- Email: [email protected]
carbon clusters composite material
Microporous and Mesoporous Materials 118 (2009) 518–522
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Short Communication
Synthesis and photocatalytic activities of MnO2-loaded Nb2O5/carbon clusters composite material H. Miyazaki a, H. Matsui a, T. Kuwamoto a, S. Ito a, S. Karuppuchamy b,*, M. Yoshihara a a b
Department of Applied Chemistry, Faculty of Science and Engineering, Kinki University, 3-4-1, Kowakae, Higashiosaka, Osaka 577-8502, Japan Molecular Engineering Institute, Kinki University, 11-6, Kayanomori, Iizuka, Fukuoka 820-8555, Japan
a r t i c l e
i n f o
Article history: Received 6 August 2008 Received in revised form 1 September 2008 Accepted 2 September 2008 Available online 11 September 2008 Keywords: Semiconductors Polymers Nanostructures Inorganic compounds Electronic structure
a b s t r a c t Nano-sized Nb2O5/carbon clusters composite material denoted as Ic, has been successfully synthesized by the calcination of NbCl5/epoxy resin complex I. MnO2-loaded Nb2O5/carbon clusters composite materials were also prepared by doping the MnO2 particles on the surface of Ic. The compositions of the synthesized composite materials were determined using inductively coupled plasma (ICP) spectroscopy, elemental analysis and surface characterization by transmission electron microscopy (TEM). The ultraviolet–visible (UV–Vis), and electron spin resonance (ESR) spectra of the composites were also measured. The reduction reaction of methylene blue with the calcined materials under the visible light irradiation has also been examined. The composite material Ic, reduced the methylene blue under the irradiation of visible light (k > 460 nm). The MnO2-loaded Ic composite material could also decompose an aqueous silver nitrate solution by visible light irradiation and give O2 and Ag with a [O2]:[Ag] ratio of 1:4. Ó 2008 Elsevier Inc. All rights reserved.
1. Introduction A charge-separated excitation under light irradiation is important in constructing new types of photo-sensitive catalysts for H2 production from water, CO2 fixation, solar cell production, catalysts for organic synthesis, and so on. Such charge-separated excitation under light irradiation could be achieved by the use of semiconductors such as TiO2 and other metal oxides [1–8]. We assume that the composites containing carbon clusters and nano-sized semiconductors may be sensitive to visible light. Therefore visible light absorption and visible light induced electron excitation could be achieved by carbon clusters and semiconductors, respectively. We have recently synthesized and reported such composite materials by the calcination of both metal-organic moiety hybrid copolymers and inorganic metal compound/organic polymer complexes. The visible light-responsive electron transfer between metal compounds and carbon phases was observed for those reported novel composite materials [9–16]. Especially, CeO2/carbon clusters/Ho2O3 composite material loaded with Pt particles have shown that the decomposition of water to H2 and O2 with a [H2]/[O2] ratio of 2 under visible light irradiation through a two-step electron transfer in the process of CeO2 ? carbon clusters ? Ho2O3 ? Pt [17]. This reveals that a smooth electron transfer could be achieved by the combination of
* Corresponding author. Tel./fax: +81 948 22 5706. E-mail address: [email protected] (S. Karuppuchamy). 1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.09.004
two metal oxide/carbon clusters composite materials with different electron moving features, i.e., (metal oxide)A ? carbon clusters and carbon clusters ? (metal oxide)B. And we expect that such an electron transfer may also be achieved by simply loading nano-sized particles of high oxidative function containing metal oxide on the surface of metal oxide/carbon clusters composite materials. We have previously reported that the electron transfer process for Nb2O5/carbon clusters composite material was found to be from carbon clusters ? Nb2O5 [18]. On the other hand, MnO2 is known to be a typical metal oxide with a high oxidation activity [19,20]. Therefore, the combination of MnO2 and Nb2O5 in carbon clusters is expected to give rise to a smooth electron transfer feature, possibly through the electron transfer process of MnO2 ? carbon clusters ? Nb2O5 (Scheme 1). In the present work, nanoparticles of MnO2 were loaded onto the surface of Nb2O5/carbon clusters composite material which was obtained by the calcination of NbCl5/ epoxy resin complex I. Finally, the electronic behavior of MnO2loaded Nb2O5/carbon clusters composite material Ic MnO2 has been examined under visible light irradiation (Scheme 2). 2. Experimental 2.1. Reagents Commercially available niobium(V) chloride, diglycidyl ether of bisphenol A (DGEBA), phthalic anhydride (PA), potassium permanganate, 1,1-diphenyl-2-picrylhydrazyl (DPPH), methylene blue,
519
H. Miyazaki et al. / Microporous and Mesoporous Materials 118 (2009) 518–522
respectively (Scheme 2). The synthesized complexes have epoxy groups and while heating at high temperature the NbCl5 is converted into Nb2O5. 2.4. MnO2-loading on calcined material Ic-500 A mixture of Ic-500 (1 g), KMnO4 (288 mg, 1.82 mmol) and ethanol (4 mL) in distilled water (100 mL) was stirred at 45 °C for 1 h. The precipitates were collected and dried at 60 °C for overnight. About 0.3 g of the obtained precipitates was heated at 300 °C for 5 min in a porcelain crucible under an air atmosphere using Bransted Thermolyne FB-1300 electric furnace to obtain MnO2-loaded material Ic-500MnO2 (Scheme 2).
