Letter pubs.acs.org/NanoLett
Chemical Stability of Graphene Fluoride Produced by Exposure to XeF2 Rory Stine,† Woo-Kyung Lee,‡ Keith E. Whitener, Jr.,‡,∥ Jeremy T. Robinson,§ and Paul E. Sheehan*,‡ †
Nova Research, 1900 Elkins Street Suite 230, Alexandria, Virginia 22308, United States Chemistry Division and §Electronic Science and Technology Division, U.S. Naval Research Laboratory, Washington, DC 20375, United States
‡
S Supporting Information *
ABSTRACT: Fluorination can alter the electronic properties of graphene and activate sites for subsequent chemistry. Here, we show that graphene fluorination depends on several variables, including XeF2 exposure and the choice of substrate. After fluorination, fluorine content declines by 50−80% over several days before stabilizing. While highly fluorinated samples remain insulating, mildly fluorinated samples regain some conductivity over this period. Finally, this loss does not reduce reactivity with alkylamines, suggesting that only nonvolatile fluorine participates in these reactions. KEYWORDS: Graphene, graphene fluoride, fluorographene, stability, xenon difluoride
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ince its initial reporting in 2010,1−3 the intriguing properties of fluorographene have been of growing research interest. Recent work has shown that by altering the concentration of fluorine, the electronic properties of graphene, including resistivity,4 band gap,5 and work function,6 can be tailored for a variety of needs. Fluorination may even produce defects in graphene with local magnetic moments.7,8 Moreover, the patterning of graphene/fluorographene using photolithography,1 lasers,9 electron beams,10 or scanning probes11,12 promises a means of fabricating conducting and semiconducting pathways on graphene sheets. As with carbon nanotubes,13 it has been found that the attachment of fluorine to graphene is a facile means to activate the carbon bonds for subsequent covalent bonding to other molecules.14−16 Other applications for fluorographene have been explored and range from the promotion of dielectric deposition on graphene17 to the promotion of neuro-induction of stem cells.18 Each of these applications, however, depends on reproducible fluorination to achieve the desired characteristics, so it is critical that a detailed study on fluorographene stability be undertaken. While fully fluorinated graphene is quite stable,2 partially fluorinated graphene (called here graphene fluoride) of the type required for tuning of electrical and chemical properties has not been extensively studied. It has been noted that the top several layers of graphite fluoride appear to defluorinate in the presence of humidity,19 and recent work showing that the composition of graphene oxide20,21 changes over time provides additional impetus for undertaking stability studies of this material. Herein, we explore the fluorination of graphene sheets on Cu, Au, and SiO2 substrates upon exposure to XeF2 gas, and the subsequent decay of the resulting graphene fluoride over time © XXXX American Chemical Society
at room temperature. X-ray photoelectron spectroscopy (XPS) directly tracked both the atomic percentage of fluorine present and the carbon 1s bonding state, while conductivity measurements indirectly measured the reformation of sp2 carbon as fluorine departs. The substrate supporting the graphene dramatically affected both the rate of fluorination and the level of achievable fluorination. We also show that, depending on storage conditions, anywhere from 50 to 80% of the initial fluorine on graphene fluoride can be lost over several days, when a stable concentration is finally reached. Some of this fluorine loss leads to regions of reformed sp2 carbon and, consequently, enhanced conductivity for lightly fluorinated samples. Most intriguingly, we find that the loss of the volatile fluorine does not reduce the extent of subsequent chemical reactions with alkylamine compounds, suggesting that only the nonvolatile fluorine activates the graphene for chemistry. Graphene films were grown inside Cu foil enclosures using low-pressure CVD as described elsewhere22 with typical partial pressures of PH2 ≈ 5 mtorr and PCH4 ≈ 40 mtorr. For the growth of “small-grain” graphene films, a PCH4 = 150 mtorr was used as well as a flat Cu foil that was not in an enclosure. The Cu foils were pretreated in an acetic acid bath at 30 °C for ∼10 min,23 then cleaned with a dry CO2 snow jet prior to loading into the growth system. For the fluorination studies, some of the graphene films were kept on the Cu growth substrate. For graphene films on SiO2 and Au, we used the conventional wet chemical transfer approach,24 where a PMMA protective layer Received: June 10, 2013 Revised: August 23, 2013
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Figure 1. Summary of XPS data showing the relative degree of fluorination for graphene samples on SiO2, Au, and Cu substrates after varying XeF2 exposure times (a). The subsequent decline of the fluorine content over several days after fluorination is also shown for SiO2 (b), Au (c), and Cu (d) for samples stored both in air and under dry N2.
