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

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Protonation of graphitic carbon nitride (g-C3N4) for electrostatically self-assembling Carbon@ g-C3N4 coreshell nanostructure toward high hydrogen evolution Longtao Ma, Huiqing Fan, Ke Fu, Shenhui Lei, Qingzhao Hu, Haitao Huang, and Geping He ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.7b01312 • Publication Date (Web): 02 Jul 2017 Downloaded from http://pubs.acs.org on July 2, 2017

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ACS Sustainable Chemistry & Engineering

Protonation of graphitic carbon nitride (g-C3N4) for electrostatically self-assembling Carbon@ g-C3N4 core-shell nanostructure toward high hydrogen evolution Longtao Maa, d, Huiqing Fan*a, Ke Fua, Shenhui Leia, Qingzhao Hub, Haitao Huangb, Geping Hec a

State Key Laboratory of Solidification Processing, School of Materials Science and

Engineering, Northwestern Polytechnical University, Xi’an 710072, China b

Department of Applied Physics and Materials Research Center, the Hong Kong

Polytechnic University, Hung Hom, Kowloon, Hong Kong c

College of Materials and Mineral Resources, Xi’an University of Architecture and

Technology, Xi’an 710055, China d

Department of Physics and Materials Science, City University of Hong Kong, 83 Tat

Chee Avenue, Kowloon, Hong Kong E-mail: [email protected]; [email protected]

ACS Paragon Plus Environment


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ABSTRACT The development of new appealing metal free photocatalyst is of great significance for photocatalytic hydrogen evolution. Herein, an electrostatically self-assemble method to form unique core-shell architecture of carbon spheres with graphitic carbon nitride (g-C3N4) colloid has been developed by 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 a stable carbon nitride colloids, which can cover the surface of carbon spheres due to the electrostatic adsorption. On account of the unique polymeric matrix of g-C3N4 and reversible hydrogen bonding, the carbon@g-C3N4 derived from sol solution showed a high mechanical stability with broadened light absorption and enhanced conductivity for charge transport. Thus, the carbon@g-C3N4 core-shell structure exhibited a remarkably enhanced photo-electrochemical performance. This polymer system is envisaged to hybrid with desirable functionalities (such as carbon nanorods) to form unique architecture for various application.

KEYWORDS: Polymeric carbon nitride; electrostatically self-assembly; Metal-free core-shell nanostructure; Broadened light absorption; Hydrogen evolution.


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INTRODUCTION Hydrogen is regarded as the most attractive clean energy and the photocatalytic hydrogen evolution reaction is one of the critical pathways to produce hydrogen from water splitting1,2. General catalysts for hydrogen evolution reaction are platinum and other noble metals, and it is challengeable to exploit cost-effective, sustainable and efficient visible-light photocatalysts with earth abundant materials to make hydrogen energy a competitive energy source3-7. Graphitic carbon nitride (g-C3N4) with a graphite-like structure as a metal-free and visible-light-responsive photocatalayst has drawn considerable attentions in hydrogen production due to its appealing electronic band structure, highly physicochemical stability and earth-abundant merits8-11. However, its catalytic performance is currently narrowed by the nonresponse on the long wavelength region, poor electrical transport, the low density of reactive sites and high recombination of photo-excited electrons-holes12-14. In addition, the stoichiometry of the as-synthesized g-C3N4 is hardly touched15,16. Carbon-based materials possessing fascinating properties such as superior electrical, mechanical and thermal properties offer a route to solve this challenge17-19. It is reported that combining g-C3N4 with carbon materials such as carbon dots, graphene and nanotubes could improve the conductivity and photo-electrochemical properties of g-C3N4 for hydrogen evolution reaction or oxygen evolution20-23. Although considerable efforts have been made toward the modification of g-C3N4 with conductive materials or heteroatoms to



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enhance its electronic and photo-electrochemical performance, few works are reported to assemble g-C3N4 into unique architectures toward highly efficient catalytic application24. Hence, we reported the rational assembly of two-dimension (2D) in situ formed g-C3N4 shell on carbon sphere to form colloidal carbon@ graphitic polymeric carbon nitride (carbon@g-C3N4) core-shell nanostructure by a simple

and

facile

chemical

depolymerization-polymerization

solution

route

characterization

owing of

to

g-C3N4.

the The

carbon@g-C3N4 is a metal-free, highly efficient, stable visible-light responsible photocatalyst, which could be applied to large-scale production. Such core-shell architecture of carbon@g-C3N4 provides a large accessible surface area, various electron transport channels, short diffusion distances, which is in favor of charge separation and transfer, and then accelerates the photo-electrochemical process. Gratifyingly, the carbon@g-C3N4 exhibits intensive light response in the

long

wavelength

of

light

spectra,

which

significantly

improves

photocatalytic performance.

RESULTS AND DISCUSSION Methods It is widely accepted that the tri-s-triazine is the fundamental building block of the carbon nitride and polymer networks. What’s more, most g-C3N4 with nitrogen-bridged poly (tri-s-triazine) structure known as “melon”


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possesses high thermodynamic stability25. It is stable up to 873 K in air or under chemical attack, and it does not dissolve in water or organic solvents. These severely render the process of constructing specific architecture 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-acceptor to form a local substructure similar to ammonium ion species

27

(the green dashed frame

of Fig.1). This implies that a colloidal suspension of g-C3N4 could be formed by a series of protonation procedure and depolymerization reactions. The protonated g-C3N4 can re-polymerize after removal of strong oxidants (as is shown inside the dashed frame of Fig.1). Besides, the structure of tri-s-triazine and g-C3N4 is similar that of carbon layer. Thus, the g-C3N4 can be grown along the surface of carbon spheres (Fig.1). For the synthesized carbon spheres, a large amount of curved layers on the surface yield a weak − stacking interaction along with their locally curved spaces, which is benefit for uniform 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 (Fig.S1. Supporting Information), which cannot be coated on the surface of carbon spheres. Gratifyingly, the nitric acid (HNO3) could be employed as the medium for the solution processing of a colloidal suspension of g-C3N4 owing to the



