3.1. Current density versus time transient and nanotube organization
Typical TiO anodization curve2 Nanotube production is shown in Figure 1a; according to the field-assisted dissolution theory, the process starts with a sudden surge in current density, followed by a rapid decrease in current density due to the formation of the barrier layer. Afterwards, the slope of the curve changes, indicating the onset of pore nucleation (S1). The barrier layer continues to increase until the current density reaches a minimum value (Jminute). The oxide thickness reached at this point limits the transport of oxygen ions across the barrier layer, which shifts the oxidation/dissolution equilibrium towards the dissolution side and leads to increased pore formation (S2); This process continues until the highest possible pore density (Jmaximum). Next, the nanopores rearrange and begin competing with each other to become nanotubes.
As the nanotube fabrication process progresses, the slope of the curve changes, marking the beginning of a new stage (S3(1)). At this stage, the nanotubes begin to grow as the process proceeds. The nucleation time was determined by Apolinaro et al. [26] as follows:n1 =t3(1) -t1where t3(1) and T1 is the time range to reach S3(1) and S1, respectively.On the other hand, according to the oxygen bubble model [27,28]the barrier layer is formed in the first part of the process; therefore, the current density decreases until Jminute.In the second stage of the process, nanotube embryos are formed thanks to oxygen bubbles acting as molds; the process begins with S1 and ends at the beginning of steady state (S3(2)). In the third stage, the nanotubes grow as the process progresses, and the electrolyte eventually reaches the bottom of the nanotubes as oxygen bubbles eject from the bottom of the nanotubes.
The nucleation time according to the oxygen bubble model can be defined as follows: tn2 =t3(2) -t1where t3(2) and T1 The time required to reach S3(2) and S1, respectively. According to Apollinaro et al. [26], the ordering and uniformity of nanotubes are closely related to the nucleation time; therefore, the longer the nucleation time, the better the ordering of the nanotubes. While keeping other anodizing parameters constant, the current density and time transient relationships of the aqueous electrolyte with and without XG are shown in Figure 1b, c. Both curves show previously reported stages of nanotube production and have the characteristic shape shown in Figure 1a. However, specific differences may occur, particularly in terms of the time required for each stage. From Figure 1b, it can be seen that for the curve of aqueous electrolyte without XG, t3(1) =t3(2) = 133 s, which means there is no difference between the S3 point of oxygen bubble and field-assisted dissolution theory, so tn1 =tn2 = 130 seconds. From Fig. 1b, for the curve of aqueous electrolyte with XG, it can be seen that tn1 = 1935 s and tn2 = 3362 seconds. Comparing the nucleation time of aqueous electrolytes with and without XG, the nucleation time of the electrolyte with XG is significantly higher than that of the electrolyte without XG.Figure 2 is the SEM photo of TiO2 Nanotubes and their respective FFT images obtained using aqueous electrolytes without and with XG. By comparing nanotubes made with XG (Fig. 2e-i) and nanotubes made without XG (Fig. 2a-d), it is evident that the organization and round shape of nanotubes made with XG-containing electrolytes higher than nanotubes made using XG. -Contains no aqueous electrolytes. The increase in viscosity caused by the addition of XG is the main reason for the increase in nucleation time, and therefore, the nanotubes generated in aqueous electrolytes containing XG have higher uniformity and organization. Viscous organic solvents are typically electrolytes used to produce highly ordered nanotubes (e.g., glycerol, dimethyl sulfoxide, and ethylene glycol).However, many authors [14,15,16,17,18,20,21,29,30,31] Concerns were raised about packaging and coating uniformity (e.g., coral-like structures, cracks, and no similar spaces between nanotubes). In contrast, the organization in aqueous electrolytes is poor (polygonal instead of circular and nanotube sizes are uneven) [4,32,33,34,35] (see Figure 2c,d)), this is due to their lower viscosity (larger ion movement), but overall, they have a higher fillability (see Figure 2a).It is important to emphasize the higher stackability exhibited by TiO2 Nanotubes were made using XG in aqueous electrolytes (see Figure 2e,f).
