Nature of Nitrogen Incorporation in BiVO_4 Photoanodes through Chemical and Physical Methods

摘要

In recent years, BiVO_4 has been optimized as a photoanode material to producephotocurrent densities close to its theoretical maximum under AM1.5 solarillumination. Its performance is, therefore, limited by its 2.4 eV bandgap.Herein, nitrogen is incorporated into BiVO_4 to shift the valence band position tohigher energies and thereby decreases the bandgap. Two different approachesare investigated: modification of the precursors for the spray pyrolysis recipeand post-deposition nitrogen ion implantation. Both methods result in a slightred shift of the BiVO_4 bandgap and optical absorption onset. Although previousreports on N-modified BiVO_4 assumed individual nitrogen atoms to substitutefor oxygen, X-ray photoelectron spectroscopy on the samples reveals thepresence of molecular nitrogen (i.e., N_2). Density functional theory calculationsconfirm the thermodynamic stability of the incorporation and reveal that N_2coordinates to two vanadium atoms in a bridging configuration. Unfortunately,nitrogen incorporation also results in the formation of a localized state of≈0.1 eV below the conduction band minimum of BiVO_4, which suppresses thephotoactivity at longer wavelengths. These findings provide important newinsights on the nature of nitrogen incorporation into BiVO_4 and illustrate theneed to find alternative lower-bandgap absorber materials for photoelectrochemicalenergy conversion applications.