Keywords

5.1 Introduction

In recent years, sustainable developments of human being are retarded by increasingly serious environmental pollution and depletion of fossil fuel resources [1]. To surmount these obstacles, various technologies have been extensively developed to reduce the environmental and energy crisis by conventional techniques including adsorption, precipitation, and osmosis [2]. Although intense endeavors have been made to impede the deterioration of these two crises, it is still challenging to develop an efficient and green technique to control and reduce the pollution growth. Photocatalysis, as a novel route to achieve deep mineralization of organic pollutants toward carbon dioxide and water under light irradiation, has attracted enormous attention on account of its great efficiency and promising potential applications [3]. Among various semiconductors, TiO2 has been determined as the most widely used photocatalyst for versatile photocatalytic applications such as hydrogen production, CO2 reduction, selective organic transformation, and nonselective organic oxidation, owing to its environmental friendliness, stable chemical and physical properties, nontoxicity, and durability [4, 5].

Up to date, a large variety of TiO2 nanostructures have been prepared for photocatalytic investigations which mainly include nanoparticles, nanowires, nanorods, nanosheets, and nanotubes [6]. Among which, TiO2 nanotube arrays (TNTAs) growing vertically from Ti foil afforded by electrochemical anodization can serve as an ideal platform for exploring new photocatalysts [7]. The structural advantages of TNTAs predominantly center on the following several aspects. Firstly, one-dimensional geometry of TNTAs with smooth surface is beneficial for efficient charge transfer, thus providing significantly shorter carrier-diffusion paths along the tube walls and minimizing the charge losses arising from electron hopping between nanoparticles in comparison with corresponding bulk TiO2 nanoparticles [8]. Secondly, owing to direct growth property of TNTAs on Ti substrate, it is convenient to separate the TNTA-based photocatalysts from reaction systems for recycling reactions, thus reducing tedious recovery procedures for practical applications [9]. Despite the developments of TNTAs in the field of heterogeneous photocatalysis during the past few years, two intrinsic drawbacks still remarkably retard their potential applications. Specifically, wide bandgap of TNTAs (i.e., 3.2 eV) requires UV light irradiation to trigger the photocatalytic reactions, which accounts for only a small fraction of (ca. 5 %) solar spectrum in comparison with visible light (52 %) and infrared light (43 %). Additionally, fast recombination of photogenerated electron–hole charge carriers over TNTAs substantially reduces the efficiency of photocatalytic reactions. In this regard, it is highly desirable to shift the optical response of TNTAs toward visible or even infrared region in conjunction with high separation efficiency of photoexcited charge carriers for boosting the photocatalytic performances of TNTA-based photocatalysts. To this end, various strategies have therefore been developed to conquer the disadvantage of TNTAs including metal or nonmetal element doping [10, 11], noble metal deposition [12], sensitization with narrow-bandgap semiconductors [13], and formation of p–n heterojunction [14], just named a few, which will be specifically elucidated in the following parts.

In this chapter, we primarily focus on the state-of-the-art developments of TiO2 nanotube with an emphasis on the TNTA-based nanocomposites, the content of which primarily contains basic introduction of TNTAs, preparation of varieties of TNTA-based heterostructures, and their diverse photocatalytic applications in a myriad of fields. Finally, a perspective on future challenges regarding the design and improvement of TNTAs for photocatalysis is afforded. It is hoped that this chapter could provide enriched information on the potential applications of TiO2 nanotube-based nanocomposites as multifunctional photocatalysts.

5.2 Basic Introduction of TNTAs

The fabrication of TNTAs via anodic oxidation of titanium foil in a fluoride-based solution was firstly reported in 2001 by Grimes and co-workers [15]. Since then, precise control over the nanotube morphology, length, pore size, and wall thickness has been the focus of many studies [16, 17]. It has been well established that electrolyte composition plays an imperative role in determining the architecture and chemical composition of nanotube arrays. In particular, the formation rate of TNTAs can be tuned by electrolyte composition and its pH value.

