The evolution of surface charge on graphene oxide during the reduction and its application in electroanalysis

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x x x ( 2 0 1 3 ) x x x –x x x

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The evolution of surface charge on graphene oxide during the reduction and its application in electroanalysis Ming-jie Li a,b,c,d,e, Chen-ming Liu He Zhao b,c,d, Yi Zhang a,b,c,d,e

b,c,d

, Yong-bing Xie

b,c,d

, Hong-bin Cao

b,c,d,* ,

a

School of Chemical Engineering & Technology, Tianjin University, Tianjin 300072, PR China Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China c National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Beijing 100190, PR China d Research Centre for Process Pollution Control, Beijing 100190, PR China e National Engineering Research Center of Distillation Technology, Tianjin University, Tianjin 300072, PR China b

A R T I C L E I N F O

A B S T R A C T

Article history:

A series of reduced graphene oxide (RGO) colloids with different amounts of surface

Received 26 February 2013

charges was prepared. The change in surface charge at different pH values was detected

Accepted 4 September 2013

by zeta potential measurement, and the evolution of oxygen-containing functional groups

Available online xxxx

attached to the RGOs was analysed by Fourier-transform infrared spectroscopy and ultraviolet-visible absorption spectroscopy. Results showed that the edge phenolic hydroxyl and carboxyl groups made more contributions to the negative surface charge compared with the basal-plane hydroxyl and epoxy groups. Electrical impedance spectroscopy results proved that the surface charge of RGOs significantly affected their electrochemical properties. Furthermore, GO and RGOs (graphene oxide materials) were also used to construct electrochemical sensors for quantitive measurement of Cu2+ by differential pulse anodic stripping voltammetry. The results revealed that the increase in negative surface charge on RGO enhanced its electrocatalytic activity for Cu2+ reduction. Thus, considering that studies on the properties of graphene oxide materials can be simplified by the surface charge, its analysis is an important means of material characterisation.  2013 Elsevier Ltd. All rights reserved.

1.

Introduction

Graphene, a two-dimensional layer of graphitic carbon, has been the focus of recent experimental and theoretical studies because of its unique nanostructure and physicochemical properties [1–6]. In particular, the extraordinary electronic transport properties and high electrocatalytic activities of graphene make it a promising material for electrochemical applications, such as energy storage [7], transparent electron-

ics [8], electrochemical supercapacitors [9] and ultrasensitive chemical sensors [10]. The chemical reduction of graphene oxide (GO) has been commonly used to mass-produce graphene, often called chemically reduced graphene oxide (RGO) [11], since GO was easily prepared from graphite with Hummers method [12]. Great efforts [11,13,14] have been directed to understanding the chemical structure of GO and RGO (graphene oxide materials). Many structure models have been proposed for GO. Among them, the most well-known one is

* Corresponding author at: Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China. Fax: +86 10 82544845. E-mail address: [email protected] (H.-b. Cao). 0008-6223/$ - see front matter  2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.09.004

