Large Pore Volume Carbons with Uniform Mesopores and

Before adsorption measurements, all the samples were outgassed for 2 h at 200 °C. Nitrogen adsorption isotherms measured on the silica templates, and...
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J. Phys. Chem. C 2007, 111, 9742-9748

Large Pore Volume Carbons with Uniform Mesopores and Macropores: Synthesis, Characterization, and Relations between Adsorption Parameters of Silica Templates and their Inverse Carbon Replicas Kamil P. Gierszal and Mietek Jaroniec* Department of Chemistry, Kent State UniVersity, Kent, Ohio 44242 ReceiVed: March 14, 2007; In Final Form: May 2, 2007

Formation of thin polymeric films on the surface of silica templates consisting of relatively large colloids creates an opportunity to synthesize carbons with extremely large pore volume and uniform spherical mesopores and macropores. Namely, resorcinol-crotonaldehyde polymeric films formed on the silica templates composed of 50 and 80 nm colloids were used to obtain mesoporous carbons with unprecedented pore volume of about 9 cm3/g. Properties of the resulting carbons can be predicted with the help of theoretical equations, which correlate basic adsorption parameters for silica templates and silica-carbon composites.

Introduction Silica-templated nanoporous carbons were synthesized for the first time by Knox et al.1 Later, this innovative idea of using porous spherical silica particles as hard templates was further developed by application of ordered mesoporous silica (OMS), which led to the discovery of ordered mesoporous carbons (OMCs) being inverse replicas of the OMS templates used.2 The inverse replication process starts with filling pores of a hard template with a carbon precursor, which often is an organic polymer such as poly(furfuryl alcohol) or polydivinylbenzene.3 The aforementioned precursor is introduced to the template pores in the form of monomer, which can be easily polymerized. Also, high molecular weight polyaromatic species (mesophase pitch) can be used as carbon precursors.4 Both kinds of carbon precursors are effective, and their selection depends on the desired properties of the resulting material. Carbonization of a polymer-template composite is usually conducted at temperature higher than 800 °C in atmosphere of neutral gas such as argon or nitrogen. The carbonaceous product, obtained after silica dissolution, is an inverse replica of the silica template used, the pores and walls of which become the walls and pores, respectively, in the resulting carbon. Thus, this fabrication process allows controlling the pore size in the resulting carbon by varying the pore wall thickness of the silica template used. Another way to prepare inverse carbon replicas involves an incomplete filling of the silica template pores with a carbon precursor in the form of a relatively thin carbon film on the surface of these pores. This approach allows one to increase porosity and at the same time to decrease the weight of the carbon matrix. OMSs are excellent templates5 for the fabrication of OMCs via either complete pore filling2 or film-type replication.6 However, they are not suitable for the synthesis of large pore volume carbons (LPVCs) due to the difficulty in controlling the pore wall thickness in OMSs, which usually is in the range from about 2 to 5 nm. To avoid this problem one can use colloidal silica or silica colloidal crystals as templates for the synthesis of carbons with tailorable pore diameter over a wide range of mesopores and macropores, which can be simply * Corresponding author. Phone: 330-672-3790. E-mail: [email protected].

achieved by varying the size of uniform silica colloids (6 nm and above).7 Moreover, a combination of colloidal silica templating and film-type replication makes the synthesis of LPVCs possible because the formation of thin carbon film on relatively large and uniform silica colloids leads to the carbons with large specific pore volumes and uniform spherical mesopores and macropores. Note that an incomplete and irregular filling of the silica pores with carbon precursor (island-type filling) followed by carbonization and template removal results in the formation of large voids in carbons because of merging the neighboring unfilled pores.7 Although this process affords LPVCs too, their pore sizes depend on the number of merged pores, which leads to the broad and irreproducible pore size distributions (PSD). Recently, we reported the synthesis of carbons with extremely large specific pore volume (6.0 cm3/g) and narrow bimodal PSD by using 24 nm colloidal silica.8 Here we present new results on the synthesis of LPVCs with extraordinary large specific volumes of uniform spherical meso/macropores, exceeding the value of 9 cm3/g. These carbons were prepared by using colloidal silicas of two different sizes (about 50 and 80 nm) as hard templates and the copolymerizing mixture of resorcinol and crotonaldehyde as carbon precursor. Note that the use of colloidal silica crystals9 as hard templates in this recipe allows one to obtain LPVCs with ordered spherical mesopores. For comparative purposes, we also prepared OMC by using hexagonally ordered silica, SBA-15, as hard template that was synthesized under microwave irradiation.10 Both types of the aforementioned carbons were fabricated by employing the filmtype replication method involving the formation of thin and uniform polymer film on the pore walls of the silica templates used, followed by polymer carbonization and template dissolution. Experimental Section Two siliceous templates composed of silica colloids of 50 and 80 nm, respectively, were obtained by simple drying of commercial monodisperse spherical colloids at 70 °C. Aqueous colloidal dispersions containing 50 wt % of silica, Nanosol 5050S and Nanosol 5080S, were donated by Precision Colloids,

