Flat Graphene-Enhanced Electron Transfer ... - ACS Publications

Subscriber access provided by UNIV TORONTO

Article

Flat Graphene Enhanced Electron Transfer Involved in Redox Reactions Meilan Pan, Yanyang Zhang, Chao Shan, Xiaolin Zhang, Guandao Gao, and Bing-Cai Pan Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b01762 • Publication Date (Web): 10 Jul 2017 Downloaded from http://pubs.acs.org on July 11, 2017

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.

Environmental Science & Technology 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 28

Environmental Science & Technology

1 2

Flat Graphene Enhanced Electron Transfer Involved

3

in Redox Reactions

4 5

Submitted to Environmental Science & Technology

6 7

May 26, 2017

8 9 10

Meilan Pan†, Yanyang Zhang‡ §, Chao Shan‡ §, Xiaolin Zhang‡ §, Guandao

11

Gao‡ § *, Bingcai Pan‡ §

12

‡ State Key Laboratory of Pollution Control and Resource Reuse, School of

13

Environment, Nanjing University, Nanjing 210023, China

14

§

15 16

Research Center for Environmental Nanotechnology (ReCENT), Nanjing University, Nanjing 210023, China † Key Laboratory of Pollution Processes and Environmental Criteria (Ministry of

17

Education), Tianjin Key Laboratory of Environmental Remediation and Pollution

18

Control, College of Environmental Science and Engineering, Nankai University,

19

Tianjin 300071, China

20 21

* Corresponding author, Guandao Gao Tel/Fax: +86-25-89681675

22

E-mail: [email protected]

23

Address: School of the Environment, Nanjing University, Nanjing 210023, China

24 25

1

ACS Paragon Plus Environment


26

Page 2 of 28

Graphical Abstract:

27

28 29 30

2


Page 3 of 28


31

ABSTRACT

32

Graphene is easily warped in the out-of-plane direction due to its high in-plane

33

Young’s modulus, and exploring the influence of wrinkled graphene on its properties

34

is essential to design graphene-based materials for environmental applications. Herein,

35

we prepared wrinkled graphene (WGN-1 and WGN-2) by thermal treatment and

36

compared their electrochemical properties with flat graphene nanosheets (FGN). FGN

37

exhibits much better activities than WGN, not only in the electrochemical oxidation

38

of methylene blue (MB) but also in the electrochemical reduction of nitrobenzene

39

(NB). Transformation ratios of MB and NB in FGN, WGN-1 and WGN-2 were

40

97.5%, 80.1%, 57.9% and 94.6%, 92.1%, 81.2%, respectively. Electrochemical

41

impedance spectroscopy (EIS) and the surface resistance of the graphene samples

42

followed the order FGN < WGN-1 < WGN-2, which suggests that the reaction

43

charges transfer faster across the reaction interfaces and along the surface of FGN

44

than that of WGN, and wrinkles restrict reaction charges transfer and reduce the

45

reaction rates. This study reveals that the morphology of the graphene (flat or wrinkle)

46

greatly affects redox reaction activities and may have important implications for

47

designing novel graphene-based nanostructures and for understanding well of

48

graphene winkles-dependent redox reactions in environmental processes.

49

3



50

Page 4 of 28

INTRODUCTION

51

Graphene is an extremely interesting material due to its unique 2D structure and

52

excellent mechanical, thermal, and electrical properties.1-4 To date, it is the only

53

atomically thin material that routinely provides stable and self-supported membranes,

54

allowing a wide span of applications ranging from nanoelectronic and optomechanical

55

devices to biology. Graphene has been employed widely in environmental

56

applications, including filtration,5 adsorption, electrochemistry, and photochemical

57

processes as well as serving as substrates for other environmentally friendly

58

composites. In recent years, the effects of surface area, number of layers, defects, and

59

oxygen-containing functional groups on the electrochemical, catalytic, adsorption and

60

separation properties of graphene6-9 have been extensively studied. As an atomically

61

thin two-dimensional layer of sp2-bonded carbon atoms, graphene has a high in-plane

