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Jinshu Wang, Qian Cai, Hongyi Li, Yuntao Cui, Hong Wang, "A Review on Nanotube Film Photocatalysts Prepared by Liquid-Phase Deposition", International Journal of Photoenergy, vol. 2012, Article ID 702940, 11 pages, 2012. https://doi.org/10.1155/2012/702940
A Review on Nanotube Film Photocatalysts Prepared by Liquid-Phase Deposition
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.
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 . 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 . 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 , 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 , gas sensors , and dye-sensitized solar cells . Titania semiconductor photocatalysts have demonstrated advantages such as transparency , wide bandgap , biological and chemical inertness [12, 13], strong oxidizing capability, and nontoxicity . 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 . 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 . 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 . 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 .
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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  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 , which provided a simple and feasible way for the preparation of the TiO2 films in a large scale.
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 . 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 .
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 :
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 , 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.
Kanawori et al. described the formation mechanism by the addition of boric acid  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 . 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 . 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 . 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.
We have studied the relationship between volume fraction of in the template and the morphology of TiO2 nanoarray . 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)).
4.3. Effect of Heat Treatment
TiO2 prepared by LPD method exhibits as amorphous phase . 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 . 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 . 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  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) .
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 .
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.  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 . 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 .
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 . 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 .
Chen et al. studied the doping effect of eight transition metal ion dopants on the crystal phase and the photoreactivity of TiO2 nanoparticles . 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 .
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 . 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 . 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 . We prepared TiO2/SiO2 composite nanotube photocatalysts by the anodic aluminum oxide (AAO) liquid-phase deposition method . 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.
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.
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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