Linkage engineering mediated carriers transfer and surface reaction over carbon nitride for enhanced photocatalytic activity

Article abstract of DOI:10.1039/d1ta03813c

Rational tailoring of the atomic structure of photocatalysts with multiple functions to enhance the carrier transfer efficiency and surface activation of carbon nitride (C₃N₄) is promising and a challenge. Here, we make the first report of a facile strategy to construct amphiphilic carbon and C-O-C chain linked terminal melem units in functional carbon nitride (COCN)viacopolymerizing formaldehyde with melem. By integrating the amphiphilic carrier bridge of carbon and C-O-C chains into the framework, the photogenerated carrier mobility and activated species (superoxide radicals, singlet oxygen) as well as surface interaction are significantly improved. Consequently, the optimal tailoring of C₃N₄attains superior photocatalytic activity for hydrogen production (34.9 μmol h^?1) and selective oxidation of sulfide to sulfoxide using air (nearly 100% conversion and selectivity after 3 h of illumination), which is about 7 times higher than that of pristine C₃N₄. This study provides deep insight into and strategies for the atomic tailoring of carrier transfer and surface reaction over organic-based photocatalysts.

Full text of DOI:10.1039/d1ta03813c

Journal of
Materials Chemistry A
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PAPER
Linkage engineering mediated carriers transfer and
surface reaction over carbon nitride for enhanced
photocatalytic activity†
Cite this: DOI: 10.1039/d1ta03813c
a
a
a
a
a
Shilian Yang,‡ Qian Wang,‡ Qiuchen Wang, Gen Li, Tianxiang Zhao,
a
a
b
Peng Chen, * Fei Liu* and Shuang-Feng Yin*
Rational tailoring of the atomic structure of photocatalysts with multiple functions to enhance the carrier
transfer efficiency and surface activation of carbon nitride (C ) is promising and a challenge. Here, we
3 4
N
make the first report of a facile strategy to construct amphiphilic carbon and C–O–C chain linked
terminal melem units in functional carbon nitride (COCN) via copolymerizing formaldehyde with melem.
By integrating the amphiphilic carrier bridge of carbon and C–O–C chains into the framework, the
photogenerated carrier mobility and activated species (superoxide radicals, singlet oxygen) as well as
3 4
surface interaction are significantly improved. Consequently, the optimal tailoring of C N attains
Received 6th May 2021
Accepted 12th July 2021
ꢀ1
superior photocatalytic activity for hydrogen production (34.9 mmol h ) and selective oxidation of
sulfide to sulfoxide using air (nearly 100% conversion and selectivity after 3 h of illumination), which is
DOI: 10.1039/d1ta03813c
3 4
about 7 times higher than that of pristine C N . This study provides deep insight into and strategies for
rsc.li/materials-a
the atomic tailoring of carrier transfer and surface reaction over organic-based photocatalysts.
a fundamental and straightforward strategy to enhance the
Introduction
15–17
performance of pristine g-C N .
For example, Ohno et al.
matrix
due to its extensive application in the eld of hydrogen evolu- using melem and barbituric acid as precursors, which signi-
tion, CO
reduction, environmental remediation, and organic cantly improved the charge separation and photocatalytic H
3
4
In recent years, photocatalysis has aroused tremendous interest reported the incorporation of C]O groups into the C N
3 4
2 2
O
2
1
18
synthesis. Highly efficient photocatalysts were key components production. Ye et al. designed a short carbon chain linked
2,3
propelling photocatalysis for commercial application. Given pristine tri-s-triazine structure in the basal plane of C
3 4
N to
their chemically stable and nontoxic features, metal-free improve its photocatalytic performance via facilitating charge
1
9
conjugated polymers of g-C
3
4
N
4
,5
are regarded as prospective carrier transfer in a convenient transport channel. Che et al.
candidates for photocatalysts. However, the high recombina- introduced a graphitic carbon ring into the in-plane of C to
tion rate of photogenerated carriers and low efficiency of reac- expedite the separation of photoexcited carriers and electron
N
3 4
20
tant activation have severely limited the photocatalytic activity transport. In addition, it is reported that localized graphitized
6
–9
of bulk g-C
3
N
4
.
