Enhanced Conversion of Light at TiO₂ Photonic Crystals to the Blue of a Stop Band and at TiO₂ Random Films Sensitized with Q-CdS: Order and Disorder

Abstract

Significant enhancements in the conversion of light to current were observed at the blue edge of TiO 2 inverse opals (i-TiO 2 -o) and at highly disordered TiO 2 films (i-TiO 2 -d) sensitized with Q-CdS in sulfide electrolyte. i-TiO 2 -o with stop bands centered at 390 and 450 nm were modified with mercaptopropionic acid-Q-CdS with absorption edges tuned to the red or to the blue of the stop-band edges. A 4.7 average enhancement factor was measured at the blue edge of the stop band when it coincided with low Q-CdS absorption, while a 1.4-1.8 average gain was measured at the red edge. The blue-edge gain can be ascribed to localized or slowed light in the low refractive index medium and was found to extend 30-70 nm to the blue of the stop-band center. Light localization effects were suppressed when the stop-band edges overlapped with appreciable absorption. A highly disordered TiO 2 film fabricated by replicating a template from 150, 190, and 243 nm diameter polystyrene spheres exhibited a similar gain per adsorbed Q-CdS in the same spectral window as the blue edge of the photonic crystal when quantum dot absorption was low. This gain is ascribed to slowed light resulting from the interference of multiple internal scattering events in the disordered medium.

Electronic structures of two types of TiO2 electrodes: inverse opal and nanoparticulate cases

“…[5][6][7][8] For the last decade much attention has been devoted to QDSCs with nanostructured TiO 2 electrodes. Photonic crystals have unique structural properties and optical characteristics, which have been utilized not only in solar cells 11,12,20,[38][39][40][41][42][43][44][45] but also in photocatalysis. The morphology of the TiO 2 electrodes, including the crystal structure and surface orientation, is important, because this has a strong inuence on the photovoltaic properties.…”

Pore confinement effects and stabilization of carbon nitride oligomers in macroporous silica for photocatalytic hydrogen production

“…increasing the path length via scattering or photonic effects. [6][7][8][9] Materials able to mediate photochemical reactions are likely to make significant contributions to future energy needs and associated chemical technologies. [1,10] The vast majority of photocatalysts are based on semiconductor inorganic oxides, principally because they exhibit significant photostability, and their structural diversity allows photon absorption and catalysis to be modified by cation and anion substitution.…”

“…Finally, the photonic structure of the mac-SiO2 host could increase light absorption via slow photon effects at the stop band edges or scattering. [6][7][8][9] However, since there is only a small refractive index difference between SiO2 (1.45), and the photocatalytic medium (water (1.00) and triethanolamine (1.48)), the optical properties of the periodic macroporous structure are unlikely to strongly modify light absorption and consequent hydrogen production.…”

Nanostructure designs for effective solar-to-hydrogen conversion

“…As shown in Figure 18 , depending on the degree of disorder in a medium, multiple elastic scattering can lead to light localization by interference of counter-propagating waves, leading to signifi cant enhancements in light harvesting. This effect would contribute to the observed gains at the disordered inverse opals, and mostly exist where absorption is low [118,134] . 3D-photonic crystal design was utilized to enhance photoactivities of some other metal oxide photoanodes.…”

“…A promising photocurrent density of 4.84 mA · cm -2 was achieved for the CdS QD sensitized TiO 2 inverse opal as photoanode at 0 V vs. Ag/AgCl under simulated solar-light illumination. On the other hand, signifi cant enhancement in photoelectrochemical performances was also observed at CdS QDs sensitized TiO 2 fi lms with highly disordered inverse opal structure, which were fabricated by replicating a template from 150, 190 to 243 nm diameter polystyrene spheres [134] . This should be ascribed to slowed light resulting from the interference of multiple internal scattering events in the disordered photonic crystal medium.…”