Nanostructured N-doped orthorhombic Nb₂O₅ as an efficient stable photocatalyst for hydrogen generation under visible...

The synthesis of orthorhombic nitrogen-doped niobium oxide (Nb₂O_5-xN_x) nanostructures was performed and a photocatalytic study carried out in their use in the conversion of toxic H₂S and water into hydrogen under UV-Visible light. Nanostructured orthorhombic Nb₂O_5-xN_x was synthesized by a simple solid-state combustion reaction (SSCR). The nanostructural features of Nb₂O_5-xN_x were examined by FESEM and HRTEM, which showed they had a porous chain-like structure, with chains interlocked with each other and with nanoparticles sized less than 10 nm. Diffuse reflectance spectra depicted their extended absorbance in the visible region with a band gap of 2.4 eV. The substitution of nitrogen in place of oxygen atoms as well as Nb-N bond formation were confirmed by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. A computational study (DFT) of Nb₂O_5-xN_x was also performed for investigation and conformation of the crystal and electronic structure. N-Substitution clearly showed a narrowing of the band gap due to N 2p bands cascading above the O 2p band. Considering the band gap in the visible region, Nb₂O_5-xN_x exhibited enhanced photocatalytic activity toward hydrogen evolution (3010 μmol h^-1 g^-1) for water splitting and (9358 μmol h^-1 g^-1) for H₂S splitting under visible light. The enhanced photocatalytic activity of Nb₂O_5-xN_x was attributed to its extended absorbance in the visible region due to its electronic structure being modified upon doping, which in turn generates more electron-hole pairs, which are responsible for higher H₂ generation. More significantly, the mesoporous nanostructure accelerated the supression of electron and hole recombination, which also contributed to the enhancement of its activity.