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Functional Nanostructured Materials (including low-D carbon)

Binary Transition Metal Oxide Hollow Nanoparticles for Oxygen Evolution Reaction Pan Peng, Xiao-Min Lin, Yuzi Liu, Alexander S. Filatov, Dongguo Li, Vojislav R. Stamenkovic, Dali Yang, Vitali B. Prakapenka, Aiwen Lei, and Elena V. Shevchenko ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06165 • Publication Date (Web): 28 Jun 2018 Downloaded from http://pubs.acs.org on June 29, 2018

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ACS Applied Materials & Interfaces

Binary Transition Metal Oxide Hollow Nanoparticles for Oxygen Evolution Reaction Pan Peng, ₸,‡ Xiao-Min Lin,‡ Yuzi Liu,‡ Alexander S. Filatov,† Dongguo Li, ≠ Vojislav R. Stamenkovic,≠ Dali Yang,₸ Vitali B. Prakapenka,#,§ Aiwen Lei,₸,* and Elena V. Shevchenko‡,* ₸

Institute of Advanced Studies (IAS), College of Chemistry and Molecular Sciences, Wuhan University,

Wuhan 430072, Hubei, P. R. China; ‡

Center for Nanoscale Materials, ≠ Materials Science Division, and §Advanced Photon Source, Argonne

National Laboratory, Argonne, Illinois, 60439 USA; †

Chemistry Department and #Center for Advanced Radiation Sources, University of Chicago, 5901 South

Ellis Avenue, Chicago, Illinois 60637, USA.

Abstract: Low-cost transition metal oxides are actively explored as alternative materials to precious metal based electrocatalysts for the challenging multistep oxygen evolution reaction (OER). We utilized the Kirkendall effect allowing the formation of hollow polycrystalline, highly disordered nanoparticles (NPs) to synthesize highly active binary metal oxide OER electrocatalysts in alkali media. Two synthetic strategies were applied to achieve compositional control in binary transition metal oxide hollow NPs. The first strategy is capitalized on the oxidation of transition metal NP seeds in the presence of other transition metal cations. Oxidation of Fe NPs treated with Ni (+2) cations allowed the synthesis of hollow oxide NPs with a 1 to 4.7 Ni to Fe ratio via an oxidation induced doping mechanism. Hollow Fe-Ni oxide NPs also reached a current density of 10 mA/cm2 at 0.30 V overpotential. The second strategy is based on the direct oxidation of iron-cobalt alloy NPs which allows the synthesis of hollow FexCo100-x-oxide NPs where x can be tuned in the range between 36 and 100. Hollow Fe36Co64- oxide NPs also revealed the current density of 10 mA/cm2 at 0.30V overpotential in 0.1 M KOH. Key words: Kirkendall effect; hollow nanoparticles, transition metals, OER, water splitting.

1.0. INTRODUCTION The synthesis of reliable and low-cost materials for efficient energy conversion and storage systems such as batteries and fuel cells remains a central research problem in the field of renewable energy.1-5 Fuel

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cells based on electrolysis of water can provide efficient conversion of the chemical energy into electricity without air pollutants.6 However, water oxidation requires a large overpotential for multistep, four electrons and four protons oxygen evolution reaction (OER).7 The most efficient electrocatalysts for OER are based on precious metals since these materials offer relatively low electrochemical overpotentials and stability.4, 8-9 However, their high cost limits the expansion of technologies based on electrolysis of water. Within the last decade, transition metal oxides were recognized as the promising materials for fabrication of inexpensive OER electrocatalysts.5, 10-13 As a result of numerous studies to guide the synthesis of efficient transition metals based electrocatalysts for OER, a few empirical rules were established. First of all, it was shown that mixed metal oxides or hydroxides had a greater performance than their single component analogues.13-18 It was also suggested that that amorphous materials and surfaces are more likely to demonstrate enhanced oxygen evolution activity.13, 19-20 Different synthetic strategies were proposed to synthesize electrochemically active films, micron- and nano-sized particles of transition metal oxides and hydroxides. Among them are electrochemical deposition,5 photochemical metal-organic deposition,13 and solution-cast deposition.21 The NPs, synthesized via solution-based approaches offer a number of advantages in terms of compositional and size control, scalability, and the possibility to fabricate highly porous electrocatalytic surfaces with an increased number of active sites, control over conductivity and efficient material utilization. Due to the significant progress in colloidal synthesis, a variety of different types of NPs became available and their electrocatalytic properties are currently actively investigated.6, 18, 22-23 The main focus in such studies is usually made on highly crystalline forms of NPs.24-25 Nevertheless, in fact, amorphous structures have been recently recognized for their enhanced activity in electrocatalytic water splitting.13,

19-20, 23, 26-28

However, thermodynamic stability of amorphous phases imposes a serious

limitation on the synthesis of amorphous NPs. As a result, there are only a few studies discussing the electrocatalytic activity of amorphous NPs. For examples, promising electrocatalytic performance of amorphous CoSe and MoP NPs was revealed in the hydrogen evolution reaction and amorphous PtNiP NPs were found to be more active than their crystalline analogue in the electrochemical oxidation of methanol.26-27,

29

Inspired by the promising activity of electrodeposited and solution-cast deposited

transition metal oxide films in challenging OER we decided to conduct a systematic study on synthesis and OER activity of transition metal oxide NPs with a high degree of structural imperfections. Moreover, in the light of recently reported new mechanism of OER by activating lattice oxygen redox reactions in


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metal oxides,30 new forms of metals oxides with potentially controlled structural defects and compositions can become particularly interesting. Here we report on the synthesis and electrocatalytic performance of highly polycrystalline hollow binary metal oxide NPs prepared via the Kirkendall effect following the compositional and structural guidance established for metal oxide films prepared by electrodeposition or solution casting.

