Figure 1 shows optical absorption spectra of bimetallic (line 2) and individual (lines 1 and 3) metal nanoparticles. The appearance of only one absorption band in the spectrum of the colloidal solution of bimetallic nanoparticles (λmax = 472 nm) positioned practically in the middle relative to the absorption band of silver (λmax = 418 nm) and gold (λmax = 520 nm) nanoparticles, indicates of the formation bimetallic alloy.
The presence of only one absorption band (Fig. 1, curve c) in the spectrum of the bimetallic sol (λmax = 472 nm), located almost in the middle relative to the absorption bands of silver (λmax = 418 nm) and gold (λmax = 520 nm), indicates that alloy particles of two metals are formed.
Figure 2 shows TEM-images and particles size distribution of the obtained colloidal nanoparticles. As one can see, Ag–Au bimetallic nanoparticles have spherical shape, narrow size distribution and the mean size of approximately 10 nm, while the nanoparticles of individual metals prepared under the same conditions are larger and less uniform in size.
In further work, the influence of the concentration of Na2EDTA on the formation of bimetallic sols was studied in more details. Figure 3 shows the λmax of the colloidal solutions of the obtained Ag–Au nanoparticles as a function of the Ag/Au molar ratio in the starting reaction mixture at different Na2EDTA concentrations. Similar studies with various reducing agents (but not Na2EDTA) were conducted earlier [15,16,17,18], where a linear dependence was observed. However, our results demonstrate that the investigated dependence is more complex and is altered considerably by the changes in the Na2EDTA concentration in the reaction mixture (Fig. 3a–c).
Figure 4 shows the λmax position in the optical absorption spectra of the colloidal solution of Ag–Au nanoparticles as a function of the reaction time at different Na2EDTA concentrations. When there is “insufficient” Na2EDTA, i.e. molar concentration of Na2EDTA is lower than the sum of molar concentrations of starting metals (Fig. 4, line 3), the reaction is inhibited and starts approximately after 5 min after the injection of all reagents. Then a slight redshift of the absorption spectrum is observed and λmax changes from 460 to 467 nm (until 20 min);after that the position of λmax remains unchanged until the end of the reaction (40 min). Moreover, judging by the value of λmax = 460 nm, bimetal nanoparticles with a predominance of silver are formed from the very beginning of the reaction.
At the high excess of Na2EDTA, i.e. when the molar concentration of Na2EDTA is 4 times higher than the sum of molar concentrations of starting metals (Fig. 4, line 1), a completely different behaviour is observed. As indicated by the visually noticeable colour change, the reaction starts several seconds after the injection of all reagents.
Judging by value of λmax = 517 nm, from the very beginning of the reaction, almost pure gold nanoparticles are formed. But over time scene changes rapidly, position of λmax shifts into the short wavelength region and stabilizes approximately after 20 min. Further increase of the reaction time (>20 min) does not result in significant changes of the λmax at all investigated Na2EDTA concentrations (Fig. 4, lines 1–3).
TEM-studies (Fig. 5) of the prepared colloids show that Na2EDTA concentration also considerably alters the average size and size distribution of the Ag–Au nanoparticles.
At lower Na2EDTA concentration the average size of Ag–Au nanoparticles increases and the size distribution becomes broader. It is worth pointing out that the stability of the prepared colloidal solutions also depends on the Na2EDTA concentration: at [Na2EDTA] ≥ 5 × 10−4 M solutions remain stable for more than 1 year, while al lower concentration ([Na2EDTA] = 1.25 × 10−4 M) solution are stable only for several weeks.
Considerable difference of λmax in the absorption spectra of the colloidal solutions prepared using different Na2EDTA concentrations suggests that nanoparticles in these solutions have different composition. To test this assumption a series of analyses were conducted using inductively coupled plasma optical emission spectrometry. Measurement results are provided in the following Table 1 along with the data obtained using optical spectroscopy and TEM.
As one can see from the table, at the same molar ratio of the starting reagents (Ag/Au = 1) as the Na2EDTA concentration decreases, the nanoparticles become enriched in silver and at the same time their size increases. It is interesting to note that such a trend, i.e. the simultaneous increase in the silver content and nanoparticle size is similar to the one reported in Ref. , although the synthetic protocol for bimetallic nanoparticles employed in that paper is completely different (reduction with oleylamine in organic medium).