Molecule Self-Assembly Synthesis of Porous Few-Layer Carbon

Jan 17, 2019 - ... Material Chemistry, Ministry of Education of the People's Republic of ... Here, we report a simple bottom-up method to synthesize p...
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Molecule Self-Assembly Synthesis of Porous Few-Layer Carbon Nitride for Highly Efficient Photoredox Catalysis Yuting Xiao,† Guohui Tian,† Wei Li,*,‡ Ying Xie,† Baojiang Jiang,† Chungui Tian,† Dongyuan Zhao,‡ and Honggang Fu*,†

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†

Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education of the People’s Republic of China, Heilongjiang University, Harbin 150080, PR China ‡ Department of Chemistry, Laboratory of Advanced Materials, Shanghai Key Lab of Molecular Catalysis and Innovative Materials, iChEM and State Key Laboratory of Molecular Engineering of Polymers, Fudan University, Shanghai 200433, PR China S Supporting Information *

ABSTRACT: Polymeric carbon nitride (C3N4) has emerged as the most promising candidate for metal-free photocatalysts but is plagued by low activity due to the poor quantum efficiency and low specific surface area. Exfoliation of bulk crystals into ultrathin nanosheets has proven to be an effective and widely used strategy for enabling high photocatalytic performances; however, this process is complicated, time-consuming, and costly. Here, we report a simple bottom-up method to synthesize porous few-layer C3N4, which involves molecule self-assembly into layered precursors, alcohol molecules intercalation, and subsequent thermalinduced exfoliation and polycondensation. The as-prepared few-layer C3N4 expose more active sites and greatly enhance the separation of charge carriers, thus exhibiting a 26-fold higher hydrogen evolution activity than bulk counterpart. Furthermore, we find that both the high activity and selectivity for the oxidative coupling of amines to imines can be obtained under visible light that surpass those of other metal-free photocatalysts so far.

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INTRODUCTION Polymeric carbon nitride (C3N4) is an attractive candidate for metal-free photocatalysts because of its appealing electronic structure, good thermal and chemical stability, environmental benignity, and facile synthesis from abundant and inexpensive starting materials.1−3 Since the first discovery of Wang and coworkers on photocatalytic hydrogen production over carbon nitride, a considerable amount of effort has been contributed to the development of polymeric carbon nitride-based photocatalysts, by virtue of their promising potential in solar energy conversion and chemical synthesis.4−8 Unfortunately, the low electronic conductivity, wide bandgap, rapid chargecarrier recombination behavior as well as the fairly small accessible surface active sites have greatly limited its application in water splitting and photosynthesis.9,10 A variety of strategies such as heteroatom doping,11,12 heterojunctions13,14 and nanostructuring design15,16 have been developed to effectively circumvent these challenges to improve the catalytic and electronic performances of C3N4. Among them, reducing the dimension from bulk crystals to nanosheets of C3N4 has attracted much attention,17−19 as the ultrathin nature can provide a larger specific surface area, more exposed active sites, and better access for electrolytes and shorter diffusion paths for both ions and electrons.20,21 More strikingly, as a result of the quantum confinement effect, the electronic band gap would be enlarged by shifting band edges in opposite © XXXX American Chemical Society

directions for contributing a stronger redox ability toward photocatalytic reactions, and the photophysical behavior would be changed for prolonging the lifetime of charge carriers. Substantial efforts in recent years have aimed at efficiently creating few-layer and even single-layer C3N4.17−19,22 Inspired by the delamination of graphene from bulk graphite, many exfoliation methods have been explored to produce C3N4 nanosheets.23−25 Nevertheless, this top-down process, in which high-quality bulk crystals need to be prepared first, is complicated, time-consuming, and costly. And the planar atomic structure would be seriously destroyed due to the weak hydrogen bond between strands of polymeric melem units, resulting in unfavorable defects that increase the recombination rate. Moreover, the ultrathin nanosheets easily tend to restack to form microsized powders or films due to the strong π−π interaction, which severely reduces the accessibility to the active sites and the rate of ion diffusion, thus leading to the unsatisfactory photocatalytic performances. In contrast, the bottom-up strategies have shown great promise toward production of high quality 2D materials, but rarely demonstrate the possibility for porous C3N4 nanosheets. Herein, we introduce a simple and low-cost bottom-up method for production of porous few-layer C3N4, obtained by Received: November 19, 2018 Published: January 17, 2019 A

