Abstract
The application of porous carbon is versatile. It is used for high-performance catalyst support, electrode material in batteries, and gas storage. In each of these application fields nanostructuring improves the material properties. Supercapacitors store a high energy density. Exactly adapted carbon structures increase the life of lithium batteries and protect catalysts with increasing reaction rate and selectivity. Most of porous carbon materials have a spherical shape. To the best of our knowledge, there is no procedure to synthesize nanostructured cylindrical porous carbon systematically. Here, template glass fibres and SiO2-tubes were modified with nanostructured SiO2/phenolic resin and SiO2/poly(furfuryl alcohol) layers by surface twin polymerization (TP) of 2,2′-spirobi[4H-1,3,2-benzodioxasiline] and tetrafurfuryloxysilane. Afterwards the SiO2/polymer layer on the template is thermally transformed into a defect-free nanostructured SiO2/carbon layer. After completely removing the SiO2 components microporous carbon tubes or rods are finally achieved. The diameters of the carbon rods and the inner as well as the outer diameter of the carbon tubes are adjustable according to the shape and size of the template. Thus, a huge variety of microporous carbon materials can be easily produced by template-assisted TP.
1. Introduction
Application of porous hollow carbon material is versatile [1–3]; it is used for high-performance catalyst support [4–6], electrode material in batteries [7, 8], gas storage [9], and filters [10]. In each of these application fields nanostructuring improves the material properties. Supercapacitors store a high energy density. Exactly adapted carbon structures increase the life of lithium batteries and protect catalysts with increasing reaction rate and selectivity.
Most pathways to produce hollow carbon rely on hard templating and solvothermal treatment [1, 2]. In literature many porous carbon bodies with different properties were described [2]. The most reported porous carbon bodies have a spherical shape [2]. Just a few publications describe the synthesis of hollow carbon vessels with wormlike structures, tubes, and hollow fibres [1, 11–15].
One requirement for producing porous carbon with pore size dimensions on the nanoscale down to 0.5 nm presuppose, the accessibility of nanostructured inorganic/organic hybrid materials which can be transformed to carbon after carbonizing the organic component and removing the noncarbon component. For this purpose nanostructured SiO2/polymer hybrid materials have been established as suitable precursors for bulk carbon materials with large pore volume [16].
The production of nanostructured inorganic oxide/polymer hybrid materials is not easy to perform, because two different components must be brought together on the molecular level in such a way that no phase separation process takes place. One established way is to polymerize two different monomers simultaneously in one procedure by adjusting the reaction conditions [17, 18]. A more elegant way is the use of monomers which contain two differently polymerizable groups in one molecule [19–21]. Each polymerizable functional group of such a heterobifunctional monomer polymerizes independently of the others and by another mechanism. Depending on the reaction conditions both processes can be done either simultaneously or consecutively. Mainly sol-gel processes are still used in inorganic component formation from hetero bifunctional monomer [17–20].
The twin polymerization (TP) is distinguished from the just mentioned methods [22–27] and was used in combination with spherical hard templates to synthesize size adjustable carbon hollow spheres with micro- or mesoporous shells [27]. TP requires the use of so-called twin monomer, an adequate inorganic/organic hybrid monomer—which undergoes polymerization to two different polymers in one process. Both polymerization processes are associated with each other and are mechanistically coupled [16, 22–25]. After the activation of a twin monomer both units polymerize simultaneously and the occurring inorganic polymer determines the structure of the organic polymer and vice versa. Thus, a nanostructured hybrid material is obtained. The silicon containing twin monomers, 2,2′-spirobi[4H-1,3,2-benzodioxasiline] (1) and tetrafurfuryloxysilane (2), yielded in nanostructured SiO2/polymer hybrid materials. Starting from this material it is possible to obtain porous carbon by carbonizing the organic phase and removing the SiO2 phase by treatment with hydrofluoric acid (Figure 1).
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In this report we present a novel route to fabricate cylindrical porous carbon compounds based on TP in combination with hard templates. Both twin monomers result in porous carbon after polymerization, carbonization, and treatment with aqueous hydrofluoric acid [16, 22].
According to Figure 1 dip-coating of a fibre template will lead to a hybrid layer on the surface after TP. Removing the SiO2 phase results in a porous carbon tube (Figure 1(c)). A tubular template will lead to a porous carbon tube (outer layer) and a porous carbon rod (inner layer) (Figure 1(d)).
