International Journal of PhotoenergyVolume 2012 (2012), Article ID 702940, 11 pagesdoi:10.1155/2012/702940

Review Article

A Review on Nanotube Film Photocatalysts Prepared by Liquid-Phase Deposition

Jinshu Wang, Qian Cai, Hongyi Li, Yuntao Cui, and Hong Wang

School of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, China

Received 15 September 2011; Revised 18 November 2011; Accepted 18 November 2011

Academic Editor: Jiaguo Yu

Copyright © 2012 Jinshu Wang 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.

TiO₂ nanotube film is a promising photocatalyst associated with its unique physical and chemical properties such as optic, electronic, high specific surface area. Liquid-phase decomposition provides a feasible way for the preparation of functional thin film. This paper reviews and analyzes the formation mechanism of TiO₂ nanotube film by liquid phase deposition. The effect of preparation parameters, such as the kinds of electrolyte solution for the preparation of anodic alumina template, volume fraction of Al₂O₃ on the template, the concentration of the deposition solution, and heat treatment, on the formation of TiO₂ nanotube film has been analyzed. The effects of doping of metallic and nonmetallic elements on the photocatalytic activity of TiO₂ nanotube have been discussed.

1. Introduction

Nowadays, humans are more and more concerned about environmental issues. Large amounts of organic pollutants are being released into the ecosystem over the past few decades and they cause a serious threat to the environment [1]. Researchers all over the world have been working on various approaches to address the issue. In the past decades, the traditional physical techniques, such as adsorption, biological treatment, coagulation, ultrafiltration, and ion exchange on synthetic resins, have been adopted for the removal of organic pollutants from wastewaters [2]. However, these methods might cause secondary pollution, and degradation of organic pollutants is usually incomplete and selective.

Since the discovery of photocatalytic splitting of water on TiO₂ electrodes by Fujishima and Honda in 1972 [3], heterogeneous photocatalysis has attracted much attention as a new purification technique for air and water [4–6]. Titania nanostructures have been widely investigated for applications in optical devices [7], gas sensors [8], and dye-sensitized solar cells [9]. Titania semiconductor photocatalysts have demonstrated advantages such as transparency [10], wide bandgap [11], biological and chemical inertness [12, 13], strong oxidizing capability, and nontoxicity [14]. So titania exhibits good performance on the degradation of organic pollutants under ultraviolet radiation. Many other photocatalysts such as CdS, WO₃, and SrTiO₃ also exhibit a certain photocatalytic activity [15–17].

Crystalline titania has many morphologies such as nanofibers, nanoparticles, nanorods, nanospheres, nanotubes, and nanowires [18–20]. Titania nanotubes are highly efficient in photocatalysis since titania nanotubes have a relatively higher interfacial charge transfer rate and surface area compared with the spherical TiO₂ particles [21]. Many approaches have been developed for the preparation of TiO₂ nanotubes, that is, chemical vapor deposition (CVD), anodic oxidation, seeded growth, the wet chemical (hydrothermal method and the sol-gel method) [22–24], and liquid-phase deposition of templates. Among these methods, liquid-phase deposition (LPD) of template method is one of the simplest and most practical one to fabricate TiO₂ nanotubes, since it has so many advantages such as low cost, mild reaction condition, simple equipment requirement and allows TiO₂ films to be deposited over large areas.

2. Liquid-Phase Deposition of Template Method: Formation of Titania Nanotubes

A variety of oxide nanohole sheets have been prepared by a liquid-phase deposition method. The liquid-phase deposition can be applied readily to the preparation of thin films on various types of substrates with large surface areas and a variety of morphologies, since the LPD is performed in an aqueous solution. In the LPD process, using anodic alumina template as a scavenger and starting materials, the metal oxide films can be fabricated by only one-step reaction. The liquid-phase deposition of template method consists of two major steps: (a) preparation of anodic aluminum oxide templates; (b) liquid-phase deposition.

Anodic alumina oxide (AAO) has been used as template for the fabrication of several kinds of functional devices with nanometer dimensions due to its unique structure, such as controllable pore diameter, extremely narrow pore size distribution, and ideally cylindrical pore shape [25–30]. Since the diameter of the cylindrical pores of the templates can be varied between 10 and 200 nm by altering the preparation conditions, the templating method has a number of interesting and useful features for the production of microscopically tailored materials [31]. The AAO films with an ordered array of holes [32–36] can be used as templates for the preparation of nanotubes, nanowires [37–42], nanodots, and nanopillars [43–45] as well as micro-electromechanical systems (MEMSs) devices [46]. The AAO films with pore diameter ranging from 4 to 250 nm, density as high as 10¹¹ pores/cm⁻¹, and film thickness varying from 0.1 to 300 μm, have been realized [37, 38]. The anodic aluminum oxide template has been widely applied in the preparation of various nanomaterials. Highly ordered polycrystalline Si nanowire arrays were synthesized with porous anodic aluminum oxide templates by chemical vapor deposition [47].

