Synthesis And Characterization Of Micro-/Nano- Silver Crystals And Their Applications In Surface Enhanced Raman Scattering

This dissertation is focused on the synthesis,morphology control and Raman enhancement performance of silver substrate for surface enhanced Raman scattering(SERS)applications.The effects of synthetic methods and parameters on silver morphology,particle size,crystal structure,growth kinetics and Raman enhancement performance were systematically discussed.The main results are as follows:(1)A facile method for fast growth of SERS active Ag substrates on Cu micro-grid surface via galvanic replacement was developed.The effect of synthetic parameters,such as reaction time and AgNO3 concentration on silver morphology and particle size were discussed.It was found that under low AgNO3 concentration(1 mmol/L),Ag particles tend to change from nanoclusters to nanoplates of gradually increased sizes with increasing reaction time,while with the increase of Ag+concentration,the morphology of Ag gradually changes from nanoplates to tower-like particles and finally to heterogeneous dendrites in a certain reaction time(30 s).The growth mechanism of Ag crystals was found to be via the oriented attachment of Ag subunits.Polyvinylpyrrolidone(PVP)additive was chosen to control the Ag morphology due to its preferential bonding to the specific faces of Ag crystal and steric hindrance effect.And PVP was observed to suppress dendrite crystallization to form closely packed nanocrystals(NCs)with uniform size.When the content of PVP is over 70%(mass ratio against AgNO3),the Ag crystals grown on the Cu surface are nearly monodispersed polyhedrons/nanoplates.The present method is facile and efficient for preparing SERS substrate,and the modified SERS substrate exhibits a high sensitivity(0.1 ?mol/L R6G),a high Raman signal enhancement factor(EF,?1.1×106)and a low relative standard deviation(RSD)value(?0.13)in the repeated detection of 10?mol/L R6G analyte.(2)Ag SERS substrates were prepared via photocatalytic method on two types of TiO2 powders,namely brookite and rutile.The photocatalytic behaviors of these TiO2 powders were studied via comparison of rarely reported flower-like spherical brookite nanoparticles with the commonly used rutile nanorods.It was found that the Ag nanocrystals(NCs)grown on brookite were larger in size and closer in distribution under otherwise identical reaction conditions.The effects of reaction parameters(AgNO3 concentration,UV power and irradiation time)on growth kinetics and morphology of Ag NCs on each type of TiO2 were systematically studied.It was found that the size of Ag NCs increases with increasing the values of reaction parameters,promoting the formation of Ag polyhedrons on both TiO2 powders.PVP was used to control the morphology of Ag product,and a new property of PVP in photochemical reaction was found,i.e.increasing the system reduction ability by forming electron acceptor-donor system with TiO2 powders.If the PVP content is increased to 50%(mass ratio against AgNO3),reducing capability of the system can be drastically improved,producing Ag nanoplates/nanopolyhedrons with closely packed distribution and large size.As to SERS capability,the EF value was found to be affected by the morphology of the Ag NCs and dependent on the synthetic parameter.The highest EF value(?3 × 105)was found for the SERS substrate prepared in the presence of a proper amount of PVP(20%)on brookite.(3)A facile one-step hydrothermal method was introduced in this work to produce rarely reported brookite containing TiO2/reduced graphene oxide(rGO)nanocomposite using TiCl3 as titanium source.And the optical bandgap of the composite was successfully reduced from 3.2 eV to 2.17 eV via a synergistic effect between rGO-TiO2 bonding and interactions of different TiO2 polymorphs.Therefore,the usage of light source was extended from the UV region to the visible region.The Ag SERS active surface with controlled morphology was photochemically prepared via carefully adjusting the constituents of different TiO2 polymorphs in the composite.The effects of reaction parameters(TiCl3 concentration,urea concentration and GO attendance)on constituents of three TiO2 phases in the composite were studied.The results show that the increase in TiCl3 concentration tends to produce more rutile,while the increase in urea and GO suppresses the formation of rutile crystals.The presence of cetyltrimethyl ammonium bromide(CTAB),on the other hand,enhances the GO stability in the TiCl3 mixture and is beneficial to the homogeneity of Ti3+and GO during the reaction.After the irradiation,it was found that the presence of rutile phase resulted in a larger size of Ag particles(?1?m),while anatase and brookite led to closely distributed smaller NCs(?200 nm)owing to the different band gaps of the three types of TiO2.As for SERS performance,the EF value is affected by the Ag particle morphologies.The SERS substrate prepared in the presence of ternary phases of TiO2 has the highest EF value.(4)A radio frequency(RF)magnetron sputtering method was applied to produce TiO2 and WO3 thin film substrates.The substrates were calcined at various temperatures,obtaining photocatalytic surfaces with different morphologies,crystallinities and crystal phases.The phase transformation process of the two types of films in the different calcination temperature regions(TiO2:300?1000?,WO3:300?800?)was studied.In the temperature range from 300 to 800?,the TiO2 film was found to be anatase.When the calcination temperature was above 800?,the film was found to be a mixture of anatase and rutile.On the other hand,it was found that at 300? the WO3 film was poorly crystalized.At 400?,the WO3 was ?-WO3 phase,and when the temperature was above 4000C the film was found to be a ?-WO3 and 8-WO3 mixture.The crystallization of both films was improved with increasing calcination temperature.Due to the difference in band gap(TiO2:3.21-2.73 eV,WO3:2.82?2.58 eV),crystallinity and surface morphology of the TiO2 and WO3 thin films,after the photochemical reaction,the resultant Ag particles were found to exhibit different morphologies.In the case of WO3,the Ag crystals were found to be extremely larger in size compared with those on TiO2 samples.However,under the same synthetic condition,the substrate made with higher calcination temperature will benefit the nucleation density of Ag crystals and reduces the spatial distance of Ag particles for both films.Meanwhile,growth of Ag along the TiO2 grain boundaries(GB)was observed,providing a direct evidence for GB enhanced photocatalytic efficiency of TiO2.For both photocatalytic substrates,Ag particle size increased with increasing AgNO3 concentration,while the photochemical reaction was enhanced by increasing UV power.As a result,a Ag SERS substrate with controlled morphology was achieved.As for SERS performance,the EF value is affected by the Ag particle morphologies.The SERS substrates prepared on the biphasic TiO2 thin film calcined at 1000? exhibits the highest EF value.