2. Experimental
2.1. Preparation
2.1.1. Monomer Synthesis
(1) Synthesis of Monomer 1: 2,2′-Spirobi[4H-1,3,2-benzodioxasiline] [16]. 35.00 g salicyl alcohol (0.28 mol) was dissolved in 300 mL toluene at 85°C under dry argon atmosphere. 0.1 mL tetra-n-butylammonium fluoride (1 M in THF) was added. Tetramethoxysilane (21.46 g, 0.14 mol) was dropped slowly into the hot solution. After 1 h, methanol was removed under reduced pressure and the clear toluene solution was decanted from viscous byproducts. Colourless crystals were obtained after evaporating of the toluene and recrystallization using hexane. Yield was 80%; m.p. 82–84°C; 1H NMR (400 MHz, CDCl3, 25°C, TMS) = 7.21–7.27 (m, 2H), 6.97–7.05 (m, 6H), 5.25–5.17 ppm (2d, 2J(H,H) = 14.2 Hz, 4H, CH2); 13C NMR (100 MHz, CDCl3, 25°C, TMS): = 152.4, 129.1, 125.7, 125.2, 122.3, 119.3, 66.3 ppm (CH2); 29Si CP-MAS-NMR (79.49 MHz, 25°C): = –78.37 ppm confirming a four-fold coordination; elemental analysis (%) calcd/found for C14H12O4Si: C (61.75/60.07), H (4.44/4.29)—deviation can occur due to possible decomposition (hydrolysis) caused by noninert measurement method; C-content confirms four-fold coordination.
(2) Synthesis of Monomer 2: Tetrafurfuryloxysilane [22]. (90 mL, 0.4 mol) and furfuryl alcohol (141 mL, 1.6 mol) were mixed, and 0.3 wt-% of KOH was added. The mixture was stirred for 3 h at 80°C and 65 mbar. Then the pressure was reduced over 1 h to 3 mbar and the by-products were distilled off. The product was obtained at 240 °C and 0.4 mbar. Light yellow crystals precipitated at 0°C and could be recrystallized from diethyl ether. Yield was 60%, m.p. 39°C, 1H NMR (400 MHz, CDCl3, 25°C) = 4.73 ppm (8 H, s), 6.32 (4H, dd), 6.25 (4H, m), 7.44 (4H, dd); 13C NMR (100 MHz, d6-Acetone), 25°C: = 58.5, 108.8, 110.8, 142.8, 155.6 ppm; elemental analysis (%) calcd/found for C20H20O8Si: C (57.00/57.70), H (4.90/4.80)—deviation can occur due to possible decomposition (hydrolysis) caused by non-inert measurement method.
2.1.2. Porous Carbon by Template-Free Synthesis (Reference Samples)
(1) Porous Carbon from Acid Induced Twin Monomers. According to literature [16] 2 g of 1 was dissolved in 50 mL of toluene and given to a solution of 240 mg methanesulfonic acid in 250 mL toluene. The mixture was stirred for 15 h. The precipitate was filtered and washed with 100 mL dichloromethane. 1.8 g (yield: 80%) of pink powder was obtained. When using 2 g of 2 in the same procedure an amount of 0.78 g (yield: 35%) of a black solid results.
After polymerization a carbonization step (heat rate: 6.2 K min−1, 800°C for 3 h) under argon followed. SiO2 was removed by treating the samples with aqueous hydrofluoric acid (40%) for 3 days.
(2) Porous Carbon from Thermally Induced Twin Monomers. The reference-hybrid derived from thermally induced TP of 1 was synthesized by heating 7 g of 1 up to 200°C under air on a heating table. After 1 h at this temperature 6.6 g (yield: 94%) of a brownish, transparent SiO2/polymer hybrid is obtained. For thermal TP of 2 a mass of 6 g monomer was heated for 1 h in a preheated (°C) muffle furnace and 3.9 g (yield: 65%) of brown hybrid material was obtained.
After polymerizing the monomers to SiO2/polymer hybrid, a carbonization step (heat rate: 6.2 K min−1, 800°C for 3 h) under argon followed. SiO2 was removed by treating the samples with aqueous hydrofluoric acid.
2.1.3. Carbon by Template-Assisted Procedure
1 g template was dipped into a solution of 1 g methanesulfonic acid in 20 mL−1 toluene for 10 s and afterwards for 10 s into a melt or solution of twin monomer. The coated template was dried in air over night at room temperature.
