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Three-dimensional porous g-C3N4 for highly efficient photocatalytic overall water splitting
Accepted Manuscript Three-dimensional porous g-C3N4 for highly efficient photocatalytic overall water splitting Xianjie Chen, Run Shi, Qian Chen, Zijian Zhang, Wenjun Jiang, Yongfa Zhu, Tierui Zhang PII:
S2211-2855(19)30198-3
DOI:
https://doi.org/10.1016/j.nanoen.2019.03.010
Reference:
NANOEN 3529
To appear in:
Nano Energy
Received Date: 2 November 2018 Revised Date:
16 February 2019
Accepted Date: 2 March 2019
Please cite this article as: X. Chen, R. Shi, Q. Chen, Z. Zhang, W. Jiang, Y. Zhu, T. Zhang, Threedimensional porous g-C3N4 for highly efficient photocatalytic overall water splitting, Nano Energy (2019), doi: https://doi.org/10.1016/j.nanoen.2019.03.010. 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.
ACCEPTED MANUSCRIPT Three-dimensional porous g-C3N4 assembled by highly crystalline and ultrathin nanosheets is successfully constructed, and could efficiently split pure water into H2 and O2 with a notable quantum yield as high as 1.4% at 420 nm. Besides, it still remains stable for more than 100 h
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of overall water splitting reaction.
ACCEPTED MANUSCRIPT 1
Three-dimensional Porous g-C3N4 for Highly Efficient Photocatalytic Overall Water
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Splitting
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Xianjie Chena, Run Shib, Qian Chenc, Zijian Zhanga, Wenjun Jianga, Yongfa Zhua, *, Tierui
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Zhangb, *
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a
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b
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Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R.
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China
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c
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Department of Chemistry, Tsinghua University, Beijing, 100084, P. R. China
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Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical
College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, P. R.
China
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* Corresponding author: [email protected] (Y. Zhu); [email protected] (T. Zhang)
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Herein, this work constructs a three-dimensional porous graphitic carbon nitride assembled by
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highly crystalline and ultrathin nanosheets (3D g-C3N4 NS). 3D g-C3N4 NS could directly
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split pure water into H2 and O2 with high evolution rate up to 101.4 and 49.1 µmol g-1 h-1
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under visible light, respectively, approximately 11.8 and 5.1 times higher than bulk g-C3N4
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and g-C3N4 NS. Besides, it achieves a notable apparent quantum yield of 1.4% at 420 nm,
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significantly superior to previously reported Pt/g-C3N4. The efficient activity of 3D g-C3N4
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NS is mainly attributed to its 3D interconnected open-framework, assembled by highly
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crystalline ultrathin nanosheet unit, provides a pathway for faster charge carrier transport.
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Moreover, benefitting from its 3D structure for preventing agglomeration of nanosheets, 3D
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g-C3N4 NS is stable for more than 100 h of overall water splitting reaction.
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Keywords: overall water splitting; graphitic carbon nitride; three-dimensional structure;
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ultrathin nanosheets; photocatalyst 1
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1. Introduction Photocatalytic overall water splitting into H2 and O2 without using sacrificial agents is
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considered as one of the most ideal ways to convert solar energy into renewable H2 energy.[1-
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6] Although a number of inorganic semiconductor have been reported to be efficient
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photocatalysts for overall water splitting, most only operation under UV light irradiation
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owing their large band-gap energy.[7-11] To further utilize solar energy to split water, it is
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therefore essential to exploit a visible-light responsive photocatalyst. In the last years,
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numerous efforts have been made to develop such a material.[12-14] Especially, Domen’s
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group have reported that certain oxynitrides, such as TaON, LaMgxTa1-xO1+3xN2-3x, and (Ga1-
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xZnx)(N1-xOx),
are quite excellent photocatalysts for overall water splitting under visible
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light.[15-18] Recently, their group reported the SrTiO3:La,Rh/Au/BiVO4:Mo system with a
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solar-to-hydrogen (STH) energy conversion efficiency of 1.1%.[19] Besides, Mi’s group have
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studied InGaN/GaN multiband nanowire heterostructures, which show highly efficient overall
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water splitting activity under visible light and have achieved a notable STH energy conversion
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efficiency of 3.3%.[20] However, the use of metal-free organic photocatalysts for overall
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water splitting in pure water has been much less reported.
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Recently, conjugated graphitic carbon nitride (g-C3N4) polymers with earth abundant
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elements and appropriate band gap have been considered as effective photocatalysts for
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overall water splitting under visible light irradiation.[21-23] Compared with traditional
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inorganic semiconductors, g-C3N4 has more superiority, including being metal-free, nontoxic
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and low-cost, and the fact that its structure and composition can also be easily tuned via
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adjusting the building blocks of versatile organic protocols.[24-30] Indeed, significant
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achievements have been obtained on the g-C3N4 based photocatalysts for enhanced overall
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water splitting activity through carbon quantum dots deposition and so on.[31-34] Even so,
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direct and efficient photocatalytic water splitting over g-C3N4 always encounters great
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challenges of low specific surface area and sluggish photogenerated carrier transfer. Thus, to
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ACCEPTED MANUSCRIPT further improve the photocatalytic activity of g-C3N4, it is essentially necessary to employ a
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new structural design to provide large specific surface area and accelerate the intrinsic
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photogenerated carrier transfer kinetics. Although two-dimensional (2D) ultrathin nanosheets
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of g-C3N4 have been considered as a novel class of nano-materials to satisfy the demand, the
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fact of structure restacking and agglomeration owing to strong van der Waals force-induced
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adhesion among ultrathin nanosheets is inevitable, which evidently reduces the specific
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surface area.[35, 36] Fortunately, three-dimensional (3D) photocatalysts assemblies of 2D
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ultrathin nanosheets can not only present great specific surface area, but also maximize the
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utilization of incident photons via the multireflection within the interconnected open-
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framework.[37-42] More importantly, 3D structure could be as supporters to prevent
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agglomeration of ultrathin nanosheets and provide a pathway for electron transport.
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Herein, we therefore conceptually design a three-dimensional porous graphitic carbon
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nitride assembled by highly crystalline and ultrathin nanosheets (3D g-C3N4 NS) via a facile
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bottom-up supramolecular self-assembly route. The prepared 3D g-C3N4 NS presents 3D
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interconnected open-framework with extremely great specific surface area, which could be as
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supporters to avoid restacking of nanosheets and provide a pathway for photogenerated
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electron transport. As a result, 3D g-C3N4 NS simultaneously realized steady and efficient
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photocatalytic overall water splitting activity with H2 and O2 evolution rate up to 102 and 51
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µmol g-1 h-1 in pure water under visible light. Besides, it achieved a notable apparent quantum
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yield (AQY) of 1.4% at 420 nm.
