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C: Surfaces, Interfaces, Porous Materials, and Catalysis
Insight Into the Enhanced Hydrogen Evolution Activity of 2,4Diaminopyrimidine-Doped Graphitic Carbon Nitride Photocatalysts Zehao Li, Siyu Zhou, Qian Yang, Zhengguo Zhang, and Xiaoming Fang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10252 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 11, 2019
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The Journal of Physical Chemistry
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Insight
into
the
Enhanced
Hydrogen
Evolution
Activity
of
2
2,4-Diaminopyrimidine-doped Graphitic Carbon Nitride Photocatalysts
3
Zehao Lia, Siyu Zhoua, Qian Yanga, Zhengguo Zhanga,b and Xiaoming Fanga,b,c*
4
a
5
of Education, School of Chemistry and Chemical Engineering, South China
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University of Technology, Guangzhou 510640, China.
7
b
8
Application, South China University of Technology, Guangzhou 510640, China.
9
c
Key Laboratory of Enhanced Heat Transfer and Energy Conservation, The Ministry
Guangdong Engineering Technology Research Center of Efficient Heat Storage and
Key Lab Fuel Cell Technology Guangdong Province, School of Chemistry and
10
Chemical Engineering, South China University of Technology, Guangzhou 510640,
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China.
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Abstract
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Molecular doping has been proven an effective way for graphite carbon nitride
25
(GCN) to extend its light harvesting and improve its charge separation and transport,
26
while little attention has been paid on its effect on the charge transfer at the interfaces
27
between
28
2,4-diaminopyrimidine (DAP), was first used to combine with urea for preparing
29
doped GCN. It is found that the optimal doped GCN sample, CN-DAP36, has a
30
narrowed band gap, reduced photoluminescent emission, and longer carrier lifetime,
31
as compared with the undoped GCN. The hydrogen evolution rate of the doped GCN
32
is found to be 2.80 mmol/(h×g), 6.09 times that of the undoped GCN (0.46
33
mmol/(h×g)) under visible light irradiation. Furthermore, according to the theoretical
34
calculations, the pyrimidine groups in DAP possess a stronger adsorption capacity for
35
the Pt particles than the tri-s-triazine of GCN does, thus leading to more Pt particles
36
deposited near the pyrimidine rings. The extending in optical absorption, the
37
reduction in charge recombination and the enhancement in charge transport, along
38
with the facilitation in the interfacial charge transfer from the doped GCN sample to
39
Pt, contribute to the enhanced photocatalytic performance of the doped GCN.
GCN
and
cocatalyst.
Herein,
a
pyrimidine-based
40 41 42 43 2
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molecule,
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1. Introduction
45
Photocatalysis is a potential solution for effectively alleviating the global energy
46
crisis and environmental pollution. Developing efficient, robust and low-cost
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photocatalysts is essential to this technology. Among the numerous photocatalysts
48
that exist, graphitic carbon nitride (GCN) is a promising non-toxic, environmentally
49
friendly, low-cost catalyst with good physicochemical stability and visible-light
50
response 1. However, some drawbacks lead to GCN exhibit a moderate photocatalytic
51
activity, which include narrow visible light response region, high recombination of
52
the photo-induced electron-hole pairs, and slow charge transport 2. Consequently, it is
53
of great significance to develop high-performance GCN based photocatalysts and
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elucidate their photocatalytic mechanisms.
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Recently, various strategies have been explored to improve the photocatalytic
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activity of GCN. These strategies can be divided into four categories: 1) combining
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with carbonaceous nanomaterials, such as graphene 3-4, carbon nanodots 5, and carbon
58
nanotubes 6; 2) constructing specific nanostructures
59
or homojunction
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17-20
61
way that can adjust its structures and properties by introducing organic molecules into
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its framework 1, 27. By now, besides some non-aromatic compounds 28-30, the organics
63
employed to dope GCN are usually benzene-based molecules 24, 31-32, thiophene-based
64
molecules
65
have strong electron attracting and capturing abilities owing to the high
13-15
and Z-Scheme
or molecular doping
23, 33-34,
21-26.
16
7-12;
3) preparing heterojunction
photocatalysts based on GCN; 4) elemental
For GCN, molecular doping is a unique and effective
and pyridine-based molecules
21, 25-26, 35.
3
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Since pyrimidine rings
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electronegativity of aromatic C=C bonds, pyrimidine-based molecules have been
67
introduced into the framework of GCN.21, 36-38 The obtained doped GCN samples have
68
been revealed to possess expanded visible light response region and altered electron
69
delocalization and arrangement and thus exhibit enhanced photocatalytic activity,
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suggesting that pyrimidine-based molecules are excellent dopants for GCN. However,
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it should be noticed that all the researches on the molecular doping of GCP have been
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found to play a positive role in improving light harvesting as well as charge separation
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and transport of GCN. While, no work has been done on its effect on the charge
74
transfer at the interface between GCN and cocatalyst through both experiment and
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theoretical calculation, which is really a critical step for realizing the surface reactions
76
to obtain products.
