Article pubs.acs.org/JACS
Identifying Key Structural Features of IrOx Water Splitting Catalysts Elena Willinger,†,‡ Cyriac Massué,†,‡ Robert Schlögl,†,‡ and Marc Georg Willinger*,‡,§ †
Max Planck Institute for Chemical Energy Conversion, Mülheim a.d. Ruhr, Germany Department of Inorganic Chemistry, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany § Max Planck Institute of Colloids and Interfaces, Department of Colloid Chemistry, Research Campus Golm, 14424 Potsdam, Germany ‡
S Supporting Information *
ABSTRACT: Hydrogen production by electrocatalytic water splitting will play a key role in the realization of a sustainable energy supply. Owing to their relatively high stability and activity, iridium (hydr)oxides have been identified as the most promising catalysts for the oxidation of water. Comprehensive spectroscopic and theoretical studies on the basis of rutile IrO2 have provided insight about the electronic structure of the active X-ray amorphous phase. However, due to the absence of long-range order and missing information about the local arrangement of structural units, our present understanding of the active phase is very unsatisfying. In this work, using a combination of real-space atomic scale imaging with atomic pair distribution function analysis and local measurements of the electronic structure, we identify key structural motifs that are associated with high water splitting activity. Comparison of two X-ray amorphous phases with distinctively different electrocatalytic performance reveals that high activity is linked to the ratio between corner- and edge-sharing IrO6 octahedra. We show that the active and stable phase consists of single unit cell sized hollandite-like structural domains that are cross-linked through undercoordinated oxygen/iridium atoms. In the less active phase, the presence of the rutile phase structural motif results in a faster structural collapse and deactivation. The presented results provide insight into the structure−activity relationship and promote a rational synthesis of X-ray amorphous IrOx hydroxides that contain a favorable arrangement of structural units for improved performance in catalytic water splitting.
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OER. 19 Indeed, differently synthesized amorphous IrO x hydroxides show different catalytic activity.19 In order to reveal the underlying catalytic function, the relevant local structural arrangement has to be identified. Activity is generally associated with the presence of high-energy sites, where the local structure differs from the thermodynamically favorable low-energy configuration.20 In the case of crystals, high-energy sites can be identified through deviations from crystalline order using direct, real-space observation methods such as electron microscopy. However, in the case of amorphous materials with similar overall composition but different catalytic performance, the identification of relevant structural motifs requires a different approach. Here we provide a detailed comparative electron microscopy study based on two IrOx hydroxide catalysts with different catalytic properties. One of them shows outstanding OER activity and stability, while the other one is a commercial benchmark IrOx with a much lower catalytic performance (Figure S1). Using low-dose imaging, we are able to provide atomistic insight into the local structure of electron beam
INTRODUCTION Efficient conversion of renewable energy to chemical fuels plays an important role in the realization of a sustainable, carbon neutral energy supply.1 Hydrogen, which will be required in large quantities as a key ingredient of chemical fuels, can be produced by electrochemical water splitting using electricity from renewable sources.2 Although the reaction has been known for many years,3 the large-scale application of water electrolyzers is seriously inhibited by the loss of catalytic efficiency at the anode side, where the oxygen evolution reaction (OER) takes place. The anode catalyst works under very harsh acidic conditions and high overpotentials, which give rise to catalyst corrosion. Facing this challenge, scientists are focusing on developing improved anode catalysts.4 Since the “X-ray amorphous” hydrous form of iridium was identified as a key ingredient of well-performing electrodes, a lot of efforts have been devoted to resolving the structure−property relationship.5−16 The coexistence of more than one iridium oxidation state in active amorphous IrOx hydroxide catalyst was revealed on the basis of in situ XAS and XPS studies.17,18 Overall, it was shown that the amount of hydroxyl groups, the average oxidation state of Ir, and the BET surface area have a strong influence on the performance of IrOx hydroxides in © 2017 American Chemical Society
Received: July 7, 2017 Published: August 9, 2017 12093
DOI: 10.1021/jacs.7b07079 J. Am. Chem. Soc. 2017, 139, 12093−12101
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Figure 1. Integral characterization of the two investigated IrOx hydroxides: (a) XRD pattern of IrOx-FHI (green) and IrOx-commercial (blue); (b, c) SEM images of IrOx-commercial and IrOx-FHI catalysts, respectively.