e-
2.5. Characterization
Nb2O5
carbon clusters
Scheme 1. Plausible electron transfer process.
citric acid, and silver nitrate were used. We have selected the DGEBA and PA to mix with NbCl5 in the view of their ability to form a solid complex with heat-treatment. 2.2. Synthesis of complex I About 4.18 g (15.5 mmol) of NbCl5, 14.1 g (41.3 mmol) of DGEBA and 12.3 g (82.8 mmol) of PA were dissolved in 50 mL of acetone. Then the acetone was evaporated and subsequently the residues were heated at 150 °C for 3 h to obtain complex I (Scheme 2). 2.3. Calcination of complex I About 3 g of complex I in a porcelain crucible was heated with a heating rate of 5/min under an argon atmosphere using Denken KDF-75 electric furnace and kept at 400, 500 and 600 °C for 1 h to obtain calcined materials denoted as Ic-400, Ic-500 and Ic-600,
Elemental analysis was performed for C and H using Yanaco MT-6, for Cl using Yanaco YS-10, and for Nb and Mn by inductively coupled plasma atomic emission spectrometry (ICP-AES) using Shimadzu ICP-7500. SEM-EDX spectra were taken with Hitachi S-4800 microscope. Transmission electron microscopy (TEM) observations were also done using Jeol JEM-3010 microscope. X-ray photoelectron spectra (XPS) were obtained using Shimadzu ESCA-850. Electron spin resonance (ESR) spectra were taken using Jeol JES-TE 200 spectrometer. UV–Vis spectra were measured using Hitachi U4000 spectrometer. Visible light was generated using Hoya-Schott Megalight 100 halogen lamp. The sharp cut filter Y-48 purchased from Hoya Candeo Optronics Co. was used. The reduction reaction of methylene blue with calcined materials Ic’s was carried out in the following way. A mixture of 3 mg of the calcined materials and 9 mL of 0.03 mmol/L methylene blue0.12 mmol/L citric acid aqueous solution was stirred in the dark for 48 h. The mixture was irradiated with visible light (k > 460 nm) and the concentration of methylene blue was determined by UV–Vis spectral analysis. The intensity of the irradiated visible light was 2 mW/cm2. The oxidation–reduction reaction of an aqueous silver nitrate solution with Ic-500MnO2 was also performed in the following way. A mixture of 10 mg of Ic-500MnO2 and 1 mL of 0.05 mmol/L AgNO3 aqueous solution was irradiated by visible light (k > 460 nm) under an argon atmosphere for 3 h, and then the evolved O2 gas was analyzed with Shimadzu GC-8A
O O
O
NbCl5 +
O
O
O
+ O PA
DGEBA O
O
NbCl5 O n
O
Nb2O5 / carbon clusters
O
O m
Ic
I KMnO4 EtOH
Nb2O5 / carbon clusters / MnO2 Ic.MnO2 Scheme 2. Synthesis of materials.
520
H. Miyazaki et al. / Microporous and Mesoporous Materials 118 (2009) 518–522
gas chromatography and the formed Ag was estimated by ICP analysis.
