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Keywords:

Hydrogen productionSolar cells

erties of modied TiO2 nanotube arrays make them excellent candidates for

splitting, solar cells and CO2 conversion.

. . . . . .lutantsotocatrays asube arr2 nanoy hete

. . . . . . . . . . . . . 29. . . . . . . . .. . . . . . . . .. . . . . . . . .

7. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

Corresponding author. Tel./fax: +86 10 89732300.E-mail address: zhouqx@cup.edu.cn (Q. Zhou).

Microporous and Mesoporous Materials 202 (2015) 2235

Contents lists available at ScienceDirect

Microporous and Mesoporous MaterialsAcknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343.3. Sample pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4. Applications in sensitized solar cells (SSCs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5. Hydrogen production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6. Photocatalytic conversion of CO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .http://dx.doi.org/10.1016/j.micromeso.2014.09.0401387-1811/ 2014 Elsevier Inc. All rights reserved.. . . . 30

. . . . 32

. . . . 333. Environmental analytical chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283.1. Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

3.1.1. Gas monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283.1.2. Detection of heavy metal ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283.1.3. Detection of organic pollutants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

3.2. Measurement of COD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29Contents

1. Introduction . . . . . . . . . . . . . . . . .2. Photocatalytic degradation of pol

2.1. TiO2 nanotube arrays as ph2.2. Modified TiO2 nanotube ar

2.2.1. Doping TiO2 nanot2.2.2. Loading on the TiO2.2.3. TiO2 nanotube arra 2014 Elsevier Inc. All rights reserved.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23alysts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25ays. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25tube arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25rojunctions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28PhotocatalysisEnvironmental analytical chemistry

use in solar cells and sensitive sensors for trace compounds, etc. This review focuses on the recentapplications of TiO2 nanotube arrays in removal of pollutants, environmental analytical chemistry, waterTiO2 nanotube arraystinct electrochemical propvailable online 29 September 2014catalytic abilities in several cases: in the degradation of environmental inorganic and organic pollutantsto less toxic compounds, water splitting, and in the reduction of atmospheric CO2 levels by incorporationof CO2 into hydrocarbons, among others. Moreover, the wide absorption spectrum characteristics and dis-d 31 December 2013d in revised form 1 September 2014ed 15 September 2014

have been demonstrated to serve as multifunctional materials which show great promise in addressingmany challenges in both environmental and energy technology elds. They have exhibited extraordinarya r t i c l e i n f o

Article history:Receive

a b s t r a c t

TiO2 nanotube arrays, novel TiO2-based nanomaterials with unique chemical and physical properties,Qingxiang Zhou , Zhi Fang, Jing Li, Mengyun WangBeijing Key Laboratory of Oil and Gas Pollution Control, College of Geosciences, China University of Petroleum Beijing, Beijing 102249, Chinavieweview

pplications of TiO2 nanotube arrays in environmental and energy elds:rejournal homepage: www.elsevier .com/locate /micromeso

Q. Zhou et al. /Microporous and Mesoporous Materials 202 (2015) 2235 231. Introduction

Due to the rapid acceleration of technological advances acrossthe globe, each year more and more products are produced tomake our lives more comfortable and convenient. Most of theseproducts originate from natural sources such as petroleum, coal,natural gas and mineral sources; meanwhile, many environmen-tal problems occur during the various steps of the extraction,transportation, and transformation of raw materials into nalproducts. These problems include CO2 emission, which contrib-utes greatly to global warming, ozone depletion, and air andwater 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. Forexample, it is reported that pesticides and heavy metals havebeen detected in remote Antarctic areas, far from their points-of-use. As a result, each country around the world is facing chal-lenges to look for better strategies to solve such energy and envi-ronmental problems.

Nanomaterials are novel material forms that emerged in the1980s and have been studied intensely in recent years. Thenano designation stems from the fact that the unit size ofthese materials is about 1100 nm along any single dimensionalscale. A new form of TiO2 nanomaterial, the TiO2 nanotube array,has attracted much attention recently. Currently, many syntheticmethods for the preparation of TiO2 nanotube arrays exist,including the most commonly-used synthetic methods: the tem-plate and anode oxidation methods. In the template method,common templates include porous alumina, zinc oxide and var-ious organic polymers [1,2]. The morphology of each resultingnanotube array depends on the size and shape of the template,and the main disadvantage is the subsequent destruction ofnanotube arrays during the downstream separation process.The anode oxidation method is an electrochemical method inwhich the Ti substrate is anodized in an electrolyte containingF ions, either with or without organic solvents. Gong et al. rstintroduced this facile method to fabricate highly-ordered verticalnanotube arrays [3], and this has been the preferred method forTiO2 nanotube array production in recent years. Many research-ers have investigated the parameters that would affect the mor-phology of prepared samples such as potential, electrolytecomposition, oxidation time and annealing temperature. TiO2nanotube arrays possess unique chemical and physical propertiessuch as chemical inertness, gas sensitivity, large surface area,biocompatibility, high photocatalytic and electrochemical activi-ties. These characteristics have led researchers to adapt themto improve upon old technologies, including solar cells, removalof pollutants, water splitting [4], sensors [5], sample pretreat-ment [6], drug delivery [7], and CO2 conversion [8]. Currently,research into new nanostructures derived from TiO2-based mate-rials holds great promise to help address many urgent globalchallenges.

Several reviews have been published which have focused onthe synthesis and applications of TiO2 nanotube arrays [9],including a short summary (only available in Chinese) byl ourgroup on the applications of TiO2 nanotube arrays to addressenvironmental challenges in 2012 [10]. Thus, the purpose of thisreview is to provide a thorough survey of recent TiO2 nanotubearray research to a global audience by focusing on severalaspects of environmental and energy eld topics including: (1)photocatalytic degradation of pollutants, (2) applications in envi-ronmental analytical chemistry, (3) hydrogen generation, (4) sen-sitized solar cells (SSCs), and (5) CO2 conversion to

hydrocarbons.2. Photocatalytic degradation of pollutants

2.1. TiO2 nanotube arrays as photocatalysts

Environmental pollution has become a growing problem,resulting in more and more research interest in this area. To thisend, photocatalytic degradation has been successfully developedand is rapidly becoming the best way to deal with environmentalpollutants. In this method, when semiconductive materials areilluminated with light with energy equal to or higher than theband gap energy of the semiconductors, the electrons in thevalence band (VB) are excited to enter the conduction band(CB), and then are transferred to the surface of particles withavailable holes in the VB. These photo-generated electronholepairs possess high redox activity. Since the band gap of anataseTiO2 is about 3.2 eV, electrons in this material can be excited byillumination using light with wavelengths less than 387 nm,allowing the electrons in the VB of TiO2 to be excited to the CBand give rise to electronhole pairs. Once these pairs reach theTiO2 surface, some pairs can recombine, releasing energy as lightor heat, while others can react with O2, H2O and OH adsorbed tothe TiO2 surface to form radicals which can oxidize macromolec-ular pollutants to form CO2 and water, etc. During the photocat-alytic process, it is of great importance to choose a suitablecatalyst. Metal oxide semiconductor materials have been success-fully developed so that these photocatalytic reactions may occurin mild conditions [11].

TiO2 powders have been used for many years in the photocata-lytic eld and their advantages have fueled ongoing interest. How-ever, this material possesses several disadvantages: it is difcult todisperse, agglomerates easily, is difcult to reuse, possesses a lowlight response, and causes environmental pollution when usedinappropriately. The introduction of TiO2 nanotube arrays, a novelform of TiO2, has solved these problems. Wender et al. [12] pre-pared TiO2 nanotube arrays employing an anode oxidation methodusing ethylene glycol (EG) electrolytes containing 1-n-butyl-3-methyl imidazolium tetrauoroborate (BMIBF4) and water(Fig. 1). The photocatalytic activity was investigated using methylorange (MO) as the model pollutant and 13% of the MO was miner-alized under UV irradiation in 150 min, indicating that the TiO2nanotube arrays possessed high photocatalytic activity. For salicylicacid (SA) and salicylaldehyde (SH), the TiO2 nanotube arrays exhib-itedmuch higher photocatalytic activity upon irradiation using UVvisible light [13]. It found that 83% of the SA was oxidized in 2 h,while SH was almost completely eliminated under the same condi-tions. These results indicated that this method can be used as apotential treatment to process SA and SH pollutants.

