- Email: [email protected]
Synthesis and characterization of hybrid nano-crystallites inside self-ordered mesoporous carbon
Microporous and Mesoporous Materials 100 (2007) 227–232 www.elsevier.com/locate/micromeso
Synthesis and characterization of hybrid nano-crystallites inside self-ordered mesoporous carbon Shenmin Zhu b
a,*
, Haoshen Zhou
b,*
, Deyue Yan
a
a State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, 1954 Huashan Road, Shanghai 200030, PR China Energy Electronics Institute, National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan
Received 19 August 2006; received in revised form 25 October 2006; accepted 25 October 2006 Available online 15 December 2006
Abstract The design and construction of nano-crystallites inside ordered mesoporous carbon is of great interest for potential applications in many fields. One of the main challenges is how to control hybrid nano-crystallites formed inside the pores. We describe a synthesis strategy of impregnation/hydrothermal method for incorporation of hybrid nano-crystallites Ru0.3Cr0.7O2 inside CMK-3 with the average size of the nano-crystallites around 2.8–3.05 nm. The texture/structures of the resultant materials have been characterized by X-ray diffraction, transmission electron microscope, and nitrogen adsorption/desorption measurements. No nano-crystallites are observed to be generated on the external surface of CMK-3. The resultant material exhibits a high specific capacitance of approximately 226 F g 1. This approach is expected to be applied to other hybrid metals oxides synthesized inside CMK-3 with specific structures and properties. Furthermore, it provides a versatile route for expanding the application of ordered mesoporous carbon with diverse pore arrangements. Ó 2006 Elsevier Inc. All rights reserved. Keywords: Mesoporous carbon; Impregnation/hydrothermal method; Hybrid; Nano-crystallites
1. Introduction Ordered mesoporous carbon has attracted great attentions for promising applications in many fields [1–10], with its regular array of uniform nanopores that exhibit high specific surface area, large adsorption capacities (1– 2 cm3 g 1), and excellent thermal and mechanical stability. Synthesis of nano-crystallites within ordered mesoporous carbon represents a constricted growth process that enables control of the size of the nanoparticles and arranges them in useful dimensions, resulting in functional materials with special physical and chemical properties. Ryoo succeeded in dispersing nanoparticles metal platinum in pipe-like nanoporous carbon with tunable pores by using impregnation/reduction method [1]. After that, Holmes and Woo *
Corresponding authors. Tel.: +86 21 62809091; fax: +86 21 62822012 (S. Zhu). E-mail address: [email protected] (S. Zhu). 1387-1811/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2006.10.038
developed Pt–C and Co–C nanocomposites independently by pyrolysis of carbon and inorganic precursors in SBA15 mesopores [11,12]. Unfortunately, large particles were observed generated on the external surface due to the limited mass transfer of the precursor in liquid phase when the loading weight reached 35 wt% [11]. Recently, we described the synthesis of nanoparticles MnO2 inside mesoporous carbon via sonochemical method [13]. However, up till now, no data concerning about hybrid nano-crystallites growing inside mesoporous carbon has been reported. It seems urgent to develop new efficient method to extend such an application. Here, we present a novel hydrothermal method for preparing nano-crystalline hybrid ruthenium chromium oxide inside CMK-3 with micropores formed for the diffusion of protons, for it use as electrochemical capacitors. Ruthenium dioxide has been receiving considerable attention due to its extraordinarily high catalytic activity, as well as its great potential for application in electrochemical
228
S. Zhu et al. / Microporous and Mesoporous Materials 100 (2007) 227–232
supercapacitors [14–16]. However, two obstacles of the low porosity and the high price of ruthenium, currently make the material unsuitable for practical applications. Therefore, substitutive systems have been considered to decrease the price, such as ruthenium–chromium oxide (RuxCr1 xO2) [17]. A decrease in particle size would facilitate the utilization of the active material due to the increase of surface area per unit volume. Therefore, the preparation of hydrous ruthenium oxide/carbon composites material is at present another object of considerable research interest [18–20]. CMK-3 [3] is constructed with uniformly sized 1-D carbon rods and has an average pore size of around 4.0 nm. The super-capacitance of RuxCr1 xO2 in combination with the special characteristics of CMK-3 is expected to improve the electrochemical performance of the resultant material in addition to reducing its cost. Here, we described a synthesis strategy to incorporate nano-crystalline hybrid material RuxCr1 xO2 into mesoporous carbon CMK-3 with interior micropores formed for the diffusion of aqueous electrolyte. The nano-crystalline hybrid material RuxCr1 xO2 was successfully prepared by an impregnation/hydrothermal treatment process. No nano-crystallites were detected on the outer surface of the host CMK-3, which could be attributed to the hydrothermal treatment’s having a preferred growth direction together with the functionalized mesoporous channels of CMK-3.
were carried out on a Hitachi-800 operated at 200 kV. Thermogravimetric analysis-differential thermal analysis (TG-DTA) was carried out on a MacScience TG-DTA 2010S. ICP was performed on a Therm Jarrell Ash IRIS/ AP. Electrodes were fabricated by mixing the sample with 5 wt% polytetrafluoroethylene (PTFE) binder pressed onto titanium foil. Electrochemical measurements were carried out with beaker-type cells employing the sample electrode as the working electrode, a saturated calomel electrode (SCE) as the reference electrode, and 1 M H2SO4 as the electrolyte. Cyclic voltammograms were recorded in the range of 0–1 V at a scan rate of 1 mV s 1. 3. Results and discussion The reaction process is described below: Firstly, CMK-3 was surface modified with –COOH by HNO3/H2SO4, making it easy for hydrophilic precursor RuCl3 enter into the mesopores. Incorporation of Ru0.3Cr0.7O2 inside CMK-3 was implemented via a hydrothermal method, resulting in the formation of nanoparticles inside mesoporous carbon.
