Article pubs.acs.org/JPCC
Atomic Oxygen on Graphite: Chemical Characterization and Thermal Reduction Rosanna Larciprete,*,† Paolo Lacovig,‡ Sandra Gardonio,‡,§ Alessandro Baraldi,∥,⊥ and Silvano Lizzit‡ †
CNR-Institute for Complex Systems, via Fosso del Cavaliere 100, I-00133 Roma, Italy Sincrotrone Trieste S.C.p.A., Strada Statale 14 Km 163.5, I-34149 Trieste, Italy § University of Nova Gorica, Vipavska 11c, 5270 Ajdovscina, Slovenia ∥ Physics Department and CENMAT, University of Trieste, Via Valerio 2, I-34127 Trieste, Italy ⊥ Laboratorio TASC CNR-IOM, Strada Statale 14 Km 163.5, I-34149 Trieste, Italy ‡
S Supporting Information *
ABSTRACT: The chemisorption of O atoms on graphite and the thermal reduction of the oxidized surface were studied by means of high energy resolution photoelectron spectroscopy with synchrotron radiation. The C 1s and O 1s core levels and the valence band spectra were used to identify the different oxidizing surface species and to evaluate the extension of the sp2 conjugation as a function of oxidation time and annealing temperature. We found that epoxy groups are the dominant species only at the low oxidation stage, and ethers and semiquinones form as oxidation proceeds. The evolution of the ether/epoxy ratio with increasing oxygen coverage provides evidence for the occurrence of C−C bond unzipping. Epoxy groups are the functionalities with the lowest thermal stability and start to desorb around 370 K, strongly affecting the desorption temperature of other functional groups. The ratio between ethers and epoxy groups determines the balance between epoxy− epoxy and epoxy−ether reactions, the latter promoting the removal of C atoms from the C backbone. Adsorbate spectroscopy during thermal annealing definitely proves the catalytic effect of the basal plane oxygen atoms on the desorption reactions.
1. INTRODUCTION Exfoliation of oxidized graphite is a cost-effective and upscalable route for the production of monolayer or few-layer graphene oxide (GO) patches which can be easily manipulated and reduced for eventual integration in electronic and optoelectronic devices.1−4 Oxygen functionalities can be removed from GO in a direct and easily accessible manner by thermal annealing.5−15 However, the issue which still hampers the performance of reduced GO is the deterioration of the transport properties, as vacancy formation and inclusion of foreign species in the C grid severely limit the conductivity with respect to that of undoped graphene.10−13 A decrease of the defect density would open the way to large-scale deposition of graphite oxide sheets for device applications. The chemistry of oxidized graphite during exfoliation and the macroscopic response of the thermally reduced GO sheets might largely vary depending on the nature and concentration of the oxidizing species attached to the honeycomb network.5,7,10,13−15 Defect density and chemical composition of the thermally treated materials are determined by the mutual interactions and by the thermal reactions taking place among the oxygenated functional groups, with each one having its characteristic desorption temperature. The need to control crystalline quality and chemical degradation in reduced GO requires an atomistic understanding of the thermal evolution of © 2012 American Chemical Society
the O-carrying species in relation to their chemical surroundings on the oxidized graphitic surface. Due to the importance of oxidized graphite as intermediate material for the production of GO flakes, in this study we have investigated the oxidation of the highly oriented pyrolitic graphite (HOPG) surface with atomic oxygen. The detailed scenario, which is possible to outline by using exclusively O atoms as oxidative agents, will facilitate the understanding of more complex systems such as GO produced by wet processing routes, which comprises also H-containing oxidative groups.16,17 Oxygen atoms bind to the basal plane of graphite in bridge position over the C−C bonds, forming epoxy groups.16,18−23 Calculations showed that in the three-member epoxy ring the length of the C−C bond located below the O atom increases from 1.42 to 1.51−1.58 Å,20,21,24,25 which is close to the sp3 bond length in diamond. Different aspects concerning epoxy configurations and stability on the hexagonal grid have been addressed theoretically. It was found that the interaction between epoxy groups is attractive up to a coverage of 50%, and epoxy clustering is energetically favored with respect to a random surface distribution.21,22 Models based on the density Received: October 12, 2011 Revised: April 18, 2012 Published: April 19, 2012 9900
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Figure 1. (a) C 1s and (b) O 1s core level spectra measured on the HOPG surface as a function of the oxidation time. For a better visualization the spectra have been vertically offset. The inset in (a) shows the epoxy peaks occurring around 286.2 eV normalized to the same height. (c−e) Decomposition of the (c, d) C 1s and (e) O 1s spectra shown in (a) and (b). The vertical scales are adjusted to better show the spectral features. In each case the black dots and the red lines reproduce the experimental and the best-fit curves, respectively. For peak assignment, see the text. (f) C 1s spectra measured at normal (θ = 0°) and grazing (θ = 60°) emission on the HOPG oxidized for 21 min. (g, h) Intensity of the O 1s and C 1s components vs the oxidation time. The main O-containing functional groups are sketched in the upper left panel.
