Synergistic Interactions Between Silver and Palladium Nanoparticles

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Synergistic interactions between silver and palladium nanoparticles in lubrication Chanaka Kumara, Harry M. Meyer, and Jun Qu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b01248 • Publication Date (Web): 10 Jul 2019 Downloaded from pubs.acs.org on July 19, 2019

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ACS Applied Nano Materials

Synergistic Interactions Between Silver and Palladium Nanoparticles in Lubrication Chanaka Kumara,1,2 Harry M Meyer III,1 Jun Qu1,* 1Materials

2Department

Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, 37831, United States

of Chemistry, University of Tennessee, Knoxville, Tennessee, 37996, United States

Abstract: The reactivity and functionality of nanoparticles (NPs) are determined by the materialand size-dependent physicochemical properties at the nanoscale. While single-type metallic NPs have been extensively studied for many potential applications, combination of multi-type NPs and the interactions among them have received less attention. Here, an interesting synergistic effect between silver and palladium NPs is presented when they are used together as lubricant additives. Dodecanethiol-modified Ag and Pd NPs were discovered to interact actively in a solution, changing their sizes, shapes, and electronic structures. A mixture of these two NPs was able to form a stable suspension in a lubricating oil and effectively improve tribological performance in the concentration range of 0.1-1 wt%. The Ag-Pd NP blend further reduced the friction and wear by >30% and >80%, respectively, compared with either NPs alone at the same total NP concentration. The NPs and worn surfaces were characterized using a set of comprehensive analytical tools and the superior lubricating behavior was attributed to the formation of two types of protective tribofilms on the contact surfaces.

Key words: Palladium, Silver, Nanoparticles, Lubricant additives, Synergistic effects, Tribofilm, Friction, wear.

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INTRODUCTION Nanoparticles (NPs) have been extensively studied in many areas of science because of their unique size-dependent physical and chemical properties at the nanoscale. They are diverse in nanostructures, compositions, and physiochemical properties, and thus have shown promises in various applications. In lubrication science, metallic, ceramic, and carbon-based NPs have been explored as potential additives to reduce friction and wear.1-11 Furthermore, some types of nanomaterials were found to provide synergistic effects when used together.12-15 Ag NPs were reported to be utilized with MoS2 or graphene to synergize the friction and wear performance.7, 16-17

Deposition of Ag NPs enhanced the graphene lubrication by preventing the nanosheet

restacking during the rubbing process. Further, the anchored Ag NPs help to exfoliate the graphene nanosheets facilitating the interlamination sliding.17 The low shear strength of Ag and laminar structure of the MoS2 jointly provided low friction. In combination of AgNP and MoS2 at high temperature (>450 oC) produced stable, self-lubricating Ag2MoO4 complex for high temperature applications.16 Pan et al. reported significant friction and wear reductions when SiO2 and MoS2 NPs were used together to lubricate an AZ31 magnesium alloy – AISI 52100 steel interface.18 Synergistic lubrication was also found between graphene and WS2 NPs for a steel-steel contact.19 In previous studies, we had developed dodecanethiol-modified Ag1 and Pd4 NPs and used them separately as lubricant additives, and either could form a stable suspension in a polyalphaolefin (PAO) lubricating oil to effectively reduce friction and wear in steel-steel sliding. Ag and Pd NPs were selected for study due to their relatively high chemical stability and robust synthesis protocol. Dodecanethiol was chosen as a ligand to enhance the NPs’ oil solubility and to prevent NPs aggregation. Combinations of Ag and Pd NPs were studied in the literature for catalytic reactions including oxygen reduction20 and hydrogen generation reactions21. The Ag-Pd alloy system


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exhibited enhanced electrocatalytic activity22-23 as a result of the interactions between the Pd and Ag NPs and the presence of Ag was believed to avoid inhibition of the Pd active sites in catalytic reactions.24-25 However, there was no reported attempts for using the Ag and Pd NPs together in lubrication. Here we present a newly discovered, interesting synergistic effect when adding both the dodecanethiol-modified Ag and Pd NPs into a lubricating oil, resulting in significant friction and wear reductions compared with using either the Ag or Pd NPs alone. The NPs were synthesized using the method developed in our previous reports,1, 4 as described in the experimental section. Both NPs were easily suspendable in organic solvents such as toluene. In this work, NP blends were prepared by mixing the Ag and Pd NPs (at 1:1 by weight) in a toluene solution at both room and elevated temperatures to allow interactions and reactions, if any, before the NPs being extracted from the toluene and then added a PAO 4 cSt base oil for tribology testing.

