J. Phys. Chem. C 2008, 112, 15157–15162
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Aligned Bundles of Carbon Nanotubes Are Easily Grown on As-Synthesized Mesoporous Silicate Substrates Stephanie Morgan and Robert Mokaya* School of Chemistry, UniVersity of Nottingham, UniVersity Park, Nottingham NG7 2RD, U.K. ReceiVed: June 18, 2008; ReVised Manuscript ReceiVed: August 10, 2008
As-synthesized mesoporous silicates have been used as substrates for the growth of aligned bundles of carbon nanotubes. The as-synthesized mesoporous silicates facilitate the growth of a rich yield of carbon nanotubes in a manner that does not occur on their calcined analogues. The growth of the nanotubes was performed via chemical vapor deposition at 800 °C with acetonitrile as precursor. The Fe metal catalyst was incorporated onto the silicate mesophases via impregnation rather than the more conventional direct-mixed-gel synthesis. Alumination, via postsynthesis grafting of the as-synthesized mesoporous silica substrates, enhances the production of carbon nanotubes and favors their growth in aligned bundles. This is the first time that assynthesized mesoporous silicates have been used as substrates to successfully grow large amounts of carbon nanotubes. The approach described is simple, avoids the need for calcining the substrate and offers further advantages including higher yields of carbon nanotubes and the attractive formation of well aligned bundles of nanotubes. 1. Introduction Carbon nanotubes show interesting physical and chemical properties that make them potentially useful for a wide range of applications including use as catalyst supports, field emitter tips in scanning probe microscopy, composites in coatings and as components of electronic devices.1,2 Carbon nanotubes (CNTs) also possess high internal surface area and large pore volume making them ideal components for energy storage, for example as hydrogen storage materials.1,2 Since the first reports in the 1990s,3 various techniques have been developed for the synthesis of CNTs including arc discharge,4 laser ablation,5 pyrolysis of hydrocarbons,6 and chemical vapor deposition (CVD).7 Laser ablation and arc discharge of carbon require high temperatures (typically above 3000 °C) and produce a range of undesirable byproduct, while chemical vapor deposition (CVD) proceeds at much lower temperatures making it more attractive for controlled CNT synthesis. Depending on the synthesis method and conditions, CNTs grow in a random or aligned manner, with aligned growth being desirable for some applications. The past decade has seen a surge in research interest focusing on simple and reliable techniques to prepare aligned CNTs. The production of aligned CNTs usually requires a careful choice of the substrate and control over the position and size of the metal particles that act as catalysts for CNT growth. Aligned CNTs can be grown on suitable metal catalystcontaining substrates via CVD.6a,7a,8 Substrates including quartz,9 silicon,10 silica,7a,11 and SiO2/Si wafers7b,d have been extensively studied. Porous supports with well ordered porous systems such as mesoporous silica and zeolites offer the opportunity to position the catalyst particles within internal space (i.e., pore channels). In particular mesoporous silicates have been extensively studied as substrates for the growth of CNTs.12-14 The positioning of the catalyst in mesoporous silica templates is achieved via either direct-mixed gel synthesis12 or postsynthesis impregnation on the calcined silica.14 In the majority of previous * To whom correspondence should be addressed. E-mail: r.mokaya@ nottingham.ac.uk.
