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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

1

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

6

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,

11

China.

12 13 14 15 16 17 18 19 20 21 22 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

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Abstract

24

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


molecule,

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44

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

47

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

54

elucidate their photocatalytic mechanisms.

55

Recently, various strategies have been explored to improve the photocatalytic

56

activity of GCN. These strategies can be divided into four categories: 1) combining

57

with carbonaceous nanomaterials, such as graphene 3-4, carbon nanodots 5, and carbon

58

nanotubes 6; 2) constructing specific nanostructures

59

or homojunction

60

17-20

61

way that can adjust its structures and properties by introducing organic molecules into

62

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


Since pyrimidine rings


66

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,

70

suggesting that pyrimidine-based molecules are excellent dopants for GCN. However,

71

it should be noticed that all the researches on the molecular doping of GCP have been

72

found to play a positive role in improving light harvesting as well as charge separation

73

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

75

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

85

recombination and transport, the interfacial electron transfer from the doped GCN

86

sample to the Pt cocatalyst was theoretically and experimentally studied. It is found

87

that the pyrimidine groups in DAP possess a stronger adsorption capacity for the Pt 4


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88

particles than the tri-s-triazine of GCN does, and more Pt particles can be thus

89

deposited near the pyrimidine rings, thereby facilitating the interfacial charge transfer

90

between the doped GCN sample with Pt. It is revealed that the molecular doping not

91

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.

93

2. Experimental section

94

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

96

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

98

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.

103

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



13C

110

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

113

adsorption–desorption isotherms and the pore size distribution plots of the obtained

114

samples were characterized using the Brunauer–Emmett–Teller (BET) method on a

115

Micrometrics ASAP 2020 apparatus. The morphologies and microstructures of the

116

samples were observed by using a scanning electron microscope (SEM, Hitachi

117

SU8220) and a transmission electron microscopes (TEM, JEOL JEM-2100F, and

118

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).

128

2.3. Photoelectrochemical measurement. The electrochemical properties of the

129

samples were tested using a Chenhua CHI660E electrochemical workstation via a

130

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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132

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

134

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

136

as-prepared slurry was coated on one piece of the clean FTO glass sheet, followed by

137

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

140

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.

143

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,

150

every reaction bottle was sealed to form a closed system, and the bottles were purged

151

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



154

Page 8 of 36

gas.

155

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

167

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:

170

AQE =

2 × 𝑡ℎ𝑒 𝑒𝑣𝑜𝑙𝑣𝑒𝑑 𝐻2 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒𝑠 𝑛𝑢𝑚𝑏𝑒𝑟 𝑡ℎ𝑒 𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡 𝑝ℎ𝑜𝑡𝑜𝑠 𝑛𝑢𝑚𝑏𝑒𝑟

× 100%

171

2.5. DFT calculations. The HOMO and LUMO energy levels of CN and

172

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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176


3. Results and discussion

177

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°

179

(100) and 27.64° (002), which correspond to the periodic array of the interlayer

180

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

182

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)

188

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



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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214

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



227

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.

248 12


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249


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

250

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



258

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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271

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



8, 41.

Page 16 of 36

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


Page 17 of 36 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


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.

17



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


Page 18 of 36

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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


as shown in Figure S10.


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.

20


Page 20 of 36

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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



Page 22 of 36

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


Page 23 of 36 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


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



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


Page 24 of 36

Page 25 of 36 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


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


the

reduced

charge


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


Page 26 of 36

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477


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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Insight into the Enhanced Hydrogen Evolution ... - ACS Publications

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