Applications of TiO2 nanotube arrays in environmental and energy fields: A review

Microporous and Mesoporous Materials 202 (2015) 22–35

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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso

Review

Applications of TiO2 nanotube arrays in environmental and energy fields: A review Qingxiang Zhou ⇑, Zhi Fang, Jing Li, Mengyun Wang Beijing Key Laboratory of Oil and Gas Pollution Control, College of Geosciences, China University of Petroleum Beijing, Beijing 102249, China

a r t i c l e

i n f o

Article history: Received 31 December 2013 Received in revised form 1 September 2014 Accepted 15 September 2014

Keywords: TiO2 nanotube arrays Photocatalysis Environmental analytical chemistry Hydrogen production Solar cells

a b s t r a c t TiO2 nanotube arrays, novel TiO2-based nanomaterials with unique chemical and physical properties, have been demonstrated to serve as multifunctional materials which show great promise in addressing many challenges in both environmental and energy technology fields. They have exhibited extraordinary catalytic abilities in several cases: in the degradation of environmental inorganic and organic pollutants to less toxic compounds, water splitting, and in the reduction of atmospheric CO2 levels by incorporation of CO2 into hydrocarbons, among others. Moreover, the wide absorption spectrum characteristics and distinct electrochemical properties of modified TiO2 nanotube arrays make them excellent candidates for use in solar cells and sensitive sensors for trace compounds, etc. This review focuses on the recent applications of TiO2 nanotube arrays in removal of pollutants, environmental analytical chemistry, water splitting, solar cells and CO2 conversion. Ó 2014 Elsevier Inc. All rights reserved.

Contents 1. 2.

3.

4. 5. 6. 7.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photocatalytic degradation of pollutants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. TiO2 nanotube arrays as photocatalysts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Modified TiO2 nanotube arrays as photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1. Doping TiO2 nanotube arrays. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2. Loading on the TiO2 nanotube arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.3. TiO2 nanotube array heterojunctions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Environmental analytical chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1. Gas monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2. Detection of heavy metal ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.3. Detection of organic pollutants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Measurement of COD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Sample pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications in sensitized solar cells (SSCs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydrogen production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photocatalytic conversion of CO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⇑ Corresponding author. Tel./fax: +86 10 89732300. E-mail address: [email protected] (Q. Zhou). http://dx.doi.org/10.1016/j.micromeso.2014.09.040 1387-1811/Ó 2014 Elsevier Inc. All rights reserved.

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Q. Zhou et al. / Microporous and Mesoporous Materials 202 (2015) 22–35

1. Introduction

2. Photocatalytic degradation of pollutants

Due to the rapid acceleration of technological advances across the globe, each year more and more products are produced to make our lives more comfortable and convenient. Most of these products originate from natural sources such as petroleum, coal, natural gas and mineral sources; meanwhile, many environmental problems occur during the various steps of the extraction, transportation, and transformation of raw materials into final products. These problems include CO2 emission, which contributes greatly to global warming, ozone depletion, and air and water pollution, which increase health risks to living things. Although these problems may only appear to impact local areas, they actually pose cumulative hazards on a global level. For example, it is reported that pesticides and heavy metals have been detected in remote Antarctic areas, far from their pointsof-use. As a result, each country around the world is facing challenges to look for better strategies to solve such energy and environmental problems. Nanomaterials are novel material forms that emerged in the 1980s and have been studied intensely in recent years. The ‘‘nano’’ designation stems from the fact that the unit size of these materials is about 1–100 nm along any single dimensional scale. A new form of TiO2 nanomaterial, the TiO2 nanotube array, has attracted much attention recently. Currently, many synthetic methods for the preparation of TiO2 nanotube arrays exist, including the most commonly-used synthetic methods: the template and anode oxidation methods. In the template method, common templates include porous alumina, zinc oxide and various organic polymers [1,2]. The morphology of each resulting nanotube array depends on the size and shape of the template, and the main disadvantage is the subsequent destruction of nanotube arrays during the downstream separation process. The anode oxidation method is an electrochemical method in which the Ti substrate is anodized in an electrolyte containing F ions, either with or without organic solvents. Gong et al. first introduced this facile method to fabricate highly-ordered vertical nanotube arrays [3], and this has been the preferred method for TiO2 nanotube array production in recent years. Many researchers have investigated the parameters that would affect the morphology of prepared samples such as potential, electrolyte composition, oxidation time and annealing temperature. TiO2 nanotube arrays possess unique chemical and physical properties such as chemical inertness, gas sensitivity, large surface area, biocompatibility, high photocatalytic and electrochemical activities. These characteristics have led researchers to adapt them to improve upon old technologies, including solar cells, removal of pollutants, water splitting [4], sensors [5], sample pretreatment [6], drug delivery [7], and CO2 conversion [8]. Currently, research into new nanostructures derived from TiO2-based materials holds great promise to help address many urgent global challenges. Several reviews have been published which have focused on the synthesis and applications of TiO2 nanotube arrays [9], including a short summary (only available in Chinese) byl our group on the applications of TiO2 nanotube arrays to address environmental challenges in 2012 [10]. Thus, the purpose of this review is to provide a thorough survey of recent TiO2 nanotube array research to a global audience by focusing on several aspects of environmental and energy field topics including: (1) photocatalytic degradation of pollutants, (2) applications in environmental analytical chemistry, (3) hydrogen generation, (4) sensitized solar cells (SSCs), and (5) CO2 conversion to hydrocarbons.

2.1. TiO2 nanotube arrays as photocatalysts

23

Environmental pollution has become a growing problem, resulting in more and more research interest in this area. To this end, photocatalytic degradation has been successfully developed and is rapidly becoming the best way to deal with environmental pollutants. In this method, when semiconductive materials are illuminated with light with energy equal to or higher than the band gap energy of the semiconductors, the electrons in the valence band (VB) are excited to enter the conduction band (CB), and then are transferred to the surface of particles with available holes in the VB. These photo-generated electron–hole pairs possess high redox activity. Since the band gap of anatase TiO2 is about 3.2 eV, electrons in this material can be excited by illumination using light with wavelengths less than 387 nm, allowing the electrons in the VB of TiO2 to be excited to the CB and give rise to electron–hole pairs. Once these pairs reach the TiO2 surface, some pairs can recombine, releasing energy as light or heat, while others can react with O2, H2O and OH adsorbed to the TiO2 surface to form radicals which can oxidize macromolecular pollutants to form CO2 and water, etc. During the photocatalytic process, it is of great importance to choose a suitable catalyst. Metal oxide semiconductor materials have been successfully developed so that these photocatalytic reactions may occur in mild conditions [11]. TiO2 powders have been used for many years in the photocatalytic field and their advantages have fueled ongoing interest. However, this material possesses several disadvantages: it is difficult to disperse, agglomerates easily, is difficult to reuse, possesses a low light response, and causes environmental pollution when used inappropriately. The introduction of TiO2 nanotube arrays, a novel form of TiO2, has solved these problems. Wender et al. [12] prepared TiO2 nanotube arrays employing an anode oxidation method using ethylene glycol (EG) electrolytes containing 1-n-butyl-3methyl imidazolium tetrafluoroborate (BMIBF4) and water (Fig. 1). The photocatalytic activity was investigated using methyl orange (MO) as the model pollutant and 13% of the MO was mineralized under UV irradiation in 150 min, indicating that the TiO2 nanotube arrays possessed high photocatalytic activity. For salicylic acid (SA) and salicylaldehyde (SH), the TiO2 nanotube arrays exhibited much higher photocatalytic activity upon irradiation using UV– visible light [13]. It found that 83% of the SA was oxidized in 2 h, while SH was almost completely eliminated under the same conditions. These results indicated that this method can be used as a potential treatment to process SA and SH pollutants. It is obvious that the structural parameters of TiO2 nanotube arrays, such as specific surface area, wall thickness, tube length and crystalline phase, have important effects on the photocatalytic activity of TiO2 nanotube arrays. Liang et al. [14] investigated these effects on the photocatalytic activity of TiO2 nanotube arrays using 2,3-dichlorophenol (2,3-DCP) as the target pollutant. They found that a larger specific nanotube surface area can lead to greater absorption of aqueous reactants, while a higher pore volume can accelerate the diffusion of various aqueous species during the photocatalytic reaction, both enhancing the reaction rate. Using optimally-calcined nanotube arrays under UV illumination, 93% of added 2,3-DCP was degraded 2.6 times more rapidly, as compared to the result obtained with TiO2 films under the same conditions. Zhang et al. fabricated TiO2 nanotube arrays on fluorine-doped tin oxide (FTO) glass [15], which contained a top porous nanoparticle layer and nanotube array bottom layer. This new material resulted in the enhancement of the photocatalytic activity in

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Q. Zhou et al. / Microporous and Mesoporous Materials 202 (2015) 22–35

Fig. 1. Schematic of TiO2 nanotube arrays synthesized in ionic liquid and used in H2 production and pollutant degradation [12].

Fig. 2. Schematic illustration of the PEC reduction of Cr(VI) with S-TNTs as the photoanode and a Ti mesh as the photocathode under UV irradiation [16].

glucose photooxidation by 60% over that of TiO2 nanotube arrays based on simple Ti foil. This effect was attributed to the fact that these nanotube arrays provided more stable electron pathways resulting in high photocatalytic activity. Wang et al. discovered that the increase of the surface area of the photocathode could greatly accelerate the photoelectrocatalytic reduction rates of Cr(VI) [16]; short TiO2 nanotube arrays (S-TNTs) had higher electron transfer efficiency than that of long TiO2 nanotube arrays (LTNTs), and nearly complete reduction of Cr(VI) using S-TNTs was achieved with UV irradiation in 60 min (Fig. 2). Their findings demonstrated that this simple and efficient method could remove Cr(VI) from aqueous samples and was a good start for developing highly-efficient methods for heavy metal pollution removal from water samples. Using a different strategy, Zhang et al. designed a photocatalytic reactor incorporating a rotating disk composed of a TiO2-nanotube (TNT)/Ti photocatalyst (Fig. 3) [17]. They investigated the effect of parameters such as the rotation velocity and irradiation time on the photocatalytic activity of the TiO2 nanotube arrays and found that the rotating velocity had an important impact on the photocatalytic activity. When the rotating velocity increased up to 30 rpm, the removal rate of rhodamine B approached 90% in 3 h, an improvement of 25–40% over results using a TiO2 nanoparticle

Fig. 3. Schematic diagram (a) and TBPC combined mechanism (b) of the rotating disk photocatalytic reactor [17].

disk. The results indicated that a higher rotational velocity increased delivery of polluted thick water film to the reaction, opening a potentially valuable new direction in the development of methodologies for environmental pollutant removal.

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Q. Zhou et al. / Microporous and Mesoporous Materials 202 (2015) 22–35 Table 1 Advances of modified TiO2 nanotube arrays as a catalyst in photocatalytic degradations. Modification

Methods

Pollutants

Light sources

Degradation rate (%)

Refs.

