Structural Evolution in a Hydrothermal Reaction between Nb_2O_5 and NaOH Solution:From Nb_2O_5 Grains to Microporous Na_2Nb_2O_6 centre dot 2/3H_2O Fibers and NaNbO_3 Cubes

摘要

Niobium pentoxide reacts actively with concentrate NaOH solution under hydrothermal conditions at as low as 120 deg C.The reaction ruptures the corner-sharing of NbO_7 decahedra and NbO_6 octahedra in the reactant Nb_2O_5,yielding various niobates,and the structure and composition of the niobates depend on the reaction temperature and time.The morphological evolution of the solid products in the reaction at 180 deg C is monitored via SEM:the fine Nb_2O_5 powder aggregates first to irregular bars,and then niobate fibers with an aspect ratio of hundreds form.The fibers are microporous molecular sieve with a monoclinic lattice,Na_2Nb_2O_6 centre dot 2/3H_2O.The fibers are a metastable intermediate of this reaction,and they completely convert to the final product NaNbO_3 cubes in the prolonged reaction of 1 h.This study demonstrates that by carefully optimizing the reaction condition,we can selectively fabricate niobate structures of high purity,including the delicate microporous fibers,through a direct reaction between concentrated NaOH solution and Nb_2O_5.This synthesis route is simple and suitable for the large-scale production of the fibers.The reaction first yields poorly crystallized niobates consisting of edge-sharing NbO_6 octahedra,and then the microporous fibers crystallize and grow by assembling NbO_6 octahedra or clusters of NbO_6 octahedra and NaO_6 units.Thus,the selection of the fibril or cubic product is achieved by control of reaction kinetics.Finally,niobates with different structures exhibit remarkable differences in light absorption and photolu-minescence properties.Therefore,this study is of importance for developing new functional materials by the wet-chemistry process.