Monodisperse Colloidal Metal Nanoparticles to Core-shell structures

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C: Physical Processes in Nanomaterials and Nanostructures

Monodisperse Colloidal Metal Nanoparticles to Core-shell structures and Alloy Nanosystems via Digestive Ripening in Conjunction with Solvated Metal Atom Dispersion: A Mechanistic Study Chirasmita Bhattacharya, and Balaji R. Jagirdar J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00874 • Publication Date (Web): 20 Apr 2018 Downloaded from http://pubs.acs.org on April 20, 2018

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The Journal of Physical Chemistry

Monodisperse Colloidal Metal Nanoparticles to Core-shell Structures and Alloy Nanosystems via Digestive Ripening in Conjunction with Solvated Metal Atom Dispersion: A Mechanistic Study Chirasmita Bhattacharya, Balaji R. Jagirdar* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India ABSTRACT Co-digestive ripening of two distinct metal nanoparticles is an exceptional method for the colloidal synthesis of core-shell heterostructures. In this report, a detailed investigation of the underlying mechanism by which surfactant molecule assisted interatomic transfer between two metal nanoparticles occurs has been described using gold/silver as a model system. Core-shell nanoparticles with gold in the core and silver in the shell in the size regime of 6.9 + 1.8 nm were obtained by conducting the co-digestive ripening of polydispersed particles of Au-pentanone and Ag-pentanone colloids in the presence of hexadecylamine as a capping agent used in the molar ratio of 1:30 with respect to metal. The progress of the formation of core-shell nanoparticles has been monitored using UV-visible spectroscopy and transmission electron microscopy. Detailed

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analysis of the nature of Au@Ag core-shell nanoparticles has been done with the help of HAADF-STEM technique, point-EDS analysis and X-ray photoelectron spectroscopy. Variation of experimental conditions such as concentration of capping agent, molar ratio of Au and Ag, and temperature in the co-digestive ripening process led to the realization that core-shell nanoparticles with thicker shell of silver can be obtained under certain reaction conditions. Alteration of the reaction conditions was also noted to affect the final Au@Ag nanoparticles with respect to average particle size and polydispersity. The as-prepared Au@Ag nanoparticles could be transformed to Au-Ag nanoalloys on being exposed to ultraviolet radiation of 254 nm. We have also attempted to elucidate the factors which dictate the formation of core-shell nanoparticles by comparing theoretical evidences from the literature with our experimental results. 1. INTRODUCTION Fabrication of assorted bimetallic heterostructures has witnessed tremendous development in the past few decades. Among the diverse range of materials explored, core-shell nanostructures have garnered significant attention owing to their promising applications in the field of catalysis,1-4 photonics,5-6 bio sensing,7 drug delivery,8 surface-enhanced Raman scattering (SERS) etc.9-10 Core-shell nanoparticles are composite materials consisting of a core made up of one material coated with a shell comprised of another material. Core and shell materials could be chosen depending on the desired application and the properties of these hybrid structures could be easily modified by changing the composition as well as core to shell ratio.11-12 Hence, an appropriate choice of shell material can prove to be beneficial in imparting colloidal, chemical, and thermal stability to any material.13-14


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Advances in the synthetic methodologies have provided ample opportunities for tailoring the size and shape of core-shell nanoparticles in order to cater to specific purposes. Various synthetic schemes for realizing different core-shell materials have been systematically presented in a review by Chaudhuri and Paria.15 Common approaches for synthesis of core-shell nanostructures include reduction of metal precursor in the presence of seeds of desired core metal, 16-17 galvanic replacement,18-19 co-reduction of metal precursors, etc.20-21 Much of the attention has been focused on the colloidal synthesis of hybrid structures of plasmonic metals. This burgeoning interest could be attributed to their absorption features in the visible region and their colloidal synthesis would enable us to maneuver their extinction properties exquisitely.22-24 To realize controlled synthesis of these materials, it is essential to manipulate the kinetics of growth of shell on the core particles. In this context, digestive ripening process offers selectivity to optimize several reaction parameters to obtain defined heterostructures.25-26 The process of digestive ripening involves inter-exchange of atoms between different particles through surface etching assisted by surface active ligands.27 Several bimetallic nanostructures, like Ag@Pd, Au@Pd, Cu@ZnO, Mg-Cu, Cu-Zn, etc. have been reported by our group using the digestive ripening approach.28-32 However, the factors that affect the core-shell structure formation by this versatile method remain undetermined. We carried out a systematic study to explore the mechanistic aspects involved in the co-digestive ripening of two different metal nanoparticles. The gold/silver metallic system was chosen as a model system to conduct this investigation since their extinction properties appear in the visible region of the electromagnetic spectrum. Hence, analysis of the reaction progress can be carried out easily by the shift and intensity variation exhibited by their surface plasmon absorption bands. Reaction parameters such as composition ratio between metals, concentration of capping agent, and temperature were varied and an


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understanding of their influence on various facets of co-digestive ripening has been presented herein. Apart from the composite structures, alloys and intermetallic nanocrystals have also been synthesized using co-digestive ripening strategy.33-34 It has been suggested that the processes of interatomic diffusion and digestive ripening simultaneously determine the bimetallic nanocrystals formed.35 Interatomic diffusion of metals becomes more feasible in the nanosize regime due to the depression in their melting points and the presence of defects at the interface of initially derived core-shell structure.36 These factors lead to considerable enhancement in the diffusion coefficients of the two metals and thereby favor the alloying process. Strategies to induce diffusion of atoms in the core-shell nanoparticles to realize their transformation into nanoalloys has also been investigated. Results of these studies have also been presented in this report. 2. EXPERIMENTAL SECTION 2.1 Materials Used Gold foil (99.95%) and silver foil (99.99%) were purchased from Arora-Matthey Limited (Kolkata, India). 3-Pentanone, t-butyl toluene and hexadecyl amine (HDA) were procured from Sigma-Aldrich. Hexadecyl amine was dried and degassed at 100 °C for 6 h. 3-Pentanone was dried using potassium carbonate (K2CO3) and distilled, and degassed by several freeze-pumpthaw cycles. Toluene was purchased from S.D. Fine Chemicals Limited, India and mesitylene, from Spectrochem Private Limited, India. Both toluene and mesitylene were dried over sodium– benzophenone, distilled, and degassed by several freeze–pump–thaw cycles prior to use in digestive ripening experiments. Tertiary butyl toluene was used as received.