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Lerf–Klinowski model [15–17], which depicts that GO sheets are terminated with hydroxyl and carboxyl groups while epoxy and hydroxyl groups are attached on their basal planes. However, even to these days no unambiguous models of graphene oxide materials exist, and more efforts will be made to investigate their chemistry of surface functional groups [18,19]. To study the surface oxygen-containing functional groups in detail, researchers have used many characterisation methods, including Fourier-transform infrared (FT-IR), ultraviolet–visible (UV–vis) absorption, X-ray photoelectron (XPS) and nuclear magnetic resonance (NMR) spectroscopy analyses and so on [17,20,21]. Sometimes, these characterisation methods must be combined with one another to analyse functional groups qualitatively or quantitatively, which is very cumbersome and easily leads to ambiguous results [22]. Actually, investigating their surface functional groups in detail aims to determine their properties for specific applications because the oxygen-containing functional groups can remarkably affect the electronic and chemical properties of graphene oxide materials [23,24]. Therefore, an easy method for evaluating the performance of graphene oxide materials instead of investigating their surface functional groups in detail should be developed. Surface charge, which consists of inherent and variable charges, is determined by the material structure. Inherent charge is mainly generated by isomorphous replacement [25], whereas variable charge is mainly caused by the ionisation and reaction of surface functional groups when materials are dispersed in water or other solutions. The latter can vary with the surface groups and solvent conditions. The zeta potential, which results from the arrangement of surface charges and balance among counter-ions in solution, is used widely to study surface charge on a variety of carbons materials [26,27]. Li et al. [28] found that the zeta potential of GO is more negative than that of RGO at the same pH value, and assumed that the surface negative charge mainly results from the ionisation of oxygen-containing functional groups. Si et al. [29] and Liu et al. [30] have respectively reported the similar findings that the zeta potential of GO decreases after the removal of oxygen-containing functional groups. Considering the close relationship between oxygen-containing functional groups and the properties of graphene oxide materials, surface charge may be able to evaluate the performance of graphene oxide materials on the whole. Furthermore, the evaluation of surface charges by zeta potential measurement is easier than the detailed determination of surface functional groups. However, there are few reports on the effect of surface functional groups on the surface charge of RGO during the reduction. In this work, a series of homogeneous RGO colloids with various quantities of surface negative charges was prepared by a chemical reduction method. The evolution of surface charge on RGO during the reduction was evaluated by zeta potential measurement. And the change in oxygen-containing functional groups was detected by FT-IR and UV–vis methods. Subsequently, the surface morphologies and structures of the as-prepared RGOs were investigated by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The electrochemical properties of RGOs with various quantities of surface charges were measured by electrical impedance

spectroscopy (EIS) in potassium chloride (KCl) supporting electrolyte containing FeðCNÞ3=4 . What’s more, the influence 6 of surface charge on the performance of RGOs in the electrochemical reduction and detection of Cu2+ was investigated by CV and differential pulse anodic stripping voltammetry (DPASV). The results showed that the surface negative charge of RGOs contributed to the electroreduction of positively charged Cu2+ ions.

2.

Experimental

2.1.

Chemicals and materials

Natural graphite flakes (99.8 wt.%, 325 mesh) were purchased from Alfa Aesar (Tianjin, China). Chemically pure hydrazine hydrate (80 wt.%) and ammonia water (28 wt.%) were purchased from Beijing Chemical Works (Beijing, China). Polishing slurries (10 wt.% aqueous suspensions of 0.5 lm and 50 nm a-Al2O3) were purchased from Tianjin Aida Hengsheng Technology Co. Ltd. (Tianjin, China). All other chemicals were analytical grade or better and used as received. All solutions used were prepared with ultrapure water (18.2 MX cm1, Milli-Q, Millipore). Cu2+ solutions with different concentrations were prepared by diluting a 0.1 M aqueous solution of Cu(NO3)2. Britton–Robison (BR) buffer with the required pH was prepared by mixing appropriate amounts of H3BO3, CH3COOH, H3PO4 and NaOH followed by diluting with ultrapure water to the appropriate volume.

2.2.

Synthesis of GO and RGOs

GO was synthesised with a modified Hummers method [12,28]. In a typical procedure, 3 g of graphite was placed in an 80 C solution of concentrated sulphuric acid (12 mL), K2S2O8 (5 g) and P2O5 (5 g) for 4.5 h. The mixture was cooled to room temperature, diluted with 1 L of water, and left undisturbed overnight. The mixture was then passed through a 0.22 lm Nylon Millipore filter and washed with water to remove residual acid, and the product was dried in air. The obtained powder and KMnO4 (15 g) were cautiously mixed with concentrated sulphuric acid (120 mL) in a round-bottom flask at 35 C for 2 h. The suspension was diluted with 1 L of water in an ice bath, and then 25 mL of 30 wt.% hydrogen peroxide was added. Finally, the mixture was washed and centrifuged five times with 10% (v/v) HCl solution and ultrapure water sequentially. The bottom sediment was resuspended in water, and a brown–yellow colloid (0.25 wt.%) was formed. After this colloid was sonicated in water (300 W, 50% amplitude) for 60 min and centrifuged at 5000 rpm for 15 min to remove the unexfoliated graphite oxide particles, a GO aqueous solution was obtained. RGO homogeneous colloids were prepared by the chemical reduction method with hydrazine. As-prepared GO aqueous solution (400 mL) was adjusted to pH 10 with 1 M ammonia water, and hydrazine solution (300 lL, 80 wt.% in water) was added under stirring. The hydrazine added solution was kept at 35 C in a water bath with refluxing. Several liquid samples were extracted with a syringe at 0, 1, 2, 6 and 12 h and marked as GO, RGO-1, RGO-2, RGO-6 and RGO-12, respectively. Part of each sample was washed and dried for FT-IR and XRD

