Coupling a titanium dioxide based heterostructure photoanode with electroless-deposited nickel-phosphorus alloy coating on magnesium alloy for enhanced corrosion protection

Abstract

• Covalent organic framework (COF) modified TiO 2 nanoarray composite was prepared. • Composite shows superior photoelectrochemical conversion and cathodic protection. • Achieved 0.42 mA cm −2 photocurrent density and 0.66 V open circuit potential drop. • Formation of a direct Z-scheme heterojunction between COF and TiO 2 is verified. • Synergistic corrosion protection of ni interlayer and composite is discussed. The utilization of photoelectrochemical cathodic protection (PECCP) enables an indirect corrosion protection of metals with low self-corrosion potential by introducing a metallic nickel interlayer. However, the ability to enhance the PECCP efficiency remains challenging because of the inherent property of the semiconductor. Herein, this ability is demonstrated by coupling a covalent organic framework (TpBD) decorated TiO 2 photoanode (TiO 2 /TpBD) with nickel coating on magnesium alloy for an effective corrosion protection. The composite photoanode showed direct PECCP for the nickel interlayer and indirect corrosion protection of the magnesium alloy. The composite structure of the nanotube array and the covalent organic framework for the photoanode were confirmed by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The enhanced photoelectrochemical conversion capability and PECCP performance of the nickel-coated Mg alloy were evidenced by the results from electrochemical and photoelectrochemical measurements including Mott-Schottky curves, photoinduced potential variations, and electrochemical impedance spectroscopy (EIS). Lastly, a corrosion protection mechanism is proposed, where the enhanced PECCP efficiency is attributed to the formation of a direct Z-scheme heterojunction, which is substantiated by the results from valence band (VB) XPS and electron spin resonance characterizations.