Comparison of the Catalytic Oxidation Reaction on Graphene Oxide

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Comparison of the Catalytic Oxidation Reaction on Graphene Oxide and Reduced Graphene Oxide Myungjin Lee, Sena Yang, Ki-jeong Kim, Sehun Kim, and Hangil Lee J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 03 Jan 2014 Downloaded from http://pubs.acs.org on January 5, 2014

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The Journal of Physical Chemistry

Comparison of the Catalytic Oxidation Reaction on Graphene Oxide and Reduced Graphene Oxide Myungjin Lee1,†, Sena Yang2, †, Ki-jeong Kim3, Sehun Kim2,*, Hangil Lee1,* 1

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Department of Chemistry, Sookmyung Women's University, Seoul 140-742, Republic of Korea

Molecular-Level Interface Research Center, Department of Chemistry, KAIST, 305-701, Republic of Korea 3

Beamline Research Division, Pohang Accelerator Laboratory (PAL), Pohang 790-784, Republic of Korea

Corresponding Author *Hangil Lee Tel: +82-2-710-9409 Fax: +82-2-2077-7321 E-mail: [email protected], Sehun Kim Tel: +82-42-350-2831 Fax: +82-42-350-2810 E-mail: [email protected] †

equally first author.

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ABSTRACT The capacities of graphene oxide (GO) and reduced graphene oxide (rGO) films grown on silicon substrate to react the aniline to azobenzene oxidation reaction were compared by using Raman spectroscopy, high-resolution photoemission spectroscopy (HRPES), and work function measurements as well as scanning electron microscopy (SEM). The oxygen carriers’ existent on GO film which includes a lot of oxygen carriers can facilitate the aniline to azobenzene oxidation reaction with slightly partial conversion of aniline to nitrobenzene, as determined by the Raman shifts and core-level spectra resulting from exposure to aniline. The work function of the GO film was found to change dramatically in comparison with rGO film, indicating that aniline exposed to a GO film produced n-type doping characteristics by electron charge transfer from GO to aniline. These results indicate that the oxygen carriers on a GO film oxidize aniline to azobenzene and show that GO film prefers to act as a reaction reagent than rGO.

KEYWORDS: Oxidation reaction, Graphene Oxide, Reduced Graphene Oxide, Raman spectroscopy, HRPES, SEM


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INTRODUCTION Since its discovery of graphene, it has been investigated for use in a variety of applications due to its high carrier mobility, good thermal stability, and favorable mechanical as well as catalytic properties.1-3 Although beneficial in some contexts, the chemical inertness of graphene limits its practical utility and significant efforts have been applied toward developing graphene surface modifications.4,5 In an effort to obtain functionalized graphene, many research groups have studied on graphene oxide (GO). GO is consist of a variety of oxygen functional groups, including epoxy (-O-), hydroxyl (OH), carbonyl (-C=O), and carboxyl groups (-COOH), suggesting that the electronic structure of graphene may be modified under selective conditions.6-8 As a point of different views, the oxygen groups existing on GO have been shown to behave as heterogeneous catalysts (or reaction reagent) in a wide range of transformations.9-11 Because of the biocompatibility, high surface area, and rich surface chemistry of GO, as well as its low cost and ease of preparation, GO may be useful in novel heterogeneous catalytic systems.12,13 On the other hand, the reduced graphene oxide (rGO) is functionalized graphene that has been chemically modified through a process involving exposure of an aqueous suspension of GO sheets to hydrazine.14 rGO film has been explored for use in electronic devices, transparent conductors, chemical sensors, thin-film transistors (TFTs), and heterogeneous catalysts, for example, in the reduction of nitrobenzene and methanol.15-17 Hence, fundamental studies of GO and rGO films can guide the design of graphene functionalization processes toward adapting graphene to particular applications. Here, we tested the reaction activities between GO and rGO films toward the oxidation of aniline to azobenzene and compared which one is more reactive as a heterogeneous catalyst (or reaction reagent). Although the method is difficult, we previously reported the photo-oxidation reaction of aniline to azobenzene in the presence of modified GO grown 6H-SiC(0001) substrate using benzoic acid.18 The study examined the reaction rate as a function of the aniline concentration, catalyst loading, airflow rate, and solvent composition. In this work, we describes an examination of the capacity of GO and rGO films to oxidize this reaction under UHV conditions maintaining 365 nm UV light exposure to enhance ACS Paragon Plus Environment

