International Journal of Photoenergy

Volume 2014 (2014), Article ID 563879, 14 pages

http://dx.doi.org/10.1155/2014/563879

Review Article

One-Dimensional Nanostructured TiO₂ for Photocatalytic Degradation of Organic Pollutants in Wastewater

Ting Feng,¹ Gen Sheng Feng,¹ Lei Yan,² and Jia Hong Pan³

¹School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China

²College of Materials, Optoelectronics and Physics, Xiangtan University, Xiangtan, Hunan 411105, China

³Department of Materials Science and Engineering, NUSNNI-NanoCore, National University of Singapore, Singapore, 117576

Received 4 April 2014; Revised 12 June 2014; Accepted 23 June 2014; Published 5 August 2014

Academic Editor: Xiwang Zhang

Copyright © 2014 Ting Feng et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The present paper reviews the progress in the synthesis of one-dimensional (1D) TiO₂ nanostructures and their environmental applications in the removal of organic pollutants. According to the shape, 1D TiO₂ nanostructures can be divided into nanorods, nanotubes, nanowires/nanofibers, and nanobelts. Each of them can be synthesized via different technologies, such as sol-gel template method, chemical vapor deposition, and hydrothermal method. These methods are discussed in this paper, and the recent development of the synthesis technologies is also presented. Furthermore, the organic pollutants, degradation using the synthesized 1D TiO₂ nanostructures is studied as an important application of photocatalytic oxidation (PCO). The 1D nanostructured TiO₂ exhibited excellent photocatalytic activity in a PCO process, and the mechanism of photocatalytic degradation of organic pollutants is also discussed. Moreover, the modification of 1D TiO₂ nanostructures using metal ions, metal oxide, or inorganic element can further enhance the photocatalytic activity of the photocatalyst. This phenomenon can be explained by the suppression of e⁻-h⁺ pairs recombination rate, increased specific surface area, and reduction of band gap. In addition, 1D nanostructured TiO₂ can be further constructed as a film or membrane, which may extend its practical applications.

1. Introduction

Organic pollutants are widely presented in wastewater, which have negative effects on environment and human health. Even developing water treatment technology calls for efficient decontamination method for complete degradation of persistent organic pollutants (POPs) [1–3]. Conventional biological, physical, and chemical processes have been employed and have showed capability in degrading most organic pollutants, while for POPs, complete degradation is still a big challenge. Advanced oxidation process (AOP) has thus developed by oxidization with hydroxyl radicals (•OH), which are provided by introducing ozone, hydrogen peroxide, and UV irradiation. However, complete degradation of POPs is still difficult due to the ease in forming disinfection by-products.

TiO₂-mediated semiconductor photocatalysis has attracted considerable attention in view of their excellences in complete degradation of organic pollutants via photocatalytic oxidation (PCO) process. A general mechanism is illustrated in Figure 1 [4]. Initially, PCO process is triggered by the excitation of TiO₂ under the irradiation of photons with energy greater than the band-gap energy of TiO₂. The photogenerated electron (e⁻) and holes (h⁺) pairs without recombination can migrate to TiO₂ surface to participate in redox reactions with adsorbed species with the possible formation of superoxide radical anion (•) and hydroxyl radical (•OH), respectively. These reactive oxygen species are mainly responsible for the degradation of organic pollutants in water. Furthermore, the excitation of TiO₂ by UV light can be displayed in the following equations [5–8]:

Figure 1: General mechanism of the photocatalysis [13].

In an organic degradation process, the formed and act as reductant and oxidant, respectively. The reaction steps are represented as follows [6].

Oxidative Reaction:

Reductive Reaction:

Current research progress has shown that TiO₂-based materials demonstrate highly active photocatalytic degradation of different organic pollutants [9–11]. For example, phenol, chlorinated aromatics, and aniline compounds are susceptible to oxidation by TiO₂, and they can form intermediate radicals that may subsequently trigger a series of radical reactions in this process. Due to the excellent oxidation ability, the resulting intermediates can be finally degraded into and . Since the oxidation process is primarily driven by electron transfer reactions at the surface of TiO₂, the specific surface area of TiO₂ is an important factor which can affect the PCO efficiency.

