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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.

This study reports the synthesis of cadmium sulfide (CdS) nanoparticles on Li + -inserted TiO 2 nanotube array (Li-TNA) to fabricate Li-TNA/CdS heterojunction electrodes for photoelectrochemical (PEC) hydrogen production under air mass (AM) 1.5 light and solar visible light (λ > 420 nm). For fabrication of the heterojunction, Li + is rapidly inserted into TNA pre-grown on Ti foil, and CdS is then photodeposited onto the Li-TNA electrodes for varying deposition times. Surface analyses reveal that sub-100-nm polycrystalline CdS particles partly cover the Li-TNA (length: ∼800 nm, pore diameter: ∼100 nm), enabling the heterojunction to utilize AM 1.5 light as well as visible light. In aqueous solutions of sulfide and sulfite, the Li-TNA/CdS exhibits an incident photon-to-current efficiency (IPCE) of ∼20% (λ = 420 nm) while generating H 2 at a Faradaic efficiency of ∼100%. This PEC performance is superior to that of TNA/CdS, which is attributed to the Li + -enhanced charge transfer at the TNA/CdS interface. Electrochemical impedance analysis shows that the charge-transfer resistance of the TNA is reduced by ∼60% by Li + insertion. Time-resolved photoluminescence decay profiles further reveal that the charge transfer in Li-TNA is completed within 0.8 ns, which is ∼33% faster than that in TNA. The sample surface is analyzed using scanning electron microscopy, X-ray diffraction, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and ultraviolet–visible spectroscopy, and the PEC behavior of the samples is discussed in detail.

Although one-dimensional TiO2 nanotube arrays (TNA) grown on Ti substrates via electrochemical anodization are extensively studied in photoelectrochemistry, the photo(electro)catalytic activity of TNA detached from the Ti substrates remains unexplored. Herein, we synthesize TNA samples with various pore sizes (40–100 nm) and tube lengths (4–15 μm) via two-step electrochemical anodization, and transfer them to transparent conducting oxide (i.e. fluorine-doped tin oxide; FTO) substrates in normal (n) alignment (front plane outward) and reverse (r) alignment (backplane outward). The front and back planes of the as-fabricated TNA film are the same based on X-ray diffraction (anatase structure), X-ray photoelectron spectroscopy (Ti and O), and UV–vis transmittance data, though the tubes are open in the front and closed in the back. Regardless of the direction of irradiation (SE: FTO → TNA vs. EE: TNA → FTO), longer tubes generate a higher photocurrent (Iph) due to the large light absorption. However, for the same alignment of TNA (either n- or r-TNA), SE irradiation leads to a very large Iph (e.g., nSE > nEE), whereas n-TNA consistently generates a larger Iph than r-TNA for a given irradiation direction (i.e., n > r). The photocatalytic decomposition of phenol follows the same tendency (n > r); however, the Faraday efficiency (based on the photocharge) is higher with EE (nEE 28%, rEE 20%) than SE (rSE 11%, nSE 7%) irradiation. These photoelectrochemical and photocatalytic behaviors are explained in terms of charge carrier generation (FTO/TNA vs. TNA/solution), dissimilar charge carrier transfer pathways (e− transfer through tube framework vs. h+ transfer via radial direction), and charge injection at the tube (open vs. clogged tube mouth)/solution interface. The time-resolved photoluminescence (TRPL) emission and incident photon-to-current efficiency (IPCE) are also studied to gain insight into the charge transfer kinetics. This research was supported by the Basic Science Research Program ( 2016R1A2B4007366 and 2018R1A6A1A03024962 ), Space Core Technology Development Program ( 2014M1A3A3A02034875 ), Framework of International Cooperation Program ( 2017K2A9A1A01092948 , FY2017 ), and Next-Generation Carbon Upcycling Project ( 2017M1A2A2043123 ) through the National Research Foundation, Korea . This publication was made possible by a grant from the Qatar National Research Fund under its National Priorities Research Program (NPRP 10-1210-160019). Appendix A Scopus