Enhanced Distribution and Anchorage of Carbon Nanofibers Grown

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Enhanced Distribution and Anchorage of Carbon Nanofibers Grown on Structured Carbon Microfibers Ping Li,*,† Qian Zhao,† Xinggui Zhou,*,† Weikang Yuan,† and De Chen‡ State Key Laboratory of Chemical Engineering, East China UniVersity of Science and Technology, Shanghai 200237, People’s Republic of China, and Department of Chemical Engineering, Norwegian UniVersity of Science and Technology, 7491 Trondheim, Norway ReceiVed: September 24, 2008; ReVised Manuscript ReceiVed: NoVember 11, 2008

Carbon nanofibers (CNFs) were immobilized on carbon microfiber (CMF) felt by chemical vapor deposition with Ni catalysts. A carbon coating, which was derived from a viscoid phenolic resin thin film involved with Ni salts, was introduced as a protect layer to stabilize the Ni nanoparticles for CNF growth, and as an interface to enhance the interaction between the CNFs and the CMFs, thus improving the uniformity and anchorage of the CNFs on the felt. The uniformly dispersed Ni nanoparticles in the carbon coating homogenously covering the CMF surfaces have resulted in a uniform layer of grown CNFs throughout the felt. The interlocked network of the entangled CNFs combined with possible penetration of the roots of CNFs into the carbon coating has enhanced the anchorage of CNFs on the surface of CMFs. 1. Introduction Recently, great efforts were made to develop engineering applications of carbon nanofibers (CNFs) and nanotubes (CNTs) to make full use of their unique properties.1,2 Growing CNFs or CNTs directly on porous structured substrates, such as graphite felt, ceramic monolith, carbon paper, metal foam, and so on,3-7 was considered as a good way of immobilization to avoid palletizing the CNFs or CNTs and to increase the contacting efficiency of active sites. A unique type of CNF composite was developed by Ledoux et al., who grew CNFs on macroscopic carbon material hosts with predefined shapes and sizes. This CNF composite combined the peculiar properties of carbon nanostructures and the easy handling of macrostructured materials, being expected to exhibit exceptional performance as promising industrial catalyst supports.3,8,9 Similarly, a CNF washcoat was prepared on a monolith support, in order to obtain superior properties of the modified ceramic monolith for catalytic reactions in liquid phase.10,11 Surface control of activated carbon fibers12,13 and glass microfibers14 was made possible through the introduction of a new catalytic surface of CNFs. It was shown that CNTs grown on carbon paper could improve the electron transportation between carbon paper and catalytic metal particles deposited on CNTs within a fuel cell electrode.15-17 Additionally, Ni or Fe in metal foams or sintered metal fiber filters were also used as intrinsically active catalysts for the formation of CNFs.6,18 Such structured CNF composites were suggested to be applied to reactions with strong thermal effects.19 Although it seems to be easy to grow CNFs on structured substrates, it is still a big challenge to control the mesostructure of the composites. As has been recognized, metal particle size affects to a great extent the diameter of CNFs. Therefore, * Corresponding authors, [email protected] (Ping Li) and [email protected] (Xinggui Zhou). Tel: +86-21-64252169. Fax: +8621-64253528. † State Key Laboratory of Chemical Engineering, East China University of Science and Technology. ‡ Department of Chemical Engineering, Norwegian University of Science and Technology.