studies, mesoporous silica substrates generally generate randomly oriented CNTs.12,14 Few studies have reported the growth of aligned CNTs on mesoporous silica substrates, and in all cases the metal catalyst was positioned via direct-mixed gel synthesis.13 On the other hand, we have recently prepared bundles of aligned N-doped CNTs via CVD using ammoniumexchanged zeolite-β as a substrate and Fe as a catalyst.15 The metal is thought to have inserted into the substrate framework by ion-exchange and initiated CNT growth in an aligned manner through cracks in the surface.15 Previous studies therefore suggest that ion-exchange may be an aiding factor for the growth of aligned CNTs on porous silicate substrates. In this report we explore the influence of ion-exchange on the growth of CNTs on mesoporous silicate substrates. Mesostructured mesophases and calcined silicates impregnated with Fe were employed as the substrate for the growth of CNTs via CVD. The specific aim was to investigate the use of assynthesized mesoporous silica mesophases as substrates for aligned CNT growth. As far as we are aware, only calcined forms of mesoporous silica substrates have previously been used to grow CNTs. The investigation was partly motivated by the need to simplify the use of mesoporous silica substrates for CNT production. We show that the use of as-synthesized mesophases (rather than calcined mesoporous silica) offers several advantages including improved CNT yields, growth of aligned CNTs and removal of the need to calcine the substrates. 2. Experimental Section 2.1. Material Synthesis. 2.1.1. Mesoporous Silicate Monolith Substrates. Mesoporous silica monoliths were prepared as previously described.16 Typically, 1 g of cetyltrimethylammonium bromide (CTAB) was dissolved in 20 mL of a mixture of ethanol (14 mL) and toluene (6 mL). The resulting solution was then added to a stirred solution of 3 g tetraethyl orthosilicate (TEOS) containing 0.74 g each of 0.1 N HCl and distilled water. After refluxing at 80 °C for 5 h, the mixture was concentrated by rotary evaporation at 50 °C. The resulting viscous liquid was transferred into a Petri dish as a thin layer and air-dried
10.1021/jp805355v CCC: $40.75 2008 American Chemical Society Published on Web 09/09/2008
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Figure 1. Powder XRD patterns of (A) as-synthesized (MSA) and calcined (MSC) silica substrates, and (B) after Fe impregnation, (C) after alumination and (D) after alumination followed by Fe impregnation.
overnight. The dry as-synthesized monolith was designated as MSA. A portion of the as-synthesized MSA monolith was calcined at 550 °C for 5 h, and designated as MSC. Al was incorporated into the monoliths via postsynthesis grafting at a Si/Al molar ratio of 20 using established procedures.17 The appropriate amount of aluminum isopropoxide in 75 mL dry hexane was heated to 50 °C to aid dispersion, and after cooling to room temperature was added to 1 g of silica in 25 mL dry hexane. After standing for 24 h at room temperature, the silica was obtained by filtration, washed in dry hexane and air-dried overnight. The aluminated silica derived from MSA was designated as Al-MSA. The aluminated monolith derived from MSC was calcined at 550 °C for 4 h to yield sample AlMSC. Fe was impregnated onto the monoliths as follows: 0.5 g of monolith was soaked in ferric nitrate nonahydrate in 2 mL ethanol at Fe/Si molar ratio of 0.05. The ethanol was removed by drying overnight at 100 °C. The Fe impregnated samples derived from MSA and MSC monoliths were designated as FMSA and FMSC, respectively, while those derived from AlMSA and Al-MSC monoliths were designated as FAl-MSA and FAl-MSC, respectively. 2.1.2. Carbon Synthesis. The silicate substrates (0.4 g) were placed in an alumina boat located in the middle (hot zone) of a flow through tube furnace. Heating, at a rate of 5 °C/min, was carried out under a flow of nitrogen up to 800 °C. Once the target temperature was achieved, the nitrogen flow was diverted through acetonitrile solution to give a flow of nitrogen saturated with acetonitrile, and maintained for 20 h. The furnace was then cooled under a flow of nitrogen only. The resulting carbon/silica composites were washed in 25% hydrofluoric (HF) acid for 3 days followed by further washing with 37 wt% hydrochloric (HCl) acid for 3 days to etch out the silicate. The resulting carbon material was dried overnight in a dry box at 50 °C. The carbon samples derived from FMSA and FMSC monoliths were designated as C-FMSA and C-FMSC respectively, while those derived from FAl-MSA and FAl-MSC monoliths were designated as C-FAl-MSA and C-FAl-MSC respectively. 2.2. Characterization Methods. Powder XRD was performed using a Philips 1830 powder diffractometer with Cu KR radiation (40 kV, 40 mA), 0.02° step size and 1 s step time. A Micromeritics ASAP2020 sorptometer was used for porosity analysis using nitrogen sorption at -196 °C. Before analysis the samples were evacuated for 24 h at 200 °C under vacuum. The BET method was applied to evaluate the specific surface area, using data in the partial pressure range 0.05-0.2. Ther-
