Oxygen Functionalities Evolution in Thermally Treated Graphene

Feb 20, 2017 - The stable GO solutions in water allow the utilization of cheap ... The presence of different oxygen groups in GO and RGO depends ... t...
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Oxygen Functionalities Evolution in Thermally Treated Graphene Oxide Featured by EELS and DFT Calculations D. D’Angelo,*,† C. Bongiorno,† M. Amato,† I. Deretzis,† A. La Magna,† E. Fazio,‡ and S. Scalese† †

CNR-IMM, Strada VIII n.5, I-95121 Catania, Italy Dip. Scienze Matematiche e Informatiche, Scienze Fisiche e Scienze della Terra, Università degli Studi di Messina, Viale F. Stagno d’Alcontres 31, I-98166 Messina, Italy

‡

S Supporting Information *

ABSTRACT: The local atomic configuration of graphene oxide (GO) was investigated by identifying the different oxygen functionalities and following their evolution induced by thermal treatments in various environments (vacuum, nitrogen or argon flow). X-ray photoelectron spectroscopy and scanning transmission electron microscopy analyses were performed, and electron energy-loss (EEL) spectra were acquired in different regions of GO and thermally reduced GO flakes. Experimental results show a series of characteristic peaks related to C and O K-edge shells and different features of GO thermally annealed at the same temperature but in different environments. In order to understand the experimental results, density functional theory calculations of core-loss EEL spectra of GO (C and O K-edges) in the presence of oxygen functional groups have been performed for different combinations and/or concentrations. Such calculations have allowed for the association of the observed experimental peaks to the presence of specific oxygen functional groups, giving the opportunity to establish the atomic configurations that prevail in different ranges of annealing temperatures and environments.

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INTRODUCTION The isolation of graphene by Geim and Novoselov opened a series of opportunities in the realization of a new class of microelectronic devices due to the elevated electrical and mechanical characteristics of single sheets of sp2-hybridized carbon atoms.1,2 Among the new typology of applications, hybrid systems composed of graphene and other materials (metals, oxides) or molecules hold great promise for future nanoscale devices.3,4 In particular, the presence of the carboxylic groups permits further derivatizations that open the intensive use in several research fields. Graphene-based materials find use in gas or biosensing,5 biomedical technology,6 photo- or electrocatalysis,7 and water purification8,9 due to the presence of oxygen-containing functional groups. For these reasons, the study of graphene oxide (GO) is currently rising. GO is composed of single or multiple sheets of graphene containing hydroxyl and epoxide groups at surfaces and hydroxyl and carboxylic groups at edges or defects.10 Chemical oxidation processes of graphite powder are able to generate graphene oxide solutions.11 The presence of the polar C−O bonds make the GO strongly hydrophilic while graphene and graphite are hydrophobic. The stable GO solutions in water allow the utilization of cheap deposition techniques as dropcasting, spraying, and spin-coating.12 Moreover, the C−O bonds open a bandgap in the GO. The size of the GO bandgap can be modulated by a reduction of the oxygen amount.3 The © 2017 American Chemical Society

realization of such reduced graphene oxide (RGO) can be achieved in several ways (through chemical or thermal treatments), and many efforts are devoted to the reduction of GO in order to massively obtain a graphene-like material.13 However, after the known reduction processes, the electrical properties of RGO are similar but worse than pure graphene. In fact, RGO shows regions where disordered carbon and oxygen functional groups are still present, mainly on edges and defect sites.14 The knowledge of the exact composition and location of the different kinds of oxygen functional groups of GO can help the understanding of the fundamental mechanisms of derivatization. The presence of different oxygen groups in GO and RGO depends on the synthesis and reduction methodologies. Usually, the content of oxygen in an as-prepared GO sample is about 40−50 at. % (mainly epoxides on the surfaces).4 This percentage can be reduced to 4−6 at. % (mainly C−OH, COOH, and CO bonds at the edges) in chemically RGO with the presence of other atomic species (∼5 at. % of nitrogen) by using hydrazine.15,16 Several studies show that the epoxides disappear after thermal treatments at temperatures between 150 and 450 °C.17,18 At temperatures higher than 700 °C, hydroxyl and carboxylic groups survive and are stable up to Received: January 9, 2017 Revised: February 16, 2017 Published: February 20, 2017 5408

