Direct measurement of Ni incorporation into Fe3O4(001)† †Electronic supplementary information (ESI) available. See DOI: 10.1039/c8cp02516a

The normal incidence X-ray standing wave (NIXSW) technique has been used to follow the evolution of the adsorption geometry of Ni adatoms on the Fe3O4(001)-(√2 × √2)R45° surface as a function of temperature.

-Depicted is the Ni 2p core level with no background subtracted. The large background peak is the bulk Fe 3s core level peak. The Shirley background for this Fe 3s peak, which was subtracted from the clean background template to give the IS and SB templates as discussed in the text, is depicted in cyan. The Nisub, Niad and satellite fitted peaks are depicted in blue, orange and red respectively. Figure S2 gives an enlarged view of this spectrum in an energy range encompassing only the Ni 2p3/2 doublet. Figure S2 -An enlarged view of Figure S1, only showing the Ni 2p3/2 doublet. No background has been subtracted from this spectrum. The Nisub, Niad and satellite fitted peaks are depicted in blue, orange and red respectively.

Soft X-ray photoelectron spectroscopy (SXPS)
Ni 2p3/2 SXP spectra (hν = 1.1 keV), without background subtraction of the Fe 3s SXP spectra, are shown in Figure  S1 for Ni deposited onto Fe3O4(001) with the substrate held at room temperature and after annealing to 425 and 875 K. Ni 2p3/2 SXP spectra for Ni deposited onto Fe3O4(001) with the substrate held at 150 K, with and without background subtraction, are shown in Figures S2 and S3 (respectively). Figure S2 also shows the Ni 2p3/2 SXP spectra for Ni deposited onto Fe3O4(001) with the substrate held at room temperature, for comparison. Figure S3 -Raw Ni 2p3/2 SXP spectra (1.1 keV incident photon energy) for as deposited Ni (blue) and deposited Ni annealed to 425 K and 875 K (red and black respectively). No background has been subtracted from these spectra and they have not been normalised. Figure S4 -Ni 2p3/2 SXP spectra for Ni deposited at 300 K and 150 K. The species present at 150 K is assigned to a subsurface species, as confirmed by the NIXSW results, which exists in conjunction with an adatom species at

Ni 2p 3/2 -SXPS
Binding energy (eV) 300 K. The sole occupation of the subsurface sites by the Ni at 150 K is possibly due to dissociative water adsorption and the subsequent loss of the SCV surface reconstruction. Figure S5 -Raw soft Ni 2p3/2 XP spectrum for the Ni metal deposited at 150 K. No background has been subtracted from this spectrum.

Computational details
All the theoretical calculations were performed using the Vienna ab initio Simulation Package (VASP) 2,3 using the Projector Augmented Wave (PAW) approach 4,5 with a basis set cut-off energy of 550 eV. The hybrid functional HSE 6 utilised the standard mixing factor 25%, and screening length (0.207 -1 Å -1 ). The k-mesh of 2 x 2 x 1 was optimised such that it delivers total energy with an accuracy of better than 1 meV, while reducing the k-mesh by a factor of 2 at PBE level leads to a change in the total energy of only 1 meV. In all cases, structures were relaxed until forces were smaller than 0.02 eV/Å.
The surface calculations utilised an asymmetric surface slab (i.e. a slab with a relaxed surface on only a single side), resulting in a significantly cheaper calculation, with 9 fixed layers and 4 relaxed layers with the subsurface cation vacancy reconstruction 7 . Due to large size of the unit-cell (~100 atoms) the adoption of a symmetric setup would be computationally prohibitive for HSE-type calculations. The vacuum gap (separation between adjacent supercells perpendicular to the surface) was set to 14 Å. The resulting relaxed structure for a Ag adatom on Fe3O4(001) and a clean Fe3O4(001) surface are shown in Tables S1 and S2 (respectively).