Optimizing the crystallization process of conjugated polymer photocatalysts to promote electron transfer and molecular oxygen activation

Article abstract of DOI:10.1016/j.jcat.2020.07.005

Photocatalytic reactive oxygen species (ROS)-induced reactions provide an appealing method to solve the environmental and energy issues, whereas the current oxidation reaction generally suffered from low efficiency and poor selectivity due to uncontrollable O₂ activation process. In view of the existence of competitive electron and energy transfer pathway, we propose that highly efficient superoxide radical anion (·O₂^?) generation can be achieved by optimizing the order degree of the photocatalyst. Herein, by taking carbon nitride polymer as an example, we optimized the crystallization process of carbon nitride polymer by selecting precursors of different polymerization degrees with a molten salt method. Benefiting from the high crystallinity, extended π-conjugated system and strong van der-Waals interactions between interlayers, the modified carbon nitride polymer exhibited accelerated charge transport and enhancement in electron induced molecular oxygen activation reactions under visible light. Consequently, the CCN-P exhibits about 1.5 times higher conversion rate in hydroxylation of phenylboronic acid and over 6-fold faster degradation rate in Rh B organic pollutants photodegradation with respect to pristine carbon nitride. This study provides an in-depth understanding on the optimization of the O₂ activation process and the design of advanced photocatalysts.

Full text of DOI:10.1016/j.jcat.2020.07.005

Journal Pre-proofs
Optimizing the Crystallization Process of Conjugated Polymer Photocatalysts
to Promote Electron Transfer and Molecular Oxygen Activation
Wei Ren, Jiajia Cheng, Honghui Ou, Caijin Huang, Masakazu Anpo, Xinchen
Wang
PII:
S0021-9517(20)30279-7
https://doi.org/10.1016/j.jcat.2020.07.005
YJCAT 13817
DOI:
Reference:
To appear in:
Journal of Catalysis
Received Date:
Revised Date:
Accepted Date:
1 June 2020
3 July 2020
4 July 2020
Please cite this article as: W. Ren, J. Cheng, H. Ou, C. Huang, M. Anpo, X. Wang, Optimizing the Crystallization
Process of Conjugated Polymer Photocatalysts to Promote Electron Transfer and Molecular Oxygen Activation,
Journal of Catalysis (2020), doi: https://doi.org/10.1016/j.jcat.2020.07.005
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Optimizing the Crystallization Process of Conjugated Polymer Photocatalysts
to Promote Electron Transfer and Molecular Oxygen Activation
Wei Ren, Jiajia Cheng,* Honghui Ou, Caijin Huang, Masakazu Anpo* and Xinchen Wang*
State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry,
Fuzhou University, Fuzhou 350116, P. R. China, http://wanglab.fzu.edu.cn.
E-mail: xcwang@fzu.edu.cn; anpo@chem.osakafu-u.ac.jp; jjcheng@fzu.edu.cn.
This article is dedicated to, and in memory of, the late Prof. Michel CHE.
Abstract: Photocatalytic reactive oxygen species (ROS)-induced reactions provide an appealing
method to solve the environmental and energy issues, whereas the current oxidation reaction
generally suffered from low efficiency and poor selectivity due to uncontrollable O activation
2
process. In view of the existence of competitive electron and energy transfer pathway, we
−
propose that highly efficient superoxide radical anion (·O ) generation can be achieved by
2
optimizing the order degree of the photocatalyst. Herein, by taking carbon nitride polymer as an
example, we optimized the crystallization process of carbon nitride polymer by selecting
precursors of different polymerization degrees with a molten salt method. Benefiting from the
high crystallinity, extended π-conjugated system and strong van der-Waals interactions between
interlayers, the modified carbon nitride polymer exhibited accelerated charge transport and
enhancement in electron induced molecular oxygen activation reactions under visible light.
Consequently, the CCN-P exhibits about 1.5 times higher conversion rate in hydroxylation of
phenylboronic acid and over 6-fold faster degradation rate in Rh B organic pollutants
photodegradation with respect to pristine carbon nitride. This study provides an in-depth
1

understanding on the optimization of the O activation process and the design of advanced
2
photocatalysts.
Key words: Crystalline carbon nitride, molecular oxygen activation, photocatalysis, electron
transfer, reactive oxygen species
2

