Graphite Carbon Nitride with Enhanced Photocatalytic

Dec 19, 2016 - Processes, Tianjin Polytechnic University, Tianjin 300387, People's Republic of China. ‡. College of Science, Tianjin University of S...
4 downloads 0 Views 4MB Size
Subscriber access provided by GAZI UNIV

Article

Fullerenes/Graphite Carbon Nitride with Enhanced Photocatalytic Hydrogen Evolution Ability Limin Song, Tongtong Li, and Shujuan Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09064 • Publication Date (Web): 19 Dec 2016 Downloaded from http://pubs.acs.org on December 19, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 32

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

The Journal of Physical Chemistry

1

Fullerenes/Graphite Carbon Nitride with Enhanced

2

Photocatalytic Hydrogen Evolution Ability Limin Songa,*; Tongtong Lia, Shujuan Zhangb,*

3 a

4

College of Environment and Chemical Engineering & State Key Laboratory of

5

Hollow-Fiber Membrane Materials and Membrane Processes, Tianjin Polytechnic

6

University, Tianjin 300387, P. R. China.

7

b

College of Science, Tianjin University of Science & Technology, Tianjin, 300457,

8

P.R. China

9

*Corresponding author. Tel./Fax: +86-22-83955458 E-mail address: [email protected]

10 11

ABSTRACT:

12

Graphite carbon nitride (g-C3N4) is an excellent photocatalytic hydrogen evolution

13

material by water splitting, but enhancing its photocatalytic activity and stability is

14

still a big challenge. In this study, by utilizing the super ability of fullerenes (C60) to

15

attract electrons, we synthesized C60/g-C3N4 photocatalytic composite materials with

16

superior electronic separation efficiency. The addition of C60 greatly improves the

17

photocatalytic hydrogen evolution ability of g-C3N4 for water splitting under visible

18

light radiation. The highest hydrogen evolution amount reached 2268.6 µmol/g over

19

15 mg/L C60/g-C3N4 after 10 h, which is 50.1 times that of g-C3N4 under the same

20

water splitting condition. The hydrogen evolution amount over 15 mg/L C60/g-C3N4 in

21

the 0.2 mol/L K2HPO4 system is 4161 µmol/g, indicating the addition of K2HPO4

22

boosts the activity of C60/g-C3N4. The apparent quantum yield after 76 h was about

1

ACS Paragon Plus Environment

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

23

5.8 %. The long-term photolysis water reaction for 76 h with rising photocatalytic

24

activity exhibits that the C60/g-C3N4 has excellent stability in water spitting. We can

25

regulate the photocatalytic activity of C60/g-C3N4 by changing the C60 concentration.

26

Photoluminescence spectrum proves the super ability of C60 to attract electrons on the

27

surface of C60/g-C3N4. Thus, C60 promotes electron separation efficiency on the

28

surface of g-C3N4 and significantly enhances photocatalytic hydrogen evolution

29

ability of g-C3N4.

30

1. Introduction

31

Graphite carbon nitride (g-C3N4), a new type of metal-free organic polymer

32

semiconductor, has great and potential applications in the field of catalysis because of

33

its excellence in water stability, thermal stability, chemical stability, light absorption

34

capability, hardness and catalytic performance.1-4 Especially, it has attracted many

35

researchers in the field of photocatalytic hydrogen evolution by photolysis water.5-9

36

Pure g-C3N4 has a band gap about 2.7 eV, corresponding to an optical wavelength

37

around 450 nm. Therefore, the photocatalytic ability of g-C3N4 can be induced

38

through excitation under visible light radiation. However, g-C3N4 usually is poorly

39

crystallized in the preparation, so the presence of numerous crystal defects would trap

40

the photo-generated charges, decelerating their migration and inhibiting the

41

photocatalytic performance of g-C3N4.10,11 To solve this problem, researchers have

42

prepared many modified g-C3N4 materials, such as mesoporous structures,3,12 ion

43

doping13-16 and composite semiconductors17-22. All these modification methods

44

significantly improve the ability of photocatalytic hydrogen evolution of g-C3N4.

2

ACS Paragon Plus Environment

Page 2 of 32

Page 3 of 32

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

The Journal of Physical Chemistry

45

Fullerenes (C60) molecules have a new type of conjugated and overlapped p-orbits.

46

The electrons of C60 with low-energy lowest unoccupied molecular orbit (LUMO)

47

are delocalized to form very large electron affinity (2.65 eV), so they have the super-

48

strong electron-withdrawing ability.11 Regarding this advantage, we make C60

49

closely couple with g-C3N4 to form composite semiconductors, which accelerates the

50

migration of photo-generated electrons and inhibits the recombination of the

51

photo-generated

52

photocatalytic ability of g-C3N4. In this study, C60/g-C3N4 hybrid materials were

53

prepared from a high-temperature calcination method, while the hydrogen evolution

54

ability of g-C3N4 was regulated by adjusting the amount of C60 in the composition.

