Self-organized TiO2 nanotubes and their photocatalytic performance measured using spin-trap ESR spectroscopy

In the present work, self-organized TiO2 nanotubes were synthesized using an anodization technique. Titanium foil was used as the substrate for growth of oxide nanotube arrays. The anodic growth experiment was performed in a two-electrode electrochemical cell using Pt as counter electrode and titanium foil as the working electrode. Effects of electrolyte concentration, anodization voltage, and anodization time on morphology of oxide layer were investigated. Aqueous NH4F in propylene glycol was used as the electrolyte at concentrations of 0.1 and 0.3 M. Anodization voltages of 20 V and 40 V were used. Anodization time was varied from 2 to 5 hours. Field emission scanning electron microscope (FE-SEM) was employed for the morphological and structural characterization of formed layers. FE-SEM micrographs of Ti foil samples in different electrolyte concentrations at 20 V for 5 hours had larger diameter tubes at lower electrolyte concentrations. Similar observations were made at a higher anodization voltage. TiO2 nanotubes made in 0.1 M NH4F had larger diameters, 60 - 80 nm. In 0.3 M NH4F, tube diameters were less than 50 nm. Nanotubes formed at a different anodization voltage (20 V vs. 40 V) had similar surface morphology but different in diameter size. However, the higher anodization voltage produced slightly shorter of nanotubes. The length of nanotubes increased with increasing anodization time. Anodization time affected thickness of oxide layers and the surface morphology of samples. Furthermore, a clear separation most nanotubes with inter-tube spacing were observed in samples prepared with longer anodization time. Morphological changes resulted in lower surface density of the tubes, which corresponding to the number of tubes per unit area. Moreover, TiO2 nanotubes samples prepared with different anodization time were evaluated the photocatalytic performance using a spin trap electron spin resonance (ESR). ESR spectra of 5,5'-dimethyl-1-pyrroline-N-oxide (DMPO) solutions on TiO2 nanotubes samples after UV irradiation was observed. The intensity of the signals varied with anodization time. The highest signal to marker (S/M) ratio was obtained for TiO2 nanotubes prepared for 2 hours, indicating that this sample generated DMPO-OH· radicals more actively than the other samples. TiO2 nanotubes prepared for 2 hours showed the highest photocatalytic performance.