Bottomup approach

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Figure 2.1 Top-down and bottom-up approaches for the preparation of nanos-tructured materials.

Figure 2.2 The template method for the preparation of nanostructured materials. (ALD: atomic layer deposition; CVD: chemical vapour deposition).

and temperature, respectively. After successful adhesion of material, the template is removed by various methods, including pyrolysis, selective etching or dissolution.

The templates can be classified into several types according to the geometry of the deposited layer.2 In instances where the precursor is deposited onto the external surface of the template (e.g. the surface of nanofibres, nanorods or spheres), the template is classified as positive. In instances where the precursor is deposited into the internal pores of the template, (e.g. anodic aluminium oxide - an AAO membrane), the template can be classified as negative. The types of template can be also divided according to the characteristic size of the nanostructure, allowing templates to be categorised into macroporous, meso-porous and microporous types.3

An example of a macroporous negative template is provided by polystyrene beads with a narrow distribution of diameter, allowing their self assembling into photonic structures with regular pores having a characteristic pore size in the range of several tens of nanometres. Sol-gel deposition of TiO2 into these pores, followed by removal of resin under calcination, results in the formation of an inversed opal4 type nanostructured TiO2 (see Figure 2.3).

Figure 2.3 Important examples of templates used to prepare nanostructured TiO2 materials. (Images are reproduced with kind permission as follows: inversed opals from ref 4. nanotubular array from ref 16. tri-block copolymer template TiO2 from ref 8. helical ribbon TiO2 from ref. 5 and MWCN-templated TiO2 nanofibres from ref. 6).

Figure 2.3 Important examples of templates used to prepare nanostructured TiO2 materials. (Images are reproduced with kind permission as follows: inversed opals from ref 4. nanotubular array from ref 16. tri-block copolymer template TiO2 from ref 8. helical ribbon TiO2 from ref. 5 and MWCN-templated TiO2 nanofibres from ref. 6).

An example of a macroporous positive template is the lyophylic organogel which can form tubular or helical nanostructures over a certain concentration range.5 The controlled hydrolysis of titanium(iv) esters in the presence of an organogel may result in the precipitation of TiO2 on the surface of the template and a reproduction of its morphology. After removal of the organic phase, the TiO2 produced has a nanotubular or helical morphology (see Figure 2.3).

Multi-walled carbon nanotubes (MWCN)6 or carbonaceous nanofibres7 can be considered as mesoporous positive templates. Sol-gel deposition of TiO2 on the external surface of MWCN or nanofibres followed by calcination in air results in the formation of TiO2 nanofibres or nanotubes (see Figure 2.3 and Table 2.1).

A new class of triblock copolymer with the general formula HO(CH2-CH2O)n(CH2CH(CH3)O)m(CH2CH2O)nH has been recently introduced. In particular, solutions above their critical micelle concentration can give rise to self-assembled, hexagonal, packed-rod like structures (see Figure 2.3). The solgel hydrolysis/condensation of titanium(iv) alkoxides in the presence of such triblock copolymers [e.g. Pluronic P123 with n = 20, m = 70 (ref. 8) or Pluronic F127 with n = 106, m = 70 (ref. 9)], followed by polymer removal at elevated temperatures, results in the formation of mesoporous TiO2 with a hexagonal pore structure.

The preparation of TiO2 nanotubes by chemical templating2 usually involves the controlled sol-gel hydrolysis of solutions of titanium containing compounds in the presence of templating agents, followed by polymerisation of TiO2 in the self-assembled template molecules or deposition of TiO2 onto the surface of template aggregates. The next stages are the selective removal of the templating agent and calcination of the sample.

The group consisting of self-assembled organic surfactant template molecules is probably the largest group, since a wide range of organic molecules can be used. Among the organic templates used for TiO2 nanotube synthesis, are the organogel of trans-(1R,2R)-1,2-cyclohexanedi(11-aminocarbonylunde-cylpyridinium)5 and dibenzo-30-crown-10-appended cholesterol10 and the hydrogel of tripodal cholamide-based materials,11 together with laurylamine hydrochloride surfactant.12

Other examples of specialised templating agents include tobacco mosaic viruses,13 precipitated platinum salts14 and electrospun polyacrylonitrile fibres.15

Porous alumina, produced by anodising of aluminium foil (anodic aluminium oxide AAO), has been widely used as a macroporous negative template for the preparation of TiO2 nanotubes. The internal surface of cylindrical pores of AAO is used for the deposition of TiO2 thin films from various precursors by sol-gel depositions16 18 or electrodeposition.19 After selective removal of alumina using a concentrated solution of NaOH, the external diameter of TiO2 hollow fibres corresponds to the diameter of alumina pores. The internal diameter of the TiO2 nanotubes depends on the synthesis conditions and the thickness of the wall (Table 2.1).

After calcination at 500 °C, the crystal structure of TiO2 nanotubes produced by templating is usually amorphous or polycrystalline anatase. The tubes have

Table 2.1 Morphological properties of Ti02 nanotubes produced by the sol-gel method in the presence of templating agents.

Precursor Template Conditions

Nano tube diameter/nm

Ref.

Ti(OBu)4

25 C. ethanol. NH4OH

50-300

25 CC, 1-butanol. benzylamine

Ti(OBu)4

CH3(CH2)IINH2 ■ HC1

25-40 C. H20

1800-6000

12

Ti(0/Pr)4

Tobacco mosaic viruses

25 C. ethanol

20

13

Ti(OBu)4

[Pt(NH3)4](HC03)

25 C. ethanol

100

14

TiCl4

polyacrylonitrile fibres

25 C. ethanol. NH4OH

220

15

Ti(OBu)4

Carbonaceous nanofibres

25 C. ethanol

20

17

Ti(0/Pr)4

AAO membrane

25 CC. pressure impregnation

60-70

16

TiF4

AAO membrane

60 C. HC1

2.5-5. 70-100

18

Ti(0/Pr)4

AAO membrane

25 C. ethanol. CH,COOH

120-140

17

TiCl,

AAO membrane

25 C. HC1. electrodeposition

70-100

19

AAO = Anodic aluminium oxide. Bu = »-butyl and /Pr = isopropyl.

AAO = Anodic aluminium oxide. Bu = »-butyl and /Pr = isopropyl.

different mean internal diameters depending on the nature of the templating agent (Table 2.1). The specific surface area for very wide tubes is not very high except for the cases where the structure of the microtube wall contains small channels.12 Nanotubes have potential uses in the photocatalytic removal of organic pollutants or in solar cells,20,21 although industrial applications may be limited by the cost of materials, insufficient material characterisation and concerns over long term instability.

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