Synthesis of 13C-,15N-Labeled Graphitic Carbon Nitrides and NMR

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Synthesis of 13C‑,15N‑Labeled Graphitic Carbon Nitrides and NMRBased Evidence of Hydrogen-Bonding Assisted Two-Dimensional Assembly Yichen Hu,†,∥ Yeonjun Shim,‡,∥ Junghoon Oh,‡ Sunghee Park,‡ Sungjin Park,*,‡ and Yoshitaka Ishii*,†,§ †

Department of Chemistry, University of Illinois at Chicago, Chicago, Illinois 60607, United States WCSL (World Class Smart Lab), Green Energy Battery Laboratory, Department of Chemistry and Chemical Engineering, Inha University, Incheon 22212, Republic of Korea § School of Life Science and Technology, Tokyo Institute of Technology, Nagatsuta 4259, Midori-ku, Yokohama 226-8503, Japan ‡

S Supporting Information *

ABSTRACT: Graphitic carbon nitride (g-C3N4) has gained great attention as a material of promise for artificial photosynthesis. In place of synthesis of traditional three-dimensional g-C3N4 via polymerization of melamine or melem, recent studies seek to establish an alternative synthetic approach for two-dimensional g-C3N4 using a smaller precursor such as urea. However, the effectiveness of such a synthetic approach and resultant polymeric forms of g-C3N4 in this approach are still largely unknown. In this study, we present that solid-state NMR (SSNMR) analysis for 13C- and 15N-labeled g-C3N4 prepared from urea offers an unparalleled structural view for the heterogeneous in-plane structure of g-C3N4 and most likely for its moieties. We revealed that urea was successfully assembled in melem oligomers, which include extended oligomers involving six or more melem subunits. SSNMR, transmission electron micrograph, and ab initio calculation data suggested that the melem oligomer units were further extended into graphene-like layered materials via widespread NH−N hydrogen bonds between oligomers.

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INTRODUCTION Graphitic carbon nitride (g-C3N4), which is the most stable form among various types of carbon nitride, has been intensely studied for broad applications associated with its remarkable photocatalytic and photoelectronic properties.1−4 The characteristic electronic structure based on an alternating arrangement of carbon and nitrogen atoms defines its semiconductor properties.5−8 Due to a suitable bandgap,9,10 the photocatalytic efficiency using three-dimensional (3D) g-C3N4 in diverse chemical reactions is improved compared to traditional catalysts,11 such as metal oxides. In addition, π-conjugation that arises from sp2 hybridization of carbons and nitrogens allows for outstanding performance of this material in photoelectronic applications, such as in light-emitting devices, optical sensors, and photocathodes. Besides the exploitation of the use of g-C3N4, modification to its electronic structure by doping12,13 or functionalization5,14,15 has been explored to further enhance © 2017 American Chemical Society

the photocatalytic and photoelectronic performance. Recently, it was reported that various chemical modifications of the C3N4 network enable the generation of solution-processable twodimensional (2D) C3N4 materials,16−20 and its hybrid materials with other 2D materials or nanoparticles.21−26 With the advantages of 2D thin materials, this progress expands the range of possible applications for 2D and 3D C3N4-based materials.27,28 In this regard, chemical structures of C3N4-based materials are of great importance in order to understand their properties and to achieve the best performances in each application. It has been suggested that g-C3N4 systems are produced by polycondensation of various precursors such as urea, melamine, Received: January 6, 2017 Revised: May 20, 2017 Published: May 22, 2017 5080

DOI: 10.1021/acs.chemmater.7b00069 Chem. Mater. 2017, 29, 5080−5089

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Chemistry of Materials

Scheme 1. Four Previously Proposed Structures of Graphitic Carbon Nitride (g-C3N4): (a) 1D Melem Polymer Model, (b) 2D Melem Polymer Model, (c) 2D Hydrogen-Bonded Melem Trimer Model, and (d) 2D Model of Polymerized Melem Incorporating Melamine in Voids via Hydrogen Bonding

(Scheme 1d). However, STEM patterns resembling the proposed structure were confirmed only in trace amounts at some particular crystalline domains. On the whole, no concrete structural models based on experimental data are available for gC3N4. In this study, we present an approach to study detailed information on an in-plane structure of C3N4 materials by advanced 1H, 13C, and 15N triple-resonance SSNMR analysis through investigating 13C- and 15N-isotope-labeled g-C3N4 material, which was produced by a thermal condensation of fully 13C- and 15N-labeled urea, in combination with STEM analysis. We elucidate the basic structural units and their chemical network for the g-C3N4 system at a molecular level. The results indicate that this synthetic method yielded extended melem oligomers, including ones that involve six (or more) melem subunits, as major species. We also demonstrate NMRbased evidence that the melem oligomers are likely to form a 2D network through hydrogen bonding for the first time. The presented approach offers a powerful means to examine various 2D C3N4 materials derived from g-C3N4 systems synthesized from urea or other small precursors that can be 13C- and 15Nlabeled.

