Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Reversibility of Graphene Photochlorination Gabriela Copetti,† Eduardo H. Nunes,‡ Guilherme K. Rolim,‡ Gabriel V. Soares,† Silma A. Correa,‡ Daniel E. Weibel,‡ and Cláudio Radtke*,‡ †
Instituto de Física and ‡Instituto de Química, UFRGS, 91509-900 Porto Alegre, Brazil
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S Supporting Information *
ABSTRACT: The reversibility of graphene photochemical chlorination was investigated. A high content of Cl is obtained through photochlorination, without changing C hybridization. To accommodate the incorporated Cl, graphene corrugation takes place. However, due to weak bonding, Cl atoms desorb during air exposure and long periods of storage. Chlorination also leads to graphene doping. When Cl is removed, doping decreases and graphene returns to its original morphology. Only a small amount of Cl (Cl/C ∼ 0.1) remains strongly bonded to graphene, most likely at grain edges and defects. Therefore, to maintain a precise doping level, Cl trapping methods are essential. Moreover, Cl removal using laser irradiation can be used to tune doping in micrometric areas, making it a promising technique to be used in applications where different doping levels are needed.
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the electronic band structure such as bandgap opening.5,6 Since graphene is a zero-gap material, bandgap engineering is crucial for applications in microelectronic devices that require materials with semiconducting behavior. Graphene is also known to be chemically inert. Functionalization with halogens increases surface reactivity, allowing the deposition of dielectric materials such as Al2O3.7 Moreover, the enhanced reactivity allows chlorinated graphene to act as an intermediate material for subsequent modifications by a great variety of organic compounds.8,9 Previous works have proposed that the nature of Cl interaction with graphene can be tuned using different techniques and processing parameters. Zhang and co-workers incorporated Cl on graphene using plasma.10 They observed that the interaction between carbon and chlorine can be tuned depending on the ion acceleration bias: both ionic and covalent bonding can be formed as well as structural defects. If Cl bonds covalently to the basal carbon atoms, a change of C hybridization from sp2 to sp3 occurs, a bandgap is opened, and graphene becomes a semiconductor.11 Li and co-workers showed that photochlorination can also be used to open a bandgap in graphene.5 The increase of the D peak in Raman spectra occurs, which is an indication of changes in hybridization. The so-called “ionic” bond, by its turn, occurs when no D peak is observed, and therefore there is no change in C hybridization or opening of the bandgap.4,10 Doping however does occur, enhancing conductivity and reducing sheet resistance. Pham and co-workers also demonstrated that Cl is lost during thermal treatments.4 For doping applications it is important to regard the stability of Cl adsorbed on graphene. It has been
INTRODUCTION Graphene is a promising material for application in electronic devices that require transparent and conductive thin films like touch screens and liquid-crystal displays (LCDs). Such devices are commonly produced using indium tin oxide (ITO). Because of the scarcity of indium reserves, the search for a new material gains further attention.1 The appropriate candidate must present a good conductivity in a relatively large area and, consequently, a low sheet resistance. Though theoretical graphene has an extraordinary carrier mobility (∼200 000 cm2/(V s)), graphene layers grown by chemical vapor deposition (CVD) present a great number of grain boundaries, wrinkles, and defects, greatly increasing the sheet resistance. Moreover, after transfer to a SiO2 substrate, graphene interacts with the substrate, being a source of carrier scattering.2 For this reason, it is essential to investigate techniques to enhance graphene’s conductivity, such as doping. There are several doping techniques, including electrostatic, substitutional, and chemical approaches.3 Electrostatic doping consists of changing the Fermi level by exposing graphene to an electric field, while substitutional doping occurs when carbon atoms are replaced by atoms with different numbers of valence electrons. Chemical doping, on the other hand, is due to the adsorption of species such as gases or liquids on graphene, in a way that these act as electron donors or acceptors. The latter is the underlying mechanism when graphene is exposed to high electronegative elements like chlorine (Cl), which pulls the electron density from the π cloud resulting in p-type doping.4 Cl incorporation can be used to modify graphene properties not only in the doping scenario. Graphene functionalization by halogenation has recently received much attention for opening paths to a great number of applications. The incorporation of halogens such as Cl gives rise to significant modifications on © XXXX American Chemical Society
Received: March 2, 2018 Revised: May 4, 2018 Published: June 20, 2018 A
DOI: 10.1021/acs.jpcc.8b02121 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C theoretically predicted that chlorinated graphene may not be stable even with Cl covalently bonded.11 Therefore, the control and understanding of the stability of Cl on graphene are crucial. The same authors proposed an interesting method to trap the physisorbed Cl dopants and keep doping stable during annealing by transferring a second graphene layer on top of the chlorinated layer.4 Nonetheless, a thorough investigation of the stability of chlorinated graphene is essential and has not been performed to the best of our knowledge. In this work, we demonstrate that Cl incorporation through photochemical chlorination leads to morphological modifications of graphene, without appreciable modification of C hybridization due to Cl incorporation. The photochlorination process results in the lowering of graphene sheet resistance due to doping. These modifications can be reversed through thermal treatment. It was observed that weakly bonded Cl desorbs even at room temperature. However, desorption can be minimized with proper storage. Nonetheless, a small amount of Cl remains more strongly attached to graphene, even after annealing. The reversibility of photochlorination allows doping to be tuned in micrometric areas by removing Cl using laser irradiation.
