Tantalum nitride (Ta3N5) is a promising photoanode material for photoelectrochemical water splitting due to its n-type doping, relatively small bandgap of 2.1 eV, and band positions that straddle the redox potentials of both the hydrogen and oxygen evolution reactions. However, there are other challenges associated with Ta3N5 including poor charge transport properties, late photocurrent onset and photodegradation. To address all of these challenges, we fabricated efficient and stable tandem core-shell Si-Ta3N5 nanowire photoanodes coated with Co and Ni-based oxygen evolution co-catalysts. Nanostructuring and scaffolding is a promising method to improve performance in charge transport limited materials. In this work, n-type Si is etched to make a semiconducting, nanowire scaffold and Ta3N5 is coated onto Si as a thin shell through which charge extraction should be efficient. Forming a tandem device with Si and Ta3N5 enables a photocurrent onset at lower applied bias due to the photovoltage gained from Si. To gain a deeper understanding of charge transport within the thin Ta3N5 shells and to optimize the electrode performance, shell thickness was varied and tested for ferrocyanide oxidation as a facile photoelectrochemical reaction. We found that in Si-Ta3N5 nanostructured devices, photocurrent decreased with increasing Ta3N5 shell thickness, which indicates charge recombination limiting the thicker Ta3N5 films. We verified this observation using absorbed photon to current efficiency (APCE) measurements with Ta3N5 deposited onto planar Si and quartz substrates. Finally, to address the challenge of Ta3N5 photodegradation and to demonstrate photoelectrochemical performance for water oxidation in basic electrolyte with the core-shell system, CoTiOx and Ni(Fe)Ox oxygen evolution catalysts were coated onto Si-Ta3N5. Devices coated with CoTiOx outperformed those coated with Ni(Fe)Ox in terms of activity, but with only ~1 nm of Ni(Fe)Ox we see a marked improvement in stability. The devices maintain 50-60% of their peak photocurrent over a 24 hour chronoamperometry test. Compared to the current Ta3N5 reports in the literature, the Ni(Fe)Ox catalyst affords noteworthy stability. Therefore, in this work, we demonstrate a heterostructured Ta3N5 system with a tandem core-shell Si-Ta3N5 configuration that enabled earlier onset of photocurrent, efficient charge extraction from thin Ta3N5 and relatively stable long term water oxidation with a Ni(Fe)Ox co-catalyst.

Nanostructured tandem Si-Ta3N5 photoanodes for solar water splitting

253rd National Meeting (2017)

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