Modulation of Oxygen Content in Graphene Surfaces Using

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Modulation of Oxygen Content in Graphene Surfaces Using Temperature Programmed Reductive Annealing: Electron Paramagnetic Resonance (EPR) and Electrochemical Study Ortal Marciano, Shmuel Gonen, Naomi Levy, Eti Teblum, Reut Yemini, Gilbert Daniel Nessim, Sharon Ruthstein, and Lior Elbaz Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02987 • Publication Date (Web): 11 Oct 2016 Downloaded from http://pubs.acs.org on October 17, 2016

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Modulation of Oxygen Content in Graphene Surfaces Using Temperature Programmed Reductive Annealing: Electron Paramagnetic Resonance (EPR) and Electrochemical Study Ortal Marcianoa,*, Shmuel Gonena,b*, Naomi Levya,b, Eti Tebluma,b, Reut Yeminia,b, Gilbert Daniel Nessima,b, Sharon Ruthsteina, and Lior Elbaza,b** a

Chemistry Department, Faculty of exact Sciences, Bar-Ilan University, Ramat Gan, 52900, Israel

b

Bar Ilan Institute for Nanotechnology and Advanced materials (BINA), Bar-Ilan University, Ramat Gan, 52900,

Israel

KEYWORDS reduced graphene oxide, EPR, surface functionalities Abstract: The oxidation level and properties of reduced graphene oxides (rGOs) were fine-tuned using temperature programmed reductive annealing. rGOs were -annealed at different temperatures (from 500°C to 1000°C) in hydrogen to modulate their oxidation levels. The surface of the rGOs was fully characterized using Electron Paramagnetic Resonance backed by Raman, XRD, and chemical analysis measurements. These experiments were used to study the changes in the surface of the reduced graphene oxide, its surface functionalities, and its defects as a function of the reduction temperature. In addition, electrochemical measurements to quantify the oxidation level of the rGOs offer a simple tool to correlate rGOs properties to their structure. Finally, we explored the effect of the different levels of reduction on the conductivity, capacitance, and surface reactivity. This research offers simple methodological techniques and routes to control and characterize the oxidation level of bulk quantities of reduced grapheneoxide.

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Introduction In recent years, graphene has been marked as a promising form of carbon due to its mechanical strength, high electrical and thermal conductivity, and very high surface area.1, 2, 3, 4 The interest and use of graphene has increased as a consequence of the simplification of its production methods, which were followed by upscaling and cost reduction.5, 6 Graphenes are used today in a wide array of applications from energy conversion and storage7,

8, 9, 10

to electronics11,

electrocatalysis12, sensors13 and medicine.14 Single crystal monolayer graphene (SCMG), which is usually produced using catalytic chemical vapor deposition (CVD)15, 16, 17 is almost impossible to produce in large scale due to its complicated synthesis and high cost.18,19,20,21 Hence, other graphene-based materials, mainly reduced graphene-oxide (GO), which resembles the structure of pure graphene, was developed. GO can be produced by oxidizing graphite flakes via synthetic routes in bulk quantities at high yields, without the need for expensive production methods, using the Hummers method and its modified protocols.22,20,23 In these methods, graphene oxide is exfoliated from graphite and reduced via a variety of synthetic routes24 to produce a graphene-like structure referred to as reduced graphene-oxide (rGO), sometimes mistakenly also called graphene (although it is not SCMG). During the reduction process, defects are formed in the GO, which are then present in the rGO (illustrated in Figure 1), thus affecting its properties.

* Equal contribution ** Corresponding author: E-mail address: [email protected]


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Figure 1. Illustration of the structural changes in GO reduced at different temperatures (to give semi-reduced GO and rGO). In black: carbon; In red: oxygen moieties.