77
In this work, 2,4-diaminopyrimidine (DAP) was first employed to copolymerize
78
with urea for preparing doped GCN. Specifically, a series of doped GCN samples
79
were prepared by mixing different amounts of DAP with urea, followed by the
80
copolymerization. The structures, optical and photoelectrochemical properties, and
81
photocatalytic performance of the obtained samples were characterized and evaluated,
82
and the suitable amount of DAP was thus determined. More importantly, in order to
83
elucidate the photocatalytic mechanism of the optimal doped GCN sample, besides
84
the investigations on the effect of the DAP doping on its light harvesting and charge
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recombination and transport, the interfacial electron transfer from the doped GCN
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sample to the Pt cocatalyst was theoretically and experimentally studied. It is found
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that the pyrimidine groups in DAP possess a stronger adsorption capacity for the Pt 4
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particles than the tri-s-triazine of GCN does, and more Pt particles can be thus
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deposited near the pyrimidine rings, thereby facilitating the interfacial charge transfer
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between the doped GCN sample with Pt. It is revealed that the molecular doping not
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only make the improvements on light harvesting as well as charge separation and
92
transport of GCN but also accelerate the charge transfer from GCN to cocatalyst.
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2. Experimental section
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2.1. Preparation of DAP doped GCN. 10 g of urea was mixed with different
95
amounts of DAP (32, 36, 40, or 44 mg), followed by dissolving into 40 mL of
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deionized water under stirring at 30 oC for 12 h. The obtained solution was then kept
97
in a refrigerator at -20 oC. Finally, the freeze-dried mixture was thermally treated at
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600 oC with a ramp rate of 10 oC/min for 2 h in air to produce GCN. The obtained
99
GCN samples were named as CN-DAPx (x represents the initial amount of DAP). In
100
addition, a pristine GCN sample was prepared by thermally treating 10 g of urea at
101
600 oC with a ramp rate of 10 oC/min for 2 h in air, and the obtained sample was
102
denoted as CN.
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2.2. Characterization. The crystal structures of the CN-DAP samples were
104
obtained through X-ray diffraction (XRD) patterns (Bruker D8 advance
105
diffractometer with Cu Kα1 radiation). The chemical structures and bonding
106
information were analyzed using Fourier transform infrared spectroscopy (FT-IR,
107
Bruker Vector 33 Fourier transform infrared spectrophotometer) and X-ray
108
photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD X-ray photoelectron
109
spectroscopy), respectively. The structures of the samples were determined using 5
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13C
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solid-state
NMR spectroscopy (Bruker Advance III 500 spectrometer).
111
Thermogravimetric analysis (TGA) was conducted on a Netzsch STA449 F3 from
112
room temperature to 800 °C at a ramp rate of 10 °C/min under an N2 atmosphere. N2
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adsorption–desorption isotherms and the pore size distribution plots of the obtained
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samples were characterized using the Brunauer–Emmett–Teller (BET) method on a
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Micrometrics ASAP 2020 apparatus. The morphologies and microstructures of the
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samples were observed by using a scanning electron microscope (SEM, Hitachi
117
SU8220) and a transmission electron microscopes (TEM, JEOL JEM-2100F, and
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JEM-1400 plus). The optical absorption properties of the samples were investigated
119
using UV-vis diffuse reflectance spectroscopy (DRS, Shimadzu UV-3600 UV-vis
120
spectrophotometer). The charge recombination of the samples was estimated using
121
photoluminescence (PL) spectroscopy (Hitachi F-4600 FL Spectrophotometer, under
122
an excitation at 370 nm). The lifetimes of the carriers were determined at room
123
temperature using time-resolved PL using an Edinburgh PLS980 spectrometer.
124
Solid-state electron paramagnetic resonance (EPR) spectroscopy was performed using
125
a Bruker model A300 spectrometer at room temperature with dark and visible light.
126
The real surface areas of CN and CN-DAP36 electrodes were observed by using
127
Atomic force microscopy (AFM, Bruker Multimode 8).
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2.3. Photoelectrochemical measurement. The electrochemical properties of the
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samples were tested using a Chenhua CHI660E electrochemical workstation via a
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conventional three-electrode system. A Pt sheet was used as the counter electrode (10
131
× 10 mm), and a Hg/ Hg2Cl2 electrode was employed as the reference electrode. The 6
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working electrode was prepared as follows. FTO glass (10 × 10 mm2) was washed by
133
sonication in acetone and ethanol for 30 min, followed by the blown dry with
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nitrogen. A slurry was obtained by dispersing 0.1 g of the photocatalyst in an ethanol
135
solution containing 0.01 g of ethylene cellulose, followed by grinding thoroughly. The
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as-prepared slurry was coated on one piece of the clean FTO glass sheet, followed by
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heating at 150 °C for 2 h. During the test, the working electrode was immersed in 70
138
mL of a Na2SO4 aqueous solution (0.5 M), and a 300 W Xe lamp with a UV-cutoff
139
filter (λ > 420 nm) was the visible light irradiation source. The periodic ON/OFF
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photocurrent response with a 1 cm2 area of the working electrode were carried out.
