Hydrothermal synthesis and optical properties of CeO2 microstructures

MENG Fan-ming, FAN Zheng-hua, SHI Guo-li, LIU Dao-rui

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

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Hydrothermal synthesis and optical properties of CeO2microstructures

MENG Fan-ming, FAN Zheng-hua, SHI Guo-li, LIU Dao-rui

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

CeO2microstructures had been successfully synthesized by a facile hydrothermal process with Ce(NO3)3·6H2O as cerium source, urea as precipitant and polyvinyl pyrrolidone (PVP, K-30) as surfactant. The obtained products were characterized by X-ray diffraction (XRD), thermogravimetry/differential thermal analysis (TG/DTA), Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), Raman spectrum, UV-visible absorption spectrum and photoluminescence (PL) spectrum. It was found that the obtained CeO2had fluorite cubic structure, and Raman spectroscopy and PL spectrum reflected the existence of the oxygen vacancies and Ce3+ions in the CeO2microstructures. Furthermore, the CeO2sample showed strong UV absorption and room temperature photoluminescence (PL), the absorption edge and Raman peak position was shifted to lower frequency which was likely associated with the surface Ce3+ions and oxygen vacancies in the sample.

optical property; CeO2microstructures; hydrothermal method; synthesis; X-ray technique

0 Introduction

In recent years, many researches have been directed toward the morphology controlling synthesis of rare-earth compounds because of their unique electronic, optical, chemical properties[1-3]. Among the family of rare-earth compounds, ceria (CeO2), as one of the most important rare-earth oxide materials, has been widely applied in catalysis[4], fuel cell[5], gas sensor[6]and as oxygen storage capacity (OSC) medium[7-8]. For example, it can be used for reducing the emission of the toxic pollutants from auto-exhaust, due to its high oxygen storage capacity and low potential between Ce3+and Ce4+[9]. However, most of excellent properties depend strongly on both morphology and size. CeO2have different morphologies such as nanoparticles, nanotubes, nanorodes, nanosheets and hollow nanocrystals that have been prepared by various methods[10-16]. Furthermore, many methods have been used to prepare complex three-dimensional (3D) structures, including the thermal oxidation process[17], the thermal reduction process[18], self-assembly of building blocks though hydrophobic interaction[19]and template-assisted synthesis[20]. To obtain high quality powder of cerium oxide, one of simple and cheap approach that has been widely employed to prepare rare-earth oxides is the thermal decomposition of cerium carbonates or carbonate hydroxides. The morphology and crystal size of cerium oxide can be easily controlled by using carbonates or carbonate hydroxides as decomposition precursors. Meanwhile, as it is very difficult for materials with isotropic structures to grow into anisotropic nanocrystal without any template, the use of surfactant as the capping agent or template is popular in solution synthesis of CeO2nanostructures[10, 15, 18, 20-21]. The surfactant can change the micro-environment of the reaction and adjust the growth habit of the particles, leading to the formation of final products with desirable morphology.

Herein, we have developed a two-step procedure to synthesize CeO2microstructures. The cerium carbonate hydroxide (CeOHCO3) precursor was prepared via a simple surfactant-assist hydrothermal method with Ce(NO3)3·6H2O as cerium source, urea as precipitant and polyvinyl pyrrolidone (PVP, K-30) as surfactant. The CeO2was obtained by thermal decomposition of cerium carbonate hydroxides. The optical properties of CeO2microstructures were discussed by various techniques.

1 Experimental procedure

1.1 Materials preparation

All chemical reagents were of analytical grade purity and used without any further purification. The as-synthesized CeOHCO3sample was synthesized by changing the dosages of Ce(NO3)3·6H2O and HNO3. The growth of sample was performed by dissolving 1.736 9 g (4 mmol) Ce(NO3)3·6H2O in one mixed solution of 15 mL distilled water and 0.45 g PVP with magnetic stirring for 30 min, and dissolving 20 mmol CO(NH2)2in another mixed solution of 15 mL distilled water and 0.45 g PVP with magnetic stirring for 30 min, followed by the mixture of the two aqueous solutions under vigorous stirring for another 1 h, forming a homogeneous solution. Then 1 mL HNO3was added dropwise to the obtained solution. The mixed solution was transferred to a 50 mL Teflon-lined stainless steel autoclave and heated at 200 ℃ for 24 h. After the autoclave was naturally cooled to room temperature, the precipitate was collected by centrifugation, washed with distilled water and ethanol, and then dried at 70 ℃ for 12 h. CeO2microstructures were obtained by calcining as-synthesized CeOHCO3simple at 500 ℃ for 5 h, accompanied by a color change from white to slight yellow.

