“Two channel” photocatalytic hydrogen peroxide production using g-C3N4 coated CuO nanorod heterojunction catalysts prepared via a novel molten salt-assisted microwave process

Article abstract of DOI:10.1039/c8nj02565g

Photocatalytic H₂O₂ production has gained increasing attention as a sustainable and eco-friendly method to replace conventional anthraquinone-based and electrochemical production processes. In this work, we report a novel molten salt-assisted microwave process to synthesize g-C₃N₄ coated CuO nanorod heterojunction photocatalysts with outstanding H₂O₂ production ability. XRD, UV-Vis, N₂ adsorption, SEM, TEM, XPS, EIS and PL were used to characterize the obtained catalysts. CN@CuO(4) with the largest heterojunction area displays the highest photocatalytic H₂O₂ production ability of 6.9 mmol L^-1, which is 7.4 and 16.4 times higher than that of individual g-C₃N₄ and CuO. A “two channel pathway” is proposed for this reaction system which causes the remarkably promoted H₂O₂ production ability. In addition, compared with a traditional polycondensation method, the catalyst prepared via a molten salt-assisted microwave process exhibits a larger heterojunction area and more effective separation rate, causing higher photocatalytic H₂O₂ production ability and stability.

Full text of DOI:10.1039/c8nj02565g

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
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ARTICLE TYPE
“
Two channel” photocatalytic hydrogen peroxide production using g-
C N coated CuO nanorods heterojunction catalysts prepared via a
3
4
novel molten salt-assisted microwave process
Jin Bai *, Yongzhen Sun, Meiyuan Li, Lina Yang, Jian Li and Shaozheng Hu*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Photocatalytic H O production has received increasing attention as a sustainable and ecoꢀfriendly
2
2
method to replace conventional anthraquinoneꢀbased and electrochemical production processes. In this
work, we report a novel molten saltꢀassisted microwave process to synthesize gꢀC N coated CuO
3
4
1
0
nanorods heterojunction photocatalyst with outstanding H O production ability. XRD, UVꢀVis, N
2 2 2
adsorption, SEM, TEM, XPS, EIS and PL were used to characterize the obtained catalysts. CN@CuO(4)
with the largest area of heterojunction displays the highest photocatalytic H O production ability of 6.9
2
2
ꢀ
1
mmol·L , which is 7.4 and 16.4 times higher than that of individual gꢀC N and CuO. “Two channel
3
4
pathway” is proposed for this reaction system which causes the remarkably promoted H O production
2
2
1
5
ability. In addition, compared with traditional polycondensation method, the catalyst prepared via molten
saltꢀassisted microwave process exhibits larger area of heterojunction and more effective separation rate,
causing the higher photocatalytic H O production ability and stability.
2
2
Introduction
heptazine ring structure and high condensation degree make it
possess many advantages, such as good chemical stability,
appropriate band gap energy and unique electronic structure.
Since its conduction band potential (ꢀ1.3 V) is more negative than
the reduction potential of O /H O (0.695 V), gꢀC N could
4
5
5
6
6
7
5
0
5
0
5
0
Hydrogen peroxide (H O ), as a highly efficient and green
2
2
2
2
3
3
4
0
5
0
5
0
oxidant, has been widely used in bleaching and disinfectant
applications, such as textile, paper pulp and medical industry.
H O has the highest content of active oxygen (47% w/w) and
2
2
2
3
4
11
2
2
reduce O to H O under visible light thermodynamically. All of
1
,2
2
2
2
only H O as the byꢀproduct. Besides that, H O is also an ideal
2
2
2
these make gꢀC N to be the best candidate for photocatalytic
3
4
energy carrier alternative to hydrogen with the low volumetric
H O production.
3,4
2
2
energy. The output potential of H O fuel cell is 1.09 V
2
2
In order to promote the electron utilization to reduce more O2
molecules, the hole scavenger is usually used. However, some
previous reports indicate that some photocatalysts can oxidize
theoretically, which is comparable with that of hydrogen fuel cell
5ꢀ7
(
1.23 V). Industrially, H O is produced by the anthraquinone
2 2
method, in which energy consumption is high because of the
multistep hydrogenation and oxidation reactions. Thus this
method is unsuitable for the current new concept of “green,
energy saving and environmentally friendly” in the chemical
industry. Recently, the direct synthesis of H O from H and O
ꢀ
12,13
OH to •OH by photogenerated holes.
