Deposition of two-dimensional (2D) materials onto catalyst surfaces is known to alter the adsorption energies of active sites due to the nanoconfinement effect. Traditionally, these 2D catalyst heterostructures were prepared by depositing a 2D material onto a pristine metallic surface. Preparing well-defined 2D monolayers, instead, on metal-oxide surfaces is challenging, although it is possible via O2 intercalation by oxidizing a metal substrate underneath. Several studies demonstrate this intercalative behavior of 2D covers, however, without the preparation of ordered structures, which are imperative for defining fundamental reaction mechanisms in confined space. We report the successful preparation and characterization of a well-defined, ultrathin cuprous oxide-like film grown between h-BN and Cu(111). The confined surface oxide adopts a "Cu2O-like" structure resembling the well-studied "44" Cu2O structure, although the oxidation temperature is surprisingly lower than its uncovered oxide counterpart and the h-BN layer remains intact following oxidation. Our experimental results, backed by theoretical simulations, outline the development of a heterostructure with an h-BN/metal-oxide interface as a model system, utilizing a preparation method likely transferable to a wide range of 2D/metal heterostructures and opening the door to new catalyst designs.

Metal hydrides are potential candidates for applications in hydrogen-related technologies, such as energy storage, hydrogen compression, and hydrogen sensing, to name just a few. However, understanding the electronic structure and chemical environment of hydrogen within them remains a key challenge. This work presents a new analytical pathway to explore these aspects in technologically relevant systems using hard x-ray photoelectron spectroscopy (HAXPES) on thin films of two prototypical metal dihydrides: YH_{2−δ} and TiH_{2−δ}. By taking advantage of the tunability of synchrotron radiation, a nondestructive depth profile of the chemical states is obtained using core-level spectra. Combining experimental valence-band (VB) spectra collected at varying photon energies with theoretical insights from density functional theory (DFT) calculations, a description of the bonding nature and the role of d versus sp contributions to states near the Fermi energy are provided. Moreover, a reliable determination of the enthalpy of formation is proposed by using experimental values of the energy position of metal s-band features close to the Fermi energy in the HAXPES VB spectra.

Magnetite $({\mathrm{Fe}}_{3}{\mathrm{O}}_{4})$ doped with earth-abundant metals has become a promising catalyst material. In particular, Ni-doped magnetite $(\mathrm{Ni}/{\mathrm{Fe}}_{3}{\mathrm{O}}_{4})$ has been demonstrated as a cheap, robust, and catalytically active material in photocatalytic and electrochemical water oxidation. Recently, the incorporation of Ni atoms in ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ single crystalline surfaces was studied extensively with scanning tunneling microscopy and density-functional theory calculations. However, because of its importance for catalytic activity, this incorporation process and the determination of the lattice sites occupied by the Ni atoms require further experimental study. In this work, we investigated the surface structure of as-grown and annealed $\mathrm{Ni}/{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$(001) as a function of Ni coverage under ultra-high vacuum conditions with temperature-dependent x-ray and ultraviolet photoelectron spectroscopy, x-ray photoelectron diffraction, and low-energy electron diffraction. We observe that octahedrally coordinated subsurface cation vacancy sites are already occupied upon Ni deposition at room temperature and that Ni atoms start to diffuse further into the octahedral subsurface sites with increasing temperature.

