An alkaline hydrothermal treatment of several types of ordered macroporous TiO2 structures, namely microtubes, sea urchin shapes and anodic nanotubes array has been investigated under stationary conditions. The effect of the size and geometry of these structures on the morphology of forming hierarchical agglomerates of titanate nanotubes have been systematically studied. It has been revealed that at sizes larger than the critical value (ca. 1 ?m), the whole geometry of the initial ordered TiO2 structure is maintained under reaction conditions leading to formation of hierarchical structures, in which bulk TiO2 is replaced with titanate nanotube agglomerates. This principle provides a convenient route for the preparation of multi-scale micro- and nanostructures of TiO2 based materials. The analysis of critical size suggests that under reaction conditions, due to the limited transport of dissolved Ti(IV) species, the growth of nanotubes occurs locally
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