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Thalassiosira pseudonana diatom as biotemplate to produce a macroporous ordered carbon-rich material Mo´nica Pe´rez-Cabero, Victoria Puchol, Daniel Beltra´n, Pedro Amoro´s* Institut de Cie`ncia dels Materials de la Universitat de Vale`ncia (ICMUV), P.O. Box 22085, 46071 Valencia, Spain
A R T I C L E I N F O
A B S T R A C T
Article history:
Ordered macroporous–mesoporous carbonaceous materials were produced as a direct rep-
Received 1 October 2007
lica of the Thalassiosira pseudonana diatom by infiltration of the skeleton with furfuryl alco-
Accepted 19 November 2007
hol. The final carbon-rich material preserves the macropores of the diatom acting as bio-
Available online 24 November 2007
template and new hierarchical macro–mesopores appears as the silica is eliminated through chemical etching. The final solid can be described as an organized array of carbon macrotubes. In order to understand the progressive silica etching and the subsequent effect on the final carbon material, different etching reagents have been used. Moreover, the similar pore topology of T. pseudonana and the well known MCM-41 mesoporous silica (hexagonal ordered arrays of non-interconnected pores), allowed us to consider this system as a micrometric model to understand in 3-D the carbon replication of MCM-41 silicas. Ó 2007 Elsevier Ltd. All rights reserved.
1.
Introduction
Porous and high-surface carbon-based materials constitute a challenging domain in chemistry that is in full expansion due to their increasing applications field [1]. In fact, porous carbons have widely and typically been used as industrial adsorbents for long time ago [2]. Nowadays, modern science and technology areas, such as water and air purification, gas separation, catalysis, chromatography, biotechnology, electrochemical capacitors and energy storage, make use of these porous carbons [3]. Traditional preparative methods, including chemical, physical, catalytic activation of carbon precursors or carbonization of polymer blends, always lead to disordered porous materials with significantly high pore size dispersion [1]. The isolation of ordered porous carbons displaying good pore size homogeneity currently implies the use of inorganic silica-based ordered porous materials (xerogels, zeolites, meso and macroporous silicas) as templates. When zeolites are used as templates, it has been traditionally reported how the pore ordering is not always well maintained in the final porous carbon structure due to the non-rigidity of
the carbon frameworks formed inside their narrow micropores [1]. However, it is possible to keep the micropore ordering when a careful infiltration of the carbon precursor is carried out [4–7]. Recently, carbon frameworks with ordered pores were reported using mesoporous templates [8–10]. In practice, the introduction of hard supramolecular assemblies as templating agents has permitted the preparation of new mesoporous compound families, with a wide variety of compositions, whose main characteristic is the presence of sharp mesopore size dispersions [11]. In particular, research into ordered mesoporous carbons (OMCs) has seen tremendous growth since the discovery of the ordered mesoporous silica of the M41’s family and the pioneering work of Ryoo and co-workers, synthesizing OMCs denoted CMK-1 through nanocasting routes by using MCM-48 silica as template [12]. Indeed, after that work, the number of successful procedures for synthesizing OMCs with different pore arrays (hexagonal, cubic) from different mesoporous silicas, and using different carbon sources, such as CMK-2 (from SBA-1), CMK-3 (from SBA-15) or SNU-2 (from HMS), has rapidly increased [8,12–16].
