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One and three dimensional mesoporous carbon nitride molecular sieves with tunable pore diameters
Recent Progress in Mesostructured Mesostructured Materials D. Zhao, S. Qiu, Y. Tang and C. Yu (Editors) © 2007 Published by Elsevier B.V.
905 905
One and three dimensional mesoporous carbon nitride molecular sieves with tunable pore diameters Ajayan Vinua*, Toshiyuki Moria, Sunichi Hishitaa, Srinivasan Anandana, Veerappan Vaithilingam Balasubramanianb and Katsuhiko Arigac "Fuel Cell Materials Center, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Ibaraki, Japan; Email: [email protected] b Department of Marine Biotechnology, Asan-City 336-745, Chungnam, Soonchunhyang University, South Korea c Supermolecules Group, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Ibaraki, Japan One- and three-dimensional mesoporous carbon nitride materials have been synthesized using SBA-15 and SBA-16 mesoporous silica hard templates, respectively. The obtained materials have been unambiguously characterized by sophisticated techniques such as XRD, HRTEM, EELS, XPS, FT-IR and nitrogen adsorption. The pore diameter of the above materials can easily be tuned by changing the pore diameter of mesoporous silica template with keeping the weight ratio of carbon and nitrogen source constant. 1. Introduction Mesoporous carbon materials [1-2] with nanoscale pore sizes prepared from periodic inorganic silica templates have been receiving much attention because of their versatile uses in size and shape selective adsorption media, chromatographic separation, catalysts, nanoreactors, battery electrodes, capacitors, energy storage and biomedical engineering. Mesoporous carbon nitride materials (MCN) with one and three dimensional pore systems promise access to an even-wider range of application possibilities because of their unique properties such as semi-conductivity, intercalation ability, hardness, etc. Until now no such materials have been reported. However, there are lots of report on the synthesis and characterization of nonporous carbon nitride materials [3]. These materials can be prepared either from molecular or chemical precursors at very high temperatures. Very recently, Gao and Giu have reported the chemical synthesis of nonporous turbostratic carbon nitride crystallites from polymerized ethylenediamine and carbon tetrachloride [4]. Here, we used the similar chemical method for the preparation of the highly ordered one and three dimensional mesoporous carbon nitride material, designated as MCN, having pores with various diameters, high specific surface area and specific pore volume [5].
906
2. Experimental Section In a typical synthesis, the calcined mesoporous silica SBA-15 or SBA-16 was added to a mixture of ethylenediamine (EDA) and carbon tetrachloride (CTC). The resultant mixture was refluxed and stirred at 90°C for 6 h. The materials prepared using SBA-15 and SBA-16 as templates were named as MCN-1 and MCN-2, respectively. Another set of samples was prepared using SBA-15 materials synthesized at different temperature and the samples were labeled as MCN-1-T where T indicates the synthesis temperature of mesoporous silica. The template-carbon nitride polymer composites were then heat treated in a nitrogen flow to carbonize the polymer. The MCN was recovered after dissolution of the silica framework in 5 wt% hydrofluoric acid. The powder X-ray diffraction (XRD) patterns of mesoporous carbon nitride materials were collected on a Rigaku diffractometer using CuKoc (A, = 0.154 nm) radiation. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method. HRTEM images were obtained with TEM JEOL JEM-2000EX2. The preparation of samples for HRTEM analysis involved sonication in ethanol for 2 to 5 min and deposition on a copper grid. The accelerating voltage of the electron beam was 200 kV. 3. Results and Discussion The ordered one-dimensional (1-D) mesoporous carbon nitride MCN-1 structure was investigated by powder XRD and nitrogen gas adsorption measurements. The XRD pattern of MCN-1 material shows three, clear peaks, which can be assigned to 100, 110, and 200 diffractions of 2-D hexagonal lattice (space group p6mm) with a lattice constant a1Oo = 9.52 nm, similar to the XRD pattern of parent template SBA-15 which consists of the hexagonal arrangement of cylindrical pores and the pores are interlinked by the micropores present in the walls, as shown in Fig. la. Such materials with 1-D mesopores are arranged in a hexagonal net are defined as 2-D mesostructure because the XRD pattern shows 2-D p6mm symmetry. The powder XRD pattern of MCN-1 before carbonization also exhibits the pattern similar to SBA-15, consisting of a week 100 reflection at low angle and two small peaks at a higher angle (not shown). However, the intensity of the low angle (100) and high angle peaks (110 and 200) decreases as compared to the parent SBA-15 material upon loading the CN matrix inside the mesopores. This can not be interpreted as a severe loss of structural order, but it is likely that larger contrast in density between the silica walls and the open pores relative to that between the silica walls and the CN matrix inside the pores is responsible for the observed decrease in intensity. Moreover, the unit cell constant of the MCN-1 material before carbonization (10.7 nm) is higher than the respective carbonized sample.
