Abstract
TiO2 nanotubes (TNTs) have drawn tremendous attention owing to their unique architectural and physical properties. Anodizing of titanium foil has proven to be the most efficient method to fabricate well-aligned TNTs, which, however, usually produces amorphous TNTs and needs further thermal annealing. Recently, a water-assisted crystallization strategy has been proposed and investigated by both science and engineering communities. This method is very efficient and energy saving, and it circumvents the drawbacks of thermal sintering approach. In this paper, we review the recent research progress in this kind of low-temperature crystallization approach. Here, various synthetic methods are summarized, and the mechanisms of the amorphous–crystalline transformation are analyzed. The fundamental properties and applications of the low-temperature products are also discussed. Furthermore, it is proved that the water-assisted crystallization approach is not only applicable to TNTs but also to crystallizing other metal oxides.
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1 Highlights
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This paper reviews the water-assisted crystallization for TiO2 nanotubes (TNTs) for the first time.
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The review summarizes various aspects of TNTs prepared by water-assisted crystallization method.
2 Introduction
Over the past few decades, titanium dioxide (TiO2) has drawn ever-increasing interest for its application to energy and environmental areas, such as photocatalysis [1, 2], dye-sensitized solar cells (DSSCs) [3, 4], Li-ion batteries [5, 6], supercapacitors [7, 8], gas sensors, and water splitting, because of its low cost, high abundance, high chemical stability, and lack of toxicity [9,10,11,12,13,14]. In particular, one-dimensional (1-D) TiO2 nanotubes (TNTs) are widely investigated because they possess the advantages of both high surface area and direct carrier transport pathways, which make them a promising candidate for various fields [15,16,17]. Four main routes have been proposed to synthesize TNTs, including sol–gel, hydrothermal, templating, and anodic oxidation methods [18,19,20,21]. The TNTs prepared through sol–gel or hydrothermal methods are generally randomly oriented, and the templating method is limited because of its complicated fabrication process and high cost [22]. Anodic oxidation not only offers the advantages of being facile and easily scaled up but also can yield highly ordered oriented nanotube (NT) arrays, resulting in markedly enhanced performance [23,24,25]. In addition, such NT characteristics as the tube diameter and thickness of the tube wall can be readily manipulated by changing the experimental parameters of the anodization process [26, 27]. Therefore, anodic oxidation is expected to be a superior method for fabricating oriented TNTs.
However, the as-anodized TNTs are generally amorphous after anodization, which is not useful for many applications such as DSSCs, where the anatase phase and higher crystallinity have been proved to be essential for enhanced performance [28]. To address this issue, thermal annealing is always carried out to obtain the desired crystalline TNTs. Specifically, the conversion of amorphous TiO2 NTs to the anatase phase occurs at above 300 °C, and mixture of anatase and rutile appears when the annealing temperature is above 550 °C [29,30,31]. Although thermal annealing is an effective method to crystallize amorphous TNTs, there are some drawbacks: (1) the annealing method requires additional energy consumption and is costly; (2) this process may facilitate the formation of a thick barrier layer that separates the NT arrays from the substrate, resulting in deterioration of electron transport; (3) the high-temperature annealing process impedes the integration of NT arrays on temperature-sensitive polymeric substrates, hindering the development of lightweight TNTs-based devices [32,33,34]. Consequently, exploring a low-temperature method for the crystallization of TNTs is significant in broadening their applications.
In fact, there are some reports concerning the fabrication of crystallized TNTs at low temperatures without annealing [35, 36]. Su et al. found that a crystalline structure formed when the anodization was carried out at a high voltage (120 V) [37]. However, the crystallinity was so low that only a weak and broad (101) peak of anatase appeared. Ali et al. fabricated crystalline TNTs with the assistance of perchloric acid electrolytes, but the TNTs fell into the electrolyte, leading to an extra centrifugation procedure [38]. Therefore, it is urgent to find a facile strategy to crystallize the as-anodized TNTs with considerable crystallinity without involvement of any hazardous substances. In 2011, Liao et al. and Wang et al. proposed a novel water-assisted crystallization (WAC) approach to crystallize the amorphous as-anodized TNTs [39, 40]. In short, after the conventional anodization (electrolyte: ethylene glycol solution containing H2O and NH4F), the as-anodized foils were simply soaked in water for a certain time, and the amorphous TNTs arrays transformed into the anatase phase. It is truly amazing that this transformation occurred with the help of only water and without any annealing treatment or additives. Because of its facile and green chemistry features, the WAC strategy arouses people’s interest, and much effort has been devoted to it [41].
Although many papers based on the WAC route have been published since 2011, no review of this field is available. Therefore, a comprehensive review could not only provide timely information for researchers but also motivate the development of TNTs. As shown in Fig. 1, this review mainly summarizes aspects of WAC for TNTs, including methods and mechanisms, fundamental properties, applications, and other materials. First, various WAC methods and the corresponding mechanisms are introduced. Second, we investigate the fundamental properties of the products after WAC treatment. Third, we introduce the main applications, including photocatalysis, DSSCs, and supercapacitors. The fourth section is mainly about other materials generated employing the WAC strategy, namely TiO2-based nanostructured materials and other metal oxides.
3 Methods and Mechanisms
It is well known that crystallization strategy plays a key role in fabricating nanomaterials significantly influencing the structure and morphology of the products. In this section, we cover the various WAC-based crystallization methods used for synthesizing TNTs. We first introduce the water-only WAC method, in which water is the only substance employed in the crystallization process. Then, some modifications are presented, such as doping with metal ions. Finally, some ingenious methods inspired by the WAC mechanism are proposed for the preparation of TNTs. The mechanisms of the abovementioned methods are also discussed.
3.1 Water-Only WAC Method
With respect to the water-only WAC method, the amorphous as-anodized TNTs are treated only with water without any other additives for crystallization. According to the different forms of water, it can be classified as a solid–liquid method or a solid–gas method. As the crystallization process involves only water, it is considered a green and cost-effective approach.
3.1.1 Solid–Liquid Method
As for the solid–liquid method, the as-anodized TNTs are simply immersed in water at different temperatures. The amorphous TNTs transform into crystalline anatase TNTs after a certain immersion time; it is schematically shown in Fig. 2. In the following sections, we mainly introduce solid–liquid methods, including room temperature (RT) water crystallization and hot water crystallization [41,42,43,44,45,46,47,48,49].
In 2011, Wang et al. prepared amorphous TNTs by anodization (electrolyte: ethylene glycol solution containing NH4F and H2O) and then immersed them in water at RT (~ 25 °C) [40]. As a result, the amorphous TNTs transformed into pure anatase phase after 3 days without occurrence of rutile or brookite phases (Fig. 3a, b). In addition, the obtained anatase TNTs were stable, and no discernible changes were observed when the immersion time was prolonged to 30 days. The amorphous TNTs transformed into the anatase phase not only by annealing but also through the water-assisted strategy at RT. Based on systematic studies, a dissolution–precipitation mechanism was proposed to explain this type of WAC phenomenon. The building blocks of the amorphous materials are randomly distributed, while the crystallized materials are long-range ordered [50]. As TiO6 octahedra are the building blocks for both amorphous TNTs and anatase TNTs, it is assumed that the amorphous–anatase transformation is a process that rearranges the TiO6 octahedra with the assistance of water. As schematically shown in Fig. 3c, two different TiO6 octahedra, which share one common vertex, first absorb a water molecule forming a bridge between the surface hydroxyl groups through the lone electrons on the oxygen (step 1). In step 2, dehydration of the abovementioned complex occurs. Two water molecules are ejected and one oxygen atom is taken away by forming a new water molecule, resulting in the linkage of octahedra by sharing one common edge. Subsequently, third octahedra proceeds in a similar hydration-dehydration process and the three octahedra become linked together by sharing their edges at a right angle (step 3). Finally, the right-angle assembly connects with another identical assembly, leading to the basic unit cell of anatase TiO2 (step 4). The reason why the rearrangement of TiO6 octahedra leads to the formation of the anatase phase and not the rutile phase can be explained as follows. It is believed that the Gibbs free energy of anatase clusters is lower than that of rutile clusters; therefore, the anatase phase is more thermodynamically stable [51]. It is concluded that the amorphous–anatase transformation is achieved through a dissolution–precipitation process, in which randomly distributed TiO6 octahedra are rearranged with the assistance of water. Owing to the dissolution and reprecipitation processes, the morphology of TNTs cannot stay the same as before, and numerous pores are formed, which will be discussed in the next chapter.
Although the dissolution–precipitation process has been developed, there are still many scientific issues to be explored. In 2016, we further investigated the amorphous–anatase transformation and supplemented the dissolution–precipitation mechanism by immersing the as-anodized TNTs in hot water (90 °C) for a certain time [52]. We named it the WAC strategy because water is the only substance involved in this crystallization process. Could any other substance also lead to this amazing amorphous–anatase transformation under the same conditions? In addition to immersing the amorphous TNTs in hot water, we also immersed them in ethanol and ethylene glycol solutions. However, no distinct anatase peaks were detected except for the Ti substrate peaks, as shown in Fig. 4b, c. We inferred that water is an essential factor in the crystallization process, confirming the water-assisted mechanism. In addition, we not only focused on the TNTs but also paid attention to the byproducts leading to deeper insight into the WAC mechanism. Specifically, after the samples were immersed for 2 h at 90 °C, the water became turbid owing to the existence of white precipitate byproducts. When the immersion time was prolonged to 20 h, white products settled to the bottom, and the water became transparent again, as shown in Fig. 4d, e. The selected area electron diffraction (SAED) results in Fig. 4f reveal that the white byproducts are composed of nanoparticles (NPs) possessing an anatase crystalline structure in line with that of the TNTs on Ti substrate. Based on this result, a supplementary dissolution–recrystallization-precipitation mechanism was proposed; its schematic diagram is presented in Fig. 4g. First, TiO6 octahedra dissolve in water forming Ti(OH) 2−6 species. Then, the Ti(OH) 2−6 species spontaneously recrystallize and precipitate in situ into TNTs, maintaining the mechanical nanotubular structure. However, a fraction of Ti(OH) 2−6 species recrystallize and precipitate into anatase TiO2 NPs, which are apart from the TNTs and suspended in water. In other words, the overall reactions of the amorphous–anatase transformation can be described as follows:
Here, TiO2(am) represents the amorphous TNTs, and TiO2(an) represents the anatase TNTs. These findings corroborate and enrich the water-assisted dissolution–precipitation mechanism and provide deeper insight into the WAC strategy.
It is worth noting that experimental parameters, such as water temperature and immersion time, have a significant influence on the properties of the obtained anatase TNTs. For example, Karine et al. employed the solid–liquid method to crystallize amorphous TNTs for various time periods at different temperatures (RT, 80, 100, and 120 °C). They found that the crystallographic properties and morphological features are distinctive under different experimental conditions [46]. Actually, the solid–liquid method to crystallize amorphous TNTs is widely recognized as a fundamental and convenient strategy. The dissolution–precipitation mechanism is not only applicable to the solid–liquid method but also serves as the theoretical basis of other derived synthetic strategies.
3.1.2 Solid–Gas Method
Although the solid–liquid WAC method is cost-effective and convenient, the full-of-water condition results in a disadvantage: TNT films may detach because of the dissolution of the bottom layer under the NTs. To circumvent this problem, a solid–gas method, in which water is gaseous, is proposed [44, 53, 54].
As schematically shown in Fig. 5a, the amorphous TNTs are put into a Teflon-lined stainless autoclave containing a small amount of water. With increasing temperature, the liquid water turns into water vapor surrounding the TNTs and crystallizes them. Liu et al. employed the solid–gas method to crystallize the amorphous TNTs at temperatures of 130–180 °C, and only 0.3 mL water was added to the autoclave [54]. The dissolution–precipitation process proceeded at the TNT/vapor interfaces leading to the crystallization of amorphous TNTs. X-ray diffraction (XRD) results showed this amorphous-anatase transformation, and the effects of vapor temperature and crystallization duration were also investigated. For comparison, they also used the solid–liquid method to crystallize amorphous TNTs by immersing them in water (maintaining other conditions the same). At temperatures of 130 or 160 °C, the nanotubular structure of the solid–gas samples was preserved, and only some NPs appeared on tube walls (Fig. 5b), which is a common morphology feature resulting from the WAC method. In contrast, the solid–liquid samples exhibited a serious collapse of TNTs, and the nanotubular architecture was destroyed (Fig. 5c), which is consistent with previous reports. These results indicate that the solid–gas method can alleviate the collapse of NTs, possibly because of relatively slow TNT/vapor interface reactions.
