Sacrificial agent-free photocatalytic H2O2evolutionviatwo-electron oxygen reduction using a ternary α-Fe2O3/CQD?g-C3N4photocatalyst with broad-spectrum response

Article abstract of DOI:10.1039/d0ta05753c

Ultrathin g-C₃N₄nanosheets have been fabricatedviaa two-step calcination regulated by melamine precursors at a high heating rate (30 °C min^?1). The resulting g-C₃N₄nanosheets were further employed as carriers for the growth of carbon quantum dots (CQDs) and (110) exposed α-Fe₂O₃through the PVP-enabled adsorption effects by a solvothermal process. It was discovered that the so fabricated ternary photocatalyst α-Fe₂O₃/CQD?g-C₃N₄presented a broad-spectrum absorption range (up to 800 nm) and particularly enhanced active sites of photogenerated electrons for highly efficient photocatalytic oxygen reduction toward H₂O₂evolution in pure water. A H₂O₂production rate of 1.16 μM min^?1could be expected for the developed photocatalyst under visible light irradiation, which is about 19 times faster than that of pure ultrathin g-C₃N₄. Herein, the loaded Fe₂O₃could transform the H₂O₂evolution from two-step single-electron reduction into one-step two-electron one, as verified by the various active species experiments and rotating ring-disk electrode tests. This work presents a new perspective in designing ultrathin g-C₃N₄through a simple method of precursor-regulated calcination, which features more outstanding advantages than the conventional exfoliation of bulk g-C₃N₄towards ultrathin g-C₃N₄. More importantly, it provides an optimized photocatalytic reaction route of two-electron oxygen reduction for efficient H₂O₂production in pure water under visible light irradiation, without the need for noble metals or organic sacrificial agents.

Full text of DOI:10.1039/d0ta05753c

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March 2018
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Journal of Materials Chemistry A
ARTICLE
Sacrificial agent-free photocatalytic H O evolution via two-
2
2
electron oxygen reduction using a ternary α-Fe O /CQD@g-C N
2
3
3
4
photocatalyst with broad-spectrum response
a,b
b
cb
a,b
b
d
Xi Chen, Wenwen Zhang, Lixiang Zhang, Luping Feng, Chunxian Zhang, Jie Jiang, Tingjiang
b
a,b,d
Yan, Hua Wang
Ultrathin g-C
heating rate (30 °C/min). The resulting g-C
quantum dots (CQDs) and (110) exposed α-Fe
The so fabricated ternary photocatalyst of α-Fe
absorption range (up to 800 nm) and especially enhanced active sites of photogenerated electrons for highly efficient
photocatalytic oxygen reduction toward H evolution in pure water. A H
production rate of 1.16 µM·min-1 could be
expected for the developed photocatalysts under visible light irradiation, which is about 19 times faster than that of pure
ultrathin g-C . Herein, the loaded Fe could transform the H evolution from two-step single-electron reduction into
one-step two-electron one, as verified by the various active species experiments and rotating ring-disk electrode tests.
This work presents a new perspective in designing ultrathin g-C through a simple method of precursor-regulated
calcination, which features the more outstanding advantages than the conventional exfoliation of bulk g-C towards
ultrathin g-C . More importantly, it provides a optimized photocatalytic reaction route of two-electron oxygen reduction
for efficient H
sacrificial agents.
3
N
4
nanosheets have been fabricated via a two-step calcination regulated by melamine precursors at a high
nanosheets were further employed as carriers for the growth of carbon
through the PVP-enabled absorption effects by a solvothermal process.
/CQD@g-C was discovered to present the broad-spectrum
3
N
4
2 3
O
2
O
3
3 4
N
2
O
2
2 2
O
Received 00th January 20xx,
Accepted 00th January 20xx
3
N
4
2
O
3
2 2
O
3 4
N
DOI: 10.1039/x0xx00000x
3 4
N
www.rsc.org/
3
N
4
2
O
2
production in pure water under visible light irradiation, without the need for noble metals or organic
large-scale industrialization.7-9 Hence, fabricating a cost-
effective and green photocatalyst for highly efficient H O
evolution in pure water has been an attractive target to
pursue.
Introduction
As an eco-friendly oxidant, hydrogen peroxide (H
been widely applied in the fields of medical disinfection,
cosmetics industry, and wastewater treatment. Recently,
photocatalytic H production techniques, especially those
2
2
2 2
O ) has
In recent years, graphitic carbon nitride (g-C
3 4
N ) has
1
-3
been increasingly employed as a green metal-free material in
2 2
O
1
0-21
the photocatalytic field.
3 4
However, g-C N in bulk format
driven by renewable and sufficient sunlight, have attracted
much attention to stand a chance of substituting the
may suffer from some inherent defects such as rapid charge-
carrier recombination, low electronic conductivity, and small
specific surface area, which may limit its large-scale
4
-6
traditional ones by anthraquinone process. However, high
yields have generally been acquired by using noble
2 2
H O
2
2-24
applications.
Alternatively, some endeavors have recently
metals or organic sacrificial agents to facilitate the efficient
separation of photogenic electron-hole pairs, which may go
against the manufacturing cost or product purification for
been devoted to prepare ultrathin or even single-layer g-
2
5-27
3 4
C N .
Inspired by the layering of graphene from bulk
graphite, the exfoliation methods have commonly been
employed to produce ultrathin g-C from bulk g-C , such
N
3 4
3 4
N
a. School of Chemistry and Chemical Engineering, Harbin Institute of Technology,
as the hyperthermia solvothermal process, hydrochloric acid
Harbin, Heilongjiang 150090, P. R. China.
Institute of Medicine and Materials Applied Technologies, College of Chemistry
28-31
protonation and alcohols sonication.
Nevertheless, these
b.
top-down methods should require the fabrication of high-
quality bulk materials, making it a more fussy and time-
consuming procedure. Therefore, the simple and efficient
and Chemical Engineering, Qufu Normal University, Qufu, Shandong 273165, P. R.
China.
School of Environment, Harbin Institute of Technology, Harbin, Heilongjiang
c.
1
50090, P. R. China.
d.
