Quantitative analysis of the volume expansion of nanotubes during constant voltage anodization
Introduction
Electrochemical synthesis and anodization have attracted more and more attention because of their simple, stable and low-cost preparation conditions [1], [2], [3], [4], [5]. Porous anodic TiO2 nanotubes prepared by anodization have regular vertical orientation geometry, which provides a path for efficient electronic transmission. Therefore, they are widely used in the fields of supercapacitors and solar cell materials [6], [7], [8], [9], [10]. In addition, potential applications have expanded to current hot fields such as electrochromic, biomedicine, sensors, and photocatalytic decomposition over the past two decades [11], [12], [13], [14], [15], [16], [17], [18]. Although many application cases have been explored, the self-organizing formation process and growth kinetics of nanotubes are still the topics that need to be discussed [19], [20], [21]. At present, there are several theories which accepted by most. One is based on the field-assisted dissolution and ejection theories, in which the F− ions dissolves the oxide layer in "a top-down mining mode" to form porous nanotubes [19,20]. Another is a viscous flow model proposed by the Manchester group [22,23], they concluded that the viscous flow is contrary to expectations of the field-assisted dissolution model of pore formation [23]. The other is the oxygen bubble model and the theories of ionic current and electronic current [21], which based on the viscous flow model and the avalanche theory. And it is believed that nanotube structure is formed by the viscous flow of barrier oxide layer around the oxygen bubble mold from bottom to top [21].
Many researchers have studied the effects of metal substrate composition, surface roughness of metal substrate, applied voltage, electrolyte temperature and electrolyte composition on the growth of nanotubes [6,[24], [25], [26], [27], [28], [29], [30], [31], [32], [33]]. Macak et al. [31] reported that anodic TiO2 nanotubes with high aspect ratio (∼450) were obtained during only 15 min of optimized anodization in NH4F electrolyte containing lactic acid. Sopha et al. [6,32] studied the different Ti substrates influenced the growth of the nanotubes. They found no significant differences between the nanotube layers prepared on the two different substrates [6]. Sometimes, it was found that grain sizes and orientations can lead to the difference of the surface morphology of the nanotubes [32]. Faraday efficiency of the nanotube growth was the same, regardless the number anodization runs and substrate roughness [32].
In recent years, based on the field-assisted ejection theory, many groups described the volume expansion during oxidation by calculating the ratio (kv) of the height of the oxide produced to the depth of the metal consumed [20,26,29]. However, the relationship between volume expansion and anodizing current was seldom studied. Li et al. [29] reported that among the three main pore-forming acid electrolytes (H2SO4, H2C2O4 and H3PO4), the volume expansion factor should be close to 1.4 for the porous alumina structure with the best porosity state. Berger et al. [20] found that the volume expansion factor of TiO2 nanotubes prepared in glycerol electrolyte ranged from 2.7 to 3.2. Many other researchers used the Pilling-Bedworth ratio (PBR) to express volume expansion, which was defined as the ratio of growing molar oxide volume (Mox) to molar metal volume consumed (Mm) , PBR= [27], [28], [29], [30]. Nielsch et al. [28] concluded that PBR was independent of the anodization conditions. However, due to the complexity of accurate measurement of oxide density and current efficiency, it was difficult to obtain accurate PBR data [19,26,28,30]. Here, we use the reaction charge quantity to calculate PBR, which can effectively avoid the above-mentioned uncertainties [34].
According to the theory of field-assisted cationic ejection loss, the growth of porous anodic alumina was carried out at a low current efficiency (ηj = 60∼93%), and the PBR value should be between 1.02 and 1.58 [26]. This is inconsistent with the results of Berger et al. [20], they found that the PBR was 2.4 at the initial stage of the barrier formation, but reached 2.7∼3.1 when the nanotube grew steadily. Schmuki et al. [35] determined the volume expansion factor of the conversion of titanium to oxide by lithography, and the results showed that PBR had a wide range of changes with the electrochemical conditions. Especially in the electrolyte with low water content (1.6 wt%), the volume expansion factor increases with the increase of anodizing voltage (1.3∼2.8) [35]. With the increase of water content, the volume expansion factor becomes less dependent on the anodizing voltage change [36]. When the water content is 5 wt%, the volume expansion factor varies from 1.6 to 2.1. When the water content reaches 10 wt%, it is almost not affected by the anodizing voltage change [36], [37], [38]. However, this conclusion is not consistent with the results calculated in the electrolyte with lower water content (2 wt%) in this study. These seemingly contradictory experimental results reflect that the root cause of the volume expansion of anodic oxide is still unknown, and it is necessary to investigate further.
In this paper, we calculated the current-time curve integral for the first time and get the total charge consumption value in the reaction process. The consumption volume of titanium substrate in the reaction process was further calculated, and PBR was calculated by using the ratio of oxide volume to consumption of titanium metal. Here, for more accurate research, we set the anodizing time as 1000 s, so that we can ignore the difference of nanotube diameter caused by too short anodizing time and the corrosion of electrolyte on the top of nanotubes. Nanotube data of five groups of NH4F concentrations under five different voltages were selected for comparison. With the support of data, this paper tries to quantitatively clarify the reasons for the volume expansion mentioned above.
Section snippets
Experimental details
The titanium foil (99.8% purity, Baoji Ronghao Titanium Co., Ltd.) with a thickness of 0.1 mm was cut into strips of 10 mm × 60 mm, and the edges of the titanium foil were burred with sandpaper (800, 1000 and 1500 mesh) until smooth. After polishing for about 10 s in the polishing solution (volume ratio HF: HNO3: H2O=1:1:2), quickly put titanium sheet into deionized water for washing to remove the residual polishing solution on the surface, and then dried and stored in the air for later use. In
Influence of anodizing voltage and NH4F concentration on the height of nanotubes
We calculated the current-time curve obtained under the condition of each group during the constant voltage anodizing process (), and got the charge quantity under each group of conditions. And the data was shown in Table 1 to observe the effect of voltage on the height of TiO2 nanotubes. Taking Fig. 1 as an example, the corresponding current-time curve was obtained in 0.3 wt% NH4F electrolyte under 50 V, and the calculated value of charge quantity was the area of the shaded part.
Fig. 2,
Conclusions
In this paper, the specific value of charge quantity in the reaction process is obtained by integrating the current-time data set. On this basis, the morphology and expansion factor of titanium oxide nanotubes in ethylene glycol electrolyte system with five different concentrations of NH4F were studied under five different anodizing voltages. With the change of NH4F concentration, the inner diameter of nanotubes almost remained unchanged, indicating that the formation of nanotubes was based on
CRediT authorship contribution statement
Yilin Ni: Methodology, Writing – original draft. Jin Zhang: Methodology, Validation, Formal analysis. Tianle Gong: Writing – review & editing, Investigation. Ming Sun: Supervision. Ziyu Zhao: Formal analysis, Investigation. Xin Li: Formal analysis. Huiwen Yu: Validation. Xufei Zhu: Conceptualization, Project administration.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51777097, 51577093), Natural Science Foundation of Jiangsu Higher Education Institutions (20KJB430040), Qing Lan Project in Colleges and Universities of Jiangsu Province, and the National Undergraduate Training Program for Innovation and Entrepreneurship (202010288034Z).
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