These •OH can
combine with each other to form additional H O , thus changing
2
2
the H O2 generation pathway from singleꢀchannel to twoꢀ
2
14,15
channel.
However, the VB positions is approximately +1.6 V
2
2
2
2
for gꢀC N . The VB holes in gꢀC N are not positive enough to
8
ꢀ10
3
4
3
4
gases has been widely studied using noble metals as catalysts.
16,17
generate •OH.
It is known that, for heterojunction catalyst,
It is considered to be an alternative and green chemistry process.
However, this method presents a potential explosion risk from
H /O2 mixtures. In contrast, photocatalytic H O production
electrons and holes will accumulate on two components
respectively by crossing the heterojunction interface.
Accordingly, the oxidation ability of VB holes may be promoted.
Molten salt method is widely used in the materials synthesis
field in recent years because it can accelerate diffusion of
2
2
2
requires only water, oxygen and visible light through twoꢀ
electron reduction from the conduction band (Reaction (1)).
However, the H O can be decomposed by reduction with e ,
which has caused the H O production rate of this photocatalytic
reduction method to be unsatisfactory to date (Reaction (2)).
ꢀ
2
2
constituent ions, control crystal growth and easily separate from
2
2
18ꢀ20
the solid product by dissolving in water.
In general, molten
salt can serve as a reaction medium for reactant dissolution and
precipitation, as soft template for tailoring micro and
mesoporosity of the materials, and as structureꢀdirecting agent in
the polycondensation and deamination reaction to obtain
graphitic materials. The features of this synthesis method are
related to the surface and interface energies between the
+
ꢀ
H O
2 2
(
1)
O + 2 H + 2 e
2
ꢀ
OH + OHꢀ
H O + e
(2)
2
2
Graphitic carbon nitride (gꢀC N ) has attracted a great deal of
interest in photocatalytic applications in recent years. The
3
4
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constituents and the salt, resulting in a tendency to minimize the
CN/CuO.
energies by forming a specific morphology. Though the electricꢀ
resistance heating moltenꢀsalt method can be used to synthesize
The XRD patterns of the prepared samples were recorded on a
Rigaku D/maxꢀ2400 instrument using CuꢀKα radiation (λ = 1.54
o
gꢀC N photocatalysts, the problems of long time consuming and 60 Å). The scan rate, step size, voltage and current were 0.05 /min,
3
4
o
5
0
5
0
5
0
5
high energy consumption in synthesis, large emission of harmful
0.01 , 40 kV and 30 mA, respectively. UVꢀVis spectroscopy was
2
1
gas and low catalyst yield are still difficult to overcome.
Recently, microwaveꢀassisted heating synthesis has also been
carried out on a JASCO Vꢀ550 model UVꢀVis spectrophotometer
using BaSO4 as the reflectance sample. The morphologies of
prepared catalyst were observed by using a scanning electron
2
2ꢀ24
widely used to prepare nanomaterials.
The microwave
treatment can transfer energy from microwave to the microwaveꢀ 65 microscope (SEM, JSM 5600LV, JEOL Ltd.). TEM images were
1
1
2
2
3
3
absorber material which induces strong heating in minutes. When
the microwave energy is absorbed by the raw material, the
molecules are orderly arrangement in the electromagnetic field of
the microwave. Then the high frequency reciprocating motion
taken on a Philips Tecnai G220 model microscope. Nitrogen
adsorption was measured at ꢀ196 °C on a Micromeritics 2010
analyser. All the samples were degassed at 393 K prior to the
measurement. The BET surface area (SBET) was calculated based
occurs inside the molecules of raw materials, causes the frequent 70 on the adsorption isotherm. ICP was performed on a Perkinꢀ
collisions between molecules, leading to the generation of a lot of
frictional heat. Under this heating method, the raw material is
rapidly heated without the presence of temperature gradient. It is
Elmer Optima 3300DV apparatus. The photoluminescence (PL)
spectra were measured with a fluorospectrophotometer (FPꢀ6300)
using Xe lamp as the excitation source. The electrochemical
impedance spectra (EIS) were recorded using an EIS
reported that gꢀC N4 can be synthesized by the microwaveꢀ
3
assisted heating method in a few minutes, suggesting this is a 75 spectrometer (ECꢀLab SPꢀ150, BioLogic Science Instruments) in
potential way in rapid synthesis of carbon nitride based
a three electrode cell by applying a 10 mV alternative signal
versus the reference electrode (SCE) over a frequency range of 1
MHz to 100 mHz. The cyclic voltammograms were measured in
2
5
materials. Besides that, the catalyst yield is high and the
emission of harmful gas is also low with this method. It is known
that the solidꢀstate reaction system needs a uniform reaction
a
0.1
M
KCl
solution
containing
2.5
mM
medium to achieve a stable and reliable condition. The moltenꢀ 80 K [Fe(CN) ]/K [Fe(CN) ] (1:1) as a redox probe at a scanning
3
6
4
6
ꢀ
1
salt process can offer a unique liquid condition that is stable and
convenient for the solidꢀstate reaction system. Thus, we
hypothesize that combination two methods mentioned above
rate of 20 mV·s in the same three electrode cell as the EIS
measurement.
