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C: Physical Processes in Nanomaterials and Nanostructures

Simulation of Colloidal Silver Nanoparticle Formation from a Precursor Complex Makoto Yoneya, and Shin-ya Sugisawa J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01360 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 9, 2019

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The Journal of Physical Chemistry

Simulation of Colloidal Silver Nanoparticle Formation from a Precursor Complex Makoto Yoneya

∗

and Shin-ya Sugisawa

National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba 305-8565, JAPAN E-mail: [email protected]

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Abstract The formation of oleylamine-stabilized colloidal silver nanoparticles (Ag-NPs), which are the most promising metal NP inks for printed electronics, was modeled and simulated to investigate the shell structure of the oleylamine coating of the Ag-NPs. Our simulation results showed that Ag-NP growth occurred by the coalescence of silver clusters. We also found that the oleylamine shell structure surrounding the Ag-NPs was not the commonly assumed radially extending structure with a single amine group adsorbed to the silver surface but rather a partially lying-down structure with multiple adsorbed groups. The latter structure results in a thinner shell and contains fewer stabilizing agents than the former, which may contribute to the low-temperature sintering characteristics of Ag-NP inks.

Introduction The development of metal nanoparticle (NP) inks and their printing techniques has been extensively studied 1–4 to realize printed electronics. 5 In these inks, metal NPs are stably dispersed in a solvent with stabilizing agents that coat the NP surface. These stabilizing agents usually have binding groups with an affinity for the NP surface and tail groups that lower the interfacial energy of the NPs and add steric stabilization against unwanted NP aggregation. The binding groups usually contain either nitrogen, oxygen, phosphorus or sulfur, and the tail groups are usually alkyl chains for apolar solvents. Among the metal NP inks, those composed of alkylamine-stabilized silver nanoparticles (Ag-NPs) are the most promising inks because they can be densely sintered on substrates at low temperatures. 6,7 In particular, oleylamine-stabilized Ag-NP inks synthesized by thermal decomposition of an oxalate-bridging silver oleylamine precursor complex are the most useful candidates due to their scalable, low-cost and high-yield characteristics. 8 The relatively weak binding strength of the alkylamine molecules coated on the silver surface was considered to realize low-temperature sintering. 7,8 However, a precise molecular level picture of the binding 2


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is not clear, even though it is crucial for understanding their low-temperature sintering characteristics. Recently, molecular simulations have been applied to investigate the coated shell structures at the molecular level. 9–12 However, these simulation studies mostly investigated strongly bound agents and surfaces, e.g., alkanethiols and gold surfaces. In these simulations, the initial shell structures are assumed to consist of the stabilizing agents radially extended with their binding groups attached to the spherical NP surface. 10,11 These assumptions can be rationalized for the case of strongly bound agents, but may not for the case of the weakly bound agents. Moreover, the coated shell structures of the weakly bound agents would be highly dependent on their formation (synthetic) process because their binding structures are more affected by the process condition than are the structures of the strongly binding agents. Thus, to investigate the shell structures of NPs with weakly bound stabilizing agents, we should take into account their formation process. However, no such simulations on metal NP formation from their metal atomic precursors have been reported for the best of our knowledge. Moreover, the formation mechanisms of the NPs themselves are still controversial, and the LaMer model, which is based on classical nucleation theory, is still seen as the basic model. 13,14 The synthesis of these oleylamine-stabilized Ag-NPs starting from silver oxalate, Ag2 (C2 O4 ) is explained as follows. 8 1) Silver oxalate was added to a mixture of oleylamine, methanol and water and then stirred for one day in the dark. 2) The methanol and water were evaporated under reduced pressure at 40 ◦ C to obtain oxalate-bridging silver oleylamine complex (Fig. 1a). 3) This precursor complex was heated to 150 ◦ C for several tens of minutes and then for an additional 10 minutes after CO2 evolution had completed. During this thermal reaction, the precursor complex was thermally decomposed as shown in Fig. 1b and the reduction of Ag+ to Ag0 occurred. Here, the oxalate ligand acts as a two-electron reducing agent. 8 4) The reaction was cooled to room temperature (R.T.). 5) Upon the addition of methanol, Ag-NPs were precipitated and centrifuged. 6) The NPs were redispersed in hexane to make

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the Ag-NP ink. [Figure 1 about here.] In this synthesis, the precursor complex is directly transformed into oleylamine-stabilized Ag-NPs with a high yield of greater than 80 % without any additional organic solvents, reducing agents or stabilizing agents. 8 In this study, we first modeled the formation process of the oleylamine-stabilized AgNPs from the oxalate-bridging silver oleylamine precursor complex using molecular dynamics (MD) simulations and then, investigated the shell structures obtained on the Ag-NP surface.

