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View Article Online,Chemical Science Edge Article, In this work we explored the SERS properties of Au Ag alloy. NPs at di erent ratios Four di erent SERS probes molecules. that yield SERS were investigated We showed that in general. the SERS enhancement found from Au Ag alloy NPs was larger. than from pure Au or Ag This behavior is in agreement with the. literature 12 21 25 However we also report that the optimal. condition alloy composition for SERS was dependent of the. chemical nature of the probe This is a very interesting obser. vation that needs to be taken into consideration for the further. development of SERS substrates from bimetallic metal alloys. DFT calculations were performed and these surprising results. were explained considering the electronic environment of the. metals in the alloys,Experimental section, Published on 30 October 2012 Downloaded on 09 06 2013 19 02 07. Synthesis of Au Ag bimetallic NPs, Fig 1 Model used in the DFT calculations a top view of Au 111 and b. Tetrachloroaurate acid HAuCl4 3H2O 99 90 silver nitrate top view of AuAg 111 and c top view of Ag 111 d the contour plot of. AgNO3 and ethylene glycol EG C2H6O2 99 were purchased Au 111 and e AuAg 111 and f Ag 111 slabs. from ACROS All the chemicals were used without any further. puri cation AuxAg10 x nanoparticles with various ratios were. prepared by the EG method In this case the EG function as of two species with similar structure the chemical structures. both reduction agent and solvent Poly N vinylpyrrolidone PVP are provided as ESI Fig S2 The SERS spectra from the. MM 58 000 g mol 1 was used to cap the metal and avoid di erent probes adsorbed on the alloys of variable composi. aggregation The metal PVP molar ratio in the reaction system tions were recorded using a Renishaw in via spectrometer. was 0 01 The PVP was dispersed in 80 ml of EG and the equipped with a microscope and a 50 objective numerical. appropriate amount proper molar ratio to produce the desired aperture of 0 75 The spectral bandwidth of the system was. composition of the alloy of HAuCl4 3H2O and AgNO3 was 2 cm 1 Typical spectral collection parameters for 632 8 nm. added to the solution The pH of the reaction mixture was were 2 s acquisition time and 10 accumulations and for. brought to 10 by the drop wise addition of saturated sodium 785 nm the exposure time was 1 s and the SERS signal was. hydroxide solution The whole reaction mixture was then accumulated 10 times as well. re uxed at 120 C for 1 h PVP is a well known steric stabiliza. tion agent for both Au and Ag nanoparticles This work shows. that it can also stabilizes well Au Ag alloy NPs at any DFT calculations. composition DFT simulations were performed using Au 111 AuAg 111 and. Ag 111 slabs shown in Fig 1 Projector augmented waves. Characterization methods PAW 29 32 generalized gradient approximation GGA 33 35 as. implemented in the Vienna ab initio simulation package. UV Vis absorption spectra were collected on an Analytik Jena. VASP 36 38 were applied in the DFT calculations In the plane. SPECORD S 100 spectrometer using quartz cuvettes High. wave calculations a cuto energy of 400 eV was applied which. resolution transmission electron microscopy HRTEM images. was automatically set by the total energy convergence calcula. were obtained with a FEI TEM 2000 instrument operating at an. tion for the Au 111 AuAg 111 and Ag 111 systems For. acceleration voltage of up to 200 kV Samples for HRTEM. modeling the Au 111 surface we adopted slabs containing. measurements were prepared by using a sonicator to disperse. three layers with four atoms per layer Fig 1a The surface was. the NPs in ethanol and placing a drop of the colloidal disper. constructed as a slab within the three dimensional periodic. sion on a Cu grid followed by drying in the oven, boundary condition and models were separated from their. images in the direction perpendicular to the surface by a 14 A. SERS measurements vacuum layer In Au 111 model the supercell dimension was. The NPs were drop casted onto glass slides and dried using a 5 767 5 767 18 709 A 3 In Ag 111 model the supercell. hotplate followed by rinsing with MeOH 10 mL methanolic dimension was 5 778 5 778 18 717 A 3 For these calcula. solution of the probe molecules were drop coated onto the NPs tions a 7 7 1 k point mesh was used in the 2 2 super cell. aggregates A er the solvent evaporated the substrates were The bottom layer was kept xed to the bulk coordinates full. further rinsed with MeOH a few times before measurement atomic relaxations were allowed for the top two layers The. Four probe molecules oxazine 720 Oxa 10 mM nile blue A atoms in the cell were allowed to relax until the forces on. NBA 10 mM thiophenol TP 1 mM and 4 hydroxythiophenol unconstrained atoms are less than 0 01 eV A 1 The contour. HTP 1 mM were all from sigma and were used without map of Fig 1d f correspond to slide models of Au 111 and. further puri cation The probe molecules represent two groups AuAg 111 and Ag 111. 510 Chem Sci 2013 4 509 515 This journal is The Royal Society of Chemistry 2013. View Article Online,Edge Article Chemical Science, Published on 30 October 2012 Downloaded on 09 06 2013 19 02 07.