It is obvious that the structural parameters of TiO2 nanotubearrays, such as specic surface area, wall thickness, tube lengthand crystalline phase, have important effects on the photocatalyticactivity of TiO2 nanotube arrays. Liang et al. [14] investigated theseeffects on the photocatalytic activity of TiO2 nanotube arrays using2,3-dichlorophenol (2,3-DCP) as the target pollutant. They foundthat a larger specic nanotube surface area can lead to greaterabsorption of aqueous reactants, while a higher pore volume canaccelerate the diffusion of various aqueous species during the pho-tocatalytic reaction, both enhancing the reaction rate. Using opti-mally-calcined nanotube arrays under UV illumination, 93% ofadded 2,3-DCP was degraded 2.6 times more rapidly, as comparedto the result obtained with TiO2 lms under the same conditions.Zhang et al. fabricated TiO2 nanotube arrays on uorine-dopedtin oxide (FTO) glass [15], which contained a top porous nanopar-ticle layer and nanotube array bottom layer. This new material

resulted in the enhancement of the photocatalytic activity in

liquid and used in H2 production and pollutant degradation [12].

sopFig. 1. Schematic of TiO2 nanotube arrays synthesized in ionic

24 Q. Zhou et al. /Microporous and Meglucose photooxidation by 60% over that of TiO2 nanotube arraysbased on simple Ti foil. This effect was attributed to the fact thatthese nanotube arrays provided more stable electron pathwaysresulting in high photocatalytic activity. Wang et al. discoveredthat the increase of the surface area of the photocathode couldgreatly accelerate the photoelectrocatalytic reduction rates ofCr(VI) [16]; short TiO2 nanotube arrays (S-TNTs) had higher elec-tron transfer efciency than that of long TiO2 nanotube arrays (L-TNTs), and nearly complete reduction of Cr(VI) using S-TNTs wasachieved with UV irradiation in 60 min (Fig. 2). Their ndings dem-onstrated that this simple and efcient method could removeCr(VI) from aqueous samples and was a good start for developinghighly-efcient methods for heavy metal pollution removal fromwater samples.

Using a different strategy, Zhang et al. designed a photocatalyticreactor incorporating a rotating disk composed of a TiO2-nanotube(TNT)/Ti photocatalyst (Fig. 3) [17]. They investigated the effect ofparameters such as the rotation velocity and irradiation time onthe photocatalytic activity of the TiO2 nanotube arrays and foundthat the rotating velocity had an important impact on the photo-catalytic activity. When the rotating velocity increased up to30 rpm, the removal rate of rhodamine B approached 90% in 3 h,an improvement of 2540% over results using a TiO2 nanoparticle

Fig. 2. Schematic illustration of the PEC reduction of Cr(VI) with S-TNTs as theorous Materials 202 (2015) 2235disk. The results indicated that a higher rotational velocityincreased delivery of polluted thick water lm to the reaction,opening a potentially valuable new direction in the developmentof methodologies for environmental pollutant removal.

photoanode and a Ti mesh as the photocathode under UV irradiation [16].

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

ns.

esop2.2. Modied TiO2 nanotube arrays as photocatalysts

In general, anatase TiO2 responds only to the UV fraction of sun-light; however, the fraction of UV light in sunlight is very small rel-ative to the fraction of visible light. Based on this fact, much efforthas been put forth to determine ways to utilize visible light in thisphotocatalytic system. The simplest successful approaches havebeen to modify the TiO2 material itself to enhance its response tovisible light via doping, depositing, loading of TiO2 nanotubearrays, and other methods [18,20,33,4652] (Table 1).

2.2.1. Doping TiO2 nanotube arraysDoping is an often-used method to change material properties

in which one or more elements or compounds are doped into thesubstrate to generate specic electrical and/or optical properties.Many studies have shown that doping TiO2 with elements suchas nitrogen, carbon, uorine, iodine and/or iron can lead to the nec-essary narrow band gap to allow a greater response to visible light,which can enhance the overall photocatalytic activity. Li et al. pre-pared N and F co-doped TiO2 nanotube arrays by anodizing a Tisubstrate in NH4F and NH4Cl solution [18]. They found that anneal-ing of the doped TiO2 nanotube arrays in an N2 atmosphere couldeffectively reduce the phenomenon of F atom replacement by Oatoms, resulting in a higher photocatalytic activity towards meth-ylene blue. In fact, annealing TiO2 nanotube arrays in a specicatmosphere has also been developed as a new doping method. Inthe process of preparing N and S co-doped TiO2 nanotube arraysthrough the annealing of TiO2 nanotube arrays in thiourea at500 C [19], NTiO and NOTi bonds were formed between Natoms and the nanotube arrays, and some of the O atom positionswere replaced by S atoms, which improved their degree of crystal-lization. Such doping markedly increased the photocatalytic activ-ity towards methylene blue, and photocatalytic activity was 1.29times higher than that of an un-doped array. Boron-doped TiO2nanotube arrays have also exhibited a phenol degradation rate

Table 1Advances of modied TiO2 nanotube arrays as a catalyst in photocatalytic degradatio

Modication Methods Pollutants

N and F Anodization MBB Electrodeposition PhenolZnTe Electrodeposition 9-AnCOOHTi Hydrothermal method Rhodamine BC Cyclic voltammetry 9-AnCOOHFe Liquid phase deposition MBAg and N Electrodeposition method AO-IICu2O Electrodeposition AO-I IC Hydrothermal treatment MBWO3 Immersion Cr(VI)

Q. Zhou et al. /Microporous and Mabout 10% higher than that of undoped arrays [20]. Owing to itslow cost and easy preparation, Fe is considered one of the mostsuitable elements for industrial applications. Doping of TiO2 withFe3+ is an effective approach to reduce electronhole recombina-tion rates and increase the photocatalytic efciency due to itssemi-full electronic conguration and an ion radius close to thatof Ti4+. Sun et al. prepared Fe-doped TiO2 nanotube arrays by anod-izing Ti in an electrolyte containing Fe(NO3)3 [21]. By controllingthe concentration of Fe(NO3)3 in the electrolyte, various concentra-tions of Fe-doped TiO2 nanotube arrays were obtained. The resultsshowed that a red shift occurred in the absorption spectrum andthe photocatalytic performance was enhanced, as expected. Wuet al. prepared Fe-doped TiO2 nanotube arrays using ultrasound-assisted impregnating and calcination [22]. The rst step of thistwo-step approach was to fabricate the TiO2 nanotube arraysdirectly on Ti foils via electrochemical anodic oxidation and thento immerse the resulting TiO2 nanotube arrays in an 0.01 MFe(NO3)39H2O aqueous solution under ultrasound assistance. Thesecond was to anneal the modied Ti foils at different tempera-tures under ambient conditions for 2 h. SEM analysis indicated thatFe2O3 nanoparticles with a size of 1020 nm were deposited ontothe TiO2 nanotubes and some Fe3+ ions were doped into the TiO2lattice. These structural modications were thought to induce thered-shift of the absorption spectral edge of the TiO2 nanotubearrays into the visible light range. These FeTiO2 nanotube arraysexhibited a much higher visible-light photocatalytic activity forthe degradation of methyl blue (MB) than undoped TiO2 nanotubearrays (Fig. 4). Electrochemical impedance spectroscopy (EIS)showed that Fe incorporation could efciently promote the separa-tion and transfer of photogenerated charge carriers, a key factor ineffecting improved photocatalytic performance. Xu et al. dopedTiO2 nanotube arrays by anodizing TiNb alloys, followed by heattreatment in a ow of ammonia gas to obtain Nb/N co-dopedTiO2 nanotube arrays [23]. They found that the Nb dopant inTiO2 nanotube arrays can enhance both the adsorption of NH3 mol-ecules and the subsequent nitrogen doping of the TiO2 nanotubearrays. These arrays demonstrated a signicantly-enhanced visiblelight response with a markedly higher visible light-induced photo-catalytic degradation of methylene blue when compared toundoped TiO2 nanotube arrays.