2. Experimental 2.1. Material synthesis The general procedure for preparation of self-ordered mesoporous silica SBA-15 is the same as that described in literature under acid conditions [21]. The process of producing carbon is similar to that described by Ryoo [3]. Ru0.3Cr0.7O2 incorporated into CMK-3 was synthesized as illustrated below: 0.5 g mesoporous carbon CMK-3 was treated with concentrated H2SO4 at 75 °C for 3 h, washed with deionized water, and dried at 80 °C. This sample is denoted as m-CMK-3. Then, m-CMK-3 impregnated with RuCl3 was added to 12 ml 0.1 M K2Cr2O7 with the pH adjusted to 7.0. Reduction was carried out by placing the mixture in a Teflon oven and heating at 150 °C for 3 h. The material thus prepared is termed CMK–Ru0.3Cr0.7O2. Heat treatment was performed on CMK–Ru0.3Cr0.7O2 under nitrogen at 400 °C for 3 h and then increased to 450 °C for another 3 h. This final sample is referred to as CMK–Ru0.3Cr0.7O2–T. 2.2. Material characterization Powder XRD patterns were recorded on a MacScienceM03XHF22 using Cu Ka irradiation. Nitrogen adsorption measurements at 196 °C were performed on a Belsorpt23SA volumetric adsorption analyzer. Transmission electron microscopy and energy-dispersive X-ray (EDX) tests
Fig. 1. Powder X-ray diffraction patterns: (a) small-angle XRD patterns of CMK-3, CMK-3 modified by HNO3/H2SO4 (m-CMK-3), Ru0.3Cr0.7O2 inside CMK-3 (CMK–Ru0.3Cr0.7O2) and after heat treatment (CMK– Ru0.3Cr0.7O2–T) and (b) wide-angle XRD patterns of CMK-3, m-CMK-3, CMK–Ru0.3Cr0.7O2 and CMK- Ru0.3Cr0.7O2–T.
S. Zhu et al. / Microporous and Mesoporous Materials 100 (2007) 227–232
After heat treatment, nanoparticles were transformed into nano-crystallites inside the mesopores, and the spaces between the nano-crystallites induced many interior micropores. Hybrid nano-crystallite RuxCr1 xO2 inside mesoporous carbon CMK-3. m-CMK-3 (CMK-3 surface-modified with H2SO4/HNO3), CMK–Ru0.3Cr0.7O2 (CMK-3 with Ru0.3Cr0.7O2 nanoparticles incorporated) together with CMK–Ru0.3Cr0.7O2–T (CMK–Ru0.3Cr0.7O2 after heat treatment under nitrogen) were characterized by X-ray diffraction (XRD) (Fig. 1a). Due to the shrinkage from hydrothermal treatment, the (1 0 0) peak of m-CMK-3 in the XRD pattern is characterized by a shift to a higher angle than that of CMK-3, with the unit length decreased to 9.8 nm. Distinct (1 0 0) peaks are detected for both CMK–Ru0.3Cr0.7O2 and CMK–Ru0.3Cr0.7O2–T throughout the range of small angles, suggesting the preservation of the mesoscopic order in the composites (Fig. 1a). No dis-
229
tinct peaks in wide-angle XRD patterns are observed for CMK–Ru0.3Cr0.7O2, illustrating the amorphous characteristics of Ru0.3Cr0.7O2 in the composite (Fig. 1b). After heat treatment, several broad but still discernible reflections display for the sample of CMK–Ru0.3Cr0.7O2– T. It can be explained by the formation of nano-crystalline Ru0.3Cr0.7O2 in CMK-3 (Fig. 1b). The reflections correspond to the tetragonal phase. This result agrees well with the analysis by radiofrequency inductively coupled plasma (ICP), which shows x in RuxCr1 xO2 to be 0.3. The fact confirms that nano-crystalline Ru0.3Cr0.7O2 is formed inside CMK-3. Based on the full-width at half-maximum of the (110) reflection, an estimation of the size of nano-crystalline Ru0.3Cr0.7O2 within CMK-3 yields about 2.8 nm using the Scherrer formula [22], about 1.3 nm smaller than the pore diameter of host m-CMK-3 (about 4.1 nm). It seems reasonable to assume that the nano-crystalline
Fig. 2. TEM image of CMK-3 (a) and CMK–Ru0.3Cr0.7O2–T along (b), perpendicular (c) to the (1 0 0) direction; the insets show the corresponding structural models.