However, nanotube order can be measured both qualitatively and quantitatively. FFT images of SEM images can be used for qualitative measurement sorting.Figure 2j,k shows the FFT image of the TiO2 SEM photo2 Nanotubes grown in aqueous electrolytes without and with XG, respectively.In our previous work [4]based on Stępniowski’s paper [36], we analyzed the form of FFT images. Therefore, in less organized nanotube structures, the FFT form can assume a variety of geometric shapes; for example, in Figure 2j, a blurry ellipse is created; however, other polygonal forms or blurry images with no defined shape may also be produced . However, nanotube structures with higher organization have rounded FFT plots (see Figure 2k). It is clear from the FFT images that the organization of nanotubes produced in aqueous electrolytes containing XG is higher than that of nanotubes produced in aqueous electrolytes without XG.Several authors have used roundness in FFT images as an indicator of nanostructured organization [4,9,36,37,38,39,40,41]. FFT images can be used to quantitatively measure TiO22 Sorting of nanotubes. Therefore, the average regularity ratio (RR) method based on FFT measurements was adopted. Its equation is:
RR = (in1/2)/(W1/2S3/2)
where n is the number of nanotubes examined, I is the radial average intensity, W1/2 is the width of the radial mean at half its height and S is the area. RR value of TiO2 Nanotubes generated in aqueous electrolytes without XG are approximately zero, as the intensity in the plot is close to zero (inset 2j); this means that these nanotubes do not exhibit any organization. On the other hand, TiO2 The RR value of nanotubes generated in aqueous electrolytes using XG is 0.69, indicating that the nanotubes are ordered. The RR values are consistent with the shapes of FFT images and SEM images.
3.2. Effect of anodizing time
SEM images of nanotube coatings produced by different anodization cycles are shown in Figure 3; other experimental parameters (pH = 3, 0.5 wt.% XG and 0.5 wt.% NaF) remained unchanged. After 1.5 h of anodization, structured nanotubes were visible on the surface, showing independent walls, which is typical of such structures; however, some nanotubes had particles at their tips.Due to the slow etching rate of viscous electrolytes, these particles remain on the surface; their origin may be related to a dense, thin titanium dioxide coating that is created early in the nanotube manufacturing process and is only partially dissolved [21,42].These particles may be related to the short anodization time, which is not long enough to dissolve them [42].Titanium dioxide2 The 3-hour anodization process produced nanotubes with no particles present at the nanotube tips; therefore, the anodization time allowed them time to dissolve. Longer anodization time (7 hours) also produced TiO2 Nanotubes covered by particles; however, corrosion of nanotubes in the electrolyte due to prolonged anodization times may produce these [43]. Regarding the nanotube organization measured by RR, the best results were obtained with nanotubes produced within 3 hours. For nanotubes formed within 1.5 hours, this is not enough time to create very organized nanotubes; on the other hand, extending the anodization time is detrimental to the uniformity and organization of the nanotubes.About the inner diameter of TiO22 Nanotubes, the value is approximately 100 nm for the three anodization times evaluated.However, there is a direct relationship between anodization time and TiO2 length2 Nanotubes were evident in the samples obtained at 1.5 and 3 h, a behavior consistent with previous reports [7,21]. On the other hand, there was no difference in the nanotube length of the samples obtained at 3 and 7 h.This result may be due to the influence of the corrosion process mentioned above [43,44].
3.3. Effect of NaF concentration
The weight percentage of fluoride in the electrolyte used to make TiO2 is typically 0.20 to 1%2 anodized nanotubes [7].According to scientific literature [7,13], lower fluoride concentrations promote the development of barrier layers rather than nanotube structures. In contrast, higher fluoride concentrations promote high dissolution rates of the oxide layer; therefore, nanotube structures cannot be formed. Specific amounts of fluoride are required to make nanotubes, although it’s worth noting that excess fluoride can affect the shape of the nanotubes and their ability to adhere to the substrate. Therefore, it is crucial to determine the minimum concentration at which nanotubes can be generated without affecting their properties.
Figure 4 shows the effect of NaF concentration on the nanotube structure while other experimental parameters (pH = 3, 0.5 wt.% XG, and 3 h) were kept constant. As can be seen in Figure 4, the anodic coating prepared at 0.25 wt.% has a layer covering the nanotubes (see SEM image at 10,000x). This layer is associated with lower fluoride concentrations that are not sufficient to dissolve this unwanted top layer. When the fluoride concentration is increased to 0.5 wt.%, this layer disappears and the nanotubes appear clean; that is, there are no particles or partially dissolved oxide layer on the nanotubes.Nanotubes produced using a fluoride concentration of 0.75 wt.% have small particles on top; these particles may be caused by the higher dissolution rate of the electrolyte [2,45]. For a concentration of 1 wt.%, no anodic coating was formed.For the three fluoride concentrations examined, the inner diameter values of TiO2 The diameter of nanotubes is approximately 100 nm. Nanotube length decreases with increasing fluoride concentration; however, nanotubes produced at 0.75 wt.% are very short compared to other fluoride concentrations.This behavior can be explained by over-etching of TiO22 nanotube. Wang et al. [45] Nanotubes were obtained using electrolytes composed of ethylene glycol, water and varying concentrations of NH4F. They found that at higher concentrations of NH4F, nanotubes become messy and shorter compared to lower concentrations of NH4The results of the study by F. Wang et al. agree with our results. According to our data, the fluoride concentration that can produce clean nanotubes is 0.5 wt.%.