Soft X-ray Absorption Spectroscopy Studies of the Electronic Structure

Article pubs.acs.org/JPCC

Soft X‑ray Absorption Spectroscopy Studies of the Electronic Structure Recovery of Graphene Oxide upon Chemical Defunctionalization Vincent Lee,† Robert V. Dennis,† Brian J. Schultz,† Cherno Jaye,‡ Daniel A. Fischer,‡ and Sarbajit Banerjee*,† †

Department of Chemistry, University at Buffalo, The State University of New York, Buffalo, New York 14260-3000, United States Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States

‡

ABSTRACT: A comparative evaluation of the efficacy of different reducing agents in defunctionalizing graphene oxide is performed using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy at carbon and oxygen K-edges. The relative intensities of π* and σ* resonances in carbon K-edge NEXAFS spectra suggest relatively high extents of recoveries of π-conjugation upon reduction using vapor-phase phenylhydrazine or hydrazine as well as neat phenylhydrazine. The use of these reducing agents in combination with modest thermal annealing appears to be particularly effective for deepoxidation and decarboxylation of graphene oxide. Polarized carbon K-edge NEXAFS spectroscopy measurements allow evaluation of the extent of warping or planarity induced upon chemical reduction. Treatment with vapor phenylhydrazine and subsequent thermal annealing entirely disrupts the alignment of reduced graphene oxide while facilitating substantial recovery of π conjugation. In contrast, annealing subsequent to treatment with vapor-phase hydrazine yields high dichroic ratio values that are substantially increased as compared to unreduced graphene oxide, indicating a greater extent of substrate alignment. Restoration of excitonic confinement of the σ* resonance is seen to be most pronounced for reduction of graphene oxide by NaBH4 and vapor-phase phenylhydrazine after annealing, suggesting relatively large conjugated domains in these materials after chemical reduction.

■

INTRODUCTION The peculiarities in the electronic structure of graphene, a monolayer of sp2-hybridized carbon atoms arrayed in a hexagonal periodic lattice, give rise to remarkable electrical transport phenomena including the half-integer quantum Hall effect, massless Dirac Fermion behavior of charge carriers, and ballistic conduction.1−3 Given the exceptional promise of graphene for device integration as the active component of sensing, high-frequency electronics, interconnect, and actuator architectures,4−6 much effort has been directed toward developing scalable routes for the fabrication and precise positioning of graphene with high crystal quality and controlled dimensions. Chemical vapor deposition7,8 and solution-phase exfoliation routes9−11 have emerged as the mainstays of these nascent nanomanufacturing efforts that have been imperative for spurring the commercialization of graphene-based products at an unprecedentedly rapid rate for a new material. As a structurally related material, graphite oxide has also been enjoying a renaissance in recent years.12−16Although the edgeand basal-plane functionalities of graphite oxide serve as scattering sites, limiting the obtained carrier mobilities and conductivities, the functional groups also endow the material with beneficial characteristics such as solution processability, chemical tunability, and matrix compatibility. The latter bears © 2012 American Chemical Society

much significance for applications of exfoliated graphene oxide (GO) as a filler material within matrices wherein it provides improved electrical conductivity, thermal conductance, and mechanical reinforcement.17−19 Solution-based approaches for preparation of graphene often start with the oxidation of graphite to graphite oxide through modifications of well-established methods pioneered by Hummers and Brodie.13,20 This oxidation process results in incorporation of oxygen-containing functionalities on the basal planes and edges of graphite leading to an increase of the interlayer separation to approximately ∼1.1 nm (depending upon the extent of hydration). The chemical structure of graphite oxide has been extensively investigated over the last several decades with the most generally accepted models being those advanced by Lerf and Klinowski and Dèkàny.21,22 The former model is particularly pertinent to our study given our choice of oxidation conditions and places epoxide and hydroxyl groups on the graphene basal planes with ketone and carboxylic acid functionalities decorating the edge sites. In a refinement of this model, recent solid-state NMR results by Ajayan and coReceived: July 1, 2012 Revised: August 24, 2012 Published: September 17, 2012 20591

dx.doi.org/10.1021/jp306497f | J. Phys. Chem. C 2012, 116, 20591−20599

The Journal of Physical Chemistry C

Article

with 45 mL of deionized water and filtered over a nitrocellulose membrane with a pore size of 25 nm. The membrane was then cut into several equivalent pieces and transferred printed onto a cleaned glass substrate following the method developed by Chhowalla et al.30 The membrane was subsequently dissolved using acetone and the resulting films were washed with successive immersion in ethanol and water. The transferprinted GO films on glass were then each individually reduced using (i) NaBH4 (2 mg/mL in water), (ii) an aqueous solution of hydrazine (3.3 mM), (iii) vapor hydrazine, (iv) liquid phenylhydrazine, and (v) vapor phenylhydrazine. The solution phase reactions were performed by immersing the GO films in the apposite solutions for 24 h. The vapor phase reductions had an additional annealing step at 150 °C in an argon glovebag to remove residual solvent. While 99.999% purity argon was used within the sealed glovebag, some residual oxygen contamination is possible. All reduced films on glass were cleaned with copious amounts of deionized water and ethanol and dried in air. NEXAFS Measurements. Carbon and oxygen K-edge NEXAFS experiments were performed at National Institute of Standards and Technology (NIST) beamline U7A located at the National Synchrotron Light Source of Brookhaven National Laboratory. A toroidal spherical grating monochromator with 600 and 1200 lines/mm was used to acquire the C and O Kedge data, respectively, yielding an energy resolution of approximately 0.1 eV with entrance and exit slits setting of 30 μm each. The spectra were acquired in partial electron yield mode (PEY) with the detection of Auger electrons facilitated by a channeltron electron multiplier detector with an entrance grid bias set at −150 V to enhance surface sensitivity. The use of a charge compensating electron gun was imperative to mitigate the effects of sample charging. Given that insulating glass is used as the substrate, substantial charging (since Auger electrons are emitted and cannot be compensated given the absence of an electrical conduction pathway to the electrical ground) and a constantly diminishing electron count is evidenced in the absence of charge compensation. The PEY signals were normalized using the incident beam intensity obtained from the photoemission yield of a freshly evaporated Au grid with 90% transmittance placed along the path of the incident X-rays to eliminate the effects of beam fluctuations and monochromator absorption features. The C K-edge spectra were calibrated to an amorphous carbon mesh with a π* transition at 285.1 eV. The O K-edge spectrum was calibrated using an oxide dip absorption feature in the I0 spectrum derived from the grating. The 531.2 eV dip has previously been internally calibrated using an O K-edge spectrum of gas-phase oxygen. Pre- and postedge normalization of the data were performed using the Athena suite of programs. Band Structure and Isosurface Calculations. The band structure of graphene and the integrated local density of states (ILDOS) of isosurfaces were modeled using density functional theory within a pseudopotential approximation (QUANTUM ESPRESSO suite of codes).31

