Phosphorus-doped porous carbon nitride for efficient sole production of hydrogen peroxide: Via photocatalytic water splitting with a two-channel pathway

Article abstract of DOI:10.1039/c9ta13929j

The production of hydrogen peroxide (H₂O₂) by photocatalytic water splitting has attracted much attention due to the wide application of H₂O₂ as a multi-functional chemical. Most of the present photocatalysts require sacrificial agents and/or noble metal cocatalysts. Herein, we report that phosphorus-doped porous carbon nitride (CPN), as a metal-free photocatalyst, achieves the photocatalytic water splitting via a two-channel pathway (water oxidation reaction and oxygen reduction reaction) with high H₂O₂ yield of 1968 μmol g^-1 h^-1 (highest yield in current records without sacrificial agent) under room temperature and normal pressure without sacrificial agent and cocatalyst. The quantum efficiency of the sole production of H₂O₂ was measured to be 1.57% under wavelength λ = 420 nm and the solar energy conversion efficiency was determined to be 0.43%. In the present reaction system, the concentration of H₂O₂ can be up to 0.171% (wt%) in a one-batch reaction using 80 mg CPN catalyst (in 15 mL oxygen saturated H₂O) after 36 h under visible light illumination. H₂O₂ obtained by this metal-free catalyst without adding a sacrificial agent is more conducive to subsequent purification in industrial production.

Full text of DOI:10.1039/c9ta13929j

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Number 12
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DOI: 10.1039/C9TA13929J
Journal Name
ARTICLE
Phosphorus-doped porous carbon nitride for efficient sole
production of hydrogen peroxide via photocatalytic water splitting
with a two-channel pathway
Received 00th January 20xx,
Accepted 00th January 20xx
§
§
Jingjing Cao, Hui Wang, Yajie Zhao, Yan Liu, Qingyao Wu, Hui Huang*, Mingwang Shao*, Yang
Liu* and Zhenhui Kang*
DOI: 10.1039/x0xx00000x
www.rsc.org/
2 2
The production of hydrogen peroxide (H O ) by photocatalytic water splitting has attracted much attention due to the wide
application of H as multi-functional chemicals. Most of present photocatalysts require sacrificial agents and/or noble
2 2
O
metal cocatalysts. Here, we report that the phosphorus-doped porous carbon nitride (CPN), as metal-free photocatalyst,
achieves the photocatalytic water splitting via two-channel pathway (water oxidation reaction and oxygen reduction
reaction) with high H
temperature and normal pressure without sacrificial agent and cocatalyst. The quantum efficiency of sole production of
was measured to 1.57 % under wavelength λ = 420 nm and determined solar energy conversion efficiency of 0.43 %.
In present reaction system, the concentration of H can up to 0.171 % (wt %) in one-batch reaction with 80 mg CPN catalyst
in 15 mL oxygen saturated H O) after 36 h under visible light illumination. H obtained by this metal-free catalyst without
adding sacrificial agent is more conducive to subsequent purification in industrial production.
2 2
O yield of 1968 μmoL/g/h (highest yield in current records without sacrificial agent) under room
2 2
H O
2 2
O
(
2
2 2
O
recombination rate of electron-hole pairs, poor solar efficiency and
1
3,14
1
. Introduction
low specific surface area limit the application of C N . A variety of
3
4
strategies have been devoted to improve the photocatalytic
Hydrogen peroxide (H
application in many areas such as chemical industry, biological
2
O ), a common oxidant, has widespread
2
1
5,16
performance of C
elemental doping
3
N
4
, such as nano-architecture of C
and design of C N -based composites.
3 4
Over the past decade, a series of photocatalysts were used in the
and BP/C
production. Both they have high
photocatalytic activity, but these catalysts are expensive or they
require sacrificial agents. Wang et al. reported that PIx-NCN can
3
N ,
4
1
7–19
20–22
1
–3
process and water purification, etc. In present, H
anthraquinone oxidation method and the direct synthesis from H
are frequently-used method in industry.4–7 However, the
and O
former way needs vast energy input and multiple steps, while the
latter carries the risk of exploding. The production of H with
photocatalytic water splitting under visible light is a promising
strategy for safe and effective. In the photocatalytic process, when
the electrons in conduction band (CB) were photoexcited from
2
O
2
production by
2
2
3
24
2
H O
2
production, such as Cu
2
(OH)PO
4
/g-C
3
N
4
3
N
4
have
2
been employed for H
2
O
2
2
O
2
effectively produce H
production rate of H
these various modification routes, the doping of non-metal (e.g. N,
S, P, B) into C can efficiently modulate the physical and chemical
Phosphorus can introduce into the catalyst as an
oxyphilic element, which can play the role of adsorbing O in the
2
O
2
in water under visible light, in which the
2
5
2
O can reach up to 1200 μmoL/g/h. Among
2
semiconductor materials, the H
reduction of O
2
O can be produced via two-electron
2
8
3
N
4
2
. In addition, the water can be oxidized to produce
,10
2
6–30
9
properties.
