Abstract

Titanium dioxide nanotube arrays (TiO2 NTAs) have attracted much interest due to their unique highly ordered array structure, outstanding properties, and potential applications in photo/photoelectrocatalytic degradation of pollutants, hydrogen production, supercapacitors, dye-sensitized solar cells, sensors, and biomedical devices. It has been fabricated via a facile electrochemical anodic oxidation of Ti substrate in fluoride-containing electrolytes. Combined with our previous work, we comprehensively review the recent progress of TiO2 TNAs on the synthesis processes, forming mechanism, and modification used to improve the photoelectric properties. In addition, this article summarizes some of the recent advances on the photo/photoelectrocatalytic degradation of pollutants. Besides, other applications and the future development of TiO2 TNAs are also briefly discussed and addressed.

1 Introduction

With the development of global economy and industrialization in the past century, there happened some serious environmental pollution problems to the world’s population. Nowadays, more and more human beings show increasing concern on the environmental problems because it threats people’s health, drinking water safety, and sustainable development [1–4]. Among the conventional water treatment method (physicochemical and biological treatment technologies), adsorption is one of the most effective strategy for decontamination of nonbiodegradable contaminants from wastewater because of its simplicity in operation and availability of a wide range of adsorbent. However, this method only realized phase transfer of pollutants without degradation [5, 6]. Obviously, photo/photoelectrocatalytic treatment, as a green technology, is promising and effective to degrade the pollutants in nature by utilizing abundant solar energy.

Titanium dioxide (TiO2), since discovered on water photolysis by Fujishima and Honda in 1972, has been paid much attention and widely used in photocatalytic degradation of pollutants, water splitting, supercapacitors, dye-sensitized solar cells, sensors, biological materials, and self-cleaning [7–17]. Moreover, much more interests were activated in the 1D dimensional tubular nanomaterials when Iijima et al. discovered carbon nanotubes (CNTs) in 1991 [18]. Within these nanotubular materials, TiO2-based nanotubes attracted wide and intensive researches due to their specific surface area, low recombination of electron/holes, and strong ion-changeable ability. In contrast to CNTs, TiO2 nanotubes were readily fabricated by template-assisted processes [19, 20], hydrothermal method [21–23], and electrochemical anodic oxidation [24–28]. Different fabrication methods have their own unique advantages and features. TiO2 nanotubes were first synthesized by Hoyer [29] by the template-assisted method. As for this method, anodic aluminum oxide (AAO) nanoporous membrane, made of an array of parallel straight nanopores with controllable diameter and length, is usually used as a template. However, the template-assisted method often encounters much difficulties of prefabrication and postremoval of the previous templates and usually results in impurities [30]. Then, Kasuga et al. discovered the hydrothermal alkaline synthesis of TiO2 nanotubes in 1998 [31]. Concerning hydrothermal treatment, the self-organized TiO2 nanotubes were obtained by putting TiO2 powders in a tightly closed high temperature and pressure vessel containing highly concentrated NaOH solution (5–10 m). The length of the nanotubes is still limited to several micrometers and with no uniform vertical alignment [32–34]. To overcome this limitation, Tang and his colleagues reported a protocol to rationally grow elongated titanate nanotubes with length up to tens of micrometers by a stirring hydrothermal method, improving the electrochemical performance of TiO2 nanotubes [35, 36]. In 1999, Zwilling et al. first reported the formation of nanoporous TiO2 by simple electrochemical anodization on Ti in a hydrofluoric acid solution [37]. Compared to the methods before, TiO2 NTAs by electrochemical anodization are recyclable, suitable for rapid production, and notable to yield uniform TiO2 nanotubes with vertical alignment [38, 39].

Since the first report on the fabrication of highly ordered TiO2 NTAs by anodic oxidation of Ti substrate, the effects of fabrication factors, doping methods, and applications have expanded tremendously [40–44]. The number of technical publications on this topic has been going up rapidly, and more than 1800 papers were published in the last decade (Figure 1). TiO2 NTAs are an excellent photocatalytic material that has been used for a long time because it is stable, nontoxic, environmentally friendly, cheap, recyclable, and easily synthesized. However, the wide application of TiO2 NTAs in some fields was limited due to its several drawbacks. Associated with a wide band gap, TiO2 (anatase: 3.2 eV, rutile: 3.0 eV) occupies only 3–5% of the total solar spectrum. Besides, fast recombination of photogenerated electron hole pairs also leads to decreased efficiency in the photo/photoelectrocatalytic activity [45–49]. Herein, this review will address different methods to modify TiO2 NTAs by suppressing the recombination of photogenerated electron hole pairs and enhancing the visible light absorption in order to improve the photo/photoelectrocatalytic activity. Besides, the effects of the fabrication factors and formation mechanism are also discussed in the article. At last, the author will envisage other potential applications and the future development on TiO2 NTAs.

Figure 1:

The number of articles published on TiO2 NTAs fabricated by electrochemical anodization from 2005 to 2015 (Data was obtained from Web of Science Database on September 16, 2015 using TiO2 nanotube and anodization as key words).

2 Electrochemical anodization of TiO2 NTAs

This section presents recent developments and achieves a preparation of self-ordered TiO2 NTAs by electrochemical anodization. First, the history and development process about TNAs on TiO2 NTAs by a self-organization process is introduced. Then, some experimental parameters of synthesis such as electrolyte, the pH of electrolyte, applied potential, time, etc., responsible for regulation of the morphology and nanostructure of TiO2 NTAs, are described. Finally, this review discussed the formation mechanisms of TiO2 TNA layers fabricated by electrochemical anodization.

2.1 History and development of TiO2 NTAs

Electrochemical anodization of the suitable metals to create decorative oxide layers on metal surfaces have been conducted for almost a century. However, only recently, self-organized titanium oxide nanotube layers have been fabricated. The first report on anodized TiO2 dates back to 1984; Assefpour-Dezfuly et al. produced porous TiO2 by performing etching in alkaline peroxide first and, then, followed by anodization in chromic acid [50]. As presented before, Zwilling and coworkers reported on the formation of nanoporous anodized titania in a fluoride containing electrolyte in 1999, making a breakthrough of work undertaken on porous/tubular anodized Ti over the last two decades [37]. Since then, a lot of researchers have made much effort on finding the optimal electrolyte and experimental parameters in order to efficiently achieve high-quality self-organized TNAs. In 2001, Gong et al. reported the preparation of self-organized TiO2 NTAs by anodization of a Ti foil in H2O/HF electrolyte at room temperature [24]. However, the nanotube length in this first synthesis generation is limited to a few hundred nanometers. Second-generation TiO2 NTAs with a length of several micrometers were produced in neutral electrolytes containing fluoride ions such as Na2SO4/NaF or (NH4)2SO4/NH4F [51–53]. The fabricated TiO2 NTAs usually have a rough external surface with rings or ripples on the walls due to the current oscillation along the anodization process. In later work, third-generation TiO2 NTAs were anodized in organic electrolyte such as formamide, dimethylsulfoxide (DMSO), ethylene glycol (EG), or diethylene glycol containing fluoride ions, allowing the construction of smooth and taller nanotubes with lengths up to approximately hundreds of micrometers [54–56]. Some attempts have been made to produce TiO2 NTAs in a fluorine-free electrolyte such as HClO4-containing electrolyte, and they are commonly considered as the fourth-generation materials [57–59]. In order to achieve highly ordered anodic TiO2 nanotube arrays, Macak [60] and Shin et al. [61] have also improved the ordering of hexagonal nanotube close packed arrays via a multistep approach, denoted as the fifth-generation materials. The SEM images of each generation process are listed in Table 1 and Figure 2. However, TiO2 NTAs with long length nanotubes show weak adhesion between nanotubes and Ti substrate. Therefore, Yu et al. [65] developed a facile method to strengthen the adhesion between nanotubes and Ti substrate by employing an additional anodization in a fluoride-free electrolyte, effectively improving the photoactivity and supercapacitor performances (Figure 3). Up to now, highly ordered anodic TiO2 nanotube arrays with long nanotubes and high adhesion between nanotubes and Ti substrate were fabricated, and the development is going on.

Table 1

Brief summary of various synthesis generations of TiO2 TNAs using different electrolytes.

TiO2NTAsElectrolyteMorphologyRefs.
1st generation (HF electrolyte)0.5 wt% HFShort nanotubes[24, 37]
Length: 200–500 nm
Diameter: 10–100 nm
Wall thickness: 13–27 nm
2nd generation1 m Na2SO4/(NH4)2SO4+0.5 wt% NH4FRough wall with wings[51, 52]
F-based buffered electrolytesLength: 0.5–2.4 μm
Diameter: 100 nm
Wall thickness: 12 nm
3rd generation0.5 wt% NH4F+2 v% H2O in ethylene glycolSmooth and ultra-long tubes[56]
Organic electrolytes containing F-Length: 5–1000 μm
Diameter: 100 nm
Wall thickness: 12 nm
4th generation0.01–3 m HClO4Disordered tubes[57]
Fluoride-free electrolytesLength: 30 μm
Diameter: 20–40 nm
Wall thickness: 10 nm
5th generation0.5 wt% NH4F+2 v% H2O in ethylene glycolSmooth and hexagonal tubes[62, 63]
Mutiple-step anodization in organic electrolytes containing F-Length: 2–10 μm
Diameter: 100 nm
Wall thickness: 15–20 nm

Figure 2:

SEM images of first generation in HF electrolyte (A), second generation in Na2SO4/NaF electrolyte (B), third generation in ethylene glycol/fluoride electrolytes containing a small amount of water (C), fourth generation in F--free electrolyte (D), fifth generation with two-step anodization in ethylene glycol/fluoride electrolytes containing a small amount of water (E) and fifth generation with three-step anodization in ethylene glycol/fluoride electrolytes containing small amount of water. Inset (A), (B), (C), (E), and (F) is the side view SEM image counterpart. (B) Reproduced from Ref. [51] with permission. Copyright Elsevier. (C) Reproduced from Ref. [64] with permission. Copyright American Chemical Society. (D) Reproduced from Ref. [57] with permission. Copyright Elsevier. (E) Reproduced from Ref. [62] with permission. Copyright American Chemical Society. (F) Reproduced from Ref. [63] with permission. Copyright American Chemical Society.

Figure 3:

SEM images of top view (left) and cross-sectional view (right) of TiO2 NTAs (A, B) 5 wt % NH4F solution in ethylene glycol and (C, D) with an additional anodization in 5 wt % H3PO4 solution in ethylene glycol. Reproduced from Ref. [65] with permission. Copyright American Chemical Society.

2.2 Influencing factors on TiO2 NTAs

2.2.1 Electrolyte and pH

Electrolyte and pH play crucial on the morphology, structure, and growth rate of the as-formed TiO2 nanotubes. It was well recognized that under the same conditions, different electrolytes may produce different electric field intensities and different chemical dissolution rates of the TiO2 oxide layer. The higher electric field intensity can induce bigger breakdown sites, which finally result in a wider diameter of TiO2 NTAs. Additionally, a fast chemical dissolution rate of the TiO2 oxide layer can lead to a short length of the nanotubes. Generally, two different types of electrolytes such as organic (or neutral) and aqueous electrolytes have been used for the synthesis of TiO2 NTAs. When the anodization takes place in an acidic aqueous solution, TiO2 nanotubes with the length of hundreds of nanometers could be grown on the Ti substrate due to the fast chemical dissolution rate of the TiO2 oxide layer. When anodized in F--based or F--free organic electrolytes, the longer nanotube length and better self-organized TiO2 NTAs layers were obtained because of the lower diffusibility and concentration of ions in organic electrolytes. Nevertheless, the typical anodic TiO2 NTAs films grown in organic electrolytes are usually covered by a nanograss [66, 67]. To obtain top-open TiO2 NTAs arrays, several approaches have been used, including ultrasonication, polishing, resistant layer techniques, multiple-step anodization, and micromechanical cleavage approach, successfully lifting off the nanograss [68–72].

The pH of the electrolyte is the key to achieve growth of the high-aspect ratio nanotubes, and it can affect the self-organization behavior of TiO2 NTAs. The difference in the pH leads to significant variations in the pore diameter and thickness of the TiO2 layer, which is decided by the oxide dissolution rate. The dissolution rate at low pH is much greater than that at a higher pH. Compared to TiO2 NTAs anodized in HF aqueous with low pH, TiO2 NTAs anodized in higher pH F--based or F--free inorganic and organic electrolytes with a higher thickness could be grown due to the slow dissolution of oxide layers and reaction. Therefore, the length of TiO2 nanotubes could be up to hundreds of micrometers with a high aspect ratio when using fluoride-free neutral electrolytes. These results were well matched with the description in section 2.1. A brief summary of the various synthesis generations of TiO2 TNAs using different electrolytes is been given in Table 1. Thereby, a selection of electrolyte medium for the TiO2 NTA fabrication is a primary concern.

2.2.2 Anodizing potential and time

The anodization voltage influences the morphology of formed nanostructures, while the anodization time mainly affects the length of TiO2 nanotubes. The applied potential determines the electric field strength across the oxide layer, ultimately affecting the nanotube diameter and length. Most experiments are performed under potentiostatic conditions, and the applied potential usually ranges from 5 to 30 V and 10 to 50 V in HF aqueous solution and F--based organic electrolytes, respectively. As shown in Figure 4, a linear relationship between the applied potential and the diameter, layer thickness of the TiO2 NTAs is generally observed up to 50 V [73–75]. However, at high anodization voltages over 50 V, breakdown events can be observed inside the nanotube, and the nanotubes are not highly organized [76, 77]. When the growth and dissolution of the oxide layers achieved equilibrium, the length of the TiO2 nanotubes did not extend even if the anodization time was prolonged. When working in HF aqueous, the conditions are far too aggressive to allow the nanotubes to grow any longer than a few micrometers. Such a thickness (hundreds of nanometers) is usually reached only needing 10–30 min. In contrast, the anodization in organic electrolytes is much gentle. In order to get highly organized and smooth nanotubes, the optimal anodization time could be adjusted from 4 to 10 h. Anodizing potential and time were determined by the electrolytes.

Figure 4:

Linear relationship between the applied potential (0–50 V) and the diameter (A), layer thickness (B) TiO2 NTAs anodized in F--based ethylene glycol electrolyte. Reproduced from Ref. [73] with permission. Copyright Elsevier.

2.2.3 Temperature and fluoride concentration

The temperature of the electrolyte affects the dissolution rate of the nanotubes, which are normally grown at 20–25°C (room temperature). Low (<5°) and high (>30°) temperatures are bad for the growth of TiO2 NTAs [78]. The viscosity of the electrolyte decreases with increasing temperature, leading to a faster etching rate. When etching becomes fast, the oxide layer gets dissolved faster and forms pores. At a lower temperature, the mobility of the fluoride ions was suppressed, which leads to a slower etching rate. Owing to the slower etching rate of the oxide layer, no regular pores were formed at lower temperatures. The concentration of H2O and F- also has an effect on the morphology of the TiO2 nanotubes grown by potentiostatic anodization in F--based organic electrolytes. The concentration of F ions determines the growth and dissolution rate of the TiO2 oxide layers, which affects the length and diameter of the TiO2 nanotubes. The appropriate content of F- is very important for the growth of nanotubes. However, each of these variables plays a different role in the formation of TiO2 nanotubes. On the one hand, a higher percentage of H2O in the electrolyte leads to a transition from a nanoporous to a nanotubular structure, as well as to a larger diameter of the tubes and a decrease in their length. In contrast, a higher F- concentration decreases the nanotube diameter and increases their length due to the insertion of F- ions into the lattice [79]. The appropriate ratio of H2O and F- is very important for the growth of nanotubes.

2.3 Growth mechanism of TiO2 NTAs

Achieving an equilibrium between field-assisted electrochemical oxidation and dissolution of the TiO2 layer is the growth mechanism of aligned TiO2 NTAs. As shown in Figure 5A, when a Ti substrate is exposed to a sufficiently anodic voltage in electrolyte, a compact oxide layer forms on the surface by the interaction between Ti and O2- or OH- ions (provided by H2O) according to Equations (1)–(3).

Figure 5:

Schematic representation of the Ti anodization characteristics of the Ti anodization (A) and (B). Typical current-time (j-t) with and without fluorides in the electrolyte (C). Corresponding evolution of the TiO2 morphology by different morphological stages (D). Reproduced from Ref. [80] with permission. Copyright Royal Society of Chemistry journals.

(1)TiTi4++4e- (1)
(2)Ti+2H2OTiO2+4H++4e- (2)
(3)Ti4++2H2OTiO2+4H+ (3)

Simultaneously, hydrogen evolution occurs on the cathode according to Equation (4).

(4)4H2O+4e-2H2+4OH- (4)

During the oxide layer formation process, the O2- migrates to the Ti substrate interface and reacts with Ti and Ti4+ forming a TiO2 layer under an electric field, while Ti4+ will be released from the metal and move to the opposition direction reaching the top of the TiO2 layer. However, the presence of F- can etch the TiO2 oxide layer to form water-soluble [TiF6]2- species [reaction (5)]. Besides, the Ti4+ react with the F- to form a soluble [TiF6]2- during the moving process at the same time [reaction (6)], as schematically shown in Figure 5B.

(5)TiO2+6F-+4H+[TiF6]2-+2H2O (5)
(6)Ti4++6F-[TiF6]2- (6)

In the absence of fluoride ions, the current usually is prone to a lot of exponential decrease as a result of the formation of a thick oxide barrier layer with low conductivity (Figure 5C), while in the F--based electrolyte, the current showed an initial decrease (stage I), followed by an increase (stage II), and then to the steady-state level (stage III) (Figure 5C). In stage I, the formation of the compact TiO2 oxide layer leads to the current decay, similar to that of fluoride-free anodization. In stage II, the TiO2 oxidation layer is etched by F-, leading to small pits and irregular pores because of the difference in the roughness of the as-formed TiO2 layer or defects stemming from impurities. Therefore, the current increases due to the pore-acting pathways available for the ion transfer. As the pores grow deeper, the electric field density across the remaining barrier layer increases, and the individual pores start interfering with each other and competing for the available current. After an equilibrium between field-assisted electrochemical oxidation and dissolution, uniform TiO2 nanotubes formed at a steady current density (stage III) [80, 81].

