Surface constrained electron-hole rich species active in the electrocatalytic water splitting


 Iridium and its oxides/hydroxides are the best candidates for the oxygen evolution reaction under harsh acidic conditions owing to the low overpotential and the high corrosion resistance observed for Ir-based anodes. Herein, by means of cutting edge operando surface and bulk sensitive X-ray spectroscopy techniques, specifically designed electrode nano-fabrication and ab initio DFT calculations, we were able to reveal the electronic structure of the active IrOx centers (i.e. oxidation state) during electrocatalytic oxidation of water in the surface and bulk of high-performance Ir-based catalysts. We found the oxygen evolution reaction is controlled by the formation of empty Ir 5d states in the surface ascribed to the formation of formally IrV species leading to the appearance of electron-deficient oxygen species bound to single iridium atoms (µ1-O and µ1-OH) that are responsible for water activation and oxidation, due to the bound oxygen’s transformation into an oxyl susceptible to nucleophilic attack water. Oxygen bound to three iridium centers (µ3-O) remains the dominant species in the bulk but do not participate directly in the electrocatalytic reaction, suggesting bulk oxidation is limited. In addition a high coverage of a µ1-OO (peroxo) species during the OER is excluded. Moreover, we provide the first photoelectron spectroscopic evidence in bulk electrolyte that the higher surface to bulk ratio in thinner electrodes enhances the material usage involving the precipitation of a significant part of the electrode surface and near-surface active species.


Electrodes Fabrication And Characterization
Two different electrodes with different surface to bulk ratios were fabricated and compared, namely a 20 nm thin-lm and 2.5 nm nanoparticles (NPs) on a graphene current collector electrodes. Fig. 1A shows a top view scanning electron microscopy (SEM) image of the sputtered thin-lm electrode. The thin-lm consists of interconnected polycrystalline islands with a thickness of 20 nm. In contrast, Fig. 1B shows the TEM image of isolated 2.5 nm IrO x NPs supported on conductive bi-layer graphene. Owing to the structural differences, the thin-lm can be considered a bulk model whereas the high surface to bulk ratio of the NPs makes them a surface model. Anodic oxidation of metallic iridium is reported to result in better performing catalysts than thermal activation, due to the existence of highly hydrated species forming an oxyhydroxide upon electro-oxidation 13 . Hence, the sputtered electrodes were oxidized/activated prior their use in OER by several potential scans between open circuit voltage (OCV) and 1.2 V at 20 mV/s scanning rate in a 50 mM H 2 SO 4 electrolyte (potentials are not iR corrected). As expected, this procedure resulted in the development of an increasingly larger anodic peak in the cyclic voltammogram (CV), where the peak height increased with the number of cycles 14 . The condition of maximum peak height corresponds to the highest electrode activity for thin-lms and NPs 15 . The TEM (Fig. 1B) images of the sputtered iridium particles on the free-standing bi-layer graphene show that they are homogeneously distributed after activation; the estimated coverage factor is around 20% of the surface and the average particle size is 2.5 nm. Contact of the IrO x NPs onto graphene is assured by a chemical bond between the surface Ir atoms and the oxygen species present at the edges and vacancies of graphene 16 . According to the SEM image in Fig. 1A, we estimate a surface area of about 1.5-2.0 monolayer (ML) for the thin-lm and about 1/3 ML for the nanoparticles. The fast Fourier transform (FFT) proves the existence of reduced metallic iridium NPs. Such beam induced reduction is common in non-stoichiometric oxides. One additional ring can be ascribed to stoichiometric IrO 2 .
The electrochemical performance of the Ir thin-lm and Ir NP electrodes are compared in Fig. 1C and 1D.
The CVs were acquired at room temperature (25°C), in de-aerated 0.1 M H 2 SO 4 with N 2 continually bubbled in the electrolyte at a scan rate of 20 mV/s using Pt and Ag/AgCl (saturated in KCl) as counter and reference electrodes, respectively. The CVs show two broad oxidation waves, labeled I and II, and two broad reduction waves, labeled IV and V, due to formally Ir III /Ir IV and Ir IV /Ir V redox couples, respectively 11,17,18 . An additional current (point III in Fig. 1C and 1D) is ascribed to the oxidation of water. The CVs in Fig. 1C and 1D indicate that both the thin-lm and bi-layer graphene coated with Ir NPs behave similarly. Therefore, it is expected that similar active species are involved during the electrocatalytic oxidation of water to dioxygen on both the thin-lm and iridium NP electrodes. It is generally accepted that these active sites are hydrated Ir-oxyhydroxides formed in situ during OER 19,20 , where the Ir oxohydroxide-based OER is stable for high-current water electrolysis under acidic conditions 21 .
