Shape evolution and enhanced sensing and functionalization of RE-OH nanostructures

Rare earth hydroxides (R(OH)₃) were synthesized by Composite mediated hydrothermal (CMH) method, a facile technique to prepare functional materials, and stoichiometric change in composition and morphology was observed. Ce(OH)3, La(OH)₃ and Nd(OH)₃ were the synthesized samples. X-ray diffraction (XRD) confirmed the hexagonal structures of the prepared samples. The crystallite size corresponding to the most intense peaks were 18, 33 and 41 nm for Nd-, La- and Ce-hydroxides. SEM revealed very interesting and fascinating morphologies. Ce(OH)₃ has belts like structures, Nd(OH)₃ has needles like structures and La(OH)₃ has wires like structures. The growth of structures can be ascribed to chemical potential, maintained through precipitating agent, the pressure inside the vessel, the temperature provided for the hydrothermal treatment and time for hydrothermal treatment time. The shape evolution can be explained by Gibbs-Curie-Wulff model which relate the shape evolution with the face energies. When the equilibrium energy is obtained for respective faces the Ostwald ripening is stopped. On heat treatment, the La(OH)₃ first converted into LaOOH at ca. 400°C and finally into La₂O₃ at ca. 600°C as observed in DSC plot. The increase of conductivity with temperature is evident from the plots. Nd(OH)₃ achieved maximum conductivity and Ce(OH)₃ acquired minimum among the three possibly due to smaller crystallite sizes in the former case. The smaller grains increase the grain boundaries and charges can pile up on boundaries which increase the conductivity. The ac conductivity showed a direct dependence on frequency (1kHz-3MHz). The corresponding dc conductivity values of Ce(OH)₃, La(OH)₃ and Nd(OH)₃ were 0.372, 6.648 and 20.369 S-cm^-1, respectively. The interesting morphologies make these materials potential candidates for sensing and functionalization.