Therefore, further improvement in g-C
3
N
4
carbon nitride with an asymmetric benzene ring could form
photocatalytic efficiency is a worthwhile mission.
separated charge carrier centers and a localized internal eld to
Along these lines, several pioneering approaches (e.g.: facilitate the separation of photogenerated carriers. It is
construction of heterojunctions and deciencies, elemental envisioned that the rational design of different chain linked
21
doping, morphology control, and cocatalyst modication) have pristine tri-s-triazine structures in C
3
N
4
could suppress carrier
10–14
been carried out to overcome these obstacles.
Notably, the recombination through forming asymmetric units. Most of the
precise and targeted atomic regulation of melem units is above work was entirely focused on the promotion of separation
and transportation of charge carriers as well as on the
construction of band structure for enhancing photocatalytic
activity. However, the surface reaction associated with reactant
a
Provincial Guizhou Key Laboratory of Green Chemical and Clean Energy Technology,
School of Chemistry and Chemical Engineering, Guizhou University, Guiyang 550025,
Guizhou, China. E-mail: pchen3@gzu.edu.cn; ce.feiliu@gzu.edu.cn
b
activation is also a signicant factor but is oen overlooked and
State Key Laboratory of Chemo/Biosensing and Chemometrics, Provincial Hunan Key rebellious for improving photocatalytic efficiency. Precise and
22
Laboratory for Cost-effective Utilization of Fossil Fuel Aimed at Reducing Carbon-
targeted control over different chain linked pristine tri-s-
triazine structures could not only suppress carrier recombina-
tion, but could also fully consider the surface reaction property.
Unfortunately, to the best of our knowledge, there has been no
dioxide Emissions, College of Chemistry and Chemical Engineering, Hunan
University, Changsha 410082, Hunan, China. E-mail: sf_yin@hnu.edu.cn
†
1
‡
Electronic supplementary information (ESI) available. See DOI:
0.1039/d1ta03813c
These authors contribute equally to this work.
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report about multiple chains linked to a pristine tri-s-triazine spectrophotometer (UV-vis, Shimadzu UV-3600 plus). The
structure in C N . photocurrent and Mott–Schottky experiments were measured
3
4
Herein, we propose a facile strategy to embed amphiphilic on an electrochemical analyser (CHI660E, Chenhua).
carbon and C–O–C chain linked melem units in functional
carbon nitride (COCN) via copolymerizing formaldehyde with
melem. Notably, by integrating an amphiphilic electron bridge
Photocatalytic selective oxidation of sulde
Photocatalytic selective oxidation of sulde to sulfoxide was
performed in a glass reactor with an air dryer and reux
condenser. Typically, 0.5 mmol of substrate, 2 mL of acetonitrile
and 0.02 g of photocatalyst were added into the reactor. Prior to
light irradiation, the reaction system was stirred for 0.5 h to
maintain a balance of absorption and desorption of air. Aer an
appropriate period of irradiation, the reaction mixture of the
catalyst and liquid was separated by centrifugation. The quali-
tative characteristics and quantication of the products were
identied by GC-MS analysis and gas chromatography (GC
into the framework, the migration of photogenerated carriers,
production of activated species (superoxide radicals, singlet
oxygen) and surface interaction capacity of the reactant are
signicantly improved. Therefore, the optimized atomic
tailoring of the sample attains superior photocatalytic perfor-
mance by achieving a 7-fold improvement for the selective
oxidation of sulde to sulfoxide using air (nearly 100%
conversion and selectivity aer 3 h of illumination) and high
ꢀ
1
hydrogen production (34.9 mmol h ) as well as good stability
compared with pristine C
3 4
N .
2014C), using biphenyl as internal standard.
Experimental section
Synthesis of COCN
Photocatalytic H evolution
2
Photocatalytic H generation performance over the as-prepared
2
The amphiphilic carbon and C–O–C chain linked melem units
catalyst was studied by employing a conventional photocatalytic
water splitting system (NBET, Beijing) with a light source (125
(COCN) were fabricated via a simple copolymerization method.
