One-Step Nickel Foam Assisted Synthesis of Holey G-Carbon Nitride

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Energy, Environmental, and Catalysis Applications

One-Step Nickel Foam Assisted Synthesis of Holey G-Carbon Nitride Nanosheets for Efficient Visible-light Photocatalytic H2 Evolution Zhenyuan Fang, Yuanzhi Hong, Di Li, Bifu Luo, Baodong Mao, and Weidong Shi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04783 • Publication Date (Web): 01 Jun 2018 Downloaded from http://pubs.acs.org on June 1, 2018

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ACS Applied Materials & Interfaces

One-Step Nickel Foam Assisted Synthesis of Holey G-Carbon Nitride Nanosheets for Efficient Visible-light Photocatalytic H2 Evolution Zhenyuan Fanga, Yuanzhi Hongb, Di Lic, Bifu Luoa, Baodong Maoa, Weidong Shia* a

School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, 212013, P. R. China

b

c

School of Materials Science and Engineering, Jiangsu University, Zhenjiang, 212013, P. R. China

Institute for Energy Research, Jiangsu University, Zhenjiang, 212013, P. R. China

ABSTRACT: Graphitic carbon nitride (g-C3N4) with layered structure represents one of the most promising metal-free photocatalysts. As yet, the direct one-step synthesis of ultrathin g-C3N4 nanosheets remains a challenge. Here, few-layered holey g-C3N4 nanosheets (CNS) were fabricated by simply introducing a piece of nickel foam over the precursors during the heating process. The as-prepared CNS with unique structural advantages exhibited superior photocatalytic water splitting activity (1871.09 µmol h-1 g-1) than bulk g-C3N4 (BCN) under visible light (λ>420 nm) (≈31 fold). Its outstanding photocatalytic performance originated from the high specific surface area (240.34 m2 g-1) and mesoporous structure, which endows CNS with more active sites, efficient exciton dissociation and prolonged charge carrier lifetime. Moreover, the obvious up-shift of the conduction band leads to a larger thermodynamic driving force for photocatalytic proton reduction. This methodology not only had the advantages for the direct and green synthesis of g-C3N4 nanosheets, but also paved a new avenue to modify molecular structure and textural of g-C3N4 for advanced applications. KEYWORDS: Graphitic carbon nitride ·Nanosheets ·Ammonia decomposition · Photocatalysis · Hydrogen evolution.

easily engineered during the polymerization because of its organic nature.39,40 In contrast to the classic routes using N, S, O or Clcontaining leaving gas to mediate the polymerization of g-C3N4 frameworks, additives of KCl, NH4Cl or KOH not only function as a soft template for the preparation of nanosheets, but serve as dopants to modify the g-C3N4 polymers.18,34-38,41-43 Meanwhile, air, H2S, H2, Ar, NH3 and H2O atmosphere were also adopted to retreat its surface morphologies and tune the connectivity patterns, respectively.31,32,22,44-48 Nevertheless, whether the synthesis or the modification of g-C3N4, the release of the toxic NH3 in the environment has long been ignored. Thus, it would be very interesting if the ammonia could be reutilized during the polymerization of the precursors. So, how about decomposing NH3 into H2O with a proper catalyst? 49-52 In principle, the H2O molecule (size≈0.25 nm) is small enough to intercalate into the layers of g-C3N4 (interlayer distance≈0.326 nm) to avoid the aggregation of g-C3N4 nanosheets; and the generated water atmosphere can be used to modify its physical and chemical properties at high temperatures through C/N-steam reforming reactions ′CNs + H Og → COg + H g + NOg′. 48,53 Importantly, the structural defects engineered by the chemoselective etching of water can not only affect the electronic structure of g-C3N4, but also help to boosting its catalytic properties.48 This reaction is well known for the largescale industrial production of hydrogen, but the combination of such reaction with ammonia decomposition for the modification of the g-C3N4 has not been reported yet. Hence, it is desired to obtain few-layered g-C3N4 nanosheets with unique surface and textural properties through reutilizing the ammonia properly and gain an intensive study of the structure-activity relationship.54,55,56 Herein, we reported a direct and green strategy to prepare fewlayered holey CNS by simply introducing a piece of nickel foam over melamine during the heating process in air. Notably, nickel is a promising catalyst for NH3 decomposition, because its low cost and still relatively high activity. The physical and chemical properties of the CNS were effectively modified and strengthened by the H2O atmosphere. The CNS enriched with in-plane na-

