Abstract
A conventional light management approach on a photo-catalyst is to concentrate photo-intensity to enhance the catalytic rate. We present a counter-intuitive approach where light intensity is distributed below the electronic photo-saturation limit under the principle of light maximization. By operating below the saturation point of the photo-intensity induced hydroxide growth under reactant gaseous H2+CO2 atmosphere, a coating of defect engineered In2O3-x(OH)y nanorod Reverse Water Gas Shift solar-fuel catalyst on an optical waveguide outperforms a coated plane by a factor of 2.2. Further, light distribution along the length of the waveguide increases optical pathlengths of the weakly absorptive green and yellow wavelengths, which increases CO product rate by a factor of 8.1-8.7 in the visible. Synergistically pairing with thinly doped silicon on the waveguide enhances the CO production rate by 27% over the visible. In addition, the persistent photoconductivity behavior of the In2O3-x(OH)y system enables CO production at a comparable rate for 2 h after turning off photo-illumination, enhancing yield with 44-62% over thermal only yield. The practical utility of persistent photocatalysis was demonstrated through outdoor solar concentrator tests, which after a day-and-night cycle showed CO yield increase of 19% over a day-light only period.
Highlights
A conventional light management approach on a photo-catalyst is to concentrate photointensity to enhance the catalytic rate
We show that the principle of light concentration yields diminishing returns on product yield, whereas applying a principle of light maximization enables significantly higher photon-to-yield efficiencies and higher catalysis rates
High efficiencies and high rates are not possible under straightforward light concentration due to the volumetric generation of charge carriers over-saturating the surface density of catalytic active sites, as well as charge carrier recombination rates increasing with increasing photo-intensities
Summary
A conventional light management approach on a photo-catalyst is to concentrate photointensity to enhance the catalytic rate. The contradiction between achieving high photon-to-yield efficiencies (for maximizing the size of solar coverage) and high product rate (for maximizing the yield per catalyst mass or area) requires light to a catalyst management approach that encompasses both optical and charges based catalysis considerations[10]. We highlight an important advantage that has to the best of our knowledge been overlooked in similar waveguide studies, which is overcoming both the photo-current and product semisaturation limit at high photo-intensities. Multiple reflection pathways of the weakly absorptive green and red wavelengths enable the band-gap trap states of the photo-catalyst to be populated[22] and compel excited charge carriers from valence and conduction band edges to relax through the surface catalyst pathway rather than through bulk recombination[23,24,25]
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