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2.2 Instrumentation Optical absorption spectral measurements were carried out at room temperature using a Shimadzu UV-2600 UV–vis–near-IR spectrophotometer. Powder X-ray diffraction data were collected using a PANalytical Empyrean diffractometer using Cu Kα (0.154 nm) radiation source. The samples for PXRD measurements were made by drop-casting the colloidal sample on a glass slide and dried under a lamp. The microscopic (TEM) analysis of the samples was carried out using a JEOL JEM-2100F field emission transmission electron microscope operating at an accelerating voltage of 200 kV. The samples were prepared on Formvar coated copper grids by slow evaporation of 1-2 μL of the diluted colloids and further drying under a lamp. The particle size distribution of the samples was calculated by measuring size of 300 particles imaged from different regions of the TEM grid using Sigma Scan Pro and Origin software. Highresolution (HR) TEM and selected-area electron diffraction (SAED) pattern were performed with the help of Digital Micrograph software. 2.3 Preparation of Au/Ag Colloids by Solvated Metal Atom Dispersion (SMAD) Method Au/Ag colloidal nanoparticles capped with 2-pentanone were prepared by the solvated metal atom dispersion (SMAD) method.37 Typically, about 100 mg of Au/Ag foil was placed in an alumina coated tungsten crucible that was connected to two water-cooled copper electrodes. A Schlenk tube containing dried, distilled, and degassed pentanone was attached to the SMAD reactor (3L, thick-walled, cylindrically shaped glass vessel) at the top through a bridge head. The entire setup was then evacuated to about 1 × 10-3 mbar. The crucible was resistively heated, which led to vaporization of the metal. The metal atoms formed were co-condensed with solvent vapor on the walls of the reactor immersed in a liquid N2 dewar. After the completion of the experiment, the frozen matrix of metal atoms and solvent molecules was allowed to warm up to


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room temperature. The as-prepared colloid was stored in a Schlenk tube under Argon atmosphere. 2.4 Digestive Ripening of Au/Ag Nanoparticles The colloidal Au/Ag nanoparticles prepared by SMAD method were subjected to a postsynthetic process termed as ‘digestive ripening’.37 In this process, the as-prepared Au/Ag colloid was taken in 100 mL round-bottom Schlenk flask equipped with a water-cooled condenser containing hexadecylamine (HDA) as a capping agent. The molar ratio of metal to capping agent is usually kept quite high in order to facilitate better interaction between them. A high boiling solvent (toluene/mesitylene/t-butyl toluene) was added to the flask and the low boiling pentanone solvent was removed under dynamic vacuum. The resulting reaction mixture was vigorously stirred at room temperature for 1 h, followed by refluxing under Argon atmosphere using a temperature-maintained oil bath/sand bath. The experiment was continued up to 30 h and aliquots were withdrawn at regular intervals of time. The progress of the reaction was monitored by analyzing aliquots by UV-visible spectroscopy and transmission electron microscopy. 2.5 Co-digestive Ripening of Au and Ag Nanoparticles In a similar fashion, Au and Ag nanoparticles and capping agent were mixed in an appropriate molar ratio and subjected to digestive ripening process using a suitable high boiling solvent. A series of experiments were conducted by varying the reaction parameters such as concentration of capping agent, molar ratio between Au and Ag, and temperature to obtain insight into the different factors that affect the co-digestive ripening process. All the experiments were monitored both microscopically and spectroscopically.


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2.6 Photolytic Treatment of Bimetallic Au-Ag Nanoparticles The as-prepared Au-Ag bimetallic nanoparticles obtained upon co-digestive ripening of Au and Ag nanoparticles were subjected to UV radiation. These experiments were carried out in a UV-photo reactor equipped with a carousel and designed such that the temperature of the reactor could be maintained below 40 ˚C. Germicidal UV lamps of 254 nm wavelength were used for our experiment. The sample was placed in a quartz cuvette on the carousel and the photolytic treatment was carried out for 6 h. The progress of the experiment was evaluated by withdrawing aliquots from the cuvette at every 2 h interval and recording their UV-visible spectra. 3. RESULTS AND DISCUSSION 3.1 Preparation and Characterization of Au/Ag Colloids Colloidal Au nanoparticles were prepared by SMAD method using 3-pentanone as a coordinating solvent. Pentanone was employed as it renders the colloid, reasonably good stability and dispersion. The as-prepared Au-pentanone colloid was deep-violet in color and homogeneous in nature. The UV-visible spectrum (Figure 1a) shows a broad band centered around 553 nm, which suggests the polydispersed nature of the colloid. The powder XRD pattern (Figure 1b) shows sharp peaks corresponding to the face-centered-cubic phase of Au(0). The polydispersed nature of the sample is also evident from the bright-field (BF) TEM image (Figure 1c). Fringes corresponding to the most intense (111) peak were noted in the HRTEM image (Figure 1d). The SAED pattern (Figure 1(d) inset) shows scattered intense spots, characteristic of a polycrystalline sample.

In a similar manner, Ag-pentanone colloid was prepared by SMAD method and further characterized. The as-prepared colloid was black in color and moderately stable towards precipitation of particles. Ag colloid was found to be less stable in comparison to Au colloid with respect to precipitation of particles upon storage.


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(111)

d= 2.350 Å

Figure 1. Colloidal Au-pentanone nanoparticles: (a) UV-visible spectrum, (b) PXRD pattern, (c) BFTEM image, (d) HRTEM image with SAED pattern The UV-visible spectrum (Figure 2a) shows a very broad feature around 401 nm and the PXRD pattern (Figure 2b) shows peaks which could be assigned to fcc phase of Ag(0). The BFTEM image (Figure 2c) shows highly polydispersed Ag nanoparticles and the HRTEM image (Figure 2d) confirms the crystalline nature of the sample due to the presence of fringes corresponding to the (111) peak with a d-spacing value of 2.383 Å. The SAED pattern (Figure 2(d) inset) suggests the polycrystalline nature of the nanoparticles.


