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Synthesis and characterization of mesoporous CeO2 nanotube arrays
Microporous and Mesoporous Materials 171 (2013) 196–200
Contents lists available at SciVerse ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Short Communication
Synthesis and characterization of mesoporous CeO2 nanotube arrays Tong Wang, Lide Zhang ⇑, Junxi Zhang, Guomin Hua Key Laboratory of Materials Physics and Anhui Key Laboratory of Nanomaterials and Nanostructures, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, 230031 Anhui, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 3 July 2012 Received in revised form 19 November 2012 Accepted 20 December 2012 Available online 28 December 2012 Keywords: Mesoporous CeO2 Nanotube arrays Reductive activity
a b s t r a c t Mesoporous CeO2 nanotube arrays were fabricated by the following procedure: ZnO nanorod arrays were prepared as templates, and then the templates were in turn submerged in the NaOH solution, deionized water and the Ce(NO3)3 solution at 60 °C for 20 times. After annealed at 500 °C for 30 min, the mesoporous polycrystalline CeO2 nanoshells covered ZnO nanorod arrays were obtained. Through HNO3 treatments, ZnO nanorods were dissolved, as a result, the mesoporous CeO2 nanotube arrays were achieved. The XPS result demonstrates that the ratio of Ce3+ ions to Ce4+ ions is about 1:2. Nitrogen adsorption–desorption experiment results give that the specific surface area is 109 m2/g. By using the mesoporous CeO2 nanotubes as the electrode in the H2O2 solution, the electrochemical experiment proves that these arrays have the reduction action. The formation mechanism of mesopores in the CeO2 nanotubes and the origin of the reduction action were discussed in detail. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction CeO2 as an important catalytic and reduction agent has been well recognized, and it has been widely used in various application fields, such as purification of exhaust gas antioxidants in biomedicine [1–3], in three-way automotive catalytic converters [4], and solid oxide fuel cells [5,6]. Many authors have made lots of works on the synthesis of nano-CeO2 and the study of catalytic properties [7–16]. Recently, Hua et al. [17] synthesize mesoporous CeO2 nanotubes. It is found that these nanotubes exhibit excellent reduction ability in the process of CO ? CO2 transition. They found that this high reduction activity was closely associated with the existence of large numbers of Ce3+ ions in the surface of mesoporous CeO2 nanotubes and formation of lots of Ce3+ ions was attributed to large numbers of oxygen vacancies and vacancy groups in these CeO2 nanotubes. Skorodumova et al. [14] theoretically suggested that oxygen-vacancy formation could induce valence transition of Ce ions in CeO2 from +4 to +3 because oxygen escapes from lattice sites, leaving vacancy sites with two electrons at each vacancy. Namely, this transition can be expressed as follows: 2Ce4+ + 2e ? Ce3+. This is why mesoporous CeO2 nanotubes possess the high reduction activity. However, easy aggregation of CeO2 nanotubes of random distribution will lead the reduction ability to be decreased, resulting in losing repeated application value. In order to overcome the aggregation disadvantage and keep the strong reduction ability, we synthesized mesoporous CeO2 nanotube arrays with macro-scale. In order to investigate the ⇑ Corresponding author. Tel.: +86 551 591420; fax: +86 551 591434. E-mail address: [email protected] (L. Zhang). 1387-1811/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2012.12.028
reduction activity of mesoporous CeO2 nanotube arrays, according to the literature [20], we used the mesoporous CeO2 nanotube arrays on the ITO layer as electrode to detect trace H2O2. In the literature [20], Ispas et al. [20] reported that they utilized the catalytic properties of CeO2 for the development of a highly sensitive, simple and inexpensive H2O2 sensor in sensing and biosensing applications. In this paper, we used ZnO nanorod arrays as the templates and synthesized the mesoporous CeO2 nanotube arrays. The formation mechanism of mesopores was discussed. The strong reduction ability was proved through H2O2 decomposition experiments. Based on this, the reduction mechanism associated with the mesoporous structure was analyzed. 2. Experimental 2.1. Synthesis The formation process of mesoporous CeO2 nanotube arrays is shown in Fig. 1. The ZnO nanorod arrays were fabricated as templates via the seeding layer and aqueous solution route [18,19]. The 5 mL of 0.005 M zinc acetate alcohol solution was deposited uniformly on ITO for 5 times. After annealed at 350 °C in air for 30 min, the seed layer was prepared. Then, the ITO with seed layer was put into an aqueous solution of zinc acetate (Zn(Ac)22H2O, 0.02 M) and hexamethylenetetramine (HMTA, C6H12N4, 0.02 M), which was stirred at 75 °C for 6 h. The product was then taken out and washed using distilled water. As a result, the ZnO nanorod arrays were obtained. The assemblies of mesoporous polycrystalline CeO2 nanoshells on the ZnO nanorod arrays were prepared
T. Wang et al. / Microporous and Mesoporous Materials 171 (2013) 196–200
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Fig. 1. Schematic illustration of formation process of the mesoporous CeO2 nanotube arrays via ZnO nanoarray template synthesis together with ultrasonic assisted SILAR covering method. (a) ZnO nanorod array; (b) ZnO/CeO2 nanorod array; (c) Mesoporous ZnO/CeO2 nanorod array; and (d) Mesoporous CeO2 nanotube array.
