Abstract
TiO2 nanotube arrays are very attractive materials by their vertically aligned porous nanostructures. They are easy to make using an anodic oxidation process in organic and inorganic electrolytes containing halogen ions, like fluorine and chlorine. Their growth tendency, various microstructures of TiO2 nanotube arrays, was fabricated and analysed with controlling of anodic oxidation process parameter like applied voltage, process time, electrolyte temperature, concentration of ammonium fluoride and DI water in ethylene glycol electrolyte, voltage applying and dropping rate, grain orientation, and size of titanium foil. Pore diameter and length were controlled in the range of 16 nm to 178 nm and 0.2 μm to 14.6 μm, respectively. Some tendencies of their growth were observed with different anodic oxidation variables. This growth tendency of TiO2 nanotube arrays is should be considered before they used in many applications, and if optimal structure for each application was fabricated and used, they will show better performance.
1. Introduction
Titanium dioxide has excellent chemical, electrical, and optical properties and chemical stability. For that reason, it is used in many different fields of study and industry, such as dye-sensitized solar cells [1–3], perovskite sensitized solar cells [4–6], photocatalysts [7], metal oxide semiconductor gas sensors [8, 9], dental implants [10, 11], interferometric sensors [12, 13], photonic crystals [14], and other applications. Typically, it is very useful materials for electron transport materials of organic solar cells due to their electronic band gap, nontoxic properties, and low product cost. Many nanostructures of TiO2 reported to improve their specific area and electron-hole recombination, like nanoparticles [1, 3, 15], nanofibers [16, 17], and nanotubes [7, 8, 10, 12, 13, 18–20]. One of them, TiO2 nanotube arrays are very attractive materials for higher electron transport materials by their vertically aligned porous nanostructures. Also, they have large specific surface area, open window, and excellent chemical and mechanical properties, and it have extended the application fields to photoanode of organic solar cells and others. TiO2 nanotube arrays are easy to make using an anodic oxidation process in organic and inorganic electrolytes containing halogen ions, like fluorine and chlorine. The fabrication of TiO2 nanotube arrays via anodic oxidation of titanium foil in fluoride-based solution was first reported in 2003 by Beranek et al. [21]. In anodic oxidation, dissolution and oxidation are very important as formation mechanism, and they are affected by process parameter such as chemical composition of electrolyte, applied voltage, processing time, and temperature. Organics or aqueous solutions have been used as an electrolyte, and they contain fluorine-based materials, such as hydrogen fluoride, potassium fluoride, and ammonium fluoride [18–20, 22]. The voltage of 10 to 60 V is usually applied, and variable voltage steps are also conducted. Processing time is very diverse according to other conditions. To apply for various application fields, a suitable microstructure for the nanotube diameter, the nanotube length, and the surface state is needed and fabricated.
In this study, to understand their growth tendency, various microstructures of TiO2 nanotube arrays were fabricated and analysed with controlling of anodic oxidation process parameter like applied voltage, process time, electrolyte temperature, concentration of ammonium fluoride and DI water in ethylene glycol electrolyte, voltage applying and dropping rate, grain orientation, and size of titanium foil. And their pore diameter and pore length are measured and observed by field emission scanning electron microscope (FE-SEM, FEI Co., Inspect F/Hitachi Ltd., SU-70). Some tendencies of their growth were observed with different anodic oxidation variables.
2. Materials and Methods
Figure 1 shows potentiostatic electrochemical system. The system consisted of a power supply (Xantrex Co., XKW 600), multimeter (Agilent Co., 34401A), constant temperature water bath, and PC. The power supply and multimeter were computer-controlled by the LabVIEW (National Instruments Co.) program for electropolishing and anodic oxidation.
To compare the microstructure with different process time, electrolyte temperature, and concentration of ammonium fluoride (NH4F, JUNSEI Co.) and DI water in ethylene glycol (JUNSEI Co.) electrolyte, titanium foils (Alfa Aesar Co., 0.89 mm thickness, 99.7% purity) were rinsed in an ultrasonic bath of DI water, ethyl alcohol, and acetone for 10 min in turn and dried at room temperature in a convection oven to remove the surface pollutant. And Ti foil was anodic oxidized in each condition, which is shown in Table 1. For the comparison of microstructure after anodic oxidation, process time, temperature, and concentration of NH4F and DI water were controlled in a range of 10 ~ 60 min, 0 ~ 30°C, 0.3 ~ 1.0 wt%, and 2.2 ~ 10.9 vol%, respectively. Other conditions of anodic oxidation were the same for each comparison.
