Figure 1 shows a sketch that summarizes the main measurements performed in the present study using APXPS ex situ and in situ and an electrochemical three electrode cell (WE: working electrode, CE: counter electrode, RE: reference electrode). The electrochemical protocol, carried out in the three electrode cell with the WE fully immersed in the electrolyte solution, consisted of a stepwise potential increase up to 1.65 V vs. RHE and then of 50 cyclic voltammograms (CVs) performed between 1 and 1.65 V vs. RHE at 100 mV/s. Ex situ measurements were carried out by pulling the WE completely out of the solution and using soft X-rays after rinsing in pure water. In this case, the electrolyte must be removed because low kinetic energy photoelectrons cannot be measured in a background pressure higher than 20 mbar. In situ measurements were carried out using tender X-rays and the so called the “dip and pull” method.17, 18, 19, 20 FIGURE 1: Schematics of the experiment. During the initial step, the sample fully dipped in the electrolyte and the electrochemical measurement are done. Once those measurements are done, in the case of the in situ experiment (tender X-rays) we pull the sample directly in the measurement position while holding the potential and proceed with XPS measurements probing through the thin electrolyte. In the case of the ex situ experiment (soft X-rays), we pull out the sample, put it in storage under a partial pressure of 26 mbar and remove the electrolyte beaker before reintroducing the sample and bringing it to XPS measurement position under a partial pressure of 1 mbar.
In this latter case, APXPS measurements could be performed under an applied potential with CE and RE fully immersed in the electrolyte and the WE partially immersed in the electrolyte with the upper part (measured spot) only covered by a thin layer of electrolyte able to maintain electrical contact. High kinetic energy photoelectrons generated by tender X-rays can be measured at background pressures higher than 20 mbar and contain information about the interface of the WE buried under the thin layer of electrolyte. The detailed experimental approach is described in the experimental methods. As shown in Fig. 1, LSCO increases its capacitive current (in the potential region between 1 and ~ 1.65 V vs. RHE) over the 50 cycles, while slightly decreasing its OER current (above ~ 1.65 V vs. RHE). This suggests that the sample surface undergoes some modifications during the CVs. To unveil such (electro)chemically driven surface modifications, the electrode has been investigated in situ and ex situ by means of APXPS.
As shown in Fig. 2a, initially the Sr 3d core level spectrum acquired in situ shows two features that can be deconvoluted in two doublets (spin orbit splitting of 1.8 eV and branching ratio of 1.5).21 The first one, at a binding energy of 133.3 eV (Sr 3d5/2, orange color) corresponds to surface-segregated strontium species. The second one, centered at 131.5 eV (Sr 3d5/2, green color) is ascribed to bulk-like (lattice) strontium.16, 22, 23 Using a higher photon excitation energy, a second core-level peak of strontium, Sr 2p, was acquired and is shown in Figure S2a. In this case, two components, corresponding to surface and bulk strontium, can be detected. Being the KEs of Sr 2p and Sr 3d in Figure S2a are approximately 3050 and 2160 eV (corresponding to 8.8 nm and 6.6 nm attenuation length), respectively, the former contains information from a larger depth. The relative intensity of the surface component in the case of Sr 2p (Srsurf/SrBulk=0.27) is lower than that calculated from Sr 3d (0.43), supporting the fact that such strontium compounds segregate at the surface. The Sr 3d photoemission signal acquired ex situ, shown in Figure S2b, was acquired with a KE of approximately 760 eV. This corresponds to 3 nm attenuation length and allows focusing the analysis on segregated strontium, whose relative intensity is maximal (Srsurf/SrBulk=2.45) among all the spectra shown in Figure S2. Upon immersion of the sample in the electrolyte solution and application of a potential of 1.65 V vs RHE, the signal of Sr 2p acquired in situ (Figure S2a) displays a clear evolution. A single peak (light green color), centered at 1938.8 eV, is present. The same line shape evolution is detected ex situ with soft X-rays (Figure S2b): a single doublet (Sr 3d5/2 at 133.0 eV, light green) is used to fit the spectrum collected after electrochemical measurements. It is important to highlight that the binding energy of the new doublet matches neither that of surface nor that of bulk strontium. This means that the electronic state of strontium is modified during CV cycles. The presence of a single component both under bulk (Sr 2p, Figure S2a) and surface (Sr 3d, Figure S2b) sensitive conditions suggests the formation of a new strontium-containing phase, promoted by the interaction with the electrolyte and OER cycles. Most likely, the local coordination environment of strontium changes after the reaction, and its binding energy is slightly affected. In addition, Fig. 2a and 2b show that, after electrochemistry, the intensity of Sr 3d decreases in a relevant way, whereas that of cobalt (Co 2p core-level peak) increases. This indicates that immersion of the electrode in the electrolyte solution followed by CV measurements leads to leaching of the segregated strontium layer, modification of its local structure, and cobalt enrichment at the surface.
