Synthetic Process of Environmentally-Friendly TiO 2 Coating on Magnesium by Chemical Conversion Treatment

Transcription

1 Materials Transactions, Vol. 47, No. 9 (2006) pp to 2340 #2006 The Japan Institute of Metals Synthetic Process of Environmentally-Friendly TiO 2 Coating on Magnesium by Chemical Conversion Treatment Takayoshi Fujino and Teppei Matzuda* Department of Applied Chemistry, Faculty of Science & Technology, Kinki University, Higashiosaka , Japan Titanium dioxide (TiO 2 ) coatings were prepared by chemical conversion treatment of magnesium in (NH 4 ) 2 [TiO(C 2 O 4 ) 2 ] with H 2 O 2, then, anatase type TiO 2 coatings were prepared by sintering. 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 compound. Furthermore, the coating analysis was carried out using X-ray diffractometry (XRD), and non-sintered coating was amorphous; however, the coating sintered at more than 573 K was anatase-type titanium dioxide. In the forming process of the conversion treatment in (NH 4 ) 2 [TiO(C 2 O 4 ) 2 ] with H 2 O 2, first, magnesium was dissolved because H þ in the bath reacted with the magnesium. Hydrogen ions on the magnesium surface were consumed to generated hydrogen gas. Thus, the ph of the interface became alkali. The hydrolysis of the peroxo-titanium compound was deposited on the magnesium because ph increased on the surface. From the XPS results and the TG-DTA results, a component of the coating is a hydrolyzation product of a peroxo-titanium compound and Mg(OH) 2. Because Mg(OH) 2 is generates in ph more than 11, it is considered that the ph on the magnesium surface is more than 11. The coating sintered at 573 K had the highest photocatalytic activity. The photocatalytic activity of the coating sintered at 623 K was lower than the coating heated at 573 K, which is attributed to growth of TiO 2 particle. This forming process of the coating is low cost because of the useless electrolytic decomposition process and increasing the speed of the treatment. It is possible to treat complicated form of the substrate metal, so this method can be expected to use in various fields. Therefore this method is expected to practical use for environmental purification. [doi:.2320/matertrans ] (Received March 6, 2006; Accepted July 12, 2006; Published September 15, 2006) Keywords: magnesium, chemical conversion treatment, forming process, titanium dioxide, photocatalyst, low cost, peroxo-titanium complex 1. Introduction Currently, steel and aluminum, etc. are used as building materials and part ingredients for electrodevice. However, there are strong demands for weight-saving measures for substrate materials, cost reductions and functionality improvements to be made in the metal surface. Amount of export of magnesium were increasing the most, because it is the lightest of the practical metals (its weight is 2/3 that of aluminum, 1/2 that of titanium and 1/4 that of iron). This means magnesium could replace aluminum in the field of building materials and as an electrodevice. 1,2) In the automotive industry, reductions in fuel-efficiency and the amount of CO 2 being discharged are being demanded from the viewpoint of global environment protection, so there is an active shift to magnesium for material parts and for weightsaving. As the demand for magnesium increases in these various fields, it is necessary to add new functionalities to the magnesium surface. As one of surface finishing, there is a chemical conversion treatment. A chemical conversion coating 3) can be prepared by only immersion of metal because the coating is formed by chemical reaction. Therefore, we expect method to reduce of costs, treat the complicated form of the substrate metal, and to increase the speed of the treatment. Recently, research on TiO 2 has advanced because TiO 2 has high oxidizability and displays super-hydrophilicity when light is irradiated to the surface of the photocatalyst. In addition, TiO 2 with high photocatalytic activity is harmless and has chemical stability. There are examples of TiO 2 s practical use for environmental purification in all fields. At present, the immobilization of TiO 2 are actively *Graduate Student, Kinki University studied, 4,5) because we hope to expand its use. In this research, TiO 2 was immobilized on magnesium because we expect will support market expansion in the future. As immobilizing TiO 2 uses a chemical conversion treatment in (NH 4 ) 2 [TiO(C 2 O 4 ) 2 ] with H 2 O 2 solution, it is possible to prepare it rapidly at low cost. 6,7) In the future, we ll also be able to create deodorization and sterilization effect, and prevent oil dirt and harmful gases. 