Materials Transactions, Vol. 47, No. 3 (26) pp. 849 to 853 #26 The Japan Institute of Metals Synthetic Process of Titanium Dioxide Coating on Aluminum by Chemical Conversion Method Takayoshi Fujino and Teppei Matzuda* Department of Applied Chemistry, Faculty of Science & Technology, Kinki University, Higashiosaka 577-852, Japan Titanium dioxide ( ) coatings were prepared by chemical conversion treatment of aluminum in (NH 4 ) 2 TiF 6 with H 2 O 2, and the sintering of the coating was prepared to immobilize the photocatalyst on aluminum. Coatings were also formed in this solution at room temperature. To identify the coating structure, coating analysis was carried out using an infrared absorption spectrum analyzer. Based on the infrared absorption results, a component of the coating was found in the hydrolysis product of peroxo titanium fluoride. Furthermore, the coating analysis was carried out using X-ray diffractometry (XRD), and non-sintered coating was amorphous; however, the coating sintered by 673 K was anatase-type titanium dioxide. In the forming process of the conversion treatment in (NH 3 ) 2 TiF 6 and H 2 O 2, first, the generated F in the bath reacted with the aluminum. At the same time, hydrogen ions on the aluminum surface were consumed because hydrogen gas was generated. Thus, the ph of the interface became alkali. The hydrolysis of the titanium peroxo fluoride was deposited on the aluminum because ph increased on the surface. The coating sintered at 473 K had the highest activity. The photocatalytic activity of the coating sintered at 623 K was lower than the coating heated at 473 K, which is attributed to aggregation This forming process of the coating is low cost because of the useless electrolytic decomposition process. Furthermore, practical industry is expected because immobilized substances on aluminum can easily be decomposed at low temperatures. (Received August, 25; Accepted January 23, 26; Published March 5, 26) Keywords: titanium dioxide, photocatalyst, chemical conversion coating, aluminum, low cost, peroxo titanium complex, forming process. Introduction Since aluminum is relatively cheap and light, and it can be added to surfaces of aluminum corrosion resistance, catalytic activity, and so on by surface finishing. Generally, aluminum treated by surface finishing is widely used as a building material and in electron devices. As one surface finishing method for aluminum, the chemical conversion treatment cost is remarkably low because the electrolytic decomposition process ) is useless compared with anodic oxidation 2) and metal plating. Furthermore, chemical conversion treatments can treat complicated forms of aluminum. In recent years, the photocatalytic properties of have been widely studied 3,4) for environmental purification. In addition, a titanium dioxide with photocatalytic activities is harmless and has high photocatalytic activity and chemical stability. Generally, a sol gel method is known as one immobilization of. 5) But this method can not preserve for long durations because the solution is a volatile organic compound in which hydrolysis of titanium compounds occurs. Furthermore, a sol gel solution consisting of titanium alkoxide is expensive, and so practical uses are still difficult. In this study, conversion coatings were prepared by dipping aluminum plates in (NH 3 ) 2 TiF 6 with H 2 O 2. The coating can be prepared by only immersion of aluminum at low temperature. Therefore, this method is expected to reduction of cost, and industrialization. 6,7) Furthermore, using aluminum could reduce waste because it can recycle. X-ray diffractometry (XRD) and infrared absorption spectrum (IR) were measured to estimate the coating structure. In addition, the depth profile of the coating was measured by secondary ion mass spectrometry (SIMS). *Graduate Student, Kinki University 2. Experimental Aluminum (plate or mesh, purity: 99.5 99.85%) was used as the basic material that had been previously treated with a surface active agent for 5 min at 323 K and then washed with distilled water. In addition, the aluminum was treated by a NaOH solution to remove a thin layer of aluminum hydroxide and increase its surface area. As a coating process by conversion treatment, coatings were prepared by the dipping aluminum in (NH 3 ) 2 TiF 6 with H 2 O 2 (primary treatment). Preparation conditioning was carried out at 293 393 K for 2 min. As a secondary treatment, the coatings were sintered at 493 773 K. Coating thickness was calculated by the mean value of five measurements. The optimum conditions of the coationg preparation follows; treatment temperature: 353 K, treatment time 3 min, concentration of H 2 O 2 :.4 kmol/m 3, concentration of (NH 4 ) 2 TiF 6 :.6 kmol/m 3, sintering temperature: 473 773 K. All the analytical samples were prepared at the optimum condition. The depth profiles of the non-sintered coating were measured by SIMS. Table shows the measurement conditions. SIMS measurement was performed using a CAME- CA IMS-6F instrument. 