Mesoporous Metallic Iridium Nanosheets - ACS Publications

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Mesoporous Metallic Iridium Nanosheets Bo Jiang, Yanna Guo, Jeonghun Kim, Andrew E. Whitten, Kathleen Wood, Kenya Kani, Alan E. Rowan, Joel Henzie, and Yusuke Yamauchi J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b05206 • Publication Date (Web): 21 Aug 2018 Downloaded from http://pubs.acs.org on August 21, 2018

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Journal of the American Chemical Society

Mesoporous Metallic Iridium Nanosheets Bo Jiang,1 Yanna Guo,1 Jeonghun Kim,2,3 Andrew E. Whitten,4 Kathleen Wood,4 Kenya Kani,3 Alan E. Rowan,3 Joel Henzie*1 and Yusuke Yamauchi*3,5 1 2 3 4 5

International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, China School of Chemical Engineering and Australian Institute for Bioengineering and Nanotechnology (AIBN), The University of Queensland, Brisbane, QLD 4072, Australia Australian Nuclear Science and Technology Organisation (ANSTO), New Illawarra Rd, Lucas Heights, NSW 2234, Australia School of Chemical Department of Plant and Environmental New Resources, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do 446-701, South Korea

ABSTRACT: Two-dimensional (2D) metals are an emerging class of cutting-edge nanostructures that have attracted enormous research interest owing to their unusual electronic and thermal transport properties. However, the generation of mesopores in ultrathin 2D metals is a largely unexplored challenge due in part to the large bulk cohesive energies of crystalline metals. Yet numerous kinds of three-dimensional (3D) mesoporous metal architectures have been synthesized with hard- and soft-templating methods. Here, we report a novel synthetic strategy to prepare an unprecedented type of 2D mesoporous metallic iridium (Ir) nanosheet. Mesoporous Ir nanosheets can be synthesized with close-packed assemblies of diblock copolymer (polystyrene-b-poly(ethylene oxide), PS-b-PEO) micelles aligned in the lateral direction of the nanosheets. This novel synthetic route opens a new dimension of control in the synthesis of 2D metals, enabling new kinds of mesoporous architectures with abundant catalytically active sites. Owing to their unique structural features, the mesoporous metallic Ir nanosheets exhibit a high electrocatalytic activity toward the oxygen evolution reaction (OER) in acidic solutions compared with commercially available catalysts.

Introduction Two-dimensional (2D) materials have generated a lot of interest in the field of catalysis because they possess unusual electronic and thermal transport properties.[1-5] For example, free-standing graphene can serve as a catalytic support and sometimes even as a catalyst itself because graphene has a high intrinsic electron mobility (~1×106 cm2·V-1·s-1) and thermal conductivity (~4×103 W·m-1·K-1) combined with exceptional mechanical/chemical stability.[6] 2D metals are a largely unexplored class of nanomaterials, but are attractive targets for catalytic applications because they have superior conductivity, and exploit the interfacial electronic effects of exposed surface atoms in architectures that could reach the highest material utilization efficiencies. Numerous metals are predicted to form stable 2D structures,[7] and non-porous Rh and Pd nanosheets have already been successfully generated with benchtop synthetic techniques.[5,8] Free-standing Rh nanosheets have highly delocalized bonding that contributes to stability and exposes unsaturated Rh surface atoms that enable efficient hydrogenation and hydroformylation reactions.[8] In a separate but parallel discovery, intermetallic PtPb substrates induce anisotropic strain on conformal Pt layers. In this supported structure system, strain optimizes the oxygen adsorption energy of the Pt layers for the oxygen reduction reaction (ORR).[9] Both examples illustrate how the intrinsic properties of the 2D metals and extrinsic properties induced by substrate or topology can influence the catalytic properties of metal nanosheets. Researchers have already used sacrificial templates to generate dimpled metal

films with improved catalytic efficiencies. [19] Extending this principle further, we sought to use soft-micelle templates to create ultrathin 2D metals with pores in the plane of the nanosheet. Such structures could utilize the material utilization efficiency of ultrathin nanosheets and further increase the availability of chemically active sites on their surface. Mesoporous chemistry has been used to generate stable 3D metallic nanoarchitectures by chemically reducing metal precursors on hard- and soft-sacrificial templates.[10-13] Removal of the sacrificial template yields free-standing mesoporous metals with highly accessible surfaces and abundant active sites for heterogenous catalysis. 3D mesoporous frameworks are most accessible using these solution-based templating methods. Some 2D-shaped mesoporous nanosheets have been fabricated under strictly controlled conditions, but in most cases the obtained compositions are limited to silica, carbon and polymer.[14-17] Previous work on metal oxide nanosheets has demonstrated the benefits of porous structure on electrocatalytic performance.[18] It shows that nanosheets arranged into highly separated, permeable porous networks to expose its surfaces to the electrolyte and take part in the catalytic reaction. To the best of our knowledge, ultrathin metallic nanosheets with mesopores that completely perforate the plane of the nanosheets with regularly spaced holes has never been reported in the literature, probably due to the large cohesive energies of unsupported or unpassivated d-block metals[7] favor morphologies that minimize its surface free energy. To overcome the inherent 1