2. Experimental Magnesium (plate or mesh, AZ91, AZ80, AZ61, etc.) was used as a basic material. Prior to use, the magnesium was treated with a surface active agent for 5 minutes at 323 K and then washed with distilled water. In addition, the magnesium was etched using alkaline and acid solution to remove a thin layer of magnesium hydroxide and to increase its surface area. As a coating process using a chemical conversion treatment, TiO 2 coatings were prepared by dipping magnesium in (NH 4 ) 2 [TiO(C 2 O 4 ) 2 ] with H 2 O 2 solution (primary treatment). This preparation conditioning was carried out at K for 120 minutes. As a secondary treatment, the coatings were sintered at K. The coating thickness was calculated using the mean value of five measurements. The optimum conditions of the coating preparation are as follows: treatment temperature: 353 K, treatment time: 30 min, concentration of H 2 O 2 : 0.04 k mol/m 3, concentration of (NH 4 ) 2 [TiO(C 2 O 4 ) 2 ]: 0.02 k mol/m 3, ph of the solution: , sintering temperature: 550 K. All the analytical samples were prepared at the optimum condition. In the solution of less than ph 2.5, the coating could not be formed as magnesium only dissolves. In the solution of more than ph 3.0, as hydrolysis of titanium ion in the solution occurs, so the solution was approximately

2 2336 T. Fujino and T. Matzuda Table 1 Working conditions on measurement of photocatalytic activity. Test piece size (cm 2 ) 0 Source of light Black light Intensity of light (mw/cm 2 ) 360 Distance from source of light to test piece (cm) Vessel size (cm) 7.0 Volume of malachite green (cm 3 ) 40 Adsorption time (min) 15 Irradiation time (min) 120 constant in ph We observed the surface of the coatings using a scanning electron microscope (SEM). As a pretreatment for SEM observation, the surface of the conversion coatings was coated with a 0.03 mm thick layer of Pt-Pd using vapor deposition equipment. The analysis of the coating was carried out using thin film X-ray diffractometry (XRD). The angles ranged from to 90 because the peak of anatase TiO 2 was detected with in this range. The absorption spectra were measured using Infrared spectroscopy (IR) analysis to estimate the coating components and the sintered coating. A KBr tablet method was used to measure the IR spectrum. The conversion coating (non-sintering) and the coating sintered at 550 K were measured using X-ray photoelectron spectroscopy (XPS) because of analysis of the compounds in the coating. In addition, the precipitation generated in the bath after chemical conversion treatment was analyzed using thermal analysis equipment (TG-DTA). We analyzed it within the range of K ( K/min) in an atmosphere of air (50 ml/min). A micro type platinum cell was used for the sample cell. We measured the UV/vis absorption spectra of the coatings within wavelengths of nm. Absorbance was defined as the ratio of the intensity of the incident and the transmitted beams. 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 solution (40 ml, 2.50 ppm) was illuminated by UV light for 60 minutes at room temperature in the dark, Table 1 shows the conditions of photocatalytic activity measurement. A UV light with a black light was used as a source light. 3. Results and Discussion Figure 1 shows the relationship between the solution concentration of (NH 4 ) 2 [TiO(C 2 O 4 ) 2 ] and film thickness in the primary treatment. The film thickness increased with increasing the concentration of (NH 4 ) 2 [TiO(C 2 O 4 ) 2 ]. Increasing the concentration of (NH 4 ) 2 [TiO(C 2 O 4 ) 2 ] caused to occur hydrogen gas remarkably, and the adhesion of the prepared coating was inferior. Figure 2 shows the dependence of H 2 O 2 concentration for the film thickness. The reactive rate increases with H 2 O 2 concentration. The film thickness increased by additions of H 2 O 2 because magnesium dissolves easily. Another reason is that the excess H 2 O 2 not only formed a peroxo-titanium complex but also worked as for oxidizing agent. Figure 3 shows the influence of the treatment temperature on film thickness. A white coating was Film thickness, th/µm Concentration of (NH 4 )[TiO(C 2 O 4 ) 2 ], c/k mol m -3 Fig. 1 Dependence of the concentration of (NH 4 ) 2 [TiO(C 2 O 4 ) 2 ] for the coating thickness (Bath temperature 353 K, treatment time 30 minutes, concentration of H 2 O k mol/m 3 ). Film thickness, th/µm Concentration of H 2 O 2, c/k mol m -3 Fig. 2 Dependence of the concentration of H 2 O 2 for the coating thickness (Bath temperature 353 K, treatment time 30 minutes, concentration of (NH 4 ) 2 [TiO(C 2 O 4 ) 2 ] 0.02 k mol/m 3 ). Film thickness, th/µm Temperature, T/K Fig. 3 Dependence of the treatment temperature for coating thickness (Treatment time 30 minutes, concentration of H 2 O k mol/m 3, concentration of (NH 4 ) 2 [TiO(C 2 O 4 ) 2 ] 0.02 k mol/m 3 ).

