Investigation of sp carbon chain interaction with silver nanoparticles by Surface Enhanced Raman Scattering

Transcription

1 Investigation of sp carbon chain interaction with silver nanoparticles by Surface Enhanced Raman Scattering A. Lucotti 1, C. S. Casari 1, M. Tommasini 1, A. Li Bassi 1, D. Fazzi 1, V. Russo 1, M. Del Zoppo 1, C. Castiglioni 1, F. Cataldo 2, C. E. Bottani 1, G. Zerbi 1 1 Dipartimento di Chimica, Materiali e Ingegneria Chimica G. Natta and NEMAS - Center for NanoEngineered MAterials and Surfaces, Politecnico di Milano, Piazza Leonardo da Vinci 32, I Milano, Italy 2 Actinium Chemical Research srl, via Casilina 1626/A, Roma, Italy and INAF Osservatorio Astrofisico di Catania, Via S. Sofia 78, Catania, Italy Abstract Linear carbon structures (namely polyynes) constituted by sp coordinated atoms represent a system with peculiar physical and chemical properties. Surface Enhanced Raman Spectroscopy (SERS) is exploited here to investigate the interaction of isolated sp carbon chains (polyynes) in a methanol solution with silver nanoparticles. Hydrogen-terminated polyynes, produced by the submerged arc discharge technique, show a strong interaction with silver colloids used as the SERS active medium (remarkable chemical SERS effect). The time evolution of polyynes SERS spectra after mixing with silver colloids allows to investigate aggregation and sedimentation effects of silver nanoparticle colloids, sp 2 phase formation as well as the evolution of the relative concentration of polyynes of different lengths occurring in the system. Experimental results are supported by density functional theory (DFT) calculations of the Raman modes and reveal shortening processes of the sp chains probably due to cross linking reactions. 1

2 2 1. Introduction Linear carbon chains with sp hybridization represent one of the simplest one dimensional systems and have therefore attracted a great interest since many years [1][2]. sp chains can display two types of carbon-carbon bonding: polyynes, chains with single-triple alternating bonds ( -C C- C C- ) and polycumulenes, chains with all double bonds ( =C=C=C= ). These linear forms of carbon are thought to be relevant in the initial stages of formation of fullerenes and nanotubes [3][4][5] and share the physics of Peierls distorsion and Kohn anomaly [6] with other polyconjugated systems such as polyacetylene [7], graphite and nanotubes [8][9]. sp carbon has been produced by means of different physical and chemical techniques [10][12][13][14][15][16] and has been also observed in the core of multiwalled carbon nanotubes [17][18]. Peculiar transport properties have been predicted and measured [19][20] and a strong non-linear optical response of finite linear carbon chains has been recently measured [21] thanks to the availability of polyynes through a new chemical synthetic route [22]. Raman spectroscopy is widely used for the investigation of sp carbon systems [23] as well as for a number of carbon-based systems [7] [24]. In addition, Surface Enhanced Raman Scattering (SERS) can be exploited to achieve a high sensitivity in detecting small amounts of sp carbon. SERS may also be exploited to follow time dependent effects, thanks to the fast spectral recording allowed by the high signal enhancement. Therefore the study of vibrational properties of sp carbon chains through SERS spectroscopy is relevant for providing unprecedented experimental data of such peculiar nanostructures, as demonstrated in our recent works dealing with polyynes produced by the submerged arc discharge method [25][26] and by other investigations of polyynes produced by laser ablation in liquids [27]. Here we use SERS to investigate time dependent effects when polyynes are interacting with a silver nanoparticle colloidal solution used as the SERS active medium. The role of hydrogenterminated polyynes in the aggregation of silver colloids, the effects of aggregation and sedimentation of silver nanoparticles on the SERS intensity and the behaviour of carbon chains of different lengths as a function of time have been investigated. The interpretation of the experimental observations are supported by density functional theory calculations of the Raman response of silver end capped polyynes (Ag-C N -Ag, 6 N 20) which have been used as simple models able to mimic the remarkable chemical SERS effect observed for these systems [25] [27]. Therefore our SERS investigations allow to observe the time evolution of processes activated by the interaction with silver nanoparticles and open the possibility to understand the structural and chemical properties of such elusive form of carbon structures.