5, 13, 21

Previously, it was shown that oxidized forms of unary transition metals (e.g. Fe2O3, CoO and Co9S8) can be obtained via direct oxidation of corresponding metal NP seeds.31-33 We propose a new robust and the highly efficient concept of design of electrochemically active binary transition metal oxide hollow NPs based on oxidation of Fe NP seeds in the presence of transition metal cations such as nickel (+2) and cobalt (+2). We also demonstrate that hollow binary transition metal oxide NPs can be synthesized by direct oxidation of corresponding metal alloy NPs. Among FexCo100-x oxide NPs synthesized by direct oxidation of binary metal NPs, hollow Fe36Co64- oxide NPs revealed the highest OER performance demonstrating current density of 10 mA/cm2 at 0.30 V overpotential in 0.1 M KOH. Similar performance has been also observed for oxidized Fe NPs treated with Ni (+2) cations.

2.0. EXPERIMENTAL SECTION Materials: All chemicals, including Fe(CO)5 (99.99%, Aldrich), Co2(CO)8 ( ≥ 90%, Aldrich), oleylamine (70%, Aldrich), 1-octadecene (90%, Aldrich), KOH (99.99%, Sigma-Aldrich) and Nafion solution (5 wt%, Sigma-Aldrich) were purchased from commercial suppliers and used without further purification. Milli-Q water with 18.25 MΩ·cm at 25 °C was used in all electrochemical experiments. Synthesis of hollow iron and iron-cobalt oxide NPs by direct oxidation of metallic NPs. Hollow Fe-Co oxide NPs with different ratios of iron to cobalt were prepared according to the modified procedure, previously developed for the synthesis of hollow iron oxide NPs.34-35 Briefly, 20 mL of 1octadecene was mixed with 0.3 mL of oleylamine and heated at 120 oC for 0.5 h to dry the solvent. To prepare FexCo100-x- oxide NPs where x is 88, 86, 81, and 36, different amounts of Co2(CO)8 were directly dissolved in Fe(CO)5 and injected into 1-octadecene at 180 oC in the presence of oleylamine (0.3 mL) and kept at this temperature for 40 min, followed by subsequent oxidation of synthesized NPs by bubbling the air into reaction mixture through a syringe needle for 60 min at the flow rate of ~25 mL/min. To prepare FexCo100-x oxide with x equals or less than 24, hexane (1 mL) was added to Fe(CO)5 to achieve solubility of Co2(CO)8. The molar ratios of Co2(CO)8 to Fe(CO)5 were used as 10:90, 12:88, 15:85, 50:50 and 80:20 to synthesize Fe88Co18-, Fe86Co14-, Fe81Co19-, Fe36Co64- and Fe24Co76- oxide NPs, respectively. The net



amount of carbonyl precursors (Co2(CO)8+Fe(CO)5) used in the synthesis of hollow NPs was 5.324 mmol. The composition of the NPs was analyzed by energy dispersive X-ray spectroscopy (EDX). Cobalt oxide NPs were synthesized by injecting of Co2(CO)8 (364 mg) dissolved in 1.5 ml hexane into 20 mL of dried 1-octadecene mixed with 0.3 mL of oleylamine at 180 oC (see Supporting Information for details). After that, the same synthetic protocol that was used for the synthesis of FexCo100-x oxide NPs was followed. After cooling down to room temperature, the obtained NPs were washed repeatedly with acetone and isopropanol and collected by centrifugation. The thoroughly purified oxide NPs were dispersed and stored in toluene. Synthesis of hollow binary transition metal (iron-nickel and iron-cobalt) oxide NPs by surface modification followed by oxidation. Fe NPs were synthesized according to the previously reported protocol35. After that 50 µl of Fe NPs in toluene (~10 mg/mL) were mixed either with 4.5 mg of Ni(acac)2 or Co(acac)2 dissolved in 200 µl of toluene containing 4.2 mg of Na(oleate) and 20 µl of oleylamine. The mixture was sonicated for 10 min. The Fe/Co2+ and Fe/Ni2+ NPs were precipitated with acetone by centrifugation and re-dispersed in 200 µl of toluene to prepare a stock solution that was further used for electrode fabrication and sample characterization. All procedures were performed under ambient conditions. Electrode Preparation: Prior to the deposition of NPs, glassy carbon (GC) disks were polished with 600 grit Carbimet SiC grinding paper (Buehler) and sonicated in 1.0 M HNO3 for 1 min. After that, GC disks were polished with 50 nm Al2O3 and sonicated subsequently for 10 min in distilled (DI) water, 10 min in acetone, 10 min in absolute ethanol and 10 min in DI water. Hollow FexCo100-x NPs where x is 100, 88, 86, 81, 36, 24 and 0 were deposited onto 6 mm diameter glassy carbon (GC) discs (HTW GmbH) by drop casting 14 µL of the corresponding stock colloidal solution. The stock colloidal solutions were prepared by dissolving of thoroughly washed and dried 1.0 mg of FexCo100-x oxide NPs in 0.2 mL of toluene. The modified Fe/Co2+ and Fe/Ni2+ NPs were deposited on the surface of GC by drying of 10 µl of corresponding stock solutions. To complete the oxidation of the deposited NPs, all electrodes were kept at 180 oC for 12 h. After that, 5 µL of Nafion solution prepared by dilution of 10 µl of 5 wt. % Nafion solution (Sigma-Aldrich) with 200 µl of ethanol was deposited on the top of NPs. The weight of the deposited NPs was measured using microbalances. After annealing, the mass of binary transition metal oxide NPs deposited on glassy carbon was in the range between 40 µg and 70 µg (Table S1). The typical electrode tested in our study consisted of several layers of densely packed NPs (Figure S1 and S2). In preliminary tests, we found that the electrochemical performance of the fabricated electrodes depended on the amount of the deposited NPs (Figure S3). This can be attributed to the difference in the coverage of the electrocatalytic NPs at the surface of glassy carbon: small amounts of NPs did not provide the optimal coverage of the glassy carbon


