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Hydrothermal synthesis of prism-like mesocrystal CeO2
Journal of Alloys and Compounds 476 (2009) 958–962
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Hydrothermal synthesis of prism-like mesocrystal CeO2 Xiaowang Lu a , Xiazhang Li b , Feng Chen a , Chaoying Ni c , Zhigang Chen a,∗ a
School of Material Science and Engineering, Jiangsu Polytechnic University, Changzhou 213164, China School of Material Science and Engineering, Jiangsu University, Zhenjiang 212013, China c Department of Material Science and Engineering, University of Delaware, Newark 19716, USA b
a r t i c l e
i n f o
Article history: Received 7 July 2008 Received in revised form 1 September 2008 Accepted 29 September 2008 Available online 30 December 2008 Keywords: Prism-like Mesocrystal CeO2
a b s t r a c t Prism-like mesocrystal CeO2 were synthesized via a hydrothermal method. TG–DSC, powder X-ray diffraction (PXRD), TEM, selected electron diffraction (SAED), and HRTEM were employed to characterize the samples. The results showed that the products had prism-like morphology and owned mesocrystal structure. The effect of the molar ratios of HMT to Ce(NO3 )3 ·6H2 O was investigated. A possible formation mechanism was put forward that the nanocrystals aggregated along with the epitaxial orientation following the manner of coherent interface. The UV–vis adsorption spectrum exhibited the red-shift phenomenon compared with bulk CeO2 presumably due to the existence of considerable defects in particular twin boundaries in this unique structure. © 2008 Published by Elsevier B.V.
1. Introduction Ceria (CeO2 ) is one of the most reactive rare earth metal oxides, which is widely used in catalysis [1,2], polishing agents [3,4], solid oxide fuel cells (SOFC) [5,6], optical devices [7,8], and ceramic materials [9,10]. As a consequence, numerous techniques have been proposed to synthesize CeO2 materials including hydrothermal synthesis [11], coprecipitation [12], decomposition of oxalate precursors [13], sol–gel [14] and microemulsion [15] methods. Up to now, more and more efforts have been directed toward controlling the shape of the particles for their potential applications. CeO2 with a variety of morphologies including prism-like [16], nanorods [17,18], nanocubes [19,20], nanotubes [21,22], nanoplates [23], triangular and rhombic microplates [24,25] nanowire [20,26] have been successfully synthesized, most of which possessed polycrystalline or single crystalline structure. Recently, Cölfen et al. developed a previously unknown solid concept: mesocrystal, which is built up from nanometer-sized (anisotropic) primary particles. Mesocrystals have been proposed to grow via oriented aggregation of individual small particles, such that the overall orientation of the resulting larger crystals are single-crystal-like [27,28]. For instance, Mo et al. [29] synthesized mesocrystal ZnO microtubules, providing fundamental model systems to load noble metal nanoparticles for catalytic and electro catalytic applications; Li et al. [30] synthesized hollow zinc oxide
∗ Corresponding author. Tel.: +86 519 86330002; fax: +86 519 86330066. E-mail address: [email protected] (Z. Chen). 0925-8388/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.jallcom.2008.09.198
mesocrystals, which may be useful for several applications in particular catalysis. However, to the best of author’s knowledge, there has been few reports regarding prism-like mesocrystal CeO2 . Herein, in this work, we report the fabrication of the prism-like mesocrystal CeO2 via a hydrothermal method as well as the exploration of its red-shift phenomenon.
