Novel covalent organic nanosheets for the construction of ultrafine and well-dispersed metal nanoparticles

The development of supported and well-dispersed ultrafine metal nanoparticles (U-MNPs) with high metal content is a research priority, yet remains a significant challenge. Herein, we propose an effective strategy involving functional covalent organic nanosheets (CONs) to construct well-dispersed U-MNPs. To construct the corresponding CON-DAI-TFP, 4,7-diamino-1H-indazole (DAI) and 1,3,5-triformylphloroglucinol (TFP) are strategically chosen as precursors. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM) clearly confirm the nanosheet structure of CON-DAI-TFP. CON-DAI-TFP is used as a substrate to immobilize Pd nanoparticles (Pd-NPs). The supported Pd-NPs are ultrafine, highly dispersed, and possess high metal content (average size, 1.74 nm; metal content, 20.47 wt%), which is superior to many of the reported supported Pd-NPs. Moreover, the supported Pd-NPs show excellent catalytic activity and recyclability in a p-nitrophenol reduction reaction. In addition to Pd-NPs, this strategy can be applied to produce other U-MNPs with similar features, including Ir-NPs and Pt-NPs (1.99 nm and 17.18 wt% for Ir-NPs, 2.25 nm and 29.46 wt% for Pt-NPs), suggesting that this strategy is universal. For comparison, a bulk COF (COF-DAI-TFB), the analogue of CON-DAI-TFP, is designed and synthesized. In contrast, many large Pd-NPs with an average size of 17.86 nm, which is more than 10 times the size of Pd-NPs on CON-DAI-TFP, are observed on the surface of the COF-DAI-TFB. This result further confirms the significance of CONs in the preparation of well-dispersed and high-loading U-MNPs.