Photocatalysts based on g-C3N4-encapsulating carbon spheres with high visible light activity for photocatalytic hydrogen evolution

Accepted Manuscript Photocatalysts based on g-C3N4-encapsulating carbon spheres with high visible light activity for photocatalytic hydrogen evolution Kui Li, Xin Xie, Wei-De Zhang PII:

S0008-6223(16)30798-9

DOI:

10.1016/j.carbon.2016.09.039

Reference:

CARBON 11321

To appear in:

Carbon

Received Date: 11 July 2016 Revised Date:

29 August 2016

Accepted Date: 17 September 2016

Please cite this article as: K. Li, X. Xie, W.-D. Zhang, Photocatalysts based on g-C3N4-encapsulating carbon spheres with high visible light activity for photocatalytic hydrogen evolution, Carbon (2016), doi: 10.1016/j.carbon.2016.09.039. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Photocatalysts based on g-C3N4-encapsulating carbon spheres with high visible light activity for photocatalytic hydrogen evolution

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Kui Li, Xin Xie, Wei-De Zhang*

School of Chemistry and Chemical Engineering, South China University of

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Technology, 381 Wushan Road, Guangzhou 510640, People’s Republic of China

*

Corresponding author. Tel and Fax: 86-20-8711 4099, E-mail address: [email protected] (W. D. Zhang). 1

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Abstract Graphitic carbon nitride-based nanocomposites hold great promise in photocatalysis.

In

this

study,

nanocomposites

based

on

graphitic carbon

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nitride-encapsulating carbon spheres (CS/g-C3N4) were facilely fabricated through polymerization of melamine from the carbon spheres. The incorporation of carbon spheres in g-C3N4 leads to enlarged contact area and strengthened interaction between

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the two π-conjugated components. The CS/g-C3N4 composites exhibit large specific

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surface area, improved visible light utilization, more negative potential of conduction band, and enhanced separation of photo-generated charge carriers. The optimized CS/g-C3N4 composite displays a hydrogen evolution rate of 50.2 µmol·h-1, which is almost 4.8 times of that over the pure g-C3N4. The structures and morphologies of the

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catalysts can be maintained after photocatalytic reaction. This approach may stimulate

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a new way to construct g-C3N4-based nanocomposites with other carbon materials.

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1. Introduction Photocatalytic hydrogen evolution over a semiconductor is economical (which can be realized without high temperature and pressure, or electricity) and

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environmentally friendly (which produces only hydrogen and oxygen) for converting solar energy to hydrogen (H2) [1, 2]. Ever since Fujishima and Honda firstly reported the photoelectrochemical water splitting under ultraviolet light on a TiO2 electrode in

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1972 [3], considerable visible light driven photocatalysts such as CdS [4], MoS2 [5, 6],

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and Fe2O3 [7] have been discovered in recent years. However, in photocatalytic reaction, these materials based on metal-H bond interactions are easily corroded and hardly activated [8]. Therefore, developing non-metal and low-cost materials with stable photo-activity to substitute these semiconductors is a continuing endeavor that

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researchers have been devoting to.

Graphitic carbon nitride polymer (g-C3N4) as a metal-free n-type semiconductor is composed of extended π-conjugated planes packed through van der Waals force [9,

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10]. The high chemical stability, appealing electronic band structure, controllable

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morphology, and environmentally friendly feature make g-C3N4 a multifunctional photocatalyst for water splitting [11-14], CO2 reduction [15] or cycloaddition [16], and of course oxidation of organics [17, 18]. However, the conventional g-C3N4 synthesized by polymerization of nitrogen-containing precursors suffered from small specific surface area, inadequate utilization of visible light, and low mobility of photo-induced charge carriers. Up to now, in order to facilitate the visible light harvesting and suppress electron-hole pair recombination, g-C3N4 has usually been 3

ACCEPTED MANUSCRIPT combined with other semiconductors such as Ag3VO4, Fe2O3, and S to construct heterojunctions [19-22]. In addition, anchoring carbonaceous materials with π-conjugated systems such as N-doped graphene [23], carbon nanotubes [24] and

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CQDs [25] not only extends the light absorption but also enhances separation of photogenerated electrons and holes. Unfortunately, preparation of graphene or CNTs is often tedious and costly. Therefore, significant challenge remains in synthesizing

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low-cost g-C3N4/carbonaceous material composites with high activity and stability for

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water evaluation.

Among various kinds of carbonaceous materials, carbon spheres (CS) have potential application as electrode materials due to good electrical conductivity and high chemical stability [26, 27]. And the CS can be easily prepared by facile

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hydrothermal carbonization processes [28]. Multiple attempts have proved that the CS is also a promising candidate as an electron-acceptor/transport material to retard the combination of charge carriers in photocatalysis. This has been proved by various

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photocatalysts, such as Fe2O3/CS [29], CS@Bi2MoO6 [30], and CS@Cu2O/Cu [31]. A

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nanocomposite of g-C3N4 and carbon nanospheres was reported to exhibit improved photocatalytic activity towards degradation of phenol [32]. However, in such a synthetic process of CN-CS, hydrothermal carbonization may change the structure and impair the performance of g-C3N4. The photocatalytic stability may be poor due to the surficial loading of CS on the g-C3N4. Optimizing the preparation method to strengthen connection of CS and g-C3N4 in the composites may further enhance the photocatalytic activity and stability. Previous reports have demonstrated that the 4

ACCEPTED MANUSCRIPT g-C3N4-based composites, such as core/shell CdS/g-C3N4 nanowires [33], Ag3PO4/g-C3N4 core-shell composite [34], and N-doped ZnO/g-C3N4 core-shell nanoplates [35], possess strong interaction in the intimate contact interfaces of the

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composites. This is beneficial to improve the transportation and separation of charge carriers. In addition, the g-C3N4 as a shell suppressed the photo-corrosion of the composite semiconductors, thus enhanced the photo-stability. Obviously, constructing

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g-C3N4-encapsulating nanostructures with carbon spheres is a promising strategy to

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enhance the interaction and photo-stability of the photocatalysts.