directly H
Carbon nitride (C
the reduction potential of O
VB, 1.9 V) positive than the oxidation potential of H
2
O
2
.
2
3
N
4
), with the bottom of CB (-0.8 V) negative than
(0.68 V) and the top of its valence band
O (1.78 V),
So, C can be
. However, the rapid
process of oxygen reduction, so as to improve the utilization rate of
oxygen in the air by the catalyst. However, in the industrial
O also need to be
considered. The addition of sacrificial agent not only raises the
consumption of raw materials, but also increases the separation
2
(
2
1
1,12
production, the purification and separation of H
2
2
becomes an attractive metal-free photocatalyst.
used as photocatalytic catalyst to produce H
3
N
4
3
1
2 2
O
difficulty of H
2
O , which will eventually lead to the production cost of
2
H
2
O .
2
Institute of Functional Nano and Soft Materials (FUNSOM), Jiangsu Key Laboratory
for Carbon-based Functional Materials and Devices, Soochow University,
Suzhou, China.
Herein, a phosphorus-doped porous carbon nitride (CPN) was
synthesized by calcined method and subsequent hydrothermal
processes. The CPN shows very high photocatalytic activity for sole
E-mail: hhuang0618@suda.edu.cn; mwshao@suda.edu.cn; yangl@suda.edu.cn;
zhkang@suda.edu.cn
These authors contributed equally to this work.
H
O
generation with 1968 μmol/g/h (highest yield in current records
without sacrificial agent), which is 3.6 times and 28.9 times than that
of phosphorus-doped carbon nitride (CP) and C , respectively.
Notably, in 80 mg CPN /15 mL O -saturated H O system, the H
2
2
§
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See DOI:
0.1039/x0xx00000x
3
N
4
2
2
2
O
2
1
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J Po lue ran sael od fo Mn aot te rai ad l js u Cs ht emm ai rs gt ri yn sA
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ARTICLE
Journal Name
concentration of CPN can up to 0.171 % (wt %) after 36 h reaction clingfilm. After that the mixture was dispersed evenly bViyewuAltrrtiaclseoOunnlinde.
without sacrificial agent. In this catalytic system, the rate of oxygen Finally, the reaction was exposed under visiDblOeI:li1g0h.1t0(3492/C09nTmA13≤92λ9≤J
reduction is higher than that of water oxidation, thus water oxidation 700 nm) for 8 h under a constant stirring. The suspension was
is the rate-determining step. The enhanced photocatalytic activity of centrifuged to remove the suspended photocatalyst. The production
CPN could be attributed to the doping of phosphorus. In addition, the of H
porous structure of CPN expands the specific surface area to solution. After the reaction, the amount of H
2
O
2
was determined by a redox titration of KMnO
was calculated by
(0.1 mol L ) with the addition of 3 mL of
solution (3 mol L ). When the solution became pink after the
addition of KMnO solution and the color of the solution was kept for
4
reagent
2 2
O
−
1
effectively improve the adsorption performance of the catalyst.
redox titration with KMnO
4
−
1
2 4
H SO
2
. Experimental section
4
1
min, the concentration of KMnO
4
solution was used to calculate to
2
.1 Materials
the concentration of H
2 2
O .
Phosphonitrilic chloride trimer (> 98%) was purchased from Aladdin.
Urea and silver nitrate were purchased from Sinopharm chemical
2
.7 Characterization
Reagent Co., Ltd. Nitrogen dioxide (> 98%) was purchased from The morphologies and microstructures of all composites were
Adamas. Methanol (>99.9%) was purchased from J&K. KMnO
characterized by Transmission electron microscopy (TEM) from a FEI-
titration standard solution (0.1 mol L ) was purchased from AKX. All Tecnai F20 microscope at 200 kV. XRD patterns were obtained by a
4
-1
the reagents were tested directly without further treatment.