1
. Introduction
The selective oxygenation of organic compounds is widely recognized as one of the most
important processes both in the laboratory and industrial synthetic chemistry [1-3]. While due to
the usually high oxidation potential of the substrates, most oxidation strategies concentrated on
the chemical process at high pressure and temperature, resulting in excessive energy
consumption and environmental pollution [4-7]. Moreover, expensive noble-metal catalysts and
hazardous stoichiometric oxidants have been employed, and the formation of undesired side
products usually leads to the troublesome product purification process [8-10]. Therefore,
developing greener reaction pathway for surmounting the energetic barrier of selective
oxygenation reactions is highly desired.
Photocatalysis that directly harvests clean and renewable solar energy provides an appealing
strategy to resolve the above problems under green and mild conditions [11-13]. In general, the
ground state triplet molecular oxygen is forbidden to react with singlet organic molecules by
Wigner’s spin selection rule [14]. Fortunately, photocatalytic activation of molecular oxygen can
be realized via the electron or energy-transfer pathway from photocatalyst to O [15, 16].
2
−
Specifically, reactive oxygen species (ROS), such as superoxide radical anion (·O ), hydroxyl
2
radical (·OH) and hydrogen peroxide (H O ), can be produced by transferring photogenerated
2
2
electrons from the surface of semiconductors to oxygen molecules. On the other hand, energy
1
transfer from the excited state of photo-sensitizer produces highly active singlet oxygen ( O ) [17,
2
1
8]. Different oxidizing abilities were observed with various ROS. The development of photo-
catalyzed controllable O activation processes to achieve efficient ROS generation is therefore
2
−
required for the sustainable oxidation chemistry. Superoxide radical anion (·O ), a typical
2
charge transfer activated ROS, has been widely used as a mild and green oxidant in the organic
3

transformations [19, 20]. For example, the photocatalyzed aerobic oxidative hydroxylation of
arylboronic acid to substituted phenol, a versatile intermediate and important building block in
the chemical and pharmaceutical industry, closely related to the generation of superoxide radical
anion [21]. Therefore, due to the existence of competitive electron and energy transfer pathway,
efficient electron transfer from the catalyst to the molecular O was essential to realize the
2
selective superoxide radical anion generation [22].
Benefiting from its peculiar physicochemical properties, high stability and moderate band
gap, the organic conjugated polymer has been widely employed in various photocatalytic
reactions using visible spectrum [23-26]. Nonetheless due to the blocked charge carrier transfer
−
in the amorphous conjugated polymer semiconductors, ·O evolution in general suffers from
2
low level of efficiency [27-28]. The electron transport in photocatalysts is closely related to the
order degree of the structures [29-31]. Accordingly, modulating the order degree of the
semiconductor structure can promote photogenerated charge carrier mobility and optimize the
electron transport. Consequently, the separation and transfer kinetics of photogenerated charges
is accelerated, and high-efficiency ROS generation could be anticipated. As a metal-free visible
light photocatalyst, the melon-based carbon nitride polymer (PCN) has attracted considerable
attention during the past few years [32-34]. The pristine CN synthesized by conventional thermal
polymerization of nitrogen containing precursors usually display an amorphous/semi-crystalline
2
D layered morphology, which provided an ideal model to study the influence of the order
degree on the charge transport and further on the O activation process [35-37]. Recently, the use
2
of molten salt as the solvent and template has been recognized as a feasible way to promote the
polymerization and further tailor the structure and properties of the semiconductor [38-40]. To
that end, we are encouraged to construct highly ordered carbon nitride with molten salt and
4

optimize the crystallization process of carbon nitride polymer to boost electron transfer for the
efficient molecular oxygen activation.
Herein, two different polymerization degrees of heptazine-based carbon nitride materials,
melem-based oligomer and melon-based polymer (noted as PCN), were first synthesized via
heating dicyandiamide molecules at different temperatures. After that, the heptazine-based
carbon nitride polymers of varying crystallinity degrees (noted as CCN-O and CCN-P,
respectively) were successfully obtained by the ionothermal synthesis of the preheated melem-
based oligomer and PCN polymer as the precursors (Scheme 1). It was found that the charge
transfer was indeed modified and the resulting O activation process could be manipulated
2
depending on the crystallinity degree of the polymer. Notably, the CCN-P sample, which
contained the more extended π-conjugated system and tighter layer-to-layer stacks (Scheme 2),
exhibits significant boosted photogenic carrier migration and superior photocatalytic
performance in subsequent photocatalytic molecular oxygen activation experiments. Benefiting
from its optimal structural characteristics, the promoted electron transfer-involved photocatalytic
processes in carbon nitride semiconductors lead to the excellent conversion of phenylboronic
acid with over 99% selectivity, as well as complete degradation of organic pollutants.
Additionally, the CCN-P shows high stability during the prolonged photochemical cycle reaction.
This work gives us an in-depth understanding for the optimization of the O activation process
2
and the design of efficient metal-free photocatalysts via crystal engineering.
2
2
. Experimental Section
.1. Preparation of precursors: 8 g of dicyandiamide (DCDA) was placed into a porcelain
o
o
crucible with a cover. Then it was thermal heated at 450 C or 550 C in the air for 4 hours with a
5

