General Metal-Ion Mediated Method for Functionalization of Graphene

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A General Metal-Ion Mediated Method for Functionalization of Graphene Fiber Li Hua, Peipei Shi, Li Li, Chenyang Yu, Ruyi Chen, Yujiao Gong, Zhuzhu Du, Jin Yuan Zhou, Huigang Zhang, Xiuzhi Tang, Gengzhi Sun, and Wei Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10057 • Publication Date (Web): 02 Oct 2017 Downloaded from http://pubs.acs.org on October 4, 2017

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ACS Applied Materials & Interfaces

A

General

Metal-Ion

Mediated

Method

for

Functionalization of Graphene Fiber Li Hua,† Peipei Shi,† Li Li,† Chenyang Yu,† Ruyi Chen,† Yujiao Gong,† Zhuzhu Du,† Jinyuan Zhou,‡ Huigang Zhang,§ Xiuzhi Tang,ǁ Gengzhi Sun*,† and Wei Huang*,†,# †

Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM),

Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, P. R. China. ‡

School of Physical Science and Technology, Lanzhou University, 222 South Tianshui Road,

Lanzhou 730000, P. R. China. §

Collaborative Innovation Center of Advanced Microstructures, College of Engineering and

Applied Sciences, Nanjing University, 22 Hankou Road, Nanjing 210093, P. R. China. ǁ

Hunan Key Laboratory of Advanced Fibers and Composites, School of Aeronautics and

Astronautics, Central South University, 605 South Lushan Road, Changsha 410083, P. R. China. #

Shaanxi Institute of Flexible Electronics (SIFE), Northwestern Polytechnical University, 127

West Youyi Road, Xi'an 710072, Shaanxi, China. KEYWORDS: graphene, fibers, functionalization, electrochemical sensors, supercapacitors

ABSTRACT: Graphene fibers (GFs) are attractive material for wearable electronics because of their lightness, superior flexibility and electrical conductivity. However, the hydrophobic nature

ACS Paragon Plus Environment

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and highly-stacked structure endow GFs similar characteristics in nature to solid carbon fibers. Therefore, the interior functionalization of GFs so as to achieve synergistic interaction between graphene nanosheets and active materials thus enhance the performance of hybrid fibers remains a challenge. Herein, a general metal-ion mediated strategy is developed to functionalize GFs and nanoparticles of Cu, Fe2O3, NiO and CoO are successfully incorporated into GFs, respectively. As proof-of-concept applications, the obtained functionalized GFs are used as electrodes for electrochemical sensors and supercapacitors. The performances of thus-devised fiber sensor and supercapacitor are greatly improved.

INTRODUCTION Electronic textiles (E-textile) that integrate various fiber-shaped devices with functions of actuating,1-3

sensing,4-6

storage/processing

16-18

energy

conversion/storage,7-14

light

emitting,15

and

data

etc., have become a rapid developing research area and shown

extraordinary promise for applications ranging from health care, clinical diagnosis, in-situ medical monitoring, smart wearable garments to military.19-21 As the key component, fiber electrodes, which primarily determine the performance of the devices, have attracted unprecedented attention in recent years.22 Originally, metallic wires were used in aforementioned applications due to their high conductivity.7 However, the adoption of metallic wires in wearable electronics is limited by their environmental instability, high weight and wearing discomfort. By contrast, carbonaceous fibers are highly preferred for their lightness, superior flexibility, chemical stability, high mechanical strength and electrical conductivity.22 As a representative example, graphene fiber (GF), which is commonly fabricated by uniaxially aligning graphene oxide (GO) nanosheets using the wet-spinning technique followed by reduction, is more


