Variation of structural disorder in Zr substituted Y2Sn2O7: Its impact on photocatalysis

Highlights: • Order-disorder phenomena in pyrochlore has been studied by XRD and Raman Spectroscopy. • ¹¹⁹Sn NMR and XPS could be successfully employed to investigate local structure of pyrochlore. • It has been observed that covalency of Sn–O bond has strong role in extension of stability of pyrochlore phase. • Surface charge, crystallite size, degree of disorder in the system has profound effect on photocatalytic dye degradation. Variation in degree of disorder and its impact on photocatalytic dye degradation has been studied in Y₂Sn_2-xZr_xO₇ series. The structural variations of the compositions were thoroughly characterized by X-ray diffraction, Raman spectroscopy, ¹¹⁹Sn MAS NMR, EXAFS and XPS. A phase transition from ordered pyrochlore to disordered fluorite could be observed in the series whilst transiting from Y₂Sn₂O₇ to Y₂Zr₂O₇ without any signature of presence of bi-phasicity. Formation of a random solid solution phase has been confirmed by ¹¹⁹Sn MAS NMR results. All the compositions until x=1.0 remain as pyrochlore whereas the compositions with x≥1.5 preferentially adopt defect-fluorite structure. X-ray photoelectron spectroscopy clearly demonstrates that the coordination environment Zr and O varies with increase in Zr content, however, the coordination environment of Sn does not alter with substitution. The phase transformation from pyrochlore to defect fluorite occurs at relatively higher Zr rich composition than expected from classical radius ratio rule due to higher covalency present in Sn–O bond. With increase in Zr content in the series crystallite size, surface charge, nature BO₆ octahedra etc. varies and it has been found that these parameters has strong bearing on catalytic property of the materials. A generic understanding for the photocatalytic degradation of cationic methylene blue dye was achieved as a function of structural disordering in the Y₂Sn_2-xZr_xO₇ system.