141
Electrochemical impedance spectroscopy (EIS) was measured by applying 20 mV
142
alternative signal over the frequency ranged from 0.01 Hz to 100 kHz in the dark.
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2.4. Evaluation of photocatalytic activity. The photocatalytic H2 evolution
144
activities of the as-prepared GCN samples were screened using a multipass light
145
catalytic reaction system (Perfectlight PCX50B Discover, Beijing). Specifically,
146
every sample (0.03 g) was well dispersed into 40 mL of an aqueous triethanolamine
147
(TEOA, 10 vol%) containing 100 μL of H2PtCl6 (Pt, 0.2% wt, as a cocatalyst). Nine
148
white LED light sources (5 W, 7.70 mW/cm2) were used for irradiation and to ensure
149
that every reaction bottle received the same reaction conditions. Before irradiation,
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every reaction bottle was sealed to form a closed system, and the bottles were purged
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and back-filled with high purity Ar three times to replace the air atmosphere. The
152
obtained gases were detected using gas chromatography coupled to a thermal
153
conductivity detector (GC7600, Tian Mei), and high purity Ar was used as the carrier 7
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gas.
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The photocatalytic hydrogen evolution rate (HER) of the doped GCN sample
156
with the best photocatalytic activity was measured and compared with that of CN
157
using on-line hydrogen production determination under visible light irradiation. A
158
sample of the material (0.03 g) was well dispersed into 100 mL of aqueous TEOA (10
159
vol%) containing 100 μL of H2PtCl6 (Pt, 0.2% wt, as a cocatalyst). Before the reaction
160
was initiated, the system was placed under vacuum to evacuate the residual air from
161
the reactor and suspension. To maintain the reaction system at 278 K, cooling water
162
was circulated through the system. Photocatalytic H2 generation was triggered by
163
irradiation with a 300 W Xe lamp (PerfectLight, PLS-SXE300C, λ > 420 nm).
164
The apparent quantum efficiency (AQE) for hydrogen revolution was evaluated
165
by using the same closed circulating system under the illumination by various
166
irradiation wavelengths: λ = 380 ± 15, 420 ± 15, 465 ± 15, 510 ± 15, 550 ± 15, and
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600 ± 15 nm. The amount of evolved hydrogen was collected after the light
168
irradiation lasted for one hour. AQE under different wavelengths was calculated by
169
the following equation:
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AQE =
2 × 𝑡ℎ𝑒 𝑒𝑣𝑜𝑙𝑣𝑒𝑑 𝐻2 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒𝑠 𝑛𝑢𝑚𝑏𝑒𝑟 𝑡ℎ𝑒 𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡 𝑝ℎ𝑜𝑡𝑜𝑠 𝑛𝑢𝑚𝑏𝑒𝑟
× 100%
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2.5. DFT calculations. The HOMO and LUMO energy levels of CN and
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CN-DAP were evaluated using the Gaussian 09 program through DFT calculations at
173
the B3LYP/ 6-31G (d, p) level of theory
174
for the calculation model. For the adsorption energies between Pt and different parts
175
of CN-DAP, the Lanl2dz pseudopotential basis set was used.
39.
The minimum trimer unit cell was used
8
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3. Results and discussion
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3.1. Structure and morphology. The XRD patterns of the samples are shown in
178
Figure 1(a). All the obtained samples exhibit the same diffraction peaks near 12.84°
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(100) and 27.64° (002), which correspond to the periodic array of the interlayer
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tri-s-triazine motif stacking of 0.68 nm and the interlayer aromatic packing structure
181
of 0.33 nm, respectively 40. These results suggest that the crystalline structure of GCN
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has not been changed by the addition of DAP. Furthermore, CN and CNx show
183
similar FT-IR spectra, as shown in Figure 1(b). Specifically, the broadband in the
184
region from 2800 to 3400 cm−1 corresponds to the N–H stretching and O–H stretching
185
vibrations from the uncondensed amine groups on the surface. The peaks located at
186
1100–1800 cm−1 are mainly ascribed to the vibration of C–N heterocycle skeleton
187
(C6N7 ring)
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triazine ring
189
Consequently, FT-IR spectroscopy is not able to confirm that DAP has been
190
successfully incorporated into GCN. In addition, the BET surface areas of CN and
191
CN-DAP have been measured by N2 adsorption–desorption isotherms. As shown in
192
Figure S1, all samples exhibit the Type IV adsorption curves 40. As listed in Table 1,
193
the surface areas of CN and CN-DAP are similar and comparable. From the typical
194
SEM and TEM images (Figure S2) of CN and CN-DAP, it can be seen that numerous
195
distorted nanosheets with coarse surfaces are clearly observed.
41. 42.