intervals in the fluorination process (0, 5, 10, and 15 min of XeF2 exposure) show a gradual increase in both surface roughness and graphene height at higher degrees of fluorination (see Supporting Information, Figures SI1 and SI2). Once the samples had reached their fluorination plateaus, the subsequent stability of the graphene fluoride was monitored with XPS. Figure 1b−d shows the rise in sample fluorination with XeF2 exposure time (left axis in each figure, taken from the data in Figure 1a and included as a guide), followed by the decline in fluorine content as the samples are stored over a number of days (right axis in each figure). Note that while the data for fluorination as a function of XeF2 exposure time was presented as the ratio of F to C peak intensities to allow direct comparison among sample sets, the data showing fluorine desorption is presented as the ratio of F to substrate peak intensity (Si 2p, Au 4f, or Cu 2p, respectively). This change was made because the build-up of adventitious carbon on the samples over several days skewed the F/C ratios and exaggerated the fluorine decline. All three sample sets, despite differing initial stoichiometries, show a similar pattern of fluorine loss over time. For all three substrate materials, samples stored under dry N2 lost roughly 50% of their initial fluorine coverage (52% for SiO2, 54% for Au, 55% for Cu) over 10 days of storage with most of the loss occurring in the first 3 days. The fluorine loss for samples stored in air was greater; 75, 80, and 61% for SiO2, Au, and Cu, respectively. The decrease in fluorine followed an exponential decay, approaching what appears to be a steady concentration over time. Starting from the stoichiometries at maximum fluorination, these losses would lead to steady state stoichiometries for samples stored in N2 of approximately C3F, C35F, and C22F for SiO2, Au, and Cu respectively, and C6F, C80F, and C26F for the respective substrates stored in air. Note that the loss of roughly half the fluorine does not depend on the duration of fluorination. Graphene samples on SiO2 that were not subjected to the
was deposited on the graphene-on-Cu sample and the Cu foil was subsequently etched using an oxygenated APS-100 Transene etchant solution.25 Graphene was then fluorinated by exposure to XeF2 gas as described previously.1 The Xactix etching system was used in pulse mode with PXeF2 = 1 Torr, PN2 = 35 Torr and pulse time = 60s. After fluorination, samples were stored either covered in air, or in a drybox purged with flowing N2. Figure 1a shows the relative increase in graphene fluorination, measured via XPS fitting of the C 1s and F 1s peaks with increasing XeF2 exposure time for graphene on SiO2, Au, and Cu substrates. Peak intensities are expressed as ratios (either to total carbon or to the main substrate material) so that valid comparisons between samples can be made. On both Au and Cu substrates, the level of fluorination increases rapidly over the first several minutes of exposure, then levels off at relatively low coverage (near C10F; ∼9 atomic % F). The samples on SiO2, however, fluorinate more rapidly at the outset and continue to fluorinate for exposures up to 15 min before finally achieving a stoichiometry close to C1.5F (∼40 atomic % F). Several factors could lead to higher fluorination of graphene on SiO2. First, the enhancement of graphene’s chemical reactivity by charge puddles in the underlying SiO2 substrate has been noted previously by Fan et al.,26 using diazonium salt chemistry. While their analysis specifically proposes charge transfer to a reactant in solution as a key step, the more general point is that the charge puddles create a broad distribution of local chemical potentials which would similarly enhance fluorination. Additionally, given the steric constraints of fluorinating graphene beyond C2F stoichiometry, fluorination of the underside of the graphene seems a necessary conclusion at the longest exposure times. It is possible that the films’ underside is more accessible on SiO2 than on the metal substrates due to oxide defects, surface roughness, or a slow etching of the SiO2 in the presence of XeF2. Atomic force microscopy (AFM) images of graphene on SiO2 taken at B
