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Electrochemical Cycling of Polycrystalline Silver Nanoparticles Produces Single Crystal Silver Nanocrystals Poonam Singh, Ray W. Carpenter, and Daniel A. Buttry Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03156 • Publication Date (Web): 03 Nov 2017 Downloaded from http://pubs.acs.org on November 6, 2017
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Electrochemical Cycling of Polycrystalline Silver Nanoparticles Produces Single Crystal Silver Nanocrystals Poonam Singh, Ray W. Carpenter and Daniel A. Buttry* School of Molecular Sciences Arizona State University Tempe, AZ 85287-1604 Corresponding author:
[email protected] Abstract Electrochemically driven phase transformations in redox active nanoparticles (NPs) are important in a number of areas including batteries and sensors. We use high resolution electron microscopy in conjunction with ex situ electrochemical experiments on TEM grids to study the oxidative conversion of polycrystalline silver NPs to amorphous silver oxide nanoparticles and their reductive conversion back to single crystal silver nanocrystals (NCs). Results show that during oxidation nucleation occurs uniformly at the NP surface, producing a Ag@Ag2O core@shell structure during growth. The images reveal polycrystalline Ag cores and amorphous Ag2O shells for these structures. Electron microscopy also showed that electrochemical reduction of Ag2O NPs can produce single crystal Ag nanocrystals, suggesting that point nucleation at the NP-electrode interface during reduction enables a growth mechanism favoring formation of single crystal nanoparticles. Introduction Metal nanoparticles (NPs) have proven to be immensely useful in a range of applications, notably including sensing and electrocatalysis.1-3 While control of size is often a key requirement in these and other areas, a number of research efforts have demonstrated that control of nanoparticle crystallinity and structure can have substantial impact on their properties. Murray and coworkers showed that Pt nanocrystals presenting different facets can have strikingly different electrocatalytic reactivities.4 Tang and Ouyang showed that single crystal silver NPs exhibit different chemical reactivity, electron-phonon interactions, and nanomechanical properties compared to their multiply-twinned counterparts.5 Tian et al. showed that single crystal Pt nanocrystals with high index facets are much more potent electrocatalysts for oxidation of small fuel molecules, such as formic acid and ethanol, than Pt nanocrystals with lower index facets.6 Chen et al. reported that electrochemically grown Fe NPs can be produced under conditions that result in single crystal NPs, and that faceting on these nanocrystals strongly influences the electrocatalytic efficiency for nitrite reduction.7 Collectively, these reports demonstrate the potent influence that nanoparticle structure, and especially crystallinity, can have on their properties. Because of the strong correlation between structure and function for metallic NPs, a number of methods have been developed that provide control over both structure and crystallinity of metallic NPs. Early work by Xia and coworkers elucidated the influence of adsorbates, especially halide ions, on the ability to produce single crystal metallic NPs.8 Tang and Ouyang described a similar strategy in which molecular precursors bearing halides could be employed to control crystallinity.5 Prucek et al. showed that smaller (approx. 30 nm diameter) Ag NPs could be “recrystallized” by treatment in solutions containing oxygen and high 1 ACS Paragon Plus Environment
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concentrations of chloride to give much larger (approx. 400 nm) Ag single crystals.9 Murray and coworkers used a “colloidal recrystallization” approach to control nanocrystal morphology and the types of facets presented at their surfaces.4 Other groups have used electrochemical approaches to control either crystallinity or structure. These methods have generally relied on two general approaches. In one, control of deposition parameters, such as the potentials at which nucleation and growth take place, provided control over the growth of the NPs.7, 10, 11 In the other, repeated oxidation and reduction cycles on Pt NPs were used to electrochemically anneal previously prepared nanospheres, producing unusual tetrahexahedral shapes with a large density of high index faces.6 A unique approach used the formation of alloys with low melting metals that subsequently could be electrochemically removed, thereby eliminating defects and promoting formation of single crystal NPs.12 These reports demonstrate that electrochemical approaches provide a great deal of control over both the initial formation of metallic NPs and their structural evolution over time during electrochemical processing. We have previously reported on the synthesis and electrochemical properties of polycrystalline silver NPs that are synthesized using adenosine triphosphate (ATP) as a (removable) capping agent.13-15 We found that the reactivity of these NPs as electrocatalysts for the oxygen reduction