907
This can be ascribed to the condensation reaction between the free carbon and nitrogen in the walls resulting in a lattice contraction. The powder XRD pattern and HRTEM of 2-D mesoporous carbon nitride materials synthesized using SBA-15 with different pore diameters prepared at different synthesis temperature are also shown in Fig. la and lb, respectively. All the materials possess a sharp 100 reflection at very low angle which is typical for hexagonally ordered mesoporous materials. It is interesting to note that the d-spacing and the unit cell size of the MCN-1 materials synthesized using different pore diameters of SBA-15 materials as templates increase in the following order : MCN-1-150 > MCN-1-130 > MCN-1-100. This is a direct evidence of pore size enlargement in the MCN-1 materials. The overall carbon to nitrogen ratio of all the materials obtained from the CHN and EELS is almost same and is found to be 4.35. EEL spectra exhibit C and N K-edges located at 284 and 401 eV. The fine structure of the edges, in particular, their left-hand 2 shoulders revealing ls-Jt* electron transitions, is a fingerprint of a sp hybridization. The FT-IR and XPS data also confirm that the materials are mainly composed of C and N with a small amount of hydrogen. The trace of H comes either from the moisture or ethanol adsorbed on the surface or NH group on the MCN-1 matrix. Higher angle XRD pattern shows that the materials are partially amorphous and possess turbostratic ordering of carbon and nitrogen atoms in the CN graphene layers. 100 100
aa -
Intensity (a.u)
10.7 10.7 nm nm
b
MCN-1-150 MCN-1-150
(0
I
110
MCN-1-130 MCN-1-130 9.51 9.51 nm nm
200
00
22
MCN-1-100 MCN-1-100
44
66
Angle θ (theta) Angle 22θ (theta)
88
10 10
Fig. 1 (a) Powder XRD patterns of mesoporous carbon nitride materials with different pore diameters and (b) HRTEM of MCN-1-130. The powder XRD pattern of MCN-2 shows a sharp 110 reflection with a broad 200 reflection and is almost similar to that for SBA-16 template (Fig. 2a), demonstrating that 3-D mesoporous cage structure is successfully replicated to the MCN-2 sample. The intensity of the 110 peak of MCN-2 is much higher than that of the silica template, indicating that the enhancement in the structural order is occurred during the replication process. The unit cell
908
3e+4
600 500
MCN-2
b
3
Intensity (cps)
-1
a
3e+4
Amount Adsorbed (cm .g STP)
parameter of the MCN-2 is calculated using the formula 2 2 dnoand is found to be 13.4 nm. The nitrogen adsorption isotherm of MCN-2 in comparison to that of the parent silica template is shown in Figure 2b. Both the materials exhibit type IV isotherm with a broad hysteresis loop which is typical for the well ordered cage type mesoporous material. It should be also noted that the specific surface area and the specific pore volume of MCN-2 are much higher than those of the template and the 2-D mesoporous carbon nitride, MCN-1, prepared from the SBA-15 template. It is also important to note that the preparation of MCN-2 failed when our previous synthesis procedure for MCN-1 was used in this study. This could mainly be attributed to difference in the pore structure and the diameter of SBA-16 as compared to SBA-15 template.
2e+4 2e+4
MCN-2
1e+4 5e+3 SBA-16
0 2
4 4
2 θ (degrees) 2θ
6 6
8
400
SBA-16
300 200 100 0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
Relative Pressure P/P0
Fig. 2 (a) powder XRD pattern and (b) nitrogen adsorption isotherms of MCN-2 in comparison with SBA-16
4. Conclusions Novel mesoporous carbon nitride materials with different structures have been synthesized using SBA-15 and SBA-16 hard templates, respectively. The obtained materials have been unambiguously characterized by various sophisticated techniques. Moreover, the pore diameter of the above materials can easily be tuned by changing the pore diameter of mesoporous silica template with keeping the weight ratio of carbon and nitrogen source constant. 5. References 1. R. Ryoo, S. H. Joo, S. Jun, J. Phys. Chem. B, 103 (1999), 7743. 2. A. Vinu, C. Streb, V. Murugesan, M. Hartmann, J. Phys. Chem. B, 107 (2003), 8297. 3. A. Vinu, M. Miyahara, K. Ariga, J. Phys. Chem. B, 109 (2005), 6436. 4. Y. Qiu, L. Gao, Chem. Commun., (2003), 2378; E. Kroke, M. Schwarz, Coordin. Chem. Rev., 248 (2004), 493. 5. A. Vinu, K. Ariga, T. Mori, T. Nakanishi, S. Hishita, D. Golberg, Y. Bando, Adv. Mater., 17(2005), 1648.