The GO shares with graphene the carbon layer lattice but does not possess the same sp2 hybridization and electrical conductivity. Hence, it is considered as a good insulator.20,24 During its reduction, the aromatic structure of graphene can be restored to some extent as well as the sp2 hybridization, thus recovering some of the desired properties in SCMG including its conductivity.20,18,24 However, defects such as vacancies that occur during the reduction as a result of the departure of oxidized carbon groups such as CO2 and CO appear.20,24,25 Although these defects are manifested in an increase in the D/G ratio in the Raman spectrum,24 the conductivity of the rGO is much higher than that of the GO from which it originated from.24,26,25 In contrast to the vast research literature on SCMGs, especially for electronic applications27, the scalable fabrication of rGO is lagging behind. In the grand scheme, low cost SCMG-like materials that could easily be fabricated on a large scale from graphite flakes are very desirable for


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applications where bulk amounts of “graphene” are required, such as electrodes for batteries, supercapacitors, and fuel cells.28, 29, 30 Using in-situ transmission infrared absorption spectroscopy, Acik et al.31 demonstrated how the annealing temperature affects the oxidation level of rGO. They investigated the effects on rGO for short annealing duration and temperatures up to 850 °C. Others32,33 annealed GO in tube furnaces up to 1000°C, but under inert Ar atmospheres (Ar or N2). In this work, we modulated the oxidation level of rGO using temperature-programmed reduction (TPR)31 using a tube furnace.34 We analyzed the reduction processes of long durations (one hour) and annealing temperatures up to 1000 °C under hydrogen. Most importantly, we characterized the unpaired free electron populations in the various rGO samples, using Electron Paramagnetic Resonance (EPR) measurements. We compare GO with rGOs at various reduction levels and we introduce a simple electrochemical tool to study their oxidation/reduction levels and to correlate the rGO oxidation level to its properties. 1. Experimental 2.1 Synthesis Graphite flakes were oxidized using the modified Hummer's method.35 In short, 9:1 mixture of concentrated H2SO4/H3PO4 (360:40 mL) was added to a mixture of graphite flakes (3.0 g) and KMnO4 (18.0 g), producing a minor exotherm to 40 °C. The reaction was then heated to 50 °C and stirred for 12 hours. The reaction was then cooled to room temperature and decanted onto ice (400 mL) with 30% H2O2 (3 mL). For workup, the mixture was washed by extensive centrifugation cycles with water (12,000 rpm for 4 h) followed by a dialysis for a week. When the solution pH reached pH = 7, it was centrifuged again (12,000 rpm 4 h), and finally dried by lyophilization, yielding 5 g of graphite oxide. The obtained graphite oxide (yellowish solid) was


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exfoliated to form graphene oxide (GO) sheets by half an hour of ultrasonication (brown solution). We used a three-zone tube furnace (chemical vapor deposition (CVD)) to anneal our GO under a flow of hydrogen (reducing agent) to perform temperature-programmed reduction (TPR) and reduce GO to form reduced graphene oxide (rGO) (black solid). The temperature was varied in the TPR annealing step to modulate the reduction of GO in order to form intermediate GOs with controlled oxidation levels. Specifically, dried GO was loaded into a quartz tube and inserted into a tube furnace and preheated to 500, 600, 700, 800, 900 and 1000 °C for an hour under a flow of a gas mixture of hydrogen (400 sccm) and argon (100 sccm), where hydrogen is the reducing agent to reduce GO into rGO.34 ` The process used the fast-heat technique,36, 37, 38 in which the quartz boat with GO inside was initially positioned in the portion of the quartz tube outside the heated zone of the furnace. While maintaining a flow of argon, a fan was blowing on the exposed section of the quartz tube outside the furnace (with the GO sample inside) to be kept at room temperature while the furnace was ramped to the desired annealing temperature. Once the equilibrium temperature was reached, the annealing gas mixture was introduced and the outside section of the quartz tube was shifted inside the furnace, thus positioning the sample in the annealing zone and starting the annealing process. 2.2 Electron Paramagnetic Resonance (EPR) Spectroscopy EPR spectra were recorded using an E500 Bruker Elexsys spectrometer operating at 9.0–9.5 GHz at room temperature. The spectra were recorded at a microwave power of 2.0 mW, a modulation amplitude of 1.0 G, and a time constant of 60 ms. The samples were measured in 3.0 mm quartz tubes. The g-values were measured using bisdiphenylene-b-phenylallyl (BDPA) and diphenyl-picrylhydrazyl (DPPH) standards. The spin concentrations were evaluated using known calibrated solutions of DPPH. ACS Paragon Plus Environment

