One-step sol–gel synthesis of hierarchically porous, flow-through carbon/silica monoliths

Abstract

Hierarchically porous, flow-through carbon/silica bicontinuous composite monoliths with ultra-high Brunauer–Emmett–Teller (BET) surface areas and tunable porosity in micro/meso/macro-structured domains, were obtained from an efficient one-step sol–gel chemistry based on the co-assembly of organic and inorganic precursors with simultaneous polymerization-induced phase separation. Without activation, the bicontinuous composites were subjected to pyrolysis and silica removal to yield crack-free hierarchically porous carbon monoliths that have large pore volumes and high BET surface areas (∼2600 m² g⁻¹). The removal of carbon from the silica/carbon composite monolith produces a microporous silica framework (BET area ∼600 m² g⁻¹). The hierarchically porous carbon monoliths were characterized in terms of their pore morphology, flow-through porosity, phase composition, mechanical strength, structural and elemental compositions, and surface wettability. The polymer monolith was determined to be hydrophobic, whereas the carbon monolith was hydrophilic in nature. The water permeability of the carbon monolith was determined to be 12 × 10⁻¹² m², and its Young's modulus was 0.42 MPa, which suggests that this monolith could be used as a potential flow-through medium. The use of the carbon monolith as a catalytic support is demonstrated by the in situ growth of silver nanoparticles, with which the hybrid exhibits excellent catalytic activity for the reduction of 4-nitrophenol (4-NP) with NaBH₄ in an aqueous medium. The hierarchically porous carbon monoliths have a plethora of potential applications owing to their mechanical stability and transport properties throughout the monolith. The method of synthesis outlined here can be easily extended to the synthesis of monolithic oxides, such as SnO₂, TiO₂, ZnO, ITO etc.

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1. Introduction

There is growing demand for nanoporous solids¹ with high surface areas,² such as carbon, porous hybrid organic–inorganic materials, and polymeric frameworks for extensive applications including electrochemical electrodes,¹ filters,² chemical adsorbents,³ gas storage,⁴ separation,⁵ membranes,⁶ structural components,⁷ structural composite matrices,⁷ medical devices,⁸ energy storage,⁹ catalysts,¹⁰ catalytic supports,¹⁰ and capacitive deionization.^11,12 Carbon with an ultra-high surface area is traditionally produced by physical and chemical activation, supercritical drying,^13–15 or self-assembled colloidal crystal templating.^16,17

In general, porous carbons are derived using a wide range of organic precursors¹¹ and a variety of synthesis techniques for simple processing and targeted functionalization for specific applications.¹⁰ However, the development of porous carbon through an economical and effective method still remains to be achieved, and this is presently an area of active research. The potential synthesis method would need to achieve large pore volumes, high surface areas, and tunable porosities of micro–meso–macro size.

Among the various approaches for the synthesis of porous carbon, templating in various forms has been popular and widely employed.^18–20 Such approaches are either based on surfactant-assisted soft templating, or include a form of hard templating which involves the casting of a precursor material over a pre-synthesized porous template that is subsequently removed.^10,13 These approaches have generally simplified the fabrication of porous carbon, but they also have their own limitations, such as the lack of control over both the pore size distribution and the tailoring of porosity. This is because hard templating requires sacrificial templates, and the resulting pore structure is limited by the pore structure of the template. In the case of soft templates, the nature of the porosity is determined by the size distribution of surfactant micelles, which have high monodispersity and consequently cannot be used to generate hierarchical structures.

Hence, we propose a relatively simple and economical template-free approach for the synthesis of porous polymer and carbon monoliths from sol–gel based precursors, with full control over the tunability of the porosity. The approach is template-free because no pre-synthesized structure is required as a template to generate the porosity. The present approach is novel because it employs the sol–gel technique. The resulting monoliths have excellent structural stability with a tailored pore structure. As the polymerization of the organic and silica precursors progresses, the system becomes unstable and undergoes phase separation due to spinodal decomposition. However, there are few reports on the synthesis of hierarchically porous carbon monoliths.^21,22 Currently, numerous methods have been developed for the synthesis of hierarchically porous polymers and carbons; however, some of these approaches consist of many complicated steps that are only feasible on a small scale.^23–27 Recently, Pauzauskie et al.²⁸ have reported the synthesis of resorcinol–formaldehyde (RF) gel derived carbon aerogel and converted it to a polycrystalline diamond aerogel where they have used sodium carbonate as a catalyst. In the same context, Schwan and her coworker²⁹ have demonstrated the preparation of flexible RF polymeric aerogels with a base catalyzed sol–gel route followed by their drying in the ambient atmosphere. Kong et al.³⁰ have fabricated RF/silica composite aerogels and transformed those to carbon/silicon carbide and carbon/silica monolithic composites where they have used supercritical drying under carbon dioxide. Interestingly, the porous carbon aerogels have also been prepared by Sharma's group³¹ through an organic–inorganic co-assembly process and demonstrated for water desalination application through capacitive deionization.

In this work, we propose a facile, scalable, highly efficient, and economical approach for the synthesis of hierarchically porous carbon and silica monoliths with an ultra-high Brunauer–Emmett–Teller (BET) surface area by the co- or self-assembly of organic and inorganic precursors during the sol–gel stage, resulting in a co-continuous network of solvent and gel phases with an interpenetrating polymer and silica framework. Owing to the mechanical stability of the system due to the hierarchical structuring of the framework, the solvent phase can be eliminated via ambient drying without disturbing the pore structure. The water permeability, wetting behaviors and mechanical strength of the carbon monolith have been evaluated experimentally. We have demonstrated a simple technique to impregnate metal nanoparticles into a carbon monolith for catalytic conversion applications with regard to the NaBH₄ mediated reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). We have demonstrated that the porous carbon monolith can function as a support material for the silver nanoparticles, without altering their catalytic activity.

