Enhanced Photoelectrochemical Response of a Composite Titania Thin Film with Single-Crystalline Rutile Nanorods Embedded in Anatase Aggregates

Abstract

A composite titania thin film consisted of quasi-aligned single-crystalline rutile nanorods embedded in sol-gel-derived anatase aggregates was fabricated and its photoelectrochemical behavior was studied in detail. A monolayer of nearly single-crystalline rutile nanorods was first deposited on metallic Ti substrates through a controlled reaction between Ti and hydrogen peroxide. The gaps among the rutile nanorods were then filled with titania nanoparticles through a sol-gel dip-coating approach, which achieved a composite thin film of single-crystalline rutile nanorods embedded in anatase aggregates, after a subsequent thermal treatment at 723 K. The X-ray diffraction measurement revealed that the composite film with thickness of 180 nm contained anatase and rutile in a nearly 50:50 ratio. The UV-visible diffusive reflectance spectra estimated a band gap of 2.99 eV for the rutile monolayer and a slightly increased value of 3.03 eV for the composite film due to the embedding of the anatase nanoparticles. Photocurrent versus potential diagrams, photocurrent transient curves, and open-circuit potential measurement supported the fact that the anatase nanoparticles possessed better inherent photoelectrochemical properties than rutile. However, electrochemical impedance spectra characterization, together with photocurrent transient curves and open-circuit potential measurement, suggests that the single-crystalline rutile nanorods exhibited a higher electron-transfer rate. The combination of the two components in such an appropriate way enhanced significantly the charge separation arising from the anatase/rutile couple, which hence combined efficiently the crystal effect and the morphological effect. A steady-state photocurrent more than double the simple sum of those generated by the two components alone was detected for the current composite film, which can be attributed to the single-crystalline rutile nanorods that provide channels for rapid electron transfer to the conductive Ti substrates under an applied bias potential. As a result, this study concludes that the present nanostructure magnified significantly the well-established mixed crystal effect, that is, an enhanced charge separation arising from a mixture of anatase and rutile.