Thermodynamic Conditions for the Nernstian Response of the Flat Band Potential of the Metal Oxide Semiconductor: A Theoretical Study

Abstract

The flat band potential (U fb) is a fundamental property of the semiconductor/electrolyte interface and is dependent on the pH of electrolyte, as described by the (non-)­Nernstian relation. However, the origin of (non-)­Nernstian dependence between the pH and flat band potential of the metal oxide semiconductor is still in dispute. Herein, we extend a continuum double-layer (CDL) model to elucidate the thermodynamic condition of Nernstian behavior. By integrating the analytical derivation and CDL simulations, we confirm that when the reaction free energies are equal for the proton adsorption and desorption at the TiO2/electrolyte interface, the pH dependence of the flat band potential follows the Nernstian relation. Specifically, we verify this proposal by coupling the CDL model with the well-known Langmuir and Frumkin adsorption isotherms, with and without considering the lateral interactions between surface species, respectively. We demonstrate well that the thermodynamic balance between the proton adsorption and desorption processes determines the Nernstian response of the flat band potential.

Electrocatalytic Mechanisms for an Oxygen Evolution Reaction at a Rhombohedral Boron Monosulfide Electrode/Alkaline Medium Interface

“…Thus, E FB corresponds to E PZC on a doped semiconductor surface. E FB depends on the pH of a solution, the morphology of a semiconductor, doping level, surface states, and specific adsorption of ions. − Therefore, herein, we assumed that the dilute carrier doping does not alter the electron chemical potential and surface dipole barrier from the undoped surface. Under this assumption, we used the relationship between the electrode potential and the OER potential, as shown in Figure .…”

Theoretical investigation on water adsorption conformations at aqueous anatase TiO2/water interfaces

“…Taking PZC as the reference, it is helpful to understand the surface charging behavior at a given pH (e.g. positively charged by the protonation of basic surface groups at pH < PZC and negatively charged by the deprotonation of adsorbed water molecules or terminal hydroxide groups at pH > PZC), [28][29][30] which has a signicant impact on the potential distribution across the anatase TiO 2 /water interface, consequently affecting the surface chemistry of the TiO 2 surface. 31 In experiment, the pK a value of an individual surface group is usually difficult to obtain.…”

Molten Salts‐Driven Discovery of a Polar Mixed‐Anion 3D Framework at the Nanoscale: Zn4Si2O7Cl2, Charge Transport and Photoelectrocatalytic Water Splitting

“…The amplitude of the anodic and cathodic currents are comparable (Figure S26A), which is characteristic of an ambipolar behavior of the semiconductor. E FB decreases linearly with increasing pH values (Figure S26B), with a slope of 23 mV pH −1 , which deviates from the Nernstian behavior met in oxides, as observed for semiconductor oxides and other oxyhalides, [51] which could be related to different free energies of proton adsorption and desorption [52] . E FB at pH 7.4 is evaluated at 0.7 V vs. RHE, respectively.…”

“…E FB decreases linearly with increasing pH values (Figure S26B), with a slope of 23 mV pH À 1 , which deviates from the Nernstian behavior met in oxides, as observed for semiconductor oxides and other oxyhalides, [51] which could be related to different free energies of proton adsorption and desorption. [52] E FB at pH 7.4 is evaluated at 0.7 V vs. RHE, respectively. As an estimation of the CBM and VBM positions, we follow the indication of ambipolarity to state that the flat band potential is in the middle of the band gap.…”