DOI: 10.1021/jacs.8b12428 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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C3N4 stripes are obtained when glycol and glycerol are used as the insertion agents (Figure S6). That may be due to the low boiling point of methanol and ethanol compared with polyols, which are easily evaporated, thus leading to insufficient exfoliation. In this regard, mixture of polyols and ethanol are explored to address the problem because the mixture would compensate the inherent shortage of each component. It was found that the C3N4 was successfully delaminated into nanosheets using the mixture of ethanol and glycerol as the insertion agents (Figure S7), suggesting the polyols can be introduced into the interlayer accompany with ethanol, simultaneously the addition of polyols facilitates the exfoliation of C3N4. In addition, the inserting temperature, time, and the volume ratio of the mixture solvents were also optimized to obtain porous few layer C3N4 (Figure S8−S10). To investigate the thermal-induced condensation and exfoliation process, thermogravimetric analysis-mass spectroscopy (TGA-MS) measurement was conducted (Figure S11). The expanded precursors show a slight weight loss (about 2.4%) below 300 °C, which is attributed to the evaporation of absorbed molecules on the surface. A dramatic weight loss was observed in the range of 300−450 °C, which is assigned to the polycondensation of the supramolecular precursors. In this process, a variety of gas released, including NH3, NO2, N2O/ CO2, C3H8O3, and C2H6O. The presence of ethanol and glycerol demonstrates the insertion of alcohol molecules, which plays a vital role in preparation of C3N4 nanosheets. For comparison, the TGA-MS of pristine precursors and melamine was carried out. There is no signal for ethanol and glycerol, but other gases are detected as the inserted ones (Figure S12 and Figure S13). Moreover, temperature- and time-dependent experiments further (Figure S14) confirm the thermal-induced condensation and exfoliation. When the calcination temperature increased from 300 to 500 °C, the microrod precursors transformed to aggregated sheets indicating the released gas assists the expansion of the stacked layers. During the calcination process, a large quantity of gaseous reaction products were produced leading to the internal gas pressure between precursors layers increases sharply, which causes the breakage of π−π interaction cohered strands of layers. Meanwhile, the released gas and volume shrinkage of precursor during thermal polycondensation process would create many pores on the layers, finally producing porous few-layer C3N4. Morphology and Structure Characterization. If not mentioned, the reported porous few-layer C3N4 was obtained by using the mixed solvent of ethanol and glycerol with a volume ratio of 3:1. As a control, the bulk C3N4 was synthesized through the widely used polycondensation method by using melamine as the precursor (Figure S15a). In addition, microtube C3N4 was obtained with cracked stripes by directly annealing layered precursors (Figure S15b). The SEM shows the representative sheet-like morphology for few-layer C3N4 with a size of tens of micrometers (Figure 2a). The sheets tend to bend and curve, which helps to lower the surface energy. The optical image confirms the volume expansion in the case of few-layer C3N4, compared with bulk and microtube C3N4 under the same mass (Figure S16). The few-layer structures can be further disclosed by the transmission electron microscopy (TEM) image. It is clear that nanosized pores ranging from ∼2 to 15 nm are distributed throughout the inplane of sheets, as shown in Figure 2b and 2c. The atomic force microscopy (AFM) image in Figure 2d reveals that the few-layer C3N4 represents the laminar structure with plentiful

a sequential molecule self-assembly, alcohol molecules intercalation, thermal-induced exfoliation, and polycondensation process. The resultant products have a high quality, a specific surface area of up to 164.2 m2 g−1, a high electrical conductivity, and an ultrathin morphology with welldistributed primarily 2 to 15 nm-width pores. We show that a remarkably increased photocatalytic hydrogen evolution activity can be obtained, which is a 26-fold higher than that of the bulk C3N4, reaching an overall apparent quantum efficiency (AQE) of approximately 9.8%. We further show that unprecedented activity and selectivity for the oxidative coupling of amines to imines can be achieved under mild conditions, which is among the best photocatalysts reported so far.