To compare with the abovementioned example, a facile solid–gas method at a low temperature of 50 °C was proposed by Andrea et al. [55]. In this case, the temperature of the TNT arrays was not the same as that of the water vapor because they were not sealed in an enclosed space. Specifically, the as-anodized samples were fixed into a clamping system and exposed to water vapor that had been previously heated to 90 °C. No intentional heating was provided to the TNTs, and their temperature reached 50 °C (measured by a Pt100 temperature detector) resulting from contact with the heated vapor. The researchers demonstrated that a thin water layer emerges in this situation and subsequently forms part of a solid/liquid/vapor interface system where the water-assisted dissolution–precipitation occurred. After exposure for only 20 min, (101) peaks belonging to the TiO2 anatase phase were detected in the XRD results, in contrast with the prior amorphous nature. This result shows that amorphous TiO2 can transform into the anatase phase even at a low temperature of 50 °C. Considering the presence of water vapor in air at RT, we may wonder if the amorphous–anatase transformation would occur when the amorphous TiO2 is exposed to air. Su et al. prepared amorphous TiO2 and placed it in air to observe the changes [56]. As a result, the amorphous TiO2 transformed into the anatase phase within 90 days. They attributed this phenomenon to the rearrangement of TiO6 octahedra with assistance from the water from moisture. Although the crystallization period was extremely long, this result still indicates that the amorphous–anatase transformation occurs in air at RT and normal atmosphere pressure without any solvent or additive.
The water-only WAC method can effectively crystallize amorphous TNTs. The dissolution–precipitation process plays an important role in this amorphous–anatase transformation in which TiO6 octahedra are rearranged with the assistance of water.
3.2 Modified WAC Method
In addition to the water-only WAC method, much effort has been devoted to investigating other modified methods that employ aqueous solutions containing various ions. In this section, we introduce the modified WAC methods involving aqueous solutions containing metal ions and nonmetal ions [57,58,59,60].
3.2.1 Aqueous Solution Containing Metal Ions
Since the first report of TNTs, many efforts have been devoted to doping TNTs with metal ions [61,62,63,64,65,66]. Although the doped TNTs exhibit satisfying performances, the complicated procedures of generating them limit their development. Inspired by the WAC strategy, researchers used aqueous solutions containing metal ions to crystallize the amorphous TNTs and dope the metal elements into TiO2 at the same time. For example, Zhang et al. fabricated MTiO3 (M = Zn, Co, Ni) NTs by a hydrothermal treatment that immersed the amorphous TNTs into aqueous solutions containing different metal acetates [67]. Considering the participation of metal acetates, the reactions in the autoclave can be described as follows:
Here, M represents the metal ions, and Ac represents acetate (CH3COO−). As mentioned above, the amorphous–anatase transformation is highly dependent on the reaction between Ti(OH) 2−6 and H+ (Eq. 2). From Eqs. 3 and 4, however, we can see that H+ is consumed because of the presence of Ac−. Consequently, the reaction in Eq. 2 is hindered, and the combining of Ti(OH) 2−6 and M2+ occurs and is promoted. As shown in Fig. 6a, the XRD results confirm the existence of ZnTiO3 when the metal acetate is Zn(Ac)2. Actually, the employment of an aqueous solution of Zn(Ac)2 not only causes the formation of ZnTiO3 but also influences the morphology of NTs. Some NPs emerge during the dissolution–precipitation process when the water-only WAC method is used because of the reaction in Eq. 2. These NPs are usually adhered on tube walls and form a NP/TNT morphological feature. In Zhang’s work, the NP/TNT structure appeared when the concentration of Zn(Ac)2 was low (0.05 M), as shown in Fig. 6b. In contrast, the nanotubular architecture remained intact, and few NPs were observed when the concentration was increased to 0.5 M (Fig. 6c). These results confirm that the addition of M(Ac)2 hinders the combining of Ti(OH) 2−6 and H+ and, subsequently, alleviates the collapse of NTs. To further investigate the reactions and obtain deep insights, additional experiments were carried out. On the one hand, crystalline anatase TNTs, instead of the amorphous TNTs, were hydrothermally treated with 0.2 M Zn(Ac)2. As expected, ZnTiO3 was not detected in this case. Because the stability of anatase TiO2 is relatively high, the TiO6 octahedra do not absorb water molecules and form Ti(OH) 2−6 groups [68,69,70]. Therefore, the reactions in Eq. 5 are restricted, and ZnTiO3 is not obtained. On the other hand, ZnTiO3 was also not obtained when Zn(Ac)2 was replaced by ZnCl2 and the other conditions were maintained, which could be attributed to the fact that strong acid radicals would not proceed the reactions, as in Eq. 4. This result demonstrates that both the amorphous nature and weak acid radicals are essential in the preparation of MTiO3. In addition, the morphology of the anatase TNTs was nearly unchanged after hydrothermal treatment in 0.2 M Zn(Ac)2 solution (Fig. 6d). Based on the above results, a schematic illustration of the reactions in the presence of anatase or amorphous TNTs is displayed in Fig. 6e.
In contrast to the metal elements used for doping, some metal ions in the solution nucleate and crystallize simultaneously with the recrystallization of amorphous TNTs. For instance, Zhao et al. fabricated Ag-TiO2 nanocomposites by immersing amorphous TNTs in an aqueous solution containing AgNO3, glucose, and ethanol at 180 °C [71]. After 2 h, the sample contained three phases, including silver, anatase TiO2, and titanium (substrate). In this reaction, Ag+ ions diffused into the amorphous TNTs and transformed into Ag NPs. The WAC of amorphous TNTs proceeded at the same time, eventually resulting in Ag-TNT nanocomposites.
3.2.2 Aqueous Solutions Containing Nonmetal Ions
As for TNTs with nonmetal ions, we mainly discuss nitrogen-doped TNTs (N-TNTs), which have many advantages in various fields [72,73,74,75]. A variety of strategies, such as ammonolysis and ion implantation methods, have been developed to implant nitrogen into TNTs [76, 77]. Unfortunately, high temperature is usually an inevitable experimental condition for obtaining N-TNTs limiting their development for many applications. Consequently, it is important to develop a convenient low-temperature method to prepare N-TNTs. Wang et al. fabricated N-TNTs by immersing the as-anodized amorphous TNTs into an aqueous solution of ammonia at 90 °C, as shown in Fig. 7a [78]. Because of the weak alkaline environment of the solution, the reactions in Eqs. 1 and 2 were accelerated. As expected, the amorphous TNTs transformed into the anatase phase after immersion, and the crystallinity increased with increasing immersion time. As shown in Fig. 7b, X-ray photoelectron spectroscopy (XPS) was carried out to investigate the influence of ammonia. Sharp peaks for Ti, O, and C were detected in both the as-anodized and ammonia-treated samples, while the N 1s peak was also observed in the latter. The N 1s peak located at approximately 399.8 eV was assigned to interstitial nitrogen with a Ti–O–N structure, which is consistent with other studies [79, 80]. The normalized Ti 2p core-level XPS spectra of the as-anodized and ammonia-treated samples are presented in Fig. 7c. The peak of the ammonia-treated sample is clearly shifted compared with that of the as-anodized sample, indicating that their Ti ions have different bonding environments. The researchers demonstrated that this redshift could be attributed to an increase in electron cloud density on Ti owing to the presence of nitrogen. The XPS results also confirm that there were more Ti3+ ions in the ammonia-treated sample, and we can conclude that oxygen vacancies (Ti3+) emerge during the ammonia solution immersion. Moreover, the N-TNTs exhibit an elevated conductivity, as shown in Fig. 7d. The conductivity of the as-anodized TNTs was only 8.96 × 10−9 S m−1, while this value of the ammonia-treated sample reached as high as 7.42 × 10−6 S m−1. The researchers demonstrated that the variation in conductivity could be attributed to the enhanced crystallinity and oxygen vacancies introduced in the ammonia-treated products.
In addition, Cui et al. immersed the as-anodized TNTs into an aqueous solution of various concentrations of (NH4)2TiF6 (0.005, 0.01 and 0.02 M) [81]. Although water molecules were considered to be the main agents in the crystallization of TNTs because of the very low concentrations of (NH4)2TiF6, the presence of [NH4]+ and [TiF6]2− markedly influenced the products, especially their morphology. When these materials were employed in supercapacitors, the specific capacitance of the (NH4)2TiF6-treated sample was three times that of the sample without (NH4)2TiF6 treatment.
3.3 Other Methods
We have introduced the main WAC methods, and a dissolution–precipitation mechanism has been adopted to explain the amorphous-anatase transformation. However, this transformation process usually requires a relatively long crystallization time, especially for the solid–liquid method. At RT, days are needed to achieve the transformation because of the low dissolution–precipitation rate. Therefore, accelerating the WAC process becomes a challenging issue. In 2017, Aijo et al. reported a very efficient technique for RT crystallization of as-anodized TNTs, which is quite different from the methods above [82]. The most attractive feature of this method is the very fast amorphous-anatase transformation, which only requires 5 min. As shown in Fig. 8a, the preparation process comprises the following steps: (1) titanium foil is anodized in an ethylene glycol solution containing ammonium fluoride and water; (2) the sample is crystallized using a two-electrode system, where the as-anodized sample is used as the working electrode and platinum acts as the counter electrode. An alternating square voltage pulse with a pulse width of 100 ms is employed, and the electrolyte used in this step is a 1 M KCl aqueous solution. The addition of KCl improves the conductivity, and this species does not participate in the reactions on either the anode or the cathode. After a pulse treatment of only 5 min, a sharp (101) XRD peak of the anatase phase was observed, suggesting successful amorphous–anatase transformation in a short time. In addition, the nanotubular structure was well maintained after pulse treatment, and few NPs were detected, in contrast with the water immersion samples, in which many NPs adhered to tube walls. As shown in Fig. 8b, an electrophilic-nucleophilic mechanism is proposed to explain the crystallization process. In the first stage, a positive pulse (+ 5 V) is applied to the amorphous TNT electrode, making it electrophilic and leading to the accumulation of OH− ions on the surface. These OH− ions act as ‘bridges’ bonding the two adjacent Ti(OH) 2−6 octahedra together. In the second stage, a negative pulse (− 5 V) is applied to the amorphous TNT electrode, making it nucleophilic and causing the attraction of H+ ions. These H+ ions ‘attack’ the ‘bridges,’ resulting in the formation of edge-shared octahedra. With the increase in pulse treatment time, this process continues until a basic unit of anatase TiO2 is formed.
In summary, there are many ways to accomplish the WAC of amorphous TNTs. The dissolution–precipitation mechanism is appropriate for most of the methods; some other mechanisms were also proposed. Finding a green, efficient, and low-cost method for crystallizing amorphous TNTs not only promote the further investigation of TiO2 but also have great significance for the exploration of crystallography.
4 Fundamental Properties
After the WAC process, some properties of the products are quite different from those of the as-anodized and high-temperature-annealed samples. In this section, we mainly introduce fundamental properties, including the morphology, surface area, crystallinity, and bandgap, of the products prepared by WAC methods.
4.1 Morphology Evolution
It is well known that the morphology of nanomaterials is the most important factor affecting their properties. Although the 1-D structure usually does not collapse completely during the WAC, there are still many changes that cannot be ignored. In the following, these new morphological features and the corresponding influence factors are summarized.
4.1.1 NT-NP/NT-NR Evolution
Treatment time Figure 9a–d shows the scanning electron microscopy (SEM) images of the morphological evolution of a sample as a function of water immersion time. From Fig. 9a, it can be seen that the as-anodized TNTs have smooth tube walls. However, when the sample is immersed in water at RT for 30 h, some NPs with a mean diameter of approximately 10 nm appear on the tube walls forming an NP/NT structure. With increasing immersion time, more NPs emerge on both the inner side and outer side of the tube walls. It should be noted that the NP/NT structure is still maintained, because the inside of the tube is not completely filled with NPs. If we further prolong the immersion time to over 72 h, the original tubular structure is no longer visible, and only the solid nanorods (NRs) are detected (Fig. 9d). The NT-NP/NT-NR evolution can be attributed to the dissolution–precipitation, in which TiO6 octahedra from the original NTs dissolve, rearrange, and precipitate as anatase TiO2 NPs on tube walls. In Fig. 9e, a scheme illustrating the transformation from amorphous NTs to anatase NRs is presented. In addition, Wang et al. found that a double-walled NT structure forms before the formation of NPs/NTs (Fig. 9b) [40]. It is believed that the large space inside the NTs facilitates water molecules to access the inner surfaces of the tubes, while the narrow space between the adjacent NTs limits the transport of water molecules. Therefore, the dissolution–precipitation process occurring on the outer surfaces of the NTs is dramatically slow compared with that on the inner side, finally forming the double-walled structure.
This type of NT-NP/NT-NR evolution has also been discovered in most experiments using WAC methods. For example, Cao et al. immersed the as-anodized TNTs into water for different duration times (up to 7 days) and investigated the morphology transformation [83]. Figure 10 shows SEM and transmission electron microscopy (TEM) images of the samples with different immersion times. For the as-anodized TNTs (Fig. 10a), the tube wall is smooth, and no lattice fringes or diffraction rings can be detected from the high-resolution TEM and SAED images, confirming the amorphous nature. After immersion for 24 h (Fig. 10b), an NP/NT architecture appears, and the diffraction rings assigned to anatase TiO2 are observed. Within 3 days (Fig. 10c), the NPs/NTs transforms into NRs composed of NPs. Both the lattice fringes and diffraction rings are clearly observed, indicating the good crystallinity of the prepared anatase TNTs. It can be noted that a higher surface area can always be obtained after WAC treatment, resulting from the formation of pores and nanoparticles during the morphology evolution process. On the other hand, losing the tubular structure also causes some drawbacks listed as follows: (1) numerous boundaries between nanoparticles lead to a slow diffusion of electrons; (2) the structure may be not as robust as before. In short, there are advantages and disadvantages of the morphology evolution, and it can be controlled to meet different requirements.
