Chinese Chemical Letters  2017, Vol. 28 Issue (3): 663-669   PDF    
Hydrothermal synthesis of triangular CeCO3OH particles and photoluminescence properties
Md.Hasan Zahira,b, ShamseldinA. Mohameda, MohammadMizanur Rahmanc, AteeqUr Rehmand     
a Center of Research Excellence in Renewable Energy, Research Institute, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia;
b Center of Research Excellence in Nanotechnology, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia;
c Center of Research Excellence in Corrosion, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia;
d Center for Engineering Research, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
Abstract: Ru/CeO2[RC] and Ru/CeO2/ethylene glycol (EG)[RCE] nanoparticles were produced by performing a simple hydrothermal reaction at 200℃ for 24 h and found to have two distinct morphologies. The RC nanoparticles are phase pure CeO2; triangular highly crystalline CeCO3OH nanoparticles are formed from the solution containing EG under the same hydrothermal reaction conditions at pH 8.5. EG plays an important role in the formation of the triangular CeCO3OH nanoparticles. The polycrystalline CeCO3OH nanoparticles retain their triangular structure even after calcination at 600℃ in air but are transformed into a pure CeO2 phase. The room temperature photoluminescence of the RC and RCE nanoparticles and of RCE calcined at 600℃[RCE-600] was also investigated. It was found that the high crystallinity triangular RCE-600 sample exhibits the highest photoluminescence intensity.
Key words: CeCO3OH particles     Ru/CeO2     Ethylene glycol     Hydrothermal process     Luminescence    
1. Introduction

In recent years, cerium compounds have been widely used in catalysis [1] and fuel cells [2], and as chemical materials [3] and luminescent materials [4] because of the specific 4f energy levels of elemental Ce [5, 6]. In particular, cerium carbonate hydroxide (CeCO3OH) is an important functional material that has attracted much attention because of its novel electronic properties, optical properties, and chemical characteristics [7, 8]. The wide variety of applications of CeCO3OH is based on its different phase structures and morphologies. In particular, CeCO3OH can be used as a template precursor for the preparation of ceria (CeO2) via simple thermal decomposition with various morphologies [9]. Note that it is possible to prepare ceria from CeCO3OH without modifying its original morphology [9, 10].

Recently, CeCO3OH with various morphologies has been synthesized by using several different approaches such as the self-assembly, sonochemical [11], hydrothermal [12, 13], and microwave-assisted hydrothermal routes [14]. A low synthesis temperature, versatile shape control, and inexpensive starting materials are possible only with the hydrothermal method [15]. For example, Han et al. reported the synthesis of triangular microplate, bundle-like, and flower-like structures of CeCO3OH with the hydrothermal method [16]. Li and Zhao synthesized singlecrystalline CeCO3OH with dendrite-like structures by using a facile hydrothermal method and obtained CeO2 by heating CeCO3OH at 500 ℃ for 6 h [17]. Zhang et al. synthesized CeCO3OH rhombic microplates with the precipitation method in the presence of 3-amino propyl triethoxysilane [18]. Moreover, CeCO3OH samples consisting of shuttles [19] and prisms [20] have been prepared by using aqueous solution processes. Recently, He et al. reported globin-like mesoporous CeCO3OH and their application in CO low-temperature oxidation [21]. It is interesting that CeO2 nanoparticles converted from CeCO3OH with various morphologies retain these morphologies even after heat treatment at high temperatures. Note that the above-mentioned methods all used the salt cerium nitrate as the starting material for CeCO3OH synthesis and that the preparations were all performed in aqueous solution.

