## 2 Materials and Methods
### XPS Analysis  
The XPS spectra of the samples collected in the five-year period were acquired with two available spectrometers, Leybold LHX1 (Leybold GmbH merged into SPECS) for most part of the research project and subsequently with the new Phoibos100 MCD5 (SPECS, Surface Nano Analysis GmbH, Berlin, Germany). Both spectrometers, equipped with double-anode, achromatic Al/Mg K\(\alpha_{1,2}\) sources (1486.6/1253.6 eV) and KE\({}^{-1}\) as the transmission function, were used with the same operating modes and in parallel for repeated analyses, after having verified the close similarity of their performances with the acquisition of standard compounds. The wide and detailed spectra were all acquired at normal emission in FAT (Fixed Analyzer Transmission) and medium area (diameter = 2 mm) modes with channel widths of 1.0 eV and 0.1 eV, respectively, and the acquisition time of 0.5 s for each channel. The energy scale was calibrated using gold, copper and silver foils as the reference set for binding energies with the Au4f7/2 signal at 84.0 eV, Cu2p3/2 at 932.7 eV and Ag3d5/2 of silver at 368.3 eV [19; 20].  
Powdered samples were prepared for XPS acquisition by pressing them with a stainless steel spatula on double-sided copper tape previously fixed on a steel sample holder. The preparation was carefully performed to ensure a uniform thin layer just thick enough to avoid any signal from the adhesive tape and to prevent loose particles from contaminating the vacuum chambers by shaking and tilting the sample holder downwards, as recommended by ISO guidelines. After this operation, when the pressure in the pre-chamber was reduced to about 10\({}^{-5}\) Pa, the sample was introduced into the analysis chamber of the spectrometer(s), where a pressure of \(\leq\)10\({}^{-7}\) Pa had been reached, as required for ultra-high vacuum (UHV) analysis.  
The acquired spectra were resolved into the component peaks and associated background using the homemade program NewGoogly, which allows the simultaneous curve-fitting of the intrinsic and extrinsic features of XPS spectra [7; 21].  
In this, as in a previous work, the figures show the spectra 'as acquired', while the tables with the curve-fitting results (of detailed regions) report the peak areas 'normalized' for the sensitivity factors (FS) associated with the orbitals of the elements under examination and the binding energies (BEs) 'corrected' for the surface electrostatic charging of the sample under analysis, referring to the aliphatic carbon, identified in all curve-fitted C1s spectra as the internal standard (IS) and setting its BE to 285.0 eV. The BE values have an instrumental uncertainty of \(\pm\)0.1 eV, but the total uncertainty, where reported (vide infra), derives from the results of repeated analysis and curve-fitting procedures.  
All the identified peaks were assigned to the various chemical speciesby comparison with the standard spectra acquired in the laboratory and with those reported in the XPS database available online ([[https://srdata.nist.gov/xps](https://srdata.nist.gov/xps)]([https://srdata.nist.gov/xps](https://srdata.nist.gov/xps)) periodically consulted, last accessed 20 February 2025).