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characterisations. The rest was stored at 4 C for other characterisations and for preparing modified electrodes.

2.3.

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50 mV and pulse period of 500 ms. The scan was terminated at 0.5 V. Prior to the next cycle, a preconditioning step (60 s at 1.0 V) was used to remove residual Cu completely.

Preparation of GO- and RGO-modified GCEs

Glass carbon electrodes (GCEs) 4 mm in diameter (Aida Hengsheng Technology Co., Ltd., Tianjin) were used as the base electrodes. Before modification, the GCEs were well polished in 0.5 lm and 50 nm aluminium oxide slurry, sonicated in ethanol for 5 min, rinsed with ultrapure water, and dried in air. Subsequently, the as-prepared GO and RGO suspensions (20 lL each) were uniformly daubed onto the surface of the GCEs. The modified GCEs were dried in air for at least 12 h and marked as GO/GCE, RGO-1/GCE, RGO-2/GCE, RGO-6/GCE and RGO-12/GCE.

2.4. Electrochemical characterisations using the GO- and RGO-modified GCEs Electrochemical measurements were carried out on a Modulab Electrochemical System (Solartron Analytical, UK) with a three-electrode electrochemical cell. The modified or bare GCEs were used as working electrodes, a platinum slice (20 mm · 20 mm · 0.1 mm) served as the auxiliary electrode, and a saturated calomel electrode (SCE) served as the reference electrode. All potentials reported hereafter refer to the potential of the reference electrode. EIS spectroscopy (Nyquist plots) was recorded over a frequency range of 0.1 Hz–105 Hz, and an amplitude of 10 mV was superimposed on the open circuit potential of each working electrode. The electrolytic cell was covered with a copper net shield, and all conducting wires were shielded to prevent electromagnetic interference. Evaluation and simulation were carried out with ZSimpWin (1:1 mixture) in 0.1 M KCl solusoftware. A 50 mM FeðCNÞ3=4 6 tion was used as an electrolyte. The CV in BR buffer containing 20 mg L1 Cu2+ was recorded and used to compare the electrochemical activity of different RGO modified electrodes. The scan rate was 50 mV s1, and the scan range was from 0.6 V to 0.8 V. All the electrochemical measurements, including the DPASV, were performed at laboratory temperature (about 25 C). Before test, all the measured solutions were deoxygenated by bubbling nitrogen (99.99% purity) for 15 min.

2.6. Nanostructural and physical characterisations of GO and RGOs The morphology of GO and RGO was observed by TEM (JEM 2100, JEOL, Japan). Before the test, the as-prepared aqueous solutions were directly dropped onto the Cu grid and dried at room temperature. FT-IR spectroscopy was carried out using a Spectra GX spectrometer (PerkinElmer, USA) operated under the transmittance mode. The spectra were acquired with a resolution yielding of 0.5 cm1 and IR traced over a wavenumber range of 370–4000 cm1. XRD patterns were obtained by an X-ray diffractometer (X’PERT-PRO MPD, PANalyt˚ ). To ical B.V., Netherlands) with Cu Ka radiation (k = 1.5418 A rule out the interference of adsorbed water, all samples were dried at 55 C for 12 h and preserved in a desiccator with a silica-gel drier before the experiment. UV–vis spectra were acquired using a spectrophotometer (UV9100A, LabTech, China) within a scan range of 200–500 nm at a scan rate of 0.5 nm s1. The samples were diluted to about 0.025 mg mL1 before the test. An optical photo was also taken to show the gradual colour change of the diluted solutions. The zeta potential, which is widely used to characterise the surface electrical properties of the materials, was measured using a particle analyser (DelsaTM Nano C, Beckman Coulter, USA). Prior to the experiment, the pH values of samples were adjusted with ammonia water and hydrochloric acid, and all samples were diluted to about 0.05 mg mL1.

