Hydrogen-Terminated Graphene by Laser Vaporization-Controlled

Article pubs.acs.org/JPCC

Hydrogen-Terminated Graphene by Laser Vaporization-Controlled Condensation of Graphite Oxide. Observation of Hydrogen-Capped Carbon Chains CnH−, CnH+, and CnH2+ (n = 2−30) in the Gas Phase Parichehr Afshani,† Isaac Attah,† Sherif Moussa,† James Terner,† and M. S. El-Shall*,†,‡ †

Department of Chemistry, Virginia Commonwealth University, Richmond, Virginia 23284-2006, United States Department of Chemistry, King Abdulaziz University, Jeddah 21589, Saudi Arabia

‡

S Supporting Information *

ABSTRACT: We report the development of a facile process for the synthesis of hydrogen-terminated graphene using the laser vaporization-controlled condensation (LVCC) method. The method allows rapid deoxygenation of bulk graphite oxide (GO) in an atmosphere of helium or a helium−hydrogen mixture to produce hydrogen-terminated graphene (HTG) nanosheets without the need for any chemical reducing agents or solvents. Direct laser vaporization/ionization (LVI) of bulk GO in a vacuum coupled with a beam expansion of a carrier gas produces the carbon cluster series Cn with n in the range 4−30, in addition to hydrogenated carbon ions with a significant enhancement in the ion intensity of the n-even ions suggesting the formation of hydrogen-capped polyyne chains corresponding to the H−(C C)n−H+ and H−(CC)n− formulas. In contrast, LVI of bulk graphite under identical experimental conditions generates mainly the carbon cluster series Cn without any significant hydrogenation in both the positive and negative ion modes. The results confirm that the LVCC method of bulk GO produces HTG nanosheets which can be ionized and dissociated in a vacuum to produce the observed hydrogen-capped polyyne chains. These species could be deposited from the gas phase to form onedimensional conducting molecular wires for a variety of potential applications in nanoelectronics, sensors, and devices.

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INTRODUCTION Graphene, the building block of all sp2-derived allotropic forms of carbon, has become one of the most important topics of research and potential technological advances during recent years.1−10 This activity is due to the exceptional properties of graphene which include high intrinsic carrier mobility, quantum Hall effect at ambient temperature,7 a bipolar electric field effect,2 tunable band gap,3 high elasticity,4 high current density, chemical inertness, high thermal conductivity, optical transmittance, and superhydrophobicity at the nanometer scale.1 In addition, graphene with an impressive surface area and tunable defect sites has shown great promise as a 2-D catalyst support for metallic and bimetallic nanoparticles for a variety of applications in heterogeneous catalysis, sensors, hydrogen storage, and energy conversion.6,11−16 In addition to the current extensive interest in graphene, carbon clusters have been a subject of intense research for several decades and continue to gain more attention due to their interesting structures, properties, and important roles they play in flames, combustion processes, soot formation, organic aerosol, astrochemistry, and astrobiology.17−25 For example, astronomical observations show that interstellar media and solar nebulae contain diverse carbon species, including high abundances of hydrogen-capped carbon chains24,26,27 and © 2013 American Chemical Society

organic molecules such as methane, acetylene, benzene, polycyclic aromatic hydrocarbons, and polycyclic nitrogencontaining aromatic hydrocarbons.28,29 The evolution of the structures and properties of carbon clusters and the transformation between different forms of graphitic materials such as carbynes, fullerenes, carbon nanotubes, graphene, and graphite are of basic interest, specifically for understanding the unique properties of carbon nanomaterials and generally for new advances in the nanoscience and nanotechnology of carbon materials. For example, the extensive studies on the formation of carbon clusters by laser ablation of graphite in the 1980s have led to the discovery of fullerenes and subsequently to the synthesis of macroscopic quantities of fullerenes and its derivatives.18−23,30,31 Surprisingly, no analogous studies have been reported to date on the formation of carbon clusters from graphene nanosheets in spite of the development of a variety of physical and chemical methods for the synthesis of macroscopic quantities of graphene.1,4−6 These studies are important not only for identifying new families of carbon clusters that could be Received: February 5, 2013 Revised: March 30, 2013 Published: April 1, 2013 9485