To significantly boost the photoenergy conversion efficiency, tailing nanostructured TiO₂ has attracted continuous research interest, among which one-dimensional (1D) nanostructures possessing large surface areas and unique physical and chemical specificities are of particular interest [12]. Various 1D nanostructures including nanorods, nanowires/nanofibers, nanotubes, and nanobelts have been successively synthesized during the past decades. Some typical synthetic methods of 1D TiO₂ nanostructures are summarized in Table 1. Nanorods have a low aspect ratio (length divided by width) ranging from 3 to 5. Nanotubes are a type of nanometer-scale tube-like structure, which have a similar size with nanorods. For the nanowires/nanofibers, they have a higher aspect ratio as compared to nanorods. Nanobelt is a nanostructure in a form of belt. The specific geometry with high aspect ratio renders dramatical enhancements in charge carrier generation, transport, and separation, boosting the photoenergy conversion efficiency [13, 14]. Up to now 1D TiO₂ has been comprehensively investigated for the degradation of organic pollutants, such as dyes, POPs (phenol and derivatives) and natural organic matters (NOMs) [15, 16].

Table 1: Typical synthetic methods of 1D TiO₂ nanostructures.

TiO₂ nanostructures
Synthesis methods
References

TiO₂ nanorods
Sol-gel template method
[20]

Hydrothermal method
[22]

Chemical vapor deposition
[23]

TiO₂ nanotubes
Electrochemical deposition
[37]

Template method
[41, 42]

Hydrothermal method
[39, 44–46]

TiO₂ nanowires/nanofibers
Hydrothermal method
[5]

Microwave
[75]

Electrospinning
[76]

TiO₂ nanobelts
Solvothermal method
[65]

Chemical vapor deposition
[68]

Hydrothermal method
[67]

2. TiO₂  Nanorods

TiO₂ nanorods have a relatively small amount of grain boundaries and can act as single crystal, which is able to reduce the grain boundary effect and provide fast electron transport [17]. Previous reports indicated that TiO₂ nanorods exhibited higher photocatalytic activity than nanoparticle counterparts due to the increased numbers of active sites and crystal plane effects [18]. Furthermore, TiO₂ nanorods have lower recombination rate of e⁻ and h⁺ as compared with TiO₂ nanoparticles, which would enhance the photocatalytic activity of the photocatalyst [19].

Various synthetic strategies have been designed for the preparation of TiO₂ nanorods, which include sol-gel template method, chemical vapor deposition (CVD), and hydrothermal method [19–23].

Attar et al. [20] developed an improved sol-gel template method for the synthesis of TiO₂ nanorods, as shown in Figure 2(a). Anodic alumina membranes (Figure 2(a)) were applied as the template with pore sizes ranging from 50 to 300 nm. Owing to the adjustable membrane pore sizes, the diameter of the synthesized TiO₂ nanorods can be controlled. Figure 2(b) shows the synthesized TiO₂ nanorods with diameter of 160–250 nm, revealing that the nanorods synthesized via this method have a uniform diameter as well as a smooth surface. However, they found that the diameter of the synthesized TiO₂ nanorods was much smaller than the pore size of the template, which was attributed to the densification and lateral shrinkage of the template during the annealing process.

Figure 2: (a) FESEM image of the template (anodic alumina membrane) [20], (b) FESEM image of the synthesized TiO₂ nanorod arrays [20], (c) FESEM image of anatase TiO₂ nanorods [21], and (d) FESEM image of TiO₂ nanorod film [22].

Wu and Yu synthesized a novel type of well-aligned rutile and anatase TiO₂ nanorods via a template-free metal-organic CVD method [21]. As shown in Figure 2(c), TiO₂ nanorods were grown directly on a silicon substrate at a temperature of 500–700°C. The single-crystalline rutile and anatase TiO₂ nanorods were formed at 630°C and 560°C, respectively, which indicated that the temperature was a key factor in the synthesis process. The disadvantage of this method is the complicated synthesis process, which is not suitable for production in large-scale.

Hydrothermal method is facile for the synthesis of 1D nanostructured TiO₂ [22]. Feng et al. [22] developed a new type of nanorod film on glass substrates via a low-temperature hydrothermal process. As shown in Figure 2(d), the nanorods have diameters of 30–60 nm and they have uniform orientation. Furthermore, the authors found that the surface of the synthesized film has switchable wettability which transferred from superhydrophobicity to superhydrophilicity under UV light irradiation. This phenomenon can be explained by the reaction between the photogenerated holes and the lattice oxygen, leading to the formation of surface oxygen vacancies.

