2 Synthesis and Applications of Titanium Oxide Nanotubes

2

Synthesis and Applications of TitaniumOxide Nanotubes

Tohru Sekino

Institute of Multidisciplinary Research for Advanced Materials (IMRAM), TohokuUniversity, Aoba-ku, Sendai 980-8577, [email protected]

Abstract Titanium oxide nanotube (TiO2 nanotube, TNT) is synthesized by thelow-temperature solution chemical method via the self-organization to form uniqueopen-end nanotubular morphology with typically 8–10 and 5–7nm in outer andinner diameters, respectively. Because of the mutual and synergy combination of itslow-dimensional nanostructure and physical-chemical characteristics of TiO2 semi-conductor, properties enhancements and novel functionalization are expected in theTiO2 nanotube. In this chapter, synthesis, nanostructures, formation mechanism,various physicochemical characteristics, and prospects of future application for theTiO2 nanotube are described in detail. In such an oxide material, property controland enhancement is possible by tuning appropriate chemical compositions, crystalstructures, and composite structures. Therefore, special emphasis is also placed tointroduce modification of the nanotubes by doping and/or nanocompositing to meetthe requirements as for the environmental friendly and energy creation systems andvarious functional devices.

2.1 Introduction

After the discovery of carbon nanotube (CNT) [1], large attention has beenpaid to this unique low-dimensional nanostructured material because of itsattractive various physical and chemical functions which arise from the syn-ergy of low-dimensional nanostructure and anisotropy of carbon network, thusknown 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, andstrengthening fillers of composites, have been extensively carried out. BesidesCNTs, 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 innatural 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 c© Springer-Verlag Berlin Heidelberg 2010

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Except natural mineral materials, fabrication of nanotubes is roughly clas-sified into two methods; one is the template or replica method, in which sometemplate materials are used to form tubular structure. Many efforts havebeen paid to fabricate tubular materials including nanotubes by attempt-ing the template/replica method [4, 5, 8, 12–14]. The other one is basedon the self-structuralization or self-organization of matter during chemicalor physical synthesis/fabrication processes. Synthetic imogolite [16], sol–gel-derived SiO2 nanotube [7], chemically prepared TiO2 nanotube [9, 10], andnanotube/nanohole arrays such as Al2O3 [17, 18] and TiO2 [11, 19] preparedby 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 oneof the promising nanostructured oxides with tubular structure. TiO2 is wellknown 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 bythe UV irradiation, inducing chemical reactions at the surface. Therefore, themost promising characteristic of TiO2 lies in its photochemical properties suchas high photocatalytic activity. Due to this reason, it has been widely studiedby many researchers from 1950s to utilize TiO2 as a photocatalyst [20–22], anelectrode 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 synthesisof nanotubular TiO2, which has open-end structure with typically 8–10 and5–7 nm in outer and inner diameters, respectively, using a simple and lowtemperature solution chemical processing. Various methods such as anodizingof metal substrates [11, 19], replica [8, 12, 13], and template methods [14] havebeen investigated to prepare tubular TiO2. However, the synthesis methoddeveloped by Kasuga et al. is based on a self-organizing and templateless routethat is achieved by low temperature process to form nanometer-sized tubularmorphology. Using this so-called Kasuga method, many related investigationshave 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 formationmechanism 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 lyingin a nanotubular semiconductor: (1) crystal structure, (2) chemical bondingand (3) physical/chemical properties of the matter, and (4) low-dimensionalnanostructures/nanospace/nanosurface and (5) self-organization/ordering ofthe structure.

In this chapter, synthesis processing, nanostructures, various propertiesand prospects of future application for the TiO2 nanotube fabricated bythe low temperature solution chemical route are described in detail. In suchan oxide material, property control and enhancement are possible by tuningappropriate 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 nanocompositing in order to meet the requirements as per theenvironmental friendly and energy creation systems and various functionaldevices.

2.2 Synthesis and Structure of TitaniumOxide Nanotubes

As mentioned before, fabrication of nanotubular TiO2 is classified into twomethods: 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 asAl2O3 prepared by anodic oxidation of Al foil, are used as the templates. TiO2

is hence often synthesized by sol–gel or precipitation methods in solution, andthen the templates are removed afterward. Therefore, the size of obtainedmaterials can be easily controlled by the size of template used. Followed bythese processing routes, however, the most as-synthesized nanotubes have anamorphous structure, and then they become nanocrystalline nanotubes afterappropriate heat treatment.

