Electrocatalytic nanoparticles that mimic the three dimensional

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Electrocatalytic nanoparticles that mimic the three dimensional geometric architecture of enzymes: Nanozymes Tania M Benedetti, Corina Andronescu, Soshan Cheong, Patrick Wilde, Johanna Wordsworth, Martin Kientz, Richard D. Tilley, Wolfgang Schuhmann, and John Justin Gooding J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08664 • Publication Date (Web): 23 Sep 2018 Downloaded from http://pubs.acs.org on September 23, 2018

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Journal of the American Chemical Society

Electrocatalytic nanoparticles that mimic the three dimensional geometric architecture of enzymes: Nanozymes Tania M. Benedetti†, Corina Andronescu‡, Soshan Cheong§, Patrick Wilde‡, Johanna Wordsworth†, Martin Kientz†, Richard D. Tilley *†§, Wolfgang Schuhmann*‡, J. Justin Gooding*†¶ †

School of Chemistry and Australian Centre for NanoMedicine, University of New South Wales, Sydney, 2052, Australia ‡Analytical Chemistry - Center for Electrochemical Sciences (CES), Faculty of Chemistry and Biochemistry, Ruhr-Universität Bochum, Universitätsstr. 150; D-44780 Bochum, Germany § Electron Microscope Unit, Mark Wainwright Analytical Centre, University of New South Wales, Sydney, 2052, Australia ¶ Australian Research Council Centre of Excellence in Convergent Bio-Nano Science and Technology, University of New South Wales, Sydney, 2052, Australia KEYWORDS electrocatalysis, enzyme, nanozyme, oxygen reduction reaction, nanoparticles

ABSTRACT: Enzymes are characterized by an active site which is typically deeply embedded within the protein shell thus creating a nano-confined reaction volume in which high turnover rates occur. We propose nanoparticles with etched substrate channels as a simplified enzyme mimic, denominated nanozymes, for electrocatalysis. We demonstrate increased electrocatalytic activity for the oxygen reduction reaction using PtNi nanoparticles with isolated substrate channels. The PtNi nanoparticles comprise an oleylamine capping layer which blocks the external surface of the nanoparticles participating in the catalytic reaction. Oxygen reduction mainly occurs within the etched channels providing a nano-confined reaction volume different from the bulk electrolyte conditions. The oxygen reduction reaction activity normalized by the electrochemically active surface area is enhanced by a factor of 3.3 for the nanozymes compared to the unetched nanoparticles and a factor of 2.1 compared to mesoporous PtNi nanoparticles that possess interconnecting pores.

Introduction With the ever increasing use of nanoparticles in catalysis and the incredible advances in nanoparticle synthesis, catalytic nanoparticles are progressively becoming more analogous to enzymes.1 Not only are nanoparticles catalysing the same reactions as performed by enzymes in nature but the optimal size for nanoparticle catalysts is frequently similar to that of enzymes, around 3-5 nm. Furthermore, we now have methods to control the position of specific individual catalytic atoms within the nanocrystalline lattice to ensure the catalytic sites are available to the reactants.2 However, there are distinct structural features not used by electrocatalysts, that give enzymes their incredible performance in terms of turnover rate, selectivity and ability to do multistep cascade reactions.1, 3 These structural features include positioning the active site deep into a substrate channel to spatially separate the redox centre from the solution environment in which the substrate is found. Such a structural approach provides control over the electronic properties of the catalytic centre, the chemical environment where the reaction proceeds and the shuttling of products to new active sites for cascade reactions, a process called substrate channelling.3 We believe catalytic nanoparticles that mimic the three dimensional geometric architectures of enzymes is an unarticulated concept in the literature. Enzymes have long provided inspiration for the design of catalysts. This is no more prevalent than in organometallic chemistry where the molecular structures of active sites have been mimicked with great success4. More recently, nanoparti-

cles that perform similar reactions to enzymes have been suggested as enzyme mimics.5-6 Furthermore, mesoporous nanoparticles, prepared using metal-organic frameworks, with multiple catalytic centres have performed cascade reactions in analogy to some enzyme reactions.7

