Influence of Anode Area and Electrode Gap on the Morphology of TiO 2 Nanotubes Arrays

In order to fabricate the titanium dioxide (TiO 2 ) nanotubes arrays which were used in the photocatalytic degradation of total volatile organic compounds (TVOC) by anodization, the influence of the electrode gap and anode area on the morphology of the titanium dioxide (TiO 2 ) nanotubes was studied. Titanium dioxide (TiO 2 ) nanotube arrays were prepared by anodization with various electrode gaps and anode areas. Field emission scanning electron microscopy was used to investigate the morphology of the TiO 2 nanotubes arrays. The results showed that the morphology of TiO 2 nanotubes arrays was influenced by electrode gap and anode area. The appropriate anode area and electrode gap were 5 cm× 2 cm and 20mm, respectively. Thus, TiO 2 nanotube arrays with better morphology (with larger dimension and uniform TiO 2 nanotubes) were successfully fabricated by anodic oxidation with 5 cm× 2 cm anode area and 20mm electrode gap at 30V.

Owing to its wide band gap (3.2 eV for anatase), anatase TiO 2 can only be excited under ultraviolet irradiation.
However, this section occupies only less than 5% of the solar irradiance at the Earth's surface.For the sake of efficient use of sunlight, enlarging the absorption band border of TiO 2 may be an appealing challenge for developing the future generation of photocatalysts.The improvement of photocatalytic efficiency by modifying TiO 2 has been focused on for a long time [32,33].Coupling TiO 2 with a small band-gap semiconductor or doping with transition metal ions such as V, Cr, Mn, Fe, Co, Ni, or Cu extends the absorption spectrum range and improves the efficiency of photocatalyst [34][35][36][37].Yang et al. [38] recently assembled Cu 2 O nanoparticles on the top surface of the TiO 2 nanotube arrays (TNAs) by anodizing method.However, these Cu 2 O nanoparticles were supported mainly on the top surface of the TNAs, leading to insufficiently effective electron pathways between the Cu 2 O and TiO 2 .Therefore, in order to deposit these nanoparticles into the inner and external walls of the nanotubes uniformly, highly structured and orderly arranged TiO 2 nanotube arrays with larger pore size must be prepared.The morphology and the structure of nanotubes are affected strongly by the electrochemical conditions (particularly the anodization voltage) and the solution parameters (in particular the HF concentration, the, pH and the water content in the electrolyte) [39].Many researchers [3,9,26,31,40,41] varied the parameters such as anodization potentials and anodization durations to fabricate the specific morphology of TiO 2 nanotubes arrays (TNAs).Kim et al. [42] and Paulose et al. [43] demonstrated that the morphology of nanotubes is strongly influenced by the applied potential, and water content of electrolytes.Several reports [17,41,[44][45][46] have shown that F − is crucial to the formation of nanotubular structures, and diluted F − electrolyte is preferred.To increase the nanotubular length, organic and viscous electrolytes have been experimented utilizing ethylene glycol or glycerol [47][48][49].A review of the literature related to TiO 2 nanotube arrays reveals no published study on the effects of the anode area and electrode gap on the morphology and the structure of the resulting TiO 2 nanotube arrays.
In this work, we report the effect of different anode areas and electrode gaps on the morphology of fabricated TiO 2 nanotubes in ethylene glycol (EG) electrolytes containing ammonium fluoride (NH 4 F) and H 2 O.The microstructures of the samples are investigated by field emission scanning electron microscopy (FE-SEM).

Experimental
2.1.Materials.Titanium foils (0.2 mm thick, 99.95% purity) were mechanically polished by different abrasive papers (600#, 1000#, 1200#, and 1800#) then followed by sonicating acetone, ethanol, and deionized water for 30 min.respectively.Then the titanium foils were dried at room temperature in air.All the other chemicals were of analytical grade and used as received without further purification.

Preparation of TiO 2 Nanotube Arrays.
A two-electrode polyethylene plastic cell ( = 300 mL) was used to perform the anodization process.The pretreated Ti foil was used as an anode and Pt sheet as the cathode.Both electrodes were immersed in 200 mL organic electrolyte containing 0.4wt.%NH 4 F, 5 vol.%H 2 O, and 95 vol.% ethylene glycol (EG).All electrolytes were prepared from analytical grade and deionized water (>18 M Ω cm).Ti foil was cut into 5 cm × 0.5 cm, 5 cm × 1 cm, 5 cm × 2 cm, and 5 cm × 4 cm, with 4 cm × 0.5 cm, 4 cm × 1 cm, 4 cm × 2 cm, and 4 cm × 4 cm areas exposed to the bulk electrolyte, respectively.The applied voltages were set to 10, 20, and 30 V by a DC power source (DH1718E-4), respectively.The electrochemical experiments were carried out at room temperature (∼28 ∘ C), and the electrolyte was kept uniform with constant magnetic stirring, the stirring speed was kept constant at ∼180 rpm.The specimens were anodized for 1 h.under different electrochemical conditions.The detailed electrochemical conditions are listed in Table 1.The anode area and electrode gap were changed for the systemic investigation of the formation of TiO 2 nanotubes.The as-anodized samples were rinsed with deionized water and dried in air.

