Photoassisted Construction of Holey Defective g-C3N4 Photocatalysts for Efficient Visible-Light-Driven H2O2 Production

Article abstract of DOI:10.1002/smll.201703142

Holey defective g-C₃N₄ photocatalysts, which are easily prepared via a novel photoassisted heating process, are reported. The photoassisted treatment not only helps to create abundant holes, endowing g-C₃N₄ with more exposed catalytic active sites and crossplane diffusion channels to shorten the diffusion distance of both reactants from the surface to bulk and charge carriers from the bulk to surface, but also introduces nitrogen vacancies in the tri-s-triazine repeating units of g-C₃N₄, inducing the narrowing of intrinsic bandgap and the formation of defect states within bandgap to extend the visible-light absorption range and suppress the radiative electron–hole recombination. As a result, the holey defective g-C₃N₄ photocatalysts show much higher photocatalytic activity for H₂O₂ production with optimized enhancement up to ten times higher than pristine bulk g-C₃N₄. The newly developed synthetic strategy adopted here enables the sufficient utilization of solar energy and shows rather promising for the modification of other materials for efficient energy-related applications.

Full text of DOI:10.1002/smll.201703142

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Photocatalysts
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Photoassisted Construction of Holey Defective
g-C3N4 Photocatalysts for Efficient Visible-Light-Driven
H2O2 Production
Li Shi, Liuqing Yang, Wei Zhou, Yanyu Liu, Lisha Yin, Xiao Hai, Hui Song, and Jinhua Ye*
1. Introduction
Holey defective g-C3N4 photocatalysts, which are easily prepared via a novel
photoassisted heating process, are reported. The photoassisted treat-
ment not only helps to create abundant holes, endowing g-C3N4 with more
Hydrogen peroxide (H2O2) has attracted
much attention as an ideal energy car-
exposed catalytic active sites and crossplane diffusion channels to shorten
the diffusion distance of both reactants from the surface to bulk and charge
carriers from the bulk to surface, but also introduces nitrogen vacancies in
the tri-s-triazine repeating units of g-C3N4, inducing the narrowing of intrinsic
bandgap and the formation of defect states within bandgap to extend the
visible-light absorption range and suppress the radiative electron–hole recom-
bination. As a result, the holey defective g-C3N4 photocatalysts show much
higher photocatalytic activity for H2O2 production with optimized enhance-
ment up to ten times higher than pristine bulk g-C3N4. The newly developed
synthetic strategy adopted here enables the sufficient utilization of solar
energy and shows rather promising for the modification of other materials for
efficient energy-related applications.
rier alternative to hydrogen, because it is
soluble in water and can be used in a one-
compartment fuel cell to generate elec-
tricity.[1,2] Semiconductor photocatalysis
is regarded as one of promising technolo-
gies for H2O2 synthesis, due to the safe
and energy-saving properties.[3–5] In the
photocatalytic process, H2O2 can be pro-
duced via two-electron reduction of O2 by
the conduction band (CB) electrons from
photoexcited semiconductor materials.[3]
Currently, numerous research studies
have mainly focused on the search and
construction of highly efficient photocata-
lysts for energy-related applications.[6–8]
Among various photocatalysts investi-
gated, graphitic carbon nitride (g-C3N4) has stimulated tremen-
dous research interest, mainly due to its unique features, such
as suitable bandgap for visible light absorption, proper band
edges for target reactions, environmental benignity, fairly good
thermal and chemical stability, and cost effective metal-free
composition.[9–16] Up to now, g-C3N4 has achieved good photo-
catalytic performances in many kinds of photocatalytic areas,
such as H2/O2 evolution from water splitting, organic pollutant
photodecomposition, and CO2 reduction.[11,17–24] Recent works
also indicated that g-C3N4 with suitable bandgap and band edge
position could promote efficient two-electron reduction of O2 to
produce H2O2 under visible light irradiation.[25–28]
Bulk g-C3N4 (BCN) has limited visible-light absorption
ability and also suffers from a low separation efficiency of
