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The Surface/Composition of Nanoparticles Au/Ag Core/ Alloy Interface Influences the Methanol Oxidation Reaction Peter N Njoki, Maurice E. D. Roots, and Mathew M. Maye ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01255 • Publication Date (Web): 17 Sep 2018 Downloaded from http://pubs.acs.org on September 19, 2018

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ACS Applied Nano Materials

The

Surface/Composition

of

Nanoparticles

Au/Ag

Core/Alloy Interface Influences the Methanol Oxidation Reaction Peter N. Njoki,a* Maurice E. D. Rootsa† and Mathew M. Mayeb a b

Department of Chemistry and Biochemistry, a† Department of Physics, Hampton University, Hampton Virginia, 23688 U.S.A. Department of Chemistry, Syracuse University, Syracuse New York, 13244 U.S.A

* Corresponding author. Tel.: +1-757 727 5833; fax: +1-757-727-5604; e-mail: [email protected]

Abstract We report on a study to investigate the surface/composition of gold-silver nanoparticles (Au/Ag NPs) via electrochemical analysis as a function of Ag thickness and synthetic temperature. Au/Ag NPs were synthesized at hydrothermal temperatures and supported on Vulcan XC-72. The shifts in the Surface Plasmon resonance (SPR) with an increase in Ag thickness, depended on whether the NPs had core/shell or core/alloy morphologies. X-ray photoelectron spectroscopy (XPS) showed different proportion of Au/Ag at various layers of Ag. Cyclic voltammetry (CV) of carbon-supported Au/Ag NPs (Au/Ag/C) was used to probe the nanostructure's surface via methanol oxidation reaction (MOR) in alkaline solution. The CVs demonstrated the differences in the redox reactions between the core/shell and core/alloy morphologies. This study shows the systematic electrochemical investigation of Au/Ag core/alloy interfacial composition in methanol oxidation reaction.

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Keywords: Gold, Silver, Nanoparticles, Core-shell, Core-alloy, Electrochemical, Methanol Oxidation Reaction, Current Density 1. INTRODUCTION Ag NPs are attractive as electrocatalysts because of their low price, high electrocatalytic activity as well as methanol tolerance.1-6 Au NPs have also attracted special attention in heterogeneous catalysis as demonstrated by Haruta7 and others.8-9 Alloying Ag NPs with other metals increases their stability and bifunctional synergistic catalytic effect.10-19 Multimetallic NPs exhibit properties and catalytic reactivities that are different from their monometallic counterparts.10-21 To understand and exploit these properties for practical applications, it is important to probe the NPs interfacial composition.4,22 Numerous researchers have probed the surface of bimetallic NPs using non-electrochemical techniques.14,23-24 Kim and co-workers characterized the morphology of Pt@Pt/Ag alloy NPs with SERS spectra of 4-aminothiophenol, which proved that the surface consisted of both Pt and Ag NPs.14 Mott and colleagues used FTIR to probe the CO adsorption on different compositions of AuPt NPs, which showed the surface to consist of bimetallic surface.23 Tian's lab characterized core−shell nanoparticles of varying thickness using enhanced Raman spectroscopy.24 Zhang and colleagues used in situ SERS and shell-isolated nanoparticle-enhanced Raman spectroscopy to elucidate the fundamental role of interfaces in catalysis.25 Few studies have probed the surface of bimetallic AuAg NPs using electrochemical methods.13,19,26-27 Gasparotto's lab elucidated the influence of the composition of gold-silver NPs on configuration and glycerol electrooxidation.19