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290.9, and 292.0 eV (all shown in green) appear due to the presence of C−F functionalities. Additionally, the sp2 carbon peak disappears almost entirely, and the blue peaks associated with C−O functionalities increase noticeably. Over time, we see a decrease in the intensities of the C−F peaks, as well as an increase in both the C−O (blue) and C−C (red) peaks, including the reformation of some sp2 bonds. Notably, some of the additional intensity of the sp3 carbon peak most likely occurs due to the adsorption of adventitious carbon, as the overall intensity of the entire C 1s peak also increases. Raman data collected over the same time period showed no discernible trend in the D/G ratio for either peak intensity or peak area (Supporting Information, Figure SI5), indicating that the relative density of defect sites in the graphene did not appreciably change as a result of fluorine loss. Representative Raman spectra are also included in the Supporting Information (Figures SI6 and SI7). The strong electron withdrawing nature of fluorine complicates assigning individual component peaks in the C 1s region of highly fluorinated samples; however, a reasonable interpretation is the presence of both CF and CF2 bonds with differing local C−F concentrations. The peaks at 289.0 and 289.9 eV are most likely both due to C−F bonds with the higher energy peak representing regions of clustered C−F functionalities (several adjacent carbon atoms with C−F bonds), while the lower energy peak is due to isolated C−F bonds. A similar assignment can be made for the peaks at 290.9 and 292.0 eV for isolated and clustered CF2 regions, respectively. Such clustering may be attributable to the charge puddle effect of the underlying SiO2 substrate mentioned previously.26 Another possible contributor could be increased reactivity at the graphene grain boundaries. To test defect reactivity, a second set of graphene films were grown under higher methane pressures to produce intentionally small grain sizes. When exposed to XeF2, these small-grained (10 μm) samples, suggesting the XeF2 reacts preferentially at these sites (Supporting Information, Figure SI8). AFM images were taken at different durations of fluorination (Supporting Information, Figures SI1 and SI2); however, the resolution was insufficient to observe differences between fluorinated and nonfluorinated regions, and the clustering phenomena suggested could not be confirmed. The increase in peak intensity associated with C−O bonds immediately after XeF2 exposure suggests that at least some fluorine functionalities are immediately lost from the graphene surface as soon as the samples are exposed to air. Analysis of the O 1s peak for samples on Au (which has no interfering surface oxide) also shows a sharp increase in O 1s peak intensity immediately after XeF2 exposure (Supporting Information, Figure SI9). These peaks continue to increase as fluorine is gradually lost from the surface, suggesting substitution of fluorine by oxygen-rich functionalities as one mechanism of fluorine loss for samples left in the ambient. However, Supporting Information Figure SI9 also shows that while the overall oxygen content of samples stored in air does increase over time, the oxygen content of the samples stored in dry N2, predictably, does not increase. As shown in Figure 1, this does not stem the loss of fluorine from these samples, so additional pathways must exist. The reformation of carbon sp2 bonds over time seen in Figure 2 suggests that fluorine loss may also occur through reaction of adjacent carbon atoms to expel fluorine and reform C−C double bonds.
maximum XeF2 exposure times also showed a similar decay pattern (see Supporting Information, Figure SI3). This suggests that the chemistries engendered on the surface do not vary with exposure but are developed from the onset. Finally, we note that fluorine loss occurs even in vacuum (see Supporting Information, Figure SI4). We considered whether the fluorine decline was an artifact caused by screening from accumulated adventitious carbon. This seems unlikely since electrons from the substrate peaks would be screened at different rates due to the differing kinetic energies. More specifically, the binding energies of the substrate peaks (Au, 84 eV; Si, 104 eV; and Cu, 932 eV) lie on either side of the fluorine peak (688 eV), and so adventitious carbon would cause the fluorine to appear to decline on the Au and SiO2 samples while increasing on the Cu samples, which was not observed. Furthermore, the calculated electron attenuation lengths for even the high binding energy (low kinetic energy) Cu peak show escape depths considerably larger than any feasible adventitious carbon layer thickness, given the relatively small increase in the overall C 1s signal. Fluorine loss appears to occur through multiple pathways. Figure 2 shows representative XPS scans of the C 1s peak for
Figure 2. Representative XPS scans of the C 1s peak for graphene on SiO2 before and after fluorination, and for several days following under ambient storage. Red peaks represent C−C species bonds, blue peaks C−O species bonds, and green peaks C−F species bonds.
