Hollow and Porous Nickel Cobalt Perselenide Nanostructured

Jul 17, 2017 - Nickel and cobalt-based chalcogenides are an interesting class of electrochemically active compounds. Herein we report a two-step synth...
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Hollow and Porous Nickel Cobalt Perselenide Nanostructured Microparticles for Enhanced Electrocatalytic Oxygen Evolution Dipak V. Shinde,† Luca De Trizio,*,† Zhiya Dang,† Mirko Prato,‡ Roberto Gaspari,†,§ and Liberato Manna*,† †

Nanochemistry Department, ‡Materials Characterization Facility, and §CompuNet, Istituto Italiano di Tecnologia, via Morego 30, 16163 Genova, Italy S Supporting Information *

ABSTRACT: Nickel and cobalt-based chalcogenides are an interesting class of electrochemically active compounds. Herein we report a two-step synthesis of uniform hollow nickel cobalt perselenide nanostructured microparticles with a corrugated surface, randomly spaced pores, and a tunable composition. In the first step, we synthesized Ni−Co acetate hydroxide prismshaped microparticles, which were subsequently used as templates for the selenization process at temperatures as low as 80 °C. The substitution of acetate and hydroxide anions with selenide anions at a low temperature eventually produced partially amorphous hollow microparticles due to the nanoscale Kirkendall effect. Thin films of the as-synthesized samples behaved as highly active catalysts for the electrochemical oxygen evolution reaction (OER). Among those, Ni0.88Co1.22Se4 hollow microparticles showed excellent performances, exhibiting a 10 mA/cm2 current density at a modest overpotential of 320 mV, a high turnover frequency of 0.146 S1−, and a long-term operational stability (over 100 h of continuous operation).

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INTRODUCTION The electrocatalytic oxygen evolution reaction (OER, 2H2O → O2 + 4H+ + 4e−) is an important half reaction in energy storage and conversion technologies, such as metal air batteries,1 fuel cells,2 and water splitting cells for the hydrogen production.3−12 The efficiency of the OER plays a critical role in the advancement of these technologies. However, as it involves a proton-coupled four electrons transfer, it is kinetically slow and requires high overpotentials to occur at practical rates (e.g., of the order of 10 mA/cm2 catalytic current, a metric relevant to solar fuel synthesis). To date, RuO2 and IrO2 are the most efficient and stable OER catalysts, and they can work in both acidic and alkaline media.10,13 However, due to their cost and low abundance, they are not economically viable. Currently, various alternative OER catalysts have been reported, including metal oxides such as NiO,14 CoO,15 NiCoO4,16 and MnO2,17 hydroxides such as Ni(OH)2,18 FeOOH,19 and Co(OH)2,20 Ni−Co layered hydroxides,21 and Fe−Co−W hydroxides.22 Unfortunately, they all suffer from poor conductivity and, therefore, from poor charge transfer. Transition-metal chalcogenides such as Ni3S2,23 NiS1.03,24 Ni3Se2,10 CoSe2,25 NiSe2,26 and NiCoS427 and phosphides such as Ni2P,11 Co2P,28 NiCoP,29 and CoMnP30 have recently emerged as new classes of highly active catalysts for OER. The better performances of these compounds, compared to those of their oxide counterparts, are thought to originate from their higher conductivities, which enable a fast charge transfer.23 The © 2017 American Chemical Society

electrical conductivity of some metal chalcogenides arises as a consequence of the covalent nature of the metal-chalcogen bonds, which are very different from the ionic metal−oxygen bonds in metal oxides. This is because chalcogen ions have d orbitals of accessible energy while oxygen anions do not, and this leads to broad valence bands and bandgaps which are narrower than those of the corresponding oxides.31 The gap actually closes for Ni and Co chalcogenides, which have a metallic character. On the other hand, even if such metal chalcogenides and pnictides can efficiently catalyze the OER without undergoing anodic oxidation, the mechanism behind their catalytic properties is still unclear. It has been proposed, for example, that these materials might act as “catalyst precursors” rather than actual catalysts, forming the corresponding oxy-hydroxide species under the high positive potentials and corrosive conditions of OER.32 Such oxy-hydroxide species formed on the surface of chalcogenide and phosphide materials are more active than the corresponding ones produced by direct synthesis.32 Overall, these recent results suggest that transition metal chalcogenides are promising catalysts for OER and that additional efforts are needed not only to further enhance their Received: June 27, 2017 Revised: July 13, 2017 Published: July 17, 2017 7032