1. INTRODUCTION Since the pioneering research by Honda and Fujishima on photocatalytic water splitting, great efforts have been devoted to explore efficient and sustainable photocatalysts for industrial applications.1-3 Hitherto, the advances of photocatalytic materials for hydrogen evolution are still restricted by some critical factors: utilization of visible light, chemical stability and cost.4,5 Recently, graphitic carbon nitride (g-C3N4) has emerged as a promising organic semiconductor photocatalyst to drive water splitting under visible light owning to its proper band structure, superior physicochemical stability and high abundance.6-8 Thus far, many strategies have been explored to boost its photocatalytic performance, such as nanostructure design for modifying the surface and textural structures,9-15 heteroatom doping (e.g., B, S, P, I)16-19 and copolymerization20-22 for tuning its optoelectronic properties, and coupling with other (semi-)conductors for facilitating charge separation.23-26 In terms of nanoarchitectures, the synthesis of ultrathin g-C3N4 nanosheets (CNS) are shown to be beneficial for the enhancement of photocatalytic efficiency due to the increased surface area with abundant active sites and efficient charge separation.27,28 Subsequently, various methods, including the approach of sonication exfoliation,29,30 thermal etching,31,32 and ball milling,33 have been employed to delaminate bulk g-C3N4 (BCN) into single or fewlayered nanosheets. In contrary to these easy controlled top-down strategies, the creation of leaving gas or the introduction of external species in the traditional precursors have been proved to be scalable synthetic approaches to template the formation of CNS.3438 Unfortunately, most of the above approaches still have the inherent shortcomings of organic or inorganic reagents inevitably needed and time consuming. Therefore, the development of a green approach for the direct assembly of precursors into twodimensional CNS remains a challenge. Similar to the surface and interface design of single crystal nanomaterials, the molecular structure and textural of g-C3N4 can be

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nopores possess high specific surface area (240.34 m2/g) and a porous 2D nanosheet morphology. Such a unique structure could expose more active edges, accelerate the exciton separation efficiency and prolong the lifetime of the photogenerated charge carrier. Moreover, the high up-shift (0.6 V) of the conduction band may further promote the photocatalytic water reduction reaction thermodynamically. As expected, the CNS show a robust visiblelight-driven (λ>420 nm) photocatalytic hydrogen production activity of 1871.09 µmol h-1 g-1, which is 31 times higher than that of BCN (59.82 µmol h-1 g-1). We also adopted dicyandiamide as the precursors to synthesize g-C3N4 nanosheets under the same condition, which still show greatly enhanced photocatalytic activity compared with the bulk sample. After that, nickel sheet and copper foam were also applied to prepare g-C3N4 respectively, demonstrating the rationality of our strategy.

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S-4800 field emission SEM (SEM, Hitachi, Japan). Transmission electron microscopy (TEM) images were collected on a transmission electron microscopy (Tecnai G2 F30 S-TWIN, America) using an accelerating voltage of 200 kV. Atomic force microscopy (AFM) image was carried out on Asylum Research MFP-30 (America). The-ray photoelectron spectroscopy (XPS) was obtained by a Thermo ESCALAB 250X (America) electron spectrometer using 150W Al Kα X-ray sources. Elemental analysis was carried out on an elemental Analyzer (FLASH1112A, America). Fourier transform infrared (FT-IR) spectra were carried out on a Nicolet NEXUS 470 spectrometer using KBr pellet technique. UV–vis absorption spectra (DRS) was collected by a Shimadzu UV–2450 spectrophotometer at room temperature. The photoluminescence(PL) spectra of samples were measured on a Perkin-Elmer LS 55 at room temperature. Time-resolved photoluminescence (T-PL) spectra were tested by a Quanta Master & Time Master Spectrofluorometer (excitation wavelength at 337 nm). The average lifetime is calculated by the following equation:

2. EXPERIMENTAL SECTION 2.1 Synthesis of Catalysts Treatment of Ni foam: Nickel foam (approximately 3 cm × 3 cm × 1 mm) was carefully cleaned with 6 M HCl solution in an ultrasound bath for 30 min in order to remove the NiO layer on the surface, and rinsed with de-ionized water and absolute ethanol, then dried at 60 °C in vacuum oven. Synthesis of Bulk g-C3N4 (BCN): 3 g melamine powder was put into an alumina crucible (50 mL) with a cover and heated from 50 °C to 550 °C with a ramp rate of 2.3 °C min-1 in a muffle furnace, and kept at 550 °C for 4 h, then cooled to room temperature. Synthesis of g-C3N4 nanosheets: 3 g melamine powder was put into an alumina crucible (50 mL) and a piece of Ni foam was put over the melamine powder (don’t forget the cover), and then it was heated from 50 °C to different temperature (500 °C, 525 °C, 550 °C) with a ramp rate of 2.3 °C min-1 in a muffle furnace, and kept for 4 h, then cooled to room temperature. It should be noted that CNS-R1-R2 (CNS-500-4, CNS-525-4, CNS-550-4) represents the reaction temperature and reaction duration of asprepared sample are R1 and R2, respectively. CNS-550-4 also was simply represented by CNS. Because it was the typical reaction temperature (550 °C) and reaction duration (4 h), CNS-550-4 also was simply represented by CNS in the full text. If the experimental conditions (such as precursors and catalysts) changed, there would be an extra annotations and instructions. In addition, copper foam and nickel sheet were also used for the preparation of g-C3N4 in order to demonstrate the decisive role of Ni foam: 3 g melamine powder was put into an alumina crucible (50 mL) and a piece of copper foam or nickel sheet was put over the melamine powder, and then was heated from 50 °C to 550 °C with a ramp rate of 2.3 °C min-1 in a muffle furnace, and kept for 4 h, then cooled to room temperature. After that, dicyandiamide is also used as precursors to synthesize g-C3N4 nanosheets with Ni foam: 3 g dicyandiamide powder was put into an alumina crucible (50 mL) and a piece of Ni foam was put over the dicyandiamide powder, and then was heated from 50 °C to 550 °C with a ramp rate of 2.3 °C min-1 in a muffle furnace, and kept for 4 h, then cooled to room temperature 2.2 Characterization X-ray diffraction (XRD) patterns were recorded by a D/MAX2500 diffractometer (Rigaku, Japan) with Cu Kα radiation (λ = 1.54178 Å) from 5.0–80º (7.0º/min) at room temperature. The nitrogen adsorption and desorption isotherms of the samples were measured by a Micromeritics ASAP 2460 analyzer (America). Scanning electron microscopy (SEM) images were obtained on an

τ

(τ1 and τ2 are the lifetime; I1 and I2 are the

relative intensity). Electron paramagnetic resonance (EPR) measurements were operated at a Bruker model A300 spectrometer with a 300 W Xenon lamp equipped with a visible light source. The Photoelectrochemical measurements were performed by using a CHI -852C (Chenhua, China) electrochemical workstation and CHI-760E (Chenhua, China) electrochemical workstation with a standard three-electrode cell at room temperature. The cleaned fluorine doped tin oxide (FTO) glass deposited with samples, a Pt wire, and a saturated calomel electrode were respectively used as working electrodes, counter electrode, and reference electrode. 2.3 Photocatalytic measurements Photocatalytic activities were evaluated by the photocatalytic hydrogen evolution from water under visible light irradiation. Typically, 20 mg photocatalyst and 10 mL triethanolamine (TEA) serving as the sacrificial electron donor were added to 90 mL ultrapure water under stirring. Then, H2PtCl6 aqueous solution was added as the precursor for the co-catalyst Pt, which was insitu photo reduced during the photocatalytic reaction (~3 wt% Pt). Next, the solution was degassed and irradiated by a 300 W Xenon lamp equipped with a 420-nm cutoff filter. The photocatalytic H2 evolution rate was determined by using a GC-7920 instrument with a thermal conductivity detector (TCD) and high-purity N2 as the carrier gas. The monochromic light was provided by using a 420 ± 15 nm band pass filter and the average intensity of monochromic light was determined by CEL-NP 2000 photoradiometer.

3. RESULTS AND DISCUSSION

Scheme 1. Illustration of the reaction route for the g-C3N4 nanosheets in this work.


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The simplified reaction route of CNS is illustrated in Scheme 1 and Figure S1. During the polymerization process of the precursor, a large amount of ammonia was released (①); Next, this environmentally harmful gases would flow upwards and be partially catalyzed by nickel foam at high temperature to H2 which would be oxidized into H2O in air atmosphere (②); Then, these tiny

water molecules could intercalate into the layers of g-C3N4 and participate in the C/N-steam reforming reactions(③) to generate more molecules (CO, NO, H2) or gas bubbles according to Wang et al’ s report,48 and avoid aggregation of nanosheets. After calcination, the nickel foam would swell and move upwards which eliminate the need for subsequent separation.

