Applied Science and Convergence Technology 2021; 30(6): 183-185
Published online November 30, 2021
Copyright © The Korean Vacuum Society.
aDepartment of Physics, Chungnam National University, Daejeon 34134, Republic of Korea
bInstitute of Quantum Systems, Chungnam National University, Daejeon 34134, Republic of Korea
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In this study, we investigated the redox process occurring on graphene with an H2SO4 electrolyte gating. The redox process occurring on graphene was studied by observing the variation in the conductance of graphene. By performing electric force microscopy measurements, we also observed the surface condition of graphene leading oxidation and its role as a seed for adsorbing undesired residues, which results in conductance suppression.
Keywords: Graphene, Graphene oxide, Electrolyte gate, Oxidation, Reduction, Redox
Selective oxidation on the surface of graphene using electrochemical techniques has been suggested for various applications, such as chemical gas sensing and biosensing due to its high sensitivity for sensing target molecules as well as the high surface-to-volume ratio of twodimensional carbon networks [1–14]. Oxidation on graphene with an electrolyte gating is one of the well-known methods for functionalization on graphene. Although various techniques are available for oxidation on graphene, electrochemical oxidation has been favored as it enables a reversible process between graphene and graphene oxide [15–20]. Since this oxidation and reduction (i.e., redox) process on graphene allows for tuning of the graphene conductance over a wide range, it has been employed to develop graphene electrodes with the required conductance [15–23]. Meanwhile, continuous conductance degradation of graphene during the redox process has been reported, and this conductance suppression should be investigated with oxidized condition [3, 19–24]. In this study, we performed the redox process on graphene using an electrolyte gating. Using a sulfuric acid (H2SO4) electrolyte, graphene was oxidized, and the resulting graphene oxide was reduced while observing the conductance variation of the graphene. The graphene surface was observed using electric force microscopy (EFM) measurements. These investigations allowed us to confirm the oxide seed formation on graphene and its role in extending the adsorption of undesired residues on graphene, which leads to a substantial conductance reduction.
Graphene was fabricated on a 280-nm-thick SiO2 substrate via mechanical exfoliation and was contacted with titanium (Ti) electrodes via a normal e-beam lithography process [Fig. 1(a)] [19–25]. Ti forms a thin oxide layer (TiO2) naturally in air, which protects the Ti electrodes from the H2SO4 electrolyte. To use one of the electrodes as electrolyte gate metal, we disconnected the metal electrode from graphene through an oxygen plasma etching process using a poly methyl methacrylate (PMMA) etch mask, as shown in Figs. 1(b) and 1(c).
The conductance of graphene was determined using two-terminal measurement with standard lock-in techniques under an H2SO4 (1 M) electrolyte-gated voltage (VEG) through the Ti electrode, which was located beside the graphene [Fig. 2(a)]. We employed H2SO4 (1 M) as the electrolyte for characterizing the redox process on graphene because this type of electrolyte gating guarantees conductance tuning as well as a reproducible redox process for graphene, as observed in our previous studies [19, 20, 22].
In Fig. 2(b), we observed a conductance of ~31.5 µS for VEG = 0 V (marked by red dashed arrows) and a conductance range of 30–32 µS when VEG varied from 0.6 to −0.4 V. Since the capacitance of the electrolyte gate (several hundreds nF/cm2) is around ten times that of 280 nm thick SiO2 (several tens of nF/cm2) [19–22], we could tune the conductance of graphene for VEGvalues less than 1 V.
Figure 3 illustrates the electrolyte-gated conductance variations and oxidation of graphene. After dropping sulfuric acid liquid solution (H2SO4, 1 M) onto the graphene and the gated electrode area, we measured the conductance of the graphene by tuning VEG as shown in Fig. 3(a). We observed that the conductance increased as VEG was gradually changed from 0 to −1.90 V; thereafter, the conductance rapidly decreased to 0 µS for VEG = −1.95 V, as shown in Fig. 3(b). Thus, the
When we applied a positive electrolyte gate voltage of VEG = 0.0 − 0.8 V, a conductance variation of approximately 12–19 µS was observed, as shown in Fig. 4(a). The EFM image still shows the electrostatic potential gradient observed in Fig. 4(b). However, upon applying a negative electrolyte gate voltage of VEG = 0.0 to −1.8 V, a conductance suppression was observed again, but the original conductance was not recovered. After this second oxidation process (3) [Fig. 4(c)], the electrostatic potential and topography images revealed adsorption of undesired residues on the graphene surface, as denoted by the white dashed arrows in Figs. 4(d) and 4(e), respectively. This indicates that the low ratio of oxidized graphene area acts as a seed point for adsorption of residues when VEG = −1.8 V, which is smaller than VEG = −1.95 V for the first oxidation shown in Fig. 3. Therefore, the oxidation process (1) depicted in Fig. 3 leads to somewhat strong oxidation areas on graphene, which cannot be recovered by the reduction process (2) depicted in Fig. 4(a); moreover, the graphene oxide acts as a seed for adsorption of undesired residues, resulting in a conductance suppression (3), as shown in Fig. 4(c). However, these undesired residues adsorbed on graphene oxide could not be confirmed to be only oxides or other residues in impure H2SO4 electrolyte. Therefore, further investigation is needed to identify these residues.
In this study, we investigated the redox process occurring on graphene with H2SO4 as an electrolyte. The graphene surface condition was observed using topography and electrostatic potential images. We observed partial oxidation of the graphene surface even after the reduction process and found that graphene oxide acts as a seed for adsorption of residues on graphene, which leads to conductance suppression. We believe that this result provides a clearer understanding of the redox process on graphene and its continuous conductance degradation.
This work was supported by the research fund of Chungnam National University.
The authors declare no conflicts of interest.