1.2 Characterizations

The crystal phase of sample was analyzed by X-ray diffractometer (XRD, XD-3) with CuKαradiation. The thermal behavior of as-synthesized products was carried out by simultaneous thermo-gravimetric analysis apparatus (TG-DTA, 449F3, Germany). Fourier transform infrared (FT-IR) spectra were recorded by Fourier transform infrared microscopy on-line system using KBr pellet (VERTEX80+HYPERION2000, Germany). The morphology was examined by scanning electron microscope (SEM, S-4800, Japan) and transmission electron microscope (TEM, JEM-2100, Japan). The Raman spectrum was recorded by an in Via-Reflex Raman spectrometer system using a laser with 532 nm excitation at room temperature. UV-Vis absorption spectroscopy was measured by an ultraviolet-visible-near-Infrared spectrophotometer (U-4100, Japan). Photoluminescence (PL) spectrum was obtained by a fluorescence spectrophotometer (HORIBA FluoroMax-4P, HORIBA Jobin Yvon) using excitation light of 340 nm.

2 Results and discussion

2.1 Structures and morphology analysis

Fig.1 shows the XRD patterns of the sample obtained under hydrothermal reaction, along with the sample calcined at 500 ℃ for 5 h. As shown in Fig.1a, all the peaks can be indexed to hexagonal structure CeOHCO3(JCPDS Card #41-0013). The sharp diffraction peaks in the XRD pattern demonstrated that the product have a good crystallinity. After a heat treatment, the hexagonal CeOHCO3structure was completely converted into fluorite cubic structure CeO2(JCPDS Card #81-0792). The diffraction peaks in Fig.1b of calcined sample indicated that the CeO2were refined after the heat treatment.

Thermal behavior of CeOHCO3microstructure was investigated using TG/DTA. TG curve for calcining in air (Fig.2a) shows 2.4% weight loss up to 400 ℃ which conforms that very little water remains in the sample after drying and a major weight loss happens rapidly in the range of 400—600 ℃, nearly keeps constant as the temperature exceed 700 ℃, which is ascribed to the loss of CO2and H2O from carbonaceous, surfactant-based material trapped within the sample. The total weight loss is about 22.5%, which is consistent with the theoretical value 23.1% calculated from the equation: 4CeOHCO3+O2→ 4CeO2+4CO2+2H2O. DTA curve (Fig.2b) shows a sharp endothermic band centered at about 460 ℃. It is well corresponding to that of the rapid weight loss in the TG curve. It suggests that an endothermic reaction involving the thermal decomposition of CeOHCO3to CeO2occurs by post-heat-process.

Fig.3 shows the FT-IR spectra of the as-synthesized CeOHCO3and CeO2obtained by calcining sample. Compared with the spectrum of the annealing sample, the spectral feature of CeOHCO3precursor is more prolific. As shown in Fig.3 as-synthesized sample, the bands in the range of 700—1 100, 1 200—1 700 and 3 000—3 700 cm-1, corresponding to carbon-involved species, carbonate species and O—H stretching of physical absorbed H2O or surface —OH groups, respectively. Vantomme et al.[22]reported that the peak 1 410 and 1 490 cm-1are assigned to the bending vibration of C—H bands and O—C—O stretching of the surfactant. The peaks at 1 070, 840, 723 cm-1are attributed toνC—O,δCO32-andνasCO2, respectively[18]. After heat-treatment, such bands originating from the organic groups or carbonate species become weaker or disappear, while the bands due to the stretching of Ce—O were seen clearly. This conclusion is in agreement with the TG/DTA result.

Fig.4 shows the scanning electron microscope (SEM) images of the as-synthesized CeOHCO3sample and the CeO2obtained by calcining sample. As shown in Fig 4a, CeOHCO3microstructures obtained displayed thin strip shape of several μm in length, about 1 μm in width, several dozen nm in thickness accompany with spherical of 1—3 μm in diameter. We can see further that the microstructures were formed by many short strip sheets which put up well-organized. Fig.4b shows the scanning electron microscope (SEM) images of CeO2microstructures obtained by calcining at 500 ℃. Obviously, it can be seen that the post heat-treatment process does not ruin the morphology of the products and the CeO2microstructures almost keep the same morphology of its counterpart. SEM images distinctly exhibit the three-dimensional structures and the texture of the CeO2microstructures[21].

2.2 Optical properties

Raman spectroscopy is a very important characterization tool to investigate the changes in the local structures. Fig.5 illustrates the Raman spectra of the CeO2sample. One strong Raman peak centered at about 462.5 cm-1dominates the spectra from the sample, which related to the F2gmode of CeO2cube structure[23-24]. This mode corresponds to symmetric breathing mode of the oxygen ions around each Ce4+cation[25].The F2gmode is very sensitive to any disorder in oxygen sub lattice due to thermal, doping and grain-size induced effect[26]. From description of the position of the Raman peak of the CeO2sample, it is found that the position of the CeO2sample is shifted to lower frequencies compared with bulk CeO2. The F2gpeak of bulk CeO2is at 464 cm-1[27]. Compared with the bulk one, the F2gRaman peak position of CeO2microstructures are 1.5 cm-1shifted to lower frequency. Meng et al.[28]have obtained a red-shifted in pure ceria. They attributed the red-shifting to the size distribution, defects, and variations in phonon relaxation. The peak near 260 cm-1can be contributed to disorder in the oxygen sub-lattice[29]. The shoulder peaks ranging from 560 to 600 cm-1can be contributed to the presence of Ce3+ions and oxygen vacancies[30]. It is likely that Ce3+ions and oxygen vacancies in the sample are responsible for the changes in the Raman scattering.

