Scalable Synthesis of Ag Networks with Optimized Sub-monolayer Au-Pd Nanoparticle Covering for Highly Enhanced SERS Detection and Catalysis.

Tianyu Li,Shaochun Tang,Sascha Vongehr,Xiangkang Meng,Yuming Dai,Xiao Huang

doi:10.1038/srep37092

Abstract

Highly porous tri-metallic AgxAuyPdz networks with a sub-monolayer bimetallic Au-Pd nanoparticle coating were synthesized via a designed galvanic replacement reaction of Ag nanosponges suspended in mixed solutions of HAuCl4 and K2PdCl4. The resulting networks’ ligaments have a rough surface with bimetallic nanoparticles and nanopores due to removal of Ag. The surface morphology and composition are adjustable by the temperature and mixed solutions’ concentration. Very low combined Au and Pd atomic percentage (1−x) where x is atomic percentage of Ag leads to sub-monolayer nanoparticle coverings allowing a large number of active boundaries, nanopores, and metal-metal interfaces to be accessible. Optimization of the Au/Pd atomic ratio y/z obtains large surface-enhanced Raman scattering detection sensitivity (at y/z = 5.06) and a higher catalytic activity (at y/z = 3.55) toward reduction reactions as benchmarked with 4-nitrophenol than for most bimetallic catalysts. Subsequent optimization of x (at fixed y/z) further increases the catalytic activity to obtain a superior tri-metallic catalyst, which is mainly attributed to the synergy of several aspects including the large porosity, increased surface roughness, accessible interfaces, and hydrogen absorption capacity of nanosized Pd. This work provides a new concept for scalable synthesis and performance optimization of tri-metallic nanostructures.

Highlights

Nanostructures of noble metals such as gold (Au), silver (Ag), platinum (Pt), and palladium (Pd) have received extensive attention due to their promising applications in catalysis[1,2,3], biosensors[4,5], optical devices[6,7], and surface-enhanced Raman scattering (SERS) detection[8,9]
We report the rational design and fabrication of highly porous trimetallic Au-Pd-Ag networks with sub-monolayer Au-Pd nanoparticle coating via a galvanic replacement reaction (GRR) in mixed solutions of chlorauric acid (HAuCl4) and potassium chloropalladite (K2PdCl4)
During a slow GRR process, well-dispersed nuclei form firstly on the surface starting from a selective replacement of the Ag ligaments at point defects

Summary

Results and Discussion

Details of the synthesis as well as the related calculations are described in the Supporting Information. According to σ≅ 3⁄4(R/r) s and x = (1−3s)/(1−2s), small differences in the amount of Ag translate into large differences in the surface coverage σ.With NPs of r = 4 nm radius, x = 0.89 has 17% more of its surface covered than x = 0.92 Both parameters, x and y/z, strongly influence the number of tri-metallic interfaces, and x influences especially the number of accessible Ag nanopores due to removed Ag. We fixed the Au/Pd ratio to the pre-optimized value of y/z = 3.55 in the mixed GRR solutions. As for the influence of the geometric factors, the galvanic replacement process results in a large increase in the surface roughness of the Ag ligaments (see Fig. 2) Another well-known acceleration of hydrogen involving reactions is due especially to the metallic Pd47 acting as a “hydrogen relay system”[56]. Catalysts Ag0.90Au0.078Pd0.022 Au@AgPd NDs Ag/Au NDs Ag/Pd NPs Ag/Pt NWs Au/Ni NSs Au-Pd/carbon Fe3O4@C@Ag-Au Ag sponges Au NPs

This work

Conclusions

Author Contributions

Additional Information