TiO₂ nanorods have been widely used in PCO process for the degradation of organics [24]. For example, organic dyes and acetone can be effectively degraded into and . Melghit and Al-Rabaniah [24] prepared rutile TiO₂ nanorods using a sol-gel method at room temperature, and the material exhibits excellent photodegradation of Congo red under sunlight. It can be explained by two aspects. (1) The Congo red is easily absorbed onto the synthesized TiO₂ nanorods, and, subsequently, the dye decomposed in a PCO process. (2) Organic dyes are capable of photosensitizing TiO₂ because of the absorption of visible light [25]. The photosensitization was considered as another way for the degradation of dyes in the presence of TiO₂ under sunlight irradiation. It was worth noting that the synthesized rutile TiO₂ nanorods which possess optimum size and shape are much more efficient for PCO process than the anatase TiO₂ [24]. Yu and coworkers [26] synthesized TiO₂ nanostructures using TiF₄ and H₃BO₃ as the precursors. The morphology characterization revealed that the synthesized material is a combination of TiO₂ nanorods and nanotubes. It is important to notice that the synthesized TiO₂ exhibited a higher PCO efficiency for the degradation of acetone than that of P25 due to the larger specific surface area and pore volume of the synthesized TiO₂. Moreover, compared with the nanosized powder photocatalysts, the prepared TiO₂ is easier to be separated from aqueous solutions after PCO process due to its long structure, and it also possesses higher photocatalytic activity. In addition, the photocatalytic activity of TiO₂ nanorods can be further improved via a thermal treatment because of the enhanced crystallization [27].

In order for facile recycling, TiO₂ nanorods have been coated onto substrates. A successful example is that well-aligned TiO₂ nanorod can be prepared on pretreated quartz substrate [28]. The quartz substrate was precoated with a thin rutile TiO₂ seed layer to facilitate the subsequent growth of the rutile TiO₂ nanorods. Owing to the excellent nucleation and growth properties, rutile TiO₂ crystal seeds are more preferable than anatase TiO₂. Thus, the pretreatment of substrates is important for the synthesized TiO₂ morphology, which would also affect the performance of the material in a PCO process. Based on the experimental results, the density of TiO₂ seeds can be controlled by the concentration of the coating colloid solution. Then the TiO₂ crystal seeds will affect the growth density, diameter distribution, and growth morphologies of TiO₂ nanorods. When the density of TiO₂ seeds is high, the seeds tend to merge together to form larger TiO₂ particles. It is shown that the degradation rates of methyl blue increased with large growth density and small diameter size of TiO₂ nanorods.

Pure TiO₂ is not an effective visible-light photocatalyst due to its wide band gap (>3.0 eV) and can be activated only by UV light at  nm. Modification of TiO₂ has been explored to extend the absorption spectra of photocatalysts to visible light range [29]. Doping of TiO₂ with various metals or nonmetals has been considered as a valid way to lower the band gap of TiO₂ and thus make the photocatalyst more active under solar light. Doping of TiO₂ would introduce allowed energy states within the band gap but very close to the energy band. The gap between the energy states and the nearest energy band is usually reduced. Thus, the electrons and holes would be more easily excited under visible light irradiation. Kerkez and Boz [30] used Cu²⁺ as a dopant to modify TiO₂ nanorod array films (Figure 3(a)), and they found that the methylene blue degradation efficiency under visible light was increased 40% with respect to the efficiency of the unmodified sample. The notable improvement could be explained by the following factors. (1) acts as a trap of photogenerated electrons. The electrons were transferred from TiO₂ to the conduction band of the CuO with little chance to return and thus increase the life time of . Thus, the presence of could retard the recombination rate of electrons and holes on the surface of the synthesized -TiO₂. (2) The band gap of -TiO₂ nanorods is lower than that of the original TiO₂, which can extend the photoresponse of the photocatalyst and make the material utilizable under both UV light and visible light.

Figure 3: (a) SEM image of Cu²⁺-TiO₂ nanorods, (b) TEM image of N-TiO₂ nanorods [31], (c) FESEM image of N-F-TiO₂ nanorods (inset: its cross section) [32], and (d) TEM image of Au-loaded TiO₂ hollow nanorods [33].

TiO₂ nanorods with 3% of nitrogen doping were prepared by Lee and coworkers, as shown in Figure 3(b) [31]. Although the N-doping process did no change the morphology of TiO₂ nanorods, it provides extra occupied states above its valence band which may enhance the photocatalytic activity. Lv et al. also reported the synthesis of N-F-doped visible light active TiO₂ nanorods via a liquid phase deposition process (Figure 3(c)) [32]. They firstly synthesized ZnO nanorod arrays on glass substrates and then combined the ZnO nanorod arrays and TiO₂ via an aqueous solution method to get the as-prepared TiO₂ nanorods. Their experimental results indicated that the doping quantity of N and F in the resultant material could be easily controlled by adjusting the calcination temperature, and the optimal temperature was found to be 450°C. Owing to the higher visible light photocatalytic activity, the obtained TiO₂ nanorods’ films exhibited higher degradation rate for methylene blue as compared to P25 films [32].