Probing the bulk species formed during the water oxidation In order to determine the nature of the active species, the electronic structure modi cation that the electrodes undergo during the electrocatalytic oxidation of water was investigated by means of operando XAS. The electrocatalysts were characterized rst using XAS in total uorescence yield (TFY) mode at the Ir L 3 -edge using a homemade in situ electrochemical cell 22 , more details can be found in the supplementary information (SI). Using a 100 nm thick SiN x membrane, which is transparent to the incoming and out-going photons, it is possible to investigate the variations in the electronic structure using photon-in/photon-out (PIPO) techniques in the hard X-ray regime. An advantage of this approach is that it enables the study of electrochemical reactions with aqueous electrolytes (i.e. 100 mM H 2 SO 4 ). Fig.   2A and 2B show the detection scheme with the thin-lm and free standing bi-layer graphene decorated with Ir NPs. For both materials it is possible to perform measurements in TFY-XAS by collecting the photons emitted during the uorescence decay follow the absorption process; this signal comes from the surface and bulk of the materials. Therefore, TFY-XAS provide information associated with the whole electrode (bulk and surface) in the case of the Ir thin-lm. However, in the case of the NPs, due to the average lm thickness of ~6 monolayers (MLs), the surface signal is enhanced, despite the methodology not being a surface sensitive technique. Moreover, the free standing graphene allows the collection of photoelectrons (by a semispherical electron analyser) from the side exposed to the electrolyte, thereby enabling the acquisition of photoelectron spectra (PES) 23,24 . Meanwhile, the thin-lm electrode (20 nm) only yields photoelectrons from the side where the electrode is exposed to the incident X-ray photons, as shown in Fig. 2A, making it impossible to investigate the electri ed solid-liquid interface of such a thick working electrode using soft PES. Note that PES is a surface sensitive technique due to the short inelastic mean free path (IMFP) of photoelectrons in solids or liquids 25,26 , making it an excellent complement to TFY-XAS.  The Ir L 3 -edge probes the   dipole allowed transitions from a core Ir 2p 3/2 electron to the partially occupied Ir 5d and Ir 6sp orbitals, which are hybridized with the O 2p orbitals 27 . Transitions to the 5d orbitals are lower in energy and well separated from transitions to the 6sp. These 2p to 5d transitions give rise to the so-called white-line.
While the large lifetime broadening (about 5 eV, see SI) does not allow the discrimination of ne structure in the white-line due to, for instance, transitions into t 2g -like and e g -like 5d orbitals, analysis of the whiteline intensity can still give insight into the electronic structure of iridium. In particular, a sum-rule relates the total number of 5d holes to the integral area of the white-line 11,27 ; that is, the white-line is linearly proportional to the iridium oxidation state 28 . Note that while the sum-rule is a property of the dipole operator and rigorously holds 29,30 for L 3 + L 2 , previous work shows no change in the L 3 :L 2 branching ratio for oxidized iridium compounds, making L 3 alone su cient for a white-line analysis 28 . The ability of the bulk sensitive TFY-XAS measurements to reveal changes in average Ir oxidation states is apparent from  [31][32][33][34] . Before turning to a quantitative evaluation of this behavior, however, the details of the proportionality between white-line intensity and Ir oxidation state must be found. To establish a connection between the white-line intensity and the number of 5d holes on Ir we analyzed a series of reference samples, including: Ir 0 , IrCl 3 , IrO 2 , and IrO x (reference samples details can be found in the SI) 34 . An iridium foil was used for Ir 0 . IrCl 3 powder was used as an Ir III reference, and IrO 2 rutile-type powder was taken as a Ir IV reference. In addition, an amorphous IrO x catalyst rich in active species 17 was used to compare with the catalysts used in this work. Note that the Ir 0 (foil) white-line can be arti cially enhanced due to the existence of a native oxide layer and the fact that this spectrum is collected in TFY mode, self-absorption effects are also possible.