nanosized particles are required to obtain nanosized carbon filaments.20,21 However, the structured substrates, like carbon paper, graphite felt, and glass microfiber felt, have low surface areas. In addition, the pristine substrates are chemically inactive and have a low affinity for the metal particles. Owing to the hydrophobic feature of the carbon, organic solvents were frequently used to help the deposition of the metal precursor on the substrates.14,22,23 Nevertheless, agglomeration of the metal particles is inevitable during calcination and reduction, which makes it difficult to control the catalyst particles size so as to control the CNFs diameter. Another crucial problem lies in the adhesion of CNFs anchored on the structured substrates. As mentioned above, the interaction between the metal particles and the substrate surface is usually weak because of the absence of sufficient functional groups on the substrate surface. During the formation of CNFs, the metal particles can possibly be lifted up by the deposited carbon and pushed away from the substrate surface in accordance with a tip-growth mechanism.24,25 Thus, only the bottom of carbon filaments interacts with the substrate surface. However, the binding forces between them are still inscrutable with the exception of van der Waals force and electrostatic force, although some researchers claimed that strong anchorage of CNFs on nonporous surface was observed.14,26,27 In order to enhance the binding of CNF filaments with the substrate as well as the spreading uniformity of CNFs, an approach by the aid of an intermediary or coating layer was proposed. As declared by Dodelet et al. a silica gel and also a sulfonated silane layer could be utilized as the intermediary.28,29 In this manner, a large density of MWCNTs was found to be firmly attached to the fibers of carbon paper. Another commonly used coating material, alumina, was employed likewise to provide a homogeneous dispersion and distribution of catalytic particles. As a result, the immobilization of CNFs on the surface of ceramic monolith and silica glass fibers, respectively, was facilitated.30,31 In the present work, carbon microfiber (CMF) felt has been selected as a structured substrate for supporting the CNFs grown by CVD method. A carbon coating (CC) formed from a resin precursor has been introduced as an intermediate, and a

10.1021/jp808474c CCC: $40.75  2009 American Chemical Society Published on Web 01/08/2009

1302 J. Phys. Chem. C, Vol. 113, No. 4, 2009 composite of CNF/CC/CMF has been synthesized in the end. The carbon coating has been devised instead of inorganic oxide layer in consideration of preserving the physicochemical similarity of carbon in the finished CNF composite, because the existence of inorganic oxides would affect the catalytic properties if the CNF composite is applied to a catalytic reaction process. The inorganic oxide impurities would also hinder electron transfer within a CNF composite electrode. The oxide intermediary or coating could be eliminated afterward through acid washing, but the linkage of the CNF filaments toward the substrate would be destroyed. On the contrary, carbon materials possess higher chemical stability than do oxides and are therefore particularly suitable for catalytic reactions in corrosive media. Furthermore, their peculiar electronic, mechanical, and thermal properties could be brought into full play if the CNF composite is wholly made of carbon materials. Therefore, our strategy is to develop a novel multiscale structured carbon composite with the CNFs uniformly distributed and tightly adhered on the carbon felt fibers. A vital step of CNF composite synthesis is overlaying a thin film of resin precursor onto the surface of structured CMFs. In this paper, the morphology of the structured CNF composite and the microstructures of the grown CNFs will be investigated, the transformation of the resin precursor to a carbon coating will be traced, and the role of the carbon coating in improving the distribution and anchorage of CNFs in the composite will be discussed. 2. Experimental Section 2.1. Materials and Synthesis. The carbon microfiber felt (CMF) from Shanghai Xinxing Carbon Co. was woven by skeins of carbon microfibers. The specific surface area of the felt was less than 1 m2 g-1. The felt was shaped as a cylinder of Φ 35 mm × 14 mm and weighed about 1.68 g. Thermosetting phenolic resin was synthesized batchwise in a three-necked flask from phenol and formaldehyde and using ammonia (27 wt % aqueous solution) as catalyst. The mole ratio of phenol to formaldehyde to ammonia was set at 1:1.3:0.068. The polymerization reaction lasted for about 1 h at 95 °C until the liquid in the flask turned turbid. The liquid was sustained in the turbid state at 95 °C for 25 min and then cooled down to room temperature. After the liquid was allowed to remain undisturbed, a yellowish phenolic resin was obtained from the underlayer of the liquid. For supporting nickel catalyst, the felt was first immersed in a solution of nickel nitrate dissolved in a mixture of ethanol and phenolic resin. The weight ratio of ethanol to resin was 2:1, and the content of the nickel in the solution was 1 wt %. Excess solution in the felt was removed by centrifugation. The wet felt was then dried overnight in air at 120 °C. The phenolic resin was solidified after thermal treating, rendering the soft felt inflexible. Prior to the growth of CNFs, the felt containing the nickel compound was put into a vertical quartz tube reactor. The temperature of the reactor was then raised to and maintained at 600 °C for 3 h in a flow of H2/Ar (1:3 (v/v)) at a flow rate of 160 mL/min. In this process, the resin polymer was converted to a carbonaceous material and meanwhile the nickel compound was reduced to nickel. The nickel loading in the felt detected by an inductively coupled plasma spectrometer (ICP, OPTIMA 2100 DV, Perkin-Elmer Instruments) was 0.75 wt %. Afterward, CNFs were grown on the CMFs at the same temperature in a flow of C2H4/H2 (2:1 (v/v), flow rate 120 mL/min) for 1-9 h. The CNF yield was defined as the ratio of the weight of the carbon finally formed (excluding the part of the carbon derived from the resin) to the initial weight of the felt.

