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3D hierarchical g-C3N4 architectures assembled by ultrathin self-doped nanosheets: extremely facile HMTA activation and superior photocatalytic hydrogen evolution Huihui Gao, Ruya Cao, Shouwei Zhang, Hongcen Yang, and Xijin Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 18 Dec 2018 Downloaded from http://pubs.acs.org on December 19, 2018

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ACS Applied Materials & Interfaces

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3D hierarchical g-C3N4 architectures assembled by ultrathin self-doped

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nanosheets: extremely facile HMTA activation and superior photocatalytic

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hydrogen evolution

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Huihui Gao,† Ruya Cao,† Shouwei Zhang,* Hongcen Yang, Xijin Xu*

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School of Physics and Technology, University of Jinan, Shandong, 250022, P. R.

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

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†These authors contributed equally to this work.

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Keywords: 3D hierarchical g-C3N4 architectures, ultrathin self-doped nanosheets,

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Tunable band structures, Photocatalytic hydrogen evolution

1 ACS Paragon Plus Environment

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

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Abstract: Photocatalytic hydrogen evolution has broad prospects as a clean solution

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for the energy crisis. However, the rational design of catalyst complex, the H2

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evolution efficiency, and the yield are great challenge. Herein, 3D hierarchical g-C3N4

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architectures assembled by ultrathin carbon rich nanosheets (3D CCNS) were

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prepared via an extremely facile hexamethylenetetramine (HMTA) activation

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approach at bulk scale, indicating the validation of scale-up production process. The

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2D ultrathin carbon rich nanosheets were several hundred nanometers in width but

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only 5-6 nanometers in thickness, and gave rise to a unique 3D interconnected

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network. The unique composition and structure of the nanosheets endow them with

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remarkably light absorption spectrum with the tunable bandgap, high electrical

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conductivity, fast charge separation and large surface areas with abundant reaction

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active sites, and thus significantly improved H2 production performance. As high as

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~7.8% quantum efficiency can be achieved by irritating 3D CCNS at 420 nm with a

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H2 evolution rate >2.7×104 μmol/g/h, which is ~31.3 times higher than the pristine g-

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C3N4. Our work introduces an extremely facile route for mass production of doping

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modified 3D g-C3N4-based photocatalyst with excellent H2 evolution performances.


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1. Introduction

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Energy shortage has been a significant threat to the sustainable development, water

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splitting using semiconductor photocatalysts has attracted increasing attention.1-3

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Although many photocatalytic materials have been developed, the photocatalytic

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efficiency is rather low and the development of novel semiconductor photocatalystic

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systems with high photocatalytic activity is still highly desirable..4-6 This has

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motivated considerable research on pursuing new type photocatalytic materials with

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high photocatalytic activity.7-11 Up to now, the photocatalytic hydrogen reaction (PHR)

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systems based on 2D nanosheets provide unique advantages in synthesizing efficient

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photocatalyst materials for hydrogen evolution.12-14

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Graphitic carbon nitride (g-C3N4) as 2D photocatalyst has received considerable

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attention due to its proper band edges position, nontoxicity, stability and easy

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preparation.15-18 Nonetheless, pristine g-C3N4 still suffers from unsatisfactory

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performance owing to its limited optical absorption, high charges recombination rate

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as well as low electrical conductivity.19-24 To enhance the photocatalytic activity of g-

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C3N4, various means have been reported, such as compositing with other

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semiconductors, conductive materials, noble metals or compounds. Among those

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approaches, the doping modulation, especially nonmetal doping, has been proven to

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be a very effective route for improving the hydrogen evolution performance of g-

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C3N4.25-27 Nonmetal doping not only improves the light absorption capacity of g-C3N4,

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but also increases the photogenerated electron-hole pairs separation and creates more 3 ACS Paragon Plus Environment


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active sites. However, although the PHR activities of g-C3N4 were significantly

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improved by doping with nonmetal, the hetero atoms doping may also have

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disadvantage such as doping asymmetry and the impurity act as the photogenerated

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charge recombination centers.22,

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absorption spectrum of g-C3N4, but also could cause the band structure change via the

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formation of delocalized N=C-N conjugation systems.33-37 Researchers have made

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great efforts for improving the hydrogen evolution efficiency of g-C3N4 through

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doping carbon elements.23,

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heterostructure was prepared by Wei, which exhibited 10 times higher increase in

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electron life time, resulting in prominent hydrogen evolution ratio (HER) up to 371

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μmol/g/h.38 The carbon-rich g-C3N4 nanosheets by successively thermal-treating g-

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C3N4 in different atmosphere prepared by Song’ group showed a high HER of 39.6

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µmol/h, 5.4 increment to g-C3N4 nanosheets.39 Other carbon implementation

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techniques to the fabrication of doped g-C3N4, such as polymerization of urea and m-

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trihydroxybenzene, hydrothermal pre-treated melamine in ethanol, etc, have also been

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reported to improve the H2 evolution performance.40, 41 Recently, researchers reported

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several strategies for narrowing the bandgap through directly thermal treatment of

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bulk g-C3N4.8,

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nanosheets.3,

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agglomeration and restacking, resulting in a significant reduction in specific surface

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area and active sites, thus resulting in deleterious effects on photocatalytic activity.