2. Experimental section

2.1. Materials

Resorcinol (R), formaldehyde solution (F), tetraethyl orthosilicate (TEOS), APTES (3-aminopropyl tri-ethoxysilane), APTMS (3-aminopropyl tri-methoxysilane), acetone, sodium hydroxide (NaOH), para-nitrophenol (4-NP), silver nitrate (AgNO₃) and sodium borohydride (NaBH₄) were obtained from Sigma-Aldrich Co., USA. The catalytic reactions were performed in HPLC grade water medium supplied by Merck Pvt. Ltd., India. All reagents were of analytical grade and used as supplied without further purification.

2.2 Sample preparation

2.2.1 Preparation of porous carbon monolith. The porous carbon monolith was prepared in the following steps as described below:
2.2.1.1 Synthesis protocol of RF/silica and carbon/silica composite monolith. RF/silica composite monoliths were synthesized by a novel one-step sol–gel process, which has been summarized in a flow diagram as shown in Fig. 1. In short, resorcinol, TEOS, APTES and APTMS were added in acetone followed by the addition of formaldehyde solution to start the sol–gel reaction. The sol was stirred for 5–10 min to ensure proper mixing in a polypropylene container. The container was sealed and kept at room temperature for around 12 h. For all the samples, the molar ratios of R : F was kept constant at 1 : 3. The tenability of porosity due to structural changes in the meso and micro scale pores were investigated by varying the proportion of APTES/APTMS. Three types of samples were prepared such as, AE (only APTES), AM (only APTMS) and AE/AM where APTES and APTMS were used in different ratios.

Fig. 1 Schematic illustration for the sol–gel synthesis and processing of monoliths.

The dried monoliths were obtained via a simplified approach in which the wet gels were subjected to ambient drying at 50 °C for 12 h in order to obtain RF/silica composite monoliths with hierarchical porosity. The resulting RF/silica composite monoliths were reddish-brown and lightweight and had typical properties with ultra-high porosities and low densities.

Carbonization of the dried composite monoliths was performed in inert N₂ atmosphere having a constant flow rate of 0.15 L min⁻¹, with heating up to 900 °C at a heating rate of 5 °C min⁻¹ and a holding for 1 h followed by ambient cooling to yield carbon/silica (C/SiO₂) composite which was black in colour. After that, the resulting carbon/SiO₂ were subjected to silica etching through 1 M aqueous NaOH treatment during which the silica phase of the composite gets eliminated by forming sodium silicate and ended with desired hierarchically porous carbon monoliths.

It is very important to note that the traditional routes of carbon aerogel preparation are tedious, time-consuming, complex and have employed more cost as compared to our developed method. For example, Lee et al.³² have been prepared carbon aerogel by using polycondensation of resorcinol polymer with formaldehyde, and also employed sodium carbonate as a catalyst. Also, Kim et al.³³ have demonstrated Na₂CO₃ catalyst mediated carbon aerogel preparation by using resorcinol–formaldehyde gel where they have dried the gel for a week to achieve the targeted monolithic structure.

2.2.1.2 Synthesis of porous carbon monoliths with impregnated Ag nanoparticles. Ag/carbon hybrid porous carbon monoliths with 0.1 equivalent silver loading was synthesized by following the same protocol as mentioned above; however; AgNO₃ salt was added to RF/silica sol–gel mixture during sample preparation.
2.2.1.3 Sample preparation of para-nitrophenol (4-NP) and sodium borohydride (NaBH₄). An amount of 70 μL of 4-NP standard solution (1 × 10⁻² M) was combined with 7.0 mL of 0.1 M NaBH₄ solution. At this stage, the appearance of a yellowish solution is indicative for of the nitrophenol molecules to nitrophenolate anion conversion. Now, the catalyst Ag/carbon hybrid monolith was immersed in the solution in a 25 mL culture tube. The initial yellow color of the solution started vanishing gradually which is an indication of the advancement of reaction which has been spectrophotometrically monitored at of 400 nm wavelength by using a UV-Vis spectrophotometer (Varian Cary 50 Bio). Three different weight of the catalyst was taken to investigate the effect of loading of silver in the monolith, which was varied from 25 mg to 50 mg and 100 mg over the catalytic reaction. During the reaction process 2.0 mL of the solution was taken in a 3 mL quartz cuvette to monitor the absorbance at several time intervals and then that was again added to the initial 25 mL solution. The total catalytic reactions were carried out in a controlled atmosphere (room temperature 24 °C and humidity level of 38%).

2.3 Characterization

The Field Emission Scanning Electron Microscopy (FE-SEM, Quanta 200, Zeiss, Germany) was used to characterize the surface morphology of porous carbon monolith and its Ag hybrid counterpart. The powder X-ray diffraction (XRD) measurements were conducted on a X'Pert Pro, PAN analytical, Netherlands, X-ray system with Cu Kα radiation to collect the structural information of the monoliths. Transmission electron microscopy (TEM) was performed on a Tecnai G², USA microscope for further structural insights into the synthesized material. For this purpose, small quantities of monoliths were taken and dispersed on a carbon coated copper grids and imaged under a TEM. Raman spectral analysis was done by a WiTec, Germany instrument using laser light of 532 nm wavelength (parameters: laser wavelength (λ) = 532 nm, power = 12 mW, spot size = 500 nm, objective lens = 20× and signal integration time = 5 s) to characterize the amorphous nature and graphitic content of the carbon material. The surface area (SBET) and the pore volumes of the composite carbon monoliths and the porous silica network were determined by BET (Quantachrome Instruments, USA) having Autosorb 1 software. The total surface area, the pore volume of the pyrolyzed hybrid samples were calculated based on the amount of nitrogen that was adsorbed and desorbed at −196.15 °C (77 K) using the Brunauer, Emmett and Teller (BET) instrument (Model: AS1-C, Quantachrome, USA). The samples were degassed at 200 °C under vacuum prior to the analysis. The total pore volume of the samples was calculated on the basis of the amount of N₂ adsorbed at a relative pressure near unity (P/P₀ = 0.999). Wetting behavior of polymer and carbon monolith surfaces were characterized by determining equilibrium water contact angle by contact angle goniometer (Rame-Hart Instruments Co., NJ, USA). For the contact angle measurements, we used 5 μL (∼3 mm spherical drop) water drops. The surface functional groups on the carbon were characterized by Fourier transform infrared attenuated total reflection spectroscopy (Bruker Optik, GmbH, Germany).

