Tailoring the morphology and structure of nanosized Zn₂SiO ₄: Mn²⁺ phosphors using the hydrothermal method and their luminescence properties

Mn²⁺-doped Zn₂SiO₄ phosphors with nanoellipsoid or nanowire morphologies were synthesized at low temperature <200 °C by a hydrothermal method without any surfactants. The morphologies of the phosphors were easily tailored by varying the amount of hydroxide ions in the precursor solution before the hydrothermal reaction. Absolute ethanol was used as a solvent instead of distilled water to explore the effects of hydroxide ions on the morphology and crystal structure. We adjusted the amount of hydroxide ions by changing the pH of the precursor solution. Sheaves of powders with an ellipsoid shape were synthesized at low pH values of 7 and 9, at which only a few hydroxide ions were present, whereas powders with a nanowire shape were produced at a high pH of 11, at which many hydroxide ions were present. In addition to its morphology, the hydroxide ions also affect the crystal structure of the synthesized powder. Whereas a Zn₂SiO ₄ phase with a willemite structure was formed at pH 7 and 9, a Zn₄Si₂O₇(OH)₂•H₂O phase with a hemimorphite structure was formed at pH 11. The as-prepared powders with a willemite structure showed an intense green emission (λ ≈ 525 nm) under 254 nm excitation, whereas the as-prepared powders with a hemimorphite structure did not show any emission. However, all of the powders showed a willemite structure while retaining their original shape after annealing at 900 °C under a reducing atmosphere. The annealed sheaves of willemite with an ellipsoid shape showed a more intense green emission with a longer decay time than the phase-transformed willemite nanowires. These results were discussed in terms of the surface defects and dopant concentration.