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2. Experimental Section
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2.1. Synthesis of 3D g-C3N4 NS
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All the chemicals were purchased from Sigma-Aldrich and used without further
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purification. 3D g-C3N4 NS was synthesized by a facile bottom-up supramolecular self-
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assembly route. In a typical synthesis, 0.01 mol melamine and 0.01 mol cyanuric acid were
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mixed in 50 mL deionized water and magnetic stirred for 12 h at room temperature. 3
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dried by vacuum freeze drying. Then, the resulting CM supramolecular precursors were
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calcined in a muffle furnace at 550 ℃ for 4 h with a heating rater of 5 ℃ min-1 to obtain light
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yellow 3D g-C3N4 NS-2. 3D g-C3N4 NS-1 also were prepared using 0.02 mol melamine and
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0.01 mol cyanuric acid. As comparison, the bulk g-C3N4 and g-C3N4 nanosheets (g-C3N4 NS)
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were synthesized according to the literature.[43, 44]
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2.2. Characterization
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Field emission scanning electron microscope (FESEM) images were performed on a
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Hitachi SU-8000 at an accelerating voltage of 5 kV. Transmission electron microscope (TEM)
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images were operated on Hitachi HT 7700 at an accelerating voltage of 100 kV. High-
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resolution transmission electron microscope (HRTEM) images were recorded on JEOL JEM-
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2100F at an accelerating voltage of 200 kV, and the images were analyzed by
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DigitalMicrograph software. Atomic force microscopy (AFM) study was carried out using
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Shimadzu SPM-960. BET surface area measurements were recorded by N2 adsorption at 77 K
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using a Micrometrics (ASAP 2010V5.02H) surface area analyzer. X-ray diffraction (XRD)
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patterns were obtained on a Rigaku D/max-2400 X-ray diffractometer using a Cu Kα1 (λ =
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0.15418 nm) at 40 kV and 200 mA, with a scan step of 0.02°. The Fourier transform infrared
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(FT-IR) spectra were acquired on Bruker V70 spectrometer. The solid-state
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magnetic resonance (NMR) spectra were recorded on a JEOL JNM-ECZ600R spectrometer.
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UV-Vis diffuse reflectance spectroscopy (DRS) were spectra were obtained on a Hitachi U-
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3010 UV-Vis spectrophotometer with BaSO4 as reference. X-ray photoelectron spectroscopy
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(XPS) measurements were performed using an ESCALAB 250Xi instrument (Thermo
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Scientific) with Al Kα radiation. The electron paramagnetic resonance (EPR) measurements
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were carried out on a JEOL JES-FA200 spectrometer. Room temperature photoluminescence
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(PL) spectra were recorded on Edinburgh F900) fluorescence spectrometer with an excitation
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C nuclear
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wavelength of 340 nm. Time-resolved photoluminescence spectra were collected on
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Edinburgh FLSP920 fluorescence spectrometer.
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2.3. Photoelectrochemical Measurements Photoelectrochemical measurements were performed on a three-electrode system using a
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CHI660E electrochemical workstation with the as-prepared samples covered ITO glass as the
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working electrodes, a Pt wires as the counter electrode, and a saturated calomel electrode
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(SCE) as reference. A 300 W xenon lamp with cutoff filter (λ≥420 nm) was used as the light
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source and the electrolyte was 0.1 mol L-1 Na2SO4 aqueous solution. For the working
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electrodes, a sample (1 mg) was dispersed in isopropanol (1 ml) to obtain a slurry. Then the
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slurry was coated onto the ITO glass and dried in an oven overnight. Subsequently, the
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working electrodes were exposed to UV light for 1 h to eliminate isopropanol and calcined at
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120 ℃ for 2 h. Electrochemical impedance spectroscopy (EIS) spectra were recorded under an
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AC perturbation signal of 10 mV over the frequency range from 100 KHz to 0.1 Hz. Mott–
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Schottky plots were collected with a scan rate of 5 mV s-1 at different frequency (1000, 800
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and 500 Hz).
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2.4. Photocatalytic Water Splitting Test
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The photocatalytic overall water splitting reaction was carried out in a Pyrex top-irradiation
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reaction vessel connected to a glass closed gas system (Labsolar-6A, PerfectLight) (Fig.
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S19c). 1 wt.% Pt and 3 wt.% IrO2 as co-catalysts were loaded on the photocatalysts by in situ
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photo-deposition method using H2PTCl6 and Na2IrCl6 as in reference.[45, 46] And 50 mg
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photocatalysts were dispersed in 100 ml distilled water without any sacrificial agent. The
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reaction solution was kept at 5 ℃ by recirculating cooling water system, then it was evacuated
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several times to remove air completely. And a 300 W Xe lamp with a cutoff filter (λ≥420 nm,
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light intensity 438 mW cm-2) was used as the light source. The amounts of gases produced
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were measured by gas chromatography equipped with a thermal conductive detector (TCD)
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and a 5Å molecular sieve column, using argon as the carrier gas.
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The apparent quantum yield (AQY) of the overall water splitting is calculated from equation as follows:
3 The solar-to-hydrogen (STH) conversion efficiency is given by:
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Eq. 1
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Eq. 2
Where R(H2), ∆Gr, P, and S represent the rate of hydrogen evolution, the Gibbs energy for
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the reaction (H2O (l) → H2 (g) + 1/2 O2 (g)), the energy intensity of the AM1.5G solar
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irradiation (100 mW cm-2) and the irradiated sample area (1 cm2), respectively.
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Several band-pass filters (FWHM=15 nm) were employed to achieve a different incident
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light wavelength under a 300 W Xe lamp (FX300, PerfectLight) for measurement of the
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quantum yield. For full spectrum measurement, a 300 W Xe lamp (FX300, PerfectLight) with
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a standard AM1.5G filter, outputting the light density of 100 mW cm-2, was used as
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irradiation light source. The average intensity was determined by an optical power meter
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(PM100D, THORLABS).
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3. Results and Discussion
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3.1. 3D g-C3N4 Assemblies of Highly Crystalline and Ultrathin Nanosheets
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Different from the top-down approach to prepare 3D structured g-C3N4 by multi-step
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method reported before,[35, 47, 48] the 3D g-C3N4 NS was fabricated via a facile bottom-up
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supramolecular self-assembly route, as shown in Fig. 1a. Firstly, the 3D network cyanuric
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acid-melamine (CM) supramoleculars (Fig. S1a) as precursor were assembled through the
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hydrogen bonding between melamine and cyanuric acid in aqueous solution, and followed by
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a freeze drying and high temperature polycondensation reaction. Finally, a light yellow 3D g-
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C3N4 NS sample was obtained. Surprisingly, the prepared 3D g-C3N4 NS sample, just like an
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aerogel, exhibits the ultralight characteristics with a quite low volume fraction of about 18.3
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mg cm-3 (Fig. S1b), and can even stand on the green bristlegrass steadily. Moreover, the
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even g-C3N4 NS (Fig. S1c). Additionally, the lighter 3D g-C3N4 NS can also be obtained by
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increase the ratio of cyanuric acid and melamine.
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Fig. 1. (a) Schematic illustration of the bottom-up supramolecular self-assembly route for
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synthesizing 3D g-C3N4 NS. (b) SEM image, (c) TEM image (HRTEM image inset) and (d)
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AFM image of 3D g-C3N4 NS-2.
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The morphology and microstructure of 3D g-C3N4 NS were further investigated with the
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scanning electron microscope (SEM) and transmission electron microscope (TEM) as shown
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in Fig. 1b and 1c. Different from the usual bulk g-C3N4 consisting of solid agglomerates with
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a size of several micrometers, 3D g-C3N4 NS presents 3D interconnected open-framework
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with an average pore width of about 20 nm (Fig. S5) fabricated by ultrathin nanosheet units in
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Fig. 1b. The ultrathin g-C3N4 nanosheets appear as floppy agglomerates with a lateral scale of
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several micrometers, similar to the graphene nanosheets,[49] and the ultrathin nanosheet unit
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trends to bend and its edges are ragged. The TEM image in Fig. 1c further confirms that the
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basic unit of 3D g-C3N4 NS is ultrathin nanosheet with a lateral scale of several micrometers.
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ultrathin nanosheet edges, indicating the high crystallinity of 3D g-C3N4 NS. The
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corresponding lattice fringes of 0.329 nm are ascribed to (002) π–π interlayer stacking motif,
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and only 4-5 lattice fringes can be observed, suggesting that 3D g-C3N4 NS consist of 4-5 g-
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C3N4 layer units. Furthermore, the atomic force microscopy (AFM) image (Fig. 1d) clearly
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shows the layer thickness of about 2.3 nm, exactly corresponding to about 5 g-C3N4 layer
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units, almost consistent with HRTEM result.
