Multiple-Heterojunction in Single Titanium Dioxide Nanoparticle for Novel Metal-Free Photocatalysis

Abstract

Despite longstanding controversy, TiO2 polymorphs heterojunction composed of anatase and rutile outperform the individual ones because of the energetic type-II band alignment at the heterojunction interface. Apart from the conventional polymorphs interface, the surface disordered TiO2 has been drawing interest and considered as an unpredictable strategy while is regarded as a fact for dramatically enhancing photocatalytic activity. However, introduction and localization of the disordered layer within conventional TiO2 nanoparticles with the heterojunction has remained a big challenge. Here we report a selective positioning of a disordered layer with different thickness ranges in between the anatase and rutile phases by a conceptually different synthetic route for highly efficient novel metal-free photocatalytic H2 production (denoted as DE-P25). With two step disorder engineering: Phase selective disordering of rutile TiO2 and thermal annealing process at temperature lower than phase transformation, the multiple heterojunction in DE-P25 single nanoparticle with the disordered layer of 2~3 nm could be synthesized which is confirmed by high resolution-TEM, electron energy loss spectroscopy (EELS) and inline electron holography mapping for the first time. EELS Ti-L2,3 spectrum geometry of DE-P25 showed distinctive oxygen-deficient variation between the two crystals which corresponds with the drastic decrease of potential and negative charge density within the disordered layer due to localized unpaired electrons. PL decay profiles exhibiting ultra-fast charge migration of DE-P25 indicate that the rutile phase incorporated with the disordered layer in the TiO2 nanoparticle induces dominant direct excition formation and reduces the self-trap of charge carriers, thus suppressing the electron/hole recombination, which perfectly coincides with the holography mapping analysis. This multiple heterojunction in single TiO2 nanoparticles composed of crystalline anatase/disordered rutile/ordered rutile demonstrate not only superior interfacial charge separation efficiency, but also novel-metal free surface reactivity, which synergistically yields ~140 times higher H2 production rate than conventional anatase and rutile single heterojunction system. Figure 1