* Corresponding author: Fax: +34 96 3543633. E-mail address:
[email protected] (P. Amoro´s). 0008-6223/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2007.11.017
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If a further step is taken, we can find the preparation of macroporous carbons, originally synthesized by carbonization of block copolymers or resins [17,18], and more recently, by means of new technological approaches such as ultrasonic spray pyrolysis [19]. However, the absence of organized templates in all these strategies leads necessarily to disordered porous carbons. Hence, the preparation of ordered macroporous carbons has only been achieved by the templating of silicon inverse opal structures [20,21]. This approach, which is effective for the preparation of ordered porous structures, presents many procedural disadvantages, such as the numerous synthetic steps and the expensive products needed. In this sense, it should be very interesting to find nonsophisticated inorganic templates to synthesize low cost ordered macroporous carbons, which could allow the control and design of the pore size, length scale and morphology in the final templated materials. Diatoms are unicellular algae which create a wide variety of three-dimensional amorphous silica shells [22,23]. These exoskeletons, the so-called frustules, consist of assembled silica nanoparticles with highly organization degree and multiple pore arrangements usually at the micrometric scale. Scarce attempts to prepare porous carbons from diatomaceous earth as template are found in the bibliography [24,25]. In both cases sucrose is utilized as carbon precursor for impregnation and, due to the over saturation of sucrose inside the template, the final materials are, as the authors stated, inverse carbon replicas. The final carbon nature not only depends on the symmetry of the silica template, but also procedural variables such as the filling degree of the carbon precursor in the pore system can modify the final structure [9]. On the one hand, when the carbon precursor is sufficient to fill up the pores, inverse carbon replicas of the silica templates are obtained, whose porosity is not related to the original pores, but with the inter-cavities generated among the packed carbon rods. On the other hand, if the pore system is only coated instead of completely filled, the result will be a more or less interconnected system of hollow carbon tubes. In this case, the silica pores are reasonably reproduced in the final carbon material, which could be properly described as a modified direct replica of the silica template. Moreover, additional pore systems are generated due to the voids among the hollow tubes in these last carbons. This strategy has been exploited for different groups to produce multimodal porous carbons denoted as CMK-5 and NCC-1 [16,26]. The inverse replica mechanism only requires the combination of a good pore filling and the presence of a certain proportion of covering faults in the carbon coating, necessary to allow the silica etching, leading to the carbon material made of carbon rods. On the contrary, the mechanism to generate direct carbon replicas seems to be slightly more complicated. A higher control in the carbon precursor infiltration is strictly necessary to avoid pore over-filling, and at the same time, to avoid over-coating of the surfaces, which would impede the template dissolution. In this work, we describe a simple and low cost approach to synthesize ordered macroporous carbons based on the use of diatoms as bio-templates. Moreover, in order to prepare carbonaceous materials with regular and homogeneous morphologies and macropore systems, we have selected a specific
diatom, Thalassiosira pseudonana, instead of commercial diatom earth, where a variety of frustules are present. The pore array dominant in the T. pseudonana frustules fits very well on ordered hexagonal pore systems at macro-scale domain, similar to what showed by MCM-41 silica [27].
2.
Experimental
2.1.
Chemicals
All the synthesis reagents are analytically pure, and were used as received from Aldrich [AlCl3 Æ 6H2O, furfuryl alcohol (FA), NaOH and HF].
2.2.
Diatom frustules isolation
T. pseudonana was supplied from the UTEX Culture Collection of Algae (UTEX#LB FD2). This widely distributed diatom was cultured in GSE medium at 20 °C using a 12 h light/12 h dark cycle as previously described in Ref. [28]. Diatoms were harvested after three weeks of culturing and cleaned using standard procedures [29]. The full removal the organic matrix covering the siliceous frustules was realized through treatments with hydrogen peroxide. This protocol allows preserving a significant amount of intact frustules. The color change from yellow-green to white indicates the organic matter removal. Finally, the white frustules were washed with water and ethanol.
2.3.
Functionalized diatom frustules
Given that we use furfuryl alcohol to coat the diatom surface, Al-acid sites at the surface are needed to catalyze the polymerization. Al was introduced as heteroelement from aqueous solution of AlCl3 as the following: 1 g of diatom was suspended in a water solution (30 mL) containing 0.60 g of AlCl3 Æ 6H2O, and kept under stirring during 1 h at 50 °C. Then, additional 50 mL of water were added, and finally, the frustules were recovered by filtration, washed with ethanol and calcined at 550 °C (heating rate of 1 °C/min) during 5 h. The modified Al-T. pseudonana frustules present an Al incorporation on the diatom surface corresponding to a Si/Al = 32 molar ratio.
2.4.