dx.doi.org/10.1021/jp401318d | J. Phys. Chem. C 2013, 117, 9485−9495

The Journal of Physical Chemistry C

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were measured on a Thermo Fisher Scientific ESCALAB 250 using a monochromatic Al KR. For the UV−vis absorption spectra, the samples (GO or HTG) were dispersed in ethanol using a sonicator bath, and the absorption spectra were recorded using a Hewlett-Packard HP-8453 diode array spectrophotometer. For the FT-IR spectra, a KBr (IR grade) disk containing either GO or HTG was prepared and scanned from 4000 to 500 cm−1 using the Nicolet 6700 FT-IR system under transmission mode. The Raman spectra were measured using an excitation wavelength of 406.7 nm provided by a Coherent Sabre DBW krypton ion laser. The laser beam was focused to a 0.10 mm diameter spot on the sample with a laser power of 1 mW. The samples were pressed into a depression at the end of a 3 mm diameter stainless steel rod, held at a 30° angle in the path of the laser beam. The detector was a Princeton Instruments 1340 × 400 liquid nitrogen CCD detector, attached to a Spex model 1870 0.5 m single spectrograph with interchangeable 1200 and 600 lines/mm holographic gratings (Jobin Yvon). The Raman scattered light was collected by a Canon 50 mm f/0.95 camera lens. Though the holographic gratings provided high discrimination, Schott and Corning glass cutoff filters were used to provide additional filtering of reflected laser light, when necessary. RTOF-MS Experiments. The LVI experiments were carried out on a reflectron time-of-flight mass spectrometer (RTOFMS) (Figure S1, Supporting Information).47 For a typical experiment, the GO target is placed 0.5−1.5 cm from the nozzle face and displaced 0.5−1.2 cm from the beam axis. Helium or neon gas pulses (ultrahigh purity, Spectra Gases, 99.99%, 4−5 atm, 250−350 μs pulse widths, 8 Hz) are interacted with the charged species produced by the LVI of the GO target utilizing the second harmonic (532 nm) of a Nd:YAG laser (Continuum-Surelite SSP-I, 8−10 ns pulse width). A high-voltage pulse (300 V/cm) is applied to the repeller plate to accelerate the ions through the ion mirrors guided by the ion optics toward the MCP detector. The delay generator utilized is a BNC, model 555 generator. Data acquisition is carried out on a digital storage oscilloscope (Lecroy 9350A, 500 MHz), averaged, and transferred to a computer for further processing.

generated from graphene but also for understanding the structures of graphene produced by different techniques. Here we report on a solution-free, laser vaporization-controlled condensation (LVCC) process for the deoxygenation and reduction of bulk graphite oxide (GO) to form hydrogenterminated graphene (HTG) nanosheets that can be ionized in a vacuum to produce hydrogen-capped linear carbon chain ions containing −(CC)n− units with n at least up to 15. These species could be deposited from the gas phase to form onedimensional conducting molecular wires for a variety of potential applications in nanoelectronics, sensors, and devices. Recent advances in the production of graphene sheets through the reduction of GO have provided efficient approaches for the large-scale production of chemically converted graphene in solution.32−35 However, chemical reduction methods suffer from the difficulty of controlling the reduction process and the potential for residual contamination by the chemical reducing agents which necessitates the use of multiple solvents and several postsynthesis treatments such as drying and thermal annealing.33,36 Other methods such as thermal annealing at high temperatures, laser photoreduction, and plasma discharge processes offer several advantages over chemical reduction including the efficient removal of oxygen from GO without the use of toxic chemical reducing agents.37−43 However, thermal annealing requires high temperatures and the use of ultrahigh-vacuum equipment for the efficient removal of oxygen from GO.37,38 On the other hand, photoreduction methods including flash photolysis and laser irradiation have been demonstrated to be effective only in GO solutions14,15,42,43 or on thin films of GO supported on substrates.39−41,44 Similarly, a plasma-assisted reduction method has been recently applied to films of GO casted on polymeric substrates.45 In this paper, we demonstrate the application of the LVCC method to bulk GO solid under an inert gas at atmospheric pressure for the large-scale production of hydrogen-terminated graphene (HTG) nanosheets without the need for chemical reducing agents, solvents, or high temperatures. We have applied a combination of different techniques including laser vaporization/ionization (LVI) of bulk GO coupled with reflectron time-of-flight (RTOF) mass spectrometry to establish the formation of abundant hydrogencapped polyyne chains from the LVI of GO in vacuum, in contrast to graphite which produces neat carbon clusters. First, we present detailed analysis of the graphene nanosheets produced by the LVCC of bulk GO, and then we provide the first direct evidence for the formation of long chain carbyne ions generated from GO nanosheets.