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Figure 1. XPS survey spectrum of a photochlorinated graphene sample evidencing a Cl/C ratio of 1.7. Si and O signals are from the underlying substrate. a.u. stands for arbitrary units.
EXPERIMENTAL DETAILS
Commercially available CVD-grown graphene/SiO2(300 nm)/Si samples (Graphene Supermarket) were submitted to photochlorination, which was performed using the system described by Li and co-workers.5 The samples are exposed to a Cl2 gas flux with UV light from a xenon−mercury lamp (80 W/cm2). Cl2 gas was produced by the MnO2(s) + 4HCl(aq) → Cl2(g) + MnCl2(aq) + 2H2O(l) reaction at 90 °C. Water vapor was removed by passing the gas through H2SO4 before reaching the sample in a quartz reactor. X-ray photoelectron spectroscopy (XPS) using a conventional Al X-ray source was used to investigate the amount of Cl incorporation. Graphene doping was verified by micro-Raman spectroscopy using a 532 nm laser and van der Pauw Hall measurements. To investigate Cl removal due to thermal treatments, samples were submitted to annealing at 330 °C in a resistively heated furnace under a 1 atm Ar atmosphere. High-resolution XPS and near-edge X-ray absorption fine structure (NEXAFS) measurements were performed at the Brazilian Synchrotron Light Laboratory, Campinas, Brazil. The PGM (Planar Grating Monochromator) beamline, which is dedicated for X-ray spectroscopy in the soft X-rays (100−1500 eV) and gives a spectral resolution (E/ΔE) from 1000 up to 25 000, was used as the monochromatic photon source. An elliptical polarization undulator with a 50 mm period was used to linearly polarize the beam. Cl areal density on highly oriented pyrolytic graphite (HOPG) submitted to chlorination was obtained by Rutherford backscattering spectrometry (RBS) using He2+ ions of 2 MeV.
Figure 2. (a) C 1s and (b) Cl 2p regions of XPS spectrum of chlorinated graphene. a.u. stands for arbitrary units.
similar to graphene samples chlorinated with plasma.10 They are a result of different amounts of Cl atoms neighboring C. Spectra of the pristine sample are shown in the Supporting Information for comparison. Polymers with high Cl content and chlorinated carbon black evidence similar high binding energy components related to C atoms bonded to two or more Cl atoms.13,14 Indeed, the 288.3 eV component increases with the Cl/C ratio (see Supporting Information). However, oxygenated functionalities (originated from contaminants) cannot be discarded since they appear at the same binding energy range. Additionally, analyzing the Cl 2p region of the XPS spectrum of the chlorinated samples (Figure 2b), one observes a Cl 2p3/2 line at energies corresponding to that of organic chlorine, i.e., Cl bonded to C.14,15 High-resolution XPS measurements using synchrotron light show that the Cl 2p spectra can be deconvoluted in three components, indicating that Cl is present in different chemical environments, also likely due to the different amounts of Cl or O neighboring atoms (see Supporting Information). These components can be assigned to organochlorides,16 confirming the interaction of Cl adsorbed atoms with the graphene layer. It was observed that UV light is essential to the chlorination process. Samples only exposed to Cl2 gas presented minimal Cl adsorption (Cl/C < 0.05). Similar chlorination experiments performed with highly oriented pyrolytic graphite (HOPG) showed that Cl is incorporated not only in the surface but also in the underlying graphene layers (see Supporting Information). This observation
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RESULTS AND DISCUSSION Photochemical chlorination allowed the incorporation of high Cl concentrations. The Cl/C ratio calculated from the XPS spectrum of a sample chlorinated for 20 min (Figure 1) is 1.7, much larger than previously reported values for photochlorinated graphene (Cl/C = 0.08).5 The C 1s region of the XPS spectrum (Figure 2) was fitted with four components. The one at lower binding energy (∼284.5 eV) is attributed to C atoms experiencing the lower influence of Cl adsorption since its binding energy is close to that of C in graphene.12 Three higher binding energy components appear following Cl incorporation B
DOI: 10.1021/acs.jpcc.8b02121 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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ideal flat sample. Chlorination of graphene resulted in a DR decrease from −0.80 to −0.07. Therefore, the high concentration of adsorbed Cl causes the graphene layer to corrugate in order to accommodate its presence. Besides, after the chlorination process, a component due to C interaction with Cl arises at 288.6 eV (Figure 3d). Following annealing, the intensity of the component due to Cl in the C K-edge spectra is reduced, and π bonds show a preferential orientation once again (DR = −0.96). Changes in the morphology of the graphene layer due to Cl adsorption are therefore reversible. Because of the high Cl electronegativity and the amount of this element incorporated into graphene, doping should be observed. Figure 4a shows Raman spectra of graphene samples
confirms that Cl can also be intercalated between graphene and SiO2. Figure 3 presents the C 1s NEXAFS spectra of a pristine sample, a sample chlorinated for 20 min, and the same chlorinated