In a typical synthesis, 5 g of melamine was added into an
ꢀ
2
mW cm ). Typically, 0.02 g of catalyst, 10 mL of triethanol-
amine (TEOA), 250 mL of H PtCl and 40 mL of water were added
into the reactor. Before irradiation, the photocatalytic H
ꢁ
alumina crucible and heated at 425 C in a muffle furnace
2
6
overnight to collect melem. The obtained powder was sus-
pended in DI water and reuxed for several hours to purify the
melem. And then, 2.182 g of melem was added to 100 g of
formaldehyde aqueous solutions (0.545 g, 37 wt%), and the pH
value adjusted to 9 using 10 wt% triethanolamine. Aer stirring
2
generation system was closed to produce an airproof system
and stirred for 0.5 h. Aer an appropriate period of irradiation,
the obtained H2 was quantied via a gas chromatography
system (GC9790, Fuli).
ꢁ
for 1.5 h at 80 C, the pH value of the mixed solution was
adjusted to 5 with 10 wt% HCl and it was subsequently heated
for 2 h under vigorous stirring. Then the solid composite
precursor could be obtained by ltering, washing, and drying.
Finally, pure COCN was fabricated by annealing treatment of
Results and discussion
Preparation and characterization
the precursor at a suitable temperature for 4 h under a nitrogen The amphiphilic carbon and C–O–C chain linked melem units
atmosphere. For comparison, COCN-1, COCN-2 and COCN-3 (COCN) have been synthesized by the simple polymerization of
were obtained by annealing treatment of the precursor at 450, formaldehyde with melem (Fig. 1a). Firstly, melem was
ꢁ
ꢁ
5
00 and 550 C for 4 h under nitrogen atmosphere, respectively. synthesized by calcination of melamine at 425 C for 12 h and
purication in boiling water. A number of gaps were discovered
in the columnar structure of the melem rod (see Fig. S1 in ESI†).
3 4
Synthesis of C N
In general, the process of the melem and formaldehyde
Typically, C
3
N
4
was generated by directly calcining melamine at
23
condensation reaction can be divided into two steps. In the
rst step, the prepared melem was treated with formaldehyde to
ꢁ
500 C for 4 h under a nitrogen atmosphere.

form trimethylohmelem under alkaline conditions. A narrow
gap had obviously appeared in the columnar structure. XRD
Catalyst characterizations
X-ray diffraction patterns (XRD, Bruker D8 Advance), Fourier (Fig. S2†), FTIR (Fig. S3†), and XPS (Fig. S4a†) analyses disclosed
transform infrared spectroscopy (FT-IR, Shimadzu IR Affinity-1) that CH OH had been successfully incorporated in the melem
2
and X-ray photoelectron spectroscopy (XPS, Thermo Fisher K- structure. Further polymerization of the copolymer was carried
Alpha Plus) were used to check the chemical structures and out to generate a short chain-like precursor under acidic
surface valence state of the obtained samples, respectively. The conditions. As a result of incomplete polymerization, there were
morphological structure of COCN was surveyed with a scanning abundant OH on the edges of the melem units (Fig. S4b†).
electron microscope (SEM, Hitachi S-4800) and transmission Finally, an oversimplied temperature-programmed heating
electron microscopy (TEM, JEM-2100F). The Brunauer– process was implemented to aggregate the short chain
Emmett–Teller (BET) and chemisorption data of the prepared precursor, resulting in embedded multiple functionalized
materials were analysed by using a Micrometrics ASAP 2020 and chains. Importantly, the melem could form C N4 without
3
AutoChem II 2920, respectively. The optical absorption prop- aggregating with formaldehyde under the same conditions
erties of COCN and C N were examined by employing a UV-vis (Fig. S5, ESI†).
3 4
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Fig. 1 (a) Schematic of COCN synthesis, (b) SEM, (c) TEM and (d–f) elemental mapping images of COCN-2.
ꢀ
1
2
As shown in Fig. 1b, the COCN-2 sample has a rod-like 3126 cm (N–H), implying the generation of N–CH –N and
1
9,27–30
appearance with a diameter of about 1500 nm. Also, the rod is C–O–C species in the structure of COCN (Fig. S3†).