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the oxidation reaction. The oxidation of aniline under these conditions is expected by an oxygen group on the GO or rGO films, and then the aniline amino group can form a new C-N bond by oxidation reaction. Therefore, we expect that azobenzene can be generated from aniline. Here, we precisely examined the capacities for oxidation of aniline on GO and rGO films using Raman spectroscopy, highresolution photoemission spectroscopy (HRPES), and scanning electron microscopy (SEM), respectively.

EXPERIMENATAL DETAILS GO and rGO films were grown on a silicon oxide substrate using standard methods.19 Aniline (Sigma Aldrich, purity, 99.5%) was purified by turbo-pumping to remove impurities prior to dosing on the GO and rGO films. A direct dozer, controlled using a variable leak valve, was used to dose the substrates. Raman spectra of the samples were collected using a home-built system equipped with an Ar+ ion laser (Spectra-Physics Stabilite 2017) as an excitation source, a spectrometer (Horiba Jobin Yvon TRIAX 550), and a CCD detector (Horiba Jobin Yvon Symphony) cooled to –133°C. The wavelength of the incident excitation beam was 514.5 nm. HRPES experiments were performed at the 8A2 beamline at the Pohang Accelerator Laboratory (PAL), which is equipped with an electron analyzer (SES100, Gamma Data Scienta). The C 1s, N 1s, and O 1s core level spectra were obtained using photon energies of 340, 510, and 600 eV, respectively. Secondary electron emission spectra (–20 V sample bias) were measured at photon energies of 80 eV. The binding energies of the core level spectra were determined with respect to the binding energies of the clean Au 4f core level and the valence band (Fermi energy) for the same photon energy. All spectra were recorded in the normal emission mode. The photoemission spectra were carefully analyzed using a standard nonlinear least squares fitting procedure with Voigt functions.20 Scanning electron microscopy (SEM) images of the samples were obtained using a field-emission scanning electron microscope (JEOL JSM-7600F) operated at an acceleration voltage of 15 kV.


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RESULTS AND DISCUSSION

FIGURE 1. Raman spectra of GO (left panel) and rGO films (right panel) exposed aniline at 300 K. GO (left panel) and rGO (right panel); (a) pristine GO, (b) 3600 L aniline on GO, (C) 18000 L aniline on GO, (d) pristine rGO, (e) 3600 L aniline on rGO, and (f) 18000 L aniline on rGO films.

Figure 1 shows the Raman spectra of aniline adsorbed on GO (left panel) and rGO (right panel). The Raman spectra of these films obtained after exposure to 3600 L or 18000 L aniline on GO film under UHV conditions maintaining 365 nm UV light exposure are shown in Figures 1(b) and (c). Two notable film features were apparent based on these spectra. First, the ID/IG ratio was sensitive to the aniline exposure levels. ID/IG ratio increased according to aniline exposure levels from 0.912 for pristine GO film to 1.257 for 18000 L aniline exposure on GO, suggesting that the GO films were reduced.21-23 Second, the D and G bands shifted toward lower wavenumber (cm–1), depending on the aniline exposure. The D and G bands at 1348.3 and 1592.1 cm–1 for the pristine GO film shifted to 1347.6 and 1570.2 cm–1 upon 18000 L aniline exposure. In addition, G band shift toward lower wavenumber (cm–1), ACS Paragon Plus Environment