55

An appropriate amount of C60 can significantly enhance the photocatalytic activity

56

and stability of g-C3N4. The photocatalytic processes and enhancement mechanisms

57

of C60/g-C3N4 were also elaborated.

58

2. Experimental

59

2.1. Synthesis of samples

electron-hole

pairs,

thereby

significantly

improving

the

60

All the reagents were bought from Sinopharm Chemical Reagent Co. Ltd. The

61

g-C3N4 was prepared as follows: 20 g of urea was added to a 30-mL crucible with

62

cover, then heated up to 550 ◦C at a rate of 1 ◦C /min in a muffle furnace, and kept for

63

3 hours. After the reaction, the furnace was cooled to room temperature, and then the

64

samples were collected and grinded into powder. The C60/g-C3N4 was synthesized as

65

follows: 10 mL of 5, 10, 15 and 20 mg/L C60 aqueous solution was added to 20 g of

66

urea, and the other steps are same to the above process. The corresponding C60/g-C3N4

3

ACS Paragon Plus Environment

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

67

samples were marked as CG5, CG10, CG15 and CG20.

68

2.2. Characterization of samples

69

The crystalline structures and phases of the samples were observed on a Rigaku

70

D/max 2500 powder X-ray diffractometer (XRD, Rigaku Corporation, Japan) from

71

2θ=10 to 60◦ using graphite monochromatized Cu Kα radiation of 1.5406 Å and

72

operating at 40 kV and 40 mA. Their morphologies were measured on a high-

73

resolution transmission electron microscope (HRTEM; JEM 2100, JEOL, Japan) at an

74

accelerating voltage of 200 kV. Their surface chemical states were analyzed on an

75

X-ray photoelectron spectroscope (XPS, Perkin-Elmer PHI5300, USA). Binding

76

energy was calibrated by setting the adventitious C 1s peak at 284.6 eV. The

77

ultraviolet visible (UV-vis) absorption spectra were measured on a UV-vis

78

spectrophotometer (BaSO4, UV2700, Shimadzu, Japan). The photoluminescence (PL)

79

spectra were recorded on an F-380 fluorescence spectrophotometer (Tianjin

80

Gangdong Sci-Tech Development Co., Ltd. China) at room temperature with an

81

excitation wavelength of 250 nm. Specific surface areas of Brunauer-Emmett-Teller

82

(BET) were determined on a Micromeritics ASAP 2020M apparatus at the liquid

83

nitrogen temperature of -196 °C.

84

2.3. Activity measurement

85

The photocatalytic H2 evolution activity was measured in a sealed 50-mL reaction

86

tube under visible-light irradiation and room temperature (λ > 420 nm). In a typical

87

process, 100 mg of a photocatalyst was added to 50 mL of an aqueous solution, which

88

contained 1 wt.‰ Pt (H2PtCl6 as the precursor) as a co-catalyst and 5 mL of

4

ACS Paragon Plus Environment

Page 4 of 32

Page 5 of 32

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

The Journal of Physical Chemistry

89

triethanolamine (TEOA, 10 vol%) as a sacrificial electron donor. A 5 W pure white

90

light-emitting diode (LED) light source (6000 K, 10 × 5 W, λ > 420 nm; visible

91

output power = 12.6 mW/cm2) was used. Prior to irradiation, the reaction system was

92

air-removed for 30 min through a vacuum pump. During irradiation, the reaction

93

system was stirred, intermittently sampled and analyzed for H2 evolution on a

94

GC7890 gas chromatograph (Rainbow Chemical Instrument Co. Ltd., Shandong

95

Lunan, China) equipped with a thermal conductivity detector and a 5 Å molecular

96

sieve column (2 m × 3 mm, Ar).

97

The apparent quantum yield (AQY) is calculated as follows:

98

AQY= 2×number of evolved H2 molecules/ number of incident photons×100%

99

3. Results and discussion

100

3.1. Characterization of photocatalysts

101

The XRD patterns of g-C3N4, C60 and CG5, CG10, CG15, CG20 characterizing

102

their crystal structures are showed in Fig. 1. Clearly, a strong diffraction peak at 27.6°

103

and a weak diffraction peak at 12.8° are found, which are assigned to (002) and (100)

104

crystal planes, respectively. No impurity is found in Fig. 1a, indicating the presence of

105

pure g-C3N4 phase. These results suggest that pure g-C3N4 can be obtained from our

106

experimental method, which is consistent with another study.23 The XRD patterns of