905 905
One and three dimensional mesoporous carbon nitride molecular sieves with tunable pore diameters Ajayan Vinua*, Toshiyuki Moria, Sunichi Hishitaa, Srinivasan Anandana, Veerappan Vaithilingam Balasubramanianb and Katsuhiko Arigac "Fuel Cell Materials Center, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Ibaraki, Japan; Email: [email protected] b Department of Marine Biotechnology, Asan-City 336-745, Chungnam, Soonchunhyang University, South Korea c Supermolecules Group, National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Ibaraki, Japan One- and three-dimensional mesoporous carbon nitride materials have been synthesized using SBA-15 and SBA-16 mesoporous silica hard templates, respectively. The obtained materials have been unambiguously characterized by sophisticated techniques such as XRD, HRTEM, EELS, XPS, FT-IR and nitrogen adsorption. The pore diameter of the above materials can easily be tuned by changing the pore diameter of mesoporous silica template with keeping the weight ratio of carbon and nitrogen source constant. 1. Introduction Mesoporous carbon materials [1-2] with nanoscale pore sizes prepared from periodic inorganic silica templates have been receiving much attention because of their versatile uses in size and shape selective adsorption media, chromatographic separation, catalysts, nanoreactors, battery electrodes, capacitors, energy storage and biomedical engineering. Mesoporous carbon nitride materials (MCN) with one and three dimensional pore systems promise access to an even-wider range of application possibilities because of their unique properties such as semi-conductivity, intercalation ability, hardness, etc. Until now no such materials have been reported. However, there are lots of report on the synthesis and characterization of nonporous carbon nitride materials [3]. These materials can be prepared either from molecular or chemical precursors at very high temperatures. Very recently, Gao and Giu have reported the chemical synthesis of nonporous turbostratic carbon nitride crystallites from polymerized ethylenediamine and carbon tetrachloride [4]. Here, we used the similar chemical method for the preparation of the highly ordered one and three dimensional mesoporous carbon nitride material, designated as MCN, having pores with various diameters, high specific surface area and specific pore volume [5].
906
2. Experimental Section In a typical synthesis, the calcined mesoporous silica SBA-15 or SBA-16 was added to a mixture of ethylenediamine (EDA) and carbon tetrachloride (CTC). The resultant mixture was refluxed and stirred at 90°C for 6 h. The materials prepared using SBA-15 and SBA-16 as templates were named as MCN-1 and MCN-2, respectively. Another set of samples was prepared using SBA-15 materials synthesized at different temperature and the samples were labeled as MCN-1-T where T indicates the synthesis temperature of mesoporous silica. The template-carbon nitride polymer composites were then heat treated in a nitrogen flow to carbonize the polymer. The MCN was recovered after dissolution of the silica framework in 5 wt% hydrofluoric acid. The powder X-ray diffraction (XRD) patterns of mesoporous carbon nitride materials were collected on a Rigaku diffractometer using CuKoc (A, = 0.154 nm) radiation. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method. HRTEM images were obtained with TEM JEOL JEM-2000EX2. The preparation of samples for HRTEM analysis involved sonication in ethanol for 2 to 5 min and deposition on a copper grid. The accelerating voltage of the electron beam was 200 kV. 3. Results and Discussion The ordered one-dimensional (1-D) mesoporous carbon nitride MCN-1 structure was investigated by powder XRD and nitrogen gas adsorption measurements. The XRD pattern of MCN-1 material shows three, clear peaks, which can be assigned to 100, 110, and 200 diffractions of 2-D hexagonal lattice (space group p6mm) with a lattice constant a1Oo = 9.52 nm, similar to the XRD pattern of parent template SBA-15 which consists of the hexagonal arrangement of cylindrical pores and the pores are interlinked by the micropores present in the walls, as shown in Fig. la. Such materials with 1-D mesopores are arranged in a hexagonal net are defined as 2-D mesostructure because the XRD pattern shows 2-D p6mm symmetry. The powder XRD pattern of MCN-1 before carbonization also exhibits the pattern similar to SBA-15, consisting of a week 100 reflection at low angle and two small peaks at a higher angle (not shown). However, the intensity of the low angle (100) and high angle peaks (110 and 200) decreases as compared to the parent SBA-15 material upon loading the CN matrix inside the mesopores. This can not be interpreted as a severe loss of structural order, but it is likely that larger contrast in density between the silica walls and the open pores relative to that between the silica walls and the CN matrix inside the pores is responsible for the observed decrease in intensity. Moreover, the unit cell constant of the MCN-1 material before carbonization (10.7 nm) is higher than the respective carbonized sample.
