Aggregation of Oxide Nanocrystals

Controlling the interface properties of nanoparticles is critical to their successful application in sensors, fuel cells, electronic and solar energy conversion devices and in catalysis in general. The proposed project aims at the characterization of particle interface effects in semiconducting oxides such as titanium dioxide or tin dioxide. In the course of particle aggregation the formation of interfaces will be explored and their effect on the overall properties of a particle ensemble will be studied. For this purpose, isolated oxide nanocrystals will be synthesized in the gasphase using the chemical vapour deposition technique. Their size, structure and morphology as well as their spectroscopic properties will be characterized. In the following step, sample dispersion in a solvent and subsequent solvent removal will be employed in order to reach the controlled aggregation of particles and the formation of a porous particle network. Its structural and spectroscopic properties will be compared with those of non aggregated materials in order to infer the consequences of particle interface formation. The particle networks will be subjected to thermal annealing under vacuum conditions to induce oxygen deficiency and thus electronic conductivity. For the investigation of charge carrier localization effects, conduction band electrons will be employed as probes for electron paramagnetic resonance and infrared spectroscopy. With regard to sensor applications, the reversible adsorption of selected probe molecules will be explored. Adsorption isotherms will be related to the adsorbate- induced localization of free charge carriers as monitored by molecular spectroscopy. Complementary temperature- programmed desorption experiments will shed light on the relative stability of adsorbates within the porous structure of the particle network. The second important motivation for this research project is that solvent-mediated aggregation of oxide particles can represent a reliable and inexpensive approach for the generation of macroscopic objects which are made of nanocrystals. Choice of the solvent, temperature and pressure during the aggregation process will be evaluated concerning their impact on the design and fabrication of monolithic particle networks with modified electronic, optical and structural properties. The formation of particle interfaces provides means for the intentional generation of so far unexplored electronic states which are expected to be relevant in conductivity-based applications such as sensing or dye-sensitized solar cells. Again, the question how particle interfaces affect the integral ensemble properties needs to be addressed because corresponding insights will allow for a more efficient exploitation of nanoparticle networks in future technologies.

The exploration of interfacial phenomena in agglomerated nanoparticle ensembles and the identification of related synergistic effects is vital to the development of particle-based photovoltaic and photoelectrochemical devices. We investigated the influence of solid-solid interfaces on the photoelectronic properties of metal oxide nanoparticle networks. Particles prepared by chemical vapor synthesis were transformed into colloidal dispersions and subsequently aggregated to yield mesoporous nanoparticle networks. Annealing in vacuum was employed to generate particle contacts and to enable the investigation of solid-gas interface effects. A comparison with sol-gel derived titania (TiO 2 ) nanostructures (aerogels) revealed critical new insights into the structure-property relationship related to mesoporous metal oxides. Pressure induced consolidation of metal oxide nanocrystal powders was also employed to generate interfaces and grain boundaries that were found to be susceptible to facilitated lattice oxygen depletion during vacuum annealing. Quantitative oxygen uptake measurements in conjunction with spectroscopy (UV-Vis-NIR diffuse reflectance and electron paramagnetic resonance) revealed that the interface region between the particles is subject to structural changes with ongoing annealing time. At the same time this procedure allowed for a quantitative assessment of the degree of nonstoichiometry of these nanostructures. Under controlled experimental conditions in terms of temperature and pressure as well as energy and flux of photons, the activation of molecular oxygen at the surface of photoexcited nanocrystals was used to compare the photoactivity of nanomaterials with different concentrations of solid-solid interfaces as well as synthesis related impurities. As a major result we found that networks of intermixed nanoparticles exhibit substantially enhanced crossections for charge separation. The superior performance holds also in comparison to homogeneous metal oxide nanoparticle networks that exhibit a higher charge recombination loss in comparison to powders of unconnected nanocrystals. Thus, the adjustment of the composition and concentration of solid-solid interfaces inside the nanoparticle network clearly determines the branching ratio between the separation and recombination of photogenerated charge carriers. In the future, the particle intermixing and aggregation cycles developed in this project may be extended to other material pairs. Advances in the development of more efficient composite nanomaterials for photochemical applications rely on comprehensive characterization work which is based on a close connection between materials` synthesis, the incisive characterization of materials` properties and physico- chemical in depth experiments which aim at key figures to quantitatively describe the various steps comprising the overall process.