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Yamanaka et al. have prepared titania nanotubes successfully with anodic aluminum oxide (AAO) templates as precursor by liquid-phase deposition method [48] and the morphology of the AAO templates and titania nanotubes formed is shown in Figure 1. The AAO templates have many pores and their mean sizes are approximately 200 nm (Figure 1(a)), and titania nanotubes formed keep the morphology of AAO with nearly the same size (Figure 1(b)). The titania nanotubes array which consist of many grains with approximately 20 nm in diameter have the thickness as high as about 50 μm, as shown in Figures 1(c) and 1(d). In the past, AAO templates had to be stripped from the aluminum substrates and removed the barrier layers. However, the AAO templates made in this way are brittle, so it is limited to fabricate large area nanotube array. Jiang et al. prepared titania nanotubes in a large scale by LPD method using AAO templates without stripping from the Al substrates [49], which provided a simple and feasible way for the preparation of the TiO₂ films in a large scale.

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Figure 1: FE-SEM photographs of the titania nanohole arrays: (a) surface morphology of anodic alumina as starting material, (b) surface morphology of titania nanohole array, (c) cross-section of titania nanohole array, and (d) cross-section detailing (c) [48].

3. Liquid-Phase Deposition of Template Method: Formation Mechanism of Titania Nanotubes

3.1. Formation Mechanism of AAO Template

The titania nanotubes are synthesized in situ on the anodic aluminum oxide templates. So the morphology of the titania nanotubes is correlated with the morphology of AAO templates. There are two types of aluminum oxide films prepared by anodizing the aluminum: barrier type and porous type [50]. The barrier-type AAO template is formed by anodizing Al foil in the electrolyte solution such as citric acid, boric acid, and glycolic acid in which Al has very low solubility. However, the porous-type AAO template can be formed by anodizing in the electrolyte solution such as sulfuric acid, oxalic acid, and phosphoric acid in which Al has better solubility. Taking oxalic acid as an example, the reaction equations can be written as below [51].

Cathode Reaction:

Anode Reaction:

At the beginning of the anodizing process, the amorphous oxide is formed. Oxide is dissolved with the electric field enhancement, leading to the formation of porous oxide.

3.2. Formation Mechanism of Titania Nanotubes

Imai et al. first reported that crystalline titania films could be deposited from aqueous solutions of TiF₄ at PH1-3 and the hydrolysis of TiF₄ in solution occurred in a stepwise manner to produce titania [31]:

Titania forms on the inner walls of the nanochannels of the alumina template through heterogeneous nucleation and then grows into nanotubes which keep the porous morphology of alumina.

Using anodic alumina films as the templates, the following chemical reactions might take place: Therefore, TiO₂ can be formed by the hydrous reaction of accompanied by an F⁻ consuming reaction, where Al₂O₃ is taken as scavenger for . The equilibrium reaction (8) is shifted to the right side by the reaction of , H⁺ and , since reacts readily with ions to give the more stable . The product deposits in situ on anodic alumina template. We present an illustration for the formation of titanium hydrous oxide nanotubes array films [52], as shown in Figure 2. When the AAO templates are immersed into (NH₄)₂TiF₆ solution, the hydrolysis of reaction of takes place at the surface. Hydrous titanium oxide deposits in situ on the AAO templates, accompanied with the consumption of AAO templates, as shown in Figure 2(b). Hydrous titanium oxide is formed from the surface to the inner part of AAO template with the prolonging of the reaction time (Figure 2(c)). After calcinations, TiO₂ nanotubes can be formed.

Figure 2: Schematic diagram of the formation of the titania nanohole array: (a) before deposition, (b) during deposition, and (c) after deposition [52].

Kanawori et al. described the formation mechanism by the addition of boric acid [43] which is used as scavenger for ion. The reactions are shown below.

Boric acid promotes reaction (10) to the right side by consuming , leading to the formation of hydrous titanium oxide.

