Protonation of Graphitic Carbon Nitride (g-C3N4) for an

Research Article pubs.acs.org/journal/ascecg

Protonation of Graphitic Carbon Nitride (g‑C3N4) for an Electrostatically Self-Assembling Carbon@g‑C3N4 Core−Shell Nanostructure toward High Hydrogen Evolution Longtao Ma,†,∥ Huiqing Fan,*,† Ke Fu,† Shenhui Lei,† Qingzhao Hu,‡ Haitao Huang,‡ and Geping He§ †

State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China ‡ Department of Applied Physics and Materials Research Center, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong § College of Materials and Mineral Resources, Xi’an University of Architecture and Technology, Xi’an 710055, China ∥ Department of Physics and Materials Science, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong S Supporting Information *

ABSTRACT: The development of new, appealing metal-free photocatalysts is of great significance for photocatalytic hydrogen evolution. Herein, an electrostatic self-assembly method to form a unique core−shell architecture of a colloid of carbon spheres with graphitic carbon nitride (gC3N4) has been developed by a one-step chemical solution route. The chemical protonation of g-C3N4 solids with strong oxidizing acids (such as HNO3) is an efficient pathway toward the sol procedure of stable carbon nitride colloids, which can cover the surface of carbon spheres via electrostatic adsorption. On account of the unique polymeric matrix of gC3N4 and reversible hydrogen bonding, the carbon@g-C3N4 derived from the sol solution showed high mechanical stability with broadened light absorption and enhanced conductivity for charge transport. Thus, the carbon@g-C3N4 core−shell structure exhibited remarkably enhanced photoelectrochemical performance. This polymer system is envisaged to hybridize with desirable functionalities (such as carbon nanorods) to form unique architectures for various applications. KEYWORDS: Polymeric carbon nitride, Electrostatic self-assembly, Metal-free core−shell nanostructure, Broadened light absorption, Hydrogen evolution

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INTRODUCTION

nating properties such as superior electrical, mechanical, and thermal properties offer a route to solve this challenge.17−19 It is reported that combining g-C3N4 with carbon materials such as carbon dots, graphene, and nanotubes could improve the conductivity and photoelectrochemical properties of g-C3N4 for the hydrogen evolution reaction or oxygen evolution.20−23 Although considerable efforts have been made toward the modification of g-C3N4 with conductive materials or heteroatoms to enhance its electronic and photoelectrochemical performance, few works are reported to assemble g-C3N4 into unique architectures toward highly efficient catalytic applications.24 Hence, we reported the rational assembly of a twodimensional (2D) in-situ-formed g-C3N4 shell on a carbon sphere to form a colloidal carbon@ graphitic polymeric carbon nitride (carbon@g-C3N4) core−shell nanostructure by a simple

Hydrogen is regarded as the most attractive clean energy source, and the photocatalytic hydrogen evolution reaction is one of the critical pathways to produce hydrogen from water splitting.1,2 General catalysts for the hydrogen evolution reaction are platinum and other noble metals, and it is challenging to exploit cost-effective, sustainable, and efficient visible-light photocatalysts with earth-abundant materials to make hydrogen energy a competitive energy source.3−7 Graphitic carbon nitride (g-C3N4) with a graphite-like structure as a metal-free and visible-light-responsive photocatalayst has drawn considerable attention in hydrogen production because of its appealing electronic band structure, highly physicochemical stability, and earth-abundant merits.8−11 However, its catalytic performance is currently narrowed by the nonresponse in the long wavelength region, poor electrical transport, low density of reactive sites, and high recombination of photoexcited electron−hole pairs.12−14 In addition, the stoichiometry of the as-synthesized g-C3N4 is hardly touched.15,16 Carbon-based materials possessing fasci© 2017 American Chemical Society

Received: April 27, 2017 Revised: June 14, 2017 Published: July 2, 2017 7093

DOI: 10.1021/acssuschemeng.7b01312 ACS Sustainable Chem. Eng. 2017, 5, 7093−7103

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ACS Sustainable Chemistry & Engineering

Figure 1. Schematic illustration of carbon-based material, tri-s-triazine, and g-C3N4 molecular structures.