The peak at 812 cm−1 is ascribed to the stretching vibration of the The main characteristic peaks of DAP and GCN are overlapped.
9
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196 197
Figure 1. XRD patterns (a) and FT-IR spectra (b) of the as-prepared samples.
198
To prove that DAP has been successfully incorporated into the GCN framework
199
in CN-DAP36, XPS and solid-state 13C NMR spectroscopy have been employed. The
200
survey spectra indicate C, N, and O existing in all the samples, as displayed in Figure
201
2(a). Both CN and CN-DAP36 exhibited similar, typical, characteristic peaks in their
202
high-resolution XPS C 1s and N 1s spectra (Figure 2, b and c). Namely, the peaks at
203
284.9 and 288.2 eV correspond to C–C and C–N bonds in the high-resolution XPS C
204
1s spectra, respectively
205
CN-DAP36 present three characteristic peaks. The three peaks at 399.4, 398.6, and
206
400.8 eV could be attributed to the bridging N atoms in N–(C)3, the sp2-bonded N
207
atoms in the triazine rings (C–N=C), and C–N–H, respectively 44. The percentages in
208
the area of these peaks are listed in Table S1. For the C 1s spectra, an obvious
209
decrease in the C–C peak ratio is found for CN-DAP in comparison with that of CN.
210
The reduction in C–C peak may suggest the incorporation of pyrimidine in the
211
conjugated network of GCN. The increase of C–N=C peaks for CN-DAP may lead to
212
the same proposition. The C/N molar ratios of the CN and DAP-doped GCN samples
213
obtained from XPS are also listed in Table 1. A gradual increase in the C/N molar
43.
The typical high-resolution XPS N 1s spectra of CN and
10
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ratio from 0.70 for CN to 0.73 for CN-DAP44 reveals the successful integration of the
215
pyrimidine ring into the GCN network. From the solid-state
216
shown in Figure 2(d), a new peak centered at 93.8 ppm is clearly observed for
217
CN-DAP36, while a sharp peak at the same position is found for DAP. These results
218
suggest the successful incorporation of a pyrimidine ring into the GCN conjugated
219
network of CN-DAP36. Furthermore, in comparison with CN, CN-DAP exhibits
220
changes in the intensities of the two peaks at 157.2 and 165.4 ppm. Note that the
221
former peak corresponds to the C–(N)3 group, and the latter one is ascribed to the
222
NH2–C(N)2 group
223
incorporated into the GCN framework of CN-DAP36.
45.
13C
NMR spectra, as
It is revealed that the pyrimidine moiety has been successfully
224 225
Figure 2. Survey (a), C 1s (b) and N 1s (c) XPS spectra of CN and CN-DAP36,
226
together with (d) solid-state 13C NMR spectra of CN, CN-DAP36, and DAP. 11
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Furthermore, the TGA curves of urea, DAP, the precursor, CN, and CN-DAP36
228
have been recorded for elucidating the formation mechanism of the doped GCN. As
229
shown in Figure S3(a), for CN and CN-DAP36, no weight losses occur before 600 °C,
230
suggesting that they are polymers; in the temperature range between 600 °C and
231
750 °C, GCN rapidly loses almost 100% of its mass because it has sublimed. For the
232
raw materials, DAP exhibits a single weight-loss step, and its onset decomposition
233
temperature is approximately 200 °C, higher than that of urea. More significantly,
234
some differences are observed between the TGA curves of urea and the precursor.
235
First, the first weight-loss step of urea occurs between 150 and 230 °C, and its ratio is
236
67.2%; by contrast, the temperatures of the first weight-loss step for the precursor
237
range from 150 to 243 °C, and the corresponding ratio is 64.8%. Second, the curve of
238
urea exhibits an inflection point at 230 °C, which is absent in the curve of the
239
precursor. These results imply that the addition of DAP changes the reaction paths.
240
Third, the transformation temperature of urea is 350 °C, whereas that of the precursor
241
is delayed to 366 °C. This result means that, with the addition of DAP into melem, a
242
higher temperature is needed for generating GCN. Urea completed its weight loss at
243
approximately 420 °C, whereas the weight loss of the precursor was not complete
244
because of the incorporation of DAP. In addition, for CN-DAP40 and CN-DAP44, the
245
percentages of the residuals are 3.59% and 7.72% above 750 °C (Figure S3b), which
246
might be that the excess DAP form carbon deposits during high-temperature
247
calcination.
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Table 1. Physicochemical properties of the as-prepared samples. Sample
C/N mole ratio
SBET (m2/g)
Pore volume (cm3/g)
Band-gap (eV)
CN
0.70
116.1755
0.2187
2.98
CN-DAP32
0.71
125.9836
0.2141
2.90
CN-DAP36
0.72
113.8914
0.1884
2.87
CN-DAP40
0.72
112.7546
0.1992
2.86
CN-DAP44
0.73
122.6019
0.2099
2.78
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Based on the above results, the formation mechanism of CN-DAP from urea and
251
DAP can be speculated 46. As illustrated in Scheme 1, first, DAP reacts with urea to
252
generate an intermediate. Then, the intermediate further reacts with urea to produce a
253
heterocyclic compound. During the thermal pyrolysis, the carbonyl groups in the
254
heterocyclic compound are aminated to generate DAP-doped GCN.