Scheme 1. The basic structures of the nanoparticles designed for this study. A Pt-Ni alloy nanoparticle is synthesized with oleylamine coating that passivates the nanoparticle. Etching of the nanoparticle leading to dealloying of the Ni forms substrate channels in the nanoparticles under formation of the nanozyme that has a recessed electrocatalytic center but

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an oleylamine-passivated external surface. The electrocatalytic performance of the nanozyme for the oxygen reduction reaction was compared to the nonetched nanoparticle with the surfactant removed, to the etched nanozyme with the surfactant removed and to an etched nanoparticle still coated in surfactant where higher Ni content results in interconnecting pores rather than individual substrate channels.

Mesoporous nanoparticles often show outstanding catalytic activity because of the high active surface area and possible electronic effects.8-9 An excellent example of this, relevant to the current paper is by Erlebacher and co-workers10 who showed that etched foils of a Pt3Ni alloy, to give a mesoporous film with recessed active sites, resulted in an enhanced specific activity compared with non-etched foils. It is important to note, that there is a distinction between a mesoporous layer of interconnected pores and isolated substrate channels in an enzyme. Mesoporous layers certainly can provide a different solution environment to the bulk but, if the pores are interconnected, the mesoporous layer is effectively similar to a partitioning membrane. In contrast, mesoporous materials composed of isolated pores exhibit higher fluxes of diffusing species to the layer, because each pore is a discrete diffusional systems, such that the material can act as a sponge to species in solution11. None of the studies mentioned above mimic the threedimensional geometric architecture of enzymes with a reactive site down an isolated substrate channel. The insight that such a strategy might give better performing electrocatalysts came from a recent paper 12 which presented a new paradigm for finding an optimal electrocatalyst. The paradigm involved counting the number of neighbouring atoms to the active site, to give a coordination-activity plot, rather than the more conventional Sabatier principle related to the strength of adsorption and desorption of key intermediates. One of the key findings of this paper was that electrocatalytic Pt nanoparticles with active sites in cavities 0.3 nm from the surface were up to 3.5 times more active for oxygen reduction reaction (ORR) than nanoparticles on which the active sites were not recessed. As such we were inspired to design electrocatalytic nanoparticles that mimic the three-dimensional architecture of enzymes. To mimic an enzyme requires enzyme sized nanoparticles (~5 nm in diameter), an electrocatalytic reaction centre down isolated substrate channels and with the exterior surface of the nanoparticle electrochemically passivated such that the electrocatalytic reaction cannot occur on the exterior surface of the nanoparticle. Because of the geometric analogy to enzymes, we call these electrocatalytic nanoparticles nanozymes. It is noted that the term nanozyme has previously been employed to describe nanoparticles that catalyse the same reactions as enzymes6, 13,14. However, considering most relevant reactions are catalysed enzymatically, we seek to redefine the definition of the word nanozymes to nanoparticles that mimic the geometric architecture of enzymes. Results To form a nanozyme will require 1) a bimetallic particle where the two metals partially phase separate into domains rich in each metal, 2) the nobility of the two metals is sufficiently different that one of them can be preferentially leached from the nanoparticle and 3) a surfactant that needs to remain on the exterior surface of the nanoparticle to passivate the nanoparticle but must be sufficiently leaky to allow corrosion on the less noble domains. The nanoparticle system we designed that fulfils these criteria is depicted in scheme 1. It is a