Metals have been incorporated into TiO₂ nanorods to form nanocomposites with a heterojunction structure. For example, TiO₂ nanorods can be coated by Au nanoparticles to form a novel heterojunction, as shown in Figure 3(d) [33]. As a model organic pollutant, methylene blue can be used to characterize the photocatalytic activity of the synthesized photocatalysts. 35% of methylene blue was degraded in the presence of Au-TiO₂ nanorods, which is much more effective than that of 15% for pure nanorods. In addition, the other metals such as Ag and Cu can also be coated onto TiO₂ nanorods [34, 35]. The synthesized nanocomposites exhibited outstanding photocatalytic activity as compared to the pure TiO₂. The enhancement in photocatalytic activity is related with the slow recombination rate of charge carriers. During PCO process, the generated electrons from TiO₂ nanorods could transfer to Au nanoparticles, leading to the longer lifetime of the pairs and therefore more reactive oxygen species produced for the degradation of organic pollutants in water. The mechanism of photocatalytic degradation of organic pollutants over metal/TiO₂ under UV light irradiation is shown in Figure 4.

Figure 4: The mechanism of photodegradation of organic pollutants over metal/TiO₂ in water under UV light irradiation.

3. TiO₂  Nanotubes

Currently, TiO₂ nanotube structures have been successfully synthesized and applied for organic pollutants degradation [36]. Hoyer [37] firstly reported the synthesis of TiO₂ nanotubes via an electrochemical deposition in a porous aluminum oxide mold. Then, an electrochemical anodic oxidation method was developed by Zwilling et al. for the synthesis of TiO₂ nanotubes [38]. Although this method is very facile to synthesize TiO₂ nanotubes with controllable pore size, good uniformity, and conformability over large areas, the high cost of fabrication apparatus and complicated operation limited its further application [39, 40]. Templating method was considered as a suitable technology to construct materials with desirable morphology [41]. TiO₂ nanotubes can be synthesized in controlled sol-gel hydrolysis of solutions containing titanium compounds and templating agents followed by polymerization or deposition of TiO₂ on the template. For example, Peng and coworkers [42] fabricated TiO₂ nanotubes via a surfactant-mediated templating method, and the fabrication process can be summarized in Figure 5. In the synthesis process, a sol-gel method was conducted for fabricating the material, and laurylamine hydrochloride (LAHC) was used as a template. Titanium alkoxide was first hydrolyzed with the addition of tetra-n-butyl-orthotitanate (TBOT), and then there is an interaction between partially charged hydrolytic species and laurylamine surfactant by H-bonding forces (Figure 5(a)). The edge part of the bilayer assemblies of LAHC was unstable and apt to combine with the interlayers, which lead to the enlargement of the layers (Figure 5(b)). Then the bilayer-like aggregates rearranged into lamellar-like liquid-crystal phases through the condensation reaction (Figure 5(c)). After the interlayer combination and crosslinking between adjacent hydrolyzed titanium alkoxide species, a mixed lamellar liquid-crystal phase membrane formed, and rodlike micelles were separated by bilayers of surfactant and water (Figure 5(d)). Upon the addition of TBOT, charge imbalance leads to the curvature of the membrane along one direction (Figure 5(e)) and then bends fully the membrane into tubules (Figure 5(f)). Moreover, the condensation of the hydrolyzed titanium species would lead to the rodlike micelles in a random arrangement (Figure 5(g)). The synthesized TiO₂ nanotubes possess a hierarchical tubules-within-tubules structure with cylindrical nanochannels walls. This structure is a combination structure of microtube and nanotube, causing the formation of porous structure. Moreover, the specific surface area of the material is also increased. The structure would be beneficial for catalysts and thus enhance the PCO efficiency. Furthermore, the scale of the synthesized TiO₂ nanotubes can be controlled by adjusting the morphology of the template [41]. However, the instability of the TiO₂ nanotubes synthesized by this method is a big issue, and the tube morphology is easily destroyed [39, 40, 43].

