“…In addition, an ultrathin fluoride-rich layer is also present at the outer walls, i.e., between individual tube walls, caused by the plastic-flow mechanism which pushes the nanotubes upward from the bottom of nanotubes/Ti interface during the tubes formation, hence promotes the F − along the tube walls (Berger et al, 2011). The fluoride-rich, double-walled morphology is well-documented with the support by EDX, XPS, High Resolution Transmission Electron Microscope (HR-TEM), High Angle Annular Dark Field Scanning TEM (HAADF-STEM) Auger Electron Spectroscopy (AES) and Time-of-Flight Secondary-ion Mass Spectrometry (ToF SIMS) depth-profiling measurements (Albu et al, 2008; Berger et al, 2011; So et al, 2017; Dronov et al, 2018).…”