Treatment temperature Regarding the solid–liquid method, its high temperature (> 100 °C) can easily lead to the collapse of TNT films [53]. In contrast, the TNT films are relatively robust under solid–gas treatment. Additionally, the treatment time plays a key role in the NT-NP/NT-NR transformation. Liu et al. demonstrated that high temperature accelerates the dissolution–precipitation rate and, subsequently, facilitates the formation of NP/NT structures [54]. After exposure of the sample to water vapor for 4 h at 130 °C, only a small number of NPs appeared, and the nanotubular structure was well preserved. Nevertheless, the nanotubular structure of the 180 °C sample started collapsing, and the NR structure was about to form.
NT lengths Although many researchers have studied the influences of treatment time and temperature, little attention has been paid to the effect of NT length. In 2013, Wang et al. used the solid–liquid WAC method to crystallize the amorphous TNTs and investigated the influence of NT lengths on the morphology [84]. When as-anodized TNTs with lengths of 3.5 µm were immersed in water for 45 min, only a few NPs were observed on surfaces, and the tube walls were still smooth. For the 6.5 µm samples, many more NPs were generated, and the tube walls became extremely rough forming a typical NP/NT structure. For the longest tubes (16.5 µm), solid NRs composed of NPs appeared. This result shows that the long NTs are more prone to NT-NP/NT-NR evolution than the short NTs, which may be attributed to the abundant titania source allowing the dissolution–precipitation process to occur. Furthermore, it was also found that the morphology near the tube bottom was different from that of the tube top. Specifically, the bottom NTs were filled with NPs and became NRs, while the top NTs still exhibited an NP/NT structure. The solid NR structure of the bottom part of NTs limits the attachment and transportation of dye molecules, which is the reason why the amount of dye loading does not increase dramatically with longer NTs.
pH effect From Eq. 2, we can see that the concentration of H+ ions plays a key role in the dissolution–precipitation and, subsequently, affects the formation of TiO2 NPs. Zhang et al. investigated the influence of pH on morphology transformation and found that the NP size is strongly related to the pH value [67]. In HCl solution (pH = 3), the NTs transformed into NRs composed of compact NPs with a diameter of approximately 80 nm. In contrast, the NRs formed in deionized water (pH = 6.5) were composed of NPs with a diameter of approximately 40 nm. When the pH was 11 (NaOH solution), smaller NPs of 20–30 nm in diameter were observed on both the top surface and tube walls. It can be concluded that the NP size decreases with increasing pH value; in other words, the NPs grow larger with a higher concentration of H+. This phenomenon can be explained by the fact that the presence of more H+ ions at low pH accelerates the reaction in Eq. 2, thus, forming larger anatase TiO2 NPs. This theory also applies to the condition when the amorphous TNTs are immersed in a solution containing weak acid radicals such as acetate (CH3COO−). CH3COO− can combine with H+ to restrict the reaction in Eq. 2; therefore, the formation of NPs is hindered, and the nanotubular structure is usually maintained.
Besides, there are many other factors influencing the NT-NP/NT-NR morphology evolution. For example, Huo et al. reported that the dissolved oxygen in water remarkably affects the tubular structure of TNTs. When the dissolved oxygen was eliminated by purging with N2, and then the WAC procedure was conducted while maintaining other conditions the same, the nanotubular architecture was well kept and the NT-NP/NT-NR evolution did not occur [43]. In addition, the residual fluorine on TNTs after anodization also affects the morphology transformation. The existence of fluorine accelerates the NT-NP/NT-NR evolution because the titanium fluoride compounds can create anatase TiO2 by hydrolysis [83, 85, 86] In summary, NP/NT and NR structures are commonly observed because of the dissolution–precipitation process during crystallization. There is no doubt that the participation of NPs would greatly increase the surface areas of products, which is beneficial for many applications.
4.1.2 Other Morphologies
Advanced NPs/NTs As the crystallization using the WAC method is always accompanied with the formation of particles, it is considered as a convenient way to decorate NPs on NTs, in contrast with the presynthesized NP method [87,88,89,90]. However, the generation of NPs is due to the sacrifice of NTs, which makes the tube walls become gradually thinner during the treatment. With this sacrifice, the nanotubular structure may completely collapse and transform into NRs. Therefore, it is a challenge to decorate NPs on tube walls without the destruction of NTs. Kurian et al. proposed a strategy of coating a secondary thin amorphous TiO2 layer on previously crystallized TNTs and then immersing them in water [45]. As illustrated in Fig. 11a, crystallized TNTs were first obtained by anodization and annealing and then transferred onto the fluorine-doped tin oxide (FTO) substrate. Second, a thin amorphous TiO2 layer (approximately 15 nm in thickness) was deposited on the TNT film by the atomic layer deposition (ALD) technique. Finally, the resultant sample was immersed in water for different durations. The primary crystallized anatase TNTs are quite stable when immersed in water [91]. Therefore, when the ALD-treated sample was immersed in water, the outer amorphous layer transformed into anatase TiO2 NPs through the dissolution–precipitation process, while the inner crystallized NTs remained unchanged. As shown in Fig. 11b, c, some crystalline NPs are decorated on the tube walls. Because the NTs are not destroyed at all, we define this type of NP-decorated structure as advanced NPs/NTs. The prepared products were employed in DSSCs, and they showed a better performance than the sample without water treatment (Fig. 11d). There are two reasons that explain this promotion: (1) the participation of NPs dramatically increases the surface area and is beneficial for higher dye loading; and (2) the primary crystallized NTs serve as backbones for electron transport. It is believed that this strategy can be applied to various 3-D structures to both increase the surface area and promote the dye loading amount without destroying the geometric architecture.
Nanoworms/NTs The water-only method usually causes NP/NT and NR structures to form, as mentioned above. Some novel morphological features appear when other ions are involved in the solution. For example, a nanoworm/NT structure was obtained when the as-anodized TNTs were immersed in the aqueous solution of ammonia at 90 °C [75]. Compared with the normal nanotubular structure of the as-anodized sample (Fig. 11e), many NPs appear on the top surface of the sample immersed in hot water (Fig. 11f), in line with previous reports. However, when the as-anodized TNTs were immersed in an aqueous solution of ammonia, many worm-like titania structures with a length of approximately 100 nm form on both the top surfaces and tube walls of TNTs, as shown in Fig. 11g. From the cross-sectional SEM image, it is clear that the diameter of the NTs decreases substantially after immersion. When the concentration of ammonia increased, the worm-like morphological feature was maintained and became denser. Researchers found that this type of nanoworm/NT structure exhibits a better performance for degrading methyl orange (MO) than the high-temperature-annealed sample.
4.2 Surface Area
One of the reasons why TiO2 NTs have been widely studied is their relatively high surface area [92]. For example, high surface area is beneficial for absorbing more dye molecules in DSSCs and, subsequently, it promotes efficiency [93]. Although the nanotubular structure contributes to a surface area that is considerably larger than that of bulk TiO2, there is still much room for improvement. On the one hand, much effort has been dedicated to increasing the surface area of TNTs by adjusting the NT length, diameter, and tube wall thickness. However, the average surface area of TNTs prepared by anodization and annealing processes is only approximately 30–40 m2 g−1 according to a Brunauer–Emmett–Teller (BET) analysis using N2 adsorption/desorption [25, 94, 95]. On the other hand, some modifications of the NT geometry, such as the fabrication of bamboo-type architecture and decoration of NTs with NPs, have been performed to improve the surface area [96,97,98]. Using the WAC method to decorate TiO2 NPs on TNTs is clearly more convenient and environmentally friendly than these methods. As shown in Table 1, some experimental parameters and their influences on TNTs properties, such as morphology and specific surface areas, are listed. As the morphology of the as-anodized NTs barely changes after annealing, the specific surface area of the annealed products is close to that of the as-anodized ones (approximately 30 m2 g−1). When the solid–liquid method is employed for crystallization, the specific surface area of the sample after water immersion remarkably increases to 104.76 m2 g−1, which is nearly four times that of the as-anodized sample. The emergence of TiO2 NPs makes a major contribution to the high specific surface area. Moreover, the specific surface area of a sample possessing a NR morphology (Ref. [40]) reaches as high as 203.3 m2 g−1. Although the long treatment time (72 h) plays a key role in forming a large specific surface area, the length and diameter of the original NTs are also important factors. In addition, the products prepared by the solid–gas method also exhibit a relatively high specific surface area.
4.3 Crystallinity
The amorphous TNTs can transform into the anatase phase through the dissolution–precipitation process with the assistance of water. There are two main factors influencing the crystallinity of the products: treatment duration and treatment temperature. Generally, the crystallinity is enhanced with increasing treatment time or treatment temperature; however, it should be noted that this upward trend is not unlimited. For example, a distinct (101) peak assigned to anatase TiO2 appeared after the sample was immersed in water for 2 days at RT. When the immersion time was prolonged to 4 days, the intensity of the (101) peak had almost no enhancement compared with that of the 2-day sample [47]. At the same time, the average crystalline size indeed decreased with the longer water treatment. Although the anatase phase can be obtained using the WAC method, the crystallinity is not as high as that of the annealed products. Fan et al. first crystallized amorphous TNTs by the solid–gas method, and then the prepared products were further annealed at 450 °C [53]. The intensity of the dominant (101) peak was significantly larger after annealing, indicating that the previously obtained TNTs were partially crystallized. As we expected, the structure and morphology were not clearly different after annealing. Therefore, some researchers first prepare rough NPs/NTs using the WAC method and then anneal them at high temperature; thus, the final products possess both high surface area and good crystallinity.
4.4 Bandgap
As TNTs are semiconductors, their bandgap is an important characteristic that can highly influence their properties and applications. Among the normal crystalline phases of TiO2 (anatase, rutile, brookite), anatase TiO2 is widely investigated because it has a better photocatalytic activity due to its relatively low charge carrier recombination rate [99]. In a conventional manner, the amorphous as-anodized NTs are annealed to obtain anatase TiO2 NTs, which usually possess a bandgap of approximately 3.2 eV [100,101,102]. Therefore, we wonder if the bandgap differs when the anatase TiO2 products are fabricated by the WAC method at low temperatures.
To assess the bandgap of the WAC-treated products, ultraviolet–visible (UV–Vis) absorption spectra of the samples were measured. Liao used a solid–liquid method to crystallize the amorphous TNTs, and Andrea used a solid–gas method. They found that the bandgap of the anatase TiO2 products was close to 3.2 eV (Fig. 12a) [39, 55]. Similar bandgap values indicate that the WAC-treated samples absorb similar numbers of photons as the annealed samples. However, in Andrea’s work, the TNTs prepared by the solid–gas method exhibited higher efficiency in degrading methylene blue than the annealed TNTs. As the abilities of these materials to absorb photons were very similar, this improvement could be attributed to a high surface area, as discussed above.
In addition, the bandgap changed remarkably and was no longer 3.2 eV when the amorphous TNTs were immersed in a solution containing specific ions. For example, the as-anodized TNTs were immersed in DI water and an aqueous solution of ammonia for crystallization [75]. The absorption spectra of the water-treated and the ammonia-treated samples are shown in Fig. 12b. The estimated bandgap of the water-treated sample is approximately 3.2 eV, which is in agreement with previous reports. However, all samples immersed in ammonia solution have higher absorption intensity than the water-treated samples. The bandgap decreased gradually with increasing ammonia concentration, indicating that more light was absorbed. When the ammonia: water concentration ratio was 1:1, the bandgap decreased to 2.84 eV, which could be attributed to an isolated localized state of N 2p (Fig. 12c). It is well known that the annealed anatase TNTs are only activated under UV light (wavelength < 387 nm), which is only a small fraction (4%) of the solar spectrum. Hence, the narrower bandgap (2.84 eV) leads to a broader absorption spectrum that includes visible light and, subsequently, enhances the material’s photocatalytic properties.