Figure 5: Proposed formation processes of the microtubules. (a) Globular aggregates containing LAHC and hydrolyzed TBOT. (b) Aggregates enlarged in size. (c) and (d) Mixed lamellar liquid-crystal phase membrane containing rodlike micelles. (e) Charge imbalance resulting in membrane curvature. (f) Further bending into tubules. (g) Random and segregated arrangement of rod micelle layers which are separated by bilayers of LAHC from bulk solution [42].

Recently, hydrothermal method has been widely used for the synthesis of high quality of TiO₂ nanotubes with diameter of about 10 nm [44]. Crystalline TiO₂ nanoparticles and highly concentrated NaOH are normally used as the precursors in a typical hydrothermal process [41]. As a necessary step, the drying or calcination process is generally conducted, leading to the transformation of titanate nanotubes to TiO₂ nanotubes. Based on previous reports [39, 45, 46], the advantages of hydrothermal process for TiO₂ nanotubes synthesis can be concluded as the following. (1) It is suitable for large scale production; (2) the modification process of TiO₂ nanotubes can be directly conducted in the synthetic system; (3) nanotubes with ultrahigh aspect ratio can be synthesized. However, hydrothermal process requires high temperature and pressure, as well as the long reaction time, which also cause a high cost. Figure 6 lists various TiO₂ nanotubes synthesized by different methods.

Figure 6: SEM image of TiO₂ nanotubes synthesized via an electrochemical deposition (a) [37] and templating methods (b) [42]; (c) FESEM (inset) and TEM images of TiO₂ nanotubes synthesized via a hydrothermal method [74].

TiO₂ nanotubes can be used for the degradation of POPs such as benzene and acetaldehyde [47, 48]. Yuan et al. [49] studied the performance of TiO₂ nanotubes for water treatment. The synthesized TiO₂ nanotubes showed complete photodegradation of humic acid in comparison to the 97.7% removal efficiency of TiO₂ P25. Moreover, the TiO₂ nanotubes can be totally separated and recovered via a membrane filtration. The stability test presented that no catalyst deactivation was observed after five consecutive PCO experiments of newly added humic acid. Yuan et al. [49] degraded humic acid using an enhanced photocatalytic process with Al and Fe codoped TiO₂ nanotubes as photocatalysts. They reported that calcination temperature, doping ions, and dosage of dopant would impact the PCO efficiency. Under an optimal condition of 1.0% codoped TiO₂ nanotubes containing 0.25 : 0.75 of Al : Fe, 79.4% of humic acid was degraded and a pseudo-first-order rate constant of 0.172 min⁻¹ was achieved. Bisphenol A (BPA) is a pervasive chemical intermediate primarily from the production of polycarbonate plastics and epoxy resin [50]. Over the past few years, considerable effort has been devoted to the development of effective treatment technologies of the removal for bisphenol A (BPA), such as Fenton’s reagent, ultrasonic cavitation, photocatalysis, and ozonation [51]. Recent research [52] displayed that a nearly complete removal of BPA was observed by Cu doped TiO₂ nanotubes. The pseudo-first-order rate constants for BPA photodegradation by Cu doped TiO₂ nanotubes at pH 7.0 were 2–5 times higher than that of pure Degussa P25. In a typical process, the generated electrons from TiO₂ with a lower conduction band could recombine with the holes in Cu, resulting in the reduction of recombination rate of electrons and holes. Then, the holes and electrons reacted with water and oxygen to form peroxyl and hydroxyl radicals, respectively. In a supposed BPA degradation process, BPA radicals were generated via an electron transfer process. Subsequently, the BPA radicals triggered a suite of reactions of radical coupling, fragmentation, substitution, and elimination, which eventually resulted in degradation of BPA [53]. However, further study is needed to prove this mechanism. BPA molecules were firstly photodegraded into some intermediates and products with smaller molecular weight, and these intermediates can be further oxidized to and by the oxidative species produced in the PCO process. N-doped TiO₂ nanotubes also showed enhanced photocatalytic activity. Based on the research work of Chen and coworkers [54], 72.5% and 89.4% of methyl orange are photodegraded in the presence of TiO₂ nanotubes and N-doped TiO₂ nanotubes, respectively. In addition, the degradation rate of methyl orange over TiO₂ nanotubes without calcination is only 17.1%, which is lower than that (45.1%) of TiO₂ nanotubes calcined at 200°C. This is due to the narrow band gap and good crystallinity [55]. However, the photocatalytic activity of the TiO₂ nanotubes calcined at 400°C decreased, which can be attributed to the agglomeration and sintering damage of nanotubes caused by calcination at high temperature [56]. The specific surface area of the calcined material may also decrease due to the destruction of the nanotube structure.