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surface, while the larger loadings of NPs resulted in the formation of the structures with the limited conductivity. It is worth mentioning that similar dependence of the current densities on the loadings was previously reported for electrochemically deposited transition metal oxides films.15,

36

We choose the

loading of NPs that provided representative electrode performance for a given type of samples. For that, several loadings were tested for each sample and the loadings giving the highest and the lowest performances were eliminated. The current density was normalized by the geometric electrode surface area so the performance of electrodes fabricated from hollow oxide NPs can be easily compared with the OER performance of electrocatalysts reported by other groups. The loading of NPs in all studied samples was in the range between 0.14 to 0.25 mg/cm2 that is comparable with the loadings reported by other groups (Table S2). Characterization: Scanning Electron Microscopy (SEM) images were obtained using JEOL FESEM 7500F high-resolution SEM operated at 10 kV. Transmission electron microscopy (TEM), energy-filtered TEM, and energy dispersive X-ray spectroscopy (EDS) data were obtained by using a JEOL JEM-2100F equipped with a Gatan GIF Quantum Energy Filters and an Oxford X-MaxN 80 TLE detector and operated at 200 kV. Synchrotron X-ray diffraction (XRD) measurements were performed at GSECARS Sector 13-ID-D at the Advanced Photon Source, Argonne National Laboratory. The photon energy was 40 keV (0.3100 Å). The X-ray absorption spectroscopy (XAS) measurements at the Fe K-edge (7112 eV), Co K-edge (7709 eV) and Ni K-edge (8333 eV) were conducted at 10-ID-B beamline at the Advanced Photon Source, Argonne National Laboratory. A rhodium coated harmonic rejection mirror was used to eliminate higher energy photons. Experiments were performed in a transmittance mode using ion chamber with Stern Heald geometry. For Fe edge, a mixture of 80% He and 20% N2 was used in the initial Io ion chamber placed before the sample and pure N2 gas was in transmission and reference ion chambers. An Ar gas was used in the fluorescence ion chamber. For Co- and Ni- edges, a pure N2 was used in the Io ion chamber and a mixture of 50% N2 and 50% Ar gas was used in the transmission and reference ion chambers. A Kr gas was used in the fluorescence ion chamber. Beamline calibration procedures were conducted with Fe, Co and Ni metal foils (7112 eV, 7709 eV and 8333 eV, respectively). X-ray photoelectron spectroscopy (XPS) experiments were performed using the Kratos Axis Nova instrument. Samples were irradiated by a monochromatic Al−Kα X-ray source with the charge-neutralization system running. The instrument work function was calibrated to give an Au 4f7/2 metallic gold binding energy of 83.95 eV. The instrument base pressure was ca. 1×10−9 Torr. The analysis area size was 0.3 × 0.7 mm2. For calibration purposes, the binding energies were referenced to C 1s peak at 284.8 eV. Electrochemical characterization of electrodes was conducted using a rotating disc electrode (RDE, Pine Research Instrumentation, Inc) in 0.1 M KOH at a scan-rate of 10 mV/s. Graphite and Ag/AgCl (saturated with KCl) were used as counter and reference electrodes, respectively. The reference potential



was calibrated by hydrogen oxidation/evolution on a Pt working electrode for reversible hydrogen electrode (RHE). For that 0.1 M KOH was saturated with hydrogen and Pt working electrode was cycled for hydrogen evolution and hydrogen oxidation reactions. The potential at which current crossed zero was taken as the value for the Ag/AgCl reference versus the thermodynamic hydrogen potential.37 The OER polarization curves were recorded with 95% iR compensation. The OER overpotentials of electrodes relative to its thermodynamic value (Eo) was calculated using the equation E = [Eo -0.138-0.059×pH], where Eo is 1.23 V.