255 256
Scheme 1. Schematic diagram for the formation mechanism of CN-DAP from urea
257
and DAP. 13
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3.2. Optical properties. The UV-vis DRS spectra of the samples are shown in
259
Figure 3(a). In comparison with CN, the CN-DAP samples present a gradual red shift
260
in their intrinsic absorption edges with the increase in the addition amount of DAP,
261
corresponding to the change in color from light yellow to yellow (the insert in Figure
262
3a). This redshift leads to the enhancement in optical absorption in the visible light
263
region. Accordingly, the band gap of the CN-DAP samples decreases with increasing
264
amounts of DAP, as displayed in Figure 3(b) and Table 1. The band gap values of CN
265
and CN-DAP36 have been estimated to be 2.98 and 2.87 eV, respectively. The
266
narrowed band gap implies that the CN-DAP samples exhibit enhanced visible light
267
absorption as compared with CN.
268 269
Figure 3. UV–vis diffuse reflectance spectra (a), plots of (αhν)1/2 versus hν (b), PL
270
emission spectra (the insert is the time-resolved fluorescence spectra of CN and 14
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CN-DAP36) (c), and Room-temperature solid-state EPR spectra of electron detected in
272
the dark and light at atmospheric conditions (d) of the samples.
273
To investigate the charge recombination behavior in the samples, their PL
274
spectra have been measured. As shown in Figure 3(c), compared with CN, the
275
CN-DAP samples exhibit obviously reduced PL emission. Specifically, as the amount
276
of DAP is increased from 32 mg to 36 mg, a reduction in PL intensity occurs. While,
277
the further in the DAP amount increase from 36 mg to 40 mg, the PL emission
278
increases. Consequently, among all the samples, CN-DAP36 has the lowest PL
279
intensity. The increase in PL emission at a higher loading of DAP may be attributed to
280
the excess DAP form carbon deposits during high-temperature calcination (Figure
281
S3b), and the carbon deposits might be the recombination sites for the photo-induced
282
charge carriers. It is revealed that the suitable addition amount of DAP should be
283
around 36 mg, at which the obtained sample shows the lowest charge recombination.
284
Furthermore, the lifetimes of the charge carriers for CN and CN-DAP36 have been
285
measured
286
double-exponential function (equation (1)) has been employed to fit the decay curves
287
47.
288
of charge carriers in CN-DAP36 is 2.85 ns with the percentage of 49.42% compared to
289
2.09 ns and 49.54% in CN. The long lifetime increase from 6.82 ns with 50.46% for
290
CN to 9.79 ns with 50.58% for CN-DAP36. It shows that the long lifetime has a great
291
influence on the carriers. Based on the weighted mean lifetime calculated by equation
292
(2) it is found that the lifetime of the photo-induced carriers for CN-DAP36 is 6.36 ns,
by
time-resolved
PL
spectroscopy
(inset
in
Figure
3c).
A
The τ1 and τ2 are short lifetime and long lifetime, respectively. The short lifetime
15
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293
longer than that of CN (4.48 ns)
294
exhibits the reduced recombination of radiative electron-hole pairs, compared to CN.
295
I(t) = 𝑓1𝑒
296
τ = (𝑓1𝜏1 + 𝑓2𝜏2).
( ―𝑡│𝜏1)
+ 𝑓2𝑒
( ―𝑡│𝜏2)
The longer lifetime suggests that CN-DAP36
(1)
+𝐼(0),
(𝑓1𝜏21 + 𝑓2𝜏22)
(2)
297
Furthermore, to explore the long pair electronic structure and charge transfer
298
properties, solid-state EPR spectra of the obtained samples have been acquired. As
299
shown in Figure 3(d), the central symmetry of the EPR peaks for all of the samples is
300
located near 3502 G (g = 2.02), which is due to the delocalized unpaired electrons of
301
the sp2 carbon atoms within the tri-s-triazine rings. The EPR signals of all the
302
CN-DAP samples are significantly stronger than that of CN, indicating that the doped
303
GCN samples have a much higher concentration of unpaired electrons. This may be
304
due to the rearrangement of π-electrons after the doping with a pyrimidine. Moreover,
305
the EPR signal of CN-DAP36 is stronger than that of the other CN-DAP samples. It is
306
revealed that CN-DAP36 has the best electronic band structure, which facilitates
307
promoting charge carriers and increasing charge carrier mobility. After visible light
308
illumination, the EPR signal intensities from all the obtained samples are increased,
309
indicating the sensitive visible light response for photo-generation of carriers.
310
CN-DAP36 also shows the highest EPR signal intensity under illumination, revealing
311
its efficient charge separation and the highest concentration of unpaired charge
312
carriers.
