Thin (single–triple) niobium oxide layers on mesoporous silica substrate

Abstract

Nb2O5 layers were constructed on a mesoporous silica (SBA-15) with a thickness at an atomic-level control through a reaction of surface silanol (SiOH) groups with Nb(OEt)5 under dry conditions followed by hydrolysis and condensation. The maximum coverage of Nb2O5 at a reaction was about a half of the surface area of SBA-15 with Nb/Si = 0.07 (13.4 wt%), which consisted of monolayer Nb2O5 sheets. By repeating the same procedure, single, double and triple layers were produced in a stepwise manner by once, twice and three times treatment, respectively, without any further growth of Nb2O5 sheets in thickness. Thus, maintaining the mesoporous structure, the surface of SBA-15 were converted to those of mesoporous Nb2O5, which consisted of thin layers. The discrete layer growth was convinced by the sharp absorption edges of UV-Vis. absorption with excitation energy of 4.1, 4.0 and 3.9 eV for single, double and triple layers, which were much higher than the band gap of bulk-type Nb2O5 (3.4 eV). The at most Nb/Si ratio of triple-layered Nb2O5 sheets was 0.23, corresponding to 33.7 wt%, and the formed Nb2O5 stayed in amorphous even after calcination at 500 °C. Thus, the acidic property of amorphous Nb2O5 survived above 500 °C. Since phase transitions of the ordinary Nb2O5 occur below 500 °C to the crystalline TT-phase (pseudohexagonal), the multilayered amorphous Nb2O5 thin film prepared in the present study would be a new type of material with high surface area (>400 m2 g-1) and uniform cylindrical pore systems.