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RESULTS AND DISCUSSION LVCC Synthesis of HTG from Bulk Graphite Oxide. The LVCC technique has been demonstrated to be an effective solvent-free method for the production of various types of nanomaterials including metal, alloy, metal oxide, metal carbide, metal nitride, and semiconductor nanocrystals.48−52 For the synthesis of HTG, the method consists of pulsed laser vaporization of a solid target of bulk GO into a selected gas mixture under controlled temperature and pressure. The GO solid is first pressed at 500 MPa using a hydraulic press in order to shape it into a cylindrical disk target. An important feature is the use of an upward diffusion cloud chamber at well-defined temperatures and pressures. A sketch of the chamber with the relevant components for the production of HTG is shown in Figure 1. The chamber consists of two horizontal, stainless steel plates, separated by a glass ring. The GO target is set on the lower plate, and the chamber is filled with a pure helium carrier gas (99.99% pure) or a mixture containing a known composition of hydrogen gas in helium (10−20% H2 in He). The GO target and the lower plate are maintained at a temperature higher than that of the upper one (temperatures are controlled by

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EXPERIMENTAL SECTION In the experiments, GO was prepared by the oxidation of highpurity graphite powder (99.999%, 200 mesh, Alfa Aesar) according to the method of Hummers and Offeman.46 After repeated washing of the resulting yellowish-brown cake with hot water, the powder was dried at room temperature under vacuum overnight. Characterization. TEM images were obtained using a Joel JEM-1230 electron microscope operated at 120 kV equipped with a Gatan UltraScan 4000SP 4K × 4K CCD camera. SEM images were carried out using a Quantum DS-130S dual stage electron microscope. The small-angle X-ray diffraction (SAXRD) patterns were measured at room temperature with an X’Pert Philips Materials Research diffractometer using Cu Kα1 radiation. The X-ray photoelectron spectroscopy (XPS) spectra 9486



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Figure 2 displays TEM and SEM images of the graphene sheets prepared by the LVCC method from GO in the presence of helium or 20% H2 in helium as a carrier gas. It is clear that LVCC produces wrinkled and partially folded sheets with a lateral dimension of up to a few micrometers in length. These images confirm that the LVCC process gently desorbs the GO sheets from the GO target where they undergo deoxygenation and reduction without evidence for the destruction of their layer structures or atomization to produce carbon nanoparticles. The XRD pattern of bulk GO (Figure 3) is characterized by a peak at 2θ = 10.9° corresponding to the (002) plane of GO

Figure 1. Schematic of the LVCC method used for the conversion of bulk graphite oxide (GO) into hydrogen-terminated graphene (HTG).

circulating fluids). The large temperature gradient between the bottom and top plates results in a steady convection current which is enhanced by using high-pressure conditions (800− 1000 Torr). The laser vaporization process (using the second harmonic of a Nd:YAG laser, λ = 532 nm, pulse width τ = 7 ns, repetition rate = 30 Hz, 40−60 mJ/pulse, Spectra-Physics LAB170-30) releases GO nanosheets into the gas phase within a very short time, typically 10−8 s, in a directional jet that allows directed deposition. The strong electron−phonon coupling within the GO nanosheets leads to a rapid local heating which results in the removal of the oxygen atoms from GO most likely in the form of CO and CO2. The remaining H atoms resulting from the dissociation of the OH and COOH groups terminate the deoxygenated GO nanosheets. The formation of the HTG nanosheets can be enhanced by the addition of 10−20% H2 to the He carrier gas during the LVCC process since the hydrogen molecules can be efficiently dissociated within the laser vaporization plume to produce reactive atomic hydrogen which can assist the reduction of GO and the formation of HTG. The resulting HTG nanosheets are carried by convection and deposited on the cold top plate of the chamber. Nichrome heater wires are wrapped around the glass ring and provide sufficient heat to prevent condensation of the HTG sheets on the ring and to maintain a constant temperature gradient between the bottom and the top plates. In a typical run, the laser operates at 30 Hz for about 30 min. Then the chamber is brought to room temperature, and the HTG sheets are collected under atmospheric conditions. Glass slides or Si wafers can be attached to the top plate when it is desired to examine the morphology of as-deposited HTG nanosheets. No materials are found on any other place in the chamber except on the top plate, thus supporting the assumption that convection carries the sheets to the top plate where deposition occurs.

