Department of Environmental Engineering and Earth Sciences , Clemson University , Clemson , South Carolina 29634 , United States.
Department of Chemical and Environmental Engineering , Yale University , New Haven , Connecticut 06520 , United States.
Environ Sci Technol. 2018 Mar 6;52(5):2973-2980. doi: 10.1021/acs.est.7b05941. Epub 2018 Feb 16.
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 TiO, 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 YSiO:Pr and YAlO:Er, 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 Pr or Er-activated systems is not a technologically appealing mechanism for visible light photocatalysis, and provide experimental guidelines for avoiding future misinterpretation of these phenomena.
上转换发光是一种通过某些工程材料实现的非线性光学过程，它允许将低能量光子转换为高能量光子。特别与环境技术相关的是，基于镧系元素的上转换荧光粉已经在数十篇出版物中出现，作为实现宽带隙半导体光催化剂（如 TiO2）可见光激活的工具，用于降解水中污染物。据称，荧光粉颗粒可将亚带隙能量光子（例如，太阳可见光）转换为更高能量的紫外光子，从而驱动催化水中污染物的降解。然而，在这里，我们重新检查了流行的可见到紫外转换器 YSiO:Pr 和 YAlO:Er 的光物理性质，发现它们的效率远不足以在报道的激发条件下引发催化降解。此外，我们的实验表明，可见到紫外上转换敏化光催化的虚假说法可能是由于在介电半导体（荧光粉-催化剂）界面存在的直接电子注入导致染料降解的偶然增强所致。这些效应与上转换无关，仅在发色团吸收带内照射的染料溶液中发生。我们得出结论，使用 Pr 或 Er 激活系统的上转换不是可见光光催化的一种有吸引力的技术机制，并为避免未来对这些现象的误解提供了实验指导。