PIXcel4b X-ray diffractometer (Empyrean, Holland Panalytical) with
Cu Kα radiation (= 0.154178 nm) to analyze the crystal structures of
composites. The UV-Vis absorption spectra were recorded using a
2
3 4
.2 Synthesis of C N
1
0 g urea was added to the porcelain crucible sealed with sealant and Cary 60 spectrophotometer (Agilent Technologies) in the wavelength
then calcined in a muffle at 550 ℃ for 3 h with a rate of 10 ℃/min. range of 200-800 nm. XPS measurements were carried out with a
Finally, the sample was cooled to room temperature to obtain the KRATOS Axis ultra-DLD X-ray photoelectron spectrometer with a
light yellow powder.
monochromatised Mg Kα X-ray (hv = 1283.3 eV). UPS was performed
by using an unfiltered HeI (21.22 eV) lamp as light source and a total
instrumental energy resolution was 100 meV. The scanning electron
microscope (SEM) was applied to characterize the surface
morphology and element contents of the samples. The Fourier
transform infrared (FTIR) spectrum of the samples was acquired from
2
.3 Synthesis of phosphorous doped carbon nitride (CP)
1
0 g urea was uniformly mixed with 0.5 g phosphonitrilic chloride
trimer. The mixture was added to the porcelain crucible sealed with
sealant and calcined in a muffle at 550 ℃ for 3 h with a rate of 10 ℃
/min. Then the sample was cooled to room temperature to obtain
a Hyperion spectrophotometer (Bruker) at the scan range of 400-
the light-yellow powder and washed three times with ultrapure
water. The as-prepared sample was named as CP.
-1
4
000 cm . All the electrochemical measurements were conducted
on a CHI 920C workstation (CH Instruments, Shanghai, China), using
a standard three-electrode system, where a platinum wire is used as
the counter electrode, a saturated calomel electrode (SCE) as the
reference electrode and a glass carbon (GC) electrode as working
electrode. Electrochemical impedance spectra (EIS) measurements
were carried out at open circuit potential as well, with a frequency
range from 1 MHz to 0.01 Hz and an AC voltage amplitude of 5 mV in
ultrapure water. The spectra were recorded at the X band using a
Bruker A300 spectrometer with a 9.79 GHz magnetic field
modulation at a microwave power level of 5.05 mW. Photocatalytic
reactions were carried out in a multichannel photochemical reaction
system (PCX50A Discover, Beijing Perfectlight Co. Ltd, China), and a
bandpass filter was used to get visible light (420 nm ≤ λ ≤ 700 nm).
The transient photovoltage (TPV) responses were carried out by a
homemade measurement system. The samples were excited with a
laser radiation pulse (wavelength of 355 nm and pulse width of 5 ns)
from a third harmonic Nd: YAG laser (Polaris II, New Wave Research,
Inc.). The TPV signal was registered by a 5-GHz digital phosphor
oscilloscope (MDO3104, Tektronix).
2
.4 Synthesis of phosphorous doped porous carbon nitride (CPN)
The CP was further treated with hydrothermal method. The obtained
CP powder was added into 20 mL hydrazine hydrate at 110 ℃ for 2
h. The resultant suspension was filtered and cleaned several times
with deionized water. Finally, the collected solid was dried under
vacuum at 60℃ overnight, and the final sample was named as CPN.
2
.5 The experimental method of transient photovoltage (TPV)
The transient photovoltage measurements were conducted under
room temperature on films with area of samples (1 cm×4 cm)
deposited on indium-tin oxide (ITO) glass substrates. The films of the
samples were prepared by drop-coating 10 mg mL aqueous
dispersions of the samples on ITO substrates, then dried in air. The
ITO glass modified with samples as the working electrode and Pt wire
-1
as the counter electrode were be wetted with N
acetonitrile and N saturated acetonitrile/H O aqueous (v/v = 10:0.5).
2 2
or O saturated
2
2
In addition, the powders of different catalysts were measured and
covered the samples on the Pt network of 1 cm×1 cm. The samples
were excited with a laser radiation pulse (wavelength 355 nm, pulse
width 5 ns) from a third-harmonic Nd: YAG laser (Polaris II, New Wave
Research, Inc.). The photocurrent was obtained by the ratio of the
photovoltage to the resistance of the test system.