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2.3 Raman Spectroscopy Raman spectroscopy of all samples was conducted in a backscattering configuration using a micro-Raman spectrometer HR 800 (Jobin Yvon Horiba), with a He–Ne laser (excitation line 632.8 nm) and a microscope objective (50×, Olympus LWD). 2.4 Electrochemistry Solution containing 1 mL solution of 2:1 (volumetric) isopropanol: deionized (D.I.) water and 0.2 %wt Nafion® (Ion Power DUPONTTM D2020 Nafion® Solution) was added to 1 mg of the graphene sample to make uniform slurry. 5 µL of the slurry was applied on a glassy carbon surface of a rotating disk electrode (disk area of 0.196 cm2) and was dried under vacuum for 10 min. This method of electrode preparation was used for all of the GO and rGO samples studied in this work. Cyclic voltammetry (CV) measurements were performed using a BioLogic VSP potentiostat. All measurements were conducted in a 0.5 M aqueous solution of H2SO4 (Acros Organics 96%). The counter electrode was a glassy carbon rod (φ = 3 mm) and the reference electrode was a homemade hydrogen reference electrode [Pt|H2(g)|H+(aq)(0.5M)]. During the experiment, the electrochemical cell was purged with Ar (99.999% purity). 2.5 Conductivity measurements The electronic conductivity of the bulk GO and rGO powders was measured using a 2-point method developed by Elbaz et al.39 A small portion of support powder (typically 25–30 mg) was placed into a Teflon die (0.25’’ dia) and compacted by two 0.25’’ diameter alumina rods with tips covered in Au foil. The alumina rods were 2-bore in configuration permitting Au wire to make contact with the Au foil. The ceramic powder was compacted using an outside micrometer that served two roles: to measure the thickness of the compacted powder and to apply the same ACS Paragon Plus Environment

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pressure of 7.5 N on the various samples using the instrument’s friction thimble. Once the powder was compacted and the sample thickness determined, the 2-point conductivity of the sample was measured using a Fluke multimeter (Fluke 87V). The conductivity of the powder was then calculated using the sample thickness and diameter. All measurements were conducted at least ten times and an average value was taken. 2.6 Elemental Analysis Determination of relative weigh of H, C, N and S was achieved using Flash EA (1112 series) device. Samples were burned at 10000C to give H2O, CO2, N2, SO3 or SO2. Determination of relative weigh of O was achieved using Flash EA (1100 series) device. The samples were burned at 10600C. 2. Results and Discussion 3.1 EPR The chemical reactions of carbon-based materials involved in the atmospheric oxidation process are complex and have been focused for decades of research.40,

41

The oxidation properties of

carbon-based samples are directly linked to their electrochemical properties. Green et al. have been investigating the oxidation properties of various carbon-based materials using EPR spectroscopy.40, 42, 43, 44 The properties and characteristics of free radicals in carbon materials are important towards understanding their chemisorption, stability, and electrochemical properties. The EPR spectrum of carbon radicals is usually characterized by a single broad resonance. In such cases, it is possible to obtain only the g-value, the line-width, and the spin concentration. The g-values of EPR spectra can be used to determine whether a radical is carbon-centered, carbon-centered with an adjacent oxygen atom, or oxygen-centered. The basic equation for the electron Zeeman interaction, E = hν = gβeB0, defines the g-value, where h is the Planck


