The Myth of Visible Light Photocatalysis Using Lanthanide Upconversion Materials

Upconversion luminescence is a nonlinear optical process achieved by certain engineered materials, which allows conversion of low energy photons into higher energy photons. Of particular relevance to environmental technology, lanthanide-based upconversion phosphors have appeared in dozens of publications as a tool for achieving visible light activation of wide-band gap semiconductor photocatalysts, such as TiO₂, for degradation of water contaminants. Supposedly, the phosphor particles act to convert sub-band gap energy photons (e.g., solar visible light) into higher energy ultraviolet photons, thus driving catalytic aqueous contaminant degradation. Herein, however, we reexamined the photophysical properties of the popular visible-to-UV converters Y₂SiO₅:Pr³⁺ and Y₃Al₅O₁₂:Er³⁺, and found that their efficiencies are not nearly high enough to induce catalytic degradations under the reported excitation conditions. Furthermore, our experiments indicate that the false narrative of visible-to-UV upconversion-sensitized photocatalysis likely arose due to coincidental enhancements of dye degradation via direct electron injection that occur in the presence of dielectric-semiconductor (phosphor-catalyst) interfaces. These effects were unrelated to upconversion and only occurred for dye solutions illuminated within the chromophore absorption bands. We conclude that upconversion using Pr³⁺ or Er³⁺-activated systems is not a technologically appealing mechanism for visible light photocatalysis, and provide experimental guidelines for avoiding future misinterpretation of these phenomena.