Synthesis of carbonaceous frustules
Initially, furfuryl alcohol was infiltrated into calcined Al-T. pseudonana (Al-Tp) frustules at room temperature (2 ml of furfuryl alcohol per g of Al-Tp) and polymerized at 95 °C for 24 h. The resultant polymer/Al-Tp composite was then placed in a ceramic boat in an horizontal quartz furnace under Ar atmosphere, and calcined at 900 °C during 3 h (heating ramp 1 °C/ min), leading to a black powder, the carbon/Al-Tp composite, from which the diatom silica skeleton was finally removed by chemical etching. In order to analyze the progressive silica dissolution and the subsequent carbon replica formation, we have used three different chemical etching treatments: (1) soft silica attack by a NaOH solution, (2) moderate treatment by using HF (5%) without stirring at different contact times
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(t = 6, 12 and 24 h), and (3) strong treatment with HF (10%) under vigorous stirring. Regardless the reagent used, the carbon replicas were recovered by filtration, washing with water and ethanol, and drying at 80 °C for further characterization.
2.5.
Characterization Techniques
All solids were analyzed for Si, Al and C by electron probe microanalysis (EPMA) using a Philips XL30 ESEM instrument. The Si/Al and Si/C averaged molar ratio values have been estimated from EPMA data (ca. 20 different measurements with a spot size resolution of 5 lm) recorded on different frustules. The samples were also analyzed by X-ray fluorescence (XRF) using a Pico TAX TXRF spectrometer. All products were characterized by X-ray powder diffraction (XRD) (Seifert 3000TT h– h) using Cu Ka radiation. Patterns were collected in steps of 0.05° (2h) over the angular range 5-50° (2h) for 5 s per step. Purified carbonaceous samples were characterized by thermogravimetric analysis (TGA) under oxygen atmosphere in a Setaram Setsys 16/18, at temperatures up to 900 °C. SEM images were acquired by using a Hitachi S-4100 FE microscope. Samples were previously coated with Au–Pd. Surface areas were calculated from nitrogen adsorption-desorption isotherms ( 196 °C) recorded on a Micromeritics ASAP-2010 automated analyzer. Calcined samples were degassed for 12 h at 110 °C and 10 6 Torr prior to analysis.
3.
Results and discussion
3.1.
Synthesis strategy
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quired when compared to previous works on this subject. So that, we have selected furfuryl alcohol as precursor according to previous works of Ryoo et al. describing a good control of the pore filling degree in SBA-15 silicas [30]. Then, furfuryl alcohol impregnation seems to be more suitable for the formation of rigid and well structured carbonaceous direct replicas of our starting material, the Al-Tp diatom. Moreover, in order to avoid over-polymerization on the silica surface and, consequently a certain proportion of filled pores, we have carried out a single impregnation protocol in our experimental procedure.
3.2. Characterization of the Carbon/Al-T. psesudonana (C/Al-Tp) composites Diatoms are eukaryotic, photosynthetic microorganisms found throughout marine and freshwater ecosystems and are responsible for as much as 20% of global primary productivity [22,23,31]. A defining feature of diatom is their ornately
As we are interested in the preparation of direct carbonaceous replicas of diatoms, a finer control of the carbon coating is re-
Fig. 1 – SEM image of a frustule of Thalassiosira pseudonana showing the hexagonal order array of macropores.
Fig. 2 – SEM images of C/Al-Tp. (a) General view of an intact frustule and (b) image along the macropore direction.
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patterned silicified cell wall or frustule, which displays such species-specific fine scale micro- and nano-structures [32]. The marine centric diatom T. pseudonana is cosmopolitan throughout the world’s oceans [31]. Then, this diatom can be considered as a low-cost silica bio-template, which could be easily cultured following well stabilized biotechnological aqua-cultural methodologies.
T. pseudonana presents frustules made of two overlapping valves joined by girdle bands. This skeleton has a circular symmetry and a pore topology similar to what showed by MCM-41 silicas [27]: a highly ordered hexagonal array of non-interconnected pores (Fig. 1). The main difference is the scale. Hence, while MCM-41 materials show mesopore sizes usually around 2–3 nm, the pore sizes in the selected diatom are around 300-1500 nm. Then, we can consider T. pseudonana frustules as a rough micrometric model to understand the carbon replication of M41’s silicas through nanocasting, even though it is obvious that the scale change from nano to micro would clearly affect parameters such as diffusion in the synthesis procedure. Fig. 2 shows SEM images of the diatom-carbon composites. From these micrographs, the formation of a carbon coating on the diatom surface is evident. Fig. 2a displays an intact frustule with a smoothing surface due to the deposited carbon. The partial opacity observed in Fig. 2b indicates that a thin carbon film is also deposited on certain frustule valves. We have used EPMA to check the chemical homogeneity of the resulting composites. We have analyzed different frustules with a spot size resolution of 5 lm and a similar Si/C molar ratio of 0.71 ± 0.10 has been obtained.