It is well known that the presence of CeO2 enhances the activities of various noble metal ions; for example, the addition of Pt and Pd into CeO2 promotes CO hydrogenation [22] and CH3OH decomposition [23]. It has been reported that Ru/CeO2 (RC) nanocomposites can be prepared that contain only CeO2 phase and no XRD peak due to Ru is observed probably Ru species are welldispersed. This well-dispersed nanocomposite is more effective in the wet oxidation of wastewater than other combinations of noble metals with CeO2 [24]. It is well established that Ru species in CeO2 are active in many catalytic reactions. However, a detailed study of RC interactions has not yet been carried out. RC catalysts have been found to exhibit fairly high catalytic activities in the oxidation and aldehydes in organic media [25]. A recent study reported the role of RC in N2O decomposition with reducing agents [26]. Guan et al. reported that RC is stable up to 550 ℃ in supercritical water, so it can be used in a supercritical water gasification (SCWG) system as part of a promising method for phenols modification [27]. Phenol gasification is required in tar removal and waste energy applications, and thus the SCWG of phenols is vital for energysaving environmental chemistry.

Wang et al. have synthesized RC catalysts with various compositions and a unique CeO2 morphology by varying the Ru loading. This morphology was found to exhibit better catalytic activity in CO2 methanation [28]. There have been no previous reports of the hydrothermal synthesis of CeCO3OH in Ru/CeO2/ ethylene glycol (RCE) or of the characterization of its photoluminescence properties. Therefore, the synthesis of CeCO3OH with new morphology of RCE might provide interesting catalytic activities and optical behaviors [29].

2. Results and discussion

Fig. 1 shows XRD patterns of the as-synthesized RC sample, the as-synthesized RCE sample, and the sample obtained after calcination of CeCO3OH at 600 ℃ for 2 h (RCE-600). Fig. 1a shows the XRD pattern of CeO2; the peaks match those of the facecentered cubic fluorite structure with space group Fm-3m (JCPDS 34-0394). The mean crystallite size was calculated with the Scherrer equation and found to be 11.9±0.09 nm. In Fig. 1a, there are no diffraction peaks for RuO2 phase near 28° or 35° or for Ru phase near 44°, which might be due to the low content and high dispersion of the Ru species. Another possibility is that the Ru species have become part of the ceria lattice. The peak position is slightly shifted to a higher value by the addition of Ru into CeO2, which probably means that a small number of Ru4+ species (Ru4+=0.62 Å) has penetrated into the fluorite-like lattice (Ce4+=0.94 Å). As a result, decreases in the lattice parameters are observed and a Ru-O-Ce structure arises to some extent [30]. In this study, small decreases in the lattice parameters are also observed after the addition of Ru into CeO2. In Fig. 1b, the diffraction pattern is that of a pure hexagonal phase of CeCO3OH with lattice constants a=7.2382 Å and c=9.9596 Å, in good agreement with the reported values (JCPDS 32-0189). Thus, the RCE system results in hexagonal phase CeCO3OH. It is important to note that the hexagonal CeCO3OH structure is more stable than the orthorhombic structure, particularly at higher temperatures [13]. Moreover, no additional peaks other than those of the CeCO3OH hexagonal phase were detected, which indicates the high purity of the final product obtained from the RCE starting materials (Fig. 1b). All the reflection peaks are very strong and sharp, which indicates that the as-synthesized products are crystalline. It is interesting to observe that the only difference between the two procedures is the presence or absence of EG, i.e. the same starting materials with the same pH produce single phase CeCO3OH under hydrothermal conditions only in the presence of EG. No previous report of the synthesis of CeCO3OH via the RCE route has been published. The diffraction peaks of RCE-600 in Fig. 1c match those of the cubic phase of ceria (Fm3m=5.41134 Å, JCPDS Card No. 34-0394).

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Figure 1. XRD patterns of (a) as-synthesized CeO2, (b) as-synthesized CeCO3OH triangular structures, and (c) CeO2, obtained from Ru/CeO2, Ru/CeO2/EG, and Ru/ CeO2/EG calcined at 600 ℃, respectively.

