High efficiency solar chemical conversion using electrochemically disordered titania nanotube arrays transplanted onto transparent conductive oxide electrodes

Free-standing, one-dimensional TiO₂ nanotube arrays (TNAs) with a disordered surface structure are synthesized on transparent conducting substrates, and their opto-physicochemical properties and photoelectrocatalytic (PEC) performances are examined in detail. A two-step anodization process is used to transplant TNAs grown on titanium foils onto fluorine-doped SnO₂ substrates (denoted as W-TNAs), after which they are electrochemically reduced for 20 and 90 s (denoted as B-TNAs-20 and 90, respectively). The as-transplanted W-TNAs exhibit low PEC activities in terms of their photocurrent, oxygen evolution reaction (OER), and oxidations of inorganic and organic substrates (iodide and urea, respectively) under simulated sunlight (AM 1.5; 100 mW cm⁻²), primarily because of the sluggish charge transfer through the poor electrically conductive TNA framework. The quick electrochemical reduction of the W-TNAs leads to an 8-fold larger photocurrent, while significantly accelerating the OER (by three times) and iodide and urea oxidation reactions (by 2 and ∼20 times, respectively). These enhanced PEC activities of the B-TNAs are attributed to the creation of Ti³⁺ and associated oxygen vacancies which strengthen their n-type characteristics and thereby increase their electrical conductivity. The time-resolved photoluminescence spectra further reveal that the lifetime (τ) of the photogenerated charge carriers in the B-TNAs (τ = 0.33 ns) is an order of magnitude shorter than that of the W-TNAs (τ = 3.63 ns). The disordered surface exhibits a lower Faradaic efficiency for multi-electron transfer oxidation reactions and a higher Faradaic efficiency for single-electron transfer oxidation reactions compared to the W-TNAs. The detailed surface characterization and PEC mechanism are discussed.