Metastable Bi2Zr2O7 with Pyrochlore-like Structure: Stabilization, Oxygen Ion Conductivity, and Catalytic Properties

Equimolar concentrations of Zr4+ and Bi3+ were chelated with ethylenediaminetetraacetic acid ligand with the purpose of using it as a precursor to generate pyrochlore-like Bi2Zr2O7. When the X-ray amorphous precursor was calcined at 750 degrees C for 3 h in air, pyrochlore-like product with superstructure reflections was identified by powder X-ray diffraction (PXRD) along with one minor reflection due to beta-Bi2O3. This phase was found to be metastable from additional experiments conducted by varying calcination conditions. Structural refinement of PXRD pattern by Le Bail method in Fd (3) over barm space group yielded cubic lattice constant of 10.8421(27) angstrom. Flower-petal-like morphology of the sample was evident in its field-emission scanning electron microscopy image and energy-dispersive X-ray analysis performed at various locations of the specimen confirmed nearly equal concentration of zirconium and bismuth. Six bands at 260, 320, 448, 531, 597, and 828 cm(-1) were observed for this sample in its Raman spectrum and supported our claim of pyrochlore-like structure. Indexation of bright spots present in selected area electron diffraction pattern and observed distances of lattice fringes in high-resolution transmission electron microscopy image were in conformity with the results from PXRD measurements. Absorbance maxima at 312, 372, and 423 nm with a broad tailing stretching up to visible region was noticed in the UV-visible spectrum of this sample. Direct band gap of 2 eV was estimated for this sample from Tauc plot. The oxygen ion conductivity of the sample in the temperature range of 333-773 K was examined, and the highest conductivity at 773 K was 3.071 X 10(-6) S/cm. From activation energy estimation and dielectric loss analysis, thermally activated process related to the mobility of oxygen ion vacancy was found responsible for the observed ionic conductivity. A similar conclusion was reached after careful analysis of dielectric spectroscopy data of this sample. High surface area (125.04 m(2)/g) and mesoporosity (pore diameter of 3.81 nm) were possessed by this sample, which paved way for studying its catalytic role in the reduction of nitroaromatics and carcinogenic Cr6+. Cyclability experiments showed the retainment of catalytic activity up to five cycles by the sample without undergoing any structural change.