2. Experimental All used reagents were of analytic purity, obtained from Shanghai Chemical Reagent Ltd. Co. of China, without further purification. Under typical synthetic conditions, 0.008 mol of Ce(NO)3 ·6H2 O(3.47 g) was dissolved into 15 ml deionized water and the quantitative amount of hexamethylenetetramine (HMT) was dissolved into 15 ml deionized water. Then the two aqueous solutions were mixed. The mixture was magnetic stirred for 1 h before being transferred into a 50-ml Teflon-lined autoclave. Then the autoclave was sealed and kept at 200 ◦ C for 10 h, and subsequently cooled down to room temperature naturally. The precipitate was filtered off, washed with deionized water and absolute ethanol three times, and dried in vacuum at 80 ◦ C for 10 h. Finally, the product was calcined with a muffle furnace at 300 ◦ C in air for 3 h. Thermogravimetric (TG) analysis and differential scanning calorimetery (DSC) were carried out by a thermal analyser (Q600-TGA/DSC TA) heating from room temperature to 600 ◦ C (10 ◦ C/min)in air atmosphere. Powder X-ray diffraction (PXRD) measurement was performed on a Rigaku X-ray diffractometer with Cu K␣ radiation (Rigaku, D/max-RB). The morphology of prism-like cerium compound was observed by transmission electron microscope (PHILIPS, Tecnai12, 120 kV), field emission transmission election microscope (JEOL, JEM2010F, 200 kV) field-emission scanning electron microscope (PHILIPS, XL-30ESEM, 30 kV). The ultraviolet–visible (UV–vis) diffuse reflectance spectra were carried out using a spectrophotometer (Shimadzu, UV-2450) and the analyzed range was 200–600 nm.
X. Lu et al. / Journal of Alloys and Compounds 476 (2009) 958–962
Fig. 1. TG–DSC curve of the cerium oxide precursor (the molar ratio of HMT to Ce(NO3 )3 ·6H2 O is 5:1).
3. Results and discussion The TG–DSC results of the thermal decomposition process for cerium oxide precursor (the molar ratio of HMT to Ce(NO3 )3 ·6H2 O
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Fig. 2. PXRD patterns of (a) as-prepared CeCO3 OH and (b) the calcined samples CeO2 .
is 5:1) is shown in Fig. 1. It can be found that there is an obvious endothermic peak around 283.7 ◦ C, associating with sharp weight loss on the TG curve, which is attributed to the decomposition of cerium oxide precursor and formation of CeO2 . We can also find that
Fig. 3. (a) TEM images of prism-like CeO2 ; (b)FESEM image; (c) HRTEM image (inset: FFT) of a particle edge; (d) SAED image of prism-like mesocrystal CeO2 (the molar ratio of HMT to Ce(NO3 )3 ·6H2 O is 5:1).
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X. Lu et al. / Journal of Alloys and Compounds 476 (2009) 958–962
there appears an endothermic peak around 91.1 ◦ C on the DSC, associating with the low weight loss on the TG curve, which is related to the dehydration of adsorption water. According to the above analysis, the calcination temperature for cerium oxide precursor was chosen to be at 300–400 ◦ C. Fig. 2 shows the PXRD patterns of the precursor and calcined samples. The precursor samples after hydrothermal treatment have the hexagonal crystal structure shown in Fig. 2(a), which have the same CeCO3 OH phases reported in JCPDS card (No. 32-0189). The samples calcined at 300 ◦ C for 3 h are consistent with TG–DSC and all the diffraction peaks shown in Fig. 2(b) are well assigned to cubic center fluorite phase CeO2 reported in JCPDS card (No. 340394). No obvious peaks for other elements or impurities were observed. Fig. 3 demonstrates the morphology and the structure images of the prism-like mesocrystal CeO2 materials. Fig. 3(a) and (b) shows the TEM and its corresponding FESEM images. It can be found that the mean diameter and length of the prism are about 300 nm and 700 nm, respectively. Fig. 3(c) shows the HRTEM image (inset: FFT) of a particle edge and Fig. 3(d) shows selected electron diffraction (SAED) pattern of prism-like CeO2 . It can be seen that the diffraction patterns are neither the typical polycrystal rings, nor the typical single crystal dots. The patterns here