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

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 alkoxidecan be used as the source materials of TNT. The raw material is refluxed in10 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 removesodium. Then 0.1 M HCl aqueous solution is added to neutralize the solutionand again treated with distilled water until the solution conductivity reached5 mS/cm. The product is then separated by filtering, centrifugation, or freezedrying technique and dried.

This synthesis is carried out under the refluxing condition so that thepressure during synthesis is the same as that of ambient atmospheric pressureof 0.1 MPa; the synthesis temperature of around 110◦C thus corresponds tothe boiling temperature of high-concentration alkaline solution.

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On the contrary, hydrothermal synthesis using an autoclave, which pro-vides closed reaction environment and hence the slightly higher pressure dur-ing the processing, can also be attempted to synthesize TNT [28]. Further-more, not only TiO2 but also Ti metal can be used as the source materialof TNT [29], in which process titanium is chemically oxidized in the alkalinesolution. The size control of TNT also has attracted much attention. Vari-ous sized, especially thick TNT can often be synthesized by the hydrothermalmethod, because it gives higher synthesis temperature than 110◦C. In ad-dition, natural mineral source is also used for the TNT synthesis that mayreduce 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 electronmicrographs are represented in Fig. 2.2. After alkaline treatment, the productmainly consists of amorphous and crystalline phase corresponding to sodiumtitanate (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 titanatedisappears completely and another crystalline phase with low crystallinity isobserved. In this step, nanometer-sized sheet-like morphology can be obtained,which is considered as the TiO2 nanosheet. Further, water washing providesfibrous product (Fig. 2.2c) with the length of several hundreds to several mi-crometers. Higher magnification TEM photograph shown in Fig. 2.2d clearlyreveals that the outer and inner diameter of the final product is around 8–10and 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 thesimilar size and morphology.

Fig. 2.1. X-ray diffraction patterns of products obtained in each chemical synthesisstep: (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 waterwashing)


Fig. 2.2. TEM images showing morphological development of the products in eachchemical synthesis step: (a) after alkaline reflux (10 M NaOH, 110◦C, 24 h), (b) after0.1 M HCl treatment, (c) final product, (d) high magnification image of obtainednanotubes

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

2.2.2 Nanostructures and Formation Mechanism

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

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Fig. 2.3. TEM image of TiO2 nanotube bundle (a) and selected area electrondiffraction 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, whichis typically found in a fibrous compound. As summarized in Table 2.1, theinterplanar spacing (d-spacing) of spots a (a′), b (b′), and d (d′) correspond tothose 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 similarcrystal structure as the anatase type of TiO2, and then the longitudinal direc-tion of the nanotube corresponds to the a-axis [(100) direction] while the crosssection is parallel to the b-plane [(010) plane] of the anatase crystal. On theother hand, the diffraction spot c (c′) provides the d-spacing of 0.87 nm, andcorresponds to the broad diffraction peak found at 2θ of around 9◦ in the XRDpatterns of Fig. 2.1d, and also corresponds to the spacing of 0.88 nm at thewall 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 notto correspond directly to the (001) of anatase structure. This large interpla-nar distance is a typical characteristic in titanium oxide nanotube and closelyrelated 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 approximately350◦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

Position d (nm) Anatase TiO2, hkl/d (nm)JCPDS

a ∼ a′′′ 0.37 (101)/0.352b, b′ 0.19 (200)/0.189d, d′ 0.28 (110)/0.268c, c′ 0.87 (001)/0.951


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 at2θ around 9◦ decreased with increasing in test temperature up to around400◦C, while the peaks corresponding to anatase structure of TiO2 becameto be the major crystalline phase and its crystallinity increased above thetemperature. Annealing temperature dependency of the specific surface areafor pure TNT is summarized in Table 2.2 (see also Fig. 2.8). High surface areawas maintained up to around 400◦C while sudden decrease occurred abovethe temperature and then reached to the value approximately 100 m2/g atan annealing temperature higher than 450◦C. From TEM investigation forthe annealed TNT, its nanotubular structure was found to be kept up toaround 450◦C. These facts imply us that the as-synthesized TNT containssome amount of hydroxyl group (–OH) and/or structure water (H2O) andhas TiO6 octahedral network structure which is similar to common anatase-type structure of TiO2 crystal or, in another words, has titanate-like structure[38]. By the heat treatment (annealing) for the as-synthesized TNT, protonis 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

Temperature (◦C) RT 200 400 450 500 550Surface area (m2/g) 322 308 228 123 101 95.0

