Crystalline tantalum-containing oxides are usually taken as the advanced photocatalysts for water splitting. How about the amorphous counterparts? In this work, a transformation of crystalline Na2Ta2O6 (C-Na2Ta2O6) to amorphous TaOx (Am-TaOx) was achieved by a facial hydrothermal method. We proposed a transformation mechanism based on nucleation-dissolution-recrystallization and further intensified the influence of base concentration on the composition, crystallinity, and morphology (CCM) as confirmed by XRD, TEM, EDS. N-2-physisorption, Raman, IR, and XPS analysis. It is found that when comparing to the crystalline counterparts, amorphous samples possessed higher surface area, abundant surface hydration layers and H+ adsorption, showing an unassisted photocatalytic water splitting with a rate of 70 +/- 7 mu mol g(-1) h(-1), much larger than that of 15 +/- 1 mu mol g(-1)h(-1) of C-Na2Ta2O6, 11 +/- 1 mu mol g(-1)h(-1) of crystalline Ta2O5 (C-Ta2O5), 30 +/- 2 mu mol g(-1)h(-1) of mixture with crystalline Ta2O5 and amorphous NaxTayOz (C-Ta2O5/Am-NaxTayOz), and even 4.6 x 10(-4) mu mol g(-1)h(-1) for commercial TiO2. This observation is beneficial from the short diffusion paths of amorphous state for charge carriers, amount of catalytic sites, and stronger reducing ability. These findings develop a novel and efficient pathway towards synthesizing the different CCM of tantalum-containing compounds under hydrothermal conditions and could open opportunities for further investigating the photocatalytic property of tantalum-containing materials. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.