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Anodic Oxidation Synthesis of One-Dimensional TiO2 Nanostructures for Photocatalytic and Field Emission Properties
One-dimensional (1D) TiO2 micro/nanostructures have received more and more attentions because of their potential applications in environmental issues. This paper reviews the most recent activities in TiO2 nanostructures with an emphasis on the authors’ own results especially on those synthesized using anodic oxidation method. The review begins with a survey of the effects of fabrication methods and the experiment conditions on the obtained TiO2 nanostructures, and then focuses on their 1D nanostructures, including the syntheses, characterizations, formation mechanisms, photocatalytic, and field emission properties. Finally, we conclude this review with the perspectives and outlooks on the future developments in this field.
Semiconductor nanomaterials have attracted great interests due to their potentials in understanding fundamental physical concepts and constructing nanoscale devices [1–4]. As an important semiconductor material, TiO2 nanostructures have been widely explored in the field of photocatalysis, dye-sensitized solar cells, sensing [5–8], self-cleaning, photolysis of water, and energy storage [9–20]. Many kinds of nanostructured TiO2 materials including spheroidal nanocrystallite , nanoparticles [22, 23], elongated nanotubes , nanosheets , nanorods [26, 27], nanocolumn arrays , and nanofibers  have been synthesized, and these nanostructures have been effectively produced by a variety of methods including template method [30–32], sol-gel methods [33–35], hydrothermal processes [35–37], and anodic oxidation method . Among these methods, anodic oxidation method has become one of the most popular methods because of its high controllability, especially for their abilities on the fabrication of 1D TiO2 nanostructures. 1D TiO2 nanostructures, such as nanotubes and nanorods, have dramatically improved their intrinsic photocatalytic properties compared to other forms of TiO2 [39–51] due to their high surface-to-volume ratios, high surface areas, and highly ordered arrangements.
This paper reviews the most recent research activities in 1D TiO2 nanostructures, with an emphasis on the authors’ own results, and on those prepared using anodic oxidation method.
2. Synthesis Routes towards 1D TiO2 Nanostructures
2.1. Template Method
In this method, TiO2 nanoarchitectures, such as nanowires, nanotubes, and nanorods, were grown in the nanoholes of the templates such as in aluminum oxide templates (AAM), the polymer templates, porous silicon templates, the protein templates, and metal materials templates [52–55]. Porous AAM with ordered nanohole arrays and polymer template are the common templates to fabricate TiO2 nanoarchitectures using electrochemical deposition, sol-gel method, sol-gel-polymerization techniques, and followed by the calcination, acid, or alkali to remove these template. Template method possesses obvious advantages, such as simplicity, low cost, and low demand of raw materials and device; however, it also has many limitations: the preparation process is very complex, the obtained nanoarchitectures depend entirely on the size and shape of the template, and the posttreatment process of the templates tends to damage the morphologies of the nanoarchitectures.
2.2. Hydrothermal Method
Hydrothermal processes use a special closed reaction vessel, in which aqueous solution is used as the reaction medium, to create a high temperature, high pressure reaction environment by heating the reaction vessel to a certain temperature.
Zhu et al.  synthesized TiO2 nanotubes for the first time with the inner diameter of about 5 nm, outer diameter about 8 nm, and length about 100 nm using hydrothermal processes in 1998. Many researchers developed the method and many different TiO2 architectures (nanotubes, nanorods, and nanoribbons) were successfully fabricated by adjusting the precursors and experimental conditions. Miyauchi and Hiromasa  sputtered 200 nm thick titanium films on the surface of the corundum by cathodic sputtering under argon atmosphere, then the corundum with Ti film was placed into the NaOH aqueous solution, and finally they obtained the highly hydrophilic, transparent, neatly arranged TiO2 nanotube arrays.
2.3. Electrochemical Anodic Oxidation Method
Electrochemical anodic oxidation method has been the foremost technique to fabricate TiO2 nanotube arrays in an F− containing solution. It is generally considered that three steps were involved. First, Ti was dissolved in F− containing acidic electrolytes and large amounts of Ti4+ were produced; second, the intensity of electric field exerted on the layer gradually increased with the formation of oxide layer on the surface, TiO2 barrier layer was partically etched, and many irregular pits were formed; third, more H− anions were gathered at the bottom of the micropores, and the TiO2 at the bottom were more easily dissolved and the Ti substrate exposed. Then the titanium substrates contacted with the electrolytes and the above three processes repeatedly occurred; eventually TiO2 nanotube arrays were formed.
