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Photo-induced properties of anodic oxide on Ti–Pd alloy prepared in acetic acid electrolyte
Journal of Alloys and Compounds 669 (2016) 91e100
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Photo-induced properties of anodic oxide on TiePd alloy prepared in acetic acid electrolyte N. Masahashi a, *, Y. Mizukoshi a, H. Inoue b, K. Ohmura a, T. Moroishi c a
Institute of for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba, Sendai, Miyagi, 980-8577, Japan Department of Materials Science, Graduate School of Engineering, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan c TiG Co.,Ltd., 4-1-32 Kawata, Higashi-Osaka, Osaka 578-0905, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 September 2015 Received in revised form 28 December 2015 Accepted 27 January 2016 Available online 4 February 2016
Photo-induced properties of titanium dioxide on TiePd alloy, prepared by anodic oxidation in acetic acid electrolytes have been studied and compared with those of the oxide on pure Ti. The photocatalytic activity of the anodic oxide on TiePd was superior to that of the anodic oxide on Ti, and the highest photocatalytic activities were found for the anodic oxide prepared in electrolytes with 1 M or 2 M acetic acid. This superiority was attributed to PdO incorporation into the anodic oxide, which facilitates charge separation. Both photocatalytic activity and hydrophilicity of the anodic oxide varied with the acetic acid concentration in the electrolyte, and were related to the surface morphology of the anodic oxides, which is controlled by the electrochemical reaction. It is concluded that photo-induced properties are imparted to the corrosion-resistant TiePd alloy by anodic oxidation in acetic acid aqueous solutions. © 2016 Elsevier B.V. All rights reserved.
Keywords: Thin films Oxide materials Chemical synthesis Catalysis Corrosion Photoelectron spectroscopies
1. Introduction Titanium dioxide (TiO2) semiconductors exhibit attractive photocatalytic characteristics [1]. These semiconductors have been applied in various fields and employed for the decomposition or sterilization of chemical substances such as endocrine disruptors, injurious volatile compounds, and organic pollutants in the environment [2,3]. Furthermore, the self-cleaning function of these materials, which results from their superhydrophilicity, leads to unique performances in the photodegradation of various noxious or malodorous chemicals [4,5]. Numerous investigations have reported that the incorporation of noble metals such as Pt [6e8], Ag [9,10], Au [11,12] and Pd [13,14] to TiO2 is an effective method to improve its photocatalytic activity. In particular, the effect of Pt incorporation on TiO2 photocatalytic activity has been widely studied, and it is considered that Pt acts as an electron sink site because of the Schottky barrier effect formed at the metal/TiO2 interface [8,15] and promotes interfacial electron transfer by facilitating charge separation [16,17]. Usually, noble metal incorporation is conducted using a photochemical deposition method,
* Corresponding author. E-mail address: [email protected] (N. Masahashi). http://dx.doi.org/10.1016/j.jallcom.2016.01.220 0925-8388/© 2016 Elsevier B.V. All rights reserved.
wherein an aqueous TiO2 suspension containing an alcohol and an acid-containing noble metal is irradiated by a mercury lamp. Noble metal-loaded TiO2 powders are obtained by filtering the TiO2 suspension and subsequent drying. The powders show a high quantum efficiency because of their large surface area and high density of surface coordination unsaturated sites [18,19]; however, the practical usage of these powders is limited because of their aggregation [20,21], and the toxicity of TiO2 poses a potential risk to human health [22e24]. In this study, anodic oxidation method was applied to a wellknown crevice corrosion resistant TiePd alloy, registered as ASTM Grade 7, to incorporate Pd into TiO2. Pd inhibits chloride corrosion of Ti due to stabilizing the passivation layer composed of titanium oxide [25]. Anodic oxidation leads to the formation of titanium oxide on the TiePd surface, and it is considered that this passivation layer, composed of a few atomic layers, is incorporated into the anodic oxide. Subsequent annealing promotes the crystallization of the oxide, and the photocatalytic characteristics of TiO2 are expected to be maintained without impairing its corrosion resistance. Anodic oxidation generates thermodynamically stable oxides on the surface of titanium and its alloys [26] with high adhesion strength to the substrate [27]. We investigated the photocatalytic properties of the anodic oxide on Ti [28e30] and Ti
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alloys such as TiNbSn [31] and Ti6Al4V [32,33]. These properties varied with the electrochemical conditions and subsequent annealing [28,30], which control TiO2 crystallinity and its chemical composition, and suppress charge recombination. Thus far, studies regarding anodic oxidation of Ti alloys containing noble metals have not been reported. The purpose of this study is to explore the photocatalytic activity and hydrophilicity of anodic oxides on TiePd alloy, with special focus on the effect of Pd on their photocatalytic activity. 2. Experimental procedures Ti-0.05 wt.% Pd alloy (Nippon Steel and Sumitomo Metals, Japan) with about 120 mm grain size was used as the anode in anodic oxidations. Microstructural and crystallographic analysis of Ti-0.05Pd (hereafter, denoted as TiePd) revealed that Pd exists as a solid solution in hcp Ti, which is consistent with the solubility limit (0.88 wt.%) of Pd in hcp Ti. A TiePd plate with dimensions of 19 9 1 mm3 was polished by emery paper and buffed using 1.0 and 0.3 mm alumina suspensions, and then washed in ethanol using an ultrasonic cleaner. A 100-mesh Pt electrode, with dimensions of 5 5 cm2 was used as the cathode. Anodic oxidations were conducted for 0.5 h using a DC power supply (Matsusada Precision, PRk 500-3.2, Japan), and electrolytes with acetic acid concentrations in the range 0.01e6 M. The applied voltage and current density were 210 V and 0.05 A/cm2, respectively. The anodized electrode was rinsed with distilled water and dried at room temperature, and subsequently annealed at 723 K for 5 h in air. Monolithic Ti (cp-Ti, grade I) was anodized in the same way using the anodic oxide on Ti as reference. Crystallographic structures were determined by means of an X-ray diffractometer (PANalytical X'Pert diffractometer, Netherlands) with CuKa radiation (0.15406 nm), at a scan rate of 1 min1, using a sample with a thin-film geometry arrangement at 0.5 glancing angle and a rotating detector. Microstructure observations were performed using scanning