Core-level and valence band XPS measurements were collected at HarwellXPS, Didcot, U.K. XPS analysis was performed using a Kratos Axis SUPRA XPS fitted with a monochromatic Al Kα X-ray source (hν = 1486.6 eV), a spherical sector analyser and 3 multichannel resistive plate, 128 channel delay line detectors. All data were recorded at 150 W and a spot size of 700 × 300 μm 2 . Survey scans were recorded at a pass energy of 160 eV, and high-resolution scans recorded at a pass energy of 20 eV. Electronic charge neutralization was achieved using a magnetic immersion lens. All sample data were recorded at a pressure below 10 -8 Torr and temperature of 294 K. Data were analysed using CasaXPS v2.3.19PR1.0. Peaks were fit with a Shirley background prior to component analysis. Mixed Gaussian-Lorentzian lineshapes (GL(50)) were used to fit components. All binding energies were measured with respect to the Fermi edge of an Ag foil reference sample.