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Figure 2. Colloidal Ag-pentanone nanoparticles: (a) UV-visible spectrum, (b) PXRD pattern, (c) BFTEM image, (d) HRTEM image with SAED pattern 3.2 Digestive Ripening of Au/Ag Nanoparticles Analysis of the as-prepared colloids of Au and Ag nanoparticles reveal that the colloids are comprised of diverse shaped and sized nanoparticles. These colloids were further subjected to a post-synthetic size modification process called digestive ripening. Presence of high concentration of a surface-active ligand like HDA in the colloid and kinetic assistance by refluxing in a highboiling solvent medium leads to surface-etching and breakdown of particles until an equilibrium


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size range is attained.27 We carried out digestive ripening of the as-prepared Au and Ag colloids separately in order to evaluate the kinetics and establish the system that undergoes faster kinetics of digestive ripening and the impact it would have during the co-digestive ripening process. In a typical experiment, digestive ripening of Au/Ag colloid was performed using mesitylene as a refluxing solvent medium and keeping metal (Au/Ag) to HDA ratio to be 1:30. The metal concentration in the reaction medium was kept at 2 mg/mL in both the experiments and the experiment was conducted for 30 h. The UV-visible spectra (Figure 3a) revealed that Au nanoparticles took about 18 h to undergo complete ripening, which was evident from the shift in the Au SPR band from 541 to 528 nm. Ripening of the Au colloid was also quite evident from the change in the color of the reaction medium from violet to wine-red. On the other hand, Ag nanoparticles got completely ripened in only 12 h (Figure 3b) and the starting black colored reaction medium turned yellow within minutes, which evidences faster ripening.

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Figure 3. Monitoring of the progress of digestive ripening by UV-visible spectroscopy: (a) Au nanoparticles and (b) Ag nanoparticles.


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Figure 4. Bright-field TEM images obtained at different stages of digestive ripening of Ag nanoparticles. These results were supported by kinetics study conducted by the analysis of full-width at half maximum (FWHM) and absorption maxima analysis (see Supporting Information). One more feature that differentiates the ripening process of these two metals is that Ag nanoparticles upon prolonged reflux undergo Ostwald ripening which is irreversible. However, polydispersity


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observed in case of prolonged refluxing of Au nanoparticles is reversible on continuing the reaction for longer duration.27 Aliquots were withdrawn from the reaction medium every 6 h during the experiment and microscopy analysis was done. The BFTEM images obtained from both the experiments are shown in Figures 4 and 5.

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Figure 5. Bright-field TEM images obtained at different stages of digestive ripening of Au nanoparticles.


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3.3 Co-Digestive Ripening of Au and Ag Nanoparticles Co-digestive ripening of Au and Ag nanoparticles was carried out in mesitylene by mixing both Au and Ag nanoparticles in a molar ratio of 1:1, keeping the concentration of metal to HDA as 1:30. Mesitylene being a high boiling solvent facilitates breakdown of particles in presence of capping agent quite effectively. In addition, we noted that the digestive ripening reaction kinetics was moderate in mesitylene solvent, which is desirable to examine the effect of other reaction parameters on the progress of the reaction. A general reaction scheme for the co-digestive ripening process is shown in Scheme 1.

Scheme 1. Schematic representation of co-digestive ripening process


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Progress of the reaction was monitored using UV-visible spectroscopy by withdrawing aliquots at regular intervals of time. Also, samples were prepared for microscopy analysis at every 6 h interval and are shown in figure 6.

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Figure 6. Bright-field TEM images obtained at different stages of co-digestive ripening of Au and Ag nanoparticles; conditions: Au: Ag 1:1, M: HDA 1:30, T = 165˚C.


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The UV-visible spectrum of a mixture containing the as-prepared Au and Ag colloids is comprised of two bands at 405 nm and 525 nm corresponding to Ag and Au nanoparticles, respectively. During the co-digestive ripening process, both Au and Ag nanoparticles undergo ripening individually as evident from the UV-visible spectrum (Figure 7). As the reaction progresses, the SPR band of Ag shifts towards higher wavelength and a blue shift was noted in the case of Au SPR band. Also, the intensity of the Ag SPR band increased and that of Au band got dampened with time. A hybrid absorption band was noted at the end of 30 h of reflux, consisting of an intense band corresponding to Ag at 435 nm and a weak broad feature corresponding to Au around 510 nm. The color of the reaction medium changed from black to brown upon completion of the experiment. Analysis of the UV-visible spectra (Figure 7) suggest that the co-digestive ripening of Au and Ag nanoparticles under these reaction conditions lead to the formation of a core-shell structure, wherein Au is in the core and Ag forms the shell.

Figure 7. UV-visible spectra recorded during co-digestive ripening of Au and Ag nanoparticles; conditions: Au: Ag 1:1, M: HDA 1:30, T = 165˚C.


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It is well documented in the literature that in case of plasmonic metals like Au and Ag, a physical mixture of Au and Ag nanoparticles would show two distinct bands in the UV-visible spectrum; alloy structure is characterized by a single band positioned in between that of Au and Ag and a core-shell structure is expected to show broad overlapping bands subject to the shell thickness over core material.33,38,39 Broadening of the band arises from the diffused electron density at the core-shell interface.40 Further, in the absence of any such interaction, we should have observed both Au and Ag bands to ripen together characterized by two distinct sharp bands observed as a result of digestive ripening of individual metal nanoparticles. A similar observation was reported by Chen et al. where it was noted that an increase in the thickness of Ag shell leads to red-shift of the band in the lower- wavelength region and blue shift of band in the higher-wavelength region.41 They also simulated the absorption spectrum of Au@Ag core-shell nanoparticle with the help of Mie theory and the calculated absorption spectrum correlated well with the experimental spectrum. Since both gold and silver have a facecentered cubic crystal structure with lattice mismatch of less than 5%, X-ray diffraction measurement cannot differentiate between a core-shell structure, an alloy or a physical mixture (see Supporting Information). Further support of the Au@Ag core-shell structure was obtained from HAADF-STEM imaging (Figure 8a) as the image contrast observed using this technique results from a difference in atomic-number of Au (79) and Ag (47).The contrast difference between the core and shell was also evident in the STEM bright-field image shown in Figure 8(b). The elemental line scan of a single core-shell particle shown in figure 8(c) further confirms that silver forms a thin shell over gold core. Also, Energy dispersive X-ray spectroscopy (EDS) was used to analyze the component nanoparticles. Point-EDS spectrum (Figure 8d) obtained by selecting a point in the core-region of a nanoparticle shows the presence of only Au, whereas, a


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point near to the shell region shows the presence of only Ag. These experimental evidences indicate that our sample is made up of a core-shell structure comprised of Au in the core and Ag in the shell.