8) The measurements were carried out with a O 2 þ beam. Absorption spectra were measured by IR analysis to estimate the component of the coating and the coating sintered. A KBr tablet method was used to measure the IR spectrum. The analysis of the coating was carried out by thin film XRD. Angles ranged from to 9 because the peak of anatase was detected in the range. The UV/vis absorption spectra of the coatings were measured within wavelengths of 3 8 nm. Absorbance was defined as the ratio of the intensity of the incident and the transmitted beams. The surface and cross section of the coating were observed by scanning electron microscope (SEM). As a
85 T. Fujino and T. Matzuda Table Measurment conditions on depth profile by SIMS. Primary beam conditions Ion species O 2 þ Accerating voltage (kv) 8 Beam current (ma) 5 Secondary beam conditions Polarity Positive Accerating voltage (kv) 4.5 Sample offset (V) (A-7) Detective area (mm ) 6 2 5 5.5..5.2.25.3.35.4 Concentration, c/kmol m -3 Table 2 Working conditions on measurement of photocatalytic activity. Test piece size (cm 2 ) 3 5 Source of light Mercury-vapor lamp (AC 3 V 4 W) Intensity of light (mw/cm 2 ) 3 Distance from source of light to test piece (cm) 7 Vessel size (cm) 7. Volume of malachite green (cm 3 ) 4 Adsorption time (min) 6 Irradiation time (min) 6 pretreatment for SEM observation, the surface and cross section of the conversion coatings were coated with a.3 mm thick layer of Pt Pd by vapor deposition equipment. The photocatalytic activity of the coatings was evaluated by measuring the decease of the malachite green concentration. After the coating was adsorbed, the solution of malachite green (4 ml, 2.5 ppm) was illuminated by UV light for 6 min at room temperature in the dark, Table 2 shows the conditions of photocatalytic activity measurement. The UV light of a mercury-vapor lamp (AC 3 V, 4 W) was used as a source light. 3. Results and Discussion Figure shows the relationship between the solution concentration of (NH 3 ) 2 TiF 6 and film thickness in the primary treatment. The film thickness increased in. kmol/m 3 or less but it was approximately constant in. kmol/m 3 or more. The roughness of the coating surface was homogeneous. But the roughness of the coating surface was heterogeneous in. kmol/m 3 above (NH 3 ) 2 TiF 6 because of high reactivity. Figure 2 shows the dependence of H 2 O 2 concentration for film thickness. The reactive rate increases with H 2 O 2 concentration. Film thickness strongly increased by additions above.6 kmol/m 3 H 2 O 2 because aluminum dissolves easily. Another reason is that excess H 2 O 2 not only formed a peroxo titanium complex but also a worked for oxidizing agent. Figure 3 shows the influence of treatment temperature on film thickness. Coatings were also formed in this solution even at room temperature. The element distribution of the conversion coating was confirmed by SIMS. Figure 4 shows the depth profiles of conversion Fig. Dependence of the concentration of (NH 4 ) 2 TiF 6 for the film thickness (Bath temperature 353 K, treatment time 3 min, concentration of H 2 O 2.4 kmol/m 3 ). 2 5 5.2.4.6.8. Concentration, c/kmol m -3 Fig. 2 Dependence of the concentration of H 2 O 2 for the film thickness (Bath temperature 353 K, treatment time 3 minutes, concentration of (NH 4 ) 2 TiF 6.6 kmol/m 3 ). 4 2 8 6 4 2 293 33 33 323 333 343 353 363 Temperature, T/K Fig. 3 Dependence of the treatment temperature for film thickness (Treatment time 3 min, concentration of H 2 O 2.4 kmol/m 3, concentration of (NH 4 ) 2 TiF 6.6 kmol/m 3 ). coating under SIMS conditions, as listed in Table. Figure 4 shows the depth profile of the coating without sintering. 48 Ti, 9 F, 67 TiF, and 4 N completely ethibited the same behavior as in Fig. 4. Figure 4 shows the depth profile of the sintered coating. 48 Ti, 9 F, 67 TiF, and 4 N completely showed the same behavior, as the depth profile of the coating without sintering. In relation to 9 F, F in the sintered coating decreased remarkably by out diffusion. Figure 5 shows the results of coating structure analysis using
