The aniline-to-azobenzene oxidation reaction on monolayer graphene or graphene oxide surfaces fabricated by benzoic acid
© Lee et al.; licensee Springer. 2013
Received: 29 May 2013
Accepted: 28 August 2013
Published: 2 September 2013
The oxidation of aniline to azobenzene was conducted in the presence of either monolayer graphene (EG) or graphene-oxide-like surface, such as GOx, under ultra-high vacuum conditions maintaining a 365-nm UV light exposure to enhance the oxidation reaction. The surface-bound products were investigated using micro Raman spectroscopy, high-resolution photoemission spectroscopy, and work function measurements. The oxygen carriers present on the GOx surfaces, but not on the EG surfaces, acted as reaction reagents to facilitate the oxidation reaction from aniline to azobenzene. Increasing the aniline concentration at 300 K confirmed that the exchange ratio from the aniline to the azobenzene was enhanced, as determined by the intensity ratio between the aniline- and azobenzene-induced N 1 s core-level spectra. The work function changed dramatically as the aniline concentration increased, indicating that the aniline on the GOx surface conveyed n-type doping characteristics at a low coverage level. A higher aniline concentration increased the p-type doping character by increasing the azobenzene concentration on the GOx surface. A comparison of the oxidation reactivity of aniline molecules on the EG or GOx surfaces revealed the role of the oxygen carriers on the GOx surfaces in the context of catalytic oxidation.
KeywordsOxidation reaction GO surface Raman spectroscopy HRPES Reaction reagent
Among the various systems composed only of carbon 5atoms, graphene (a two-dimensional allotrope of carbon) presents a framework for understanding of the properties of all other carbon allotropes. The properties of graphene, including a high intrinsic mobility[1, 2], a large theoretical specific surface area, and a high chemical stability, are potentially useful in applications ranging from chemical sensors to transistors[3–8]. Toward exploiting these unique properties of graphene, several research groups have attempted to fabricate large-scaled graphene oxide sheets[9–12]. Graphene oxide (GO) is a layered material consisting of hydrophilic oxygenated graphene oxide sheets bearing oxygen functional groups on their basal planes and edges. It is a useful platform for fabricating functionalized graphene that can potentially confer improved mechanical, thermal, or electronic properties. The numerous chemical functionalities on a GO surface are expected to readily lend themselves to further chemical functionalization. Graphene-based materials, therefore, show promise in a variety of technological applications. The use of GO surfaces as catalysts of synthetic transformations is a relatively new research area with outstanding potential. Current efforts are directed toward harnessing the oxygen carriers present on GO surfaces as heterogeneous catalysts[14–16].
In this study, we systematically compared and investigated the oxidation of aniline to form azobenzene on monolayer graphene (EG) or graphene-oxide-like (GOx) surfaces fabricated with benzoic acid. Moreover, we focus on examining the difference between EG and GOx surfaces in one substrate, simultaneously. Raman spectroscopy and high-resolution photoemission spectroscopy (HRPES) were used to characterize the surface-bound products. The carboxyl groups introduced onto the graphene surface upon oxidation by benzoic acid to GOx allowed aniline to react with the oxygen carriers. The oxidation of aniline proceed via a reaction between the aniline amine groups and the oxygen groups on the GOx surface under ultra-high vacuum (UHV) conditions maintaining a 365-nm UV light exposure.
Generally, it is hard to distinguish the difference between EG and GOx surfaces in one substrate due to the large size of the HRPES beam. Hence, no previous systematic experimental studies have examined the oxidation of aniline on a GOx surface. However, this study is meaningful with regards to indicating this distinctive difference using the feature of micro Raman spectroscopy.
A Si-terminated 6H-SiC(0001) substrate (Cree Research, Durham, NC, USA) was used to fabricate EG. The substrate was degassed, annealed at 1,200 K under a Si flux (1 Å/min), and graphitized at temperatures up to 1,500 K (for 2 min) to produce a monolayer of graphene (EG). The annealing temperature was monitored using an infrared pyrometer (with an emissivity of 0.9).