2.2.2. Loading on the TiO2 nanotube arraysThe noble metals have lower fermi levels than that of TiO2.

Thus, they can absorb the electrons excited from TiO2, effectivelyreducing the recombination rate of photogenerated electronholepairs, ultimately increasing photocatalytic activity. Xie et al. pre-pared highly-dispersed Ag nanoparticles on TiO2 nanotube arraysusing a pulse current deposition technique [24]. The TiO2 nanotubearrays with Ag particles under electrodeposited charge densities of1800 mC cm2 resulted in the highest absorption peak. In the deg-radation experiment using methyl orange (MO) under visible lightirradiation, the photocatalytic kinetic rate constant of the Ag/TiO2

Light sources Degradation rate (%) Refs.

Visible light 90 [18]Visible light 66 [20]Simulated solar light 100 [33]UV light 80 [46]Simulate solar light 100 [47]Visible light 50 [48]Visible light 37 [49]Visible light 90 [50]Visible light 80 [51]UV light 100 [52]

orous Materials 202 (2015) 2235 25nanotube array was 5.16 times that of the undoped TiO2 nanotubearray, indicating that the TiO2 nanotube array modied with Agnanoparticles could efciently inhibit electronhole recombina-tion. This method could also be used to modify other metal nano-particles on the TiO2 nanotube arrays. Liu et al. dispersed Agnanoparticles onto the surface of TiO2 nanotube arrays using anelectrodeposition method [25]. The uniform Ag nanoparticlesincreased the separation efciency of electrons and holes. In addi-tion, the modied TiO2 nanotube arrays led to the highest photo-catalytic activity towards MO when the electrodeposition timewas 60 min. It was postulated that Ag nanoparticles acted as elec-tron reservoirs to suppress the electronhole recombination, mak-ing more holes available for the oxidation reactions. Tan et al.fabricated Pd-functionalized TiO2 nanotube arrays [26], whichwere utilized to photodegrade methylene blue (MB) and stearicacid (SA). The photodegradation efciency of 76% (MB) wasachieved under UV irradiation in 4 h, while only 62% was

und

sopdecomposed when TiO2 nanotube arrays without Pd coating wereused. These results were attributed to the higher surface area ofmodied arrays and to the catalytic activity of Pd nanoparticlesand to the effective separation of the electronhole pairs. Thisgroup also investigated the photocatalytic activity for solid con-taminants such as SA lm. The results indicated that the TiO2 nano-tube arrays demonstrated much better photocatalytic activity thanthat of TiO2 lm under the same conditions. Liu et al. deposited CdSnanoparticles onto a TiO2 nanotube array surface using a chemicalbath deposition method [27]. This modied photocatalyst showed

Fig. 4. Schematic of illustrating the separation and transport of charge carriers

26 Q. Zhou et al. /Microporous and Meexcellent photocatalytic performance and cycling stability undervisible light. The photodegradation rate of MO was up to 96.7%higher under visible light irradiation using CdS/TiO2 nanotubearrays after 180 min. Xiao et al. found that gold nanoparticle-func-tionalized TiO2 nanotube arrays could also effectively enhance thephotocatalytic performance of TiO2 nanotube arrays [28,29]. Theydiscovered that Au nanoparticles acted as electron traps, thusprolonging the separation lifetime of photoexcited electronholecharge carriers. Iron oxide-modied TiO2 nanotube arrays weresynthesized and the photocatalytic activity was investigated with2-naphthol as the model pollutant [30]. The results indicated thatthe modied TiO2 nanotube arrays obtained by annealing at 873 Kafter anodization showed high 2-naphthol degradation efciency,and the degradation rate was 4.26 times higher than that ofunmodied TiO2 nanotube arrays. This was due to iron oxide medi-ating the transmission of electrons in the VB and CB of TiO2. Theelectrons then effectively reduced the adsorbed O2 to form radicals,which had high oxidizability and rapidly degraded the 2-naphtholinto small molecular compounds. Chen et al. developed Au nano-particles and used reduced graphene oxide (RGO) co-modiedTiO2 nanotube arrays to serve to photocatalyze the degradationof methyl orange [31]. The results exhibited that, in addition toAu, RGO could also capture the photoinduced electrons of TiO2nanotube arrays to suppress the recombination of the electronhole pairs. Owing to the simultaneous electron transfer of TiO2nanotube arrays to Au and RGO, the minimal recombination ofphotogenerated charges in Au/RGO-TiO2 nanotube arrays resultedin more effective charge separation, which made it a good photo-catalyst for the degradation of organic pollutants. The results alsodemonstrated that the prepared photocatalyst displayed high cat-alytic activity, excellent stability, and easy recyclability.

Different semiconductors possess distinct band gaps in whichphotocarriers can transfer light energy between semiconductors,and a coupling effect would then be expected to occur under illu-mination [32]. ZnTe is a semiconductor with a narrow band gap of2.232.28 eV, which means that this material will be excited undervisible light. So if this type of semiconductor is introduced intoTiO2 nanotube arrays, the visible light response will be increasedand lead to signicant enhancement of photocatalytic ability. Liu

er visible light irradiation for TiO2 nanotube arrays, Fe3+/Fe4+ and a-Fe2O3 [22].

orous Materials 202 (2015) 2235et al. fabricated ZnTe-modied TiO2 nanotube arrays using an elec-trodeposition method, and investigated the resulting photocata-lytic activity under visible light radiation. ZnTe-modied TiO2nanotube arrays displayed much higher photocatalytic activitytowards 9-AnCOOH [33]. The mechanism for the enhanced photo-catalytic activity was demonstrated in Fig. 5. Under the illumina-tion of both visible and UV light, the photogenerated electrons inthe conduction band (CB) of ZnTe were transferred to the conduc-tion band (CB) of TiO2 and then reacted with O2 to produce O2 rad-icals, which combined with H+ from hydrogen peroxide (H2O2). Asthe holes of the valence band of TiO2 moved to the valence band ofZnTe, 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-modied TiO2 nanotube arrays as thephotoelectrocatalytic catalyst [34]. Their results showed that thecomposites had stronger absorption in the visible region and muchhigher photocatalytic efciency than the undoped TiO2 nanotubearrays with rhodamine B as the model pollutant. The modicationgave rise to a synergistic effect between the lowered electronholerecombination rate and the wider spectral response. ZnFe2O4/TiO2nanotube arrays were prepared by an electrodeposition method[35], in which Zn2+ and Fe3+ were reduced to Zn and Fe, and thentreated with oxidation to form the proposed catalyst. From the sur-face photovoltaic spectra (SPV), it was found that the adsorptionarea of modied TiO2 nanotube arrays extended from the UV tothe visible light region, when compared to the unmodied one.Under UV illumination, the degradation efciency of 4-chlorophe-nol (4-CP) by loaded TiO2 nanotube arrays was 1.08 times greaterthan that of unmodied TiO2 nanotube arrays, and at a bias

Zhang et al. modied TiO2 nanotube arrays with reduced grapheneoxide (RGO) and PbS nanoparticles (NP) in one step [40]. Theresults showed that PbS was successfully dispersed inside and out-

O2 n

Fig. 6. UVvis diffuse reectance absorption spectra of the (a) plain TiO2 nanotubearrays, (b) CdS/TiO2 nanotube arrays, and (c) sonication-CdS/TiO2 nanotube arrays[38].

esoppotential of 0.8 V, all contaminants could be degraded by loadedTiO2 nanotube arrays. TiO2 nanotube arrays loaded with theBi2O3 nanoparticles also achieve better photoelectrochemicalproperties [36], and the experimental results showed that the pho-tocurrent could be easily observed in the Bi2O3/TiO2 nanotubearrays, due to Bi2O3 enhancement of the photocurrent. Theremoval rate of 4-CP with Bi2O3/TiO2 nanotube arrays was 2 timesthan that of unmodied arrays under the same conditions. Theband gap of Bi2O3 is 2.85 eV and can be excited by the light withwavelength less than 435 nm, but the photocatalytic activity ofBi2O3 is very low due to the high recombination rate of electronand hole pairs in Bi2O3. However, the electrons of the VB of TiO2can be transferred to the VB of Bi2O3 under visible light, and holesgenerated in TiO2 can initiate the photocatalytic reactions, whichenhance the photocatalytic activity of the catalyst.

