XPS measurements were performed using a K-Alpha+ spectrometer (Thermo Fisher Scientific, East Grinstead, UK) operated in the constant analyzer energy mode and equipped with a micro-focused and monochromatic Al K X-ray source (1486.6 eV, spot size diameter: 400 m) and a flood gun (charge compensation system for insulating samples). The samples were clipped onto the sample holder and pumped under vacuum overnight. The step size and pass energy used were set to 1 eV/200 eV and 0.1 eV/40 eV for the survey and the high resolution regions, respectively. Data acquisition and processing were done using Avantage software, version 5.9902. The surface chemical composition was determined by using the manufacturer sensitivity factors.