Vacancy Concentration Variation In Nanoporous Gold And The Size-dependent Effect

Nanoporous materials have open porous structure, nano-scale size, large volume-surface area and tunable ligament size. Nanoporous materials can be used as catalysts, sensors, actuators, filters and electrochemical catalysts. Dealloying is a corrosion process has been studied for decades, so it is very important to study the dealloying process and the thermal coarsening process for tuning the nanoporous structure. In this study, we investigated the dealloying process and thermal coarsening process of nanoporous metals, and the size-dependent vacancy concentration of thin films and nanoparticles. The main results are summarized bellow:(1) The vacancies and small voids evolution within the ligaments of nanoporous (np) Au(Pt) in dealloying processes were investigated by positron annihilation spectroscopy. The np Au(Pt) samples with 5-8 nm ligaments were prepared by electrochemical dealloying of (Au0.95Pt0.05)25Ag75 ingots. The positron lifetime of as-prepared np Au(Pt) consists of the two main components. One is assigned to the positron annihilation at vacancies within the nano-ligaments and the other to the positron annihilation in the enclosed voids in the ligaments. Vacancy concentration increased significantly when silver atoms leached out further from the np structure, which supports the vacancy-mediated lattice diffusion mechanism of porosity evolution.(2) Vacancies and encased voids evolution within the nanoporous (np) AuAg(Pt) ligaments during thermal coarsening was investigated by positron annihilation spectroscopy and scanning electron microscopy (SEM). Partially dealloyed ternary np AuAg(Pt) samples with 5-8 nm ligaments were prepared by electrochemical dealloying of the (Au0.95Pt0.05)25Ag75 ingots. Positron lifetime experiments demonstrated that nano-ligaments are stable against coarsening below 350 ?. Vacancies play an essential role during the coarstning process. Lifetime parameters changed above 350 ?, associated with a large number of vacancies and divacancies formed via coarsening with large enclosed voids formed at the same time. SEM images demonstrate ligaments growing non-uniformly during the thermal coarsening process.(3) Solid films are considered as typical model systems to study size effects on thermal vacancies concentration in nanomaterials. By combining the generalized Young-Laplace equation with the chemical potential of vacancies, a strict size-dependent thermodynamic model of vacancies, which includes the surface intrinsic elastic parameters of the eigenstress, Young's modulus and the geometric size of the solid films, was established. The vacancy concentration changes in the film with respect to the bulk value, depending on the geometric size and surface stress sign of the solid films. Atomistic simulations of Au and Pt films verify the developed thermodynamic model. These results provide physical insight into the size-dependent thermal vacancy concentration in nanomaterials.(4) Size-dependent thermodynamic model is presented to analyze simultaneously stress, vacancy concentration and their coupling behavior in spherical shaped metal nanoparticles. A nanoparticle is treated as a composite with a core lattice and a surface shell, and both core and shell possess their corresponding thermodynamic properties. Thermodynamic analysis about metallic particles of Ni, Cu, Au and Pt shows the similar results that both the vacancy concentration in the core and surface shell could be greatly decreased as the particle size decreased. It has also been found that the apparent vacancy concentration in the whole nanoparticle increases as the particle size decreases, and then reaches a maximum at a small particle size. This conclusion provides comprehensive understanding of the vacancy concentration in nanomaterials, which had been represented by the two opposite views in the last decades.