Figure 3. X-ray diffraction data of (a) graphite, (b) graphite oxide (GO), and (c) graphene produced by the LVCC of graphite in helium. (d, e) HTG produced by LVCC of GO in helium and 20% H2−helium mixture, respectively.

which exhibits a larger d-spacing of 8.14 Å (as compared to the typical value of 3.34 Å in graphite), resulting from the insertion of hydroxyl and epoxy groups between the carbon sheets and the carboxyl groups along the terminal and lateral sides of the sheets as a result of the oxidation process of graphite.33,35 As

Figure 2. (a, b) TEM images of HTG nanosheets prepared by the LVCC of GO in pure He and 20% H2−He carrier gases, respectively. (c) SEM image of the as-deposited HTG prepared by LVCC of GO in He. 9487



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shown in Figure 3, following the LVCC process of GO in the presence of He or He−H2 carrier gas, the 2θ = 10.9° peak disappears, thus confirming the loss of a long-range order in the GO stacked layers and implying the deoxygenation of GO and the restoration of the sp2 carbon sites in the resulting HTG nanosheets.33,35,42 For comparison, we also carried LVCC of a bulk graphite target in the presence of helium carrier to directly remove graphene nanosheets as was shown previously.53 It is clear that the XRD of the graphene produced by the LVCC of graphite show the disappearance of the peak at 2θ = 27.9° corresponding to the (002) plane of hexagonal graphite as displayed in Figure 3. Therefore, the LVCC of both GO and graphite results in loss of the long-range orders associated with the 10.9° and 27.9° 2θ peaks, respectively. The UV−vis spectra of GO and graphene prepared by the LVCC method in the presence of He or He−H2 carrier gas and dispersed in ethanol are shown in Figure 4. The UV absorption

Figure 5. (a) FTIR spectra of graphite, graphite oxide (GO), and graphene prepared by the LVCC of graphite in He, GO in He, GO in a 10% H2−He mixture, and GO in a 20% H2−He mixture. (b) FTIR spectra of graphene prepared by the LVCC of GO in a 10% H2−He mixture using different laser powers (532 nm) as indicated.

cm−1, the O−H deformations of the C−OH groups at 1350− 1390 cm−1, the C−O stretching vibrations at 1060−1100 cm−1, and the epoxide groups (1230 cm−1).42,55 In addition, a strong and broad O−H stretching vibration at 3420 cm−1 is observed resulting from water adsorbed on GO. These bands are completely absent from the spectrum of graphene prepared in He or H2−He carrier gas, thus confirming the deoxygenation of GO following the LVCC process. The strong similarity between the FT-IR spectra of the graphene produced by the LVCC of graphite or GO and the spectrum of bulk graphite and the absence of the oxygen-related features confirm that the graphene sheets produced by the LVCC process are free of oxygen functional groups. Furthermore, the incorporation of hydrogen in the reduced GO is clearly evident from the spectrum of the HTG prepared in a 10% H2 in He shown in Figure 5a. The spectrum clearly shows aliphatic C−H stretching bands in the 2840−2950 cm−1 region. Similar results have been reported by Subrahmanyam et al., who used Birch reduction to hydrogenate few-layer graphene samples.56 Interestingly, the intensity of the observed C−H stretching bands in our experiments decreases significantly with increasing the laser vaporization power as shown in Figure 5b. This suggests that at higher temperatures caused by higher laser powers the incorporation of hydrogen within the reduced GO nanosheets decreases. This observation is consistent with the results of Subrahmanyam et al. where thermal heating to 500 °C or laser irradiation of the hydrogenated graphene samples using a KrF excimer laser resulted in dehydrogenation of the samples and disappearance of the C−H stretching bands in the IR spectra.56 Reversible hydrogenation of graphene and the formation of graphane have been demonstrated by Elias et al. using a dc plasma produced in a 10% H2 in Ar gas mixture to generate atomic hydrogen that reacts with graphene.57 In our experiments, the plasma generated within the laser vaporization plume dissociates the H2 molecules present in the He carrier gas to produce atomic hydrogen that assists the reduction of GO and terminates the resulting graphene sheets.

