Edge-Enhanced Oxygen Evolution Reactivity at Ultrathin, Au

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Article

Edge-Enhanced Oxygen Evolution Reactivity at Ultrathin, Au-Supported FeO Electrocatalysts 2

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Douglas R. Kauffman, Xingyi Deng, Dan C. Sorescu, Thuy-Duong Nguyen-Phan, Congjun Wang, Chris M. Marin, Eli Stavitski, Iradwikanari Waluyo, and Adrian Hunt ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b01093 • Publication Date (Web): 02 May 2019 Downloaded from http://pubs.acs.org on May 3, 2019

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Edge-Enhanced Oxygen Evolution Reactivity at Ultrathin, Au-Supported Fe2O3 Electrocatalysts Douglas R. Kauffman,*,† Xingyi Deng,*,†,‡ Dan C. Sorescu,*,† Thuy-Duong NguyenPhan,†,‡ Congjun Wang,†, ‡ Chris M. Marin,†,‡ Eli Stavitski,§ Iradwikanari Waluyo,§ and Adrian Hunt§.

†

National Energy Technology Laboratory, United States Department of Energy,

Pittsburgh, Pennsylvania, 15236, USA.

‡

Leidos Research Support Team, 626 Cochrans Mill Road, P.O. Box 10940, Pittsburgh,

PA 15236-0940, USA.

§

Photon Sciences Division, National Synchrotron Light Source II, Brookhaven National

Laboratory, Upton, New York 11973, USA

ACS Paragon Plus Environment

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ABSTRACT. Transition metal oxides have gained attention as promising oxygen

evolution reaction (OER) electrocatalysts in alkaline electrolytes, but heterogeneities in

typical catalyst samples often obscure key structure-property relationships that are

essential for developing higher-performance materials. Here, we have combined

ultrahigh vacuum surface science techniques, electrochemical measurements, and

density functional theory (DFT) to quantify structure-dependent OER activity in a series

of well-defined electrocatalysts. We describe a direct correlation between the population

of Fe edge-site atoms and the OER activity of ultrathin Fe2O3 nanostructures (~0.5 nm apparent height) grown on Au(111) substrates. Hydroxylated Fe atoms residing at edge-

sites along the catalyst/support interface were spectroscopically identified as key


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reaction centers, and these Fe edge-site atoms were estimated produced OER turnover

frequencies approximately 150 times higher than Fe atoms on the catalyst surface at an applied potential of 1.8V vs. the reversible hydrogen electrode. Impressively, ultrathin

Fe2O3/Au nanostructures with a high density of catalytically active Fe edge-site atoms outperformed an ultrathin IrOx/Au catalyst at moderate overpotentials. DFT calculations revealed more favorable OER at edge sites along the Fe2O3/Au interface, with lower predicted overpotentials due to beneficial modification of intermediate binding. Our

results demonstrate how a combination of surface science, electrochemistry, and

computational modeling can be used to identify key structure-property relationships in a

well-defined electrocatalytic system.

KEYWORDS. Oxygen Evolution Reaction, Well-defined catalysts, Density Functional

Theory, Electrocatalysis, Scanning Tunneling Microscopy, Surface Science,

Electrochemistry, Structure Property Relationships.

INTRODUCTION.


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The oxygen evolution reaction (OER) plays a crucial role in renewable fuels synthesis

as the anodic process for electrochemical CO2 reduction, H2 evolution, and N2 reduction reactions.1-3 Metal oxides containing Fe, Ni, Co and/or Mn have been known to promote the alkaline OER for several decades,2 however, the role of the catalyst morphology and the influence of the support are still subjects of active debate.1-6 For example,

reports have suggested that gold supports and gold/catalyst interfaces can improve the OER activity of transition metal oxides,7-13 but the fundamental phenomena behind Au-

enhanced performance remains unclear. These uncertainties stem from heterogeneities

in the shape, size, and crystallographic orientation of typical catalysts that can obscure

key structure-property relationships. Understanding these atomic-level details is

essential for developing the low cost, high activity catalysts needed for large-scale

deployment of renewable fuels technology.

Ultrahigh vacuum (UHV) surface science techniques and atomically-resolved

scanning tunneling microscopy (STM) can provide deep insight into the structure-

property relationships of well-defined catalyst systems. However, applying these

techniques to electrocatalytic systems has been challenging due to the requirement of


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atomically-flat substrates, potential instability of UHV-synthesized materials during

electrochemical reactions, and inherent difficulties associated with post-reaction STM

imaging of samples tested in aqueous environments. To date, only a few UHV-

compatible electrochemical systems have been reported, including two-dimensional MoS214 and cobalt oxide15 catalysts on Au(111), Ni-incorporated Fe2O3 films on Pt(111),16 as well as PtAg surface alloys on Pt(111) and Pt islands and on Ru(0001).17

