Hollow graphene with apparent potential difference to boost charge directional transfer for photocatalytic H2 evolution

Journal Pre-proof Hollow graphene with apparent potential difference to boost charge directional transfer for photocatalytic H2 evolution Zhexing Lin, Chengtian Shao, Shujuan Jiang, Chuanzhi Sun, Shaoqing Song

PII:

S0926-3373(20)30157-0

DOI:

https://doi.org/10.1016/j.apcatb.2020.118742

Reference:

APCATB 118742

To appear in:

Applied Catalysis B: Environmental

Received Date:

31 October 2019

Revised Date:

17 January 2020

Accepted Date:

7 February 2020

Please cite this article as: Lin Z, Shao C, Jiang S, Sun C, Song S, Hollow graphene with apparent potential difference to boost charge directional transfer for photocatalytic H2 evolution, Applied Catalysis B: Environmental (2020), doi: https://doi.org/10.1016/j.apcatb.2020.118742

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Hollow graphene with apparent potential difference to boost charge directional transfer for photocatalytic H2 evolution

Zhexing Lina#, Chengtian Shaoc#, Shujuan Jiang*a, Chuanzhi Sun*b, Shaoqing Song*a

a

School of Material Science and Chemical Engineering, Ningbo University, Fenghua

b

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Road 818, Ningbo 315211, P. R. China

College of Chemistry, Chemical Engineering and Materials Science, Shandong

Normal University, Jinan 250014, PR China c

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Department of Chemistry, Chung Yuan Christian University, Taoyuan City, Taiwan

Corresponding Authors

These authors contributed equally.

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#

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E-mail: [email protected]; [email protected]; [email protected];

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Graphical abstract

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Highlights

Apparent potential difference of HG promotes charge directional transfer.

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Deviation of graphene into HG results in the activation of carbon π electrons.

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g-C3N4 was rooted into hollow graphene (g-C3N4@HG).

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g-C3N4@HG shows much high H2 evolution activity without any noble metals.

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

Abstract: g-C3N4 was in situ rooted into hollow graphene (g-C3N4@HG) for

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photocatalytic H2O dissociation into H2 by vacuum-filling thermal polymerization.

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Characterizations and DFT calculations reveal that deviation of graphene (GR) into HG results in the activation of carbon π electrons, and the apparent potential difference

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between inner and outer surfaces of HG boosts charge directional transfer from the

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rooted g-C3N4 to HG. H2 evolution efficiency of g-C3N4@HG is 1.43 mmol∙g-1h-1 without any noble metal as cocatalyst under visible irradiation, which is even more than

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2.86, and 1.72 times that of g-C3N4 with 3 wt.% Pt, and g-C3N4/GR with 1 wt.% Pt, respectively. Results in the field of solar energy conversion supply a novel strategy to

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boost directional charge transfer for H2 evolution by utilizing apparent potential difference of HG.

Keywords: hollow graphene, g-C3N4, hydrogen evolution, directional electron transfer, apparent potential difference

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1. Introduction Photocatalytic H2O dissociation into H2 over semiconductor photocatalysts under light irradiation is considered to be one of the most promising strategies to develop clean and green energy for future energy demand [1-3]. In the past few decades, numerous semiconductor materials, e.g., CdS [4-7], TiO2 [8,9], ZnO [10], Cu2O [11],

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SrTiO3 [12], g-C3N4 [13-28], have been performed to dissociate H2O into H2,

nevertheless, H2 efficiency is still low. In photocatalytic H2O dissociation process, the efficiency of semiconductor is subject to three factors: (i) light absorption capability;

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(ii) directional transfer of charge carriers; and (iii) H2 evolution overpotential [29,30].

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The former requires more photon energy to excite the photogenerated charges. The second factor dynamically reflects a competition between the relative slow utilization

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process and the fast recombination of e- and h+ [31,32]. Moreover, e- cannot react with

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H2O directly owing to the large H2 evolution overpotential [33-35]. These factors severely restrict the utilization of photogenerated charges.