workers also suggest the presence of five- and six-membered lactol rings.23 Highly functionalized domains with predominantly sp3-hybridized carbons are thought to coexist with and remain structurally linked to relatively pristine sp2-hybridized enclaves.12−16 The increase in the interlayer separation induced by oxidation allows for facile exfoliation of graphite oxide to single-layered GO upon ultrasonication or thermal expansion. GO is electrically insulating, but some degree of conductivity can be restored through the use of chemical or thermal reduction. Restoration of graphene’s electronic structure remains of much interest for a plethora of practical applications, and consequently substantial effort has been directed at investigating various chemical defunctionalization routes.14,24,25 The clear need for chemical (and not just thermal or electrochemical) defunctionalization routes arises from the requirements of processing GO thin films deposited onto plastic and glass substrates as well as in situ within GOembedded polymer matrices. To the best of our knowledge, a comparative evaluation of the restoration of graphene electronic structure upon chemical reduction remains unexplored. Here, we have used near-edge X-ray absorption fine structure (NEXAFS) spectroscopy to examine electronic structure recovery, extent of planar alignment, and the surface chemistries of reduced graphene oxide (RGO) derivatives obtained by treating GO with a number of different reducing agents. NEXAFS is an especially powerful element-specific probe of graphene’s band structure above the Fermi level.25−29 Based on dipole-selection rules, polarized NEXAFS measurements allow evaluation of electronic corrugations in graphene based on the directionality of the graphene π cloud.25 In recent work, Papakonstantinou and co-workers have carefully examined the evolution of NEXAFS spectra at C K- and O K-edges during the thermal defunctionalization of graphene.29 In previous work, we have used angle-resolved NEXAFS spectroscopy to examine the extent of substrate alignment of electrophoretically deposited and CVD-grown graphene thin films.25,26 Here, we delineate the differences in structural distortion (crumpling) and restoration of π-conjugated sp2-hybridized domains upon treatment of GO with different reducing agents.

■

EXPERIMENTAL SECTION Preparation of GO. Graphite was purchased from Bay Carbon Inc. (Michigan, U.S.A.). A modified version of Hummer’s method was used to synthesize graphite oxide.13 Typically, 5.0 g of natural graphite powder and 2.5 g of NaNO3 were added to 125 mL of H2SO4 in a flask cooled by an ice bath. Subsequently, 15.0 g of KMnO4 was added slowly in small increments to prevent the temperature of the mixture from exceeding 20 °C. The mixture was then heated to 35 °C and kept at that temperature for 30 min. At this point, 230 mL of deionized water was added to the reaction vessel, which resulted in the temperature increasing to 98 °C. The reaction mixture was then maintained at this temperature for 15 min. Subsequently, an additional 230 mL of deionized water was added, followed by the addition of an aqueous solution of 30% hydrogen peroxide until cessation of gas evolution. The resulting solution was filtered and washed with copious amounts of water. The recovered solid was left to dry overnight under vacuum. The obtained graphite oxide was then added to water at a concentration of 1 mg/mL and sonicated to obtain a visibly clear solution. Next, 5 mL of this solution was mixed

■

RESULTS AND DISCUSSION Raman spectroscopy, differential scanning calorimetry, and ζpotential data for our GO samples have been published previously.25 Figure 1A depicts a digital micrograph of a GO film transfer-printed onto glass. We have attempted to maintain a film thickness close to ca. 300 nm in each case (as verified by profilometry). The GO film displays a light brown color before 20592