photoexcited charge carriers, resulting in low photocatalytic
activity.[29] Aiming at improving the photocatalytic perfor-
mance of g-C3N4, various strategies have been developed, such
as heteroatoms doping,[30,31] nanostructure design,[32–35] dye
sensitization,[36,37] and coupling with other semiconductors
or conductors.[38–41] Particularly, the introduction of nitrogen
defects into the g-C3N4 framework was recently developed as
an effective strategy to significantly enhance the photocatalytic
activity of g-C3N4 under visible light.[42–45] The introduction of
nitrogen defects in the tri-s-triazine repeating units of g-C3N4
can not only modify the electronic structure by narrowing the
bandgap to promote more visible light harvesting, but also act
as specific reactive sites for photocatalytic reactions.[42] The
Dr. L. Shi, Dr. X. Hai, H. Song, Prof. J. Ye
Graduate School of Chemical Sciences and Engineering
Hokkaido University
Sapporo 060-0814, Japan
E-mail: Jinhua.YE@nims.go.jp
Dr. L. Shi, L. Yang, Dr. L. Yin, Dr. X. Hai, H. Song, Prof. J. Ye
International Center for Materials Nanoarchitectonics (WPI-MANA)
National Institute for Materials Science (NIMS)
1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
Prof. W. Zhou, Y. Liu
Department of Applied Physics
Tianjin Key Laboratory of Low Dimensional Materials Physics
and Preparing Technology
Faculty of Science
Tianjin University
Tianjin 300072, P. R. China
Prof. J. Ye
TU-NIMS Joint Research Center
School of Materials Science and Engineering
Tianjin University
92 Weijin Road, Nankai District, Tianjin 300072, P. R. China
Prof. J. Ye
Collaborative Innovation Center of Chemical Science and Engineering
Tianjin 300072, P. R. China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/smll.201703142.
DOI: 10.1002/smll.201703142
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Figure 1. a) Illustration of the preparation process of holey DCN, and the insets are their simulated structure (H, C, and N atoms are represented
by the gray, red, and blue balls). b) The relationship between light intensity and current output; c) the increase in temperature of g-C3N4 under light
irradiation at different current output.
positive role of nitrogen defects in affecting the photocatalytic
activities of g-C3N4 for H2 evolution and organic molecular
2. Results and Discussion
degradation has been actively reported, but rare for H2O2 pro-
duction. What is more, approaches used to date for the intro-
duction of nitrogen defects in g-C3N4 frameworks are usually
based on the solid-state thermal polymerization or hydro-
thermal process heated at high temperature in an electric
furnace in the presence of reducing atmosphere, which have
inherent drawbacks such as high energy consumption.[45–47]
Therefore, it is desirable to explore more facile and efficient
approaches toward the construction of nitrogen defective
g-C3N4 (DCN).
The recently emerged rapid heating of materials by photon
energy[48] inspired us to seek new methods for preparing
modified g-C3N4. In this work, we describe a novel one-step
photoassisted route to prepare holey DCN by using hydrazine
as a reducing agent (Figure 1a). In sharp contrast to the con-
ventional approaches based on heating in an electric furnace,
the photoassisted approach enables the rapid heating of mate-
rials within few minutes owing to the remarkable and prompt
increase in the local temperature of materials under light irra-
diation. The introduction of nitrogen defects in g-C3N4 can not
only narrow its intrinsic bandgap, but also create abundant
defect states within bandgap, greatly increasing the visible-
light absorption range and suppressing radiative electron–hole
recombination. Moreover, holey structure can be built after this
photoassisted reduction process, which allows for exposing
more active sites and facilitates the accessibility of reactants
to the active surface. As a result, the holey DCN shows much
improved photocatalytic activity for H2O2 production than pris-
tine BCN.
Direct irradiation of the BCN sample with hydrazine by a 300 W
Xe lamp (the current outputs are set from 7 to 21 A) can produce
DCN (χ A; χ indicates the current output). The light intensity
of Xe lamp can be controlled by adjusting the current output
(Figure 1b). The changes in temperature of BCN powder upon
light irradiation at different intensities are depicted in Figure 1c.