Fig. 2 shows SEM images of as-synthesized RC and assynthesized RCE. The SEM image of RC shows that it contains fine amorphous particles. The particles look like clouds floating in the air. In contrast, the RCE morphology consists mainly of triangular particles with diameters in the range 2-3 μm. The energy dispersive X-ray spectroscopy (EDS) results shown in Fig. 2c indicate the presence of Ru, Ce, and O. The EDS data were obtained from the position on the surface marked by a circle in Fig. 2b. The same elements were also found in different locations. Fig. 3a shows a TEM image of the as-synthesized RC sample; the morphology consists of cube-type CeO2 elements with an almost uniform size of approximately 10-20 nm. This value is consistent with the result obtained by using the Scherrer equation. Fig. 3b shows a HRTEM image of CeO2; the corresponding fast Fourier transform (FFT) image is shown in the inset. The spacing between the lattice fringes was found to be 0.280±0.004 nm (Fig. 3b) Furthermore, the SAED ring patterns (inset in Fig. 3a) are the (111), (200), (220), (311), (222), and (400) planes of the CeO2 fcc phase. Fig. 3c shows a TEM image of RCE, which shows that EG plays an important role in the formation of CeCO3OH with triangular structures. The SAED pattern (Fig. 3c inset) obtained from a triangular microplate confirms that it is crystalline. The SAED pattern also contains discontinuous rings, which suggests that it consists of polycrystals with an oriented crystallographic axis. Most of the triangular particles look spongy and some possess sharp corners and smooth surfaces (Fig. 3c') attached to nanoparticles, which are CeO2 based on their SAED ring patterns, at their periphery (Fig. 3c). Fig. 3c" shows a HRTEM image of CeO2, which was obtained from pure hexagonal CeCO3OH by calcination at 600 ℃ for 2 h. The CeCO3OH morphology was retained after thermal conversion of CeCO3OH to CeO2, as shown in Fig. 3c". Fig. 3d shows a HRTEM image of triangular CeCO3OH; the spacing between the lattice fringes is 0.317±0.003 nm.

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Figure 2. SEM images: (a) as-synthesized CeO2 and (b) as-synthesized triangular microplates of CeCO3OH obtained from Ru/CeO2 and Ru/CeO2/EG respectively. (c) EDS spectrum for the region marked by a circle in (b).

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Figure 3. (a) TEM image of CeO2; the inset is the SAED pattern. (b) HRTEM image of CeO2. TEM images: (c) triangular CeCO3OH microplates, (c') triangular CeCO3OH microplates with sharp corners, and (c") Ce (OH) CO3 triangular microplates calcined in air at 600 ℃ for 2 h. (d) HRTEM image of the sample in (c).

Thermogravimetric analysis (TGA) profiles of the as-synthesized RC and RCE samples are presented in Fig. 4. The initial weight loss in the profile of the as-synthesized RC sample (Fig. 4a) might be that of hydrated materials, i.e. the decomposition of Ce (OH)3 or Ce (OH)4/CeO2·2H2O. Most of the weight loss, almost 16%, occurs up to 400 ℃. However, for complete dehydration a much higher temperature of~500 ℃ seems necessary. The thermal decomposition of triangular CeCO3OH (obtained from RCE) to CeO2 was also investigated with TG analysis (Fig. 4b). Almost no weight loss is evident up to 280 ℃ for the RCE sample, which indicates that it contains negligible amounts of adsorbed water, has good purity, and retains its Ru species. The slight weight loss at temperatures between 20 ℃ and 200 ℃ might be due to the loss of H2O and trapped solvent. The sharp weight loss from 250 ℃ to 380 ℃ must be due to the removal of organic residues and the thermal conversion of CeCO3OH.

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Figure 4. TGA patterns of the as-synthesized (a) CeO2 and (b) CeCO3OH samples obtained from Ru/CeO2 and Ru/CeO2/EG respectively.