Figure 2. Schematic illustrations of (a) the depolymerization−repolymerization process of g-C3N4, and (b) the preparation process of the carbon@ g-C3N4 core−shell photocatalyst.

water or organic solvents. These properties severely contribute to the process of constructing specific architectures of g-C3N4 with other nanostructured materials through conventional liquid chemistry.26 Enjoyably, the melon units are stacked together through hydrogen bonds between the strand nitrogen atoms and NH/NH2 groups. The melon units possessing a larger numbers of active sites act as proton-acceptors to form a local substructure similar to ammonium-ion species27 (the green dashed frame of Figure 1). This implies that a colloidal suspension of g-C3N4 could be formed by a series of a protonation procedure and depolymerization reactions. The protonated g-C3N4 can repolymerize after the removal of strong oxidants (as is shown inside the dashed frame of Figure 1). Additionally, the structure of tri-s-triazine and g-C3N4 is similar to that of the carbon layer. Thus, the g-C3N4 can be grown along the surface of carbon spheres (Figure 1). For the synthesized carbon spheres, a large amount of curved layers on the surface yield a weak τ−τ stacking interaction along with their locally curved spaces, which are benefits for the uniform

and facile chemical solution route because of the depolymerization−polymerization characterization of g-C3N4. Carbon@gC3N4 is a metal-free, highly efficient, stable visible-lightresponsive photocatalyst, which could be applied to largescale production. Such a core−shell architecture of carbon@gC3N4 provides a large accessible surface area, various electron transport channels, and short diffusion distances; these properties favor charge separation and transfer, and then accelerate the photoelectrochemical process. Gratifyingly, carbon@g-C3N4 exhibits an intensive light response in the long wavelengths of light spectra, which significantly improves photocatalytic performance.

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RESULTS AND DISCUSSION Methods. It is widely accepted that tri-s-triazine is the fundamental building block of carbon nitride and polymer networks. Additionally, most g-C3N4 with nitrogen-bridged poly(tri-s-triazine) structures known as “melon” structures possess high thermodynamic stabilities.25 It is stable up to 873 K in air or under chemical attack, and it does not dissolve in 7094


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electron microscopy (SEM) and transmission electron microscopy (TEM). Uniform carbon spheres with homogeneous distributions display smooth surfaces (Figure 4a). With their placement into g-C3N4 colloidal solution and the removal of the molecular liquid, the carbon spheres are uniformly dispersed in g-C3N4 gel (Figure 4b). After calcination of the carbon-protonated g-C3N4 mixture at 625 K for 2 h to completely remove molecular HNO3, the uniform carbon@gC3N4 core−shell structure is successfully obtained (Figure 4c,d). When the concentration of g-C3N4 is appropriate, the gC3N4 adheres to carbon spheres perfectly (Figure S3a in the Supporting Information). Interestingly, with the increase of sol concentration, some g-C3N4 nanosheets are located on the surface of the carbon@g-C3N4 core−shell structure and show a layered morphology, with a thickness of a few nanometers in the middle of the carbon spheres. This is ascribed to the growth fronts of the g-C3N4 layer located on the edge of carbon spheres and to the electrostatically repulsive interaction between the electrostatic layer on the surface of the carbon spheres and the protonated g-C3N4. This result can be further confirmed by the excess g-C3N4 in Figures S3b and S4 (Supporting Information). In addition, no g-C3N4 aggregates are observed on the surface of the carbon@g-C3N4 core−shells, indicating a strong binding of the g-C3N4 layer and carbon spheres and a graphite-like structure (Figure S5 in the Supporting Information). The structure contributes to its growth along with the layer direction. The high-magnification TEM images further confirm the nanospheres with diameters of around 200 nm (Figure 4e). The g-C3N4 layer with a thickness of approximately 3 nm is clearly distinguished from the carbon@g-C3N4 core−shell structure (Figure 4f), which further confirms the formation of the carbon@g-C3N4 core−shell structure. In addition, this structure can increase the surface area of the photocatalyst, and the specific surface area is an important factor for photocatalytic efficiency. Figure 5 exhibits the nitrogen adsorption−desorption isotherms for the carbon spheres, pristine g-C3N4, and carbon@g-C3N4. The carbon spheres obtain a Brunauer−Emmett−Teller (BET) surface area of 92.8 cm2 g−1, and the carbon@g-C3N4 core−shell structure shows a relatively high surface area (126.2 cm2 g−1), which is much higher than that of pristine g-C3N4 (12.8 cm2 g−1). The increase in surface area is ascribed to the unique architecture of coating colloid g-C3N4 on the surface of carbon spheres. The result indicates that the carbon spheres can effectively integrate with g-C3N4 to form a core−shell structure and alleviate the self-aggregation of g-C3N4 units. The crystal structures of carbon@g-C3N4, the carbon− protonated g-C3N4 mixture, and pristine g-C3N 4 were characterized by X-ray diffraction (XRD) pattern. The XRD characteristic peaks of carbon@g-C3N4 at 13.1° and 27.3° are assigned to (001) and (002) of a typical g-C3N4,27,31 which unambiguously disclose the existence of g-C3N4 (Figure 6a). It is widely accepted that the (002) plane is related to the interlayer reflection of graphene-like construction and that the (001) plane originates from the in-plane repeating motifs of the aromatic system.32,33 As shown in Figure 6a, the carbonprotonated g-C3N4 mixture only shows the (002) peak, which indicates that the in-plane aromatic systems are destroyed in the protonation process through the breakage of hydrogen bonds. The slight shift of the (002) peak from 27.3° to 27.1° is attributed to the loosely layered structure being swollen after hydrogen-ion insertion.28 The destroyed crystal structure is