FIGURE 3: O 1s XPS spectra from the in situ experiment (a) and the ex situ experiment (b) with red, purple and black curves corresponding to the “As prepared”, “Under 1.65 V vs RHE/ After 1.65 V vs RHE” and “After Electrochemistry” states, respectively. The green, orange, red, light blue and dark blue components correspond to the perovskite bulk, surface, hydroxyls, water gas phase and electrolyte signal, respectively.
The evolution of the Sr 3d/Co 2p ratio (corrected by photon flux and photo-ionization cross sections), evaluated both during in situ and ex situ measurements, is reported in Fig. 2b and Table S3. After introduction in the analysis chamber (“As prepared”), the electrode shows a ratio of 3.95 and 3.54, measured in situ and ex situ, respectively. After EC, such values decrease to 0.75 and 0.77. In both experiment the system ends up with more cobalt exposed at the surface, in fair agreement with previous observations.21 The fact that the Sr/Co ratios acquired in situ and ex situ show similar values (within the error), suggests that strontium leaching takes place within the probing depth of in situ measurements. Co 2p photoemission peaks are shown in Fig. 2a, and display the line shape of Co(III) in the lattice of perovskites (Co 2p3/2 centered at approximately 780 eV).24, 25, 26, 27 When looking for chemical changes on the cobalt due to EC, a slight modification of the line shape is observed between the “As prepared” and “After EC” conditions (see also Figure S3a) with a more pronounced bump in the satellite structure present at a binding energy around 785–790 eV while the main peak at 780 eV remains unchanged. This possibly indicates a slight evolution of the surface. However, this change is too small to speculate on the nature of such modification. Ex situ experiments (Figure S3b and S3c), have been used to cross check the in situ results. Both the Co 2p and Co L2,3 edges show negligible line shape modifications, suggesting that negligible changes of the electronic state of cobalt take place within the probed depth, which is 2.2 nm and 2.8 nm for Co 2p and Co L2,3, respectively.
Figure 3 displays the comparison between O 1s measurements performed in situ (Fig. 3a) and ex situ (Fig. 3b). The signal of oxygen is of particular importance because it contains information about lattice components (oxygen in the lattice of LSCO surface termination) and surface species that are present or form during reaction. O 1s spectra of the “as prepared” sample were separated into two components. The first one, at a binding energy of 528.9 eV, corresponds to oxygen in the perovskite lattice, as commonly reported in the literature.22, 28, 29 The second component, at a binding energy of 531.5 eV, is assigned to oxygen in the termination layer.28 In situ measurements (Fig. 3a) were performed with two excitation energies (2300 and 5000 eV), in order to vary the information depth within the same measurement. The relative intensity of the 531.5 eV component shows an increasing trend passing from 5000 to 2300 eV, and becomes the main component of O 1s acquired ex situ (Fig. 3b). This proves that oxygen in the termination layer has a thickness distribution comparable/limited to the probing depth of ex situ measurements (2.2 nm). In situ measurements performed during EC show a complex line shape due to the presence of a thin liquid layer on top of the WE. As reported in Fig. 3a, the peak at 532.6 eV corresponds to condensed water and that at 535.0 eV to water vapor in equilibrium with the liquid (background in the analysis chamber). Liquid water is not present during ex situ measurements (Fig. 3b), because spectra were acquired at a 1 mbar water vapor background, without a stabilized electrolyte layer on top of the electrode and no potential applied. Two further components, centered at 529.0 eV and 530.6 eV, were used to deconvolute O 1s acquired in situ during EC. While the former (green color) is still assigned to oxygen in the lattice of LSCO, the latter (red color) reflects the presence of new oxygen-containing species, different from those in the termination layer observed after sample introduction. Interestingly, the O 1s spectrum acquired ex situ shows only this new component, at a binding energy of 531.0 eV. Once again, we want to stress that O 1s acquired ex situ has a KE of approximately 480 eV, whereas the KE in situ is 4460 eV. This corresponds to a relevant change of the probed depth from 2.2 nm to 6 nm, and demonstrates that newly formed species are located at the surface. We tentatively attribute the new component to hydroxyls, which form at the surface of the sample during electrochemistry.