Model and methods Silver oxalate and oleylamine in an oxalate-bridged silver oleylamine complex (Fig. 1a) were modeled with a flexible detailed-atom model with the exception that all bond-stretching degrees of freedom were constrained to the equilibrium bond lengths. For the inter and intramolecular interactions, a general AMBER force field (GAFF) 15 was used in combination with aliphatic CHn united atom parameters from a reoptimized united atom force field. 16 The Lennard-Jones (LJ) parameters developed by Heinz et al. 17 were applied for the silver atoms. GAFF standard Lorentz-Berthelot mixing rules 18 were also applied for interactions of the silver atoms with other atoms. The implementation of Heinz silver LJ model in GAFF force field was reported to give comparably valid results with the other silver model. 19 The restrained electrostatic potential (RESP) charges 20 were obtained using ab initio molecular orbital calculations at the B3LYP level with 6-31G* and DGDVZP(Ag) basis sets using the Gaussian09 program. 21 The atomic charge on the silver atoms after the thermal reaction (Fig. 1b) were set to zero. The MD simulations were conducted using the GROMACS program package (version 5.0.7) 22 under a 3-D periodic boundary condition. The GROMACS molecular topology file, which includes atomic partial charges, non-bonded LJ and all the other FF parameters, 4


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is found in the Supplementary Information. A cutoff distance of 0.9 nm was applied to the van der Waals interactions. The value (0.9 nm) is same as in the developments and evaluations of the force-field parameters 16 applied in this study. The long-range Coulombic interactions were treated using a particle-mesh Ewald (PME) summation with a real-space cutoff of 0.9 nm under the default settings (Fourier spacing of 0.12 nm and PME order of 4). Trajectories were produced using GROMACS with leapfrog time integration and LINCS bond constraints. 23 The time integration step was set to 4 fs due to the stability of the LINCS algorithm. A combination of the bond constraints and hydrogen mass repartitioning scheme (to be four times heavier while maintaining the total molecular mass) 24 enabled a 4 fs time integration step to be used with reasonably good energy conservation 25 and without affecting the thermodynamics. 26 The simulation temperature and pressure were controlled with the velocity rescaling thermostat of Bussi et al. 27 and Berendsen barostat, 28 respectively. Coupling time constants of 0.5 ps and 1.0 ps were applied for the thermostat and the barostat, respectively. We checked validity of the oleylamine model by comparing its melting/freezing temperature and density with their experimental values. We found that deviation of the melting/freezing temperature and the density were 2 % and 9.1 %, respectively (details are in the Supplementary Information). We think these deviations were acceptable for the current purpose.

Results and discussion Simulation of the precursor complex system We first simulated the system of the pure precursor complex (Fig. 1a) under the assumption that all the methanol and water molecules were evaporated by the end of step 2 of the Ag-NP synthetic protocol. A set of precursor complex models was built as shown in Fig. 1a, and then, the energy was minimized. A total of 1200 sets of the resultant complex model were 5



randomly inserted (using GROMACS utility program “insert-molecules”) into a cubic MD cell with a cell length of 21.5 nm to produce the initial configuration of the system (Fig. 2a). The system contained 1200 silver oxalate and 2400 oleylamine molecules. After running the steepest descent energy minimization, 10 ns MD runs were performed at a temperature of 40 ◦ C and a reduced pressure (0.1 atm). Snapshots after the 10 ns MD runs are shown in Fig. 2c. [Figure 2 about here.] Fig. 2 shows that the initial randomly generated structure (Fig. 2a) was self-assembled into nanosegregated structure (Fig. 2c), i.e., the silver oxalate components and oleylamine components were spatially segregated. We suppose that this nanosegregated structure assists Ag-NP growth in the following processes.

