Size Modulation of Colloidal Au Nanoparticles via Digestive Ripening

Jul 14, 2014 - Digestive ripening, a postsynthetic treatment of colloidal nanoparticles, is a versatile method to produce monodisperse nanoparticles a...
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Size Modulation of Colloidal Au Nanoparticles via Digestive Ripening in Conjunction with a Solvated Metal Atom Dispersion Method: An Insight Into Mechanism Srilakshmi P. Bhaskar, Megha Vijayan, and Balaji R. Jagirdar* Department of Inorganic & Physical Chemistry, Indian Institute of Science, Bangalore 560012, India S Supporting Information *

ABSTRACT: Digestive ripening, a postsynthetic treatment of colloidal nanoparticles, is a versatile method to produce monodisperse nanoparticles and to prepare various bimetallic nanostructures. The mechanism of this process is largely unknown. Herein, we present a systematic study conducted using Au nanoparticles prepared by a solvated metal atom dispersion method to probe the mechanistic aspects of digestive ripening. In our study, experimental conditions such as concentration of capping agent, reaction time, and temperature, were found to influence the course of the digestive ripening process. Here it is shown that, during digestive ripening under reflux, nanoparticles within an optimum size window are conserved, and surface etching facilitated mass transfer resulted in monodisperse nanoparticles. Overall, digestive ripening can be considered as a kinetically controlled thermodynamic process.

solvent.19 Owing to the broad applicability of digestive ripening, it has been extended to numerous systems at various temperatures with different digestive ripening agents.20−23 Apart from the well-known application of digestive ripening as a postsynthetic method to narrow down the size distribution of a polydisperse colloid, it has also been employed to synthesize different bimetallic nanostructures.24−29 Digestive ripening is believed to involve mass transfer between differently sized particles until an equilibrium state is reached with uniform particles. A number of studies have been conducted for understanding the process in detail. A theoretical model put forth by Hwang and co-workers proposed that the electrostatic energy of charged particles acts as a balancing driving force against the curvature effect. 30 A recent experimental study with Au, Ag, and Pd has shown that the final particle size and size distribution are influenced by the metallic system and digestive ripening agent used.31 This size variation was ascribed to the difference in interaction strength between the metal and the digestive ripening agent, which in turn is governed by hard soft−acid base principles. In other studies, it has been reported that the size of nanoparticles can be focused further by controlling parameters such as ligand interaction strength and temperature.32,33 However, the underlying mechanism of this inverse Ostwald ripening process has not been revealed to date. Moreover, the previous reports on digestive ripening largely dealt with the initial and final stages of this process, whereas the intermediate and extended

1. INTRODUCTION Uniform nanoparticles have been attracting immense scientific interest because the material properties change drastically with size and shape.1−4 These size-dependent properties, arising largely due to quantum confinement and increased surface to volume ratio, contribute to many intriguing applications in diverse fields, such as optoelectronics, biomedicine, and catalysis.5−8 Hence, over the past few years great efforts have been paid to develop synthetic strategies capable of producing monodisperse nanoparticles along with better tunability of size to the desired range.9−12 Several chemical methods of synthesis of monodisperse nanoparticles have been reported based on the concept of separation of nucleation and growth. Methods like thermal decomposition and hot injection are based on this concept wherein discrete homogeneous nucleation is established for a short period of time followed by controlled growth.13,14 Apart from these, monodisperse nanoparticles are also produced via techniques like reduction in reverse micelles,15 polyol process,16 and seed mediated growth.17 In this context, a combination of solvated metal atom dispersion (SMAD)18 and digestive ripening19 is an attractive prospect for synthesis of uniform nanoparticles with high reproducibility and yield with fine control. Other advantages of the SMAD method include easy scaleup, formation of no byproducts, and avoidance of tedious purification processes. The SMAD method is basically a physical method that involves the vaporization of bulk material into atoms followed by the growth of clusters from atoms in low-temperature matrices.18 Digestive ripening is a temperature-induced size modification process that involves the refluxing of polydisperse colloid in the presence of capping agent near the boiling point of the © 2014 American Chemical Society

Received: May 25, 2014 Revised: July 12, 2014 Published: July 14, 2014 18214

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X-ray diffractometer using a Cu Kα source. The samples were prepared by drop-coating the Au colloid on a glass slide and dried under a lamp. The transmission electron microscopy measurements were conducted on a JEOL JEM-2100F field emission transmission electron microscope operating at 200 kV. The samples were prepared on copper grids by placing 1−3 μL of the diluted gold colloid and drying under a lamp. The size distribution of samples during digestive ripening under reflux was determined by measuring at least 500 spherical or nearly spherical particles from different regions of the TEM grid using Sigma Scan Pro and Origin software.

stages of digestive ripening have not been explored. No detailed studies have been conducted on this process at room temperature. Herein, we report our first attempt to systematically study different stages of the digestive ripening process at both room temperature and reflux conditions to shed some more light onto the mechanistic aspects using gold nanoparticles prepared by the SMAD method.