Zhu et al. fabricated CdS/TiO2 nanotube arrays by an electro-chemical atomic layer deposition method [37] in which CdS wasloaded on the inner and outer of the walls of TiO2 nanotube arrays.This method could also reduce the CdS deposited at the entrance ofthe nanotube to avoid pore-clogging. In addition, this coaxial het-erogeneous structure signicantly enlarged the contact areabetween CdS/TiO2 and the CdS/electrolyte, which decreased thetravel distance that electrons and holes must move to react withpollutants, thus increasing the adsorption of protons and the pho-tocurrent of modied TiO2 nanotube arrays. The results showedthat the introduction of CdS increased the photocatalytic activityof TiO2 nanotube arrays by 5-fold over that of unmodied TiO2

Fig. 5. Illustrative diagrams of the electron and hole transfer in ZnTe/Ti

Q. Zhou et al. /Microporous and Mnanotube arrays. Xie et al. [38] developed a new sonication-assisted chemical batch electrodeposition approach to prepareCdS quantum dots (QDs) sensitized TiO2 nanotube arrays. Theresults demonstrated that the absorption spectrum of the modiedarrays signicantly moved into the visible region approaching550 nm (Fig. 6). When compared with a conventional sequentialchemical bath deposition method, this method can modify theCdS QDs both on the nanotube and on the tube walls, which caneffectively reduce clogging. The photocatalytic degradation ratetowards methyl orange was 1.158 times higher than that ofunmodied TiO2 nanotube arrays. The band gap of the sonica-tion-CdS/TiO2 nanotube arrays was calculated to be 2.20 eV. Itwas much lower than the band gap of TiO2 nanotube arrays, whichmeant that it could absorb more visible light. Wang et al. loadeddifferent amounts of Cu2O nanoparticles onto TiO2 nanotube arraysusing ultrasonication-assisted sequential chemical bath deposition[39]. Cu2O nanoparticles with narrow band gaps acted as the sen-sitizer to promote the charge transfer to TiO2, which led to efcientphotogenerated charge carrier separation. The modied TiO2 nano-tube array composite showed enhanced absorption of visible lightand improved separation of photogenerated electrons and holes.anotube arrays and the mechanism of photocatalysis degradation [33].

orous Materials 202 (2015) 2235 27side the walls of TiO2 nanotube arrays and a reduced grapheneoxide lm was formed on the top of the surface of TiO2 nanotubearrays. Almost 100% of added pentachlorophenol was removed in120 min vs. only 61% using bare TiO2 nanotube arrays. The reducedgraphene oxide surface lm could successfully suppress the photo-corrosion of PbS, which led to high photoactivity of these modiedTiO2 nanotube arrays. They also replaced PbS nanoparticles withAg nanoparticles and investigated their photocatalytic activitytowards 2,4-dichlorophenoxyacetic acid. The obtained catalyst alsoachieved good photocatalytic activity and 93% of the target pollu-tant was degraded [41].

TiO2 nanoparticles have also exhibited good photocatalyticactivity, although their structure is different from TiO2 nanotubearrays. The combination of these two distinct types of TiO2 materi-als has been shown to exhibit excellent photocatalytic perfor-mance. Zhang et al. prepared novel high-activity TiO2nanoparticle-lled TiO2 nanotube arrays using vacuum-assistedlling methods [42]. These novel arrays demonstrated theexpected properties and 4-fold higher degradation rate for MOthan that of unmodied TiO2 nanotube arrays, presumably due toan increase in reaction sites contributed by highly active TiO2

and 1000 ppm of H2. Meanwhile, Pd-modied TiO2 nanotube

chain. In order to evaluate environmental safety, it is important

sopnanoparticles on the surface of TiO2 nanotube. Deng et al. reportedBi2WO6-modied TiO2 nanotube arrays synthesized by hydrother-mal deposition [43]. The addition of Bi2WO6 led to the shifting ofthe absorption band edges to higher wavelengths and exhibitedstronger light absorption in both UV and visible light regions.Due to the enhanced separation of photogenerated electronholepairs, the photocurrent of the modied TiO2 nanotube arrays was4 times higher than that of the unmodied annealed TiO2 nanotubearrays. The photocatalytic activity of Bi2WO6/TiO2 nanotube arrayswas 2 times higher than that of the unmodied TiO2 nanotubearrays under visible light irradiation, and was about 1.5 times asmuch as that with Bi2WO6.

2.2.3. TiO2 nanotube array heterojunctionsA Z-scheme system, by mimicking natural photosynthesis, was

proposed to enhance photocatalytic efciency, which combinestwo photoexcitation systems and an electron-transfer mediator[44]. Xie et al. reported a Z-scheme type CdSAgTiO2 nanotubearray [45]. They found that the recombination rate of photogener-ated carriers was reduced on CdS or Ag nanoparticle-modied TiO2nanotube arrays, and also for the CdSAgTiO2 nanotube array sys-tem. The photocurrent density of CdSAgTiO2 nanotube arraysexceeded that of the AgTiO2, CdSTiO2, and TiO2 nanotube arraysystems under UV light irradiation at 2 h. For the degradation ofMB, the removal rate was about 63% under UV irradiation by 2 h,and only 23% could be degraded when using pure TiO2 nanotubearrays. The enhanced performance could be explained by a two-step excitation of CdS and TiO2, and Ag as a mediator which ledto efcient charge separation of the photogenerated electronholepairs.

Wang et al. synthesized graphite-like carbon-modied TiO2nanotube arrays by impregnating TiO2 nanotube array lms in asucrose solution [53]. They found that the photoresponse initiallyincreased with increasing sucrose concentration and decreasedwhen the sucrose concentration exceeded 0.01 g mL1, achievingtwice the photoresponse of TiO2 nanotube arrays under the sameconditions. The EIS spectra and HR-TEM image of these modiedTiO2 nanotube arrays demonstrated that the graphite-like carbonlayer that formed on the sample surface and nanotube walls playeddifferent roles in photoelectrocatalytic responses. The two graph-ite-like carbon layers on the nanotube walls could decrease thedepleting and Helmholtz layer resistance and then enhance theseparation of charges. On the contrary, the graphite occulesmainly acted as a light blocking layer, decreasing photoabsorption.The optimum photoresponse of TiO2 nanotube arrays was observedunder UV illumination when the sucrose concentration was0.1 mg L1, the response was twice as high as that of pure TiO2nanotube arrays and three times higher than that of pure TiO2nanotube arrays under the illumination of visible light. The syner-getic effect between carbon and TiO2 nanotube arrays resulted inthe high efciency of the charge separation process, thus enhanc-ing photoelectrocatalytic activity.

Although many improvements on TiO2 nanotube arrays asphotocatalysts have been achieved, there are still some deciencieswhich need to be resolved. Until now, the approach to enhance theutilization of visible light has gained some progress and opens thedoor to greater prospects, but there is still much work ahead tosolve practical difculties in wastewater treatment methodologies.Meanwhile the most widely-researched model pollutants are sim-ple dyes such as MO and MB, which differ from real industrial pol-lutants that are released into the environment. Moreover, manycomplex pollutants and persistent organic pollutants haveattracted much more attention and seriously threaten the environ-

28 Q. Zhou et al. /Microporous and Mement and human health. Thus, nanotube arrays and other methodsshould be applied to develop methods to achieve degradation ofthese compounds.to develop rapid and sensitive methods for monitoring them atlow levels. Many methods have been developed based on ameatomic absorption spectrometry (FAAS), graphite furnace atomicabsorption spectrometry (GFAAS), inductively coupled plasmamass spectrometry (ICP-MS), atomic uorescence spectrometry(AFS), and inductively coupled plasma atomic emission spectrom-etry, etc. In addition to these methods, sensors provide a noveltechnique for monitoring heavy metal pollutants.