4. Factors Influencing the Formation and Photocatalytic Activity of Titania Nanotube Film

There are several factors which affect the formation of titania nanotubes: (a) the electrolyte solution used to prepare AAO templates (such as phosphoric acid, oxalic acid, and sulfuric acid), (b) oxidation temperature, (c) anodizing voltage, (d) oxidation time, (e) the concentration of the deposition solution, (f) deposition temperature, (g) deposition time, and (h) the temperature of heat treatment. These factors can be summarized as (1) the preparation parameters of AAO template, (2) liquid-phase deposition parameters, (3) Heat treatment. The characteristics and morphology of titania nanotubes such as specific surface area, film thickness, crystal structure and others are dependent on the selected above-mentioned conditions.

4.1. Effect of Preparation Parameters of AAO Template on the Morphology of TiO₂ Nanoarrays

The AAO templates are prepared with a modified two-step anodization process. The pore size, pore density, and the thickness of the AAO template are approximately proportional to the anodization voltage, but the quality of the ordering in the AAO structure also highly depends on the first anodization time [34]. Zhang et al. anodized aluminum in a 0.3 M oxalic acid solution with outer voltage of 40 V at 0°C for different time. After removing the preliminary oxidization layer, the second anodization was carried out at 0°C for about 1 h under the same conditions as the first anodization step. Afterward, the AAO template was immersed into a 5 wt% phosphoric acid solution to widen the nanochannels. The outer diameter of the AAO pore is 180 nm, and the wall thickness of the AAO pore is 55 nm [53]. Jiang et al. used 10% phosphoric acid instead of oxalic acid to oxidize aluminum with outer voltage of 120 V at room temperature for 1 h. Then the aluminum was immersed in the 1.8% H₂CrO₄ : 6% H₃PO₄ = 1 : 1(volume ratio) mixed solution to remove preliminary oxidization layer. The second anodization step was the same as the first anodization step but for 4 h [49]. After widening the nanochannels, the AAO templates were obtained with larger aperture than the AAO templates made in oxalic acid. We also find in our work that the anodic alumina templates prepared in oxalic acid have relatively small pore diameter, as shown in Figure 3(a). The mean outside diameter and inside diameter of that prepared in oxalic acid are about 100 nm and 25 nm, respectively. On the other hand, the templates prepared in phosphoric acid have large pore diameter, as shown in Figure 3(b), that is, the mean outside diameter of the tubule and inside diameter are about 250 nm and 150 nm, respectively. The pores in all the templates are uniform and arranged regularly. The pore size formed in the membrane is related to the anodizing voltage applied in the electrolyte. It is reported that the maximum anodizing voltage applied in the electrolyte is in the order phosphoric acid > oxalic acid. The anodizing process will be blocked when a voltage higher than is employed. To obtain the template with large pores, high anodizing voltage should be applied in the electrolyte; therefore, anodic alumina template prepared with phosphoric acid has the larger pore size.

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Figure 3: FE-SEM of titanium oxide nanotubes array films: (a) AAO membrane (oxalic acid), (b) AAO membrane (phosphoric acid), (c) surface morphology of titanium oxide nanotubes array films (oxalic acid), and (d) surface morphology of titanium oxide nanotubes array films (phosphoric acid) [54].

We have studied the relationship between volume fraction of in the template and the morphology of TiO₂ nanoarray [54]. When the volume fraction of in the template is more than 0.71, such as the template prepared in oxalic acid, the ordered aligned titania nanorods can be obtained (Figure 3(c)) whereas the ordered aligned titania nanotubes can be synthesized when the volume fraction of in the template (e.g., the template prepared in phosphoric acid) is less than 0.71, as shown in Figure 3(d). Therefore, the morphology of TiO₂ nanoarray depends on the volume fraction of in the template which can be controlled by the preparation parameters of AAO templates.

4.2. Effect of Concentration of the Deposition Solution

The concentration of the deposition solution should be controlled in an appropriate value. In our study, it is found that 0.1 mol/L (NH₄)₂TiF₆ is suitable for the deposition, and the morphology of TiO₂ nanotube is shown in Figure 4(b). If the concentration of the deposition solution is too low, only a small number of TiO₂ nanotubes can be produced with a large amount of alumina remained, as shown in Figure 4(a). If the concentration of the deposition solution is too high, the reaction is so severe that parts of TiO₂ nanotubes will be destroyed (Figure 4(c)).

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Figure 4: SEM micrographs of TiO₂ nano tubes prepared with different concentration of deposition solution ((NH₄)₂TiF₆): (a) 0.05 mol/L, (b) 0.1 mol/L, (c) 0.3 mol/L.

