Tuning electrochemically driven surface transformation in atomically flat LaNiO3 thin films for enhanced water electrolysis

Abstract

Structure–activity relationships built on descriptors of bulk and bulk-terminated surfaces are the basis for the rational design of electrocatalysts. However, electrochemically driven surface transformations complicate the identification of such descriptors. Here we demonstrate how the as-prepared surface composition of (001)-terminated LaNiO3 epitaxial thin films dictates the surface transformation and the electrocatalytic activity for the oxygen evolution reaction. Specifically, the Ni termination (in the as-prepared state) is considerably more active than the La termination, with overpotential differences of up to 150 mV. A combined electrochemical, spectroscopic and density-functional theory investigation suggests that this activity trend originates from a thermodynamically stable, disordered NiO2 surface layer that forms during the operation of Ni-terminated surfaces, which is kinetically inaccessible when starting with a La termination. Our work thus demonstrates the tunability of surface transformation pathways by modifying a single atomic layer at the surface and that active surface phases only develop for select as-synthesized surface terminations.

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Fig. 1: Standing wave XPS analysis.
Fig. 2: Electrochemical performance.
Fig. 3: Operando UV–vis spectroelectrochemistry of LNO.
Fig. 4: OER overpotentials for LNO surfaces and oxide|LNO interfaces.

Data availability

The DFT data are available at https://www.catalysis-hub.org/publications/BaeumerTuning2020 (ref. 76). The experimental data are available via https://doi.org/10.26165/Juelich-Data/BMNAGT (ref. 77).

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Acknowledgements

This project was funded by the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement no. 796142. C.B., Q.L., J.T.M., A.Y.-L.L. and W.C.C. gratefully acknowledge financial support through the Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under contract no. DE-AC02-76SF00515. J.L. and M.B. acknowledge support by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, Electrocatalysis Science Program to the SUNCAT Center for Interface Science and Electrocatalysis. We also acknowledge funding from DFG (German Science Foundation) within the collaborative research centre SFB 917. H.P.M. has been supported for salary by the US Department of Energy (DOE) under Contract No. DE-SC0014697. Part of this work was performed at Stanford Nano Shared Facilities (SNSF)/Stanford Nanofabrication Facility (SNF), supported by the National Science Foundation under award ECCS-1542152. We thank Helmholtz-Zentrum Berlin for the allocation of synchrotron radiation beamtime. This research used resources of the Advanced Light Source, a DOE Office of Science User Facility under contract no. DE-AC02-05CH11231. We thank A. Küppers (Forschungszentrum Jülich, ZEA-3) for the ICP-MS measurements and F. Hausen, M. Weber and S. Karthäuser for helpful discussions and help in the LEED measurements. We thank F. El Gabaly for supplying an air-free transfer vessel for the SW-XPS analysis and M. Blum for beamtime support. The authors acknowledge the use of the computer time for the m2997 allocation at the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the US Department of Energy under Contract no. DE-AC02-05CH11231.

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Contributions

C.B., Q.L., S.N., J.T.M. and W.C.C. conceived and designed the experiments. J.L. and M.B. designed, performed, analysed, and interpreted the DFT simulations; C.B., Q.L., S.N., J.T.M., and W.C.C. contributed through in-depth discussion and comparison with the experiments during all stages. C.B., Q.L., A.Y.-L.L. and M.A.W. prepared the samples and performed thin-film and electrochemical characterization. C.B., S.M.G. and S.N. performed the standing-wave photoemission spectroscopy experiments. Q.L. modelled, analysed and interpreted the standing-wave photoemission spectroscopy results; C.B., S.M.G. and S.N. contributed through discussion and interpretation. A.Y.-L.L. performed the UV–vis spectroelectrochemistry analysis. H.P.M. performed the XPS cluster calculations. L.J. performed the STEM analysis. T.D. performed the photoemission electron microscopy measurements and M. Giesen analysed and quantified the resultant data using PCA. M. Glöß and C.B. performed the LEED measurements. C.B., E.E.P. and J.T.M. analysed the electrochemical performance of the thin films in the context of previous reports. R.D. and F.G. advised on the epitaxy approach to the electrolysis and sample fabrication. C.B., J.L., Q.L. and M.B. wrote the manuscript with contributions from all the authors. R.D., F.G., R.W., M.B., S.N., J.T.M. and W.C.C. supervised the research. C.B., R.W., M.B., S.N., J.T.M. and W.C.C. jointly determined the research direction.

Corresponding authors

Correspondence to Christoph Baeumer or Michal Bajdich or Slavomír Nemšák.

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Peer review information Nature Materials thanks Scott Chambers and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 LaNiO3 growth at different temperatures.

a, RHEED intensity oscillations during growth at different Tgrowth. b, RHEED diffraction patterns after growth of 20 nm, confirming predominantly two-dimensional growth. c-f, AFM morphology in the as-prepared states for different Tgrowth. Scale bars are 1 µm, step height is ~0.4 nm. g-j, AFM morphology after cyclic voltammetry with 52 cycles with a maximum potential ~1.6 V vs. RHE and 2 cycles with maximum potential of ~1.9 V vs. RHE, revealing similar morphologies and similarly low roughness compared to the as-prepared state.

Extended Data Fig. 2 Electrical transport characterization.

Hall resistivity (top), mobility (middle) and carrier concentration (bottom). Error bars indicate standard error from triplicate measurements.