N and F B ZnTe Ti C Fe Ag and N Cu2O C WO3

Anodization Electrodeposition Electrodeposition Hydrothermal method Cyclic voltammetry Liquid phase deposition Electrodeposition method Electrodeposition Hydrothermal treatment Immersion

MB Phenol 9-AnCOOH Rhodamine B 9-AnCOOH MB AO-II AO-I I MB Cr(VI)

Visible light Visible light Simulated solar light UV light Simulate solar light Visible light Visible light Visible light Visible light UV light

90 66 100 80 100 50 37 90 80 100

[18] [20] [33] [46] [47] [48] [49] [50] [51] [52]

2.2. Modified TiO2 nanotube arrays as photocatalysts In general, anatase TiO2 responds only to the UV fraction of sunlight; however, the fraction of UV light in sunlight is very small relative to the fraction of visible light. Based on this fact, much effort has been put forth to determine ways to utilize visible light in this photocatalytic system. The simplest successful approaches have been to modify the TiO2 material itself to enhance its response to visible light via doping, depositing, loading of TiO2 nanotube arrays, and other methods [18,20,33,46–52] (Table 1). 2.2.1. Doping TiO2 nanotube arrays Doping is an often-used method to change material properties in which one or more elements or compounds are doped into the substrate to generate specific electrical and/or optical properties. Many studies have shown that doping TiO2 with elements such as nitrogen, carbon, fluorine, iodine and/or iron can lead to the necessary narrow band gap to allow a greater response to visible light, which can enhance the overall photocatalytic activity. Li et al. prepared N and F co-doped TiO2 nanotube arrays by anodizing a Ti substrate in NH4F and NH4Cl solution [18]. They found that annealing of the doped TiO2 nanotube arrays in an N2 atmosphere could effectively reduce the phenomenon of F atom replacement by O atoms, resulting in a higher photocatalytic activity towards methylene blue. In fact, annealing TiO2 nanotube arrays in a specific atmosphere has also been developed as a new doping method. In the process of preparing N and S co-doped TiO2 nanotube arrays through the annealing of TiO2 nanotube arrays in thiourea at 500 °C [19], N–Ti–O and N–O–Ti bonds were formed between N atoms and the nanotube arrays, and some of the O atom positions were replaced by S atoms, which improved their degree of crystallization. Such doping markedly increased the photocatalytic activity towards methylene blue, and photocatalytic activity was 1.29 times higher than that of an un-doped array. Boron-doped TiO2 nanotube arrays have also exhibited a phenol degradation rate about 10% higher than that of undoped arrays [20]. Owing to its low cost and easy preparation, Fe is considered one of the most suitable elements for industrial applications. Doping of TiO2 with Fe3+ is an effective approach to reduce electron–hole recombination rates and increase the photocatalytic efficiency due to its semi-full electronic configuration and an ion radius close to that of Ti4+. Sun et al. prepared Fe-doped TiO2 nanotube arrays by anodizing Ti in an electrolyte containing Fe(NO3)3 [21]. By controlling the concentration of Fe(NO3)3 in the electrolyte, various concentrations of Fe-doped TiO2 nanotube arrays were obtained. The results showed that a red shift occurred in the absorption spectrum and the photocatalytic performance was enhanced, as expected. Wu et al. prepared Fe-doped TiO2 nanotube arrays using ultrasoundassisted impregnating and calcination [22]. The first step of this two-step approach was to fabricate the TiO2 nanotube arrays directly on Ti foils via electrochemical anodic oxidation and then to immerse the resulting TiO2 nanotube arrays in an 0.01 M

Fe(NO3)39H2O aqueous solution under ultrasound assistance. The second was to anneal the modified Ti foils at different temperatures under ambient conditions for 2 h. SEM analysis indicated that Fe2O3 nanoparticles with a size of 10–20 nm were deposited onto the TiO2 nanotubes and some Fe3+ ions were doped into the TiO2 lattice. These structural modifications were thought to induce the red-shift of the absorption spectral edge of the TiO2 nanotube arrays into the visible light range. These Fe–TiO2 nanotube arrays exhibited a much higher visible-light photocatalytic activity for the degradation of methyl blue (MB) than undoped TiO2 nanotube arrays (Fig. 4). Electrochemical impedance spectroscopy (EIS) showed that Fe incorporation could efficiently promote the separation and transfer of photogenerated charge carriers, a key factor in effecting improved photocatalytic performance. Xu et al. doped TiO2 nanotube arrays by anodizing Ti–Nb alloys, followed by heat treatment in a flow of ammonia gas to obtain Nb/N co-doped TiO2 nanotube arrays [23]. They found that the Nb dopant in TiO2 nanotube arrays can enhance both the adsorption of NH3 molecules and the subsequent nitrogen doping of the TiO2 nanotube arrays. These arrays demonstrated a significantly-enhanced visible light response with a markedly higher visible light-induced photocatalytic degradation of methylene blue when compared to undoped TiO2 nanotube arrays. 2.2.2. Loading on the TiO2 nanotube arrays The noble metals have lower fermi levels than that of TiO2. Thus, they can absorb the electrons excited from TiO2, effectively reducing the recombination rate of photogenerated electron–hole pairs, ultimately increasing photocatalytic activity. Xie et al. prepared highly-dispersed Ag nanoparticles on TiO2 nanotube arrays using a pulse current deposition technique [24]. The TiO2 nanotube arrays with Ag particles under electrodeposited charge densities of 1800 mC cm2 resulted in the highest absorption peak. In the degradation experiment using methyl orange (MO) under visible light irradiation, the photocatalytic kinetic rate constant of the Ag/TiO2 nanotube array was 5.16 times that of the undoped TiO2 nanotube array, indicating that the TiO2 nanotube array modified with Ag nanoparticles could efficiently inhibit electron–hole recombination. This method could also be used to modify other metal nanoparticles on the TiO2 nanotube arrays. Liu et al. dispersed Ag nanoparticles onto the surface of TiO2 nanotube arrays using an electrodeposition method [25]. The uniform Ag nanoparticles increased the separation efficiency of electrons and holes. In addition, the modified TiO2 nanotube arrays led to the highest photocatalytic activity towards MO when the electrodeposition time was 60 min. It was postulated that Ag nanoparticles acted as electron reservoirs to suppress the electron–hole recombination, making more holes available for the oxidation reactions. Tan et al. fabricated Pd-functionalized TiO2 nanotube arrays [26], which were utilized to photodegrade methylene blue (MB) and stearic acid (SA). The photodegradation efficiency of 76% (MB) was achieved under UV irradiation in 4 h, while only 62% was

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Fig. 4. Schematic of illustrating the separation and transport of charge carriers under visible light irradiation for TiO2 nanotube arrays, Fe3+/Fe4+ and a-Fe2O3 [22].

decomposed when TiO2 nanotube arrays without Pd coating were used. These results were attributed to the higher surface area of modified arrays and to the catalytic activity of Pd nanoparticles and to the effective separation of the electron–hole pairs. This group also investigated the photocatalytic activity for solid contaminants such as SA film. The results indicated that the TiO2 nanotube arrays demonstrated much better photocatalytic activity than that of TiO2 film under the same conditions. Liu et al. deposited CdS nanoparticles onto a TiO2 nanotube array surface using a chemical bath deposition method [27]. This modified photocatalyst showed excellent photocatalytic performance and cycling stability under visible light. The photodegradation rate of MO was up to 96.7% higher under visible light irradiation using CdS/TiO2 nanotube arrays after 180 min. Xiao et al. found that gold nanoparticle-functionalized TiO2 nanotube arrays could also effectively enhance the photocatalytic performance of TiO2 nanotube arrays [28,29]. They discovered that Au nanoparticles acted as ‘‘electron traps’’, thus prolonging the separation lifetime of photoexcited electron–hole charge carriers. Iron oxide-modified TiO2 nanotube arrays were synthesized and the photocatalytic activity was investigated with 2-naphthol as the model pollutant [30]. The results indicated that the modified TiO2 nanotube arrays obtained by annealing at 873 K after anodization showed high 2-naphthol degradation efficiency, and the degradation rate was 4.26 times higher than that of unmodified TiO2 nanotube arrays. This was due to iron oxide mediating the transmission of electrons in the VB and CB of TiO2. The electrons then effectively reduced the adsorbed O2 to form radicals, which had high oxidizability and rapidly degraded the 2-naphthol into small molecular compounds. Chen et al. developed Au nanoparticles and used reduced graphene oxide (RGO) co-modified TiO2 nanotube arrays to serve to photocatalyze the degradation of methyl orange [31]. The results exhibited that, in addition to Au, RGO could also capture the photoinduced electrons of TiO2 nanotube arrays to suppress the recombination of the electron– hole pairs. Owing to the simultaneous electron transfer of TiO2 nanotube arrays to Au and RGO, the minimal recombination of photogenerated charges in Au/RGO-TiO2 nanotube arrays resulted in more effective charge separation, which made it a good photocatalyst for the degradation of organic pollutants. The results also

demonstrated that the prepared photocatalyst displayed high catalytic activity, excellent stability, and easy recyclability. Different semiconductors possess distinct band gaps in which photocarriers can transfer light energy between semiconductors, and a coupling effect would then be expected to occur under illumination [32]. ZnTe is a semiconductor with a narrow band gap of 2.23–2.28 eV, which means that this material will be excited under visible light. So if this type of semiconductor is introduced into TiO2 nanotube arrays, the visible light response will be increased and lead to significant enhancement of photocatalytic ability. Liu et al. fabricated ZnTe-modified TiO2 nanotube arrays using an electrodeposition method, and investigated the resulting photocatalytic activity under visible light radiation. ZnTe-modified TiO2 nanotube arrays displayed much higher photocatalytic activity towards 9-AnCOOH [33]. The mechanism for the enhanced photocatalytic activity was demonstrated in Fig. 5. Under the illumination of both visible and UV light, the photogenerated electrons in the conduction band (CB) of ZnTe were transferred to the conduction band (CB) of TiO2 and then reacted with O2 to produce O 2 radicals, which combined with H+ from hydrogen peroxide (H2O2). As the holes of the valence band of TiO2 moved to the valence band of ZnTe, they then reacted with H2O2 to form hydroxyl radicals (OH). The OH radicals then oxidized 9-AnCOOH into the end-products. Zhu et al. prepared BiFeO3-modified TiO2 nanotube arrays as the photoelectrocatalytic catalyst [34]. Their results showed that the composites had stronger absorption in the visible region and much higher photocatalytic efficiency than the undoped TiO2 nanotube arrays with rhodamine B as the model pollutant. The modification gave rise to a synergistic effect between the lowered electron–hole recombination rate and the wider spectral response. ZnFe2O4/TiO2 nanotube arrays were prepared by an electrodeposition method [35], in which Zn2+ and Fe3+ were reduced to Zn and Fe, and then treated with oxidation to form the proposed catalyst. From the surface photovoltaic spectra (SPV), it was found that the adsorption area of modified TiO2 nanotube arrays extended from the UV to the visible light region, when compared to the unmodified one. Under UV illumination, the degradation efficiency of 4-chlorophenol (4-CP) by loaded TiO2 nanotube arrays was 1.08 times greater than that of unmodified TiO2 nanotube arrays, and at a bias

