Facile Soft-Templated Synthesis of High-Surface Area and Highly

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A Facile Soft-Templated Synthesis of High Surface Area and Highly Porous Carbon Nitrides Maryam Peer, Marcella Lusardi, and Klavs F. Jensen Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b03570 • Publication Date (Web): 26 Jan 2017 Downloaded from http://pubs.acs.org on January 26, 2017

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Chemistry of Materials

A Facile Soft-Templated Synthesis of High Surface Area and Highly Porous Carbon Nitrides Maryam Peer†a, Marcella Lusardi‡a and Klavs F. Jensen†‡* †Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139 (USA) ‡ Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139 (USA) a

These authors contributed equally to this work.

Abstract: Mesoporous carbon nitride is synthesized in a one-pot approach using different nonionic surfactants (Pluronic F-127, Pluronic P-123 and Triton X-100) and melamine cyanurate hydrogen-bonded complex using just water as the solvent. We obtain three-dimensional (3D) assembled nanostructures from low dimensional carbon nitride sheets, by taking advantage of supra-molecular assembly of melamine and cyanuric acid, moderate interactions between surfactant and precursors, structure directing effects of the surfactants, and the good thermal stability of the melamine cyanurate sheets formed around the micelles. Different morphologies including sheet-like, hollow spherical and tubular or highly porous network result depending upon the synthesis approach and the surfactant-to-precursor ratio. Pseudo-ternary phase diagrams map the composition of the starting solution to the resultant carbon nitride morphology. Increasing the surfactant amount leads to higher carbon residue (C/N ~1) and large BET surface areas (up to 300 m2/g). Further tuning of the synthesis parameters as well as addition of HCl produces uniformly porous nanostructures with high porosity (0.4-0.5 cc/g), high surface area (>200 m2/g) and yet stoichiometric C/N ratio (~0.75). The synthesized high surface area carbon nitrides show improved light absorption and enhanced photocatalytic activity in a Rhodamine B dye degradation reaction under visible light irradiation compared to bulk melamine-derived carbon nitride.

1 INTRODUCTION Carbon-based nanoporous materials have attracted increasing attention due to their desired physiochemical properties along with the possibility for doping and functionalization with heteroatoms such as boron and nitrogen.1–5 Among different doped carbonaceous materials, carbon nitrides, which comprise a class of materials with C/N ratio ~0.75 (at/at) and varying degrees of crystallinity based on synthesis conditions, have been widely studied during the last decade.1–10 Desired electronic properties such as bands positioned for water reduction/oxidation, a band gap in the visible light region, and high thermal and chemical stability make carbon nitrides promising metal-free semiconductor candidates in a variety of applications including energy conversion and storage, CO2 adsorption, catalysis and hydrogen production.11–16 Bulk carbon nitrides are typically synthesized by direct condensation of nitrogen-rich precursors such

as cyanamide, dicyandiamide and melamine.17,18 Despite its suitable band gap and visible-light driven photocatalytic activity, bulk carbon nitride suffers from low efficiency in photocatalytic applications owing to fast recombination of photo-generated electron and hole pairs and a low specific surface area. For example, bulk carbon nitride derived from melamine condensation has a BET surface area of around 10 m2/g.3 One strategy to improve the charge carrier separation and hence photocatalytic efficiency is to dope carbon nitride with heteroatoms or to make composites by combining a second semiconductor or metal nanoparticle with carbon nitride.13,14,19–21 Modifying the textural and morphological properties through copolymerization of different precursors is another tactic to improve electronic and optical characteristics.22–24 Hence, synthesis of carbon nitride with high surface area and tunable textural properties (pore size and pore volume) has been of particular interest.25–28

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Most studies of high surface area, 3D structured porous carbon nitrides have focused on hard templating (nanocasting) techniques using mesoporous silica or porous anodic aluminum oxides as the sacrificial template.25 Although nanocasting results in high surface area, it is a multistep process, which involves hazardous reagents such as hydrogen fluoride, ammonium bifluoride or strong sodium hydroxide solution for template removal.6,26,27 Soft-templating, on the other hand, can offer a facile, one-step route towards high surface area porous carbon nitride.3,25,28 Exploration of soft-templating has resulted in improvement in surface area and porosity, but the strong interaction between the surfactant molecule and carbon nitride precursors, and hence incomplete condensation of the precursor and high carbon residue, remain challenges.3 The choice of the carbon nitride precursor is as important as the choice of surfactant since the interaction between the precursor and the template affects the degree of polymerization. Recently, it was shown that pre-organization of melamine and cyanuric acid in the form of melamine cyanurate sheets through hydrogen bonding led to 3D carbon nitride structures with tunable morphology depending on the solvent used in the process.22,23 However, the surface area of the synthesized carbon nitrides was limited to 40-60 m2/g. Post-synthesis calcination in air has been used to increase the surface area of carbon nitrides up to 130 m2/g.25,28 Herein we present a simple one-step soft templating approach using just water as the solvent, taking advantage of both melamine-cyanuric acid supramolecular assembly through hydrogen bonding and the structure-directing character of different surfactants, to synthesize high surface area, high porosity carbon nitrides. A systematic study is conducted to develop pseudo-ternary phase diagrams for the surfactant/water/precursor system and determine the condition at which both surface area and C/N ratio are optimized. Different morphologies are obtained by altering the surfactant-to-precursor ratio. It is shown that the carbon nitride samples synthesized through this method have BET surface areas as high as 160-270 m2/g while possessing the desired C/N ratio (0.75-0.77). Along with high surface area and porosity, the synthesized carbon nitrides display desirable optical properties: band gap and improved visible light absorption potentially rendering them suitable in photocatalytic applications. Other carbonaceous materials synthesized in this study, having higher C/N ratio (=0.8-1), are still rich in nitrogen, have high surface area (200-400 m2/g) and show bimodal hierarchical porosity. These could be promis-