As can be seen, upon light irradiation, an instant and remarkable
increase in the temperature of sample was observed (reaching
the maximum temperature within only about 5 min). The fast
and controllable photoassisted treatment enables the heating
of g-C3N4 within few minutes, showing a desirable and effec-
tive synthetic approach for the modification of g-C3N4. At a light
intensity of 774 mW cm−2 (full arc with a maximum input power
of 300 W), the temperature of BCN reaches up to 350 °C. The
good photothermal property of BCN indicates its efficient uti-
lization of light energy. Under such high temperature, hydra-
zine can be evaporated, and also some part of hydrazine can be
decomposed into reductive products NH3 and H2 according to
the reaction equation (Equation S1, Supporting Information).
The formation of NH3 and H2 can be evidenced by gas chro-
matography–mass spectrometry (GC-MS) results, as shown in
Figure S1 in the Supporting Information. Hydrazine, NH3, and
H2 are strong reducing agents, which would react with BCN
at such high temperature under circulation system (Figure S2,
Supporting Information) to produce DCN samples. To the best
of our knowledge, this should be the first report on the utiliza-
tion of photoenergy for the modification of g-C3N4.
The evolution of X-ray diffraction (XRD) patterns of the
products obtained as a function of the current output is given
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Figure 2. a) XRD patterns and b) FTIR spectra of BCN and DCN samples.
in Figure 2a. All the DCN samples show two typical diffraction
peaks centered at 13.1° and 27.6°, which are consistent with the
pristine BCN sample. Those two diffraction peaks of 13.1° and
27.6° are associated with the in-plane repeating units of the con-
tinuous heptazine framework and the stacking of the conjugated
aromatic structure along c-axis of g-C3N4, respectively.[49] It indi-
cates that the crystal structure of g-C3N4 is largely retained after
the photothermal reduction process. Fourier transform infrared
(FTIR) spectroscopy was used to further investigate the micro-
structure evolution of the g-C3N4. FTIR spectra (Figure 2b) also
show no much change in the feature bands between 1900 and
700 cm−1 attributed to the trigonal N(C)3/bridging HN(C2) units
and heptazine ring of the melon unit.[50] This suggests that the
original g-C3N4 backbone remains almost unchanged, which is
inconsistent with the XRD results. The results of XRD patterns
and FTIR spectra prove that the conjugated backbone structures
of DCN are well maintained, which is essential for π-delocalized
electronic systems to generate and transport photoexcited elec-
tron–hole pairs for the subsequent redox reactions.
Brunauer-Emmett-Teller (BET) surface area and pore volume
compared with pristine BCN (Figure S6 and Table S1, Sup-
porting Information). Such a dramatically increased BET sur-
face area and the formation of abundant holes in DCN could
allow for exposing more active sites and facilitate the accessi-
bility of reactants to the active surface, which should be more
favorable for photocatalysis.[51,52]
The successful introduction of defects into g-C3N4 via pho-
tothermal treatment can be evidenced by X-ray photoelectron
spectroscopy (XPS). High-resolution XPS spectra (Figures S7
and S8, Supporting Information) give similar C 1s and N 1s
core electron peak regions of the carbon nitride products,
indicating that this photothermal treatment has little influ-
ence on the C–N aromatic systems. The deconvolution of the
overlapped N 1s peaks has produced three peaks centered at
around 398.1, 398.8, and 400.2 eV, which can be ascribed to sp2-
hybridized aromatic nitrogen atoms bonded to carbon atoms
 

(C N C), tertiary nitrogen (N (C)3) groups, and amino func-
[49]
 
tional groups(C N H), respectively. The elemental analysis
In contrast to the unchanged crystal structure, the mor-
phology of the g-C3N4 after photothermal reduction reaction
is greatly changed, as demonstrated by the scanning elec-
tron microscopy (SEM) images in Figure 3. The pristine BCN
sample displays the smooth surface; however, after photo-
thermal treatment, the surfaces of resultant samples have been
corroded and appear as irregular holey structures (Figure 3).