3 4
synthesis of ultrathin g-C N nanosheets is a challenging
School of Marine Science and Technology, Harbin Institute of Technology at
Weihai, Weihai, Shandong 264209, P. R. China.
issue. Moreover, recent years have witnessed the rapid
development of another type of eco-friendly materials of
carbon quantum dots (CQDs) as the active sites of
*
4
†
Corresponding Author: E-mail address: huawang@qfnu.edu.cn; Tel: +86 537
456306; Web: http://wang.qfnu.edu.cn.
Electronic
Supplementary
Information
(ESI)
available.
See
DOI: 10.1039/x0xx00000x
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Journal of Materials Chemistry A
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ARTICLE
Journal Name
photocatalysis, due to they possess the properties of fast
charge transfer, excellent photogenerated electron capture,
View Article Online
DOI: 10.1039/D0TA05753C
Nanorod-like precursors
A
D
B
C
3
2,33
and efficient up-conversion.
2 3
Besides, α-Fe O especially
the one with exposed (110) has been demonstrated with an
3
4
2
excellent O adsorption capacity.
3 4
In the present work, ultrathin g-C N nanosheets were
fabricated via the two-step calcination of melamine
precursors without the need of complicated pre- and post-
5
00 nm
50 nm
Calcination
processing steps. The obtained g-C
further used as carriers for the growth of CQDs and α-Fe
via a solvothermal process in the presence of PVP, yielding
the ternary photocatalysts of α-Fe /CQD@g-C , with the
3 4
N nanosheets were
Ultrathin g-C
3
N
4
Layered precursors
2 3
O
O
2 3
3 4
N
main procedure schematically illustrated in Scheme 1.
Calcination
Thereinto, CQDs were employed to remedy the drawback of
5
0 nm
50 nm
ultrathin g-C
with exposed (110) might serve as the photocatalytic active
centers for the H evolution. The so established carrier
transfer pathway of α-Fe /CQD@g-C was further
3 4 2 3
N regarding the broad band gap. Also, α-Fe O
Fig. 1 TEM images of the products yielded in the step-by-step calcination
procedure towards ultrathin g-C from (A) nanorod-like precursors with (B)
amplified part, (C) layered precursors, and (D) ultrathin g-C
2 2
O
3 4
N
O
2 3
3 4
N
3 4
N .
confirmed by the removal experiments of reactive species
and DMPO electron spin resonance, in addition to the
explanation by the energy band structure analysis. Of note,
the hydrolysis of melamine at room temperature showing a
mesoporous structure with long interlamellar distance, as
disclosed clearly in the amplified part (Fig. 1B). During the
calcination at 450 °C with a rapid heating, the layered
precursors were achieved due to the airflow produced from
glycol-intercalated cyanuric acid derivatized with abundant -
3 4
herein, the construction of ultrathin g-C N was achieved
through the precursor-regulated direct calcination route
serving as the carriers for the growth of CQDs and α-Fe
2
O
O
3
toward a superior ternary photocatalyst for efficient H
2
2
production under visible light, without the need of noble
metals or organic scavengers.
2
NH and -OH groups, resulting in the decreased interlamellar
distance (Fig. 1C). Furthermore, the thermal-polymerization
of layered precursors could occur during the continuous
calcination at higher temperature of 550 °C, yielding the
3 4
ultrathin g-C N nanosheets (Fig. 1D).
Synthesis and characterization of α-Fe
Herein, the α-Fe /CQD@g-C composites were prepared
alternatively through decorating with (110) exposed α-Fe
and CQDs on g-C nanosheets via a solvothermal process in
the presence of PVP, as illustrated in Scheme 1. Fig. 2
manifests the TEM images of α-Fe /CQD@g-C
nanosheets so produced. As can be seen from Fig. 2A, the
ultrathin g-C consists of thin layers with low density.
2 3 3 4
O /CQD@g-C N
2
O
3
3 4
N
2 3
O
3 4
N
2
O
3
3 4
N
3 4
N
Scheme 1 Schematic illustration for the synthesis of α-Fe
ternary photocatalysts via the solvothermal process, where CQDs and (110)
exposed α-Fe were decorated onto g-C nanosheets in the presence of
PVP using IPA and Fe(NO as the precursors.
2 3 3 4
O /CQD@g-C N
Herein, the curled edges of the nanosheets may demonstrate
the ultrathin structure owing to the minimization of surface
2
O
3
3 4
N
3
5
energy, as also revealed by the transparent character of the
3 3
)
image (Fig. 2B and Fig. S1). Additionally, Fig. S2 displays the
close connection among α-Fe O , CQDs, and g-C N
2 3 3 4
nanosheets, which can be witnessed more clearly in the
magnification-amplified parts. PVP as a nonionic polymer
Results and discussion
Preparation and characterization of ultrathin g-C
compound was employed to can integrate the g-C
3 4
N with α-
3 4
N
2 3
Fe O and CQDs so as to conduct the synergistic effect
Ultrathin g-C nanosheets were prepared directly by
3
N
4
achieving the boosted carrier transfer. Moreover, as is shown
calcining melamine precursors through a two-step rapid
heating process (30 °C/min), which may be completely
differed from the conventional top-down exfoliation way. The
products yielded in the step-by-step calcination procedure
were monitored by TEM imaging with the data shown in Fig.
in Fig. 2C, the quasicubic α-Fe O could be observed with a
2
3
spacing of 0.25 nm on the surface of g-C N nanosheets, thus
3
4
3
6,37
validating the exposure of (110) facet.