Article

unoccupied states of π* and σ* symmetry that are probed in a NEXAFS experiment. For pristine graphene (as for crystalline graphite), the π* and σ* set of resonances are thought to be of excitonic origin with a characteristic double-peaked structure for the σ* resonance in high-crystalline quality samples.34,35 Figure 3B−D plot the integrated local density of states corresponding to the π* and the two components of the σ* resonance. Three additional spectral features are discernible centered at 286.7, 288.7, and ∼289.8 eV (labeled A, C, and D, respectively) in the intermediate region between the π* and σ* resonances along with a shoulder at ∼287.5 eV (labeled B). Given the spectral resolution and errors in fitting, a ±0.2 eV precision is estimated in the assignment of the intermediate resonances (the π* peak position is estimated within ±0.1 eV). Conflicting assignments of NEXAFS resonances in this intermediate region have been reported in the literature.28,29,36−38 A possible origin of absorption features in this region are transitions to interlayer states with free-electron character located between graphene basal planes, analogous to transitions in crystalline graphite.33,39 However, in recent work, we have used ab initio density functional theory (DFT) calculations to demonstrate that freestanding single-layered graphene does not exhibit any characteristic interlayer absorptions.27 Certainly, the GO sheets are aggregated within the thin films but the large interlayer separation and disordered stacking suggests that a pronounced absorption feature of interlayer origin is improbable. The resonances in the intermediate region (between the π* and σ* peaks) can thus be attributed to functional groups. Fourier transform infrared (FTIR) spectroscopy studies have permitted a qualitative and semiquantitative description of functional group distributions within graphene oxide and their transformation under thermal annealing.40 Acik et al. have noted the need to exercise caution in assignment of FTIR peaks given the interactions between randomly arranged oxygen functionalities, which tend to substantially alter their vibrational frequencies from the characteristic frequencies expected for isolated functional groups. Notably, FTIR spectroscopy does not permit evaluation of the electronic structure of graphene oxide. Table 1 summarizes NEXAFS functional group assignments reported previously in the literature that are generally consistent with our assignments noted below. The features at ∼289.8 and ∼288.7 eV can be assigned to the excitation of C 1s core electrons to states of π* symmetry primarily localized at CO bonds of carboxylic acid (−COOH) and carbonyl (C O) moieties, respectively.29,37 The feature at ∼286.7 eV can be attributed to transitions to π* states of C−O bonds derived from hydroxyl groups.25,37 Furthermore, the feature at ∼287.5 eV seen in the GO film before reduction can be attributed to the transition of C 1s core level electrons to antibonding C−O states derived from epoxide groups on the basal plane of GO.37 The assigned functional group distribution is consistent with the Lerf-Klinowski model of GO and places epoxide and hydroxyl groups on the graphene basal planes with ketone and carboxylic acid functionalities decorating the edge sites.21,23 The assignment of NEXAFS resonances at energies between the graphene π* and σ* features to unoccupied π* CO and π* C−O states centered on functional groups reflects the relative energies of these electronic states. Given the uncertainty in the bond connectivities in the geometric structure of GO, a detailed electronic structure calculation in not available but Mkhoyan et al. have calculated the 2p projected density of states (which is of most relevance for C K-

Figure 1. Digital photographs of (A) a GO film transfer printed onto glass and (B) GO reduced by liquid phenylhydrazine. Optical micrographs of (C) a GO film transfer printed onto glass and (D) GO reduced by liquid phenylhydrazine.

reduction, consistent with the color of electrophoretically deposited GO films prior to chemical reduction.25 This suggests a partial disruption of the conjugated sp2-hybridized graphitic framework. In contrast, after reduction with phenylhydrazine, the film turns much darker (Figure 1B), which suggests significant restoration of the π-conjugated graphitic framework (resulting in the absorption maxima being shifted to higher wavelengths) and increased carrier concentration.32 The films are macroscopically smooth except for some blisters induced during transfer from the membrane to the glass as seen in the optical micrographs before (Figure 1C) and after (Figure 1D) chemical reduction. NEXAFS spectroscopy allows for probing of alignment on a much shorter length scale (vide infra). Figure 2 shows the C K-edge NEXAFS spectrum of a GO film transfer printed onto glass, acquired at 54.7° (magic angle)

Figure 2. Magic angle (54.7° incidence) C K-edge NEXAFS spectrum acquired for a GO film transfer printed onto glass.

incidence of the X-ray beam to eliminate effects of preferential orientation. The lowest energy peak closest to the Fermi level at ∼285.2 eV (labeled π*) can be assigned to the transition of C 1s core level electrons into states of π* symmetry around the M and K points of the graphene Brillouin zone. The higher energy sharp peak centered at ∼293.2 eV (labeled σ*) can be attributed to the excitation of C 1s core level electrons into dispersionless states of σ symmetry.25,26,28,33 Figure 3A shows the calculated band structure of graphene indicating the 20593



Article

Figure 3. (A) Calculated ground state band structure for a pristine graphene sheet where the unoccupied levels that are probed in a NEXAFS experiment are highlighted with a gray box. (B−D) Integrated local density of states of isosurfaces of the π* and the two σ* resonances, respectively. The excited (photoabsorbing) atom is in the center in each case.

Table 1. Functional Group Assignments of Spectral Features Observed in C K-Edge NEXAFS Spectra of GO as Reported in the Literature (refs 29, 37, and 38) peak position (eV)

assignment

ref

286.3 286.7 287.0 287.2 288.1 288.2 288.7 288.9

hydroxyl/epoxide hydroxyl hydroxyl/epoxide epoxide carboxyl/carbonyl carboxylic acid carboxylic acid+carbonyl carboxylic acid

38 37 29 37 38 36b 29 37

Figure 4. Magic angle (54.7° incidence) C K-edge NEXAFS spectra for eight GO and RGO films: (a) GO film with no reduction; (b) GO film reduced by NaBH4; (c) GO film reduced by immersion in 3.3 mM aqueous hydrazine; (d) GO film reduced by vapor hydrazine under vacuum without subsequent annealing; (e) GO film reduced by vapor hydrazine under vacuum followed by annealing under an argon atmosphere to 150 °C; (f) GO film reduced by immersion in liquid phenylhydrazine; (g) GO film reduced by vapor phenylhydrazine under vacuum without any further annealing; (h) GO film reduced by vapor phenylhydrazine under vacuum followed by annealing under an argon atmosphere to 150 °C.