fluorinated graphene on SiO2 stored in air for different lengths of time. Prior to fluorination, we see the large peak at 284.5 eV indicative of the sp2 carbon of the graphene lattice (shown in red). Additional peaks at 285.5 eV (also in red), 286.9 eV (in blue), and 287.8 eV (also in blue) show the presence of sp3 carbon, C−OH groups, and CO groups, respectively. These peaks most likely arise from residual PMMA after transfer, adventitious carbon, and/or oxidized defects within the graphene film. Immediately after XeF2 exposure, we see a drastic change in the C 1s peak. Four peaks at 289.0, 289.9, C
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Additionally, we tracked the resistivity of graphene samples on SiO2 over time for various initial levels of fluorination (Figure 3). Samples with a high degree of fluorine (XeF2
Figure 3. Measurements of graphene resistivity over several days after fluorination for samples at different XeF2 exposure times, stored in both air and under dry N2. Figure 4. Summary of XPS data tracking the amount of EDA that is attached to fluorinated graphene (via the N 1s peak intensity) for samples reacted several days after fluorination (red line) as well as the stability of the EDA on the surface for several days after reaction (blue line).
exposure ≥9 min; >30 at% F) have high sheet resistivity (>1 TΩ/□) that did not measurably change with time, regardless of storage conditions (ρ/ρ0 remains constant at 1). Lightly fluorinated samples (XeF2 exposure = 3−6 min; 10−20 atom % F), however, showed resistivities that gradually decreased over time. This trend was particularly pronounced for samples that were stored in ambient (samples that also showed the largest decline in fluorine content in Figure 1) with resistivity dropping 40−50% from initial values immediately after XeF2 exposure (note that the average sheet resistivity of 3 min fluorinated samples stored in air for 20 days was 0.86 ± 0.66 MΩ/□, which is >400× the resistivity of the pristine graphene). This, along with the increase in sp2 carbon seen in the XPS data, suggests the presence of reformed conduction pathways as fluorine is lost. Taken together, these results should enable the choice of the desired level of fluorination or conductivity by first overshooting the extent of fluorination and then allowing the system to relax into the desired stoichiometry. Beyond the interest in graphene fluoride for electronics, there is significant interest in using chemically modified graphenes for subsequent chemical reactions. In carbon chemistry, fluorine is a powerful mediator for activating chemical bonds.13 Consequently, one final aspect we examined was the reactivity of graphene fluoride to other molecules over various points during the decay cycle. Ethylenediamine (EDA) was chosen as the reactive molecule, as we have shown previously that it forms dense brushes when reacted with graphene fluoride.14 The EDA concentration on graphene was tracked via XPS spectra of the N 1s and C 1s regions. Two different aspects of the reaction were investigated and are depicted in Figure 4. The first was to fluorinate graphene and then wait a number of days before reacting with EDA (red data points). Notably, despite the significant decline in fluorine content over time, the concentration of EDA attached to the graphene remained constant. As noted in our previous report,14 the amount of EDA that attaches to graphene fluoride is only about 30% of the total number of C−F sites for “fully” fluorinated graphene samples. That the same areal concentration of EDA on graphene is attained even after several days of fluorine loss suggests that only the nonvolatile fluorine (i.e., the C−F sites that remain after the initial fluorine loss) react with EDA. The second aspect of the EDA reaction investigated was the stability of the EDA on graphene. Here, EDA was
reacted with graphene fluoride immediately after XeF 2 exposure, and its concentration was monitored over a number of days (blue data points). Again, we see that the amount of EDA on the graphene surface remains stable over the length of the experiment and that it matches the areal concentrations of the previous experiment. Thus, unlike when engineering the conductivity of graphene, it is the initial number of binding sites generated during fluorination that is the critical control. While the nonvolatile versus volatile fluorination sites are likely the result of differing electron density within the C−F bonds, the exact nature of the bonding state is somewhat contentious. A “semi-ionic” binding state for fluorine has been proposed for graphite27 and carbon nanotubes28 and extended to graphene6 based largely on the position of the F 1s peak in XPS data.6,28 However, this interpretation has been called into question by Sato et al. on the basis of X-ray diffraction data which does not show the presence of any such “semi-ionic” bond. This group attributes the varying peak positions in XPS to differences in the local chemical environment, namely local degree of fluorination.29 We also see a shift in the peak position of the