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Tominaga and colleagues used CV to study the surfaces of different composition of AuAg NPs with electrolytic product being a function of the amount of Ag in the alloy.26 In this paper, we describe the electrochemical analysis of the interfacial composition of Au/Ag NPs synthesized at hydrothermal temperatures (TH) = 120 °C and 160 °C and then assembled on carbon black powder (Vulcan XC-72). The fabricated NPs exhibited core/shell and core/alloy morphologies when synthesized at 120 °C and 160 °C respectively, as reported in our previous work.28-30 The different interfaces were experimentally observed by an initial SPR blue shift at lower Ag thickness and subsequently a red shift as the Au SPR was screened and Ag SPR at ~400 nm predominated for the core/shell while the core/alloy exhibited a prominent hump at ~480 nm accompanied by another SPR wavelength at ~425 nm.28 In this work, our goal was to use CV to further probe the surface composition of the NPs since the surface elements are electroactive.27 The presence or absence of the redox waves for Au/Ag during electrochemical analysis gives an indication of the metal composition on the surface. The CVs of Au/Ag/C were carried out with/without methanol in 0.5 M KOH with the results exhibiting contrasting reactivity towards MOR depending on the surface morphology and composition. 2. MATERIALS AND METHODS 2.1. Chemicals Hydrogen tetrachloroaurate (III) trihydrate (HAuCl4•3H2O, 99.9+ %), trisodium citrate dihydrate (C6H5Na3O7•2H2O, 99 %), silver nitrate (AgNO3, 99.9+ %), Nafion (5 wt %), and methanol (CH3OH, >99.8 %) were procured from Sigma Aldrich. Carbon black powder (Vulcan XC-72R) was purchased from Cabot. Silver nanoparticles (Ag NPs, 20


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nm) were purchased from Ted Pella Inc. Water was purified to 18 MΩ cm using a Millipore Milli-Q® water purification system. 2.2. Synthesis of Au/Ag Nanoparticles. We synthesized Au/Ag NPs using a hydrothermal process that employed microwave irradiation.28 Au NPs (15.2 ± 1.0 nm) were first made following a modified citrate method and used as cores for making Au/Ag NPs.28,31 To synthesize Au/Ag, we used a calculated amount of the pre-synthesized citrate-capped Au cores and deposited a shell (n) of Ag in a systematic pattern as reported in our prior work.28 After the synthesis, the NPs were characterized via UV-vis, TEM and XPS. Figure 1 illustrates the synthesis of Au/Ag NPs at hydrothermal temperatures and electrochemical analysis of the Au/Ag/C catalysts in methanol oxidation reaction.

Figure 1. An idealized schematic representation for the fabrication of Au/Ag NPs and electrochemical characterization of the surface of the core/alloy interface. 2.3. Electrochemical Characterization. Before electrochemical analysis, the NPs were cleaned by multiple washings to eliminate excess reactants, by-products and capping agent.32 Thermal gravimetric analysis (TGA, Q-500 TGA Analyzer) was done to


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determine the NPs metal concentration in each sample. Typical TGA samples containing ~20 µL of NPs were heated from 25 °C to 800 °C in a Pt pan in N2 at a rate of 10 °C/min. The assembly of Au/Ag NPs on Carbon black powder support was accomplished following our previously reported protocol.33 The resulting Au/Ag/C mixture was gathered and dried through N2 purge. Due to Ag propensity to oxidation and possible change in nanostructures, the carbon-loaded NPs were not thermally treated. The Au/Ag/C catalyst ink was prepared in water, ethanol and 5 wt % Nafion (200:50:1) giving an ink concentration of 1 µg/1 µL. 4 µL of ink was then deposited on the cleaned shiny glassy surface of carbon electrodes (0.3 cm in diameter and a geometric surface area of 0.07 cm2, Bioanalytical Systems, Inc., USA) and allowed to dry in air. Catalytic activity was probed by CV measurements (Galvanostat, PAR-Model 263A) using three electrodes set-up in an electrochemical cell as reported in our previous work.33 2.4. Instrumentation. Microwave Synthesizer. A Discover-S (CEM Inc.) microwave was used to synthesize Au/Ag NPs. The microwave operates at preset power, temperature and pressure with the reactants held in pressurized vials as reported by Wu and colleagues.28 Ultra-violet visible spectrophotometry (UV-vis). We collected the UV-vis spectra using a Varian Cary 100 Bio UV-vis spectrophotometer at the wavelength range of 200 900 nm as we previously reported. 30 Transmission Electron Microscopy (TEM) and X-Ray Photoelectron Spectroscopy (XPS). The TEM and XPS measurements were performed at Cornell Center for Materials Research following the protocols in our previous work.28,30