Synthetic Process of Titanium Dioxide Coating on Aluminum by Chemical Conversion Method 85 Intensity/CPS 8 6 4 2 27Al 9F 4N 48Ti Ti F 8 6 4 2 48Ti 27Al 4N 9F.8.6 2.4 3.2 4..8.6 2.4 3.2 4. Depth, d/µm Depth, d/µm Ti F Fig. 4 Depth profiles of the conversion coatings by SIMS. Non-sintered coating, Sintered coating. Intensity/a.u. Al T/% 5 95 9 4 35 3 25 2 5 5 893 cm - 4 cm - 8 2 3 4 5 6 7 8 9 2θ 6 4 35 3 25 2 5 5 - Wavenumber, σ /cm Fig. 5 X ray diffraction pattern of coatings. Non-sintered coating, Coating sintered at 673 K. Fig. 6 FT-IR spectra of the coating on aluminum. Non-sintered coating, Coating sintered at 673 K. thin film XRD. Non-sintered coating [Fig. 5] was amorphous, however, the composite of the coating sintered by 673 K [Fig. 5] was anatase-type titanium dioxide to crystallize. Immobilized substance on aluminum decompose at low temperatures. Figure 6 shows the IR of the coating obtained. From Fig. 6 in non-sintered coating, infrared absorption resulting from a free peroxide (O O bond) stretching mode was detected at 893 cm. Broad peaks at 27 3667 cm and a clear peak at 628 cm are attributed to water molecules. A peak at 4 cm is due to the stretching vibration of N H bonds in NH 4 þ. A peak at 236 cm is attributed to KBr. The UV/vis spectra of the non-sintered coating and the coating sintered at 673 K are shown in Fig. 7. The wavelength range was 3 8 nm. The absorption spectrum of the non-sintered coating was characterized by a strong band at 52 nm. The position of the absorption was strongly influenced by the peroxo titanium compound. The position of the absorption of the coating sintered at 673 K was below 38 nm, which is the same absorbance peak as the peak of existing anatase-type. But the position of the absorption of the coating sintered at 723 773 K shifted slightly to the visual region. It is expected that this shift caused by F dope. In addition, Fig. 8 shows a SEM image at the surface. From Fig. 8, Crystals of were grown with increasing sintering temperatures. Figure 9 shows a SEM image at the surface [Fig. 9] and the cross section [Fig. 9]. A large number of loosely adhered granularity spherical crystals (anatase-type titanium dioxide) was observed on the surface. An interface layer was observed between the coating and aluminum. 9) Structure analysis of the interface layer was carried out using thin film XRD after the removal of the top layer of coating. Figure illustrates that the interface layer consists of AlF 3 and. Based on the results obtained, a forming process can therefore be presumed, as shown in Fig.. (NH 3 ) 2 TiF 6 is a colorless, transparent liquid, but the color of the (NH 4 ) 2 TiF 6 solution changed to yellow by adding H 2 O 2. Peroxo-titanic
852 T. Fujino and T. Matzuda acid is formed by the addition of excessive H 2 O 2.,) It can be also estimated that a peroxo titanium bond brings similarity to this solution. Absorbance/a.u. (c) 35 45 55 65 75 Wavelength, l/nm Fig. 7 UV vis spectra of the conversion coatings at wavelength 3 8 nm. Non-sintered coating, Coating sintered at 673 K, (c) Coating sintered at 723 K. O Ti O Peroxo titanium complex After aluminum was immersed in this solution, hydrogen gas was immediately generated from the aluminum surface. The forming process follows. First, the generated F [Formula ()] reacted with the aluminum, and AlF 3 was brought to the aluminum surface [Formulas (2) and (3)]. At the same time, the hydrogen ions on the aluminum surface were consumed. Thus, ph increased [Formula (4)]. The hydrolysis product of the titanium peroxo fluoride was deposited because ph increased on the surface [Formula ()]. From SEM images [Fig. 8], a layer of AlF 3, AlOF, and Al(OH) 3 was observed between the and the aluminum plate. Amorphous coating of titanium peroxo fluoride was transformed to a crystalline phase by sintering at Non-treatment 473 K 673 K Fig. 8 Scanning electron micrographs image of the coating surface (magnification: 5 3 ). layer Interface layer Al plate Fig. 9 Scanning electron micrographs image. Surface (magnification: 5 4 ), Cross section (magnification: 4 ).