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Generally, it is considered that graphene can be hopefully utilized to solve these aforementioned constraints due to its excellent characteristics, e.g., fast carrier mobility,

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high optical transparency, unique mechanical flexibility and strength. Graphene (GR) based photocatalysts have been widely utilized to eliminate organic pollutants and heavy metal ions, dissociate H2O, and split CO2 [36,37]. However, monolayer GR demonstrates insufficient photo-conductivity property because of the weak interaction between photon and GR [38]. Study has revealed that monolayer GR only absorbs 2.3%

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of visible and near-infrared light, and absorptivity increases linearly with the number of layers [38]. Although monolayer GR presents exceedingly good mobility of charge carriers (2×105 cm2 V-1 s-1), layer superposition results in the limited electron transport capability of GR along with the increased grain boundary density [39]. Additionally, there is always a large interface barrier between GR and semiconductor, which also severely influences the efficiency of charge transfer at their interface [32,40,41].

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Theoretical prediction indicates that graphene is deviated from two-dimensional plane, the π-electron density shifts from concave to convex of GR with hybridization

intermediate between sp2 and sp3, and finally this uneven distribution of electrons will

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result in an apparent potential difference [42,43]. Beyond that, the deviated GR

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increases its surface density, and the undulating surface endows higher photosensitivity and absorbance [44,45]. Herein, two-dimensional GR was deviated into three-

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dimensional hollow graphene (HG) with using chemical vapor deposition, and g-C3N4

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is in situ rooted into HG (g-C3N4@HG) by a vacuum-filling thermal polymerization process. Theoretical prediction and experimental characterizations display that the

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deviation of GR into HG can not only results in the localization distribution of surface charge and the activation of carbon π electrons, but also introduce an apparent potential

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difference between inner and outer surfaces which may be the driving force for the directional transfer of charge from g-C3N4 to HG. In photocatalytic H2O dissociation into H2, g-C3N4@HG presents highly efficient and sustainable H2 evolution under simulated solar excitation, which is distinctly higher than that of g-C3N4/GR with 1 wt.% Pt and g-C3N4 with 3 wt.% Pt. The strategy of rooting g-C3N4 into HG suggests a new

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perspective for boosting photocatalytic activity of g-C3N4 and can be suitable for other semiconductor materials.

2. Experimental 2.1 Construction of g-C3N4@HG, and g-C3N4/GR samples Preparation of HG sample: HG was synthesized at 750 oC with using CH4Mg2O6

as

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template and benzene as precursor [46]. The as-prepared HG was uniformly dispersed in 50 ml of hydrochloric acid (37%) and refluxed at 80 oC for 6 h to remove MgO,

which thus opens the HG. The mixture was filtered and then washed with the deionized

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H2O and ethanol, ultimately dried at 80 oC for 2 h.

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HG (10 mg) was placed in vacuum reactor and evacuated to 0.67 Pa. 100 mL of urea aqueous solution (1 g.mL-1) was rapidly injected into the vacuum reactor, and the

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vacuum process lasted for 30 min. Subsequently, the mixture was magnetically stirred

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for 1 h and dried at 60 oC for 2 h. The sample was ground and then immediately heated to 520 oC at a rate of 5 oC/min and maintained there for 2 h. In this way, urea was

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polymerized into g-C3N4 within HG, and grew from HG cavity. The obtained sample is named as g-C3N4@HG-10. When the weight of HG increases to 5 and 15 mg, and the

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accordingly obtained samples were named as g-C3N4@HG-5 and g-C3N4@HG-15, respectively.

For comparison, pristine g-C3N4 was prepared by direct heat treatment with using urea as precursor at 520 oC for 2 h. Additionally, g-C3N4 was grown on the outer surface of GR or HG by impregnating graphene or HG into urea solution. After impregnation,

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the same drying process and polymerization was performed, and g-C3N4/GR or gC3N4/HG was obtained with 10 mg GR or HG.