2. EXPERIMENTAL SECTION 2.1. Materials. Gold foil (99.95%) of thickness 0.5 mm was purchased from Arora-Matthey (Kolkata, India). 2-Butanone, dodecylamine (DDA), hexadecylamine (HDA), and octadecylamine (ODA) were purchased from Sigma-Aldrich. Amines were dried and degassed for 12 h at 100 °C, and 2-butanone was dried using K2CO3 as drying agent followed by several freeze−pump−thaw cycles to degas the solvent prior to SMAD synthesis. Mesitylene was purchased from Spectrochem Pvt. Ltd. (Mumbai, India) and used as received. 2.2. Synthesis of Gold−Butanone Nanoparticles by the SMAD Method. Au nanoparticles in butanone solvent were prepared by coevaporating Au foil and butanone in the SMAD reactor, which was kept at 77 K using a liquid nitrogen filled Dewar surrounding the reactor.34 Once the evaporation was complete, the liquid nitrogen Dewar was removed and the frozen matrix of Au atoms and butanone was allowed to warm up to room temperature under argon atmosphere. The colloid was stirred well, siphoned out into a Schlenk tube, and stored under argon. The violet-colored colloid obtained was homogeneously dispersed and stable. 2.3. Synthesis of Amine-Capped Au Nanoparticles. The syntheses of various amine-capped Au nanoparticles were performed in a similar manner as described for Au−butanone nanoparticles. In a typical experiment, the SMAD reactor was loaded with HDA at the bottom, and then, coevaporation and cocondensation of Au and butanone were carried out. The ratio of Au to HDA was maintained as 1:10. After the warm up process and before the siphoning of colloid into a Schlenk tube for storage, the reaction mixture was stirred vigorously with HDA for 1 h under argon. The colloid obtained was wine red in color and stable under argon. The experiment was carried out with DDA and ODA as capping agents following the same procedure. 2.4. Digestive Ripening of Au Nanoparticles. Digestive ripening was performed on Au−butanone nanoparticles with various amines as capping agents, added externally after the SMAD synthesis. In a typical experiment, HDA was added in a Au:HDA molar ratio of 1:10 into 10 mL of Au−butanone colloid taken in a round-bottomed flask. Butanone was removed in vacuo and the residue was redispersed in 10 mL of mesitylene. The colloid was stirred well at room temperature for 1 h followed by heating with constant stirring in a preheated oil bath maintained at ∼165 °C. Refluxing was monitored for 30 h both spectroscopically and microscopically by withdrawing aliquots from the reaction mixture at certain time intervals. Digestive ripening reactions were also carried out with different Au to HDA ratios (1:1, 1:20, and 1:30) to study the effect of concentration of capping agent, and a similar set of experiments was conducted replacing HDA with DDA and ODA as capping agents to understand the effect of chain length. 2.5. Instrumentation. The UV−visible absorption spectra were collected using a PerkinElmer Scan Lambda 750 UV− visible spectrometer in the range 400−850 nm. X-ray diffraction measurements were carried out using a PANalytical Empyrean

3. RESULTS AND DISCUSSION Au nanoparticles were prepared by the SMAD method in butanone solvent. The colloid obtained was violet in color and showed remarkable stability toward precipitation of particles, even in the absence of any capping agent. The detailed characterization of this as-prepared colloid was carried out and is shown in Figure 1.

Figure 1. Au−butanone colloid: (a) UV−visible spectrum, (b) PXRD pattern, (c) bright-field (BF) TEM image, and (d) high-resolution (HR) TEM image with SAED pattern (inset).

As indicated by the violet color, a broad band around 540 nm was observed in the UV−visible spectrum, which is due to the surface plasmon resonance of gold nanoparticles. The structural characterization conducted with powder XRD confirmed the presence of the face-centered cubic phase of Au(0). The morphological analysis performed using TEM showed a large extent of aggregation among the particles, and the majority of the particles were nonspherical with random shapes. The extreme polydispersity in size and shape could be attributed to an uncontrolled growth that occurred during the synthesis, since the synthetic scheme was devoid of any capping agent. Consequently, we could perform a systematic study to understand the role of capping agent exclusively on the digestive ripening process with the as-prepared Au−butanone as starting material. Digestive ripening of Au−butanone colloid was carried out with different long chain primary amines as digestive ripening agents, and mesitylene was used as refluxing medium following the procedure given in Scheme 1. Mesitylene was chosen as solvent to provide a high temperature for digestive ripening, and that was expected to facilitate the breakdown of particles. 18215

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Scheme 1. Reaction Scheme of Digestive Ripening Process

Figure 2. UV−vis spectra recorded with time of refluxing with various amines taken in different concentrations.

correlate the change in absorption spectra to the change in particle size, shape, and/or interparticle distance. When refluxing was performed with lower amine concentration (Au:HDA ratio of 1:1), precipitation of particles occurred largely during refluxing, leading to incomplete digestive ripening process (Supporting Information). When the Au:HDA ratio was increased to 1:10, we could observe precipitation of particles during the initial period of refluxing, but it was negligible compared to that for the Au:HDA ratio of 1:1. Hence we optimized the minimum Au:HDA ratio as 1:10 for effective digestive ripening to occur, preventing the precipitation of particles during refluxing. The UV−vis spectra recorded for different time intervals for different ratios of Au to amine such as 1:10, 1:20, and 1:30 are shown in Figure 2.

In all cases, when butanone was replaced with mesitylene, the color of colloids changed from violet to blue, and further, in the reaction mixture we could observe a color change from blue to pinkish violet immediately upon refluxing. This color gradually turned pink and finally to wine red. Careful spectroscopic and microscopic analysis of different stages of digestive ripening was carried out by withdrawing aliquots at certain time intervals; it is discussed in detail in the following sections. 3.1. Effect of the Chain Length of the Capping Agent. UV−vis absorption spectroscopy is a prominent tool to study the progress of reactions involving plasmonic nanoparticles. According to Mie theory, the absorption spectra of colloidal nanoparticles largely depend on the size and shape of particles, interparticle distance, and dielectric constant of the surrounding medium.35−37 Hence, for a given refluxing reaction, we can 18216