DOI: 10.1021/acs.chemmater.7b02666 Chem. Mater. 2017, 29, 7032−7041

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performances but also to understand the mechanisms underlying their catalytic properties. Regarding the morphology of the catalysts, hollow and porous nanostructured materials are particularly interesting for electrocatalysis, owing to their unique properties: (1) they are composed of nanosized building blocks that ensure not only a high electrochemically active surface area but also a reduced diffusion length for ions and electrons; (2) the cavities in the hollow structures provide extra void space and thus a high electrolyte−electrode contact area for the fast diffusion of evolved gases.30,33 Indeed, one of the best-performing OER catalysts is α-Ni(OH)2 in the form of micron-sized hollow spheres composed of nanosized building blocks. It achieved 10 mA/cm2 (the current density expected for integrated water splitting devices operating at 10% solar to hydrogen efficiency)34 at 1.56 V vs RHE.18 Also, the porous and hollow nanostructures of Ni and Co based sulfides have been successfully synthesized by several groups and employed in high-performance electrochemical devices.35−40 Among those, the most promising materials are ternary Ni−Co−S compounds, which have also been widely explored as electrode materials in batteries and supercapacitors.27,41−44 Their excellent pseudocapacitive and electrocatalytic properties have been ascribed not only to their high electrical conductivity but also to a synergy between Ni and Co metals. While considerable efforts have been made to develop hollow ternary metal sulfide materials, not much has been reported on the synthesis and the electrocatalytic properties of their selenide counterparts.45−47 Herein, we report a two-step synthesis of partially amorphous hollow nickel cobalt perselenide (NCP) particles with sizes around ∼700 nm and a tunable composition, which were tested as catalysts for the OER (see Scheme 1). In the first

Article

EXPERIMENTAL SECTION

Chemicals. Nickel(II)acetate tetrahydrate (98%), cobalt(II) acetate tetrahydrate (99%), polyvinylpyrrolidone (PVP, molecular weight- 55 000), selenourea (98%), ruthenium(IV) oxide (99.9%), absolute ethanol, 2-propanol, nafion solution (5 wt % in lower aliphatic alcohols and 15−20% water), potassium hydroxide (90%), and fluorine-doped tin oxide glass substrates (FTO) of surface resistivity ∼7 Ω/sq were purchased from Sigma-Aldrich. All the chemicals were used without further purification. FTO substrates were cleaned first with detergent in water, followed by rinsing with distilled water. Eventually, the substrates were dipped in a 1:1 solution of 2propanol and acetone and ultrasonicated for 30 min and dried under air. Synthesis of Ni−Co Acetate Hydroxide Particles. NCAH MPs were synthesized following the procedure reported by Du et al. with minor modifications.48 Initially, 4 g of PVP and a fixed amount of nickel(II)acetate tetrahydrate and cobalt(II)acetate tetrahydrate were dissolved in 90 mL of ethanol by continuous stirring. The reaction solution was then refluxed at 80 °C for 4 h. The precipitate was collected by centrifugation and washed with ethanol five times. Then the product was dried in a hot air oven at 60 °C for 5 h. In order to obtain MPs with different compositions, various Ni/Co precursors ratios were used while keeping the total Ni+Co amount fixed (2 mmol): 1/1, 2/1, and 1/2. Synthesis of Hollow Nickel Cobalt Perselenide (Ni1−xCo1+xSe4) MPs. NCAH MPs (22 mg) were dispersed in 40 mL of ethanol in a glass vial by sonication in a N2 filled glovebox. A solution of 60 mg of selenourea in 5 mL of ethanol was then added to the MPs dispersion, which was subsequently heated up to 80 °C using a hot plate, and kept at that temperature under continuous stirring for 2.5 h. To study the intermediate products, 1 mL aliquots were collected at different time intervals and mixed with 1 mL of ethanol (kept at room temperature) to halt the reaction. The final products were collected by centrifugation and washed with ethanol for three times to remove impurities. The resulting products were dried in a hot air oven at 60 °C for 2 h. The dried powders were then stored in a glass vial in air for further characterizations. At least 5 different syntheses in each case were conducted to confirm the reproducibility of the synthetic procedure. Selected samples were treated in an Ar filled tube furnace at 400 °C for 30 min to check the effects of the annealing on the crystallization and the OER activity of the corresponding products. To remove surface oxide/hydroxide species for XPS analysis, some NCP samples were treated with 5 mL of a 0.5 M H2SO4 solution for 30 min, washed with ethanol, dried, and stored in the glovebox. Structural Characterization and Elemental Analysis. Scanning Electron Microscopy (SEM). The samples for the SEM analysis were prepared by dropping solutions of MPs on thin glass substrates followed by coating them with a 10 nm layer of gold by sputtering. The imaging was performed on a FEI NanoLab 600 dual beam system operating at 20 kV. Transmission Electron Microscopy (TEM). Samples were prepared by dropping dilute solutions of MPs in ethanol onto carbon-coated 200 mesh copper grids for low-resolution TEM or ultrathin carbon/ holey carbon coated 400 mesh copper grids for high-resolution TEM (HRTEM) and energy dispersive X-ray spectroscopy (EDS) measurements. Low-resolution TEM measurements were conducted on a JEOL JEM-1100 transmission electron microscope operating at an acceleration voltage of 100 kV. HRTEM was performed with a JEOL JEM-2200FS microscope equipped with a 200 kV field emission gun, a CEOS spherical aberration corrector for the objective lens, enabling a spatial resolution of 0.9 Å, and an in-column image filter (Ω-type). The HRTEM images were obtained with a direct electron detection camera (K2 Summit, Gatan Inc.) with 7420 × 7676 pixels, which is able to capture images on a large field of view with atomic resolution. The selective area electron diffraction (SAED) patterns were acquired with a CCD camera with 2048 × 2048 pixels (UltraScan 1000, Gatan Inc.). The chemical composition of the MPs was determined by EDS analysis performed on the same microscope in high-angle annular dark field scanning TEM (HAADF-STEM) mode with a Bruker Quantax

