Photocatalytic Activity of Anodized Titanium Sheets under Ultra-Violet and Visible Light Irradiation

A commercially pure titanium sheet precipitated TiC in the surface layer was fabricated by anodic oxidation in NH4NO3 solution and heat treatment in air. The fabricated sheet showed relatively high photocatalytic activity in 0.1 M KI solution, which was close to the activity level of the P-25 particle made by Degussa Corporation. It exhibited photocatalytic activity in antifungal and antivirus tests under black light irradiation. The better photocatalytic activity under black light irradiation is considered to be related to the formation of anatase and rutile type titanium dioxides and rough surface. It also showed photocatalytic activity under visible light irradiation, which is considered to be attributable to carbon and nitrogen doping in titanium dioxide.


Introduction
Since Honda and Fujishima discovered the electrolysis of water by titanium dioxide under irradiation [1], the interest in titanium dioxide has been widened to environmental purification [2] [3] antibacterial [4] [5] and other uses.The various methods for making titanium dioxide with high photocatalytic activity have been investigated.
Anodic oxidation of titanium substrate is commercially used to produce colored titanium sheets.The color of the anodized titanium substrate can be changed by varying the thickness of surface titanium oxide layer.And colored titanium sheets are mainly used as architectural materials.
Recently, the anodic oxidation method has been applied for making titanium dioxide with enhanced photocatalytic activity under ultra-violet (UV) or visual light irradiation.K. Onoda et al. reported that the anodization of a pre-nitridated titanium substrate in a mixed electrolyte composed of H 2 SO 4 , H 3 PO 4 , and H 2 O 2 resulted in photocatalytic activity [6].Furthermore, N. Ohtsu et al. investigated the effects of anions in various ammonium salts electrolytes and reported on titanium substrate anodized in ammonium salts such as (NH 4 ) 2 SO 4 , (NH 4 ) 2 PO 4 and (NH 4 ) 2 O 5 B 2 O 3 and it was identified that S, P and B were incorporated into oxide layers, which affected photocatalytic activity [7].
Concerning visual light response, Y. Mizukoshi et al. conducted research on the visible light response of sulfur-doped rutile titanium dioxide photocatalysts fabricated by anodic oxidation [8].N. Ohtsu et al. reported that the visible-light-responsive titanium dioxide layer was fabricated by anodizing a titanium plate in aqueous nitric acid solutions, followed by annealing [9].
However, a few studies for effects of material factors on photocatalytic activity of anodized titanium substrate have been investigated.The authors have investigated the effects of a TiC precipitated in surface layer of titanium sheet on discoloration of titanium due to the growth of titanium oxide layer and it was found that TiC in surface layer enhanced the growth of titanium oxide layer on the titanium substrate under an atmospheric environment [10] [11].In this study, based on the prior studies, the effects of TiC precipitated in the surface layer on the photocatalytic activity of anodized titanium sheets were investigated.Concerning an electrolyte, NH 4 NO 3 solution was applied based on previous studies by N. Ohtsu et al. [7] [9].Photocatalytic activity of anodized titanium sheets was investigated under black light and visual light irradiation.