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42

33

28-32

Self-doping could not only optimize the

For example, a two-dimensional CN-based in-plane

However, these methods always give a low yield of g-C3N4 However, ultrathin nanosheets usually suffer from serious


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Building up novel 3D hierarchical structures from 2D nanosheets is an effective

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strategy to inhibit the aggregation or stacking of photocatalysts. This structure could

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effectively enhance photocatalytic performance due to their interconnected and easily

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accessible network and large surface area that would provide extra active sites for

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photocatalytic H2 evolution reactions. Beyond that, it has strengthened light

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harvesting within interconnected network, enhanced H+ adsorption and facilitated

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transport and migration of H+ to the reactive sites. Consequently, an upgraded

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fabrication technique is proposed to integrate the merits of the favorable

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characteristics (self-doping, tunable band structures, mass production and 3D

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hierarchical structure) in g-C3N4 photocatalyst from a simple strategy by controlling

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the synthetic conditions, such as carbon source, preparation process, etc.

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Recently, several appealing approaches have been demonstrated to realize rational g-

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C3N4 nanostructures for PHR. For example, Wang constructed g-C3N4 nanospheres

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composed of nanosheets.43 Yang reported the construction of well-defined 3D ordered

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macroporous g-C3N4 by a simple SiO2 template method.44 A macroscopic 3D porous

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architecture of 2D g-C3N4 has been reported by thermal polymerization of urea and

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commercial melamine sponge.45 Moreover, excessive inorganic salts were found to

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act as a bubble-soft template and give rise to porous g-C3N4 products.46 Although

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some exciting advances have been achieved in these successful examples,

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photocatalytic hydrogen evolution activity could be further enhanced by optimizing

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the g-C3N4 morphology. First, indispensable corrosive reagent for template removal 5 ACS Paragon Plus Environment


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(HF, NH4HF2, etc.) brings the possible risk for environment. Second, these methods

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often require complex templates to achieve nanostructure control, which is

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detrimental to large-scale production in industry. Third, the common 3D g-C3N4 is

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usually tens or even hundreds of nanometers thick, which severely hinders the light

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absorption and photogenerated charges separation. Forth, most of the dopants for g-

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C3N4 only can minor increase in the onset for optical absorption of tens of

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nanometers, the expanded light harvesting capability is rather limited.

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Hexamethylenetetramine (HMTA) is one of the commonly used precursors to

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synthesize nitrogen-doped carbon materials through dehydration and carbonization.

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Noticeably, it is an easy sublimation decomposition process to produce

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carbon/nitrogen species upon heating, and these carbon species are easy to get into the

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interior considering that the galleries between layers can act as diffusion paths,

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namely homogeneous self-modification. Moreover, the HMTA sublimation

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decomposition temperature (~280 oC) is embedded in the range of the temperature for

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thermal oxidation etching of bulk g-C3N4, thus offering an opportunity to large-scale

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syntheses of homogeneous self-modification ultrathin g-C3N4 nanosheets.

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Here, we successfully developed an extremely facile HMTA activation strategy to

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fabricate 3D hierarchical g-C3N4 architectures assembled by ultrathin carbon rich

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nanosheets (3D CCNS) with tunable band structures and high yield. Specifically, 3D

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CCNS was prepared by directly thermal-treatment of HMTA and pristine g-C3N4

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mixture in air. Consequently, 3D CCNS achieved a much higher H2 evolution 6 ACS Paragon Plus Environment

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efficiency of >2.7×104 μmol/g/h, that of pristine g-C3N4. Four scientific issues,

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including modulated compositions, active surface area and porosity, solar light

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harvesting, charge separation and transportation, were investigated to explore the

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relationship between the structure and photocatalytic H2 evolution activity for 3D

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CCNS.

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2. Experimental Section

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2.1 Synthesis of 3D hierarchical g-C3N4 nanosheets

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Firstly, the pristine g-C3N4 nanosheets were prepared by a previously reported two-

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step calcination thermal-etching method. Typically, urea (~30 g) was filled into a

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crucible with cover and warmed up to 550 oC (2.3 oC/min) for 4 h. The obtained faint

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yellow powder (bulk g-C3N4) was further heated to 500 oC (5 oC/min) for 2 h. The

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crucible is not covered during the second calcination process. After cooling to room

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temperature, the pristine g-C3N4 nanosheets were obtained. Secondly, a designed

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dosage (i.e. ~50 mg) of HMTA was mixed with 1 g pristine g-C3N4 nanosheets in the

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state of solid, and then the homogenous mixture was tranferred to a crucible without

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cover and annealed at 500 oC (5 oC/min) for 2 h in air atmosphere. After being cooled

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down, a light brown powder carbon-rich g-C3N4 nanosheets was obtained. Products

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were denoted as 3D CCNS-H (where H = 30, 40, 50, 70, 100 and 200 and corresponds

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to the mass (mg) of HMTA used). CCNS was also prepared by direct one-step

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calcination of urea together with HMTA, defined as one step-CCNS.

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2.2 Characterizations 7 ACS Paragon Plus Environment


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The X-ray diffraction (XRD) were measured using a Rigaku D/max-2000

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diffiractometer. The Fourier transform infrared (FT-IR) spectra were recorded on a

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Bruker Tensor 27 spectrometer. The surface morphology of the materials was

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investigated using scanning electron microscope (SEM, Hitachi S-4800). The

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structure and the elemental distribution were determined using transmission electron

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microscope (TEM, JEOL, JEM-2100F). X-ray photoelectron spectroscopy (XPS)

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measurement was carried out on an ESCALAB MKII instrument equipped with a Mg

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Kα source. UV-Vis absorption spectroscopy (Shimadzu UV-3600) was conducted to

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investigate the optical properties of the samples. Brunauer-Emmett-Teller (BET)

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specific surface areas were evaluated at 77 K by nitrogen adsorption-desorption

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analyses (Micromeritics ASAP 2020). Photoelectrochemical experiments were

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measured using a electrochemical workstation (CHI 660E).