Synthetic Process of Titanium Dioxide Coating on Aluminum by Chemical Conversion Method 853 Adsorption time Ultraviolet irradiation time Intensity/a.u. AlF 3 2 4 6 8 2θ Concentration/ppm 2.5 2.5.5 : 673 K : 373 K : 573K : 473 K Fig. X ray diffraction pattern of interface layer observed between the coating and aluminum. 6 7 8 9 2 Time, t/min AlF 3 Fig. 2 Photocatalytic activity of the conversion coatings sintered. Aluminum Hydrolysate of Peroxotitanium fluoride high temperature. The obtained at 473 673 K was white. Figure 2 shows the results of the photocatalitic activity of the sintered coatings. coatings were confirmed by sintering at 673 K. The coating was anatase-type titanium dioxide from XRD data (Fig. 5). The coating sintered at 473 K had the highest activity. The activity of the coating sintered at 673 K was lower than the coating sintered at 473 K. This is probably due to the decrease of the surface area of the coating because crystals were grown with increasing sintering temperatures. 4. Conclusion Etching (NH 4 ) 2 TiF 6 + H 2 O 2 + nh 2 O Annealing Chemical conversion (NH 4 ) 2 TiF 6 + H 2 O 2 (NH 4 ) 2 Ti(O 2 )F 4 n (OH) n + (2+n)H + + (2+n)F () Al Al 3+ + 3e (2) Al 3+ + 3F AlF 3 (3) 3H + + 3e 3/2H 2 (4) Fig. Forming process of chemical conversion coating. () The coating was prepared by treating an aluminum plate in a solution of (NH 4 ) 2 TiF 6 and H 2 O 2. Coating was formed in this solution at room temperature. (2) The coating component was found to be a hydrolysis product of peroxo titanium fluoride. The coating was anatase-type titanium dioxide by sintering at 473 773 K. (3) This method is expected of industrialization because it is only immersed aluminum regardless of the electrolytic decomposition process, and the coating was prepared at low temperature (293 K) and the coating was crystallized at low temperature (473 K). (4) The coating sintered at 473 K had the highest activity. Acknowledgement This work was University-Industry Joint Research project for Private Universities and supported by funding from the Ministry of Education, Culture, Sports, Science and Technology private school grant, 22 26. The authors thank them for their immense assistance in the development of functional chemical conversion coatings and its applicability. REFERENCES ) H. Kaneko: Aruminiumu no kaseisyori, (Kallos publishing Co., 23) pp. 5 6. 2) T. Fujino and T. Nara: Keikinzoku 53 (23) 4 45. 3) T. Fujino, M. Simokado and H. Noguchi: Keikinzoku 442 (22) 52. 4) H. Yamaoka: Hyoumen gijutsu 55 (24) 33 334. 5) M. Sato, H. Hara, T. Zishide and Y. Sawada: J. Mater. Chem. 6 (996) 767 77. 6) E. Okada and H. Hosono: Hyoumen Gijutu 55 (24) 79 724. 7) T. Fujino, M. Miyamoto and H. Noguchi: Keikinzoku 5 (2) 486 49. 8) T. Fujino, H. Mizumoto and H. Noguchi: Keikinzoku 48 (998) 38 394. 9) H. Kaneko: Aruminiumu no kaseisyori, (Kallos publishing Co., 23) pp. 7 7. ) J. Muehlebach, K. Mueller and G. Schwarzenbach: Inorg. Chem. 9 (97) 238 239. ) D. Schwarzenbach: Inorg. Chem. 9 (97) 239 2397.