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Thermal gravimetric analysis (TGA). TGA measurements were performed on a Q-500 TGA Analyzer to determine the NPs metal concentration in each sample. Samples containing ~20 µL of NPs were heated in a Pt pan in N2 at a rate of 10 °C/min. 3. RESULTS AND DISCUSSION In this section, we report our results by comparing the optical properties and electrochemical results of Au/Ag/C NPs fabricated at hydrothermal temperature of 120 °C and 160 °C. The difference in synthesis temperature resulted in Au/Ag NPs with varied morphology.28-30 We first report the XPS analysis of Au/Ag NPs containing various Ag layers (n) and fabricated at different hydrothermal temperatures (Table 1). The results revealan elevated Au content in the shell at the angle of 55°. As more Ag is added, we observed a correlation with the XPS data. This correlation was found to be influenced by the morphology of the Au/Ag NPs.28 Table 1: XPS results of Au/Ag NPs prepared at hydrothermal temperatures of 120 °C and 160 °C, Ag shells n = 3, 7 and 10. TH (oC) 120 160

n 3 7 10 3 7 10

Au (%) 32 14 13 29 19 20

Ag (%) 68 86 87 71 81 80

Figure 2 (left column) displays TEM morphology of Au and Au/Ag NPs with the NPs increasing in size range of 17.9-22.6 nm from a core of 15.2 nm, as shown in Figure S1. Figure 2 (middle column) shows UV-vis of Au/Ag (n = 10 (a), 7 (b) and 3 (c)) and


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Au seeds (n = 0 (d)) respectively. As reported in our prior work,28 the Au SPR band blue shift from 522 nm to 410 nm as Ag was deposited at TH = 120 °C. This SPR response corresponds with the formation of Ag-rich structure, as evidenced by the blue shift with an increase in Ag layers, shown in Figure 2 (middle column).

Figure 2. Morphological, UV-vis and electrochemical observations for Au/Ag core/shell NPs synthesized at TH = 120 °C for Ag layers n = 10 (a), 7 (b), 3 (c) and 0 (d). TEM micrographs (left), UV-vis (middle), and CVs (right) comparing n = 0, 3, 7 and 10. The NPs were loaded on carbon with ~20 wt%. Electrochemical analysis done in 0.5 M KOH in presence (solid curves) and absence (dashed curves) of 1.0 M methanol at a scan rate of 100 mVs−1 in room temperature. Figure 2 (right column) shows the CVs results for the Au/C and Au/Ag/C electrodes in 0.5 M KOH with and without 1.0 M methanol. The CV for Au NPs (n = 0)


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with and without methanol, shows oxidation wave at ~0.38 V and a reduction wave at ~0.1 V, which are ascribed to oxidation of Au and reduction of the oxide in absence of methanol. The oxidation peak at ~-0.1 V is attributed to the chemisorption of OH- and the formation of pre-oxidized groups on the Au surface as reported by Liu's lab.34 As more Ag is deposited on Au, at n = 3, Au features are still observed as the surface is not fully covered with Ag and the reduction peak is split into two. The formation of an alloy skin at lower Ag layers,28 may explain the double peaks between -0.1 V and 0.1 V. With deposition of more Ag at n = 7 and 10, the Ag signatures are observed.35 In methanol at n = 7 and 10, we observe oxidation waves at ~0.32 V and reduction peaks at ~0.08 V that correspond to the formation of Ag oxides and their reduction. Raj and colleague reported a similar phenomenon for Au NPs in methanol oxidation.36 The current density is more at n = 10 and sharper due to increased thickness of Ag layer. The oxidation wave at n = 7, is broader suggesting some Au signature which can also be observed from the UV-vis spectrum in Figure 2 (middle column). The CV for Ag/C in methanol (Figure S3 in the Supporting Information) shows an oxidation wave at ~0.33 V associated with the formation of Ag2O and an ascending second peak corresponding to electrooxidation of methanol appears at ~0.6 V. The reduction peak that appears at ~0.02 V is assigned to reduction of Ag2O to metallic silver. Orozco and colleagues reported similar results.37 The comparisons between the CVs show a shift towards the cathodicside as the Ag content increases in Au/Ag/C catalysts. Lima and colleagues17 reported similar shifts in oxygen reduction reactions for AgCo/C in alkaline medium. When no methanol was added, the CV for Au/Ag/C at n = 7 and 10 shows characteristic Ag oxidation and reduction peaks,5 with less current density as