electron microscopy (SEM) (Keyence VE-8800, Japan) at an operating voltage of 20 kV, and laser microscopy (Shimazu, Nano Search® Microscope SFT3500(S), Japan). The absorption spectra were measured by using a UVevis spectrophotometer (Jasco V-550, Japan). X-ray photoelectron spectroscopy (XPS) measurements were carried out with an electron spectrometer (Shimazu, Kratos AXIS-Ultra DLD, Japan) equipped with monochromated Al Ka radiation at a base pressure of 3.0 107 Pa. The full width at half maximum intensity of the Ag 3d5/2 peak was 0.73 eV, and the base pressure of the spectrometer was 6.5 108 Pa. The photocatalytic activities of the anodic oxide were evaluated by measuring the amount of CO2 produced from the decomposition of CH3CHO. An anodized specimen was placed in a cylindrical Pyrex vessel with a quartz window, and CH3CHO (50 mL) was introduced into the vessel. After confirming that there was no change in the amounts of CO2, the specimen was illuminated by UV light using a beam lamp (Asahi Spectra, MAX-302, Japan; 1.1 mW/cm2), and the evolved CO2 was measured by gas chromatography (Shimadzu GC2014, Japan). The hydrophilicity of the anodic oxides was evaluated by measuring the contact angle of distilled water using a goniometer system (Kyowa Interface Science Co Ltd., CA-X, Japan). The average of three measurements for each sample, illuminated by a beam lamp (Seric Co Ltd., SOLAX XC-100B, Japan; 0.1 mW/cm2), was used as the contact angle. 3. Results 3.1. Microstructure and crystallographic structure Fig. 1 shows the SEM images of the annealed anodic oxides on
TiePd, prepared in electrolytes with acetic acid concentrations of 0.01 M (a), 1 M (b), 2 M (c), and 6 M (d). When an acetic acid concentration of 0.01 M was used (a), a few micron-sized pores were observed on the weakly corroded surface. With the increase in the acetic acid concentration to 1 M, the surface of the anodic oxide was severely corroded, as shown in (b). A further increase in concentration to 2 M generated large-sized hollows due to the coalescence of the micron-sized pores and the corroded cavities (c), and its frequency of appearance (marked by arrows) increased with an increase in the acetic acid concentration to 6 M (d). The microstructure variation of the anodic oxide on Ti with the acetic acid concentration in the electrolyte is similar to that for the anodic oxide on TiePd. EDX analysis did not detect Pd in any of the anodic oxides, probably due to the low concentration of Pd. Fig. 2 shows the variation in the surface area ratio and surface roughness of the annealed anodic oxides on Ti and TiePd against the acetic acid concentration in the electrolyte (a), and the laser microscope images of the annealed anodic oxide on TiePd prepared using acetic acid concentrations of 0.01 M (b) and 2 M (c), respectively. Here, the surface area ratio is defined as the ratio of the measured surface area to the projected surface area, which becomes unity in a perfectly flat surface. The roughness and surface area ratio increase with the acetic acid concentration up to 2 M and then decrease irrespective of the substrate. The laser microscope images reveal that the surface morphology of the anodic oxide prepared in 2 M (c) is rougher than that of the anodic oxide prepared in 0.01 M (b). These results imply that the corrosion resistance of the anodic oxide depends on the acetic acid concentration in the electrolyte. Fig. 3 shows the XRD profiles of the annealed substrates and anodic oxides on Ti (a) and TiePd (b). The main peak of anatase TiO2 is found in the case of all annealed anodic oxides regardless of the acetic acid concentration, and weak peaks attributed to rutile TiO2 are seen. On the other hand, the XRD profiles of annealed Ti and TiePd exhibit anatase with weak rutile in addition to hcp-Ti main peaks. This suggests that annealing promotes rutile TiO2 formation on Ti and TiePd, whereas it enhances crystallization to anatase structure in the anodized oxide. Thick anodic oxides are formed on Ti as compared to TiePd as evidenced from the comparison of the peak intensity ratios of anatase 101 TiO2 (2q ¼ 25.3 ) to hcp Ti 101 diffracted peak (2q ¼ 40.04 ). 3.2. Absorption behavior Fig. 4 shows the absorption spectra of the annealed anodic oxide on Ti (a) and TiePd (b) prepared in 0.01 M, 1 M, 2 M and 6 M acetic acid electrolytes. The spectra of the anodic oxide prepared in 0.01 M exhibit oscillations due to the thin thickness of the anodic oxide layer. When the acetic acid concentration increases, the spectra show a steep absorbance decrease with increasing wavelength, which is typical of semiconductors. Further increase in the acetic acid concentration up to 6 M leads to a gentle decrease in the absorbance when wavelength increases. This absorbance decrease is lower than that corresponding to the samples prepared in 1 M or 2 M acetic acid electrolytes. No distinctive difference in the spectra of the anodic oxide between Ti and TiePd substrates was found. 3.3. XPS analysis The survey XPS spectra of the annealed anodic oxides on Ti and TiePd show peaks due to Ti, Pd (only for TiePd), O, and C, along with a weak peak corresponding to N. The existence of carbon and nitrogen in the anodic oxide is attributed to contamination due to air exposure during sample preparation. Ti 2p spectrum of the anodic oxide is dominated by species in Ti4þ oxidation state, at a binding energy of 458.8 eV, irrespective of the acetic acid
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Fig. 1. SEM images of the annealed anodic oxides on TiePd, prepared in electrolytes with acetic acid concentrations of 0.01 M (a), 1 M (b), 2 M (c), and 6 M (d).
Fig. 2. Variation in the surface area ratio and surface roughness of the annealed anodic oxides on Ti and TiePd against the acetic acid concentration in the electrolyte (a), and the laser microscope images of the annealed anodic oxide on TiePd prepared using acetic acid concentrations of 0.01 M (b) and 2 M (c), respectively.
concentration and the substrate employed. This implies that the surface of the anodic layer is composed of titanium oxide. Fig. 5 shows Pd 3d XPS of TiePd (a) and the annealed anodic oxides on TiePd prepared in electrolytes with acetic acid concentrations of
0.01 M (b), 2 M (c) and 6 M (d). The two single peaks observed in (a), at a binding energy of approximately 340.1 and 334.8 eV, correspond to the Pd 3d3/2 and 3d5/2 contributions of metal Pd, implying that Pd is not oxidized. The peaks of the spectra in (b), at a binding
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Fig. 3. XRD profiles of the annealed anodic oxides prepared in the electrolyte with an acetic acid concentration of 0.01 M, 1 M, 2 M and 6 M on Ti (a) and TiePd (b). The profiles for the annealed Ti and TiePd are also inserted.