In a typical thermal induced TP synthesis 1 g template was dipped into 2 g of monomer melt ( = 90°C, = 50°C), heated to 200°C, and held at this temperature for 15 minutes. After thermal polymerization a carbonization step (heat rate: 6.2 K min−1, 800°C for 3 h) under argon followed. SiO2 was removed by treating the samples with aqueous hydrofluoric acid.
2.2. Chemicals
Tetraethoxysilane (Acros, 98%) and furfuryl alcohol (Acros, 99%) were distilled and stored under argon. 2-Hydroxybenzyl alcohol (ABCR, 99%), methanesulfonic acid (Alfa Aesar, 98+%), tetra--butylammonium fluoride (1 M in THF), tetramethoxysilane (Fluka, 98%), and aqueous hydrofluoric acid (Prolabo, 40%) are commercially available and were used without further purification.
Glass fibres were provided by Cordus GmbH and ORMOCERs by Fraunhofer-Institut Würzburg. Glass capillaries from Lab Logistics Group GmbH were used. All substrates were used without purification.
2.3. Physicochemical Characterization
The carbon samples were imaged by a scanning electron microscope Philips SEM 515 and a NovaNanoSEM (FEI), the latter having Schottky field emission gun.
The pore texture was analyzed by measuring nitrogen-sorption-isotherms with a Sorptomatic 1990 (Fisons). The pore-size distribution plots of the carbon materials were calculated from nitrogen-sorption-isotherms by the method of Horvath and Kawazoe [26] according to DIN 66135.
To measure the length scale of the pore texture of the carbon materials made from 1 and 2, small angle X-ray scattering (SAXS) measurements were used. All SAXS measurements were performed on a setup using an X-ray generator (rotating anode) from Rigaku. The flight path of the instrument was fully evacuated and the scattered radiation was detected by a 2D Bruker Hi-Star multiwire proportional chamber. Aluminum discs with a thickness of 1 mm and a central hole of 1 mm diameter, into which the powder-like samples were stuffed, served as sample holder. Measurements were taken at two different sample-detector distances in order to cover a broad range of scattering vectors . The intensity scale of the two data sets was adjusted such that the data overlapped in the intermediate -range. The first few data points (around 3 10−2 Å−1) of the second data set (higher ) are affected by the beam stop and should not be evaluated. A background subtraction was not necessary as the scattering signal from the sample was much stronger than any background intensity.
Data analysis was based on the Debye-Bueche expression for a random two-phase system, . Here is a constant and is the correlation length, with , where and are the average chord length in phases 1 and 2, respectively; that is, the average is taken over the carbon phase as well as the pore phase [28, 29]. The correlation length gives therefore an average size of the two phases, weighted with the volume fractions.
3. Results
3.1. Verification of Carbon from Template-Free Procedure
The silicon based twin monomers 1 and 2 were polymerized after induction by an acid or thermal activation. Thereby, 1 reacts to a nanostructured SiO2/phenolic resin and 2 to a nanostructured SiO2/poly(furfuryl alcohol) hybrid material, as known from literature (see also Figure 1) [16, 22]. Starting from those materials porous carbon was synthesized by carbonizing the polymer matrix and removing the SiO2 phase. The obtained carbons do not contain any residual SiO2; thermogravimetric analyses (TGA) show a complete mass loss, while a carbon content of 85 wt-% is measured by elementary analysis. The Raman spectra of carbon derived from 1 and 2 are nearly congruent and contain characteristic D- and G-bands, which indicate amorphous carbon as well as X-ray powder diffraction did (see supplementary information, (Figures SI 1 and SI 2) in the Supplementary Material available online at http://dx.doi.org/10.1155/2013/872019).
Template-free acid induced polymerization of 1 leads to porous carbon, which contains mostly (according to literature: 72 vol-%) micropores. In case of 2 mostly (mesopore content: 41 vol-%) mesoporous carbon results from the same procedure (Figure 2).
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Small angle X-ray scattering (SAXS) measurements support the results from gas sorption experiments. The scattering curves implicate pore texture on two length scales. The power law scattering at lower is attributed to the larger mesopores and the external surfaces of the powder-like sample. The part of the data described by the Debye-Bueche expression reflects the small scale microporosity of the sample (higher ). The results shown in Figure S2 clearly indicate that the microporous structure in carbon from 1 has a smaller length scale (correlation length Å) than porosity in carbon from 2 ( Å). Thus, the results by nitrogen-sorption-isotherm and SAXS differ slightly, but both methods confirm consistently that the pore-size distributions depend on the starting twin monomer.