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2.3 Evaluation of photocatalytic activity

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The hydrogen evolution activities of photocatalysts were evaluated under the 300 W

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Xe lamp irradiation. During the photocatalytic hydrogen evolution reaction, the

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samples (0.01 g) were added into triethanolamine/H2O solution (100 mL, 10%

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triethanolamine). ~3 wt.% Pt was photo-deposited loaded on photocatalyst using UV-

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vis irradiation. At every 30 min time interval, H2 production was analyzed by gas

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chromatography. The apparent quantum efficiency (AQE) was also measured similar

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conditions. AQE is estimated according to a typical equation,


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AQE

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Number reacted electron  100% Number of incidengt photons 2  NH2   100% I  A t   hc

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N is the number of electrons, I represents the light intensity (5.54 mW/cm2), A

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(Irradiation area)= 26.4 cm2 (r = 2.9 cm), t (time)=3600 s, λ=420 nm, h=6.62×10-34 J·s,

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and c=3.0×108 m/s. The reported data were the average of triplicates and the relative

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errors were about 5%.

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3. Results and Discussion

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3.1. Structure and Property Analysis

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The schematic illustration of the preparation of the 3D CCNS was shown in Figure 1.

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In brief, the prisitine g-C3N4 nanosheet (CNS) is obtained via thermal oxidation

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exfoliation of bulk g-C3N4, then activated and doped to form 3D CCNS with a help of

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HMTA flame. This novel approach to produce 3D CCNS in gram scale is carried out

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in solid-phase.

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The crystal structure of the pristine CNS and a series of 3D CCNS-H were recorded

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by XRD. As illustrated in Figure 2A, all the diffraction peaks are the characteristic

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peaks of g-C3N4. It is obvious that both the crystalline phase and orientation are not

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altered. For pristine g-C3N4, the strong peak observed at 27.65o is typical (002) peak

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of the interlayer stacking of aromatic segments, while the weak peak (13.05°) can be

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interpreted as the in-planar structural packing motif, i.e. (100) peak.47, 48 Moreover,

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the (002) peak shows slightly shift toward higher angles (from 27.65o for 3D CCNS-





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30 to 27.77o for 3D CCNS-200) with increasing HMTA usage, indicating a decrease

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in the interlayer stacking distance, which is advantageous for the photogenerated

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charge separation and transportation.47, 49 FT-IR measurement was shown in Figure

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2B. For pristine CNS, the broad peaks between ~3000 and ~3400 cm-1 correspond to

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the N-H stretching vibration and the O-H band. The peaks in the ~1200-1700 cm-1

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region are assigned to sp2 C=N stretching vibration modes and the aromatic sp3 C-N

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bonds.35, 43-52 Moreover, the intense peak at ~807 cm-1 can be due to the out-of-plane

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skeletal bending modes of triazine. It is found that there are no obvious changes

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between the pristine CNS and a series of 3D CCNS-H, indicating that the HMTA

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treatment did not change the skeleton structure of g-C3N4.

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The specific surface area and porous nature of pristine CNS and 3D CCNS were

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further investigated by N2 adsorption/desorption isotherms. As presented in Figure 2C

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and S1, all the samples exhibit a similar shape of type IV isotherm. The specific

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surface area of pristine CNS is ~101.3 m2/g with a well-defined micropore and

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mesopore distribution. As a comparison, minor deviations of the BET specific surface

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areas were observed from 3D CCNS-30 (~104.49 m2/g) to 3D CCNS-200 (~107.65

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m2/g), suggesting negligible influence on the surface of the as-prepared photocatalysts.

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However, although all samples show similar pore-size distribution in ~1-50 nm

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(Figure 2D), a gradually decreased peak at ~1.5 nm can be clearly observed from

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pristine CNS to 3D CCNS-200, reflecting a corresponding loss in the well-defined

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microporous structure seen for pristine CNS. Thus, it can be concluded that HMTA 10 ACS Paragon Plus Environment

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activation retain the outstanding large surface area and mesoporosity properties of

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nanosheets.

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The FESEM and TEM images of 3D CCNS-50 were shown in Figure 3. After heating

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mixture of pristine CNS and HMTA in air atmosphere, the 3D CCNS-50 maintain the

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fluffy appearance of pristine CNS, but with a deep brown color caused by the rich

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carbon in g-C3N4 nanosheets, corresponding to the narrowed bandgap. Apparently, a

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3D porous configuration with vertically aligned nanowalls was observed (Figure 2A).

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These nanosheets gave rise to a unique interconnected network, may lead to a high

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dispersity in water, which is essential for photocatalytic H2 evolution.39 The Figure S2

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showed the digital photograph of 25.0 mL of homogeneous CCNS-50 aqueous

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dispersion with the concentration of 10 mg/mL stockpiled for one week,

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demonstrating the uniform dispersion of CCNS-50 in water and the good stability.

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The excellent water dispersibility of CCNS-50 could greatly promote proton reduction

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reaction for H2 evolution.The structure were further studied, revealing a 3D

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hierarchical morphology comprising of crumpled nanosheets with numerous wrinkles

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and folds (Figure 3B), which were several hundred nanometers in width but only 5-6

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nanometers in thickness, no particles or other aggregations on the surface. TEM

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image of the one step-CCNS was shown in Figure S3. Obviously, the one step-CCNS

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obviously displayed thicker nanosheets structures. The TEM result verified that

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ultrathin 3D CCNS nanosheets can be obtained by the several steps rather than one

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step. To explore the thickness of as-synthesized CCNS nanosheets, the samples were 11 ACS Paragon Plus Environment


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investigated by means of Atomic force microscopy (AFM). As shown in Figure S4,

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the average thickness of CCNS-50 is around 5-6 nm, which directly prove the

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presence of ultrathin CCNS nanosheets. TEM images of the pristine CNS and 3D

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CCNS-50 are shown in Figure 3C-D and S5. After HMTA activation, 3D CCNS-50

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displays obvious thin nanosheet structures, which significantly differed from the

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mainly dense and stacked sheets of the pristine CNS. The SAED pattern reveals that

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3D CCNS are polycrystalline (inset of Figure 3D), consistent with the XRD result.