The main data analysis was done with Wavemetrics Igor Pro 7.08 using user-defined macros. Data files given in *.itx format are human-readable text files that can be opened in Igor Pro. User-defined macros are available from the authors upon reasonable request. Static spectra are measured with the proprietary "Croissant" software for the channeltron analyzer and saved in human-readable *.plsp format, or SpecsLab Prodigy 4.60.1 for the 2D analyzer and saved in the proprietary SPECS *.sle format or exported into *.itx format. Time-resolved spectra are measured with a proprietary LabView program and exported in the binary HDF5 *.h5 file format. ******************************************** Fig. 1 LEED and He Ia ARPES ******************************************** LEED images taken with SBIG STF-8300 CCD Camera, the SBIG format is a 16-bit grayscale bitmap with metadata. Fig. 1a: LEED image at 120 eV LEED220210_120eV_Cu111_hBN_Cu2O.SBIG Fig. 1b: LEED image at 48 eV LEED220210_048eV_Cu111_hBN_Cu2O.SBIG other energies (not shown in the figure): see Fig. S2/S3 Fig. 1c: He Ia spectrum second derivative as function of parallel momentum and binding energy Spectra measured with VG ESCALAB 220 channeltron hemispherical analyzer by tilting the sample at two fixed azimuthal angles and using a Gammadata VUV 5050 monochromated helium lamp. The azimuthal angles correspond to the M and K directions, respectively, as determined by x-ray photoelectron diffraction of the Cu(111) surface. Measurement parameters are given in the files. Positive parallel momentum: M direction, VG2Z220628N015.plsp Negative parallel momentum: K direction, VG2Z220628N016.plsp Combined ARPES spectrum as a function of parallel momentum and binding energy: VG2Z220628N015_N016.itx Second derivative along energy direction: VG2Z220628N015_N016d.itx; smoothed: VG2Z220628N015_N016d_smth.itx ************************************************ Fig. 2 He IIa ARPES ************************************************ All ARPES spectra measured with SPECS Phoibos 150 WAL hemispherical analyzer (2D detector) using SpecsLab Prodigy software and non-monochromated helium lamp. Fig. 2a: Detail of He IIa spectrum measured on h-BN/Cu(111) Full angle-resolved spectrum (intensity as function of angular coordinate and kinetic energy), summed over all energy channels and scans, exported from SpecsLab Prodigy as Igor Text. All measurement parameters are given in the file. Spectrum HeIIa h-BN Cu111 20201023.itx Spectrum scale converted into parallel momentum and binding energy: Spectrum HeIIa h-BN Cu111 20201023 k2.itx Fig. 2b: Detail of He IIa spectrum measured on h-BN/Cu2O/Cu(111) Full angle-resolved spectrum (intensity as function of angular coordinate and kinetic energy), summed over all energy channels and scans, exported from SpecsLab Prodigy as Igor Text. All measurement parameters are given in the file. Spectrum HeIIa h-BN Cu2O Cu111 20220211.itx Spectrum scale converted into parallel momentum and binding energy: Spectrum HeIIa h-BN Cu2O Cu111 20220211 k2.itx Fig. 2c: Spectra integrated over given parallel momentum range Intensity as a function of binding energy Spectrum HeIIa h-BN Cu111 20201023 k2 042_092.txt Spectrum HeIIa h-BN Cu2O Cu111 20220211 k2 041_091.txt ************************* Fig. 3 2PPE spectra of Cu(111), h-BN/Cu(111) and h-BN/Cu2O/Cu(111) ************************* Spectra measured with SPECS Phoibos 150 WAL hemispherical analyzer (2D detector) using SpecsLab Prodigy software and exported as Igor Text. All measurement parameters are given in the files. 3eV wavelength was 412nm, p-polarized. A -10V bias voltage was applied to the sample. Cu(111) 2PPE: P=1mW, Cu111 2022-04-14_20h47m45s.itx Cu(111) 3PPE: P=3mW, Cu111 2022-04-14_21h09m29s.itx h-BN/Cu(111) 2PPE: P=1.0mW, 0.5mm entrance slit, Cu111 hBN Spectrum3eV_2B_ppol.itx h-BN/Cu(111) 3PPE: P=0.4mW, 3.0mm entrance slit, Cu111 hBN Spectrum3eV_4B_ppol.itx h-BN/Cu2O/Cu(111) 2PPE: P~0.1mW, Cu111 hBN Cu2O 2022-02-11_17h07m34s.itx h-BN/Cu2O/Cu(111) 3PPE: P~0.1mW, Cu111 hBN Cu2O 2022-02-11_16h58m49s.itx ************************************************* Fig. 4 Delay Scan ************************************************* Delay scans are recorded with proprietary LabView software and saved in binary HDF5 format as a 3D stack of detector images (intensity as function of angular coordinate and kinetic energy) as a function of pump-probe delay. Bias -5V, 3eV=413nm 0.3mW p-pol, 6eV=208nm 1nA p-pol, Ekin=11.9eV, Epass=20eV, 1mm slit, exposure 10x500ms, 20 scans, 10fs steps Raw data, 256 angular pixels x 348 energy pixels x 201 delays x 20 scans: WAL_20220215_UZH_JB_dscan_040_0to9.h5 WAL_20220215_UZH_JB_dscan_040_10to19.h5 Sum of all scans (transposed): WAL_20220215_UZH_JB_dscan_040_sum.h5 Cropped to active detector window and applied distortion correction and correct scaling: dscan_20220215_040_data3Dcorrected.h5 Integrated over +-10° angular window: dscan040.itx Background averaged over delay positions 0-19 subtracted and energy and delay scales corrected: Fig. 4a: dscan040bgi0.itx Delay scan with 50ps range and 0.2ps steps, not shown in figure but analyzed the same way: dscan041bgi.itx Fig. 4b: Intensity as a function of intermediate state energy 50 fs, delay positions 40-50: dscan040t0mbg.txt 1 ps, delay positions 140-150: dscan040t2mbg.txt 10 ps, delay positions 55-65 in dscan041bgi: dscan041t3mbg.txt Fig. 4c: detector image obtained by averaging images at delay positions 40-50 and subtracting the background image, then correcting the angular distortion by normalizing the intensity at the Fermi energy dscan040image_diff0.itx ************************************************* Fig. 5 Fit Curves ************************************************* The Igor Pro batch fitting procedure was used with a custom fitting function to fit the delay scan dscan040bgi binned in 0.1 eV intervals with index 0 at -0.3 eV. The displayed curves have index 13 (1.0 eV), 6 (0.3 eV) and 4 (0.1 eV). Binned delay scan: dscan040bgi_pix.itx Fit parameters: dscan040bgi_fitparams.txt Fit result: dscan040bgi_pixRateFits.itx Fast component: dscan040bgi_FastComponent.itx Slow component: dscan040bgi_SlowComponent.itx ******************************************* Fig. 6 Fit Results Comparison ******************************************* Relaxation times are extracted from the batch fit results of different delay scans. 3 nJ pump: dscan040bgi_fitparams.txt dscan_220215_040 20 nJ pump: dscan010_BG4_fitparams.txt dscan220703_010 (data: dscan010_BG4.itx, binned: dscan010_BG4_pix.itx, fits: dscan010_BG4_pixFit.itx) h-BN/Cu(111): dscan033_fitparams.txt (binned data: dscan044_hBN_side_pix1.itx, fits: dscan033_hBN_side_pixFits.itx) Lisowski et al.: LifetimesLisowski.txt (data from M. Lisowski, P. A. Loukakos, U. Bovensiepen, and M. Wolf, Femtosecond Dynamics and Transport of Optically Excited Electrons in Epitaxial Cu Films on Si(111)-7 x 7, Appl. Phys. A 79, 739 (2004)) Extrapolation: fit_LifetimesLisowski.txt, using power law tau=0.054797*E^(-1.1419) **************************************************** Fig. S1 XPS **************************************************** Preparation 1 before oxidation: Preparation 1 after oxidation: Preparation 2 after oxidation: B 1s: VG2Z220209N005.pesp B 1s: VG2Z220210N023.pesp B 1s: VG2Z220628N002.pesp N 1s: VG2Z220209N006.pesp N 1s: VG2Z220210N024.pesp N 1s: VG2Z220628N003.pesp C 1s: VG2Z220209N007.pesp C 1s: VG2Z220210N025.pesp C 1s: VG2Z220628N004.pesp O 1s: VG2Z220209N008.pesp O 1s: VG2Z220210N026.pesp O 1s: VG2Z220628N005.pesp Cu 2p:VG2Z220209N009.pesp Cu 2p:VG2Z220210N027.pesp Cu 2p:VG2Z220628N006.pesp **************************************************** Fig. S2 LEED **************************************************** 48 eV: LEED220209_048eV_Cu111_hBN.SBIG 70 eV: LEED220209_070eV_Cu111_hBN.SBIG 100 eV: LEED220209_100eV_Cu111_hBN.SBIG not shown in figure: 40 eV: LEED220209_040eV_Cu111_hBN.SBIG 120 eV: LEED220209_120eV_Cu111_hBN.SBIG **************************************************** Fig. S3 LEED **************************************************** 40 eV: LEED220210_040eV_Cu111_hBN_Cu2O.SBIG 48 eV: LEED220210_048eV_Cu111_hBN_Cu2O.SBIG 70 eV: LEED220210_070eV_Cu111_hBN_Cu2O.SBIG 100 eV: LEED220210_100eV_Cu111_hBN_Cu2O.SBIG 120 eV: LEED220210_120eV_Cu111_hBN_Cu2O.SBIG 150 eV: LEED220210_150eV_Cu111_hBN_Cu2O.SBIG **************************************************** Fig. S4 LEED **************************************************** 40 eV: LEED220628_040eV_Cu111_hBN_Cu2O.SBIG 48 eV: LEED220628_048eV_Cu111_hBN_Cu2O.SBIG 70 eV: LEED220628_070eV_Cu111_hBN_Cu2O.SBIG 100 eV: LEED220628_100eV_Cu111_hBN_Cu2O.SBIG 110 eV: LEED220628_110eV_Cu111_hBN_Cu2O.SBIG 140 eV: LEED220628_140eV_Cu111_hBN_Cu2O.SBIG 180 eV: LEED220628_180eV_Cu111_hBN_Cu2O.SBIG not shown in figure: 120 eV: LEED220628_120eV_Cu111_hBN_Cu2O.SBIG 150 eV: LEED220628_150eV_Cu111_hBN_Cu2O.SBIG ********************************************* Fig. S5 Work function maps ********************************************* Ekin=10.4eV, Epass=20eV, 6eV=208.5nm 0.2nA p-pol, 1mm slit, -5V bias, exposure 1x500ms, 0.1mm steps, 81x81 pixels Preparation 1: Data cube after lens correction and correct scaling: dscan_220221_003_data3Dcorrected.h5 Integrated over all angles and brought into raster format: dscan_220221_003_raster.h5 Work function map: dscan_220221_003_rasterWF.itx Inset: Photograph of the sample after preparation 1, "Photograph hBN Cu2O Cu.png" Preparation 2: Data cube after lens correction and correct scaling: dscan_220704_011_data3Dcorrected.h5 Integrated over all angles: dscan_220704_011_raster.h5 Work function map: dscan_220704_011_rasterWF.itx ************************************************ Fig. S6 Delay Scans ************************************************ See description of Fig. 4 for processing details. Pump power was measured with a thermal powermeter and probe power was measured with a Thorlabs SM05PD7A GaP-photodiode with 14.4 mA/W sensitivity at 208nm. The thermal powermeter has an accuracy of +-0.1mW. 1.0mW pump power at 100kHz repetition rate equals 10nJ pulse energy. 1.0nA photodiode current equals approximately 70nW probe power or 0.7 pJ probe energy. Common settings: Bias voltage -5V, exposure time 20x500ms, 20 scans, 20fs steps, entrance slit size 1mm, Pass energy 20eV, Kinetic energy 11.9 eV, pump wavelength 413nm, pump and probe beam are p-polarized. Differing settings are written for each dataset. Fig. S6a: dscan_220215_040/dscan040bgi0.itx, processed raw data: dscan_220215_040_data3Dcorrected.h5 0.3mW pump, 208nm 1.0nA probe, 10fs steps, exposure 10x500ms Fig. S6b: dscan_220217_045/dscan045bgi.itx, processed raw data: dscan_220217_045_data3Dcorrected.h5 0.3mW pump, 208nm 1.0nA probe, 10fs steps, Ekin=13.3eV, Epass=30eV, 7mm slit, 50 scans Fig. S6c: dscan_220216_041/dscan041bgi.itx, processed raw data: dscan_220216_041_data3Dcorrected.h5 0.3mW pump, 208nm 1.0nA probe, 200fs steps, exposure 10x500ms, 10 scans ************************************************ Fig. S7 Delay Scans ************************************************ Common settings: same as in Fig. S6 Fig. S7a: dscan_220701_020/dscan020bgi0.itx, processed raw data: dscan_220701_020_data3Dcorrected.h5 0.5mW pump, 209nm 0.9nA probe, 3mm slit Fig. S7b: dscan_220712_029/dscan029_BG.itx, processed raw data: dscan_220712_029_data3Dcorrected.h5 1.5mW pump, 209nm 1.0nA probe Fig. S7c: dscan_220703_010/dscan010_BG4.itx, processed raw data: dscan_220703_010_data3Dcorrected.h5 2.0mW pump, 209nm 0.9nA probe Fig. S7d: dscan_220417_033/dscan033_hBN_side0.itx, processed raw data: dscan_220417_033_data3Dcorrected.h5 1.0mW pump, 208nm 0.1nA probe, Ekin=11.8eV, 40 scans *********************************************** Fig. S8 Beam profiles *********************************************** 8-bit CCD images were acquired with a Basler puA1280-54um CCD camera and Basler pylonViewer 5.0 acquisition software. Sensor resolution: 1280x960, pixel size: 3.75x3.75 micrometers 3 eV image: 10 microseconds exposure time, Spot3eV 10us 20mm.bmp 6 eV image: 100 milliseconds exposure time, Spot6eV 100ms 20mm.bmp