Figure 1. (a) SEM images of BCN. (b) SEM images of CNS. (c, d) TEM images of CNS. The morphology and microstructure of BCN and CNS were investigated with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The SEM images in Figure 1a revealed that BCN possessed dense and stacked structure, but the CNS in Figure 1b obviously showed sheet structure. Moreover, TEM images (Figure 1c and d and Figure S2) showed that the surface of CNS was much rougher and homogeneously dispersed with numerous in-plane nanopores with the size from several nanometers to about 50 nm, which could be related to the steam reforming reactions or the oxidation etching.31,32,48 Such an porous nanosheets architecture would accommodate plenty of active sites and also benefit for the rapid diffusion of photogenerated carriers from interior to the surface sites.38 Atomic force microscopy (AFM) was employed to measure the thickness of the CNS. It revealed that the thickness of the CNS was approximately 4 nm (Figure S5), which mean that the gas atmosphere effectively avoided the aggregation of nanosheets. Brunauer-Emmett-Teller (BET) surface areas and pore structures of CNS were accessed from N2 adsorption–desorption measurements at 77.4 K. As shown in the Figure 2a, a typical type-IV isotherm with H1-type hysteresis loop at a high relative pressure ranging from 0.5 to 1 was observed, suggesting the exist-

ence of mesopores and micropores, consistent with TEM observation. The BET surface area and pore volume were calculated to be 240.34 m2/g and 0.813 cm3/g, respectively, which were much higher than that of BCN (19.31 m2/g and 0.049 cm3/g).16,57 This unique characteristic could result from the gas atmosphere, which effectively prevent the stacking of g-C3N4 nanosheets and engineer its surface with abundant pores. As for pore size distribution, the curve for CNS was broad and showed a sharp peak at 1.35 nm, while that of BCN was not obvious, clearly indicating the porous structure of CNS. Moreover, the pore property of CNS was studied through the cumulative pore volume and cumulative surface area which were calculated by original density functional theory (ODFT) model in Figure 2b.32 It showed that the mesoporous account for the maximum ratio of 81.19 % for the total volume, and also possess the maximum ratio of 54.23 % for surface area. While the micropores account for the minimum ratio of 3.52 % for the total volume, but still account for high ratio of 26.13 % for surface area. The plenty of surface pores endow CNS with more active sites for the photocatalytic reaction and facilitate the kinetics of the interfacial reaction by promoting photogenerated electrons and substances transfer.17,32,44-48



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Figure 2. (a) Nitrogen adsorption–desorption isotherms, with BCN as a reference sample. Inset: pore size distribution curves. (b) cumulative surface area and Cumulative pore volume of CNS calculated by using ODFT model. The formation of graphitic carbon nitride could be directly clarified by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS) analysis and elemental analysis. For XRD analysis, the crystal structure was studied by two characteristic peaks at approximately 13.1o (100) and 27.4o (002), which could be ascribed to the in-plane structural packing motif and stacking of the conjugated aromatic layers, respectively.19 In comparison with BCN, CNS showed broader and weaker peaks and its (002) peak had slightly shift to high degree from 27.4o to 27.9o (Figure 3a). This could be attributed to the significantly reduced correlation length of interlayer periodicity of the tri-s-triazine building blocks,15,31 which was consistent with the AFM results. FT-IR spectra (Figure 3b) of both BCN and CNS processed the characteristic breathing peaks of the heptazine ring system at 810 cm-1, the skeletal vibrations of aromatic C-N heterocycles at 1200-1600 cm-1, and the N-H or OH stretch between 3100 and 3500 cm-1. The peaks for CNS were sharper and showed evident enhanced absorption. It was worth noting that there was a unique peak at 1082 cm-1 corresponding to C-O vibration for CNS (Figure 3b and S12), indicating the existence of O-containing group.58 This should be caused by the steam reforming reactions or the oxidation etching.31,32,48 The chemical groups and oxidation state of g-C3N4 were investigated by X-ray photoelectron spectroscopy (XPS). Only C, N, and O elements were detected, which matched with the elemental mappings and the energy dispersive X-ray (EDX) spectrum (Figure S3 and S4). The C 1s and N 1s spectra were almost same for both samples, but the intensities for CNS were stronger (Figure S6), further suggesting the influence from H2O or O2. There were three kinds of C species, including the graphitic carbon (sp2 C=C bonds) in 285.0 eV, the sp2-hybridized carbon in the N-containing aromatic ring (N–C=N) in 288.5 eV and oxide carbon in 288.7 eV (Figure S7). The peak of C-O at 288.7 eV showed increased intensity implying the partial oxidation of carbon, consistent with the FT-IR results. And the peak area ratio of C=C to N–C=N decreased from 0.091 to 0.050 in CNS, indicating the great loss of graphitic carbon species (Table S2).60 For the N 1s spectrum (Figure S8), it could be fitted to four N species, including 399.0 eV for sp2-hybridized nitrogen in triazine rings (C–N=C), 400.5 eV for tertiary nitrogen N–(C)3 groups, 401.6 eV for amino function scaring nitrogen (C–NHx) and 404.7 eV caused by π excitation of the heterocycles. Compared to BCN, the peak-area ratio of N-(C)3 were obviouslyly decreased, indicating that the loss of N atoms