RESULTS AND DISCUSSION The behavior of the NP mixture was studied by mixing the Ag and Pd NPs in a toluene solution (1:1 weight ratio) at both room temperature (RT) and 80 C. Initial characterization was done using UV-visible spectroscopy. The Ag NPs alone exhibited a broad band centered at around 450 nm due to surface plasmonic resonance (SPR), which is a collective oscillation of the valance band free electron in response to the incident radiation (Figure 1a). In contrast, the Pd NPs alone showed a featureless spectrum in the UV-visible range. Optical spectra of the NP mixture were monitored over time at room temperature, as shown in Figures 1a and S1. The SPR peak of the Ag NPs gradually became broader and blue-shifted after mixing with the Pd NPs. The peak shifting, and broadening indicated modification of the electronic structure of the SPR of the Ag NPs, possibly


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by Pd atom doping. The SPR peak eventually dampened and blue-shifted from 452 to 425 nm after 8 days. The size distribution of the as-synthesized Ag and Pd NPs are 3–6 nm and 2–4 nm (Figure S2), respectively.1, 4 Upon mixing and after 8 days of storage, the NPs evolved to be relatively large particles of up to 10 nm, as shown in the transmission electron microscopy (TEM) images in Figure 1b. The evolutions in both the optical spectrum and particle morphology suggested interactions between the Ag and Pd NPs. The NP mixture was further studied by heating another sample of freshly mixed Ag and Pd NPs in toluene at 80 C and holding the temperature for 30 min. As shown in Figures 1c, S3, and S4, heating evidently caused faster interactions between the Ag and Pd NPs, as reflected by more significantly blue-shifting (to 402 nm) and broadening of the Ag NPs’ plasmonic band, compared with the SPR at RT. Interestingly, the SPR peak of the Ag NPs alone was blue-shifted (452 to 422 nm) and narrowed with the heat treatment as shown in Figures 1a (right) and this shift can be attributed to the heat-induced Ag NPs’ aggregation/size increase. The size change of the Pd NPs seemed to be insignificant after heating. The large NPs (~20 nm) in quasi-spherical shape appeared in the Ag-Pd NPs’ mixture after 30-min heating. The quasi-spherical shapes and broad size distribution of the NPs in the heat-treated mixture may be due to several causes: (1) Inter-NP atom diffusion: Having similar atomic radii and crystal lattices, Ag and Pd NPs may interact and form AgPd alloy NPs. Recently, inter-NP atom exchange phenomena have been revealed by several research groups both theoretically and experimentally.26 The plasmonic peak shifting and particle size and shape changes are possibly due to diffusion between the Ag and Pd NPs; (2) Galvanic replacement reaction (GRR). Silver NPs consist of Ag (0) in the metallic core and Ag (+1) at the interface in staple motif. Similarly, Pd NPs have metallic Pd in the core and the oxidized form (2+) at the interface. The difference in the standard reduction potentials (Pd=0.987 eV and Ag=0.799


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eV) may lead to a redox reaction. The Pd in the NP outer-shell (Pd2+) may be reduced to the metallic form (Pd0) by oxidizing the metallic Ag in the core. This galvanic reaction was further evidenced by X-ray photoelectron spectroscopy (XPS) analysis. A similar GRR phenomenon has previously been reported to form AgPd alloy NPs.23, 27-29


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C. 80oC

Figure 1. UV-visible spectra and TEM morphology of the NPs. (a) UV-visible spectra of the Ag NPs (red), Pd NPs (blue), and AgNP + PdNP mixture (1:1 by weight) at room temperature (RT) and 80 oC. TEM image of Ag NPs, Pd NPs, and AgNP + PdNP mixture; (b) After 8 days stored at room temperature (RT); (c) Heat treated at 80 C for 30 minutes. Optical spectra were measured in toluene solution and spectra are offset vertically for clarity.


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The two metallic Ag and Pd NPs were mixed into a PAO base oil to test their lubricating properties. Silver and Pd NPs have been previously reported as effective lubricant additives when used alone.1-2, 7 While the Pd NPs alone remained stably suspended in the PAO base oil for more than 6 months, the Ag NPs alone would start to precipitate after 2–3 months in the oil, perhaps because of impact of visible light. In contrast, the combination of Ag and Pd NPs appeared to form a kinetically stable colloidal suspension in the oil which remains for >18 months (Figure S5) without observable precipitation or color change. The viscosity of the oils without and with NPs were measured at 23, 40 and 100 oC and results (see Table S1) showed insignificant change (

Synergistic Interactions Between Silver and Palladium Nanoparticles

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