The key process responsible for anodic preparation of TNTAs mainly includes the following four processes: (1) oxide growth on the metal surface owing to the interaction of metal with O2− or OH− ions. The in situ formed anions during the formation of an initial oxide layer can migrate through the oxide layer approaching the metal/oxide interface where they react with the metal [18]. (2) Ti4+ migrated from the metal at the interface will be rapidly released under an exerted external electric field and move toward the oxide/electrolyte interface. (3) The third process is the field-assisted dissolution of the oxide at the oxide/electrolyte interface [19]. The Ti–O bond is weakened under applied electric field giving rise to dissolution of the Ti4+ which dissolves into the electrolyte and interacts with the free O2− anions at the interface [20, 21]. (4) Chemical dissolution of titania in the HF electrolyte also takes place during the anodization process. Based on the above mechanism, highly ordered and self-aligned TNTAs were thus prepared. The experimental setup for preparing TNTAs was illustrated in Scheme 5.1.

Scheme 5.1
scheme 1

Experimental setup for preparing the TNTAs

Full size image

5.3 TNTA-Based Photocatalysts

5.3.1 TNTAs with Nonmetal Element Doping

Vertically oriented TNTAs have been regarded as an ideal candidate for photocatalytic applications, such as water splitting to produce H2 and photocatalytic degradation of organic dye pollutants, owing to their high catalyst/electrolyte interface area, electrolyte percolation, and remarkably enhanced separation of photogenerated electron–hole charged careers [22]. Nonetheless, the applications of TNTAs were retarded by the wide bandgap (Eg) energy (ca. 3.2 eV) of TiO2 which can only be excited by UV light irradiation. Hence, it is essential to shift the photoresponse of TiO2 from UV region to visible scope which may significantly promote the photocatalytic and photoelectrochemical properties of TiO2-based nanomaterials [23]. To this end, modification of the electronic structure of TNTAs with nonmetal element doping to narrow its bandgap energy has provided a convenient way to solve the disadvantage of TiO2, most of which centers on carbon [24], nitrogen [25, 26], phosphorus–fluorine [27], and nitrogen–fluorine–iodine doping [28], which result in remarkably enhanced visible-light-driven photocatalytic performances of the materials.

N-doped TNTA nanocomposites have been fabricated for photocatalytic explorations under visible light irradiation [29]. The N-modified TNTAs can be prepared by annealing TNTAs in a urea atmosphere or by anodization in nitrogen-containing electrolytes [30]. The N-implanted TNTAs have been well demonstrated to be the most efficient visible-light-driven photocatalyst among various nonmetal element-doped TNTAs since the nitrogen p states placed just above the valence band maximum of TiO2 may contribute to the bandgap narrowing without substantial increase of the charge career recombination.

Sulfur doping can also facilitate similar bandgap narrowing; nevertheless, the ionic radius of sulfur was found to be too large to be introduced into TiO2 lattice as proved by larger formation energy need for the substitution of sulfur than that need for the substitution of N. For example, Tang and Li fabricated S-doped TNTAs by annealing TNTAs in a H2S atmosphere, and the result showed that sulfur-modified TNTAs demonstrate more pronounced response over the visible light scope leading to significantly enhanced photoelectrochemical signals [31]. Carbon-doped TNTAs were fabricated by oxidation of TNTAs in a burning flame [32] or in CO gas flow [33] or under an argon and acetylene gas mixture flow [34], which endows TNTAs with improved photocatalytic performances under visible light irradiation.

Boron doping was also found to reinforce the visible-light-driven photocatalytic activities, as reported by Lu and co-workers [35] who prepared B-doped TNTAs via chemical vapor deposition approach, in which trimethyl borate was used as boron source and N2 as carrier gas. It was found that UV–vis spectra of B-doped TNTA nanostructures exhibited a relatively small absorption edge shift toward visible region (385–405 nm) when compared with the large shift observed in N-doping and C-doping TNTAs [36].

5.3.2 TNTAs with Metal Element Doping

Another efficient way to extend the photoresponse of TiO2 to visible region is doping of TiO2 with transitional metal ions or rare early metal ions [37, 38]. Extrinsic energy levels in the bandgap of TiO2 can be formed by incorporation of metal ions into TiO2 lattice, which may greatly influence the transfer of photogenerated electrons and holes. Generally, doped metal ions locate near TiO2 surface to facilitate charge transferring in which the energy level of metal ion reduction should be less negative than the conduction band edge of TiO2 and the energy level of metal oxidation should be less positive than the valence band edge of TiO2. Noteworthily, there exists the optimum concentration for metal ion doping, above which the photoactivity decreases owing to recombination of charge carriers in the site of metal ion [39, 40]. With respect to the fabrication of metal ion-doped TNTAs, it is essential to develop applicable approach to efficiently introduce metal ions into TNTAs without changing the vertically oriented morphology of TNTAs. For instance, Zn-doped TNTAs were achieved by immersing TNTAs in Zn-containing solution followed by calcination, as carried out by Yang and co-workers [41]. Zr-doped TNTAs can be fabricated via an electrochemical strategy based on their similar atomic radii (Ti, 2 Å; Zr, 2.16 Å) and belonging to both IV B elements and tetravalentine (+4) elements. W-doped TNTAs were prepared by anodization of tungsten titanium foil, and the red shift of the absorbance edge and a reduction in bandgap (0.14 eV) result in enhanced photocurrent density in comparison with blank TNTAs [42]. Alternatively, Cr-doped TNTAs attained by ion implantation exhibited a significant enhancement in photocurrent response within both UV and visible regions [43].

