Flat-Band Potentials of Molecularly Thin Metal Oxide Nanosheets

Exfoliated nanosheets derived from Dion-Jacobson phase layer perovskites (TBA_xH_1-xA₂B₃O₁₀, 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 (V_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 V_FB. The measured values of V_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 (J. Solid State Chem. 1996, 126 (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.