Design Of ZnO-based Photonic Crystals With Binder-free Interfaces For High Efficient Photoelectrochemical Performances

With a rising demand for clean and renewable energy,photoelectrochemical?PEC?water splitting using photoelectrode conversion of solar energy into hydrogen and oxygen have attracted considerable attention.An efficient solar-to-hydrogen?STH?conversion involves several sequential steps:the generation of electron-hole pairs by absorbed photons,the separation and migration of charges to the electrode surface,and subsequent water oxidation/reduction reaction with holes/electrons.Specifically,three-dimensional?3D?inverse opal structures are used to improve the PEC efficiency of photoanodes due to its high specific surface area and porosity surface water redox reaction sites.The light in this structure undergoes strong coherent multiple scattering and diffraction,leading to a delay and storage of light in photonic materials.This phenomenon called a slow-light effect slow-light effect can considerably result in an increased light absorption as well as the enhanced light utilization.Among the various semiconductors,ZnO as a classic and widely used semiconductor in PEC field,it is considered as one of the most promising materials due to their environmental-friendly,non-toxicity,morphology controllable preparationand earth abundance.However,owing to its large band gap of about 3.2 eV,pristine ZnO almost can't respond to visible region of solar spectrum and can absorb only up to 4%of the solar power spectrum.At the same time ZnO possesses the drawback of short lifetime of the charge carriers and photocorrosion effect under illumination condition which extremely damage its photochemical stability and photoelectrochemical efficiency.Which greatly restrict its practical applications?To solve these problems,great efforts have been made including heterojunction structure formation and tailoring the morphology.Binder-free Interfaces structured inverse opal electrodes are expected to bring better PEC performances by the improving charge separation and increasing light absorption.The specific results are as follows:First,the monodispersed polystyrene spheres were self-assembled into a 3Dhexagonal close-packing structure on the substrate layer by layer,then theprecursor solution of ZnO was dropped on the template,finally the dried samples were calcined to remove the PS template,then the highly ordered ZnO inverse opal were fabricated.The ZnFe₂O₄ heterojunction products were obtained through a solid reaction.The effect of different immersion times on the morphology and properties of the photonic crystals is also analyzed.The optimum performance parameters are explored.Here,we design and fabricate the three-dimensional ordered macroporous ZnO/ZnFe₂O₄ inverse opals with binder-free heterojunction interfaces that improve the separation of photogenerated charge carriers.The well-defined ZnO/ZnFe₂O₄ inverse opal displays a high photoconversion efficiency of 0.81%at 0.38 V versus RHE without any co-catalyst assistance,which is over 6 times higher than that of the pristine ZnO sample?0.13%at 0.65 V vs.RHE?.Furthermore,the photoanode is quite stable even after 10 h continuous illumination with slight photocurrent decay of 4.9%?91.0%loss for the pristine ZnO?.The high performances of the ZnO/ZnFe₂O₄ inverse opals can be attributed to the improved interfacial charge carrier separation based on electrochemical analysis and first-principle calculation.The ZnO@CdSe photonic crystals were obtained through a ion exchange in very mild conditions ZnO photonic crystals are very high with full spectral response.The spectra harvesting by ZnO/CdSe composited photonic crystals with 3D-porous structure is extended from the ultraviolet region to the near infrared region,enhancing the transferal and separation of the photogenerated carriers due to the low interfacial resistance.Under the irradiation of light??<400 nm?,the photocurrent of the ZnO/CdSe composited photonic crystals is as high as 17.5 mA cm^-2,possessing the satisfactory photostability.The hydrogen production rate of ZnO/CdSe composited photonic crystals is 148?mol cm^-2 h^-1,,meanwhile the Faraday efficiency is over 95%.Compared to the reported results in the similar environment,the synthesized ZnO/CdSe composited photonic crystals with 3D-porous structure provide more excellent competitivities.This work may open up more insights in the design of high efficient photoanodes for photoelectrochemical applications.