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As is evident from the UV−vis spectra, in all the cases the reaction mixture before refluxing, indicated as 0 h, is characterized by a broad SPR band with low intensity. With time of refluxing, the SPR band arising from gold nanoparticles became sharper with increased intensity, except for reactions wherein DDA was employed as digestive ripening agent. In those cases, continuous precipitation of particles was observed during the experiment and resulted in decreased intensity of the SPR band. In the case of the Au:DDA ratio of 1:10, complete precipitation of particles occurred within 3 h of refluxing, resulting in a colorless supernatant solution that corresponds to no SPR band in the absorption spectrum. However, digestive ripening occurred for those particles in solution with Au:DDA ratios of 1:20 and 1:30, which is indicated by a gradual color change from blue to red. The TEM analysis of these solutions showed the presence of spherical particles with an average size in the range 10−12 nm (Supporting Information). To have a comparison with DDA, it is to be noted here that the average sizes of spherical particles obtained with HDA and ODA as capping agents were found to be in the range 7.5−9.5 and 10− 13 nm, respectively. The relatively ineffective digestive ripening observed in case of DDA in comparison with ODA and HDA could be due to the difference in chain length. It is well-known that, for hydrophobic colloids in nonpolar solvents, the longrange van der Waals force of attraction between the particles largely determines the colloidal stability.14,38 Here the van der Waals forces of attraction between particles would be larger than the steric barrier provided by DDA capping, leading to the aggregation of particles, which in turn resulted in the precipitation of larger ones. Hence, it is necessary to use a longer alkyl chain capping agent for effective digestive ripening, depending on the degree of polydispersity and size of the particles. Prasad et al. reported a trend of increasing mean particle size with increasing alkyl chain length when different long chain alkyl thiols were used as digestive ripening agents.38 They attributed this trend to the preference of long chain capping agent toward a less curved surface so that it can be oriented in such a way so as to attain a state of minimum free energy. A similar study with different long chain alkylamines showed that the average particle size varies with the chain length, but a trend of increasing size with chain length was not observed.39 Hence, it was proposed that a balance of two opposing factors, such as curvature-dependent surface energy and ligand−metal binding energy, plays a key role in determining the final size.39,40 In our study also we found a certain size range for a particular metal−ligand combination, and one could focus the size by controlling polydispersity via digestive ripening. 3.2. Kinetics of Digestive Ripening. A kinetic study with respect to concentration of digestive ripening agent was conducted by comparing the change in full width half-maxima (fwhm) and absorption maxima of SPR band with time of refluxing, obtained from the UV−vis spectra recorded at certain time intervals for each Au:amine ratio. From the plot of fwhm vs time shown in Figure 3, it is clear that the kinetics of digestive ripening showed a strong dependency on the concentration of the digestive ripening agent. The change of fwhm showed the fastest and steepest decrease in the case of the Au:amine ratio of 1:30 with both HDA and ODA; this could be ascribed to a rapid decrease of polydispersity of nanoparticles during refluxing. In contrast, fwhm decreased steadily but was slowest in the case of a Au:amine ratio of 1:10, and an intermediate rate was observed

Figure 3. Plot of fwhm vs time of refluxing for various Au:amine ratios.

for a Au:amine ratio of 1:20. Apart from the rapid kinetics, we could also predict a better size distribution from these plots upon increasing the concentration of digestive ripening agent, since there is a lowering of fwhm for a certain time of refluxing. A similar trend in kinetics was noted in the plot of absorption maximum vs time, which is shown in Figure 4. From the plot, it

Figure 4. Plot of absorption maximum vs time of refluxing for various Au:amine ratios.

could be inferred that the position and the shape of the SPR band got stabilized within a shorter refluxing period with an increase in the concentration of the digestive ripening agent. Thus, the concentration of capping agent has a determining role in the kinetics of digestive ripening. With an increasing concentration of capping agent, there would be more amine molecules to interact with the nanoparticle surface to facilitate the digestive ripening process, which is believed to involve mass transfer among the differently sized particles. 3.3. Effect of Room Temperature Stirring. As per the reaction procedure given in Scheme 1, Au nanoparticles were subjected to 1 h of vigorous room temperature stirring with capping agent under argon atmosphere to ensure a better interaction with the nanoparticle surface. The morphological analysis of thus obtained colloids for various Au:HDA ratios is shown in Figure 5. As is evident from the BF TEM images, for all three Au:HDA ratios, the colloids remained polydisperse, with randomly distributed particles having different size and morphology. In addition to that, particles seem to be largely agglomerated with poor interparticle separation. We calculated the size and size distribution of nearly spherical particles present in the colloids from the TEM images and found that there is a decrease in the average particle size with an increase in the concentration of HDA after room temperature stirring. In case of a Au:HDA ratio of 1:10, the average particle size was found to be 9.2 ± 2.2 nm, whereas those obtained for Au:HDA ratios of 1:20 and 1:30 were 8.7 ± 1.9 and 7.9 ± 2.2 nm, respectively. Though there was no noticeable variation in the size distribution with a change in concentration of HDA, a greater number of spherical particles and much better separated regions were well-evident in the TEM images analyzed from different regions of the TEM 18217

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From the TEM images we could presume qualitatively that there is an increment in the number of such particles with an increase in the concentration of amines. Taking all the points discussed so far into account, it can be concluded that digestive ripening of nanoparticles does take place, although not completely, at room temperature by stirring with capping agent. Hence, kinetic assistance via refluxing the colloid is essential in this case to render the process to achieve monodispersity. It is to be noted in this context that synthesis of monodisperse nanoparticles via simple room temperature stirring with HDA during the SMAD process for certain metals like Mg,41 Ca,42 Al,43 and Co44 was reported by our group. The distinct role of HDA in this ripening process is not very clear at this moment. To investigate the ability of amines in the room temperature ripening process further, we performed the synthesis of Au nanoparticles using the SMAD method in the presence of various amines as capping agents taken within the SMAD reactor, as discussed in detail below. 3.4. Room Temperature Ripening during SMAD Synthesis. In this case, coevaporation and cocondensation of Au and butanone were carried out in the presence of HDA taken in the molar ratio of 1:10 (Au:HDA) placed at the bottom of the reactor. Thus, the formed frozen matrix of evaporated atoms and solvent was allowed to melt down, warmed up to room temperature, and subsequently subjected to rigorous stirring with HDA for 1 h under argon atmosphere. This colloid was wine red in color and homogeneously dispersed. It showed a sharp absorption band at 524 nm in the UV−vis spectrum, as shown in Figure 7. The TEM

Figure 5. BF TEM images of Au−mesitylene−HDA colloid after 1 h room temperature stirring with various Au:HDA ratios: (a) 1:10, (b) 1:20, and (c) 1:30.

grids. Again it should be noted that the size and size distribution mentioned here represent only those particles with at least nearly spherical morphology and were present only in minor region of the polydisperse colloids. Hence, the distribution given above does not represent the degree of polydispersity of the entire colloid. However, the greater number of spherical particles and separated regions suggest that there is an improvement of size dispersion with an increase in amine concentration. This is further supported by the analysis of SPR absorption arising from the whole colloid after 1 h stirring at room temperature. From the plot of fwhm vs time given in Figure 3, it is clear that there is a decrease in the fwhm value of SPR band with an increase in HDA concentration. Moreover, a distinct blue shift in the absorption maxima was also observed with an increase in amine concentration, as shown in Figure 4, which suggests a decrease in the size of particles. With ODA as digestive ripening agent, we did not note any significant concentration dependency on room temperature stirring. The average particle sizes obtained from TEM images as well as spectroscopic data obtained were comparable for all three Au:ODA ratios. Another impact of room temperature stirring was the appearance of extremely small dot particles of size less than 2 nm in the TEM images obtained after stirring with digestive ripening agent. TEM images with dot particles are shown in Figure 6 with a Au:HDA ratio of 1:30. Such dot particles were formed at all Au to capping agent ratios. The absence of dot particles in Au−butanone colloid confirms that those were generated only because of the interaction of capping agent on the nanoparticle surface, leading to slow dissolution of small clusters from the surface.

