A Novel Investigation of the Formation of Titanium Oxide Nanotubes on Thermally Formed Oxide of Ti-6Al-4V
Traditionally, titanium oxide (TiO2) nanotubes (TNTs) are anodized on Ti-6Al-4V alloy (Ti-V) surfaces with native TiO2 (amorphous TiO2); subsequent heat treatment of anodized surfaces has been observed to enhance cellular response. As-is bulk Ti-V, however, is often subjected to heat treatment, such as thermal oxidation (TO), to improve its mechanical properties. Thermal oxidation treatment of Ti-V at temperatures greater than 200°C and 400°C initiates the formation of anatase and rutile TiO2, respectively, which can affect TNT formation. This study aims at understanding the TNT formation mechanism on Ti-V surfaces with TO-formed TiO2 compared with that on as-is Ti-V surfaces with native oxide. Thermal oxidation–formed TiO2 can affect TNT formation and surface wettability because TO-formed TiO2 is expected to be part of the TNT structure. Surface characterization was carried out with field emission scanning electron microscopy, energy dispersive x-ray spectroscopy, water contact angle measurements, and white light interferometry. The TNTs were formed on control and 300°C and 600°C TO-treated Ti-V samples, and significant differences in TNT lengths and surface morphology were observed. No difference in elemental composition was found. Thermal oxidation and TO/anodization treatments produced hydrophilic surfaces, while hydrophobic behavior was observed over time (aging) for all samples. Reduced hydrophobic behavior was observed for TO/anodized samples when compared with control, control/anodized, and TO-treated samples. A method for improved surface wettability and TNT morphology is therefore discussed for possible applications in effective osseointegration of dental and orthopedic implants.
Schematic with labeled parts of the (a) Lindberg furnace, (b) anodization setup, (c) nanotubes, and (d) experimental protocol.
Field emission scanning electron microscopy (FESEM) images of before and after anodization treatments. (a, d) Control and control/anodized (CTNT) sample, respectively. (b, e) 300°C thermal oxidation (TO)–treated (TO300) and 300°C TO/anodized (300TNT), respectively. (c, f) 600°C TO-treated (TO600) and 600°C TO/anodized (600TNT), respectively. Effect of TO treatment before anodization is observed in the form of reduced and smaller surface pores (300TNT and 600TNT samples, images e and f, respectively). On most 600TNT surfaces, a cover layer is present as seen in image f. In images d and e, circled areas represent dissolution of vanadium-rich areas.
Figure 3. White light interferometry (WLI) data show an increase in roughness for TO600 samples over control and TO300 samples. The data corroborate with field emission scanning electron microscopy images of thermal oxidation–treated surfaces (Figure 2). Statistical markers: a = control, c = significant difference between TO600 and control samples if P < .05. Figure 4. Field emission scanning electron microscopy images of TiO2 nanotubes (TNT) from (a) control/anodized (CTNT), (b) 300TNT, and (c) 600TNT samples. Longer TNTs were observed on 300TNT samples as compared with CTNT and 600TNT samples.
Figure 5. Energy dispersive x-ray spectroscopy of TiO2 nanotubes (TNTs) taken from a 600TNT sample. Titanium, aluminum, vanadium, nitrogen, and fluorine are labeled. Ti, Al, and V are from Ti-V components, with the oxygen from TNTs, and residual fluorine and nitrogen from electrolyte and/or presence of fluorine on TNTs in the form of TiF6−2. Spectra from all anodized samples showed calculated oxygen to titanium ratio of 2:1 (TiO2). Figure 6. Dimensions of TNTs. (a) ImageJ was used to measure the TNT length, surface inner diameter (SID), and inner/outer diameters under the surface (ID and OD). (b) Graph of TNT dimensions. (Inset) Blowup of OD, SID, and ID. Statistical markers if P < .05: a = control, b = significant difference in TNT lengths between CTNT and 300TNT samples, c = significant difference in SID between CTNT and 600TNT samples. Figure 7. Water contact angle (WCA) measurements taken after thermal oxidation (TO) and TO/anodization treatments. TO and TO/anodization treatments result in significantly hydrophilic surfaces; WCA increases significantly over time (aging) for all groups, and WCA decreases significantly after deionized water wash for all groups except for 600TNT samples. Lowercase letters are assigned for statistical analysis within each group, and uppercase letters are assigned for analysis between groups. Statistical markers if P < .05: a = control, # = as-prepared and aged control samples are the same, b = significant difference between as-prepared and aged samples, c = significant difference between aged and washed-after-aging samples, d = significant difference between washed-after-aging and anodized samples, e = significant difference between anodization and 1-day aging, f = significant difference between 1- and 2-day aging, g = significant difference between 2- and 7-day aging, h = significant difference between 7-day and 14-day aging, and i = significant difference between 14-day aging and wash after aging. Figure 8. Schematic of anodization process. (a) Oxidation at the electrolyte and Ti-V surface leads to formation of barrier oxide layer (BOL) and accumulation of H+ ions that attract F− ions, resulting in etching or dissolution of BOL; (b) etching leads to formation of nanopores, which grow into nanotubes; and (c) as oxidation/dissolution steps in part A and B reach equilibrium at the Ti-V bulk and nanotube interface, nanotubes extend into Ti-V bulk.
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