Subscriber access provided by BUFFALO STATE
Energy, Environmental, and Catalysis Applications
Microstructural Evolution of Au@Pt Core-shell Nanoparticles under Electrochemical Polarization Wei Hong, and Christina W. Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b10158 • Publication Date (Web): 31 Jul 2019 Downloaded from pubs.acs.org on August 1, 2019
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Materials & Interfaces
Microstructural Evolution of Au@Pt Core-shell Nanoparticles under Electrochemical Polarization Wei Hong and Christina W. Li* Department of Chemistry, Purdue University, 560 Oval Dr., West Lafayette, Indiana 47907, United States Supporting Information Placeholder
Keywords: electrocatalysis, colloidal synthesis, core-shell, bimetallic, structural evolution ABSTRACT: Understanding the microstructural evolution of bimetallic Pt nanoparticles under electrochemical polarization is critical to developing durable fuel cell catalysts. In this work, we develop a colloidal synthetic method to generate core-shell Au@Pt nanoparticles of varying surface Pt coverage in order to understand how as-synthesized bimetallic microstructure influences nanoparticle structural evolution during formic acid oxidation. By comparing the electrochemical and structural properties of our Au@Pt core-shells with bimetallic AuPt alloys at various stages in catalytic cycling, we determine that these two structures evolve in divergent ways. In core-shell nanoparticles, Au atoms from the core migrate outwards onto the surface, generating transient “single atom” Pt active sites with high formic acid oxidation activity. Metal migration continues until Pt is completely encapsulated by Au, and catalytic reactivity ceases. In contrast, AuPt alloys undergo surface dealloying and significant leaching of Pt out of the nanoparticle. Elucidating the dynamic restructuring processes responsible for high electrocatalytic reactivity in Pt bimetallic structures will enable better design and predictive synthesis of nanoparticle catalysts that are both active and stable.
INTRODUCTION Bimetallic Pt nanoparticles are an important class of catalysts for electrochemical reactions, designed to maximize Pt utilization as well as tune the energetics and ensemble geometry of exposed Pt atoms for high catalytic reactivity and selectivity.1-4 As-synthesized, bimetallic nanoparticles adopt a variety of morphologies including core-shell structures, solid solution alloys, intermetallic alloys, and phase-segregated structures, many of which are metastable under electrochemical conditions.5-13 Understanding the microstructural evolution of bimetallic structures under electrochemical polarization is critical to developing catalysts that are durable for long-term operation in electrolytic devices. AuPt bimetallic alloys have been studied extensively for electrochemical formic acid and alcohol oxidation, and previous studies have shown that Au-rich compositions exhibit enhanced electrocatalytic activity.14-21 Layered or decorated Pt on Au structures have also been synthesized via electrodeposition or chemical reduction of Pt onto metallic Au, and similar enhancements in activity were observed at low coverage of Pt on the surface.22-27 Extensive characterization of AuPt bimetallic structures has been carried out to identify the surface speciation of Pt responsible for enhanced formic acid oxidation activity, and several authors have postulated that isolated Pt atoms embedded within the Au lattice show enhanced direct dehydrogenation of formic acid and suppressed formation of the catalyst poison carbon monoxide.14,23,28,29 The majority of these studies have focused on structureactivity relationships based upon the as-synthesized bimetallic structure, but a few authors have observed that thin films of Pt on polycrystalline Au undergo significant electrochemical changes over repeated scanning.23,30 Direct observation of the physical structure of these catalysts corresponding to the