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G band stiffening, is associated with n-type doping effects. Here, such effects were induced by charge transfer from the GO film to the adsorbed aniline (n-type character).24-27 The D band of GO appears around 1350 cm–1 and shifts toward lower wavenumber (cm–1) due to functionalization.25,26 Furthermore, the adsorption of aniline on the GO film introduced a new D′ band, indicating an increase in defects on the GO film. The intensity of D′ increased with increasing aniline exposure. Hence, we can explain that the Raman spectra of the GO films were consistent with the occurrence of a surface reaction upon aniline exposure. The rGO film, on the other hand, displayed an initially higher ID/IG value (1.761) than one for the GO film (0.912), as shown in Figure 1(d), indicating that smaller and more numerous sp2 domains partially covered the π-conjugated rGO film surface. The ID/IG ratio was consistent with the values reported previously.21-23,28 Interestingly, the ID/IG ratio and Raman shifts of the rGO films did not change upon either level of aniline exposure (see Figures 1(e) and (f)). This observation suggests that most of the GO surface oxygen groups had previously been removed during reduction to form rGO, making them unavailable for reaction with aniline.29 Another possibility is that the remaining oxygencontaining groups on the rGO film were incapable of oxidizing aniline. The ID/IG values for the GO and rGO films are summarized in Table 1.

Table 1 The relative intensity values (ID/IG) for the GO and rGO films according to the aniline exposures (ID/IG) D (cm-1) G (cm-1) D′ (cm-1) GO 0.912 1348.3 1592.1 3600L aniline on GO 1.156 1347.9 1582.3 1655.0 18000L aniline on GO 1.257 1347.6 1570.2 1655.8 rGO 1.761 1297.9 1580.2 3600L aniline on rGO 1.763 1295.4 1579.4 1628.9 18000L aniline on rGO 1.762 1295.1 1578.2 1627.5


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FIGURE 2. N 1s core-level spectra upon oxidation of aniline to form azobenzene on a GO film (top panel: (a) – (c)) or an rGO film (bottom panel: (d) – (f)), as a function of aniline exposure. (a) and (d) indicate 3600 L aniline exposure at 300 K; (b) and (e) indicate 10800 L aniline exposure at 300 K; (c) and (f) indicate 18000 L aniline exposure at 300 K. In the right panel, it displays the molecular structures of aniline (marked as N1), azobenzene (marked as N2), and nitrobenzene (marked as N3). The gray, blue, red, and white balls indicate a carbon (C), nitrogen (N), oxygen (O), or hydrogen (H), respectively.

Figure 2 presents the N 1s core-level spectra characterizing the nitrogen bonding features to obtain the direct evidence for oxidation reaction from aniline to azobenzene on GO or rGO films, which are sensitive to the transformation of aniline to azobenzene or nitrobenzene under UHV conditions maintaining 365 nm UV light exposure to enhance the oxidation reaction. Exposure of the GO and rGO films to aniline at 300 K revealed three distinct nitrogen peaks at 398.4 eV (marked N1), 400.3 eV (marked N2), and 405.2 eV (marked N3) with different intensity values, as shown in Figure 2. These peaks were assigned to aniline (N1), azobenzene (N2), and nitrobenzene (N3), in view of the binding energies and material characteristics.30-32 Interestingly, the GO film displayed distinct aniline-loadingdependent spectra upon exposure of the GO film to aniline, as shown in Figures 2 (a)-(c). The intensity of the nitrogen bonding feature corresponding to azobenzene (N2) increased with increasing aniline


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exposure whereas the intensity of aniline peak (N1) decreased slightly. These results demonstrate that aniline may be fully oxidized to azobenzene not only under aqueous conditions, but also on a GO film under UHV conditions maintaining 365 nm UV light exposure.33 By contrast, Figures 2 (d)-(f) show that the oxidation reaction did not occur on the rGO film, regardless of the aniline exposure level (3600–18000 L).

FIGURE 3. HRPES measurements of the GO (left panel) and rGO (right panel) films, before and after aniline exposure. C 1s and O 1s core-level spectra of (a) and (b) the GO film; (c) and (d) exposure of the GO film to 18000 L aniline; (e) and (f) the rGO film; (g) and (h) exposure of the rGO film to 18000 L aniline.