Figure 7. Comparison of UV−vis absorption spectra of Au−butanone nanoparticles with (a) HDA added externally after SMAD synthesis and (b) HDA added internally during SMAD synthesis.

characterization of the as-prepared colloid shown in Figure 8 revealed the presence of monodisperse spherical nanoparticles with a size of 5.2 ± 0.7 nm. This confirms that ripening of

Figure 6. BF TEM images of Au−HDA−mesitylene colloid after stirring at room temperature for 1 h.

Figure 8. Au−HDA−butanone colloid: (a) BF TEM image, (b) histogram showing size distribution, and (c) HR TEM image. 18218

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size 9.0 ± 1.2 nm. When the synthesis was carried out with ODA as capping agent, we obtained a colloid that showed an absorption band at 524 nm in the UV−vis spectrum. The morphology of nanoparticles as revealed from TEM images was nearly spherical particles having an average size of 6.4 ± 1.3 nm. In the SMAD method of synthesis, during the melting and warm up stages of evaporated atoms embedded in frozen matrix of solvent, the nucleation of nanoparticles gets initiated. This is followed by the growth stage wherein these nuclei would tend to grow fast to minimize the surface energy.45 At this stage, if capping agent is available, it can come into contact with small clusters of nanoparticles and attempt to stabilize them. This will provide better control over random growth processes, depending on the extent of stabilization provided by the capping agent. This explains the observation of small particle size when capping agent was present during SMAD synthesis compared to bigger size obtained in postsynthetic addition of amine followed by digestive ripening under reflux. 3.5. Digestive Ripening under Reflux with HDA. When HDA was used as digestive ripening agent, monodisperse nanoparticles were produced from the starting polydisperse colloid under reflux condition in all the Au:HDA ratios (1:10, 1:20, and 1:30). A detailed microscopic characterization was conducted using TEM for Au−mesitylene−HDA colloid for certain time intervals during refluxing, and representative TEM images obtained for a Au:HDA ratio of 1:10 is shown in Figure 10.

nanoparticles took place to give uniform size via simple stirring with HDA at room temperature. When compared with Au−butanone colloid, there is a prominent blue shift of 16 nm in the UV−vis spectrum of asprepared HDA-capped nanoparticles. To understand the effect of adding HDA after SMAD synthesis, we carried out another experiment wherein we added HDA in the same molar ratio (1:10) to the as-prepared Au−butanone colloid and kept vigorously stirring for 1 h at room temperature under Ar. There was no apparent change in the color or dispersity of the colloid throughout this period. The UV−vis spectrum was recorded after 1 h of stirring and compared with that of Au−butanone as shown in Figure 7. Since the SPR absorption band is highly sensitive, especially to the change in size and shape of nanoparticles, it is inferred from the UV−vis spectra that HDA could not make any interaction with Au nanoparticles. One possible reason could be that the nanoparticle surface is inaccessible for the capping agent because of the initial stabilization provided by butanone. Once butanone was removed in the presence of capping agent and redispersed in mesitylene, digestive ripening got initiated in 1 h, even for room temperature stirring, as discussed above. This points out that choosing suitable solvent is an important parameter for enhancing the digestive ripening ability of the digestive ripening agent. Coming back to the ripening of nanoparticles occurring during SMAD synthesis, we repeated similar experiments in the presence of DDA and ODA taken at the same Au:amine molar ratio of 1:10. The siphoned colloids, after 1 h room temperature stirring with DDA and ODA, were dark red and wine red in color, respectively, and were homogeneously dispersed. Detailed spectroscopic and morphological characterizations conducted for these as-prepared colloids are given in Figure 9. With DDA as capping agent, Au nanoparticles showed

Figure 10. BF TEM images showing gradual conversion of polydisperse colloid to monodisperse with the corresponding HR TEM image in the inset (scale bar 5 nm in all the HR TEM images).

Figure 9. (a) UV−vis spectrum, (b) BF TEM image with HR TEM inset of Au−DDA−butanone colloid, (c) UV−vis spectrum, (d) BF TEM image with HR TEM inset of Au−ODA−butanone colloid.

As discussed earlier, the colloid after stirring for 1 h at room temperature remained polydisperse with mainly agglomerated particles with no definite shape. Two distinct changes were apparent at first glance from the BF TEM images analyzed from different regions of the TEM grid, acquired for different times of refluxing: (a) the gradual transformation of agglomerated regions into well-separated particles and (b) the conversion of

a sharp SPR absorption at 521 nm. BF TEM images evidenced well-separated particles with only spherical morphology without any aggregated region. In particle size analysis, we observed a bimodal size distribution and showed size segregated regions of monodisperse spherical particles with a size of 5.9 ± 0.8 nm along with slightly larger monodisperse spherical particles of 18219

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nonspherical particles into nearly spherical morphology. Overall there was an increase in the number of spherical particles with uniform size with a progressive time of refluxing. The preference of spherical or nearly spherical polyhedral shape over other morphologies during digestive ripening might be arising because of the maximum surface available for capping for a given size, so that more capping agent molecules could attach to the nanoparticle surface, which in turn decreases the overall energy. To gain more insight into the process, we measured the particle size of nearly spherical particles, and histograms were constructed to get the trend in size distribution with refluxing time. It is important again to note that the histograms shown in Figure 11 do not represent the size distribution of nonspherical as well as agglomerated regions present in the images.

Figure 12. BF TEM images at different time intervals of digestive ripening (Au:HDA 1:20).