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NO3-’s strong oxidizing property28. In addition, the protons could easily intercalate into the layered structure of g-C3N4 solids. They are selectively immobilized to the strand nitrogen atoms in similar basic interaction (Fig.2a). Thus, this reaction greatly swells the bulk matrix and makes it ready for depolymerization

29

. Zhang, Niu et al, reported that the structural hydrogen

bonds among melon units are allowed to be broken by strong oxidant (such as KMnO4 and HNO3), which facilitates the process towards depolymerizing g-C3N4 solids

28 30

.In addition, a homogeneous g-C3N4 colloid can form in a

concentrated HNO3 solution at 353 K and the colloids can re-polymerize to larger ones by re-establishing the extended π-conjugation systems after complete removal of HNO3 by at 625 K due to the soft nature of the polymeric matrix and the reversible formation of hydrogen-bonds. This unique colloidal behavior of g-C3N4 is in favor of the formation of 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 Fig.2b. On accounts of protonation process, the g-C3N4 changes from negatively charged surface to a positively charged surface. As is shown in Fig.3, zeta potential value of protonated g-C3N4 is +17.2 mV as compared to that of -15.7 mV for pristine g-C3N4. The carbon spheres exhibit negatively charged

surface

(

-32.6

mV

of

zeta

potential

value),

owing

to

oxygen-containing groups. It is evident that the spontaneous self-assemble


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process between positively charged protonated g-C3N4 and negatively charged carbon spheres occurs due to electrostatic and π − π stacking interaction. In detail, the g-C3N4 is dispersed in concentrated HNO3 to form conjugated g-C3N4 sol solution (Fig.S2. 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 thermal re-polymerization of g-C3N4 during the process of removal HNO3 and the carbon@ g-C3N4 obtained.

Fig.1 Schematic illustration of carbon-based materials, tri-s-triazine and g-C3N4 molecular structure. Materials characterizations The

surface

morphology

of

as-synthesized

carbon@g-C3N4

was

investigated by scanning electron microscope (SEM), transmission electron microscope (TEM). Uniform carbon spheres with the homogeneous distribution



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display smooth surface (Fig4a). Putting them into g-C3N4 colloidal solution and removing the liquid molecular, the carbon spheres are dispersed in g-C3N4 gel uniformly (Fig.4b). After calcination of the carbon-protonated g-C3N4 mixture at 625 K for 2h to remove HNO3 molecular completely, the uniform carbon@g-C3N4 core-shell structure is obtained successfully (Fig.4c, d). When the concentration of g-C3N4 is appropriate, the g-C3N4 adheres to carbon spheres perfectly (Fig.3a. Supporting Information). Interestingly, with the increase of sol concentration, some g-C3N4 nanosheets are located on the surface of carbon@g-C3N4 core-shell structure and show a layered morphology, with thickness of a few nanometers in the middle of carbon spheres. It is ascribed to the growth fronts of g-C3N4 layer located on the edge of carbon spheres and electrostatically repulsive interaction between electrostatic layer on surface of carbon spheres and protonated g-C3N4. This result can be further confirmed by excess g-C3N4 in Fig.S3b and Fig.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 g-C3N4 layer and carbon spheres and graphite-like structure (Fig.S5. Supporting Information). The structure contributes to its growth along with layer direction. The high magnification TEM images further confirm the nanospheres with diameter of around 200 nm (Fig.4e). The g-C3N4 layer with a thickness of approximately 3 nm is clearly distinguished from carbon@g-C3N4 core-shell structure (Fig.4f), which further confirms the formation of carbon@g-C3N4 core-shell structure.


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

Fig.3 Zeta potential of the carbon spheres, pristine g-C3N4 and protonated g-C3N4



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Fig.4 SEM images of (a) carbon spheres, (b) complexes of carbon spheres and C3N4 gel. (c) carbon@g-C3N4 core-shell photocatalyst. (d) TEM image, (e) HR-TEM image of carbon@g-C3N4 core-shell photocatalyst.

In addition, this structure can increase the surface area of the photocatalyst and the specific surface area is an important factor for photocatalytic efficiency. Fig.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


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g-C3N4 on the surface of carbon spheres. The result indicates that the carbon spheres can effectively integrate with g-C3N4 to form core-shell structure and alleviates self-aggregation of g-C3N4 units.

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

The crystal structures of carbon@g-C3N4, carbon-protonated g-C3N4 mixture and pristine g-C3N4 were characterized by X-ray diffraction pattern (XRD). The XRD characteristic peaks of carbon@g-C3N4 at 13.1o and 27.3o are assigned to (001）and (002) of a typical g-C3N4 27,31, which unambiguously disclose the existence of g-C3N4 (Fig.6a). It is widely accepted that the (002)



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plane is related to the interlayer reflection of graphene-like construction and the (001) plane originates from the in-plane repeating motifs of the aromatic system32,33. As shown in Fig.6a, carbon-protonated g-C3N4 mixture only exists (002) peak, which indicates the in-plane aromatic systems are destroyed in the protonation process through the breakage of hydrogen bonds. The slightly shift of (002) peak from 27.3o to 27.1o is attributed to the loose layered structure swollen after hydrogen ions insertion28. The destroyed crystal structure is prone to recover after removing HNO3 molecular at 625 K. In addition, the characteristic (001) and (002) peaks are consistent with that of pristine g-C3N4, indicating that the g-C3N4 has been successfully regenerated34,35. The chemical structure of the as-synthesized products was characterized by FT-IR spectroscopy (Fig.6a).