distribution of carbon spheres in the g-C3N4 sol. Hence, a perfect carbon@g-C3N4 core−shell structure can be achieved. The bulk g-C3N4 accumulates with a large amount of aggregates (Figure S1 in the Supporting Information), which cannot be coated on the surface of carbon spheres. Gratifyingly, nitric acid (HNO3) could be employed as the medium for the solution processing of a colloidal suspension of g-C3N4 because of the strong oxidizing property of NO3−.28 In addition, the protons could easily intercalate into the layered structure of gC3N4 solids. They are selectively immobilized to the strand nitrogen atoms in a similar basic interaction (Figure 2a). Thus, this reaction greatly swells the bulk matrix and makes it ready for depolymerization.29 Zhang and Niu et al. reported that the structural hydrogen bonds among melon units are allowed to be broken by a strong oxidant (such as KMnO4 and HNO3), which facilitates the process toward depolymerizing g-C3N4 solids.28,30In addition, a homogeneous g-C3N4 colloid can form in a concentrated HNO3 solution at 353 K, and the colloids can repolymerize to larger ones by re-establishing the extended πconjugation systems after the complete removal of HNO3 by 625 K because of the soft nature of the polymeric matrix and the reversible formation of hydrogen bonds. This unique colloidal behavior of g-C3N4 favors the formation of the carbon@g-C3N4 core−shell photocatalyst. The representatively synthetic procedure of g-C3N4 coating on carbon spheres for the carbon@g-C3N4 core−shell structured photocatalyst is displayed in Figure 2b. On account of the protonation process, the g-C3N4 changes from a negatively charged surface to a positively charged surface. As is shown in Figure 3, the ζ potential value of protonated g-C3N4

Figure 3. ζ potentials of the carbon spheres, pristine g-C3N4, and protonated g-C3N4.

is +17.2 mV as compared to −15.7 mV for pristine g-C3N4. The carbon spheres exhibit a negatively charged surface (−32.6 mV of ζ potential value), because of oxygen-containing groups. It is evident that the spontaneous self-assembly process between positively charged protonated g-C3N4 and negatively charged carbon spheres occurs via electrostatic and τ−τ stacking interactions. In detail, the g-C3N4 is dispersed in concentrated HNO3 to form conjugated g-C3N4 sol solution (Figure S2 in the Supporting Information). Then, the acid active sites are formed inside of colloidal carbon spheres to induce the disproportionation of g-C3N4 sol on the surface of carbon spheres. The carbon spheres and g-C3N4 are consolidated along with the thermal repolymerization of g-C3N4 during the process of removal of HNO3 and the obtained carbon@g-C3N4. Material Characterizations. The surface morphology of as-synthesized carbon@g-C3N4 was investigated by scanning 7095


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Figure 4. SEM images of (a) carbon spheres, (b) complexes of carbon spheres and C3N4 gel, and (c) carbon@g-C3N4 core−shell photocatalysts. (d) TEM and (e) HR-TEM images of carbon@g-C3N4 core−shell photocatalyst. (f) Close-up of part e, detailing the width of the g-C3N4 shell.