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attractive because of its easy fabrication and cost-effectiveness over traditional carbon fiber and newly developed carbon nanotube fiber.23-25 However, the restacking of graphene nanosheets during fabrication and the chemical inertness of graphene nanosheets endow GFs similar characteristics in nature to solid carbon fibers, thus seriously restricting their multifunctional applications. In order to extend the applications of GFs and enhance their electrochemical performance, numerous attempts have been made to functionalize GFs using electrochemical deposition and co-spinning.26-28 However, both methods failed to functionalize the interior of GFs so as to achieve synergistic interaction between graphene nanosheets and electrochemically active materials. For instance, by using electrochemical deposition, active materials are primarily deposited on the outer surface of GF because of its highly-stacked structure and hydrophobic characteristic as a result of the reduction.26 In this manner, the poor conductivity of the electrochemically active materials is not effectively improved and only the outmost materials contribute to the capacitive improvement. On the other hand, co-spinning, which usually involves three steps of 1) dispersing pre-synthesized active materials in GO solution, 2) preparing concentrated suspension from high-speed centrifugation and 3) wet-spinning, is developed to prepare graphene-based hybrid fibers. Although this process is low-cost and shows the potential for large-scale production, there are two main issues that are not resolved so far. First of all, the good dispersion of active materials in GO solution is hard to achieve because nanoscale materials intend to aggregate as a result of their ultrahigh surface area, while microscale materials cannot be well dispersed due to gravity.27 Secondly, high-speed centrifugation unavoidably leads to phase separation and the coagulation of active materials in the mixed suspension because of the density difference between the pre-synthesized active


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materials and GO nanosheets.27 Therefore, a facile fabrication route is highly desired to functionalize GFs and concurrently realize the effective interaction between graphene nanosheets and electrochemically active materials. In this study, we develop a general metal-ion mediated approach to functionalize GFs by preimpregnating GO hydrogel fibers in a transition metal ion-containing solution followed by thermal annealing. Because of the hydrophilic nature of GO nanosheets, metal ions can easily penetrate into the GO hydrogel fiber and anchor onto GO nanosheets through electrostatic interaction between metal ions and negatively charged oxygen-containing functionalities on GO nanosheets. As a result, by tuning annealing condition, nanoparticles of Cu, Fe2O3, NiO and CoO are successfully incorporated into GFs, respectively. The obtained hybrid fibers are further used as electrodes for electrochemical sensors and supercapacitors. RESULTS AND DISCUSSION The proposed mechanism for functionalization of GFs is schematically depicted in Figure 1. The large aspect ratio and highly hydrophilic characteristic of GO nanosheets render their aqueous suspension liquid crystalline behavior, which enables the alignment of GO nanosheets into a macroscopic hydrogel fiber by using a high throughout wet-spinning method (Step 1). The collected GO hydrogel fibers are pre-impregnated in a transition metal ions-containing solution. The negatively charged functional groups on GO nanosheets allow the dynamic penetration of metal ions into the interspaces between GO nanosheets (Step 2) because of the electrostatic force. The obtained ion-incorporated hydrogel fibers are then thermally annealed (Step 3) to reduce GO and transform metal ions into electrochemically active materials (either metal


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nanoparticles or metal oxide depending on the annealing conditions) which are uniformly distributed in GFs. In a typical experiment, GO nanosheets were prepared by a modified Hummers method according to our previous study.29-35 The transparent characteristic shown in transmission electron spectroscopy (TEM) indicates its ultrathin feature (Figure S1a). The height of GO nanosheet is approximately 1.0 nm (Figure S1b) determined by atomic force microscopy (AFM), reflecting the monolayer feature. This thickness is higher than that of single layer graphene due to the existence of oxygen-containing functionalities, which is confirmed by X-ray photoelectron spectroscopy (XPS) in Figure S2. The concentrated suspension of GO nanosheets exhibits liquid crystalline behavior (Figure S3), enabling the alignment of GO nanosheets into macroscopic fibers via a simple wet-spinning process as schematically illustrated in Figure S4a. Briefly, the aqueous GO suspension (20 mg mL-1) was injected into a KOH saturated ethanol solution by a syringe.

