Insights into the synthesis of porous nanoparticles: from sputtering onto liquids to dealloying
Synthesizing high purity metal nanoparticles (NPs), having a well define shape and size distribution remains very challenging. Nowadays, NPs are mainly produced either by chemical or a physical methods. However, both methodologies suffer from drawbacks, i.e. the purity of NPs obtained by chemical methods is low while the control over the NP size, dispersion, and shape when using physical methods is limited . In this context, the combination of both approaches seems promising to overcome these drawbacks and sputtering onto liquid appears to be an efficient alternative. In this process, ionized argon (ions) are accelerated from the plasma towards a metallic target to induce the ejection (i.e. the sputtering) of metal atoms or clusters into the vacuum chamber. These particles coalesce into NPs once they reach the liquid substrate . This technique has been successfully used for the production of metal NPs and the simultaneous co-sputtering of two metal targets enabled the synthesis of homogenous alloy NPs . Such alloy NP can be the dealloyed to make nanoporous NPs  with enhanced properties as, for example, in cancer therapy applications . The dealloying, or selective leaching, deals with the removing of the less noble metal of an alloy to leave behind a skeleton made of the more noble element. This procedure has been investigated for thin films but not so much for NPs [4, 6].
In this contribution, we first present the production of Au-Cu NPs by magnetron co-sputtering of gold and copper targets onto a liquid substrate (pentaerythritol ethoxylate). Second, the coalescence of these NPs by thermal annealing is discussed because it is a necessary to get big enough NP to allow the dealloying process to occur. Finally, the formation of nanoporous gold NPs by dealloying is demonstrated. Overall, this study implies the use of transmission electron microscopy, small angle X-ray scattering, DFT calculations and UV-Vis absorption spectrophotometry.