Compared to other
crystal facets of Fe O , of note, the (110) facet with a
2
3
2
relatively higher density of iron atom (10.1 atoms/nm ) might
present stronger interaction with oxygen so as to enhance
the surface oxygen contents of the photocatalysts,
1
. As shown in Fig. 1A, nanorod-like precursors derived from
glycol-intercalated cyanuric acid were first obtained through
2
| J. Name., 2012, 00, 1-3
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Journal of Materials Chemistry A
Journal Name
ARTICLE
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A
α-Fe O /CQD@g-C N
Ultrathin g-C N
A
α-Fe O /CQD@g-C N
B
DOI: 10.1039
α
/
-
D
Fe
0
O
T A/ C
0
Q
5
D
7
5
g-C N
3
C
2
3
3
4
B
D
3
4
2
3
3
4
2
3
3 4
N 1s
CQD@g-C N
3
4
ultrathin g-C N
3
4
O 1s
C 1s
Fe 2p
5
00 nm
20 nm
C
(110) exposed α-Fe
2
O
3
CQDs
20
40
60
80
0
200
400
600
800
1000
1200
2
θ / °
Binding energy / eV
C 1s
D
N 1s
C
E
2
C=N-C
sp -bonded C
d(110) = 0.25 nm
d(101) = 0.20 nm
graphitic C
N-(C)3
-NH2
1
nm
5 nm
1 nm
5 nm
3
sp -bonded C
π-π* satellite
Fig. 2 TEM images of (A) α-Fe
amplified views of (B) ultrathin g-C
2
O
3
/CQD@g-C
3
N
O
4
, showing the magnification-
and (D) CQD.
3 4
N , (C) α-Fe
2
3
2
95
290
285
280
410
405
400
395
Binding energy / eV
Binding energy / eV
which might improve the photocatalytic H
2
O
2
production via
Fe 2p
O 1s
F
3
4,38
Fe 2p3/2
the oxygen reduction reactions.
Furthermore, Fig. 2D
adsorbed oxygen
Fe 2p1/2
confirms the existence of CQDs with a spacing of 0.20 nm,
showing the graphitic-like structure with the size of 6 nm in
diameter.33 These above morphological characteristics
lattice oxygen
(
α-Fe O )
satellite Fe
730
3+
2 3
indicate that the α-Fe O /CQD@g-C
N
3 4
composites were
2
3
successfully synthesized.
5
45
540
535
530
740
720
710
Binding energy / eV
Binding energy / eV
Phase structure and surface element composition
Fig. 3 (A) XRD patterns of different samples; (B) XPS spectra of α-Fe
2 3
O /CQD@g-
X-ray diffraction (XRD) was conducted to determine the
3 4
C N , with (C) C1s, (D) N1s, (E) O1s, and (F) Fe2p.
phase structure of the α-Fe
comparing with pure g-C
results shown in Fig. 3A. The diffraction peaks at 2θ values of
2
3
O /CQD@g-C
3
N
4
composites by
3
N
4
and CQD@g-C
3 4
N , with the
developed composite, respectively (Fig. 3C).40 Also, the
signals of C≡N and C-C should be ascribed to the bonding
1
C
3.2° and 27.3° correspond to the (100) and (002) facets of g-
, respectively, which can separately represent the in-
manner of g-C
that the peaks situated at 398.4, 399.4, 400.7 and 404.1 eV
are obtained referring to C=N-C, N-(C) , -NH and π-π*
satellite, respectively, revealing the characteristic signal of
3 4
N . Furthermore, one can find from Fig. 3D
3 4
N
plane packing motif and interlayer stacking, indicating the
phase structure of the carriers can be well indexed to
3
2
3
9
graphite carbon nitride (JCPDS No. 87-1526). Moreover, the
4
1
nitrogen from g-C
the binding energy of 528.9 eV should be assigned to the
lattice oxygen of α-Fe in the composites. Fig. 3F manifests
3 4
N . As can be observed from Fig. 3E that
significant broadening of main diffraction peak from 15° to
3
5° may embody the small size of the samples, suggesting the
successful formation of ultrathin g-C . It is a pity that there
is no diffraction peak of CQDs or α-Fe can be observed,
2 3
O
3 4
N
that the peaks at binding energies of 710.0 and 722.4 eV are
the split signals of Fe2p3/2 and Fe2p1/2, respectively, where
the satellite at 717.9 eV should be related to the trivalent of
2 3
O
presumably due to that they might present the uniform
distribution in composites with the peaks largely covered by
4
2
2 3
Fe element. Besides, the bulk loading amount of Fe O was
the broadening peak of g-C
composites could exhibit the g-C
of pure g-C and CQD@g-C , implying that the structures
of g-C carriers could be well sustained after the
integration with CQDs and α-Fe
The chemical states of the elements contributed in α-
/CQD@g-C composites were explored by X-ray
N
3 4
2 3 3 4
. Yet, the α-Fe O /CQD@g-C N
calculated to be about 12.3% according to the XPS
measurements for the elemental contents in the composites.
3 4
N
pattern similar to those
N
3 4
3 4
N
3 4
N
Light absorption properties and charge carriers behaviors
2 3
O .
It is commonly recognized that the light harvest capacity is
one of the vital evaluations for the performance of
photocatalysts. Herein, the UV–vis diffuse reflectance
spectra were recorded for the different products (Fig. 4A).
Fe
2
O
3
3 4
N
photoelectron spectroscopy (XPS). As shown in Fig. 3B, C, N,
O, and Fe elements are spread on the surface of the
composites, with the relative contents shown in Table S1.
Moreover, the peaks with binding energies of 284.8, 285.7,
3 4
One can see that ultrathin g-C N can present an absorption
edge at about 441 nm, which may exhibit a noticeable red-
3
and 287.9 eV are indexed to graphitic carbon, sp -
2 3
shift once loaded with α-Fe O . Fig. 4B discloses the up-
2
coordinated carbon bonds, and sp -bonded carbon in
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A
ultrathin g-C N4
B9
441 nm (λmax
)
DOI: 10.1039/D0TA05753C
3
A
ultrathin g-C N4
B
00 nm
875 nm
850 nm
3
CQD@g-C N4
445 nm (EM)
3
α-Fe O3
2
α-Fe O /CQD@g-C N
4
2
3
3
825 nm
2
α-Fe O3
800 nm (EX)
775 nm
750 nm
725 nm
700 nm
670 nm
650 nm
4
41 nm (λmax
)
2.17 eV
-1.74 V
-0.35 V
2.81 eV
ultrathin g-C N4
α-Fe O
3
2
3
400
500
600
Wavelength / nm
700
800
350
400
450
500
550
2.0
2.5
hv / eV
3.0
-2.0
-1.5
-1.0
-0.6
-0.4
-0.2
0.0
Emission wavelength / nm
Potential / V
5
4
3
2
1
0
Fig. 4 (A) UV-Vis DRS spectra of g-C
3
N
4
, CQD@g-C
3
N
4
, α-Fe
2
O
3
/CQD@g-C
3
N
4
,
g-C N4
3
C
0.9
0.6
D
g-C N
and α-Fe . (B) Up-conversion fluorescence spectra of CQDs.