o
-1
o
o
heating rate of 5 C min (450 C for oligomer, 550 C for PCN polymer). The yield of oligomer
was 56%, for PCN polymer was 49%.
2
.2. Preparation of CCN-O and CCN-P: 600 mg of corresponding precursor (melem-based
oligomer or PCN polymer) and a eutectic mixture of LiCl (45 wt. %, 2.7 g) and KCl (55 wt. %,
.3 g) were ground together. The mixed powder was placed in a covered ark and transferred into
3
o
-1
o
-1
a tube furnace and heated to 550 C with nitrogen flow (200 mL min ) of 12 C min . After
keeping the temperature for 4 hours, the tube furnace naturally cooled to room temperature. The
o
yellow-green product was washed with distilled water to remove residual salt, and dried at 60 C.
The CCN polymers obtained from different precursor were marked as CCN-O (O= melem-based
oligomer) and CCN-P (P= PCN polymer), respectively. The yield of CCN-O was 92%, for CCN-
P is 98%.
2
.3. Photocatalytic hydroxylation of phenylboronic acid: The photocatalytic hydroxylation of
phenylboronic acid was carried out as following: Catalyst (10 mg) and phenylboronic acid
0.5mmol) was poured into the mixture solution with triethylamine (TEA, 1.5mmol) dissolved in
(
the acetonitrile (3 mL). Then, the mixture was transferred into a 10 mL Pyrex glass bottle filled
with pure oxygen at atmospheric pressure. The suspensions were irradiated by a blue LED lamp.
After the reaction, the mixture was centrifuged to completely remove the catalyst particles. The
1
filtered solution was analyzed by H NMR spectrum using 1, 3, 5-Trimethoxybenzene as the
internal standard.
2
.4. Photodegradation of organic pollutants: The photodegradation of organic pollutants was
carried out using rhodamine B (noted as Rh B) as example. 20 mg of photocatalyst was mixed
-5
-1
with 80 mL Rh B solution (10 mol L ) and ultrasonic uniform. Before irradiation, the mixture
was stirred in dark for 30 min to attain an adsorption-desorption equilibrium. After that, it was
6

exposed to irradiation using a 300 W Xe-arc lamp equipping with cutoff filters (λ ≥ 420 nm)
under constant magnetic stirring at 300 rpm. At several minutes’ intervals, a 1.5 mL aliquot was
taken out and centrifuged to get the supernatant. The supernatant was detected by UV-vis
spectroscopy at a wavelength of 554 nm. And C/C represents the degradation percentage, where
0
C is the initial concentration of Rh B solution, and C represents the concentration at different
0
irradiation time.
2
.5. Photocatalysts Characterizations: X-ray diffraction patterns (XRD) were collected on a
Bruker D8 Advance diffractometer with Cu Kα1 radiation (λ=1.5406 Å). The field emission
scanning electron microscopy (SEM) was done on a Hitachi SU8010 microscope. Transmission
electron microscopy (TEM) measurement was conducted on a FEI Tencai 20 microscope. The
1
3
solid-state C cross polarization (CP)/magic angle spinning (MAS) nuclear magnetic resonance
NMR) spectra were measured on a Bruker Advance III 500 spectrometer. X-ray photoelectron
(
spectroscopy (XPS) analysis was conducted on a Thermo ESCALAB250 instrument with a
monochromatized Al Kα line source (200 W). BET surface area was measured by nitrogen
adsorption and desorption at 77 K using a Micromeritics ASAP 3020. The Fourier transform
infrared (FTIR) spectra were recorded on a BioRad FTS 6000 spectrometer. The UV-visible
diffuse reflectance spectra (UV-vis DRS) of the powders were obtained on a Varian Cary 500
Scan UV-visible system. The photocurrent performance analysis and Photoelectrochemical
measurements were conducted using a BioLogic VSP-300 electrochemical system with a three-
electrode cell: work electrode (photocatalyst-coated FTO), counter electrode (Pt), and reference
electrode (Ag/AgCl). And Na SO aqueous solution (0.2 M) as electrolyte at +0.6 V bias
2
4
potential vs. Ag/AgCl. Electron paramagnetic resonance (EPR) was carried out on a Bruker
1
model A300 spectrometer (Bruker Instruments, Inc.). H NMR spectrum was measured on a
7