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The elemental mapping images show homogeneous distribution of C and N in 3D

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CCNS (Figure 3E-H). As we know, the synergetic effects of different dimensional

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levels often lead to significant improvements in photocatalytic performance. In our

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case, the 2D nanosheets can not only provide high surface area for hosting numerous

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active sites, but also can serve as “electron-transport channels”, which can effectively

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reduce the recombination of photogenerated charges. The 3D hierarchical

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architectures not only enhance the efficiency of light harvesting, but also facilitate the

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interfacial transport or dispersion of active sites at different scales and shorten

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diffusion paths.

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The surface compositions of the pristine CNS and 3D CCNS were further investigated

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by XPS. Three sharp peaks were found for all apples, of which peaks at ~284 eV and

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~398 eV (Figure 4A) are assigned to C 1s and N 1s, respectively. As observed from

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high-resolution C 1 s spectrum (Figure 4B), pristine CNS showed three obvious peaks

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located at ~284.4, ~285.7 and ~288.3 eV, due to the graphitic carbon (C=C) (C1), C-N 12 ACS Paragon Plus Environment

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(C2) and sp2-hybridized carbon in N=C-N conjugation systems (C3), respectively.17, 53

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Comparatively, the C1 and C2 peaks in 3D CCNS-50 (for example) slightly shift

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toward higher binding energy to ~284.7 and ~286.1 eV, respectively.54 With

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increasing HMTA usage during the activation process, the relative intensities of the

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C1 component of 3D CCNS increased and the C2 component decreased, confirming

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the successful preparation of carbon doped g-C3N4. For the N 1s spectra (Figure 4C),

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the peak can be deconvoluted into three peaks at~398.6, ~399.9 and ~401.4 eV, which

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could be assigned to C-N=C sp2-bonded N atoms in triazine rings (N1), tertiary (N-

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(C)3) groups (N2) and C-N-H group (N3).55, 56 The carbon-rich was further confirmed

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by the increased C/N ratios determined with the increase of HMTA dosage by

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elemental analysis (Figure 4D and Table S1). For example, C/N atomic ratio in 3D

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CCNS-50 increased to 0.72 from 0.63 for pristine CNS, indicating the successful

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introduction of carbon into g-C3N4 via HMTA activation process.54 Similar

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conclusion can be further supported by the EDS results in Figure S6. These results

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clearly demonstrate the successful formation carbon rich embedded in g-C3N4.

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The optical properties of the photocatalysts are shown in Figure 5. We can see that all

17

photocatalysts have strong absorption in the visible light region (Figure 5A). For

18

CCNS-H, the absorption bands showed obvious red shifts and the light absorption

19

broadens to ~800 nm, suggesting that carbon doping could influence the optical

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absorption of the as-prepared g-C3N4.57, 58 Consistent with absorption results, the 3D

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CCNS suffers a matching variation from light yellow to deep brown as the HMTA 13 ACS Paragon Plus Environment


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usage increases (Figure 5B). To prove their suitability for potential large-scale

2

production, the large amounts of 3D CCNS-50 (~150 g) have been synthesized

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through this extremely facile synthesis route as shown in Figure 5C. The bandgaps of

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the samples determined from the transformed Kubelka-Munk formula progressively

5

narrowed from ~2.62 to ~2.06 eV (Figure 5D). It can be clearly observed that carbon

6

rich CNS can systematically reduce the bandgap of g-C3N4 and improve its spectrum

7

absorption ability. Furthermore, the band edges of the samples were also examined by

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valence band (VB) XPS spectra and presented in Figure 5E. The VB maximum was

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similar for both pristine CNS and 3D CCNS (~ -1.76 eV) through this HMTA

10

activation process. Therefore, the band alignments of the pristine CNS and 3D CCNS-

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H are schematically given in Figure 5F. Obviously, all samples have suitable

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conduction band valence band positions to reduce H2O to H2. The narrower band gaps

13

of 3D CCNS-H in agreement with the redshift of the intrinsic absorption edge in UV-

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Vis spectra. The above results indicate that an appropriate HMTA activation can

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significantly adjust the band structure of the g-C3N4, and thus the photocatalytic

16

efficiency.

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3.2. Photocatalytic H2 evolution activity

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The photocatalytic H2 evolution performance of the pristine CNS and the 3D CCNS-

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H was evaluated by investigating at room temperature. No obvious H2 was detected

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over all samples in the blank experiments, suggesting the essential H2 generation

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during the photocatalytic process. The rates of H2 evolution over pristine CNS and 3D 14 ACS Paragon Plus Environment

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CCNS-H exhibit a linear increase (Figure 6A), which suggests that all samples have

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excellent photostability under visible light irradiation. Meanwhile, it is obvious that

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one step-CCNS showed much lower H2 production rates than CCNS-50 (Figure S2).

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The average HER over pristine CNS and 3D CCNS-H were calculated and displayed

5

in Figure 6B. Obviously, the HER of 3D CCNS-50 is up to ~27035.23 μmol/g/h,

6

which is higher than the pristine g-C3N4 (~864.47 μmol/g/h) by a factor of ~31.3.

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Furthermore, the turnover frequency (TOF) of 3D CCNS-50 is calculated to be

8

~178.24 h-1, enhancement ~31.6 times compare with pristine g-C3N4. (Figure S7).