TABLE 1: Textural Properties of Mesoporous Silicate Substrates and Carbon Materials sample
surface area (m2/g)
pore volume (cm3/g)
MSC FMSC Al-MSC C-FMSA C-FMSC C-FAl-MSC C-FAl-MSA
1540 1398 1467 365 974 626 427
0.70 0.61 0.32 0.28 0.59 0.49 0.30
mogravimetric analysis was performed using a SDT Q600 TGA Analyzer at a heating rate of 5 °C/min in a static-air environment. SEM images were recorded using JEOL JSM-6400 and JEOL XL30 microscopes. Samples were prepared by mounting a small amount on conductive carbon double-sided sticky tape. A thin (∼10 nm) layer of gold was deposited onto the samples to prevent charge developing on the surface. 3. Results and Discussion 3.1. Silicate Substrates. The XRD patterns of the silicate substrates are shown in Figure 1. The patterns are typical of well ordered hexagonal pore channel mesostructures.18 The pattern for (as-synthesized) monolith MSA (Figure 1A) exhibits a peak at low 2θ values, which may be ascribed to the basal (100) diffraction of hexagonal pore channel structure (space group P6mm),18 corresponding to a d100 spacing of 3.8 nm. The calcined silica monolith MSC (Figure 1A) exhibits an intense basal (100) diffraction peak, corresponding to a d100 spacing of 3.2 nm indicating a slight contraction of the lattice after calcination. Two other low intensity peaks in the 2θ range of 3-10° can be assigned to higher index, (110) and (200), diffractions of two-dimensional hexagonal structure with moderate level of long-range ordering. The intensity of the basal peak is higher for MSC than MSA due to increased phase contrast in the absence of surfactant in the pores.18 As shown by the XRD patterns in Figure 1C, alumination of both the assynthesized and calcined silica monoliths does not affect the mesostructural ordering. Furthermore, the XRD patterns in Figure 1B and D indicate that mesostructural ordering is preserved after Fe impregnation. Besides the typical reflections due to mesostructural ordering, no additional peaks are observed for the Fe impregnated substrates in the 2θ range of 10 - 50°, which confirm that in general no large particulates of crystalline Fe species are formed. As shown in Table 1, the surface area and pore volume of the silica monoliths (data for sample MSC
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Figure 2. Representative SEM images of carbons prepared from Fe impregnated silica substrates: C-FMSA (a,b), C-FMSC (c,d).
Figure 3. TGA curves (A) and corresponding DTG profiles (B) for carbon prepared from Fe-impregnated substrates.
is provided) are high and consistent with previous reports.18 The data in Table 1 and Figure 1 confirm that mesostructural ordering is retained in all the silicate substrates used in this study. 3.2. Carbon from Fe Impregnated Silica Templates. Fe impregnated silicas, FMSA and FMSC, were used as substrates for carbon formation via CVD. Thermal analysis of the resulting composites (Supporting Information, Figure 1) indicates a high carbon yield (28 - 34 wt %) for both substrates with the assynthesized mesoporous silica generating more carbon. SEM images of the carbon materials after HF treatment of the resulting carbon/silica composites are shown in Figure 2. The SEM images reveal striking differences in the nature of the carbons; the C-FMSA carbon generated from the as-synthesized silica substrate (FMSA) is made up of carbon nanotubes while the calcined substrate (FMSC) yields particulate/monolithic carbon (sample C-FMSC) with hardly any nanotubes. The C-FMSA nanotubes generated by the as-synthesized silica substrate appear to be diffuse with no preferred growth direction. This is the first time that as-synthesized mesoporous silica substrates have been used to grow large amounts of CNTs.12-14 Furthermore, the formation of large amounts of CNTs from silica substrates for which the metal catalyst has been impreg-