Bruker Advance spectrometer in the solvents indicated. The total organic carbon (TOC) values
were determined using a Shimadzu TOC-Vcph total organic carbon analyzer.
3
. Results and discussion
The crystallinity and the layered-stacking mode of all the photocatalysts were firstly
analyzed by powder XRD patterns. As displayed in Figure 1a, the pristine PCN presents the
typical diffraction pattern of melon-based carbon nitride polymer, two domain peaks located at
o
o
1
3.1 and 27.6 corresponding to (100) in-plane repeating heptazine units and (002) interlayer
stacking peak [41]. For CCNs, an evident shift and narrowing of the two peaks are observed,
suggesting the enhanced periodicity and crystallinity of the samples. Specifically, the shift of the
o
o
(
100) peak of CCNs to the lower degree (8.1 for CCNs, 13.1 for PCN) could be attributed to
the extended distance of the interplanar repeat unit, indicating a more complete polymerization
of CCNs after molten salt treatment [42]. In contrast, the narrowed and intensified (002) peaks of
o
o
o
CCNs was shifted to higher degree (Figure 1b, 28.4 for CCN-P, 28.2 for CCN-O, 27.6 for
PCN), reflecting a more tightly stacked interlayer distance caused by the stronger π-π
interactions between the neighboring heptazine layers [43]. Furthermore, the CCN-P constructed
from melon-based PCN polymer exhibits shorter interlayer distance than CCN-O (constructed
from melem-based oligomer), suggesting that the convergence degree of the precursor may have
an impact on the crystallization process. HR-TEM (Figure 1c) further offers an intuitive
observation for the crystallinity and interlayer stacking distance of the CCNs. In comparison with
the pristine PCN sample (Figure S1), the clear two lattice fringes are observed on both CCNs.
The measured lattice distances corresponding to 0.31 nm for CCN-O, 0.28 nm for CCN-P likely
originated from the (002) interlayer space, and the other corresponding to 0.93 nm for CCN-O,
0
.94 nm for CCN-P, which may be assigned as the (100) in-plane periodicity. The distances of
8

these lattice fringes are consistent with the XRD results. Combining with the above results, it
clearly indicates that the obtained CCN samples show more extended π-conjugated system and
more compact layer interaction than pristine carbon nitride. Especially, the CCN-P has tighter
stacked layers than CCN-O. Those results also proved that the CCN-P constructed from melon-
based PCN polymer is easy to form the more extended conjugated system than CCN-O which
constructed from melem-based oligomer. Therefore, the extended in-plane conjugate system and
decreased interlayer distance can significant promote the photoelectron transfer within the stacks
and reduce the potential barrier on the surface, enhancing the photocatalytic performance
accordingly [43].
The chemical structure of the CCNs was characterized at the molecular level by the solid-
1
3
13
state C CP-MAS NMR, FT-IR and XPS spectra. In the C solid-state NMR patterns (Figure
a), both CCN-O and CCN-P have two similar distinct responses for carbon species. The high-
intensity peak at 161.2 ppm could be attributed to the (NH )-CN groups (C (e)) and the peak at
2
X
2
1
54.8 ppm corresponds to CN groups (C (i)). The results confirmed that CCNs maintain the core
3
structure of heptazines. In the FTIR spectra (Figure 2b), the characteristic peaks associated with
-1
the heptazine units are observed, such as the breathing modes located around 800 cm and the
-1
stretching vibration modes in the region of 1200-1700 cm . The distinct peaks of CCN samples
reveal the promoted degree of polymerization and crystallinity degree. In addition to these
-1
-1
retained absorption peaks, two new peaks located around 987 cm and 1151 cm are observed
with the CCNs samples, which could be assigned to the symmetric and asymmetric vibrations of
NC bonds in metal-NC groups, respectively (the presence of K could further be verified in the
2
2
-1
subsequent XPS measurement) [44]. And a new peak appears at around 2150 cm , is attributed
to the typical cyano peak. This may connect with the defects in the re-condensation of terminal
9