62

Young’s modulus.10 Definitely, it is easily warped in the out-of-plane direction,

63

similar to a piece of flexible paper.11-13 Three-dimensional deformations naturally

64

occur in graphene, as membranes are intrinsically wrinkled to achieve thermodynamic

65

stability,14 and solution-chemistry processes usually produce flakes and multiply

66

wrinkles.15

67

self-aggregation of aromatic nanosheets and generate both adsorptive sites and

68

micropore volume, thus enhancing the adsorption capability and kinetics for efficient

69

pollutants removal.6 However, the effects of graphene wrinkles on other important

70

environmental reactions, such as the classical redox reactions containing electrons

71

transfer which are sensitive to the graphene surface, remain unexplored.

Morphological

wrinkles

of

graphene

nanosheets

relieve

the

72

Redox reaction rates occurring on a graphene surface depend on both electrons

73

migration across the reaction interfaces and the subsequent transfer onto graphene

74

sheets. Studies on the correlation between electron transfers across / along wrinkled 4


Page 5 of 28


75

graphene and the oxidation-reduction reactions occurred in graphene are absent.

76

Meanwhile, electron conditions and transfers on wrinkled graphene, ranging from

77

theoretical to experimental studies,16-20 have been extensively conducted for

78

nanoelectronic and optomechanical fields. Generally, wrinkles degrade the physical

79

properties and electrical conductance of the corresponding electronic devices,21 22 and

80

flat graphene provides a large surface area and facilitates the migration of electrons,23

81

which is in agreement with recent theoretical predictions reporting that wrinkles led to

82

a sizeable band gap increasing electron-hole charge fluctuations24 and reduce in-plane

83

stiffness of graphene from 340 N m-1 for flat graphene to only 20-100 N m-1

84

resulting in strong height fluctuations,26 as well as limit the quantum transport by a

85

density-of-states bottleneck and interlayer tunneling across the collapsed bilayer

86

region.27 These results benefit to understand the role of wrinkles in environmental

87

applications of graphene.

25

88

Herein, we prepared wrinkled graphene nanosheets (WGN) by thermal treatment

89

with morphological alteration and flat graphene nanosheets (FGN).6 Methylene Blue

90

(MB) and Nitrobenzene (NB), as model pollutants, were electrochemically oxidized

91

and reduced with WGN or FGN as the anode and cathode, respectively, to probe the

92

effect of graphene wrinkles on redox reactions. Afterwards, the influence of the

93

surface morphology of graphene nanosheets on redox activities and the involved

94

interaction mechanisms are also inferred. Results indicate that the surface morphology

95

of graphene nanosheets (flat or wrinkle) mainly affects redox reaction activities by

96

accelerating or impeding reaction electron transfer based on electrochemical

97

impedance spectroscopy (EIS) which represents the reaction charges transfer

98

resistance and surface ohmic resistance analysis. The findings of this study are

5



99

Page 6 of 28

important for the design of novel graphene-based nanostructures and the

100

understanding of winkle-dependent roles of graphene in environmental processes.

101

EXPERIMENTAL SECTION

102

Materials and Reagents.

103

All chemicals, including methylene blue (MB), sodium sulfate (Na2SO4),

104

dimethylsulfoxide (DMSO), ethanol, potassium hydroxide (KOH), hydrazine hydrate,

105

potassium ferricyanide (K3Fe(CN)6), nitrobenzene (NB), and aniline (AN), were

106

reagent grade products purchased from Sigma-Aldrich, except DMSO. The graphene

107

oxide (GO) was purchased from Plannano Energy Technologies Co. ltd, Tianjin,

108

China.

109

Preparation and Treatment of GO with Varying Wrinkles.

110

As-received GO (25 mg) was dispersed in 45 mL of ethanol solution with

111

continuous ultrasonic treatment (Branson, Sonifier S450D) for 30 min to obtain a

112

dispersed GO suspension in advance. Then, 0 mg, 25 mg and 125 mg of KOH was

113

added into the as-prepared suspension and stirred until completely dissolved and 2 mL

114

of hydrazine hydrate was added into the suspension. After stirring for 30 min, the

115

mixed solution was transferred to a 50-mL Teflon-lined autoclave, which was sealed

116

in a stainless-steel tank and heated at 180 °C for 18 h for the reduction of GO.