sensitive iridium hydroxides for the first time. By combining results of high-angle annular dark field scanning transmission electron microscopy (HAADF STEM) with electron pair distribution function (ePDF) analysis, we are able to identify key structural features that are relevant for a good catalytic performance in water splitting. Consequences of different short-range ordering on the electronic structure are revealed by electron energy loss spectrometry (EELS) and related to the electronic structure of reference phases. On the basis of the generated insights, we propose a mechanism involving switching between corner- and edge-sharing units in the red− ox process that is involved in the water splitting reaction.
higher OER activity. However, our previous studies showed that IrOx-FHI catalysts synthesized with very low BET surface areas (7−26 m2/g) still had a better OER performance than the IrOx-commercial catalyst.19 This indicates that the structure of the catalyst is more relevant for good catalytic performance than the surface area. In order to reveal further details about a structural difference between the two “X-ray amorphous” phases, atomic scale imaging was performed using low-dose HAADF STEM. HAADF STEM Analysis. Due to the high sensitivity of the hydroxides against electron beam irradiation, careful studies of electron dose effects were required in order to identify low-dose conditions that allow artifact free imaging. Figure 2 shows a comparison of low-dose HAADF images recorded from IrOx-FHI and IrOx-commercial catalysts,
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STRUCTURAL ANALYSIS General Characterization of IrOx Catalysts Using XRD and SEM. Figure 1a shows a comparison of the XRD pattern recorded from the benchmark IrOx hydroxide from Alfa Aesar (IrOx-commercial) and an IrOx hydroxide synthesized at the Department of Inorganic Chemistry of the Fritz Haber Institute (IrOx-FHI). The sharp diffraction maxima of the commercial sample are due to metallic Ir impurities. According to quantitative XRD analysis, the content of metallic Ir is below 2 wt %.18 The XRD pattern of the IrOx-FHI shows the presence of a barely visible intensity due to a trace amount of Ir metallic phase as well. Since metallic Ir is known to be inactive in OER, it can be considered as an irrelevant impurity phase in the following discussion. The OER active IrOx hydroxide phase of both catalysts gives rise to broad diffuse halos, indicating their “X-ray amorphous” structure. The diffuse halos are very similar, and besides the metallic impurities, the XRD analysis does not reveal any significant difference between the two hydroxides. A first indication for a difference between the two samples is provided by secondary electron microscopy (SEM). Recorded images such as those shown in Figure 1b and c clearly reveal different morphologies of particle aggregates in the two samples. While the commercial material shows compact, rodlike aggregates, the FHI sample consists of thin platelet- or needle-like aggregates and a minor portion of spherical ones. The morphological difference of aggregates is confirmed by BET measurements, which shows that the surface area measures ∼30 m2/g for the commercial IrOx catalyst and about 100 m2/g for the IrOx-FHI sample. The higher BET surface area of the IrOx-FHI sample could be a reason for
Figure 2. HAADF STEM images of the IrOx-commercial catalyst (a, c) and the IrOx-FHI sample (b, d). Images c and d were Fourier filtered in order to reduce the noise in the original images (see the Supporting Information for the original images (Figure S2)). 12094
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Figure 3. Comparison of tunnel-like structural types with iridium clusters revealed by HAADF.
form a coherent intergrowth with romanechite, which is a [3 × 2] tunnel structure.23 Figure 3 shows representations of the rutile, hollandite, and romanechite structures viewed along the [001] direction together with examples of observed Ir clusters. Despite the absence of a long-range order, some of the Ir clusters observed in the HAADF image show similarities in terms of the size and arrangement of Ir atoms with structural features associated with the tunnels of the hollandite and romanechite structures. However, the amount of K found in the IrOx-FHI is much lower than that in hollandite. No K is present in the IrOxcommercial sample. According to the literature, tunnels in the α-MnO2 hollandite structure are only partially occupied by K.24 Besides K, the channels can contain alternative mono- or divalent cations or water in the form of hydroxonium H3O+ ions.25,26 The presence of cations in the tunnels is required to stabilize the mixed 3+ and 4+ oxidation states of the octahedrally coordinated