Electronic properties of the interface between p-CuI and anatase-phase n-TiO2 single crystal and nanoparticulate surfaces: A photoemission study

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We present a study of the growth of the p-type inorganic semiconductor CuI on n-type TiO2 anatase single crystal (101) surfaces and on nanoparticulate anatase surfaces using synchrotron radiation photoemission spectroscopy. Core level photoemission data obtained using synchrotron radiation reveal that both the substrate (TiO2) and the overlayer (CuI) core levels shift to a lower binding energy to different degrees following the growth of CuI on TiO2. Valence band photoemission data show that the valence band maximum of the clean substrate differs from that of the dosed surface which may be interpreted qualitatively as due to the introduction of a new density of states within the band gap of TiO2 as a result of the growth of CuI. The valence band offset for the heterojunction n-TiO2p-CuI has been measured using photoemission for both nanoparticulate and single crystal TiO2 surfaces, and the band energy alignment for these heterojunction interfaces is presented. With the information obtained here, it is suggested that the interface between p-CuI and single crystal anatase-phase n-TiO2 is a type-II heterojunction interface, with significant band bending. The measured total band bending matches the work function change at the interface, i.e., there is no interface dipole. In the case of the nanoparticulate interface, an interface dipole is found, but band bending within the anatase nanoparticles remains quite significant. We show that the corresponding depletion layer may be accommodated within the dimension of the nanoparticles. The results are discussed in the context of the functional properties of dye-sensitized solid state solar cells.