Scheme 1. Template-Assisted Synthesis of Hollow Nanoporous NiCoSe4 MPs

step, Ni−Co acetate hydroxide (NCAH) microparticles (MPs) were synthesized using a simple precipitation process. These MPs were then reacted with selenourea at a relatively low temperature (i.e., 80 °C) to form the final Ni1−xCo1+xSe4 product. The resulting MPs preserved the outer morphology of the parent particles, developing, at the same time, a hollow interior and a porous surface. The analysis of the intermediate steps of the selenization process revealed that the reaction starts from the surface of the precursor MPs with the flux of ingoing selenide anions being much slower than that of outgoing anion species, thus resulting in hollow structures. This low-temperature soft solution-based strategy produced MPs exhibiting excellent catalytic activity and stability for over 100 h in 1 M KOH (pH ∼ 14) when employed as OER catalysts. The NCP MPs developed in this work exhibited one of the highest turnover frequency (TOF) values among all chalcogenide OER catalysts reported to date. 7033

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Chemistry of Materials 400 system with a 60 mm2 XFlash 6T silicon drift detector (SDD), using the thin film approximation (Cliff−Lorimer) on the Co Kα, Ni Kα, and Se Kα lines. X-ray Diffraction (XRD) Analysis. The samples for the XRD analysis were prepared by drop casting the MPs dispersions onto zero diffraction silicon wafers. The diffraction patterns were collected in air at room temperature on a PANalytical Empyrean X-ray diffractometer with a 1.8 kW Cu Kα ceramic X-ray tube and PIXcel3D 2 × 2 area detector operating at 45 kV and 40 mA. The patterns were collected using a parallel-beam (PB) geometry and a symmetric reflection mode. XRD data analysis was performed using the HighScore 4.1 software from PANalytical. Inductively Coupled Plasma Atomic Emission Spectroscopy (ICPAES). A known quantity of samples was dissolved in HCl/HNO3 3:1 (v/v), digested overnight and diluted with deionized water. The elemental analysis was conducted using an iCAP 6500 Thermo spectrometer. Raman Analysis. The samples were prepared by drop-casting MPs dispersions in ethanol on glass substrates. The measurements were performed under an inert atmosphere. For this purpose, N2 was fluxed through a closed chamber (from Linkam) coupled with a Renishaw InVia spectrometer. Data were acquired at λ = 532 nm with a 50× objective using a nominal power of 25 mW and an integration time of 30 s. X-ray Photoelectron Spectroscopy (XPS). The samples were prepared by drop-casting a few microliters of NC solutions onto a graphite substrate (HOPG, ZYB quality, NT-MDT), then transferred to the XPS setup. Measurements were conducted with a Kratos Axis Ultra DLD spectrometer, using a monochromatic Al Kα source (15 kV, 20 mA). Wide scans were acquired at an analyzer pass energy of 160 eV. High-resolution narrow scans were performed at a constant pass energy of 20 eV in 0.1 eV steps. Photoelectrons were detected at a takeoff angle Φ of 0° with respect to the surface normal. The pressure in the analysis chamber was maintained below 5 × 10−9 Torr for data acquisition. The data were converted to VAMAS format and processed using CasaXPS version 2.3.16. The binding energy scale was internally referenced to the C 1s peak (BE for C−C of 284.8 eV). Computational Modeling. Density functional theory (DFT) calculations were performed on bulk models of stoichiometric NiCoSe4,49 using the primitive cubic unit cell. Ni sites were fully replaced with Co sites, and vice versa, to model the CoSe2 and NiSe2 systems. We used the Perdew−Burke−Enzerhof functional,50 ultrasoft pseudo potentials for all elements 51 and the pwscf code. 52 Respectively, 30 and 240 Ryd were used as a cutoff for the plane waves and charge density. A Monkhorst−Pack 6 × 6 × 6 mesh was used for Brillouin Zone sampling. All the systems investigated displayed a metallic ground state. In order to eliminate the possibility of a wrong prediction of the metallic state, a strong Hubbard correction,53 favoring a semiconducting state, U = 10 eV, was added on the d orbitals of Ni/Co and on the p orbitals of Se. The use of U also on the p orbitals appeared to be necessary as p orbitals contribute to the electronic density at the Fermi level.54 The state of the system remained metallic even after the Hubbard correction. The experimental geometry of the system was used for the wave function optimization. Using the same level of theory employed for the NiCoSe4 systems, plain DFT calculations were performed on bulk CoO.55 The electron density of CoO and NiCoSe4 systems were finally used to obtain Bader charges. Electrochemical Measurements. The OER catalytic activity measurements were performed in a three-electrode electrochemical cell with a platinum wire as a counter electrode, Ag/AgCl (3 M NaCl) as a reference electrode and NiCoSe4 MPs deposited on FTO as a working electrode, on a IVIUM Compactstat potentiostat. The catalyst ink was prepared by dispersing 5 mg of catalyst powder in 1 mL of a solution containing 0.8 mL of deionized water, 0.2 mL of isopropanol and 10 μL of 5 wt % Nafion solution, followed by ultrasonication for 30 min. This ink was then drop-cast on precleaned FTO substrates with 1 cm2 of geometric surface area, and allowed to dry in ambient air. Linear sweep voltammograms were recorded in 1 M KOH solution (pH = 13.60) at a sweep rate of 2 mV/S. The scans were repeated