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RESULTS AND DISCUSSION Synthetic Strategy of Porous Few-Layer Carbon Nitride. A facile molecule self-assembly strategy is developed here to prepare porous few-layer C3N4, which is totally different with traditional top-down exfoliation methods (Figure 1). Briefly, layered microrod precursors were first fabricated

Figure 1. Schematic representation of few-layer C3N4.

through the self-assembly of melamine molecules and hydrolyzed product (cyanuric acid) (Figure S1−S2),26 which possess a large interlamellar distance (0.315 nm) and abundant functional groups (e.g., −NH2, −OH) between the tri-striazine layer. Such unique features enable the facile intercalation of small polar molecules into the layered microrod precursors. We carried out the first-principle density functional theory (DFT) calculations to study the interactions between the precursor and different polar alcohol molecules (Figure S3 and Table S1), which demonstrate the feasibility of insertion. The X-ray diffraction (XRD) patterns (Figure S4 and Table S2) show that the (002) peaks of the layered precursors shift to lower angles after the intercalation of methanol and ethanol, which are much lower than that of polyols (e.g., glycol, glycerol) based ones. As shown in Table S1 and Table S3, although the glycerol has the minimum adsorption energy than other alcohols, the relatively large molecule size and high viscosity impedes the insertion of glycerol due to the low driving force. After thermal treatment at 500 °C under air, the scanning electron microscopy (SEM) images (Figure S5) show that aggregated C3N4 nanosheets can be obtained with methanol and ethanol intercalation, while agglomerates of B

DOI: 10.1021/jacs.8b12428 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Figure 2. (a) SEM image of the few-layer C3N4, (b) TEM image of the few-layer C3N4, (c) Magnified TEM image of the few-layer C3N4. (d,e) AFM image and corresponding height profiles along the white line in d of the few-layer C3N4. (f) XRD patterns, (g) solid-state 13C MAS NMR spectra, (h) N 1s XPS spectra, and (i) N2 sorption isotherms of bulk C3N4, microtube C3N4 and few-layer C3N4.

is ascribed to the emergence of nitrogen vacancy. To further confirm the formation of nitrogen vacancies, the electron paramagnetic resonance (EPR) spectra (Figure S19) are performed. The EPR signal with g value of 2.003 is ascribed to unpaired electrons on the carbon atoms of the heptazine rings within π-bonded nanosized cluster.31 The peak intensity is much higher than that of bulk C3N4, which demonstrates the introduction of nitrogen vacancies in few-layer C3N4 since the loss of nitrogen atoms would leave excess electrons, which are allocated to their nearest carbon atoms and delocalized among the π-conjugated networks of the C3N4, and thus leading to the increased concentration of lone pair electrons. The introduction of nitrogen vacancies in few-layer C3N4 was evidenced by X-ray photoelectron spectroscopy (XPS) and elemental analysis (EA). As observed in XPS survey spectra (Figure S20a,b), few-layer C3N4 is mainly composed of C, N, and no phosphorus species are detected, which is similar to that of bulk and microtube C3N4. The appearance of very small O 1s peaks is probably due to surface adsorbed oxygen-containing species. Notably, the surface N to C atom ratio drastically decreases from 1.33 (bulk C3N4) to 0.83 (few-layer C3N4, Table S4), which agrees with the bulk ratio determined by EA results (Table S5), illustrating the preferred loss of nitrogen atoms in the layer structure during the pyrolysis process. To further determine the position of nitrogen vacancy, high