Photocatalytic Reaction
should be an effective strategy to synthesize gꢀC N4 based
3
materials with high performance.
The photocatalytic H O production ability of the samples were
2 2
In this work, gꢀC N4 coated CuO nanorods heterojunction 85 evaluated. For these experiments, 0.2 g of photocatalyst was
3
catalyst with outstanding photocatalytic H O production ability
added to 200 mL deionized water. The suspension was dispersed
using an ultrasonicator for 10 min. During the photoreaction
under visible light irradiation, the suspension was exposed to a
250 W highꢀpressure sodium lamp with main emission in the
2
2
was synthesized via a novel molten saltꢀassisted microwave
process. Under “Zꢀscheme” mechanism, CuO/gꢀC N4
heterojunction catalyst can change photocatalytic H O2
3
2
production pathway from “single channel” to “two channel”. The 90 range of 400 to 800 nm, and O was bubbled at 80 mL/min
2
possible reaction mechanism is proposed.
through the solution. The UV light portion of the sodium lamp
was filtered by a 0.5 M NaNO solution. All runs were conducted
2
at ambient pressure and 30 °C. At given time intervals, 5 mL
aliquots of the suspension were collected and immediately
centrifuged to separate the liquid samples from the solid catalyst.
Experimental
Preparation and characterization
95
2
6
CuO nanorods were prepared according to previous work. In a
typical process, eutectic mixture of KClꢀLiCl (1:1 weight ratio)
was selected as a solvent. The mixture of melamine and CuO
The concentration of H O was analyzed by normal iodometric
2 2
2
7,28
4
4
5
5
0
5
0
5
method.
(
1
with a weight ratio melamine/CuO = 2, 4 and 8) were added into
00 g of eutectic salts and finely ground in a mortar, then
Results and discussion
In this work, gꢀC N4 coated CuO nanorods heterojunction
3
transferred to an alumina crucible and treated by microwave for
30 min in a normal microwave oven (G70D20CN1PꢀD2, Galanz).
The obtained catalyst was denoted as CN@CuO(x), where x
stands for the weight ratio of melamine to CuO. Neat gꢀC N was
prepared following the same procedure mentioned above in the
absence of CuO, and denoted as GCN. For comparison, CuO/gꢀ
1
1
1
00 catalyst (CN@CuO) was synthesized via a novel molten saltꢀ
assisted microwave process. Compared with traditional
polycondensation method, CN@CuO exhibits larger area of
heterojunction and more effective separation rate. According to
3
4
“Zꢀscheme” mechanism, not only the CB electrons of CN@CuO
−
05 reduce O to form H O , but the VB holes can oxidize OH to
2
2
2
C N
heterojunction catalyst was also prepared by
3
4
form •OH, which subsequently react with each other to form
H O . Such “two channel pathway” causes the remarkably
polycondensation method. 10g of melamine and CuO with a
weight ratio of 4:1 were added into 30 mL of deionized water.
The suspension was stirred for 20 min and was heated to 100 C
under stirring to remove the water. The solid product was dry at
80 C in oven, followed by milling and annealing at 520 C for 2
2
2
o
promoted H O production ability.