230
S. Zhu et al. / Microporous and Mesoporous Materials 100 (2007) 227–232
Ru0.3Cr0.7O2 formed would be inside the pore channels of m-CMK-3. This suggestion is corroborated directly by transmission electron microscope (TEM) images. Wellordered channels are clearly arranged in a hexagonal pattern after formation of nano-crystallites inside CMK-3 (Figs. 2b and c). High temperature treatment at 450 °C does not cause the collapse of the ordered mesoporous structure, indicating the high thermal stability of the carbon framework. Compared with the original CMK-3 in Fig. 2a and b displays the presence of nano-crystalline Ru0.3Cr0.7O2 as dark rod-like objects along the mesoporous channels. Furthermore, nano-crystalline Ru0.3Cr0.7O2 formed around the reverse hexagonal pores of CMK-3 can be also discerned from TEM images perpendicular to the direction of the hexagonal pore arrangement (Fig. 2c). Selected-area electron diffraction patterns recorded on the image area of Fig. 2c give characteristic diffuse electron diffraction rings that can be indexed to the formation of the nano-crystalline phase of Ru0.3Cr0.7O2 (not shown). Traditional TEMs have been extensively used for microanalytical investigations, which give the composition information, but only simultaneous acquisition of the image and analytical information on the same length-scale. Alternatively, scanning transmission electron microscope (STEM)
[23] provides the optimum images for spatially resolved microanalysis: the micro-analysis can be performed by the smallest probe on the specimen surface with sufficient beam current, and the proper scattering angles can be selected. Since the image resolution in STEM mode is mostly determined by the electron probe size that is then scanned across the specimen surface, the most essential part of STEM imaging is the formation of the smallest possible probe. Therefore, STEM provides a powerful tool to characterize the bulk structure and also the composition of nanocomposites. The STEM micrograph in Fig. 3a reveals that nano-crystallites are diffused very well inside the pores of CMK-3 in a large scale (Fig. 3a). Shown in Fig. 3b, hybrid nano-crystallites Ru0.3Cr0.7O2 (white wire) are observed distinctly inside the mesoporous CMK-3 (gray area) with hydrothermal method, and the impact arrangement of nano-crystallites forms a series of nanowires in the pores. On the contrary, large particles of nano-crystallites outside the pores of the carbon are apparent up to almost 20 nm diameter for the sample prepared via the traditional method (Fig. 3c). This further confirms that hydrothermal method described here is an efficient method with incorporating hybrid nano-crystallites inside mesoporous carbon. In addition to XRD, further information regarding the composition comes from transmission electron energy-dis-
Fig. 3. STEM images of hybrid nano-crystallites Ru0.3Cr0.7O2 inside CMK-3 prepared by hydrothermal method (a, b) and the traditional impregnation/ redox method (c).
S. Zhu et al. / Microporous and Mesoporous Materials 100 (2007) 227–232
persive X-ray analysis spectroscopy (TEM-EDX), which reveals the presence of Ru, Cr, O and C in CMK– Ru0.3Cr0.7O2–T. The appearance of Cu peak is related to the copper grid. In comparison to CMK-3 (990 m2 g 1), the specific surface area of m-CMK-3 is reduced slightly to 895 m2 g 1 (Fig. 4a), whereas the average pore diameter of m-CMK3 (Fig. 4b) increases to around 4.1 nm compared to that of the original CMK-3 (around 3.9 nm) (DH method). Surface modification of CMK-3 using H2SO4/HNO3 could contribute to the small changes in the pore diameter and surface area. The corresponding average pore size distributions of CMK-3 (around 3.9 nm) and m-CMK-3 are calculated using the DH method (around 4.1 nm) [24], and CMK–Ru0.3Cr0.7O2 calculated using the HK method (around 1.05 nm and 1.3 nm) [25]. All of the data are calculated from the desorption isotherm. No distinct hysteresis loop is observed after synthesis of nanoparticles Ru0.3Cr0.7O2 inside CMK-3 (Fig. 4a). In other words, the formation of nanoparticles inside CMK3 results in the disappearance of most of the mesopores. A t-plot was carried out to further investigate the reason, and the results are compiled in Table 1. The total surface
Fig. 4. Nitrogen adsorption/desorption isotherms (a) and pore-size distribution plots (b) for CMK-3 mesoporous carbon, m-CMK-3, and nanocomposite CMK–Ru0.3Cr0.7O2. The plot for CMK-3 is shifted by 200 ml STP g 1.
231
Table 1 Surface areas of m-CMK-3 and CMK–Ru0.3Cr0.7O2 calculated from t-plot Sample
Total surface area (m2 g 1)
Mesoporous surface area (m2 g 1)
Microporous surface area (m2 g 1)
m-CMK-3 CMK–Ru0.3Cr0.7O2
895 384
822 67
73 317
area is found to decrease significantly from 895 m2 g 1 for m-CMK-3 to 384 m2 g 1 for CMK–Ru0.3Cr0.7O2. The surface area owing to the mesopores decreases from 822 m2 g 1 (m-CMK-3) to 67 m2 g 1 (CMK– Ru0.3Cr0.7O2). Meanwhile, it is interesting to find that the surface area attributable to the micropores increases from 73 m2 g 1 (m-CMK-3) to 317 m2 g 1 (CMK–Ru0.3Cr0.7O2) (Table 1). Most of the mesopores disappear after the reaction of RuCl3 with K2Cr2O7, while at the same time the surface area due to micropores increases greatly. Thus, it can be postulated that the mesopores in m-CMK-3 are occupied by nanoparticles of Ru0.3Cr0.7O2 after formation of CMK–Ru0.3Cr0.7O2. This assumption can be validated by comparing the average pore diameter of m-CMK-3 with that of CMK–Ru0.3Cr0.7O2. The average pore size of CMK–Ru0.3Cr0.7O2 is estimated to be mainly around 1.05 nm and 1.3 nm using the HK method (Fig. 4b), which means that the formation of nanoparticles inside CMK-3 is maintained at about 3.05 nm and 2.8 nm. As mentioned above, after heat treatment, the average particle size of the nano-crystalline Ru0.3Cr0.7O2 is approximately 2.8 nm, slightly smaller than that of Ru0.3Cr0.7O2 nanoparticles (average sizes around 2.8 nm to 3.05 nm) in CMK– Ru0.3Cr0.7O2, due to the shrinkage of nanoparticles during the process of crystallization inside CMK-3. This analysis deduces a conclusion that the incorporation of nano-crystalline Ru0.3Cr0.7O2 is inside CMK-3. The spaces between the sites occupied by Ru0.3Cr0.7O2 nanoparticles in mesopores form micropores (about 1.05 nm and 1.3 nm), which
Fig. 5. Capacitor performances of m-CMK-3 and CMK–Ru0.3Cr0.7O2 at 1 mV s 1 in 1 M H2SO4 and a physical mixture of carbon and metal oxide as a comparison.