Our previous investigations identified facile water dissociation at the edge sites of

ultrathin Fe-oxides grown on single crystal Au(111) surfaces under UHV and near ambient conditions.18,19 These findings prompted us to consider a similar system for

electrocatalytic OER, and here we’ve evaluated the structure-dependent OER activity of

a series of ultrathin

-Fe2O3 nanostructures grown on Au(111) substrates. The

nanostructures were grown in a UHV chamber via metal evaporation, and all samples

contained a consistent thickness of ~0.5 nm (apparent height in STM) that corresponded to a bilayer stacking of the O-Fe-Fe-O unit in -Fe2O3.20 The morphology of the nanostructures, hereafter abbreviated as 2L-Fe2O3/Au, were systematically varied between discrete nanoparticles and near monolayer coverages by controlling the total


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loading of Fe2O3 on the Au(111) substrate. This synthetic procedure created a series of well-defined catalysts with equivalent thickness and controlled variations in the ratio

between surface and edge sites. The 2L-Fe2O3/Au catalysts were characterized with Xray photoelectron spectroscopy (XPS) and STM,20 they were removed from UHV for

electrocatalytic OER activity measurement in 0.1M KOH electrolyte, and then

transferred back into the UHV chamber for post-reaction analysis with XPS and STM.

Our experimental data linearly correlated the OER activity with the relative population

of Fe edge-site atoms at the 2L-Fe2O3/Au interface, and Fe edge-site atoms promoted the OER at rates approximately 150 times faster than Fe atoms on the extended surface of the 2L-Fe2O3/Au structures at an applied potential of 1.8 V vs. the reversible hydrogen

electrode

(RHE).

Complementary

DFT

calculations

supported

the

experimental results by predicting lower OER overpotentials at the edge sites of the 2L-

Fe2O3/Au structure than on the extended surface. Despite the fact that bulk Fe2O3 is not typically considered a particularly active OER catalyst due to its electrically insulating nature,4-6 we show that ultrathin 2L-Fe2O3/Au catalysts with a high density of Fe edgesite atoms can outperform an ultrathin, Au-supported IrOx catalyst at moderate


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overpotentials. Our results demonstrate that a combination of surface science

techniques, electrochemical measurements, and DFT calculations can provide atomic-

level insight into key structure-property relationships in a well-defined electrocatalytic

system.

METHODS.

Catalyst Synthesis and Characterization. The growth and characterization of 2L-Fe2O3 on Au(111) substrates were carried out in a commercial UHV chamber from Omicron Nanotechnology GmbH (base pressure 2 ×10-10 mbar), as described in detail in our previous work.20 The UHV chamber is equipped with a fast entry lock allowing quick

sample transfer between UHV and atmosphere. The surface of the single crystal Au(111) (10 × 10 × 1 mm3, 99.999% purity, Princeton Scientific Corp.) was cleaned by Ar+ sputtering (1.5 keV) at room temperature and annealed at 700 K for 15 min each

time prior to metal evaporation. Fe nanostructures were first grown on the clean Au(111) surface via room temperature metal evaporation using a 2 mm Fe rod (99.99%


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purity, Goodfellow) and an e-beam assisted evaporator (Omicron EFM3T). Fe2O3 was then formed by oxidizing the Fe nanostructures in NO2 (P = 1×10-7 mbar, Research Grade) at T = 700 K for 5 min.

The Fe2O3 coverages (monolayer equivalent, MLE) on Au (111) were determined by XPS.20 This MLE coverage is essentially the numeric ratio of the deposited Fe atoms to

the surface Au(111) atoms and was used to estimate the Fe2O3 loadings by assuming an atomic packing density of Au(111) of 1.387 × 1015/cm2 (ref. 21). Further details on

estimating Fe2O3 loadings from XPS are provided in the supporting information. An ultrathin IrOx catalyst (~0.25 nm apparent height in STM) was also grown on a clean Au(111) surface via reactive deposition of Ir in NO2 (P = 1×10-7 mbar, Research Grade) at room temperature, followed by annealing at 550 K for 10 min. Ir was evaporated from

a rod material (Goodfellow, 2.0 mm, 99.9% purity) and the loading was determined

using XPS.

All XPS data were collected using an Mg

M X-ray source (1253.6 eV, 300 W) and a

hemispheric analyzer with a pass energy of 20 eV, and the corresponding binding

energies (BE) were calibrated using the Au4f7/2 peak at BE = 83.8 eV. The O 1s XPS


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data were analyzed by subtracting a nonlinear background (Shirley) and fitting the

spectral peaks using mixed Gaussian-Lorentzian functions. STM experiments were

conducted in constant current mode at room temperature using etched tips from

Omicron, and all STM images were processed with plane corrections using scanning

probe imaging processor (SPIP, Imagemet) software. STM images were analyzed with ImageJ software (version 1.51j8),22 and details on estimating the relative fraction of Fe

edge site atoms are presented in the Supporting Information section.