4.3. Effect of Heat Treatment

TiO₂ prepared by LPD method exhibits as amorphous phase [57]. It is well known that it shows poor photocatalytic activity since amorphous TiO₂ has defects in its crystal structure. To prepare anatase TiO₂ with a high surface area and good crystallization is essential to improve the photocatalytic activity. Calcination is a simple way for the crystallization of TiO₂. When annealing at 400°C for 2 h, the amorphous TiO₂ films change to anatase phase [52, 58]. With increasing calcination temperature, the photocatalytic activity increases due to the formation of anatase TiO₂ and the improvement of crystallization [59]. With further increase in the calcination temperature from 600 to 800°C, the photocatalytic activity rapidly decreased due to the vanishing of anatase phase, collapse of nanotube structures, and decrease of surface areas [60]. TiO₂ has three types of crystal structures: anatase, rutile, and brookite. Among them, the anatase films show best photocatalytic property. It is also found that a mixed phase of anatase and rutile or brookite shows excellent photocatalytic ability [61] due to the reduction of the combination probability of the hole-electron pairs.

Recently, vapor-thermal treatment can be applied to the crystallization of TiO₂. In vapor-thermal treatment, the as-prepared TiO₂ nanotube array film was placed on the support to avoid direct contact with the water, which was then placed into a 100-mL stainless steel autoclave with a 100 mL Teflon linear. Two mL distilled water was added into the linear. This method is different from hydrothermal method, in which the TiO₂ nanotube array film was put into Teflon-lined autoclave, which was then filled with water up to 80% of the total volume. The vapor-thermal-treated films have better crystallization than the calcined films and remained tubular structures compared with the hydrothermal-treated samples. So the vapor-thermal treatment exhibits better photocatalytic activity than the calcined and hydrothermal treated films (Figure 5) [55].

Figure 5: Comparison of photocatalytic activity of the TNs samples before and after treatment for the photocatalytic decomposition of MO in water: (a) untreated, (b) calcination, (c) vapor thermal, and (d) hydrothermal. (A) Apparent rate constants. (B) C/C₀-t curve. C and C₀ denote the reaction and initial concentration of MO in the system, respectively [55].

5. Modification of Titania Nanotube Photocatalysts

In order to improve the photocatalytic activity, various approaches, such as nonmetal anions doping, surface improvement with noble metal, transition metal cation doping, and semiconductor composite, have been attempted to hamper the recombination of the photogenerated hole-electron pairs in the photocatalysis [62].

Nonmetal dopants, such as N, S, C, and P have been applied in the photocatalysis to broaden the utilization of solar energy in visible region [63–65]. Asahi et al. [66] found that the substitutional doping of N was the most effective for its contribution to the bandgap narrowing by mixing its p states with O 2p states. Although doping with S shows a similar band-gap narrowing, it is difficult to incorporate it into the TiO₂ lattice. The states introduced by C and P are too deep in the gap to overlap sufficiently with the band states of TiO₂ to transfer photoexcited carriers to reactive sites at the catalyst surface within their lifetime. However, sulfur doping has been reported to have better photoabsorption as compared to nitrogen doping [63]. Compared with the single-element doping, the codoped TiO₂ can provide better photocatalytic performance. Synergistic effect of doped S and N forms a new band above the valence band and narrows the band-gap of the photocatalyst, leading to photo-absorption and catalytic activity in the visible light region [67].

Some researchers fabricated metal-doped TiO₂ films. Zhao et al. successfully prepared Zn-doped titania nanotubes, which was about 20 nm red shift in the spectrum of UV-vis absorption compared with TiO₂ nanotubes [68]. Fe-doped titania nanotubes improve the photocatalytic ability of titania by hindering the recombination of photogenerated hole electron and a stronger absorption in the 410–650 nm range [69, 70]. The photocatalytic performance of a series of Pt/RE/TiO₂ photocatalysts has been investigated. The activities of all rare-earth-doped TiO₂ samples have been increased compared to those of pure TiO₂ in the order: La/TiO₂ > Sm/TiO₂ > Eu/TiO₂ > Dy/TiO₂ > Er/TiO₂. It is shown that the transformation from anatase to rutile has been prevented, which can enhance the activities of the photocatalysts. The data of lattice distortion implies that Ti⁴⁺ can enter (antidope) into rare earth oxide that exists on the surface of titanium dioxide. The flat-band potential of conduction of RE/TiO₂ has been shifted negatively since the lattice distortion raises the Fermi level, which causes flat-band potential of the conduction of TiO₂. As a result, the photoinduced electrons of the conduction band have stronger reduction capability and thus the photocatalytic activity is improved [71].