are elongated dots, implying that the particles are comprised of many small crystals which attached with each other on the same orientation; it is termed as mesocrystal according to the literature [27,28]. As can be known, the morphology, the crystal size and the physicochemical nature of cerium oxide can be easily controlled by using hydroxycarbonates as decomposition precursors. When heating above 150 ◦ C, HMT hydrolyze to form NH3 and CO3 2− , the carbonate and hydroxide ion react with Ce3+ to form CeCO3 OH. In the hydrothermal system, trivalent Ce3+ (aq) has a strong affinity with OH− (aq). The cation thus combine with OH− (aq), forming the Ce(OH)2+ (aq)polyatomic group. At elevated temperatures, CO3 2− bond with the positive-charged groups to yield the solid CeCO3 OH at super saturation [31] (Eq. (1)) Ce3+ + CO3 2− + OH− → CeCO3 OH
(1)
The phase transformation of CeCO3 OH into CeO2 after calcination can be elucidated by the following equation: 4CeCO3 OH + O2 → 4CeO2 + 4CO2 + 2H2 O
(2)
The HMT quantities cause the transformation from irregular shape to prism-like and subsequently to spheres. When the molar ratio of HMT to Ce(NO3 )3 ·6H2 O is 3:1, the samples are all irregular shape CeO2 (Fig. 4(a)) the corresponding electron diffraction pattern exhibits that they are polycrystalline; however, when the molar ratio reaches 8:1, the samples are all spherical CeO2 materials (Fig. 4(b)),the corresponding selected area electron diffraction pattern shows they are polycrystalline as well. We consider that HMT behaves not only as a mineralizer but also as a surfactant in the hydrothermal process. HMT hydrolyzes to form NH3 , subsequently hydrolyzes to form OH− ions, which are responsible for the shape evolution. Agglomeration of nanocrystals is a very common occurring phenomenon because the nanocrystals tend to decrease the exposed surface in order to lower the surface energy. In principle, when two crystals are in contact, likely they tend to rotate with each other to minimize the interface strain energy. Therefore, the same types of crystal plane tend to align with each other, forming a coherent interface to decreasing interface energy [32]. According to the thermodynamic theory [33], the supersaturation degree has a significant influence on crystal nucleation rate and crystal growth rate. As the molar ratio of HMT to Ce(NO3 )3 ·6H2 O being 3:1, namely with
Fig. 4. (a) TEM image of irregular CeO2 and the corresponding electron diffraction pattern (the molar ratio of HMT to Ce(NO3 )3 ·6H2 O is 3:1). (b) TEM image of spherical CeO2 and the corresponding electron diffraction pattern (the molar ratio of HMT to Ce(NO3 )3 ·6H2 O is 8:1).
the relatively low concentration of OH− ions, the crystal nucleation rate and crystal growth rate are low, and there is not sufficient driven force in the reaction system, resulting that the small particles aggregate disorderly, as they are shown to be polycrystals. On the contrary, when the molar ratio reaches 8:1, the solution involves abundant OH− ions, which consequently has much higher crystal nucleation rate due to the existence more nucleus clusters in solution, however, crystal growth rate is inhibited, leading that the small crystals aggregate randomly as well correspondingly exhibiting polycrystals. Therefore, the molar ratio of 5:1 was the optimal parameter for crystal nucleation rate and crystal growth rate, the individual small crystals aggregate oriented along epitaxial direction, which form prism-like mesocrystal CeO2 . Nevertheless, the formation mechanism of prism-like mesocrystal CeO2 needs further investigation.
X. Lu et al. / Journal of Alloys and Compounds 476 (2009) 958–962
Fig. 5. UV–vis absorption spectra of CeO2 .
The UV–vis diffuse reflectance spectra of the samples are shown in Fig. 5. The UV–vis absorption spectra of spectrally pure CeO2 powder (∼500 nm) referred to as bulk CeO2 and the prism-like CeO2 are shown in Fig. 5. An adsorption band centred at 325 nm for bulk CeO2 is broadened and red shifts about 20–345 nm for prism-like CeO2 .