2
O
3
3
4
α-Fe O
2 3
α-Fe O
light on light off
CQD@g-C N4
2
3
3
CQD@g-C N
α-Fe O @g-C N
3
4
2
3
3
4
α-Fe O @g-C N
α-Fe O /CQD@g-C N
conversion fluorescence spectra of CQDs under the visible
light and near infrared light (i.e., 800 nm) irradiation. As
expected, CQDs can feature the highest fluorescence
emission intensity at about 445 nm, which is very close to the
absorption edge of carbon nitride (441 nm). The data indicate
2
3
3
4
2
3
3
4
α-Fe O /CQD@g-C N
4
2
3
3
0
.3
.0
0
0
.0
0.5
1.0
1.5
2.0
2.5
20
30
40
50
60
70
80
90
4
that the minimum excitation wavelength of α-Fe
2
O
3
/CQD@g-
Z' / Ω × 10
Time / s
C
3
N
4
composite might be expanded to the near infrared
Fig. 5 (A) Plots of (αhv)1/2 versus energy (hv) of g-C N and α-Fe O ; (B) Mott-
3 4 2 3
range (i.e., 800 nm) on account of the up-conversion of CQDs
loaded.
3 4 2 3
Schottky plots of g-C N and α-Fe O obtained at 1000 Hz; (C) electrochemical
impedance spectra and (D) transient photocurrent responses of different
samples.
The band structure of α-Fe
investigated aiming to comprehend the charge carriers
behaviors (Fig. 5). Herein, the band gaps of g-C and α-
Fe were estimated according to the following formula: αhv
A(hv - E
in Fig. 5A. Moreover, based on the Nernst equation: ERHE
O
2 3
/CQD@g-C
3
N
4
was
H
2
O
2
production experiments were carried out comparably
under visible light irradiation. As can be seen from Fig. 6A,
the H O production rates were 0.03, 0.005, 0.01, 0.15, 0.73
3 4
N
2 3
O
2
43,44
=
g
) ,
with the results of 2.81 and 2.17 eV shown
2 2
-1
and 0.91 μM·min for g-C
3 4 2 3 3 4
N , α-Fe O , CQD, CQD@g-C N , α-
=
0
45,46
Fe O @g-C N , and α-Fe O /CQD@g-C N in O -equilibrated
E
Ag/AgCl + 0.05916 pH + E Ag/AgCl
,
Mott-Schottky test was
2
3
3
4
2
3
3
4
2
water as well as being 0.06, 0.02, 0.03, 0.31, 0.96 and 1.16
carried out to explore the conduction band potential of g-
(Fig. 5B). It is found that the positive slope of plots
indicates g-C is n-type semiconductor, of which
conduction band nearly verges on the flat band. Also, the
potential of conduction band (ECB) g-C is calculated to be -
.13 eV (vs. NHE). Similarly, the ECB of α-Fe is calculated to
be 0.26 eV (vs. NHE). The band potentials of α-Fe and g-
are ultimately listed in Table S2.
Fig. 5C describes the data of electrochemical impedance
spectroscopy (EIS) for confirming the charge carriers
behaviors of the α-Fe /CQD@g-C composites by the
route described previously. One can see that the semicircle
radii of CQD@g-C is smaller than that of pure g-C
indicating that CQDs have efficiently improved the charge
transfer capacity of ultrathin g-C . Also, the analogous
result was received for α-Fe /CQD@g-C , which should
μM·min- for those in O
1
2
-saturated water (Fig. 6B-C),
3 4
C N
respectively. One can be note that a higher concentration of
dissolved oxygen in water could effectively render an
3 4
N
improved production rate of H
photocatalytic performances of α-Fe
production were compared to those of some
2 2
O . More importantly, the
3 4
N
2 3
O
/CQD@g-C for
3 4
N
1
2 3
O
2 2
H O
2 3
O
photocatalysts reported elsewhere, with the data
summarized in Table S3. It is noted that the developed
3 4
C N
ternary composites can present the H
1
2
O
2
yield rate of
38.6 μmol·g ·h at 5.0 mg photocatalysts, which is much
higher than those of most current photocatalysts like g-C N
-1
-1
2
O
3
3 4
N
4
7
3
4
composites, and comparable to those of the ones with noble
metal cocatalysts. In addition, the process of photocatalytic
N
3 4
3 4
N ,
H
2
O
2
production usually accompanies with the formation and
decomposition of H . In consequence, the zero-order
kinetic (H formation) and first-order kinetic (H
decomposition) were established to assess the behaviors of
3 4
N
2 2
O
O
2 3
3 4
N
O
2 2
2 2
O
provide an evidence of heterojunctions with accelerated
carriers transfer due to its smallest semicircle radii. Moreover,
the carrier separation efficiencies were investigated with the
4
8
H
2
O
2
production using the kinetic model shown as followed:
Kf
results shown in Fig. 5D. Compared with CQD@g-C
3 4
N , the

H O2



1 exp
 Kdt
2
Kd
Fe @g-C presents a more efficient separation of
2
O
3
3 4
N
f d 2 2
Where K and K represent the H O formation and
photogenerated electron-hole pairs, suggesting that the
photoelectrons in ternary composite can be readily
transferred from g-C N to Fe O .
3 4 2 3
-1
decomposition rate (μM·min ), which calculated by fitting
the data in Fig. 6B based on above kinetic model, with the
results displayed in Fig. 6D. The K
prominently enhanced by the addition of α-Fe
value was not. The consequences reveal that the combination
of g-C with α-Fe could dramatically accelerate the H
f
3 4
values of g-C N has
2
O
3
, but the K
d
Photocatalytic performances
To evaluate the photocatalytic activities of different samples,
N
3 4
O
2 3
2 2
O
4
| J. Name., 2012, 00, 1-3
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Page 5 of 11
Journal of Materials Chemistry A
Journal Name
ARTICLE
6
4
3
1
0
80
60
40
20
0
activity with 17.80% of AQY at 420 nm, even far more than
that of TiO (2.28% at 420 nm).