Simulation of Ag-NP growth In the previous sections, we obtained the structure corresponding to just after completion of step 2 of the Ag-NP synthetic protocol in the introduction. We then proceeded to step 3 of the protocol. In step 3, the precursor complex thermally decompose as depicted in Fig. 1, and the reduction of Ag+ to Ag0 occurs. Due to the molecular geometry of the precursor complex (Fig. 1a), i.e., the complex of 2Ag+ with oxalate ion C2 O2− 4 , which acts as the reducing agent for Ag+ , this precursor reduction is highly probable to occur very quickly without extensive mixing of the reaction system. Thus, we simply model the reaction as the instantaneous removal of CO2 and neutralization of the silver atoms of the final simulated structure in the previous step, i.e., Fig. 2b. Our model corresponds to an infinitely fast precursor reduction, and consequently, the reduction of silver is separated from NP growth. NP syntheses with these characteristics are defined as a category 1 synthesis by Polte. 13 The resultant structure after the removal of CO2 becomes the initial structure for the high temperature MD run for step 3 of the Ag-NP synthetic protocol. Thus, 100 ns MD runs were performed at a temperature of 150 ◦ C under normal pressure (1 atm). 6


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Snapshots after 1 ns, 10 ns and 100 ns MD runs are shown in Fig. 3. [Figure 3 about here.] The corresponding time evolution of the total number of silver clusters and their average and maximum sizes (number of atoms per cluster) are shown in Fig. 4. [Figure 4 about here.] Here, the cutoff for determining clustering was 0.35 nm. Fig. 4 shows that the cluster sizes increased almost monotonically, and after merging of the initial (t = 0) ca. 1200 clusters, only six clusters remained at the end of the simulation (t = 100 ns). Fig. 5 shows the time evolution of silver cluster sizes for five selected clusters. [Figure 5 about here.] The stepwise increase in the cluster sizes implies that cluster growth occurred by the coalescence of clusters. We noted above that the characteristics of our model (fast precursor reduction separated from NP growth) corresponds to those of a category 1 synthesis as characterized by Polte. 13 Recent time-resolved experimental investigations of these category 1 syntheses have revealed that the NP growth in this category is driven by coalescence. 13,29 Our simulation results agree with these experimental results. Fig. 6 shows the time evolution of the cluster size distributions for the initial 10 ns MD run (corresponding plot extended to 100 ns MD run is in the the Supplementary Information). [Figure 6 about here.] The initial narrow distribution of cluster sizes of only a few atoms changed into a broad distribution, including clusters having over 300 atoms, within the first 10 ns of the simulation even though the growth processes started at the same time. The persistent appearance of a distribution peak around a cluster size of ca. 50 atoms (shown as a broken line) is also shown in Fig. 6. 7



Fig. 7 shows the radial distribution functions (RDFs) of the silver atoms for the structure after 100 ns MD runs (blue line). [Figure 7 about here.] For comparison, the corresponding RDFs for the initial structure (black line) and a spherical face-center cubic (FCC) NP (red broken line) are also shown. Here, the FCC NP was modeled as an ideal silver FCC crystal by clipping atoms within a radius of 5 nm (i.e., diameter of 10 nm). The figure implies that the Ag-NPs at the end of the MD run (t = 100 ns) were not amorphous but rather nanocrystallines. The Ag-NP sizes at the end of the 100 ns MD simulation were 2-3 nm and were still growing. Experimentally, for synthesis using the oxalate-bridging silver oleylamine precursor complex, Ag-NPs are expected to continue growing until their averaged particle size (diameter) is ca. 11 nm. 8 However, a particle size of 11 nm and the expected time scale needed for this growth are outside the reachable range of the current atomistic MD simulations. Thus, we stopped simulating the Ag-NP growth at this stage (100 ns) and moved to the latter synthetic steps (step 4 and further) by assuming that the local shell structure of the oleylamine-stabilized Ag-NPs is not substantially different between the Ag-NPs with sizes of 2-3 nm and those of 11 nm. Compared to the polymer stabilizing agents, whose sizes could be between 2-3 nm and 11 nm, this assumption would be acceptable for the oleylamine stabilizing agents whose size is ca. 1.5 nm, i.e., smaller than 2-3 nm and also 11 nm. We note that the most of simulated Ag-NPs have non-spherical rod-like shapes as seen in Fig. 3f. As we stated in above, simulated growth of Ag-NPs was non-completed at this stage because of short simulation time. Then, we can not discuss the completed Ag-NP shape from the results in this study. However, we can try some control MD simulations to investigate how the shape of Ag-NPs in this growing stage are affected with changing oleylamine stabilizing agent to 9-nonadecene (C9 H19 -CH=CH-C8 H17 ), simple linear-hydrocarbon without any binding group. Details of the control MD simulation are in the Supplementary Information. We found no remarkable difference in the shape of Ag-NPs between the original 8