The C 1s and O 1s core-level spectra were collected before and after exposure of the GO and rGO films to 18000 L aniline to characterize the aniline oxidation products and to confirm the change of oxygen carriers on GO and rGO films after reaction with aniline. At first, five distinct deconvoluted peaks were observed in the C 1s core-level spectra of the GO (Figures 3(a) and (c)) and rGO (Figure 3(e) and (g)) films. The peak centered at a binding energy of 284.5 eV was assigned to a C-C bond with sp2 character (marked as C0). Other peaks clearly assigned to epoxy (C-O-C, 285.6 eV: marked as C1), hydroxyl (-C-OH, 286.7 eV: marked as C2), carbonyl (-C=O, 288.5 eV: marked as C3), and carboxyl (ACS Paragon Plus Environment

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COOH, 290.1 eV: marked as C4) groups using HRPES.34 The C 1s core-level spectra indicated that the carbon features depended on the aniline exposure. Figures 3(a) and (c) show the C 1s core-level spectra of a GO film before and after 18000 L aniline exposure. The intensities of all carbon features, except for the C0 peak (graphene peak), decreased upon increasing aniline exposure, and the peak intensity ratios varied. Among four oxygen bonded carbon peaks (C1-C4), the C 1s peaks corresponding to epoxy (marked as C1) and hydroxyl (marked as C2) groups remarkably decreased by 50% upon exposure to aniline (see Figure 3(c)), suggesting that these groups were the main contributors to the surface reactions with aniline. The surface reaction on rGO film differed from that on GO film. Figures 3(e) and 3(g) show that the carbon peaks did not change significantly upon exposure to aniline except for those corresponding to epoxy groups. The epoxy peak intensity (C1 in Figure 3(e)) only decreased by 10%. Figure 3(b) shows the O 1s core-level spectrum of the GO film. The core-level spectra indicated the presence of four functional groups; epoxy, hydroxyl, carbonyl, and carboxyl groups on the GO film. Based on the relative electronegativity of these groups, the four peaks can be assigned to –O– (at 534.4 eV: marked as O1 peak), –OH (at 533.1 eV: marked as O2 peak), –C=O (at 532.0 eV: marked as O3 peak), and –COOH (at 531.2 eV: marked as O4 peak) bonding features. This interpretation is consistent with Lerf’s model.7 The O 1s core-level spectra after exposure of the GO film to 18000L aniline at 300 K is shown in Figure 3(d). Two bonding features changed dramatically over the course of the reaction, indicating 50% reduction in the epoxy (O1) and hydroxyl (O2) groups and suggesting that these groups dominated the surface reaction. These results suggested an oxidation reaction at 300 K, in agreement with the C 1s core-level spectra. The O 1s core-level spectra for the rGO films did not display any significant changes, as shown in Figures 3(f) and (h), with the exception of a slight decrease in the number of oxygen carrier groups. These results indicate that the GO film reacted with aniline via surface epoxy and hydroxyl groups whereas they were less numerous on the rGO film. The Raman data shown in Figure 1 and the C 1s and O 1s core-level spectra (see Figure 3) indicated that the reactivity of the GO film was higher than that ACS Paragon Plus Environment

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of the rGO film because the numerous oxygen carriers on the GO surface reacted with the aniline, whereas the rGO film presented only a fraction of the oxygen carriers of the GO film and did not react extensively.

FIGURE 4. The relative intensity ratio of azobenzene (red circles) to aniline (black rectangles) on (a) GO or (b) rGO films, as a function of the aniline exposure level.

Figure 4 plots the intensity ratio of azobenzene to aniline immobilized on GO and rGO films, as a function of the aniline exposure level. The azobenzene existent on the GO film increased as the aniline exposure increased, whereas this value remained constant at all aniline exposure levels on the rGO film. The paucity of oxygen carriers on the rGO film relative to the GO film reduced the total number of sites available to oxidize aniline, which well matched with the results obtained from Raman spectra and C 1s, O 1s core-level spectra. The Raman peaks of the GO film shifted toward lower wave numbers upon aniline exposure, indicating n-type doping and surface stiffening. In addition, we investigated the doping characteristics by measuring the work functions for the GO and rGO films.


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Figure 5. Top panel: Secondary edge measurements as a function of aniline exposure of the (a) GO and (b) rGO films under a sample bias of –20 V. (a) Pristine GO film, (b) GO film exposed to 3600 L aniline, (c) GO film exposed to 18000 L aniline, obtained at 300 K. (d) Pristine rGO, (e) rGO film exposed to 3600 L aniline, and (f) rGO film exposed to 18000 L aniline, obtained at 300 K, respectively. Bottom panel: Band diagrams of work function change from GO to aniline on GO (left panel) and from rGO to aniline on rGO film (right panel).