Figure 4. UV−vis spectra of graphite oxide (GO) and graphene prepared by the LVCC of GO in pure He, in a 10% H2−He mixture, and in a 20% H2−He mixture.

spectrum of GO shows significant absorption below 400 nm with the characteristic shoulder at 305 nm attributed to n → π* transitions of CO bonds.42,54 It is clear that this shoulder disappears after the LVCC process of GO, and the absorption peak of GO at 230 nm red-shifts to 270 and 274 nm, in the presence of He and He−H2 carrier gases, respectively. This indicates the presence of π → π* transitions of extended aromatic C−C bonds, thus suggesting that the electronic conjugation within the deoxygenated sheets is restored following the LVCC of GO.42,54 It also indicates that the presence of H2 in the He carrier gas enhances the reduction of GO during the LVCC process. Figure 5a compares the FT-IR spectra of graphite, GO, and graphene prepared by the LVCC process of graphite and GO. The graphite spectrum shows no significant peaks since graphite is an IR inactive solid. Similarly, the spectrum of the graphene produced by LVCC of graphite shows no significant peaks. The GO spectrum shows strong bands corresponding to the CO stretching vibrations of the COOH groups at 1730 9488



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oxygen atoms most likely in the form of CO, CO2, and H2O vapors. However, in the presence of 10% H2 and 20% H2 in the He carrier gas, the resulting graphene samples show significant increases in the carbon atomic percent (88% and 89.7%, respectively), which result in C/O ratios of about 7.3 and 8.7, respectively. This indicates that the presence of H2 under the LVCC process plays an important role in the reduction of the GO nanosheets. This can be explained by the formation of atomic hydrogen within the laser vaporization plume which can act as an efficient reducing agent of GO. It should be noted that further increase in the amount of H2 up to 50% of the He carrier gas did not result in further increase of the carbon atomic percent of the resulting graphene sheets. Therefore, the 10−20% H2 in helium was used in the preparation of all the graphene samples by the LVCC method. The C/O ratios in the graphene samples prepared in He and H2−He carrier gases can be further increased to 5.7 and 11.5, respectively, by annealing the samples in at 400 °C in helium for 3 h. The increase in the C/O ratio is most likely due to the removal of the remaining oxygen functional groups left in the graphene samples following the LVCC process. In addition to the XRD, UV−vis, FT-IR, and XPS data, the successful conversion of GO into graphene using the LVCC method was further verified by Raman spectroscopy as shown in Figure 7 which displays Raman spectra of graphite, GO, and graphene prepared by the LVCC method of graphite and GO. The graphite spectrum (Figure 7a, top) shows the typical Gband at ∼1568 cm−1, which is common to all sp2 carbon materials as it arises from vibrations of sp2-hybridized carbon atoms. The spectrum of graphene prepared by the LVCC of graphite (Figure 7a, bottom) shows a broader G-band almost at the same frequency as that of bulk graphite, in addition to the disorder-induced D-band at∼1363 cm−1, and a D/G intensity ratio of ∼0.40. The D-band requires defects for its activation, and therefore, its intensity provides a measure for the amount of disorder in graphene.59,60 The intensity ratio of the D-band to the G-band is usually used as a measure of the quality of the graphitic structures since for highly ordered pyrolitic graphite, this ratio approaches zero.59,60 The Raman spectrum of GO shows a broadened and blue-shifted G-band (1584 cm−1) (as compared to graphite) and the D-band at 1372 cm−1 (Figure 7b, top).59 The spectrum of the HTG prepared by the LVCC of GO in the presence of 10% H2 in He (Figure 7b, bottom) shows a strong G-band around 1571 cm−1, almost at the same frequency as that of graphite, and a relatively weak D-band around 1372 cm−1 with a D/G intensity ratio of ∼0.50 as compared to 0.67 in GO, thus indicating a decrease in the degree of disorder and defect sites following the LVCC process of GO.59,60 In addition, the spectrum of the HTG shows a 2D band around 2742 cm−1 with a significantly reduced intensity, indicating the presence of a few-layer graphene.60 The overall Raman spectrum of the HTG prepared by the LVCC of GO in the presence of 10% H2 in He (Figure 7b, bottom) appears very similar to that of the few-layer graphene produced by arc evaporation of graphite under hydrogen.56 The decrease of the intensity of the 2D band relative to the G band and the appearance of sharp D peak in the HTG spectrum (Figure 7b, bottom) is also consistent with the changes observed in the Raman spectra of hydrogenated graphene prepared using dc plasma in a 10% H2 in Ar gas mixture.57 Laser Vaporization/Ionization (LVI) of Bulk GO in a Vacuum. To demonstrate the presence of hydrogen in the graphene sheets prepared by the LVCC of GO and to be