epoxy groups and epoxy pairs have been proposed by theory.25,31,32 Moreover, for partially oxidized GO a phase separation in contiguous oxidized and bare graphene domains is commonly predicted,16,31−33 as in the case of partially hygrogenated graphene.34 Conversely, the most energetically stable configurations provide strain compensation by O adsorption on the opposite graphene sides,25,32,31,35 which is unrealistic on the surface of bulk graphite. It must be noted that the ratio between zipped and unzipped epoxy groups might influence the reduction modality and the rate of thermal deoxygenation. The release of CO and CO2, which above 600− 670 K36−38 arises from the desorption of O atoms bonded to graphitic edges or vacancy sites, can be dramatically anticipated by the presence of nearby epoxy groups.26,29,39−41 These species, once diffusing on the surface, might recombine and desorb as O2 without causing lattice damage,29 but have also the propensity to act as reacting agents able to catalyze reducing reactions and gasification in oxidized graphitic materials.26,29,39−41 In this study we followed the room temperature (RT) oxidation of HOPG with oxygen atoms up to the extensive disruption of the sp2 network. We used high energy resolution
functional theory (DFT) approach indicated that on small graphene patches aligned rows of epoxy oxygens in a thirdneighbor configuration are the most stable,21 whereas on larger graphene patches such an arrangement is disfavored and epoxy oxygens prefer to adsorb as second neighbors.25,26 The former model establishes that, due to the cumulative cleaving force exercised on the underlying C−C bonds,21,27 the aligned epoxy groups tend to unzip into ethers being definitely incorporated into the C basal plane. On the contrary, according to the second model, the second-neighbor configuration is more stable and the epoxy assemblies maintain the unzipped geometry.26 The experimental results are also intriguing: unzipping of aligned epoxy groups has been proposed as responsible for initiating the fault lines observed in GO,21 but epoxy defect lines were not observed in the transmission electron microscopy (TEM) images of reduced GO monolayers.28 Moreover, on corrugated graphene grown on Ir(111), the epoxy groups are stable up to the saturation of O atom adsorption and the concentration of ethers, fingerprints of the occurrence of unzipping, is negligible.29,30 For graphene partially or totally oxidized exclusively by O atoms, a few structural motifs comprising zipped and unzipped 9901
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grazing incidence (50°), and photoelectrons were collected at an emission angle of 20° from the surface normal. Two different procedures were used in the thermal desorption experiments: in the fast XPS acquisition mode the O 1s and C 1s spectra were measured while the oxidized HOPG was annealed. Alternatively, the sample was heated in steps and each time high resolution photoemission spectra were acquired at RT.
X-ray photoelectron (XPS) spectroscopy with synchrotron radiation to monitor the chemisorption of O atoms on HOPG and to identify the species residing on the surface as a function of the oxidation time. Then, for samples oxidized at different levels, fast XPS was exploited to monitor the thermal reduction obtained by controlled thermal annealing up to nearly complete deoxygenation. We found that epoxy is the dominant surface species only at the low oxidation stage. With increasing oxidation level other functionalities such as ethers, semiquinones, and lactones form, which require the presence of unsaturated C−C bonds at the edges of the basal plane or at the periphery of vacancies.42 Epoxy groups are found to be the least thermally stable species: depending on the oxidation level they desorb around 370 K and strongly affect the thermal stability of the other functional groups.
