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Plating Precious Metals on Nonprecious Metal Nanoparticles for Sustainable Electrocatalysts Lei Wang, Zhenhua Zeng, Cheng Ma, Yifan Liu, Michael Giroux, Miaofang Chi, Jian Jin, Jeffrey Greeley, and Chao Wang Nano Lett., Just Accepted Manuscript • Publication Date (Web): 05 May 2017 Downloaded from http://pubs.acs.org on May 8, 2017
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Plating Precious Metals on Nonprecious Metal Nanoparticles for Sustainable Electrocatalysts Lei Wang,1 Zhenhua Zeng2, Cheng Ma3, Yifan Liu1, Michael Giroux1, Miaofang Chi3, Jian Jin4, Jeffrey Greeley2, Chao Wang1,* 1
Department of Chemical and Biomolecular Engineering, Johns Hopkins University, Baltimore,
Maryland 21218, USA; 2
Davison School of Chemical Engineering, Purdue University, West Lafayette, IN 47907, USA;
3
Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge,
Tennessee 37831, USA; 4
Nano-Bionics Division and i-Lab, Collaborative Innovation Center of Suzhou Nano Science
and Technology, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, Jiangsu 215123, China;
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Abstract Precious metals have broad applications in the modern industry and renewable energy technologies. The high cost and limited availability of these materials, however, have caused a grand challenge for sustainability. Here we reported on the plating of a precious metal on nonprecious metal nanoparticles for the development of sustainable electrocatalysts. Cobalt/platinum core/shell (denoted as Co@Pt) nanoparticles were synthesized via seed-mediated growth. The Co seeds were first synthesized by thermal decomposition of cobalt carbonyl, and the Pt shell was overgrown in situ by adding platinum acetylacetonate (Pt(acac)2). Galvanic replacement reaction between Co and the Pt precursor was successfully suppressed by taking advantage of CO (generated from the decomposition of cobalt carbonyl) as the stabilizing ligand and/or reducing agent. The obtained Co@Pt nanoparticles were further found to exhibit enhanced catalytic activity for the oxygen reduction reaction (ORR).
Keywords: core/shell nanoparticles, cobalt, platinum, precious metal catalysts, oxygen reduction reaction
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Precious metals play an indispensable role in modern industry, with broad applications in electronics, optics and catalysis.1, 2 The recent development of renewable energy technologies enabled by electrochemical energy conversion and storage, such as water electrolyzers, fuel cells and metal-air batteries, has further led to growing demands for precious metals, in particular, platinum (Pt), for electrocatalytic applications.3, 4 The availability of these critical materials is, however, limited due to their low abundances on earth, which represents a grand challenge for sustainability.5 Significant effort has thus been dedicated to improve the efficiency of using precious metals or develop their earth-abundant substitutes in various applications. Plating has long been studied for decoration and modification of metal surfaces, probably originating from ancient Egypt where it was used to give copper a gold or silver finish.6-8 Plating in the nanoscale, e.g., coating a nonprecious metal (NPM) nanoparticle with a precious metal (PM) shell, is highly desirable for the development of cost-effective nanomaterials for catalytic and other applications. The formation of NPM/PM core/shell nanostructures may not only increase the specific surface areas in terms of precious metals, but also take advantage of the interactions between the two metals to enhance the catalytic activity (e.g., via ligand and strain effects9, 10). It is noticed that core/shell catalysts have been extensively studied in the literature, but mostly limited to those with both the core and shell consisting of precious metals.11-19 Growth of NPM/PM core/shell nanoparticles has largely remained challenging, primarily due to the surface of NPM nanoparticles being prone to oxidation20 and/or galvanic replacement reactions that usually takes place between precious metal salts and non-precious metals21. Here we report on the synthesis of Co@Pt nanoparticles in organic solutions. Plating of Co with Pt is achieved by in situ overgrowth of Pt on Co seeds (Scheme 1, see also the Supporting Information for more details). The nanostructures are characterized by scanning transmission