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and control MD simulations up to simulated 100 ns. From this results, we suppose a role of oleylamine stabilizing agents in the shape control of Ag-NPs, if it exists, would be working in the later stage, e.g., closer to their final grown-up size.

Simulation of the colloidal dispersion We modeled the later synthetic steps, i.e., step 4 (cooling to R.T.), 5 (addition of methanol and centrifuging), and 6 (redispersion in hexane), in a simplified manner as follows. The final simulated structure of the previous section, Fig. 3e, was sandwiched between hexane (colored blue) layers with thicknesses of ca. 5 nm, as shown in Fig. 8a. [Figure 8 about here.] Here, the hexane molecule was modeled in the same manner as the oleylamine molecule. Then, we conducted the MD run at 30 ◦ C under normal pressure form this structure. Here, we set the simulation temperature a bit higher than R.T., the experimental condition, 8 to avoid terminal chain freezing of our oleylamine model because the model has a bit higher melting/freezing temperature (ca. 27 ◦ C) than the real oleylamine (see Supplementary Information for details). As shown in the snapshots shown in Fig. 8b and c, the free (from the Ag-NPs) oleylamine molecules are dispersed into the hexane layers. Oleylamine molecules dispersed beyond the top and bottom quarters of the MD cell length (in the z-direction) were subtracted every 1 ns to model longer distance dispersion than the MD cell size. Thus, the number of oleylamine molecules decreased from 2400 (in Fig. 8a) to 254 (in Fig. 8c). In Fig. 8d, we note that the oleylamine molecules look partially crystallized within the confined space between the Ag-NPs. Crystallization would be possible in this simulation at R.T. (293 K) under the nanoconfinement, considering the melting point of the oleylamine (ca. 294 K).

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Shell structure of colloidal Ag-NPs Next, the structure after 50 ns MD runs, shown in Fig. 8c, was analyzed to clarify the shell structure of the oleylamine-stabilized Ag-NPs. In Fig. 8c (and 8d), the oleylamine molecules seem not be radially extended with their binding (amine) groups attached to the Ag-NP surfaces but rather are partially lying down on the Ag-NP surfaces. Then, we analyzed the distribution of the number of atoms from the oleylamine molecules that were adsorbed onto the Ag-NP surface. Here, the cutoff for the determination of adsorption onto the silver surface was 0.31 nm for nitrogen atoms and 0.33 nm for carbon atoms. The result are shown in Fig. 9. [Figure 9 about here.] In this figure, the origin of the horizontal axis (number of adsorbed atoms) corresponds to the oleylamine molecules that are free from the Ag-NPs, and 1 corresponds to the adsorption of a single group (atom) of oleylamines onto the Ag-NP surface. Among the single-group adsorption populations shown in Fig. 9, only the small blue-colored portion corresponds to the adsorption of a single amine nitrogen atom onto the Ag-NP surface, as depicted in the inset. Thus, Fig. 9 shows that the oleylamine molecules are mostly adsorbed as multiple groups on the Ag-NP surfaces. This is logically plausible because if the adsorption energy of the binding group was relatively close to that of the alkyl methylene groups, selective (single) adsorption of the binding groups may not occur, and multiple adsorptions, including of the alkyl methylene groups, may occur. Multiple adsorptions of the monomer units are commonly assumed for polymer stabilizing agents 30 because of the identical adsorption energy among the monomer units. The partially lying-down shell structure results in thinner shell thicknesses than those of the radially extending shell structure. Additionally, the number of stabilizing agents within the shell surrounding the Ag-NPs is much lower in the partially lying-down shell structure than in the radially extending structure. The contrast between these two shell