3. RESULTS AND DISCUSSION 3.1. Oxidation of Graphite. Figure 1a,b shows the C 1s and O 1s spectra measured on the HOPG samples exposed to atomic oxygen for increasing time. The C 1s curve measured on the clean sample, reported at the bottom of Figure 1a, consists of the sp2 component located at 284.3 eV. The interaction with O atoms for 3 min determines the appearance of a peak around 286.2 eV. With increasing oxidation time (tox) the main peak becomes progressively asymmetric, gains a high binding energy (BE) shoulder, and finally manifests a second shoulder on the low BE side. The peak at ∼286.2 eV also broadens and smears out with oxidation time (see inset in Figure 1a). The O 1s spectra reported in Figure 1b show, for low O exposure times, the presence of nearly a single peak at 531.2 eV. With increasing oxidation a second component appears at ∼533 eV and later a shoulder grows around 529.7 eV. The C 1s and O 1s core level spectra were fitted with Doniach−Šunjić functions convoluted with Gaussians (see the Supporting Information). The C 1s spectra were decomposed into six components (C2−C6) besides the main sp2 peak named C1 (see Figure 1c,d). The comparison between the surface-sensitive (θemiss = 60°) and the bulk-sensitive (θemiss = 0°) C 1s spectra measured at tox = 21 min (see Figure 1f) shows a strongly higher relative intensity of the C2−C7 components in the spectrum measured at grazing emission, indicating for all of them a surface localization with respect to C1. Component separation was carried out also for the O 1s spectra, which were decomposed into three peaks (O1−O3), as shown in Figure 1e. The intensities of the C 1s and O 1s components as a function of the oxidation time are displayed in Figure 1, parts g and h, respectively. The C 1s spectra in Figure 1c show that in the very early oxidation stage the main sp2 peak at 284.3 eV undergoes a line shape modification that cannot be reproduced by a Gaussian broadening of the component C1. This is accounted for by the superposition of C1 and a new component, C2. The two components overlap for tox = 3 min and then split under the effect of the rising oxidation. The component C1, which almost keeps the original line shape, represents sp2 C−C bonds in the regions containing only unoxidized C rings at the surface or in deeper layers. The broader C2 component arises from the C−C bonds surrounding or in the close vicinity of the chemisorbed O atoms that, according to DFT calculations, manifest C 1s shifted components45 in agreement with the structural modification25 and the charge redistribution46 in the sp2 lattice. The C2 intensity as a function of tox (see Figure 1g) indicates at first the expansion of the oxidized domains, which comprise a rising number of C−C bonds perturbed by O chemisorption, and later the decay of these C−C bonds, as they progressively transform into C−O bonds. The components C4 and O1 that appear for tox = 3 min at 286.2 and 531.2 eV, respectively, are fingerprints for the formation of epoxy groups.29,30,45,47 With increasing oxidation C3 and O2 emerge with BE shifts of 0.9 and 1.4 eV with respect to C4 and O1, respectively. As similar
2. EXPERIMENTAL SECTION The experiments were performed in the ultrahigh vacuum (UHV) chamber (base pressure 8 × 10−11 mbar) of the SuperESCA beamline at the Elettra synchrotron radiation facility (Trieste, Italy). The HOPG sample was mounted in a folded Ta frame with a 6 × 6 mm2 front side window to expose the HOPG surface. The mechanical and electric contact between the Ta foil and the HOPG was secured by two Ta clips. The HOPG surface was peeled off in air and immediately after was loaded into the vacuum chamber and annealed to ∼970 K. The sample was heated by electron bombardment on the back of the Ta holder, and the temperature was measured with an uncertainty of ±5 °C by a K-type thermocouple spotwelded to the Ta frame. The cleanliness of the sample was carefully checked, taking care that no contaminant was detected in the XPS spectra. Atomic oxygen was produced by a radio frequency plasma source (TECTRA, Gen2) equipped with an ion suppressing grid and an ion trap. The atomic source current and the O2 pressure were kept constant at 20 mA and 8 × 10−6 mbar, respectively. The sample was kept at RT and exposed to atomic oxygen for increasing time. The geometry usually chosen for oxygen functionalization is with the atomic source facing the back of the sample: this configuration reduces the atomic flux on the sample surface but on the other hand minimizes the damage induced by energetic ions.29 In this case, due to the low adsorption efficiency of O atoms on the flat graphite,43,44 oxidation was performed with the source at a grazing angle with the HOPG surface. For low oxidation levels we checked that, by properly selecting the exposure time, dosing from the back or at the grazing angle produced equivalent effects on the sample and similar surface concentrations of the different oxygen functionalities were observed. After each oxidation step the