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electron microscopy (STEM)-based imaging and element mapping. Mechanisms for inhibiting (or mitigating) the galvanic replacement reaction is discussed based on the control experiments with different amounts of CO present in the solution for Pt overgrowth. The obtained Co@Pt nanoparticles are further subjected to electrochemical studies for demonstration of the catalytic applications. Cobalt nanoparticles were first prepared by thermal decomposition of cobalt carbonyl in dichlorobenzene (DCB) in the present of dioctylamine and oleic acid as the surfactants.22 Pt was then overgrown on these Co seeds by adding platinum acetylacetonate (Pt(acac)2) in situ. The Co nanoparticles possessed a polycrystalline nature and sphere-like shape23 (Figure 1a), which had directed the growth of the core-shell nanostructure into pseudospherical particles (Figures 1 b and c; see the Supporting Information for more TEM images). The obtained Co@Pt nanoparticles have an overall dimeter of ~10 nm and a uniform shell thickness of ~1 nm, with most of the lattice fringes exhibited on the higher-resolution transmission electron microscopy (HRTEM) images corresponding to the (111) planes of face-centred cubic (fcc) Pt (Figures 1 d – f). The elemental line profiles reveal that the change of composition from the Co core to the Pt shell is rather smooth (Figure 1f), indicating the presence of a compositional gradient at the interface rather than a sharp boundary, which could be a result of the interfacial diffusion of elements during the synthesis (at ~160 oC). The XRD pattern collected for the Co@Pt nanoparticles only shows a weak peak at ~40.4o, which can also be assigned to the (111) plane of Pt (Figure S5). The synthesis reported here is different from the previously reported method using metal carbonyls to initiate the nucleation and ensure the conformal growth of Pt-alloy shells on metal seeds.16, 19 It is also noted that the strategy of seeded growth by just adding Pt(acac)2 in situ has previously been used for coating PtNi nanoparticles with Pt, where the suppression of self-
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nucleation and island growth was achieved by controlling the rate of Pt deposition to be slower than that for Pt diffusion on the surface.24 During the growth of the Co seeds, a significant amount of CO was generated from the decomposition of cobalt carbonyl. These CO species are believed to play a crucial role in enabling the plating of Pt on Co. Ex situ growth using washed Co seeds resulted in Pt multipods attached to the Co nanoparticles (Figure 2a). CO has previously been used as both reducing agent and stabilizing ligand in shape-controlled synthesis of Pt and Pt-alloy nanoparticles, where CO was either introduced by flowing or generated by decomposition of metal carbonyls in situ.25-28 Such a dual role can also be effective in the growth of Co@Pt nanoparticles reported here. It is likely that certain amount of CO residues from the growth of Co seeds adsorbed on the surface and/or left in the solution (even after blowing with Ar, see the Supporting Information for the experimental methods), which served as ligands to protect the Co seeds from galvanic replacement reaction and also as reductant to facilitate the initial deposition of Pt on the Co surface. Once these CO species were consumed, oleylamine could further reduce the Pt salt (Pt(acac)2) to enable continual growth of the Pt shell. Here it should be noted that the presence of extra CO (without blowing with Ar) during the Pt overgrowth was found to be detrimental and led to the formation of separate Pt nanoparticles (Figure 2b), which is probably due to the fast reduction of Pt(acac)2 when too much CO is present in the solution. Besides CO, the reaction temperature and the solvent used for adding the Pt precursor were found to be also important for ensuring the complete coating of Co with Pt (Figures S6 – S12). The growth reported here is also distinct from the galvanic replacement that has widely been adopted for overgrowth of a noble metal onto a less-noble metal nanoparticle,29-31 with the latter usually resulting in hollow or porous nanoparticles. A recent report by Yang et al. demonstrated galvanic replacement-free growth of Ag@Au core-shell nanoparticles, where the