2.2 Characterization Micromorphology was observed by transmission electron microscopy (JEM2010HR). Raman spectra were recorded at room temperature using a micro-Raman

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spectrometer (Renishaw InVia) in the backscattering geometry with a 514.5 nm Ar+ laser as an excitation source. Equivalent series resistance (ESR) spectra were recorded on MEX-nano, Bruker. Chemical composition and chemical state of samples were

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examined by X-ray photoelectron spectroscopy (XPS) (ESCALAB250xi, Thermon

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Scientific). Water contact angle measurements were performed as follows: the powder sample was first dispersed into ethanol and then droped onto glass slide repeatedly until

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a compact film was formed. And then the slides with film were dried at 80 °C for 12 h,

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the evolution of the water contact angle was recorded by the contact angle measurement system (OCA 30, DataPhysics Instruments GmbH). UV-visible diffuse reflectance

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spectra were recorded on a UV-visible spectrophotometer (UV-2600, Shimadzu, Japan), using Ba2SO4 as the reference standard. Electrochemical impedance spectra (EIS),

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transient photo-current responses, and linear sweep voltammetry (LSV) were examined on electrochemical workstation (CHI 660C Chenhua Instruments) in Na2SO4 and KNO3 solutions. Time-resolved photo-luminescence spectra were performed on FLS920 fluorescence lifetime spectrophotometer (Edinburgh, Instruments, UK). Specific surface area and pore volume were measured with using N2-adsorption apparatus

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(Micromeritics ASAP 2020).

2.3 DFT calculations All geometry models were optimized using the B3LYP density functional method in Gaussian 09 package. The standard 6-31G(d) basis set was performed on H, C and N atoms. The bending or planar graphene were all consist of 56 carbon atoms. The same

boundary C or N atoms to eliminate boundary influence.

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2.4 Photocatalytic H2 evolution

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amount of carbon atoms facilitates for comparing. H atoms were compensated on the

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H2 evolution was performed in a three-neck flask (100 mL) equipped with siliconerubber membrane under anaerobic condition. 30 mg of sample was dispersed into 75

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mL of aqueous solution including 10 vol.% triethanolamine. A xenon lamp (350 W)

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with a 420 nm filter as a light source is used to excite photocatalytic material. Before light excitation, high-purity N2 gas (99.999 %) was passed into the photocatalytic

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system for 30 min to dislodge dissolved O2 with continuous stirring. H2 evolution efficiency was surveyed with using GC-2014C (Shimadzu, Japan, TCD, 5 Å molecular

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sieve column). In a typical experiment, 50 mg of as-prepared photocatalysts were dispersed in 72 mL of deionized water with ultrasonic treatment. Next, 8 mL of triethanolamine as the scavengers was added into above suspension under rapid agitations. 3 wt.% and 1 wt.% Pt was deposited on g-C3N4 or g-C3N4/GR by photochemical reduction method. In addition, a LED lamp with monochromatic light

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(λ = 420 nm) was used as the light source to carry out the experiments related to apparent quantum yield.

3. Results and discussion 3.1. Morphology and composition of the photocatalysts The morphology of samples was observed by SEM, TEM and HRTEM (Figure 1 and

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Figure S1). In Figure 1A, HG demonstrates an inner cavity size of 50-70 nm and wall

thickness of 3~6 nm with convex and concave surface, and g-C3N4 present typical nanosheets with size of 200-400 nm (Figure 1B). For g-C3N4@HG-10, it is observed

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that g-C3N4 nanosheet with the size of 100-150 nm is rooted into the cavity of HG,

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which forms the mosaic-type structure between g-C3N4 nanosheets and HG (Figure 1C). It is seen that layer spacing of HG is 0.34 nm, corresponding to its (002) facet, and the

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intimate interaction between g-C3N4 and HG is confirmed because no obvious

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boundary is observed between them as shown in HRTEM image (Figure 1D). For gC3N4/GR, two-dimensional planar structures of g-C3N4 and GR overlaps (Figure S1).