Few works involving the preparation of Fe-doped TNTAs have been reported, except that Sun and co-workers [44] prepared Fe3+-doped TNTAs by anodization of Ti foil in HF-aqueous electrolyte containing ferric nitrate. Similarly, Li and co-workers [45] prepared Fe-doped TNTAs by anodization of Ti foil in F− containing aqueous electrolytes of ferrous sulfate. It was found that the Fe-doped TNTAs demonstrate enhanced photocurrent response as compared with blank TNTAs.

5.3.3 TNTA/Noble Metal Nanocomposites

It has been ascertained that modification of TNTAs with noble metal nanoparticles (NPs) renders TNTA/metal nanocomposites efficient photocatalysts leading to significantly enhanced photocatalytic and photoelectrochemical performances [46]. Up to now, TiO2 has been deposited with various metal NPs such as Pt [47, 48], Au [49, 50], Pd [51], Ag [52, 53], and Co–Ag–Pt [54]. Deposition of metal NPs on the TNTA substrate is beneficial for retarding the recombination of photoexcited electron and hole charge carriers, in which metal NPs can serve as “electron reservoirs” for capturing the photogenerated electrons, thus giving rise to improved photocatalytic performances. For example, with respect to Ag-loaded TNTA nanocomposites, the conduction band (CB) of TiO2 is lower than the Fermi level of metallic Ag which allows for the transport of photogenerated electrons from CB of TiO2 to Ag forming a Schottky barrier between TNTAs and Ag NPs [55, 56]. The photogenerated electrons trapped by Ag NPs could be transferred to the adsorbed oxygen on the Ag surface resulting in O2 − active species which favor the photocatalytic process. Simultaneously, holes collected in the valence band of TiO2 could react with water to yield hydroxyl radicals which oxidize pollutants to CO2 and water, thereby fulfilling the whole photocatalytic cycle.

On the other hand, with regard to the preparation of metal/TNTA nanocomposites, the challenges of synthetically controlling the monodispersivity and homogeneous site distribution of metal NPs on the TNTAs still met with limited success. In particular, uniform deposition of metal NPs on the interior surfaces of TNTAs has been evidenced to be rather difficult than that on the outer surface [57]. Meanwhile, conventional synthetic methods such as complicated photoreduction [58–60] or chemical reduced approach [61] continued to plague the fabrication of well-defined metal/TNTA nanomaterials owing to poor repeatability. Therefore, achieving monodispersed deposition of metal NPs on the TNTAs is highly desirable. To this end, a facile, precisely controlled, and repeatable layer-by-layer (LBL) assembly route was developed to fabricate hierarchically ordered metal/TNTA heterostructures, M/TNTAs (M = Au, Ag, Pt). It was found that the tailor-made metal (Au, Ag, Pt) colloidal NPs were uniformly deposited on the TNTAs through the self-assembly monolayer (SAM) of LBL buildup, as shown in Fig. 5.1, which is afforded by substantial electrostatic attractive interaction between metal NPs and polyelectrolytes. Moreover, photoactivity of these well-defined heterostructures can be tuned by deposition cycles in the LBL process [62, 63]. Apart from the LBL assembly approach, another facile and green deposition strategy was also developed to achieve the preparation of Au/TNTAs, in which the negatively charged surface ligands of Au NPs were used as linking medium to facilitate the uniform deposition of Au NPs on the positively charged TNTA framework, as shown in Fig. 5.2 [64]. The as-assembled Au/TNTA heterostructure demonstrates significantly enhanced photocatalytic performances under UV light irradiation in comparison with blank TNTAs, for which Au NP deposited intimately on the TNTA substrate is speculated to serve as “electron trap” leading to enhanced separation of photogenerated electron–hole pairs. In another similar work, a facile self-assembly approach based on chemical bonding was developed to synthesize Au/TNTA binary nanostructures, in which Au NPs capped with dodecanethiol (DDT) linker were tethered to the interior and exterior surfaces of TNTAs via 3-mercaptopropionic acid (MPA) molecular which possesses bifunctional groups, as shown in Fig. 5.3. The ensemble of results indicated that the Au/TNTA nanocomposite obtained via the self-assembly approach exhibits significantly enhanced photocatalytic performance as compared to the counterparts of blank TNTAs, P25 particulate film, flat anodic TiO2 layer (FTL), and Au/FTL owing to the well-dispersed deposition of Au on the TNTA matrix, in which Au components play crucial roles as “electron reservoirs” and, simultaneously, the conducting Ti substrate beneath the nanotubes is conducive to electron transport, thus concurrently reinforcing the separation of photogenerated electron–hole pairs. This work affords a facile and efficient strategy to prepare a large variety of metal/TNTA nanocomposites via similar surface modification.