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Moreover, the stability of 3D CCNS-50 was also tested and shown in Figure 6C. It is

10

apparent that the HER remains fairly stable even after120 h, indicating its excellent

11

stability for H2 evolution. The AQE of 3D CCNS-50 on various monochromatic light

12

irradiations was examined in detail. The 3D CCNS-50 has an AQE of ~7.8% at 420

13

nm (Figure 6D), higher than most g-C3N4 related photocatalysts, implying a high

14

utilization efficiency of photogenerated charges. As expected, the AQE of 3D CCNS-

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50 is tested to be ~6.3% - ~0.2% in wavelength range of 450-600 nm, indicating that

16

the 3D CCNS still exhibit good solar energy conversion efficiency. The wavelength-

17

dependent PHR activity matched its optical absorption spectrum, indicating that the

18

H2 evolution reaction is primarily driven by photogenerated electrons in g-C3N4. The

19

photogenerated electron-hole pairs separation and transfer properties were further

20

investigated. For the pristine CNS, the PL spectrum of pristine g-C3N4 shows a strong

21

emission peak at ~456 nm (Figure S8), while the signal of the 3D CCNS decreased 15 ACS Paragon Plus Environment


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dramatically upon the introduction of carbon rich states, which suggests that the

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photogenerated charge could be efficiently separated. Moreover, the narrowing

3

bandgaps from pristine CNS to 3D CCNS can be further confirmed by the redshift of

4

the emission peak. Furthermore, time-resolved PL studies were applied to better gain

5

a quantitative knowledge of the photogenerated charges lifetime.37, 59 In Figure 6E, the

6

decay curves were fitted by the following equation:

I (t )  A  B1 ( 7

t t )  B2 ( ) 1 2

8

where B1 and B2 are the pre-exponential functions. τ1 and τ2 are the corresponding

9

lifetimes, respectively. The intensity-average lifetime τ was estimated from the

10

equation as follows,

11

B112  B2  22  B11  B2  2

12

The lifetime (τ) of 3D CCNS-50 (~5.75 ns) was undoubtedly higher than that of

13

pristine CNS (~2.19 ns), suggesting that the HMTA activation approach could

14

effectively elongate the lifetime of charge carriers. The prolonged lifetimes indicate

15

that some favorable defect states, e.g., carbon rich states, can act not only as

16

photogenerated charge traps, which promotes charge separation and enhances

17

photocatalytic activity.42

18

promote the photogenerated charges separation and transfer, resulting in a much

19

longer lifetime of the charge carriers. The improved separation efficiency of charge

20

pairs was also verified by EIS Nyquist plot (Figure S9). It can be clearly seen that 3D

That is, the 3D hierarchical structure can significantly


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1

CCNS-50 has a smaller semicircle radius than the pristine g-C3N4, indicating

2

significantly

3

measurement (i-t curves) further confirmed the transfer efficiency of charge carriers in

4

the samples. As presented in Figure 6F, the samples show the greatly increased

5

transient photocurrent of 3D CCNS-50 over pristine CNS due to the promoting charge

6

separation and transfer. These results provide solid evidence that rapid separation and

7

transfer of photogenerated charges is an important for improving photocatalytic

8

performance. The calculated HER of 3D CCNS is higher than most nanostructured g-

9

C3N4 (Figure 6G and Table S2).

improved

electronic

conductivity.43-46

Transient

photocurrent

10

For photocatalysis, three crucial steps constitute the basic photocatalytic process: (1) a

11

fair response to visible light, (2) photogenerated charge separation and transfer, (3)

12

more active sites for H2 evolution. Unfortunately, photogenerated carriers are easy to

13

recombine via the long-distance transportation. Therefore, novel structural design

14

should be shortened charge migration distance, thus accelerating the photo-generated

15

charge separation. In our case, unique nanostructures endow some unique

16

characteristics for the photocatalytic H2 evolution (Figure 7): (1) 3D hierarchical

17

nanostructures can produce multiple light scattering, which facilitates light harvesting;

18

(2) ultrathin nanosheets can reduce the charge-to-surface migration distance, thus fast

19

photogenerated charge separation can be obtained; (3) 3D nanostructures provides

20

more exposed active catalytic sites for surface reactions, which acts a pivotal part in

21

shortening the charge migration distance; (4) carbon rich states effectively tunable the 17 ACS Paragon Plus Environment


1

band structure of 3D CCNS and extends its visible light response up to 800 nm. These

2

structure-induced favorable features enable more effective photogenerated charge

3

separation, faster transfer onto its surfaces for reaction with H2O, and promote mass

4

transport in 3D CCNS, definitely favoring the proceeding of PHR. Meanwhile, the

5

band structure of 3D CCNS is well optimized. These electronic structure-induced

6

favorable features allow more light absorption and significantly enhanced

7

photogenerated charge separation, thus an enhancement of the H2 evolution ability.

8

Meanwhile, carbon-rich induced electrical conductivity increase would favor the

9

transfer of photogenerated electrons to the surface, which was confirmed by i-t curves

10

and EIS.43-46 The homogeneous carbon doping favorable feature could further

11

improve the light responses and conductivity, and thus enhancement of the

12

photocatalytic hydrogen evolution performance.