Pt-Ni bimetallic nanoparticle catalysing the oxygen reduction reaction (ORR). The different lattice spacing of Pt and Ni fulfil criteria 1.2 The Ni is far less noble than Pt and hence can be acid etched out of the nanoparticle to form the nanozyme, as in criteria 2, which is how nanoporous electrocatalysts can be prepared using Pt-Ni bimetalllic materials.15. Finally we expected the oleylamine surfactant used in the synthesis to remain bound to the exterior surface of the nanoparticle as in criteria 3. Additionally, because Pt-Ni nanoparticles exhibit the highest electrocatalytic performance for the ORR, and hence are the most studied, this nanoparticle system is a robust test of the nanozyme concept.16 The nanozymes were prepared by partially removing Ni domains from Pt-Ni nanoparticles (SI, Figure S1) via acid etching, where the etched sites formed the substrate channels of the nanozymes (experimental details and the synthesis of Pt-Ni nanoparticles are presented in the SI). The synthesized nanozymes, as shown in Figure 1a, are near-spherical in shape and have a mean diameter of 8.4 nm (s.d. = 1.4 nm, n = 311). Close examination of the nanozymes reveals channels within the nanoparticles as indicated by the concave outlines and small regions with much lighter contrast. Figure 1b shows a high-resolution TEM (HRTEM) image of a typical nanozyme, with a concave surface on the left and a light contrast spot (~1.2 nm across) on the right. Observations in HRTEM also show that the nanozymes are single crystals and retain the fcc crystal structure of the pre-etched Pt-Ni nanoparticles.

Figure 1. (a) TEM image of nanozymes. (b) HRTEM image of a nanozyme; inset: FFT of the image indicating a projection of an fcc crystal structure. (c) HAADF-STEM image of a nanozyme and EDX line scan profile of Pt and Ni across the particle along the direction indicated by the arrow in the STEM image, showing there is Pt and Ni at the bottom of the substrate channel.

The presence of substrate channels in the nanozymes is more apparent when observed under high-angle annular dark field (HAADF) conditions in scanning-TEM (STEM) imaging mode (Figure 1c and SI Figure S2). Each nanozyme typically has several channels that have a maximum width and depth of ~2 nm that have a darker contrast compared to the brighter core and the surrounding regions. The nanozyme as shown in


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Journal of the American Chemical Society the STEM image in Figure 1c has two channels exhibiting a dark contrast. An energy dispersive X-ray (EDX) line scan profile in Figure 1c of both Pt and Ni across the nanozyme shows a sharp drop of both signals corresponding to the dark contrast regions, where the channels are located. Importantly, both intensities do not decrease to zero, which indicates that the substrate channels do not penetrate through the nanozyme. This is in agreement with the HRTEM observation of lattice fringes across the channels, such as that shown in Figure 1b. EDX spectral analysis performed over multiple areas gives an elemental composition of 1.00:0.64 for Pt:Ni (SI Figure S3a), which is close to the 1.00:0.59 ratio obtained from ICP-OES analysis. The ratio for the Pt:Ni nanoparticles prior to etching was determined to be 1.00:1.33 (SI Figure S3b), indicating that half of the Ni atoms was removed during the etching process. Selective etching on bimetallic nanoparticles has previously led to nanoframes,2 nanocages17 and porous nanoparticles.15 Crucially, in this study, the formation of substrate channels via etching of Ni relies on the unique structure of the Pt-Ni nanoparticles that have multiple, small, segregated domains of Pt and Ni across the particles as shown in SI, Figures S2 and S4 The existence of such domains is consistent with the lattice mismatch between the two metals and we suggest such lattice mismatching is key to forming nanozymes in this way.2 Depending on the nanoparticle compositions and etching conditions, nanozymes with isolated substrate channels or mesoporous nanoparticles with interconnecting pores can be produced (Figure S2c). The mesoporous nanoparticles were synthesized by starting with PtNi2.5 nanoparticles followed by two etching cycles to give a composition of 1.00:0.56 for Pt:Ni from ICP-OES.