117

Afterward, the mixtures were left undisturbed to cool to room temperature, then was

118

washed by filtering the mixture through a polycarbonate membrane (0.22 µm, Welch

119

Material, Inc), finally it was freeze-dried and annealed at 800 °C for 1 h under an Ar

120

atmosphere. The as-obtained simples, reduced graphene oxide (rGO), were denoted as

121

FGN, WGN-1, and WGN-2.

122

Structural Characterization of rGO with Wrinkles.

6


Page 7 of 28


123

The microscopic features and morphology of the samples were characterized by

124

atomic force microscopy (AFM, Bruker Mult Imode), scanning electron microscopy

125

(SEM, Hitachi S-3400N) and transmission electron microscopy (TEM, JEOL

126

JEM-200CX). The samples were dispersed in the ethanol solution by ultrasonic

127

(Branson, Sonifier S450D, pulse mode: pulse on 30s and pulse off 30s), then they

128

were observed after drying. Furtherly, three kinds of samples were prepared for the

129

following characterization by sonication and vaccum same to rGO film preparation in

130

the next section. The BET specific surface area and pore distribution of the samples

131

were determined by N2 adsorption/desorption analysis using an Autosorb-IQ-MP

132

(Quantachrome) surface area analyzer. X-ray diffraction (XRD) analysis of the

133

samples was carried out with an X-ray diffractometer (Shimazduo 6000) using Cu Kα

134

radiation and a scanning rate (2θ/min) of 2°. Raman spectra were obtained with a

135

LabRAM Aramis Raman spectrometer (Horiba Scientific, Japan), and laser excitation

136

was provided by an Ar+ laser at 523 nm. The surface functional groups were observed

137

by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared

138

spectroscopy (FTIR). The XPS data were collected by a PHI 5000 VersaProbe

139

(U1VAC, Japan) with a resolution below 0.2 eV, and the C1s peak spectra were

140

analyzed using XPS Peak 4.1 software. The FTIR spectra were recorded on a Thermo

141

Fisher spectrometer (Nicolet iS5) in the 4000-500 cm-1 region with a resolution of 4

142

cm-1 in transmission mode.

143

Assessment and Characteristics of the Electrochemical Redox Performance of

144

rGO with Wrinkles

145

The rGO filters were produced by dispersing 10 mg of the as-prepared graphene in

146

DMSO by probe sonication (Branson, Sonifier S450D) for 15 min at an applied power

147

of 400 W L-1. Then, 30 mL of the sample-DMSO dispersion was filtered onto a 0.22 7



Page 8 of 28

148

µm PTFE membrane (Welch Material, Inc). The sample filters were subsequently

149

washed with 100 mL of EtOH, 100 mL of 1:1 DI-H2O: EtOH, and 100 mL of DI-H2O

150

to remove any residual DMSO. Finally, the prepared filter was loaded into an

151

electrochemically modified filtration casing to explore the electrochemical redox

152

performance by flowing MB or NB ([MB] = 10.8 µM, pH= 5.66; [NB] = 40 µM, pH=

153

5.86) in 10 mM Na2SO4 unless otherwise specified. During the operation, the sample

154

filter was utilized as the anode or cathode electrically connected via a titanium ring

155

and wire to the DC power supply of 3 V. The appropriate influent solution was then

156

peristaltically pumped (Masterflex) through the sample filter at a flow rate of 1.0 mL

157

min-1. Sample aliquots were collected directly from the filter casing outlet and

158

analyzed immediately, as described in previous studies. 28-30 The influent and effluent

159

MB and NB were measured at various time points during electrolysis by a UV-visible

160

spectrophotometer (Agilent) at 662 nm and by a High Performance Liquid

161

Chromatography (HPLC) (Waters) at 262 nm, respectively. The mobile phase was

162

30:70 (DI-water : methanol) with a flow rate of 1 mL min-1.