cations in the hollandite structure. The latter is of relevance to the structures investigated here, since a mixed oxidation state between 3+ and 4+ was found for both IrOx hydroxides.18,19 In the case of a strongly disordered structure, tunnels are truncated to pores. The average size of homogeneously dispersed pores is of the same scale as the tunnels in the hollandite and romanechite structures. HAADF imaging thus indicates similarities in the short-range ordering. The presence of tunnel-like motifs in the disordered hydroxides might facilitate the transport and exchange of relevant species and play a role in the formation of a high catalytic activity. Despite the apparent similarity between the isolated, oriented clusters in the HAADF images and the tunnel-like structural motifs in the crystalline iridium hydroxides, it is impossible to conclude on the degree of structural similarity and the characteristic atomic arrangements on the basis of 2D images. In order to deduce more quantitative 3D structural information, ePDF analysis was performed. ePDF Analysis. Structural characterization of amorphous materials can be carried out using the pair-distribution function (PDF).27,28 The PDF G(r) represents the probability of finding a pair of atoms with an interatomic distance r, weighted by the scattering power of the individual atoms. The PDF is thus very sensitive to the local structural arrangement. The reduced pair distribution function, G(r), is obtained by the Fourier transform of powder diffraction data, according to27
respectively. Contrast in HAADF images is due to high-angle Rutherford scattering between beam electrons and the Ir (Z = 77), O (Z = 16), and H (Z = 1). Due to the low scattering power of hydrogen and oxygen and different angular distribution of the scattered electrons, only iridium atoms can be observed as bright spots in images generated with the HAADF detector. The observed variations in brightness are caused by local thickness and density variations, indicating porosity and the presence of regions that are enriched with hydroxyl groups (see red arrows in Figure 2a,b). These nanoscale pores are ∼1 nm in size and homogeneously distributed in both samples, but they are characteristically more abundant in the FHI sample. At first glance, the high-resolution HAADF images exhibit randomly distributed iridium atoms for both catalysts, confirming their structural disorder and XRD amorphous nature. However, more detailed inspection of the HAADF data reveals that both samples contain well-defined iridium clusters. Some of them are highlighted by circled regions in Figure 2c and d. They appear as a single (∼7 Å) or double (aspect ratio 10/7 Å) cluster of Ir structural units. A summary of observed Ir structural units is shown in Figure S3. Some of the Ir clusters contain a localized bright contrast in their center. Judging by the contrast, the central feature could be due to Ir atoms localized above, below, or even inside the ring structure. Clusters without the bright contrast in the center are generally found at the edge of the sample or in very thin regions. It is thus likely that the central contrast in the 2D projection is caused by Ir atoms that are located either above or below the ring structure. In order to determine whether the observed Ir clusters are related to known crystalline iridium oxides, a crystallographic analysis with comparison of structural motifs was performed. Crystallographic Insight into Ir Clusters. According to quantitative energy-dispersive X-ray spectroscopy (EDX) performed during TEM investigations (Figure S4), the ratio of O/Ir is about 2 for both IrOx hydroxides. Besides Ir and O, the IrOx-FHI sample contains trace amounts of K, which was used during the synthesis.19 There are two known iridium oxide structures that contain Ir and O in a 2/1 ratio: IrO2-rutile and K0.25IrO2-hollandite.21,22 Both structures are built of [IrO6] octahedra but differ in the way in which the octahedra are interconnected through corner- and edge-sharing. A characteristic feature of the resulting structure is the presence of channels or tunnels with different dimensionalities. The rutile phase can thus be described as a [1 × 1] tunnel structure and the hollandite as a [2 × 2] tunnel structure (see Figure 3). High-resolution TEM studies on similarly structured α-MnO2 (hollandite-type) showed that the hollandite tunnels frequently
G (r ) =
2 π
∫Q
Q max
Q [S(Q ) − 1] sin(Qr ) dQ
min
where Q is the scattering vector and S(Q) is the structure function that is experimentally obtained by powder diffraction measurements. 12095
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Figure 4. SAED patterns and corresponding ePDFs of IrO2-rutile (red), IrOx-FHI (green), and IrOx-commercial (blue) samples.