313
3.3. Photocatalytic activity for hydrogen evolution. Figure 4(a) shows the
314
photocatalytic performance of all the obtained samples under the 5 W white LED 16
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315
light irradiation. After the light irradiation for 5 h, the samples exhibit different
316
hydrogen evolution rates (HER). Specifically, HER increases with the DAP amount
317
ranging from 32 mg to 36 mg, but decreases with the further increase to 40 mg and 44
318
mg. Consequently, CN-DAP36 exhibits the highest hydrogen evolution, which is
319
781.54 µmol/(0.03 g catalyst), 4.22 times that of CN (185.23 µmol/(0.03 g catalyst)).
320
Although CN-DAP36 does not exhibit the highest optical absorption, it shows the
321
lowest PL emission and the strongest EPR signal among all the CN-DAP samples. It
322
is revealed that, compared with the optical absorption, the charge separation and
323
transport plays a more important role in determining the photocatalytic performance
324
of the doped GCN samples. The doping with DAP at a suitable dosage has the
325
functions of reducing the charge recombination and increasing the concentration of
326
unpaired electrons. Moreover, the HER values of CN and CN-DAP36 have also been
327
evaluated using on-line hydrogen production determination under visible light
328
irradiation (λ > 420 nm). In Figure 4(b), the HER of CN-DAP36 is 2.80 mmol/(h×g),
329
which is 6.09 times that of CN (0.46 mmol/(h×g)). The increased fold of CN-DAP36
330
to CN for multipass light catalytic reaction system is different from that of on-line
331
hydrogen production determination. The reasons for this difference are that the
332
multipass light catalytic reaction system is LED white light (5 W, 7.70 mW/cm2),
333
while the on-line hydrogen production determination is visible light irradiation
334
(λ>420 nm), and in the visible region, the optical absorption of CN-DAP36 is stronger
335
than that of CN.
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336 337
Figure 4. (a) HER of the GCN samples under LED white light irradiation for 5 h. (b)
338
HER of CN and CN-DAP36 under visible light irradiation (λ>420 nm). (c)
339
Wavelength-dependent of apparent quantum efficiency of CN-DAP36. (d)
340
Wavelength-dependent photocurrent response plots of CN-DAP36.
341
To elucidate the relations between optical absorption and hydrogen evolution, a
342
comparison between the DRS spectra and wavelength dependence of the AQE for
343
CN-DAP36 has been carried out. As plotted in Figure 4(c), the AQE and the optical
344
absorption region are found to match well. The AQE at λ = 380 ± 15, 420 ± 15, and
345
465 ± 15 nm is 11.82%, 4.88%, and 3.88%, respectively. The AQE gradually
346
decreases with the increase in the light wavelength, revealing that the absorption for
347
photons is the primary force to induce the photocatalytic reaction for hydrogen
348
evolution. Interestingly, CN-DAP36 exhibits a low photocatalytic activity at λ = 510 ±
349
15 and 560 ± 15 nm, suggesting its extended optical response range. Furthermore, the 18
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350
photoresponse range of CN-DAP36 has been further analyzed by testing the
351
photocurrent response plots at different wavelengths with band-pass filters. As shown
352
in Figure 4d, a relatively small photocurrent response corresponding to the ON/OFF
353
illumination cycles is detected at λ = 600 ± 15 nm for the CN-DAP36 electrode. This
354
indicates that the photoresponsive range of CN-DAP36 for photocatalytic hydrogen
355
evolution has been extended to 600 nm. In addition, to evaluate the photocatalytic
356
stability of CN-DAP36, the time course of the hydrogen evolution reaction has been
357
monitored (Figure S4). The HER of CN-DAP36 is slightly reduced but still maintains
358
at a high rate after 4 cycles. The result suggests the good photocatalytic stability of the
359
material. As shown in Figure S5, S6, S7 and S8, the XRD pattern, FT-IR spectrum,
360
XPS and solid-state
361
before and after being subjected to the photocatalytic reaction. It is observed from the
362
HRTEM image (Figure S9b) that some Pt particles can be clearly observed to be well
363
dispersed onto the surface of the photocatalyst.
13C
NMR spectra of CN-DAP36 did not change significantly
364
3.4 Photocatalytic mechanism. In order to figure out why CN-DAP36 exhibits a
365
much enhanced hydrogen evolution rate than CN, the relative positions for the
366
valence and conduction bands (VB and CB), of CN and CN-DAP36, have been
367
estimated by XPS valence band spectroscopy and Mott–Schottky (MS) plots. The
368
prerequisites for MS analysis has been verified by EIS
369
The real surface areas of CN and CN-DAP36 electrodes have been observed by using
370
AFM, as shown in Figure S11. The MS plots for CN-DAP36 performed in 0.5 M
371
Na2SO4 (pH = 6.8) at 1, 5, 10 and 15 kHz are displayed in Figure 5(a). Based on these
47-48,
19
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372
results, the flat band potentials of CN and CN-DAP36 are estimated to be −0.65 and
373
−0.56 V vs SCE (equivalent to −0.43 and −0.34 V vs NHE), respectively, as shown in
374
Figure 5(a). Since the flat band potential is nearly not changed under different pH
375
values, the value of the flat band potential can be considered approximately equal to
376
the Fermi level 49. Furthermore, based on the VB-XPS spectra (Figure 5b) of CN and
377
CN-DAP36, the energy gap between the VB and Fermi level could be calculated. The
378
VB positions of CN and CN-DAP36 are found to be 2.15 and 2.11 eV, respectively.
