Controlled Alloying of Au@Ag Core–Shell Nanorods Induced by Femtosecond Laser Irradiation

Abstract

are essential components in innumerable materials and structures such as buildings, airplanes, computers, or medical implants. The allure of alloys stems from the improved mechanical properties (e.g., ductility or hardness) and enhanced chemical behavior (e.g., oxidation resistance) that can emerge from the combination of two or more metals. [1] In nanotechnology, the formation of alloy nanocrystals promotes drastic changes in their optical, electrical, and magnetic features and/or their performance as catalysts. [2][3][4][5][6][7][8] However, most of these attributes are susceptible to subtle variations in the size, shape, and composition of the nanoparticles, thereby compromising their practical application. Thus, achieving control over the alloying process and nanocrystal growth would provide a great potential in this direction. Among the wide variety of available synthetic methods, colloid chemistry is highly versatile in terms of nanoparticle shape and composition, in particular for noble metals such as platinum, silver, palladium, or gold. [9][10][11][12][13] For example, different kinds of mesoporous alloy nanoparticles have been synthesized using this method. [14][15][16] The synthesis of colloidal alloy nanocrystals is typically accomplished via co-reduction or thermal decomposition of suitable metal precursors. [9][10][11][12][13][17][18][19][20][21] Other strategies based Bimetallic nanoparticles display unique physical and chemical properties, including improved chemical stability, enhanced optical properties, or higher catalytic activity. Here, a synthetic methodology is described to obtain bimetallic heterostructures and alloyed plasmonic nanocrystals through the irradiation of colloidal Au@Ag core-shell nanorods (Au@Ag NRs) with femtosecond laser pulses. Depending on the energy deposited on the Au@Ag NRs, different morphologies and degrees of alloying are obtained, such as hot-dog-like and rice-like (partially alloyed) NRs, as well as fully alloyed nanospheres. By using advanced electron microscopy techniques and energy-dispersive X-ray spectroscopy (EDX) tomography, both the morphology and the elemental distribution of the irradiated nanoparticles can be disclosed, and correlated to detailed investigations of their optical properties using electromagnetic simulations. The wide variety of bimetallic species provided by the proposed approach is a clear indication of the potential of combining synthetic colloidal methods with fs-pulsed laser irradiation for the fabrication of unique multielemental nanoparticles. The resulting control over size and composition raises promising prospects for catalytic, plasmonic, and magnetic applications of multimetallic nanocrystals.

Multimode Electron Tomography Sheds Light on Synthesis, Structure, and Properties of Complex Metal‐Based Nanoparticles

“…[ 31 ] An example is the work by González‐Rubio et al, who tracked the influence of femtosecond laser irradiation on core@shell Au@Ag nanorods, to investigate the potential of thermal relaxation to induce alloying. [ 107 ] Au@Ag NRs were thus irradiated with 2.3, 6.1, and 33.2 J m –1 fluxes. Irradiation resulted in the selective removal of Ag from the tips of the core@shell NRs, leaving an unexposed Au core (2.3 J m –1 ), followed by the nanorod tip becoming increasingly rounded into an ellipsoidal morphology (6.1 J m –1 ), and finally, upon extreme irradiation, the formation of spherical NPs no longer displaying any evidence of a core@shell arrangement (33.2 J m –1 ).…”

Recent advances of Au@Ag core–shell SERS‐based biosensors

“…Furthermore, the key advantages of Au/Ag alloys are their chemical and thermal stability and their improved mechanical qualities. [ 83 ] They also generate a greater SERS signal than pure gold and silver platforms.…”

Composition‐Controlled Laser‐Induced Alloying of Colloidal Au–Cu Hetero Nanoparticles

“…Due to the isochoric melting process, the composition of the individual particles does not change compared to the initial hetero nanoparticles. Thus, the composition (gold to copper ratio) of the laser-induced nanoalloys can be previously adjusted and controlled by wet chemical synthesis, which opens the door for controllable alloying processes of a variety of further metal systems such as Au-Ag, [21][22][23]25] Ag-Bi, [26] Au-Fe, [20] Au-Zn, [27] Fe-Pt, [28] and Ag-Pt, [29] whose alloy formation could already be observed by LAL and LFL methods. In principle, the implementation of this method is conceivable for all alloy-forming metal systems as long as they exhibit suitable absorption properties in the range of the laser wavelength.…”