3.

Results and discussion

3.1.

Zeta potential analysis

To investigate the change in surface charge during the reduction process, zeta potentials of all samples at different pH levels were measured. Fig. 1 shows that the zeta potentials of the

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2.5. Electrochemical detection of Cu2+ using the GO- and RGO-modified GCEs

0

DPASV is widely used in the quantitative detection of metal ions because of its convenience and high sensitivity. In this work, DPASV was used to compare the modified electrodes with different quantities of surface negative charges in BR buffer containing 1 mg L1 Cu2+. The influence of the BR buffer pH on the stripping peak current was also studied. As a positively charged model, Cu2+ was then determined to characterise the sensing capabilities of these RGO/GCEs. In the electrodeposition process, the modified electrode was soaked in BR buffer with the required Cu2+ concentration at 0.6 V without stirring for 300 s. After hibernating for 60 s at open circuit, a voltammogram was recorded by applying a positive-going differential pulse voltammetry scan with a potential step height of 5 mV, pulse width of 0.2 s, pulse amplitude of

Zeta potential (mV)

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pH value 0.5±0.1 4.1±0.1 1.0±0.1 7.0±0.1 2.5±0.1 10.0±0.1

-60

Reduction time (h)

Fig. 1 – Zeta potentials of GO and RGOs as a function of reduction time at different pH values. (A colour version of this figure can be viewed online.)

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GO, b

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lue) is generally regarded as a critical value that represents sufficient mutual repulsion to guarantee the stability of a dispersion [31]. The successful dissolution of RGO in solution enables solution-phase chemists to modify graphene oxide materials for new functionalities and directly modify electrodes by the drop-coating method. Thus, a series of homogeneous and stable RGO colloids with various quantities of surface negative charge can be prepared by adjusting the reaction time and pH value.

RGO-12

Transm ittance (% )

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3.2. Oxygen-containing functional group analyses by FTIR and UV–vis spectroscopy 3500

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)

Fig. 2 – FT-IR spectra of (a) GO, (b) RGO-1, (c) RGO-2, (d) RGO-6 and (e) RGO-12. (A colour version of this figure can be viewed online.)

graphene oxide materials at the same reduction degree increase with increased pH. For example, the zeta potential of GO increases from 12.2 mV at pH 0.5 to 30 mV at pH 4.1 and 35 mV at pH 7.0, and reaches 52.8 mV when the pH approaches 10.0. This result well agrees with that of Li et al. [28]. As pH increases, more carboxylic acid and hydroxyl groups are ionised to form negatively charged radicals. As a result, the surface charge of the graphene oxide materials becomes more negative. On the other hand, both GO and RGOs are negatively charged over a very wide pH range, and the magnitude of the zeta potentials of RGOs are lower than that of GO at the same pH. For example, at pH 10.0, the zeta potential of GO drops from 52.8 mV to 34.5 mV sharply after 2 h of reduction, and then fluctuates around 30 mV after 6 h of reduction or longer. This evolution of surface charge during the reduction may result from the assumption that different oxygen-containing functional groups are removed at different reduction time and they may make different contributions to the surface charge. A zeta potential >30 mV (absolute va-

(a)

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Absorbance

(b)