Fig. 4. Absorption spectrum of the annealed anodic oxides on Ti (a) and TiePd (b) prepared in 0.01 M, 1 M, 2 M and 6 M acetic acid electrolytes.
energy of approximately 342.1, 340.1, 336.8 and 334.8 eV, correspond to Pd oxide 3d3/2, Pd metal 3d3/2, Pd oxide 3d5/2 and Pd metal 3d5/2, respectively, implying that Pd is partially oxidized in the anodic oxide. The Pd XPS of the anodic oxide prepared in 2 M exhibits weak spectral peaks at 342.2 and 336.9 eV due to Pd oxide 3d3/2 and 3d5/2 (c). In the spectrum of the anodic oxide prepared in 6 M acetic acid, two single peaks at approximately 342.2 and 336.9 eV corresponding to Pd oxide 3d3/2 and 3d5/2 are observed. Estimates of 0.20, 0.04, 0.04 and 0.08 wt.% for the fraction of Pd in the anodic oxides prepared in electrolytes with 0.01 M, 1 M, 2 M and 6 M concentrations of acetic acid, respectively, were obtained from the XPS analysis.
Fig. 6 shows O 1 s XPS of TiePd alloy (a) and the annealed anodic oxides prepared in electrolytes with acetic acid concentrations of 0.01 M (b), 1 M (c) and 6 M (d). The profiles exhibit an asymmetrical shape accompanied by a shoulder peak extending toward higher binding energies, which is ascribed to hydroxyl groups. The hydroxyl group fraction of (a), (b), (c), and (d) is calculated as 32.3%, 9.6%, 9.1% and 10.0%, respectively. This implies that the amount of hydroxyl groups on the TiePd alloy decreases with anodic oxidation; no distinctive difference in the fraction of hydroxyl groups is found among the anodic oxides. The fraction of hydroxyl groups in the anodic oxides on Ti prepared in the electrolytes with acetic acid concentrations of 0.01 M, 1 M, and 6 M are calculated as 8.3%, 8.9%
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Fig. 5. Pd 3d XPS of TiePd alloy (a) and the annealed anodic oxides on TiePd prepared in electrolytes with acetic acid concentrations of 0.01 M (b), 1 M (c) and 6 M (d).
and 8.8%, respectively, which are slightly lower than those for the anodic oxides on TiePd. 3.4. Photo-induced properties Fig. 7 shows the amount of the evolved CO2, decomposed from CH3CHO, against the illumination duration for the as-anodized (a, b) and annealed oxides (c, d) on Ti (a, c) and TiePd (b, d). The photocatalytic activities of the annealed anodic oxides are superior to those of the as-anodized oxides irrespective of the acetic acid concentration and substrate employed. The CO2 evolution rates in the annealed anodic oxides on TiePd prepared in the electrolytes with acetic acid concentrations of 0.01 M, 1 M, 2 M and 6 M were calculated as 6.64, 14.56, 13.93 and 9.81 mmol/h, respectively. Lower rates were measured for the same reaction on Ti (4.33, 11.03, 13.34 and 9.55 mmol/h, respectively), therefore indicating that the anodic oxides on TiePd have better photocatalytic activities than those on Ti. The contact angles plotted against the duration of UV light illumination when a water droplet falls on the as-anodized (a, b) and annealed oxide (c, d) on Ti (a, c) and TiePd (b, d) are shown in Fig. 8. The data for substrates (without anodic oxides) are also plotted. The contact angle for every substrate decreases with anodic oxidation. Annealed oxides exhibit contact angles lower than those of as-anodized oxides irrespective of the acetic acid concentration in the electrolyte and the substrate. The contact angles of the
annealed anodic oxides on Ti are lower than the corresponding ones on TiePd, and the anodic oxide prepared in 2 M acetic acid shows the lowest contact angle among the examined samples irrespective of the substrate employed. 4. Discussion The photocatalytic activities of the anodic oxides on TiePd were superior to those of the ones on Ti, and the highest activities were found for the anodic oxides prepared in the electrolytes with 1 M and 2 M acetic acid. The obvious difference in the photocatalytic activity of the anodic oxides on Ti and TiePd substrates proves a positive effect of Pd on this property. The XPS analysis revealed that Pd incorporates into the anodic oxide while the chemical state of Pd was oxide (PdO). Generally, anodic oxides on Ti alloy are composed of TiO2 and additional oxides of the alloying constituent depending on the standard oxidation free-energy of the constituent. When the charge transfer between the semiconductors occurs based on the electronic band structure, the photocatalytic activity of the materials is improved, as already reported for TiO2eV2O5 [34], TiO2eCdS [35] and SnO2eV2O5 [36]. However, if the aforementioned additional oxides exhibit poor or no photoactivity, the photoactivity of the material is weakened because the oxides cover the photosensitive surface of TiO2. The free-energy of PdO is 85 kJ mol1 at room temperature [37], and PdO is formed by annealing Pd at temperatures above about 1073 K [38]. This suggests that a high
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Fig. 6. O 1 s XPS of TiePd alloy (a) and the annealed anodic oxide on TiePd prepared in electrolytes with acetic acid concentrations of 0.01 M (b), 1 M (c) and 6 M (d).
thermal energy is necessary for PdO formation and the present anodic oxidization condition satisfies this requisite. Several previous studies on the photocatalytic properties of PdO have been reported. Mesoporous PdOeTiO2 nanocomposites, prepared by the solegel method, exhibit high and efficient photocatalytic activities for CH3OH oxidation to HCHO. These high photocatalytic activities (~4 and 2 times than TiO2eP25 and Pd/TiO2eP25 respectively [39]) have been attributed to the fact that the Pd atoms incorporated act as local electron reservoirs and prevent the recombination of the photo-generated holes and electrons; however, Pd but not PdO has been the state observed in this study. Liu et al. reported a photocatalytic disinfection performance of PdO loaded TiO2 hollow spheres superior to that of Pd loaded TiO2 hollow spheres, and concluded that this is due to the electron/hole pair separation caused by PdO nanoparticles [40]. According to their study, the photocatalytic disinfection performance of the PdO loaded TiO2 is ascribed to the higher work function of PdO (~7.9 eV) in comparison to that of Pd (~5.12 eV). The work function of TiO2 is ~4.2 eV. Therefore, the photo-generated electrons could transfer from TiO2 to PdO more easily than from TiO2 to Pd. This mechanism was derived from the intrinsic work function of individual substances; however, the position of the valance and conduction bands of every substance in the composites should be considered to verify the transfer of the electrons generated in TiO2 to Pd or PdO. PdO is a semiconductor with a small direct band gap [41]; however, its electronic structure and band gap energy are not
established. The experimental band gap measured varies with the method employed: 0e1 eV from XPSeUPS analysis [42], 0.8 eV from optical transmittance measurements [43,44], 1.5 eV from electric conductivity measurements [45], 2.13 eV from optical density measurements, and 2.67 eV from photoconductivity measurements [46]. Values from theoretical investigations are also scattered: 0.1 eV were calculated using the local-densityapproximation of the augmented spherical wave method [47], 0.6 eV using the linear-muffin-tin-orbital (LMTO) method [48], 0.9 eV using the screened-exchange hybrid functional HSE (HeydScuseria-Ernzerhof) as implemented in the Vienna ab initio simulation package, and no band gap was found when density functional theory (DFT) was employed [49]. Park et al. calculated the band structure using full-potential linearized augmented-plane-waves and full-potential LMTO methods [50]. According to their results, a hybridized Pd 4d - O 2p band overlapped the valence and conduction bands resulting in no gap at the Fermi level, and PdO comes out to be metallic. A similar result was reported using DFT calculations [51]. Here, two possibilities are considered to explain the effect of Pd on the photocatalytic activity. First, the existence of an excited charge transfer between TiO2 and PdO that could separate the charge carriers and retard recombination. This would agree with the lower band gap energy reported for PdO in comparison to that found for TiO2. The precise position of PdO valence and conduction bands is not clear; however, it is plausible that the position of the upper state of the valence band of PdO be located at higher
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Fig. 7. Plot of amount of the evolved CO2 decomposed from CH3CHO against UV illumination duration for the as-anodized (a, b) and annealed oxides (c, d) on Ti (a, c) and TiePd (b, d).