nanoscale pores. The randomly measured thicknesses of the nanosheet is approximately 1 nm (Figure 2e and Figure S17), which corresponds to three atomic layers of C−N. As shown in the XRD patterns (Figure 2f), the few-layer C3N4 gives two consistent peaks with bulk and microtube C3N4, suggesting that they have the same crystal structure. The weaker and broader peak at (002) demonstrates the few-layer nature of C3N4 nanosheets. The peak at 13.1°, stemmed from the in-plane structural packing motif of (100) lattice plane becomes much weaker in the nanosheets, which maybe result from in-plane porous structures.27,28 The Fourier-transform infrared (FT-IR) spectroscopy (Figure S18) confirms that the few-layer C3N4 possesses a similar chemical structure with bulk and microtube C3N4. The adsorption at 810 cm−1 for breathing mode of tri-s-triazine ring and in region of 1200− 1600 cm−1 for characteristic stretching modes of CN heterocyclic implies short-range structure is well maintained. The broad peaks in the 3000−3600 cm−1 range are assigned to N−H stretching.29 The presence of heptazine structure can be further confirmed by the 13C MAS NMR spectra (Figure 2g). The NMR spectra of all samples show two distinct peaks at chemical shifts of 156.7 and 164.6 ppm, which is assigned to C(1) atoms and C(2) atoms in the melem units, respectively.30 For few-layer C3N4, the peak intensity at 156.7 ppm declines markedly in comparison with bulk and microtube C3N4, which C

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Figure 3. (a) UV−vis absorption spectra and (inset) corresponding band gap energies of bulk C3N4 (blue curve) and few-layer C3N4 (red curve). (b) Mott−Schottky plots of bulk C3N4 (blue curve) and few-layer C3N4 (red curve) collected at various frequencies versus the saturated Ag/AgCl reference electrode (pH = 7). (c) Electronic band structure, (d) steady state photoluminescence spectra, and (e) transient state photoluminescence spectra of bulk C3N4 and few-layer C3N4. (f) EIS Nyquist plots of bulk C3N4 and few-layer C3N4 electrodes.

edge compared with bulk C3N4, suggesting the enlarged band gap (Figure 3a). The bandgaps are 2.66 eV for bulk C3N4 and 2.75 eV for few-layer C3N4, respectively. The increased bandgap can be ascribed to the strong quantum confinement effect caused by the nature of ultrathin nanosheets.23,35 The conduction band (CB) positions of few-layer C3N4 and bulk C3N4 were measured by Mott−Schottky plots at frequencies of 1.2, 1.8, and 2.4 kHz (Figure 3b). Both bulk C3N4 and fewlayer C3N4 exhibit positive slope, which is consistent with the character of typical n-type semiconductor. The flat band positions derived from the intersection is ∼ −1.12 V vs Ag/ AgCl for bulk C3N4 and ∼ −0.97 V vs Ag/AgCl for few-layer C3N4. Since it is generally accepted that the CB potential in ntype semiconductors is approximately equal to the flat band potential,36,37 the CB edges of bulk C3N4 and few-layer C3N4 is determined to be −1.12 eV and −0.97 eV, respectively. The downshift of the CB for few-layer C3N4 is 0.15 eV compared with bulk C3N4, which is mainly due to the formation of defect states caused by nitrogen vacancies.33 Although the CB potential of the few-layer C3N4 downshifted, which is still more negative than the reduction potential of H+/H2.38 The potentials of the valence bands of bulk C3N4 and few-layer C3N4 are calculated to be 1.54 and 1.78 eV by a combined analysis of bandgaps and CB edges, respectively. The corresponding band structure alignments of bulk and fewlayer C3N4 are schematically depicted in Figure 3c. In addition to affect the bandgap structure of few-layer C3N4, the morphology and electronic structure engineering can also remarkably inhibit the recombination of photoexcited charge carriers. Steady state and the time-resolved photoluminescence (PL) spectroscopy were employed to reveal photophysical characteristics of the photogenerated electron− holes. The PL intensity of few-layer C3N4 decreases drastically in comparison with that of bulk C3N4 (Figure 3d), implying that the electron−hole recombination rate can be efficiently

