Nanoscience research has made a significant progress in the synthesis of high-quality colloidal nanoparticles of metals and semiconductors. However, some fundamental questions regarding variations in the stability of colloidal nanoparticles in different colloidal systems, the role of surfactants and ligands in protection of particles, as well as the surface architecture of the nanoparticles, have yet to be answered.
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The synthesis of atomically precise and molecular pure nanoparticles has the potential to answer these questions, since these particles can be assigned with a definite formula, unlike colloidal nanoparticles that are typically limited in their nanoparticle size range requirements.
The total structure of molecularly pure nanoparticles, such as atomically precise gold (Au) nanoparticles, can be determined by performing X-ray crystallography of a macroscopic single crystal. In fact, researchers have been able to incorporate a specific number of foreign atoms into the gold core by doping and alloying methods, which can also be used to study structural changes following doping. The doping of atomically precise nanoparticles allows researchers to understand the fundamental properties of these types of nanoparticles at atomic levels, which is crucial in the development of new applications.
Au Nanoclusters with Thiolate (SR) substitutions
The cluster mass of thiolate substituted gold nanoparticles, such as Au25(SR)18, Au38(SR)24 and Au144(SR)60, can be accurately determined by electrospray ionization mass spectrometry (EMI-MS). Furthermore, the structure of these nanoparticles can be determined by various structural analysis techniques, such as X-ray crystallography of a macroscopic single crystal. In the Au25(SR)18 nanocluster, the Au25 metal core is composed of an inner core or kernel with 13 Au atoms arranged in an icosahedron (magneta) fashion and an exterior shell composed of 12 surface Au atoms. The kernel is composed of a central Au atom, as well as 12 Au atoms in the icosahedral shell. Eighteen SR ligands protect the entire Au25 particle by arranging themselves into an inner and outer chemical environment.
Three different absorption bands observed in Au25(SR)18 nanoparticles were found to correspond to single-electron excitations in the visible wavelengths of 400, 440 and 680 nm. Whereas, a single surface plasmon band corresponding to collective electron excitation (plasmon) is observed in the case of larger and spherical Au nanoparticles.
Doping of Gold Nanoparticles with Other Metals
Researchers have able to substitute gold in Au25(SR)18 nanoparticle with heteroatoms like Palladium (Pd), Platinum (Pt) and Silver (Ag) to study changes in structure, as well as the electronic and optical properties of Au25(SR)18 nanoparticles.
Doping with Palladium (Pd)
A mono Pd substituted pure Pd1Au24(SC2H4Ph)18 nanoclusters isolated by solvent extraction and size exclusion chromatography demonstrated that the Pd1Au24(SC2H4Ph)18 nanoclusters were found to be charge neutral, whereas the native Au25(SR)18 nanoparticle carried a 1-charge.
Differences in the optical spectra of Pd1Au24(SC2H4Ph)18 and Au25(SR)18 nanoparticles were also found. To this end, the three absorption peaks in the Pd1Au24(SC2H4Ph)18 appeared to be less pronounced in Pd1Au24(SC2H4Ph)18 as compared to those of the Au25(SR)18 nanoparticle. Despite these electronic and optical differences when doped with Pd, the Pd1Au24(SC2H4Ph)18 nanocluster adopted the same shape as the native Au25(SR)18 nanoparticles.
Doping with Platinum (Pt)
Pt1Au24(SC2H4Ph)18 formed by a single Pt substitution of the central Au atom in the Au25(SR)18, appeared to be most energetically stable as compared to the other heteroatom substitutions of Au25(SR)18 with Pd, Ag and Cu. Pt1Au24(SC2H4Ph)18 appeared to be charge neutral, which is similar to the Pd1Au24(SC2H4Ph)18 nanoparticles. While this may be true, there were significant differences in the absorption spectrum of Pt1Au24(SC2H4Ph)18 as compared to Pd1Au24(SC2H4Ph)18 and Au25(SR)18. A distinct absorption peak at a visible wavelength of 590 nm was observed in Pt1Au24(SC2H4Ph)18. Various structure analysis techniques also showed that replacement of the central Au atom by Pt in Pt1Au24(SC2H4Ph)18 resulted in a structure that is similar to what was originally seen in Au25(SR)18.
Alloying with Silver (Ag)
Unlike the mono substitutions possible with Pd and Pt, researchers found that multiple Au atoms in the Au25(SR)18 nanoparticles can be replaced with Ag atoms by increasing the ratio of [AgNO3]:[HAuCl4]. However, the structure of various Ag-Au substitutions in AgxAu25-x(SC12H25)18 retained a similar structure to Au25(SR)18 nanoparticles. Despite these similarities, there were significant differences in the absorption spectrum between Au25(SR)18 and the Ag substituted AgxAu25-x(SC12H25)18.
Conclusion
Various levels of substitution of Au with heterometals can be achieved by doping or alloying. The optical and electronic properties appeared to vary with the type of metal doping and the degree of doping achieved. However, the native structure of the gold nanoparticles appeared to be preserved in various doped nanoclusters, suggesting that the fashion of the atomic packing in these nanoclusters is the reason for their extraordinary stability.
Reference
- Jin, R., & Nobusada, K. (2013). Doping and alloying in atomically precise gold nanoparticles. Nano Research. DOI: 10.1007/s12274-014-0403-5.
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