3. Results and discussion

3.1 Surface morphology

The as-synthesized porous carbon monoliths were observed by field emission scanning electron microscopy (FE-SEM). The FE-SEM images in Fig. 2 revealed the surface pore morphology of the hierarchically porous carbon structures. The carbon matrix exhibits pores distributed over three types of size. The carbon monolith is comprised of mesopores with an average diameter of <50 nm, and micropores with an average diameter of <2 nm. The overall bulk consists of macro-pores with an average diameter of ≥50 nm. No cracking was observed in the monolith.

Fig. 2 Surface morphology of the porous carbon monolith: FE-SEM micrographs of (a) AE derived porous carbon monolith, (b) higher magnification image of ‘a’ showing micropores, (c) AE/AM (0.5 : 0.5) derived porous carbon monolith and (d) higher magnification image of ‘c’ showing meso and micropores, (e) AM derived carbon monoliths and (f) higher magnification image of ‘e’ showing micro and macropores.

3.2 Pore morphology

The pore morphologies observed in the three types of monoliths are discussed below:

AE: it has a pore structure dominated by micro and mesoporosity (71% and 27%), respectively. Its macroporosity (2.5%) was eliminated because of the fast gelation. The micro- and meso-structures are preserved, and hinder the formation of macroporosity. The FE-SEM images in Fig. 2a and b show the micrographs of the AE carbon monoliths.

AE/AM: Fig. 2c and d show the FE-SEM micrographs of the AE/AM carbon monoliths. It can be observed that micro-, meso-, and macropores (68%, 27%, and 6%, respectively) co-exist in a tree-like hierarchical structure. The late gelation allows time for phase separation to occur, which results in the development of the macropores.

AM: the structure is dominated by micro- and mesopores (84% and 12.5%, respectively). The macropores (3.42%) are eliminated. Gelation occurs very slowly; therefore, major fractions of the mesopores are converted into micropores because of the phase separation. The micro- and mesopore structures hinder the formation of the macropores. The FE-SEM images in Fig. 2e and f reveal the pore morphology of the AM carbon monoliths.

Here, resorcinol–formaldehyde/silica composite monoliths were synthesized by a one-step sol–gel mediated process. In summary, resorcinol, tetraethyl orthosilicate (TEOS), 3-aminopropyl tri-ethoxysilane (APTES), and 3-aminopropyl tri-methoxysilane (APTMS) were added to an acetone solvent, followed by the addition of a formaldehyde solution to initiate the sol–gel reaction. First, resorcinol was dissolved in the solvent (acetone), followed by the drop-wise addition of a silica precursor, TEOS, and its catalyst, APTES and/or APTMS. This forms a transparent solution which consists of a miscible mixture of the three reactants. Subsequently, when formaldehyde is added, a reaction is initiated between the resorcinol and formaldehyde, forming a gel. Phase separation is initiated between the resorcinol–formaldehyde gel, acetone, and the silica precursor, TEOS.

According to the theories of microphase separation and gel formation.^34,35 the precursor molecules (resorcinol) polymerize and create a branched polymeric network in which the polymer (resorcinol) and the solvent (acetone) are mixed on a molecular scale. When the degree of branching and/or the molecular weight increases, the solubility of the polymer in the solvent decreases. This causes the polymer chains to progressively fold, forming locally dense structures which expel the solvent. The skeleton of the gel is initially huge with a polymeric structure; however, it contains a large amount of solvent in the form of very small pores of almost molecular size. During the gel structuring process, these pores progressively disappear, and the largest pores outside the skeleton increase in size. Simultaneously, the structure becomes more ordered through microphase separation which generally occurs on a well-defined scale of length that depends on the degree of branching of the macromolecules. Here, the silica precursor, TEOS, along with its catalyst, APTES/APTMS, also exist with the acetone solvent within the polymeric skeleton of the resorcinol. The TEOS, in the form of silica nanoparticles, prevents the collapse of the microspores and further orders the polymer network when the solvent, acetone, initially evaporates from the polymeric skeleton. This explains why the resorcinol–formaldehyde/silica (∼122.2 m² g⁻¹) derived carbon network produces a high BET surface area (∼2600 m² g⁻¹) following the silica etching.

In our case, the phase separation is beneficial for the enhancement of the porosity, and creates an ultra-high BET surface area through the following three important steps: (1) following its addition during this single-step sol–gel process, the formaldehyde immediately reacts with the resorcinol, resulting in gelation between the resorcinol and formaldehyde. Subsequently, phase separation commences between the resorcinol, acetone, and TEOS. (2) The micro/nano phase separation assists the formation of pores in the polymer monoliths. These pores are formed because of the evaporation of the acetone solvent from the resorcinol–formaldehyde polymeric network. (3) The phase-separated APTES/APTMS-catalyzed TEOS-derived silica particles are trapped within the polymer network. This achieves an ordered structure due to the silica backbone, and also produces further porosity after the removal of the silica from the resorcinol–formaldehyde/silica or carbon/silica monolith.

We also studied the morphologies of the monolith prepared by three different approaches, AE, AE/AM, and AM, and can confirm that the carbon monolith exhibits a homogeneous architecture. To verify this, a small quantity of the monolith was removed and mixed with methanol. Following careful drying, it was placed on a freshly cleaned silicon wafer for imaging. The FE-SEM images revealed that the porous structure is hierarchically present throughout the carbon matrix (Fig. 2a–f). The AM monolith (prepared with APTMS catalyst) exhibits a BET surface area of 941.40 m² g⁻¹, which further increases with the APTES/APTMS catalyst ratio. When the APTES/APTMS ratio was 0.50 : 0.5 (i.e., AE/AM monoliths), the BET surface area increased to 1832 m² g⁻¹ (BET adsorption isotherm of the AE/AM carbon monolith is shown in Fig. 3 along with the pore size distribution in the inset). A further increase in the APTES/APTMS ratio to 0.75 : 0.25, produces a BET surface area of 1930 m² g⁻¹, and finally, a BET surface area of 2630 m² g⁻¹ is achieved in the case of the pure AE-derived monoliths. The total micropore and mesopore volumes of the AE monoliths were calculated to be 71% and 27% of the total pore volume of 1.558 cm³ g⁻¹, respectively. The monoliths also possess macropores which constitute 2.5% of the total pore volume, and coexist with the distribution of micro- and mesopores. The porosity distributions of the synthesized composite monoliths are presented by a histogram in Fig. S1.†

Fig. 3 BET adsorption isotherm of the AE/AM (0.5 : 0.5) carbon monolith. Inset showing the pore size distribution was calculated from the adsorption of the nitrogen isotherm by the BJH method.