Q. Zhou et al. / Microporous and Mesoporous Materials 202 (2015) 22–35

27

Fig. 5. Illustrative diagrams of the electron and hole transfer in ZnTe/TiO2 nanotube arrays and the mechanism of photocatalysis degradation [33].

potential of 0.8 V, all contaminants could be degraded by loaded TiO2 nanotube arrays. TiO2 nanotube arrays loaded with the Bi2O3 nanoparticles also achieve better photoelectrochemical properties [36], and the experimental results showed that the photocurrent could be easily observed in the Bi2O3/TiO2 nanotube arrays, due to Bi2O3 enhancement of the photocurrent. The removal rate of 4-CP with Bi2O3/TiO2 nanotube arrays was 2 times than that of unmodified arrays under the same conditions. The band gap of Bi2O3 is 2.85 eV and can be excited by the light with wavelength less than 435 nm, but the photocatalytic activity of Bi2O3 is very low due to the high recombination rate of electron and hole pairs in Bi2O3. However, the electrons of the VB of TiO2 can be transferred to the VB of Bi2O3 under visible light, and holes generated in TiO2 can initiate the photocatalytic reactions, which enhance the photocatalytic activity of the catalyst. Zhu et al. fabricated CdS/TiO2 nanotube arrays by an electrochemical atomic layer deposition method [37] in which CdS was loaded on the inner and outer of the walls of TiO2 nanotube arrays. This method could also reduce the CdS deposited at the entrance of the nanotube to avoid pore-clogging. In addition, this coaxial heterogeneous structure significantly enlarged the contact area between CdS/TiO2 and the CdS/electrolyte, which decreased the travel distance that electrons and holes must move to react with pollutants, thus increasing the adsorption of protons and the photocurrent of modified TiO2 nanotube arrays. The results showed that the introduction of CdS increased the photocatalytic activity of TiO2 nanotube arrays by 5-fold over that of unmodified TiO2 nanotube arrays. Xie et al. [38] developed a new sonicationassisted chemical batch electrodeposition approach to prepare CdS quantum dots (QDs) sensitized TiO2 nanotube arrays. The results demonstrated that the absorption spectrum of the modified arrays significantly moved into the visible region approaching 550 nm (Fig. 6). When compared with a conventional sequential chemical bath deposition method, this method can modify the CdS QDs both on the nanotube and on the tube walls, which can effectively reduce clogging. The photocatalytic degradation rate towards methyl orange was 1.158 times higher than that of unmodified TiO2 nanotube arrays. The band gap of the sonication-CdS/TiO2 nanotube arrays was calculated to be 2.20 eV. It was much lower than the band gap of TiO2 nanotube arrays, which meant that it could absorb more visible light. Wang et al. loaded different amounts of Cu2O nanoparticles onto TiO2 nanotube arrays using ultrasonication-assisted sequential chemical bath deposition [39]. Cu2O nanoparticles with narrow band gaps acted as the sensitizer to promote the charge transfer to TiO2, which led to efficient photogenerated charge carrier separation. The modified TiO2 nanotube array composite showed enhanced absorption of visible light and improved separation of photogenerated electrons and holes.

Fig. 6. UV–vis diffuse reflectance absorption spectra of the (a) plain TiO2 nanotube arrays, (b) CdS/TiO2 nanotube arrays, and (c) sonication-CdS/TiO2 nanotube arrays [38].

Zhang et al. modified TiO2 nanotube arrays with reduced graphene oxide (RGO) and PbS nanoparticles (NP) in one step [40]. The results showed that PbS was successfully dispersed inside and outside the walls of TiO2 nanotube arrays and a reduced graphene oxide film was formed on the top of the surface of TiO2 nanotube arrays. Almost 100% of added pentachlorophenol was removed in 120 min vs. only 61% using bare TiO2 nanotube arrays. The reduced graphene oxide surface film could successfully suppress the photocorrosion of PbS, which led to high photoactivity of these modified TiO2 nanotube arrays. They also replaced PbS nanoparticles with Ag nanoparticles and investigated their photocatalytic activity towards 2,4-dichlorophenoxyacetic acid. The obtained catalyst also achieved good photocatalytic activity and 93% of the target pollutant was degraded [41]. TiO2 nanoparticles have also exhibited good photocatalytic activity, although their structure is different from TiO2 nanotube arrays. The combination of these two distinct types of TiO2 materials has been shown to exhibit excellent photocatalytic performance. Zhang et al. prepared novel high-activity TiO2 nanoparticle-filled TiO2 nanotube arrays using vacuum-assisted filling methods [42]. These novel arrays demonstrated the expected properties and 4-fold higher degradation rate for MO than that of unmodified TiO2 nanotube arrays, presumably due to an increase in reaction sites contributed by highly active TiO2

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nanoparticles on the surface of TiO2 nanotube. Deng et al. reported Bi2WO6-modified TiO2 nanotube arrays synthesized by hydrothermal deposition [43]. The addition of Bi2WO6 led to the shifting of the absorption band edges to higher wavelengths and exhibited stronger light absorption in both UV and visible light regions. Due to the enhanced separation of photogenerated electron–hole pairs, the photocurrent of the modified TiO2 nanotube arrays was 4 times higher than that of the unmodified annealed TiO2 nanotube arrays. The photocatalytic activity of Bi2WO6/TiO2 nanotube arrays was 2 times higher than that of the unmodified TiO2 nanotube arrays under visible light irradiation, and was about 1.5 times as much as that with Bi2WO6. 2.2.3. TiO2 nanotube array heterojunctions A ‘Z-scheme’ system, by mimicking natural photosynthesis, was proposed to enhance photocatalytic efficiency, which combines two photoexcitation systems and an electron-transfer mediator [44]. Xie et al. reported a Z-scheme type CdS–Ag–TiO2 nanotube array [45]. They found that the recombination rate of photogenerated carriers was reduced on CdS or Ag nanoparticle-modified TiO2 nanotube arrays, and also for the CdS–Ag–TiO2 nanotube array system. The photocurrent density of CdS–Ag–TiO2 nanotube arrays exceeded that of the Ag–TiO2, CdS–TiO2, and TiO2 nanotube array systems under UV light irradiation at 2 h. For the degradation of MB, the removal rate was about 63% under UV irradiation by 2 h, and only 23% could be degraded when using pure TiO2 nanotube arrays. The enhanced performance could be explained by a twostep excitation of CdS and TiO2, and Ag as a mediator which led to efficient charge separation of the photogenerated electron–hole pairs. Wang et al. synthesized graphite-like carbon-modified TiO2 nanotube arrays by impregnating TiO2 nanotube array films in a sucrose solution [53]. They found that the photoresponse initially increased with increasing sucrose concentration and decreased when the sucrose concentration exceeded 0.01 g mL1, achieving twice the photoresponse of TiO2 nanotube arrays under the same conditions. The EIS spectra and HR-TEM image of these modified TiO2 nanotube arrays demonstrated that the graphite-like carbon layer that formed on the sample surface and nanotube walls played different roles in photoelectrocatalytic responses. The two graphite-like carbon layers on the nanotube walls could decrease the depleting and Helmholtz layer resistance and then enhance the separation of charges. On the contrary, the graphite floccules mainly acted as a light blocking layer, decreasing photoabsorption. The optimum photoresponse of TiO2 nanotube arrays was observed under UV illumination when the sucrose concentration was 0.1 mg L1, the response was twice as high as that of pure TiO2 nanotube arrays and three times higher than that of pure TiO2 nanotube arrays under the illumination of visible light. The synergetic effect between carbon and TiO2 nanotube arrays resulted in the high efficiency of the charge separation process, thus enhancing photoelectrocatalytic activity. Although many improvements on TiO2 nanotube arrays as photocatalysts have been achieved, there are still some deficiencies which need to be resolved. Until now, the approach to enhance the utilization of visible light has gained some progress and opens the door to greater prospects, but there is still much work ahead to solve practical difficulties in wastewater treatment methodologies. Meanwhile the most widely-researched model pollutants are simple dyes such as MO and MB, which differ from real industrial pollutants that are released into the environment. Moreover, many complex pollutants and persistent organic pollutants have attracted much more attention and seriously threaten the environment and human health. Thus, nanotube arrays and other methods should be applied to develop methods to achieve degradation of these compounds.

3. Environmental analytical chemistry 3.1. Sensors Environmental pollutants have attracted much attention due to their toxic effects on human health and environment. In order to evaluate their environmental safety and prevent human exposure to naturally and industrially generated inorganic and organic contaminants, it is of great value to develop rapid, sensitive, and lowcost monitoring methods and devices. To date, many useful and robust methods have been developed. Sensors are important and valuable tools for the detection of hazardous pollutants. TiO2 nanotube arrays have great potential to play a role in development of new sensors. Recently, electrochemical sensors based on TiO2 nanotube arrays have been employed for a variety of applications, including monitoring of heavy metals [54], amines [55], SO2 [56], glucose [57] and hydrogen [58]. 3.1.1. Gas monitoring In 2003, Grimes et al. found that TiO2 nanotube arrays could be used as a good sensor for H2 and showed a sensitivity between 104 and 1000 ppm of H2. Meanwhile, Pd-modified TiO2 nanotube arrays have the potential to significantly increase the sensitivity of this sensor by up to 107-fold with a shorter response time [59]. Due to its good insulating performance and arc extinction, SF6 has been widely used in gas-insulated switchgears (GIS). When insulation faults occur in GIS, discharging electrical energy causes the SF6 gas to undergo a decomposition reaction and then generate gases such as SOF4, SOF2, SO2F2, and SO2. Thus, there is an increasing interest to develop such gas sensors. Zhang et al. [56] used TiO2 nanotube arrays as a sensor to detect SO2. They found that the sensitivity of the sensor increased with increase in temperature, and in addition, the response time decreased, because higher temperatures accelerated the movement and diffusion of the molecules. At the optimal working temperature of 200 °C, they also found that the higher the concentration of SO2 gas, the higher the response (sensitivity). There was a good linear relationship between the response signal and the concentration over the range of 10– 50 ppm with a relation coefficient of 0.992, which indicated that this sensor could be used to determine SO2 gas at low levels. 3.1.2. Detection of heavy metal ions Toxic heavy metal ions in water and soils pose a serious threat to the environment and human beings when they enter the food chain. In order to evaluate environmental safety, it is important to develop rapid and sensitive methods for monitoring them at low levels. Many methods have been developed based on flame atomic absorption spectrometry (FAAS), graphite furnace atomic absorption spectrometry (GFAAS), inductively coupled plasma mass spectrometry (ICP-MS), atomic fluorescence spectrometry (AFS), and inductively coupled plasma atomic emission spectrometry, etc. In addition to these methods, sensors provide a novel technique for monitoring heavy metal pollutants. A DNA-modified TiO2 nanotube array sensor was reported for the determination of Pb2+ in water samples [54]. The determination procedure involved two steps. First, Pb2+ was applied to the sensor by immersing it into the sample solution. Second, the electrode was rinsed and then transferred to an electrolytic cell without Pb2+, and a differential pulse anodic stripping voltammetry (DPASV) method was utilized to determine Pb2+concentration. The results showed that the experimental values detected by this sensor were in agreement with that of AAS for real samples (Fig. 7). Arsenic is an important inorganic pollutant, requiring rapid and sensitive detection methods. Currently there are many useful analytical methods based on different principles. Yang et al. fabricated

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Fig. 7. Schematic illustration for DNA/C-TiO2 nanotube arrays construction and its Pb ion monitoring [54].