4. TiO₂ Nanowires/Nanofibers

TiO₂ nanowires/nanofibers are common nanostructures of TiO₂, as shown in Figure 7. Fujishima et al. [5] prepared two kinds of TiO₂ nanowires (TNW₁₀ and TNW₂₀) for the degradation of humic acid. The investigation of photocatalytic activities of the synthesized materials showed that TNW₁₀ performed better than TiO₂ P25 while TNW20 was as good as P25. This is due to the high ratio aspect of the synthesized TiO₂ nanowires. Owing to the incomplete degradation, the degradation rates of total organic carbon content are lower than the removal rate of humic acid. This problem can be addressed by extending reaction time. Furthermore, the nanowires can be totally separated by a commercial microfiltration membrane with negligible membrane fouling. Piril and coworkers [57] studied the photocatalytic degradation of butanol in aqueous solutions using commercial TiO₂, N-Pt-doped TiO₂ nanofibers, and N-Pd-doped TiO₂ nanofibers. The experimental results showed that the N-Pd-doped TiO₂ nanofibers had high efficiency in the degradation of butanol under UV irradiation as compared to the N-Pt-doped TiO₂ nanofibers and the commercial TiO₂. It probably has relationship with the proton formation caused by better radical formation ability. GC-MS analysis revealed that butanol converted to some decomposition products such as aldehydes and ethanol [58]. Moreover, the synthesized materials had very similar BET surface area whereas they expressed relatively different reaction rate of the photocatalytic degradation, and it can be concluded that the specific surface area of the photocatalysts is not a key factor for the photocatalytic efficiency, while the doped metals played an important role in promoting the PCO reaction. In addition, doped TiO₂ nanofibers can be synthesized by electrospinning followed by calcination process [59]. The different dosage of would affect the photodegradation rate of methylene blue. For example, 96.1% of methylene blue was removed in the presence of TiO₂ nanofibers with 2 at.% , which was considered as an optimum doping dosage. Furthermore, the synthesized nanofibers were successfully recycled and reused for five times with little photocatalytic activity reduced. TiO₂ nanowires can also combine with other nanostructures, which can enhance the photocatalytic activity. Fe₂O₃ nanoparticles have been grafted onto TiO₂ nanowires by Qin and coworkers via a facile impregnation–solvothermal method [60]. The synthesized heterojunctions exhibited remarkable photocatalytic activity for photocatalytic oxidation of Direct Red 4BS in the presence of H₂O₂. Moreover, the material showed good tolerance with respect to organic matter poisoning due to the synergetic effect of TiO₂ nanowires and Fe₂O₃ nanoparticles. The size of Fe₂O₃ also affected the performance of the material, and it can be controlled by adjusting the impregnation duration time in the synthesis process. The possible reaction pathways are shown as follows [60]:

where the 4BS stands for the dye molecule and the stands for the produced intermediates.

Figure 7: (a) FESEM of TiO₂ nanowires synthesized via a hydrothermal process [5], (b) FESEM of TiO₂ nanowires synthesized via an electrospun process [63].

The heterojunction of TiO₂ nanofibers and nanoparticles can promote the separation of photogenerated e⁻ and . The conduction band of is more active as compared to that of TiO₂, leading to the electrons transfer from TiO₂ to and the further conversion to . Since promotes the decomposition of to OH [61], the photocatalytic activity was enhanced in this charge transfer process.

Liu and coworkers [62] prepared another kind of core-shell heterojunctions (TiO₂-B nanowires and anatase nanocrystals) using a combination of hydrothermal and calcination methods. Similarly, the charge recombination rate was suppressed. The synthesized material tends to separate the and into two different regions of the catalyst and thus enhances photocatalytic efficiency [63].

Recently, TiO₂ nanowire membranes were fabricated and applied for organics degradation in water purification process due to their photocatalytic activity, excellent chemical resistance, and thermal stability [64–66].