2000 1500
3. Results and discussion
1000
Table 1 Elemental analysis of complex I and calcined materials Ic’s Materials
I Ic-400 Ic-500 Ic-600
Found (%) Nb
C
H
Cl
5.19 10.68 10.84 11.25
59.26 68.55 68.91 69.55
4.87 4.25 2.87 2.08
8.02 0 0 0
500
Intensity
The results of the elemental analysis of the materials are shown in Table 1. The elemental analysis of complex I (Table 1) showed the presence of Nb atom. The SEM-EDX analysis of I revealed that Nb atom was uniformly dispersed in the matrix (figure is not shown). This observation indicates the formation of complex I. The calcination of complex I at various temperatures such as 400, 500 and 600 °C produced the black-colored materials denoted as Ic-400, Ic-500 and Ic-600, respectively. The H content was decreased with the increase of calcination temperature, suggesting that the carbonization of the materials was successfully proceeded. The XPS measurements of Ic’s were found to give a binding energy at 207.1–207.3 eV due to the Nb3d orbital of Nb2O5. The TEM observation (Fig. 1) for Ic-400 and Ic-500 shows the presence of particles in carbon phases with the diameters of ca. 1 nm. The particle size of ca. 5–10 nm is seen for Ic-600 and we assume that the observed particles in all these calcined materials may be Nb2O5. The results suggest that the calcined materials were composed of nano-sized Nb2O5 and carbon clusters. The electronic behaviors of the calcined materials were also examined. Fig. 2 is the ESR spectra of Ic’s. A peak signal at g = 337 mT (g = 2.003) was observed and the highest peak intensity was obtained for Ic-500. The radical spin quantities (rsq) of calcined materials Ic-400, Ic-500 and Ic-600 were determined by a double integrating calculation of the differential absorption line with the use of DPPH and the values to be 1.7 1019, 1.3 1020 and 5.98 1019 spins/g, respectively (Table 2). Our understanding is that an electron transfer between Nb2O5 particles and carbon clusters takes place to form a free electron on carbon clusters and the highest electron transfer appeared for Ic-500. Fig. 3 shows the ESR spectra of Ic-500 in the presence of either an oxidant (1,4benzoquinone) or a reductant (pyrogallol) under the irradiation of
0 -500 -1000 -1500 -2000 333
335
337
339
341
Field / mT Fig. 2. ESR spectra of calcined materials Ic’s.
Table 2 Radical spin quantities (rsq) and reduction activities (ra) of calcined materials Ic’s Materials
rsq (spin/g)
ra (lmol/g h)
Ic-400 Ic-500 Ic-600
1.70 1019 1.30 1020 5.98 1019
1.68 5.30 1.63
light (k > 460 nm). The signal intensity was increased with the addition of the oxidant but decreased with the addition of the reductant, indicating that the signal is due to a radical cation. It is thus deduced that an electron transfer from carbon clusters to Nb2O5 particles takes place to form an oxidation site at carbon clusters and a reduction site at Nb2O5 part. The photocatalytic abilities of the calcined materials were also examined. Fig. 4 is the UV–Vis spectra of methylene blue in the presence of Ic-500 under the irradiation of visible light (k > 460 nm). The absorption band of methylene blue decreased with increase of the irradiation time, indicating that the calcined materials have visible light-responsive reduction ability. The reduction activities (ra) of the calcined materials in the reduction reaction of methylene blue were determined by the equation ra = (the amount of methylene blue) (g of the calcined material) 1
Fig. 1. TEM images of calcined materials Ic’s.
521
H. Miyazaki et al. / Microporous and Mesoporous Materials 118 (2009) 518–522
addingpyrogallol
adding 1,4-benzoquinone light irradiation
in the dark
1500
1500
1000
1000
500
500
Intensity
Intensity
in the dark
0
light irradiation
0
-500
-500
-1000
-1000
-1500
-1500 333
335
337
339
341
Field / mT
333
335
337
339
341
Field / mT
Fig. 3. ESR spectra of Ic-500 in the presence of 1,4-benzoquinone and pyrogallol.
(hour) 1, and the results are also shown in Table 2. Here again, the highest ra value was obtained for Ic-500, indicating that Ic-500 have the highest photo-reduction ability. Further, we assume that the reason for the less photocatalytic activity of Ic-600 compared to Ic-400 is may be due to the formation of aggregated particles in Ic-600. As can be seen in Fig. 1, the TEM image of Ic-400 is having uniformly dispersed particles without aggregates and Ic-600 shows the aggregated particles. Therefore the Ic-400 has more reduction ability than Ic-600 even though it has higher rsq. Simi-
2
Absorbance
1.5
1
larly, Ic-500 shows better photocatalytic performance compared to the other two calcined materials (Ic-400 and Ic-600) and that may be due to the formation of uniformly dispersed particles without aggregation (Fig. 1 and 5). MnO2 particles with a high oxidation ability were loaded on the surface of Ic-500 according to the procedure described in experimental Section 2.4 to obtain MnO2-loaded material Ic-500MnO2. The Elemental analysis of these composite indicates the presence of respective atoms and the percentage of Nb, Mn, C and H are 16.56, 7.91, 56.25, and 2.42, respectively. The ratio of the atom is [Nb]:[Mn]:[C] = 1:0.81:26. The SEM-EDX analysis of Ic-500MnO2 showed that Mn atom was uniformly dispersed on the surface of the matrix. The XPS spectra measurement of Ic-500MnO2 shows a peak at 642 eV due to the Mn2p orbital of MnO2. The TEM observation of Ic-500MnO2 revealed the presence of particles with a diameters of ca. 50 nm, possibly MnO2 on the surface of the matrix (Fig. 5). The visible light-irradiated oxidation–reduction reaction of an aqueous silver nitrate solution with Ic-500 and Ic-500MnO2 was performed and the results are also shown in Table 3. The amounts of O2 and Ag formed for Ic-500MnO2 were found to be higher than those for Ic-500. Here, if a four electron
0.5
0 550
600
650
700
Wavelength / nm
0 min 30 min 60 min
90 min 120 min
150 min 180 min
Fig. 4. UV–Vis spectra of methylene blue in the presence of Ic-500 under the irradiation of light (k > 460 nm).
