Amplified Photochemistry with Slow Photons

Abstract

Slow photon, or light with reduced group velocity, is a unique phenomenon found in photonic crystals that theoreticians have long suggested to be invaluable for increasing the efficiency of light-driven processes. This thesis demonstrates experimentally the feasibility of using slow photons to optically amplify photochemistry of both organic and inorganic systems. The effect of photonic properties on organic photochemistry was investigated by tracing out the wavelength-dependent rate of photoisomerization of azobenzene anchored on silica opals. The application of slow photons to inorganic photochemical processes was realized by molding nanocrystalline titania into an inverse opal structure and investigating its photodegradation efficiency in relation to the photonic properties. Changes in the photodegradation efficiency were directly linked to modifications of the electronic band gap absorption as a result of the photonic properties.The highest enhancement of twofold was achieved when the energy of the slow photons overlaps with the electronic band gap absorption, such that the loss of light due to photonic stop-band reflection was significantly reduced. In addition, the strength of slowphoton amplification with respect to the macroscopic structural order was studied by ii introducing controlled disorder via the incorporation of guest spheres into the opal templates. For the first time, a correlation between structural order, photonic properties and a photochemical process was established. The ability to combine slow-photon optical amplification with chemical enhancement was further achieved by incorporating platinum nanoparticles in inverse titania opals where the platinum nanoparticles increased the lifetimes of the higher population of electron-hole pairs arising from slow photon.Overall, various important factors governing the slow photon enhancement were investigated in detail, including the energy of the photonic stop band, angle dependence, thickness of the film, degree of structural order, filling fraction of the dielectric material and diffusion of a second medium if present. Theoretical calculations based on scalarwave approximation in support of the experimental findings were provided wherever possible. The findings provide a blueprint for achieving optical amplification using slow photons in the broad range of photochemical or photophysical processes.iii

Sandwich-structured TiO2 inverse opal circulates slow photons for tremendous improvement in solar energy conversion efficiency

“…An order of magnitude difference in photocurrent density between the inverse opal TiO 2 and ncTiO 2 is owing to the open macroporous structure, which increases the contact surface area between photoanode and electrolyte and better light penetration, 50,51 as well as the slow photon effect. 52 Therefore, to understand the effect of sandwich-structure and heterostructure, our comparison baseline is the monolithic TiO 2 IO, specifically M-IO-B. e v i s e d m a n u s c r i p t The observation of M-IO-B performing better than M-IO-R under white light irradiation is consistent with previous observations.…”

“…e v i s e d m a n u s c r i p t The observation of M-IO-B performing better than M-IO-R under white light irradiation is consistent with previous observations. 22,37 When the red edge of the stopband coincides with the electronic bandgap of TiO 2 (M-IO-R), although the slow photons concentrate in the high dielectric medium, namely the TiO 2 skeleton, the fact that the stopband rejects a considerable amount of light centred at 340 nm reduces the overall usage of light. The 10% increase in photocurrent generated by H-IO-TiO 2 in comparison to M-IO-B can be explained by the scattering effect at the interface, consistent with transmission measurement displayed in Fig.…”

“…[18][19][20] By matching the red or blue edge of photonic stopbands with the electronic excitation energy of the material, more than 2 fold enhancement of photocatalytic efficiency of TiO 2 has been demonstrated. [21][22][23][24][25][26] But, have we reached the limit of PC's amplification function in photochemistry? Can the effective optical path length of the 'slow photons' be further extended?…”

“…It should be noted that the 'mirrors' in this Fabry-Pérot microcavity are colloidal PCs that generate slow photons which are confined in the low refractive index phase, namely air spheres. 22 In addition, the cavity itself is another PC with a R e v i s e d m a n u s c r i p t different lattice constant, which could further slowdown the slow photons by circulating them in the middle layer PC, since the slow photon energy coincides with the blue edge of middle layer PC's stopband. Such a sandwich-structured PC design has much-increased light trapping effect, which can give rise to the extraordinary light absorbance of the TiO 2 without adding metallic materials such as gold, silver, aluminium and graphene or CdS Inverse Opals.…”

A colloidoscope of colloid-based porous materials and their uses

“…442,443,452,456,457 In a photonic crystal such as an inverse or direct opal, light near the photonic bandgap has increased absorption due to a longer lifetime in the structure. 467 This additional absorption can improve the efficiency. For example, it accounted for a >50% increase in the efficiency of a DSSC, 447 and can be seen clearly in Figure 30 for a photoanode made from WO 3 , where the efficiency varies as a function of wavelength depending on the periodicity of the structure.…”