(621au) Optoelectronic Properties of Tantalum Nitride (Ta3N5) for Photoelectrochemical (PEC) Water Splitting: A Theoretical and Experimental Study

The photoelectrochemical (PEC) splitting of water is a renewable means of generating hydrogen for energy applications. Tantalum nitride (Ta₃N₅) has recently attracted attention as a visible light absorber for solar water splitting due to itsfavorable optical band gap of 2.1 eV with conduction and valence bands that straddle the redox potentials for water reduction/oxidation, which thermodynamically enable Ta₃N­₅ to perform unassisted water splitting under solar illumination.¹ As synthesized, Ta₃N₅ is naturally n-type and is commonly used as a photoanode. If photons incident on the surface of the semiconductor are  all converted to electron/hole pairs and are utilized at 100% efficiency for the desired reaction, then Ta₃N₅ could produce a photocurrent density of 12.5 mA/cm² which, if coupled with an appropriate photocathode in a tandem PEC device, could perform unassisted water splitting at a solar-to-hydrogen efficiency of ~15%.² To date, the record Ta₃N₅ device produces only 7 mA/cm² and onsets relatively late with reference to the hydrogen evolution reaction potential (0 V vs. RHE).³ Therefore, understanding the materials limitations of Ta­₃N­₅ by studying its optoelectronic properties is important for designing and engineering a device architecture that maximizes Ta₃N₅ performance as a photoanode for PEC water splitting. An important materials property for efficient light absorption is the direct or indirect nature of the optical bandgap—a matter that has inconclusive results in the literature. For our work, the goal was to definitively assign direct or indirect interband transitions to two distinct absorption features in Ta₃N₅ UV-Vis spectra occurring at 2.1 and 2.5 eV⁴ using theoretical and experimental techniques.  High-level Density Functional Theory (DFT) calculations were used to model the electronic band structure and dielectric function of Ta₃N₅. Spectroscopic ellipsometry was conducted to determine the dielectric function of Ta₃N₅ experimentally. Theoretical and experimental results were in good agreement and enabled the identification of both of the transitions, occurring at 2.1 and 2.5 eV, as direct interband transitions.⁶ A direct optical band gap semiconductor is advantageous because a thin absorber layer can be used while maintaining efficient light absorption. DFT calculations also showed high effective masses of electrons and holes in multiple directions indicative of low charge carrier mobilities, which is consistent with experimental findings that photocurrent density scales with increasing Ta₃N₅ surface area.^5,6

References:
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(2) Seitz et al, ChemSusChem 2014, 7, 1372-1385.
(3) Li et al, Nat. Commun. 2013, 4, 2566.
(4) Pinaud et al, Chem. Mater. 2014, 26, 1576-1572.
(5) Pinaud et al, J. Phys. Chem. C 2012, 116(3), 15918-15924.
(6) Morbec et al, Phys. Rev. B. 2014, 90, 155204.