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ing candidates for different types of metal-free basecatalyzed reactions. 2 EXPERIMENTAL 2.1 Materials and Methods Cyanuric acid and melamine from Sigma-Aldrich were used as precursors for carbon nitride synthesis. Deionized water was the solvent in all syntheses. Pluronic F-127, Pluronic P-123 and Triton X-100 were purchased from Sigma-Aldrich and used as received as the structure-directing agents. A commercial melamine cyanurate sample (MC-4500), kindly provided by Nissan Chemicals, was used as the control sample in the FTIR and Raman studies. In a typical synthesis using the first procedure, we mixed 1 g of the surfactant with 40 ml deionized water and subsequently added 506 mg cyanuric acid. The solution was stirred for one hour. In a separate beaker, an equimolar amount of melamine (494 mg) was added to 40 ml deionized water and stirred for one hour. The melamine solution was subsequently added to the cyanuric acid solution, and stirring was continued overnight at room temperature. The resulting milky solution was placed on a hot plate at 90°C to evaporate all water. We dried the paste-like residue in the oven at 70°C and heat-treated the subsequent white solid to 500°C at a rate of 2.3°C/min and soaked at that temperature for 4 hours. These samples are denoted as F127-F-x or Tri-F-x depending on the surfactant used in the synthesis. “F” refers to the first synthesis methodology and “x” is the weight ratio of surfactant to precursor. In the second approach, a given amount of surfactant was dissolved in 40 ml water followed by addition of cyanuric acid while heating to 60°C. After one hour of mixing, an equimolar amount of melamine was added and stirring was continued at 60°C for an additional 4 hours. We filtered the milky solution, washed it with an excess of water and dried it in an oven at 70°C before heat treatment in the furnace. The samples synthesized using this procedure are denoted as F127-S-x or Tri-S-x, in which “S” refers to the second synthesis approach. In the third approach, we adapted a synthesis route analogous to mesoporous silica preparation.27 In a typical synthesis, 1 g Pluronic P-123 (or other surfactants) was dissolved in a mixture of 8 g water and 30 g of 2M HCl and stirred at 35°C overnight. Then, 1.04 g melamine and 1.06 g cyanuric acid were added and mixed for 1 hour. After initial mixing, the milky solution underwent static aging in a capped vial in an oil bath at 35°C for 24 hours. The temperature was in-


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creased to 70°C for a second step of aging for another 24 hours. Finally, the solution was cooled down to room temperature, filtered and washed with a copious amount of deionized water. We dried the collected white cake in the oven and performed heat treatment in the furnace at 500°C following a similar heating program used in the first two approaches. For chemical and textural comparison with the synthesized soft-templated samples, a bulk carbon nitride sample was prepared via condensation of melamine at 500°C. The same temperature program used for the three soft-templating recipes was employed. This sample is identified as “M-CN” for melaminederived carbon nitride. The photocatalytic activities were evaluated by monitoring Rhodamine B (RhB) degradation under visible light irradiation. Experiments were conducted in a capped Pyrex flask as the reaction vessel. A 1.05×10-5 M solution of RhB was prepared by dissolving 0.2 mg of the dye in 40 ml of deionized water. To this solution, we added 10 mg of the synthesized catalyst and continued stirring the suspension in the dark. After one hour a sample was collected to assess the degradation rate in the dark. Then, the flask was illuminated by visible light generated using a solar simulator (Xenon lamp, with UV cut-off filter (λ > 400 nm) and recirculating coolant to remove the IR portion). Samples were collected at given time intervals (1 hour) and separated from the photocatalyst using filtration. The absorption spectra of the samples were measured on a Shimadzu UV-3101PC spectrophotometer. 2.2 Characterization techniques We employed different techniques to fully characterize the synthesized materials. A Zeiss Merlin highresolution scanning electron microscope (SEM) was used to examine the morphology of the synthesized samples. We assessed the microstructure and morphology using a FEI Tecnai transmission electron microscope (TEM, G2 Spirit TWIN). Nitrogen physisorption measurements at 77 K, performed using an Autosorb iQ (Quantachrome Instruments), determined the textural properties and BET surface areas. All the samples were degassed at 150°C for 9 hours prior to the adsorption measurement. Pore size distributions were obtained using the desorption branch of the sorption isotherms and the BarrettJoyner-Halenda (BJH) model for mesoporous materials. Bulk elemental analysis was conducted using a Vario EL CHNS elemental analyzer instrument (Elementar Germany). X-ray photoelectron spectroscopy, (XPS, Physical Electronics Versaprobe II) was used to de-