mainly occurred at the N-(C)3 lattice sites (Table S3). Moreover, XPS composition analysis showed that the C/N molar ratio of CNS (0.634) was smaller than that of BCN (0.658), suggesting the presence of carbon vacancies in CNS. Elemental analysis (Table S1) also revealed that the CNS has a smaller C/N atomic ratio (0.64) than that of BCN (0.65).48,61,62 The O 1s peak in 532.6 eV (Figure S9) could be from the absorbed H2O on the surface of the samples,63,64 but the peak of CNS was obviously broader which means the existence of different oxygen.65-68 It was inevitable that the water atmosphere or oxygen played a vital role in chemoselective etching of carbon atoms and modifying the molecular structure of CNS.48 These results suggested that this direct synthesis approach assisted by nickel foam was different from that of those delamination mechanisms.29-33,48

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Figure 4. (a) Time-resolved PL spectra collected under an excitation of 337 nm. (b) Room temperature EPR spectra for BCN and CNS. (c) Transient photocurrent response. (d) EIS plot. The time-resolved PL spectra were used to study the lifetime and separation efficiency of the photogenerated charge carriers for both samples. The information of average lifetime and its contribution by the multi-exponential fitting were listed in the timeresolved PL spectra in Figure 4a. The average fluorescence lifetime of radiative charge carriers for CNS (9.36 ns) was 1.68 times longer than that for BCN (5.56 ns), which is beneficial for free charges to participate in the surface photocatalytic reaction.17,31,4448 In addition, the room-temperature electron paramagnetic resonance (EPR) was further used to identify above results in Figure 4b. Only one single Lorentzian line with a g value of 2.0034, which can be assigned to lone pair electrons on the carbon atoms of the triazine rings, was detected for both CNS and BCN.19 The stronger EPR signal for CNS was observed, demonstrating the much higher concentration of unpaired electrons and consistent with the higher density of surface sites (SBET ≈240.34 m2/g).69 This was very beneficial for the photogeneration of active radical pairs for catalytic reaction. After irradiated with visible light, the intensity for CNS was greatly enhanced, further indicating the efficient generation of radical pairs in the semiconductors.21,69 In accordance with the EPR spectra, an obvious higher photocurrent density (Figure 4c) under visible light radiation and smaller diameter of the electrochemical impedance spectroscopy (Figure 4d) were observed for CNS, further demonstrating more efficient charge separation and faster interface charge transport, respectively. These differences of CNS may be resulted by a large number of micropores and mesoporous short the diffusion length of photogenerated charge and facilitate the charge transfer.31 The optical properties of CNS were greatly altered by the water atmosphere compared with BCN. The electronic band structures of the sample were studied by UV–vis absorption spectra (Figure 5). The volume of the CNS with the same weight was much larger than that of BCN in Figure 5a, indicating the as-prepared CNS were much more looser and with greater specific surface area. An obviously blue-shift of the absorption edge was observed in the

UV–vis diffuse reflectance spectrum (DRS) for CNS. This was consistent with the lighter color of CNS. Meanwhile, the ~34 nm blue-shift in the photoluminescence (PL) spectra further proved the larger bandgap by 0.35 eV of CNS (Figure S10). This could be

Figure 5. (a) UV-vis absorption spectra (Inset: the comparison of stacking density between BCN and CNS). (b) The corresponding Tauc plots.



