Ag@CoxP Core–Shell Heterogeneous Nanoparticles as Efficient

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Ag@CoxP Core@Shell Heterogeneous Nanoparticles as Efficient Oxygen Evolution Reaction Catalysts Yuhui Hou, Yipu Liu, Ruiqin Gao, Qiuju Li, Huizhang Guo, Anandarup Goswami, Radek Zboril, Manoj B. Gawande, and Xiaoxin Zou ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02341 • Publication Date (Web): 05 Sep 2017 Downloaded from http://pubs.acs.org on September 5, 2017

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Ag@CoxP Core@Shell Heterogeneous Nanoparticles as Efficient Oxygen Evolution Reaction Catalysts Yuhui Hou,† Yipu Liu,‡ Ruiqin Gao,‡ Qiuju Li,‡ Huizhang Guo,*, § Anandarup Goswami,¶, £ Radek Zboril, ¶ Manoj B. Gawande,*, ¶ and Xiaoxin Zou*, ‡ †

Institute for Catalysis, Hokkaido University, Kita 21 Nishi 10, Kita-ku, Sapporo, Japan State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, International Joint Research Laboratory of NanoMicro Architecture Chemistry, College of Chemistry, Jilin University, Changchun 130012, China £ Centre of Excellence in Advanced Materials, Manufacturing, Processing and Characterization (CoExAMMPC) and Division of Chemistry, Department of Sciences and Humanities, Vignan’s Foundation for Science, Technology and Research University (VFSTRU; Vignan’s University), Vadlamudi, Guntur 522 213, Andhra Pradesh, India ‡

§

Wood Materials Science, Institute for Building Materials, ETH Zürich, Stefano-Franscini-Platz 3, 8093 Zürich, Switzerland Regional Centre of Advanced Technologies and Materials, Faculty of Science, Department of Physical Chemistry, Palacký University Olomouc, Šlechtitelů 27, 783 71, Olomouc, Czech Republic KEYWORDS: core–shell nanomaterials, metal phosphide, oxygen evolution reaction, electrocatalysis ¶

ABSTRACT: We present a facile synthetic method that yields Ag@CoxP core@shell-type heterogeneous nanostructures with excellent (oxygen evolution reaction) OER activity. This nanocatalyst can deliver a current density of 10 mA/cm2 at a small overpotential of 310 mV, and exhibits high catalytic stability. Additionally, the catalytic activity of Ag@Co xP is eight times higher than that of the Co2P nanoparticles, owing primarily to the strong electronic interaction between the Ag core and the CoxP shell.

Electrocatalytic water-splitting for the production of hydrogen and oxygen is considered a promising and sustainable – alternative to natural gas and fossil fuel energy sources.1 3 Water-splitting consists of two half-reactions, namely the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), which occur on the cathode and anode, respectively.4 The overall success of the electrocatalytic process is often limited by the energy-intensive OER, owing mainly to the multiple proton-coupled electron-transfer (PCET) steps involved and the slow corresponding kinetics. To date, IrO2 or RuO2 are the most efficient water-oxidation catalysts.5,6 However, the low abundance and high cost of noble metals have prompted researchers to investigate other catalysts. These studies have identified 3d transition metal-based (e.g., Fe, Co, Ni) catalysts as promising candidates for stable, inexpensive OER electrocatalysts.7–10 Superior HER activity (often comparable with the activities of some of the best catalytic systems) has been obtained recently for transition metal phosphides (TMP), especially those composed of several earth-abundant elements.11–14 However, to date, the OER of TMP has rarely been investigated and reported.15–17 Furthermore, the structure-activity relationship of TMP catalysts must be elucidated in order to optimize their OER activity. The properties of the interface between the active materials and the matrix play a key role in the performance of the catalysts.18,19 Therefore, some core-shell nanostructures have been fabricated with the aim of achieving conducive interfaces that yield improved catalytic activity.20 Compared with their monometallic counterparts, composite

nanostructures usually exhibit unique catalytic and electrocatalytic21 properties, due to the synergy between two or more distinct components. Despite their recent prevalence in (electro)catalytic applications, particularly in core–shell nanomaterials,22 similar hybrid systems are used relatively sporadically in OER applications. Au is the most prevalent core metal for tuning the activity of OER, but, owing to its low abundance, is scarcely used in other applications.23,24 In addition, to date, the shell is usually composed of metal oxide and, hence, other composites (e.g., TMP) have to be explored, to broaden the spectrum of core–shell electrocatalysts. For improving the performance of TMP catalysts, highly efficient interfaces on TMP are desired, using cheaper core metals than those currently employed. Nevertheless, to the best our knowledge, successful preparation of well-defined and high-quality metalTMP core–shell nanocatalysts has yet to be achieved. Herein, we designed core–shell Ag@CoxP OER nanoelectrocatalysts and prepared them via in situ growth of TMP using Ag seed. The unique Ag core allows successful formation of the surrounding CoxP shell and enhances the OER performance, owing to its distinct electronic nature. The possible surface interactions and catalytically active sites are identified using a combination of several experimental techniques that allow correlation of these interactions and sites with the electrocatalytic properties.