The main data analysis was done with Wavemetrics Igor Pro 7.08 using user-defined macros. General Remarks The calibration factor for our electron analyzer in the laboratory source XPS measurements was determined based on the peak positions of Au 4f7/2 Ag 3d5/2, and Cu 2p3/2 acquired from freshly prepared metallic Au, Ag, and Cu samples. For a pass energy of 50 eV: CKE=(MKE-0.58267)/0.99913 where CKE and MKE represent calibrated and measured kinetic energies, respectively. The XPD and FSM patterns throughout this dataset are presented in a file structure: tab-separated YXZT. To obtain the patterns as they were represented in the manuscript, first plot Y vs. X in marker mode. Assign Z and T as marker size and color, respectively. %%% Figure 1 %%% Figure 1.a and Figure 1.b The high symmetry points: |ΓL|= √3π/a |ΓX|= 2π/a |ΓW|= √5π/a |ΓK|= √3π/2a |ΓU|= √3π/2a Figure 1.c The SBZ boundaries for the bulk and reconstructed cell: |ΓX|bulk= √2π/a |ΓX|rec= π/a Figure 1.d File: "Fig1d.tiff" LEED Image (Energy=20 eV) Figure 1.e File: "Fig1e.txt" File structure(Left-to-right): Tab separated YXZT The Fermi surface of a pristine Fe3O4(001), excitation: He Ia (21.22 eV) Figure 1.f File: "Fig1f.txt" File structure(Left-to-right): Tab separated YXZT The Fermi surface of a pristine Fe3O4(001), excitation: He IIa (40.80 eV) %%% Figure 2 %%% Angular distribution maps of photoelectrons excited from their fixed initial-state energy Ei (EF, 0.22 eV, 0.43 eV, 1.29 eV, 1.49 eV and 1.70 eV) mapped versus parallel momentum. Figure 2.a Files: "Fig2a_EF.txt, Fig2a_0p22eV.txt, Fig2a_0p43eV.txt, Fig2a_1p29eV.txt, Fig2a_1p49eV.txt, Fig2a_1p70eV.txt" File structure(Left-to-right): Tab separated YXZT The measurements were acquired with a He Ia excitation. Figure 2.b Files: "Fig2b_EF.txt, Fig2b_0p22eV.txt, Fig2b_0p43eV.txt, Fig2b_1p29eV.txt, Fig2b_1p49eV.txt, Fig2b_1p70eV.txt" File structure(Left-to-right): Tab separated YXZT The simulations, which were obtained with the RSMS code, were acquired to mimic He Ia excitation. Figure 2.c Files: "Fig2c_EF.txt, Fig2c_0p22eV.txt, Fig2c_0p43eV.txt, Fig2c_1p29eV.txt, Fig2c_1p49eV.txt, Fig2c_1p70eV.txt" File structure(Left-to-right): Tab separated YXZT The measurements were acquired with a He IIa excitation. Figure 2.d Files: "Fig2d_EF.txt, Fig2b_0p22eV.txt, Fig2d_0p43eV.txt, Fig2d_1p29eV.txt, Fig2d_1p49eV.txt, Fig2d_1p70eV.txt" File structure(Left-to-right): Tab separated YXZT The simulations, which were obtained with the RSMS code, were acquired to mimic He IIa excitation. %%% Figure 3 %%% Figure 3.a File: "Fig3a.txt" "File structure: Tab-separated YYYX, where the first three columns (from left to right) represent spectra taken from the pristine, as-grown, and post-annealed samples, respectively. Column X (Ni 2p) denotes the binding energy. Figure 3.b File: "Fig3b.txt" "File structure: Tab-separated YYYX, where the first three columns (from left to right) represent spectra taken from the pristine, as-grown, and post-annealed samples, respectively. Column X (Fe 2p) denotes the binding energy. Figure 3.c-e File: "Fig3c.txt, Fig3d.txt, Fig3e.txt " File structure(Left-to-right): Tab separated YXZT %%% Figure 4 %%% Figure 4.a File: "Fig4a_010.txt, Fig4a_110.txt, Fig4ac_kpar.txt" File structure (Left-to-right): YY….YY (Fig4a_010.txt, Fig4a_110.txt) vs. X (Fig4ac_kpar.txt) Parallel momentum range from k=0 Å-1 to k=1.83 Å-1. Energy range Ebin= 0 eV to Ebin= 2.6 eV. Plot type: MDC, Sample: Pristine Fe3O4(001), Crystal directions: [010] and [110] Note that the data presented in the manuscript were given with a Y-offset. Figure 4.b File: "Fig4b_010.txt, Fig4b_110.txt, Fig4bd_en.txt" File structure (Left-to-right): YY….YY (Fig4b_010.txt, Fig4b_110.txt) vs. X (Fig4bd_en.txt) Parallel momentum range from k=0 Å-1 to k=1.83 Å-1. Energy range Ebin= 0 eV to Ebin= 2.6 eV. Plot type: EDC, Sample: Pristine Fe3O4(001), Crystal directions: [010] and [110] Note that the data presented in the manuscript were given with a Y-offset. Figure 4.c File: "Fig4c_010.txt, Fig4c_110.txt, Fig4ac_kpar.txt " File structure (Left-to-right): YY….YY (Fig4e_010.txt, Fig4e_110.txt) vs. X (Fig4ac_kpar.txt) Parallel momentum range from k=0 Å-1 to k=1.83 Å-1. Energy range Ebin= 0 eV to Ebin= 2.6 eV. Plot type: MDC, Sample: Post-annealed 26-ML Ni/Fe3O4(001), Crystal directions: [010] and [110] Note that the data presented in the manuscript were given with a Y-offset. Figure 4.d File: "Fig4d_010.txt, Fig4d_110.txt , Fig4bd_en.txt" File structure (Left-to-right): YY….YY (Fig4d_010.txt, Fig4d_110.txt) vs. X (Fig4bd_en.txt) Parallel momentum range from k=0 Å-1 to k=1.83 Å-1. Energy range Ebin= 0 eV to Ebin= 2.6 eV. Plot type: EDC, Sample: Post-annealed 26-ML Ni/Fe3O4(001), Crystal directions: [010] and [110] Note that the data presented in the manuscript were given