3.0. RESULTS AND DISCUSSION We explored the possibility to utilize the Kirkendall effect to synthesize hollow binary transition metal oxide NPs with the compositions expected to reveal promising performance in OER.5, 13, 21 Two synthetic strategies were applied to achieve compositional control in binary transition metal oxide hollow NPs. The first approach is based on oxidation of transition metal NP seeds in the presence of cations of other transition metals. The second approach is based on the direct oxidation of bimetallic NPs in which the composition of the final oxide NPs is determined by the composition of the initial metal NP seeds. Below we discuss the structural properties and the OER performance of hollow binary transition metal oxide NPs obtained via these two oxidation induced routes. Cation assisted synthesis of hollow binary transition metal oxide NPs. Previously, we have demonstrated that doping of oxidized forms of NPs can be achieved via oxidation induced mechanism.38 We have shown that such mechanism allowed high levels of dopant (molybdenum) in iron and cobalt oxide and sulfide NPs. Inspired by these results, we decided to utilize similar idea to obtain hollow binary transition metal oxide NPs. For that, we dissolved as-synthesized Fe NP seeds in the toluene solutions containing Co (+2) and Ni (+2) cation precursors. The Fe NPs exposed to cations of Co (+2) and Ni (+2) will be further referred as Fe/Co2+- and Fe/Ni+2 NPs. After 10 min, Fe NPs were thoroughly washed by addition of a non-solvent followed by centrifugation and were re-dispersed in toluene. In order to complete oxidation of Fe NPs,34 Fe/Co2+- and Fe/Ni+2 NPs were deposited on the substrate and were annealed at 180 oC for 12 h. The TEM studies revealed a strong effect of Co (+2) and Ni (+2) cations on the structure and composition of the oxidized Fe/Co2+- and Fe/Ni+2 NPs (Figure 1). Note, that the same Fe NP seeds were used to prepare both Co (+2) and Ni (+2) surface modified hollow oxide NPs. The difference in the morphology of oxidized Fe/Co2+ and Fe/Ni2+ NPs can be attributed solely to the difference in cation coalescence kinetics upon oxidation.39 The Fe NPs treated with Co (+2) converted into hollow NPs (Figures 1b-f) consisting of mainly cobalt atoms, as it is indicated by the elemental mapping data (Figure


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1d-1f). Oxidation of Fe NPs treated with Ni (+2) cations resulted in the formation of more uniform hollow shell NPs (Figure 1g, h) that consisted mainly of Fe atoms (Figures 1i-1k). HRTEM images (Figures 1c, h) show very distorted lattices of oxidized Fe/Co2+ and Fe/Ni2+ NPs. However, oxidized Fe/Co2+ NPs are highly polycrystalline with the multiple crystalline domains, while oxidized Fe/Ni2+ NPs are nearly amorphous with an only short range of the order of atoms.

Figure 1. TEM images of Fe NPs (a); binary transition metal hollow oxide NPs synthesized by oxidation of Fe/Co2+ and Fe/Ni2+ NPs (b and g, respectively). HRTEM images of oxidized Fe/Co2+ and Fe/Ni2+ NPs (c and h, respectively). Figures (f) and (k) demonstrate the false color superposition of Fe (blue) and Co (red) and Fe (blue) and Ni (green) reconstructed from EDX mapping images of individual elements (d, e, i, j).

The high-resolution elemental mapping of oxidized Fe/Co2+ and Fe/Ni2+ NPs (Figure 2) indicated the random distribution of iron, cobalt and nickel atoms in the shells of the oxidized Fe/Ni2+ and Fe/Co2+ NPs. The core of the oxidized Fe/Co2+ NPs contained mainly cobalt atoms. This is rather surprising observation since these NPs were obtained by oxidation of Fe NP seeds in the presence of Co (+2) precursor. However, iron atoms were found to be present in the hollow shell in low concentration (Figure 2d). The size of the oxidized Fe/Co2+ is 23.2±2.5 nm that is about 13% larger than the size of oxidized 20.1±0.8 nm Fe/Ni2+ NPs. The oxidation of Fe NPs in the absence of any cations resulted in the formation of 17.8±0.7 nm hollow γ-Fe2O3 oxide NPs. The difference in the morphology of oxidized Fe/Ni2+ and Fe/Co2+ NPs (hollow shell vs. core/hollow shell) and in their sizes can be attributed to the difference in their oxidation rates that affects the coalescence process of the cation vacancies.39 The synthesis of Fe-Ni



hollow oxide NPs can be explained very well in terms of oxidation induced doping mechanism previously reported for iron oxide NPs doped with molybdenum.38 In oxidized Fe/Ni2+ NPs EDX analysis indicated 1 to 4.7 atomic ratio for Ni to Fe atoms. Such concentration of randomly distributed Ni atoms in oxide NPs can be explained by internalization of Ni atoms adsorbed at the surface of Fe NPs in the lattice of the growing iron oxide upon oxidation. In oxidized Fe/Co2+ NPs, the atomic ratio of Co to Fe was found to be ~10 to 1 indicating that the majority of Fe atoms leached out. The composition of oxidized Fe/Co2+ NPs suggests that they were formed in the solution upon their exposure to the precursor of Co (+2) cations. The formation of cobalt-rich oxidized Fe/Co2+ NPs can be still explained by oxidation induced doping mechanism accompanied by leaching of iron atoms upon oxidation. On the other hand, we cannot fully exclude the substitution40 of Fe atoms with Co upon introducing of cobalt precursor to the toluene solution of Fe NPs. The detailed understanding of the oxidation of the transition metal NPs in the presence of different metal cations requires further detailed studies.

