Crystalline-to-amorphous transformation of tantalum-containing oxides for a superior performance in unassisted photocatalytic water splitting

Abstract 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 Na 2 Ta 2 O 6 (C Na 2 Ta 2 O 6 ) to amorphous TaO x (Am-TaO x ) 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 μmol g −1  h −1 , much larger than that of 15 ± 1 μmol g −1 h −1 of C Na 2 Ta 2 O 6 , 11 ± 1 μmol g −1 h −1 of crystalline Ta 2 O 5 (C Ta 2 O 5 ), 30±2 μmol g −1 h −1 of mixture with crystalline Ta 2 O 5 and amorphous Na x Ta y O z (C Ta 2 O 5 /Am-Na x Ta y O z ), and even 4.6 × 10 −4  μmol g −1 h −1 for commercial TiO 2 . 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.