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rsc.li/sustainable-energy
Sustainable Energy & FuelsInterdisciplinary research for the development of sustainable energy technologies
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rsc.li/sustainable-energy
Volume 1 Number 1 2017 Pages 1–100
Sustainable Energy & FuelsInterdisciplinary research for the development of sustainable energy technologies
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This article can be cited before page numbers have been issued, to do this please use: I. A. Abdelhafeez,
Q. Yao, C. Wang, Y. Su, X. Zhou and Y. Zhang, Sustainable Energy Fuels, 2019, DOI:
10.1039/C9SE00263D.
Journal Name
ARTICLE
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Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
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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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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