Facile structure design based on C3N4 for mediator-free Z-scheme water splitting under visible light

Article abstract of DOI:10.1039/c5cy00379b

In this work, two photocatalysts (i.e., C₃N₄ and WO₃) were successfully combined into a heterojunction structure by a facile hydrothermal method for mediator-free overall water splitting, analogous to the natural photosynthesis over a two-step photoexcitation Z-scheme system. Hydrogen and oxygen are evolved with a 2:1 ratio by irradiating the C₃N₄-WO₃ composites loaded with Pt under visible light (λ > 420 nm) without any redox mediator. Introducing reduced graphene oxide (rGO) into the C₃N₄-WO₃ composites enhances the water splitting efficiency. Through optimizing the mass ratio in the C₃N₄-WO₃ composites, rGO content, amount of loaded Pt and pH value of the reacting system, the highest H₂/O₂ evolution rates of 2.84 and 1.46 μmol h^-1 can be obtained, with a quantum yield of 0.9%. Our findings demonstrate that the hydrothermal method is a promising strategy for constructing intimate heterostructures for Z-scheme water-splitting systems without using any redox mediator, and that rGO can be used to further enhance the performance in optimized conditions.

Full text of DOI:10.1039/c5cy00379b

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ARTICLE TYPE
Facile Structure Design Based on C N for Mediator-free Z-Scheme
3
4
Water Splitting under Visible Light
Guixia Zhao,‡ Xiubing Huang,‡ Federica Fina, Guan Zhang and John T.S. Irvine*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
In this work, two photocatalysts (i.e., C N and WO ) were successfully combined into a heterojunction
3
4
3
structure by a facile hydrothermal method for mediator-free overall water splitting, analogous to the
natural photosynthesis over two-photo excited Z-scheme system. Hydrogen and oxygen are evolved with
a 2:1 ratio by irradiating the C N -WO composites loaded with Pt under visible light (λ > 420 nm)
3
4
3
1
0
without any redox mediator. Introducing reduced graphene oxide (rGO) to the C N -WO composites
3 4 3
enhances the water splitting efficiency. Through optimizing the mass ratio in the C N -WO composites,
3
4
3
rGO content, amount of loaded Pt and pH value of the reacting system, the highest H /O evolution rates
2
2
-
1
of 2.84 and 1.46 ꢀmol h can be obtained, with quantum yield of 0.9%. Our findings demonstrate that
hydrothermal method is a promising strategy for constructing intimate heterostructures for Z-scheme
water-splitting system without using any redox mediator, and that rGO can be used to further enhance the
performance in optimized conditions.
1
5
1
1
1
. Introduction
In Iwase et al’s report, it has been shown that photoreduced
graphene oxide (PRGO) was able to function as a solid electron
With the increasing energy demand and concerns about
environmental issues, hydrogen gas is considered as a promising
fuel because of its no carbon emission nor other harmful by-
5
5
6
6
7
7
0
5
0
5
0
5
mediator for Z-scheme overall water splitting, where SrTiO :Rh
was used as H evolution photocatalyst, with 0.7 wt% Ru as co-
catalyst, and BiVO was used as O evolution photocatalyst. The
PRGO was synthesized by photoreduction, i.e., irradiating the
mixture of graphene oxide and the photocatalyst (Ru/SrTiO :Rh
or BiVO ) in the presence of methanol as electron donor. For the
3
2
2
2
3
3
4
4
0
5
0
5
0
5
1
4
2
products. The artificial photosynthesis for hydrogen generation
through water splitting under visible light is an important
approach, among which, the powdered-type two-step
photoexcitation system, i.e., Z-scheme photocatalysis system, is
the most similar to natural photosynthesis. Normally, a Z-
scheme system contains two photocatalysts (one for H evolution,
and the other for O evolution) and usually an electron mediator,
which transfers the electrons from the excited O photocatalyst to
the holes generated in the H photocatalyst.
3
4
combined PRGO/BiVO and Ru/SrTiO :Rh under pH 3.5, the H
2
2
-6
4
3
-1
evolution rate is ca. 8 ꢀmol h , while under the neutral condition,
the performance is much lower, which is considered due to the
bad physical interaction between the PRGO/BiVO4 and
2
2
2
Ru/SrTiO :Rh under neutral conditions. In total, the reported Z-
7
3
2
scheme based on PRGO is complicated in preparation as the
noble metals (i.e., Ru and Rh) are used as co-catalyst and dopant,
respectively, and the performance is dependent on the acidity.
It is suggested that electron transfer is the rate-determining
8
process. The commonly used electron mediators are ionic redox
couples such as IO /I ,
these ionic redox couples, solid electron shuttle is more advanced
for the recycle of the photocatalyst and cleaning of water. The
challenges for the efficient H /O evolution lie in the electron-
accepting and -donating abilities of the mediator, which should
retain a dynamic equilibrium and keep stable during reactions.
Furthermore, interface contact in the ternary system also plays an
important role in maintaining continuous electron flow between
the two photocatalysts. Considering the extremely high
conductivity and specific surface area of graphene, it may be a
good candidate as electron mediator for Z-scheme system.
