(Single)Plasmonic Nanostructures Enhance Electrochemistry Of Metal Organic Frameworks(MOFs)

Due to the special structure of noble nanoparticles,the valence electrons on their surface will be collectively oscillate under incident light,and generating efficiently separated hot electrons and holes on the corresponding nanostructures surface,that is,the plasmon.When the plasmonic nanostructures are in close contact with a semiconductor photocatalyst,the generated hot electrons or holes with sufficient energy can be transferred to the photocatalyst,and regulated the electronic structure of photocatalyst,and then promoted chemical and electrochemical reaction.Plasmon enhanced electrochemistry（PEEC）has been widely used in carbon dioxide（CO₂）reduction,hydrogen evolution reaction（HER）,etc,exhibiting potential application values in electrocatalysis,energy storage,transformation and other fields.Traditional nanoparticles electrochemistry is an electrochemical investigation of nanoparticle population at macroscope scale.Due to the heterogeneity of structure and function of nanoparticles,such the exploration of average cannot accurately describe the basic physical and chemical properties of nanoparticles,such as electron transfer,material transport and transformation at microscale.In recent years,the development of electrochemical analysis and detection method for single nanoparticles is of great significance to establish and understand the structure-activity relationship of nanoparticles and reveal the physical and chemical properties of nanoparticles at the nanoconfined scale.If the study of the PEEC process can be realized and the heterogeneity and correlation of nanoparticles structure and function can be revealed at a single nanoparticles scale,which is huge of vital to understand the mechanism of PEEC and high-performance PEEC system and design high-performance PEEC systems.Nevertheless,limited by the research technology,the current exploration of PEEC is mainly carried out at the macroscale by electrochemical equipment,and there is still a lack of effective research on the PEEC process at the scale of single plasmonic resonance structure.Based on the above background and problems,this thesis uses metal organic frameworks（MOFs）as model semiconductor catalyst to carry out a systematic study on the enhancement of electrochemical catalytic behavior by plasmonic nanostructures.Firstly,we observed that the plasmon of gold nanorods（Au NRs）to accelerate the performance of hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）for Co-MOFNs at the conventional modified electrodes.The mechanism of the enhancement effect was further studied by the density functional theory（DFT）,and a new OER process which is different from the common cognition was found.Subsequently,the improvement of HER electrocatalytic activity for Co-MOFNs by plasmonic gold and silver nanoparticles（Au NPs and Ag NPs）with the same morphology and size was observed at the single nanoparticle electrochemistry.It is found that the contribution of Au NPs and Ag NPs to enhanced electrocatalysis in the mixed syetem were effectively distinguished by the difference of intrinsic plasmon enhancement effect produced by dissimilar nanoparticles at single nanoparticles collisions electrochemical experiments under different optimal light irradiation.We also studied the PEEC of silver nanowires（Ag NWs）,silver nanospheres（Ag NSs）and silver nanoplates（Ag NTs）with different morphology at the single nanoparticles level,and it was discovered that the plasmon of Ag NWs,Ag NSs and Ag NTs enhanced HER electrocatalytic activity for Co-MOFNs with a trend of Ag NWs<Ag NSs<Ag NTs,which indicated that the dependence of plasmon on the structure of nanoparticles.The main research work is divided into the following three aspects:（1）Based on the advantages of high specific surface area and abundant unsaturated metal active sites on the surface of metal organic frameworks,and combined with the plasmon of Au NRs,we construct Au NRs decorated Co-MOFNs to form Au NRs/Co-MOFNs composites by a simple ultrasonic method,and it was found that the plasmon of Au NRs could simultaneously accelerate the electrocatalytic activityof HER and OER for Co-MOFNs.Experimental results show that Au NRs/Co-MOFNs exhibit superior electrocatalytic activity with a decreased overpotential of 218 m V and 130 m V,and the Tafel slopes are decreased from 185 to 133 m V/dec and 130 to 102 m V/dec,respectively,for the HER and OER,as compared to the pristine Co-MOFNs under light irradiation.According to the density functional theory（DFT）,the HER enhancement relies on the direct injection of hot electrons from plasmonic Au NRs into the condution band of Co-MOFNs under light irradiation.This charge transfer results in an upraised Fermi level of Co-MOFNs to match better with the energy level of H₂O/H₂potential,and then enhance the performance of HER for Co-MOFNs.Regarding the promotion of the OER,the partial density of states（PDOS）calculation deduces that the process is different from the general mechanism widely accepted:the holes formed in Co-MOFNs should majorly locate on the surface oxygen atoms,which may also serve as active positions working jointly with neighboring Co atoms in oxidizing OH^-,so as to enhance the OER electrocatalytic activity of Co-MOFNs.We also confirm the generality of this strategy by observing a similar enhancement toward Au NRs/Ni-MOFNs and Au NRs/Ni Co-MOFNs catalysts in this research.（2）The PEEC process was first explored at single plasmonic nanostructures using single nanoparticles collisions electrochemistry.In the experimental,the plasmonic Au NPs and Ag NPs with the same morphology and a diameter of ca.50 nm were used as the research objects,and the Co-MOFNs electrocatalyzed the HER as a model reaction.This combination can effectively inhibit the electron-hole recombination on Au NPs and Ag NPs surfaces,and make the"hot electrons"transfer to the conduction band of Co-MOFNs and regulate its electronic structure,thus improving the electrocatalytic activity of Co-MOFNs for HER.Firstly,the significant enhancement of HER for Co-MOFNs by plasmonic Au NPs and Ag NPs under the optimal light irradiation（532 nm and 420 nm）at the modified electrode system was observed.Afterwards,similar PEEC process were detected at single nanoparticles electrochemical level.Meanwhile,we found that the consistent dependence of plasmon is strongly dependent on wavelength and has a rapid and reversible response to light in both systems.In addition,based on the above exploration,we also conducted the PEEC test on the mixed gold and silver nanoparticles system with the same morphology and size at single nanoparticles level.It was discovered that by analyzing the effective integral charge of the single Au NPs and Ag NPs with different plasmon to enhance the HER electrocatalytic activity of Co-MOFNs under different optimal light irradiation,and the contribution of Au NPs and Ag NPs to enhanced electrocatalysis in the mixed syetem could be effectively distinguished.Based on the work（2）,we further tested and studied the enhancement effect of of HER for Co-MOFNs by three kinds of plasmonic silver nanosrtuctuers with the same composition and different morphologies at single nanoparticles electrochemical method.Three kinds of plasmonic silver nanostructures were successfully synthesized,which are relatively uniform silver nanowires（Ag NWs）,silver nanospheres（Ag NSs）and silver nanoplates（Ag NTs）with the same composition.In the period of electrochemical test,it was observed that the corresponding plasmon of three kinds of silver nanostructures enhanced HER performance for Co-MOFNs on the modified electrode system,where the trend of enhancement is Ag NWs<Ag NSs<Ag NTs under light irradiation of optimal absorption wavelength.In the meantime,the same acceleration law was also detected at the single nanoparticle electrochemistry level.This experimental result well illustrates that the dependence of the plasmon on the structure of nanoparticles.The work（2）and work（3）provide a novel perspective for PEEC research and a new research pattern for in-depth understanding of PEEC by exploring the intrinsic properties and structure-activity relationship of plasmon nanoparticles at the single nanoparticles level.