Based on the aforementioned consideration, we herein propose a novel CS/g-C3N4 nanocomposite synthesized via thermal polymerization of melamine from the CS. This structure suggests that the CS was wrapped by g-C3N4 nanosheets,

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resulting in strong interaction with enlarged contacting interface. Due to similar electronic structures of carbon rich materials and g-C3N4, the combination of two π-conjugated systems not only benefits the separation of photo-generated charge

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carriers but also increases the light absorption of g-C3N4 [36]. The as-prepared

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CS/g-C3N4 composites show remarkably enhanced visible light absorption and separation of photo-excitation charge carriers, which consequently contribute to higher photocatalytic hydrogen evolution performance and photo-stability under visible light. In addition, the morphology of the sample was maintained during the photocatalytic

reaction.

We

also

successfully

synthesized

CS/g-C3N4

nanocomposites using other precursors, like dicyandiamide, urea, or dicyandiamide with ammonium chloride. The result indicates that the thermal polymerization of 5

ACCEPTED MANUSCRIPT various precursors in the existence of CS is helpful for constructing nanocomposites with strong interaction and improving the photocatalytic performance of g-C3N4.

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2. Experimental 2.1. Materials

Melamine (98%) was purchased from Tianjin Kemiou Chemical Reagent Co. Ltd.

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(Tianjin, China), and glucose anhydrous (C6H12O6) was obtained from Shanghai Bio

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Science & Technology Co. Ltd. (Shanghai, China). Dicyandiamide (98%) was acquired from Lingfeng Chemical Reagent Co. Ltd. (Shanghai, China). Urea (99%) was purchased from Tianjin Fuchen Chemical Reagents Factory (Tianjin, China). Ammonium chloride (NH4Cl, 99.5%) was obtained from Guangzhou Chemical

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Reagent Factory (Guangzhou, China). Triethanolamine (85%) was adopted from Tianjing Fuyu Fine Chemical Co. Ltd. (Tianjing, China). Chloroplatinic acid (H2PtCl6·6H2O, Pt ≥ 37.5%) was obtained from Aladdin Chemistry Co. Ltd.

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purification.

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(Shanghai, China). All the chemicals were used as received without further

2.2. Synthesis of photocatalysts Carbon spheres were synthesized by hydrothermal carbonization of glucose [36].

Typically, 4.458 g glucose anhydrous was dissolved in 30 mL deionized water. After stirring for 30 min, the clear and colorless solution was transferred into a 50 mL Teflon-lined stainless steel autoclave. The hydrothermal carbonization was conducted at 180 °C for 12 h. Then, the obtained precipitates were filtrated and washed by DI 6

ACCEPTED MANUSCRIPT water for five times, followed by drying under vacuum overnight. The CS/g-C3N4 composites were prepared by thermal polymerization of melamine in the existence of CS. In detail, 2 g melamine and a certain amounts (5, 10,

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15, 20 or 25 mg) of CS were dispersed into 100 mL DI water under sonication for 30 min. Then, the mixture was heated at 100 °C in an oil bath upon stirring to remove the water. The obtained grey solids were ground finely. Finally, the homogeneous

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powders were placed in a crucible with a cover and calcined at 550 °C for 4 h in a

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muffle furnace at a heating rate of 3 °C/min. After cooling down to room temperature, the CS/g-C3N4 samples were collected for further use. The pristine g-C3N4 was synthesized without adding CS and labeled as g-C3N4. The CS/g-C3N4 composites prepared by adding different amounts of CS were labeled as CS/g-C3N4-5,

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CS/g-C3N4-10, CS/g-C3N4-15, CS/g-C3N4-20 and CS/g-C3N4-25, respectively. According to the adding amount of CS, the mass ratios of CS in the above composites were estimated to be 0.73, 1.47, 2.30, 3.09 and 3.98%, respectively. In addition, the

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CS/g-C3N4 composites using other nitrogen-containing precursors were synthesized. 2

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g dicyandiamide, 15 g urea, or 3 g dicyandiamide with 15 g NH4Cl substituted 2 g melamine. The obtained samples were named as CS/DCN, CS/UCN, and CS/CNNS, respectively.

2.3. Characterization

X-ray powder diffraction (XRD) patterns were collected on a Bruker GADDS diffractometer (Cu Kα radiation). Fourier transform infrared (FTIR) spectra were recorded on an IR Affiniy-1 FTIR spectrometer from 4000 to 400 cm-1 and used KBr pellets as the matrix. UV-vis diffuse reflectance spectra (DRS) of the samples were 7

ACCEPTED MANUSCRIPT obtained on a UV-vis spectrometer (Hitachi U-3010, Japan) using BaSO4 as a reference. X-ray photoelectron spectroscopy (XPS) analysis was carried out on an X-ray photoelectron spectroscope (Thermo ESCALAB 250XI, USA). Nitrogen adsorption and desorption isotherms at 77 K were conducted by a static volume

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method with a specific surface analysis instrument (Beishide, 3H-2000PSI, China). The morphologies and structures of the samples were observed by a field emission scanning electron microscope (SEM, Merlin, Zeiss) at an acceleration voltage of 5 kV and a transmission electron microscope (TEM, JEM-2100F) at an acceleration voltage

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of 300 kV. 13C solid-state NMR measurements were conducted on a Bruker Advance III HD 400 spectrometer. Photoluminescence (PL) was measured on an F-4500

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fluorescence spectrophotometer at room temperature using 350 nm as the excitation wavelength. Photoelectrochemical experiments were carried out on a CHI 660C electrochemical workstation with a conventional three-electrode system (Chenhua Instruments, China). A platinum plate and an Ag/AgCl (saturated KCl) electrode were used as counter electrode and reference electrode, respectively. The working

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electrodes were prepared as follows: 10 mg samples were ground with 1 mL ethanol. The obtained slurry was transferred onto a 1 cm × 0.5 cm FTO glass by spin-coating and dried at 80 °C. 0.5 M Na2SO4 aqueous solution was used as the electrolyte and

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the oxygen was removed by purging with nitrogen before any test. The visible light irradiation was provided by a 500 W Xe lamp (CHF-XM, Beijing Changtuo Technology Co. Ltd, China) with an ultraviolet cutoff filter (λ > 420 nm). The on/off