dicyandiamide, and/or their mixtures under high-temperature treatment.1,29−35 However, verifying the synthesis results has posed a challenge because the noncrystalline and heterogeneous nature of the systems has made it difficult to elucidate the chemical structures of g-C3N4 and its variants in general at the molecular level.29 A handful of different structural models for gC3N4 have been proposed previously. Solid-state nuclear magnetic resonance (SSNMR) has been an effective tool for various noncrystalline carbon and other materials.36−44 An SSNMR study on the reaction intermediates of g-C3N4, which were isolated from the heat treatment of various organic precursors such as cyanide and melamine, revealed the generation of tris-triazine units such as melem.45 However, this work provided structural information on intermediates rather than on bulk 3D g-C3N4. Lotsch et al.46 proposed a planar sheet model made of a hydrogen-bonded one-dimensional (1D) melem-containing polymer (melon) using melamine-driven materials based on experimental data by SSNMR and electron diffraction experiments (Scheme 1a), but direct proof of the intermolecular hydrogen-bond formation was lacking. Although detailed high-resolution SSNMR analysis with the magic-anglespinning (MAS) technique was performed, use of 15N and 13C cross-polarization (CP) methods made quantification of different chemical moieties difficult. Sehnert et al.47 presented a theoretical study on a group of possible structures based on melamine or melem subunits and their NMR parameters. They proposed 2D polymers of melem (Scheme 1b) or melamine. This 2D polymer model, however, does not explain the presence of NH2 or NH species indicated in the 15N SSNMR data in this reference. Thus, they proposed an alternate model of the hydrogen-bonded cyclic melon subunits, each of which is made of three melem groups; this model forms an oblique unit cell (Scheme 1c). Therein, the hydrogen bonds between melon units were postulated, yet without any experimental evidence. In a later electron diffraction study for a carbon nitride material produced by pyrolysis of melamine at temperature above 630 °C, Döblinger et al.48 proposed a planar network of polymerized melem that incorporates melamine molecules in the voids

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METHODS

Preparation of Isotope-Labeled and Naturally Abundant gC3N4 Materials. The g-C3N4 samples were separately synthesized from urea-13C, 15N2 (99.0 at. % 13C, 98.0 at. % 15N, Sigma-Aldrich), and urea (ACS reagent, 99.0%, Sigma-Aldrich) for 13C- and 15N-labeled and naturally abundant g-C3N4 samples, respectively. The urea precursor was placed in an alumina crucible with cover, and this crucible was placed in the middle of a quartz tube furnace (TFP-80-3, Dongseo Science Co., Ltd., Korea). The furnace became the vacuum state using a vacuum pump and then was filled with N2 gas (99.9%). This procedure was repeated three times. The precursor was calcined at 300 °C for 30 min at a heating rate of 3 °C/min in N2 gas flow; then, it was heated up (3 °C/min) to the designated temperature (550 °C) and held at the temperature for 1 h in N2 gas flow. SSNMR Spectroscopy. All of the SSNMR experiments were performed at a static magnetic field of 11.7 T with a Bruker Avance III 5081


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program (Revision B.01)50 on the Extreme cluster at UIC Research Computing. The structural models were first geometry optimized at the B3LYP/6-31G* level of theory.42,51−53 Chemical shielding tensors for 15 N of interest were calculated using GIAO54 with BPW91 and a 6311+G* basis set, which was optimized from a previous study on aminopyrimidines and aminobenzenes.55 The chemical shielding tensors for 15N were converted to the chemical-shift tensors as follows. An isolated ammonia molecule (15NH3) in a gas state was used as the theoretical reference compound. The isotropic chemical shielding of 15 N in an isolated ammonia molecule was calculated at the BPW91/6311+G* level of theory, after geometry optimization at the B3LYP/631G* level of theory. The calculated shielding for ammonia gas was converted to the shielding of liquid ammonia by adding the experimentally confirmed difference (−20.7 ppm) between the chemical shift of gas-phase ammonia at zero pressure and that of liquid-phase neat ammonia at 25 °C.56,57 This gave the calculated shielding of neat liquid ammonia, which was 236.6 ppm. The calculated shielding tensor values of each nitrogen species of our interest were then subtracted from the calculated isotropic shielding of liquid ammonia at the same level of theory in order to calculate the 15N chemical-shift tensor values. The average of the principal values of the chemical-shift tensor yielded the isotropic 15N chemical-shift value. 1H and 13C chemical shifts were calculated in a similar manner using a reference compound of an isolated methane.42 On the basis of the same structures geometry optimized at the B3LYP/6-31G* level, 1H and 13C GIAO chemical-shift calculations were conducted at the level of B3PW91/6-311G(d,p) and B3LYP/6311G, respectively. The shielding values calculated for methane were 31.72 ppm for 1H and 184.2 ppm for 13C. Instruments for Chemical Analysis. X-ray photoelectron spectroscopy (XPS) measurements were performed on an angle-resolved Xray photoelectron spectrometer (Theta probe, Thermo Fisher Scientific, UK). X-ray diffraction (XRD) data were obtained using a Rigaku D/max lllC (3 kW) diffractometer. Diffraction patterns were collected using a λ = 1.5406 Å X-ray beam in the range 5−90° (2θ). The Fourier-transforminfrared (FT-IR) spectra were obtained from KBr pellets containing gC3N4 powder samples using an FT-IR vacuum spectrometer (Bruker VERTEX 80 V, Bruker, Germany). Raman spectra were measured with a Horiba LabRAM HR Evolution spectrometer using a 1064 nm laser source. The energy dispersive X-ray spectroscopy (EDX) data were obtained using a high-resolution scanning electron microscope (HRSEM, SU8010, Hitachi) combined with an EDX analyzer (EX-250, HORIBA). Scanning transmission electron microscopy (STEM) analysis of the g-C3N4 samples was carried out on JEOL JEM-3100 instrument with an electron beam voltage of 300 kV and a relatively weak current density of 150 pA/cm2 at the UIC Research Resource Center. Approximately 2 mg of the sample in fine powder was added to 3 mL of methanol, and then, the suspension was sonicated for 5 min. A drop of the supernatant was delivered on the porous carbon grid and dried in ambient environment for a subsequent STEM analysis performed under vacuum.