The pore morphologies of the composite carbons and carbon/silica composites were characterized by nitrogen adsorption–desorption isotherms. Fig. 3 clearly shows that the nitrogen sorption isotherms do not reach saturation (i.e., P/P₀ = 1) at a relative pressure close to unity; however, they reveal a hysteresis, which is typically associated with capillary condensation in the mesopores (pore diameter ∼ 2–50 nm). The BET surface area, pore size distribution, and average pore diameter were calculated from the BET data (shown in Table 1). From the low-pressure section of the curve (BET isotherm in Fig. 3), one can determine that the isotherms relate to type IV behavior for the mesopores, and type II behavior for the macropores of the materials. The distribution of the pores in the composite carbon monolith also indicate the existence of a porous morphology, ranging from the microporous region (pore diameter < 2 nm) and extending to the macroporous (pore diameter > 50 nm) region. As the isotherms do not reach saturation for the nitrogen adsorption, the vapor pressure is underestimated; thus, it can be confirmed that the values calculated by this method are promising and are comparable with previously reported data for other types of carbon aero-gels.^36,37 By observing the microporous surface areas of the silica/carbon monolith, it is evident that its micropores are much more accessible than those of other resorcinol–formaldehyde/silica aerogels.^36,37 The microporous surface area of the silica/carbon composite increases as the initial catalyst/polymer (APTES/resorcinol) ratio decreases, indicating that the micropores primarily develop during the carbonization of the resorcinol–formaldehyde component; this may result in a path for evolving volatile elements. The silica provides structural support to the monolith, and its removal further increases the mesoporosity. The removal of the silica generates mesoporosity and serves the purpose of activation, which is confirmed by the BET data within the histogram plot of Fig. S1.† During thermal activation, part of the carbon from the silica monolith is removed by the use of activating gases; however, this approach results in irregular activation throughout the monolith and requires high temperatures. To generate mesoporosity throughout the monolith, it is easier and more effective for activation to occur through the removal of silica from the carbon–silica nanocomposite.

Table 1 Pore size distribution from BET data

Sample detail	Total pore volume (cm^3 g^−1)	SBET area (m^2 g^−1)	Micropore volume (cm^3 g^−1)	Mesopore volume (cm^3 g^−1)	Macropore volume (cm^3 g^−1)	Average pore diameter (nm)
AE	1.558	2630	1.111	0.418	0.028	2.4
AE/AM(0.75 : 0.25)	1.079	1930	0.8684	0.1827	0.028	2.2
AE/AM(0.5 : 0.5)	1.121	1832	0.765	0.298	0.058	2.5
AM	0.5101	941.4	0.4291	0.0635	0.0174	2.1
AE/SiO2	0.3237	525.30	0.241	0.014	0.069	3.4
AM/SiO2	0.3928	633.60	0.309	0.016	0.0701	2.2
RF	0.258	122.2	0.041	0.1917	0.025	4.2
SiO2	0.4928	679.30	0.2123	0.028	0.253	2.9

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It has been demonstrated that metal nanoparticles can be successfully integrated into these porous carbon monoliths. For example, we have added silver nanoparticles into the AE monoliths (shown in Fig. 4a and b) and explored their catalytic behavior. The FE-SEM micrographs (Fig. 4c and d) clearly show the existence of ∼30 nm diameter silver nanoparticles and beads that are well dispersed over the porous carbon monoliths (histogram in Fig. 4e shows the particle size distribution of the silver). The FE-SEM images show that the silver nanoparticles are embedded underneath the monolith layers and upon the surfaces. Interestingly, the initial impregnating material, silver nitrate, decomposes into NO₂, O₂, and metallic silver, which are subsequently converted into silver nanoparticles via the heat treatment during carbonization.³⁸

Fig. 4 FE-SEM micrographs of (a) AE derived porous carbon monolith, (b) higher magnification image of ‘a’ showing macro and meso porous structures, (c) carbon monoliths with silver loading, (d) higher magnification image of ‘c’ showing silver nanoparticles distributed over the monolith, (e) histogram for the estimation of average silver particle size.

Further, the elemental composition of the silver nanoparticles was determined by selected area EDX analysis (Fig. S2, ESI†) along with the quantitative results of the silver-impregnated carbon monolith, as shown in Table T1 (ESI†). Fig. 5a and b show the transmission electron microscopy (TEM) images of the carbon monoliths following the silica removal. Microporous structures can be observed in these TEM images. There was no crystallinity observed in the porous carbon, which was also confirmed by the diffused selected area electron diffraction (SAED) pattern, as shown in the inset of Fig. 7b. The FE-SEM and TEM images also showed that bead-like monodispersed silver nanoparticles with an average diameter of ∼30 nm were embedded in the structures of the monolith layers, as shown in Fig. 5c and d. The crystalline nature of the face centered cubic (FCC) silver nanoparticles was also confirmed by the concentric rings of the SAED pattern, as shown in the inset of Fig. 5d.

Fig. 5 TEM micrograph of (a) porous carbon monoliths, (b) higher magnification image of ‘a’ showing pore morphology of the monoliths, (c) confirmation of silver/carbon porous monoliths, and (d) higher magnification image of ‘c’ showing the presence of silver nanoparticles in the porous carbon monoliths. Inset of images in figure ‘b’ and ‘d’ show SAED patterns.