Li et al. 2.2. Characterization. The morphology of the samples was observed by scanning electron microscopy (SEM) on a JEOL JSM-6360LV scanning electron microscope. High-resolution transmission electron microscopy (HRTEM) investigation was conducted using a JEOL JEM-2010 transmission electron microscope. The samples prepared for HRTEM investigation were first ground into fragments and then dispersed in ethanol under ultrasound. After that, a drop of the sample-ethanol solution was transferred onto a carbon-coated copper grid. Thermogravimetric (TG) analysis in a 1:9 (v/v) H2/Ar flow at a heating rate of 10 °C/min to 700 °C was carried out on a TA SDT-Q600 thermobalance. The nickel dispersion was detected using H2-pulse chemisorption on Micromeretics Autochem II 2920. The textural property of the samples was analyzed by cryogenic N2 physisorption on Micromeretics ASAP 2010. Specific surface area of the samples was calculated with the BET equation, and their pore volume was determined from N2 desorption isotherm using the BJH method. 3. Results and Discussion 3.1. Morphology of CNF/CC/CMF Composite. The evolution of CNF/CC/CMF composite forming from the starting CMF substrate to the final product of carbon-coated CMF covered with grown CNFs was tracked by SEM. As illustrated in Figure 1a, the individual CMFs of the felt have a mean diameter around 20 µm, and their surface seems rather smooth except for some stripes. After the felt was impregnated with resin-ethanol solution containing Ni precursor and cured at 120 °C, a continuous thin film is uniformly coated onto the whole surface even including the end planes of the carbon microfibers, as can be seen in Figure 1b. It is also seen that there is some resin material standing like bridges among adjacent microfibers which bind the microfibers together. After the carbon-containing gas C2H4 is introduced into the reactor at 600 °C, fluffy filaments appear and grow up from the surface of carbon microfibers (Figure 1c). Because the phenolic resin can be carbonized at 600 °C, a carbon coating layer transformed from the resin film should underlie the dense filaments. The bridges, where carbon filaments spread around, still connect the interlacing microfibers and become a component part of the structured carbon composite. After prolonged time of exposure in the carboncontaining gas, every carbon microfiber is wrapped with a very thick layer of CNFs, among which much longer nanofilaments are discernible (Figure 1d). The texture property of the formed CNF/CC/CMF composites with different CNF yields is evaluated and listed in Table 1. The composites possess large specific surface area but lack in micropores, making them a promising catalytic material.4,32 A high-magnification SEM image is displayed in Figure 2, revealing that there are two forms of CNFs distinct in diameter. The diameters of thick CNFs vary in the range of 50-100 nm while those of thin CNFs are around 10-20 nm. HRTEM pictures in panels a and b of Figure 3 demonstrate that both forms of CNFs possess rather disordered graphene layers that align considerably parallel at an approximately vertical angle to the c-axis of the nanofiber. Such orientation of the graphene layers is the characteristic of the CNFs behaving a specific microstructure like platelet.33,34 A relationship between the two forms of CNFs is captured in Figure 3c. It seems that CNFs of small diameters are gestated from the big matrix of CNFs. The branched CNFs have also been observed on Ni/AC (activated carbon) when C2H4 was used as the carbon source.33 It is suggested that the original Ni particle, on which CNFs of big sizes are formed, may be fragmentized, forming small Ni