edge NEXAFS spectra given the 1s → 2p nature of the transition) for a carbon sheet with a bonded oxygen atom. A clear maxima appears in the partial DOS plot at an intermediate energy between the π* and σ* states for the graphene sheet with a bonded oxygen atom, which is thus consistent with the assignments of intermediate-energy resonances here to functional groups.41 This assignment of relative energy levels is also consistent with electron energy loss spectroscopy data reported by the authors. The assignment of π* CO and π* C−O states as being relatively lower in energy than C−C σ* states is further analogous to accepted spectral assignments of resonances in the C K-edge NEXAFS spectra of oxidized single-walled carbon nanotubes.42 Kuznetsova et al. have assigned NEXAFS resonances between 287 and 292 eV to functional groups in oxidized single-walled carbon nanotubes based on the “building block” model wherein they invoke comparisons to fused-ring aromatic hydrocarbon molecular standards bearing the same functional groups. Consistent with such an assignment, thermal annealing results in a diminution of peak intensity in this region, which has been ascribed as the removal of functional groups from carbon nanotube surfaces. A similar diminution in intensities of these intermediate-energy resonances has been observed upon thermal annealing of graphene oxide.29 Figure 4 contrasts the C K-edge NEXAFS spectra acquired for eight films, variously reduced using NaBH4, hydrazine, and phenylhydrazine. To facilitate comparisons of electronic structure recovery, the spectra have been acquired at magic angle (54.7°) incidence of the X-ray beam where the peak intensities are independent of the angular symmetry dependence. In the initial GO films, the intensity of the π* feature is

much diminished in comparison to the σ* resonance indicating substantial disruption of π-conjugation and a diminution in the size of sp2-hybridized domains. The relative intensities of the π* and σ* resonances (Iπ*/Iσ*) at 54.7° incidence of the X-ray beam provide a measure of the restoration of electronic structure upon chemical defunctionalization of GO. For a perfect single-layered graphene sample, the calculated Iπ*/Iσ* ratio is 1.30 (ratio of integrated peak areas is 1.49).27 However, since the σ* intensity is superimposed on a broad background arising from π* C−O and σ* C−O absorptions as well as the photoionization continuum, a more rigorously quantitative analysis across different samples is instead facilitated by comparing the integrated π* intensities of pre- and postedge normalized spectra, which thus relate the relative proportion of sp2-hybridized carbon atoms to a unitary carbon absorption cross-section. These values are listed in Table 2.27 Upon reduction with vapor phenylhydrazine, the Iπ*/Iσ* ratio is substantially enhanced and the integrated π* intensity increases from 0.33 to 2.04 (Table 2), indicating significant recovery of π-conjugation in the reduced graphene oxide (RGO) sheets . Still further enhancement of the Iπ*/Iσ* ratio is evidenced upon annealing the vapor-phenylhydrazine-treated 20594



Article

Table 2. Comparison of the Integrated π* Intensity at 54.7° and Dichroic Ratio for the Eight Distinct Reduced GO Films on Glassa reducing agent vapor phenylhydrazine annealed vapor phenylhydrazine not annealed liquid phenylhydrazine vapor hydrazine annealed vapor hydrazine not annealed aqueous hydrazine sodium borohydride no reducing agent

integrated π* intensity at 54.7°

DR

2.28

−0.04 ± 0.10

2.04

−0.47 ± 0.05

2.76 2.09 1.44 1.91 1.11 0.33

−0.33 −0.69 −0.33 −0.61 −0.67 −0.42

± ± ± ± ± ±

with removal of epoxide and hydroxyl groups located at the edges and interiors of aromatic domains.24 Phenylhydrazine and hydrazine are particularly efficient at de-epoxidation of 1,2epoxide functionalities located within the interior of aromatic domains.24 Since these moieties are especially abundant and disruptive of π conjugation, their removal facilitates substantial recovery of the graphene electronic structure. In the vapor phase, phenyhydrazine appears to be more efficient at inducing defunctionalizaiton of GO with or without annealing (Table 2). The phenyl groups may facilitate interactions with remnant conjugated domains of GO; moreover, the decomposition of hydrazines at surfaces is sensitive to pH, temperature, and presence of trace metals (in this instance, possibly Mn from the Hummer’s oxidation process); the increased reduction achieved with phenylhydrazine may be reflective of its increased stability under these conditions. Nagase and co-workers predict a mechanistic pathway wherein hydrazine attacks a sp2-hybridized carbon atoms located either ortho or meta to the epoxide group with concomitant H-atom transfer and opening of the threemembered epoxide ring.24 Another H-atom transfer from the incipient hydrazino group to the proximal alcohol moiety, followed by removal of H2O and cis-diazene (N2H2) leads to restoration of π-conjugation at the original buckled epoxide site.15,24 A reaction scheme based on Nagase’s DFT results is depicted as Scheme 1.

0.04 0.08 0.05 0.07 0.03 0.03

a The π* intensities have been calculated for pre- and post-edge normalized spectra and are therefore all normalized to a unitary carbon absorption cross-section. The post-edge intensity at 330.0 eV is set at 1.0.

samples under an argon ambient at 150 °C; indeed, the peak height of the π* resonance surpasses that of the σ* feature (the integrated π* intensity increases to 2.28). Immersion in liquid phenylhydrazine has an analogous pronounced effect on GO defunctionalization. Concomitantly with an increase in the Iπ*/ Iσ* ratio (integrated π* intensity of 2.76), the intensity in the intermediate functional group region between ∼286 and 289 eV is observed to be the most strongly diminished for the sample immersed in neat phenylhydrazine, again suggesting effective defunctionalization. For all three samples, the prominent 289.8 eV carboxylic acid feature observed for graphene oxide is shifted to ∼288.8 eV, which is more characteristic of carbonyl or quinone moieties. Interestingly, a recent DFT study has determined that carbonyl groups located at the edges of aromatic domains are essentially unreactive to hydrazine and are expected to persist even upon successive hydrazine reduction and annealing.24 If these groups are indeed located at the edges of aromatic domains, they are not expected to have a very deleterious impact on electron transport within an individual graphene domain although they would influence electrical transport between adjacent RGO sheets within a thin film. Analogously, the phenylhydrazine-reduced samples show a broad absorption in the 287.2287.4 eV region, which can be attributed to remnant epoxides, such as perhaps the moieties located at the edges of aromatic domains and also predicted to be especially stable to hydrazine reduction by Gao et al. (although these groups can react to form hydrazino alcohols).24 Similar to previous results noted for hydrazine reduction of GO, it is expected that more efficacious reduction occurs upon treatment with liquid phenylhydrazine (as compared to vapor phase phenylhydrazine) because the negatively charged pendant carboxylic acid groups separating the GO sheets through electrostatic repulsion can be better reduced through neutralization with phenylhydrazinium cations. Under aqueous conditions and low concentrations of phenylhydrazine delivered in the vapor phase, the carboxylic acid moieties inhibit the reaction of GO with the reducing agents.43,44 For treatment with both hydrazine and phenylhydrazine, annealing to 150 °C clearly helps to further restore the πconjugated structure as indicated by the increase in the relative π* intensity values to 2.09 and 2.28, respectively (Table 2). Annealing, even at this low temperature, is expected to facilitate decarboxylation of −COOH groups at edges as well as assist