The work functions of the GO and rGO films were monitored while measuring the center of low kinetic energy cutoff as a function of the aniline exposure level, as shown in Figures 5(a) and (d). Figure 5(a) corresponds to the reference spectrum of the pristine GO film with a work function of 4.92 eV, which was consistent with previous results.19,33 The work function of the GO film exposed to 3600 L aniline (Figure 5(b)) demonstrated that the secondary electron edge was shifted by 380 meV (∆φ = -0.38 eV) toward lower kinetic energies relative to the corresponding values of the GO film, indicating an ntype character due to the reaction of oxygen carriers on the GO film or charge transfer from the GO film to the immobilized aniline molecules. Either situation supports the oxidation of the aniline to yield azobenzene. The work functions were measured after exposure of the GO film to 18000 L aniline at 300 K (saturation deposition; see Figure 5 (c)). The secondary electron edge in this spectrum was shifted by ACS Paragon Plus Environment

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560 meV (∆φ = -0.56 eV) toward lower kinetic energies relative to the GO, indicating n-type doping due to the oxidation reaction process. On the other hand, the work function value of the rGO film was measured with 4.62 eV, which was the similar result.35,36 As expected, its value did not change significantly, as shown in Figures 5(d)-(f), even upon increasing the aniline exposure level. These results for work function changes of GO and rGO films indicate the direct evidence that the oxidation reaction clearly depended on the number of oxygen carriers on the GO film or rGO film. As a result, the GO film, alone, provided sufficient numbers of surface oxygen groups to engage in the aniline oxidation reaction.

Figure 6. SEM images (2.5 µm × 2.0 µm) of (a) a pristine GO film, (b) a GO film onto which had been deposited 18000 L aniline, (c) a pristine rGO film, and (d) an rGO film onto which had been deposited 18000 L aniline. The images were obtained on a JEOL field emission SEM at an acceleration voltage of 15 kV. (Scale bar = 500 nm).

We finally acquired the SEM images of GO and rGO films to monitor the respective surface properties after exposure to 18000 L aniline at 300 K. Figures 6 (a) and (c) show 50 nm thick SEM images of pristine GO and rGO films grown on silicon oxide substrates and Figures 6 (b) and (d) show SEM images of GO and rGO films after exposure to 18000 L aniline. The pristine and aniline-exposed surfaces displayed several notable differences. As shown in Figures 6 (b) and (d), we can observe the occurrence of white small particles after aniline exposure. There is a difference between GO and rGO, though. The number of white particles on GO is greater than that of rGO. From the SEM images, we ACS Paragon Plus Environment

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can confirm that oxidation reaction of aniline is more active on GO substrate than on rGO. Therefore, this result can support the Raman spectra and HRPES results that aniline reacted to a greater extent with the GO film than with the rGO film.

Scheme 1. A schematic diagram showing the oxidation reaction from aniline (A) to azobenzene (C) on the GO film including the expected intermediate molecule (B; C6H5NO.). The green, gray, blue, red, and white balls indicate a graphene substrate, carbon (C), nitrogen (N), oxygen (O), or hydrogen (H), respectively.

CONCLUSION In conclusion, aniline was successfully converted to azobenzene on GO via an oxidation process (see Scheme 1) under UHV conditions, with slightly partial conversion of aniline to nitrobenzene. This process did not occur on the rGO film due to the absence of oxygen carriers, as measured using Raman and high-resolution photoemission spectroscopy. In addition, the work functions of the surfaces measured at various aniline exposure levels, were shifted toward low kinetic energies. The major product conveyed an n-type doping character on the GO film, consistent with the reduction in the number of oxygen carriers due the oxidation of aniline to azobenzene.

ACKNOWLEDGMENT This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2013ACS Paragon Plus Environment

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021127). Additionally, it was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 20090083525). ABBREVIATIONS Oxidation reaction, Graphene Oxide, Reduced Graphene Oxide, Raman spectroscopy, HRPES, SEM


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Table of Contents

SEM image and N 1s core level spectra obtained after aniline exposed on GO film (left panel) and rGO film (right panel).


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Comparison of the Catalytic Oxidation Reaction on Graphene Oxide

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