until reproducible CV curves were obtained. The long-term stability of the electrodes was measured by chronoamperometry. Impedance spectra of the electrodes were recorded at open circuit potentials in a frequency range of 0.1 to 100 000 Hz at a sinusoidal amplitude of 5 mV. To fairly compare the catalytic activity of different catalysts, CV curves were corrected for ohmic losses throughout the system. The series resistance for each sample was obtained from Nyquist plots and subtracted from raw CV data following Ohm’s law, thus obtaining the iR-corrected data. Potentials were converted to the reversible hydrogen electrode (RHE) scale using the following formula: ERHE = EAg/AgCl + (0.0591 × pH). Calculation of the Turnover Frequency (TOF). The TOF was calculated by assuming that every metal atom took part in the OER. More specifically, the lower limit of TOF was calculated as follows:18 TOF =

j· Sgeo 4F · n

in which j is the current density at 350 mV overpotential, Sgeo is the geometric surface area (1 cm2) of the FTO electrode, 4 indicates the number of electrons transferred per mole of O2, F is the Faraday’s constant (96 485 C/mol) and n is the number of moles of metal, calculated on the basis of the weight of the catalyst loading on the FTO surface.

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RESULTS AND DISCUSSION The two-step synthesis that we developed to prepare hollow NCP MPs is sketched in Scheme 1. In the first step, hydrated Ni and Co acetates were allowed to react in ethanol in the presence of PVP, using different Ni/Co precursors ratios; namely 1/1, 1/2 and 2/1. SEM and TEM images of the products show that prism shaped particles with a length of ∼700 nm and a width of ∼200 nm were formed in all the experiments (see Figure 1A and Figure S1 of the Supporting

Figure 1. (A) Representative TEM images showing the temporal evolution of the reaction between a typical NCAH MP (top left panel) and selenium anions to produce a hollow nanoporous NiCoSe4 MP (bottom right panel). The scale bar in all the images is 100 nm. (B) XRD patterns of the products collected at various time intervals of the selenization process. The inset of B is a zoom of a selected region of (B) showing the main diffraction peak of NCAH MPs at various stages of the transformation. The NCAH MPs used in the selenization process illustrated in this figure were prepared using a precursor ratio of Ni/Co = 1.

Information, SI). The ICP elemental analysis confirmed that, in each experiment, Ni and Co atoms were both present in the final product and that the Ni/Co ratio in the MPs was always lower than the Ni/Co precursors ratio used in their synthesis (see Table 1). These findings suggest a lower reactivity of the Ni precursor compared to that of the Co precursor under our synthetic conditions. 7034

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Chemistry of Materials Table 1. Elemental Compositions of the Products Obtained by ICPa Ni/Co ratio Ni/Co feed ratio

NCAH MPs

NCP MPs

composition of NCP MPs

1/1 1/2 2/1

0.76 0.42 1.72

0.72 0.45 1.78

Ni0.88Co1.22Se4 Ni0.62Co1.38Se4 Ni1.28Co0.72Se4

a

The error in the ICP measurement was 5%.

The XRD patterns of the MPs prepared with the three different Ni/Co ratios, shown in Figure 1B (black line) and Figure S2 of the SI, are well matching with those of cobalt acetate hydroxide crystals reported by Du et al.48 (see Figure S2B of the SI). This material, with the formula Co5(OH)2(CH3COO)8·2H2O, crystallizes in a tetragonal structure with a = b = 23.693 Å and c = 11.565 Å.48,56 As shown by Kuhlman et al., such a structure can be visualized as a three-dimensional network of edge-sharing CoO6 polyhedra (see Figure S2C and D of the SI for details).51 The profound similarities, in terms of shape and XRD pattern, between our NCAH MPs and the crystals synthesized by Du et al. suggest that the product of our synthesis has a Co5(OH)2(CH3COO)8•2H2O like structure, in which Co2+ and Ni2+ ions are randomly distributed in the octahedral sites. The NCAH MPs were then reacted with selenourea in ethanol at 80 °C for 2.5 h. For all three samples, the color of the reaction solution gradually changed from pale pink to dark black, indicating the formation of Ni−Co−Se products. Our ICP elemental analysis showed that in all three cases, the final products contained Ni, Co, and Se, with a resulting Ni1−xCo1+xSe4 composition that depended on that of the starting NCAH MPs (see Table 1). Indeed, the Ni/Co ratio observed in the NCAH MPs was preserved after the selenization process, suggesting that no preferential loss of metal cations took place in this step. Typical TEM images of the particles collected at the different stages of the selenization process are shown in Figure 1A. It is clear that the starting bulky NCAH MPs slowly transformed into hollow porous prism shaped microstructures. Selenide ions, formed by the thermal decomposition of selenourea, first reacted with the surface of precursor MPs forming a thin shell (1 h) that became thicker over time at the expense of the inner NCAH material (1.5 and 2 h) eventually forming a hollow structure (see Figure 1A 2.5 h and Figure S3 of the SI). The presence of a cavity inside the product MPs was further confirmed by SEM micrographs, in which MPs with partially broken walls had an internal void (see inset of Figure 2A and Figure S4 of the SI). The MPs were also characterized by randomly spaced pores throughout the surface. Our XRD analysis evidenced that, during the reaction with selenium anions, the XRD peaks of the starting NCAH compounds gradually disappeared, with the final products exhibiting weak XRD reflections, which could not be univocally ascribed to a specific Ni−Co−Se crystal structure (see Figure 1B and Figures S5 of the SI). It is interesting to note that samples prepared with Ni/Co precursors ratios of 2 and 0.5 had stronger diffraction peaks than those synthesized starting with a Ni/Co precursors ratio of 1. The presence of broad and weak XRD peaks in all three of the Ni1−xCo1+xSe4 samples suggested that the products are poorly crystalline, presumably as a consequence of the low temperature of the selenization process, which allowed little or no crystallization to occur.