termine surface C/N ratios in comparison with the bulk. X-ray diffraction (XRD) patterns were collected on a PANalytical XPert instrument, in the range of 5° to 80°, to elucidate the crystalline structure of the samples. We studied the chemical structures and bonds of the synthesized samples by conducting Fourier Transform Infrared spectroscopy (FTIR) using a Thermo Fisher FTIR6700. The chemical structure was further probed by performing Raman spectroscopy using a Horiba labRAM (HR800) system equipped with a 785 nm laser source. Optical absorbance of the synthesized samples was studied using a UV-Vis spectrophotometer (Lambda1050 NIR) equipped with an integrated sphere to collect total diffusive reflectance. 3. RESULTS AND DISCUSSION 3.1 Morphology Evolution 3.1.1 First assembly

approach:

evaporation-induced

self-

Figures 1a and b show the pseudo-ternary phase diagrams for the samples synthesized using Pluronic F127 and Triton X-100, respectively, as the structuredirecting agents. The maximum concentration of surfactant used was about 12 wt%, since working at higher surfactant concentrations resulted in irregular carbonaceous structures with no specific textural properties. This behavior is attributed to the inhibitive effect of a large amount of surfactant for the self-assembly of melamine and cyanuric acid, through side reactions between precursors and surfactant.3 Figure 2 shows the SEM images of F127-F-x samples synthesized at different surfactant-to-precursor mass ratios. At very low ratios of surfactant to precursor (x≤0.5, Figure 2a, b and c), sheet-like structures with no preferential curvature and texture are obtained. At very low surfactant concentrations (lower than the Critical Micelle Concentration, CMC: ~1 wt% at 25°C), Pluronic F-127 is mainly present in the solution as unimers.29 Consequently, no specific morphological feature is observed for the samples synthesized at surfactant concentrations smaller than the CMC. However, as the ratio of surfactant to precursor increases, spherical and cylindrical micelles are formed. Presumably, weak hydrogen bonding between melamine cyanurate and polyethylene oxide (PEO) blocks of the surfactant leads to the preferential growth around the micelles.3 At x=1 (Figure 2d), spherical and tubular hollow structures are formed. Each micelle serves as a locus for nucleation and



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(a)

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(a)

(b)

200 nm

200 nm

(c)

(d)

500 nm 500 nm

B

A C

400 nm

1 µm

(e)

(f)

1 µm

200 nm

(b) (g)

1 µm

B

200 nm

Figure 2. SEM images of F127-F-x samples synthesized with 1.2 wt% precursor and (a) x=0.1, (b) x=0.25, (c) x=0.5, (d) x=1, (e) x=2, (f) x=3 and (g) x=4.

A C

Figure 1. Pseudo-ternary phase diagrams of the samples synthesized with (a) Pluronic F-127 and (b) Triton X-100, using the first approach (green arrow shows the transition from sheet-like to globular and curved (region “A”) followed by irregular structures (region “B” in both diagrams). Samples synthesized in region “C” have a sheet-like structure.

growth of the polymeric sheets. It should be noted that the size of the spherical and tubular features are larger compared to what is expected from a single micelle, due to aggregation and growth of the micelles during the solvent evaporation step. As the number density of micelles increases, the features formed become smaller (Figure 2e). By further increasing the amount of surfactant at a constant precursor concentration (higher x values), the micelles’ hard sphere diameter decreases and the interaction between closely packed micelles leads to aggregation and formation of an interconnected micellar network producing a 3D reticulated porous structure at x=3 (Figure 2f ). Eventually, at higher ratios of surfactant to precursor, an irregular dense structure (Figure

2g) emerges as a result of high carbon residue from the surfactant. Figure 3a and b show high-resolution TEM images of samples F127-F-1 and F127-F-3, respectively. The hollow spherical and tubular structure of F127-F-1 and sponge-like porous morphology of F127-F-3 can be clearly seen in these images.

Figure 3. High-resolution TEM images of (a) F127-F-1 and (b) F127-F-3.

