In-Depth Investigation of the Mechanism of Dehydration-Induced Phase Transformation from Nb 3 O 7 (OH) to H-Nb 2 O 5 : A Theoretical and Experimental Approach.

H-Nb 2 O 5 is a promising energy material, which can be typically obtained from any other polymorph after conducting high temperature calcination (∼1273 K). Recently, a low-temperature dehydration from Nb 3 O 7 (OH) was employed to prepare H-Nb 2 O 5 at 723 K for 2 h, and yet the transformation mechanism has remained unclear in the literature. Here, the dehydration kinetic and phase transformation mechanism of the Nb 3 O 7 (OH) were investigated for the first time by experiments, density functional theory, and molecular dynamics calculations. After dehydration, the orthorhombic Nb 3 O 7 (OH) initially transformed into an intermediate Nb-O compound with dislocations, preserving parent structure, and subsequently transformed into monoclinic H-Nb 2 O 5 . The activation energy for the transformation from Nb 3 O 7 (OH) to H-Nb 2 O 5 was as low as 1.35 eV, compared to that of T-Nb 2 O 5 to H-Nb 2 O 5 (3.60 eV). Furthermore, the defect-rich H-Nb 2 O 5 obtained from Nb 3 O 7 (OH), does not exhibit pristine bound exciton state due to severe recombination of photogenerated carriers, resulting in poor photocatalytic activity.