Regulating Impurity in Amorphous TiO₂ for Ultra-Stable Photoanode Protection

On demand of sustainable fuels under irreversible consumption of fossil fuels crisis, hydrogen production from photoelectrochemical (PEC) water splitting can be promising approach to converting solar light into clean hydrogen energy. Silicon (Si) is a feasible semiconductor with small band gap and high industrial maturity, but bare Si suffers deleterious corrosion during photoelectrochemical reaction which largely limited practical application ^[1]. Amorphous TiO₂ (a-TiO₂) thin film emerged as an excellent protection layer to avoid Si exposure to corrosive electrolyte directly. However, in comparably thicker a-TiO₂ films, intermediates exhibited conductivity heterogeneity leading to failure in short term ^[2]. By lowering the deposition temperature, crystallization behavior could be suppressed to form fully amorphous film but impurities from incomplete ligand exchange reaction were also buried into amorphous film to affect film quality.<br/>In this work, we analyzed the low temperature grown ALD a-TiO₂ film failure process and conducted a water annealing treatment to partially remove chlorine (Cl) impurity to achieve ultra-stable Si photoanode protection performance more than 600 hours. In pristine TiO₂ film, Cl impurity was uniformly distributed into whole TiO₂ and could lead to pinhole formation during electrochemical reaction to allow hydroxide groups to attack Si wafer. As a result, photocurrent density exhibited a continuous decay in PEC stability test. With water annealing treatment on pristine TiO₂ films, cross-sectional scanning transmission electron microscopy (STEM) showed it maintained amorphous phase without any crystallization. X-ray photoelectron spectroscopy (XPS) and long-term electron dispersive spectroscopy (EDS) indicated partial Cl impurity was removed at the top of TiO₂ film to form more stoichiometry TiO₂ film as an excellent protection layer. This work provides a promising approach to obtaining high-quality amorphous TiO₂ film at low temperature deposition and points a pathway to achieve maximum protection of ALD a-TiO₂ coating.<br/><b>Reference: </b><br/>[1] Y. Yu, Z. Zhang, X. Yin, A. Kvit, Q. Liao, Z. Kang, X. Wang, Enhanced photoelectrochemical efficiency and stability using a conformal TiO₂ film on a black silicon photoanode. <i>Nature Energy</i>, 2,1-7 (2017).<br/>[2] Y. Yu^†, C. Sun^†, X. Yin, J. Li, S. Cao, C. Zhang, P. Voyles*, X. Wang*, Metastable Intermediates in Amorphous Titanium Oxide: A Hidden Role Leading to Ultra-Stable Photoanode Protection, Nano Lett. 18, 5335−5342 (2018).