wide-bore NMR spectrometer with a Bruker 3.2 mm triple-resonance Efree MAS probe at a 1H NMR frequency of 500.25 MHz. The uniformly 13 C- and 15N-labeled g-C3N4 sample of 16.6 mg was packed in a MAS rotor for SSNMR. All of the 13C chemical shifts were referenced to neat TMS using the secondary reference of the adamantane CH2 peak at 38.48 ppm,49 and the 15N chemical shifts were referenced to liquid 15 NH3 at 0 ppm using the indirect referencing of a 13C signal. The temperature of VT cooling air was set to 275 K at a flow rate of 1200 L/h. The MAS frequency was set to 22 000 ± 10 Hz. For 1H decoupling of 13 C or 15N SSNMR, a TPPM scheme was employed with an RF field strength of 90.9 kHz with pulse flip angle of 5.5 μs and phase switching between 0° and 25°. The 1D 13C and 15N MAS spectra in Figure 2a,c were collected with excitation by a π/2-pulse of 5 and 6 μs widths, respectively. For each of these 1D experiments, a rotor-synchronous echo scheme was used with an interval of two rotation periods (90.9 μs) prior to the signal acquisition after the excitation π/2-pulse, and a π-pulse was applied in the middle of the interval. 13C and 15N signals of 16 and 32 scans were, respectively, accumulated with or without 1H RF decoupling. The spectra were apodized with line broadening of 100 Hz for 15N while no line broadening was applied for 13C. Recycle delays of 1000 s were employed for both 13C and 15N. Specifically, for the 15N 1D data used for component deconvolution in Figure 3b, recycle delays of 5000 s were used, and signals of 112 scans were accumulated. Lorentzian broadening of 100 Hz was applied for this 15N spectrum. The 13C/15N CP-MAS spectra in Figure 2b,d were collected with 4 scans for 13C and 16 scans for 15N. The 1H−13C and 1H−15N contact times were 2.2 and 1.4 ms, respectively. RF intensity for the initial 1H 90° pulse was 62.5 kHz for both experiments. During the CP period for 13 C CP experiments, RF intensities were 57.9 and 37.1 kHz for 1H and 13 C, respectively, while for 15N CP experiments, the RF intensities were 55.0 and 35.2 kHz for 1H and 15N, respectively. No line broadening was used for the 13C spectrum; Lorentzian broadening of 100 Hz was applied to the 15N spectrum. For the 2D 13C−15N correlation experiment in Figure 3a, 80 t1 complex points were acquired with 32 scans for each real or imaginary t1 data point with a t1 increment of 34.09 μs. The carrier frequencies for the 13 C, 15N, and 1H channels were set at 100, 75, and 0 ppm, respectively. The 1H−15N CP mixing time was 2 ms, and the 15N−13C mixing time was 1 ms. During the 1H−15N CP, RF intensities were 55.0 and 35.2 kHz for 1H and 15N, respectively. For the latter 15N−13C CP, the intensities were 35.2 kHz for 15N and 52.1 kHz for 13C. For both the t1 and t2 periods, 1H RF decoupling was applied. During the t2 period, 13C signals were collected in an acquisition period of 5.12 ms. The spectra were processed without any line broadening. The total experimental time was 7.1 h. For the 2D 15N−1H correlation experiment in Figure 6a, 72 t1 complex points were collected using a mixing time of 0.2 ms with 16 scans for each real or imaginary t1 data point with an increment of 11.36 μs. For the CP part of the sequence, the 1H RF field was 54.5 kHz, while the 15N RF intensity was 33.0 kHz. During the t2 period, 15N signals were collected in an acquisition period of 3.84 ms with 1H decoupling. The carrier frequencies for 15N and 1H were set at 50 and 0 ppm, respectively. No line broadening was applied in processing. The total experimental time was 3.2 h. For the 2D NHHN experiments in Figure 5a and Supporting Information Figure 2a, 96 t1 complex points were collected with 32 scans for each real or imaginary t1 point with an increment of 45.45 μs and recycle delays of 5 s. The initial 1H−15N CP period to prepare 15N polarization was 2 ms. The second and third 15N−1H CP periods and the 1 H−1H mixing time were specified in the caption of each figure. A unified RF field condition for all CP periods was 54.5 kHz for 1H and 33.0 kHz for 15N. During the t2 period, 15N signals were collected in an acquisition period of 5.12 ms with 1H decoupling. The carrier frequencies for 15N and 1H were set at 80 and 3.5 ppm, respectively. The spectra were processed with Lorentzian broadening of 200 Hz for both 15N dimensions. The total experimental time was 8.5 h each. Ab Initio Calculations. Structural optimizations and 15N, 1H, and 13 C chemical-shift calculations were performed using the Gaussian 09

