Photoluminescence of titanium-doped zinc spinel blue-emitting nanophosphors

• Single-phase ZnAl2O4:Ti nanocrystals have been synthesized by a sol–gel process.
• Under UV excitation at 280 nm, the blue emission centered at 435 nm is observed.
• Blue emission is attributed to a charge-transfer transition involving the Ti4+ ions.

A blue-emitting phosphor of titanium-doped zinc spinel (ZnAl2O4:Ti; Ti=0–6.0 mol% in relation to Al) nanopowders was prepared by a simple sol–gel method. On annealing at 1000 °C, single-phase ZnAl2O4:Ti powders had primary particles of 25–30 nm in size and most Ti ions in the form of Ti4+. Under UV excitation at 280 nm, a strong and broad blue emission centered at 435 nm was observed. The sources of the excitation and emission were assigned to the charge-transfer excitation and recombination between O2––Ti4+ and Ti3+–O– ion pairs. Optimum brightness occurred at a doping of 2.0 mol% Ti. The decay lifetime of ZnAl2O4:2%Ti was calculated to be 3.0 ms for the blue emission with CIE coordinates of x=0.168 and y=0.153. The results suggest that ZnAl2O4:Ti is a promising candidate for application as a blue component phosphor for UV-converting white light-emitting diodes.

The absorption band around 270 nm is associated with the charge-transfer processes between octahedral Ti4+ and O2− ions. The excitation band around 280 nm corresponds to the charge-transfer excitations from O2–(2p)6 electrons to Ti4+ (3d0). Under 280 nm excitation, the PL spectrum shows a strong blue emission with a peak at around 435 nm.Figure optionsDownload as PowerPoint slide