A DNA-modied TiO2 nanotube array sensor was reported forthe determination of Pb2+ in water samples [54]. The determina-tion procedure involved two steps. First, Pb2+ was applied to thesensor by immersing it into the sample solution. Second, the elec-trode was rinsed and then transferred to an electrolytic cell with-out 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 thissensor were in agreement with that of AAS for real samples (Fig. 7).arrays have the potential to signicantly increase the sensitivityof 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). Wheninsulation faults occur in GIS, discharging electrical energy causesthe SF6 gas to undergo a decomposition reaction and then generategases such as SOF4, SOF2, SO2F2, and SO2. Thus, there is an increas-ing interest to develop such gas sensors. Zhang et al. [56] used TiO2nanotube arrays as a sensor to detect SO2. They found that the sen-sitivity of the sensor increased with increase in temperature, and inaddition, the response time decreased, because higher tempera-tures accelerated the movement and diffusion of the molecules.At the optimal working temperature of 200 C, they also found thatthe higher the concentration of SO2 gas, the higher the response(sensitivity). There was a good linear relationship between theresponse signal and the concentration over the range of 1050 ppm with a relation coefcient of 0.992, which indicated thatthis sensor could be used to determine SO2 gas at low levels.

3.1.2. Detection of heavy metal ionsToxic heavy metal ions in water and soils pose a serious threat

to the environment and human beings when they enter the food3. Environmental analytical chemistry

3.1. Sensors

Environmental pollutants have attracted much attention due totheir toxic effects on human health and environment. In order toevaluate their environmental safety and prevent human exposureto naturally and industrially generated inorganic and organic con-taminants, it is of great value to develop rapid, sensitive, and low-cost monitoring methods and devices. To date, many useful androbust methods have been developed. Sensors are important andvaluable tools for the detection of hazardous pollutants. TiO2 nano-tube arrays have great potential to play a role in development ofnew sensors. Recently, electrochemical sensors based on TiO2nanotube 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 monitoringIn 2003, Grimes et al. found that TiO2 nanotube arrays could be

used as a good sensor for H2 and showed a sensitivity between 104

orous Materials 202 (2015) 2235Arsenic is an important inorganic pollutant, requiring rapid andsensitive detection methods. Currently there are many useful ana-lytical methods based on different principles. Yang et al. fabricated

reproducibility [66]. The new TiO2 nanotube array method also has

ube

esopAu shrubs-modied TiO2 nanotube arrays as a novel sensor todetect the concentration of arsenic [60]. This composite possesseda high surface area compared with other modied TiO2 nanotubearrays. The results showed a high sensitivity between currentchanges 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 whenAu lm-modied TiO2 nanotube arrays were used, which indicatedthat more surface area and the unique 3D structures accounted forthe high performance.

3.1.3. Detection of organic pollutantsCai et al. reported a sensor composed of molecularly imprinted

polymer-modied TiO2 nanotube arrays for peruorooctane sulfo-nate in water samples. The direct detection of peruorooctane sul-fonate by electrocatalytic reduction reaction was fullled usingmodied TiO2 nanotube arrays with a detection limit of86 ng mL1. The selectivity of this sensor was also very good[61]. Cai et al. developed an octachlorostyrene (OCS) photoelectro-chemical (PEC) immunosensor by cross-linking anti-OCS antibodyonto CdTe/CdS-sensitized TiO2 nanotube arrays [62]. The PECimmunosensor exhibited high specicity and high sensitivity witha limit of detection of 2.58 pM, and a linear range from5 pM 50 nM. Due to the excellent photoelectronic performanceof the CdTe/CdSTiO2 nanotube arrays, the label-free PEC immuno-sensor showed a highly sensitive and selective response to OCS.The testing time was 4 min. Compared with conventional opticalmethods, the PEC immunoassay was simpler in instrumentationand more easily miniaturized.

3.2. Measurement of COD

Chemical oxygen demand (COD) is an important parameter forthe evaluation of water pollution and is also the most commonitem in water monitoring. The most commonly-used method todetermine COD is the potassium bichromate method, which hasadvantages such as reliable results and good reproducibility, butthe defects are also obvious. For example, the procedure takes arelatively long time (24 h), and consumes a large quantity ofexpensive and poisonous agents, such as Ag2SO4 and HgSO4, etc.In recent years, many novel technologies have been developed todetect COD [63,64] based on electrochemical methods, photocata-lytic oxidation, and photoelectrochemical oxidation. Recently, aTiO2 nanotube array was used to develop a new determinationmethod for COD. Using photoelectrochemical oxidation, Zhanget al. developed a new determination method for COD using TiO2

Fig. 7. Schematic illustration for DNA/C-TiO2 nanot

Q. Zhou et al. /Microporous and Mnanotube arrays as the work electrode [65]. They found that TiO2nanotube arrays, which were prepared in the solution with 1%HF electrolyte (pH = 2) at the anodic potential of 20 V and annealedat 450 C, showed the highest photocurrent density. The principleof COD determination is based on Faradays law, and the COD valuecould be calculated using the following equation:

CODmg=L of O2 nC4 3200 Q4FV

3200 KQ

During the COD determination experiments, they found thatinterferences such as pH variation and coexisting ions such asNH4+ and Cl had no effect on the determination of COD. Thisadvantages such as short experiment time and is free from sometoxic reagents, which are often used in the conventional method.The key to obtaining the real COD value is to oxidize the organiccomponents completely, and the excellent photocatalytic oxidabil-ity of TiO2 nanotube arrays makes them ideal electrode materialsfor determination of COD. Chen et al. assessed performance of thismethod using water samples containing some refractory and lowconcentration organic compounds [67]. A comparison was madebetween the theoretical chemical oxygen demand (ThCOD) andresponse COD, and the related equation was used, as follows:

COD a ThCOD:They chose recalcitrant organic compounds including sugars,

benzene derivatives, organic acids, alcohols, amino acids and pyri-dines, etc. The COD values achieved using the photoelectrochemi-cal sensor were larger than those obtained using the CODCrmethod, especially in determination of the COD values of pyridineand amino acid solutions, where the CODCr method showed alower a value of 4.5% and 0.5%, while the photoelectrochemicalmethod obtained a higher a value of 95.1% and 97.6%, which exhib-ited the super-oxidation capability of TiO2 nanotube arrays. Thisnew COD determination method based on TiO2 nanotube arrayspossessed many advantages such as effective catalysis, fast masstransport, large effective surface area, and good control over theelectrode microenvironment; these properties make it applicableto water monitoring. A Cu2O-loaded TiO2 nanotube arrays elec-trode was fabricated by an electrodeposition process and used asa sensor to detect COD value [68]. By modication with Cu2O,TiO2 nanotube arrays showed high absorption intensity in the vis-ible light region and a much higher sensitivity to visible light. Add-ing a positive bias potential of 0.3 V in visible light achieved a lowdetection of limit of 15 mg L1 and a good linear range of 20300 mg L1. However, environmental samples often have unknownor changing matrix characteristics, which will result in difculty inachieving precise concentrations of focused pollutants. This chal-lenge will impact the development of measurement devices [69].

3.3. Sample pretreatment

With the technological advances, trace pollutants havemethod worked within a linear range of 0850 mg L1. Comparedwith the conventional K2Cr2O7 method, this method, based onhighly ordered structure of TiO2 nanotube arrays, resulted in good

arrays construction and its Pb ion monitoring [54].

orous Materials 202 (2015) 2235 29attracted more attention. However, it is also difcult to achieveaccurate detection due to the varieties of pollutants in water, soiland air, low concentrations of the pollutants and strong matrixeffects. Thus, a sample pretreatment step is necessary in the ana-lytical procedure. In general, it is estimated that more than 60%of analysis time is spent on the sample pretreatment, especiallyon trace and ultra analysis. Enrichment is an effective method thatconcentrates trace targets to the concentration that matches thedetection sensitivity of instruments. The sample pretreatment stepnot only concentrates the targets, but also cleans the samples,which decreases matrix effects and increases the accuracy of ana-lytical methods.