FT-IR spectrum of pristine g-C3N4 displays

the presence of 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 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 structural O-H groups at 3416 cm-1 clearly increases, because there is few g-C3N4 coating on the carbon spheres. In addition, the characteristic peaks of carbon@g-C3N4 to recognize g-C3N4 is identical to the pristine g-C3N4, implying the tri-s-triazine unites are not destroyed in the process of depolymerization-repolymerization34,37. In addition, there is a wide band bone


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at 600-800 cm-1 in carbon@g-C3N4 core-shell structure, which demonstrates that an interaction between carbon spheres and g-C3N4 occurs owing to the electrostatic self-assemble route. It is consistent with the zeta potential and TEM results. The chemical status and chemical composition of the constituent elements were measured by X-ray photoelectron spectroscopy (XPS). As is displayed in Fig.7a, the C, N and O elements are detected on the pristine g-C3N4 and the carbon@g-C3N4

photocatalyst.

The

C1s

high-resolution

spectra

of

carbon@g-C3N4 can be deconvoluted into five peaks according to Gaussian fitting approach (Fig.7b). The peaks at 284.6 eV and 288.4 eV are assigned to the presence of sp2 C-C bonds of graphitic carbon and sp2 -hybridized carbon in N-containing aromatic systems (N-C=N) configuration, respectively, which is corresponding to that of pristine g-C3N438. The weak peaks at 286.1 eV 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 sp2 -hybridized carbon atoms or sp3 atoms in carbon spheres39. Compared with pristine g-C3N4, the emerging peaks at 289.3 eV originates from HO-C=O bonds of carbon spheres. In the O1s spectrum (Fig.7c), the peaks centered at 533.1 and 531.4 eV owe to the existence of C-OH and HO-C=O stretching, which is coincided well with high-resolution C 1s spectrum. In addition, a broad O 1s peak at 530.2 eV in the pristine g-C3N4 derives from the adsorbed H2O or CO2.



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Fig.6 (a) XRD pattern of carbon@g-C3N4 core-shell, carbon-g-C3N4 mixture and g-C3N4. (b) FT-IR spectra of the pristine g-C3N4 and the carbon@g-C3N4, (c) enlarged FT-IR spectra corresponding to the gray ellipse image in panel (b).

The N1s high-resolution spectrum of carbon@ g-C3N4 is deconvoluted to three peaks, which is the assumption of four species (Fig.7d). In details, the weak peak at 404.1 eV owes to the protonation of g-C3N4, which makes carbon nitride heterocycles and cyano groups positively charged, which is in consistent


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with zeta potential40. The dominant peak at 399.2 eV is characteristics of the bridging N atoms in N-(C)3 or sp2 -bonded N atoms in 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-C3N441. 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 polymerization42. Besides, the weak peak centered at 404.2 eV is attributed to the charging effects or positive charge location at the heterocycles and cyano groups43. 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.



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Fig.7 (a) XPS survey spectra and high-resolution spectra of (b) C1s, O1s and N1s core levels in carbon@ g-C3N4 and pristine g-C3N4. 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 Fig.8a, with the introduction of carbon spheres, the carbon@g-C3N4 reveals broad background absorption and light absorption intensity in both the UV and visible light regions increases, especially in visible-light range, which is attributed to the superior light absorption in barely entire wavelength of the narrower gap of sp2 carbon cluster embedded in carbon spheres44,45. The existence of abundant sp2 carbon cluster can be confirmed by Raman spectra, due to the peak dispersion of the D and G speaks (Fig.S6. Supporting Information). This is a typical behaviour of carbon-based materials37,46-48. For pristine g-C3N4, a response cut-off wavelength of ≈460 nm portrayed a high selectivity to the visible spectrum range corresponding to 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 smeared out to longer wavelengths, whilst the carbon spectrum has changed46,49. The bandgap changes were attributed to interface interaction between g-C3N4 and carbon spheres, the formation of chemical bonds for C=N-C and –CN (Fig.7d)47. The enhanced photocurrent further gives expression of increased UV-vis absorption (Fig.S7. Supporting Information).


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The trend of photo-induced electrons intensity is in agreement with that of UV-vis adsorption. Besides, the transient light response exhibited a very good stability for photo-detection and the photocurrent density of carbon@g-C3N4 is 4 times higher than that of pristine g-C3N4(Fig.8c). These results illustrate that the constructive effect of carbon@g-C3N4 core-shell structure promotes electron shuttling and suppresses charge recombination, which can be further confirmed by PL spectra (Fig.9a).

Fig.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,

Carbon@g-C3N4

heterojunction and carbon spheres under lager 475nm light.


core-shell


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To get further information about the transport behaviours of the photo-generated charge carries, the λ>475 LSV techniques were employed. The intensity of LSV response of the carbon spheres and carbon@g-C3N4 is higher than that of g-C3N4. Under λ>475 light irradiations, the LSV intensity of carbon@g-C3N4 is much stronger than that of pristine g-C3N4, implying the higher separation efficiency of the photo-generated charge carries in the carbon@g-C3N4 due to 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 photo-excited electrons transfer to g-C3N4, which causes the efficient separation of electron–hole pairs, resulting in the high response in λ>475 light irradiations. This is in good agreement with the above transient photocurrent response measurement. The photo-electrochemical properties were investigated by I-V curves and EIS spectra (Fig.9). The typical I-V curves of as-synthesized products are displayed in Fig.9b. The carbon@g-C3N4 exhibits around three times higher photocurrent density at potential from -3 V to 3 V than that of pristine g-C3N4. This can be attributed to the fact that the unique architecture increases the contact area and shortens the diffusion distance of photo-generated charges between reactive sites, which hinders the recombination of photo-induced electron-hole pairs effectively50. In addition, the incorporation of carbon


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significantly increases the efficiency of photo-generated electron-hole pairs, owing to the enhancement of light-harvesting capability. To further get insight into

the

charge

transport

properties,

the

electrochemical

impedance

spectroscopy (EIS) was conducted in dark and light irradiation (Fig.9c). The radius of the arc is related to the charge transfer behavior at the interface of electrode-electrolyte51. The carbon@g-C3N4 displays smaller charge-transfer resistance than pristine g-C3N4 electrode both in dark and light irradiation. The results further confirm that this unique construction possesses effective shuttling of charges between the electrode and electrolyte, and a faster interfacial charge transfer52.