nated rather than introduced via direct-mixed gel synthesis is unusual.12-14 Using the as-synthesized silica substrate appears to have clear advantages over the calcined substrate.13 A higher retention or distribution of the Fe catalyst may explain the much greater formation of CNTs on the assynthesized FMSA substrate. We propose that the Fe is better retained on the FMSA substrate via some exchange with surfactant molecules. It is known that the formation of the assynthesized substrates requires charge balancing/matching between the surfactant species (as CTA+) and silicate framework.18 The surfactant CTA+ species can therefore be removed/displaced by other positively charged species in a process similar to ionexchange. We envisage that the Fe species can also replace some of the surfactant species. In such a scenario the Fe will be initially located within the pores or near the pore mouths of the silica substrate. Subsequent heating to 800 °C, during the CVD process, not only removes the surfactant molecules but also causes the Fe species to migrate to the surface.14d,15,19 This increases the surface distribution of Fe particles with perhaps a greater number of particles of suitable size to catalyze CNT growth. It is known that CNTs can only grow from catalyst particles with a radius smaller than a critical value.20 On the other hand, deposition of Fe species on calcined (MSC) silica is minimal due to absence of ion exchange. Any Fe retained is via simple grafting, which appears to be less efficient in forming Fe species capable of catalyzing significant growth of CNTs. Thermogravimetric analysis (TGA) curves of the C-FMS carbons, shown in Figure 3A, indicate that most of the inorganic phase was removed during the washing with HF. The residual weight at 1000 °C typically varied between 0 and 3 wt %. The residual weight was however always higher (g3 wt %) for carbon C-FMSA, which we attribute to a higher content of retained Fe in the as-synthesized MSA silicate as discussed above. The differential thermogravimetric (DTG) profiles in Figure 3B indicate that carbon combustion of sample C-FMSC occurs over a wider temperature range and at a lower temperature (centered at ca. 510 °C). Carbon C-FMSA exhibits a higher combustion temperature (539 °C), which is an indication of relatively higher level of graphitisation. The thermal properties are clearly related to the amount of CNTs in the carbon yield. Carbon materials prepared from calcined silica substrates (C-
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Figure 4. Representative SEM images of carbon/Fe-aluminosilicate composites derived from FAl-MSA (a-c) and FAl-MSC (d) substrates.
FMSC) contain low amounts of CNTs, while the use of assynthesized substrates (C-FMSA) generates significant amounts of CNTs. The assumption here is that CNTs are more graphitic compared to amorphous carbon and therefore exhibit higher thermal stability. Powder XRD patterns of C-FMS carbons (Supporting Information, Figure 2) show that the graphitic peak (at 2θ ) 26°) is sharper for C-FMSA (indicating higher level of graphitisation and greater CNT content), which is consistent with the SEM images (Figure 2) and TGA data (Figure 3). Nitrogen sorption data (Supporting Information, Figure 2) of the carbons indicates adsorption into small mesopores for sample C-FMSC, while micropore filling is predominant for sample C-FMSA. The mesoporosity of sample C-FMSC arises from particulate/ monolithic carbon for which the mesoporous silica acts as template rather than substrate. As shown in Table 1, the surface area (974 m2/g) and pore volume (0.59 cm3/g) of sample C-FMSC are high due to the presence of mesoporous particulate carbon. The surface area (365 m2/g) and pore volume (0.28 cm3/ g) of sample C-FMSA are lower, which is consistent with the presence of a higher proportion of CNTs. For the pure silica substrates it appears therefore that the optimal CNT growth is achieved by using an as-synthesized silica substrate. The Fe species can undergo an exchange process with the surfactant molecules within the pores of the as-synthesized substrate. This generates Fe species within the pores as well as on the external