Figure 2. (a) 15th scan of cyclic voltammetry for catalyst activation at 0.2 V s-1 in HClO4 0.1 mol L-1 with currents normalized by the Pt mass based on ICP-OES analysis of the different samples. (b) Ratio between ECSA obtained by stripping of CuUPD and HUPD monolayers. (c) Scheme to show the access of H+ and Cu2+ in the nanozyme particle. Nanozyme (black), non-etched particles without surfactant (blue), etched particles without surfactant (green) and mesoporous particles with surfactant (red)

We next showed that there was surfactant remaining on the exterior surface of the nanozymes that electrochemically passivated the surface. Surfactant remaining on the surface of the nanozymes is vital to ensure that the electrocatalytic activity can be attributed solely to the catalytic sites inside the substrate channels as is the case of enzymes. To achieve this, the carbon supported nanoparticles for electrochemical measurements were first deposited on electrodes and then activated by cycling the potential between 0.03 V and 1.0 V in 0.1 M HClO4 until no changes in the potentiodynamic profile were observed (figure 2a). The nanozyme particles were compared with non-etched nanoparticles with the surfactant removed, etched nanoparticles with the surfactant removed and mesoporous nanoparticles with surfactant as controls as illustrated in scheme 1. The HUPD region of the voltammograms (figure 2a), between 0.3 V and 0.03 V, shows that the currents increased in the order of nanozyme < < non-etched without surfactant < etched without surfactant < mesoporous particles. The smallest



HUPD region, and hence ECSA, (figure 2a, Table S1 in SI) in the voltammograms for the nanozymes indicates the surfactant coating on the nanozymes was not removed in the etching process. The positive vertex potential however was important. More positive vertex potentials than the +1.0 V vs. RHE yielded larger charge at the HUPD regions (Figure 5S and Table S1 in SI). This suggests in turn, that more of the surfactantpassivating layer was beginning to be electrochemically removed, perhaps due to the formation of Pt oxide. To determine the electrochemical active surface area (ECSA) we used CuUPD 18 rather than HUPD because of concerns about the permeability of H+ through the surfactant layer on the nanozyme which would affect the determined HUPD of the particles leading to an overestimation of the ECSA. This is evident in ECSA-CuUPD/ECSA-HUPD ratios presented in figure 2b and Table S1 where unetched nanoparticles without surfactant and etched particles without surfactant CuUPD/ECSA-HUPD ratios were 1.4 and 1.25, respectively (reflecting an underestimation of the ECSA from the HUPD in agreement with a previous report19,20) whilst for the nanozyme particles the ratio is only 0.17. Taken together, these results point to the surfactant limiting access of Cu2+ to only the non-passivated region within the substrate channel as suggested in figure 2c. This notion is further supported by the fact that increasing the positive vertex potential during the activation to values where more surfactant is removed, the ECSA-CuUPD/ECSA-HUPD ratio increases (SI Figures S5c and table S1). The ECSA of the three particle types as determined from CuUPD is shown in Table S1. The nanozymes exhibit an ECSA of 2.3 m2 g-1 which is an order of magnitude lower than that of the etched particles without surfactant. This strong evidence for the surfactant remaining on the nanozymes is supported by thermal gravimetric analysis coupled with mass spectrometry (TGAMS) showing the presence of the oleylamine surfactant on the nanozyme particles before and after an electrochemical activation step (SI Figure S6). Taken together, electron microscopy, electrochemistry and surface characterisation data provide evidence for the function of a nanozyme as illustrated in Scheme 1 with a substrate channel where electrocatalysis can primarily occur at the otherwise passivated nanoparticle. The excellent electrocatalytic performance of the nanozymes for the ORR was demonstrated by comparing the nanozymes to unetched particles, etched particles both without surfactant and mesoporous particles containing surfactant (Figure 3a and b). Figure 3a shows the conventional presentation of voltammograms for the nanozymes, unetched particles, etched particles without surfactant and for the mesoporous particles with the same Pt mass loading. The reduction currents normalized to the geometric area of the glassy carbon surface are shown in Figure 3a. The same diffusion limited current in all cases is naturally expected when there is overlapping diffusion layers arising to the nanoparticles such that mass transport, and not the number of active sites, is limiting. As such the concurrence of the diffusion limiting currents shows that the coverage of the carbon loaded electrocatalytic nanoparticles is comparable for all particle types. Naively, the data in figure 3a would be interpreted as showing the nanozyme having inferior activity relative to the nanoparticles without surfactant because the control particles show a steeper increase in the magnitude of the reduction current at low overpotentials. However, such an interpretation ignores the fact that the nanozymes are mostly coated in a passivating surfactant layer with only the substrate channel accessible for ORR.