5 Applications
5.1 Degradation of Pollutants
Recently, various photocatalytic semiconductor materials have been widely investigated [103,104,105]. Among them, TiO2 has received increasing attention because of its excellent photocatalytic performance in degrading organic pollutants [106,107,108]. A schematic illustration of the degradation process of TiO2 is shown in Fig. 13a. First, UV irradiation promotes electrons from the valence band (VB) to the conduction band (CB), and the corresponding electrons (e−) and holes (h+) reach the TiO2-environment interfaces. In the CB, the electrons on the surface of TiO2 are easily captured by oxygen dissolved in the solution, forming O2·− species. O2·− has a high oxidizing power and, thus, plays a vital oxidative role in the degradation process [109, 110]. At the VB, a fraction of photogenerated holes can directly oxidize the pollutants adsorbed on the surface of TiO2, and the other holes can react with water molecules to form the hydroxyl radical (OH·). Because O ·−2 and OH· possess high oxidative activities, the pollutants are effectively degraded to harmless substances. The related reactions are shown as follows:
Liao et al. crystallized amorphous TNTs by immersing them in hot water, and their photocatalytic properties were investigated through the degradation of MO aqueous solution, as shown in Fig. 13b, c [39]. The results showed that the TNTs that received hot water treatment for 6 h exhibited a slightly higher performance than the annealed (450 °C) products. Because the crystallinity of the hot water-treated TNTs was not as high as that of the annealed samples, the approximately equal degradation efficiency was attributed to their elevated surface area. With increasing immersion time, the degradation efficiency remarkably increased. The efficiency of a 35-h-immersed sample was nearly four times that of an annealed sample. Hou et al. fabricated anatase TiO2 by immersing amorphous TNTs in an aqueous solution of ammonia [75]. As expected, the ammonia-treated products also presented a significant improvement over the annealed sample. Although a high surface area is an important factor that notably promotes the contact of TiO2 and MO aqueous solution, the narrower bandgap of the ammonia-treated products also plays a key role. Such bandgap allows the ammonia-treated products to absorb visible light more efficiently than the annealed sample and, consequently, contributes to their excellent degradation performance.
5.2 DSSCs
In recent years, DSSCs have attracted worldwide attention as promising candidates for next-generation photovoltaics [111,112,113]. For conventional DSSCs, the photoanode is usually a mesoporous thin film composed of randomly distributed TiO2 NPs [114]. Unfortunately, the numerous boundaries between NPs cause a slow diffusion of photogenerated electrons in this system. To circumvent this issue, a variety of 1-D structures, including NTs, nanowires, and nanofibers, have been investigated to optimize the electron transport [115,116,117]. In particular, vertically oriented TNTs have been widely studied as an alternative because of the fast transport of electrons and ions through their TiO2 layers [118,119,120]. However, the smooth tube walls often have insufficient surface area for dye adsorption and, hence, a relatively poor light harvesting ability, which limits the improvement of conversion efficiency [121, 122]. Therefore, it is important to increase the surface area of TNTs and maintain the nanotubular structure at the same time [123, 124]. Clearly, the WAC method introduced in this review is more convenient and efficient than other methods for obtaining NPs/NTs that both possess high surface area and exhibit rapid transport. In the following section, we will introduce the basic principles of DSSCs and discuss related studies employing WAC-treated products.
A schematic illustration of the configuration of DSSCs is presented in Fig. 14 [125]. Incident photons are absorbed by dye molecules adsorbed on the TiO2 NT walls, and electrons are excited from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). The excited electrons are injected into the CB of TiO2 and then travel through the NTs via diffusion toward the back contact, finally reaching the counter electrode through the circuit. Meanwhile, the oxidized dye on the surface accepts electrons from I− in the electrolyte, leading to the regeneration of the ground state of the dye and generation of I3−. Then, I3− diffuses toward the counter electrode and reduces back to I−, completing the cycle [126, 127].
A high surface area and nanotubular structure are important factors that ensure that more dye molecules can be absorbed and the recombination rate is restricted. Therefore, the products synthesized by WAC are considered suitable for DSSCs. For example, as-anodized TNTs were immersed in water for several days at RT, transferred, and then applied in DSSCs [47]. After 2 days of immersion, the amount of absorbed dye increased remarkably (by 38.9%) compared with that adsorbed by the as-anodized TNTs. Correspondingly, the DSSCs based on TNTs immersed in water for 2 days exhibited the excellent solar energy conversion efficiency (η) of 6.06%, which is a 33% improvement compared with that of a sample employing untreated TNTs. When the immersion time was prolonged to 3 days, the dye-adsorption ability of the products increased. However, the conversion efficiency of the 3-day sample decreased to 83.3% of the 2-day sample value. Similarly, Zeng et al. fabricated NP/NT products by the solid–gas method and employed them in DSSCs [44]. The samples synthesized at 180 °C showed the highest conversion efficiency (up to 8.11%), while the efficiency of the 200 °C sample was only 6.4%. This decline was attributed to the elevated recombination rate due to the relatively collapsed nanotubular architecture, which was generated using either a long immersion time or a high temperature. We can conclude that not only a high surface area but also a good architecture is beneficial for efficiency. In Table 2, some photovoltaic parameters of DSSCs based on various photoanodes are listed. Specifically, Jsc, Voc, FF, and η represent the short-circuit current density, open-circuit voltage, fill factor, and solar energy conversion efficiency, respectively. SL, SG, and MW represent the solid–liquid, solid–gas, and modified WAC methods, respectively.
5.3 Supercapacitors
It is well known that TNT supercapacitors are a type of double-layer capacitors, where a large surface area corresponds to an excellent areal specific capacitance [128,129,130,131]. Therefore, enhancing the NT surface area is considered to be an effective approach to improve the capacitance, and numerous research efforts have been dedicated to it. Hybrid NPs/NTs possessing high surface areas prepared by the WAC method are clearly promising candidates for high-performance capacitors.
Fan et al. first used the hydrothermal solid–gas (HSG) method to crystallize the as-anodized TNTs and adjust their morphology; the HSG-treated sample was then annealed to yield fully crystallized products [53]. As shown in Fig. 15a, b, the as-anodized products had smooth tube walls, while the final HSG-treated products possessed a NP/NT structure, leading to a high surface area. For the supercapacitor performance, all the HSG-treated samples exhibit larger integrated areas and higher current responses than the direct-annealed TNTs without HSG treatment, indicating a significant enhancement of capacitance by HSG treatment (Fig. 15c). From Fig. 15d, the calculated areal capacitance of the HSG-180 sample (180 μL water in a 50 mL Teflon liner) was highest, up to 41.04 mF cm−2, which is 2.96 times that of the direct-annealed TNTs without HSG treatment. The surface area of the HSG-180 sample is 3.16 times that of the direct-annealed TNTs without HSG treatment. The high similarity of these values suggests that the improvement in capacitance could be fully attributed to the enlarged surface area. Furthermore, when the HSG-treated sample was annealed in argon atmosphere rather than air, the areal capacitance of the HSG-180 sample further increased to 50.39 mF cm−2. It is believed that the oxygen vacancies (Ti3+) formed in argon atmosphere lead to superior electrical conductivity and thereby promote areal capacitance. In addition to using deionized water, immersing the as-anodized TNTs in the aqueous solution containing some ions is also an appealing approach. In Cui’s work, the specific capacitance of the products prepared through immersion in (NH4)2TiF6 solution was 2 times higher than that of a sample treated in deionized water under the same conditions [81].
In summary, WAC-treated TNTs are suitable for many applications owing to their advantages of high surface area and structures that can be easily controlled by adjusting experimental parameters, such as immersion time and treatment temperature. In addition to the applications mentioned above, TNTs are also widely used in water splitting cells, gas sensors, biomedical coatings, drug delivery, etc. [132,133,134,135,136,137]. Thus, we foresee the WAC strategy to be employed in these fields in the future.
6 Other Materials
In the above sections, we have detailed the WAC strategy mainly as it regards TiO2 NTs. The WAC method can be used for many other materials that can be classified as TiO2-based materials and other metal oxides.
6.1 TiO2-Based Materials
In addition to TiO2 NTs, there are a variety of nanostructured TiO2 materials, such as TiO2 nanospheres (TNSs) and nanofibers (TNFs). Different from the TNTs usually prepared from Ti substrate by anodization, TNSs and TNFs can be synthesized by various methods, including sol–gel, solvothermal, hydrothermal, and electrospinning methods [138,139,140,141,142]. Despite the large differences in synthetic methods and morphologies, the WAC method is also considered suitable for crystallization and controlling the morphology of TNSs and TNFs.
TNSs As shown in Fig. 16a, Li et al. used the WAC method to fabricate porous anatase TNSs based on amorphous nanospheres prepared by the sol–gel technique [143]. Figure 16b shows a TEM image of pristine nanospheres just after the sol–gel process. The as-prepared nanospheres were sticky and tended to aggregate because of the presence of oligomers formed during hydrolysis. After the WAC, the original nanospheres became quite porous with the emergence of numerous NPs (Fig. 16c). When the water treatment time was prolonged, the nanospheres became more porous, and the aggregation tendency was notably relieved. In this transformation process, water mainly plays the following roles: (1) water dissolves the oligomers on the surface of as-prepared nanospheres restricting aggregation, and (2) water assists dissolution–precipitation, which eventually causes the formation of anatase grains, which are responsible for the pores of the finally obtained TNSs. The crystallinity and surface area of the TNSs were controlled by adjusting either the water treatment time or temperature. With increasing water treatment time, the crystallinity enhanced (Fig. 16d). In particular, crystalline TNSs were highly porous, reaching a surface area of as high as 647 m2 g−1 under appropriate conditions (treatment temperature: 75 °C), which is remarkably higher than the values reported in previous reports (Fig. 16e). Because of their high surface area and good crystallinity, the porous TNSs show better performance in phosphoprotein enrichment than commercial anatase products.
Joo et al. reported that porous, hollow TNSs can also be synthesized employing the WAC strategy [144]. As shown in Fig. 16f, the synthetic procedure was as follows: (1) a SiO2@TiO2 core–shell structure was first prepared by a sol–gel technique; (2) the as-prepared SiO2@TiO2 nanospheres were crystallized in water, resulting in a porous and crystalline TiO2 shell; and (3) the SiO2 core was removed, resulting in the successful fabrication of porous hollow TNSs. The corresponding morphology transformation is presented in Fig. 16g, h; it shows that porous hollow nanospheres were successfully prepared. Moreover, XRD results confirmed the transition from an amorphous form to the anatase phase after water reflux treatment. In addition, many other relevant studies have also been reported, and the WAC method is considered to be cost-effective and convenient in preparing porous TiO2 nanospheres [145,146,147,148].
TNFs TNFs, as one of the most important 1-D nanomaterials, have received extensive attention, and various methods have been developed for synthesizing them [149, 150]. Among the methods, the electrospinning technique is considered to be very promising owing to its versatility and flexibility [151,152,153]. Although TNFs have been investigated systematically in many ways, there are few reports about the effects of water on electrospinning TNFs. As water plays an important role in adjusting the properties of TNTs and TNSs, it can be speculated that electrospinning TNFs would also be affected by the presence of water.
In 2017, Jin et al. fabricated porous and crystalline TNFs by a simple water steam treatment without any template agents [154]. Similar to the previous WAC results, the morphology of the nanofibers greatly changed after water steam treatment and was highly dependent on treatment temperature. SEM images of the TNFs under water steam treatment at various temperatures are presented in Fig. 17. When the precursor nanofibers were treated at 150 °C for 2 h, the surface of the nanofibers became rough, and some pores emerged (Fig. 17a, b). With increasing temperature, the surface became rougher, and the diameter of the pores increased (Fig. 17c–f). Specifically, the average pore sizes of the 150, 350, and 550 °C samples were 3.63, 8.84, and 13.95 nm, respectively. After steam treatment, the original nanofibers became crystalline, and the crystallinity of the obtained TNFs increased with the temperature elevation (Fig. 17g), similar to the behavior of the TNTs mentioned above. Figure 17h, i presents TEM results of a sample prepared at 550 °C; they show that the rough nanofibers were composed of anatase NPs with a diameter of 30 nm. Because the vapor-treated nanofibers were composed of NPs, the porous TNFs exhibited higher surface areas than the annealing sample (Fig. 17j). The highest surface area of the water steam-treated TNFs was 128.07 m2 g−1, while that of the TNFs that were annealed in air was only 9.85 m2 g−1. This large difference can be attributed to the dissolution–precipitation process under water steam. Sun et al. also prepared TiO2 nanoflowers through the dissolution–precipitation mechanism [155]. It can be concluded that the WAC strategy is applicable to TiO2-based materials.
6.2 Other Metal Oxides
The as-anodized amorphous TNTs can be transformed into the anatase phase with water treatment through the dissolution–precipitation process, in which the TiO6 octahedra rearrange and form the unit cell of anatase TiO2. Although this type of WAC mechanism can be extended to other TiO2 nanomaterials, their basic building blocks are still TiO6 octahedra. Therefore, we wondered whether the WAC mechanism is applicable to metal oxides that contain no TiO6 octahedra.