SEM images of the Tri-F-x samples show a similar morphological transition for Triton X-100 (Figure 4). However, the transition from sheet-like to globular structures occurs at higher Triton X-100 concentrations compared to Pluronic F-127. The lower molecular weight and shorter hydrophilic PEO chains of the


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surfactant, which subsequently provide less precursor-surfactant interaction especially at higher temperatures during the solvent evaporation step, could be an explanation for the late morphological transition compared to the F127-F-x samples. 3.1.2 Second approach: mild mixing and filtration Figure S1a and b show the pseudo-ternary phase diagrams for the samples made with the second recipe using Pluronic F-127 and Triton X-100, respectively. For both surfactants, increasing the surfactant-toprecursor ratio results in a trend similar to that observed for the first approach. However, samples prepared using Pluronic F-127 have a less uniform structure (Figure 5). The sheet-like structure at low surfactant concentrations transforms to partially globular and then a highly porous structure at higher surfactant concentrations. This structural evolution might be attributed to the elimination of the evaporation-induced self-assembly step. The solvent evaporation step at 90°C implemented in the first method appears to promote the formation and partial hardening of melamine cyanurate sheets. A similar

(b)

(a)

300 nm

(a)

300 nm

(c)

200 nm

(b)

200 nm

(d)

200 nm

Figure 5. SEM images of F127-S-x samples synthesized using 2.4 wt% precursor and (a) x=0.5, (b) x=1, (c) x=3 and (d) x=4.

On the other hand, samples synthesized using Triton X-100 through the second recipe (Tri-S-x) showed more defined structures and interesting morphological features (Figure 6). A mild synthesis condition (mixing at 60°C and filtration) seems to better stabilize the micellar structure in

(a)

(b)

200 nm

200 nm

(c)

(d)

200 nm

200 nm

300 nm

(d)

(c)

300 nm

1 µm

(e)

(e)

(f)

200 nm

1 µm

200 nm

Figure 4. SEM images of Tri-F-x samples synthesized with 1.2 wt% precursor and (a) x=0.1, (b) x=0.25, (c) x=0.5, (d) x=3 and (e) x=4.

effect of solvent evaporation on network hardening has been observed for synthesis of mesoporous carbon and silica materials using Pluronic F-127.30–32

Figure 6. SEM images of Tri-S-x samples synthesized with 2.4 wt% precursor and (a) x=0.05, (b) x=0.2, (c) x=0.5, (d) x=2, (e) x=3 and (f) x=4.



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the case of Triton X-100 and hence result in spherical and tubular structures. The yellow/light brown color of the samples, as well as the elemental analysis data, confirmed the absence of undesired side reactions and thus lower carbon residue. Moving to very high concentrations of Triton X-100 (x>3) led to the formation of irregular carbonaceous structures, as expected. The small sphere-like nanostructures in the TEM image of the sample Tri-S-0.05 (Figure 7a) are in agreement with the features observed in the corresponding SEM image (Figure 6a). The curved highly porous nanostructures formed in the case of sample F127-S-1 (Figure 7b) are clear indications of the templating effect of the high molecular weight surfactant. As a general conclusion, the first recipe is more suitable for Pluronic F-127, while samples synthesized using Triton X-100 have more uniform structures when synthesized through the second procedure. SEM images of the two samples prepared using Pluronic F-127 with the two different approaches and constant ratio of surfactant to precursor of 0.5, (Figure S2a and b), indicates the difference.

Figure 7. High resolution TEM images of (a) Tri-S-0.05 and (b) F127-S-1.

3.1.3 Third approach: HCl-assisted templating In an effort to synthesize more uniform nanostructured carbon nitride with a well-defined porosity, we adapted a third recipe using Pluronic P-123 (unless otherwise mentioned) along with HCl to direct the growth of carbon nitride sheets.

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This combination (Pluronic P-123/HCl) has been commonly employed to synthesize mesoporous silica materials.33 In addition to the templating effect of Pluronic P-123, HCl, as shown previously, could form an adduct with the basic carbon nitride framework and fill the nitridic in-plane pores in the graphitelike sheets formed by condensation of melamine and cyanuric acid.34 Figures 8a and d show SEM and TEM images, respectively, of the samples synthesized using the third approach with a surfactant-to-precursor ratio of 0.5 (P123-T-0.5). Both images verify the highly porous structure of this sample. Decreasing the Pluronic P-123 concentration (P123-T-0.25) results in a less ordered porosity but still a highly porous network with voids smaller than the P123-T-0.5 sample (Figure 8b). As observed in the figures, with HCl added, a uniform porous network with thin walls results. This is in contrast to the irregular and crumpled sheet-like structures formed without HCl (P123T-0.5-a) (Figure 8c) and with a control sample synthesized without surfactant. In addition to its effect of filling in-plane nitridic pores, HCl can have a significant effect on the stability and size of Pluronic P123 micelles. When the acid concentration increases, the hydrogen bonding between PEO blocks of the surfactant and water strengthens by formation of hydronium ions. The PEO blocks of the surfactant stretch and thus the micelles’ size increases. Moreover, the excess positive charge in the micelles’ corona due to the presence of hydronium ions stabilizes them, producing a uniform array of micelles.35 The samples synthesized with Triton X-100 and Pluronic F-127 using the third method (Tri-T-0.5, F127-T-0.5) resulted in similar highly porous nanostructures. It is noteworthy that using melamine as the only starting material (P123-T-M-0.5) results in sheet-like structures. This observation, along with the effect of surfactant on the morphology, further signifies the role of the pre-organization of the melamine cyanurate through hydrogen bonding and the desired interaction of the sheets with the micelles.22,23 To isolate the role that the thermal condensation plays in the resultant morphology compared to that of the surfactant, and further highlight the critical role of the micelles, we synthesized control samples using the second (X-S-0) and third (X-T-0) approaches without a template. Prior to heat treatment, these samples show large, extended sheets (Figures S3 a, b, e and f ), while the templated samples display faceted and flake-like features (Figures S3 c, d, g and h). After thermal condensation, the non-templated samples are in the form of irregular structures and crumpled sheets, lacking the small, defined


