X-ray Absorption Spectroscopy Investigation of Platinum–Gadolinium Thin Films with Di ﬀ erent Stoichiometry for the Oxygen Reduction Reaction

: Alloys of platinum and lanthanides present a remarkable activity for the oxygen reduction reaction—both in the form of extended surfaces and nanoparticulate catalysts. Co-sputter-deposited thin ﬁlm catalysts based on platinum and gadolinium show great oxygen reduction activity improvement over pure Pt. The sputter-deposition technique represents a viable and versatile approach for investigating model catalyst systems with di ﬀ erent compositions. In this work, co-sputtered Pt 5 Gd and Pt 7.5 Gd thin ﬁlms were investigated using X-ray absorption spectroscopy as well as standardized electrochemical techniques. These investigations revealed the importance of forming alloys with speciﬁc stoichiometry, supporting the need of forming compressively strained Pt overlayers in order to achieve optimum catalytic performances. presented in combination with electrochemical characterization of the co-sputtered Pt x Gd thin ﬁlms. While our XAS measurements were carried out at Stanford Synchrotron Radiation Lightsource (SSRL), all the fabrication and electrochemical testing was performed


Introduction
Hydrogen-fueled energy conversion devices such as low-temperature fuel cells are expected to play a key role in the transition towards a sustainable economy [1]. Pt-based electrocatalysts hold great promise for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs) [1]. However, a high loading of 0.2 mg Pt /cm 2 is required at the cathode [2], where the ORR takes place. Since the amount of Pt available is very scarce [3], we need to reduce the Pt-loading at the cathode, while enhancing the ORR catalytic activity. Substantial progress [4] has been made in recent years to improve the sluggish ORR kinetics [5]. This rapidly progressing research field has resulted in a wide range of intricate principles for the rational design of Pt-based oxygen electroreduction nanocatalysts [6][7][8][9][10][11].
Elucidating the factors governing ORR is essential to design more efficient electrocatalysts. The ORR activity on Pt-based catalysts follows a Sabatier volcano relationship with the binding energy of the oxygen-containing reaction intermediates, e.g., OH. This has been thoroughly corroborated from extensive density functional theory (DFT) calculations [12,13]. These calculations show that the optimum activity value (the top of the volcano) should be achieved by an electrocatalyst which binds OH ca. 0.-eV weaker than pure Pt [13]. Design principles from a combination of model studies on well-defined electrode surfaces [14][15][16][17][18] and DFT calculations have facilitated the development of Figure 1. Illustration of the experimental methods carried out to investigate 10-nm PtxGd thin films. The thin films were prepared by co-sputtering [37], some of them were then subjected to electrochemical testing in a standard three-electrode setup. Both as-prepared and electrochemically tested samples were studied using X-ray absorption spectroscopy (XAS).

Results and Discussion
PtxGd thin films were produced by means of co-sputtering Pt and Gd [37]. PtxGd thin films form catalytically active Pt overlayers under electrochemical conditions [37,38]. To investigate the structure of the Pt overlayers, ex situ XAS was employed on 10-nm-thick PtxGd samples, both asprepared and after electrochemical cycling in 0.1-M HClO4, see Materials and Methods for details.
Our earlier investigations on the formation of Pt and PtxGd thin films [37] revealed that thin films of thicknesses <27 nm were prone to pinhole formation during sputtering. Hence, a range of thin films of ~50-nm-thickness was made in parallel with the 10-nm thin films. The 50-nm thin films were the basis for the electrochemical ORR activity and stability measurements, while the 10-nm thin films were the basis for the XAS investigations. Using the small thickness films for XAS was a way of obtaining surface sensitive XAS as thicker 50-nm samples would give information on the bulk structure of the thin films. Hence, 10-nm Pt7.5Gd and Pt5Gd co-sputter-deposited thin films were transferred to a N2-saturated 0.1-M HClO4 containing three-electrode electrochemical cell (see Figure  1). PtxGd thin films were then cycled from 0.6 to 1.0 V vs. RHE until stable CVs were formed (see Figure 2a). By means of CO-stripping measurements and H-adsorption areas from the base CVs, we were able estimate the roughness factor ( г / . / . / ) of the thin films, where ( ) designates the geometric electrode area. Although the diameter of the electrode was 5.0 mm, the outer 0.05 mm around the rim was covered by a mask. However, we neglect this small difference, corresponding to a ~4% smaller sputter area on the electrode (see Table 1). Figure 1. Illustration of the experimental methods carried out to investigate 10-nm Pt x Gd thin films. The thin films were prepared by co-sputtering [37], some of them were then subjected to electrochemical testing in a standard three-electrode setup. Both as-prepared and electrochemically tested samples were studied using X-ray absorption spectroscopy (XAS).