ammonia group [45]. Obviously, the cyano peak intensity in CCN-P is lower than CCN-O,
indicating more sufficient convergence of CCN-P compared with CCN-O.
The chemical state of the samples was further analyzed by the XPS measurements. As
shown in Figure 2c, the C 1s spectrum of CCNs can be divided into peaks centered at 288.4 and
2
2
84.8 eV, attributed to the sp -bonded carbon (-N-C=N) and the standard reference carbon. A
new peak located at 286.4 eV can be assigned to the C species in cyano group (-C≡N) [29].
Obviously, the area ratio of this peak in CCN-O is higher than in CCN-P (Table S1), which
agrees well with FTIR result. The N 1s spectra of all samples (Figure 2d) can be resolved into
2
four peaks which are assigned to the sp hybridized N in C=N-C groups (398.8 eV), the tertiary
N (-N-(C) ) in the center of heptazine units (400.1 eV), the N species in the terminal amino
3
groups (401.2 eV), and the charging effects in heptazine units (404.6 eV), respectively [46].
Clearly, compared to PCN, the ratio of N species in terminal amino groups in CCN-O decrease
to 8.6% and further minimized to 6.2% in CCN-P, suggesting the gradually enhanced
polymerization degree (Table S2). Furthermore, none of Cl or Li species is introduced into the
polymeric structure after LiCl/KCl salt melt treatment, except small amount of K ions was
observed which may be used to neutralize a handful of terminal negative groups (Figure S2). On
the basis of the above results, it can be concluded that the CCNs have the similar core chemical
skeleton with amorphous PCN, while they exhibit much more complete polymerization and more
tight stacked layers. Moreover, it is worth to mention that the higher degree of polymerization in
the precursor, the higher crystallinity was observed in the obtained CCN samples. The
morphological structure of the synthesized samples was implemented by SEM. As presented in
Figure S3, the PCN exhibits a typical bulk-like morphology but CCNs exhibit a nanorod-shaped
structure. Also notable is that the formation of nanorods is more complete in CCN-P than CCN-
1
0

O. This specific morphology of CCNs can efficiently increase the surface areas, which has been
certified by the nitrogen absorption isotherms tests (Figure S4). The CCNs exhibited increased
2
-1
nitrogen adsorption capacity and pore volume. The BET surface area of CCN-P is 85 m g ,
2
-1
2 -1
which is larger than CCN-O (59 m g ) and pristine CN (15 m g ).
The structure of the ordered plane and tight interlayer stacking is supposed to greatly affect
charge carrier separation and transfer which play crucial factors in determining the photocatalytic
performance. As the radiation decay of excitons leads to fluorescence, the optical excitation
processes of CCNs were investigated by room temperature PL measurements [42]. In Figure 3a,
the CCNs exhibit significant decreased emission intensity with respect to PCN, illustrating the
greatly suppressed recombination of the photogenerated charges for CCNs. Moreover, time-
resolved fluorescence spectra detected at the corresponding emission peaks give the mean
radiative lifetime of 3.02 ns, 0.98 ns and 0.78 ns for PCN, CCN-O and CCN-P, respectively
(Figure 3a, inset). The significantly reduced singlet excitons lifetime of CCN samples, clearly
indicates the enhanced separation of photogenerated charge carriers. The PL quenching can be
attributed to the optimized structural characteristics of CCNs for charge transfer, such as more
unfolded intralayer structure, shorter interlayer distance, and lower surface defect density which
mostly acting as charge recombination centers [29]. Furthermore, the transient photocurrent
response (Figure 3b) shows about 1.5 times stronger photocurrent transient response of CCNs
than that of the PCN. Meanwhile, as evidenced in Figure 3c, the electrochemical impedance
spectroscopy (EIS) Nyquist plot of both CCNs exhibit a much smaller arc radius than that of
PCN, which means the CCNs possess both lower electrochemical impedance and remarkably
enhanced electron migration [47]. These results clearly indicate more efficient carrier separation
and faster electron transfer on the surface of the obtained highly crystalline CCNs.
1
1

