2019 Volume 60 Issue 9 Pages 1814-1820
A two-step thermal oxidation process was applied to Ti–xNb binary alloys (x = 0, 1, 10, 15, and 30 at%) to prepare anatase-containing TiO2 layers, and their photocatalytic activities were evaluated by measuring the water contact angle and decomposition of methylene blue (MB) under UV irradiation. The condition of the first-step treatment was fixed as heating in Ar–1%CO atmosphere at 1073 K for 3.6 ks, and the subsequent second-step treatment was conducted in air at 673–1073 K for 10.8 ks. The reaction layer formed after the two-step thermal oxidation consisted of TiO2. The anatase fraction of the TiO2 layers increased with decreasing second-step temperature and increasing Nb content of the Ti–Nb alloys. In addition, Nb and carbon were introduced into the TiO2 layers. A water contact angle of around 5° was observed on the TiO2 layers formed at the second-step temperatures of 673–973 K. The rate constant of MB decomposition showed a maximum for an anatase fraction of 0.6–0.8 at which the recombination of exited electrons and holes are suppressed. The TiO2 layer formed on the Ti–10 at%Nb alloy exhibited a higher rate constant of MB decomposition compared with Ti–30 at%Nb, in which the TiNb2O7 phase formed. These results indicate that Nb is an effective alloying element for producing a photocatalytically active TiO2 layer on Ti by the two-step thermal oxidation process. Nevertheless, the presence of an anatase-rich TiO2 layer and an appropriate Nb content in TiO2 are required for achieving high photocatalytic activities.
Ti and its alloys are used as implant materials in orthopedic and dental fields.1) Although they can be directly connected to a living bone at an optical microscopic level (osseointegration),2,3) a relatively long period is required by these implants to integrate with the bone tissue and the fixation depends on the state of the bone. Therefore, surface treatment of Ti and its alloys is required to further improve their bone compatibility.4) Recently, an improvement in bone compatibility by forming a TiO2 layer on the Ti surface through UV irradiation has been reported,5–7) which is considered to be due to the cleaning of surface by decomposition of hydrocarbon contaminants through the photocatalytic activity of the TiO2 layer.6) In addition, the organic decomposition ability is related to antibacterial property. Thus, photocatalytically active TiO2 coatings help improve the bone-forming ability as well as antibacterial activity of Ti implants.
High-temperature oxidation of Ti can produce a TiO2 layer with high crystallinity and excellent adhesion to Ti; however, the formed TiO2 layer generally consists of only the rutile phase. It is known that anatase exhibits excellent photocatalytic activity as compared to rutile.8) Therefore, we developed a two-step thermal oxidation process for the formation of anatase-rich TiO2 on Ti substrates.9–11) In this process, the Ti substrates are treated first in a CO-containing atmosphere (first step) and subsequently in air (second step). An anatase-rich TiO2 layer is formed on commercially pure (CP) Ti and Ti alloys using this process,9–12) indicating that the carbon dissolution in TiO2 and/or the similarity between anatase and NaCl-type Ti(C,O) crystal structures stabilized the anatase phase.9) The anatase-rich TiO2 layer exhibited photocatalytic activity.9,12)
In a previous study, a TiO2 layer formed on the surface of a Ti–25 mass%Nb alloy (Ti–14.6 at%Nb) exhibited excellent photocatalytic activity.9) However, the effect of Nb content of the Ti–Nb alloy on the phase constitution and photocatalytic activity of the TiO2 layer formed by the two-step thermal oxidation process has not been revealed. Nb is an alloying element widely used for biomedical Ti alloys, such as TNTZ (Ti–29 mass%Nb–13 mass%Ta–4.6 mass%Zr) alloy,13) and the information on TiO2 layers formed on Ti–Nb alloys is expected to provide basic data for applying TiO2 coatings on the Ti implants for biomedical applications.
In this study, TiO2 layers were formed on Ti–xNb alloys (x = 0, 1, 10, 15, and 30 at%) by a two-step thermal oxidation process, and the photocatalytic activity of TiO2 layer under UV irradiation was investigated by measuring the water contact angle and decomposition of methylene blue (MB).
