Upconverting nanorods in silica film for use in CO2 -optodes: Synthesis and characterization of β-NaYF4:Yb,Er

Abstract

The upconverting material of β-NaYF4:Yb,Er powder has been successfully made by a solvothermal syntheses method. The morphology of the main sample is hexagonal rods of about 1.2 µm in length. The samples show visible green upconversion by IR excitation at 980 nm. Four different syntheses methods were applied, and the final one developed by the author showed the most uniform particles. The samples were characterized by SEM, XRD and optical spectroscopy. 2100C and 24h proved to be the ideal synthesis parameters for producing the material together with the surfactant oleic acid. Oleylamine was also investigated as a surfactant, however to poor results. SEM and XRD investigations show that there is also NaF present in the samples. Unfortunately the optical measurements do not support the observed luminescence for all of the samples. This can be assigned as a result of scattering or absorption of both the laser and the emission of the material by NaF. The poor colloidal stability of the samples in cyclohexane should also be remarked upon. Furthermore the heat-up strategy for producing core-shell particles of β-NaYF4:Yb,Er@β-NaYF4:Nd was investigated and the samples briefly investigated by TEM and XPS as well as SEM and XRD. The results indicate a core-shell structure, however further studies are needed. The optical spectroscopy results are inconclusive for IR excitation at 808 nm. However, it can be observed that the samples still show green luminescence by excitation at 980 nm. Finally the particles were tried incorporated in a sensing element of silica also containing the chromophore BTB. The sol-gel synthesis of the film was not successful and the NaYF4:Yb,Er powder was not compatible with the sol. A sensing element of silica with CP was successfully made which also showed a sensitivity towards dissolved CO2 in water.