Characterization Technique.
The surface morphologies of obtained samples were observed using field emission scanning electron microscope (FE-SEM, Hitachi S-4800).Direct SEM cross-sectional observations were carried out on flat and cracked samples.

Influence of Anode Area on the Top Morphology of TiO 2
Nanotube.The TiO 2 nanotubes formation mechanism includes the following processes [50][51][52].
(i) Formation of oxide layer at the surface of the metal takes place due to interaction of the metal with O 2− or OH − ions.After the development of an initial oxide layer, these ions move through the oxide layer towards the metal/oxide interface where they react with the Ti metal.The reaction can be represented as follows: (ii) Field-assisted dissolution of Titanium oxideMetal ion (Ti 4+ ) transfer from the Ti metal at the metal/oxide interface by ejection under application of an electric field and migrate towards the oxide/electrolyte interface.Ti 4+ cations dissolve into the electrolyte, and the free O 2− anions transfer towards the metal/oxide interface to interact with the Ti metal.The fieldassisted dissolution of titanium oxide is represented by the following reaction: (iii) Chemical dissolution of the Ti metal or oxide also occurs due to the acidic and fluorine contained electrolyte, which then causes more etching at the porebottom than at the opening and the walls.Therefore, in the early stages of the anodization process, small pits are formed.The formation of these small pits is represented by the following reaction: Therefore, the growth and propagation of the pores occur by inward movement at the oxide/metal interface, which is determined by the amount of fluorine on the anode surface and the degree of electric field.Figure 1 shows the surface images of the samples during anodization of titanium at 10 V in 0.4wt% NH 4 F + 5wt% H 2 O EG solution with different anode areas.With 5 cm × 0.5 cm anode area, the top of the sample is covered by much debris, and no nanotube is found on the surface (Figure 1(a)).We suppose that with a smaller anode area, the current density and the field effect are too high.Thus, the higher electric field would polarise and weaken Ti-O bond resulting in dissolution of oxide rapidly before small pores formation.In addition, the rate of the chemical dissolution on the anodes of the smaller area is higher due to the more fluoride ions per anode unit area, resulting in dissolution of oxide rapidly before small pores formation.With 5 cm × 1 cm anode area, the surface morphologies consist of ring-like structures whose the openings are around 20 nm in diameter, and numerous crannies are observed on the surface (Figure 1(b)).The samples prepared with 5 cm × 2 cm anode at 10 V (Figure 1(c)) consist of a ringlike structure with diameter of less than 20 nm, no debris and crannies are found on the surface.At the initial stage of anodisation, the rate of the field-assisted dissolution on the anodes of different area is equal due to the same applied voltage, while the rate of the chemical dissolution on the anodes of the larger area is lower due to the less fluoride ions per anode unit area, which results in the oxide thickness reduction being slower on the larger area anode [53].As the anodisation proceeds and oxide thickens, the rate of the fieldassisted dissolution on the larger area anode decreases faster than that of the smaller area anode [53].So, we can speculate that with the increase of the anode area, the electric fieldassisted dissolution of the oxide is weakened, the chemical dissolution dominates electric field-assisted dissolution, and the growth and propagation of the tubes occur by inward movement at the oxide/metal interface, thereby forming nanotubes.At the meantime, when the current density is beyond certain value, electric field-assisted dissolution is believed to occur too rapidly, polarizing and weakening the Ti-O band not only at the bottom part of the nanotubes but also along the longitudinal direction of the nanotubes as well; hence more crannies are produced.
As shown in Figure 1(d) for the 5 cm × 4 cm anode area, the TiO 2 layer is more compact and only some small pits disperse sparsely, no nanotubes are observed; however, Ti is partly etched.Under this condition, the anode area is too large, resulting in the fact that the amount of fluorine ions per square centimeter at the anode surface is less, the rate of the reaction (3) decreases rapidly [54]; thus, the TiO 2 layer is thicker, which results in less electric field dissolution [53].Therefore, the formation of nanotubes by anodization fails.The appreciate anode area for TiO 2 nanotube formation is, therefore, around 5 cm × 2 cm in the 200 mL 0.4wt% NH 4 F + 5wt% H 2 O EG solution.