by the successive ionic layer adsorption and reaction (SILAR) method, along with annealing treatment. The detailed procedure is as follows: ZnO nanorod arrays were submerged in a 0.05 M NaOH solution for 20 seconds. Subsequently, the nanorod arrays were further submerged in deionized water for 10 seconds. Then, the nanorod arrays were further submerged in a 0.05 M Ce(NO3)3 solution for 20 seconds. After that, the ZnO nanorod arrays were submerged in deionized water for 10 seconds. The above operations were taken in 60 °C in the thermostatic water tank, and the process is referred to as one deposition cycle. To obtain the ideal CeO2 nanoshells covered ZnO nanorod arrays, the above deposition cycle was repeated for about 20 times. As a result, the Ce(OH)3 covered ZnO nanorod arrays were obtained. This deposition product was annealed at 500 °C for 30 min to get mesoporous polycrystalline CeO2 nanoshells covered ZnO nanorod arrays. Then they were treated by 5% HNO3 solution to remove the ZnO nanorod array core and were purged in deionized water several times to obtain final products. Characterization by several techniques proves that the finally obtained product is mesoporous CeO2 nanotube arrays.
by mixing stock solutions of Na2HPO4 and KH2PO4. The mesoporous CeO2 nanotube array was used as the working electrode. All the potentials were recorded and reported vs. a saturated calomel electrode and all the experiments were carried out at room temperature. Cyclic voltammetry experiments were performed in unstirred air-saturated solutions at room temperature at a scan rate of 50 mV/s. Scanning range from 1.5 V to 1.5 V. 3. Results and discussion X-ray diffraction patterns of the products obtained are shown in Fig. 2, it is found that the ZnO nanorod arrays are of the wurtzite structure, furthermore, the obtained mesoporous CeO2 nanotube
2.2. Characterization Phase identification was performed with a powder X-ray diffractometer (XRD, Philips X’Pert) using Cu-Ka (0.15419 nm) radiation. Field emission scanning electron microscopy (FESEM, Sirion 200) was used to observe the morphologies. Transmission electron microscopy (TEM JEM-2010) was used to examine microstructures. The nitrogen adsorption–desorption experiment was carried out on a Surface Area and Porosity Analyzer (Omnisorp 100X), and from obtained adsorption–desorption isotherms, the specific surface area and the pore size distribution in the CeO2 nanotube walls, and inner diameters of the CeO2 nanotubes were obtained by calculation. Energy dispersive X-ray (EDX, Inca Oxford) analysis was conducted to determine the element composition. High resolution X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB 250. Thermal analysis was performed on a Thermogravimetry/Differential thermal analyzer (TGA–DTA, DTG-60H) and a Differential scanning calorimeter (DSC-60) to analyze the formation process of mesoporous CeO2 nanotube arrays from the Ce(OH)3 covered ZnO nanorod arrays. 2.3. Reductive activity measurement The reductive activity measurements were conducted in the H2O2 solution with electrochemical workstation (Zahner IM6ex, Germany). A 1/15 M phosphate buffer (PB) solution was prepared
Fig. 2. X-ray diffraction patterns of ZnO nanorod arrays obtained by the seeding layer and aqueous solution route and the mesoporous CeO2 nanotube arrays after the removal of ZnO nanorod arrays, respectively.