Fig.6 shows the UV-Vis absorption spectra of CeO2nanostructures. The sample exhibits a strong absorption below 400 nm with two peaks in the UV region that originated from the charge-transfer transitions from O 2p to Ce 4f[31]. The direct band gap energy (Eg) for the ceria nanostructure were determined by fitting the absorption data to the direct transition equation[32]: (αhν)2= (hν-Eg), whereαis the absorption coefficient,hνthe photon energy,Egthe direct band gap. The plot (αhν)2versushνis given in inset Fig.6, the extrapolation of linear portion of the curve towards absorption equal to zero givesEgfor direct transition. The estimated direct band gap of the cerium oxide was found to be 3.02 eV, compared with bulk ceria energy band of 3.19 eV, which suggests that the energy band of ceria occurs red-shifting.

It was found that at the outermost nanocrystals surface, Ce4+ions coexist with Ce3+ones when the cluster size decreases. At the same time, there is an electrostatic potential effect due to a cerium valence change, which results in the red-shift of the absorption edge. Thus, the charge transition of Ce ion (Ce3+-Ce4+) may play an important role in the decrease of band gap for cerium oxide sample. Chen et al.[33]have observed a red-shifted phenomenon for the CeO2nanoneedles, which is attributed to the shape effect. Chen et al.[34]found that the red-shift of optical band gap of CeO2film is orrelated with the increase of Ce3+ions content, and defects are directly proportional to the concentration of Ce3+ions. Furthermore the Raman and PL results indicate the microstructures possess high oxygen vacancies. The localized states of band gap result from the defects will advance with the increase of Ce3+ions, leading to red-shift. More investigations need to be carried out in order to clarify the origin of this phenomenon.

Fig.7 shows the room temperature photoluminescence (PL) spectrum of CeO2microstructures at room temperature at an excitation wavelength of 340 nm. It is obvious that at 400—500 nm there is a wide emission band. Generally, the broad PL emission peaks are observed because there are many defects energy level between Ce 4f and O 2p level. It is known that CeO2is a wide band gap semiconductor, whose Ce 4f energy levels localize at forbidden band and lie about 3 eV above the valence band (O 2p) with width of 1.2 eV[35]. Its defect levels localized between the Ce 4f band and O 2p band can lead to wider emission bands. For example, Maensiri et al.[36]observed a wide PL emission band from 325 to 550 nm in CeO2nanoparticles, and the considered the surface defects are responsible for the broad emission. Sun et al.[37]also observed two emission peaks centered at 415 and 435 nm from the flowerlike CeO2nanoparticles, which were attributed to the hopping from different defect levels to O 2p band. Thus, the blue emission band for our sample is ascribed to hipping from different defect levels to O 2p band. This conclusion is in agreement with the Raman scattering result.

3 Conclusions

In summary, CeO2microstructures were synthesized in aqueous solution by a facile hydrothermal method using Ce(NO3)3·6H2O as cerium source and CO(NH2)2as precipitant. CeOHCO3microstructures were considered to be formed by the self-assembly of the small size nanocrystal. The CeO2microstructures showed a broad strong emission in the near UV region, which may mainly originate from defect state existing extensively between the Ce 4f band and O 2p band. The controllable morphologies, microstructures, and optical properties of CeO2should make the nanomaterials excellent candidates for applications in oxygen transportation, catalysts, fuel cells, ultraviolet blocks and luminescent materials.

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(責(zé)任編輯 鄭小虎)

CeO2微結(jié)構(gòu)的水熱合成和光學(xué)性質(zhì)

孟凡明, 范拯華, 史國利, 劉道瑞

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

以Ce(NO3)3·6H2O作為鈰源、尿素作為沉淀劑、K-30型PVP作為表面活性劑，采用水熱法成功制備二氧化鈰微結(jié)構(gòu).采用X射線衍射儀(XRD)、熱重/差熱分析(TG/DTA)、傅里葉變換紅外譜(FT-IR)、掃描電子顯微鏡(SEM)、拉曼光譜(Raman)、紫外可見近紅外分光光度計(UV-Vis)和熒光分光光度計(PL)對此CeO2樣品進行表征，結(jié)果表明：制備的CeO2樣品具有螢石立方結(jié)構(gòu)，樣品中存在氧空位和Ce3+離子，樣品具有較強的紫外吸收和室溫光致發(fā)光性能.吸收邊和拉曼峰位的移動與樣品中氧空位和Ce3+離子有關(guān).

光學(xué)性質(zhì); CeO2微結(jié)構(gòu);水熱法;合成；X射線技術(shù)

10.3969/j.issn.1000-2162.2015.06.007

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

TQ174 Document code:A Article ID:1000-2162(2015)06-0037-08

Received date:2015-06-29

Author’s brief:MENG Fan-ming (1966-), male, born in Hefei of Anhui province, professor of Anhui University.