3.2. Glass Fibre Template
Pure SiO2-fibres are very rare and expensive. Thus, we chose commercially available glass fibres as a very cheap SiO2-rich but impure substrate. According to Figure 1(c) a functional SiO2/polymer hybrid layer on the template surface should result in porous carbon tubes after removing the SiO2 phase.
The polymerization of 1 and 2 can be induced by acidic catalysis or by heating to 200°C under air. Both methods lead to a coated glass fibre (see supplementary information, Figure SI 3). Acidic catalysis can take place in solution or melt. Both monomers lead to very thin hybrid layers at the surface of the glass fibres, even by dip-coating into melt. The maximum carbon content of glass fibre, coated by acid induced polymerization, amounts to 15%. Those thin layers are not stable enough to overcome the carbonization and etching procedure. They break down after carbonization and removing the SiO2 phase. It is not possible to increase the thickness of the hybrid layers, because inhomogeneous material results by acid activation of high concentrated monomer solution or a monomer melt.
Thus, it is necessary to induce the polymerization of the twin monomers thermally on the surface of the glass fibre. In this way a thick SiO2/polymer hybrid layer can be generated on the surface of the template. The maximum carbon content of 33% was achieved by dip-coating into a monomer melt and thermal induction. The thick SiO2/polymer hybrid shell leads to an SiO2/C layer after carbonization, which results in stable carbon tubes with a porous shell after HF treatment (Figure 3).
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Since glass fibre templates consist of bundles and not of isolated fibres, the resulting carbon tubes are agglomerated. The inner diameter of those carbon tubes is about 8.4 μm (Figure 3(c)), according to the template diameter (see supplementary information, Figure SI 3(a)). The outer diameter of the carbon tubes amounts to about 9 μm (Figure 3(a)). Calcium fluoride is visible as white crystals in Figure 3(b) and verified by energy dispersive X-ray analysis (Figure 3(b)). Calcium fluoride formation occurs because industrial glass fibre is an impure SiO2 template. It contains also calcium oxide, aluminum oxide, and in traces magnesium oxide (see EDX-analysis, supplementary information, Figure SI 4). Due to the treatment with hydrofluoric acid, calcium oxide is converted to insoluble calcium fluoride. According to thermal gravimetric analysis, the whole material consists of 25 m-% calcium fluoride. The synthesis of pure porous carbon bodies is suitable by the use of pure SiO2 templates (see next section).
There are no significant differences between the SEM images and Raman spectra of carbon tubes received either from 1 or from 2 (see supplementary information, Figures SI 1 and SI 5). In contrast to carbon derived from acidic induced twin monomers (Figure 2(a)) the nitrogen-sorption-isotherms are nearly congruent if the monomers were thermally induced (Figure 4(a)).
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In general, the nitrogen-sorption-curves of carbon from thermally induced twin monomers are both type I isotherms, corresponding to microporous carbon [30]. Thus, the pore-size distribution plot shows mostly micropores (carbon from 1: 73 vol-%, carbon from 2: 75 vol-%) and the specific surface areas of those carbons are 710 m2 g−1 (1) and 930 m2 g−1 (2). The specific surface area of carbon decreases dramatically by using glass fibre template (510 m2 g−1 and 310 m2 g−1) and the pore-size distribution (Figure 4(b)) shows mostly mesopores (carbon from 1: 50 vol-%, carbon from 2: 46 vol-%). However, the reason for different pore-size distribution lies in the occurrence of by-product and not in the type of induction. According to TGA measurements the carbon from polymerization of 1 on glass fibre consists of 25 m-% CaF2 and the product of using 2 consists of 75 m-% CaF2. Thus, the nitrogen-sorption experiments and the resulting pore-size distributions are distorted. If CaF2 could be released from the mixture, the porous carbon should have a higher specific surface area and more micropores should be measured. To avoid CaF2 as by-product a template should be used, which can be removed completely.
3.3. ORMOCER Templates
In order to overcome the drawbacks from standard impure SiO2-fibres we chose ORMOCERs as pure fibrous hollow SiO2 templates provided in small lots. Their use in the template assisted-TP of 1 will lead to pure porous carbon tubes without the formation of metal fluorides as by-product (Figure 5).