Results and Discussion
Pt x Gd thin films were produced by means of co-sputtering Pt and Gd [37]. Pt x Gd thin films form catalytically active Pt overlayers under electrochemical conditions [37,38]. To investigate the structure of the Pt overlayers, ex situ XAS was employed on 10-nm-thick Pt x Gd samples, both as-prepared and after electrochemical cycling in 0.1-M HClO 4 , see Materials and Methods for details.
Our earlier investigations on the formation of Pt and Pt x Gd thin films [37] revealed that thin films of thicknesses <27 nm were prone to pinhole formation during sputtering. Hence, a range of thin films of~50-nm-thickness was made in parallel with the 10-nm thin films. The 50-nm thin films were the basis for the electrochemical ORR activity and stability measurements, while the 10-nm thin films were the basis for the XAS investigations. Using the small thickness films for XAS was a way of obtaining surface sensitive XAS as thicker 50-nm samples would give information on the bulk structure of the thin films. Hence, 10-nm Pt 7.5 Gd and Pt 5 Gd co-sputter-deposited thin films were transferred to a N 2 -saturated 0.1-M HClO 4 containing three-electrode electrochemical cell (see Figure 1). Pt x Gd thin films were then cycled from 0.6 to 1.0 V vs. RHE until stable CVs were formed (see Figure 2a). By means of CO-stripping measurements and H-adsorption areas from the base CVs, we were able estimate the roughness factor (Γ CO/H ad. = A CO/H ad. /A) of the thin films, where (A) designates the geometric electrode area. Although the diameter of the electrode was 5.0 mm, the outer 0.05 mm around the rim was covered by a mask. However, we neglect this small difference, corresponding to ã 4% smaller sputter area on the electrode (see Table 1).  Two as-prepared and two electrochemically conditioned 10-nm-thick Pt7.5Gd and Pt5Gd thin films were then transferred to SSRL and ex situ EXAFS spectra were obtained for these four samples (see Figure 3).
EXAFS spectra at the Gd L3 and Pt L3 line-edges for the four 10-nm PtxGd samples were obtained and fitted (see S.I.). Both the electrochemical data and EXAFS results from Figures 2 and 3 are summarized in Table1. Real-space transforms of EXAFS data can be found in Figures S1 and S2.
The results summarized in Table 1 suggest that, when the PtxGd thin films are formed, significant oxygen is present in the surface layer, as observed by the significant Gd-O coordination. This suggests that the oxophilic nature of Gd [37] results in a significantly oxidized surface termination (see S.I.). Moreover, the electrochemical conditioning of the thin films significantly reduces the Gd-O coordination suggesting gadolinium-oxides in the surface dissolve completely during cycling in the acidic electrolyte. Similar observations were made from earlier X-ray photoelectron spectroscopy (XPS) studies [37].  Table 1. Fitted extended X-ray absorption fine structure (EXAFS) and electrochemical characterization data for Pt x Gd (10 nm) samples as prepared and after electrochemical conditioning, the latter denoted by "*". All EXAFS strain data relied on a Pt foil reference measurement [21] in which a relaxed Pt-Pt bond length of 2.775 Å was established, see S.I. EXAFS data are given with ±0.01 Å. This corresponds to a strain error of about 0.4%.

