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rsc.li/sustainable-energy

Sustainable Energy & FuelsInterdisciplinary research for the development of sustainable energy technologies

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Volume 1 Number 1 2017 Pages 1–100

Sustainable Energy & FuelsInterdisciplinary research for the development of sustainable energy technologies

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Q. Yao, C. Wang, Y. Su, X. Zhou and Y. Zhang, Sustainable Energy Fuels, 2019, DOI:

10.1039/C9SE00263D.

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DOI: 10.1039/x0xx00000x

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Green synthesis of ultrathin edge-activated foam-like carbon nitride nanosheets for enhanced photocatalytic performance

under visible light irradiation

Islam A. Abdelhafeez,ab‡ Qiufang Yao,a‡ Cixuan Wang,c Yiming Su,ad Xuefei Zhou*ac and Yalei Zhang*ac

Fabrication of few-layered polymeric carbon nitride (PCN) photocatalyst has attracted increasing attention due to its

substantial enhancement of the photocatalytic performance. A green strategy for synthesis of ultrathin PCN nanosheets

with integration of high-efficiency and enriched active edges with minimizing chemicals and energy input is overwhelmingly

required and still challenging. Herein, we report a green, cost-effective and template-free synthesis approach for ultrathin

foam-like PCN nanosheets with enriched active sites in wet atmosphere via “three-in-one” strategy. This strategy relies on

coupling of melem segments polymerization with condensed layers delamination by water molecules and introducing of

new termial groups solely in one-pot without any other additives. The obtained melem-derived PCN (MFCN-wet) shows high

loosen and extremely light characteristic with formation of ultrathin few-layers. Furthermore, it exhibits high specific surface

area and pore volume. Most importantly, enrich active sites with fast charge carrier transfer drastically enhance the

hydrogen evolution rate and rhodamine B (RhB) degradation with high stability under visible light irradiation compared with

that as-synthesized materials in nitrogen and air atmosphere. Such a sustanaible strategy would pave new opportunities for

further environmental and energy applications.

1. Introduction

In recent years, with increasing urbanization and

industrialization growth and rapid fossil fuels consumption, the

environmental pollutions and energy crisis have become the

most critical issues to the world.1–3 The use of solar energy for

environmental remediation and sustainable chemical

production has been regarded to be a promising approach as it

is easily accessible and widely available.4,5 Recently, polymeric

carbon nitride (PCN), as 2D free-metal sustainable

semiconductor, has drawn intensive attention since it has been

investigated as hydrogen evolution photocatalyst in 2009.6 PCN

has been introduced as a promising visible-light absorption

candidate due to its extraordinary properties, suitable visible-

light driven band gap, unique electronic features, superior

physiochemical and photochemical stability even under strong

acid or base due to the strong covalent bonds between carbon

and nitrogen atoms, low density, non-toxicity, low-cost and

ease to prepare for large-scale applications.7–10 Generally, PCN

is typically prepared by thermal condensation of nitrogen-rich

precursors such as urea, thiourea, melamine, cyanamide and so

forth,2,11 and has been widely used for a variety of applications

including organic pollutants degradation, H2 production, water

splitting, CO2 reduction, selective organic synthesis and

disinfection of bacteria under visible-light irradiation.12–15

However, in bulk format, the aforementioned merits of PCN

diminish by several factors such as the low specific surface

areas, high recombination rate of photon-generated electron-

hole pairs, slow charge transport, limited active sites which

hamper its practical applications.8,16–18

To enhance and improve this promising photocatalyst,

fabrication of ultrathin PCN nanosheets has been recognized as

novel approach and may act an ideal way to achieve the

promising photocatalyst.18,19 Few layers with small thickness in

nanometre scale lead to unique and exceptional electronic and

optical features associated with large specific surface area and

higher reduction potential of photogenerated of electrons that

suppress the high recombination rate of photoexcited charge

carriers.20–24 To achieve ultrathin nanosheets, there are two

conventional synthesis approaches: top-down and bottom-up.

In the top-down approach, the bulk PCN is efficiently exfoliated

via ultrasonication-assisted liquid exfoliation, thermal oxidation

exfoliation, hydrothermal delamination, or gaseous stripping.25–

27 However, this strategy is limited by its long-time processing

a. State Key Laboratory of Pollution Control and Resources Reuse, Tongji University, Shanghai 200092, China

b. Soil, Water and Environment Research Institute, Agricultural Research Center, Giza, Egypt

c. Key Laboratory of Yangtze Water Environment for Ministry of Education, Tongji University, Shanghai 200092, China

d. Department of Civil and Environmental Engineering, University of California, Los Angeles, CA 90095, USA

‡ The authors contributed to the work equally. †Electronic Supplementary Information (ESI) available: SEM and TEM images, Mott-Schottky plot, Gaussian fitting peak, AQY values, UV-visible absorption spectra, TOC removal plot, ESR signals, RhB photodegradation in different water mat rixes, Normalization of RhB degradation with surface area and comparison table with the previous methods of PCN synthesis.