However, the oxide layer is not a problem since it is thin compare to the bulk with minor contribution to the white-line intensity.  (Fig. 3B), see SI for more details (Fig. S3). From the white-line integration of the experimental data, Fig. 3A2 shows the percent increase in the Ir 5d density of electron-holes referred to IrCl 3 and the equivalent oxidation state. Therefore, an oxidation state of +4 correspond to an increase of 22% of the white-line intensity with respect to +3 (IrCl 3 sample), and a change of ~32% for the case of IrO x corresponding to a +4.7 average oxidation state. This increase is supported by its comparison with Ir 0 , where an increase of a 52% of the white-line intensity is found for +4 and 65% for IrO x , corresponding to a +4.6 average oxidation state. Performing a similar quanti cation using the simulated data shows the maximum white-line intensity changes linearly with the Löwdin charge on Ir in the bulk phases as well as across various surface Ir species bound to O, OH, OOH, and OO on IrO 2 surfaces spanning formal oxidation states from <IV to >VII (see SI for details, Fig. S4 and S5), suggesting no signi cant changes in intensity variation should be expected at higher oxidation states for the iridium oxyhydroxides. One can also consider that the change in the oxidation can often be correlated to the shift in the peak position, using this approach an average experimental oxidation state of +4.  collected under chronoamperometric (CA) control, and ~1 hour was required to recorded each Ir L 3 -edge spectrum. This fact, together with the constant current observed under the applied potential, veri es that the spectra where collected under steady state conditions, which is important to assure their delity. The white-line intensity, and hence Ir oxidation state, can be seen to remain constant up to 0.7 V, above which the materials are oxidized. From the Ir Pourbaix diagram, Ir 0 is expected below 0.73 V, 35 making it tempting to assign the starting state of the lm and NPs to Ir 0 . However, the surfaces likely remain partially oxidized after the CV activation steps 36 making an assignment to Ir III more appropriate, as consequence of surface irreversible oxidation state, as con rmed by XPS below. At 1.0 V Ir IV has become the stable phase on the Pourbaix diagram, which is further supported by the CVs of the lm and NPs showing 1.0 V is above the Ir III /Ir IV redox couple, but below the transition assigned to Ir IV /Ir V . At 1.0 V the near-surface region is then expected to be Ir IV . The higher white-line intensities con rm oxidation, with thẽ 30% increase for the Ir NPs in line with an increase to +4.5, assuming Ir III to start. The higher white-line intensity of the NPs compared to the thin-lms is likely due to incomplete oxidation of the bulk of the 20 nm thin-lms 18,37,38 . At 1.6 V, though no further increase in white-line intensity is observed for the thinlms, whereas the NPs show an additional ~4% increase above that observed at 1.0 V suggesting the coexistence of Ir IV /Ir V , or higher oxidation states, on the catalyst surface. To gain a better understanding of what these species may be, we turned to ab initio calculations.
In order to provide a better description of the observed changes in the white-line spectra, these reference and operando measurements were compared with the DFT calculated Ir L 3 -edge spectrum (details can be found in the SI). We begin with the lowest energy, in vacuum, IrO 2 surface, (110), as a model for the DFT calculations 39 . The surface was rst fully hydrogenated as shown in Fig. 5. It was then successively oxidized by following a series of proton coupled electron transfers to explore the adsorbates argued to be present under OER conditions 39  Removing H + and efrom µ 1 -H 2 O on the (110) surface yields a surface with µ 1 -OH and µ 2 -OH that is predicted to be stable up to 0.7 V. 39 The average formal surface oxidation state of Ir on this surface is +IV, in-line with the computed L 3 -edge white-line intensity, Fig. 5. At 0.7 V the surface termination is predicted to transform into µ 1 -OH and µ 2 -O, with an average Ir surface oxidation state of +4.5, in general agreement with the experimental results on the NPs. Above 1.2 V both a surface µ 1 -O or µ 1 -OO have been predicted to be stable 17,39 . Of these, the µ 1 -O appears more likely on the NP surface owing to the small increase in white-line intensity observed experimentally between 1.0 V to 1.6 V, which is consistent with the increase in iridium formal oxidation state and the computed white-line intensity for transitioning from µ 1 -OH to µ 1 -O. The µ 1 -OO (and µ 1 -OOH) both show reduced white-line intensities relative to µ 1 -OH and µ 1 -O, and while we observed no evidence for such a reduction in intensity, it has been previously observed on IrO x NPs at high applied potential 40 .