X-ray absorption spectroscopy (XAS). Fe L-edge and K-edge XAS were collected

using beamlines 23-ID-2 (IOS) and 8-ID ISS of the National Source Light Source II at

Brookhaven National Laboratory. Additional details are provided in the Supporting

Information.

Electrochemistry. Electrochemical experiments were performed using a high-density

polyethylene (HDPE) cell, a Biologic SP-150 potentiostat, N2-purged 0.1M KOH electrolyte, a commercially available RHE reference electrode (Hydroflex), and a Pt-wire

counter electrode. The Au(111) substrate functioned as the working electrode by

pressing it against a hole in the bottom of the HDPE cell using a 6 mm inner diameter


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Viton O-ring. This configuration created a liquid-tight seal for measuring a consistent substrate area (0.283 cm2) and eliminated spurious contributions from the sides of the

Au(111)

substrate.

The

uncompensated

resistance

was

determined

using

electrochemical impedance spectroscopy and potentials were automatically corrected at

85% iR compensation using the instrument software. Cyclic voltammograms (CVs) were measured between +0.8V and +1.8V vs. RHE at a scan rate of 50 mV/sec until stable

activity was obtained (typically 5-15 scans; Figure S1). Trace Fe contaminants in the

0.1M KOH electrolyte were below the detection limit of inductively coupled mass

spectrometry and optical emission spectroscopy (ICP-MS; ICP-OES). Further purification23 using Ni(OH)2 or Co(OH)2 was not pursued because these techniques introduced significant Ni (>10,00 ppb) and Co (>100 ppb) contaminants into the

electrolyte that could influence the apparent OER activity of 2L-Fe2O3/Au catalysts and/or hinder post-reaction STM imaging and XPS analysis. A control experiment using

an uncoated carbon cloth as a counter electrode (The Fuel Cell Store; item 7302003)

produced nearly identical mass activity and ruled out potential influences of the Pt-wire

counter electrode.


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The background current associated with exposed regions of Au(111) substrate was

subtracted from voltammograms to isolate the current from the 2L-Fe2O3 catalysts, as described in the Supporting Information. Mass activities (mA/mgFe) were calculated based on the XPS-determined mass of Fe on the Au(111) surface. Turnover

frequencies (TOF: O2/s/atomFe) were also calculated by dividing the 2L-Fe2O3 current by Faraday’s constant (96485 C/mol e-), the number of electrons involved in the reaction (4 e-), and the total XPS-determined moles of Fe in the electrochemically-

probed area. We point out that these experimentally measured TOFs represent a

sample-wide average based on total Fe content, and the main text describes that

specific active sites at the catalyst edge or on the catalyst surface may show much

higher or lower site-specific TOFs.

Density Functional Theory Calculations. The OER was described using a reaction

mechanism and computational methodology similar to those introduced by Rossmeisl and coworkers24-26 and also considered by Liao et al.27 and Zhang et al.28,29 in their

studies of OER on various hematite Fe2O3 surfaces. Additional computational details of


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the Fe2O3 models used and of the OER mechanism analysis are provided in the Supporting Information.

RESULTS and DISCUSSION

Characterization and OER Activity. A series of 2L-Fe2O3/Au catalyst structures were grown in UHV via metal evaporation onto Au(111) substrates. Catalyst morphologies

were varied between discrete, 5-6 nm diameter nanoparticles to near monolayer coverages by controlling the XPS-determined Fe loadings between 2.0x10-10 and 2.9x10-9 molFe/cm2 (Table S1).20 The Fe2O3 composition was confirmed with both XPS30 and X-ray absorption spectroscopy (Figures S2 and S3),31,32 and STM imaging showed

that all structures formed with a consistent apparent height of ~0.5 nm (Figure S4) that corresponds to a bilayer stacking of the O-Fe-Fe-O unit in the 2L-Fe2O3/Au catalysts.20 The 2L-Fe2O3/Au catalysts were transferred out of the UHV chamber for electrochemical testing under ambient conditions in 0.1M KOH electrolyte. Figure 1a

presents OER voltammograms of 2L-Fe2O3/Au catalysts at different loadings alongside


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200 nm x 200 nm STM images. The back-ground current from the Au(111) substrate

was subtracted from the voltammograms and the activity was normalized to the mass of

Fe present in the electrochemically-probed area (Figure S5).

Figure 1a shows that the OER mass activity increased as the 2L-Fe2O3/Au morphology transitioned from near monolayer coverages to small nanoparticles. The 2.0x10-10 molFe/cm2 voltammogram represents the average of two independently synthesized samples, and the shaded region represents the standard deviation. These

results

demonstrate

the

reproducibility

of

our

synthetic

and

electrochemical

measurement techniques, and a maximum mass activity of ~2.5x105 mA/mgFe was obtained at an applied potential of 1.8V vs. RHE. We point out that one of the 2.0x10-10 molFe/cm2 samples was tested using a carbon counter electrode, which indicates the use of a Pt counter electrode did not impact the apparent OER activity. All 2L-Fe2O3/Au catalyst loadings produced consistent apparent OER onset potentials (1.58±0.01 V vs.