Extended Data Fig. 3 TEM images of SW-XPS samples and SW-XPS variation of information depth.

a, and b, Low-resolution high-angle annular dark-field HAADF STEM images of LaNiO3 top layers deposited at 450 °C and 650 °C, respectively. The super lattice was deposited at 550 °C in each case. c, High-resolution image of the super lattice revealing the four-unit-cell periodicity. d,e, The calculated standing-wave profile of electric field intensity (|E2|) as a function of depth and incident angle for the standing wave samples with LaNiO3 top layers deposited at 450 °C and 650 °C, respectively. f,g, Calculated mean information depth as a function of depth and incident angle based on the optimized structure for the standing wave samples with LaNiO3 top layers deposited at 450 °C and 650 °C, respectively (Fig. 2b,c of the main text), derived through multiplication of electric field intensity (|E2|) and photoelectron attenuation. Varying the incident angle changes the information depth deterministically. Simulating all XPS core level rocking curves simultanteously therefore allows to extract the compositional depth profile with atomic layer resolution.

Extended Data Fig. 4 XPS surface composition.

a, La 4d laboratory XP spectra and b, Ni 3p spectra for different Tgrowth. Open circles show the data points, solid lines indicate the fitting result obtained (see Supplementary Note 4 for fitting details). Exemplary components used for the fit are shown for one spectrum each. c, Cation ratio as a function of Tgrowth for different mean escape depths d. Error bars indicate the maximum deviation from several measurements for a few identical samples measured under the same conditions.

Extended Data Fig. 5 Rotating disc electrode setup for epitaxial thin films.

a, Schematic of the rotating disc setup. b, Schematic of the sample contact. The Pt plug of the rotating shaft is connected to the sample back side. c, d, Optical images of the sample front and back side. A 50 nm Pt layer connects the sample back side to the substrate front side, forming ohmic electrical contact (R < 10 Ω) with the back side of the LaNiO3 catalyst layer. The electrode area is defined by a perfluoroelastomer (FFKM) O-ring fitted to the PEEK sample adapter. All experiments are performed without silver paste or epoxy adhesives. This leads to electrochemical investigation with a minimized amount of contaminating species.

Extended Data Fig. 6 AFM investigation before and after OER.

a, AFM morphology after 54 CV cycles b, Exemplary height profile extracted along the line in a, confirming unit cell height for all steps, indicating single termination. c, AFM morphology of the as-prepared state. b, Exemplary height profile extracted along the line in c, confirming unit cell height for all steps, indicating single termination.

Extended Data Fig. 7 Sweep rate dependence of the redox peaks for samples with different growth temperature.

a-d, Sweep rate-dependent cyclic voltammetry. e,f, Peak current as a function of sweep rate. Red lines indicate a power-law fit to the data with b-values of 0.90, 0.91, and 1.08 for panels a-c, respectively, indicating a surface-limited process. h, Redox behavior in cyclic voltammetry at 10 mV/s after various treatments. For LNO-La, the redox peak does not appear after 29 sweeps in cyclic voltammetry (green curve, 27 cycles with a maximum potential ~1.6 V vs. RHE and 2 cycles with maximum potential of ~1.9 V vs. RHE), nor after chronoamperometry (orange curve, potential stepped from 0.9 V up to 1.6 V in 50 mV steps and back down to 0.9 V in 100 mV steps; each step took 40 min, total time ~16 hrs). Similar results were obtained after a 38 h chronoamperometry at 1.63 V vs. RHE. For comparison, the redox peak for LNO-Ni after 20 sweeps in cyclic voltammetry is also shown (red curve).

Extended Data Fig. 8 Comparison to literature.

OER current density at 400 mV overpotential for different temperatures in comparison to activities previously reported for RENiO3 epitaxial thin films. Literature references are listed in Supplementary Table 2.

Extended Data Fig. 9 UV-Vis spectroelectrochemistry of LaNiO3 and Ni(OH)2.

UV-Vis spectra of a, a 20 nm LNO-Ni film and c, a 50 nm electrodeposited Ni(OH)2 film at various potentials, with optical density scaled relative to OCV (0.9 V vs. RHE). Note that the features around λ = 480 nm, 570–620 nm, and 660 nm are introduced during background subtraction as these are the emission lines from the deuterium light source. c,d, Changes in optical density at λ = 500 nm and 700 nm during potential holds plotted alongside the cyclic voltammetry results for LaNiO3 and Ni(OH)2, respectively. e, f, Spectral decomposition into surface and bulk spectra of a 20 nm LNO film.

Extended Data Fig. 10 Surface Pourbaix diagram of LNO-La and NiO2|LNO-La.

a, Surface Pourbaix diagram of LNO-La with corresponding (2 × 2) structures as insets. Under OER conditions (U~1.68 V), LNO-La is fully covered with 4 OH* (1 ML) at the La-O-La bridge sites. b, Surface Pourbaix diagram of NiO2|LNO-La with corresponding structures as insets. Under OER conditions (U~1.68 V), the clean (full ML of O3c) surface (S5) and ¼ monolayer (ML) OH covered surface followed by (S6) are the most favorable.

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Supplementary Figs. 1–22, Tables 1–5 and Notes 1–11.

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Baeumer, C., Li, J., Lu, Q. et al. Tuning electrochemically driven surface transformation in atomically flat LaNiO3 thin films for enhanced water electrolysis. Nat. Mater. (2021). https://doi.org/10.1038/s41563-020-00877-1

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