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Fig. 2. (a) N2 adsorption-desorption isotherms, (b) XRD patterns, (c) FT-IR spectra, and (d) schematic band structure diagrams of bulk g-C3N4, g-C3N4 NS and 3D g-C3N4 NS.
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Benefiting from 3D porous interconnected open-framework assemblies of ultrathin
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nanosheet, 3D g-C3N4 NS exhibit quite large specific surface area, up to about 130.00 m2 g-1,
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and is approximately 13 and 1.4 times larger than bulk g-C3N4 and g-C3N4 NS (Fig. 2a),
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which could provide more active sites and improve the solar light harvesting. In X-ray 8
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27.4° corresponding to (100) in-plane structural packing motif and (002) interlayer stacking
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of aromatic segments appear in all g-C3N4 samples. With respect to bulk g-C3N4, the (002)
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diffraction peak in 3D g-C3N4 NS-2 is shifted from 27.38° to 27.77°, implying a shortened
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gallery distance between the basic layer unit, and the significantly reduced full width at half
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maximum further demonstrates the higher crystallinity of 3D g-C3N4 NS. In Fourier transform
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infrared (FTIR) spectroscopy (Fig. 2c), the sharp peak at 811 cm-1 is assigned to a signature of
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the formation of tri-s-triazine. The strong absorption bands in the region of 1100-1700 cm-1
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are ascribed to the typical stretching vibration modes of C-N heterocycles, these absorption
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bands become sharper probably owing to the more ordered packing of tri-s-triazine units.[50]
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The wide peaks between 3000 and 3500 cm-1 are assigned to O-H and N-H stretching
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vibration modes. Moreover, in the solid-state
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C3N4 NS, two peaks at 155.6 (C1) and 164.3 ppm (C2) are ascribed to the C(i) atoms of
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melem (CN3) and C(e) atoms of [CN2(NHx)][44], respectively, further confirming the
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existence of tri-s-triazine in 3D U-C3N4 NS. Additionally, the element composition of C, N
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and O in 3D g-C3N4 NS was also investigated by XPS analysis (Fig. S7). In C 1s spectra (Fig.
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S7c), the peak centered at 288.5 eV is assigned to sp2-hybridized carbon in the N-containing
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aromatic ring (N=C-N), and the decrease in the peak intensity at 285.0 eV of 3D g-C3N4 NS
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suggests that the fewer impurities of graphitic carbon species (C(sp2)-C(sp2) bonds)
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originating from defective polymerization, further demonstrating the more ordered packing of
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tri-s-triazine units relative to bulk g-C3N4. The fewer graphitic carbon species is helpful for
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the optimization of the aromatic π-conjugated system for charge separation, since too many
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graphitic carbon centers doped in the g-C3N4 conjugated system will act as defect centers for
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charge recombination.[51]
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C CP-MAS NMR spectra (Fig. S6) of 3D g-
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Then, the redox potential of 3D g-C3N4 NS was calculated according to the UV-Vis diffuse
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reflectance spectra and Mott-Schottky plots (Fig. S8 and Fig. S9)[52, 53], as shown in Fig. 2d. 9
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and the valence band potential is more positive than the oxidation potential of O2/H2O, which
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completely meets the thermodynamic energy criterion of direct water splitting. Additionally,
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With respect to bulk g-C3N4, the conduction band potential moves up obviously, whereas the
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valence band potential of 3D g-C3N4 NS has little change, meaning a stronger capability of
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3D g-C3N4 NS for overall water splitting.
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3.2. Highly Efficient and Stable Water Splitting Performance
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Fig. 3. (a) Time-dependent overall water splitting over 3D g-C3N4 NS-2, (b) overall water
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splitting rate over bulk g-C3N4, g-C3N4 NS, and 3D g-C3N4 NS, (c) the wavelength-dependent
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AQY of overall water splitting over 3D g-C3N4 NS-2, and the repeated cycles of overall water
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splitting over 3D g-C3N4 NS-2. Reaction condition: 1wt.% Pt and 3wt.% IrO2 as co-catalysts;
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distilled water (pH 7.0, 100 mL); light source, 300 W Xe lamp equipped with a visible light
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filter (λ≥420 nm) or various band-pass filters; All reaction were carried out at 278 K and 3
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kPa. 10
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pure water under visible light irradiation, and Pt and IrO2 as co-catalysts were photoloaded in
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situ on samples to boost H2 and O2 generation in advance[44, 46], respectively. The typical
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time course of gas evolution over 3D g-C3N4 NS-2 is shown in Fig. 3a. Notably, both H2 and
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O2 were generated simultaneously under light irradiation, and the evolved H2 to O2 ratio was
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very close to the expected 2:1 stoichiometry for overall water splitting. The average H2 and
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O2 evolution rate is 101.4 and 49.1 µmol g-1 h-1 over 3D g-C3N4 NS-2, respectively,
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approximately 11.8 and 5.1 times higher than that of bulk g-C3N4 and g-C3N4 NS (Fig. 3b),
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significantly superior to a great many previously reported g-C3N4-based photocatalysts (Table
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S5). To further confirm that the photocatalytic overall water splitting reaction was driven by
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the absorption of an incident photon, the wavelength-dependent overall water splitting
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experiments were also carried out. As shown in Fig. 3c, the AQY values for overall water
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splitting over 3D g-C3N4 NS-2 is well coincident with its UV-Vis DRS, and the AQY at 420
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nm was calculated to be 1.4%, significantly higher than that of Pt/g-C3N4 (0.3% at 405 nm) by
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Wang’s group[21]. Then, the solar-to-hydrogen (STH) energy-conversion efficiency of 3D g-
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C3N4 NS was also evaluated under AM 1.5G simulated sunlight (100 mW cm-2, 1 cm2). After
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6 h illumination, a total of 5.73 µmol H2 was produced, and the STH value of 3D g-C3N4 NS
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was determined to be 0.06% (Fig. S12).
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In addition, no notable deactivation emerges over 3D g-C3N4 NS-2 during a continuous
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more than 100 h measurement (Fig. 3d), indicating its robust resistance to water and light
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corrosion at the soft interface. However, the overall water splitting performance of g-C3N4 NS
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dropped sharply during the repeated cycles (Fig. S13), and its gas evolution rate was
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significantly reduced by 44% after 102 h reaction. As shown in Fig. S14, serious
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agglomeration of nanosheets occurred over g-C3N4 NS after the continuous cycles, which
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should be the main reason for the decrease of its water splitting activity. In contrast, 3D g-
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could be as supporters to prevent agglomeration of ultrathin nanosheets.
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2.3. Faster Charge Carriers Separation over 3D g-C3N4 NS
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Fig. 4. (a) EPR spectra, (b) PL spectra (time-resolved PL spectra collected under an excitation
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of 330 nm inset), (c) electrochemical impedance spectra (The equivalent circuit impedance
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model inset), and (d) transient photocurrent density of bulk g-C3N4, g-C3N4 NS and 3D g-
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C3N4 NS-2.