Figure 13. Histograms showing particle size distribution during digestive ripening (Au:HDA 1:20). Figure 11. Histograms showing the trend of size distribution of nearly spherical particles during digestive ripening (Au:HDA 1:10).

out on an aliquot withdrawn at 10 h, we observed spherical particles of average size 8.5 ± 2.3 nm. When compared to 5 h, a slight decrease in the average particle size was apparent at 10 h, but the partial presence of nonspherical and agglomerated particles was also noted. So it can be inferred from spectral and microscopic data that digestive ripening got completed in 5 h. Images obtained for the 20 h sample resembles that of 10 h with a slight increase in the average particle size. When an aliquot was imaged for a sample after 30 h of reflux, we could observe well-separated spherical particles without any agglomerated and nonspherical particles. They tend to form a shortrange 2D arrangement throughout (Figure 12). To gain more insight into the effect of concentration against refluxing time, we compared the spectral and microscopic data obtained for different time intervals of refluxing with a Au:HDA ratio of 1:30. The UV−vis spectrum of colloid before reflux was characterized by a broad band around 535 nm, and immediately after 10 min of reflux, it got shifted to 530 nm. Since fast kinetics of digestive ripening was evident from the UV−vis spectra, we obtained the TEM images for samples within 10 min of refluxing wherein we noted monodisperse nanoparticles with an average size of 7.9 ± 1.7 nm along with negligible agglomerated and nonspherical particles. TEM images obtained for certain time intervals and corresponding histograms are shown in Figures 14 and 15, respectively. After 1 h and until 5 h of refluxing, a notable change in UV− vis spectra in terms of fwhm and SPR band was not apparent. When imaging was done for samples at 1 and 3 h, we noted that the majority of the particles were separated spherical particles throughout the grid. Compared to the sample at 10 min, there was a slight increase in the average particle size at 1 h, which was found to be 8.3 ± 2.1 nm. However, with further refluxing, at 3 h the particle size decreased to 7.9 ± 1.7 nm. When refluxing was continued we observed an increase in

TEM images obtained after up to 5 h of refluxing showed the coexistence of both separated spherical particles and agglomerated and nonspherical particles. However, after 10 h the presence of well-separated spherical particles with an average size of 9.0 ± 2.1 nm was observed. There was no apparent change in the sample on further refluxing up to 30 h. These results are consistent with the spectral data showing a distinct stepwise blue shift in the absorption maximum from 557 to 533 nm after 10 h of refluxing and remained with no further shift thereafter. 3.6. Effect of Prolonged Refluxing vs Concentration of Amine. As discussed above, when refluxing was continued for 30 h after observing monodisperse nanoparticles within 10 h of reflux with a Au:HDA ratio of 1:10, there were no significant changes in spectral data as well as in particle size and size dispersion obtained from TEM images. However, when digestive ripening was carried out with higher amine concentration, refluxing time was found to affect the dispersion of nanoparticles. With a Au:HDA ratio of 1:20, BF TEM images obtained at different time intervals of refluxing and corresponding histograms are shown in Figures 12 and 13, respectively. With a Au: HDA ratio of 1:20, the UV−vis spectrum recorded before refluxing showed a broad band around 548 nm. It got blue-shifted to 533 nm within 5 h of refluxing, and the corresponding TEM image also revealed the presence of separated spherical nanoparticles with an average size of 9.0 ± 2.2 nm throughout the grid along with slightly agglomerated and nonspherical particles. When refluxing was continued up to 10 h, the SPR band slightly shifted to 534 nm and got stabilized thereafter. The corresponding change in fwhm was also found to be less after 10 h. When morphological analysis was carried 18220

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and the reversible nature of the dispersion of size and shape, we can think of digestive ripening as progressing toward a dynamic equilibrium state, and mass transfer at this stage is facilitated via a surface etching process. The role of dot particles generated via surface etching in a mass transfer process, is discussed later. 3.7. Digestive Ripening with ODA. Similar to HDA, digestive ripening was carried out with ODA as capping agent in three Au:ODA ratios, 1:10, 1:20, and 1:30, and a detailed morphological analysis was conducted for samples up to 30 h of refluxing. TEM images obtained with a Au:ODA ratio of 1:10 and histograms showing size distribution are shown in Figure 17. From the UV−vis spectrum as well as TEM image obtained before refluxing it was apparent that the colloid is polydisperse in nature. The BF TEM image particularly showed that particles are highly elongated in shape. Upon refluxing, a broad SPR band around 552 nm was observed with a Au:ODA ratio of 1:10, which became sharper with an absorption maximum at 535 nm in 3 h of refluxing. The corresponding TEM image is characterized with well-separated particles of less spherical morphology without any agglomerated regions. When refluxing was continued further, there was no significant change in the SPR band position; however, there was a considerable decrease in fwhm for samples up to 10 h. We also observed a decrease in the average particle size and size distribution; moreover, the morphology of the particles became more spherical. After 10 h of refluxing, significant changes were not seen in the BF TEM images. Monitoring of digestive ripening using TEM for a Au:ODA ratio of 1:20 is shown in Figure 18 and the corresponding histograms in Figure 19. With a Au:ODA ratio of 1:20, the initial broad band around 547 nm blue-shifted to 535 nm within 1 h of reflux, and the TEM images evidenced the transformation of elongated particles into well-separated, nearly spherical particles. The effect of prolonged refluxing was not well-pronounced in the case of ODA as capping agent compared to that in the case of HDA. However, we could observe a slight variation in the particle size similar to that in the case of HDA on further refluxing. Though we observed the presence of polyhedral particles with no definite shape in some regions, such particles were negligible compared to the polydispersity observed in the case of HDA. When the Au:ODA ratio was raised further to 1:30, 10 min of refluxing resulted in uniform particles that were well-separated, and the UV−vis spectra showed a sudden shift from 548 to 532 nm. After 1 h of refluxing, the change in fwhm was less and the SPR band gradually got shifted to 535 nm after 30 h of refluxing. Correspondingly, a slight increase in average particle size was also observed and found to be 12.6 ± 1.7 nm. The TEM images at certain time intervals and the corresponding histograms are shown in Figures 20 and 21, respectively. It is obvious from all the histograms constructed for digestive ripening with HDA and ODA that the average particle size remained more or less in the same size regime throughout the digestive ripening period. Subsequently, it can be concluded that there is an equilibrium size range preferred and stabilized for a particular metal-capping agent under a given set of experimental conditions. Hence during digestive ripening, a change in size occurred only to those particles that are outside this preferred size window along with a conversion of nonspherical morphology to nearly spherical. In summary, digestive ripening resulted in the modification of size, shape, and separation of particles away from the equilibrium

Figure 14. BF TEM images at different time intervals of digestive ripening (Au:HDA 1:30).

