N-doped ZrO2/TiO2 bimetallic materials synthesized in supercritical CO2: Morphology and photocatalytic activity

A series of N- and N/Zr-doped titanium nanomaterials were synthesized via an acetic acid modified sol–gel route using supercritical carbon dioxide (scCO2) as both the synthesis and drying medium. Titanium isopropoxide (TIP), and triethylamine (TEA) w/wo zirconium propoxide precursors were reacted with acetic acid in a polycondensation reaction in scCO2. The effects of N and N/Zr doping on the morphology, phase structure, mean crystallite size, textural properties, thermal and crystallization behavior, and photocatalytic degradation of methylene blue was investigated. SEM and TEM analysis showed that pure titania formed nanofibers from TIP and acetic acid whereas the doped samples gave a flake-like structure. The SEM and TEM images showed that a porous material consisting of ca. 10–15 nm crystals were formed. XPS spectra indicated that the N1s peak for both N-doped titania and Ti–Zr binary metal oxide were centered at 400 eV, indicating effective doping of nitrogen in the TiO2 matrix. From the XRD analysis, it was observed that a small amount of nitrogen and zirconia inhibited the crystal growth, resulting in smaller crystallite materials. The BET analysis of the N2 isotherm data revealed that small amount of zirconia and nitrogen (0.4 at%) increased the surface area. All synthesized doped samples gave superior photocatalytic degradation of methylene blue compared to P25. These results show that scCO2 is a new promising route to provide N- and N/Zr-doped advanced photocatalytic nanomaterials.