Fig. 5.1
figure 1

(a) Panoramic, (b) bottom, and (c) magnified cross-sectional SEM images of TNT substrate post-treated by calcination at 450 °C in air for 3 h with corresponding overall cross-sectional view in the inset of (c). Top-view SEM images of (d) Au/TNT, (e) Ag/TNT, and (f) Pt/TNT hybrid nanostructures prepared via LBL assembly method (Reprinted with the permission from Ref. [62]. Copyright 2012, American Chemical Society)

Full size image
Fig. 5.2
figure 2

(a) Panoramic SEM and (b–c) TEM views of blank TNTs with cross-sectional image in the inset of (a), (d) SEM and (e) TEM views of the Au/TNT heterostructure. (f) Photocatalytic performances of TNT, Au@DTDTPA/TNT, and Au/TNT heterostructure and (g) transient photocurrent response of TNT and Au/TNT heterostructure in 0.1 M Na2SO4 aqueous solution under UV light irradiation (365 ± 15 nm). The potential of the working electrode was set at 0.0 V versus the Pt counter electrode (Reprinted with permission from Ref. [64]. Copyright 2012, Royal Society of Chemistry)

Full size image
Fig. 5.3
figure 3

Typical panoramic SEM views of (a) vertically oriented TNTAs post-treated with calcination at 450 °C in air for 3 h with cross-sectional image in the inset and (b) Au/TNTAs (1.14 wt%) with additional detailed image in the inset; magnified cross-sectional SEM images of (c) TNTAs and (d) Au/TNTAs; HRTEM images of (e) TNTAs and (f) Au/TNTAs with corresponding SAED patterns in the inset (Reprinted with permission from Ref. [91]. Copyright 2012, Royal Society of Chemistry)

Full size image

5.3.4 TNTA/Plasmonic Metal or Metal Cluster Nanocomposites

In recent years, plasmonic energy conversion was proposed as a promising route to conventional electron–hole separation in semiconductors [65]. The key to utilize the plasmonic effect of metal component in photocatalysis is the controlled spatial distribution of monodispersed metal NPs on the semiconductor matrix. Till now, there have been some promising results on the applications of plasmonic effect to enhance the photoactivities of TNTAs, such as visible-light-driven Ag/AgCl/TNTA nanocomposite [66]. In such a ternary heterostructure, a new surface-plasmon-induced photocatalytic mechanism was presented for the remarkably enhanced photocatalytic performance of Ag/AgCl/TNTA nanocomposite, in which Ag NPs with a mean diameter of 20 nm were speculated to be photoexcited owing to plasmonic resonance, followed by the transfer of photogenerated electrons from Ag NPs to the conduction band of TiO2 and, simultaneously, the transfer of compensative electrons from electron donor (Cl−) to the Ag NPs, thereby resulting in pronouncedly enhanced photoactivities of the ternary nanostructure [67].