To observe the specific activity advantage of the nanozymes requires the voltammograms to be normalised to the ESCA of each particle type and focussing on the low overpotential kinetic region of the voltammograms where the process is limited by the charge transfer rate at the active sites as shown in Figure 3b. The specific activities of the nanozyme are two times higher than that of the mesoporous particles with surfactant and more than three times higher than that of the non-etched particles without surfactant, (see Figure 3b at 0.95 V this is 421 µA cm2 for the nanozyme, 199 µA cm-2 for the mesoporous particles with surfactant, 140 µA cm-2 for the etched particles without surfactant, and 125 µA cm-2 for the non-etched particles without surfactant). Each experiment was performed at least 3 times and the maximum standard deviation was 8 %, 9 % and 14 % for the particles without surfactant, nanozyme and porous particles respectively, for the activities obtained at 0.95 V. It is worth noting that the specific activity for the particles without surfactant are comparable to recently reported hollow PtNi nanoparticles at the same potential which are among the best nanoparticle electrocatalysts reported for the ORR.21 As such, the nanozymes with three times higher specific activity, show the incredible potential of mimicking the threedimensional geometry of enzymes in nanoparticles for electrocatalysis. The superior specific activity of the nanozymes is further emphasized by the turnover numbers for the nanozymes, so commonly used to describe enzymes, versus the other nanoparticles, as presented in Figure 3c. The nanozymes have turnover numbers 3.6 times higher than the nonetched nanoparticles and 3 times higher than the etched particles with the surfactant removed. Note with the porous particles that have interconnected pores rather than substrate channels, the improvement in the turnover number over unetched particles is only 1.6 times which suggests the confinement in the substrate channels plays a key role in the improved electrocatalytic performance of the nanozymes. The electrocatalytic results suggest that the kinetics of the reaction inside the channels are much faster than at the outside surface. If one considers the ORR, the two potential dependent steps are the first proton-electron transfer to O2 and the last proton-electron transfer in which adsorbed OH- is converted to H2O. Recessing the active site under formation of a concave shaped geometry is expected to alter the adsorption energies due to electronic effects of the underlying Ni and strain at the active site.12 Alternatively, the availability of oxygen and protons close to the active site may be altered by the confinement of the substrate channel. The fact that the enhanced electrocatalytic performance of the mesoporous nanoparticles with interconnecting nanopores over the unetched nanoparticles is much less significant than with the nanozymes does provide a strong hint here. This observation suggests that the impact of the nanoconfinement on the solution environment inside the substrate channel of the nanozyme plays a major role in the performance advantage of the nanozymes, as is the case of enzymes. With the interconnecting pores of the mesoporous nanoparticles, there is similarly concaved shaped geometries but much more mixing of solution with the bulk.


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Journal of the American Chemical Society

Figure 3. Background subtracted and iR corrected linear sweep voltammograms under rotation (1600 rpm) and in O2 saturated electrolyte. The scans were performed in 0.1 mol L-1 HClO4 as electrolyte in the cathodic direction at a scan rate of 50 mV s-1. Nanozyme (black), non-etched particles without surfactant (blue), etched particles without surfactant (green). Currents are normalized by (a) geometric area and by (b) ECSACuUPD