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Figure 8. SEM images of (a) P123-T-0.5, (b) P123-T-0.25 and (c) P123-T-0.5-a. (d) TEM image of P123-T-0.5.

features evident in those synthesized with surfactant (Figure S4). 3.2 Physiochemical characterization FTIR (Figure S5) and Raman (Figure S6) spectroscopy confirmed that the templated melamine and cyanuric acid precursors formed a melamine cyanurate complex prior to high temperature condensation. High-resolution XPS data collected for the thermally treated synthesized samples confirmed the presence of carbon, nitrogen and a small amount of oxygen (Figure S7). Deconvoluted C1 spectra displayed two main contributions (Figure S7c and d and S8a and b). The major carbon species with a binding energy of

288.2 eV is assigned to sp2-bonded carbon in C=N coordination. The second peak at 284.9 eV corresponds to graphitic carbon.4,36 The intensity of the graphitic carbon peak rises as the carbon residue in the sample increases verifying the partial replacement of nitrogen with carbon in the graphite-like framework. Four main contributions appear in the N1s deconvoluted spectra (Figure S7e and f and S8c and d). The signals at 398.7 and 400.1 eV correspond to pyridinic nitrogen (bonded to two neighboring carbons) and graphitic nitrogen (bonded to three neighboring carbons), respectively. The two weaker peaks at 401.3 eV and 404.5 eV are attributed to uncondensed terminal amino groups.4,36 The ratio of pyridinic to graphitic nitrogen is close to the expected theoretical value of 3, for the samples with high carbon residue (Figure S7e). This ratio decreases (e.g., 2.5 for P123-T-0.5-a) as pyridinic nitrogen atoms are partially substituted by carbon (Figure S7f ). Table 1 summarizes physisorption and elemental analysis results for selected samples synthesized in this study. Moreover, absolute C, H, N and O content is reported in Table S1. The measured elemental composition for the samples agrees well with the values reported for carbon nitrides and is expected based upon the partially condensed and defective structures of our samples.10 For samples prepared with Triton X-100, the C/N ratio increases with surfactant amount due to carbon residue and the inhibitive effect of the template for stoichiometric condensation of melamine cyanurate pre-organized sheets. The presence of excess surfactant typically decreases the extent of condensation, leading to the formation of smaller volatile domains. These are prone to decomposition during heat treatment, and therefore lower the nitrogen content of the final material.3 The increase in the C/N ratio is accompanied by an increase in the BET surface area and total porosity, which is the result of decomposition of the excess surfactant and volatile domains during heat treatment, leaving behind voids. The samples synthesized using Triton X-100 show an increase in the C/N ratio with an increase in the surfactant concentration (Table 1). For Pluronic F127, however, black carbon with a high C/N ratio results even at low template concentrations, while increasing the Pluronic F-127 concentration (x=1) lowers the C/N ratio to 0.74-0.8. There is an optimum range for the Pluronic F-127 concentration to achieve a high degree of monomer condensation as well as a proper templating effect. This trend could be attributed to the presence of surfactant molecules as unimers at low concentrations as opposed to the



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Table 1. BET surface areas, pore volumes and C/N ratio of nanostructured carbon nitrides synthesized using Pluronic F-127 and Triton X-100 through the first and second recipes.