Figure 2. (A) Representative low-resolution SEM image of as synthesized hollow Ni0.88Co1.22Se4 particles. The inset shows a SEM image of a broken particle highlighting its hollow interior. (B) Azimuthal integration of SAED pattern shown in Figure S5 of the SI together with the reference card of cubic NiCoSe4. (C) Representative low-resolution TEM image of a single hollow particle. (D) Highresolution TEM of the highlighted region in panel (C). (E) Enlarged image of a selected crystalline domain of panel (D). (F) STEM-EDS elemental maps of some Ni0.88Co1.22Se4 particles showing uniform distribution of Ni, Co, and Se.

Analogous amorphous hollow particles have been previously observed in sulfurization processes performed at 160 °C under hydrothermal conditions.44 To gain further insights into the structure/nature of the hollow NCP MPs, we conducted HRTEM analysis, as reported in Figure 2C−E. The analysis of Ni0.88Co1.22Se4 MPs evidenced that the hollow structures were composed of very small nanoparticles, among which some exhibited crystallinity with lattice fringes corresponding to the cubic pyrite-like NiCoSe4 phase (ICSD number 624483, see Figure 2C−E). The SAED patterns of such particles confirmed that the sample was weakly crystalline and the azimuthal integration of the diffraction pattern well-matched that of the cubic NiCoSe4 phase (see Figure 2B and Figure S6 of the SI). EDS elemental analysis revealed that Co, Ni and Se elements were homogeneously distributed throughout the hollow MPs with a mean composition of Ni0.97Co1.14Se4, in agreement with the results of the ICP elemental analysis (see Figure 2F and Table 1). These findings suggest that the reaction of NCAH MPs with selenourea produced, at the early stages, NCAH@Ni−Co−Se core@shell structures which evolved into hollow Ni1−xCo1+xSe4 MPs as a consequence of the faster out-diffusion of the OH− and CH3COO− anions compared to the slower in-diffusion of the selenide anions during the anion exchange reaction. This “Kirkendall effect” has been largely exploited for the formation of different hollow nanostructures.33,37,57−61 It is also worth mentioning that the anion exchange reaction occurring here (i.e., the selenization process) is driven by the difference in the solubility product constants of the initial and final materials.33,58 Being the solubility product constant of NCAH precursor much higher than that of the NCP product, the reaction is strongly favored and occurs even at the low 7035

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material is a solid solution of NiSe2 and CoSe2, compatible with the NiCoSe4 phase. The surface chemical composition and oxidation states of the metal ions in the starting NCAH MPs (Ni/Co = 0.76) and in the pristine and annealed Ni0.88Co1.22Se4 MPs was studied by XPS. The data collected on the energy regions typical for Ni 2p, Co 2p and Se 3d peaks are displayed in Figure 4. Ni and Co

temperature used here. On the other hand, different possible precursors, such as nickel oxide/hydroxide materials, having poor solubility in polar solvents, would have required much higher selenization temperatures to undergo full anion exchange.33 In order to increase the crystallinity of our products and, especially, to compare partially amorphous and crystalline samples for OER, a portion of the Ni1−xCo1+xSe4 MP samples was annealed at 400 °C for 30 min in an Ar atmosphere. As determined by the ICP measurements, NCP MPs maintained their composition after the annealing treatment. The XRD patterns of the annealed samples were characterized by sharp reflections that well matched with those of the pyrite-like NiCoSe4 phase (see Figure 3A and Figure S7 of the SI). No

Figure 4. XPS spectra of NCAH MPs (black curves) together with those of pristine and annealed NCP MPs before (blue and red curves, respectively) and after (pink and light blue curves, respectively) the acid treatment with H2SO4 in Ni 2p, Co 2p, and Se 3d regions.

Figure 3. XRD patterns (A) and (B) Raman spectra of pristine and annealed Ni0.88Co1.22Se4 MPs deposited on glass substrates. The bulk reflections of cubic NiCoSe4 are also reported by means of the blue bars.