2 2
Figure 1a shows the XRD patterns of asꢀprepared catalysts.
10 CuO matches well with standard diffraction data (JCPDS file no.
o
o
2
9,30
0
5ꢀ0661).
GCN has two characteristic peaks, which are
o
ꢀ1
h (at a rate of 5 C·min ). The prepared catalyst was denoted as
located at 2θ = 13.1° and 27.5°, respectively. The diffraction peak
2
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at 27.5° is the characteristic peak of the interlayer stacking of the
µm long is observed. Besides that, it is noted that the surface of
aromatic segments with a Miller index of (002). The diffraction 60 CuO is smooth. For CN@CuO(4) (Figure 2c), it can be seen that
peak at 13.1° is assigned to the melonꢀlike structure with a Miller
index of (100). These two characteristic peaks are in agreement
the nanorodꢀlike CuO is coated by the GCN. The CuO surface is
not as smooth as shown in Figure 2b but very rough, confirming
the GCN coating. Highꢀresolution transmission electron
microscopy (HRTEM) analysis was performed to get information
1
6
5
0
5
0
5
0
5
0
5
0
5
with that reported in previous literature. In the case of
composite materials, the crystal phase of CuO and gꢀC N does
3
4
not change. The diffraction peak intensities of CuO in CuO/gꢀ 65 on the microstructure of asꢀprepared CN@CuO(4) (Figure 2d).
C N composite materials become weaker when increasing the
The observed lattice fringe spacing of 0.324 nm corresponds to
the (002) crystal plane of gꢀC N (JCPDS 87ꢀ1526). The lattice
3
4
mass percentage of gꢀC N . In addition, no other impurity peak
3
4
3
4
1
1
2
2
3
3
4
4
5
5
exits in the XRD patterns, indicating that CuO and gꢀC N are the
only two components in the composites. The CuO contents of asꢀ
fringe of 0.252 nm in the HRTEM image should be assigned to
the (111) plane of CuO. Smooth and intimate interface is clearly
3
4
3
2
prepared catalysts, measured by ICP, are 52.5, 32.8 and 18.7 70 observed between GCN and CuO in Figure 2d, which confirms
wt.% for CN@CuO(2), CN@CuO(4) and CN@CuO(8),
respectively. The specific surface area of catalyst is an important
factor that determines the photocatalytic performance. To
characterize the specific surface area of asꢀprepared catalysts, the
nitrogen adsorption and desorption isotherms of GCN, CuO and
heterojunction photocatalysts were measured (Figure 1b). The
isotherms of all the samples are of classical type IV. The BET
specific surface areas (SBET) of GCN, CuO, CN@CuO(2),
CN@CuO(4) and CN@CuO(8) are calculated to be 9.5, 20.1,
the formation of gꢀC N /CuO heterojunction. Such strong
3 4
interaction can result in the higher interfacial charge transfer rate
and H O production ability.
2
2
75
80
85
90
2
ꢀ1
1
7.9, 17.2 and 13.8 m ·g .
(
a)
GCN
CN@CuO(8)
CN@CuO(4)
CN@CuO(2)
CuO
10
15
20
25
30
35
40
45
50
2
theta / degree
250
(b)
GCN
(c)
GCN
CuO
CuO
2
00
CN@CuO(2)
CN@CuO(4)
CN@CuO(8)
CN@CuO(2)
CN@CuO(4)
CN@CuO(8)
Figure 2. SEM images of GCN (a), CuO (b), CN@CuO(4) (c)
150
and HRTEM of CN@CuO(4) (d).
100
(a)
284.6 eV
(b)
C 1s
N 1s
398.5 eV
398.2 eV
50
9
5
0
288.6 eV
400.2 eV
99.9 eV
400
0
.0
0.2
0.4
0.6
0.8
1.0
CN@CuO(4)
200
300
400
500
600
700
800
CN@CuO(4)
Rlative pressure (P/P
0
)
Wavelength / nm
3
Figure 1. XRD (a), N adsorption (b) and UVꢀVis spectra (c) of
asꢀprepared catalysts.