10.1021/jp072067f CCC: $37.00 © 2007 American Chemical Society Published on Web 06/12/2007

Large Pore Volume Carbons with Uniform Pores SCHEME 1: Illustration of the Synthesis of Carbons with Extremely Large Volume of Uniform Pores by Using Colloidal Silica Templatesa

a t denotes the thickness of carbon film in the silica-carbon composite, which is equivalent to the pore wall thickness in the resulting carbon.

LLC (U.S.A.). After complete water evaporation at 70 °C, the drying process was repeated at 120 °C for 3 h. The synthesis of large pore SBA-15 silica involved two steps: gel formation and subsequent hydrothermal treatment.10 The initial gel formation was done under acidic conditions using poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer (EO20PO70EO20, Pluronic P-123, BASF) as a template and tetraethyl orthosilicate (TEOS, Aldrich) as a silica source at 40 °C for 2 h. The molar composition was as follows: 1.0 TEOS/0.0167 P123/190 H2O/5.82 HCl. The hydrothermal treatment was carried out under microwave conditions at 160 °C for 3 h as reported elsewhere.10 The sample was filtered, washed with water, dried in a furnace for 24 h at 90 °C, and finally calcined in air at 540 °C for 5 h with the heating rate of 5 °C/min. Scheme 1 illustrates the synthesis of LPVCs by using colloidal silica template. This procedure involves the formation of polymer film on the accessible surface of silica colloids, followed by carbonization of this film and silica dissolution. The first step in the synthesis of colloidal silica-templated carbons was carried out by impregnation of 1 g of colloidal silica template with 40 mg of 98% anhydrous oxalic acid (catalyst; purchased from Acros Organics and used as received). In the next step, the oxalic acid-impregnated template was infiltrated by using an incipient wetness method with 120 mg of 98% resorcinol (purchased from Acros Organics and used as received) and the specified amount of 99% crotonaldehyde (2-butenal; purchased from Acros Organics and used as received) to fill the available pore volume of the template. After impregnation, the sample was held at 60 °C for 0.5 h in an open container. An initial prepolymerization was carried out at 120 °C for 6 h (the sample was of yellow-orange color) and continued at 200 °C for 2 h. The resulting polymer-silica composite (having soft brown color) was placed in a tube furnace for carbonization under nitrogen at 900 °C for 2 h with the heating rate of 2 °C/min. More than 99 wt % of the silica was removed from the silica-carbon composite by using ∼15% HF acid, which was confirmed by thermogravimetric analysis.