3 Modification of TiO2 NTAs

TiO2 nanomaterials are widely used due to its low cost, good physical and chemical properties [82–84]. However, associated with a wide band gap (anatase: 3.2 eV, rutile: 3.0 eV), TiO2 can only absorb ultraviolet light (3–5% solar light). Besides, the electron/holes are recombined fast. These drawbacks make TiO2 inactive under visible light (Figure 6A) and limit it in wide applications. For the anodized TiO2 nanotube array samples, there still exist several shortcomings that need to be improved and solved. On the one hand, compared to TiO2 nanoparticles and TiO2 nanotubes synthesized by the hydrothermal method, it has a lower surface area. On the other hand, when used in photocatalysis, photoelectrocatalysis, or solar cells, the light can only illuminate one side of the anodized TiO2 nanotube arrays, leading to lower power conversion efficiency. Therefore, it is very essential to deal with these drawbacks. Considerable efforts have been put in extending the light absorption to visible light, enhancing surface area, and suppressing the combination of electron/holes over the past years [85–87]. Table 2 summarized the common modification strategies and their corresponding purposes, such as thermal treatment, doping (non-metal and metal elements), noble metal particle decoration, and heterostructure construction. As for the doping strategy, three essential benefits are expected: (1) doping with metal to act as a surface plasmon resonance (SPR) photosensitizer for driving the visible light (Figure 6B), (2) doping with non-metal to prevent the recombination of electron/holes (Figure 6C), (3) doping with semiconductor to form heterojunction that provides suitable energy levels for synergic absorption and charge separation for enhanced utilization of solar energy (Figure 6D).

Figure 6:

Schematic of energy level and electron-hole pair separation/transfer of pure TiO2 (A) and corresponding modification with metal (B), non-metal (C), and semiconductors (D).

Table 2

Various modification strategies and corresponding purposes.

StrategyPurpose
Thermal treatment (annealing in O2, Ar, or other atmospheres)To form unsaturated Ti3+ cations and O2- vacancies for enhanced light absorption and suppressing the recombination of electron/holes
Doping with non-metal (N, C, S, etc.) and metal elements (Fe, V, Cu, Ni, etc.)To prevent the recombination of electrons/holes and enhance light absorption
Decoration with noble metal particles (Pt, Ag, Pd, etc.)To act as a surface plasmon resonance (SPR) photosensitizer for driving the visible light and facilitating the transfer of charges
Construction of heterostructure composites (NiO, Cu2O, Bi2O3 and perovskite, etc.)To form a heterojunction that provides suitable energy levels for synergic absorption and charges separation

3.1 Thermal treatment

TiO2 exists naturally mainly in three crystalline phases: anatase, rutile, and brookite. Moreover, TiO2 NTAs by electrochemical anodization are usually in an amorphous form. Among the different polymorphs, rutile is generally considered to be thermodynamical in most stable bulk phase, while at the nanoscale, anatase is considered to be more stable, although there are some arguments in literature [88, 89]. The amorphous TiO2 NTAs can be converted into anatase or rutile by annealing in air or O2 [90]. The as-anodized amorphous TiO2 NTAs annealed in other gas such as Ar, N2, etc., can form unsaturated Ti cations such as Ti3+, Ti2+, and Ti+ or O2- vacancies in TiO2 NTAs, resulting in enhanced light absorption and improved electrochemical activity [91, 92], and it will be discussed in section 3.2. The crystal phases have a significant effect on the mechanical, electrochemical properties, and applications [93–95]. As displayed in Figure 7, no obvious peak was observed in the XRD pattern of the TiO2 NTAs before annealing. Conversion of the amorphous TiO2 NTAs into anatase begins at around 325°C. The relative intensity of the anatase peaks increases with annealing temperature rising from 325°C to 500°C. When annealed at 600°C, the TiO2 NTAs consisted of an anatase and rutile mixed phase, indicating that a part of the anatase phase changed into a rutile phase as the temperature was increased. During the temperature up to 800°C, the peak intensity of rutile increases, while decreasing in the anatase. Besides, the TiO2 NTAs with thermal treatment (<700°C) show better photoelectric properties than that of amorphous TiO2 NTAs [96–98].

Figure 7:

XRD patterns of TiO2 NTAs without heat treatment (WHT) and annealed at different temperatures (indicated in the figure) for 0.5 h. Reproduced from Ref. [95] with permission. Copyright Elsevier.

Besides, the nanotube morphology and the conductivity of TiO2 NTAs are affected by the annealing temperature during calcination. As displayed in Figure 8, TiO2 NTAs nanotubes are obviously unchanged when the temperature rises to 550°C. The nanotubes are open and smooth with an average pore diameter of 80–120 nm and a wall thickness of 15 nm. The surface emerged with agglomeration when the temperature was raised to 600°C (see the black circle). However, the tubes started collapsing at higher temperatures (higher than 700°C) [99]. As shown in Figure 9, the resistivity of TiO2 NTAs increases below 200°C due to the evaporation of water from the surface. The conductivity becomes good because the amorphous TiO2 NTAs transform from amorphous into anatase between 200°C and 500°C, and thus, it owns the lowest resistivity around 450°C. The resistivity of the TiO2 NTAs increases again due to the formation of the rutile layer underneath the TiO2 NTAs with higher temperature. The presence of the rutile layer has negatively influenced the electronic properties and some applications of TiO2 NTAs [100].

Figure 8:

SEM images of TiO2 NTAs by electrochemical anodization in F--based Na2SO4 electrolyte (A) and TiO2 NTAs annealed at different temperatures 400°C (B), 500°C (C), 550°C (D), 600°C (E), and 700°C (F). Reproduced from Ref. [99] with permission. Copyright Elsevier.

Figure 9:

Two-point conductivity measurements for TiO2 NTAs anodized in F--based ethylene glycol electrolyte annealed for 2.5 h at different temperatures in air. Reproduced from Ref. [100] with permission. Copyright Elsevier.

3.2 Doping with non-metal and metal elements

As we all know, photoelectric performances of TiO2 nanomaterials are related to the structure and chemical composition. Therefore, incorporating a secondary electronically active species into the lattice is efficient for sensitizing TiO2 to visible light and suppressing the recombination of electrons and holes. Asahi et al. first reported that the nitrogen-doped TiO2 by sputtering in nitrogen containing gas mixture enhanced the photoelectric reactivity under visible light irradiation [101]. Since then, other non-metals such as carbon, including graphene and its derivatives [82, 102–104], B [105], S [106, 107], N [108], F [109], have also been introduced into the TiO2 by various methods. These results indicated that non-metal/TiO2 NTAs not only can narrow the band gap of TiO2 and facilitate the transfer of photogenerated carries but can also enhance the visible light absorption and improve the photochemical conversion efficiency. The conventional technique includes thermal treatment of TiO2 NTAs in a gas atmosphere (N2, CO, Ar, etc.), plasma ion implantation or sputtering in an atmosphere with the doping species, Ti alloy anodization, and electrodeposition. Evidently, ion implantation or sputtering in an atmosphere of doping species following an annealing process has been verified to be an effective doping method [110, 111]. However, this method needs high energy accelerators in a high vacuum environment, and the doping depth is limited to several micrometers. Thermal treatment of TiO2 NTAs in special atmosphere and electrodeposition are recognized as facile doping means. Compared to bare TiO2 NTAs, non-metal/TiO2 NTAs show significant improved photoresponse and photochemical performances.

Among all the non-metal materials, doping with carbon and nitrogen has attracted much interest. Lai et al. successfully obtained N-doped TiO2 NTAs by immersing in NH3·H2O solution and then annealing in a muffle furnace under ambient atmosphere for 2 h at 450°C [108]. Besides, Schmuki et al. doped nitrogen into the TiO2 lattice by annealing in NH3 atmosphere [112]. Direct electrochemical anodization of a TiN alloy or growing TNAs in a solution containing N species is also a promising N-doping approach [113, 114]. Carbon doping is typically achieved by electrodeposition [115] or thermal treatment in CO [116], acetylene [117], or Ar gas [118]. Graphene, a one-atom-thick sp2-hybridized carbon material, has been widely used to modify TiO2 due to its superior mechanical, electrical, and thermal properties [119–123]. Luo and his coworkers deposited graphene films onto TiO2 NTAs by the electrodeposition technique assisted with photoreduction [124, 125]. As shown in Figure 10, the covered area and thickness of graphene were determined by adjusting the cyclic voltammetric cycles. A closed graphene film layer was uniformly dispersed on the TiO2 NTA surface at optimized 26 cycles. Thus, photo-assisted reduction was used to further improve the crystallinity and the charge transport of the graphene. Recently, our group has synthesized reduced graphene oxide/TiO2 NTA composite via a combination of electrodeposition and carbonation techniques [62]. The graphene/TiO2 NTA composite showed improved photocatalytic activity for the degradation pollutants under solar light. Huo et al. constructed a coaxial carbon/TiO2 NTA structure by hydrothermal treatment in a glucose solution and then carbonization under N2 at 700°C [126]. As depicted in Figure 11, TiO2 nanotubes were vertically grown on the Ti substrate with an average pore diameter of 70–80 nm and a wall thickness of 15–20 nm. After the hydrothermal treatment in glucose, the thickness of nanotube increases to 35–40 nm. Further carbonization in N2 atmosphere at 700°C led to the morphology of well-aligned NTAs that is preserved, and the wall thickness has no obvious change, indicating that the thickness of coaxial carbon coating is about 20 nm. From the Roman spectra, the two peaks at 1345 and 1598 cm-1 ascribed to the D and G bands demonstrated the formation of C/TiO2 NTA nanocomposites. Carbon-endowed coaxial C/TiO2 NTAs composites have better conductivity and improved electrochemical properties. What is more, Huo et al. also developed other methods to construct coaxial C/TiO2 NTAs structure by anodizing in F--based ethylene glycol electrolyte and then carbonizing under Ar atmosphere at 500°C with the residual ethylene glycol (EG) absorbed on the nanotube wall serving as the carbon source [93]. Besides, Tang et al. constructed a coaxial core-shell-structured TiO2@carbon nano-rod arrays through a bio-inspired method [127]. These results all demonstrated that doping with non-metal into TiO2 lattice can enhance the light absorption and suppress the recombination of electrons and holes, making TiO2 more widely used.

Figure 10:

SEM images of TiO2 NTAs (A), and TiO2 NTAs covered by graphene electrodeposited with five cycles (B), 11 cycles (C), 17 cycles (D), 26 cycles (E), and 32 cycles (F). Partial graphene islands are marked by the dashed lines for the eye guide. The inset (F) is a typical FFT image from photoreduced graphene sheets. Reproduced from Ref. [124] with permission. Copyright Elsevier.

Figure 11:

SEM images of TiO2 NTAs annealed in air at 450°C (A), hydrothermal treated TiO2 NTAs in glucose solution at 200°C (B), and subsequently annealed sample under N2 at 700°C for 3 h (C). The Raman spectra of the samples (A) and (C) are shown in (D). Reproduced from Ref. [126] with permission. Copyright Elsevier.

At the same time, TiO2 doped with transition metal cations such as Fe, Cu, V, and Mn [128–135] have also been verified to widen visible light absorption and enhanced the conversion efficiency by suppressing the recombination of photogenerated electrons and holes. It is noted that when the doping content is excessively high, it may partially block the channels of TiO2 nanotubes and act as recombination centers, rather than facilitate electron-hole separation, resulting in the decrease in the photo/photoelectrocatalytic activity. This adverse effect could be avoided by a suitable doping amount or annealing the doped TiO2.

3.3 Decoration with noble metal particles

Another promising approach is to decorate TiO2 NTAs with noble metal nanoparticles (Au, Ag, Pt, Pd, or alloys) [136–143]. Modification with noble metals has been proven to restrain the recombination of electron/hole pairs, leading to the enhanced photo/photoelectrocatalytic activity. Upon visible light illumination, the noble metal nanoparticles could be photo-excited and generate a lot of electrons on its surface due to the surface plasmon resonance. Besides, a Schottky barrier was formed at noble metal/TiO2 NTAs interfaces due to the large work function, so charge separation was accompanied by photo-excited electrons easily transfer from the noble metal to the conduction band of TiO2 NTAs. Simultaneously, the SPR effect can form a strong local electronic field to enhance the energy of trapped electrons, making them transfer and react with electron acceptors more easily [64, 144–146]. Moreover, the noble metal can also improve the photoresponse in the visible light due to the surface plasmon resonance (SPR) effect [63, 147–151]. Many strategies have been adopted to decorate TiO2 NTAs with noble metal nanoparticles by UV irradiation reduction, plasma sputtering, electrodeposition, and hydrothermal method. Wu et al. adopted the electrodeposition method to construct highly dispersed Au nanoparticles on the TiO2 nanotube arrays [152]. Both the particle size and loading amount were facilely controlled via adjusting electrochemical parameters. The Au/TiO2 NTAs showed much higher photocatalytic degradation of MO under visible light. Schmuki et al. successfully deposited Au layer on to the TiO2 NTAs by sputtering [153]. Sun et al. fabricated Ag/TiO2 NTAs nanocomposites by ultrasound-aided photoreduction technique [154]. The amount of Ag introduced into TiO2 can be controlled by changing the concentration of AgNO3. In view of these methods exhibiting low control over the metal particle size and dispersion, Ye and his colleagues reported a facile hydrothermal strategy for crafting TiO2 NTAs sensitized by Pd QDs (PdQDs@TiO2 NTAs) with superior performance in photoelectrocatalytic water splitting [154]. As shown in Figure 12, the nanotube arrays were crack-free and smooth with an average tube diameter of 80 nm and a wall thickness of 30 nm after a three-step electrochemical anodization (Figure 12A,B). After hydrothermal reaction, Pd quantum dots were uniformly dispersed over the entire surface of the nanotubes, both inside and outside of the nanotubes with very small particle size of 3.3±0.7 nm (Figure 12C–F). TiO2 NTAs modified with noble metal can effectively prevent electron/hole recombination and enhance visible light absorption due to the SPR effect, showing higher photo/photoelectrocatalytic degradation rate of pollutants. Besides, ternary Ag/AgCl/TiO2 NTAs and Ag/AgBr/TiO2 NTAs were developed to enhance the visible light absorption and facilitate the transfer of photocarries [155–158].

Figure 12:

Top and cross-sectional SEM images of pure TNAs (A), (B) and Pd QDs@TiO2 NTAs (C), (D). The insets show the corresponding magnified images. TEM (E) and HRTEM (F) images of TiO2 NTAs coated with Pd QDs. Reproduced from Ref. [154] with permission. Copyright American Chemical Society.

3.4 Modification with semiconductor composites

Owing to the wide band gap of TiO2 and low utilization of solar light, it is essential to couple TiO2 NTAs with a small band semiconductor (CdS, CdTe, PbS, Cu2O, Bi2O3, etc.) [159–166] to construct a heterostructure for visible light harvest and effective separation of electrons and holes. Under visible light irradiation, photogenerated electrons are prone to be injected from the conduction band of the semiconductor to that of the TiO2, inhibiting the recombination of photogenerated charge carriers. At the same time, the holes from the valance of TiO2 will move to that of the semiconductor and then oxidize the targeted pollutants.

Cadmium sulfide (CdS) is a well-known semiconductor and is widely used in photocatalytic systems [167–172]. The narrow band gap about 2.4 eV, matching well with the spectrum of sunlight, makes CdS absorb low-energy photons up to 520 nm. Besides, its conduction band is more negative than TiO2 facilitating the transfer of the photo-generated electrons at the interface between CdS and TiO2. Yin et al. developed a constant current electrochemical deposition route to decorate TiO2 NTAs with CdS nanoparticles [173]. It showed higher photocurrent and hydrogen production activity under visible light. Hsu et al. modified TiO2 NTAs with CdS nanoparticles by sequential chemical bath deposition (S-CBD) [174]. What is more, Xie et al. decorated TiO2 NTAs with CdS quantum dots (QDs) by a sonication-assisted sequential chemical bath deposition [175]. As shown in Figure 13, compared to the conventional S-CBD method, this method prevented CdS QDs from aggregating at the entrance of TiO2 NTAs, and CdS QDs were uniformly dispersed on and inside TiO2 nanotubes. The CdS QDs/TiO2 NTAs samples exhibited an enhanced photocurrent generation and photocatalytic efficiency under visible illumination due to efficient separation of photogenerated electrons and holes. Recently, researchers have developed ternary TiO2 composites such as CdS/Pt/TiO2 and CdS/CdSe/TiO2 [176–178], yielding a higher photocatalytic efficiency.

Figure 13:

Top view (A) and side view (B) SEM images of TiO2 NTAs. SEM image (C) of CdS QDs/TiO2 NTAs. TEM (D) and HRTEM (E) images of CdS QDs/TiO2 NTAs. EDX spectra (F) of CdS QDs/TiO2 NTAs fitted to the TEM. Reproduced from Ref. [175] with permission. Copyright American Chemical Society.

