Graphene hydrogenation by molecular hydrogen in the process of graphene oxide thermal reduction V. M. Mikoushkin, S. Yu. Nikonov, A. T. Dideykin, A. Ya. Vul', D. A. Sakseev et al. Citation: Appl. Phys. Lett. 102, 071910 (2013); doi: 10.1063/1.4793484 View online: http://dx.doi.org/10.1063/1.4793484 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v102/i7 Published by the American Institute of Physics.
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APPLIED PHYSICS LETTERS 102, 071910 (2013)
Graphene hydrogenation by molecular hydrogen in the process of graphene oxide thermal reduction V. M. Mikoushkin,1,a) S. Yu. Nikonov,1 A. T. Dideykin,1 A. Ya. Vul’,1 D. A. Sakseev,1 M. V. Baidakova,1 O. Yu. Vilkov,2,3 and A. V. Nelyubov2,3 1
Physical Technical Institute, Russian Academy of Sciences, St. Petersburg 194021, Russia Insnitut f€ ur Festk€ orperphysik, Technische Universit€ at Dresden, D-01062 Dresden, Germany 3 Fock Institute of Physics, St. Petersburg State University, St. Petersburg 198904, Russia 2
(Received 7 December 2012; accepted 12 February 2013; published online 22 February 2013) Thermal reduction in molecular hydrogen of the graphene oxide films has been studied by X-ray photoelectron spectroscopy using synchrotron radiation. The restoration process was revealed to be accompanied by hydrogenation due to collisionally induced interaction of molecular hydrogen with carbon atoms. One side hydrogenated graphene films consisting of 20 lm one monolayer flakes were fabricated on SiO2/Si surface with hydrogen concentration as far as 40 at. %, at which the 0.3 eV bandgap opening was observed. It was shown that both H-coverage and bandgap width of C 2013 American the films can be controlled by varying the temperature of the heat treatment. V Institute of Physics. [http://dx.doi.org/10.1063/1.4793484] Since the discovery of graphene,1,2 a lot of effort has been devoted to developing different technological approaches to graphene fabrication including graphite cleavage, SiC heattreatment, chemical vapor deposition of carbon-containing gases, and chemical or thermal graphene oxide (GO) reduction.2–6 The methods of GO reduction seem to be attractive due to relative simplicity, low cost, and capability for largescale production of graphene and graphene-like films. Effective reduction of GO has been demonstrated recently with laser based7,8 and electron-beam techniques.9 As it has been shown in Ref. 9, both these techniques can be used for nano-lithography purposes. Controlled reduction of GO gives an opportunity to tune its band structure,10 which is needed for device applications. Unfortunately, GO is not thermally stable enough for many of the applications. Therefore, hydrogenation is considered to be more perspective way for graphene functionalization.11 The hydrogenation of graphene was studied with plasma12–14 and atomic hydrogen beams15–19 on metallic surfaces13,15–19 and on dielectric SiO2 substrate12,14 minimally affecting graphene properties. High efficiency of chemical hydrogenation via Birch reduction mechanism was demonstrated by attachment to a few-layer graphene as far as 5 wt. % of hydrogen atoms.20 X-ray photoelectron spectroscopy (XPS) was employed to quantitatively evaluate the H-coverage in many of these studies.13,14,16–19 We have shown with high resolution XPS that hydrogenated graphene films can be fabricated directly in the process of GO thermal reduction in molecular hydrogen due to collisionally induced chemical reaction, which essentially simplifies the technology. Another advantage of the revealed process is that the large scale film can be fabricated directly on dielectric SiO2/Si (or any) substrate and that molecular hydrogenation employed at relatively low temperatures is expected to be low-destructive. The initial GO was synthesized via oxidation of the natural crystalline graphite by potassium permanganate in sulfuric acid in the presence of sodium nitrate, using the
procedure described in Ref. 6. Then, sample films were prepared on the silicon substrates with SiO2 surface sublayer by evaporating a drop of colloidal GO suspension. Finally, the films were annealed for 2 h at different temperatures including T ¼ 750 C (sample GR750) and T ¼ 800 C (sample GR800) in a quartz reactor that was filled with hydrogen. Fig. 1 shows a scanning electron microscopy (SEM) image of a typical flake with characteristic size of about 20 lm. The image illustrates also solidity and adsorption strength of the flake, checked by a steel scriber whose diagonal traces are seen on the substrate surface. The scriber forced the SiO2 substrate even through the graphene flake but did not damage or remove it. The chemical composition and electron structure of the samples were studied by XPS at the Russian–German synchrotron radiation beamline of the BESSY electron storage ring.21 The photoelectron spectra were measured using a VG SPEC hemispherical analyzer with a spectral resolution of DE ¼ 0.15 eV. Prior to XPS measurements, the surface of