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Although the specific surface area of 3D g-C3N4 NS-2 is slightly higher than that of g-C3N4
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NS, even that of 3D g-C3N4 NS-1 is lower, 3D g-C3N4 NS still shows significantly superior
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overall water splitting performance, indicating that the specific surface area doesn’t mainly
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account for the highly efficient activity of 3D g-C3N4 NS. It’s well known that the
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photocatalyst activity is closely related to its photogenerated charge carrier separation
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efficiency. To further understand why the 3D g-C3N4 NS exhibited superior photocatalytic 12
ACCEPTED MANUSCRIPT overall water splitting performance, the charge recombination behavior of all samples was
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investigated. In EPR spectra (Fig. 4a), all samples show one single Lorentzian line with a g
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value of 2.0036 that is induced by unpaired electrons on sp2-carbon atoms of the heptazine
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rings.[54] Compared to bulk g-C3N4 and g-C3N4 NS, the stronger EPR spin intensity of 3D g-
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C3N4 NS-2 implies the obviously higher concentration of unpaired electrons and greater
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ability to delocalize electrons, which is favorable to the photogenerated electron-hole pair
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separation. The excitonic process of 3D g-C3N4 NS-2 was also monitored by
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photoluminescence (PL) measurements. In Fig. 4b, 3D g-C3N4 NS-2 exhibit an obviously
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decreased PL emission intensity with respect to bulk g-C3N4 and g-C3N4 NS, which further
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demonstrates its suppressed electron-hole pair recombination rate. Meanwhile, time-resolved
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PL spectra monitored at the corresponding emission peaks provide the average radiative
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lifetime (Aτ) of the charge carriers. The Aτ of bulk g-C3N4, g-C3N4 NS, and 3D g-C3N4 NS-2
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are 4.24 ns, 5.38 ns, and 6.69 ns, respectively. As this increased exciton lifetime of 3D g-C3N4
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NS-2 significantly implies enhanced exciton dissociation, and the longer diffusion length and
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lifetime of charge carriers are further confirmed.
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Furthermore, as shown in Fig. 4c, the charge transfer resistance (Rsc) of 3D g-C3N4 NS-2
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estimated via fitting the hemicycle radius on electrochemical impedance spectroscopy (EIS)
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was only 4.134 Ω, much lower than that of bulk g-C3N4 and g-C3N4 NS (Table S7),
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demonstrating a faster electron transfer kinetics in 3D g-C3N4 NS-2. Additionally, the
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enhanced photocurrent response (Fig. 4d) further reveals the significantly improved mobility
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of charge carriers in 3D g-C3N4 NS-2. Except the 3D structure of 3D g-C3N4 NS-2, g-C3N4
22
NS and 3D g-C3N4 NS-2 both presents ultrathin nanosheet structure, and their layer thickness
23
and specific surface area are very close. However, 3D g-C3N4 NS-2 exhibits longer lifetime of
24
charge carriers, much lower charge transfer resistance and significantly enhanced
25
photocurrent response than that of g-C3N4 NS, which fully demonstrates that the 3D structure
26
of 3D g-C3N4 NS-2 provides a pathway for electron transports.
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ACCEPTED MANUSCRIPT Therefore, benefitting from the 3D porous interconnected open-framework to prevent
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agglomeration of nanosheets and provide a pathway for electron transports., the suppressed
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photogenerated electron-hole pair recombination rate and a faster charge carrier transfer
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kinetics emerge in 3D g-C3N4 NS with respect to bulk g-C3N4 and g-C3N4 NS, resulting to its
5
superior photocatalytic overall water splitting activity.
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4. Conclusion
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In summary, 3D g-C3N4 assembled by highly crystalline and ultrathin nanosheets were
8
successfully fabricated through a facile bottom-up supramolecular self-assembly route. The
9
unique 3D porous interconnected open-framework makes 3D g-C3N4 NS exhibit quite large
10
specific surface area, being as supporters to prevent agglomeration of nanosheets and provides
11
a pathway for electron transports. As a consequence, 3D g-C3N4 NS realized highly efficient
12
and stable overall water splitting under visible light, and achieved considerable AQY as high
13
as 1.4% at 420 nm. Briefly, this work throws light on designing polymer photocatalysts with
14
3D porous structure assemblies of low-dimensional nanomaterials for improved solar energy
15
capture and conversion.
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Acknowledgements
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This work was financially supported by the National Natural Science Foundation of China (grant numbers 21437003, 21673126, 21761142017, and 21621003) and the Collaborative
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Innovation Center for Regional Environmental Quality.
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Conflict of Interest
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The authors declare no conflict of interest. Appendix A. Supplementary material Supplementary data associated with this article can be found in the online.
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ACCEPTED MANUSCRIPT 1
Vitae
2 Xianjie Chen is currently a Ph.D. candidate under the supervision of Prof. Yongfa Zhu from
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the Department of Chemistry at Tsinghua University. His research interest is focused on the
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design and fabrication of highly efficient photocatalysts and their application for overall water
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splitting and environmental remediation.
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Run Shi studied materials science and engineering at the Tianjin Polytechnic University
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where he received his B.Sc. in 2012. He received his Ph.D. in materials science at the
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Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, in the lab of
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Prof. Tierui Zhang. His current research interest focuses on controlled synthesis of noble-
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metal-free photocatalysts for hydrogen evolution and CO2 reduction.
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Qian Chen is now pursing her M.S. degree under the supervision of Prof. Qin Kuang and Prof.
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Zhaoxiong Xie at College of Chemistry and Chemical Engineering, Xiamen University. Her
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research interests mainly focus on the design and fabrication of nano-sized photocatalysts and
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their application for CO2 reduction and water splitting.
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1 Zijian Zhang is a Ph.D. student at the Department of Chemistry, Tsinghua University. He
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received his bachelor degree from the College of Chemistry and Molecular Sciences, Wuhan
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University. His research focuses on the design and fabrication of organic photocatalysts and
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their application for environmental pollution control and the resource utilization of solar
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energy.
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Wenjun Jiang received his Ph.D. student in 2018 under the supervision of Prof. Yongfa Zhu at
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the Department of Chemistry, Tsinghua University. His research focuses on the design and
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fabrication of nano-sized photocatalysts and their applications for solar energy conversion and
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environmental remediation.
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Prof. Yongfa Zhu received his B.S. degree in 1985 from Nanjing University and obtained his
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M.S. degree in 1988 from the Peking University. He has studied and worked at Tsinghua
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University since 1992 and received his Ph.D. degree in 1995. He is currently a full professor
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at Tsinghua University. His current research is focused on photocatalysis, environmental
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catalysis and nanomaterials.
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1 Prof. Tierui Zhang obtained his Ph.D. degree in Chemistry in 2003 at Jilin University, China.
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He then worked as a postdoctoral researcher in the labs of Prof. Markus Antonietti, Prof.
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Charl F. J. Faul, Prof. Hicham Fenniri, Prof. Z. Ryan Tian, Prof. Yadong Yin and Prof.
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Yushan Yan. His current scientific interests are focused on the surface and interface of
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micro/nano-structures, including photocatalysis, electrocatalysis, thermocatalysis, and the
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controllable synthesis, self-assembly, and surface modification of colloidal inorganic
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nanostructures.
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ACCEPTED MANUSCRIPT Highlights Three-dimensional porous g-C3N4 nanosheets (3D U-C3N4 NS) assembled by highly crystalline and ultrathin nanosheets was successfully fabricated via a facile route.
and 49.1 µmol g-1 h-1 under visible light, respectively.
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3D g-C3N4 NS could directly split pure water into H2 and O2 with high evolution rate up to 101.4
3D U-C3N4 NS achieved a notable quantum yield of 1.4% at 420 nm, significantly superior to
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previously reported Pt/g-C3N4.
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3D U-C3N4 NS could still remains stable for more than 100 h of overall water splitting reaction.