Parameter
Pt 7.5 Gd (10 nm) Pt 5 Gd (10 nm) Two as-prepared and two electrochemically conditioned 10-nm-thick Pt 7.5 Gd and Pt 5 Gd thin films were then transferred to SSRL and ex situ EXAFS spectra were obtained for these four samples (see Figure 3).
EXAFS spectra at the Gd L 3 and Pt L 3 line-edges for the four 10-nm Pt x Gd samples were obtained and fitted (see S.I.). Both the electrochemical data and EXAFS results from Figures 2 and 3 are summarized in Table 1. Real-space transforms of EXAFS data can be found in Figures S1 and S2.
The results summarized in Table 1 suggest that, when the Pt x Gd thin films are formed, significant oxygen is present in the surface layer, as observed by the significant Gd-O coordination. This suggests that the oxophilic nature of Gd [37] results in a significantly oxidized surface termination (see S.I.). Moreover, the electrochemical conditioning of the thin films significantly reduces the Gd-O coordination suggesting gadolinium-oxides in the surface dissolve completely during cycling in the acidic electrolyte. Similar observations were made from earlier X-ray photoelectron spectroscopy (XPS) studies [37]. As expected, the nearest Pt-Pt distance from Table 1 increases after electrochemical cycling. The Pt-Pt distance of the pure Pt overlayer lies between that of the PtxGd alloy and bulk Pt [36]. Thus, when the thin film is transformed from mostly alloy to part alloy and part overlayer, the average Pt-Pt distance is increased. This is also evident from the raw data in Figure 3a: In reciprocal space, the frequency increases due to longer average distances in real space. After the electrochemical cycling, a compressively strained Pt overlayer is formed from Gd leaching, in agreement with earlier studies of Pt rare-earths and lanthanide alloys [22,35,37,38]. The formation of the overlayer was supported by an additional measurement on the Pt5Gd sample at a shallower angle (at 0.2° which increases the signal contribution from the surface), in which we observe marginally increased Pt-Pt distances. Table 1 also shows that relatively high roughness factors are found both from H-adsorption and CO-stripping area measurements. This strongly suggests that extensive pinhole is formed due to the low thickness of 10 nm (see S.I.). Unfortunately, concluding on any compressively strain difference (seen from the Pt-Pt distance) between the Pt7.5Gd and Pt5Gd sample is not straightforward, given the extensive roughness. From our earlier work on well-defined extended surfaces [22], we expect Pt5Gd to exhibit superior ORR activity performance, as its Pt overlayer is more compressively strained than that of Pt7.5Gd. Indeed, we recently observed that the overlayer strain on polycrystalline Pt5Gd leads to an activity that approaches the top of the experimental volcano [22]. We note that, according to theoretical calculations [13,32], the OH weakening leading to an optimal ORR activity corresponds to a Pt overlayer under a compressive strain of ca. 3% relative to pure Pt [22].
To further correlate Pt:Gd stoichiometry with ORR activity, 50-nm-thick Pt7.5Gd and Pt5Gd thin films were produced and electrochemically characterized. We must note that we carried out the electrochemical measurements on 50-nm thin films as we could not use thinner films due to the presence of pinholes and excess of under-coordinated sites associated (see Figure S3) [37]. Following earlier established electrochemical characterization procedures on Pt5Gd thin films [37], a minimum of four samples of each composition were tested using standard rotating disk electrode (RDE) measurements in 0.1-M HClO4 electrolyte using a three-electrode setup, as shown in Figure 1. The base CVs on PtxGd (50-nm) thin films before and after accelerated stability tests (see Section 3 Materials and Experimental Methods) were evaluated in conjunction with CO-stripping experiments and CVs under ORR conditions. Representative base CVs and CO oxidation can be seen in Figure  4a,b, respectively.
From the base CVs in Figure 4a, it is noted that features typically associated with H and OH adsorption and desorption [16,40] do not change significantly after accelerated test. This suggests the formation of stable films under the presented electrochemical conditions. Conversely, there is a small shift in the position of the CO stripping peak. In this context, it should be noted that CO can induce Pt surface roughening/smoothening [41][42][43] and general reorganization [44]. The CO-stripping As expected, the nearest Pt-Pt distance from Table 1 increases after electrochemical cycling. The Pt-Pt distance of the pure Pt overlayer lies between that of the Pt x Gd alloy and bulk Pt [36]. Thus, when the thin film is transformed from mostly alloy to part alloy and part overlayer, the average Pt-Pt distance is increased. This is also evident from the raw data in Figure 3a: In reciprocal space, the frequency increases due to longer average distances in real space. After the electrochemical cycling, a compressively strained Pt overlayer is formed from Gd leaching, in agreement with earlier studies of Pt rare-earths and lanthanide alloys [22,35,37,38]. The