163

For electrochemical characterization, a potentiostat was used to perform cyclic

164

voltammetry (CV) and electrochemical impedance spectroscopy (EIS) with a CHI

165

604D electrochemical workstation (CHI, USA). The samples were employed as the

166

working electrode, a stainless-steel cathode was used as the counter electrode, and a 1

167

M Ag/AgCl solution was used as the reference electrode. Thus, all anode potentials

168

listed in the text and figures are with respect to 1 M Ag/AgCl. Flow and solution

169

conditions similar to those of the electrolysis experiments were used for the

170

electrochemical characterization. Cyclic voltammetry (CV) was performed at a scan

171

rate of 10 mV s-1. EIS was completed at a potential amplitude of 5 mV scanned over

172

the frequency range of 0.1-106 Hz. The resultant data were modeled with Nyquis plots.

8


Page 9 of 28


173

Zview software (Scribner, Southern Pines, NC) was used for quantitative analysis of

174

the EIS data to determine individual capacitor and resistor values.

175 176

RESULTS AND DISCUSSION

177

The Flat/Wrinkled Surfaces of Graphene

178

The microstructures of the as-prepared graphene were characterized by SEM,

179

TEM and AFM, and representative images are shown in Figure 1 and Figure 2. The

180

selected area electron diffraction (SAED) showed a hexagonal pattern, indicating the

181

amorphous nature and the regular carbon framework of graphene and that the three

182

kinds of graphene samples were presented in the form of monolayer graphene

183

nanosheets (the inset in Figure 1). Without KOH treatment, the graphene exhibited

184

typical ripples loosely distributed on the flat surface of FGN (Figure 1a, d), and the

185

thickness of the sheet was approximately 0.3 nm ~ 0.5 nm, equivalent to single-layer

186

graphene (Figure 2a, d). After KOH treatment, the flat surface tended to be wrinkled

187

and easily reunited to form the curved and porous 3D structure characteristic of

188

WGN-1 and WGN-2 (Figure 1b, e).Because the highly crystal structure of graphene

189

was altered after KOH treament, and self-aggregation of graphene layers were

190

evidently relieved, which resulted in the produce of wrinkles. With the increase of

191

KOH from WGN-1 to WGN-2, WGN-2 warped more heavily than WGN-1, and a

192

more wrinkled surface is shown in the SEM and TEM images (Figure 1c, f). Root

193

Mean Square (RMS) analysis in AFM is used to evaluate the roughness of graphene,

194

and it is also further used to semi-quantitatively assess the degree of wrinkling.31, 32

195

The RMS roughness of FGN was approximately 0.52±0.05 nm, lower than WGN-1

196

and WGN-2, which had values of 0.84±0.42 nm and 1.66±0.11 nm, respectively, in

197

accordance with the SEM and TEM observation results (More SEM and TEM images 9



Page 10 of 28

198

see Figure S1 in Supporting Information). Surface morphological analysis indicates

199

that KOH treatment and heating of graphene is a mild and suitable approach to

200

produce wrinkles on the graphene surface6 compared to other preparative strategies,

201

including photocatalytic oxidation,33 copolymer lithography,34 steam etching,35 and

202

other chemical activations.36

203

The Effect of the Flat/Wrinkled Surfaces of Graphene on Electrochemical Redox

204

Activities

205

To evaluate the effect of the degree of wrinkling of graphene on the properties of

206

graphene materials comprehensively, we carried out an electrochemical experiment in

207

the two parts: electrochemical oxidation and electrochemical reduction with MB and

208

NB as model pollutants on WGN or FGN as the anode or cathode, respectively.

