Synthesis and Applications of TitaniumOxide Nanotubes

Synthesis and Applications of Titanium Oxide Nanotubes Tohru Sekino Institute of Multidisciplinary Research for Advanced Materials IMRAM, Tohoku University, Aoba-ku, Sendai 980-8577, Jap

Synthesis and Applications of Titanium

Oxide Nanotubes

Tohru Sekino

Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, Aoba-ku, Sendai 980-8577, Japan

sekino@tagen.tohoku.ac.jp

Abstract Titanium oxide nanotube (TiO2 nanotube, TNT) is synthesized by the low-temperature solution chemical method via the self-organization to form unique open-end nanotubular morphology with typically 8–10 and 5–7nm in outer and inner diameters, respectively Because of the mutual and synergy combination of its low-dimensional nanostructure and physical-chemical characteristics of TiO2 semi-conductor, properties enhancements and novel functionalization are expected in the TiO2 nanotube In this chapter, synthesis, nanostructures, formation mechanism, various physicochemical characteristics, and prospects of future application for the TiO2 nanotube are described in detail In such an oxide material, property control and enhancement is possible by tuning appropriate chemical compositions, crystal structures, and composite structures Therefore, special emphasis is also placed to introduce modification of the nanotubes by doping and/or nanocompositing to meet the requirements as for the environmental friendly and energy creation systems and various functional devices

2.1 Introduction

After the discovery of carbon nanotube (CNT) [1], large attention has been paid to this unique low-dimensional nanostructured material because of its attractive various physical and chemical functions which arise from the syn-ergy of low-dimensional nanostructure and anisotropy of carbon network, thus known as graphene structure Till now, large numbers of not only funda-mental studies on the structure, electrical, optical, mechanical, and physico-chemical properties but also application-oriented research and development, such as single-electron transistor device, field emission device, fuel cells, and strengthening fillers of composites, have been extensively carried out Besides CNTs, various inorganic nanotubular materials have been reported in non-oxide compounds, boron nitride (BN) [2] and molybdenum disulfide (MoSi2) [3]; in oxides such as vanadium oxide (V2O5) [4–6], aluminum oxide (Al2O3) [6], silicon dioxide (SiO2) [6, 7], titanium oxide (TiO2) [8–14]; and also in natural minerals like imogolite [15, 16]

T Kijima (Ed.): Inorganic and Metallic Nanotubular Materials.

Topics in Applied Physics 117, 17–32 (2010)

DOI 10.1007/978-3-642-03622-4 2  Springer-Verlag Berlin Heidelberg 2010c

Except natural mineral materials, fabrication of nanotubes is roughly clas-sified into two methods; one is the template or replica method, in which some template materials are used to form tubular structure Many efforts have been paid to fabricate tubular materials including nanotubes by attempt-ing the template/replica method [4, 5, 8, 12–14] The other one is based

on the self-structuralization or self-organization of matter during chemical

or physical synthesis/fabrication processes Synthetic imogolite [16], sol–gel-derived SiO2 nanotube [7], chemically prepared TiO2 nanotube [9, 10], and nanotube/nanohole arrays such as Al2O3 [17, 18] and TiO2 [11, 19] prepared

by electrochemically using anodic oxidation of metal films are the typical sys-tems fabricated by the self-organizing process

Among them, titanium oxide nanotube (TiO2 nanotube, TNT) is one

of the promising nanostructured oxides with tubular structure TiO2 is well known as a wide gap semiconductor oxide It is, however, inexpensive, chem-ically stable, and harmless and has no absorption in the visible light region Instead, it is UV light responsible; electron and hole pair is generated by the UV irradiation, inducing chemical reactions at the surface Therefore, the most promising characteristic of TiO2lies in its photochemical properties such

as high photocatalytic activity Due to this reason, it has been widely studied

by many researchers from 1950s to utilize TiO2as a photocatalyst [20–22], an electrode of dye-sensitized solar cell [23], a gas sensor [24], and so on