The

formed

GO

hydrogel

fiber

was

continuously

wound

around

a

polytetrafluoroethylene (PTFE) rod (Figure S4b). Subsequently, the collected GO hydrogel fibers were pre-impregnated in a mixed water/ethanol solution (volume ratio of 1:2) containing Cu2+ and Fe3+, respectively, to form ion-incorporated GO hydrogel fibers (Figure S5). The Cu2+-incorporated GO hydrogel fiber was annealed at 800 oC in forming gas (5 wt% H2 and 95 wt% Ar) to obtain Cu/GF800. Similar to GF800 (Figure S6), Cu/GF800 is composed of aligned graphene nanosheets with plenty of wrinkles formed during coagulation (Figure S7a). Enlarged scanning electron microscopic (SEM) image shows that numerous Cu nanoparticles are loaded on the surface of graphene nanosheets (Figure 2a), which is further confirmed by energy dispersive spectroscopy (EDS) mapping (Figure 2a), TEM imaging (Figure S7b), XPS surface surveying (Figure S7c) and XRD pattern (Figure S7d). The peaks at 932.4 and 952.2 eV in the


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XPS spectrum of Cu 2p can be well indexed to metallic Cu (Figure 2b).36 The loading amount of Cu nanoparticles in the Cu/GF800 is determined approximately 74.2 wt% by thermo gravimetric analysis (TGA). The reduction of GO was evidenced by the XPS spectrum of C 1s (Figure S7e), in which the C-O and C=O peaks (at 286.5 and 288.2 eV) are dramatically reduced compared to those of the GO fiber (Figure S2). This result is consistent with the Raman spectra. Upon reduction, the intensity ratio between D (1328 cm-1) and G (1589 cm-1) bands is slightly increased (Figure 2c) due to the creation of new edges in graphene nanosheets accompanying the removal of oxygen-containing groups.37 The electrical conductivity of Cu/GF800 is 29.7 S m-1 which is higher than that of GF800 (~11.5 S m-1) because of the incorporation of Cu nanoparticles.38 In contrast, the strength of Cu/GF800 (170.2 MPa) is slightly compromised compared to GF800 (194.9 MPa) as shown in Figure S7f. Cu nanostructures have been reported highly active for electrocatalysis,39,

40

which might

suggest Cu/GF800 is of potential for electrochemical sensing. As a proof-of-concept application, the electrode based on Cu/GF800 is used for non-enzymatic detection of glucose and H2O2. The bare GF800 electrode shows no obvious electrochemical response to glucose (Figure 3a), the detection of which is of importance to the clinical diagnosis of diabetes.41 In comparison, due to the high catalytic activity of Cu towards the oxidation of glucose,42 the responsive current on Cu/GF800 electrode increases with the addition of glucose in a concentration-dependent manner (Figure 3b). At the fixed potential of +0.60 V, the amperometric response of the Cu/GF800 electrode to the successive addition of glucose was measured (Figure 3c), and an obvious amperometric response can be triggered by the addition of glucose to an equilibrium concentration of 10 µM (inset in Figure 3c). The linear calibration of the amperometric current response to the concentration of glucose is plotted in Figure 3d, showing an extraordinary


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sensitivity of 0.6 mA mM-1 cm-2 in the linear response range from 10 µM to 1.6 mM. Such a high sensitivity predicts a remarkable limit of detection (LOD) of 0.1 µM with S/N = 3. This detection limit is superior to Cu/N-graphene (1.3 µM),43 Cu/graphene (0.5 µM),42 and Cu/MWCNTs (1.0 µM).44 H2O2 is an essential mediator in food, pharmaceutical, clinical, industrial, and environmental analyses.45 Therefore, its reliable, accurate, and rapid determination is of practical importance. Compared to GF800 (Figure 4a), Cu/GF800 electrode exhibits a stronger reduction current and lower onset potential due to the high catalytic activity of Cu to the reduction of H2O2 (Figure 4b). These characteristics allow Cu/GF800 electrode the possibility to detect H2O2 with higher sensitivity and stability. As shown in Figure 4b, the reduction current of H2O2 increases linearly in a concentration-dependent manner. The amperometric response of the Cu/GF800 electrode on a successive addition of H2O2 into a stirred 0.1 M PBS (pH 7.4) was performed at an applied potential of -0.15 V (Figure 4c). The Cu/GF800 electrode exhibits a fast response with 90% of the steady current achieved within 2.1 s. In addition to the high catalytic activity of Cu, the fast response could be partially attributed to the fact that H2O2 can rapidly diffuse to the Cu/GF800 electrode due to the advantageous structure of present electrode (the merit of microelectrode). The fiber electrode shows a linear response to H2O2 ranging from 50 to 650 µM and a LOD of 0.5 µM based on S/N = 3 (Figure 4d), which is superior to Cu/chitosan/CNTs (LOD of 20.0 µM),46 Ag/graphene (LOD of 7.0 µM),47 and Cu2O microcubes (LOD of 1.5 µM).48 The above comparison reveals that the Cu/GF800 electrode possesses superior electrocatalytic activity towards the detection of glucose and H2O2 than that of GFs, which can be ascribed to: 1) the well aligned and interconnected graphene nanosheets provide highly conductive channels for electron transportation; 2) the incorporated Cu nanoparticles offer high catalytic sites for the


