Sulfate Exchange of the Nitrate-Type Layered Hydroxide Nanosheets of Ln(2)(OH)(5)NO(3)·nH(2)O for Better Dispersed and Multi-color Luminescent Ln(2)O(3) Nanophosphors (Ln = Y(0.98)RE(0.02), RE = Pr, Sm, Eu, Tb, Dy, Ho, Er, and Tm)

Through restricting thickness growth by performing coprecipitation at the freezing temperature of ~4 °C, solid-solution nanosheets (up to 5-nm thick) of the Ln(2)(OH)(5)NO(3)·nH(2)O layered hydroxide (Ln = Y(0.98)RE(0.02); RE = Pr, Sm, Eu, Tb, Dy, Ho, Er, and Tm, respectively) were directly synthesized without performing conventional exfoliation. In situ exchange of the interlayer NO(3)(−) with SO(4)(2−) produced a sulfate derivative [Ln(2)(OH)(5)(SO(4))(0.5)·nH(2)O] of the same layered structure and two-dimensional crystallite morphology but substantially contracted d(002) basal spacing (from ~0.886 to 0.841 nm). The sulfate derivative was systematically compared against its nitrate parent in terms of crystal structure and phase/morphology evolution upon heating. It is shown that the interlayer SO(4)(2−), owing to its bonding with the hydroxide main layer, significantly raises the decomposition temperature from ~600 to 1000 °C to yield remarkably better dispersed oxide nanopowders via a monoclinic Ln(2)O(2)SO(4) intermediate. The resultant (Y(0.98)RE(0.02))(2)O(3) nanophosphors were studied for their photoluminescence to show that the emission color, depending on RE(3+), spans a wide range in the Commission Internationale de l’Eclairage (CIE) chromaticity diagram, from blue to deep red via green, yellow, orange, and orange red.