Scheme 1. Phenylhydrazine De-Epoxidation of GO Based on the Reaction Pathway Proposed by Nagase et al.24

Vapor phase hydrazine is observed to bring about some recovery of electronic structure, evidenced as an increase of the Iπ*/Iσ* ratio with or without annealing. The integrated π* intensity increases from 0.33 for graphene oxide to 1.44 for the hydrazine-treated sample without annealing and is further enhanced to 2.04 upon annealing (Table 2). Note that treatment with neat hydrazine causes extensive reduction but leads to delamination and disintegration of the GO films on glass and thus this set cannot be compared to the other reducing conditions.43 In the vapor-hydrazine-reduced samples, the σ* feature shows a distinctive splitting and the appearance of a characteristic double-peaked structure that has been ascribed to an excitonic state in graphite oxide and graphene.34,35 The manifestation of a strong σ* excitonic feature has been correlated to carbonaceous materials with good local ordering and well-formed (less defective) C−C bonds.45 In contrast, for the phenylhydrazine set of samples, such excitonic confinement does not appear to be substantially restored, suggesting a relatively smaller size (but greater abundance) of the restored π-conjugated domains. The slopes to the σ* resonance vary depending on the specific reducing conditions used, which likely alters the functional group distribution. Integration of the σ* intensity is also complicated by overlap with C−O 20595



Article

spectra are characterized by two well-separated sets of absorption features at ∼531.5 and ∼538 eV. The feature at ∼531.5 eV can be attributed to transitions from O 1s core level electrons to π* CO states derived from carboxylic acid and ketone groups at the GO edge sites.25,29,37,47 The feature centered at ∼538 eV can be attributed to several transitions including to O−H, C−O, and CO states of σ* symmetry.25,47 Notably, the intensities of both transitions are affected by the defunctionalization process. The lineshapes of these features are altered depending on the specific reducing agent although the O K-edge NEXAFS literature is not rich enough to allow more definitive assignments to specific functional groups, as has also been noted by Papakonstantinou and co-workers.29 Remnant water is observed at ∼558 eV for GO films reduced using vapor hydrazine (before and after annealing) and liquid phenylhydrazine. As noted above, angle-resolved NEXAFS spectroscopy can serve as an excellent probe of the anisotropy and extent of substrate alignment of graphene.25,26 To evaluate the extent of local short-range lattice distortion and crumpling accompanying chemical reduction, we have performed a series of polarized C K-edge NEXAFS experiments for the variously reduced samples as shown in Figures 6 and 7. At normal incidence of the X-ray

resonances that add spectral weight, particularly to the lowenergy tail of this resonance. The GO samples treated with vapor phase hydrazine show a pronounced absorption feature at ca. 287.6 eV in addition to the presumptive carbonyl peak at 288.7 eV. These features are again consistent with unreacted epoxides and carbonyl groups located at edges of aromatic domains although in the absence of definitive first-principles assignment of spectral peak positions, much of the assignments in this region must remain primarily empirical. The relative efficacies of different reducing conditions in facilitating de-epoxidation and decarbonylation are clearly different although a quantitative description of the evolution of functional group concentrations is precluded by the overlapping of their spectral signatures. In contrast to phenylhydrazine and vapor phase hydrazine reduction, more modest enhancements in the Iπ*/Iσ* ratio are observed for aqueous hydrazine and NaBH4 under our conditions. Although NaBH4 reduction does not bring about as large of an enhancement in the integrated π* intensity (1.11) as hydrazine or phenylhydrazine, Figure 3 does indicate recovery of the σ* excitonic feature for this sample. The π* resonances for these two samples are further seen to be substantially broadened in contrast to GO and other reduced GO films. One possible origin for the broadening is the presence of a broader distribution in the π-conjugation lengths of different incipient reduced graphene domains in these relatively less defunctionalized samples. The characteristic peak position of π* resonances depends on the extent of conjugation-a broader distribution would thus be reflected in a wider fwhm of the π* resonance.46 Further corroboration of the general trends in electronic structure recovery noted above is seen in the O K-edge NEXAFS spectra shown in Figure 5. The spectra have only

Figure 5. Magic angle (54.7° incidence) O K-edge NEXAFS spectra for eight distinct films: (a) GO film with no reduction; (b) GO film reduced by NaBH4; (c) GO film reduced by immersion in 3.3 mM aqueous hydrazine; (d) GO film reduced by vapor hydrazine under vacuum with no annealing; (e) GO film reduced by vapor hydrazine under vacuum followed by annealing under argon at 150 °C; (f) GO film reduced by immersion in liquid phenylhydrazine; (g) GO film reduced by vapor phenylhydrazine under vacuum without subsequent heat treatment; (h) GO film reduced by vapor phenylhydrazine under vacuum followed by annealing under argon at 150 °C.