On the other hand, Kasuga et al [9, 10] have succeeded in the synthesis

of nanotubular TiO2, which has open-end structure with typically 8–10 and 5–7 nm in outer and inner diameters, respectively, using a simple and low temperature solution chemical processing Various methods such as anodizing

of metal substrates [11, 19], replica [8, 12, 13], and template methods [14] have been investigated to prepare tubular TiO2 However, the synthesis method developed by Kasuga et al is based on a self-organizing and templateless route that is achieved by low temperature process to form nanometer-sized tubular morphology Using this so-called Kasuga method, many related investigations have been extensively carried out on structural analysis, process optimization, properties evaluation, and so on [25–27]

As mentioned above, not only fundamental interests in the formation mechanism and the unique nanotubular structures but also functions’ en-hancements and novel functionalization are hence expected in the TiO2 nan-otube because of the mutual and synergy combination of various factors lying

in a nanotubular semiconductor: (1) crystal structure, (2) chemical bonding and (3) physical/chemical properties of the matter, and (4) low-dimensional nanostructures/nanospace/nanosurface and (5) self-organization/ordering of the structure

In this chapter, synthesis processing, nanostructures, various properties and prospects of future application for the TiO2 nanotube fabricated by the low temperature solution chemical route are described in detail In such

an oxide material, property control and enhancement are possible by tuning appropriate chemical compositions and crystal structures Therefore, special

2 Synthesis and Applications of Titanium Oxide Nanotubes 19 emphasis is also placed to introduce modification of the nanotubes by dop-ing and/or nanocompositdop-ing in order to meet the requirements as per the environmental friendly and energy creation systems and various functional devices

2.2 Synthesis and Structure of Titanium

Oxide Nanotubes

As mentioned before, fabrication of nanotubular TiO2 is classified into two methods: template/replica route [8, 12–14] and direct synthesis (i.e., template-less) route In the former method, some materials, such as organic, inorganic, and metal nanowires/nanorods/whiskers or nanotube/nanohole arrays such as

Al2O3prepared by anodic oxidation of Al foil, are used as the templates TiO2

is hence often synthesized by sol–gel or precipitation methods in solution, and then the templates are removed afterward Therefore, the size of obtained materials can be easily controlled by the size of template used Followed by these processing routes, however, the most as-synthesized nanotubes have an amorphous structure, and then they become nanocrystalline nanotubes after appropriate heat treatment

The latter (direct) synthesis route includes low temperature solution chem-ical method [9, 10] and electrochemchem-ical oxidation route from metal sub-strate or foil, i.e., anodic oxidation of titanium or titanium alloy [19] that also gives amorphous nanotubes In the case of solution chemical route, crystalline TiO2 nanotube based on the TiO6 octahedron network can be obtained In this section processing and structures of the TNT will be given

2.2.1 Low Temperature Solution Chemical Processing

Typical TNT is synthesized by the solution chemical route using high-concentration alkaline solution [9, 10] Various titanium oxide powders in-cluding anatase- or rutile-type titania, their mixture, or titanium alkoxide can be used as the source materials of TNT The raw material is refluxed in

10 M NaOH aqueous solution at around 110◦C for 20 h or longer The re-sultant product is washed many times by distilled water in order to remove sodium Then 0.1 M HCl aqueous solution is added to neutralize the solution and again treated with distilled water until the solution conductivity reached

5 mS/cm The product is then separated by filtering, centrifugation, or freeze drying technique and dried

This synthesis is carried out under the refluxing condition so that the pressure during synthesis is the same as that of ambient atmospheric pressure

of 0.1 MPa; the synthesis temperature of around 110◦C thus corresponds to the boiling temperature of high-concentration alkaline solution

On the contrary, hydrothermal synthesis using an autoclave, which pro-vides closed reaction environment and hence the slightly higher pressure dur-ing the processdur-ing, can also be attempted to synthesize TNT [28] Further-more, not only TiO2 but also Ti metal can be used as the source material

of TNT [29], in which process titanium is chemically oxidized in the alkaline solution The size control of TNT also has attracted much attention Vari-ous sized, especially thick TNT can often be synthesized by the hydrothermal method, because it gives higher synthesis temperature than 110◦C In ad-dition, natural mineral source is also used for the TNT synthesis that may reduce the production cost of the TNT [30]