Figure 5. N2 adsorption−desorption isotherms of (a) carbon spheres, (b) carbon@g-C3N4, and (c) pristine g-C3N4.

those of pristine g-C3N4, implying that the tri-s-triazine units are not destroyed in the process of depolymerization− repolymerization.34,37 In addition, there is a wide band zone at 600−800 cm−1 in the carbon@g-C3N4 core−shell structure, which demonstrates that an interaction between carbon spheres and g-C3N4 occurs because of the electrostatic self-assemble route. It is consistent with the ζ potential and TEM results. The chemical status and chemical composition of the constituent elements were measured by X-ray photoelectron spectroscopy (XPS). As displayed in Figure 7a, the C, N, and O elements are detected on the pristine g-C3N4 and the carbon@ g-C3N4 photocatalyst. The C 1s high-resolution spectra of carbon@g-C3N4 can be deconvoluted into five peaks according to the Gaussian fitting approach (Figure 7b). The peaks at 284.6 and 288.4 eV are assigned to the presence of sp2 CC bonds of graphitic carbon and sp2-hybridized carbon in Ncontaining aromatic system (NCN) configurations,

prone to recover after the removal of molecular HNO3 at 625 K. In addition, the characteristic (001) and (002) peaks are consistent with those of pristine g-C3N4, indicating that the gC3N4 has been successfully regenerated.34,35 The chemical structure of the as-synthesized products was characterized by FT-IR spectroscopy (Figure 6a). The FT-IR spectrum of pristine g-C3N4 displays the presence of a triazine ring sextant at 810 cm−1, tri-s-triazine (CNHC and CN stretching) at 1200−1620 cm−1, and NH stretching at 3000−3500 cm−1.36 Compared with that of pristine g-C3N4, the intensity of triazine-related groups in the carbon@g-C3N4 core−shell products significantly decreases, while that of the polar oxygen-containing functional groups at 1060 cm−1 (C OC stretching) appears and that of structural OH groups at 3416 cm−1 clearly increases, because there is little g-C3N4 coating on the carbon spheres. In addition, the characteristic peaks of carbon@g-C3N4 to recognize g-C3N4 are identical to 7096


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spectrum. In addition, a broad O 1s peak at 530.2 eV in pristine g-C3N4 derives from the adsorbed H2O or CO2. The N 1s high-resolution spectrum of carbon@g-C3N4 is deconvoluted to three peaks, which assume four species (Figure 7d). In detail, the weak peak at 404.1 eV is due to the protonation of g-C3N4, which makes carbon nitride heterocycles and cyano groups positively charged, which is consistent with the ζ potential.40 The dominant peak at 399.2 eV is characteristic of the bridging N atoms in N(C)3 or sp2bonded N atoms in the triazine system (CNC). The intensity of this peak is higher than that of g-C3N4, which is caused by the strong interaction between carbon spheres and N atoms in g-C3N4.41 In addition, the peak at 401.3 eV is assigned to the terminal amino groups (CNH), because of incomplete condensation in the process of thermal polymerization.42 Additionally, the weak peak centered at 404.2 eV is attributed to the charging effects or positive charge located at the heterocycles and cyano groups.43 These results imply that an electrochemical interaction occurs between carbon spheres and g-C3N4 in the carbon@g-C3N4 core−shells, corresponding to the results of FT-IR analysis. The introduction of carbon and the resultant heterojunction might provide novel optoelectronic properties. The spectral response characteristic is investigated using UV−vis diffuse reflectance spectra (DRS). As shown in Figure 8a, with the introduction of carbon spheres, carbon@g-C3N4 reveals that broad background absorption and light absorption intensity in both the UV- and visible-light regions increase, especially in the visible-light range, which is attributed to the superior light absorption in almost the entire wavelength of the narrower gap of the sp2 carbon cluster embedded in carbon spheres.44,45 The existence of an abundant sp2 carbon cluster can be confirmed by Raman spectra, because of the peak dispersion of the D and G peaks (Figure S6 in the Supporting Information). This is a typical behavior of carbon-based materials.37,46−48 For pristine g-C3N4, a response cutoff wavelength of ≈460 nm portrayed a high selectivity to the visible spectrum range corresponding to

Figure 6. (a) XRD pattern of the carbon@g-C3N4 core−shell, carbong-C3N4 mixture, and g-C3N4. (b) FT-IR spectra of pristine g-C3N4 and carbon@g-C3N4. (c) Enlarged FT-IR spectra corresponding to the gray ellipse image in part b.

