Titanium dioxide (TiO 2 ) nanoparticles were synthesized by the homogeneous precipitation method using titanium (IV) butoxide as Ti precursor. The solution was prepared by dissolving 26 ml of titanium (IV) butoxide in a mix of ethanol, glacial acetic acid, and sulfuric acid (53:43:4 ml, volume proportion), under constant magnetic stirring at 55 °C, until having a white precipitate. This precipitate was centrifuged, decanted, and finally dried at 100°C for two hours. The resultant product was thermally annealed at different temperatures and times, namely, 450, 550, 600, and 650 °C for 6, 12 and 24 h, respectively. Finally the powders were milled at 400 rpm in a planetary ball milling at different times, 2, 6 and 8 h. All powders were characterized by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). All the X-ray spectra confirmed the existence of anatase phase. From the XRD spectra, the crystallites size was estimated by using the Scherrer's fomula, giving values in an interval of 18 to 22 nm. The SEM micrographs showed a uniform particle distribution, with a spherical shape. The particle size was measured by TEM and SEM, which oscillated from 20 to 35 nm.

A set of opaque films were prepared with Degussa P25® or Hombikat UV100® TiO2 powders by the doctor blade method on glass slides with different compositions of polyethylene glycol of 20 kDa (PEG20), and they were characterized by spectroscopy, microscopy and photochemical kinetics measurements. After annealing treatment at 450 °C, about 5–7% C atom was incorporated into the films, as a consequence of the degradation of the organic complexing agents, inducing a small reduction of the energy band gap of TiO2 (i.e. 3.02 ≤ Eg (eV) ≤ 3.08). All films were about 15 ± 2 μm thick but their micro‐morphological characteristics depended on the content of PEG20, showing different patterns of cracks and aggregates that produce intense light scattering and retransmission phenomena with the result of a three‐dimensional excitation of the TiO2 particles in the thick film. Back‐face excitation with UVA light (365 ± 42 nm) of the opaque films in contact with an aqueous solution produced both surface‐bound and free hydroxyl radicals (HO•), as detected using a coumarin solution as a radical dosimeter. The photogeneration efficiency of HO• decreased with the surface roughness of the films, which varied between 135 and 439 nm depending on the film's composition.

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