Besides, it is also challenging to choose appropriate
photocatalysts for realizing the intimate physical interaction with
graphene, the efficient charge carrier separation, the H /O2
evolution on the H /O photocatalysts and then finally preventing
-
- 2, 3, 9
3+
2+ 4, 9, 10
or Fe /Fe .
Compared with
8
3
Although Sasaki et al. reported the interparticle electron transfer
without any electron mediator, the water splitting can only
proceed efficiently by acidifying the aqueous solution to realize
the effective contact through the aggregation of the two
2
2
photocatalysts, Ru/SrTiO :Rh and BiVO . Thus, more sturdy
1
1
3
4
contacts between the H /O evolution phototcatalysts need to be
2
2
found.
Since the introduction of graphitic carbon nitride (C N ) into
heterogeneous catalysis in 2006,
consisting of tri-s-triazines interconnected via tertiary amines has
attracted increasing interest in material research especially in H -
evolution photocatalysis under visible light.
C N is ca. 2.7 eV, with the CB and VB positions located at ca. -
.1 eV and ca. +1.6 eV vs normal hydrogen electrode (NHE),
3
4
14, 15
this yellow polymer
11
2
16-18
The band gap of
3
4
2
1
2
2
16, 19
respectively.
More importantly, the preparation of C N is
3 4
1
2, 13
water formation due to the backward reaction of H and O .
2
2
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low cost since it only consists two elements (C and N). For O2-
were used in the presence of triethanolamine (TEOA) as
sacrificial agent. For overall water splitting test by C N -rGO-
evolution photocatalysis from water, tungsten trioxide (WO ) has
3
3
4
been widely investigated because of its suitable bandgap, high 60 WO and C N -WO composites, these Pt-loaded samples were
3
3
4
3
2
0
electron mobility, abundance and low cost.
further reduced by NaBH solution in Ar protection. For example,
4
5
0
5
Herein, we introduced a novel method to generate a close
contact between C N and WO by the reaction of Na WO ·2H O
200 mg of C N -rGO-150-WO 400-260 loaded with 1 wt% Pt
was dissolved in 30 mL of deionized water and purged with Ar
3
4
3
3
4
3
2
4
2
and C N under hydrothermal conditions. To enhance the
for 30 min before adding 5 mg of NaBH . After 2 h for reduction,
3
4
4
performance, rGO was also used to function as electron mediator. 65 the suspention was centrifugated and washed with distilled water
By calcining the mixture of rGO and melamine, the graphene
for several times. The resulted slurry was then used for
photocatalytic test.
Material Characterizations: Specimens were characterized by
XRD on a PANalytical Empyrean Reflection diffractometer with
1
layer was able to be homogeneously mixed with C N4 layers.
3
Then through hydrothermal method, the C N -rGO composites
3
4
were loaded with WO particles, which function as O evolution
3
2
photocatalyst. It is demonstrated that the binary (in the case of no 70 a Cu Kα source (λ = 1.541 Å). TEM analysis was performed on a
rGO) or ternary (in the case of containing rGO) composites were
JEM-2011. XPS were tested by an ESCALAB 250 and all
binding energies were referenced to the C 1s peak at 284.6 eV. A
UV-Vis spectrophotometer (JASCO-V550) was used for testing
the optical absorbance spectra.
Photocatalytic measurement: The measurement was performed
with a home-made Teflon reactor with top window sealed by a
1
able to give out H and O under visible light irradiation (λ > 420
2
2
nm) with relatively high performance when loaded with Pt as co-
catalyst, and appropriate amount of rGO will enhance the
performance.
75
2
. Experimental Section
pyrex glass. In H -evolution test by C N -rGO composites loaded
2 3 4
with 1 wt% Pt, 200 mg of photocatalyst was dispersed into 100
mL of water and 10 mL of TEOA was added as sacrificial agent.
In O -evolution test by WO ·1/3H O, 100 mg of WO ·1/3H O
2 3 2 3 2
20
25
30
35
40
45
50
55
Synthesis of C N -rGO composite and pure C N : Graphene oxide
3
4
3
4
was synthesized according to the modified Hummers method as
8
8
9
9
0
5
0
5
2
1
described in our previous work. The as-prepared GO was then
reduced by hydrothermal method. 60 mg of GO was dissolved in
powder was dispersed into 100 mL of water with 10 mmol/L of
AgNO as an electron acceptor. The reactor was firstly purged
3
3
0 mL of H O by moderate sonication and the pH was adjusted to
2
with pure Ar to remove air from the reaction system, then the
solution was irradiated by a 250 w iron doped metal halide UV-
Vis lamp (UV Light Technology Limited, UK) with a UV cut-off
about 7 using NaOH solution before transferred into a Teflon-
o
lined steel autoclave, which was heated in an oven at 140 C for
1
8 h. The obtained rGO was collected and washed with distilled
filter (λ > 420 nm) and the amount of H or O produced was
o
2
2
water and ethanol several times, and dried at 90 C for 2 h. C N -
3
4
determined by gas chromatography (Agilent 3000 Micro Gas
Chromatograph with a TCD detector, using argon carrier gas).