[Figure caption was corrected after online publication February 10, 2016. Text before amendment: […] SEM image of CdS QDs/TiO2 NTAs. TEM (C) and HRTEM (D) images of CdS QDs/TiO2 NTAs. EDX spectra of CdS QDs/TiO2 NTAs fitted to the TEM (F). Reproduced from Ref. [174] with permission. […]]

As TiO2 is an n-type semiconductor, the construction of a p-n junction is believed to be one of the most effective strategies due to the existence of an internal electric field in the interface [179–184]. Liu et al. decorated the n-type TiO2 with p-type BiOI by S-CBD method [185]. The annealed TiO2 NTAs samples are immersed in Bi(NO3)3·5H2O and NaI by turns. The amount and thickness of BiOI nanoflakes are decided by the repeated cycles. As displayed in Figure 14, nanoflakes grow perpendicular to the wall of nanotubes, which is beneficial for the increase in the specific surface area at five repeated cycles (Figure 14B–D). A large amount of BiOI nanoflakes loaded on both the outer and inner walls of TiO2 nanotubes can serve as the light-transfer paths for distribution of the photo energy onto the deeper surfaces. Accordingly, the light utilization can be significantly enhanced due to the multiple reflections of light among the BiOI nanoflakes. Besides, the internal electric field caused by p-n BiOI/TiO2 heterojunction effectively prevented the recombination of electrons and holes. Consequently, BiOI/TiO2 NTAs photoelectrodes exhibited a more effective photoconversion efficiency than single TiO2 nanotubes. Furthermore, BiOI/TiO2 NTAs photoelectrodes also possessed superior photoelectrocatalytic activity and stability in methyl orange under visible light irradiation (Figure 14E,F). Other p-type semiconductors such as NiO, Bi2O3, CuO, Cu2O, etc., are widely used to modify TiO2 NTAs to construct the p-n heterojunction to enhance the visible light absorption and facilitate the transfer of photocarries [166, 186–188].

Figure 14:

SEM images of TiO2 NTAs (A) and BiOI/TiO2 NTAs with repeated times for five cycles (B). Insets (A), (B) are the side view images counterparts. TEM and HRTEM images of BiOI/TiO2 NTAs. Inset (C) is the EDX spectra. Photoelectrocatalytic degradation rate of MO of TiO2 NTAs and BiOI/TiO2 NTAs with different repeated deposition times (E). Cycling degradation curve of BiOI/TiO2 NTAs after four cycles (F). Reproduced from Ref. [185] with permission. Copyright Royal Society of Chemistry journals.

[Figure caption was corrected after online publication February 10, 2016. Text before amendment: […] Inset (A), (B) are the side view images counterpart. TEM and HRTEM images of BiOI/TiO2 NTAs. Inset (C) is the EDX spectra. Photoelectrocatalytic degradation rate of MO of TiO2 NTAs and BiOI/TiO2 NTAs with different repeated deposition times. Cycling degradation curve of BiOI/TiO2 NTAs after four cycles. […]]

Decorating TiO2 NTAs with an n-type semiconductor to form an n-n heterojunction is also beneficial for the separation of electrons and holes [189–192]. Ye et al. decorated the TiO2 NTAs with TiO2 nanoparticles by hydrolysis of TiCl4 solution, significantly increasing the surface area and improving the solar cell efficiency [193]. Li et al. successfully constructed the n-n type CuInS2/TiO2 NTA heterostructure via an ultrasonication-assisted cathodic electrodeposition strategy [194]. The CuInS2 particles were uniformly decorated on the inner and external walls of the TiO2 NTA electrode (Figure 15). The n-type CuInS2 could extend the visible-light response range and enhance the visible light photoactivity. Moreover, it facilitated the transfer of photocarriers and prevented the recombination of electrons and holes. The results indicated that the as-prepared CuInS2/TiO2 NTAs electrode displayed much higher photoelectrocatalytic degradation efficiency of 2-chlorophenol under visible light irradiation than that of the pure TiO2 NTA electrode. Besides, loading TiO2 NTAs with n-type ZnO can achieve higher photocatalytic activity than pure TiO2 NTAs [195–197].

Figure 15:

SEM image of TiO2 NTAs (A). Top view (B), side view images (C), and EDX spectra (D) of CuInS2/TiO2 NTAs. TEM (E) and HRTEM (F) images of CuInS2/TiO2 NTAs. Reproduced from Ref. [194] with permission. Copyright Elsevier.

3.5 Formation of perovskite MTiO3/TiO2 heterostructure

A large variety of perovskite-type oxides MTiO3 (M=Sr, Ca, Ba, Mg etc.) have been widely studied in photocatalysis, water splitting, solar cells, and dielectric devices due to good solar light absorption ability [198–204]. It is promising to construct a perovskite MTiO3/TiO2 heterostructure by hydrothermal treatment in the respective cation-containing solutions for photo/photoelectrocatalytic degradation of pollutants. When the amorphous TiO2 are hydrothermally treated in metal cation-containing aqueous solution, the amorphous and hydrothermally unstable TiO2 or TiOx (1<x≤2) species absorb water molecules to form soluble species of Ti(OH)62-. Then, the soluble species Ti(OH)62- can react with the metal cation to form perovskite oxide/TiO2-heterostructured NTAs [205]. The overall reaction is described by Equations (7)–(9).

(7)TiOx+(1-x/2)O2+4H2OTi(OH)62-+2H+(1<x2) (7)
(8)Ti(OH)62-+2H+TiO2+H2O (8)
(9)x/2Ti(OH)62-+MX+MTiO3+H2O (9)

During the hydrothermal reaction process, the Ti precursor can be the TiO2 nanotubes itself or introduced TiO2 nanoparticles. As shown in Figure 16, Huo et al. constructed the SrTiO3/TiO2 NTAs heterostructure by the one-step hydrothermal method using the TiO2 nanotubes as the Ti precursor [206]. The amount of SrTiO3 was decided by the hydrothermal reaction time. The wall of TiO2 nanotubes thickened from 12 to 21 nm when the reaction time increased from 30 min to 6 h due to crystal cell expansion induced by the transformation from TiO2 to SrTiO3. SrTiO3/TiO2 NTAs showed good photocatalytic activity because SrTiO3 can enhance the light absorption and effectively suppress the recombination of electrons/holes. Meanwhile, Sun et al. modified the TiO2 NTAs with SrTiO3/TiO2 hetero-nanoparticles by the two-step hydrothermal method using TiO2 nanoparticles as Ti precursor [207]. As displayed in Figure 17, TiO2 nanoparticles were uniformly dispersed on the TiO2 nanotube arrays when immersed in the TiCl4 solution (Figure 17B). Then, part of the TiO2 nanoparticles transformed into SrTiO3 by a hydrothermal reaction (Figure 17C,D). The hydrothermal time affected the amount of SrTiO3. Compared to pure TiO2 NTAs or SrTiO3/TiO2 NTAs, TiO2 NTAs modified with SrTiO3/TiO2 hetero-nanoparticles showed higher hydrogen production activity. ZnTiO3, BaTiO3, and CaTiO3/TiO2 NTA heterostructure have also been prepared by this simple hydrothermal technique [205, 208–210]. Recently, perovskite materials coupling TiO2 is a hot topic on photocatalysis, water splitting, and solar cells due to its excellent performance.

Figure 16:

Top view (A) and side view (B) images of TiO2 NTAs. SEM images of SrTiO3/TiO2 prepared by hydrothermal reaction for 0.5 h (C), 1 h (D), 3 h (E), and 6 h (F). Reproduced from Ref. [206] with permission. Copyright American Ceramic Society.

Figure 17:

SEM images of TiO2 NTAs (A), TiO2 nanoparticles modified TiO2 NTAs (B), and SrTiO3/TiO2 hetero-nanoparticles modified TiO2 NTAs (C). TEM image of SrTiO3/TiO2 hetero-nanoparticles modified TiO2 NTAs (D). Hydrogen production rate (E) of TiO2 NTAs, TiO2 nanoparticles modified TiO2 NTAs (T-TiO2 NTAs), SrTiO3/TiO2 NTAs (S-TiO2 NTAs) and SrTiO3/TiO2 hetero-nanoparticles modified TiO2 NTAs (ST-TiO2 NTAs). Cycling hydrogen production cure of ST-TiO2 NTAs (F). Reproduced from Ref. [207] with permission. Copyright Elsevier.

4 Photo/photoelectron-catalytic degradation of pollutants

With the fast development of economy, the environmental problems are more and more serious, especially clean water reduction and water contamination. Human beings have urgently called up dealing with these problems. Since then, TiO2 has shown to be an excellent photocatalyst due to low-cost, nontoxicity, long-term stability, and a strong-enough oxidizing power to be useful for the decomposition of unwanted organic compounds.

4.1 Photocatalysis

4.1.1 Pure TiO2 nanotube arrays for photocatalytic degradation

Compared to TiO2 nanoparticles and nanowires, TiO2 NTAs vertically oriented on the Ti substrate by electrochemical anodization have been paid more attention due to the oriented charge transfer channel, the large interfacial area, and especially recyclable ability [211–214]. However, associated with wide band gap (anatase: 3.2 eV, rutile: 3.0 eV) and the fast recombination of photogenerated electron-hole pairs, TiO2 NTAs can only absorb UV light (3–5% solar light), limiting its wide application [215–219]. As shown in Figure 18, upon UV illumination, electrons at the valance band transfer to the conduction band. Therefore, the electrons at the conduction band can react with oxygen to generate superoxide radicals, and holes at the valance band can react with water to generate hydroxyl radicals at the same time, resulting in degrading pollutants.

Figure 18:

Schematic diagram for photocatalytic degradation of pollutants under UV irradiation.

The effects of some structure factors of TiO2 NTAs on photocatalytic activity have also been discussed (Table 3). Zhuang et al. studied the effect of TiO2 nanotubes with different thicknesses and tube diameters on the photocatalytic degradation of methyl orange (MO) [220]. It was found that the thickness of nanotube arrays had a profound influence on the photocatalytic efficiency as a result of its surface area and the separation efficiency of photogenerated electron-hole pairs within the nanotube arrays, while the tube diameter showed only a minor effect on photocatalytic activity. Besides, Alsawat et al. studied the effect of annealing temperature on the photocatalytic degradation [88]. Compared to TiO2 NTAs annealed at 250°C, 450°C, and 850°C, TiO2 NTAs annealed at 650°C with a slight ratio mixture of anatase and rutile showed the fastest degradation rate because the rutile is generally considered to have better thermodynamical stability than anatase.

Table 3

Summary of the degradation of pollutants on TiO2 NTAs samples with different nanotube dimensions and crystallinity.

Nanotube length (μm)Pore diameter (nm)Annealing temperature (°C)Phase composition (%)Rate constant (min-1)Refs.
0.4100450Anatase0.0178[21]
1.5100450Anatase0.0188
2.5100450Anatase0.0222
3.1100450Anatase0.0210
3.5100450Anatase0.0223
3.555450Anatase0.0257
3.5100450Anatase0.0222
3.5125450Anatase0.0233
45120250Anatase0.014[88]
45120450Anatase0.018
4512065086.5% Anatase+13.5% Rutile0.023
4512085012.1% Anatase+87.9% Rutile0.004

4.1.2 Modified TiO2 nanotube arrays for photocatalytic degradation

Photocatalytic degradation of a toxic pollutant is one of the most practical applications of TiO2. However, low solar light absorption and fast recombination of electrons/holes limited its wide applications. Compared to bare TiO2 NTAs, modified TiO2 NTAs showed much better photocatalytic activity because modification can narrow the band of TiO2 NTAs, enhance visible light absorption, and suppress the recombination of electron/holes. Xiao et al. developed a facile layer-by-layer self-assembly technique to deposit noble metal (Au, Ag, Pt) on TiO2 NTAs [221]. The amount of noble metal (Au, Ag, Pt) was decided by the deposition cycles. As shown in Figure 19, Au, Ag, and Pt were uniformly dispersed on the TiO2 NTAs. Decoration with noble metal can effectively separate the photoelectron/holes pairs and enhance the light absorption. Therefore, noble metal (Au, Ag, Pt)/TiO2 NTAs showed much higher photocatalytic degradation efficiency of methyl orange under UV light irradiation. Owing to the SPR effect of noble metal, decoration TiO2 with noble metal can also enhance visible light absorption. Pan et al. doped TiO2 with metal (Ag, Pt, Pd), and they showed better photocatalytic activity under visible light illumination [222]. Liu et al. decorated TiO2 nanotubes with Ag nanoparticles via electrochemical technique, and the photocatalytic degradation rate of MO of Ag/TiO2 NTAs was 10 times higher than pure TiO2 NTAs [223]. Besides, Sim et al. performed photocatalytic decomposition of a model organic pollutant (methylene blue) with ternary GO/Ag/TiO2 NTAs structure [224]. First, photoreduction method was adopted to deposition Ag nanoparticles on TiO2 NTAs. Then, GO/Ag/TiO2 NTAs was constructed by dip-coating in graphene oxide solution. As displayed in Figure 20, GO sheets and Ag nanoparticles were uniformly dispersed on TiO2 NTAs. When TiO2 NTAs and Ag nanoparticles are in contact, there is a Schottky barrier between the surface TiO2 NTAs and Ag nanoparticles. Ag nanoparticles can overcome this barrier and inject electrons from Ag nanoparticles to the conduction band of TiO2 NTAs. The TiO2 NTAs function as an electron reservoir by capturing the electrons transferred from the GO and Ag, while GO served as an electron-accepting mediator between the MB and Ag NPs. Therefore, the photoelectron and holes are effectively separated. The photogenerated holes with strong oxidizing ability can degrade the organic pollutants (MB) to smaller chain segments or inorganic molecules, e.g. CO2 and H2O. Notably, GO/Ag/TiO2 three-component nanotube array system exhibited superior synergistic effect on both photoelectrochemical and photocatalytic activities compared to those of the pure TiO2 and Ag or GO-modified TiO2 systems. These results all demonstrated that modified TiO2 NTAs showed better photocatalytic degradation activity of pollutants under both UV and visible light.

Figure 19:

SEM images of TiO2 NTAs by two-step electrochemical anodization (A), Au/TiO2 NTAs (B), Ag/TiO2 NTAs (C), and Pt/TiO2 NTAs (D). Photocurrent of TiO2 NTAs and TiO2 NTAs decorated with Au, Ag, Pt (E). Photocatalytic degradation rate of MO under UV irradiation (F). Reproduced from Ref. [221] with permission. Copyright American Ceramic Society.

Figure 20:

SEM (A) and TEM (B) images of GO/Ag/TiO2 NTAs (A). Inset (A) is the EDX spectra. (C) Raman spectra of bare TNTs (a), GO-Ag/TNTs (b), Ag/TNTs (c). Photocatalytic degradation rates of MB for bare TNTs, Ag/TNTs, GO/TNTs, and GO-Ag/TNTs under visible light irradiation (D). Schematic diagrams of electron transfer and degradation mechanism of MB (E). Reproduced from Ref. [224] with permission. Copyright Royal Society of Chemistry journals.

4.2 Photoelectrocatalysis

4.2.1 Pure TiO2 nanotube arrays for photoelectrocatalytic degradation

Except for these modifications, photoelectrocatalysis (PEC) has been proven as a facile and promising route to solve the difficult problem of the fast recombination between photogenerated electron and holes, which can be beneficial for improving the photocatalytic degradation efficiency of the TiO2 NTAs [225–227]. As depicted in Figure 21, when a low bias potential was applied on the TiO2 NTAs, it significantly facilitated the transfer of photocarriers and suppressed the recombination of photogenerated electrons and holes. Upon UV light irradiation, the electrons can leap up the valance band of TiO2 to the conduction band, and then driven to the counter electrode via the external circuit, left the holes on the surface of the TiO2 NTA electrode. Meanwhile, a large number of active species were produced. The electrons at the counter electrode and at the conduction band can react with oxygen to generate superoxide radicals, while the holes at the valance band can react with water to generate hydroxyl radicals at the same time, resulting in degrading pollutants [219, 228–230].

Figure 21:

Schematic diagram for photoelectrocatalytic degradation of pollutants under UV irradiation.

Zhang et al. studied the effects of TiO2 nanotubes with different thicknesses, annealing temperature, and applied bias potential on the photoelectrocatalytic degradation of salicylic acid (Figure 22A–C) [231]. As can be seen, the length of TiO2 nanotubes and annealing temperature had profound influence on photoelectrocatalysis, while external bias potential had little effect on photoelectrocatalysis. The photocatalytic degradation activity increased when prolonging the length of nanotubes because the larger surface area is good for photon absorption, and the light can easily penetrate and scatter in such regular nanostructures. But photoelectrocatalytic activity started to decrease when the length increased to 8 μm. It is speculated that beyond a certain range, the diffusion of reacting species inside the inner tubes plays a key role in controlling the overall reaction rates. The photoelectrocatalytic degradation efficiency shows slight discrepancy from 400°C to 600°C, whereas at 700°C, a significant drop happens. Too much amount of rutile was bad for degradation of pollutants. Therefore, we can conclude that photoelectrolytic activity is not only related with crystalline structure but also closely associated with the micromorphology. There was a little enhancement in degradation of salicylic acid from 0.5 to 0.8 V and even higher potential. Besides, compared with photocatalysis, photolysis, and electrocatalysis, photoelectrocatalysis achieved the best degradation efficiency, which could be ascribed to the synergetic effect of light irradiation and external electric field (Figure 22D).

Figure 22:

Effect of TiO2 NTAs length (A) and annealing temperature (B) on salicylic acid degradation in photoelectrocatalysis process at 0.5 V. Effect of bias potential on salicylic acid degradation in photoelectrocatalysis process with 6 μm length and annealed at 500°C (C). Comparison of salicylic acid degradation ratios under different situations (D). C0 is initial concentration, Ct is the concentration after a certain reaction time. Reproduced from Ref. [231] with permission. Copyright Elsevier.