Ingots of Ti–xNb alloys (x = 1, 10, 15, and 30 at%) with a size of 70 × 40 × 25 mm were prepared using a non-consumable W electrode Ar arc melting furnace (ACM-05-A, DIAVAC Ltd., Yachiyo, Japan). A CP Ti plate with 4 mm thickness (JIS Gr. 2) and a Nb plate with 1 mm thickness (99.9%) were used as raw materials for the melting after pickling them in a solution with the composition HF:HNO3:H2O = 2:13:85 (vol%). The ingots were subjected to hot forging at 1373 K and a subsequent second hot forging at 1273 K to shape them into bars with a diameter of 12 mm. Coin-shaped alloys with 1 mm thickness were cut from the bars. The Nb content of the alloys was quantitatively analyzed by inductively coupled plasma-mass spectrometry (ICP-MS, Agilent8800, Agilent Technologies, CA, USA). The Nb content of the alloys and the alloy designations are summarized in Table 1. For the alloy without added Nb, a coin-shaped CP Ti with a diameter of 12 mm and thickness of 1 mm, which was cut from the CP Ti bar (JIS Gr. 2, UEX Co., Ltd., Tokyo, Japan), was used. Hereafter, the Ti–xNb alloys (x = 0, 1, 10, 15, and 30 at%) are referred to as 0Nb, 1Nb, 10Nb, 15Nb, and 30Nb, respectively. The specimens were mirror polished and ultrasonically cleaned in ethanol and ultrapure water for 0.3 ks each.
The coin-shaped Ti–xNb alloys were subjected to two-step thermal oxidation, which is schematically shown in Fig. 1. The first-step treatment in Ar–1%CO gas atmosphere was conducted at 1073 K for 3.6 ks. The second-step treatment in air was conducted at temperatures ranging from 673 K to 1073 K for 10.8 ks. The phase of the reaction layers formed on the Ti–xNb alloy surface was analyzed by α-2θ X-ray diffraction (XRD, RU-200B, Rigaku Co., Tokyo, Japan) with an incident angle of 0.3° and Cu Kα radiation, and Raman spectroscopy (NRS-5100AMS, JASCO Co., Tokyo, Japan) with a laser wavelength of 532 nm. The cross section of the reaction layers was observed using a scanning electron microscope (SEM, JSM-7800F, JEOL Ltd., Tokyo, Japan). The reaction layer was analyzed by radio-frequency glow discharge optical emission spectroscopy (GD-OES, GD-Profiler 2, Horiba, Ltd., Kyoto, Japan), X-ray photoelectron spectroscopy (XPS, Theta Probe, Thermo Fisher Scientific K.K., Tokyo, Japan), and transmission electron microscopy (TEM, JEM-2100, JEOL Ltd., Tokyo, Japan).
Schematic diagram depicting the heating pattern of the two-step thermal oxidation process.
The water contact angles on the reaction layers of the Ti–xNb alloys were measured under UV irradiation using a contact angle gauge (DM-501, Kyowa Interface Science Co., Ltd., Niiza, Japan). A UV lamp (127B-BL, Raytronics Co., Fujimino, Japan) with a peak wavelength of 351 nm was used for UV irradiation. The irradiance of the UV light source was set as 1.0 mW·cm−2 at the surface of the specimen, which was measured using a UV radiometer (UVR-300, Topcon Technohouse Co., Tokyo, Japan) with a wavelength range of 310–400 nm in the light-receiving section. Before the water contact angle measurements, the specimens were ultrasonically cleaned in ultrapure water and ethanol for 0.6 ks each. The water contact angle was measured at five different points on the reaction layers after various UV irradiation periods of up to 7.2 ks. The average of five values was used as the water contact angle for each irradiation time.
The organic decomposition ability of the reaction layer on the Ti–xNb alloys under UV irradiation was evaluated by the decomposition test of MB (Fujifilm Wako Pure Chemical Co., Osaka, Japan) based on JIS R1703-2: 2007. Before UV irradiation, the specimen was placed in a 0.02 mM MB solution and stored under dark for 86.4 ks to reach an absorption equilibrium for MB on the reaction layer. After this treatment, the specimen was immersed in 2.5 mL of 0.01 mM MB solution and irradiated with UV light with an irradiance of 1.0 mW·cm−2. The change in MB concentration on the TiO2 layer under UV irradiation was evaluated using a UV–vis spectrophotometer (V-650, JASCO Co., Tokyo, Japan) at 663 nm. The concentration of MB was measured every 1.2 ks of UV irradiation up to 10.8 ks. The detailed procedures of evaluating photocatalytic activities have been reported elsewhere.9)
The α-2θ XRD patterns of the reaction layers on the Ti–xNb alloys after the first-step and second-step treatments are shown in Fig. 2. The presence of Ti(C,O) was confirmed after the first-step treatment, and the TiO2 phase was found to be the main phase of the reaction layer after the second-step treatment. A single phase of anatase was detected at the second-step temperature of 673 K, while the rutile phase was formed at 873 K and became dominant at 1073 K. The powder diffraction pattern of the anatase phase (PDF#21-1272) is presented in Fig. 2(b). No peak shift is detected for the anatase phase formed on the alloys. In addition to TiO2, a minor TiNb2O7 phase was detected in the case of 30Nb at 1073 K. The presence of TiNb2O7 phase was also confirmed by Raman spectroscopy and TEM analysis. When the Ti–xNb alloys were subjected to air oxidation at 673–873 K for 10.8 ks, i.e., without the first-step treatment, only rutile phase was detected as an oxidation product.