379
Based on the band gap values, the CB positions of CN and CN-DAP36 are −0.83 and
380
−0.76 eV, respectively. Consequently, the energy bands of CN and CN-DAP36 are
381
illustrated in Figure 5(c). These results reveal that the doping of DAP into GCN leads
382
to a positive shift in the CB as well as a remarkable negative shift in the VB, thereby
383
resulting in a reduction in the band gap. Although the CB of CN-DAP36 is
384
positive-shifted, it is still more negative than the hydrogen reduction potential.
385
Therefore, CN-DAP36 not only is a photocatalyst with the extended optical absorption
386
range but also possesses the photocatalytic activity for hydrogen evolution.
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The Journal of Physical Chemistry
387 388
Figure 5. Mott-Schottky plots (a), VB-XPS spectra (b), and schematic band structures
389
(c) of CN and CN-DAP36.
390
Furthermore, DFT has been employed to elucidate the electron distribution and
391
energy level structures of CN and CN-DAP. As shown in Figure 6(a), the HOMO of
392
the CN trimer is distributed over the combination of nitrogen pZ orbitals, and the
393
LUMO is primarily attributed to C–N bond orbitals 31, 50. Correspondingly, because of
394
the electronegativity of pyrimidine, the electrons of CN-DAP are redistributed. As
395
illustrated in Figure 6(b), the LUMO of CN-DAP is mainly distributed in the
396
pyrimidine ring, whereas the HOMO of CN-DAP remains in the tri-s-triazine subunit.
397
Therefore, in comparison with CN, CN-DAP exhibits shifts in HOMO from −6.10 to
398
−5.96 eV and LUMO from −2.22 to −2.35 eV, as listed in Table 2, resulting in the
399
narrow by 0.27 eV in the band gap. The trend of the change is consistent with the
400
above results from MS plots and the XPS VB spectra. These results reveal that the 21
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401
aromatic C=C bonds with high electronegativity in DAP account for the variations in
402
the electronic band structure of CN-DAP.
403 404
Figure 6. Electronic structures of polymeric models for CN (a) and CN-DAP (b),
405
together with their optimized HOMO and LUMO.
406
Table 2. The optimized HOMO and LUMO of samples. Name
LUMO(eV)
HOMO(eV)
Gap(eV)
CN
-2.22
-6.10
3.88
CN-DAP
-2.35
-5.96
3.61
CN-DAP/Pt1
-3.22
-5.72
2.50
CN-DAP/Pt2
-3.41
-5.98
2.57
407
Notes: CN-DAP/Pt1 is the interaction between Pt and triazine unit, and CN-DAP/Pt2
408
is the interaction between Pt and pyrimidine ring for CN-DAP.
409
Moreover, the photoelectrochemical properties of CN and CN-DAP36 have been
410
studied using transient photocurrent-time plots and Nyquist plots. Notably, the
411
photocurrent value of CN-DAP36 is twice that of CN (as shown in Figure 7a), which
412
may be ascribed to the DAP modification that enhances the visible light absorption 22
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22, 51.
413
and reduces the charge recombination
From the EIS measurement data (Figure
414
7b) it can be seen that the radius of the semicircular Nyquist plot for CN-DAP36 is
415
smaller than that of CN, implying the resistant of charge transfer in CN-DAP36 is
416
lower than that in CN. The increase in the photocurrent and the facilitation in charge
417
transfer suggests that more photogenerated charge carriers can take part in the
418
photocatalytic reaction over CN-DAP36.
419 420
Figure 7. (a) Transient photocurrent response in 0.5 M Na2SO4 electrolyte under
421
visible light irradiation (λ>420 nm). (b) EIS Nyquist plots in the dark.
422
Note that the interaction between the photocatalyst and the cocatalyst has an
423
important impact on the hydrogen evolution reaction. The adsorption energies (Eads)
424
of different parts in CN-DAP for the cocatalyst (Pt) have been calculated by DFT.
425
Specifically, Pt was first placed on the triazine unit and pyrimidine ring, and then the
426
energy changes before and after Pt adsorption were calculated. As shown in Figure 8
427
and Table 3, the Eads of the pyrimidine ring for Pt is smaller than that of the triazine
428
unit, revealing that Pt is more readily adsorbed on the pyrimidine ring than on the
429
triazine unit. Moreover, from HRTEM it can be observed that there are more Pt
430
particles on the surface of CN-DAP36 than that of CN (as shown in Figure S9). 23
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431
Furthermore, the DFT calculations indicate that the energy level structure of
432
CN-DAP/Pt2 is different from that of CN-DAP, as shown in Figure 8 (d) and Table 2.