formation of the overlayer was supported by an additional measurement on the Pt 5 Gd sample at a shallower angle (at 0.2 • which increases the signal contribution from the surface), in which we observe marginally increased Pt-Pt distances. Table 1 also shows that relatively high roughness factors are found both from H-adsorption and CO-stripping area measurements. This strongly suggests that extensive pinhole is formed due to the low thickness of 10 nm (see S.I.). Unfortunately, concluding on any compressively strain difference (seen from the Pt-Pt distance) between the Pt 7.5 Gd and Pt 5 Gd sample is not straightforward, given the extensive roughness. From our earlier work on well-defined extended surfaces [22], we expect Pt 5 Gd to exhibit superior ORR activity performance, as its Pt overlayer is more compressively strained than that of Pt 7.5 Gd. Indeed, we recently observed that the overlayer strain on polycrystalline Pt 5 Gd leads to an activity that approaches the top of the experimental volcano [22]. We note that, according to theoretical calculations [13,32], the OH weakening leading to an optimal ORR activity corresponds to a Pt overlayer under a compressive strain of ca. 3% relative to pure Pt [22].
To further correlate Pt:Gd stoichiometry with ORR activity, 50-nm-thick Pt 7.5 Gd and Pt 5 Gd thin films were produced and electrochemically characterized. We must note that we carried out the electrochemical measurements on 50-nm thin films as we could not use thinner films due to the presence of pinholes and excess of under-coordinated sites associated (see Figure S3) [37]. Following earlier established electrochemical characterization procedures on Pt 5 Gd thin films [37], a minimum of four samples of each composition were tested using standard rotating disk electrode (RDE) measurements in 0.1-M HClO 4 electrolyte using a three-electrode setup, as shown in Figure 1. The base CVs on Pt x Gd (50-nm) thin films before and after accelerated stability tests (see Section 3 Materials and Experimental Methods) were evaluated in conjunction with CO-stripping experiments and CVs under ORR conditions. Representative base CVs and CO oxidation can be seen in Figure 4a Table 2, reveals that; i) Quite rough thin film surfaces form. ii) Pt7.5Gd thin films are less rough than Pt5Gd thin films, likely due to the lower degree of Gd dissolution by acid leaching [35]. iii) The potential of the maximum CO oxidation peak increases after stability tests and the UCO-peak value attains similar values for both the Pt5Gd and Pt7.5Gd samples, suggesting a similar structure is formed after aging.  The ORR activity of the PtxGd thin films was also electrochemically evaluated. Representative anodic sweeps during ORR can be seen in Figure 5a. It should be noted that the ORR activities were evaluated using the Koutecky-Levich relation, which identifies the mass-transport corrected kinetic component (jk) of the oxygen reduction current density (j) taking into account the diffusion limiting current density (jdiff): [45]. The logarithm of the kinetic current density was plotted against the RHE potential to obtain a Tafel plot, as seen in Figure 5b.
The electrochemically active surface areas derived from the CO-stripping charges were utilized to estimate the kinetic current densities in Figure 5b, whereas current densities in Figures 4a,b and 5a were normalized by the geometric electrode area of ca. 0.196 cm 2 [37]. From the electrochemical characterization (Figures 4 and 5) key parameters such as CO and H-adsorption charge (Q) and associated active electrodes areas (A) and roughness factors (г could be extracted. Moreover, kinetic current densities for ORR and CO oxidation peak potential are summarized in Table 2. Table 2 shows that Pt5Gd (50 nm) and Pt7.5Gd (50 nm) samples present differences in ORR activity, Pt5Gd being more active than Pt7.5Gd, as expected from a higher Pt-Pt compressive overlayer strain [22]. From the base CVs in Figure 4a, it is noted that features typically associated with H and OH adsorption and desorption [16,40] do not change significantly after accelerated test. This suggests the formation of stable films under the presented electrochemical conditions. Conversely, there is a small shift in the position of the CO stripping peak. In this context, it should be noted that CO can induce Pt surface roughening/smoothening [41][42][43] and general reorganization [44]. The CO-stripping experiments from Figure 4b, presented in Table 2, reveals that; (i) Quite rough thin film surfaces form. (ii) Pt 7.5 Gd thin films are less rough than Pt 5 Gd thin films, likely due to the lower degree of Gd dissolution by acid leaching [35]. (iii) The potential of the maximum CO oxidation peak increases after stability tests and the U CO-peak value attains similar values for both the Pt 5 Gd and Pt 7.5 Gd samples, suggesting a similar structure is formed after aging. Table 2. Electrochemical data for the 40 nm Pt as-prepared and 50 nm Pt 5 Gd and Pt 7.5 Gd thin films before and after AST. Specific activities were evaluated at 0.9 V vs. RHE and are shown both using the area derived from CO oxidation and hydrogen adsorption [37].