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structures should be more remarkable for stabilizing agents with longer alkyl chains. A longer terminal-chain length often results in smaller final NP size (e.g., alkyl-carboxylic acid stabilizing agents) because the thick shell suppresses NP growth. 31 In contrast, the terminal-chain length of alkyl-amine stabilizing agents does not affect the final NP size. 31,32 This difference can be explained by the fact that the shell thickness does not change with increasing terminal-chain length for the partially lying-down shell structure.

Conclusions We modeled and simulated the formation of colloidal oleylamine-stabilized Ag-NPs from a oxalate-bridging silver oleylamine precursor complex. Assuming an infinitely fast precursor reduction separated from the NP growth, we simply modeled the thermal reaction of the precursor complex as the instantaneous removal of CO2 and neutralization of the silver atoms. We found that the simulated NP growth occurred by the coalescence of silver clusters in this model. This growth behavior agrees with the results of time-resolved experiments for NP synthesis, which are characterized by a fast precursor reduction separated from the NP growth (category 1 synthesis). 13,29 For these category 1 syntheses, a novel NP growth model based on colloidal stability has been presented recently which is in contrast to the commonly applied LaMer model. 13 Our simulation study is the first (for the best of our knowledge) to support this novel NP growth model for the initial synthetic stage. We also analyzed the simulated colloidal Ag-NP structure to clarify the shell structure of the oleylamine-stabilized Ag-NPs. Our simulation results showed that the oleylamine shell structure around the Ag-NPs was not the commonly assumed radially extending structure with a single adsorbed amine group but rather a partially lying-down structure with multiple adsorbed groups. The latter partially lying-down shell structure results in thinner shell thickness than that of the former radially extending shell structure. Additionally, the number of stabilizing agents within the shell on the Ag-NPs is much lower in the partially lying-down

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shell structure than in the radially extending structure. Both of these (e.g., thinner shell and fewer stabilizing agents) are considered to contribute to the low-temperature characteristics of the oleylamine-stabilized Ag-NPs. We hope our results stimulate further investigations on the formation mechanism of colloidal metal NPs and their shell structures.

Acknowledgments We thank Mr. Keisuke Aoshima, and Prof. Tatsuo Hasegawa of the University of Tokyo for their valuable discussions. We also thank Prof. Masao Kurihara of Yamagata University for information regarding Ag-NP inks. This work was supported by JSPS KAKENHI Grant Number JP17K05151 and partially supported by the Tanaka Kikinzoku Kogyo K.K.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXX.

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a) The oxalate-bridging silver oleylamine complex (R=C8 H16 -CH=CH-C8 H17 ) and b) its products after thermal reaction. . . . . . . . . . . . . . . . . . . a) Initial structure of the simulation of the precursor complex system. Carbon and nitrogen atoms are colored gray and blue, respectively. The silver atoms are colored magenta and shown in the space-filling representations. The green box corresponds to the MD cell. b) Ibid, with only silver atoms. c) Snapshots after the 10 ns MD runs. d) Ibid, with only silver atoms. . . . . . . . . . . a) Snapshot after 1 ns MD runs. Carbon and nitrogen atoms are colored gray and blue, respectively. The silver atoms are colored magenta and shown in the space-filling representation. The green box corresponds to the MD cell. b) Ibid, with only silver atoms. c) Snapshot after 10 ns MD runs. d) Ibid, with only silver atoms. e) Snapshot after 100 ns MD runs. f) Ibid, with only silver atoms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Time evolution of the total number of silver clusters (black line) and their average (blue) and maximum (red) sizes (number of atoms per cluster). . . Time evolution of silver cluster sizes for five selected clusters (black, red, green, blue, and cyan). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Time evolution of the cluster size distributions for the initial 10 ns. . . . . . Radial distribution functions (RDFs) of the silver atoms for the initial structure (black line), the structure after 100 ns MD runs (blue line) and the spherical face-center cubic NP (red broken line. The NP is shown in the inset). a) Initial structure of simulation of the Ag-NP dispersion. Carbon and nitrogen atoms are colored gray and blue, respectively. The silver atoms are colored magenta and shown in the space-filling representation. The green box corresponds to the MD cell. Hexane molecules are colored with blue. b) Snapshots after 1 ns MD runs. c) Snapshots after 50 ns MD runs. d) Enlarged image for the red boxed area in c) (hexane molecules are omitted for clarity). Distribution of the number of adsorbed atoms. The small blue-colored portion corresponds to the adsorption of the single amine group onto the Ag-NP surfaces as depicted in the inset. . . . . . . . . . . . . . . . . . . . . . . . .