values than that of TiO2 because, as illustrated in Fig. 9, the upper edge of the valence band in TiO2 is located in a negatively deep state (7.2 eV). Second, PdO, similar to Pt [16,17], could serve as an electron sink site and promote interfacial electron transfer, in agreement with theoretical studies reporting the existence of metallic PdO with no band gap [49e51] and instable PdO surfaces due to the low vacancy formation energy [52] required to induce PdO decomposition into metallic Pd [42]. The nature of Pd contribution to the photocatalytic activity of the anodic oxide is still a subject of investigation, but we speculate that the effect of Pd on the photocatalytic performance can be attributed to the incorporated PdO, retarding charge recombination. On the other hand, the variation in the photocatalytic activity of the anodic oxides against the acetic acid concentrations in the electrolytes is related to the surface roughness and surface area of the anodic oxides. Acetic acid is a weak acid (dissociation constant 4.76 [53]), and Ti and Ti alloy do not generally corrode in acetic acid aqueous solutions. In order to explore the corrosion behavior in the electrolytes, the electrochemical reactions that occur in the presence of for Ti and TiePd were explored by monitoring the current obtained by applying an anodic voltage of 210 V. A three-electrode configuration, containing a counter electrode and a Luggin capillary with Ag/AgCl as the reference electrode, was used for this experiment. Fig. 10 shows the current density change of Ti (a) and TiePd
(b) in the electrolyte during anodization. High current densities, which gradually decayed to a minimum, were found for 1 M and 2 M acetic acid electrolytes. The decrease of the current densities is due to the formation of insulate oxide, and the subsequent increase, after the recording of the minimum, is attributed to the dissolution of the anodic oxide. At the minimum current density, both reactions (dissolution and oxidation) are in equilibrium. The low current densities found in 0.01 M are due to the high chemical resistance of Ti and TiePd in the electrolytes, and that measured in 6 M is due to the low dissociation of the electrolyte. The variation in the photocatalytic activity of the anodic oxides against the acetic acid concentration in the electrolytes is attributable to the surface morphology of the anodic oxide, which is controlled by this electrochemical behavior. While it is accepted that hydroxyl groups on the surface of TiO2 are responsible for the hydrophilicity [54e57], the corresponding mechanism has not been delineated yet. The fractions of hydroxyl groups in the annealed anodic oxides on TiePd prepared in electrolytes with acetic acid concentrations of 0.01 M, 1 M, 2 M and 6 M were estimated as 9.6, 9.1, 8.9 and 10.0%, respectively, from the XPS analysis, whereas the corresponding ones on Ti were 8.3, 8.9, 8.2 and 8.8%. A distinct difference in the fraction of hydroxyl groups between the anodic oxides was not found and, therefore, a convincing explanation for the results shown in Fig. 8 cannot be
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Fig. 8. Contact angle variation against the duration of UV light illumination for the as-anodized (a, b) and annealed oxides (c, d) on Ti (a, c) and TiePd (b, d). The same variation of Ti and TiePd without oxide (w/o oxide) is plotted as references.
Fig. 9. Schematic illustration for the charge separation of the PdO incorporated anodic oxide.
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Fig. 10. Current density change of Ti (a) and TiePd (b) in the electrolytes with acetic acid concentrations of 0.01 M, 1 M, 2 M and 6 M during anodization.
provided. Another factor affecting hydrophilicity is surface roughness [58,59]; an increase in the surface roughness of a hydrophilic surface results in a significant decrease in the contact angle. The surface roughness of the annealed anodic oxide (Fig. 2) provides a reasonable explanation for the contact angle variation with the increase in the acetic acid concentration of the electrolyte. The roughness of the anodic oxide on TiePd is slightly higher than that of the one on Ti. Consequently, the reason why the contact angles of the anodic oxide on Ti are lower than those of the anodic oxide on TiePd is still unclear. These findings allow us to conclude that the photo-induced properties of the anodic oxide on TiePd are provided by the incorporation of PdO in the anodic oxide, and that the surface morphology is controlled by the electrochemical behavior in the acetic acid electrolytes. 5. Conclusion The photo-induced properties of anodic oxides on TiePd alloy prepared in acetic acid aqueous solutions were investigated and compared with those of the anodic oxides on pure Ti. The photocatalytic activity of the anodic oxides on TiePd was found to be superior to that of the ones on Ti, which was explained by the incorporation of PdO, retarding charge recombination. The variation of both photocatalytic activity and hydrophilicity of anodic oxides against the acetic acid concentrations used in the electrolytes was attributed to the surface morphology of the anodic oxides, which is controlled by the electrochemical behavior in the electrolyte. It is concluded that the photo-induced properties of TiePd alloy are provided by the anodic oxidation in acetic acid electrolytes. Acknowledgments The authors wish to acknowledge Dr. A. Kuroda from Nippon Steel and Sumitomo Metals for providing TiePd alloy, Ms. Y. Matsuda and Mr. S. Sugiyama from IMR, Tohoku Univ. for sample preparations and characterizations. This work is a cooperative
program (Proposal No. 15G0415) of the Cooperative Research and Development Center for Advanced Materials, Institute for Materials Research, Tohoku University. One of the authors (N.M.) acknowledges the Grant-in-Aid for Scientific Research (B) (No. 15H04138) from the Ministry of Education, Science, Sports, and Culture, Japan. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Photo-induced properties of anodic oxide on TiePd alloy prepared in acetic acid electrolyte N. Masahashi a, *, Y. Mizukoshi a, H. Inoue b, K. Ohmura a, T. Moroishi c a
Institute of for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba, Sendai, Miyagi, 980-8577, Japan Department of Materials Science, Graduate School of Engineering, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan c TiG Co.,Ltd., 4-1-32 Kawata, Higashi-Osaka, Osaka 578-0905, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 September 2015 Received in revised form 28 December 2015 Accepted 27 January 2016 Available online 4 February 2016
Photo-induced properties of titanium dioxide on TiePd alloy, prepared by anodic oxidation in acetic acid electrolytes have been studied and compared with those of the oxide on pure Ti. The photocatalytic activity of the anodic oxide on TiePd was superior to that of the anodic oxide on Ti, and the highest photocatalytic activities were found for the anodic oxide prepared in electrolytes with 1 M or 2 M acetic acid. This superiority was attributed to PdO incorporation into the anodic oxide, which facilitates charge separation. Both photocatalytic activity and hydrophilicity of the anodic oxide varied with the acetic acid concentration in the electrolyte, and were related to the surface morphology of the anodic oxides, which is controlled by the electrochemical reaction. It is concluded that photo-induced properties are imparted to the corrosion-resistant TiePd alloy by anodic oxidation in acetic acid aqueous solutions. © 2016 Elsevier B.V. All rights reserved.