433
In comparison with CN-DAP, the bandgap of CN-DAP/Pt2 is narrower, which may
434
be due to the synergistic action of Pt and pyrimidine. CN-DAP/Pt2 has a
435
positive-shifted LUMO as compared with CN-DAP. Consequently, the electrons
436
generated by CN-DAP would be transferred to the LUMO of CN-DAP/Pt2, which can
437
improve the utilization of the photo-generated electrons. As shown in Figure S12, the
438
PL emission spectra of CN/Pt and CN-DAP36/Pt are weaker than those of CN and
439
CN-DAP36, respectively, revealing the recombination of photogenerated electron-hole
440
pairs has been further suppressed after the loading of Pt. The heterogeneous-like
441
structure of CN-DAP/Pt may be the reason why the photo-responsive range of
442
CN-DAP36 for photocatalytic hydrogen evolution could be extended to near 600 nm.
443 444
Figure 8. Electronic structures of CN-DAP and Pt including the optimized HOMO 24
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The Journal of Physical Chemistry
445
and LUMO for CN (a), CN-DAP (b), Pt (c), together with a schematic illustration of
446
the proposed mechanism for charge transfer in CN-DAP (d).
447
Table 3. The binding energy of CN-DAP with Pt. R1
E1(eV)
R2
E2(eV)
P1
E3(eV)
Eads(eV)
CN-DAP
-125279.33
Pt
-3239.70
CN-DAP/Pt1
-128516.90
2.14
CN-DAP
-125279.33
Pt
-3239.70
CN-DAP/Pt2
-128518.47
0.57
448
CN-DAP/Pt1 is the interaction between Pt and triazine unit, and CN-DAP/Pt2 is the
449
interaction between Pt and pyrimidine ring for CN-DAP.
450
In summary, compared with CN, the optimized DAP doped GCN sample
451
displays enhanced visible light harvesting and narrowed band gap, which make more
452
photoinduced
453
recombination and the decreased charge-transport resistance lead to more
454
photogenerated carriers get to the surface of the doped sample. Finally, at its surface,
455
more Pt particles tend to be distributed over the pyrimidine rings. Owing to the
456
synergistic action of Pt and pyrimidine, more photogenerated electrons rush to the
457
pyrimidine rings, followed by the rapid transfer from the doped GCN sample to the Pt
458
particles to take part in the photocatalytic hydrogen evolution reaction, as shown in
459
Scheme 2.
carriers
generated
in
it.
Subsequently,
25
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the
reduced
charge
The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
460 461
Scheme 2. Schematic illustration for the photocatalytic process of CN-DAP under
462
visible light irradiation.
463
4. Conclusions
464
In this work, a simple approach for synthesizing pyrimidine doped CN
465
(CN-DAP) has been developed by copolymerization of urea and DAP. In comparison
466
with CN, the CN-DAP photocatalysts displayed reduced bandgaps, enhanced
467
absorption in the visible region, decreased PL intensities, and improved EPR signals.
468
Owing to the extraordinary electron affinity of pyrimidine in DAP and the strong
469
interaction with Pt particles, the photo-generated electrons from tri-s-triazine units
470
rush to the pyrimidine rings and quickly transfer to the surface of the Pt particles for
471
participation in the photocatalytic hydrogen evolution reaction. The HER for
472
CN-DAP36 was 2.80 mmol/(h×g), which was 6.09 times that of CN (0.46
473
mmol/(h×g)) under visible light irradiation (λ > 420 nm). Furthermore, the
474
photoresponsive range of photocatalytic hydrogen evolution for CN-DAP36 could be
475
extended to near 600 nm.
476
ASSOCIATED CONTENT 26
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477
The Journal of Physical Chemistry
Supporting Information
478
Additional data, including BET of samples; SEM images and TEM images of CN
479
and CN-DAP36; TG curves of urea, DAP, precursor, CN, CN-DAP36, CN-DAP40,
480
and CN-DAP44; time course of H2 release for CN-DAP36; XRD patterns, FTIR
481
spectra, XPS spectra, and Solid-state
482
CN-DAP36; EIS and AFM images of CN electrode and CN-DAP36 electrode; PL
483
emission spectra of CN, CN/Pt, CN-DAP36, and CN-DAP36/Pt; XPS peak area
484
ratio of samples.
13C
NMR spectra of the fresh and used
485
AUTHOR INFORMATION
486
Corresponding author
487
*Tel:
488
Notes
489
The authorship declare no competing financial interest.
490
Acknowledgements
491
This work was financially supported by the National Natural Science Foundation of
492
China (21276088 and U1507201) and Natural Science Foundation of Guangdong
493
Province (2014A030312009).
86 20 87112997, Fax: 86 20 87113870, Email:
[email protected] 494 495
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