Fig 2 Representative TEM images of the AuxAg10 x NPs A Ag B Au1Ag9 C Au5Ag5 D Au7Ag3 E Au9Ag1 and F Au Insets show the corresponding HRTEM. Results and discussion not signi cantly a ect the trend of the SERS e ect on the. composition because the NPs were heavily aggregated dried on. Fig 2 presents typical transmission electron microscopic TEM glass slides using a hot plate which minimized the size e ect. images of the AuxAg10 x NPs The subscripts in AuxAg10 x Although this di erence in size should be noted the presence. indicate the apparent ratio which is de ned by the amount of of the pure gold data point in the SERS intensity trend plots is. Au and Ag salts added to the reaction mixture during the not essential to support the main conclusions of this work. synthesis The corresponding high resolution TEM HRTEM The composition of the bimetallic NPs can be monitored. images of these NPs are shown as insets The AuxAg10 x NPs through UV Vis extinction measurements as shown in Fig 3. were well dispersed with particle sizes around 3 5 nm An Two important aspects of Fig 3 are 1 the maximum in. exception was for the pure Au NPs In this case the synthesis extinction shi s linearly with the NPs composition from. yielded NPs with mean particle size of about 20 25 nm with a 400 nm pure Ag to 530 nm pure Au The extinction. relative narrow size distribution Several attempts to prepare Au maximum is related to the excitation of localized surface plas. NPs with diameters in the 3 5 nm range using the PVP method mons LSPs from these structures and the resonance wave. were unsuccessful In any case the SERS investigations were length is expected to increase with the gold ratio The linear. carried out for the Au NPs and the results compared with the SP energy shi with the composition has been assigned to a. alloy NPs We would like to argue that this size di erence will homogeneous distribution of metals within the alloy 16 2 there. were no clear contribution which should manifest as. additional extinction peaks from the pure metals in any of. the UV Vis of the alloys 20 suggesting that just one type of NPs. were present in the suspensions Although the particle size also. a ects the position of the LSPR this e ect is less dramatic. than the composition of the alloys 39, Four di erent probe molecules were used their structures. are presented in Fig S2 to evaluate the SERS characteristics of. the NPs deposited on glass The probe molecules were selected. considering their chemical characteristics Two thiols HTP and. TP with similar structure the only di erence between then is. an extra hydroxyl group in HTP and two cation dyes Oxa and. NBA were used HTP and TP can be considered as Lewis bases. that bind to the metal through their thiolate groups On the. other hand Oxa and NBA are positively charged heterocyclic. Fig 3 UV Vis extinction spectra of the AuxAg10 x NPs From top to bottom Ag. aromatic amines and are expected to interact with the metal. Au1Ag9 Au5Ag5 Au7Ag3 Au9Ag1 and Au The spectra were taken after diluting surface as Lewis acids The objective of using two groups of. the NPs in methanol chemicals with similar structures was to highlight the e ect of. This journal is The Royal Society of Chemistry 2013 Chem Sci 2013 4 509 515 511. View Article Online,Chemical Science Edge Article, Published on 30 October 2012 Downloaded on 09 06 2013 19 02 07. Fig 4 Dependence of the SERS signal on the Au Ag ratio a HTP and Oxa b TP and NBA The error bars are the results of 5 measurements 632 8 nm excitation. the binding site on the SERS trend with the NPs composition that 1 in agreement to previous reports on both core shell. The use of two species of the same family e g two thiols and alloy bimetallic NPs systems the combination of the two. provided some redundancy that can be used to con rm the metals leads to a better SERS performance when compared to. reliability of the experimental trends Another important aspect the pure metallic NPs 12 14 19 2 the two SERS probes tested. is that the main electronic transitions for both HTP and TP are demonstrated distinct enhancement trends with the composi. in the UV range while Oxa and NBA absorbs in the red around tion of the NPs which is a very