Angle resolved XPS studies on an anodic oxide formed on Ti–Nb–Sn alloy and the photo-induced change in carbon contaminants adsorbed on its surface

Applied Surface Science 258 (2012) 6052–6055

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Angle resolved XPS studies on an anodic oxide formed on Ti–Nb–Sn alloy and the photo-induced change in carbon contaminants adsorbed on its surface Naofumi Ohtsu a,∗ , Naoya Masahashi b , Yoshiteru Mizukoshi b a b

Instrumental Analysis Center, Kitami Institute of Technology, Kitami 090-8507, Japan Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

a r t i c l e

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Article history: Received 21 June 2011 Received in revised form 20 February 2012 Accepted 25 February 2012 Available online 3 March 2012 Keywords: XPS TiO2 Photocatalysis Anodic oxidation

a b s t r a c t Anodic oxide formed on a Ti–Nb–Sn alloy was analyzed by angle-resolved X-ray photoelectron spectroscopy (XPS) to determine the chemical state and composition of the uppermost surface. The anodic oxide formed on the alloy consisted of TiO2 , SnO2 , and Nb2 O. The cationic fractions of Nb2 O5 and SnO2 were lower than the atomic fractions of Nb and Sn in the alloy, and the uppermost surface contained higher concentrations of these oxides. The photo-induced change in the amount of carbon contaminants adsorbed on its surface was also analyzed by XPS combined with in situ ultraviolet (UV) light illumination. The variations in the chemical state induced by the UV light illumination were consistent with those on monolithic TiO2 . © 2012 Elsevier B.V. All rights reserved.

1. Introduction Anodic oxidation on a valve metal is a widely used industrial technique for producing a metallic oxide layer for the purpose of improving properties such as corrosion resistance, wear resistance, and coloring. The homogeneous oxide layer strongly adheres to the substrate because the reaction occurs in thermodynamic equilibrium. For instance, an alumina (Al2 O3 ) layer strongly enhances the hardness and wear resistance of an aluminum alloy substrate [1,2]. Anodic oxidation has also proven useful for producing photocatalytic titanium dioxide (TiO2 ) layers on titanium (Ti) and on its alloy substrate [3–6]. The TiO2 layer, which has a rutile-type structure, is produced when the Ti substrate is anodized in a high-concentration sulfuric acid (H2 SO4 ) electrolyte under a high applied potential, and it exhibits photocatalytic activity for the bleaching of methylene blue (MB) under ultraviolet (UV) or visible-light illumination [6,7]. We recently attempted to prepare a photocatalytic layer on a Ti–Nb–Sn alloy developed by Hanada et al. [8]. Anodic oxide grows on a substrate in thermodynamic equilibrium, resulting in high adhesive strength with a substrate. This alloy consists of noncytotoxic elements and has a small Young’s modulus, comparable to that of human cortical bone, making it a potential novel biomaterial for applications to bone plates, dental implants, and artificial joints [9]. Our aim was to achieve photocatalytic self-sterilization of the alloy through anodization. The anodic oxide layer on the alloy

∗ Corresponding author. Fax: +81 157 26 9563. E-mail address: [email protected] (N. Ohtsu). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2012.02.132

substrate showed photocatalytic activity and superhydrophilicity under UV illumination, albeit less than anodic TiO2 on pure Ti substrate. X-ray photoelectron spectroscopy (XPS) measurements showed that the anodic oxide on the alloy consisted of TiO2 , Nb2 O5 , and SnO2 . Oxides other than TiO2 could not be identified by crosssectional STEM images with EDS mapping [10] because XPS, unlike EDS in STEM, can probe to a depth of several nanometers. Consequently, it is possible that the decrease in the photocatalytic activity of the anodic oxide on the alloy substrate is due to the existence of Nb2 O5 and SnO2 near the uppermost surface. Further XPS analysis should provide valuable information for clarifying the role played by these oxides in photocatalytic activity. The anodic oxide on the Ti–Nb–Sn alloy has already been analyzed by XPS using a conventional method [10]. In the present study, we perform angle-resolved XPS to analyze the chemical states and composition of the anodic oxide in detail. Furthermore, to investigate the photocatalytic reaction that occurs at the anodic oxide, we also analyzed the photo-induced change in the amount of naturally adsorbed carbon contaminant using a combination of XPS and in situ UV illumination. 2. Experimental procedures A Ti–16 at.%Nb–5.5 at.%Sn alloy ingot was fabricated by arcmelting in Ar atmosphere, and the ingot was cold-rolled to a thickness of 1.5 mm (see Ref. [10] for details). The alloy plate, of dimensions 20 mm × 20 mm × 1 mm, was polished with SiC paper of 1500 grid for use as a substrate. The alloy plate was anodically oxidized in 1.2 M H2 SO4 aqueous solution for 0.5 h at room