ascribed to the well-known quantum confinement effect by the conduction and valence band edges shift in reversed directions. After that, the prepared CNS showed stronger fluorescence emission intensity in comparison to that of BCN which could be attributed to higher surface defect density (surface defects proved to be recombination centers for the separated electronhole).29,31,47,54,58 In addition, Tauc plots obtained from the Kubelka–Munk function showed a large bandgap energy increase from 2.43 eV for BCN to 2.83 eV for CNS. The potentials of valence band determined by XPS valence band spectra (Figure S11) were 2.37 V for BCN and 2.12 V for CNS. It revealed that the CNS have a higher valence band (VB) edge of 0.25 V than that of the BCN. Based on the bandgap and valence band edge, the potentials of the conduct band (CB) (Figure 6a) were calculated to be -0.11 V (BCN) and -0.71 V (CNS), respectively. The 0.60 V up-shift of the conduction band, which may be caused by carbon vacancies and the nanosheet structure, indicating a much high water reduction ability.30,33 The photocatalytic activity of CNS was evaluated by photocatalytic water splitting to produce hydrogen under visible light (λ>420 nm), by using 3 wt% Pt as cocatalysts and 10 vol% triethanolamine as a sacrificial electron donor. As shown in Figure 6b, the hydrogen evolution rate (HER) of CNS (1871.09 µmol h-1 g-1) was over 31 times higher than that of BCN (59.82 µmol h-1 g-1). But in the absence of Pt cocatalysts, no H2 were detected for all

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samples after four hours irradiation (Figure S13). The stability of CNS was evaluated under the same reaction conditions for four consecutive reactions (Figure 6c). There was no obvious deactivation under continuous visible light radiation in 20 h, which results from the excellent structure stability. After 20 h operations, 748.43 µmol hydrogen was produced, exceeding the amount of the catalyst (110 µmol). The corresponding turnover number respecting to the polymer and Pt atoms were estimated to be 6.80 and 242.99, respectively.32 The significantly enhanced photocatalytic activity of the CNS could be attributed to its unique nanosheet and surface structures. As illustrated in Figure 6d, the few-layered holey nanosheets could be simplified as several stacked, single-layered nanosheets with plentiful in-plane nanopores.46 The abundant surface or subsurface nanopores and edges are particularly crucial for the enhanced intralayer or interlayer photogenerated charges separation, and further contribute to collection of electrons on the deposited Pt nanoparticles catalyzing H2 evolution.48,70 Moreover, the high-density nanopores not only provide more exposed edges and new active sites, but also can serve as efficient transport channels for substances.The photocatalytic performances for CNS and some previous g-C3N4 nanosheets in recent years were listed in Table 1. In order to achieve a better photocatalytic performance, most of the reported works adopted two-step or multistep strategies, and thus there were few reports for the direct preparation of g-C3N4 nanosheets.

Figure 6. (a) Schematic illustration of the band structures for BCN and CNS. (b) Hydrogen generation rate of CNS, and DCNS and DBCN prepared by dicyandiamide with or without Ni foam, and the samples prepared with Cu foam and Ni sheet, respectively, with BCN as a reference sample (by a 300 W Xenon lamp using a long-pass cutoff filter allowing λ > 420 nm). (c) Cycling test of CNS for photocatalytic hydrogen generation under visible light irradiation. (d) A schematic illustration highlights the structural benefits of the few-layered nanosheets catalyst during hydrogen evolution process. In addition, dicyandiamide was also used as precursors to synthesize g-C3N4 nanosheets (called as DCNS) by the same condition. The XRD, UV-visible absorption and FT-IR were shown in the Figure S14. Expectedly, the DCNS showed obviously enhanced photocatalytic activity than the bulk sample (called as

DBCN) in Figure 6b and S14. The hydrogen evolution rate (1005.49 µmol h-1 g-1) was about 3.6 times higher than that of DBCN (278.01 µmol h-1 g-1). In order to demonstrate the decisive role of the nickel foam, copper foam and nickel sheet were also used for the preparation of g-C3N4 in the same conditions. After 4


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h illumination of visible light, hydrogen evolution rate was calculated to 184.19 µmol h-1 g-1 and 425.97 µmol h-1 g-1 for CN-Cu foam and CN-Ni sheet, respectively (Figure 6b and S15). This was consistent with the lower catalytic activity of copper for ammonia decomposition. Compared with the CN-Ni sheet, hydrogen evolution rate of CN-Ni foam (also called as CNS) was significantly higher, which meant that the high surface area was very conducive to the decomposition reaction and thus the modification from H2O was more sufficient. To further confirm the role of

nickel and oxygen, argon atmosphere was used as a modifying gas to prepare g-C3N4. The crystal structure of the used nickel foam was analyzed by XRD in the Figure S16. It was noted that Ni2H was generated in argon atmosphere and NiO was generated in air atmosphere, which could demonstrate the conversion between ammonia and water indirectly. All these results were consistent with the purpose of ammonia reutilization for the preparation of g-C3N4 nanosheets.