X-ray diffraction patterns showing phase development during the chemi-cal processing are shown in Fig 2.1, and corresponding transmission electron micrographs are represented in Fig 2.2 After alkaline treatment, the product mainly consists of amorphous and crystalline phase corresponding to sodium titanate (Na2TiO3, Fig 2.1b), but the shapeless matter is obtained (Fig 2.2a) After the water and HCl treatment (Figs 2.1c and 2.2b), sodium titanate disappears completely and another crystalline phase with low crystallinity is observed In this step, nanometer-sized sheet-like morphology can be obtained, which is considered as the TiO2 nanosheet Further, water washing provides fibrous product (Fig 2.2c) with the length of several hundreds to several mi-crometers Higher magnification TEM photograph shown in Fig 2.2d clearly reveals that the outer and inner diameter of the final product is around 8–10 and 5–7 nm, respectively, and it has an open-end structure The size of ob-tained TNT does not depend on the kind of raw materials used In addition, when KOH is used as a reaction solution, TNT can also be produced with the similar size and morphology

Fig 2.1 X-ray diffraction patterns of products obtained in each chemical synthesis

step: (a) anatase-type TiO2 raw material, (b) after alkaline reflux (10 M NaOH,

110◦C, 24 h), (c) after water washing, (d) final product (after 0.1 M HCl and water washing)

2 Synthesis and Applications of Titanium Oxide Nanotubes 21

Fig 2.2 TEM images showing morphological development of the products in each

chemical synthesis step: (a) after alkaline reflux (10 M NaOH, 110◦C, 24 h), (b) after 0.1 M HCl treatment, (c) final product, (d) high magnification image of obtained nanotubes

The surface area of the typical TNT is approximately 300∼ 350 m2/g, and the value is in good agreement with the calculated theoretical surface area, 345 m2/g, by assuming the tubular structure, the observed size, and the density of TiO2 crystal However, recent investigation has revealed that the larger TNT with more than 10 nm in diameter can be obtained when larger titanium oxide powders with particle diameter in micrometer is used and when hydrothermal synthesis method is utilized

2.2.2 Nanostructures and Formation Mechanism

On the contrary to layered compounds like graphite, TiO2 has rigid crystal structure in which a lattice spreads out isotropically and three dimension-ally, so that its crystal shape is usually equiaxial However, solution chemical synthesis described above gives anisotropic and open-end nanotube structure

in TiO2 In order to identify the structural characteristic and also to under-stand the formation mechanism of TNT in relation to its synthesis process, much efforts for the structural analyses have been paid by using X-ray and

Fig 2.3 TEM image of TiO2 nanotube bundle (a) and selected area electron diffraction pattern (b)

neutron diffraction and high-resolution electron microscopy coupled with elec-tron diffraction technique [9, 10, 28–39]

In the selected area electron diffraction (SAED) pattern of TNT bundle (Fig 2.3), some diffraction spots with belt-like spreading are found, which

is typically found in a fibrous compound As summarized in Table 2.1, the

interplanar spacing (d-spacing) of spots a (a  ), b (b  ), and d (d ) correspond to those of (101), (200), and (100) of typical anatase crystal of TiO2, respectively [38] From these facts, it is considered that the TNT basically has the similar crystal structure as the anatase type of TiO2, and then the longitudinal

direc-tion of the nanotube corresponds to the a-axis [(100) direcdirec-tion] while the cross section is parallel to the b-plane [(010) plane] of the anatase crystal On the other hand, the diffraction spot c (c  ) provides the d-spacing of 0.87 nm, and corresponds to the broad diffraction peak found at 2θ of around 9 ◦in the XRD patterns of Fig 2.1d, and also corresponds to the spacing of 0.88 nm at the wall in Fig 2.2d The reflection of anatase crystal near to this value is (001)

with d = 0.951 nm (Table 2.1); however, there is a slightly large deviation (ap-proximately 8.5%) between these values and hence the spot c(c ) seems not

to correspond directly to the (001) of anatase structure This large interpla-nar distance is a typical characteristic in titanium oxide nanotube and closely related to the formation of the structures as described in the latter part Thermogravimetry coupled with mass spectroscopic analysis for the as-synthesized TNT exhibited the weight loss continued up to approximately