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The Raman spectrum of HG demonstrates two peaks at 1590 and 1325 cm-1, corresponding to G and D bands of GR structure (Figure 2A) [47]. Raman signals of

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pristine g-C3N4 are located at 523 and 1059 cm-1, which is assigned to the in-plane symmetrical stretching and stretching vibrations of C-N in heptazine heterocycles [48,49], respectively. For g-C3N4/GR, the two characteristic peaks of g-C3N4 (523/1059 cm-1) shift to 547/1079 cm-1 which further blue shift to 559/1096 cm-1 for g-C3N4@HG10. The blue shift of C-N vibrations in heptazine heterocycles is influenced by the size

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of the rooted g-C3N4 sheet [42]. The result suggests that the length of C-N becomes short, and the overlap of electron cloud in the delocalization system of heptazine heterocycle is enhanced, which will thus promote the electron transfer from g-C3N4 to HG. In ESR spectra, g-C3N4 presents a Lorentzian line with a g value at 2.0034, which reflects the unpaired electrons of π-conjugated structure (Figure 2B) [50,51]. As g-C3N4 combined with GR or rooted into HG, the peak intensity of ESR increase, and g-

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C3N4@HG-10 shows the strongest ESR signal, revealing the extended-π conjugated system by π-π* interaction between the rooted g-C3N4 and HG. Additionally, the

chemical environment and electron cloud of C and N of the rooted g-C3N4 were also

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studied by XPS. C 1s peaks for g-C3N4 can be divided into 284.6, and 288.0 eV,

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corresponding to sp2 C-C bond and C=N-C [52,53]. The binding energy of sp2 C=N-C bond in g-C3N4 shifts positively from 288.0 eV to 288.2 eV in g-C3N4@HG-10 (Figure

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2C). N 1s peak of g-C3N4 can be splitted into three species at 398.4, 399.9, and 401.2

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eV, which can be considered as sp2-hybridized N (C=N-C), bridging N (C3-N), and NH2/NH, respectively (Figure 2D) [53,54]. The binding energy of sp2-hybridized N in

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g-C3N4 shifts negatively to 398.2 eV after rooted in HG (Figure 2D) [54,55]. For gC3N4/GR, the binding energies of C and N show no obvious changes, suggesting the

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weak interaction between GR and g-C3N4. Therefore, the outer electron cloud and chemical environment of N and C of the rooted g-C3N4 have been influence by the internal and external electronic characteristics and electric field of HG, which is also confirmed by DFT predictions.

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3.2. DFT study results According to the morphology and microstructure, the models of HG, g-C3N4@HG as well as g-C3N4/GR sheet were designed for DFT studies in Figure 3. For GR nanosheet, the equipotential curves are symmetrically distributed in GR plane (Figure 3A), which is much different from the case of HG. It is seen that the deformation of sp2 hybridization system in GR results in the shift of π electron density from concave to

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convex of HG as presented by the dense distribution of equipotential curves outside of the HG (Figure 3B). Therefore, an apparent potential difference is generated which can work as a driving force to promote electrons to flow from inner to outer surface of HG.

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Mulliken charge distribution analysis of GR or HG before and after interacting with g-

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C3N4 were shown in Figure 3C-F and Figure S2. It is found that the positive and negative charges are uniformly distributed in GR plane, and the whole sp2 carbon

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system shows electricalneutrality (Figure 3E and Figure S2D). For HG, the deviation

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of GR from the plane breaks the electroneutrality of the sp2 carbon system, and the uneven charge distribution is demonstrated on the surface of HG (Figure 3F and Figure

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S2D). When g-C3N4 combines with GR, the charge distribution and electroneutrality of the combined GR shows no obvious change (Figure 3E), sugesting that the interaction

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between GR and g-C3N4 is weak. For g-C3N4@HG, the charge on each carbon atom oscillates, indicating the strong interaction between the rooted g-C3N4 and HG (Figure 3F). Moreover, it is noted that, both C electropositivity and N electronegativity of the rooted g-C3N4 become smaller (Figure S2E), so it is easier for electron excitation from N to C of the rooted g-C3N4 [54]. Therefore, the deviation of GR into HG results in the

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activation of carbon π electrons, and the apparent potential difference between inner and outer surfaces of HG boosts charge directional transfer from the rooted g-C3N4 to HG.

