DSpace at VNU: Laser-Induced Synthesis of Au-Ag Alloy Nanoparticles in Polyvinylpyrrolidone (C6H9NO)(n) Solution tài liệ...
J Clust Sci DOI 10.1007/s10876-015-0877-7 ORIGINAL PAPER Laser-Induced Synthesis of Au–Ag Alloy Nanoparticles in Polyvinylpyrrolidone (C6H9NO)n Solution The Binh Nguyen1 • Thanh Dinh Nguyen1 • Trong Duc Tran1 • Thu Hanh Nguyen Thi1 Received: 30 December 2014 Ó Springer Science+Business Media New York 2015 Abstract Au–Ag alloy nanoparticles (NPs) were prepared in Polyvinylpyrrolidone (C6H9NO)n (PVP) solution by laser irradiation method Au and Ag NPs were separately prepared in PVP solution by laser ablation The mixture of Au and Ag NP colloids was irradiated by the second harmonic (532 nm) of pulsed Nd:YAG laser with different laser fluencies and exposure times The Plasmon resonance absorption spectrum, morphology and structure of Au–Ag alloy NPs were observed by a UV2450 spectrometer (Shimadzu), a high resolution transmission electron microscope (TEM Tecnai G2 20 S-TWIN/FEI) The results show that the resulting Au–Ag alloy NPs are homogeneous alloyed particles and dispersed in PVP solution with average diameter of nm without sintered structure Au–Ag alloy NPs were produced by 532 nm laser with different Au/Ag molar ratio of the initial mixed solution to give the plasmon resonance absorption peak in the intermediate positions between about 420 and 520 nm Keywords Au–Ag alloy NPs Á Laser ablation Á Laser-induced synthesis Introduction Bimetallic nanoparticles (NPs) have received enormous attention for many years due to their composition-dependent optical and catalytic properties Au–Ag bimetallic NPs are attractive materials because of their fascinating localized surface plasmon resonance absorption (LSPR) in the visible spectral region Combination of desirable features of Ag, Au NPs and Au–Ag bimetallic NPs is useful to a diverse range of applications related to photonics, catalysis, information storage, chemical/ & The Binh Nguyen thebinh@vnu.edu.vn Department of Physics, University of Science, VNUHN, Hanoi, Vietnam 123 T B Nguyen et al biological sensing, and surface-enhanced Raman scattering (SERS) [1–4] In Au– Ag bimetallic systems, alloy and core–shell NPs often show different optical properties, although the composition of Au and Ag within the nanostructures is the same [5, 6] The surface plasmon resonance (SPR) absorption peaks, in consequences, the optical properties of core–shell NPs depend critically on the relative composition and thickness of the core and shell [7, 8] Meanwhile, the SPR absorption peaks of the alloy NPs depend crucially on relative composition of nanoalloys So far, several methods and combination techniques have been developed to synthesis metal alloy NPs such as biological [9, 10], chemical [11, 12] and laserbased method [13–17] Chemical syntheses of homogenous alloy NPs of noble metals require high temperature above 100 °C Beside this, aqueous co-reduction methods lead to phase separation [18] In contrast, laser-based method allows synthesis of alloy NPs with totally homogeneous structure in aqueous media at room temperature without reducing agents or organic ligands The laser-based Au–Ag alloy NP synthesis has been successfully produced by post-irradiation (laser induced synthesis) of Ag and Au NP colloids generated during pulsed laser ablation in liquids (PLAL), by PLAL of Ag or Au bulk targets in Au or Ag NP colloidal solutions or by PLAL of bulk alloy targets [18] Post-irradiation of a colloidal mixture of previously generated Ag- and Au-NPs is an extensively studied method to prepare Au–Ag alloy NPs Though there are many reports in literature, to date, the dominant alloy formation process during laser-induced Au–Ag alloy NP synthesis is not fully understood [18] Depending on the pulse length, the laser wavelength, laser fluence fragmentation of the NPs and subsequent sintering, melting and diffusion processes are discussed to elucidate the formation of bimetallic NPs [19].Compagnini et al proposed that re-irradiation first leads to core–shell NPs, later (t [ 10 min) succeeded by alloy NP formation [20] Another study showed that post-irradiation of a colloidal mixture of Au and Ag resulted in alloy NP formation after 25 [21] For a colloidal mixture of Au and Ag NPs exposed to laser irradiation, the absorption spectra change as the irradiation time is increased They expect that the Au and Ag SPR absorption peaks will disappear and a new SPR absorption peak located at an intermediate position between the Au and