232
S. Zhu et al. / Microporous and Mesoporous Materials 100 (2007) 227–232
explains why the surface area attributable to micropores increases greatly after the incorporation of nanoparticles of Ru0.3Cr0.7O2 inside CMK-3 (CMK–Ru0.3Cr0.7O2). The capacitor performance of CMK–Ru0.3Cr0.7O2 was examined via cyclic voltammetry (CV) measurements shown in Fig. 5. m-CMK-3 gives a specific capacitance of 92 F g 1 at a scan rate of 1 mV s 1 in 1 M H2SO4 electrolyte. The specific capacitance is estimated to be 226 F g 1 for the composite material CMK–Ru0.3Cr0.7O2 under the same conditions. The amount of Ru0.3Cr0.7O2 in CMK–Ru0.3Cr0.7O2 is calculated by thermogravimetric analysis-differential thermal analysis (TG-DTA) to be 50%, meaning that 46 F out of the total 226 F can be attributed to 0.5 g of m-CMK per 1 g CMK–Ru0.3Cr0.7O2. As a comparison, the specific capacitance of a physical mixture of carbon and metal oxide at the same ratio of CMK–Ru0.3Cr0.7O2 is measured of 125 F g 1, 45% lower than that of CMK–Ru0.3Cr0.7O2 here reported. That is to say nano-crystalline Ru0.3Cr0.7O2 inside CMK-3 results in an increase of specific capacitance. It is worth mentioning that the shape of cyclic voltammetry shows somewhat deformed even though the composite CMK–Ru0.3Cr0.7O2 demonstrates a relatively high electrochemistry performance. This is possibly due to the existence of large amount of Cr in Ru0.3Cr0.7O2 as described by Manthiram [17]. 4. Conclusion In conclusion, hybrid nano-crystallites can be incorporated into CMK-3 in a controlled way by modifying the CMK-3 pore surface then using the impregnation/hydrotherm method. The composite materials were well characterized by XRD and TEM, particularly the pore size distribution clearly shows that nanometer sized ruthenium clusters are well dispersed all over the pores of CMK-3. The formation of micropores among the nano-crystallites or nanoparticles creates preferred sites that provide a path for the aqueous electrolyte for electrochemical use. This synthesis strategy allows us to combine the advantages of ordered mesoporous carbon together with those of the con-
trolled nano-crystallites formed inside the pores, leaving a porous interior to provide pathways. By inserting other metal oxides and hybrid nano-crystallites into ordered mesoporous carbon, it is expected to selectively design composite materials to act as high performance catalysts for gas or solutions passing through the interior or to place alloys inside for hydrogen storage systems. References [1] S.H. Joo, S.J. Choi, I. Oh, J. Kwak, Z. Liu, O. Terasaki, R. Ryoo, Nature 412 (2001) 169. [2] M. Kruk, M. Jaroniec, R. Ryoo, S.H. Joo, J. Phys. Chem. B 104 (2000) 960. [3] S. Jun, S.H. Joo, R. Ryoo, M. Kruk, M. Jaroniec, Z. Liu, T. Ohsuna, O. Terasaki, J. Am. Chem. Soc. 122 (2000) 10712. [4] S. Han, T. Hyeon, Chem. Commun. (1999) 1955. [5] A.C. Dillon, Nature 386 (1997) 377. [6] J.C. Hulteen, C.R. Martin, J. Mater. Chem. 7 (1997) 1075. [7] S. Mann, G.A. Ozin, Nature 382 (1996) 313. [8] G.D. Stucky, J.E. MacDougall, Science 247 (1990) 669. [9] J. Lee, S. Yoon, T. Hyeon, S.M. Oh, K.B. Kim, Chem. Commun. (1999) 2177. [10] R. Ryoo, S.H. Joo, S. Jun, J. Phys. Chem. B 103 (1999) 7743. [11] S.M. Holmes, P.E. Foran, P.L. Roberts, J.M. Newton, Chem. Commun. (2005) 1912. [12] W.C. Choi, S.I. Woo, M.K. Jeon, J.M. Sohn, M.R. Kim, H.J. Jeon, Adv. Mater. 17 (2005) 446. [13] S. Zhu, H. Zhou, M. Hibino, I. Honma, M. Ichihara, Adv. Funct Mater. 15 (2005) 381. [14] S. Trasatti, Electrochim. Acta 36 (1991) 225. [15] D.R. Rolison, P.L. Hagans, K.E. Swider, J.W. Long, Langmuir 15 (1999) 774. [16] J.P. Zheng, P.J. Cygan, T.R. Jow, J. Electrochem. Soc. 142 (1995) 2699. [17] Y.U. Jeong, A. Manthiram, Electrochem. Solid State Lett. 3 (2000) 205. [18] J.M. Miller, B. Dunn, J. Electrochem. Soc. 144 (1997) L309. [19] H. Kim, B.N. Popov, J. Power Sources 104 (2002) 52. [20] Y. Sato, K. Yomogida, T. Nanaumi, Y. Ohsawa, M. Kawai, Electrochem. Solid State Lett. 3 (2000) 113. [21] M. Kruk, M. Jaroniec, C.H. Ko, R. Ryoo, Chem. Mater. 12 (2000) 1961. [22] W. Xu, Y. Liao, D.L. Akins, J. Phys. Chem. B 106 (2002) 11127. [23] R.F. Klie, Y. Zhu, Micron 36 (2005) 219. [24] D. Dollimore, G.R. Heal, J. Appl. Chem. 14 (1964) 109. [25] G. Horvath, K. Kawazoe, J. Chem. Eng. Jpn. 16 (1983) 470.