Fig.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>420nm).



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Electrochemical characterization of (c) Nyquist plots in dark and light irradiation, and (d) bode plots for pristine g-C3N4 and carbon@ g-C3N4.

The lifetime of photo-generated charges is investigated by bode plots of EIS spectra (Fig.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. τ = 1/(2π )

(1)

where, the f is the frequency and the τ is the lifetime of charges. The characteristic peak of carbon@g-C3N4 shifts to lower frequency, indicating a great electron transport capability, prolonged lifetime (42.59 ms and 17.26 ms for carbon@g-C3N4 and pristine g-C3N4, respectively). These properties can be further confirmed by time-resolved fluorescence decay spectrum measurement (Fig.10). As is shown in Fig.10, the emission decay data could be fitted by tri-exponential function and three components are derived. By fitting the decay spectra, the radiative lifetimes with different percentages could be summarized in Table 1. Fascinatingly, compared with pristine g-C3N4, the carbon@g-C3N4 yield 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) for correlated to the fast-carried charge recombination of g-C3N4 excited state, whereas the corresponding of τ1 is prolonged to 0.97 ns with the addition of carbon sphere, which could be


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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 captured by reactive substrates to initiate the photocatalytic reactions53,54.

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



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carbon@g-C3N4. (b) Room-temperature EPR spectra for pristine g-C3N4 and carbon@ g-C3N4.

Table 1 The radiative fluorescence lifetimes and the relative percentages of photo-induced charge carriers in g-C3N4 and Carbon@g-C3N4. Lifetime Samples

g-C3N4

Intensity

Components

Average lifetime(ns) (ns)

(%)

τ1

0.864

12.88

τ2

3.196

54.87

τ3

10.9449

42.24

τ1

0.9768

15.96

τ2

4.2218

40.23

Carbon@g-C3N4

5.39

9.91 τ3

11.795

31.83

τ4

35.8388

11.99

The extension of the covalent system was further evaluated by Electron Paramagnetic Resonance (EPR) measurement (Fig.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 conjugation51, 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 nano-sized clusters53, 54. In addition, compared


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with 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 conduction band after the electrons 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 efficient photochemical generation of radical pairs in the photocatalyst55,56.

Evaluation of photocatalytic activity for hydrogen evolution The photocatalytic activity of as-synthesized 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 sacrificial agent and the H2 evolution is confirmed by gas chromatography. The hydrogen evolution of as-synthesized photocatalyst increases along with the time of irradiation (Fig.11a). The hydrogen evolution rate depending on amount of trithanolamine (TEOA) is shown in Fig.S5. It is seen that the hydrogen evolution rate increases with the increase of amount of TEOA and it is up to maximum when the TEOA is 10% (Fig.S8. Supporting Information). Thus, the following reaction is conducted under this condition. In addition, the hydrogen evolution rate (HER) of carbon@g-C3N4 increase along with the increase of g-C3N4 (Fig.S9. Supporting Information) and that is up to 129 mol h-1 under visible light irradiation (λ>420nm), when the thickness of the



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g-C3N4 layer is 3 nm. The HER of it is eight times than that of pristine g-C3N4 (16 mol h-1) (Fig.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 HER of nanosheets are higher than that of pristine g-C3N4 as we previous reported57, 58. In addition, the pristine g-C3N4 and carbon@g-C3N4 photocatalysts manifest a great stability after three cycle measurement (Fig.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 (Fig.11d). In fact, the active wavelength of carbon@g-C3N4 is extended to 600 nm. Hence, the enhanced absorption ability is in favour of improvement of photocatalysis for hydrogen evolution. A comparison with other reported carbon nitrides for hydrogen production is shown in Table.S1 (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 carbon@g-C3N4 core-shell structure is one of the most effective method 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


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formation of carbon@g-C3N4 core-shell structure significantly improve the light absorption and benefit the separation of photo-generated 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.

Fig.11 (a) H2 evolution over pristine g-C3N4, carbon@g-C3N4 and carbon spheres photocatalysts. (b) Production yield of H2 over pristine g-C3N4, carbon@g-C3N4 and carbon spheres 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.

Mechanisms of photocatalysis enhancement in carbon@g-C3N4 core-shell photocatalyst



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To further verify the mechanism of the carbon@g-C3N4 core-shell photocatalysts for hydrogen evolution under visible light irradiation, the generated ·OH in 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 nm59,60. The quantity of ·OH group can be assessed by the intensity of fluorescence peak. As Fig.12 shown, a strong fluorescence is obtained under UV-Vis light irradiation, indicating a large amount of ·OH due to the photo-excited 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 the carbon@g-C3N4 during the photocatalytic reaction. However, there is barely no fluorescence detected for the carbon@g-C3N4 under visible light irradiation (λ > 475 nm), implying a little formed ·OH (Fig.12). This is assigned that a little “electron–hole” pairs are generated from g-C3N4 under visible light 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 the carbon@g-C3N4 serve as a photosensitizer, like organic dyes, and sensitize g-C3N4 through the newly formed C–N–C bond between the carbon spheres and carbon@g-C3N4 already evidenced by the above FT-IR and XPS measurements.


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It is widely accepted that the light absorption capability of the photocatalyst and the capability of separation for photo-generated electron-hole pairs are crucial for photocatalytic reaction. There are two mechanisms being discussed to analyse the enhancement of photocatalytic performance of carbon@g-C3N4 core-shells. In the first, there are large amount of carbon clusters that exhibits the superior light absorption in nearly entire wavelength, owing to the narrower gap of sp2 carbon cluster. With the introduction of carbon spheres, the carbon@g-C3N4 reveals broad background absorption, and the light absorption intensity in both the UV and visible light regions increases, especially in visible-light range. This provides more opportunities for photo-induced charge carriers, which is in favor of improvement of photocatalytic activity.