surface was analyzed by photoemission. At the end of each oxidation−reduction cycle the HOPG sample was extracted from the UHV chamber and peeled off several times in order to remove the top layers and regenerate the surface. C 1s and O 1s core level spectra were measured at photon energies of 400 and 650 eV, respectively, corresponding to comparable kinetic energy values of ∼110−113 eV. The energy resolution was of 80 and 150 meV for C 1s and O 1s measurements, respectively. The valence band was measured at a photon energy of 130 eV with an energy resolution of 60 meV. For each spectrum the binding energy position was calibrated by measuring the Fermi level position of an Au reference sample in good electrical contact with HOPG. The measurements were performed with the beam impinging at 9902
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BE shifts with respect to the epoxy peaks were measured45,47 and calculated48 for ether groups, C3 and O2 are assigned to C and O atoms forming in-plane C−O−C ether bonds. The C5 and C6 components appearing at higher tox are attributed to C atoms forming CO and O−CO bonds with O atoms respectively, likely in semiquinones, carbonyls, and lactone groups,29 and in the O 1s spectrum are paralleled by the O3 component representing the CO bonds. The component C7 emerging at BEs lower than that for C1 (see Figure 1d) can be related to atoms surrounding bare and oxidized C vacancies, which exhibit negative chemical shifts up to ∼1.1 eV.45 The proliferation of vacancies after a long exposure to the oxygen plasma is due to the action of the energetic atoms and ions that activate the erosion of the oxidized sp2 lattice even at RT. The oxygen coverage θox for the sample dosed for 3 min was estimated from the C 1s line shape to be ∼0.14 ML (1 ML = 1 monolayer = 3.8 × 1015 atoms/cm2) (see the Supporting Information). The total coverage at the end of the oxidation, evaluated from the intensity of the O 1s spectra, is θox ∼ 0.53 ML. For tox ≥ 6 min the BEs of all C 1s and O 1s components start to move and the shifts increase with oxidation time (see Figure S1 in the Supporting Information). The C2−C6 components drift all together to higher BE, maintaining a nearly constant relative energy separation, and all the O 1s components follow a similar behavior. On the other hand, the main sp2 component C1 moves toward lower BEs. Note that we have not observed any charging effect or instability nor any effect induced by the high X-ray photon flux. Moreover, the interface charge configuration built up during each oxidation step remained stable in the sample as far as it did not undergo other treatments. Therefore, the observed behavior manifests a dipole formation at the interface between the positively charged C grid and the negatively charged graphite oxide domains. The low BE shift of the sp2 peak reveals the hole doping due to charge transfer to the oxidized domains, whereas the high BE shifts of the components arising from the oxidized domains likely reflect also a work function increase.49 The O 1s spectra in Figure 1 indicate that at low O coverage the bridge sites over the C−C bonds are almost the exclusive adsorption sites for O atoms which form solely epoxy structures,29,30 but with increasing oxidation time the relative concentration of ether groups rises considerably. Ether groups might reside in the graphite basal plane or at the vacancy and edge sites: in the first case they arise from the inclusion of O atoms into seven member or larger rings in the C network, likely via unzipping processes, whereas in the second case ethers result from the oxidation of undercoordinated C atoms. In order to ascertain the origin of the ethers revealed in the HOPG surface, it should be considered that the latter route, that is, the oxidation of defect sites, implies that the increase of the ether concentration is accompanied by a simultaneous increase of the number of semiquinones, as both groups quickly saturated bare C vacancies.42,45 This is well exemplified in Figure 2, where the C 1s and O 1s spectra measured after the pristine HOPG surface (pr-HOPG) is oxidized for 15 min are compared with the spectra measured after the same oxidation time on the HOPG surface (d-HOPG) where the defect density had been increased on purpose by means of oxidation− reduction cycles. The ratio between the ether and epoxy intensities IO2/IO1 rises from 0.36 to 0.55 when going from prHOPG to d-HOPG (see Figure 2c). In the latter case the relative increase of O2 is accompanied by the appearance of the
Figure 2. (a) C 1s and (inset) O 1s spectra measured on pristine HOPG (pr-HOPG) and on defected HOPG (d-HOPG) oxidized for 15 min. (b) Decomposition of the O 1s spectra shown in the inset of (a); the black dots and the blue lines reproduce the experimental and the best-fit curves, respectively. (c) Ratio between ether intensity IO2 and epoxy intensity IO1 measured on the pr-HOPG vs oxidation time. The red point shows the IO2/IO1 value measured on d-HOPG for tox = 15 min.