S2211-2855(19)30198-3
DOI:
https://doi.org/10.1016/j.nanoen.2019.03.010
Reference:
NANOEN 3529
To appear in:
Nano Energy
Received Date: 2 November 2018 Revised Date:
16 February 2019
Accepted Date: 2 March 2019
Please cite this article as: X. Chen, R. Shi, Q. Chen, Z. Zhang, W. Jiang, Y. Zhu, T. Zhang, Threedimensional porous g-C3N4 for highly efficient photocatalytic overall water splitting, Nano Energy (2019), doi: https://doi.org/10.1016/j.nanoen.2019.03.010. 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.
ACCEPTED MANUSCRIPT Three-dimensional porous g-C3N4 assembled by highly crystalline and ultrathin nanosheets is successfully constructed, and could efficiently split pure water into H2 and O2 with a notable quantum yield as high as 1.4% at 420 nm. Besides, it still remains stable for more than 100 h
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of overall water splitting reaction.
ACCEPTED MANUSCRIPT 1
Three-dimensional Porous g-C3N4 for Highly Efficient Photocatalytic Overall Water
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Splitting
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Xianjie Chena, Run Shib, Qian Chenc, Zijian Zhanga, Wenjun Jianga, Yongfa Zhua, *, Tierui
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Zhangb, *
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a
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b
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Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R.
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China
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c
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Department of Chemistry, Tsinghua University, Beijing, 100084, P. R. China
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Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical
College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, P. R.
China
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* Corresponding author: [email protected] (Y. Zhu); [email protected] (T. Zhang)
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Herein, this work constructs a three-dimensional porous graphitic carbon nitride assembled by
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highly crystalline and ultrathin nanosheets (3D g-C3N4 NS). 3D g-C3N4 NS could directly
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split pure water into H2 and O2 with high evolution rate up to 101.4 and 49.1 µmol g-1 h-1
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under visible light, respectively, approximately 11.8 and 5.1 times higher than bulk g-C3N4
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and g-C3N4 NS. Besides, it achieves a notable apparent quantum yield of 1.4% at 420 nm,
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significantly superior to previously reported Pt/g-C3N4. The efficient activity of 3D g-C3N4
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NS is mainly attributed to its 3D interconnected open-framework, assembled by highly
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crystalline ultrathin nanosheet unit, provides a pathway for faster charge carrier transport.
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Moreover, benefitting from its 3D structure for preventing agglomeration of nanosheets, 3D
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g-C3N4 NS is stable for more than 100 h of overall water splitting reaction.
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Keywords: overall water splitting; graphitic carbon nitride; three-dimensional structure;
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ultrathin nanosheets; photocatalyst 1
ACCEPTED MANUSCRIPT 1
1. Introduction Photocatalytic overall water splitting into H2 and O2 without using sacrificial agents is
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considered as one of the most ideal ways to convert solar energy into renewable H2 energy.[1-
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6] Although a number of inorganic semiconductor have been reported to be efficient
5
photocatalysts for overall water splitting, most only operation under UV light irradiation
6
owing their large band-gap energy.[7-11] To further utilize solar energy to split water, it is
7
therefore essential to exploit a visible-light responsive photocatalyst. In the last years,
8
numerous efforts have been made to develop such a material.[12-14] Especially, Domen’s
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group have reported that certain oxynitrides, such as TaON, LaMgxTa1-xO1+3xN2-3x, and (Ga1-
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xZnx)(N1-xOx),
are quite excellent photocatalysts for overall water splitting under visible
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light.[15-18] Recently, their group reported the SrTiO3:La,Rh/Au/BiVO4:Mo system with a
12
solar-to-hydrogen (STH) energy conversion efficiency of 1.1%.[19] Besides, Mi’s group have
13
studied InGaN/GaN multiband nanowire heterostructures, which show highly efficient overall
14
water splitting activity under visible light and have achieved a notable STH energy conversion
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efficiency of 3.3%.[20] However, the use of metal-free organic photocatalysts for overall
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water splitting in pure water has been much less reported.
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Recently, conjugated graphitic carbon nitride (g-C3N4) polymers with earth abundant
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elements and appropriate band gap have been considered as effective photocatalysts for
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overall water splitting under visible light irradiation.[21-23] Compared with traditional
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inorganic semiconductors, g-C3N4 has more superiority, including being metal-free, nontoxic
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and low-cost, and the fact that its structure and composition can also be easily tuned via
22
adjusting the building blocks of versatile organic protocols.[24-30] Indeed, significant
23
achievements have been obtained on the g-C3N4 based photocatalysts for enhanced overall
24
water splitting activity through carbon quantum dots deposition and so on.[31-34] Even so,
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direct and efficient photocatalytic water splitting over g-C3N4 always encounters great
26
challenges of low specific surface area and sluggish photogenerated carrier transfer. Thus, to
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ACCEPTED MANUSCRIPT further improve the photocatalytic activity of g-C3N4, it is essentially necessary to employ a
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new structural design to provide large specific surface area and accelerate the intrinsic
3
photogenerated carrier transfer kinetics. Although two-dimensional (2D) ultrathin nanosheets
4
of g-C3N4 have been considered as a novel class of nano-materials to satisfy the demand, the
5
fact of structure restacking and agglomeration owing to strong van der Waals force-induced
6
adhesion among ultrathin nanosheets is inevitable, which evidently reduces the specific
7
surface area.[35, 36] Fortunately, three-dimensional (3D) photocatalysts assemblies of 2D
8
ultrathin nanosheets can not only present great specific surface area, but also maximize the
9
utilization of incident photons via the multireflection within the interconnected open-
10
framework.[37-42] More importantly, 3D structure could be as supporters to prevent
11
agglomeration of ultrathin nanosheets and provide a pathway for electron transport.
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Herein, we therefore conceptually design a three-dimensional porous graphitic carbon
13
nitride assembled by highly crystalline and ultrathin nanosheets (3D g-C3N4 NS) via a facile
14
bottom-up supramolecular self-assembly route. The prepared 3D g-C3N4 NS presents 3D
15
interconnected open-framework with extremely great specific surface area, which could be as
16
supporters to avoid restacking of nanosheets and provide a pathway for photogenerated
17
electron transport. As a result, 3D g-C3N4 NS simultaneously realized steady and efficient
18
photocatalytic overall water splitting activity with H2 and O2 evolution rate up to 102 and 51
19
µmol g-1 h-1 in pure water under visible light. Besides, it achieved a notable apparent quantum
20
yield (AQY) of 1.4% at 420 nm.
21
2. Experimental Section
22
2.1. Synthesis of 3D g-C3N4 NS
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All the chemicals were purchased from Sigma-Aldrich and used without further
24
purification. 3D g-C3N4 NS was synthesized by a facile bottom-up supramolecular self-
25
assembly route. In a typical synthesis, 0.01 mol melamine and 0.01 mol cyanuric acid were
26
mixed in 50 mL deionized water and magnetic stirred for 12 h at room temperature. 3
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dried by vacuum freeze drying. Then, the resulting CM supramolecular precursors were
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calcined in a muffle furnace at 550 ℃ for 4 h with a heating rater of 5 ℃ min-1 to obtain light
4
yellow 3D g-C3N4 NS-2. 3D g-C3N4 NS-1 also were prepared using 0.02 mol melamine and
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0.01 mol cyanuric acid. As comparison, the bulk g-C3N4 and g-C3N4 nanosheets (g-C3N4 NS)
6
were synthesized according to the literature.[43, 44]
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2.2. Characterization
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Field emission scanning electron microscope (FESEM) images were performed on a
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Hitachi SU-8000 at an accelerating voltage of 5 kV. Transmission electron microscope (TEM)
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images were operated on Hitachi HT 7700 at an accelerating voltage of 100 kV. High-
11
resolution transmission electron microscope (HRTEM) images were recorded on JEOL JEM-
12
2100F at an accelerating voltage of 200 kV, and the images were analyzed by
13
DigitalMicrograph software. Atomic force microscopy (AFM) study was carried out using
14
Shimadzu SPM-960. BET surface area measurements were recorded by N2 adsorption at 77 K
15
using a Micrometrics (ASAP 2010V5.02H) surface area analyzer. X-ray diffraction (XRD)
16
patterns were obtained on a Rigaku D/max-2400 X-ray diffractometer using a Cu Kα1 (λ =
17
0.15418 nm) at 40 kV and 200 mA, with a scan step of 0.02°. The Fourier transform infrared
18
(FT-IR) spectra were acquired on Bruker V70 spectrometer. The solid-state
19
magnetic resonance (NMR) spectra were recorded on a JEOL JNM-ECZ600R spectrometer.