4.2.2 Modified TiO2 nanotube arrays for photoelectrocatalytic degradation

Though applied bias potential can effectively suppress the recombination of electron and holes, absorption of UV light still limited its photoelectrocatalytic application. Modified TiO2 nanotube arrays displayed much better photoelectrocatalytic degradation of pollutants than pure TiO2 NTAs under both UV and visible light irradiation [232–234]. For example, Wang et al. prepared Cu2O/TiO2 NTAs p-n heterojunction photoelectrodes via an ultrasonication-assisted sequential chemical bath to yield the p-type Cu2O nanoparticles on n-type TiO2 nanotube arrays [235]. The Cu2O nanoparticles were uniformly deposited on TiO2 nanotubes (Figure 23A–D). The p-n heterojunction effectively improved separation of photogenerated electrons and holes and enhanced the absorption of visible light. Consequently, p-n Cu2O/TiO2 heterojunction photoelectrodes exhibited a more effective photoconversion capability than single TiO2 nanotubes (Figure 23E). Furthermore, compared to bare photocatalysis and electrocatalysis, Cu2O/TiO2 composite photoelectrodes also possessed superior photoelectrocatalytic activity and stability in rhodamine B degradation with a synergistic effect between electricity and visible light irradiation. Besides, Zhang et al. constructed p-n Cu2O/TiO2 NTAs heterostructure by electrodeposition [166]. When compared with pure TNAs, the Cu2O/TiO2 heterojunction composites exhibit considerably higher photocurrent density and enhanced photoelectrocatalytic activity for the visible-light-driven photodegradation of methyl orange. Moreover, Cu2O/TiO2 composite photoelectrodes also possessed superior photoelectrocatalytic activity than photocatalytic performance, which was consistent with Wang’s results. The enhanced photoelectrocatalytic activity can be attributed to reducing the recombination rate of the photoexcited electron/hole pairs in TiO2 NTAs when coupled with Cu2O nanoparticles.

Figure 23:

SEM image of TiO2 NTAs (A) and Cu2O/TiO2 NTAs (B, C). TEM image (D) of Cu2O/TiO2 NTAs. Photoresponse (E) of Cu2O/TiO2 NTAs with (curve A) or without (curve C) a bias potential of 0.5 V (vs. SCE) and TiO2 NTAs with (curve B) or without (curve D) a bias potential of 0.5 V (versus SCE) under visible light irradiation. (F) Comparison of photocatalytic, electrocatalytic, and photoelectrocatalytic degradation of RhB under visible light irradiation for Cu2O/TiO2 NTAs. (G) Comparison of RhB degradation efficiency using Cu2O/TiO2 NTAs in the process of photoelectrocatalysis (right) and with the sum of photocatalysis and electrolysis (left). (H) Schematic illustration of photoelectrocatalytic degradation of RhB under visible light irradiation. Reproduced from Ref. [235] with permission. Copyright Royal Society of Chemistry journals.

[Figure caption was corrected after online publication February 10, 2016. Text before amendment: […] TEM image of Cu2O/TiO2 NTAs (D). Photoresponse of Cu2O/TiO2 NTAs with (A) or without (C) a bias potential of 0.5 V (vs. SCE) and TiO2 NTAs with (B) or without (D) a bias potential of 0.5 V (versus SCE) under visible light irradiation. Comparison of photocatalytic, electrocatalytic, and photoelectrocatalytic degradation of RhB under visible light irradiation for Cu2O/TiO2 NTAs (F). Comparison of RhB degradation efficiency using Cu2O/TiO2 NTAs in the process of photoelectrocatalysis (right) and with the sum of photocatalysis and electrolysis (left) (G). Schematic illustration of photoelectrocatalytic degradation of RhB under visible light irradiation (H). […]]

Besides, doping TiO2 NTAs with non-metal materials such as graphene, boron showed much higher photoelectrocatalytic activity [236, 237]. Zhai et al. developed an electrodeposition method to make Pt flowers and reduced graphene oxide (RGO) uniformly dispersed on highly ordered TiO2 NTAs (Figure 24) [238]. The RGO acted as electron conductor, while modification of Pt enhanced the absorption of visible light, improving separation of photogenerated electrons and holes. Consequently, Pt/RGO/TiO2 NTA photoelectrodes exhibited a more effective photoconversion capability than single TiO2 nanotubes and Pt/TiO2 NTAs. Furthermore, compared to bare photocatalysis and electrocatalysis, Pt/RGO/TiO2 NTA photoelectrodes also possessed superior photoelectrocatalytic activity and stability in MB degradation. In conclusion, it is easy operation for photocatalysis, but photoelectrocatalysis showed promising prospects for much higher degradation efficiency than photocatalysis in the future.

Figure 24:

SEM images of TiO2 NTAs (A, B) and Pt/RGO/TiO2 NTAs (C, D). Photoelectrocatalytic activities of TiO2 NTAs, Pt/TiO2 NTAs, and Pt/RGO/TiO2 NTAs under visible light irradiation (E). Comparison of photocatalytic, electrocatalytic, and photoelectrocatalytic degradation of MB under visible light irradiation for Pt/RGO/TiO2 NTAs (F). Reproduced from Ref. [238] with permission. Copyright American Ceramic Society.

5 Other main promising applications

5.1 Solar cells

One of the most promising applications of TiO2 today is in solar cells, particularly dye-sensitized (Figure 25A), quantum dots sensitized [239–241]. Compared to TiO2 nanoparticles, TiO2 nanotube arrays by anodization obtained high-energy conversion efficiency due to improved charge-collection efficiency and short pathway for the photogenerated excitons along the vertically aligned tubes [242–245]. However, it showed low power conversion efficiency under solar light due to the wide band and low utilization of solar light. Therefore, sensitized with organic dyes or inorganic narrow band gap semiconductors makes TiO2 absorb solar light and convert solar energy into electrical energy at the same time. The dye-sensitized solar cell (DSSC), a concept first introduced by O’Regan and Grätzel in 1991 [246], is widely studied for achieving the goal of high efficiency and low cost in the utilization of solar energy [247–249]. At an early stage of the development on DSSC, Mor’s group first applied TiO2 nanotube arrays coated with TiO2 nanoparticles to dye-sensitized solar cells and displayed a power conversion efficiency (PCE) of 2.9% [250]. Besides, Shankar et al. fabricated long, vertically aligned TiO2 nanotubes arrays with lengths between 10 and 220 μm by electrochemical anodization method. Dye-sensitized solar cells containing these arrays yielded a maximum power conversion efficiency of 6.89% [55]. In order to further increase the overall photo conversion efficiency, surface modification and morphology design have been explored to enhance the electron transfer and separation ability of TiO2 NTAs. So et al. constructed hierarchical DSSC structures based on “single-walled” TiO2 nanotube arrays coupled with TiO2 nanoparticles by layer and layer deposition, and it reached a solar light conversion efficiency over 8% [251].

Besides, noble metals such as Ag, Au, Pt, and Pd are used to modify TiO2 NTAs for enhancing the solar light absorption, and the hybrids showed much improved photoelectric conversion efficiency (Figure 25B and C [252, 253]. In addition, quantum dot-sensitized solar cells (QDSSC) have been more and more popular and are considered as promising alternatives to DSSCs due to higher light absorption, low-cost, and long cycling stability [254–257]. Coupling TiO2 NTAs with other semiconductor quantum dots (CdS, CdSe, CdTe, PbS, carbon, etc.) provide new opportunities for harvesting light ranging from the visible to the infrared regions of solar light, suppressing the recombination of electrons/holes and facilitating the transfer of photoelectrons. In the cases of the QDSSC, upon light irradiation, quantum dots can absorb photons and excited electrons move from the valence band of quantum dots to their conduction band. Owing to the good match of energy levels, electrons on the conduction band of quantum dots then flowed into that of TiO2. Then, the photoelectrons in the CB are collected by transparent conductive oxide, flow through the external circuit. Finally, holes are transported to the counter electrode and react with a redox couple. During this process, photogenerated electrons and holes are effectively separated [258–260]. As we all know, these quantum dots can be easily deposited on the surface of one-dimensional (1D) TiO2 NTAs at room temperature by successive ionic layer-by-layer adsorption and reaction (SILAR) [261], chemical bath deposition (CBD) [262], and electrodeposition method [263]. Recently, Lai et al. fabricated 1D CdSe/CdS@TiO2 core-shell nanotube array for quantum dots co-sensitized solar cell application. Under optimum conditions, the CdS/CdSe co-sensitized QDSSC demonstrated conversion efficiency of 2.40% under simulated sunlight [264]. Besides, Sun et al. sensitized TiO2 TNAs with CdS quantum dot photoelectrodes by a sequential chemical bath deposition technique, and the PCE of QDSSC efficiency increased up to 4.15% under solar light due to the fast and efficient transfer of the photogenerated electrons and enhanced light harvest (Figure 25D–F) [265]. These results clearly demonstrate that the synergistic effect of unique nanotube structure and quantum dots can facilitate the separation and transfer of photogenerated charges.

Figure 25:

Backside illuminated DSSC based on TiO2 NTAs on Ti substrate (A). SEM image of Pt/TiO2 NTAs (B). J-V curves of DSSCs with Pt/FTO and Pt/TiO2 NTAs/Ti mesh as counter electrode (C). Sketch showing the nanostructure of CdS modified TiO2 nanotube film electrode and charge-transfer processes between CdS and TiO2 (D). SEM image of CdS/TiO2 NTAs (E). Photocurrent versus voltage spectra (F) in 1-M Na2S solution under solar light illumination for plain (a) and CdS quantum dot-modified TiO2 nanotube film electrode (b). (c, d) Corresponding currents in the dark for curves a and b. Reproduced from Ref. [30] with permission. Copyright Hindawi Publishing Corporation. Reproduced from Ref. [252] with permission. Copyright Elsevier. Reproduced from Ref. [265] with permission. Copyright American Ceramic Society.

5.2 Supercapacitors

In recent years, supercapacitors have attracted much interests for use in energy storage due to high power density, fast rates of charge/discharge, reliable cycling life, and safe operation [266–269]. Salari et al. compared the capacitance of TiO2 NTAs and TiO2 power, and TiO2 NTAs (911 μF·cm-2) exhibited much higher capacitance than TiO2 powder due to tubular channel paths and highly active surface sites for ion diffusion and charge transfer [270]. However, the charge storage ability of TiO2 nanotube arrays is too low to be the ideal electrode material of supercapacitors due to their low conductivity. Therefore, assembling TiO2 NTAs with metal oxides (RuO2, MnO2, NiO etc.) [271–273], conducting polymers (PANI) [274], or carbon materials [275] is an efficient approach to obtaining a low-cost, high-performance supercapacitors. Lu et al. reported a new and general strategy for improving the capacitive properties of TiO2 materials for supercapacitors, involving the synthesis of hydrogenated TiO2 nanotube arrays by calcination in hydrogen atmosphere. The H-TiO2 NTAs prepared at 400°C yields the largest specific capacitance of 3.24 mF·cm-2 at a scan rate of 100 mV·s-1, which is 40 times higher than the capacitance obtained from air-annealed TiO2 NTAs at the same conditions [91]. As described before, Gao et al. fabricated MnO2-C/TiO2 NTAs ternary structure by a combination of carbonization and electrodeposition strategy (Figure 26) [126]. The MnO2-C/TiO2 NTAs electrode showed a high specific capacitance of 580 F·g-1 at a current density of 2.6 F·g-1, an excellent rate capability with capacitance retention of 60% when the scanning rate increased 50 times, as well as a good cycling stability. As a new category of supercapacitor electrodes, more efforts are underway to further improve the specific capacitance and performance of TiO2 nanostructured materials.

Figure 26:

The fabrication process of the well-aligned MnO2-TiO2/C NTAs (A). SEM image of MnO2-TiO2/C NTAs (B). GCDs (C) and gravimetric capacitance (D) at different scanning rates and current densities of the MnO2-TiO2/C NTAs. (E) Cycling stability of the MnO2-TiO2/C NTAs at a current density of 13 F·g-1 for 1000 cycles. Reproduced from Ref. [126] with permission. Copyright Elsevier.

[Corrections added after online publication February 10, 2016: Figure 26: Within the caption (H) was changed into (E).]

6 Conclusion and outlook

In addition, photo/photoelectrocatalytic degradation of pollutants, TiO2 nanotube arrays are widely used in water splitting, supercapacitors, dye-sensitized solar cells, and biological materials due to excellent physical and chemical properties. Until now, a large number of fundamental studies on developments and applications are extensively carried out by many researchers for this 1D nanomaterial. This work has presented the recent progress of preparation, modification, and photo/photoelectrocatalysis on the electrochemically anodized TiO2 NTA materials. However, extensive challenges on fabrication of high-quality TiO2 nanotube arrays continued. On the one hand, it is urgent to seek new modification method to improve the transfer efficiency of photocarriers and suppress the recombination of electrons and holes. On the other hand, it is vital to improve the photo and electron conversion efficiency. As applied bias potential can significantly facilitated the transfer of photocarriers and suppressed the recombination of photogenerated electrons and holes, photoelectrocatalysis showed much better photoelectric activity than bare photocatalysis due to the synergetic effect of light irradiation and external electric field. Suffered from serious environmental problems and the shortage of fuel fossils, photoelectrocatalysis for degradation of pollutants and hydrogen production showed promising prospects to solve these problems in the future.


Corresponding author: Yue-Kun Lai, National Engineering Laboratory for Modern Silk, College of Textile and Clothing Engineering, Soochow University, Suzhou 215123, China; and Research Center of Cooperative Innovation for Functional Organic/Polymer Material Micro/Nanofabrication, Soochow University, Suzhou 215123, China, e-mail:
aMing-Zheng Ge and Chun-Yan Cao: These authors contributed equally to this publication.

Acknowledgments

The authors thank the National Natural Science Foundation of China (21501127; 51502185), Natural Science Foundation of Jiangsu Province of China (BK20130313; BK20140400). We also acknowledge the funds from the project of the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and Project for Jiangsu Scientific and Technological Innovation Team (2013).

References

[1] Litter MI. Heterogeneous photocatalysis – transition metal ions in photocatalytic systems. Appl. Catal. B-Environ. 1999, 23, 89–114. Search in Google Scholar

[2] Malato S, Fernández-Ibanez P, Maldonado MI, Blanco J, Gernjak W. Decontamination and disinfection of water by solar photocatalysis: recent overview and trends. Catal. Today 2009, 147, 1–59. Search in Google Scholar

[3] Pronina N, Klauson D, Moiseev A, Deubener J, Krichevskaya M. Titanium dioxide sol-gel-coated expanded clay granules for use in photocatalytic fluidized-bed reactor. Appl. Catal. B-Environ. 2015, 178, 117–123. Search in Google Scholar

[4] Zhang XY, Deng YJ, Liu JK, Lu Y, Yang XH. Mass preparation and novel visible light photocatalytic activity of C and Ag Co-modified ZnO nanocrystals. J. Colloid Interf. Sci. 2015, 459, 1–9. Search in Google Scholar

[5] Vimonses V, Jin B, Chow CWK, Saint C. An adsorption-photocatalysis hybrid process using multi-functional-nanoporous materials for wastewater reclamation. Water Res. 2010, 44, 5385–5397. Search in Google Scholar

[6] Yap PS, Lim TT. Effect of aqueous matrix species on synergistic removal of bisphenol-A under solar irradiation using nitrogen-doped TiO2/AC composite. Appl. Catal. B-Environ. 2011, 101, 709–717. Search in Google Scholar

[7] Fujishima A, Honda K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972, 238, 37–38. Search in Google Scholar

[8] Ji Y, Zhang M, Cui J, Lin KC, Zheng H, Zhu JJ, Samia ACS. Highly-ordered TiO2 nanotube arrays with double-walled and bamboo-type structures in dye-sensitized solar cells. Nano Energy 2012, 1, 796–804. Search in Google Scholar

[9] Macak JM, Zlamal M, Krysa J, Schmuki P. Self-organized TiO2 nanotube layers as highly efficient photocatalysts. Small 2007, 3, 300–304. Search in Google Scholar

[10] Lai YK, Pan F, Xu C, Fuchs H, Chi LF. In situ surface-modification-induced superhydrophobic patterns with reversible wettability and adhesion. Adv. Mater. 2013, 25, 1682–1686. Search in Google Scholar

[11] Huang JY, Lai YK, Pan F, Yang L, Wang H, Zhang KQ, Fuchs H, Chi LF. A multifunctional superamphiphobic TiO2 nanostructure surfaces with facile wettability and adhesion engineering. Small 2014, 10, 4865–4873. Search in Google Scholar

[12] Seger B, Pedersen T, Laursen AB, Vesborg PC, Hansen O, Chorkendorff I. Using TiO2 as a conductive protective layer for photocathodic H2 evolution. J. Am. Chem. Soc. 2013, 135, 1057–1064. Search in Google Scholar

[13] Lai YK, Lin LX, Pan F, Huang JY, Song R, Huang YX, Lin CJ, Fuchs H, Chi LF. Bioinspired patterning with extreme wettability contrast on TiO2 nanotube array surface: a versatile platform for biomedical applications. Small 2013, 9, 2945–2953. Search in Google Scholar

[14] Wang H, Lai YK, Zheng RY, Bian Y, Zhang KQ, Lin CJ. Tuning the surface microstructure of titanate coatings on titanium implants for enhancing bioactivity of implants. Int. J. Nanomedicine 2015, 10, 3887–3896. Search in Google Scholar

[15] Xiong BT, Zhou BX, Bai J, Zheng Q, Liu YB, Cai WM, Cai J. Light scattering of nanocrystalline TiO2 film used in dye-sensitized solar cells. Chinese Phys. B 2008, 17, 3713–3719. Search in Google Scholar

[16] Wang LN, Jing M, Zheng YD, Guan YP, Lu X, Luo JL. Nanotubular surface modification of metallic implants via electrochemical anodization technique. Int. J. Nanomedicine 2014, 9, 4421–4435. Search in Google Scholar

[17] Zheng Q, Zhou BX, Bai J, Li LH, Jin ZJ, Zhang JL, Li JH, Liu YB, Cai WM, Zhu XY. Self-organized TiO2 nanotube array sensor for the determination of chemical oxygen demand. Adv. Mater. 2008, 20, 1044–1049. Search in Google Scholar

[18] Iijima S. Helical microtubules of graphitic carbon. Nature 1991, 354, 56–58. Search in Google Scholar

[19] Sander MS, Cote MJ, Gu W, Kile BM, Tripp CP. Template-assisted fabrication of dense, aligned arrays of titania nanotubes with well-controlled dimensions on substrates. Adv. Mater. 2004, 16, 2052–2057. Search in Google Scholar