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compared to Ag/C (Figure S3 in the Support Information). In addition, the CV for Au/Ag/C at n = 10 is different from that of Ag/C (Figure S3) suggesting that Au in the core modified the surface of Au/Ag due to an electronic effect thought to emanate from heterometallic bond around the surface as reported by Patra's38 and Ma's39 labs. Figure 3 (left column) gives the TEM morphology of Au and Au/Ag NP with the NPs increasing in size range of 17.1-20.4 nm from a core of 15.2 nm, see the histograms in Figure S2. Compared to the Au/Ag NPs fabricated at 120 °C (Figure 2), a SPR peak with a shoulder is observed at ~480 nm and a second peak appears at ~425 nm. The Au/Ag core/alloy in Figure 3 (middle column), exhibited a small increase in molar absorption , which is expected for an alloy shell.27-28 The CVs for 160 °C system (Figure 3, right column) showed significantly less current density than that for 120 °C system (Figure 2), an indication of alloy structure. The Ag oxidation waves were observed at ~0.35 V for n = 10 and at ~0.33 V for n = 7 in presence of methanol. The reduction peaks for n = 10 and 7 exhibited shoulders at ~0.08 V and ~-0.05 V respectively, suggesting the presence of Au and Ag on the surface either from alloy formation or incomplete Ag coverage. A second reduction wave at -0.34 V ascribed Ag-OHad reduction is noticed for n = 10. At n = 3, a broad oxidation peak is seen at ~0.43 V and reduction peak at -0.03 V. In absence of methanol the redox waves are less pronounced, but the metals signatures are evident.


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Figure 3. Morphological, UV-vis and electrochemical observations for Au/Ag core/alloy NPs prepared at TH = 160 °C with Ag layers n = 10 (a), 7 (b), 3 (c) and 0 (d). TEM micrographs (left), UV-vis (middle), and CVs (right) comparing n = 0, 3, 7 and 10. The NPs were loaded on carbon with ~20 wt%. Electrochemical analysis done in 0.5 M KOH in presence (solid curves) and absence (dashed curves) of 1.0 M methanol at a scan rate of 100 mVs−1 in room temperature. To further compare the electroactivity of the two morphologies (n = 10), we determined the peak potential, peak current, mass activity, and specific activity from CV measurements following a protocol used by Ansari's lab.40 Our results are tabulated in table 2. Generally, Au/Ag core/shell NPs showed superior (72% higher) catalytic activity over similarly sized core/alloy NPs.


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Table 2. Evaluation of catalytic activity for Au/Ag core/shell and Au/Ag core/alloy from CVs performed at a scan rate of 100 mV/s in methanol. Nanostructures

Peak current Ip/(mA)

Peak potential Ep/V

Au/Ag Core-shell Au/Ag Core-alloy

Specific activity j/mA./cm2

Mass activity mA/mg-Au/Ag

0.322

0.0268

0.382

33.5

0.348

0.0101

0.145

12.63

The electrochemical characterization of NPs synthesized at 120 °C and 160 °C confirmed the existence of different surface compositions. Au/Ag NPs at TH = 120 °C exhibited more Ag signature confirming a core/shell structure. This was evident as more Ag was added. At n = 10 the surface of Au is fully covered by Ag while at n = 3 there is an alloy skin of AuAg (double reduction peaks). For TH = 160 °C system, Au/Ag core/alloy exhibits a small Ag2O oxidation wave and the reduction wave has a shoulder from Au on the surface. The overall oxidation reaction of methanol in alkaline medium is given below CH 3OH + 6OH − → CO 2 + 5H 2O + 6e −

(1)