107

C60 are exhibited in Fig. 1b. Those crystal planes of (111), (220), (311), (222), (331),

108

(024), (422) and (333) can be belonged to cubic phase of C60 (JCPDS No. 82-0505,

109

space group: Pm3[202], a=b= c= 14.26 Å). CG5-20 show similar XRD patterns as

110

g-C3N4 in Figs. 1c-1f. In addition to the two peaks at 27.6° and 12.8°, no other

5

ACS Paragon Plus Environment

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

111

impurity is found in Fig. 1c-1f, which suggests the addition of C60 does not introduce

112

new impurities into g-C3N4. Interestingly, the two main peaks of CG5-20 slightly shift

113

to left, which suggests that additional C60 in urea may affect the crystal structure and

114

performance of g-C3N4 in the calcination. The changeable structure and performance

115

of g-C3N4 and additional C60 may significantly affect the photocatalytic activity of CG.

116

No diffraction peaks of C60 are found in CG5 and CG10. This is because the addition

117

of too little C60 does not result in the XRD diffraction peaks of C60. However, it is

118

found that the CG15 and CG20 with excessive C60 has a weak peak from the (111)

119

plane of C60 except the two main peaks at 27.6° and 12.8° of g-C3N4. The result

120

exhibits that the C60 in CG keeps an original form.

121

The morphologies of CG5-20 surveyed via TEM are shown in Fig. 2. The CG15

122

has very thin cicada-wing-like sheets (Fig. 2a), which stack together layer by layer

123

and extend to all directions. The thin-sheet structures of GC15 are very uniform (Fig.

124

2b). These thin sheets provide large surface areas (201 m2/g) and abundant surface

125

active sites, which help to improve the photocatalytic ability of CG15. The TEM

126

images of CG5, CG10 and CG20 are shown in Fig. 2c, 2d and 2e. They have the

127

morphologies like-sheet, which are similar to CG15. Those thin-sheets also stack

128

together layer by layer. Their BET surface areas are 162.9, 199.9 and 202 m2/g for

129

CG5, CG10 and CG20, respectively. The high-resolution image of CG15 exhibits the

130

feature without obvious lattice fringes (Fig. 2f), which is consistent with the XRD

131

result of weak crystallinity in Fig. 1. The weak crystallinity of CG15 is further proved

132

by the corresponding diffuse diffraction rings in Fig. 2g. It is found that many small

6

ACS Paragon Plus Environment

Page 6 of 32

Page 7 of 32

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

The Journal of Physical Chemistry

133

black spots are distributed on the surface of g-C3N4 evenly in Fig. 2f. We can clearly

134

distinguish the lattice of small black dots in Fig. 2h. The lattice is consistent with (137)

135

crystal planes of C60. It is clear that C60 particles are high dispersion on g-C3N4 under

136

high temperature calcination. According to the energy-dispersive X-ray (EDX)

137

spectra (Fig. 2i), the composition of CG15 is CN1.25, but the atom ratio of N/C is

138

lower than that of g-C3N4 (1.25 vs. 1.33), indicating the excess of C element in CG15.

139

The UV–vis absorption spectra of g-C3N4, CG5, CG10, CG15, and CG20 (Fig. 3)

140

show a very broad and intensive absorption from the ultraviolet to visible region. The

141

pure g-C3N4 shows a visible-light absorption edge of 464.4 nm. The spectrum below

142

464.4 nm suggests a charge-transfer process from the valence band produced by the

143

orbitals of Nn- to the conduction band caused by the orbitals of Cn+.23 The band gap

144

value of g-C3N4 was estimated to be 2.67 eV. Moreover, spectral shapes of the

145

absorption edges of CG5, CG10, CG15, and CG20 are not significantly different from

146

that of g-C3N4 (Fig. 3). This result suggests that a small addition of C60 does not

147

significantly affect the optical properties of g-C3N4.

148

The molecular structures of g-C3N4 and CG15 measured by FT-IR are showed in

149

Fig. 4. The peaks of g-C3N4 at 815, 1060, 1160-1700, and 3000-3700 cm-1 are

150

assigned to the bending vibration of tristriazine heterocycle; additional CO32-; the

151

skeletal stretching vibration of aromatic C-N heterocycles; the adsorbed H2O and the

152

terminal amino groups (-NH2 and -NH) in g-C3N4, respectively.24,25 The characteristic

153

peaks of C60 at 527, 577, 1183 and 1428 cm-1 overlap those of g-C3N4 in Fig. 4. We

154

cannot distinguish the peaks of C60 in CG15, but for CG15 compared with g-C3N4, the

7

ACS Paragon Plus Environment

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

155

intensities of all peaks are decreased and all peak positions shift to short wavelength,

156

indicating the addition of C60 affects the electronic environment of g-C3N4.