In the
1
3
made up of numerous mesopores and macropores (Fig. 1c). The
C NMR spectrum (Fig. S8†), two peaks at 161.9 and 154.4 ppm
and N]C–N bonds, respectively.
than that of the C N composite (11.7 m g ) (Fig. S6, ESI†). It This result indicated that the heptazine units of carbon nitride
2
ꢀ1
31
specic surface area of COCN-2 is (79.4 m g ), much larger are assigned to the C–NH
x
2
ꢀ1
3 4
is speculated that the hierarchical pores are conducive to light were not damaged. Two smaller peaks at 67.7 and 55.5 ppm
absorption and product/reactant migration. In addition, the belong to the C–O–C and carbon chains, suggesting that C–O–C
element mapping images reveal that the dispersion of C, N and and carbon chains have been inserted into the framework of
32
O elements in COCN is uniform (Fig. 1d–f and S7†).
COCN. In addition, two peaks around 107.5 and 115.2 ppm are
Successful synthesis of COCN was tested by XRD, FT-IR and ascribed to unsaturated carbon, which derives from the disso-
1
3
33
C NMR spectral analyses. As shown in Fig. S2,† the two ciation of C–O–C at high temperature. In order to obtain the
ꢁ
ꢁ
characterization peaks at 13.1 and 27.8 are ascribed to the in- thermal behavior of the as-prepared samples, TG measure-
plane repetition of (100) and the p–p reection of (002), ments were carried out (Fig. S9†). A small weight loss can be
24
ꢁ
respectively. Compared with C N , the peak of (002) in COCN- observed below 600 C, which was assigned to the decomposi-
3
4
2
moves to a higher angle, displaying a decrease in the interlayer tion of the inserted chains in COCN, indicating that C–O–C and
distance, which could signicantly improve interlayer exciton carbon species exist in the framework. According to the results
2
5
ꢁ
13
splitting and charge transfer. Some new sharp peaks at 12.4 , acquired from XRD, IR, and C NMR measurements, we can
1
ꢁ ꢁ
9.6 and 21.8 appeared for COCN-1 and COCN-2, implying the conrm that the heptazine units of carbon nitride had not
formation of new chains. Interestingly, heat treatment at changed, coupled with the insertion of C–O–C and carbon
a higher temperature caused the disruption of new chains to chains into the framework of COCN.
form a conventional tri-s-triazine linked framework. The FTIR
XPS survey spectra were implemented to further test the
spectrum of COCN shows characteristic broad peaks at 811 and surface chemical states of COCN. As shown in Fig. 2a, the C 1s
1
300–1700 ascribed to the out-of-plane bending of the triazine of COCN-2 possesses four peaks at 284.1 eV, 284.6 eV, 286.1 eV,
26
2
6 7 2
ring and the heptazine heterocyclic ring (C N ) units. Small 287.2 eV and 287.8 eV corresponding to C–H , the sp hybrid-
peaks are observed at 993 (C–N), 1088 (C–O–C), 1495 (C–H
2
) and ized carbon atoms, C–NH, C–O–C and N]C–N bond,
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Fig. 2 XPS survey spectra of COCN: (a) C 1s, (b) N 1s, and (c) O 1s. (d) UV-vis absorption performance, (e) the corresponding Tauc plots (f) and
schematic diagram of electronic band structure of fabricated samples.
2
7,34–37
respectively.
compared with C
been changed. Moreover, it was found that the peaks of the increased via the rise in calcination temperature (Table S3,
C–O–C bonds of COCN-1 shied to a slightly higher position for ESI†), suggesting C–NH on the edges of the heptazine units.
COCN-3, indicating oxidation had occurred in COCN-3 Three characteristic peaks at 531.6, 532.7 and 533.7 eV can be
There was some shi toward a lower position peak area of C–NH decreased from 12.07% of COCN-1 to 8.73%
3
N
4
, implying the chemical environment has of COCN-3, while the peak area of N–(C ) and C–N]C in C N
3
3 4
x
(
Fig. S10, ESI†). As revealed in Table S2 (ESI†), COCN-1 observed from the O 1s XPS spectra of COCN, assigned to
39
possesses plenty of oxygen species and COCN-2 has the most surface adsorbed O and H O, and C–O–C groups, respectively.