Synthesis and characterization of hybrid nano-crystallites inside self-ordered mesoporous carbon Shenmin Zhu b
a,*
, Haoshen Zhou
b,*
, Deyue Yan
a
a State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, 1954 Huashan Road, Shanghai 200030, PR China Energy Electronics Institute, National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan
Received 19 August 2006; received in revised form 25 October 2006; accepted 25 October 2006 Available online 15 December 2006
Abstract The design and construction of nano-crystallites inside ordered mesoporous carbon is of great interest for potential applications in many fields. One of the main challenges is how to control hybrid nano-crystallites formed inside the pores. We describe a synthesis strategy of impregnation/hydrothermal method for incorporation of hybrid nano-crystallites Ru0.3Cr0.7O2 inside CMK-3 with the average size of the nano-crystallites around 2.8–3.05 nm. The texture/structures of the resultant materials have been characterized by X-ray diffraction, transmission electron microscope, and nitrogen adsorption/desorption measurements. No nano-crystallites are observed to be generated on the external surface of CMK-3. The resultant material exhibits a high specific capacitance of approximately 226 F g 1. This approach is expected to be applied to other hybrid metals oxides synthesized inside CMK-3 with specific structures and properties. Furthermore, it provides a versatile route for expanding the application of ordered mesoporous carbon with diverse pore arrangements. Ó 2006 Elsevier Inc. All rights reserved. Keywords: Mesoporous carbon; Impregnation/hydrothermal method; Hybrid; Nano-crystallites
1. Introduction Ordered mesoporous carbon has attracted great attentions for promising applications in many fields [1–10], with its regular array of uniform nanopores that exhibit high specific surface area, large adsorption capacities (1– 2 cm3 g 1), and excellent thermal and mechanical stability. Synthesis of nano-crystallites within ordered mesoporous carbon represents a constricted growth process that enables control of the size of the nanoparticles and arranges them in useful dimensions, resulting in functional materials with special physical and chemical properties. Ryoo succeeded in dispersing nanoparticles metal platinum in pipe-like nanoporous carbon with tunable pores by using impregnation/reduction method [1]. After that, Holmes and Woo *
Corresponding authors. Tel.: +86 21 62809091; fax: +86 21 62822012 (S. Zhu). E-mail address: [email protected] (S. Zhu). 1387-1811/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2006.10.038
developed Pt–C and Co–C nanocomposites independently by pyrolysis of carbon and inorganic precursors in SBA15 mesopores [11,12]. Unfortunately, large particles were observed generated on the external surface due to the limited mass transfer of the precursor in liquid phase when the loading weight reached 35 wt% [11]. Recently, we described the synthesis of nanoparticles MnO2 inside mesoporous carbon via sonochemical method [13]. However, up till now, no data concerning about hybrid nano-crystallites growing inside mesoporous carbon has been reported. It seems urgent to develop new efficient method to extend such an application. Here, we present a novel hydrothermal method for preparing nano-crystalline hybrid ruthenium chromium oxide inside CMK-3 with micropores formed for the diffusion of protons, for it use as electrochemical capacitors. Ruthenium dioxide has been receiving considerable attention due to its extraordinarily high catalytic activity, as well as its great potential for application in electrochemical
228
S. Zhu et al. / Microporous and Mesoporous Materials 100 (2007) 227–232
supercapacitors [14–16]. However, two obstacles of the low porosity and the high price of ruthenium, currently make the material unsuitable for practical applications. Therefore, substitutive systems have been considered to decrease the price, such as ruthenium–chromium oxide (RuxCr1 xO2) [17]. A decrease in particle size would facilitate the utilization of the active material due to the increase of surface area per unit volume. Therefore, the preparation of hydrous ruthenium oxide/carbon composites material is at present another object of considerable research interest [18–20]. CMK-3 [3] is constructed with uniformly sized 1-D carbon rods and has an average pore size of around 4.0 nm. The super-capacitance of RuxCr1 xO2 in combination with the special characteristics of CMK-3 is expected to improve the electrochemical performance of the resultant material in addition to reducing its cost. Here, we described a synthesis strategy to incorporate nano-crystalline hybrid material RuxCr1 xO2 into mesoporous carbon CMK-3 with interior micropores formed for the diffusion of aqueous electrolyte. The nano-crystalline hybrid material RuxCr1 xO2 was successfully prepared by an impregnation/hydrothermal treatment process. No nano-crystallites were detected on the outer surface of the host CMK-3, which could be attributed to the hydrothermal treatment’s having a preferred growth direction together with the functionalized mesoporous channels of CMK-3.
were carried out on a Hitachi-800 operated at 200 kV. Thermogravimetric analysis-differential thermal analysis (TG-DTA) was carried out on a MacScience TG-DTA 2010S. ICP was performed on a Therm Jarrell Ash IRIS/ AP. Electrodes were fabricated by mixing the sample with 5 wt% polytetrafluoroethylene (PTFE) binder pressed onto titanium foil. Electrochemical measurements were carried out with beaker-type cells employing the sample electrode as the working electrode, a saturated calomel electrode (SCE) as the reference electrode, and 1 M H2SO4 as the electrolyte. Cyclic voltammograms were recorded in the range of 0–1 V at a scan rate of 1 mV s 1. 3. Results and discussion The reaction process is described below: Firstly, CMK-3 was surface modified with –COOH by HNO3/H2SO4, making it easy for hydrophilic precursor RuCl3 enter into the mesopores. Incorporation of Ru0.3Cr0.7O2 inside CMK-3 was implemented via a hydrothermal method, resulting in the formation of nanoparticles inside mesoporous carbon.
