Peering into Carrier Transport Mechanism of Anatase/Rutile Core/Shell TiO_2 NFs Photocatalysts by Photo-Kelvin Probe Force Microscopy

摘要

Photocatalysis has raised a lot of interests due to the contribution to renewable energy production and organic pollutant decomposition. Among various photocatalysts, TiO_2 is widely used due to its advantages, such as highly active photocatalytic property, chemical, and thermal stability, environment-friendly, non-toxic and low cost. Generally, anatase TiO_2 is much active than rutile TiO_2, the anatase/rutile mixed-phase TiO_2 seems to have superior photocatalytic performance. It is known that the band alignment between two phases is a crucial factor for carrier separation after photogeneration. In this study, we demonstrate a method for measuring surface potential shifts of core/shell TiO_2 nanofibers (NFs) by photo-Kelvin probe force microscopy (photo-KPFM) to reveal assessment of surface charge accumulation after light irradiation. According to the results, core-shell TiO_2 facilitate the separation of electron-hole pair, in which anatase acts as an electron acceptor and rutile acts as a hole trap. The Pt nanoparticles decorated anatase/rutile core/shell TiO_2 NFs could promote an obvious accumulation of electron on the surface and give rise to a large surface potential shift. The surface potential shift is well correlated to the photodegradation activity. Based on the studies, we can construct a band structure model of anatase/rutile core/shell TiO_2 NFs. For the anatase and rutile TiO_2, the aligned conduction bands allow the electron injecting between two phases seamlessly. For the valence band edges, the small offset between anatase TiO_2 and rutile TiO_2 impedes the holes inject from rutile to anatase. The decorated Pt NPs which contact with rutile TiO_2 forms a Schottky interface; therefore, the electrons can further inject from the semiconductor to the metal.