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RESULTS AND DISCUSSION The 13C- and 15N-isotope-labeled g-C3N4 powder was produced by heat treatment of 13C- and 15N-labeled urea at 550 °C under a N2 environment (see Methods section). For comparison, naturally abundant g-C3N4 was also produced by the same method using unlabeled urea. The XPS, XRD, FT-IR, EDX, and Raman spectroscopy measurements of naturally abundant gC3N4 showed characteristic patterns for typical g-C3N4 materials (see detailed discussion for Supporting Information Figures 3− 5). XPS and FT-IR spectra of the labeled sample showed the peak assignments identical to those of the natural one. These analyses support that the g-C3N4 network was successfully generated in the isotope-labeled sample. To capture microscopic morphologies of the system, STEM images of the g-C3N4 material with different magnifications were obtained (Figure 1). The sample was sonicated in methanol prior to the STEM experiment 5082


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delays. In the spectrum, two peaks with equal integral intensity at C chemical shifts of 164 and 157 ppm were detected. This result agrees reasonably well with 13C chemical shifts suggested by our ab initio calculations for the melem subunit in g-C3N4 polymer. The calculated shifts of 13Ce and 13Ci of melem (see Figure 2e) by our ab initio calculations are, respectively, in the ranges of 161−168 and 149−153 ppm, as discussed below. The possibility of generation of a melamine-like structure in our gC3N4 (see Figure 2f) was ruled out as only one type of carbon species predicted from the melamine structure contradicts our observation of two major carbon species at an equal molar ratio as shown in Figure 2a. The profile of the 13C spectrum remained the same when the 1H decoupling power was turned off (Figure 2a, red). Removing 1H decoupling generally introduces line broadening for protonated 13C species due to strong 13C−1H dipolar spin−spin couplings, which are proportional to 1/R3, where R denotes an internuclear 1H−13C distance. Thus, this result indicates that these two carbon species are not protonated, which is consistent with the structure of the melem subunit. In a 13 C CP spectrum with 1H decoupling (Figure 2b, blue), the signal intensity ratio varies due to different proximities to 1H species as 13C spin polarization that generates NMR signals is transferred from neighboring 1H species in the CP method via a 1 H−13C dipolar coupling. Again, this spectral profile remained the same without 1H decoupling. The more intense peak at 164 ppm should originate from Ce; the signal at 157 ppm was assigned to Ci, given that Ce is generally closer to protons of nonpolymerized NH2 or partially polymerized NH species. An example of polymerized g-C3N4 having terminated NH2 and NH groups at the outside edges is shown in Figure 3c. Next, we examined a 15N SSNMR spectrum for this sample (Figure 2c) that was collected by π/2-pulse excitation with a rotorsynchronous echo pulse sequence. Note that all the 15N shifts in this work were referenced to ammonia in the liquid state.49,58 The spectrum in Figure 2c was collected with long recycle delays of 1000 s, which is comparable to the 15N T1 values (615−816 s). Again, without 15N labeling, an equivalent experiment for 15N species at low natural abundance (0.36%) would take ∼80 years. The spectrum indicates four distinct 15N peaks. The ab initio calculated 15N chemical shifts (presented in a later part) allowed us to assign each peak to a nitrogen species. The strongest peak at 200 ppm was easily assigned to Nc, while the peak at 157 ppm was assigned to Ni. The other two nitrogen peaks at 137 and 110 ppm were attributed to the NH and NH2 moieties, respectively. These assignments are consistent with signal intensities observed for 15 N CP spectra (with 1H decoupling, Figure 2d, blue), in which the NtH and NtH2 peaks were pronounced (due to more efficient CP transfer from 1H) compared with the nonprotonated Nc peak. In the CP spectrum, the signal was very weak for Ni (orange arrow) due to its remote position from any protons. Additionally, we confirmed that both NtH and NtH2 peak intensities were attenuated without 1H decoupling (Figure 2d, red) because of broadening; the attenuation was notable especially for the NH2 resonance. We also confirmed the assignment for NtH and NtH2 groups based on the protonation state by constant time 15N−1H rotational echo double resonance (REDOR) experiments (see Supporting Information Figure 1). Although the assignments from the data in Figure 2 are reasonable, these 1D experiments do not offer information on the chemical connectivity between C and N atoms. Thus, we elucidated the connectivity of 15N and 13C species by 2D 15 N−13C correlation experiments for the 13C- and 15N-labeled g13

Figure 1. STEM micrographs of 13C- and 15N-labeled g-C3N4 sample in three scales. (a) A typical region representing the dimension and morphology of network. (b) Edge architecture where a few layers are stacked. (c) A monolayer flake with distinguishable pattern of constituent subunits.