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constant, ν is the constant microwave frequency applied in the experiment, B0 is the resonance magnetic field, βe is the Bohr magneton.45 The g-value for a free electron is 2.00232. Variations in the g-value from the 2.00232 are related to magnetic interactions involving the orbital angular momentum of the unpaired electron and its chemical environment. Organic radicals usually have g-values close to the free electron g value (2.0023), which depends on the location of the free radical in the organic matrix. Carbon-centered radicals have g-values that are close to the free electron g-value of 2.0023. Carbon-centered radicals with an adjacent oxygen atom have higher g-values in the range of 2.003–2.004, while oxygen centered radicals have g-values that are >2.004. The g-value of 2.0034–2.0039 is characteristic of carbon-centered radicals in a nearby oxygen heteroatom resulting in increased g-values over that of a purely carbon-centered radical.46, 47, 48 In this study, we compared the properties of various reduced graphene oxides (rGOs) that were reduced at different temperatures, between 500 to 1000 °C. The rGO samples were recorded by continuous wave EPR (CW-EPR). The spectra are presented in Figure 2A. Several interesting features were observed in the EPR spectra. First, the EPR spectrum rGO reduced at lower temperature (i.e. rGO @ 500ºC) is much broader than rGO reduced at higher temperature (i.e. rGO@ 1000ºC). Such broad signal can be owing to the presence of Mn2+ paramagnetic ions, which might be generated while preparing the rGO samples.49 However, we have not succeeded to observe the six-lines signal, typical for Mn2+, therefore, we believe that owing to long dialysis process, we succeeded to get rid of most of the manganese. Hence, we assign the broadening of the spectrum toanisotropy of the g-tensor. Anisotropy signal is usually obtained for radicals that are not distributed uniformly in the bulk. Inductively coupled plasma (ICP) measurements were used to check if manganese residues in the samples. Small amounts of manganese were detected


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in the GO and rGO samples (up to 0.017 mM in the rGO) which are below the detection level of the EPR.


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Second, a change in the g-value was detected for the various rGO. A decrease in g-value occurs, as the reduction temperature increases. (g=2.06 for rGO@500 °C, vs. g=2.004 for rGO@1000 °C). The decrease in the g-value suggests that upon reduction at higher temperatures, there are less oxygen molecules on the graphene oxide surface, and a shift from oxygen-centered radicals to carbon-centered radicals occurs. In addition, a narrower signal was observed for the rGO@1000 °C, a compared to the samples reduced at lower temperatures. The line-width of the EPR signal is governed by the spin–lattice relaxation process. Interaction between adjacent


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radicals or between radical and molecular oxygen results in a decrease in the spin–lattice relaxation time, and hence, an increase in the line-width.50, 51 Therefore, the comparable narrow line width for rGO@1000 °C indicates that the free radicals are more spread apart upon reduction. In addition, the number of spins for rGO at higher temperatures is much higher than the number of spins for rGO reduced at lower temperatures (Table 1), and the g-value of these radicals are correlated with carbon centered radicals. Hence, upon reduction, there is transfer from oxygen centered redicals to carbon

Figure 2. (A) CW-EPR spectra of rGO reduced at various temperatures between 500°C to 1000°C. (B) Schematic presentation of oxygen centered radicals (red) for rGO reduced at 500°C and carbon centered radicals (light blue) distribution for rGO reduced at 1000°C.


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centered radicals, and the amount of the latter is higher. In summary, the EPR data indicates that rGO reduced at lower temperature, is characterized by lower amount of oxygen-centered radicals on the surface of the graphene oxide (Figure 2B), manifested by broad anisotropic spectrum of g>2.05, owing to the two-dimensional distribution of spins at oxidized environment. The rGO reduced at higher temperatures is characterized by homogeneous distribution of carbon-centered radicals, and therefore it's spectrumis characterized by a sharp isotropic pronounced singlet line at around g=2.004(see Figure 2B). These differences between the radical nature and distribution of spins in the rGO samples is expected to affect, among other properties, such as the electrochemical behavior of the samples. Table 1: Number of spins (#/cm3) of the various rGO samples, at various times after exposure to nitrogen atmosphere.1 0s