3 Synthetic Process of Environmentally-Friendly TiO 2 Coating on Magnesium by Chemical Conversion Treatment 2337 prepared by processing within a temperature range of 323 K from 298 K. We assumed that the main ingredient of the coating was Mg(OH) 2. A yellow coating was obtained at a temperature of more than 333 K, and the titanium content was increased. Figure 4 shows the results of coating structure analysis using thin film XRD. The non-sintered coating (Fig. 4) was amorphous, however, the composite of the coating sintered at 550 K (Fig. 4) was an anatase-type titanium dioxide to crystallize. Scherrer formula TðnmÞ ¼ 0:9=B cos (B: half-value width, : Wavelength of CuK ¼ 0:15414 nm). From the formula indicated above, we established that the grain diameter of the TiO 2 crystal in the coating was 27 nm. Figure 5 shows an SEM image of the coating (non-sintering) and the sintered coating. The conditions of the coating preparation for SEM observation are as follows: treatment temperature: 353 K, treatment time: 30 min, concentration of H 2 O 2 : 0.04 k mol/m 3, concentration of (NH 4 ) 2 [TiO(C 2 O 4 ) 2 ]: 0.02 k mol/m 3, ph of the solution: 2.7, sintering temperature: 550 K. We observed a large number of adhered granular spherical crystals on the surface. As Fig. 5, shows, the TiO 2 crystals (diameter of particle: approximately 30 nm) were deposited on magnesium. Figure 6 shows the IR of the Intensity /a.u. Anatase TiO θ /deg Fig. 4 X ray diffraction pattern of coatings. Non-sintered coating, Coating sintered at 550 K. coating. From Fig. 6 in non-sintered coating, we detected infrared absorption resulting from a free peroxide (O-O bond) stretching mode at 904 cm 1. The broad peaks observed at Non-sintered coating Sintered coating at 550 K 1 µm 500 nm 1 µm 500 nm Fig. 5 Scanning electron micrographs image of the coating surface. Non-sintered coating, Coating sintered at 550 K.

