Specsurf version 1

XPS measurements were conducted at room temperature (~298 K) using a JPS 9010 TR spectrometer (JEOL, Ltd., Japan) equipped with an ultrahigh vacuum chamber and an Al Kα X-ray source (1486.6 eV). The pass energy was 10 eV, the energy resolution (estimated from the Ag 3d_5/2 peak width of a clean Ag sample) was 0.635 eV, and the binding energy uncertainty was ±0.05 eV. The sample was placed on an Au surface (Au sheet, Au-173421, 99.95%, Nilaco Co. Ltd.). The Shirley background was subtracted from the spectrum using SpecSurf version 1.8.3.7 (JEOL, Ltd., Japan). The charge build-up in the sample (due to the incomplete contact of the Au sheet with the sample holder of the apparatus) caused a slight shift to higher binding energies for those spectra. Therefore, we calibrated the charge build-up based on the Au 4f_7/2 peak as 84.0 eV.

To characterize the sample, XPS measurement was performed using a JPS 9010 TR spectrometer (XPS; JPS 9010 TR, JEOL, Ltd., Tokyo, Japan) equipped with an ultrahigh vacuum chamber and an Mg Kα X-ray source (1253.6 eV). The pass energy was 10 eV, the energy resolution (estimated from the Ag 3d5/2 peak width of a clean Ag sample) was 0.635 eV, and the binding energy uncertainty was ±0.05 eV. The sample was placed on a graphite tape. The Shirley background was subtracted from the spectrum using SpecSurf version 1.8.3.7 (JEOL, Ltd., Japan). The same software was used for estimating the quantitative atomic ratio of Ni and B of the reaction product, based on the peak area and sensitivity factors. The charge build-up in the sample (because of the incomplete contact of the graphite tape with the sample holder) resulted in a slight shift to higher binding energies for those spectra. Therefore, we calibrated the charge build-up based on the C1s peak of the graphite tape as 284.6 eV. The charge states of boron were judged based on the B 1s core level peak position by comparing the results with those in the literature (listed in the Supplementary Information of Ref. [46 (link)]).