J. Phys. Chem. C, Vol. 111, No. 27, 2007 9743 The resulting carbon was rinsed with butanol and hexane (Acros Organics) and dried in a vacuum oven at 120 °C for 5 h. In the case of SBA-15-templated carbons, the synthesis procedure was the same as that described above except the amounts of oxalic acid and resorcinol, 400 mg and 450 mg, respectively. The silica templates used, carbon-silica composites, and the resulting carbons were denoted as X-50, X-80, and X-SBA-15, where X refers to the silica (S), composite (SC), or carbon (C), respectively, and 50 and 80 refer to the size of the silica colloids used. Nitrogen adsorption isotherms were measured using a Micrometrics ASAP 2010 volumetric adsorption analyzer (Norcross, GA). Before adsorption measurements, all the samples were outgassed for 2 h at 200 °C. Nitrogen adsorption isotherms measured on the silica templates, and the corresponding carbon replicas were used to evaluate the BET specific surface area, single-point pore volume, and PSD. The BET surface area was calculated in the range of relative pressures from 0.04 to 0.25. The single-point pore volume was evaluated from the amount adsorbed at relative pressure of about 0.99. The PSDs were calculated from adsorption branches of the isotherms by using the Kruk-Jaroniec-Sayari (KJS) method,11 which represents a significant improvement of the well-known Barrett-JoynerHalenda (BJH) method. Note that this method was elaborated for cylindrical pores, while pore geometries of the colloidal silica templates and the corresponding carbon replicas are different. Thermogravimetric analysis (TGA) was carried out in air using a high-resolution TGA 2950 thermogravimetric analyzer (TA Instruments, Inc., New Castle, DE) with the heating rate of 2 °C min-1. Theoretical Relations In this section we present a set of theoretical relations between adsorption parameters of the silica template, silica-carbon composite, and the resulting carbon replica (derivations of these relations are in the Supporting Information). Although this work is focused on the carbon replicas of the colloidal silica templates, several of the proposed relations are applicable for any type of template (made from silica or other material) and the corresponding inverse replica (made from carbon or other material), if the correct material’s density is applied. The aforementioned relations are derived by assuming the structural invariance of the silica template during carbonization and silica dissolution processes. Since silica templates are usually calcined at lower temperatures than the temperatures used in the carbonization process, they may shrink that is usually reflected by a decrease in the specific surface area, which in turn can lead to a less accurate prediction of the carbon replica properties. While this shrinkage can be quite significant in the case of OMSs used as hard templates, its extent is much smaller when colloidal silica templates are used for the synthesis of mesoporous carbons. Also, the structure shrinkage caused by thermal treatment (carbonization) of the silica templates filled with carbon precursors can be smaller than that of the silica templates alone. The packing degree of silica colloids in the colloidal silica template, x, defined as the ratio of the solid matrix volume to the total volume of the solid with pores, can be determined by eq 1 using the volume of primary pores assessed by gas adsorption and the true density of silica:

x)

1 Vsds + 1

(1)

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where Vs and ds are the pore volume and true density of the silica template, respectively. Equation 1 can be applied to any porous material by using the true density and specific pore volume for a given material instead of those for silica. Note that x ) 1 - , where  is the material’s porosity. The size of spherical silica colloids (wcoll) can be estimated by eq 2 using the specific surface area (Ss; obtained by gas adsorption using, for instance, the BET method) and the true density of silica:

Ss )

6 wcollds

(2)

Derivation of eq 2 was done under assumptions that silica colloids are spherical, uniform, and interconnected via singlepoints only. Equation 2 provides only an approximate estimation of the colloid size due to the following reasons: (i) difficulties in assessing an accurate value of the specific surface area, (ii) nonuniformity of colloids, and (iii) coalescence of colloids that causes a decrease in the accessible surface area. The maximum average carbon film thickness, tc(max), formed on the template surface by complete filling of pores (voids) between silica colloids can be estimated by dividing the pore volume of the colloidal silica template (Vs) by its specific surface area (Ss):

tc(max) )

Vs Ss

(3)

Note that the carbon film thickness estimated by eq 3 is equivalent to the maximum average pore wall thickness in the carbon obtained by complete filling of pores in the colloidal silica template. The minimum total pore volume of the carbon prepared by complete filling of pores in the template, Vc(min), which corresponds to tc(max), is given by eq 4:

Vc(min) )

1 Vsdsdc

Vs + Vc )

(

wcollds Vs + Vc )

(5)

Derivation of eq 5 can be found in our previous publication.8 The function Vc(tc) expressed by eq 5 reveals the relation between the experimental volume of primary pores in the carbon replica, Vc, (the total pore volume reduced by the volume of micropores and secondary pores) and the pore wall thickness. Note that eq 5 was derived by assuming the lack of micropores in the silica template, which increases its specific surface area and, consequently, provides much less accurate prediction of Vc and related parameters. Also, the excessive amount of carbon on the external surface of the silica template diminishes the prediction of carbon replica properties on the basis of adsorption parameters of the template.

)

1 ds

6tcdc

-

1 dc

(6)

Another formula for the estimation of the carbon film thickness can be obtained on the basis of eq 7, which expresses the pore volume of the silica-carbon composite, Vsc, in terms of Ss, Vs, and ds (the specific surface area, pore volume, and true density of the silica template, respectively), true carbon density, dc, and the carbon film thickness.

