Super-Nernstian pH Sensor Based on Anomalous Charge Transfer Doping of Defect-Engineered Graphene.

The conventional pH sensor based on the graphene ion-sensitive field-effect transistor (Gr-ISFET), which operates with an electrostatic gating at the solution-graphene interface, cannot have a pH sensitivity above the Nernst limit (∼59 mV/pH). However, for accurate detection of the pH levels of an aqueous solution, an ultrasensitive pH sensor that can exceed the theoretical limit is required. In this study, a novel Gr-ISFET-based pH sensor is fabricated using proton-permeable defect-engineered graphene. The nanocrystalline graphene (nc-Gr) with numerous grain boundaries allows protons to penetrate the graphene layer and interact with the underlying pH-dependent charge-transfer dopant layer. We analyze the pH sensitivity of nc-Gr ISFETs by adjusting the grain boundary density of graphene and the functional group (OH-, NH2-, CH3-) on the SiO2 surface, confirming an unusual negative shift of the charge-neutral point (CNP) as the pH of the solution increases and a super-Nernstian pH response (approximately -140 mV/pH) under optimized conditions.

Unraveling the Factors Affecting the Electrochemical Performance of MoS2-Carbon Composite Catalysts for Hydrogen Evolution Reaction: Surface Defect and Electrical Resistance of Carbon Supports.

In MoS2-carbon composite catalysts for hydrogen evolution reaction (HER), the carbon materials generally act as supports to enhance the catalytic activity of MoS2 nanosheets. The carbon support provides a large surface area for increasing the MoS2 edge site density, and its physical structure can affect the electron transport rate in the composite catalysts. However, despite the importance of the carbon materials, direct observation of the effects of the physical properties of the carbon supports on the HER activity of MoS2-carbon composite catalysts has been hardly reported. In this work, we conduct an experimental model study to find the fundamental and important understanding of the correlation between the structural characteristics of carbon supports and the HER performance of MoS2-carbon composite catalysts using surface-modified graphitic carbon shell (GCS)-encapsulated SiO2 nanowires (GCS@SiO2 NWs) as support materials for MoS2 nanosheets. The surface defect density and the electrical resistance of GCS@SiO2 NWs are systematically modulated by control of H2 gas flow rates during the carbon shell growth on the SiO2 NWs. From in-depth characterization of the model catalysts, it is confirmed that the intrinsic catalytic activity of MoS2-carbon composites for the HER is improved linearly with the conductance of the carbon supports regardless of the MoS2 edge site density. However, in the HER polarization curve, the apparent current density increases in proportion to the product of the number of MoS2 edge sites and the conductance of GCS@SiO2 NWs.