Nanostructured tin oxides (SnO2) synthesized by anodizing tin foils have been widely investigated in various areas [156,157,158]. Similar to TiO2, the as-anodized SnO2 nanomaterials are amorphous and not suitable for many applications, such as gas sensing and energy storage [159, 160]. Because the melting point of the Sn substrate is approximately 230 °C, using the conventional annealing method (500 °C) to crystallize the as-anodized SnO2 is impracticable. Some researchers focused on the interesting amorphous–anatase transformation of the as-anodized TiO2 NTs and developed a convenient way to obtain crystalline SnO2 at low temperatures. In 2017, Bian et al. used the WAC method to crystallize as-anodized SnO2 for the first time [161]. To be specific, the anodization of tin was first carried out with tin foil as the anode in an oxalic acid aqueous solution. Then, the amorphous as-anodized SnO2 was immersed in deionized water at various temperatures (25, 40, 80, or 100 °C) for different durations. Although no clear change was observed when the as-anodized sample was immersed at 25 °C for the short time of 2 h (Fig. 18a, b), prolonging the immersion time to 168 h effectively crystallized the amorphous sample. In addition, the higher treatment temperature accelerated the crystallization. Both digital photographs and SEM images of the products prepared at 60 °C are quite different from those of the as-anodized and RT-treated samples. As the building blocks of SnO2 (amorphous and rutile) are SnO6 octahedra, similar to TiO6 octahedra in TiO2, the Bian group proposed a mechanism explaining that the transformation from amorphous to rutile SnO2 was assisted by water soaking (Fig. 18c). This mechanism also includes a hydration-dehydration process similar to that of TiO2, as discussed in Fig. 3c. As shown in Fig. 18e, TEM and SAED images reveal the porous structure and amorphous nature of the as-anodized SnO2. After the samples were soaked in water at 60 °C for 2 h, clear and strong diffraction rings assigned to rutile SnO2 appear (Fig. 18f), indicating successful amorphous–crystalline transformation with the assistance of water. The WAC-treated SnO2 products were applied in sodium-ion storage and delivered excellent performance, as shown in Fig. 18g.
In addition, the WAC method was also proven to be applicable to the as-anodized iron oxides in our previous work [162]. The facile WAC method is appropriate for the crystallization of not only TiO2 NTs but also many other metal oxides at low temperatures.
7 Conclusion and Outlook
The WAC strategy has been extensively studied and it is considered to be an efficient and convenient approach to crystallizing amorphous TiO2 NTs at low temperatures. In this review, we have summarized various aspects of the recent progress in using this strategy. The basic WAC method is simply immersing as-anodized TNTs in water, during which the disordered TiO6 octahedra are rearranged with the assistance of water. Although many modified methods have been developed, water molecules still play the most important role in causing the amorphous-anatase transformation. In addition, the WAC strategy can also be employed for doping metal or nonmetal elements into TNTs by using solutions containing different ions. Because of the dissolution–precipitation process, some crystalline TiO2 NPs emerge on the tube walls, leaving a rough surface feature, which is quite different from the tube walls of the annealed products. Because of the numerous NPs, the WAC-treated products usually possess a high surface area. Furthermore, we can control the morphology of the samples by adjusting experimental parameters, such as the immersion time and treatment temperature. Combining the merits of 1-D architecture and high surface area, the WAC-treated products show excellent performance in many applications, including photocatalysis, DSSCs, and supercapacitors. It should be noted that the WAC mechanism is not only applicable to amorphous as-anodized TNTs; it was proven that many other TiO2 nanomaterials prepared by various techniques can also be crystallized through this type of WAC method.
Despite many clear advantages of the WAC strategy for crystallization, there are still some drawbacks, such as the relatively long crystallization time and low degree of crystallinity. Therefore, optimizing the WAC method to overcome these challenges requires further study. Although we have shown that the WAC strategy is also applicable to as-anodized tin or iron oxides, there are still many metal oxides that have not been investigated in this sense. Hence, another topic that must be further studied is the expansion of the WAC method to other metal oxides. We hope that this review will motivate the development of WAC strategies in more fields and inspire researchers in low-temperature crystallization.
References
K. Nakata, A. Fujishima, TiO2 photocatalysis: design and applications. J. Photochem. Photobiol. C 13(3), 169–189 (2012). https://doi.org/10.1016/j.jphotochemrev.2012.06.001
S.G. Kumar, L.G. Devi, Review on modified TiO2 photocatalysis under UV/Visible light: selected results and related mechanisms on interfacial charge carrier transfer dynamics. J. Phys. Chem. A 115(46), 13211–13241 (2011). https://doi.org/10.1021/jp204364a
W.T. Sun, Y. Yu, H.Y. Pan, X.F. Gao, Q. Chen, L.M. Peng, CdS quantum dots sensitized TiO2 nanotube-array photoelectrodes. J. Am. Chem. Soc. 130(4), 1124–1125 (2008). https://doi.org/10.1021/ja0777741
K. Zhu, T.B. Vinzant, N.R. Neale, A.J. Frank, Removing structural disorder from oriented TiO2 nanotube arrays: reducing the dimensionality of transport and recombination in dye-sensitized solar cells. Nano Lett. 7(12), 3739–3746 (2007). https://doi.org/10.1021/nl072145a
B.L. He, B. Dong, H.L. Li, Preparation and electrochemical properties of Ag-modified TiO2 nanotube anode material for lithium-ion battery. Electrochem. Commun. 9(3), 425–430 (2007). https://doi.org/10.1016/j.elecom.2006.10.008
H. Liu, W. Li, D. Shen, D. Zhao, G. Wang, Graphitic carbon conformal coating of mesoporous TiO2 hollow spheres for high-performance lithium ion battery anodes. J. Am. Chem. Soc. 137(40), 13161–13166 (2015). https://doi.org/10.1021/jacs.5b08743
X. Lu, G. Wang, T. Zhai, M. Yu, J. Gan, Y. Tong, Y. Li, Hydrogenated TiO2 nanotube arrays for supercapacitors. Nano Lett. 12(3), 1690–1696 (2012). https://doi.org/10.1021/nl300173j
H. Wu, D. Li, X. Zhu, C. Yang, D. Liu, X. Chen, Y. Song, L. Lu, High-performance and renewable supercapacitors based on TiO2 nanotube array electrodes treated by an electrochemical doping approach. Electrochim. Acta 116, 129–136 (2014). https://doi.org/10.1016/j.electacta.2013.10.092
X. Wang, D. Zhang, J. Li, Z. Zhong, L. Jia, T. Wen, H. Zhang, Y. Liao, A novel sol-gel method for preparing favorable TiO2 thin film. Mater. Res. Express 3(1), 016401 (2016). https://doi.org/10.1088/2053-1591/3/1/016401
H. Yu, S. Zhang, H. Zhao, G. Will, P. Liu, An efficient and low-cost TiO2 compact layer for performance improvement of dye-sensitized solar cells. Electrochim. Acta 54(4), 1319–1324 (2009). https://doi.org/10.1016/j.electacta.2008.09.025
G.K. Mor, O.K. Varghese, M. Paulose, K. Shankar, C.A. Grimes, A review on highly ordered, vertically oriented TiO2 nanotube arrays: fabrication, material properties, and solar energy applications. Sol. Energy Mater. Sol. Cells 90(14), 2011–2075 (2006). https://doi.org/10.1016/j.solmat.2006.04.007
B. Karunagaran, P. Uthirakumar, S.J. Chung, S. Velumani, E.K. Suh, TiO2 thin film gas sensor for monitoring ammonia. Mater. Charact. 58(8), 680–684 (2007). https://doi.org/10.1016/j.matchar.2006.11.007
J. Moon, J.A. Park, S.J. Lee, T. Zyung, I.D. Kim, Pd-doped TiO2 nanofiber networks for gas sensor applications. Sens. Actuator B-Chem. 149(1), 301–305 (2010). https://doi.org/10.1016/j.snb.2010.06.033
M. Ni, M.K.H. Leung, D.Y.C. Leung, K. Sumathy, A review and recent developments in photocatalytic water-splitting using TiO2 for hydrogen production. Renew. Sustain. Energy Rev. 11(3), 401–425 (2007). https://doi.org/10.1016/j.rser.2005.01.009
B.X. Lei, J.Y. Liao, R. Zhang, J. Wang, C.Y. Su, D.B. Kuang, Ordered crystalline TiO2 nanotube arrays on transparent FTO glass for efficient dye-sensitized solar cells. J. Phys. Chem. C 114(35), 15228–15233 (2010). https://doi.org/10.1021/jp105780v
H. Xiong, M.D. Slater, M. Balasubramanian, C.S. Johnson, T. Rajh, Amorphous TiO2 nanotube anode for rechargeable sodium ion batteries. J. Phys. Chem. Lett. 2(20), 2560–2565 (2011). https://doi.org/10.1021/jz2012066
Z. Zhang, L. Zhang, M.N. Hedhili, H. Zhang, P. Wang, Plasmonic Gold nanocrystals coupled with photonic crystal seamlessly on tio2 nanotube photoelectrodes for efficient visible light photoelectrochemical water splitting. Nano Lett. 13(1), 14–20 (2013). https://doi.org/10.1021/nl3029202
J. Wang, Z. Lin, Freestanding TiO2 nanotube arrays with ultrahigh aspect ratio via electrochemical anodization. Chem. Mater. 20(4), 1257–1261 (2008). https://doi.org/10.1021/cm7028917
J. Qiu, Z. Jin, Z. Liu, X. Liu, G. Liu, W. Wu, X. Zhang, X. Gao, Fabrication of TiO2 nanotube film by well-aligned ZnO nanorod array film and sol-gel process. Thin Solid Films 515(5), 2897–2902 (2007). https://doi.org/10.1016/j.tsf.2006.08.023
S. Xu, J. Ng, X. Zhang, H. Bai, D.D. Sun, Adsorption and photocatalytic degradation of Acid Orange 7 over hydrothermally synthesized mesoporous TiO2 nanotube. Colloid Surf. A-Physicochem. Eng. 379(1), 169–175 (2011). https://doi.org/10.1016/j.colsurfa.2010.11.032
M. Paulose, K. Shankar, S. Yoriya, H.E. Prakasam, O.K. Varghese, G.K. Mor, T.A. Latempa, A. Fitzgerald, C.A. Grimes, Anodic growth of highly ordered TiO2 nanotube arrays to 134 μm in length. J. Phys. Chem. B 110(33), 16179–16184 (2006). https://doi.org/10.1021/jp064020k
N. Liu, X. Chen, J. Zhang, J.W. Schwank, A review on TiO2-based nanotubes synthesized via hydrothermal method: formation mechanism, structure modification, and photocatalytic applications. Catal. Today 225, 34–51 (2014). https://doi.org/10.1016/j.cattod.2013.10.090
D. Kuang, J. Brillet, P. Chen, M. Takata, S. Uchida, H. Miura, K. Sumioka, S.M. Zakeeruddin, M. Grätzel, Application of highly ordered TiO2 nanotube arrays in flexible dye-sensitized solar cells. ACS Nano 2(6), 1113–1116 (2008). https://doi.org/10.1021/nn800174y
Z. Liu, M. Misra, Dye-sensitized photovoltaic wires using highly ordered TiO2 nanotube arrays. ACS Nano 4(4), 2196–2200 (2010). https://doi.org/10.1021/nn9015696
M. Paulose, H.E. Prakasam, O.K. Varghese, L. Peng, K.C. Popat, G.K. Mor, T.A. Desai, C.A. Grimes, TiO2 nanotube arrays of 1000 μm length by anodization of titanium foil: phenol red diffusion. J. Phys. Chem. C 111(41), 14992–14997 (2007). https://doi.org/10.1021/jp075258r
D. Regonini, C.R. Bowen, A. Jaroenworaluck, R. Stevens, A review of growth mechanism, structure and crystallinity of anodized TiO2 nanotubes. Mater. Sci. Eng. R 74(12), 377–406 (2013). https://doi.org/10.1016/j.mser.2013.10.001