Chen et al. studied the doping effect of eight transition metal ion dopants on the crystal phase and the photoreactivity of TiO₂ nanoparticles [72]. Among all the eight doping metal ions of Zn²⁺, Fe³⁺, Cu²⁺, Co²⁺, Ni²⁺, Cr³⁺, V⁵⁺, and Mn²⁺, and ions doping can improve the photocatalytic activity of TiO₂ effectively. In general, red shift occurs to Ni-doped TiO₂ nanoparticles. Among the ions investigated, Ni-doped TiO₂ nanoparticles have shown highest photoreactivity at the concentration of 0.002 at.%, about 1.9 times that of the pure TiO₂. Ion doping is shown to reduce the diameter and influence the fraction of anatase. Data also indicates that the combination of anatase diameter and ion radius might play an important role in the photoreactivity of TiO₂ nanoparticles. Apart from the transition metal ions shown above, the noble metal nanoparticles such as silver can also improve the photocatalytic activity of TiO₂ [73].

Coupling TiO₂ films with other semiconductors is considered as a good way because coupling two semiconductors with different redox energy levels can increase the charge separation for their corresponding conduction and valence bands [74, 75]. WO₃ is an appropriate material to couple with TiO₂ because WO₃ has a suitable conduction band potential to allow the transfer of photogenerated electrons from TiO₂ facilitating effective charge separation [76]. However, coupled WO₃ did not shift the optical absorption to the visible region. The improvement of photocatalytic activity is attributed to the increased surface acidity, better separation between photoinduced carriers, and higher content of anatase [77]. BiOI is an attractive p-type semiconductor with a narrow bandgap of 1.94 eV, which is introduced to prepare p-n junction BiOI/TiO₂ nanotube arrays. After being coated with BiOI, the space between individual nanotube of TiO₂ has been filled and the wall thickness of TiO₂ nanotubes increases by ca. 8 nm (Figure 6). Meanwhile, the Fermi level of BiOI is moved up, while the Fermi level of TiO₂ is moved down until an equilibrium state is formed (Figure 7). Thus, the photogenerated electron-hole pairs will be separated effectively by the p-n junction formed in the p-BiOI/n-TiO₂ interface, and the recombination of electron-hole pairs can be reduced. So the p-n junction BiOI/TiO₂ nanotube arrays can display much greater photoelectrocatalytic activity under visible-light irradiation [56]. We prepared TiO₂/SiO₂ composite nanotube photocatalysts by the anodic aluminum oxide (AAO) liquid-phase deposition method [78]. The Ti-O-Si bonds are formed on the surface and the surface hydroxyl concentration is increased, resulting in enhanced photocatalytic activity, being about 20% higher than pure TiO₂ films. Moreover, the TiO₂/SiO₂ composite exhibits a wider conduction band which would effectively prohibit recombination of photogenerated electrons and holes.

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Figure 6: SEM (a, b) and TEM (c, d) images of self-organized TiO₂ NTs (a, c) and BiOI/TiO₂ NTs (b, d). (a) Top and side (inset) view SEM images of the unmodified TiO₂ NTs. (b) Top SEM image of BiOI/TiO₂ NTs. (c) TEM images of the unmodified TiO₂ NTs. (d) TEM images of BiOI/TiO₂ NTs [56].

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Figure 7: Schematic diagrams for (a) energy bands of p-BiOI and n-TiO₂ before contact and (b) the formation of a p-n junction and its energy band diagram at equilibrium and transfer of photoinduced electrons from p-BiOI to n-TiO₂ under visible-light irradiation [56].

6. Conclusions

This paper reviews a serial study of TiO₂ nanotube films prepared by liquid-phase deposition based on the template-based growth. The formation mechanisms of anodic alumina template and TiO₂ nanotube film have been discussed. The morphology of TiO₂ is affected by the morphology of anodic alumina. High anodizing voltage and phosphoric acid are favorable for the formation of large pore alumina template. The volume of alumina in the template affects the morphology of TiO₂ nanoarray films. Using the template with a certain volume fraction of Al₂O₃ (less than 0.71), the ordered aligned titania nanotubes can be obtained. Proper concentration of deposition solution (0.1 mol/L (NH₄)₂TiF₆) and proper calcinations temperature (400°C) are favorable for the production of TiO₂ nanotube films. Doping of metal and nonmetal elements can improve the photocatalytic activity.

Acknowledgments

This work is financially supported by Beijing Municipal Commission of Education Key Foundation (KZ2010100050001); Beijing Innovation Talent Project (PHR201006101); Beijing New Century Hundred, Thousand and Ten Thousand Talent Project, State key Laboratory of Electronic Thin Films and Integrated Devices (UESTC: KFJJ201001) and Guangxi Natural Science Foundation (2010GXNSFB013009).

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