RHE) and Tafel slopes (76±7 mV/dec). This result indicates a common reaction

mechanism for all catalyst loadings (Table S1 and Figure S6), but an unambiguous


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assignment of the rate determining step from this specific Tafel slope value is not straightforward.33

The results in Figure 1a can be compared to the heterostructured Au (~5 nm) @ FeOx (~ 11 nm) nanoparticles reported by Jaramillo and coworkers,7 which demonstrated an OER onset potential of ~1.6V vs. RHE and produced a maximum mass activity of 580 mA/mgFe at 1.68 V vs. RHE in 1.0M KOH. Their results suggested that Au@FeOx systems containing a large density interfacial FeOx/Au sites could produce high OER mass activity. The smallest 2L-Fe2O3/Au structures considered in this study (2.0x10-10 molFe/cm2; 5-6 nm Fe2O3 diameter), which should possess a high density of catalyst/Au interface sites, produced approximately 57-times higher mass activity (~3.3x104 mA/mgFe at 1.68V vs. RHE) than the above noted Au@FeOx heterostructures at the same applied potential.

The small 2L-Fe2O3/Au nanostructures also demonstrated higher OER activity than an ultrathin IrOx/Au catalyst at moderate overpotentials. For example, Figure S7 compares the 2.0x10-10 molFe/cm2 2L-Fe2O3/Au samples to a UHV-synthesized, ultrathin IrOx/Au catalyst (~0.25 nm apparent height) with a similar metal loading (2.5x10-10 molIr/cm2).


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The ultrathin IrOx/Au catalyst demonstrated a lower apparent OER onset potential of approximately 1.52 V vs. RHE, but the 2L-Fe2O3/Au catalyst produced equivalent or higher mass activity above 1.6V vs. RHE. The OER activity of the IrOx/Au catalyst is well within the range reported for other Ir-based materials (Figure S7),34-39 which

supports the accuracy of our activity normalization method and demonstrates the high

OER activity of small, ultrathin 2L-Fe2O3/Au nanostructures.

Structure-Activity Relationships. STM images were analyzed to estimate the

population of Fe atoms residing at edge-sites relative to the total amount of Fe atoms

present in the 2L-Fe2O3/Au structure (Figure S8 and Table S1). Lower catalyst loadings produced small 2L-Fe2O3/Au structures, and we determined a maximum Fe edge-site atom population of ~14% for the two samples containing 2x10-10 molFe/cm2. Higher catalyst loadings produced larger 2L-Fe2O3/Au structures and near monolayer coverages with a minimum Fe edge-site atom population of 0.9%.

The experimentally-measured OER mass activity, turnover frequency (TOF:

O2/s/atomFe; based on total XPS-determined moles of Fe), and specific current density


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(based on STM-determined Fe2O3 surface area) all increased linearly with the relative population of Fe edge-site atoms (Figures 1b, S8 and Table S1). Importantly, the

relationship between the size, number density, and OER activity of the 2L-Fe2O3/Au samples ruled out activity differences based on mass transport limitations or the relative flux of reactant molecules to individual catalyst structures (Figure S9).40

The linear correlation between OER TOFs and the relative population of Fe edge-site

atoms allowed us to extrapolate beyond the range of experimentally tested samples and

estimate site-specific TOFs for Fe edge-site atoms and Fe atoms on the extended surface. We estimated an upper-bound TOF of 242 O2/s/atomFe at 1.8V vs. RHE for Fe edge-site atoms based on a 2L-Fe2O3/Au structure that contained 100% Fe edge-site atoms. A lower-bound TOF of 1.6 O2/s/atomFe was estimated at 1.8V vs. RHE for Fe surface atoms based on an infinite 2L-Fe2O3/Au surface with 0% Fe edge-site atoms, and assuming Fe atoms in the bottom layer of this structure could not participate in the

OER due to electrochemical inaccessibility. These estimates do not consider potential

mass transfer limitations at extremely high TOFs; however, they indicate that Fe edge-


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site atoms in the 2L-Fe2O3/Au structure provide a much larger contribution to the measured OER activity compared with Fe atoms on the extended surface. Experimentally-measured specific current densities at 1.8V vs. RHE ranged between ~7 mA/cm2Fe2O3 for near monolayer 2L-Fe2O3/Au structures to ~19 mA/cm2Fe2O3 for small 2L-Fe2O3/Au nanoparticles that contained the highest population of Fe edge-site atoms (Figure S8). Based on the linear relationship between Fe edge-site population

and specific current density, we estimated that an infinite, edge-free 2L-Fe2O3/Au extended surface should produce a specific current density of 5.8 mA/cm2Fe2O3 at 1.8V

vs. RHE in 0.1M KOH. In comparison, bulk Fe-oxide films formed through the anodic polarization of Fe foils have been reported to produce current densities between 6-10 mA/cm2 at 1.8V vs. RHE in 0.1M NaOH electrolyte,41 which agrees well with our

estimate for an infinite, edge-free 2L-Fe2O3/Au surface. We calculated bulk Fe-oxide TOFs between 0.03-0.06 O2/s/atomFe at 1.8V vs. RHE based on the reported current density and anodic charge required to form the oxide film. These bulk Fe-oxide TOFs

are lower than those estimated for Fe atoms on the extended surface of 2L-Fe2O3/Au,