(details in the Methods section). Large 2D materials of up to a few square microns were observed (Figure 1a). The material was not perfectly planar; rather, it showed some characteristics of corrugation, as will be reproduced below in an optimized ab initio model (Supporting Information Figure 6). Another STEM image with a higher magnification (Figure 1b) indicated a possible stacking of multiple sheets, where several sheets partially overlap. However, the interlayer affinity is likely not so strong since it was not difficult to find single sheets at some STEM images (Figure 1c). To examine the molecular-level architecture of the g-C3N4 system, we performed a series of SSNMR analyses of the isotopelabeled g-C3N4 sample. First, we collected a 13C MAS SSNMR spectrum using π/2-pulse excitation, a rotor-synchronous echo pulse, and 1H decoupling (Figure 2a, blue). It should be noted

Figure 2. SSNMR spectra of 13C- and 15N-labeled g-C3N4 and structures of candidate subunits. (a, b) 13C and (c, d) 15N 1D SSNMR MAS spectra obtained with (blue) and without (red) 1H dipolar decoupling by (a, c) π/2-pulse excitation and (b, d) cross-polarization (CP) from 1H to 13 C/15N spins. (e, f) Chemical structures of (e) melem with notations of carbon and nitrogen species and (f) melamine. Experimental times were (a) 4.4 h, (b) 20 s, (c) 8.9 h, and (d) 1.3 min for each of the blue and red spectra.

that by taking advantage of the 13C- and 15N-labeled system, we collected the data with long recycle delays of 1000 s for a quantitative spectral analysis. From a series of experiments with different delays, the 13C T1 values of the sample were found to be in the range 269−277 s. The spectral analysis is quantitative as the delays were set more than 3 times that of the 13C T1 values, which guarantees a full recovery of all the signals (>95%) without variations from species to species. Without 13C labeling, an equivalent experiment for 1% naturally abundant 13C would take over 4 years (i.e., 104 times longer). Since previous studies were performed on unlabeled samples, lack of sensitivity made it nearly impossible to employ such quantitative analysis with long 5083


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Figure 3. SSNMR 13C−15N connectivity identification and 15N component decomposition. (a) A 2D 15N−13C correlation SSNMR MAS spectrum of the 13C- and 15N-labeled g-C3N4 sample. (b) A 1D 15N SSNMR MAS spectrum of the same sample obtained by a π/2-pulse excitation (blue) with spectral deconvolution into 5 spectral components (green, Nc1, Nc2, Ni, NtH, and NtH2). The spectrum was obtained with a rotary echo using long recycle delays of 5000 s/scan. The red spectrum denotes a sum of the deconvoluted subspectra, which fits nearly perfectly to the experimental data. (c) An example of the polymerized network based on melem. Colored dashed bonds represent atomic connectivity corresponding to the dashed lines in part a.

Table 1. Comparison of Nitrogen Species Ratiosa

a

exptl data or type of models

Nc

Ni

NtH

NtH2

Nt‑tert

no. of melem subunits

exptl (Figure 3b) structural models (Figure 4)

5.994 (±0.044) 6.000 6.000 6.000 6.000 6.000 6.000

1.000 (±0.007) 1.000 1.000 1.000 1.000 1.000 1.000

1.154 (±0.009) 0.000 1.000 1.000 1.333 0.900 1.000

0.634 (±0.005) 3.000 1.000 0.500 0.333 0.300 1.000

0.019 (±0.009) 0.000 0.000 0.167 0.000 0.300 0.000

1 3 6 9 10 ∞

(a) (b) (c) (d) (e) (f)

Ratio for deconvoluted experimental spectrum (top) and ratios calculated from the models (a−f) in Figure 4 are listed.

C3N4 sample (Figure 3a). The signal for Ce at ∼164 ppm exhibits strong spatial correlations with NtH2, NtH, and Nc species as each of these nitrogen species is chemically bonded to Ce in the melem subunit. Interestingly, the 2D 15N−13C spectrum suggests that the 13Ce species exhibits two distinctive 13C shifts at 164 and 166 ppm, which, respectively, correspond to the 13C shifts for 13Ce bonded to NtH and NtH2; pink and orange dashed lines in Figure 3a clearly indicate the connectivity from 13Ce to 15NtH at 136 ppm (pink) and 15NtH2 peaks at 113 ppm (orange), respectively. Also, the two 13Ce peaks at 164 and 166 ppm, respectively, show connectivity to two 15Nc peaks at 200 and 190 ppm, which were assigned to 15Nc species that are directly bonded to Ce and are two bonds away from either NtH or NtH2, respectively. We initially expected connectivity from 13Ce to nonprotonated 15Nt (Nt‑tert), which should have a similar shift to 15Nc at 190 ppm according to ab initio calculations as discussed later. However, no such isolated correlation was observed presumably because CP polarization transfer from 1H did not produce strong signals for nonprotonated Nt‑tert (arrows in Figure 3c), which should be at least seven bonds away from 1H as shown in Figure 3c. The two discrete Ci−Nc cross peaks at (ωC, ωN) of (157 ppm, 200 ppm) and (156 ppm, 190 ppm) also suggest a relatively mild effect on the 13Ci shift of γ-substitution of NH/NH2. Due to longer distances between Ci and NH/NH2 (three bonds away) and a relatively short contact time between 15N to 13C (1 ms), only a weak 13Ci−15NH cross peak was observed at (ωC, ωN) of (157 ppm, 137 ppm), while a 13Ci−15NH2 cross peak was not observed. Finally, a very weak cross peak between 13Ci and 15Ni was observed at (ωC, ωN) of (157 ppm, 157 ppm) presumably because Ni is remote from protons (five bonds away from NH or