Table 2 BET surface areas, pore volumes and C/N ratio of nanostructured carbon nitrides synthesized using the third approach. BET surface 2 area (m /g)

Pore volume

P123-T-0.25

164

0.55

0.73

1.6

P123-T-0.5

158

0.54

0.70

0.51

0.74

P123-T-0.5-a*

283

0.84

0.83

137

0.4

0.78

F127-T-0.5

155

0.5

0.71

F127-S-0.5

385

1.1

0.8

Tri-T-0.5

88

0.22

0.68

F127-S-1

163

0.6

0.7

F127-S-2

275

0.76

1.1

Tri-F-0.2

159

0.46

1.1

Tri-S-0.05

268

0.73

0.70

Tri-S-0.2

236

0.81

0.72

Tri-S-0.5

186

0.6

0.77

Tri-S-1

192

0.7

0.78

Tri-S-2

305

0.94

0.85

Sample

a

a

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BET surface 2 area (m /g)

Pore volume

M-CN

10

< 0.1

0.66

F127-F-0.5

200

0.6

F127-F-1

145

F127-F-3

C/N

Sample

(cc/g)

atomic ratio determined by elemental analysis

formation of micelles at concentrations higher than the CMC. In the third synthesis method, aging the starting solution in the presence of HCl promotes the selfassembly of melamine cyanurate sheets around the template structure. The outcome is a high surface area uniformly porous network possessing low C/N ratio of ~0.7-0.8 (Table 2, P123-T-0.25 and 0.5). In the absence of HCl, the undesired interaction between precursors and surfactant leads to a carbon-rich residue nanostructure with high surface area. The BET surface areas and pore volumes of samples prepared at optimized synthesis conditions are significantly higher than previously reported values for mesoporous carbon nitrides synthesized through softtemplating.3,25 The high surface area (>150 m2/g) and high pore volume (0.5-0.6 cc/g), along with the low C/N ratio, is a unique combination that could potentially improve the catalytic and photocatalytic performance. XPS measurements were consistent with the elemental composition data (see Figures S7 and S8). Pore size distributions of the samples, measured by nitrogen physisorption, provide further insight into the templating effect of the surfactants. The tem-

C/N

(cc/g)

*P123-T-0.5-a represents the sample synthesized similar to P123-T-0.5 but without HCl

plated samples have adsorption isotherms typical of porous materials containing both meso- and macropores (Figure S9).37 For the samples synthesized using the second approach and Triton X-100, larger pores become more prevalent as a result of increased surfactant concentration (Figure 9a). For samples prepared through the third approach using Pluronic P-123, pores of size ~4 nm persist in samples synthesized at different surfactant-to-precursor ratios, while the secondary mesopores size shifts from ~30 nm (P123-T-0.25) to ~60 nm (P123-T-0.5) by increasing the ratio from 0.25 to 0.5 (Figure 9b). With no HCl added, two broad peaks appear (P123-T-0.5a), confirming the critical role of HCl along with the surfactant in directing the assembly. A similar effect of surfactant templating was observed for the samples synthesized using Pluronic F-127. Figure S10 displays the pore size distribution of F127-S-1, with two distinct modes of porosity (4 and 35 nm). X-ray diffraction patterns (Figure S11) display a strong (002) peak at around 27°, characteristic of interlayer stacking of aromatic graphite-like domains. The peak broadening observed in the templated samples indicates an amorphous structure and the presence of more defects compared to the M-CN sample. We also ran FTIR on the samples with the stoichiometric C/N ratio (0.75) and compared the results with the samples with higher C/N ratios and melaminederived bulk carbon nitride (M-CN). IR spectra of samples with lower C/N ratios along with melamine-derived nitride show peaks in the wavenumber range of 1200-1700 cm-1, which is typical of C-N heterocycle stretching (Figure 10). The peak observed at 800 cm-1 is attributed to the breathing mode of tri-s-triazine units.22,36 The broad peak at


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3.0 Tri-S-0.2 Tri-S-0.5 Tri-S-1

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Wavenumber (cm ) Figure 10. FTIR transmission spectra of the synthesized carbon nitride samples (a: Tri-S-0.1, b: F127-S-1, c: F127-F-3, d: F127-F-0.5, e: P123-T-0.5, f: Tri-S-0.5, g: F127-F-1, h: M-CN).

2.0 1.5 1.0 0.5

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Pore Diameter, D (nm) Figure 9. Pore size distributions of samples synthesized using (a) the second approach with Triton X-100 and (b) the third approach with Pluronic P-123.

~3000-3600 cm-1 corresponds to the N-H stretching mode and is assigned to uncondensed amine groups and water.34 This peak is more pronounced for the nanostructured samples compared to M-CN, suggesting the presence of more defects in these materials. The IR results confirm the formation of C-N-C bonds; but the IR spectra for samples with higher C/N ratio differ from those with the lower molar ratio of carbon to nitrogen (e.g., 0.75). As an example, in the IR spectrum of one of those samples (F127-F0.5) with the carbon-to-nitrogen ratio of 1.7, the peak at 800 cm-1 has almost vanished. The bonds in the range of 1200-1600 cm-1 are significantly weaker and the peak at 1600 cm-1, assigned to C-C in-ring stretching, is stronger. The appearance of peaks

