Dual‐channel charge transfer doping of graphene by sulfuric acid

Two-dimensional materials represented by graphene and transition metal dichalcogenides undergo charge transfer (CT) processes and become hole-doped in strong mineral acids. Nonetheless, their mechanisms remain unclear or controversial. This work proposes and verifies two distinctive CT channels in sulfuric acids, respectively driven by oxygen reduction reaction involving O2/H2O redox couples and reduction of bisulfate or related species. Acid-induced changes in the charge density of graphene were in-situ quantified as a function of oxygen content using Raman spectroscopy. At acid concentrations lower than 6 M, the former channel is operative, requiring dissolved O2. Above 6 M, the degree of CT was even higher because the former is cooperated with by the latter channel, which does not need dissolved oxygen. The mechanism revealed in this study will advance our fundamental understanding of how low-dimensional materials interact with chemical environments. Surface charge transfer (CT) doping or chemical doping refers to the charge injection by adsorbed dopant molecules (electron donor or acceptor). The method, demonstrated early for conductive polymers, is highly effective for low-dimensional materials because of their high fraction of surface atoms. Graphene, a representative two-dimensional (2D) material, strongly interacts with I2, Br2, NO2 and alkali metals, and undergoes substantial changes in its charge density and thus Fermi level. Such changes enabled graphene to be used in the single-molecule detection, pH sensor, photodetector, solar cell and so on. Similar chemical doping has also been successfully exploited for semiconducting 2D materials. 10 Despite the wide functional tunability and potential applications enabled by CT doping, however, its mechanistic details still remain unclear or unexplored except for simple dopants that do not involve bondbreaking chemistry during the doping process. For example, single-entity dopants like atomic K and molecular halogens inject charges upon adsorption and remain as monovalent ionic adsorbates. In contrast, the O2-mediated hole doping of 2D materials had been controversial for a considerable period because of the intertwined roles of oxygen, water and substrates. 12, 13, 14 Recent studies showed that O2/H2O redox couples drive the oxygen reduction reaction (ORR: O2 + 4H + 4e ↔ 2H2O) serving as composite hole dopants in graphene and 2D semiconductors. 14 Although chemical doping with mineral acids has been widely used to enhance electrical conductivity in graphene 15, , the CT process itself has not been scrutinized until recently. Because the ORR consumes electrons of graphene efficiently at low pH as confirmed for HCl solution, the same process should occur in other acids. However, there may be an additional doping channel where their conjugate bases play a role. For example, chemical doping by H2SO4 solution may be more complex than that by HCl because of bisulfate ions (HSO4) and other potentially redox-active species. Notably, the parent and dissociated forms of sulfuric acids collaboratively induce CT doping in graphite and form intercalation compounds in the form of (C24 HSO4 2.5 H2SO4) in the presence of oxidizing agents or electrical activation. Despite the lack of consideration of solvation effect, theoretical calculations predicted sizable CT doping in graphene by HSO4 radicals unlike H2SO4. In this work, we report the concentration-dependent dual-channel CT mechanism of graphene in H2SO4. In-situ Raman spectroscopy was employed with an optical liquid cell to directly monitor the charge density of graphene in a real-time manner. It was revealed that the CT is mainly driven by O2/H2O redox couples in the concentration lower than 6 M, and possibly bisulfate species give an additional contribution at a higher concentration. Single-layer graphene samples were prepared by mechanical exfoliation using kish graphite and Si substrates with 285 nmthick SiO2 layers. To generate nanopores on the graphene surface, the samples were thermally oxidized at 500 °C for 30 min in a quartz tube furnace containing Ar:O2 gas mixture (flow rate = 200:50 mL/min). 22 AFM (atomic force microscopy) topography was obtained under ambient conditions in a noncontact mode with Si tips with a radius of 8 nm. Raman spectra were obtained using a 514 nm laser with a spectrometer with a spectral resolution of 6.0 cm. 20 The average power on samples was maintained below 200 μW to avoid potential photo-induced artifacts. In-situ Raman measurements in sulfuric acid solutions were performed using a customized optical liquid cell with a Teflon body and quartz window. The concentration of dissolved O2 was varied by sparging Ar or O2 gas into the solutions at a rate of 250 mL/min. Figures 1a and 1b show the optical micrograph and AFM height image of a representative nano-perforated sample. The oxidationinduced nanopores were created to facilitate otherwise sluggish molecular diffusion at the graphene-substrate interface and enhance the spatial homogeneity of the CT reaction. Whereas either surface of graphene can accommodate CT, the reaction on the one facing substrates is limited by the interfacial diffusion of redox species. 14 The inhomogeneity-derived G-peak splitting in sulfuric acids could be removed by introducing nanopores. The oxidation procedure typically led to a set of nanopores of ~45 nm in diameter and ~80 μm in density as shown in Figure 1b.