Changes in the crystal structure and grain modifications in titanium oxide ͑TiO 2 ͒ thin films were observed during the photocatalytic oxidation of hydrocarbons. When the hydrocarbon and collodion films were irradiated, single crystalline titanium oxide transformed into polycrystals. The titanium oxide films gradually became network aggregates. These changes were analyzed with a dedicated in situ transmission electron microscope and observed three dimensionally by electron tomography. A detailed analysis of electron energy loss spectra of the samples also revealed that the changes were associated with the loss of oxygen atoms in the TiO 2 crystal lattice. Correlations between the polycrystalline grain size of TiO 2 and its catalyst activity were discussed based on the measured data.

Titanium dioxide (TiO 2) materials with useful self-cleaning, deodorizing and antibacterial functions by help of ultraviolet (UV) light have been a strong subject of investigation since their discovery of photo-dissolving effect 30 years ago[1]. These kinds of materials have been investigated in separated viewpoints of the structures and properties for a long time, but the relationship with the photocatalytic reaction is now necessary to be elucidated in atomic or molecular level. In the present study, for studying the potocatalytic reaction in atomic level by physical science techniques, we have developed a new kind of transmission electron microscope (TEM) system. We introduced UV light into the column by using an optical guide. This system enables us to observe the photocatalytic reactions dynamically. We observed photocatalytic decomposition reactions of hydrocarbons on TiO 2 films by using the in-situ HRTEM system[2].

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