3 3 2. Experimental 2.1 Polyyne production Polyynes were produced by electric arc discharge between two graphite electrodes submerged in 100 ml of methanol in a three-necked round bottomed flask. The electric arc was conducted under the usual conditions of 10 A and electrodes arranged in a V geometry with external cooling in a water/ice bath [14][28][29]. The arc was prolonged for 30 min. and then the crude mixture was subjected to high performance liquid chromatography (HPLC) analysis, after filtration through a polyvinylidenefluoride (PVDF) filter [30]. Individual polyynes separated by the HPLC column were identified both on the basis of their retention time and their electronic absorption spectra [30]. The concentration of each species was measured on the basis of the absorbance of their most intense peak in the electronic absorption spectra by using the Lambert-Beer law and the molar extinction coefficients reported in the literature [31]. Typical distributions of chain lengths H-C N -H range from N = 6 up to N = 16 with a relative abundance maximum for N = 8 [25]. 2.2 Preparation of silver colloids AgNO 3 (99%) and trisodium citrate (98%) from Aldrich chemicals were used without further purification. A modified Lee and Meisel [32] procedure was implemented to obtain highly concentrated silver colloids. 200 mg of AgNO 3 were dissolved in 500 ml of distilled water and brought to boiling. 20 ml of a 3% trisodium citrate solution were added and maintained at boiling until the color turned to orange. The solution was then cooled to room temperature and placed in a sealed ampoule for about 1 month. After this time, highly concentrated colloids can be extracted from the bottom of the flask. Plasmon resonance of silver colloids has been investigated by UV-Vis absorption spectroscopy using a V-570 Jasco spectrophotometer. Scanning transmission electron microscopy (STEM) images of silver nanoparticles were taken with a Zeiss Supra 40 field emission SEM equipped with a STEM detection module. The STEM sample was prepared by drying a droplet of colloidal solution on a TEM grid covered by a thin carbon layer. 2.3 SERS experiments Silver colloids were added to the methanolic polyyne solution in order to perform SERS measurements. SERS spectra were recorded with a Nicolet NXR9650 FT-Raman (resolution 4 cm - 1 ) equipped with a InGaAs detector and a Nd:YVO 4 laser providing a 1064 nm excitation line. SERS spectra have been recorded on aqueous solutions containing silver nanoparticles, methanol (used as reference) and polyynes (sample molecules). The polyyne solution (~ 10-5 M) has been mixed with the silver colloid keeping a 1 to 1 volume ratio. For direct comparison all the spectra

4 4 have been recorded with the same experimental conditions (backscattering geometry, laser power about 0.3 W at the sample, collection time 30 sec.). We estimated a SERS intensification between 10 5 and 10 6 [25]. 3. Theory Density functional theory calculations of the off-resonance Raman response have been carried out using the pure Perdew-Becke-Ernzerhof (PBE) exchange and correlation functional [33]. We have selected the 6-311G* basis set for carbon atoms and the 3-21G* basis set for silver atoms. The theoretical method considered in this work employs the same basis sets used in a previous study [25] but the exchange-correlation functional PBE (state-of-the-art among generalized gradient corrected functionals) instead of BPW91 [34]. The comparison between these two theoretical approaches shows that the differences are minor and do not imply changes in the interpretation of the data. DFT calculations provide helpful information which nevertheless has to be considered with some care. In fact, it has been recently pointed out that while DFT can correctly predict the observed trend of the Raman response of polyynes, it is not able to account quantitatively for the observed red shifts of the strong Raman lines of hydrogen capped polyynes with increasing chain length. Suitable scaling procedures have been introduced to overcome this limitation [35], [36]. This inaccuracy of DFT calculations is likely to be an issue also for silver end-capped polyynes, but to date the proposed scaling procedures have not been adapted to these polyynic systems. 4. Results and discussion As prepared SERS active silver colloids show a plasmon resonance centered at about 450 nm (see Fig. 1-a) and consist of both spherical shaped (50-80 nm size) and rod-shaped (200 nm long and nm wide) Ag nanoparticles as shown in the STEM image reported in Fig. 2. This distribution of size and shape can account for the broadness ( nm FWHM) of the observed plasmon peak. Furthermore the plasmon resonance can be strongly affected by aggregation since a dipole coupling takes place when isolated nanoparticles are brought at a close distance [37]. In fact a color change takes place when polyynes are mixed in the colloidal solution and UV-Vis-NIR absorption spectra show that the initial surface plasmon resonance peaked at 450 nm changes position and width due to dipole couplings induced by aggregation (Fig. 1). It has to be noticed that aggregation is usually induced by modifying the ionic strength of the solution and hence the nanoparticle electric double layer, for instance by adding NaCl, or by introducing linker molecules. Polyynes seem to act as a strong linker as shown in Fig. 1 where aggregation induced by adding NaCl is also reported for comparison. In both cases an evident modification of the absorption spectrum takes place and a broadening of the surface plasmon peak is observed. It is worth noticing that in many cases a broadening of the absorption spectrum is a desired effect since it helps to take