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oxidation of glucose and reduction of H2O2; 3) the microsize of fiber electrode in diameter further enhances the mass transportation of analytes to the electrode surface. The same protocol was also employed to prepare transition metal oxide/GFs. For example, Fe3+-incorporated GO hydrogel fiber was thermally annealed at 400 oC in Ar to form Fe2O3/GF400 (Figure S8a). Similar to Cu/GF800, Fe2O3 nanoparticles are successfully loaded on graphene nanosheet (Figure S8b) and the loading amount of Fe2O3 nanoparticles in the Fe2O3/GF400 is approximately 44.6 wt% determined by TGA. EDS mapping (Figure 5a), XPS surface surveying (Figure S8c) indicate the uniform incorporation of Fe in the Fe2O3/GF400. The peaks at 724.7 and 711.2 eV in XPS spectrum of Fe 2p (Figure 5b) can be indexed to the Fe 2p1/2 and Fe 2p3/2 spin–orbit peaks of Fe2O3, respectively.49, 50 In addition, the satellite peak which centers at 718.9 eV (circled in red) is the characteristic of Fe2O3.49 The XPS spectrum of O 1s peaking at 529.2 eV also reveals the formation of metal oxide (Figure S8d).49 These results are in good agreement with the observation in the XRD pattern (Figure S8e) and Raman spectra (Figure 5c). Compared to GF400, Fe2O3/GF400 shows several characteristic peaks in the low frequency region, i.e., Eg modes (at 222, 288, 405 and 485 cm-1) and A1g mode at (606 cm-1), that can be ascribed to the presence of Fe2O3 phase (Figure 5c).51-53 As the annealing temperature is relatively low, GO nanosheets are mildly reduced compared to Cu/GF800, which is evidenced by XPS spectrum of C 1s (Figure S8f). Therefore, a conductivity of 0.9 S m-1 is obtained for Fe2O3/GF400. It is similar to that of GF400 (~ 0.5 S m-1), but lower than that of Cu/GFs. The strength of Fe2O3/GF400 is 124.7 MPa (Figure S8g) which is slightly compromised compared to GF400 (149.8 MPa). By using the same strategy, other transition metal oxide (CoO and NiO) were also successfully incorporated in GFs, respectively, which have been well identified by SEM, EDS mapping, XPS spectra and XRD (Figure S9-10). In addition, according to Figure S11, the


