Characterization of potential nanoporous sodium titanate film formation on Ti6Al4V and TiO2 microspherical substrates via wet-chemical alkaline conversion

The authors present novel insights into the formation of nanoporous, wet-chemically produced sodium titanate films onto microspherical substrates of varying composition. Microspheres of Ti6Al4V (atomised; ca. 20–50 μm), which were utilised due to their ubiquitous industrial usage relative to metallic titanium, were compared with TiO2 microspheres (flame spheroidised anatase and rutile powders; average ca. 30–40 μm). These were then suspended in 5 M NaOH solutions (60 °C, 24 h), and then characterized (SEM, EDS, XRD, XPS) to determine the extent of sodium titanate generation, and the potential inhibition of formation due to oxygen content. It was found that excessive oxygen content (flame-spheroidised rutile and anatase powders) resulted in inhibition of nanoporous titanate formation, apart from the top few nanometres of the surface, since a diffusion barrier of TiO2 prevents further conversion. The characteristic nanoporous titanate structures were formed on the Ti6Al4V microspheres, ca. 1 μm (999 ± 25 nm) in thickness, whereas no visible alteration to the TiO2 microspheres were seen. High surface concentration (ca. 9.5–17.6 at.%) of Na was seen in all samples via XPS, including the TiO2 microspheres (despite no morphological change), however, only the Ti6Al4V microspheres exhibited moderate Na content (ca. 4.7 at.%) via EDS, illustrating a diffusion gradient during formation. The confirmation of these structures onto microspherical substrates opens the possibility for application in biomaterials, water treatment, and energy fields