The optical absorptions of the polymers are investigated by the UV-vis DRS measurement.
The Figure S5a shows that both CCNs display evident enhancement visible light absorption and
a slight red-shift absorption edge than PCN. The band gaps of CCN-P and CCN-O determined
according to the Tauc plot approach are 2.72 and 2.75 eV, respectively (Figure S5c and S5d),
which are narrower than PCN (2.77 eV) (Figure S5b) [48]. The optimized optical absorption of
CCNs would definitely benefit for the utilization of more incident photons in subsequent
photoreaction [42]. The Mott-Schottky curves indicate typical n-type semiconductor behavior for
all the samples. And the flat potential of CCN-P, CCN-O and PCN was determined to be -1.12V,
-1.31V and -1.48 V vs Ag/AgCl (pH 6.6), respectively (Figure S6). In combination with their
band gaps (Figure 3d), the CCNs and PCN exhibit appropriate CB (conduction band) and VB
(valence band) positions that thermodynamic enable the rapid reduction of molecular oxygen.
By virtue of the promoted photogenic carrier migration and suitable band position, the
CCNs are expected to be a promising photocatalyst for molecular oxygen activation reaction.
Phenols serve as important building blocks in the chemical and pharmaceutical industries [49-
5
1]. Photocatalytic oxidation of phenylboronic acid is an effective and direct method for the
preparation of various substituted phenols [52-54]. Thus, the transformation of phenylboronic
acid to phenol was selected as the model reaction to evaluate molecular oxygen activation
capacity of the polymers. The PCN and CCNs photo-catalyzed hydroxylation of phenylboronic
acid using triethylamine (Et N) as the sacrificial reagent and molecular oxygen as the green
3
oxidant was first investigated. The factors affecting this reaction were comprehensively explored
through control experiments, and the results are listed in Table 1. The reaction takes place
smoothly with either PCN or CCNs as the photocatalyst, resulting in the desired phenol product
(entries 1-3). Obviously, phenylboronic acid was completely oxidized with CCN-P and the yield
1
2

(
>99%) is about 1.5 times higher than that with PCN (68%). When N was used instead of O ,
2 2
almost no products could be detected (entry 4). This result clearly demonstrates that oxygen
activation is the vital process for this reaction. In the absence of light, photocatalyst or Et N,
3
negligible conversion of phenylboronic acid was observed, indicating the key role of visible
light, catalyst and amine in the reaction (entry 5-7).
To further verify the generality of the CCN catalyst for the aerobic oxidative hydroxylation,
the electronic and steric effect of different substituents on aryl and alkyl boronic acid were
investigated (Table 2). Obviously, bearing whatever the electron-deficient (entries 2-8) or
electron-rich substituent on the phenyl moiety of the boronic acid (entry 9), the CCN-P always
exhibited excellent photocatalytic performance (yield: > 90%). Besides, the substitution position
on the benzene ring only had a little effect on the reaction outcome (entries 4 and 5). The
disubstituted phenyl boronic acid, naphthylboronic acid has also been examined, and all of them
are effectively converted into the corresponding phenol derivatives (entries 10 and 11).
Furthermore, an alkylboronic acid was well tolerated under the optimal conditions and the
corresponding product was obtained in satisfactory conversion (entry 12). To our delight,
phenylboronic acid pinacol ester also proved to be a suitable candidate for the transformation
(Figure 4, Eq. (1)). Moreover, with potassium phenyltrifluoroborate as the substrate, the
reaction was able to convert completely into phenol after 12 h under the same reaction conditions
as described above (Figure 4, Eq. (2)). Meanwhile, Figure S7 shows the catalytic cycle test with
CCN-P as the photocatalyst. After four repeated cycles, no apparent deactivation in catalytic
performance was observed, indicating the high stability of CCN-P during the prolonged
photochemical reaction process. The high stability of the photo-catalyst is also demonstrated by
the XRD patterns and FT-IR spectra for the used sample after the photo-reaction (Figure S9).
1
3