O3 component due to CO bonds and by a higher photoemitted intensity around 284 eV in the C 1s spectrum, in correspondence with the C7 component arising from C atoms surrounding oxidized and bare vacancies (see Figure 1d). Therefore, in the presence of defects both ethers and semiquinones become accessible adsorption configurations in competition with epoxy sites. For that reason at least up to tox = 15 min, the absence of CO bonds (C5, O3) in the C 1s and O 1s spectra (see Figure 1) suggests that the observed ethers are not due to defect oxidation but result from unzipping processes. It has been recently shown that the formation of unzipped C−O−C bonds is driven by the local strain induced by the O adspecies.50 Although at RT the oxygen diffusion is not active,24,27 long graphene exposures to O atoms increase the probability for the adsorption of epoxy oxygens as first neighbors of other epoxy groups already residing on graphene, thus forming the precursors for unzipping. The rise of the IO2/ IO1 ratio with oxidation time in Figure 2c indicates that the stability of the epoxy groups on the flat graphitic surface drops with the increase of their surface density. Conversely, when O atoms adsorb on graphene supported by Ir(111), the epoxy groups remain stable without unzipping into ethers up to O saturation coverage, likely because the strain is conveniently released by the more flexible layer structure modulated by the moirè corrugation.29,30 The other reaction route yielding ethers, that is, the oxidation of defect sites, dominates at longer oxidation times upon the occurrence of severe damaging in the C network. The C 1s spectra in Figure 1d show that for tox ≥ 27 min the component C7 increases dramatically, revealing a high defect density in the graphitic lattice. Vacancy proliferation manifests in Figure 2c 9903
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with the remarkable increase of the ratio IO2/IO1 for tox ≥ 27 min. The detrimental effect of O adsorption on the π conjugation is pointed out by the evolution of the valence band (VB) spectra with oxidation time, which are shown in Figure 3 as
core levels the most noticeable trend evident in Figure 4a,c is the decay of the features due to epoxy groups (peaks at 286.2 and 531.2 eV) which around 370 K start to decrease rapidly with increasing temperature.46,29 The whole thermal evolution of the C 1s core level is illustrated by the intensity plot in Figure 4b obtained from the sequence of spectra acquired while heating the sample with a rate of 0.5 K/s. The curve on the left side of Figure 4b shows the temperature dependence of the C 1s intensity at 286.2 eV and depicts the rapid epoxy decay between 370 and 570 K. An analogous behavior is revealed by the analysis of the O 1s spectra. The decomposition of the O 1s core levels measured before and after annealing at 523 and 873 K is summarized in Figure 4d, whereas Figure 4e displays the O 1s component intensities vs temperature. As observed for the C 1s spectra, the epoxy component O1 drops between 370 and 570, and simultaneously the ether component O2 increases. Slightly above 470 K the component O3 due to CO bonds emerges together with a new component, O4. The small energy shift of 0.6 eV between O4 and O3 suggests that both represent double CO bonds in different chemical surroundings. Whereas O3 appears at 470 K and then stays almost constant up to 870 K, O4 shows a slight increase and then a decrease with the temperature, indicating that it is part of a surface intermediate, and on the basis of the following discussion can be related to the formation of unstable lactone-containing surface structures. The modification of the chemical surroundings for the epoxy groups during reduction is manifested by the progressive high BE shift of the peak at 286.2 eV in the C 1s spectra (inset in Figure 4a). Moreover, in agreement with the behavior of the component O2 in Figure 4e, starting from 486 K the C 1s spectra show an increase of the spectral intensity around 285.5 eV, in correspondence with the ether component C3. Thermal annealing to 1373 K is necessary to restore the C 1s line shape of the pristine HOPG. However, even after this treatment the presence of residual topological