Besides, a new class of nanomaterial-metal cluster, consisting of precise number of metal atoms protected by thiolate ligands, has recently emerged as a novel photosensitizer to extend the photoactivity of TiO2. In striking contrast to conventional bulk metallic NPs, metal clusters exhibit (e.g., Aux clusters) several distinct properties, such as unique atom-packing mode, strong electron energy quantization induced by the ultra-small cluster size, sizable bandgap, and controllable catalytic properties. More recently, glutathione-capped Aux clusters and highly ordered nanoporous layer-covered TNTAs (NP-TNTAs) were employed as nanobuilding blocks for the construction of well-defined Aux/NP-TNTA heterostructures via a facile electrostatic self-assembly strategy. Versatile photocatalytic performances of the Aux/NP-TNTA heterostructure which acts as a model catalyst, including photocatalytic oxidation of organic pollutants, photocatalytic reduction of aromatic nitro compounds, and photoelectrochemical (PEC) water splitting under simulated solar light irradiation, were systematically exploited [68]. It was found that synergistic interaction stemming from monodisperse coverage of Aux clusters on NP-TNTAs in combination with hierarchical nanostructure of NP-TNTAs reinforces light absorption of Aux/NP-TNTA heterostructure especially within visible region, hence contributing to the significantly enhanced photocatalytic and PEC water splitting performances.

5.3.5 TNTA/Semiconductor Nanocomposites

A formidable challenge still remains in reducing the quick recombination rate of photogenerated electron–hole pairs over TiO2. Although transformation of TiO2 morphology to nanotubular structure may open a convenient avenue to improve the photocatalytic efficiency, it cannot tackle the central issue of photocatalysis [69]. Therefore, various research activities have devoted to reinforcing the photocatalytic properties by synthesizing TNTA/semiconductor hybrid nanostructures [70, 71]. For example, nanosized judicious coupling of TiO2 and ZnO has been well established to remarkably enhance the separation efficiency of photoexcited charge carriers due to the formation of heterojunction structure between them [72, 73], thereby boosting quantum efficiency and photostability of the hybrid photocatalyst [74, 75].

Inspired by this, combined with structural advantages of vertically aligned TNTA framework (e.g., high specific surface area and excellent chemical stability) as starting nanobuilding blocks, hierarchical ordered ZnO/TNTA composite photocatalysts with promising photocatalytic performances could be attained. Numerous chemical, electrochemical, and physical approaches have been developed to fabricate the ZnO/TNTA hybrid nanomaterials, including template-assisted strategy [76–78], hydrothermal method [79, 80], electrodeposition approach [81], and filtered cathodic-vacuum-arc technique [82]. Besides, Xiao et al have developed an efficient one-step pyrolysis route to fabricate 1D hierarchical ZnO/TNTA heterostructures, by which in situ formed ZnO nanocrystals were uniformly grafted on the framework of TNTAs [83]. The results show that enhanced separation of electron–hole pairs and improved photostability of the ZnO/TNTA heterostructures were achieved. The morphologies of the hierarchical nanostructures of ZnO/TNTA heterostructure were illustrated in Fig. 5.4.

Fig. 5.4
figure 4

Panoramic views of (a) TNTs fabricated via two-step anodization approach and the as-prepared (b) ZnO/TNT heterostructure; cross-sectional images of (c and e) TNT and (d and f) ZnO/TNT heterostructure (15 %); (e) photocatalytic activities of TNT, ZnO film, and ZnO/TNT heterostructure (15 %), and P25 particulate film for photodegradation of RhB aqueous solution under ambient conditions. (f) Photocatalytic performance of the ZnO/TNT heterostructure with varied deposition percentage of Zn(NO3)2 precursor in ethanol aqueous solution (Reprinted with the permission from Ref. [83]. Copyright 2012, American Chemical Society)

Full size image

In another work, spatially hierarchically ordered ZnO nanorod (NR)-decorated NP-TNTA (ZnO NR/NP-TNTA) nanocomposites have been prepared by an efficient, two-step anodization route combined with an electrochemical deposition strategy, by which monodispersed one-dimensional (1D) ZnO NRs were uniformly grown on the framework of NP-TNTA substrate, as shown in Fig. 5.5 [84]. It was found that the ZnO NR/NP-TNTA heterostructure exhibits significantly enhanced photocatalytic and photoelectrocatalytic performances, along with favorable photostability toward degradation of organic pollutants under UV light irradiation, as compared to the single-component counterparts. The remarkably enhanced photoactivity of ZnO NR/NP-TNTA heterostructure is ascribed to the intimate interfacial integration between ZnO NR and NP-TNTA substrate imparted by the unique spatially branched hierarchical structure, thereby contributing to the efficient transfer and separation of photogenerated electron–hole charge carriers.