Vsc )

Vs - Sstc Sstcdc + 1

(7)

In contrast to eq 5, which predicts the carbon film thickness on the basis of adsorption parameters for the silica template and carbon replica, eq 7 uses the pore volume of the silica-carbon composite to predict the aforementioned quantity. Since the pore volume of the composite can be small and less accurately determined, eq 7 seems to be less attractive for the tc estimation. The specific surface area of the film-type carbon replica, Sc, corresponding to the carbon film thickness, tc, is given by

Sc )

2 t c dc

(8)

In the case of the complete filling of the template the specific surface area is twice smaller, and the replacement of tc by eq 3 gives

Sc )

(4)

where ds and dc are the true (skeleton) densities of silica (2.2 g/cm3) and carbon. Equation 4 was derived for the carbon replica having the silica-templated (primary) mesopores only, i.e., no additional micropores or secondary pores have been created during carbonization process. When the specific surface area and pore volume of the silica template is known, the carbon wall thickness, tc, can be estimated by rearranging eq 5 with respect to tc:

1 - Sstc ds Sstcdc

Substitution of the specific surface area of the silica template, Ss, in eq 5 by eq 2 gives eq 6, which allows one to relate the volume of primary pores in the carbon replica, Vc, to the size of silica colloids used, wcoll:

Ss Vsdc

(9)

Since all the above equations are derived under assumption of the lack of additional microporosity in the carbon film, its thickness can be somewhat underestimated, and the extent of this underestimation depends on the fraction of microporosity. To obtain more accurate results it is necessary to use the density defined as the mass of carbon divided by the total volume of carbon skeleton and micropores in it. In the case of a microporous carbon film, the aforementioned density can be much smaller than the true carbon density, which leads to much higher carbon film thickness. For phenolic resin-based carbons, which were carbonized at 900 °C, the carbon density is ∼1.5 cm3/g.12 It is noteworthy that all equations derived for the filmtype carbon replicas are applicable for the samples without excess of carbon deposited on the external surface of the template. Results and Discussion Scheme 1 illustrates the use of colloidal silica template for the synthesis of mesoporous carbons. This synthesis involves the deposition of catalyst (oxalic acid) on the surface of the silica template, infiltration of resorcinol and crotonaldehyde into oxalic acid-modified pores of the template, thermal treatment of the filled template at 60 °C to initiate the formation of polymeric film, polymerization of resorcinol and crotonaldehyde at 200 °C associated with simultaneous decomposition of oxalic acid catalyst, and carbonization of the silica-polymer composite

Large Pore Volume Carbons with Uniform Pores

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Figure 1. Nitrogen adsorption isotherms at -196 °C and the corresponding PSDs for 50 and 80 nm colloidal silica templates.

TABLE 1: Total Pore Volume (Vt), BET Specific Surface Area (SBET), Pore Size (w), and the Ratio of the Solid Volume to the Total Volume of Solid with Pores (x) for 50 and 80 nm Colloidal Silicas and SBA-15 Silica sample

Vta [cm3/g]

SBETb [m2/g]

wc [nm]

xd

S-50 S-80 S-SBA-15

0.20 0.20 1.24

59 46 520

19.2 24.4 11.0

0.694 0.694 0.268

Figure 2. Nitrogen adsorption isotherm at -196 °C and the corresponding PSD for SBA-15 silica template.

a Calculated at the relative pressure of 0.99. b Calculated in the relative pressure range of 0.04-0.25. c Calculated at the maximum of PSD obtained by the KJS method for cylindrical pores (ref 11); the pore size for the SBA-15 silica was estimated using the improved KJS method (ref 13). d Note that x ) (1 - porosity).

followed by the silica dissolution. The film-type replica of the colloidal silica template features two kinds of pores: spherical mesopores of diameter of the colloids used and those that are formed between the spheres in the result of an incomplete filling of the template by carbon film. In the case of SBA-15 OMS template, the carbon replica, CMK-5,6 represents the structure of interconnected hexagonally ordered nanopipes. CMK-5 carbon has two types mesopores: mesopores left after incomplete filling of SBA-15 channels by carbonized polymeric film (interior of nanopipes) and mesopores formed between hexagonally ordered nanopipes. Figure 1 shows adsorption isotherms for the templates composed of 50 and 80 nm silica colloids. The single-point pore volume of these templates was ∼0.2 cm3/g (see Table 1), which gives the packing degree of monodisperse silica spheres (estimated according to eq 1) equal to 0.694; note that the maximum possible packing degree for a perfect colloidal crystal is ∼0.74. Their BET specific surface area decreased expectedly with increasing colloid size (Table 1). This surface area can be also estimated by means of eq 2 using the size of silica colloids. This equation shows that the decrease in the specific surface area follows the inverse proportionality of this quantity to the size of silica colloids. For the same packing degree of highly packed colloidal silica templates, the size of pores between spherical colloids increases with increasing the colloid diameter. For the samples studied the largest pores (of about 24 nm) were obtained for the template composed of 80 nm colloids (see Table 1 and PSD curves in the inset of Figure 1). The nitrogen adsorption isotherm for the SBA-15 silica and the corresponding PSD are shown in Figure 2. Adsorption