is broader in the case of both hydroxides. Furthermore, its position is shifted to higher r distances. The second PDF peak at 3.14 Å of the rutile structure is due to the edge-sharing Ir−Ir pairs. A similar peak position is observed for the IrOxcommercial sample. In the case of the IrOx-FHI sample, the peak is shifted to lower r distances (3.07 Å) and its intensity is higher compared to the commercial sample. The third PDF peak of the rutile structure at 3.55 Å is due to corner-sharing Ir−Ir pairs. The corresponding peak is broadened and shifted toward higher r for both hydroxides. The observed deviation of PDF peak positions and their significant broadening for both hydroxides can be ascribed to the presence of water in these samples and mixed Ir oxidation state (Ir3+, Ir4+), which in turn results in a broader dispersion of Ir−O and Ir−Ir distances. Furthermore, the observed different intensity ratio of the edgeand corner-sharing Ir−Ir peaks for the two hydroxides indicates different numbers of contributing Ir−Ir pairs in the two different hydroxides. While IrOx-FHI shows comparable intensities of peaks associated to corner- and edge-sharing, the IrOx-commercial shows a dominant fraction of cornershared Ir−Ir pairs. In the case of crystalline iridium oxide phases, it is known that the IrO2-rutile structure has a dominant fraction of the corner-sharing Ir−Ir octahedra with a coordination number of 8 and only two pairs of edge-sharing Ir−Ir octahedra, while the hollandite phase contains the same number, namely, 4, of both pair types.21,22 In order to illustrate how this difference influences the resulting PDF functions, we used PDFgui36 to calculate xPDFs of the IrO2-rutile and K0.25IrO2-hollandite structures with cluster sizes of 7 Å (Figure S5). A clear difference in the respective corner- and edge-shared Ir−Ir peak intensities can be seen in these two reference phases. In addition, the Ir−Ir edge-sharing peak position in hollandite is shifted to lower r distances (3.1 Å) compared to the rutile phase (3.14 Å). This can be explained by the fact that the hollandite structure contains two edge-sharing Ir−Ir pairs in the (ab) plane with 3.07 Å interatomic distance and two additional Ir−Ir pairs perpendicular to this plane at a distance of 3.14 Å. This deviation of Ir−Ir interatomic distances is linked to the presence of a mixed Ir oxidation state of Ir3+ and Ir4+ in the hollandite structure. This, in turn, results in a higher anisotropy and dispersion of Ir−O distances and, correspondingly, an
Traditionally, this method was applied on data generated at X-ray synchrotron and neutron sources. In recent years, however, studies have demonstrated that PDFs obtained from electron diffraction (ePDF) can successfully be applied for quantitative structural characterization of nanoparticulated samples.29−31 Here, we use the ePDF technique to abstract information about the local and intermediate-range order in disordered IrOx hydroxides. Figure 4 shows the electron diffraction pattern and corresponding ePDFs of a reference, nanocrystalline IrO2-rutile (red curve) sample together with the diffraction and ePDF data obtained from IrOx-commercial (blue curve) and IrOx-FHI (green) samples. A comparison of the IrO2-rutile PDF with that of the two X-ray amorphous IrOx hydroxides clearly shows a similar short-range order. However, while the G(r) of the nanocrystalline rutile exhibits well defined peaks across the shown region, the pair correlation peaks of both hydroxides start to fade very fast with increasing r. The latter is a consequence of the small size of the respective coherent domains, which is in the range of ∼7−8 Å32 and thus in agreement with the dimensions of the distinct Ir clusters observed by HAADF STEM. The local-range order of the rutile phase and the two hydroxides can be described by the first three discrete sharp PDF peaks up to ∼4 Å. The pair correlations at higher r represent the intermediate-range order of the samples. Despite the fact that the associated PDF peaks are strongly damped for both hydroxides, they are still present. In the case of truly amorphous materials, all pair correlations above the first shells disappear due to severe broadening. The presence of peaks in the intermediate range thus points out that both are not truly amorphous but composed of very tiny coherent domains. The same conclusions were drawn for Ru-, Co-, and Mn-based hydroxides on the basis of X-ray PDF (xPDF) and extended X-ray absorption fine structure (EXAFS) studies.33−35 The position and full width at half-maximum of peaks are structural descriptors that can easily be abstracted from the PDF function. The peak width contains information about the dispersion of the respective interatomic distance, while its integral intensity provides information about the number of atomic pairs. The first peak of the rutile phase at 2.0 Å is due to Ir−O pairs within IrO6 octahedra. One can see that the width of this peak 12096
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Figure 5. ePDF refinement. (a) Comparison between the experimental G(r) of IrOx-commercial (blue line) and the calculated G(r) (red line). (b) Comparison between the experimental G(r) of IrOx-FHI (green line) and the calculated G(r) (black line). The difference G(r) is shown as a dark gray line.
