2nanoparticles of 12—15, 8—12 and 4—8 nm in diameter were synthesized when the pH value is 2, 4 and 6. Obviously, the diameter decreases with the increase of pH value. Compared with the lattice constants calculated from the XRD patterns we can conclude that the size of the nanoparticles synthesized under acid condition increase with the increase of lattice parameter.

Fig.2　SEM images for CeO2 nanoparticles synthesized at pH=2 (a), 4 (b), 6 (c)

2.2Optical properties

The UV-Vis absorption spectra of CeO2samples are shown in Fig.3a. All of them illustrate strong absorption bands in the UV region. Generally, the absorption of ceria in the UV region originated from the charge-transfer transition is much stronger than the 4f1-5d1transition from the Ce3+species in the mixed valence ceria system[14]. So the absorption of ceria in the UV region mainly originates from the charge-transfer transition between the O2-(2p) and Ce4+(4f) orbit in CeO2[15].

The relation between absorption coefficient and band gap (Eg) can be written as (αhv)n∝hv-Eg, whereαis the absorption coefficient,Egis the band gap for direct transitions, andnis 2 for a direct transition[16]. The plots of (αhν)2vs. photon energy of CeO2nanoparticles are shown in Fig.3b. As a result, the band gap energy of the samples prepared at the pH value of 2, 4 and 6 are 2.97, 3.02 and 3.04 eV, respectively. Obviously,Egis increase with the increase of pH. And these values are smaller than that of the bulk CeO2(3.19 eV)[17-18].

Fig.3　　　　UV-Vis absorption spectra(a) and plots (b) of (ahv)2 as function of energy of 　　　CeO2 nanoparticles synthesized at pH=2, 4, 6

Fig.4　PL spectra of CeO2 nanoparticles synthesized at pH=2 (a), 4 (b), 6 (c)

Photoluminescence (PL) study can discern the defect-related transition with the location and intensity of some bands related to the oxygen-vacancy density[19]. Fig.4 shows the PL spectra of CeO2nanoparticles synthesized at different pH values, and seven emission peaks located at 373, 450, 468, 482, 493, 565 and 648 nm are observed. CeO2is a wide band gap compound semiconductor, whose gap is about 3 eV. The Ce 4f energy levels localize at the forbidden band and lie about 3 eV above the valence band (O 2p) with 1.2 eV width[20-23]. Usually, it is easy to observe the hopping from Ce 4f to O 2p (>3 eV). In addition, the defect levels localized between the Ce 4f band and the O 2p band can also lead to wider emission bands (<3 eV)[24]. So it can be concluded that the 373 nm emission peak related to the hopping from the localized Ce 4f state to the O 2p valence band. And the wide emission band ranging from 400 to 500 nm can be attributed to the hopping from different defect levels to the O 2p band[25]. The sample synthesized at pH=2 shows the higher intensity than the other two sample, this must be caused by the maximum concentration oxygen vacancy existed in the sample when pH=2, and the peak located at 565 and 648 nm can be attributed to the hopping between the defect levels.

Fig.5 shows room temperature Raman spectra.Raman peak at 462.7 cm-1can be attributed to a symmetrical stretching mode of the Ce-8O vibrational unit corresponding to the triply degenerate mode of the fluorite crystal structure of CeO2, this mode should be very sensitive to any disorder in the oxygen sublattice resulting from thermal and doping or grain size induced non-stoichiometry[26]. In bulk CeO2this frequency is 465 cm-1. Several factors may have caused this slight red shift, including phonon confinement, strain, broadening associated with the size distribution, defects, and variations in phonon relaxation with particle size[27]. Based on the date of XRD and PL, it can be confirmed that it is the Ce3+ions and oxygen vacancies contribution to this change[28]. The peak near 589 cm-1can be attributed to oxygen vacancies and defects caused by small size effect[29-30]. The intensity increases with the increase of oxygen defects, so we can confirm that the sample synthesized at pH=2 has the maximum concentration oxygen vacancy and this is corresponding with the PL spectra. The second-order features at 1 172 cm-1are very prominent for the samples synthesized at pH value of 2 and 4, but it is hard to see this peak for sample synthesized at pH=6. It can be attributed to the second-order Raman mode of surface superoxide species (O2-), and has little additional contributions from F2gsymmetry[31]. As for the peak at 2 935 cm-1, this is a very interesting and novel phenomenon and needed further investigation.

