Charge separation in layered titanate nanostructures: effect of ion exchange induced morphology transformation.

Morphology changes induced by surface chemistry can provide important insights into photoexcitation processes on solids which are critical to photovoltaic and photocatalytic applications. This area is of particular relevance for TiO2based nanomaterials, which have become available as sheets, wires, tubes, and rods only recently. We investigated charge-separation processes on Na2Ti3O7 nanowires and scrolled-up H2Ti3O7 nanotubes, two types of morphologies which can reversibly be transformed into each other by acid/ base treatment. A complementarity between efficient charge separation and the radiative recombination of photoexcited states clearly demonstrates that morphology and interlayer composition have a critical influence on the photoelectronic properties of layered oxide nanostructures. Layered transition-metal oxide structures can be described as stacked polyanion sheets of interconnected [MO6] n octahedra with intercalated cations in the interlayer region. While the negatively charged metal oxide sheet exhibits strong intrasheet covalent bonds, those between the sheets are actually relatively weak. For this reason, surface chemistry can induce sheet exfoliation upon formation of two-dimensional solids with sometimes unexpected properties, such as strongly enhanced acidity or surface structures that do not exist in corresponding three-dimensional compounds. Phenomena that prevail on anisotropic and morphologically well-defined nanostructures have become accessible for exploration thanks to recent advances in materials chemistry. For the production of Na2Ti3O7 nanowires like those shown in Figure 1a, we heated commercial TiO2 (Alfa Aesar no. 36199) in an aqueous solution of 10n NaOH at reflux (380 K). With lengths of several hundreds of nanometers, Na2Ti3O7 nanowires represent layered and massive structures with diameters between 10 and 100 nm. Corresponding Raman spectra show only bands that are consistent with those reported for titanate nanostructures. Highresolution transmission electronmicrographs reveal relatively smooth surfaces and parallel-oriented lattice fringes with interlayer spacings of 7.5 0.7 9 (Figure S1b in the Supporting Information). Inconsistent with other reports, acidic treatment of nanowires was essential in our work for their transformation into tubes. Washing Na2Ti3O7 nanowires with 0.1n HCl causes replacement of the sodium ions by protons and, at the same time, produces much smaller structures than nanowires (for comparison, Figure 1b characterizes a sample in which both qualities are present). Figure 1c shows a typical high-resolution image of scrolled and multiwalled H2Ti3O7 nanotubes with approximately 4 and 12 nm as inner and outer diameters, respectively. As a major result of this study, we found that the [*] A. Riss, T. Berger, S. Stankic, E. Kn"zinger, O. Diwald Institute of Materials Chemistry Vienna University of Technology Veterin2rplatz 1/GA, 1210 Vienna (Austria) Fax: (+43)1-25077-3890 E-mail: [email protected] Homepage: http://www.imc.tuwien.ac.at