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which may result from the bulk films containing electrochemically inaccessible or

electrically insulated Fe atoms that did not participate in the OER.

Spectroscopic Identification of OER Active Sites. Post-reaction STM imaging provided

direct evidence that 2L-Fe2O3/Au structures retained their general size and morphology during OER (Figures 2a,b and S10), which supports the above noted structural analysis. In contrast, bulk Fe2O3 films have been shown to dissolve during alkaline OER,4 which suggests the Au(111) substrate may have stabilized the 2L-Fe2O3/Au catalysts against anodic dissolution. Post-reaction XPS revealed changes in the O 1s and Fe 2p spectral

regions characteristic of Fe2O3 hydroxylation during OER (Figures 2c and S2). Figure 2c presents representative post-reaction O 1s spectra for low, intermediate, and high

loading 2L-Fe2O3/Au samples, where the spectra were fit with three peaks that correspond to lattice oxygen (OLattice), hydroxyl groups (OH), and carbonate species.18 Pre-reaction O 1s spectra collected from 2L-Fe2O3/Au samples before removal from the UHV chamber contained a single peak at ~529.6 eV that corresponds to OLattice species (Figure S2).


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Smaller 2L-Fe2O3/Au structures that contained a larger relative population of Fe edgesite atoms also showed a larger relative signature for OH species in the post-reaction O

1s XP spectra. The data in Figure 2d quantifies this relationship and presents a linear

correlation between the structure’s relative population of Fe edge-site atoms and the

post-reaction OH-to-OLattice peak area ratio (OH/OLattice). This result supports our previous observation of preferential hydroxyl formation along the edge of ultrathin Feoxide nanostructures,18,19 and the post-reaction OH-to-Olattice ratio serves as a spectroscopic descriptor for the relative population of hydroxylated Fe edge-site atoms.

Finally, the data in Figure 2e directly correlates electrochemical OER activity with the

spectroscopically-determined OH-to-Olattice ratio, which identifies hydroxylated edgesites as key reaction centers for the OER at 2L-Fe2O3/Au catalysts.

Computational Modeling. Ab initio DFT calculations were used to model OER at

unsupported and Au(111)-supported -Fe2O3 (0001) structures (Figures 3a and S11). Details on model construction are provided in the Computational Methods section of the


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Supporting Information. The Au(111)-supported 2L-Fe2O3/Au model structure was ~0.7 nm thick and demonstrated Fe-Fe distances of approximately 0.5 nm, which closely

reproduces the experimentally-tested 2L-Fe2O3/Au samples (Figure S4 and reference 20). The O-terminated surfaces were hydroxylated to mimic the structure of the

experimentally-tested 2L-Fe2O3/Au catalysts during OER conditions, as indicated by the XPS data in Figures 2c and S2, and reference 18. Among various OER pathways proposed in the literature,33 we assumed the validity of the mechanism introduced by Rossmeisl and coworkers24-26 and found by others27-29 to

closely reproduce OER at the (0001) and other crystallographic surfaces of Fe2O3. The mechanistic steps considered below are fully consistent with those previously used by Liao et al.27 to describe water oxidation on pure and doped hematite:

(1) H2O + * T H2O* (2) H2O* T OH* + H+ + e(3) OH* T O* + H+ + e(4) H2O + OH* T OOH* + H+ + e(5) OOH* T * + O2 (g) + H+ + e-


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Within this OER mechanism, initial water adsorption at a vacant active site (*) is

followed by four proton/electron transfer steps that include bound OH*, O*, and OOH* intermediates. By assuming the computational electrode model,24-26 where solvated

protons and electrons are in equilibrium with H2 in gas phase, the chemical potentials of protons and electrons can be expressed for a given pH level as function of the applied

potential. The Gibbs free energy change ( G) for the above set of reactions provided

the expected OER formal potential of 1.23V when divided by the total number of

electrons (Table S2). Within the set of potential-dependent reactions (2-5), the reaction

step with the largest free energy change ( Gmax) represents the so-called potential determining step (PDS) and corresponds to the minimum electrochemical potential

required to make all the voltage-dependent steps have

G 0. The theoretical

electrochemical overpotential can be evaluated from the PDS as:

theor

= ( Gmax/e-) -

1.23 V. Rossmeisl and coworkers have shown that theoretical overpotentials calculated

in this manner are valid for comparing catalysts experimentally evaluated in either acidic or alkaline electrolytes.25