In summary, a new concept in electrocatalysis is proposed, namely the development of electrocatalytic nanoparticles that mimic the three dimensional geometric architecture of enzymes. In such a geometry, the catalytic centre is located down a narrow substrate channel so that the solution environment is substantially different to the bulk solution environment in which the substrate of the catalytic reaction is found. Importantly, in this concept the substrate channels are isolated channels that do not connect to other channels in the catalytic

material which makes the concept fundamentally different from a mesoporous layer of interconnecting pores. The difference is that isolated channels serve as discrete chemical systems which can enhance diffusion into them whereas connected mesopores essentially serve as a partition barrier. The electrocatalytic activity of these nanozyme electrocatalysts was impressive with a 3 times higher specific catalytic efficiency over the state-of-the-art Pt-Ni nanoparticles for the ORR if normalised by the ECSA. This initial study on the nanozyme concept shows that there is incredible potential in the idea of locating catalytic sites down isolated channels as enzymes do. Although this idea is demonstrated using ORR the scope of the idea is much broader presumably being applicable to a wide range of reactions in the field of electrocatalysis. There is considerable research to be done to understand the reasons behind the enhanced catalytic performance so that it can be fully exploited for technological applications. Once the improved catalytic performance is understood opportunities exist to design nanozymes for cascade reactions and to explore strategies to further mimic enzymes by controlling the chemistry inside the substrate channel to confer greater selectivity. Methods Nanozyme synthesis: The nanozyme particles were prepared by acid dealloying of carbon supported Pt-Ni nanoparticles stabilized with oleylamine. The PtNi2 nanoparticles were obtained with synthesis method adapted from literature22. Briefly, a 2:1 molar ratio of Ni(acac)2 and Pt(acac)2 were dissolved in oleylamine at 100oC, degassed and reacted for one hour at 300oC under argon. The resulting nanoparticles were then washed with ethanol to remove excess of surfactant. The nanoparticles were further supported on carbon Vulcan (XC 72R Cabot) by mixing them after dispersed in hexane with the carbon powder under sonication for 2 h. The hexane was then removed and the carbon supported nanoparticles were washed with ethanol and dried at room temperature. For etching, the supported nanoparticles were dispersed in HNO3 70% and kept under sonication for 1 min, followed by centrifugation, washing 3 times with water and 2 times with ethanol to remove excess of acid, Ni2+ and surfactant and drying at room temperature. Non-etched nanoparticles without the surfactant as well as etched nanoparticles without surfactant were used as control. The surfactant was removed in a tube furnace at 200oC for 5 h under air flow. The porous particles were obtained by etching PtNi2.5 particles twice, using similar procedures as described above. Inductively coupled plasma-optical emission spectrometry (ICP-OES): First, the carbon supported particles were digested in aqua-regia for 1 h at 80oC to dissolve the metals and then analysed using an Optima7300DV- ICP-OES Perkin Elmer instrument at the selected wavelengths of Ni 231.604 and Pt 265.945 to give the composition and weight percentage of the metals. Scanning/transmission electron microscopy (S/TEM) and energy-dispersive X-ray spectroscopy (EDX): TEM and STEM imaging, and EDX elemental analysis were performed on a JEOL JEM-F200 (200 kV, cold field emission gun) equipped with an annular dark-field (ADF) detector and a JEOL windowless 100 mm2 silicon drift X-ray detector. STEM images were acquired with a convergence semi-angle of 8.2 mrad, and an ADF inner collection angle of 20 mrad (for STEM-EDX mapping) or 62 mrad (for achieving highangle Z-contrast conditions). EDX data processing and analy-