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Fig. 4. TEM images of mesoporous CeO2 nanotubes. (a) Low magnification TEM image; (b) High-magnification image; (c) The corresponding SAED patterns.
Fig. 3. (a) SEM images of ZnO nanorod arrays; (b) TEM image of the dispersive ZnO nanorods; (c) SEM images of mesoporous CeO2 nanotubes.
arrays are only the fluorite CeO2 phase. The morphologies of the ZnO nanorod arrays and the as-prepared mesoporous CeO2 nanotube arrays are shown in Fig. 3. It is clear that the diameter of ZnO nanorods is about 20 nm. Fig. 4a and b present the TEM images of nanostructured CeO2. From Fig. 4b, it can be seen that the inner diameter and the wall thickness of each CeO2 nanotube are about 25 and 6 nm, respectively. The nanostructured CeO2 is in the form of nanotube, and the inner diameter of nanotubes and wall thickness of each CeO2 nanotube are about 25 and 6 nm, respectively. As shown in the high-magnification TEM image in Fig. 4b, walls of CeO2 nanotube arrays show mesoporous structures. The selected area electron diffraction (SAED) pattern reveals that the mesoporous CeO2 nanotube arrays are polycrystalline (Fig. 4c). Steps and corners are formed on the surfaces of nanotubes. The exposed major facets are the (1 1 1) planes and (1 0 0)
Fig. 5. The structural feature of ceria nanotubes after calcinations at 500 °C. (a) Nitrogen adsorption–desorption isotherms; (b) Mesopore size distribution calculated by the adsorption branch of the isotherms.
planes. More importantly, the high density of grain boundaries is observed. The existence of mesopores in mesoporous CeO2 nanotube walls can be further proved by the nitrogen adsorption– desorption experiment. The nitrogen adsorption–desorption iso-
T. Wang et al. / Microporous and Mesoporous Materials 171 (2013) 196–200
Fig. 6. XPS spectra of mesoporous CeO2 nanotube arrays. The Ce 3d core level spectrum curve A and fitting results, curve B represents the background of the spectra, curves C–G denote Ce 3d 5/2, and curves H–L denote Ce 3d 3/2, where the fitting peaks of curves C, E, H and J belong to Ce3+ ions, and fitting peaks of curves D, F, G, I, K and L belong to Ce4+ ions.
Fig. 7. Thermal analysis of the formation process of mesoporous CeO2 nanotube arrays from the Ce(OH)3 covered ZnO nanorod arrays.
therms for CeO2 nanoarrays are shown in Fig. 5a. The H2-type hysteresis loop in the adsorption–desorption isotherm suggests the mesoporous feature of CeO2 nanotubes. Fig. 5b gives the pore size distribution calculated by the adsorption branch of the isotherms. This result indicates that the pore sizes are mainly distributed between 2 and 30 nm. And the pores with <20 nm in diameter are much more than those with 20–30 nm in diameter. Fig. 4b indicates that inner diameter of CeO2 nanotubes is about 25 nm, and thus it can be deduced that the 20–30 nm pore diameter corresponds to the inner diameter of nanotube. Therefore, the pores with <20 nm in diameter correspond to the pores in CeO2 nanotube walls. The specific surface area of CeO2 nanotubes is 109 m2/g, which was obtained by calculation according to the Brunauer–Emmett–Teller (BET) method for the nitrogen adsorption–desorption isotherms of CeO2 nanotubes after calcinations at 500 °C. Clearly, this large specific surface area is closely related to the existence of large numbers of mesopores in the tube walls. Therefore, it can be deduced that these small pores are located in the nanotube walls. In Fig. 4b, many pores in the nanotube wall can be seen. This further proves that many mesopores exist in the nanotube walls. Fig. 6 demonstrates the X-ray Photoelectron Spectroscopy (XPS) spectra of the mesoporous CeO2 nanotube arrays. The Ce3d core level spectrum (curve A) and fitting results are as follows, C
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Fig. 8. Cyclic voltammogram of the CeO2 nanotube arrays for the H2O2 solution with different concentrations, 0, 0.2, 2.0 and 10 m ML 1, respectively.