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View Article OnlineDOI: 10.1039/C9SE00263D


ARTICLE Journal Name

2 | J. Name. , 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx



and high energy consumption. On the other hand, bottom-up

approach is based on the growth and assembly of precursor

molecules via template-assisted, heteroatoms-mediates or

additive-mediated synthesis including soft and hard templates,

harsh solvents or complex processes. As a consequence, the

production costs and the secondary environmental pollution

increase which restrict its utilization in terms of clean synthesis

and practical application.27,28 Thence, it is still a huge

challenging to develop clean and cost-effective synthetic

strategy of ultrathin PCN nanosheets with high-efficiency and

stability.

In this manuscript, we demonstrate a straightforward, versatile,

environmentally friendly and cost-effective approach to

synthesize (bottom-up process) ultrathin foam-like PCN

nanosheets in wet atmosphere via “three-in-one” strategy. This

strategy depends on coupling of tri-s-triazine segments

polymerization with stripping of stacked layers through water

molecules and introducing of new terminal active groups

merely in one-pot without any template assistant or pre-

treatment. The as-prepared material exhibits high specific

surface area with ultrathin few-layers and short-order

polymerization morphology. In addition, the visible light

photocatalysis activities for H2 evolution and RhB degradation

are intrinsically enhanced.

2. Materials and methods

2.1. Materials

Melamine, rhodamine B (RhB), triethanolamine (TEOA), CaCl2

and NaCl were purchased from Aladdin, China. Isopropanol

(IPA), MgCl2, MgSO4 and KCl were purchased from Sinopharm

chemical reagent, China. P-benzoquinone (BQ) was obtained

from Shanghai Macklin biochemical company, China. All

chemicals were used as received without further purification.

2.2. Synthesis of foam-like PCN nanosheets

10 g of melamine was transferred to alumina crucible with cover

and heated in a muffle furnace (SX2-2, China) at 450 °C for 2 h

with an elevation temperature rate of 5 °C min-1. After cooling

to room temperature, the white bulk melem agglomerates were

milled well to fine powder by agate mortar and transferred

again to alumina crucible for next step. Melem powder was

moved to tube furnace (OTF-1200X, China) to heat at 550 °C for

4 h. Before switching on the furnace, the tube insulation plugs

were wetted with water and then left to comb the extra water

out. Thereafter, the moist plugs transferred to tube furnace and

the trapped air inside the tube was removed by evacuated and

introduced nitrogen gas, followed by heating at 550 °C for 4 h

with temperature rate 5 °C min-1. After cooling to room

temperature, the fine and loose white powder was obtained, as

shown in Fig. S1, with the final product yield of 14%. For

comparison, the melem powder was separately heated in air

and nitrogen atmosphere at 550 °C for 4 h without wet

atmosphere. The obtained powders are donated as MFCN-wet,

MCN-air, and MCN-N2 for wet, air and N2 atmosphere,

respectively.

2.3. Characterization

The X-ray diffraction (XRD) analysis was performed on X-ray

Powder Diffractometer (D8 ADVANCE, 40 mV and 40 mA).

Fourier transform infrared (FTIR) spectra were recorded on

Nicolet 6700 FTIR Spectrometer (Thermo Fisher Scientific). X-

ray photoelectron spectroscopy (XPS) was performed on

EscaLab 250 Xi, (Thermo Fisher Scientific). The morphology of

as-synthesized materials was characterized by Scanning

Electron Microscopy (SEM) (Phenom Pro), Transmission

Electron Microscopy (TEM) (Tecnai G2F20S-TWIN, 200 KV) and

Atomic Force Microscope (AFM) (Bruker Dimension Icon). The

specific surface area, pore volume and pore diameter were

acquired from Automatic Specific Surface Area and Pore

Analyzer (Micromeritics Instrument Corporation, ASAP2460).

Element analysis was measured by Elemental Analyzer

(Elementar Vario EL Cube). The UV–vis diffuse reflectance

spectra (DRS) was performed on SHIMADZU UV-2600 UV-visible

Spectrophotometer. Steady-state photoluminescence (PL)

spectroscopy was measured on Steady State and Transient

State Fluorescence Spectrometer (Edinburgh Instruments

FLS980). The total organic carbon (TOC) of the collected samples

was measured by Multi N/C 2100 (Analytikjena). Electron spin

resonance (ESR) was recorded by JEOL JES Spectrometer (JES-

FA200). Zeta potential was conducted with Zeta Analyzer

(Malvern Zetasizer Nano ZS90). The electrochemical impedance

spectroscopy (EIS) test was conducted with a CHI760E

electrochemical workstation (Chenhua, Instruments, Shanghai,

China) in a standard three-electrode cell in 0.5 M Na2SO4. In

detail, the working electrode was prepared by ultrasonication

of 5 mg of photocatalyst with 1 mL of absolute ethanol and 0.1

mL of nafion solution (0.05 wt%) to form homogeneous slurry.

Then, 10 μL of the slurry was dropped on glassy carbon

electrode. After evaporation of ethanol, the photocatalytic was

adhered on the glass surface. A Pt plate and a saturated Ag/AgCl

electrode were selected as counter and reference electrode,

respectively. The EIS test was carried out in the frequency range

of 0.01 Hz to 106 Hz with an AC voltage amplitude of 5 mV.

2.4. Photocatalytic hydrogen evolution measurement

Photocatalytic hydrogen evolution was carried out in a Pyrex

top irradiation reaction vessel connected to a closed glass gas

system. In detail, 20 mg of as-prepared materials with

appropriate amount of H2PtCl6 (3 wt% Pt) as cocatalyst were

separately dispersed in 100 mL of aqueous solution, as well as

20 mL of triethanolamine (10 vol%) as sacrificial hole scavenger.