3 A b so rb a n ce

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The evolution of oxygen-containing functional groups attached to RGOs was determined by FT-IR (Fig. 2) and UV–vis spectroscopy (Fig. 3). As shown in Fig. 2, the FT-IR absorption bands gradually change during the chemical reduction. An intense absorption band at about 3400 cm–1 corresponding to alcohol or phenolic hydroxyl (OH) vibrations gradually decreases with prolonged reduction time, and this decrease becomes very obvious after 12 h reduction. The absorption band at 1040 cm–1 are typically assigned to epoxide (C–O–C) group. The adsorption intensity of this band weakens slowly during the first 6 h, and observable decrease is detected after 12 h of reduction. Notably, the carboxyl (–COOH) stretching vibration at 1730 cm–1 significantly decreases. After 2 h of reduction or longer, this band almost disappears, demonstrating the efficient reduction of carboxyl groups generally considered to exist on the edges of GO sheets. Another band at 1390 cm–1 likely corresponding to anther edge groups of phenolic hydroxyl (C– O–H) decreases distinctly during the first 2 h, and the band intensity becomes weak after reduction of 6 h or longer. The FT-IR result indicates that oxygen functional groups are partially removed by hydrazine hydrate. However, it is difficult to remove them completely under the reaction conditions. As shown in Fig. 3a, the gradual transformation of GO to RGO is also confirmed by UV–vis spectroscopy. Along with the reduction, the main absorption peak caused by the p–p* transitions of C@C appearing at 215 nm (GO) gradually red shifts to 269 nm (RGO-12), demonstrating that the electronic

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300 350 400 Wavelength (nm)

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Wavelength (nm) Fig. 3 – (a) UV–vis spectra and (b) optical photo of GO and RGOs suspensions (about 0.025 mg mL1). The inset in (a) is the UV– vis spectroscopy of GO at a high concentration (about 0.04 mg mL1). (A colour version of this figure can be viewed online.) Please cite this article in press as: Li M-j et al. The evolution of surface charge on graphene oxide during the reduction and its application in electroanalysis. Carbon (2013), http://dx.doi.org/10.1016/j.carbon.2013.09.004

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conjugation in graphene was restored [28]. A shoulder peak attributed to the n–p* transition of C@O around 310 nm [28] is observed in GO and RGO-1. The inset is the GO spectrum at a high concentration. With prolonged reduction time for more than 2 h, this peak almost disappears because of the effective removal of carboxyl groups, consistent with the results of FT-IR analysis. As shown in Fig. 3b, a significant colour change from light yellow to dark brown occurs during the reduction. From the FT-IR and UV–vis analyses in evolution of functional groups during the reduction of GO, it can be concluded that edge groups, such as carboxyl and phenolic hydroxyl, are easy and first to remove under this condition while to remove the basal-plane hydroxyl and epoxy groups needs a longer reduction period. That may be because the existing position

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of edge groups makes them subject to exposure to the reducing environment of hydrazine hydrate. In the zeta potential analysis, we have found that the surface charge of GO decreased notably in the first 2 h during which most of the edge oxygencontaining groups were removed. Thus, it might be concluded that edge oxygen-containing groups can make more contribution to the surface charge of graphene oxide materials.

3.3. RGOs

Morphology and structure characterisation of GO and

3.3.1.

TEM images of GO and RGOs

The morphological change of GO after different reduction time was characterised by TEM as shown in Fig. 4. GO exhibits a flat sheet with some crinkles on its surface (Fig. 4a), how-

Fig. 4 – TEM images of GO and RGOs: (a) GO, (b) RGO-1, (c) RGO-2, (d) RGO-6 and (e) RGO-12. Please cite this article in press as: Li M-j et al. The evolution of surface charge on graphene oxide during the reduction and its application in electroanalysis. Carbon (2013), http://dx.doi.org/10.1016/j.carbon.2013.09.004

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RGO-12

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the different parts of one RGO lamella because the solution is very dilute (about 0.5 mg mL–1). Consequently, instead of agglomerated with each other, the RGO sheets fold over themselves.

3.3.2.

