To allow for the use of graphene in various nanoelectronic applications, the methods for the large-scale production of graphene with controllable electrical properties need to be developed. Here, we report the results of a fundamental study on the remarkable conversion between n- and p-type reduced graphene oxide (rGO) with changes in the thermal annealing temperature. It was found that the charge carriers in rGO for temperatures of 300–450 °C and 800–1000 °C are electrons (n-type), whereas for temperatures of 450–800 °C, they are holes (p-type). This is because the individual oxygen functional groups present on rGO are determined by the annealing temperature. We found that the predominance of electron-withdrawing groups (i.e., carboxyl, carbonyl, and sp3-bonded hydroxyl, ether, and epoxide groups) resulted in p-type rGO, although that of electron-donating groups (sp2-bonded hydroxyl, ether and epoxide groups) lead to n-type rGO. In addition, as a proof of concept, a flexible thermoelectric device consisting of GO-700 and GO-1000 as p-type and n-type components, respectively, was fabricated. This device, which contained eight pairs of the two components, exhibited an output voltage of 4.1 mV and an output power of 41 nW for ΔT = 80 K. These results demonstrate that the carrier characteristics of rGO can be altered significantly by changing the functional groups present on it, thus allowing it to be used in various applications including flexible thermoelectrics.

Electrochemical CO 2 reduction is always accompanied by a competitive hydrogen evolution reaction as water is used as a hydrogen source. In addition to intrinsic activity control, geometrical factors of electrocatalysts such as their porous structure have been demonstrated to affect the reaction selectivity, but understanding its origin is still important. Herein, we demonstrate that reduced graphene oxide layers can effectively control the Faradaic efficiency for CO production of porous zinc nanoparticle electrocatalysts. Simply tuning the coverage of graphene oxide dramatically varies Faradaic efficiency for CO production from 66 to 94% even in the bicarbonate electrolyte at the same biased potential, in which the hydrogen evolution rate was notably suppressed without sacrificing CO 2 reduction to CO production rate unlike many Zn-based electrocatalysts. The graphene oxide layers are revealed to play roles in providing geometric barriers for the mass transport channels of reactants rather than changing the chemical states of the Zn-based electrocatalysts according to in situ X-ray absorption spectroscopic analysis and electrochemical reaction kinetic studies. In addition, computational fluid dynamics simulation studies estimate the Faradaic efficiency dependence on the surface coverage and suggest that the selective suppression of H 2 evolution is associated with the larger increment in local pH compared to that in local pCO 2 at the porous electrocatalyst surfaces. Decoupling between these reactant concentrations is originated from the higher consumption rate and lower bulk concentration of proton compared to those of CO 2 , and the surface coating with graphene oxide can be an effective way to control mass transport channel.

We investigated the thermoelectric properties of reduced graphene oxide (rGO) as a function of the oxidation level of rGO. rGO among graphene derivatives was selected as a thermoelectric material since rGO bucky paper shows low thermal conductivity due to the phonon scattering at the junctions of rGO nanoplatelets. The oxdation level of rGO was controlled by differing the amount of chemical reductant (hydrazine) used to reduce graphene oxide (GO) to rGO, which correspondingly affected the electrical conductivity and Seebeck coefficient of rGO film. In this study, the maximum of figure of merit (ZT) was found to reach to 1.1×10 -4 at 298 K for rGO reduced with the hydrazine of 1000 μL. These results provide the first experimental evidence that the thermoelectric performance of graphene and its derivative can be controlled by the oxidation level of graphitic nanoplatelets.

For the utilization of graphene in various energy storage and conversion applications, it must be synthesized in bulk with reliable and controllable electrical properties. Although nitrogen-doped graphene shows a high doping efficiency, its electrical properties can be easily affected by oxygen and water impurities from the environment. We here report that boron-doped graphene nanoplatelets with desirable electrical properties can be prepared by the simultaneous reduction and boron-doping of graphene oxide (GO) at a high annealing temperature. B-doped graphene nanoplatelets prepared at 1000 °C show a maximum boron concentration of 6.04 ± 1.44 at %, which is the highest value among B-doped graphenes prepared using various methods. With well-mixed GO and g-B2O3 as the dopant, highly uniform doping is achieved for potentially gram-scale production. In addition, as a proof-of-concept, highly B-doped graphene nanoplatelets were used as an electrode of an electrochemical double-layer capacitor (EDLC) and showed an excellent specific capacitance value of 448 F/g in an aqueous electrolyte without additional conductive additives. We believe that B-doped graphene nanoplatelets can also be used in other applications such as electrocatalyst and nano-electronics because of their reliable and controllable electrical properties regardless of the outer environment.