reaction (ORR) was influenced by the facets exposed at the NP surface.13 This dependence of reactivity on surface structure is consistent with a broader body of work that has emerged over time.16 We also showed that these Ag NPs could be electrochemically converted between Ag and Ag2O (as well as various silver halides) via non-topotactic phase transformations that preserved the identity of the individual NPs.14 In other words, the individual NPs were converted directly from one phase to the other during the electrochemical oxidation and reduction reactions that drove the phase transformations, with no dissolution or redeposition of the NPs during the interconversions. This behavior led us to investigate the mechanisms by which these phase transformations take place. We report here the results of a high resolution electron microscopy study of the electrochemical oxidation of Ag NPs to produce Ag2O NPs and their subsequent reduction to regenerate Ag NPs. We show that the oxidation process occurs by homogeneous nucleation of the oxide over the entire surface of the Ag NP, as evidenced by the presence of Ag@Ag2O core@shell structures as intermediates. In contrast, the subsequent reduction of Ag2O NPs to Ag NPs occurs via point nucleation of the Ag metal at the interface where the Ag2O NP is attached to the electrode surface. Because of this point nucleation, the resulting Ag NPs grow into single crystals. Thus, this study describes a unique way in which single crystal silver NPs may be prepared. Experimental NP synthesis and purification All chemicals including silver nitrate (AgNO3, 99.999%), adenosine 5′-triphosphate disodium salt hydrate (Na3ATP, 99 %), sodium borohydride (NaBH4, 99.9%) and poly(diallyldimethylammonium) chloride (PDDA) were purchased from Sigma-Aldrich. Deionized (DI) water of resistivity 18.3 MΩ from a Millipore purification system was used for all solutions. ATP capped silver NPs were prepared as previously described.14 Briefly, AgNO3 and Na3ATP were added to DI water in a sealed glass vial under bubbling N2, followed by addition of freshly prepared NaBH4. A light yellow color appeared immediately upon addition of NaBH4, indicating the formation of ATP capped silver nanoparticles. The formation and growth of the NPs was allowed to proceed for 4-5 hours, during which the solution darkened to an amber 2 ACS Paragon Plus Environment
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color. Following this, the NP solution was concentrated on a rotary evaporator. Solid NPs were obtained by addition of isopropanol to the concentrated NP solution, which caused precipitation of the NPs. Further, the NPs were isolated by centrifugation. The solid NPs can be readily redispersed in water and precipitated again using isopropanol. This process was repeated five times to eliminate free ATP from the NP sample. Characterization Both electrochemical and electron microscopy methods were used to examine ATP capped silver NPs in layer-by-layer (LbL) films on Au TEM grids coated with a thin film of amorphous carbon (aC). These LbL films were prepared using a modification of the method previously described.14 The modification comprised assembling the films on Au TEM grids coated with amorphous carbon (aC) rather than the previously reported gold-coated Au TEM grids. After coating the Au grids with amorphous carbon in a thermal evaporator, a brief exposure to an air plasma was used to produce carboxylic acid functionalities on the carbon surface that could be deprotonated to impart a net negative charge on the surface of the aC film. The remainder of the LbL film formation followed the previous procedure.14 Briefly, a 5 minute exposure of the grids to 0.1 M NaOH was used to deprotonate the acid groups. Then, cationic PDDA chains were electrostatically adsorbed to the negatively charged carboxylate groups by soaking in a PDDA solution. This was followed by attachment of the negatively charged ATP capped Ag NPs to the cationic PDDA layer by soaking in the NP solution. Finally, a second layer of cationic PDDA was electrostatically adsorbed onto the Ag NP layer. In this way, electrostatic attraction between the cationic PDDA layers and the negatively charged ATP capped Ag NPs allowed for the stable immobilization of the Ag NPs at the electrode surface, where they could be interrogated using electrochemical or HRTEM methods. We note that, as previously described, electrochemical oxidation of the Ag NPs from the metallic state to the oxide state and back causes loss of the ATP capping ligands, preventing the ATP ligands from influencing the behavior described here.13-15 A JEOL ARM200F STEM-aberration-corrected analytical electron microscope equipped with both an x-ray spectrometer and an electron spectrometer was used for all microscopy. NPs supported in LbL films on aC-coated Au support grids were examined by bright field (BF) phase contrast or high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) at 80 kV. Single or polycrystal character of NPs was confirmed by computed Fourier nanodiffraction patterns. Contrast in the HAADF images is proportional to Zx , where Z is the average atomic number of the target and 1.8