Microfiltration and ultrafiltration membranes have been successfully synthesized by Zhang and coworkers. The membranes were fabricated by TiO₂ nanowires with different diameters. In a typical synthesis process, The TiO₂ nanowires’ suspension was first filtered via a vacuum filtration setup to form a porous functional layer. After drying at 105°C, a free-standing membrane can be peeled off from the filter. Finally, the membrane was pressurized under 5 bar at 120°C via a hot-press before being calcined at 550°C. As shown in Figure 8(a) [64], the synthesized membranes were robust and flexible and they possess multifunctions. In continuous experiments, the synthesized TiO₂ microfiltration nanowire membrane achieved near 100% and 93.6% removal rate of humic acid and total organics carbon, respectively. It is important to notice that the transmembrane pressure of the nanowire membrane did not change under UV light irradiation (Figure 8(b), [66]), indicating the antifouling and self-cleaning property of the synthesized TiO₂ nanowire membrane. The organic pollutants can be degraded concurrently during the filtration process. For a TiO₂ nanowire ultrafiltration membrane, the membrane showed a higher separation ability as well as excellent photocatalytic activity due to the enhanced selectivity. The rejection rate of humic acid by the ultrafiltration membrane can be achieved till 65% without UV irradiation, and even the bacteria (E. coli) can be intercepted by the membrane. In addition, the synthesized membranes were capable of overcoming the polymeric membrane problems such as membrane fouling and high-temperature applications. The antifouling property would facilitate the regeneration of the membrane and thus lower the cost for membrane cleaning.

(a)

(b)

Figure 8: (a) TiO₂ nanowire membrane. (b) The transmembrane pressure change of TiO₂ nanowire membrane during filtration: (i) with UV irradiation and (ii) without UV irradiation.

TiO₂ nanowires membrane can also apply for the degradation of pharmaceuticals in water [65]. The membrane was directly fabricated by hydrothermal growth on Ti substrates at 180°C with the assistance of some organic solvents. Experimental details revealed that various pharmaceuticals such as norfluoxetine, atorvastatin, lincomycin, and fluoxetine were almost completely removed in a concurrent filtration and PCO process.

5. TiO₂  Nanobelts

TiO₂ nanobelts can be synthesized via CVD, solvothermal, and hydrothermal methods [67–69]. Gao and coworkers [69] reported the self-catalytic growth of codoped TiO₂ nanobelts via a metallorganic CVD method, and they found that rutile structure is dominant in the synthesized material. Furthermore, the material exhibited a magnetic anisotropy with a high coercive field value at room temperature. This property may facilitate the separation of the synthesized material after PCO process.

A solvothermal process was carried out to synthesize nitrogen-fluorine codoped TiO₂ (N-F-TiO₂) by He et al. [67]. Amorphous titania microspheres were used as precursors, which were prepared by the hydrolysis of Ti(OBu)₄. The synthesized N-F-TiO₂ nanobelts showed higher photocatalytic activity for the degradation of methyl orange as compared to that of TiO₂ P25. As shown in Figure 9, the codoped nanobelts photocatalyst exhibited the highest decrease in COD values for the degradation of organic compounds under both visible light and UV irradiation. This phenomenon can be attributed to the porous structures of the synthesized N-F-TiO₂, the increased specific surface area, and enhanced light adsorption. The N-F doping induced oxygen vacancy and led to the red shift in optical energy gap. Moreover, the nanobelt structure of the synthesized material would facilitate the capture of photogenerated photons and thus promote the formation of .

(a)

(b)

Figure 9: COD removal of the methyl orange solution after the photocatalytic degradation under UV and visible light irradiations [67].

Hydrothermal methods are more commonly used for TiO₂ nanobelts synthesis [70–72]. A commercial P25 TiO₂ can be used as a precursor, which is subsequently hydrothermally treated for nanobelts synthesis, as shown in Figure 10(a) [70]. However, the photocatalytic activity of the synthesized TiO₂ nanobelts was lower than that of P25. The degradation rates of methyl orange are 35% and 95% in the presence of TiO₂ nanobelts and P25, respectively. To further enhance the performance of the material for organics degradation, NiO nanoparticles were successfully deposited onto the nanobelts, as shown in Figure 10(b). The synthesized materials were applied for the decomposition of methyl orange under ultraviolet light irradiation. Experimental results indicated that heterojunction NiO-TiO₂ nanobelts showed better performance for the methyl orange removal as compared to both pure NiO nanoparticles and TiO₂ nanobelts. It can be attributed to the increased specific surface area and extended e⁻-h⁺ lifetime.

Figure 10: (a) SEM image of TiO₂ nanobelts synthesized via a hydrothermal process [70], (b) SEM image of NiO/TiO₂ heterojunctions [64], (c) SEM image of the Cu₂O/TiO₂ heterojunctions [71], and (d) TEM image of the synthesized CeO₂/TiO₂ nanobelt heterojunction [72].