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diameter quartz tube at 200 °C in N2 flow (pressure of 10 mbar) or on a hot plate in air at atmospheric pressure or at 700 °C in Ar flow (pressure of the chamber 14 mbar). All the grids were analyzed by a JEOL JEM 2010F equipped with a 200 keV field emission gun emitter and a Gatan GIF 2001 EEL spectrometer. The EEL spectra are acquired in scanning transmission electron microscopy (STEM) mode with a low dose configuration. The selected energy dispersion of the spectrometer was 0.1 eV/pixel in the case of the near-edge analysis and 0.5 eV/pixel for the quantitative analysis. In both cases, the final spectrum is the sum of 10 or 20 acquisitions at 0.2 s each, acquired during a continuous fast scan of 50 nm × 50 nm area, using 1 nm step and 10 μs per step. In this configuration, the selected area is scanned 80 or 160 times for each final spectrum (2 or 4 s in total), which hence is an average spectrum in that area. Although the total electron current used for the complete acquisition is high, the dose on each point is very low, considering that the sample is not continuously hit in the same point during the scan. In order to verify that the continuous fast scan does not modify the analyzed region, we compared a spectra series acquired in the same area as a function of time. No substantial difference in the spectrum after 1 min scanning was observed (not shown). The relative O:C atomic ratio of the samples was calculated considering the ratio between the areas of C 1s and O 1s core signals normalized for the corresponding photoemission cross sections, averaging the results of more than five different regions in the sample. The XPS analysis was performed by using a Thermo Scientific K-Alpha system with a monochromatic Al Kα (1486.6 eV) source. The photoelectron spectra were collected by a CAE analyzer using a pass energy of 200 and 50 eV for the survey and high-resolution spectra, respectively. A flood gun was used to decrease the effect of surface electrical charging. Density functional theory calculations were performed for graphene oxide systems with different oxygen functional groups in order to compute core-loss (C and O K-edges) EEL spectra. We used the WIEN2K24 full-potential augmented-plane-wave code within the Perdew−Burke−Ernzerhof implementation of the generalized gradient approximation.25 The structural models were based on a spatial repetition of periodic (2 × 2) and (3 × 3) graphene supercells functionalized with either a single type or a combination of epoxides (C−O−C) and C− OH groups. Calculations were also performed for O atoms in the proximity of vacancies. A plane-wave cutoff parameter of RMTKmax = 7 was considered, where RMT is the muffin tin radius (the smallest radius considered in the unit cell) and Kmax is the plane wave cutoff, along with and a sampling of the hexagonal Brillouin zone with 100 k-points for the electronic accuracy of the calculations. A vacuum space of 15 Å was introduced along the c-axis in order to minimize the interactions between the periodic replicas of the slab. All structures were fully relaxed until forces were less than 0.04 eV/Å. EEL spectra were computed with the TELNES3 postprocessing program using a Gaussian broadening of 0.5 eV.

1000 °C.16 The main characterization techniques used to determine the kind and the amount of oxygen functional groups are X-ray photoelectron spectroscopy (XPS) and infrared (IR) spectroscopy.16,18,19 However, these techniques provide an average information since they are coming from large regions (generally about 0.4 mm of diameter for XPS and 0.5 cm for IR spectroscopy). Recently, we have reported how it is possible to identify and localize at atomic resolution the different oxygen functionalities on a graphene surface by electron energy-loss spectroscopy (EELS) analyzing the C and O K-edges spectra.20 Usually, the C K-edge EEL spectrum of GO shows a multipeak structure in the 1s−π* transition. We found different EEL peaks of the C K-edge region with intensities depending on the presence of C−OH or C−O−C on the graphene surface, as confirmed by density functional theory calculations of the electronic structure with different oxygen functional groups. Other papers report the simulated and experimental EELS spectra at the C K-edge of GO in comparison with graphite or amorphous carbon10 or when oxygen atoms are located at the edge of a graphene sheet.21 Recently, Tararan et al. showed the characteristic peaks of the C 1s EEL spectra for GO and RGO sheets at different C:O atomic ratio22 acquired at liquid nitrogen temperature and low electron dose. They attributed the C 1s EEL peaks to a high concentration of C−OH in the as-prepared GO. Nevertheless, the used dehydration process and redispersion in ethanol can modify the original concentration of hydroxyl groups of GO.23 Moreover, we found a multipeak structure in the O K-edge EEL spectrum of GO, and we attributed the peaks at about 530.5 and 533 eV to oxygen bound at graphene divacancies and to epoxides, respectively.20 Nevertheless, experimental EEL spectra of the O 1s region at different concentration levels after reduction processes have not been reported yet in the literature. The preparation and reduction strategies of GO affect the presence of different oxygen functional groups and their identification requires the use of a technique able to characterize GO at atomic resolution. In this work, we study the modification and evolution of the EEL spectra features of both C and O K-edges of GO after thermal processes in a range of temperatures between 200 and 700 °C, in different environments (ultrahigh vacuum, N2 or Ar flow, atmospheric air). X-ray photoelectron spectroscopy supports the determination of the type of oxygen functionalities present in the processed GO samples. Density functional theory (DFT) calculations are performed in order to compute core-loss (C and O K-edges) EEL spectra of graphene oxide containing epoxides (C−O−C) and hydroxyls (C−OH) with variable concentrations and synthesis in order to identify the different oxygen functional groups in the GO surface at atomic resolution.