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photocurrent responses were recorded at -0.2 V versus Ag/AgCl bias potential under visible light irradiation. Electrochemical impedance spectroscopy (EIS) plots were measured at a bias voltage of -0.5 V in the dark. The amplitude is 5 mV and the frequency ranges from 10000 to 0.01 Hz. 2.4. Photocatalytic evolution of H2 The photocatalytic generation of H2 was performed in a Pyrex top-irradiation reaction vessel with a closed glass gas circulation system (Labsolar III AG, Beijing

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ACCEPTED MANUSCRIPT Perfectlight Technology Co. Ltd, China). The visible light source came from a 300 W Xe lamp (PLS-SXE 300/300UV, Beijing Perfectlight Technology Co. Ltd, China) with a 420 nm cut-off filter. The intensity was tuned to 100 mW·cm-2, which was

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measured by a PL-MW2000 Photoradiometer (Beijing Perfectlight Technology Co. Ltd, China). 50 mg catalyst was dispersed into an aqueous solution (100 mL) containing triethanolamine (10 vol%), and 3 wt% Pt was deposited on the surface of

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the sample by in situ photo-reduction using H2PtCl6 as a precursor. The closed gas

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circulation system connecting to the solution was vacuumed before the photocatalytic reaction. The released gases were analyzed by a gas chromatography (GC7806, Beijing Shiweipuxin Analytical Instruments Co. Ltd, China) equipped with a thermal

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conductive detector and a 5A molecular sieve column. N2 was used as a carry gas.

3. Results and discussion

3.1. Characterization of photocatalysts

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The fabrication strategy of the CS/g-C3N4 composites is presented in Fig. 1. In

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brief, glucose is carbonized to CS in hydrothermal process. After recrystallization of melamine with CS suspension, the CS is fully incorporated onto the melamine. The melamine features irregular particles, as indicated in Fig. S1A. Fig. S1B shows the microscopic morphology of CS/melamine. It can be seen that CS is embedded on the surface of the bulk melamine. After thermal polymerization of CS/melamine, the melem is formed and connected to the CS firstly. Further condensation of melem along the surface of CS produces CS/g-C3N4 nanocomposites. 9

ACCEPTED MANUSCRIPT SEM and TEM images in Fig. 2A and 2B show that the CS has a spherical morphology with uniform size. The diameter of the particles is evaluated to be about 400 nm. On the other hand, the pristine g-C3N4 is of aggregated irregular particles,

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which is clearly revealed by both SEM and TEM images (Fig. 2C and 2D). The SEM images of CS/g-C3N4-15 disclose that CS with about 400 nm diameters are embedded by g-C3N4 (Fig. 3A and 3B). High-magnification SEM image shows that the surface

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of the CS become rough, manifesting that the CS is encapsulated by g-C3N4. TEM

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images of CS/g-C3N4 presented in Fig. 3C and 3D indicate that the CS is clearly wrapped by lamellar g-C3N4. The TEM images of CS/DCN (A and D), CS/UCN (B and E) and CS/CNNS (C and F) are also shown in Fig. S2. All the composites exhibit similar nanostructures with good contact, indicating that the precursors can also be

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polymerized onto the surface of CS and encompass them.

To further analyze the textural properties of the prepared samples, nitrogen adsorption-desorption isotherm at 77 K was measured to evaluate the specific surface

the

g-C3N4

and

CS/g-C3N4-15

show

typical

type

IV

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classification,

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area and pore size distribution. As shown in Fig. 4A, according to the IUPAC

adsorption-desorption isotherms. The Brunauer-Emmett-Teller (BET) specific surface area (SBET) of the pure g-C3N4 is 49.0 m2·g-1, which is lower than that of CS/g-C3N4-15 (66.4 m2·g-1). The pore-size distributions of the two samples range from 2 to 50 nm, which confirm the existence of mesopores (Fig. 4B). The mean pore diameter and pore volume of the g-C3N4 are 26 nm and 0.33 cm3·g-1, increasing to 30 nm and 0.51 cm3·g-1 for the CS/g-C3N4-15, respectively. Thus, incorporation of CS 10

ACCEPTED MANUSCRIPT leads to increment of both SBET and pore size. In the presence of CS, melamine is polymerized against the surface of CS, rather than a condensed bulk structure. It is conducive to provide large surface area and active sites for photocatalytic reaction.

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XRD patterns of the pristine g-C3N4 and CS/g-C3N4 composites with different amounts of CS are displayed in Fig. 5A. Two characteristic signals are reflected on the samples, which can be attributed to the two typical diffraction peaks of g-C3N4.

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The sharp diffraction peak at 27.7° relates to the (002) diffraction plane, revealing the

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graphitic structure with a typical interlayer stacking peak of aromatic segment [37]. Specially, the intensity of (002) peak of the CS/g-C3N4 composites is weakened compared to that of g-C3N4. This is because a part of melamine was polymerized along the surface of CS, causing less complete crystallization of interlayer networks

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in g-C3N4 [38]. The XRD patterns of CS/DCN, CS/UCN and CS/CNNS composites also display weakened intensity in (002) peak compared to the corresponding pure g-C3N4 (Fig. S3). The diffraction peak at 13.1° is indexed to the (100) plane,

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indicating the inter-planar structural packing of tri-s-triazine units. No other

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diffraction peaks were detected in addition to these two signals in the CS/g-C3N4. The result demonstrates that incorporation with little amount of CS does not destroy the crystal structure of g-C3N4. Fig. 5B shows the FTIR spectra of CS, g-C3N4 and CS/g-C3N4-15. In the case of

pure CS, the broad band at 3000-3700 cm-1 is assigned to stretching vibrations of O−H (hydroxyl or carboxyl). The sharp band at 1701 cm-1 can be attributed to C=O (carbonyl, quinone, ester, or carboxyl), whereas the bands at 1620 and 1510 cm-1 11

ACCEPTED MANUSCRIPT correspond to the C=C vibrations [25, 39]. The CS with carboxyl groups is prone to connect with the amino groups (−NH2) of melamine and form CS/melamine composite. The characteristic peaks of g-C3N4 and CS/g-C3N4-15 are almost the same.