RC and RCE were also characterized by measuring their nitrogen adsorption desorption isotherms. Fig. 5a shows the Ⅳ isothermwith an apparent H4-type hysteresis loop in the P/P0 range 0.6-1 for the RC sample. This type of hysteresis loop clearly indicates that the RC powders are mainly mesoporous. The BET areas of RC and RCE were found to be 168 and 5 m2/g respectively. The maximum pore radius of the RC sample is 13 nm. The pore volume of RCE (Fig. 5b) is significantly lower, which is attributed to the incorporation of EG species into the stacking pores of CeO2.

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Figure 5. N2 adsorption/desorption isotherms for as-synthesized (a) CeO2 and (b) CeCO3OH samples obtained from Ru/CeO2 and Ru/CeO2/EG respectively.

Fig. 6a shows the FTIR spectral features of an as-synthesized RC sample. A broad band at 3436 cm-1, a sharp peak at 1627 cm-1, and some peaks with low intensity below 800 cm-1 can be observed for the RC sample. The peaks at 3436 and 1627 cm-1 correspond to Hbonded water molecules and δ(OH) respectively. As-synthesized CeO2 samples usually contain residual water and/or hydroxyl groups regardless of the synthesis method [31]; heat treatment eliminates such bands. The peaks below 800 cm-1 are probably due to the δ(Ce-O-C) mode. Fig. 6b shows the FTIR spectrum of an as-synthesized RCE sample; bands due to the O-H stretching of surface-adsorbed H2O and the OH groups of EG molecules are evident in the range 3500-3700 cm-1. The two peaks at 2923 and 2845 cm-1 are assigned to -CH2 asymmetric stretches in EG molecules [32]. The presence of these peaks clearly demonstrates that EG molecules are adsorbed onto the crystal surfaces and act as capping agents during crystallization. The three peaks at 1078, 840, and 720 cm-1 are attributed to the ν(C-O), δ(CO32-), and ν(as CO2) modes, respectively [29]. The peaks in the range 1400-1500 cm-1 are ascribed to carbonate species. These carbonaterelated peaks all disappear after heat treatment due to the thermal decomposition of CeCO3OH to form CeO2 (Fig. 6c). The FTIR spectrum of RCE-600 is quite similar to that of CeO2 (Fig. 6a); the only difference is in the intensity of the highest peak, which is decreased, probably because the calcination was performed at a higher temperature. The FTIR spectrum of Fig. 6c also confirms that CeCO3OH is transformed to CeO2 by calcination.

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Figure 6. FTIR spectra of (a) as-synthesized CeO2, (b) as-synthesized CeCO3OH triangular structures, and (c) CeO2, obtained from Ru/CeO2, Ru/CeO2/EG, and Ru/ CeO2/EG calcined at 600 ℃, respectively.

We also examined the surface composition of the RCE sample by recording its XP spectrum. Fig. 7 shows the only signals in the XPS spectrum are those of Ru, Ce, oxygen, and carbon impurities. At high resolution, two Ce 3d XPS peaks can be distinguished at 885.5 and 905.2 eV. The Ru 3p pattern contains two peaks due to Ru species, i.e. the different oxidative states of Ru give rise to peaks at 464.2 eV (Ru4+), and 483.2 eV (Ru4+ (hydrate)). These results confirm that Ru species are present in the RCE i.e., CeCO3OH compound. The surface concentrations of Ru were 1.44 atomic%, analyzed by XPS. Three types of carbon species were found in the C 1s spectrum (Fig. 7d). A peak of C-type was observed, which is due to carbon-oxygen bonds. The intensity of the C-type peak in the spectrum of RCE is higher than that due to RC because of the presence of EG molecules [13]. The C-type peak is due to carbon contamination on the surface of the sample. A broad O1s XPS peak was observed at 532.5 eV for the as-prepared sample. The lattice oxygen of the RCE sample produced a peak in the low binding energy region at 530.5 eV. This peak is shifted toward a lower value for RCE i.e., CeCO3OH, which is probably due to the change in charge from Ce3+ to Ce4+.