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Gold Silver

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Figure 8. Au@Ag core-shell particle: (a) HAADF-STEM image, (b) bright field STEM image, (c) elemental line scan analysis, and (d) point-EDS spectrum recorded by scanning a point in the core and another in the shell.


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Figure 9. Histograms obtained by calculation of average size of nanoparticles at different time intervals during co-digestive ripening of Au and Ag nanoparticles; conditions: Au: Ag 1:1, M: HDA 1:30, T = 165˚C


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Figure 10. Bright-field TEM images obtained at earlier stages of co-digestive ripening of Au and Ag nanoparticles; conditions: Au: Ag 1:1, M: HDA 1:30, T = 165˚C.


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The histograms obtained from a measurement of the average size of the spherical particles (in Figure 6) at different time intervals are shown in Figure 9. The sample before co-digestive ripening was largely comprised of aggregated particles with non-spherical morphology therefore, size distribution histogram was not constructed. The size and shape evolution of these nanoparticles upon digestive ripening was studied using TEM of samples prepared from aliquots taken during the initial stages of the ripening process. Analysis of the TEM micrographs shown in Figure 10 reveal that size focusing was attained at a short duration of 2 h. However, the reaction was continued up to 30 h in an attempt to understand the effect of prolonged reflux on the formation of bimetallic nanoparticles. 3.4 Effect of Concentration of Capping Agent Role of capping agent in the digestive ripening process was elucidated earlier, in the case of monometallic nanoparticles. In a study conducted by our group, it was noted that the kinetics of digestive ripening becomes more rapid on increasing the concentration of capping agent. 27 High concentration of capping agent facilitates better interaction between metal nanoparticles and ligand molecules. This leads to faster etching of clusters from the surface of nanoparticles by the digestive ripening agent, thereby enhancing mass transfer and exchange among particles. When two different metal nanoparticles are present in the refluxing medium, it is expected that the metal nanoparticles which undergo faster ripening and hence, faster surface etching should deposit over metal nanoparticles that undergo slower rate of ripening or remain within the medium. In the case of co-digestive ripening of Au and Ag nanoparticles, these observations indeed were made. Moreover, as-prepared silver nanoparticles are highly polydispersed in comparison to gold nanoparticles and thus, Au nanoparticles proved to be seeds over which atom clusters of


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Ag undergo deposition. In the case of lower metal to amine ratios, such a hybrid absorption band was observed after prolonged reflux. This supports our assumption that a high concentration of capping agent would lead to enhanced reaction rates and hence, formation of Ag shell on Au nanoparticles was accomplished in very short time duration. Another observation from a comparison of the UV-visible spectral data (Figure 11) was a great extent of dampening of the band corresponding to Au when higher metal to amine ratio was used. This could be ascribed to the presence of a thick and uniform shell of silver.39

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Figure 11. UV-visible spectra recorded during co-digestive ripening of Au and Ag nanoparticles by varying metal to HDA concentration ratios: (a) 1: 15, and (b) 1:45 The reaction progress of experiments with M:HDA ratios of 1:15 and 1:45 was monitored using TEM (Figures 12a and 12b). The final particle sizes calculated by considering over 300 nanoparticles are 5.5+1.2 nm, 6.9+1.8 nm, and 6.4+1.4 nm in experiments with M:HDA ratios of 1:15, 1:30, and 1:45 respectively. Histograms obtained corresponding to the experiments with M:HDA ratios of 1:15 and 1:45 are shown in Figures 13a and 13b. Although, no regular trend in the final particle sizes was noted, analysis of bright-field images recorded at different stages reveal that the samples become more polydisperse upon increasing the concentration of HDA.


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This is expected as faster reaction dynamics in the case of M:HDA ratio of 1:45 induces a variable size modification of particles, leading to increased polydispersity.

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Figure 12 (a). Bright-field TEM images obtained at different stages of co-digestive ripening of Au and Ag nanoparticles; conditions: Au: Ag 1:1, M: HDA 1:15, T = 165˚C.


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Figure 12 (b). Bright-field TEM images obtained at different stages of co-digestive ripening of Au and Ag nanoparticles; conditions: Au: Ag 1:1, M: HDA 1:45, T = 165˚C.


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Figure 13 (a). Histograms showing average particle size at different time intervals during codigestive ripening of Au and Ag nanoparticles; conditions: Au:Ag 1:1, M:HDA 1:15, T = 165˚C.


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Figure 13 (b). Histograms showing average particle size at different time intervals during codigestive ripening of Au and Ag nanoparticles; conditions: Au:Ag 1:1, M:HDA 1:45, T = 165˚C. 3.5 Effect of Varying the Molar Ratio of Au and Ag Nanoparticles Au@Ag core-shell nanoparticles could be obtained via co-digestive ripening of Ag and Au nanoparticles mixed in a molar ratio of 1:1 at 165 ˚C as described above. Next, we studied the effect of varying the molar ratio of core and shell metals on co-digestive ripening. A higher


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composition of Ag nanoparticles over Au could either lead to the formation of a thicker shell of Ag on Au core, or, there could be a limitation to further deposition of Ag so that excess Ag remains in the medium. Intrigued by these questions, we conducted the co-digestive ripening experiments by varying the Au: Ag molar ratio as 1:0.5, 1:1, and 1:2 at 165˚C. Metal to amine ratio was chosen to be 1:30 since reasonably rapid kinetics was noted in this case. In a similar manner, aliquots were withdrawn from the reaction medium at different intervals of time and studied by both spectroscopy and microscopy, the details of which are presented in Figures 14 and 15 respectively.