We also considered the possibility that the NPs facet during anodic polarization, as above 1.1 V the (111) surface is thermodynamically favored 39 . Following the same methodology as above, Fig. 5 shows the Ir L 3 -edge white-line intensities as a function of the number of electrons transferred starting from a fully hydroxylated (111) surface, see SI for structures (Fig. S4). Here, though more points are included owing to the presence of four µ 2 -OH species in the unit cell, a similar trend emerges as found for the (110) 39 While we see no drop in white-line intensity on the NPs supporting the formation of µ 1 -OO, we cannot completely rule out the appearance of Ir VI from XAS alone.
Thus, we turned to XPS.

Probing The Surface Species Formed During The Water Oxidation
As a surface sensitive technique, XPS compliments the bulk sensitive XAS and offers the opportunity to gain deeper knowledge about the mechanism mediating the electrochemical oxidation of water on iridium oxide. The electronic structure of the IrO x NPs on graphene was investigated at the Ir 4f core level using the facilities and in situ electrochemical ow cells described in the SI 24 . The Ir 4f spectra depending on the applied potential are shown in Fig. 6A, as well as its comparison with the reference samples (bottom spectra) and the computed line shapes for the relevant species inferred through the XAS analysis (see Fig. S6 and S7). Computed XPS binding energies are summarized in Table S1.
At, and below, 0.7 V the NPs are composed of predominantly bulk Ir III , with an Ir 4f binding energy of 62.3 eV. 41 Once the electrode is polarized to 1.0 V, we know from XAS that the NPs are oxidized to Ir IV , and possibly higher. From the simulations we nd that Ir atoms on the surface of IrO 2 show normal Ir 4f binding energy shifts up to formal Ir oxidation states of, at least, 7.3, see Table S1, with the average formally Ir V and Ir VI species appearing near ~62.2 eV and ~62.7 eV, respectively. Thus, the Ir 4f shift is a good measure of iridium oxidation state for Ir atoms in a conductive matrix. However, this conductive matrix also in uences the Ir 4f lineshape, making accurate speciation challenging. The computed Ir 4f spectra all show complex lineshapes due to conduction band screening, see predictions based on ab initio atomistic thermodynamics 39 . Details of the parameters used for tting the spectra can be found in the SI (Table S2), where a Shirley background is used.
Under OER conditions the species at 62.5 eV becomes the dominant species and no higher binding energy component is found. This suggests surface oxidation has occurred when increasing the bias from 1.0 V to 1.6 V, as shown in Fig. 6A, in agreement with the XAS -TFY operando measurements. With XPS, however, we can rule out the possibility of Ir oxidation states of Ir VI and above since these would appear at higher binding energies than those observed. Thus, taking XPS and XAS together we tentatively assign the species appearing at 62.5 eV to a formally Ir V species bound to µ 1 -O on the (110) surface. While small amounts of the Ir at this high binding energy has also been observed in both ex situ 43 and in situ 44 studies, the surface model system employed in this work allows us to show the formally Ir V species is the dominant surface species under the OER, a key aspect in materials with a high surface to bulk ratio 45 When combined with ab intio simulation, this allows a more de nitive assignment to be made. Moreover, from the simulations we see that at such high formal Ir oxidation states, the µ 1 -O species bound to Ir V may also be referred to as O Iowing to the strong radical character. This electron de ciency on µ 1 -O makes the oxygen susceptible to nucleophilic attack 11,17,42,46 , suggesting the high activity of the Ir NPs is linked to electron hole-enrichment on µ 1 -O.
When the XPS and XAS measurements are compared another important aspect of the active state of the material can be seen, the relative thickness of the electron hole-enrichment beyond Ir IV is con ned to the near surface region, while Ir IV may extend through the bulk. XPS shows surface and near-surface of the NPs are dominated by the formation of active Ir V+ (or O I-) species that are active in the electrocatalytic oxidation of water. These active species contribute ca. ~4% to the overall Ir L 3 white-line intensity observed for the NPs when increasing the bias from 1.0 V to 1.6 V, while the Ir IV bound to µ 3 -O species located in the bulk dominate the ~30% increase in white-line intensity seen when increasing the bias from 0.6 V to 1.0 V. By contrast, for the 20 nm lms, surface oxidation past Ir IV cannot be discerned from the bulk oxidation to Ir IV owing to the overwhelming bulk signal.