3
. Results and Discussion
The morphological features of CPN were characterized by using
electron scanning microscopy (SEM) and transmission electron
microscopy (TEM). As displayed in Figure 1a, porous structure can be
2
.6 Experiment of photocatalysis
1
0 mg catalyst was added to a 40 mL glass bottle, then 15 mL observed clearly in CPN. Furthermore, the energy dispersive
ultrapure water was added and sealed with a rubber stopper and spectrometer (EDS) elemental mapping images of CPN in Figure 1(b
2
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to e) confirm the existence of C, N and P and the elements are highly
Figure 2b displays Fourier Transform Infrared spectroscopy (FTIR)
View Article Online
DOI: 10.1039/C9TA13929J
dispersed on the surfaces of CPN. The EDS elemental mappings and spectra of C
XPS results are used to confirm the content of P element in as-
prepared catalyst in table S1 and S2, the content of P element can be cm are attributed to the stretching modes of CN heterocycles, while
3 4
N (black line), CP (red line) and CPN (blue line). For the
-1
3 4
C N (black line), the absorption bands between 1150 cm and 1700
-1
-1
-1
calculated to be about 4-6 %. The SEM images and element mappings the absorption peaks at 803 cm , 3156 and 3437 cm correspond to
of C and CP are given for comparison in Figure S1 and S2, which characteristic vibration mode of triazine rings and the stretching
show that both CP and C are lamellar structures. The TEM images vibration of N-H, respectively. The FTIR spectrum of CP (Figure 2b,
exhibit that CPN possesses porous structure in Figure 1f and 1g. red line) shows the similar characteristic peaks with that of C . For
However, no porous structure was observed for C and CP in Figure the CPN (Figure 2b, blue line), the stretching modes of CN
S1 and S2. The porous structure may be caused by the additional heterocycles’ peak strength decreased obviously. This can be
gases produced during the calcination of hydrazine hydrate. At the attributed to that phosphorus was doped into the C successfully
same time, the nitrogen adsorption-desorption isotherms and pore and bonded to the nitrogen atoms of CN heterocycles, thereby
3 4
N
3 4
N
3 4
N
3 4
N
3 4
N
3
5,36
size of CPN (black line) and CP (red line) are shown in Figure S3a and altering the structure of CN heterocycles.
S3b, reflecting the specific surface area of CPN is larger than that of
CP.
Fig. 2 (a) XRD patterns for C
spectra of C (black line), CP (red line) and CPN (blue line). (c) UV–Vis DRS spectra of
(black line), CP (red line) and CPN (blue line). (d) The band gaps of C (blue line)
and CPN (red line).
3 4
N (black line), CP (red line) and CPN (blue line). (b) FTIR
3
N
4
C
3
N
4
3 4
N
Fig. 1 (a) and (b) SEM images of CPN. (c-e) The corresponding EDS elemental mappings
of constituent elements for CPN. (f) and (g) TEM images of CPN.
As shown in Figure 2c, a typical absorption band at the 440-450 nm
region is observed for the CPN (blue line), indicating the doping of
phosphorus can tune the optical property of CPN. The CPN has higher
absorption capacity in the visible region. Figure S4 shows that the
The XRD pattern of the C
peaks at 13.8° and 27.9° in Figure 2a, which can assign to the
100) and (002) diffraction peaks of C , respectively. The (002)
diffraction peak of C
is a interplanar structural packing.32,33
The (100) diffraction peaks of C is interlayered space of
graphitic structure.34 However, the CP (red line) sample shows C N was used to determine the valence band energy in Figure S5a,
3 4
N (black line) displays two diffraction
band gaps of C
That also proves that the CPN has a narrower band gap than that of
the pure C , which can improve the utilization efficiency of solar
energy. And the ultraviolet photo-electron spectroscopy (UPS) of
3 4
N and CPN are 2.81 eV and 2.58 eV, respectively.
(
3 4
N
3 4
N
3 4
N
3 4
N
3
4
weak intensity at 27.9° compared to that of the C
be attributed to the lower polymerization of C
3 4
3 4
N , which may and the valence band of C N was determined to be 2.51 V, according
3
7
3 4
N after to (EC = Ev - Eg). While the conduction band value of C
3 4
N was
phosphorous doping. No peaks corresponding to phosphorus measured to -0.3 V. And the UPS was used to determine the valence
was found in the XRD pattern due to the low phosphorous band energy of CPN in Figure S5b, which was calculated to be 2.08 V.