[20] Lee J, Kim DH, Hong SH, Jho JY. A hydrogen gas sensor employing vertically aligned TiO2 nanotube arrays prepared by template-assisted method. Sensors and Actuat. B-Chem. 2011, 160, 1494–1498. Search in Google Scholar

[21] Tian ZRR, Voigt JA, Liu J, McKenzie B, Xu HF. Large oriented arrays and continuous films of TiO2-based nanotubes. J. Am. Chem. Soc. 2003, 125, 12384–12385. Search in Google Scholar

[22] Sun XM, Li YD. Synthesis and characterization of ion-exchangeable titanate nanotubes. Chem. Eur. J. 2003, 9, 2229–2238. Search in Google Scholar

[23] Bavykin DV, Friedrich JM, Walsh FC. Protonated titanates and TiO2 nanostructured materials: synthesis, properties, and applications. Adv. Mater. 2006, 18, 2807–2824. Search in Google Scholar

[24] Gong D, Grimes CA, Varghese OK, Hu WC, Singh RS, Chen Z, Dickey EC. Titanium oxide nanotube arrays prepared by anodic oxidation. J. Mater. Res. 2001, 16, 3331–3334. Search in Google Scholar

[25] Grimes CA. Synthesis and application of highly ordered arrays of TiO2 nanotubes. J. Mater. Chem. 2007, 17, 1451–1457. Search in Google Scholar

[26] Lau YK, Gao XF, Zhuang HF, Huang JY, Lin CJ, Jiang L. Designing superhydrophobic porous nanostructures with tunable water adhesion. Adv. Mater. 2009, 21, 3799–3803. Search in Google Scholar

[27] Macak JM, Schmuki P. Anodic growth of self-organized anodic TiO2 nanotubes in viscous electrolytes. Electrochim. Acta 2006, 52, 1258–1264. Search in Google Scholar

[28] Roy P, Berger S, Schmuki P. TiO2 nanotubes: synthesis and applications. Angew. Chem. Int. Ed. 2011, 50, 2904–2939. Search in Google Scholar

[29] Hoyer P. Formation of a titanium dioxide nanotube array. Langmuir 1996, 12, 1411–1413. Search in Google Scholar

[30] Huang JY, Zhang KQ, Lai YK. Fabrication, modification, and emerging applications of TiO2 nanotube arrays by electrochemical synthesis: a review. Int. J. Photoenergy 2013, 2013, 761971. Search in Google Scholar

[31] Kasuga T, Hiramatsu M, Hoson A, Sekino T, Niihara K. Formation of titanium oxide nanotube. Langmuir 1998, 14, 3160–3163. Search in Google Scholar

[32] Bavykin DV, Parmon VN, Lapkin AA, Walsh FC. The effect of hydrothermal conditions on the mesoporous structure of TiO2 nanotubes. J Mater. Chem. 2004, 14, 3370–3377. Search in Google Scholar

[33] Parayil SK, Razzaq A, Park SM, Kim HR, Grimes CA, In SI. Photocatalytic conversion of CO2 to hydrocarbon fuel using carbon and nitrogen co-doped sodium titanate nanotubes. Appl Catal. A-Gen. 2015, 498, 205–213. Search in Google Scholar

[34] Qin GH, Zhang J, Wang CY. Constructing robust TiO2-V2O5/C nanostructures decorated by multi-walled carbon nanotubes for high performance lithium ion batteries. J. Alloy. Compd. 2015, 635, 158–162. Search in Google Scholar

[35] Tang YX, Zhang YY, Deng JY, Wei JQ, Tam HL, Chandran BK, Dong ZL, Chen Z, Chen XD. Mechanical force-driven growth of elongated bending TiO2-based nanotubular materials for ultrafast rechargeable lithium ion batteries. Adv. Mater. 2014, 26, 6111–6118. Search in Google Scholar

[36] Tang YX, Zhang YY, Deng JY, Qi DP, Leow WR, Wei JQ, Yin SY, Dong ZL, Yazami R, Chen Z, Chen XD. Unravelling the correlation between the aspect ratio of nanotubular structures and their electrochemical performance to achieve high-rate and long-life lithium-ion batteries. Angew. Chem. Int. Ed. 2014, 53, 13488–13492. Search in Google Scholar

[37] Zwilling V, Darque-Ceretti E, Boutry-Forveille A, David D, Perrin MY, Aucouturier M. Structure and physicochemistry of anodic oxide films on titanium and TA6V alloy. Surf. Interface Anal. 1999, 27, 629–637. Search in Google Scholar

[38] Li HQ, Lai YK, Huang JY, Tang YX, Yang L, Chen Z, Zhang KQ, Wang XC, Tan LP. Multifunctional wettability patterns prepared by laser processing on superhydrophobic TiO2 nanostructured surfaces. J. Mater. Chem. B 2015, 3, 342–347. Search in Google Scholar

[39] Seong WM, Kim DH, Park IJ, Park G D, Kang K, Lee S, Hong KS. Roughness of Ti substrates for control of the preferred orientation of TiO2 nanotube arrays as a new orientation factor. J. Phys. Chem. C 2015, 119, 13297–13305. Search in Google Scholar

[40] Chen CL, Dong CL, Chen CH, Wu JW, Lu YR, Lin CJ, Liou SYH, Tseng CM, Kumar K, Wei DH, Guo J, Chou WC, Wu MK. Electronic properties of free-standing TiO2 nanotube arrays fabricated by electrochemical anodization. Phys. Chem. Chem. Phys. 2015, 17, 22064–22071. Search in Google Scholar

[41] Cummings FR, Muller TFG, Malgas GF, Arendse CJ. Investigation of the growth and local stoichiometric point group symmetry of titania nanotubes during potentiostatic anodization of titanium in phosphate electrolytes. J. Phys. Chem. Solids 2015, 85, 278–286. Search in Google Scholar

[42] Passalacqua R, Perathoner S, Centi G. Use of modified anodization procedures to prepare advanced TiO2 nanostructured catalytic electrodes and thin film materials. Catal. Today 2015, 251, 121–131. Search in Google Scholar

[43] Mor GK, Varghese OK, Paulose M, Shankar K, Grimes CA. A review on highly ordered, vertically oriented TiO2 nanotube arrays: fabrication, material properties, and solar energy applications. Sol. Energ. Mat. Sol. C. 2006, 90, 2011–2075. Search in Google Scholar

[44] Rani S, Roy SC, Paulose M, Varghese OK, Mor GK, Kim S, Yoriya S, LaTempa TJ, Grimes CA. Synthesis and applications of electrochemically self-assembled titania nanotube arrays. Phys. Chem. Chem. Phys. 2010, 12, 2780–2800. Search in Google Scholar

[45] Su YF, Wu Z, Wu YN, Yu JD, Sun L, Lin CJ. Acid Orange II degradation through a heterogeneous Fenton-like reaction using Fe-TiO2 nanotube arrays as a photocatalyst. J. Mater. Chem. A 2015, 3, 8537–8544. Search in Google Scholar

[46] Wu Z, Wang YY, Sun L, Mao YX, Wang MY, Lin CJ. An ultrasound-assisted deposition of NiO nanoparticles on TiO2 nanotube arrays for enhanced photocatalytic activity. J. Mater. Chem. A 2014, 2, 8223–8229. Search in Google Scholar

[47] Lai YK, Wang LN, Liu DW, Chen Z, Lin CJ. TiO2-based nanomaterials: design, synthesis, and applications. J. Nanomater. 2015, 2015, 250632. Search in Google Scholar

[48] Zhao ZH, Tian J, Sang YH, Cabot A, Liu H. Structure, synthesis, and applications of TiO2 nanobelts. Adv. Mater. 2015, 27, 2557–2582. Search in Google Scholar

[49] Tian J, Zhao ZH, Kumar A, Boughton RI, Liu H. Recent progress in design, synthesis, and applications of one-dimensional TiO2 nanostructured surface heterostructures: a review. Chem. Soc. Rev. 2014, 43, 6920–6937. Search in Google Scholar

[50] Assefpourdezfuly M, Vlachos C, Andrews EH. Oxide morphology and adhesive bonding on titanium surfaces. J. Mater. Sci. 1984, 19, 3626–3639. Search in Google Scholar

[51] Macak JM, Sirotna K, Schmuki P. Self-organized porous titanium oxide prepared in Na2SO4/NaF electrolytes. Electrochim. Acta 2005, 50, 3679–3684. Search in Google Scholar

[52] Yasuda K, Schmuki P. Control of morphology and composition of self-organized zirconium titanate nanotubes formed in (NH4)2SO4/NH4F electrolytes. Electrochim. Acta 2007, 52, 4053–4061. Search in Google Scholar

[53] Cai QY, Paulose M, Varghese OK, Grimes CA. The effect of electrolyte composition on the fabrication of self-organized titanium oxide nanotube arrays by anodic oxidation. J. Mater. Res. 2005, 20, 230–236. Search in Google Scholar

[54] Yoriya S, Grimes CA. Self-assembled TiO2 nanotube arrays by anodization of titanium in diethylene glycol: approach to extended pore widening. Langmuir 2010, 26, 417–420. Search in Google Scholar

[55] Shankar K, Mor GK, Prakasam HE, Yoriya S, Paulose M, Varghese OK, Grimes CA. Highly-ordered TiO2 nanotube arrays up to 220 μm in length: use in water photoelectrolysis and dye-sensitized solar cells. Nanotechnology 2007, 18, 065707. Search in Google Scholar

[56] Paulose M, Prakasam HE, Varghese OK, Peng L, Popat KC, Mor GK, Desai TA, Grimes CA. TiO2 nanotube arrays of 1000 μm length by anodization of titanium foil: phenol red diffusion. J. Phys. Chem. C 2007, 111, 14992–14997. Search in Google Scholar

[57] Hahn R, Macak JM, Schmuki P. Rapid anodic growth of TiO2 and WO3 nanotubes in fluoride free electrolytes. Electrochem. Commun. 2007, 9, 947–952. Search in Google Scholar

[58] Umebayashi Y, Mitsugi T, Fukuda S, Fujimori T, Fujii K, Kanzaki R, Takeuchi M, Ishiguro SI. Lithium ion solvation in room-temperature ionic liquids involving bis(trifluoromethanesulfonyl) imide anion studied by Raman spectroscopy and DFT calculations. J. Phys. Chem. B 2007, 111, 13028–13032. Search in Google Scholar

[59] Richter C, Wu Z, Panaitescu E, Willey RJ, Menon L. Ultrahigh- aspect-ratio titania nanotubes. Adv. Mater. 2007, 19, 946–948. Search in Google Scholar

[60] Macak JM, Albu SP, Schmuki P. Towards ideal hexagonal self-ordering of TiO2 nanotubes. Phys. Status Solidi RRL 2007, 1, 181–183. Search in Google Scholar

[61] Shin Y, Lee S. Self-organized regular arrays of anodic TiO2 nanotubes. Nano Lett. 2008, 8, 3171–3173. Search in Google Scholar

[62] Ge MZ, Li SH, Huang JY, Zhang KQ, Al-Deyab SS, Lai YK. TiO2 nanotube arrays loaded with reduced graphene oxide films: facile hybridization and promising photocatalytic application. J. Mater. Chem. A 2015, 3, 3491–3499. Search in Google Scholar

[63] Ye MD, Gong JJ, Lai YK, Lin CJ, Lin ZQ. High-efficiency photoelectrocatalytic hydrogen generation enabled by palladium quantum dots-sensitized TiO2 nanotube arrays. J. Am. Chem. Soc. 2012, 134, 15720–15723. Search in Google Scholar

[64] Hua H, Hu CG, Zhao ZH, Liu H, Xie X, Xi Y. Pt nanoparticles supported on submicrometer-sized TiO2 spheres for effective methanol and ethanol oxidation. Electrochim. Acta 2013, 105, 130–136. Search in Google Scholar

[65] Yu DL, Zhu XF, Xu Z, Zhong XM, Gui QF, Song Y, Zhang SY, Chen XY, Li DD. Facile method to enhance the adhesion of TiO2 nanotube arrays to Ti substrate. ACS Appl. Mater. Interfaces 2014, 6, 8001–8005. Search in Google Scholar

[66] Lai YK, Zhuang HF, Sun L, Chen Z, Lin CJ. Self-organized TiO2 nanotubes in mixed organic-inorganic electrolytes and their photoelectrochemical performance. Electrochim. Acta 2009, 54, 6536–6542. Search in Google Scholar

[67] Zhu K, Vinzant TB, Neale NR. Frank AJ. Removing structural disorder from oriented TiO2 nanotube arrays: reducing the dimensionality of transport and recombination in dye-sensitized solar cells. Nano Lett. 2007, 7, 3739–3746. Search in Google Scholar

[68] Lin J, Chen XF. Synthesis of high-aspect-ratio, top-open and ultraflat-surface TiO2 nanotubes through double-layered configuration. Phys. Status Solidi RRL 2012, 6, 28–30. Search in Google Scholar

[69] Vaenas N, Stergiopoulos T, Kontos AG, Likodimos V, Boukos N, Falaras P. Sensitizer activated solar cells based on self-organized TiO2 nanotubes. Microelectron. Eng. 2012, 90, 62–65. Search in Google Scholar

[70] Roy P, Albu SP, Schmuki P. TiO2 nanotubes in dye-sensitized solar cells: higher efficiencies by well-defined tube tops. Electrochem. Commun. 2010, 12, 949–951. Search in Google Scholar

[71] Chanmanee W, Watcharenwong A, Chenthamarakshan CR, Kajitvichyanukul P, de Tacconi NR, Rajeshwar K. Formation and characterization of self-organized TiO2 nanotube arrays by pulse anodization. J. Am. Chem. Soc. 2008, 130, 965–974. Search in Google Scholar

[72] Liao YL, Zhang DN, Wang Q, Wen TL, Jia LJ, Zhong ZY, Bai FM, Tang LH, Que WQ, Zhang HW. Open-top TiO2 nanotube arrays with enhanced photovoltaic and photochemical performances via a micromechanical cleavage approach. J. Mater. Chem. A 2015, 3, 14279–14283. Search in Google Scholar

[73] Sun Y, Yan KP. Effect of anodization voltage on performance of TiO2 nanotube arrays for hydrogen generation in a two-compartment photoelectrochemical cell. Int. J. Hydrogen Energy 2014, 39, 11368–11375. Search in Google Scholar

[74] Regonini D, Satka A, Jaroenworaluck A, Allsopp DWE, Bowen CR, Stevens R. Factors influencing surface morphology of anodized TiO2 nanotubes. Electrochim. Acta 2012, 74, 244–253. Search in Google Scholar

[75] Atyaoui A, Cachet H, Sutter EMM, Bousselmi L. Effect of the anodization voltage on the dimensions and photoactivity of titania nanotubes arrays. Surf. Interface Anal. 2013, 45, 1751–1759. Search in Google Scholar

[76] Huang JY, Lai YK, Wang LN, Li SH, Ge MZ, Zhang KQ, Fuchs H, Chi LF. Controllable wettability and adhesion on bioinspired multifunctional TiO2 nanostructure surfaces for liquid manipulation. J. Mater. Chem. A 2014, 2, 18531–18538. Search in Google Scholar

[77] Albu SP, Schmuki P. TiO2 nanotubes grown in different organic electrolytes: two-size self-organization, single vs. double-walled tubes, and giant diameters. Phys. Status Solidi RRL 2010, 4, 215–217. Search in Google Scholar

[78] Regonini D, Bowen CR, Jaroenworaluck A, Stevens R. A review of growth mechanism, structure and crystallinity of anodized TiO2 nanotubes. Mat. Sci. Eng. R: Rep. 2013, 74, 377–406. Search in Google Scholar

[79] Acevedo-Pena P, Lartundo-Rojas L, Gonzalez I. Effect of water and fluoride content on morphology and barrier layer properties of TiO2 nanotubes grown in ethylene glycol-based electrolytes. J. Solid State Electr. 2013, 17, 2939–2947. Search in Google Scholar

[80] Huo KF, Gao B, Fu JJ, Zhao LZ, Chu PK. Fabrication, modification, and biomedical applications of anodized TiO2 nanotube arrays. RSC Adv. 2014, 4, 17300–17324. Search in Google Scholar

[81] Bai J, Zhou BX, Li LH, Liu YB, Zheng Q, Shao JH, Zhu XY, Cai WM, Liao JS, Zou LX. The formation mechanism of titania nanotube arrays in hydrofluoric acid electrolyte. J. Mater. Sci. 2008, 43, 1880–1884. Search in Google Scholar

[82] Li YC, Wang YQ, Kong JH, Jia HZ, Wang ZS. Synthesis and characterization of carbon modified TiO2 nanotube and photocatalytic activity on methylene blue under sunlight. Appl. Surf. Sci. 2015, 344, 176–180. Search in Google Scholar

[83] Yin HY, Wang XL, Wang L, N QL, Zhao HT. Self-doped TiO2 hierarchical hollow spheres with enhanced visible-light photocatalytic activity. J. Alloy. Compd. 2015, 640, 68–74. Search in Google Scholar

[84] Setyawati MI, Tay CY, Chia SL, Goh SL, Fang W, Neo MJ, Chong HC, Tan SM, Loo SCJ, Ng KW, Xie JP, Ong CN, Tan NS, Leong DT. Titanium dioxide nanomaterials cause endothelial cell leakiness by disrupting the homophilic interaction of VE-cadherin. Nat. Commun. 2013, 4, 1673. Search in Google Scholar

[85] Zhang X, Wang F, Huang H, Li H, Han X, Liu Y, Kang ZH. Carbon quantum dot sensitized TiO2 nanotube arrays for photoelectrochemical hydrogen generation under visible light. Nanoscale 2013, 5, 2274–2278. Search in Google Scholar

[86] Chen SG, Paulose M, Ruan C, Mor GK, Varghese OK, Kouzoudis D, Grimes CA. Electrochemically synthesized CdS nanoparticle-modified TiO2 nanotube-array photoelectrodes: preparation, characterization, and application to photoelectrochemical cells. J. Photoch. Photobio. A 2006, 177, 177–184. Search in Google Scholar