The H - and O -evolution half reaction results shown in
rGO composites were obtained by calcining the mixtures of
melamine and rGO with different mass ratios. These C N -rGO
3
4
2
2
composites are noted as C N -rGO-x, where x represents the mass
3
4
supporting information demonstrated their activity under visible
light. The mediator-free Z-scheme overall water splitting reaction
trials were performed in the same top-irradiation system. During
ratio of melamine to rGO during the synthesis process. For
example, the composite C N -rGO-150 was prepared by calcining
3
4
o
the mixture of 20 mg of rGO and 3000 mg of melamine at 500 C
for 3 h with a 5 C/min increasing rate. Pure C N was also
obtained by calcining melamine at 500 C for 3 h.
Synthesis of C N -rGO-WO and C N -WO composites: The
these trials, 200 mg of Pt-loaded C N -rGO-WO or C N -WO
3
o
3
4
3
3
4
3
4
composites were suspended in 100 mL of deionized water, but no
TEOA was added. The evolved gas composition in the sealed
reactor was measured by gas chromatography.
Quantum yield efficiency measurement: The apparent quantum
yield (AQY) of the Z-scheme water splitting with a two-photon
excitation process was measured using the same reactor with the
o
3
4
3
3
4
3
ternary composites are noted as C N -rGO-x-WO m-n, m
3
4
3
representing the initial reacting mass of C N -rGO-x in mg, n
3
4
representing that of Na WO ·2H O. Taking C N -rGO-150-WO
3
2
4
2
3
4
4
00-260 for example, 400 mg of C N -rGO-150 and 260 mg of
3 4
1
00 addition of a band-pass filter (420 nm, ∆λ1/2 = 10 nm). The AQY
Na WO ·2H O (99%, Aldrich) were mixed in 30 mL of
22
2
4
2
values were calculated using the following equation:
deionized water and kept stirring at ca. 300 rpm for 12 h. Then
the pH value was adjusted to 2 using diluted HCl solution. After
further stirring at ca. 300 rpm for another 12 h, the suspension
was transferred into a Teflon-lined steel autoclave and kept at 160
AQY (%) = [4 × n(H )]/n(photons) × 100
(1)
2
where n(H ) and n(photons) represent the quantities of evolved
2
H molecules and the number of incident photons, respectively.
2
1
05 The total number incident photons were determined using a
grating spectroradiometer (Apogee Instruments, Inc, Model MQ-
o
C for 18 h. The C N -WO composite was obtained by the same
3
4
3
method except for using pure C N4 instead of C N -rGO
3
3
4
2
00) and the the incident photon power in the apparent quantum
composites. After hydrothermal reaction, the resulted composites
were separated by centrifugation at 4000 rpm and washed with
-2 -1
yield measurement is about 120 ꢀmol m s .
o
distilled water for several times before dried at 90 C for 3 h.
3. Results and Discussion
Pt-loading on the C N -rGO, C N -rGO-WO and C N -WO
3
3
4
3
4
3
3
4
composites: Pt was firstly loaded onto the specimen by 110 Figure 1 shows the typical XRD patterns of WO and C N -rGO-
3
3
4
impregnation method: proper amount of H PtCl solution (ca. 4
150-WO composites resulting from the hydrothermal reaction of
2
6
3
mL) was impregnated into the sample powders by evaporating
Na WO ·2H O without or with different contents of C N -rGO-
2
4
2
3
4
o
the solvent of the mixed suspension at 90 C until dry. For H -
150. The XRD pattern in Figure 1a denotes the presence of
orthorhombic tungsten oxide one third hydrate (WO ·1/3H O,
2
evolution test by C N -rGO composites, these Pt-loaded samples
3
4
3
2
2
| Journal Name, [year], [vol], 00–00
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JCPDS Card no. 54-1012), and the characteristic peak from C N4
particles are not loaded on the surface of C N -rGO-150 but must
3 4
3
located at around 27 degree increased with the increment of
C N -rGO-150 content, as shown in Figure 1b and 1c. There are
be independent of it, while in C N -rGO-150 1000-260, almost
3 4
no independent WO particles exist. By loading Pt as co-catalyst,
3
4
3
no obvious diffraction peaks for WO ·1/3H O in Figure 1c, 30 efficient H -evolution performance can be realized. Figure 2c and
3
2
2
5
which may be due to the large molar ratio of C N to WO in
Figure S1 (see in Supporting Information) show the TEM images
3
4
3
C N -rGO-150-WO 1000-260 (ca. 13.8 calculated from the
of C N -rGO-150 400-260 loaded with 1 wt% Pt after several
3
4
3
3
4
added amount). If considering the smaller density and bigger
particle size of C N than WO nanoparticles in C N -rGO-150-
periods of photocatalytic water splitting tests, indicating that Pt is
mainly distributed onto the surface of C N maybe due to its
3
4
3
3
4
3
4
+
WO3 1000-260, the XRD diffraction density of C N4 is much 35 stronger chelating ability to Pt . The typical UV-Vis light
3
1
0
stronger than that of WO3 nanoparticles. Therefore, the
diffraction peaks of WO ·1/3H O were properly overlapped by
absorbance spectra in Figure 2d also indicate that there were no
obvious differences in the absorbance between the composites
3
2
that of C N . In addition, the crystallinity of WO may be also
with different WO contents, while adding rGO indeed increased
3
4
3
3
poor because of the dispersion of C N particles.
the absorption toward the visible light.