NH2 protons). These 2D data have confirmed that a melem group is nearly a sole constituent subunit of the g-C3N4 material, confirming that our synthetic route with 13C- and 15N-labeled urea successfully produced a well-defined 13C- and 15N-labeled gC3N4 system. The data also have demonstrated that protonated amine groups (either NtH or NtH2) are present at the terminus of the subunits. As discussed below, despite the indication that relatively large melon involving 6 or more melem subunits is formed via polymerization, nonprotonated Nt‑tert appears to be only a minor species. To further examine the polymerization state of the melem group, we performed a quantitative analysis of the 1D 15N MAS SSNMR spectrum of the g-C3N4 sample in Figure 3b (blue). It should be noted that the 1D 15N SSNMR spectrum was collected with very long recycle delays of 5000 s, which is greater than 5 times the 15N T1 values (615−816 s) and guarantees 99.3% signal recovery; thus, the 15N spectrum is suited for a quantitative spectral analysis. The obtained spectrum was deconvoluted into four components (Nc + Nt‑tert, Ni, NtH, NtH2) (Figure 3b, green), where “Nc + Nt‑tert” denotes a sum of the overlapping signals of 15 Nc and 15Nt‑tert, which are both nonprotonated. In Figure 3a, we did not identify Nt‑tert. As discussed below, we found that only a very limited amount of Nt‑tert species is present in this sample. On the basis of the 2D spectrum, the Nc (including Nt‑tert) components were represented by two separate peaks at 190 and 200 ppm. The normalized integral intensities of (I(Nc) + I(Nt‑tert), I(Ni), I(NtH), I(NtH2)) were found to be (6.013, 1.000, 1.154, 0.634), where I(NX) denotes a normalized integral signal intensity for a species NX. The signal was normalized with respect to Ni as each melem subunit has one 15Ni. Considering 5084


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a mixture of the 1D polymer model and the oligomeric models. Thus, we excluded this model (f) from the following analysis. The presence of a unit like model c also cannot be excluded as a small fraction of Nt‑tert is likely to exist on the basis of our quantitative analysis. Therefore, the g-C3N4 system should involve relatively large oligomeric units involving six or more melem subunits such as ones shown in Figure 4c,d as major species. The results also explain the elemental C/N ratio that was calculated from XPS measurements (Supporting Information Figure 3a,b). For the model in part c or d in Figure 4, its theoretical C/N ratio is 0.692 while the ratio for the model in part b is 0.667. The ratio of fully polymerized g-C3N4 is 0.750. In the XPS analysis, that of our urea-driven g-C3N4 is 0.67, indicative of the major presence of the six or more melem subunits in g-C3N4. Finally, we will examine a detailed molecular-level mechanism of the 2D sheet formation for the g-C3N4 system. Previous studies hypothesized that self-assembly of melem-based cyclic oligomers (Figure 4b) through intermolecular hydrogen bonding may result in an extended sheet structure.47 However, an experimental basis of such intermolecular interactions is scarce. In order to examine intermolecular contacts of the melem oligomer, we conducted 2D NHHN correlation experiments61 for different 1H−1H mixing times. In this experiment, 15N polarization is transferred to directly bonded 1H by CP, and then during the 1H−1H mixing time, the 1H polarization is transferred to a remote 1H species; the signal is detected at 15N that is directly bonded to the remote 1H after another 1H−15N CP period. Thus, the presence of 1H−1H contacts is detected as 15 N−15N cross peaks. Specifically, our aim is to detect shortrange intermolecular contacts between NtH and NtH2 groups. Figure 5b represents an ab initio structurally optimized model of two interacting triangular assemblies (model c in Figure 4) for modeling a 2D assembly of the melem oligomers. The intermolecular N−N distances between NH and NH2 (solid orange arrows) were identified to be short (∼2.7 Å). On the other hand, the intramolecular N−N distances between NH and NH2 within the same oligomer unit are all greater than ∼7 Å (dotted orange arrows in Figure 5b). These intramolecular contacts should not yield any cross peaks with a short 1H−1H mixing time of 0.5 ms, for which 1H−1H contacts within 3 Å typically produce cross peaks.61 Short N−H or H−N CP contact times of 0.2 ms were used in order to ensure the transfer of