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EXPERIMENTAL AND THEORETICAL METHODS GO in water solution (2 g/L) was acquired by Sigma-Aldrich. The solution was diluted at a concentration of 100 mg/L in deionized water and was sonicated for 30 min. Drops of GO solution were placed into separated copper grids on a glass slide. The grids were dried in a desiccator for 2 days. Successively, some copper grids containing the GO suspension were loaded into the column of a JEOL JEM 2010F transmission electron microscope at ultrahigh vacuum (base pressure of about 10−10 mbar) and annealed in situ for 30 min at different temperatures (200, 300, and 430 °C). Some other copper grid samples were processed for 30 min in a 2 in.

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RESULTS AND DISCUSSION In a previous experimental and theoretical work, we have studied the EEL spectra in order to discriminate at atomic resolution the presence of hydroxyl or epoxy groups on the graphene surface by analyzing the peak position of C and O Kedges.20 5409

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Figure 1. Electron energy-loss spectra of carbon (a) and oxygen (b) K-edges for as-prepared GO, thermally treated GO in the TEM chamber (from 200 to 430 °C) or in a furnace at 700 °C in Ar flow.

and it needs to be investigated as we will discuss after the theoretical calculations of EELS spectra in the presence of different oxygen functional groups. The C and O K-edge electron energy-loss spectra of GO annealed at 700 °C in Ar flow are also reported in Figure 1 (cyan curves). Usually, this thermal treatment induces serious changes in the GO structures. The epoxides disappear, and only hydroxyl, carboxyl, and carbonyl groups survive.15,19 Indeed, we observe that the signals at 287 and 533 eV correlated to the presence of epoxides disappear. The EEL spectra are normalized to the intensities of σ* peak positions, and in the case of O K-edge the signal-to-noise ratio is very low for annealed GO at 700 °C due to the reduction of the oxygen atomic content after the annealing. Therefore, in the O K-edge only the signal at about 539 eV survives, and the signal-to-noise ratio is very low due to a strong reduction of the oxygen content. The strong peak at about 285−286 eV in the C K-edge can be attributed to hydroxyls, but also to sp2 and sp3 carbon and to other oxygen functionalities of GO that are stable at edges or defects as we will explain below. Figure 2 shows the C and O K-edges energy-loss spectra of GO samples processed at the same temperature (200 °C) but in different environments. Each spectrum of Figure 2 is the sum