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Compared with the absorption and emission spectra of nanomaterials, a blue-shift phenomenon has been observed which could be explained by the quantum confinement effect. As for the red-shift, it could be attributed to electron–phonon coupling. Fundamentally, the electron–phonon coupling coefficients are proposed to increase with decreasing size of semiconductors [34]. In certain systems, electron–phonon coupling could be strong enough to overcome the spatial confinement to determine the energy of excitons. It determines or modifies the effective mass of carriers and the style of carrier scattering by the lattice, leading to a red-shift of the emission band [35]. According to the literature, CeO2 have a strong electron–phonon coupling effect [36]. Prism-like mesocrystal CeO2 have specific oriented aggregation of individual nanocrystals, leading to the existence of a large number of defects. In particular, a twin boundary is also formed between the two nanocrystals, which tend to have lower energy than the conventional grain boundary especially for the highly oriented nanocrystals [32]. These may explain the red-shift of the absorption edges of the samples in the UVdiffuse reflectance spectra. An estimate of the optical band gap Eg can be determined by the following equation for a semiconductor: (˛h)n = B(h − Eg ), where h is the photon energy, ˛ is the absorption coefficient, B is a constant relative to the material, and n is 2 for a direct transition. The plots of (˛h)2 versus photon energy for the samples are shown in Fig. 6(a) and (b). It reveals that the band gap of the prism-like mesocrystal CeO2 is about 3.02 eV, which is smaller than the value for the bulk CeO2 (Eg = 3.19 eV). It could be ascribed to the coexistence of abundant defects in such prism-like mesocrystal CeO2 . Prism-like mesocrystal CeO2 may have a potential application in the optical devices and necessarily need to be further studied [36,37]. 4. Conclusions In summary, prism-like mesocrystal CeO2 are successfully synthesized via a hydrothermal method. The effect of the molar ratios of HMT to Ce(NO3 )3 ·6H2 O on the morphology of the products is investigated. On the optimal molar ratio of 5:1, the samples are prism-like mesocrystal CeO2 . However, when the molar ratio is 3:1 the samples are all irregular shape CeO2 , which own polycrystals structure. When the molar ratio of the two reaches 8:1 the samples are all spherical shape CeO2 and they possess polycrystals as well. A possible formation mechanism of prism-like mesocrystal CeO2 is concluded on account of the nanocrystals aggregation along with the epitaxial orientation. The UV–vis absorption spectra of prism-like CeO2 shows unusual red-shift compared with that of bulk CeO2 . The existence of considerable defects especially twin boundaries may be responsible for this phenomenon and prismlike mesocrystal CeO2 may offer a potentiality application in optical apparatus. Acknowledgment This work was financially supported by National Natural Science Foundation of China (NSFC) under the contract (NO. 20771047). References
Fig. 6. (a) Plots of (˛h)2 vs. photon energy for spectrally pure CeO2 powder and (b) plots of (˛h)2 vs. photon energy for prism-like CeO2 .
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Hydrothermal synthesis of prism-like mesocrystal CeO2 Xiaowang Lu a , Xiazhang Li b , Feng Chen a , Chaoying Ni c , Zhigang Chen a,∗ a
School of Material Science and Engineering, Jiangsu Polytechnic University, Changzhou 213164, China School of Material Science and Engineering, Jiangsu University, Zhenjiang 212013, China c Department of Material Science and Engineering, University of Delaware, Newark 19716, USA b
a r t i c l e
i n f o
Article history: Received 7 July 2008 Received in revised form 1 September 2008 Accepted 29 September 2008 Available online 30 December 2008 Keywords: Prism-like Mesocrystal CeO2
a b s t r a c t Prism-like mesocrystal CeO2 were synthesized via a hydrothermal method. TG–DSC, powder X-ray diffraction (PXRD), TEM, selected electron diffraction (SAED), and HRTEM were employed to characterize the samples. The results showed that the products had prism-like morphology and owned mesocrystal structure. The effect of the molar ratios of HMT to Ce(NO3 )3 ·6H2 O was investigated. A possible formation mechanism was put forward that the nanocrystals aggregated along with the epitaxial orientation following the manner of coherent interface. The UV–vis adsorption spectrum exhibited the red-shift phenomenon compared with bulk CeO2 presumably due to the existence of considerable defects in particular twin boundaries in this unique structure. © 2008 Published by Elsevier B.V.