surface. Heating to 800 °C, during CVD, removes the surfactant molecules and the Fe species migrate to the surface,14d,15,19 which enhances the surface distribution of Fe particles that then function as catalysts for CNT growth. 3.3. Carbon from Aluminosilicate Substrates. The XRD patterns in Figure 1C and D confirm that alumination does not have any deleterious effect on the mesostructural ordering of the silicate substrates. As shown in Table 1 for sample Al-MSC, the surface area (1467 m2/g) and pore volume (0.32 cm3/g) confirm that the silicate substrate retains high porosity after alumination. The grafted aluminosilicate silicates, after impregnation with Fe, were used as substrates for CNT growth. Thermal analysis of the carbon/Fe-aluminosilicate composites (Supporting Information, Figure 3) indicated a carbon content of ca. 33 wt %, which is similar to that observed for equivalent silica substrates (Supporting Information, Figure 1). The combustion of carbon from the aluminosilicate substrates is however centered at a slightly higher temperature of 530 °C compared to ca. 510 °C for the silica substrates. This indicates an overall higher level of graphitization for the carbon generated by aluminosilicate substrates. SEM images of the carbon/Fealuminosilicate composites (Figure 4) show much greater amounts of CNTs for the as-synthesized FAl-MSA substrate compared to the calcined FAl-MSC substrate. Bundles of aligned CNTs are observed on substrate FAl-MSA (Figure 4 a,b). Although isolated bundles grow in various directions, the
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Figure 5. Representative SEM images of carbon materials derived from Fe impregnated aluminosilicate substrates; C-FAl-MSA (a,c) and C-FAlMSC (b,d).
Figure 6. TGA curves (A) and corresponding DTG profiles (B) for carbon materials prepared from Fe impregnated aluminosilicate substrates.
nanotubes in any given bundle are well aligned. This contrasts with the random growth of the few nanotubes on the calcined FAl-MSC substrate (Figure 4d). The use of a noncalcined substrate therefore not only enhances the formation of CNTs but also favors their growth in an aligned manner. The SEM images of the carbons after HF treatment are shown in Figure 5. The SEM images confirm that sample C-FAl-MSA contains only CNTs and no other carbon particles were observed
(Figure 5a), while sample C-FAl-MSC contains significantly less CNTs and a substantial quantity of particulate/monolithic carbon (Figure 5b). Furthermore, the alignment of the CNTs in sample C-FAl-MSA is retained after removal of the substrate; highly aligned CNTs are observed (Figure 5c). It is noteworthy that while the CNTs in sample C-FAl-MSC grow in a random manner to form a mesh of entangled tubes (Figure 5d), those for sample C-FAl-MSA grow in aligned bundles away from the substrate (Figure 4a-c) and are preserved after substrate removal to give a reasonable quantity of highly aligned nanotubes (Figure 5a,c). The alignment which was absent for the equivalent pure silica template (carbon C-FMSA) must originate from a change in location or distribution of the Fe catalyst species. The presence of both surfactant molecules and grafted Al increases the ion exchange capability of the aluminosilicate substrate, and thus the ability to retain Fe. It is therefore likely that during the CVD process, the concentration of Fe species on/near the surface of the noncalcined Fealuminosilicate substrate is relatively high. Consequently CNTs grow in dense arrays which lead to their alignment. If the nanotube density is sufficiently high, then neighboring tubes interact by van der Waals forces and support each other via the crowding effect, which leads to improved alignment.21 The XRD patterns of the C-FAl-MS carbons (Supporting Information, Figure 4) show sharper graphitic peaks for sample
15162 J. Phys. Chem. C, Vol. 112, No. 39, 2008 C-FAl-MSA compared to C-FAl-MSC, which is consistent with a higher level of graphitization (and consequently a higher CNT content) in the former. Porosity analysis (Supporting Information, Figure 4) indicated that both carbon materials possess micropores and mesopores, with sample C-FAl-MSC exhibiting considerably greater mesoporous character. This may be a consequence of the presence of particulate/monolithic carbon that is mesoporous in nature. This assumption is consistent with the higher surface area and pore volume (Table 1) of sample C-FAl-MSC compared to sample C-FAl-MSA. Overall, when compared to Fe-silica substrates, there is an improvement in CNT production for the Fe-aluminosilicate substrates. The TGA curves of the carbons derived from the Fe-aluminosilicate substrates are shown in Figure 6. The residual weight for both carbons was typically