Figure 4. (a) Raman spectra of graphene/SiO2/Si samples with different Cl/C ratios obtained by submitting samples to chlorination for 0, 5, and 10 min. (b) Raman spectra of a sample chlorinated for 20 min before and after being submitted to laser irradiation at 20 mW/μm2. a.u. stands for arbitrary units. Figure 3. Angle-resolved C K-edge NEXAFS spectra measured for pristine (a), chlorinated (b), and annealed (c) samples. The annealing of the chlorinated sample was performed at 330 °C under an Ar atmosphere. Indicated angles are set between incident beam direction and sample plane. The dashed line indicates the energy position of the component due to Cl incorporation. (d) Comparison between spectra obtained at 50° for each sample.
with different Cl/C ratio that were produced varying the photochlorination time from 0 to 10 min. Interestingly, the D peak does not increase with the Cl/C ratio, indicating that neither modifications in C hybridization nor defect formation occur.18 This confirms that Cl is ionically bonded to C, since other authors have shown that weak ionic Cl−C bonds in chlorinated graphene lead to similar XPS spectra without changing in C hybridization.4,10 Additional peaks appear at 1160, 1290, and 1500 cm−1 in the spectra of highly chlorinated samples. Raman measurements on poly(vinyl chloride) (PVC) show peaks at 1100 and 1500 cm−1.19 Also, similar signals (1128, 1264, and 1466 cm−1, using a 514 nm laser) appear in graphene layers highly doped with K and Rb, being attributed to graphene folding.20 This last observation agrees with the present NEXAFS data, which demonstrate film corrugation. The relation between these signals and Cl incorporation can be further ratified by Cl removal using laser irradiation as shown in Figure 4b. A more detailed analysis of Raman signals further evidenced the effects of graphene chlorination. Previous works21,22 have shown that graphene p-type doping results in (i) blue-shift of the Raman G and 2D peaks, (ii) the decrease of the full width at half-maximum of the G peak (FWHM(G)), and (iii) the decrease of the ratio between the intensities of the 2D and G peaks. The blue-shift of the G peak occurs in both p and n-type doping, since the change in the Fermi level moves the Kohn anomaly (which is responsible for decreasing phonon frequency) away from phonons with wave vector q = 0. Additionally, since the number of transitions that allow electron−hole pair formation and recombination is reduced, the energy dispersion is smaller and the FWHM(G) decreases. The intensity of the G peak (I(G)) is increased at high doping levels since the occupancy modification of electronic states excludes electronic
sample following annealing in Ar (1 atm, 330 °C) for 30 min. In these specific conditions, the Cl/C ratio is reduced to about 0.1 as shown by XPS. Spectra have been acquired at different sample geometries, resulting in four beam incident angles with respect to sample plane. Resonance peaks in these absorption spectra are a result of electron transitions from core levels to unoccupied states. The main resonances are at 285.2 eV (labeled π*) and 291.5 eV (labeled σ*), corresponding to transitions from C 1s core levels to π* and σ* states, respectively.17 The directional properties of these resonances depend upon the molecular orientation in relation to the electric-field vector of the synchrotron radiation. Observing the absorption curves, one can see that the peak due to the C 1s → π*CC transition of the pristine sample varies with the incident angle, its intensity being maximum when this angle tends to zero. This indicates that the π bonds are parallel to the sample plane. A large band due to interlayer states as well as contaminants such as oxygen and hydrogen is also observed. After chlorination, the lack of angular dependence shows that the π bonds in graphene have no longer a preferential orientation. In order to obtain a numeric scale for graphene puckering, we calculated the dichroic ratio (DR),17 using the π* peak area (which varies linearly with cos2 θ, as shown in the Supporting Information). A value of 0 is expected for samples with no preferential orientation while −1 for an C
DOI: 10.1021/acs.jpcc.8b02121 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C
Annealing the sample in Ar (1 atm, 200 °C), part of the Cl was removed, decreasing greatly the hole density and increasing sheet resistance almost to their original values. This demonstrates that the doping due to Cl is greatly reversible. The stability of chlorinated graphene was investigated following the Cl/C ratio during a month by performing a series of XPS measurements. We observed that Cl desorbs rather easily during ambient exposure. Figure 7 shows how a chlorinated
transitions, resulting in the suppression of destructive interference. The 2D peak blue-shifts in virtue of a decrease in the lattice constant that causes phonon frequency to increase: the high electronegativity of Cl pulls electrons from carbon, reducing its atomic radius. Raman spectra were analyzed in several points of two chlorinated samples to demonstrate the variation of these parameters with the Cl/C ratio, which can be observed in the histograms of Figure 5. All the consequences of p-type doping
Figure 7. Cl/C ratio as a function of the number of weeks exposed to atmospheric air. The respective C 1s and Cl 2s regions of XPS spectra are shown in the inset.