respectively, which correspond to those of pristine g-C3N4.38 The weak peaks at 286.1 and 287.1 eV are ascribed to CN and CN coordination, respectively. This indicates that the N atoms in the lattice of g-C3N4 bond with the defect sp2hybridized carbon atoms or sp3 atoms in carbon spheres.39 Compared with pristine g-C3N4, the emerging peaks at 289.3 eV originate from the HOCO bonds of carbon spheres. In the O 1s spectrum (Figure 7c), the peaks centered at 533.1 and 531.4 eV are due to the existence of COH and HOCO stretching, which coincide well with the high-resolution C 1s

Figure 7. (a) XPS survey spectra and high-resolution spectra of (b) C 1s, (c) O 1s, and (d) N 1s core levels in carbon@g-C3N4 and pristine g-C3N4. 7097


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Figure 8. (a) UV−vis absorption spectra, (b) Kubelka−Munk transformation of UV−vis absorption spectra, (c) periodic on/off photocurrent response, and (d) comparison of LSV curves of g-C3N4 and the carbon@g-C3N4 core−shell heterojunction under longer 475 nm light.

Figure 9. (a) PL spectra of pristine g-C3N4 and carbon@g-C3N4. (b) I−V curves of pristine g-C3N4 and carbon@g-C3N4 under visible-light irradiation (λ > 420 nm). Electrochemical characterization of (c) Nyquist plots in dark and light irradiation, and (d) Bode plots for pristine g-C3N4 and carbon@g-C3N4.

its band gap (2.6 eV, 460 nm), and an evident red-shift of the absorption edge is observed for the carbon@g-C3N4 core−shell structure. These results infer that the g-C3N4 bandgap is clearly spread out to longer wavelengths, while the carbon spectrum has changed.46,49 The bandgap changes were attributed to an interface interaction between g-C3N4 and carbon spheres and the formation of chemical bonds for CNC and CN (Figure 7d).47 The enhanced photocurrent gives further expression of increased UV−vis absorption (Figure S7 in the Supporting Information). The trend of photoinduced electron intensity is in agreement with that of UV−vis adsorption. Additionally, the transient light response exhibited very good

stability for photodetection, and the photocurrent density of carbon@g-C3N4 is 4 times higher than that of pristine g-C3N4 (Figure 8c). These results illustrate that the constructive effect of the carbon@g-C3N4 core−shell structure promotes electron shuttling and suppresses charge recombination, which can be further confirmed by PL spectra (Figure 9a). For further information about the transport behaviors of the photogenerated charge carriers, the λ > 475 nm LSV techniques were employed. The intensity of the LSV response of the carbon spheres and carbon@g-C3N4 is higher than that of gC3N4. Under λ > 475 nm light irradiations, the LSV intensity of carbon@g-C3N4 is much stronger than that of pristine g-C3N4, 7098


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ACS Sustainable Chemistry & Engineering implying the higher separation efficiency of the photogenerated charge carriers in carbon@g-C3N4 because of the presence of the carbon spheres acting as electron reservoirs. Under visible-light irradiation, the π-conjugated carbon clusters, like organic dyes, can be excited, and then the photoexcited electrons transfer to g-C3N4, which causes the efficient separation of electron−hole pairs, resulting in the high response in λ > 475 nm light irradiations. This is in good agreement with the above transient photocurrent response measurement. The photoelectrochemical properties were investigated by I− V curves and EIS spectra (Figure 9). The typical I−V curves of the as-synthesized products are displayed in Figure 9b. Carbon@g-C3N4 exhibits around 3 times higher photocurrent density at potentials from −3 to 3 V than pristine g-C3N4. This can be attributed to the fact that the unique architecture increases the contact area and shortens the diffusion distance of photogenerated charges between reactive sites, which hinders the recombination of photoinduced electron−hole pairs effectively. 50 In addition, the incorporation of carbon significantly increases the efficiency of photogenerated electron−hole pairs, because of the enhancement of the lightharvesting capability. For further insight into the charge transport properties, electrochemical impedance spectroscopy (EIS) was conducted in dark and light irradiation (Figure 9c). The radius of the arc is related to the charge-transfer behavior at the electrode−electrolyte interface.51 Carbon@g-C3N4 displays a smaller charge-transfer resistance than the pristine g-C3N4 electrode both in dark and light irradiation. The results further confirm that this unique construction possesses the effective shuttling of charges between the electrode and electrolyte, and a faster interfacial charge transfer.52 The lifetime of photogenerated charges is investigated by the Bode plots of EIS spectra (Figure 9d), which could elucidate the electron-transfer capability, lifetime of charges, and recombination rate. The frequency from 1 to 103 Hz is correlated to the lifetime of charges as follows:

τ = 1/(2πf )

Figure 10. (a) Time-resolved fluorescence decay spectra of pristine gC3N4 and carbon@g-C3N4 monitored at 460 nm by single-photon counting and excited by incident light of 380 nm from a picosecondpulsed light-emitting diode at room temperature. The insert is the average lifetime of pristine g-C3N4 and carbon@g-C3N4. (b) Roomtemperature EPR spectra for pristine g-C3N4 and carbon@g-C3N4.

Table 1. Radiative Fluorescence Lifetimes and Relative Percentages of Photoinduced Charge Carriers in g-C3N4 and Carbon@g-C3N4 sample g-C3N4

carbon@g-C3N4

(1)

where the f is the frequency and the τ is the lifetime of charges. The characteristic peak of carbon@g-C3N4 shifts to a lower frequency, indicating a great electron transport capability and prolonged lifetime (42.59 and 17.26 ms for carbon@g-C3N4 and pristine g-C3N4, respectively). These properties can be further confirmed by a time-resolved fluorescence decay spectrum measurement (Figure 10). As is shown in Figure 10, the emission decay data could be fit by a triexponential function, and three components are derived. With the fit of the decay spectra, the radiative lifetimes with different percentages could be summarized in Table 1. Fascinatingly, compared with pristine g-C3N4, carbon@g-C3N4 yields a longer decay time, implying an accelerated charge-transfer mechanism induced by the introduction of carbon spheres and the unique architecture. The short lifetime of g-C3N4 (take τ1, as an example) correlated to the quickly carried charge recombination of the g-C3N4 excited state, whereas the corresponding τ1 is prolonged to 0.97 ns with the addition of the carbon sphere, which could be attributed to efficient electron transfer from the excited g-C3N4 to the EF of carbon spheres. The average lifetime of charge carriers also increases from 5.39 ns for pristine g-C3N4 to 9.91 ns for carbon@g-C3N4. This prolonged lifetime would lead to the increased probability of charge carriers, which can be

component

lifetime (ns)

intensity (%)

average lifetime (ns)

τ1 τ2 τ3 τ1 τ2 τ3 τ4

0.864 3.196 10.9449 0.9768 4.2218 11.795 35.8388

12.88 54.87 42.24 15.96 40.23 31.83 11.99

5.39 5.39 5.39 9.91 9.91 9.91 9.91

captured by reactive substrates to initiate the photocatalytic reactions.53,54 The extension of the covalent system was further evaluated by electron paramagnetic resonance (EPR) measurements (Figure 10b). The structure of g-C3N4 derived from the condensation of s-triazine units into an extended conjugated system, and the EPR could be used to investigate the extent of conjugation.51,52 Both samples show only one Lorentzian line with a g value of 2.0034, which is attributed to delocalized electrons on sp2 carbon atoms of the aromatic rings within bonded nanosized clusters.53,54 In addition, compared with that from pristine g-C3N4, the EPR intensity of the resonance from carbon@g-C3N4 is remarkably enhanced correlating very well with the increased surface activity. The result demonstrates the increased density state of the conduction band after the electron donation from the carbon spheres. A significantly enhanced EPR intensity is detected when g-C3N4 and carbon@ g-C3N4 are irradiated with visible light, indicating the efficient photochemical generation of radical pairs in the photocatalyst. Evaluation of Photocatalytic Activity for Hydrogen Evolution. The photocatalytic activity of the as-synthesized 7099


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Figure 11. (a) H2 evolution over pristine g-C3N4, carbon@g-C3N4, and carbon sphere photocatalysts. (b) Production yield of H2 over pristine gC3N4, carbon@g-C3N4, and carbon sphere photocatalysts to evaluate reproducibility by cycle test. (c) Stability test and (d) wavelength dependence of H2 evolution of pristine g-C3N4 and carbon@g-C3N4 photocatalysts.