α-2θ XRD patterns of the reaction layers on Ti–xNb alloys after (a) first-step treatment and (b–d) second-step treatment. The second-step temperatures of (b), (c), and (d) are 673, 873, and 1073 K, respectively.
Figure 3 shows the phase fraction of TiO2 as a function of Nb content in the alloys and the second-step temperature. The anatase fraction (fA) was calculated using eq. (1).14)
| \begin{equation} f_{\text{A}} = I_{\text{A}}/(I_{\text{A}} + 1.26I_{\text{R}}) \end{equation} | (1) |
Phase fraction of TiO2 layers formed on Ti–xNb alloys after second-step treatment.
The depth profiles of oxygen, carbon, and Nb of the reaction layers of 10Nb after the second-step treatment at 673, 873, and 1073 K, which were measured by GD-OES, are shown in Fig. 4. The region with the high intensity of oxygen is considered to correspond to the TiO2 layer. SEM observation demonstrated that the thickness of the TiO2 layers on 10Nb at 673 and 873 K was 1 µm and that at 1073 K was 2.5 µm. A high oxygen intensity in the GD-OES profile was detected up to sputtering times of around 200 s for 673 and 873 K and around 600 s for 1073 K, respectively; this is consistent with the thickness of the TiO2 layers, as determined by SEM observation. Compared with the intensity of the TiO2 region in the GD-OES profile, it is suggested that the carbon content decreased and the Nb content increased with increasing second-step temperature. The chemical state of carbon in the TiO2 layers was not investigated in this study; however, in our previous study,9) both carbon dissolution and the existence of amorphous carbon/disordered graphite in the anatase layer were suggested after two-step thermal oxidation.
GD-OES spectra of the reaction layers of 10Nb alloy after second-step treatment at (a) 673, (b) 873, and (c) 1073 K.
Figure 5 shows the initial and final water contact angles after UV irradiation on the TiO2 layers for 7.2 ks as a function of the second-step temperature. In the second-step temperature range of 673–973 K, the final water contact angle decreased to around 5°. On the other hand, at the second-step temperature of 1073 K, the final water contact angle depended on the Nb content of the alloys: a final water contact angle of around 5° was obtained for 10Nb and 15Nb, while that of 1Nb and 30Nb were 20° and 10°, respectively. No decrease in the final water contact angle was detected for 0Nb.
Initial and final water contact angles on TiO2 layers formed on (a) 0Nb, (b) 1Nb, (c) 10Nb, (d) 15Nb, and (e) 30Nb alloys after UV irradiation for 7.2 ks as a function of the second-step temperature.
The changes in the normalized concentration of MB with UV irradiation time for 0Nb, 10Nb, and 30Nb are shown in Fig. 6. The vertical axis shows the logarithm of C/C0, where C is the MB concentration during UV irradiation and C0 is the initial MB concentration before UV irradiation. A decrease in MB concentration was detected for all the alloys subjected to the second-step treatment in the temperature range of 673–973 K and the organic decomposition ability of the TiO2 layer was confirmed. Among the alloys subjected to the second-step treatment at 973 K, only 10Nb showed a decrease in MB concentration. On the other hand, none of the alloys showed a decrease in MB concentration when the alloys were subjected to the second-step treatment at 1073 K.
Decomposition of methylene blue on TiO2 layers of Ti–xNb alloys after second-step treatment at (a) 673, (b) 773, (c) 873, (d) 973, and (e) 1073 K.
