Flower-like TiOsub.2 nanostructures were obtained by chemical oxidation of Ti foil using Hsub.2Osub.2 combined with subsequent annealing. This paper offers an analysis of the phase transformation of 3D flower-like titanium dioxide nanostructures. The role of the annealing atmosphere, sample thickness, grain shape, and nanoflower size are discussed. The nanostructures were examined using SEM, XRD, and Raman spectroscopy. Due to the nature of these two processes, the morphology of these nanomaterials is complex, and is obtained through a reaction involving Ti foil and Hsub.2Osub.2 at 80°C. A distinction is made between the layer composed of small grains at the substrate/oxide interface, elongated crystal-like structures, and outer spongle-like film. The annealing parameters, such as atmosphere (air or argon) and temperature (450 or 600 °C), affect phase composition. The photoelectrochemical performance of the anode based on flower-like TiOsub.2 has been shown. The thickness and phase composition of the anodes are factors that strongly affect the photocurrent. The multiphase heterojunctions proposed for 3D flower-like TiOsub.2 photoanodes in photoelectrochemical (PEC) cells suppose that the conduction band of anatase should be above rutile. The highest photoelectrochemical performance was obtained for a photoanode composed of 20-40% anatase and an associated thickness of 0.75-1.5 µm.