Conventional sample treatment methods have been used formany years such as liquidliquid extraction (LLE) and Soxhletextraction, etc. However, the typical disadvantage of these meth-ods is the usage of a large quantity of organic reagents, which willgenerate secondary pollution. In recent years, some novel pretreat-ment methods have been developed to address this issue. Solid-phase micro-extraction (SPME) is a novel pretreatment methodbased on partition equilibrium, which allows analytes to reachequilibrium between stationary phase and liquid phase. Jianget al. fabricated TiO2 nanotube arrays on a Ti wire, which was usedas the extraction ber in SPME [70]. The highly ordered TiO2 nano-tube arrays showed high selective adsorption ability to differentanalytes. The results indicated that PAHs and alkanes could beeffectively adsorbed on a TiO2 nanotube array with enrichmentfactors in the range of 82.696.9, while lower adsorption abilities

to the TiO2 nanotube. Large holes would decrease the specic sur-face area, reducing the absorption ability, and a potential of 20 Vwas nally chosen. The other parameters that affected the l-SPEEprocedure included the eluting solvent, pH value, salt effect, equi-librium time and desorption time. After optimization of conditions,the LODs were in the range of 0.0180.073 lg L1 (S/N = 3) and thelinear ranges were in the range of 0.180 lg L1 for pyrethroidsbifenthrin, fenpropathrin and fyhalothrin, and 0.2160 lg L1 and0.3210 lg L1 for fenvalreate and deltamethrin, respectively.When this method was used for the determination of ve fungi-cides, lower limits of detection were obtained in contrast to thecommonly-used SPE method [80].

The surfactants could be adhered onto TiO2 nanotube arraysthrough electrostatic interactions between the charges of metallicoxide and opposing charges of the surfactants to form micelles

(PAHs) [82]. The results showed that when the concentration ofCTAB was 90 mg L1, the maximum adsorption capacity was

30 Q. Zhou et al. /Microporous and Mesoporous Materials 202 (2015) 2235were found for anilines and phenols. Limits of detection were inthe range of 0.0010.1 lg L1 for the targeted PAHs. When realwater samples were analyzed, the recoveries were in the rangeof 78.57119.28%. These results demonstrated that TiO2 nanotubearray bers had many advantages over commercial SPME berssuch as high rigidity, long lifetime and good resistance to pH vari-ation and high temperature conditions.

Due to the excellent properties of TiO2 nanotube arrays, ourgroup has focused on them and developed a new type of pretreat-ment method called micro-solid phase equilibrium extraction (l-SPEE) technique (Fig. 8) [6,7174]. Previously, we investigatedthe applicability of TiO2 nanotube powders as the adsorbent inSPE and developed many enrichment and determination methodsfor monitoring copper [66], nickel [75] and cadmium [76], ben-zoylurea insecticides [77], paraquat and diquat [78] and DDTsand their main metabolites [79] in water samples. The resultsshowed that TiO2 nanotubes had better enrichment capabilitiesand the analytical methods exhibited low detection limits for tar-geted analytes. The l-SPEE method developed recently was alsobased on the equilibrium principle, and the TiO2 nanotube arrayswere used as the adsorbent. The procedures of adsorption anddesorption of pollutants on sorbent occurred at the same time.When the rates of adsorption and desorption was equal, the TiO2nanotube array was taken out and then a little solvent was usedto elute analytes adsorbed on the TiO2 nanotube array. The nalsolution was dried and redissolved with an appropriate solventand analyzed with selected analytical instruments such as GC,HPLC and AES, etc. We then used this developed method to deter-mine trace levels of pyrethroids in environmental water samples[71]. We rst investigated the effect of anodic potential on theadsorption performance of a TiO2 nanotube array. The nanotubearray prepared under low potential possessed a small-diameterhole, which made pollutants difcult to transfer from liquid phaseFig. 8. The principle of lobtained. With this modied TiO2 nanotube array as the l-SPEEsorbent, a rapid and sensitive determination method was devel-oped and the detection limits of 16 PAHs were obtained in therange of 0.0260.82 lg L1. Pan et al. reported a new method forthe determination of PAHs with TiO2 nanotube arrays fabricatedon Ti wire and modied with Au nanoparticles and n-octadeca-nethiol, and the detection limits of selected PAHs were in the rangeof 0.13.0 ng L1 [83]. TiO2 nanotube and modied TiO2 nanotubearrays will have many applications, and our efforts will focus ondevelopment of highly selective, rapid, reliable and sensitiveenrichment and determination methods for trace pollutants suchas new persistent organic pollutants and typical pollutants in theenvironment.

4. Applications in sensitized solar cells (SSCs)

Solar energy is a great renewable source and would serve as theideal future clean energy supply for the world, due to its accessibil-ity when compared with wind, nuclear and biomass energy. Ef-cient solar cells must absorb enough light over a broad spectralrange from visible to near-infrared (near-IR) wavelengths (350950 nm) and convert the incident light effectively into electricitywhen the concentration of the surfactant reached critical micelleconcentration (CMC). As a result, the metallic oxide surface coatedwith surfactants became hydrophobic, useful for the enrichment ofpollutants. Niu et al. used a cetyltrimethyl ammonium bromide(CTAB)-coated titanate nanotube as the adsorbent for solid phaseextraction to enrich phthalate esters in water samples, obtaininggood results [81]. We prepared CTAB-coated TiO2 nanotube arraysand investigated the adsorption of polyaromatic hydrocarbons-SPEE method [82].

esop[84]. Among many solar cells devices, the dye-sensitized solar cell(DSSC) is a new solar cell with special properties. ORagan et al.rst developed this sensitized solar cell using a TiO2 particle lmas the photoanode [85]. Many efforts were dedicated to enhancethe light conversion of these cells. Owing to the advantages suchas their low cost, easy manufacture and high efciency, DSSC-based TiO2 materials have been shown to be a potential alternativeto conventional solid-state solar cells [8690]. As one of the impor-tant components of solar cells, the photoanode has a great effect onlight conversion efciency. The TiO2 nanotube array, a new mate-rial with unique properties, has been used in photovoltaic studieswhere many experiments have also veried desirable properties[9194] such as highly ordered nanotube structure which leadsto accelerated electron transport and an ordered surface whichincreases sensitizer absorption [95]. Park et al. transplanted TiO2nanotube arrays onto FTO (uorine-doped tin oxide) glass [96],which can improve the performance of the photoresponse whencompared with TiO2 nanoparticles. The efciency was enhancedto 5.36% by a post-treatment with a TiCl4 solution. The studyshowed treatment with TiCl4 could inhibit the recombination ofcharges and accelerated the move of electrons, which increasedthe charge density in the photoanode. They also fabricated a nano-porous layer-covered TiO2 nanotube array in TiCl4 solution [97].Both the electron transport rate and electron lifetime wereimproved, and more surface defects were found on the surface ofTiO2 nanotube arrays over conventional arrays, which alsoincreased the performance of the DSSCs. Wang et al. rst depositedTi lm on the FTO and then prepared TiO2 nanotube arrays on it foruse as the photoanode in the DSSCs [98]. Their study showed thatthe adhesive force between Ti lm and FTO depended on the tem-perature of the sputtering process. At lower temperatures, TiO2nanotube arrays were easily peeled off, while the stability wasimproved in higher temperatures. This transparent photoanodeexhibited a high conversion of light to electricity. A bamboo-likestructured TiO2 nanotube array was fabricated by altering theanodization voltage [99]. They found that the bamboo rings couldprovide much larger surface area for dye loading which led to ahigh conversion efciency. Wang et al. fabricated a bamboo-typeTiO2 nanotube array by using a square-wave voltage [100]. TheEIS measurements showed reduced interfacial resistance andincreased the interfacial capacitance in the bamboo-type TiO2nanotube arrays compared with the smooth type arrays. Anincrease in surface area of the bamboo-type TiO2 nanotube arraysresulted in dye loading in both the inner and outer walls, whichincreased the conversion efciency by 7.36% when compared withsmooth TiO2 nanotube arrays.