around 2880 cm-1 is attributed to the C-H stretching vibration typical of the graphite structure.38 The IR results confirm the defective and incomplete condensation of melamine cyanurate and the presence of high carbon residue in the framework for sample F127-F-0.5. The Raman and FTIR data for the samples prior to thermal condensation (Figures S5 and S6, respectively) and high resolution XPS and FTIR data for the carbon nitrides support the formation of melamine cyanurate complex prior to heat treatment and the presence of C-N bonds and tri-s-triazine building blocks in the resulting materials, respectively. In order to study the electronic structure and the effect of morphological and textural variations on the light absorption properties of the synthesized porous carbon nitrides, we converted UV-Vis diffuse reflectance spectra to absorption spectra through the Kubelka-Munk function. Figure 11a, b and c show UVVis absorption spectra for samples synthesized using Pluronic F-127, Triton X-100 and Pluronic P-123, respectively. The results indicate that the band gap of the templated carbon nitrides (those with lower C/N ratio) slightly shifted presumably due to the variation in morphology and conjugation length and the presence of defects in the templated polymeric materials compared to the melamine-derived bulk carbon



M-CN F127-F-1 F127-F-2 F127-F-3

(a) M-CN Tri-S-0.05 Tri-S-0.1 Tri-S-0.2 Tri-S-0.5 Tri-S-1 Tri-S-2

Absorbance (a.u.)

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(b) M-CN P123-T-0.25 P123-T-0.5 F127-T-0.5 Tri-T-0.5 P123-T-M-0.5

(c) 300

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Figure 11. UV-Vis absorption spectra of porous carbon nitrides synthesized using (a) Pluronic F-127, (b) Triton X-100 and (c) the third approach.

nitride (Figure S12). The obtained band gap energies (2.6-3 eV) lie within the visible range. A similar trend of a slight shift in the band gap has been observed for porous carbon nitrides synthesized through templating or supra-molecular pre-organization of different precursors.22,23,25

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The UV-Vis spectra also display significant improvement in the light absorption in both the UV and visible ranges. The enhancement in light absorption is attributed to multiple transmission and reflection of light due to the existence of structural defects and large BET surface area of the carbon nitrides synthesized in this study. The increase in light absorption correlates with the BET surface area of the synthesized samples. High surface areas, low C/N ratios and improved light absorption in the UV and visible range could make these materials promising candidates for photocatalytic applications. The photograph of a select few of the samples prepared using the one-pot soft templating technique is shown in Figure S13. 3.3 Photocatalytic activity We further studied the enhanced light absorption and photocatalytic activity of the synthesized carbon nitrides by performing Rhodamine B (RhB) degradation under visible light irradiation. Figures 12a, b and c show the time-dependent UV-Vis spectra of the RhB aqueous solution during the phototcatalytic dye degradation over two of the synthesized samples, TriS-0.2 and F127-T-0.5, as well as M-CN for comparison. As illustrated in this figure, the λmax value for the main peak, initially observed at 554 nm, shifts gradu ally to shorter wavelengths and eventually reaches 496-498 nm, corresponding to rhodamine (completely de-ethylated RhB).39 The gradual peak shift is accompanied by a continuous decrease in the maximum absorbance with time. The main absorption peak disappears more quickly for the reactions run with Tri-S-0.2 and F127-T-0.5, while a significant amount of dye is left even after 6 hours of visible light irradiation for the M-CN-catalyzed reaction. The rather fast discoloration of RhB solution for the photocatalytic evaluation of F127-T-0.5 is shown in Figure S14. The normalized absorption (an indication of the degradation rate) as well as peak shifts are shown in Figure 13. A faster decomposition rate is clearly observed for the high surface area materials compared to M-CN. The shift in the wavelength of the main absorbance peak is assigned to the simultaneous deethylation of RhB and its intermediates.39 The control experiment with no catalyst confirmed insignificant dye degradation via photolysis over a six hour period of irradiation with visible light. The observed decrease in absorption during the one hour of dark can be attributed to the adsorption-desorption


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equilibrium.38,39 Moreover, the peak shift is continuous and gradual for the reaction on M-CN pertaining to slow and simultaneous de-ethylation and degradation.40,41

peak shift data both confirm the facilitated RhB degradation as a result of high surface area of the synthesized nanostructured carbon nitrides compared to low surface area M-CN.

However, the shift in λmax is larger during the initial stages of the reaction for the high surface area nanostructured materials, due to accelerated deethylation of RhB. The de-ethylation pathway is considered as a surface reaction initiated by photosensitization of the adsorbed dye.41,42 The degradation and

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Figre 13. Normalized UV-Vis absorption (closed symbols) of the Rhodamine B solution and wavelength of maximum absorption (open symbols) during visible light irradiation with no catalyst (□), M-CN (○), Tri-S-0.2 (△), and F127-T-0.5 (▽).