4 2338 T. Fujino and T. Matzuda Peroxo titanium complex Anatase TiO 2 Mg(OH) 2 MgO + H 2 O 8.0 T (%) 904 cm cm -1 DTA, /uv H 2 O MgCO 3 MgO + CO TGA, /mg Wavenumber, σ /cm peroxo titanium complex decomposition Temperature, T/K Fig. 6 FT-IR spectra of the coating on aluminum. Non-sintered coating, Coating sintered at 550 K. Fig. 8 TG-DTA curves of chemical conversion coating. Intensity / a.u. Ti 2p Before sintering Sample:coating O 1s Before sintering Sample:coating (c) O 1s Before sintering Sample:Powder Binding energy, E/eV Fig. 7 XPS of Ti 2p and O 1s electron binding energy of the coating. 2p spectrum of the coating, O 1s spectrum of the coating, (c) O 1s spectrum of the powder cm 1 and the clear peak at 1628 cm 1 are attributed to water molecules. The peak at 1401 cm 1 is due to the stretching vibration of N-H bonds in NH þ 4. The peak at 2361 cm 1 is attributed to KBr. Those peaks were disappeared by sintering as shows Fig. 6. Figure 7 shows the XPS data of the coating and powder as well as the spectrums of Ti 2p and O 1s. The spectra must be calibrated with the standard sample, such as C 1s. Normally, the simple calibration is done by C 1s (approximately ev) spectrum of adventitious carbon that exists on all samples. Figure 7 shows that Ti 2p was detected in two peaks, and ev in the sintered coating. From the two peaks, the coating composite sintered at 550 K was an anatase-type TiO 2 to crystallize. The O 1s spectrum of the coating is indicated in Fig. 7. The chemical conversion coating (non-sintering) contained Mg (OH) 2 from a peak of ev. And, the peak at ev is characteristic of titanium oxides. And then, in sintering coating, O 1s spectrum at ev is attributed to anatase TiO 2. After the coating on magnesium had been removed, the coating was made a fine particle. And, O 1s spectrum of the sample was measured by XPS. The spectrum shows in Fig. 7(c). The powder contained MgO was confirmed that from a peak of ev after sintering. In addition, the existence of MgCO 3 was also confirmed from a peak of ev. These peaks were not detected with XRD because it was an amorphous substance. MgCO 3 brings in coating because Mg(OH) 2 reacts with CO 2 in the air or water. The deposit in the solution after chemical conversion treatment was analyzed using a TG- DTA measurement. The measurement results are indicated in Fig. 8. We confirmed the peak of the wide endothermic reaction around 376 K in a DTA spectrum, This is considered according to the dehydration reaction. In a TG spectrum, decrease of nineteen percent was detected to temperatures of about 523 K, because this is regarded as the cutting of the dehydration reaction and the resolution of the peroxo-

5 Synthetic Process of Environmentally-Friendly TiO 2 Coating on Magnesium by Chemical Conversion Treatment 2339 Absorbance, /a.u Wavelength, I/nm Fig. 9 UV-vis spectra of the conversion coatings at wavelength nm. Non-sintered coating, Coating sintered at 550 K. titanium complex in the coating. An endothermic peak at 493 K is a dehydration reaction when Mg(OH) 2 changes into MgO. The exothermic peak was confirmed at 550 K. Therefore, an amorphous peroxo-titanium hydrate crystallizes to an anatase type TiO 2 is at 550 K. The exothermic peak at 690 K was a change point from MgCO 3 to MgO. The exothermic peak at 73 K was a crystal change to a rutile-type titanium dioxide from an anatase type TiO 2. The UV/vis spectra of the non-sintered coating and the coating sintered at 550 K are shown in Fig. 9. The wavelength range was nm. The absorption spectrum of the non-sintered coating was characterized by a strong band at 520 nm. The position of the absorption was strongly influenced by the peroxo titanium compound. The position of the coating absorption sintered at 550 K was below approximately 400 nm, which is the same absorbance peak as the peak of the existing anatase-type TiO 2. Based on the results obtained, we can presume that there was a forming process, as shown in Fig.. (NH 4 ) 2 [TiO(C 2 O 4 ) 2 ] is a colorless, transparent liquid, but the color of the (NH 4 ) 2 [TiO(C 2 O 4 ) 2 ] solution changed to yellow when H 2 O 2 was added. Peroxo-titanic acid is formed by the addition of excessive amounts of H 2 O 2. 