Au shrubs-modified TiO2 nanotube arrays as a novel sensor to detect the concentration of arsenic [60]. This composite possessed a high surface area compared with other modified TiO2 nanotube arrays. The results showed a high sensitivity between current changes and concentration of arsenic with a value of 25.7 lA cm2 at 5 lg L1 As3+. However, only 10.6 lA cm2 was obtained when Au film-modified TiO2 nanotube arrays were used, which indicated that more surface area and the unique 3D structures accounted for the high performance. 3.1.3. Detection of organic pollutants Cai et al. reported a sensor composed of molecularly imprinted polymer-modified TiO2 nanotube arrays for perfluorooctane sulfonate in water samples. The direct detection of perfluorooctane sulfonate by electrocatalytic reduction reaction was fulfilled using modified TiO2 nanotube arrays with a detection limit of 86 ng mL1. The selectivity of this sensor was also very good [61]. Cai et al. developed an octachlorostyrene (OCS) photoelectrochemical (PEC) immunosensor by cross-linking anti-OCS antibody onto CdTe/CdS-sensitized TiO2 nanotube arrays [62]. The PEC immunosensor exhibited high specificity and high sensitivity with a limit of detection of 2.58 pM, and a linear range from 5 pM  50 nM. Due to the excellent photoelectronic performance of the CdTe/CdS–TiO2 nanotube arrays, the label-free PEC immunosensor showed a highly sensitive and selective response to OCS. The testing time was 4 min. Compared with conventional optical methods, the PEC immunoassay was simpler in instrumentation and more easily miniaturized. 3.2. Measurement of COD Chemical oxygen demand (COD) is an important parameter for the evaluation of water pollution and is also the most common item in water monitoring. The most commonly-used method to determine COD is the potassium bichromate method, which has advantages such as reliable results and good reproducibility, but the defects are also obvious. For example, the procedure takes a relatively long time (2–4 h), and consumes a large quantity of expensive and poisonous agents, such as Ag2SO4 and HgSO4, etc. In recent years, many novel technologies have been developed to detect COD [63,64] based on electrochemical methods, photocatalytic oxidation, and photoelectrochemical oxidation. Recently, a TiO2 nanotube array was used to develop a new determination method for COD. Using photoelectrochemical oxidation, Zhang et al. developed a new determination method for COD using TiO2 nanotube arrays as the work electrode [65]. They found that TiO2 nanotube arrays, which were prepared in the solution with 1% HF electrolyte (pH = 2) at the anodic potential of 20 V and annealed at 450 °C, showed the highest photocurrent density. The principle of COD determination is based on Faraday’s law, and the COD value could be calculated using the following equation:

CODðmg=L of O2 Þ ¼

nC Q  3200 ¼  3200 ¼ KQ 4 4FV

During the COD determination experiments, they found that interferences such as pH variation and coexisting ions such as NH+4 and Cl had no effect on the determination of COD. This

method worked within a linear range of 0–850 mg L1. Compared with the conventional K2Cr2O7 method, this method, based on highly ordered structure of TiO2 nanotube arrays, resulted in good reproducibility [66]. The new TiO2 nanotube array method also has advantages such as short experiment time and is free from some toxic reagents, which are often used in the conventional method. The key to obtaining the real COD value is to oxidize the organic components completely, and the excellent photocatalytic oxidability of TiO2 nanotube arrays makes them ideal electrode materials for determination of COD. Chen et al. assessed performance of this method using water samples containing some refractory and low concentration organic compounds [67]. A comparison was made between the theoretical chemical oxygen demand (ThCOD) and response COD, and the related equation was used, as follows:

COD ¼ a  ThCOD: They chose recalcitrant organic compounds including sugars, benzene derivatives, organic acids, alcohols, amino acids and pyridines, etc. The COD values achieved using the photoelectrochemical sensor were larger than those obtained using the CODCr method, especially in determination of the COD values of pyridine and amino acid solutions, where the CODCr method showed a lower a value of 4.5% and 0.5%, while the photoelectrochemical method obtained a higher a value of 95.1% and 97.6%, which exhibited the super-oxidation capability of TiO2 nanotube arrays. This new COD determination method based on TiO2 nanotube arrays possessed many advantages such as effective catalysis, fast mass transport, large effective surface area, and good control over the electrode microenvironment; these properties make it applicable to water monitoring. A Cu2O-loaded TiO2 nanotube arrays electrode was fabricated by an electrodeposition process and used as a sensor to detect COD value [68]. By modification with Cu2O, TiO2 nanotube arrays showed high absorption intensity in the visible light region and a much higher sensitivity to visible light. Adding a positive bias potential of 0.3 V in visible light achieved a low detection of limit of 15 mg L1 and a good linear range of 20– 300 mg L1. However, environmental samples often have unknown or changing matrix characteristics, which will result in difficulty in achieving precise concentrations of focused pollutants. This challenge will impact the development of measurement devices [69]. 3.3. Sample pretreatment With the technological advances, trace pollutants have attracted more attention. However, it is also difficult to achieve accurate detection due to the varieties of pollutants in water, soil and air, low concentrations of the pollutants and strong matrix effects. Thus, a sample pretreatment step is necessary in the analytical procedure. In general, it is estimated that more than 60% of analysis time is spent on the sample pretreatment, especially on trace and ultra analysis. Enrichment is an effective method that concentrates trace targets to the concentration that matches the detection sensitivity of instruments. The sample pretreatment step not only concentrates the targets, but also cleans the samples, which decreases matrix effects and increases the accuracy of analytical methods.

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Conventional sample treatment methods have been used for many years such as liquid–liquid extraction (LLE) and Soxhlet extraction, etc. However, the typical disadvantage of these methods is the usage of a large quantity of organic reagents, which will generate secondary pollution. In recent years, some novel pretreatment methods have been developed to address this issue. Solidphase micro-extraction (SPME) is a novel pretreatment method based on partition equilibrium, which allows analytes to reach equilibrium between stationary phase and liquid phase. Jiang et al. fabricated TiO2 nanotube arrays on a Ti wire, which was used as the extraction fiber in SPME [70]. The highly ordered TiO2 nanotube arrays showed high selective adsorption ability to different analytes. The results indicated that PAHs and alkanes could be effectively adsorbed on a TiO2 nanotube array with enrichment factors in the range of 82.6–96.9, while lower adsorption abilities were found for anilines and phenols. Limits of detection were in the range of 0.001–0.1 lg L1 for the targeted PAHs. When real water samples were analyzed, the recoveries were in the range of 78.57–119.28%. These results demonstrated that TiO2 nanotube array fibers had many advantages over commercial SPME fibers such as high rigidity, long lifetime and good resistance to pH variation and high temperature conditions. Due to the excellent properties of TiO2 nanotube arrays, our group has focused on them and developed a new type of pretreatment method called micro-solid phase equilibrium extraction (lSPEE) technique (Fig. 8) [6,71–74]. Previously, we investigated the applicability of TiO2 nanotube powders as the adsorbent in SPE and developed many enrichment and determination methods for monitoring copper [66], nickel [75] and cadmium [76], benzoylurea insecticides [77], paraquat and diquat [78] and DDTs and their main metabolites [79] in water samples. The results showed that TiO2 nanotubes had better enrichment capabilities and the analytical methods exhibited low detection limits for targeted analytes. The l-SPEE method developed recently was also based on the equilibrium principle, and the TiO2 nanotube arrays were used as the adsorbent. The procedures of adsorption and desorption of pollutants on sorbent occurred at the same time. When the rates of adsorption and desorption was equal, the TiO2 nanotube array was taken out and then a little solvent was used to elute analytes adsorbed on the TiO2 nanotube array. The final solution was dried and redissolved with an appropriate solvent and analyzed with selected analytical instruments such as GC, HPLC and AES, etc. We then used this developed method to determine trace levels of pyrethroids in environmental water samples [71]. We first investigated the effect of anodic potential on the adsorption performance of a TiO2 nanotube array. The nanotube array prepared under low potential possessed a small-diameter hole, which made pollutants difficult to transfer from liquid phase

to the TiO2 nanotube. Large holes would decrease the specific surface area, reducing the absorption ability, and a potential of 20 V was finally chosen. The other parameters that affected the l-SPEE procedure included the eluting solvent, pH value, salt effect, equilibrium time and desorption time. After optimization of conditions, the LODs were in the range of 0.018–0.073 lg L1 (S/N = 3) and the linear ranges were in the range of 0.1–80 lg L1 for pyrethroids bifenthrin, fenpropathrin and fyhalothrin, and 0.2–160 lg L1 and 0.3–210 lg L1 for fenvalreate and deltamethrin, respectively. When this method was used for the determination of five fungicides, lower limits of detection were obtained in contrast to the commonly-used SPE method [80]. The surfactants could be adhered onto TiO2 nanotube arrays through electrostatic interactions between the charges of metallic oxide and opposing charges of the surfactants to form micelles when the concentration of the surfactant reached critical micelle concentration (CMC). As a result, the metallic oxide surface coated with surfactants became hydrophobic, useful for the enrichment of pollutants. Niu et al. used a cetyltrimethyl ammonium bromide (CTAB)-coated titanate nanotube as the adsorbent for solid phase extraction to enrich phthalate esters in water samples, obtaining good results [81]. We prepared CTAB-coated TiO2 nanotube arrays and investigated the adsorption of polyaromatic hydrocarbons (PAHs) [82]. The results showed that when the concentration of CTAB was 90 mg L1, the maximum adsorption capacity was obtained. With this modified TiO2 nanotube array as the l-SPEE sorbent, a rapid and sensitive determination method was developed and the detection limits of 16 PAHs were obtained in the range of 0.026–0.82 lg L1. Pan et al. reported a new method for the determination of PAHs with TiO2 nanotube arrays fabricated on Ti wire and modified with Au nanoparticles and n-octadecanethiol, and the detection limits of selected PAHs were in the range of 0.1–3.0 ng L1 [83]. TiO2 nanotube and modified TiO2 nanotube arrays will have many applications, and our efforts will focus on development of highly selective, rapid, reliable and sensitive enrichment and determination methods for trace pollutants such as new persistent organic pollutants and typical pollutants in the environment.