F 1s spectra with the peak initially appearing at 688.6 eV for samples on SiO2 (it appears slightly lower at 688.3 eV on metal substrates) and then shifting to a lower binding energy of 687.4 eV as the fluorine content decreases and stabilizes (Supporting Information, Figure SI10). For a possible explanation of this effect, we looked at XPS studies of polyfluorinated benzenes.30 In these studies, Clark et al. found that for molecules of the form C6H6−nFn, the F 1s binding energy increases linearly with n. They used CNDO/2 semiempirical calculations to determine that the partial negative charge on fluorine was lower for more highly fluorinated species as well as for fluorines that were in close proximity to other fluorines. In other words, the F 1s binding energy increased as the polarization of the C−F bond decreased. In turn, the polarization of the C−F bond decreases in the highly fluorinated species because multiple strongly electron-withdrawing fluorines compete for electron density. Conversely, in singly fluorinated benzene the fluorine can withdraw electrons D
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without competition from neighboring electronegative species, causing the C−F bond to be more highly polarized. Applying the observations of Clark et al. and Sato et al. to our own system gives us an approximate explanation of the observed XPS. Upon initial fluorination, a large number of fluorines compete for electron density from graphene. This causes the C−F bonds to be less polarized, thus increasing the binding energy of the F 1s peak. As volatile fluorine is removed from graphene, the remaining fluorines have less competition for electron density and the C−F bonds become more polarized, thus decreasing the binding energy of the F 1s peak. However we cannot attribute the shift in F 1s peak position solely to globally decreasing fluorine content, as the initial peak position at 688.6 eV and the percent loss and halflife of fluorine on SiO2-supported graphene were unchanged regardless of the initial XeF2 exposure time and degree of fluorination. We propose instead that the F 1s peak shift occurs because of local changes in fluorine content. Fluorination occurs preferentially near appropriately biased charge puddles,26 defect sites, or other anomalies in the graphene lattice, so these sites fluorinate at a much faster rate than pristine sites on the graphene basal plane. Once maximally fluorinated, however, all sites desorb fluorine at roughly equal rates. This model explains why the same XPS peak shift, percent fluorine loss, and half-life are observed independent of initial degree of fluorination. In conclusion, we have shown that to engineer a desired level of fluorination in CVD graphene using XeF2 the supporting substrate material, sample storage conditions, and initial graphene domain must all be taken into account. Graphene on SiO2 fluorinates to a considerably higher degree than does graphene on metal substrates, and the total fluorine content of all samples declines steeply over several days following XeF2 exposure before reaching a final, stable stoichiometry. This decline is exacerbated for samples left under atmospheric conditions, as exposure to air and ambient humidity appears to speed fluorine loss. Lightly fluorinated samples begin to regain a degree of conductivity after several days, and this is particularly pronounced in samples stored in ambient, which experienced the most fluorine loss. Finally, chemical modification of the graphene requires a different design rule since the amount of EDA that can be covalently attached to fluorinated graphene does not vary as total fluorine content declines, suggesting that only the nonvolatile C−F sites participate in this reaction.
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AUTHOR INFORMATION
Corresponding Author
* E-mail:
[email protected]. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest. ∥ National Research Council Postdoctoral Fellow.
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ACKNOWLEDGMENTS This work was supported by the Naval Research Laboratory Base Program and the Defense Threat Reduction Agency under MIPR number B112609M. K.E.W. appreciates the support of the National Research Council.
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ASSOCIATED CONTENT
S Supporting Information *
AFM images showing change in graphene height (Figure SI1) and surface roughness (Figure SI2) as a function of fluorination, data for graphene fluoride stability on samples that did not reach maximum fluorination (Figure SI3), data for graphene fluoride stability in vacuum (Figure SI4), data for Raman data during graphene fluoride decay (figure SI5), typical Raman spectra (Figures SI6 and SI7), data for fluorinated graphene samples with smaller grain sizes (Figure SI8), data for oxygen content of graphene fluoride during decay process (Figure SI9), XPS F 1s peak positions during decay process (Figure SI10). This material is available free of charge via the Internet at http://pubs.acs.org. E
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