The diameters of holes range from several nanometers to dozens
of nanometers. The formation of holy structure should be from
the hydrazine etching at high temperature, since no obvious
morphology change is observed after photoassisted treatment
without adding hydrazine (Figure S3, Supporting Informa-
tion). The transmission electron microscopy (TEM) images,
as shown in Figure S4 in the Supporting Information, further
confirm that the DCN samples show holey structure, which are
in good agreement with the SEM observation. The high angle
annular dark field-scanning transmission electron microscopy
(HAADF-STEM) image of DCN-15A further indicates its holey
structure, and the corresponding elemental mapping images
show that both C and N elements are homogeneously distrib-
uted in a holey way similar to the shape of the HAADF-STEM
image (Figure S5, Supporting Information). Because of the for-
mation of holey structure, DCN samples show the increased
manifests that the C/N atomic ratio gradually increases from
0.686 of BCN to 0.737 of DCN-21A (Table S2, Supporting Infor-
mation), suggesting the preferential loss of N atoms during
photothermal treatment. The C/N atomic ratio of DCN-21A
sample becomes closer to the value of the stoichiometric carbon
nitride (0.750). It is well known that the existence of residual
amine groups is responsible for the higher concentration of
nitrogen in the commonly available g-C3N4 than that in the
stoichiometric one.[50] Moreover, it has been demonstrated that
the loss of nitrogen in the amine groups was much easier
than that in the N(sp2) and N(sp3) groups for g-C3N4.[43,50] There-
fore, the increased C/N atomic ratio observed could be caused
by the partial loss of the amine groups from the commonly
available g-C3N4, which can be confirmed by the decreased peak
area ratio and slightly increased binding energy of the amino
 
functional groups (C N H) in the N 1s spectra of the carbon
nitrides (Table S3, Supporting Information). This is also con-
firmed by the increased intensity of the 285.5 eV peak origi-

nating from the N C bonding structure in the high resolution
C 1s spectra of the carbon nitrides with the increase of cur-
rent output (Figure S8 and Table S4, Supporting Information),
which results from the defect-containing sp2-bonded carbon
atoms.[53,54] The measured H amount decreased from 2.27 wt%
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Figure 3. SEM images of a) BCN, b) DCN-7A, c) DCN-11A, d) DCN-15A, e) DCN-19A, and f) DCN-21A samples, respectively.
in BCN to 1.82 wt% in DCN-21A. The proposed schematic for
the formation of this DCN by the photothermal treatment of
BCN sample is illustrated in Figure S9 in the Supporting Infor-
mation, and the BCN loses one NH2 group to form nitrogen
vacancies.
is more reliable. Figure 4c shows the optical properties of BCN
and all DCN samples, as measured by UV–vis absorption
spectra. As expected, the absorption edges of the photother-
mally treated g-C3N4 samples display remarkable redshifts in
comparison with the pristine bulk sample, which renders the
narrowing of the intrinsic bandgaps. Consistent with these
absorption results, the color of the powders suffers a matching
variation from yellow to brown as the current output increases
(insets in Figure 4c). The plots (Figure S11, Supporting Infor-
mation) of the transformed Kubelka–Munk function versus
the light energy show a gradual intrinsic bandgap narrowing
from 2.75 eV of BCN to 2.55 eV of DCN-21A. The bandgap nar-
rowing can be well explained as a consequence of the changed
electronic structure caused by nitrogen vacancies, which is con-
sistent with the DFT calculation results. To further investigate
the influence of nitrogen vacancies on the relative positions of
the conduction band minimum and valence band maximum
of the samples, valence band X-ray photoelectron spectroscopy
(VB-XPS) were employed to determine the electronic structure.