3. Influence Factors in Anodic Oxidation Method
3.1. Anodizing Potential
The oxidation potential is an important factor to form the nanotube arrays. The highly ordered TiO2 nanotube arrays can be formed at the right potential range, which is generally called potential window . When the oxidation potential is lower than the potential window, only nanoporous film is formed on the Ti film; however, when the potential is too high, a sponge oxide layer [59, 60] is formed. The potential window generally ranges from 10 V to 40 V. The upper limit of the potential window can reach 220 V in some special solution system. With the increase in oxidation potential, the diameters TiO2 nanotubes increase dramatically.
3.2. Potential Rising Rate
Potential rising rate (PRR) means the rising rate for the applied potential to the predetermined one from the open circuit potential in anodic oxidation process. The PRR affects the morphologies of oxide layer greatly; if it is too large, the disorder porous TiO2 layers are formed. The PRRs are different in different solutions, and it is generally larger in the organic solutions than that in aqueous solutions.
3.3. The Oxidation Time
The oxidation time (OT) is another important factor for the nanotube formation. If the OT is too short, the electrochemical reaction has not yet reached a final equilibrium state, then the resulting product is a disordered porous oxide layer, and no ordered TiO2 nanotube arrays can be formed. When the OT is too long, the tubes’ top and bottom will collapse due to excessive corrosion, and the wires  or porous walls  will be formed, so an appropriate OT is important for the tube formation.
3.4. PH and Concentrations of F−
The impacts of pH and concentrations of F− determine the final morphologies to a certain extent in the dissolution process. When the pH is lower and F− concentrations are higher than those of the electrolyte solution, the dissolution rate will be faster, so the electrolyte solutions with acidic or weakly alkaline () are generally chosen for the formation of ordered TiO2 nanotube arrays. With the lower pH, the oxidation time is short; and with the higher pH, the oxidation time needed will be longer for the tube formation. Furthermore, if the F− concentration is too low, it is impossible to obtain TiO2 nanotube arrays; if the F− concentration is too high; the reaction is too severe to form TiO2 nanotube arrays.
Aqueous solution and the organic solution are used in the fabrication of TiO2 nanotube arrays by anodic oxidation method. Generally, the TiO2 architectures dissolve faster in aqueous solutions than that in organic solution. Compared with aqueous solution, TiO2 nanotubes arrays prepared in organic solution have uniform shape and are very regular, and the extra-long tubes with smooth surface can be obtained.
4. Synthesis, Characterization, and Properties of Different TiO2 Nanoarchitectures
Recently, various TiO2 nanoarchitectures have been fabricated; microstructured-TiO2 nanotubes were synthesized using a LUCE laser source, processing the Ti substrates in an electrolyte for 2 h at room temperature . Bamboo-type TiO2 nanotubes were electrochemically prepared by controlling anodization of Ti in an electrolyte consisting of 0.2 mol·L−1 HF in ethylene glycol [64–66], and bamboo leaf-like structured TiO2 layer on TiO2 nanotube arrays was synthesized via simple electrochemical anodization with wet chemistry pretreatment . Furthermore, thick anatase TiO2 mesosponge layers were synthesized by voltage-step anodization of a Ti metal foil at 50 V for 48 h in a 10 wt% containing glycerol electrolyte followed by a chemical etch in 30 wt% H2O2 ; highly ordered TiO2 nanolace was synthesized in an organic electrolyte with Ti pressed together with a Cu plate against an O-ring in an electrochemical cell . Large-diameter multipodal TiO2 nanotubes were synthesized via processing the TiO2 nanotubes into 0.1 M HCl acid for an hour , and conical-shaped TiO2 nanotube arrays were prepared using anodic oxidation of Ti foil at 80 V in a 3 : 97 (v/v) mixture of H2O2 and ethylene glycol solution containing 0.5 wt% NH4F at 20°C ; multibranched TiO2 nanotubes were controllably fabricated via a two-step anodization method . Other TiO2 nanotubes have been successfully synthesized via various methods.