3.3 H2O adsorption and photoelectric properties H2O adsorption on the surface of samples were revealed by contact angle tests of

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H2O (Figure 4). g-C3N4 presents a contact angle of 45o which is unchanged for 30 s or

longer time. Comparatively, g-C3N4/GR and g-C3N4@HG-10 are hydrophilic with the

initial contact angles of 30o and 5o, particularly, the droplet on g-C3N4@HG-10 is

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entirely absorbed within 5 s. In Figure S3, the BET specific surface area and pore

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volume of g-C3N4@HG-10 and g-C3N4@GR are almost the same. Thus, this result suggests that the obviously improved H2O adsorption of g-C3N4@HG-10 has been

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achieved due to the broken electroneutrality of the sp2 carbon system for HG [57,58].

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Furthermore, rooting g-C3N4 into HG will tremendously strengthen light absorption by photon scattering. In Figure 5A, absorbing boundary of sunlight over g-C3N4 is 455 nm.

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It is noteworthy that the absorption spectroscopies of g-C3N4@HG-10 samples present a more obvious red shift with stronger absorption intensity compared with g-C3N4/GR,

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suggesting that the hollow structure of HG is conducive to light absorption owing to confinement of incident light within HG by multiple light scattering [59]. g-C3N4@HG10 and g-C3N4/GR display a tremendous difference in the charge carrier transfer efficiency reflected by the different semicircle radius in EIS Nyquist plots [60-63]. In Figure 5B, the conduction resistances are tested to be 1002 and 1207 Ω for g-

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C3N4@HG-10, and g-C3N4/GR, respectively. It is seen that g-C3N4@HG-10 provides better conductivity in comparison with g-C3N4/GR, which will efficiently restrain the photo-induced carrier recombination under light irradiation. As a result, the current intensity (J) for g-C3N4@HG-10 is distinctly higher than that of g-C3N4/GR under visible-light illumination in the transient photo-current test (Figure 5C) [64-66]. In addition, linear sweep voltammetry (LSV) tests show that the onset potentials are -0.26,

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-0.74, and -1.10 V for g-C3N4@HG-10, g-C3N4/GR, and g-C3N4, respectively (Figure

5D). Under the influence of the apparent potential difference of HG, the strong transfer and injection ability of photoelectron over g-C3N4@HG can reduce its hydrogen

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production overpotential [66]. Based on the above characterizations and theoretical

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affecting hydrogen production.

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predictions, the strategy of rooting g-C3N4 into HG overcomes the three factors

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3.4. Photocatalytic H2 evolution

Accordingly, H2 production efficiency of samples were investigated by H2O

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dissociation under Xenon-lamp irradiation with an optical filter of λ ≥ 420 nm (Figure 6). In Figure 6A, H2 evolution efficiency of g-C3N4 with 3.0 wt.% Pt as cocatalyst is

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0.50 mmol∙g-1h-1. When g-C3N4 combined with GR, H2 evolution efficiency over gC3N4/GR with 1.0 wt.% Pt as cocatalyst is 0.83 mmol∙g-1h-1. Interestingly, H2production rates of 0.77, 1.43, and 1.10 mmol∙g-1h-1 are achieved over g-C3N4@HG-5, g-C3N4@HG-10, and g-C3N4@HG-15 without any noble metal as cocatalysts. It is seen that g-C3N4@HG-10 demonstrates the optimal performance, and the demonstrated H2-