Secondly, some titanium foil was conducted in anodic oxidation process for different applied voltage, voltage applying rate, and dropping rate. Applied voltage was controlled in a range of 3 ~ 60 V at 30°C for 4 hr in ethylene glycol electrolyte with 0.5 wt% NH4F and 10.9 vol% DI water; Table 2 shows voltage applying rate and dropping rate conditions, and they are controlled in a range of 0.1 ~ 60.0 V/s, immediately drop ~0.1 V/s anodic oxidized at 60 V and 30°C for 10 min in ethylene glycol electrolyte with 0.5 wt% NH4F and 2.2 vol% DI water.
To compare the microstructure with different grain size and orientation, Ti foil was annealed in an electric furnace for 1 hr at 500, 800, 950, and 1100°C. After annealing, to observe the grain size and orientation, Ti foils were mechanically polished by sand paper and were electropolished for 20 V and 5 min in methyl alcohol (J.T.Baker. Co.) with 38 wt% ethylene glycol and 10 wt% perchloric acid (JUNSEI Co., 70%) at 0°C and stirred at 800 rpm. Then, polished titanium foils were etched for 20 sec in DI water with 10 vol% nitric acid and 10 vol% hydrofluoric acid. Grain structure and orientation of etched titanium foils were observed by FE-SEM and X-ray diffractometer (Rigaku D/MAX-RC, Cu Kα radiation). After electropolishing, titanium foils were anodized at 60 V and 30°C for 40 min in ethylene glycol electrolyte with 0.5 wt% NH4F and 8.2 vol% DI water. Also, after anodic oxidation process, all anodic oxidized samples were immersed in ethyl alcohol and dried in a dry oven. Then, their microstructures were observed by FE-SEM.
3. Results and Discussion
The various microstructures of TiO2 nanotube arrays were obtained with anodic oxidation process time from 10 min to 60 min at 60 V and 30°C in ethylene glycol electrolyte with 0.5 wt% NH4F and 2.2 vol% DI water. Figures 2 and 3 show FE-SEM image and their microstructure change of anodic-oxidized TiO2 nanotube arrays with different process time. With the increase of process time, higher pore diameter and pore length were observed. The pore diameter and pore length of 10 min anodic-oxidized TiO2 nanotube arrays were 46 nm and 3.4 μm, and the pore diameter and pore length of 60 min anodic-oxidized TiO2 nanotube arrays were 90 nm and 14.6 μm, respectively.
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Figure 4 shows FE-SEM image and their microstructure change of anodic-oxidized TiO2 nanotube arrays with different electrolyte temperature. Titanium foils were conducted for different electrolyte temperature from 0°C to 30°C at 60 V for 4 hr in ethylene glycol electrolyte with 0.5 wt% NH4F and 10.9 vol% DI water. With the increase of electrolyte temperature, both pore diameter and pore length were increased. The pore diameter and pore length of TiO2 nanotube arrays which were formed at 0°C were 143 nm and 2.3 μm, respectively, and the pore diameter and pore length of TiO2 nanotube arrays which were formed at 30°C were 178 nm and 4.1 μm, respectively. It resulted from higher ion diffusivity in higher temperature of electrolyte.
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Figure 5 shows FE-SEM image and their microstructure change of anodic-oxidized TiO2 nanotube arrays with different concentration of NH4F in electrolyte. For the comparison with different concentration of NH4F, titanium foils were conducted at 60 V and 30°C for 4 hr in ethylene glycol electrolyte with 0.3 ~ 1.0 wt% NH4F and 10.9 vol% DI water. With the increase of concentration of NH4F in electrolyte, pore diameter was increased but pore length was decreased. The pore diameter and pore length of TiO2 nanotube arrays which were formed in 0.3 wt% NH4F-contained electrolyte were 140 nm and 5.8 μm, respectively, and the pore diameter and pore length of TiO2 nanotube arrays which were formed in 1.0 wt% NH4F-contained electrolyte were 174 nm and 3.7 μm, respectively. NH4F in electrolyte acted as an etchant for TiO2 nanotubes during anodic oxidation, and the pores were widened and shortened.