20
UV-Vis diffuse reflectance spectroscopy (DRS) were spectra were obtained on a Hitachi U-
21
3010 UV-Vis spectrophotometer with BaSO4 as reference. X-ray photoelectron spectroscopy
22
(XPS) measurements were performed using an ESCALAB 250Xi instrument (Thermo
23
Scientific) with Al Kα radiation. The electron paramagnetic resonance (EPR) measurements
24
were carried out on a JEOL JES-FA200 spectrometer. Room temperature photoluminescence
25
(PL) spectra were recorded on Edinburgh F900) fluorescence spectrometer with an excitation
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C nuclear
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wavelength of 340 nm. Time-resolved photoluminescence spectra were collected on
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Edinburgh FLSP920 fluorescence spectrometer.
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2.3. Photoelectrochemical Measurements Photoelectrochemical measurements were performed on a three-electrode system using a
5
CHI660E electrochemical workstation with the as-prepared samples covered ITO glass as the
6
working electrodes, a Pt wires as the counter electrode, and a saturated calomel electrode
7
(SCE) as reference. A 300 W xenon lamp with cutoff filter (λ≥420 nm) was used as the light
8
source and the electrolyte was 0.1 mol L-1 Na2SO4 aqueous solution. For the working
9
electrodes, a sample (1 mg) was dispersed in isopropanol (1 ml) to obtain a slurry. Then the
10
slurry was coated onto the ITO glass and dried in an oven overnight. Subsequently, the
11
working electrodes were exposed to UV light for 1 h to eliminate isopropanol and calcined at
12
120 ℃ for 2 h. Electrochemical impedance spectroscopy (EIS) spectra were recorded under an
13
AC perturbation signal of 10 mV over the frequency range from 100 KHz to 0.1 Hz. Mott–
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Schottky plots were collected with a scan rate of 5 mV s-1 at different frequency (1000, 800
15
and 500 Hz).
16
2.4. Photocatalytic Water Splitting Test
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The photocatalytic overall water splitting reaction was carried out in a Pyrex top-irradiation
18
reaction vessel connected to a glass closed gas system (Labsolar-6A, PerfectLight) (Fig.
19
S19c). 1 wt.% Pt and 3 wt.% IrO2 as co-catalysts were loaded on the photocatalysts by in situ
20
photo-deposition method using H2PTCl6 and Na2IrCl6 as in reference.[45, 46] And 50 mg
21
photocatalysts were dispersed in 100 ml distilled water without any sacrificial agent. The
22
reaction solution was kept at 5 ℃ by recirculating cooling water system, then it was evacuated
23
several times to remove air completely. And a 300 W Xe lamp with a cutoff filter (λ≥420 nm,
24
light intensity 438 mW cm-2) was used as the light source. The amounts of gases produced
25
were measured by gas chromatography equipped with a thermal conductive detector (TCD)
26
and a 5Å molecular sieve column, using argon as the carrier gas.
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The apparent quantum yield (AQY) of the overall water splitting is calculated from equation as follows:
3 The solar-to-hydrogen (STH) conversion efficiency is given by:
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Eq. 1
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Eq. 2
Where R(H2), ∆Gr, P, and S represent the rate of hydrogen evolution, the Gibbs energy for
7
the reaction (H2O (l) → H2 (g) + 1/2 O2 (g)), the energy intensity of the AM1.5G solar
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irradiation (100 mW cm-2) and the irradiated sample area (1 cm2), respectively.
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Several band-pass filters (FWHM=15 nm) were employed to achieve a different incident
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light wavelength under a 300 W Xe lamp (FX300, PerfectLight) for measurement of the
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quantum yield. For full spectrum measurement, a 300 W Xe lamp (FX300, PerfectLight) with
12
a standard AM1.5G filter, outputting the light density of 100 mW cm-2, was used as
13
irradiation light source. The average intensity was determined by an optical power meter
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(PM100D, THORLABS).
15
3. Results and Discussion
16
3.1. 3D g-C3N4 Assemblies of Highly Crystalline and Ultrathin Nanosheets
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Different from the top-down approach to prepare 3D structured g-C3N4 by multi-step
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method reported before,[35, 47, 48] the 3D g-C3N4 NS was fabricated via a facile bottom-up
19
supramolecular self-assembly route, as shown in Fig. 1a. Firstly, the 3D network cyanuric
20
acid-melamine (CM) supramoleculars (Fig. S1a) as precursor were assembled through the
21
hydrogen bonding between melamine and cyanuric acid in aqueous solution, and followed by
22
a freeze drying and high temperature polycondensation reaction. Finally, a light yellow 3D g-
23
C3N4 NS sample was obtained. Surprisingly, the prepared 3D g-C3N4 NS sample, just like an
24
aerogel, exhibits the ultralight characteristics with a quite low volume fraction of about 18.3
25
mg cm-3 (Fig. S1b), and can even stand on the green bristlegrass steadily. Moreover, the
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even g-C3N4 NS (Fig. S1c). Additionally, the lighter 3D g-C3N4 NS can also be obtained by
3
increase the ratio of cyanuric acid and melamine.
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Fig. 1. (a) Schematic illustration of the bottom-up supramolecular self-assembly route for
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synthesizing 3D g-C3N4 NS. (b) SEM image, (c) TEM image (HRTEM image inset) and (d)
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AFM image of 3D g-C3N4 NS-2.
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The morphology and microstructure of 3D g-C3N4 NS were further investigated with the
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scanning electron microscope (SEM) and transmission electron microscope (TEM) as shown
10
in Fig. 1b and 1c. Different from the usual bulk g-C3N4 consisting of solid agglomerates with
11
a size of several micrometers, 3D g-C3N4 NS presents 3D interconnected open-framework
12
with an average pore width of about 20 nm (Fig. S5) fabricated by ultrathin nanosheet units in
13
Fig. 1b. The ultrathin g-C3N4 nanosheets appear as floppy agglomerates with a lateral scale of
14
several micrometers, similar to the graphene nanosheets,[49] and the ultrathin nanosheet unit
15
trends to bend and its edges are ragged. The TEM image in Fig. 1c further confirms that the
16
basic unit of 3D g-C3N4 NS is ultrathin nanosheet with a lateral scale of several micrometers.
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ultrathin nanosheet edges, indicating the high crystallinity of 3D g-C3N4 NS. The
3
corresponding lattice fringes of 0.329 nm are ascribed to (002) π–π interlayer stacking motif,
4
and only 4-5 lattice fringes can be observed, suggesting that 3D g-C3N4 NS consist of 4-5 g-
5
C3N4 layer units. Furthermore, the atomic force microscopy (AFM) image (Fig. 1d) clearly
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shows the layer thickness of about 2.3 nm, exactly corresponding to about 5 g-C3N4 layer
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units, almost consistent with HRTEM result.