5 5 advantage from the effect of Raman enhancement driven by plasmon resonance, using excitation lines far from the intrinsic plasmon resonance of silver colloids (located at 450 nm for our colloids). This allows to have plasmonic resonance also at 1064 nm thus allowing to carry out SERS with excitation wavelength in the near IR. During our measurements we observed that SERS spectra were quite sensitive to the time interval elapsed since the mixing of silver colloids (the SERS active medium) with the methanol solution of polyynes. In order to quantify these modifications we recorded SERS spectra at different times after mixing (between 2 and 65 minutes). All spectra are plotted on a common scale in Fig. 3 where the methanol peak at 1020 cm -1 has been used for normalization. SERS spectra show two common features in different ranges. The first feature is related to CC stretching vibrations of sp carbon ( cm -1, [25]). The large part of the SERS intensity associated to the second feature ( cm -1 ) can be attributed to both sp 2 carbon containing molecular species and amorphous carbon. Hereafter we will concisely name sp 2 this latter feature of the SERS spectra. We observed a good reproducibility of the evolution with time of the SERS spectra. After mixing of polyynes with silver colloids, aggregation occurs and indeed the SERS signal increases in intensity at early times (2-7 minutes) and decreases for longer times (>10 minutes). Inspection of Fig. 3 also reveals that the sp 2 /sp ratio rises with time. To better follow the time evolution of the SERS intensities, the sp and sp 2 signals has been fitted with a double exponential decay law, as shown in Fig. 4: I(t) = A 1 exp(-t/τ 1 ) + A 2 exp(-t/τ 2 ) + I 0 This fit is used here to analyze the evolution of sp and sp 2 signals, while a quantitative evaluation of sp and sp 2 content is extremely difficult due to the unknown SERS cross sections. The total SERS intensity (sp+sp 2 ) increases with a characteristic time (τ 1 ) of about 2 minutes while the decrease of the overall signal has a time constant (τ 2 ) of about 26 minutes (see Table 1). The asymptotic value (I 0 = 0.47) represents about half the value of the maximum intensity (I = 0.94 at t = 8 min). The observed behavior reveals a competition between two different effects: silver colloid aggregation on one side and precipitation of large aggregates from the colloidal solution on the other. These two phenomena (aggregation and sedimentation) have opposite effects on the overall SERS signal. Aggregation enhances the SERS signal by providing a higher number of hot spots [38] and better resonance conditions with respect to the 1064 nm excitation line. On the other hand, precipitation weakens the SERS signal due to segregation of the SERS active aggregates away from the probed volume. In any case, the two effects have different characteristic times: the increase is observed at early times during the initial stage of aggregation when the SERS effect builds up due