In this section, we will present recent progresses achieved by us in relation to the synthesis, characterizations, and properties of TiO2 nanostructure by oxidation method.
4.1. Tube-in-Tube TiO2 Nanotubes with Porous Walls 
In our previous work, we successfully fabricated tube-in-tube (TIT) TiO2 nanostructures as shown in Figure 1, which have coarse, porous walls increasing the surface areas compared with the conventional TiO2 nanotubes. The TIT TiO2 nanostructures cover the entire substrate surface, and the enlarged scanning electron microscopy (SEM) images (Figures 1(b) and 1(c)) clearly show numerous TIT TiO2 nanotube-like structures with the outer tube diameter within the array are of about 200 nm (Figures 1(b) and 1(c)). The nanotubes lateral profiles are coarse (Figure 1(d)), which means they have a larger contact surface area compared to conventional structures.
The as-fabricated TIT nanostructures are amorphous and the crystallinity is improved when the thermal annealing process was carried out at 550°C. It can be seen that the nanostructures crystallize in an anatase phase after the heat treatment, which are confirmed by Raman spectroscopy (Figure 2). TEM and HRTEM images (Figure 3) show that the nanostructures have uniform diameters throughout their lengths. The TIT morphologies are preserved after the thermal treatment.
4.2. Ridged TiO2 Nanotubes 
Ridged TiO2 nanotubes were fabricated and their growth mechanism together with their structures and morphology transformations is analyzed. The nanotubes turn into an anatase phase from the amorphous state after an annealing process. It should be mentioned that the annealed nanotubes become decorated with nanoparticles opposed to the starting ridged tube surfaces as shown in Figure 4.
From the TEM observations, the as-prepared TiO2 nanotubes have the ridged surface structures (Figures 5(a) and 5(b)), and it is amorphous confirmed by the FFT in Figure 5(b). However, the annealed nanotubes change into nanoparticle-decorated morphologies (Figures 5(c) and 5(d)), and the ridged structures disappear. Furthermore, the annealed ones become anatase (Figure 5(e)), with spacing between the lattice fringes being ca 0.35 nm (JCPDS card Number 21-1272).
The photocatalytic activities (Figure 6) show that the crystalline nanotubes with nanoparticle-decorated morphologies have a stronger photocatalytic activity than the amorphous samples.
We have carried out FE measurements on the dependence of the FE properties on the distances between the anodes and the samples. It is observed that the Eto decreases from 96.2 to 34 V μm−1 during a voltage increase when the distance increases from 10 to 25 μm and increase (61.8, 115.6, 214.2, and 255.5 for , 15, 20, and 25 μm, resp.) (Figure 7).
4.3. Anatase TiO2 Nanotubes with Rod-Formed Walls 
Besides the TIT nanostructures, the TiO2 nanostructures with rod-formed single walls (RWTN) (Figure 8) were also fabricated. The fabrication of RWTN nanostructures is similar to the process of TIT . In brief, the pretreated Ti foils were anodized at a constant potential of 40 V for 72 h in the mixture of HF and DMSO and followed by the annealing process at 550°C for 3 h.
The SEM results show that the nanostructures have a porous structure and the tube walls are coarse. Compared to TIT, RWTN nanostructures are single-walled with the rod-formed morphologies and coarse walls characteristics (Figures 8(a)–8(d)), which endow them larger surface areas compared with the conventional TiO2 nanotubes. The rods with the length of about 20 nm constitute the tube walls, and the outer and inner diameters are of about 200 nm (Figures 8(e) and 8(f)) and 100 nm, respectively.
TEM images of a single RWTN (Figures 9(a) and 9(c)) show that the outer diameter is about 200 nm, and the tube walls are assembled by many small nanorods (Figure 9(c)) with the length of about 20 nm. HRTEM image (Figure 9(b)) shows the lattice fringe separation of 0.35 nm, corresponding to the  zone axis lattice orientation. FFT pattern (inset) further confirms that the crystalline structure grows along the  direction. EDS (Figure 9(d)) indicates that the RWTN consists of Ti and O in a stoichiometric 1 : 2 composition.