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Figure 7. Typical XPS spectra of as-synthesized RCE: (a) survey spectrum, (b) Ce 3d region; (c) Ru 3p region (d) C 1s region and (e) O 1s region.

Then why does CeCO3OH form upon the addition of 40 mL EG into RC? As we know, after the addition of cerium nitrate into deionized water, [Ce (H2O)n]3+ ions are likely to form; these ions are converted into [Ce (OH)(H2O)n]2+ in aqueous solution. Finally the formation of CeCO3OH could result from the reaction between [Ce (OH)(H2O) n-1]2+ and CO32. The CO32- ions derive from the presence of (NH4)2CO3 in the aqueous solution. The level of CO32- formation is very low in the absence of EG, and as a result ceria is obtained rather than CeCO3OH. It has been reported that trivalent Ce3+ is strongly attracted to OH- under hydrothermal conditions. The Ce3+ ions probably react with OH-(aq) to form the polyatomic group Ce (OH)2+ (aq) [11]. Under hydrothermal conditions at high supersaturation and high temperatures, the CO32- ions could bond with the positively charged groups to yield solid triangular particles of Ce (OH) CO3. It is likely that the carbonate ligands form through chelation or bridging in which each carbonate ion is bound to two metal centers [33]. Ligand formation was confirmed with FTIR analysis by the presence of the peak at 588 cm-1, which is due to the stretching of CO32- and O-Ce-O, in the spectrum of CeCO3OH (Fig. 6b). Of course, ethylene glycol possesses plentiful hydroxyl groups and therefore, the hydroxyl groups can selectively adsorb on top of the crystal surfaces via chemical forces and/or hydrogen-bond interactions [34]. Self-assembly can then take place because of the neutralization of the surface charges by the EG molecules. As a result, the formed nanoparticles are capped by the organic agent and finally aggregate to form triangular anisotropic building blocks.

Fig. 8 displays the luminescence spectra of as-synthesized RC, as-synthesized RCE, and RCE-600. After excitation at 280nm for 30s at room temperature, highly intense emission peaks can be seen for all three samples at 380nm. This emission peak is due to transitions within the 4f shell. In addition, we have to keep in mind that the host lattices can react with impurities, which often affects luminescence behavior through the crystal field [35]. Three emission peaks centered at 380nm were observed for the three samples. The most intense peak arose for the RCE-600 sample. The emission intensity is slightly lower in the case of RCE, but note however that the emission intensity of the same sample increases just after calcination at 600 ℃ for 2h in air. Guo et al. reported the maximum emission bands for CeCO3OH at 360nm when the excitation wavelength was 300nm at room temperature [9]. Han et al. also reported strong luminescence bands for CeCO3OH with a pure orthorhombic phase for wavelengths in the range 365nm-373nm upon excitation at 300nm [16]. Very strong emission peaks centered at 332nm and 347nm are evident for the CeCO3OH nanoparticles with a pure orthorhombic crystalline phase [36, 37]. In contrast, we found sharp peak characteristic of rare earth compounds for these CeCO3OH products with a pure hexagonal crystalline phase. Although the emission and excitation wavelengths are different, the sharp characteristic peaks might be due to the morphology of the CeCO3OH products. It is a common phenomenon that the luminescence properties of a crystal can change if its morphology changes. As a newly discovered material, our understanding of CeCO3OH is only at a preliminary stage. The luminescence properties of CeCO3OH powders prepared from RCE need further investigation. These nanostructured triangular particles with crystalline hexagonal phase are expected to exhibit enhanced activities in gas sensors, electrode materials, and catalysts because of their new morphology.

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Figure 8. Luminescence spectra of (a) as-synthesized CeO2, (b) as-synthesized CeCO3OH triangular structures, and (c) CeO2, obtained from Ru/CeO2, Ru/CeO2/EG, and Ru/CeO2/EG calcined at 600 ℃, respectively.