4. Applications in sensitized solar cells (SSCs) Solar energy is a great renewable source and would serve as the ideal future clean energy supply for the world, due to its accessibility when compared with wind, nuclear and biomass energy. Efficient solar cells must absorb enough light over a broad spectral range from visible to near-infrared (near-IR) wavelengths (350– 950 nm) and convert the incident light effectively into electricity

Fig. 8. The principle of l-SPEE method [82].

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[84]. Among many solar cells devices, the dye-sensitized solar cell (DSSC) is a new solar cell with special properties. O’Ragan et al. first developed this sensitized solar cell using a TiO2 particle film as the photoanode [85]. Many efforts were dedicated to enhance the light conversion of these cells. Owing to the advantages such as their low cost, easy manufacture and high efficiency, DSSCbased TiO2 materials have been shown to be a potential alternative to conventional solid-state solar cells [86–90]. As one of the important components of solar cells, the photoanode has a great effect on light conversion efficiency. The TiO2 nanotube array, a new material with unique properties, has been used in photovoltaic studies where many experiments have also verified desirable properties [91–94] such as highly ordered nanotube structure which leads to accelerated electron transport and an ordered surface which increases sensitizer absorption [95]. Park et al. transplanted TiO2 nanotube arrays onto FTO (fluorine-doped tin oxide) glass [96], which can improve the performance of the photoresponse when compared with TiO2 nanoparticles. The efficiency was enhanced to 5.36% by a post-treatment with a TiCl4 solution. The study showed treatment with TiCl4 could inhibit the recombination of charges and accelerated the move of electrons, which increased the charge density in the photoanode. They also fabricated a nanoporous layer-covered TiO2 nanotube array in TiCl4 solution [97]. Both the electron transport rate and electron lifetime were improved, and more surface defects were found on the surface of TiO2 nanotube arrays over conventional arrays, which also increased the performance of the DSSCs. Wang et al. first deposited Ti film on the FTO and then prepared TiO2 nanotube arrays on it for use as the photoanode in the DSSCs [98]. Their study showed that the adhesive force between Ti film and FTO depended on the temperature of the sputtering process. At lower temperatures, TiO2 nanotube arrays were easily peeled off, while the stability was improved in higher temperatures. This transparent photoanode exhibited a high conversion of light to electricity. A bamboo-like structured TiO2 nanotube array was fabricated by altering the anodization voltage [99]. They found that the bamboo rings could provide much larger surface area for dye loading which led to a high conversion efficiency. Wang et al. fabricated a bamboo-type TiO2 nanotube array by using a square-wave voltage [100]. The EIS measurements showed reduced interfacial resistance and increased the interfacial capacitance in the bamboo-type TiO2 nanotube arrays compared with the smooth type arrays. An increase in surface area of the bamboo-type TiO2 nanotube arrays resulted in dye loading in both the inner and outer walls, which increased the conversion efficiency by 7.36% when compared with smooth TiO2 nanotube arrays. Luo et al. immersed the TiO2 nanotube arrays in deionized water to remove Ti foil and then formed free-standing membrane nanotube arrays, and then coupled them with TiO2 nanoparticles, FTO and electrolyte to form solar cells [101]. The improved light scattering performance and improved I 3 diffusion were observed in this DSSC. Wang et al. modified the TiO2 nanotube arrays with Ru(dcbpy)2(NCS)2 as a dye-sensitized solar cell [102]. They found that the TiO2 nanotube arrays fabricated for 50 h showed the highest conversion efficiency, however the morphology of the material prepared for 60 h was destroyed, which would provide more recombination centers for electrons, and cause the reduction in efficiency. Wang et al. [92] reported results after TiO2 nanoparticles were deposited onto TiO2 nanotube arrays. They found that deposition of TiO2 nanoparticles can remarkably improve the absorption of N3 dye with an enhancement of 47.2% due to the increasing surface area of nanostructure. A conversion efficiency of 6.28% was achieved for DSSC with TiO2 nanoparticles as the flexible photoanodes. This electrodeposition method of nanoparticles has great potential uses, such as in photovoltaic, photodegradation and sensor applications.

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Liu investigated the effect of the length of nanotube on the performance of solar cells [103]. The results indicated that a longer nanotube has a positive effect on photocurrent density and conversion efficiency. Meanwhile, the open-circuit photovoltage was decreased, which offset the short-circuit photocurrent density, so the overall performance was improved. Many studies have shown that the use of a photonic crystal (PC) layer on top of a mesoporous TiO2 layer can enhance light harvesting [104,105]. Huang et al. developed a single-step method to couple a PC layer to TiO2 nanotube arrays [106]. The TiO2 nanotube arrays layer was obtained by normal electrochemical anodization and the TiO2 PC layer was fabricated by a periodic current pulse anodization. This bi-layer structure DSSC showed a significantly enhanced power conversion efficiency (PCE) of 50% over that of single layer DSSC. They proposed a novel photonic crystal-based photoanode composed of a TiO2 nanoparticle (TiO2 NP) absorption layer and a thin TiO2 nanotube array photonic crystal (TiO2 NT PC). The TiO2 NP worked as the adsorbing layer and TiO2 NT PC conferred the PC effect and acted as the scattering layer. Compared with conventional TiO2 NP-based DSSCs, the PCE increased by 39.5% due to the combined effects [107]. Wang et al. reported a spiral structure of TiO2 nanotube arrays on Ti wire and found that it could efficiently trap scattered light [108]. Alivov et al. transformed TiO2 nanotube arrays to form TiO2 nanoparticle films using similar annealing conditions as used to create the photoanodes in DSSC applications [109]. With a nanoparticle size of 65 nm, a maximum nominal efficiency of 9.05% was observed. The lowest efficiency of 1.48% was observed for DSSCs when nanoparticle size was 350 nm, which indicated that the nanoparticle size had a significant influence on the performance of the solar cells. Bandara et al. [110] fabricated solid-state dyesensitized solar cells with different thicknesses of transparent TiO2 nanotube array electrodes coupled with a Ru-(II)-donor antenna dye. A power conversion efficiency of 1.94% was obtained. They also found that a linear increase in the cell current was observed with the increase in length of the TiO2 nanotube arrays. Mirabolghasemi et al. fabricated single-walled TiO2 nanotube arrays and demonstrated that a significant gain in electrical and photoelectrochemical properties could be reached with these unique tubes [111]. They also found that the short circuit current density and efficiency of the solar cell were higher than those for double-walled nanotube ones. However, the cost of preparing dye-sensitized solar cells is relatively high, which interferes with the commercialization of DSSCs. This, it is necessary to look for substitutes that replace dye in the conversion of light energy to electrical energy. New sensitized solar cells such as quantum dot-sensitized solar cells and heterojunction solar cells have been introduced. Semiconductors are widely used in the modified TiO2 nanotube arrays to improve the photoelectrochemical properties by broadening the light region that is absorbed and by reducing the recombination rate of the photogenerated electron–hole pairs. In recent years, the semiconductor material has received more attention and has been used as the sensitizer, replacing the dye sensitizer [112–114]. Hossain et al. synthesized CdSe nanoclusters on highly-ordered TiO2 nanotube arrays using chemical bath deposition [112]. That study showed that parameters such as photocurrent, photovoltage, filling factor, and conversion efficiency were enhanced significantly by increasing the deposition time due to increased deposition of CdSe onto TiO2 nanotube arrays under that condition. Because CdSe is a narrow band gap material, it could be used to absorb the long wavelength light in the visible spectrum. The results exhibited that higher conversion efficiency was achieved when compared with other forms of CdSe QDs [115]. CdS nanoparticles adsorbed onto TiO2 nanotube arrays could be also used in solar cells [116]. The results indicated that the method effectively decreased the

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Table 2 TiO2 nanotube arrays as a photoanode in solar cells.

a

Photoanode

Jsca

Voca

FFa

Conversion efficiency (%)

Refs.

TiO2NA/N719 TiO2NA/N719/ TiO2NA/N719 TiO2 NA/N719 TiO2NA/Ti wire/N719 TiO2NA/CdSe TiO2NA/N719 P3HT@CdS@TiO2NA TiO2NA/Nanocrystalline CdS TiO2 NA/N719 TiO2NA based on mash/N719 TiO2NA/FTO/N719 TiO2NA/CdS/CdSe/ZnS TiO2NA/N719

6.48 7.85 12.39 7.63 10.9 7.19 7.21 3.00 5.17 15.00 12.4 15.46 13.52 13.5

0.76 0.77 0.637 0.68 0.522 0.438 0.65 0.7 0.77 0.59 0.68 0.814 0.48 0.7

0.58 0.61 0.5549 0.708 0.48 0.495 0.45 0.55 0.47 0.49 0.6 0.641 0.53 0.7

3.18 3.7 4.38 3.68 2.78 1.56 2.13 1.16 1.87 4.29 5 8.070 3.44 6

[33] [94] [98] [101] [103] [112] [120] [121] [122] [123] [124] [125] [126] [127]

Jsc = short circuit photocurrent density (mA cm2); Voc = open circuit photovoltage (V); FF = fill factor; TiO2NA = TiO2 nanotube arrays.

aggregation during modification and the photoelectric conversion efficiency increased by 65.8% when compared with the sequential chemical bath-deposition method. Ren et al. detached TiO2 nanotube arrays from the TiO2 substrate and then fabricated a TiO2/ ZnO nanotube array heterojunction [117]. The results indicated that the DSSCs based on the heterojunction exhibited a better short circuit current density of 8.67 mA cm2 and a higher PCE of 3.98% under AM 1.5 illumination. Fan et al. reported results after affixing Ag–Ag2S hybrid nanoparticles onto TiO2 nanotube arrays, which exhibited a photocurrent density of 2.76 mA cm2 and approximately 92 times greater photoelectric efficiency than that of bare TiO2 nanotube arrays. The excellent performance of the composite was attributed to the surface plasmonic resonance effect, which was further enhanced by an Ag2S outer-layer [118]. Wu et al. reported that CdS-sensitized ZnO nanorod arrays on the TiO2 nanotube arrays could be used to generate hydrogen from water photoelectrolysis [119]. The existence of the one-dimensional structure of TiO2 nanotube arrays, acting as an electron collector and transporter, could provide a direct and quick electron pathway for photoinjected electrons along the photoanode and reduce electron– hole recombination. The applications of TiO2 nanotube arrays used as the photoanode are shown in Table 2.