Table 1 Summary of the previously reported g-C3N4 nanosheets photocatalysts for hydrogen evolution rate (HER), increased HER fold than bulk g-C3N4 and surface area (N/A: no applicable). Samples HER Increased BET Preparation Ref -1 -1 2 -1 (µmol h g ) factors vs. (m g ) condition Bulk Holey 1871 31 240 One step This work CNS (>420 nm) CN600

N/A

N/A

96.6

One step

[35]

g-C3N4 nanosheets

N/A

N/A

N/A

One step

[37]

CN600

N/A

N/A

109.9

One step

[36]

Porous g-C3N4

N/A

N/A

60

One step

[11]

g-C3N4 nanosheets

9000 (>420 nm)

13

52.9

One step

[38]

g-C3N4 nanosheets

3410 (>400 nm)

3

306

Two steps

[31]

g-C3N4 nanosheets

1860 (>420 nm)

9.3

384

Two steps

[30]

PCN-U nanosheets

3390 (>400 nm)

N/A

210

Two steps

[48]

GCN2.5

524 (>420 nm)

9

170

Two steps

[40]

MonolayerC3N4

230 (>420 nm)

2.6

205.8

Multistep process

[54]

2860 (>400 nm)

22

278

Multistep process

[32]

CNHS g-C3N4 nanomesh

8510 (>420 nm)

5.5

331

Multistep process

[55]

1060 (>420 nm)

2.5

203

Multistep process

[59]

CCNNSs

beneficial to expose plenty of catalytic active sites and improve charge separation efficiency. And the up-shift in the CB of the CNS further led to a larger thermodynamic driving force for photocatalytic hydrogen production. Thus, the CNS showed excellent photocatalytic activity. Overall, this work highlights the importance of partial reutilization of ammonia to environmental protection, as well as the surface engineering of the 2D semiconductor nanosheets with high photocatalytic activities.

4. CONCLISION In summary, the holey g-C3N4 nanosheets with high photocatalytic activity were directly fabricated by a highly environmentalfriendly, nickel-foam assisted synthesis method. The generated H2O atmosphere not only avoided the aggregation of nanosheets, but also engineered its surface with aboundant nanopores. Based on the above results, the porous g-C3N4 nanosheets obtained had large specific surface area and porous structure, which were