Fig. 5. TEM image of Ic-500MnO2.
522
H. Miyazaki et al. / Microporous and Mesoporous Materials 118 (2009) 518–522
Table 3 Amounts of O2 and Ag formed in the photo-decomposition of an aqueous silver nitrate solution with Ic-500 and Ic-500-MnO2 Materials
Ic-500 Ic-500MnO2
lmol
Ratio
O2
Ag
[O2]:[Ag]
0.29 1.85
3.34 7.35
1:8.4 1:4.0
oxidation–reduction reaction takes place, then a [O2]:[Ag] ratio is given to be 1:4. The [O2]:[Ag] ratio of Ic-500MnO2 was obtained to be 1:4, but that of Ic-500 was 1:8.4. Our opinion is that the MnO2-loading onto the surface of Ic-500 caused a smooth electron transfer through MnO2 ? carbon clusters ? Nb2O5 to enhance the degree of either an oxidation ability at MnO2 particles or a reduction ability at Nb2O5 particles, thus facilitating the decomposition of a AgNO3 solution. References [1] K. Fujihara, T. Ohno, M. Matsumura, J. Chem. Soc. Faraday Trans. 94 (1998) 3705. [2] K. Domen, J.N. Kondo, M. Hara, T. Takata, Bull. Chem. Soc. Jpn. 73 (2000) 1307.
[3] R. Konta, T. Ishii, H. Kato, J. Phys. Chem. B 108 (2004) 8992. [4] T. Ohno, D. Haga, K. Fujihara, K. Kaizaki, M. Matsumura, J. Phys. Chem. B 101 (1997) 6415. [5] A. Kudo, H. Kato, I. Tsuji, Chem. Lett. 33 (2004) 1534. [6] N. Kobayashi, T. Narumi, R. Morita, Jpn. J. Appl. Phys. 44 (2005) 784. [7] N. Arai, N. Saito, H. Nishiyama, Y. Inoue, K. Domen, K. Sato, Chem. Lett. 35 (2006) 796. [8] S. Karuppuchamy, M. Iwasaki, H. Minoura, Vacuum 81 (2007) 708. [9] S. Yamamoto, H. Matsui, S. Ishiyama, S. Karuppuchamy, M. Yoshihara, Mater. Sci. Eng. B 135 (2006) 120. [10] H. Matsui, T. Kawahara, H. Kajita, Y. Seguchi, M. Yoshihara, J. Mater. Sci. 41 (2006) 7302. [11] T. Furukawa, H. Matsui, H. Hasegawa, S. Karuppuchamy, M. Yoshihara, Solid State Commun. 142 (2007) 99. [12] S. Yamamoto, H. Matsui, S. Ishiyama, S. Karuppuchamy, M. Yoshihara, Mater. Chem. Phys. 103 (2007) 127. [13] H. Matsui, S. Karuppuchamy, J. Yamaguchi, M. Yoshihara, J. Photochem. Photobio. A Chem. 189 (2007) 280. [14] S. Yamamoto, H. Matsui, Y. Kanae, S. Karuppuchamy, M. Yoshihara, J. Chem. Eng. Jpn. 40 (2007) 329. [15] H. Matsui, T. Kawahara, R. Kudo, M. Uda, S. Karuppuchamy, M. Yoshihara, J. Alloy Compd. 462 (2008) L20. [16] H. Matsui, K. Kira, S. Karuppuchamy, M. Yoshihara, Curr. Appl. Phys., in press. [17] H. Matsui, K. Otsuki, H. Yamada, T. Kawahara, M. Yoshihara, J. Colloid Interface Sci. 297 (2006) 672. [18] T. Kuroda, H. Matsui, S. Matsumura, K. Sugiyama, M. Yoshihara, Mater. Technol. 25 (2007) 274. [19] L. Singoredjo, R. Korver, F. Kapteijn, J. Moulijn, Appl. Catal. B 1 (1992) 297. [20] L. Wang, Y. Liu, M. Chen, Y. Cao, H. He, K. Fan, Phys. Chem. C 112 (2008) 6981.