3
4
1
5
Figure 1. XRD patterns of pure WO
3
(a), C
3
N
4
3
-rGO-150-WO 400-260
(
b) and C -rGO-150-WO 1000-260 (c).
3
N
4
3
4
4
0
5
Figure 3. The performance of the cycling test for C
4
3
N
4
-rGO-150-WO
00-260 without (a) and with (b) Pt under pH = 7; the C -rGO-150-
400-260 without Pt before photo-reaction (c) and after six cycle of
-rGO-150-WO 400-260 with Pt after
six cycles of photo-reaction (e).
3
3 4
N
WO
3
photo-reaction (d); the sample C
3
N
4
3
We used the as-prepared C N -rGO-150-WO 400-260 without
3
4
3
loading Pt as photocatalyst for cycling water-splitting test under
visible light (Figure 3). As shown in Figure 3a, in the initial four
cycles, the performance increased but then decayed on further
5
5
6
0
5
0
cycling. The best performance is only about 12 ꢀmol H in 20
hours' irradiation. After loading Pt as co-catalyst, the case is
2
much different, as shown in Figure 3b. The H -evolution
2
performance is very stable from the second cycle and the final
amount of H produced in 20 hours is as high as 35 ꢀmol and the
2
produced O is about half in ꢀmol compared with that of H . It is
2
2
also noted that without loaded Pt, the photocatalyst underwent
obvious color change during the photo-reaction as shown in
Figure 3c and 3d, where the milk-white suspension changed into
blue after six cycles of test. According to much reported research
Figure 2. TEM images of C
50-WO 1000-260 (b), C
3
N
4
-rGO-150-WO
3 3 4
400-260 (a), C N -rGO-
1
3
3
N
4
-rGO-150-WO 400-260 loaded with 1 wt%
3
on the photochromogenic WO , when irradiated at a photo energy
3
2
0
Pt after photocatalytic test (c); The UV-Vis light absorbance spectra
converted from the corresponding reflectance spectra) of three typical
samples loaded with 1 wt% Pt (d).
higher than its band gap (2.8 eV), photochromism would occur,
in which the photo-generated electron-hole would lead to the
(
6+
5+ 23-28
reduction of W into blue-colored W .
with Pt nanoparticles, the photocatalyst became just a little darker
65 comparing with that of the milk-white C N -rGO-150-WO 400-
While after loaded
Figure 2a and 2b show the TEM images of C N -rGO-150-
3
4
WO 400-260 and C N -rGO-150-WO 1000-260, respectively. It
3
3
4
3
3
4
3
2
5
is obvious that in the hydrothermal reaction, the higher
Na WO ·2H O/C N -rGO-150 ratio means that the more WO
2
60, and no color changed during the photo-reaction, as shown in
2
4
2
3
4
3
This journal is © The Royal Society of Chemistry [year]
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a
Table 1. Overall water splitting under visible light by C
3 4 3
N -WO system in the presence and absence of rGO
Hydrothermal conditions
Activity after 20 h irradiation
b
c
No.
C N or C N -rGO
Na WO ·2H O
Amount of loaded Pt
pH
H2
O2
3
4
3
4
2
4
2
Type of C
3 4 3 4
N or C N -rGO
(mg)
(mg)
(ꢀmol)
(ꢀmol)
1
2
3
4
5
6
7
8
9
Pure C
3
N
4
400
400
0
1 wt% Pt
1 wt% Pt
1 wt% Pt
1 wt% Pt
1 wt% Pt
1 wt% Pt
1 wt% Pt
1 wt% Pt
3 wt% Pt
3 wt% Pt
1 wt% Pt
1 wt% Pt
1 wt% Pt
1 wt% Pt
7
7
7
7
7
7
7
7
7
7
3
5
8
trace
3.3
Trace
1.1
C
C
3
3
N
4
4
-rGO-150
-rGO-150
0
N
400
260
260
35.1
26.1
20.8
30.3
29.9
48.1
6.0
18.5
14.1
11.2
14.8
14.5
24.5
3.3
C
3
3
N
4
-rGO-80
-rGO-50
400
C
N
4
400
260
260
260
260
260
260
260
260
260
260
Pure C
Pure C
3
N
4
400
3
N
4
1000
1000
400
C
3
N
4
-rGO-150
3 4
C N -rGO-50
1
1
1
1
1
0
1
2
3
4
C
3
3
3
3
3
N
4
N
4
N
4
N
4
N
4
-rGO-150
-rGO-150
-rGO-150
-rGO-150
-rGO-150
400
11.0
11.4
29.0
53.5
56.6
5.0
C
C
C
C
1000
1000
1000
1000
30.5
15.1
26.8
29.2
10
a)
Conditions: photocatalyst (200 mg in total) in water (100 mL); light source, 250 W iron-doped metal halide ultraviolet-visible lamp, with a cut-off filter
b)
(λ > 420 nm); top-irradiation cell with a quartz glass. Pt was loaded by first impregnation of H
2
6 4
PtCl , the lateral reduction with NaBH solution and the
c)
last washing for several times before using as photocatalyst in muddy; The pH of the suspension in the photocatalystic test system.