The heating process during carbonization at high temperature induced the loss of volatile materials and consolidates of porous solids during the synthesis of the composite monolith. Fig. S3 (ESI†) shows a comparison between polymer composite and carbon monolith obtained after pyrolization. The shrinkage in area of the monolith after carbonization was estimated (shows in Fig. S3(a) and (b)†) and summarized in a histogram plot in S3(c). It was observed around 89–94% weight loss with 25–40% shrinkage in area of the three types of monoliths, whereas, AE monoliths suffer highest weight loss and larger shrinkage compared to AE/AM and AM. The polymer silica composite monoliths Fig. S3 (ESI†) represents the analysis of weight loss and shrinkage in surface area undertaken for the RF/silica composite monoliths. We have also calculated the yield for the synthesis of the monolith from sol–gel process. The estimated conversion of this process has been summarized as 6.63 ± 2.1%, 8.15 ± 1.8% and 10.51 ± 1.6% in the case of AE, AE/AM and AM derived porous carbon respectively.

3.3 Synthesis of hierarchical porous SiO₂

Further, hierarchical porous SiO₂ was synthesized by burning the carbon from carbon/silica composite monoliths in air. Calcination of the composite allows the carbon to remove from monoliths in the form of CO₂ and left with the porous silica skeleton. The synthesized SiO₂ particles exhibit 50% macroporosity, 43% microporosity and 7% mesoporosity as shown by histogram in Fig. S1 (ESI†). This further confirmed the role of SiO₂ for creating mesiporosity in carbon monoliths.

The yield for synthesis of SiO₂ from carbon/silica has been found 24.92 ± 2.5%. Fig. 6 shows the bulk (‘a’ and ‘b’ showing monoliths before and after calcinations) and nano (micro structures of SiO₂ shown in ‘c’ and ‘d’ gives an idea about the diameter of the silica particles) surface morphology of the synthesized porous SiO₂ particles.

Fig. 6 Images: (a) synthesized carbon/silica composite monoliths, (b) porous silica skeleton after calcinations of the monoliths in ‘a’, (c) FE-SEM micrograph of the porous silica, and (d) higher magnification image of ‘c’ shows spherical silica nanoparticles.

3.4 Raman and XRD analysis of the porous monoliths

Following carbonization, the phase composition and graphitic content of the monolith samples were analyzed. Raman spectroscopy was performed on the samples, as shown in Fig. 7a. The Raman spectrum of the porous carbon monolith exhibited two major characteristic peaks at approximately 1338 and 1589 cm⁻¹, corresponding to the fundamental D and G-bands for carbon, respectively.

Fig. 7 (a) Raman spectroscopic analysis of the porous carbon and Ag/carbon monoliths. Inset: magnified view of D and G peak for the calculation of area under peaks, (b) XRD pattern of the porous carbon and silver NPs impregnated carbon monoliths. Inset shows the (111) lattice plane of carbon.

Another broad peak or hump at approximately 2800 cm⁻¹ was also observed, which corresponds to the 2D-band of the carbon material. The 2D-band or G'-band represents a second-order LO phonon scattering process, and its frequency is almost double that of the D-band, which is normally located between the wavenumbers of 2500 to 2900 cm⁻¹.

The graphitic nature of the carbon is determined by the Raman D/G band intensity ratio. The ratio of the area under the D-peak over that of the G-peak (i.e., A_D/A_G) ratio is <2.0 for amorphous carbon without defects.^39,40 Here, the A_D/A_G ratio for the porous carbon monolith was calculated as ∼1.62, which indicates partial graphitization within the amorphous domain of the carbon samples.

The Raman spectrum of the silver/carbon hybrid monolith is almost identical to that of the porous carbon monolith, as demonstrated in Fig. 7a (red line). This is because the silver nanoparticles do not bond with the carbon, and are physio-adsorbed onto the carbon monoliths. As the silver metal does not produce a Raman signal, it can be confirmed that the silver in the carbon matrix exists in an Ag° state and is strongly retained by the porous monolithic support. The A_D/A_G ratio of the porous carbon monolith impregnated with the silver nanoparticles is calculated as ∼1.50, which indicates enhanced graphitization compared to the bare carbon monolith. Here, the silver nanoparticles possibly perform some function in the localized heating of the surrounding carbon particles in the monolith, which consequently enhances the graphitization.

The intensity ratio of the D-peak over the G-peak, denoted by R = I_D/I_G, progressively decreases with the carbonization temperature; however, the positions of the D and G-peaks are unaffected by the carbonization temperature.⁴¹ Knight and White⁴² demonstrated that the value of R depends on the size of the in-plane graphite crystallite (denoted by L_c), expressed by an empirical formula, L_c = 4.4/R. Using this equation, L_c was calculated as ∼4.36 and 4.38 nm for the porous carbon monolith and silver/carbon monoliths, respectively, corresponding to R values of 1.009 and 1.003, respectively.⁴³ Therefore, the size of the in-plane graphite crystallite increases when silver is impregnated into the carbon monolith, which indicates that a greater amount of graphitization occurs when metallic silver nanoparticles are present in the carbon monoliths.

Fig. 7b presents the wide-angle X-ray diffraction (XRD) patterns for the synthesized hierarchical porous carbon monolith following the removal of silica from the resorcinol–carbon precursors via NaOH treatment. The XRD pattern exhibits distinct broad peaks at around 2θ = 23° and 44°, which correspond to the (002) and (101) reflections characteristic for partial graphitic domains.¹³ These two peaks correlate with the results reported in literature. The inset of Fig. 7b shows another peak at 2θ = 10.3°, which corresponds to the (111) plane of the carbon. The XRD data was compared with Pearson's crystal data (File no. 1819831). These broad XRD peaks and the SAED pattern (inset of Fig. 5b) observed during the TEM imaging (Fig. 5b) show a diffused pattern instead of a ring-like structure, which further confirms the amorphous nature of the porous carbon monolith.