The VB-XPS spectra show that the DCN samples have lower VB
The above microstructure modification caused by nitrogen
vacancies has a great influence on the electronic bandgap struc-
ture of g-C3N4, which can be evidenced by density functional
theory (DFT) studies. As shown in Figure 4a,b, the calculated
density of states of DCN shows an obvious decrease of intrinsic
bandgap compared with BCN. Moreover, it can be seen that
defect states appear below the CB of BCN in Figure 4b. The
defect states can accommodate the electrons photoexcited
from the valence band (VB) of BCN, which greatly contribute
to the absorption of photons with energies smaller than the
bandgap, promoting to more light harvesting. No defect states
can be theoretically obtained if we use H substituting the N site
(one NH2 group is replaced by an H atom) as the model struc-
ture (Figure S10, Supporting Information), indicating that the
simulated structure in Figure S9 in the Supporting Information
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Figure 4. Density of states of a) BCN and b) DCN calculated from the model of Figure S8 in the Supporting Information. c) UV–vis absorption spectra
and corresponding pictures of BCN and DCN samples, from left to right: BCN, DCN-7A, DCN-11A, DCN-15A, DCN-19A, and DCN-21A. d) ESR spectra
of BCN and DCN samples.
edges than the BCN (Figure S12, Supporting Information). The
CB position of BCN can be obtained by investigating the elec-
trochemical Mott– plot (Figure S13, Supporting Information).
The flat band potential of BCN is found to be at −1.17 V (vs
normal hydrogen electrode (NHE), pH 7). Based on the differ-
ences in bandgaps and VB edges between the BCN and DCN,
their band alignments are depicted in Figure S14, in the Sup-
porting Information. The nature of the bandgap narrowing of
the DCN is that nitrogen vacancies lead to the downshift of
both the VB and the CB. The CB positions of DCN samples are
still more negative than the reduction potential of O2 to H2O2
(Figure S14, Supporting Information).[26]
that the photothermal treatment of g-C3N4 maintains its semi-
conducting properties.
Obviously, as shown in Figure 4c, broader and stronger
absorption tails extending to 600 nm are also clearly observed
for DCN samples. Usually, the absorption tail is attributed to
the defect-related states located within the bandgap, which
is consistent with the theoretical prediction (Figure 4b).
Increasing the photothermal treatment temperature leads to
a gradually increased band-tail-related absorption, indicating
that higher density of defect states can be created at a higher
temperature. The introduction of defect states leads to substan-
tial suppression of radiative electron–hole recombination in
carbon nitrides, as indicated by the greatly weakened photolu-
minescence (PL) peak in Figure 5a. For the pristine BCN, the
spectrum shows a strong emission peak at 456 nm, which is
originated from the band-to-band recombination of electrons
and holes. The intensity of this band-to-band emission peak
decreased dramatically upon the introduction of defect states,
which means that the band-to-band recombination of elec-
trons and holes was effectively inhibited. Moreover, the emis-
sion peak experiences a gradual redshift, which is attributed
to the narrowing of bandgaps for DCN samples. To further
reveal the photophysical characteristics of the photogenerated
charge carriers, the time-resolved PL spectra were investigated
and displayed in Figure 5b, which were monitored at the wave-
length of the emission peak of each sample. By the triexponen-
tial fitting, the average lifetimes of the samples are calculated
and presented in Figure 5b. The average PL lifetime gradually
decreases from 15.85 ns in the pristine BCN to 5.04 ns in the
To monitor the evolution of the electronic band structure
for DCN with nitrogen vacancies, the room-temperature elec-
tron spin resonance (ESR) spectra were recorded. As shown
in Figure 4d, all the samples exhibit an ESR signal with a g
value of 2.003, which can be attributed to unpaired elec-
trons on the sp2-carbon atoms of the aromatic rings within
π-bonded nanosized clusters.[49] Compared with pristine BCN,
the gradually enhanced spin intensity of DCN samples gives
evidence for the promoted formation of unpaired electrons
after the introduction of nitrogen vacancies. This is probably
due to the fact that the loss of nitrogen atoms would leave
extra electrons, which are redistributed to their nearest carbon
atoms delocalized among the big π-bonds of the g-C3N4, and
thus lead to the formation of more unpaired electrons.[42]
A
slightly enhanced ESR signal under visible light illumination
of DCN-21A provides the clue of promoted photochemical
generation of radical pairs on its CB (Figure 4d), indicating
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DCN-21A. The decreased lifetimes indicate that the depopu-
lation of the excited states in the obtained DCN occurs more
readily through nonradiative pathways, presumably by charge
transfer of the electrons to some favorable defect states, e.g.,
surface states, and thus point to the improved charge transfer
and separation.