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properties of these metals, it is necessary to find a surfactant that can generate metal nanosheets and also accommodate poredirecting agents during the reduction step. Iridium (Ir) metal is an important electrocatalyst in the oxygen evolution reaction (OER) under acidic conditions because it can operate at low overpotentials and still maintain high chemical stability.[20-22] As a result, various Ir-based materials such as nanoframes,[23] core-shell structures,[24] and lamellar superstructures[25] have been examined in OER schemes to find an electrocatalyst structure that has high performance and utilizes the least amount of expensive material. In this paper, we describe a simple synthetic method to generate Ir nanosheets perforated with uniformly-sized mesopores. Polystyrene-b-poly(ethylene oxide) (PSb-PEO) micelles were used as sacrificial pore-directing agents to generate the mesopores, while formic acid serves as both a reducing agent and shape-directing agent (Figure 1a). In the reaction, formic acid plays such a central role in the formation of the 2D morphology because it degrades into carbon monoxide (CO). CO is known to bind strongly to the basal (111) facets of many catalytic metals[5] and enables the metal nanosheets to grow and form around the micelles. In addition, CO as a surface passivating agent is beneficial for catalytic applications because it can be removed at relatively low temperatures and pressures to generate

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relatively clean metal surfaces. This new mesoporous Ir architecture highlights the unique catalytic behavior of 2D metals by maximizing exposed surface atoms. Electrode structures functionalized with mesoporous Ir nanosheets exhibited very high OER performance with a low overpotential of 240 mV at 10 mA cm−2 (η10) vs. RHE and a low Tafel slope of 49 mV dec-1 in 0.5 M H2SO4 electrolyte. To the best of our knowledge, these Ir nanosheets are one of the highest performing OER electrocatalysts in acidic conditions that have been reported in the current scientific literature. Experimental section Chemicals and materials. IrCl3·6H2O, Pluronic®F127 and Nafion solution (5 wt%) were purchased from Sigma-Aldrich. Poly(ethylene oxide)-b-polystyrene (PEO(2200)-b-PS(5000)) and poly(ethylene oxide)-b-poly(methyl methacrylate) (PEO(10500)-bPMMA(18000)) was purchased from Polymer Source Inc. The molecular weight for each block is shown in subscripted parentheses. Formic acid, N,N-dimethylformamide (DMF), acetone, ethanol and H2SO4 solution (0.5 M), ascorbic acid were purchased from Nacalai Tesque, Inc. Commercial Ir black (Ir/B) and IrO2 was purchased from Alfa Aesar and Sigma-Aldrich, respectively. These materials were used as the reference catalysts for the electro-

Figure 1. Scheme showing the formation mechanism, with SEM and TEM images showing mesoporous Ir nanosheets synthesized by a soft-templating method. (a) Formation mechanism of the mesoporous Ir nanosheets. (b and c) Low- and high-magnification SEM images showing the macroporous and mesoporous structure of the nanosheets. (d) TEM image and (e) HAADF-STEM image showing the structures collected from the samples which are planar nanosheets with near-perfect hexagonally arranged mesopores. The inset image in (e) is an SAED pattern and matches the electron diffraction pattern for polycrystalline Ir. (f) An HR-TEM image shows the walls of the mesopores are crystalline Ir and the lattice spacing of d = 0.22 nm matches the (111) plane. (g, h, i and j) Mesoporous Ir nanosheets were synthesized using the standard procedure and aliquots were collected during the reaction at 30 min (g), 33 min (h), 36 min (i), and 39 min (j) and measured in TEM. Revised Ms_2


