Micro/nanoscale mesoporous Nb2O5 particles: Effect of synthesis conditions and doping with N, C, or S on their properties

A simple methodology was designed to obtain mesoporous niobium pentoxide (Nb2O5) solids functionalized with nitrogen, carbon, or sulfur. Dopant precursors were urea (N), citric acid (C) and thiourea (S). The doped solids were characterized using IR, Raman and diffuse reflectance UV–Vis spectroscopies, powder X-ray diffraction, adsorption–desorption isotherms, transmission electron and scanning electron microscopies. Depending on the amount of surfactant (cetyltrimethyl ammonium bromide — CTAB) used during synthesis, the solid presented mainly an orthorhombic crystal structure or an amorphous phase (for the sample with 0.25 mol/L CTAB). The adsorption–desorption isotherms of the synthesized solids showed hysteresis loops ( H1 andH4 types) characteristic of mesoporous solids, while transmission electron micrographs showed a disordered structure of pore channels known as “wormholes”. There was considerable interaction between the dopant anion (mainly N and S) and the solid surface, generating new surface chemical species that clogged the pores, reducing specific surface and pore volume. The doped mesoporous Nb2O5 materials were polycrystalline, formed by primary monocrystalline particles of nanometric size (10–20 nm). Doping with N, C and S caused an important effect on the absorption of visible light, reducing the band gap energy value to 2.95 eV when doped with N, and to ∼2.8 eV with S, relative to that of the undoped sample (3.03 eV). This result is important considering the potential use of mesoporous Nb2O5 materials in photocatalytic processes.