Conclusions
In summary, the combination of ab initio calculations, XA and PE spectroscopies and nano-fabrication of thin-lm IrO x and free standing graphene decorated IrO x NP electrodes provided relevant information related to the active sites of iridium-based electrocatalysts in the kinetically sluggish OER. It was found that the electrocatalytic activity of IrO x is ascribed to the formation of formally Ir V species bound to µ 1 -O, where, due to the electron de ciency of these Ir sites the µ 1 -O on the surface of the electrocatalyst transforms into an oxyl that is susceptible to nucleophilic attack by water. Our results show that the potential-dependent oxidation state changes in the IrO x extend through the bulk for oxidation states below Ir V but are constrained to the near-surface region for higher oxidation states, suggesting bulk oxidation is limited. Thus, the higher surface to bulk ratio of nano-structured materials enhances iridium usage and the participation of a signi cant larger amount of surface and near-surface active µ 1 -O oxyls.
In addition we provide evidence using bulk electrolyte for these changes using surface sensitive XPS, which is in good agreement with the bulk sensitive XAS-TFY measurements yielding a direct link between surface and bulk effects in electrocatalytic OER.

Electrode preparation
Graphene was grown by chemical vapor deposition (CVD) on a 20 µm thick Cu foil (Alfa Aesar 99.8%) as catalysts and CH 4 (diluted in Ar and H 2 ) at 1000 °C using an Aixtron BM Pro (4 inch) reactor yielding a continuous polycrystalline lm with grain size in the range of ~20 µm, which was con rmed by scanning electron microscopy. The graphene layer was xed to 500 nm of Poly(methyl methacrylate) PMMA (4 wt.% in anisole, 950k molecular weight) deposited by spin coating. After that, the copper support was eliminated by oating on a 50 mM aqueous solution of (NH 4 ) 2 S 2 O 8 . The graphene/PMMA layer was oated in deionized water (DI-water) and transferred onto another graphene/copper foil and dried at 50 °C for 5 minutes. The resulting sample was oated again in the (NH 4 ) 2 S 2 O 8 solution to remove the copper foil before rinsing in DI-water. The PMMA/BLG layer was transferred to a Norcada ® Si 3 N 4 TEM grid with 500 nm diameter holes or onto a SiN x membrane 100 nm thick for the TFY measurements 47,48 . Finally, the PMMA was eliminated with acetone and the adherence between the BLG and the substrate is due to Van der Waals interaction ensuring stability for the electrodes. It produces a continuous lm that can be used as an electrode in aqueous environments as electrocatalytic applications among others 49,50 . Fig. S1 A and B show the SEM measurements of the free standing graphene on the Si 3 N 4 grid. The in uence of holes is suppressed by the addition of a second graphene layer. Raman spectroscopy was used to check the lattice vibrations of the crystallized BLG which is sensitive in order to determine the graphitic character of this sample. Fig. S1 C shows the Raman spectra of different reference samples such as HOPG, single layer graphene (G), graphene oxide (GO), reduced graphene oxide (RGO), and the transferred BLG used in this study. The origin of the graphitic Raman spectrum is well established and is accepted that it presents three rst order bands between 1000 cm -1 and 3000 cm -1 : D band at ~1350 cm -1 , G band at ~1580 cm -1 with a shoulder at ~1620 cm -1 . 51,52 The D band is associated with defects that perturb the breathing modes of carbon rings. The G band is due to the in-plane phonons at the Brillouin zone centre. The 2D band is due to excitation of two phonons with opposite momentum in the highest optical branch near the K point and is sensitive to the number of graphene layers 53 . The Raman spectrum of HOPG does not show a D band, which attributed to the small number of points defects, and attenuated 2D due to the excitation of two phonons with opposite momentum in the underlying layers. On the other hand, the graphene reference spectrum does not show a D band and the ratio of the 2D and G peak intensities, i.e., I 2D /I G is approximately ~2 indicating a good graphitization and likely the absence of more underlying layers 54 . Graphene oxide (GO) shows a I 2D /I G ratio decreases and the D peak intensity increases, indicating an increase in the defect concentration. This trend in the peak intensity is not due to the presence of oxygen, as the similar behavior of the