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Fig. 2.5. (010) projection of H2Ti3O7 unit cell (a) and structure model of nanotubeby 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 ataround 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 wastitanate with the chemical formula of H2Ti3O7 and proposed the structuremodel as shown in Fig. 2.5. On the other hand, Ma et al. [32, 33] showed it waslepidocrocite which was one of the defect-containing titanate with the formulaof HxTi2−x/4�x/4O4. Besides these structures, various compositions were re-ported, Na2Ti2O4(OH)2 or its protonated titanate of H2Ti2O4(OH)2 [34] andH2Ti4O9 [35]. These compounds, however, basically contain OH group and/orH2O and can be described as (TiO2)n·(H2O)m, which reasonably explains thefact that H2O is released by the heat treatment of as-synthesized TNT asmentioned above. The reason why many plausible composition models arereported 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 alsoits crystallinity is rather low as shown in Fig. 2.1 by comparing with usualTiO2 crystalline particles. Furthermore, a large number of titanates are knownin the series, and most of them have a layered structure with the similarstructure.

As mentioned before, TNT can be fabricated by using not only NaOHbut also KOH, while the nanotubular matter is not synthesized in the caseof LiOH solution; in this case more stable crystalline LiTiO2 is formed [38].These facts imply us that the formation of alkaline titanate like Na2TiO3 orits amorphous matter (see Fig. 2.1b) is an important intermediate compound


for the formation of the nanotube. By considering these facts the formation ofthe TNT is thus regarded as follows: at first, titanate-containing alkali metals(alkali titanates) is formed during the solution chemical treatment. Then thealkali metal element is ion exchanged, and protonated titanate is formed as ananosheet. 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 modelsand formation mechanisms for the TiO2 nanotubes [37–39], and related inves-tigations such as process development for controlling nanotubes length anddiameter and extended research toward nanowires/nanorods, are continuedby many research groups. Nevertheless, it should be noted that the crystalstructure based on the three-dimensional framework of TiO6 polyhedron andlow-dimensional nanostructure formation for the TiO2 nanotube is a quiteunique and different from those of the carbon nanotube, which is built fromthe two-dimensional graphene sheet (carbon network).

2.3 Functions of Titanium Oxide Nanotubes

Similar to common TiO2 powder, the TNT is also white colored powder. Theoptical bandgap energy calculated from the ultraviolet–visible light absorp-tion spectra by assuming indirect transition of TiO2 is approximately 3.41∼ 3.45 eV for chemically synthesized TNT [38], which value is slightly largerthan that of anatase (3.2 eV) and rutile (3.0 eV) crystals. This blue shift of theabsorption edge wavelength is attributed to the quantum size effect of TiO2

semiconductor [40] in TNT because of very thin nanotube wall thickness ofaround 1 ∼ 2 nm. Recent materials design strategy of TiO2 nanoparticles fo-cuses on the developed visible light responsible TiO2 photocatalyst [41] sothat the enlarged bandgap seems to be disadvantageous; nevertheless TNTexhibits 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 lightirradiation on TNT using time-resolved diffuse reflectance spectroscopy. Theyobserved 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-stateirradiation of UV light could exist for longer time on the TNT surface, whichphenomenon was usually not confirmed in TiO2 nanoparticles, and also theevidence of rapid reaction of trapped e− with organic halide pollutants such asCCl4. These features are considered mainly due to the unique one-dimensional

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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 moveto the surface, and then charge recombination is inhibited due to its longone-dimensional structure, clearly suggesting morphological advantage of theTNT on the charge recombination dynamics. These may also be advantageousfor 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 diffusioncoefficient of electron in TNT has been reported when it has used for theelectrode of dye-sensitized solar cell [43].

As mentioned before, anatase-type TiO2 is well known as a promising pho-tocatalytic material due to its photochemical characteristic. Figure 2.6 showsvariation of hydrogen generation by UV light irradiation to as-synthesized andannealed TNTs and commercial TiO2 nanoparticles in water/methanol mixedsolution (so-called water splitting test) [38]. As can be seen from the figure,as-synthesized TNT shows lower photocatalytic activity than the commercialTiO2 powders (P-25 and ST01). This low activity is considered due to theexistence of many hydroxyls (–OH) and/or structural water (H2O) and lowcrystallinity of the as-synthesized TNT. On the other hand, annealed (400◦C)TNT can generate approximately two to three times higher amount of H2 thanthat of nanoparticles, when compared to H2 amount per unit mass of TiO2

photocatalyst. The enhanced hydrogen evolution performance of the annealedTNT is caused by the improved crystallinity (see Fig. 2.4) with maintainingits nanotubular structure and higher surface area, around 230 m2/g (Table 2.2and Fig. 2.8), than that of TiO2 nanoparticle (approximately 50 m2/g). How-ever, by comparing the generated amount of H2 per unit surface area of thecatalysts, TNT exhibits around 44–65 % of nanoparticle system. This factindicates that an approximately half of the surface may not act as for theactive site of hydrogen generation, and hence the inner wall of the nanotube