XRD analysis

Generally, the layer-to-layer distance expansion (d-spacing) in the process of graphite oxidation is caused by the accommodation of various oxygen-containing functional groups [33,34]. XRD is an effective method widely used to characterize the oxidation levels of graphite. As shown in Fig. 5, a very intense and narrow peak of GO ascribed to X-ray reflection on the (0 0 2) planes is located at 10.98. According to the Bragg ˚ , which equation [35], it corresponds to a d-spacing of 8.01 A is evidence of the presence of oxygen-containing functional groups attached on both sides of the GO sheets compared ˚ [36]. with graphite whose theoretical layer distance is 3.37 A During the reduction, this peak gradually shifts from 10.98

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Fig. 7 – CV plots obtained in BR buffer (pH 1.81) containing 20 mg L1 Cu2+ using (a) GO/GCE, (b) RGO-1/GCE, (c) RGO-2/ GCE, (d) RGO-6/GCE, (e) RGO-12/GCE and (f) GCE. The inset is a magnified view of the CV of GCE. All data were obtained from the second cycle. (A colour version of this figure can be viewed online.)

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ever, with prolonged reduction time, it is clearly seen that GO sheet begins to fold starting from the edges to the base panel, and a self-stacked structure emerges after 6 h of reaction (Fig. 4b–e). This interesting phenomenon may be attributed to the different removal orders and different contributions to the surface charge of the surface oxygen-containing functional groups. The relatively flat morphology of GO mostly results from the electrostatic repulsion because oxygencontaining functional groups can make graphene oxide materials negatively charged as a result of ionisation in aqueous solution. During the reduction, the electrostatic repulsion decreases and folding occurs from the edges because the edge phenolic carbonyl and carboxyl are firstly removed under applied reduction conditions. With the extended reduction time, the removal of basal-plane groups makes the further decrease in electrostatic repulsion so the RGO folds. Furthermore, the folded structure of the RGO might be partly attributed to p–p attractive interactions after the electronic conjugation within the graphene sheets was restored [28,32], which was also confirmed by the UV–vis spectroscopy results. This interaction among independent RGO sheets is weaker than that among

GO RGO-1 RGO-2 RGO-6 RGO-12

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Fig. 5 – XRD spectra of GO and RGOs. (A colour version of this figure can be viewed online.)

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GO

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Fig. 6 – (a) Nyquist plots of GO/GCE, RGO-1/GCE, RGO-2/GCE, RGO-6/GCE and RGO-12/GCE in 50 mM FeðCNÞ63=4 (1:1 mixture) and 0.1 M KCl within a frequency range of 0.1 Hz to 105 Hz (inset: equivalent circuit for EIS). (b) Simulation results of Rct, Rs and W for EIS. (A colour version of this figure can be viewed online.) Please cite this article in press as: Li M-j et al. The evolution of surface charge on graphene oxide during the reduction and its application in electroanalysis. Carbon (2013), http://dx.doi.org/10.1016/j.carbon.2013.09.004

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7

30

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GO/GCE RGO-1/GCE RGO-2/GCE RGO-6/GCE RGO-12/GCE GCE

GO/GCE

20 15 GCE

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Fig. 8 – Stripping voltammograms of the different modified electrodes in BR buffer containing 1 mg L1 Cu2+. From top to bottom: GO/GCE, RGO-1/GCE, RGO-2/GCE, RGO-6/GCE, RGO12/GCE and GCE. (A colour version of this figure can be viewed online.)

to 15.92, 16.39, 19.03 and 23.72 after 1, 2, 6 and 12 h of reduction according to a d-spacing of 5.56, 5.40, 4.66 and ˚ , respectively, due to the removal of oxygen functional 3.75 A groups at various reduction stages. It is easy to find that the d-spacing decreases observably even in the removal stage of edge oxygen functional groups. Therefore, edge oxygen functional groups can also make contributions to the d-spacing between the sheets of graphene oxide materials.

3.4.

