In-situ Sers Study Of Plasmon-mediated Catalytic Reactions On Single-particle Ag Microsphere

Surface-enhanced Raman spectroscopy(SERS)was always deemed as a non-destructive noninvasive technique for chemical detection,which means the analytes will remain intact under irradiation of the light source(i.e.,laser)in Raman spectroscopy.Thus chemical enhancement mechanisms were interpreted from the modified Raman spectrum of 4-aminothiophenol(4-ATP).The new peaks(i.e.―b2‖mode)obtained from Raman spectrum were considered as a favorable experimental evidence for charge transfer mechanisms.p,p’-Dimercaptoazobenzene(DMAB)transformed from 4-ATP by a surface catalytic coupling reaction on Ag nanoparticles was predicted theoretically and confirmed by experimental results.Then,it was discovered that DMAB can also be produced from 4-nitrothiophenol(4-NTP)with the assistance of surface-plasmon resonance.However,the surface plasmonic catalysis of 4-ATP as well as 4-NTP are not clear yet,and still has a dispute.In this research,the plasmonic reactions were further investigated to get a deep insight.The plasmonic reactions of N719 was sequentially studied using the same method,which was expected to explain the instability of dye-sensitized solar cells from the viewpoint of stability of dye sensitizer.In this work,the Ag microspheres with relative large size were firstly synthesized with dicarboxylic acids as the directing agents.The relationships among the molecular structure of dicarboxylic acids,the size and morphology of Ag particle were studied.The results showed that the particle size distribution of the Ag microspheres increased with the increasement of the number of C atoms in dicarboxylic acid.The as-prepared Ag microspheres can be clearly observed in the optical microscope of the microscopic confocal Raman spectro meter.In this case,the single Ag particles can be exploited as the excellent SERS substrate,which is convenient to investigate the plasmonic reactions in the following experiments.The plasmonic reaction of 4-ATP in air was studied by virtue of the in situ SP-SERS.Given the same experimental conditions,the ―b2 mode‖ cannot be observed in the SP-SERS of 2-ATP and 3-ATP.This suggests that the ―b2 mode‖ is substantially from newly produced DMAB,instead of chemical enhancement of 4-ATP.And the position of-NH2 on phenyl ring could affect the plasmon related reaction.A gas control flow cell was prepared to study the influence of the component of air on the surface plasmon-driven reaction of 4-ATP.Interestingly,when N2 was introduced into the atmosphere control apparatus,the so-called ―b2 mode‖ did not present in the SERS spectrum of 4-ATP even upon the wavelength of 532 nm and 633 nm laser irradiation for a long period.This results further confirm that the ―b2 mode‖ is responsible for the plasmonic reaction rather than the chemical enhancement.By controling the different oxidizing / reducing atmospheres,the reversible transformation of 4-ATP-DMAB in the presence of surface plasmonic effect could be achieved.Dimerization of 4-ATP into DMAB is actually induced by efficient energy transfer(plasmonic heating)from surface plasmon resonanc e to the surface adsorbed 4-ATP.In this system,O2 is a necessary oxidant and the reaction can be dramatically accelerated by H2 O.However,in the reversed reaction,DMAB is reduced to 4-ATP,resulting from hot electron induction at the metal surface to drive the reduction of the azo group.Here H2 O or H2 can act as a protonsource.The plasmonic reaction of 4-NTP was also investigated.The dimerization of 4-NTP in different atmospheres was studied by in situ SP-SERS.Under the same laser conditions,the conversion of 4-NTP molecules in N2 is faster than that in air,but the reaction in pure O2 is slower than that in air.This result indicates that O2 can effectively quench the ―hot electron‖ of the metal surface,leading to inhibit the reduction reaction from 4-NTP to DMAB.It is further verified that the so-called ―b2 mode‖ of 4-ATP is derived from DMAB.On the basis of SERS spectra and density functional theory(DFT)calculations,the broadening and splitting of the ν(C-C)band of 4-NTP during the plasmon-driven reactions have been interpreted and highlighted.The reduction of 4-NTP to 4-ATP via DMAB in reducing atmospheres was demonstrated based on the above findings.The surface plasmonic reaction of N719 was studied based on the in situ SP-SERS technique.It was found that the ν(C-C)peak at 1545 cm-1 in SERS spectrum of N719 broadened and splitted upon the illumination of a high-energy laser(488 nm and 532 nm).The integration of the findings of the plasmonic reactions of 4-NTP with density functional theory calculations verified that the changes of Raman peaks in SERS of N719 are related to the decarboxylation of N719,which may be one of the reasons for the instability of N719-based dye-sensitized solar cells and provides a theoretical reference for the study of this kind of solar cells.And the results showed that in-situ plasmonic reactions could be applied for the study of the reaction mechanisms.