2. Experimental 2.1. Material synthesis The general procedure for preparation of self-ordered mesoporous silica SBA-15 is the same as that described in literature under acid conditions [21]. The process of producing carbon is similar to that described by Ryoo [3]. Ru0.3Cr0.7O2 incorporated into CMK-3 was synthesized as illustrated below: 0.5 g mesoporous carbon CMK-3 was treated with concentrated H2SO4 at 75 °C for 3 h, washed with deionized water, and dried at 80 °C. This sample is denoted as m-CMK-3. Then, m-CMK-3 impregnated with RuCl3 was added to 12 ml 0.1 M K2Cr2O7 with the pH adjusted to 7.0. Reduction was carried out by placing the mixture in a Teflon oven and heating at 150 °C for 3 h. The material thus prepared is termed CMK–Ru0.3Cr0.7O2. Heat treatment was performed on CMK–Ru0.3Cr0.7O2 under nitrogen at 400 °C for 3 h and then increased to 450 °C for another 3 h. This final sample is referred to as CMK–Ru0.3Cr0.7O2–T. 2.2. Material characterization Powder XRD patterns were recorded on a MacScienceM03XHF22 using Cu Ka irradiation. Nitrogen adsorption measurements at 196 °C were performed on a Belsorpt23SA volumetric adsorption analyzer. Transmission electron microscopy and energy-dispersive X-ray (EDX) tests
Fig. 1. Powder X-ray diffraction patterns: (a) small-angle XRD patterns of CMK-3, CMK-3 modified by HNO3/H2SO4 (m-CMK-3), Ru0.3Cr0.7O2 inside CMK-3 (CMK–Ru0.3Cr0.7O2) and after heat treatment (CMK– Ru0.3Cr0.7O2–T) and (b) wide-angle XRD patterns of CMK-3, m-CMK-3, CMK–Ru0.3Cr0.7O2 and CMK- Ru0.3Cr0.7O2–T.
S. Zhu et al. / Microporous and Mesoporous Materials 100 (2007) 227–232
After heat treatment, nanoparticles were transformed into nano-crystallites inside the mesopores, and the spaces between the nano-crystallites induced many interior micropores. Hybrid nano-crystallite RuxCr1 xO2 inside mesoporous carbon CMK-3. m-CMK-3 (CMK-3 surface-modified with H2SO4/HNO3), CMK–Ru0.3Cr0.7O2 (CMK-3 with Ru0.3Cr0.7O2 nanoparticles incorporated) together with CMK–Ru0.3Cr0.7O2–T (CMK–Ru0.3Cr0.7O2 after heat treatment under nitrogen) were characterized by X-ray diffraction (XRD) (Fig. 1a). Due to the shrinkage from hydrothermal treatment, the (1 0 0) peak of m-CMK-3 in the XRD pattern is characterized by a shift to a higher angle than that of CMK-3, with the unit length decreased to 9.8 nm. Distinct (1 0 0) peaks are detected for both CMK–Ru0.3Cr0.7O2 and CMK–Ru0.3Cr0.7O2–T throughout the range of small angles, suggesting the preservation of the mesoscopic order in the composites (Fig. 1a). No dis-
229
tinct peaks in wide-angle XRD patterns are observed for CMK–Ru0.3Cr0.7O2, illustrating the amorphous characteristics of Ru0.3Cr0.7O2 in the composite (Fig. 1b). After heat treatment, several broad but still discernible reflections display for the sample of CMK–Ru0.3Cr0.7O2– T. It can be explained by the formation of nano-crystalline Ru0.3Cr0.7O2 in CMK-3 (Fig. 1b). The reflections correspond to the tetragonal phase. This result agrees well with the analysis by radiofrequency inductively coupled plasma (ICP), which shows x in RuxCr1 xO2 to be 0.3. The fact confirms that nano-crystalline Ru0.3Cr0.7O2 is formed inside CMK-3. Based on the full-width at half-maximum of the (110) reflection, an estimation of the size of nano-crystalline Ru0.3Cr0.7O2 within CMK-3 yields about 2.8 nm using the Scherrer formula [22], about 1.3 nm smaller than the pore diameter of host m-CMK-3 (about 4.1 nm). It seems reasonable to assume that the nano-crystalline
Fig. 2. TEM image of CMK-3 (a) and CMK–Ru0.3Cr0.7O2–T along (b), perpendicular (c) to the (1 0 0) direction; the insets show the corresponding structural models.