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strength of CoO/GF400 and NiO/GF400 are 104.6 MPa and 123.1 MPa, respectively. Fe2O3 nanostructures have been shown excellent electrochemical performance in applications of energy storage devices and electrocatalysis with the advantages of low cost and environmental friendliness.49, 54 As a proof-of-concept application, the as-prepared Fe2O3/GF400 was used as electrodes for supercapacitor and electrochemical sensor. The capacitive performances of GF400 and Fe2O3/GF400 electrodes were tested in a three-electrode system. Due to the densely restacked structure, GF400 electrode shows stable CV behavior (Figure 6a). In comparison, the capacitance of Fe2O3/GF400 electrode increases from 63.4 to 142.5 F cm-3 during cycling (Figure 6b-d) which can be attributed to the intercalation of Li+ into the enlarged interlayer spaces between graphene nanosheets because the incorporation of Fe2O3 nanoparticles (Figure 6e). Once the Fe2O3/GF400 electrode reaches steady, it is compared with GF400 electrode in Figure 7a. GF400 electrode exhibits a slightly distorted rectangular shape, suggesting that the typical electric double-layer capacitance (EDLC) behavior dominates the charge/discharge process. In contrast, the Fe2O3/GF400 electrode has a much higher current density and a distorted CV curve owing to the pseudocapacitive contribution from the incorporated Fe2O3 nanoparticles. The specific capacitances of GF400 and Fe2O3/GF400 electrodes calculated from the GCD curves at various current densities (Figure S12 and Figure 7b) are compared in Figure 7c. The maximum capacitance of 230.0 F cm-3 (corresponding to gravimetric capacitance of 201.8 F g-1) for Fe2O3/GF400 electrode is obtained at a current density of 0.5 A cm-3 which is much higher than that of GF400 electrode. This value is also higher than that of PPy/MWCNT/RGO fiber electrode (29.5 F cm-3)55 and MnO2@MWCNT fiber electrode (~63.9 F cm-3).10 However, the rate performance of Fe2O3/GF400 electrode is moderate as a result of the low conductivity of Fe2O3/GF400. The cyclic stability of the Fe2O3/GF400 electrode was evaluated. It is found that the


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specific capacitance of the Fe2O3/GF400 electrode decays slightly within the first 600 cycles, then remains stable up to 3000 cycles (Figure 7d). The improved supercapacitive performance of Fe2O3/GF400 can be attributed to the strong coupling between graphene nanosheets and the incorporated Fe2O3 nanoparticles. Firstly, the incorporated Fe2O3 nanoparticles serve as spacers to alleviate restacking between graphene nanosheets. Therefore, Li+ can intercalate into and deintercalate from Fe2O3/GF400 electrode easily. Secondly, the incorporated Fe2O3 nanoparticles provide large pseudo-capacitance. Finally, the interconnected graphene network offers EDLC and highly conductive channels for charge transfer and transport. The Fe2O3/GF400 electrode was also used for electrochemical detection of H2O2. It exhibits higher electrocatalytic activity than that of GF400 (Figure S13a-b) attributable to the incorporation of Fe2O3 nanoparticles. Figure S13c shows a typical amperometric response of the Fe2O3/GF400 electrode on a successive addition of H2O2 into stirred 0.1 M PBS (pH 7.4) at an applied potential of -0.50 V. The Fe2O3/GF400 electrode exhibits sensitive response to H2O2 with a linear range from 50 µM to 1.2 mM and a LOD of 0.5 µM based on S/N = 3 (Figure S13d). CONCLUSIONS A general metal-ion mediated strategy has been developed to fabricate functionalized GFs and nanoparticles of Cu, Fe2O3, CoO and NiO have been successfully incorporated into GFs, respectively. Particularly, by employing Cu/GF800 and Fe2O3/GF400 as examples, fiber electrodes are fabricated for electrochemical sensors and supercapacitors. Due to the well-aligned and interconnected graphene nanosheets, the uniform incorporation of electrochemically active nanomaterials, and the microsize of hybrid fiber in diameter, the fiber electrodes exhibit superior electrocatalytic activity and much improved capacitance in comparison to GFs.