EIS analysis of GO- and RGO-modified GCEs

EIS is one of the most important electrochemical methods to measure electrode process kinetics, electrode surface phenomena, conductivity and diffusion behaviour [37–39]. The GO and RGO modified GCEs with different surface charges were investigated by EIS. As shown in Fig. 6a, a typical Faradaic impedance spectrum presents as a Nyquist plot comprised of a semicircle in the high-frequency area and a linear line in the low-frequency area. The Randles circuit (inset of Fig. 6a) was chosen to fit the obtained impedance data containing Rs, Rct, W and Cdl [40]. Rs is the resistance of the electrolyte, including the lead resistance and contact resistance. Rct is the resistance of charge transfer usually used to

Fig. 10 – Calibration curves of the different modified electrodes for the detection of trace Cu2+ in BR buffer (pH 4.10): (a) GO/GCE, (b) RGO-1/GCE and (c) RGO-12/GCE. (A colour version of this figure can be viewed online.)

characterize the electrochemical activity and electron conductivity of the electrode material [41–43]. W is the Warburg impedance (corresponding to a slope of about 45 portion in the spectrum) resulting from the diffusion of ions in the electrolyte to the electrode interface [44]. Cdl is the electrical double-layer capacitance caused by the separation of positive and negative ions towards the electrode interface in the electric field. The simulation results are shown in Fig. 6b. Rct and Rs decrease drastically with prolonged reduction time. This result can be attributed to the progressive removal of surface oxygen-containing functional groups and the restoration of a graphitic network of sp2 bonds [45]. W increases with prolonged reduction time and peaks at 2 h. The W value is about 2.5 · 103 S s1/2 at RGO-12/GCE, which is more than twice that at GO/GCE (about 1.2 · 103 S s1/2). This phenomenon ions can be attributed to the interaction between FeðCNÞ3=4 6 and the surface charge of the graphene oxide materials. diffuse and When the negatively charged ions of FeðCNÞ3=4 6 react onto the negatively charged surface of the modified electrodes, electrostatic repulsion inevitably occurs and ions to diffuse onto the modified electrode blocks FeðCNÞ3=4 6 surface. So, the W value of GO/GCE is much lower than other electrodes. With the prolonged reduction time, the negative surface charge of the electrode materials weakens, thereby resulting in the increase of W value.

Fig. 9 – Stripping voltammograms in BR buffers containing 5 mg L1 Cu2+ at different pH values (1.81, 2.36, 2.87, 3.29, 4.10, 4.78, 5.33, 6.09 and 7.00): (a) GO/GCE, (b) RGO-1/GCE and (c) RGO-12/GCE. (A colour version of this figure can be viewed online.) Please cite this article in press as: Li M-j et al. The evolution of surface charge on graphene oxide during the reduction and its application in electroanalysis. Carbon (2013), http://dx.doi.org/10.1016/j.carbon.2013.09.004

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3.5. Electrochemical reduction and detection of Cu2+ using the GO- and RGO-modified GCEs 3.5.1. CVs in BR buffer containing Cu2+ using the GO- and RGO-modified GCEs GCE and the electrodes modified with graphene oxide materials were tested using CV in acidic BR buffer containing Cu2+. The CV plots are presented in Fig. 7. As shown in Fig. 7a, GO/GCE exhibits a reduction peak with a highest peak current of about 90 lA among all tested electrodes. Furthermore, GO/GCE exhibits a more positive reduction potential (about 4.3 mV) than the other modified electrodes. Generally, RGO shows a higher electrochemical catalytic activity than GO because of its excellent electrical conductivity [46]. However, GO is an undesirable electron conductor with an advantageous electrochemical sensitivity towards Cu2+. The increase in magnitude of the reduction peak current and the right shift in reduction potential could be attributed to the presence of more negative charge and oxygen-containing groups on the GO surface. In acidic aqueous solution, Cu2+ is likely to form a kind of simple water-complex ion [Cu(H2O)4]2+. Because GO possesses more hydrophilic oxygen-containing groups, GO/GCE might bind more [Cu(H2O)4]2+ ions by electrostatic forces. For comparison, CV was also performed for bare GCE (inset of Fig. 7). GCE presents two oxidation peaks probably representing a successive oxidation of Cu(0) to Cu(I), then to Cu(II). This phenomenon is not observed at the electrodes modified with graphene oxide materials. The one-step oxidation of Cu(0) to Cu(II) demonstrated better catalytic activity of graphene oxide materials than GCE.