2
View Article Online
DOI: 10.1039/D0TA05753C
A
B
g-C N4
g-C N4
3
3
5
0
5
0
2
α-Fe O3
2
α-Fe O3
The stability tests of photocatalytic H evolution were
2
O
2
CQD
CQD@g-C N4
CQD
CQD@g-C N4
carried out by using α-Fe /CQD@g-C N under the same
2
O
3
3 4
3
3
α-Fe O @g-C N
α-Fe O @g-C N
2
3
3
4
2
3
3
4
photocatalytic conditions (Fig. S3A). To our surprise, only 4%
deactivation was observed at the fifth run, indicating that the
developed ternary composite can well sustain an excellent
photostability for the long-term application. Not only that, it
is found that the XRD patterns (Fig. S3B) of the composite
exhibit no obvious variation after several photocatalytic
reactions, inferring that this ternary photocatalyst can still
remain the satisfactory crystal structure.
α-Fe O /CQD@g-C N
α-Fe O /CQD@g-C N
2 3 3 4
2
3
3
4
0
15
30
45
60
0
15
30
t / min
45
60
t / min
1.5
1.0
0.5
0.0
20
15
10
5
2
.0
.5
.0
C
α-Fe O /CQD@g-C N
4
2
3
3
D
α-Fe
α-Fe O @g-C N
4
2
O /CQD@g-C N
3
3
4
(
1.16)
α-Fe O @g-C N
2
3
3
4
1
1
(
0.96)
2
3
3
CQD@g-C N
3
4
Active species experiments
CQD@g-C N4
3
0.5
0.0
Superoxide radicals (·O -
conducted to affirm the active species during the
(
0.31)
g-C
3
N
4
2
) removal experiments were first
CQD
α-Fe O
3
g-C N4
2
3
α-Fe O3 CQD
2
(0.06)
(
0.02) (0.03)
4
9,50
0
photocatalytic reactions.
photocatalytic activity of CQD@g-C
restrained by p-benzoquinone (BQ), showing that the
As shown in Fig. 7A, the
Different samples
Different samples
3 4
N
could be completely
8
0
0
0
0
0
80
60
40
20
0
E
F
AQY @420 nm
α-Fe O /CQD@g-C N : 17.80%
generation of H
2
O
2
could just be depended on the
/CQD@g-C could
production in the presence
was not primarily generated
6
2
3
3
4
-
abundant ·O
2
. In contrast, the α-Fe
2
O
3
3 4
N
Commercial TiO : 2.28%
2
2 2
well sustain a highly efficient H O
4
2
Apparent quantum yields (AQY)
17.80%
of BQ, indicating that the H
O
2 2
-
by ·O
2
in this system. Not only that, DMPO spin-trapping ESR
6.49%
2
.51%
spectra were given to further substantiate the results of the
0
.92%
5
1
active species experiments. Fig. 7B exhibits the signals of
CF420
DT420
DT450
DT600
DT700
α-Fe O /CQD@g-C N
Commercial TiO2
2
3
3
4
-
Different light exposure conditions / nm
DMPO-·O
Fe
visible light for H
peaks were observed in CQD@g-C
produced by the CQD@g-C
were detected after 10-min irradiation of
prodcution. Four strong characteristic
, whereas the quite
N
and α-
2
3
4
2 3 3 4
O /CQD@g-C N
Fig. 6 (A) Photocatalytic H
O
O
2
O
2
production amounts of different samples in (A)
-equilibrated water and (B) O -saturated water; (C) H production rates in
-saturated water; (D) K and K values for H productions of various
2 2
O
2
2
2 2
O
3 4
N
2
f
d
2 2
O
samples; (E)
H
2
O
2
production and apparent quantum yields (AQY) of α-
under different monochromatic light; (F) comparison of
weak signals were found in the ternary composite,
Fe /CQD@g-C
2
O
3
3
N
4
-
demonstrating that ·O
Fe /CQD@g-C
To further testify the active species of photocatalytic
H O production by CQD@g-C N and α-Fe O /CQD@g-C N .
2
was not the main active species of α-
2 2
H O production and AQYs of the samples under visible light @420 nm.
O
2 3
3 4
N during photocatalysis.
formation and inhibit H
construction of efficient heterojunction.
Fig. 6E illustrates the H production capacities and
apparent quantum yields (AQY) of α-Fe /CQD@g-C
under different monochromatic light exposures. It can be
seen that, the composite features a favorable yield of H
with the 17.80% of AQY at 420 nm. What's more, this
composite also embodies the considerable photocatalytic
activities, of which absorption edge is greater than that of
2 2
O decomposition due to the
2
2
3
4
2
3
3 4
-
The superoxide radical (·O ) evolution could be investigated
2
-
O
2 2
by NBT, which was employed as a ·O2 scavenger, and
-
52
O
2 3
N
3 4
subsequently degraded by ·O during the photocatalysis. It
2
was discovered in Fig. 7C that the NBT was more effectively
O
2 2
degraded by CQD@g-C N than by α-Fe O /CQD@g-C N .
3
4
2
3
3 4
Only very few ·O2- were evolved by the developed ternary
composite, although it could present a higher H O2
2
production. The results reveal that the α-Fe O has converted
2 3
pure g-C
3
N
4
(441 nm), ascribing to the excellent up-
/CQD@g-C
2 2 2 3 3 4
the H O production pathway of α-Fe O /CQD@g-C N from
the two-step single-electron reduction into the direct one-
step two-electron reduction.
conversion of CQD. Especially, α-Fe
2
O
3
N
3 4
processes a 6.49% of AQY at 450 nm, which is referred to the
maximum up-conversion intensity of CQD under light
irradiation (Fig. 4B), even sustains a 0.92% of value at 700 nm.
These results attest that the up-conversion of CQD
observably enhances the light utilization efficiency of
comparable investigations in Fig. 6F indicate that the
Considering that H O may be acquired through the
2 2
reaction of hydroxyl radicals (·OH), the ·OH removal
experiments were carried out (Fig. 7D), which may help to
understand the contribution of ·OH for photocatalytic H O
2
2
5
3,54
generation.