21

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Page 22 of 46

The extended 2L-Fe2O3 (0001) surface contained Fe atoms that were six-fold coordinated to O species, and the OER active site corresponds to a surface with one O

vacancy site in the top layer. The deprotonation of OH* into O* (step 3) was found to be

the PDS for OER reactions on extended surfaces of both the unsupported and Au

(111)-supported 2L-Fe2O3 structures (Figure S11), which is consistent with previous theoretical PDS determinations for a number of other unsupported Fe2O3 surfaces.28,29 The unsupported 2L-Fe2O3 structure had a predicted theoretical overpotential of

theor

=

0.58 V (Figure 3b). The presence of the Au support increased the free energies of

formation for OH* and OOH* species and slightly decreased the free energy of

formation for the O* species (PDS) at the Au(111)-supported 2L-Fe2O3/Au structure relative to unsupported 2L-Fe2O3. These changes decreased supported 2L-Fe2O3/Au extended surface to

theor

theor

for the Au(111)-

= 0.56 V, as shown in Figure 3b.

Additional calculations on more “bulk-like”, four-layer -Fe2O3 (0001) extended surfaces predicted weaker stabilization of bound O* intermediates, larger overpotentials (

0.63 V), and practically no influence of the Au(111) support on

Calculated G and

theor

theor

theor

=

(Figure S11).

values are summarized in Figure S11 and Table S2.


22

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ACS Catalysis

We also considered the edge of the Au(111)-supported 2L-Fe2O3 as a potential active site for OER. This structure is denoted as 2L-Fe2O3(edge)/Au in Figure 3a. OER intermediates associated with reaction steps 2-4 formed at a bridge site between two Fe

edge-site atoms located in the top and bottom layers of the 2L-Fe2O3/Au structure (Figure S12). This binding site increased the free energies of forming both OH* and

OOH* species while decreasing the free energy of forming O* species relative to the

Au-supported extended surface. The increased stabilization of O* at the edge bridge

site lowered its free energy of formation and shifted the PDS to reaction step 4 (OOH*

formation). As a result, the overall theoretical OER overpotential was lowered to

theor

=

0.51V, as shown in Figure 3b. Additional OER calculations for edge sites on an

unsupported 2L-Fe2O3 structure predicted an overpotential of 0.54 V that was larger than the corresponding value for the edge site of the Au(111)-supported 2L-

Fe2O3(edge)/Au

structure.

Overall,

these

theoretical

results

indicate

a

thermodynamically more favorable OER process at the edge sites along the 2L-

Fe2O3/Au interface due to beneficial modification of intermediate binding and a lowering of the free energy of formation for the O* species.


23

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Page 24 of 46

The free energy for reaction step 3 (O* formation) is directly correlated to the difference between the Gibbs adsorption energies of O* and OH* species, i.e. G3= GO- GOH. Based on the scaling relationships demonstrated by Man et al.,25 the GO- GOH quantity can be used as a useful descriptor for OER activity. A volcano dependence between the negative theoretical overpotential (-

theor)

and

GO- GOH

descriptor value can be identified for broad classes of oxides, with a minimum -

theor

expected near the optimum GO- GOH value of 1.6 eV. Figure 3c compares the results obtained for the Fe2O3 systems modeled in this study (red symbols) with previous data reported for different crystallographic surfaces of bulk Fe2O3 (black symbols).28,29 A clear trend develops for Fe2O3-based catalysts where sites demonstrating O* formation as the PDS fall on the right side of the plot, and sites with OOH* formation as the PDS

fall on the left side of the plot. Besides the good correlation between the results reported in this and previous studies,28,29 Figure 3c also demonstrates that the overpotential

obtained for the 2L-Fe2O3(edge)/Au structure is among the smallest theoretical values reported for various crystallographic surface orientations of Fe2O3. The favorable modification of intermediate binding at edge sites between 2L-Fe2O3 and Au support


24

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ACS Catalysis

produced an important reduction in OER overpotential and placed the 2L-

Fe2O3(edge)/Au structure closer to the top of the volcano curve. Taken together, the DFT results indicate the importance of interfacial edge sites

impacting the binding of the intermediate species relative to sites on the extended

surface. Experimentally,

smaller 2L-Fe2O3/Au nanostructures demonstrated higher

OER activity because they contained a higher density of OER-active edge sites along

the catalyst/support interface. Extending this insight suggests that ultrathin, Au-

supported Fe2O3 nanowires with a large population of interfacial edge-sites may be a promising OER catalyst candidate worth future consideration. Beyond this study, our

findings suggest that some previous examples of Au-enhanced OER activity in transition metal oxides,7-13 especially Au-supported Fe-oxides,7 may have contained

strong contributions from perimeter sites at the catalyst/Au interface.