Fig. 2.6. Hydrogen generation by the water splitting during UV irradiation tovarious TiO2 photocatalysts (P-25 and ST01, commercial TiO2 nanopowders, as-prepared TNT, and annealed TNT at 400◦C)


may not contribute to the photocatalytic reaction, which is probably due tothe diffusion limit of molecules within the inner part of the nanotubes duringthe reaction. Nevertheless, it is expected that the TNT would be one of thepromising candidate as the high-performance energy creation materials andsystems such as the excellent hydrogen generation catalyst.

2.3.2 Novel Environmental Purification Functions

Photocatalytic performance is often evaluated by the removal test of organicmolecules in water system. Figure 2.7 represents the variation of methyleneblue (MB) concentration in TiO2 dispersed water system under dark and UVlight irradiation conditions. In the case of commercial TiO2 nanoparticles, MBconcentration is quickly decreased under UV irradiation while is not changedwithout the UV irradiation (hence under the dark condition). This clearlyindicates that the TiO2 nanopowder is an excellent photocatalyst. However, inthe case of as-synthesized TNT, MB decrease can be confirmed even under thedark condition and is enhanced further under the UV light irradiation. Thisfact indicates that the TNT has a molecule adsorption characteristic, and it ismore obvious than the photocatalytic degradation under the UV irradiation.When TNT is annealed, the MB degradation under the dark condition isreduced; however, the photodegradation is higher than that of as-synthesizedTNT. It is again considered that the increased crystallinity can enhance itsphotocatalytic performance.

Generally, TiO2 including nanopowder has very low molecule adsorptioncapability compared to typical adsorbent materials such as zeolite, activatedcarbon, and clay minerals. Therefore, development of composite materialsof TiO2 photocatalyst and some other adsorbents such as mesoporous silica

Fig. 2.7. Variation of methylene blue concentration under the dark and UVlight irradiation conditions for the TiO2 nanoparticle, as-synthesized and an-nealed TiO2 nanotubes dispersed water system, and schematic drawing of adsorp-tion/photochemical behaviors for the TNT

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[44] has been investigated. On the other hand, TNT has not only excellentphotocatalytic property but also high capability for the molecule adsorptionas a single phase material. This novel multifunctionality, hence the synergy ofmolecule adsorption and photocatalytic properties, is attributed to its uniquecrystal and nanostructures as well as material’s photochemical characteristic;as described before, TNT has a high surface area and layered compound-likestructure such as clay minerals. These structural characteristics might be thereason of the high adsorption capability of the TiO2 nanotube. Therefore,TNT is regarded as a novel multifunctional nanostructured material and thenexpected to be an excellent candidate as for the advanced high-performanceenvironmental purification system.

2.3.3 Multi-functionalized Titanium Oxide Nanotubes

In order to enhance properties and/or to functionalize materials, doping someelements and/or compositing with the other materials is often utilized. Forinstance, doping to silicon can control its semiconductive properties and hencevarious devices are widely developed and used. In the case of TiO2 nanotube,these are also applicable techniques. For instance, when TNT is consideredto be used as the chemical sensing device, electrode of solar cell, and so on,control and improvement of electrical properties are necessary and required toobtain higher conductivity, i.e., good carrier transfer properties and resultantbetter device performance. For this purpose, various metal cations have beendoped into TNT via the chemical synthesis process [45]. When transition metalcations such as Cr3+, Mn3+, Co2+, Nb5+, and V5+ were doped, morphology,surface area, and optical bandgap of the doped TNT were almost as sameas those of pure TNT. However, electrical conductivity of the doped TNTwas around 1–2 orders of magnitude higher, for example, 1.0× 10−4 S/cm for0.08 mol% Cr-doped TNT, than those of TiO2 nanopowder (2.6×10−6 S/cm)or pure TNT (3.0 × 10−6 S/cm).