209

Figure 3a shows the MB adsorption breakthrough curve in the absence of an

210

electrochemical reaction and the electrochemically mediated desorption and/or

211

oxidation of MB at a 3 V voltage for three graphene nanosheets of varying degrees of

212

wrinkling, where the black squares, red circles, and blue triangles represent FGN,

213

WGN-1 and WGN-2, respectively. The MB initially run through the graphene filter,

214

are adsorbed to form monolayer coverage on the graphene surface, and finally reach

215

adsorption saturation.37 The MB adsorption capacities of the three graphene samples

216

followed the order FGN (27.7 mg g-1) > WGN-1 (20.9 mg g-1) > WGN-2 (18.0 mg

217

g-1). Plots of electrochemically mediated desorption and/or oxidation of adsorbed MB

218

are shown in the right part of Figure 3a. At t = 0, marked by the dashed line in Figure

219

3a, a 3 V voltage was applied to the electrochemical cell during MB filtration. The

220

first aliquot of effluent collected contained a greater concentration of MB than the

221

influent, [MB]eff/[MB]in > 1, suggesting that the adsorbed MB is electrostatically

222

desorbed on the anode based on the fact that MB is positively charged at the 10


Page 11 of 28


223

experimental condition. Subsequently, if the adsorbed species is an undesirable

224

contaminant, a higher potential could be applied to oxidatively degrade the adsorbed

225

compound. The absence of dye breakthrough under anodic potential indicates that the

226

primary MB loss mechanism is oxidation. In all cases, upon continued electrolysis,

227

the [MB]eff/[MB]in quickly decreased until it achieved equilibrium values of 97.5%

228

for FGN, 80.1% for WGN-1, and 57.9% for WGN-2 (Figure 3a), suggesting that the

229

flat graphene has a better oxidation activity than the wrinkled graphene.

230

Figure 3b shows the electrochemical reduction of adsorbed NB at 3 V, where the

231

black squares, red circles, and blue triangles represent FGN, WGN-1 and WGN-2,

232

respectively. At t = 0, marked by the dashed line in Figure 3b, a 3V voltage was

233

applied to the electrochemical cell with graphene nanosheets as the cathode during the

234

NB electrochemical reduction. The removal ratios of NB followed the order, FGN

235

([NB]eff/[NB]in = 94.6%) > WGN-1 ([NB]eff/[NB]in = 92.1%) > WGN-2

236

([NB]eff/[NB]in = 81.2%) (Figure 3b). AN was the main product, and the amount of

237

generated AN followed the order FGN (35.9 µM) > WGN-1 (34.6 µM) > WGN-2

238

(29.2 µM), as shown in Figure S2. Interestingly, the adsorption of NB on the three

239

graphene samples was negligible different from that of MB. This also indicates that

240

adsorption interaction of NB on graphene is not requirement to electrochemical

241

reactions. Generally, the electrochemical redox performance of three graphene

242

samples, electron injection and escape, all follow the order FGN > WGN-1 > WGN-2,

243

which correlates with the degree of graphene wrinkles. The concerns as to whether

244

wrinkles are the main factor affecting the performance of the samples will be explored

245

in the next section.

246

Other Potential Properties of Graphene That Possibly Influence Its Redox

247

Performance 11



Page 12 of 28

248

The electrochemical activities of graphene likely depend on the layers, surface

249

areas, oxygen-containing functional groups and defects. The measured parameters are

250

listed in Table 1 and Figures 4 and 5. First, we confirmed that the three graphene

251

samples all consist of single-to-few graphene nanosheets and exclude the factor of

252

additional graphene layers based on the SEM, TEM and AFM images in Figures 1

253

and 2. The BET specific surface area (SSA) values of FGN, WGN-1 and WGN-2

254

were 508.6±10.9, 529.4±13.9 and 540.8±20.6 m2g-1, respectively. These values are

255

approximately equivalent (Table 1, Figure S3), which is consistent with other

256

wrinkle-dependent studies.6 The composition of the surface functional groups for the

257

obtained graphene samples were examined by FTIR spectra and XPS (Figure 4 and

258

Figure S4 in information supporting). The XPS surface O/C ratio of the FGN was

259

0.0289, which is no significant difference to the values of 0.0227 for WGN-1 and

260

0.0262 for WGN-2. An in-depth examination of the C1s and O1s specific binding

261

energies indicated that the three samples had almost no difference in the distribution

262

of specific surface oxy-functional groups. Deconvolution of the C1s peak showed the

263

presence of C=C/C-C (~284.6 eV) and C-O (~286.2 eV) groups (Figure 4a-c). The

264

FTIR spectra of the graphene samples indicated that there were no measureable

265

differences among the graphene samples with varying degrees of wrinkling, which

266

agreed well with XPS analysis of the graphene surface functional groups. Moreover,

267

they are equally hydrophobic based on their equal contact angle in Figure S5.