Figure 15. Histograms showing size distribution during digestive ripening (Au:HDA 1:30).

polydispesity of the colloid until 20 h. It should be noted that the histograms represent the size distribution of spherical particles present and an increase in polydispersity is contributed largely by particles with random shapes. TEM images showing the coexistence of spherical as well as nonspherical particles imaged after 10 and 20 h of refluxing are shown in Figure 16.

Figure 16. BF TEM images showing polydisperse nanoparticles with a Au:HDA ratio of 1:30 on prolonged refluxing.

On further refluxing at 30 h, TEM images showed spherical particles of size 8.8 ± 1.9 nm in majority, and a considerable decrease in polydispersity was also observed. The TEM data are consistent with UV−vis absorption spectral data wherein the SPR band started to broaden at 10 h of refluxing and there was a decrease in fwhm after 30 h of reflux. In summary, results discussed for samples refluxed for 30 h with different Au:HDA ratio point out that prolonged refluxing after attaining monodispersity has the adverse effect of rendering the sample again polydisperse under a high concentration of amines. However, due to the reversible nature of digestive ripening, continuation of refluxing can transform the polydisperse sample back to monodisperse. Hence, an optimum balance of digestive ripening time and concentration of digestive ripening agent is essential to attain the minimum size distribution. Considering the slight variation in average particle size of spherical particles 18221

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Figure 17. BF TEM images and corresponding histograms obtained at different time intervals during digestive ripening with a Au:ODA ratio of 1:10.

Figure 18. BF TEM images at different time intervals of digestive ripening (Au:ODA 1:20).

Figure 21. Histograms showing size distribution during digestive ripening (Au:ODA 1:30).

conditions. Though there was slight variation in average particle size on refluxing after reaching monodispersity, it remained within the optimum size window preferred for each metal capping agent system. One possible reason for this observed variation of particle size could be the etching of the nanoparticle surface with capping agent. 3.8. Surface Etching of Nanoparticles. In Figure 6 is shown the presence of extremely small particles (dot particles) in the TEM images taken after 1 h of room temperature stirring with HDA. Under refluxing condition we could see a tremendous increase in the number of such particles and these further increased with an increase in the Au to amine ratio. Figure 22 shows the presence of a large number of dot particles observed in the sample after 30 h of refluxing in the case of a Au:HDA ratio of 1:30. It is to be noted that the dot particles were observed with a Au:amine ratio of 1:10 and 1:20 as well. The appearance of such particles in significant numbers was quite random and suggests that transformation is taking place to such particles within a short period of time. The observed slight variation in average particle size after attaining monodispersity on continuing refluxing could be ascribed to the effect of surface etching. One possibility is that the etched particles can deposit directly over existing particles, resulting in an increase in average particle size. On the other hand, further etching can result in a slight decrease of particle size. This process can continue as long as a dynamic equilibrium is maintained, and the effect is more pronounced when the rate of etching is high.

Figure 19. Histograms of particle size distribution during digestive ripening (Au:ODA 1:20).

Figure 20. BF TEM images at different time intervals of digestive ripening (Au:ODA 1:30).

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Scheme 2. Schematic Showing the Role of Etched Particles in the Digestive Ripening Process

Figure 22. BF TEM image of a sample after 30 h of reflux with HDA as capping agent showing dot particles (lighter contrast).

In our experiments, temperature and amine concentration were found to increase the etching rate. Also with ODA as digestive ripening agent, we observed dot particles in all the ratios at random time intervals, and the representative images in the case of a Au:ODA ratio of 1:30 are shown in Figure 23. According to the LaMer plot, homogeneous nucleation can occur when the solution is supersaturated with monomer concentration.46−48 On the other hand, when concentration of monomer is less, it can precipitate over existing nuclei, resembling heterogeneous nucleation, since it is more energetically favorable. Here it is reasonable to consider small clusters of Au dissolved in solution as monomers, and once the dissolution rate is fast enough to reach supersaturation, homogeneous nucleation occurs to generate dot particles. Dot particles would grow with one another to decrease the surface energy, and capping agent can direct the growth to equilibrium size. However, if the rate of dissolution is slow, redisposition of Au clusters over existing nanoparticles can be expected. So surface etching will assist the process to reach equilibrium, but extended refluxing under increased etching rate can increase polydispersity shortly, as we showed with a Au:HDA ratio of 1:30.

Figure 23. Dot particles observed with ODA as capping agent.

3.9. Insight into the Mechanism. On the basis of the experimental evidence discussed above, some reaction pathways likely to be involved in digestive ripening process are discussed below. Digestive ripening involves mass transfer among particles that is facilitated only with the interaction of the capping agent with the nanoparticles under suitable experimental conditions. Different factors such as digestive ripening agent, solvent, temperature, reaction time, and concentration of reactants are found to affect the course of the digestive ripening process drastically. In this particular case, digestive ripening is an extremely slow process at room temperature and kinetic assistance is required to carry out the process within reasonable time. When butanone is removed in the presence of capping agent, two opposing factors begin to operate: (a) rapid aggregation of nanoparticles due to the removal of the stabilization provided by butanone molecules and (b) amine molecules tend to cap the nanoparticle surface, since capped nanoparticles are energetically more favorable than bare nanoparticles. The capping agent’s continuous efforts to increase the surface area either by breaking the particles or by increasing the curvature would initiate the digestive ripening process, where the driving force for further steps would be the minimum energy that can be achieved with particles of optimum size. This is validated by the experimental evidence showing the stabilization of a certain size range for a particular metal-capping agent combination in a given experimental condition. Hence, during digestive ripening the nanoparticles in the optimum size regime are almost preserved, whereas modification in size and shape occurs to the rest. Mass transfer is further facilitated by dissolving small clusters of Au into solution via etching the surface by capping agent. The outcome of surface etching is schematically illustrated in Scheme 2.

