Regularity control of porous anodic alumina and photodegradation activity of highly ordered titania nanostructures
LIU Xiang-zhi(刘祥志), XU Ming-xia(徐明霞), TIAN Yu-ming(田玉明),
SHANG Meng(尚 萌), ZHANG Ping(张 平)
School of Materials Science and Engineering, Key Laboratory for Advanced Ceramics and
Machining Technology of Ministry of Education, Tianjin University, Tianjin 300072, China
Received 10 April 2006; accepted 25 April 2006
Abstract: A two-step anodizing process was used to prepare wide-range highly ordered porous anodic alumina membrane (PAA) in the electrolyte of oxalic acid. The effects of anodic voltage, anodizing time, size of aluminium foil and additives on the regularity of PAA membrane were also studied in the process of two-step anodization. The template method was combined with the sol-electrophoresis deposition and sol-gel method respectively to prepare highly ordered titania nanostructures. The diameter and length of the obtained nanostructures were determined by the pore size and depth of the PAA template. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to characterize the morphology and phase structure of the PAA template and the titania nanostructures. The results show that the anodizing time and the additive of ethanol have a great effect on the regularity of PAA template. This can be explained from the self-organized process and the current density theory. A theoretical model based on the self-organized process was established to discuss the formation mechanism of PAA template from the chemical perspective. The titania nanostructures prepared with this method has a high specific surface area. Furthermore, the photocatalytic activity of titania nanostructures on methyl orange were studied. Compared with ordinary titania membranes, the titania nanostructures synthesized with this method have higher photodegradation activity.
Key words: PAA template, titania structures, theoretical model, photocatalytic activity
1 Introduction
Nowadays, nanosized materials exhibit electronic and optical properties somewhat different from the same bulk materials because of their quantum tunable electronic properties, which emerge as a function of size, the so-called quantum size effect. Recently, the synthesis and the preliminary exploration of the properties of novel nano-structures or nanometer constitutional units have attracted much interest in the nanometer technological field.
Titania is non-toxic, harmless and non-corrosive, and can be used repeatedly and also can mineralize the organic contaminants completely in water and inorganic ions without secondary pollution. It is a type of environment-friendly material with potential wide applications[1, 2]. Titania prepared with conventional methods has low specific surface area. To overcome this defect, researchers are developing new methods to prepare titania with high specific surface areas. In recent years, some special nano structures such as nanotubes[3], nanorods[4], nanowires[5, 6] and sheet-layer of titania have been reported.
Templates with nano-pores combined with different deposition technologies can be used to prepare nano-structures in pores directly. This method is compatible with the idea that nano-structures begin to grow from atom or molecule level. Of many kinds of templates, porous alumina template (PAA) is widely used because of its remarkable hardness, uniform pore size, high pore density, low cost and relative ease of their preparation. The template method combined with electrodeposition or sol-gel methods which are considered as the best two methods for preparing titania nano-structures[7].
In this work, the effects of anodizing time and additives on the regularity of PAA membrane were studied. Sol-electrodeposition technology was used to prepare titania nano-array systems in the PAA template. This method is very simple and can achieve large area growth of nano-arrays. The new type of titania can be greatly applied perspective in the field of photocatalysis and photon crystal.
2 Experimental
2.1 Preparation of PAA
High purity (99.999%) aluminium foils were used as the starting material. Prior to anodizing process, the aluminium foil was annealed under nitrogen atmosphere at 500 ℃ for 2 h. Then the foils were electropolished in a 4∶4∶2 electrochemical solution by volume mixture of perchloric acid, ethanol and distilled water for about 90 s.
The two-step anodizing process[8] was carried through in a thermally isolated electrochemical cell. We used platinum-titanium net as cathode material and aluminium plate as anode material. The aluminium was anodized for the first time for 2 h in the electrolyte of oxalic acid. The oxide membrane was removed in the solution (6% H3PO4 and 1.8% H2CrO4 in mass fraction) at 60 ℃ for 2 h. The last step was the second anodizing process. It was almost the same as the first anodizing process except for the different anodizing time (10 h) corresponding to the need of pore depth.
2.2 Preparation of TiO2 nanowire-array system
The tetrabutyl titanate was used as raw material. The absolute alcohol and acetylacetone were chosen as solvent and stabilizer respectively. The pH value of the sol was adjusted to 5-6 by 1mol/L hydrochloric acid and then the homogeneous titania sol was obtained after sufficient agitation [9]. The platinum-titanium net was chosen as anode. The PAA template was used as cathode. They were put in the electrolytic cell filled with titania sol. The voltage of 2 V was imposed between the anode and cathode. After the electrophoresis period for 3 min, the PAA template was taken out under the electriferous condition and dried in the air. Then it was calcined at 500 ℃ for 30 min. The surface of template was first rubbed and then corroded in the sodium hydroxide solution. The titania nanowire arrays were obtained.
2.3 Characterization
The LEO 1530VP Field Emission SEM was used to characterize the surface and cross section of PAA template. PHILIPS XL30 SEM was used to characterize the surface and cross section of the titania nanowires. BDX3300 X-Ray Diffraction instrument was used to characterize the phase structure of the PAA template and TiO2 nanowires.
2.4 Photocatalytic properties
The photocatalytic-activity of TiO2 nanowires was carried out for the degradation of the methyl orange aqueous solution with a concentration of 20 mg/L. The 25 W ultra-violet bactericidal lamp with 254 nm emission as the main wavelength was used as light source. The concentrations of methyl orange were measured using UV-vis spectrophotometer (Exact Science Apparatus Ltd. of Shanghai, 7 230 G). All the photocatalytic experiments were carried out at room temperature.