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Journal of the American Chemical Society chemical experiments. All the chemicals were used without further purification. Preparation of mesoporous Ir nanosheets. The synthesis of mesoporous nanosheets is based on a wet chemical reduction process in solution phase (Fig. 1a). In a typical synthesis, 5 mg of poly(ethylene oxide)-b-polystyrene (PEO-b-PS) was completely dissolved in 1 mL DMF. Then, 1 mL of deionized water, 1 mL of aqueous 40 m M IrCl3 solution, and 1 mL formic acid were added to the DMF solution in sequence. Finally, the reaction solution was kept in a water bath for 5 hours at 80 oC to complete the reaction. Reduction of the Ir precursor into Ir caused the color to change from light-brown to black. Samples were collected by centrifugation at 14,000 rpm for 20 minutes and the residual PEOb-PS was removed by 5 consecutive washing/centrifugation cycles with acetone and water and then re-dispersed in ethanol. The mesoporous Ir nanosheets were completely converted to the Ir fcc metal by annealing them at 150 °C for 15 minutes in an N2 atmosphere. Characterization. A field-emission scanning electron microscope (FESEM, HITACHI SU-8000) was used to initially characterize the morphology of mesoporous Ir nanosheets operating at an accelerating voltage of 5 kV. High-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100F) images were collected at an operation voltage of 200 kV. The samples for TEM and HRTEM measurements were prepared by depositing a drop of a diluted colloidal suspension on a TEM grid. Powder X-ray diffraction (XRD) measurements were collected with a Smart lab Xray diffractometer (RIGAKU) at a scanning rate of 1° min-1 using a Cu Kα radiation (40 kV, 30 mA) source. Small angle X-ray scattering (SAXS) measurements (Rigaku NANO-Viewer) were used to evaluate the pore-to-pore distance. The SAXS instrument used a Cu Kα radiation (40 kV, 30 mA) source with a camera length at 700 mm. X-ray photoelectron spectroscopy (XPS) was performed on a JPS-9010TR (JEOL) instrument with an Mg Kα X-ray source. All the binding energies were calibrated using the C 1s binding energy peak (284.5 eV) as a reference. Samples were corrected for background measurements and placed on an absolute scale using standard procedures. Inductively coupled plasma optical emission spectrometer (ICP-OES) was performed on a Hitachi model SPS3520UV-DD. Nitrogen adsorption-desorption isotherms were acquired by using a BELSORP-mini (BEL, Japan) at 77 K and surface area was estimated using the Multipoint Brunauer-Emmett-Teller (BET) method. Thermal gravity (TG) analysis was obtained with a Hitachi HT-Seiko Instrument Exter 6300 TG/DTA. Fourier transform infrared spectroscopic (FTIR) measurements were recorded on a thermoscientific Nicolet 4700 using KBr pellets. Small angle neutron scattering (SANS) measurements were performed at the Australian Nuclear Science and Technology Organisation on the BILBY instrument.[26] For SANS experiments, deuterated solvents were used where possible to enhance the scattering contrast. The instrument was used in time of flight mode with 0.3 < λ < 2 nm and ∆λ = 12-16% where λ is the neutron wavelength. The detectors were positioned at 8 m (rear), 2 m (horizontal curtains) and 1 m (vertical curtains) from the sample position. Round Hellma cells with a 2 mm pathlength were used in the experiments. Data was reduced and put on an absolute scale relative to the direct beam using Mantid[27] and then the influence of the solvent was subtracted from the data.

Electrochemical measurement. 3 mg of the as-prepared nanosheets (before thermal treatment) was dispersed in 5 mL ethanol/acetone mixture by ultrasonic treatment for 2 hours. Then 4 mg of an ethanol suspension containing Vulcan XC-72 carbon (1 mg mL-1) (Premetek Co.) was mixed with the above nanosheet solution and then this mixture was vortex at 900 rpm for 15 hours (ThermoMixer C). The carbon supported Ir samples were collected by centrifugation (12,000 rpm for 20 minutes) and then washed several times with ethanol to remove impurities and calcinated at 150°C under N2 atmosphere for 15 minutes. The actual amount of Ir in both carbon-supported mesoporous Ir nanosheets and nonporous Ir sample was ~32 wt% according to ICP-OES. The various carbon supported samples (i.e. mesoporous Ir nanosheets and non-porous Ir sample) and commercial Ir/B and IrO2 catalysts were dispersed in a mixture containing water, acetone, ethanol and Nafion (5 %) and then sonicated for 1 hour to form a homogeneous catalyst ink. Catalyst-supported electrodes were prepared with a glassy carbon electrode (GCE; diameter = 3 mm). The GCE was carefully polished and washed, then the catalyst suspension (6 µL) was deposited on its surface. After the catalyst ink dried at room temperature, the GCE was used as the working electrode. The loading amount of Ir on the glassy carbon (GC) electrode was ∼9.6 µg for the Ir-based electrocatalysts. Electrochemical investigations were collected using an electrochemical workstation (BioLogic Science Instruments; VMP3) that measured cyclic voltammograms and steady-state linearsweep voltammetry (LSV) measurements. The samples measured were mesoporous Ir nanosheets, non-porous Ir bulk, commercially-available Ir/B and IrO2. The three-electrode cell consists of a reference electrode (saturated calomel electrode, SCE), a counter electrode (carbon rod) and a working electrode (GCE). The modified GCE was coated with the samples (6.0 µL) and dried at room temperature following the method above. Prior to CO stripping measurements, the GCE was electrochemically activated by cycling the potential between -0.2 V and +1.0 V (vs. SCE) in 0.5 M H2SO4 until the CVs were stable. Then the 0.5 M H2SO4 electrolyte solution was purged with CO gas for 20 minutes, followed with a purge using N2 gas for 10 minutes to remove any excess CO gas from solution. The ECSAs were estimated using COad stripping coulometry assuming that the electrooxidation of a COad monolayer requires 420 µC per cm2. OER measurements were performed in a solution of 0.5 M H2SO4 solution at a scan rate of 5 mV·s-1. All measured potentials are relative to a reversible hydrogen electrode (RHE) following the equation: ERHE = ESCE + (0.241 + 0.059 × pH). The ohmic potential drop (iR) losses that arise from the solution resistance were all corrected. Tafel plots of the overpotential vs. the log(current density) were recorded with linear portions at low overpotentials fitted to the Tafel equation: η = a + blog j, where η is the overpotential, b is the Tafel slope, j is the current density, and a is the exchange current density. Stability tests were performed via continuous cyclic voltammetry (CV) measurements in the potential range from 1.0 V to 1.6 V vs. RHE in 0.5 M H2SO4 with a scan rate of 100 mV·s-1 for 2000 cycles. The long-term durability of the catalysts was examined with chronopotentiometry in 0.5 M H2SO4 at a constant current density of 10 mA cm-2 over 8 h.