Figure 6. (A) Polarized C K-edge spectra for an as-prepared GO film. (B) Polarized C K-edge spectra for a GO reduced by vapor phenylhydrazine with no annealing.

beam, the electric-field vector E is aligned along the intermolecular bond axis of a hypothetically flat graphene (or graphite) sample, and thus exclusively probes states of σ symmetry. In contrast, at glancing incidence, E is perpendicular to the intermolecular σ-bond axis, and will have a large component suitable for coupling to states of π* symmetry. Given the directionality of both π (out-of-plane) and σ (inplane) bonding in graphene, the corresponding π* and σ* resonances show contrasting dependences on the angle of incidence of the X-ray radiation. For an aligned graphene sample, the π* resonance is expected to monotonically decrease with increasing angle of incidence as E has a successively

been pre-edge normalized to compare the relative edge jump heights of the GO and reduced GO films. The intensities beyond 555 eV are reflective of the relative oxygen concentrations in each sample. The GO sample clearly has the highest concentration of oxygenated functional groups. The lowest concentrations of oxygen are found for GO treated with vapor phase phenylhydrazine or hydrazine. The O K-edge 20596



Article

likely disrupts sheet-to-sheet hydrogen bonding networks within the GO thin films, resulting in 3D warping to accommodate the strains of heterogeneously functionalized regions within individual reduced graphene oxide sheets. Indeed, defunctionalization can also lead to unzipping of larger graphene flakes into smaller domains and the creation of holes spanning several C−C bond lengths.14,16 Notably, the lack of planar alignment does not necessarily need deleteriously alter the electrical conductivitythe distortions of the graphene platelets and removal of edge functional groups indeed might allow for better intergrain conductivity to be established in RGO films, facilitating easier variable range hopping in these materials. In contrast, in pristine single-layered graphene, ballistic conduction mechanisms can be disrupted by quenched rippling with the corrugations essentially acting as Coulombic scattering sites.1,27,49 In contrast to the phenylhydrazine-reduced set of samples, the DR deduced for the GO film reduced by vapor hydrazine followed by annealing at 150 °C is −0.69 ± 0.08, which denotes a significant improvement in substrate alignment over GO. The DR calculated for the GO film reduced by vapor hydrazine with no annealing is −0.33 ± 0.05. We postulate that effective basalplane defunctionalization followed by thermal annealing allows more rigidly planar sp2-hybridized graphene domains to orient parallel to the substrate. Some remnant edge-functional groups (−COOH, CO, and hydrazine alcohols) likely facilitate hydrogen bonding within the films (in contrast to the vaporphenylhydrazine case wherein complete disruption of the hydrogen bonding network and vaporization of excess reducing agent leads to 3D warping and loss of planarity). The DR values for the reduced GO films are listed in Table 2 and have been deduced from the plots of integrated π* intensities depicted in Figure 6. Chemical reduction is seen to shift all the plots to higher π* values relative to GO. The π* versus cos2 θ curve shows the highest slope (indicative of increased planarity and substrate alignment) after reduction with vapor-phase hydrazine and annealing and the lowest slope (indicative of 3D warping) after reduction with vapor-phase phenylhydrazine and annealing.

Figure 7. Integrated intensity of the π* resonance versus the angle of incidence for eight distinct films: (a) graphene oxide film with no reduction; (b) graphene oxide film reduced by NaBH4; (c) graphene oxide film reduced by dipping into 3.3 mM aqueous hydrazine; (d) graphene oxide film reduced by vapor hydrazine under vacuum with no annealing; (e) graphene oxide film reduced by vapor hydrazine under vacuum followed by annealing; (f) graphene oxide film reduced by dipping into liquid phenylhydrazine; (g) graphene oxide film reduced by vapor phenylhydrazine under vacuum followed with no annealing; (h) graphene oxide film reduced by vapor phenylhydrazine under vacuum followed by annealing.

smaller component that is out-of-plane. The converse is true for the σ* resonance as also illustrated by the integrated local density of plots corresponding to these resonances depicted in Figure 3B−D. Since the σ* resonance resides atop the photoionization continuum and the functional group resonances, it is typical to use the integrated intensity of the π* resonance to quantitate the degree of alignment of graphene or graphitic materials.25,26,48 Evaluation of the relative intensity of the π* transition at ∼285.2 eV at different angles thus allows for determination of the extent of alignment of individual graphene platelets within the film. As a measure of alignment and flatness of the graphene samples, a dichroic ratio (DR)25,26 can be defined as DR =

(I⊥ − I ) (I⊥ + I )

(1)

■

A DR value of 0 is expected from a sample with no alignment and a DR value of −1 is expected for a perfectly aligned graphene sample with π* orbitals oriented vertically to the substrate surface. Figure 6A plots the angle-resolved NEXAFS spectra for GO prior to chemical reduction, and in Figure 7, the integrated π* intensities are plotted against the square of the cosine of the angle of incidence. A DR value of −0.42 ± 0.03 has been deduced for the transfer-printed GO film (based on extrapolated π* intensities at θ = 0° and θ = 90°) prior to chemical reduction, suggesting a moderate alignment and retention of at least some locally planar sp2-hybridized regions (Table 2). Upon exposure to vapor phenylhydrazine, there is no substantial change in the DR value within the limits of experimental error. However, upon annealing to 150 °C, the sample shows considerable surface roughness with the DR value reduced to −0.04, suggestive of little or no alignment (Table 2 and Figure 7). As noted above, the annealing step likely facilitates further decarboxylation, de-epoxidation, and dehydroxylation, allowing substantial recovery of the conjugated graphene π structure (as also suggested by the increase in the Iπ*/Iσ* ratio). Removal of edge functional groups as well as loss of trapped excess reducing agent and water molecules

