(353d) Tandem Core-Shell Si-Ta3N5 Photoanodes for Photoelectrochemical Water Oxidation

Tantalum nitride (Ta₃N_5­) 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.¹ 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 core-shell heterostructured Si-Ta₃N₅ nanowire photoanodes coated with CoTiO_x and NiO oxygen evolution co-catalysts.

Previous device architecture strategies for metal oxide semiconductors with charge transport limitations have included nanostructuring or scaffolding. In this work, a similar approach was applied to Ta₃N₅ to make a nanostructured core-shell Si-Ta₃N₅ device where the Si is a semiconducting, nanowire scaffold and the Ta₃N₅ is coated onto Si as a thin shell through which charge extraction should be efficient. With n-type Si as the scaffold, a heterojunction between Si and Ta₃N₅ is formed enabling a dual-absorber configuration resulting in a 200 mV cathodic shift for photocurrent onset due to the photovoltage gained from Si and therefore improves the overall performance of the Si-Ta₃N₅ device. The Si nanostructures were fabricated by a pseudo-Bosch deep reactive ion etching process, then tantalum oxide (Ta₂O₅) was coated onto Si by atomic layer deposition (ALD), and finally the Ta₂O₅ was nitrided in pure ammonia gas at 900 °C.

To gain a deeper understanding of charge transport within the thin Ta₃N₅ shells and to optimize the electrode performance, we varied the thickness of the shell between 10 and 70 nm and tested for ferrocyanide oxidation as a facile photoelectrochemical reaction. We found that in a core-shell Si-Ta₃N₅ nanostructured device, photocurrent decreased with increasing Ta₃N₅ thickness, which indicates that charge extraction becomes problematic with thicker Ta₃N₅ films. To verify that decreasing photocurrent was due to poor charge transport in thicker films, we performed absorbed photon to current efficiency (APCE) measurements with Ta₃N₅ deposited on planar Si and quartz substrates. Again, we found that APCE decreased once the films were thicker than 30 nm, demonstrating that the minority carrier diffusion length is likely tens of nanometers.

Finally, to address the problem of Ta₃N₅ photodegradation and to demonstrate photoelectrochemical performance for water oxidation in basic electrolyte with the core-shell system, CoTiO_x and NiO oxygen evolution catalysts were coated onto Si-Ta₃N₅ by dipcoating from a sol-gel and ALD, respectively. In terms of activity, we found that CoTiO_x interfaces well with Ta₃N₅ and yields a high photocurrent density compared to NiO.With NiO we were able to achieve a photocurrent density close to that of CoTiO_x after Fe incorporation into NiO (henceforth called Ni(Fe)O).² However, with CoTiO_x, the device is not stable; but with only ~1 nm of Ni(Fe)O we see a marked improvement in stability. The device loses 40-50% of its peak photocurrent over a 24 hour chronoamperometry test. Compared to the current Ta₃N₅ reports in the literature, the Ni(Fe)O catalyst affords noteworthy stability.^3-5 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 Ta₃N₅ and relatively stable long term water oxidation with a Ni(Fe)O co-catalyst.

1. Chun, W.-J.; Ishikawa, A.; Fujisawa, H.; Takata, T.; Kondo, J. N.; Hara, M.; Kawai, M.; Matsumoto, Y.; Domen, K. The Journal of Physical Chemistry B 2003, 107, (8), 1798-1803.

2. Trotochaud, L.; Young, S. L.; Ranney, J. K.; Boettcher, S. W. J Am Chem Soc 2014, 136, (18), 6744-6753.

3. Li, Y.; Zhang, L.; Torres-Pardo, A.; González-Calbet, J. M.; Ma, Y.; Oleynikov, P.; Terasaki, O.; Asahina, S.; Shima, M.; Cha, D.; Zhao, L.; Takanabe, K.; Kubota, J.; Domen, K. Nat Commun 2013, 4.

4. Liao, M.; Feng, J.; Luo, W.; Wang, Z.; Zhang, J.; Li, Z.; Yu, T.; Zou, Z. Advanced Functional Materials 2012, 22, (14), 3066-3074.

5. Liu, G.; Fu, P.; Zhou, L.; Yan, P.; Ding, C.; Shi, J.; Li, C. Chemistry â?? A European Journal 2015, 21, (27), 9624-9628.