[87] Mahshid S, Li CC, Mahshid SS, Askari M, Dolati A, Yang LX, Luo SL, Cai QY. Sensitive determination of dopamine in the presence of uric acid and ascorbic acid using TiO2 nanotubes modified with Pd, Pt and Au nanoparticles. Analyst 2011, 136, 2322–2329. Search in Google Scholar

[88] Alsawat M, Altalhi T, Shapter JG, Losic D. Influence of dimensions, inter-distance and crystallinity of titania nanotubes (TNTs) on their photocatalytic activity. Catal. Sci. Technol. 2014, 4, 2091–2098. Search in Google Scholar

[89] Muscat J, Swamy V, Harrison NM. First-principles calculations of the phase stability of TiO2. Phys. Rev. B 2002, 65, 224112. Search in Google Scholar

[90] Varghese OK, Gong DW, Paulose M, Grimes CA, Dickey EC. Crystallization and high-temperature structural stability of titanium oxide nanotube arrays. J. Mater. Res. 2003, 18, 156–165. Search in Google Scholar

[91] Lu XH, Wang GM, Zhai T, Yu MH, Gan JY, Tong YX, Li Y. Hydrogenated TiO2 nanotube arrays for supercapacitors. Nano Lett. 2012, 12, 1690–1696. Search in Google Scholar

[92] Salari M, Konstantinov K, Liu HK. Enhancement of the capacitance in TiO2 nanotubes through controlled introduction of oxygen vacancies. J. Mater. Chem. 2011, 21, 5128–5133. Search in Google Scholar

[93] Zhang XM, Huo KF, Wang HR, Zhang WR, Chu PK. Influence of structure parameters and crystalline phase on the photocatalytic activity of TiO2 nanotube arrays. J Nanosci. Nanotechno. 2011, 11, 11200–11205. Search in Google Scholar

[94] Roguska A, Pisarek M, Andrzejczuk M, Dolata M, Lewandowska M, Janik-Czachor M. Characterization of a calcium phosphate-TiO2 nanotube composite layer for biomedical applications. Mat. Sci. Eng. C-Mater. 2011, 31, 906–914. Search in Google Scholar

[95] Acevedo-Pena P, Carrera-Crespo JE, Gonzalez F, Gonzalez I. Effect of heat treatment on the crystal phase composition, semiconducting properties and photoelectrocatalytic color removal efficiency of TiO2 nanotubes arrays. Electrochim. Acta 2014, 140, 564–571. Search in Google Scholar

[96] Li D, Cheng XW, Yu XJ, Xing ZP. Preparation and characterization of TiO2-based nanosheets for photocatalytic degradation of acetylsalicylic acid: influence of calcination temperature. Chem. Eng. J. 2015, 279, 994–1003. Search in Google Scholar

[97] Ku Y, Fan ZR, Chou YC, Wang WY. Effects of TiO2 nanotube array dimension and annealing temperature on the Acid Red 4 degradation in aqueous solution by photocatalytic process. Water Sci. Technol. 2010, 61, 2943–2949. Search in Google Scholar

[98] Zhu K, Neale NR, Halverson AF, Kim JY, Frank AJ. Effects of annealing temperature on the charge-collection and light-harvesting properties of TiO2 nanotube-based dye-sensitized solar cells. J. Phys. Chem. C 2010, 114, 13433–13441. Search in Google Scholar

[99] Palmas S, Da Pozzo A, Mascia M, Vacca A, Ardu A, Matarrese R, Nova I. Effect of the preparation conditions on the performance of TiO2 nanotube arrays obtained by electrochemical oxidation. Int. J. Hydrogen Energy 2011, 36, 8894–8901. Search in Google Scholar

[100] Tighineanu A, Ruff T, Albu S, Hahn R, Schmuki P. Conductivity of TiO2 nanotubes: influence of annealing time and temperature. Chem. Phys. Lett. 2010, 494, 260–263. Search in Google Scholar

[101] Asahi R, Morikawa T, Ohwaki T, Aoki K, Taga Y. Visible-light photocatalysis in nitrogen-doped titanium oxides. Science 2001, 293, 269–271. Search in Google Scholar

[102] Wen W, Wu JM, Jiang YZ, Yu SL, Bai JQ, Cao MH, Cui J. Anatase TiO2 ultrathin nanobelts derived from room-temperature-synthesized titanates for fast and safe lithium storage. Sci. Rep. 2015, 5, 11804. Search in Google Scholar

[103] Sang YH, Zhao ZH, Tian J, Hao P, Jiang HD, Liu H, Claverie JP. Enhanced photocatalytic property of reduced graphene oxide/TiO2 nanobelt surface heterostructures constructed by an in situ photochemical reduction method. Small 2014, 10, 3775–3782. Search in Google Scholar

[104] Tian J, Leng YH, Zhao ZH, Xia Y, Sang YH, Hao P, Zhan J, Li MC, Liu H. Carbon quantum dots/hydrogenated TiO2 nanobelt heterostructures and their broad spectrum photocatalytic properties under UV, visible, and near-infrared irradiation. Nano Energy 2015, 11, 419–427. Search in Google Scholar

[105] Li H, Xing JH, Xia ZB, Chen JQ. Preparation of extremely smooth and boron-fluorine co-doped TiO2 nanotube arrays with enhanced photoelectrochemical and photocatalytic performance. Electrochim. Acta 2014,139, 331–336. Search in Google Scholar

[106] Yan GT, Zhang M, Hou J, Yang JJ. Photoelectrochemical and photocatalytic properties of N plus S co-doped TiO2 nanotube array films under visible light irradiation. Mater. Chem. Phys. 2011, 129, 553–557. Search in Google Scholar

[107] Zhou W, Leng YH, Hou DM, Li HD, Li LG, Li GQ, Liu H, Chen SW. Phase transformation and enhanced photocatalytic activity of S-doped Ag2O/TiO2 heterostructured nanobelts. Nanoscale 2014, 6, 4698–4704. Search in Google Scholar

[108] Lai YK, Huang JY, Zhang HF, Subramaniam VP, Tang YX, Gong DG, Sundar L, Sun L, Chen Z, Lin CJ. Nitrogen-doped TiO2 nanotube array films with enhanced photocatalytic activity under various light sources. J. Hazard. Mater. 2010, 184, 855–863. Search in Google Scholar

[109] Ramanathan R, Bansal V. Ionic liquid mediated synthesis of nitrogen, carbon and fluorine-codoped rutile TiO2 nanorods for improved UV and visible light photocatalysis. RSC Adv. 2015, 5, 1424–1429. Search in Google Scholar

[110] Ghicov A, Macak JM, Tsuchiya H, Kunze J, Haeublein V, Kleber S, Schmuki P. TiO2 nanotube layers: dose effects during nitrogen doping by ion implantation. Chem. Phys. Lett. 2006, 419, 426–429. Search in Google Scholar

[111] Ghicov A, Macak JM, Tsuchiya H, Kunze J, Haeublein V, Frey L, Schmuki P. Ion implantation and annealing for an efficient N-doping of TiO2 nanotubes. Nano Lett. 2006, 6, 1080–1082. Search in Google Scholar

[112] Vitiello RP, Macak JM, Ghicov A, Tsuchiya H, Dick LFP, Schmuki P. N-Doping of anodic TiO2 nanotubes using heat treatment in ammonia. Electrochem. Commun. 2006, 8, 544–548. Search in Google Scholar

[113] Kim D, Fujimoto S, Schmuki P, Tsuchiya H. Nitrogen doped anodic TiO2 nanotubes grown from nitrogen-containing Ti alloys. Electrochem. Commun. 2008, 10, 910–913. Search in Google Scholar

[114] Kim D, Tsuchiya H, Fujimoto S, Schmidt-Stein F, Schmuki P. Nitrogen-doped TiO2 mesosponge layers formed by anodization of nitrogen-containing Ti alloys. J. Solid State Electr. 2012, 16, 89–92. Search in Google Scholar

[115] Wang Y, Li Z, Tian YF, Zhao W, Liu XQ, Yang JB. A facile way to fabricate graphene sheets on TiO2 nanotube arrays for dye-sensitized solar cell applications. J. Mater. Sci. 2014, 49, 7991–7999. Search in Google Scholar

[116] Park JH, Kim S, Bard AJ. Novel carbon-doped TiO2 nanotube arrays with high aspect ratios for efficient solar water splitting. Nano Lett. 2006, 6, 24–28. Search in Google Scholar

[117] Hahn R, Ghicov A, Salonen J, Lehto VP, Schmuki P. Carbon doping of self-organized TiO2 nanotube layers by thermal acetylene treatment. Nanotechnology 2007, 18, 105604. Search in Google Scholar

[118] Hu LS, Huo KF, Chen RS, Gao B, Fu JJ, Chu PK. Recyclable and high-sensitivity electrochemical biosensing platform composed of carbon-doped TiO2 nanotube arrays. Anal. Chem. 2011, 83, 8138–8144. Search in Google Scholar

[119] Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y, Dubonos SV, Grigorieva IV, Firsov AA. Electric field effect in atomically thin carbon films. Science 2004, 306, 666–669. Search in Google Scholar

[120] Williams G, Seger B, Kamat PV. TiO2-graphene nanocomposites. UV-assisted photocatalytic reduction of graphene oxide. ACS Nano 2008, 2, 1487–1491. Search in Google Scholar

[121] Zhang H, Lv XJ, Li YM, Wang Y, Li JH. P25-graphene composite as a high performance photocatalyst. ACS Nano 2010, 4, 380–386. Search in Google Scholar

[122] Zhang XY, Li HP, Cui XL, Lin Y. Graphene/TiO2 nanocomposites: synthesis, characterization and application in hydrogen evolution from water photocatalytic splitting. J. Mater. Chem. 2010, 20, 2801–2806. Search in Google Scholar

[123] Liu YB, Lee JHD, Xia Q, Ma Y, Yu Y, Yung LYL, Xie JP, Ong CN, Vecitis CD, Zhou Z. A graphene-based electrochemical filter for water purification. J. Mater. Chem. A 2014, 2, 16554–16562. Search in Google Scholar

[124] Liu CB, Teng YR, Liu RH, Luo SL, Tang YH, Chen LY, Cai QY. Fabrication of graphene films on TiO2 nanotube arrays for photocatalytic application. Carbon 2011, 49, 5312–5320. Search in Google Scholar

[125] Tang YH, Luo SL, Teng YR, Liu CB, Xu XL, Zhang XL, Chen L. Efficient removal of herbicide 2,4-dichlorophenoxyacetic acid from water using Ag/reduced graphene oxide co-decorated TiO2 nanotube arrays. J. Hazard. Mater. 2012, 241, 323–330. Search in Google Scholar

[126] Gao B, Li XX, Ma YW, Cao Y, Hu ZY, Zhang XM, Fu JJ, Huo KF, Chu PK. MnO2-TiO2/C nanocomposite arrays for high-performance supercapacitor electrodes. Thin Solid Films 2015, 584, 61–65. Search in Google Scholar

[127] Tang HL, Xiong M, Qu DY, Liu D, Zhang ZJ, Xie ZZ, Wei X, Tu WM, Qu DY. Enhanced supercapacitive performance on TiO2@C coaxial nano-rod array through a bio-inspired approach. Nano Energy 2015, 15, 75–82. Search in Google Scholar

[128] Wang QY, Qiao JL, Xu XH, Gao SM. Controlled synthesis of Cu nanoparticles on TiO2 nanotube array photoelectrodes and their photoelectrochemical properties. Mater. Lett. 2014, 131, 135–137. Search in Google Scholar

[129] Smith YR, Gakhar R, Merwin A, Mohanty SK, Chidambaram D, Misra M. Anodic titania nanotube arrays sensitized with Mn- or Co-doped CdS nanocrystals. Electrochim. Acta 2014, 135, 503–512. Search in Google Scholar

[130] Piskunov S, Lisovski O, Begens J, Bocharov D, Zhukovskii YF, Wessel M, Spohr E. C-, N-, S-, and Fe-doped TiO2 and SrTiO3 nanotubes for visible-light-driven photocatalytic water splitting: prediction from first principles. J. Phys. Chem. C 2015, 119, 18686–18696. Search in Google Scholar

[131] Kim SH, Choi SY. Fabrication of Cu-coated TiO2 nanotubes and enhanced electrochemical performance of lithium ion batteries. J. Electroanal. Chem. 2015, 744, 45–52. Search in Google Scholar

[132] Liu K, Song Y, Chen SW. Defective TiO2-supported Cu nanoparticles as efficient and stable electrocatalysts for oxygen reduction in alkaline media. Nanoscale 2015, 7, 1224–1232. Search in Google Scholar

[133] Chen ML, Zhang FJ, Zhang K, Meng ZD, Oh WC. Fabrication of M-CNT/TiO2 (M=Cr, Mn and Fe) composites and the effect of transition metals on their photocatalytic activities. J. Chem. Res. 2010, 34, 283–287. Search in Google Scholar

[134] Ranjitha A, Muthukumarasamy N, Thambidurai M, Velauthapillai D, Balasundaraprabhu R, Agilan S. Fabrication of Ni-doped TiO2 thin film photoelectrode for solar cells. Sol. Energy 2014, 106, 159–165. Search in Google Scholar

[135] Zhang W, Zhou WD, Wright JH, Kim YN, Liu DW, Xiao XC. Mn-doped TiO2 nanosheet-based spheres as anode materials for lithium-ion batteries with high performance at elevated temperatures. ACS Appl. Mater. Interfaces 2014, 6, 7292–7300. Search in Google Scholar

[136] Truong NN, Altomare M, Yoo J, Schmuki P. Efficient photocatalytic H2 evolution: controlled dewetting-dealloying to fabricate site-selective high-activity nanoporous Au particles on highly ordered TiO2 nanotube arrays. Adv. Mater. 2015, 27, 3208–3215. Search in Google Scholar

[137] Tong X, Yang P, Wang YW, Qin Y, Guo XY. Enhanced photoelectrochemical water splitting performance of TiO2 nanotube arrays coated with an ultrathin nitrogen-doped carbon film by molecular layer deposition. Nanoscale 2014, 6, 6692–6700. Search in Google Scholar

[138] Zhang L, Pan NQ, Lin SW. Influence of Pt deposition on water-splitting hydrogen generation by highly-ordered TiO2 nanotube arrays. Int. J. Hydrogen Energy 2014, 39, 13474–13480. Search in Google Scholar

[139] Qin YH, Yang HH, Lv RL, Wang WG, Wang CW. TiO2 nanotube arrays supported Pd nanoparticles for ethanol electrooxidation in alkaline media. Electrochim. Acta 2013, 106, 372–377. Search in Google Scholar

[140] Bian HD, Shu X, Zhang JF, Yuan B, Wang Y, Liu LJ, Xu GQ, Chen Z, Wu YC. Uniformly dispersed and controllable ligand-free silver-nanoparticle-decorated TiO2 nanotube arrays with enhanced photoelectrochemical behaviors. Chem. Asian J. 2013, 8, 2746–2754. Search in Google Scholar

[141] Huang YX, Sun L, Xie KP, Lai YK, Liu BJ, Ren B, Lin CJ. SERS study of Ag nanoparticles electrodeposited on patterned TiO2 nanotube films. J. Raman Spectrosc. 2011, 42, 986–991. Search in Google Scholar

[142] Lai YK, Gong JJ, Lin CJ. Self-organized TiO2 nanotube arrays with uniform platinum nanoparticles for highly efficient water splitting. Int. J. Hydrogen Energy 2012, 37, 6438–6446. Search in Google Scholar

[143] Xie KP, Sun L, Wang CL, Lai YK, Wang MY, Chen HB, Lin CJ. Photoelectrocatalytic properties of Ag nanoparticles loaded TiO2 nanotube arrays prepared by pulse current deposition. Electrochim. Acta 2010, 55, 7211–7218. Search in Google Scholar

[144] Wang Y, Yu JG, Xiao W, Li Q, Microwave-assisted hydrothermal synthesis of graphene based Au-TiO2 photocatalysts for efficient visible-light hydrogen production. J. Mater. Chem. A 2014, 2, 3847–3855. Search in Google Scholar

[145] Zhan ZY, An JN, Zhang HC, Hansen RV, Zheng LX. Three-dimensional plasmonic photoanodes based on Au-embedded TiO2 structures for enhanced visible-light water splitting. ACS Appl. Mater. Interfaces 2014, 6, 1139–1144. Search in Google Scholar

[146] Chen YH, Bian JJ, Qi LL, Liu EZ, Fan J. Efficient degradation of methylene blue over two-dimensional Au/TiO2 nanosheet films with overlapped light harvesting nanostructures, J. Nanomater 2015, 905259. Search in Google Scholar

[147] Xiao FX, Hung SF, Miao JW, Wang HY, Yang HB, Liu B. Metal-cluster-decorated TiO2 nanotube arrays: a composite heterostructure toward versatile photocatalytic and photoelectrochemical applications. Small 2015, 11, 554–567. Search in Google Scholar

[148] Li JG, Zhao TT, Chen TK, Liu YB, Ong CN, Xie JP. Engineering noble metal nanomaterials for environmental applications. Nanoscale 2015, 7, 7502–7519. Search in Google Scholar

[149] Liu H, Ye F, Yao QF, Cao HB, Xie JP, Lee JY, Yang J. Stellated Ag-Pt bimetallic nanoparticles: an effective platform for catalytic activity tuning. Sci. Rep. 2014, 4, 3969. Search in Google Scholar

[150] Chen BX, Zhang WF, Zhou XH, Huang X, Zhao XM, Wang HT, Liu M, Lu YL, Yang SF. Surface plasmon enhancement of polymer solar cells by penetrating Au/SiO2 core/shell nanoparticles into all organic layers. Nano Energy 2013, 2, 906–915. Search in Google Scholar