For the silver/carbon monoliths, the peaks (red line in Fig. 7b) correspond with the Pearson's crystal data (File no. 261046). The peak at 2θ = 38.11° and 44.31° relate to the (111) and (200) planes, respectively, where the first peak is the most intense. The other peaks at 57.43°, 64.45°, and 77.48°, correspond to the (222), (220), and (311) planes of silver, respectively. The aforementioned carbon peaks, along with these silver peaks, confirm that silver nanoparticles are present in the porous carbon monolith. All these XRD diffraction peaks correspond to the characteristic FCC silver planes, which is also confirmed by the ring-like SAED pattern recorded by the TEM imaging (in the inset of Fig. 5d). The average crystallite size or mean particle size of the silver nanoparticles (mean particle size = D) was determined from the XRD diffraction data using the Debye–Scherrer equation: L_c (or D) = 0.90λ/β × cos(θ), where β is equal to the full peak width at half maximum (FWHM) corresponding to the diffraction angle (θ), and 0.90 is Scherrer's constant.⁴⁴ The average diameter or mean particle size of the silver nanoparticles was determined to be ∼33.73 nm, corresponding to the (111) silver plane; this is also supported by the FE-SEM and TEM results, as previously discussed.

3.5 Characterization for the carbon monolith as flow through electrodes

The wettability and surface chemistry of these carbon monoliths was studied in terms of Fourier transform infrared (FTIR) spectral analysis (attenuated total reflection mode) and contact angle measurement. Fig. S4† shows FTIR spectra of RF/SiO₂ derived carbon monolith after etching the silica particles. The spectra were recorded from 450–4000 cm⁻¹ at a scan rate 0.2 cm S⁻¹. Presence of stretching bond vibration 3100–3600 cm⁻¹ attributes the presence of –OH vibration which arises due to surface hydroxylic groups and chemisorbed water. The asymmetric nature of tis bond at lower wavenumbers confirms the presence of string hydrogen bonds. The peaks situated at 2880 and 2970 cm⁻¹ stands for absorption band characteristic of –CH₂– and –CH₃ structures which suggests the existence of some aliphatic species on the carbon monolith.⁴⁵ Also, oxygen-containing functional group such as carbonyl is seen at 1652 cm⁻¹.⁴⁶ The band at 1560 and 1737 cm⁻¹ attributed to conjugated system like diketone, keto-esters and keto–enol structures whereas 1737 cm⁻¹ band stands for carboxylic, ester, lactonic or anhydride groups. Since the carbon monoliths are undergoes silica etching, it helps in chemical activation and causes water molecules to sorbed on the surface of the carbon. This happens because of specific and nonspecific interactions like hydrogen bonds, chemisorption due to surface oxide hydration and physical adsorption which can be described by OH binding vibrations in 1500–1600 cm⁻¹ region. Besides, 1390–1470 cm⁻¹ band region suggest deformation vibration of surface carboxylic groups and in-plane vibration of C–H in many C=C–CH structures.⁴⁷ The absorption band in 1000–1265 cm⁻¹ region ascribes ether-like, epoxide and phenolic (vibration at 1180 cm⁻¹) structures.⁴⁵

Water contact angle (WCA) measurement was performed on both the RF/SiO₂ polymer monoliths (Fig. S5a₁†) and carbon monoliths obtained from the polymeric precursor. Contact angle measurement shows that the water droplets exhibit contact angles as high as 128 ± 1.5° on the polymer surface, which is revealing of hydrophobicity (see Fig. S5a₂†). The WCA still approximately same (127 ± 1°, Fig. S5a₃†) when measured after 10 min. This observed hydrophobicity is probably because of the synergistic effect of two reasons: firstly, the geometrical surface rough surface of the porous monolithic structure and secondly, the existence of methyl groups on the polymer surface. The silica precursor TEOS monomer and APTES catalyst each of them comprise of a non-hydrolysable methyl group (–CH₃).⁴⁸ The attachment of these hydrolytically stable –CH₃ groups to the siloxane skeleton during sol–gel process causes very small solid–liquid interfacial energies, which results in a hydrophobic polymer monolith.

On the other hand, Fig. S5b₁† summarizes the contact angle measurements on RF/SiO₂ derived carbon monolithic surface. Fig. S5† show the change in contact angle values with time which was measured with different time intervals (shown in Fig. S5b₂–b₄†). The carbon monolith surface shows a very strong hydrophilicity (WCA 19 ± 1.5°). The observed hydrophilicity is probably because of the presence of various surface hydroxylic groups which were observed in FTIR studies and discussed earlier.

Water flow rates through the carbon monolith under variable pressure gradients were measured at room temperature in a flow through experiment setup, shown in Fig. S6†. Fig. S6a† shows different parts of the device and complete device is shown in Fig. S6b.† The setup is based on flows through an electrode made of a porous carbon monolith (shown in inset of Fig. S6b†). A cylindrical specimen, 0.6 cm in diameter and 2 cm long, was used. The experimental data for the four different preforms is shown in Fig. S7.† The good-ness of the fit of linear regression (R² = 0.943) results shows the validity of Darcy's law.⁴⁹ The Darcy's equation is given by Q = −(k/μ)ΔP/L, where Q, μ, ΔP, L and k are the volumetric flow rate, viscosity of water, pressure difference, length of the frit, and water permeability of the monolith, respectively. Hence, the slopes of the linear water flow rate versus pressure gradient curve were used to get the permeability of the carbon monolith. Repeat experiments were performed and good reproducibility was found. The permeability of the carbon monolith used as a flow through medium was calculated ∼11.75 ± 1.45 × 10⁻¹² m² and found comparable with the literature report.⁴⁹ This result suggests that this highly porous carbon and the experimental setup could be useful for filtration, catalytic reactor, electro osmotic pump and drug delivery systems.

