Niobium pentoxide prepared by soft chemical routes: morphology, structure, defects and quantum size effect

TLDR

Niobium pentoxide was synthesized via soft‑chemical digestion in ammonia or hydrazine solutions and benchmarked against a commercial reference. Characterization revealed that the soft‑chemical Nb₂O₅ shows higher phase‑transition temperatures, method‑dependent textural properties, defect‑induced visible absorption, with ammonia‑derived samples exhibiting larger pore volume and better sintering resistance, hydrazine‑derived, sonicated particles displaying smaller size and higher surface area, and a quantum‑size‑induced band‑gap increase from 3.4 to 4.2 eV.

Abstract

Niobium pentoxide (Nb2O5) is prepared by soft chemical routes (digestion either in ammonia or in hydrazine solutions) and is compared to a commercial sample. According to various characterization methods (XRD, HRTEM, DTA-TG, UV-visible diffuse reflectance, Raman and EPR spectroscopies), Nb2O5 shows particular bulk and surface properties. The phase transformation temperatures (amorphous → pseudo-hexagonal, pseudo-hexagonal → orthorhombic and orthorhombic → monoclinic) for the synthetic Nb2O5 are about 100–150 °C higher than for the commercial sample. The textural properties depend strongly on the preparation method. After calcination at 400 °C, the sample prepared in ammonia has a larger pore volume (0.22 cm3 g−1) and a better resistance to sintering at 600 °C. The preparation in hydrazine gives the following advantages only if the gel is sonically redispersed in ethanol: low particle size, narrow size distribution, and higher resistance to sintering (140 m2 g−1 at 600 °C). Preparation in ammonia or in hydrazine favors the formation of defects, characterized by a significant absorption in the visible range adjacent to the interband transition (3.4 eV, 360 nm). These defects, which disappear upon heating in oxygen, are identified as Nb4+ species and ionized oxygen vacancies as confirmed by EPR measurements. Finally, the nanoparticles obtained by sonication in ethanol (average particle size 4.5 nm) shows a significant band gap increase (from 3.4 eV to 4.2 eV) which is assigned to a quantum size effect.