157

The surface compositions and states of g-C3N4 and CG15 characterized by XPS are

158

shown in Fig. 5. The spectrum of CG15 (Fig. 5A) shows that its surface elements

159

mainly include N, C and O. The additional O element may result from the O

160

contamination in air. No other impurities are found in Fig. 5A. The XPS peaks of

161

g-C3N4 at 288.4 and 284.8 eV can be assigned to C1s in Fig. 5B, and they may result

162

from the C-N structure in the g-C3N426 and the C standard peak, respectively. The C1s

163

peaks of CG15 are fully consistent with those of g-C3N4 in Fig. 5B, indicating C60

164

added to g-C3N4 does not affect the C1s electronic structure of g-C3N4 because of the

165

mechanical mixing of C60 and g-C3N4. The N1s peaks of g-C3N4 at 398.5, 399.7 and

166

401.1 eV in Fig. 5C belong to C-N-C, N-(C)3 and C-N-H, respectively.26 Moreover,

167

the N1s peaks of CG15 are also fully consistent with those of g-C3N4 in Fig. 5C. The

168

semiquantitative surface compositions of g-C3N4 and CG15 determined by XPS are

169

CN1.08 and CN1.16, respectively. Two values are similar. The CN1.08 and CN1.16 values

170

are slightly lower than that of the CG15 composition obtained by EDX (CN1.25).

171

Different measurement methods and semiquantitative analysis result in the above fact.

172

3.2 Photocatalytic activity

173

The photocatalytic H2 evolution activities detected over g-C3N4 and CG in 50-mL

174

sealed reaction tubes under visible-light irradiation (λ > 420 nm) are showed in Fig. 6.

175

The H2 evolution amounts for g-C3N4, CG5, CG10, CG15 and CG20 are 45.3, 400.8,

176

1286.5, 2268.6 and 197.5 µmol/g, respectively (Fig. 6A). All the CG samples show

8

ACS Paragon Plus Environment

Page 8 of 32

Page 9 of 32

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

The Journal of Physical Chemistry

177

higher H2 evolution activity than that of g-C3N4, indicating that addition of C60

178

obviously improves the H2 evolution ability of g-C3N4 under visible-light irradiation.

179

The activity of g-C3N4 is improved with the addition of C60 from 5 to 15 mg/L, and

180

then is reduced after the addition of 20 mg/L. The H2 evolution amount of CG15 is

181

50.1 times that of g-C3N4, suggesting that an appropriate concentration of C60 favors

182

the hydrogen production of g-C3N4. However, excessive C60 coating on the surface of

183

g-C3N4 prevents photon absorption and close contact with H2O molecules, and

184

therefore weakens the g-C3N4 activity. In order to further enhance the hydrogen

185

production ability of CG15, we added 0.2 mol/L K2HPO4 to the reaction system. As

186

shown in Fig. 6B, the hydrogen evolution amount is 4161 µmol/g, which is 1.83 times

187

that of CG15. The improvement can be attributed to the synergy between enhanced

188

proton reduction and TEOA photooxidation.5 The H2 evolution rates for g-C3N4, CG5,

189

CG10, CG15 and CG20 after 10 h are 4.5, 40, 128, 227, and 20 µmol/g/h, respectively

190

(Fig. 6C). The rate of CG15 is 50.1 times that of g-C3N4. All the C60-modified g-C3N4

191

samples show a significant improvement compared with g-C3N4, which is consistent

192

with their hydrogen evolution amounts in Fig. 6A. The rate of CG15 in the K2HPO4

193

system is 92.9 times that of g-C3N4, which further proves the phosphorylation of

194

K2HPO4 in the g-C3N4 hydrogen production. The AQYs of g-C3N4 and CG samples

195

are exhibited in Fig. 6D. It is very clear that the AQYs of all CG samples are higher

196

than that of g-C3N4. The AQY of CG15 is 50.2 times that of g-C3N4. In order to

197

investigate the lifetime and reusability of the C60-modified g-C3N4, we measured the

198

long-term and reusability activity of CG15 in Fig. 7 and Fig. 8. It is observed that the

9

ACS Paragon Plus Environment

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

199

hydrogen production amount of CG15 increases with time and does not decease

200

during 76 h (Fig. 7A). The hydrogen production amount reaches 7420.4 µmol/g. The

201

corresponding AQY of CG15 after 76 h is 5.8% (Fig. 7B). The above results suggest

202

that the C60/g-C3N4 has long lifetime under the photocatalytic condition because

203

g-C3N4 and C60 are resistant against corrosions from light, acids and bases. The

204

reusability experimental result is shown in Fig. 8. The hydrogen production amount

205

CG15 has reached 1105 µmol/g after the first test (after 4 h). The data don’t reduce

206

and increased to 1657.2 µmol/g after the fifth round in Fig. 8, suggests the CG15 has

207

a high reusability.