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All of these signals observed in Fig. 5B belong to the typical molecular structure of graphitic carbon nitride. The broad bands in the 3000-3500 cm-1 region are attributed to primary amine group (−NH2) stretches. And the bands of the CS/g-C3N4-15

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decreases obviously compared to the g-C3N4. It is because that the fringe N atoms in

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the g-C3N4 are connected to the C in the CS to form the sp2 C=N−C bonds. The sharp absorption bands located at 1200-1700 cm-1 correspond to the stretching vibrations of aromatic C−N heterocycles. Specially, the peaks at 1327 and 1244 cm-1 represent the stretching vibrations of N−(C)3 (full condensation) and C−N−C (partial condensation)

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in the connected units, respectively. The bands located at 1639, 1574 and 1410 cm-1 are attributed to the typical modes of heptazine derived from repeating units [40]. The breathing mode at 812 cm-1 is ascribed to the out-of-plane bending vibration of

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triazine units, which is weakened in the CS/g-C3N4-15. The introduction of CS altered

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the periodicity of the peaks in the g-C3N4-based composites. The result is in consistent with the XRD analysis. The characteristic bands of CS are not observed in CS/g-C3N4-15, which is mainly due to low content of CS and the carboxyl groups in CS were destroyed at high temperature during preparation of the composites. The chemical states of component elements on the surface of the samples were investigated by XPS. The survey spectra of the samples in Fig. 6A indicate that the CS/g-C3N4-15 and g-C3N4 are mainly composed of C, N and low content of O. The 12

ACCEPTED MANUSCRIPT high resolution XPS spectra of C 1s and N 1s of CS, g-C3N4 and CS/g-C3N4-15 are shown in Fig. 6B and 6C, respectively. Only a distinct peak at 284.8 eV in the C 1s spectrum of CS is detected. It is the typical signal of C−C bonds of graphitic carbon

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[41]. In the C 1s spectrum of g-C3N4, two de-convoluted peaks at 284.8 and 288.3 eV are identified as the graphitic carbon and sp2-hybridized carbon in the N-containing aromatic ring (N−C=N) [42]. Compared to g-C3N4, the peak of C−C at 284.8 eV

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becomes stronger after the incorporation of CS, and the C-C/Call atomic ratio

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increases from 0.065 to 0.099 (Table 1). The N 1s binding energy of g-C3N4 can be de-convoluted into four peaks in Fig. 6C. The peaks located at 398.8, 399.6, 401.1 and 404.8 eV correspond to sp2 hybridized aromatic N atoms in sp2 C=N−C, tertiary N atoms in N−(C)3 or H−N−(C)2, quaternary N in the aromatic cycles and the π

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excitations in the polymeric g-C3N4 structure, respectively [43]. The atomic ratio of sp2 C=N−C bonds at 398.8 eV to the sum of N atoms remarkably increases from 0.668 (g-C3N4) to 0.691 (CS/g-C3N4-15). It indicates that the carbon in CS is

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connected to the nitrogen of the aromatic ring of g-C3N4 in the condensation process,

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which manifests the interaction between the CS and g-C3N4 is strengthened. The result is in consistent with the FTIR result. H2 evolution performance of a semiconductor photocatalyst is related to the photo

excitation, band gap of the catalysts, energy level of photo-generated electrons, and separation efficiency of the photo-induced charge carriers [44]. The optical properties of the g-C3N4 and CS/g-C3N4 composites were qualitatively investigated by UV-vis DRS and PL spectroscopy. As shown in Fig. 7A, one can see that the CS/g-C3N4 13

ACCEPTED MANUSCRIPT composites show similar onset visible light absorption. The light harvesting ability is enhanced with increasing content of the CS. When the amount of CS reaches beyond 15 mg, the absorption band edges of CS/g-C3N4 composites show a slight red-shift,

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while the edges of the remaining composites almost coincide with the pure g-C3N4. The band gaps are derived from Tauc plots (Fig. 7B), in which (αhν)1/2 is plotted versus hν, where α is the diffuse absorption coefficient, h is Planck constant and ν is

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the light frequency [13]. The band gaps of CS/g-C3N4-5, CS/g-C3N4-10 and

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CS/g-C3N4-15 are almost the same as that of the g-C3N4 (2.69 eV). The band gaps of CS/g-C3N4-20 and CS/g-C3N4-25 are 2.65 and 2.61 eV, which are narrowed by 0.04 and 0.08 eV, respectively. The decreased band gaps are induced by the introduction of more amount of CS in the composite catalysts, in which more carbon atoms of CS

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bonded with nitrogen atoms of g-C3N4. Although narrowed band gap allows the composite capture more visible photons, the potential of the conduction band (CB) will be more positive accordingly, which impairs the photo-reduction property of

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electrons. There must be a reasonable alignment of the band gap width and position of

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CB. XPS VB spectra were measured to investigate the position of the valence band (VB) and CB of g-C3N4 and CS/g-C3N4-15. As shown in Fig. 7C, the VB potential of the CS/g-C3N4-15 raises 0.34 eV compared to pure g-C3N4. According to the width of band gap, the CB potential of CS/g-C3N4-15 is conformed to be upshifted by 0.34 eV. The band positions of the g-C3N4 and CS/g-C3N4-15 are schematically displayed in Fig. 7D. The more negative CB potential of the CS/g-C3N4-15 results in a larger thermodynamic driving force in the photocatalytic reduction of protons [13, 45]. 14

ACCEPTED MANUSCRIPT Furthermore, the UV-vis DRS of CS/DCN, CS/UCN, and CS/CNNS composites displayed in Fig. S4 also show enhanced visible light harvesting, indicating the CS is a suitable additive to adjust the light absorption of g-C3N4.

