Constructing Multiple Component Photonic Crystals For Photocatalytic Hydrogen Evolution

Hydrogen production from photocatalytic water splitting has been a promising strategy to release the global pressure from energy crisis and environmental pollution.To improving the solar-to-chemical energy conversion efficiency by fine designing the structure and component has been a hot research topic.Photonic crystals,as a kind of metamaterials are periodic ordered structures constructed by multiple composites with different refractive indes.Due to their photonic bandgap structure,hierarchically porous property,structure color and lotus effect,photonic crystals have been widely used in optoelectronic devices,microwave technology,catalysis and biomimetic fields.Photonic crystals have been regard as the semiconductor of 21century.Inverse opal structure（also called three-dimensionally ordered macroporous structure,3DOM）,as a typical photonic crystal is endowed with excellent photocatalytic and photoelectrocatalytic properties due to the enhanced light harvesting,interconnected porous structure,large surface area and high assessibility of components.Aiming at the existing low light absorption efficiency and high recombination rate of charge carries of single comoponet photonic crystals in photocatalystic water splitting,this paper constructed multiple component photonic crystals by regulating the basic structure and composites to greatly promote the solar-to-chemical energy conversion efficiency.Titanium dioxide（TiO₂）inverse opal was synthesized by sol-gel method.As pure TiO₂ only absorbs ultraviolet light due to its large band gap and suffers the high recombination of photogenerated electrons and holes,the as-fabricated TiO₂ inverse opal was then deposited with gold nanoparticles（Au NPs）and cadmium sulfide（Cd S）to form the ternary photocatalyst 3DOM TiO₂-Au-Cd S.The as-prepared 3DOM TiO₂-Au-Cd S could effectively promote photocatalystic hydrogen evolution performance as the 3DOM structure enhanced light harvesting and increased the diffusion efficiency of sacrificial agents,thus restrained the photocorrosion of Cd S.The existing Cd S and TiO₂ formed theⅡtype heterostructure,which facilitated the separation of photogenerated electrons and holes of Cd S under different incident light irradiation.Besides,Au NPs between Cd S and TiO₂ attributed to highly accelerating the electron transfer to enhance the quantum efficiency of ternary photocatalyst.Incident photons at the edges of photonic bandgap（red edge or blue edge of the reflection spectrum）are with much reduced group velocity and this part of photons is called as slow photons.If the energy of these slow photons overlaps the intrinsic absorption of framework component,the incident light absorption would be much enhanced and this phenomenon is the well-known slow photon effect.As the slow photon effect could greatly improve the light harvesting of photocatalyst,ternary photocatalysts 3DOM TiO₂-Au-Cd S with different pore sizes were fabricated according the modified Bragg diffraction equation by controlling the diameter of colloidal template.The energy of slow photons was regulated to overlap the intrinsic absorption of Cd S by changing the macroporous size and fine designing the inverse opal structure.Slow photon effect in powdered samples was firstly revealed to greatly promote photocatalytic hydrogen evolution performance and the photocatalytic enhancement by blue-edge slow photon effect was better than that of red-edge slow photon effect.The size effect of Au NPs was then systematically investigated by changing the size of Au NPs between Cd S and TiO₂ to utilize the surface plasmon resonance（SPR）effect to further improve the light harvesting of ternary photocatalyst 3DOM TiO₂-Au-Cd S.The existing Au NPs between Cd S and TiO₂ has been revealed to play a dual role during photocatalytic hydrogen evolution process.First,Au NPs acted as photosensitizer to provide hot electrons and extent the light absorption region.Second,Au NPs served as electron transfer media to effectively separate the photogenerated electrons and holes of Cd S.The above two effects of Au NPs were revealed to be greatly dominated by their intrinsic size.As the size of Au NPs decreased,the SPR effect gradually dominated while the electron transfer resistance first decreased and then greatly increased.The ternary photocatalyst exhibited the highest photocatalytic activity and apparent quantum efficiency as the size of Au NPs was around 10 nm.Common inverse opals are always constructed by nanoparticles and the numerous grain boundaries greatly boost the recombination of photogenerated electrons and holes.To further improve the separation efficiency of photogenerated charge carriers and extent the basic morphology of inverse opal structure,hierarchically macro-meso-microporous nanosheet-structured calcium titanate（Ca TiO₃）inverse opal was fine designed and successfully fabricated.This kind of hierarchically porous structure not only facilitated the mass diffusion property but also endowed the as-fabricated photocatalyst with large specific surface area.The two-dimensional nanosheet structure could greatly reduce the recombination rate of photogenerated electrons and holes due to their much limited transfer path of charge carriers.To further improve the quantum efficiency,carbon quantum dots（CQDs）were decorated onto the surface of 3DOM Ca TiO₃ through the bottom-up strategy to replace traditional noble-metal co-catalysts.The as-fabricated composites exhibited relatively good photocatalytic hydrogen evolution performance and excellent activity and selectivity in photoconversion of acetone and photoredution of carbon dioxide.