1

1000s

3000s

rGO@5000C

1.776 ∙ 10

2.148 ∙ 10 1.936 ∙ 10

rGO@6000C

2.186 ∙ 10

2.378 ∙ 10 3.350 ∙ 10

rGO@7000C

4.219 ∙ 10

3.575 ∙ 10 3.439 ∙ 10

rGO@8000C

3.672 ∙ 10

3.028 ∙ 10 3.952 ∙ 10

rGO@9000C

4.721 ∙ 10

4.384 ∙ 10 4.776 ∙ 10

rGO@10000C

10.1 ∙ 10

9.439 ∙ 10 8.021 ∙ 10

The relative error is 0.05-0.1%

In order to explore the stability of the various rGO samples to oxidizing environment, the change in the EPR line shape as a function of exposure to anaerobic environment was studied (nitrogen atmosphere).

The standard measurement was composed of three stages: (1)

establishing a baseline under air atmosphere for several scans (2) nitrogen gas is flowed at a pressure of 1.2 atm and the sample is continuously measured until equilibrium is reached (3) the ACS Paragon Plus Environment

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flow is stopped and the sample is exposed to air. A comparison between the change in the linewidths and intensity of the four samples under N2 is shown in Figure 3. It is clear that we obtained different ranges of line-width, as the maximal line- width is for rGO@500 °C sample. The line-width decreases gradually, to the lowest values for rGO@1000 °C. This decrease in line-width indicates a decrease in dipolar interaction between the radicals.

Additionally,

although all samples show small changes in linewidth and signal’s intensity, rGO samples that were reduced at lower temperatures, exhibit larger fluctuations in the linewidth. The fact that no significant changes are observed for rGO samples when exposed to nitrogen, after adsorbed oxygen is removed, suggests that the surface of the rGO samples is quite packed and dense, and there are relatively few adsorbed oxygen molecules on the surface. This is in contrast to coal samples, where adsorbed oxygen was clearly detected by EPR.40,

43

Interestingly, during the

nitrogen exposure, a new narrow signal (g=2.002) arises with time (in nitrogen atmosphere) for the rGO@500 °C sample (see Figure 4A). The intensity of this signal increases, until a steady state is achieved around 600 sec (Figure 4B). Naturally, under the experimental conditions it is not likely that bonds are being cleaved. In addition, the spin concentration was found to be constant during the exposure to the gas for all rGO samples (Table 1). Hence, the g = 2.002 signal must be due to Heisenberg spin exchange interaction that extremely broadens the signal under atmospheric conditions. When exposed to the gas flow, the Heisenberg exchange interaction is reduced and this species is resolved. Therefore, it indicates that for rGO@500 °C sample, some oxygen is adsorbed on the surface of carbon atoms. When this oxygen is removed under nitrogen atmosphere, the carbon-centered radicals (g=2.002) are exposed (Figure 4C).


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Figure 3. The change in the line width (A) and EPR signal’s intensity (B) for the various rGO samples as a function of exposure time to N2 gas.

Figure 4. A. CW-EPR spectra of rGO reduced at 500°C, at various times after exposure to N2 gas. (B) The change in the signal’s intensity of the narrow component signal at g=2.002, during exposure to N2 atmosphere. (C) The physical separation model of the carbon radicals on the rGO surface by nitrogen gas.

3.2 Raman Spectroscopy


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Raman spectroscopy was measured for all of the samples and presented in Figure 5. The spectrums of the various rGOs have two well-defined peaks, D and G, at 1325 and 1592 cm−1, respectively. The latter peak, G, is due to Eeg C═C stretching vibrations in graphene planes of the cluster. Whereas, the D peak is attributed to a breathing mode of A1g symmetry, which is considered as forbidden in perfect crystalline graphite and only becomes active when it is disordered.52 When analyzing the D and G bands of the reduced graphene oxide samples, it appears that the higher the temperature of reduction, the higher the disorder of the graphene structure. This is also consistent with the EPR results that showed a more ordered two-dimensional distribution of radicals in rGO samples reduced at lower temperatures.