The resulting fit shows that the size of coherent hollandite clusters in both hydroxides is approximately 10 Å and contains some deficiency of Ir positions. Additionally, the obtained values of Ir Debye−Waller factors along the c direction are about 1 order of magnitude higher than that of the published hollandite structure, indicating a high degree of structural distortions along the double hollandite chains. Overall, the ePDF modeling of the two hydroxide samples reveals their distinct structural difference. It shows that the ratio between corner- and edge-sharing IrO6 units in the amorphous IrOx-FHI is the same as that in crystalline hollandite. This indicates that not only the identified coherent domains but also the interconnection between hollandite-like structural domains follows the same connecting scheme. Due to the small size of the domains, the structural framework between them makes up a significant portion of the volume and would otherwise alter the overall ratio of edge- to corner-sharing units. The presence of a thermodynamically stable and more rigid rutile structural motif with monovalent Ir4+ is most likely the key reason for the lower OER performance of the commercial sample. Electronic Structure (EELS Analysis). As mentioned above, IrOx hydroxides contain Ir cations in a mixed oxidation state of 3+ and 4+. As a consequence, the average Ir oxidation state varies for different synthesis routes. In order to obtain information about the Ir oxidation state in the IrOx-FHI and IrOx-commercial catalysts, EELS measurements were performed. Since the binding energy of electrons is influenced
increased width and a slightly higher r position of the corresponding xPDF peak in hollandite (Figure S5). A comparison of the calculated xPDFs with the experimental ePDFs of the two iridium hydroxides is shown in Figure S6. A clear similarity between the calculated xPDF of nanostructured hollandite and the experimental ePDF of the IrOx-FHI sample is observed in terms of the Ir−Ir pair peak ratio (Figure S6a). In contrast, the experimental ePDF of the IrOx-commercial sample does not show a high correspondence with either the xPDF of the hollandite or the rutile structures and most probably comprises a mixture of both phases. In order to abstract further structural details, modeling of the experimental ePDFs was performed on the basis of the abovediscussed reference phases using the PDFgui software (for more details, see the Experimental Section). As shown in Figure 5, the hollandite structural motif describes very well the ePDF of the IrOx-FHI sample with a quite good “goodness-of fit” parameter, Rw ∼ 0.21. Compared to the IrOx-FHI sample, the ePDF refinement of the commercial hydroxides shows the presence of both hollandite and rutile structural motifs at a ratio of 80 to ∼20 wt %, respectively, at a similar “goodness-of-fit” parameter of Rw ∼ 0.18. It is worth noting that the hollandite and rutile structures have very different unit cell volumes of ∼318 and 64 Å3, respectively. Expressed in terms of unit cells, this ratio corresponds to 60% hollandite and 40% rutile unit cells. This, in turn, indicates that nearly each hollandite cluster in the IrOxcommercial sample is adjoined by the rutile structural motif. 12097
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Figure 6. EELS spectra of IrCl3 (magenta), IrO2-rutile (red), K0.25IrO2-hollandite (black), IrOx-commercial (blue), and IrOx-FHI (green) samples. (a) EELS spectra of the Ir O2,3-edge (the 2d derivative of the Ir O2,3-edge is shown as an inset). (b) EELS spectra of the Ir oxygen K-edge.
refinements, the IrO6 coordination is getting even more distorted in the case of IrOx hydroxides with nonequivalent Ir−O bonds. The near-edge fine structure of the Ir O2,3-edge is also sensitive to changes in the local IrO6 coordination. It is known for oxide materials with different degrees of octahedral distortions that the pre-edge feature in metal K- and L-edges (EELS or XANES spectroscopy) appears and grows in intensity with decreasing metal−oxygen octahedral symmetry.45,46 Thus, the observed pre-edge feature in the Ir O2,3-edge of the hollandite and both IrOx hydroxides might be attributed to their higher octahedral distortion. Figure 6b shows oxygen K-edge EELS spectra recorded from the hydroxides together with reference spectra of rutile and hollandite. The oxygen K-edge exhibits two pronounced peaks which can be assigned to transitions into π*- and σ*antibonding states that are formed between the metal and oxygen ligands. One can clearly see that the spectra of rutile and hollandite show a substantial difference in the intensity ratio between π*- and σ*-peaks. The lower intensity of the π*feature in hollandite can be ascribed to the presence of Ir3+ and Ir4+ species. It results in a higher occupancy of π*-antibonding states compared to the case of rutile, which only contains Ir4+ ions. The reduced oxidation state leads to a reduction of available final states for the transition measured by EELS. Since the oxygen K-edges of the iridium hydroxides show a π*/σ*peak ratio similar to hollandite, we assume the presence of Ir3+ and Ir4+ species. Additional intensity is observed at the low-energy side of the π*-feature of the hydroxides. NEXAFS studies by Pfeifer et al. have shown that this low-energy component is directly related to the high OER catalytic activity of IrOx hydroxides.17,18 For mixed transition metal oxides, it was shown that the presence of two different metals with completely and partially filled π*antibonding states can give rise to similar low-energy components in the O K-edge.44,47 Disorder and structural inhomogeneity combined with the presence of pores and associated surface terminations provide additional channels for excitations and, thus, intensity in the pre-edge region. Saturation of terminal bonds, either between interconnected domains or at surfaces, most likely occurs via hydroxyl groups. They give rise to a local reduction of the Ir oxidation state. Since it was reported that dehydration of iridium oxohydroxide leads to partial deactivation,48 we decided to monitor the effect of dehydration in situ in the TEM.49