268

The XRD patterns of the graphene materials are shown in Figure 5a. FGN

269

exhibited broad reflections corresponding to d0002 for graphite (2θ ~ 27.0 °) indicating

270

an regular spacing of the graphene sheets. WGN-1 and WGN-2 showed a weak broad

271

002 diffraction peak at approximately 2θ of 27.0 °. No more defects formed after the

272

graphene nanosheets wrinkled, and the three kinds of graphene samples had similar 12


Page 13 of 28


273

defects. Raman spectroscopy (Figure 5b) has usually been used to investigate the

274

structural characteristics of carbon materials providing useful information on the

275

defects (D band ~ 1350 cm-1), including the in-plane stretching vibration of the sp2

276

carbon atoms (G band ~ 1580 cm-1) as well as the stacking order (2D band). The

277

intensity ratio of the D and G bands (ID/IG) is often used to estimate the number of

278

defects on carbon materials, and the ID/IG will decrease with the decay of the size of

279

the perfect graphene nanosheets.6, 38 The ID/IG values of FGN, WGN-1 and WGN-2

280

were 1.18, 1.15 and 1.10, respectively, meaning that the addition of KOH resulted in

281

reduced size of perfect graphene nanosheets while restore the sp2 structure with lower

282

defects, and it’s in consistent with the results of XRD pattern.6, 38

283

Mechanistic Aspects of the Redox Activities of Graphene with Flat/Wrinkled

284

Surfaces

285

Our experimental results show that FGN exhibits much higher activities, not only

286

in the electrochemical oxidation of MB but also in the electrochemical reduction of

287

NB compared to the WGN. Obviously, other potential properties of graphene possibly

288

resulting in

289

surface area, oxygen-containing functional groups and defects, are excluded carefully

290

one by one, and the graphene wrinkles are affirmed as the main aspect affected its

291

redox performance. The manner in which graphene wrinkles affect the redox reaction

292

occurring on its surface and the potential interaction mechanism need be explored.

their redox performance difference, such as the number of layers,

293

We speculate that electrons transferring across the reaction interfaces between

294

graphene and reactants, and/or along the graphene surface may be restricted when

295

passing the wrinkle sites of graphene, which subsequently decreases the reaction rate.

296

The transfer rate of reaction electrons passing between the reaction interfaces and/or

13



Page 14 of 28

297

along the graphene surface can be assessed by electrochemical impedance

298

spectroscopy and measuring the surface resistance of the graphene. Here,

299

electrochemical impedance spectroscopy (EIS) was performed in the electrochemical

300

oxidation of MB and electrochemical reduction of NB on FGN, WGN, and WGN-2.

301

Data were also simulated using Zview software and are presented as the dashed line in

302

Figure 6a and 6b with the corresponding fitted parameters in Table S2 and S3 based

303

on model circuit (1). Rs is the solution resistance in ohms, Rct is the charge-transfer

304

resistance in ohms, Wmt is the mass-transfer resistance in ohms and CPEdl is the

305

double-layer capacitance in microfarads in model circuit (1).39

(1)

306

307

Rct values of the electrochemical oxidation of MB on FGN, WGN-1 and WGN-2

308

followed the order 0.86 Ω, 4.11 Ω and 5.30 Ω, respectively. Interestingly, Rct values

309

of the electrochemical reductive of NB were also in the same order, FGN (48.91 Ω)

Flat Graphene-Enhanced Electron Transfer ... - ACS Publications

Recommend Documents