Influence of Electrode Gap on the Top Morphology of TiO 2
Nanotubes.Figure 2 shows the surface images of TiO 2 nanotubes prepared with different electrode gaps.When the electrode gap is 5 mm, the morphology which is covered by the thicker oxide layer with severely localized shedding is observed (Figure 2(a)).Because the smaller electrode gap is, the solution resistance between electrodes is lower, the rate O 2− or OH − ions to the anode increase; therefore, the rate of the reactions (1a), (1b), (1c), and (1d) is higher; that is, the formation of TiO 2 layer is faster.On the other hand, the F − ions quickly reach the anode and the Ti 4+ ions quickly leave the anode, resulting in the fact that the chemical dissolution and electric field dissolution of the TiO 2 layer rapidly occur.So the oxide layer on the sample surface is etched severely.Samples anodized at a 10 mm electrode gap consist of a ringlike structure with a diameter of <30 nm and some crannies (Figure 2(b)).With increased electrode gap, the rate of the TiO 2 layer formation and dissolution is slowed down.Under this condition, the reaction (3) on the bottom of the pits continues, so lots of ring-like structures occur, but the higher the electric field intensity due to the smaller electrode gap results in stronger field-assisted dissolution of TiO 2 on the sample surface.Therefore, there are lots of TiO 2 crannies on the sample surface.Increasing the electrode gap to 20 mm, Figure 1(c) shows the ring-like structure without any cranny on the sample surface.The inward movement rate of the oxide/metal interface is approximately equal to the rate of the reaction (3).So, a discrete, hollow-like cylindrical structure was formed which developed into the nanotubular structure with diameters of <20 nm.At the meantime, with decreased electric field intensity, the electric field dissolution of the oxide occurs much slower, thus without forming crannies on the surface.At 40 mm electrode gap (Figure 2(c)), the TiO 2 layer is more compact and many pit structures are observed.Under this condition, the higher electrical resistance due to the larger electrode gap results in that the inward movement rate of the F − ions is decreased, only small pits are formed without any nanotubes on the surface.And the weaker   electric field dissolution due to the smaller electric field intensity does not generate crannies on the sample surface.
According to the aforementioned experiments, it is inferred that samples anodized with 20 mm electrode gap will suffer from appreciate chemical and electric field dissolution forming nanotube structure with smaller diameter with the ratio of electrode area to electrolyte volume being 0.05 cm −1 .Under this condition, anodisation experiments at higher voltages (20 V, 30 V) were carried out to seek the possibility of increasing the length and diameter of the nanotubes further.Surface morphologies of TiO 2 formed at 20 V and 30 V are shown in Figures 3(a) and 3(b) when the samples are anodized for 1 h.The surface morphologies consist of ringlike structures related to the opening of nanotubes.The insets are the cross-sectional images and higher magnifications of the same sample.The length of the nanotubes increases from 1 m to 2.4 m, and the diameter of the nanotubes increases from 70 nm to 200 nm for samples anodized at 20 and 30 V, respectively.As reported by Lockman et al. [53], pore growth occurs at a higher voltage, thereby increasing the length and diameter of the nanotubes, but the nanotubes structure starts to be deteriorated at 25 V and is completely lost at 30 V. Therefore, the joint effect of electrolysis conditions such as voltage, electrode area, electrode gap, composition of the electrolytic solution, and temperature on the nanotubes morphologies needs to be studied further.

Conclusion
In conclusion, it is demonstrated that the surface morphology of TiO 2 nanotube arrays which were used in the photocatalytic degradation of total volatile organic compounds (TVOC) can be modified by anodization of Ti foils in EG solution consisting of 0.4% NH 4 F and 5% wt H 2 O using the appropriate anode area and electrode gap.The effect of anode area and electrode gap on surface morphology of TiO 2 nanotube arrays was investigated and the results of SEM show that the highly ordered TiO 2 nanotube arrays cannot be formed with inappropriate anode area and electrode gap.Moreover, a detailed theoretical interpretation of the influence of anode area and electrode gap on the morphology of TNAs was introduced.With appropriate anode area 5 cm × 2 cm and electrode gap 20 mm, larger-dimension, and uniform TiO 2 nanotube arrays were successfully formed after 1 h. of anodization of pure titanium foil in a two-electrode bath with 200 mL electrolyte solution at 30 V.

Figure 2 :
Figure 2: FE-SEM images of the surface morphology of TiO 2 nanotube arrays formed at 10 V in ethylene glycol containing 0.4wt% NH 4 F and 5wt% H 2 O electrolyte for 1 h.with different electrode gaps: (a) 5 mm, (b) 10 mm, and (c) 40 mm.

Figure 3 :
Figure 3: FE-SEM images of the surface morphology of TiO 2 nanotube arrays formed in ethylene glycol containing 0.4wt% NH 4 F and 5wt% H 2 O electrolyte at different voltages for 1 h.(a) 20 and (b) 30 V.

Table 1 :
Electrochemical conditions and the morphologies of the samples.