In order to explore the origin of oxygen in the product phenol, isotope labeling experiments
1
8
is conducted. Excessive amounts of H O was added to the system under the optimal reaction
2
condition, and none of 18O-labeled products were detected (Figure 4, Eq. (3)). This result
indicated the oxygen atom in the product probably come from O . Therefore, to investigate the
2
main reactive oxygen species for inducing the photoreactions over CCN-P, the control
experiments with p-benzoquinone (BQ), t-butyl alcohol (TBA), NaN , catalase and KI employed
3
-
1
+
as the corresponding •O , •OH, O , H O and h scavengers were performed. Figure 4a
2
2
2
2
illustrates that the oxidation of phenylboronic acid is significantly inhibited in the presence of
BQ or KI. However, there only caused a slight decrease in the reaction efficiency when TBA,
-
NaN or catalase was added to the reaction mixture. The results provided that the •O was the
3
2
main oxygen-containing active species for the oxidation of phenylboronic acid in our system.
-
Therefore, the generation of •O over PCN and CCNs under visible-light irradiation was verified
2
in an EPR spin-trapping experiment with DMPO at room-temperature. The EPR signal with a
characteristic intensity ratio of 1:1:1:1 was observed, which is in line with that of DMPO-•O2-
adduct (Figure 4b). Additionally, the notable difference of the signal intensity of PCN and
-
CCNs clearly confirm the enhanced •O generation in CCNs system. Consistent with the
2
-
conjugation and interlayer distance of their structures, the CCN-P presents stronger •O signals
2
-
+
than CCN-O. The above results demonstrated that the •O and h are the main active species in
2
photooxidation of phenylboric acid with the CCN-P photocatalyst. The enhanced electron
-
migration of CCN-P is beneficial for the generation of •O during the photocatalytic molecular
2
oxygen activation.
The photocatalytic molecular oxygen activation performance of the PCN and CCNs are
further evaluated by the degradation of Rh B under visible light (λ ≥420 nm) irradiation. Before
1
4

the visible light illumination, all the samples are stirred for 30 minutes in dark to achieve the
equilibrium of adsorption and desorption (Figure S10). After that, the photodegradation tests
with PCN and CCNs were conducted. As displayed in Figure 5a, both CCN photocatalysts
exhibit faster downtrend in Rh B concentrations than PCN polymer. As for CCN-P sample, after
6
0 minutes of visible light irradiation, over 94% of Rh B is photodegraded, while only 48% for
pristine PCN. In contrast, when conducting this reaction without catalyst, no obvious change
could be observed, indicating the stable nature of Rh B molecule. Furthermore, the
mineralization rate by determining the total organic carbon (TOC) values was about 24.2% in the
CCN/RhB system (Figure S11), suggesting that most of RhB was broken up into small
molecules and harsher conditions may be required to realize the complete mineralization of RhB
[55].
It is generally accepted that the photodegradation process of Rh B on the photocatalyst is
indexed to the first-order kinetics with an equation as following ln (C /C) = kt
0
-1
(
k: Rate constant (min ); C : Initial concentration of Rh B; C: Actual concentration of Rh B)
0
[18]. We have therefore calculated the rate constant k of PCN and CCNs to give a direct
comparison of the photodegradation activity. As shown in Figure 5b, the k valves increased in
-1
-1
-1
the order of PCN (0.008 min ), CCN-O (0.031 min ) and CCN-P (0.050 min ). The k value of
CCN-P is shown to be more than 6 times higher than that of PCN, which is in accordance with
the effective charge mobility across the compact layer-to-layer stacks and more conjugate planes.
Moreover, CCN-P maintains the high degradation efficiency (over 90%) throughout three
photocatalytic cycles (Figure 5c). When we pump N into the reaction system to get the oxygen
2
out of the air, the degrading rate of Rh B depresses clearly. It proved that the O is essential for
2
the degradation to proceed smoothly. Furthermore, when BQ was added into the system, the
1
5

-
degradation rate was significantly inhibited, indicating the •O is also playing key role in the
2
photodegradation of Rh B over CCN-P (Figure 5d).
Based on the above studies, a proposed working mechanism for the hydroxylation of
phenylboronic acid in CCN photocatalytic system is illustrated in Scheme 3. Briefly, the reaction
starts with excitation of the photocatalyst CCN-P via irradiation, and single electron transfer
-
(
SET) from excited CCN-P to the molecular O leads to the formation of •O . Subsequently, a
2
2
-
peroxide radical intermediate was formed via reaction of the boronic acid (Ar-B(OH) ) with •O ,
2
2
which undergo rearrangement and hydrolysis to deliver the final product [56]. In other hand, the
photocatalyst CCN-P is regenerated in the presence of electron donor to close the photoredox
cycle.
4
. Conclusion
In summary, we have synthesized highly ordered carbon nitride by using precursors with
different degrees of polymerization, which presented the advantages of the increased
polymerization degree, more unfolded intralayer structure and shorter interlayer distance. The
combination of fluorescence study, electrochemical analysis and EPR spectral observations
demonstrated the significantly promoted photogenic carrier separation and transfer process,
−
−
facilitating •O evolution. Benefiting from the increased •O production, the CCNs exhibit the
2
2
superior activity in photocatalytic selective oxidations, including hydroxylation of phenylboronic
acid and Rh B degradation. This study gives us an in-depth understanding on the advantages of
CCN in molecular oxygen activation and a practical method to achieve more ROS-induced
reactions by using CCN photocatalyst.
1
6

