# Melamine Foams Decorated with In-Situ Synthesized Gold and Palladium Nanoparticles
## 3 Results and Discussion
### ME/PDMS/Au and ME/PDMS/Pd Foams
Previous studies have shown that the functional components of the PDMS are inducing the formation of noble metal NPs on their surface upon chemical reduction of the metallic precursors adsorbed. In particular, the noble metal nanoparticles formation is expected to happen in non-reacted cross-linking Si-H sites [10; 34; 35; 30]. In order to prove, in the present case, the transformation of the adsorbed Au or Pd ions into NPs onto the surface of the foams in the presence of PDMS, detailed HRSEM and EDS studies were performed. As shown in Figure 4, both ME/PDMS/Au and ME/PDMS/Pd foams present small NPs and aggregates on the surface of their struts. Moreover, EDS mapping of the struts of the ME/PDMS/Au and ME/PDMS/Pd foams confirmed that both the small NPs and bigger aggregates are attributed to the presence of Au or Pd NPs (Figure 4c,d, respectively). The presence of Au or Pd NPs in their crystalline form was further confirmed by X-ray Diffraction analysis in all cases (XRD, see Supplementary Information, Figure S3). It should be mentioned that the presence of Au or Pd NPs is strictly related to the PDMS layer, as the EDS elemental maps obtained by HRSEM analysis of ME and ME/PDMS foams proved. In fact, the NPs are present only on the latter foams (see Supplementary Information, Figure S4). In contrast, pure ME foams subjected to the same dipping procedure presented no modifications on their surface. Regarding the effect of the reaction time for a fixed concentration of 0.350 mg/mL of HAuCl4, quite constant average sizes between 18 and 20 nm and similar size distributions with most of the NPs ranging from 10 to 30 nm were found for reaction times between 1 and 48 h (Figure 5 and Figure S6, see Supplementary Information). This result indicates that the significant increase of Au transferred to the foams occurring in that range, from 0.1 to 1 wt.% (Figure 3), is related to the rise of the number of  
Figure 4: HRSEM micrographs of the struts of the ME/PDMS/Au (**a**) and ME/PDMS/Pd foams (**b**) showing the presence of small nanoparticles (NPs) and aggregates. EDS maps of the micrographs showing of the ME/PDMS/Au (**c**) and ME/PDMS/Pd (**d**) showing the Au (**c**) or Pd (**d**) signal of the NPs. Au NPs synthesized and not to their size increase. Longer reaction times provided a slightly larger average size of about 25 nm and a broadening of the size distribution with a noticeable rise in the presence of NPs over 30 nm (Figure 5). Therefore, taking into account the results of the amount of Au transferred and the NPs sizes the optimal conditions to produce the ME/PDMS/Au foams are a HAuCl4 concentration of 0.350 mg/mL and 48 h of reaction time, obtaining 1 wt.% of Au NPs with sizes about 18 nm synthesized on the foams. The achieved amount of Au NPs synthesized on the foams is comparable to the results obtained by [PERSON] et al. [33], about 0.9 wt.%, and significantly higher than those of [PERSON] and [PERSON] [35] (0.06 wt.%). On the contrary, the reaction time between the ME/PDMS foams and the 0.500 mg/mL Pd precursor solutions present a stronger influence on the Pd NPs size than in the case of Au NPs. Reaction times from 1 to 24 h provided NPs average sizes about 27 to 30 nm (Figure 6 and Figure S7, see Supplementary Information), while longer times (48 and 120 h) produced Pd NPs with average sizes about 42 to 45 nm (Figure 6 and Figure S7, see Supplementary Information). In all the cases, the NPs size distributions present similar widths, with no clear relationship with the reaction time. Accordingly, in this case, it is possible to obtain ME/PDMS/Pd foams with similar Pd loads of about 1.0 to 1.1 wt.% (Figure 3) and different particle sizes, about 30 or 45 nm, by using reaction times of 24 or 48 h, respectively. The obtained results, in terms of the amount of Pd transferred to the foams, are lower than previous results reported in the literature using porous polymeric substrates (6.7 wt.% using Poly(styrene/DVB)/PolyHIPE and 3 wt.% using Polysulfone membranes) [36; 37].