Supporting Information

Transcription

1 Supporting Information Surface Composition and Crystallinity of Coalescing Silver-Gold Nanoparticles Eirini Goudeli and Sotiris E. Pratsinis* Particle Technology Laboratory, Institute of Process Engineering, Department of Mechanical and Process Engineering, ETH Zürich, Sonneggstrasse 3, CH-8092 Zürich, Switzerland. Ph ; Fax Figure S1. Melting point of Ag (diamonds 21 ), Au (orange line 23 ; circles 22 ) and Ag-Au segregated (squares) and alloyed nanoparticles (triangles) as function of particle diameter, d p,0, as obtained by the evolution of Lindemann index. The size-dependent Ag and Au melting points are compared to those of bulk Ag (dotted line) and Au (broken line, filled circle). The melting point decreases with decreasing particle size consistent with Buffat and Borel (1976). Pure Ag and Au nanoparticles as well as Ag-Au nanocomposites exhibit similar melting points. 1

2 Figure S2. The effect of composition on the surface area evolution of Ag-Au composites. Increasing their Aucontent gradually shifts the surface area evolution (sintering or coalescence rate) from that of pure Ag to pure Au. 2

3 Figure S3. Characteristic sintering time, τ s, as a function of temperature for Ag-Au segregated nanoparticles of d p,0 = 3 (triangles), 3.5 (diamonds) and 4 nm (squares). Increasing temperature or decreasing particle size results in smaller characteristic sintering times (faster sintering). The τ s of such Ag-Au segregated particles can be bracketed inbetween the sintering rate of Ag (Buesser and Pratsinis, 2015) and Au nanoparticles (Goudeli and Pratsinis, 2016). 3

4 Figure S4. Evolution of the Lindemann index of Ag (grey line), Au (orange line) and Ag-Au segregated (black line) and alloyed nanoparticles (blue line) having initial diameter of d p,0 = 3 nm during sintering at T = 800 K. The trend of the Lindemann index is similar to that of the average disorder variable for coalescing 3 nm-nanoparticles (Figure 2b )exhibiting higher values during particle adhesion and the first stages of sintering (0.01 ns <t< 1 ns) while leveling off later on when particle re-shaping takes place from oval- to sphere-like (t > 1 ns). Pure Au nanoparticles exhibit higher Lindemann index values than pure Ag or Ag-Au nanocomposites, in contrast to the average disorder variable, D (Fig. 2b), which indicates that alloyed Ag-Au nanoparticles are the least crystalline throughout all stages of sintering. Furthermore, even though there is a clear increase of D during particle adhesion (Fig. 2b: t = ns), consistent with the literature 22, indicating an increase in particle disorder, the Lindemann index does not capture as accurately such small changes in crystalline structure as it does during particle melting (Fig. 1a in ref. 22). Itshould be noted that the Lindemann index ranges from 0.01 up to 0.06 during particle sintering (Fig. R2) while its range is an order of magnitude larger during melting (about 0-0.3) as the entire particle becomes amorphous. Thus, D is chosen here for better quantification of nanoparticle crystallinity, increasing mostlyby atoms on the surface or near the neck region between the two coalescing particles. 4

5 Figure S5. Evolution of normalized surface area by coalescence of two alloyed Ag-Au nanoparticles with d p,0 = 3 nm at T = 800 K that adhere to each other at different initial facet orientations. 5

6 Figure S6. Evolution of normalized surface area of two segregated Ag-Au nanoparticles with d p,0 = 4 nm at T = 800 K. The two particles adhere to each other either by a Ag (100) terrace witha Au (100) terrace (orientation 1, circles) or by a Ag (100) terrace with a Au edge (orientation 2, crosses). 6

7 Figure S7. Evolution of the surface Ag fraction of Ag-Au segregated nanoparticles with d p,0 = 3 nm during sintering at T = 600 (red lines), 700 (green line) and 800 K (blue lines). During sintering, the surface Ag composition increases from initially ~0.5 up to about 0.64 (for T = 800 K) similarly to larger composites (Fig. 1). The Ag surface fractionof the sintered particle increases with temperature. The shaded region represents the variation of three simulations. 7

8 Figure S8. Snapshots of Ag-Au nanoparticles with (a)d p,0,ag = 2 and d p,0,au = 4nm and (b)d p,0,ag = 4 and d p,0,au = 2 nmcoalescing at T = 800 K at t = 0, 0.1, 1 and 10ns. The Ag atoms are blue while the Au atoms are yellow. Their corresponding cross-section snapshots are shown in Fig. 4. 8

9 FigureS9. Evolution of the XRD diameter of Ag (grey lines), Au (orange lines), Ag-Au segregated (black lines) and alloyed (blue lines) nanoparticles with d p,0 = 4 nm coalescing at 800 K. Alloyed nanoparticles exhibit the smallest crystallite size than segregated or pure Au and Ag nanoparticles similarly to Fig. 8. 9