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Figure 14. UV-visible spectra recorded during co-digestive ripening of Au and Ag nanoparticles by using different Au:Ag molar ratios, (a) 1:0.5, and (b) 1:2 On careful observation of the UV-visible spectra, it was noted that when the concentration of the core metal (Au) is greater than that of shell metal (Ag), reflux time duration of even 30 h did not lead to appreciable dampening of the Au SPR band. As expected, with an increase in the concentration of Ag in the reaction medium from 1:0.5 to 1: 2 with respect to gold, formation of a thicker shell of Ag was evident from the dampening of the Au SPR band.


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Also, the evolution of the absorption bands of Au and Ag with time when Au:Ag molar ratio is taken as 1:2 correlates very well with a previously described experiment where M:HDA was 1:45. In this case as well, the absorption spectrum recorded at 30 h showed a highly intense Ag band with almost complete dampening of the Au SPR band. Comparison of spectroscopic data alone evidences the thickening of the shell when Ag composition is increased in relation to that of Au. Shore et al. noted a limiting behavior wherein the thickness of Ag shell could not be increased even on increasing the quantity of AgOAc precursor used.42 This limiting behavior was traced to the differences in the binding of ligand to metal, possible oxidation of the Ag shell, or electronic charging. They noted that with the growth of Ag shell, further coating of Ag becomes energetically less viable. To achieve stoichiometric control and desired product composition, they increased the composition of silver in the reactant. Yang and coworkers obtained Au-Ag@Ag nanoparticles by a low temperature ripening method using Au:Ag ratio as 1:1 whereas, only Au-Ag alloy nanoparticles were obtained using lower Ag to Au ratios.43 They also noted faster ripening when Ag nanoparticles were in excess. Our observation was similar to theirs. Dampening of the Au band on increasing the Ag composition in the absorption spectrum of Au@Ag nanoparticles was also noted by some other groups.39,44,45 Further insight into the mechanism was obtained by analyzing the electron microscopy data (Figure 15 a and b) and the corresponding histograms (Figures 16 a and b).


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Figure 15 (a). Bright-field TEM images obtained at different stages of co-digestive ripening of Au and Ag nanoparticles; conditions: Au: Ag 1:0.5, M: HDA 1:30, T = 165˚C.


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Figure 15 (b). Bright-field TEM images obtained at different stages of co-digestive ripening of Au and Ag nanoparticles; conditions: Au:Ag 1:2, M:HDA 1:30, T = 165˚C. Average particle size in case of experiments with Au:Ag molar ratios of 1:1, 1:0.5, and 1:2 were found to be 6.9 + 1.8 nm, 7.5 + 3.5 nm, and 6.7 + 3.2 nm respectively. These values


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indicate that particle size and polydispersity both are enhanced on either increasing or decreasing the Au to Ag ratio.

Figure 16 (a). Histograms obtained by calculation of average size of nanoparticles at different intervals of time during co-digestive ripening of Au and Ag nanoparticles; conditions: Au:Ag 1:0.5, M:HDA 1:30, T = 165˚C.


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Figure 16 (b). Histograms obtained by calculation of average size of nanoparticles at different intervals of time during co-digestive ripening of Au and Ag nanoparticles; conditions: Au:Ag 1:2, M:HDA 1:30, T = 165˚C.


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3.6 Effect of Temperature One of the important parameters that has a significant influence on the digestive ripening process is temperature. In the case of monometallic nanoparticles, it has been demonstrated that high temperature is essential to provide kinetic assistance for the breakdown and dissolution of particles in order to attain monodispersity. Rate of etching of surface atoms from nanoparticles increases at high temperature. The etched atoms either get re-deposited on the surface of existing nuclei or simply be present within the medium.27 To investigate the effect of temperature on the formation of core-shell nanoparticles, co-digestive ripening experiment was carried out at three different temperatures, 110˚C, 165˚C, and 195˚C in toluene, mesitylene, and t-butyl toluene as solvents. Au and Ag nanoparticles were mixed in a ratio of 1:1 and M:HDA ratio of 1:30 was used. The UV-visible spectra recorded for the reaction conducted at 110˚C and 195˚C at different time intervals have been presented in Figure 17.

(b)

(a)

Figure 17. UV-visible spectra recorded during co-digestive ripening of Au and Ag nanoparticles at (a) 110˚C and (b) 195˚C


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0h

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Figure 18 (a). Bright-field TEM images obtained at different stages of co-digestive ripening of Au and Ag nanoparticles; conditions: Au: Ag 1:1, M:HDA 1:30, T = 110˚C.


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Figure 18 (b). Bright-field TEM images obtained at different stages of co-digestive ripening of Au and Ag nanoparticles; conditions: Au:Ag 1:1, M:HDA 1:30, T=195˚C. As is evident from the spectra, the SPR band corresponding to Ag red-shifted to a greater extent at 195˚C in comparison to that at 110˚C. In fact, we could observe a regular trend in the


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shift of this absorption band position: a least shift in case of toluene (110˚C) followed by mesitylene (165˚C) and then the most in case of t-butyl toluene (195 ˚C). Furthermore, almost complete dampening of Au SPR band was noted on completion of codigestive ripening carried out at 195˚C. This could be attributed to deposition of a thick Ag shell over Au core. However, the absorption band in the higher wavelength region is quite prominent even after prolonged refluxing at 110˚C. Morphological evolution of particles during the ripening carried out at 110˚C and 195˚C is shown in Figure 18. The average particle size calculated by carrying out the co-digestive ripening experiments at 110˚C, 165˚C, and 195˚C are 5.5 + 1.2, 6.9 + 1.8 nm, and 5.3 + 0.9 nm respectively. Highly polydispersed and aggregated regions that could be noted in the TEM samples prepared from an experiment conducted at 110˚C were completely absent in those prepared from an experiment performed at 195˚C. However, prolonged reflux at 110˚C also resulted in an improvement in the size dispersion of particles. Bright-Field TEM images of nanoparticles in Figure 18 (b) evidence highly monodispersed particles when the medium is tbutyl toluene. These uniformly spherically-shaped nanoparticles form a three-dimensional layered arrangement. We also noted that the perfectly spherical particles observed at 6 h attain cuboidal morphology after 12 h of reflux. However, continued refluxing further changes their shape to the energetically favorable spherical form. A certain degree of polydispersity was noted upon refluxing for 24 h leading to observation of particles with average size of 11.1 + 2.1 nm, which further become monodisperse on continued refluxing for 30 h with particles in the size regime of 5.3 + 0.9 nm. These observations were also evident from the analysis of the histograms (Figures 19 a and b) constructed at different time intervals during the co-digestive ripening process.