that NtH2 belongs to a single melem unit, the NH group is shared by two melem subunits, and Nt‑tert is shared by three of them; the network of a melem oligomer unit requires a relationship of 3I(Ni) = I(NtH2) + 2I(NtH) + 3I(Nt‑tert). With this relationship and the experimentally measured integral intensities, we deduced I(Nt‑tert) of 0.019 ± 0.009 and I(Nc) of 5.994 ± 0.044. These results are summarized in Table 1. The ratio of I(Nc)/I(Ni) of 5.994 is consistent with the expected value of 6.000 for ideal melem units, confirming that the quantitative analysis is most likely to be accurate. Thus, it is likely that the Nt‑tert is barely present in the sample. This finding is inconsistent with a model of infinite 2D polymerization such as that in Scheme 1b. The results suggest that the degree of polymerization is likely to be finite despite large 2D networks observed in the STEM image (Figure 1) as discussed below. In Figure 4, possible models of oligomeric

Figure 4. Possible structural arrangements of the g-C3N4 network having different connectivities of melem subunits. The chemical structure of the melem subunit is shown at the top.

units and an infinitely elongated 1D polymer model made of melem subunits59,60 are listed. None of the 15N species ratios from these possible models can precisely reproduce the deconvolution ratios (see Table 1). Among these models, models b−d yield the ratios that are close to the experimental values. For example, a one-to-one mixture (w/w) of b and d would reproduce the experimentally obtained ratios in Table 1 reasonably well. A 1D polymeric carbon nitride59,60 model connected by hydrogen bonds in Figure 4f does not reproduce the ratio for NtH2 well. As the 1D polymer model appears to be structurally incompatible with any other models in Figure 4, it is difficult to explain the 2D sheet formation via self-assembly from

Figure 5. Interoligomer NH to NH2 contact measurement and schematic illustration. (a) 2D NHHN spectra of the 13C- and 15N-labeled g-C3N4 sample with 1H−1H mixing times of 0 ms (black) and 0.5 ms (red). The H−N and N−H contact times for the NHHN transfer were set to 0.2 ms to minimize long-range polarization transfer from 1H to 15N and that from 15N to 1H, respectively. (b) A diagram showing two interacting triangular melem oligomers with suggested intermolecular contacts between NH and NH2 groups (solid orange arrows). Corresponding intramolecular contacts (dotted orange arrows) are too long to explain the cross peak in part a. Hydrogen bonds are denoted by green dashed lines. 5085


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Figure 6. 15N−1H 2D SSNMR spectrum of g-C3N4 and ab initio simulated spectra for two possible models. (a) An experimental 2D 15N−1H correlation spectrum compared with (b, c) corresponding simulated 2D spectra for (b) a monomeric model and (c) a dimeric model, which are shown in Figure 7a,b, respectively.

Figure 7. Ab initio chemical-shift calculations of two fully relaxed models after geometry optimization. Evident 15N and 1H shift changes that, respectively, exceed 2.0 and 0.20 ppm are highlighted in bold as detailed below. (a) Monomeric and (b) dimeric models with 15N (light blue) and 1H (red) shifts. For a comparison with the shifts in part a, the shift values in part b are displayed only when 15N and 1H shifts deviate more than 2.0 and 0.20 ppm, respectively. (c) Monomeric and (d) dimeric models with 13C chemical-shift values (black).

polarization between directly bonded 15N−1H pairs for N−H and H−N−H groups during the N−H polarization transfer periods.62 When the 1H−1H mixing time was set to 0.0 ms, only diagonal peaks and relatively weak cross peaks can be detected (Figure 5a, black). With an increased mixing time of 0.5 ms (Figure 5a, red), cross peaks between NH and NH2 were identified (orange arrows); the results suggest short 1H−1H contacts within 3 Å,61 which cannot be explained by intramolecular contacts within a melem oligomer unit. Therefore, the intense cross peaks provide evidence of interoligomer interactions. The close contact was likely realized via hydrogen bonds between NtH or NtH2 of one oligomer and Nc of another. Two triangular oligomers can be matched up in planar geometry with each other with alternating hydrogen bonds shown in green

dashed lines in Figure 5b. Additional 2D NHHN data also supported the contact (Supporting Information Figure 2). To the best of our knowledge, this is the first experimental evidence of intermolecular hydrogen-bonding interactions for g-C3N4. It should be noted that the optimized structure is not perfectly planar (Supporting Information Figure 6), which rather indicates some modulation. The results are consistent with long-range corrugation observed in our STEM image (Figure 1). Additional evidence of intermolecular hydrogen bonding in gC3N4 was obtained from a 1H chemical-shift analysis. Figure 6a shows a 2D 15N−1H correlation experiment using a very short CP contact time of 0.2 ms. The spectrum indicates cross peaks at (ωN, ωH) of (136 ppm, 8.9 ppm), and (113 ppm, 8.9 ppm) within 15N−1H and 15N−1H2 groups, respectively. To evaluate 5086


