| One-dimensional anodic titanium oxide nanotube (TONT) arrays provide a direct pathway for charge transport, and thus hold great potential as working electrodes for electrochemical energy conversion and storage devices. As for the application in photoelectrochemical (PEC) water splitting, the TiO2 nanotube surface area is minor which is against with the improvement of photoelectric conversion efficiency. And surface area is in direct proportion to the nanotube length, thus to increase the nanotube length is one method to impove the water splitting performance. On the other hand, the recombination of electron-hole also limits the photoelectric conversion efficiency.In this work, we use the anodic anodization method to obtain the TiO2 nanotube, by morphology characterization, the TiO2 nanotube is highly ordered and closely packed. Then we analysis the current-time curve and nanotube formation mechanism. The obtained TONT films were employed as anodes for PEC water splitting. By PEC test of the TONT with different length oxidized for various time (10 min-180 min), we found that the photocurrent density reached a peak value with the anodization time of 2 h. As the time extended to 3 h, the photocurrent density showed a decrease trend, which can be attributed to the following reason: when Ti foil was anodized in fluoride-containing electrolyte for a long time, undesired etching-induced "nanograss" would inevitably generate on the top of anodic TiO2 nanotubes. The nanograss will hinder the ions transport and in turn yield depressed (photo) electrochemical performance. In order to obtain nanograss-free nanotubes, a modified three-step anodization and two-layer nanostructure of TONT were designed to avoid the nanograss. The first layer (L1) nanotubes were obtained by the conventional two-step anodization. After washing and drying processes, the third-step anodization was carried out with the presence of L1 nanotubes. The L1 nanotubes, serving as a sacrificed layer, was etched and transformed into nanograss, while the ultralong nanotubes (L2) were maintained underneath the L1. The bi-layer nanostructure of the nanograss/nanotubes (L1/L2) was then ultrasonically rinsed in deionized water to remove the nanograss (L1 layer). Then much longer nanotubes (L2 layer) with intact nanotube mouths could be obtained. Using this novel approach, the ultralong nanotubes without nanograss can be rationally controlled by adjusting the anodizing times of two layers.On the other hand, the prominent surface recombination due to the large amount surface defects hinders the performance improvement. In this work, the surface states of TONTs were passivated by conformal coating of high-quality Al2O3 onto the tubular structures (denoted as TONT-A) using atomic layer deposition (ALD). The photocurrent (0.5 V vs Ag/AgCl) recorded under air mass 1.5 global illumination presented 0.8 times enhancement on the electrode with passivation coating. The reduction of surface recombination rate is responsible for the substantially improved performance, which is proposed to have originated from a decreased interface defect density in combination with a field-effect passivation induced by a negative fixed charge in the Al2O3 shells. These results not only provide a physical insight into the passivation effect, but also can be utilized as a guideline to design other energy conversion devices. |
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