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Fig. 2. (a) N2 adsorption-desorption isotherms, (b) XRD patterns, (c) FT-IR spectra, and (d) schematic band structure diagrams of bulk g-C3N4, g-C3N4 NS and 3D g-C3N4 NS.
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Benefiting from 3D porous interconnected open-framework assemblies of ultrathin
12
nanosheet, 3D g-C3N4 NS exhibit quite large specific surface area, up to about 130.00 m2 g-1,
13
and is approximately 13 and 1.4 times larger than bulk g-C3N4 and g-C3N4 NS (Fig. 2a),
14
which could provide more active sites and improve the solar light harvesting. In X-ray 8
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2
27.4° corresponding to (100) in-plane structural packing motif and (002) interlayer stacking
3
of aromatic segments appear in all g-C3N4 samples. With respect to bulk g-C3N4, the (002)
4
diffraction peak in 3D g-C3N4 NS-2 is shifted from 27.38° to 27.77°, implying a shortened
5
gallery distance between the basic layer unit, and the significantly reduced full width at half
6
maximum further demonstrates the higher crystallinity of 3D g-C3N4 NS. In Fourier transform
7
infrared (FTIR) spectroscopy (Fig. 2c), the sharp peak at 811 cm-1 is assigned to a signature of
8
the formation of tri-s-triazine. The strong absorption bands in the region of 1100-1700 cm-1
9
are ascribed to the typical stretching vibration modes of C-N heterocycles, these absorption
10
bands become sharper probably owing to the more ordered packing of tri-s-triazine units.[50]
11
The wide peaks between 3000 and 3500 cm-1 are assigned to O-H and N-H stretching
12
vibration modes. Moreover, in the solid-state
13
C3N4 NS, two peaks at 155.6 (C1) and 164.3 ppm (C2) are ascribed to the C(i) atoms of
14
melem (CN3) and C(e) atoms of [CN2(NHx)][44], respectively, further confirming the
15
existence of tri-s-triazine in 3D U-C3N4 NS. Additionally, the element composition of C, N
16
and O in 3D g-C3N4 NS was also investigated by XPS analysis (Fig. S7). In C 1s spectra (Fig.
17
S7c), the peak centered at 288.5 eV is assigned to sp2-hybridized carbon in the N-containing
18
aromatic ring (N=C-N), and the decrease in the peak intensity at 285.0 eV of 3D g-C3N4 NS
19
suggests that the fewer impurities of graphitic carbon species (C(sp2)-C(sp2) bonds)
20
originating from defective polymerization, further demonstrating the more ordered packing of
21
tri-s-triazine units relative to bulk g-C3N4. The fewer graphitic carbon species is helpful for
22
the optimization of the aromatic π-conjugated system for charge separation, since too many
23
graphitic carbon centers doped in the g-C3N4 conjugated system will act as defect centers for
24
charge recombination.[51]
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C CP-MAS NMR spectra (Fig. S6) of 3D g-
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Then, the redox potential of 3D g-C3N4 NS was calculated according to the UV-Vis diffuse
26
reflectance spectra and Mott-Schottky plots (Fig. S8 and Fig. S9)[52, 53], as shown in Fig. 2d. 9
ACCEPTED MANUSCRIPT The conduction band potential 3D g-C3N4 NS is more negative than the reduction of H+/H2,
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and the valence band potential is more positive than the oxidation potential of O2/H2O, which
3
completely meets the thermodynamic energy criterion of direct water splitting. Additionally,
4
With respect to bulk g-C3N4, the conduction band potential moves up obviously, whereas the
5
valence band potential of 3D g-C3N4 NS has little change, meaning a stronger capability of
6
3D g-C3N4 NS for overall water splitting.
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3.2. Highly Efficient and Stable Water Splitting Performance
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Fig. 3. (a) Time-dependent overall water splitting over 3D g-C3N4 NS-2, (b) overall water
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splitting rate over bulk g-C3N4, g-C3N4 NS, and 3D g-C3N4 NS, (c) the wavelength-dependent
11
AQY of overall water splitting over 3D g-C3N4 NS-2, and the repeated cycles of overall water
12
splitting over 3D g-C3N4 NS-2. Reaction condition: 1wt.% Pt and 3wt.% IrO2 as co-catalysts;
13
distilled water (pH 7.0, 100 mL); light source, 300 W Xe lamp equipped with a visible light
14
filter (λ≥420 nm) or various band-pass filters; All reaction were carried out at 278 K and 3
15
kPa. 10
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2
pure water under visible light irradiation, and Pt and IrO2 as co-catalysts were photoloaded in
3
situ on samples to boost H2 and O2 generation in advance[44, 46], respectively. The typical
4
time course of gas evolution over 3D g-C3N4 NS-2 is shown in Fig. 3a. Notably, both H2 and
5
O2 were generated simultaneously under light irradiation, and the evolved H2 to O2 ratio was
6
very close to the expected 2:1 stoichiometry for overall water splitting. The average H2 and
7
O2 evolution rate is 101.4 and 49.1 µmol g-1 h-1 over 3D g-C3N4 NS-2, respectively,
8
approximately 11.8 and 5.1 times higher than that of bulk g-C3N4 and g-C3N4 NS (Fig. 3b),
9
significantly superior to a great many previously reported g-C3N4-based photocatalysts (Table
10
S5). To further confirm that the photocatalytic overall water splitting reaction was driven by
11
the absorption of an incident photon, the wavelength-dependent overall water splitting
12
experiments were also carried out. As shown in Fig. 3c, the AQY values for overall water
13
splitting over 3D g-C3N4 NS-2 is well coincident with its UV-Vis DRS, and the AQY at 420
14
nm was calculated to be 1.4%, significantly higher than that of Pt/g-C3N4 (0.3% at 405 nm) by
15
Wang’s group[21]. Then, the solar-to-hydrogen (STH) energy-conversion efficiency of 3D g-
16
C3N4 NS was also evaluated under AM 1.5G simulated sunlight (100 mW cm-2, 1 cm2). After
17
6 h illumination, a total of 5.73 µmol H2 was produced, and the STH value of 3D g-C3N4 NS
18
was determined to be 0.06% (Fig. S12).
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In addition, no notable deactivation emerges over 3D g-C3N4 NS-2 during a continuous
20
more than 100 h measurement (Fig. 3d), indicating its robust resistance to water and light
21
corrosion at the soft interface. However, the overall water splitting performance of g-C3N4 NS
22
dropped sharply during the repeated cycles (Fig. S13), and its gas evolution rate was
23
significantly reduced by 44% after 102 h reaction. As shown in Fig. S14, serious
24
agglomeration of nanosheets occurred over g-C3N4 NS after the continuous cycles, which
25
should be the main reason for the decrease of its water splitting activity. In contrast, 3D g-
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2
could be as supporters to prevent agglomeration of ultrathin nanosheets.
3
2.3. Faster Charge Carriers Separation over 3D g-C3N4 NS
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Fig. 4. (a) EPR spectra, (b) PL spectra (time-resolved PL spectra collected under an excitation
6
of 330 nm inset), (c) electrochemical impedance spectra (The equivalent circuit impedance
7
model inset), and (d) transient photocurrent density of bulk g-C3N4, g-C3N4 NS and 3D g-
8
C3N4 NS-2.