(880.5 eV), D (882.7 eV), E (885.5 eV), F (889.2 eV), and G (898.3 eV) peaks correspond to Ce 3d 5/2, and H (899.1 eV), I (901.2 eV), J (903.6 eV), K (907.4 eV), and L (916.8 eV) peaks correspond to Ce 3d 3/2, where the fitting peaks of C, E, H and J curves belong to Ce3+ ions, and fitting peaks of D, F, G, I, K and L curves belong to Ce4+ ions. The concentration of Ce3+ ions reaches 34% of the total cerium ions, and the ratio of Ce3+ to Ce4+ is about 1:2. The high concentration of Ce3+ ions in mesoporous CeO2 nanotube arrays is related to the negatively charged active oxygen species. From Fig. 7, the formation process of mesoporous CeO2 nanotube arrays can be analyzed as follows: when the Ce(OH)3 covered ZnO nanorod arrays were heated from room temperature to 600 °C, the TG curve presents two obvious weight loss processes. They appear at about 100 and 250 °C, respectively, and correspond to the endothermal peak and the exothermal peak, respectively, at the DTA curve. This suggests that at about 100 °C, the adsorption water in the sample is desorbed and the exothermal peak and the obvious weight loss appeared at about 250 °C are caused by transformation from Ce(OH)3 into CeO2. From the DSC curve, the above formation process can be analyzed in detail. It is clear that in 170–600 °C, there exist three exothermal peaks. Their peak positions correspond to 200, 275 and 475 °C, respectively. The peak at 200 °C is caused by OH ion escape and vacancy aggregation. OH ion escape will leave lots of vacancies and the Ce(OH)3 changes into CeO2. The vacancy formation process belongs to the exothermal process, whose production energy is much higher than endothermal energy induced by OH ion escape. Therefore, the peak at 200 °C is an exothermal peak. When the temperature reaches about 275 °C, a weak exothermal peak occurs. The 275 °C peak can be explained as follows. According to the Ostwald ripening phenomenon [21], the pores in a system can form aggregates under the action of the temperature driven force, as a result, the free energy of the system decreases to minimum, and thus this system become very stable. Therefore, it can be supposed that the 275 °C peak is caused by aggregation of large numbers of vacancies and vacancy groups leading to formation of mesopores in CeO2 nanotube walls, which is driven by the temperature. Namely, the formation of mesopores in CeO2 nanotube arrays can also be attributed to Ostwald ripening. At 475 °C, the formation of cubic fluorite structure CeO2 is responsible for the exothermal peak appearance. By using mesoporous CeO2 nanotube arrays on the ITO layer as the electrode, the electrochemistry measurement was carried out in an extremely low concentration H2O2 solution. The result shows that the redox peaks can be detected, as shown in Fig. 8. This reveals that the mesoporous CeO2 nanotube arrays have the reduction action. This result is similar to that of disordered distribution of mesoporous CeO2 nanotube [17]. Therefore, we consider that Ce3+ ions are responsible for this reduction action.
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4. Conclusion
[4] [5] [6] [7]
In summary, mesoporous polycrystalline CeO2 nanotube arrays have been synthesized. The ratio of Ce3+ ions and Ce4+ ions in the surface of mesoporous CeO2 nanotube arrays reaches about 1:2. The formation of mesopores in CeO2 nanotubes arises from OH ions’ escape forming vacancies and the vacancy aggregation. The strong reduction action is attributed to the existence of Ce3+ ions in CeO2 nanotubes, which is closely associated with the mesoporous structure.