defects, likely nonhexagonal rings or irregular lattice regions resulting from C−C rebonding,28,53 attenuates the intensity of the π band in the VB spectrum of the reduced HOPG (see Figure S2 in the Supporting Information). The thermal behavior of the HOPG sample with θox ∼ 0.53 ML (tox= 36 min) is summarized in Figure 5. In this case the sample surface comprises, in addition to epoxy groups, a large amount of ethers and a considerable concentration of CO bonds. Figure 5a shows the O 1s intensity plots obtained from the series of XPS spectra measured while heating the oxidized HOPG at a rate of 0.5 K/s. The high resolution O 1s and C 1s spectra measured before thermal annealing and on the sample heated at 660 and 1020 K are also shown. Figure 5c shows the analysis of the O 1s spectra, whereas the component intensities vs temperature are reported in Figure 5b. For the strongly oxidized graphite as well, the epoxy groups appear highly unstable and desorption initiates around 345 K. It must be noted that the reason for the downshift in temperature of a few tens of degrees of the O1 and O4 curves in Figure 5b with respect to the corresponding curves reported in Figure 4e has to be found in the different procedure followed to heat the sample that was done in steps (Figure 4c−e) or following a temperature ramp (Figure 5b). At variance with the surface exposed to low oxygen dose, for the strongly oxidized HOPG also the ether component O2 reveals a high thermal instability: ethers start to desorb simultaneously with epoxides and decay with a nearly constant rate up to 870 K.
Figure 3. Valence band spectra of the oxidized HOPG surface measured as a function of the oxidation time at a photon energy of 130 eV and at an emission angle of 20°. For a better visualization the spectra have been vertically offset. The region of the π band is magnified in the inset.
measured at an emission angle of 20°. The VB of the pristine HOPG exhibits the π and the σ bands peaking at 2.8 and 16.5 eV, respectively.51 Both features appear moderately attenuated after tox = 3 and 6 min, which also induces the appearance of the O 2s peak at 26.2 eV and the growth at ∼8 eV of the structure A attributed, on the basis of the O 1s core level spectra, to the out-of-plane C−O−C σ bonds in the epoxy structures. The π conjugation of the graphitic network is progressively lost with increasing O coverage, and after the highest O dose the π bond feature has almost completely vanished due to bond rearrangement and disorder. The additional B and C structures appearing at BEs of 7.6 and 5.5. eV, respectively, in the VB spectrum measured for tox = 36 min, on the basis of the comparison with the C 1s and O 1s spectra, can be tentatively ascribed in order to C−O−C σ bonds in ether groups and to the formation of strongly oxidized species characterized by the presence of CO and OCO bonds. As already observed for the O 1s and C 1s core levels, with increasing O coverage the spectral features of the VB band related to the oxidized domains manifest a shift of ∼1 eV to higher BE, whereas the π and the σ bands, while decaying, move slightly toward the opposite direction. 3.2. Thermal Evolution of Oxidized Graphite. The thermal evolution of the HOPG with θox ∼ 0.14 ML (tox = 3 min) containing almost exclusively epoxy groups is summarized in Figure 4, which shows selected C 1s and O 1s spectra measured as a function of the annealing temperature. For both 9904
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Figure 4. (a) C 1s spectra measured on the HOPG with θox ∼ 0.14 ML (tox = 3 min) before and after annealing at increasing temperatures. The inset shows the thermal evolution of the epoxy peak at 286.2 eV. The C 1s spectrum measured after annealing at 1373 K is down shifted on purpose. (b) Two-dimensional intensity plot of the C 1s spectra during thermal annealing. The bottom and top panels show the first and the last C 1s spectra of the series, whereas the curve on the left side shows the intensity at 286.2 eV, which is on the epoxy peak, vs temperature. (c) O 1s spectra measured on oxidized HOPG before and after annealing at increasing temperatures. (d) Decomposition of selected O 1s spectra. (e) Intensity of the O 1s components vs temperature.