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To further elucidate the promoted performance of the CS/g-C3N4 composites, steady-state and time-resolved spectroscopic measurements were conducted to characterize the separation rates of charge carriers. The PL spectra of g-C3N4 and

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CS/g-C3N4 composites are shown in Fig. 8A. A broad emission peak at ca. 480 nm is

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detected under the excitation light of 350 nm, which results from the charge transfer emission. It is observed that the intensity of PL emission apparently decreases upon the increasing content of CS. Obviously, incorporation of CS in the g-C3N4 effectively suppresses the recombination of charge carriers, which is beneficial for photocatalytic

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reaction. The PL intensity of CS is very low because the amount of the photo-induced electron-hole pairs is low under the same irradiation condition [46]. Meanwhile, the intensity of PL emission at 480 nm of the CS/DCN, CS/UCN and CS/CNNS

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composites decreases compared to the corresponding g-C3N4. Thus, recombination of

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charge carriers in g-C3N4 can be significantly suppressed through introduction of CS. Time-resolved photoluminescence decay spectra of g-C3N4 and CS/g-C3N4-15 were measured to further identify the lifetime of photo-induced charge carriers. As shown in Fig. 8B, the calculated average lifetime of 2.49 ns in the g-C3N4 increases to 2.57 ns in CS/g-C3N4-15. The prolonged lifetime of charge carriers is caused by the increasing captured probability of photo-generated electrons by the CS. This is another hint that the photogenerated charge carriers are effectively separated. 15

ACCEPTED MANUSCRIPT The excitation and transfer efficiency of photo-generated electron and hole pairs under visible light irradiation were further measured by transient photocurrent responses. As shown in Fig. 8C, the CS/g-C3N4-15 exhibits improved photocurrent

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response for three on-off cycles, disclosing that the separation of charge carriers in CS/g-C3N4-15 is more effective than that in the g-C3N4. The electrochemical impedance spectra (EIS) were recorded to investigate the charge-transfer resistance of

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g-C3N4 and CS/g-C3N4-15 in the dark. In Fig. 8D, the arc radius of the Nyquist plot of

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CS/g-C3N4-15 is much smaller than that of g-C3N4, indicating the lower resistance and higher mobility of charge transfer comparing with the g-C3N4. Electron paramagnetic resonance (EPR) was employed to investigate the electronic property of the modified g-C3N4. As shown in Fig. 9, only one single

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Lorentzian line centered at g-value of 1.999 is observed, which is attributed to unpaired electrons in the aromatic rings of carbon atoms and π-bonded nanosized clusters on the surface of the g-C3N4 [47, 48]. Compared with pure g-C3N4, the

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intensity of EPR signal increases after introduction of CS, causing broader

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delocalization range of electrons in CN heterocycles. In addition, the EPR intensity of CS/g-C3N4 is extremely amplified under visible light irradiation (λ > 420 nm), indicating the efficient generation of radicals [49]. 3.2. Photocatalytic H2 evolution of the CS/g-C3N4 composites The photocatalytic hydrogen evolution under visible light over the g-C3N4 and CS/g-C3N4 composites are displayed in Fig. 10A. All the CS/g-C3N4 composites show improved photocatalytic hydrogen evolution performance. As depicted in Fig. 10B, 16

ACCEPTED MANUSCRIPT the optimized hydrogen evolution rate of CS/g-C3N4-15 is 50.2 µmol·h-1, which is about 4.8 times as that of g-C3N4 (10.5 µmol·h-1). When the incorporating amount of CS is more than 15 mg, the hydrogen evolution rate of the CS/g-C3N4 composites

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decreases. It is mainly because the excess amount of CS impairs the visible light absorption of g-C3N4 and becomes the recombination sites of charge carriers. Furthermore, the stability and durability of CS/g-C3N4-15 was evaluated during 4

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consecutive runs (16 h) under the same conditions (Figure 10C). There is no obvious

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deactivation in the fourth cycle, revealing high stability of the photocatalyst in photocatalytic evolution. The TEM images of CS/g-C3N4-15 after photocatalyic reaction are shown in Fig. 11A and 11B. It can be seen that the reaction does not change the morphology. The Pt nanoparticles are uniformly dispersed on the surface

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of CS/g-C3N4-15. In addition, the structures and visible light responses of the fresh and used CS/g-C3N4-15 were carried out by XRD, FTIR, DRS and PL, respectively. As indicated in Fig. S7A and S7B, the XRD patterns and FTIR spectra show that the

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phase and structure of the catalyst remained unchanged after photocatalytic reaction.

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The light response ability is also maintained based on the DRS (Fig. S7C). Fig. S7D displays that the PL intensity is slightly weakened after reaction. It is because the deposition of Pt nanoparticles on the CS/g-C3N4 could suppress the combination of charge carriers. The hydrogen evolution rate of the CS/g-C3N4 composites prepared by other precursors such as dicyandiamide, urea, or dicyandiamide are also illustrated in Fig. S6. One can see that the CS/DCN, CS/UCN and CS/CNNS all present improved photocatalytic hydrogen evolution performance compared to the 17

ACCEPTED MANUSCRIPT corresponding g-C3N4. The curves of wavelength-dependent H2 evolution over the g-C3N4 and CS/g-C3N4-15 were shown in Fig. 12. The photocatalytic performance of the pure

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g-C3N4 is closely accordant with its optical absorption. The hydrogen evolution rate of the g-C3N4 is 2.4 µmol·h-1 at 450 nm (λ0 ± 20 nm). Interestingly, in the case of CS/g-C3N4-15, the hydrogen evolution rates match well with the optical absorption

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spectrum under 450 nm. When the irradiation wavelength is above 450 nm, the

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photocatalytic activity improved indistinctively compared with the g-C3N4. The hydrogen evolution rate of CS/g-C3N4-15 is 13.1 µmol·h-1 at 450 nm. It indicates that incorporation of CS in g-C3N4 promotes the separation of photo-induced charge carriers and photocatalytic reduction power of electrons.

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Based on the above experimental results, the superior photocatalytic performance of H2 evolution over the CS/g-C3N4 composites can be attributed to the following reasons. First, as being proved by DRS, the CS as a sensitizer would be the origin of

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improved visible light absorption [50, 51]. Second, the CB position of CS/g-C3N4 is

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more negative, thus the driving force in photocatalytic reduction of hydrogen protons is enhanced. Third, according to previous report, the work function of CS is about -0.1 V, which is more positive than the CB potential of g-C3N4 [37]. The excited electrons from the CB of g-C3N4 tend to be trapped by the CS or Pt nanoparticles. These photo-generated electrons diffuse to the surface and act as active species for the reduction of protons. The photo-induced holes in the VB of g-C3N4 can be captured by TEOA, which is oxidized to TEOA+. From the analysis of PL and 18

ACCEPTED MANUSCRIPT photoelectrochemical measurements, the recombination of photo-generated electrons and holes are restricted in the CS/g-C3N4. The transfer mechanism of photo-induced charge carriers is illustrated in Fig. 13. In brief, due to the encapsulated nanostructure

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of CS/g-C3N4 with closely contacted interface, the CS serves as electron trappers to efficiently improve the separation of photo-generated charge carriers. Thus, the

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photocatalytic performance of g-C3N4 is enhanced.