Thin Film Fabrication
PtxGd and Pt thin films were produced by a (co)sputtering method described elsewhere [37]. A polished and clean glassy-carbon stub (HTW, Ø×h = 5 × 4 mm) was placed in an ultra-high vacuum system. The system base pressure was 10 −9 mbar. Thin films of Pt-Gd alloys were co-sputtered atop using Ar (N5, 50 sccm) and powers of 180 W and 17-20 W for the Pt and Gd targets, respectively (Kurt-Lesker, 99.995%). The film thickness was monitored using a quartz microbalance; the deposition temperature was maintained at 300 °C.

Electrochemical Conditioning and Characterization of PtxGd Co-sputtered Thin Films
The co-sputtered Pt7.5Gd and Pt5Gd thin films (10-nm-thick) used for the EXAFS investigation were transferred to a N2-saturated 0.1 M HClO4 (Merck, Suprapur ® , 70%) containing three-electrode The electrochemically active surface areas derived from the CO-stripping charges were utilized to estimate the kinetic current densities in Figure 5b, whereas current densities in Figure 4a,b and Figure 5a were normalized by the geometric electrode area of ca. 0.196 cm 2 [37]. From the electrochemical characterization (Figures 4 and 5) key parameters such as CO and H-adsorption charge (Q) and associated active electrodes areas (A) and roughness factors (Γ) could be extracted. Moreover, kinetic current densities for ORR and CO oxidation peak potential are summarized in Table 2. Table 2 shows that Pt 5 Gd (50 nm) and Pt 7.5 Gd (50 nm) samples present differences in ORR activity, Pt 5 Gd being more active than Pt 7.5 Gd, as expected from a higher Pt-Pt compressive overlayer strain [22].

Thin Film Fabrication
Pt x Gd and Pt thin films were produced by a (co)sputtering method described elsewhere [37]. A polished and clean glassy-carbon stub (HTW, Ø×h = 5 × 4 mm) was placed in an ultra-high vacuum system. The system base pressure was 10 −9 mbar. Thin films of Pt-Gd alloys were co-sputtered atop using Ar (N5, 50 sccm) and powers of 180 W and 17-20 W for the Pt and Gd targets, respectively (Kurt-Lesker, 99.995%). The film thickness was monitored using a quartz microbalance; the deposition temperature was maintained at 300 • C.