3. Conclusions

We prepared nanostructured CeO2 and CeCO3OH particles with hydrothermal processes from aqueous solutions of Ru/CeO2 and Ru/CeO2/EG with added (NH4)2CO3. Homogeneous CeO2 consisting of nanoparticles with high surface areas and sizes of approx.10nm were obtained from precursor solutions containing Ru and CeO2 under basic conditions. In contrast, triangular CeCO3OH precipitates were obtained from precursor solutions containing EG. The nanostructured CeCO3OH particles are converted to CeO2 particles with the same morphology by heat treatment at 600 ℃.

4. Experimental

Hydrothermal method: In the syntheses of the RC and RCE nanoparticles, 3 wt% RuCl3·H2O and 97 wt% Ce (NO3)3·6H2O were used as raw materials. The Ru and Ce salts were purchased from Sigma Aldrich. The appropriate amounts of the two starting salts were dissolved separately in distilled water. An appropriate amount of (NH4)2CO3 solution (4.1 mol/cm3, purchased from Merck) was added to the solutions to co-precipitate the metallic ions. The pH of the solutions was kept at approx. 8.5 and then the solutions were vigorously stirred for 12 h. For the RCE sample, 40 mL EG was added to the solution. Each precursor suspension was transferred into a plastic container with an inner volume of 500 cm3 held in a steel vessel. N2 gas was flushed through the suspension for 10 min. The mouth of the vessel was closed, and the hydrothermal reaction was performed at 200 ℃ for 24 h. An autoclave with a capacity of 500 mL and a magnetically driven stirrer were used in these hydrothermal reactions. The resulting powders were washed three times with alcohol and deionized water and dried at 110 ℃. These powders are identified here as the "as-synthesized" samples.

Analysis: X-ray diffraction (XRD) analysis was performed on a Bruker D8 Advance diffractometer system with an operating voltage of 40 kV and a current of 40 mA by using Cu Kα radiation (λ=1.5405 Å) and a graphite monochromator. The samples were investigated in the 2θ range of 10-80° at a scanning speed of 2 min-1. Fourier transformed infrared (FT-IR) spectra taken on a Bruker FT-IR spectrometer using KBr pellet technique. The morphologies and sizes of the resulting products were determined by field emission scanning electron microscope (TESCAN LYRA3, Czech Republic). The TEM images were recorded by using a transmission electron microscope (JEOL, JEM 2011) operated at 200 kV with a 4k × 4k CCD camera (Ultra Scan 400SP, Gatan). Energy dispersive X-ray spectra (EDS) were recorded using an X-mass detector, Oxford Instruments, equipped with the Lyra3 TESCAN FE-SEM. Photoluminescence (PL) measurements were performed with a spectrofluorometer (Fluorolog FL3-iHR, HORIBA Jobin Yvon, France). Thermal gravimetric analysis (TGA) was carried out in a thermogravimetric analyzer (Discovery, TA, USA). 5.0 mg sample was placed in an aluminum pan and heated from 30 ℃ to 600 ℃ under N2 at a heating rate of 10 ℃/ min. BET surface area measurements were carried out with a Tristar Ⅱ 3020 system. The powders were evacuated for 3 h at 300 ℃, and the N2 adsorption isotherms of the catalysts were obtained in liquid N2 (-196 ℃). The pore size distributions were obtained with the Barrett-Joyner-Halenda (BJH) formula. The chemical composition of the samples was investigated by X-ray photoelectron spectroscopy (XPS) using an X-ray photoelectron spectrometer (ESCALAB-250, Thermo-VG Scientific) with Al-Kα radiation (1486.6 eV).

Acknowledgment

The author would like to acknowledge the support provided by King Abdulaziz City for Science and Technology (KACST) through the Science & Technology Unit at King Fahd University of Petroleum & Minerals (KFUPM) for funding this work through project No. AT-32-21.

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