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Acknowledgement
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This work is financially supported by the National Basic Research Program of China (2012CB932303). References [1] J.P. Chen, S. Patil, S. Seal, J.F. McGinnis, Nat. Nanotechnol. 1 (2006) 142–150. [2] G.A. Silva, Nat. Nanotechnol. 1 (2006) 92–94. [3] C. Mandoli, F. Pagliari, S. Pagliari, G. Forte, P.D. Nardo, S. Licoccia, E. Traversa, Adv. Funct. Mater. 20 (2010) 1617–1624.
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Contents lists available at SciVerse ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Short Communication
Synthesis and characterization of mesoporous CeO2 nanotube arrays Tong Wang, Lide Zhang ⇑, Junxi Zhang, Guomin Hua Key Laboratory of Materials Physics and Anhui Key Laboratory of Nanomaterials and Nanostructures, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei, 230031 Anhui, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 3 July 2012 Received in revised form 19 November 2012 Accepted 20 December 2012 Available online 28 December 2012 Keywords: Mesoporous CeO2 Nanotube arrays Reductive activity
a b s t r a c t Mesoporous CeO2 nanotube arrays were fabricated by the following procedure: ZnO nanorod arrays were prepared as templates, and then the templates were in turn submerged in the NaOH solution, deionized water and the Ce(NO3)3 solution at 60 °C for 20 times. After annealed at 500 °C for 30 min, the mesoporous polycrystalline CeO2 nanoshells covered ZnO nanorod arrays were obtained. Through HNO3 treatments, ZnO nanorods were dissolved, as a result, the mesoporous CeO2 nanotube arrays were achieved. The XPS result demonstrates that the ratio of Ce3+ ions to Ce4+ ions is about 1:2. Nitrogen adsorption–desorption experiment results give that the specific surface area is 109 m2/g. By using the mesoporous CeO2 nanotubes as the electrode in the H2O2 solution, the electrochemical experiment proves that these arrays have the reduction action. The formation mechanism of mesopores in the CeO2 nanotubes and the origin of the reduction action were discussed in detail. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction CeO2 as an important catalytic and reduction agent has been well recognized, and it has been widely used in various application fields, such as purification of exhaust gas antioxidants in biomedicine [1–3], in three-way automotive catalytic converters [4], and solid oxide fuel cells [5,6]. Many authors have made lots of works on the synthesis of nano-CeO2 and the study of catalytic properties [7–16]. Recently, Hua et al. [17] synthesize mesoporous CeO2 nanotubes. It is found that these nanotubes exhibit excellent reduction ability in the process of CO ? CO2 transition. They found that this high reduction activity was closely associated with the existence of large numbers of Ce3+ ions in the surface of mesoporous CeO2 nanotubes and formation of lots of Ce3+ ions was attributed to large numbers of oxygen vacancies and vacancy groups in these CeO2 nanotubes. Skorodumova et al. [14] theoretically suggested that oxygen-vacancy formation could induce valence transition of Ce ions in CeO2 from +4 to +3 because oxygen escapes from lattice sites, leaving vacancy sites with two electrons at each vacancy. Namely, this transition can be expressed as follows: 2Ce4+ + 2e ? Ce3+. This is why mesoporous CeO2 nanotubes possess the high reduction activity. However, easy aggregation of CeO2 nanotubes of random distribution will lead the reduction ability to be decreased, resulting in losing repeated application value. In order to overcome the aggregation disadvantage and keep the strong reduction ability, we synthesized mesoporous CeO2 nanotube arrays with macro-scale. In order to investigate the ⇑ Corresponding author. Tel.: +86 551 591420; fax: +86 551 591434. E-mail address: [email protected] (L. Zhang). 1387-1811/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2012.12.028