Ag SPR peaks will be observed due to the formation of Au–Ag alloy NPs Peng et al observed that elimination of the Ag SPR band needs very long irradiation times when a mixture of Au and Ag NPs (molar ratio 1:1) was irradiated by a 532 nm laser [15] Even in a laser irradiation times of 30 min, the Ag SPR absorption intensity (at 406 nm) increases without obvious spectral shift and an absorption shoulder appeared at 460 nm They supposed that alloy NPs may be obtained by two modes: (i) formation of alloy NPs with a larger size or a sintered structure, and (ii) explosion into smaller alloy NPs depending on the energy absorbed by the NPs Both two modes were observed in that report Morphology, size distribution of PNs prepared by laser-based method depend on several factors such as laser wavelength, pulse duration, energy per pulse, irradiation time, liquid environments and composition of initial mixture in case of laser induced synthesis Laser ablation and laser induced synthesis methods depend on several factors such as laser wavelength, pulse duration, energy per pulse, irradiation time, liquid 123 Laser-Induced Synthesis of Au–Ag Alloy Nanoparticles… environments and composition of initial mixture in case of laser induced synthesis They expected such parameters can be used to control the shape and size distribution of the NPs [22] However, variations of the laser parameters during synthesis alone are not suitable for the size control during PLAL One characteristic feature of PLAL method is to give a dispersed size distribution (2–400 nm) [16, 17].The possible strategy for size control may be post-processing of the laserfabricated NPs such as centrifugation at varying speed, yielding different size fractions [23] or re-irradiation with pulsed lasers, namely pulsed laser fragmentation in liquid (PLFL) [24] In this paper we report some results in our study to prepare Au–Ag alloy NPs in PVP solution by a laser based method where Au and Ag NPs are prepared by PLAL and then the mixture of Au and Ag NPs is irradiated by a 532 nm laser The preparation regime of Au and Ag NPs in PVP by PLAL was studied by ourselves and reported elsewhere Laser fluence, laser exposure time and PVP concentration in water were chosen so that Au NPs can get smaller size than Ag NPs The composition of Au and Ag within the nanostructures and accordingly the SPR absorption peak of Au–Ag alloy NPs can be varied depending molar ratio of Au and Ag NPs in the initial mixture The impact of laser fluence and laser irradiation time on the formation of Au–Ag alloy NPs in PVP was studied and reported This method does not need Au–Ag bulk alloys with different compositions which are expensive to have Au–Ag alloy NPs with any composition By the laser based method we can prepare metal alloy NPs in aqueous media at room temperature without reducing agents or chemical additives Experimental A pulsed Nd:YAG (Quanta Ray Pro 230-Spectra Physics USA) was used to prepare both Au, Ag NPs and Au–Ag alloy NPs Au and Ag NPs were separately prepared by laser ablation of corresponding metal plates (purity of 99.99 %), which was placed in the glass vessel filled with 10 mL of 0.02 M PVP solution in water The laser ablation was performed by laser pulses of ns duration at 10 Hz repetition rate and 1064 nm wavelength The laser beam was focused by a lens with a focal length of 100 mm The mixtures of the Ag and Au NPs colloidal solutions with different molar ratios were irradiated by laser beam at 532 nm wavelength The absorption spectra of colloidal solutions were recorded by a UV-2450 spectrometer (Shimadzu) in the 300–800 nm range The morphology of the Au and Ag NPs and their alloys was studied by TEM observation The HR-TEM images and energy-dispersive X-ray (EDX) spectra were obtained using a transmission electron microscope (TEM Tecnai G2 20 S-TWIN/ FEI; EXD Genesis XM2, EDAX) The structure of materials was examined by a X-ray diffraction (XRD) instrument D5005, Bruker, using CuKa radiation ˚ ) The metal concentration of NP colloids was determined by (k = 1.540656 A F-AAS technique using a Perkin Elmer AAS-3300 atomic absorption spectroscopy system The average laser power was measured by a 13 PEM 001 (Melles Griot) power meter 123 T B Nguyen et al The size of NPs was determined by ImageJ 1.46r of Wayne Rasband (National institutes of Heath, USA) The size distribution was obtained by measuring the diameter of more than 500 particles and using Origin 8.5.1 software Results and Discussion Preparation of Au and Ag NPs by Laser