U. Hörmann

High surface area crystalline titanium dioxide: potential and limits in electrochemical energy storage and catalysis

Titanium dioxide is one of the most intensely studied oxides due to its interesting electrochemical and photocatalytic properties and it is widely applied, for example in photocatalysis, electrochemical energy storage, in white pigments, as support in catalysis, etc. Common synthesis methods of titanium dioxide typically require a high temperature step to crystallize the amorphous material into one of the polymorphs of titania, e.g. anatase, brookite and rutile, thus resulting in larger particles and mostly non-porous materials. Only recently, low temperature solution-based protocols gave access to crystalline titania with higher degree of control over the formed polymorph and its intra- or interparticle porosity. The present work critically reviews the formation of crystalline nanoscale titania particles via solution-based approaches without thermal treatment, with special focus on the resulting polymorphs, crystal morphology, surface area, and particle dimensions. Special emphasis is given to sol-gel processes via glycolated precursor molecules as well as the miniemulsion technique. The functional properties of these materials and the differences to chemically identical, non-porous materials are illustrated using heterogeneous catalysis and electrochemical energy storage (battery materials) as example.

TiO2 Anatase Nanoparticle Networks: Synthesis, Structure, and Electrochemical Performance

Nanocrystalline anatase TiO(2) materials with different specific surface areas and pore size distributions are prepared via sol-gel and miniemulsion routes in the presence of surfactants. The samples are characterized by X-ray diffraction, nitrogen sorption, transmission electron microscopy, and electrochemical measurements. The materials show a pure anatase phase with average crystallite size of about 10 nm. The nitrogen sorption analysis reveals specific surface areas ranging from 25 to 150 m(2) g(-1) . It is demonstrated that the electrochemical performance of this material strongly depends on morphology. The mesoporous TiO(2) samples exhibit excellent high rate capabilities and good cycling stability.

Nanostructured, Glassy-Carbon-Supported Pt/GC Electrodes: The Presence of Secondary Pt Nanostructures and How to Avoid Them

Nanostructured, glassy carbon GC supported Pt/GC electrodes, with Pt nanostructures nanodisks of controlled size 100–140 nm in diameter and separation homogeneously distributed on a planar GC substrate, were recently shown to be interesting model systems for electrocatalytic reaction studies M. Gustavsson, H. Fredriksson, B. Kasemo, Z. Jusys, C. Jun, and R. J. Behm, J. Electroanal. Chem., 568, 371 2004. We present here electron microscopy and electrochemical measurements which reveal that the fabrication of these nanostructured electrodes via colloidal lithography, in addition to the intended nanodisks, results in a dilute layer of much smaller Pt nanoparticles diameter 5n m on the GC surface in the areas between the Pt nanodisks. We further demonstrate that by using the developed, related method of hole-mask colloidal lithography HCLH. Fredriksson, Y. Alaverdyan, A. Dmitriev, C. Langhammer, D. S. Sutherland, M. Zach, and B. Kasemo, Adv. Mater. (Weinheim, Ger.), 19, 4297 2007, similar electrodes can be prepared which are free from these Pt nanoparticles. The effect of the additional small Pt nanoparticles on the electrochemical and electrocatalytic properties of these nanostructured electrodes, which is significant and can become dominant at low densities of the Pt nanodisks, is illustrated and discussed. These results leave HCL the preferred method for the fabrication of nanostructured Pt/GC electrodes, in particular, of low-density Pt/GC electrodes.

Structure and luminescence of sol-gel synthesized anatase nanoparticles

Two samples of mesoporous anatase nanoparticles, prepared by the sol-gel method, were characterised by Cs-corrected high resolution transmission electron microscopy (HRTEM), X-ray powder diffraction (XRD) and Raman spectroscopy. Statistical evaluation of TEM data showed an average diameter of these crystallites of 8.8 nm and 11.1 nm, respectively. Optical spectroscopy by cathodoluminescence (CL) in a scanning electron microscope (SEM) showed free exciton transitions related to the direct and the indirect band gap of anatase TiO2. From the analysis of the excited states of the free excitons an exciton binding energy of 10 meV and a Bohr radius of 2.35 nm is obtained. The small Bohr radius could explain the absence of quantum confinement in the particles presented in this study.

Structural properties of sol-gel synthesized Li+-doped titania nanowhisker arrays

Nanostructured titania is of particular interest for applications in photo-catalysis due to its high catalytic activity. Moreover, these structures are of particular interest for many applications due to their electronic properties, e.g. anti-reflection layers, sensors, vacuum microelectronics. The band gap of the nanoscaled semiconducting anatase is size dependent. The band gap increases in the size range of 15 nm to 3.9 eV [1], compared to the bulk value of 3.2 eV [2], suggesting already a quantum confinement effect. Nanowhiskers, grown in even smaller dimensions as in this study are prospective candidates for showing a transition to the quantum confinement effect.

Complementary EM study on highly active nanodendritic Raney-type Ni catalysts with hierarchical build-up

Nanostructured Raney-type Ni catalysts have been used in industry since the 1920s for the production of a wide range of chemicals. [1] In the EU supported project IMPRESS it has been shown that by using gas atomisation processing high surface area particles with significantly increased catalytic activity in hydrogenation reactions can be produced. [2,3] Structural investigations with complementary methods of electron microscopy in combination with X-ray powder diffractometry have enabled the link between processing, structure and catalytic activity to be explored. [4]