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FIG. 1. SEM image of hydrogenated flake on silicon substrate. 102, 071910-1
C 2013 American Institute of Physics V
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samples was cleaned by prolonged (for many hours) keeping in vacuum followed by 1 h low-temperature (T ¼ 250 C) heating in high vacuum. To retain only chemically bounded hydrogen, samples were heated at higher temperatures (T ¼ 430–550 C) for 20-40 min. The average film thickness (l) was controlled by comparing the intensities of C1s line in the spectra of studied film and a massive highly oriented pyrolytic graphite (HOPG) sample.10 Assuming the mean free paths of C1s photoelectrons (h ¼ 450 eV) in studied films (k) and HOPG (kG) to be equal to k kG ¼ 0.73 nm 2.2 ML (monolayers),22 the following estimates were obtained: l(GR750) ¼ 1.4 ML and l(GR800) ¼ 1.1 ML. These values confirm the SEM data that studied films consist mainly of one monolayer rarely overlapping flakes with relatively small contributions of the flake aggregates and uncovered substrate surface. Taking k ¼ 0.35 nm from Ref. 19, we should admit that graphene flakes studied here do not cover essential part of the substrate surface. Photoelectron spectra of C1s core level excited by x-ray photons with an energy of h ¼ 450 eV were analyzed to identify the chemical composition of the samples. Fig. 2 shows the spectra of HOPG and the films annealed at T ¼ 750 and 800 C in the electron binding energy scale (EBE). Minor contribution of hydroxyl group –OH evidences for virtually complete restoration of initial GO film and its transformation into graphene film with increase in the annealing temperature. Second derivative of the GR750 spectrum revealed the peculiarity at EBE ¼ 285.0 eV related to carbon atoms chemically bounded with hydrogen atoms. This binding energy for C-H bond is approximately known (e.g., Refs. 14 and 17). But direct and independent measurement of the C-H binding energy enabled us to obtain more precise estimation of the C-H coverage described below. Fig. 2 inset shows decomposed spectrum of the GR750 film. It reveals unexpectedly effective C-H bond creation in the molecular hydrogen atmosphere. H-coverage (toned contribution) strongly depends on
FIG. 2. C1s photoelectron spectra of HOPG (dotted curve) and the films annealed in hydrogen at different temperatures: T ¼ 800 C (dashed curve) and T ¼ 750 C (solid curve). Second derivative of the GR-750 C spectrum is shown by dashed-dotted curve. The inset shows decomposed GR-750 spectrum.
Appl. Phys. Lett. 102, 071910 (2013)
the temperature and reaches the value H/C ¼ 40 6 4 at. % (5 wt. %) in the GR750 film. The analysis conducted also shows that C1s electron binding energies for C ¼ C (C1) and C-H (C3) states essentially depend on H-coverage or on the relationship of sp2 and sp3 hybridization of carbon sheet. This fact can be explained by exhaustion of the p-electron subsystem due to hydrogenation, resulting in extra-atomic relaxation energy diminution which causes the binding energy rise independently on the type of the chemical bond. Therefore, C1s binding energies for both C1 and C3 chemical states are higher for more hydrogenated GR750 film by 0.2–0.1 eV. Different H-coverage may be one of the reasons for notable spread in the binding energy values published in literature for hydrogenated graphene. Efficient hydrogenation of studied films is confirmed by near-edge x-ray adsorption fine structure spectra (NEXAFS) of the GR750 film (solid line) and HOPG (dotted line) presented in Fig. 3. Decrease in the GR750 p* peak intensity by 25%-30% as compared to that of HOPG is a result of hydrogen atoms attachment to graphene sheet and transformation of the corresponding part of sp2 C @ C p-bonds into sp3 C-H r-bonds. This estimate qualitatively corresponds to H-coverage obtained from the XPS data (40%), though systematic errors connected with low contrast graphene signal treatment makes NEXAFS estimate less accurate. Comparison of the GR750 film spectra measured at normal (u ¼ 0 ) and sloping (u ¼ 60 ) incidences shows strong angular dependence analogous to that for HOPG. This fact indicates strong p-states alignment evidencing one plane orientation and low amorphization of graphene sheets in GR750 film. Physical properties of studied films were characterized by measuring valence band (VB) photoelectron spectra at the photon energy h ¼ 130 eV. The mean free path of the VB photoelectrons does not exceed the thickness of the dielectric SiO2 substrate sublayer. Due to wide bandgap, SiO2 layer does not contribute to the top of the VB spectra and brings no errors typical for the case of graphene grown on the conductive substrates. Fig. 4 presents VB spectra of HOPG and the heat-treated films. The VB edge in the GR800 film
FIG. 3. C1s NEXAFS spectra for HOPG (dotted curve) and GR750 film (dotted curve) measured at sloping incidence (u ¼ 60 ) and GR750 film spectrum (dashed curve) measured at normal incidence (u ¼ 60 ).