Figure 2. HAADF-STEM images (a, e) of oxidized Fe/Co2+ and oxidized Fe/Ni2+ NPs synthesized via oxidation of Fe NPs treated with Co (+2) and Ni (+2) cations, respectively. High-resolution EDX elemental maps (b, c, f, g) corresponding to Fe (blue), Ni (green), and Co (red) elements, and elemental mapping overlays of Fe and Ni (d), and Fe and Co (h) elements.

The pre-edge peak positions of the of XANES spectra indicates the presence of Fe (+3) atoms in all Fe/Co2+ and Fe/Ni2+ NPs, that we attributed to the fact that all manipulations with as-synthesized Fe NP seeds were conducted under ambient conditions (Figure S4a). Oxidative annealing at 180 oC of Fe/Co2+


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and Fe/Ni2+ NPs resulted in the higher overall oxidation state of iron atoms as it is evidenced by the shift of their white lines toward higher energies as compared to their reference samples (Figure S4a). XANES spectra measured at cobalt and nickel K-edge indicated Co (+2) and Ni (+2) in all Fe/Co2+ and Fe/Ni2+ NPs (Figures S4b, S4c). The XPS study revealed the very low peak intensities of the XPS spectra of the Fe2p level for oxidized Fe/Co2+ and oxidized Fe/Ni2+ NPs (Figure S5a) and hence, both samples are likely to have low concentrations of surface iron (Figure S5a). The low peak intensities of Fe 2p signals blend in with Auger signals from Co or Ni that does not allow quantative analysis of these spectra. The XPS spectra of Co 2p and Ni 2p levels obtained for oxidized Fe/Co2+ and Fe/Ni2+ NPs are typical for Co(+2) and Ni(+2) compounds, correspondingly (Figures S5b, S5c).

Figure 3. Current density – potential curves for hollow iron-nickel and iron-cobalt NPs synthesized by oxidation of the Fe NPs surface in the presence of Ni (+2) and Co (+2) precursors. The NPs deposited on glassy carbon were cycled in 0.1 M KOH at a scan rate of 10 mV/s. Grey dashed curve represents the performance of hollow Fe36O64-x oxide NPs prepared by direct oxidation of metal alloy NPs. Inset demonstrates the electrode potentials at 5 mA/cm2 (solid squares) and 10 mA/cm2 (solid circles) current densities as a function of NP composition.

To study the electrochemical performance, Fe/Co2+ and Fe/Ni2+ NPs were deposited on the surface of glassy carbon followed by their oxidation at 180 oC for 12 h. The sharp increase in current observed for studied samples in the voltage range between 1.45 V and 1.5 V corresponds to the electrocatalytic water oxidation (Figure 3). In alkaline media (0.1 M KOH), the oxidized Fe/Ni2+ NPs demonstrated a promising5, 16, 23, 41-42 OER overpotential of 0.30 V at 10 mA/cm2 current density (Figure 3). Oxidized Fe/Co2+ NPs showed slightly higher OER overpotential of 0.32 V at the same current density (Figure 3).