Keywords: Thin films Oxide materials Chemical synthesis Catalysis Corrosion Photoelectron spectroscopies
1. Introduction Titanium dioxide (TiO2) semiconductors exhibit attractive photocatalytic characteristics [1]. These semiconductors have been applied in various fields and employed for the decomposition or sterilization of chemical substances such as endocrine disruptors, injurious volatile compounds, and organic pollutants in the environment [2,3]. Furthermore, the self-cleaning function of these materials, which results from their superhydrophilicity, leads to unique performances in the photodegradation of various noxious or malodorous chemicals [4,5]. Numerous investigations have reported that the incorporation of noble metals such as Pt [6e8], Ag [9,10], Au [11,12] and Pd [13,14] to TiO2 is an effective method to improve its photocatalytic activity. In particular, the effect of Pt incorporation on TiO2 photocatalytic activity has been widely studied, and it is considered that Pt acts as an electron sink site because of the Schottky barrier effect formed at the metal/TiO2 interface [8,15] and promotes interfacial electron transfer by facilitating charge separation [16,17]. Usually, noble metal incorporation is conducted using a photochemical deposition method,
* Corresponding author. E-mail address: [email protected] (N. Masahashi). http://dx.doi.org/10.1016/j.jallcom.2016.01.220 0925-8388/© 2016 Elsevier B.V. All rights reserved.
wherein an aqueous TiO2 suspension containing an alcohol and an acid-containing noble metal is irradiated by a mercury lamp. Noble metal-loaded TiO2 powders are obtained by filtering the TiO2 suspension and subsequent drying. The powders show a high quantum efficiency because of their large surface area and high density of surface coordination unsaturated sites [18,19]; however, the practical usage of these powders is limited because of their aggregation [20,21], and the toxicity of TiO2 poses a potential risk to human health [22e24]. In this study, anodic oxidation method was applied to a wellknown crevice corrosion resistant TiePd alloy, registered as ASTM Grade 7, to incorporate Pd into TiO2. Pd inhibits chloride corrosion of Ti due to stabilizing the passivation layer composed of titanium oxide [25]. Anodic oxidation leads to the formation of titanium oxide on the TiePd surface, and it is considered that this passivation layer, composed of a few atomic layers, is incorporated into the anodic oxide. Subsequent annealing promotes the crystallization of the oxide, and the photocatalytic characteristics of TiO2 are expected to be maintained without impairing its corrosion resistance. Anodic oxidation generates thermodynamically stable oxides on the surface of titanium and its alloys [26] with high adhesion strength to the substrate [27]. We investigated the photocatalytic properties of the anodic oxide on Ti [28e30] and Ti
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alloys such as TiNbSn [31] and Ti6Al4V [32,33]. These properties varied with the electrochemical conditions and subsequent annealing [28,30], which control TiO2 crystallinity and its chemical composition, and suppress charge recombination. Thus far, studies regarding anodic oxidation of Ti alloys containing noble metals have not been reported. The purpose of this study is to explore the photocatalytic activity and hydrophilicity of anodic oxides on TiePd alloy, with special focus on the effect of Pd on their photocatalytic activity. 2. Experimental procedures Ti-0.05 wt.% Pd alloy (Nippon Steel and Sumitomo Metals, Japan) with about 120 mm grain size was used as the anode in anodic oxidations. Microstructural and crystallographic analysis of Ti-0.05Pd (hereafter, denoted as TiePd) revealed that Pd exists as a solid solution in hcp Ti, which is consistent with the solubility limit (0.88 wt.%) of Pd in hcp Ti. A TiePd plate with dimensions of 19 9 1 mm3 was polished by emery paper and buffed using 1.0 and 0.3 mm alumina suspensions, and then washed in ethanol using an ultrasonic cleaner. A 100-mesh Pt electrode, with dimensions of 5 5 cm2 was used as the cathode. Anodic oxidations were conducted for 0.5 h using a DC power supply (Matsusada Precision, PRk 500-3.2, Japan), and electrolytes with acetic acid concentrations in the range 0.01e6 M. The applied voltage and current density were 210 V and 0.05 A/cm2, respectively. The anodized electrode was rinsed with distilled water and dried at room temperature, and subsequently annealed at 723 K for 5 h in air. Monolithic Ti (cp-Ti, grade I) was anodized in the same way using the anodic oxide on Ti as reference. Crystallographic structures were determined by means of an X-ray diffractometer (PANalytical X'Pert diffractometer, Netherlands) with CuKa radiation (0.15406 nm), at a scan rate of 1 min1, using a sample with a thin-film geometry arrangement at 0.5 glancing angle and a rotating detector. Microstructure observations were performed using scanning electron microscopy (SEM) (Keyence VE-8800, Japan) at an operating voltage of 20 kV, and laser microscopy (Shimazu, Nano Search® Microscope SFT3500(S), Japan). The absorption spectra were measured by using a UVevis spectrophotometer (Jasco V-550, Japan). X-ray photoelectron spectroscopy (XPS) measurements were carried out with an electron spectrometer (Shimazu, Kratos AXIS-Ultra DLD, Japan) equipped with monochromated Al Ka radiation at a base pressure of 3.0 107 Pa. The full width at half maximum intensity of the Ag 3d5/2 peak was 0.73 eV, and the base pressure of the spectrometer was 6.5 108 Pa. The photocatalytic activities of the anodic oxide were evaluated by measuring the amount of CO2 produced from the decomposition of CH3CHO. An anodized specimen was placed in a cylindrical Pyrex vessel with a quartz window, and CH3CHO (50 mL) was introduced into the vessel. After confirming that there was no change in the amounts of CO2, the specimen was illuminated by UV light using a beam lamp (Asahi Spectra, MAX-302, Japan; 1.1 mW/cm2), and the evolved CO2 was measured by gas chromatography (Shimadzu GC2014, Japan). The hydrophilicity of the anodic oxides was evaluated by measuring the contact angle of distilled water using a goniometer system (Kyowa Interface Science Co Ltd., CA-X, Japan). The average of three measurements for each sample, illuminated by a beam lamp (Seric Co Ltd., SOLAX XC-100B, Japan; 0.1 mW/cm2), was used as the contact angle. 3. Results 3.1. Microstructure and crystallographic structure Fig. 1 shows the SEM images of the annealed anodic oxides on