by the oxidation state and chemical environment of the respective atom, the energetic onset of core-level ionization edges recorded in X-ray absorption (XANES/NEXAFS) or EELS can be used to estimate the oxidation state.37,38 The threshold energy of transition metal K edges can shift to higher energy with increasing valence if the core-level shift due to the reduced screening of the nuclei’s positive Coulomb potential is higher than the change in the position of the Fermi level. A close to linear relationship between oxidation state and edge position was found for nickel and vanadium, with shifts of, respectively, 1.5 or 2.5 eV per unit increase in valence.39,40 Assuming a similar linear dependence in the case of Ir, the position of the edge can be used to estimate the oxidation state. This was already demonstrated for the Ir-L3 edge at around 11220 eV using XANES.38,41,42 However, since the energetic position of the Ir-L3 edge is beyond the accessible range of the EELS spectrometer, we concentrate on the Ir O2,3-edge caused by 5p → 5d, 6s electron transitions. Although the initial state is different, it contains similar information about the valence state. For reference, O2,3 edges were also recorded from compounds containing iridium at different oxidation states, namely, IrCl3 (Ir3+, magenta spectrum), K0.25IrO2 (Ir3.75+, black spectrum), and IrO2 (Ir4+, red spectrum). The observed energy shifts nicely correlate with the increasing oxidation state (Figure 5a). Maxima in the second derivative were used as a reference point for the measurement of the energy shifts (inset in Figure 5a). Using the simplified picture according to which the energetic position of the edge is a linear function of the oxidation state, we derive an average oxidation state of ∼3.5+ for the IrOx-FHI and ∼3.6+ for the IrOx-commercial catalysts (Figure S7). The values are in close agreement with the values abstracted from the temperature programmed reduction data (Ir3.51+Ox-FHI and Ir3.62+Ox-commercial).18,19 Furthermore, a comparative analysis of the fine structure of the Ir O2,3-edge shows the presence of a low energy feature, which is labeled as “a” in Figure 6a. It is located slightly below 49 eV and found in the spectra of both hydroxides and in the one of hollandite. Recently, it was reported that OER activity is related to the IrO6 octahedral distortion. The distortion leads to a broadening of the Ir 5d band and a stronger orbital overlap with O 2p states, which influences the oxygen adsorption energy and further affects catalytic activity.43,44 The K0.25IrO2hollandite structure has a higher anisotropy of Ir−O bond lengths compared to rutile IrO2. According to the ePDF 12098
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Journal of the American Chemical Society Therefore, energy-loss spectra of the O K- and Ir O2,3-edge were measured at elevated current densities. Figure S8a shows the O K-edge of the IrOx-commercial sample. It is evident that the low-energy π* component vanishes upon exposure to higher e-beam intensity. In order to correlate beam-induced changes observed in the oxygen K-edge fine structure with underlying structural and compositional changes, electron diffraction patterns were recorded from the exposed areas and the corresponding ePDF generated (Figure S8b). Two pronounced structural modifications were detected. First, the appearance of metallic Ir and, second, a shift of ePDF peaks to lower r distances. The latter is a consequence of the hydrogen depletion, which is accompanied by an increase of the Ir oxidation state. Indeed, EELS analysis of the Ir O2,3-edge spectra confirms that the Ir oxidation state slightly increased from ∼3.6+ to ∼3.75+ due to e-beam induced reduction (Figure S8c,d). The simultaneous formation of metallic Ir indicates that e-beam induced hydrogen depletion gives rise to a disproportionation of the original hydroxide. Some fraction of the iridium hydroxide undergoes a complete reduction to metallic Ir, while, in other portions, the local structure stays intact but shrinks slightly due to an increased oxidation state of Ir. HRTEM images recorded at higher e-beam intensity confirm the formation of metallic Ir at the surface, while the disordered structure is preserved in the inner part of the exposed region (Figure S9). Overall, the EELS study provides valuable insight about the electronic structure of IrOx hydroxides and shows a good agreement with the structural model revealed by ePDF analysis. In addition, the observation of electron beam induced changes confirms the important role of the surface terminating hydroxyl groups in the formation of the pre-edge features of the oxygen K-edge, which have been associated with high catalytic activity.