Luo et al. immersed the TiO2 nanotube arrays in deionizedwater to remove Ti foil and then formed free-standing membranenanotube arrays, and then coupled them with TiO2 nanoparticles,FTO and electrolyte to form solar cells [101]. The improved lightscattering performance and improved I3 diffusion were observedin this DSSC. Wang et al. modied the TiO2 nanotube arrays withRu(dcbpy)2(NCS)2 as a dye-sensitized solar cell [102]. They foundthat the TiO2 nanotube arrays fabricated for 50 h showed the high-est conversion efciency, however the morphology of the materialprepared for 60 h was destroyed, which would provide morerecombination centers for electrons, and cause the reduction inefciency. Wang et al. [92] reported results after TiO2 nanoparti-cles were deposited onto TiO2 nanotube arrays. They found thatdeposition of TiO2 nanoparticles can remarkably improve theabsorption of N3 dye with an enhancement of 47.2% due to theincreasing surface area of nanostructure. A conversion efciencyof 6.28% was achieved for DSSC with TiO2 nanoparticles as the ex-

Q. Zhou et al. /Microporous and Mible photoanodes. This electrodeposition method of nanoparticleshas great potential uses, such as in photovoltaic, photodegradationand sensor applications.Liu investigated the effect of the length of nanotube on the per-formance of solar cells [103]. The results indicated that a longernanotube has a positive effect on photocurrent density and conver-sion efciency. Meanwhile, the open-circuit photovoltage wasdecreased, which offset the short-circuit photocurrent density, sothe 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 harvest-ing [104,105]. Huang et al. developed a single-step method to cou-ple a PC layer to TiO2 nanotube arrays [106]. The TiO2 nanotubearrays layer was obtained by normal electrochemical anodizationand the TiO2 PC layer was fabricated by a periodic current pulseanodization. This bi-layer structure DSSC showed a signicantlyenhanced power conversion efciency (PCE) of 50% over that ofsingle layer DSSC. They proposed a novel photonic crystal-basedphotoanode composed of a TiO2 nanoparticle (TiO2 NP) absorptionlayer and a thin TiO2 nanotube array photonic crystal (TiO2 NT PC).The TiO2 NP worked as the adsorbing layer and TiO2 NT PC con-ferred the PC effect and acted as the scattering layer. Comparedwith conventional TiO2 NP-based DSSCs, the PCE increased by39.5% due to the combined effects [107].

Wang et al. reported a spiral structure of TiO2 nanotube arrayson Ti wire and found that it could efciently trap scattered light[108]. Alivov et al. transformed TiO2 nanotube arrays to formTiO2 nanoparticle lms using similar annealing conditions as usedto create the photoanodes in DSSC applications [109]. With a nano-particle size of 65 nm, a maximum nominal efciency of 9.05% wasobserved. The lowest efciency of 1.48% was observed for DSSCswhen nanoparticle size was 350 nm, which indicated that thenanoparticle size had a signicant inuence on the performanceof the solar cells. Bandara et al. [110] fabricated solid-state dye-sensitized solar cells with different thicknesses of transparentTiO2 nanotube array electrodes coupled with a Ru-(II)-donorantenna dye. A power conversion efciency of 1.94% was obtained.They also found that a linear increase in the cell current wasobserved with the increase in length of the TiO2 nanotube arrays.Mirabolghasemi et al. fabricated single-walled TiO2 nanotubearrays and demonstrated that a signicant gain in electrical andphotoelectrochemical properties could be reached with theseunique tubes [111]. They also found that the short circuit currentdensity and efciency of the solar cell were higher than those fordouble-walled nanotube ones.

However, the cost of preparing dye-sensitized solar cells is rel-atively high, which interferes with the commercialization of DSSCs.This, it is necessary to look for substitutes that replace dye in theconversion of light energy to electrical energy. New sensitizedsolar cells such as quantum dot-sensitized solar cells and hetero-junction solar cells have been introduced. Semiconductors arewidely used in the modied TiO2 nanotube arrays to improve thephotoelectrochemical properties by broadening the light regionthat is absorbed and by reducing the recombination rate of thephotogenerated electronhole pairs. In recent years, the semicon-ductor material has received more attention and has been usedas the sensitizer, replacing the dye sensitizer [112114]. Hossainet al. synthesized CdSe nanoclusters on highly-ordered TiO2 nano-tube arrays using chemical bath deposition [112]. That studyshowed that parameters such as photocurrent, photovoltage, llingfactor, and conversion efciency were enhanced signicantly byincreasing the deposition time due to increased deposition of CdSeonto TiO2 nanotube arrays under that condition. Because CdSe is anarrow band gap material, it could be used to absorb the longwavelength light in the visible spectrum. The results exhibited thathigher conversion efciency was achieved when compared with

orous Materials 202 (2015) 2235 31other forms of CdSe QDs [115]. CdS nanoparticles adsorbed ontoTiO2 nanotube arrays could be also used in solar cells [116]. Theresults indicated that the method effectively decreased the

tube arrays from the TiO2 substrate and then fabricated a TiO2/ZnO nanotube array heterojunction [117]. The results indicated

age

sopthat the DSSCs based on the heterojunction exhibited a better shortcircuit 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 afxingAgAg2S hybrid nanoparticles onto TiO2 nanotube arrays, whichexhibited a photocurrent density of 2.76 mA cm2 and approxi-mately 92 times greater photoelectric efciency than that of bareTiO2 nanotube arrays. The excellent performance of the compositewas attributed to the surface plasmonic resonance effect, whichwas further enhanced by an Ag2S outer-layer [118]. Wu et al.reported that CdS-sensitized ZnO nanorod arrays on the TiO2 nano-tube arrays could be used to generate hydrogen from water photo-electrolysis [119]. The existence of the one-dimensional structureof TiO2 nanotube arrays, acting as an electron collector and trans-porter, could provide a direct and quick electron pathway for pho-toinjected electrons along the photoanode and reduce electronhole recombination. The applications of TiO2 nanotube arrays usedas the photoanode are shown in Table 2.

5. Hydrogen production

Hydrogen is regarded as an ideal energy resource and is mainlyaggregation during modication and the photoelectric conversionefciency increased by 65.8% when compared with the sequentialchemical bath-deposition method. Ren et al. detached TiO2 nano-

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

Photoanode Jsca Voca

TiO2NA/N719 6.48 0.76TiO2NA/N719/ 7.85 0.77TiO2NA/N719 12.39 0.637TiO2 NA/N719 7.63 0.68TiO2NA/Ti wire/N719 10.9 0.522TiO2NA/CdSe 7.19 0.438TiO2NA/N719 7.21 0.65P3HT@CdS@TiO2NA 3.00 0.7TiO2NA/Nanocrystalline CdS 5.17 0.77TiO2 NA/N719 15.00 0.59TiO2NA based on mash/N719 12.4 0.68TiO2NA/FTO/N719 15.46 0.814TiO2NA/CdS/CdSe/ZnS 13.52 0.48TiO2NA/N719 13.5 0.7

a Jsc = short circuit photocurrent density (mA cm2); Voc = open circuit photovolt

32 Q. Zhou et al. /Microporous and Meproduced from natural gas via steam methane reforming [128].However, hydrogen production from the splitting of water hasbeen considered as another effective way to solve the currentenergy shortage. Highly-ordered TiO2 nanotube arrays have beenexplored as a catalyst for hydrogen production [129]. The experi-mental results exhibited that the Pt nanoparticles loaded on theTiO2 nanotube arrays by a microwave irradiation methodenhanced hydrogen generation rate up to 0.613 ml h1 cm2 com-pared with 0.313 ml h1 cm2 of the unmodied one. The reasonwas that the Pt nanoparticles loaded onto the TiO2 nanotube arrayswould trap the photoinduced charge carriers and accelerate theinterfacial charge-transfer process, which increased the photocata-lytic reaction rate. Kei reported a catalyst membrane that consistedof TiO2 nanotube arrays and a Pd layer. The Pd layer could be usedto separate H2 from other by-products [130]. The effect of theannealing temperature of TiO2 nanotube arrays [131] on hydrogenproduction was also investigated. The annealing temperature had asignicant effect on the crystal phase, morphology and photoelect-rochemical properties of TiO2 nanotube arrays. Low annealing tem-perature had no effect on the crystal phase and morphology of pre-existing material. With further increase in temperature, the crystalphase transferred from anatase to the rutile phase, resulting indestruction of the tubular structure for vectorial charge transfer,resulting in a sharp decrease in photocurrent. TiO2 nanotube arraysannealed at 450 C showed the highest photoconversion efciencyof 4.49% and the highest hydrogen production reported, a rate of122 lmol h1 cm2. Sang et al. reported that TiO2 nanotube arrayscreated via sonoelectrochemical anodic oxidation and annealed indifferent gases showed distinct photoelectrochemical properties inhydrogen production [132], and they fabricated different element-doped TiO2 nanotube arrays in their studies [133]. C-doped TiO2nanotube arrays demonstrated more surface-active sites and anegative at band potential, which improved the photoelectro-chemical properties for hydrogen production. Smith et al. reporteda facile method to synthetize hierarchical TiO2 nanotube arrays byrst etching them in a solution of HF and HNO3, followed by anod-ization [134]. The unique morphology provided an increased sur-face area for light utilization and accelerated the separation ofelectron and charge pairs. Water splitting efciencies were 0.34%and 0.15% at 1.23 (RHE) using hierarchical TiO2 nanotube arraysgrown on wires and foils, respectively. This hydrogen generationrate was increased by over 40% for hierarchical TiO2 nanotubesarrays vs. plain TiO2 nanotube arrays, and over 25% increased forwire substrate vs. foil.