Figure 6 displays the XPS C 1s spectra of GO and graphene prepared by the LVCC method using different carrier gases.

Figure 6. XPS C 1s spectra of graphite oxide (GO) and graphene prepared by the LVCC of GO in pure He and in H2−He carrier gases.

The GO spectrum shows peaks corresponding to oxygencontaining groups between 285.5 and 289 eV, in addition to the sp2-bonded carbon CC at 284.5 eV. Typically, peaks at 285.6, 286.7, 287.7, and 289 are assigned to the C 1s of the C− OH, C−O, CO, and HO−CO groups, respectively.42,43 The XPS spectra of the graphene clearly indicate that most of the oxygen-containing groups in GO are removed after the LVCC process in the presence of the H2−He carrier gas mixture. Based on the XPS curve fitting using a Gaussian− Lorenztian peak shape analysis, the ratio of carbon to oxygen was estimated by dividing the area of the C 1s and O 1s peaks and multiplying by the corresponding photoionization cross sections.58 The calculated atomic percent and C/O atomic ratio for the GO target used in our experiments and the graphene samples prepared using pure He, 10% H2 in He, and 20% H2 in He as carrier gases in the LVCC experiments are shown in Table 1. It is clear that the C/O ratio increases from about 1.9 in the GO target to 4.7 in the graphene prepared in pure He during the LVCC process. This indicates that laser vaporization of the GO sheets is accompanied by a significant removal of Table 1. XPS Analysis of GO and HTG Samples sample GO HTG HTG HTG HTG HTG a

from from from from from

GO GO GO GO GO

(He) (Hea) (10% H2−He) (20% H2−He) (20% H2−Hea)

C (%)

O (%)

C/O

65.7 82.5 85.0 88.0 89.7 92.0

34.3 17.5 15.0 12.0 10.3 8.0

1.9 4.7 5.7 7.3 8.7 11.5

Sample was annealed in at 400 °C in helium for 3 h. 9489



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Figure 7. Raman spectra of (a) graphite and graphene prepared by the LVCC of a graphite target in helium carrier gas and (b) graphite oxide (GO) and HTG prepared under a 10% H2/He atmosphere by the LVCC method. The excitation wavelength was 406.7 nm produced by a krypton ion laser.

(i) GO and (iii). For example, the ion intensity ratio CnH−/Cn− reaches a maximum of 7 for n = 14 for the carbon clusters generated by the LVI of the GO target in He (inset of (i) in Figure 8a) while the maximum CnH−/Cn− ratio in the clusters produced from the graphite target in He is 0.3 for n = 10 (inset of (iii) in Figure 8a). This can clearly be observed in the highresolution mass spectra shown in Figure 8b. The insignificant CnH−/Cn− ratio observed in the clusters generated from the graphite target is slightly higher than the 13C natural isotope ratio and therefore does not represent a significant abundance of carbynes produced from graphite. Thus, it seems that the CnH− ions with even n are produced specifically from the laser vaporization/ionization of the GO target. This is a direct evidence for the retention of hydrogen within the graphene nanosheets following the laser vaporization and deoxygenation of the GO sheets. These hydrogen atoms stabilize the resulting graphene sheets by the attachment to the terminal carbon atoms. In a vacuum, the conversion of the laser multiphoton energy to photochemical reactions involving excitation and deoxygenation of GO with the removal of CO and CO2 gases and formation of hydrogen-terminated graphene sheets followed by dissociation and ionization leads to formation of the observed carbyne ions. To further confirm the generation of long-chain carbyne ions from GO, we investigated the positive ion mass spectra resulting from the LVI of bulk GO using He or Ne carrier gas as shown in Figure 9a. In the presence of helium carrier gas, the formation of CO+, HCO+, and C3H3O+ ions is clearly observed in the mass spectrum of (i) (Figure 9a) and provides direct evidence for the deoxygenation of GO under the laser vaporization/ionization process. The observed C3H3O+ ion most likely has the structure of the CH2CH−CO+ ion, which is known to dissociate into the observed HCO+ ion and C2H2 (acetylene).62 In addition to the small oxygen-containing ions, the mass spectrum, in (i) (Figure 9a) is dominated by carbon clusters (Cn+, n = 1−20) and small CnH+ series with n = 3, 5, 7, and 9. However, under the more efficient cooling carrier gas (Ne), the spectrum is dominated by both the CnH+ series with n = 3, 5, 7, 9, and 11 and a new CnH2+ series with n = 4, 6, 8, 10, 12, 14, and 16 as shown in Figure 9a (ii). The even-n CnH2+

