Tuning Surface Reactivity and Electric Field Strength via Intermetallic Alloying

Many electrosynthesis reactions, such as CO2 reduction to multicarbon products, involve the formation of dipolar and polarizable transition states during the rate-determining step. Systematic and independent control over surface reactivity and electric field strength would accelerate the discovery of highly active electrocatalysts for these reactions by providing a means of reducing the transition state energy through field stabilization. Herein, we demonstrate that intermetallic alloying enables independent and systematic control over d-band energetics and work function through the variation of alloy composition and oxophilic constituent identity, respectively. We identify several intermetallic phases exhibiting properties that should collectively yield higher intrinsic activity for CO reduction compared to conventional Cu-based electrocatalysts. However, we also highlight the propensity of these alloys to segregate in air as a significant roadblock to investigating their electrocatalytic activity.


Sputter Deposition
PdGe, Pd x Sn 1-x , and PdIn thin films were prepared by sputter deposition using an AJA ATC Orion magnetron sputtering system with 4 individual targets (Pd, Ge, Sn, and In).All sputtering was performed in Ar at a pressure of 2 mTorr.The thin films were deposited onto Si(100) wafers, which were cleaned by Ar sputtering for 5 min immediately before the deposition.Pd-based intermetallic thin films were deposited by co-sputtering Pd (99.95% Kurt J. Lesker) with either Ge (99.999%Kurt J. Lesker), Sn (99.994%Kurt J. Lesker), or In (99.998%Kurt J. Lesker) at a combined rate of roughly 1 Å/s to a thickness of 100 nm.

Energy Dispersive Spectroscopy
The bulk compositions of the thin films were measured using an FEI Quanta FEG 200 scanning electron microscope (SEM) equipped with a Oxford Instruments energy dispersive spectrometer (EDS).
Elemental quantification was conducted by measuring the x-ray emission from the Pd, Sn, and In L edges and the Ge K edge upon excitation by an electron beam (10 kV).Each sample was analyzed at 10 distinct positions in order to assess the spatial uniformity of the measured bulk composition, which was <1 at.% for all thin films investigated.

X-Ray Diffraction
The crystal structures of the thin films were analyzed using a PAN-Analytical Empyrian x-ray diffractometer (XRD) using Cu Kα radiation (40 kV, 40 mA).The diffractometer was equipped with parallel beam optics during all grazing incidence XRD measurements, which were performed with an incident angle of 0.25.Grazing incidence diffractograms were recorded using a step size of 0.01 and a dwell time of 5 s.Phase identification was performed using Malvern Panalytical's HighScore Plus software.

X-Ray Photoelectron Spectroscopy
The near-surface compositions of the thin films were measured using a Thermo Scientific ThetaProbe x-ray photoelectron spectrometer (XPS).XPS was performed using monochromatized Al Kα radiation.Core level spectra were recorded using a pass energy of 100 eV, a step size of 0.05 to 0.1 eV, a dwell time of 50 ms, and at least 20 integrated sweeps.Valence band spectra were recorded using a pass energy of 20 eV, a step size of 0.025 eV, a dwell time of 50 ms, and at least 50 integrated sweeps.Ar sputtering of the sample surface was performed using an Ar pressure of 2 x 10 -7 mbar and a beam energy of 4 kV.The ion beam was rastered over a 4 x 4 cm 2 area during sputtering.Angle-resolved XPS (ARXPS) was performed by measuring photoelectrons ejected at angles of 20 to 80° relative to the surface normal using a 2D detector, which was divided into 32 separate signal channels.Thus, each signal output was derived from photoelectrons emitted from a 1.875 window.The energy scale of the observed spectra was calibrated by setting the C 1s binding energy to 284.8 eV.The Pd 3d, Sn 3d, and In 3d spectral features were fit to a single component, the Ge 3d spectral features were fit to either one or two individual components (Ge 0 and Ge 4+ ) using the Thermo Avantage software.Elemental quantification was performed by integrating the signals from the Pd 3d, Pd 3p, Ge 3d, Sn 3d, and In 3d regions using a Shirley background and normalizing them by an internally calibrated relative sensitivity factor.Pd 3p was used for quantification in samples containing Ge due to the overlap of Pd 3d with a Ge auger electron.The penetration depth of the ARXPS measurements were calculated assuming the penetration depth of photoelectrons emitted normal to the surface was 10 nm, which is roughly equivalent to 3 mean free paths of the photoelectrons investigated herein.

Ultraviolet Photoelectron Spectroscopy
The work functions of the thin films were measured using a Thermofisher Scientific NEXSA XPS.
Ar sputtering of the sample surface was performed using an Ar pressure of 2 x 10 -7 mbar and a beam energy of 4 kV.The ion beam was rastered over a 3 x 3 cm 2 area during sputtering.The ultraviolet photoemission spectra are acquired using a helium discharge lamp, with principal photon energies at 21.2 eV (HeI) and 40.8 eV (HeII), while applying a negative bias of -5V to the sample to deconvolute the work function of the surface from that of the energy analyzer.Spectra were acquired using a pass energy of 2.0 eV, a step size of 0.01 eV, a dwell time of 50 ms, and at least 10 integrated sweeps.The energy scale of the observed spectra was calibrated using the inflection point of the Fermi edge.

Low Energy He Ion Scattering
The surface compositions of the thin films were measured using low energy He ion scattering (He-LEIS) performed in the same instrument above.He-LEIS was performed using a He pressure of 2 x 10 -7 mbar and beam energy of 1 kV.The beam was not rastered during the measurement.Spectra were acquired using a retard ratio of 2.5, a step size of 1 eV, a dwell time of 50 ms, and a single sweep.

Temperature Programmed Desorption
The CO temperature programmed desorption measurements were carried out in a custom-built chamber.The sample is placed in a copper holder, which is cooled using liquid nitrogen.Isotopically labeled C 18 O is dosed until full coverage is achieved.The sample is heated using a tungsten alloy filament at a rate of 0.5 K/s.The desorbed gas is measured using a quadrupole mass spectrometer (QMA 125, Pfeiffer Vacuum Technology AG).

Figure S6 -Figure S11 -
Figure S6 -Raw XPS data of the Pd x Sn 1-x valence band region.Linear correlation between the integrated area of the d-band portion of the valence band and the nearsurface Pd content of the Pd x Sn 1-x thin films.

Extended X-Ray Photoelectron Spectroscopy of Ar Sputtered Intermetallic Thin Films
Figure S2 -Grazing incidence x-ray diffractograms of Pd x Sn 1-x , PdGe, and PdIn intermetallic thin films.