CONCLUSIONS These current results extend our group’s18,19 and other’s42 previous UHV and near

ambient pressure-based observations of edge-enhanced water dissociation at Au-


25

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Page 26 of 46

supported, ultrathin metal oxides by quantifying the edge-enhanced OER activity of Au-

supported, ultrathin Fe2O3 electrocatalysts. Experimental data directly correlated the OER activity of 2L-Fe2O3/Au structures with the relative population of hydroxylated Fe edge-site atoms. Extrapolated TOFs, based on the linear relationship between OER

activity and Fe edge-site population, suggest that the OER rate at Fe edge-site atoms is ~150 times higher at 1.8V vs. RHE than at surface Fe atoms on the extended surface of 2L-Fe2O3/Au (242 vs. 1.6 O2/s/atomFe, respectively). DFT calculations supported this observation by predicting the lowest OER overpotentials at edge sites along the 2L-

Fe2O3/Au interface, compared with other Fe2O3 structures, due to beneficial modification of intermediate binding. Our results demonstrate that a combination UHV

surface science techniques, experimental electrocatalytic activity measurements, and

DFT modeling can identify important structure-property relationships and provide

atomic-level details for well-defined electrocatalyst systems. Beyond this study, we

hypothesize that a similar combination of surface science, electrochemistry, and

computational modeling should also help identify key structure-property relationships in


26

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ACS Catalysis

other well-defined electrocatalyst systems for CO2 or O2 reduction, H2 evolution, and/or NH3 synthesis reactions.

ASSOCIATED CONTENT

AUTHOR INFORMATION

Corresponding Author

*[email protected]; *[email protected];

*[email protected]

Supporting Information. Experimental and computational methods, additional XPS, XAS, STM, electrochemical, and computational data. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENT


27

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Page 28 of 46

The authors thank Dr. Christopher Matranga and Dr. Junseok Lee for useful

discussions. Portions of this work were performed in support of the National Energy

Technology Laboratory’s ongoing research under the RSS contract

89243318CFE000003. This research used resources of the 23-ID-2 (IOS) beamline and

8-ID (ISS) beamline of the National Synchrotron Light Source II, a U.S. Department of

Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by

Brookhaven National Laboratory under Contract No. DE-SC0012704. This report was

prepared as an account of work sponsored by an agency of the United States

Government. Neither the United States Government nor any agency thereof, nor any of

their employees, makes any warranty, express or implied, or assumes any legal liability

or responsibility for the accuracy, completeness, or usefulness of any information,

apparatus, product, or process disclosed, or represents that its use would not infringe

privately owned rights. Reference herein to any specific commercial product, process,

or service by trade name, trademark, manufacturer, or otherwise does not necessarily

constitute or imply its endorsement, recommendation, or favoring by the United States

Government or any agency thereof. The views and opinions of authors expressed


28

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ACS Catalysis

herein do not necessarily state or reflect those of the United States Government or any

agency thereof.

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38

Fe Loading (mol / cm2) 2.5

Low Loading

2.0x10-10 3.1x10-10 4.7x10-10 7.8x10-10 1.2x10-9 1.8x10-9 2.3x10-9 2.9 x10-9

2.0

1.5

1.0

0.5 High Loading

0.0 1.2

1.3

1.4

1.5

1.6

1.7

1.8

High Loading

(b)

Low Loading

40 1.8V

2.5

35 30

2.0

25

1.5

1.75V 20

15

1.0

10

0.5

0.0

iR Corrected Potential (V vs. RHE)

1.68V

0

2

4

6

8

10

12

14

16

TOF (O2 / s / atomFe )

(a) Mass Activity (105 mA / mgFe )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Mass Activity (105 mA / mgFe )

Page 39 of 46

5 0

Fe Edge-Site Population (atomic %)

Figure 1. (a) OER voltammograms of 2L-Fe2O3/Au at different loadings in N2 purged 0.1M KOH. STM images detail the transition from near-monolayer coverage at high

loading to discrete nanostructures at low loadings (STM conditions: I = 5 pA, V = 2.0 V; image sizes: 200 nm x 200 nm). The 2.0x10-10 molFe/cm2 voltammogram represents the average of two independently synthesized samples, and the shaded region represents

the standard deviation. (b) Mass activity and turnover frequency (TOF) versus the

relative population of Fe edge-site atoms at three iR-corrected electrochemical potentials (V vs. RHE).


39

ACS Catalysis

(b)

(d)

OH

carbonate

-9

2.0 1.5 1.0 0.5

0

2

4

6

8

10

12

14

16


(e)

2.0x10-10 mol 540 538 536 534 532 530 528 526

Figure 2.

