To include particle attachment and porosity in the discussion of the electronic properties of nanostructured oxide materials is indispensable for deeper insights into electronic conduction across grain boundaries and thus essential to electronics, sensor technology and photovoltaics. We investigated the aggregation of isolated TiO₂-nanocrystals which results from the application of a simple hydration-dehydration cycle. After contact with water and subsequent dehydration and adsorbate removal under high vacuum conditions the powder containing isolated nanocrystals [1] is transformed into a monolithic solid which consists of a mesoporous particle network. These monoliths show significant changes in the optical absorption properties as investigated by UV-diffuse reectance spectroscopy. Vacuum annealing at temperatures T > 870 K induces oxygen anion vacancy formation on isolated TiO₂-nanocrystals. The electrons left behind form paramagnetic states which can be tracked by electron paramagnetic resonance (EPR) spectroscopy. On aggregated TiO₂-nanocrystals EPR measurements reveal that vacancy formation occurs at significantly lower temperatures than T = 870 K. In addition, polarizable conduction band electrons [2] have only been observed in the network of adjoined TiO2-nanocrystals [3]. Changes in the spectroscopic properties resulting from solvent-mediated particle aggregation and particle network formation as well as the facilitated defect formation process will be discussed in the light of the associated structural data.

[1] T. Berger, M. Sterrer, O. Diwald, E. Knözinger, Chem.Phys.Chem. 2005, 6, 2104

[2] E. Serwicka, M. W. Schlierkamp, R. N. Schindler, Z. Naturforschung 1981, 36a, 226

[3] M. J. Elser, T. Berger, D. Brandhuber, J. Bernardi, O. Diwald, E. Knözinger, J. Phys. Chem. B 2006, 110, 7605

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