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4 CONCLUSIONS Using three different recipes, nanostructured porous carbon nitride was synthesized through a one-pot soft-templating approach utilizing melamine cyanurate hydrogen-bonded complex and different nonionic surfactants. We controlled the morphology and textural properties of the carbon nitrides by adjusting the surfactant and precursor amounts and developed pseudo-ternary phase diagrams for the surfactant/precursor/ solvent system. High surface area (up to 260 m2/g) and highly porous materials with low C/N ratio (0.75-0.8) were obtained by working at low to medium surfactant-to-precursor ratios (x=0.05-1) depending upon the synthesis procedure and the surfactant employed. Triton X-100 at very low concentrations resulted in the highest BET surface area and pore volume while maintaining a low C/N ratio. Further control over morphology and porosity was realized by using HCl along with Pluronic P-123. The synthesized high surface area carbon nitrides showed improved light absorption in both the UV and visible ranges, in addition to enhanced activity in a photocatalytic degradation of RhB under visible light irradiation, underscoring the importance of controlling morphological and textural properties.

Figure 12. UV-Vis spectral changes of visible light-irradiated RhB solution over (a) M-CN, (b) Tri-S-0.2 and (c) F127-T-0.5.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Carlsson, J. M. Graphitic carbon nitride materials: variation of structure and morphology and their use as metal-free catalysts. J. Mater. Chem. 2008, 18, 4893.

ASSOCIATED CONTENT Electronic Supplementary Information (ESI) available: PseudoTernary phase diagrams, SEM images, nitrogen physisorption isotherms, XPS spectra, pore size distribution, band gap plot, Raman and IR spectra, XRD patterns, elemntal analysis results, photographs of the carbon nitride samples and the Rhodamine B solution. This material is available free of charge via the Internet at http://pubs.acs.org.”

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Lee, W. J.; Maiti, U. N.; Lee, J. M.; Lim, J.; Han, T. H.; Kim, S. O. Nitrogen-doped carbon nanotubes and graphene composite structures for energy and catalytic applications. Chem. Commun. 2014, 50, 6818–6830.

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Zheng, Y.; Liu, J.; Liang, J.; Jaroniec, M.; Qiao, S. Z. Graphitic carbon nitride materials: controllable synthesis and applications in fuel cells and photocatalysis. Energy Environ. Sci. 2012, 5, 6717–6731.

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Zhu, J.; Xiao, P.; Li, H.; Carabineiro, A. C. Graphitic carbon nitride: Synthesis, properties, and applications in catalysis. ACS Appl. Mater. Interfaces 2014, 6, 16449–16465.

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Zhao, Z.; Sun, Y.; Dong, F. Review Graphitic carbon nitride based nanocomposites: a review. Nanoscale 2014, 7 (1), 15–37.

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Sun, J.; Zhang, J.; Zhang, M.; Antonietti, M.; Fu, X.; Wang, X. Bioinspired hollow semiconductor nanospheres as photosynthetic nanoparticles. Nat. Commun. 2012, 3, 1139.

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Schwinghammer, K.; Tuffy, B.; Mesch, M. B.; Wirnhier, E.; Martineau, C.; Taulelle, F.; Schnick, W.; Senker, J.; Lotsch, B. V. Triazinebased carbon nitrides for visible-light-driven hydrogen evolution. Angew. Chemie - Int. Ed. 2013, 52, 2435–2439.

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Oh, Y.; Le, V.; Maiti, U. N.; Hwang, J. O.; Park, W. J.; Lim, J. Selective and regenerative carbon dioxide capture by highly polarizing porous carbon nitride. ACS Nano 2015, 9, 9148–9157.

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Xu, H.; Yan, J.; She, X.; Xu, L.; Xia, J.; Xu, Y.; Song, Y.; Huang, L.; Li, H. Graphene-analogue carbon nitride: novel exfoliation synthesis and its application in photocatalysis and photoelectrochemical selective detection of 2+ trace amount of Cu . Nanoscale 2014, 6, 1406– 1415.

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Hu, S.; Li, F.; Fan, Z.; Wang, F.; Zhao, Y.; Lv, Z. Band gap-tunable potassium doped graphitic carbon nitride with enhanced mineralization ability. Dalt. Trans. 2015, 44, 1084–1092.

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Kuriki, R.; Sekizawa, K.; Ishitani, O.; Maeda, K. Visible-light-driven CO2 reduction with carbon

AUTHOR INFORMATION Corresponding Author Correspondence should be addressed to Klavs F. Jensen: Email: [email protected]; Tel: 617-253-4589; Fax: 617-2588992.

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. M.P and M.L equally contributed to this manuscript.

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Funding Sources We gratefully acknowledge the financial support from MIT Energy Initiative through its seed fund program. This work made use of the MRSEC Shared Experimental Facilities at MIT, supported by the National Science Foundation under award number DMR-1419807.

Acknowledgements We appreciate the technical help from the MIT CMSE staff, the members of the Stephanopoulos group, and Lisi Xie for help with UV-Vis spectrometry. We are also grateful to the Román group and Strano group at MIT for sharing their instrumentation. We thank Dr. Baris Unal for helpful discussions regarding carbon nitride materials and templating techniques in the early parts of this effort.

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Facile Soft-Templated Synthesis of High-Surface Area and Highly

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