Size Dependence of Cubic to Trigonal Structural Distortion in Silver Micro- and Nanocrystals under High Pressure

Abstract

Silver micro-and nanocrystals with sizes of -2-3.5 llm and -50-100 nm were uniaxially compressed under nonhydrostatic pressures (strong deviatoric stress) up to -30 GPa at room temperature in a symmetric diamond-anvil cell and studied in situ using angle-dispersive synchrotron X-ray diffraction. A cubic to trigonal structural distortion along a 3-fold rotational axis was discovered by careful and comprehensive analysis of the apparent lattice parameter and full width at half maximum, which are strongly dependent upon the Miller index and crystal size. 1 In the past fifteen years, much attention has been paid to the high pressure behavior of nanomaterials, especially the pressure-induced structural transformations in semiconductor 1O nanocrystals. I -In contrast, there are few reports on structural transformations and distortions in noble metals under high pressure due to their close-packed structures. It is generally accepted that sub millimeter crystals of silver (~149 !lm) and palladium (~250 Jlll1) are stable in the face-centered cubic (fcc) structure at room temperature up to at least 91. Two samples of conventional, irregularly-structured Ag crystals were purchased from Aldrich and used in our comparative study. They have reported average sizes and purities of2-3.5 Jlll1 and 99.9+%and <100 nm and 99.5%, respectively and are referred to as micro-and nano-Ag powders from hereon. Figure 1 shows representative scanning and transmission electron micrographs (SEM and TEM, respectively) of the Ag samples. These samples were uniaxially compressed under nonhydrostatic pressures up to -30 OPa in a symmetric diamond-anvil cell (DAC) and studied in situ using angle-dispersive synchrotron X-ray diffraction. Two ruby grains, located in the center and at the edge of the samples, respectively, were used for pressure calibration and radial pressure gradient estimation. 17 Detailed experimental procedures are similar to our previous report. 162 Figure 2 shows X-ray diffraction patterns, up to ~30 GPa, for the micro-and nano-Ag powders.From first analysis, it appeared that no phase transfonnations or distortions occurred because no splits, disappearances, or appearances of the diffraction peaks were observed. But when the d-spacing of the (200) peak was carefully compared with that of the (111) The apparent lattice parameters a200 and alll and their ratio a200/al II for the nano-Ag powder are shown in Figure 3a and Figure 3b, respectively. It is well-known that ahkl of Ag with the idealized fcc structure is not dependent upon the Miller index; however, according to Figure 3, it becomes apparent that ahktCP) in the nano-Ag is actually strongly dependent upon the Miller index when P > 1.9 OPa. In general, a200, was always larger than the corresponding alII and the higher the pressure, the larger the relative difference (within the pressure range of 0 to 11.2 GPa). Specifically, the relative difference at ambient pressure was less than 0.03% but increased to 0.37% at a pressure of 11.2 GPa. Beyond 11.2 GPa, this difference d...

Stress-induced phase transformation and optical coupling of silver nanoparticle superlattices into mechanically stable nanowires

“…Mechanical property. As discussed here and in previous work [12][13][14][15][16]18 , high-pressure stress can effectively induce Ag microstructure phase transformations and potentially manipulate the mechanical stability of Ag nanoparticles. Gu et al 12 found an unexpected high stiffness of Ag and gold (Au) nanoparticles under high pressure.…”

“…Many chemical methods use such defect structures (also the most reactive sites) to achieve oriented growth of Ag nanostructures; for example, oriented attachment was used to grow chain-like Ag nanostructures 11 . It has been shown elsewhere that an external stress induced by high-pressure compression can alter the mechanism of defect motion in atomic lattices through induced phase transformations [12][13][14] . Depending on the pressure condition, different Ag phase and structural transformations were observed: under hydrostatic pressure up to 10 GPa, Ag nanoparticles with sizes of 5-10 nm underwent a reversible, linear, pressure-dependent structural transformation 14 .…”

“…Depending on the pressure condition, different Ag phase and structural transformations were observed: under hydrostatic pressure up to 10 GPa, Ag nanoparticles with sizes of 5-10 nm underwent a reversible, linear, pressure-dependent structural transformation 14 . At higher uniaxial compressions, disordered Ag microstructure was rearranged through phase transformations to form Ag nanoparticles with much higher stiffness 12,13,15 . Recent transmission electron microscopy (TEM) studies further confirmed these phase transformations in Ag microstructures and proposed that external stress can effectively alter the defect mode of Ag microstructures through relaxation of surface and internal strain 15,16 .…”

Optical and thermodynamic studies of silver nanoparticles stabilized by Daxad 19 surfactant

“…During the last twod ecades,muche ffort havebeen devoted toinvestigating the opticalpropertiesof silvernanoparticless ynthesized viac hemicalreductions [11,12], gas condensation [13], laserirradiation [14], sonochemicaldeposition [15]a nd nanostructured templates [7].All these advanceshaveadvanced scientificknowledge of the nature of silvernanoparticles.Aheadamong others,c hemicalreduction techniquesofferanovel,simple proceduref ort he synthesisof silvernanoparticlescompared toothers which required high production cost and moreprocess timing becausetheyu sed complexs et-upsfors ynthesis [16][17][18]. Well-defined particle size,shape and sized istribution are controlled byjudiciouslychoosing the reaction temperature,the orderof addition of reagents and the ratio of protectiveagent/metalcompound whicharethe factors actuallygovernthe plasmonresonancee xcitation of nanoparticles.…”

Multiple‐Step Phase Transformation in Silver Nanoplates Under High Pressure

“…This reversible distortion is consistent with a mechanism that considers the bond‐length distribution in idealized, multiply twinned icosahedral particles. Guo et al compared the Ag nanocrystals (with size of ≈50–100 nm) and microcrystals (with size of ≈2–3.5 μm) with irregular morphologies under uniaxial compression (i.e., nonhydrostatic pressure) up to ≈30 GPa, and found a cubic‐to‐trigonal structural distortion along a three‐fold rotational axis of the fcc lattice 14. These studies show an obvious controversy regarding the pressure‐dependent structural variation in Ag nanoparticles.…”