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Smetana et al. obtained Au-Ag alloy nanoparticles by co-digestive ripening of Au and Ag nanoparticles in t-butyl toluene but we observed core-shell nanoparticles only even upon prolonged reflux at the same temperature.33 This difference in structure could be attributed to a better etching ability of thiol ligand in their case compared to amine in our case which renders efficient interatomic diffusion of Au and Ag resulting in a homogenous nano-alloy formation.


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Figure 19 (a). Histograms obtained by calculation of average size of nanoparticles at different intervals of time during co-digestive ripening of Au and Ag nanoparticles; conditions: Au:Ag 1:1, M:HDA 1:30, T = 110˚C.

Figure 19 (b). Histograms obtained by calculation of average size of nanoparticles at different intervals of time during co-digestive ripening of Au and Ag nanoparticles; conditions: Au:Ag 1:1, M:HDA 1:30, T = 195˚C.


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3.7 Kinetics Study A plot of the absorption maxima of Ag SPR band in the UV-visible spectrum versus time provided the kinetics data of the digestive ripening reaction. This study was conducted with respect to silver SPR band alone; the broad and diffused nature of the Au SPR band precluded the determination of its absorption maximum. For an experiment with Au:Ag composition of 1:1, metal to HDA ratio of 1:30, carried out at 165˚C, detailed kinetics data revealed that two different processes take place during digestive ripening. The two major processes involved in a digestive ripening process are dissolution of large particles to form small atom clusters and deposition of these clusters on any existing nuclei or other small clusters present in the reaction medium. Based on our understanding of the system supported by experimental data, we expect the silver nanoparticles (which undergo much faster ripening compared to gold) to breakdown more rapidly than gold nanoparticles and the etched atoms of silver to deposit on either gold or silver particle eventually. From the analysis of UV-visible spectra recorded during the course of reaction, deposition of Ag atoms on Au nuclei instead of Ag appears to be a more probable scenario as we noted a shift in the Ag SPR band towards higher wavelength along with dampening of Au SPR band. Hence, the process undergoing faster kinetics was traced to etching of nanoclusters from the particle surface, whereas, the slower process, to the deposition of these etched atoms on any existing nuclei (Au/Ag). The plots of absorption maxima vs. time at various temperatures, molar ratios of Au and Ag, and concentration of HDA are shown in Figures 20 a, b, and c respectively. The rate of etching as well as deposition of Ag nanoparticles increased progressively on variation of the reaction temperature from 110˚C to 165˚C. While the etching rates at 110˚C and 165˚C


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were not very different from one another, the reaction carried out at 195˚C exhibited much faster etching.

(a)

(b)

(c) Figure 20. Plot of absorption maxima vs time obtained by variation of (a) temperature, (b) molar ratio of Au and Ag, and (c) concentration of capping agent in co-digestive ripening of Au and Ag nanoparticles. This result corroborates well with the observation of a hybrid absorption band attributed to the core-shell structure in just 2 h of co-digestive ripening at 195˚C. Also, faster rates of deposition of Ag nanoclusters on Au led to a considerable dampening of the Au SPR band at this temperature. Analysis of the kinetics data revealed a greater tendency of deposition of Ag nanoparticles over Au core compared to etching of atoms from the surface in experiments wherein Ag was


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taken in lower concentration compared to that of Au. Reactions in which Au and Ag were taken in equimolar concentrations exhibited rates that are comparable for both etching and deposition processes. On the other hand, doubling the concentration of Ag in relation to Au led to formation of a thick Ag shell over Au core resulting from a faster deposition rate over etching process. A comparative analysis of change in the absorption maximum of Ag SPR band in co-digestive ripening experiment as well as individual ripening experiments under similar experimental conditions has been deposited in the Supporting Information. Kinetics of ripening was found to be very rapid for high M:HDA ratios in case of Au. 27 A similar observation was noted here as well, faster kinetics when the M:HDA ratio was changed from 1:15 to 1:30. On the other hand, metal to HDA ratio of 1:45 resulted in very rapid codigestive ripening wherein both the rate of etching and deposition are comparable. Although a thick shell formation could be expected based on the rates obtained by either carrying out the reaction at 195˚C, or by taking a high concentration of silver, this observation was noted only in the case of M:HDA ratio of 1:45. 3.8 Stability of Core-shell Nanoparticles In order to further establish the composition and stability of the core-shell nanoparticles, a simple chemical test was carried out wherein the bimetallic nanoparticles were subjected to a drop-wise treatment with tetrahydrofuran (THF) containing peroxide.46 The final sample isolated at the end of each experiment was flocculated with dried and degassed ethanol and redispersed in dry THF (devoid of any water/ peroxide molecules). The presence or absence of peroxide in THF was established by treatment with acidic KI solution.


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(b)

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Figure 21. (a) UV-visible spectrum recorded upon addition of wet THF to final sample obtained at 195˚C, (b) change in color of the sample observed after the test, and (c) BFTEM image of the final sample. As anticipated, complete oxidation of Ag shell leading to exposure of Au core was noted in the cases where we observed considerable dampening of the Au band in the Au@Ag core-shell systems. This was evident by a color change of the sample from yellow/brown to pink/violet and further supported by UV-visible spectroscopy (Figure 21 a) in which Ag SPR band disappeared completely to re-evolve the Au SPR band. The PXRD data further showed the presence of silver oxide (see Supporting Information). In all the other experiments where a thin shell of Ag over Au core was present, only a partial oxidation of Ag shell was noted resulting in dampening of the


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Ag SPR band (Figure 22). Data obtained for other samples have been deposited in the Supporting Information. This simple test provided a lot of insight into the core-shell structural aspects in co-digestive ripening process.