3
0-32
5
0
5
0
5
0
5
Figure 3e. It is now discovered that the loaded Pt is able to inhibit 40 scattering.
An appropriate amount of graphene in the C N -
3 4
the color changing, which may be due to the efficient electron
capture by the loaded Pt nanoparticles. The gas generation
amount dependence on reaction time shown in Figure S2 for
C N -rGO-150-WO 400-260 loaded with 1 wt% Pt indicates that
rGO composite would improve the electron transfer and further
prevent the charge carrier-recombination,
enhancement occurred for C N -rGO-150 composite with Pt as
co-catalyst.
The results for No. 3 and 8 in Table 1 show that sample C N -
rGO-150-WO 1000-260 displays a higher performance by 37%
than that of C N -rGO-150-WO 400-260 under the same
condition (i.e., loaded with 1 wt% Pt in pH = 7), indicating too
much WO3 deposited on the surface of C N -rGO-150 will
33-35
thus a little
3
4
3
4
3
1
1
2
2
3
3
gaseous products evolve linearly for the next 12 h with a ratio of
45
3
4
-
1
ca. 2:1 (H and O : 1.52 and 0.74 ꢀmol h ) under visible light.
2
2
3
To investigate the effects of graphene content, the mass ratio of
C N (or C N /rGO) and WO , the pH in the photocatalytic
3
4
3
3
4
3
4
3
system and the amount of loaded Pt on the water-splitting
3
4
performance, we tested the H /O evolution amount with different 50 prevent the incident light reaching to the C N -rGO-150 surface,
2
2
3
4
photocatalysts and in different conditions. As indicated by No. 1
and 2 in Table 1, when using only C N loaded with 1 wt% Pt in
which results in the weakened photoexcitation of C N -rGO-
3 4
150. However, for pure C N4 and WO3 system, there is no
obvious change in the water-splitting performance with different
mass ratios of C N4 to WO3 (No. 6 and 7 in Table 1). These
36
3
4
3
the nonsacrificial condition, no water splitting occurred, while
when using C N -rGO-150 composite loaded with 1 wt% Pt, the
3
4
3
produced H is about 3.3 ꢀmol in 20 hours' irradiation. In Jorge et 55 results show that WO3 amount has more obvious influence in
2
al’s work, C N was used for H and O evolution from water
C N -rGO-150 system than pure C N system, further suggesting
3 4 3 4
3
4
2
2
+
half-splitting with methanol and Ag as sacrificial agent,
that rGO is able to play a significant role in the Z-scheme water-
splitting system because of its good conductivity and its function
as promising electron mediator for transferring the photo-
2
9
respectively. In our photocatalytic reaction test, No. 2, the little
water splitting activity should come out from the reduction and
oxidation of water due to the improved electro-hole separation by 60 generated electrons from WO3 to the photo-generated holes in
graphene. In No. 3-6, it is also shown that different rGO contents
in the C N -rGO composites have great effect on the water-
C N . The effect of loaded Pt amount is also revealed from
3 4
comparison between No. 5 and 9 or No. 3 and 10. Too much Pt
has negative effect for the gas evolution, as proved by other
3
4
splitting activity. Both no rGO or too much rGO in the C N -rGO
3
4
3
7-39
composites gives worse performance for the H /O evolution than
reports.
According to previous research on Z-scheme photocatalyst
C N -rGO-150 composite only displays an enhancement of 15.8% system, to ensure the charge transfer, intimate physical
2
2
that of the C N -rGO-150 composite (No. 3), even though the
65
3
4
3
4
for the H /O evolution over pure C N and WO system (No. 6).
interaction between the two photocatalysts was often realized by
the aggregation of the two photocatalysts under some appropriate
In our case, the effect of pH on the water-splitting activity
using TEOA as sacrificial agent and Pt as co-catalyst (see more 70 was also investigated using C N -rGO-150-WO 1000-260 as
2
2
3
4
3
We have also investigated the rGO effect by testing the H2-
evolution performance using different C N -rGO composites
8, 11
pH.