So, the conduction band value of CPN was measured to -0.5 V. The
values of the XPS valence band spectra and UPS spectra are
consistent. Based on these data, the band structure diagram of CPN
content. With the further hydrothermal treatment with
hydrazine hydrate, it is found that the peak of (002) diffraction
peak decreased significantly and the peak of (100) diffraction
peak almost disappeared for CPN (blue line), contributing to the
production of porous structures. That also indicates that
(
red line) and C
3
N
4
(blue line) are proposed as shown in Figure 2d.
, which is helpful
CPN has a narrower band gap than that of pure C
3 4
N
for improving the light absorbance and the photocatalytic efficiency.
3 4
phosphorus was doped into C N successfully.
The full X-ray photoelectron spectroscopy (XPS) spectrum of CPN
offered in Figure 3a exhibits the peaks of elements C, N and P in CPN.
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The high-resolution XPS spectrum of C 1s shows three deconvoluted reaction rate due to the phosphorus can play the role of oxygen
View Article Online
DOI: 10.1039/C9TA13929J
peaks at 288.04 eV, 285.6 eV and 284.6 eV in Figure 3b. The major C adsorption. The oxygen adsorption curve of the catalyst is shown in
2
peak at 288.04 eV corresponds to the sp hybridized carbon (N- C=N), the Figure S8. Meanwhile, the porous structure is conducive to
while the peak at binding energy of 284.6 eV should assign to (C-C) provide more active sites for the photocatalytic process. The
3
8,39
in CPN.
As shown in Figure 3c, the N 1s peak at 398.7 eV is quantum efficiency of CPN catalyst was measured to 1.57 % under
2
assigned to the sp hybridized aromatic N bonded to carbon atoms wavelength λ= 420 nm in Figure S9. As shown in Figure 4b, the
(C=N - C) and the peak at 400.3 eV corresponds to the P-N species. catalyst shows good stability within six cycles for each cycle lasted 8
Compared to the C in Fig. S6, the peaks are assigned to quaternary h. The stability experiment was carried out through centrifuging the
3 4
N
4
0
N bonded to three carbon atoms in CPN are disappeared, which photocatalyst from the reaction solution, then washed it three times
should ascribe to the phosphorus replaces carbon and forms P-N with ultra-pure water. Finally, the solid powder was dried at 60 ℃
with N atom. As shown in Figure 3d, the XPS spectrum of P 2p in CPN overnight.
shows three peaks, assigning to P-N at 133.2 eV, P=N at 134 eV and
4
1–43
P=O at 135 eV.
The P−N bond may be attributed to P replace C
in the C−N frameworks. The existence of P=N can be attributed to
the phosphonitrilic chloride trimer is not completely broken, that
resulting in the presence of P=N. From the comparison of P 2p
spectra of CP in Figure S7d and CPN in Figure 3d, the content of P=N
decreases after the treatment of hydrazine hydrate, and more P-N
bonds are formed, indicating that more phosphorus was doped into
3 4
C N . Moreover, according the percentage of the content of P=O
bond in all samples, the content of P=O bond also decreased greatly
with hydrazine hydrate in table S3, indicating that hydrazine hydrate
as a good reducing agent can effectively inhibit the oxidation of
phosphorus. Therefore, hydrazine hydrate can effectively induce
more phosphorus doping into C N , changing the light absorption
3 4
performance of the catalyst and carrier recombination.
Fig. 4 (a) The evolution of H
catalysts for different time. (b) The cyclic stability of CPN as catalyst for H
c) The dependence of H concentration (wt %) on catalyst contents. (d) Dependence
of H concentration (wt %) on the different time at 80 mg CPN. (e) Comparison of H
evolutions with CPN as catalyst at different gas atmosphere conditions in 8 h. (f)
Comparison of H evolution of CPN as catalyst with/without different scavengers in 8
h (0.45 mL CH OH and 5 mg AgNO ). (a-b) and (e-f) photocatalytic condition: 10 mg
catalyst in 15 mL pure water under visible light irradiation (420 nm ≤ λ ≤ 700 nm) and
2
O
2
with CPN (black line), CP (red line) and C
3
N
4
(blue line) as
2
2
O evolution.
(
2 2
O
2
O
2
2 2
O
2
O
2
3
3
normal pressure. (c-d) photocatalytic condition: in 15 mL oxygen saturated H
2
O under
visible light irradiation for 36 h.