Similar with this kind of heterojunction, other meal oxides nanoparticles and TiO₂ nanobelts can also be combined together for the improvement of photocatalytic activity. Zhang et al. prepared Cu₂O/TiO₂ heterojunctions using a combination process of hydrothermal method and chemical precipitation method [71]. The TiO₂ nanobelt acted as a substrate, and the dosage of Cu₂O can be controlled by adjusting the concentration of the precursor (copper sulfate). Figure 10(c) shows that Cu₂O nanoparticles were evenly grafted onto the TiO₂ nanobelts. Moreover, the synthesized heterojunction possessed higher adsorbability as compared to pure TiO₂ nanobelt. Although the photocatalytic activity of the synthesized material showed a little decrease, the decoloration activity of the material for organics was enhanced. This phenomenon is attributed to the strong adsorption of methyl orange molecules on the surface of Cu₂O with exposure of facets.

Tian et al. synthesized novel CeO₂/TiO₂ nanobelt heterojunctions via a facile hydrothermal process, as shown in Figure 10(d) [72]. Compared with TiO₂, the synthesized CeO₂/TiO₂ exhibited enhanced photocatalytic performance for the degradation of methyl orange due to the proposed capture-photodegradation release mechanism. During PCO process, methyl orange molecules were captured by CeO₂ nanoparticles, then degraded by photogenerated radicals, and finally released to the solution. Since the PCO process is mainly conducted at the surface of photocatalysts, the adsorption capacity of CeO₂ nanoparticles was found to be important for the performance of PCO process. The synthesized CeO₂/TiO₂ nanobelt heterojunctions possessed good stability, high activity, and recyclable properties.

TiO₂ nanobelts have been applied to photocatalyze the oxidation of pharmaceutical contaminants in wastewater. Liang and coworkers [73] prepared anatase phase TiO₂ nanobelts with 30–100 nm in width and 10 μm in length via a high temperature hydrothermal method. Their results showed that persistent pharmaceuticals such as malachite green, naproxen, carbamazepine, and theophylline can be efficiently photodegraded in the presence of the synthesized TiO₂ nanobelts. The photogenerated active oxygen species such as hydroxyl radical, , and hydrogen peroxide were determined and it was proven that they played an important role in the PCO process. Investigation on operation parameters presented that photodegradation of the pharmaceuticals was evidently dependent on pH, illumination time, temperature, and concentrations of contaminants. This work heralds a pathway towards the photodegradation of organics in actual wastewater. TiO₂ nanobelts can have potential in applications of industrial wastewater.

6. Conclusions

This review paper overviews the recent development in synthesis of 1D TiO₂ nanostructures and their photocatalytic applications for organics degradation. Thanks to the development of nanotechnology, significant progress has been achieved in controllable synthesis of 1D TiO₂ materials with different aspect ratio and inner structure (solid nanowire, nanorod, nanobelt, and hollow nanotube). Various synthetic strategies have been developed, prompting the subsequent exploration of their photocatalytic properties and the practical environmental applications. Under the excitation of UV light, efficient degradation of organic pollutants, such as NOMs, POPs, dyes, phenols, and pharmaceuticals, has been achieved by numerous groups. On contrast, solar-light-active 1D TiO₂ material is still in its infant stage. Since solar energy is economical, solar photocatalysis using 1D TiO₂ nanostructures would be a promising pathway for the degradation of organic pollutants. It is important to note that the application of 1D TiO₂ in industry cannot be successful without solar energy assistance. Thus, many future works should concentrate on the synthesis of solar-light-active TiO₂ materials. Doping of 1D TiO₂ is considered as a possible way to lower the band gap of TiO₂ and thus enhance the activity of TiO₂ under solar light. Moreover, sensitization of 1D TiO₂ nanostructures with other narrow-band-gap semiconductor and noble metals (e.g., Au or Ag) may further enhance the photocatalytic activity by suppressing the charge recombination rate.

Alternatively, 1D nanostructured TiO₂ materials can be used as building blocks to assemble active and integrated nanosystems, such as TiO₂ nanorod, nanotube, or nanowire membrane or film. The coupling of semiconductor photocatalysis of 1D nanomaterials and engineering design such as self-cleaning membrane filtration and microreactor construction might significantly broaden the application range of 1D TiO₂ photocatalysts. Rapid progress is expected and will mainly occur in developing economic and scalable synthetic strategy of 1D TiO₂ nanostructures and their industry-scale applications in water treatment process.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

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