C–H bonds. This can be explained by the fact that the precursor According to the XPS, FTIR and C NMR spectrum results, high
2
2
1
3
2
of carbon and C–O–C chains will be recombined at high temperature will break down the carbon and C–O–C, resulting
temperature to form a traditional tri-s-triazine framework. in COCN-1 and COCN-2 possessing the maximum number of
Three peaks at 398.4, 399.7 and 400.9 eV were obtained by C–O and carbon chains, respectively.
deconvoluting the spectrum of COCN-2 and related to the C–
The optical absorption properties of bulk C
3 4
N and COCN
38
N]C, N–(C
3
) and C–NH bonds, respectively. In addition, the were checked by employing UV-vis diffuse reectance
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spectroscopy. As shown in Fig. 2d, COCN exhibits a slightly blue a negligible conversion (15%) of thioanisole was acquired over
shied absorption band but higher photoabsorption in C N , whereas carbon and C–O–C chain modied COCN
3
4
comparison with bulk C N . Such a slight blue shi might be exhibited markedly increased activities. Among these, near
3
4
attributed to the intensive quantum connement effect or complete conversion and selectivity were achieved over COCN-2
40
delocalization of the p-conjugation system. The bandgap of aer 3 hours of illumination, which was 7 times higher than
the as-prepared samples is calculated from the Kubelka–Munk that of pristine C . In addition, it also had the highest
equation. It displayed a narrowing of the bandgap width from formation rates among the reported catalysts (Table S5, ESI†).
.58 eV (g-C ) to 2.48 eV (COCN-3). To identify the possible We also studied the stability of COCN-2 in ten successive runs
3 4
N
2
3 4
N
positions of the valence and conduction bands, Mott–Schottky and detected almost no loss of photoactivity or structure
measurements were investigated at different frequencies (Fig. S12, ESI†), indicating the high stability of COCN-2.
(1000 Hz, 1500 Hz, 2000 Hz) and are shown in Fig. S11.† There Furthermore, COCN-2 was extended to the oxidation of
was a positive slope and the corresponding conduction bands various thioether compounds. Thioanisole derivatives in the
were estimated as ꢀ1.12, ꢀ1.24, ꢀ1.36 and ꢀ1.37 vs. RHE for presence of either electron-withdrawing or electron-donating
3 4
the n-type semiconductors of C N , COCN-1, COCN-2 and substituent groups were successfully oxidized to the corre-
COCN-3, respectively. According to bandgap measurements of sponding sulfoxides with excellent conversion and selectivity.
the UV-vis spectrum, the band gap of the as-prepared samples is The excellent conversion rate of different substituted thio-
obtained as shown in Fig. 2f. There are negative positions of the anisoles is in the order ortho < para, which indicates the pres-
41
conduction band and mild oxidizing capacity by embedding ence of a steric effect. In general, COCN-2 is a promising
carbon and C–O–C chains, which favours hydrogen evolution photocatalyst for industrial applications of the selective oxida-
and photocatalytic selective oxidation.
tion of suldes.
Behind such an outstanding performance, it is more signif-
icant to decode the key scientic factors that affect the photo-
catalytic activity of COCN. Hence, we are concerned with the
Photocatalytic selective oxidation of suldes and mechanisms
In this study, the selective oxidation of methyl phenyl sulde relationship of photogenerated charge carriers with or without
was implemented as a model reaction using air as an oxidizing modied chains in the C N framework. In general, steady-
3
4
agent at room temperature and under visible light irradiation. state/time-resolved photoluminescence (PL) spectroscopy is an
Parallel experiments were carried out under N and CO atmo- important method to monitor the recombination ability of
2
2
spheres or without light illumination and/or photocatalyst, photogenerated carriers. As shown in Fig. 3c, the photocurrent
resulting in no products being generated, suggesting that the density of COCN-2 is nearly three times higher than that of
selective oxidation reaction is photocatalysed and O
2
3 4
from the C N . Meanwhile, EIS measurements suggest that the incorpo-
air is compulsory. As shown in Table 1, it was observed that ration of carbon and C–O–C bonds into g-C N can effectively
3
4
enhance the electronic conductivity and charge transportation
of the polymer matrix. Overall, we can reasonably conclude that
Table 1 Selective oxidation of sulfides over different photocatalysts the bridged carbon and C–O–C bonds act as an electron trans-
a
under visible-light irradiation
porter to dramatically accelerate the separation and transfer of
photocarriers. Furthermore, the maximum number of carbon
chains is more conducive to carrier transmission. In particular,
due to the different distances migrated by the charge carriers
according to the appropriate ratio of carbon and C–O–C bridges,
a built-in electric eld could be formed to enlarge the separa-
tion of photogenerated carriers.