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EXPERIMENTAL SECTION Preparation of GO fibers by wet-spinning. GO nanosheets were prepared according to our previous studies.29-32 A wet-spinning method was used to fabricate GO hydrogel fibers. In brief, GO suspension with a concentration of 20 mg mL-1 was injected into the coagulation bath at a rate of 20 µL min-1. The coagulation bath used in this study was KOH saturated ethanol solution. GO hydrogel fiber was continuously wound around a PTFE rod. The collected GO hydrogel fiber was transferred into the mixed solution of water and ethanol (volume ratio of 1:2) to wash away the residual coagulation solution. Preparation of Cu/GF800. The obtained GO hydrogel fiber was pre-impregnated into the Cu(NO3)2 solution (the solvent was the mixture of H2O and ethanol with a volume ratio of 1:2) for 30 min. Then, the Cu2+-incorporated GO hydrogel fiber was washed and thermally annealed in forming gas (5 wt% H2 and 95 wt% Ar) at 800 oC for 2 h to obtain Cu/GF800. For comparison, GO hydrogel fiber was also thermally annealed under the same condition, which is named as GF800. Preparation of Fe2O3/GF400, CoO/GF400 and NiO/GF400. The obtained GO hydrogel fiber was pre-impregnated into the Fe(NO3)3 solution (the solvent was the mixture of H2O and ethanol with a volume ratio of 1:2) for 7 h. Then, the Fe3+-incorporated GO hydrogel fiber was washed and thermally annealed in Ar at 400 oC for 2 h to form Fe2O3/GF400. For comparison, GO hydrogel fiber was also thermally annealed under the same condition, which is named as GF400. The obtained GO hydrogel fibers were pre-impregnated into the Co(NO3)2 and Ni(NO3)2 solution for 30 min, respectively. Then, the Co2+- and Ni2+-incorporated GO hydrogel fibers were washed and thermally annealed in Ar at 400 oC for 2 h to obtain CoO/GF400 and NiO/GF400,


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respectively. Characterizations. Raman spectra were recorded on a WITeck CRM200 confocal Raman system with a laser of 514.5 nm. The morphology of the composite was examined by scanning electron microscopy (FESEM, JSM-6700F, JEOL) and transmission electron microscopy (TEM) (JEOL JEM-1400F). X-ray photoelectron microscopy (XPS) was recorded on PHI Quantera spectrometer. The loading amounts of electrochemically active nanomaterials in the functionalized GFs were determined by thermo gravimetric analysis (TGA, METTLER TOLEDO TGA2). The Cu/GF800 was tested at 800 oC for 1 h in O2 with the heating rate of 5 oC min-1 while the Fe2O3/GF400 was tested at 400 oC for 2 h in O2 with a ramping rate of 3 oC min-1. Polarized optical microscopy (POM) was performed on a Nikon LV100, and the liquid samples were loaded on a glass slide. Atomic force microscopy (AFM) was carried out using JEOL JSPM-5200. Electrochemical Measurements. Cyclic voltammetry (CV) and amperometry measurements were performed using a CHI660D electrochemical workstation (Chenhua, Shanghai). The functionalized GFs serve as the working electrode, while a platinum foil electrode and an Ag/AgCl (saturated KCl) electrode were used as the counter and reference electrodes, respectively. The fiber electrodes were fabricated according to our previous study.56 In brief, the hybrid fiber was attached to a polyethylene terephthalate (PET) substrate with one end electrically connected to a Cu wire by silver paste and the other end fixed by a scotch tape. Both ends were subsequently sealed using silicone rubber (Dow Corning). The fiber electrodes with length of 0.5 and 1 cm were used for sensing and supercapacitor measurements, respectively. The gravimetric capacitance Fe2O3/GF400 was calculated based on its weight which was measured by using a high resolution balance with an accuracy of 0.01 mg. The volume of


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Fe2O3/GF400 was calculated according to the equation of V=Scross-section × Lfiber, where Scross-section is the cross-sectional area of Fe2O3/GF400 that was calculated according to cross-sectional SEM image of Fe2O3/GF400, and Lfiber refers to the length of Fe2O3/GF400 electrode.

FIGURES:

Figure 1. Schematic illustration of the fabrication of functionalized GFs.


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Figure 2. (a) SEM image and EDS mapping of a Cu/GF800. (b) XPS spectrum of Cu 2p. (c) Raman spectra of GF800 and Cu/GF800, respectively.


