Supporting Information: Time- and Size-Resolved Plasmonic Evolution with nm Resolution of Galvanic Replacement Reaction in AuAg Nanoshells Synthesis

Transcription

1 Supporting Information: Time- and Size-Resolved Plasmonic Evolution with nm Resolution of Galvanic Replacement Reaction in AuAg Nanoshells Synthesis Lorenzo Russo, Florind Merkoçi, Javier Patarroyo, Jordi Piella, Arben Merkoçi, Neus G. Bastús and Victor Puntes Figure S1 Monodispersed and high-quality silver pseudo-spherical NCs are used as sacrificial templates for the GRR. Left: TEM dark field images of silver templates of c.a 80 nm size (SD < 10% ) showing a high degree of crystallinity and multifaceted surface. Right: The seeded-growth synthesis of silver templates is followed by UV-Vis spectroscopy (above) and normalized (below). The inset shows LSPR intensity and maximum wavelength trends during synthesis. 1

2 Figure S2 GRR carried out in absence of PVP. UV-Vis spectra evolution end up in a black solution with flat absorbance profile (left). TEM bright and dark field images (right) show particles with uneven hollowing degree and a rough granular surface. Milli-Q Water NaCl HCl Figure S3 Final morphology (above) and UV-Vis spectra (below) obtained when carrying out GRR in Milli-Q water, 10 mm NaCl and 10 mm HCl respectively. When both chlorides and proton are present, a hollow morphology characterized by a thin shell and large void is observed. When using an equinormal NaCl solution instead, that is with the same amount of chlorides as protons in the previous case, the degree of hollowing of the forming structure is decreased strongly. In these conditions only chlorides and AuIII compete for the oxidative etching of Ag from the core of the particle, making the hollow morphology still recognizable at the final stages of GRR but with a smaller void. Carrying out the reaction in absence of both co-etchers lowers the reaction rate so significantly that, for equivalent HAuCl4 volumes added, no relevant shift in LSPR is observed. 2

3 Figure S4 GRR carried out in absence of HCl. UV-Vis spectra evolution reveals a significant delay in GRR initiation (left). TEM bright and dark field images show particles with random pitting corrosion events (right). GRR with HCl GRR with HNO 3 Figure S5 GRR carried out with different co-etchers: with HCl (left) a smoother surface and larger void are obtained; with HNO 3 (right) rough, granular Au deposition on the templates surface is observed, while the degree of hollowing is significantly lower. 3

4 Figure S6 Study of the reproducibility of the optimized GRR for the selective synthesis of hollow AuAg NSs with large voids and thin, smooth shells. Three different AuAg NSs syntheses were compared by monitoring the LSPR band shift of three solutions of monodisperse Ag NCs (d = 80 nm) titrated with HAuCl 4 1 mm during the last GRR phase (total templates volume used = 25 ml). Inset a shows the time-dependent absorbance spectra of a 80 nmsized solution of Ag NCs titrated with increasing volumes of HAuCl 4, each absorption spectrum corresponding to 25 µl of a 1 mm HAuCl 4 aqueous solution injected at 10 µl/min (spectra for earlier GRR are not shown for clarity). Inset b reports the LSPR maximum wavelength position during the titration, three different syntheses (black, red and grey curves) are compared and their average plotted against their corresponding HAuCl 4 volume added (vertical error bars represent standard deviation for LSPR maximum wavelength; horizontal error bars represent standard deviation for volume of HAuCl 4 solution added; N = 3). For all the cases (main plot), the same linear trend between the amount of Au III added and the LSPR position was observed revealing an almost complete reproducibility (R 2 = or linear trend). Each synthesis was carried out onto a different Ag NCs solution, whose slight variability in templates concentration is responsible for the relatively small differences between each GRR. 4

5 Figure S7 Time-resolved evolution of maximum LSPR wavelength (red) and intensity (black) of Ag templates solutions of different sizes titrated with increasing volumes of HAuCl 4 (each absorption spectrum corresponding to 25 µl of a 1 mm HAuCl 4 aqueous solution upon titration at 10 µl/min). Four optical regimes are identified for bigger particles (150, 100 and 80 nm) while for smaller ones the hollow structure collapses at earlier stages. Differences in total HAuCl4 volumes added are probably caused by slight variation in templates concentration due to their synthetic protocol. (Bastús, N.G., Merkoçi, F., Piella, J., Puntes, V.F., Synthesis of Highly Monodisperse Citrate-Stabilized Silver Nanoparticles of up to 200 nm: Kinetic Control and Catalytic Properties. Chem. Mater. 26, ) In addition to the dipole mode, particles larger than 60 nm exhibited quadrupole plasmon resonance modes, found at 400, 410 and 425 nm for 80, 100 and 150 nm-sized particles respectively. The quadrupole and dipole peaks both shifted to longer wavelengths with increased Au content in AuAg NSs, the extent of this shift depending on the interplay between the thickness of the Au shell and the internal void size. While this quadrupole has been previously reported in solid Ag NCs, its observation after GRR with Au III species is challenging because of the relatively weaker optical activity of Au. However, the extent of the quadrupole peak shift ( λ MAX 100 nm) is less than that of the dipole peak shift ( λ MAX 250 nm). Considering that a quadrupole peak can be observed on single metal Ag NCs of 80 nm size but cannot be found on Au NCs of the same size, we conclude that the quadrupole peak originated from the formation of hollow AuAg NSs is a further indicator of the presence of Ag in the hollow NSs, since the phase retardation of oscillating surface electrons needs to be established in order to observe a higher order surface plasmon resonance band. The Ag component in Au matrix and the free electrons on the hollow shell may induce effectively such retardation in hollow AuAg NSs. 5

6 AuIII AuI Figure S8 GRR carried out onto 30 nm silver templates: only when Au I precursor is used (above right, below) the hollow morphology is reached, even if slightly incomplete. When using instead the normal Au III precursor the hollow morphology collapses before reaching completion. Figure S9 a) Calculated extinction efficiency for 80 nm Ag spheres surrounded by a thin Au shell of growing diameter. b) HRTEM image of AuAg NS at its last GRR stage, displaying a thin outer shell of about 10 nm. Figure 6 a) represents the calculated spectra evolution for 80 nm Ag templates with the initial deposition of a thin growing Au shell, modeled by adding a second Au layer of varying thickness onto the initial Ag core. The calculated spectrum for the analogous Ag NC exhibited a dipole mode at ca. 430 and a quadrupole mode at ca. 380 nm, showing the good agreement with the experiment (dipole and the quadrupole modes found at ca. 440 and 400 nm, respectively in Figure 1 A). In accordance with experimental results previously described, the calculated spectra reveal how the presence of this Au layer is translated into a red-shift of the position of the LSPRs of the Ag templates from 446 nm (Ag NC templates) to 460 nm (2 nm thickness), 517 nm (5 nm thickness), 537 nm (10 nm thickness) and 568 nm (16 nm thickness). Remarkably, the quadrupolar peak of Ag NCs templates rapidly attenuate and vanishes as the thickness of the Au shell increases. The second stage involves the voiding of the core/shell AgAu NC, modeled by adding an inner H 2O sphere of varying diameter surrounded by two shells: one of Ag, which corresponds to the original Ag template not dissolved, and one of Au corresponding to initial deposition already described (Figure 6 a)) 6