sis were carried out using the Thermo Scientific Pathfinder Xray Microanalysis Software. The ‘drift-compensation’ mode was applied when performing elemental mapping to minimize the effect of specimen drift. EDX results were filtered to remove background counts and Bremsstrahlung radiation, and quantified using the standardless Cliff-Lorimer method. TEM specimens were prepared by placing a drop of nanoparticle dispersion in hexane or isopropanol onto carbon coated copper grids and allowed to dry under ambient conditions. Thermo gravimetric analysis coupled with mass spectroscopy (TGA-MS): Was performed using a Cahn TG-2131 thermobalance coupled with an Omnistar quadrupole mass spectrometer (Pfeifer Vacuum). TG curves were registered in He atmosphere from RT to 800 °C with 5 K/min heating rate. Different m/z fragments were registered in the 18 - 100 range. Electrochemical characterization: Working electrode preparation: 2.5 mg of the carbon supported nanoparticles were dispersed in 750 µL of H2O, 249 µL of isopropanol and 1 µL of Nafion 5% solution to give an ink. 10 µL of the ink was placed on a RDE glassy carbon disk (0.196 cm2) and dried at 120oC for 3 min to give a thin and uniform film. Electrochemical set-up: All experiments were performed using a µAutolab potentiostat controlled with Nova 2.1.2 software using an electrochemical cell with three-electrode assembly consisting of a Pt mesh and a Ag|AgCl|3 mol L-1 NaCl as the counter and reference electrodes, respectively. The reference electrode was separated from the main cell compartment using a fritted double-junction filled with the electrolyte to avoid chloride contamination. As the electrolyte 0.1 mol L-1 HClO4 (Suprapur Merck) was used and the cell was kept under N2 or O2 purging during the experiments. All potentials are referred against the reversible hydrogen electrode (RHE) and were converted by measuring the potential difference in the electrolyte between the reference electrode used for the measurements and a fresh RHE prepared prior to the experiments. Electrochemical activation: Catalysts activation was done by cyclic voltammetry under N2 in the potential range from 0.04 V to 1.0 V (etched particles) and 0.04 V to 1.4 V (non-etched particles) at a scan rate of 200 mV s-1 until no differences in the voltammograms were observed. Electrochemically active surface area (ECSA): Two different methods were used: a) Integration of the reduction current at the HUPD region (after excluding the capacitive current) of the last voltammogram cycle during activation to give the charge that was further converted into surface area (using 210 µC cm-2 as correlation). b) Electrodeposition of a Cu monolayer at 0.39 V for 3 min using 5 mM CuSO4 in 0.1. mol L-1 HClO4 as electrolyte followed by its oxidation by linear sweep voltammetry at 100 mV s-1 up to a potential of 0.9 V. The background subtracted oxidation currents were then integrated to give the charge that was further converted into surface area (using 420 µC cm-2 as correlation). Oxygen reduction reaction electrocatalysis: The measurements were done by linear sweep voltammetry in the anodic direction from 0.04V to 1.0V at 50mV s-1. The electrolyte was saturated with O2 and the electrode was rotate at 1600 rpm using a PAR RDE rotator (model 616). Background measurements were done at the same conditions under saturated N2 and the current was subtracted from the currents under O2. The potentials were also corrected for iR drop by measuring the solution resistance at OCP by Electrochemical Impedance Spectroscopy.

The Supporting Information is available free of charge on the ACS Publications website. HRTEM, HDAAF/STEM and EDX analysis of the nanoparticles, ECSA and activities of etched nanoparticles and nanozyme obtained from activation going to different positive potential limits, TGA analysis of nanozyme before and after activation (PDF)

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected] *[email protected]

ACKNOWLEDGMENT The authors acknowledge the UNSW Mark Wainwright Analytical Centre including facilities supported by AMMRF in the Electron Microscope Unit as well as funding from the ARC Centre of Excellence in Convergent Bio-Nano Science and Technology (JJG, CE140100036), the ARC Laureate Fellowship (JJG, FL150100060) program and a National Health and Medical Research Council development grant (JJG APP1075628). The Deutsche Forschungsgemeinschaft (DFG) is acknowledged for financial support in the framework of the cluster of excellence RESOLV (EXC1069) and the BMBF in the framework of the project NeMeZU (FKZ 03SF0497B). P.W. acknowledges the “Fonds der Chemischen Industrie” for a “Chemiefonds-Stipendium”

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