Measurements performed after EC give other interesting information about the evolution of the sample surface. Photoemission peaks can be separated into three components, assigned to oxygen in the lattice of LSCO (529.0 eV, green color), surface hydroxyl groups (530.6 eV) and oxygen in the termination layer of LSCO (531.6 eV), which are detected in both measurement conditions. The reappearance of oxygen in the termination layer of LSCO suggests that hydroxyls may originate from such species during OER while part of them converts back under UHV conditions. It is generally assumed that for LSCO perovskites, cobalt is the active site for OER.2, 30 Experimental data described above suggest the formation of hydroxyls during reaction, while the surface is enriched in cobalt. Therefore, the formation of a cobalt oxyhydroxide phase during reaction can be speculated. Such species are well visible in the O 1s spectrum,
but not in the Co 2p. Based on the literature, line shape modifications of Co 2p upon formation of hydroxide are extremely small and mostly affect the structure of shake-up satellites.31, 32
To further support our hypothesis, theoretical simulations of the core electron binding energies of different oxygen species on LSCO have been performed and compared with the experimental data.
FIGURE 4: Oxygen binding energy simulation results compared to the measured results. Colored binding energy values correspond to the calculated data, with green being the bulk perovskite oxygen (O2-), orange being the surface signal corresponding to adsorbed carbonate species on the strontium segregated layer, and red being the hydroxyls groups formed during electrochemistry.
Figure 4 shows the spectrum of O1s acquired ex situ after EC measurements, deconvoluted using the same peak components described above. The core electron binding energy value calculated for a bulk LSCO structure (Fig. 4, right) oscillates between 528.8 and 529.0 eV (O atoms computationally split in two groups, depending on their location with respect to La and Sr, see Figure S4 in SI), in good agreement with the position of the green component detected experimentally (528.9–529.0 eV). The structure of strontium carbonate was used to simulate oxygen in the termination layer of LSCO (strontium segregations soluble in water, Fig. 4, left), which should correspond to the highest BE peak (orange color in the O 1s). Simulations estimate a binding energy value between 531.6 and 531.9, in fair agreement with the experimental value of 531.5 eV. A series of possible defects that could form on cobalt oxide-terminated LSCO during EC have been simulated adsorbing hydroxyl groups on the CoO2 topmost layer. Three possible structures are presented in Fig. 4 (top) and correspond to different configurations of the adsorbed hydroxyl. These structures represent some of the possible geometrical structures of the surface hydroxyl groups, and correspond to the following situations: a) formation of the surface OH− group in the place of a surface oxygen defect; b) adsorption of the OH− group on the CoO2-terminated surface; and c) formation of multiple OH− groups in the place of multiple surface oxygen defects. Whereas these three structures by no means represent a comprehensive set of possible surface structures (some additional structures are presented in Figures S5 and S6 in SI), they provide sufficient variability to check whether they may in principle correspond to the observed experimental data. The simulated binding energy of OH− in the three structures varies in the 530.2531.0 eV range. The centroid of the new O 1s peak component detected experimentally is at 530.6 eV (red color), at the center of the simulated BE range. Most likely, a collection of different hydroxyl species form on top of the electrode during EC, thus the experimental peak is a convolution of different species displaying similar BE values. In summary, an excellent agreement between the experiment and theory is found, and theory supports the hypothesis of cobalt oxyhydroxide active phase formation during reaction