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4. Conclusions

In summary, CS/g-C3N4 nanocomposites have been successfully synthesized by the polymerization process of melamine from the carbon spheres. The slight amount of CS embedded in the g-C3N4 is believed to be responsible for strong interaction,

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visible light harvesting, enlarged specific surface area and photo-induced electrons trapping, and thus promote the separation of charge carriers. Meanwhile, incorporation of CS induces negative-shift of conduction band potential, resulting in

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enhanced proton reduction property of photo-electrons. Thus, the CS/g-C3N4

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composites exhibit significantly improved photocatalytic hydrogen evolution performance under visible light irradiation, and the CS/g-C3N4-15 shows the optimal hydrogen evolution rate of 50.2 µmol·h-1, which is 4.8 folds of pure g-C3N4. This study also demonstrates that polymerization of dicyandiamide, urea, or dicyandiamide with CS can also construct CS/g-C3N4 nanocomposites with improved photocatalytic hydrogen evolution performance. Other carbon materials/g-C3N4

nanostructures

could also be synthesized based on the present strategy, which may be promising 19

ACCEPTED MANUSCRIPT photocatalysts for H2 production in energy industry.

Acknowledgements

of

China

(21273080)

and

Guangdong

Natural

Science

Foundation

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(2014A030311039).

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This work was financially supported by the National Natural Science Foundation

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References

[1] K. Maeda, K. Domen, Photocatalytic water splitting: recent progress and future challenges, J. Phys. Chem. Lett. 1 (2010) 2655-2661.

[2] R. Kuriki, H. Matsunaga, T. Nakashima, K. Wada, A. Yamakata, O. Ishitani, K. Maeda, Nature-inspired, highly durable CO2 reduction system consisting of a

TE D

binuclear Ruthenium (II) complex and an organic semiconductor using visible light, J. Am. Chem. Soc. 138 (2016) 5159-5170.

[3] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37-38.

EP

[4] Y. Liu, Y. X. Yu, W. D. Zhang, Carbon quantum dots-doped CdS microspheres with enhanced photocatalytic performance, J. Alloy Compd. 569 (2013) 102-110.

AC C

[5] Y. Liu, Y. X. Yu, W. D. Zhang, MoS2/CdS heterojunction with high photoelectrochemical activity for H2 evolution under visible light: the role of MoS2, J. Phys. Chem. C 117 (2013) 12949-12957. [6] J. He, L. Chen, F. Wang, Y. Liu, P. Chen, C. T. Au, S. F. Yin, CdS nanowires decorated with ultrathin MoS2 nanosheets as an efficient photocatalyst for hydrogen evolution, ChemSusChem 9 (2016) 624-630. [7] Y. Liu, Y. X. Yu, W. D. Zhang, Photoelectrochemical properties of Ni-doped Fe2O3 thin films prepared by electrodeposition, Electrochim. Acta 59 (2012) 121-127. [8] Y. Zhao, F. Zhao, X. P. Wang, C. Y. Xu, Z. P. Zhang, G. Q. Shi, L. T. Qu, Graphitic 20

ACCEPTED MANUSCRIPT carbon nitride nanoribbons: graphene-assisted formation and synergic function for highly efficient hydrogen evolution, Angew. Chem. Int. Ed. 53 (2014) 13934-13939. [9] Y. Zheng, L. H. Lin, X. J. Ye, F. S. Guo, X. C. Wang, Helical graphitic carbon nitrides with photocatalytic and optical activities, Angew. Chem. Int. Ed. 53 (2014)

RI PT

11926-11930. [10] M. K. Bhunia, K. Yamauchi, K. Takanabe, Harvesting solar light with crystalline carbon nitrides for efficient photocatalytic hydrogen evolution, Angew. Chem. Int. Ed. 53 (2014) 11001-11005.

SC

[11] Y. J. Zhou, L. X. Zhang, W. M. Huang, Q. L. Kong, X. Q. Fan, M. Wang, J. L. Shi, N-doped graphitic carbon-incorporated g-C3N4 for remarkably enhanced

M AN U

photocatalytic H2 evolution under visible light, Carbon 99 (2016) 111-117. [12] S. Qiao, R. W. Mitchell, B. Coulson, D. V. Jowett, B. R. Johnson, R. Brydson, M. Isaacs, A. F. Lee, R. E. Douthwaite, Pore confinement effects and stabilization of carbon nitride oligomers in macroporous silica for photocatalytic hydrogen production, Carbon 106 (2016) 320-329.

TE D

[13] Q. Han, B. Wang, J. Gao, Z. H. Cheng, Y. Zhao, Z. P. Zhang, L. T. Qu, Atomically thin mesoporous nanomesh of graphitic-C3N4 for high-efficiency photocatalytic hydrogen evolution, ACS Nano 10 (2016) 2745-2751.

EP

[14] X. C. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M. Carlsson, K. Domen, M. Antonietti, A metal-free polymeric photocatalyst for hydrogen production from water under visible light, Nat. Mater. 8 (2009) 76-80.