6 6 to the formation of highly SERS active aggregates and the decrease of the SERS signal occurs at much later times (after approximately 10 minutes) due to precipitation of aggregates. Strong interaction with silver was already reported in these systems. In particular by comparing Raman and SERS spectra we observed shifts of sp features and appearance of new peaks revealing a chemical effect [25]. Moreover silver plays a key role both in catalyzing sp chain formation from PTFE under laser irradiation [39] and in stabilizing already formed polyynes in a solid state sample [26]. The analysis of I sp and I sp² shows that I sp (τ 1 = 1.3 min., τ 2 = 20 min.) has a faster evolution than I sp² (τ 1 = 2.8 min., τ 2 = 35 min.) and the asymptotic values show a substantial decrease of the sp signal with respect to the sp 2 (see Table 1). This could be due to degradation of sp phase into sp 2 phase due to chain cross linking [40]. Such behavior is in agreement with the tendency of sp phase to undergo transition towards the more stable sp 2 phase [13]. Focusing on the sp band, it looks structured in distinct peaks, as already reported [25]. In addition to the evolution of the overall sp signal, the behavior as a function of time of these peaks has been investigated here. In order to make easier the analysis of the data relative to the sp carbon Fig. 5 presents the same SERS spectra of Fig. 3 in a reduced spectral range ( cm -1 ). All the spectra are normalized with respect to the total sp signal in order to normalize the influence of the aggregation/precipitation of silver colloids on the SERS intensity and to follow the spectral changes of the peaks which give rise to the sp band. Of course this procedure hides any possible change of the total sp intensity due, for instance, to degradation of the sp chains or to their conversion into sp 2 carbon, already discussed. Three main features are observed in the sp band, namely the peaks ν 1 = 1910 cm -1, ν 2 = 2020 cm -1, ν 3 = 2116 cm -1. The time evolution of the peak intensities has been investigated performing a three gaussians fit of the normalized SERS spectra reported in Fig. 5. The results are reported in Fig. 6 where we plot the evolution of the intensities as a function of time. This fit technique has to be considered merely an expedient for attributing a global (approximated) intensity to each one of the three main features in the SERS spectra and to help reading the evolution with time of the three bands. The evolution of the intensity of these bands has been fitted with an exponential decay law: I(t) = A exp(-t/τ) + I 0 From the inspection of Fig. 6 we observe the increase of the ν 3 band and the relative decrease of the ν 1 and ν 2 bands (values are reported in table 2). The chemical interaction between the colloidal silver nanoparticles and the hydrogen capped polyynes [25] [26] has been modeled considering linear carbon chains capped at both ends by silver atoms. Simulated Raman spectra are dominated by just one (in some cases two) bands attributed to

7 7 collective CC stretching vibrations (see Fig. 5). The inspection of the associated nuclear displacements has shown that these vibrations, similarly to the case of hydrogen capped polyynes, can be associated to longitudinal optical phonons of the corresponding infinite polyyne (for a detailed discussion of the vibrational dynamics and Raman response of carbon linear chains see [6], [35], [41],[42]). The results from DFT calculations indicate that, because of the dispersion of the strong Raman band with size, the three bands ν 1, ν 2, ν 3, can be interpreted as due to the convolution of the signal produced by chains of various length. This behavior is indeed confirmed by recent SERS experiments carried out on hydrogen capped polyynes of selected chain lengths [27]. The SERS data reported in [27] do not allow for a precise assignment of bands ν 1, ν 2, ν 3 to specific chain lengths, due to the difference between the SERS active substrate in the present case (silver colloid) with respect to [27] (silver islands films) and the essential role of the substrate-analyte interaction in SERS. Anyway, we can assign the observed ν 1 SERS band to the fraction of longer carbon chains existing in the sample, the ν 3 band to the fraction of shorter chains and the ν 2 band to intermediate lengths. Therefore, one can ascribe the observed decrease of the ν 1 and ν 2 bands as due to the decrease in the relative concentration of the long polyynes with respect to the short ones. Based on the available experimental data, we can state that the spectra of Fig. 3 and Fig. 5 indicate cross linking processes [40] which convert long polyynes into shorter ones and sp 2 carbon phase. These results are somehow in agreement with the observation of C 8 H 2 as the most abundant species in solution while longer chains are more difficult to be produced [23]. Low stability of sp chains with conversion to sp 2 was observed also in other sp carbon systems. For instance high energy release is observed when isolated sp chains embedded in solid inert gas matrices interact to form sp 2 network [13][43]; formation of graphitic nano-domains induced by thermal treatments in sp-sp 2 amorphous carbon films deposited by low energy cluster beam deposition has been also reported [10].