[151] Lian ZC, Wang WC, Xiao SN, Li X, Cui YY, Zhang DQ, Li GS, Li HX. Plasmonic silver quantum dots coupled with hierarchical TiO2 nanotube arrays photoelectrodes for efficient visible-light photoelectrocatalytic hydrogen evolution. Sci. Rep. 2015, 5, 10461. Search in Google Scholar

[152] Wu L, Li F, Xu YY, Zhang JW, Zhang DQ, Li GS, Li HX. Plasmon-induced photoelectrocatalytic activity of Au nanoparticles enhanced TiO2 nanotube arrays electrodes for environmental remediation. Appl. Catal. B-Environ. 2015, 164, 217–224. Search in Google Scholar

[153] Yoo J, Lee K, Schmuki P. Dewetted Au films form a highly active photocatalytic system on TiO2 nanotube-stumps. Electrochem. Commun. 2013, 34, 351–355. Search in Google Scholar

[154] Sun L, Li J, Wang CJ, Li SF, Lai YK, Chen HB, Lin CJ. Ultrasound aided photochemical synthesis of Ag loaded TiO2 nanotube arrays to enhance photocatalytic activity. J. Hazard. Mater. 2009, 171, 1045–1050. Search in Google Scholar

[155] Hou Y, Li XY, Zhao QD, Chen GH, Raston CL. Role of hydroxyl radicals and mechanism of Escherichia coli inactivation on Ag/AgBr/TiO2 nanotube array electrode under visible light irradiation. Environ. Sci. Technol. 2012, 46, 4042–4050. Search in Google Scholar

[156] Tang YX, Subramaniam VP, Lau TH, Lai YK, Gong DG, Kanhere PD, Cheng YH, Chen Z, Dong ZL. In situ formation of large-scale Ag/AgCl nanoparticles on layered titanate honeycomb by gas phase reaction for visible light degradation of phenol solution. Appl. Catal. B-Environ. 2011, 106, 577–585. Search in Google Scholar

[157] Wang QY, Qiao JL, Jin RC, Xu XH, Gao SM. Fabrication of plasmonic AgBr/Ag nanoparticles-sensitized TiO2 nanotube arrays and their enhanced photo-conversion and photoelectrocatalytic properties. J. Power Sources 2015, 277, 480–485. Search in Google Scholar

[158] Yu JG, Dai GP, Huang BB. Fabrication and characterization of visible-light-driven plasmonic photocatalyst Ag/AgCl/TiO2 nanotube arrays. J. Phys. Chem. C 2009, 113, 16394–16401. Search in Google Scholar

[159] Feng H, Tran TT, Chen L, Yuan LJ, Cai QY. Visible light-induced efficiently oxidative decomposition of p-Nitrophenol by CdTe/TiO2 nanotube arrays. Chem. Eng. J. 2013, 215, 591–599. Search in Google Scholar

[160] Yang HH, Fan WG, Vaneski A, Susha AS, Teoh WY, Rogach AL. Heterojunction engineering of CdTe and CdSe quantum dots on TiO2 nanotube arrays: intricate effects of size-dependency and interfacial contact on photoconversion efficiencies. Adv. Funct. Mater. 2012, 22, 2821–2829. Search in Google Scholar

[161] Cai FG, Yang F, Zhang Y, Ke C, Cheng CH, Zhao Y, Yan G. PbS sensitized TiO2 nanotube arrays with different sizes and filling degrees for enhancing photoelectrochemical properties. Phys. Chem. Chem. Phys. 2014, 16, 23967–23974. Search in Google Scholar

[162] Ge MZ, Cao CY, Li SH, Zhang SN, Deng S, Huang JY, Li QS, Zhang KQ, Al-Deyab SS, Lai YK. Enhanced photocatalytic performances of n-TiO2 nanotubes by uniform creation of p-n heterojunctions with p-Bi2O3 quantum dots. Nanoscale 2015, 7, 11552–11560. Search in Google Scholar

[163] Lai YK, Lin ZQ, Chen Z, Huang JY, Lin CJ. Fabrication of patterned CdS/TiO2 heterojunction by wettability template-assisted electrodeposition. Mater. Lett. 2010, 64, 1309–1312. Search in Google Scholar

[164] Lin ZQ, Lai YK, Hu RG, Li J, Du RG, Lin CJ. A highly efficient ZnS/CdS@TiO2 photoelectrode for photogenerated cathodic protection of metals. Electrochim. Acta 2010, 55, 8717–8723. Search in Google Scholar

[165] Yu X, Zhang J, Zhao ZH, Guo WB, Qiu JH, Mou XN, Li AX, Claverie JP, Liu H. NiO-TiO2 p-n heterostructured nanocables bridged by zero-bandgap rGO for highly efficient photocatalytic water splitting. Nano Energy 2015, 16, 207–217. Search in Google Scholar

[166] Zhang JF, Wang Y, Yu CP, Shu X, Jiang L, Cui JW, Chen Z, Xie T, Wu YC. Enhanced visible-light photoelectrochemical behaviour of heterojunction composite with Cu2O nanoparticles-decorated TiO2 nanotube arrays. New J. Chem. 2014, 38, 4975–4984. Search in Google Scholar

[167] Liu LJ, Lv J, Xu GQ, Wang Y, Xie K, Chen Z, Wu YC. Uniformly dispersed CdS nanoparticles sensitized TiO2 nanotube arrays with enhanced visible-light photocatalytic activity and stability. J. Solid State Chem. 2013, 208, 27–34. Search in Google Scholar

[168] Bessekhouad Y, Robert D, Weber J. Bi2S3/TiO2 and CdS/TiO2 heterojunctions as an available configuration for photocatalytic degradation of organic pollutant. J. Photoch. Photobiolo. A 2004, 163, 569–580. Search in Google Scholar

[169] Jing DW, Guo LJ. A novel method for the preparation of a highly stable and active CdS photocatalyst with a special surface nanostructure. J. Phys. Chem. B 2006, 110, 11139–11145. Search in Google Scholar

[170] Korake PV, Achary SN, Gupta NM. Role of aliovalent cation doping in the activity of nanocrystalline CdS for visible-light-driven H2 production from water. Int. J. Hydrogen Energy 2015, 40, 8695–8705. Search in Google Scholar

[171] Bai J, Li JH, Liu YB, Zhou BX, Cai WM. A new glass substrate photoelectrocatalytic electrode for efficient visible-light hydrogen production: CdS sensitized TiO2 nanotube arrays. Appl. Catal. B-Environ. 2010, 95, 408–413. Search in Google Scholar

[172] Liu YB, Zhou HB, Zhou BX, Li JH, Chen HC, Wang JJ, Bai J, Shangguan WF, Cai WM. Highly stable CdS-modified short TiO2 nanotube array electrode for efficient visible-light hydrogen generation. Int. J. Hydrogen Energy 2011, 36, 167–174. Search in Google Scholar

[173] Yin YX, Jin ZG, Hou F. Enhanced solar water-splitting efficiency using core/sheath heterostructure CdS/TiO2 nanotube arrays. Nanotechnology 2007, 18, 495608. Search in Google Scholar

[174] Hsu MC, Leu IC, Sun YM, Hon MH. Fabrication of CdS@TiO2 coaxial composite nanocables arrays by liquid-phase deposition. J. Cryst. Growth 2005, 285, 642–648. Search in Google Scholar

[175] Xie Y, Ali G, Yoo SH, Cho SO. Sonication-assisted synthesis of CdS quantum-dot-sensitized TiO2 nanotube arrays with enhanced photoelectrochemical and photocatalytic activity. ACS Appl. Mater. Interfaces 2010, 2, 2910–2914. Search in Google Scholar

[176] Gao HZ, Wang HG, Jin YL, Lv J, Xu GQ, Wang DM, Zhang XY, Chen Z, Zheng ZX, Wu YC. Controllable fabrication of immobilized ternary CdS/Pt-TiO2 heteronanostructures toward high-performance visible-light driven photocatalysis. Phys. Chem. Chem. Phys. 2015, 17, 17755–17761. Search in Google Scholar

[177] Li H, Xia ZB, Chen JQ, Lei L, Xing JH. Constructing ternary CdS/reduced graphene oxide/TiO2 nanotube arrays hybrids for enhanced visible-light-driven photoelectrochemical and photocatalytic activity. Appl. Catal. B-Environ. 2015, 168, 105–113. Search in Google Scholar

[178] Lv J, Wang HG, Gao HZ, Xu GQ, Wang DM, Chen Z, Zhang XY, Zheng ZX, Wu YC. A research on the visible light photocatalytic activity and kinetics of CdS/CdSe co-modified TiO2 nanotube arrays. Surf. Coat. Tech. 2015, 261, 356–363. Search in Google Scholar

[179] Zhang Z, Shao CL, Li XH, Wang CH, Zhang MY, Liu YC. Electrospun nanofibers of p-type NiO/n-type ZnO heterojunctions with enhanced photocatalytic activity. ACS Appl. Mater. Interfaces 2010, 2, 2915–2923. Search in Google Scholar

[180] He HC, Xiao P, Zhou M, Zhang YH, Lou Q, Dong XZ. Boosting catalytic activity with a p–n junction: Ni/TiO2 nanotube arrays composite catalyst for methanol oxidation. Int. J. Hydrogen Energy 2012, 37, 4967–4973. Search in Google Scholar

[181] Chen C, Cai WM, Long MC, Zhou BX, Wu YH, Wu DY, Feng YJ. Synthesis of visible-light responsive graphene oxide/TiO2 composites with p/n heterojunction. ACS Nano 2010, 4, 6425–6432. Search in Google Scholar

[182] Mor GK, Varghese OK, Wilke RHT, Sharma S, Shankar K, Latempa TJ, Choi KS, Grimes CA. p-type Cu-Ti-O nanotube arrays and their use in self-biased heterojunction photoelectrochemical diodes for hydrogen generation. Nano Lett. 2008, 8, 1906–1911. Search in Google Scholar

[183] Tang YB, Chen ZH, Song HS, Lee CS, Cong HT, Cheng HM, Zhang WJ, Bello I, Lee ST. Vertically aligned p-type single-crystalline GaN nanorod arrays on n-type Si for heterojunction photovoltaic cells. Nano Lett. 2008, 8, 4191–4195. Search in Google Scholar

[184] Yang LX, Luo SL, Li Y, Xiao Y, Kang Q, Cai QY. High efficient photocatalytic degradation of p-nitrophenol on a unique Cu2O/TiO2 p-n heterojunction network catalyst. Environ. Sci. Technol. 2010, 44, 7641–7646. Search in Google Scholar

[185] Liu JQ, Ruan LL, Adeloju SB, Wu YC. BiOI/TiO2 nanotube arrays, a unique flake-tube structured p-n junction with remarkable visible-light photoelectrocatalytic performance and stability. Dalton Trans. 2014, 43, 1706–1715. Search in Google Scholar

[186] Sinatra L, LaGrow AP, Peng W, Kirmani AR, Amassian A, Idriss H, Bakr OM. A Au/Cu2O-TiO2 system for photo-catalytic hydrogen production. A pn-junction effect or a simple case of in situ reduction? J. Catal. 2015, 322, 109–117. Search in Google Scholar

[187] Li L, Li WS, Ji A, Wang Z, Zhu CY, Zhang LJ, Yang JF, Mao LF. Anisotropic relaxation of a CuO/TiO2 surface under an electric field and its impact on visible light absorption: ab initio calculations, Phys. Chem. Chem. Phys. 2015, 17, 17880–17886. Search in Google Scholar

[188] Liu YB, Zhou HB, Li JH, Chen HC, Li D, Zhou BX, Cai WM. Enhanced photoelectrochemical properties of Cu2O-loaded short TiO2 nanotube array electrode prepared by sonoelectrochemical deposition. Nano-Micro Lett 2010, 2, 277–284. Search in Google Scholar

[189] Lin L, Yang YC, Men L, Wang X, He DN, Chai YC, Zhao B, Ghoshroy S, Tang QW. A highly efficient TiO2@ZnO n-p-n heterojunction nanorod photocatalyst. Nanoscale 2013, 5, 588–593. Search in Google Scholar

[190] Hossain MA, Park J, Ahn JY, Park C, Kim Y, Kim SH, Lee D. Investigation of TiO2 nanotubes/nanoparticles stacking sequences to improve power conversion efficiency of dye-sensitized solar cells. Electrochim. Acta 2015, 173, 665–671. Search in Google Scholar

[191] Cui XY, Gu HM, Guan Y, Ren GP, Ma Z, Yin YY, Liu JL, Cui XG, Yao LY, Yin YK, Wang D, Jin G, Rong SZ, Tong L, Hou JF, Li MJ. Fabrication of AgInS2 nanoparticles sensitized TiO2 nanotube arrays and their photoelectrochemical properties. Sol. Energ. Mat. Sol. C. 2015, 137, 101–106. Search in Google Scholar

[192] Maheswari D, Venkatachalam P. Performance enhancement in dye-sensitized solar cells with composite mixtures of TiO2 nanoparticles and TiO2 nanotubes. Acta Metall. Sin. Eng. 2015, 28, 354–361. Search in Google Scholar

[193] Ye MD, Xin XK, Lin CJ, Lin ZQ. High efficiency dye-sensitized solar cells based on hierarchically structured nanotubes. Nano Lett. 2011, 11, 3214–3220. Search in Google Scholar

[194] Li TT, Li XY, Zhao QD, Shi Y. Teng W. Fabrication of n-type CuInS2 modified TiO2 nanotube arrays heterostructure photoelectrode with enhanced photoelectrocatalytic properties. Appl. Catal. B-Environ. 2014, 156, 362–370. Search in Google Scholar

[195] Xiao FX. Construction of highly ordered ZnO-TiO2 nanotube arrays (ZnO/TNTs) heterostructure for photocatalytic application. ACS Appl. Mater. Interfaces 2012, 4, 7054–7062. Search in Google Scholar

[196] Yang Y, Wang XH, Sun CK, Li LT. Photoluminescence of ZnO nanorod-TiO2 nanotube hybrid arrays produced by electrodeposition. J. Appl. Phys. 2009, 105, 094304. Search in Google Scholar

[197] Li JH, Lv SB, Liu YB, Bai J, Zhou BX, Hu XF. Photoeletrocatalytic activity of an n-ZnO/p-Cu2O/n-TNA ternary heterojunction electrode for tetracycline degradation. J. Hazard. Mater. 2013, 262, 482–488. Search in Google Scholar

[198] Bera A, Wu K, Sheikh A, Alarousu E, Mohammed OF, Wu T. Perovskite oxide SrTiO3 as an efficient electron transporter for hybrid perovskite solar cells. J. Phys. Chem. C 2014, 118, 28494–28501. Search in Google Scholar

[199] Jayabal P, Sasirekha V, Mayandi J, Jeganathan K, Ramakrishnan V. A facile hydrothermal synthesis of SrTiO3 for dye sensitized solar cell application. J. Alloys Compd. 2014, 586, 456–461. Search in Google Scholar

[200] Liu RC, Liang FL, Zhou W, Yang YS, Zhu ZH. Calcium-doped lanthanum nickelate layered perovskite and nickel oxide nano-hybrid for highly efficient water oxidation. Nano Energy 2015, 12, 115–122. Search in Google Scholar

[201] Xu JS, Pan CS, Takata T, Domen K. Photocatalytic overall water splitting on the perovskite-type transition metal oxynitride CaTaO2N under visible light irradiation. Chem. Commun. 2015, 51, 7191–7194. Search in Google Scholar

[202] Wang W, Tade MO, Shao ZP. Research progress of perovskite materials in photocatalysis- and photovoltaics-related energy conversion and environmental treatment. Chem. Soc. Rev. 2015, 44, 5371–5408. Search in Google Scholar

[203] Kato H, Kudo A. Visible-light-response and photocatalytic activities of TiO2 and SrTiO3 photocatalysts codoped with antimony and chromium. J. Phys. Chem. B 2002, 106, 5029–5034. Search in Google Scholar

[204] Qin P, Paulose M, Dar MI, Moehl T, Arora N, Gao P, Varghese OK, Gatzel M, Nazeeruddin MK. Stable and efficient perovskite solar cells based on titania nanotube arrays. Small 2015, 11, 5533–5539. Search in Google Scholar

[205] Zhang XM, Gao B, Hu LS, Li LM, Jin WH, Huo KF, Chu PK. Hydrothermal synthesis of perovskite-type MTiO3 (M=Zn, Co, Ni)/TiO2 nanotube arrays from an amorphous TiO2 template. Crystengcomm 2014, 16, 10280–10285. Search in Google Scholar

[206] Zhang XM, Huo KF, Hu LS, Wu ZG, Chu PK. Synthesis and photocatalytic activity of highly ordered TiO2 and SrTiO3/TiO2 nanotube arrays on Ti substrates. J. Am. Ceram. Soc. 2010, 93, 2771–2778. Search in Google Scholar

[207] Wu Z, Su YF, Yu JD, Xiao W, Sun L, Lin CJ. Enhanced photoelectrocatalytic hydrogen production activity of SrTiO3-TiO2 hetero-nanoparticle modified TiO2 nanotube arrays. Int. J. Hydrogen Energy 2015, 40, 9704–9712. Search in Google Scholar

[208] Cai YY, Ye YX, Tian ZF, Liu, Liu YS, Liang CH. In situ growth of lamellar ZnTiO3 nanosheets on TiO2 tubular array with enhanced photocatalytic activity. Phys. Chem. Chem. Phys. 2013, 15, 20203–20209. Search in Google Scholar

[209] Li QY, Li R, Zong LL, He JH, Wang XD, Yang JJ. Photoelectrochemical and photocatalytic properties of Ag-loaded BaTiO3/TiO2 heterostructure nanotube arrays. Int. J. Hydrogen Energy 2013, 38, 12977–12983. Search in Google Scholar

[210] Li R, Li QY, Zong LL, Wang XD, Yang JJ. BaTiO3/TiO2 heterostructure nanotube arrays for improved photoelectrochemical and photocatalytic activity. Electrochim. Acta 2013, 91, 30–35. Search in Google Scholar