After that, the solution was degassed several times to

completely remove the air and irradiated by a 300 W Xenon

Lamp with a 420 nm cut-off filter for 4 h. The temperature of

the reaction system was maintained at room temperature with

flowing of water. The evolved H2 gas was detected by GC

instrument with thermal conductive detector (TCD) using high-

purity N2 gas as gas carrier. The apparent quantum yield (AQY)

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for H2 evolution was measured under different monochromatic

light. The AQY was calculated as following29:

AQY (%) =2 × number of evolved H2 molecules

number of incident photons× 100%

(1)

2.5. Photocatalytic degradation of RhB

The photocatalytic test was carried out to evaluate the RhB

degradation under visible light irradiation. In particular, 20 mg

of photocatalyst was added to 50 mL of RhB solution (10 mg L-

1). Prior to irradiation, the suspension was magnetically stirred

in the dark for 30 min to achieve the adsorption/desorption

equilibrium between the catalyst and RhB. After that, the

suspension was irradiated by a 300 W xenon lamp (PLS-

SXE300C, Perfect Light Limited, Beijing) with a 420 nm cutoff

filter provided visible light irradiation and the distance between

the light source and the sample was 12 cm. At certain time

intervals, aliquots of the suspension were collected and

centrifuged at 10,000 rpm for 15 min to remove the catalyst.

The concentrations of RhB were measured by UV-visible 6000

spectroscopy at wavelength of 554 nm.

2.6. RhB photodegradation in different water matrixes

The stability of MFCN-wet for degradation of RhB under visible

light irradiation with different water matrixes was carried out as

previous photocatalytic test but with different artificial water

environments (fresh, sea and produced water). To synthesize of

artificial fresh water, the prepared water was simulated to the

salts available in Yangtze River, China, and made with 0.44 mM

NaCl, 0.47 mM MgCl2, 0.7 mM CaCl2, 0.047 mM KCl.30 On the

other hand, the artificial sea and produced water were made as

described in the previous report.31 The artificial seawater was

made with 0.68 M NaCl, 0.013 M CaCl2, 0.019 M MgSO4 and 0.03

M MgCl2, whereas the artificial produced water was made with

1.27 M NaCl, 0.17 M CaCl2, 0.004 M MgSO4 and 0.018 M MgCl2.

3. Results and discussion

3.1. As-prepared materials characterization

The phase structure of as-prepared samples was investigated by

X-ray diffraction (XRD). As shown in Fig. 1a, the XRD pattern of

melem exhibits sharp peaks at different angles (12.2°, 13.4°,

25.9°, 26.4°, and 27.9°) which refers to melem (C6N10H6) units

and aligns with the previous reports.32–34 All PCN samples

exhibit two distinguished diffraction peaks which refer to

polymeric carbon nitride. MCN-air and MCN-N2 have strong

diffraction peaks at 27.36° and 27.44°, respectively, indicating

to (002) interplanar stacking. However, in regards to MFCN-wet,

the main diffraction peak is much weaker intensity which could

decrease the crystallinity and weaken the long-range order in

the atomic arrangement of MFCN-wet and slightly shifted to

27.78° which reflects a reduction in the stacking distance

between layer planes. This feature is represented formation of

ultrathin nanosheets.35 Furthermore, all as-prepared samples

exhibit weak peak at 13° indicting to (100) in-plane c-axis and

tri-s-triazine motif. It’s clearly obvious that this peak is

weakened for MFCN-wet implying to the decrease of planar size

of the layers.24

To investigate the functional groups on the obtained materials,

FTIR spectra were carried out. For melem spectrum, as shown

in the Fig. 1b, there are three sharp peaks at 1612, 1467, and

803 cm-1 ascribing to the characteristic absorption of melem.

Additionally, there are small two peaks at 3424 and 3466 cm -1

referring to stretching vibration of -N-H terminal amine group

due to the existence of partly condensed melamine in melem

oligomer.34,36,37 For as-synthesized PCN, the observed bands in

1200-1638 cm-1 region are attributed to typical stretching

vibration modes of either trigonal N-(C3) or bridging (-C-NH-C-)

in the s-triazine heterocyclic ring (C6N7) units. It is noticeable

that this region is more intensively for MFCN-wet which may

suggest the well-order in-plane structural packing motif.38

Furthermore, a new small peak appears at 1543 cm-1 which

pointing to aromatic nitroso group (-C-N=O) or nitro group (-C-

Fig. 1 (a) XRD pattern, and (b) FTIR spectra of melem and synthesized PCN in different atmosphere (wet, air and N2 atmosphere).


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NO2) stretching mode. While the peak at 807 cm-1 with blue

shifting for MFCN-wet at 814 cm-1 is assigned to out-of-plane

bending vibration of triazine rings. Moreover, new peak appears

at 1071 cm-1 in MFCN-wet originated from (-CO) stretching.