Revised Ms_3



Results and Discussion The shape and porous structure of the Ir nanostructures was initially characterized with scanning electron microscopy (SEM) (Fig. 1b-c). According to low-magnification SEM image, the material appears to be a network of thin structures that display the mesoporous morphology, and each structure has a lateral size of a few hundred nanometers (Fig. 1b). Surprisingly, the nanosheets are riddled with tiny mesopores ~15 nm in diameter, indicating that micelles are incorporated within the 2D nanosheet structure during growth (Fig. 1c). Looking more closely, it became clear that the structures were actually composed of very thin nanosheets (Fig. 1c). Some of the sample suspension solution was transferred to a solid substrate for AFM observation. The AFM height profile confirms that the thickness of the nanosheet is ca. 10 nm (Fig. S1). Transmission electron microscopy (TEM) images show that the mesopores (ca. 15 nm diameter) are uniformly distributed throughout the nanosheets (Fig. 1d-e). The selected-area electron diffraction (SAED) patterns of sample have concentric diffraction rings (Inset, Fig. 1e) that match the spacings of a polycrystalline face-centered cubic (fcc) Ir crystal. According to high resolution TEM (HRTEM) (Fig. S2a) the nanosheets have pore walls that are composed of numerous tiny Ir crystals. HRTEM image (Fig. 1f) shows that the pore wall has 0.22 nm lattice fringes that correspond to the (111) plane of a fcc Ir crystal. The mesoporous structure of the nanosheets and its uniformity throughout the entire sample was further characterized by lowangle X-ray diffraction (XRD) and nitrogen (N2) adsorptiondesorption isotherm measurements. A single diffraction peak located at 2θ = 0.38o (d = 23.3 nm) can be observed in XRD pattern, demonstrating that the mesoporous structure of the material is periodic (Fig. 2a). In an ideal hexagonal packing, the pore-topore distance of the mesopores should be ~26.9 nm (23.3 nm × 2/√3), which matches the dimensions observed in TEM (Fig. S2bc). The mesostructural domain size of the Ir nanosheets is small, so it is very challenging to detect higher ordered diffraction peaks from the mesoporous structure in XRD pattern.[14] The N2 adsorption-desorption isotherms can be categorized as type IV with a hysteresis loop (Fig. 2b). The Brunauer-Emmett-Teller (BET) surface area was ca. 42 m2 g-1, which is much higher than commercial Ir/B catalyst (20 m2 g-1) (Fig. S3). The pore size distribution derived from the BJH method shows the presence of mesopores with an average diameter of 15 nm (Inset of Figs. 2b and S3). The specific surface area value of mesoporous Ir nanosheets is in the reasonable range for mesoporous materials with large mesopores (> 10 nm), as shown in Table S1, considering the high density of Ir (22.56 g cm-3). The wide-angle XRD pattern proves that the nanosheets match the metallic Ir fcc crystal and that no other impurities or iridium oxide diffraction peaks could be detected (Fig. 2c). X-ray photoelectron spectroscopy (XPS) measurements collected on the Ir nanosheet sample and commercial Ir/B have peaks with binding energies of 63.9 eV and 60.9 eV matching metallic Ir 4f5/2 and 4f7/2, respectively, which are different from the IrO2 standard sample (64.6 eVand 61.8 eV) (Figs. 2d and S4). As illustrated in Fig. 1a, block polymer PEO-b-PS completely dissolves as a unimer in DMF because hydrophilic PEO and hydrophobic PS segments are soluble in polar aprotic solvents. Adding water to the mixture causes the hydrophobic PS segments to

