Auriga microscope

Scanning Electron Microscopy (SEM) measurements were conducted using an Auriga microscope (Zeiss, Oberkochen, Germany) equipped with an Energy Dispersive X-ray Spectroscopy (EDX) device (Apollo XP from EDAX, Ametek GmbH, Wiesbaden, Germany). The SEM images were obtained at an accelerating voltage of 2 kV with the SE2—detector. Cross sections of the SC and negative control samples (SNC) were prepared by ion beam milling at 30 keV using a gallium focused ion beam (FIB) attached to the Auriga. To prevent damage to the cells and the corrosion layer as well as to obtain a precise cut along the degradation layer, a layer of platinum was deposited on the surface by the Gas-Injection-System (GIS). Five EDX—line scans were performed directly after FIB milling at 15 keV to define the vertical element distribution of the degradation layer on the cross-sections. The EDX scans were conducted in 8 regions of interest in the respective EDX spectrum, i.e., carbon (C), nitrogen (N), oxygen (O), sodium (Na), Mg, phosphorus (P), sulphur (S) and calcium (Ca), complemented by the alloying elements of the respective sample, i.e., Ag and Gd. Each degradation layer cross section was analysed using this method. To obtain a high counting rate for the EDX analysis, the SEM aperture (120 μm diameter) was used in high-current mode.

The crystal structure of the as-synthesized samples was determined by X-ray powder diffraction (XRD) using a Bruker D8 Advance diffractometer equipped with a Cu Kα radiation (1.5417 Å) source, a LYNXEYE super speed detector and a Ni filter. X-ray diffraction data was collected in the 2θ range between 20° and 80° using a scan rate of 0.05° by 2 s. The light absorption properties were measured by using a UV-Vis diffuse reflectance spectrophotometer (Perkin Elmer Lambda 950 UV-Vis) with a wavelength range of 250–800 nm. A Zeiss Auriga microscope equipped with energy-dispersive X-ray (EDX) detector was used to perform scanning electron microscopy (SEM) and EDX analysis.
X-ray photoelectron spectroscopy (XPS) from SPECS system (Germany) was used to identify Mn and Fe oxidation states on the samples. The instrument was equipped with XR50 duel anode source (Al operated at 150 W) and a Phoibos MCD-9 detector. All measurements were done under the vacuum (pressure 5 × 10⁻⁹ mbar) and the hemispherical analyzer was set at the pass energy 25 eV while the high resolution spectra step size was set at 0.1 eV. Casa XPS program (Casa Software Ltd., UK) was used for the data analysis.

SEM provides morphological images in the microscopic range through a beam of electrons that react with atoms on the surface of a sample, leading to the emission of electrons from the surface. While scanning the surface, variations in the signals of the backscattered electrons occur according to the surface topology, thus providing information on the sample surface morphology and composition [67 ]. This technique was used for MP shape characterization in Milli-Q water and synthetic seawater. For SEM analysis, MP and NP solutions with concentrations of 0.02 mg/mL (dispersed in Milli-Q water and synthetic seawater) were applied on a carbon-coated adhesive, dried at room temperature, and examined using a scanning electron microscope (Auriga microscope, Carl Zeiss, Jena, Germany) with a 5 kV field and aperture size of 30 microns.
The experimental design and statistical analysis are shown in Appendix A.