208

3.3 Photocatalytic process and mechanism

209

In order to study the photocatalytic process and mechanism, the hydrogen

210

production amount of 1 wt.‰ Pt/C60 (15 mg/L C60), C60/g-C3N4 (15 mg/L C60) and 1

211

wt.‰ Pt/C60/g-C3N4 (CG15) is survived under the same experimental condition.

212

Their activities are 40.1, 136.6 and 2268.6 µmol/g in Fig. 9, respectively. The

213

hydrogen production amount of 1 wt.‰ Pt/C60 is 40.1 µmol/g, shows that C60 can

214

absorb visible light and have the weak photocatalytic ability. It is found that

215

C60/g-C3N4 has a higher activity than that of 1 wt.‰ Pt/g-C3N4 (10.1µmol/g). The

216

hydrogen production amount of 1 wt.‰ Pt/C60/g-C3N4 greatly raises compared with

217

C60/g-C3N4 and 1 wt.‰ Pt/g-C3N4, which suggests that the synergies of Pt and C60

218

significantly enhance the photocatalytic ability of g-C3N4. The collaborative

219

electron-absorbing of C60 and Pt greatly facilitates the separation of photo-generated

220

charges, and improves the utilization efficiency of photo-generated electrons, so as

10

ACS Paragon Plus Environment

Page 10 of 32

Page 11 of 32

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

The Journal of Physical Chemistry

221

to enhance hydrogen production activity of the catalyst g-C3N4.

222

The PL spectra of g-C3N4 and CG15 prove the above result in Fig. 10. The

223

excitation wavelength is 250 nm in the PL spectra. Clearly, g-C3N4 shows two strong

224

emission peaks centered at 470 and 655 nm. Compared with CG15, the peak

225

intensity of CG15 is markedly quenched. It is clear that C60 has a very large electron

226

affinity of 2.65 eV. 11 At the same time, C60 shows a larger work function (4.81 eV)

227

than that of g-C3N4 (4.31 eV), which makes C60 have the super-strong

228

electron-withdrawing ability.

229

photo-generated electron transfer from g-C3N4 to C60. It is sure that the addition of

230

C60 is crucial to promoting the separation of photo-generated charges.

Therefore,

the

PL result suggests a

quick

231

In order to investigate the process of photocatalytic hydrogen production over

232

CG15, we measured the concentrations of H2O2 in the photocatalytic reaction system

233

of g-C3N4 and CG15. The concentration of H2O2 in the CG15 system after 10 h is

234

2.57 times that of g-C3N4 (3.941 vs. 1.536 mg/L), indicating that the plenty H2O2

235

results from splitting water. The typical process is showed as follows:

236 237

(1) 2H2O → H2O2 + H2 According to reference,17 it is a two-electron two step pathway (2) and (3):

238

(2) 2H+ + 2e- → H2

239

(3) 2H2O → H2O2 + 2H+ + 2e-

240

(4) H2O2 → 1/2O2 + H2O

241

The existence of abundant H2 and H2O2 in the CG15 reaction system proves a

242

two-step process. Equation 4 shows an exothermic process and needs to be induced

∆G = 106.1 kJ/mol

11

ACS Paragon Plus Environment

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

243

by a catalyst to split water to form H2O2. However, no O2 is generated in the CG15

244

photocatalytic reaction, indicating C60 does not induce the reaction, resulting in the

245

presence of abundant H2O2.

246 4. Conclusions 247

C60 is an efficient cocatalyst over g-C3N4 to enhance photocatalytic H2 production

248

under visible light irradiation. The H2 evolution amount of C60/g-C3N4 is 50.1 times

249

that of g-C3N4. The strong electron-absorbing action of C60 over g-C3N4 plays a

250

remarkable role in facilitating the separation and transfer of photogenerated charges.

251

Acknowledgements

252

This work was supported by Natural Science Foundation of Tianjin of China (Grant

253

14JCYBJC20500) and Graduate Program of Science and Technology Innovation of

254

Tianjin Polytechnic University (16114).

255

Reference:

256

(1) Groenewolt, M.; Antonietti, M. Synthesis of g-C3N4 nanoparticles in mesoporous

257

silica host matrices. Adv. Mater. 2005, 17, 1789-1792.

258

(2) Goettmann, F.; Fischer, A.; Antonietti, M.; Thomas, A. Chemical synthesis of

259

mesoporous carbon nitrides using hard templates and their use as a metal-free catalyst

260

for friedel-crafts reaction of benzene. Angew. Chem. Int. Ed. 2006, 45, 4467-4471.