Page 8 of 10

(12) Lin, Z.; Wang, X. Nanostructure engineering and doping of conjugated carbon nitride semiconductors for hydrogen photosynthesis. Angew. Chem. Int. Ed. 2013, 52, 1735-1738. (13) Liu, J.; Huang, J.; Zhou, H.; Antonietti, M. Uniform graphitic carbon nitride nanorod for efficient photocatalytic hydrogen evolution and sustained photoenzymatic catalysis. ACS Appl. Mater. Interfaces 2014, 6 (11), 8434-8440. (14) Shi, L.; Chang, K.; Zhang, H.; Hai, X.; Yang, L.; Wang, T.; Ye, J. Drastic Enhancement of Photocatalytic Activities over Phosphoric Acid Protonated Porous g-C3N4 Nanosheets under Visible Light. Small 2016, 12, 4431-4439. (15) Liang, Q.; Li, Z.; Yu, X.; Huang, Z. H.; Kang, F.; Yang, Q. H. Macroscopic 3D porous graphitic carbon nitride monolith for enhanced photocatalytic hydrogen evolution. Adv. Mater. 2015, 27 (31), 4634-4639. (16) Wang, Y.; Zhang, J.; Wang, X.; Antonietti, M.; Li, H. Boron‐and Fluorine‐Containing Mesoporous Carbon Nitride Polymers: Metal‐Free Catalysts for Cyclohexane Oxidation. Angew. Chem. Int. Ed. 2010, 49 (19), 3356-3359. (17) Liu, G.; Niu, P.; Sun, C.; Smith, S. C.; Chen, Z.; Lu, G. Q.; Cheng, H.-M. Unique electronic structure induced high photoreactivity of sulfur-doped graphitic C3N4. J. Am. Chem. Soc. 2010, 132 (33), 11642-11648. (18) Zhu, Y. P.; Ren, T. Z.; Yuan, Z. Y. Mesoporous Phosphorus-Doped g-C3N4 Nanostructured Flowers with Superior Photocatalytic Hydrogen Evolution Performance. ACS Appl. Mater. Interfaces 2015, 7 (30), 16850-16856. (19) Zhang, G.; Zhang, M.; Ye, X.; Qiu, X.; Lin, S.; Wang, X. Iodine modified carbon nitride semiconductors as visible light photocatalysts for hydrogen evolution. Ad. Mater. 2014, 26 (5), 805-809. (20) Che, W.; Cheng, W.; Yao, T.; Tang, F.; Liu, W.; Su, H.; Huang, Y.; Liu, Q.; Liu, J.; Hu, F. Fast Photoelectron Transfer in (Cring)–C3N4 Plane Heterostructural Nanosheets for Overall Water Splitting. J. Am. Chem. Soc. 2017, 139 (8), 3021-3026. (21) Wang, Y.; Bai, X.; Qin, H.; Wang, F.; Li, Y.; Li, X.; Kang, S.; Zuo, Y.; Cui, L. Facile One-Step Synthesis of Hybrid Graphitic Carbon Nitride and Carbon Composites as HighPerformance Catalysts for CO2 Photocatalytic Conversion. ACS Appl. Mater. Interfaces 2016, 8 (27), 17212-17219. (22) Zhang, J.; Chen, X.; Takanabe, K.; Maeda, K.; Domen, K.; Epping, J. D.; Fu, X.; Antonietti, M.; Wang, X. Synthesis of a carbon nitride structure for visible‐light catalysis by copolymerization. Angew. Chem. Int. Ed. 2010, 49 (2), 441-444. (23) Wang, Y.; Shi, R.; Lin, J.; Zhu, Y. Enhancement of photocurrent and photocatalytic activity of ZnO hybridized with graphite-like C3N4. Energy Environ. Sci. 2011, 4 (8), 29222929. (24) Chen, J.; Zhao, D.; Diao, Z.; Wang, M.; Guo, L.; Shen, S. Bifunctional Modification of Graphitic Carbon Nitride with MgFe2O4 for Enhanced Photocatalytic Hydrogen Generation. ACS Appl. Mater. Interfaces 2015, 7 (33), 18843-18848. (25) Hong, Y.; Fang, Z.; Yin, B.; Luo, B.; Zhao, Y.; Shi, W.; Li, C. A visible-light-driven heterojunction for enhanced photocatalytic water splitting over Ta2O5 modified g-C3N4 photocatalyst. Inter. J. Hydrogen Energy 2016, 42 (10), 6738-6745. (26) Zheng, Y.; Yu, Z.; Ou, H.; Asiri, A. M.; Chen, Y.; Wang, X. Black Phosphorus and Polymeric Carbon Nitride Heterostructure for Photoinduced Molecular Oxygen Activation. Adv. Fun. Mater. 2018, 28, 1705407. (27) Dong, X.; Cheng, F. Recent development in exfoliated two-dimensional g-C3N4 nanosheets for photocatalytic applications. J. Mater. Chem. A 2015, 3 (47), 23642-23652. (28) Hong, Y.; Li, C.; Fang, Z.; Luo, B.; Shi, W. Rational synthesis of ultrathin graphitic carbon nitride nanosheets for ef-

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website. Experimental sections include the preparation of g-C3N4 nanosheets; photochemical measurement; additional Figures S1−S15, Table S1−S3, and more discussions.

AUTHOR INFORMATION Corresponding Author * Email: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported financially by the National Natural Science Foundation of China (21522603), the Henry Fok Education Foundation (141068), Six Talents Peak Project in Jiangsu Province (XCL-025), and the Chinese-German Cooperation Research Project (GZ1091).

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Table of Content Few-layered holey g-C3N4 nanosheets (CNS) are prepared by simply introducing a piece of nickel foam over the precursors. The released NH3 are partially decomposed into H2O for the modification of g-C3N4. The holey CNS with strengthened surface properties and abundant active cites thus show excellent photocatalytic activity under visible light.


One-Step Nickel Foam Assisted Synthesis of Holey G-Carbon Nitride

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