reduction activity of mesoporous CeO2 nanotube arrays, according to the literature [20], we used the mesoporous CeO2 nanotube arrays on the ITO layer as electrode to detect trace H2O2. In the literature [20], Ispas et al. [20] reported that they utilized the catalytic properties of CeO2 for the development of a highly sensitive, simple and inexpensive H2O2 sensor in sensing and biosensing applications. In this paper, we used ZnO nanorod arrays as the templates and synthesized the mesoporous CeO2 nanotube arrays. The formation mechanism of mesopores was discussed. The strong reduction ability was proved through H2O2 decomposition experiments. Based on this, the reduction mechanism associated with the mesoporous structure was analyzed. 2. Experimental 2.1. Synthesis The formation process of mesoporous CeO2 nanotube arrays is shown in Fig. 1. The ZnO nanorod arrays were fabricated as templates via the seeding layer and aqueous solution route [18,19]. The 5 mL of 0.005 M zinc acetate alcohol solution was deposited uniformly on ITO for 5 times. After annealed at 350 °C in air for 30 min, the seed layer was prepared. Then, the ITO with seed layer was put into an aqueous solution of zinc acetate (Zn(Ac)22H2O, 0.02 M) and hexamethylenetetramine (HMTA, C6H12N4, 0.02 M), which was stirred at 75 °C for 6 h. The product was then taken out and washed using distilled water. As a result, the ZnO nanorod arrays were obtained. The assemblies of mesoporous polycrystalline CeO2 nanoshells on the ZnO nanorod arrays were prepared
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Fig. 1. Schematic illustration of formation process of the mesoporous CeO2 nanotube arrays via ZnO nanoarray template synthesis together with ultrasonic assisted SILAR covering method. (a) ZnO nanorod array; (b) ZnO/CeO2 nanorod array; (c) Mesoporous ZnO/CeO2 nanorod array; and (d) Mesoporous CeO2 nanotube array.
by the successive ionic layer adsorption and reaction (SILAR) method, along with annealing treatment. The detailed procedure is as follows: ZnO nanorod arrays were submerged in a 0.05 M NaOH solution for 20 seconds. Subsequently, the nanorod arrays were further submerged in deionized water for 10 seconds. Then, the nanorod arrays were further submerged in a 0.05 M Ce(NO3)3 solution for 20 seconds. After that, the ZnO nanorod arrays were submerged in deionized water for 10 seconds. The above operations were taken in 60 °C in the thermostatic water tank, and the process is referred to as one deposition cycle. To obtain the ideal CeO2 nanoshells covered ZnO nanorod arrays, the above deposition cycle was repeated for about 20 times. As a result, the Ce(OH)3 covered ZnO nanorod arrays were obtained. This deposition product was annealed at 500 °C for 30 min to get mesoporous polycrystalline CeO2 nanoshells covered ZnO nanorod arrays. Then they were treated by 5% HNO3 solution to remove the ZnO nanorod array core and were purged in deionized water several times to obtain final products. Characterization by several techniques proves that the finally obtained product is mesoporous CeO2 nanotube arrays.
by mixing stock solutions of Na2HPO4 and KH2PO4. The mesoporous CeO2 nanotube array was used as the working electrode. All the potentials were recorded and reported vs. a saturated calomel electrode and all the experiments were carried out at room temperature. Cyclic voltammetry experiments were performed in unstirred air-saturated solutions at room temperature at a scan rate of 50 mV/s. Scanning range from 1.5 V to 1.5 V. 3. Results and discussion X-ray diffraction patterns of the products obtained are shown in Fig. 2, it is found that the ZnO nanorod arrays are of the wurtzite structure, furthermore, the obtained mesoporous CeO2 nanotube
2.2. Characterization Phase identification was performed with a powder X-ray diffractometer (XRD, Philips X’Pert) using Cu-Ka (0.15419 nm) radiation. Field emission scanning electron microscopy (FESEM, Sirion 200) was used to observe the morphologies. Transmission electron microscopy (TEM JEM-2010) was used to examine microstructures. The nitrogen adsorption–desorption experiment was carried out on a Surface Area and Porosity Analyzer (Omnisorp 100X), and from obtained adsorption–desorption isotherms, the specific surface area and the pore size distribution in the CeO2 nanotube walls, and inner diameters of the CeO2 nanotubes were obtained by calculation. Energy dispersive X-ray (EDX, Inca Oxford) analysis was conducted to determine the element composition. High resolution X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB 250. Thermal analysis was performed on a Thermogravimetry/Differential thermal analyzer (TGA–DTA, DTG-60H) and a Differential scanning calorimeter (DSC-60) to analyze the formation process of mesoporous CeO2 nanotube arrays from the Ce(OH)3 covered ZnO nanorod arrays. 2.3. Reductive activity measurement The reductive activity measurements were conducted in the H2O2 solution with electrochemical workstation (Zahner IM6ex, Germany). A 1/15 M phosphate buffer (PB) solution was prepared
Fig. 2. X-ray diffraction patterns of ZnO nanorod arrays obtained by the seeding layer and aqueous solution route and the mesoporous CeO2 nanotube arrays after the removal of ZnO nanorod arrays, respectively.