261

(3) Thomas, A.; Fischer, A.; Goettmann, F.; Antonietti, M.; Muller, J. O.; Schloglb,

262

R.; Carlsson, J. M. Graphitic carbon nitride materials: variation of structure and

263

morphology and their use as metal-free catalysts. J. Mater. Chem. 2008, 18,

264

4893-4908.

12

ACS Paragon Plus Environment

Page 12 of 32

Page 13 of 32

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

The Journal of Physical Chemistry

265

(4) Wang, Y.; Yao, J.; Li, H. R.; Su, D. S.; Antonietti, M. Highly selective

266

hydrogenation of phenol and derivatives over a Pd@Carbon nitride catalyst in

267

aqueous media. J. Am. Chem. Soc. 2011, 133, 2362-2365.

268

(5) Liu, G. G.; Wang, T.; Zhang, H. B.;

269

Kako, T.; Ye, J. H. Nature-inspired environmental “phosphorylation” boosts

270

photocatalytic H2 p over carbon nitride nanosheets under visible-light irradiation.

271

Angew. Chem. Int. Ed. 2015, 12 , 13765-13769.

272

(6) Wang, X. C.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G. ; Carlsson, J. M.;

273

Domen, K.; Antonietti, M. A. metal-free polymeric photocatalyst for hydrogen

274

production from water under visible light, Nature Mater. 2009, 8, 76-80.

275

(7) Shin, W. H.; Yang, S. H.; Choi, Y. J.; Jung, H. M.; Song, C. O.; Kang, J. K.

276

Charge polarization-dependent activity of catalyst nanoparticles on carbon nitride

277

nanotubes for hydrogen generation. J. Mater. Chem. 2009, 19, 4505-4509.

278

(8) Sun, J. H.; Zhang, J. S.; Zhang, M. W.; Antonietti, M.; Fu, X. Z.; Wang, X. C.

279

Bioinspired hollow semiconductor nanospheres as photosynthetic nanoparticles.

280

Nature comm. 2012, 3, 1139-1145.

281

(9) Li, Y. X. Wang, H. Peng, S. Q. Tunable Photodeposition of MoS2 onto a composite

282

of reduced graphene oxide and CdS for synergic photocatalytic hydrogen generation.

283

J Phys. Chem. C, 2014, 118, 19842-19848.

284

(10) Knupfer, M. Exciton binding energies in organic semiconductors, Appl. Phys. A:

285

Mater. Sci. Pro. 2003, 77, 623-626.

286

(11) Haugeneder, A.; Neges, M.; Kallinger, C. Exciton diffusion and dissociation in

Meng, X. G.; Hao, D.; Chang, K.; Li, P.;

13

ACS Paragon Plus Environment

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

Page 14 of 32

287

conjugated polymer/fullerene blends and heterostructures. Phys. Rev. B. 1999, 59,

288

15346-15351.

289

(12) Su, F. Z.; Mathew, S. C.; Lipner, G.; Fu, X. Z.; Antonietti, M.; Blechert, S.;

290

Wang, X. C. mpg-C3N4-catalyzed selective oxidation of alcohols using O2 and visible

291

light. J. Am. Chem. Soc. 2010, 132, 16299–16301.

292

(13) Liu, Q.; Guo, Y. R.; Chen, Z. H.; Zhang, Z. G.; Fang, X. M. Constructing a

293

novel

294

visible-light driven photocatalyticactivity via interfacial charge transfer effect. Appl.

295

Catal. B: Environ. 2016, 183, 231-241.

296

(14) Di, Y.; Wang, X. C.; Thomas, A. Making metal-carbon nitride heterojunctions

297

for improved photocatalytic hydrogen evolution with visible light. Chem.Cat.Chem.

298

2010, 2, 834–838.

299

(15) Liu, G.; Niu, P.; Sun, C.H.; Smith, S. C.; Chen, Z.G.; Lu, G. Q.; Cheng, H. M.

300

Unique electronic structure induced high photoreactivity of sulfur-doped graphitic

301

C3N4. J. Am. Chem. Soc. 2010, 132, 11642-11648.

302

(16) Chen, X. F.; Zhang, J. S.; Fu, X. Z.; Antonietti, M.; Wang, X. C.

303

Fe-g-C3N4-catalyzed oxidation of benzene to phenol using hydrogen peroxide and

304

visible light. J. Am. Chem. Soc. 2009, 131, 11658-11659.

305

(17) Liu, J.; Liu, Y.; Liu, N. Y.; Han, Y. Z.; Zhang, X.; Huang, H.; Lifshitz, Y.; Lee,

306

S. T. ; Zhong, J.; Kang, Z. H. Metal-free efficient photocatalyst for stable visible

307

water splitting via a two-electron pathway. Science 2015, 347, 970-974.