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Short Communication
Synthesis and photocatalytic activities of MnO2-loaded Nb2O5/carbon clusters composite material H. Miyazaki a, H. Matsui a, T. Kuwamoto a, S. Ito a, S. Karuppuchamy b,*, M. Yoshihara a a b
Department of Applied Chemistry, Faculty of Science and Engineering, Kinki University, 3-4-1, Kowakae, Higashiosaka, Osaka 577-8502, Japan Molecular Engineering Institute, Kinki University, 11-6, Kayanomori, Iizuka, Fukuoka 820-8555, Japan
a r t i c l e
i n f o
Article history: Received 6 August 2008 Received in revised form 1 September 2008 Accepted 2 September 2008 Available online 11 September 2008 Keywords: Semiconductors Polymers Nanostructures Inorganic compounds Electronic structure
a b s t r a c t Nano-sized Nb2O5/carbon clusters composite material denoted as Ic, has been successfully synthesized by the calcination of NbCl5/epoxy resin complex I. MnO2-loaded Nb2O5/carbon clusters composite materials were also prepared by doping the MnO2 particles on the surface of Ic. The compositions of the synthesized composite materials were determined using inductively coupled plasma (ICP) spectroscopy, elemental analysis and surface characterization by transmission electron microscopy (TEM). The ultraviolet–visible (UV–Vis), and electron spin resonance (ESR) spectra of the composites were also measured. The reduction reaction of methylene blue with the calcined materials under the visible light irradiation has also been examined. The composite material Ic, reduced the methylene blue under the irradiation of visible light (k > 460 nm). The MnO2-loaded Ic composite material could also decompose an aqueous silver nitrate solution by visible light irradiation and give O2 and Ag with a [O2]:[Ag] ratio of 1:4. Ó 2008 Elsevier Inc. All rights reserved.
1. Introduction A charge-separated excitation under light irradiation is important in constructing new types of photo-sensitive catalysts for H2 production from water, CO2 fixation, solar cell production, catalysts for organic synthesis, and so on. Such charge-separated excitation under light irradiation could be achieved by the use of semiconductors such as TiO2 and other metal oxides [1–8]. We assume that the composites containing carbon clusters and nano-sized semiconductors may be sensitive to visible light. Therefore visible light absorption and visible light induced electron excitation could be achieved by carbon clusters and semiconductors, respectively. We have recently synthesized and reported such composite materials by the calcination of both metal-organic moiety hybrid copolymers and inorganic metal compound/organic polymer complexes. The visible light-responsive electron transfer between metal compounds and carbon phases was observed for those reported novel composite materials [9–16]. Especially, CeO2/carbon clusters/Ho2O3 composite material loaded with Pt particles have shown that the decomposition of water to H2 and O2 with a [H2]/[O2] ratio of 2 under visible light irradiation through a two-step electron transfer in the process of CeO2 ? carbon clusters ? Ho2O3 ? Pt [17]. This reveals that a smooth electron transfer could be achieved by the combination of
* Corresponding author. Tel./fax: +81 948 22 5706. E-mail address: [email protected] (S. Karuppuchamy). 1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.09.004
two metal oxide/carbon clusters composite materials with different electron moving features, i.e., (metal oxide)A ? carbon clusters and carbon clusters ? (metal oxide)B. And we expect that such an electron transfer may also be achieved by simply loading nano-sized particles of high oxidative function containing metal oxide on the surface of metal oxide/carbon clusters composite materials. We have previously reported that the electron transfer process for Nb2O5/carbon clusters composite material was found to be from carbon clusters ? Nb2O5 [18]. On the other hand, MnO2 is known to be a typical metal oxide with a high oxidation activity [19,20]. Therefore, the combination of MnO2 and Nb2O5 in carbon clusters is expected to give rise to a smooth electron transfer feature, possibly through the electron transfer process of MnO2 ? carbon clusters ? Nb2O5 (Scheme 1). In the present work, nanoparticles of MnO2 were loaded onto the surface of Nb2O5/carbon clusters composite material which was obtained by the calcination of NbCl5/ epoxy resin complex I. Finally, the electronic behavior of MnO2loaded Nb2O5/carbon clusters composite material Ic MnO2 has been examined under visible light irradiation (Scheme 2). 2. Experimental 2.1. Reagents Commercially available niobium(V) chloride, diglycidyl ether of bisphenol A (DGEBA), phthalic anhydride (PA), potassium permanganate, 1,1-diphenyl-2-picrylhydrazyl (DPPH), methylene blue,
519
H. Miyazaki et al. / Microporous and Mesoporous Materials 118 (2009) 518–522
respectively (Scheme 2). The synthesized complexes have epoxy groups and while heating at high temperature the NbCl5 is converted into Nb2O5. 2.4. MnO2-loading on calcined material Ic-500 A mixture of Ic-500 (1 g), KMnO4 (288 mg, 1.82 mmol) and ethanol (4 mL) in distilled water (100 mL) was stirred at 45 °C for 1 h. The precipitates were collected and dried at 60 °C for overnight. About 0.3 g of the obtained precipitates was heated at 300 °C for 5 min in a porcelain crucible under an air atmosphere using Bransted Thermolyne FB-1300 electric furnace to obtain MnO2-loaded material Ic-500MnO2 (Scheme 2).