3 Results and discussion
3.1 Effect of anodizing time on pore regularit
The anodizing time has a great effect on the regularity of PAA template. The formation of PAA is a self-organized process. The pores adjust their positions and shapes with each other and this process needs a certain period. If the anodizing time is short, the pores do not have enough time to arrange regularly. As a result, the regularity of the PAA template will decrease. However, the anodizing time can not be too long, or the alumina pores will be partly dissolved and become distorted.
Fig.1 shows that the regularity of the template anodized for 10 h is better than that anodized for 6 h. In addition, the pore depth increases with the anodizing time in a certain range. In our experiments, the anodizing time of 10 h was mainly used because of the relatively
Fig.1 SEM photographs at different anodizing time: (a) 6 h; (b) 10 h
good regularity and suitable pore depth.
3.2 Effect of organic additives on PAA regularity
Generally speaking, in a certain range, the pore diameter can be adjusted by applying different anodic voltages. However, the current density of the system increases greatly when the anodic voltage reaches 60 V. Under this condition, the anodizing process will give off a lot of heat and the temperature will increase to a very high value. Consequently, the high current density is bad for the regularity of PAA and preparation of titania nanowires[10]. In our work, the additive of ethanol was added to the oxalic acid electrolyte. Because ethanol could lower the conductivity of the electrolyte and the current density of the system, the anodizing process could be carried out in a larger range of voltage.
Fig.2(a) shows the photograph of the PAA template prepared in the oxalic acid electrolyte without ethanol at 40 V. Fig.2(b) shows the PAA template prepared in the oxalic acid-ethanol system at 80 V. As shown in Fig.2(b), the regularity of PAA is still good. By this technology, PAA templates with good regularity can be obtained in a much larger range of voltage and the pore diameter can be adjusted to a wider range.
Fig.2 SEM photographs of PAA in different electrolytes: (a) Without additive; (b) With ethanol additive
3.3 SEM characterization of titania nanowires
The nanowires prepared by the sol-electrophoresis deposition template method are shown in Fig.3. This indicates that the diameter of the nanowires is about 75 nm, the length is about 20 mm and the distance of neighbouring two nanowires is about 100 nm. It could also be seen that the nanowires are parallel to each other. The nanowires keep the regularity of the template. The diameter of the nanowires is uniform and consistent with the pore diameter of the template, suggesting that the diameter of nanowires is controlled by the diameter of the template. The length of nanowires is equal to the depth of pores. This indicates that the length of nanowires is controlled by the pore depth. The periodic concave-convex structures are found in each nanowire and the shape looks like strings of candied haws. Obviously, the nanowires with this shape have larger specific surface area. This new type of material is expected to play an important role in the field of photocatalysis.
Fig.3 SEM photograph of titania nanowires
3.4 XRD Characterization
The XRD patterns of the PAA template and titania nanometer arrays are shown in Fig.4.
Fig.4 X-ray diffraction patterns: (a) PAA template; (b) Titania nanowires
Fig.4(a) shows the XRD pattern of PAA template after heat treatment at 500 ℃ for 30 min. The obtain-
ed PAA template has a single diffraction peak. This indicates that the PAA template is of single crystal structure. From the PDF23-1009, we can know that the diffraction peak is corresponding to the (-112) face of θ-Al2O3, belonging to the monoclinic system. The XRD pattern of titania nanowire arrays within the template is overlapping with each other (Fig.4(b)). Compared with Fig.4(a), there are two diffraction peaks in the pattern. One is the peak of θ-Al2O3; the other peak is corresponding to the (101) face of titania from the PDF1-562. The phase structure of nanowires is anatase.
3.5 Photocatalytic properties
Fig.5 shows the decoloration of methyl orange with different photocatalysts consisting of TiO2 nanowires and TiO2 films. As shown in Fig.5, the TiO2 nanowires have much better photocatalytic activity compared with TiO2 films. As for TiO2 nanowires, the decoloration rate of methyl orange solution reaches 93.6% after irradiated for 60 min and reaches nearly 100% after 120 min which implie that the methyl orange is completely oxidized to H2O, CO2 and other inorganic materials. As for TiO2 films, the decoloration rate is only 53.4% after 60min and 84.4% after 120 min. It is well known that the photocatalysis is a kind of surface reaction. The TiO2 nanowires have much higher specific surface area than TiO2 films, so TiO2 nanowires show high photoactivities.
Fig.5 Photodegradation of methyl orange solution with different photocatalysts: (a) TiO2 nanowires; (b) TiO2 films; (c) Blank sample
4 Conclusions
1) The anodizing time has a great effect on the regularity of PAA template.
2) By the technology of adding ethanol to the electrolyte, PAA templates with good regularity can be obtained in a much larger range of voltage(30-100V) and the pore diameter can be adjusted to a wider range.
3) PAA template after heat treatment at 500 ℃ for 30 min is of single crystal structure corresponding to the (-112) face of θ-Al2O3. The phase structure of nanowires is anatase.
4) The template method is combined with the sol-electrophoresis deposition to prepare titania nanometer-array systems. The nanometer-array systems have larger specific surface area than membranes prepared with routine methods. Compared with the TiO2/glass film prepared by the same sol, the titania nanowire arrays have much higher photocatalytic activity.
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(Edited by HE Xue-feng)
Foundation item: Project (20030056001) supported by the Doctor Foundation of Ministry of Education, China
Corresponding author: XU Ming-xia; Tel.: +86-22-27890489; Fax: +86-22-27404724; E-mail: xumingxia@tju.edu.cn