13

To further illustrate the contribution of specific surface area and doping to enhanced

14

photocatalytic activity, we compared the photocatalytic activity before and after being

15

normalized with surface area (Table S3). The photocatalytic activity enhancement

16

times between bulk g-C3N4 and pristine CNS decreased from ~4.63 to ~2.43 after

17

normalization with surface area, suggesting that higher surface areas contributed to

18

the photocatalytic performance enhancement. However, though both pristine CNS and

19

CCNS-50 have similar surface area, the photocatalytic activity enhancement times of

20

CCNS-50 is significantly increased after normalization with surface area, suggesting

21

that the major contribution of photocatalytic activity enhancement could be the carbon 18 ACS Paragon Plus Environment

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1

self-doping rather than higher surface area. That is, the surface area may not be the

2

only factor affecting the photocatalytic activity of photocatalysts after the surface is

3

increased to a certain extent. Considering that pristine CNS and CCNS-50 have

4

similar chemical compositions and surface areas, we assume that the following factors

5

may be responsible for the much greater photocatalytic activities of CCNS-50. First,

6

3D hierarchical nanostructures can produce multiple light scattering, which facilitates

7

light harvesting. Second, ultrathin nanosheets can reduce the charge-to-surface

8

migration distance, thus fast photogenerated charge separation can be obtained. Third,

9

carbon rich states effectively tunable the band structure of 3D CCNS and greatly

10

extended its light absorption range. Fourth, optimsticed band structure and enhance

11

the delocalized π bonds, thus enhancing the conductivity and photogenerated charge

12

transfer. Therefore, to further enhance the photocatalytic performance, we should not

13

only improve the specific surface area of the photocatalysts, but also further improve

14

the light absorptive ability, conductivity and morphology, etc.

15

As we known, g-C3N4 with sp2 C-N rings has a similar structure to that of graphene

16

with sp2 C-C ring. Graphene are prepared predominantly by chemical vapor

17

deposition (CVD) on metal substrates. In this process, the metal substrates could serve

18

as catalyst and template to decompose the carbonaceous gaseous species and deposit

19

graphene on its surfaces. Similarly, the ultrathin g-C3N4 nanosheets could also be

20

selected as template to fabricate the few-layer carbon doped g-C3N4 nanosheets.

21

HMTA can be easily sublimated to produce carbon/nitrogen species upon heating, and 19 ACS Paragon Plus Environment


1

these carbon gaseous species can permeate to the interior considering that the galleries

2

between layers can act as diffusion paths, then deposited on g-C3N4 surface. This

3

process is called a simple in-air CVD method since the entire preparation process is in

4

the muffle furnace under air atmosphere. The HMTA is easily sublimation

5

decomposition to produce carbon/nitrogen species upon heating, providing a carbon-

6

rich atmosphere and forming homogeneous carbon doping into ultrathin nanosheets.

7

This method utilizes ultrathin g-C3N4 nanosheets as a template and HMTA as a CVD

8

precursor for the deposition of a thin layer of carbon layer on the template surface.

9

Therefore, compared with previous reports for 3D g-C3N4 nanostructure, our method

10

presents some advantages: (i) a facile solid phase mixing technique at room

11

temperature and air calcination that warrants low cost manufacturability; (ii) a mild

12

fabrication process that avoid the usage of any strong acid or hard template; (iii) a

13

guarantee of scale-up fabrication and reproducibility; (iv) a homogeneous process to

14

deposited carbon on the entire g-C3N4 surface, resulting in tunable band structures and

15

extends its visible light response up to 800 nm.

16

4. Conclusion

17

In summary, In summary, a series of carbon rich 3D CCNS photocatalysts were

18

successfully fabricated via facile HMTA activation process without introducing other

19

heterocomponents. The 2D ultrathin carbon rich nanosheets were several hundred

20

nanometers in width but only 5-6 nanometers in thickness, and gave rise to a unique

21

3D interconnected network. Comparing to the pristine CNS, the unique rich carbon 20 ACS Paragon Plus Environment

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1

structure exhibit narrowed bandgaps from ~2.62 to ~2.06 eV, greatly extended visible

2

light absorption. Meanwhile, the resulting 3D CCNS induces stronger optical

3

absorption and more prolonged lifetime of photogenerated charge as compared with

4

pristine CNS, while maintaining the chemical structure similar to that of pristine CNS.

5

This unique structure also endows 3D CCNS with efficient multidirectional

6

photogenerated charge transfer, thus resulting in greatly improved photogenerated

7

charge separation, and surface reaction rate. As a result, the optimized 3D CCNS-50

8

exhibits an unprecedented H2 evolution performance, with a notable AQE of ~7.8% at

9

420 nm and a prominent HER up to ~27035.23 µmol/g/h, with an enhancement of

10

~31.3 times higher as compared with pristine CNS. The proposed synthetic route

11

provides an extremely facile strategy toward the development of highly efficient

12

photocatalysts for large-scale exploration and practical applications.

13

ASSOCIATED CONTENT

14

Supporting Information: N2 adsorption-desorption isotherms of all the samples; The

15

TEM image of pristine CNS; The EDS of 3D CCNS-50; AFM image of as-prepared

16

CCNS-50 nanosheets; Digital photographs of the photocatalysts aqueous dispersion;

17

Turnover frequency of pristine CNS and 3D CCNS-H; The photoluminescence

18

emission spectra comparison of pristine CNS and 3D CCNS-H; C/N atomic ratios of

19

pristine g-C3N4 and CCNS-H, etc.. This material is available free of charge via the

20

Internet at http://pubs.acs.org.

21

AUTHOR INFORMATION 21 ACS Paragon Plus Environment


1

Corresponding Author

2

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

3

Author Contributions

4

†These authors contributed equally to this work.

5

Acknowledgements

6

The work was fnancially supported by National Natural Science Foundation of China

7

(Grant No. 21707043) and the Natural Science Foundation of Shandong Province

8

(Grant No. ZR2017BEE005).

9

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Dong, G. P. 3D Foam Strutted Graphene Carbon Nitride with Highly Stable

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Optoelectronic Properties. Adv. Funct. Mater. 2017, 27(42), 1703711.