The reactions occurring at the electrode-electrolyte interface41 are

Metal + OH − → Metal − (OH )ad + e −

Metal − (OH )ad + OH − → Metal − O + H 2 O + 2e −

(2)

where metal is Au and/or Ag. The adsorption of OH- on Ag (core/shell) leads to formation of Ag2O during the anodic wave (equation 2) and the reduction of the oxide to metallic silver on reversing the potential. In presence of methanol, the CH3OH adsorbs on Ag, dehydrogenate and


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gets oxidized to CO2, H2O and electrons. Montoya’s lab42 and others43 reported that CH3OH adsorbs weakly on Ag (111) and the existence of surface OH- promotes the O-H scissions in CH3OH leading to formation of water. Moreover, the OH- adsorption on Au and Ag on the surface of Au/Ag alloy leads to formation of Au2O3 and Ag2O, which are further, reduced to Au and Ag respectively in the reduction wave. In the presence of methanol, CH3OH adsorbs on Au and Ag and gets oxidized to CO2, H2O and electrons. Since Au/Ag NPs formed different surfaces with NPs at TH = 120 °C producing core/shell and those at TH = 160 °C producing alloy, the electrochemical characterizations exhibit catalytic results consistent with varied surface atoms' coordination environments.44 Nonetheless, more work is required to investigate the poor electrocatalytic activity of Au/Ag NPs towards methanol oxidation. A possible explanation for the poor activity, as reported by Hernandez and colleagues,45 is the adsorption of chemicals that prevented methanol oxidation. Furthermore, in many NPs catalysis experiments, heat treatment is frequently used to eliminate the organic capping layer and activate the NPs. In our case, we did not heat treat the NPs, as this would have resulted in the restructuring of the nanostructures through redistribution of the Au and Ag NPs in the core/alloy surface. In the future, before electrochemical analysis, we will explore methods that can remove the capping layer without distorting the surface/composition of the NPs. These results have reinforced our previous high resolution TEM, scanning TEM and selective area energy-dispersive X-ray investigation that Au/Ag formed core/shell and core/alloy nanostructures at hydrothermal temperatures of 120 °C and


160 °C

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respectively.28 These morphological observations are significant in the fabrication of active electrocatalysts for fuel cells. CONCLUSIONS Au/Ag NPs were successfully synthesized by microwave-assisted irradiation at two hydrothermal temperatures. We used electrochemical analysis to probe the morphology of the resulting Au/Ag NPs. The electrochemical characterization confirmed the core/shell nature of Au/Ag NPs fabricated at TH = 120 °C and core/alloy nature for those at TH = 160 °C. The core/shell feature was demonstrated by the absence of Au signature during the electrochemical characterization suggesting that Ag had coated the Au core while the core/alloy structure exhibited both Au and Ag signatures. The core/shell Au/Ag exhibited higher current density than the core/alloy Au/Ag in methanol oxidation. The electrochemical analysis can be used to characterize the surface/composition morphologies of other electroactive metallic NPs like Pd and Pt, which is part of our current research. ASSOCIATED CONTENT Supporting Information: The following files are available free of charge Figs. S1-S3 (Histograms and Silver nanoparticles CV). AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel: +1-757-727-5833 Fax: +1-757-727-5604 ORCID ID Peter N. Njoki: 0000-0001-5085-1178 Mathew M. Maye: 0000-0002-5320-1071


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was partly supported by National Science Foundation HBCU undergraduate program ACE implementation Award, HRD-1238838 (P.N.N. and M.E.D.R.) and by the ACS Petroleum Research Fund (5130-DN110, M.M.M.). This work made use of the Cornell Center for Materials Research Facilities supported by the National Science Foundation under Award Number DMR-1719875. We thank all the Nanoscience at Hampton University (NanoHU) Pioneers who participated in the synthesis.


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G.

Electrochemical

Behavior

of

Gold–Silver

Alloy

Nanoparticles.

ChemElectroChem 2016, 3, 1039–1043. 28. Wu, W.; Njoki, P. N.; Han, H.; Zhao, H.; Schiff, E. A.; Lutz, P. S.; Solomon, L.; Matthews, S.; Maye, M. M. Processing Core/Alloy/Shell Nanoparticles: Tunable


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Alloy Nanoparticles

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