The photocatalytic activities of the prepared samples were
evaluated by H2O2 production from O2 reduction. The photo-
catalytic reactions were performed with isopropanol (IPA) as
electron and proton donor, and the setup of the experiment
was shown in Figure S15 in the Supporting Information. An
IPA/water mixture containing g-C3N4 photocatalyst was photo-
irradiated by simulated sunlight with an L-42 cutoff filter
(AM1.5, λ > 420 nm) under stirring and O2-equilibrated condi-
tions. The H2O2 is produced via a two-electron reduction of O2,
and the reaction equations can be described as follows[25]
discussed later. After photocatalytic reaction for 2.5 h, about
12.1 µmol H2O2 was produced over DCN-15A, which is
10.1 times higher than that of the pristine BCN. DCN-15A shows
higher photocatalytic activity for H2O2 production than some
reported g-C3N4-based photocatalysts (Table S5, Supporting
Information). Moreover, DCN-15A also shows higher activity
than graphitic carbon nitride nanosheet, H3PO4 protonated
porous graphitic carbon nitride nanosheet, and DCN under
the same reaction conditions (see more details in Figures S18
and S19, Supporting Information), which indicates that the
introduction of nitrogen vacancies via photothermal treatment
should represent an effective approach to improve the photocat-
alytic H2O2 production activity for g-C3N4-related photocatalysts.
The wavelength-dependent apparent quantum efficiency
(AQE) of H2O2 production was then further investigated to
prove whether or not the reaction proceeds through light
absorption. Figure 6c shows the UV–vis diffuse reflectance
spectra of the DCN-15A along with the AQE of H2O2 evolu-
tion as a function of the incident light wavelength. The trend of
AQE almost agrees with the absorption spectrum of DCN-15A,
indicating that photoexcitation of DCN-15A promotes H2O2 for-
mation. The photocatalytic stability of this sample was evalu-
ated by collecting and reusing the DCN-15A for three times. As
shown in Figure 6d, the DCN-15A can retain its good photocat-
alytic activity after three cycles of reaction. XRD patterns were
further employed to characterize the recovered catalyst after
the third cycling reactions (Figure S20, Supporting Informa-
tion). As expected, any obvious changes in structure were not
observed, again supporting the robust nature of DCN-15A.
The above information clearly shows the advantages of the
photothermal-treated g-C3N4 for photocatalytic H2O2 produc-
tion. We are now in a position to try to understand the mecha-
nisms behind the functions. The photocatalytic efficiency of a
photocatalyst is usually determined by the synergistic effects of
three basic steps of photocatalysis, (i) light absorption ability,
(ii) charge separation efficiency, and (iii) surface transfer of
photogenerated charge carriers.[50] On the one hand, the higher
BET surface area combined with the unique holey structure
provides DCN samples with more exposed active catalytic sites
and channels, which play an important role in shortening the
diffusion lengths of both the photoexcited electrons from the
interior to edges of the holes and reactants from the surface to
CH3CHOHCH3 + 2h+ → CH3COCH3 + 2H+
O2 + 2H+ + 2e− → H2O2
(1)
(2)
The generated H2O2 can be monitored via N,N-diethyl-p-phe-
nylenediamine (DPD)–horseradish peroxidase (POD) method
(see more details in Experimental Section). The time-dependent
absorption spectra of the DPD/POD solution with different
photocatalysts were shown in Figure S16 in the Supporting
Information. The concentration of H2O2 can be monitored by
the absorption intensity of the solution at 551 nm, which can be
calculated through the prepared standard curves (Figure S17,
Supporting Information). Figure 6a shows the time-dependent
evolution of H2O2 produced by BCN and DCN samples. The
concentration of H2O2 increases almost linearly as irradiation
time prolongs, indicating that all the samples are able to pro-
duce H2O2 in the photocatalytic reaction system. The produc-
tion of H2O2 was negligible in a control experiment when O2
was replaced by Ar in the photocatalytic reaction, indicating
that H2O2 was indeed generated from photocatalytic O2 reduc-
tion. All the DCN samples show a much superior activity to the
pristine BCN sample. The photocatalytic H2O2 production rate
of the samples increases with increasing current output up to
15 A. Further increasing the current output to 19 or 21 A gives
rise to a gradually decreased activity but they are still superior
to the pristine sample. The reason for this trend will be
Figure 5. a) PL spectra and b) decay time spectra of BCN and DCN samples.