As expected, the photocatalytic activities of
composite, thus raising the H
2
O
2
production. Furthermore, α-
could exhibit > 7.5 times higher amount
evolution than commercially available TiO under
these photocatalysts above might not be obviously affected
by the ·OH quencher of isopropanol (IPA). Therefore, the
water should be directly oxidized to H O mainly by the
Fe
2
3
O /CQD@g-C
N
3 4
of H O
2 2
2
2
2
visible light irradiation, showing the excellent photocatalytic
photogenerated holes, without formation of ·OH.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
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Journal of Materials Chemistry A
Page 6 of 11
ARTICLE
Journal Name
6
0
5
0
5
0
View Article Online
DOI: 10.1039/D0TA05753C
A
B
0.6
0.4
0.2
A
CQD@g-C N4
α-Fe O /CQD@g-C N
3
3.5
B
2
3
3
4
3
CQD@g-C N4
α-Fe O /CQD@g-C N
4
3
1
2
3
3
4
n = 3.12
CQD@g-C N +BQ
CQD@g-C N
3 4
3
4
3.0
2.5
2.0
1.5
α-Fe O /CQD@g-C N
in dark
2
2
3
3
3
4
Ring
Disk
α-Fe
O
/CQD@g-C
N
+BQ
3
4
0.0
3
CQD@g-C N4
n = 2.25
α-Fe O /CQD@g-C N
2
3
3
4
-
1.0
2.0
in dark
326
-
0
15
30
t / min
45
60
318
0
320
322
324
-0.4
-0.2
0.0
0.2
0.4
0.6
-0.4
-0.3
-0.2
-0.1
0.0
Magnetic / mT
Potential / V vs.RHE
Potential / V vs. RHE
6
4
3
1.0
0.8
0.6
0.4
0
.00
C
D
C
D
5
0
3
CQD@g-C N4
-
0.05
CQD@g-C N +IPA
3
4
α-Fe O /CQD@g-C N
2
3
3
4
-0.10
-0.15
α-Fe O /CQD@g-C N +IPA
2
3
3
4
g-C N4
3
CQD@g-C N4
15
0
α-Fe O3
3
2
-
0.20
α-Fe O /CQD@g-C N
CQD@g-C N4
3
2
3
3
4
α-Fe O /CQD@g-C N
2
3
3
4
-0.25
0
15
30
t /min
45
60
0
15
30
45
60
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
t / min
Potential / V vs. RHE
Fig.
Fe
7
(A) Reactive species trapping results of CQD@g-C
3
N
4
and α-
Fig. 8 (A) RRDE measured curves of α-Fe
0.10 M PBS; (B) electron transfer numbers at different potentials calculated
from RRDE data; (C) LSVs of different products measured on RDE under 1200
rpm in 0.10 M PBS under light irradiation; (D) the photocatalytic H O
2 2
2 3 3 4
generation mechanism of α-Fe O /CQD@g-C N featuring the Z-scheme charge
2
O
3
/CQD@g-C
3
N
4
under 1600ꢀrpm in
2
O
3
/CQD@g-C
N
3 4
in the absence and presence of BQ (1.0 mM) in O
2
-
-
equilibrated water; (B) DMPO-·O
and α-Fe /CQD@g-C for
illumination; (C) photocatalytic NBT decomposition (2.5 mM) of CQD@g-C
and α-Fe /CQD@g-C in H
trapping results of CQD@g-C
presence of IPA (1.0 mM) in O
2
spin-trapping ESR spectra of CQD@g-C
N
3 4
2
O
3
N
3 4
H
2
O
2
production before and after 10 min
N
3 4
2
O
3
N
3 4
2
O
2
production process; (D) reactive species
and α-Fe /CQD@g-C in the absence and
-equilibrated water.
transfer route.
3
N
4
2
O
3
3 4
N
2
3
irradiation using AgNO as photoelectron quencher (Fig. S4).
Accordingly, an O
2
yield of 7.52 μmol in 4 h was obtained
Photocatalytic mechanism
To affirm the pathway of photocatalytic towards H
-
under visible light, corresponding to the rate of 62.7 μmol·g
1
-1
2 2
O , the
·h in average, indicating that the developed composite
rotating ring-disk electrode (RRDE) technique was carried out
during the oxygen reduction reaction (ORR), which was
should conduct the photocatalytic water oxidation reactions
so as to release the protons in water.
considered the main product as H
similar to the photocatalytic condition. The measured disk
and ring currents of and α-Fe /CQD@g-C were
displayed in Fig. 8A, and the apparent electron transfer
number (n) could be calculated from Fig. 8A using the
following equations:55
2
O
2
in neutral solution
Rotating disk electrode (RDE) technique was first carried
out to investigate the oxygen reduction abilities of different
photocatalytic materials under light irradiation, with the
responses of linear sweep voltammetry (LSV) shown in Fig. 8C.
2
O
3
3 4
N
Accordingly, the α-Fe
highest oxygen reduction ability, in contrast to α-Fe
showing the lowest one. Especially, the results show that the
electrons in α-Fe /CQD@g-C might be transferred from
α-Fe to g-C under light irradiation. Moreover,
DMPO-·O electron spin resonance (ESR) tests were
comparably carried out for α-Fe /CQD@g-C and
CQD@g-C by the photo-degradation of methanol under
visible light irradiation, with the results shown in Fig. S5. One
can note that the developed α-Fe /CQD@g-C can
display four stronger characteristic peaks than the CQD@g-
under the same testing conditions, implying that the
photo-generated electrons in the ternary composites might
be transferred from α-Fe to g-C . Furthermore, the
2 3 3 4
O /CQD@g-C N could present the
2 3
O
4
J
D
n 
J
R
O
2 3
3 4
N
JD 
N
O
2 3
3 4
N
-
Where J
D
and J
R
are the disk and ring current densities,
2
respectively. The officially provided collection efficiency (N) is
determined to be 37%. As shown in Fig. 8B, the α-
O
2 3
3 4
N
3 4
N
Fe
to 2, thereby going through the 2-electron pathway with
primary reduction products as H . However, the n value of
CQD@g-C was calculated to be 3, which was greatly
exceeded 2, indicating its difficult production of H via 2-
2 3 3 4
O /CQD@g-C N could perform a calculated n value close
O
2 3
3 4
N
2 2
O
N
3 4
3 4
C N
2 2
O
electron pathway. These results were in well agreement with
those of active species experiments, further demonstrating
that the developed ternary composite could feature two-
O
2 3
3 4
N
water oxidation abilities of different photocatalysts were
explored by the DMPO-·OH electron spin resonance tests
under light irradiation (Fig. S6). One can note that very weak
characteristic peaks of DMPO-·OH could be observed for
2 2
electron transfer mechanism for H O production in neutral
solution.