graphene sample stored in air had its C 1s signal modified as Cl was lost during this time period. After an initially rapid Cl desorption, Cl loss becomes much slower, maintaining a Cl/C ratio between 0.2 and 0.1, which indicates that some amount of Cl is more strongly bonded to graphene than others. This is corroborated by the fact that after being dipped in deionized water for 1 min chlorinated samples also remained with a 0.1 Cl/C ratio, which would again correspond to more tightly bonded Cl. To investigate the influence of the ambient in Cl desorption, samples were stored for a week in vacuum (10−8 mbar), nitrogen (1 atm), atmospheric air (1 atm), oxygen (1 atm), and water vapor (20 mbar) (Figure 8). Maintaining graphene in an inert
Figure 5. Histograms showing (a) G peak position (Pos(G)), (b) 2D peak (Pos(2D)) position, (c) the ratio between the 2D and G intensities (I(2D)/I(G)), and (d) the FWHM of the G peak for samples with different Cl/C ratios.
cited above were observed in our samples. Special attention can be given to the narrowing of the G peak, since, though it has been demonstrated through electrostatic doping,3,22 it was never shown experimentally in the scenario of dopant incorporation to our knowledge. It is therefore clear that doping was successfully achieved with Cl incorporation. We used the van der Pauw method to estimate hole density in a pristine sample, a chlorinated sample, and a chlorinated sample after annealing (Figure 6). It is important to note that measurements were
Figure 8. Cl percentage removed from samples kept for a week in four different storage atmospheres: nitrogen gas, vacuum, atmospheric air, water vapor, and oxygen.
Figure 6. Carrier density and sheet resistance of a commercial sample prior and after chlorination (Cl/C ∼ 0.2) obtained by the van der Pauw method. Annealing the sample in 1 atm of Ar for 30 min at 200 °C; the carrier density was reduced, and the sheet resistance was increased almost to their original values.
atmosphere greatly reduced the Cl loss. Vacuum also suppressed Cl desorption but is less effective. This is expected since lower pressures should facilitate desorption. However, it is clear that air components, like water and oxygen, have an important role in accelerating the Cl removal process. It was observed that after 1 week in 20 mbar of water vapor, almost 90% of the Cl content was lost. Therefore, it is extremely important to take into account sample exposure to air and humidity during device production.
performed one month after chlorination (as will be shown later, Cl/C is reduced from above 1.0 to about 0.2). Carrier density and sheet resistance were observed to be elevated in the commercial graphene used, probably due to its intrinsic low quality and long storage period in atmospheric air. However, it is clear from Figure 6 that after chlorination hole density increases and the sheet resistance is greatly reduced. D
DOI: 10.1021/acs.jpcc.8b02121 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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to Cl adatom and induces negligible distortion in the underlying planar graphene. This study also shown that migration of a single chlorine adatom on the surface of perfect graphene takes place almost without barrier. These theoretical results points to the weak bonding configuration of Cl and graphene. Moreover, different bonding types were obtained with plasma treatments of graphene under different dc bias conditions.10 At low bias, no pronounce D peak was observed in the Raman spectra after chlorination, while at higher bias values a substantial D peak is observed. That observation evidence that Cl interacts differently with graphene depending on the chlorination procedure. In view of that, different bonding configurations are obtained. In our case, Cl radicals did not substantially distort bonding in graphene, resulting in a weak D peak. NEXAFS measurements show that graphene corrugates to accommodate this weakly bonded Cl. The extent of corrugation spans orbital bending and nanometric puckering of the graphene layer. Nonetheless, Cl could form covalent bonds at grain boundaries and defects without giving origin to a D peak and would correspond to the more strongly bonded Cl. To remove these more strongly bonded Cl, more sophisticated techniques such as excitation at energies above the Cl 2p edge are required (see Supporting Information).