3.3.
Fig. 3 – Thermogravimetric analysis of purified C/Al-Tp after etching with (a) NaOH and (b) HF (10%).
Carbon purification
In order to observe as the progressive silica etching affects the morphology and organization of the final carbons we have applied three chemical etching protocols with increasing capability for the silica removal in the C/Al-Tp composites: NaOH, HF(5%) and HF(10%). The final solids have been studied by TGA to determine the purity degree expressed in wt% of inorganic residue. Their general combustion behavior (Fig. 3) proceeded through two exothermal steps, at temperatures around 317 °C and 470 °C, which indicates heterogeneity in the nature of the carbonaceous material (either structural or textural). Significant differences in the amount of inorganic
Fig. 4 – XRD patterns of (a) C/Al-Tp, and after etching with (b) NaOH, (c) HF (5%) and (d) HF (10%).
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residue are observed depending on the etching media selected. When a soft treatment is applied (NaOH), a significant amount of silica (80 wt%) remains after the etching (Fig. 3a). Then, this sample could correspond to the initial silica elimination and so, to the first step in the carbon purification. On the contrary, when HF is used as reagent, the etching process leads to rich carbon materials with inorganic residue in the 25–30 wt% range (Fig. 3b). The presence of a certain amount of inorganic residue after combustion of samples treated in HF indicates a hindered etching process probably due to a homogeneous and thick carbon coating. Good silica replication and low inorganic residues work in opposite sense as the template evolution requires of a significant proportion of covering faults in the carbon coating. EPMA also confirms at micrometric level the gradual decrease in the silica proportion when harder etchings protocols are used. In fact, Si/C molar ratios at the composite surface decrease from 0.80 to 0.03 for NaOH and HF treatments, respectively. The achieved silica residues are significantly higher than the values usually achieved for M41’s silicas (where less than 5% silica is retained). As above mentioned, the different scale between diatoms and M41’s silicas is the responsible for this behavior. In fact, while the replication of nanotubes requires only small carbon amounts on the silica surfaces as thin deposits, the replication of micrometric tubes typical of diatoms imply thicker carbon deposits, with the subsequent trapping of certain silica domains. Moreover, the leaching behavior will also be significantly different, since the transport length are orders of magnitude longer. In any case, the carbon deposition from furfuryl alcohol decomposition and the subsequent silica etching have been optimized to produce a good diatom replica. The XRD patterns of the starting composite (C/Al-Tp) and the purified carbons through etching are shown in Fig. 4. Although the diatom frustule is build up by aggregation of amorphous silica particles, the high temperature (900 °C) used to carbonize the polymer/Al-Tp composite induce a certain crystallization degree. Then, we can observe in the XRD patterns together with a broad signal centered at ca. 20°(2h), typical of amorphous SiO2, some peaks that can be attributed to a incipient crystallization of quartz and crystoballite. The intensity of these diffraction peaks decreases as the etching capability of the reagents (NaOH and HF) increases. Small peaks remains even after etching in HF (10%). Then, according to TGA and EPMA data, the evolution of the XRD pattern of the composite also reveals a progressive decrease in the silica content. SEM images of the purified carbons clearly show the silica etching progress. The sample after NaOH treatment corresponds to the initial silica elimination step. We can observe in Fig. 5 that the macroporous structure of the frustule is preserved. The solid is a rich silica material and only a soft chemical attack in the grain boundaries is observed. In these regions is the formation of carbon macrotubes as good replicas of the frustule pores is evidenced. At this stage the solid must be considered as carbon macrotubes embedded in a slightly degraded silica-based matrix. The evolution of the etching process at different reaction times when a more aggressive reagent as HF (5%) is used is showed in the SEM micrographs included in Fig. 6. After short
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Fig. 5 – SEM images of C/Al-Tp after etching with NaOH. contact times (6 h) (Fig. 6a and b), the morphology of the composite and the silica degradation degree is practically indistinguishable from the samples treated in NaOH. However, for larger contact times a progressive silica etching is clearly observed in SEM images (Fig. 6c–f). Fig. 6e,f correspond to very fine replicas of the frustule structure of T. pseudonana. This type of frustule fragments are the dominant motives after etching with HF 5%. A good carbon replication of the two overlapped valves, and also of the macropores which are connecting them, is achieved. Carbonaceous replicas with averaged thick around 1000 nm are obtained. This value fits very well with the typical distance between the epitheca and the hypotheca valves of T. pseudonana. While this distance is approximately fixed by the diatom type, the remaining dimensions of the carbonaceous frustules seem to be related to the etching conditions used. Hence, without stirring, we have observed that relatively large valve areas of ca. 4–6 lm2 are preserved. Then, it is the carbon deposition on the frustule valves the responsible of the conservation of the macrostructure by fixing the carbon tubes.