e-
2.5. Characterization
Nb2O5
carbon clusters
Scheme 1. Plausible electron transfer process.
citric acid, and silver nitrate were used. We have selected the DGEBA and PA to mix with NbCl5 in the view of their ability to form a solid complex with heat-treatment. 2.2. Synthesis of complex I About 4.18 g (15.5 mmol) of NbCl5, 14.1 g (41.3 mmol) of DGEBA and 12.3 g (82.8 mmol) of PA were dissolved in 50 mL of acetone. Then the acetone was evaporated and subsequently the residues were heated at 150 °C for 3 h to obtain complex I (Scheme 2). 2.3. Calcination of complex I About 3 g of complex I in a porcelain crucible was heated with a heating rate of 5/min under an argon atmosphere using Denken KDF-75 electric furnace and kept at 400, 500 and 600 °C for 1 h to obtain calcined materials denoted as Ic-400, Ic-500 and Ic-600,
Elemental analysis was performed for C and H using Yanaco MT-6, for Cl using Yanaco YS-10, and for Nb and Mn by inductively coupled plasma atomic emission spectrometry (ICP-AES) using Shimadzu ICP-7500. SEM-EDX spectra were taken with Hitachi S-4800 microscope. Transmission electron microscopy (TEM) observations were also done using Jeol JEM-3010 microscope. X-ray photoelectron spectra (XPS) were obtained using Shimadzu ESCA-850. Electron spin resonance (ESR) spectra were taken using Jeol JES-TE 200 spectrometer. UV–Vis spectra were measured using Hitachi U4000 spectrometer. Visible light was generated using Hoya-Schott Megalight 100 halogen lamp. The sharp cut filter Y-48 purchased from Hoya Candeo Optronics Co. was used. The reduction reaction of methylene blue with calcined materials Ic’s was carried out in the following way. A mixture of 3 mg of the calcined materials and 9 mL of 0.03 mmol/L methylene blue0.12 mmol/L citric acid aqueous solution was stirred in the dark for 48 h. The mixture was irradiated with visible light (k > 460 nm) and the concentration of methylene blue was determined by UV–Vis spectral analysis. The intensity of the irradiated visible light was 2 mW/cm2. The oxidation–reduction reaction of an aqueous silver nitrate solution with Ic-500MnO2 was also performed in the following way. A mixture of 10 mg of Ic-500MnO2 and 1 mL of 0.05 mmol/L AgNO3 aqueous solution was irradiated by visible light (k > 460 nm) under an argon atmosphere for 3 h, and then the evolved O2 gas was analyzed with Shimadzu GC-8A
O O
O
NbCl5 +
O
O
O
+ O PA
DGEBA O
O
NbCl5 O n
O
Nb2O5 / carbon clusters
O
O m
Ic
I KMnO4 EtOH
Nb2O5 / carbon clusters / MnO2 Ic.MnO2 Scheme 2. Synthesis of materials.
520
H. Miyazaki et al. / Microporous and Mesoporous Materials 118 (2009) 518–522
gas chromatography and the formed Ag was estimated by ICP analysis.