3
4
3
4
3
details in Figure S3 and Table S1 in Supporting Information). It is
implied that rGO has two synergetic effects, one is to prevent the
hole-electron recombination by transferring electrons, the other is
to prevent the efficient irradiation due to its opacity and light
target. It is found that with the increasing of pH, the gas evolution
rate is gradually increased (No. 8, 11-14). When the pH equals to
3, the produced H is as low as 11.4 ꢀmol in 20 hours. Since WO
2
3
is inherently instable at high pH, we did not test the performance
4
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DOI: 10.1039/C5CY00379B
under higher pH conditions. Thus the pH effect should not come
260 (loaded with 1 wt% Pt) after several cycling tests with stable
performance. Interestingly, after dried in air at 80 C for 4 hours,
o
from the aggregation level but from the kinetic rate of the H and
2
O evolution reactions, which can be expressed as followings:
the sample showed a lower activity compared with its stable
40 performance before drying, and only after 3 cycles, the
2
+
-
2
H + 2e → H
(2)
(3)
2
+
+
5
2H O + 4h → O + 4H
performance was recovered, as shown in Figure 4. Then the
2
2
o
It can be inferred that although pH has the conflicting effect for
Equation 2 and 3, a higher pH is beneficial to the whole
performance. This is despite that under acidic condition, H2-
sample was dried again at 80 C for 30 hours, after which the
H /O evolution became even worse and also recovered after 4
2
2
cycles (Figure 4a). We investigated the XPS of the sample dried
o
evolution reaction is relatively favorable, the associated negative 45 in air at 80 C for 30 hours and the fresh wet sample dried in Ar.
1
1
2
2
0
5
0
5
effect for the O evolution would trade off the little benefit since
the water oxidation depends on the adsorbed OH ions onto the
The XPS survey spectrum shown in Figure S4 (see in Supporting
Information) demonstrated the existence of C, N, O, Pt and W,
but no Na was observed. Although both of the samples showed a
high content of PtO (indicated by the Pt 4f and Pt 4f peaks
2
-
WO3 surface and their following oxidation by multiple photo-
4
0
generated holes. Thus under more alkaline conditions, the final
x
7/2
5/2
41
H /O -evolution performance turns out higher, and these results 50 centered at ca. 73.0 eV and 76.3 eV, respectively), the metallic
2
2
indicate that the O -evolution reaction is the rate-determining
Pt content in the sample dried in Ar is obviously higher than that
dried in air (indicated by the Pt 4f7/2 and Pt 4f5/2 peaks centered at
ca. 71.0 eV and 74.3 eV, respectively), as shown in Figure 4b and
2
process.
We also prepared a C N -WO composite via two other
3
4
3
42
methods, one by calcining WO nanoparticles and C N mixture
4c. Therefore, it is inferred that the evolved H would reduce
2
3
3
4
o
at 350 C and the other by hydrothermal treatment of WO3 55 PtO into Pt metal during the irradiation and the activity of the
nanoparticles and C N mixture at 160 C. Both of the as-
x
o
sample is quite sensitive to the Pt metal content.
3
4
prepared mixture were loaded with
1
wt% Pt before
The evolution of H from water splitting was further confirmed
2
photocatalytic test. And for the two cases, no H was detected. It
by using D O as the solvent in the suspension of photocatalyst.
2
2
is confirmed that the hydrothermal condition results in the
The evolved gas mixture was then detected by the mass spectra
intimate contact between C N (or C N -rGO) and WO due to 60 and the results were shown in Figure S5 (see in Supporting
3
4
3
4
3
the special high-temperature and high-pressure environment. And
Information). The extra signal located at 4 amu is due to evolved
the in-situ formation of WO particles on C N framework also
D , which is absent in that of the background gas. We also
2
3
3
4
contributes the intimate interaction between the two
photocatalysts.
measured the apparent quantum yield efficiency under
monochromatic light at 420 nm. The sample C N -rGO-150-WO
3
4
3
6
7
7
8
8
9
5
0
5
0
5
0
1000-260 (loaded with 1 wt% Pt) under neutral condition shows
an AQY efficiency of ca. 0.9% while the homologous sample
without rGO shows
a smaller value (i.e., 0.7%), further
demonstrating the positive effect of rGO.
10
Although Martin et al., recently reported a similar Z-scheme
water splitting on the base of C N and WO (or BiVO ), their
3
4
3
4
performance is strongly dependent on the redox mediator type
-
-
(
i.e., I /IO ) and the pH. However, in our C N -rGO (or C N )
3 3 4 3 4
and WO composite system, no redox mediator is needed for the
3
water-splitting photocatalytic test, which further infers that the
intimate contact resulted from hydrothermal condition is of
benefit to the H /O evolution in the two-photo excited Z-scheme
2
2
system. Figure 5 shows a tentatively proposed mechanism for the
H /O evolution by the C N -WO composite with or without the
2
2
3
4
3
mediating of rGO. According to previous study, the CB and VB
4
3
positions of WO are located about +0.41 eV and +3.18 eV,
3
1
9
while those of C N are about -1.13 eV and +1.57 eV. It is not
3
4
difficult to infer that under irradiation, O could be evolved from
2
WO3 surface and H2 from C N4 surface. Considering the Pt
3
function as discussed above, H is more readily evolved from the
2
surface of Pt than that of C N . Therefore, water is oxidized by
3
4
the photo-induced holes in the VB of WO and reduced by the
3
electrons on Pt trapped from the CB of C N . The electrons on
3
4
3
0
Figure 4. The cycling performance of the sample (C
3
N
4
-rGO-150-WO
3
CB of WO are able to combine with the holes in VB of C N at
3 3 4
1
000-260 loaded with 1 wt% Pt) dried after several photo-test cycles as
the interface due to the intimate contact. rGO successfully
functions as a positive electron mediator because of its good
conductivity for electron migration, its sturdy contact with C N
fresh slurry (a); XPS of the sample dried in air for 30 hours (b); XPS of
the fresh wet sample dried in Ar (c).