(a)

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(c)

Figure 22. (a) UV-visible spectrum recorded upon addition of wet THF to the final sample obtained at 165˚C, (b) change in color of the sample observed after the test, and (c) BFTEM image of the final sample. 3.9 Transformation of Core-shell Nanoparticles into Nano-alloys: Heat Treatment and Photolytic Treatment Fabrication of Au-Ag nano-alloys was accomplished by several methodologies, including laser ablation of bulk alloy, co-reduction of metal ion precursors, laser synthesis method, galvanic replacement reactions, etc.38,47-51 These nanoalloys are of immense research interest due primarily to the possibility of tuning their absorption properties between 400 nm and 500 nm by


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simply varying their composition and size. Smetana and co-workers first demonstrated the synthesis of Au-Ag nanoalloys by digestive ripening.33 This was followed up by few other reports where modified-ripening strategy was adopted to obtain Au-Ag alloy nanoparticles.42,43

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Figure 23. UV-visible spectrum recorded during (a) heat treatment of core-shell nanoparticles at 195˚C and (b) photolytic treatment by exposure to 254 nm UV radiation. In an attempt to transform the Au@Ag core-shell nanoparticles into Au-Ag nano-alloys, we isolated the sample obtained after co-digestive ripening and carried out further heat and photolytic treatments. Refluxing the core-shell nanoparticles obtained at 165˚C at a higher temperature of 195˚C for 36 h did not lead to their transformation into a nano-alloy, which would be evident by a single SPR band in the UV-visible spectrum. Instead, dampening of the SPR band of Au nanoparticles by an intense Ag SPR band was noted (Figure 23 a). To stimulate interatomic diffusion between the core and shell, high energy UV radiation of 254 nm was used. The core-shell nanoparticles that exhibited a hybrid absorption band upon exposure to UV radiation transformed into an alloy which was evident from the single sharp absorption band at 419 nm (Figure 23 b). Although a single SPR band could be observed within 2 h itself, we


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continued the experiment up to 6 h to ensure homogeneous diffusion of atoms of the core and the shell. A color change from brown to canary yellow was noted after 6 h (Figure 24 a). The final sample obtained upon photolysis was highly stable towards precipitation of particles. Characterization of the sample was achieved by point-EDS technique (Figure 24 c), wherein elemental analysis of a typical single nanoparticle at different points showed the presence of both Ag and Au. Furthermore, line scan of the particle showed homogeneous distribution of both the metals (Figure 24 d). We further investigated the core-shell and alloy nanoparticles using X-ray photoelectron spectroscopy (XPS). The XPS spectra (Figure 25) for both the core-shell and alloy nanoparticles confirmed the presence of both Au(0) and Ag(0) in the samples. In case of coreshell nanoparticles, the binding energies of Au 4f7/2 and 4f5/2 peaks were observed to be 84.3 eV and 88.1 eV, and Ag 3d5/2 and 3d3/2 peaks were noted at 368.7 eV and 374.7 eV. However, in the case of alloy nanoparticles, we observed a shift of 1.3 eV and 1.0 eV in the binding energies corresponding to Au 4f7/2 and Ag 3d5/2 peaks with respect to core-shell nanoparticles, which suggests a change in the electronic structure upon transformation of core-shell nanoparticles into nano-alloys. Similarly, all the other Au@Ag core-shell samples were transformed to nanoalloys (see Supporting Information), except in the cases where M:HDA ratio was taken to be 1:15 and the one where the reaction was carried out at 110˚C. It is not clear as to why these samples behave differently but we ascribe it to the instability of the core-shell morphology under UV-light that only partial alloying was attained.


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UV radiation 254 nm

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Figure 24. (a) Change in color of the sample after photolysis, (b) BF-TEM image of the final sample, (c) Point-EDS spectrum analysis of alloy nanoparticle, and (d) Elemental line scan analysis of a single alloy nanoparticle (cyan trace for Au and red trace for Ag).

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Figure 25. XPS spectrum showing Au 4f peak and Ag 3d peak in case of (a) core-shell nanoparticles, and (b) alloy nanoparticles. 3.10 Discussion on the Mechanistic Aspects The absorption properties of plasmonic metal based nanostructures arise from the interaction of the electromagnetic field generated by the surface plasmons. This interaction results in the hybridization of the plasmon energies, which could be visualized by the shifts observed in the UV-visible spectrum.52 A concentric nanoshell consisting of four layers, i.e., a dielectric core,


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metal shell, dielectric spacer layer, and a second metallic shell was analyzed to understand the plasmon hybridization phenomenon.52 It was suggested that the effect of the strength of the interaction between the individual metal shells and that between the sphere and cavity plasmons in combination plays crucial roles in dictating the plasmon response of the structure. Chen et al. incorporated this plasmon hybridization theory to investigate the case of Au@Ag core-shell nanostructures and explained the observed shifts in the absorption band positions.41 The higher wavelength region band corresponding to Au, as observed in the experimental as well as the calculated absorption spectrum of Au@Ag nanostructures, was predicted to be an antibonding mode arising as a result of hybridization of dipolar plasmon modes of core and the bonding dipolar mode of a thin shell. Simulation of the absorption spectrum of core-shell nanoparticles by using Mie theory was also carried out by them. They also noted a red-shift of the lower wavelength band with an increase in thickness of shell. However, when core and shell are in contact with each other, the extent of the peak shift is expected to decrease as a result of charge transfer. The results depicted by the study of the plasmon hybridization model complements well with our experimental observations. A generic trend that was prevalent in all the co-digestive ripening experiments was the progressive shift of silver surface plasmon band towards higher wavelength and concurrent intensification. Periodic analysis of this band position revealed small fluctuations during the reflux process. Since digestive ripening is a dynamic process, formation of small clusters of silver as a result of continuous etching is inevitable. Thus, the lower wavelength band observed corresponding to core-shell structure is anticipated to have contribution from both coreshell as well as the small clusters present in the reaction mixture. Depending on the concentration of these small clusters, shifts in the band position also varies.