Figure 7. Schematic representation of a redox process of IrOx hydroxides.
the rotation does not require a concerted movement of extended Ir chains. In order to confirm the involvement of such a mechanism, operando TEM investigations under simulated redox conditions should be conducted. Compared to the flexible hollandite-like framework, the presence of the thermodynamically more stable rutile structural motifs results in a faster structural collapse and deactivation. In addition, the residual presence of K ions in the IrOx-FHI catalyst might play an important role in catalytic stability, as it stabilizes the hollandite-like open framework structure. Indeed, it is known for other isostructural compounds, such as MnO2, that the hollandite structure shows exceptional catalytic performance and can be stabilized only with certain sizes of cations inside the large tunnels.50,51 It is important to take into account that the present structural and spectroscopic analysis was performed in the vacuum of a TEM column. The actually active phase is immersed in an electrolyte and attached to an electron collector. How well can the vacuum description then fit to the actually active state? A comparison of the presented EELS spectra of this work with NEXAFS data recorded from electrochemically oxidized metallic Ir under in situ conditions confirms the close agreement in the electronic structure of the active phases.52 Furthermore, operando EXAFS studies of IrO2 and Mn oxides (i.e., layered and tunnel structured) did not show a change of the overall structural motif under reaction conditions.6,53 However, an increase of the average oxidation state was observed under applied potential, which could be explained by the proposed “paddle-wheel”-like redox mechanism. Overall, the presented results promote a rational synthesis of IrOx hydroxides that contain a favorable arrangement of structural units for improved performance in catalytic water splitting.
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DISCUSSION AND CONCLUSION On the basis of a detailed comparative investigation of the atomic arrangement in two different X-ray amorphous iridium hydroxides, we have shown that the hollandite-like structural motif is related to high catalytic activity and stability in the oxygen evolution reaction. The less active and unstable reference catalyst contains, besides hollandite-like structural units, a significant portion of thermodynamically more stable rutile units. The finding reflects the results of a comparative study on crystalline IrO2-rutile and K0.25IrO2-hollandite phases according to which hollandite shows a higher water splitting activity.41 On the basis of previous studies on the IrO6 octahedral distortion and its correlation with water splitting activity, we assume that the higher octahedral distortion of the hollandite structural motif is beneficial for catalytic activity (for further detail, see Supporting Information Figure S10). Furthermore, we find that high water splitting activity is linked to the interconnected hollandite-like structural domains which are, in the absence of long-range order, cross-linked through undercoordinated oxygen/iridium atoms. The active structural motif contains a ratio between corner- and edgesharing units that is similar to the one of hollandite. The mixed Ir oxidation state in the open three-dimensional network suggests a redox mechanism, in which the changing from corner- to edge-sharing, and vice versa, provides a path for release and uptake of oxygen atoms. The process could be similar to the so-called “paddle-wheel” mechanism and involve a coupled rotational motion of structural IrO6 units such as that indicated in Figure 7. Due to the absence of long-range order,
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EXPERIMENTAL SECTION
Transmission Electron Microscopy (TEM). The local chemical composition, microstructure, and electronic structure of the samples were investigated by analytical electron microscopy using a double Cscorrected JEOL JEM-ARM200CF scanning transmission electron microscope that was operated at 200 kV. The microscope is equipped with a high angle Silicon Drift EDX detector with a solid angle of 0.98 sterradians from a detection area of 100 mm2. For electron microscopy, samples were prepared by drop deposition of iridium hydroxide dispersed in isopropanol onto a copper grid with a holey carbon support film. All TEM images were measured at low-dose illumination conditions with current densities in the range of 0.3 pA/cm2 measured on the small viewing screen. 12099