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Fig.12 Time dependences 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.

The second mechanism is that the carbon spheres act as sensitizer and transfer the photo-generated electrons to the g-C3N4. To confirm the relative position of the conduction band (CB) and valence band (VB), the total density 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 eV to -1.67 eV after construction of carbon@ g-C3N4 core-shell structure as observed in Fig. S10a (Supporting Information). From Fig.S10b (Supporting Information), compared to bulk-g-C3N4, layered-g-C3N4 in carbon@ g-C3N4 core-shell structures show a VB maximum down-shift from 1.38 eV to 1.15 eV. The increase of the VB width narrows the distance of potential between carbon nitride and water, enhancing the redox ability of the charge carries and photocatalytic performance. For carbon, there are amount of carbon cluster and it show various energy levels. In the photocatalytic process of carbon@ g-C3N4 structure, the photo-generated electrons are injected into the conduction band of the g-C3N4, allowing for the reaction with hydrogen ions of adsorbed water molecular. The schematic illustration of this mechanism is displayed in Fig.13a. In addition, the photo-induced electrons are generated from the valence to the conduction band of g-C3N4 by the excitement of the high-energy photons. The


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photo-generated electrons formed in the space-charge regions are transferred into the carbon spheres, and the holes remain on the g-C3N4 to participate in the redox reaction. The schematic illustration of this mechanism is displayed in Fig.13b. This process significantly improves the efficiency toward separation of photo-generated

electron-hole

pairs.

In

summary,

the

increased

photo-absorption capability, the carbon spheres act as sensitizer and reservoir to improve separation efficiency of photo-generated electrons-holes and the enhance the redox ability of the charge carries.

Fig.13 The proposed mechanism for the carbon spheres-mediated enhancement of photocatalysis. (a) the electron-hole pairs in the carbon spheres are generated under visible-light irradiation. Based on the relevant position of the band, the electrons inject in the g-C3N4.

(b) Carbon spheres as an electron sink, and

scavenge away the electrons, thus hindering recombination of electron-hole pairs.

CONCLUSIONS



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Page 30 of 41

The high efficient carbon@g-C3N4 core-shell structure has been successfully

fabricated

through

protonation

of

g-C3N4,

depolymerization-repolymerization procedure of g-C3N4 and electrostatic self-assembly route on the surface of carbon spheres, owing to the soft nature of the polymeric matrix and the reversible formation of hydrogen-bond. The results show that the 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 carriers transport capability, prolonged electron lifetime and lower recombination of electron-hole pairs. To be mentioned, 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.

SUPPORTING INFORMATION Experimental details; SEM image of pristine g-C3N4, carbon@g-C3N4 at lower concentration of g-C3N4 sol and higher concentration of g-C3N4 sol, TEM image of carbon@g-C3N4 after at higher concentration of g-C3N4 sol and the photo of g-C3N4 sol; Ideal g-C3N4 structure; Raman spectra; transient photo-response; XPS high revolution; Motte-Schottky spectra for g-C3N4 and carbon@g-C3N4 core-shell structure.


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ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation (51672220), the 111 Program (B08040) of MOE, the National Defense Science Foundation (32102060303), and the Fundamental Research Funds for the Central Universities (3102014JGY01004) of China.

References (1)

Ye, G. L.; Gong, Y. J.; Lin, J. H.; Li, B.; He, Y. M.; Pantelides, S. T.; Zhou, W.; Vajtai, R.; Ajayan, P. M. Defects Engineered Monolayer MoS2 for Improved Hydrogen Evolution Reaction. Nano Letters 2016, 16 (2), 1097. DOI: 10.1021 /acs.nanolett.5b04331

(2)

Morales-Guio, C. G.; Liardet, L.; Mayer, M. T.; Tilley, S. D.; Gratzel, M.; Hu, X. L. Photoelectrochemical Hydrogen Production in Alkaline Solutions Using Cu2O Coated with Earth-Abundant Hydrogen Evolution Catalysts. Angew

Chem Int Edit 2015, 54 (2), 664. DOI:10.1002/anie.201410569 (3)

Voiry, D.; Yamaguchi, H.; Li, J. W.; Silva, R.; Alves, D. C. B.; Fujita, T.; Chen, M. W.; Asefa, T.; Shenoy, V. B.; Eda, G.et al. Enhanced catalytic activity in strained chemically exfoliated WS2 nanosheets for hydrogen evolution. Nat.

Mater. 2013, 12 (9), 850. DOI:10.1038/nmat3700



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

(4)

Helm, M. L.; Stewart, M. P.; Bullock, R. M.; DuBois, M. R.; DuBois, D. L. A Synthetic Nickel Electrocatalyst with a Turnover Frequency Above 100,000 s(-1) for H-2 Production. Science 2011, 333 (6044), 863. DOI: 10.1126/ science. 1205864

(5)

Hou, Y. D.; Abrams, B. L.; Vesborg, P. C. K.; Bjorketun, M. E.; Herbst, K.; Bech, L.; Setti, A. M.; Damsgaard, C. D.; Pedersen, T.; Hansen, O.et al. Bioinspired molecular co-catalysts bonded to a silicon photocathode for solar hydrogen evolution. Nat. Mater. 2011, 10 (6), 434. DOI:10.1038/nmat3008

(6)

Du, P. W.; Knowles, K.; Eisenberg, R. A homogeneous system for the photogeneration of hydrogen from water based on a platinum(II) terpyridyl acetylide chromophore and a molecular cobalt catalyst. J. Am. Chem. Soc. 2008, 130 (38), 12576. DOI:10.1021/ja804650g

