Abstract
Converting solar energy to chemical energy in the form of hydrogen via water splitting is one of the promising strategies to solve the global energy crisis. Hematite, a traditional semiconducting oxide photoelectrode, can only absorb UV and visible parts of the solar spectrum, losing 40% infrared energy. In this paper, we report a novel plasmonic enhanced water splitting photoanode based on hematite-lanthanide upconversion nanocomposites to harvest lost photons below the bandgap of hematite. NaYF4:Er, Yb upconversion nanoparticles can upconvert photons from 980 nm to 510 nm-570 nm within the bandgap of hematite. More importantly, a gold nanodisk array with a plasmonic peak centered ∼1000 nm can further boost the photocurrent by 93-fold. It is demonstrated that the excitation process of lanthanide upconversion nanoparticles can be significantly enhanced by plasmonic nanostructures and can thus improve the water oxidation activity via plasmonic enhanced upconversion and hot electron injection, respectively. This new promising strategy will pave the way for plasmonic enhanced lost photon harvesting for applications in solar energy conversion.
Highlights
Solar light is a reliable and persistent alternative energy source to fossil fuels
Like other photoanode materials, hematite suffers from a limitation of bandgap that prevents it from harvesting photons outside the UV–Vis region, limiting the overall efficiency of the water splitting device
Since 980 nm incident photons have energy far below hematite’s bandgap, the only plasmonic enhancement mechanism that could contribute to the photocurrent is hot electron injection (HEI)
Summary
Solar light is a reliable and persistent alternative energy source to fossil fuels. Development of devices for effective solar energy harvesting is a critical technical challenge. While converting solar energy directly to electricity with solar voltaic panels is one of the most recognized methods for solar energy harvesting, it is more favorable to store the harvested solar energy in the form of fuel to achieve a much higher energy density and to be compatible with existing fuel-based devices. Like other photoanode materials, hematite suffers from a limitation of bandgap that prevents it from harvesting photons outside the UV–Vis region, limiting the overall efficiency of the water splitting device.. We focused on harvesting photons in the NIR region which are the lost photons in conventional photoanodes To collect these lost photons, we designed a plasmonic array (AuND) coupled to UCNPs to achieve light harvesting below the bandgap of hematite through two mechanistic pathways (Fig. 2). By introducing AuNDs matching the excitation wavelength of UCNPs, the plasmonic effect can provide a dramatic boost to the efficiency, making a significant enhancement to the upconversion of NIR 980 nm incident photons, to ∼530 nm. Three orders of magnitude photocurrent enhancement was observed using the novel designed composite photoanodes
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