of the most effective methods to improve photocatalytic performance, and the enhanced photocatalytic activity is ascribed to the efficient light absorption and the formation of the unique core−shell structure. Importantly, this method is simple and cost-efficient, and can be applied to large-scale production. In detail, the introduction of carbon spheres and the formation of the carbon@g-C3N4 core−shell structure significantly improve the light absorption and benefit the separation of photogenerated electron−hole pairs through the internal electrostatic field in the interlayer of carbon spheres and g-C3N4; thus, more charge carriers participate in the photocatalytic reaction. Mechanisms of Photocatalysis Enhancement in Carbon@g-C3N4 Core−Shell Photocatalysts. For further verification of the mechanism of the carbon@g-C3N4 core− shell photocatalyst for hydrogen evolution under visible-light irradiation, the generated •OH in the photocatalytic reaction process was detected. In detail, the •OH could react with terephthalic acid to produce 2-hydroxy terephthalic acid, and the 2-hydroxy terephthalic acid shows a fluorescence peak at 426 nm. The quantity of •OH groups can be assessed by the intensity of the fluorescence peak. As Figure 12 shows, a strong fluorescence is obtained under UV−vis-light irradiation, indicating a large amount of •OH groups via the photoexcited holes from g-C3N4 in the carbon@g-C3N4 photocatalysts. In addition, a good linear relationship between the fluorescent intensity and irradiation time reveals a good stability of carbon@g-C3N4 during the photocatalytic reaction. However, there is barely any fluorescence detected for carbon@g-C3N4 under visible-light irradiation (λ > 475 nm), implying a small amount of •OH formed (Figure 12). This is a sign that a few “electron−hole” pairs are generated from g-C3N4 under visiblelight irradiation (λ > 475 nm), and the visible-light-driven photocatalytic hydrogen-evolution behavior of carbon@g-C3N4 should be attributed to the photosensitization effect of carbon spheres. The π-conjugated carbon clusters of carbon spheres in carbon@g-C3N4 serve as a photosensitizer, like organic dyes, and sensitize g-C3N4 through the newly formed C−N−C bond

products was evaluated by investigating the hydrogen evolution from water splitting under visible-light irradiation. The reaction was conducted in a system using triethanolamine (TEOA) as a sacrificial agent, and the H2 evolution is confirmed by gas chromatography. The hydrogen evolution of the as-synthesized photocatalyst increases along with the time of irradiation (Figure 11a). The hydrogen evolution rate dependence on the amount of trithanolamine (TEOA) is shown in Figure S5. It is seen that the hydrogen evolution rate increases with the increase in the amount of TEOA, and it reaches a maximum when the TEOA is 10% (Figure S8 in the Supporting Information). Thus, the following reaction is conducted under this condition. In addition, the hydrogen evolution rate (HER) of carbon@g-C3N4 increases along with the increase of g-C3N4 (Figure S9 in the Supporting Information), and that is up to 129 mol h−1 under visible-light irradiation (λ > 420 nm), when the thickness of the g-C3N4 layer is 3 nm. The HER of it is 8 times than that of pristine g-C3N4 (16 mol h−1, Figure 11b). With the increasing introduction of g-C3N4, the HER of carbon@g-C3N4 declines, but the HER is higher than that of pristine g-C3N4. This is due to the emergence of g-C3N4 nanosheets among the core−shell structure, and the HERs of nanosheets are higher than that of pristine g-C3N4 as we previous reported. In addition, the pristine g-C3N4 and carbon@g-C3N4 photocatalysts manifest great stability after three cycle measurements (Figure 11c). The wavelength dependence of HER for g-C3N4 and carbon@g-C3N4 is correlated with the optical absorption spectrum and portrayed a stable improvement in HER of carbon@g-C3N4 over the whole range of optical activity (Figure 11d). In fact, the active wavelength of carbon@g-C3N4 is extended to 600 nm. Hence, the enhanced absorption ability favors the improvement of photocatalysis for hydrogen evolution. A comparison with other reported carbon nitrides for hydrogen production is shown in Table S1 (Supporting Information). The photocatalytic activity of the presented carbon@g-C3N4 core−shell photocatalyst is significantly superior to those previously reported. In fact, the construction of the carbon@g-C3N4 core−shell structure is one 7100