Based on the same principles of photocatalytic oxidization men-tioned in the pollutant- degradation section above, it is of greatimportance to decrease the recombination rate of electronholepairs within the TiO2 nanotube arrays during hydrogen production.

FFa Conversion efciency (%) Refs.

0.58 3.18 [33]0.61 3.7 [94]0.5549 4.38 [98]0.708 3.68 [101]0.48 2.78 [103]0.495 1.56 [112]0.45 2.13 [120]0.55 1.16 [121]0.47 1.87 [122]0.49 4.29 [123]0.6 5 [124]0.641 8.070 [125]0.53 3.44 [126]0.7 6 [127]

(V); FF = ll factor; TiO2NA = TiO2 nanotube arrays.

orous Materials 202 (2015) 2235Many researchers have made great strides in achieving this goal byloading semiconductors or metals onto TiO2 nanotube arrays suchas Cu2O/Cu [135], and CdS (Fig. 9) [136]. When CdS-modied TiO2nanotube arrays were used as electrodes for hydrogen production[129], the presence of CdS nanoparticles led to improved photo-electrochemical reactivity, which efciently facilitated the separa-tion and transfer of photogenerated electronhole pairs. Lai et al.used an impregnation method to load WO3 onto the TiO2 nanotubearrays [137]. By controlling the soaking time, different amounts ofWO3 loaded TiO2 nanotube arrays were obtained. Their studyshowed that a small amount of WO3 could signicantly improvethe PEC water splitting performance, which was approximately1.5 times higher than that of pure TiO2 nanotube arrays. An exces-sive amount of WO3 would decrease the photocatalytic activity dueto formation of agglomerates. Palladium quantum dots (Pd QDs)-sensitized TiO2 nanotube arrays were prepared by hydrothermalmethod. When this material was used as the photoanode, a hydro-gen production rate of 592 lmol h1 cm2 was obtained, andwhich was 1.6 times than that of unmodied one. This enhance-ment was ascribed to the synergetic effects between TiO2 and PdQDs, and Pd QDs acted as electron sinks and catalytic centers,

s ele

esoporous Materials 202 (2015) 2235 33which reduced the recombination rate of photogenerated electronsand holes and increased the rate of water decomposition [138].Smith et al. fabricated TiO2 nanotube arrays onto Ti wire, whichacted as the catalyst to split water into hydrogen. An enhancementof 40% was observed when compared with TiO2 nanotube arrays onTi foil [135]. Zhang et al. loaded carbon quantum dots onto the TiO2nanotube arrays using a deposition method and prepared a carbonquantum dots-sensitized catalyst for hydrogen production [139]. Itwas found that enhanced optical absorption was seen in both thevisible and NIR (near infrared) regions. The photocurrent densityof the composite was 4 times higher than that of unmodiedTiO2 nanotube arrays. The hydrogen production rate was about14.1 mmol h1 for a carbon quantum dots-sensitized TiO2 nano-tube array under simulated sunlight illumination (AM 1.5G,100 mW cm2). Zhang et al. [140] used Cu(OH)2 modied TiO2nanotube arrays as the catalyst for hydrogen production and foundthat the H2-production yield was 20.3 times higher than that withunmodied TiO2 nanotube arrays due to the synergistic effectbetween Cu(OH)2 and TiO2 nanotube arrays. Wang et al. usedCdSe/CdS/TiO2 nanotube arrays for hydrogen production [141].The results showed a 7-fold enhancement in photoconversion ef-ciency and its band gap generated an obvious red shift to broadenthe visible light response.

Hydrogen also can be generated from the decomposition of

Fig. 9. Conguration model consists of CdS/TNAs glasQ. Zhou et al. /Microporous and Malcohols like ethanol. A gas phrase photocatalytic decompositionof alcohols in high vacuum conditions was reported [142]. TheH2 production of 2.8 108 Torr was obtained by using Pt-modi-ed TiO2 nanotube arrays. The experimental results showed thata longer TiO2 nanotube was favorable for hydrogen generationdue to its ability to provide more reaction sites.

6. Photocatalytic conversion of CO2

The main energy sources currently are fossil fuels such as petro-leum, natural gas and coal because of their usability, stability andhigh caloric values. The large amounts of CO2 emissions are pro-duced during the burning or transforming of these fossil fuels, withresulting greenhouse effects. How to effectively reduce CO2 is animportant issue facing the countries around the world. In addition,the shortage of fossil fuels and the need to nd alternative renew-able and sustainable fuels are triggering increasing interest in thephotocatalytic reduction of CO2 [143145].

TiO2 has been successfully incorporated in methodologies forphotocatalytic conversion of CO2. Different crystalline phases ofTiO2 have been evaluated for their performance in photocatalyticreactions. It has been known that increasing the annealing temper-ature changes the crystalline phase of TiO2, causing an increase inthe composition of the rutile phase and decrease in anatase phase.Because the highest photocatalytic reactivity was observed in crys-tals composed of a mixture of anatase and rutile phases, controlover the ratio of the phases using temperature annealing, and thusthe overall reaction rate, was observed [146]. For CO2 reduction,methane production rate in visible light reached the highest levelswith annealing temperature at 480 C and 680 C (Fig. 10) [147]. At480 C, crystals were mostly (91%) anatase phase, while at 680 C,the crystals were mostly rutile phase (91%). Thus, methane produc-tion was highest when both phases were present but one predom-inated. It was postulated that crystals containing anataserutileinterfaces possess more numerous active sites for photoreductionthan in crystals with only one phase.

Varghese et al. also studied reduction of CO2 to methane, butused actual sunlight and modied TiO2 nanotube arrays as a cata-lyst [148]. They found that co-catalysis using Pt nanoparticles andCu nanoparticles that were loaded onto the surface of nanotubearrays had an important effect on the conversion rate, and couldalso effectively drive the total conversion. The product species var-ied owing to the amounts of loaded co-catalysts on the TiO2 nano-tube arrays. Ultrane Pt nanoparticles distributed on TiO2nanotube arrays acting as co-catalysts could provide many reac-tion sites for CO2 conversion reaction, enhancing the transforma-

ctrode and Pt cathode for hydrogen generation [136].tion rate from CO2 and H2O to methane [149]. In general theconversion rate was lower than 5% when using pure TiO2 nanotubearrays as the catalyst, while Pt nanoparticle-modied TiO2 nano-tube arrays exhibited a higher conversion rate of 25%. Meanwhile

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

Acknowledgements

sopThis work was nancially supported by the National NaturalScience Foundation of China (21377167), Program for New CenturyExcellent Talents in University (NCET-10-0813) and ScienceFoundation of China University of Petroleum, Beijing (KYJJ2012-01-15).

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7. Conclusions

The new architecture of vertically-aligned TiO2 nanotube arrayshas gained much interest during recent decades due to their novelphysical attributes as well as their numerous potential applicationsin various elds. The rapidly-developing techniques based on TiO2nanotube arrays have provided new solutions to address chal-lenges in the removal of environmental pollutants, greater utiliza-tion of solar energy, decreases in greenhouse gases, creation of newenergy sources and others. However, there is still much work to do,including increasing the conversion efciency of solar energy tocreate electrical and chemical energy, signicantly improvingcatalysis in response to visible light, and developing new catalystswith higher photocatalytic activities. TiO2 nanotube arrays holdgreat promise in playing a key role in the development of newtechnologies to address growing challenges that must be overcometo assure a bright future for our world.

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