Because of the Cl loss observed at different atmospheres at room temperature, one should expect its removal by other methods involving energy transfer to the sample. An example is sample irradiation during XPS and Raman analyses. Indeed, we have observed that X-rays and laser radiation could remove incorporated Cl if the power density was high enough. Previous work has already shown that laser irradiation can remove Cl incorporated on graphene.23 This characteristic can be explored for tuning the doping level of graphene: by increasing the power density of the Raman laser from 2 to 20 mW/μm2, irradiation was used to remove Cl and lower graphene doping while obtaining the Raman spectra. After each irradiation step, the position of the G and 2D peaks red-shifts, the G peak broadens, and the I(2D)/I(G) ratio increases (Figure 9). Additionally, the
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CONCLUSIONS The reversibility of graphene chlorination, achieved by the photochemical method, was investigated. Cl does not change the hybridization of the basal carbon atoms, evidencing ionic aspect of the C−Cl bonds. In order to accommodate the great amount of incorporated Cl, graphene layer corrugation takes place. Graphene doping due to Cl is verified and can be reversed through thermal annealing. However, due to its weak binding, Cl desorbs during air exposure and long periods of storage. After Cl desorption, graphene returns to its original morphology. Some Cl atoms (Cl/C ∼ 0.1), however, remain more strongly bonded to graphene, most likely at grain edges and defects. It is important to regard X-ray and laser power density in the study of chlorinated graphene, since it was observed that they can cause Cl removal during characterization procedures, such as XPS and Raman, if no proper care is taken. Therefore, due to the lack of stability of the Cl−C bonds, for maintaining a precise doping level, Cl trapping methods are necessary. On the other hand, the easy removal of Cl with laser irradiation makes it a promising method for doping tuning in micrometric areas, allowing the architecture of pathways with different doping levels and paving the way to several applications where variable doping is needed.
Figure 9. Variation of Pos(G), Pos(2D), FWHM(G), and I(2D)/I(G) with each irradiation step at a laser power density of 20 mW/μm2. For comparison, the median values of these parameters from the histograms of the pristine sample in Figure 5 are marked with stars.
peaks at 1160, 1290, and 1500 cm−1 disappear after laser irradiation as was seen in Figure 4b, demonstrating that the effects of Cl incorporation were reversed. Therefore, laser irradiation allows the formation of micrometric areas with different doping levels. Complete undoping was not achieved probably due to the more strongly bonded Cl atoms. We concluded from Raman measurements that the Cl−C interaction at the basal plane is not strong enough to be considered a classic covalent bond, which changes carbon hybridization. Therefore, it is expected that a great majority of the Cl atoms forms only weak ionic bonds to graphene, which is corroborated by the rapid initial Cl loss during air exposure. Density functional theory (DFT) calculations24 evidenced various states when one side of graphene is exposed. In the initial reaction stage, it forms Cl−graphene charge-transfer complex, where the C orbitals keep sp2 hybridization and the graphene is p-type doped, in agreement with the present results. Another theoretical investigation25 also proposed that the bonding of a single Cl atom is ionic through the transfer of charge from graphene
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b02121.
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Variation of the C 1s 288.3 eV component with Cl/C ratio; high-resolution XPS of Cl 2p region; integrated areas of π* peaks as a function of cos2 θ; chlorine removal by inner-shell excitation; HOPG chlorination; XPS survey spectra before and chlorination (PDF)
AUTHOR INFORMATION
Corresponding Author
*Tel +55 51 33086204; e-mail
[email protected] (C.R.). E
DOI: 10.1021/acs.jpcc.8b02121 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C ORCID
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Daniel E. Weibel: 0000-0002-9104-4791 Cláudio Radtke: 0000-0003-3469-4920 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the financial support of INCT Namitec, INCT INES, MCT/CNPq, CAPES, and FAPERGS. This research used resources of the Brazilian Synchrotron Light Laboratory (LNLS), an open national facility operated by the Brazilian Centre for Research in Energy and Materials (CNPEM) for the Brazilian Ministry for Science, Technology, Innovations, and Communications (MCTIC). The PGM beamline staff is acknowledged for the assistance during experiments.
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