Materials and Photocatalytic Activity Measurement
Commercially pure (CP) titanium sheets cold rolled to the thickness of 0.4 mm were heat treated in vacuum annealing furnace.The holding temperature and time were 873 K and 6 hours, respectively.And average raising rate was approximately 100 K/hour and cooling rate was nearly 50 K/hour.The concentrations of impurity elements in the CP titanium sheets are shown in Table 1.
The titanium sheets were rinsed in acetone and immersed in 0.06 M NH 4 NO 3 solution for anodic oxidation.The anodic oxidation was controlled by galvanostatically at 15 V, 24 V, 45 V and 80 V for 120 sec at room temperature.After the anodization they were rinsed in distilled water and air-dry.After these treatments, they were heated in air at 803 K for 3600 sec and air cooled.The average raising rate was 315 K/min and the average cooling rate 136 K/min, following which they were cut into 15 mm × 25 mm and 50 mm × 50 mm samples to be used for measuring photocatalytic activity.
Photocatalytic activity was measured using KI solution [12].The 15 mm × 25 mm test samples were dipped in 4 cc of 0.1 M KI solution at room temperature and illuminated with black light for 1800 sec or white light emission diode (LED) for 21,600 sec.The intensity of the black light at 365 nm was 1010 μW/cm 2 , and the intensity of the white LED was 47,000 lx.After irradiation with black light or LED, the peak intensity of 0.1 M KI solution at 287 nm was measured by absorption spectrophotometer (U2910, HITACHI).In addition, P-25 titanium dioxide particles made by Degussa corp.were also tested.The photocatalytic activity of a tape measuring 15 × 25 mm in size to which P-25 particles were attached was also measured under black light illumination as same method as described above.And I − is oxidized by dissolved oxygen without a phtocatalyst, thus 0.1 M KI solution without test specimen were also measured as control.
Photocatalytic activity of the anodized Ti samples was also investigated in antifungal and antivirus tests based on Japanese Industrial Standard (JIS) R-1705 (2008) [13] and JIS R-1702 [14] using black light irradiation.The antifungal test was carried out at Japan Food Research Laboratories.Three anodized titanium samples of 50 mm  The sample "Blank" was a control performed by irradiating a 0.1 M KI solution containing no samples with black light for the prescribed time.As shown in Figure 2, Blank showed about 0.2 of absorbance at 287 nm.It was considered that I − was oxidized by dissolved oxygen without phtocatalyst.For the sample anodized at 15 V, an absorbance value slightly increased but large.But above 20 V, absorbance values increased largely with applied voltages.And anodiezed samples at 45 V and 80 V showed about 5 times higher that of Blank, which were close to that of the P-25 particle.It is considered that thicker titanium oxide layer and rough surface might enhance photocatalytic activity above 20 V of applied voltage.

Photocatalytic Activity under Blacklight Illumination
In order to investigate crystal structure and surface morphogy of anodized titanium samples, XRD measurement and SEM observation were carried out for the titanium sample anodized at 45 V.As Figure 3 shows result of the XRD measurement, strong peaks from anatase type TiO 2 were observed.And peaks from rutile type TiO 2 were also observed.Weak peaks of titanium metal were from titanium substrate.It is generally accepted that TiO 2 in anatase type shows the highest photocatalytic activity.
On the other hand, it has been considered that dual phase of anatase and rutile had better photocatalytic activity [15] [16].As shown in Figure 3, anodized CP titanium sheets followed by heat treatment had dual phase of anatase and rutile.Dual phase might contribute better photocatalytic activity of anodized CP titanium sample.
Next a surface morphology of the anodized and heat treated titanium sample was observed by SEM. Figure 4 shows SEM photographs of the titanium sample, and as can be seen in the photographes, the surface of the sample was relatively rough.In addition to the formation of dual phase of anatase and rutile TiO 2 , surface roughness on titanium surface is also considered to enhance photocatalytic activity.
In order to investigate the photocatalytic activity of the anodized titanium sample at 45 V and heat treated in regards to fungal growth, an antifungal test was carried out.The obtained results were shown in Figure 5 and Figure 6.The spore count on the glass sheet used as a control changed little during the test.On the other hand, spore count of the titanium sample remarkably decreased to below 10.As shown in Figure 6, it is clear that the titanium sample under black light irradiation effectively terminates Aspergillus niger NBRC 105649.From Figure 5 and Figure 6, it was found that the anodized titanium sheets had better photocatalytic activity in relation to fungal growth.Next, antivirus test was carried out.
Figure 7 shows the antivirus test results for a glass sheet as a control and titanium sample.After the test, the infectivity titer of the virus for the glass sheet was over 6000.On the other hand, it was undetectable for the titanium sample.Thus, it was also found that the anodized titanium sample had better photocatalytic activity in relation to the influenza A virus.These results are in good agreement with the results shown in Figure 2.

