Flat-Band Potentials of Molecularly Thin Metal Oxide Nanosheets

Abstract

Exfoliated nanosheets derived from Dion–Jacobson phase layer perovskites (TBA_<i>x</i>H_1–<i>x</i>A₂B₃O₁₀, A = Sr, Ca, B = Nb, Ta) were grown layer-by-layer on fluorine-doped tin oxide and gold electrode surfaces. Electrochemical impedance spectra (EIS) of the five-layer nanosheet films in contact with aqueous electrolyte solutions were analyzed by the Mott–Schottky method to obtain flat-band potentials (<i>V</i>_FB) of the oxide semiconductors as a function of pH. Despite capacitive contributions from the electrode–solution interface, reliable values could be obtained from capacitance measurements over a limited potential range near <i>V</i>_FB. The measured values of <i>V</i>_FB shifted −59 mV/pH over the pH range of 4–8 and were in close agreement with the empirical correlation between conduction band-edge potentials and optical band gaps proposed by Matsumoto (<i>J. Solid State Chem.</i> <b>1996</b>, <i>126</i> (2), 227–234). Density functional theory calculations showed that A-site substitution influenced band energies by modulating the strength of A–O bonding, and that subsitution of Ta for Nb on B-sites resulted in a negative shift of the conduction band-edge potential