with a Y-offset. %%% Figure 5 %%% Figure 5.a File: "Fig5a.txt" File structure: Tab-separated YYX(Mg Ka) YYX(He Ia), where Y represents spectra taken from the pristine and post-annealed samples from left to right, respectively, and X denotes the binding energies. Figure 5.b File: "Fig5b.txt" File structure : Tab separated YX-YYYX-YX YX(Measurement) Left-to-right (Y: The spectrum from the pristine sample, X: Binding energy) YYYX(TDOS) Left-to-right (Y: Spin up, Y: Spin down, Y: Spin Tot, X: Energy(X-offset: 4.943 eV) for NixFe3-xO4 (x=0)) YX(Gaussian-broadened TDOS) Broadening:0.5 eV Left-to-right (Y: Spin Tot, X: Energy) Figure 5.c File: "Fig5c.txt" File structure: Tab separated YX-YYYX-YX YX(Measurement) Left-to-right (Y: The spectrum from the post-annealed sample, X: Binding energy) YYYX(TDOS) Left-to-right (Y: Spin up, Y: Spin down, Y: Spin Tot, X: Energy(X-offset: 5.343 eV) for NixFe3-xO4 (x=1)) YX(Gaussian-broadened TDOS) Broadening:0.5 eV Left-to-right (Y: Spin Tot, X: Energy) Figure 5.d File: "Fig5d.txt" File structure: Tab separated YX-YYYX-YX YX(Measurement) Left-to-right (Y: XP spectrum from the post-annealed sample, X: Binding Energy) YYYX(TDOS) Left-to-right (Y: Spin up, Y: Spin down, Y: Spin Tot, X: Energy(X-offset: 5.593 eV) for NixFe3-xO4 (x=2)) YX(Gaussian-broadened TDOS) Broadening:0.5 eV Left-to-right (Y: Spin Tot, X: Energy) Figure 5.e File: "Fig5e.txt" File structure: Tab separated Y-YY-YY-X Left-to-right (Y:Fe Total PDOS, Y: Fetet PDOS spin up, Y: Fetet PDOS spin down , Y: Feoct PDOS spin up, Y: Feoct PDOS spin down, X: Energy for for NixFe3-xO4 (x=0)) Gaussian Broadening: 0.2 eV Figure 5.f File: "Fig5f.txt" File structure: Tab separated Y-YY-YY-YY-X Left-to-right (Y: Ni and Fe Total PDOS, Y: Nioct PDOS spin up, Y: Nioct PDOS spin down , Y: Fetet PDOS spin up, Y: Fetet PDOS spin down, Y: Feoct PDOS spin up, Y: Feoct PDOS spin down, X: Energy(X-offset: 1.93 eV) for NixFe3-xO4 (x=1)) Gaussian Broadening: 0.2 eV Figure 5.g File: "Fig5g.txt" File structure: Tab separated Y-YY-YY-X Left-to-right (Y: Ni and Fe Total PDOS, Y: Nioct PDOS spin up, Y: Nioct PDOS spin down, Y: Fetet PDOS spin up, Y: Fetet PDOS spin down, X: Energy(X-offset: 2.12 eV) for NixFe3-xO4 (x=2)) Gaussian Broadening: 0.2 eV %%% Figure 6 %%% Figure 6.a File: "Fig6a.txt" File structure(Left-to-right): Tab separated Y-Y-Y-X (Y: 120 ML, Y:50 ML, Y: 10 ML, X: Energy) (X-offset: 2 eV) Figure 6.b File: "Fig6b.txt" File structure(Left-to-right): Tab separated Y-Y-Y-Y-X (Y: 120 ML, Y:50 ML, Y: 10 ML, Y:Pristine, X: Ni 2p Energy) %%% Figure 7 %%% Figure 7.a File: "Fig7a.txt" File structure(Left-to-right): Tab separated Y-Y-Y-Y-X (Y: 120 ML, Y:50 ML, Y: 10 ML, Y:Pristine, X: Energy) (X-offset: 2.8 eV) Figure 7.b File: "Fig7b.txt" File structure(Left-to-right): Tab separated Y-Y-Y-Y-X (Y: 120 ML, Y:50 ML, Y: 10 ML, Y:Pristine, X: Fe 2p Energy) %%% Figure 8 %%% Figure 8.a File: "Fig8a.txt" File structure(Left-to-right): Tab separated Y-Y-Y-Y-X (Y: 120 ML, Y:50 ML, Y: 10 ML, Y:Pristine, X: Energy) (X-offset: 1.1 eV) Figure 8.b File: "Fig8b.txt" File structure(Left-to-right): Tab separated Y-Y-Y-X (Y: (120 ML-Pristine), Y: (50 ML-Pristine), Y: (10 ML-Pristine), X: Energy) %%% SUPLLEMENTARY INFORMATION %%% %%% Figure SI3 %%% File: "FigSI3_pristine.txt, FigSI3_26ML_AG.txt, FigSI3_26ML_PA.txt" File structure (Left-to-right): XY1-XY2-XY3… XY9-XY10-XY11-XY12 (AG: As-grown, PA: Post-annealed) XY1 – XY9 (Y: Fit Peak, X: Energy) XY10 (Y: Cumulative Fit Peak, X: Energy) XY11 (Y: Residue, X: Energy) XY12 (Y: Shirley BG Subtracted XPS Measurement, X: Energy) %%% Figure SI4 %%% Figure SI4.a File: "FigSI4_oct.txt" File structure(Left-to-right): YYYYYY1-YYYYYY2-YYYYYY3… YYYYYY4-X YYYYYY1 (Y: PDOS eg down, PDOS Tot down, PDOS t2g down, PDOS eg up, PDOS Tot up, PDOS t2g up) Layer: S YYYYYY2 (Y: PDOS eg down, PDOS Tot down, PDOS t2g down, PDOS eg up, PDOS Tot up, PDOS t2g up) Layer: S-2 YYYYYY3 (Y: PDOS eg down, PDOS Tot down, PDOS t2g down, PDOS eg up, PDOS Tot up, PDOS t2g up) Layer: S-6 YYYYYY4 (Y: PDOS eg down, PDOS Tot down, PDOS t2g down, PDOS eg up, PDOS Tot up, PDOS t2g up) Layer: S-8 X: Energy Figure SI4.b File: "FigSI4_tet.txt" File structure(Left-to-right): YYYYYY1-YYYYYY2-YYYYYY3… YYYYYY4-X YYYYYY1 (Y: PDOS eg down, PDOS Tot down, PDOS t2g down, PDOS eg up, PDOS Tot up, PDOS t2g up) Layer: S-1 YYYYYY2 (Y: PDOS eg down, PDOS Tot down, PDOS t2g down, PDOS eg up, PDOS Tot up, PDOS t2g up) Layer: Interstitial Tet YYYYYY3 (Y: PDOS eg down, PDOS Tot down, PDOS t2g down, PDOS eg up, PDOS Tot up, PDOS t2g up) Layer: S-3 YYYYYY4 (Y: PDOS eg down, PDOS Tot down, PDOS t2g down, PDOS eg up, PDOS Tot up, PDOS t2g up) Layer: S-7 X: Energy Channeltron positions: 0.00 eV, 0.22 eV, 0.43 eV, 1.29 eV, 1.49 eV and 1.70 eV %%% Figure SI5 %%% Figure SI5.a File: "FigSI5a_EF.txt, FigSI5a_0p22eV.txt, FigSI5a_0p43eV.txt, FigSI5a_1p29eV.txt, FigSI5a_1p49eV.txt, FigSI5a_1p70eV.txt " File structure(Left-to-right): Tab separated YXZT Figure SI5.b File: "FigSI5b_EF.txt, FigSI5b_0p22eV.txt, FigSI5b_0p43eV.txt, FigSI5b_1p29eV.txt, FigSI5b_1p49eV.txt, FigSI5b_1p70eV.txt " File structure(Left-to-right): Tab separated YXZT Figure SI5.c File: "FigSI5c_EF.txt, FigSI5c_0p22eV.txt, FigSI5c_0p43eV.txt, FigSI5c_1p29eV.txt, FigSI5c_1p49eV.txt, FigSI5c_1p70eV.txt " File structure(Left-to-right): Tab separated YXZT %%% Figure SI6 %%% Figure SI6.a File: "FigSI6a_EF.txt, FigSI6a_0p22eV.txt, FigSI6a_0p43eV.txt, FigSI6a_1p29eV.txt, FigSI6a_1p49eV.txt, FigSI6a_1p70eV.txt " File structure(Left-to-right): Tab separated YXZT Figure SI6.b File: "FigSI6b_EF.txt, FigSI6b_0p22eV.txt, FigSI6b_0p43eV.txt, FigSI6b_1p29eV.txt, FigSI6b_1p49eV.txt, FigSI6b_1p70eV.txt " File structure(Left-to-right): Tab separated YXZT Figure SI6.c File: "FigSI6c_EF.txt, FigSI6c_0p22eV.txt, FigSI6c_0p43eV.txt, FigSI6c_1p29eV.txt, FigSI6c_1p49eV.txt, FigSI6c_1p70eV.txt " File structure(Left-to-right): Tab separated YXZT %%% Figure SI7 %%% Figure SI7.a File: "FigSI7a.tif" LEED Image (Energy=20 eV), Sample: Pristine Fe3O4(001) Figure SI7.b File: "FigSI7b.tif" LEED Image (Energy=20 eV), Sample: As-grown 26 ML Ni/Fe3O4(001) Figure SI7.c File: "FigSI7c.tif" LEED Image (Energy=20 eV), Sample: Post-annealed 26 ML Ni/Fe3O4(001) %%% Figure SI8 %%% File: "FigSI8_XPS_and_UPS.txt, FigSI8_Fe3O4_DOS.txt, FigSI8_NiFe2O4_DOS.txt, FigSI8_Ni2FeO4_DOS.txt" Experiment: FigSI8_XPS_and_UPS.txt File structure(Left-to-right): Tab separated YYX(Mg Ka) YYX(He Ia), where Y are the spectra taken from the pristine and post-annealed sample left to right, respectively, and X donate the binding energies. DOS (Fe3O4): FigSI8_Fe3O4_DOS.txt File structure: YYYX(TDOS) - YX(Gaussian-broadened TDOS) Broadening:0.5 eV (Y: Spin Tot, Y: Spin up, Y: Spin down, X: Energy) (X-offset: 4.943 eV) (Y: Spin Tot, X: Energy) DOS (NiFe2O4): FigSI8_NiFe2O4_DOS.txt File structure: YYYX(TDOS) - YX(Gaussian-broadened TDOS) Broadening:0.5 eV (Y: Spin Tot, Y: Spin up, Y: Spin down, X: Energy) (X-offset: 5.343 eV) (Y: Spin Tot, X: Energy) DOS (NiFe2O4): FigSI8_Ni2FeO4_DOS.txt File structure: YYYX(TDOS) - YX(Gaussian-broadened TDOS) Broadening:0.5 eV (Y: Spin Tot, Y: Spin up, Y: Spin down, X: Energy) (X-offset: 5.593 eV) (Y: Spin Tot, X: Energy) %%% Figure SI9 %%% Figure SI9.a File: "Fig9a.txt" File structure: YYYYYY1-YYYYYY2-X YYYYYY1 (Y: PDOS eg down, PDOS Tot down, PDOS t2g down, PDOS eg up, PDOS Tot up, PDOS t2g up) Coordination: Fe_oct YYYYYY2 (Y: PDOS eg down, PDOS Tot down, PDOS t2g down, PDOS eg up, PDOS Tot up, PDOS t2g up) Coordination: Fe_tet X: Energy Figure SI9.b File: "Fig9b.txt" File structure: YYYYYY1-YYYYYY2-YYYYYY3-X YYYYYY1 (Y: PDOS eg down, PDOS Tot down, PDOS t2g down, PDOS eg up, PDOS Tot up, PDOS t2g up) Coordination: Fe_oct YYYYYY2 (Y: PDOS eg down, PDOS Tot down, PDOS t2g down, PDOS eg up, PDOS Tot up, PDOS t2g up) Coordination: Fe_tet YYYYYY3 (Y: PDOS eg down, PDOS Tot down, PDOS t2g down, PDOS eg up, PDOS Tot up, PDOS t2g up) Coordination: Ni_oct X: Energy Figure SI9.c File: "Fig9c.txt" File structure(Left-to-right): YYYYYY1-YYYYYY2-X YYYYYY1 (Y: PDOS eg down, PDOS Tot down, PDOS t2g down, PDOS eg up, PDOS Tot up, PDOS t2g up) Coordination: Fe_tet YYYYYY2 (Y: PDOS eg down, PDOS Tot down, PDOS t2g down, PDOS eg up, PDOS Tot up, PDOS t2g up) Coordination: Ni_oct X: Energy %%% Figure SI10 %%% File: "FigSI10_010.txt, FigSI10_110.txt, FigSI10_Energy.txt" File structure(Left-to-right): YY….YY (FigSI10_010.txt, FigSI10_110.txt) vs. X (FigSI10_Energy.txt) Parallel momentum range from k=0 Å-1 to k=1.83 Å-1. Plot type: EDC, Sample: Pristine Fe3O4(001), Crystal directions: [010] and [110] Note that the data presented in the manuscript were given with a Y-offset. %%% Figure SI11 %%% Figure SI11.a File: "Fig SI11a.txt" File structure(Left-to-right): Tab separated Y-Y-Y-X (Y: 120 ML, Y:50 ML, Y: 10 ML, X: Energy) (X-offset: 2 eV) Figure SI11.b File: "Fig SI11b.txt" File structure(Left-to-right): Tab separated Y-Y-Y-Y-X (Y: 120 ML, Y:50 ML, Y: 10 ML, Y:Pristine, X: Ni 2p Energy) Figure SI11.c File: "FigSI11c.txt" File structure(Left-to-right): Tab separated Y-Y-Y-Y-X (Y: 120 ML, Y:50 ML, Y: 10 ML, Y: Pristine, X: Energy) (X-offset: 2.8 eV) Figure SI11.d File: "FigSI11d.txt" File structure(Left-to-right): Tab separated Y-Y-Y-Y-X (Y: 120 ML, Y:50 ML, Y: 10 ML, Y:Pristine, X: Fe 2p Energy) Figure SI11.e File: "FigSI11e.txt" File structure(Left-to-right): Tab separated Y-Y-Y-Y-X (Y: 120 ML, Y:50 ML, Y: 10 ML, Y: Pristine, X: Energy) (X-offset: 1.1 eV) Figure SI11.f File: "FigSI11f.txt" File structure(Left-to-right): Tab separated Y-Y-Y-Y-X (Y: 120 ML, Y:50 ML, Y: 10 ML, Y:Pristine, X: O 1s Energy)