Photocatalytic Activity under Visual Light Irradiation
Next, photocatalytic activity under visible light such as white color LED light was investigated by using 0.1 M KI solution.The results obtained are shown in Figure 8.
The absorbance of Ti sample was clearly higher than blank (no sample), which indicates photocatalytic activity under visual light irradiation.Thus, a surface analysis of the titanium sample before and after anodization was carried out by GDS. Figure 9 shows GDS analysis of titanium sample before anodization.
As shown in Figure 8, the anodized titanium sample showed photocatalytic activity under visual light irradiation.Concerning visible light response, R. Asahi et al. indicated that doping of anions such as carbon, nitrogen, fluorine and sulfur to titanium dioxide was effective for enhancing visual light response [17].
As shown in Figure 9 carbon is enriched in the surface layer of titanium sheets which forms TiC.Formation mechanism of TiC has been described elsewhere [10].Figure 10 shows GDS analysis result for a titanium sample anodized at 45 V and heat treated.As shown in Figure 10, carbon existing in the titanium oxide layer which is considered to be derived from TiC and is attributable to visual right response.In Figure 10 nitrogen was also observed in the titanium oxide later, although the concentration was lower than that of carbon.In Figure 11(b) (C-H)n bonding was observed which was presumably caused by a contamination.However, a weak peak other than (C-H)n was also observed which indicated carbide.And a weak peak from carbide was also observed in depth of 230.5 nm.Concerning a chemical state of nitrogen, a peak from nitride was observed from only near the outmost surface layers.
As R. Asahi et al. reported that doping of carbon and nitrogen into titanium oxide induced visible light response [17].Thus, it is believed that doped carbon and nitrogen in anodized titanium oxide layer induced visible light response.Nitride in titanium oxide layer is considered to derive from electrolyte, NH 4 NO 3 solution, for anodic oxidation.

Conclusion
A commercially pure titanium sheet precipitated TiC in the surface layer fabricated by anodic oxidation in

Figure 1
Figure1shows applied voltage vs. current density of CP titanium sheets in 0.06 M NH 4 NO 3 solution.Current density of CP titanium sheet in NH 4 NO 3 solution was almost same below 20 V, however it increased largely above 20 V and it was increased with applied voltage.And above 20 V, surface of anodized titanium sheets became rough.Based on the results shown in Figure1, phtocatalytic activity for anodized titanium samples at 15 V, 24 V, 45 V and 80 V and heat treated at 803 K were evaluated in KI solution.Obtained results were shown in Figure2.The sample "Blank" was a control performed by irradiating a 0.1 M KI solution containing no samples with black light for the prescribed time.As shown in Figure2, Blank showed about 0.2 of absorbance at 287 nm.It was considered that I − was oxidized by dissolved oxygen without phtocatalyst.For the sample anodized at 15 V, an absorbance value slightly increased but large.But above 20 V, absorbance values increased largely with applied voltages.And anodiezed samples at 45 V and 80 V showed about 5 times higher that of Blank, which were close to that of the P-25 particle.It is considered that thicker titanium oxide layer and rough surface might enhance photocatalytic activity above 20 V of applied voltage.In order to investigate crystal structure and surface morphogy of anodized titanium samples, XRD measurement and SEM observation were carried out for the titanium sample anodized at 45 V.

Figure 1 .
Figure 1.Applied voltage vs. current density of titanium sheet in 0.06 M NH 4 NO 3 solution.

Figure 2 .
Figure 2. Photocatalytic activity test results using 0.1 M KI solution under black light irradiation.

Figure 3 .
Figure 3. XRD pattern of the titanium sample anodized at 45 V and heat treated.

