Photoelectrochemical and Photocatalytic Behaviors of Hematite-Decorated Titania Nanotube Arrays: Energy Level Mismatch versus Surface Specific Reactivity

Abstract

Nanocrystalline hematite particles (alpha-Fe2O3) were electrodeposited on the TiO2 nanotube (TiNT) arrays that were fabricated via anodization of Ti foils. The short precontact time (1 h) of aqueous ferric ions (Fe3+) on TINT resulted in formation of hematite particles selectively on the mouth surface of TINT (hematite@1 h/TiNT), whereas the long precontact time (24 h) resulted in complete filling of the TINT inside and an even full-covering of the TiNT top surface with the hematite particles (hematite@24 h/TNT). For comparison, hematite particles were also electrodeposited on TiO2-nanoparticulate films obtained via oxidative annealing of Ti foil resulting in hematite fully covered TiO2 nanoparticles (hematite/TiNP). Photoelectrochemical (PEC) study with AM 1.5 light (UV + Vis) indicated that the PEC activity of TINT decreased by ca. 40% and almost completely vanished when hematite covered the full surface of TiNT (hematite@ 24 h/TiNT) and loaded on the mouth surface of TiNT (hematite@ h/TiNT), respectively. The relatively higher PEC activity of hematite@24 h/TiNT was further observed under varying visible light conditions (400 nm < lambda < 500 nm). Hematite/TiNP also has ca. 40%-reduced PEC activity as compared to TiNP under AM 1.5 light, the tendency of which is similar to hematite@24 h/TiNT. Photocatalytic (PC) activities of TNT and hematite/TINT for degradation of aqueous phenol under AM 1.5-light were also compared, which indicates that the PC activity of TiNT vanishes almost completely with hematite@1 h/TiNT, whereas it is recovered at a moderate level with hematite@24 h/TiNT. All of these PEC and PC behaviors of TNT and hematite/TiNT were discussed in terms of hematite-induced charge recombination due to an energy level mismatch between TiO2 and hematite, as well as surface-specific photoactivity of TINT (i.e., mouth surface vs interwall and/or underlying base layer). Various surface analysis techniques (XRD, XPS, TEM, UV-vis diffuse reflectance) were employed to understand the surface states of TiNT and hematite/TINT. Finally, more detailed charge transfer mechanism was proposed.