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Figure 6 shows FE-SEM image and their microstructure change of anodic-oxidized TiO2 nanotube arrays with different concentration of DI water in electrolyte. For the comparison with different concentration of DI water, titanium foils were conducted at 60 V and 30°C for 30 min in ethylene glycol electrolyte with 0.5 wt% NH4F and 2.2 ~ 10.9 vol% DI water. With the increase of concentration of DI water in electrolyte, pore diameter was increased but pore length was decreased. The pore diameter and pore length of TiO2 nanotube arrays which were formed in 2.2 vol% DI water-contained electrolyte were 72 nm and 7.6 μm, respectively, and the pore diameter and pore length of TiO2 nanotube arrays which were formed in 10.9 wt% NH4F-contained electrolyte were 104 nm and 1.0 μm, respectively. Pore length formed in aqueous electrolyte is shorter than that in organic electrolytes such as ethylene glycol and glycerol [23–25]. The DI water increased the effect of aqueous electrolyte, and the pore length was decreased with DI water.
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With 3 V to 60 V range of applied voltage, various microstructures of TiO2 nanotube arrays were obtained for 4 hr and 30°C in ethylene glycol electrolyte with 0.5 wt% NH4F and 10.9 vol% DI water. Figures 7 and 8 show FE-SEM image and their microstructure change of anodic-oxidized TiO2 nanotube arrays with different applied voltage. With the increase of applied voltage, higher pore diameter and pore length were observed. The pore diameter and pore length of TiO2 nanotube arrays which were fabricated at 60 V were 178 nm and 4.07 μm, respectively, and the pore diameter and pore length of TiO2 nanotube arrays which were fabricated at 3 V were 16 nm and 0.2 μm, respectively.
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Figures 9 and 10 show FE-SEM image and microstructure of anodic-oxidized TiO2 nanotube arrays with different voltage applying rate and dropping rate. For the comparison with different concentration of DI water, titanium foils were conducted at 60 V and 30°C for 10 min in ethylene glycol electrolyte with 0.5 wt% NH4F and 2.2 vol% DI water. At 0.5 V/s ~ 30.0 V/s applying rate, there was no obvious difference or slightly changed in pore diameter and pore length, but lower pore diameter and higher pore length were observed at V/s applying rate, and higher pore diameter and lower pore length were observed at 60 V/s applying rate. With the increase of dropping speed, lower pore diameter and pore length were observed. The pore diameter and pore length of TiO2 nanotube arrays fabricated at 0.5 V/s applying rate were 59 nm and 1.1 μm, respectively. And the pore diameter and pore length of TiO2 nanotube arrays fabricated at 30.0 V/s applying rate were 83 nm and 1.1 μm, respectively. Also, 73 nm, 1.1 μm and 82 nm, 1.7 μm of pore diameter and pore length were observed, respectively, at 1.0 V/s applying rate, immediately dropped and 1.0 V/s applying rate, 0.1 V/s dropping rate.
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Figures 11 and 12 show grain structure, their size, and crystal orientation. To compare with different grain size, orientation, and their anodic oxidation behavior, titanium foil was annealed in an electric furnace for 1 hr at 500, 800, 950, and 1100°C. With the increase of annealing temperature, grain size was increased and their orientation aligned for (110), (012), and (013) plane of α-Ti. Also, each titanium foils were anodic oxidized at 60 V and 30°C for 40 min in ethylene glycol electrolyte with 0.5 wt% NH4F and 8.2 vol% DI water. Figure 13 shows their FE-SEM image and microstructure changes. With the change of grain structure and orientation, there was no obvious change. They have about 110 nm of pore diameter and 3.3 μm of pore length.
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4. Conclusions
To fabricate the various microstructured TiO2 nanotube arrays by anodic oxidation, some process parameters such as applied voltage, process time, electrolyte temperature, concentration of ammonium fluoride and DI water in ethylene glycol electrolyte, voltage applying and dropping rate, grain orientation, and size of titanium foil were controlled. With the increase of applied voltage, process time, temperature, and voltage dropping time, higher pore diameter and pore length were observed, and with the increase of ammonium fluoride and DI water concentration, higher pore diameter and lower pore length were observed. But with the increase of grain size and the change of crystal orientation of titanium, pore diameter and pore length were almost not changed. Typically, larger specific area with smaller pore diameter and longer pore length of TiO2 nanotube arrays leads to a better performance of many devices, like photovoltaics, sensors, and photocatalysts. This growth tendency of TiO2 nanotube arrays is should be considered before they used in many applications, and if optimal structure for each application was fabricated and used, they will show better performance.
Data Availability
The data used to support the findings of this study are included within the article.
Conflicts of Interest
There is no conflict of interest regarding the publication of this paper.
Acknowledgments
This research was financially supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP), the Ministry of Trade, Industry & Energy (MOTIE) (Grant No. 20181110200070), and the Local University Excellent Scientist Supporting Program through the National Research Foundation of Korea (Grant No. 2017R1D1A3B03036068).