Xiulan Tan

Template-dealloying synthesis of ultralow density Au foams with bimodal porous structure

Noble metallic foams with low density are desirable in high energy-density physical experiments. In this study, Au foams with ultralow density were successfully synthesized from a template-dealloying approach. Microspherical polystyrene (PS) was employed as the sacrificial template. Au and Ag nanoparticles were sequentially coated onto the PS surface from a three-step deposition process. Cylindrical PS/Au/Ag monoliths were formed by filter-casting of the nanoparticle coated PS suspension. The templates were removed and Au–Ag alloy foams were generated after calcination at 400 °C. Self-supported Au foams with bimodal porous structure were produced by dealloying of the less noble Ag component. Au foam with density as low as 0.36 g cm−3 can be achieved, which demonstrates high surface area of 23.6 m2 g−1 and pore volume of 0.15 m3 g−1.

Convenient synthesis of silver nanoplates with adjustable size through seed mediated growth approach

Silver nanoplates, with average thickness about 5 nm and average tunable size from 40 to 500 nm, were synthesized via a simple room-temperature solution-phase chemical reduction method in the presence of appropriate concentration of trisodium citrate and silver seeds. The optical in-plane dipole plasmon resonance bands of these silver plates could be tuned from 520 to 1100 nm. Control experiments were explored for understanding of the growth mechanism. It is found that both the amount of citrate ions and the small silver seeds added to the growth solution are the key to controlling the silver nanoplates without changing their thickness and crystal structure. Small silver seeds are found to play an important role in the formation of large thin silver nanoplates when poly(vinylpyrrolidone) (PVP) are used as capping agent.

Mesoporous gold sponges: electric charge-assisted seed mediated synthesis and application as surface-enhanced Raman scattering substrates

Mesoporous gold sponges were prepared using 4-dimethylaminopyridine (DMAP)-stabilized Au seeds. This is a general process, which involves a simple template-free method, room temperature reduction of HAuCl4·4H2O with hydroxylamine. The formation process of mesoporous gold sponges could be accounted for the electrostatic interaction (the small Au nanoparticles (~3 nm) and the positively charged DMAP-stabilized Au seeds) and Ostwald ripening process. The mesoporous gold sponges had appeared to undergo electrostatic adsorption initially, sequentially linear aggregation, welding and Ostwald ripening, then, they randomly cross link into self-supporting, three-dimensional networks with time. The mesoporous gold sponges exhibit higher surface area than the literature. In addition, application of the spongelike networks as an active material for surface-enhanced Raman scattering has been investigated by employing 4-aminothiophenol (4-ATP) molecules as a probe.

Multi-Scale Modeling for Predicting the Stiffness and Strength of Hollow-Structured Metal Foams with Structural Hierarchy

This work was inspired by previous experiments which managed to establish an optimal template-dealloying route to prepare ultralow density metal foams. In this study, we propose a new analytical–numerical model of hollow-structured metal foams with structural hierarchy to predict its stiffness and strength. The two-level model comprises a main backbone and a secondary nanoporous structure. The main backbone is composed of hollow sphere-packing architecture, while the secondary one is constructed of a bicontinuous nanoporous network proposed to describe the nanoscale interactions in the shell. Firstly, two nanoporous models with different geometries are generated by Voronoi tessellation, then the scaling laws of the mechanical properties are determined as a function of relative density by finite volume simulation. Furthermore, the scaling laws are applied to identify the uniaxial compression behavior of metal foams. It is shown that the thickness and relative density highly influence the Young’s modulus and yield strength, and vacancy defect determines the foams being self-supported. The present study provides not only new insights into the mechanical behaviors of both nanoporous metals and metal foams, but also a practical guide for their fabrication and application.

Linear magnetoresistance in gold foams

The electrical transport properties of metal foams may not only be of fundamental research interest but can also lead to wide application in sensors. We report the magnetotransport properties of gold foams prepared by a chemical template–dealloying method. The temperature dependence of the resistivity of gold foams represents a metallic behavior with zero magnetic field. With increasing magnetic field, a crossover from quadratic to linear dependence of the magnetoresistance in gold foams is observed in the studied temperature range (2–50 K). The physical mechanisms resulting in this observation may be ascribed to the classical linear magnetoresistance theory based on the spatial mobility fluctuations or current distribution effect. However, further evidence is required to determine the exact mechanism.