907
This can be ascribed to the condensation reaction between the free carbon and nitrogen in the walls resulting in a lattice contraction. The powder XRD pattern and HRTEM of 2-D mesoporous carbon nitride materials synthesized using SBA-15 with different pore diameters prepared at different synthesis temperature are also shown in Fig. la and lb, respectively. All the materials possess a sharp 100 reflection at very low angle which is typical for hexagonally ordered mesoporous materials. It is interesting to note that the d-spacing and the unit cell size of the MCN-1 materials synthesized using different pore diameters of SBA-15 materials as templates increase in the following order : MCN-1-150 > MCN-1-130 > MCN-1-100. This is a direct evidence of pore size enlargement in the MCN-1 materials. The overall carbon to nitrogen ratio of all the materials obtained from the CHN and EELS is almost same and is found to be 4.35. EEL spectra exhibit C and N K-edges located at 284 and 401 eV. The fine structure of the edges, in particular, their left-hand 2 shoulders revealing ls-Jt* electron transitions, is a fingerprint of a sp hybridization. The FT-IR and XPS data also confirm that the materials are mainly composed of C and N with a small amount of hydrogen. The trace of H comes either from the moisture or ethanol adsorbed on the surface or NH group on the MCN-1 matrix. Higher angle XRD pattern shows that the materials are partially amorphous and possess turbostratic ordering of carbon and nitrogen atoms in the CN graphene layers. 100 100
aa -
Intensity (a.u)
10.7 10.7 nm nm
b
MCN-1-150 MCN-1-150
(0
I
110
MCN-1-130 MCN-1-130 9.51 9.51 nm nm
200
00
22
MCN-1-100 MCN-1-100
44
66
Angle θ (theta) Angle 22θ (theta)
88
10 10
Fig. 1 (a) Powder XRD patterns of mesoporous carbon nitride materials with different pore diameters and (b) HRTEM of MCN-1-130. The powder XRD pattern of MCN-2 shows a sharp 110 reflection with a broad 200 reflection and is almost similar to that for SBA-16 template (Fig. 2a), demonstrating that 3-D mesoporous cage structure is successfully replicated to the MCN-2 sample. The intensity of the 110 peak of MCN-2 is much higher than that of the silica template, indicating that the enhancement in the structural order is occurred during the replication process. The unit cell
908
3e+4
600 500
MCN-2
b
3
Intensity (cps)
-1
a
3e+4
Amount Adsorbed (cm .g STP)
parameter of the MCN-2 is calculated using the formula 2 2 dnoand is found to be 13.4 nm. The nitrogen adsorption isotherm of MCN-2 in comparison to that of the parent silica template is shown in Figure 2b. Both the materials exhibit type IV isotherm with a broad hysteresis loop which is typical for the well ordered cage type mesoporous material. It should be also noted that the specific surface area and the specific pore volume of MCN-2 are much higher than those of the template and the 2-D mesoporous carbon nitride, MCN-1, prepared from the SBA-15 template. It is also important to note that the preparation of MCN-2 failed when our previous synthesis procedure for MCN-1 was used in this study. This could mainly be attributed to difference in the pore structure and the diameter of SBA-16 as compared to SBA-15 template.
2e+4 2e+4
MCN-2
1e+4 5e+3 SBA-16
0 2
4 4
2 θ (degrees) 2θ
6 6
8
400
SBA-16
300 200 100 0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
Relative Pressure P/P0
Fig. 2 (a) powder XRD pattern and (b) nitrogen adsorption isotherms of MCN-2 in comparison with SBA-16
4. Conclusions Novel mesoporous carbon nitride materials with different structures have been synthesized using SBA-15 and SBA-16 hard templates, respectively. The obtained materials have been unambiguously characterized by various sophisticated techniques. Moreover, the pore diameter of the above materials can easily be tuned by changing the pore diameter of mesoporous silica template with keeping the weight ratio of carbon and nitrogen source constant. 5. References 1. R. Ryoo, S. H. Joo, S. Jun, J. Phys. Chem. B, 103 (1999), 7743. 2. A. Vinu, C. Streb, V. Murugesan, M. Hartmann, J. Phys. Chem. B, 107 (2003), 8297. 3. A. Vinu, M. Miyahara, K. Ariga, J. Phys. Chem. B, 109 (2005), 6436. 4. Y. Qiu, L. Gao, Chem. Commun., (2003), 2378; E. Kroke, M. Schwarz, Coordin. Chem. Rev., 248 (2004), 493. 5. A. Vinu, K. Ariga, T. Mori, T. Nakanishi, S. Hishita, D. Golberg, Y. Bando, Adv. Mater., 17(2005), 1648.