(7)

Cheng, N. C.; Stambula, S.; Wang, D.; Banis, M. N.; Liu, J.; Riese, A.; Xiao, B. W.; Li, R. Y.; Sham, T. K.; Liu, L. M.et al. Platinum single-atom and cluster catalysis of the hydrogen evolution reaction. Nat. Commun. 2016, 7, 9. DOI: 10. 1038/ncomms13638

(8)

Niu, P.; Zhang, L. L.; Liu, G.; Cheng, H. M. Graphene-Like Carbon Nitride Nanosheets for Improved Photocatalytic Activities. Advanced Functional

Materials 2012, 22 (22), 4763. DOI:10.1002/adfm.201200922 (9)

Yang, S. B.; Gong, Y. J.; Zhang, J. S.; Zhan, L.; Ma, L. L.; Fang, Z. Y.; Vajtai, R.; Wang, X. C.; Ajayan, P. M. Exfoliated Graphitic Carbon Nitride Nanosheets as Efficient Catalysts for Hydrogen Evolution Under Visible Light.


Page 32 of 41

Page 33 of 41

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


Adv. Mater. 2013, 25 (17), 2452. DOI:10.1002/adma.201204453 (10)

Cao, S. W.; Yu, J. G. g-C3N4-Based Photocatalysts for Hydrogen Generation.

J. Phys. Chem. Lett. 2014, 5 (12), 2101. DOI:10.1021/jz500546b (11)

Zheng, Y.; Lin, L. H.; Wang, B.; Wang, X. C. Graphitic Carbon Nitride Polymers toward Sustainable Photoredox Catalysis. Angew Chem Int Edit 2015, 54 (44), 12868. DOI:10.1002/anie.201501788

(12)

Wang, Y.; Wang, X. C.; Antonietti, M. Polymeric Graphitic Carbon Nitride as a Heterogeneous Organocatalyst: From Photochemistry to Multipurpose Catalysis to Sustainable Chemistry. Angew Chem Int Edit 2012, 51 (1), 68. DOI:10.1002/anie.201101182

(13)

Dong, F.; Zhao, Z.; Sun, Y.; Zhang, Y.; Yan, S.; Wu, Z. An Advanced Semimetal–Organic Bi Spheres–g-C3N4 Nanohybrid with SPR-Enhanced Visible-Light Photocatalytic Performance for NO Purification. Environmental

Science & Technology 2015, 49 (20), 12432. DOI:10.1021/acs.est.5b03758 (14)

Ni, Z.; Dong, F.; Huang, H.; Zhang, Y. New insights into how Pd nanoparticles influence the photocatalytic oxidation and reduction ability of g-C3N4 nanosheets. Catalysis Science

& Technology

2016, 6 (16), 6448.

DOI:10.1039/C6CY00580B (15)

Jun, Y. S.; Lee, E. Z.; Wang, X. C.; Hong, W. H.; Stucky, G. D.; Thomas, A. From Melamine-Cyanuric Acid Supramolecular Aggregates to Carbon Nitride Hollow Spheres. Advanced Functional Materials 2013, 23 (29), 3661. DOI: 10.1002/adfm.201203732



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

(16)

Zheng, D. D.; Pang, C. Y.; Liu, Y. X.; Wang, X. C. Shell-engineering of hollow g-C3N4 nanospheres via copolymerization for photocatalytic hydrogen evolution. Chemical Communications 2015, 51 (47), 9706. DOI: 10.1039/ c5cc03143e

(17)

Liao, G. Z.; Chen, S.; Quan, X.; Yu, H. T.; Zhao, H. M. Graphene oxide modified g-C3N4 hybrid with enhanced photocatalytic capability under visible light irradiation. Journal of Materials Chemistry 2012, 22 (6), 2721. DOI: 10.1039/ c1jm13490f

(18)

Ge, L.; Han, C. C. Synthesis of MWNTs/g-C3N4 composite photocatalysts with efficient visible light photocatalytic hydrogen evolution activity. Appl.

Catal. B-Environ. 2012, 117, 268. DOI:10.1016/j.apcatb.2012.01.021 (19)

Pei, Z.; Li, H.; Huang, Y.; Xue, Q.; Huang, Y.; Zhu, M.; Wang, Z.; Zhi, C. Texturing in situ: N,S-enriched hierarchically porous carbon as a highly active reversible oxygen electrocatalyst. Energy & Environmental Science 2017, 10 (3), 742. DOI:10.1039/C6EE03265F

(20)

Fang, S.; Xia, Y.; Lv, K. L.; Li, Q.; Sun, J.; Li, M. Effect of carbon-dots modification on the structure and photocatalytic activity of g-C3N4. Appl.

Catal. B-Environ. 2016, 185, 225. DOI:10.1016/j.apcatb.2015.12.025 (21)

Guo, F.; Shi, W. L.; Guan, W. S.; Huang, H.; Liu, Y. Carbon dots/g-C3N4/ZnO nanocomposite as efficient visible-light driven photocatalyst for tetracycline total degradation. Sep. Purif. Technol. 2017, 173, 295. DOI:10.1016/ j.seppur. 2016.09.040


Page 34 of 41

Page 35 of 41

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


(22)

Li, G. S.; Lian, Z. C.; Wang, W. C.; Zhang, D. Q.; Li, H. X. Nanotube-confinement induced size-controllable g-C3N4 quantum dots modified single-crystalline TiO2 nanotube arrays for stable synergetic photoelectrocatalysis.

Nano

Energy

2016,

19,

446.

DOI:10.1016/j.nanoen.2015.10.011 (23)

Chen, K.; Chai, Z. G.; Li, C.; Shi, L. R.; Liu, M. X.; Xie, Q.; Zhang, Y. F.; Xu, D. S.; Manivannan, A.; Liu, Z. F. Catalyst-Free Growth of Three-Dimensional Graphene Flakes and Graphene/g-C3N4 Composite for Hydrocarbon Oxidation.