The tubular template is coated at the inner and the outer sides by thermally induced TP. After carbonization a functional SiO2/C layer remains at the inner and at the outer sides of the ORMOCER. Treatment with aqueous hydrofluoric acid completely removes the SiO2 of the ORMOCER, but the outer carbon layer breaks down (Figure 5(1b)), too. In this way porous solitary carbon tubes with a smooth surface were obtained (Figures 5(1a) and 5(1c)). The outer diameters of the carbon tubes are 100 μm and 130 μm and match exactly the inner diameters of ORMOCER 540 and ORMOCER 1051 templates. The inner diameters of the synthesized carbon tubes are about 70 and 90 μm.
The pore texture of the resulting carbon tubes is comparable with those of carbon tubes using glass fibre as template. The specific surface area is also reduced to 450 m2 g−1. Thus, not only the formation of CaF2 but also the shape of carbon tubes should be a reason for the lower specific surface areas.
As seen, dip-coating into a hot melt (90°C) leads to porous carbon tubes (Figure 5), but it is not only possible to create the described species of carbon tubes by using ORMOCERs as template. The whole inner space of the ORMOCER tubes can be filled with monomer by dip-coating into an undercooled (50°C) melt. The viscosity of the monomer melt increases with decreasing temperature and so the monomer melt has a honey-like viscosity at 50°C and does not flow out of the template. Thus, the completely filled inner template space results in porous carbon rods after polymerization, carbonization, and treatment with hydrofluoric acid.
The products from this procedure are porous carbon rods with diameters of 100 μm and 130 μm by using ORMOCER 540 (Figure 5(2a)) and ORMOCER 1051 (Figure 5(2b)) as templates. Larger porous carbon rods ( μm) can be produced by using glass capillary as cheap template (Figure 5(2c)). In comparison to the use of glass fibre templates just a very small amount of calcium fluoride occurs as by-product, because the carbon rods slide out of the glass capillary after carbonization.
3.4. Combination of Templates
It is possible to combine a glass capillary as outer template with ORMOCERs or glass fibre as inner templates. In this procedure the smaller template has been put into the glass capillary, and both templates were dipped into the monomer melt and heated up to induce the polymerization. After carbonization and treating with aqueous hydrofluoric acid, porous carbon tubes with a defined outer diameter and inner diameter result (Figures 5(3a), 5(3b), and 5(3c)).
In case of using glass fibres as inner template a small channel of about 40 μm is visible in the 580 μm thick porous carbon tube (Figure 5(3a)). The channel size is increasing by using ORMOCER 540 (Figure 5(3b)) and ORMOCER 1051 (Figure 5(3c)) up to 160 μm and 220 μm.
4. Summary
Porous cylindrical carbon bodies with a tailored shape were synthesized by template-assisted TP. ORMOCERs, glass fibres, and glass capillaries with various inner and outer diameters were coated by surface TP of two different twin monomers at their surfaces. Porous carbon tubes of well tailored morphology result after carbonization and release of SiO2. The route of template-assisted TP allows the synthesis of porous carbon of any shape in only one procedure. The outer diameter of carbon rods or tubes can be controlled by an outer template, like a glass tube, and the inner diameter by an inner template like glass fibre or ORMOCERs.
This work points out the high potential of the template-assisted TP, which could be used to create hollow carbon bodies of almost any shape or size, according to desired properties for application. Currently, more monomers and templates are under investigation.
Conflict of Interests
The paper describes original research. The authors used several templates to produce nanostructure carbon. ORMOCER is one of those templates. The research is not influenced by a secondary interest, such as financial gain.
Acknowledgments
The authors gratefully acknowledge the Deutsche Forschungsgemeinschaft for financial support DFG (Project: Sp 392/31-1) and Cordus GmbH for providing glass fibre. The substance classis called ORMOCER was provided by Fraunhofer-Institut für Silicatforschung ISC in Würzburg to support the research project. Thus, the authors thank Dr. Ladislav Kido and Dr. Thomas Ballweg for providing this template without any financial interests.
Supplementary Materials
Figure 1: Raman-spectra of synthesized carbon.
Figure 2: Representative XRPD of synthesized carbon.
Figure 3: Photographs and SEM images of glass fibre and coated glass fibre.
Figure 4: EDX analysis of pure glass fibre.
Figure 5: SEM images of carbon tubes synthesized by TP on glass fibre.