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Chemistry of Materials the obtained chemical shifts, we constructed two structural models shown in Figure 7 for (a) an isolated triangular oligomer and (b) two interacting triangular oligomers, the latter of which represents a simple model for possible two-dimensional layer formation. Both models showed stability in geometry optimization. Then, 1H, 13C, and 15N ab initio chemical-shift calculations were performed for the fully stabilized models and their chemical network.42 Figure 6b,c shows simulated peak positions for a 2D 15N−1H correlation experiment from directly bonded groups (i.e., Nt−H and H−Nt−H) and hydrogenbonded N···H contacts that only exist in the dimeric triangular model. The results clearly show consistency with the experimental data only for the dimeric model with the simulated 15 N−1H 2D correlation map displayed in Figure 6c. In NtH2 groups associated with hydrogen-bonded neighboring unit (Figure 7b), the calculated 15N chemical shift is ∼100 ppm (light blue values), which reasonably reproduces the experimental value (∼110 ppm). In contrast, in the non-hydrogenbonded NtH2 groups (Figure 7a), the 15N chemical shifts of ∼85 ppm notably deviate from that of a major component of the 15 NtH2 peak at ∼110 ppm in Figure 6a. In addition, 1H ab initio chemical-shift values (red values in Figure 7b) also verify the presence of H-bonds, by which all protons in the H-bonds exhibit a shift around 9 ppm while the non-H-bonded 1H shows a shift of ∼4.5 ppm. As a result, the simulated 2D 15N−1H spectrum for an isolated melen (Figure 6b) does not reproduce the experimental data. These data further confirm that a majority of the NtH2 groups are involved in the hydrogen-bonding interactions. These findings explain well the primary 1H component at around 9 ppm with the shoulder resonance at ∼4.4 ppm in the 1D spectrum and the cross peak for 15NtH2 and 1H correlation that possesses a side component for ∼4.5 ppm 1H in the 15N−1H 2D spectrum (Figure 6a). Moreover, the ab initio 13C chemical shifts (Figure 7c,d) are consistent with 1D 13C experimental spectra in Figure 2a,b by which a subunit of melem was proposed, although 13C shifts are insensitive to the hydrogen bonding. Overall, these results suggest that hydrogen bonds are extensive in the material, and they offer the cohesive force to construct this expansive network.

extensive 2D g-C3N4 network as a consequence of ubiquitous hydrogen bonds should be favored in studies associated with enhancing the efficiency in photocatalytic reactions, and generating flexible electronic and optical devices. Detailed structural knowledge of the g-C3N4 material from the study will enable us to understand the relationship between physical properties and chemical structures and would offer a novel path for doping and functionalization of g-C3N4 for altered electronic structures. Although other synthetic approaches may produce different types of g-C3N4 structures,63 the presented framework of NMR-based structural analyses with a novel isotope-labeling scheme for g-C3N4 is most likely applicable to a variety of gC3N4-based systems in studies for their functionalization.

CONCLUSIONS In this work, we have investigated the structural characteristics of a g-C3N4 material synthesized from a urea precursor by thermal condensation. By SSNMR analyses together with STEM, XPS study, and ab initio calculations, we observed evidence indicating that melem is likely the underlying and exclusive subunit for the g-C3N4 material. Melem subunits were polymerized to produce oligomers that were composed of 3−9 melem subunits. We discovered that the material reproducibly assembled into a large 2D C3N4-based structure via prevailing hydrogen bonds between oligomers. 15N and 1H SSNMR data and ab initio calculations provide a convincing proof of the extensive formation of hydrogen bonds from NH or NH2 at the edges of melem-based oligomers to Nc atoms from neighboring oligomers. Our analysis indicated that a majority of the Nt groups are protonated and involved in a hydrogen-bonding bridge with neighboring melem oligomers, leaving a minor portion of Nt groups that are not involved in hydrogen bonding. Despite the oligomeric nature of the subunits, the assembled system retains a 2D structure composed of sp2 C and N atoms, which is consistent with the term graphitic carbon nitride. The STEM images in Figure 1 indicate stacking of the sheets. The

Funding

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b00069. Additional NMR and characterization details (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +82-32-860-7677. *E-mail: [email protected]. Phone: +81-45-924-5817. ORCID

Sungjin Park: 0000-0002-1447-4536 Yoshitaka Ishii: 0000-0002-7724-6469 Author Contributions ∥

Y.H. and Y.S. contributed equally to this work. Y.H. conducted SSNMR and STEM experiments, ab initio simulations, and data analyses for the SSNMR experiments and simulations under the supervision of Y.I. Y.I. analyzed the SSNMR and STEM data and directed the characterization project. Y.S, J.O., Sunghee Park, and Sungjin Park established and performed the synthesis of the gC3N4 samples, and Y.S. conducted XPS, XRD, and FT-IR analysis under the supervision of Sungjin Park. Y.H., Y.S., Y.I., and Sungjin Park wrote the paper.

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Y.H. and Y.I. were supported in this study by funding from the U.S. National Science Foundation (NSF) for Y.I. (CHE 1310363). Y.S, J.O., Sunghee Park, and Sungjin Park were supported by grants from the Center for Advanced Soft Electronics as a Global Frontier Project (CASE2013M3A6A5073173) and Basic Science Research Program (NRF-2015R1C1A1A02036958) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Alan Nicholls at the UIC Research Resource Center-Electron Microscopy Service Facility for his assistance in the STEM analysis.

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REFERENCES

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