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become less solvated, which drives the assembly of PEO-b-PS micelles composed of PS cores surrounded by PEO shells (Fig. S5). The micelles are required to form the 2D mesoporous Ir structures. In order to directly visualize micelle structure, polymeric micelles were imaged in TEM using phosphotungstic (PW) acid in a common staining technique. The TEM images reveal the formation of narrowly dispersed spherical micelles composed of PS cores with an average diameter of ~15 nm, although some of aggregated micelles (likely cylindrical micelles) were also observed during imaging (Fig. 3a and Fig. S6a-b). After addition of Ir metal precursor and formic acid, the size of the micelle did not change significantly (Fig. 3b and Fig. S6c-d).

Figure 2. Structural characterizations of mesoporous Ir nanosheets. (a) The low-angle XRD pattern of the Ir nanosheets has a peak at 2θ = 0.38° which is equivalent to a pore-to-pore spacing of 23.3-nm. (b) The N2 adsorption-desorption isotherm is type IV with a BET surface area of ca. 42 m2 g-1. The inset of (b) shows the BJH estimated average diameter is 15-nm. (c) A wideangle XRD pattern of the mesoporous Ir nanosheet sample. (d) XPS spectrum of the Ir 4f peaks. The peaks match the binding energies of Ir(0) metal reference (see Figure S4) and the Ir 4f peaks are moderately asymmetric in magnitude. This result shows that the sample surface is primarily Ir metal but there is some oxidized Ir on the surface of the sample. To further investigate the micelle structure, small angle neutron scattering (SANS) was performed on solution samples at room temperature (Fig. 3c and Table S2). In all the SANS measurements presented here, the signal is due to the templating polymer. Based on the TEM result describing the dimensions of the micelles above, all SANS data was modeled as a sum of spheres and cylinders with the NIST Igor Pro data analysis macros.[28] To minimize fitting parameters, the diameter of the spheres was constrained to be within 1% of the diameter of the cylinders. Fig. 3c shows the SANS data for samples containing: (i) PEO-b-PS + DMF + D2O + IrCl3 + formic acid, (ii) PEO-b-PS + DMF + D2O + IrCl3, and (iii) PEO-b-PS + DMF + D2O + formic acid. The data is well described by the model, and the extracted diameter of the micelles can be determined, however it is not possible to discern Revised Ms_4


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Journal of the American Chemical Society the length of the cylinders. Solution (i) has micelles with a diameter of ~15.4 nm at room temperature, which matches our observations in TEM. The micellar diameters extracted from other two solutions are slightly smaller (ii = 13.6 nm; iii = 13.8 nm), indicating that a decrease in the polarity of the solvent mixture can cause the micellar diameter to shrink (Fig. 3c and Table S2).

Figure 3. TEM observation and in-situ characterization of the synthetic method using SANS of the polymer micelles. TEM images of the polymeric micelles (a) before and (b) after the addition of formic acid and IrCl3. (c) Small-angle neutron scattering (SANS) patterns of polymeric micelle solutions at room temperature with varying reagent solutions: (i) contains PEO-b-PS + DMF + D2O + IrCl3 + formic acid, (ii) contains PEO-b-PS + DMF + D2O + IrCl3, and (iii) contains PEO-b-PS + DMF + D2O + formic acid. (d) Reagent solution (i) (i.e. PEO-b-PS + DMF + D2O + IrCl3 + formic acid) was heated to 80°C its SANS patterns were collected over time. To study the influence of the micellar structure during the formation of the nanosheets, solution (i) was heated to 80°C and in-situ SANS setup and the data was collected as a function of time. The same fitting procedure was used to extract a radius of the PEO-b-PS (Fig. 3d). During the first 10-minutes of the experiment, the diameter of the micelles slightly increases from 15.4 nm to 18 nm, and then the value remains almost constant for the duration of the experiment (Fig. S7). The SANS data proves that the micellar structure is not destroyed during the heating process because the SANS curves do not change significantly over time (Fig. 3d). As control experiments, the Ir deposition was carried out without the PEO-b-PS template (Fig. S8a). The resulting sample had a sheet-like morphology without any mesopores. Based on this systematic study of the micelle structure with SANS, we conclude that the self-assembled micelles serve as pore-directing agents and as a steric barrier to help prevent the aggregation of Ir nanosheets during and after synthesis (Fig. S9). Based on these structural measurements, we propose the following mechanism. During the reaction (Fig. 1a), tiny Ir nanocrystals are reduced from the precursor solution and stabilized by the micelle EO chains in the presence of the formic acid reducing agent at 80 oC. The Ir precursor is reduced and the formic acid decomposes into carbon dioxide (i.e., 2IrCl3 + 3HCOOH → 2Ir + 6HCl + 3CO2). However, it is known that formic acid selfdecomposes into carbon monoxide (CO) in the presence of noble