In the method used here for the

reduction of GO, where the exposure time to the reducing environment is very short, as the temperature of reduction is increased, larger amount of the oxygen groups detaches from the GO surface, leaving behind highly reactive “dangling bonds”. These “dangling bonds”, if not stabilized at high temperatures for a substantial time to form a homogenous aromatic structure, will cause a disordered, sp3-rich material. Hence the increase of the D band at higher temperatures of reduction and an increase of the D/G ratio (inset of Figure 5). The oxygen functional groups are usually located on the carbon vacancies and boundaries, during the reduction, some of the oxygen leaves, and the defects are formed on the vacant positions. This phenomana was also observed by us in the past.42 For coal samples with higher oxygen content, the second carbon radical species (g~2.002), shown in Figure 4, are more pronounced. Together with some more recent findings,53 this might suggest that carbon-based materials with higher oxygen content suffer from more defects on their surfaces, this initiates higher activity, and the formation of radicals on the surfaces of the carbon based materials.


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Normalized Raman Intensity (a.u)

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D

1.0

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1.5 1.0 0.5 0.0

Sample

G

GO rGO 500 rGO 600 rGO 700 rGO 800 rGO 900 rGO 1000

0.8 0.6 0.4 0.2 0.0 1200

1300

1400

1500

1600 -1

1700

1800

Wave Number (cm ) Figure 5. Raman spectra of rGO reduced at varying temperatures (Inset: the ratio between the D and G bands).

3.3 X-Ray Diffraction XRD measurements were conducted with the GO and the rGO powders (Figure 6), using Cu Kα as the X-ray. A GO sample has a typical (002) peak at around 2θ = 10°.54,55,56All of the reduced samples show typical broad (002) peaks at 2θ = 25°.54, 56 The d-spacing, calculated from Bragg’s law,57,58 slightly decreases as the temperature of reduction is increased (see Table 2). This trend can be explained by the decrease in the oxygen moieties on the graphene surface, as the temperature of reduction increases, and is in good agreement with the EPR, Raman and elemental analysis results (given in Table 3) As a consequence, as the layers get closer to each other, the electrical conductivity between the layers is expected to improve.59,60 The larger the amount of oxygen groups on the graphene surface, the more distant the layers are. This is also supported by the EPR measurements. In addition, the EPR measurements demonstrated that rGO samples reduced at lower temperatures can rapidly react with nitrogen and air, this suggests that there are larger spaces between the rGO layers, and that the GO and rGO reduced at lower temperatures are expected to show enhanced carbocatalysis when compared to the relatively packed rGO, reduced at higher temperatures. Moreover, the EPR ACS Paragon Plus Environment

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spectrum of rGO at lower temperatures was characterized by an anisotropic spectrum compared to the isotropic spectrum observed for rGO reduced at higher temperatures. This suggests that the layers are independent of each other, and thus the spectrum reassemble more of a twodimensional graphene surface rather than a three-dimensional homogeneous distribution of spins. Based on these observations, the average width of the stacked graphene layers was determined from the (002) peak’s FWHM by the Scherrer equation (Table 2),

58,61

and the number of

graphene layers can hence be estimated from the crystallite width and the d-spacing between the layers according to Bragg’s equation58: 2d sin θ =n λ where n is the diffraction order and equals to one and λ is the X-ray wavelength and is equal to 0.154 nm. The height of the stacked graphene layers, L, was calculated using the Scherrer’s equation: =

cos

where B is the ∆ (2θ) (peak breadth) in radians, K is the shape factor and will be considered as 0.9. From this we were able to extract the number of graphene layers shown in Table 2.


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7000 O

rGO @ 1000 C

6000 O

rGO @ 900 C

5000

Intensity a.u.