Fig. 3 (a) XPS survey spectrum of CPN. High-resolution XPS spectra of CPN for (b) C 1s, (c)
N 1s, and (d) P 2p.
Generally, the EIS spectrum with a smaller semicircle arc means a
lower electron-transfer value, as shown in Figure S10, CPN has the
smallest semicircle diameter, indicating that the photogenerated
electrons have less resistance to transport, so that the electrons and
2 2
Figure 4a shows the time-dependent change in the amounts of H O
formed on the different catalysts. The production rate of CPN (black holes can be separated well. Figure S11 describes the current-time
line) was calculated to 1968 μmol/g/h (highest yield in current curve of CPN, which can be seen the photocurrent declines little,
records without sacrificial agent), CP (red line) was 547 μmol/g/h and indicating that H
(blue line) was 68 μmol/g/h after 8 h reaction, respectively. The catalyst. The FTIR spectra and XRD patterns of CPN for before (black
CPN exhibits highest photocatalytic activity, and the rate of H
line) and after (red line) photocatalytic reaction presented in Figure
production is 28.9 times higher than that of pure C
. The doping of S12, where the similar peaks can also indicate the good stability of
phosphorus not only can improve the light absorption of the catalyst, CPN. Figure 4c shows the dependence of H concentration (wt %)
but also can provide oxygen for the reaction to accelerate the on catalyst contents. Typically, different amounts of catalysts (20, 40,
2 2
O on the surface of the catalyst will not poison the
3 4
C N
2 2
O
3 4
N
2 2
O
4
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6
0, 80 and 100 mg) were dispersed into 15 mL water with saturated consume electrons, and then the reaction rate of water oxidation to
View Article Online
DOI: 10.1039/C9TA13929J
oxygen under visible light (420 nm≤ λ ≤700 nm) for 36 h. The CPN can
produce H with concentrations of 0.171 % (The industrial method
usually generates H mixtures with concentrations of 1 to 2 weight
(wt %)). The evolution of H decreased with the addition of 100
2 2
H O was estimated to be 746 μmol/g/h.
2 2
O
2 2
In order to demonstrate the pathway of H O production, the
DMPO spin-trapping EPR technique was employed to measure the
2 2
O
%
2 2
O
production of radical in photocatalytic process. As shown in EPR, the
mg catalyst, which can be ascribed to the excessive catalysts
affecting the light absorption of the reaction system. The
dependence of H
was exhibited in Figure 4d, that reveals the evolution of H
catalyzed by CPN increased steadily within 36 h. With the extension
-
characteristic peaks of •O
2
and •OH can be observed in CPN systems.
-
It is obvious the •O
2
signals can be observed in the CPN from Figure
2 2
O concentration (wt.%) on time with 80 mg CPN
5
O
a, indicating the photogenerated electrons can be used to reduce
2 2
O
-
-
2
to •O
2
, and the •O
2
2 2
can be further reduced to H O .
of illuminated time, the yield of H reaches a plateau due to the
2
O
2
balance between the amount of H O degraded and produced.
2 2
Fig. 6 (a) RRDE i-t curves of CPN under dark and light in N
mol L ) with light irradiation of λ>420 nm (scan rate: 50 mV s ). (b) Proposed mechanism
2
-saturated NaSO
4
solution (0.1
-1
-1
2 2
of H O evolution with CPN as photocatalyst system.
Moreover, •OH also can be observed in the CPN from Figure 5b,
confirming that generated holes from CPN could oxidize H O to
produce •OH, and the •OH can transform to H O through
2
Fig. 5 (a) DMPO spin-trapping EPR spectra recorded for •O -
nm irradiation). (b) DMPO spin-trapping EPR spectra recorded for •OH in the CPN under combination with each other. The •O
in the CPN. (under λ > 420
2 2
2
-
2
and •OH, as intermediate
3 4
anaerobic condition. (c) Transient photovoltage curves of C N (black line), CP (red line)
products in the process of hydrogen peroxide generation, can be
detected in EPR under visible light. Therefore, CPN as catalyst can
achieve the production of hydrogen peroxide via photocatalytic
water splitting with a two-channel pathway (water oxidation
reaction and oxygen reduction reaction). As shown in Figure 5a and
and CPN (blue line). (d) Transient photovoltage curves of CPN in different reaction
conditions (oxygen saturated MeCN (black line), nitrogen saturated MeCN (red line) and
nitrogen saturated MeCN with H O (blue line)).