Figure 3. Nitrogen adsorption isotherms at -196 °C and the corresponding PSDs for the carbon replicas of 50 and 80 nm colloidal silicas.

analysis for this sample synthesized under microwave conditions revealed a relatively high total pore volume, low volume of micropores (∼0.04 cm3/g), low specific surface area, and large width of uniform cylindrical pores (Table 1). The PSD curve in Figure 2 does not reveal porosity in the pore range between ∼2.5 and 6 nm, indicating that the size of interconnecting pores in the SBA-15 sample studied is larger than 6 nm. The presence of larger pores connecting hexagonally ordered mesopores in SBA-15 is advantageous for the formation of carbon replicas, because those pores (after introducing carbon precursors) provide stronger connections for the resulting hexagonally ordered carbon nanopipes. Nitrogen adsorption isotherms and the corresponding PSDs for the silica-carbon composites obtained by carbonization (at 900 °C) of resorcinol-crotonaldehyde film formed on the surface of 50 and 80 nm silica colloids as well as on SBA-15 are shown in the Supporting Information Figures 1S and 2S. Figure 3 shows nitrogen adsorption isotherms for the carbons synthesized using hard templates composed of 50 and 80 nm silica colloids. They exhibit very high BET specific surface area (Table 2), which is particularly pronounced (1620 m2/g) in the case of the carbon obtained by using 50 nm colloids. A most significant feature of those carbons is an extremely large pore

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TABLE 2: Total Pore Volume (Vt), Porosity, BET Specific Surface Area (SBET), Theoretical Specific Surface Area (Sc, eq 8), Pore Width (w), and Theoretical Carbon Wall Thickness (tc, eq 5) for the Carbon Replicas Obtained by Using 50 and 80 nm Colloidal Silicas and SBA-15 as Hard Templates Vt [cm3/g]a

porosity [%]

C-50

8.6

94.8

C-80

9.3

95.1

C-SBA-15

2.8

85.5

sample

SBET (80% SBET) [m2/g]b 1620 (1296) 1420 (1136) 1960 (1568)

Sc [m2/g]c

(Sc - 0.8SBET)/ (0.8SBET) × 100%

1333

2.9

1130

-0.5

1709

9.0

w [nm]d

tc [nm]

11.0; ∼70

1.00

20.0; ∼105

1.18

4.7; 7.5

0.78

a Calculated at the relative pressure of 0.99. b Calculated in the relative pressure range of 0.04-0.25 (the values in brackets refer to SBET reduced by 20%). c Calculated by eq 8 using the carbon density of 1.5 cm3/g. d Pore widths estimated at the maximum of both peaks of bimodal PSD (ref 11); the first peak reflects the unfilled space of pores in the silica template after formation of the carbon film, whereas the second peak reflects the spherical pores formed after dissolution of silica colloids.