3
4
resulted from its calcination with the precursor (i.e., melamine),
It is worth to note that the photocatalysts used in Table 1 were
and its good contact with WO particles under the hydrothermal
3
5
fresh wet samples from the reduction by NaBH without drying.
3
4
conditions.
And we also tried to dry the sample C N -rGO-150-WO 1000-
3
4
3
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DOI: 10.1039/C5CY00379B
2
.
R. Abe, T. Takata, H. Sugihara and K. Domen, Chem. Commun.,
005, 3829-3831.
2
4
4
5
5
6
6
7
7
8
8
0
5
0
5
0
5
0
5
0
5
3. M. Tabata, K. Maeda, M. Higashi, D. Lu, T. Takata, R. Abe and K.
Domen, Langmuir, 2010, 26, 9161-9165.
4
5
.
.
H. Kato, M. Hori, R. Konta, Y. Shimodaira and A. Kudo, Chem.
Lett., 2004, 33, 1348-1349.
R. M. Navarro, M. C. Alvarez-Galvan, J. A. Villoria de la Mano, S.
M. Al-Zahrani and J. L. G. Fierro, Energy Environ. Sci., 2010, 3,
1
865-1882.
6
7
8
9
.
.
.
.
S. J. A. Moniz, S. A. Shevlin, D. J. Martin, Z.-X. Guo and J. Tang,
Energy Environ. Sci., 2015, 8, 731-759.
Y. Miseki, S. Fujiyoshi, T. Gunji and K. Sayama, Catal. Sci. Techno.,
2013, 3, 1750-1756.
Y. Sasaki, H. Nemoto, K. Saito and A. Kudo, J. Phys. Chem. C,
2
009, 113, 17536-17542.
M. Higashi, R. Abe, K. Teramura, T. Takata, B. Ohtani and K.
Domen, Chem. Phys. Lett., 2008, 452, 120-123.
10. D. J. Martin, P. J. T. Reardon, S. J. A. Moniz and J. Tang, J. Am.
Chem. Soc., 2014, 136, 12568-12571.
1
1. A. Iwase, Y. H. Ng, Y. Ishiguro, A. Kudo and R. Amal, J. Am. Chem.
Soc., 2011, 133, 11054-11057.
1
2. K. Yamaguti and S. Sato, J. Chem. Soc. Faraday Trans. 1: Phys.
Chem. Condensed Phases, 1985, 81, 1237-1246.
Figure 5. Proposed mechanism for the H
2
/O
2
evolution by C
3
N
4
-WO
3
13. K. Maeda, K. Teramura, D. Lu, N. Saito, Y. Inoue and K. Domen,
composite with or without the mediating of rGO.
Angew. Chem. Int. Edit., 2006, 45, 7806-7809.
1
4. F. Goettmann, A. Fischer, M. Antonietti and A. Thomas, Angew.
Chem. Int. Edit., 2006, 45, 4467-4471.
4
. Conclusions
15. F. Goettmann, A. Fischer, M. Antonietti and A. Thomas, Chem.
Commun., 2006, 4530-4532.
5
0
5
0
In summary, we have introduced a mediator-free Z-scheme
system design based on C N by utilizing WO3 as O2
1
6. X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M.
Carlsson, K. Domen and M. Antonietti, Nat. Mater., 2009, 8, 76-80.
7. X. Wang, K. Maeda, X. Chen, K. Takanabe, K. Domen, Y. Hou, X.
Fu and M. Antonietti, J. Am. Chem. Soc., 2009, 131, 1680-1681.
3
4
photocatalyst for the first time, where C N and WO3 are
3
4
1
combined as composites through a facile hydrothermal method.
Successful water splitting under visible light (λ > 420 nm) can be
realized without any redox mediator when Pt is loaded on the
surface of the composites. More importantly, rGO is introduced
into these composites to form C N -rGO-WO composites and
18. Z. Zhao, Y. Sun and F. Dong, Nanoscale, 2015, 7, 15-37.
1
9. S. C. Yan, S. B. Lv, Z. S. Li and Z. G. Zou, Dalton Trans., 2010, 39,
488-1491.
1
1
2
1
2
0. H. Kominami, K.-i. Yabutani, T. Yamamoto, Y. Kera and B. Ohtani,
3
4
3
J. Mater. Chem., 2001, 11, 3222-3227.
21. G. Zhao, J. Li, X. Ren, C. Chen and X. Wang, Environ. Sci. Technol.,
2011, 45, 10454-10462.
22. Q. Wang, T. Hisatomi, S. S. K. Ma, Y. Li and K. Domen, Chem.
Mater., 2014, 26, 4144-4150.