5. Hydrogen production Hydrogen is regarded as an ideal energy resource and is mainly produced from natural gas via steam methane reforming [128]. However, hydrogen production from the splitting of water has been considered as another effective way to solve the current energy shortage. Highly-ordered TiO2 nanotube arrays have been explored as a catalyst for hydrogen production [129]. The experimental results exhibited that the Pt nanoparticles loaded on the TiO2 nanotube arrays by a microwave irradiation method enhanced hydrogen generation rate up to 0.613 ml h1 cm2 compared with 0.313 ml h1 cm2 of the unmodified one. The reason was that the Pt nanoparticles loaded onto the TiO2 nanotube arrays would trap the photoinduced charge carriers and accelerate the interfacial charge-transfer process, which increased the photocatalytic reaction rate. Kei reported a catalyst membrane that consisted of TiO2 nanotube arrays and a Pd layer. The Pd layer could be used to separate H2 from other by-products [130]. The effect of the annealing temperature of TiO2 nanotube arrays [131] on hydrogen production was also investigated. The annealing temperature had a significant effect on the crystal phase, morphology and photoelectrochemical properties of TiO2 nanotube arrays. Low annealing temperature had no effect on the crystal phase and morphology of preexisting material. With further increase in temperature, the crystal

phase transferred from anatase to the rutile phase, resulting in destruction of the tubular structure for vectorial charge transfer, resulting in a sharp decrease in photocurrent. TiO2 nanotube arrays annealed at 450 °C showed the highest photoconversion efficiency of 4.49% and the highest hydrogen production reported, a rate of 122 lmol h1 cm2. Sang et al. reported that TiO2 nanotube arrays created via sonoelectrochemical anodic oxidation and annealed in different gases showed distinct photoelectrochemical properties in hydrogen production [132], and they fabricated different elementdoped TiO2 nanotube arrays in their studies [133]. C-doped TiO2 nanotube arrays demonstrated more surface-active sites and a negative flat band potential, which improved the photoelectrochemical properties for hydrogen production. Smith et al. reported a facile method to synthetize hierarchical TiO2 nanotube arrays by first etching them in a solution of HF and HNO3, followed by anodization [134]. The unique morphology provided an increased surface area for light utilization and accelerated the separation of electron and charge pairs. Water splitting efficiencies were 0.34% and 0.15% at 1.23 (RHE) using hierarchical TiO2 nanotube arrays grown on wires and foils, respectively. This hydrogen generation rate was increased by over 40% for hierarchical TiO2 nanotubes arrays vs. plain TiO2 nanotube arrays, and over 25% increased for wire substrate vs. foil. Based on the same principles of photocatalytic oxidization mentioned in the pollutant- degradation section above, it is of great importance to decrease the recombination rate of electron–hole pairs within the TiO2 nanotube arrays during hydrogen production. Many researchers have made great strides in achieving this goal by loading semiconductors or metals onto TiO2 nanotube arrays such as Cu2O/Cu [135], and CdS (Fig. 9) [136]. When CdS-modified TiO2 nanotube arrays were used as electrodes for hydrogen production [129], the presence of CdS nanoparticles led to improved photoelectrochemical reactivity, which efficiently facilitated the separation and transfer of photogenerated electron–hole pairs. Lai et al. used an impregnation method to load WO3 onto the TiO2 nanotube arrays [137]. By controlling the soaking time, different amounts of WO3 loaded TiO2 nanotube arrays were obtained. Their study showed that a small amount of WO3 could significantly improve the PEC water splitting performance, which was approximately 1.5 times higher than that of pure TiO2 nanotube arrays. An excessive amount of WO3 would decrease the photocatalytic activity due to formation of agglomerates. Palladium quantum dots (Pd QDs)sensitized TiO2 nanotube arrays were prepared by hydrothermal method. When this material was used as the photoanode, a hydrogen production rate of 592 lmol h1 cm2 was obtained, and which was 1.6 times than that of unmodified one. This enhancement was ascribed to the synergetic effects between TiO2 and Pd QDs, and Pd QDs acted as electron sinks and catalytic centers,

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Fig. 9. Configuration model consists of CdS/TNAs glass electrode and Pt cathode for hydrogen generation [136].

which reduced the recombination rate of photogenerated electrons and holes and increased the rate of water decomposition [138]. Smith et al. fabricated TiO2 nanotube arrays onto Ti wire, which acted as the catalyst to split water into hydrogen. An enhancement of 40% was observed when compared with TiO2 nanotube arrays on Ti foil [135]. Zhang et al. loaded carbon quantum dots onto the TiO2 nanotube arrays using a deposition method and prepared a carbon quantum dots-sensitized catalyst for hydrogen production [139]. It was found that enhanced optical absorption was seen in both the visible and NIR (near infrared) regions. The photocurrent density of the composite was 4 times higher than that of unmodified TiO2 nanotube arrays. The hydrogen production rate was about 14.1 mmol h1 for a carbon quantum dots-sensitized TiO2 nanotube array under simulated sunlight illumination (AM 1.5G, 100 mW cm2). Zhang et al. [140] used Cu(OH)2 modified TiO2 nanotube arrays as the catalyst for hydrogen production and found that the H2-production yield was 20.3 times higher than that with unmodified TiO2 nanotube arrays due to the synergistic effect between Cu(OH)2 and TiO2 nanotube arrays. Wang et al. used CdSe/CdS/TiO2 nanotube arrays for hydrogen production [141]. The results showed a 7-fold enhancement in photoconversion efficiency and its band gap generated an obvious red shift to broaden the visible light response. Hydrogen also can be generated from the decomposition of alcohols like ethanol. A gas phrase photocatalytic decomposition of alcohols in high vacuum conditions was reported [142]. The H2 production of 2.8  108 Torr was obtained by using Pt-modified TiO2 nanotube arrays. The experimental results showed that a longer TiO2 nanotube was favorable for hydrogen generation due to its ability to provide more reaction sites.

ature changes the crystalline phase of TiO2, causing an increase in the composition of the rutile phase and decrease in anatase phase. Because the highest photocatalytic reactivity was observed in crystals composed of a mixture of anatase and rutile phases, control over the ratio of the phases using temperature annealing, and thus the overall reaction rate, was observed [146]. For CO2 reduction, methane production rate in visible light reached the highest levels with annealing temperature at 480 °C and 680 °C (Fig. 10) [147]. At 480 °C, crystals were mostly (91%) anatase phase, while at 680 °C, the crystals were mostly rutile phase (91%). Thus, methane production was highest when both phases were present but one predominated. It was postulated that crystals containing anatase–rutile interfaces possess more numerous active sites for photoreduction than in crystals with only one phase. Varghese et al. also studied reduction of CO2 to methane, but used actual sunlight and modified TiO2 nanotube arrays as a catalyst [148]. They found that co-catalysis using Pt nanoparticles and Cu nanoparticles that were loaded onto the surface of nanotube arrays had an important effect on the conversion rate, and could also effectively drive the total conversion. The product species varied owing to the amounts of loaded co-catalysts on the TiO2 nanotube arrays. Ultrafine Pt nanoparticles distributed on TiO2 nanotube arrays acting as co-catalysts could provide many reaction sites for CO2 conversion reaction, enhancing the transformation rate from CO2 and H2O to methane [149]. In general the conversion rate was lower than 5% when using pure TiO2 nanotube arrays as the catalyst, while Pt nanoparticle-modified TiO2 nanotube arrays exhibited a higher conversion rate of 25%. Meanwhile

6. Photocatalytic conversion of CO2 The main energy sources currently are fossil fuels such as petroleum, natural gas and coal because of their usability, stability and high calorific values. The large amounts of CO2 emissions are produced during the burning or transforming of these fossil fuels, with resulting greenhouse effects. How to effectively reduce CO2 is an important issue facing the countries around the world. In addition, the shortage of fossil fuels and the need to find alternative renewable and sustainable fuels are triggering increasing interest in the photocatalytic reduction of CO2 [143–145]. TiO2 has been successfully incorporated in methodologies for photocatalytic conversion of CO2. Different crystalline phases of TiO2 have been evaluated for their performance in photocatalytic reactions. It has been known that increasing the annealing temper-

Fig. 10. Methane production under 365 nm irradiation of TiO2 nanotube arrays annealed in different temperature [147].

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a microwave-assisted modification method has been developed and has been very useful for fabricating other nanoparticle- modified TiO2 nanotube arrays. Ping et al. used modified TiO2 nanotube arrays as the photocatalyst for CO2 conversion [150]. The band gap of TiO2 nanotube arrays decreased by approximately 0.2 eV when compared to TiO2 nanoparticles, which induced the nanotube red shifted absorption edge and enhanced photocatalytic activities of TiO2 nanotube arrays. The products were predominately methanol and ethanol, and the production rates of methanol and ethanol were calculated to be about 10 and 9 nmol cm2 h1. The advances of CO2 transformation technologies will have a far-reaching effect on environmental protection and sustainable development. It makes sense that CO2 conversion to hydrocarbon fuels would not only reduce the quantity of CO2 in the atmosphere, but would also trap it in organic matter and speed up the overall recycling of carbon. Although much progress has been achieved in recent years, the low utilization of solar energy, easy recombination of electrons and holes, and poor absorption of CO2 by TiO2 nanotube arrays are problems that must be addressed before we will see a high conversion rate of CO2 which will have a great impact on the world. 7. Conclusions The new architecture of vertically-aligned TiO2 nanotube arrays has gained much interest during recent decades due to their novel physical attributes as well as their numerous potential applications in various fields. The rapidly-developing techniques based on TiO2 nanotube arrays have provided new solutions to address challenges in the removal of environmental pollutants, greater utilization of solar energy, decreases in greenhouse gases, creation of new energy sources and others. However, there is still much work to do, including increasing the conversion efficiency of solar energy to create electrical and chemical energy, significantly improving catalysis in response to visible light, and developing new catalysts with higher photocatalytic activities. TiO2 nanotube arrays hold great promise in playing a key role in the development of new technologies to address growing challenges that must be overcome to assure a bright future for our world. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21377167), Program for New Century Excellent Talents in University (NCET-10-0813) and Science Foundation of China University of Petroleum, Beijing (KYJJ201201-15). References [1] X. Chen, S.S. Mao, Chem. Rev. 107 (2007) 2891–2959. [2] G.K. Mor, O.K. Varghese, M. Paulose, K. Shankar, C.A. Grimes, Sol. Energy Mater. Sol. Cells 90 (2006) 2011–2075. [3] D. Gong, C. Grimes, O.K. Varghese, W. Hu, R. Singh, Z. Chen, E.C. Dickey, J. Mater. Res. 16 (2001) 3331–3334. [4] S.K. Mohapatra, K.S. Raja, V.K. Mahajan, M. Misra, J. Phys. Chem. C 112 (2008) 11007–11012. [5] P. Benvenuto, A.K.M. Kafi, A. Chen, J. Electroanal. Chem. 627 (2009) 76–81. [6] Q. Zhou, Y. Huang, G. Xie, J. Chromatogr. A 1237 (2012) 24–29. [7] Y.-Y. Song, F. Schmidt-Stein, S. Bauer, P. Schmuki, J. Am. Chem. Soc. 131 (2009) 4230–4232. [8] T.J. LaTempa, S. Rani, N. Bao, C.A. Grimes, Nanoscale 4 (2012) 2245–2250. [9] P. Roy, S. Berger, P. Schmuki, Angew. Chem. Int. Ed. 50 (2011) 2904–2939. [10] Z. Fang, Q.X. Zhou, Acta Chim. Sinica 70 (2012) 1767–1774. [11] F. Al Momani, N. Jarrah, Environ. Technol. 30 (2009) 1085–1093. [12] H. Wender, A.F. Feil, L.B. Diaz, C.S. Ribeiro, G.J. Machado, P. Migowski, D.E. Weibel, J. Dupont, S.R. Teixeira, ACS Appl. Mater. Interfaces 3 (2011) 1359– 1365. [13] M. Tian, B. Adams, J. Wen, R.M. Asmussen, A. Chen, Electrochim. Acta 54 (2009) 3799–3805.