resolution N 1s spectrum (Figure 2h) of the few-layer C3N4 was carried out, which displays three peaks located at 398.6, 399.7, and 401.0 eV, assigning to CNC, N−(C)3, and C− NH, respectively.32 Note that the peak area ratio between C NC and N−(C)3 significantly decreased from 2.63 of bulk C3N4 to 1.72 of few-layer C3N4, suggesting that the nitrogen vacancies are majorly at CNC site.32 In addition, the C 1s spectra (Figure S20c) show the signal intensity of NCN decreases and a new peak centered at 286.1 eV arises for fewlayer C3N4, further confirming the loss of lattice nitrogen and the formation of nitrogen vacancy.33 The introduction of nitrogen defects may act as capturing sites, which can greatly reduce the recombination possibility of photogenerated electron−hole pairs in few-layer C3N4 by trapping the photoinduced electrons, and thus favoring the enhancement of photoredox process. N2 sorption isotherms of the few-layer C3N4 shows a representative type-IV curve with a typical H3-type hysteresis loop (Figure 2i), which indicates the highly porous structure.34 The Brunauer−Emmett−Teller (BET) specific surface area of few-layer C3N4 is calculated to 164.2 m2 g−1 with a 14-fold and nearly 4-fold increasement relative to bulk C3N4 (11.2 m2 g−1) and microtube C3N4 (42.5 m2 g−1). In addition, the pore size distribution curve of few-layer C3N4 discloses the well-defined micromesopore nature, as shown in Figure S21, which is consistent with the TEM analysis. The pore volume of bulk C3N4 is much smaller than that of few-layer C3N4; the small overall pore volume should be caused by the platelet-like structure in bulk phase. The highly porous structure of fewlayer C3N4 can not only provide a large surface area for accommodating active sites, but also shorten diffusion path for reactants and photogenerated charge carriers. Band Structure and Charge Carrier Behavior. The UV−vis absorption spectra show both samples absorb visible light, few-layer C3N4 exhibits a blue shift of the absorption D