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Figure 1. Schematic illustration of the Ag@CoxP core–shell nanoparticle formation process.

Figure 2. (a), (c) TEM and STEM images, respectively, of the as-synthesized nanoparticles; (b), (d) HRTEM and HAADF-STEM images, respectively, of a Ag@CoxP core– shell nanoparticle; (e), (f), and (g) elemental maps of Ag, Co, and P, respectively, from the single particle shown in (d); (h) integrated elemental map of Ag, Co, and P; (i) EDS line-scan profile and HAADF image (inset) of an individual particle. The Ag@CoxP core–shell nanoparticles were synthesized via the one-pot method, which involved the formation of Ag seeds, growth of the Co shell, and subsequent phosphating, as illustrated in Figure 1. Monodisperse Ag nanoparticles (diameter: 6 nm) were fabricated by reducing AgNO3 in OAm solution using TPP (triphenylphosphine) as surfactants (Figures S1, S2).25 A mixed solution of OAm (oleylamine) and Co(ac)2 was used as the Co source, which was injected into the flask after the Ag nanoparticles were formed. During heating, Co ions were reduced to metallic Co and formed, via heterogeneous nucleation, a shell layer on the surface of the nanoparticles.26 The phosphating process occurred during hightemperature aging, using TPP as the P source. The XRD pattern of Ag@CoxP core-shell nanoparticles (Figure S3) shows the existence of cubic Ag (PDF No. 04-0783) and orthorhombic Co2P (PDF No. 89-3030). An image (Figure 2a) obtained via transmission electron microscopy (TEM) reveals the nearly spherical morphology of the particles (average diameter: 23.7±3.3 nm, see Figure S4). Furthermore, the high-resolution

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(HR)TEM image (Figure 2b) reveals lattice fringes of the {111} facets of the metallic Ag in the core. In addition, the distribution of components in the core and the shell is revealed by an image (Figure 2c, z2 contrast) obtained via high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The Ag core has a larger atomic number than the CoxP shell and, hence, appears in brighter contrast in this image. Moreover, the Ag core (size: 14 nm) is larger than the isolated Ag seeds. The elemental distribution of an individual nanoparticle is further investigated via energydispersive spectroscopy (EDS) analysis, and the resulting spectrum (see Figure S5) indicates that the as-synthesized nanomaterial powder consists of Ag, Co, and P. The atomic ratio of Co to P is determined to be 2.5 by EDS. Elemental mapping of the core–shell nanoparticle (see Figure 2d) reveals that Ag is localized in the center of the particle, whereas Co and P are mainly distributed in the outer region (Figure 2e–h). A line-scan across a nanoparticle shows that the EDS counts for Ag occur only in the center (red line, see Figure 2i), whereas counts for Co and P occur throughout the particle, with the strongest signal occurring at the edges (blue and red lines). These scans clearly reveal the core–shell structure of the particles. Ag@CoxP janus nanostructures are obtained if Co(acac)2 is used as the precursor in the second step, and the aging temperature after injection of the OAm solution of Co(acac)2 is increased to 300 C. The results of Ag@CoxP janus-nanostructure characterization are shown in Figures S6 and S7. The X-ray diffraction (XRD), scanning electron microscopy (SEM), and TEM results of the Co2P nanoparticles are shown in Figures S8–S10.