reduced graphene oxide (RGO) spectrum proves, indicating that the origin of this behavior is the existence of defects in the graphene lattice. The absence of the D band con rms the BLG is of high-quality with a low density of defects in the graphene lattice. In addition, the reduction in the I 2D /I G ratio to approximately ~1.7 suggests the presence of an underlying graphene layer due to the excitation of phonons with opposite momentum, which reduces the intensity of the 2D. In situ electrochemical cell The EC-cell used for hard X-ray measurements is shown in Fig. S2A, where the ow of liquid was assured with a peristaltic micro pump. This cell is based on a 100 nm thick SiN x membrane, transparent to the incoming X-ray photons, possible photon-in/photon-out (PIPO) techniques possible in the hard X-ray regimen, thereby enabling the study of electrochemical reactions with aqueous electrolyte. The SiN x window is 100 nm thick, with an area of 5 mm x 5mm and silicon frame of 1 cm x 1 cm and 200 µm thick 22 .
For the in situ XPS characterization, the liquid ow cell 55,56

Electrolyte preparation
The electrolyte was prepared by diluting 9.8 g of H 2 SO 4 (purity 98%, Alfa Aesar, Massachusetts, USA) in 1 L of Milli-Q water (18.2 MΩ) at room temperature (RT), 25 °C. The electrolyte was continuously saturated by bubbling pure N 2 gas, which minimizes the presence of other dissolved gases in the electrolyte. The electrolyte is acidic with pH~1.

Potentiostat
Potentiometric control was assured with a Biologic SP-300 (Seyssinet-Pariset, France) allowing different potentiometric and amperometric controls. The experiments were performed in the presence of aqueous electrolyte where the measurements were acquired using the electrochemically activated IrO x electrode in presence of 100 mM H 2 SO 4 aqueous electrolyte (pH~1) saturated in N 2 (gas) at the same time that the electrolyte was continuously refreshed by a peristaltic pump (Pt counter electrode and Ag/AgCl reference), which minimizes the possibility to form trapped gas bubbles that can potentially in uence the electrochemical cell performance.

Reference samples
Commercially available iridium powders were purchased from Sigma-Aldrich (99.9% trace metals basis) and AlfaAesar (Premion, 99.99% trace metals basis). In addition, Ir foil was purchased from Mateck (99.99% metals basis). The AlfaAesar powder was used as received and the SigmaAldrich powder was washed in Milli-Q water and calcined at 800 °C in O 2 for 50 hours. The AlfaAesar powder is X-ray amorphous (IrO x ) wile the calcined sample of SigmaAldrich exhibits the rutile-type structure (IrO 2 ). More details of the sample preparation/characterization can be found elsewhere 10 . The Ir foil was subjected to several cycles of sputtering in Ar + atmosphere and annealing in H 2 atmosphere and shows a purely metallic phase.

Calculations
Density functional theory calculations were performed at the PBE level using the Quantum ESPRESSO package 57 using pseudopotentials from the PSLibrary 58 with a kinetic energy (charge density) cutoff of 60 Ry (600 Ry). For bulk rutile-type IrO 2 calculations a (12 × 12 × 12) k-point mesh was used with Marzari-Vanderbilt cold smearing using a 0.01 Ry smearing parameter 59 . The structure for bulk Ir 2 O 5 was found using USPEX 60 with Quantum ESPRESSO. A starting population of 28 individuals was employed with a xed composition of Ir 2 O 5 . The search was allowed to run until the lowest energy structure remained unchanged for 5 generations. The resulting low energy C2/m structure from this search was further optimized with Quantum ESPRESSO using a (6 × 6 × 6) k-point mesh and is shown in Fig. S3A. The structure for bulk IrO 3 was taken from the Materials Project (ID:mp-1097041). 61 The orthorhombic Cmcm structure was further optimized Quantum ESPRESSO using a (6 × 6 × 6) k-point mesh and is shown in Fig. S3B. Rutile-type (110) surfaces were modeled with symmetric 5-layer slabs separated by 20 Å of vacuum using a (6 × 12 × 1) k-point mesh and 0.01 Ry cold smearing. Rutile-type (111) surfaces were modeled with 11-layer slabs separated by 20 Å of vacuum using a (4 × 4 × 1) k-point mesh and 0.01 Ry cold smearing. Surface calculations included spin-polarization. A 7-layer rutile-type IrO 2 (001) surface was also included to access higher Ir surface oxidation states and was computed using a (6 × 6 × 1) kpoint mesh and 0.01 Ry cold smearing with 20 Å of vacuum separating periodic images.
