New progress on probing charge separation at interface of TiO2 phase junction

503Group POSTED:2017-03-20

     Recently, our group published new result on directly probing the energy band alignment across the interface of a model rutile/anatase phase junction on nanometer scale. This work is now published in the journal of J. Phys. Chem. Letters (DOI: 10.1021/acs.jpclett.7b00285, JPCL, 2017, 8, 1419–1423.).
    Advancing artificial photocatalysts towards high light to energy conversion efficiency requires efficient separation of photogenerated electrons and holes. In 2008, our group found that the photocatalytic activity of TiO2 can be greatly enhanced when anatase TiO2 nanoparticles were highly dispersed on the surface of rutile TiO2 to form anatase–rutile surface-phase junctions (Angew. Chem. Int. Ed., 2008, 120, 1766-1769). Based on this concept, we further found the surface phase junction on Ga2O3 can significantly improve photocatalytic overall water splitting into H2 and O2 (Angew. Chem. Int. Ed., 2012, 51, 13089-13092). In recent years, the concept has been successfully extended to the fabrication of PV devices (Nano Energy, 2015, 15, 406-412) and PEC electrode (Chem. Sci., 2016, 7, 6076-6082). The fabrication of phase junction structures has been proved to be an effective strategy for promoting charge separation in solar fuel production.  
    In recent years, as a crucial factor to determine the photogenerated charges dynamics, there is an increasingly hot debate about the energy band alignment across the interface of phase junction. To make this issue clear, a direct probe of energy band alignment at the interface of TiO2 phase junction at nanoscale is highly desirable but challenging, because the phase junction structures are often buried at interface and therefore high spatial resolution techniques are required. In this work, we give the first report on directly probing the energy band alignment across the interface of well-defined TiO2 phase junction. A built-in electric field up to 1 kV/cm across the rutile/anatase interface was clearly revealed. Moreover, a home-built spatially resolved surface photovoltage spectroscopy (SRSPS) directly revealed that photogenerated electrons transfer from rutile nanorods (NRs) to anatase nanoparticles (NPs) under UV light illumination. We found that the size of anatase NPs in the phase junction significantly affect the surface photovoltage and charge transfer process due to the variation of charge depletion layer and built-in electric field at the interface. A quantitative understanding of charge layer width will further broaden the application of phase junction in photocatalysts where interface engineering is required to optimize the photocatalytic activity.