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oxidation of Ag seeds by the gold precursor (chloroauric acid, HAuCl4) was supressed (or mitigated) by introducing a strong reducing agent (ascorbic acid) and optimizing the power of reduction via tuning of solution pH.32 The growth mechanism of Co@Pt nanoparticles discussed above could be in line with this report, where an appropriate concentration of CO left in the solution might give rise to the desired reduction power for the Pt overgrowth. The CO-mediated growth has produced core/shell nanoparticles with Co preserved in the core after the growth of the Pt shell, as evidenced by the central Co peak observed in the composition line profile (Figure 1f). This could be important for maintaining the intermetallic coupling effects for catalytic enhancement. The obtained Co@Pt nanoparticles were loaded on carbon black and evaluated as electrocatalysts for the ORR. The cyclic voltammogram (CV) of Co@Pt exhibits more pronounced peaks for underpotential deposition of hydrogen (Hupd) (at E < 0.4 V) than Pt. (Figure 3a). The specific electrochemically active surface area (ECSA) estimated from the Hupd charges are 54 and 43 m2/gPt for Co@Pt and Pt, respectively. Although the overall particle size of Co@Pt is significantly larger than Pt (~5 nm), the Co@Pt catalyst has a higher specific surface area owing to the presence of non-precious cobalt in the particle core. From the CVs, it can be seen that Co@Pt displays a positive shift of 40 mV as compared to Pt for the adsorption peak of oxygenated species (e.g., OHad, at 0.8 – 0.9 V) in the anodic scans, albeit the peak being more pronounced which can be ascribed to the higher ECSA of the core/shell catalyst at the same loading (ca. 20 μgPt/cm2). A similar shift can also be seen in the cathodic scans. These observations suggest that the surface of Co@Pt is less oxophilic and has weaker binding to the oxygenated species than the surface of pure Pt, which is desired for lowering the kinetic barrier of the ORR.33 Such an anticipation is confirmed by the ORR polarization curves shown in Figure 3b, where Co@Pt exhibits a positive shift of 68
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mV in half-wave potential as compared to Pt, and the Tafel plots shown in Figure 3c, with Co@Pt having a lower Tafel slope (43 mV/dec) than Pt (55 mV/dec). The kinetic current densities (specific activities) were calculated to be 2.24 and 0.36 mA/cm2 (at 0.9 V) for Co@Pt and Pt, respectively (Figure 3d). Correspondingly, the mass activity of Co@Pt achieved 1.21 A/mgPt, which represents an improvement factor of 8.1 versus the commercial Pt catalyst (Figure 3e). In addition to the high catalytic activity observed in the initial electrochemical studies, further activation of the Co@Pt catalysts was observed after potential cycling in the ORR-relevant regime. After 5,000 cycles between 0.6 and 1.0 V, the specific activity was raised to 3.02 mA/cm2, while negligible loss was observed in ECSA (Figures 3 a and d). As a result, the mass activity achieved 1.45 A/mgPt after the potential cycling, corresponding to an improvement factor of 9.7 versus Pt (Figure 3e). The loss in catalytic activity after further potential cycling was found to be insignificant (e.g., up to 10,000 cycles, Figure S16). “Activation” by potential cycling has previously been reported by Cui et al. on Pt-Ni alloy nanoparticles, which was ascribed to compositional segregation and/or surface restructuring induced by dealloying.34 It is also noticed that the compositional changes near the surface could have a large impact on the surface properties and catalytic activities.35-37 To examine potential nanoscale and surface restructuring in the Co@Pt nanoparticles, we have collected STEM images and element maps for the core/shell catalyst before and after the potential cycling. It is found that the thermal annealing for the catalyst preparation (at 185 oC in air to remove the organic surfactants, see the Supporting Information for the experimental methods) has caused diffusion of Co outward, forming a Pt-Co alloy shell, although the core/shell structure was still visible from the contrasts in the STEM images (Figure 4a). After 5,000 cycles between 0.6 and 1.0 V, the core/shell nanostructure was still preserved, but the Co species in the near-surface region has been leached
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out, forming a Pt-rich surface layer of 0.5 – 0.6 nm in thickness that is corresponding to about two or three atomic layers of Pt (Figure 4b, see more TEM images in the Supporting Information). It seems that the Pt shell has protected the Coin the core from further leaching out, as evidenced by the very small drop in Co content (