Ablation Gold and silver were separately ablated in 0.02 M PVP solution by 1064 nm fundamental wavelength of the Nd:YAG laser with average laser power density of 16 W/cm2 and exposure time of 15 for gold and for silver We take the longer exposure time for gold to get higher concentration of gold in comparison of silver Our AAS measurement shows that the concentrations of Au NPs and Ag NPs in 0.02 M PVP solution are 112.77 and 42.04 mg/L respectively The absorption spectra of Au and Ag NPs in PVP solution are shown in Fig The characteristic SPR absorption peaks are at about 521 nm for Au NPs and 401 nm for Ag NPs TEM image and size distribution of Au NPs and Ag NPs prepared in PVP solution are presented in Figs and respectively The TEM images show both Au and Ag NPs are rather spherical in shape The Au NPs have average diameter of 13 nm with size ranging from to 43 nm The Ag NPs have average diameter of 34 nm with size ranging from to 110 nm The crystalline structure of Au and Ag NPs was examined by XRD measurement The XRD pattern of Au and Ag NPs showed 03 peaks at around the same position 2h = 38.1°; 44.4° and 64.4° corresponding to diffraction from the (111), (200) and (220) planes respectively of a face centered cubic structure This result was predictable because of the very similar lattice constants between Au and Ag (Au, ˚ ; Ag, 4.0862 A ˚ ) [25] 4.0786 A Fig SPR absorption spectra of Au NPs (a) and Ag NPs (b) in PVP solution 123 Laser-Induced Synthesis of Au–Ag Alloy Nanoparticles… Fig TEM image (a) and size distribution (b) of Au NPs prepared in PVP solution Fig TEM image (a) and size distribution (b) of Ag NPs prepared in PVP solution Laser Induced Synthesis of Au–Ag Alloy NPs A mixture of the Au and Ag NPs in 0.02 M PVP solution was prepared with 1:1 volume ratio and irradiated by 532 nm wavelength of Nd:YAG laser The average laser power density was set at 1.6 W/cm2 and the exposure time was 20 The Fig shows the UV–Vis absorption spectrum of the Au–Ag NPs mixtures before and after laser irradiation Notice that the characteristic SPR absorption peak positions of pure Au NPs and Ag NPs are shifted in Au–Ag colloidal mixture The SPR peaks for the Au and Ag NPs in the mixed solution before laser irradiation located at 516 and 421 nm, respectively (Fig 4a) They disappeared in the Au–Ag colloidal mixture after laser irradiation and only one SPR absorption peak was observed at 488 nm This result indicates that the resulting colloid contains Au–Ag bimetallic NPs (core–shell or alloy structure) The single SPR absorption peak in the intermediate positions between Au and Ag SPR peaks is not enough convinced evidence for Au–Ag alloy 123 T B Nguyen et al Fig The absorption spectrum of the mixed solution of Au and Ag NPs (volume ratio 1:1) before and after laser irradiation Fig The TEM image (a) and the size distribution (b) of Au–Ag bimetallic NPs structure Core–shell NPs could also give the similar results depending shell thickness [26] We investigated the structure of resulting NPs by HR-TEM observation The TEM image and the size distribution of Au–Ag bimetallic NPs were taken and shown in Fig As can be seen in the Fig 5, the Au–Ag bimetallic NPs are isolated and dispersed in PVP solution without sintered structure Their shape is rather spherical and size ranges from to 10 nm with average diameter of nm 123 Laser-Induced Synthesis of Au–Ag Alloy Nanoparticles… The structure and chemical composition of selected Au–Ag bimetallic NPs was investigated by HR-TEM image analysis and EDX spectroscopy The HR-TEM image of asingle isolated Au–Ag bimetallic NP of nm diameter is given in Fig 6a EDX microanalysis demonstrated that the composition of resulting NPs consists of Au and Ag The HR-TEM image (Fig 6a) indicates that the resulting NP has crystalline structure as Au–Ag alloy NPs but not core–shell structures We can estimate from the HR-TEM image that the lattice plane spacing is 0.24 nm which is ˚ ) of XRD pattern in Fig 6b corresponding to the (111) plane spacing (2.3488 A Using Scherrer equation we can estimate diameter D of the NP D¼k k ; bcosh ˚ = 1.54056 10-10 m; h = 19.1°; b = 1.44° = where: k = 0.9; k = 1.54056 A 0.025 rad (Fig 6b) It results that: D = 5.9 nm The XRD pattern (Fig 6b) of the resulting NPs showed 03 peaks corresponding to diffraction from the (111), (200) and (220) planes respectively of a face centered cubic structure that is similar with Au and Ag NPs due to the very similar lattice constants between Au and Ag Under irradiation with an intense pulsed