Plasmonic photocatalysis has facilitated rapid progress in enhancing photocatalytic efficiency under visible light irradiation. Poor visible-light-responsive photocatalytic materials and low photocatalytic efficiency remain major challenges. Plasmonic metal-semiconductor heterostructures where both the metal and semiconductor are photosensitive are promising for light harvesting catalysis, as both components can absorb solar light. Efficiency of photon capture can be further improved by structuring the catalyst as a photonic crystal. Here we report the synthesis of photonic crystal plasmonic photocatalyst materials using Au nanoparticlefunctionalized inverse opal (IO) photonic crystals. A catalyst prepared using a visible light responsive semiconductor (V2O5) displayed over an order of magnitude increase in reaction rate under green light excitation (=532 nm) compared to no illumination. The superior performance of Au-V2O5 IO was attributed to spectral overlap of the electronic band gap, localized surface plasmon resonance and incident light source.Comparing the photocatalytic performance of Au-V2O5 IO with a conventional Au-TiO2 IO catalyst, where the semiconductor band gap is in the UV, revealed that optimal photocatalytic activity is observed under different illumination conditions depending on the nature of the semiconductor. For the Au-TiO2 catalyst, despite coupling of the LSPR and excitation source at =532 nm, this was not as effective in enhancing photocatalytic activity compared to carrying out the reaction under broadband visible light, which is attributed to improved photon adsorption in the visible by the presence of a photonic band gap, and exploiting slow light in the photonic crystal to enhance photon absorption to create this synergistic type of photocatalyst.
By forming anatase TiO2 inverse opals by infiltration of an opal photonic crystal, we demonstrate that the optical response and angle-resolved blue-shift of the band-gap of the inverse opal structure is defined by a particular three-dimensional structure of the infilled voids. The optical structure of TiO2 inverse opals usually displays significant deviation from its physical structure and from theoretically predicted position of the photonic band-gap. Following rigorous structural characterization of the parent opal template and TiO2 inverse opals, alternative explanations for the signature of optical transmission through inverse opals is proposed. These approaches posit that, for light-matter interaction, an inverse opal is not precisely the inverse of an opal. Accurate parameters for the structure and material properties can be obtained by invoking a Bragg FCC selection rule-forbidden (-211) plane, which is not a realistic model for diffraction in the IO. Alternatively, by assuming optical interactions with just the periodic arrangement of tetrahedral filled interstitial sites in the structure of inverse opal, a complete reconciliation with spectral blue-shift with angle, photonic band gap and material parameters are obtained when a reduced unit cell is defined based on interstitial void filling. The analysis suggests a reduced interplanar spacing (d = 1/√3 D, for pore diameter D), based on the actual structure of an inverse opal in general, rather than a definition based on the inverse of an FCC packed opal. This approach provides an accurate and general description for predicting the spectral response and material parameters of ordered inverse opal photonic crystal materials.
Understanding the nature of light transmission and the photonic bandgap in inverse opal photonic crystals is essential for linking their optical characteristics to any application. This is especially important when these structures are examined in liquids or solvents. Knowledge of the true correlation between the nature of the inverse opal (IO) photonic bandgap, their structure, and the theories that describe their optical spectra is surprisingly limited compared to colloidal opals or more classical photonic crystal structures. We examined TiO2 and SnO2 IOs in a range of common solvents to solve the conflict between Bragg-Snell theory, optical and physical measurements by a comprehensive angle-resolved light transmission study coupled to microscopy examination of the IO structure. Tuning the position of the photonic bandgap and index contrast by solvent infiltration of each inverse opal requires a modification to the Bragg-Snell theory and the photonic crystal unit cell definition. We also demonstrate experimentally and theroetically that low fill factors are caused by less desne material infilling all interstitial vancancies in the opal template to form an IO. By also including an optical interference condition for inverse opals with an effective refractive index greater than its substrate, and an alternative internal refraction angle in the substrate, angle-resolved transmission spectra for inverse opals are now consistent with physical measurements. This work now allows an accurate correlation between the true response of an IO to the index contrast with a solvent, how an IO is infilled, and the directionality and bandwidth of the photonic bandgap. As control in functional photonic materials becomes more prevalent outside of optics and photonics, such as biosensing and energy storage, for example, a comprehensive and consistent correlation between photonic crystals structures and their primary optical signatures is a fundamental requirement for application.
The effect of particle size, morphology and Crates on 3D structured Co 3 O 4 inverse opal conversion mode anode materials
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