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Journal of the American Chemical Society restrained for few-layer C3N4. In addition, the blue shift of the PL emission peak from 460 to 450 nm is observed for few-layer C3N4, which agrees fairly well with the change in bandgap. The time-resolved PL decay spectra were also carried out to validate the improvement of charge separation efficacy in fewlayer C3N4, as illustrated in Figure 3e and Table S6. The fewlayer C3N4 exhibits a slower exponential decay with average lifetime of 5.70 ns, which is 2 times longer than that of bulk C3N4 (2.69 ns). The prolonged charge carrier lifetime in fewlayer C3N4 suggests the photocarrier is more likely to involve in the subsequent surface redox reactions, which is beneficial for improving photocatalytic performance. Transient photocurrents were measured for few-layer C3N4 and bulk C3N4 electrodes to gain deeper understanding of charge transport efficiency. As displayed in Figure S22, both electrodes exhibit rapid and uniform photocurrent responses under intermittent visible light irradiation, and such photoresponsive phenomenon is well repeated in six on−off cycles. Compared with bulk C3N4, the few-layer C3N4 photoelectrode shows a much higher photocurrent response, which indicates the transfer and separation of charge carriers were greatly enhanced, in consistence with the PL analysis. The significantly improved charge separation efficiency for few-layer C3N4 is attributed to the tailored ultrathin nature and nitrogen defects. A similar tendency was obtained in electrochemical impedance (EIS) measurement. Few-layer C3N4 shows a much smaller arc radius than that of bulk C3N4 (Figure 3f) indicating its lower resistance for fast interfacial charge carrier transfer, which results from the enhanced diffusion mobility of electrons. Therefore, combined with the above spectroscopic results, it is inferred that the few-layer C3N4 delivers promoted separation capacity of photoinduced electron−hole pairs, and enhanced accessibility to the target reactant molecules adsorbed on the surface. Benefiting from these features, the few-layer C3N4 shows outstanding competence for diverse photocatalytic application. Photocatalytic Performances. The photocatalytic activity of few-layer C3N4 for hydrogen evolution was checked by using TEOA as sacrificial electron donor and Pt as cocatalyst under visible light irradiation. As presented in Figure 4a, the average hydrogen evolution rate of the few-layer C3N4 achieves 159.8 μmol h−1, which is over 26 times higher than that of bulk C3N4 (6.1 μmol h−1) and 4 times higher than that of microtube C3N4 (40.2 μmol h−1), respectively. Moreover, there is no noticeable attenuation in H2 production rate after 5 cycling tests within the 20 h photocatalytic period, suggesting the high stability of few-layer C3N4 under the applied reaction conditions. Similar enhancement of hydrogen evolution activity was observed for other C3N4 nanosheets obtained by the proposed strategy with other alcohol molecules intercalation (Figure S23), which strongly demonstrates the proposed strategy is efficient for fabrication of C3N4 with excellent photocatalytic performances. Wavelength-dependent H2 evolution of few-layer C3N4 reveals that the variation tendencies of AQE curves coincided with its optical absorption spectrum indicating that the H2 production is dominantly driven by photogenerated electrons (Figure 4b). The AQE of few-layer C3N4 at 420 nm is estimated to be about 9.8%, and the performance of few-layer C3N4 is also higher than most of the existing C3N4-based photocatalysts as summarized in Table S7. Compared with bulk C3N4, the few-layer C3N4 has several unique merits as depicted in Figure 4c that accounts for the remarkable enhanced H2 production activity. First of all, few-

Figure 4. (a) Time course of H2 evolution for bulk, microtube, and few-layer C3N4 under visible light irradiation (λ > 420 nm). In a typical experiment, 20 mg of photocatalyst coloaded with 1 wt % Pt was used with the triethanolamine (TEOA) as the sacrificial agent in deionized water. (b) Wavelength-dependent efficiency of H 2 evolution over few-layer C3N4 with 1 wt % Pt (left axis), UV/vis light absorption spectrum of few-layer C3N4 (right axis). (c) Charge transfer and separation mechanism of few-layer and bulk C3N4 under visible light irradiation.

layer C3N4 displays enlarged surface area for accommodating massive absorption. For another, the ultrathin nanosheets can greatly reduce the diffusion path of charges and protons from the interior to the surface for photocatalysis. Meanwhile, the high planar conductivity of ultrathin layered structure enables the fast electron migration and thus reducing the recombination with holes. Moreover, the porous structure can afford abundant edge active sites to participate in the interfacial photocatalytic hydrogen evolution. The fantastic photoredox activity of the few-layer C3N4 was further evaluated by the oxidative coupling of amines, which shows great potential application in the field of organic synthesis including pharmaceutical and biological active compounds, fine chemicals and organic transformations.39 Control experiments (Table 1, entries 1−3) reveal the critical role of catalyst, visible light irradiation and oxygen in oxidative coupling reaction. Thus, various substituted benzylamines were selected to conduct oxidative coupling reaction with few-layer C3N4 under the optimized conditions. As listed in Table 1, amines are transformed to imines efficiently by few-layer C3N4 with high conversion and selectivity (Table 1, entries 4, 8−13) demonstrating its superior photoactivity and the applicability. In contrast, the TOF of benzylamine obtained by bulk and microtube C3N4 (Table 1, entries 14, 15) is 7.6 and 16.5, respectively, much lower than that of few-layer C3N4 (33.3) under identical conditions. To best of our knowledge, the performance of few-layer C3N4 is superior to the most reported photocatalysts (Table S8) and better than series of C3N4 obtained via the same method with other alcohol molecules as insertion agents (Table S9). The effects of various substitutions on the oxidative coupling processes are also investigated. It can be concluded that electron donating groups such as methyl (Table 1, entries 10−12), methoxy (Table 1, entry 13) led to higher conversion rates than the electron withdrawing groups such as Cl (Table 1, entry 8) or Br (Table 1, entry 9). Above results illustrated that the E

