High energy conversion efficiency for solar fuel generation through photocatalytic water splitting necessitates visible light absorbing, high quantum efficiency materials. Historically, TiO2 has become a widely studied ‘model’ system for this application at the expense of visible light absorption. In 2009, photocatalytic degradation of methylene blue under visible light using TiO2-supported CeO2 was demonstrated and attributed to a ‘coupled semiconductor’ mechanism1. Here, the supported CeO2 absorbs visible light photons (due Ce3+ at the grain boundaries) and transfers photoexcited electrons to TiO2 due to its more negative conduction band minimum. More recent experimental evidence showing Ce3+ enrichment at the CeO2-TiO2 interface suggests a mixed-metal-oxide (MMO) mechanism wherein partially occupied Ce-4f levels introduce a donor state into TiO2’s bandgap, effectively reducing the bandgap energy2. However, structure-activity relationships regarding the impact of increasing Ce3+ concentration on O2/H2 evolution rates remain inconsistent, possibly due to the inability to distinguish Ce3+ at the interface vs. in the bulk of CeO2 particles2,3.
Using monochromated electron energy-loss spectroscopy (EELS) coupled to annular dark field scanning transmission electron microscopy (ADF-STEM), we aim to directly characterize the electronic structure of the CeO2-TiO2 MMO interface and correlate these properties to photocatalytic performance. For example, similar to previous work by our research group looking at Pr-doped CeO2, a joint density of states approach could be applied to valence EELS data to deduce the energy position and width of bandgap states4. By applying this technique to valence EELS at the CeO2-TiO2 interface, we may be able to elucidate the electronic structure of this MMO and correlate it to Ce3+ concentration providing direct evidence of this mechanism. To this end, we have synthesized ‘model’ CeO2-TiO2 nanoparticles with 6 wt.% Ce-loading. The relatively high loading, well-defined shape, and small size of the TiO2 support (<30 nm) should provide clean interfaces suitable for STEM-EELS at 60-kV.
To assess the photocatalytic performance, we plan to use a continuously-stirred photoreactor with recirculated Ar as a carrier gas so that the small nominal H2/O2 produced (due to the small amount of MMO interface per weight of sample) can be sampled every 2.5 minutes with a gas chromatograph. Fast sampling of produced gas will reveal transient photocatalytic behavior, if any, present in this system.
 G. Magesh et al. Indian J. Chem. 2009, 48A, 480-88.  S. Kundu et al. J. Phys. Chem. C 2012, 116, 14062-70.  S. Luo et al. J. Phys. Chem. C 2015, 119, 2669-79.  W.J. Bowman et al. Ultramicroscopy 2016, 167, 5-10.  We gratefully acknowledge support of DOE grant DE-SC0004954, ASU’s John M. Cowley Center for High Resolution Electron Microscopy and ASU’s Center for Solid State Science.