C.C. Chen, H.W. Chung, C.H. Chen, H.P. Lu, C.M. Lan, S.F. Chen, L. Luo, C.S. Hung, E.W.G. Diau, Fabrication and characterization of anodic titanium oxide nanotube arrays of controlled length for highly efficient dye-sensitized solar cells. J. Phys. Chem. C 112(48), 19151–19157 (2008). https://doi.org/10.1021/jp806281r
K. Zhu, N.R. Neale, A. Miedaner, A.J. Frank, Enhanced charge-collection efficiencies and light scattering in dye-sensitized solar cells using oriented TiO2 nanotubes arrays. Nano Lett. 7(1), 69–74 (2007). https://doi.org/10.1021/nl062000o
D. Wang, B. Yu, C. Wang, F. Zhou, W. Liu, A novel protocol toward perfect alignment of anodized TiO2 nanotubes. Adv. Mater. 21(19), 1964–1967 (2009). https://doi.org/10.1002/adma.200801996
J. Yu, B. Wang, Effect of calcination temperature on morphology and photoelectrochemical properties of anodized titanium dioxide nanotube arrays. Appl. Catal. B-Environ. 94(3), 295–302 (2010). https://doi.org/10.1016/j.apcatb.2009.12.003
S. Sreekantan, R. Hazan, Z. Lockman, Photoactivity of anatase-rutile TiO2 nanotubes formed by anodization method. Thin Solid Films 518(1), 16–21 (2009). https://doi.org/10.1016/j.tsf.2009.06.002
J.M. Macak, M. Zlamal, J. Krysa, P. Schmuki, Self-organized TiO2 nanotube layers as highly efficient photocatalysts. Small 3(2), 300–304 (2007). https://doi.org/10.1002/smll.200600426
N.K. Allam, K. Shankar, C.A. Grimes, A general method for the anodic formation of crystalline metal oxide nanotube arrays without the use of thermal annealing. Adv. Mater. 20(20), 3942–3946 (2008). https://doi.org/10.1002/adma.200800815
J.H. Yang, Y.S. Han, J.H. Choy, TiO2 thin-films on polymer substrates and their photocatalytic activity. Thin Solid Films 495(1), 266–271 (2006). https://doi.org/10.1016/j.tsf.2005.08.195
K. Lee, A. Mazare, P. Schmuki, One-dimensional titanium dioxide nanomaterials: nanotubes. Chem. Rev. 114(19), 9385–9454 (2014). https://doi.org/10.1021/cr500061m
P. Roy, S. Berger, P. Schmuki, TiO2 nanotubes: synthesis and applications. Angew. Chem. Int. Ed. 50(13), 2904–2939 (2011). https://doi.org/10.1002/anie.201001374
Z. Su, L. Zhang, F. Jiang, M. Hong, Formation of crystalline TiO2 by anodic oxidation of titanium. Prog. Nat. Sci. 23(3), 294–301 (2013). https://doi.org/10.1016/j.pnsc.2013.04.004
S. Ali, S.P. Hannula, Titania nanotube powders obtained by rapid breakdown anodization in perchloric acid electrolytes. J. Solid State Chem. 249, 189–198 (2017). https://doi.org/10.1016/j.jssc.2017.03.007
Y. Liao, W. Que, P. Zhong, J. Zhang, Y. He, A facile method to crystallize amorphous anodized TiO2 nanotubes at low temperature. ACS Appl. Mater. Interfaces 3(7), 2800–2804 (2011). https://doi.org/10.1021/am200685s
D. Wang, L. Liu, F. Zhang, K. Tao, E. Pippel, K. Domen, Spontaneous phase and morphology transformations of anodized titania nanotubes induced by water at room temperature. Nano Lett. 11(9), 3649–3655 (2011). https://doi.org/10.1021/nl2015262
N. Liu, S.P. Albu, K. Lee, S. So, P. Schmuki, Water annealing and other low temperature treatments of anodic TiO2 nanotubes: a comparison of properties and efficiencies in dye sensitized solar cells and for water splitting. Electrochim. Acta 82, 98–102 (2012). https://doi.org/10.1016/j.electacta.2012.06.006
B.M. Rao, S.C. Roy, Water assisted crystallization, gas sensing and photo-electrochemical properties of electrochemically synthesized TiO2 nanotube arrays. RSC Adv. 4(90), 49108–49114 (2014). https://doi.org/10.1039/C4RA06842D
H. Kaifu, W. Hairong, Z. Xuming, C. Yue, P.K. Chu, Heterostructured TiO2 nanoparticles/nanotube arrays: in situ formation from amorphous TiO2 nanotube arrays in water and enhanced photocatalytic activity. ChemPlusChem 77(4), 323–329 (2012). https://doi.org/10.1002/cplu.201200024
T. Zeng, H. Ni, X. Su, Y. Chen, Y. Jiang, Highly crystalline Titania nanotube arrays realized by hydrothermal vapor route and used as front-illuminated photoanode in dye sensitized solar cells. J. Power Sources 283, 443–451 (2015). https://doi.org/10.1016/j.jpowsour.2015.02.150
S. Kurian, P. Sudhagar, J. Lee, D. Song, W. Cho, S. Lee, Y.S. Kang, H. Jeon, Formation of a crystalline nanotube-nanoparticle hybrid by post water-treatment of a thin amorphous TiO2 layer on a TiO2 nanotube array as an efficient photoanode in dye-sensitized solar cells. J. Mater. Chem. A 1(13), 4370–4375 (2013). https://doi.org/10.1039/c3ta01339a
K. Assaker, C. Carteret, B. Lebeau, C. Marichal, L. Vidal, M.-J. Stébé, J.-L. Blin, Water-catalyzed low-temperature transformation from amorphous to semi-crystalline phase of ordered mesoporous titania framework. ACS Sustain. Chem. Eng. 2(2), 120–125 (2014). https://doi.org/10.1021/sc400323w
J. Lin, X. Liu, M. Guo, W. Lu, G. Zhang, L. Zhou, X. Chen, H. Huang, A facile route to fabricate an anodic TiO2 nanotube-nanoparticle hybrid structure for high efficiency dye-sensitized solar cells. Nanoscale 4(16), 5148–5153 (2012). https://doi.org/10.1039/c2nr31268a
W. Krengvirat, S. Sreekantan, A.F. Mohd Noor, N. Negishi, G. Kawamura, H. Muto, A. Matsuda, Low-temperature crystallization of TiO2 nanotube arrays via hot water treatment and their photocatalytic properties under visible-light irradiation. Mater. Chem. Phys. 137(3), 991–998 (2013). https://doi.org/10.1016/j.matchemphys.2012.11.013
X. Keyu, G. Min, L. Wei, H. Haitao, Aligned TiO2 nanotube/nanoparticle heterostructures with enhanced electrochemical performance as three-dimensional anode for lithium-ion microbatteries. Nanotechnology 25(45), 455401 (2014). https://doi.org/10.1088/0957-4484/25/45/455401
T.C. Hufnagel, Finding order in disorder. Nat. Mater. 3, 666 (2004). https://doi.org/10.1038/nmat1227
M.P. Finnegan, H. Zhang, J.F. Banfield, Phase stability and transformation in titania nanoparticles in aqueous solutions dominated by surface energy. J. Phys. Chem. C 111(5), 1962–1968 (2007). https://doi.org/10.1021/jp063822c
Y. Liao, X. Wang, Y. Ma, J. Li, T. Wen, L. Jia, Z. Zhong, L. Wang, D. Zhang, New mechanistic insight of low temperature crystallization of anodic TiO2 nanotube array in water. Cryst. Growth Des. 16(4), 1786–1791 (2016). https://doi.org/10.1021/acs.cgd.5b01234
H. Fan, H. Zhang, X. Luo, M. Liao, X. Zhu, J. Ma, Y. Song, Hydrothermal solid-gas route to TiO2 nanoparticles/nanotube arrays for high-performance supercapacitors. J. Power Sources 357, 230–240 (2017). https://doi.org/10.1016/j.jpowsour.2017.05.009
J. Liu, Z. Liu, T. Zhang, J. Zhai, L. Jiang, Low-temperature crystallization of anodized TiO2 nanotubes at the solid-gas interface and their photoelectrochemical properties. Nanoscale 5(13), 6139–6144 (2013). https://doi.org/10.1039/c3nr01286g
A. Lamberti, A. Chiodoni, N. Shahzad, S. Bianco, M. Quaglio, C.F. Pirri, Ultrafast room-temperature crystallization of TiO2 nanotubes exploiting water-vapor treatment. Sci. Rep. 5, 7808 (2015). https://doi.org/10.1038/srep07808
J. Su, X. Zou, G.D. Li, Y.M. Jiang, Y. Cao, J. Zhao, J.S. Chen, Room-temperature spontaneous crystallization of porous amorphous titania into a high-surface-area anatase photocatalyst. Chem. Commun. 49(74), 8217–8219 (2013). https://doi.org/10.1039/c3cc43772h
C. Zhao, D. Zhu, S. Cao, Amorphous TiO2 nanotube-derived synthesis of highly ordered anatase TiO2 nanorod arrays. Superlattices Microstruct. 90, 257–264 (2016). https://doi.org/10.1016/j.spmi.2015.12.037
Y. Cai, Y. Ye, S. Wu, J. Liu, C. Liang, Simultaneous Cu doping and growth of TiO2 nanocrystalline array film as a glucose biosensor. RSC Adv. 6(81), 78219–78224 (2016). https://doi.org/10.1039/C6RA15014D
Y. Liao, J. Brame, W. Que, Z. Xiu, H. Xie, Q. Li, M. Fabian, P.J. Alvarez, Photocatalytic generation of multiple ROS types using low-temperature crystallized anodic TiO2 nanotube arrays. J. Hazard. Mater. 260, 434–441 (2013). https://doi.org/10.1016/j.jhazmat.2013.05.047
Z. Li, Y. Chen, J. Shen, X. Cui, Facile synthesis of a heterogeneous Li2TiO3/TiO2 nanocomposite with enhanced photoelectrochemical water splitting. New J. Chem. 41(14), 6305–6314 (2017). https://doi.org/10.1039/C7NJ00198C
K. Huo, X. Zhang, H. Wang, L. Zhao, X. Liu, P.K. Chu, Osteogenic activity and antibacterial effects on titanium surfaces modified with Zn-incorporated nanotube arrays. Biomaterials 34(13), 3467–3478 (2013). https://doi.org/10.1016/j.biomaterials.2013.01.071
Y. Xin, J. Jiang, K. Huo, T. Hu, P.K. Chu, Bioactive SrTiO3 nanotube arrays: strontium delivery platform on ti-based osteoporotic bone implants. ACS Nano 3(10), 3228–3234 (2009). https://doi.org/10.1021/nn9007675
H.J. Lin, T.S. Yang, M.C. Wang, C.S. Hsi, Structural and photodegradation behaviors of Fe3+-doping TiO2 thin films prepared by a sol-gel spin coating. J. Alloys Compd. 610, 478–485 (2014). https://doi.org/10.1016/j.jallcom.2014.05.053
X. Li, X. Zou, Z. Qu, Q. Zhao, L. Wang, Photocatalytic degradation of gaseous toluene over Ag-doping TiO2 nanotube powder prepared by anodization coupled with impregnation method. Chemosphere 83(5), 674–679 (2011). https://doi.org/10.1016/j.chemosphere.2011.02.043
Y.F. Tu, S.Y. Huang, J.P. Sang, X.W. Zou, Preparation of Fe-doped TiO2 nanotube arrays and their photocatalytic activities under visible light. Mater. Res. Bull. 45(2), 224–229 (2010). https://doi.org/10.1016/j.materresbull.2009.08.020
H.A. Hamedani, N.K. Allam, H. Garmestani, M.A. El-Sayed, Electrochemical fabrication of strontium-doped TiO2 nanotube array electrodes and investigation of their photoelectrochemical properties. J. Phys. Chem. C 115(27), 13480–13486 (2011). https://doi.org/10.1021/jp201194b
X. Zhang, B. Gao, L. Hu, L. Li, W. Jin, K. Huo, P.K. Chu, Hydrothermal synthesis of perovskite-type MTiO3 (M = Zn Co, Ni)/TiO2 nanotube arrays from an amorphous TiO2 template. CrystEngComm 16(44), 10280–10285 (2014). https://doi.org/10.1039/C4CE00992D
A.L. Castro, M.R. Nunes, A.P. Carvalho, F.M. Costa, M.H. Florêncio, Synthesis of anatase TiO2 nanoparticles with high temperature stability and photocatalytic activity. Solid State Sci. 10(5), 602–606 (2008). https://doi.org/10.1016/j.solidstatesciences.2007.10.012
Z. Wei, S. Fanfei, P. Kai, T. Guohui, J. Baojiang, R. Zhiyu, T. Chungui, F. Honggang, Well-ordered large-pore mesoporous anatase TiO2 with remarkably high thermal stability and improved crystallinity: preparation, characterization, and photocatalytic performance. Adv. Funct. Mater. 21(10), 1922–1930 (2011). https://doi.org/10.1002/adfm.201002535
J. Ye, W. Liu, J. Cai, S. Chen, X. Zhao, H. Zhou, L. Qi, Nanoporous anatase TiO2 mesocrystals: additive-free synthesis, remarkable crystalline-phase stability, and improved lithium insertion behavior. J. Am. Chem. Soc. 133(4), 933–940 (2011). https://doi.org/10.1021/ja108205q
C. Zhao, D.C. Zhu, X.Y. Cheng, S.X. Cao, Highly ordered Ag-TiO2 nanocomposited arrays with high visible-light photocatalytic activity. Front. Mater. Sci. 11(3), 241–249 (2017). https://doi.org/10.1007/s11706-017-0386-8