Carboxylic acids bind to titanium dioxide (TiO2) dissociatively, forming surface superstructures that give rise to a (2 × 1) pattern detected by low-energy electron diffraction. Exposing this system to water, however, leads to a loss of the highly ordered surface structure. The formate-covered surface was investigated by a combination of diffraction and spectroscopy techniques, together with static and dynamic ab initio simulations, with the conclusion that a dynamic equilibrium exists between adsorbed formic acid and water molecules. This equilibrium process is an important factor for obtaining a better understanding of controlling the self-cleaning properties of TiO2, because the formic acid monolayer is responsible for the amphiphilic character of the surface.

Magnetite (Fe3O4) doped with earth-abundant metals has become a promising catalyst material. In particular, Ni-doped magnetite (Ni/Fe3O4) has been demonstrated as a cheap, robust, and catalytically active material in photocatalytic and electrochemical water oxidation. Recently, the incorporation of Ni atoms in Fe3O4 single crystalline surfaces was studied extensively with scanning tunneling microscopy and density-functional theory calculations. However, because of its importance for catalytic activity, this incorporation process and the determination of the lattice sites occupied by the Ni atoms require further experimental study. In this work, we investigated the surface structure of as-grown and annealed Ni/Fe3O4(001) as a function of Ni coverage under ultra-high vacuum conditions with temperature-dependent x-ray and ultraviolet photoelectron spectroscopy, x-ray photoelectron diffraction, and low-energy electron diffraction. We observe that octahedrally coordinated subsurface cation vacancy sites are already occupied upon Ni deposition at room temperature and that Ni atoms start to diffuse further into the octahedral subsurface sites with increasing temperature.