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production efficiency of g-C3N4@HG-10 is more than 2.86 and 1.72 times of g-C3N4 with 3.0 wt.% Pt and g-C3N4/GR with 1.0 wt.% Pt, respectively. Also, the value for gC3N4@HG-10 is among the highest H2 evolution rates of all g-C3N4 with noble metals and/or GR so far under similar reaction conditions (Table S1). When g-C3N4 combined with HG (g-C3N4/HG, Figure S5), H2 evolution rate declines sharply from 1.43 (gC3N4@HG-10) to 0.12 (g-C3N4/HG) mmol∙g-1h-1. Effects of irradiation time on H2

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production efficiency over g-C3N4, g-C3N4/GR, and g-C3N4@HG photocatalysts were displayed in Figure 6B, and the durative H2 evolution is achieved with no decline for

all samples. However, H2 production efficiency over g-C3N4 with 3% Pt increases

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tardily under visible-light irradiation, while steep H2 evolution efficiency curves over

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g-C3N4/GR with 1.0% Pt and g-C3N4@HG can be obtained, especially g-C3N4@HG10 without Pt shows superior evolution rate of 2.86 mmol∙g-1 at 2 h. Meanwhile, the

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apparent quantum yield (AQY) of g-C3N4@HG-10 is 3.56% at wavelength of 420 nm,

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which is 2.02 times higher than that of g-C3N4/GR with 1 wt.% Pt (1.76%) (Figure S5). Additionally, from the light absorption spectra of Figure 5A, g-C3N4@HG-10 presents

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obvious absorption capability from visible to near-infrared (NIR) region, and thus the excitation-wavelength effect on H2 evolution efficiency over g-C3N4@HG-10 was

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investigated as well. Action spectrum of pristine HG is consistent with its absorption spectrum in Figure S6, and AQY of 0.23% at 780 nm is obtained, indicating HG is excited under visible-NIR light. When g-C3N4 is in situ rooted into HG, the two structures are closely linked and integrated. Under irradiation with λ ≥ 560 nm or ≥ 780  nm, g-C3N4@HG-10 presents H2 evolution rates of 1.03 or 0.33 mmol∙g-1h-1 in Figure

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6C, respectively. Therefore, g-C3N4@HG-10 performs the efficient H2 evolution in the visible to NIR region, promising the more efficient utilization of long-wavelength solar energy in the process of H2 evolution. Moreover, the recycling stability of g-C3N4@HG10 was tested (Figure 6D). After five recycles, g-C3N4@HG-10 sample also presents efficient H2 evolution performance under visible-light irradiation, and the structure of the collected g-C3N4@HG-10 was well kept, suggesting its high photocatalytic and

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structural stability (Figure S7).

3.5. Kinetics of photogenerated electron transfer and its mechanism

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Kinetically, time-resolved fluorescence spectrum was performed to probe the

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separation, transfer and utilization of photo-induced carries [67,68]. In Figure 7A, the decay spectra of g-C3N4, g-C3N4/GR, and g-C3N4@HG-10 have been fitted into two

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different lifetime processes. τ1, and τ2 demonstrate radioactive and non-radioactive

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energy transfer, respectively [65,67,69-71]. In the inset of Figure 7A, τ1 and τ2 for gC3N4 and g-C3N4/GR are shorter than those of g-C3N4@HG-10, and the percent of

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charge carrier decreases from 45.3% and 32.5% to 29.5% over g-C3N4@HG-10, respectively. The above characterizations and DFT calculation results reveal that the

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deviation of GR from the plane results in the decrease of C electropositivity and N electronegativity of g-C3N4 rooted into the cavity of HG, thus electrons are excited from N to C of the rooted g-C3N4 easily. Under the apparent potential difference of HG, the excited electrons will be directionally transferred from g-C3N4 to HG surface, reflecting the lengthened τ1, and τ2 as well as the increased proportion of non-radioactive energy