4. CONCLUSIONS In this work, we conducted a systematic study of digestive ripening to reveal the mechanistic aspects using Au nanoparticles prepared by the SMAD method, in the presence of alkyl amines of different chain length as capping agents. We experimentally showed that, during digestive ripening, capping agent urges the system toward an equilibrium size of lowest free energy which is characteristic of a particular metal-capping agent combination under a given set of experimental conditions. The conservation of equilibrium sized nanoparticles with least modification until digestive ripening was accomplished is a new insight. Prolonged refluxing under conditions favoring surface etching has adverse effect on monodispersity. In our study, we proposed that the point of equilibrium is achieved by operating two simultaneous processes. First is the digestion of bigger particles, which is achieved either by a breakdown process in which nonspherical particles are converted into spherical particles or by dissolution of small clusters from the surface of nanoparticles. Second is the ripening process in which either dissolved clusters precipitate as nuclei and transform to equilibrium size by rapidly growing with one another or dissolved clusters redeposit over existing particles, which would lead to the minute growth of such particles. The favorable pathway of ripening depends on the experimental conditions employed to achieve monodispersity. 18223

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(12) Yang, Y.; Yan, Y.; Wang, W.; Li, J. Precise Size Control of Hydrophobic Gold Nanoparticles Using Cooperative Effect of Refluxing Ripening and Seeding Growth. J. Nanotechnology 2008, 19, 175603−175612. (13) Hyeon, T.; Lee, S. S.; Park, J.; Chung, Y.; Na, H. B. Synthesis of Highly Crystalline and Monodisperse Maghemite Nanocrystallites without a Size-Selection Process. J. Am. Chem. Soc. 2001, 123, 12798− 12801. (14) Murray, C. B.; Norris, D. J.; Bawendi, M. G. Synthesis and Characterization of Nearly Monodisperse CdE (E = S, Se, Te) Semiconductor Nanocrystallites. J. Am. Chem. Soc. 1993, 115, 8706− 8715. (15) Tanori, J.; Pileni, M. P. Control of the Shape of Copper Metallic Particles by Using a Colloidal System as Template. Langmuir 1997, 13, 639−646. (16) Teranishi, T.; Miyake, M. Size Control of Palladium Nanoparticles and Their Crystal Structures. Chem. Mater. 1998, 10, 594−600. (17) Wilcoxon, J. P.; Provencio, P. P. Heterogeneous Growth of Metal Clusters from Solutions of Seed Nanoparticles. J. Am. Chem. Soc. 2004, 126, 6402−6408. (18) Klabunde, K. J.; Timms, P. L.; Skell, P. S.; Ittel, S. Introduction to Metal Atom Syntheses. Inorg. Synth. 1979, 19, 59−86. (19) Lin, X. M.; Sorensen, C. M.; Klabunde, K. J. Digestive Ripening, Nanophase Segregation and Superlattice Formation in Gold Nanocrystal Colloids. J. Nanopart. Res. 2000, 2, 157−164. (20) Prasad, B. L. V.; Stoeva, S. I.; Sorensen, C. M.; Klabunde, K. J. Digestive-Ripening Agents for Gold Nanoparticles: Alternatives to Thiols. Chem. Mater. 2003, 15, 935−942. (21) Shankar, R.; Wu, B. B.; Bigioni, T. P. Wet Chemical Synthesis of Monodisperse Colloidal Silver Nanocrystals Using Digestive Ripening. J. Phys. Chem. C 2010, 114, 15916−15923. (22) Cingarapu, S.; Yang, Z.; Sorensen, C. M.; Klabunde, K. J. Synthesis of Indium Nanoparticles: Digestive Ripening under Mild Conditions. Inorg. Chem. 2011, 50, 5000−5005. (23) Cingarapu, S.; Yang, Z.; Sorensen, C. M.; Klabunde, K. Synthesis of CdSe Quantum Dots by Evaporation of Bulk CdSe Using SMAD and Digestive Ripening Processes. Chem. Mater. 2009, 2, 1248−1252. (24) Kalidindi, S. B.; Jagirdar, B. R. Synthesis of Cu@ZnO Core− Shell Nanocomposite through Digestive Ripening of Cu and Zn Nanoparticles. J. Phys. Chem. C 2008, 112, 4042−4048. (25) Smetana, A. B.; Klabunde, K. J.; Sorensen, C. M.; Ponce, A. A.; Mwale, B. Low-Temperature Metallic Alloying of Copper and Silver Nanoparticles with Gold Nanoparticles through Digestive Ripening. J. Phys. Chem. B 2006, 110, 2155−2158. (26) Jose, D.; Jagirdar, B. R. Au@Pd Core−Shell Nanoparticles through Digestive Ripening. J. Phys. Chem. C 2008, 112, 10089− 10094. (27) Kalidindi, S. B.; Jagirdar, B. R. Magnesium/Copper Nanocomposite through Digestive Ripening. Chem.Asian J. 2009, 4, 835− 838. (28) Arora, N.; Jagirdar, B. R. From (Au5Sn + AuSn) Physical Mixture to Phase Pure AuSn and Au5Sn Intermetallic Nanocrystals with Tailored Morphology: Digestive Ripening Assisted Approach. Phys. Chem. Chem. Phys. 2014, 16, 11381−11389. (29) Arora, N.; Jagirdar, B. R.; Klabunde, K. J. Digestive Ripening Facilitated Atomic Diffusion at Nanosize Regime: Case of AuIn2 and Ag3In Intermetallic Nanoparticles. J. Alloys Compd. 2014, 610, 35−44. (30) Lee, D. K.; Park, S. I.; Lee, J. K.; Hwang, N. M. A Theoretical Model for Digestive Ripening. Acta Mater. 2007, 55, 5281−5288. (31) Sahu, P.; Prasad, B. L. V. Effect of Digestive Ripening Agent on Nanoparticle Size in the Digestive Ripening Process. Chem. Phys. Lett. 2012, 101−104, 525−526. (32) Jose, D.; Matthiesen, J. E.; Parsons, C.; Sorensen, C. M.; Klabunde, K. J. Size Focusing of Nanoparticles by Thermodynamic Control through Ligand Interactions. Molecular Clusters Compared with Nanoparticles of Metals. J. Phys. Chem. Lett. 2012, 3, 885−890.