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Journal of the American Chemical Society HAADF STEM, FFT. Low electron beam exposure is crucial in order to obtain data without detectable electron beam induced artifacts. One of the consequences is the low signal-to-noise-ratio of acquired images. Fourier-filtering was thus applied to reduce the noise level. A comparison of original and filtered data is provided in Figure S2. EELS spectra were acquired in diffraction mode with 0.05 eV/ channel dispersion using a 2.5 mm spectrometer entrance aperture and a camera length of 30 cm. To obtain accurate threshold energies for ionization edges, the dual EELS mode was used. The energy resolution according to the full width at half-maximum measured at the zero-loss was about 0.55 eV at an emission current of 2−3 μA. All electron diffraction patterns were recorded on a Gatan Orius CCD instrument. Integration of two-dimensional electron diffraction was done using the freely available RDFtools package for the GATAN Digital micrograph software.54 Subsequent conversion of the 1D diffraction pattern into the reduced pair-distribution function G(r) was done using the SUePDF software.55 Structural refinement of the obtained ePDFs was carried out using the PDFgui software.36 For modeling, we followed the procedure described for disordered nanocrystalline materials.56 Although this software was written for modeling of PDFs obtained from X-ray and neutron diffraction, it can also be used for PDFs obtained from electron diffraction, as recently shown by Abeykoon et al.57 The camera length calibration and instrument resolution parameter Qdamp were determined using a nanocrystalline SnO2 sample with a narrow particle size dispersion (around 7 nm) as calibration. For ePDF modeling of disordered IrOx hydroxide samples, the instrument resolution parameter, Qdamp, was fixed to the value refined from the calibration SnO2 sample. The camera length was adjusted to reach a reasonable Q value of ∼20 Å−1. X-ray Diffraction (XRD). XRD data were measured at room temperature (RT) by using a Bruker AXS D8 Advance θ/θ diffractometer employed in Bragg−Brentano geometry using Nifiltered Cu Kα radiation and a position sensitive LynxEye silicon strip detector. All diffraction patterns were measured with a step size of 0.0197° and exposure time of 120 s per step. Scanning Electron Microscopy (SEM). SEM images were acquired with an FIB FEI G3 secondary electron microscope operated at 3 kV. Samples were prepared by drop deposition of iridium hydroxide dispersed in isopropanol onto a SEM aluminum holder preliminarily coated with carbon double side adhesive tape. This sample preparation procedure was applied to avoid big agglomerates of IrOx particles. Surface Area Determination. Surface area determination was carried out in a volumetric N2-physisorption setup (Autosorb-6-B, Quantachrome) at the temperature of liquid nitrogen. The sample was degassed in a dynamic vacuum at a temperature of 80 °C for 2 h prior to adsorption. Full adsorption and desorption isotherms were measured. The linear range of the adsorption isotherm (P/P0 = 0.05−0.3) was considered to calculate the specific surface area according to the BET method. Sample Preparation. The IrOx-commercial sample was synthesized by Alfa Aesar company, and the IrOx-FHI hydroxide was synthesized by microwave-assisted hydrothermal synthesis. Aqueous KOH solutions were prepared from Milli-Q filtered water and KOH (AppliChem, p.a.). The solutions were kept under an inert atmosphere via constant Ar bubbling. K2IrCl6 (Alfa Aesar, Ir 39% min.) was then added in order to obtain a final Ir concentration of 10−2 mol/L and left stirring at RT for 1 h. On the basis of the above protocol, the precursor solution has a KOH:Ir ratio of 5:1. After 1 h of aging, 4 × 62 mL of a precursor solution was added to four 100 mL PTFE-lined vessels. The four vessels were then placed inside the microwave reactor (Anton Paar, Multiwave PRO). The solutions were heated up using a 10 K/ min ramp from RT to a selected temperature of 250 °C, under constant stirring using a magnetic PTFE stirrer and maintained at the selected temperature for 1 h. After the treatment, the vessels were left to cool down to RT. The resulting black product was centrifuged at 8000 rpm for 10 min, further dissolved in Millipore-filtered water,
sonicated for 5 min, and recentrifuged until the conductivity of the supernatant was measured below 0.05 mS/cm. The solid product was subsequently dried at 80 °C for 12 h and ground in a mortar.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b07079. Linear sweep voltammetry profiles, HAADF STEM images, high resolution transmission electron microscopy (HRTEM) images, PDF analysis, and EELS and EDX analysis (PDF)
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AUTHOR INFORMATION
Corresponding Author
*
[email protected] ORCID
Cyriac Massué: 0000-0002-1575-5057 Marc Georg Willinger: 0000-0002-9996-7953 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We would like to acknowledge financial support by the Max Planck Society and MaxNet Energy research initiative. REFERENCES
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