In turn, hollow iron oxide NPs synthesized by oxidation of untreated Fe NP seeds (further referred as Fe100-oxide) were electrochemically inactive in OER. The precatalytic redox small feature was observed only for oxidized Fe/Ni2+ NPs (Figure S6) can be associated with Ni(+2) oxidation.40 Previously, it has been reported for NiOx films prepared via a solution-cast method that the conversion of NiO into NiOOH with Fe impurities led to a remarkable decrease of OER overpotential.21 Also, it has been demonstrated that mixed Ni-Co oxide films with high concentrations of hydroxides allowed lower OER overpotential while CoOx films with a high concentration of oxide groups demonstrated the highest OER overpotential.21 Our study revealed the similar trend for hollow binary metal oxide NPs. The XPS data on the O1s level (Figure S5) showed that the oxidized Fe/Co2+ and Fe/Ni2+ NPs have significant amounts of hydroxides on their surfaces. For example, hydroxide/oxide ratios for oxidized Fe/Co2+ and Fe/Ni2+ NPs are 7.46 and 7.75, respectively, while in the case of hollow iron oxide this ratio is found to be 0.65 (Table S3). Thus cation assisted synthesis based on the oxidation of one type of transition metal NPs in the presence of other types of transition metals allows synthesis of hollow binary transition metal oxide NPs that reveal promising electrocatalytic properties for OER in alkaline media. Oxidation of transition metal seed NPs in the presence of different types of metal cations is proposed as a novel approach toward further advancements toward the efficient design of active electrocatalysts. Binary transition metal oxide hollow NPs synthetized by direct oxidation of bimetallic NPs. In order to estimate the efficiency of the cation assisted synthesis discussed above, we also explored more traditional strategy based on the direct oxidation of synthetized bimetallic transition metal NPs. The synthesis of Fe NP seeds is based on thermal decomposition of Fe(CO)5.32, 35, 43 In order to minimize the variables in the synthetic protocol of Fe NPs, we decided to use metal carbonyls as a source of transition metals. Since the handling of Ni(CO)4 presents a higher hazard as compared to both Fe(CO)5 and Co2(CO)5, we limit our study to FexCo100-x oxide NPs only. The details of synthesis are given in the Experimental Section. Representative TEM images of FexCo100-x oxide NPs are demonstrated in Figure 4. The ratio of iron to cobalt in FexCo100-x oxide NPs was similar to the ratio of iron to cobalt in FexCo100-x NP seeds. In turn, the composition of FexCo100-x NP seeds was controlled by the ratio of Fe(CO)5 to Co2(CO)8. The TEM overview of Co NPs prepared according to the previously reported procedures44-45 is given in the Supporting Information (Figure S7). As we mentioned above, hollow Fe100- oxide NPs demonstrated almost no OER activity. Introduction of ~19% of cobalt atoms into iron oxide NPs significantly improved their performance; however, Co100oxide NPs still outperformed Fe81Co19- oxide sample (Figure 5). Among all studied FexCo100-x oxide NPs


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samples, hollow Fe36Co64- oxide NPs demonstrated the highest OER activity revealing the lowest OER overpotential of ~0.30 V at 10 mA/cm2 current density (Figure 5, insert). Further increase of cobalt concentration in hollow FexCo100-x oxide NPs was accompanied by the lowering of their performance in OER (Figure S8). The sizes of Fe100- and NPs Fe88Co12- oxide NPs are slightly larger than the sizes of other FexCo100-x- oxide NPs; however, the significant difference in OER activity of similar sized Fe86Co14, Fe81Co19-, and Fe36Co64- oxide NPs points out to prevailing effects of composition and structure rather than size. As it is shown in Figure S9, the electrodes demonstrated good stability in performed 20 cycles. The scanning electron microscopy (SEM) study of the electrode surfaces before and after electrochemical cycling revealed almost no alteration in the morphology of the hollow NPs (Figures S1, S2). However, more extended cycling (200 cycles) resulted in the delamination of the fragments of the deposited NP films. No correlation between the composition and the degree of delamination was established.

Figure 4. TEM images of hollow γ-Fe2O3 oxide NPs and hollow binary FexCo100-x- oxide NPs obtained by direct oxidation of bimetallic FexCo100-x NP seeds.



Figure 5. Current density – potential curves for hollow FexCo100-x- oxide NPs and Fe100- and Co100- oxide NPs deposited on glassy carbon measured by cycling the electrodes in 0.1 M KOH at a scanrate of 10 mV/s. The cyclic voltammetry curves obtained for Fe100- and Fe88Co12- oxides coincide with each other. Inset demonstrates the potentials at different current densities as a function of NP composition.

In order to gain insights into compositional and structural effects on the electrochemical activity of hollow FexCo100-x oxide NPs in OER we conducted a detailed characterization of the inactive (Fe100oxide), the slightly active (Fe81Co19- oxide) and the highly active Fe36Co64- oxide hollow NPs. Elemental mapping analysis of hollow FexCo100-x- oxide NPs revealed a uniform elemental distribution of cobalt and iron and no elemental segregation within individual NPs in the samples with cobalt to iron ratio up to 2 (Figure 6). However, a detectable inhomogeneity of iron distribution was observed in the case of hollow NPs prepared with higher cobalt to iron ratio (Figure S10) that also resulted in the lower OER activity as compared to Fe36Co64- oxide hollow NPs. The increase of the cobalt concentration in the composition of FexCo100-x- oxide NPs is associated with the changes of the XRD peak intensities (Figure S11). This can be explained by a modification of the cation sites occupancies in the spinel lattice or by the superposition of diffraction peaks of γ-Fe2O3 and CoO. Both γ-Fe2O3 and CoO have cubic lattices. The peak positions of Fe81Co19- and Fe36Co64- oxide NPs matches those of Fe100- oxide NPs and we can expect that these compositions adopt the spinel structure of γ-Fe2O3. In turn, the peak positions of XRD spectrum of hollow oxide NPs with higher concentration of cobalt and pronounced compositional non-uniformity, are visibly shifted, that in combination with