metals (HCOOH → CO + H2O).[29,30] Previous experiments have shown that CO binds to the (111) surface of Ir metal and likely affects the growth kinetics in colloidal reactions.[31] Chemisorption of CO on metals has been previously used to synthesize 2D metal nanomaterials because CO suppresses the growth of the metal orthogonal to the crystallographic facet that CO binds most strongly.[5,32-34] To confirm the proof-of-concept, we collected samples during the reaction and observed in TEM how the Ir begins as tiny crystals that eventually fuse together and grow anisotropically to form nanosheets with hexagonal pores (Fig. 1g-j). Gas chromatography measurements of the post-synthesis solution confirm that CO is released during the reaction (Fig. S10). To prove the presence of CO on the surface of the Ir metal, asprepared mesoporous Ir nanosheets (Fig. S11) before thermal treatment were measured with Fourier transform infrared spectroscopy (FTIR) (Fig. S12a). The FTIR spectra of the nanosheets have an absorption band at ~2060 cm-1 that is typically assigned to the stretching mode of CO adsorbed on metal atoms (Fig. S12a).[35-38] As-prepared non-porous Ir sample generated without PEO-b-PS micelles also have the same absorption band in FTIR. Another absorption band between 1300 and 1400 cm-1 observed in the FTIR spectra matches the symmetric stretching modes of the carboxylic group ν(COO) (Fig. S12b),[39] indicating that some formic acid molecules also adsorb to the surface of Ir metals during the reaction.[34] However, the thermal treatment at 150 °C under nitrogen causes these FTIR absorptions to disappear, indicating the loss of surface adsorbed CO, formate and its derivatives. Thermogravimetric analysis (TGA) shows that the material loses ~25% of its weight when the temperature increases to 350 °C, which can be attributed to the removal of CO, formate and its derivatives (Fig. S13). Going further, we compared the C 1s and O 1s XPS peaks of the as-prepared mesoporous Ir nanosheets (before thermal treatment) with commercial Ir/B and IrO2 catalysts. The Ir nanosheets have new peaks at 286.3 and 287.5 eV in the C 1s spectrum, and a significant shift in the O 1s peak compared to the standard commercial catalysts, indicating that the species of carbon atoms bonded to oxygen atoms adsorb on the surface of metals (Fig. S14).[40,41] Considering all the data together, we concluded that formic acid enables the formation of 2D nanosheets by serving as a reducing agent and as a source of CO (Additional experiment using various reducing agents is shown in Fig. S15.). The CO drives the anisotropic growth of the Ir/micelle superstructure as tiny Ir nanoparticles nucleate, condense and grow around the PEO-b-PS micelles. The resulting 2D Ir nanosheets are polycrystalline and riddled with 15 nm diameter mesopores. The cohesive energy of perfectly crystalline 2D Ir nanosheets expected to be larger than most other d-block metals.[7] Surface adsorbed CO may serve to passivate higher energy atomic steps, Ir surface atoms and grain boundaries of the polycrystalline Ir nanosheets. The polycrystallinity of the nanosheets and the topology created by the mesopores may also play some role in the stability of the structure or prevent the growth of 3D Ir structure. Unlike polymeric surface directing agents like PVP polymer, CO is easy to remove by heating at relatively low temperatures. This is beneficial for catalysis because removal of CO results in pristine metallic Ir surfaces that retain the original mesoporous nanosheet morphology of the synthesized material. Our concept based on micelles can be applied to Revised Ms_5