Hydrogen as a fuel plays a crucial role in driving the transition to net zero greenhouse gas emissions. To realise its potential, obtaining a means of efficient storage is paramount. One solution is using metal hydrides, owing to their good thermodynamical absorption properties and effective hydrogen storage. Although metal hydrides appear simple compared to many other energy materials, understanding the electronic structure and chemical environment of hydrogen within them remains a key challenge. This work presents a new analytical pathway to explore these aspects in technologically relevant systems using Hard X-ray Photoelectron Spectroscopy (HAXPES) on thin films of two prototypical metal dihydrides: YH_2-δ and TiH_2-δ. By taking advantage of the tunability of synchrotron radiation, a non-destructive depth profile of the chemical states is obtained using core level spectra. Combining experimental valence band spectra collected at varying photon energies with theoretical insights from density functional theory (DFT) calculations, a description of the bonding nature and the role of d versus sp contributions to states near the Fermi energy are provided. Moreover, a reliable determination of the enthalpy of formation is proposed by using experimental values of the energy position of metal s band features close to the Fermi energy in the HAXPES valence band spectra.

Stable single metal adatoms on oxide surfaces are of great interest for future applications in the field of catalysis. We studied iridium single atoms (Ir1) supported on a Fe3O4(001) single crystal, a model system previously only studied in ultra-high vacuum, to explore their behavior upon exposure to several gases in the millibar range (up to 20 mbar) utilizing ambient-pressure X-ray photoelectron spectroscopy. The Ir1 single adatoms appear stable upon exposure to a variety of common gases at room temperature, including oxygen (O2), hydrogen (H2), nitrogen (N2), carbon monoxide (CO), argon (Ar), and water vapor. Changes in the Ir 4f binding energy suggest that Ir1 interacts not only with adsorbed and dissociated molecules but also with water/OH groups and adventitious carbon species deposited inevitably under these pressure conditions. At higher temperatures (473 K), iridium adatom encapsulation takes place in an oxidizing environment (a partial O2 pressure of 0.1 mbar). We attribute this phenomenon to magnetite growth caused by the enhanced diffusion of iron cations near the surface. These findings provide an initial understanding of the behavior of single atoms on metal oxides outside the UHV regime.

Experimental and computational, archived in a single zip file

Cu2O has appealing properties as an electrode for photoelectrochemical water splitting, yet its practical performance is severely limited by inefficient charge extraction at the interface. Using hybrid DFT calculations, we investigate carrier capture processes by oxygen vacancies (V-O) in the experimentally observed (root 3 x root 3)R30 degrees reconstruction of the dominant (111) surface. Our results show that these V-O are doubly ionized and that associated defects states strongly suppress electron transport. In particular, the excited electronic state of a singly charged V-O plays a crucial role in the nonradiative electron capture process with a capture coefficient of about 10(-9) cm(3)/s and a lifetime of 0.04 ps, explaining the experimentally observed ultrafast carrier relaxation. These results highlight that engineering the surface V-O chemistry will be a crucial step in optimizing Cu2O for photoelectrode applications.