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Figure 6. a) Time-dependent evolution of H2O2 produced by BCN and DCN samples. b) photocatalytic H2O2 activity comparison of different samples.
c) UV–vis absorbance spectrum and wavelength-dependent AQE of DCN-15A. d) Recycling test of DCN-15A. e) Schematic of mechanisms underlying
the photoexcited dynamics involved in photocatalytic H2O2 evolution over BCN and DCN-15A.
the interior of g-C3N4 particle along interconnected holes. These
structure-induced favorable features can lower the bulk recom-
bination probability of electrons and holes, and also promote
mass transport in DCN, definitely favoring the proceeding of
photocatalytic reactions. On the other hand, the introduction
of nitrogen vacancies via photothermal reduction reaction can
extend visible-light absorption range to increase the low-energy
photon excitations, whose origin is the decreased intrinsic
bandgap and the formation of defect states. The introduction
of defect states in DCN suppresses the radiative electron–
hole pairs recombination as demonstrated by Figure 5, which
is due to the CB-to-defect states charge transfer processes
induced by the increased defects/localized states within the
bandgaps (Figure 6e), thus point to the improved charge sepa-
ration. These electronic structure-induced favorable features
also favor the proceeding of photocatalytic reactions. However,
the introduction of nitrogen vacancies would also induce the
downshift of CB positions and defect states (Figure S21, Sup-
porting Information), which would decrease the reductive
ability of photogenerated electrons, exerting a negative effect on
the photocatalytic H2O2 production. Moreover, the overforma-
tion of bulk defects beyond 15 A could serve as recombination
centers for electrons and holes. Those two main reasons can
explain why the photocatalytic activities decrease beyond 15 A.
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Photocatalytic Experiments: The photocatalytic H2O2 production
reactions were carried out with 50 mg photocatalyst suspending in an
O2-equilibrated aqueous solution (60 mL solution containing 20 vol%
IPA as a sacrificial electron donor) in a glass reaction cell (Figure S15,
Supporting Information). A simulated sunlight with an L-42 cutoff
filter (AM1.5, λ > 420 nm) was used as the visible light source. The
evolved amount of H2O2 was measured by using the colorimetric DPD
method. The DPD method is based on the oxidation of N,N-diethyl-p-
phenylenediamine (DPD) catalyzed with horseradish peroxides (POD)
by H2O2. Briefly, 0.1 g of DPD was dissolved in 10 mL 0.05 m H2SO4,
denoted as solution A. 10 mg of peroxidase (POD) was dissolved in
10 mL water, denoted as solution B. 10 mL of 0.1 m Na2HPO4 solution and
90 mL of 0.1 m NaH2PO4 solution were mixed together to obtain solution
C. Then 3 mL deionized water, 0.3 mL solution C, 30 µL solution A and
30 µL solution B were mixed together to obtain solution D. The time-
dependent H2O2 generation was measured as follows: 1 mL filtration
from reaction solution was dispersed into solution D, which was
monitored by the absorption of the solution at 551 nm and determined
with UV–vis spectrophotometer. The AQE was measured by using an
AM1.5 lamp combined with wavelength-dependent band-pass filters
(MIF-W, Optical Coatings Japan Co., Japan). A radiant power energy
meter (Ushio Spectroradiometer, USR-40) was applied to measure the
number of incident photons. The AQE was calculated according to the
following equations
3. Conclusion
In summary, holey DCN was successfully constructed via a facile
photoassisted heating process. The obtained DCN products
possess not only tunable electronic structures controlled by
nitrogen vacancies, but also holey structure with abundant
holes formed throughout the whole particles. The narrowed
bandgap and the rationally created defect states enable DCN to
harvest visible light and suppress radiative electron–hole recom-
bination more efficiently than pristine BCN. In addition, the
abundant holes can provide DCN more reductive reaction sites
and facilitate the accessibility of reactants to the active surface.
