H2 and O2 photocatalytic production on TiO2 nanotube arrays : effect of the anodization time on structural features and photoactivity

TiO2 nanotube (NT) arrays of different lengths were prepared by electrochemical anodization of ca. 10 cm2 area titanium disks in NH4F-H2O-formamide solution for different times, followed by annealing at 450°C. After Pt deposition on the opposite side of the disk, the so obtained Ti-supported crystalline anodic oxides were employed as photoactive electrodes in a two compartments cell for separate H2 and O2 production through water photosplitting and characterized by SEM, XRD analysis and photocurrent measurements. The anodization time affected the phase composition and morphology of the growing NTs, which strictly influenced their photocatalytic activity. Short anodization times (40-60 min) resulted in well aligned short tubes composed of an anatase-rutile mixed phase. Longer anodization (> 2.5 h) yielded thicker NT arrays covered on top by a preferentially oriented anatase layer that limited their photoactivity. Photocurrent intensity measurements perfectly paralleled the water splitting activity results obtained with the different NT arrays. In particular, a square-shaped fast photoresponse was recorded with ordered and fully top-open nanotubular structures. On the other hand, clogged tubes not only yielded low current densities, but also showed delayed photocurrent transient signals due to the reduced mobility of the charge carriers within the preferentially oriented anatase layer. NT arrays obtained under optimized conditions had a ca. 80:20 anatase:rutile composition and ensured a hydrogen production rate of 83 mmol h-1 m-2 (i.e. 1.9 NL h-1 m-2) in the absence of any hole scavenger or external bias.