In order to explore the behavior of water oxidation, the
experiments for the photocatalytic O evolution with the
developed composite was performed under the visible light
CQD@g-C
obvious enhancement of DMPO-·OH ESR signal was obtained
for α-Fe /CQD@g-C . It was found that the developed
3 4
N after being illuminated for 10 min. However, an
2
2
O
3
3 4
N
6
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
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Page 7 of 11
Journal of Materials Chemistry A
Journal Name
ARTICLE
photocatalyst of α-Fe
much stronger water oxidation capability than CQD@g-C
The ESR results are in good agreement with those of the RDE
measurements, indicating that the as-fabricated α-
2 3
O /CQD@g-C
N
3 4
could present the
could exhibit a apparent quantum yield of 17.80V%iewatAr4tic2le0Onnlmine,
even far more than commercially availa
study opens a door towards the preparation of ultrathin g-
nanosheets simply by the green precursor-regulated
calcination route. Also, it proposes an optimized
photocatalytic pathway for the two-electron oxygen
D
b
O
leI: 1
T
0
iO.10
2
3
(
9
2
/D.2
0
8
T
%
A0).57
This
5
3
C
3 4
N .
3 4
C N
Fe
transfer mechanism for the efficient photocatalytic reactions
of H generation.
Based on the above investigations,
2 3 3 4
O /CQD@g-C N might feature the Z-scheme charge
2 2
reduction for H O production in pure water, without the use
2 2
O
of any noble metal cocatalysts or organic scavengers. The
further catalytic applications of the ternary photocatalyst in
the medical and environmental field like the disinfection and
wastewater treatment will be conducted in future work.
a
possible
photocatalytic H
Fe /CQD@g-C
2
O
2
generation pathway was proposed for α-
composites as schematically illustrated in
2
O
3
3
N
4
Fig. 8D. One can find that CQDs with the splendid up-
conversion capacity could harvest the light at a long
wavelength up to 800 nm to shine a short wavelength (< 445
Experimental section
Materials and instruments
nm), which can correspond to the absorption edge of g-C
as aforementioned. When exposing α-Fe /CQD@g-C
the visible light range (420 - 800 nm), as shown in Fig. 8D, the
excited electrons might be transferred from the CB of α-Fe
to the VB of g-C due to the Z-scheme charge transfer
pathway between g-C and α-Fe . Furthermore, the
excited electrons from the CB of g-C (-1.13 V vs NHE)
3 4
N
O
2 3
3 4
N to
Melamine (C
3 6 6
H N , 99.0%), iron nitrate nonahydrate (98.5%),
N,N-Dimethylformamide (DMF, 99.5%), ethylene glycol (EG,
2 3
O
9
9
9.0%), isopropanol (IPA, 99.7%), nitroblue tetrazolium (NBT,
8.0%), and commercial titanium dioxide (TiO , 98.0%) were
3 4
N
2
N
3 4
2 3
O
obtained from Sinopharm Chemical Reagent Co. Ltd.
Shanghai, China). Polyvinylpyrrolidone (PVP ≈ WM 58000,
9.5%) and p-benzoquinone (BQ, 97%) were purchased from
Sigma-Aldrich Co. Ltd. (Shanghai, China). Peroxidase
horseradish (HRP), p-hydroxyphenylacetic acid (C ),
potassium hydrogen phthalate (C KO ), and ethanol
OH) were used without further purification. Deionized
water ( 18.2 M Ω /cm) was used for all experiments (Pall,
USA). Muffle furnace (Sx -10-12) with high output (380 V/10
3 4
N
(
9
would be captured by CQDs, which might serve as electron
acceptors so as to efficiently retard the recombination of
charge carriers. Subsequently, the excited electrons on CQDs
9 10 3
H O
can directly restore O
that, owing to the EVB (2.43 V vs. NHE) of α-Fe
positive than the E(H /H O) (1.76 V vs. NHE), the
photogenerated holes localized on α-Fe might oxidize H
into H . What’s more, when exposing α-Fe /CQD@g-
to the near infrared light range (≥ 800 nm), the photo-
electrons might be transferred from the excited CQD to the
un-excited g-C , and further to the un-excited α-Fe
Ultimately, the oxygen might be restored by the electrons of
α-Fe to yield H in the photocatalytic reactions with the
2
into H O
2 2
(0.69 V vs NHE). Not only
H
8 5
4
2 3
O
is more
2 5
(C H
O
2 2
2
O
2 3
2
O
2
O
2 2
2 3
O
kW) was brought from Qixin Instruments Co. Ltd. (Shanghai,
China).
3 4
C N
N
3 4
2 3
O .
Synthesis of photocatalytic materials
O
2 3
2 2
O
Synthesis of (110) exposed α-Fe
O
2 3
. α-Fe
modified solvothermal method previously
·9H
2 3
O was fabricated
developed composites.
through
a
3
1
reported. Typically, an aliquot of 0.404 g of Fe(NO
)
3 3
2
O
and 0.60 g of PVP (WM 58000) were dissolved in 36 mL DMF.
Then, the solution was transferred into a 70 mL of Teflon-
lined stainless steel autoclave, and heated at 180 °C for 12 h.