In this work, we investigate the effect of annealing temperature and environment on the O functional groups present on the GO sheets. Figure 1 shows the carbon (a) and oxygen (b) K-edge energy-loss spectra of GO before (black curve) and after 30 min of in situ annealing in the TEM chamber at 200, 300, and 430 °C (red, green, and blue curves, respectively) and after 30 min of annealing in furnace in Ar flow at 700 °C (cyan curve). All the spectra reported in Figure 1 are obtained as the sum of at least ten single EEL spectra of each sample acquired in different regions in order to draw an average situation of each processed GO sample. The C K-edge region (Figure 1a) shows two main peaks: the first one between 284 and 288 eV and the second one at 292 eV due to the 1s−π* and 1s−σ* electronic transitions, respectively. The C K-edge region of pristine graphene (not shown) shows two main peaks at 285.2 and 292.0 eV due to the 1s−π* and 1s−σ* electronic transition, respectively. The multipeak structure of the 1s−π* region of GO was explained in our previous paper with a prevalence of different oxygen groups.20 The peak at 287 eV is the indication of the presence of epoxides on the GO surface, and the peak at 285.5 eV is related mainly to hydroxyl groups. The epoxide peak is more intense in GO (black curve of Figure 1a), and after thermal treatments the intensity of this peak decreases until it almost disappears at 430 °C, whereas the hydroxyl peak is still evident. The O K-edge spectra (Figure 1b) show three main contributions at about 530.5, 533.5, and 538.5 eV. The two peaks at lower energy loss can be assigned to oxygen atoms bound to carbon vacancies (or at edges) and to epoxides, respectively.20 All the samples show the main contribution at 538.5 eV. While the shape of the O K-edge energy loss is the same for GO and GO processed up to 300 °C in situ in the TEM chamber, the peak at 533.5 eV disappears in the EEL spectrum of GO thermally treated at 430 °C (blue dotted curve of Figure 1b) and a weak signal appears at about 532.5 eV. Red stars in the graph of Figure 1b highlight the positions of the observed O 1s peaks. The data of Figure 1 show that for an annealing temperature of GO in UHV at 430 °C the epoxide signal is no longer detectable in the C and O K-edge energy-loss spectra. This is in agreement with the thermogravimetric and IR analyses of GO annealed under vacuum18,19 reported in the literature, showing that GO loses epoxides at temperatures between 150 and 400 °C. Moreover, the peak at 532.5 eV of annealed GO at 430 °C has not yet been observed in the literature, to our knowledge,

Figure 2. C and O K-edge electron energy-loss spectra of GO annealed at 200 °C in situ TEM (black curve), in air (red curve) on a hot plate, and in N2 flow (green curve) in a quartz tube for 30 min. 5410

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Figure 3. C 1s and O 1s XPS signals of GO (black curves), annealed GO at 200 °C in air (red curves) or N2 flow (green curves), and annealed GO at 700 °C in Ar (blue curves). The positions of the different kinds of bonds are reported in the graph.

graphs at the corresponding binding energy. The C 1s emissions of samples annealed at 200 °C are very similar, while the O 1s signal of sample annealed in N2 flow has a shift in the peak position with respect to the sample annealed in air at the same temperature. For the annealing at 700 °C strong variations take place for both C 1s and O 1s spectra. In order to get more detailed information, we report in Table 1 the relative intensities (%) of the deconvoluted XPS peaks for

of several spectra (>10) acquired in different areas of the same sample. The previous procedure assures the representation of the average situation of each sample. The profile of the C Kedge energy loss of GO annealed at 200 °C in air (red curve of Figure 2) is different from GO annealed in situ in the TEM chamber (UHV, black curve of Figure 2) and from GO annealed in N2 flow (green curve of Figure 2). The shape of the EEL profiles shows many differences. After processing in N2, the peak at 285 eV is more intense than the one observed in the sample processed in UHV or in air while the peak attributed to epoxides at 287 eV disappears as well as the one at 533 eV. The EEL spectra of GO annealed in air (red curves of Figure 2) appear similar to a nondefined carbon structure (amorphous carbon), and the O K-edge profile is quite similar to GO annealed in situ in the TEM chamber. The experimental EEL spectra of Figure 2 suggest that the ambient influences the kind of oxygen functionalities left after annealing process at the same temperature. In order to understand the different oxygen functionalization after thermal annealing, we reported in Figure 3 the C 1s and O 1s XPS spectra of GO (black curves), annealed GO at 200 °C in air (red curves) or N2 flow (green curves) and annealed GO at 700 °C in Ar (blue curves). The intensities of the spectra of Figure 3 are normalized to the XPS peak intensities at 284.5 and 532 eV of GO for C 1s and O 1s emissions, respectively, for a better visualization. The XPS C 1s emission (Figure 3a) of GO shows two main peaks at about 284.5 and 286.5 eV that can be attributed to CC and C−O bonds, respectively. In detail, according to the literature data,15,16,18 the C 1s XPS signal of GO can be deconvoluted considering many contributions: sp2 carbon bonds at 284.2 eV, sp3 carbon bonds between 284.5 and 285.0 eV, C−H, C−OH, and C−N bonds at about 286.0 eV, C−O (mainly epoxides) at about 286.5−287.0 eV, CO (carbonyl) at 288.7 eV, and COOH (carboxyl) at 291.0 eV. In some cases, the distinction between epoxides and hydroxyls is not discussed, and a unique peak at about 286.5 eV is reported for C−O bonds.4,15 The O 1s XPS feature of GO (Figure 3b) can be reproduced using four characteristic contributions, located at about 535, 533.5, 532, and 531 eV, due to O−H bonds (presence of water), O bound to an aromatic carbon atom (phenols, OH at the edge in the case of graphene oxide), O bound to one carbon atom (both hydroxyl and epoxides on the GO surface), and OC bonds, respectively.18 The different kinds of bonds are indicated in the