23. C. Bechinger, G. Oefinger, S. Herminghaus and P. Leiderer, J. Appl.
Phys., 1993, 74, 4527-4533.
this shows positive effect on the performance in an optimized
amount. The highest H /O evolution rate is about 2.84 and 1.46
2
2
-
1
ꢀmol h under visible light (λ > 420 nm) with the quantum yield
of 0.9% at 420 nm. Although the resulted performance is not in
the highest level for water splitting, the newly proposed
methodology for constructing heterostructure through
hydrothermal method will pave a way for more efficient overall
water splitting with two-photo excited Z-scheme system in the
absence of redox mediator.
24. J. G. Zhang, D. K. Benson, C. E. Tracy, S. K. Deb, A. W. Czanderna
and C. Bechinger, J. Electrochem. Soc., 1997, 144, 2022-2026.
2
5. Y. He, Z. Wu, L. Fu, C. Li, Y. Miao, L. Cao, H. Fan and B. Zou,
Chem. Mater., 2003, 15, 4039-4045.
2
6. W. Morales, M. Cason, O. Aina, N. R. de Tacconi and K. Rajeshwar,
J. Am. Chem. Soc., 2008, 130, 6318-6319.
Acknowledgements
27. G. Hodes, D. Cahen and J. Manassen, Nature, 1976, 260, 312-313.
2
8. M. A. Butler, R. D. Nasby and R. K. Quinn, Solid State Commun.,
1976, 19, 1011-1014.
The authors gratefully thank the Engineering and Physical
Sciences Research Council (EPSRC) platform grant
EP/K006800/1, EP/K036769/1 and EP/K022237/1 for financial
support.
90
29. A. B. Jorge, D. J. Martin, M. T. S. Dhanoa, A. S. Rahman, N.
Makwana, J. Tang, A. Sella, F. Corà, S. Firth, J. A. Darr and P. F.
McMillan, J. Phys. Chem. C, 2013, 117, 7178-7185.
25
3
0. W. Ho, J. C. Yu, J. Lin, J. Yu and P. Li, Langmuir, 2004, 20, 5865-
95
5869.
Notes and references
31. J. Yu, Y. Hai and M. Jaroniec, J. Colloid Interf. Sci., 2011, 357, 223-
28.
2
School of Chemistry, University of St Andrews, St Andrews, Fife, UK:
Fax: +44 (0)1334463808; Tel:+44(0)1334463817; E-mail: jtsi@st-
andrews.ac.uk.
3
3
2. J. Yu, X. Zhao and Q. Zhao, Thin Solid Films, 2000, 379, 7-14.
3. X.-H. Li, J.-S. Chen, X. Wang, J. Sun and M. Antonietti, J. Am.
Chem. Soc., 2011, 133, 8074-8077.
3
0
5
1
1
00
†
Electronic Supplementary Information (ESI) available: [H
performance by C -rGO composites with TEOA as sacrificial agent
under visible light, rGO contents in different C -rGO composites, and
the mass spectra results using D O]. See DOI: 10.1039/b000000x/
‡ These authors contributed equally to this work.
2
evolution
3
3
4. Q. Xiang, J. Yu and M. Jaroniec, J. Phys. Chem. C, 2011, 115, 7355-
3
N
4
7
363.
5. Y. Li, H. Zhang, P. Liu, D. Wang, Y. Li and H. Zhao, Small, 2013, 9,
336-3344.
05 36. P. Zhou, J. Yu and M. Jaroniec, Adv. Mater., 2014, 26, 4920-4935.
3
N
4
2
3
3
3
7. M. Hara, J. Nunoshige, T. Takata, J. N. Kondo and K. Domen, Chem.
Commun., 2003, 3000-3001.
1
.
N. S. Lewis, Science, 2007, 315, 798-801.
6
| Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

Page 7 of 8
Catalysis Science & Technology
View Article Online
DOI: 10.1039/C5CY00379B
3
3
8. N. Bao, L. Shen, T. Takata and K. Domen, Chem. Mater., 2007, 20,
10-117.
9. K. Maeda, X. Wang, Y. Nishihara, D. Lu, M. Antonietti and K.
41. G. M. Bancroft, I. Adams, L. L. Coatsworth, C. D. Bennewitz, J. D.
Brown and W. D. Westwood, Anal. Chem., 1975, 47, 586-588.
42. G. Johansson, J. Hedman, A. Berndtsson, M. Klasson and R. Nilsson,
J. Electron Spectrosc. Relat. Phenom., 1973, 2, 295-317.
43. S. J. Hong, S. Lee, J. S. Jang and J. S. Lee, Energy Environ. Sci.,
2011, 4, 1781-1787.
1
Domen, J. Phys. Chem. C, 2009, 113, 4940-4947.
10
5
40. Y. Zhong, K. Ueno, Y. Mori, X. Shi, T. Oshikiri, K. Murakoshi, H.
Inoue and H. Misawa, Angew. Chem., 2014, 126, 10518-10522.
1
5
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DOI: 10.1039/C5CY00379B
A mediatorꢀfree Zꢀscheme overall water splitting photocatalyst system is achieved
through the use of C N (H generation catalyst) and WO (O generation catalyst)
3
4
2
3
2
composite from a hydrothermal method, and Pt as coꢀcatalyst under visible light (λ >
20 nm).
4