Clearly, compared with MCN-air and MCN-N2, MFCN-wet

exhibits broadened band between 2900 and 3600 cm-1 (the red

dashed rectangle), indicating to -NH and -OH stretching due to

the free terminal amino and hydroxyl groups. This broaden

band may be indicated of MFCN-wet with enlarged open-up

surfaces.39,40 Furthermore, the strong peak at 3070 cm-1 refers

to primary and secondary amines, which implies the formation

of melon-based carbon nitrides.41,42

XPS was performed to further investigate the chemical and

surface structure analysis of MFCN-wet. As displayed in Fig. 2a,

XPS survey spectrum of MFCN-wet clearly shows the presence

of strong N 1s peak at 398.58 eV and weak O 1s signal at 323 eV.

High resolution spectrum of C 1s could be fitted by three peaks

at 284.68, 288.1 and 293.78 eV corresponding to aromatic

carbon atoms, sp2-bonded carbon atom bonds to N within the

s-triazine ring, and π excitation, respectively (Fig. 2b). The

corresponding N 1s spectrum can be devaluated to four peaks

(Fig. 2c). The strongest one appears at 398.58 eV which assigns

to -C-N=C- nitrogen atoms within triazine rings, and additional

two peaks appeared at 400.1 eV and 400.98 eV are attributed

to N atoms bonded with H atoms or the central of N atoms

bridging between three s-triazine rings (N-C3 units) and amino

function group (-C-N-H), respectively. The weak peak at 404.28

eV could be ascribed to π excitation and nitro or nitroso

group.43,44 The presence of C-O and N-O bonds is confirmed by

the peaks at 532.9 and 531.9 eV, respectively, in the O 1s

spectrum (Fig. 2d).43,45

Elemental analysis of the as-prepared materials was conducted

to determine the elemental content of C, N and O percentage

and C/N ratios as shown in Table 1. Compared with MCN-N2 and

MCN-air, the elemental analysis quantitatively shows that the

average value of C/N mass or atomic ratio of MFCN-wet is

slightly decreased (0.664) attributed to uncompleted

Fig. 2 XPS patterns of MFCN-wet: (a) Survey pattern, (b) High resolution pattern of C 1s, (c) High resolution pattern of N 1s, and (d) High resolution pattern of O 1s.

Fig. 3 Nitrogen adsorption-desorption isotherms, the inset is the volume of same weight (100 mg) of MFCN-wet, MCN-air and MCN-N2.


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condensation of terminal amino group in s-triazine rings,

whereas the oxygen content increase (4.82%) confirming of

oxidation occurrence of amino group to nitro or nitroso group

or substitution with hydroxyl group.

Specific surface area of as-prepared materials was measured by

N2 adsorption-desorption measurements at 77 k. As clearly

shown from Fig. 3, The volume of the MFCN-wet nanosheets is

much larger than that of MCN-air and MCN-N2 with the same

weight, indicating the fluffy state of the nanosheets. MFCN-wet

exhibits a typical IV isotherm with a hysteresis loop at a relative

pressure (p/p0) in the range of 0.5-1 proofing the existence of

mesoporous. The specific surface area is calculated to be 96 m2

g-1 which is much larger 8.2 and 11.3 times than that of MCN-air

and MCN-N2, respectively (11.77 and 8.5 m2 g-1, respectively).

Fig. 4 (a) SEM morphology of MFCN-wet, b) TEM image, (c and d) High magnification of TEM and, (e) AFM image of MFCN-wet.

(a) (b)

(c) (d)

(e)

Table 1 Specific surface area, pore properties and elemental analysis of MFCN-wet, MCN-air and MCN-N2.

Sample

Nitrogen sorption analysis Elemental analysis

SBET (m2 g-1) Pore volume (cm3 g-1) Pore size (nm) C (wt%) N (wt%) O (wt%) C/N ratio (atomic)

MFCN-wet 96 0.167 7 33.816 59.442 4.824 0.664

MCN-air 11.77 0.0195 6.6 34.615 60.265 3.433 0.670

MCN-N2 8.5 0.0165 7.7 34.99 60.285 3.053 0.677


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The average of pore diameter is around 7 nm for all samples,

whereas the pore volume is much enhanced in MFCN-wet

(0.167 cm3 g-1) with 8.6 and 10.12 times higher than that of

MCN-air and MCN-N2, respectively (0.0195 and 0.0165 cm3 g-1,

respectively). The large specific area and pore volume of MFCN-

wet can efficiently promote the kinetics of photocatalysis

process due to introducing further active sites and enhancing

mass transfer.19,46 The BET results are shown in Table 1.

The morphology of as-prepared materials was characterized by

SEM and TEM. As depicted in Fig. 4, MFCN-wet appears as

nanoflakes with laminar morphology (Fig. 4a). Meanwhile,

MCN-air and MCN-N2 have a blocky morphology with

agglomeration of irregular lamellar structures (Fig. S2). Clearly,

TEM image of MFCN-wet (Fig. 4b) shows formation of 2D velvet-

like nanoarchitecture with curled edges and low degree of

polycondensation. Further magnification of TEM (Fig. 4c and d)

reveals that MFCN-wet exhibits uniformly ultrathin nanosheets

with high wrinkled and rippled edges which this is likely due to

the role of water and elevated gases to weaken the interaction

between the packing layers and cracking the hydrogen bonds

between melon units. On the other hand, TEM images of MCN-

air and MCN-N2 show blocky morphology with agglomerates of

lamellar nanosheets structure and sharp edges (Fig. S2).