AC C

[15] J. Lin, Z. Pan, X. Wang, Photochemical reduction of CO2 by graphitic carbon nitride polymers, ACS Sustainable Chem. Eng. 2 (2014) 353-358. [16] D. H. Lan, H. T. Wang, L. Chen, C. T. Au, S. F. Yin, Phosphorous-modified bulk graphitic carbon nitride: Facile preparation and application as an acid-base bifunctional and efficient catalyst for CO2 cycloaddition with epoxides, Carbon 100 (2016) 81-89. [17] M. Xiong, L. Chen, Q. Yuan, J. He, S. L. Luo, C. T. Au, S. F. Yin, Controlled synthesis of graphitic carbon nitride/beta bismuth oxide composite and its high visible-light photocatalytic activity, Carbon 86 (2015) 217-224. 21

ACCEPTED MANUSCRIPT [18] M. Xiong, L. Chen, Q. Yuan, J. He, S. L. Luo, C. T. Au, S. F. Yin, Facile fabrication

and

enhanced

photosensitized

degradation

performance of the

g-C3N4–Bi2O2CO3 composite, Dalton Trans. 43 (2014) 8331-8337. [19] S. Z. Wu, K. Li, W. D. Zhang, On the heterostructured photocatalysts

RI PT

Ag3VO4/g-C3N4 with enhanced visible light photocatalytic activity, Appl. Surf. Sci. 324 (2015) 324-331.

[20] Y. Liu, Y. X. Yu, W. D. Zhang, Photoelectrochemical study on charge transfer properties of nanostructured Fe2O3 modified by g-C3N4, Inter. J. Hydrogen Energy 39

SC

(2014) 9105-9113.

[21] Y. Xu, W. D. Zhang, Sulfur/g-C3N4 composites with enhanced visible light

M AN U

photocatalytic activity, Sci. Adv. Mater. 6 (2014) 2611-2617.

[22] P. Zhou, J. Yu, M. Jaroniec, All-solid-state Z-scheme photocatalytic systems, Adv. Mater. 26 (2014) 4920-4935.

[23] J. J. Duan, S. Chen, M. Jaroniec, S. Z. Qiao, Porous C3N4 nanolayers/N-graphene films as catalyst electrodes for highly efficient hydrogen evolution, ACS Nano 9

TE D

(2015) 931-940.

[24] J. S. Zhang, M. W. Zhang, L. H. Lin, X. C. Wang, Processing of conjugated carbon nitride powders for thin-film fabrication, Angew. Chem. Int. Ed. 54 (2015),

EP

6297-6301.

[25] K. Li, F. Y. Su, W. D. Zhang, Modification of g-C3N4 nanosheets by carbon quantum dots for highly efficient photocatalytic generation of hydrogen, Appl. Surf.

AC C

Sci. 375 (2016) 110-117. [26] N. P. Wickramaratne, J. Xu, M. Wang, L. Zhu, L. Dai, M. Jaroniec, Nitrogen enriched porous carbon spheres: Attractive materials for supercapacitor electrodes and CO2 adsorption, Chem. Mater. 26 (2014) 2820-2828. [27] Y. Liu, L. K. Pan, T. Q. Chen, X. T. Xu, T. Lu, Z. Sun, D. H. Chua, Porous carbon spheres via microwave-assisted synthesis for capacitive deionization, Electrochim. Acta 151 (2015) 489-496. [28] Q. Wang, H. Li, L. Q. Chen, X. J. Huang, Monodispersed hard carbon spherules with uniform nanopores, Carbon 39 (2001) 2211-2214. 22

ACCEPTED MANUSCRIPT [29] A. Boudjemaa, A. Rebahi, B. Terfassa, R. Chebout, T. Mokrani, K. Bachari, N. J. Coville, Fe2O3/carbon spheres for efficient photo-catalytic hydrogen production from water and under visible light irradiation, Sol. Energy Mater. Sol. Cells 140 (2015) 405-411.

RI PT

[30] J. L. Li, X. J. Liu, X. Q. Piao, Z. Sun, L. K. Pan, Novel carbon sphere@Bi2MoO6 core-shell structure for efficient visible light photocatalysis, RSC Adv. 5 (2015) 16592-16597

[31] Y. H. Li, M. Y. Zhao, N. Zhang, R. J. Li, J. X. Chen, Synthesis and photoactalytic

SC

activity of carbon spheres loaded Cu2O/Cu composites, J. Alloys Compd. 643 (2015) 106-110.

M AN U

[32] H. Q. Sun, G. L. Zhou, Y. X. Wang, A. Suvorova, S. B. Wang, A new metal-free carbon hybrid for enhanced photocatalysis, ACS Appl. Mater. Interfaces 6 (2014) 16745-16754.

[33] J. Y. Zhang, Y. H. Wang, J. Jin, J. Zhang, Z. Lin, F. Huang, J. G. Yu, Efficient visible-light photocatalytic hydrogen evolution and enhanced photostability of

TE D

core/shell CdS/g-C3N4 nanowires, ACS Appl. Mater. Interfaces 5 (2013) 10317-10324.

[34] L. Liu, Y. H. Qi, J. R. Lu, S. L. Lin, W. J. An, Y. H. Liang, W. Q. Cui, A stable

EP

Ag3PO4@g-C3N4 hybrid core@shell composite with enhanced visible light photocatalytic degradation, Appl. Catal. B 183 (2016) 133-141. [35] S. Kumar, A. Baruah, S. Tonda, B. Kumar, V. Shanker, B. Sreedhar,

AC C

Cost-effective and eco-friendly synthesis of novel and stable N-doped ZnO/g-C3N4 core-shell nanoplates with excellent visible-light responsive nanoplates with excellent visible-light responsive, Nanoscale 6 (2014) 4830-4842. [36] S. Patnaik, S. Martha, An overview of the modification of g-C3N4 with high carbon containing materials for photocatalytic applications, Inorg. Chem. Front. 3 (2016) 336-347. [37] X. C. Wang, K. Maeda, X. F. Chen, K. Takanabe, K. Domen, Y. D. Hou, X. Z. Fu, M. Antonietti, Polymer semiconductors for artificial photosynthesis: hydrogen evolution by mesoporous graphitic carbon nitride with visible light, J. Am. Chem. Soc. 23

ACCEPTED MANUSCRIPT 131 (2009) 1680-1681. [38] R. C. Cheng, L. X. Zhang, X. Q. Fan, M. Wang, M. L. Li, J. L. Shi, One-step construction of FeOx modified g-C3N4 for largely enhanced visible-light photocatalytic hydrogen evolution, Carbon 101 (2016) 62-70.