Aniline (Sigma Aldrich, purity, 99.9%) was purified by turbo pumping to remove impurities prior to dosing onto the GOx surfaces. A direct doser, controlled by means of 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, Santa Clara, CA, USA) as an excitation source; a spectrometer (Horiba Jobin Yvon TRIAX 550, Kyoto, Japan), and a CCD detector (Horiba Jobin Yvon Symphony) cooled to 140 K. The wavelength of the incident excitation beam was 514.5 nm. HRPES experiments were performed at the 8A2 beamline at the Pohang Accelerator Laboratory, which was equipped with an electron analyzer (SES100, Gamma Data Scienta, Uppsala, Sweden). The N 1 s core-level spectrum was obtained using photon energies of 460 eV. Secondary electron emission spectra (−20 V sample bias) and valence band spectra 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.
Results and discussion
Figure 2b shows an optical image of a GOx surface that had been freshly fabricated by treatment with benzoic acid (see Figure 1). Contrasting with Figure 2a, the GOx surface clearly displayed two regions: a bottom region and a particle region. As with the EG surface, the Raman spectra were collected at these two positions. As expected, the particle position (marked (D)) yielded a distinct Raman spectrum, whereas the bottom position (marked (C)) displayed a typical EG surface spectrum, with the G band at 1,597.6 cm–1. Figure 2f shows that the graphene oxide spectrum was measured with a high intensity. Note that the G band (1,613.1 cm–1) obtained from the particle position was shifted toward higher wavenumbers relative to the G bands of graphene and graphite. The ratio of the D and G band intensities, ID/IG, is inversely proportional to the average size of the sp2 domains. The Raman D/G intensity ratio for the GOx surface was found to be 0.92, similar to the results reported previously for graphene oxide.
A Raman spectrum similar to the spectrum of GO surface indicated that benzoic acid treatment successfully yielded a GOx surface. The EG and GOx surfaces were used in the subsequent experiments involving the oxidation of aniline, which is difficult to oxidize in general. We hypothesized that only the GOx surface would be able to oxidize aniline if the oxidation process is possible. Because the oxidation of aniline on a GOx surface could not be fully characterized by micro Raman spectroscopy alone, we obtained the core-level spectra of the N 1 s peak, which is an indicator of the overall molecular electronic properties. The morphological discrepancies observed between the optical images could only be explained in terms of a surface reaction, as supported by the HRPES results.
Intensity measurements indicating relative aniline and azobenzene coverage
Aniline exposure (L)
Relative intensity of aniline (N1)
Relative intensity of azobenzene (N2)
Figure 4c shows the valence band spectra of the five samples. The spectra are colored as in Figure 4a. The black spectrum corresponds to the valence band spectrum of the monolayer EG obtained at 80 eV. The five distinct peaks corresponding to monolayer EG were clearly observed. The magnified Fermi edge spectrum (Figure 4d) revealed the typical characteristics of monolayer EG. The red spectrum, obtained from the GOx surface, displayed remarkable insulating properties, as demonstrated by the band gap at 0.25 eV. The magnified valence band spectra indicated the presence of a band gap, and the insulating properties resulted from the high oxide character of the substrate. Other spectra were obtained after depositing at various coverages. These figures showed that the valence band spectra were similar to the spectra obtained from the GOx surface, even at higher coverage deposition. The oxidation process did not appear to affect the structure of the GOx surface, suggesting that the oxygen groups present on the GOx surface supplied oxygen atoms during the oxidation reaction.
The Raman spectra and HRPES experiments further supported the conclusion that the oxidation reaction occurred on the GOx surface. The work function of the surface was monitored as the doping characteristic changed from p-type to n-type due to charge transfer from the GOx surface to the adsorbed aniline or azobenzene. The doping characteristic changed from n-type to p-type as the oxidation reaction proceeded from aniline to azobenzene.
The oxidation of aniline to azobenzene was investigated on a GOx surface prepared using benzoic acid. Micro optical images and their corresponding Raman spectra, HRPES measurements, and work function measurement were conducted from the samples prepared under a variety of conditions. The Raman images revealed the structure of the GOx surface prepared using benzoic acid. The HRPES measurements indicated that the relative concentration of aniline and azobenzene varied with the aniline surface coverages. The work functions of the samples were measured as a function of the aniline surface coverage to identify the major product of the surface reaction. n-Type doping was observed at high aniline concentrations (at lower aniline deposition), whereas p-type doping was observed at high azobenzene concentrations (at higher aniline deposition) on the GOx surface. The oxygen carriers present on the GOx surface were found to act as the reaction reagents.
- GOx surface:
High-resolution photoemission spectroscopy.
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2013–021127). The experiments at the PLS were supported in part by MEST, POSTECH, XFEL project.
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