GCN
2
GCN
1
00
283
284
285
286
287
288
289
290
396
397
398
399
401
402
Binding Energy / eV
Binding Energy / eV
The UVꢀVis spectra of the asꢀprepared photocatalysts are
shown in Figure 1c. GCN displays an absorption edge at
approximately 460 nm. The band gap energy, which is calculated
(d)
(
c) Cu 2p
GCN
CuO
9
34
CN/CuO
3
1
according to the method of Oregan and Gratzel, is 2.7 eV. CuO
+
1.45 V
shows obviously improved visible light absorption ability 105
compared with GCN. Its absorption edge is observed at 718 nm,
and the corresponding band gap is estimated to be 1.72 eV. For
the asꢀprepared heterojunction photocatalysts, benefitting from
the addition of CuO, the asꢀprepared composite catalysts display
9
33.7
CN@CuO(4)
+
2.36 V
9
28 932 936
940
944
948
952
956
960
0
5
10
15
20
25
30
Binding Energy / eV
Binding Energy / eV
enhanced absorption in the visible light region. The higher CuO 110 Figure 3. XPS spectra of asꢀprepared catalysts in the region of C
content, the stronger light absorption.
1s (a), N 1s (b), Cu 2p (c) and VB XPS (d).
The morphologies of the representative samples were
examined by using SEM analysis. Figure 2a indicates that asꢀ
prepared GCN is composed of a large number of irregular
XP spectra are used to investigate the structure of the asꢀ
prepared catalysts. In Figure 3a, the spectra of both two catalysts
particles. Those particles exhibit layer structure that is similar to 115 in C 1s region can be fitted with two contributions which located
analogue graphite. In Figure 2b, the nanorodꢀlike CuO with 1~2
at 284.6 and 288.6 eV. The sharp peak around 284.6 eV is
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4
0,41
attributed to the pure graphitic species in the CN matrix. The
gꢀC N4 and CuO.
CN@CuO(4) displays the smallest arc
3
peak with binding energy of 288.6 eV indicates the presence of 60 radius, confirming that
a more effective separation of
2
sp C atoms bonded to aliphatic amine (–NH or –NH–) in the
photogenerated electronꢀhole pairs and a faster interfacial charge
transfer occur on the CN@CuO(4) surface.
Figure 5a displays the photocatalytic H O production ability
2
3
3,34
aromatic rings.
In Figure 3b, the main N 1s peak of GCN
2
5
0
5
0
5
0
5
0
5
0
5
located at 398.2 eV can be assigned to sp ꢀhybridized nitrogen
2
2
2
(C=NꢀC), thus confirming the presence of sp ꢀbonded graphitic
over asꢀprepared catalysts. It is clearly seen that the H O2
2
carbon nitride. The peak at a higher binding energy of 399.9 eV is 65 concentration of asꢀprepared catalyst increases with time for
3
5,36
attributed to tertiary nitrogen (Nꢀ(C)3) groups.
For
about 4 h when it reaches a constant level. This level corresponds
to the steadyꢀstate where the rate of H O production is equal to
GCN@CuO(4), a 0.3 eV shift to higher binding energy is
observed, indicating the decreased electron density of nitrogen
atoms. In Cu 2p region (Figure 3c), the CN/CuO shows the main
2
2
4
2,43
1
1
2
2
3
3
4
4
5
5
the rate of decomposition.
concentration, 0.93 and 0.42 mmol·L . In the case of asꢀprepared
GCN and CuO display low H O
2 2
ꢀ
1
2
+
binding energy of 934 eV, which is assigned to the Cu of 70 heterojunction catalyst, the H O2 production ability promotes
2
3
7
CuO. In the case of CN@CuO(4), the binding energy in Cu 2p
region shifts to 933.7 eV, hinting the increased electron density of
Cu atoms. Combined with the binding energy shift in N 1s
region, it is deduced that the strong electronic interaction between
obviously. CN@CuO(4) shows the highest H O equilibrium
2 2
ꢀ
1
concentration of 6.9 mmol·L , which is 7.4 and 16.4 times higher
than that of GCN and CuO. Figure 5b shows the H O production
2
2
ability of CN@CuO(4) under different reaction conditions. The
ꢀ
1
the gꢀC N and CuO exists in CN@CuO(4). The electronꢀrich gꢀ 75 addition of AgNO3 (10 mmol· L ) as an electron scavenger
3
4
C N transfer of partial electrons to CuO, leading to the change of
should suppress the H O production ability. However,
2 2
3
4
electron densities in N 1s and Cu 2p regions. Figure 3d shows the
VB XPS of GCN and CuO. The EVB is +1.45 and +2.36 V for
GCN and CuO, respectively. Combine with the UVꢀVis result, it
interestingly, the H O2 production ability of CN@CuO(4)
2
ꢀ
1
increases to 7.7 mmol· L when AgNO is added. Still 3.2 mmol·
3
ꢀ
1
L
H O is obtained in the absence of O . These results indicate
2 2 2
is deduced that the ECB for GCN and CuO is ꢀ1.25 and +0.64 V. 80 that H O is not only produced by O2 reduction over
2
2
3,38
These results are close to the previous report.