Figure 3. XPS spectra associated with the (a) Co 2p, (b) Ag 3d, (c) P 2p regions of the as-prepared Ag@CoxP, Ag, and Co2P, respectively. (d) Schematic illustration of the electron transfer between the CoxP shell and the Ag core. X-ray photoelectron spectroscopy (XPS) measurements were performed on the as-prepared Ag@CoxP, Co2P, and Ag nanoparticles. In the Ag@CoxP spectrum, the characteristic peaks of Co-P corresponding to Co 2p3/2 and Co 2p1/2 occur at 778.8 and 793.6 eV, respectively.27,28 The Co-P of Ag@CoxP can also be determined from the P 2p spectrum shown in Figure 3b, where the peaks occurring at 129.4 and 130.1 eV are associated with Co-P. The peaks at 782.2 and 798.2 eV (see Figure 3a) and the two satellite peaks at 786.8 and 803.7 eV,

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are associated with oxidized Co species; those occurring at 133.2 eV (see Figure 3b) arise from oxidized P species.29,30 The occurrence of oxidized Co and P indicates that the surface of CoxP is slightly oxidized,31,32 as is typically the case for phosphides. The peaks occurring at 368.0 and 374.0 eV in the Ag 3d spectrum (Figure 3c) are associated with Ag 3d5/2 and Ag 3d3/2 core levels, respectively, of metallic Ag0.33 Furthermore, the binding energy is significantly shifted compared with the signature of Ag@CoxP, Ag, and Co2P. Due to the formation of core–shell Ag@CoxP, the binding energy of the Ag 3d decreases, whereas the binding energy of the Co 2p increases relative to that of the Co2P nanoparticles. These shifts confirm the significant electronic interaction between Co and Ag, resulting in electron-density migration (to some degree) from Co to Ag (Figure 3d).34,35 This interaction might facilitate the formation of high valent CoIV species due to the protoncoupled conversion of CoIII−OH to CoIV−O during OER, prior to oxygen evolution.15 The electrocatalytic activity of the Ag@CoxP towards oxygen evolution was investigated using a typical three-electrode system in 1 M KOH at room temperature. The corresponding polarization curves (see Figure 4a) were obtained at a scan rate of 1 mV/s. As the figure shows, for the considered appliedvoltage range, Ag nanoparticles have negligible catalytic effect on OER and the current density of Ag@CoxP is considerably higher than that of Co2P. For example, at a current density of 1.6 V vs. RHE, Ag@CoxP and Co2P reach current densities of 32 mA/cm2 and only 3.7 mA/cm2, respectively. This indicates that the activity of core–shell Ag@CoxP is almost eight times higher than that of Co2P. We further compare the overpotentials at the current density of 10 mA/cm2, which is equivalent to a solar energy conversion efficiency of 10%. Ag@CoxP core–shell nanoparticles reach a current density of 10 mA/cm2 at an overpotential of 310 mV, which is significantly lower than that of Co2P (420 mV). Moreover, the activity of Ag@CoxP nanoparticles is better than those associated with most of the reported powder phosphide catalysts31,36–41 and Au-based core–shell OER catalysts42 (Tables S1). In addition, as shown in Figure S11, the activity of Ag@CoxP janus nanoparticles is lower than that of Ag@CoxP core–shell nanoparticles. These results possibly from the fact that more interfaces contribute to OER in the core–shell structure than in the janus structure (i.e., the Ag nanoparticles in the core–shell structure are all used for tuning the electronic structure of the CoxP shell and enhancing the activity of CoxP). The stability of Ag@CoxP was determined from the I-t curve obtained via potentiostatic measurements performed at an overpotential of 274 mV. As shown in the inset of Figure 4a, the current density of Ag@CoxP remains stable for up to 20 h during the OER electrocatalysis, indicative of the excellent durability of Ag@CoxP.

Figure 4. (a) Polarization curves obtained for Ag@CoxP and Co2P (inset shows the I-t curve obtained at an overpotential of 274 mV). (b) Electrochemical impedance spectroscopy (EIS) Nyquist plots of Ag@CoxP and Co2P (inset shows the corresponding equivalent circuit model). (c) Tafel slope determined from the polarization curves. (d) The linear relationship between Δj and the scan rate. Electrochemical impedance spectroscopy (EIS) analysis was also performed on Ag@CoxP and Co2P. Figure 4b shows the Nyquist plots of Ag@CoxP and Co2P, which were fitted using an equivalent circuit model. The resulting chargetransfer resistance (Rs) of Ag@CoxP (2.0 Ω) is smaller than that of Co2P (3.5 Ω), indicating that the core–shell Ag@CoxP favors the charge-transfer process. The OER kinetics of Ag@CoxP and Co2P for OER are determined from the Tafel plots and fitted to the Tafel equation, yielding Tafel slopes of 76.4 and 214.3 mV/dec. Therefore, OER occurs at a considerably higher rate in Ag@CoxP than in Co2P. The aforementioned results indicate that the OER performance of the core– shell Ag@CoxP is far superior to that of Co2P. Furthermore, the Tafel slope (76.4 mV/dec) is approximately the same as that of Co-oxide catalysts and, hence, we attribute the activity of the Ag@CoxP to the oxidized Co species. Further oxidation of the CoxP is confirmed via XPS analysis (Figure S12). Compared with that of the as-prepared Ag@CoxP, the intensity of the characteristic Co-P peaks of the sample decreases significantly after OER testing. Therefore, during the OER process, CoxP is partially in situ electrochemically transformed into CoOx, which serves as an active center for OER. In order to compare the catalytic properties of Ag@CoxP and Co2P further, their electrochemically active surface areas are measured by a widely adopted capacitive method. The specific activities are obtained by normalizing the current with respect to the electrochemical surface area of the material. The results, presented in Table S2, SI, reveal that the specific activity (measured by the normalized current densities) of Ag@CoxP is about 3.3 times higher than that of Co2P. Thus, it is concluded that the excellent activity of Ag@CoxP must be attributed to the higher intrinsic catalytic activity of the active site, compared with that of Co2P. This higher intrinsic catalytic property results from the electronic changes induced by the Ag core. The core may serve as an electron acceptor, which facilitates electron extraction and thereby helps to stabilize Co at high oxidation levels, leading to enhanced activity of CoxP.23,43