electrochemical evolution and identification of the in-situ surface structure responsible for high reactivity remains elusive. In this work, we have developed a colloidal synthetic method for the controlled deposition of submonolayer and monolayer Pt layers on Au nanoparticles in order to study the evolution of Au@Pt core-shell structures under formic acid oxidation (FAO) conditions. We find that the FAO activity of single monolayer Au@Pt nanoparticles increases dramatically over the first 100 cyclic voltammetry (CV) scans as the surface transforms from a pure Pt surface in the as-synthesized core-shell structure to a mixed Au-Pt surface. This process is followed by slow deactivation over an additional 1100 cycles as Pt is further depleted from the surface. Detailed transmission electron microscopy coupled with energy dispersive X-ray spectroscopy suggests that the initial fast activation may be due to formation of isolated Pt ensembles via the outward migration of Au atoms, and the deactivation process involves a slower restructuring of the mixed Au-Pt outermost layers. EXPERIMENTAL SECTION Synthesis of Au@Pt Nanoparticles. 16 nm Au-citrate nanoparticles were synthesized via a literature method.31 In a typical synthesis of Au@Pt core-shell nanoparticles, 400 µL of 5 mM K2PtCl4 solution was added into 40 mL of 0.5 mM Aucitrate nanoparticle solution. The mixture was stirred at room temperature for 1 min and then transferred to a pre-heated water bath at 70 °C and stirred for 1 hour. After cooling down to room temperature, the nanoparticles were collected by centrifugation and washed two times with water to remove excess K2PtCl4. The nanoparticles were redissolved in 2 mL water, and 10 eq. of ascorbic acid with respect to Pt was added. The solution was sonicated for 10 min, and excess ascorbic acid was then washed away. The resulting nanoparticles were dissolved in 2 mL water to give a nominal 10 mM solution by Au atom%. This synthesis
ACS Paragon Plus Environment
ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
results in 0.9 ML Pt on Au. To vary the thickness of the Pt shell, the amount of Pt precursor added was varied between 4 mol% and 40 mol% with respect to Au. Physical Characterization. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and EDS mapping were obtained on an FEI Talos F200X S/TEM with a 200 kV X-FEG field-emission source and a super X-EDS system. Inductively-coupled plasma ionization mass spectrometry (ICP-MS) data was collected using a Thermo Fisher ELEMENT 2. To obtain characterization data for samples at max activity and post catalysis, the glassy carbon electrode coated with Au@Pt/C after electrochemical cycling was immersed in 0.5 mL isopropanol and sonicated until all of the powder came off the electrode. The isopropanol suspension was then centrifuged at 14000 rpm for 8 min, and the supernatant was removed. The solid was dried in air prior to analysis. Electrochemical Methods. Electrochemical experiments were conducted on a Pine WaveDriver 20 Bipotentiostat and Gamry Interface 1010B potentiostat. The working electrode was prepared by drop casting 2 µL of a 10 mM colloidal nanoparticle solution or 1.8 µL of Au@Pt/C suspension. The samples were air dried and coated with 2 µL 0.5 wt% Nafion solution. The counter electrode was a Pt mesh gauze. Currents are reported with anodic current as positive and cathodic current as negative. Potentials were measured against a Ag/AgCl reference (3.5 M KCl) and converted to the RHE reference scale using: E (vs. RHE) = E (vs. Ag/AgCl) + 0.210 V + 0.0591 V*pH Initial electrochemical characterization of Au@Pt was carried out on unsupported nanoparticles, drop cast directly from the colloidal solution. All subsequent extended electrochemical cycling was carried out on nanoparticles supported on carbon. Cyclic voltammetry in 0.1 M HClO4 under N2 at 50 mV/s was carried out to ascertain PtO and Au(OH)3 reduction peak positions and relative surface coverages. The electrode was scanned from 0.0 V to 1.7 V vs. RHE, and the metal oxide reduction areas were integrated assuming two and three electron processes for Pt and Au respectively.32,33 Electrochemical CO stripping was performed to evaluate the electronic structure and surface area of Pt in 0.1 M HClO4. The electrode was held at 0.22 V vs. RHE in CO-saturated 0.1 M HClO4 solution for 5 min and then purged with N2 for 15 min to completely remove dissolved CO. Linear sweep voltammetry at 50 mV/s was subsequently carried out from 0.22 V to 1.27 V vs. RHE. One additional CV was recorded to obtain the baseline under N2. Formic acid oxidation reactions were carried out using CV between 0.11 V and 1.3 V vs. RHE in a 0.5 M HCOOH + 0.1 M HClO4 solution at a scan rate of 50 mV/s. The solution was purged with N2 for at least 15 minutes prior to the experiment. Only the anodic scan of the FAO voltammogram is shown in the following data. The electrolyte was replaced every 20 CV cycles before samples reached maximum catalytic activity and every 30 CV cycles after max activity for Au@Pt core-shell nanoparticles due to the consumption of formic acid during cycling. FAO cycling experiments were repeated three times for each Au@Pt/C sample, and the current density at maximum activity is reported as an average over those three runs. Electrochemical surface area measurements, CVs in 0.1 M HClO4, and CO stripping experiments were carried out after
