Less-noble Metal Nanoparticles

The synthesis of metal nanoparticles is the more challenging the lower the electrochemical potential of the respective metal is. Thus, less noble metals are highly reactive; they are instantaneously reoxidized by oxygen or water. Due to the huge specific surface area nanoscale less-noble metals, furthermore, are much more reactive than the respective bulk metals. As a consequence, specific strategies of synthesis are necessary for preparation. Moreover, the complete analytical characterization, including sample transfer from synthesis to analytical devices, essentially need inert conditions. Our activities address three strategies of synthesis: polyol-mediated synthesis, liquid-ammonia-in-oil microemulsions, and synthesis in liquid ammonia. They are shortly discussed in the following:

Metal nanoparticles including Bi⁰, In⁰, Ni⁰, Co⁰, Cu⁰, or intermetallic phases such as Cu₁₁In₉ are prepared via a polyol-mediated synthesis. Here, the polyol (e.g. diethylene glycol) allows controlling particle size and size distribution of the nanoparticles that are typcially obtained with diameters of 10–30 nm. In principle, the synthesis is straight forward and easy-to-scale-up. Often the nanoparticles can be easily redispersed even after separation as a powder sample. As a typcial example of application, In⁰, Cu⁰, and Cu₁₁In₉ can be used as precursors for copper-indium-selenide (CIS) thin-film solar cells. Together with our partners, we have realized CIS solar cells via printing of nanoparticle-containing inks that show power-conversion efficiencies up to 7 % (Figure 1).

Figure 1: Synthesis of Cu₁₁In₉ nanoparticles and its use as a precursor for printing thin-film CIS solar cells.

To obtain even less-noble metals on the nanoscale, we have developed a microemulsion containing liquid ammonia a the polar micelle phase. Such liquid-ammonia-in-oil microemulsions (a/o-microemulsion) have been shown and used in the research group for the first time (Figure 2). This is even more surprising since microemulsion techniques belong to the most widely applied strategies for preparing high-quality nanoparticles with about 1,000 papers appearing each year. These standard microemulsions, however, contain water as the polar micelle phase (water-in-oil or w/o-microemulsion). Thus, a synthesis of less-noble metals is excluded herein. Knowing how to establish an a/o-microemulsion, this thermodynamically stable system can be reproducibly obtained and handled as easy as a standard w/o-microemulsion  except for the lower temperature of 40 °C needed for liquid ammonia. In between, the metals Bi0, Re0, Fe0, as well as the nitrides CoN and GaN were obtained as nanoparticles exhibiting a mean diameter in the 18 nm range (Figure 5). Surprisingly, crystalline nanoparticles can be readily obtained without any additional thermal treatment. Some nanomaterials, e.g. GaN, show characteristic quantum size effects. Perspectively, the here established a/o-microemulsion can open the door to many more reactive nanomaterials (e.g., less-noble metals, Zintl phases, metal nitrides) and allows studying their fundamental properties (e.g. quantum-confinement effects) and their potential application (e.g., catalysis, high-power batteries, solar cells).

Figure 2: Scheme displaying the synthesis of nanoparticles in liquid-ammonia-in-oil microemulsions with some examples shown: Re⁰, Bi⁰, Fe⁰, CoN, GaN.

Besides microemulsions with liquid ammonia, nanoparticle synthesis can be as well performed in pure liquid ammonia. For instance, W⁰ nanoparticles are prepared via reduction of WCl₆ with elemental sodium in liquid ammonia as the solvent (Figure 6). The solvated electrons - generated by dissolving elemental sodium in liquid ammonia - provide an extremely strong reducing agent (E₀ = -2.25 V) and can give access to many additional less-noble metals. The as-prepared W⁰ nanoparticles exhibit a diameter of 1-2 nm. Due to quantum-confinements effects, the nanoparticles exhibit a grayish orange color in suspension (Figure 3).

Figure 3: Scheme illustrating the synthesis strategy of W0 nanoparticles in liquid ammonia via sodium-driven reduction as well as TEM image and photo of the nanoparticles in suspension

For more information see:

C. Kind, C. Feldmann*, One-pot Synthesis In⁰ Nanoparticles with Tuned Particle Size and High Oxidation Stability, Chem. Mater. 2011, 23, 4982–4987.

C. Kind, C. Feldmann*, A. Quintilla, E. Ahlswede*, Citrate-capped Cu₁₁In₉ Nanoparticles and Its Use for Thin-film Manufacturing of CIS Solar Cells, Chem. Mater. 2011, 23, 5269−5274.

F. Gyger, P. Bockstaller, D. Gerthsen, C. Feldmann*,Ammonia-in-Oil-Microemulsions and Their Application, Angew. Chem. Int. Ed. 2013, 52, 12443–12447.

C. Schöttle, P. Bockstaller, D. Gerthsen, C. Feldmann*, Tungsten Nanoparticles from Liquid-Ammonia-based Synthesis, Chem. Commun. 2014, in press.