CN@CuO(4). The detailed explanation is given below. The H O2
2
concentration is trivial in the absence of photocatalyst, indicating
that H O is produced via a photocatalytic process. No H O is
generated when using dimethylformamide (DMF, aprotic solvent)
8
(
a)
GCN
CuO
CN@CuO(2)
CN@CuO(4)
CN@CuO(8)
(b)
GCN
CuO
CN@CuO(2)
CN@CuO(4)
CN@CuO(8)
7
6
5
4
3
2
1
0
2
2
2
2
8
5
instead of water, confirming that H O is necessary as the proton
2
source for H O production.
2
2
8
7
6
5
4
3
2
1
0
8
7
6
5
4
3
2
1
0
(a)
(b)
1CN@CuO(4)
2CN@CuO(4) + AgNO3
CN@CuO(4)
3without O
2
350
400
450
500
550
600
650
700
750
0
1
2
3
4
5
6
7
8
9
10
6
Wavelength / nm
ZRe / 10 ohm
4without CN@CuO(4)
5with DMF
CN@CuO(8)
CN@CuO(2)
90
Figure 4. PL (a) and EIS (b) spectra of asꢀprepared catalysts.
GCN
CuO
PL and EIS were used to characterize chargeꢀcarrier migration
and confirm the interfacial charge transfer effect of the asꢀ
prepared heterojunction catalysts. Figure 4a shows the PL spectra
of asꢀprepared catalysts using excitation at 255 nm. For GCN,
broad PL band around 460 nm is observed with the energy of
light approximately equal to the band gap of gꢀC N . CuO
0
1
2
3
4
5
6
7
8
time / h
1
2
3
4
5
95
Figure 5. The photocatalytic H O production ability over asꢀ
2
2
prepared catalysts (a) and H O production ability of
2
2
CN@CuO(4) under different reaction conditions (b).
3
4
exhibits emission peak which intensity is lower than that of GCN.
1
0
8
6
4
2
0
semiconductor 1
ꢀ
semiconductor 1
(b)
(
a)
a: without scavenger
b: with AgNO
c: with EDTA-2Na
c
e
e
ꢀ
In the case of asꢀprepared heterojunction catalysts, the PL spectra 100
show the similar shape to that of GCN, whereas the intensities are
obviously decreased. CN@CuO(4) shows the lowest PL intensity,
hinting its most effective separation rate of electrons and holes.
This is reasonable because, with this gꢀC N /CuO mass ratio, gꢀ
3
b
a
eꢀ
e
ꢀ
h+
h
+
c
h+
h
+
3
4
a
semiconductor 2
Zꢀscheme mechanism
a
b
semiconductor 2
2
ꢀ1
c
b
double charge
transfer mechanism
C N and CuO have the approximate S
(9.5 and 20.1 m ·g
105
3
4
BET
CuO
GCN
CN@CuO(4)
for GCN and CuO, and the yield of GCN is approximately 50%
by this microwave process). They can contact with each other as
much as possible, leading to the formation of the maximum area
of the heterojunction. As shown in Figure 4b, the asꢀprepared
Figure 6. The influence of various scavengers on the H O2
production ability over asꢀprepared catalysts (a) and the sketch
map of double charge transfer mechanism and Zꢀscheme
2
heterojunction catalysts exhibit a decreased arc radius compared 110 mechanism over heterojunction catalyst (b).
