Plasma-Enhanced ALD Deposition of Mixed CoNiOx Protective Layer for Ta3N5 Water Splitting Photoanode
Authors: Pastukhova Nadiia , Li Yanbo 
Published: December 2020

The photoelectrochemical (PEC) water splitting using the visible light-responsive photoelectrodes is a promising method for converting solar energy into hydrogen fuel. The overall process consists of three steps: 1) Absorption of photons and excitation to generate electron−hole pairs; 2) Migration of the generated electrons/holes to the semiconductor-electrolyte interface; 3) The surface reaction to form hydrogen or oxygen. Although numerous materials are capable to split water, the overall efficiency is still far from practical applications. The performance is always limited by the semiconductor properties such as bandgap and band positions, charge separation and recombination, and the slow kinetics of the surface reaction.
The water splitting reaction using Ta3N5-based photo-electrodes has been extensively investigated because Ta3N5 has suitably positioned valence and conduction band, promotes water splitting under visible light up to 600 nm (1), and generates photocurrent up to 12 mA cm-2 (2, 3). Unfortunately, the Ta3N5 based photo-anodes have a strong tendency to corrode under oxidative conditions, which is further enhanced by the self-oxidative deactivation due to the accumulation of the photogenerated holes on the surface during operation (4). A thin oxide layer that is formed in this way completely suppresses the photocurrent. Thus, to produce a stable photo-electrode, Ta3N5 must be isolated from the electrolyte.
We use Plasma-Enhanced Atomic Layer Deposition (PE-ALD) to design a protective-hole-extraction-layer to improve stability and performance. The main advantage of PE-ALD over convenient thermal ALD is the deposition at significantly lower temperatures with better film adhesion and consistency, delivering no oxidative damage to Ta3N5.
Nickel and cobalt oxides are very promising materials for the protection layer for Ta3N5 semiconductors due to their high resistance for photo-corrosion, excellent optical transmittance, and high hole conductivity. Besides, nickel-cobalt oxides demonstrate decent catalytic activity and have gained considerable attention as water oxidation catalysts. Using PE-ALD, we deposited mixed CoNiOx on the 700 nm thick layer of n-type Ta3N5 photo-anode. PE-ALD allows atomic-scale mixing of the metal ions at modest temperature and precise control of the oxide composition that makes it possible to tune the band positions and type of conductivity. While both materials are p-type semiconductors, only cobalt oxide has its conduction band positioned in proximity of the Ta3N5 conduction band. We show that a thin film of Co-oxide on Ta3N5 promotes hole-extraction, resulting in higher photocurrent, while pure Ni-oxide on Ta3N5 suppresses photocurrent. However, a thin film of Ni-oxide provides superior stability compared to Co-oxide. Tuning the ratio of Co and Ni metal in the mixed CoNiOx thin layer on Ta3N5 we were able to prepare stable photo-anode with photocurrent decay of less than 10 % over 500 minutes runtime.
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