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nonhomogeneous elementals distribution revealed by EDX mapping, points out to the presence of two phases (Figure S11). According to XANES spectra of FexCo100-x oxide NPs acquired at Fe K-edge, the energy positions of the pre-edge peak representing the mixing of the d-states of the iron atoms with the p-states of the surrounding oxygen atoms are at around 7114.3 eV (Figure S12a). This indicates Fe (+3) in all studied samples.46 The Co K-edge XANES spectra of FexCo100-x- oxide revealed the edge shoulder and the maximum of absorption (white line) attributed to the 1s→4p transitions at ~7709 eV and ~7726 eV, respectively, that is expected for CoO (Figure S12b).47 Systematic lowering of intensity of the edge shoulder and the increase of white line intensity in XPS spectra from hollow Fe81Co19- to Fe36Co64- and to non-uniform Fe24Co76- oxide NPs can be associated with the increasing of the signal from Co2+ in the studied samples as a result of cobalt concentration increase in such NPs. XANES data also indicate that Co100- oxide NPs have some amount of metallic cobalt even after prolonged oxidation, which is in agreement with XRD data (Figure S11).47 Since the cobalt is present in oxidation state (+2), we can exclude the presence of the spinel Co2O3.

Figure 6. TEM (a,b) and EDX mapping images obtained by acquiring EDX spectra including the Fe L series (c,d) and Co L series (e,f) X-rays on hollow Fe81Co19- (c,e) and Fe36Co64- (d,f) oxide NPs. Superposition maps of Fe (blue) and Co (red) obtained for Fe81Co19- and Fe36Co64- oxide NPs are given in (g) and (h), respectively. EDX maps of individual elements (Figures 6c-6f) are given using the same color pallets for better perception of elemental distribution in the samples.

The XPS analysis of the series of FexCo100-x- oxide NPs showed that the oxidation states of Fe and Co do not change and stay the same within the series (Figure S13). The detailed analysis of low



concentrations of Fe in mixed oxides is complicated by the appearance of Co Auger signals in the Fe 2p region. At lower concentrations, such as Fe24Co76, the low peak intensities of Fe 2p signals blend in with Auger signals from Co. The presence of the pronounced satellite peak at ~786 eV relative to the main peak at ~780 eV pointed out to (+2) oxidation state of cobalt atoms at the surface in all studied hollow FexCo100-x oxide NPs (Figure S12b). Examination of O 1s region revealed that all oxide NPs have both oxide and hydroxide groups on their surface (Figure S13, Table S3).48 The hydroxide/oxide ratio ranges from approximately 8.9:1 to 0.65:1 from pure Co oxide to pure Fe oxide and it is within this range for the mixed FexCo100-x- oxide NPs. For example, hydroxide/oxide ratio for Fe36Co64 NPs is 2.4 to 1. Thus we can argue that the formation of mixed metal NPs lead to the modified hydrophilicity of the surface interface of NPs, making it better suited for OER. The electrocatalytic OER involves three adsorbed intermediates such as OH(ad), O(ad) and OOH(ad)8, 49 and it is reasonable to assume that the ratio of oxide to hydroxide can play an important role in binding of OER intermediates at the electrocatalytic surface. As it has been previously stated, it is hard to characterize the active surface phases responsible for OER, since the electrocatalytic surface can undergo structural and electronic changes in electrolyte upon polarization.42 However, the initial concentrations of oxides and hydroxides at the surface of electrocatalysts can determine the ability of the interface to alter in a favorable way for OER. The performance of hollow FexCo100-x- oxides synthesized by direct oxidation of corresponding bimetallic NPs in OER was found to be significantly better as compared to OER performance of solid NPs.12, 18, 50-51 Thus, current density of 10 mA/cm2 at 0.30 V overpotential can be achieved in 0.1 M KOH using hollow Fe36Co64- oxide NPs. Analysis of the XRD and XANES data indicated the high concentration of cation vacancies in the structure of hollow FexCo100-x- oxide NPs; however, it revealed no unambiguous correlation between the type and concentration of the cation vacancies in the volume of the lattice of NPs and OER performance. However, OER activity of hollow FexCo100-x- oxides NPs correlated with the hydroxide/oxide ratio at their surfaces. This ratio is likely to be depended on the NP composition. The higher hydroxide/oxide ratio in hollow binary transition metal oxide NPs can be indeed can be associated with their lower OER overpotential; however, the Co100- oxide NPs characterized by the highest hydroxide/oxide ratio among all studied samples demonstrated higher OER overpotential as compared to hollow Fe24Co76-, Fe36Co64- oxide NPs obtained via direct oxidation bimetallic NPs and oxidized Fe/Co2+ and Fe/Ni2+ NPs. Thus, we can conclude that high concentrations of hydroxyl groups allowed to achieve lower OER overpotentials; however, the presence of surface oxides at the surface of FexCo100-x- oxide NPs in low concentrations seems to be beneficial for this reaction at lower