to that of GCN and CuO. In general, the radius of the arc in the
EIS spectra reflects the reaction rate on the surface of the
electrode. The reduced arc radius indicates a diminished
resistance of the working electrodes, suggesting a decrease in the
Besides the O reduction to form H O , another channel to
2 2 2
3
9
15
produce H O is reported by Dong et al. It is reported that
2 2
−
photogenerated
holes
can
oxidize
OH
to
•OH
solidꢀstate interface layer resistance and the charge transfer 115 thermodynamically, as shown in Reaction (3). Two •OH can form
resistance across the solidꢀliquid junction on the surface between
H O though combination with each other, as shown in Reaction
2
2
4
| Journal Name, [year], [vol], 00–00
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Page 5 of 7
New Journal of Chemistry
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DOI: 10.1039/C8NJ02565G
(
4). In order to investigate the reaction mechanism of asꢀprepared
microwave process, CN/CuO heterojunction catalyst was also
prepared by polycondensation method. Figure 7a clearly displays
that the asꢀprepared CN/CuO is composed of bulk gꢀC N and
heterojunction catalyst, the influence of various scavengers on the
H O production ability is carried out and shown in Figure 6a.
AgNO and EDTAꢀ2Na are used as the electrons (e ) and hole
2
2
3
4
ꢀ
nanorodꢀlike CuO. CuO nanorods seem to stick to the gꢀC N4
3
3
+
44
5
0
5
0
(h ) scavenger, respectively. When AgNO is added to trap the 60 surface. This interaction between two components is poor. Figure
3
electrons, H O is still formed with the concentration of 0.3
7b compares the PL spectra of CN/CuO and CN@CuO(4). It can
be seen that the PL intensity of CN/CuO is remarkably stronger
than that of CN@CuO(4), hinting its much lower separation rate
of electronsꢀholes. This should be due to that the cladding
2
2
ꢀ
1
mmol·L over CuO. It is known that the redox potential for
−
45
•
OH/OH is +1.99 V. Whereas the VB of CuO is +2.36 V. The
VB holes in CuO is positive enough to generate •OH. Therefore,
H O should be produced by Reaction (4). When EDTAꢀ2Na is 65 structure of CN@CuO(4) can form the maximum area of
2 2
1
1
2
added to trap the holes, although the utilization rate of electrons is
promoted, the H O equilibrium concentration over CuO
heterojunction, which results in an increase in the migration
efficiency.
2
2
ꢀ
1
decreases to 0.12 mmol·L . This is probably due to the close
position between reduction potential of O /H O (+0.695 V) and
(b)
CN/CuO
CN@CuO(4)
2
2
2
CB of CuO (+0.64 V), leading to the poor driving force. In the
case of GCN, the CB and VB positions are ꢀ1.25 V and +1.45 V,
respectively. Thus, when AgNO is added, no H O is produced
70
75
80
3
2
2
due to that the VB holes in gꢀC N are not positive enough to
3
4
4
00
500
600
700
generate •OH. Without any doubt, the H O production ability of
2
2
Wavelength / nm
8
7
6
5
4
3
2
1
0
gꢀC N is obviously promoted by adding EDTAꢀ2Na to trap the
Cu 2p
d)
3
4
(c)
933.9 eV
(
holes
CN@CuO(4)
h+ + OHꢀ
(3)
(4)
OH
H O
reused CN/CuO
933.6 eV
OH + OH
2
2
CN/CuO
In general, there are two typical working mechanisms for
heterojunction catalyst, double charge transfer mechanism and Zꢀ
reused CN@CuO(4)
0
10
20
30
40
50
925
930
935
940
945
950
955
960
965
970
time / h
Binding Energy / eV
4
6ꢀ48
scheme mechanism, as shown in Figure 6b.
If CN@CuO(4)
2
3
3
4
4
5
5
5
0
5
0
5
0
5
heterojunction catalyst follows the double charge transfer
Figure 7. SEM image of CN/CuO (a), PL spectra of CN/CuO and
CN@CuO(4) (b), photocatalytic H production ability of
When CB electrons is trapped by AgNO , VB holes in gꢀC N are 85 CN/CuO and CN@CuO(4) (c) and XPS spectra of reused
mechanism, holes should be transferred to the VB of gꢀC N .
2
O
2
3
4
3
3
4
not positive enough to generate •OH. Thus H O should be not
CN/CuO and CN@CuO(4) in Cu 2p region (d).