cations in NCAH samples exhibit main XPS peaks at binding energies of 855.6 ± 0.3 eV (Ni 2p3/2) and 780.8 ± 0.3 eV (Co 2p3/2) that are in agreement with the typical values reported for metal oxide or hydroxide species63,64 (see Figure 4, black curves and Table 2). After reacting the NCAH MPs with selenourea, the samples were characterized by different XPS signals that did not evolve significantly after the annealing step (see Figure 4): both pristine and annealed NCP MPs samples had Ni 2p and Co 2p profiles characterized by the presence of two groups of three peaks (2p3/2 and 2p1/2 components). The binding energies of these peaks are summarized in Table 2. In line with what was reported for Ni and Co perselenide compounds, the Ni 2p peak at 853.4 eV and the Co 2p signal at 778.7 eV have been assigned to NiCoSe4.25,26,65−68 On the other hand, the peaks at higher binding energies, present also in the NCAH MPs, have been assigned to Ni and Co oxides and/ or hydroxides species that are, most likely, populating the surface of our NCP MPs.63,64 The presence of such species on the surface of Ni or Co perselenide materials, already observed by others,69 is not surprising considering that hydrated metal acetate hydroxide MPs were used as templates/precursors. In analogy with what was reported by Zhu et al.,69 a mild acid treatment of NCP samples with dilute H2SO4 led to the complete removal of such oxide and/or hydroxide surface species and to “pure” NiCoSe4 (see Figure 4, light blue and pink curves, and Figure S10 of the SI). The XPS spectra of the annealed NCP MPs were very similar to those of pristine ones with the main difference being the Se 3d region. In this regard, while the as synthesized samples were characterized by a single broad peak centered at (54.9 ± 0.3) eV (see Figure 4, blue curve), the annealed samples exhibited a well resolved Se 3d doublet, having the main Se 3d5/2 component centered at (54.9 ± 0.2) eV (see Figure 4, red

extra peaks ascribable to any secondary product were detected by our XRD analysis. The TEM images of the annealed samples revealed that the size of the crystallites forming the MPs increased after the thermal treatment (see Figure S9 of the SI). With the aim to better understand the effects of the annealing, we also performed Raman spectroscopy on pristine and annealed hollow Ni0.88Co1.22Se4 MPs (see Figure 3B). The as-synthesized sample exhibited a broad peak at 189 and a shoulder at 230 cm−1, while the annealed one was characterized by a sharp peak at 200 and a shoulder at 242 cm−1. Since no Raman spectra for the pyrite-like NiCoSe4 phase are available in literature, we compared our spectra with those of pyrite-like CoSe2 and NiSe2 materials, which have the same crystal structure as that of our NCP MPs (see Figure S8 of the SI for details). These materials are characterized by a sharp Raman peak at 190 and a shoulder at 250 cm−1 in the case of CoSe2, or a peak at 215 and a shoulder at 240 cm−1 in the case of NiSe2, which, in both cases, were ascribed to the Ag and Tg symmetry phonon modes of the Se−Se pairs, respectively.26,62 On the basis of these observations, we can assign the broad peaks in as-synthesized particles to the Se−Se stretching modes of the cubic NiCoSe4. The narrowing of the Raman peaks upon annealing not only further confirmed the improved crystallinity of the annealed sample but also supported our findings that the as-synthesized hollow MPs are composed of partially amorphous Ni−Co−Se nanoparticles which crystallize upon thermal treatment. In addition, the position of the Ag Raman signal of the annealed MPs between 190 and 215 cm−1 and the Tg signal between 240 and 250 cm−1 suggest that the final 7036

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Chemistry of Materials Table 2. Ni 2p and Co 2p Binding-Energy Values Found in NCAH, Pristine, and Annealed NCP Samples sample NCAH NCP annealed NCP

Ni 2p (eV) 853.4 ± 0.2 853.4 ± 0.2

855.6 ± 0.3 855.9 ± 0.3 855.9 ± 0.3

Co 2p (eV) 860.7 ± 0.3 861.1 ± 0.4 861.1 ± 0.4

curve).22 The invariance of the Se 3d peak position upon annealing suggested that the Se chemical environment was preserved upon the thermal treatment. On the other hand, the sharper Se 3d peaks observed in the annealed samples (see Figure 4, red curve) are indicative of their higher crystallinity, in agreement with the XRD and Raman data. It is noteworthy that Ni and Co XPS peaks in Ni1−xCo1+xSe4 are located very close to the expected values for metallic Ni (852.6 eV)70 and metallic Co (778.1 eV),59 suggesting a low charge on the Ni and Co ions.64 This was actually corroborated by DFT calculations of the partial atomic charges of metal ions: the calculated charge of Co sites in CoSe2, for example, was 0.45 e. On the other hand, the charge of Co ions in more ionic compounds, such as CoO, was calculated to be 1.75 e (see the Experimental Section for details). DFT calculations also showed that NiCoSe4 is metallic with a broad band at the Fermi level, characterized by the hybridization of Nid+Cod+Sep orbitals (see Figure S11 of the SI). The metallic nature of our samples was also confirmed by current−voltage measurements on films of MPs deposited on glass substrate, which showed ohmic behavior (see Figure S15 of the SI). The as-synthesized hollow MPs were then used as catalysts for OER. Thin films of the three NCP samples with different Ni/Co ratios were prepared by drop-casting dilute MP inks on FTO substrates. The SEM analysis of the particles deposited on FTO substrates indicated that the particles were uniformly distributed over the electrode surface and coated with Nafion binder (see Figure S16A of the SI). The OER activity was then measured by soaking such films in 1 M KOH electrolyte. Linear sweep voltammograms of electrodes with various catalysts are displayed in Figure 5A.