RI PT

[39] J. V. Ibarra, E. MuÇoz, R. Moliner, FTIR study of the evolution of coal structure during the coalification process, Org. Geochem. 24 (1996) 725-735.

[40] J. Liu, T. Zhang, Z. Wang, G. Dawson, W. Chen, Simple pyrolysis of urea into graphitic carbon nitride with recyclable adsorption and photocatalytic activity, J Mater

SC

Chem 21 (2011) 14398-14401.

[41] Y. Wang, X. Cui, Y. Li, L. Chen, H. Chen, L. Zhang, J. Shi, A co-pyrolysis route

M AN U

to synthesize nitrogen doped multiwall carbon nanotubes for oxygen reduction reaction, Carbon 68 (2014) 232-239.

[42] D. H. Lan, H. T. Wang, L. Chen, C. T. Au, S. F. Yin, Phosphorous-modified bulk graphitic carbon nitride: Facile preparation and application as an acid-base bifunctional and efficient catalyst for CO2 cycloaddition with epoxides, Carbon 100

TE D

(2016) 81-89.

[43] J. Liu, Y. Liu, N. Y. Liu, Y. Z. Han, X. Zhang, H. Huang, Y. Lifshitz, S. T. Lee, J. Zhong, Z. H. Kang, Metal-free efficient photocatalyst for stable visible water splitting

EP

via a two-electron pathway, Science 347 (2015) 970-974. [44] S. Cao, J. Low, J. Yu, M. Jaroniec, Polymeric photocatalysts based on graphitic carbon nitride, Adv. Mater. 27 (2015) 2150-2176.

AC C

[45] Q. Han, C. G. Hu, F. Zhao, Z. P. Zhang, N. Chen, L. T. Qu, One-step preparation of Iodine-doped graphitic carbon nitride nanosheets as efficient photocatalysts for visible light water splitting, J. Mater. Chem. A 3 (2015) 4612-4619. [46] J. Z. Ma, C. X. Wang, H. He, Enhanced photocatalytic oxidation of NO over g-C3N4-TiO2 under UV and visible light, Appl. Catal. B 184 (2016) 28-34. [47] J. Zhang, G. Zhang, X. Chen, S. Lin, L. Möhlmann, G. Dołega, G. Lipner, M. Antonietti, S. Blechert, X. Wang, Co-monomer control of carbon nitride semiconductors to optimize hydrogen evolution with visible light, Angew. Chem. Int. Ed. 124 (2012) 3237-3241. 24

ACCEPTED MANUSCRIPT [48] K. Li, X. Xie, W. D. Zhang, Porous graphitic carbon nitride derived from melamine-ammonium oxalate stacking sheets with excellent photocatalytic hydrogen evolution activity, ChemCatChem 8 (2016) 2128-2135. [49] J. N. Qin, S. B. Wang, H. Ren, Y. D. Hou, X. C. Wang, Photocatalytic reduction

RI PT

of CO2 by graphitic carbon nitride polymers derived from urea and barbituric acid, Appl. Catal. B 179 (2015) 1-8.

[50] P. Zabek, J. Eberl, Kisch, On the origin of visible light activity in carbon-modified titania, H. Photochem. Photobiol. Sci. 8 (2009) 264-269.

SC

[51] Y. Hu, Y. Liu, H. S. Qian, Z. Q. Li, J. F. Chen, Coating colloidal carbon spheres with CdS nanoparticles: microwave-assisted synthesis and enhanced photocatalytic

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activity, Langmuir 26 (2010) 18570-18575.

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ACCEPTED MANUSCRIPT Table 1 Area ratios of the C atoms in graphitic carbon (C−C) and N−C=N of the g-C3N4 and CS/g-C3N4-15. C 1s

284.8

C-C

288.3

N=C-N

284.8

C-C

288.3

N=C-N

C/Call (at%) 6.5

93.5

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CS/g-C3N4-15

assign

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g-C3N4

position

26

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Captions for figures: Fig. 1 Schematic illustration of the synthesis process of CS/g-C3N4 composites.

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Fig. 2 SEM and TEM images of CS (A and B) and g-C3N4 (C and D).

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Fig. 3 SEM (A and C) and TEM (B and D) images of CS/g-C3N4-15.

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Fig. 4 (A) N2 adsorption-desorption isotherm and (B) pore size distribution curves obtained for g-C3N4 and CS/g-C3N4-15.

Fig. 5 (A) XRD patterns of g-C3N4 and CS/g-C3N4 composites; (B) FTIR spectra of

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CS, g-C3N4 and CS/g-C3N4-15.

Fig. 6 (A) XPS survey scans, high resolution (B) C 1s and (C) N 1s XPS spectra of

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g-C3N4 and CS/g-C3N4-15.

Fig. 7 (A) UV-vis diffuse reflectance absorption spectra, (B) Tauc plots, (C) XPS valence band spectra and (D) electronic band structure of g-C3N4 and CS/g-C3N4-15.

Fig. 8 (A) Fluorescence emission spectra and (B) time-resolved photoluminescence decay spectra of g-C3N4 and CS/g-C3N4-15; (C) Transient photocurrent response under visible light irradiation and (D) electrochemical impedance spectroscopy (EIS) 27

ACCEPTED MANUSCRIPT Nyquist plots of g-C3N4 and CS/g-C3N4-15 in the dark.

Fig. 9 Solid-state EPR spectra of g-C3N4 and CS/g-C3N4-15 under visible light (λ >

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420 nm) at room temperature with a g-value 1.999. The experimental parameters: center field 324.006 mT, frequency 9070.224 MHz, power 0.998 mW.

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Fig. 10 (A) Photocatalytic hydrogen evolution performance and (B) hydrogen

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evolution rate of g-C3N4 and CS/g-C3N4 composites; (C) Stability test of H2 evolution on CS/g-C3N4-15.

Fig. 11 (A) Low and (B) magnified TEM images of CS/g-C3N4-15 after photocatalyic

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reaction.

Fig. 12 Wavelength dependence of hydrogen evolution rate on g-C3N4 and

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CS/g-C3N4-15.

Fig. 13 Schematic illustration of the mechanism for the improved photocatalytic hydrogen evolution performance over CS/g-C3N4 composites.

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