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O

rGO @ 800 C

4000 O

rGO @ 700 C

3000 O

rGO @ 600 C

2000 O

1000

rGO @ 500 C GO

0 10

15

20

25

30

35

Two Theta Figure 6. XRD measurements of GO reduced at various temperatures.

Table 2: crystallographic analysis of the various rGO samples Sample

2θ

d (nm)

L (nm)

Layers

GO

10.90

0.811

9.727

12

rGO-500

24.82

0.358

3.111

8-9

rGO-600

24.93

0.357

2.970

8-9

rGO-700

25.28

0.352

3.410

9-10

rGO-800

25.43

0.350

2.935

8-9

rGO-900

25.53

0.349

3.028

8-9

rGO-1000

25.53

0.349

3.035

8-9

Table 3. Elemental Analysis (% of carbon and oxygen) for the various rGO samples.


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3.4 Electrochemistry Carbonaceous materials usually have quinone / hydroquinone-like moieties on their surface which are formed when they are oxidized. These moieties have well-known redox activity according to:

In the case of GO and rGO, this redox activity can be exploited using simple electrochemical techniques in order to quantify the amount of surface oxides on the GO or rGO, which can be tied to their properties (conductivity, capacitance, etc.). The GO and rGO samples in this work were characterized using cyclic voltammetry (CV). Since in this work the GO and rGO were studied in bulk quantities and not as monolayers, all available surface area had to be reached by the electrolyte in the solution, and in order to assure it will, relatively large quantity of polyelectrolyte, Nafion, was added to the studied GO and rGOs suspensions as described in the experimental section, afterwards, the samples were dried on glassy carbon electrodes and


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undergone potential sweeps from 1.2 to 0.0 V vs. RHE at a scan rate of 200 mV/s. A broad peak between 0.4 V and 0.8 V vs. RHE was observed in the CVs of all samples (Figure 7), which is attributed to the redox of the Quinone/Hydroquinone groups on the material’s surface.62,63,64

0.1

I (mA)

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0.0 rGO 500 rGO 600 rGO 700 rGO 800 rGO 900 rGO 1000 GO

-0.1

-0.2 0.0

0.2

0.4

0.6

0.8

1.0

1.2

E (V vs.RHE)

Figure 7. CVs of GO and GO reduced at different temperatures on glassy carbon electrode measured in deaerated (with Ar) 0.5 M H2SO4 at 200 mV/s (all measurments were conducted at room temperature).

The peak charge was calculated for each of the samples and is presented in Figure 8. The larger the peak area, the higher the concentration of quinone groups on the surface. There is a clear decrease in the peak charge as the reduction temperature rises. This is in good agreement with the EPR, Raman, XRD and elemental analysis results. It is important to note that even the rGO reduced at the highest temperature is not fully reduced and has significant amounts of oxygen and therefore, oxidized carbon. The electrochemical measurements conducted with the GO and rGOs can also give information regarding the capacitance of the rGO; from the voltammograms we can conclude that the capacitance, also decreases when the reduction temperature of the GO rises as was expected64, indicating a loss in the total surface area of the rGO.65 Two electrons are taking part


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in each quinone/hydroquinone (Q/H2Q) redox; therefore, we can also quantify the amount of these groups on the carbon surface.

Figure 8. The amount of charge integrated from the Q/H2Q peaks in Figure 7.

