Figure 2: Morphology and elemental composition of electrodeposited TO films on austenitic-grade steel.

(a) SEM image of a bare AISI 304 stainless-steel surface, (b,c) SEM images of TO films deposited by square wave-pulsed deposition on stainless steel revealing the deposited film thickness and hierarchically structured island morphology: (b) top view, (c) tilted view (70°). Scale bar, 200 nm. (d) Energy-dispersive X-ray analysis spectra of bare stainless steel and after deposition of TO film. (e–h) Raman (e), FTIR (f) and high-resolution XPS spectra of W 4f (g) and O 1s (h) regions of as-deposited TO films on stainless steel (red lines) compared with the corresponding spectra from the bare stainless-steel substrate (black lines). The stretching mode of the terminal W=O bond (947 cm⁻¹) is clearly observed in both Raman and the FTIR spectra characteristic of hydrated TO (e,f). The Raman spectrum exhibits a single broadband centred at 695 cm⁻¹ associated with the O–W–O stretching modes of the bridging oxygen atoms and a band at 370 cm⁻¹ is assigned to the W–OH₂ translational motion (e). The OH stretching and H–O–H bending vibrations of the water molecules are clearly identified in the FTIR spectrum around 3,340 and 1,620 cm⁻¹, respectively (f). The XPS spectra (g) consist of a single doublet at binding energies 35.7 eV for the W 4f_7/2 and 37.8 eV for the W 4f_5/2 corresponding to the W(VI) oxidation state (see also Supplementary Fig. 3A). The O 1s broad peak of bare stainless steel (h) is associated with the O²⁻ (∼530.2 eV) and the OH⁻ (∼531.4 eV) from the native oxides and hydroxides formed on stainless-steel surface. The O 1s spectrum of deposited films shows slightly asymmetric peak centred at 530.9 eV (H) that has been deconvoluted into three components: 530.8 eV (69.4%), 531.8 eV (29.0%) and 532.7 eV (1.6%) corresponding to the O²⁻, OH⁻ and H₂O, respectively, in the hydrated TO (see Supplementary Fig. 3B).