3.6 Mechanical strength of the monoliths

The mechanical strength of the monoliths was also measured by a universal testing machine for compression tests of materials in monolithic geometry. The Young's modules of the porous carbon, RF/SiO₂ polymer and silica monolithic samples were obtained from the slope of the stain vs. strain curve for each sample.⁵⁰ The Fig. S8 (ESI†) shows a stress vs. strain curve for a carbon monolith and the estimated Young's modulus of the samples were summarized in Table T2 (ESI†). The carbon monolith shows a Young's modulus 0.42 MPa which is almost 4.5 times higher than polymer monolith. Interestingly, this modulus value is twice greater than that reported for a nanocast carbon monolith (0.19 MPa)⁵¹ and the soft-templated carbon monolith (0.2 MPa).⁵² Our synthesized carbon monolith possesses few salient features such as tree like hierarchical porous morphology and fully interconnected macropores, mesopores and micropores and high mechanical strength. It can be easily prepared in a timesaving manner without involvement of supercritical drying. Additionally, the silica monolith obtained by burning the carbon from C/SiO₂ network was very fragile and has a Young's modulus 1.1 kPa.

3.7 Catalytic activity of the silver impregnated porous carbon monoliths

The silver/carbon monolith was used for the catalytic reduction of 4-NP with excess NaBH₄ in an aqueous medium. An aqueous 4-NP solution has a maximum absorption wavelength (λ_max) of 317 nm, as shown by trace ‘A’ in Fig. 8a. It is considered that, following the addition of a freshly-prepared aqueous NaBH₄ solution, the 317 nm peak red shifts to 400 nm because of the 4-NP. This peak at 400 nm appeared because of the formation of 4-nitrophenolate ions that were created by the addition of the NaBH₄ to the 4-NP. The position of this peak remained unchanged for several days. Owing to the absence of any catalyst, the thermodynamically favorable reduction of 4-NP is not initiated. As the hierarchically porous carbon–silver nanoparticle nanocomposite catalysts (25 mg, 50 mg, and 100 mg) are added to, and mixed with the reaction mixture, the yellowish color of the 4-NP solution gradually fades and ultimately vanishes; this is owed to the steady conversion of the nitrophenolate ions with the addition of the silver catalyst, and was indicative of the progress of the catalytic reaction. Further degradation could be envisaged by the gradual reduction of the height of the 400 nm peak with the simultaneous initiation of a 295 nm peak (traces B, Fig. 8a), confirming the creation of 4-AP.

Fig. 8 Catalytic activity: (a) Uv-vis spectra of 4-NP and NaBH₄, (b) degradation of 4-NP to 4-AP for the 50 mg Ag/carbon monolith loading, (c) control study of 4-NP reduction without Ag particles for the carbon monoliths, (d) calculation of rate constants for three different catalyst loading.

The role of the silver nanoparticles during the catalytic reaction can be determined with the aid of electrochemical current–potential measurements.⁵³ The silver nanoparticles intensely catalyze the redox reactions. First, BH₄⁻ and 4-NP adsorb onto the catalyst surface, and subsequently an electron is transferred from the BH₄⁻ to the 4-NP. The electron transfer process on the surface of the catalyst can be explained by the following mechanism:

i. Adsorption of 4-nitrophenolate ions onto the surface of the silver nanocatalysts.

ii. Desorption of 4-aminophenolate on the exterior of the silver surface by an interfacial electron transfer process.

The adsorption occurs on the substrate through the chemical interaction (chemisorption) between the catalyst surface and the substrate. The adsorption of the 4-nitrophenolate ions onto the surfaces of the catalyst nanoparticles consequently provides support to overcome the kinetic energy barrier of the catalytic reaction. However, the rate of desorption from the particle surfaces is slower than the rate of electron transfer i.e., the rate constant for desorption is lesser than the rate constant for electron transfer.

The catalytic reaction of the porous carbon monoliths impregnated with the silver nanoparticles was studied with regard to the catalytic hydrogenation of 4-NP assisted by an excess of sodium borohydride. This catalytic reaction is widely used as a model reaction to evaluate the catalytic activity of various nanoparticles. Also, various techniques have been developed for the removal of nitrophenol compounds. These nitrophenol compounds are known organic pollutants that originate from various industrial and agricultural wastewaters. Moreover, the catalytically reduced product, 4-AP, is a key intermediate used by pharmaceutical industries. 4-AP is used as an ingredient for the synthesis of various analgesic and antipyretic drugs. Furthermore, the final hydrogenated product, 4-NP, is used for many other applications, such as corrosion inhibitors, photographic development, hair-dye chemicals, and anticorrosion-lubricants. Therefore, the development of a method for the catalytic hydrogenation of 4-NP to 4-AP under mild conditions is very important from industrial and environmental perspectives.

Here, the catalytic reaction commenced immediately when the silver/carbon monolith was added to the 4-NP/NaBH₄ solution, without an induction period. It was important that there was significant enhancement in the 4-NP degradation reaction rate when a greater amount of the carbon monoliths impregnated with the silver nanoparticles were employed (Fig. 8b). Fig. 8c demonstrates that no 4-AP peak is generated, which confirms that the catalytic conversion of 4-NP to 4-AP does not occur without the presence of silver in the carbon support; just a small amount of physical adsorption causes a change in the absorption intensity. This clearly demonstrates the essential role of the silver nanoparticles for the catalytic degradation process. Moreover, the silver nanoparticles were securely incorporated into the porous carbon monolith through physical adsorption, and this attachment was sufficiently robust that the silver nanoparticles remained in the surface of the monolith throughout the reaction.