N. Ohtsu et al. / Applied Surface Science 258 (2012) 6052–6055

Fig. 1. Survey XPS spectrum of the anodic oxide formed on the Ti-Nb-Sn alloy.

temperature. The anodic oxidation was controlled galvanostatically with a constant current density of 50 mA cm−2 . The anodized plate was rinsed with distilled water, dried at room temperature, and annealed at 723 K for 5 h in air. The XRD pattern showed that the anodic oxide predominantly consisted of TiO2 with rutile structure, and the cross-sectional STEM image revealed that the thickness of the oxide is approximately 6–9 ␮m [10]. The XPS apparatus was equipped with an atmospherecontrolled chamber (see Ref. [11] for the apparatus geometry). The anodized alloy was placed in the XPS analysis chamber (<5.0 × 10−8 Pa) and XPS spectra were measured at take-off angles (TOAs) of 15◦ , 35◦ , and 90◦ . The TOA is defined as the angle between the specimen surface and the spectrometer slit. The alloy was then transferred to the atmosphere-controlled chamber (<2 × 10−6 Pa) without being exposed to air, and high-purity oxygen gas (99.999%) was introduced into the chamber to a pressure of 10 kPa. UV light of wavelength 365 nm (SX-UI501HQ, Ushio Inc., Japan) and intensity 20 mW cm2 illuminated the alloy surface for 120 min. The alloy was then transferred to the analysis chamber without being exposed to air, and the spectra were measured again at a TOA of 35◦ . XPS measurements were conducted using a Surface Science Laboratory spectrometer (M-Probe SSX-100). Photoelectrons were excited by monochromatized Al K␣ radiation (1486.6 eV) with a spot size of ca. 300 ␮m × 500 ␮m and analyzed using a concentric hemispherical analyzer (CHA) with a multi-channel plate (MCP) detector. In this system, the binding energies of Au 4f7/2 and Cu 2p3/2 were 84.0 and 932.6 eV, respectively, and the full width at half maximum (FWHM) of the Au 4f7/2 peak was 1.1 eV. The spectral regions of Survey, C 1s, O 1s, Ti 2p–Sn 3d, and Nd 3d, were subsequently measured, and the measurement time for each specimen was ca. 150 min. 3. Results and discussion 3.1. Angle-resolved XPS analysis of the anodic oxide The survey XPS spectrum of the anodic oxide formed on the Ti–Nb–Sn alloy is shown in Fig. 1. The photoelectron peaks in the spectrum can be attributed to C, O, Ti, Nb, Sn, and S. The binding energy of the C 1s peak was 284.8 eV, indicating that carbon contaminant is adsorbed onto the surface. The sulfur originates from the electrolyte. The angle-resolved XPS spectra of the O 1s, Nb 3d, Ti 2p, and Sn 3d regions for the anodic oxide, obtained at TOAs of 15◦ and 90◦ ,

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are shown in Fig. 2; these spectra are normalized by the spectral intensity of the Ti 2p3/2 peak. The XPS spectra for standard TiO2 , Nb2 O5 , and SnO2 , obtained from the thermally oxidized metallic surface, are also shown. Spectral shapes of Ti 2p, Nb 3d, and Sn 3d coincide with those of standard TiO2 , Nb2 O5 , and SnO2 for all TOAs, whereas spectral intensities of the Nb 3d and Sn 3d peaks vary with the TOA. The cationic fractions of Ti4+ , Nb5+ , and Sn4+ in the anodic oxide were calculated from the spectral intensities at each TOA, as shown in Fig. 3. The calculation used the sensitivity factors programmed into the instrument, and the background of the spectra was subtracted by the Shirley method. At a TOA of 90◦ , the cationic fractions of Nb5+ and Sn4+ (corresponding to Nb2 O5 and SnO2 ) are 0.09 and 0.03, respectively, which are smaller than those of Nb and Sn in the alloy. These fractions increase with decreasing TOA, attaining 0.16 for Nb5+ and 0.07 for Sn4+ at a TOA of 15◦ . The angle-resolved XPS measurements reveal that the uppermost surface of the anodic oxide is rich in Nb2 O5 and SnO2 . Anodic oxide grows both at the alloy/oxide interface, owing to the penetration of oxygen ions, and at the oxide/electrolyte interface, due to the emanation of cations [12,13]. Thus, non-uniformity in the oxide species is often explained in terms of differences in the cation migration rates. However, the migration rate of Ti and Nb ions should be similar because the energy of the Ti4+ O bond almost equals that of the Nb5+ O bond, namely, 328 [14] and 323 kJ mol−1 [15], respectively. Furthermore, Semboshi et al. reported that the anodic oxide produced on a Nb–Ti alloy has an almost uniform distribution of constituent elements [16]. Therefore, the higher contents of Nb2 O5 and SnO2 at the uppermost surface cannot be explained in terms of migration rates. Mattsson et al. reported that doping of anatase TiO2 with Nb2 O5 resulted in a disordered anatase structure and in a unit cell expansion with pentavalent Nb oxide in the lattice with a surface enrichment of Nb [17]. Kubacka et al. studied a Ti-M (M = V, Nb, Mo) mixed-metal oxide powder and showed that the atomic concentration of the M at the uppermost surface was higher than the corresponding chemical composition measured using inductively coupled plasma (ICP) [18]. Based on these results, we propose that the higher contents of Nb2 O5 and SnO2 in the uppermost surface are not due to anodic oxidation but to surface segregation of the Nb and Sn incorporated in TiO2 . 3.2. Photo-induced change of the contaminant carbon The change in the C 1s spectrum induced by UV illumination is shown in Fig. 4. The spectral intensities are normalized to that of the Ti 2p3/2 peak. The spectral intensity of the main peak around 285 eV decreases with increasing UV illumination, whereas no change is seen in the higher energy range from 286 to 288 eV. These results indicate that a carbon species corresponding to the main C 1s peak is specifically removed by UV illumination. The C 1s spectra can be deconvoluted into four Gaussian functions, corresponding to C C, C O, C O, and O C O bonds, as shown in Fig. 5(a) [19]. Gaussian parameters such as the peak energy and the FWHM are labeled in the figure. The atomic ratio of C to Ti ([C]/[Ti]) for each chemical bond changes with the illumination time, as shown in Fig. 5(b). The [C]/[Ti] ratio for the C C bond decreases monotonously with the illumination time, whereas that for the O C O bond increases slightly. The changes observed in the chemical bonds on the current anodic oxide are consistent with those on monolithic TiO2 prepared by anodizing pure Ti [11]. The XPS results indicate that the doping of Nb2 O5 and SnO2 in TiO2 does not affect the photo-induced variation of the chemical state. On the other hand, the photocatalytic activity cannot be estimated from the quantitative change in adsorbed carbon because it is impossible to control the initial amount of adsorbate. The activity must be considered from another perspective.