interesting e ect that has not. 600 nm Therefore an additional resonance Raman contribu been reported before. tion is expected for Oxa and NBA under 632 8 nm excitation The adsorption of ionic species to charged metallic surfaces. The resonance Raman e ect should increase the SERS e ciency can be accomplished through ion pair interactions A control. for these dyes for red excitation but should not a ect the experiment was performed to demonstrate that this e ect does. dependence of the scattering on the composition of the NPs not signi cantly alter the observed trend In the control exper. Fig 4 shows the variation of SERS intensities with the iment the NPs were mixed with excess KCl equal volumes of. composition of the Au Ag alloys obtained under 632 8 nm NP suspension containing Oxa and 0 1 M KCl solution before. excitation The scattering from the molecules adsorbed on the drying The NPs dried in the excess of KCl presented a slightly. pure metal NPs were also plotted for comparison The SERS better SERRS e ciency see Fig S5 but the only spectral. intensities in Fig 4 correspond to the height of the strongest di erence was the presence of a strong metal halide stretching. band in the spectrum of each species The ring breathing modes in the low frequency range In terms of SERRS intensity trend. were used for both HTP 1078 cm 1 and TP 1074 cm 1 and with the NPs composition the presence of excess KCl shi ed. phenoxazine ring stretching vibrations were used for Oxa the position of the maximum SERRS to the 9Au 1Ag in the case. 596 cm 1 and NBA 593 cm 1 The full spectrum from of OXA relative to 7Au 3Ag as shown in Fig 4 This indicated. each species is available in Fig S3 that even the presence of excess halides might a ect the. Fig 4a shows that the SERS intensity from HTP at Au1Ag9 is composition of optimal SERRS performance although the. larger 4 times than from HTP at pure Ag NPs This shows qualitative trend for this species best SERRS enhancement. that a relative small amount of gold 10 in the alloy has a factor at higher gold compositions remained the same. dramatic e ect on the SERS intensity in that particular case In order to further con rm the results from Fig 4a the. A er Au1Ag9 the SERS signal of HTP decreases as the Au Ag experiments were repeated with two di erent probe molecules. ratio increases Finally the HTP SERS signal from pure Au NPs TP and NBA The TP and NBA binding to the metal surfaces. is similar in magnitude than from pure Ag NPs The observed should be equivalent to HTP and Oxa respectively since they. result agrees well with the trends that have been published in have very similar structures The use of di erent molecules with. the literature 12 19 A small amount of Au in Au Ag alloys has been similar chemical characteristics increase the con dence that. shown to enhance the SERS performance but the SERS the trends are not related to contaminations or artifacts The. response deteriorates as the amount of Au increases in the alloy experimental results for both TP and NBA are shown in Fig 4b. However this trend is not followed by Oxa Fig 4a Similarly Not surprisingly TP showed almost exactly the same trend as. to the HTP case the addition of 10 gold into the alloy Au1Ag9 HTP In the case of NBA the optimum Au Ag ratio did not. results in an increase of the Oxa SERRS signal However in exactly match the observed for Oxa However the overall trend. contrast to HTP further increase in the amount of Au in the was still the same higher gold ratio led to better SERS signal. alloy composition leads to an increase in the SERRS signal until The results in Fig 4 are very interesting because they show the. a maximum is reached at Au7Ag3 although it is important to e ect of the chemical characteristics of the probe on the SERS. point out that the SERRS intensities are quite close for Au Ag e ciency of the substrate This demonstrates that an opti. ratios equals to 5 5 7 3 and 9 1 It is then clear from Fig 4a mized substrate for HTP and TP is achieved at di erent NPs. nanoparticles should consider the nature of the probes and the electronic induced 111 AuAg 111 and Ag 111 systems For Chemical Science Edge

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