components O4 and O2 can be related to the formation and the decomposition of a limited number of lactone−ether structures. This reaction channel, by eliminating CO and CO2, leaves vacancies which are rapidly saturated by ethers and semiquinones,26 and is thus responsible for the appearance of the small O3 component. Moreover, for the softly oxidized graphite which is vacancy-free at RT, the number of ethers might also be increased by the occurrence of unzipping promoted by the epoxy diffusion.50 The O2 curve in Figure 4e shows that the sum of these processes makes the number of ethers at 870 K higher than that at RT. This confirms that the ether−epoxy reactions in this case are secondary in comparison with epoxy−epoxy recombination. The permanence of C−O− C and CO bonds up to almost 900 K is consistent with the high thermal stability of isolated ethers and semiquinones.26,42 Conversely, at high ether surface density in the sample with θox ∼ 0.53 ML, the simultaneous drop of epoxides and ethers demonstrates that epoxy−ether interactions become highly effective. The O2 curve reveals that graphite gasification catalyzed by the reactive epoxides starts at a temperature as low as 350 K. As in the previous case, the component O4 indicates the CO bonds that form during the reducing reactions in thermally unstable configurations, i.e., in lactonebased structures, as well as that temporarily saturate the vacancies in the vicinity of other oxidized groups. As in the case of low O coverage, the O3 component maintains an overall constant intensity up to 800 K and then declines slightly, confirming the high stability of isolated semiquinones, carbonyls, and lactones.26,42
The different ether behavior observed at low and high O coverages can be reconciled considering the different processes that are activated by the rising temperature. The key factor in the thermal reduction is the epoxy diffusion which is regulated by an energy barrier of 0.73 eV.26,29 It has been shown that two epoxy oxygens diffusing on the hexagonal grid can undergo a cycloaddition reaction and desorb as molecular O2 with a barrier of 1.13 eV,29 which is activated around 350 K. This explains most of the epoxy loss from the HOPG surface. However, diffusing epoxides might combine among them or with ethers to form unstable surface structures.26 According to recent DFT calculations the lowest-energy configurations resulting from the interaction of multiple O adsorbates always contain lactones either in lactone−ether or in ether−lactone− ether configurations.26 In particular, lactone−ether pairs, which eliminate CO and CO2 with a barrier of less than 1 eV, are formed by the interaction among epoxides and ethers with an activation barrier of 1.10 eV,29 comparable to that limiting the cycloaddition reaction. Similarly, ether−lactone−ether structures, once formed, decompose releasing CO and CO2 with activation energies as low as 0.5−0.6 eV.26 Thus elimination of O2 and reduction of the lactone-containing surface structures, being limited by comparable energy barriers, occur at similar temperatures, with the difference that in the first case only O adatoms are desorbed, whereas in the second the thermal reduction consumes the C lattice and removes, at different rates, both epoxides and ethers. For the HOPG with θox ∼ 0.14 ML, with strongly minority surface ethers, the thermal reduction determines the rapid loss of the epoxy groups. The rise and the decrease of the 9905
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Figure 5. Thermal dependence of O 1s spectra taken on the HOPG with θox ∼ 0.53 ML (tox = 36 min). (a) The left panel shows the 2D intensity plot of O 1s spectra measured during sample annealing; the middle and right panels show the high resolution O 1s and C 1s spectra, respectively, measured before annealing and after heating the sample to 660 and 1020 K. The light blue curve shows for comparison the C 1s spectrum measured on clean HOPG. (b) Temperature dependence of O 1s component intensities. (c) Decomposition of O 1s spectra shown in (a). (d) Magnification of C 1s spectra shown in (a).