consistent with the LVCC method, we applied the LVI technique (Figure S1, Supporting Information) to the GO target in a vacuum using a beam expansion of a carrier gas such as helium or neon in the absence of hydrogen gas. In this case, the detection of hydrogen-containing carbon clusters would confirm that the main source of hydrogen is the OH and COOH groups present in GO nanosheets. The comparison of the LVI experiments of GO and graphite targets provides further support that the source of hydrogen is the OH and COOH groups of GO since in the case of a graphite target the amounts of hydrogen-containing carbon clusters were significantly reduced in comparison with the GO target. This can be clearly seen in Figure 8a, which compares the negative ion mass spectra obtained from the LVI experiments of GO and graphite targets. The mass spectrum produced from the LVI of GO in the presence of He (i) or Ne (ii) expansion is dominated by carbon clusters Cn with a significant enhancement of the hydrogenated even-n clusters for n = 4−24. The enhanced ion intensity observed when using Ne as a carrier gas instead of He is due to the more efficient cooling effect of Ne as compared to He. The beam expansion of the carrier gas cools the hot ions generated by the LVI of the GO target. The most prominent feature is the mainly even-n CnH− series which results in a strong even/odd alternation in the intensity ratio of the CnH−/ Cn− clusters. This reflects the extra stability of the even CnH− clusters which most likely represent linear carbon chains containing −(CC)n− units with a hydrogen atom at one end of the chain and the electron on the other end. These species are interesting due to the detection of larger hydrocarbons in space environments such as the homologous series of acetylenic carbon chain radicals CnH, unsaturated carbenes CnH2, and cyclic and polycyclic compounds.17,26,27 The CnH− ion series are usually generated by the plasma discharge ionization of small hydrocarbon clusters such as acetylene.27,61 Interestingly, in the present work, the long-chain carbynes are produced in significant abundance from the LVI of GO but not from graphite under identical experimental conditions as evidenced from the comparison of the mass spectra of (i) and (iii) in Figure 8a although the abundance of the carbon clusters series Cn appears to be comparable in the two mass spectra shown in 9490



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Figure 8. (a) Negative ion TOF mass spectra generated from the LVI experiments of (i) GO target in helium expansion, (ii) GO target in neon expansion, and (iii) graphite target in helium expansion. Insets show the ion intensity CnH−/Cn− ratios for the three mass spectra. (b) Highresolution negative ion TOF mass spectra generated from the LVI experiments of (i) GO target in helium expansion, (ii) GO target in neon expansion, and (iii) graphite target in helium expansion.

deoxygenation of the GO sheets. A comparison of the negative and positive ion mass spectra shown in Figure 9b confirms that stable long-chain carbyne ions can be generated by the LVI of bulk GO under an efficiently cooling carrier gas such as Ne. These results are consistent with the carbon clustering reactions observed by laser ablation of graphite in Smalley-type sources where the carbon plasma produced by laser ablation is allowed to react with reactive gases such as H2, H2O, and NH3 before it expands into vacuum.22,63 The formation of long carbon-chain molecules capped with H atoms at the ends was also observed by laser ablation of graphite in organic liquids such as hexane,

series most likely represents linear carbon chains containing −(CC)n− units and terminated by H atoms at the ends to form the stable polyynes. These results are consistent with mass spectrum of the negative ions shown in Figure 8a (ii). On the other hand, the positive ion mass spectrum generated from the LVI of a graphite target using He as a carrier gas (Figure 9a (iii)) shows only a weak signal due to carbon clusters (Cn+, n = 1−8) in addition to some impurity peaks assigned to N2 and O2 gases. This again confirms that the even-n CnH2+ ions are produced specifically from the laser vaporization/ionization of the GO target utilizing the hydrogen atoms formed by the 9491