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I. N.; Wu, L. Z.; Tung, C. H.; Zhang, T. R., Alkali-Assisted Synthesis of Nitrogen

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Deficient Graphitic Carbon Nitride with Tunable Band Structures for Efficient

Guo, Q. Y.; Zhang, Y. H.; Zhang, H. S.; Liu, Y. J.; Zhao, Y. J.; Qiu, J. R.;

Yu, H. J.; Shi, R.; Zhao, Y. X.; Bian, T.; Zhao, Y. F.; Zhou, C.; Waterhouse, G.

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Visible-Light-Driven Hydrogen Evolution. Adv. Mater. 2017, 29 (16), 1605148.

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Assisted Synthesis of Holey G-Carbon Nitride Nanosheets for Efficient Visible-Light

13

Photocatalytic H2 Evolution. ACS Appl. Mater. Interfaces 2018, 10 (24), 20521-20529.

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"Seaweed" Architecture for Enhanced Hydrogen Evolution. Angew. Chem. Int. Edit.

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2015, 54 (39), 11433-11437.

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L.; Dai, B.; Jia, X., Morphology and defects regulation of carbon nitride by

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hydrochloric acid to boost visible light absorption and photocatalytic activity. Appl.

20

Catal. B: Environ. 2017, 217, 629-636.

Fang, Z.; Hong, Y.; Li, D.; Luo, B.; Mao, B.; Shi, W., One-Step Nickel Foam

Han, Q.; Wang, B.; Zhao, Y.; Hu, C. G.; Qu, L. T., A Graphitic-C3N4

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Yang, H.; Zhou, Y.; Wang, Y.; Hu, S.; Wang, B.; Liao, Q.; Li, H.; Bao, J.; Ge,

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G.; Jia, S., Three-dimensional flower-like phosphorus-doped g-C3N4 with a high

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surface area for visible-light photocatalytic hydrogen evolution. J. Mater. Chem. A

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2018, 6 (34), 16485-16494.

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Jiang, J.-J.; Wang, H.-P.; Pan, M.; Su, C., A Facile Method for Scalable Synthesis of

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Ultrathin g-C3N4 Nanosheets for Efficient Hydrogen Production. J. Mater. Chem. A

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10

Intermediate-mediated strategy to horn-like hollow mesoporous ultrathin g-C3N4 tube

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with spatial anisotropic charge separation for superior photocatalytic H2 evolution.

12

Nano Energy 2017, 41, 738-748.

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8

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Environ. Sci. 2018, 11 (3), 566-571.

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Page 32 of 40

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


1 2

Figure 1. Schematic illustration of the preparation of the 3D CCNS.

3 4



B

A

27.77

12.96

CCNS-200

CCNS-200

Intensity (a.u.)

Intensity (a.u.)

CCNS-100 CCNS-70 CCNS-50 CCNS-40


CCNS-30

10

30

pristine CNS

40

2 Theta (degree)

3500

2800

2100

1400

Wavenumber (cm-1)

450 300

CNNS-100 CNNS-50 CNNS-30

150

0.2

0.4

0.6

0.8

Relative pressure (P/P0)

Pore volume (cc/g/nm)

CNNS-200 CNNS-70 CNNS-40 pristine CNNS

700 D

C

600

0 0.0

1

20

CCNS-30

pristine CNS

27.65

13.05

Volume adsorbed (cm3/g)

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

Page 34 of 40

1.0

0.03

CNNS-200 CNNS-70 CNNS-40 pristine CNNS

0.02

CNNS-100 CNNS-50 CNNS-30

0.01

0.00

0

10

20 30 40 Pore width (nm)

50

2

Figure 2. (A) XRD patterns, (B) FTIR spectra, (C) N2 adsorption/desorption

3

isotherms and (D) pore size distribution of pristine CNS and 3D CCNS-H.

4 5


200 100 70 50 40 30 0

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


A

B

C

D

(002)

(100)

E

1

F

G

Overlapping

H

C-K

2 3

Figure 3. (A and B) SEM, (C) TEM and (D) HRTEM images of 3D CCNS-50. (E-H)

4

STEM images of 3D CCNS-50 and the corresponding elemental mapping images.

5

The inset (D) shows the SAED pattern of 3D CCNS-50.

6


N-K


A

B CCNS-200

Intensity (a.u.)

Intensity (a.u.)

CCNS-200 CCNS-50 CCNS-30

CCNS-50

CCNS-30

pristine CNS

0

200

400

600

Binding Energy (eV)

pristine CNS

800

282

1000 0.9

C

50

284

286

288

Binding Energy (eV)

D

290

CCNS-200

0

200

CCNS-50

CCNS-30

C/N atomic ration

CCNS-200

Intensity (a.u.)

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

Page 36 of 40

0.8 CCNS-100 CCNS-70

0.7

CCNS-40 CCNS-30

CNNS-50

30

pristine CNS

396

1

399

402

Binding Energy (eV)

0.6

405

pristine CCNS

2

Figure 4. (A) XPS survey spectra and high-resolution C 1s (B) and N 1s (C) of

3

pristine CNS and 3D CCNS-H. (D) The C/N ratio characterized by elemental analysis

4

of pristine CNS and 3D CCNS-H.

5

.


70

40

50

0

Page 37 of 40

Absorbence (a.u.)


200 mg

0 mg CNS+HMTA

300

400

500

600

CCNS50 CCNS200 CCNS40 CCNS100 CCNS30 CCNSpristine CNS

700

Wavelength (nm)

pristine CCNS-50 ~150 g CNS

E

Intensity (a.u.)