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Figure 1. Typical SEM images illustrating the evolution of CNF/CC/CMF composite formation from starting carbon microfiber felt to its final state with growing carbon nanofibers in the presence of carbon coating: (a) pristine carbon microfibers of felt; (b) carbon microfibers covered with a thin film of resin polymer; (c) fluffy carbon nanofibers grown for 1 h on the surface of carbon microfibers coated with carbon; (d) longer carbon nanofibers emerging after the resin-covered carbon microfiber felt was exposed in C2H4 containing gases at 600 °C for 9 h. Note: arrows in (b) and (c) showing the resin and the carbon bridges, respectively, connecting adjacent carbon microfibers; a circle in (b) shows a thin film of resin polymer wrapped on the end sections of carbon microfibers where the thickness of the thin film can be determined.

TABLE 1: Texture Property of CNF/CC/CMF Composite with Different CNF Yield

sample CNF-0.6a CNF-1.5 CNF-2.4 CNF-1.5-Tb CNF-2.4-T

BET area pore volumec (m2/g) (cm3/g) 23.00 56.80 75.74 57.12 74.96

0.071 0.134 0.156 0.135 0.154

micropore volume mean pore (cm3/g) diameter (nm) ∼0 ∼0 ∼0 ∼0 ∼0

8.4 8.1 7.1 8.0 7.1

a CNF-0.6, CNF-1.5, and CNF-2.4 refer to the yield of CNFs in the composite of 0.6, 1.5, and 2.4 (wt/wt), respectively. b CNF-1.5-T and CNF-2.4-T are referred to the samples of CNF-1.5 and CNF-2.4 after treated in ultrasound for 120 min., respectively. c Pore volume is calculated by the BJH method which is referred to the volume of the pores smaller than 300 nm.

Figure 2. High-magnification SEM image displaying two dimensions of growing carbon nanofibers distinct in diameter.

particles and generating secondary smaller nanofibers.20 Weak metal-support interaction may favor the metal fragmentation.35 Because the interaction between the metal particles and the carbon materials is generally weak especially when the carbon materials have a low degree of graphitization,36 the fragmentation of Ni particles supported on the CMF substrate coated with a carbon thin film is hence expected. Nevertheless, the possibility of two forms of CNFs produced separately from Ni particles of different sizes at the very beginning of carbon growth cannot be excluded.

3.2. Formation and Role of Carbon Coating. TG analysis was used to study the mechanism of carbonization of phenolic resin. The weight change of phenolic resin with and without Ni precursor was measured at high temperatures. The samples for TG analysis were cured at 120 °C to remove the volatile substances, e.g., alcohol, and to solidify the resin. 10% (v) H2/ Ar was used as the carrier gas to simulate the reduction atmosphere in the whole process of growth.

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Figure 3. HRTEM pictures of the microstructure of carbon nanofibers: (a) arrangement of graphene layers in a carbon nanofiber with large diameter; (b) arrangement of graphene layers in a thin carbon nanofiber; (c) a thin carbon nanofiber grown on the surface edge of a thick nanofiber.

Figure 4. TG analysis results of solidified resin polymer (a) with and (b) without Ni precursor in a reduction atmosphere.

Figure 4 shows the TG curves of the resin with and without Ni precursor. Both of them undergo several weight loss steps, but only few differences exist, indicating the slight influence of Ni precursor on thermal behavior of the resin. As is wellknown, phenolic resin is a thermosetting polymer which undergoes condensation and polymerization at elevated temperatures and leaves a carbon framework.37 Therefore, the first obvious 3% weight loss appearing between 150 and