The peculiar result arising from the comparison between the thermal evolution of the two samples is that the competition between the epoxy−epoxy and the epoxy−ether reactions drives the different evolution of the ethers: these remain locked in the C grid even above 800 K (see Figure 4e) unless reactive epoxides catalyze gasification channels that remove most of them from the C backbone. For oxidized graphene/Ir(111), being the ether surface density limited even for O coverages close to saturation, the competition between epoxy−epoxy recombination and the reactions triggered by epoxy which consume the backbone can be observed only by revealing the gas phase products by means of temperature programmed desorption. In the case of graphite instead, the strong variation of the ether/epoxy ratio with oxidation allows the epoxy−epoxy and ether−epoxy reactions to be monitored directly by adsorbate spectroscopy The balance between epoxy−epoxy and epoxy−ether reactions determines the efficiency of the thermal reduction. The curves in Figure 5b show that at 870 K almost 90% of the oxygen present at RT has desorbed, whereas for the sample oxidized for 3 min the different thermal evolution leaves at the same temperature nearly 30% of the initial oxygen on the surface (see Figure S3 in the Supporting Information). The different reduction modalities exhibited by the two samples, which however at 870 K contain similar amounts of oxygen, imply that at low O coverage mobile epoxides partly convert into thermally stable functional groups, whereas at high O
coverage the epoxy reactivity promotes the removal of thermally stable species accelerating the gasification of the HOPG surface (see Figure S3 in the Supporting Information). The C 1s spectra in Figure 5a,d show that the thermal reduction is accompanied by a large modification of the main peak at 284 eV, which after annealing manifests a severe line width narrowing and a shift toward higher BEs. On the contrary, the features at 289 and 286.5 eV due to oxidized C atoms move to lower BEs while decaying (see Figure 5d). Both behaviors indicate that the loss of oxygen progressively reequilibrates the charge distribution at the graphite−graphite oxide interface and all spectral features exhibit trends opposite to those shown during oxidation. Moreover, defect annealing strongly damps the shoulder occurring at BEs lower than 284 eV due to the C atoms surrounding vacancies, and the main sp2 peak tends to recover the height and width of the spectrum measured on the O-free graphite surface. However, after heating at 1020 K, the low quantity of O still residing at the HOPG surface and the incomplete defect annealing make the C 1s line shape still quite different from the spectrum measured on pristine HOPG shown for comparison in Figure 5a,d. This spectrum cannot be fully recovered even after annealing to 1370 K. It has been reported that, during the deoxidation of substoichiometric GO nanosheets, heating above 420 K leads to the development of π conjugated nanostructures that percolate the nanosheet, allowing bandlike transport of the 9906
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charge carriers.52 For the strongly oxidized graphite the C 1s and VB spectra show that annealing at 660 K, although corresponding to the total desorption of epoxy groups, allow only a moderate intensity increase for the sp2 features after a very limited restoration of the π conjugation (see Figure S2 in the Supporting Information). In this case the extended topological defects resulting from the oxidation−reduction cycle are so invasive that the crystalline hexagonal framework cannot be recovered, in agreement with the TEM observation of solely long-range orientational order in reduced GO monolayers.52
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4. CONCLUSIONS We have shown that oxygen chemisorption on graphite begins with the adsorption of epoxy groups and that the presence of ethers, observed even in the absence of evident nucleation of vacancies and surface defects, seems indicative for the occurrence of epoxy unzipping. At high oxidation levels lattice damage is proven by the appearance of CO bonds which accompanies the enhancement of the ether concentration, with both arising from O saturation of vacancies. The oxidized HOPG surface starts to lose epoxy groups around 370 K. The epoxy−ether concentrations drive also the destiny of ethers, which desorb together with epoxides or remain trapped in the C grid depending on whether the epoxy−ether interactions are predominant over the epoxy−epoxy recombination. Our results indicate that the anticipated CO and CO2 emission observed between ∼400 and 470 K while thermally expanding oxidized graphite5 or during GO reduction7 can be at least in part regarded as the effect of the abrupt ether consumption catalyzed by the presence of epoxides. Desorption of oxygens bonded to undercoordinated C atoms consumes the graphitic network, leaving extended bond disorder which, for heavily oxidized graphite, cannot be adequately annealed by thermal processing. In this case the limited restoration of the sp2 conjugation hints at the difficulty of recovering satisfactory transport properties in thermally reduced GO.
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ASSOCIATED CONTENT
* Supporting Information S
Core level spectra fit parameters; binding energies of the C 1s and O 1s spectra as a function of the oxidation time; estimation of the oxygen coverage; valence band spectra of the oxidized graphite before and after thermal reduction; total oxygen coverage vs annealing temperature. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank S. Fabris and T. Sun for fruitful discussions. The support of the COST Action MP0901 “NanoTP” is gratefully acknowledged.
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