308

(18) Yan, Z. P.; Sun, Z. J.; Liu, X.; Jia, H. X.; Du, P. W. Cadmium sulfide/graphitic

ternary

Fe(III)/graphene/g-C3N4compositephotocatalyst

14

ACS Paragon Plus Environment

with

enhanced

Page 15 of 32

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

The Journal of Physical Chemistry

309

carbon nitride heterostructure nanowire loading with a nickel hydroxide cocatalyst for

310

highly efficient photocatalytic hydrogen production in water under visible light.

311

Nanoscale 2016, 8, 4748-4756.

312

(19) Xiang, Q. J.; Yu, J. G.; Jaroniec, M. Preparation and enhanced visible-light

313

photocatalytic H2-production activity of graphene/C3N4 composites. J. Phys. Chem. C

314

2011, 115, 7355-7363.

315

(20) Li, X. H.; Chen, J. S.; Wang, X. C.; Sun, J. H.; Antonietti, M. Metal-free

316

activation of dioxygen by graphene/g-C3N4 nanocomposites: functional dyads for

317

selective oxidation of saturated hydrocarbons. J. Am. Chem. Soc. 2011, 133,

318

8074-8077.

319

(21) Xu, J. Y.; Li, Y. X.; Peng, S. Q.; Lu, G. X.; Li, S. B. Eosin Y-sensitized graphitic

320

carbon nitride fabricated by heating urea for visible light photocatalytic hydrogen

321

evolution: the effect of the pyrolysis temperature of urea, Phys. Chem. Chem. Phys.

322

2013, 15, 7657-7665.

323

(22) Xu, J. Y.; Li, Y. X.; Peng, S. Q. Photocatalytic hydrogen evolution over

324

erythrosin B-sensitized graphitic carbon nitride with in situ grown molybdenum

325

sulfide cocatalyst. Int J. Hydrogen Energy, 2015, 40, 353-362.

326

(23) Fang, S.; Xia, Y.; Lv, K.; Li, Q.; Sun, J.; Li, M. Effect of carbon-dots

327

modification on the structure and photocatalytic activity of g-C3N4. Appl. Catal. B:

328

Environ. 2016, 185, 225-232.

329

(24) Hebard, A.F.; Haddon, R.C.; Fleming, R. M.; Kortan, A. R. Deposition and

330

characterization of fullerene films. Appl. Phys. Lett. 1991, 59, 2109-2111.

15

ACS Paragon Plus Environment

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

331

(25) Cheng, R.L.; Zhang, L.X.; Fan, X.Q.; Wang, M.; Li, M. L.; Shi, J. L. One-step

332

construction of FeOx modified g-C3N4 for largely enhanced visible-light

333

photocatalytic hydrogen evolution. Carbon 2016, 101, 62-70.

334

(26) Zhang, Z .Z.; Xua, M. K.; Hob, W. K.; Zhang, X. W.; Yang, Z. Y.; Wang, X. X.

335

Simultaneous excitation of PdCl2 hybrid mesoporous g-C3N4 molecular/solid-state

336

photocatalysts for enhancing the visible-light-induced oxidative removal of nitrogen

337

oxides. Appl. Catal. B: Environ. 2016, 184, 174-181.

16

ACS Paragon Plus Environment

Page 16 of 32

Page 17 of 32

Figure:

180000

(100)

(002)

30000

160000

f

o

25000

140000

e d

20000

Intensity (a.u.)

120000

c

100000

15000 80000

60000

10000

40000

(311)

a (331) (024) (422) (333)

(222)

(220)

5000

(111)

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

The Journal of Physical Chemistry

20000

b

0

0 10

20

30

40

50

60

70

80

2Theta/degree

Fig. 1. X-ray diffraction patterns of samples. (a) g-C3N4, (b) C60 (c) CG5, (d) CG10, (e) CG15, (f) CG20. o: the (111) crystal plane of C60.

17

ACS Paragon Plus Environment

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

18

ACS Paragon Plus Environment

Page 18 of 32

Page 19 of 32

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

The Journal of Physical Chemistry

19

ACS Paragon Plus Environment

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

20

ACS Paragon Plus Environment

Page 20 of 32

Page 21 of 32

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

The Journal of Physical Chemistry

21

ACS Paragon Plus Environment

The Journal of Physical Chemistry

1000

C

i

800

Intensity (a.u.)

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

Page 22 of 32

N

600

400

200

0 0

1

2

3

4

5

Energy KeV

Fig. 2. TEM images of (a) and (b) CG15, (c) CG5, (d) CG10, (e) CG20, (f) and (h) HRTEM images of GC15, (g) The SAED pattern of CG15, (i) The EDX pattern of CG15.