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Appl. Phys. Lett. 102, 071910 (2013)
and low amorphization of graphene sheets. Analysis of XPS spectra revealed almost 40 at. % H-coverage ( 5 wt. %) and the band gap opening at annealing temperature T ¼ 750 C. The bandgap proved to be narrow enough (Eg 0.3 eV) to evidence for a one-sided hydrogenation, which also agrees with the assumed hydrogenation mechanism including collisionally induced interaction of molecular hydrogen with carbon atoms. It was also shown that the created C-H chemical bond in the hydrogenated graphene films remains stable to heat at rather high temperatures T ¼ 430-550 C. Hence, the activation energy was estimated to exceed Ea 3.4 eV. Thus, the GO thermal reduction technology was shown to allow obtaining stable hydrogenated graphene on insulator with the bandgap varied by tuning the annealing temperature.
FIG. 4. Valence band spectra of HOPG (dotted curve) and the films annealed in hydrogen at different temperatures: T ¼ 800 C (dashed curve) and T ¼ 750 C (solid curve).
annealed at higher temperature T ¼ 800 C is very close to the Fermi level, indicating occurrence of metallic conductivity similar to the case of HOPG. (The VB edge was estimated at the 1% intensity of the Q*(p) peak.) On the contrary, the VB edge in the GR750 film shows the bandgap opening. The distance between the VB edge and the Fermi level was estimated to be 0.3 eV. This distance is close to the bandgap width (Eg) in the case of GO derived materials.10 The narrow bandgap observed in this work evidences for a one-sided hydrogenation which may give only the gap Eg ¼ 0.46 eV according to theoretical calculations in the case of graphene with one side 100% H-coverage.23 Hydrogenation observed may be explained by collisionally induced interaction of “hot” H2 molecules corresponding to the high energy tail of the Maxwell-Boltzman distribution with carbon atoms: hot H2 molecule passes over the barrier in the C-H potential energy curve, hydrogen atom approaches carbon atom to the C-H bond length (0.109 nm), H2 molecule decays due to excitation in collision and creates C-H bond. In this model, the kinetic energy of hot molecule EK(H2) must not be comparable with enormous H2 bond dissociation energy (BDE ¼ 4.12 eV). It must only exceed the defect energy (ED) which is the difference between the bond energies for molecular hydrogen and C-H bond: EK(H2) >ED ¼ EBDE(H2) – Ea(C-H) ¼ 4.12 – 3.4 ¼ 0.72 eV. The activation energy Ea (C-H) 3.4 eV was estimated in this work using the fact that the sample GR800 annealed in the spectrometer at T ¼ 550 C for 20 min retained an essential part of hydrogen. The assumed model explains both growth of the H-coverage with decreasing the temperature due to gain in C-H bond lifetime and one-side hydrogenation by low-energy H2 molecules. In summary, the research showed that hydrogenated graphene films can be obtained directly in the process of GO thermal reduction in molecular hydrogen. The revealed process essentially simplifies fabrication of the hydrogenated graphene. Hydrogenated graphene films consisting of relatively large (20 lm) rarely overlapping one-monolayer flakes were fabricated on SiO2/Si substrate. NEXAFS analysis showed strong p-states alignment evidencing one plane orientation
This study was supported in part by BESSY II, the Russian Foundation for Basic Research (Project No. 10-0700508-a), and the Ministry of Education and Science (Contract No. 14.740.11.0593).
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