On the other hand, the surface properties of COCN are
crucial factors affecting the photocatalytic performance. Firstly,
hydrophilicity/hydrophobicity plays a very important role in the
surface reaction for nanomaterials. To probe the surface prop-
erties of the as-prepared samples, the contact angle of thio-
anisole was measured. As shown in Fig. S13,† the contact angle
Conversion
(%)
Selectivity
(%)
Entry
1
R
1
R
2
Time (h)
Ph
Ph
Ph
Ph
Ph
Ph
P-MeOPh
P-ClPh
P-BrPh
O-BrPh
CH
CH
CH
CH
CH
CH
CH
Ph
CH
CH
CH
CH
CH
CH
3
3
3
3
3
3
3
3
3
3
3
3
5
8
8
5
5
99
95
87
15
33
99
99
99
99
99
99
99
>99
>99
>99
92
b
2
c
3
d
2
e
3
96
3 4
of COCN is in the order C N < COCN-3 < COCN-1 < COCN-2,
4
5
6
7
8
9
>99
>99
>99
>99
>99
>99
>99
implying a relatively strong interaction with thioanisole. This
is in accordance with the content of non-polar functional
groups (carbon chains) on the surface of COCN, resulting in the
surface being under hydrophobic conditions. In addition, the
3
3
3
3
3
3
3
adsorption of O
reaction. From Fig. S14,† an increase in intensity in the adsor-
bed oxygen molecule occurred in O -TPD, implying the ability to
activate O was improved by embedding chains.
2
plays an important role in the oxidation
1
0
3
CH
2
CH
2
a
Reaction conditions: substrate (0.5 mmol), photocatalyst (COCN-2, 20
>420 nm.
2
mg), acetonitrile (2 mL),c air, room temperature,
l
b
d
e
Photocatalyst: COCN-1.
precursor of COCN.
COCN-3.
Photocatalyst:
C
3
N
4
.
The
2
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Fig. 3 (a) PL (lEx ¼ 380 nm), (b) transient fluorescence, (c) photocurrent and (d) electrochemical impedance spectroscopy of as-prepared
samples.
ꢀ
To determine the crucial electron transfer pathway of the efficient O
2
c
trapping agent was added to the DPBF decom-
photocatalyst, we used different kinds of quenching agents to position process in the presence of a COCN photocatalyst. The
test the nature of the reaction. In this investigation, tert-butanol decomposition efficiency of DPBF was almost halved, suggest-
(
TBA), L-tryptophan (LT), p-benzoquinone (BQ), potassium per- ing that both energy transfer and charge transfer played
1
sulfate (KSO) and ammonium oxalate (AO) served as hydroxyl important roles in
radical (cOH), singlet oxygen ( O ), superoxide radical (O
electron (e ) and hole (h ) scavengers.
results (Fig. S15†), singlet oxygen, and photogenerated elec- to 3500 cm were observed under an O atmosphere before
O
2
generation. In addition, in situ FT-IR
c ), measurements were conducted for COCN-2 to study the pho-
According to the tocatalytic mechanism (Fig. S17†). Only weak peaks from 3000
1
ꢀ
2
2
ꢀ
+
42,43
ꢀ
1
2
trons and holes were considered to be essential active species light irradiation, indicating the chemical adsorption of oxygen
whereas superoxide radicals participated but less signicantly. on the surface of COCN-2. Under light illumination, the char-
Therefore, EPR analysis and radical trapping experiments were acteristic peaks of carbon and C–O–C appeared. This can be
carried out for the qualitative and quantitative characterization explained by the excitation and transmission of electrons and
ꢀ
1
44,45
of O
2
c
and O
2
.
