Study on the Production Mechanism of Fluorescent Species and Its Application to White LED of NH4-Form Y-Type Zeolite
Authors: Kubota Koki , Miyajima Naoya , Sakane Hideto 
Published: December 2024

Introduction

Zeolites are aluminosilicates in which all vertices of SiO4 and AlO4 tetrahedra (TO4) are shared to build a three-dimensional framework and are classified on the basis of the framework structure. Assuming that SiO4
4− in neutral SiO2 is partially replaced with AlO4
5−, the zeolite framework becomes negatively charged by the same amount of Al3+. Various cations can be introduced into the framework cages to compensate for this negative charge. The charge-compensating cation is primarily an alkali metal ion such as Na+ or K+. This cation can be easily ion-exchanged for other monovalent and polyvalent cations. Zeolites are ideal host materials because they can easily give fluorescent light by introducing luminescent centers as charge-compensating cations and are composed only of ubiquitous elements.

Hayashi et al., reported that NH4-form Y-type zeolite exhibited intense fluorescence upon calcination in air atmosphere without any addition of dopants. In their study, the following fluorescence mechanism was proposed: On the halfway conversion to the H-form by heating, NH4-form zeolite was decomposed to produce oxygen vacancies. Then new excited levels appeared leading to fluorescence. This zeolite phosphor does not contain any main transition-metal nor rare-earth elements and produces bright white emission, so it will become an ecological and economical fluorescent material.

In this study, the dependence of the fluorescence properties of NH4-form Y-type zeolites on calcination atmosphere and temperature was investigated and the production mechanism of fluorescent species was elucidated from detailed analyses of the calcination samples. White LEDs were fabricated from the most fluorescent samples. It is demonstrated that they can be applied practical white phosphors without rare-earth nor main transition-metal elements.

Experimental

The thermal properties of a commercially available NH4-form Y-type zeolite (Sigma-Aldrich, ammonium Y zeolite) underwent thermogravimetric/differential thermal analysis under air, oxygen, nitrogen, and 4% hydrogen/nitrogen mixture atmospheres at a flow rate of 150 mL/min or in
vacuo at approximately 27 Pa using a rotary pump. From the TG curves, 130, 230, 280, 350, 460, 500, and 600 °C were selected as the calcination temperatures. The zeolite was calcined for 2 h at these temperatures under the above-mentioned atmospheres in a tubular furnace.

The following measurements were performed on the calcined zeolite samples; X-ray diffraction (XRD), fluorescence, ultraviolet-visual (UV-Vis) absorption, electron spin resonance (ESR), Fourier transform infrared (FT-IR) absorption, and 29Si and 27Al magic-angle spin nuclear magnetic resonance (MAS-NMR) by solid state.

The phosphors with the most fluorescence intensities were mixed with silicone rubber sealant in 2:1 mass ratio and were applied to a near-ultraviolet (405 nm) LED. The coated LED was then heated at 80 ºC overnight for curing. This process was repeated three times to fabricate the white LED. These LED were driven at 3.0 V, 16 mA by a stabilized power supply and illuminance and emission spectrum were measured.

Results and discussion

The fluorescence intensity was higher when the zeolite was calcined at 350 °C for 2 h in nitrogen atmosphere than in other atmospheres. In atmospheres containing oxygen, quenching was observed at the higher temperatures. These results indicate that calcination caused oxygen vacancies under every atmosphere which brought fluorescence properties. The spectra of ESR showed two signals corresponding to the E'-center and the non-bridging oxygen hole center (NBOHC) which were generated by the collapse of the SiO4 tetrahedron. In FT-IR spectra, the absorptions assigned to Si–O vibration decreased in the fluorescent samples. Figure shows the production mechanism of the fluorescent species. The oxygen vacancy on the SiO4 tetrahedron created SiO3 and/or SiO2 radicals and brought the zeolites to generate fluorescence.

The white LED constructed with the zeolite phosphors. Driving this LED with a regulated power source could obtain illumination with color temperature of 7500 K and the tristimulus values determined xy chromaticity coordinates parameters to x = 0.272 and y = 0.341. It will be a guidelines to be able to produce a practical white phosphor that does not contain rare-earth nor transition-metal elements.

Figure 1