other surfactants and block copolymer systems (Figs. S8b-d). F127 has a smaller molecular weight, causing the mesopores to decrease in size to ~8 nm (Figs. S8c-d). We expect that the mesoporous Ir nanosheets would perform well in OER catalysis, because mesoporous architectures expose high surface areas and abundant active sites. Moreover, the 2D nanosheet topology of the Ir metal may also present unique electronic surface states that are intrinsic to 2D systems or strained 2D systems and are not found on bulk Ir metal. Although various Earth-abundant 3d transition-metal-based (e.g., Co, Ni, Fe, Mn) compounds have been recently tested as catalysts in electrochemical water splitting reactions, they are still susceptible to corrosion and require high overpotentials in acidic conditions.[21,42,43] To examine their OER performance, as-prepared mesoporous Ir nanosheets were loaded onto carbon black and annealed in a N2 atmosphere at 150 °C for 15 minutes to generate a pristine metallic fcc Ir phase (Fig. S16a). SEM images confirm that the overall morphology of the samples is unaffected by thermal treatment (Fig. S16b). Non-porous metallic Ir sample was also loaded onto carbon black using the same method (Fig. S16c). The electrochemically active surface area (ECSA) of the mesoporous Ir nanosheets (Fig. S16b), non-porous Ir sample (Fig. S16c) and commercial Ir/B (Fig. S16d) was determined to be 88 m2 g-1, 34 m2 g-1, and 22 m2 g-1, respectively (Fig. S17). The larger ECSA of mesoporous Ir nanosheets confirms that our method generates highly accessible surfaces compared to that of non-porous Ir bulk and commercial Ir/B. Interestingly, the onset and the peak potential of the adsorbed CO oxidation of mesoporous Ir nanosheets is slightly lower than the non-porous Ir sample and commercial Ir/B (Fig. S17), indicating that mesoporous Ir nanosheets may have a better resistance to CO poisoning.[13,44]

Figure 4. Electrochemical characterization of mesoporous Ir nanosheets versus non-porous Ir bulk, commercial Ir black and IrO2 catalysts as OER catalysts. (a) Polarization curves, (b) bar graph showing the overpotential (η10) to drive 10 mA cm-2 and Ir mass activity (jm) at 1.5 V (vs. RHE) and (c) Tafel plots. All catalysts were examined using 0.5 M H2SO4 as the electrolyte at the scan rate of 5 mV s−1. (d) Chronopotentiometry curves of the catalysts at a constant current density of 10 mA cm-2.

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Linear-sweep voltammetry (LSV) curves were measured using 0.5 M H2SO4 as the electrolyte to compare the electrocatalytic OER performance of mesoporous Ir nanosheets versus various Ircontaining catalysts. The polarization curves of the materials were recorded at a slow scan rate of 5 mV s-1 (Fig. 4a). The comparison of overpotentials to drive 10 mA cm-2 (η10) in the OER polarization curves (Fig. 4a) and Ir-mass activities at 1.5 V (vs RHE) reveal that mesoporous Ir nanosheets possess the highest activity compared with other Ir-containing catalysts (Fig. 4b and Table S3). Additionally, the mesoporous Ir nanosheets have a sharp onset potential at 1.41 (V vs. RHE) and a low Tafel slope (49 mV·dec-1) (Fig. 4c). The mesoporous Ir nanosheets have a lower Tafel slope than other materials we tested, indicating it has better kinetics for water oxidation.[45] This enhanced catalytic activity is attributed to the formation of an abundant and well-distributed mesoporous structure combined with the 2D morphology.[18,46] In addition, the overpotential was 240 mV at a current density of 10 mA cm-2, which is small compared to other works in the literature (Table S4). The durability of the catalyst was examined with continuous CV measurements at a potential range from between 1.0 and 1.6 V (vs. RHE) using a scan rate of 100 mV·s-1 for 2,000 cycles. The polarization curves show only a slight decay in activity over the initial 2,000 cycles (Fig. S18). The long-term durability of the catalysts was also examined via chronopotentiometry in 0.5 M H2SO4 at a constant current density of 10 mA cm-2 over 8 h (Fig. 4d). Our mesoporous Ir nanosheets show good stability for OER with a potential nearly unchanged even after 8 h, compared with commercial available Ir/B and IrO2. In addition, the electrocatalytic OER performance of the as-prepared mesoporous Ir nanosheets prior to thermal treatment were also examined in Fig. S19. These untreated nanosheets possess a low activity and stability compared to those that have been thermally treated to remove the adsorbed molecules. Finally, the 2D mesoporous Ir nanosheets were collected after the OER durability test and carefully examined with SEM, TEM, and XPS to assess the impact of OER on the morphology of the nanostructured material. The SEM and TEM (Fig. S20a-b) images show no obvious change in the morphology and mesoporous structure of the material after the durability tests. However, XPS analysis shows that the surface of mesoporous Ir nanosheets is partially oxidized after the 8 hour OER reaction (Fig. S20c), due to the oxidation environment in the acidic solution, which is consistent with the previously reported results.[23,25,47] Conclusion We have described the first method to synthesize 2D mesoporous metal nanosheets. The method leverages the tools of mesoporous chemistry to assemble Ir metal around a close-packed array of soft polymeric micelle templates. Formic acid serves as both a reducing agent and shape-directing agent by generating CO molecules that drive the 2D growth of the Ir metal around the polymeric micelles. In-situ SANS measurements show that the structure of the original polymeric micelles does not change during the reaction, proving that the micelles function as a stable and robust pore-directing agent during the metal reduction reaction. The mesoporous metallic Ir nanosheets were stable to thermal treatment and retain both the mesoporous structure and original highRevised Ms_6