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enhancing the redox ability of the charge carriers and photocatalytic performance. For carbon, there are amounts of carbon clusters, and they show various energy levels. In the photocatalytic process of the carbon@g-C3N4 structure, the photogenerated electrons are injected into the conduction band of g-C3N4, allowing the reaction with hydrogen ions of adsorbed molecular water. The schematic illustration of this mechanism is displayed in Figure 13a. In addition, the photoinduced electrons are generated from the valence band to the conduction band of g-C3N4 by the excitement of the high-energy photons. The photogenerated electrons formed in the space-charge regions are transferred into the carbon spheres, and the holes remain on g-C3N4 to participate in the redox reaction. The schematic illustration of this mechanism is displayed in Figure 13b. This process significantly improves the efficiency toward separation of photogenerated electron−hole pairs. In summary, with an increased photoabsorption capability, the carbon spheres act as a sensitizer and reservoir to improve the separation efficiency of photogenerated electron−hole pairs and enhance the redox ability of the charge carriers.

Figure 12. Time dependencies of the fluorescence intensity at 426 nm by irradiating the carbon@g-C3N4 suspension containing 3 mM terephthalic acid under UV−vis-light irradiation and visible-light (λ > 475 nm) irradiation for 40 min.

between the carbon spheres and carbon@g-C3N4 already made evident by the above FT-IR and XPS measurements. It is widely accepted that the light absorption capability of the photocatalyst and the capability of separation for photogenerated electron−hole pairs are crucial for photocatalytic reaction. There are two mechanisms being discussed to analyze the enhancement of the photocatalytic performance of carbon@g-C3N4 core−shells. In the first, there are large amounts of carbon clusters that exhibit superior light absorption in nearly the entire wavelength, because of the narrower gap of the sp2 carbon cluster. With the introduction of carbon spheres, carbon@g-C3N4 reveals broad background absorption, and the light absorption intensity in both the UVand visible-light regions increases, especially in the visible-light range. This provides more opportunities for photoinduced charge carriers, which favor the improvement of photocatalytic activity. The second mechanism is that the carbon spheres act as a sensitizer and transfer the photogenerated electrons to g-C3N4. For confirmation of the relative position of the conduction band (CB) and valence band (VB), the total densities of states of the VB and Mott−Schottky plots are carried out. The Mott− Schottky plots confirm that the CB minimum up-shifts from −1.28 to −1.67 eV after construction of the carbon@g-C3N4 core−shell structure as observed in Figure S10a (Supporting Information). From Figure S10b (Supporting Information), compared to bulk g-C3N4, layered g-C3N4 in carbon@g-C3N4 core−shell structures shows a VB maximum down-shift from 1.38 to 1.15 eV. The increase of the VB width narrows the distance of potential between carbon nitride and water,

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CONCLUSIONS The highly efficient carbon@g-C3N4 core−shell structure has been successfully fabricated through the protonation of g-C3N4, the depolymerization−repolymerization procedure of g-C3N4, and the electrostatic self-assembly route on the surface of carbon spheres, because of the soft nature of the polymeric matrix and the reversible formation of the hydrogen-bond. The results show that carbon@g-C3N4 exhibits superior photocatalytic activity and remarkably better light absorption capability than pristine g-C3N4. This outstanding photocatalytic activity can be attributed to the narrower gap sp2 carbon cluster embedded in carbon spheres, the promoted charge-carrier transport capability, prolonged electron lifetime, and lower recombination of electron−hole pairs. The high flexibility and compatibility of the g-C3N4 colloidal solution provide a path for the polymer system to decorate desired functionalities, and it is a new strategy toward decorating carbon materials with g-C3N4.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01312.

Figure 13. Proposed mechanism for the carbon-spheres-mediated enhancement of photocatalysis. (a) Electron−hole pairs in the carbon spheres are generated under visible-light irradiation. On the basis of the relevant position of the band, the electrons inject in g-C3N4. (b) Carbon spheres as an electron sink scavenge away the electrons, thus hindering the recombination of electron−hole pairs. 7101


Research Article


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Experimental details, SEM and TEM images, ideal gC3N4 structure, Raman spectra, transient photoresponse, XPS high revolution, and Mott−Schottky spectra (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. ORCID

Longtao Ma: 0000-0002-2942-3221 Shenhui Lei: 0000-0001-6591-2228 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation (51672220), the 111 Program (B08040) of MOE, the National Defense Science Foundation (32102060303), the Fundamental Research Funds for the Central Universities (3102014JGY01004), the Xi’an Science and Technology Foundation, the Shaanxi Provincial Science Foundation, and the NPU Gaofeng Project of China.

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Protonation of Graphitic Carbon Nitride (g-C3N4) for an

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