volume. To the best of our knowledge, this is the first publication after our previous report8 on the exceptionally LPVCs possessing uniform spherical pores, the size of which can be easily tailored by selecting silica colloids of desired diameter. Both carbon samples have the highest ever reported values of the total pore volume of about 9 cm3/g. The maximum pore volume can be even further increased by using a larger size of colloids in the template and keeping the same carbon film thickness. To elucidate the dependence between the pore volume and the size of silica colloids one needs to reconsider the meaning of the specific pore volume, which represents the volume of liquid adsorbate needed to fill pores per gram of the porous material. Thus, this volume can be enlarged by reducing the thickness of the carbon film on the template surface and by using larger silica colloids to create the large spherical pores in the carbon replica. The reduction of the carbon wall thickness causes that the porosity increases while the carbon mass decreases. However, the thickness of the carbon wall has limits. The upper limit is related to the complete filling of pores in the template. For example, when an ordered template is used such as SBA-15 silica, the upper limit of this thickness is the radius of the primary cylindrical mesopores. The general formula for the average maximum carbon film thickness, tc(max), is given by eq 3. On the other hand, the lower limit refers to the thinnest possible carbon film being able to support a porous structure of the inverse replica obtained for a given silica template. For porous carbons synthesized by using hard templates and having pore walls of the thickness close to the lower limit, the only way to increase their pore volume is by making their pores larger. In the case of the carbons prepared by colloidal silica templating, their large pore volume can be achieved by employing silica colloids of larger diameter. This issue can be better clarified by analyzing eq 5, which expresses the total pore volume, Vc, as a function of the carbon wall thickness, tc (see Figure 4). Since the function, Vc(tc), is inversely proportional to the specific surface area of the silica template, Ss, to obtain LPVC, one needs to employ the template having low surface area (eq 2 indicates that the low surface area is obtained for larger colloids). Note that the Vc(tc) curves in Figure 4 for C-50 and C-80 carbons show different values of the maximum film thickness, which correspond to the complete filling of the same pore volume in the silica templates. Moreover, for a very thin film thickness such as 0.5-0.7 nm (which is probably close to the smallest possible value for a self-supportive carbon structure), the pore volume in a carbon replica can be enlarged by using larger silica colloids (see eq 6). A particularly useful form of eq 5 can be obtained by its rearrangement to estimate the carbon film thickness on the basis

Figure 4. Theoretical dependence of the total pore volume on the carbon film thickness for the carbon replicas of SBA-15 and the silica templates composed of 50 and 80 nm colloids. The pore volume curves were calculated according to eq 5 using the BET specific surface area for the silica template reduced by 20% and the carbon density equal to 1.5 g/cm3.

of the carbon pore volume and the silica template parameters. The values of the film thickness for the carbons studied are listed in Table 2. Additional experimental examples of the filmtype carbon replicas are shown in the Supporting Information (see Figure 3S and Tables 1S and 2S). These values along with the relation between Vc and tc shown in Figure 4 allow one to understand the increase in the carbon pore volume with increasing diameter of colloids. An alternative way for the estimation of the carbon film thickness is provided by eq 7, which was derived for the silicacarbon composite. Nitrogen adsorption isotherms and PSDs for all the composites are shown in the Supporting Information. The inset in Figure 3 shows bimodal PSDs for the colloidtemplated carbons. Neglecting the relatively small peaks positioned at ∼2 nm that reflect intrinsic microporosity, PSDs reveal two kinds of primary mesopores (see Table 2). The small mesopores are related to the incomplete filling of the voids between colloids during film formation, whereas the large ones were formed after dissolution of silica colloids. PSDs for the colloid-templated carbons studied show some spikes around the main peak, the position of which does not match the size of colloid used. This behavior is caused by (i) the inadequateness of the PSD method, which was derived for cylindrical mesopores, (ii) the presence of large pore connections, and (iii) a smaller accuracy of adsorption measurements at high pressures close to the saturation pressure. The carbon replica of SBA-15 is an example of well-defined OMC structure synthesized by resorcinol-crotonaldehyde po-

Large Pore Volume Carbons with Uniform Pores

Figure 5. Nitrogen adsorption isotherms at -196 °C and the corresponding PSD for the carbon replica of SBA-15.