Table 1. Peak Positions and Intensities (Calculated as % Area) of the Deconvoluted XPS C 1s (a) and O 1s (b) Peaks of Figure 3a area % (a) C 1s peak 2

sp C sp3 C C−OH C−O CO COOH

position [eV]

GO

200 °C air

284.2 285.0 286.0 286.8 287.7 289.1

12.9 33.6 5.9 40.3 4.8 2.5

4.1 47.8 16.7 18.1 9.3 4.0

200 °C N2 6.9 45.8 17.0 18.1 8.3 3.9 area %

700 °C Ar 20.2 51.5 14.3 6.9 3.6 3.5

(b) O 1s peak

position [eV]

GO

200 °C air

200 °C N2

700 °C Ar

OC O−C O−C aromatic O−H

531.0 532.0 533.0 535.0

20.6 72.0 6.5 0.9

11.4 33.5 48.6 6.50

5.8 70.8 19.2 4.2

18.7 10.8 62.0 8.5

a

The deconvoluted spectra are reported in Figures S1 and S2 of the Supporting Information.

GO, annealed GO at 200 °C in N2 flow or in air and annealed GO at 700 °C in Ar. The deconvoluted spectra are reported in Figures S1 and S2 of the Supporting Information. The deconvolution confirms that after the annealing process at 700 °C the signal due to C−O (epoxides) decreases significantly, while the signal related to hydroxyls (phenols, OH at edges in the case of GO) shows an increase. In this case, a correlation between OH at edges observed in the XPS O 1s signal can be correlated to the peak at 286 eV of the XPS C 1s signal, while the peaks at 286.8 and 532.5 eV should be assigned to C−O (both epoxides and hydroxyls on the surface of GO). Carboxyl and carbonyl groups show less dramatic 5411

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∼450 °C. The hydroxyl groups are smaller in number in the asprepared GO and are stable at temperatures up to 1000 °C.15 Particularly, the data of Figure 4 show two main effects: the first one is the reduction of the oxygen content by increasing the temperature, as expected for the thermal reduction of GO; the second one is the dependence of the total oxygen content on the annealing ambient for processes at the same temperature. The O/C ratios of GO annealed at 200 °C in air are below the values measured in GO annealed at the same temperatures but in N2 flow (10 mbar) or in UHV (10−10 mbar), respectively. In particular, the O/C ratio of GO annealed at 200 °C in air is lower than in the sample annealed in UHV and similar to the O/C value of the sample annealed in UHV at 300 °C (see Figure 4). Therefore, the reduction of GO is decreased in an inert ambient. This effect needs further studies. However, the experimental peaks of carbon K-edges at 287 eV and oxygen K-edges at 533 eV (epoxides) are clearly defined and more intense in the 1s−π* region of the EEL spectrum when the O/C ratio is higher than 0.15 (see Figures 1a and 4). The value of the O/C ratio of GO processed at 700 °C is about 0.075. Considering the data obtained by the XPS analysis of GO processed at 700 °C, the oxygen content is nearly all due to hydroxyl, carbonyl and carboxylic groups at this temperature. Moreover, Figure 2 shows that the EELS peaks at lower energies in the C and O K-edges follow the same energy differences and signal profiles. Peculiarly, the similar shapes of the C 1s and O 1s EELS signals of GO annealed at 200 °C in N2 flow or at 200 °C in air (respectively green and red curves in Figure 2) require further deduction. In order to explain in detail the observed experimental EEL spectra, Figure 5 shows the calculated spectra for GO with different types as well as concentrations of oxygen functionalities. The O-related contributions in the C K-edge peak are positioned above the energy loss value of ideal sp2 carbon (285.2 eV), except for the case of oxygen bound to vacancies (or edges) of the sample. For epoxide and hydroxyl groups instead, the main factor that influences the C K-edge resonance is the O concentration. Hence, upon equal concentration, hydroxyl groups give rise to peaks with a lower resonance with respect to epoxides (285.9 and 287.3 eV for 12.5% of O/C ratio, respectively). However, by increasing the oxygen percentage, a general shift of the C K-edge toward higher energies is observed for all types of oxygen functionalities. Therefore, it becomes clear that a definite attribution of an EELS peak for the C K-edge spectrum of GO is plausible only when the concentrations of the various functionalities are known. Similar considerations can be made for the O K-edge spectrum: peaks at ∼530 eV can be only attributed to oxygen groups bound to defects or edges, while intense peaks at ∼540 eV should mainly have a hydroxyl character with a high O concentration. In the region between 532 and 534 eV, both epoxides and hydroxyl groups give rise to core-loss peaks, with the 533 eV peak having mainly an epoxide character. Finally, as in the case of the C K-edge, we point out the importance of the O concentration on the shape of the EEL spectrum. The graph of Figure 4 shows that after thermal annealing the concentration of O is about 13 at. %. In this case the consideration of one O atom for a 2 × 2 graphene supercell fits well with the experimental O/C atomic ratio for GO annealed in the temperature range between 200 and 450 °C (see Figure 4). Moreover, the calculated EEL spectra show similar shape and the same distance between the positions of the peaks in the O and C K-edges for a specific configuration at O/C ratio of