Page 2 of 20
various FAO cycling endpoints. FAO current densities are normalized using the capacitive surface area. Electrochemical Surface Area Measurements. Capacitive surface area was obtained by recording CVs at scan rates of 10, 20, 50, 80, 100 and 120 mV/s between 0.32 V and 0.52 V vs. RHE, a potential region where no Faradaic processes occur. Capacitive current density (jcap) at 0.42 V vs. RHE was plotted versus scan rate, and the double layer capacitance, Cdl (mF/cm2) was determined from the slope of the fitted line. Representative examples are given in Fig. S13. Roughness factors and normalized current densities were calculated by referencing to a sample of pure Vulcan carbon, and the values are given in Table S3. Cu underpotential deposition was also performed to determine metal-only electrochemical surface area. The electrode was transferred to 0.1 M HClO4 + 1 mM Cu(NO3)2 electrolyte and electrochemically cleaned by holding at 0.90 V vs. RHE for 5 s. After polarizing the electrode at 0.30 V vs. RHE for 100 s, two CVs were recorded from 0.30 V to 0.90 V vs. RHE at a scan rate of 10 mV/s. A representative example of a Cu-UPD scan is given in Fig. S14, and measured values are given in Table S4. These two methods of electrochemical surface area determination resulted in very similar roughness factors. RESULTS AND DISCUSSION Our group has previously demonstrated a colloidal ligandexchange synthesis of Au@Pd core-shell nanoparticles, in which submonolayer and monolayer Pd surfaces were generated through the self-limiting adsorption of tetrachloropalladate anions onto metallic Au surfaces.34 Adapting this colloidal ligand-exchange method, we have deposited controlled submonolayers of Pt onto colloidal Au core nanoparticles, ranging in coverage from 0.3 to 0.9 monolayer (ML) (Fig. 1). Starting with 16 nm citrate-capped Au nanoparticles, submonolayer Pt0 is deposited onto the Au
Figure 1. TEM characterization of as-synthesized (a) Au-citrate nanoparticles and (b, c, d, e) Au@Pt core-shell nanoparticles. (f, g, h, i) HAADF-STEM and EDS mapping of 0.9 ML Au@Pt nanoparticles.
ACS Paragon Plus Environment
Page 3 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Materials & Interfaces
surface by reacting potassium tetrachloroplatinate with residual citrate on the surface (Fig. S1-S4). By varying the amount of K2PtCl4 in solution from 4 mol% to 40 mol% with respect to Au, we are able to achieve a range of submonolayer coverages. At the 4 mol%, 6 mol%, and 10 mol% Pt loadings, the majority of the PtCl42– in solution is deposited onto the surface, resulting in 0.3 ML, 0.5 ML, and 0.8 ML surface coverage based on integration of the PtO and Au(OH)3 reduction peaks in the CV (Fig. 2a) and 3%, 4%, and 6% average Pt percentage relative to Au based on inductively coupled plasma-mass spectrometry (ICP-MS) (Table S1). A close-packed monolayer of Pt atoms on a 16 nm Au core is calculated to have 9% Pt relative to Au. Comparing the ICP-MS atomic percentage to the surface coverage obtained from CV confirms that the majority of the deposited Pt remains electrochemically accessible on the surface of the Au nanoparticle. At the highest deposition loading (40 mol% with respect to Au), a large excess of free Pt complex remains at the end of the ligand adsorption step and must be removed through multiple centrifugation and washing steps. An atomic percentage of 8% Pt by ICP-MS and 0.9 ML surface coverage by CV is obtained, clearly indicating that no
secondary nucleation or thick shell growth of Pt is observed even with an excess of K2PtCl4 in solution. Transmission electron microscopy (TEM) images show that the morphology and diameter of the initial Au-citrate nanoparticles are retained after Pt deposition (Fig. 1a-e). Scanning transmission electron microscopy coupled to energydispersive X-ray scattering (STEM-EDS) of the 0.3 ML and 0.5 ML Au@Pt samples reveal that Pt is evenly distributed on Au with no secondary nucleation of individual Pt nanoparticles though the shell layer is not clearly discernible at this resolution (Fig. S5, S6). The higher coverage Au@Pt samples (0.8 ML and 0.9 ML) show a clear shell of Pt that is