8 8 5. Conclusions The SERS technique has been successfully used to investigate the interaction of sp carbon chains (polyynes) with silver colloids (the SERS active medium) as a function of time. Thanks to the high enhancement achieved in SERS, we have been able to follow the evolution in the minutes range of the distribution of chain lengths when a solution of polyynes in methanol is mixed with silver colloids. Under these conditions and due to aggregation induced by the strong interaction of polyynes with silver, several reactions take place. For instance cross linking among linear chains occurs and chain concatenation mediated by the surface of silver nanoparticles may be invoked. We can obtain information on these processes by examining both the sp 2 /sp ratio and the internal ratios within the sp region of the SERS signal. We have shown that under our operative conditions (methanolic solution of polyynes mixed with silver aqueous colloids) the sp 2 /sp ratio increases and the chain length distribution of polyynes converges towards presumably more stable shorter chains. This evidence supports the hypothesis of a cross link process rather than other chemical reactions. Even if additional studies and experiments are still necessary to further clarify the interesting interaction of polyynes with silver nanoparticles, we demonstrated that SERS is a powerful technique to investigate the structure and the stability of sp carbon chains also giving access to the time evolution of complex processes involving such elusive form of carbon structures. Acknowledgments This work has been partly supported by grants from the Italian Ministry of Education, University and Research through FIRB projects Molecular compounds and hybrid nanostructured materials with resonant and non resonant optical properties for photonic devices (RBNE033KMA) and Carbon based micro and nano structures (RBNE019NKS), by project PRIN Molecular materials and nanostructures for photonics and nanophotonics ( ) and by FlagProject "ProLife mobilità sostenibile" funded by the Milano city administration. The authors acknowledge A. Bonetti and S. Salvatore for the contribution given during their undergraduate thesis project. Franco Cataldo acknowledges the Italian Space Agency for support under the contract n. I/015/07/0 (Studi di Esplorazione Sistema Solare).

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12 12 Captions to figures and tables Fig. 1. (a) Extinction (absorption + scattering) spectra of silver colloids measured before and after aggregation induced by mixing colloids with a polyynes in methanol. (b) Extinction (absorption + scattering) spectra of silver colloids measured at increasing NaCl concentrations showing the aggregation of nanoparticles (see text). Fig. 2. STEM image of Ag nanoparticles used as the SERS active medium Fig. 3. Time evolution of SERS spectra recorded on a colloidal solution of silver nanoparticles and polyynes in methanol. Time is measured after the mixing of polyynes with the concentrated silver colloid. The arrow indicates a methanol peak at 1020 cm -1. Fig. 4. Evolution with time of the integrated SERS signal of sp and sp 2 from Fig. 3. Fig. 5. Time evolution of SERS spectra recorded on a colloidal solution of silver nanoparticles and polyynes obtained with the submerged arc-discharge method in methanol. Time is measured in minutes after the mixing of polyynes with the concentrated silver colloid. The spectra are all normalized in such a way that the integral over the frequency range cm -1 is constant. Bars represent calculated frequency and intensity from first-principles calculations of off-resonance Raman response of silver end capped polyynes Ag-C N -Ag (6 N 20). Fig. 6. Change with time of the SERS intensity of the three main features ν 1, ν 2, ν 3 according to the three gaussians fit of Fig. 5. Table 1: Fitting values for a double exponential decay fit of SERS intensity of sp and sp 2 bands reported in Fig. 4 Table 2: Fitting values for exponential decay fit of SERS intensity of ν 1, ν 2, ν 3 peaks in the sp band reported in Fig. 6

13 (a) Absorbance Wavelength (nm) (b) Absorbance Wavelength (nm) 1250 Fig. 1.

14 Fig

15 Fig

16 SERS intensity (arb. units) sp+sp 2 sp 2 sp Time (min.) Fig. 4.

17 17 ν 3 ν 2 ν 1 Fig. 5.

18 SERS Intensity (arb. units) ν 2 ν 3 ν Time (min.) Fig. 6.

19 19 τ 1 (min) τ 2 (min) I 0 sp sp sp+sp Table 1 τ (min) I 0 ν ν ν Table 2