778.7 ± 0.2 778.7 ± 0.2

780.8 ± 0.3 781.1 ± 0.2 781.1 ± 0.2

785.7 ± 0.3 786.0 ± 0.3 786.0 ± 0.3

Mechanistic insights into the OER process could be gained through Tafel plots deduced from current−voltage curves (see Figure 5B). The three NCP samples were characterized by different values of Tafel slopes, indicating their different OH− adsorption capacity.71 The lowest Tafel slope of 78 mV/dec was observed for the Ni0.88Co1.22Se4 MP electrodes, demonstrating its faster OER kinetics. In order to better understand the origin of the different performances of these catalysts, we performed an impedance spectroscopy analysis. The Nyquist plots measured at open circuit potentials in all cases, reported in Figure S12 of the SI, exhibited a depressed semicircle in the high frequency region, which corresponds to the charge-transfer resistance (Rct) at the electrode−electrolyte interface, and a quasi-sloping line at low frequencies corresponding to mass-transfer resistance.18 From these plots, we concluded that the electrodes comprising Ni0.88Co1.22Se4 MPs were characterized by the lowest chargetransfer resistance, as evident from the smallest size of the semicircle in the high frequency range, as well as the lowest value of series resistance. Interestingly, the OER performances of such MPs after the annealing were lower than those recorded on pristine Ni0.88Co1.22Se4 MPs with similar catalyst loading amounts (see Figure S13 of the SI). This was tentatively attributed to a reduced active surface area of the annealed NCP MPs, connected to the increase in crystallites’ size. Also, as reported by Tour et al.,72 amorphous metal oxides with numerous surface defects and lattice dislocations exhibit a higher OER activity compared to their crystalline counterparts, which is also applicable here. On the basis of such observations, we selected the assynthesized Ni0.88Co1.22Se4 MPs as the catalyst with the highest performance, and we studied the dependence of its OER performance as a function of the amount of catalyst loading. Linear sweep voltammograms of electrodes comprising of Ni0.88Co1.22Se4 MPs with various catalyst loadings (24, 48, 72, 96, 192, and 288 μg/cm2) are shown in Figure 6A. For convenience, a bar plot of the overpotential needed to achieve a 10 mA/cm2 current density (calculated by subtracting 1.23 V from the observed potential vs RHE) vs catalyst loading is also reported in Figure 6B. The OER activity was maximized at a catalyst loading of 72 μg/cm2, while the lowest performances

Figure 5. Oxygen evolution activity of NCP MPs measured in 1 M KOH electrolyte. (A) Linear sweep voltammograms of electrodes comprising of NiCoSe4 particles with various Ni/Co ratios with the (B) corresponding Tafel plots. The catalyst loading in each case was 96 μg/cm2. Error bars in B were calculated as the standard deviation from three measurements.

Among the three samples tested, the Ni0.88Co1.22Se4 MPs exhibited the highest catalytic activity, while the lowest performance was registered for the sample with the highest nickel content. Working with Ni0.88Co1.22Se4 MPs, a 10 mA/ cm2 current density was reached at an overpotential of 340 mV, while for the other two samples, using similar catalyst loading amounts, the same catalytic current was observed at 350 mV (Ni0.62Co1.38Se4 MPs) and 370 mV (Ni1.28Co0.72Se4 MPs).

Figure 6. (A) Linear sweep voltammograms of electrodes comprising of Ni0.88Co1.22Se4 MPs with various catalyst loading amounts, measured in 1 M KOH at a sweep rate of 2 mV/s. (B) A bar plot of Ni0.88Co1.22Se4 catalyst loading vs overpotential needed to achieve a 10 mA/cm2 current density calculated from CV plots shown in (A). 7037

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toward the OER with a lower Tafel slope, low overpotential to achieve a 10 mA/cm2 catalytic current and high TOF values. This can be attributed to their metallic nature, the synergy between Co and Ni metals, the porous surface and the hollow structure, which allow for numerous accessible electrochemical active sites and for a fast diffusion of the developing oxygen bubbles. This is further supported by the impedance spectroscopy data reported in Figure 7C, which revealed a smaller charge-transfer resistance in addition to a lower series resistance for Ni0.88Co1.22Se4 MPs electrode compared to the RuO2 electrode. In order to verify whether the operative conditions during the OER reaction induced any chemical transformation in our material at the electrode surface, we further conducted a XPS and SEM “post-mortem” investigation on our Ni0.88Co1.22Se4 MPs after OER measurements. Our analysis revealed that the high binding energy components in both Ni and Co 2p XPS spectra, ascribable to Ni2+ and Co2+ in oxide/hydroxide compounds, strongly increased after OER, while the intensity of the Se 3d, Ni and Co 2p signals, related to the perselenide material, was strongly reduced (see Figure S17 of the SI). On the other hand, our SEM characterization evidenced that the starting Ni0.88Co1.22Se4 MPs retained their original morphology after the reaction, as shown in Figure S16B of the SI. Considering that the XPS analysis is surface sensitive, our results suggested that hollow perselenide MPs undergo partial oxidation during the OER, forming a surface layer of highly active Ni−Co oxide/hydroxide species. Such species, which have been shown to form also on the surface of sulfide and phosphide OER electrocatalysts, are thought to be the active catalytic sites for water oxidation.11,30,32,73,74 Since both nickeland cobalt-based hydroxides are active catalysts for OER, we can hypothesize here that in perselenide derived hydroxides, both the Ni and Co centers act as active sites. On the other hand, the metallic perselenide “core”, most likely, is responsible of the fast charge transport. To further assess the stability of our NCP MPs electrodes, which is the most important parameter in determining their viability for practical OER, we tested them by chronoamperometry by applying a constant potential of 1.55 V vs RHE for over 100 h. As it can be seen in Figure 7D, the catalytic current did not evidence any sign of degradation even after continuous 110 h of operation, highlighting the robustness of our hollow NCP MPs eletrodes.