Fig.5　Room temperature Raman spectra of CeO2 nanoparticles synthesized at pH=2 (a), 4 (b), 6 (c)

2.3Room temperature ferromagnetism

Fig.6 shows the M-H curve of CeO2nanoparticles synthesized at pH=2. RTFM could be observed for the CeO2sample. It can be seen that the saturation magnetization (Ms) is about 1.58×10-3emu·g-1, residual magnetization (Mr) is 0.3×10-3emu·g-1and coercivity (Hc) is 190 Oe. The value ofHcis larger than those in previous reports[32]andMsandMrare smaller than those in previous reports[33-34]. According to the previous studies, it can be concluded that the presence of Ce3+ions and oxygen vacancies in CeO2nanoparticles enhanced the hybridization between Ce 4f and O 2p and results in the charge transfer transition between Ce 4f and O 2p bands through double-exchange mechanism, and this is FM order favored. Based on XRD and Raman analysis, it can be concluded that Ce3+and oxygen vacancy existed in CeO2deposits, and this must be the origin of the RTFM.

Fig.6　Magnetic hysteresis loops of CeO2 nanoparticles synthesized at pH=2

3Conclusions

According to the XRD analysis, it can be seen that CeO2nanoparticles with fluorite structure have been successfully synthesized by a facile hydrothermal method under acidic condition. The pH value influences the crystallite size and lattice parameter of the samples. The samples are all ball-like, and the band gaps determined by the UV-Vis absorption increase with the increase of pH value. RTFM could be observed from the sample synthesized at pH=2, itsMs,MrandHcare about 1.58×10-3emu·g-1, 0.3×10-3emu·g-1and 190 Oe, respectively. Raman and PL show that the Ce3+ions and oxygen vacancies existed in the samples, and this is likely the origin of the RTFM.

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(責(zé)任編輯鄭小虎)

doi:10.3969/j.issn.1000-2162.2016.04.007

Received date:2015-12-19

Foundation item:Supported by Anhui Provincial Natural Science Foundation (1508085SME219)， College Students Innovation Training Program of Anhui University (201510357349，201510357125) of China

CLC number:TQ174

Document code:AArticle ID:1000-2162(2016)04-0037-07

酸性水熱合成CeO2納米顆粒的光學(xué)和磁學(xué)性質(zhì)

孟凡明,李慧杰，何廣遠(yuǎn)，高鑫，劉道瑞

(安徽大學(xué) 物理與材料科學(xué)學(xué)院，安徽 合肥 230601)

摘要:以CeCl3·7H2O為鈰源、NH3·H2O為礦化劑、HCl為酸性調(diào)節(jié)劑，利用水熱法成功制備了二氧化鈰納米顆粒.采用X射線衍射儀(XRD)、掃描電子顯微鏡(SEM)、紫外-可見(jiàn)分光光度計(jì)(UV-Vis)、熒光分光光度計(jì)(PL)、拉曼光譜(Raman)和振動(dòng)樣品磁強(qiáng)計(jì)(VSM)等分析測(cè)試手段，對(duì)CeO2納米顆粒的晶相、形貌、光學(xué)和磁學(xué)性質(zhì)進(jìn)行了表征.XRD測(cè)試結(jié)果表明樣品的晶格參數(shù)略高于塊狀CeO2的，并隨著pH值的增大而減小.所有樣品的形貌均為球形.從紫外-可見(jiàn)吸收光譜中，估計(jì)出的樣品直接帶隙值小于塊狀CeO2的，且此值隨pH值的增加而增加.pH值為2時(shí)合成樣品具有室溫鐵磁性，此性質(zhì)的出現(xiàn)可能與Ce3+和氧空位的存在有關(guān).

關(guān)鍵詞:CeO2;納米顆粒;酸性環(huán)境;光學(xué)性質(zhì)；磁性

Author’s brief:MENG Fanming (1966-), male, born in Hefei of Anhui province, professor of Anhui University, E-mail: mrmeng@ahu.edu.cn.