4
92
D. Zhao et al. / Journal of Catalysis 352 (2017) 491–497
photocatalytic enhancement could be attributed to the small thick-
ness of CNNS, rendering promoted charge transfer and separation.
However, these CNNS with thickness of 2–3 nm still contain 6–9
Bruker Vextex 70 FTIR spectrometer using the KBr pellet technique.
The synchrotron X-ray spectroscopic (XAS) measurements at C and
N K-edge were performed at BL20A, at the National Synchrotron
1
CAN atomic layers (theoretical value of monolayer g-C
3
N
4
is
Radiation Research Center, Taiwan. Solid-state
H
cross-
0
.33 nm [36]), which led to confined charge transport in CNNS,
polarisation magic angle spinning nuclear magnetic resonance
(CP-MAS-NMR) spectra were recorded by a Bruker Avance 400
spectrometer operating at 12 KHz resonance frequency. X-ray pho-
toelectron spectroscopy (XPS) data were obtained on a Kratos Axis-
limiting the further improvement of photocatalytic activity. It
could be thus expected that the CNNS with smaller thickness,
namely, less atomic layers, could exhibit more efficient photocat-
alytic hydrogen evolution. Recently, some chemical exfoliation
strategies have been developed to synthesize ultrathin and even
monolayer CNNS, which achieved significantly enhanced photocat-
alytic performance for hydrogen evolution as expected [31,37,38].
However, these developed chemical methods are still facing prob-
lems of long time and/or multiple-step synthesis procedure as well
as relatively low product yields (<10%). Thus, it is still essential to
develop a facile, rapid and scalable method to synthesize ultrathin
CNNS with high photocatalytic activities.
Ultra DLD instrument with a monochromatized Al K
a line source
(150 W). All the binding energies were referenced to the C 1 s peak
at 284.8 eV. The steady-state photoluminescence (PL) emission
spectra and time-resolved transient PL decay spectra were carried
out at room temperature using a PTI QM-4 fluorescence spec-
trophotometer. The charge decay kinetics calculation methods
were depicted in Supplementary material (Decay kinetics calcula-
tion methods). Electron paramagnetic resonance (EPR) experi-
ments were performed on a Bruker EMX X-band spectrometer
Herein, by breaking and then repolymerizing the heptazine
units in BCN, we propose a two-step ultrasonication-calcination
method to prepare ultrathin CNNS with thickness of only 1 nm (3
atomic layers) in relatively high product yield (ꢀ24%). Remarkably,
with the unique disordered layer structure for interlayer tunneled
charge transport and broken in-plane CAN bonds in CNNS for effi-
cient electron excitation, the resultant 1 nm-thick ultrathin CNNS
exhibit excellent activity and stability for photocatalytic hydrogen
evolution under visible light.
and microwave frequency = 9.40 GHz at room temperature. N
2
adsorption-desorption isotherms were conducted at 77 K using
an Accelerated Surface Area and Porosimetry Analyzer (ASAP
2020, Micromeritics) after degassing the samples at 150 °C for
4 h. The specific surface areas were determined by the Brunauer-
Emmett-Teller (BET) methods. UV–vis diffuse reflectance spectra
were recorded on a Cary Series UV–vis-NIR spectrophotometer
(Agilent Technologies).
2.4. Photocatalytic measurements
2
. Experimental
Photocatalytic hydrogen evolution reactions were performed in
2
.1. Synthesis of bulk g-C
3
N
4
a 230-mL gas-closed Pyrex reactor. In a typical photocatalytic reac-
tion, 50 mg photocatalysts were dispersed in 200 mL aqueous solu-
tion of triethanolamine (TEOA, 10 vol%) by a magnetic stirrer with
constant rotational velocity. 1 wt% Pt as cocatalyst was in situ pho-
All chemicals in the present study are of analytical grade and
used as received without further purification. Deionized water
was used throughout the experiments. Bulk g-C (BCN) was syn-
3
N
4
todeposited on the photocatalyst from the precursor of H
2 6
PtCl -
thesized by the well reported thermal polymerization method [39].
Typically, 2 g of melamine was put into an alumina crucible with a
cover, and calcined at 520 °C in air for 4 h with a ramping rate of
Á6H O. Nitrogen was purged through the reactor for 20 min
2
before reaction to remove the residual air. A 300 W Xe lamp with
a 420 nm cutoff filter was used to trigger the photocatalytic hydro-
gen generation. The temperature of the reaction solution was kept
at 35 °C via a circulating water pump during the whole experiment.
Evolved hydrogen was measured through a thermal conductivity
detector (TCD) gas chromatograph (NaX zeolite column, nitrogen
as the carrier gas). Blank experiments revealed no appreciable
hydrogen evolution without irradiation or photocatalysts. The
measurement and calculation of apparent quantum yield (AQY)
and turnover number (TON) was depicted in details in Supplemen-
tary material.
5
°C/min.
2
.2. Synthesis of g-C
3
N
4
nanosheets
4
nanosheets (denoted as CN-UC, where U
The ultrathin g-C
3
N
means ultrasonication and C means calcination) were synthesized
by an ultrasonication-calcination two-step process. In detail, 0.5 g
of as-prepared BCN was added into 200 mL of deionized water in
a 250 mL glass beaker. After high-powered ultrasonication treat-
ment (900 watts) under stirring for 2 h, the obtained product
was centrifugalized and dried in air at 80 °C for 8 h. The resultant
powder, denoted as CN-U, was collected and calcined at 520 °C in
air for 4 h with a ramping rate of 5 °C/min. Then the light yellow
powder of CN-UC was obtained. The quantity of as-prepared CN-
UC was approximately 0.12 g, which corresponds to a high product
yield of 24%. The reference sample of CN-C was acquired by a direct
calcination of BCN using the same temperature.
3
. Results and discussion
Starting from BCN prepared by thermal polymerization of mel-
amine [39], ultrathin CNNS were successfully obtained via an
ultrasonication-calcination two-step process (see Experimental
Section for details and Fig. S1 for preparation process in Supple-
mentary material). As shown in Fig. 1, the as-prepared BCN, aggre-
gated as large thick sheets (Fig. 1a), was shattered and broken into
small and thin pieces (Fig. 1b) through the first ultrasonication step
2.3. Characterization
(
denoted as CN-U, U means ultrasonication). Interestingly, the fol-
Transmission electron microscopy (TEM) images were recorded
lowing calcination step then induced the repolymerization of small
CN-U pieces into ultrathin CNNS (denoted as CN-UC, where U and
C means ultrasonication and calcination, respectively) with thick-
ness of only 1 nm (3 atomic layers) (Fig. 1c and d). In comparison,
much thicker CNNS were obtained via a thermal oxidation exfolia-
tion process by calcination of BCN in air (denoted as CN-C), which
has larger thickness of 3 nm (9 atomic layers) (Fig. 1e and f), agree-
ing well with the previous study [32]. Moreover, CN-UC was
on a FEI Tecnai G2 F30 S-Twin transmission electron microscope at
an accelerating voltage of 300 kV. Atomic force microscopy (AFM)
images were obtained from a NT-MDT Solver Next atomic force
microscope. X-ray diffraction (XRD) patterns were collected on a
PANalytical X’pert MPD Pro diffractometer operated at 40 kV and
4
0 mA using Ni-filtered Cu K
a irradiation (Wavelength = 1.5406 Å).
Fourier transform infrared (FTIR) spectra were recorded on a