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Fig. 6 – SEM images of C/Al-Tp after etching with HF (5%) at different contact times. (a and b) t = 6 h. (c and d) t = 12 h. (e and f) t = 24 h. This morphology of well organized carbon macrotubes generates additional porosity due to the voids spaces among adjacent tubes. The formation of new pore systems in the carbon replicas is further confirmed by N2 adsorption-desorption measurements. As it is shown in Fig. 7, additional pores appear as the etching process progresses. Hence, the carbon purified with HF 5% during 24 h displays an isotherm having a pronounced and complex hysteresis loop at 0.5 < P/P0 < 0.9 relative pressures (Fig. 7c). The latter demonstrates the existence of a multimodal pore system combining the typical frustule macropores (pores in the inner part of the carbon tubes; not observed by this technique) with the pores left by
the diatom template (in the meso and macropore domains). As consequence of the changes both in composition (SiO2 for carbon) and pore topology, (with internal and external macrotube surfaces) the surface area is significantly improved from the silica diatom (13 m2/g) (Fig. 7a) to the carbon replica (169 m2/g) (Fig. 7c). Finally, we have tested the use of HF 10% (under vigorous stirring) as etching media. The increase of concentration (when compared to the previous HF 5%) does not imply significant purity improve. In fact, similar inorganic residues around 25–30 wt% are obtained in both cases. Although the media severity and the stirring should favor quicker silica dis-
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solution, these experimental conditions lead to a significant disassembling of the carbon macrotubes. As it can be observed in Fig. 8, isolated or non-organized carbon macrotubes are the dominant morphologies at micrometric level.
4.
Fig. 7 – N2 adsorption-desorption isotherms of (a) Al-Tp, (b) C/Al-Tp after etching with NaOH and (c) C/Al-Tp after etching with HF (5%).
Concluding remarks
New ordered macroporous carbonaceous materials as a direct replica of the T. pseudonana diatom have been prepared from infiltration with furfuryl alcohol as carbon precursor. The final carbon-rich material presents a hierarchical porosity: macropores associated to the inner part of the carbon macrotubes, and meso/macropores related to the voids among carbon tubes. The use of a specific diatom as bio-template opens the possibility of synthesizes accurate (direct or inverse) carbon replicas with well defined and predetermined morphologies and macropore sizes and shapes. The large variety of available diatoms with a variety of macropores that extend from the nano to the micrometric domains can be considered as a wide catalogue of bio-templates to produce ordered macroporous carbon-based materials. The extension of classical application fields of the diatoms to hydrophobic media constitutes an obvious utility of these new materials.
Acknowledgements This research was supported by the Ministerio de Educacio´n y Ciencia (under Grant CTQ2006-15456-C04-03/BQU). M.P.-C. thanks the MEC for a Juan de la Cierva contract.
R E F E R E N C E S
Fig. 8 – SEM images of C/Al-Tp after etching with HF (10%).
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