Flat Graphene-Enhanced Electron Transfer Involved in Redox

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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

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Environmental Science & Technology

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Flat Graphene Enhanced Electron Transfer Involved

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in Redox Reactions

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Submitted to Environmental Science & Technology

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May 26, 2017

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

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Gao‡ § *, Bingcai Pan‡ §

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

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Environment, Nanjing University, Nanjing 210023, China

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§

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

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Education), Tianjin Key Laboratory of Environmental Remediation and Pollution

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

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Tianjin 300071, China

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

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E-mail: [email protected]

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

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1

ACS Paragon Plus Environment


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Graphical Abstract:

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ABSTRACT

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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greatly affects redox reaction activities and may have important implications for

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designing novel graphene-based nanostructures and for understanding well of

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graphene winkles-dependent redox reactions in environmental processes.

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INTRODUCTION

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

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

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

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allowing a wide span of applications ranging from nanoelectronic and optomechanical

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devices to biology. Graphene has been employed widely in environmental

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

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processes as well as serving as substrates for other environmentally friendly

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

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

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

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

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

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

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

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

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wrinkles.15

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

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

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

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environmental reactions, such as the classical redox reactions containing electrons

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

Morphological

wrinkles

of

graphene

nanosheets

relieve

the

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

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migration across the reaction interfaces and the subsequent transfer onto graphene

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


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

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

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

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nanoelectronic and optomechanical fields. Generally, wrinkles degrade the physical

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

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

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

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

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

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

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

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

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applications of graphene.

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

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

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

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

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

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surface morphology of graphene nanosheets on redox activities and the involved

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

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

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accelerating or impeding reaction electron transfer based on electrochemical

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impedance spectroscopy (EIS) which represents the reaction charges transfer

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

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

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

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EXPERIMENTAL SECTION

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Materials and Reagents.

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

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

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

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

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

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China.

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Preparation and Treatment of GO with Varying Wrinkles.

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

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

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

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

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

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

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

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

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

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

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

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FGN, WGN-1, and WGN-2.

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Structural Characterization of rGO with Wrinkles.

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The microscopic features and morphology of the samples were characterized by

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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cm-1 in transmission mode.

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Assessment and Characteristics of the Electrochemical Redox Performance of

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rGO with Wrinkles

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

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

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



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

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

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

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electrochemically modified filtration casing to explore the electrochemical redox

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

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

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

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

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

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

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

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

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

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

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

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For electrochemical characterization, a potentiostat was used to perform cyclic

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

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

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

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

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

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

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

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

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

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

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the EIS data to determine individual capacitor and resistor values.

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RESULTS AND DISCUSSION

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The Flat/Wrinkled Surfaces of Graphene

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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



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

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

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

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

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other chemical activations.36

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

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Activities

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

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graphene materials comprehensively, we carried out an electrochemical experiment in

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

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

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

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electrochemical reaction and the electrochemically mediated desorption and/or

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

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

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

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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adsorption interaction of NB on graphene is not requirement to electrochemical

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reactions. Generally, the electrochemical redox performance of three graphene

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

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

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

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in the next section.

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Other Potential Properties of Graphene That Possibly Influence Its Redox

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Performance 11



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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


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

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structural characteristics of carbon materials providing useful information on the

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

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

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

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

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

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

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

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

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

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Surfaces

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

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

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

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resulting in

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

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

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

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

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

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We speculate that electrons transferring across the reaction interfaces between

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

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

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

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along the graphene surface can be assessed by electrochemical impedance

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

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electrochemical impedance spectroscopy (EIS) was performed in the electrochemical

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

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

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

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

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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)

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

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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 Involved in Redox

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