TiePd, prepared in electrolytes with acetic acid concentrations of 0.01 M (a), 1 M (b), 2 M (c), and 6 M (d). When an acetic acid concentration of 0.01 M was used (a), a few micron-sized pores were observed on the weakly corroded surface. With the increase in the acetic acid concentration to 1 M, the surface of the anodic oxide was severely corroded, as shown in (b). A further increase in concentration to 2 M generated large-sized hollows due to the coalescence of the micron-sized pores and the corroded cavities (c), and its frequency of appearance (marked by arrows) increased with an increase in the acetic acid concentration to 6 M (d). The microstructure variation of the anodic oxide on Ti with the acetic acid concentration in the electrolyte is similar to that for the anodic oxide on TiePd. EDX analysis did not detect Pd in any of the anodic oxides, probably due to the low concentration of Pd. Fig. 2 shows the variation in the surface area ratio and surface roughness of the annealed anodic oxides on Ti and TiePd against the acetic acid concentration in the electrolyte (a), and the laser microscope images of the annealed anodic oxide on TiePd prepared using acetic acid concentrations of 0.01 M (b) and 2 M (c), respectively. Here, the surface area ratio is defined as the ratio of the measured surface area to the projected surface area, which becomes unity in a perfectly flat surface. The roughness and surface area ratio increase with the acetic acid concentration up to 2 M and then decrease irrespective of the substrate. The laser microscope images reveal that the surface morphology of the anodic oxide prepared in 2 M (c) is rougher than that of the anodic oxide prepared in 0.01 M (b). These results imply that the corrosion resistance of the anodic oxide depends on the acetic acid concentration in the electrolyte. Fig. 3 shows the XRD profiles of the annealed substrates and anodic oxides on Ti (a) and TiePd (b). The main peak of anatase TiO2 is found in the case of all annealed anodic oxides regardless of the acetic acid concentration, and weak peaks attributed to rutile TiO2 are seen. On the other hand, the XRD profiles of annealed Ti and TiePd exhibit anatase with weak rutile in addition to hcp-Ti main peaks. This suggests that annealing promotes rutile TiO2 formation on Ti and TiePd, whereas it enhances crystallization to anatase structure in the anodized oxide. Thick anodic oxides are formed on Ti as compared to TiePd as evidenced from the comparison of the peak intensity ratios of anatase 101 TiO2 (2q ¼ 25.3 ) to hcp Ti 101 diffracted peak (2q ¼ 40.04 ). 3.2. Absorption behavior Fig. 4 shows the absorption spectra of the annealed anodic oxide on Ti (a) and TiePd (b) prepared in 0.01 M, 1 M, 2 M and 6 M acetic acid electrolytes. The spectra of the anodic oxide prepared in 0.01 M exhibit oscillations due to the thin thickness of the anodic oxide layer. When the acetic acid concentration increases, the spectra show a steep absorbance decrease with increasing wavelength, which is typical of semiconductors. Further increase in the acetic acid concentration up to 6 M leads to a gentle decrease in the absorbance when wavelength increases. This absorbance decrease is lower than that corresponding to the samples prepared in 1 M or 2 M acetic acid electrolytes. No distinctive difference in the spectra of the anodic oxide between Ti and TiePd substrates was found. 3.3. XPS analysis The survey XPS spectra of the annealed anodic oxides on Ti and TiePd show peaks due to Ti, Pd (only for TiePd), O, and C, along with a weak peak corresponding to N. The existence of carbon and nitrogen in the anodic oxide is attributed to contamination due to air exposure during sample preparation. Ti 2p spectrum of the anodic oxide is dominated by species in Ti4þ oxidation state, at a binding energy of 458.8 eV, irrespective of the acetic acid
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Fig. 1. SEM images of the annealed anodic oxides on TiePd, prepared in electrolytes with acetic acid concentrations of 0.01 M (a), 1 M (b), 2 M (c), and 6 M (d).
Fig. 2. Variation in the surface area ratio and surface roughness of the annealed anodic oxides on Ti and TiePd against the acetic acid concentration in the electrolyte (a), and the laser microscope images of the annealed anodic oxide on TiePd prepared using acetic acid concentrations of 0.01 M (b) and 2 M (c), respectively.
concentration and the substrate employed. This implies that the surface of the anodic layer is composed of titanium oxide. Fig. 5 shows Pd 3d XPS of TiePd (a) and the annealed anodic oxides on TiePd prepared in electrolytes with acetic acid concentrations of
0.01 M (b), 2 M (c) and 6 M (d). The two single peaks observed in (a), at a binding energy of approximately 340.1 and 334.8 eV, correspond to the Pd 3d3/2 and 3d5/2 contributions of metal Pd, implying that Pd is not oxidized. The peaks of the spectra in (b), at a binding
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Fig. 3. XRD profiles of the annealed anodic oxides prepared in the electrolyte with an acetic acid concentration of 0.01 M, 1 M, 2 M and 6 M on Ti (a) and TiePd (b). The profiles for the annealed Ti and TiePd are also inserted.