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Journal of the American Chemical Society surface area nanosheet topology. As a result, the mesoporous Ir nanosheets exhibited superior electrocatalytic performance in OER in acid solutions compared to other Ir-based bulk and nanomaterial catalysts. We believe that this method may serve as an efficient and scalable method to further explore ultrathin 2D metals with mesoporous topologies and examine how the pores influence the electronic, structural and catalytic properties of the metal, which may be tuned by the dimensions and chemistry of the polymeric micelles. ASSOCIATED CONTENT Experimental details and additional data are available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *[email protected]; [email protected] Funding Sources This work was supported by the Australian Research Council (ARC) Future Fellow (FT150100479), JSPS KAKENHI (17H05393 and 17K19044), and the research fund by the Suzuken Memorial Foundation. The authors would like to thank New Innovative Technology (NIT) for helpful suggestions and discussions. Notes The authors declare no competing financial interest. REFERENCES [1] Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183-191. [2] Zhang, X.; Xie, Y. Chem. Soc. Rev. 2013, 42, 8187-8199. [3] Nicolosi, V.; Chhowalla, M.; Kanatzidis, M. G.; Strano, M. S.; Coleman, J. N. Science 2013, 340, 1226419. [4] Chhowalla, M.; Shin, H. S.; Eda, G.; Li, L-J.; Loh, K. P.; Zhang, H. Nat. Chem. 2013, 5, 263-275. [5] Huang, X.; Tang, S.; Mu, X.; Dai, Y.; Chen, G.; Zhou, Z.; Ruan, F.; Yang, Z.; Zheng, N. Nat. Nanotechnol. 2011, 6, 28-32. [6] Hu, M.; Yao, Z.; Wang, X. Ind. Eng. Chem. Res. 2017, 56, 3477-3502. [7] Nevalaita, J.; Koskinen, P. Phys. Rev. B 2018, 97, 035411. [8] Duan, H.; Yan, N.; Yu, R.; Chang, C.-R.; Zhou, G.; Hu, H.-S.; Rong, H.; Niu, Z.; Mao, J.; Asakura, H.; Tanaka, T.; Dyson, P. J.; Li, J.; Li, Y. Nat. Commun. 2014, 5, 3093. [9] Bu, L.; Zhang, N.; Guo, S.; Zhang, X.; Li, J.; Yao, J.; Wu, T.; Lu, G.; Ma, J.-Y.; Su, D.; Huang, X. Science 2016, 354, 14101414. [10] Attard, G. S.; Goltner, C. G.; Corker, J. M.; Henke, S.; Templer, R. H. Angew. Chem. In. Ed. Engl. 1997, 36, 1315-1317. [11] Wang, H.; Jeong, H. Y.; Imura, M.; Wang, L.; Radhakrishnan, L.; Fujita, N.; Castle, T.; Terasaki, O.; Yamauchi, Y. J. Am. Chem. Soc. 2011, 133, 14526-14529. [12] Jiang, B.; Li, C.; Tang, J.; Takei, T.; Kim, J. H.; Ide, Y.; Henzie, J.; Tominaka, S.; Yamauchi, Y. Angew. Chem. Int. Ed. 2016, 55, 10037-10041. [13] Jiang, B.; Li, C.; Dag, Ö.; Abe, H.; Takei, T.; Imai, T.; Hossain, M. S. A.; Islam, M. T.; Wood, K.; Henzie, J.; Yamauchi, Y. Nat Commun. 2017, 8, 15581. [14] Fang, Y.; Lv, Y.; Che, R.; Wu, H.; Zhang, X.; Gu, D.; Zheng, G.; Zhao, D. J. Am. Chem. Soc. 2013, 135, 1524-1530. [15] Fang, Y.; Lv, Y.; Gong, F.; Elzatahry, A. A.; Zheng, G.; Zhao, D. Adv. Mater. 2016, 28, 9385-9390. [16] Liu, S.; Zhang, J.; Dong, R.; Gordiichuk, P.; Zhang, T.; Zhuang, X.; Mai, Y.; Liu, F.; Herrmann, A.; Feng, X. Angew. Chem. Int. Ed. 2016, 55, 12516-12521.

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Mesoporous Metallic Iridium Nanosheets - ACS Publications

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