CONCLUSIONS Polarized NEXAFS spectroscopy investigations allow comparative evaluation of the restoration of π conjugation in GO thin films reduced with different reducing agents. Treatment with neat or vapor-phase phenylhydrazine or vapor-phase hydrazine are observed to result in the highest obtained intensities of the normalized π* intensity. Absorptions ascribed to functional groups in the ∼286289.5 eV region of the C K-edge spectrum are also greatly diminished upon reduction of GO with these reducing agents. Extensive decarboxylation and deepoxidation are evidenced with the remnant functional groups likely being edge carbonyls and edge epoxides. Polarized NEXAFS spectroscopy results suggest extensive 3D warping of reduced graphene oxide sheets upon vapor phenylhydrazine reduction and annealing but increased planarity and rigidity of sp 2-hybridized domains upon hydrazine reduction and annealing. Restoration of excitonic confinement of the σ* resonance is seen to be most pronounced for reduction of GO by NaBH4 and vapor-phase phenylhydrazine after annealing, suggesting relatively large conjugated domains in these materials after chemical reduction. 20597



■

Article

(23) Gao, W.; Alemany, L. B.; Ci, L.; Ajayan, P. M. Nat. Chem. 2009, 1, 403−8. (24) Gao, X.; Jang, J.; Nagase, S. J. Phys. Chem. C 2010, 114, 832− 842. (25) Lee, V.; Whittaker, L.; Jaye, C.; Baroudi, K. M.; Fischer, D. a.; Banerjee, S. Chem. Mater. 2009, 21, 3905−3916. (26) Lee, V.; Park, C.; Jaye, C.; Fischer, D. A.; Yu, Q.; Wu, W.; Liu, Z.; Bao, J.; Pei, S.-S.; Smith, C.; Lysaght, P.; Banerjee, S. J. Phys. Chem. Lett. 2010, 1, 1247−1253. (27) Schultz, B. J.; Patridge, C. J.; Lee, V.; Jaye, C.; Lysaght, P. S.; Smith, C.; Barnett, J.; Fischer, D. A.; Prendergast, D.; Banerjee, S. Nat. Commun. 2011, 2, 372. (28) Pacilé, D.; Papagno, M.; Rodríguez, A.; Grioni, M.; Papagno, L.; Girit, Ç .; Meyer, J.; Begtrup, G.; Zettl, A. Phys. Rev. Lett. 2008, 101, 066806. (29) Ganguly, A.; Sharma, S.; Papakonstantinou, P.; Hamilton, J. J. Phys. Chem. C 2011, 115, 17009−17019. (30) Eda, G.; Fanchini, G.; Chhowalla, M. Nat. Nanotechnol. 2008, 3, 270−4. (31) Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I.; Dal Corso, A.; de Gironcoli, S.; Fabris, S.; Fratesi, G.; Gebauer, R.; Gerstmann, U.; Gougoussis, C.; Kokalj, A.; Lazzeri, M.; Martin-Samos, L.; Marzari, N.; Mauri, F.; Mazzarello, R.; Paolini, S.; Pasquarello, A.; Paulatto, L.; Sbraccia, C.; Scandolo, S.; Sclauzero, G.; Seitsonen, A. P.; Smogunov, A.; Umari, P.; Wentzcovitch, R. M. J. Phys.: Condens. Matter. 2009, 21, 395502. (32) Stankovich, S.; Piner, R. D.; Chen, X.; Wu, N.; Nguyen, S. T.; Ruoff, R. S. J. Mater. Chem. 2006, 16, 155. (33) Fischer, D.; Wentzcovitch, R.; Carr, R.; Continenza, A.; Freeman, A. Phys. Rev. B: Condens. Matter. 1991, 44, 1427−1429. (34) Ågren, H.; Vahtras, O.; Carravetta, V. Chem. Phys. 1995, 196, 47−58. (35) Brühwiler, P.; Maxwell, A.; Puglia, C.; Nilsson, A.; Andersson, S.; Mårtensson, N. Phys. Rev. Lett. 1995, 74, 614−617. (36) (a) Jeong, H.-K.; Noh, H.-J.; Kim, J.-Y.; Colakerol, L.; Glans, P. -A.; Jin, M.; Smith, K.; Lee, Y. Phys. Rev. Lett. 2009, 102, 099701. (b) Jeong, H.-K.; Colakerol, L.; Jin, M. H.; Glans, P.-A.; Smith, K. E.; Lee, Y. H. Chem. Phys. Lett. 2008, 460, 499−502. (37) Jeong, H.-K.; Noh, H.-J.; Kim, J.-Y.; Jin, M. H.; Park, C. Y.; Lee, Y. H. EPL 2008, 82, 67004. (38) Zhan, D.; Ni, Z.; Chen, W.; Sun, L.; Luo, Z.; Lai, L.; Yu, T.; Wee, A. T. S.; Shen, Z. Carbon 2011, 49, 1362−1366. (39) Strocov, V.; Blaha, P.; Starnberg, H.; Rohlfing, M.; Claessen, R.; Debever, J.-M.; Themlin, J.-M. Phys. Rev. B 2000, 61, 4994−5001. (40) Acik, M.; Lee, G.; Mattevi, C.; Pirkle, A.; Wallace, R. M.; Chhowalla, M.; Cho, K.; Chabal, Y. J. Phys. Chem. C 2011, 115, 19761−19781. (41) Mkhoyan, K. A.; Contryman, A. W.; Silcox, J.; Stewart, D. A; Eda, G.; Mattevi, C.; Miller, S.; Chhowalla, M. Nano Lett. 2009, 9, 1058−63. (42) Kuznetsova, A.; Popova, I.; Yates, J. T.; Bronikowski, M. J.; Huffman, C. B.; Liu, J.; Smalley, R. E.; Hwu, H. H.; Chen, J. G. J. Am. Chem. Soc. 2001, 123, 10699−10704. (43) Tung, V. C.; Allen, M. J.; Yang, Y.; Kaner, R. B. Nat. Nanotechnol. 2009, 4, 25−9. (44) Fowler, J. D.; Allen, M. J.; Tung, V. C.; Yang, Y.; Kaner, R. B.; Weiller, B. H. ACS Nano 2009, 3, 301−6. (45) Coffman, F. L.; Cao, R.; Pianetta, P. a.; Kapoor, S.; Kelly, M.; Terminello, L. J. Appl. Phys. Lett. 1996, 69, 568. (46) Francis, J. T.; Hitchcock, A. P. J. Phys. Chem. 1992, 96, 6598− 6610. (47) Pacilé, D.; Meyer, J. C.; Fraile Rodríguez, A.; Papagno, M.; Gómez-Navarro, C.; Sundaram, R. S.; Burghard, M.; Kern, K.; Carbone, C.; Kaiser, U. Carbon 2011, 49, 966−972. (48) Hemraj-Benny, T.; Banerjee, S.; Sharadha, S.; Mahalingam, B.; Fischer, D. A.; Eres, G.; Alexander, P. A.; Geohegan, D. B.; Lowndes, D. H.; Weiqiang, H.; Misewich, J. A.; Wong, S. S.; Sambasivan, S.; Balasubramanian, M.; Puretzky, A. A.; Han, W. Small 2006, 2, 26−35.