Y.K. Lai, J.Y. Huang, H.F. Zhang, V.P. Subramaniam, Y.X. Tang et al., Nitrogen-doped TiO2 nanotube array films with enhanced photocatalytic activity under various light sources. J. Hazard. Mater. 184(1), 855–863 (2010). https://doi.org/10.1016/j.jhazmat.2010.08.121
M. Sathish, B. Viswanathan, R.P. Viswanath, C.S. Gopinath, Synthesis, characterization, electronic structure, and photocatalytic activity of nitrogen-doped TiO2 nanocatalyst. Chem. Mater. 17(25), 6349–6353 (2005). https://doi.org/10.1021/cm052047v
H. Tokudome, M. Miyauchi, N-doped TiO2 nanotube with visible light activity. Chem. Lett. 33(9), 1108–1109 (2004). https://doi.org/10.1246/cl.2004.1108
X. Hou, C.W. Wang, W.D. Zhu, X.Q. Wang, Y. Li et al., Preparation of nitrogen-doped anatase TiO2 nanoworm/nanotube hierarchical structures and its photocatalytic effect. Solid State Sci. 29, 27–33 (2014). https://doi.org/10.1016/j.solidstatesciences.2014.01.007
S. Hoang, S.P. Berglund, N.T. Hahn, A.J. Bard, C.B. Mullins, Enhancing visible light photo-oxidation of water with tio2 nanowire arrays via cotreatment with H2 and NH3: synergistic effects between Ti3+ and N. J. Am. Chem. Soc. 134(8), 3659–3662 (2012). https://doi.org/10.1021/ja211369s
R.P. Antony, T. Mathews, K. Panda, B. Sundaravel, S. Dash, A.K. Tyagi, Enhanced field emission properties of electrochemically synthesized self-aligned nitrogen-doped TiO2 nanotube array thin films. J. Phys. Chem. C 116(31), 16740–16746 (2012). https://doi.org/10.1021/jp302578b
C.W. Wang, W.D. Zhu, J.B. Chen, X. Hou, X.Q. Zhang, Y. Li, J. Wang, F. Zhou, Low-temperature ammonia annealed TiO2 nanotube arrays: synergy of morphology improvement and nitrogen doping for enhanced field emission. Thin Solid Films 556, 440–446 (2014). https://doi.org/10.1016/j.tsf.2014.01.066
W. Chenglin, W. Mengye, X. Kunpeng, W. Qi, S. Lan, L. Zhiqun, L. Changjian, Room temperature one-step synthesis of microarrays of N-doped flower-like anatase TiO2 composed of well-defined multilayer nanoflakes by Ti anodization. Nanotechnology 22(30), 305607 (2011). https://doi.org/10.1088/0957-4484/22/30/305607
S. Karthik, T. Kong Chhay, K.M. Gopal, A.G. Craig, An electrochemical strategy to incorporate nitrogen in nanostructured TiO2 thin films: modification of bandgap and photoelectrochemical properties. J. Phys. D-Appl. Phys. 39(11), 2361 (2006). https://doi.org/10.1088/0022-3727/39/11/008
H. Cui, Y. Chen, S. Lu, S. Zhang, X. Zhu, Y. Song, TiO2 nanotube arrays treated with (NH4)2TiF6 dilute solution for better supercapacitive performances. Electrochim. Acta 253, 455–462 (2017). https://doi.org/10.1016/j.electacta.2017.09.080
John K. Aijo, Manju Thankamoniamma, Joaquim Puigdollers, R. Anuroop, B. Pradeep, Thoudinja Shripathie, Rachel Reena Philip, Rapid room temperature crystallization of TiO2 nanotubes. CrystEngComm 19(12), 1585–1589 (2017). https://doi.org/10.1039/C6CE02526A
C. Cao, J. Yan, Y. Zhang, L. Zhao, Stability of titania nanotube arrays in aqueous environment and the related factors. Sci. Rep. 6, 23065 (2016). https://doi.org/10.1038/srep23065
X. Wang, L. Sun, S. Zhang, X. Wang, K. Huo, J. Fu, H. Wang, D. Zhao, A composite electrode of TiO2 nanotubes and nanoparticles synthesised by hydrothermal treatment for use in dye-sensitized solar cells. RSC Adv. 3(27), 11001–11006 (2013). https://doi.org/10.1039/c3ra23482g
H. Imai, Y. Takei, K. Shimizu, M. Matsuda, H. Hirashima, Direct preparation of anatase TiO2 nanotubes in porous alumina membranes. J. Mater. Chem. 9(12), 2971–2972 (1999). https://doi.org/10.1039/a906005g
I.D. Tevis, S.I. Stupp, Patterning of periodic high-aspect-ratio nanopores in anatase titanium dioxide from titanium fluoride hydrolysis. Nanoscale 3(5), 2162–2165 (2011). https://doi.org/10.1039/c0nr01010c
I. Paramasivam, J.M. Macak, P. Schmuki, Photocatalytic activity of TiO2 nanotube layers loaded with Ag and Au nanoparticles. Electrochem. Commun. 10(1), 71–75 (2008). https://doi.org/10.1016/j.elecom.2007.11.001
Q. Kang, S. Liu, L. Yang, Q. Cai, C.A. Grimes, Fabrication of PbS nanoparticle-sensitized TiO2 nanotube arrays and their photoelectrochemical properties. ACS Appl. Mater. Interfaces 3(3), 746–749 (2011). https://doi.org/10.1021/am101086t
Y.Y. Song, Z.D. Gao, P. Schmuki, Highly uniform Pt nanoparticle decoration on TiO2 nanotube arrays: a refreshable platform for methanol electrooxidation. Electrochem. Commun. 13(3), 290–293 (2011). https://doi.org/10.1016/j.elecom.2011.01.006
Y. Zhang, Y. Yang, P. Xiao, X. Zhang, L. Lu, L. Li, Preparation of Ni nanoparticle-TiO2 nanotube composite by pulse electrodeposition. Mater. Lett. 63(28), 2429–2431 (2009). https://doi.org/10.1016/j.matlet.2009.08.019
X.Q. Gong, A. Selloni, Reactivity of anatase TiO2 nanoparticles: the role of the minority (001) surface. J. Phys. Chem. B 109(42), 19560–19562 (2005). https://doi.org/10.1021/jp055311g
D.J. Yang, H. Park, S.J. Cho, H.G. Kim, W.-Y. Choi, TiO2-nanotube-based dye-sensitized solar cells fabricated by an efficient anodic oxidation for high surface area. J. Phys. Chem. Solids 69(5), 1272–1275 (2008). https://doi.org/10.1016/j.jpcs.2007.10.107
P. Roy, D. Kim, K. Lee, E. Spiecker, P. Schmuki, TiO2 nanotubes and their application in dye-sensitized solar cells. Nanoscale 2(1), 45–59 (2010). https://doi.org/10.1039/B9NR00131J
D. Kowalski, D. Kim, P. Schmuki, TiO2 nanotubes, nanochannels and mesosponge: self-organized formation and applications. Nano Today 8(3), 235–264 (2013). https://doi.org/10.1016/j.nantod.2013.04.010
J.R. Jennings, A. Ghicov, L.M. Peter, P. Schmuki, A.B. Walker, Dye-sensitized solar cells based on oriented TiO2 nanotube arrays: transport, trapping, and transfer of electrons. J. Am. Chem. Soc. 130(40), 13364–13372 (2008). https://doi.org/10.1021/ja804852z
X. Luan, D. Guan, Y. Wang, Facile synthesis and morphology control of bamboo-type TiO2 nanotube arrays for high-efficiency dye-sensitized solar cells. J. Phys. Chem. C 116(27), 14257–14263 (2012). https://doi.org/10.1021/jp305280q
P. Albu Sergiu, D. Kim, P. Schmuki, Growth of aligned TiO2 bamboo-type nanotubes and highly ordered nanolace. Angew. Chem. Int. Ed. 120(10), 1942–1945 (2008). https://doi.org/10.1002/ange.200704144
D. Kim, A. Ghicov, S.P. Albu, P. Schmuki, Bamboo-type TiO2 nanotubes: improved conversion efficiency in dye-sensitized solar cells. J. Am. Chem. Soc. 130(49), 16454–16455 (2008). https://doi.org/10.1021/ja805201v
D.A.H. Hanaor, C.C. Sorrell, Review of the anatase to rutile phase transformation. J. Mater. Sci. 46(4), 855–874 (2011). https://doi.org/10.1007/s10853-010-5113-0
K. Yang, Y. Dai, B. Huang, Study of the nitrogen concentration influence on n-doped TiO2 anatase from first-principles calculations. J. Phys. Chem. C 111(32), 12086–12090 (2007). https://doi.org/10.1021/jp067491f
J. Zhang, P. Zhou, J. Liu, J. Yu, New understanding of the difference of photocatalytic activity among anatase, rutile and brookite TiO2. Phys. Chem. Chem. Phys. 16(38), 20382–20386 (2014). https://doi.org/10.1039/C4CP02201G
W. Ren, Z. Ai, F. Jia, L. Zhang, X. Fan, Z. Zou, Low temperature preparation and visible light photocatalytic activity of mesoporous carbon-doped crystalline TiO2. Appl. Catal. B-Environ. 69(3), 138–144 (2007). https://doi.org/10.1016/j.apcatb.2006.06.015
C. Song, X. Wang, J. Zhang, X. Chen, C. Li, Enhanced performance of direct Z-scheme CuS-WO3 system towards photocatalytic decomposition of organic pollutants under visible light. Appl. Surf. Sci. 425, 788–795 (2017). https://doi.org/10.1016/j.apsusc.2017.07.082
M.A. Mahadik, G.W. An, S. David, S.H. Choi, M. Cho, J.S. Jang, Fabrication of A/R-TiO2 composite for enhanced photoelectrochemical performance: solar hydrogen generation and dye degradation. Appl. Surf. Sci. 426, 833–843 (2017). https://doi.org/10.1016/j.apsusc.2017.07.179
X. Li, W. Zhao, J. Zhao, Visible light-sensitized semiconductor photocatalytic degradation of 2,4-dichlorophenol. Sci. China Ser. B-Chem. 45(4), 421–425 (2002). https://doi.org/10.1360/02yb9054
Z. Zhang, G. Yuan, Y. Shi, L. Fang, H. Liang, L.Jin Ding, Photoelectrocatalytic activity of highly ordered TiO2 nanotube arrays electrode for azo dye degradation. Environ. Sci. Policy 41(17), 6259–6263 (2007). https://doi.org/10.1021/es070212x
H.C. Liang, X.Z. Li, Effects of structure of anodic TiO2 nanotube arrays on photocatalytic activity for the degradation of 2,3-dichlorophenol in aqueous solution. J. Hazard. Mater. 162(2), 1415–1422 (2009). https://doi.org/10.1016/j.jhazmat.2008.06.033
Z. Liu, X. Zhang, S. Nishimoto, M. Jin, D.A. Tryk, T. Murakami, A. Fujishima, Highly ordered TiO2 nanotube arrays with controllable length for photoelectrocatalytic degradation of phenol. J. Phys. Chem. C 112(1), 253–259 (2008). https://doi.org/10.1021/jp0772732
L. Cheng, Q. Xiang, Y. Liao, H. Zhang, CdS-Based photocatalysts. Energy Environ. Sci. (2018). https://doi.org/10.1039/C7EE03640J
J. Fu, B. Chang, Y. Tian, F. Xi, X. Dong, Novel C3N4-CdS composite photocatalysts with organic-inorganic heterojunctions: in situ synthesis, exceptional activity, high stability and photocatalytic mechanism. J. Mater. Chem. A 1(9), 3083–3090 (2013). https://doi.org/10.1039/c2ta00672c
B. O’Regan, M. Grätzel, A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films. Nature 353(6346), 737–740 (1991). https://doi.org/10.1038/353737a0
J. Qian, P. Liu, Y. Xiao, Y. Jiang, Y. Cao, X. Ai, H. Yang, TiO2-coated multilayered SnO2 hollow microspheres for dye-sensitized solar cells. Adv. Mater. 21(36), 3663–3667 (2009). https://doi.org/10.1002/adma.200900525
B.E. Hardin, H.J. Snaith, M.D. McGehee, The renaissance of dye-sensitized solar cells. Nat. Photonics 6(6), 162–169 (2012). https://doi.org/10.1038/nphoton.2012.22
W.Y. Cheng, J.R. Deka, Y.C. Chiang, A. Rogeau, S.Y. Lu, One-step, surfactant-free hydrothermal method for syntheses of mesoporous TiO2 nanoparticle aggregates and their applications in high efficiency dye-sensitized solar cells. Chem. Mater. 24(16), 3255–3262 (2012). https://doi.org/10.1021/cm3017616
S.H. Kang, S.H. Choi, M.S. Kang, J.Y. Kim, H.S. Kim, T. Hyeon, Y.E. Sung, Nanorod-based dye-sensitized solar cells with improved charge collection efficiency. Adv. Mater. 20(1), 54–58 (2007). https://doi.org/10.1002/adma.200701819
C.T. Yip, M. Guo, H. Huang, L. Zhou, Y. Wang, C. Huang, Open-ended TiO2 nanotubes formed by two-step anodization and their application in dye-sensitized solar cells. Nanoscale 4(2), 448–450 (2012). https://doi.org/10.1039/C2NR11317A
M. Adachi, Y. Murata, J. Takao, J. Jiu, M. Sakamoto, F. Wang, Highly efficient dye-sensitized solar cells with a titania thin-film electrode composed of a network structure of single-crystal-like TiO2 nanowires made by the oriented attachment mechanism. J. Am. Chem. Soc. 126(45), 14943–14949 (2004). https://doi.org/10.1021/ja048068s
Q. Chen, D. Xu, Large-scale, noncurling, and free-standing crystallized TiO2 nanotube arrays for dye-sensitized solar cells. J. Phys. Chem. C 113(15), 6310–6314 (2009). https://doi.org/10.1021/jp900336e