335 °C on the TG curve of the resin without Ni precursor can be assigned to the intermolecular dehydration of phenolic resin. In the range of 335-445 °C, further intramolecular condensation proceeds, resulting in another 3% weight loss. With respect to the resin with Ni precursor, a slightly large weight loss is observed before 500 °C owing to the decomposition and reduction of Ni precursor. Between 445 and 700 °C, about 20% of original resin weight is lost, releasing a large amounts of gases such as CH4, H2O, CO, etc. For the resin with and without Ni precursor at 700 °C, the carbon-like residues are 72.8% and 71.5% in weight, respectively. Because phenolic resin is a popular binding agent of excellent adhesive property, strong chemical and physical interactions could be expected to occur at the microfiber-resin interface. In fact, good wettability of the carbon microfibers in the phenolic resin solution has been established in the experiment owing to the hydrophobic nature of both substances. The effect of the resin adhesion has been perused by SEM as demonstrated in Figure 1b. No visible fracture or exfoliation of resin film can be observed on the surface of microfibers. In fact, the perfect ductility of the resin film makes it fitting well to the shape of each carbon microfiber. Apparently, this behavior of the resin film is crucial to the uniform dispersion of Ni particles and,

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Figure 5. SEM images of (a) resin-derived carbon containing Ni particles obtained in the absence of C2H4, and (b) long carbon filaments among resin-derived carbon obtained in the presence of C2H4.

consequently, the distribution of CNFs within the composite. It should be mentioned that the thickness of the resin film can be estimated from the silklike film wrapped over the cross section of microfibers. The dimension of the film thickness is around 100 nm, which will determine the final thickness of the derived carbon coating. An effort to adjust the thickness of carbon coating is being made by changing the ratio of resin to ethanol for the purpose of precise control of CNF growth and the properties of the composite concerned. The Ni dispersion on CMF was detected by H2 adsorption. The results show that a much higher dispersion of Ni particles is obtained on the CMF covered with a resin-derived carbon coating as comparing to that on the pristine surface of CMF. More specifically, 6.6% (corresponding to 15.3 nm) of Ni dispersion is obtained on CMF with a carbon coating versus 0.9% (117.8 nm) of Ni on the pristine surface of CMF. As extensively reported in the literature, nanosized metal particles can be stabilized in liquid media against aggregation by surfactants or linear polymers such as ethylene glycol and poly(vinylpyrrolidone).38,39 It is difficult, however, to inhibit the sintering of metal nanoparticles after losing above thermolabile stabilizers at high temperatures.40 Therefore, the improvement of Ni dispersion by the use of phenolic resin in this paper is of significance. The resin acts indeed as a polymer protector for the Ni precursor dissolved in it. During the thermal treating, the migration and the gathering of the Ni ion clusters are effectively hindered by the solidified phenolic resin. While stepping up to higher temperatures in a reducing atmosphere, Ni nanoparticles are transformed gradually from compounds through reduction. The pores formed in the phenolic resin by gas release provide the route for H2 approach to the embedded Ni ions. Meanwhile, carbothermic reduction between carbon and Ni compounds may also proceed. Owing to the obstruction of carbonaceous material derived from the phenolic resin, the Ni nanoparticles are confined on sites and prevented from sintering.41 In order to further verify that the Ni particles are accessible to the reaction gases and the CNFs can grow easily in the resinderived carbon, an experiment was devised. Two materials were synthesized deliberately. Both used solidified phenolic resin containing Ni salt as the substrate, and the steps for subsequent processing resembled those for growing CNFs. But one of the substrates was heat-treated in the absence of C2H4. The SEM images of the resulting materials are illustrated in Figure 5a and 5b, respectively. As can be seen in Figure 5a, granular carbon material has been formed from the solidified phenolic

Figure 6. Changes of weight loss with ultrasonic vibration time for different composite samples: (a) original carbon microfiber felt; (b) carbon-coated felt; (c) felt with the growth of carbon nanofibers yielding 1.5; (d) carbon-coated felt with the growth of carbon nanofibers yielding 0.6; carbon-coated felt with the growth of carbon nanofibers (e) yielding 1.5 and (f) yielding 2.4.