[211] Mohamed AER, Rohani S. Modified TiO2 nanotube arrays (TNTAs): progressive strategies towards visible light responsive photoanode, a review. Energy Environ. Sci. 2011, 4, 1065–1086. Search in Google Scholar

[212] Lai YK, Sun L, Chen YC, Zhuang HF, Lin CJ, Chin JW. Effects of the structure of TiO2 nanotube array on Ti substrate on its photocatalytic activity. J. Electrochem. Soc. 2006, 153, D123–127. Search in Google Scholar

[213] Gong JJ, Lai YK, Lin CJ. Electrochemically multi-anodized TiO2 nanotube arrays for enhancing hydrogen generation by photoelectrocatalytic water splitting. Electrochim. Acta 2010, 55, 4776–4782. Search in Google Scholar

[214] Liu DW, Xiao P, Zhang YH, Garcia BB, Zhang QF, Guo Q, Champion R, Cao GZ. TiO2 nanotube arrays annealed in N2 for efficient lithium-ion intercalation. J. Phys. Chem. C 2008, 112, 11175–11180. Search in Google Scholar

[215] Liu CB, Cao CH, Luo XB, Luo SL. Ag-bridged Ag2O nanowire network/TiO2 nanotube array p-n heterojunction as a highly efficient and stable visible light photocatalyst. J. Hazard. Mater. 2015, 285, 319–324. Search in Google Scholar

[216] Zhu YX, Wang YF, Chen Z, Qin LS, Yang LB, Zhu L, Tang P, Gao T, Huang YX, Sha ZL, Tang G. Visible light induced photocatalysis on CdS quantum dots decorated TiO2 nanotube arrays. Appl. Catal. A-Gen. 2015, 498, 159–166. Search in Google Scholar

[217] Su LL, Lv J, Wang HG, Liu LJ, Xu GQ, Wang DM, Zheng ZX, Wu YC. Improved visible light photocatalytic activity of CdSe modified TiO2 nanotube arrays with different intertube spaces. Catal. Lett. 2014, 144, 553–560. Search in Google Scholar

[218] Zhong JS, Wang QY, Zhu X, Chen DQ, Ji ZG. Sovolthermal synthesis of flower-like Cu3BiS3 sensitized TiO2 nanotube arrays for enhancing photoelectrochemical performance. J. Alloys Compd. 2015, 641, 144–147. Search in Google Scholar

[219] Shin SW, Lee JY, Ahn KS, Kang SH, Kim JH. Visible light absorbing TiO2 nanotube arrays by sulfur treatment for photoelectrochemical water splitting. J. Phys. Chem. C 2015, 119, 13375–13383. Search in Google Scholar

[220] Zhuang HF, Lin CJ, Lai YK, Sun L, Li J. Some critical structure factors of titanium oxide manotube array in its photocatalytic activity. Environ. Sci. Technol. 2007, 41, 4735–4740. Search in Google Scholar

[221] Xiao FX. An efficient layer-by-layer self-assembly of metal-TiO2 nanoring/nanotube heterostructures, M/T-NRNT (M=Au, Ag, Pt), for versatile catalytic applications. Chem. Commun. 2012, 48, 6538–6540. Search in Google Scholar

[222] Pan XY, Xu YJ. Defect-mediated growth of noble-metal (Ag, Pt, and Pd) nanoparticles on TiO2 with oxygen vacancies for photocatalytic redox reactions under visible light. J. Phys. Chem. C 2013, 117, 17996–18005. Search in Google Scholar

[223] Liu X, Liu ZQ, Lu JL, Wu XL, Xu B, Chu W. Electrodeposition preparation of Ag nanoparticles loaded TiO2 nanotube arrays with enhanced photocatalytic performance. Appl. Surf. Sci. 2014, 288, 513–517. Search in Google Scholar

[224] Sim LC, Leong KH, Ibrahim S, Saravanan P. Graphene oxide and Ag engulfed TiO2 nanotube arrays for enhanced electron mobility and visible-light-driven photocatalytic performance. J. Mater. Chem. A 2014, 2, 5315–5322. Search in Google Scholar

[225] Zhang ZH, Yuan Y, Shi GY, Fang YJ, Liang LH, Ding HC, Jin LT. Photoelectrocatalytic activity of highly ordered TiO2 nanotube arrays electrode for azo dye degradation. Environ. Sci. Technol. 2007, 41, 6259–6263. Search in Google Scholar

[226] Quan X, Yang SG, Ruan XL, Zhao HM. Preparation of titania nanotubes and their environmental applications as electrode. Environ. Sci. Technol. 2005, 39, 3770–3775. Search in Google Scholar

[227] Mohapatra SK, Raja KS, Mahajan VK, Misra M. Efficient photoelectrolysis of water using TiO2 nanotube arrays by minimizing recombination losses with organic additives. J. Phys. Chem. C 2008, 112, 11007–11012. Search in Google Scholar

[228] Zheng XZ, Li DZ, Li XF, Yu LH, Wang P, Zhang XY, Fang JL, Shao Y, Zheng Y. Photoelectrocatalytic degradation of rhodamine B on TiO2 photonic crystals. Phys. Chem. Chem. Phys. 2014, 16, 15299–15306. Search in Google Scholar

[229] Cheng XW, Liu HL, Yu XJ, Chen QH, Li JJ, Wang P, Umar A, Wang Q. Preparation of highly ordered TiO2 nanotube array photoelectrode for the photoelectrocatalytic degradation of methyl blue: activity and mechanism study. Sci. Adv. Mater. 2013, 5, 1563–1570. Search in Google Scholar

[230] Ojani R, Khanmohammadi A, Raoof JB. Photoelectrocatalytic degradation of p-hydroxybenzoic acid at the surface of a titanium/titanium dioxide nanotube array electrode using electrochemical monitoring. Mat. Sci. Semicon. Proc. 2015, 31, 651–657. Search in Google Scholar

[231] Zhang Q, Zhu JW, Wang Y, Feng JT, Yan W, Xu H. Electrochemical assisted photocatalytic degradation of salicylic acid with highly ordered TiO2 nanotube electrodes. Appl. Surf. Sci. 2014, 308, 161–169. Search in Google Scholar

[232] Huang B, Yang WJ, Wen YW, Shan B, Chen R. Co3O4-modified TiO2 nanotube arrays via atomic layer deposition for improved visible-light photoelectrochemical performance. ACS Appl. Mater. Interfaces 2015, 7, 422–431. Search in Google Scholar

[233] Pan JQ, Li XY, Zhao QD, Li TT, Tade M, Liu SM. Construction of Mn0.5Zn0.5Fe2O4 modified TiO2 nanotube array nanocomposite electrodes and their photoelectrocatalytic performance in the degradation of 2,4-DCP. J. Mater. Chem. C 2015, 3, 6025–6034. Search in Google Scholar

[234] Zhang M, Yang CZ, Pu WH, Tan YB, Yang K, Zhang JD. Liquid phase deposition of WO3/TiO2 heterojunction films with high photoelectrocatalytic activity under visible light irradiation. Electrochim. Acta 2014, 148, 180–186. Search in Google Scholar

[235] Wang MY, Sun L, Lin ZQ, Cai JH, Xie KP, Lin CJ. p-n Heterojunction photoelectrodes composed of Cu2O-loaded TiO2 nanotube arrays with enhanced photoelectrochemical and photoelectrocatalytic activities. Energy Environ. Sci. 2013, 6, 1211–1220. Search in Google Scholar

[236] Bessegato GG, Cardoso JC, Boldrin Zanoni MV. Enhanced photoelectrocatalytic degradation of an acid dye with boron-doped TiO2 nanotube anodes. Catal. Today 2015, 240, 100–106. Search in Google Scholar

[237] Zhai CY, Zhu MS, Lu YT, Ren FF, Wang CQ, Du YK, Yang P. Reduced graphene oxide modified highly ordered TiO2 nanotube arrays photoelectrode with enhanced photoelectrocatalytic performance under visible-light irradiation. Phys. Chem. Chem. Phys. 2014, 16, 14800–14807. Search in Google Scholar

[238] Zhai CY, Zhu MS, Bin D, Wang HW, Du YK, Wang CY, Yang P. Visible-light-assisted electrocatalytic oxidation of methanol using reduced graphene oxide modified Pt nanoflowers-TiO2 nanotube arrays. ACS Appl. Mater. Interfaces 2014, 6, 17753–17761. Search in Google Scholar

[239] Bach U, Lupo D, Comte P, Moser JE, Weissortel F, Salbeck J, Spreitzer H, Gratzel M. Solid-state dye-sensitized mesoporous TiO2 solar cells with high photon-to-electron conversion efficiencies. Nature 1998, 395, 583–585. Search in Google Scholar

[240] Liu B, Aydil ES. Growth of oriented single-crystalline rutile TiO2 nanorods on transparent conducting substrates for dye-sensitized solar cells. J. Am. Chem. Soc. 2009, 131, 3985–3990. Search in Google Scholar

[241] Wang ZS, Kawauchi H, Kashima T, Arakawa H. Significant influence of TiO2 photoelectrode morphology on the energy conversion efficiency of N719 dye-sensitized solar cell. Coordin. Chem. Rev. 2004, 248, 1381–1389. Search in Google Scholar

[242] Ren JB, Que WX, Yin XT, He YC, Javed HMA. Novel fabrication of TiO2/ZnO nanotube array heterojunction for dye-sensitized solar cells. RSC Adv. 2014, 4, 7454–7460. Search in Google Scholar

[243] Rho WY, Jeon H, Kim HS, Chung WJ, Suh JS, Jun BH. Recent progress in dye-sensitized solar cells for improving efficiency: TiO2 nanotube arrays in active layer. J. Nanomater. 2015, 247689. Search in Google Scholar

[244] Lin J, Liu XL, Zhu S, Chen XF. TiO2 nanotube structures for the enhancement of photon utilization in sensitized solar cells. Nanotechnol. Rev. 2015, 4, 209–238. Search in Google Scholar

[245] Zhang YY, Jiang ZL, Huang JY, Lim LY, Li WL, Deng JY, Gong DG, Tang YX, Lai YK, Chen Z. Titanate and titania nanostructured materials for environmental and energy applications: a review. RSC Adv. 2015, 5, 79479–79510. Search in Google Scholar

[246] Oregan B, Grätzel M. A low-cost, high-efficiency solar-cell based on dye-sensitized colloidal TiO2 film. Nature 1991, 353, 737–740. Search in Google Scholar

[247] He ZL, Que WX, Sun P, Ren JB. Double-layer electrode based on TiO2 nanotubes arrays for enhancing photovoltaic properties in dye-sensitized solar cells, ACS Appl. Mater. Interfaces 2013, 5, 12779–12783. Search in Google Scholar

[248] Ho SY, Su CC, Kathirvel S, Li CY, Li WR. Fabrication of TiO2 nanotube-nanocube array composite electrode for dye-sensitized solar cells. Thin Solid Films 2013, 529, 123–127. Search in Google Scholar

[249] Ma XT, Shen Y, Wu QS, Shen T, Cao M, Gu F, Wang LJ. Free-Standing TiO2 nanotube arrays for front-side illuminated CdS quantum dots sensitized solar cells. J. Inorg. Organomet. Polym. 2013, 23, 798–802. Search in Google Scholar

[250] Mor GK, Shankar K, Paulose M, Varghese OK, Grimes CA. Use of highly-ordered TiO2 nanotube arrays in dye-sensitized solar cells. Nano Lett. 2006, 6, 215–218. Search in Google Scholar

[251] So S, Hwang I, Schmuki P. Hierarchical DSSC structures based on “single walled” TiO2 nanotube arrays reach a back-side illumination solar light conversion efficiency of 8%. Energy Environ. Sci. 2015, 8, 849–854. Search in Google Scholar

[252] Bao ZQ, Xie HX, Rao J, Chen, L, Wei Y, Li HF, Zhou XF. High performance of Pt/TiO2-nanotubes/Ti mesh electrode and its application in flexible dye-sensitized solar cell. Mater. Lett. 2014, 124, 158–160. Search in Google Scholar

[253] Zhao YL, Kim HS, Lee SH, Jung S, Suh JS, Hahn YB, Jun BH. Front-illuminated dye-sensitized solar cells with Ag nanoparticle-functionalized freestanding TiO2 nanotube arrays. Chem. Phys. Lett. 2014, 614, 78–81. Search in Google Scholar

[254] Kamat PV. Quantum dot solar cells. Semiconductor nanocrystals as light harvesters. J. Phys. Chem. C 2008, 112, 18737–18753. Search in Google Scholar

[255] Lee H, Wang M, Chen P, Gamelin DR, Zakeeruddin SM, Graetzel M, Nazeeruddin MK. Efficient CdSe quantum dot-sensitized solar cells prepared by an improved successive ionic layer adsorption and reaction process. Nano Lett. 2009, 9, 4221–4227. Search in Google Scholar

[256] Gyori Z, Konya Z, Kukovecz A. Visible light activation photocatalytic performance of PbSe quantum dot sensitized TiO2 Nanowires. Appl. Catal. B-Environ. 2015, 179, 583–588. Search in Google Scholar

[257] Jiao S, Shen Q, Mora-Sero I, Wang J, Pan ZX, Zhao K, Kuga Y, Zhong XH, Bisquert J. Band engineering in core/shell ZnTe/CdSe for photovoltage and efficiency enhancement in exciplex quantum dot sensitized solar cells. ACS Nano 2015, 9, 908–915. Search in Google Scholar

[258] Mora-Sero I, Bisquert J. Breakthroughs in the development of semiconductor-sensitized solar cells. J. Phys. Chem. Lett. 2010, 1, 3046–3052. Search in Google Scholar

[259] Wu MX, Lin X, Wang YD, Ma TL. Counter electrode materials combined with redox couples in dye- and quantum dot-sensitized solar cells. J. Mater. Chem. A 2015, 3, 19638–19656. Search in Google Scholar

[260] Concina I, Vomiero A. Metal oxide semiconductors for dye- and quantum-dot-sensitized solar cells. Small 2015, 11, 1744–1774. Search in Google Scholar

[261] Li Z, Yu LB, Liu YB, Sun SQ. Efficient quantum dot-sensitized solar cell based on CdSxSe1-x/Mn-CdS/TiO2 nanotube array electrode. Electrochim. Acta 2015, 153, 200–209. Search in Google Scholar

[262] Tao L, Xiong Y, Liu H, Shen WZ. High performance PbS quantum dot sensitized solar cells via electric field assisted in situ chemical deposition on modulated TiO2 nanotube arrays. Nanoscale 2014, 6, 931–938. Search in Google Scholar

[263] Fu H, Liu H, Shen WZ. A composite CdS thin film/TiO2 nanotube structure by ultrafast successive electrochemical deposition toward photovoltaic application. Nanoscale Res. Lett. 2014, 9, 631. Search in Google Scholar

[264] Lai YK, Lin ZQ, Zheng DJ, Chi LF, Du RG, Lin CJ. CdSe/CdS quantum dots co-sensitized TiO2 nanotube array photoelectrode for highly efficient solar cells. Electrochim. Acta 2012, 79, 175–181. Search in Google Scholar

[265] Sun WT, Yu Y, Pan HY, Gao XF, Chen Q, Peng LM. CdS quantum dots sensitized TiO2 nanotube-array photoelectrodes. J. Am. Chem. Soc. 2008, 130, 1124–1125. Search in Google Scholar

[266] Wang HL, Gao QM, Jiang L. Facile approach to prepare nickel cobaltite nanowire materials for supercapacitors. Small 2011, 7, 2454–2459. Search in Google Scholar

[267] Zhang X, Lai YK, Ge MZ, Zheng YX, Zhang KQ, Lin ZQ. Fibrous and flexible supercapacitors comprising hierarchical nanostructures with carbon spheres and graphene oxide nanosheets. J. Mtaer. Chem. A 2015, 3, 12761–12768. Search in Google Scholar

[268] Ghosh S, Inganas O. Conducting polymer hydrogels as 3D electrodes: applications for supercapacitors. Adv. Mater. 1999, 11, 1214–1218. Search in Google Scholar

[269] Niu ZQ, Zhang L, Liu L, Zhu BW, Dong HB, Chen XD. All-solid-state flexible ultrathin micro-supercapacitors based on graphene. Adv. Mater. 2013, 25, 4035–4042. Search in Google Scholar

[270] Salari M, AboutalebI SH, Konstantinov K, Liu HK. A highly ordered titania nanotube array as a supercapacitor electrode. Phys. Chem. Chem. Phys. 2011, 13, 5038–5041. Search in Google Scholar

[271] Kim JH, Zhu K, Yan YF, Perkins CL, Frank AJ. Microstructure and pseudocapacitive properties of electrodes constructed of oriented NiO-TiO2 nanotube arrays. Nano Lett. 2010, 10, 4099–4104. Search in Google Scholar

[272] Yang Y, Kim D, Yang M, Schmuki P. Vertically aligned mixed V2O5-TiO2 nanotube arrays for supercapacitor applications. Chem. Commun. 2011, 47, 7746–7748. Search in Google Scholar

[273] Di J, Fu XC, Zheng HJ, Jia Y. H-TiO2/C/MnO2 nanocomposite materials for high-performance supercapacitors. J. Nanopart. Res. 2015, 17, 255. Search in Google Scholar

[274] Huang H, Gan MY, Ma L, Yu L, Hu HF, Yang FF, Li YJ, Ge CQ. Fabrication of polyaniline/graphene/titania nanotube arrays nanocomposite and their application in supercapacitors. J. Alloys Compd. 2015, 630, 214–221. Search in Google Scholar

[275] Shao Z, Li HJ, Li MJ, Li CP, Qu CQ, Yang BH. Fabrication of polyaniline nanowire/TiO2 nanotube array electrode for supercapacitors. Energy 2015, 87, 578–585. Search in Google Scholar

Received: 2015-9-18
Accepted: 2015-11-18
Published Online: 2016-1-7
Published in Print: 2016-2-1

©2016 by De Gruyter

This article is distributed under the terms of the Creative Commons Attribution Non-Commercial License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.