The narrow band gap and significant separation of photogenerated carriers are essential aspects in practical photocatalytic applications. Nitrogen doping usually narrows the band gap of semiconductor oxides, and it enhances photocatalytic activity. Nitrogen-doped Nb₂O₅ was prepared by a multiple hydrothermal method. The non-metal element N inside the nanostructure, working as the trapping sites for the holes, which were effectively incorporated into the crystal lattice of Nb₂O₅ semiconductor oxide, remarkably shorten the band gap (3.1 eV) to enhance the visible light response, effectively reducing the photoinduced electron–hole pair recombination and prolonging carrier lifetime. The multilayer coating structure with a gradient concentration distribution and the type of nitrogen doped is favorable for the migration of photoexcited carriers in the bulk of catalysts. The unique multi-layer coating with the micro-concentration gradient of doped nitrogen provides a fast separation channel and jump steps for the separation of electron–hole pairs.

To effectively improve photocatalytic activity, the morphology and crystallinity of semiconductor photocatalysts must be precisely controlled during the formation process. Self-aligned Nb₂O₅ nanotube arrays have been successfully fabricated using the electrochemical anodization method. A novel growth mechanism of Nb₂O₅ nanotubes has been proposed. Starting from the initial oxidation process, the “multi-point” corrosion of fluoride ions is a key factor in the formation of nanotube arrays. The inner diameter and wall thickness of the nanotubes present a gradually increasing trend with increased dissociative fluorine ion concentration and water content in the electrolyte. With dehydroxylation and lattice recombination, the increased crystallinity of Nb₂O₅ represents a reduction of lattice defects, which effectively facilitates the separation and suppresses the recombination of photo-generated carriers to enhance their catalytic degradation activity.