Lithium ion-inserted TiO₂ nanotube array photoelectrocatalysts

A quick electrochemical Li ion insertion into TiO₂ nanotube arrays (TNTs) markedly enhances the photoelectrochemical and photoelectrocatalytic performance. Potential pulses (-1.0~-1.7V_SCE for 1-11s in 1M LiClO₄) to pre-annealed TNTs effectively insert Li ions (pre-annealed Li-TNTs) into the mouth/wall and bottom TiO₂ depending on the insertion condition. Pre-annealed Li-TNTs prepared under an optimal Li ion insertion condition (-1.4V_SCE for 3s) exhibit ~70%-enhanced photocurrent generation, ~2.5 fold-higher incident photon-to-current efficiency, and an improved photoelectrocatalytic activity for the degradation of phenolic compounds in 1M KOH electrolyte. A change in photoluminescence (PL) emission spectra and decrease in charge transfer resistance by Li ion insertion suggest that the inserted Li ions play a role in inhibiting charge recombination by compensating for the photogenerated Ti³⁺ charges (Li⁺-Ti³⁺-OH). However, as KOH concentration is diluted such enhanced Li⁺ effects gradually vanish primarily due to liberation of reversibly inserted Li ions. To insert Li ions irreversibly, the potential pulses were applied to non-annealed TNTs followed by annealing (post-annealed Li-TNTs). Comparison between pre-annealed and post-annealed Li-TNTs in circum-neutral pH (0.1M Na₂SO₄ at pH ~6) indicates that the former exhibits a similar performance to bare TNTs (absence of Li ion effect), whereas the latter shows a superior performance with ca. 2.5-fold higher photoelectrochemical and photoelectrocatalytic activities. Detailed surface analyses (XPS, XRD, PL, SEM, ICP-MS, etc.) and Li⁺-induced reaction mechanism were discussed.