2
5
b, no signals were detected in dark condition, indicating that the
production of radicals is related to light. To better understand the
catalytic mechanism of CPN with H production via two-channel
Figure 4e shows the amount of H
gas atmospheres. The 10 mg CPN was added into 15 mL ultra-pure
water to both systems, then N and O were introduced to both
systems until saturated. The amount of H decreases under N
saturation, indicating that the catalyst can also oxidize water after
the exclusion of O and generate H . The amount of H increases
under oxygen-saturated H O, which also indicates that the
generation of H is related to the amount of O . The oxygen
reduction reaction will consume electrons, and water oxidation to
produce H will consume holes. The co-existence of the two
processes will inhibit the combination of electron and hole, and it can
be considered that the two processes can be mutually sacrificial
2 2
agents. That will effectively increase the production of H O without
sacrificial agents. As shown in Figure 4f, 3 vol % methanol (0.45 mL)
was added in the experimental process as hole sacrifice agent, in
order to detect the rate of oxygen reduction to H O , enough oxygen
2 2
2 2
O produced by CPN in different
2 2
O
2
2
pathway, the in situ TPV (transient photovoltage measurements) test
was employed to probe the charges distribution of samples and the
rate of two-channel pathway (water oxidation reaction and oxygen
reduction reaction). As shown in Figure 5c, CPN (blue line) shows the
2
O
2
2
2
2
O
2
2 2
O
2
3 4
highest photocurrent intensity among C N (black line), CP (red line)
2
O
2
2
and CPN (blue line) powders, indicating that CPN produces the
maximum surface charges. It also suggested that CPN may realize the
rapid separation of photo-induced electrons and holes and should
has a very high photocatalytic activity. In addition, we further studied
the changes of photocurrent intensity of CPN under three different
reaction conditions, shown in Figure 5d. The photocurrent intensity
exhibits a decline in 5% H
when compared with that in N
the vast majority of holes on CPN just oxidize H
visible light irradiation. While, the photocurrent intensity of CPN
shows a significant decline in O -saturated MeCN (black line)
compared with that in N -saturated MeCN (red line). It indicates that,
2 2
O
2
O filled with N
-saturated MeCN (red line), indicating
O molecules under
2
-saturated MeCN (blue line)
2
2
was injected to the experiment and ensure enough reactant can
react with photogenerated electrons. Finally, the rate of oxygen
reduction was determined to 2456 μmol/g/h. At the same time, 5 mg
2
2
3
AgNO was added in the system as electron sacrificial agent to
in present reaction system, the oxygen reduction rate is higher than
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ARTICLE
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4
K. Mase, M. Yoneda, Y. Yamada and S. FuVkieuwzAurtmiclie,ONnlianet
that of water oxidation process, and then water oxidation process is
the rate-determining step. As shown in figure 6a, the electron
transfer number of CPN is about 2 via the rotating ring disk electrode
Commun, 2016, 7, 11470.
DOI: 10.1039/C9TA13929J
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(
RRDE) collection experiment, that can prove this photocatalytic
process is the two-electron oxidation path forming H from H O,
instead of the concerted four-electron process for O evolution.
Based on all above results, we proposed a mechanism for the
efficient two-channel production of H , shown in Figure 6b. Briefly,
1
3454–13459.
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CPN surface, oxygen was reduced by the photogenerated electrons
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2 2 2
O , at the same time, the oxidation of H O by the
7
80.
photogenerated holes also produce H
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O .
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. Conclusions
In summary, a porous metal-free photocatalyst was prepared with
doping of phosphorus into C by simple calcination and
subsequent hydrothermal treatment. This catalyst shows a very high
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3 4
N
2
O
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achieves a quantum efficiency of 1.57 % under wavelength λ= 420
nm and the solar energy conversion efficiency of 0.43 %. In 80 mg
1
1
2 2 2
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There are no conflicts to declare
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Acknowledgements
This work is supported by National MCF Energy R&D Program
(2018YFE0306105), the National Natural Science Foundation of
China (51725204, 21771132, 21471106, 51972216), the Natural
Science Foundation of Jiangsu Province (BK20190041, BK20190828),
Key-Area Research and Development Program of GuangDong
Province (2019B010933001), Collaborative Innovation Center of
Suzhou Nano Science & Technology, the Priority Academic Program
Development of Jiangsu Higher Education Institutions (PAPD), and
the 111 Project.
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