2000 1500
3. Results and discussion
1000
Table 1 Elemental analysis of complex I and calcined materials Ic’s Materials
I Ic-400 Ic-500 Ic-600
Found (%) Nb
C
H
Cl
5.19 10.68 10.84 11.25
59.26 68.55 68.91 69.55
4.87 4.25 2.87 2.08
8.02 0 0 0
500
Intensity
The results of the elemental analysis of the materials are shown in Table 1. The elemental analysis of complex I (Table 1) showed the presence of Nb atom. The SEM-EDX analysis of I revealed that Nb atom was uniformly dispersed in the matrix (figure is not shown). This observation indicates the formation of complex I. The calcination of complex I at various temperatures such as 400, 500 and 600 °C produced the black-colored materials denoted as Ic-400, Ic-500 and Ic-600, respectively. The H content was decreased with the increase of calcination temperature, suggesting that the carbonization of the materials was successfully proceeded. The XPS measurements of Ic’s were found to give a binding energy at 207.1–207.3 eV due to the Nb3d orbital of Nb2O5. The TEM observation (Fig. 1) for Ic-400 and Ic-500 shows the presence of particles in carbon phases with the diameters of ca. 1 nm. The particle size of ca. 5–10 nm is seen for Ic-600 and we assume that the observed particles in all these calcined materials may be Nb2O5. The results suggest that the calcined materials were composed of nano-sized Nb2O5 and carbon clusters. The electronic behaviors of the calcined materials were also examined. Fig. 2 is the ESR spectra of Ic’s. A peak signal at g = 337 mT (g = 2.003) was observed and the highest peak intensity was obtained for Ic-500. The radical spin quantities (rsq) of calcined materials Ic-400, Ic-500 and Ic-600 were determined by a double integrating calculation of the differential absorption line with the use of DPPH and the values to be 1.7 1019, 1.3 1020 and 5.98 1019 spins/g, respectively (Table 2). Our understanding is that an electron transfer between Nb2O5 particles and carbon clusters takes place to form a free electron on carbon clusters and the highest electron transfer appeared for Ic-500. Fig. 3 shows the ESR spectra of Ic-500 in the presence of either an oxidant (1,4benzoquinone) or a reductant (pyrogallol) under the irradiation of
0 -500 -1000 -1500 -2000 333
335
337
339
341
Field / mT Fig. 2. ESR spectra of calcined materials Ic’s.
Table 2 Radical spin quantities (rsq) and reduction activities (ra) of calcined materials Ic’s Materials
rsq (spin/g)
ra (lmol/g h)
Ic-400 Ic-500 Ic-600
1.70 1019 1.30 1020 5.98 1019
1.68 5.30 1.63
light (k > 460 nm). The signal intensity was increased with the addition of the oxidant but decreased with the addition of the reductant, indicating that the signal is due to a radical cation. It is thus deduced that an electron transfer from carbon clusters to Nb2O5 particles takes place to form an oxidation site at carbon clusters and a reduction site at Nb2O5 part. The photocatalytic abilities of the calcined materials were also examined. Fig. 4 is the UV–Vis spectra of methylene blue in the presence of Ic-500 under the irradiation of visible light (k > 460 nm). The absorption band of methylene blue decreased with increase of the irradiation time, indicating that the calcined materials have visible light-responsive reduction ability. The reduction activities (ra) of the calcined materials in the reduction reaction of methylene blue were determined by the equation ra = (the amount of methylene blue) (g of the calcined material) 1
Fig. 1. TEM images of calcined materials Ic’s.
521
H. Miyazaki et al. / Microporous and Mesoporous Materials 118 (2009) 518–522
addingpyrogallol
adding 1,4-benzoquinone light irradiation
in the dark
1500
1500
1000
1000
500
500
Intensity
Intensity
in the dark
0
light irradiation
0
-500
-500
-1000
-1000
-1500
-1500 333
335
337
339
341
Field / mT
333
335
337
339
341
Field / mT
Fig. 3. ESR spectra of Ic-500 in the presence of 1,4-benzoquinone and pyrogallol.
(hour) 1, and the results are also shown in Table 2. Here again, the highest ra value was obtained for Ic-500, indicating that Ic-500 have the highest photo-reduction ability. Further, we assume that the reason for the less photocatalytic activity of Ic-600 compared to Ic-400 is may be due to the formation of aggregated particles in Ic-600. As can be seen in Fig. 1, the TEM image of Ic-400 is having uniformly dispersed particles without aggregates and Ic-600 shows the aggregated particles. Therefore the Ic-400 has more reduction ability than Ic-600 even though it has higher rsq. Simi-
2
Absorbance
1.5
1
larly, Ic-500 shows better photocatalytic performance compared to the other two calcined materials (Ic-400 and Ic-600) and that may be due to the formation of uniformly dispersed particles without aggregation (Fig. 1 and 5). MnO2 particles with a high oxidation ability were loaded on the surface of Ic-500 according to the procedure described in experimental Section 2.4 to obtain MnO2-loaded material Ic-500MnO2. The Elemental analysis of these composite indicates the presence of respective atoms and the percentage of Nb, Mn, C and H are 16.56, 7.91, 56.25, and 2.42, respectively. The ratio of the atom is [Nb]:[Mn]:[C] = 1:0.81:26. The SEM-EDX analysis of Ic-500MnO2 showed that Mn atom was uniformly dispersed on the surface of the matrix. The XPS spectra measurement of Ic-500MnO2 shows a peak at 642 eV due to the Mn2p orbital of MnO2. The TEM observation of Ic-500MnO2 revealed the presence of particles with a diameters of ca. 50 nm, possibly MnO2 on the surface of the matrix (Fig. 5). The visible light-irradiated oxidation–reduction reaction of an aqueous silver nitrate solution with Ic-500 and Ic-500MnO2 was performed and the results are also shown in Table 3. The amounts of O2 and Ag formed for Ic-500MnO2 were found to be higher than those for Ic-500. Here, if a four electron
0.5
0 550
600
650
700
Wavelength / nm
0 min 30 min 60 min
90 min 120 min
150 min 180 min
Fig. 4. UV–Vis spectra of methylene blue in the presence of Ic-500 under the irradiation of light (k > 460 nm).