Conclusions
3 4
In summary, ultrathin g-C N nanosheets were successfully
fabricated by a two-step precursor-regulated calcination
route at a high heating rate of 30 °C/min, serving as the
carriers for decorating carbon QDs and (110) exposed α-Fe
to yield a ternary photocatalyst of α-Fe /CQD@g-C
composite. Herein, the introduction of carbon QDs could
extend its light absorption edge to the near infrared range
After cooling, the precursor solution of α-Fe
2 3
O was
preserved in the dark. Besides, the α-Fe precursor solution
2 3
O
2
O
N
3
was centrifuged and washed separately with deionized water
and ethanol for several times. The obtained brown red
products were dried at 60°C to be stored for future usage.
2 3
O
3
4
(
i.e., 800 nm). Meantime, the loading of (110) exposed α-
Fe could provide the active centers of the composite for
the photocatalytic H generation. More importantly, it is
evident that the (110) exposed α-Fe has converted the
generation route of α-Fe /CQD@g-C from the two-
3 4 3 4
Synthesis of ultrathin g-C N . Ultrathin g-C N nanosheets
2 3
O
were fabricated by the two-step calcination regulated by
melamine precursors at a high heating rate in the air.
Typically, an aliquot of 0.50 g melamine was dispersed in 15
2 2
O
2 3
O
H
2
O
2
2
O
3
3 4
N
3
mL ethylene glycol. Then, 0.10 mM HNO solution (30 mL)
step single-electron reduction into the direct one-step two-
electron reduction. The improved charge carriers behaviors
could thus be expected for the significant enhancement of
was slowly dropped into the mixture. The obtained white
precipitate was centrifuged and washed with ethanol for
several times, followed by being grinded to powder in agate
mortar after vacuum drying at 60°C for 24 h. Subsequently,
the powder was heated in a muffle furnace under ambient
2 3 3 4
photocatalytic activities of the α-Fe O /CQD@g-C N
composite. Moreover, the developed green ternary
photocatalyst could display highly efficient photocatalysis for
the H
visible light irradiation, which is about 19 times faster than
that of pure ultrathin g-C . Also, the developed composite
-1
O
2 2
production with a rate of 1.16 µM·min under the
-1
pressure at 450°C for 2 h (30°C min of heating rate) in the
air. After cooled to room temperature, the resulting light
yellow powder was further annealed at 550°C for another 2 h
3 4
N
This journal is © The Royal Society of Chemistry 20xx
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Journal Name
-
1
(
30°C min of heating rate) in the air, forming the final yellow
product of ultrathin g-C nanosheets.
Synthesis of CQD@g-C . CQD@g-C
prepared through the growth of carbon QDs in situ onto g-
nanosheets. Typically, an aliquot of 0.10 g g-C
The absorbance of NBT was determined by the UV–vis
spectrophotometer (Shimadzu/UV-3600) at its characteristic
absorbance peak around 260 nm.
View Article Online
DOI: 10.1039/D0TA05753C
3
N
4
3
N
4
3 4
N composites were
The measurements of apparent quantum yields (AQY)
were simultaneously conducted during the H
of α-Fe /CQD@g-C with various band-pass filters (420
nm, 450 nm, 600 nm, and 700 nm). The AQYs were calculated
2 2
O production
C
3
N
4
3 4
N
O
2 3
3 4
N
nanosheets was dispersed into the solution containing 13.6
mL DMF and 16.4 mL IPA under ultrasonic condition. After
stirring for 30 min, the mixture was turned into a 70 mL
autoclave to be heated at 180°C for 8 h. The yielding yellow
5
2
using the following equation:
Ne
Np
N(H O )2
N(photos)
2 M  NA hc
S  Pt 
2
2
AQY 
100% 
100% 
CQD@g-C
3
N
4
composites were collected by centrifugation.
After being washed with ethanol for several times, the
product was eventually dried at 60 ° C for 12 h. (The
preparation of carbon QDs can be seen in electronic
supplementary information.)
Where N
the incident photons, M refers to the amount of H
molecules, is the light intensity determined by
photoradiometer (PL-MW 2000/PerfectLight), S is the light
irradiation area, N represents the Avogadro constant, c is
the speed of light, is the wavelength of the
e p
represents the reaction electrons, N represents
2 2
O
P
Synthesis of α-Fe
procedure of α-Fe
2
O
3
/CQD@g-C
3
N
4
.
The preparation
nanocomposite was
A
2
O
3
/CQD@g-C
3
N
4
λ
schematically illustrated in Scheme 1. Typically, an aliquot of
.10 g of ultrathin g-C was dispersed into 16.4 mL IPA.
Afterwards, 13.6 mL of α-Fe precursor solution was
monochromatic light, h is the Planck constant, and t
represents the photoreaction time.
0
3 4
N
2 3
O
introduced into above mixed solution under vigorous stirring.
After the ultrasonic dispersion for 30 min, the mixture was
transferred into a 70 mL Teflon-lined autoclave to be heated
Conflicts of interest
2 3 3 4
at 180 ° C for 8 h. Subsequently, the α-Fe O /CQD@g-C N
nanocomposites were collected by centrifugation and
washed for three times with ethanol, followed by being dried
There is no conflicts of interest.
2 3 3 4
at 60°C overnight (the preparation of α-Fe O @g-C N can be
seen in the Electronic supplementary information).
Acknowledgements
This work is supported by the National Natural Science
Foundations of China (No. 21675099), Major Basic Research
Program of Natural Science Foundation of Shandong Province
(ZR2018ZC0129), and Science and Technology Development
Project of Weihai City (2015DXGJZD002), P. R. China.
Photocatalytic tests
The photocatalytic testing apparatus composing of glass
reactor and external illumination source was utilized in the
photocatalytic experiments according to the procedure
previously reported.56 A 300 W Xe-lamp (420 nm cut-off) was
chosen as visible light source (XL-300/Yirida Technology Co.
Ltd.), with a distance of 20 cm apart from the reaction
solution. Briefly, an aliquot of 5.0 mg photocatalyst was first
dispersed in 10 mL water under sonication. All of the
photocatalytic tests were proceeded in pure water with the
pH value at 7. The mixtures were stirred for 1 h in darkness so
as to reach the adsorption-desorption equilibrium between
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The developed α-Fe O /CQD@g-C N can perform a direct one-step two-electron reduction
2
3
3
4
for efficient photocatalytic H O evolution in pure water.
2
2