were registered both at either lower or higher loadings. This trend was somehow unexpected considering that the catalytic current for oxygen evolution at a given overpotential should increase linearly with the catalyst loading amount; provided that the mass-transport, ionic resistances within the electrolyte and the electrical resistance of the catalyst film are marginal. In the present case, however, the MPs on the surface of the electrode might have a poor contact with the FTO substrate at higher catalyst loadings. Also, the oxygen bubbles that form from the particles inside the film near the FTO substrate may have difficulty escaping, thereby decreasing the current at a given overpotential. After optimizing the catalyst loading, we compared the activity of our MPs to that of the commercial RuO2 catalyst. Figure 7A shows linear sweep voltammograms of

Figure 7. Water oxidation activity of electrodes comprising Ni0.88Co1.22Se4 MPs and RuO2 particles deposited on FTO as a current collector in 1 M KOH. (A) Linear sweep voltammograms of electrodes measured at a sweep rate of 2 mV/s.; (B) corresponding Tafel plots; (C) impedance spectra of electrodes measured at open circuit potential; (D) stability of Ni0.88Co1.22Se4 MPs electrode measured by chronoamperometry at a constant applied potential of 1.55 V vs RHE in 1 M KOH. Error bars in B represent standard deviation from three measurements.

Ni0.88Co1.22Se4 MPs with a loading amount of 72 μg/cm2 and commercial RuO2 with an optimized loading of 96 μg/cm2 (see Figure S14 of the SI for details). The RuO2 electrode exhibited a lower onset potential for the OER compared to the one made of Ni0.88Co1.22Se4 MPs. However, the latter was characterized by a steeper catalytic current slope, achieving a current density of 10 mA/cm2 at an overpotential of only 320 mV. The OER kinetics was estimated by plotting Tafel plots, as displayed in Figure 7B. The Ni0.88Co1.22Se4 MPs electrode exhibited a Tafel slope of only 58 mV/dec, much smaller than that of RuO2 (97 mV/dec), which is an indication of the fast OER kinetics of our porous and hollow NCP MPs.18 The intrinsic OER activity of the catalysts was further characterized by calculating the TOF at an overpotential of 350 mV. It is noteworthy that the calculated TOFs represent the lowest limit since not all the metal sites are accessible for the electrocatalysis reaction. The Ni0.88Co1.22Se4 MPs were characterized by a high TOF value (0.146 S1−), in comparison not only to the reference RuO2 (0.067 S1−) but also to most of the reported chalcogenide OER electrocatalysts (see Table S1 of the SI for a detailed comparison). The partially amorphous NCP MPs fabricated here exhibit favorable kinetics

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CONCLUSIONS We have developed a low-temperature two-step chemical route to fabricate porous and hollow Ni1+xCo1−xSe4 MPs with different compositions. In the first step, prism-shaped NCAH MPs were synthesized at room temperature. Successively, these MPs were reacted with Se2− anions to produce the final NCP MPs. As evidenced by our TEM analysis, such hollow structures formed as a consequence of the fast outward diffusion of OH− and CH3COO− species together with a slow inward diffusion of selenide anions during the anion exchange process, a classic case of Kirkendall effect. HRTEM and XRD characterizations evidenced that the as-formed MPs were made mostly of amorphous nanoparticles. When employed as catalysts for OER, the NCP MPs exhibited excellent catalytic activity, with a 10 mA/cm2 current density at a small overpotential of 320 mV, and a Tafel slope of 58 mV/dec. XPS analysis revealed that after OER, a surface Ni−Co oxide/hydroxide layer formed on the surface of perselenide particles. Such a layer is believed to 7038

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be the actual catalyst for OER, while the metallic perselenide “core” is responsible for the fast charge transport. The excellent catalytic activity was attributed not only to the synergy between Ni and Co, which was optimized for the Ni0.88Co1.22Se4 composition, but also to the hollow and porous structure of the MPs, which offers numerous electrochemical active sites and, at the same time, ensures a fast diffusion of the formed oxygen bubbles. The low temperature synthesis strategy developed here has the potential to create new opportunities in the fabrication of porous and hollow structures for various applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b02666. SEM and TEM images, XRD characterization, SAED patterns, structural models, density of states calculated by DFT, electrochemical characterization of both Ni1−xCo1+xSe4 MPs and reference RuO2, comparison of synthesis methods/conditions and catalytic activity of various OER catalysts (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Dipak V. Shinde: 0000-0002-1197-8472 Mirko Prato: 0000-0002-2188-8059 Liberato Manna: 0000-0003-4386-7985 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank F. Drago (Materials Characterization Facility − IIT) for support in the ICP analysis and P. Rastogi for the SEM measurements. The research leading to these results has received funding from the seventh European Community Framework Programme under Grant Agreement No. 614897 (ERC Consolidator Grant “TRANSNANO”).

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REFERENCES

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DOI: 10.1021/acs.chemmater.7b02666 Chem. Mater. 2017, 29, 7032−7041

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DOI: 10.1021/acs.chemmater.7b02666 Chem. Mater. 2017, 29, 7032−7041