Scheme 1. Schematic diagram for synthesis of PCN, CCN-O and CCN-P from precursors with
different polymerization degree (Dicyandiamide, Melem-based oligomer and Melon-based PCN
polymer).
1
7

Scheme 2. Proposed interlayer charge carrier transfer of (left) pristine and (right) optimized
CCN with shortened layer stacking distance.
1
8

Figure 1. The XRD patterns a) and enlarged XRD patterns b) of PCN and CCNs. c) TEM
images of CCNs.
1
9

1
3
Figure 2. Solid-state C CP-MAS NMR a), FTIR spectra b) and High-resolution XPS spectrum
of c) C 1s and d) N 1s for PCN and CCNs.
2
0

Figure 3. a) Room temperature PL spectra of PCN and CCNs under 395 nm excitation. Inset:
Time-resolved fluorescence kinetics monitored at the corresponding emission peaks of PCN and
CCNs. Transient-state photocurrent response b) and Nyquist plots of electrochemical impedance
spectroscopy c) of PCN and CCNs at +0.6 V vs. Ag/AgCl reference electrode using Pt as counter
electrode. d) Schematic diagram showing the band structure of PCN, CCN-O and CCN-P versus
Ag/AgCl, pH = 6.6.
2
1

Table 1. Photocatalytic hydroxylation of arylboronic acids.
B(OH)2
Visible light, Photocatalyst
OH
Et N, MeCN, O
3
2
Deviation from
standard condition
Entry a Photocatalyst
Yield b
1
2
3
4
5
6
7
PCN
-
68
92
>99
<1
<1
6
CCN-O
CCN-P
CCN-P
CCN-P
-
-
-
N2
No light
-
CCN-P
No Et N
<1
3
[
a] Reaction conditions: phenylboronic acid (0.5 mmol), photocatalyst (10 mg), Et N (1.5 mmol),
3
1
3
mL CH CN, and 1 atm O for 4 h. [b] Determined by H-NMR using 1,3,5-Trimethoxybenzene
3
2
as an internal standard. MeCN = Acetonitrile.
2
2

Table 2. The oxidative hydroxylation of phenylboronic acid and alkenylboronic acid using
CCN-P as the photocatalyst under standard conditions.
OH
CCN-P, Blue LED
Ar
B
Ar OH
OH
Et N, MeCN, O
3 2
Entry a
1
Ar
Yield. b
>99
Entry a
7
Ar
Yield. b
>99
O2N
O
F
2
3
>99
97
8
9
>99
>99
Br
Cl
MeO
F
4
98
10
>99
Cl
Cl
5
6
93
98
11
12
>99
>99
NC
[
a] Reaction conditions: phenylboronic acid (0.5 mmol), photocatalyst (10 mg), Et N (1.5 mmol),
3
1
3
mL CH CN, and 1 atm O for 4 h. [b] Determined by H-NMR using 1,3,5-Trimethoxybenzene
3
2
as an internal standard.
2
3

Figure 4. a) Trapping experiments using active species scavengers (BQ, TBA, NaN , catalase
3
-
1
+
and KI employed as the corresponding •O , •OH, O , H O and h scavengers) in phenylboronic
2
2
2
2
-
acid hydroxylation for CCN-P. b) EPR spectra of DMPO-•O adducts in the system of PCN and
2
CCNs under visible light irradiation.
2
4

Figure 5. a) Photocatalytic activities for Rh B degradation of PCN and CCNs. b) Relationship of
degradation rate (k) and the light irradiation time for PCN and CCNs. c) Cycling runs for Rh B
degradation in the presence of CCN-P sample. d) Effect of quencher additives on the
-
+
photocatalytic activity of CCN-P in RhB degradation. BQ: •O scavenger; KI: h quencher.
2
2
5

Scheme 3. The proposed mechanism for the oxidation of phenylboronic acid in CCN-P
photocatalytic system.
2
6

Conflicts of interest
The authors declare no conflicts of interest.
Acknowledgment: This work was financially supported by the National Natural Science
Foundation of China (U1905214, 21861130353, 21961142019, 21761132002 and 21425309),
the Science Foundation of the Fujian Province (2019J01203), the National Key R&D Program of
China (2018YFA0209301), the National Basic Research Program of China (2013CB632405), the
Chang Jiang Scholars Program of China (T2016147) and the 111 Project (D16008).
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