Further, the images prove the formation of long-order

condensation of melon frameworks and further pores in the

MCN-air nanosheets. For the insight into the thickness of MFCN-

wet nanosheets, atomic force microscopy (AFM) was acquired

as shown in Fig. 4e and further confirms the ultrathin

nanosheets formation. The thickness fluctuation throughout

the nanosheet determined with the average thickness ~3.8 nm.

According to the previous data, we can hypothesize that there

are three main paths during the process: polycondensation,

oxidation and delamination. High temperature here (550 °C)

plays an essential role for condensation of melem segments to

melon. Whereas, water molecules and ammonia atmosphere

(resulting from melem condensation) play another role. Water

molecules attack C-N bond in uncondensed terminal -NH2 in tri-

s-triazine and deamination with hydroxyl group. Furthermore,

it is assumed that some of the amino group may be oxidized to

nitro and nitroso groups under wet atmosphere at high

temperature. Additionally, water molecules, during melem

condensation, attack and break hydrogen bonds between tri-s-

triazine units which in turn forming short-turn order of the

interplanar structure packing and the ultrathin nanosheets

where the MFCN-wet become more loosen. Released ammonia

represents an important role in enhancing specific surface area

and pore volume owing to its etching ability to nanosheets.47

After cooling and open the tube furnace, the smell of ammonia

was clearly detected and there were crystals sublimated on the

isolated plug as shown in Fig. 5 which assumes formation of

urea aerosol crystals. The formation of urea crystals could be

explained as previously reported in the following equations48,49:

2NH3 + CO2 ↔ NH2COONH4 (2)

NH2COONH4 + heat ↔ NH2CONH2 +H2O (3)

The overall process can be summarized and illustrated in the

Fig. 5 and wet nitrogen atmosphere flow rate is shown as view

video in Video S1.

3.2. Optical and electrochemical properties

To investigate the electronic structure and photoelectronic

properties of as-prepared materials, The UV-visible reflectance

spectroscopy (DRS) and photoluminescence spectra (PL) were

measured. In DRS measurement, as shown in Fig. 6a, the

Fig. 5 Schematic illustration of melem-derived foam-like PCN nanosheets synthesis process (scale bar is 200 μm).


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absorption edges of MCN-air and MCN-N2 appear at about 470

and 473 nm, respectively. In regards to MFCN-wet, the

absorption edge exhibits a remarkable blue shift at ca. 420 nm

due to the decrease of layers thickness.21 Furthermore, this shift

is consistent with white colour and low polycondensation

degree. Moreover, the absorbance curve of MFCN-wet shows a

new peak at ca. 310 nm which confirms that the wet

atmosphere can greatly improve the ability of MFCN-wet to

harvest light.17 The absorbance curve at ~368 nm is ascribed to

π*-π transitions that is commonly observed in conjugated rings

in the heterocyclic aromatics.50 Consequently, the derived

electronic band gaps (Eg) are obtained from plotting of

transformed Kubelka–Munk function (F(R)hν)1/2 versus the

exciting light energy (hν) (Fig. 6b). The band gaps are estimated

to be 2.59, 2.61 and 2.95 eV for MCN-N2, MCN-air and MFCN-

wet, respectively. The larger band gap of MFCN-wet (2.95 eV) is

associated with the well-known quantum confinement effect

caused by smaller crystalline domains. This larger band gap

increases the redox ability of charge carriers generated in the

nanosheets.19,51,52 As shown in Fig. S3, the electrochemical

Mott-Schottky plot of MFCN-wet measured at different

frequencies shows the typical n-type semiconducting character

owing to the positive slope of the linear plots. Furthermore, the

derived flat-band potential for MFCN-wet nanosheets is about -

1.44 V (vs Ag/AgCl), which thermodynamically endows them the

ability for photocatalytic reduction of water (H+/H2: -0.59 V vs.

Ag/AgCl).53 Because the flat-band potential of n-type

semiconductors is approximately equal to the lowest potential

of the conduction band (CB).54 Thus, The CB potential of MFCN-

wet is calculated to be -1.22 V versus the normal hydrogen

electrode (NHE). According to the band gap of MFCN-wet, the

valence band then could be deduced to be 1.73 V versus NHE.

PL emission spectra was used to investigate the separation,

transfer and recombination of processes of photogenerated

charge carrier. As shown in Fig. 6c, PL spectra, at the excitation

wavelength of 325 nm, exhibit a broad emission band for both

MCN-air and MCN-N2 with the centre of PL spectra at ~460 nm.

For MFCN-wet, it is clearly observed the blue shift and narrow

of its PL spectrum at ~433 nm. This shift may be due to the

reduction of polycondensation degree of MFCN-wet compared

with MCN-air and MCN-N2. To further understand the overall

transition pathways in the MFCN-wet sample, Gaussian fitting

of the PL peak is plotted (Fig. S4). The fitting shows that there

are intrinsically ternary excitation emission pathways centred at

430, 451 and 479 nm. It is worth to be mentioned that the

bandgap states of PCN consist of σ band of sp3 C-N, π band of

Fig. 6 (a) UV-visible diffuse reflectance spectra, (b) The corresponding band-gap plots, (c) Photoluminescence spectra, and (d) EIS Nyquist plots of MFCN-wet, MCN-air and MCN-N2.


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sp2 C-N and lone pairs (n) state of the bridge nitride atom.55 The

three excitation emissions at 430, 451 and 479 nm are

originated from the three different pathways of transitions: σ*-

n, π*-n, and π*-π, respectively.50,55,56

Electrochemical impedance spectroscopy (EIS) is a valid

electrochemical approach to illustrate the efficiency of

interfacial charge transfer performance. Generally, the smaller

of arc radius of semi-circular EIS Nyquist plot, the lower of

electron transfer resistance. As shown in Fig. 6d, the semicircle

arc of MFCN-wet is much smaller than that of MCN-air and

MCN-N2, implying the inhibition of interfacial electron transfer

resistance on the electrode surface, which confirms that the

MFCN-wet nanosheets can boost the photogenerated charge

separation and transfer via active edges and hence enhance the

photocatalytic activity.

3.3. Photocatalytic activity

The photocatalytic water splitting to produce H2 gas under

visible light illumination (λ ≥ 420 nm) was investigated to

evaluate the performance of as-synthesized materials. TEOA

was used as sacrificial hole scavenger and 3 wt% Pt was used to

enhance H2 evolution process. As indicated in Fig. 7a, MFCN-wet

nanosheets exhibit superior H2 evolution activity in comparison

with MCN-N2 and MCN-air. The highest HER rate after 4 h is

reported, as shown in Fig. 7b, for MFCN-wet (1880.2 μmol g-1 h-

1), which is 5.53 times higher than that of MCN-N2 (339.75 μmol

g-1 h-1) and 9.66 times higher than that of MCN-air (194.69 μmol

g-1 h-1). It’s clearly observed that the wet atmosphere

tremendously boosts the activity of melem-derived nanosheets

for H2 evolution. Furthermore, as illustrated in Fig. S5, the

apparent quantum yield (AQY) of MFCN-wet was calculated to

be 8.74% at 365 nm, 5% at 380 nm, 1.3% at 420 nm and 0.51%

at 450 nm, which is well matched with the UV-visible absorption

spectrum of MFCN-wet nanosheets. The stability of materials in

photocatalytic process is critical issue in the large-scale

applications. As demonstrated in Fig. 7c, MFCN-wet was

examined with prolonged exposure for visible light irradiation

under the same conditions for 4 sequential runs, and no obvious

deterioration is observed after 16 h of irradiation indicating the

high stability of the photocatalyst. The slight decline after 16 h

may be attributed to decrease of sacrificial agent since no

additional amount of TOEA was added to the reaction system.

The preservation of MFCN-wet nanosheets morphology after 4

recycling tests was confirmed by SEM analysis in Fig. S6.

To evaluate photocatalytic activity of MFCN-wet for dyes

degradation under visible light irradiation, the photocatalytic

degradation of RhB was investigated as an example. As

described in Fig. 8a, after adsorption equilibrium, the MFCN-

wet sample has ability to adsorb about 12% of RhB, and the

photodegradation efficiency is significantly enhanced where

most of RhB molecules decompose after 20 min (98.22%) and

photodegrade completely in 30 min and further confirmed by

time-dependent UV-visible absorption spectra as evidenced in

Fig. S7, whereas the MCN-air and MCN-N2 samples exhibit low

photodegradation efficiency (26.23% and 15.5%, respectively,

after 30 min), implying the excellent photocatalytic

performance of MFCN-wet. Furthermore, to study the kinetic of

RhB degradation and related reaction constant (k), the obtained

photocatalysis data were fitted with pseudo-first-order as

shown in the following equation:

‒ ln(C/C0) = kt (4)

Where C0 is the initial concentration of RhB (mg L-1) and C is the

remaining concentration after irradiated time t (mg L-1), and k is

the corresponding kinetic rate constant (min-1). From the Fig.

8b, it is clearly observed that the rate constant (k) of MFCN-wet

is much higher than MCN-air and MCN-N2. The k value of MFCN-

wet (0.15467 min-1) is 16 times higher than MCN-air (0.00966

min-1) and 28.9 times higher than MCN-N2 (0.00536 min-1),

which attributed to the enhanced separation efficiency of

photogenerated species and the higher surface area of the

photocatalyst.

To explore the ability of MFCN-wet for mineralization of RhB,

the TOC removal rates of RhB were conducted with TOC

analyser (Fig. S8). The results show that the TOC removal rate

exhibits the faster mineralization degree during 10 min with

removal rate of 35% and further mineralized with slow rate up

Fig. 7 (a) Hydrogen generation per gram of as-prepared materials under visible light irradiation (λ ≥ 420 nm) in aqueous solution with TEOA as sacrificial hole agent and Pt as cocatalyst, (b) Hydrogen evolution rate (HER) comparison between as-prepared materials, (c) The stability of MFCN-wet for H2 production for prolonged visible light irradiation under the same conditions (4 runs, 16 h).


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to 49% after protracted irradiation (60 min), suggesting that the

RhB may be further mineralized posterior to longer irradiation

times.

The reusability of MFCN-wet is also evaluated and five

consecutive photocatalytic runs are measured. Due to the

significant low density of MFCN-wet (29 mg cm-3), compared

with MCN-air (372 mg cm-3) and MCN-N2 (362 mg cm-3), and

high dispersity in water, amount of the photocatalyst lost during

repeat washing by water is inevitable, so we resort to a simple

method depending on continuous degradation of RhB under

visible light for certain time (about 1.5 h) to ensure the

complete degradation and then evaporate the remain solution

and finally dry the run sample in the oven. As seen in Fig. 8c, the

photoactivity of MFCN-wet for RhB degradation demonstrates

high stable and reusability performance. After 5 experiment

runs, no significant deactivation of the photocatalyst is

observed, which can keep more than 95% degradation

efficiency during 30 min, suggesting the high ability to apply in

the practical fields. The stability of MFCN-wet nanosheets

morphology after 5 runs of RhB degradation was further

confirmed by SEM analysis in Fig. S9.

To elucidate the function of the reactive species generated in

the photocatalysis process of RhB degradation, electron spin

resonance (ESR) trapping technique was employed using 5, 5-

dimethyl-1-pyrroline N-oxide (DMPO) as radical trapper with

MFCN-wet photocatalyst. As shown in Fig. S10, no ESR signal is

observed in both DMPO-•O2- and DMPO-•OH in the dark. While,

after 12 min of visible light irradiation, the ESR signal of DMPO-•O2

- in methanol solution with 6 crack peaks is clearly observed,

indicating that the •O2- radical is produced during the

photocatalytic process. Besides, four-cracked ESR signal of

DMPO-•OH in aqueous solution is also observed with an

intensity ratio of 1:2:2:1, suggesting the formation of •OH

radical in the photocatalytic process.

To further detect the dominant radicals during the RhB

photocatalytic degradation process, scavenger experiments

were carried out with the addition of p-benzoquinone (BQ, 1

mM), triethanolamine (TEOA, 1 mM), and isopropanol (IPA, 10

mM), separately, as the scavenger of superoxide (•O2-), hole

(h+), and hydroxyl (•OH) radicals, respectively. As illustrated in

Fig. 8d, the addition of BQ exhibits the highest suppression of

RhB degradation rate (44.37%) implying that the dominant

oxidative species in the photodegradation process is (•O2-)

radical. Additionally, TEOA inhibits the RhB degradation rate

(70.5%), but less than BQ, suggesting that photogenerated h+

radical is the second main reactive species in photocatalysis

process. Whereas IPA exhibits the lower trapped effect on

Fig. 8 (a) Degradation efficiency of RhB under visible light irradiation with MFCN-wet, MCN-air, MCN-N2 and without irradiation, (b) First-order kinetic plots for the degradation, (c) Recyclability of MFCN-wet for the degradation of RhB under visible light irradiation, (d) Photocatalytic activity of MFCN-wet for the degradation of RhB in the presence of different scavengers.


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photodegradation of RhB (96.36%) showing the hydroxyl radical

(•OH) is the minor reactive species.

To investigate the potential of MFCN-wet nanosheets for

applying in the real scale, we have simulated the different water

environments including fresh, sea and produced water. As

shown in Fig. S11, MFCN-wet exhibits high ability in

photodegradation of RhB under low and high ionic strength

(fresh and seawater, respectively) and no significant decline in

degradation rate. After 30 min, the photodegradation efficiency

of RhB slightly declines to 98.32% and 98.11% for fresh and

seawater, respectively, affirming the potential of MFCN-wet to

apply under high ionic strength. In contract, under harsh ionic

strength, the photodegradation rate of RhB detrimentally

decreases to 46.14% in 30 min and 80.22% in 60 min due to the

massive saline concentration in produced water. The

probability of this inhibition may be caused by the competitive

adsorption between extreme-concentrated salts and RhB

molecules on the surface of MFCN-wet.57 Furthermore, the

interaction between the component ions of the salts and

photogenerated species may play another role in the inhibition.

For example, chloride ions (Cl-) and sulphate ions (SO42-) may

penetrate through the surface of nanosheets and scavenge of

oxidizing agents such as holes, causing the decline of RhB

photodegradation.58

3.4. Photocatalytic mechanisms

According to the above results, the overall reaction mechanism

of H2 evolution and RhB degradation over MFCN-wet under

visible light irradiation is illustrated in Fig. 9a. For H2 evolution

mechanism, the incident irradiation is absorbed by nanosheets

and excites the electrons, where transfer from the valence band

to the conduction band, meanwhile, the positive holes remain

in the valence band. The excited electrons migrate from the

conduction band to the surface of the cocatalyst Pt due to its

electron-sink function,59 where the H2O/H+ is reduced to H2

molecules. While the holes remaining on the valence band will

react and be consumed by the sacrificial agent (TEOA).

Furthermore, high surface area and coordinating terminal

group including nucleophilic groups may attribute to enhance

the hydrogen generation rate of MFCN-wet nanosheets through

well-attached with Pt4+, where the negatively charged surface

of MFCN-wet was confirmed by zeta analyser where zeta

potential ranges between -26.2 and -37.9 mV.

For RhB photodegradation mechanism, after absorption of the

photons from light source, the electron excitation from valence

bands to conduction bands, forming electron-hale pairs. The

photogenerated electron with higher reduction ability could

reduce the oxygen molecules to superoxide radicals (•O2-) which

in turn degrade and mineralize the RhB molecules (Fig. 9a). On

the other hand, the photogenerated holes transfer rapidly to

the surface of the nanosheets as oxidizing agent and oxidize the

RhB molecules (Fig. 9a). According to the band structure of

MFCN-wet as mentioned above, the potential of valence band

is lower than that of •OH/H2O (2.27 eV) and •OH/OH- (1.99 eV).

Thus, the photogenerated holes on the MFCN-wet surface could

not oxidize H2O or OH- adsorbed on the surface of nanosheets

to produce (•OH) radicals.60 Thus, the (•OH) radicals can be

produced from the superoxide radicals.

It is worth to be mentioned that RhB degradation over the

photocatalyst occurs via two mechanisms: N-deethylation and

decomposition of the conjugated xanthene ring in RhB.

According to time-dependent UV-visible absorption spectra

(Fig. S7), there is no hypsochromic shift during 10 min indicating

to the highly efficient of MFCN-wet to destruction of xanthene

ring and RhB mineralization. After that, the absorption shift is

observed after 20 min referring to the formation of N-

deethylation species and decrease the mineralization rate until

it is completely degraded after 30 min.61 These results are

further confirmed by and aligned with TOC analysis (Fig. S8).

Furthermore, it is hypothesized that introducing of nucleophilic

groups such as hydroxyl and amino groups on the surface of

PCN nanosheets may play an important role in further

degradation of RhB through electrostatic attraction between

these groups and the cationic molecule and the adsorption of

RhB molecules on the surface by the hydrogen bond (Fig. 9b).

The negatively charged surface of the nanosheets confirmed by

zeta potential emphasizes the suggestion of RhB adsorption on

the surface of nanosheets through electrostatic attraction.

Additionally, we hypothesize that the formed nitroso or nitro

groups over the nanosheet surface as electron withdrawing

may have the ability to capture electron and enhance the

photogenerated separation. In addition, specific surface area

Fig. 9 (a) Schematic illustration for hydrogen evolution and photodegradation of RhB over MFCN-wet under visible light irradiation, (b) Schematic illustration of the proposed electrostatic attraction and adsorption mechanism of RhB with hydroxyl and amino groups.


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plays another important factor affecting on RhB degradation. By

normalizing with degradation rate of as-prepared materials

after 30 min (Fig. S12), it is manifestly observed that surface

area increasing of MFCN-wet dramatically enhances the RhB

degradation compared with MCN-air and MCN-N2 due to

increasing of active sites. Moreover, the higher surface area can

not only present more abundant photocatalytic active centres,

but also enhances the probability of adsorption and diffusion

process of RhB on the surface of nanosheets.60

The overall reaction of photocatalytic process of H2 evolution

and RhB degradation can be simplified as in the following

equations:

MFCN-wet + hν → MFCN-wet (h+ + e-) (5)

2H+ + 2e- → H2 (6)

TEOA + h+ → TEOA+ (7)

e- + O2 → •O2- (8)

RhB + •O2- → Degraded and mineralized products (9)

RhB + h+ → RhB+ → Degraded and mineralized products (10)

Summing up, the superior photocatalytic activity of MFCN-wet

over MCN-air and MCN-N2 could be summarized in main points:

(1) the geometric structure of MFCN-wet (short-order

polymerization and ultrathin layers), compared with bulky

structure of MCN-air and MCN-N2, promotes the visible light

harvesting and fosters the photogenerated charge separation

and transfer (2) higher surface area and extremely light

characteristic of MFCN-wet nanosheets enhance the

photocatalytic process through expanded area and well-

dispersion of nanosheets and increase the probability of

interaction with more active sides (3) appearance of new

terminal groups as active sides including hydroxyl and nitro

groups boosts the photocatalytic performance via adsorption

and attraction and raises electron–hole transfer efficiency.

By comparison with previous metal-free carbon nitride

synthetic processes, as shown in Table S1, the present PCN

synthesis does not only show the green advance of ultrathin

PCN nanosheets synthesis with less consumption of time,

chemicals and energy, but also shows high efficiency with enrich

and diverse active sites endowing it the ability to scale up. Such

fascinating properties meet the standard criteria of sustainable

approach.

4. Conclusion

In conclusion, foam-like PCN ultrathin nanosheets from melem

has been successfully prepared in wet atmosphere via “three-

in-one” strategy without any other additives. The derived

material exhibits loosen and very lightweight features. MFCN-

wet shows higher photocatalytic activity for H2 evolution than

that of MCN-N2 and MCN-air. Furthermore, MFCN-wet

nanosheets demonstrates high stability of H2 evolution rate

over 4 runs within 16 h. The ultrathin nanosheets display

extraordinary degradation of RhB with high degradation rate in

different water matrixes and high recyclability. The activation of

the nanosheets edge with active groups via wet atmosphere

significantly contributes in boosting of the photocatalytic

process. These findings offer clear evidence that the foam-like

PCN nanosheets from melem under wet atmosphere has the

potential to be applied in the large-scale applications and can

be employed as green platform for further modification to

cover broad and diverse environmental and energy fields.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

The authors gratefully acknowledge support from National

Natural Science Foundation of China (No. 41671488, 51878465

& 21707103）and National Key R and D Program of China (No.

2016YFE0123800). The first author thanks the support of

Marine Scholarship of China.

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