Nature of Nitrogen Incorporation in BiVO₄ Photoanodes through Chemical and Physical Methods

In recent years, BiVO₄ has been optimized as a photoanode material to produce photocurrent densities close to its theoretical maximum under AM1.5 solar illumination. Its performance is, therefore, limited by its 2.4 eV bandgap. Herein, nitrogen is incorporated into BiVO₄ to shift the valence band position to higher energies and thereby decreases the bandgap. Two different approaches are investigated: modification of the precursors for the spray pyrolysis recipe and post-deposition nitrogen ion implantation. Both methods result in a slight red shift of the BiVO₄ bandgap and optical absorption onset. Although previous reports on N-modified BiVO₄ assumed individual nitrogen atoms to substitute for oxygen, X-ray photoelectron spectroscopy on the samples reveals the presence of molecular nitrogen (i.e., N₂). Density functional theory calculations confirm the thermodynamic stability of the incorporation and reveal that N₂ coordinates 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 BiVO₄, which suppresses the photoactivity at longer wavelengths. These findings provide important new insights on the nature of nitrogen incorporation into BiVO₄ and illustrate the need to find alternative lower-bandgap absorber materials for photoelectrochemical energy conversion applications.