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The formation of bimetallic structures of Au and Ag is much expected owing to the relatively similar energies of Au-Au, Ag-Ag, and Au-Ag bonds as well as considering their isomorphous structures.36 Digestive ripening process involves transfer of clusters of metal atoms until minimum surface energy is achieved by tailoring the particles in an optimum size regime. In the absence of sufficient energy to induce homogeneous interatomic diffusion, atoms of one element only get deposited on the surface of particles of another element, thereby leading to the formation of a core-shell structure. As mentioned earlier, Au nanoparticles are more uniform compared to Ag and thus, Ag nanoparticles which undergo faster ripening have a propensity to deposit on Au nuclei to afford Au@Ag nanostructure rather than remaining within the system. However, presence of very few particles with the inverse core-shell structure (Ag@Au core-shell structure) or the existence of stand-alone particles in the medium owing to the dynamic nature of digestive ripening process cannot be completely ruled out. Detailed analysis of variable reaction conditions on co-digestive ripening suggest us that a thick shell of silver on gold nanoparticles is formed either on increasing the concentration of capping agent to as high as 1:45, conducting the reaction at 195˚C, or, by keeping the silver composition double with respect to that of gold. This could be interpreted in a manner that a higher concentration of HDA and a higher temperature would cause the etching of silver clusters to increase, which in turn would affect the quantity of silver deposited on gold. Also, an increased silver composition would favor further deposition of additional silver particles on gold core other than staying dormant in the reaction medium. For achieving a homogeneous mixing of two metal nanoparticles, their diffusion coefficient should be sufficiently larger than that of their bulk counterparts.53,54 Gold and silver metals are miscible in the bulk as well as in the nanosize-regime. Shibata et al. studied the room


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temperature transformation of Au-Ag core shell nanoparticles into an alloy.36 They noted that the diffusion of two metal nanoparticles in a core-shell structure at room temperature is dependent on the core size as well as the total particle size. While smaller core-shell nanoparticles spontaneously get alloyed, interdiffusion in the case of larger particles remains restricted to the core-shell boundary only. This self-diffusion was explained to be a result of both the decrease in the melting point with size decrease as well as due to defects present at the core-shell interface. These imperfections could be a result of the presence of capping agent molecules at the junction during growth of particles. Percolation of the defects from the interface to the surface and the concurrent migration of atoms into the vacancies were predicted to be the driving forces for alloying to occur. Hence, it would only be reasonable to consider similar kind of dislocations to be present in our system as HDA molecules participate actively in the etching and deposition processes. Although room temperature alloying was not observed in our study, formation of alloys with graded composition could be expected. Irradiation with UV light facilitates interatomic diffusion at the core-shell interface resulting in Au-Ag nanoalloys with higher concentration of silver atoms at the surface. This justifies the observation of a single SPR band close to the absorption band corresponding to silver nanoparticles. In the case of formation of an alloy with homogeneously mixed atoms and equal composition of both the metals, a single surface plasmon band is expected to be observed in between the band positions corresponding to Ag and Au. This understanding of the mechanistic aspects involved in the co-digestive ripening of Au and Ag nanoparticles could be effectively extended to other bimetallic systems as well. 4.0 CONCLUSIONS A comprehensive investigation was carried out to get an insight into the mechanism involved in the co-digestive ripening process of gold and silver nanoparticles. Digestive ripening of


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individual metal nanoparticles revealed faster kinetics of ripening in the case of silver nanoparticles compared to gold. These nanoparticles, on being subjected to co-digestive ripening lead to the formation of a core-shell structure, wherein silver forms a thin shell over gold core. Parameters such as concentration of capping agent, molar ratio of constituent metals and temperature influence the core-shell structure formation. Kinetics of the formation of core-shell nanoparticles revealed that both the rate of etching of nanoclusters from the surface as well as rate of re-deposition on existing nuclei influence the structure significantly. A thick shell of silver, which leads to almost complete dampening of the gold surface plasmon band was proposed under reaction conditions which tend to enhance the etching rate. Exposure to high energy UV radiation transformed the as-prepared core-shell nanoparticles into alloys. The synthetic strategy established by this rigorous investigation could be extended to the realization of core-shell nanoparticles of several other metallic systems. In addition, a better understanding of the co-digestive ripening process would prove to be beneficial in realizing desired heterostructures of tailored size and composition. We have demonstrated co-digestive ripening as an exceptional strategy to design core-shell nanostructures in order to cater to desired applications. ASSOCIATED CONTENT Supporting Information Plots of change in full-width at half maximum (FWHM) and absorption maxima for individual ripening of Au and Ag colloids (M:HDA 1:30, 165˚C), XRD pattern showing peaks corresponding to Au@Ag core-shell nanoparticles, Au, and Ag nanoparticles, plots showing change in the absorption maximum of Ag SPR band during co-digestive ripening experiment as well as individual ripening experiments under similar experimental conditions for all the cases,


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UV-visible spectra and BFTEM images corresponding to individual ripening of Ag nanoparticles, UV-visible spectra, PXRD patterns and TEM images corresponding to stability tests conducted with all the samples using wet-THF, UV-visible spectra and TEM images corresponding to all the photolysis experiments, lower magnification TEM images obtained at different time intervals corresponding to co-digestive ripening of Au and Ag nanoparticles under different reaction conditions. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Both authors contributed equally. ACKNOWLEDGMENTS We gratefully acknowledge financial support from the Council of Scientific & Industrial Research, India. We also thank the Indian Institute of Science for funding the procurement of a 200 kV FETEM. We thank Prof. S. Ramakrishnan (Department of Inorganic and Physical Chemistry, IISc) for providing access to the photoreactor. We also thank Mr. Aman Jindal


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(Department of Inorganic and Physical Chemistry, IISc) for helpful discussions regarding the kinetics data study. C. B. thanks IISc for a fellowship.

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TOC Graphic (For Table of contents entry)


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Monodisperse Colloidal Metal Nanoparticles to Core-shell structures

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