variations, and an increase of the CC contribution is observed at 284.2 eV. In the case of annealing at 200 °C in N2 flow, the XPS O 1s signal suggests the presence of a large contribution due to oxygen atoms single bonded to one carbon atom (both epoxides and hydroxyls on the GO surfaces). Here the strong peak at about 285 eV of the C K-edge EEL spectrum in conjunction with the high percentage of O in the sample suggests the presence of a large part of hydroxyls with respect to epoxides. The high percentage of O−H for the sample annealed in air at atmospheric pressure suggests the capture of water from the atmosphere, but the shape of O 1s XPS signal is very similar to that of initial GO. In order to understand the correlation between oxygen concentration of GO and the detection range of the different peaks associated with the oxygen functionalities, we have calculated the O and C concentrations of the GO samples from EELS and XPS spectra, as explained in the Methods section. We note that the spatial resolution of XPS is about 400 μm × 400 μm, while the spatial resolution of the experimental EEL spectra is of the order of a few angstroms (we considered for EELS characterization an average of at least 10 or 20 spectra in different selected areas to achieve an average situation). Figure 4 shows the O/C atomic ratio of GO as a function of the annealing temperature of the thermally treated samples for

Figure 4. O/C atomic ratio of thermally treated GO at different temperatures and ambients, obtained by EELS and XPS analyses.

30 min in different ambient: in situ TEM (black circles), in Ar flow (blue triangle), in N2 flow (green triangle), in air on a hot plate (red square). The O/C ratio of GO without any thermal treatment is reported at 25 °C in the graph. The O/C ratio obtained by the XPS analysis (open symbols) of the initial GO and GO annealed at 700 °C in Ar flow are reported for comparison with the values calculated from the EELS analysis. The O/C atomic ratio of as prepared GO is 35.2 ± 4.5%. XPS analysis reports a concentration of 40.6 ± 3.0%. The O/C ratio of GO annealed at 700 °C is 9.1 ± 0.6% and 8.6 ± 0.5% for the EELS and XPS analyses, respectively. Hence, XPS confirms the O/C values obtained by the EELS spectra within the statistical error. Usually, several studies report that at room temperature the GO surface can retain gas and water. The water and gas are removed from the GO surface at temperatures higher than 100−120 °C. The epoxides are the main oxygen group of asprepared GO, and they are reactive species surviving up to 5412

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Figure 5. Simulated C (a) and O (b) K-edges electron energy loss of different graphene supercells. Bridge configuration represents the epoxides. The notation 12.5% and 25% (value of O/C ratio) denote the presence of one or two oxygen functionalities on the 2 × 2 graphene supercell, respectively. The configuration labeled “vacancies” refers to an oxygen atom at a divacancy of a 3 × 3 graphene supercell.