Fig. 4. Absorption spectrum of the annealed anodic oxides on Ti (a) and TiePd (b) prepared in 0.01 M, 1 M, 2 M and 6 M acetic acid electrolytes.
energy of approximately 342.1, 340.1, 336.8 and 334.8 eV, correspond to Pd oxide 3d3/2, Pd metal 3d3/2, Pd oxide 3d5/2 and Pd metal 3d5/2, respectively, implying that Pd is partially oxidized in the anodic oxide. The Pd XPS of the anodic oxide prepared in 2 M exhibits weak spectral peaks at 342.2 and 336.9 eV due to Pd oxide 3d3/2 and 3d5/2 (c). In the spectrum of the anodic oxide prepared in 6 M acetic acid, two single peaks at approximately 342.2 and 336.9 eV corresponding to Pd oxide 3d3/2 and 3d5/2 are observed. Estimates of 0.20, 0.04, 0.04 and 0.08 wt.% for the fraction of Pd in the anodic oxides prepared in electrolytes with 0.01 M, 1 M, 2 M and 6 M concentrations of acetic acid, respectively, were obtained from the XPS analysis.
Fig. 6 shows O 1 s XPS of TiePd alloy (a) and the annealed anodic oxides prepared in electrolytes with acetic acid concentrations of 0.01 M (b), 1 M (c) and 6 M (d). The profiles exhibit an asymmetrical shape accompanied by a shoulder peak extending toward higher binding energies, which is ascribed to hydroxyl groups. The hydroxyl group fraction of (a), (b), (c), and (d) is calculated as 32.3%, 9.6%, 9.1% and 10.0%, respectively. This implies that the amount of hydroxyl groups on the TiePd alloy decreases with anodic oxidation; no distinctive difference in the fraction of hydroxyl groups is found among the anodic oxides. The fraction of hydroxyl groups in the anodic oxides on Ti prepared in the electrolytes with acetic acid concentrations of 0.01 M, 1 M, and 6 M are calculated as 8.3%, 8.9%
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Fig. 5. Pd 3d XPS of TiePd alloy (a) and the annealed anodic oxides on TiePd prepared in electrolytes with acetic acid concentrations of 0.01 M (b), 1 M (c) and 6 M (d).
and 8.8%, respectively, which are slightly lower than those for the anodic oxides on TiePd. 3.4. Photo-induced properties Fig. 7 shows the amount of the evolved CO2, decomposed from CH3CHO, against the illumination duration for the as-anodized (a, b) and annealed oxides (c, d) on Ti (a, c) and TiePd (b, d). The photocatalytic activities of the annealed anodic oxides are superior to those of the as-anodized oxides irrespective of the acetic acid concentration and substrate employed. The CO2 evolution rates in the annealed anodic oxides on TiePd prepared in the electrolytes with acetic acid concentrations of 0.01 M, 1 M, 2 M and 6 M were calculated as 6.64, 14.56, 13.93 and 9.81 mmol/h, respectively. Lower rates were measured for the same reaction on Ti (4.33, 11.03, 13.34 and 9.55 mmol/h, respectively), therefore indicating that the anodic oxides on TiePd have better photocatalytic activities than those on Ti. The contact angles plotted against the duration of UV light illumination when a water droplet falls on the as-anodized (a, b) and annealed oxide (c, d) on Ti (a, c) and TiePd (b, d) are shown in Fig. 8. The data for substrates (without anodic oxides) are also plotted. The contact angle for every substrate decreases with anodic oxidation. Annealed oxides exhibit contact angles lower than those of as-anodized oxides irrespective of the acetic acid concentration in the electrolyte and the substrate. The contact angles of the
annealed anodic oxides on Ti are lower than the corresponding ones on TiePd, and the anodic oxide prepared in 2 M acetic acid shows the lowest contact angle among the examined samples irrespective of the substrate employed. 4. Discussion The photocatalytic activities of the anodic oxides on TiePd were superior to those of the ones on Ti, and the highest activities were found for the anodic oxides prepared in the electrolytes with 1 M and 2 M acetic acid. The obvious difference in the photocatalytic activity of the anodic oxides on Ti and TiePd substrates proves a positive effect of Pd on this property. The XPS analysis revealed that Pd incorporates into the anodic oxide while the chemical state of Pd was oxide (PdO). Generally, anodic oxides on Ti alloy are composed of TiO2 and additional oxides of the alloying constituent depending on the standard oxidation free-energy of the constituent. When the charge transfer between the semiconductors occurs based on the electronic band structure, the photocatalytic activity of the materials is improved, as already reported for TiO2eV2O5 [34], TiO2eCdS [35] and SnO2eV2O5 [36]. However, if the aforementioned additional oxides exhibit poor or no photoactivity, the photoactivity of the material is weakened because the oxides cover the photosensitive surface of TiO2. The free-energy of PdO is 85 kJ mol1 at room temperature [37], and PdO is formed by annealing Pd at temperatures above about 1073 K [38]. This suggests that a high
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Fig. 6. O 1 s XPS of TiePd alloy (a) and the annealed anodic oxide on TiePd prepared in electrolytes with acetic acid concentrations of 0.01 M (b), 1 M (c) and 6 M (d).
thermal energy is necessary for PdO formation and the present anodic oxidization condition satisfies this requisite. Several previous studies on the photocatalytic properties of PdO have been reported. Mesoporous PdOeTiO2 nanocomposites, prepared by the solegel method, exhibit high and efficient photocatalytic activities for CH3OH oxidation to HCHO. These high photocatalytic activities (~4 and 2 times than TiO2eP25 and Pd/TiO2eP25 respectively [39]) have been attributed to the fact that the Pd atoms incorporated act as local electron reservoirs and prevent the recombination of the photo-generated holes and electrons; however, Pd but not PdO has been the state observed in this study. Liu et al. reported a photocatalytic disinfection performance of PdO loaded TiO2 hollow spheres superior to that of Pd loaded TiO2 hollow spheres, and concluded that this is due to the electron/hole pair separation caused by PdO nanoparticles [40]. According to their study, the photocatalytic disinfection performance of the PdO loaded TiO2 is ascribed to the higher work function of PdO (~7.9 eV) in comparison to that of Pd (~5.12 eV). The work function of TiO2 is ~4.2 eV. Therefore, the photo-generated electrons could transfer from TiO2 to PdO more easily than from TiO2 to Pd. This mechanism was derived from the intrinsic work function of individual substances; however, the position of the valance and conduction bands of every substance in the composites should be considered to verify the transfer of the electrons generated in TiO2 to Pd or PdO. PdO is a semiconductor with a small direct band gap [41]; however, its electronic structure and band gap energy are not
established. The experimental band gap measured varies with the method employed: 0e1 eV from XPSeUPS analysis [42], 0.8 eV from optical transmittance measurements [43,44], 1.5 eV from electric conductivity measurements [45], 2.13 eV from optical density measurements, and 2.67 eV from photoconductivity measurements [46]. Values from theoretical investigations are also scattered: 0.1 eV were calculated using the local-densityapproximation of the augmented spherical wave method [47], 0.6 eV using the linear-muffin-tin-orbital (LMTO) method [48], 0.9 eV using the screened-exchange hybrid functional HSE (HeydScuseria-Ernzerhof) as implemented in the Vienna ab initio simulation package, and no band gap was found when density functional theory (DFT) was employed [49]. Park et al. calculated the band structure using full-potential linearized augmented-plane-waves and full-potential LMTO methods [50]. According to their results, a hybridized Pd 4d - O 2p band overlapped the valence and conduction bands resulting in no gap at the Fermi level, and PdO comes out to be metallic. A similar result was reported using DFT calculations [51]. Here, two possibilities are considered to explain the effect of Pd on the photocatalytic activity. First, the existence of an excited charge transfer between TiO2 and PdO that could separate the charge carriers and retard recombination. This would agree with the lower band gap energy reported for PdO in comparison to that found for TiO2. The precise position of PdO valence and conduction bands is not clear; however, it is plausible that the position of the upper state of the valence band of PdO be located at higher
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Fig. 7. Plot of amount of the evolved CO2 decomposed from CH3CHO against UV illumination duration for the as-anodized (a, b) and annealed oxides (c, d) on Ti (a, c) and TiePd (b, d).