resin with Ni particles involved in the absence of C2H4. By contrast, as shown in Figure 5b, a mass of carbon filaments with high aspect ratios emerge and distribute uniformly on the substrate treated in the presence of C2H4. This implies that the Ni particles surrounded by the resin-derived carbon are not only accessible to C2H4 for CNF growth but also well spread in the carbon layer. Therefore, there is no wonder that the threedimensional distribution of grown CNFs is uniform throughout the CNF/CC/CMF composite thanks to the enhanced Ni distribution by the overspread carbon coating on the surface of CMFs. To examine the mechanical stability of the CNF/CC/CMF composite, it was immersed into an ethanol solution and then vibrated by ultrasound. The weight loss of the composite after vibration for a certain period of time was measured as a criterion.5,6,42 Figure 6 shows the change of weight loss with the duration of ultrasonic vibration for CNF/CC/CMF composites with different amounts of CNF formation. The comparative results of the original carbon felt, the carbon-coated felt (CC/ CMF), and the CNFs directly grown on the felt without coating (CNF/CMF) are also shown in this figure. The weight loss of the original carbon felt can be attributed to the detaching off some microfiber pieces during the ultrasonic treatment, which can be prevented to some extent by the carbon bridges between the interlaced microfibers transformed from the resin or by the interlocked network formed by the curling CNF filaments. It is obvious that the existence of either the carbon coating or the

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Figure 7. Schematic diagram of CNF growth model on the bare and the carbon-coated microfiber surfaces.

CNFs is beneficial to reduce the weight loss of the carbon composite. Moreover, the combination of carbon bridge and CNF network has resulted in a remarkable increase in the mechanical stability of the composite, as is manifested by lines b and e in Figure 6, which have used the same carbon coating on microfibers, and lines c and e, which have the same amount of CNFs formation. Because the carbon coating lies between the CNFs and CMFs as an intermediary, there is no doubt that the carbon coat has helped to enhance the anchorage of the CNFs on the CMFs, by embracing the roots of the CNFs. As shown in this figure, the weight loss of the CNF/CC/CMF composites with the CNFs yields of 1.5 and 2.4, respectively, is so small that one can suppose that the CNFs are firmly anchored on the carbon-coated microfibers. The textural property of the CNF/CC/CMF composites after the ultrasonic treatment for 120 min is also tested and listed in Table 1. The results are scarcely affected by the long time treatment, indicating the starting structure of the composites is retained and the connection of CNFs with the carbon-coated microfibers is still unchanged. For an explanation of the role of carbon coating in the interaction of CNFs with microfiber, a suggested model of CNF growth on the bare and the carbon-coated microfiber surfaces is schematically depicted in Figure 7. For carbon-coated felt, Ni nanoparticles are embedded in the carbon coating prior to CNF growth and the grown CNFs can be rooted in the holes left behind by metal particles. The strong anchorage of CNFs in the carbon layer, together with the interlocked CNFs network, is responsible for the strong mechanical stability of the composite. 4. Conclusion A novel structured CNF composite has been synthesized with the aid of a carbon layer covered on the surface of CMFs to improve the distribution and anchorage of the CNFs in the composite. The carbon layer, which is transformed from a thin film of liquid resin containing predissolved Ni precursor, is well spread on the CMFs and has a uniform coverage thanks to the outstanding adhesive and ductile features of the liquid resin. The resin has acted as a polymer protector inhibiting the immigration of the Ni compounds during thermal treating at high temperatures, so to increase the Ni dispersion and the uniformity of CNFs growth. Because of the uniform coverage of the carbon coating and the dispersion of the Ni particles, a homogeneous threedimensinal distribution of CNFs is achieved throughout the composite. The mechanical stability of the composite is increased by the interlocked network of the CNFs and also by the firm roots of CNFs in the carbon coating. By all accounts,