Fig. 5.5
figure 5

Panoramic FESEM images of (a) NP-TNTA and (c) spatially branched hierarchical ZnO NR/NP-TNTA heterostructure, and cross-sectional images of (b) NP-TNTA and (d) ZnO NR/NP-TNTA heterostructure. (e) Photocatalytic performances of blank NP-TNTAs, pure ZnO NR film, and spatially branched hierarchical ZnO NR/NP-TNTA heterostructure toward degradation of RhB under UV light irradiation (365 ± 15 nm), (f) photocatalytic and photoelectrocatalytic activities of ZnO NR/NP-TNTA heterostructure under UV light irradiation (365 ± 15 nm) (Reprinted with permission from Ref. [84]. Copyright 2014, Royal Society of Chemistry)

Full size image

Moreover, many other narrow-bandgap semiconductors have also been used to sensitize the NP-TNTAs, for example, a hierarchically ordered CdSe/NP-TNTA hybrid nanostructure was fabricated through a facile electrochemical deposition strategy, by which the CdSe ingredients, consisting of clusters of quantum dots (QDs), were uniformly assembled on the inner and outer surfaces of the NP-TNTA framework, as revealed by Fig. 5.6 [85]. It was demonstrated that the as-prepared CdSe/NP-TNTA heterostructure could serve as an efficient photoanode for photoelectrochemical water splitting, and, moreover, it could also be used as a multifunctional photocatalyst for photoredox applications, including photocatalytic oxidation of organic dye pollutants and selective reduction of aromatic nitro compounds under visible light irradiation. Similarly, tailor-made negatively charged CdS QDs were evenly deposited on a hierarchical framework of NP-TNTAs by modulating surface charge properties of constituents, as displayed in Fig. 5.7. It has been demonstrated that the CdS/NP-TNTA hybrid nanostructures exhibit promising visible-light-driven photoactivity toward photooxidation of organic dye pollutants and photocatalytic reduction of nitrophenol derivatives as a result of monodisperse deposition of CdS QDs on the well-defined NP-TNTA scaffold [86].

Fig. 5.6
figure 6

FESEM images of (a and b) NP-TNTA and (c and d) CdSe/NP-TNTA heterostructure prepared via electrochemical deposition with deposition time of 1600s. (e) Photocatalytic performances of different samples and (f) photocatalytic reduction of 4-NA over different samples under visible light irradiation (λ > 420 nm) with the addition of ammonium formate as quencher for photogenerated holes and N2 purge under ambient conditions (Reprinted with permission from Ref. [85]. Copyright 2014, Royal Society of Chemistry)

Full size image
Fig. 5.7
figure 7

Panoramic FESEM images of (a) NP-TNTAs and (d) CdS QD/NP-TNTAs and cross-sectional FESEM images of (b and c) NP-TNTAs, (e and f) CdS QD/NP-TNTA heterostructure (Reprinted with permission from Ref. [86]. Copyright 2013, Royal Society of Chemistry)

Full size image

5.4 Photocatalytic Applications of TNTA-Based Nanocomposites

5.4.1 Nonselective Degradation of Organic Dye Pollutants

Nonselective photocatalysis has been extensively investigated owing to its great significance to environmental remediation [5, 87–89], by which contaminants are mineralized to less toxic inorganic compounds, such as water, carbon dioxide, and salts [90]. A series of 1D noble metal/TiO2 nanocomposites (with mean diameter of 14.7, 6.3, and 3.1 nm for Au, Ag, and Pt, respectively.) have been prepared via a facile and easily accessible electrostatic self-assembly approach by tuning surface charge properties of the hierarchically ordered TNTAs, as shown in Fig. 5.8 [62–64, 91]. These 1D hybrid nanostructures exhibited substantially enhanced photocatalytic performances toward degradation of organic dye pollutants owing to the Schottky barrier effect of noble metal NPs, which provides a new strategy to design highly ordered metal/1D semiconductor binary nanocomposites based on electrostatic interaction.