[14] H.-C. Liang, X.-Z. Li, J. Hazard. Mater. 162 (2009) 1415–1422. [15] S. Zhang, J. Qiu, J. Han, H. Zhang, P. Liu, S. Zhang, F. Peng, H. Zhao, Electrochem. Commun. 13 (2011) 1151–1154. [16] Q. Wang, J. Shang, T. Zhu, F. Zhao, J. Mol. Catal. A 335 (2011) 242–247. [17] A. Zhang, M. Zhou, L. Han, Q. Zhou, J. Hazard. Mater. 186 (2011) 1374–1383. [18] Q. Li, J.K. Shang, Environ. Sci. Technol. 43 (2009) 8923–8929. [19] G. Yan, M. Zhang, J. Hou, J. Yang, Mater. Chem. Phys. 129 (2011) 553–557. [20] J. Li, N. Lu, X. Quan, S. Chen, H. Zhao, Ind. Eng. Chem. Res. 47 (2008) 3804– 3808. [21] L. Sun, J. Li, C.L. Wang, S.F. Li, H.B. Chen, C.J. Lin, Sol. Energy Mater. Sol. Cells 93 (2009) 1875–1880. [22] Q. Wu, J. Ouyang, K. Xiea, L. Sun, M. Wang, C. Lin, J. Hazard. Mater. 199 (2012) 410–417. [23] Z. Xu, W. Yang, Q. Li, S. Gao, J.K. Shang, Appl. Catal. B 144 (2014) 343–352. [24] K. Xie, L. Sun, C. Wang, Y. Lai, M. Wang, H. Chen, C. Lin, Electrochim. Acta 55 (2010) 7211–7218. [25] X. Liu, Z. Liu, J. Lu, X. Wu, B. Xu, W. Chu, Appl. Surf. Sci. 288 (2014) 513–517. [26] L.K. Tan, M.K. Kumar, W.W. An, H. Gao, ACS Appl. Mater. Interfaces 2 (2010) 498–503. [27] L. Liu, J. Lv, G. Xu, Y. Wang, K. Xie, Z. Chen, Y. Wu, J. Solid State Chem. 208 (2013) 27–34. [28] F. Xiao, J. Mater. Chem. 22 (2012) 7819–7830. [29] F.-X. Xiao, RSC Adv. 2 (2012) 12699–12701. [30] Y. Muramatsu, Q. Jin, M. Fujishima, H. Tada, Appl. Catal. B 119 (2012) 74–80. [31] Y. Chen, Y. Tang, S. Luo, C. Liu, Y. Li, J. Alloys Compd. 578 (2013) 242–248. [32] M. Yang, J. Xu, J. Wei, J.-L. Sun, W. Liu, J.-L. Zhu, Appl. Phys. Lett. 100 (2012) 253113. [33] Y. Liu, X. Zhang, R. Liu, R. Yang, C. Liu, Q. Cai, J. Solid State Chem. 184 (2011) 684–689. [34] A. Zhu, Q. Zhao, X. Li, Y. Shi, ACS Appl. Mater. Interfaces 6 (2014) 671–679. [35] Y. Hou, X.-Y. Li, Q.-D. Zhao, X. Quan, G.-H. Chen, Adv. Funct. Mater. 20 (2010) 2165–2174. [36] X. Zhao, H. Liu, J. Qu, Appl. Surf. Sci. 257 (2011) 4621–4624. [37] W. Zhu, X. Liu, H. Liu, D. Tong, J. Yang, J. Peng, J. Am. Chem. Soc. 132 (2010) 12619–12626. [38] Y. Xie, G. Ali, S.H. Yoo, S.O. Cho, ACS Appl. Mater. Interfaces 2 (2010) 2910– 2914. [39] M. Wang, L. Sun, Z. Lin, J. Cai, K. Xie, C. Lin, Energy Environ. Sci. 6 (2013) 1211–1220. [40] X. Zhang, Y. Tang, Y. Li, Y. Wang, X. Liu, C. Liu, S. Luo, Appl. Catal. A 457 (2013) 78–84. [41] Y. Wang, Y. Tang, Y. Chen, Y. Li, X. Liu, S. Luo, C. Liu, J. Mater. Sci. 48 (2013) 6203–6211. [42] Z. Zhang, D. Pan, J. Feng, L. Guo, L. Peng, C. Xi, J. Li, Z. Li, M. Wu, Z. Ren, Mater. Lett. 66 (2012) 54–56. [43] F. Deng, Y. Liu, X. Luo, D. Chen, S. Wu, S. Luo, Sep. Purif. Technol. 120 (2013) 156–161. [44] D. Walker, New Phytol. 121 (1992) 325–345. [45] K. Xie, Q. Wu, Y. Wang, W. Guo, M. Wang, L. Sun, C. Lin, Electrochem. Commun. 13 (2011) 1469–1472. [46] K. Huo, H. Wang, X. Zhang, Y. Cao, P.K. Chu, ChemPlusChem 77 (2012) 323– 329. [47] C. Liu, Y. Teng, R. Liu, S. Luo, Y. Tang, L. Chen, Q. Cai, Carbon 49 (2011) 5312– 5320. [48] Y.-F. Tu, S.-Y. Huang, J.-P. Sang, X.-W. Zou, Mater. Res. Bull. 45 (2010) 224– 229. [49] S. Zhang, F. Peng, H. Wang, H. Yu, S. Zhang, J. Yang, H. Zhao, Catal. Commun. 12 (2011) 689–693. [50] S. Zhang, S. Zhang, F. Peng, H. Zhang, H. Liu, H. Zhao, Electrochem. Commun. 13 (2011) 861–864. [51] H. Yang, C. Pan, J. Alloys Compd. 501 (2010) L8–L11. [52] L. Yang, Y. Xiao, S. Liu, Y. Li, Q. Cai, S. Luo, G. Zeng, Appl. Catal. A 94 (2010) 142–149. [53] Y. Wang, J. Lin, R. Zong, J. He, Y. Zhu, J. Mol. Catal. A 349 (2011) 13–19. [54] M. Liu, G. Zhao, Y. Tang, Z. Yu, Y. Lei, M. Li, Y. Zhang, D. Li, Environ. Sci. Technol. 44 (2010) 4241–4246. [55] Z. Xu, J. Yu, Nanotechnology 21 (2010) 245501–245506. [56] X. Zhang, J. Zhang, Y. Jia, P. Xiao, J. Tang, Sensors 12 (2012) 3302–3313. [57] Z. Zhang, Y. Xie, Z. Liu, F. Rong, Y. Wang, D. Fu, J. Electroanal. Chem. 650 (2011) 241–247. [58] J. Lee, D.H. Kim, S.-H. Hong, J.Y. Jho, Sens. Actuators B 160 (2011) 1494–1498. [59] G.K. Mor, K. Shankar, O.K. Varghese, C.A. Grimes, J. Mater. Res. 19 (2004) 2989–2996. [60] L. Yang, S. Luo, F. Su, Y. Xiao, Y. Chen, Q. Cai, J. Phys. Chem. C 114 (2010) 7694– 7699. [61] T.T. Tran, J. Li, H. Feng, J. Cai, L. Yuan, N. Wang, Q. Cai, Sens. Actuators B 190 (2014) 745–751. [62] J. Cai, P. Sheng, L. Zhou, L. Shi, N. Wang, Q. Cai, Biosens. Bioelectron. 50 (2013) 66–71. [63] J.X. Qiu, S.Q. Zhang, H.J. Zhao, J. Hazard. Mater. 211 (2012) 381–388. [64] C.A. Almeida, P. Gonzalez, M. Mallea, L.D. Martinez, R.A. Gil, Talanta 97 (2012) 273–278. [65] J. Zhang, B. Zhou, Q. Zheng, J. Li, J. Bai, Y. Liu, W. Cai, Water Res. 43 (2009) 1986–1992. [66] Q. Zheng, B. Zhou, J. Bai, L. Li, Z. Jin, J. Zhang, J. Li, Y. Liu, W. Cai, X. Zhu, Adv. Mater. 20 (2008) 1044–1049.