1. Introduction Ceria (CeO2 ) is one of the most reactive rare earth metal oxides, which is widely used in catalysis [1,2], polishing agents [3,4], solid oxide fuel cells (SOFC) [5,6], optical devices [7,8], and ceramic materials [9,10]. As a consequence, numerous techniques have been proposed to synthesize CeO2 materials including hydrothermal synthesis [11], coprecipitation [12], decomposition of oxalate precursors [13], sol–gel [14] and microemulsion [15] methods. Up to now, more and more efforts have been directed toward controlling the shape of the particles for their potential applications. CeO2 with a variety of morphologies including prism-like [16], nanorods [17,18], nanocubes [19,20], nanotubes [21,22], nanoplates [23], triangular and rhombic microplates [24,25] nanowire [20,26] have been successfully synthesized, most of which possessed polycrystalline or single crystalline structure. Recently, Cölfen et al. developed a previously unknown solid concept: mesocrystal, which is built up from nanometer-sized (anisotropic) primary particles. Mesocrystals have been proposed to grow via oriented aggregation of individual small particles, such that the overall orientation of the resulting larger crystals are single-crystal-like [27,28]. For instance, Mo et al. [29] synthesized mesocrystal ZnO microtubules, providing fundamental model systems to load noble metal nanoparticles for catalytic and electro catalytic applications; Li et al. [30] synthesized hollow zinc oxide
∗ Corresponding author. Tel.: +86 519 86330002; fax: +86 519 86330066. E-mail address: [email protected] (Z. Chen). 0925-8388/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.jallcom.2008.09.198
mesocrystals, which may be useful for several applications in particular catalysis. However, to the best of author’s knowledge, there has been few reports regarding prism-like mesocrystal CeO2 . Herein, in this work, we report the fabrication of the prism-like mesocrystal CeO2 via a hydrothermal method as well as the exploration of its red-shift phenomenon.
2. Experimental All used reagents were of analytic purity, obtained from Shanghai Chemical Reagent Ltd. Co. of China, without further purification. Under typical synthetic conditions, 0.008 mol of Ce(NO)3 ·6H2 O(3.47 g) was dissolved into 15 ml deionized water and the quantitative amount of hexamethylenetetramine (HMT) was dissolved into 15 ml deionized water. Then the two aqueous solutions were mixed. The mixture was magnetic stirred for 1 h before being transferred into a 50-ml Teflon-lined autoclave. Then the autoclave was sealed and kept at 200 ◦ C for 10 h, and subsequently cooled down to room temperature naturally. The precipitate was filtered off, washed with deionized water and absolute ethanol three times, and dried in vacuum at 80 ◦ C for 10 h. Finally, the product was calcined with a muffle furnace at 300 ◦ C in air for 3 h. Thermogravimetric (TG) analysis and differential scanning calorimetery (DSC) were carried out by a thermal analyser (Q600-TGA/DSC TA) heating from room temperature to 600 ◦ C (10 ◦ C/min)in air atmosphere. Powder X-ray diffraction (PXRD) measurement was performed on a Rigaku X-ray diffractometer with Cu K␣ radiation (Rigaku, D/max-RB). The morphology of prism-like cerium compound was observed by transmission electron microscope (PHILIPS, Tecnai12, 120 kV), field emission transmission election microscope (JEOL, JEM2010F, 200 kV) field-emission scanning electron microscope (PHILIPS, XL-30ESEM, 30 kV). The ultraviolet–visible (UV–vis) diffuse reflectance spectra were carried out using a spectrophotometer (Shimadzu, UV-2450) and the analyzed range was 200–600 nm.
X. Lu et al. / Journal of Alloys and Compounds 476 (2009) 958–962
Fig. 1. TG–DSC curve of the cerium oxide precursor (the molar ratio of HMT to Ce(NO3 )3 ·6H2 O is 5:1).
3. Results and discussion The TG–DSC results of the thermal decomposition process for cerium oxide precursor (the molar ratio of HMT to Ce(NO3 )3 ·6H2 O
959
Fig. 2. PXRD patterns of (a) as-prepared CeCO3 OH and (b) the calcined samples CeO2 .
is 5:1) is shown in Fig. 1. It can be found that there is an obvious endothermic peak around 283.7 ◦ C, associating with sharp weight loss on the TG curve, which is attributed to the decomposition of cerium oxide precursor and formation of CeO2 . We can also find that
Fig. 3. (a) TEM images of prism-like CeO2 ; (b)FESEM image; (c) HRTEM image (inset: FFT) of a particle edge; (d) SAED image of prism-like mesocrystal CeO2 (the molar ratio of HMT to Ce(NO3 )3 ·6H2 O is 5:1).