Thermal treatments of GO induce a modification of oxygen evolution, causing a decrease of the O/C ratio as the annealing temperature is increased. At temperatures below 400 °C, for a O/C ratio higher than 0.15, the EELS peak of epoxides are more intense. Samples annealed at the same temperature (200 °C) but in various environments show a similar O/C atomic ratio but different EELS peaks at the C and O K-edges. XPS analyses confirm the presence of OH at GO edges (phenols) evidenced by a strong peak at 533 eV in the O 1s XPS signal and a peak at 286.0 eV in the C 1s. The C 1s and O 1s XPS peaks at 286.8 and 532.5 eV can be attributable to the signals due to carbon single bonded to oxygen atoms (both C−OH and C−O−C) at the GO surface. EELS analyses suggest that after thermal treatments of GO at 200 °C in N2 flow the oxygen functionalities evolution leads to the formation of hydroxyls, while for the same thermal annealing performed in air or in vacuum epoxides prevail. Further investigations are required to clarify these behaviors. In general, the simulation of the EELS spectra in the presence of different oxygen functionalities at various oxygen content on the GO surface helps the understanding of the experimental results. For example, we have shown that by increasing the oxygen percentage, a general shift of the C Kedge toward higher energies is observed for all types of oxygen functionalities. Similar considerations can be made for the O Kedge spectrum for the hydroxyl functionalities. The correlation between experimental EEL spectra and DFT calculations allowed us to correlate some experimental features observed for O K-edge spectrum to a local enrichment of OH functional groups on the GO surface, as theoretically calculated by Lu et al.26

25%. This effect is more evident in the case of high concentration of oxygen in the bridge 25% + OH configuration (two epoxides and one OH on the 2 × 2 graphene supercell). Finally, the weak experimental peak at about 532.5 eV of GO annealed at 430 °C in UHV (blue curve of Figure 1b) is found to be centered at an energy-loss position between the one related to the simulated EEL spectrum in the presence of OH with a O/C ratio of 12.5% (531.9 eV) and of 25% (532.7 eV) on GO. This result could be consistent with a reduction of epoxides making the hydroxyls peak evident in the O K-edge EELS signal. The position of this peak is very close to the one associated with a high concentration of OH groups on GO. Theoretical ab initio calculations26 show that OH functional groups on the GO surface tend to agglomerate; hence, even though the average total oxygen concentration decreases down to a value of 11.5 at. % for GO annealed at 430 °C (O/C ratio of about 0.13; see Figure 4), the O K-edge EELS feature can originate from regions with higher local O concentration.

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CONCLUSIONS A combined EELS and XPS study of the oxygen functionalities evolution of GO after thermal treatments in various environments is reported. A series of characteristic peaks in the C and O K-edges of GO detected at resolution of the EELS probe size (a few angstroms) can be correlated to the presence of different oxygen functionalities. The experimental results are supported by theoretical calculations of the EEL spectra of various atomic configurations of oxygen functional groups, for O/C ratio values of 12.5% and 25% in graphene supercells. The different oxygen functionalities contribute to the C and O K-edges EEL spectra as follows: oxygen in vacancies (or edges), hydroxyls, and epoxides produce signals at 284−285 and 530.5 eV, at 285.5 and 532 eV, and at 287 and 533 eV, respectively. 5413

DOI: 10.1021/acs.jpcc.7b00239 J. Phys. Chem. C 2017, 121, 5408−5414

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The Journal of Physical Chemistry C

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b00239. Deconvolution of the C 1s and O 1s XPS spectra shown in Figure 3; Figure S1: GO and GO annealed for 30 min in Ar flow at 700 °C; Figure S2: relative to GO annealed at 200 °C in N2 flow or in air (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (D.D.). ORCID

D. D’Angelo: 0000-0003-3534-1633 I. Deretzis: 0000-0001-7252-1831 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.D. acknowledges the funding support of the PON R&C 2007−2013 national programme, Project “Hyppocrates− Sviluppo di Micro e Nano-Tecnologie e Sistemi Avanzati per la Salute dell’uomo” (PON02 00355). M.A. acknowledges the Project ‘‘Formazione di tecnologi esperti in materiali, processi e modellizzazione per l ‘elettronica su supporti flessibili− PLAST_Ics’’ (PON02_00355_3416798-F).

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DOI: 10.1021/acs.jpcc.7b00239 J. Phys. Chem. C 2017, 121, 5408−5414