XAS spectra were computed with a resolvent-based Bethe-Salpeter Equation (BSE) approach 62 using the wavefunctions from Quantum ESPRESSO with the core-level BSE solver in the OCEAN package 63 . For these calculations normconserving pseudopotentials were used with a kinetic energy cutoff of 120 Ry. Empty bands were included to up to 200 eV above the Fermi energy (E F ). Other parameters matched the total energy calculations. All XAS spectra were broadened with a 5.2 eV wide Lorentzian to capture the lifetime broadening at the Ir L 3 edge 64 . Spectra were aligned using ΔSCF calculations 65 . The excited state of each absorbing atom was computed separately using a 2p core-hole on the absorbing atom, and the excited electron was included in the simulation. For bulk IrO 2 ΔSCF calculations were performed using a (4 × 4 × 4) supercell, while for the (110) and (111) surfaces (3 × 6) and (3 × 3) supercells were used, respectively. The spectra were aligned to experiment using bulk rutile-type IrO 2 .
XPS spectra were computed using a Hop eld perturbation model 66 . Following reference 67 , the initial density of states (DOS) of the photoexcited atom were taken from the project Ir 5d DOS of the ground state, and the nal density of states were taken from the project Ir 5d DOS of the atom with an Ir 4f corehole using the same supercells as the ΔSCF calculations used for XAS alignment. Constant Gaussian broadening (0.3 eV) was included along with energy dependent Lorentzian broadening. Lorentzian broadening used an empirical Seah-Dench like model 68 , where the energy dependent line width above E F is given by: Γ(E) = Γ 0 + Γ max [1/2+1/π arctan(e-1/e 2 )] with e = (E -E F )/(E c -E F ). The parameters Γ max , and E c were taken as 22 and 8 eV, respectively, while Γ 0 was taken as the tabulated natural line width, 0.3 eV. 69 The N 6 and N 7 edges were broadened separately and the spin-orbit splitting was taken from the free atom value. The Ir 4f shifts were computed by way of ΔSCF calculations using the total energy differences from the ground state and excited calculations. The spectra were aligned to experiment using bulk rutiletype IrO 2 . Figure 1 A SEM image of the sputtered thin-lm IrOx electrode. B TEM characterization of the sputtered IrOx NPs onto the freestanding bi-layer graphene obtained by CVD. CVs of the C thin-lm IrOx electrode and D IrOx NPs in 100 mM H2SO4 with Pt counter and Ag/AgCl reference electrodes, respectively. Schematic detection scheme using A a thin-lm IrOx electrode or B free standing CVD bi-layer graphene electrode decorated with IrOx NPs. Spectra comparison at different potential for the C thin-lm IrOx electrode (20 nm) and free-standing CVD bi-layer graphene electrode decorated with IrOx 2.5 nm NPs.  Schematic detection scheme using A a thin-lm IrOx electrode or B free standing CVD bi-layer graphene electrode decorated with IrOx NPs. Spectra comparison at different potential for the C thin-lm IrOx electrode (20 nm) and free-standing CVD bi-layer graphene electrode decorated with IrOx 2.5 nm NPs.   Operando measurements in 100 mM H2SO4 of the free standing bi-layer graphene decorated with IrOx NPs A Ir 4f B simulated Ir 4f spectra of species identi ed in the Ir L3 edge analysis with the formal oxidation state in brackets.

Figure 6
Operando measurements in 100 mM H2SO4 of the free standing bi-layer graphene decorated with IrOx NPs A Ir 4f B simulated Ir 4f spectra of species identi ed in the Ir L3 edge analysis with the formal oxidation state in brackets.

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