Photoelectrochemical (PEC) solar energy conversion utilizes sunlight to produce high purity hydrogen fuel via water splitting, thus providing clean energy on demand. In a PEC device at least one electrode (here the anode) is semiconducting in order to absorb radiation and generate electron/hole (e/h) pairs; holes are injected from the anode to the electrolyte, carrying out water oxidation, while electrons travel to the cathode, performing hydrogen reduction. In this talk we will discuss two distinct semiconductor photoanodes for solar water oxidation.

TiO₂ nanotube arrays (NTs) made by electrochemical anodization are stable in operation and feature low cost, but the wide bandgap (3.2 eV) limits the photocurrent, and the low mean free path of the charge carriers increases recombination. In order to improve performance we carried out various modifications to the pristine material, including (i) Li intercalation to limit e/h pairs recombination[1], (ii) addition of electrocatalysts to amplify the water oxidation rate[2], (iii) formation of sub-stoichiometric TiO_2-x by H₂ annealing to enhance oxygen vacancies as active sites, or annealing in gaseous ammonia to implement nitrogen doping, resulting in visible absorption. All these modifications result in a significant increase in the PEC response, showing a sizeable cathodic shift of the photocurrent onset (~ 100 mV) and up to a 5-fold photoresponse increase at 1.23V_RHE.

Gallium Arsenide (GaAs) on the other hand exhibits a direct bandgap of 1.43eV, resulting in efficient photon-to-electron conversion and excellent PEC performance. GaAs however suffers from fast photocorrosion in aqueous media, leading to semiconductor dissolution or oxidation. Electrodeposition of a thin Ni layer on GaAs using a chemistry that leads to self-terminating growth and affords continuous coverage at few nm thickness[3] provides a passivating layer through which electrons/holes may tunnel. The effect of Ni films of up to 4.4 nm on n-doped GaAs was studied; the dependence of thickness on the deposition time at 10 s and 60 s was observed via X-ray reflectometry, confirming growth inhibition during deposition. Ni-coated GaAs showed enhanced photocorrosion resistance compared to a bare GaAs substrate, while self-limiting Ni films on GaAs retains a high value of photocurrent (8.5mA cm^-2 at 1.23V vs. RHE) compared to non-self-limited deposited sample (6.5 mA cm^-2 at 1.23V vs. RHE), owed to a full coverage of the Ni layer and high photo-transmissivity due to a low coalescence thickness (~3.5nm).

References

[1] Tsui, L. K., Saito, M., Homma, T., & Zangari, G. (2015). Journal of Materials Chemistry A, 3(1), 360-367.

[2] Tsui, L. K., Xu, Y., Dawidowski, D., Cafiso, D., & Zangari, G. (2016). Journal of Materials Chemistry A, 4(48), 19070-19077.

[3] Vanpaemel, J. et al., (2015) Langmuir, 30, 2047–2053.