Photochemically induced deposition of protective alumina coatings onto UV emitting phosphors for Xe excimer discharge lamps

• A UV-reactor for the pH induced precipitation of inorganic material is described.
• The photolysis of Azide (N3−) leads to a steady pH increase used for precipitation.
• A UV induced Al(OH)3 precipitation is used to craft Al2O3 coatings onto YPO4:Bi.
• The influence of Al2O3 coated onto YPO4:Bi with different thicknesses is discussed.
• SEM, VUV-spectroscopy and ESA measurements were performed on Al2O3 coated samples.

This work concerns the particle coating of the UV-C emitting phosphor YPO4:Bi, targeting a stability enhancement of the phosphor material for Xe excimer lamp operation. To this end, the material is coated by the wide band gap material Al2O3. In order to obtain a thin and homogeneous coating layer, a novel process based on the photochemical cleavage of NaN3 in water was developed. This results in a slow and continuous enhancement of the pH value due to ongoing NaOH formation, which results in the precipitation of Al(OH)3 from an Al2(SO4)3 × 18H2O solution. It turned out that the obtained particle coatings are of much better quality, i.e. homogeneity, compared to coatings made from a wet-chemical homogeneous precipitation process. The morphology and electrochemical properties of Al2O3 coated YPO4:Bi are discussed on the basis of optical spectroscopy, ESA measurements, and SEM/EDX investigations.

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