Hanaor and Sorrell15) reviewed the effect of doping elements in TiO2 on the irreversible transformation from anatase to rutile and showed that the elements having large ionic radii and valence in TiO2 inhibit the transformation. Nb is classified as an inhibitor of transformation. The addition of Nb to the alloy, that is, the introduction of Nb to the TiO2 layer increased the anatase fraction under the same second-step temperature, as shown in Fig. 3. Figure 7 shows the XPS spectra of the TiO2 layer formed on 10Nb at the second-step temperature of 873 K. The presence of Nb5+ and Nb4+ ions in the TiO2 layer was confirmed. It was reported that Nb addition to Ti decreased its oxidation rates at high temperatures.16–18) The reduction in oxygen vacancy concentration in TiO2 through eq. (2) is considered the mechanism through which the oxidation resistance of Ti is improved.19)
| \begin{equation} \textit{Nb}_{2}O_{5} + V_{O}^{\bullet\bullet} \xrightarrow{\textit{Rutile}}2\textit{Nb}_{\textit{Ti}}^{\bullet} + 2\textit{TiO}_{2} \end{equation} | (2) |
Nb 3d XPS spectra of the reaction layer of 10Nb alloy after second-step treatment at 873 K.
The rate equation of MB decomposition is represented by eq. (3).9,22)
| \begin{equation} \ln(C/C_{0}) = -k\cdot t \end{equation} | (3) |
Change in rate constant of decomposition of methylene blue (MB) under UV irradiation as a function of anatase fraction of TiO2 layers.
A comparison of the k values of 0Nb and 10Nb with an anatase fraction of around 0.5 reveals that 10Nb exhibits higher k values than 0Nb does. The Nb5+ ions introduced into TiO2 could trap the excited electrons (eq. (5)), thereby suppressing the recombination of the excited electrons and holes.26)
| \begin{equation} \text{Nb$^{5+}$} + \mathrm{e}^{-} = \text{Nb$^{4+}$} \end{equation} | (4) |
On the other hand, the k values of 30Nb are less than those of 10Nb and 0Nb. The effect of Nb addition to TiO2 on its photocatalytic activity has been studied.27,28) It was reported that Nb addition improves the photocatalytic activity of TiO2 up to a certain concentration; however, further addition of Nb deteriorates the photocatalytic activity, which agrees with the present results. Our study suggests that the TiNb2O7 formed in TiO2 possibly acts as a site of recombination of the excited electrons and holes in the case of 30Nb.
As shown in Fig. 5, 10Nb and 15Nb after the second-step treatment at 1073 K exhibited a low final water contact angle of around 5° despite the formation of single-phase rutile or rutile-rich TiO2 on them. The mechanism responsible for the low final water contact angle of TiO2 under UV irradiation is still under discussion.29) Therefore, presently, it is difficult to clarify the reason for the low water contact angle of around 5° of rutile-rich TiO2 with added Nb under UV irradiation. It is reported that the addition of Nb oxide to TiO2 increases the surface acidity of TiO2,30) which might contribute to the low water contact angle of the rutile-rich TiO2 layer by inducing an interaction between the outer water molecules and the surface hydroxyl groups.31)
Figure 9 shows the relationship between the final water contact angle and the rate constant of MB decomposition (k values). A low final water contact angle of around 5° does not necessarily lead to high k values. These results suggest that the presence of an anatase-rich TiO2 layer and an appropriate Nb content in TiO2 are required for achieving high photocatalytic activities. The hydrophilicity of TiO2 is reportedly related to the ability of organic decomposition and change in surface structure of TiO2 during UV irradiation.29) The relationship shown in Fig. 9 suggests that the change in TiO2 surface structure cannot be ignored for expression of hydrophilicity, which would not contribute to the expression of organic decomposition ability.
Relationship between final water contact angle and rate constant of methylene blue (MB) decomposition.
TiO2 layers were formed on Ti–xNb alloys (x = 0, 1, 10, 15, and 30 at%) using a two-step thermal oxidation process, in which the alloys are treated in Ar–1%CO gas atmosphere in the first step and in air in the second step. The water contact angle and the decomposition rate of MB on the reaction layer mainly consisting of TiO2 were measured under UV irradiation, and the following results were obtained.
The authors would like to thank Dr. K. Kobayashi, Ms. K. Omura, Dr. N. Akao, Ms. M. Nemoto, and Ms. Y. Nakano of Tohoku University for TEM, XPS, GD-OES, Raman spectroscopy, and ICP-MS analyses, respectively. This study was partially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan (No. 18H01718), the Iketani Science and Technology Foundation, and The Light Metal Educational Foundation, Inc.