Here, NaBH₄ was used as the reducing agent and its concentration was very high (∼1000 times) compared with the 4-NP concentration. Hence, the concentration of the NaBH₄ remained constant throughout the reaction. Therefore, the reaction followed pseudo-first-order kinetics with respect to the 4-NP. As soon as the NaBH₄ was added, the silver nanoparticles transferred electrons from the BH₄⁻ (donor) to the 4-NP (acceptor) just after their adsorption onto the catalyst surfaces, and the catalytic reduction was subsequently initiated. The degradation of the BH₄⁻ was retarded by the excess NaBH₄, which increased the pH of the reaction medium. The BH₄⁻ released hydrogen and purged out the air, which prohibited the aerial oxidation of the formed 4-AP product. The generation of small H₂ gas bubbles surrounding the silver catalysts helped to stir the solution during the reaction, and hence the catalyst particles were well-dispersed in the reaction mixture. Thus, a favorable environment was achieved for a smooth reaction to occur. Generally, for a micro-heterogeneous catalysis reaction,⁵⁴ the rate of the reaction linearly increases with the quantity of the catalyst.⁵⁵ Here, the amount of catalyst was varied, and all the other parameters were kept constant to investigate the effect of the catalyst loading on the reaction rate. Interestingly, it was observed that the rate also increased linearly as the catalyst loading was increased. The rate constant (K_t) for the catalytic reaction of 4-NP to 4-AP was estimated from the slope of the ln(C_t/C₀) vs. time ‘t’ plot (Fig. 8d), where C_t and C₀ are the 4-NP concentrations at ‘t’ and ‘0’ time, respectively. As the concentration and absorbance are linearly proportional, the ratio (C_t/C₀) was measured from the respective absorbance at 400 nm. The ln(C_t/C₀) vs. time plot (Fig. 8d) follows a linear relationship. This suggests that the catalytic reaction follows pseudo-first-order kinetics.^53,55 The rate constant values for the various catalyst loadings are summarized in Table T2 (ESI†). The amount of silver in the silver/carbon monolith was calculated by considering the decomposition of the AgNO₃ into metallic silver nanoparticles (summarized in Table T1†). The calculated rate constants were very high and comparable with previous reports.^56,57 These findings confirm that the reaction rate is very much dependent on the amount of silver loading in the porous carbon monolith, which is also evident in earlier works.^54,55,58 It was also verified by comparing the FE-SEM images (Fig. 5c and d) of the composite monolith before and after the complete catalytic degradation of the 4-NP (Fig. 9a and b), which suggests that the silver particles were sufficiently attached to the carbon monolith.

Fig. 9 (a) FE-SEM image after catalytic reduction of 4-NP, (b) higher magnification image of ‘a’ shows the retained silver nanoparticles after catalysis, (c) reusability of the Ag/carbon composite nanocatalyst for the reduction of 4-NP with NaBH₄.

It was determined that the porous silver/carbon monolith is a very effective catalyst for the 4-NP to 4-AP conversion reaction. The silver nanoparticles remained catalytically active even at the end of the reaction, and they were subsequently removed from the 4-AP product. Following each reaction, the used catalysts were washed with distilled water and dried, and subsequently tested with regard to reusability. It was determined that the catalyst was reusable for the catalytic reduction of 4-NP, even when they were used six times (reusability is shown in Fig. 9c).

It is considered that the impregnated silver nanoparticles in the hierarchical porous carbon monoliths perform a function for the electron transfer during the redox reaction. The BH₄⁻ ions donate electrons to the catalyst, whereas the nitrophenols capture the electrons from the catalytic metal particles. This is because the nitrophenols are naturally electrophilic owing to the electron extracting effect of the nitro groups. The concurrent adsorption of the 4-NP and BH₄⁻ onto the surfaces of the metal nanoparticles can be explained by the observed activity of the nanoparticles. Thus, the porous carbon monoliths impregnated with the silver nanoparticles exhibit catalytic activity owing to the presence of the silver nanocatalysts within its matrix. The porous carbon monolith acts as a robust catalyst support which helps to prevent the agglomeration of the silver nanoparticles throughout the reaction, and also facilitates unobstructed mass transport over its porous network structures.

This cost-effective one-step sol–gel derived porous polymer, carbon monolith fabrication process can be easily scaled up for various industrial applications including an efficient catalytic supporter. This demonstrated fabrication technique is superior to the other traditional techniques because it does not involve any expensive chemicals or processes, such as supercritical drying, hydrothermal techniques, or colloidal silica-based templating methods. Interestingly, the fabricated monoliths exhibit hierarchical porosity distribution and excellent mechanical stability; hence, the synthesis procedure does not require a complicated drying procedure, and is therefore cost- and time-effective. Moreover, when the carbon is burned, the composite monolith produces a pure microporous silica structure, which also has many interesting applications, such as inorganic membranes for hydrogen purification, water adsorption, flow-through membrane, microfluidic devices for advanced surface-based bioanalysis, and lab-on-a-chip devices.

4. Conclusions

We have developed a hierarchically porous carbon material with an ultra-high BET surface area of 2630 m² g⁻¹, and with superior material properties in terms of mechanical stability, pore size distribution, material transport throughout the monolith, and catalytic support. The pore size of the monolith can be easily tailored by changing the APTES/APTMS catalyst loading. In this work, the carbon monoliths have been synthesized with a hierarchical pore structure and superior transport properties, and can be easily produced in large quantities owing to the elimination of the costly and complex supercritical drying method and its replacement, the subcritical ambient drying method. In terms of synthesis, the present approach is simple and inexpensive on both laboratory and industrial scales. The good water permeability of the monolith suggests that it could be used as a filtration membrane as well as other applications, such as drug delivery and catalytic reactors. We have also successfully demonstrated the impregnation of metallic silver nanoparticles into the porous carbon monolith to enhance its functionality for catalytic reactions. The catalytic potential of the silver/carbon hybrid towards the NaBH₄ mediated reduction of 4-NP was evaluated. The hybrid monolith showed good catalytic activity. The interconnected pore structures and the ultra-high surface areas of the carbon monolith could facilitate interesting applications in fields such as Li-ion battery electrodes, adsorbents, catalyst supports, and supercapacitors.

Author contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the DST Unit of Excellence on Soft Nanofabrication at Indian Institute of Technology Kanpur from the Department of Science and Technology, New Delhi, India. Authors thank Mr Srujan Singh for his M.Tech thesis, July, 2015 at IIT Kanpur and interesting discussions regarding permeability measurement setup.

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Footnotes

† Electronic supplementary information (ESI) available: Experimental details and characterization of the prepared carbon monoliths. See DOI: 10.1039/c5ra26503g

‡ These authors have contributed equally to this work.

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