350◦C and detected major species was H2O High-temperature XRD results

Table 2.1 Interplanar (d) spacing observed for TiO2 nanotube bundle (Fig 2.3)

and corresponding plane and d-spacing of anatase-type TiO2

2 Synthesis and Applications of Titanium Oxide Nanotubes 23

Fig 2.4 High-temperature X-ray diffraction patterns of synthesized TiO2

nanotubes and corresponding structure change

(Fig 2.4) demonstrated that the typical diffraction peak intensity found at

2θ around 9 ◦ decreased with increasing in test temperature up to around

400◦C, while the peaks corresponding to anatase structure of TiO2 became

to be the major crystalline phase and its crystallinity increased above the temperature Annealing temperature dependency of the specific surface area for pure TNT is summarized in Table 2.2 (see also Fig 2.8) High surface area was maintained up to around 400◦C while sudden decrease occurred above the temperature and then reached to the value approximately 100 m2/g at

an annealing temperature higher than 450◦C From TEM investigation for the annealed TNT, its nanotubular structure was found to be kept up to around 450◦C These facts imply us that the as-synthesized TNT contains some amount of hydroxyl group (–OH) and/or structure water (H2O) and has TiO6 octahedral network structure which is similar to common anatase-type structure of TiO2crystal or, in another words, has titanate-like structure [38] By the heat treatment (annealing) for the as-synthesized TNT, proton

is released as H2O and then the nanotube becomes to be the stoichiometric

Table 2.2 Variation of surface area on the annealing temperature for the TiO2

nanotubes The surface area is measured by the BET method

Fig 2.5 (010) projection of H2Ti3O7unit cell (a) and structure model of nanotube

by assuming a chemical composition as H2Ti3O7(b and c) proposed by Chen et al.

[31] Reprinted with permission from [31]

TiO2 nanotube with an anatase structure as its base crystal structures at around 400◦C

Detailed structure analyses have been carried out extensively Chen et al [31] investigated the structure of chemically prepared TNT by using high-resolution transmission electron microscopy and reported that the TNT was titanate with the chemical formula of H2Ti3O7 and proposed the structure model as shown in Fig 2.5 On the other hand, Ma et al [32, 33] showed it was lepidocrocite which was one of the defect-containing titanate with the formula

of HxTi2−x/4x/4O4 Besides these structures, various compositions were re-ported, Na2Ti2O4(OH)2or its protonated titanate of H2Ti2O4(OH)2[34] and

H2Ti4O9[35] These compounds, however, basically contain OH group and/or

H2O and can be described as (TiO2)n ·(H2O)m, which reasonably explains the fact that H2O is released by the heat treatment of as-synthesized TNT as mentioned above The reason why many plausible composition models are reported is considered as follows; synthesized TNT usually has a small di-ameter and hence the wall thickness is quite thin, around 1–2 nm, and also its crystallinity is rather low as shown in Fig 2.1 by comparing with usual TiO2crystalline particles Furthermore, a large number of titanates are known

in the series, and most of them have a layered structure with the similar structure

As mentioned before, TNT can be fabricated by using not only NaOH but also KOH, while the nanotubular matter is not synthesized in the case

of LiOH solution; in this case more stable crystalline LiTiO2 is formed [38] These facts imply us that the formation of alkaline titanate like Na2TiO3 or its amorphous matter (see Fig 2.1b) is an important intermediate compound

2 Synthesis and Applications of Titanium Oxide Nanotubes 25 for the formation of the nanotube By considering these facts the formation of the TNT is thus regarded as follows: at first, titanate-containing alkali metals (alkali titanates) is formed during the solution chemical treatment Then the alkali metal element is ion exchanged, and protonated titanate is formed as a nanosheet In the final step, the nanosheet converts to be a tubular structure (Fig 2.5) by scrolling process in order to lower the surface energy