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X. Lu et al. / Journal of Alloys and Compounds 476 (2009) 958–962
there appears an endothermic peak around 91.1 ◦ C on the DSC, associating with the low weight loss on the TG curve, which is related to the dehydration of adsorption water. According to the above analysis, the calcination temperature for cerium oxide precursor was chosen to be at 300–400 ◦ C. Fig. 2 shows the PXRD patterns of the precursor and calcined samples. The precursor samples after hydrothermal treatment have the hexagonal crystal structure shown in Fig. 2(a), which have the same CeCO3 OH phases reported in JCPDS card (No. 32-0189). The samples calcined at 300 ◦ C for 3 h are consistent with TG–DSC and all the diffraction peaks shown in Fig. 2(b) are well assigned to cubic center fluorite phase CeO2 reported in JCPDS card (No. 340394). No obvious peaks for other elements or impurities were observed. Fig. 3 demonstrates the morphology and the structure images of the prism-like mesocrystal CeO2 materials. Fig. 3(a) and (b) shows the TEM and its corresponding FESEM images. It can be found that the mean diameter and length of the prism are about 300 nm and 700 nm, respectively. Fig. 3(c) shows the HRTEM image (inset: FFT) of a particle edge and Fig. 3(d) shows selected electron diffraction (SAED) pattern of prism-like CeO2 . It can be seen that the diffraction patterns are neither the typical polycrystal rings, nor the typical single crystal dots. The patterns here are elongated dots, implying that the particles are comprised of many small crystals which attached with each other on the same orientation; it is termed as mesocrystal according to the literature [27,28]. As can be known, the morphology, the crystal size and the physicochemical nature of cerium oxide can be easily controlled by using hydroxycarbonates as decomposition precursors. When heating above 150 ◦ C, HMT hydrolyze to form NH3 and CO3 2− , the carbonate and hydroxide ion react with Ce3+ to form CeCO3 OH. In the hydrothermal system, trivalent Ce3+ (aq) has a strong affinity with OH− (aq). The cation thus combine with OH− (aq), forming the Ce(OH)2+ (aq)polyatomic group. At elevated temperatures, CO3 2− bond with the positive-charged groups to yield the solid CeCO3 OH at super saturation [31] (Eq. (1)) Ce3+ + CO3 2− + OH− → CeCO3 OH
(1)
The phase transformation of CeCO3 OH into CeO2 after calcination can be elucidated by the following equation: 4CeCO3 OH + O2 → 4CeO2 + 4CO2 + 2H2 O
(2)
The HMT quantities cause the transformation from irregular shape to prism-like and subsequently to spheres. When the molar ratio of HMT to Ce(NO3 )3 ·6H2 O is 3:1, the samples are all irregular shape CeO2 (Fig. 4(a)) the corresponding electron diffraction pattern exhibits that they are polycrystalline; however, when the molar ratio reaches 8:1, the samples are all spherical CeO2 materials (Fig. 4(b)),the corresponding selected area electron diffraction pattern shows they are polycrystalline as well. We consider that HMT behaves not only as a mineralizer but also as a surfactant in the hydrothermal process. HMT hydrolyzes to form NH3 , subsequently hydrolyzes to form OH− ions, which are responsible for the shape evolution. Agglomeration of nanocrystals is a very common occurring phenomenon because the nanocrystals tend to decrease the exposed surface in order to lower the surface energy. In principle, when two crystals are in contact, likely they tend to rotate with each other to minimize the interface strain energy. Therefore, the same types of crystal plane tend to align with each other, forming a coherent interface to decreasing interface energy [32]. According to the thermodynamic theory [33], the supersaturation degree has a significant influence on crystal nucleation rate and crystal growth rate. As the molar ratio of HMT to Ce(NO3 )3 ·6H2 O being 3:1, namely with
Fig. 4. (a) TEM image of irregular CeO2 and the corresponding electron diffraction pattern (the molar ratio of HMT to Ce(NO3 )3 ·6H2 O is 3:1). (b) TEM image of spherical CeO2 and the corresponding electron diffraction pattern (the molar ratio of HMT to Ce(NO3 )3 ·6H2 O is 8:1).