values than that of TiO2 because, as illustrated in Fig. 9, the upper edge of the valence band in TiO2 is located in a negatively deep state (7.2 eV). Second, PdO, similar to Pt [16,17], could serve as an electron sink site and promote interfacial electron transfer, in agreement with theoretical studies reporting the existence of metallic PdO with no band gap [49e51] and instable PdO surfaces due to the low vacancy formation energy [52] required to induce PdO decomposition into metallic Pd [42]. The nature of Pd contribution to the photocatalytic activity of the anodic oxide is still a subject of investigation, but we speculate that the effect of Pd on the photocatalytic performance can be attributed to the incorporated PdO, retarding charge recombination. On the other hand, the variation in the photocatalytic activity of the anodic oxides against the acetic acid concentrations in the electrolytes is related to the surface roughness and surface area of the anodic oxides. Acetic acid is a weak acid (dissociation constant 4.76 [53]), and Ti and Ti alloy do not generally corrode in acetic acid aqueous solutions. In order to explore the corrosion behavior in the electrolytes, the electrochemical reactions that occur in the presence of for Ti and TiePd were explored by monitoring the current obtained by applying an anodic voltage of 210 V. A three-electrode configuration, containing a counter electrode and a Luggin capillary with Ag/AgCl as the reference electrode, was used for this experiment. Fig. 10 shows the current density change of Ti (a) and TiePd
(b) in the electrolyte during anodization. High current densities, which gradually decayed to a minimum, were found for 1 M and 2 M acetic acid electrolytes. The decrease of the current densities is due to the formation of insulate oxide, and the subsequent increase, after the recording of the minimum, is attributed to the dissolution of the anodic oxide. At the minimum current density, both reactions (dissolution and oxidation) are in equilibrium. The low current densities found in 0.01 M are due to the high chemical resistance of Ti and TiePd in the electrolytes, and that measured in 6 M is due to the low dissociation of the electrolyte. The variation in the photocatalytic activity of the anodic oxides against the acetic acid concentration in the electrolytes is attributable to the surface morphology of the anodic oxide, which is controlled by this electrochemical behavior. While it is accepted that hydroxyl groups on the surface of TiO2 are responsible for the hydrophilicity [54e57], the corresponding mechanism has not been delineated yet. The fractions of hydroxyl groups in the annealed anodic oxides on TiePd prepared in electrolytes with acetic acid concentrations of 0.01 M, 1 M, 2 M and 6 M were estimated as 9.6, 9.1, 8.9 and 10.0%, respectively, from the XPS analysis, whereas the corresponding ones on Ti were 8.3, 8.9, 8.2 and 8.8%. A distinct difference in the fraction of hydroxyl groups between the anodic oxides was not found and, therefore, a convincing explanation for the results shown in Fig. 8 cannot be
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Fig. 8. Contact angle variation against the duration of UV light illumination for the as-anodized (a, b) and annealed oxides (c, d) on Ti (a, c) and TiePd (b, d). The same variation of Ti and TiePd without oxide (w/o oxide) is plotted as references.
Fig. 9. Schematic illustration for the charge separation of the PdO incorporated anodic oxide.
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Fig. 10. Current density change of Ti (a) and TiePd (b) in the electrolytes with acetic acid concentrations of 0.01 M, 1 M, 2 M and 6 M during anodization.
provided. Another factor affecting hydrophilicity is surface roughness [58,59]; an increase in the surface roughness of a hydrophilic surface results in a significant decrease in the contact angle. The surface roughness of the annealed anodic oxide (Fig. 2) provides a reasonable explanation for the contact angle variation with the increase in the acetic acid concentration of the electrolyte. The roughness of the anodic oxide on TiePd is slightly higher than that of the one on Ti. Consequently, the reason why the contact angles of the anodic oxide on Ti are lower than those of the anodic oxide on TiePd is still unclear. These findings allow us to conclude that the photo-induced properties of the anodic oxide on TiePd are provided by the incorporation of PdO in the anodic oxide, and that the surface morphology is controlled by the electrochemical behavior in the acetic acid electrolytes. 5. Conclusion The photo-induced properties of anodic oxides on TiePd alloy prepared in acetic acid aqueous solutions were investigated and compared with those of the anodic oxides on pure Ti. The photocatalytic activity of the anodic oxides on TiePd was found to be superior to that of the ones on Ti, which was explained by the incorporation of PdO, retarding charge recombination. The variation of both photocatalytic activity and hydrophilicity of anodic oxides against the acetic acid concentrations used in the electrolytes was attributed to the surface morphology of the anodic oxides, which is controlled by the electrochemical behavior in the electrolyte. It is concluded that the photo-induced properties of TiePd alloy are provided by the anodic oxidation in acetic acid electrolytes. Acknowledgments The authors wish to acknowledge Dr. A. Kuroda from Nippon Steel and Sumitomo Metals for providing TiePd alloy, Ms. Y. Matsuda and Mr. S. Sugiyama from IMR, Tohoku Univ. for sample preparations and characterizations. This work is a cooperative
program (Proposal No. 15G0415) of the Cooperative Research and Development Center for Advanced Materials, Institute for Materials Research, Tohoku University. One of the authors (N.M.) acknowledges the Grant-in-Aid for Scientific Research (B) (No. 15H04138) from the Ministry of Education, Science, Sports, and Culture, Japan. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
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