Another effect of cation doping to the TNT was found in the thermalstability improvement as shown in Fig. 2.8; structural degradation of the nan-otube and the resultant decrease of surface area began at around 400◦C forthe pure TNT (refer also to Table 2.2). However, the critical temperature wasenhanced approximately 50 (Mn3+, Co2+, Nb5+, V5+) to 100◦C (Cr3+) forthe doped TNT [38]. These facts indicate that the cation doping can enhanceboth electrical conductivity and thermal stability of the nanotube, which isregarded as one of the advantages when the TNT will be used as variousdevices, because most of these devices are fabricated by the pasting and thefollowing sintering of the material to form films on the appropriate substrates.

Furthermore, loading various metals and/or compounds into inside of thenanotubes and/or onto the surfaces in nanometer scale is possible. Figure 2.9shows TEM images of metals and sulfide compound-loaded TNT nanocom-posites which were prepared by using various physicochemical processing.


Fig. 2.8. Temperature dependence of BET surface area of cation-doped TNTs(cation concentration ca. ∼ 0.1 mol%) and corresponding morphology change forpure and Cr-doped TNTs at 500◦C

Fig. 2.9. Various TNT-metal nanocomposites. (a) Pd-loaded TNT prepared bysonochemical method, (b) Ag nanoparticles formed inside of the TNT, (c) Ninanoparticles inside of the TNT, (d) ZnS-loaded TNT prepared by solution chemicalroute

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When Pd nanoparticles are composed with the TNT, hydrogen generationperformance was much enhanced due to the promoting effect of the loadednoble metals [38]. For the properties enhancement and further multifunction-alization, nanocompositing of TNT with the other materials is suitable andadvantageous method.

2.4 Conclusion and Prospects

In this chapter, chemical processing, structure, physical, and chemical prop-erties of TiO2 nanotube that can be prepared by the solution chemicalroute have been reviewed. Till now, a large number of fundamental stud-ies and application-oriented researches and developments are extensively car-ried out by many researchers for this low-dimensional nanomaterial, becausenot only enhancement of various properties of TiO2 but also multifunction-alization due to the harmonization of materials properties and unique low-dimensional nanostructure is expected. As for the application of TNT, it hasbeen used as the oxide electrode of the dye-sensitized solar cell, and bet-ter cell efficiency and structure-related characteristics on the charge trans-port phenomenon have been reported [43]. Also it is reported that TNTexhibits proton intercalation/de-intercalation and resultant electrochromism,size-selective adsorption of molecules [46], anion doping to develop visiblelight responsible TNTs [47], and biocompatibility [48, 49]. On the other hand,extensive challenges to develop various oxide and compound nanotubes havebeen continued. For instance, rare earth oxide nanotubes have recently beensynthesized [50]. (Details on the variety of nanotube materials are introducedin other chapters.) All these facts imply us that the oxide nanotubes in-cluding TNT have multifunctionalities owing to the structure–property cor-relations. As mentioned before, one of the future research direction of theTiO2 nanotube might lie toward the application as the environmental and/orenergy creating systems, which would become more important in the nearfuture.

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Index

Ag nanoparticles in TNT, 29

Carbon nanotube (CNT) discovery, 17Cation doping to TNT, 28

Direct synthesis of TNT, 19

Environmental purification functionsof TNT, 27–28

Formation mechanism of TNT, 21–25

High-temperature X-ray diffractionpatterns of synthesized TNT, 23

Hydrothermal synthesis of TNT, 20

Interplanar spacing in TNT, 22

Kasuga method, 18

Low temperature solutionchemical processing synthesisof TNT, 18–21

Ni nanoparticles in TNT, 29

Pd-loaded TNT, 29Photocatalytic functions of TNT, 25–27

Photochemical properties of TNT,25–27

Selected area electron diffraction(SAED) pattern of TNT, 22

Self-structuralization or self-organization, 18

Surface area variation on annealingtemperature for TNT, 23

TEM images of TNT synthesis, 21–22Temperature dependence of BET

surface area of cation-dopedTNTs, 29

Template or replica method, 18Titanium oxide nanotube (TNT) mutual

and synergy combination, 18Titanium oxide nanotube (TNT)

properties, 18TNT-metal nanocomposites, 29

Water splitting test, 26

X-ray diffraction patterns of TNTsynthesis, 20

ZnS-loaded TNT, 29

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2 Synthesis and Applications of Titanium Oxide Nanotubes