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Fig. 4. TEM images of mesoporous CeO2 nanotubes. (a) Low magnification TEM image; (b) High-magnification image; (c) The corresponding SAED patterns.
Fig. 3. (a) SEM images of ZnO nanorod arrays; (b) TEM image of the dispersive ZnO nanorods; (c) SEM images of mesoporous CeO2 nanotubes.
arrays are only the fluorite CeO2 phase. The morphologies of the ZnO nanorod arrays and the as-prepared mesoporous CeO2 nanotube arrays are shown in Fig. 3. It is clear that the diameter of ZnO nanorods is about 20 nm. Fig. 4a and b present the TEM images of nanostructured CeO2. From Fig. 4b, it can be seen that the inner diameter and the wall thickness of each CeO2 nanotube are about 25 and 6 nm, respectively. The nanostructured CeO2 is in the form of nanotube, and the inner diameter of nanotubes and wall thickness of each CeO2 nanotube are about 25 and 6 nm, respectively. As shown in the high-magnification TEM image in Fig. 4b, walls of CeO2 nanotube arrays show mesoporous structures. The selected area electron diffraction (SAED) pattern reveals that the mesoporous CeO2 nanotube arrays are polycrystalline (Fig. 4c). Steps and corners are formed on the surfaces of nanotubes. The exposed major facets are the (1 1 1) planes and (1 0 0)
Fig. 5. The structural feature of ceria nanotubes after calcinations at 500 °C. (a) Nitrogen adsorption–desorption isotherms; (b) Mesopore size distribution calculated by the adsorption branch of the isotherms.
planes. More importantly, the high density of grain boundaries is observed. The existence of mesopores in mesoporous CeO2 nanotube walls can be further proved by the nitrogen adsorption– desorption experiment. The nitrogen adsorption–desorption iso-
T. Wang et al. / Microporous and Mesoporous Materials 171 (2013) 196–200
Fig. 6. XPS spectra of mesoporous CeO2 nanotube arrays. The Ce 3d core level spectrum curve A and fitting results, curve B represents the background of the spectra, curves C–G denote Ce 3d 5/2, and curves H–L denote Ce 3d 3/2, where the fitting peaks of curves C, E, H and J belong to Ce3+ ions, and fitting peaks of curves D, F, G, I, K and L belong to Ce4+ ions.
Fig. 7. Thermal analysis of the formation process of mesoporous CeO2 nanotube arrays from the Ce(OH)3 covered ZnO nanorod arrays.
therms for CeO2 nanoarrays are shown in Fig. 5a. The H2-type hysteresis loop in the adsorption–desorption isotherm suggests the mesoporous feature of CeO2 nanotubes. Fig. 5b gives the pore size distribution calculated by the adsorption branch of the isotherms. This result indicates that the pore sizes are mainly distributed between 2 and 30 nm. And the pores with <20 nm in diameter are much more than those with 20–30 nm in diameter. Fig. 4b indicates that inner diameter of CeO2 nanotubes is about 25 nm, and thus it can be deduced that the 20–30 nm pore diameter corresponds to the inner diameter of nanotube. Therefore, the pores with <20 nm in diameter correspond to the pores in CeO2 nanotube walls. The specific surface area of CeO2 nanotubes is 109 m2/g, which was obtained by calculation according to the Brunauer–Emmett–Teller (BET) method for the nitrogen adsorption–desorption isotherms of CeO2 nanotubes after calcinations at 500 °C. Clearly, this large specific surface area is closely related to the existence of large numbers of mesopores in the tube walls. Therefore, it can be deduced that these small pores are located in the nanotube walls. In Fig. 4b, many pores in the nanotube wall can be seen. This further proves that many mesopores exist in the nanotube walls. Fig. 6 demonstrates the X-ray Photoelectron Spectroscopy (XPS) spectra of the mesoporous CeO2 nanotube arrays. The Ce3d core level spectrum (curve A) and fitting results are as follows, C
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Fig. 8. Cyclic voltammogram of the CeO2 nanotube arrays for the H2O2 solution with different concentrations, 0, 0.2, 2.0 and 10 m ML 1, respectively.