laser at 532 nm which is near SPR absorption peak of Au NPs, the Au NPs in the mixed colloidal solution were excited and heated to their boiling point The absorbances for Au and Ag NPs at 532 nm are 0.81 and 0.23, respectively So upon laser irradiation at 532 nm, Au NPs will be more efficiently excited than Ag NPs of the same size However, the melting point of Ag is lower than Au (Ag: 961.78 °C; Au: 1064.18 °C) It is possible that Ag NPs with lager size can also be heated to meting point at the same time at 532 nm as Au Fig HR-TEM image of a selected Au–Ag bimetallic nanoparticle of nm diameter (a) and XRD pattern (b) 123 T B Nguyen et al NPs [27, 28] These heated Au and Ag NPs may be fragmented into smaller NPs by releasing atoms and clusters The hot released atoms and clusters have a strong tendency to rapidly recombine into Au–Ag alloy particles on a nanosecond time scale [29] Also, single atoms or small clusters might diffuse to and aggregate on the surface of preexisting NPs [30] Since Au and Ag have almost identical lattice constants, mixing of the two metals is a thermodynamically favorable process Therefore, alloying process may take place easily to form homogeneous Au–Ag alloy particles (as indicated in Fig 6a) In the presence of PVP—(C6H9NO)n, the C=O group of the polymer interacts with metal atoms on the surface of NPs The oxygen atoms of C=O group are attached to the metal atoms and create local surface state that protect metal NPs against growth and aggregation [31] It explains that the Au–Ag alloy NPs prepared in PVP solution have small size and disperse in solution without sintered structure (as indicated in Fig 5) The formation process of Au–Ag alloy NPs depends strictly on laser fluence and laser exposure time A set of experiments was carried out to determine the influence of laser fluence and laser exposure time in the preparation of Au–Ag alloy NPs The mixture of Au–Ag NPs in 0.02 M PVP solution (volume ratio 1:1) was irradiated by 532 nm laser with different laser fluences (0.8, 1.2 and 1.6 W/cm2) during different exposure times (10, 20 and 30 min) The SPR absorption spectra of the resulting colloidal solutions are shown in Figs 7, and Before exposure to laser light, two distinct absorbance maxima at 516 and 421 nm, corresponding to pure Au and Ag SPR peaks, respectively For solutions exposed to laser irradiation at 532 nm, the absorption spectra changed This variation in the absorption spectra clearly implies changes in the colloidal properties In laser exposure time of 10 min, a new single absorption peak only starts to be formed at 492 nm when average laser power density increases up to 1.6 W/cm2 Fig The absorption spectra of Au–Ag NP colloidal mixtures irradiated during 10 by different average laser power densities of 0.8, 1.2 and 1.6 W/cm2 123 Laser-Induced Synthesis of Au–Ag Alloy Nanoparticles… Fig The absorption spectra of Au–Ag NP colloidal mixtures irradiated during 20 by different average laser power densities of 0.8, 1.2 and 1.6 W/cm2 Fig The absorption spectra of Au–Ag NP colloidal mixtures irradiated during 30 by different average laser power densities of 0.8, 1.2 and 1.6 W/cm2 In laser exposure time of 20 min, the Au SPR absorption intensity decreases and shifts to the blue, while the Ag SPR absorption intensity decreases and shifts to the red when laser fluence increases Further increasing laser fluence, a new single SPR absorption peak appears with increasing intensity and slightly shifts from 490 to 123 T B Nguyen et al 488 nm when laser power density increase from 1.2 to 1.6 W/cm2.This SPR peak can be contributed to the formation of Au–Ag alloy NPs In laser exposure time of 30 min, two SPR absorption peaks of pure Au and Ag NPs disappear, only one new SPR absorption peak is formed at 490 nm with increasing intensity and slightly shifts to 488 nm when laser power density increases from 1.2 to 1.6 W/cm2 Our results support that by using intense laser fluence and Ag NPs of large enough size Ag NPs can be heated to meting point at the same time at 532 nm as Au NPs and fragmented into smaller NPs by releasing atoms and clusters The hot released atoms and clusters rapidly recombine into Au–Ag alloy NPs on a nanosecond time scale According to this explanation, the number of Ag and Au NPs not increase and consequently the intensities of Ag and Au SPR peaks decrease as shown in Figs and The absorption spectra with two peaks in Figs and are the sum of Ag NP, Au NP and Au–Ag alloy NP absorption bands By further increasing irradiation