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Figure 3. (a) CV curves of a GF800 electrode in the absence and presence of 3.0 mM glucose in 0.1 M NaOH. Scan rate: 100 mV s−1. (b) CV curves of a Cu/GF800 electrode in the presence of glucose with different concentrations (0, 1.0, 3.0, 5.0, 7.0 and 9.0 mM) in 0.1 M NaOH. Scan rate: 100 mV s−1. Inset: the linear calibration of the amperometric current response as a function of the concentration of glucose at +0.60 V. (c) Amperometric current response to the successive addition of glucose into 0.1 M NaOH at an applied potential of +0.60 V. Inset: amplification of the marked rectangle region. (d) The linear calibration of the amperometric current response as a function of the concentration of glucose.


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Figure 4. (a) CV curves of a GF800 electrode in the absence and presence of 2.0 mM H2O2 in 0.1 M PBS (pH 7.4). Scan rate: 50 mV s−1. (b) CV curves of a Cu/GF800 electrode in the presence of H2O2 with different concentrations (0, 1.0, 3.0, 5.0, 7.0 and 9.0 mM) in 0.1 M PBS (pH 7.4). Scan rate: 50 mV s−1. Inset: the linear calibration of the amperometric current response as a function of the concentration of H2O2 at -0.15 V. (c) Amperometric current response to the successive addition of H2O2 into 0.1 M PBS at an applied potential of -0.15 V. Inset: amplification of the marked rectangle region. (d) The linear calibration of the amperometric current response as a function of the concentration of H2O2.


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Figure 5. (a) SEM image and EDS mapping of a Fe2O3/GF400. (b) XPS spectrum of Fe 2p. The satellite peak circled in red centers at 718.9 eV. (c) Raman spectra of GF400 and Fe2O3/GF400.


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Figure 6. (a) CV curves of a GF400 electrode obtained at different cycle numbers. (b) CV curves of a Fe2O3/GF400 electrode obtained at different cycle numbers. (c) Galvanostatic charge/discharge curves of the Fe2O3/GF400 electrode at 2.0 A cm-3. (d) The dependence of capacitance on cycle numbers. (e) The intercalating mechanism of Li+ during charge/discharge cycling.


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Figure 7. (a) CV curves of GF400 and Fe2O3/GF400 electrodes in 1.0 M LiCl, respectively. (b) Galvanostatic charge/discharge curves of the Fe2O3/GF400 electrode. (c) The variation of specific capacitances versus current density for GF400 and Fe2O3/GF400, respectively. (d) Cycling performance of Fe2O3/GF400 electrode at 2.0 A cm-3.

ASSOCIATED CONTENT Supporting Information The Supporting Information provides 1) TEM and AFM images of a single layer GO nanosheet; 2) XPS C 1s spectrum of GO nanosheets; 3) Polarized optical microscopic image of concentrated GO aqueous suspension (20 mg mL-1); 4) Schematic illustration of the wet-spinning of GO


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hydrogel fiber and a photograph of a single GO hydrogel fiber wound around a PTFE rod; 5) Photograph of GO hydrogel fibers pre-impregnated in a mixed water/ethanol solution (volume ratio of 1:2) containing Cu2+ and Fe3+, respectively, and the photographs of the obtained ionincorporated GO hydrogel fiber; 6) SEM images of a GF thermally annealed at 800 oC; 7) SEM, TEM, XPS characterizations, XRD pattern and typical stress-strain curve of Cu/GF800; 8) SEM, TEM, XPS characterizations, XRD pattern and typical stress-strain curve of Fe2O3/GF400; 9) SEM, XPS characterizations and XRD pattern of CoO/GF400; 10) SEM, XPS characterizations and XRD pattern of NiO/GF400; 11) Typical stress-strain curves of CoO/GF400 and NiO/GF400 fibers; 12) Galvanostatic charge/discharge curves of the GF400 electrode; 13) The performance of Fe2O3/GF400 electrode for electrochemical detection of H2O2. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by NanjingTech Start-Up Grant (3983500150), Jiangsu SpeciallyAppointed Professor program (54935012) and the Natural Science Foundation of Jiangsu


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General Metal-Ion Mediated Method for Functionalization of Graphene

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