(880.5 eV), D (882.7 eV), E (885.5 eV), F (889.2 eV), and G (898.3 eV) peaks correspond to Ce 3d 5/2, and H (899.1 eV), I (901.2 eV), J (903.6 eV), K (907.4 eV), and L (916.8 eV) peaks correspond to Ce 3d 3/2, where the fitting peaks of C, E, H and J curves belong to Ce3+ ions, and fitting peaks of D, F, G, I, K and L curves belong to Ce4+ ions. The concentration of Ce3+ ions reaches 34% of the total cerium ions, and the ratio of Ce3+ to Ce4+ is about 1:2. The high concentration of Ce3+ ions in mesoporous CeO2 nanotube arrays is related to the negatively charged active oxygen species. From Fig. 7, the formation process of mesoporous CeO2 nanotube arrays can be analyzed as follows: when the Ce(OH)3 covered ZnO nanorod arrays were heated from room temperature to 600 °C, the TG curve presents two obvious weight loss processes. They appear at about 100 and 250 °C, respectively, and correspond to the endothermal peak and the exothermal peak, respectively, at the DTA curve. This suggests that at about 100 °C, the adsorption water in the sample is desorbed and the exothermal peak and the obvious weight loss appeared at about 250 °C are caused by transformation from Ce(OH)3 into CeO2. From the DSC curve, the above formation process can be analyzed in detail. It is clear that in 170–600 °C, there exist three exothermal peaks. Their peak positions correspond to 200, 275 and 475 °C, respectively. The peak at 200 °C is caused by OH ion escape and vacancy aggregation. OH ion escape will leave lots of vacancies and the Ce(OH)3 changes into CeO2. The vacancy formation process belongs to the exothermal process, whose production energy is much higher than endothermal energy induced by OH ion escape. Therefore, the peak at 200 °C is an exothermal peak. When the temperature reaches about 275 °C, a weak exothermal peak occurs. The 275 °C peak can be explained as follows. According to the Ostwald ripening phenomenon [21], the pores in a system can form aggregates under the action of the temperature driven force, as a result, the free energy of the system decreases to minimum, and thus this system become very stable. Therefore, it can be supposed that the 275 °C peak is caused by aggregation of large numbers of vacancies and vacancy groups leading to formation of mesopores in CeO2 nanotube walls, which is driven by the temperature. Namely, the formation of mesopores in CeO2 nanotube arrays can also be attributed to Ostwald ripening. At 475 °C, the formation of cubic fluorite structure CeO2 is responsible for the exothermal peak appearance. By using mesoporous CeO2 nanotube arrays on the ITO layer as the electrode, the electrochemistry measurement was carried out in an extremely low concentration H2O2 solution. The result shows that the redox peaks can be detected, as shown in Fig. 8. This reveals that the mesoporous CeO2 nanotube arrays have the reduction action. This result is similar to that of disordered distribution of mesoporous CeO2 nanotube [17]. Therefore, we consider that Ce3+ ions are responsible for this reduction action.
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4. Conclusion
[4] [5] [6] [7]
In summary, mesoporous polycrystalline CeO2 nanotube arrays have been synthesized. The ratio of Ce3+ ions and Ce4+ ions in the surface of mesoporous CeO2 nanotube arrays reaches about 1:2. The formation of mesopores in CeO2 nanotubes arises from OH ions’ escape forming vacancies and the vacancy aggregation. The strong reduction action is attributed to the existence of Ce3+ ions in CeO2 nanotubes, which is closely associated with the mesoporous structure.
[9] [10]
Acknowledgement
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