3.5.2. DPASV analysis of electrochemical Cu2+ detection using the GO- and RGO-modified GCEs DPASV was also used to investigate the electrochemical behaviour of Cu2+ on the different electrodes modified with GO and RGOs. The striping voltammograms are depicted in Fig. 8. A sharp and high stripping peak current appears on the GO-modified electrode at about 0.1 V. With prolonged reduction time, this peak current gradually decreases with the deterioration of the peak shape. The current at GO/GCE (25.12 lA) is about two times higher than that at RGO-12/ GCE (7.86 lA). The zeta potential measurement results revealed that GO possessed more negative charges on its surface. Preferably, the GO/GCE surface could bind more Cu2+ by electrostatic attraction. Consequently, enhanced copper growth occurred in the electrodeposition process, which led to increased peak current in the stripping process. Compared with GO, the electrostatic attraction of RGOs towards Cu2+ was much weaker after the partial removal of oxygen-containing functional groups. Thus, their stripping peak currents diminished. In electrochemical analysis, the properties of the supporting electrolyte directly affect the performance of the electrode and the electrochemical response of the analyte. In this work, the influence of the BR buffer pH on the stripping peak current was studied, and the results are shown in Fig. 9. The stripping peak currents of Cu2+ on the modified electrodes increase with increased pH. However, these currents decrease when the pH is higher than 4.10 where the

highest stripping peak emerges. This signal enhancement can be ascribed to the increased surface negative charge resulting from the further ionisation of oxygen functional groups with increased pH. At the same time, Cu(OH)2 may have formed in the test solution when the pH increased above 4.10. Consequently, the amount of positively charged Cu2+ or [Cu(H2O)4]2+ decreased and the response signal inevitably decreased. Fig. 10 shows the calibration curves of the different modified electrodes for trace Cu2+ detection using GO/GCE, RGO-1/ GCE and RGO-12/GCE. The resulting calibration plots of the three electrodes are linear within the ranges of 0.05–1.00, 0.1–1.00 and 0.1–1.00 mg L1. Furthermore, the gradients of the calibration curve slope obtained by DPASV are 22.75, 14.66 and 7.15 lA mg1. Therefore, negative charges can likely improve the sensitivity of chemical sensors constructed with graphene oxide materials in detecting positively charged Cu2+ ions.

4.

Conclusion

A series of water-soluble RGOs with different amounts of surface charges was prepared by controlling the reduction time at a low temperature. Zeta potential analysis revealed graphene oxide materials are negatively charged over a very wide pH range. FT-IR and UV–vis spectroscopy analyses indicated that the edge phenolic hydroxyl and carboxyl groups could make more contributions to the negative charge than the basal-plane hydroxyl and epoxy groups. The surface charge affected the morphology and structure of the graphene oxide materials as confirmed by TEM and XRD characterisations. The EIS in electrolyte containing FeðCNÞ3=4 6 demonstrated surface charge significantly affected the electrochemical properties of graphene oxide materials. And the graphene oxide materials with abundant surface negative charges were favourable for detecting positively charged Cu2+ even though the conductivity under the experimental conditions was very low. In conclusion, the surface charge can be used to reflect the performance of graphene oxide materials to some extent. Furthermore, graphene oxide materials contain many oxygen functional groups on their surface. Thus, the electronic structure of the graphene oxide materials is easily modified by chemical reaction to graft electron-withdrawing or electron-donating substituents. And surface charge is much more easily determined than the type and quantity of surface-grafted functional groups. Thus, surface charge can serve as an indicator in material design for building specific functions, such as molecular electronics and chemical sensors incorporated in circuitry or actuators. The surface charge quantified graphene oxide materials may have potential applications.

Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51102238 and 51108441). Thanks are also extended to Prof. Zhuangjun Fan for his helpful suggestion to improve the quality of our manuscript.

Please cite this article in press as: Li M-j et al. The evolution of surface charge on graphene oxide during the reduction and its application in electroanalysis. Carbon (2013), http://dx.doi.org/10.1016/j.carbon.2013.09.004

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Please cite this article in press as: Li M-j et al. The evolution of surface charge on graphene oxide during the reduction and its application in electroanalysis. Carbon (2013), http://dx.doi.org/10.1016/j.carbon.2013.09.004