Field-emission properties (Figure 10) were studied and the results showed that the annealed RWTN arrays have a better FE property, with a lower turn-on field of 96.8 V μm−1 and higher field-enhancement factors (120.9) than those of the as-prepared arrays (102.5 V μm−1 for turn-on voltage, 86.7 for ). The F-N plots (Figure 10(b)) exhibit approximately linear relationship, indicating that the electron emission from as-prepared and annealed RWTN nanostructures follows the F-N behavior.
4.4. TiO2 Nanorods and Porous Layers 
By adjusting the anodized time, we successfully fabricated TiO2 with different morphologies, such as nanotubes, nanorods, and porous layers with one-step anodization. In this case, Ti sheets and Ti films on the Si substrates were used, which were anodized in 0.5% HF solution and the anodization is conducted using a 20 V DC voltage at 25°C. Different anodization time (such as 10 min, 30 min, and 60 min) was used for the TiO2 nanostructures with different morphologies, respectively, and 20 min for the Ti film on the Si substrate.
XRD patterns (Figure 11) of synthesized nanostructures under different anodic oxidation conditions show that Ti peaks dominate, caused by the Ti substrates. The main reasons is the poor crystallinities of the TiO2 nanostructures, as mentioned in our previous reports on the TIT and ridged nanotubes [61, 62]. However, the Raman results prove the formation of anatase TiO2 (Figure 12), as the anatase TiO2 phase at 198, and 640 cm−1  can be clearly observed.
The SEM images of TiO2 nanostructures under different anodization time (Figure 13) show that the tubular nanostructure is formed on the surface of the Ti sheet when the time is 10 min, when the oxidation time is extended to 30 min and 60 min, the synthesized samples are rod-like nanostructure, and the length of the rod is increased with the increasing anodization time (Figures 13(b) and 13(c)). This is first report on the synthesis TiO2 nanorod structure by the simple anodic oxidation method.
Meanwhile, the porous TiO2 layers were also fabricated when Ti films on Si substrate were anodized (Figure 14), which were different to the morphologies of discrete TiO2 nanotubes and rod-like structures. The main reason is that the Ti films deposited on the Si substrate are not so compact; then the “soft” parts are easy to be anodized and finally these special morphologies were formed in the anodization process.
5. Conclusion and Outlook
In conclusion, this paper briefly reviews the authors’ recent efforts with respect to the TiO2 nanoarchitectures syntheses using anodic oxidation method. TiO2 nanotubular structures were synthesized in different electrolytes and its field emission; photocatalytic properties are briefly discussed. The fascinating achievements towards the practical applications of TiO2 nanotubular structures should inspire more and more research efforts to address the remaining challenges in the fundamental and technological fields.
Of course, we know this brief review is unable to cover all the progresses, but we do hope this paper can offer some help to the researchers in this field. The investigation of these TiO2 nanostructures will be continuously exciting and highly rewarding. Undoubtedly, more and more innovations and developments will happen toward their practical applications and many issues need to be solved before that.(1)Much more challenges still exist to develop stable, efficient, and low-cost anodic oxidation technology to synthesize high-performance TiO2 nanoarchitectures; the existing problems include the choice of the electrolytes, the time of anodic oxidation, the stability, and the crystalline of the nanostructures.(2)Doping and composites of the TiO2 nanotubular structures have become an important route for strengthening the performance in many possible applications. Different doping of these nanostructures, such as metal, oxide, sulfide, and element, could improve the electrical, optical, and photocatalytic properties.(3)Sensitivity, selectivity, and stability of the TiO2 nanotubular photoelectric device and photocatalysts will require significant improvements in order to meet the great demands in a variety of fields.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Thanks are due to University of Jinan (UJN) for the support on new staff, and the project supported by the Taishan Scholar (no. TSHW20120210), the National Natural Science Foundation of China (Grant nos. 11304120 and 61106059), the Encouragement Foundation for Excellent Middle-aged and Young Scientist of Shandong Province (Grant nos. BS2012CL005, BS2011NJ003, and BS2013CL020), the Science-Technology Program of Higher Education Institutions of Shandong Province (Grant no. J11LA10), and the Research Foundation of University of Jinan (Grant no. XKY1127).
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