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