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N. Ohtsu et al. / Applied Surface Science 258 (2012) 6052–6055

Fig. 2. Angle-resolved XPS spectra of the O 1 s, Nb 3d, Ti 2p, and Sn 3d regions for the anodic oxide, obtained at the TOAs of 15◦ and 90◦ . The XPS spectra for standard TiO2 , Nb2 O5 and SnO2 are also shown. The spectra are normalized by the spectral intensity of Ti 2p3/2 peak.

We previously reported the photocatalytic activity of the anodic oxide on the alloy, as witnessed by the bleaching of MB under UV illumination, and concluded that this activity is inferior to that of anodic TiO2 formed on pure Ti [10]. Mattsson et al. also reported that the doping of Nb2 O5 induces the formation of recombination sites, resulting in a decrease in photocatalytic activity [17]. These results suggest that Nb2 O5 and SnO2 in

the current anodic oxide suppress the photocatalytic activity of TiO2 through the formation of electron–hole recombination sites but do not affect the reaction mechanism involving photocatalytic decomposition. Nb2 O5 and SnO2 in the current anodic oxide are segregated at the uppermost surface, thereby highlighting the influences of recombination in the photocatalytic activity.

Fig. 3. Cationic fractions of Ti4+ , Nb5+ and Sn4+ in the anodic oxide calculated form the spectral intensities at each TOA.

Fig. 4. Change in the C 1 s spectra of the anodic oxide induced by UV illumination. The spectra are obtained at the TOA of 35◦ .

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Fig. 5. (a) Typical peak donconvolution of C 1 s spectra with Gaussian function. (b) Change of the [C]/[Ti] ratio for each chemical bond with the illumination time.

4. Conclusions Anodic oxide formed on a Ti–16 at.%Nb–5.5 at.%Sn alloy consists of TiO2 , SnO2 , and Nb2 O5 . The uppermost surface has higher contents of SnO2 and Nb2 O5 resulting from the segregation of these oxides. The cationic fractions of Nb2 O5 and SnO2 are lower than the atomic fractions of Nb and Sn in the alloy. When UV light is illuminated onto the oxide in oxygen atmosphere, the hydrocarbon content due to the adsorption of carbon contaminants is drastically reduced, whereas the carboxyl carbon content is slightly increased. These variations observed on the current anodic oxide are consistent with those observed on monolithic TiO2 . Acknowledgements The authors gratefully acknowledge Prof. S. Hanada for providing the Ti–Nb–Sn alloy. The authors also acknowledge Prof. M. Oku for his advises in the XPS analysis and Dr. Semboshi for his valuable comment in the anodic oxidation. This work was performed under the inter-university cooperative research program of the Laboratory for Analytical Science, Institute for Materials Research, Tohoku University. References [1] Y. Nakamura, T. Sakai, H. Hirano, K.S. Ravi Chandren, Int. J. Fatigue 32 (2010) 621–626. [2] E. Girik, K. Genel, Surf. Coat. Technol. 202 (2008) 5190–5201.

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