Low Loading

0.0

2.3x10 mol

7.8x10-10 mol

Post Reaction

High Loading 2.5

OH to OLattice Ratio

OLattice

Binding Energy (eV)

40 1.8V

2.5

35 30

2.0

25 1.5

1.75V 20

15

1.0

10 0.5

1.68V 5

0.0 0.5

1.0

1.5

2.0

2.5


Pre Reaction

Post-Reaction O 1s


(c)

(a)

Counts (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 46

0 3.0


(a,b) Pre- and post-reaction STM images of a 2L-Fe2O3/Au

sample

containing 5x10-10 molFe/cm2 (STM conditions: I = 5 pA, V = 2.0 V; image sizes: 200 nm x 200 nm). Additional pre- and post-reaction STM images are presented in Figure S10.

(c) Representative post-reaction O1s X-ray photoelectron spectra at low, medium, and

high loadings. (d) Variation in post-reaction OH to OLattice peak area ratio (OH/OLattice) vs. the population of Fe edge-site atoms. (e) Correlation between the XPS-determined ratio

of OH to OLattice species and the electrocatalytic OER activity at three iR-corrected electrochemical potentials (V vs. RHE). The linear fits in panels d and e serve as guides

for the eye.


40

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ACS Catalysis


41

ACS Catalysis

(a)

2L-Fe2O3

2L-Fe2O3 / Au

2L-Fe2O3 (edge) / Au

H Fe O Au

(c)

(b) 0.60

-0.2

2L-Fe2O3(edge) / Au

-0.4

(100)L

-0.6

2L-Fe2O3 / Au

(V)

0.56

2L-Fe2O3

theor

(V)

0.58

theor

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 46

2L-Fe2O3 / Au 2L-Fe2O3 4L-Fe2O3 / Au (210) (101) (021)

-0.8 (100)

-1.0

0.54

-1.2 0.52


-1.6 1.2

0.50

(211)

-1.4 1.4

Fe2O3 Structure

1.6

1.8

2.0

2.2

2.4

2.6

2.8

GO - GOH (eV)

Figure 3. (a) DFT-optimized structures of unsupported and Au (111) supported, two-

layer thick -Fe2O3 (0001) structures. The O-terminated surfaces were hydroxylated to mimic the experimentally-tested 2L-Fe2O3/Au system, and the corresponding active sites are described in the main text. (b) Theoretical OER overpotentials (

theor)

different systems investigated. (c) Negative of the theoretical overpotential (-

function of the OER descriptor

for the

theor)

as a

GO- GOH for the systems analyzed in this study (red

filled circles) and previously published data for bulk Fe2O3 surfaces with different


42

Page 43 of 46 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

crystallographic orientations (black filled circles; data from references 28 and 29). The

corresponding G and

theor values

are summarized in Table S2 and Figure S11.

For Table of Contents Only: Fe O H Au

O2

2 H2O 10 nm


43

ACS Catalysis

Low Loading

-10

2.0x10 3.1x10-10 4.7x10-10 7.8x10-10 1.2x10-9 1.8x10-9 2.3x10-9 2.9 x10-9

2.0

1.5

1.0

0.5 High Loading

0.0 1.2

(b)

1.3

1.4

1.5

1.6

1.7

1.8

iR Corrected Potential (V vs. RHE)

High Loading

Low Loading 1.8V

2.5

35 30

2.0

25 1.5

1.75V 20

15

1.0

10 0.5 0.0

1.68V

0

2

4

6

8

10

12

14

16



40

5 0


2.5

Fe Loading (mol / cm2)


(a) Mass Activity (105 mA / mgFe )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 44 of 46

(b)

OH

OLattice

carbonate

Low Loading

2.5

2.3x10-9 mol

2.0 1.5 1.0 0.5 0.0

0

2

4

6

8

10

12

14

16


(e) 7.8x10-10 mol

2.0x10-10 mol

Post Reaction

High Loading

540 538 536 534 532 530 528 526

Binding Energy (eV)


40 1.8V

2.5

35 30

2.0

25 1.5

1.75V 20

15

1.0

10

0.5 0.0

1.68V 5

0.5

1.0

1.5

2.0

2.5


0 3.0


Pre Reaction

Post-Reaction O 1s

(d)


(c)

(a)

Counts (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

ACS Catalysis


Page 45 of 46

ACS Catalysis

(a)

2L-Fe2O3

2L-Fe2O3 / Au


H Fe O Au

(c)

(b)

-0.2 2L-Fe O (edge) / Au 2 3

0.60

0.56

-0.4

2L-Fe2O3 2L-Fe2O3 / Au

theor(V)

0.58

theor (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 46 of 46

0.54

(100)L

2L-Fe2O3 / Au 2L-Fe2O3

-0.6

4L-Fe2O3 / Au

-0.8

(210) (101) (021)

-1.0

(100)

-1.2 0.52


0.50

Fe2O3 Structure

(211)

-1.4 -1.6 1.2

1.4

1.6

1.8

2.0

2.2

GO - GOH (eV)


2.4

2.6

2.8

Edge-Enhanced Oxygen Evolution Reactivity at Ultrathin, Au

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