it is the carbon coating that plays the key role in improving the uniformity and anchorage of the grown CNFs. Acknowledgment. The authors are grateful to the support from the Chinese Education Ministry 111 project (B08021) and Creative Team Development project (IRT0721), the Natural Science Foundation of China project NSFC-20518001/RGCN_HKUST620/05, and the NSFC/China-Petro major project (20490200). References and Notes (1) Rodriguez, N. M.; Chambers, A.; Baker, R. T. K. Langmuir 1995, 11, 3862–3866. (2) De Jong, K. P.; Geus, J. W. Catal. ReV.sSci. Eng. 2000, 42, 481– 510. (3) Ledoux, M. J.; Pham-Huu, C. Catal. Today 2005, 102-103, 2–14. (4) Li, P.; Li, T.; Zhou, J. H.; Sui, Z. J.; Dai, Y. C.; Yuan, W. K.; Chen, D. Microporous Mesoporous Mater. 2006, 95, 1–7. (5) Garcı´a-Bordeje´, E.; Kvande, I.; Chen, D.; Rønning, M. AdV. Mater. 2006, 18, 1589–1592. (6) Chinthaginjala, J. K.; Seshan, K.; Lefferts, L. Ind. Eng. Chem. Res. 2007, 46, 3968–3978. (7) Su, D. S.; Chen, X. W.; Liu, X.; Delgado, J. J.; Schlo¨gl, R.; Gajovic, A. AdV. Mater. DOI 10.1002/adma.200800323. (8) Vieira, R.; Pham-Huu, C.; Keller, N.; Ledoux, M. J. Chem. Commun. 2002, 954–955. (9) Ledoux, M. J.; Vieira, R.; Pham-Huu, C.; Keller, N. J. Catal. 2003, 216, 333–342. (10) Jarrah, N.; van Ommen, J. G.; Lefferts, L. Catal. Today 2003, 7980, 29–33. (11) Garcia-Bordeje, E.; Kvande, I.; Chen, D.; Rønning, M. Carbon 2007, 45, 1828–1838. (12) Lim, S.; Yoon, S. H.; Shimizu, Y.; Jung, H.; Mochida, I. Langmuir 2004, 20, 5559–5563. (13) Tzeng, S. S.; Hung, K. H.; Ko, T. H. Carbon 2006, 44, 859–865. (14) Keller, N.; Rebmann, G.; Barraud, E.; Zahraa, O.; Keller, V. Catal. Today 2005, 101, 323–329. (15) Wang, C.; Waje, M.; Wang, X.; Tang, J. M.; Haddon, R. C.; Yang, Y. Nano Lett. 2004, 4, 345–348. (16) Tang, H.; Chen, J.; Nie, L.; Liu, D.; Deng, W.; Kuang, Y.; Yao, S. J. Colloid Interface Sci. 2004, 269, 26–31. (17) Smiljanic, O.; Dellero, T.; Serventi, A.; Lebrun, G.; Stansfield, B. L.; Dodelet, J. P.; Trudeau, M.; Desilets, S. Chem. Phys. Lett. 2001, 342, 503– 509. (18) Jarrah, N.; van Ommen, J. G.; Lefferts, L. J. Catal. 2006, 239, 460–469. (19) Tribolet, P.; Kiwi-Minsker, L. Catal. Today 2005, 102-103, 15– 22. (20) Rodriguez, N. M. J. Mater. Res. 1993, 8, 3233–3248. (21) Ochoa-Ferna´ndez, E.; Chen, D.; Yu, Z.; Tøtdal, B.; Rønning, M.; Holmen, A. Catal. Today 2005, 102-103, 45–49. (22) Otsuka, K.; Ogihara, H.; Takenaka, S. Carbon 2003, 41, 223–233. (23) Downs, W. B.; Baker, R. T. K. J. Mater. Res. 1995, 10, 625–633. (24) Melechko, A. V.; Merkulov, V. I.; Lowndes, D. H.; Guillorn, M. A.; Simpson, M. L. Chem. Phys. Lett. 2002, 356, 527–533. (25) Song, I. K.; Cho, Y. S.; Choi, G. S.; Park, J. B.; Kim, D. J. Diamond Relat. Mater. 2004, 13, 1210–1213. (26) Vieira, R.; Ledoux, M. J.; Pham-Huu, C. Appl. Catal. A 2004, 274, 1–8. (27) Delgado, J. J.; Su, D. S.; Rebmann, G.; Keller, N.; Gajovic, A.; Schlo¨gl, R. J. Catal. 2006, 244, 126–129.

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