S. Yoriya, C.A. Grimes, Self-assembled TiO2 nanotube arrays by anodization of titanium in diethylene glycol: approach to extended pore widening. Langmuir 26(1), 417–420 (2010). https://doi.org/10.1021/la9020146
T. Stergiopoulos, A. Ghicov, V. Likodimos, D.S. Tsoukleris, J. Kunze, P. Schmuki, P. Falaras, Dye-sensitized solar cells based on thick highly ordered TiO2 nanotubes produced by controlled anodic oxidation in non-aqueous electrolytic media. Nanotechnology 19(23), 235602 (2008). https://doi.org/10.1088/0957-4484/19/23/235602
L. Sun, S. Zhang, X. Sun, X. He, Effect of the geometry of the anodized titania nanotube array on the performance of dye-sensitized solar cells. J. Nanosci. Nanotechnol. 10(7), 4551–4561 (2010). https://doi.org/10.1166/jnn.2010.1695
M. Macak Jan, S. Aldabergerova, A. Ghicov, P. Schmuki, Smooth anodic TiO2 nanotubes: annealing and structure. Phys. Status Solidi A 203(10), R67–R69 (2006). https://doi.org/10.1002/pssa.200622214
D. Kim, P. Roy, K. Lee, P. Schmuki, Dye-sensitized solar cells using anodic TiO2 mesosponge: improved efficiency by TiCl4 treatment. Electrochem. Commun. 12(4), 574–578 (2010). https://doi.org/10.1016/j.elecom.2010.02.003
P. Roy, D. Kim, I. Paramasivam, P. Schmuki, Improved efficiency of TiO2 nanotubes in dye sensitized solar cells by decoration with TiO2 nanoparticles. Electrochem. Commun. 11(5), 1001–1004 (2009). https://doi.org/10.1016/j.elecom.2009.02.049
J. Wang, Z. Lin, Dye-sensitized TiO2 nanotube solar cells with markedly enhanced performance via rational surface engineering. Chem. Mater. 22(2), 579–584 (2010). https://doi.org/10.1021/cm903164k
J.E. Boercker, E. Enache-Pommer, E.S. Aydil, Growth mechanism of titanium dioxide nanowires for dye-sensitized solar cells. Nanotechnology 19(9), 095604 (2008). https://doi.org/10.1088/0957-4484/19/9/095604
K.E. Lee, M.A. Gomez, S. Elouatik, G.P. Demopoulos, Further understanding of the adsorption mechanism of N719 sensitizer on anatase TiO2 films for DSSC applications using vibrational spectroscopy and confocal raman imaging. Langmuir 26(12), 9575–9583 (2010). https://doi.org/10.1021/la100137u
H. Zhou, Y. Zhang, Electrochemically self-doped TiO2 nanotube arrays for supercapacitors. J. Phys. Chem. C 118(11), 5626–5636 (2014). https://doi.org/10.1021/jp4082883
W. Hui, X. Chen, X. Jing, L. Linfeng, F. Zhiyong, C. Xiaoyuan, S. Ye, L. Dongdong, Enhanced supercapacitance in anodic TiO2 nanotube films by hydrogen plasma treatment. Nanotechnology 24(45), 455401 (2013). https://doi.org/10.1088/0957-4484/24/45/455401
D. Yu, X. Zhu, Z. Xu, X. Zhong, Q. Gui, Y. Song, S. Zhang, X. Chen, D. Li, Facile method to enhance the adhesion of TiO2 nanotube arrays to Ti substrate. ACS Appl. Mater. Interfaces 6(11), 8001–8005 (2014). https://doi.org/10.1021/am5015716
Z. Shao, H. Li, M. Li, C. Li, C. Qu, B. Yang, Fabrication of polyaniline nanowire/TiO2 nanotube array electrode for supercapacitors. Energy 87, 578–585 (2015). https://doi.org/10.1016/j.energy.2015.05.025
Y.Y. Song, F. Schmidt-Stein, S. Bauer, P. Schmuki, Amphiphilic TiO2 nanotube arrays: an actively controllable drug delivery system. J. Am. Chem. Soc. 131(12), 4230–4232 (2009). https://doi.org/10.1021/ja810130h
S. Lin, D. Li, J. Wu, X. Li, S.A. Akbar, A selective room temperature formaldehyde gas sensor using TiO2 nanotube arrays. Sens. Actuator B 156(2), 505–509 (2011). https://doi.org/10.1016/j.snb.2011.02.046
Y. Lai, L. Lin, F. Pan, J. Huang, R. Song, Y. Huang, C. Lin, H. Fuchs, L. Chi, Bioinspired patterning with extreme wettability contrast on TiO2 nanotube array surface: a versatile platform for biomedical applications. Small 9(17), 2945–2953 (2013). https://doi.org/10.1002/smll.201300187
C. Moseke, F. Hage, E. Vorndran, U. Gbureck, TiO2 nanotube arrays deposited on Ti substrate by anodic oxidation and their potential as a long-term drug delivery system for antimicrobial agents. Appl. Surf. Sci. 258(14), 5399–5404 (2012). https://doi.org/10.1016/j.apsusc.2012.02.022
K. Huo, X. Li, B. Gao, L. Wang, Q. Li, X. Peng, X. Zhang, J. Fu, P.K. Chu, Self-supporting and binder-free anode film composed of beaded stream-like Li4Ti5O12 nanoparticles for high-performance lithium-ion batteries. ChemElectroChem 3(9), 1301–1305 (2016). https://doi.org/10.1002/celc.201600215
K. Huo, Y. Li, R. Chen, B. Gao, C. Peng, W. Zhang, L. Hu, X. Zhang, P.K. Chu, Recyclable non-enzymatic glucose sensor based on Ni/NiTiO3/TiO2 nanotube arrays. ChemPlusChem 80(3), 576–582 (2015). https://doi.org/10.1002/cplu.201402288
A. Ranga Rao, V. Dutta, Low-temperature synthesis of TiO2 nanoparticles and preparation of TiO2 thin films by spray deposition. Sol. Energy Mater. Sol. Cells 91(12), 1075–1080 (2007). https://doi.org/10.1016/j.solmat.2007.03.001
J. Archana, M. Navaneethan, Y. Hayakawa, Solvothermal growth of high surface area mesoporous anatase TiO2 nanospheres and investigation of dye-sensitized solar cell properties. J. Power Sources 242, 803–810 (2013). https://doi.org/10.1016/j.jpowsour.2013.05.126
H. Li, W. Zhang, B. Li, W. Pan, Diameter-dependent photocatalytic activity of electrospun TiO2 nanofiber. J. Am. Chem. Soc. 93(9), 2503–2506 (2010). https://doi.org/10.1111/j.1551-2916.2010.03841.x
H.-E. Wang, H. Cheng, C. Liu, X. Chen, Q. Jiang et al., Facile synthesis and electrochemical characterization of porous and dense TiO2 nanospheres for lithium-ion battery applications. J. Power Sources 196(15), 6394–6399 (2011). https://doi.org/10.1016/j.jpowsour.2011.03.085
I.D. Kim, A. Rothschild, B.H. Lee, D.Y. Kim, S.M. Jo, H.L. Tuller, Ultrasensitive chemiresistors based on electrospun TiO2 nanofibers. Nano Lett. 6(9), 2009–2013 (2006). https://doi.org/10.1021/nl061197h
N. Li, Q. Zhang, J.B. Joo, Z. Lu, M. Dahl, Y. Gan, Y. Yin, Water-assisted crystallization of mesoporous anatase TiO2 nanospheres. Nanoscale 8(17), 9113–9117 (2016). https://doi.org/10.1039/C6NR01892K
J.B. Joo, M. Dahl, N. Li, F. Zaera, Y. Yin, Tailored synthesis of mesoporous TiO2 hollow nanostructures for catalytic applications. Energy Environ. Sci. 6(7), 2082–2092 (2013). https://doi.org/10.1039/c3ee41155a
Z. Chen, K. Zhou, Surface morphology, phase structure and property evolution of anodized titanium during water vapor exposure. Surf. Coat. Technol. 263, 61–65 (2015). https://doi.org/10.1016/j.surfcoat.2014.12.056
K. Fischer, M. Grimm, J. Meyers, C. Dietrich, R. Gläser, A. Schulze, Photoactive microfiltration membranes via directed synthesis of TiO2 nanoparticles on the polymer surface for removal of drugs from water. J. Membr. Sci. 478, 49–57 (2015). https://doi.org/10.1016/j.memsci.2015.01.009
L.L. Lai, W. Wen, J.M. Wu, Room-temperature hydrolysis of potassium titanyl oxalate and water-assisted crystallization for TiO2 with high photocatalytic activity. ChemistrySelect 2(18), 5025–5031 (2017). https://doi.org/10.1002/slct.201700372
Q. Luo, Y. Chen, D. Wang, J. An, X. Li, R. Yin, L. Shi, A facile method to prepare mesoporous anatase TiO2 materials in water at lower temperatures. Mater. Res. Bull. 67, 140–145 (2015). https://doi.org/10.1016/j.materresbull.2015.03.020
L.W. Zhu, L.K. Zhou, H.X. Li, H.F. Wang, J.P. Lang, One-pot growth of free-standing CNTs/TiO2 nanofiber membrane for enhanced photocatalysis. Mater. Lett. 95, 13–16 (2013). https://doi.org/10.1016/j.matlet.2013.01.004
Y. Suzuki, S. Pavasupree, S. Yoshikawa, R. Kawahata, Direct synthesis of an anatase-TiO2 nanofiber/nanoparticle composite powder from natural rutile. Phys. Status Solidi A 204(6), 1757–1761 (2007). https://doi.org/10.1002/pssa.200675312
L. Yang, W.F. Leung Wallace, Application of a bilayer TiO2 nanofiber photoanode for optimization of dye-sensitized solar cells. Adv. Mater. 23(39), 4559–4562 (2011). https://doi.org/10.1002/adma.201102717
X. Zhang, S. Xu, G. Han, Fabrication and photocatalytic activity of TiO2 nanofiber membrane. Mater. Lett. 63(21), 1761–1763 (2009). https://doi.org/10.1016/j.matlet.2009.05.038
K. Mondal, M.A. Ali, V.V. Agrawal, B.D. Malhotra, A. Sharma, Highly sensitive biofunctionalized mesoporous electrospun TiO2 nanofiber based interface for biosensing. ACS Appl. Mater. Interfaces 6(4), 2516–2527 (2014). https://doi.org/10.1021/am404931f
X. Jin, K. Yuan, C. Xu, X. Wang, L. Zhu, G. Zhang, D. Xu, Water steam modified crystallization and microstructure of mesoporous TiO2 nanofibers. Ceram. Int. 44(2), 2158–2164 (2018). https://doi.org/10.1016/j.ceramint.2017.10.168
J. Sun, W. Wen, J.M. Wu, Low-temperature transformation of titania thin films from amorphous nanowires to crystallized nanoflowers for heterogeneous photocatalysis. J. Am. Chem. Soc. 96(7), 2109–2116 (2013). https://doi.org/10.1111/jace.12378
J.W. Lee, S.J. Park, W.S. Choi, H.C. Shin, Well-defined meso- to macro-porous film of tin oxides formed by an anodization process. Electrochim. Acta 56(17), 5919–5925 (2011). https://doi.org/10.1016/j.electacta.2011.03.144
M. Wang, Y. Liu, D. Xue, D. Zhang, H. Yang, Preparation of nanoporous tin oxide by electrochemical anodization in alkaline electrolytes. Electrochim. Acta 56(24), 8797–8801 (2011). https://doi.org/10.1016/j.electacta.2011.07.085
A. Palacios-Padros, M. Altomare, A. Tighineanu, R. Kirchgeorg, N.K. Shrestha, I. Diez-Perez, F. Caballero-Briones, F. Sanz, P. Schmuki, Growth of ordered anodic SnO2 nanochannel layers and their use for H2 gas sensing. J. Mater. Chem. A 2(4), 915–920 (2014). https://doi.org/10.1039/C3TA13704J
I. Paulowicz, V. Hrkac, S. Kaps, V. Cretu, O. Lupan et al., Three-dimensional SnO2 nanowire networks for multifunctional applications: from high-temperature stretchable ceramics to ultraresponsive sensors. Adv. Electron. Mater. 1(8), 1500081 (2015). https://doi.org/10.1002/aelm.201500081
H.C. Shin, J. Dong, M. Liu, Porous tin oxides prepared using an anodic oxidation process. Adv. Mater. 16(3), 237–240 (2004). https://doi.org/10.1002/adma.200305660
H. Bian, R. Dong, Q. Shao, S. Wang, M.F. Yuen et al., Water-enabled crystallization of mesoporous SnO2 as a binder-free electrode for enhanced sodium storage. J. Mater. Chem. A 5(45), 23967–23975 (2017). https://doi.org/10.1039/C7TA08228B
X. Wang, Y. Liao, H. Zhang, T. Wen, D. Zhang et al., Low temperature-derived 3D hexagonal crystalline Fe3O4 nanoplates for water purification. ACS Appl. Mater. Interfaces 10(4), 3644–3651 (2018). https://doi.org/10.1021/acsami.7b17582
Acknowledgements
This work was financially supported by the National R&D Program of China under No. 2017YFA0207400, National Key Research and Development Plan under No. 2016YFA0300801, National Natural Science Foundation of China under Nos. 51502033, 61571079, 61131005 and 51572042.
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Wang, X., Zhang, D., Xiang, Q. et al. Review of Water-Assisted Crystallization for TiO2 Nanotubes. Nano-Micro Lett. 10, 77 (2018). https://doi.org/10.1007/s40820-018-0230-4
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DOI: https://doi.org/10.1007/s40820-018-0230-4