Acs Nano 2016, 10 (3), 3665. DOI:10.1021/acsnano.6b00113 (24)

Pei, Z.; Zhao, J.; Huang, Y.; Huang, Y.; Zhu, M.; Wang, Z.; Chen, Z.; Zhi, C. Toward enhanced activity of a graphitic carbon nitride-based electrocatalyst in oxygen reduction and hydrogen evolution reactions via atomic sulfur doping.

Journal of Materials Chemistry A 2016, 4 (31), 12205. DOI:10.1039/ C6TA03588D (25)

Wang, X.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.; Domen, K.; Antonietti, M. A metal-free polymeric photocatalyst for hydrogen production from water under visible light. Nat Mater 2009, 8 (1), 76. DOI:10.1038/nmat2317

(26)

Thomas, A.; Fischer, A.; Goettmann, F.; Antonietti, M.; Müller, J.-O.; Schlögl, R.; Carlsson, J. M. Graphitic carbon nitride materials: variation of structure and morphology and their use as metal-free catalysts. Journal of Materials

Chemistry 2008, 18 (41), 4893. DOI:10.1039/b800274f



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

(27)

Wang, X.; Blechert, S.; Antonietti, M. Polymeric Graphitic Carbon Nitride for Heterogeneous Photocatalysis. ACS Catalysis 2012, 2 (8), 1596. DOI:10.1021/ cs300240x

(28)

Zhang, J. S.; Zhang, M. W.; Lin, L. H.; Wang, X. C. Sol Processing of Conjugated Carbon Nitride Powders for Thin-Film Fabrication. Angew Chem

Int Edit 2015, 54 (21), 6297. DOI: 10.1002/anie.201501001 (29)

Ma, T. Y.; Tang, Y.; Dai, S.; Qiao, S. Z. Proton-functionalized two-dimensional graphitic carbon nitride nanosheet: an excellent metal-/label-free biosensing platform. Small 2014, 10 (12), 2382. DOI:10.1002/smll.201303827

(30)

Niu, P.; Zhang, L.; Liu, G.; Cheng, H.-M. Graphene-Like Carbon Nitride Nanosheets for Improved Photocatalytic Activities. Advanced Functional

Materials 2012, 22 (22), 4763. DOI:10.1002/adfm.201200922 (31)

Groenewolt, M.; Antonietti, M. Synthesis of g-C3N4 Nanoparticles in Mesoporous Silica Host Matrices. Adv Mater 2005, 17 (14), 1789. DOI:10.1002/adma.200401756

(32)

Bai, X.; Wang, L.; Zong, R.; Zhu, Y. Photocatalytic Activity Enhanced via g-C3N4Nanoplates to Nanorods. The Journal of Physical Chemistry C 2013,

117 (19), 9952. DOI:10.1021/jp402062d (33)

Yang, S.; Gong, Y.; Zhang, J.; Zhan, L.; Ma, L.; Fang, Z.; Vajtai, R.; Wang, X.; Ajayan, P. M. Exfoliated Graphitic Carbon Nitride Nanosheets as Efficient Catalysts for Hydrogen Evolution Under Visible Light. Adv Mater 2013, 25 (17), 2452. DOI:10.1002/adma.201204453


Page 36 of 41

Page 37 of 41

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


(34)

Ong, W.-J.; Tan, L.-L.; Chai, S.-P.; Yong, S.-T.; Mohamed, A. R. Surface charge modification via protonation of graphitic carbon nitride (g-C3N4) for electrostatic self-assembly construction of 2D/2D reduced graphene oxide (rGO)/g-C3N4 nanostructures toward enhanced photocatalytic reduction of carbon dioxide to methane. Nano Energy 2015, 13, 757. DOI:10.1016/ j.nanoen. 2015.03.014

(35)

Liang, J.; Zheng, Y.; Chen, J.; Liu, J.; Hulicova-Jurcakova, D.; Jaroniec, M.; Qiao, S. Z. Facile Oxygen Reduction on a Three-Dimensionally Ordered Macroporous Graphitic C3N4/Carbon Composite Electrocatalyst. Angew Chem

Int Edit 2012, 51 (16), 3892. DOI:10.1002/anie.201107981 (36)

Li, J. J.; Zhang, Y. M.; Zhang, X. H.; Han, J. C.; Wang, Y.; Gu, L.; Zhang, Z. H.; Wang, X. J.; Jian, J. K.; Xu, P.et al. Direct Transformation from Graphitic C3N4 to Nitrogen-Doped Graphene: An Efficient Metal-Free Electrocatalyst for Oxygen Reduction Reaction. Acs Appl Mater Inter 2015, 7 (35), 19626. DOI: 10.1021/acsami.5b03845

(37)

Su, F. Z.; Mathew, S. C.; Lipner, G.; Fu, X. Z.; Antonietti, M.; Blechert, S.; Wang, X. C. mpg-C3N4-Catalyzed Selective Oxidation of Alcohols Using O-2 and Visible Light. J. Am. Chem. Soc. 2010, 132 (46), 16299. DOI:10.1021/ ja102866p

(38)

Ma, T. Y.; Dai, S.; Jaroniec, M.; Qiao, S. Z. Graphitic Carbon Nitride Nanosheet-Carbon Nanotube Three-Dimensional Porous Composites as High-Performance Oxygen Evolution Electrocatalysts. Angew Chem Int Edit



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

Page 38 of 41

2014, 53 (28), 7281. DOI: 10.1002/anie.201403946 (39)

Chen, K.; Chai, Z.; Li, C.; Shi, L.; Liu, M.; Xie, Q.; Zhang, Y.; Xu, D.; Manivannan, A.; Liu, Z. Catalyst-Free Growth of Three-Dimensional Graphene Flakes and Graphene/g-C3N4 Composite for Hydrocarbon Oxidation.