Figure 4 .
Figure 4. SEM microphotographs of titanium sample anodized at 45 V and heat treated.

Figure 5 .
Figure 5. Antifungal test results of glass sheet and titanium sample anodized at 45 V and heat treated.

Figure 6 .
Figure 6.Appearance after antifungal test of glass sheet (a) and titanium sample anodized at 45 V and heat treated (b).

Figure 7 .
Figure 7. Antivirus test results of glass sheet and the titanium sample anodized at 45 V and heat treated.

Figure 8 .
Figure 8. Photoactivity test results under white color LED irradiation.

Figure 9 .
Figure 9. GDS analysis result for titanium sample before anodization.

Figure 10 .
Figure 10.GDS analysis result for anodized titanium sample at 45 V, followed by heat-treatment.

Figure 11
Figure11 shows XPS analysis of surface layers of the anodized titanium sample at 45 V and followed by heat treatment.From the Figure11(a), titanium oxide exists at 37 nm and 230.5 nm in depth from the outmost surface.And chemical states of carbon were analyzed at the depth above.Obtained results were shown in Figure11(b)and Figure11(c).In Figure11(b) (C-H)n bonding was observed which was presumably caused by a contamination.However, a weak peak other than (C-H)n was also observed which indicated carbide.And a weak peak from carbide was also observed in depth of 230.5 nm.Concerning a chemical state of nitrogen, a peak from nitride was observed from only near the outmost surface layers.As R. Asahi et al. reported that doping of carbon and nitrogen into titanium oxide induced visible light response[17].Thus, it is believed that doped carbon and nitrogen in anodized titanium oxide layer induced visible light response.Nitride in titanium oxide layer is considered to derive from electrolyte, NH 4 NO 3 solution, for anodic oxidation.

Figure 11 .
Figure 11.XPS analysis of anodized titanium sheet at 45 V and followed by heat treatment: (a) O1s; (b) C1s at 37 nm; (c) C1s at 230.5 nm; (d) N1s.NH 4 NO 3 solution at 45 V and 80 V and heat treatment in air at 803 K showed relatively high photocatalytic activity in 0.1 M KI solution which was close to the absorbance value of P-25 particle.The anodized sample at 45 V and heat treated, showed good photocatalytic activity in antifungal and antivirus tests under black light irradiation.The better photocatalytic activity under black light irradiation is considered to be related formation of dual phase of anatase and rutile type titanium dioxide and rough surface.It also showed photocatalytic activity under visual light irradiation.It is considered that visible light response was caused by doping of carbon and nitrogen titanium dioxide.

Table 1 .
Concentration of impurity elements of Ti sheets tested (mass %).size were used.Same size glass sheets were used as control.Aspergillus niger NBRC 105649 was used in the test.Intensity of black light was 0.8 mW/cm 2 and irradiation time was 24 hour.And the antivirus test was conducted at Kitasato Research Center for Environmental Science.Anodized titanium sample of 50 mm × 50 mm size were used and glass sheets of the same size were used as a control.Influenza A virus was used in the test and the intensity of the black light was 0.1 mW/cm 2 and the irradiation time was 8 hour.For XRD measurement, incident angle of X-ray beam to surface was 1 degree to obtain structure of anodized oxidized layer on titanium substrate.Surface morphology of the anodized titanium sample was observed by scanning electron microscope (SEM; JIS-7000F, Jeol).Glow Discharge Spectroscopy (GDS; GD Profiler2, HORIBA Jobin Yvon) with Ar ion sputtering was conducted to investigate content of elements in oxide layers.X-ray photoelectron spectroscopy (XPS; Physical Electronics Quantum 2000 Scanning ESCA Microprobe, ULVAC-PHI Inc.) with Ar ion sputtering was conducted using monochromatized Al Kαradiation (1486.6 eV) to investigate the chemical states of O, C and N in anodized titanium oxide layer on titanium substrate.