As shown in Fig. 4a and b, all the prepared holes on the surface of COCN-2. According to the previous
1
ꢀ
2
c . Noticeably, the response discussion, we can further suppose that the embedded new
samples could generate O and O
intensity of O and O c signicantly increased in the order chains serve as a carrier transmission channel for oxygen
2
1
ꢀ
2
2
C N < COCN-3 < COCN-1 < COCN-2 by incorporating carbon activation.
3
4
and C–O–C chains in C
3
N
4
. We also chose nitro blue tetrazolium
Based on the above analysis and the principles reported in
46,47
(
NBT) and 1,3-diphenylisobenzofuran (DPBF) as scavengers to the literature,
a plausible mechanism for this photocatalytic
under visible light oxidation over COCN-2 is proposed in Fig. S18.† Under light
irradiation (Fig. S16†), respectively. An enhancement of about illumination, COCN-2 was easily photoexcited and produced
ꢀ
1
quantify the concentration of O c and O
2 2
ꢀ
1
11 and 9 times, respectively, in the amount of O
2
c
and O
2
electron and hole pairs. Photogenerated electrons will react
ꢀ
generated from COCN-2 was achieved compared to pristine with the surface of O
2
to form a radical species O
2
c . Subse-
ꢀ
+
1
C N . It is deduced that high oxygen adsorption and excellent quently, a quantity of O2c was attacked by h to form O .
3
4
2
ꢀ
electron-transfer efficiency can be achieved to form O c and Finally, the sulfoxide product could be obtained from further
2
1
1
O
2
over COCN-2. To investigate whether
2
O formation oxidizing sulde using singlet oxygen. Another minor pathway
undergoes a charge transfer process, p-benzoquinone (BQ) as an is that the photogenerated holes are activated with sulde to
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ꢀ
1
Fig. 4 (a) DMPO spin trapping and (b) TEMP spin trapping ESR technique, (c) O
2
c
and (d) O
2
formation during the photoreaction, and (e) time-
2 2
dependent photocatalytic H production over different catalysts. (f) Cycling performance of photocatalytic H production over COCN-2.
produce the corresponding intermediate, followed by reaction liquid contact angle follows the order C
3
N
4
< COCN-3 < COCN-2
ꢀ
with O
2
c
to generate sulfoxide.
< COCN-1, which is in harmony with the content of polar
groups. It should be noted that COCN-1 possessed abundant
C–O–C groups but had less carrier mobility. Therefore, the
excellent photocatalytic activity in hydrogen production could
be ascribed to ameliorated hydrophilicity and the enhancement
of photoinduced charge separation. The zeta potential was
measured to reect the status of the surface charge. As shown in
Table S6,† the absolute value of the zeta potential of COCN-2 is
Photocatalytic hydrogen production
3 4
The photocatalytic activities of COCN and C N were also
studied for H
average H generation rates over C
COCN-3 were ca. 5.6, 24.6, 34.9 and 15.9 mmol h , respectively
Fig. 4e). The COCN-2 photocatalyst shows about six times the
activity of C . The hydrophilic properties on the surface of all
2
evolution under visible light illumination. The
2
3 4
N
, COCN-1, COCN-2 and
ꢀ
1
(
+
highest among the prepared samples, indicating that H can be
signicantly adsorbed over COCN by the electronegative
3 4
N
the catalysts can also be observed in Fig. S19.† The trend of the
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clusters. Besides the photocatalytic performance, the cycling
stability of COCN-2 was studied in eight repeated tests of pho-
tocatalytic hydrogen production and each test was maintained
for 4 h (Fig. 4f). There was no signicant deactivation and no
structural collapse was detected (Fig. S20†), evidencing the
outstanding stability of the inherent structure of COCN-2.
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