Fig. 5.8
figure 8

(a) Panoramic view of T-NRNT attained via the 2nd anodization with cross-sectional view in the inset, (b) high-resolution images of Au/T-NRNT with cross-sectional view in the inset, (c) Ag/T-NRNT, and (d) Pt/T-NRNT fabricated via a LBL self-assembly approach. (e) Photocatalytic performances of M/T-NRNT (M = Au, Ag, Pt) and (f) transient photocurrent responses of T-NRNT and M/T-NRNT (M = Au, Ag, Pt) heterostructures in 0.1 M Na2SO4 aqueous solution under UV light irradiation (365 ± 15 nm) (Reprinted with permission from Ref. [63]. Copyright 2012, Royal Society of Chemistry)

Full size image

5.4.2 Selective Organic Transformation

Selective organic transformation is of great industrial importance owing to the extensive use of organics in diverse applications. In particular, photocatalytic selective organic transformation is a green and promising technique, exhibiting intrinsic merits including mild reaction conditions and the possibility to reduce the generation of undesired by-products. More recently, TNTAs modified with ultra-small Aux clusters has been utilized as an efficient photocatalyst for selective reduction of a series of aromatic nitro compounds to corresponding amino compounds under simulated solar light irradiation [68]. It was speculated that the photogenerated electrons photoexcited from the HOMO (highest occupied molecular orbital) to LUMO (lowest unoccupied molecular orbital) of Aux clusters under simulated solar light irradiation transfer to the conduction band of TiO2 which reduces the nitro compounds absorbed on the scaffold of TNTAs to amino compounds, as clearly displayed in Fig. 5.9. Noteworthily, the selective photocatalytic reactions were performed in a N2 atmosphere, and photogenerated holes were completely quenched by hole scavengers; thus all photoinduced electrons in the reaction system were involved in the selective photoreduction reactions. Similar organic transformation reactions were also observed on the CdS QDs/NP-TNTAs which demonstrated significantly enhanced photoreduction performances under visible light irradiation in comparison with blank NP-TNTAs [86].

Fig. 5.9
figure 9

Photocatalytic reduction of substituted aromatic nitro compounds over blank NP-TNTA and Aux/NP-TNTA heterostructure (with dipping time for 96 h) under simulated solar light irradiation, with the addition of ammonium formate as quencher for photogenerated holes and N2 purge under ambient conditions: (a) 4-nitroaniline (4-NA), (b) 3-nitroaniline (3-NA), (c) 2-nitroaniline (2-NA), (d) 1-chloro-4-nitrobenzene (4-Cl), (e) 1-bromo-4-nitrobenzene (4-Br), and (f) 4-nitroanisole (4-OCH3). (g and h) Panoramic FESEM images of Aux/NP-TNTA heterostructure. (i) Low-magnified TEM image of Aux/NP-TNTA heterostructure (Reproduced from Ref. [68] by permission of John Wiley & Sons, Ltd)

Full size image

5.4.3 CO2 Reduction

Solar-energy-driven conversion of CO2 into hydrocarbon fuels can simultaneously generate chemical fuels to meet energy demand and mitigate rising CO2 levels. Thus far, diverse ranges of 1D nanostructures have been used for photoreduction of CO2 into chemical fuels [92–94], among which TNTAs were found to efficiently convert CO2 and water vapor into methane and other hydrocarbons under outdoor sunlight irradiation [92]. Specifically, Shankar’s group demonstrated an approach that is able to achieve high-rate sunlight-driven conversion of diluted CO2 to light hydrocarbons in which an optimized combination of a Cu–Pt coating and modulated-diameter TiO2 nanotube was used as photocatalyst. Their results showed that at least fourfold improvement in CO2 conversion rates over prior art using a catalyst consisting of coaxial Cu–Pt bimetallic shells supported on a periodically modulated double-walled TNTA (PMTiNT) core. Under AM 1.5 one-sun illumination, a hydrocarbon production rate of 3.51 mL g−1 h−1 or 574 nmol cm−2 h−1 using 99.9 % CO2 was attained, as shown in Fig. 5.10. Moreover, the periodic modulation of the diameters of the TNTAs increased the surface area and improved the utilization of light, while the bimetallic coating increased catalyst activity and specificity [92]. In another work, nitrogen-doped TNTAs were used for photocatalytic conversion of CO2 and water vapor to hydrogen fuels. Using outdoor global AM 1.5 sunlight (100 mW/cm2), a hydrocarbon production rate of 111 ppm cm−2 h−1, or ≈ 160 μL/(g h), was obtained when the nanotube arrays were loaded with both Cu and Pt nanoparticles [95, 96]. This work suggested that high-rate photocatalytic conversion of CO2 can be achieved using sunlight and high-surface-area TNTAs, with a nanotube wall thickness less than or in the range of the minority carrier-diffusion length, in combination with co-catalyst nanoparticles coated on the nanotube array surface. These two works open new avenues for carbon recycling using renewable sources.