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Although the specific surface area of 3D g-C3N4 NS-2 is slightly higher than that of g-C3N4
10
NS, even that of 3D g-C3N4 NS-1 is lower, 3D g-C3N4 NS still shows significantly superior
11
overall water splitting performance, indicating that the specific surface area doesn’t mainly
12
account for the highly efficient activity of 3D g-C3N4 NS. It’s well known that the
13
photocatalyst activity is closely related to its photogenerated charge carrier separation
14
efficiency. To further understand why the 3D g-C3N4 NS exhibited superior photocatalytic 12
ACCEPTED MANUSCRIPT overall water splitting performance, the charge recombination behavior of all samples was
2
investigated. In EPR spectra (Fig. 4a), all samples show one single Lorentzian line with a g
3
value of 2.0036 that is induced by unpaired electrons on sp2-carbon atoms of the heptazine
4
rings.[54] Compared to bulk g-C3N4 and g-C3N4 NS, the stronger EPR spin intensity of 3D g-
5
C3N4 NS-2 implies the obviously higher concentration of unpaired electrons and greater
6
ability to delocalize electrons, which is favorable to the photogenerated electron-hole pair
7
separation. The excitonic process of 3D g-C3N4 NS-2 was also monitored by
8
photoluminescence (PL) measurements. In Fig. 4b, 3D g-C3N4 NS-2 exhibit an obviously
9
decreased PL emission intensity with respect to bulk g-C3N4 and g-C3N4 NS, which further
10
demonstrates its suppressed electron-hole pair recombination rate. Meanwhile, time-resolved
11
PL spectra monitored at the corresponding emission peaks provide the average radiative
12
lifetime (Aτ) of the charge carriers. The Aτ of bulk g-C3N4, g-C3N4 NS, and 3D g-C3N4 NS-2
13
are 4.24 ns, 5.38 ns, and 6.69 ns, respectively. As this increased exciton lifetime of 3D g-C3N4
14
NS-2 significantly implies enhanced exciton dissociation, and the longer diffusion length and
15
lifetime of charge carriers are further confirmed.
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Furthermore, as shown in Fig. 4c, the charge transfer resistance (Rsc) of 3D g-C3N4 NS-2
17
estimated via fitting the hemicycle radius on electrochemical impedance spectroscopy (EIS)
18
was only 4.134 Ω, much lower than that of bulk g-C3N4 and g-C3N4 NS (Table S7),
19
demonstrating a faster electron transfer kinetics in 3D g-C3N4 NS-2. Additionally, the
20
enhanced photocurrent response (Fig. 4d) further reveals the significantly improved mobility
21
of charge carriers in 3D g-C3N4 NS-2. Except the 3D structure of 3D g-C3N4 NS-2, g-C3N4
22
NS and 3D g-C3N4 NS-2 both presents ultrathin nanosheet structure, and their layer thickness
23
and specific surface area are very close. However, 3D g-C3N4 NS-2 exhibits longer lifetime of
24
charge carriers, much lower charge transfer resistance and significantly enhanced
25
photocurrent response than that of g-C3N4 NS, which fully demonstrates that the 3D structure
26
of 3D g-C3N4 NS-2 provides a pathway for electron transports.
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2
agglomeration of nanosheets and provide a pathway for electron transports., the suppressed
3
photogenerated electron-hole pair recombination rate and a faster charge carrier transfer
4
kinetics emerge in 3D g-C3N4 NS with respect to bulk g-C3N4 and g-C3N4 NS, resulting to its
5
superior photocatalytic overall water splitting activity.
6
4. Conclusion
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In summary, 3D g-C3N4 assembled by highly crystalline and ultrathin nanosheets were
8
successfully fabricated through a facile bottom-up supramolecular self-assembly route. The
9
unique 3D porous interconnected open-framework makes 3D g-C3N4 NS exhibit quite large
10
specific surface area, being as supporters to prevent agglomeration of nanosheets and provides
11
a pathway for electron transports. As a consequence, 3D g-C3N4 NS realized highly efficient
12
and stable overall water splitting under visible light, and achieved considerable AQY as high
13
as 1.4% at 420 nm. Briefly, this work throws light on designing polymer photocatalysts with
14
3D porous structure assemblies of low-dimensional nanomaterials for improved solar energy
15
capture and conversion.
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Acknowledgements
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This work was financially supported by the National Natural Science Foundation of China (grant numbers 21437003, 21673126, 21761142017, and 21621003) and the Collaborative
20
Innovation Center for Regional Environmental Quality.
21
Conflict of Interest
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The authors declare no conflict of interest. Appendix A. Supplementary material Supplementary data associated with this article can be found in the online.
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ACCEPTED MANUSCRIPT 1
Vitae
2 Xianjie Chen is currently a Ph.D. candidate under the supervision of Prof. Yongfa Zhu from
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the Department of Chemistry at Tsinghua University. His research interest is focused on the
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design and fabrication of highly efficient photocatalysts and their application for overall water
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splitting and environmental remediation.
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Run Shi studied materials science and engineering at the Tianjin Polytechnic University
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where he received his B.Sc. in 2012. He received his Ph.D. in materials science at the
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Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, in the lab of
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Prof. Tierui Zhang. His current research interest focuses on controlled synthesis of noble-
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metal-free photocatalysts for hydrogen evolution and CO2 reduction.
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Qian Chen is now pursing her M.S. degree under the supervision of Prof. Qin Kuang and Prof.
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Zhaoxiong Xie at College of Chemistry and Chemical Engineering, Xiamen University. Her
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research interests mainly focus on the design and fabrication of nano-sized photocatalysts and
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their application for CO2 reduction and water splitting.
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ACCEPTED MANUSCRIPT
1 Zijian Zhang is a Ph.D. student at the Department of Chemistry, Tsinghua University. He
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received his bachelor degree from the College of Chemistry and Molecular Sciences, Wuhan
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University. His research focuses on the design and fabrication of organic photocatalysts and
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their application for environmental pollution control and the resource utilization of solar
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energy.
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Wenjun Jiang received his Ph.D. student in 2018 under the supervision of Prof. Yongfa Zhu at
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the Department of Chemistry, Tsinghua University. His research focuses on the design and
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fabrication of nano-sized photocatalysts and their applications for solar energy conversion and
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environmental remediation.
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Prof. Yongfa Zhu received his B.S. degree in 1985 from Nanjing University and obtained his
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M.S. degree in 1988 from the Peking University. He has studied and worked at Tsinghua
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University since 1992 and received his Ph.D. degree in 1995. He is currently a full professor
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at Tsinghua University. His current research is focused on photocatalysis, environmental
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catalysis and nanomaterials.
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ACCEPTED MANUSCRIPT
1 Prof. Tierui Zhang obtained his Ph.D. degree in Chemistry in 2003 at Jilin University, China.
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He then worked as a postdoctoral researcher in the labs of Prof. Markus Antonietti, Prof.
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Charl F. J. Faul, Prof. Hicham Fenniri, Prof. Z. Ryan Tian, Prof. Yadong Yin and Prof.
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Yushan Yan. His current scientific interests are focused on the surface and interface of
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micro/nano-structures, including photocatalysis, electrocatalysis, thermocatalysis, and the
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controllable synthesis, self-assembly, and surface modification of colloidal inorganic
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nanostructures.
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ACCEPTED MANUSCRIPT Highlights Three-dimensional porous g-C3N4 nanosheets (3D U-C3N4 NS) assembled by highly crystalline and ultrathin nanosheets was successfully fabricated via a facile route.
and 49.1 µmol g-1 h-1 under visible light, respectively.
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3D g-C3N4 NS could directly split pure water into H2 and O2 with high evolution rate up to 101.4
3D U-C3N4 NS achieved a notable quantum yield of 1.4% at 420 nm, significantly superior to
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previously reported Pt/g-C3N4.
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3D U-C3N4 NS could still remains stable for more than 100 h of overall water splitting reaction.