|Fig. 1: Device for photocatalysis of water to produce hydrogen and oxygen. (Source: Wikimedia Commons)|
The use of fossil fuels damages our health and environment, driving a shift toward renewable, green energy technologies. Photocatalysis is a way to harvest solar energy, where irradiation of UV, visible, and near-infrared (NIR) radiation from the sun causes photocatalysts to generate free radicals.  These highly reactive molecules have diverse applications. For instance, they can be used to split water for generation of hydrogen fuel, or to treat hazardous waste water by reacting with the toxic compounds.  Nanoparticle-based photocatalysts could also be coated onto structural materials to self-clean the surfaces and minimize degradation. 
Photocatalysts are typically semiconducting materials with a band gap that corresponds to the wavelength of the source. These materials include titania, zinc oxide, cadmium sulfide, and bismuth tungsten oxide. Mechanistically, an electron is excited across the bandgap to generate electron-hole pairs which produce hydroxyl radicals and superoxide ions.  These radicals attack pollutants to break them down into water and carbon dioxide.  The performance of a photocatalytic material is restricted by its need to absorb light with energy at least as high as the bandgap, which necessarily excludes some of the solar spectrum.  Thus, to increase the efficiency of photocatalysts, progress must be made to increase the range of light that they may absorb.
Lanthanide-based materials can upconvert, where two or more photons of lower energy are absorbed and transformed into one, higher-energy emitted photon. By tuning the exact composition of these materials, the absorbed wavelength can be sub-bandgap and emitted above-bandgap, thus increasing the amount of light that can be absorbed.  This then increases the efficiency of photocatalytic systems. Lanthanide-based upconversion occurs as a result of intra-4f electronic dipole transitions. These transitions are typically forbidden due to quantum mechanical selection rules. However, orbital mixing that breaks symmetry permits these transitions with long decay times, which allows for the sequential absorption that is characteristic of upconverting materials. 
Wang et al. created the first photocatalyst that can utilize visible light by combining a lanthanide-based upconverting material, erbium oxide, with titania.  Then, to utilize NIR light in photocatalytic reactions, Qin et al coated titania with ytterbium- and terbium-doped yttrium fluoride particles, which converts NIR to UV light.  These ideas were put into practice by Shi et al, where an erbium-doped strontium titanate photocatalyst used visible light to produce hydrogen by splitting water, when normally only UV light could be used.  These studies demonstrate the applicability of upconversion for photocatalysis.
While initial results have been exciting, many issues with upconverting material-based photocatalysis remain. A major concern is that upconverting materials have low quantum efficiency as a result of narrow absorption bands and low probability of converting absorbed photons into emitted ones.  In fact, many papers do not even cite the overall quantum efficiency (energy utilized divided by input energy); a recent report by Wisser et al., however, reports record quantum efficiencies in sub-25 nm, unshelled particles of 0.074%, indicating that overall device architectures would undoubtedly achieve even lower efficiencies.  In addition, the selection of reasonably efficient absorption band wavelengths is restricted to 800 and 980 nm of ytterbium.  There have been studies investigating coupling upconversion mechanisms with others, include photonic crystal engineering, coupling with surface plasmons, or sensitizing with broad wavelength-absorbing dyes, but these technologies are still years away from practical application.  Nonetheless, though these issues will require diverse efforts to solve them, this field remains full of potential.
© Derek Wang. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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