These experimental results confirm that digestive ripening is a thermodynamic process under kinetic control. Room temperature ripening with amine during SMAD synthesis was also demonstrated and it resulted in smaller size and better distribution compared to digestive ripening under refluxing condition. Hence, this further provides a means to modulate the size and size dispersion of nanoparticles.

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ASSOCIATED CONTENT

S Supporting Information *

BF TEM images (with histograms showing size distribution) obtained during digestive ripening reaction using DDA as digestive ripening agent, fwhm vs time of refluxing plotted for 30 h of refluxing with various Au: amine ratios, and UV−vis spectra and BF TEM images of Au−mesitylene−HDA colloid (Au:HDA ratio 1:1) recorded with certain time intervals of refluxing. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the Council of Scientific and Industrial Research, India. We also thank the Indian Institute of Science for funding the procurement of a 200 kV FETEM. S.P.B thanks the CSIR for a fellowship.

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REFERENCES

(1) Fendler, J. H. Nanoparticles and Nanostructured Films; WileyVCH: Weinheim, Germany, 1998. (2) Klabunde, K. J. Nanoscale Materials in Chemistry; WileyInterscience: New York, 2001. (3) Roduner, E. Size Matters: Why Nanomaterials Are Different. Chem. Soc. Rev. 2006, 35, 583−592. (4) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chemistry and Properties of Nanocrystals of Different Shapes. Chem. Rev. 2005, 105, 1025−1102. (5) El-Sayed, M. A. Small Is Different: Shape-, Size-, and Composition-Dependent Properties of Some Colloidal Semiconductor Nanocrystals. Acc. Chem. Res. 2004, 37, 326−333. (6) Talapin, D. V.; Lee, J. S.; Kovalenko, M. V.; Shevchenko, E. V. Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications. Chem. Rev. 2010, 110, 389−458. (7) Bruchez, M., Jr.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Semiconductor Nanocrystals as Fluorescent Biological Labels. Science 1998, 281, 2013−2016. (8) Rioux, R. M.; Song, H.; Grass, M.; Habas, S.; Niesz, K.; Hoefelmeyer, J. D.; Yang, P.; Somorjai, G. A. Monodisperse Platinum Nanoparticles of Well-Defined Shape: Synthesis, Characterization, Catalytic Properties and Future Prospects. Top. Catal. 2006, 39, 167− 174. (9) Cui, H.; Feng, Y.; Ren, W.; Zeng, T.; Hongying, L.; Yanfei, P. Strategies of Large Scale Synthesis of Monodisperse Nanoparticles. Recent Pat. Nanotechol. 2009, 3, 32−41. (10) Park, J.; Joo, J.; Kwon, S. G.; Jang, Y.; Hyeon, T. Synthesis of Monodisperse Spherical Nanocrystals. Angew. Chem., Int. Ed. 2007, 46, 4630−4660. (11) Zheng, N.; Fan, J.; Stucky, G. D. One-Step One-Phase Synthesis of Monodisperse Noble-Metallic Nanoparticles and Their Colloidal Crystals. J. Am. Chem. Soc. 2006, 128, 6550−6551. 18224

dx.doi.org/10.1021/jp505121b | J. Phys. Chem. C 2014, 118, 18214−18225

The Journal of Physical Chemistry C

Article

(33) Sahu, P.; Prasad, B. L. V. Fine Control of Nanoparticle Sizes and Size Distributions: Temperature and Ligand Effects on the Digestive Ripening Process. Nanoscale 2013, 5, 1768−1771. (34) Stoeva, S. I.; Klabunde, K. J.; Sorensen, C. M.; Dragieva, I. Gram-Scale Synthesis of Monodisperse Gold Colloids by the Solvated Metal Atom Dispersion Method and Digestive Ripening and Their Organization Into Two- and Three-Dimensional Structures. J. Am. Chem. Soc. 2002, 124, 2305−2311. (35) Link, S.; El-Sayed, M. A. Size and Temperature Dependence of the Plasmon Absorption of Colloidal Gold Nanoparticles. J. Phys. Chem. B 1999, 103, 4212−4217. (36) Scaffardi, L. B.; Tocho, J. O. Size Dependence of Refractive Index of Gold Nanoparticles. Nanotechnology 2006, 17, 1309−1315. (37) Ghosh, S. K.; Nath, S.; Kundu, S.; Esumi, K.; Pal, T. Solvent and Ligand Effects on the Localized Surface Plasmon Resonance (LSPR) of Gold Colloids. J. Phys. Chem. B 2004, 108, 13963−13971. (38) Prasad, B. L. V.; Stoeva, S. I.; Sorensen, C. M.; Klabunde, K. J. Digestive Ripening of Thiolated Gold Nanoparticles: The Effect of Alkyl Chain Length. Langmuir 2002, 18, 7515−7520. (39) Sun, Y.; Jose, D.; Sorensen, C.; Klabunde, K. J. Alkyl and Aromatic Amines as Digestive Ripening/Size Focusing Agents for Gold Nanoparticles. Nanomaterials 2013, 3, 370−392. (40) Chikan, V.; Kelley, D. F. Size-Dependent Spectroscopy of MoS2 Nanoclusters. J. Phys. Chem. B 2002, 106, 3794−3804. (41) Kalidindi, S. B.; Jagirdar, B. R. Highly Monodisperse Colloidal Magnesium Nanoparticles by Room Temperature Digestive Ripening. Inorg. Chem. 2009, 48, 4524−4529. (42) Sanyal, U.; Datta, R.; Jagirdar, B. R. Colloidal Calcium Nanoparticles: Digestive Ripening in the Presence of a Capping Agent and Coalescence of Particles Under an Electron Beam. RSC Adv. 2012, 2, 259−263. (43) Arora, N.; Jagirdar, B. R. Monodispersity and Stability: Case of Ultrafine Aluminium Nanoparticles (