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Journal of the American Chemical Society Table 1. Oxidative Homocoupling of Aminesa

catalytic conditions entry

catalyst

O2

light

additive

R

conv./select. (%)b

TOFc

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

no catalyst Few-layer C3N4d Few-layer C3N4e Few-layer C3N4 Few-layer C3N4 Few-layer C3N4 Few-layer C3N4 Few-layer C3N4 Few-layer C3N4 Few-layer C3N4 Few-layer C3N4 Few-layer C3N4 Few-layer C3N4 Bulk C3N4 Microtube C3N4

+ + − + + + + + + + + + + + +

+ − + + + + + + + + + + + + +

− − − − hole scavenger superoxide scavenger singlet oxygen scavenger − − − − − − − −

H H H H H H H P-Cl p-Br p-Me o-Me m-Me P-OMe H H

0/− 0/− 6/− 99/99 12/− 17/− 69/− 87/98 82/96 99/99 99/99 95/99 98/99 24/95 51/97

− − − 33.3 − − − 28.4 26.2 33.3 33.3 31.4 32.3 7.6 16.5

a

General reaction conditions: 0.5 mmol substrate; 15 mg photocatalyst; 5 mL of acetonitrile as solvent; 0.1 MPa O2; reaction time 1 h under irradiation visible light (λ > 420 nm); 25 ◦C. bConversion data were obtained by GC. cTOF = mmol of amine converted per gram of catalyst per hour. dWithout light irradition. eBubbled with N2. Ammonium oxalate as hole scavenger, benzoquinone as superoxide scavenger and sodium azide as singlet oxygen scavenger.

inductive effect of substituent groups plays a crucial role in conversion rate, and the electron donating groups (e.g., methoxy and methyl group) can increase the electron density, thus promoting the oxidative coupling reaction. In addition, the stability test results demonstrate that few-layer C3N4 retained excellent conversion and selectivity for 5 runs (Figure S24). No noticeable change in the crystal and morphology structure (Figure S25) can be observed for the few-layer C3N4 catalyst after cycling test, indicating the good stability. To get insight into the oxidative coupling mechanism of benzylic amines to form imines over the few-layer C3N4, we first conduct the active species trapping experiments to clarify the contribution of different active species to the reaction. Accordingly, ammonium oxalate (AO), benzoquinone (BQ) and sodium azide (NaN3), which are the scavengers for hole, superoxide radical (O2•− ), and singlet oxygen ( 1 O 2), respectively, are selected to identify the active species. It was found that the conversion of benzylamine drastically decreased after adding AO and BQ (Table 1, entries 5−6), and the transformation was also suppressed to some extent by NaN3 (Table 1, entry 7). Hence, the hole and superoxide radical are determined as the major reactive species for oxidative coupling of benzylamines, and singlet oxygen could promote the proceeding of this reaction as well. Then, spin trap EPR experiments were also performed to give direct proofs for the involved reactive species. As shown in Figure 5a, 5,5-dimethyl1-pyrroline-N-oxide (DMPO) is employed as trapping agents for O2•−, typical signal for oxidation product of DMPO was observed in the presence of few-layer C3N4, whereas much weaker signal for bulk C3N4 indicate trace of O2•− yield. In addition, the EPR spectra with 2,2,6,6-tetramethylpiperidine (TEMP) as 1O2 trapping agents (Figure 5b), the signal intensity exhibits the same trend with O2•−. Above results readily elucidate that the few-layer C3N4 possesses stronger capability of activating O2 molecules into O2•− species and 1O2 than bulk C3N4 and microtube C3N4.