As a consequence of these favorable features, the DCN with an
optimized nitrogen vacancies introduction gives a significantly
enhanced photocatalytic H2O2 production activity under visible
light. It is expected that the simple and highly effective strategy
adopted here could be extended to modify other materials for
improved solar energy capture and conversion.
4. Experimental Section
Preparation of the Photocatalysts: BCN was prepared by heating
dicyandiamide (Wako Co.) in a covered crucible to 550 °C for 4 h in air
at a heating rate of 5 °C min−1. DCN was prepared by photoassisted
treatment of BCN with hydrazine in a batch-type reaction system with
a total volume of about 330 mL (Figure S2, Supporting Information). In
brief, 0.2 g BCN powder was dispersed on a porous quartzose film in the
reaction cell, followed by injecting 3 mL hydrazine solution. The reaction
system was evacuated for a few minutes to remove air completely
(no N2 and O2 could be detected), and then was heated by a 300 W Xe
lamp with adjustable current output (from 7 to 21 A) for 30 min. After
photoassisted treatment, the obtained DCN samples were cooled down
to room temperature and collected by washing with water for several
times, and dried under vacuum at 60 °C.
number of reacted electrons
number of incident photons
AQE (%) =
× 100%
number of evolvedhydrogenperoxide molecules × 2
=
× 100%
(3)
number of incudent photons
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Characterization: The XRD patterns of the prepared photocatalysts were
measured by powder XRD on an X’Pert PRO diffractometer (PANalytical).
The UV–vis spectra of the photocatalysts powder were obtained on
a UV−visible spectrophotometer (SHIMADZU UV-2600) and BaSO4
was used as the reflectance standard reference. The SEM images of the
as-prepared photocatalysts were taken on a scanning electron microscope
(S4800, Hitachi Co., Japan). TEM, HAADF-STEM and elemental mapping
images were taken on a transmission electron microscope (2100F,
JEOL Co., Japan). Nitrogen adsorption–desorption isotherms and BET
surface area were recorded using a BEL SORP-mini II (BEL Japan INC.,
Japan) at 77 K. The PL spectra were recorded by using JASCO FP-6500
spectrofluorometer, and the excitation wavelength was 370 nm. The decay
time measurements were carried out on a compact fluorescence lifetime
spectrometer (Quantaurus-Tau, C11367, HAMAMATSU), and an LED
lamp (365 nm) was used as an excitation source. XPS (PHI Quantera
SXM, ULVAC-PHI Inc., Japan) was used to analyze the compositions and
chemical state of the samples. The FTIR spectra of the samples were
studied by Thermoscientific Nicolet 4700. The ESR measurements were
carried out with JEOL JES-FA-200 at room temperature in the air.
Acknowledgements
This work received financial support from the World Premier
International Research Center Initiative (WPI Initiative) on Materials
Nano-architectonics (MANA), MEXT (Japan), the National Basic
Research Program of China (973 Program, 2014CB239301), and the
National Natural Science Foundation of China (21633004).
Conflict of Interest
The authors declare no conflict of interest.
Keywords
g-C3N4, nitrogen defects, photocatalyst, photoassisted
DFT Calculations: All calculations here were performed with the
Vienna ab-initio Simulation Package based on the DFT. The Projector-
augmented wave with generalized gradient approximation (GGA)-PBE
function was used for the electron–ion interactions. The number of k
points and the cutoff energy were increased until the calculated total
energy converged within an error of 1 × 10−5 eV atom−1. Therefore, a
cutoff energy of 500 eV with 3 × 3 × 1 k points was set for the slab model
with a 14 Å thickness of vacuum layer. The energy convergence tolerance
was set to below 1 × 10−5 eV atom−1. The lattice and atomic coordinates
were relaxed until the Hellmann–Feynman force on each atom was
Received: September 11, 2017
Revised: November 27, 2017
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