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transfer. On the basis of these results and the calculated band parameters (Figure S8), we proposed a suitable principle for spatial seperation and utilization of the photogenerated electrons in photocatalytic H2 production over g-C3N4/GR and gC3N4@HG (Figure 7B and Figure S9). When samples are irradiated, photo-irradiated electrons from VB position (1.52 V) of g-C3N4 are excited to CB position (-1.18 V), and holes remain in VB position. For g-C3N4/GR with 1 wt.% Pt, although GR has

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almost zero band gap (5A), suitable reduction level position (-0.8 V vs. RHE) and excellent electron transport property, photo-irradiated electrons are still difficult to

cross nanolayers of g-C3N4 due to the barrier between layers. Consequently, photo-

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irradiated charge carriers easily recombine and only few electrons and holes participate

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in the photocatalytic reaction. When GR is deviated into HG, an apparent potential difference between inner and outer surfaces of HG can be used as the driving force for

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the directional transfer of charge from g-C3N4 to HG. In this case, photo-irradiated

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electron from VB of g-C3N4 efficiently transfer to CB and further to the reduction level position of HG (-0.6 V vs. RHE). Thus, spatial separation and directional migration of

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photogenerated electrons and holes have been achieved, resulting in the significant

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enhancement of the photocatalytic activity.

4. Conclusions In summary, g-C3N4 has been in situ rooted in the curved hollow graphene for photocatalytic H2O dissociation into H2. The constructed g-C3N4@HG without Pt demonstrates much higher H2 evolution performance than g-C3N4 with 3% Pt and g-

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C3N4/GR with 1% Pt. Theoretical calculation and experimental analysis confirm that the deviation of graphene into HG results in the the activation of carbon π electrons for H2O adsorption, and the introduced apparent potential difference works as the driving force for charge directional transfer from the rooted g-C3N4 to HG. The research develops an effective strategy for promoting solar energy successive conversion by rooting g-C3N4 into HG, which can also be suitable for rooting other semiconductor

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materials.

Declaration of interests

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The authors declare that they have no known competing financial interests or personal relationships

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Credit author statement

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that could have appeared to influence the work reported in this paper.

Zhexing Lin: Conducting experiments, Collecting data; Chengtian Shao: Conducting supplementary experiments, Writing original draft;

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Shujuan Jiang: Ideas; Revising manuscript;

Chuanzhi Sun: Theoretical calculation and data analysis;

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Shaoqing Song: Ideas; Formulation or evolution of overarching research goals and aims.

Acknowledgements Study was jointly supported by the National Natural Science Foundation of China

(21871155 and 51972177) and Natural Science Foundation of Ningbo City (2018A610067), the K. C. Wong Magna Fund in Ningbo University, Fan 3315 Plan, and Yongjiang Scholar Plan. 16

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Figure 1. TEM images for (A) HG, (B) g-C3N4, and (C,D) g-C3N4@HG-10.

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Figure 2. (A) Raman spectra, (B) ESR, and (C, D) XPS subspectra of (C) C 1s and (D)

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Figure 3. (A, B) Equipotential curve distribution for GR (A) and HG (B); (C, D) g-

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C3N4/GR (C) and g-C3N4@HG (D) models; (E, F) Mülliken charge distribution changes over C atoms of GR and HG (E), and C, N, H atoms of g-C3N4 before and after combining with GR or rooting into HG (F).

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Figure 4. Optical photographs for contact angle tests on (A) g-C3N4@HG-10, (B) g-

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C3N4, and (C) g-C3N4/GR.

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Figure 5. (A) UV-vis spectra, (B) EIS Nyquist plots, (C) transient photocurrent

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response, and (D) LSV curves for samples.

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Figure 6. Photocatalytic performances of g-C3N4 with 3 wt.% Pt, g-C3N4/GR with 1 wt.% Pt, g-C3N4@HG-10, and g-C3N4/HG. (A) H2 production efficiency; (B) effects of

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Figure 7. (A) Time-resolved fluorescence spectra for the samples, and (B) schematic illustration of spatial seperation and utilization of the photogenerated electrons on g-

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