Fig. 5. TEM image of Ic-500MnO2.
522
H. Miyazaki et al. / Microporous and Mesoporous Materials 118 (2009) 518–522
Table 3 Amounts of O2 and Ag formed in the photo-decomposition of an aqueous silver nitrate solution with Ic-500 and Ic-500-MnO2 Materials
Ic-500 Ic-500MnO2
lmol
Ratio
O2
Ag
[O2]:[Ag]
0.29 1.85
3.34 7.35
1:8.4 1:4.0
oxidation–reduction reaction takes place, then a [O2]:[Ag] ratio is given to be 1:4. The [O2]:[Ag] ratio of Ic-500MnO2 was obtained to be 1:4, but that of Ic-500 was 1:8.4. Our opinion is that the MnO2-loading onto the surface of Ic-500 caused a smooth electron transfer through MnO2 ? carbon clusters ? Nb2O5 to enhance the degree of either an oxidation ability at MnO2 particles or a reduction ability at Nb2O5 particles, thus facilitating the decomposition of a AgNO3 solution. References [1] K. Fujihara, T. Ohno, M. Matsumura, J. Chem. Soc. Faraday Trans. 94 (1998) 3705. [2] K. Domen, J.N. Kondo, M. Hara, T. Takata, Bull. Chem. Soc. Jpn. 73 (2000) 1307.
[3] R. Konta, T. Ishii, H. Kato, J. Phys. Chem. B 108 (2004) 8992. [4] T. Ohno, D. Haga, K. Fujihara, K. Kaizaki, M. Matsumura, J. Phys. Chem. B 101 (1997) 6415. [5] A. Kudo, H. Kato, I. Tsuji, Chem. Lett. 33 (2004) 1534. [6] N. Kobayashi, T. Narumi, R. Morita, Jpn. J. Appl. Phys. 44 (2005) 784. [7] N. Arai, N. Saito, H. Nishiyama, Y. Inoue, K. Domen, K. Sato, Chem. Lett. 35 (2006) 796. [8] S. Karuppuchamy, M. Iwasaki, H. Minoura, Vacuum 81 (2007) 708. [9] S. Yamamoto, H. Matsui, S. Ishiyama, S. Karuppuchamy, M. Yoshihara, Mater. Sci. Eng. B 135 (2006) 120. [10] H. Matsui, T. Kawahara, H. Kajita, Y. Seguchi, M. Yoshihara, J. Mater. Sci. 41 (2006) 7302. [11] T. Furukawa, H. Matsui, H. Hasegawa, S. Karuppuchamy, M. Yoshihara, Solid State Commun. 142 (2007) 99. [12] S. Yamamoto, H. Matsui, S. Ishiyama, S. Karuppuchamy, M. Yoshihara, Mater. Chem. Phys. 103 (2007) 127. [13] H. Matsui, S. Karuppuchamy, J. Yamaguchi, M. Yoshihara, J. Photochem. Photobio. A Chem. 189 (2007) 280. [14] S. Yamamoto, H. Matsui, Y. Kanae, S. Karuppuchamy, M. Yoshihara, J. Chem. Eng. Jpn. 40 (2007) 329. [15] H. Matsui, T. Kawahara, R. Kudo, M. Uda, S. Karuppuchamy, M. Yoshihara, J. Alloy Compd. 462 (2008) L20. [16] H. Matsui, K. Kira, S. Karuppuchamy, M. Yoshihara, Curr. Appl. Phys., in press. [17] H. Matsui, K. Otsuki, H. Yamada, T. Kawahara, M. Yoshihara, J. Colloid Interface Sci. 297 (2006) 672. [18] T. Kuroda, H. Matsui, S. Matsumura, K. Sugiyama, M. Yoshihara, Mater. Technol. 25 (2007) 274. [19] L. Singoredjo, R. Korver, F. Kapteijn, J. Moulijn, Appl. Catal. B 1 (1992) 297. [20] L. Wang, Y. Liu, M. Chen, Y. Cao, H. He, K. Fan, Phys. Chem. C 112 (2008) 6981.