lymerization on the walls of SBA-15 channels. Presented in Figure 5, the nitrogen adsorption isotherm for this sample exhibits two capillary condensation steps (for the relative pressure of ∼0.48 and ∼0.68) related to two different kinds of mesopores. This property is better visible on the PSD plot in the inset of Figure 5 that shows the well-separated peaks corresponding to the pore sizes of 4.7 and 7.5 nm. Interesting properties of this carbon replica are high specific surface area (1960 m2/g) and total pore volume (2.8 cm3/g) that appear to be particularly pronounced for such structure of the ordered carbon. The carbon wall thickness calculated by applying the rearranged eq 5 had a low value of 0.78 nm. The Vc(tc) curve for the SBA-15 silica studied shows that its fully filled carbon replica, compared to the other materials presented in Figure 3, can reach much lower pore volume (eq 4), which is a consequence of relatively high pore volume and surface area of the template. Since the Vc(tc) function for the SBA-15templated carbon shows a very steep increase in the pore volume corresponding to extremely thin pore walls (∼0.5 nm), it is difficult to produce LPVC replica (over 3 cm3/g) for this template. A comparison of the calculated carbon wall thickness and the size of mesopores in SBA-15 and in its carbon replica indicates a substantial shrinkage of the carbon structure during synthesis process. Figure 6 shows the plot of the function Sc(tc) (eq 8) as well as the points reflecting the BET specific surface area reduced by 20% (the BET method overestimates the surface area by about 20%)14 in relation to the corresponding carbon wall thickness (eq 5 and Table 2). The values of the surface area for the C-50, C-80, and C-SBA-15 samples fit very well the theoretical curve, which was obtained on the basis of eq 8 applicable for all types of carbon inverse replicas of any silica template, for which the density and thickness of the film are known. Note that the rearranged form of eq 8, tc ) 2/(Scdc), represents an alternative to eq 5 for the tc estimation. The latter quantity can be determined by using the specific surface area of the carbon replica only and gives very good results for the replicas having no micropores at all or when their amount is negligible. In conclusion, the formation of thin carbon film on the surface of colloidal silica template by copolymerization and subsequent carbonization of resorcinol and crotonaldehyde carbon precursors afforded LPVCs with uniform spherical meso- or macropores. The proposed formulas indicate that a suitable way to increase the total pore volume of porous materials created by inverse replication is the use of a template featuring low surface area, small framework density, and high pore volume. Additionally,

J. Phys. Chem. C, Vol. 111, No. 27, 2007 9747

Figure 6. Theoretical dependence of the specific surface area on the carbon wall thickness (determined by using carbon density, dc, in eq 8 equal to 1.5 g/cm3) for all types of carbon inverse replicas of any template (the solid curve and the dotted curve). The dotted curve is plotted in the range of the carbon wall thickness below 0.68 nm (the latter value reflects the double graphene sheet thickness, below which it is extremely difficult to obtain a stable OMC sample). The points correspond to the BET specific surface area reduced by 20% and the calculated carbon wall thickness (eq 5) for the carbon replicas of 50 and 80 nm colloidal silica and SBA-15 templates. The value of the surface area for the C-22 carbon is listed in the Supporting Information Table 2S.

it is important to keep the carbon pore wall as thin as possible. This wall thickness should be reduced as much as possible, i.e., to the value, which does not cause the structure collapse after silica template removal. Therefore, to obtain LPVCs it is necessary to use silica colloids of larger size and/or to reduce the carbon film thickness. For comparative purposes, also large pore volume film-type OMC was synthesized using resorcinolcrotonaldehyde and SBA-15 as silica template. The formulas shown in this work allow one to predict various properties of the inverse carbon replicas on the basis of the parameters for the porous template and the corresponding silica-carbon composite. In particular, the average carbon film thickness can be estimated on the basis of adsorption parameters such as specific surface area and total pore volume. Acknowledgment. This research was supported in part through subcontract under the NIRT DMR-0304508 Grant from NSF awarded to the Carnegie Mellon University. Precision Colloids LCC is acknowledged for donation of the research samples of Nanosol. Supporting Information Available: Derivations of eqs 1-9, two figures with nitrogen adsorption isotherms and the corresponding pore size distributions for the silica-carbon composites, as well as a supplement showing experimental verification of the aforementioned equations, which includes information about silica templates and additional carbon replica samples, two tables with adsorption parameters, and one figure with nitrogen adsorption isotherms for the carbon replica samples. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Knox, J. H.; Kaur, B.; Millward, G. R. J. Chromatogr. 1986, 352, 3. (2) (a) Ryoo, R.; Joo, S. H.; Jun, S. J. Phys. Chem. B 1999, 103, 7743. (b) Jun, S.; Joo, S. H.; Ryoo, R.; Kruk, M.; Jaroniec, M.; Liu, Z.; Ohsuna, T.; Terasaki, O. J. Am. Chem. Soc. 2000, 122, 10712. (3) (a) Ryoo, R.; Joo, S. H.; Kruk, M.; Jaroniec, M. AdV. Mater. 2001, 13, 667. (b) Lu, A.-H.; Schmidt, W.; Spliethoff, B.; Schu¨th, F. AdV. Mater.

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