Acs Nano 2016, 10 (3), 3665. DOI:10.1021/acsnano.6b00113 (40)

Dante, R. C.; Martín-Ramos, P.; Correa-Guimaraes, A.; Martín-Gil, J. Synthesis of graphitic carbon nitride by reaction of melamine and uric acid.

Materials

Chemistry

and

Physics

2011,

130

(3),

1094.

DOI:10.1016/j.matchemphys. 2011. 08.041 (41)

Liu, Q.; Zhang, J. Graphene Supported Co-g-C3N4 as a Novel Metal– Macrocyclic Electrocatalyst for the Oxygen Reduction Reaction in Fuel Cells.

Langmuir 2013, 29 (11), 3821. DOI:10.1021/la400003h (42)

Hou, Y.; Wen, Z. H.; Cui, S. M.; Guo, X. R.; Chen, J. H. Constructing 2D Porous Graphitic C3N4 Nanosheets/Nitrogen-Doped Graphene/Layered MoS2 Ternary Nanojunction with Enhanced Photoelectrochemical Activity. Adv.

Mater. 2013, 25 (43), 6291. DOI: 10.1002/adma.201303116 (43)

Liang, Q.; Li, Z.; Huang, Z.-H.; Kang, F.; Yang, Q.-H. Holey Graphitic Carbon Nitride Nanosheets with Carbon Vacancies for Highly Improved Photocatalytic Hydrogen Production. Advanced Functional Materials 2015, 25 (44), 6885. DOI:10.1002/adfm.201503221

(44)

Pan, D.; Zhang, J.; Li, Z.; Wu, C.; Yan, X.; Wu, M. Observation of pH-, solvent-, spin-, and excitation-dependent blue photoluminescence from carbon


Page 39 of 41

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


nanoparticles. Chemical Communications 2010, 46 (21), 3681. DOI:10.1039/ C000114G (45)

Xia, X.; Deng, N.; Cui, G.; Xie, J.; Shi, X.; Zhao, Y.; Wang, Q.; Wang, W.; Tang, B. NIR light induced H2 evolution by a metal-free photocatalyst.

Chemical Communications 2015, 51 (54), 10899. DOI:10.1039/C5CC02589C (46)

Zhang, Y. H.; Tang, Z. R.; Fu, X. Z.; Xu, Y. J. TiO2-Graphene Nanocomposites for Gas-Phase Photocatalytic Degradation of Volatile Aromatic Pollutant: Is TiO2-Graphene Truly Different from Other TiO2-Carbon Composite Materials.

Acs Nano 2010, 4 (12), 7303. DOI: 10.1021/nn1024219 (47)

Zhang, H.; Lv, X. J.; Li, Y. M.; Wang, Y.; Li, J. H. P25-Graphene Composite as a High Performance Photocatalyst. Acs Nano 2010, 4 (1), 380. DOI: 10.1021/ nn901221k

(48)

Tong, Z.; Yang, D.; Shi, J.; Nan, Y.; Sun, Y.; Jiang, Z. Three-Dimensional Porous Aerogel Constructed by g-C3N4 and Graphene Oxide Nanosheets with Excellent Visible-Light Photocatalytic Performance. ACS Applied Materials &

Interfaces 2015, 7 (46), 25693. DOI:10.1021/acsami.5b09503 (49)

Zhang, Y. H.; Tang, Z. R.; Fu, X.; Xu, Y. J. Engineering the Unique 2D Mat of Graphene to Achieve Graphene-TiO2 Nanocomposite for Photocatalytic Selective Transformation: What Advantage does Graphene Have over Its Forebear Carbon Nanotube. Acs Nano 2011, 5 (9), 7426. DOI: 10.1021/ nn202519j

(50)

Meng, F.; Li, J.; Cushing, S. K.; Zhi, M.; Wu, N. Solar Hydrogen Generation



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

Page 40 of 41

by Nanoscale p–n Junction of p-type Molybdenum Disulfide/n-type Nitrogen-Doped Reduced Graphene Oxide. Journal of the American Chemical

Society 2013, 135 (28), 10286. DOI:10.1021/ja404851s (51)

Hou, Y.; Zuo, F.; Dagg, A.; Feng, P. Y. A Three-Dimensional Branched Cobalt-Doped alpha-Fe2O3 Nanorod/MgFe2O4 Heterojunction Array as a Flexible Photoanode for Efficient Photoelectrochemical Water Oxidation.

Angew Chem Int Edit 2013, 52 (4), 1248. DOI: 10.1002/anie.201207578 (52)

Hou, Y.; Zuo, F.; Dagg, A.; Feng, P. Visible Light-Driven α-Fe2O3 Nanorod/Graphene/BiV1–xMoxO4

Core/Shell

Heterojunction

Array

for

Efficient Photoelectrochemical Water Splitting. Nano Letters 2012, 12 (12), 6464. DOI:10.1021/nl303961c (53)

Cheng, H.; Hou, J.; Takeda, O.; Guo, X.-M.; Zhu, H. A unique Z-scheme 2D/2D nanosheet heterojunction design to harness charge transfer for photocatalysis. Journal of Materials Chemistry A 2015, 3 (20), 11006. DOI:10.1039/ C5TA01864A

(54)

Zhang, Z.; Huang, J.; Zhang, M.; Yuan, Q.; Dong, B. Ultrathin hexagonal SnS2 nanosheets coupled with g-C3N4 nanosheets as 2D/2D heterojunction photocatalysts toward high photocatalytic activity. Applied Catalysis B:

Environmental 2015, 163, 298. DOI:10.1016/j.apcatb.2014.08.013


Page 41 of 41

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Table of Contents

Description: The carbon@g-C3N4 core-shell photocatalyst with great light adsorption and high charge separation efficiency shows superior hydrogen evolution from water splitting under visible-light irradiation.


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

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