Single-step preparation of rutile-type CrNbO4 and CrTaO4 oxides from oxalate precursors-characterization and properties

Chromium niobate and tantalate (CrNbO4 and CrTaO4) were synthesized by pyrolysis of the oxalate-based heterometallic complexes [Cr-2(bpy)(4)(mu-O)(4)Nb-2(C2O4)(4)]center dot 3H(2)O (Cr-Nb) and [Cr(bpy)(2)(H2O)(mu-O)Ta(C2O4)(3)](2)center dot 3.5H(2)O (Cr-Ta) (bpy = 2,2'-bipyridine). Compared to conventional solid-state synthesis, herein studied oxides are prepared at lower temperatures, in one step without repeating grinding procedures. The structure, morphology, and optical properties of the as-synthesized oxides were characterized using powder X-ray diffraction (PXRD), field emission scanning electron microscopy (SEM), the thermogravimetric and differential thermal analysis (TG/DTA) and UV-vis diffuse reflectance spectroscopy (UV-vis DRS). The determined band gap energies of CrNbO4 and CrTaO4 are 2.39 and 2.82 eV, respectively, which prompted us to investigate photocatalytic activity of these oxides in degradation of dyes. Microscopy studies show that the particles of both oxides began to aggregate into bigger particles, leading to an increase in grain size. Additionally, magnetization measurements on both oxides revealed spin-glass behavior at low temperatures.