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论文题目： Thermodynamics Model for Mechanochemical Synthesis of Gold Nanoparticles: Implications for Solvent-Free Nanoparticle Production

发表时间：Published online 11 February 2021

刊源：ACS Applied Nano Materials [ 点击下载PDF ]

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Fig. (2) shows the free energy profile of the system as a function of c and thus as it transitions from initial disordered stage to final AuNPs stage, with γ= 0 and γ= 0.1.Fig. (2) confirms visually the trends explained above. Specifically, the reaction is favoured, both kinetically and thermodynamically by the ballistic energy input (γ > 0).Without the ballistic term, the activation barrier is ∆G_a = 0.0354, while with = 0.1, the activation barrier reduces to ∆G_a = 0.0250, which represents a percentage reduction of f = 29.28% of the nucleation barrier compared to its value in the absence of ballistic energy.As shown in the analysis of Fig. 2, turning on the ballistic energy serves to lower the activation energy barrier, which facilitates the precipitation of Au nanoparticles through normal thermal fluctuations. Comparison of Sample 2 with Sample 3 was found to give different morphology for the final precipitated products. Since in Sample 3 simulations the ballistic term is deactivated at timestep = 430, no new nuclei can form after that (and some nuclei also dissolve into the disordered matrix), leaving the nuclei remaining to continue to grow, thus attaining larger NPs sizes compared to NPs in Sample 2 where new nuclei continue to form in the matrix past = 430 timesteps.

Figure 2: Dimensionless free energy landscape in ε × c × γ space of a Au(0)- octadecylamine (C18) system.The blue curves correspond to γ= 0 and the green curves correspond to γ= 0.1. The free energy landscapes for both γ have been subtracted by their maximum value, aligning them all at ε = 0. The black vertical dash line at c = 0.167 marks the reaction starting point. The difference in the green and blue vertical lines indicates ∆G_a, the energy change required (relative to the maximum energy) to activate the precipitation of the ordered phase from the disordered phase.

Fig. 4 shows the result of the microstructure simulations in the Sample 2 case. In the early stages of NP synthesis (figures (a) to (c)), Au is precipitated from the matrix, forming amorphous clusters and crystal seeds. However, not all of the initial clusters evolve into fully developed Au particles, as shown in frames (d) to (e). Some of these small initial clusters (and some small nuclei) are dissolved back into the disordered phase, or swallowed by other stably growing nuclei. This observation suggests the Ostwald ripening process at play, however to verify this, more statistics and larger simulation sizes are required. The final stage (f) comprises very stable AuNPs, which will continue to grow and amalgamate with each other to form large Au plates over the time scale of the simulations

Figure 4: Simulated mechanosynthesis of AuNPs from a disordered heptadecylamine (C17) – Au(0) matrix, with the ballistic term continuously active (γ = 0.1 and R = 9.7a). The black / white pixels represent the dimensionless density n and the colourbar readings represent the composition of Au. (a) The initial timestep = 0 configuration is randomly distributed with average reduced density n_timestep=0 = 0.05 and composition c_timestep=0 = 0.167. (b-c) show the nucleation process. (d-f) show the post-nucleation growth of NP precipitates.

AuNPs were found to precipitate in all samples, with some typical late-time precipitate configurations for each case shown in Fig. 7.The trend that emerges in the dynamical simulations of Fig. 7 is consistent with the predictions made in subsection 4.2.1, namely, as the ligands become longer and heavier, the average AuNP size becomes smaller. Table 1 summarizes the average diameters of AuNPs for each case and the results are compared to those in the reference experimental work. The trend in the average NP diameter as a function of ligand size and mass is the same as those observed experimentally. In particular, as the ligands become longer and heavier, the AuNP diameters become smaller. It is noted that the simulated AuNP diameters are quite different quantitatively from the experimental values.

Figure 7: Simulated late-stage configurations of AuNPs precipitated from a disordered ligand -Au(0) phase. The black / white pixels represent the dimensionless density n, and the colourbar readings represent the composition of Au. AuNPs for Four ligand types are shown: (a) pentadecylamine (C15), (b) hexadecylamine (C16), (c) heptadecylamine (C17), and (d) octadecylamine (C18)