Fig. 5.10
figure 10

(a and b) Plan-view and (c and d) cross-sectional SEM images of the as-prepared PMTiNT platform (a and c) and Cu–Pt loaded nanotubes (b and d). (e and f) Plots of hydrocarbon (□), CH4 (△), and C2H4 (○) solar-driven generation rates against fraction of Cu in Cu–Pt bimetallic system using 0.998 % (e) and 99.9 % (f) CO2. (g) Comparison of the C2H4 (○) and C2H6 (▽) solar-driven generation rates against fraction of Cu in the Cu–Pt bimetallic system using 99.9 % CO2. (h) Comparison of hydrocarbon generation activities of Cu0.33–Pt0.67 nanotube-loaded PMTiNTs (I, III) and regular TiNTs (II, IV) using 99.9 % (I, II) and 0.998% (III, IV) CO2 (Reproduced from Ref. [92] by permission of John Wiley & Sons, Ltd)

Full size image

5.4.4 Photoelectrochemical (PEC) Water Splitting

The aligned porosity, crystallinity, and oriented nature of the nanotubular structure make TNTAs an ideal candidate for promising PEC applications. The intrinsic configuration of TNTAs affords significantly shorter carrier-diffusion paths along the nanotube wall and minimizes the occurrence of charge losses arising from the electron hopping between nanoparticles [97, 98]. Moreover, the freestanding TNTAs are grown vertically on Ti substrate which can be directly used as photoelectrodes. In the past few years, construction of TNTA-based photoelectrode for PEC water splitting has been gaining continuous interest. For example, a visible-light-responsive plasmonic photocatalytic composite photoelectrode was constructed by rationally selecting Au nanocrystals (20 nm) and assembling them onto TNTA-based photonic crystal substrate with a pore diameter of 200 nm (Fig. 5.11) [99]. The rational design of the composite materials remarkably increased the SPR intensity of Au and consequently promoted the hot-electron injection from Au nanocrystals into CB of TiO2, thus resulting in substantially enhanced PEC water splitting performances under visible light irradiation (λ > 420 nm). In another work, TNTAs modified with palladium quantum dots (Pd QDs, 3.3 ± 0.7 nm) demonstrated substantially increased monochromatic incident photon-to-electron conversion efficiency (IPCE) of nearly 100 % at λ = 330 nm [100]. It was speculated that the synergistic interaction between nanotubular structure of TNTAs and uniformly dispersed Pd QDs facilitated the charge transfer of photogenerated electrons from TNTAs to Pd QDs and, simultaneously, the high activity of Pd QDs acting as catalytic centers contributed to the high-efficiency PEC hydrogen production. Apart from TNTAs, multicomponent nanotube arrays such as vertically oriented Ti–Pd mixed oxynitride nanotube arrays [101], Ti–Fe–O nanotube arrays [102], Ti–Nb–Zr–O mixed oxide nanotube arrays [103], and Ta3N5 nanotube arrays [104] have also been studied for PEC water splitting under simulated solar light or visible light irradiation.

Fig. 5.11
figure 11

(a) Schematic diagram showing the fabrication procedure for Au/TiO2 nanotube photonic crystals (NTPC). (b) SEM images of TiO2 NTPC, top right inset shows a cross-sectional view and bottom left inset shows high magnification of tilt 15° cross-sectional view of top PC layer, and (c) SEM image of Au/TiO2 NTPC. (d) Linear-sweep voltammograms of the samples under chopped AM 1.5G irradiation with a scan rate of 5 mV/s, and (e) photoconversion efficiency as a function of applied potential (Reprinted with the permission from Ref. [99]. Copyright 2013, American Chemical Society)

Full size image

5.5 Conclusions

In summary, TNTA-based nanostructures have emerged as promising photocatalysts for utilizing solar energy in the field of photocatalysis owing to their structural advantages including high surface-to-volume ratios and unique nanotube array structures. This chapter briefly introduces the state-of-the-art developments of TNTAs which mainly concentrate on the modification strategies and photocatalytic applications of TNTA-based nanocomposites. These judiciously modified TNTA nanostructures exhibit improved UV and visible light absorption, thus harvesting an increased portion of solar spectrum and reducing the recombination of photogenerated electron–hole pairs. Future research efforts may be directed to fabricate TNTAs of different chemical compositions with good chemical and physical stabilities to absorb a broader solar spectrum, especially the infrared region which accounts for a large portion of the solar spectrum. It is anticipated that TNTA-based hybrid nanostructures could afford more versatile potential applications in a myriad of fields.