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18

19

20 21 22 23

24

25

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O

O R H2N

Ag+ O

Ag+ NH2 R O (a)

2CO2 (g) R H2N

Ag0

Ag0 NH2 R (b)

Figure 1: a) The oxalate-bridging silver oleylamine complex (R=C8 H16 -CH=CH-C8 H17 ) and b) its products after thermal reaction.

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(a)

(b)

(c)

(d)

Figure 2: a) Initial structure of the simulation of the precursor complex system. Carbon and nitrogen atoms are colored gray and blue, respectively. The silver atoms are colored magenta and shown in the space-filling representations. The green box corresponds to the MD cell. b) Ibid, with only silver atoms. c) Snapshots after the 10 ns MD runs. d) Ibid, with only silver atoms.

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(a)

(b)

(c)

(d)

(e)

(f)

Figure 3: a) Snapshot after 1 ns MD runs. Carbon and nitrogen atoms are colored gray and blue, respectively. The silver atoms are colored magenta and shown in the space-filling representation. The green box corresponds to the MD cell. b) Ibid, with only silver atoms. c) Snapshot after 10 ns MD runs. d) Ibid, with only silver atoms. e) Snapshot after 100 ns MD runs. f) Ibid, with only silver atoms.

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Page 21 of 27

80

800

60

600

40

400

20

200

0 0.0

20.0

40.0

60.0

80.0

number of atoms

1000

1200

number of clusters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 100.0

time (ns) Figure 4: Time evolution of the total number of silver clusters (black line) and their average (blue) and maximum (red) sizes (number of atoms per cluster).

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500

400

number of atoms

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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300

200

100

0

0.0

20.0

40.0

60.0

80.0

100.0

time (ns) Figure 5: Time evolution of silver cluster sizes for five selected clusters (black, red, green, blue, and cyan).

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Page 23 of 27

number of atoms

800

200 150 100

600

50 0

400

200

0.0 2.0

time

4.0

(ns)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6.0 8.0 10.0

0

50

150

100

200

250

300

cluster size Figure 6: Time evolution of the cluster size distributions for the initial 10 ns.

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160.0

120.0

RDF

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 27

80.0

40.0

0.0

0.2

0.4

0.6

0.8

1.0

1.2

r (nm) Figure 7: Radial distribution functions (RDFs) of the silver atoms for the initial structure (black line), the structure after 100 ns MD runs (blue line) and the spherical face-center cubic NP (red broken line. The NP is shown in the inset).

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(a)

(b)

(c)

(d)

Figure 8: a) Initial structure of simulation of the Ag-NP dispersion. Carbon and nitrogen atoms are colored gray and blue, respectively. The silver atoms are colored magenta and shown in the space-filling representation. The green box corresponds to the MD cell. Hexane molecules are colored with blue. b) Snapshots after 1 ns MD runs. c) Snapshots after 50 ns MD runs. d) Enlarged image for the red boxed area in c) (hexane molecules are omitted for clarity).

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0.20

0.15

fraction

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 27

N

0.10

0.05

0.00

0

1

2

3

4

5

6

7

8

9 10 11 12 13 14

number of adsorpted atoms Figure 9: Distribution of the number of adsorbed atoms. The small blue-colored portion corresponds to the adsorption of the single amine group onto the Ag-NP surfaces as depicted in the inset.

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202x157mm (72 x 72 DPI)


Simulation of Colloidal Silver Nanoparticle ... - ACS Publications

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