Electrochemical Conditioning and Characterization of Pt x Gd Co-sputtered Thin Films
The co-sputtered Pt 7.5 Gd and Pt 5 Gd thin films (10-nm-thick) used for the EXAFS investigation were transferred to a N 2 -saturated 0.1 M HClO 4 (Merck, Suprapur ® , 70%) containing three-electrode electrochemical cell (see Figure 1). The samples were immersed into the electrochemical cell under potential control and cycled at 50 mV/s between 0.05 and 1.00 V vs. the RHE until stable cyclic voltammograms (CVs) were obtained. The charge associated with H adsorption (Q H ad. ) can be used to estimate the electrochemically active surface area (ECSA) of Pt catalysts [37,[46][47][48]. Once stable CVs had formed, CO-stripping measurements were conducted at 10 mV/s [37,49]. The CO-stripping procedure consisted of sample being cycled twice in Ar-saturated electrolyte at 10 mV/s. Then, the electrode was maintained at 0.05 V vs. RHE while bubbling CO for 2 min, saturating the electrode surface. The electrode potential was maintained at 0.05 V vs. RHE for 15 min while bubbling Ar gas purging the remaining CO from the electrolyte. This allowed CO to be adsorbed on the electrode surface, while allowing sufficient time for residual CO to be purged from the cell. The potential was then swept between 0.05 V and 1.00 V vs. RHE at 10 mV/s. The CO oxidation charge (Q CO ) derived from the CO-stripping procedure was also used to evaluate the ECSA [44,50,51]. From our earlier work [37,49], the typical CO oxidation charge per area for polycrystalline Pt is~349 µC/cm 2 . This charge, along with the hydrogen adsorption/desorption value of~191 µC/cm 2 [37], was used to estimate the ECSA by CO stripping and hydrogen underpotential deposition (H UPD ), respectively. We must note that, although evaluating the H UPD area is a standard method of ECSA evaluation on Pt electrodes, H binding may be modified on Pt alloys due to electronic effects such as strain effects [37]. Fifty-nanometer Pt x Gd thin films were used to evaluate electrochemical performance and for direct comparison with the 10-nm samples. We also include reference data of 40-nm-thick pure Pt thin films for comparison. Pt x Gd samples were initially cycled from 0.0 to 1.0 V vs. RHE in N 2 -saturated 0.1-M HClO 4 at 200 mV/s and 400 rpm until CVs were stable. The base CVs at 50 mV/s were then gathered at 400 rpm. The ORR activity was evaluated in O 2 -saturated electrolyte at 1600 rpm. The samples were subjected to electrochemical impedance spectroscopy (EIS) and Ohmic drop was evaluated at double-layer region at 0.5 V vs. RHE. The Ohmic drop was evaluated by linear regression of the resulting Nyquist plot in the 60 to 400 Hz region [37]. All electrochemical data in this work were presented using a Pt counter electrode and plotted relative to RHE. The RHE potential was evaluated by bubbling H 2 while rotating the working electrode at 1600 rpm and cycling between −0.74 and −0.70 V vs. the Hg (l) |HgSO 4(g) reference electrode (Schott Instruments, 0.6-M K 2 SO 4 ). The value of the RHE potential can be read as the intercept with the current axes [37]. Finally, a base CV in N 2 -saturated electrolyte at 50 mV/s and CO-stripping CVs at 10 mV/s were obtained. Accelerated stability tests (AST) were also conducted, consisting of additional 10,000 cycles in O 2 -staurated electrolyte between 0.6 and 1.0 V vs. RHE at 100 mV/s without any electrode rotation [52]. After this extended cycling, the electrolyte was changed to a fresh one and the earlier described characterization steps were repeated.

X-ray Absorption Spectroscopy Measurements
XAS experiments were carried out ex situ at SSRL (Stanford Synchrotron Radiation Lightsource, CA, USA). Both as prepared and electrochemically conditioned samples, were transferred under ambient conditions in a sample holder. EXAFS measurements were carried out with a gracing incidence of 2 • (unless otherwise stated) at beam line 11-2 at Stanford Synchrotron Radiation Lightsource (SSRL). Spectra were obtained of both Pt L 3 and Gd L 3 edges and fitted in R-space using a k 2 -weighing, see Figure 3 and supporting information (S.I.). The spectra were processed and fitted using the software package SIXPack (v1.40, SAMware, Stanford, CA, USA, 2016), using scattering paths from FEFF8 [53].

Conclusions
Thin film Pt x Gd electrocatalysts with different stoichiometry were fabricated using a co-sputter-deposition technique. Both 50 nm Pt 5 Gd and Pt 7.5 Gd thin films are highly active catalysts for ORR. The increased activity compared to pure Pt and the difference in activity between Pt 5 Gd and Pt 7.5 Gd can be explained by the inherent effect of overlayer strain. This was observed using 10-nm-thick thin film equivalents by EXAFS. We were able to detect compressive strain in the thin films and reveal that the overlayer on Pt 5 Gd is more compressively strained, in good agreement with ORR activity measurements on Pt x Gd nanoparticles [21] and Pt 5 Gd extended surfaces [22,31,35]. Although ORR experiments were carried out on 50-nm films and EXAFS measurements were performed on 10-nm thin films, the compressive strain values from EXAFS can be used to explain the differences in