8 ) We can also estimate that a peroxo-titanium bond brings similarity to this solution ) After magnesium was immersed in this solution, hydrogen gas was immediately generated from the magnesium surface. The forming process was as follows: Mg! Mg 2þ þ 2e ð1þ 2H þ þ 2e! H 2 " ð2þ Mg 2þ þ 2OH! Mg(OH) 2 ð3þ (NH 4 ) 2 [TiO(C 2 O 4 ) 2 ] þ H 2 O 2 þ 2H 2 O! (NH 4 ) 2 TiO(O 2 )(OH) 2 þ 4H þ þ 2(C 2 O 4 ) 2 ð4þ First, magnesium was dissolved in a treatment solution and the hydrogen ions on the magnesium surface were consumed. Thus, the ph of the interface was increased because H 2 gas brings (Formulas (1) and (2)) and then, Mg(OH) 2 was brought to the magnesium surface (Formulas (3)). At the same time the hydrolysis product of the titanium peroxo compound was deposited with the separation of Mg(OH) 2 because the ph increased on the surface (Formula (4)). Because Mg(OH) 2 is generates in ph more than 11, it is considered that the ph on the magnesium surface is more than 11. An amorphous titanium-peroxo compound in the coating was transformed to a crystalline phase by sintering. The TiO 2 coating obtained at K was white. From XRD and XPS result, the component of the coating was anatase type TiO 2. In addition, from XPS (Fig. 8), we detected Mg(OH) 2 in the coating (non-sintered coating), then the Mg(OH) 2 changed into MgO and MgCO 3 by sintering more than 493 K. Figure 11 shows the results of the photocatalitic activity of the sintered coatings. TiO 2 coatings were confirmed by sintering at 673 K. The activity of the coating sintered at 673 K was lower than the coating sintered at 573 K. This is probably due to the decrease of the surface area of the coating because TiO 2 crystals were grown with increasing sintering temperatures. 4. Conclusion (1) The coating was prepared by treating a magnesium plate in a solution of (NH 4 ) 2 [TiO(C 2 O 4 ) 2 ] and H 2 O 2. Magnesium Etching Peroxo titanium compound Anatase TiO 2, Chemical conversion treatment in (NH 4 ) 2 [TiO(C 2 O 4 ) 2 ] H 2 O 2 Mg(OH) 2 MgCO 3, MgO Fig. Forming process of chemical conversion coating.

6 2340 T. Fujino and T. Matzuda Concentration, /ppm Adsorption time Ultraviolet irradiation time : Blank : 573 K : 673 K Acknowledgements 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, The authors thank them for their immense assistance in the development of functional chemical conversion coatings and its applicability Fig. 11 Time, t/min Photocatalytic activity of conversion coatings. The coating was formed in this solution at room temperature. (2) The coating component was found to be a hydrolysis product of peroxo-titanium compound. The coating was an anatase-type titanium dioxide achieved by sintering at 550 K. From the XPS results of the measurement and the TG-DTA result, a component of the coating is a hydrolyzation product of a peroxo-titanium compound and Mg(OH) 2. The hydrolyzation product of peroxotitanium was crystallized to an anatase type TiO 2 by sintering more than 550 K. (3) We expect this method to be industrialized because it is only immersed magnesium at low cost regardless of the electrolytic decomposition process, and the anatase type TiO 2 coating was crystallized at a low temperature (550 K). REFERENCES 1) M. Sato, T. Kaziya and T. Yashiro: Keikinzoku 54 (2004) ) M. Takatsu: Keikinzoku 54 (2004) ) H. Kaneko: Aruminiumu no kaseisyori, (Kallos publishing Co., 2003) ) A. Fujishima: Hyoumen gijutsu 55 (2004) 3. 5) Yoshie Ishikawa and Yasumichi Matsumoto: Electrochimica Acta 46 (2001) ) T. Fujino: kinki aruminiumu hyomensyori kenkyukai 2 (2001) 1. 7) T. Fujino, M. Miyamoto and H. Noguchi: keikinzoku 50 (2000) ) A. Piccini and C. R. Aad: Sci. 97 (1883) 64. 9) J. Muehlebach, K. Mueller and G. Schwarzenbach: Inorganic Chemistry 9 (1970) ) D. Schwarzenbach: Inorganic Chemistry 9 (1970) ) Y. Gao, Y. Matsuda, Z. Peng, T. Yonezawa and K. Koumoto: Chem. Mater. 13 (2003) ) Y. Masuda, Y. Jinbo, T. Yonezawa and K. Koumoto: Chem. Mater. 14 (2002) ) K.-J. Kim, K. D. Benkstein, J. V. Lagemaat and A. J. Frank: Chem. Mater. 14 (2002) ) M. Tada, Y. Yamashita, V. Petrykin, M. Osada, K. Yoshida and M. Kakihana: Chem. Mater. 14 (2002) ) M. Kakihana, M. Tada, M. Shiro, V. Petrykin, M. Osada and Y. Nakamura: Chem. Mater. 40 (2001)