2.0 1.5 1.0 0.5

2.06 eV

0.0

2.0

-1

2.5

3.0

Energy (eV) Ec = -0.86

2.62 eV

1 2

Ev = +1.76 pristine CNS


pristine CNS 35

3.5

30

-1.76 eV 25

20

15

10

Binding energy (eV)

5

0

F -0.65

-0.62

0 Eg = 2.62 eV

CCNS-200

CCNS-30

Eg

1

70

C

D

2.5

Kubelka-Monk

B

A

pristine CNS CCNS-30 CCNS-50

200

Potential (V vs. RHE)

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


Eg = 2.41 eV

+1.76 CCNS-30

Eg = 2.38 eV +1.76 CCNS-40

-0.59

-0.51

-0.35

Eg = 2.27 eV Eg = 2.35 eV Eg = 2.11 eV

+1.76 CCNS-50

+1.76 CCNS-70

+1.76 CCNS-100

-0.3 H+/H2 Eg = 2.06 eV O2/H2O +1.76 CCNS-200

2

Figure 5. (A) The UV-Vis diffuse reflectance spectra of pristine CNS and 3D CCNS-

3

H; (B) photographs of pristine CNS and 3D CCNS-H; (C) large amount of 3D CCNS-

4

50 synthesized by this extremely facile route; (D) Kubelka-Munk function curves of

5

pristine CNS and 3D CCNS-H; (E) VB-XPS spectra for pristine CNS and 3D CCNS-

6

H; (F) Schematic energy level diagrams of the pristine CNS and 3D CCNS-H.

7


1

2

Time (h)

3

4

Absorbance (a.u.)

D 10

1

4 2 400

500

0

3

7.0x10

0.0 CN

S CC

N

30 S-

CC

N

40 S-

CC

N

50 S-

CC

N

Storage for 120 hours

1x105 0

0 0 20 10 SSN N CC CC

70 S-

0

5

10 15

165

Time (h)

1.0x10-5

E

Average life time CCNS-50: 5.75 ns CCNS-70: 4.61 ns CCNS-30: 3.61 ns CNS: 2.19 ns

2x105

170

CNS CCNS-30

8.0x10

6.0x10-6

CCNS-70 CCNS-50

4.0x10-6 2.0x10-6

10

20

30

Time (ns)

40

0.0

200

250

300

Time (s)

350

G

ref. S42 ref. S43 ref. S44 ref. S45 ref. S46 ref. S47 ref. S48 ref. S49

s 1 " s 3 4 x s 4 4 0 0 1 2 4 ) 1 S 4 4 x s 0 1 S S 6 4 s 4 P S S TA N N N CN N + S N4 -2 0. H N SC -S UB N N N 0 1. ts ts 0- % .0 N CN CN S CN -9 N et N CN M - ets N N C et s- ed ℃ P M N N M -C -C 3 A 3 4 N 3 N C N - N N N PC 3 3 -2 C ee ee 0 t -0 C G - N L 0 3 he 3 P PCNPCNshe CN -TCN4@she-CN we -500 CN U -C3 a-C H pg Pt g-C D-C C3N CN s-C C CN C CN C C C de/ /g-C-g-C CN G osh osh 4-5 8 w Nx H H HR IGC N18 g-Cnos -g-C o a G P P o 3 4 e n P C ot i n p a S U ed g ure m s a D n n N .1 3 B 2/ t% 2D na P t/g-C I n HE " C3N ph oP A s LC m h n N na na 3 (0 -C CD s os C -PD oS .16wO3/ C T SA 4 4 g-C 2+ g ba P e g h P A N N t M 0 e2 C ap ed -P om re U 1C3 -C3 z 4 n t% α-F o i U a g g N N w C n id 5 PC C3 ox 0. gly al m er th

2

Figure 6. (A) Reaction time profile of the H2 evolution reaction and (B) rates of H2

3

evolution of pristine CNS and 3D CCNS-H; (C) stable hydrogen evolution by 3D

4

CCNS-50; (D) wavelength dependence of H2 evolution rate of 3D CCNS-50; (E)

5

time-resolved PL spectra and (F) transient photocurrent response of pristine CNS and

6

3D CCNS-H; (G) hydrogen evolution rate for 3D CCNS-50 in comparison with other

7

g-C3N4-based catalysts (References in SI, defined as ref. Sx (x=1, 2, 3,.......,49)).


pristine CNS: 8.64 mol/h CCNS-30: 172.13 mol/h CCNS-40: 198.61 mol/h CCNS-50: 270.35 mol/h CCNS-70: 229.01 mol/h CCNS-100: 152.20 mol/h CCNS-200: 56.75 mol/h

175

F

-6

ref. S41

Wavelenth (nm)

600

1.4x10

C

3x105

ref. S3 ref. S4 ref. S5 ref. S6 ref. S7 ref. S8 ref. S9 ref. S10 ref. S11 ref. S12 ref. S13 ref. S14 ref. S15 ref. S16 ref. S17 ref. S18 ref. S19 ref. S20 ref. S21 ref. S22 ref. S23 ref. S24 ref. S25 ref. S26 ref. S27 ref. S28 ref. S29 ref. S30 ref. S31 ref. S32 ref. S33 ref. S34 ref. S35 ref. S36 ref. S37 ref. S38 ref. S39 ref. S40

6x104 5x104 4x104 3x104 2x104 1x104 0

6

ref. S1 ref. S2

300

8

4

 evolution (mol/g)

300

2.1x104

Page 38 of 40

Current (A)

600

B

2.8x104

Intensity (a.u.)

CCNS-30 CCNS-50 CCNS-100

 evolution rate (mol/g/h)

900

A pristine CNS CCNS-40 CCNS-70 CCNS-200

Quantum efficiency (%)

1200

0 0

 evolution rate (mol/g/h)

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

 evolution (mol)


B D F H

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


1 2

Figure 7. Graphical illustration of the structural advantages and mechanism of

3

photocatalytic hydrogen evolution for 3D CCNS under visible light irradiation.



1

Graphical Abstract

2


Page 40 of 40

extremely facile HMTA activation and superior

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