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overpotentials. This observation is in agreement with the data presented for Ni-based hydroxides nanowire arrays. Doping of Ni(OH)2 nanowires with Fe atoms resulting in the improved OER performance was accompanied by increase of surface oxide species.52 CONCLUSIONS Multicomponent transition metal oxide NPs is a promising class of low-cost materials allowing the design of highly porous electrocatalytic surfaces with efficient compositional and size control. We presented two strategies for the synthesis of a series of binary transition metal oxide NPs utilizing the Kirkendall effect. Previously, we have shown that hollow metal oxide shell with thickness more than 2.5 nm is not penetrable for the solutions and gases.43 Thus, we can assume that only external surface is available for electrochemical reactions and hence, we can rule out the surface area factor (assuming the similar loadings of similar sized NPs).We show that hollow polycrystalline, highly disordered mixed metal oxide NPs can be formed by the direct oxidation of metal alloy NPs. The composition of iron-cobalt NPs can be tuned by control over the ratio of the transition metal precursors. Hollow NPs with a high content of iron demonstrated low activity in OER in alkali media. Our study revealed that higher content of Co in the series of hollow FexCo100-x- oxide NPs results in their better performance in anodic OER. Among NPs obtained by direct oxidation of metal alloy NPs, hollow Fe36Co64-oxide NPs revealed the best performance in OER and a current density of 10 mA/cm2 was achieved at ~0.30 V overpotential. The pronounced non-uniformity of the elemental distribution in series of FexCo100-x- oxide NPs with a low concentration of iron (e.g. Fe24Co76- oxides NPs) pointed out to the limitation of synthesis of binary metal oxide NPs via direct oxidation of bimetallic alloys. Generally speaking, the synthesis of oxidized forms of multicomponent metal NPs with desired compositions faces a number challenges. First of all, it requires the synthesis of multicomponent metal seed NPs with the desired stoichiometry. Since the oxidation of the multicomponent seeds can be accompanied by leaching of certain cations, establishing of the seed composition, needed to synthetize the desired compositions of the oxidized forms of transition metals, can be labor intense. Moreover, in the case of different diffusion rates of elements during the oxidation process, the morphology of NPs can be affected or elemental segregation can take place. Even though in the case of Fe-Co based NPs uniform FexCo100-x- oxide NPs were synthesized for x above 24 (most likely due to the similar diffusion rates of cobalt and iron cations), the above mentioned considerations can be rather serious for other elemental compositions. Therefore, we proposed an alternative approach to synthesize multicomponent transition metal oxide NPs that could potentially allow uniform distribution of low concentrations of desired cations in the matrix of a different transition metal oxide NPs. This approach is based on the oxidation of as-synthesized Fe NP seeds in the presence of other transition metal cations followed by oxidative annealing. We showed that this approach also allowed the



synthesis of hollow binary transition metal oxides such as iron-cobalt and iron-nickel with a promising performance in the OER. In fact, the performance of oxide NPs prepared by oxidation of Fe NPs modified with Ni (+2) and Co (+2) cations was comparable with the performance of hollow Fe36Co64-oxide NPs obtained by direct oxidation of binary metallic NPs. For example, oxidized hollow Fe/Ni2+ NPs with 4.7 to 1 ratio of Fe to Ni also demonstrated current density of 10 mA/cm2 at 0.30 V. Therefore, we propose cation assisted synthesis as a promising way to design different functional oxidized forms of transition metals that are of interest for a broad range of application in catalysis, fuel cells, electrolyzers, and batteries. Our data show that cobalt- and nickel-rich surfaces are more active, however, the presence of surface iron is also critical. This observation is in agreement with previously reported data on lowering of OER overpotential of nickel oxide/(oxy)hydroxide by introducing iron atoms.42, 53 Our study also demonstrates that the ratio of oxide to hydroxide at the nanoparticle surface can play an important role. We believe that the presented here synthetic approaches can be efficiently utilized to tune the compositions of oxidized forms of multicomponent transition metal NPs enabling the control over the spin states and lattice site occupancies of the cations that, in turn, will further promote the design of efficient OER electrocatalysts.54-55 ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: SEM images of electrode prepared from FexCo100-x- and Co100- oxide NPs before and after electrochemical cycling; effect of the loading on the electrochemical performance of NP electrodes; mass loadings of the studied samples; XANES and XPS data on the studied samples; description of Co100-oxide samples; additional data on electrochemical performance of binary hollow FexCo100-x- oxide NPs and Fe100- and Co100- oxide NPs; XRD patterns for Fe81Co19-, Fe36Co64-, and Fe24Co76- oxide NPs and control hollow iron oxide and cobalt oxide NPs; XPS data of the O1s levels for hollow FexCo100-x-, Fe100- and Co100- oxide NPs and oxidized Fe/Co2+ and Fe/Ni2+NPs.

AUTHOR INFORMATION Corresponding authors: *E-mail: Aiwen Lei ([email protected]) and Elena Shevchenko ([email protected]).


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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS Work at the Center for Nanoscale Materials, Advanced Photon Source, and Electron Microscopy Center was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC0206CH-11357. XRD study was performed at GeoSoilEnviroCARS (The University of Chicago, Sector 13), Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation - Earth Sciences (EAR - 1634415) and Department of Energy- GeoSciences (DE-FG02-94ER14466). The research efforts of Pan Peng were supported by the National Natural Science Foundation of China (21390402 and 21520102003), the Ministry of Science and Technology of China (2012YQ120060), the Fundamental Research Funds for the Central Universities and the Program of Introducing Talents of Discipline to Universities of China (111 Program) and a scholarship from the China Scholarship Council (CSC) (201505990325).

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TABLE OF CONTEST


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Binary Transition-Metal Oxide Hollow ... - ACS Publications

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