AUTHOR INFORMATION

Corresponding Author

*E-mail: sb244@buffalo.edu. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We gratefully acknowledge Dr. David Prendergast of The Molecular Foundry for helpful discussions. This work was primarily supported by the National Science Foundation under DMR 0847169. Certain commercial names are presented in this manuscript for purposes of illustration and do not constitute an endorsement by the National Institute of Standards and Technology. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC0298CH10886

■

REFERENCES

(1) Castro Neto, A.; Guinea, F.; Peres, N.; Novoselov, K.; Geim, A. Rev. Mod. Phys. 2009, 81, 109−162. (2) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183−91. (3) Geim, A. K. Science 2009, 324, 1530−4. (4) Wu, Y.; Lin, Y.; Bol, A. A; Jenkins, K. A; Xia, F.; Farmer, D. B.; Zhu, Y.; Avouris, P. Nature 2011, 472, 74−8. (5) Morozov, S.; Novoselov, K.; Katsnelson, M.; Schedin, F.; Elias, D.; Jaszczak, J.; Geim, A. Phys. Rev. Lett. 2008, 100, 11−14. (6) Chen, J.-H.; Jang, C.; Xiao, S.; Ishigami, M.; Fuhrer, M. S. Nat. Nanotechnol. 2008, 3, 206−9. (7) Li, X.; Cai, W.; An, J.; Kim, S.; Nah, J.; Yang, D.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; Banerjee, S. K.; Colombo, L.; Ruoff, R. S. Science 2009, 324, 1312−4. (8) Lee, Y.; Bae, S.; Jang, H.; Jang, S.; Zhu, S.-E.; Sim, S. H.; Song, Y. I.; Hong, B. H.; Ahn, J.-H. Nano Lett. 2010, 10, 490−493. (9) Dikin, D. A.; Stankovich, S.; Zimney, E. J.; Piner, R. D.; Dommett, G. H. B.; Evmenenko, G.; Nguyen, S. T.; Ruoff, R. S. Nature 2007, 448, 457−460. (10) Park, S.; An, J.; Piner, R. D.; Jung, I.; Yang, D.; Velamakanni, A.; Nguyen, S. T.; Ruoff, R. S. Chem. Mater. 2008, 20, 6592−6594. (11) Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y.; Wu, Y.; Nguyen, S. T.; Ruoff, R. S. Carbon 2007, 45, 1558−1565. (12) Loh, K. P.; Bao, Q.; Eda, G.; Chhowalla, M. Nat. Chem. 2010, 2, 1015−24. (13) William, S.; Hummers, R. E. O. J. Am. Chem. Soc. 1958, 80, 1339−1339. (14) Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S. Chem. Soc. Rev. 2010, 39, 228−40. (15) Hossain, M. Z.; Johns, J. E.; Bevan, K. H.; Karmel, H. J.; Liang, Y. T.; Yoshimoto, S.; Mukai, K.; Koitaya, T.; Yoshinobu, J.; Kawai, M.; Lear, A. M.; Kesmodel, L. L.; Tait, S. L.; Hersam, M. C. Nat. Chem. 2012, 4, 305−9. (16) Bagri, A.; Mattevi, C.; Acik, M.; Chabal, Y. J.; Chhowalla, M.; Shenoy, V. B. Nat. Chem. 2010, 2, 581−7. (17) Kuilla, T.; Bhadra, S.; Yao, D.; Kim, N. H.; Bose, S.; Lee, J. H. Prog. Polym. Sci. 2010, 35, 1350−1375. (18) Stankovich, S.; Dikin, D. a; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. a; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Nature 2006, 442, 282−6. (19) Bai, H.; Li, C.; Shi, G. Adv. Mater. 2011, 23, 1089−115. (20) Brodie, B. C. Phil. Trans. R. Soc. London 1859, 149, 249−259. (21) Lerf, A.; He, H.; Forster, M.; Klinowski, J. J. Phys. Chem. B 1998, 102, 4477−4482. (22) Szabó, T.; Berkesi, O.; Forgó, P.; Josepovits, K.; Sanakis, Y.; Petridis, D.; Dékány, I. Chem. Mater. 2006, 18, 2740−2749. 20598



Article

(49) Katsnelson, M. I.; Geim, a K. Philos. Trans. R. Soc. A 2008, 366, 195−204.

20599


Soft X-ray Absorption Spectroscopy Studies of the Electronic Structure

Recommend Documents