230
S. Zhu et al. / Microporous and Mesoporous Materials 100 (2007) 227–232
Ru0.3Cr0.7O2 formed would be inside the pore channels of m-CMK-3. This suggestion is corroborated directly by transmission electron microscope (TEM) images. Wellordered channels are clearly arranged in a hexagonal pattern after formation of nano-crystallites inside CMK-3 (Figs. 2b and c). High temperature treatment at 450 °C does not cause the collapse of the ordered mesoporous structure, indicating the high thermal stability of the carbon framework. Compared with the original CMK-3 in Fig. 2a and b displays the presence of nano-crystalline Ru0.3Cr0.7O2 as dark rod-like objects along the mesoporous channels. Furthermore, nano-crystalline Ru0.3Cr0.7O2 formed around the reverse hexagonal pores of CMK-3 can be also discerned from TEM images perpendicular to the direction of the hexagonal pore arrangement (Fig. 2c). Selected-area electron diffraction patterns recorded on the image area of Fig. 2c give characteristic diffuse electron diffraction rings that can be indexed to the formation of the nano-crystalline phase of Ru0.3Cr0.7O2 (not shown). Traditional TEMs have been extensively used for microanalytical investigations, which give the composition information, but only simultaneous acquisition of the image and analytical information on the same length-scale. Alternatively, scanning transmission electron microscope (STEM)
[23] provides the optimum images for spatially resolved microanalysis: the micro-analysis can be performed by the smallest probe on the specimen surface with sufficient beam current, and the proper scattering angles can be selected. Since the image resolution in STEM mode is mostly determined by the electron probe size that is then scanned across the specimen surface, the most essential part of STEM imaging is the formation of the smallest possible probe. Therefore, STEM provides a powerful tool to characterize the bulk structure and also the composition of nanocomposites. The STEM micrograph in Fig. 3a reveals that nano-crystallites are diffused very well inside the pores of CMK-3 in a large scale (Fig. 3a). Shown in Fig. 3b, hybrid nano-crystallites Ru0.3Cr0.7O2 (white wire) are observed distinctly inside the mesoporous CMK-3 (gray area) with hydrothermal method, and the impact arrangement of nano-crystallites forms a series of nanowires in the pores. On the contrary, large particles of nano-crystallites outside the pores of the carbon are apparent up to almost 20 nm diameter for the sample prepared via the traditional method (Fig. 3c). This further confirms that hydrothermal method described here is an efficient method with incorporating hybrid nano-crystallites inside mesoporous carbon. In addition to XRD, further information regarding the composition comes from transmission electron energy-dis-
Fig. 3. STEM images of hybrid nano-crystallites Ru0.3Cr0.7O2 inside CMK-3 prepared by hydrothermal method (a, b) and the traditional impregnation/ redox method (c).
S. Zhu et al. / Microporous and Mesoporous Materials 100 (2007) 227–232
persive X-ray analysis spectroscopy (TEM-EDX), which reveals the presence of Ru, Cr, O and C in CMK– Ru0.3Cr0.7O2–T. The appearance of Cu peak is related to the copper grid. In comparison to CMK-3 (990 m2 g 1), the specific surface area of m-CMK-3 is reduced slightly to 895 m2 g 1 (Fig. 4a), whereas the average pore diameter of m-CMK3 (Fig. 4b) increases to around 4.1 nm compared to that of the original CMK-3 (around 3.9 nm) (DH method). Surface modification of CMK-3 using H2SO4/HNO3 could contribute to the small changes in the pore diameter and surface area. The corresponding average pore size distributions of CMK-3 (around 3.9 nm) and m-CMK-3 are calculated using the DH method (around 4.1 nm) [24], and CMK–Ru0.3Cr0.7O2 calculated using the HK method (around 1.05 nm and 1.3 nm) [25]. All of the data are calculated from the desorption isotherm. No distinct hysteresis loop is observed after synthesis of nanoparticles Ru0.3Cr0.7O2 inside CMK-3 (Fig. 4a). In other words, the formation of nanoparticles inside CMK3 results in the disappearance of most of the mesopores. A t-plot was carried out to further investigate the reason, and the results are compiled in Table 1. The total surface
Fig. 4. Nitrogen adsorption/desorption isotherms (a) and pore-size distribution plots (b) for CMK-3 mesoporous carbon, m-CMK-3, and nanocomposite CMK–Ru0.3Cr0.7O2. The plot for CMK-3 is shifted by 200 ml STP g 1.
231
Table 1 Surface areas of m-CMK-3 and CMK–Ru0.3Cr0.7O2 calculated from t-plot Sample
Total surface area (m2 g 1)
Mesoporous surface area (m2 g 1)
Microporous surface area (m2 g 1)
m-CMK-3 CMK–Ru0.3Cr0.7O2
895 384
822 67
73 317
area is found to decrease significantly from 895 m2 g 1 for m-CMK-3 to 384 m2 g 1 for CMK–Ru0.3Cr0.7O2. The surface area owing to the mesopores decreases from 822 m2 g 1 (m-CMK-3) to 67 m2 g 1 (CMK– Ru0.3Cr0.7O2). Meanwhile, it is interesting to find that the surface area attributable to the micropores increases from 73 m2 g 1 (m-CMK-3) to 317 m2 g 1 (CMK–Ru0.3Cr0.7O2) (Table 1). Most of the mesopores disappear after the reaction of RuCl3 with K2Cr2O7, while at the same time the surface area due to micropores increases greatly. Thus, it can be postulated that the mesopores in m-CMK-3 are occupied by nanoparticles of Ru0.3Cr0.7O2 after formation of CMK–Ru0.3Cr0.7O2. This assumption can be validated by comparing the average pore diameter of m-CMK-3 with that of CMK–Ru0.3Cr0.7O2. The average pore size of CMK–Ru0.3Cr0.7O2 is estimated to be mainly around 1.05 nm and 1.3 nm using the HK method (Fig. 4b), which means that the formation of nanoparticles inside CMK-3 is maintained at about 3.05 nm and 2.8 nm. As mentioned above, after heat treatment, the average particle size of the nano-crystalline Ru0.3Cr0.7O2 is approximately 2.8 nm, slightly smaller than that of Ru0.3Cr0.7O2 nanoparticles (average sizes around 2.8 nm to 3.05 nm) in CMK– Ru0.3Cr0.7O2, due to the shrinkage of nanoparticles during the process of crystallization inside CMK-3. This analysis deduces a conclusion that the incorporation of nano-crystalline Ru0.3Cr0.7O2 is inside CMK-3. The spaces between the sites occupied by Ru0.3Cr0.7O2 nanoparticles in mesopores form micropores (about 1.05 nm and 1.3 nm), which
Fig. 5. Capacitor performances of m-CMK-3 and CMK–Ru0.3Cr0.7O2 at 1 mV s 1 in 1 M H2SO4 and a physical mixture of carbon and metal oxide as a comparison.