the relatively low concentration of OH− ions, the crystal nucleation rate and crystal growth rate are low, and there is not sufficient driven force in the reaction system, resulting that the small particles aggregate disorderly, as they are shown to be polycrystals. On the contrary, when the molar ratio reaches 8:1, the solution involves abundant OH− ions, which consequently has much higher crystal nucleation rate due to the existence more nucleus clusters in solution, however, crystal growth rate is inhibited, leading that the small crystals aggregate randomly as well correspondingly exhibiting polycrystals. Therefore, the molar ratio of 5:1 was the optimal parameter for crystal nucleation rate and crystal growth rate, the individual small crystals aggregate oriented along epitaxial direction, which form prism-like mesocrystal CeO2 . Nevertheless, the formation mechanism of prism-like mesocrystal CeO2 needs further investigation.
X. Lu et al. / Journal of Alloys and Compounds 476 (2009) 958–962
Fig. 5. UV–vis absorption spectra of CeO2 .
The UV–vis diffuse reflectance spectra of the samples are shown in Fig. 5. The UV–vis absorption spectra of spectrally pure CeO2 powder (∼500 nm) referred to as bulk CeO2 and the prism-like CeO2 are shown in Fig. 5. An adsorption band centred at 325 nm for bulk CeO2 is broadened and red shifts about 20–345 nm for prism-like CeO2 .
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Compared with the absorption and emission spectra of nanomaterials, a blue-shift phenomenon has been observed which could be explained by the quantum confinement effect. As for the red-shift, it could be attributed to electron–phonon coupling. Fundamentally, the electron–phonon coupling coefficients are proposed to increase with decreasing size of semiconductors [34]. In certain systems, electron–phonon coupling could be strong enough to overcome the spatial confinement to determine the energy of excitons. It determines or modifies the effective mass of carriers and the style of carrier scattering by the lattice, leading to a red-shift of the emission band [35]. According to the literature, CeO2 have a strong electron–phonon coupling effect [36]. Prism-like mesocrystal CeO2 have specific oriented aggregation of individual nanocrystals, leading to the existence of a large number of defects. In particular, a twin boundary is also formed between the two nanocrystals, which tend to have lower energy than the conventional grain boundary especially for the highly oriented nanocrystals [32]. These may explain the red-shift of the absorption edges of the samples in the UVdiffuse reflectance spectra. An estimate of the optical band gap Eg can be determined by the following equation for a semiconductor: (˛h)n = B(h − Eg ), where h is the photon energy, ˛ is the absorption coefficient, B is a constant relative to the material, and n is 2 for a direct transition. The plots of (˛h)2 versus photon energy for the samples are shown in Fig. 6(a) and (b). It reveals that the band gap of the prism-like mesocrystal CeO2 is about 3.02 eV, which is smaller than the value for the bulk CeO2 (Eg = 3.19 eV). It could be ascribed to the coexistence of abundant defects in such prism-like mesocrystal CeO2 . Prism-like mesocrystal CeO2 may have a potential application in the optical devices and necessarily need to be further studied [36,37]. 4. Conclusions In summary, prism-like mesocrystal CeO2 are successfully synthesized via a hydrothermal method. The effect of the molar ratios of HMT to Ce(NO3 )3 ·6H2 O on the morphology of the products is investigated. On the optimal molar ratio of 5:1, the samples are prism-like mesocrystal CeO2 . However, when the molar ratio is 3:1 the samples are all irregular shape CeO2 , which own polycrystals structure. When the molar ratio of the two reaches 8:1 the samples are all spherical shape CeO2 and they possess polycrystals as well. A possible formation mechanism of prism-like mesocrystal CeO2 is concluded on account of the nanocrystals aggregation along with the epitaxial orientation. The UV–vis absorption spectra of prism-like CeO2 shows unusual red-shift compared with that of bulk CeO2 . The existence of considerable defects especially twin boundaries may be responsible for this phenomenon and prismlike mesocrystal CeO2 may offer a potentiality application in optical apparatus. Acknowledgment This work was financially supported by National Natural Science Foundation of China (NSFC) under the contract (NO. 20771047). References
Fig. 6. (a) Plots of (˛h)2 vs. photon energy for spectrally pure CeO2 powder and (b) plots of (˛h)2 vs. photon energy for prism-like CeO2 .
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