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Figure 9. Positive ion TOF mass spectra generated from the LVI experiments of (i) GO target in helium expansion, (ii) GO target in neon expansion, and (iii) graphite target in helium expansion. Comparison of negative and positive ion mass spectra generated from the LVI experiments of GO target in neon expansion.

benzene, and toluene.64 In these studies, the observation of carbon-chain molecules capped with H, N, or CN groups at the ends was attributed to the generation of reactive Cn radicals which can readily add H, N, or CN at the ends to form relatively stable polyynes and cyanopolyynes.22,63,64 What is unique about the current results is that the long-chain carbyne ions are directly generated by the laser vaporization/ionization of bulk GO under inert gas atmospheres without the addition of reactive gases or the expansion of the vaporized carbon species through a cluster source. In our experiments, laser desorption and deoxygenation of the GO sheets take place first to form hydrogen-terminated graphene sheets which are then ionized and dissociated to generate the long-chain carbyne ions. Furthermore, the observation of these ions provides direct evidence for the formation of hydrogenated graphene sheets by the LVCC of bulk GO.

The mechanism of the LVCC of GO could involve a combination of both electronic and thermal processes. Initial excitation by the absorption of two 532 nm photons produces electron−hole plasma within the solid GO. The strong electron−phonon coupling within GO sheets is expected to lead to rapid local heating which could result in the removal of the oxygen atoms from GO most likely in the form of CO and CO2. The remaining H atoms resulting from the dissociation of the OH and COOH groups terminate the deoxygenated GO nanosheets. The extent of deoxygenation and reduction of GO can be enhanced by using a hydrogen−helium mixture as a carrier gas for the LVCC process. In this case, the plasma generated by the laser vaporization of the GO target dissociates the H2 molecules on the surface of the target and the resulting reactive H atoms are used to further reduce the deoxygenated GO sheets by the removal of remaining oxygen functional 9492



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groups. The observation of CO, COH, and C3H3O in the mass spectrum of the LVI of GO (Figure 9a (i)) provides evidence for the removal of the oxygen functional groups from the GO sheets.

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CONCLUSIONS In conclusion, we developed a facile process for the synthesis of hydrogen-terminated graphene using the laser vaporizationcontrolled condensation method. The method allows rapid deoxygenation of bulk graphite oxide in an atmosphere of helium or a helium−hydrogen mixture to produce hydrogenterminated graphene nanosheets without the need for any chemical reducing agents or solvents. We have applied a combination of different techniques including laser vaporization/ionization time-of-flight (LVI-TOF) mass spectrometry to characterize the formation of graphene by the LVCC of bulk GO. These experiments allow us, for the first time, to establish the formation of abundant hydrogen-capped linear carbon chains containing −(CC)n− units with n at least up to 15 from the LVI of GO in a vacuum. In contrast, LVI of bulk graphite under identical experimental conditions generates mainly the carbon cluster series Cn without any significant hydrogenation in both the positive and negative ion modes. The observation of hydrogen-capped carbon chains CnH−, CnH−, and CnH2+ with n = 2−30 confirms that the LVCC method of bulk GO produces HTG nanosheets which can be ionized and dissociated in a vacuum to produce the observed hydrogen-capped polyyne chains. These species could be deposited from the gas phase to form one-dimensional conducting molecular wires for a variety of potential applications in nanoelectronics, sensors, and devices.

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1 showing the setup for the laser vaporization/ ionization (LVI) experiment with description of the reflectron time-of-flight mass spectrometer (RTOF-MS) used for the detection of the carbon ions generated from bulk graphite oxide. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Foundation (CHE-0911146) and NASA (NNX08AI46G) for support. REFERENCES

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Hydrogen-Terminated Graphene by Laser Vaporization-Controlled

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