3.5 Conductivity Although most work conducted to date relating to the conductivity of graphenes and their derivatives focused on single atomic layers, the bulk conductivities may become more and more interesting since these materials are expected to be implemented in energy technologies such as capacitors, batteries and fuel cells. Considering that the single-layer properties of these materials cannot be preserved in bulk due to the layer-layer interactions shown in the XRD and EPR measurements, it is important to study their bulk properties. In order to demonstrate a general trend for the conductivity of the rGOs produced in this work, as an example for how their properties could be controlled and fine-tuned, we measured the bulk-conductivity of GO and rGOs using a technique developed and used by us in earlier studies.39 As expected, the


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conductivity of the rGOs increased with the annealing temperature, up to 7.27 S/cm at 1000°C. This is over two orders of magnitude when compared to the conductivity of the rGO that was annealed at 500°C (0.06 S/cm), and almost seven orders of magnitude more conductive than pristine GO (3.64*10-7 S/cm) (Figure 9). The EPR data also showed a decreased amount of free radicals for sample reduced at lower temperatures, where most of the free radicals are located at the surface of the graphene and not in the bulk (Table 1). This significant change is attributed to the formation of new sp2 bonds during the reduction process.24 In fact, a significant increase in conductivity is already reached at relatively low reduction temperature with rGO@ 500 °C that shows 5 orders of magnitude increase in the conductivity. This is also in good agreement to the electrochemical measurements and elemental analysis results shown earlier. 8 7 6

Conductivity (S/cm)

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5 4 3

Sample Conductivity S/cm GO 3.64E-07 rGO 500 0.06 rGO 600 0.13 rGO 700 0.18 rGO 800 1.65 rGO 900 2.72 rGO 1000 7.27

2 1 0

GO

rGO 500 rGO 600 rGO 700 rGO 800 rGO 900 rGO 1000

sample Figure 9. Conductivity of bulk GO and GO reduced at varying temperatures.

3. Conclusions Although very important, studies with bulk quantities of GO and rGO are not as common as those for monolayers of graphene and GO. Using very simple methods we have shown fine ACS Paragon Plus Environment

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control over the oxidation level of rGO, characterized it, and controlled its properties. The GO studied in this work was reduced in a controlled manner to obtain the desired oxidation levels which translated to its defects, conductivity, capacitance, and surface reactivity. Using EPR spectroscopy, we demonstrated how rGO annealed at low temperature exhibited smaller amount of free radicals, which were distributed in a non-uniform way. The fact that lowtemperature rGOs are more sensitive to the atmosphere suggests that the layers are more spread apart. This is consistent with the Raman analysis, also showing that rGO reduced at lower temperature was more ordered, and with XRD showing that the separation between the graphene layers gets smaller as the amount of oxygen groups on the surface decreases with the rise in the annealing temperature of the rGO. Lastly, the conductivity measurements confirmed that when there are less free radicals distributed in a two-dimensional ordered phase (i.e. rGO @ 500°C) the conductivity is lower. The Raman spectra shows the existence of more defects in the rGOs reduced at higher temperatures. The activity of the rGO increased with the reduction temperature. We observed that more oxygen groups departed from the GO surface which resulted in higher concentration of defects (carbon radicals) and increased the rGO activity. From the EPR measurements, it is apparent that in these rGOs there is an increase in the concentration of carbon radicals which replaced the oxygen radicals during the annealing process. The carbon radicals are electrically conductive, in contrast to the oxygen radical which are more dominant in the rGOs treated at lower temperatures. This phenomenon can explain the trends in the conductivity, in addition to the narrowing of d-spacing. Electrochemical methods were used to quantify the surface oxidation levels, which correspond to the amount quinone-like molecules: A simple technique for the study of rGO oxidation level.


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This work is a significant advance to make rGO a suitable material for applications that require graphene-like materials in bulk quantities such as electrodes for batteries, supercapacitors, and fuel cells.

Acknowledgments LE would like to thank the Israeli Ministry of Energy and Space for partially funding this project (grant no. 024-11-215). LE and GDN would also like to thank the Israel Science Foundation and Israel Prime minister’s Office fuel alternatives initiative for partial funding of this work under the Israel Research center for Electrochemical Propulsion (INREP) (Grant no. ISF 2797/11). References 1.

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Graphical Abstract

rGO reduced at different temperatures show significant changes in surface chemistry and properties: EPR and Electrochemical characterization


Modulation of Oxygen Content in Graphene Surfaces Using

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