2
2
produced. However, the H O with the concentration of 7.7
2
2
ꢀ
1
mmol·L is formed over CN@CuO(4) when AgNO is added.
Figure 7c compares the photocatalytic H
of CN/CuO and CN@CuO(4). CN/CuO shows the H O
2 2
2 2
O production ability
3
Similarly, if CN@CuO(4) heterojunction catalyst follows the
ꢀ
1
double charge transfer mechanism, the electrons should be 90 concentration of 1.8 mmol·L , much lower than that of
ꢀ
1
transferred to the CB of CuO. The H O production ability should
CN@CuO(4) (6.9 mmol·L ). In addition, the H O production
2 2
2
2
be low by adding EDTAꢀ2Na to trap the holes because of the
ability for CN@CuO(4) keeps stable after 48 h reaction.
Whereas, the activity for CN/CuO decreases obviously with
increasing the reaction time, hinting its poor photocatalytic
poor driving force of CB electrons. Whereas, the H O2
2
equilibrium concentration over CN@CuO(4) is promoted to 8.9
ꢀ
1
mmol·L when EDTAꢀ2Na is added. Based on the above results, 95 stability. Figure 7d compares the XPS spectra of reused CN/CuO
it is deduced that not double charge transfer mechanism but Zꢀ
scheme mechanism with “two channel pathway” is proposed.
Under visible light irradiation, the photogenerated electronꢀhole
pairs are formed in both components. The electrons in the CB of
and CN@CuO(4) in Cu 2p region. The binding energies locate at
933.9 and 933.6 eV for reused CN/CuO and CN@CuO(4). Such
difference in binding energy confirms that the strong electronic
interaction between gꢀC N and CuO in CN@CuO(4) still exist
3 4
CuO combine with the holes in the VB of gꢀC N at the interface 100 after reaction. All of these results confirm that, compared with
3
4
of the heterojunction. Therefore, the holes tend to stay in the VB
of CuO and the electrons accumulate in the CB of gꢀC N ,
traditional polycondensation method, the gꢀC N coated CuO
3 4
nanorods heterojunction catalyst prepared via molten saltꢀassisted
microwave process exhibits larger area of heterojunction and
more effective separation rate, causing the higher photocatalytic
3
4
leading to the enhanced separation rate of electronꢀhole pairs. The
CB electrons in gꢀC N can reduce O to form H O , as well as
3
4
2
2
2
−
the VB holes in CuO can oxidize OH to form •OH, which 105
subsequently react with each other to form H O . Such “two
H O production ability and stability.
2 2
2
2
channel pathway” causes the remarkably promoted H O2
Conclusions
2
production ability. When AgNO (or EDTAꢀ2Na) is added to trap
3
In this work, gꢀC N4 coated CuO nanorods heterojunction
3
the electrons (or holes), the separation efficiency of catalyst is
catalyst with outstanding photocatalytic H O production ability
2
2
promoted, leading to the enhanced H O2 production ability
2
was synthesized. Compared with traditional polycondensation
10 method, the catalyst prepared via molten saltꢀassisted microwave
process exhibits larger area of heterojunction and more effective
(Figure 6a).
1
In order to prove the advance of this gꢀC N4 coated CuO
3
nanorods heterojunction catalyst prepared via molten saltꢀassisted
separation rate, causing the higher photocatalytic H O2
2
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 5

New Journal of Chemistry
Page 6 of 7
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DOI: 10.1039/C8NJ02565G
production ability and stability. CN@CuO(4) with the largest
area of heterojunction displays the highest photocatalytic H O
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−
5
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2
“
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2
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| Journal Name, [year], [vol], 00–00
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Page 7 of 7
New Journal of Chemistry
View Article Online
DOI: 10.1039/C8NJ02565G
H O
2 2
-e-
e
-
e
CB
-
1.25 V
e-
e-
O + H O
Two Channel Pathway
2
2
+
0.64 V
hγ
+
0.695 V
hγ
(O
2
/H
2
O
2
)
+
1.45 V
2.36 V
OH-
VB
h+
+
+
1.99 V
h
gꢀC N
-
3
4
OH/OH h+
+
h+
OH + OH
H O
2 2
CuO
This work reports a g-C
3 4
N coated CuO nanorods catalysts with outstanding
photocatalytic H production ability via “two channel pathway”.
2
O
2