Q. Zhou et al. / Microporous and Mesoporous Materials 202 (2015) 22–35 [67] H. Chen, J. Zhang, Q. Chen, J. Li, D. Li, C. Dong, Y. Liu, B. Zhou, S. Shang, W. Cai, Anal. Methods 4 (2012) 1790–1796. [68] C. Wang, J. Wu, P. Wang, Y. Ao, J. Hou, J. Qian, Sens. Actuators B 181 (2013) 1–8. [69] C.M.A. Brett, Pure Appl. Chem. 73 (2001) 1969–1978. [70] H. Liu, D. Wang, L. Ji, J. Li, S. Liu, X. Liu, S. Jiang, J. Chromatogr. A 1217 (2010) 1898–1903. [71] Y. Huang, Q. Zhou, J. Xiao, Analyst 136 (2011) 2741–2746. [72] Q. Zhou, Y. Huang, J. Xiao, G. Xie, Anal. Bioanal. Chem. 400 (2011) 205–212. [73] Q. Zhou, Z. Fang, RSC Adv. 4 (2014) 7471–7475. [74] Q. Zhou, W. Wu, G. Xie, Y. Huang, Anal. Methods 6 (2014) 295–301. [75] X.N. Zhao, Q.Z. Shi, G.H. Xie, Q.X. Zhou, Chin. Chem. Lett. 19 (2008) 865–867. [76] Q.-X. Zhou, X.-N. Zhao, J.-P. Xiao, Talanta 77 (2009) 1774–1777. [77] Y. Huang, Q. Zhou, G. Xie, H. Liu, H. Lin, Microchim. Acta 172 (2011) 109–115. [78] Q. Zhou, J. Mao, J. Xiao, G. Xie, Anal. Methods 2 (2010) 1063–1068. [79] Q. Zhou, Y. Ding, J. Xiao, G. Liu, X. Guo, J. Chromatogr. A 1147 (2007) 10–16. [80] Y. Huang, Q. Zhou, G. Xie, Chemosphere 90 (2013) 338–343. [81] H. Niu, Y. Cai, Y. Shi, F. Wei, S. Mou, G. Jiang, J. Chromatogr. A 1172 (2007) 113–120. [82] Y. Huang, Q. Zhou, G. Xie, J. Hazard. Mater. 193 (2011) 82–89. [83] D. Pan, C. Chen, F. Yang, Y. Long, Q. Cai, S. Yao, Analyst 136 (2011) 4774–4779. [84] M.M. Lee, J. Teuscher, T. Miyasaka, T.N. Murakami, H.J. Snaith, Science 338 (2012) 643–647. [85] B. O’Regan, M. Gratzel, Nature 353 (1991) 737–740. [86] S. Ko, H. Choi, M.-S. Kang, H. Hwang, H. Ji, J. Kim, J. Ko, Y. Kang, J. Mater. Chem. 20 (2010) 2391–2399. [87] J.Y. Kim, S. Lee, J.H. Noh, H.S. Jung, K.S. Hong, J. Electroceram. 23 (2009) 422– 425. [88] I. Kartini, D. Menzies, D. Blake, J.C.D. da Costa, P. Meredith, J.D. Riches, G.Q. Lu, J. Mater. Chem. 14 (2004) 2917–2921. [89] J. Dewalque, R. Cloots, F. Mathis, O. Dubreuil, N. Krins, C. Henrist, J. Mater. Chem. 21 (2011) 7356–7363. [90] B.W. Jing, M.H. Zhang, T. Shen, Chin. Sci. Bull. 42 (1997) 1937–1948. [91] C. Xu, P.H. Shin, L. Cao, J. Wu, D. Gao, Chem. Mater. 22 (2010) 143–148. [92] S. Wang, J. Zhang, S. Chen, H. Yang, Y. Lin, X. Xiao, X. Zhou, X. Li, Electrochim. Acta 56 (2011) 6184–6188. [93] S. Li, Y. Liu, G. Zhang, X. Zhao, J. Yin, Thin Solid Films 520 (2011) 689–693. [94] C. Rho, J.-H. Min, J.S. Suh, J. Phys. Chem. C 116 (2012) 7213–7218. [95] J. Lee, K.S. Hong, K. Shin, J.Y. Jho, J. Ind. Eng. Chem. 18 (2012) 19–23. [96] H. Park, W.-R. Kim, H.-T. Jeong, J.-J. Lee, H.-G. Kim, W.-Y. Choi, Sol. Energy Mater. Sol. Cells 95 (2011) 184–189. [97] J.-H. Park, J.-Y. Kim, J.-H. Kim, C.-J. Choi, H. Kim, Y.-E. Sung, K.-S. Ahn, J. Power Sources 196 (2011) 8904–8908. [98] H. Wang, H. Li, J. Wang, J. Wu, Mater. Lett. 80 (2012) 99–102. [99] D. Kim, A. Ghicov, S.P. Albu, P. Schmuki, J. Am. Chem. Soc. 130 (2008) 16454– 16455. [100] S. Wang, X. Zhou, X. Xiao, Y. Fang, Y. Lin, Electrochim. Acta 116 (2014) 26–30. [101] J. Luo, L. Gao, J. Sun, Y. Liu, RSC Adv. 2 (2012) 1884–1889. [102] S. Wang, W. Tan, J. Zhang, Y. Lin, Chin. Sci. Bull. 57 (2012) 864–868. [103] Z. Liu, M. Misra, ACS Nano 4 (2010) 2196–2200. [104] S. Nishimura, N. Abrams, B.A. Lewis, L.I. Halaoui, T.E. Mallouk, K.D. Benkstein, J. van de Lagemaat, A.J. Frank, J. Am. Chem. Soc. 125 (2003) 6306–6310. [105] J.I.L. Chen, G. von Freymann, V. Kitaev, G.A. Ozin, J. Am. Chem. Soc. 129 (2007) 1196–1202. [106] C.T. Yip, H. Huang, L. Zhou, K. Xie, Y. Wang, T. Feng, J. Li, W.Y. Tam, Adv. Mater. 23 (2011) 5624–5628. [107] M. Guo, K. Xie, J. Lin, Z. Yong, C.T. Yip, L. Zhou, Y. Wang, H. Huang, Energy Environ. Sci. 5 (2012) 9881–9888. [108] Y. Wang, Y. Liu, H. Yang, H. Wang, H. Shen, M. Li, J. Yan, Curr. Appl. Phys. 10 (2010) 119–123. [109] Y. Alivov, Z.Y. Fan, J. Mater. Sci. 45 (2010) 2902–2906. [110] J. Bandara, K. Shankar, J. Basham, H. Wietasch, M. Paulose, O.K. Varghese, C.A. Grimes, M. Thelakkat, Eur. Phys. J. Appl. Phys. 53 (2011) 20261–20266. [111] H. Mirabolghasemi, N. Liu, K. Lee, P. Schmuki, Chem. Commun. 49 (2013) 2067–2069. [112] M.F. Hossain, S. Biswas, Z.H. Zhang, T. Takahashi, J. Photochem. Photobiol. A 217 (2011) 68–75.

35

[113] S. Huang, Q. Zhang, X. Huang, X. Guo, M. Deng, D. Li, Y. Luo, Q. Shen, T. Toyoda, Q. Meng, Nanotechnology 21 (2010) 375201–375207. [114] Y. Xie, S.H. Yoo, C. Chen, S.O. Cho, Mater. Sci. Eng. B 177 (2012) 106–111. [115] L. Wonjoo, K. Soon Hyung, K. Jae-Yup, B.K. Govind, S. Yung-Eun, H. SungHwan, Nanotechnology 20 (2009) 335706. [116] X. Ma, Y. Shen, G. Wu, Q. Wu, B. Pei, M. Cao, F. Gu, J. Alloys Compd. 538 (2012) 61–65. [117] J. Ren, W. Que, X. Yin, Y. He, H.M.A. Javed, RSC Adv. 4 (2014) 7454–7460. [118] W. Fan, S. Jewell, Y. She, M.K.H. Leung, Phys. Chem. Chem. Phys. 16 (2014) 676–680. [119] L. Wu, J. Li, S. Zhang, L. Long, X. Li, C. Cen, J. Phys. Chem. C 117 (2013) 22591– 22597. [120] H.Y. Hwang, A.A. Prabu, D.Y. Kim, K.J. Kim, Sol. Energy 85 (2011) 1551–1559. [121] Y. Hao, Y. Cao, B. Sun, Y. Li, Y. Zhang, D. Xu, Sol. Energy Mater. Sol. Cells 101 (2012) 107–113. [122] J.-Y. Hwang, S.-A. Lee, Y.H. Lee, S.-I. Seok, ACS Appl. Mat. Interfaces 2 (2010) 1343–1348. [123] P. Zhong, W. Que, J. Chen, X. Hu, J. Power Sources 210 (2012) 38–41. [124] C.S. Rustomji, C.J. Frandsen, S. Jin, M.J. Tauber, J. Phys. Chem. B 114 (2010) 14537–14543. [125] B.-X. Lei, J.-Y. Liao, R. Zhang, J. Wang, C.-Y. Su, D.-B. Kuang, J. Phys. Chem. C 114 (2010) 15228–15233. [126] H.J. Lee, J. Bang, J. Park, S. Kim, S.-M. Park, Chem. Mater. 22 (2010) 5636– 5643. [127] P.-T. Hsiao, Y.-J. Liou, H. Teng, J. Phys. Chem. C 115 (2011) 15018–15024. [128] J.A. Turner, Science 305 (2004) 972–974. [129] K.-C. Sun, Y.-C. Chen, M.-Y. Kuo, H.-W. Wang, Y.-F. Lu, J.-C. Chung, Y.-C. Liu, Y.Z. Zeng, Mater. Chem. Phys. 129 (2011) 35–39. [130] M. Hattori, K. Noda, K. Matsushige, Appl. Phys. Lett. 99 (2011). [131] Y. Sun, K. Yan, G. Wang, W. Guo, T. Ma, J. Phys. Chem. C 115 (2011) 12844– 12849. [132] L.X. Sang, Z.Y. Zhang, C.F. Ma, Int. J. Hydrogen Energy 36 (2011) 4732–4738. [133] L.-X. Sang, Z.-Y. Zhang, G.-M. Bai, C.-X. Du, C.-F. Ma, Int. J. Hydrogen Energy 37 (2012) 854–859. [134] Y.R. Smith, B. Sarma, S.K. Mohanty, M. Misra, Int. J. Hydrogen Energy 38 (2013) 2062–2069. [135] Z. Li, J. Liu, D. Wang, Y. Gao, J. Shen, Int. J. Hydrogen Energy 37 (2012) 6431– 6437. [136] J. Bai, J. Li, Y. Liu, B. Zhou, W. Cai, Appl. Catal. A 95 (2010) 408–413. [137] C.W. Lai, S. Sreekantan, Int. J. Hydrogen Energy 38 (2013) 2156–2166. [138] M. Ye, J. Gong, Y. Lai, C. Lin, Z. Lin, J. Am. Chem. Soc. 134 (2012) 15720–15723. [139] X. Zhang, F. Wang, H. Huang, H.T. Li, X. Han, Y. Liu, Z.H. Kang, Nanoscale 5 (2013) 2274–2278. [140] S. Zhang, H. Wang, M. Yeung, Y. Fang, H. Yu, F. Peng, Int. J. Hydrogen Energy 38 (2013) 7241–7245. [141] H. Wang, W. Zhu, B. Chong, K. Qin, Int. J. Hydrogen Energy 39 (2014) 90–99. [142] M. Hattori, K. Noda, K. Kobayashi, K. Matsushige, Gas phase photocatalytic decomposition of alcohols with titanium dioxide nanotube arrays in high vacuum, in: S. Fujita (Ed.), Physica Status Solidi C: Current Topics in Solid State Physics, vol. 8, WILEY-VCH Verlag GmbH, Pappelallee 3, W-69469 Weinheim, Germany, 2011, pp. 549–551. [143] G. Liu, K. Wang, N. Hoivik, H. Jakobsen, Sol. Energy Mater. Sol. Cells 98 (2012) 24–38. [144] S.A.A. Yahia, L. Hamadou, A. Kadri, N. Benbrahim, E.M.M. Sutter, Catal. Today 185 (2012) 263–269. [145] N. Ahmed, M. Morikawa, Y. Izumi, Catal. Today 185 (2012) 263–269. [146] J. Yu, B. Wang, Appl. Catal. B 94 (2010) 295–302. [147] K.L. Schulte, P.A. DeSario, K.A. Gray, Appl. Catal. B 97 (2010) 354–360. [148] O.K. Varghese, M. Paulose, T.J. LaTempa, C.A. Grimes, Nano Lett. 9 (2009) 731– 737. [149] X. Feng, J.D. Sloppy, T.J. LaTemp, M. Paulose, S. Komarneni, N. Bao, C.A. Grimes, J. Mater. Chem. 21 (2011) 13429–13433. [150] G. Ping, C. Wang, D. Chen, S. Liu, X. Huang, L. Qin, Y. Huang, K. Shu, J. Solid State Electrochem. 17 (2013) 2503–2510.