Molecular reactivity is determined by the energy levels and spatial extent of the frontier orbitals. Orbital tomography based on angle-resolved photoelectron spectroscopy is an elegant method to study the electronic structure of organic adsorbates, however, it is conventionally restricted to systems with one single rotational domain. In this work, we extend orbital tomography to systems with multiple rotational domains. We characterise the hydrogen evolution catalyst Co-pyrphyrin on an Ag(110) substrate and compare it with the empty pyrphyrin ligand. In combination with low-energy electron diffraction and DFT simulations, we fully determine adsorption geometry and both energetics and spatial distributions of the valence electronic states. We find two states close to the Fermi level in Co-pyrphyrin with Co 3d character that are not present in the empty ligand. In addition, we identify several energetically nearly equivalent adsorption geometries that are important for the understanding of the electronic structure. The ability to disentangle and fully elucidate multi-configurational systems renders orbital tomography much more useful to study realistic catalytic systems.

We study electronic properties and adsorption geometries of the molecular charge-transfer-complex tetrathiafulvalene-dipyrazine on Ag(110). Using a combination of angle-resolved photoemission and electron diffraction, supported by DFT-based simulations, renders a comprehensive picture of this interesting system. We find low interaction between the substrate and the molecule and thus little changes of the molecular geometry upon adsorption, as compared to the free gas phase molecule. Five electronic valence states can be unambiguously assigned owing to their distinctive photoemission patterns. The molecules adsorb aligned with the Ag rows in the first layer, while they are slightly rotated in the second layer. Additional intensity of the molecular photoemission signal near the Fermi energy indicates partial charge-transfer into formerly unoccupied states, most likely of intermolecular origin.

To achieve high power conversion efficiencies in photovoltaic or photoelectrochemical cells, an understanding of the dynamical processes in the electrode materials upon photoexcitation is crucial. Antimony sulfide is a promising candidate material. The authors present time-resolved two-photon photoemission measurements from a Sb${}_{2}$S${}_{3}$(100) single crystal surface. A first laser pulse generates a population of free charge carriers that are probed by a second pulse. Ultrafast relaxation towards the conduction band minimum is followed by a fast decay within 1.3 ps into two longer-lived trap states, self-trapped electrons and self-trapped excitons states, with lifetimes of 27 and 63 ps, respectively. The results support a polaronic self-trapping mechanism by optical phonons.

The strategy of anchoring molecular catalysts on electrode surfaces combines the high selectivity and activity of molecular systems with the practicality of heterogeneous systems. The stability of molecular catalysts is, however, far less than that of traditional heterogeneous electrocatalysts, and therefore a method to easily replace anchored molecular catalysts that have degraded could make such electrosynthetic systems more attractive. Here, we apply a non-covalent “click” chemistry approach to reversibly bind molecular electrocatalysts to electrode surfaces via host-guest complexation with surface-anchored cyclodextrins. The host-guest interaction is remarkably strong and allows the flow of electrons between the electrode and the guest catalyst. Electrosynthesis in both organic and aqueous media was demonstrated on metal oxide electrodes, with stability on the order of hours. The catalytic surfaces can be recycled by controlled release of the guest from the host cavities and readsorption of fresh guest. This strategy represents a new approach to practical molecular-based catalytic systems.

Carbon contamination is a notorious issue that has an enormous influence on surface science experiments, especially in near-atmospheric conditions. While it is often mentioned in publications when affecting an experiment’s results, it is more rarely analyzed in detail. We performed ambient-pressure x-ray photoelectron spectroscopy experiments toward examining the build-up of adventitious carbon species (both inorganic and hydrocarbons) on a clean and well-prepared surface using large-scale (50 × 10 mm2) rutile TiO2(110) single crystals exposed to water vapor and liquid water. Our results highlight how various factors and environmental conditions, such as beam illumination, residual gas pressure and composition, and interaction with liquid water, could play roles in the build-up of carbon on the surface. It became evident that beam-induced effects locally increase the amount of carbon in the irradiated area. Starting conditions that are independent of light irradiation determine the initial overall contamination level. Surprisingly, the rate of beam-induced carbon build-up does not vary significantly for different starting experimental conditions. The introduction of molecular oxygen in the order of 10 mbar allows for fast surface cleaning during x-ray illumination. The surface carbon contamination can be completely removed when the oxygen partial pressure is comparable to the partial pressure of water vapor in the millibar pressure range, as was tested by exposing the TiO2(110) surface to 15 mbar of water vapor and 15 mbar of molecular O2 simultaneously. Furthermore, our data support the hypothesis that the progressive removal of carbon species from the chamber walls by competitive adsorption of water molecules takes place following repeated exposure to water vapor. We believe that our findings will be useful for future studies of liquid-solid interfaces using tender x rays, where carbon contamination plays a significant role.

The thermal oxidation of Ru(0001) has been extensively studied in the surface science community to determine the oxidation pathway towards ruthenium dioxide (RuO2(110)), improving the knowledge of Ru(0001) surface chemistry. Using time-lapsed ambient-pressure x-ray photoelectron spectroscopy (APXPS), we investigate the thermal oxidation of single-crystalline Ru(0001) films toward rutile RuO2(110) in situ. APXPS spectra were continuously collected while the Ru(0001) films were exposed to a fixed O2 partial pressure of 10−2 mbar and the sample temperature was increased stepwise from room temperature to 400 °C. We initially observe the removal of adventitious carbon and subsequent formation of a chemisorbed oxygen overlayer at 250 °C. Further annealing to 300 °C leads to an increase in thickness of the oxide layer and a shift in the Ru–O component of the Ru 3d spectra, indicating the presence of a metastable O–Ru–O trilayer structure. A rapid formation of the RuO2 rutile phase with an approximate thickness of at least 2.6 nm is formed about four minutes after stabilizing the temperature at 350 °C and subsequent annealing to 400 °C, signaled by a distinct binding energy shift in both the Ru 3d and O 1s spectra, as well as quantitative analysis of XPS intensities. This observed autocatalytic oxidation process agrees well with previous theoretical models and experimental studies, and the data provide the unambiguous spectral identification of one proposed metastable precursor required for full oxidation to rutile RuO2(110). Further ex situ characterization of the grown oxide with x-ray photoelectron diffraction confirms the presence of three rotated domains of rutile RuO2(110) and reveals their orientation relative to the substrate lattice.