22

ACS Paragon Plus Environment

Page 23 of 32

1.4

1.2

CG10 1.0

Absorbtion (a.u.)

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

The Journal of Physical Chemistry

CG20

0.8

g-C3N4 CG15

0.6

CG5

0.4

0.2

0.0 200

300

400

500

600

700

Wavelength (nm)

Fig. 3. UV-vis absorption spectra of the as-synthesized samples.

23

ACS Paragon Plus Environment

800

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

Transmittance (a.u.)

The Journal of Physical Chemistry

Page 24 of 32

CG15

g-C3N4

0

500

1000

1500

2000

2500

3000

3500

-1

Wavenumber (cm )

Fig. 4. FT-IR spectra of the as-synthesized samples.

24

ACS Paragon Plus Environment

4000

4500

Page 25 of 32

Intensity (a.u.)

N1s

A

C1s O1s

0

200

400

600

800

1000

1200

1400

Bing energy (eV)

398.9 eV B

N1s CG15 Intensity (a.u.)

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

The Journal of Physical Chemistry

g-C3N4

399.7 eV 401.1 eV

390

392

394

396

398

400

402

404

406

408

410

412

Bing energy (eV)

288.4 eV C1s

C

284.8 eV g-C3N4

CG15 280

284

288

292

296

Bing energy (eV)

Fig. 5. XPS spectra of the as-synthesized g-C3N4 and CG15 samples. (A) Survey, (B) N1s, (C) C1s. 25

ACS Paragon Plus Environment

The Journal of Physical Chemistry

2500

A H2 production amount (µmol/g)

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

Page 26 of 32

CG15

2000

1500

CG10

1000

CG5

500

CG20 0

g-C3N4 0

2

4

6

8

Time (h)

26

ACS Paragon Plus Environment

10

12

Page 27 of 32

4500

B

H2 production amount (µmol/g)

4000 3500

CG15 + K2HPO4 3000 2500 2000

CG15

1500 1000 500 0

2

4

6

8

10

12

Time (h)

450

CG15 + K2HPO4

C

400 350 300

CG15

250 200

CG10

150 100

CG5

50

CG20

g-C3N4 0 1

2

3

4

5

6

0.018

CG15

D

0.016

The apparent quantum yields (%)

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

The Journal of Physical Chemistry

0.014 0.012 0.010

CG10

0.008 0.006 0.004

CG5 0.002

CG20

0.000

g-C3N4 0

2

4

6

8

10

12

Time (h)

Fig. 6. (A) The H2 production amount of samples with different contents of C60. (B)The H2 production amount of samples with different contents of CG15 and CG15 27

ACS Paragon Plus Environment

The Journal of Physical Chemistry

+ K2HPO4. (C) The H2 production rate of samples. (D) The H2 production AQYs of samples.

8000

A

H2 production amount (µmol/g)

7000

CG15

6000 5000 4000 3000 2000 1000 0 0

10

20

30

40

50

60

70

80

50

60

70

80

Time (h)

0.06

B The apparent quantum yields (%)

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

Page 28 of 32

0.05

0.04

0.03

0.02

0.01

0.00 0

10

20

30

40

Time (h)

Fig. 7. (A) and (B) The long-term stability for CG15. 28

ACS Paragon Plus Environment

Page 29 of 32

1800

CG15

1600

H2 production amount (µmol/g)

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

The Journal of Physical Chemistry

1400 1200 1000 800 600 400 200 0 0

5

10

15

20

Time (h)

Fig. 8. The H2 production amount of CG15 after 5 cycles.

29

ACS Paragon Plus Environment

25

30

The Journal of Physical Chemistry

2500

C60/g-C3N4

100

2000

Pt/C60/g-C3N4 1500

50 1000

Pt/C60

500

g-C3N4

0 0

2

4

6

8

10

Time (h)

Fig. 9. The H2 production amount of samples.

30

ACS Paragon Plus Environment

12

H2 production amount (µmol/g)

150

H2 production amount (µmol/g)

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

Page 30 of 32

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

The Journal of Physical Chemistry

Intensity (a.u.)

Page 31 of 32

g-C3N4

CG15 400

500

600

700

800

Wavelength (nm)

Fig. 10. The PL emission spectra of g-C3N4 and CG15 at the excitation wavelength of 250 nm.

31

ACS Paragon Plus Environment

The Journal of Physical Chemistry

TOC graphic

338

2500

C60/g-C3N4

100

2000

Pt/C60/g-C3N4 1500

50 1000

Pt/C60

500

g-C3N4

0 0

2

4

6

8

10

Time (h)

32

ACS Paragon Plus Environment

12

H2 production amount (µmol/g)

150

H2 production amount (µmol/g)

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

Page 32 of 32