Till now a large number of discussions on the actual structure models and formation mechanisms for the TiO2nanotubes [37–39], and related inves-tigations such as process development for controlling nanotubes length and diameter and extended research toward nanowires/nanorods, are continued

by many research groups Nevertheless, it should be noted that the crystal structure based on the three-dimensional framework of TiO6polyhedron and low-dimensional nanostructure formation for the TiO2 nanotube is a quite unique and different from those of the carbon nanotube, which is built from the two-dimensional graphene sheet (carbon network)

2.3 Functions of Titanium Oxide Nanotubes

Similar to common TiO2powder, the TNT is also white colored powder The optical bandgap energy calculated from the ultraviolet–visible light absorp-tion spectra by assuming indirect transiabsorp-tion of TiO2 is approximately 3.41

∼ 3.45 eV for chemically synthesized TNT [38], which value is slightly larger

than that of anatase (3.2 eV) and rutile (3.0 eV) crystals This blue shift of the absorption edge wavelength is attributed to the quantum size effect of TiO2

semiconductor [40] in TNT because of very thin nanotube wall thickness of around 1∼ 2 nm Recent materials design strategy of TiO2 nanoparticles fo-cuses on the developed visible light responsible TiO2 photocatalyst [41] so that the enlarged bandgap seems to be disadvantageous; nevertheless TNT exhibits unique and excellent photochemical properties which contribute en-hanced environmental purification performance

2.3.1 Photochemical Properties and Photocatalytic Functions

In order to clarify the photochemical characteristic of TNT, Tachikawa et al [42] investigated the photocatalytic one-electron oxidation reaction of an or-ganic molecule and related charge recombination dynamics during UV light irradiation on TNT using time-resolved diffuse reflectance spectroscopy They observed remarkably long-lived radical cation and trapped e−for the TNT, ap-proximately five times or more long lifetime than those for the nanoparticles Further, they have observed that the electron generated by the steady-state irradiation of UV light could exist for longer time on the TNT surface, which phenomenon was usually not confirmed in TiO2 nanoparticles, and also the evidence of rapid reaction of trapped e−with organic halide pollutants such as CCl4 These features are considered mainly due to the unique one-dimensional

nanostructure of the TNT and are the reason of the good photocatalytic prop-erties; TNT has very thin wall so that generated carriers can effectively move

to the surface, and then charge recombination is inhibited due to its long one-dimensional structure, clearly suggesting morphological advantage of the TNT on the charge recombination dynamics These may also be advantageous for the use of TNT as for the electrode of solar cell in which transfer charac-teristic is very important In fact, longer lifetime while the similar diffusion coefficient of electron in TNT has been reported when it has used for the electrode of dye-sensitized solar cell [43]

As mentioned before, anatase-type TiO2is well known as a promising pho-tocatalytic material due to its photochemical characteristic Figure 2.6 shows variation of hydrogen generation by UV light irradiation to as-synthesized and annealed TNTs and commercial TiO2nanoparticles in water/methanol mixed solution (so-called water splitting test) [38] As can be seen from the figure, as-synthesized TNT shows lower photocatalytic activity than the commercial TiO2 powders (P-25 and ST01) This low activity is considered due to the existence of many hydroxyls (–OH) and/or structural water (H2O) and low crystallinity of the as-synthesized TNT On the other hand, annealed (400◦C) TNT can generate approximately two to three times higher amount of H2than that of nanoparticles, when compared to H2 amount per unit mass of TiO2

photocatalyst The enhanced hydrogen evolution performance of the annealed TNT is caused by the improved crystallinity (see Fig 2.4) with maintaining its nanotubular structure and higher surface area, around 230 m2/g (Table 2.2 and Fig 2.8), than that of TiO2nanoparticle (approximately 50 m2/g) How-ever, by comparing the generated amount of H2 per unit surface area of the catalysts, TNT exhibits around 44–65 % of nanoparticle system This fact indicates that an approximately half of the surface may not act as for the active site of hydrogen generation, and hence the inner wall of the nanotube

Fig 2.6 Hydrogen generation by the water splitting during UV irradiation to

various TiO2 photocatalysts (P-25 and ST01, commercial TiO2 nanopowders, as-prepared TNT, and annealed TNT at 400◦C)