232
S. Zhu et al. / Microporous and Mesoporous Materials 100 (2007) 227–232
explains why the surface area attributable to micropores increases greatly after the incorporation of nanoparticles of Ru0.3Cr0.7O2 inside CMK-3 (CMK–Ru0.3Cr0.7O2). The capacitor performance of CMK–Ru0.3Cr0.7O2 was examined via cyclic voltammetry (CV) measurements shown in Fig. 5. m-CMK-3 gives a specific capacitance of 92 F g 1 at a scan rate of 1 mV s 1 in 1 M H2SO4 electrolyte. The specific capacitance is estimated to be 226 F g 1 for the composite material CMK–Ru0.3Cr0.7O2 under the same conditions. The amount of Ru0.3Cr0.7O2 in CMK–Ru0.3Cr0.7O2 is calculated by thermogravimetric analysis-differential thermal analysis (TG-DTA) to be 50%, meaning that 46 F out of the total 226 F can be attributed to 0.5 g of m-CMK per 1 g CMK–Ru0.3Cr0.7O2. As a comparison, the specific capacitance of a physical mixture of carbon and metal oxide at the same ratio of CMK–Ru0.3Cr0.7O2 is measured of 125 F g 1, 45% lower than that of CMK–Ru0.3Cr0.7O2 here reported. That is to say nano-crystalline Ru0.3Cr0.7O2 inside CMK-3 results in an increase of specific capacitance. It is worth mentioning that the shape of cyclic voltammetry shows somewhat deformed even though the composite CMK–Ru0.3Cr0.7O2 demonstrates a relatively high electrochemistry performance. This is possibly due to the existence of large amount of Cr in Ru0.3Cr0.7O2 as described by Manthiram [17]. 4. Conclusion In conclusion, hybrid nano-crystallites can be incorporated into CMK-3 in a controlled way by modifying the CMK-3 pore surface then using the impregnation/hydrotherm method. The composite materials were well characterized by XRD and TEM, particularly the pore size distribution clearly shows that nanometer sized ruthenium clusters are well dispersed all over the pores of CMK-3. The formation of micropores among the nano-crystallites or nanoparticles creates preferred sites that provide a path for the aqueous electrolyte for electrochemical use. This synthesis strategy allows us to combine the advantages of ordered mesoporous carbon together with those of the con-
trolled nano-crystallites formed inside the pores, leaving a porous interior to provide pathways. By inserting other metal oxides and hybrid nano-crystallites into ordered mesoporous carbon, it is expected to selectively design composite materials to act as high performance catalysts for gas or solutions passing through the interior or to place alloys inside for hydrogen storage systems. References [1] S.H. Joo, S.J. Choi, I. Oh, J. Kwak, Z. Liu, O. Terasaki, R. Ryoo, Nature 412 (2001) 169. [2] M. Kruk, M. Jaroniec, R. Ryoo, S.H. Joo, J. Phys. Chem. B 104 (2000) 960. [3] S. Jun, S.H. Joo, R. Ryoo, M. Kruk, M. Jaroniec, Z. Liu, T. Ohsuna, O. Terasaki, J. Am. Chem. Soc. 122 (2000) 10712. [4] S. Han, T. Hyeon, Chem. Commun. (1999) 1955. [5] A.C. Dillon, Nature 386 (1997) 377. [6] J.C. Hulteen, C.R. Martin, J. Mater. Chem. 7 (1997) 1075. [7] S. Mann, G.A. Ozin, Nature 382 (1996) 313. [8] G.D. Stucky, J.E. MacDougall, Science 247 (1990) 669. [9] J. Lee, S. Yoon, T. Hyeon, S.M. Oh, K.B. Kim, Chem. Commun. (1999) 2177. [10] R. Ryoo, S.H. Joo, S. Jun, J. Phys. Chem. B 103 (1999) 7743. [11] S.M. Holmes, P.E. Foran, P.L. Roberts, J.M. Newton, Chem. Commun. (2005) 1912. [12] W.C. Choi, S.I. Woo, M.K. Jeon, J.M. Sohn, M.R. Kim, H.J. Jeon, Adv. Mater. 17 (2005) 446. [13] S. Zhu, H. Zhou, M. Hibino, I. Honma, M. Ichihara, Adv. Funct Mater. 15 (2005) 381. [14] S. Trasatti, Electrochim. Acta 36 (1991) 225. [15] D.R. Rolison, P.L. Hagans, K.E. Swider, J.W. Long, Langmuir 15 (1999) 774. [16] J.P. Zheng, P.J. Cygan, T.R. Jow, J. Electrochem. Soc. 142 (1995) 2699. [17] Y.U. Jeong, A. Manthiram, Electrochem. Solid State Lett. 3 (2000) 205. [18] J.M. Miller, B. Dunn, J. Electrochem. Soc. 144 (1997) L309. [19] H. Kim, B.N. Popov, J. Power Sources 104 (2002) 52. [20] Y. Sato, K. Yomogida, T. Nanaumi, Y. Ohsawa, M. Kawai, Electrochem. Solid State Lett. 3 (2000) 113. [21] M. Kruk, M. Jaroniec, C.H. Ko, R. Ryoo, Chem. Mater. 12 (2000) 1961. [22] W. Xu, Y. Liao, D.L. Akins, J. Phys. Chem. B 106 (2002) 11127. [23] R.F. Klie, Y. Zhu, Micron 36 (2005) 219. [24] D. Dollimore, G.R. Heal, J. Appl. Chem. 14 (1964) 109. [25] G. Horvath, K. Kawazoe, J. Chem. Eng. Jpn. 16 (1983) 470.