time or laser fluence, the number of Au–Ag alloy NPs emerges and only SPR absorption peak of Au–Ag alloy NPs is observed The formation of Au–Ag alloy NPs by 532 nm laser in this regime of preparation was not obtained by two mode as proposed by several reports in literature [15, 20, 21] In our experiments the elimination of the Ag SPR band and accordingly the formation of Au–Ag alloy NPs occur after 10 irradiation by 532 nm laser at average power density of 1.6 W/cm2 Using laser power density of 1.6 W/cm2, the SPR absorption band is almost unchanged when laser exposure time increases from 20 to 30 The suitable values of laser power density and exposure time for Au–Ag alloy NPS preparation in our experiment are 1.6 W/cm2 and 20 min, respectively The SPR absorption peak of Au–Ag alloy NPs can be varied between 421 and 516 nm when changing the molar ratio in the initial mixed solution of Au and Ag NPs Figure 10 shows the absorption spectra of mixed solutions of Au and Ag NPs with initial Au/Ag molar ratio of 2/3 (Fig 10a) and 3/2 (Fig 10b), respectively Fig 10 The absorption spectra of mixed solutions of Au and Ag NPs with initial Au/Ag molar ratio of 2/3 (a) and 3/2 (b), respectively before and after 532 nm laser irradiation 123 Laser-Induced Synthesis of Au–Ag Alloy Nanoparticles… before and after 532 nm laser irradiation by the same average power density of 1.6 W/cm2and laser exposure time of 20 The SPR absorption peak of Au–Ag alloy NPs shifts from 488 to 471 nm when initial Ag/Au molar ratio decreases from 3/2 to 2/3 This result supports that Ag and Au can be miscible in all proportions to give different SPR absorption peak wavelengths in the intermediate positions between Au and Ag SPR absorption peaks In order to prove the role of PVP solution in water we prepared Au–Ag alloy NPs in distilled water by the same process Au and Ag NPs were separately prepared in distilled water and the mixture of Au and Ag NP colloids was irradiated by 532 nm laser to produce Au–Ag alloy NPs The average power density of laser was set at 1.6 W/cm2 Figure 11 shows the absorption spectra of the colloidal Au–Ag mixture in distilled water with different irradiation times With an irradiation time of 15 min, only one SPR absorption peak of Au–Ag alloy NPs appeared at 448 nm (Fig 11d) This peak shifted to 458 nm and kept almost unchanged when laser irradiation time increased from 30 to 45 (Fig 11e, f) TEM image and the size distribution of Au–Ag alloy NPs were given in Fig 12 Fig 11 The absorption spectra of mixed solutions of Au and Ag NPs in distilled water with initial Au/Ag molar ratio of 2/3 by different irradiation times Fig 12 TEM image and size distribution of Au–Ag alloy NPs in distilled water 123 T B Nguyen et al There are two types of morphology corresponding to formation of isolated spherical NPs and sintered structure, meanwhile no sintered structure when prepared in PVP solution The Au–Ag alloy NPs have diameters ranging from 8.4 to 75.3 nm and their average diameter is of 29.4 nm that is much greater than Au–Ag alloy NPs prepared in PVP solution by the same average power density of laser and irradiation time Conclusion We found that Au–Ag alloy NPs could be easily produced by laser irradiation at 532 nm wavelength The diameter of Au–Ag alloy NPs synthesized by postirradiation of Ag NPs (3–110 nm diameter) and Au NPs (2–43 nm diameter) in PVP solution ranges from to 10 nm with average diameter of nm Formation of homogeneous alloyed particles was clearly demonstrated by UV–Vis absorption spectroscopy and HR-TEM and EDX measurements The Au–Ag alloy NP formation depends strictly on laser fluence (or average laser power density) By the laser based method we can prepare Au, Ag NPs and Au–Ag alloy NPs with different Au/Ag molar ratios The preparation procedure is simple and versatile with respect to the kinds of metals and liquids employed 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Au–Ag alloy NPs Though there are many reports in literature, to date, the dominant alloy formation process during laser-induced Au–Ag alloy NP synthesis is not fully understood [18] Depending on... energy per pulse, irradiation time, liquid 123 Laser-Induced Synthesis of Au–Ag Alloy Nanoparticles? ?? environments and composition of initial mixture in case of laser induced synthesis They expected... surface of NPs The oxygen atoms of C=O group are attached to the metal atoms and create local surface state that protect metal NPs against growth and aggregation [31] It explains that the Au–Ag alloy