In-situ TEM Study On The Mechanical And Oxidation Behavior Of Nanosized Silver

As a noble metal,silver(Ag)and Ag-based materials have wide applications in industry and our lives.Recently,micro-and nanosized Ag have attracted numerous attentions due to their potential applications in micro-nano electromechanical systems,electronics and catalyst etc.In the real applications,the Ag nanocomponents will be exposed to different kinds of loadings,such as mechanical,electronic and thermal loading,which may result in the structural change and thus degradation of these nanocomponents.It is thus necessary to investigate the structural evolution of Ag nanocomponents during these processes.Optimal design of micro-nano electromechanical devices requires tuning both the size and length scales of their nanocomponents,however,the length effect on mechanical behavior of small crystals remains poorly understood.Here,nano-mechanical deformation behavior of Ag nanowires with the same diameter but different lengths was researched via electron beam induced amorphous carbon deposition.The in situ nano-mechanical testing reveals a remarkable length-dependent mechanical property in silver nanowires,where both the strength and ductility display a net increase with the length reduction.Such length-dependent plasticity originated from the change of slip mode and the unusual length effect on strengthening resulted from the difference of frequency and stability of surface flaws on the nanowires.Quantitative analysis shows that reduction of crystal size and crystal length could induce a completely-opposite strengthening in metallic nanowires due to the distinct operating mechanisms.Specifically,with the reduction of aspect ratio,the strength of small metals shows a net increase if the sample diameter is constant,but it decreases dramatically if the sample length keeps unchanged These findings provide new insights into the size-dependent deformation in metallic nanowires,holding important implications for designing durable nanostructures and nanodevices.Oxidation is a critical issue that influences the performance of metallic materials in their applications.During the in situ nanomechanical testing,electron irradiation damage was observed on the freshly-formed surface of Ag nanowires,which was identifies as the oxidation of Ag by high resolution transmission electron microscopy(TEM).To further understand the oxidization dynamics,we conducted an in situ oxidation experiment of nanosized Ag inside TEM.It revealed that the oxidation preferentially occurs at the edge of(111)surface steps in Ag,and the formation and growth of these surface oxides involves a solid-solid transformation.The DFT simulation results verity the experimental results that the O binding energy on the edge of(111)surface step in Ag nanocrystals is much higher than that on the surface facets,favoring the nucleation of Ag2O.Similar oxidation behavior also occurs in nanosized of palladium(Pd).Besides,ex situ characterization of the Ag exposed in air for few days shows that numerous atomic steps exist at the Ag/Ag2O oxidation interface,suggesting that the oxidation mechanism we observed is also likely applicable in the natural oxidation conditions.These findings provided new experimental evidence for understanding the initial oxidation behavior of metals.