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Similar phenomena have been rarely reported especially in the context of OER activity. For example, Bell and co-workers42 showed that (i) a sub-monolayer deposition of Cooxide/hydroxide on Au film significantly enhances the OER activity and (ii) the surface plasmon property of Au could be used to realize increased OER activity through photoactivation.24 In summary, we report a new Ag@CoxP core–shell electrocatalyst obtained through the controlled addition of Co(ac) 2 to a AgNO3 solution, where the uniform CoxP shell formed around the Ag core enables OER activity. This report provides a simple and straightforward synthetic strategy for the preparation of Ag@CoxP phosphide catalysts, allowing the investigation of synergistic interactions between the Ag core and the shell. The Ag@CoxP core–shell nanoparticles are more efficient and sturdy catalysts for OER than the reference and other reported catalysts. We believe that the Ag core modulates the electronic structure of CoxP, thereby leading to enhanced catalytic activity. We believe that these findings can encourage other researchers to explore similar synthetic approaches to make other sustainable materials with advanced properties for renewable energy-related catalytic applications.

ASSOCIATED CONTENT Supporting information. Experimental details; Characterizations; Electrocatalytic activity; TEM image of Ag seed (Figure S1); XRD pattern of Ag seed (Figure S2); XRD of Ag@CoxP (Figure S3); Diameter of Ag@CoxP (Figure S4); EDS spectrum of Ag@CoxP (Figure S5); TEM and HRTEM images of Ag@CoxP (Figure S6); EDS line-scan profile of Ag@CoxP and HAASF image (Figure S7); XRD pattern of Co2P (Figure S8); SEM image of Co2P (Figure S9); TEM image of Co2P (Figure S10); Polarization curves of Ag@CoxP and Ag@CoxP janus (Figure S11); XPS spectra of Co 2p and P 2p of Ag@CoxP undergoing OER for ~20 h (Figure S12); Summary of several reported catalysts (Table S1); Comparison of electrochemical surface areas (ECSA) and catalytic activities of Ag@CoxP and Co2P (Table S2). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] * E-mail: [email protected] * E-mail: [email protected]

Author Contributions All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT The authors gratefully acknowledge the support from the Ministry of Education, Youth and Sports of the Czech Republic (project LO1305) and the assistance provided by the Research Infrastructure NanoEnviCz under project LM2015073. X. Zou acknowledges the financial support from the NSFC 21401066, Jilin Province Science and Technology Development Plan 20150520003JH, and Science and Technology Research Program of Education Department of Jilin Province [2016] No. 410. H. Guo acknowledges the financial support by ETH Career Seed Grant SEED-14 17-1.

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TOC Ag@CoxP Core@Shell Heterogeneous Nanoparticles as Efficient Oxygen Evolution Reaction Catalysts Yuhui Hou,† Yipu Liu,‡ Ruiqin Gao,‡ Qiuju Li,‡ Huizhang Guo,*, § Anandarup Goswami,¶, £ Radek Zboril,¶ Manoj B. Gawande,*, ¶ and Xiaoxin Zou*, ‡

Ag@CoxP core@shell heterogeneous nanostructures with excellent OER activity are reported. The catalytic activity of this nanocatalyst is eight times higher than that of the Co2P nanoparticles, owing primarily to the strong electronic interaction between the Ag core and the CoxP shell.

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