Figure 5. (a) EPR spectra of different samples in the presence of DMPO. (b) EPR spectra of different samples in the presence of TEMP. (c) Proposed reaction mechanism of oxidative coupling of benzylamine over few-layer C3N4. ISC: intersystem crossing.

On the basis of the above discussions and previous reports, a plausible reaction mechanism (benzylamine has been taken as a model) is proposed and presented in Figure 5c. Under visible light irradiation, electrons and holes are produced by excitation of the few-layer C3N4. The band structure manifests that the few-layer C3N4 possesses sufficiently enough reductive and oxidative potentials to reduce molecular oxygen into its active species (Ered = −0.24 vs Ag/AgCl) and oxidize benzylamine into its cationic radical form (Eoxi = +0.56 vs Ag/AgCl), respectively.40 As a consequence, the photogenerated electron reduces molecular oxygen to produce O2•− and singlet oxygen 1 O2. Benzylamine is oxidized to its cationic radical by the holes in the valence band, then the obtained active oxygen species would react with intermediate to form the benzyliminium and F

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H2O2.41 And H2O2 reacted immediately with another benzylamine molecule to obtain (phenyl)methanimine. The obtained (phenyl)methanimine reacts with another benzylamine molecule to give the N-benzyl-1-phenylmethanediamine as intermediate.42 Then, after elimination of ammonia from intermediate under hole-assisted, the final coupled product is obtained.43,44 The remarkable improvement of the photocatalytic performance of few-layer C3N4 is the result of integrated optimization of the structural and electronic properties. First, the unique porous ultrathin configuration endows few-layer C3N4 with considerable merits for photoredox reactions, such as more exposed surface catalytic sites, shortened migration distance from interior to surface, and facilitated transfer of reactants and products. Besides, the ultrathin thickness of few-layer C3N4 can greatly improve the charge mobility along the in-plane direction. Furthermore, the defect states induced by nitrogen vacancies can act as separation centers to temporarily capture photoinduced electrons from the CB and activate the molecular oxygen more effectively. Finally, the VB potential of few-layer C3N4 is more positive than bulk C3N4, which enables the few-layer C3N4 with stronger oxidation capacity.

ACKNOWLEDGMENTS This work was supported by the National Nature Science Foundation of China (No. 21631004). WL thanks the support from the Major State Basic Research Development Program (2016YFA0204000) and the NSFC of China (Grant No. 21603036). This work sponsored by Shanghai Rising-Star Program.

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CONCLUSION In summary, our study affords a new strategy for facile and efficient fabrication of porous few-layer C3N4 via a sequential molecular assembly, alcohol molecules intercalation, thermalinduced exfoliation, and polycondensation. The obtained fewlayer C3N4 exhibits superior photocatalytic activity and stability for photocatalytic H2 production and oxidative coupling of benzylic amines to form imines under visible light irradiation. The unprecedented performance is due to the synergic advantages of enlarged surface area, increased access to reaction sites, facilitated diffusion of reactants and products, and remarkably improved charge carrier transfer and separation. In addition, this work also paves the way for construction of few-layer C3N4 materials with excellent optical and electrical properties, which can be anticipated for versatile applications in the fields of photocatalysis, photoelectrocatalysis, and optoelectronic devices. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b12428.

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Supplementary experimental methods, characterization of the materials and products, supporting tables and figures (PDF)

AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected], [email protected] ORCID

Wei Li: 0000-0002-4641-620X Chungui Tian: 0000-0003-4846-3605 Dongyuan Zhao: 0000-0001-8440-6902 Honggang Fu: 0000-0002-5800-451X Notes

The authors declare no competing financial interest. G

DOI: 10.1021/jacs.8b12428 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/jacs.8b12428 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX