G Model
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D. Ni et al. / Chinese Chemical Letters xxx (2019) xxx–xxx
protonation/deprotonation process during the conventional ther-
mal condensation [40]. However, the high cost or/and the
complicated multiple processes hindered the high synthetic yields
and its large-scale applications.
showed a much narrowed FWHM of (002) peak and increased
relative intensity ratio of peak (002) and peak (100), suggesting an
improved stack along the c-axis. Other ramping rates, namely
ꢀ
increasing temperature from room temperature to 550 C in 1 h
Herein, we report a highly effective and facile way to directly
regulate the crystallinity of bulk CN merely by a faster ramping rate
in thermal condensation. It was revealed that the promoted
condensation kinetics had a profound favorable influence on the
crystallinity of the as-prepared bulk CN and the photocatalytic
activities as well. No additional chemicals or other complements
were needed, the modified condensation recipe was even simpler
than the conventional one, but led to an improved photocatalytic
and 2 h, were also investigated. It was found the trend of the higher
ramping rate the more narrowed FWHM of (002) peak was not
changed (Fig. S1a in Supporting information). Moreover, the
ꢀ
different holding time at 550 C were also studied. As shown in
Fig. S1b in Supporting information, the FWHM of (002) peak did
not altered as evidently, indicating that compared to engineer the
ꢀ
ramping rate, the holding time at 550 C during the polymerization
had a less influence on the polymerization kinetics.
H
2
evolution up to 3 times. This work would be promising to
To identify the chemical structure of the obtained different
carbon nitrides, Fourier transform infrared (FT-IR) spectra was
undertook (Fig. 1d and Fig. S2 in Supporting information). All
greatly pave the large-scale industrial application of bulk CN in
sustainable energy, environmental and biosensing applications.
CN was generally synthesized from dicyandiamide (DCDA) by
thermal condensation under 550 C (Fig. 1a) [41]. Previous reports
demonstrated that tuning the condensation temperature would
introduce more defects and improve the photocatalytic activity
ꢁ1
samples showed the sharp peak at 807 cm , which belonged to
the breathing mode of tri-s-triazine system; meanwhile the peaks
ꢀ
ꢁ1
between 1200 and 1700 cm , characteristic of C
¼N and C ꢁꢁ N
stretching vibration modes, were not changed, which suggested
the pristine conjugated structure of CN was preserved. The
elemental analysis (Table S1) showed that the C/N atomic ratio
and the N content for both CN-4 h-4 h and CN-20 min-4 h were
almost same. Besides, the UV–vis spectra of CNs also kept almost
identical (Fig. S3 in Supporting information), which indicates that
the electronic properties and the light absorb ability of these CNs
were similar. Therefore, the different CNs had similar tri-s-
triazine-based chemical structures and electronic structures, but
graphitic crystallinity became higher when increasing the ramping
rate.
[
42]. In this study, we argued that the intermediate reactions
during the heating process (as shown in Fig. 1b) would also affect
the final structure of the as-obtained CN. To confirm this
hypothesis, different CN was firstly synthesized from DCDA
through changing the ramping rate and then holding at 550 C
for 4 h in air atmosphere. The ramping time were 4 h, 2 h, 1 h,
ꢀ
2
2
0 min, in which the obtained CN are denoted as CN-4 h-4 h, CN-
h-4 h, CN-1 h-4 h, and CN-20 min-4 h, respectively.
In the first set of experiments, the crystal structure of CN
obtained at the different ramping rates were characterized by X-
ray diffraction (XRD) as shown in Fig. 1c. CN-4 h-4 h gave two
As rapid ramping rate may result in a quick gas (e.g. NH
product release, which was supposed to generate higher porosity,
the N adsorption–desorption isotherms of obtained CNs were
3
) by-
ꢀ
ꢀ
typical diffraction peaks at around 13.2 and 27.4 with a good
agreement with previous reports, which were originated from the
in-plane structural packing motif and periodic stacking of layers
along the c-axis, respectively. The CN-20 min-4 h synthesized by
high ramping rate exhibited similar XRD patterns, but the
2
further measured. As shown in Table S1 and Fig. S4 (Supporting
information), The Brunauer-Emmett-Teller (BET) surface area of
2
CN-20 min-4 h was calculated to be 17.86 m /g, which is ca. twice
ꢀ
2
diffraction peak at 27.4 was much sharper and more pronounced.
of that of CN-4 h-4 h (8.68 m /g). It confirmed that CN-20 min-4 h
According to the Debye-Scherrer formula, smaller full-width at
half maximum (FWHM) indicating larger grain size. For further
comparison, the full-width at half maximum (FWHM) and the
relative intensity ratio of (100) and (002) peak for the different CNs
are summarized and listed in Table S1 (Supporting information). It
is noted that by increasing the ramping rate, the resultant CN
with the high ramping rate exhibited higher surface area. The
scanning electron microscope (SEM) images were also undertaken
to get the morphology information of the CNs (Fig. 2). It was found
the as-obtained CNs exhibited similar morphology, i.e. lamellar
particles with rough surface, and CN-20 min-4 h exhibit slightly
larger particles than that of CN-4 h-4 h from the SEM image.
Fig. 1. (a) Scheme of general thermal condensation processes for idealized bulk CN and (b) typical heating programs for bulk CN. (c) XRD patterns and (d) FT-IR spectra of CN-
2
0 min-4 h and CN-4 h-4 h.
Ni, Dongya
