Metal Oxide Nanotubes

Aluminium oxide nanotubes. The anodic oxidation of aluminium in acidic electrolytes, resulting in the formation of a porous film of anodic aluminium oxide (AAO) consisting of nanotubes, was discovered in a pre-nanoscience era.28 In a typical method, pure aluminium is anodised in the presence of oxalic acid (0.3 mol dm-3), at a constant cell voltage of 40 V. A long period of ano-disation can improve the regularity of the tube arrangement, towards a hexagonal array of nanotubes.29 AAO films are currently widely used as a template for the preparation of other materials with a nanostructured morphology.

Aluminium oxide nanorods are usually produced by the hydrolysis of aluminium chloride during electrospinning30 or in hydrothermal conditions.31 In a typical hydrothermal procedure, 0.724 g of pure AlCl3 • 6H2O is dissolved in 30 cm3 of water at room temperature, followed by the slow addition of 15 cm3 of aqueous Na2B4O7. 10H2O (0.1moldm~3) with vigorous stirring. The transparent solution is then transferred into a PTFE-lined autoclave and hydrothermally treated at 200 °C for 24 h. The chemical composition of the nanorod bundles produced is similar to that of boehmite (g-AlOOH).

Barium titanate nanotubes (BaTiO3 NT) can be produced via a hydrothermal reaction of a TiO2 nanotube array with excess of aqueous Ba(OH)2 (0.05moldm-3) at 150 °C for 2 h.32 The morphology of barium titanate nanotubes is similar to that of the original TiO2 nanotubes.

Bismuth oxyhalides nanotubes (Bi24O31Br10 NT) can be produced by the hydrolysis of Bi(NO3)3 (0.5 g) in the presence of CTAB (0.5 g) with the addition of NaOH to a pH of 10, followed by hydrothermal treatment at 100-120 °C for 2-4 h.33 Bismuth oxyhalide nanotubes are characterised by a multilayer wall structure, with a diameter of 3-8 nm and a length of between 2 and 5 mm. An increase in hydrothermal temperature or the synthesis time, results in the formation of nanofibres rather than nanotubes.

Cobalt oxide nanotubes (CoO and Co3O4 NT) can be synthesised by the slow addition of 10 cm3 of a NH3 • H2O solution (0.1moldm-3) to 10 cm3 of a Co(NO3)2 solution (0.025 moldm-3). After stirring for 15 min, the precipitate is washed with water followed by filtration. The wet precipitate is mixed with a water-methanol mixture (1 : 1 v/v) and 0.3 g NaNO3 is added for the preparation of Co3O4 nanotubes (otherwise CoO nanotubes are formed), followed by hydrothermal treatment at 250 °C for 24 h.34 Both nanotubes are characterized by a multilayer wall structure with a 0.7 nm interlayer spacing. The outer diameter of nanotubes ranges from 10 to 20 nm and the nanotubes are up to several micrometres long.

Germanium oxide nanofibres (GeO2) can be synthesised by the simple hydrothermal recrystallisation of 1 g of bulk GeO2 in 48 cm3 of distilled water at 450 °C and 8.5-9.3 MPa, and stirring with a rotating speed of 100 rpm for 24 h.35 After the hydrothermal process, the nanofibres can be collected from the internal surface of the autoclave. The nanofibres are characterised by a single crystal structure of 30-300 nm diameter, and a length > 10 mm.

Hafnium oxide nanotubes (HfO2 NT) can be prepared by the electrochemical oxidation of hafnium in a fluoride-containing acid electrolyte. In a typical procedure, pure hafnium foil is degreased by sonicating it in acetone, iso-propanol and methanol sequentially, followed by rinsing with water and drying in a nitrogen atmosphere. Electrochemical anodization takes place in H2SO4 (1 moldm-3) with an aqueous NaF addition to the electrolyte (0.2 wt%), at room temperature with a cell voltage of 10 to 60 V.36 The nanotubes obtained have an internal diameter in the range of 15 to 90 nm (depending on the voltage), and are several tenths of a micrometre in length.

Iron oxide nanotubes (a-Fe2O3 NT, hematite) can be synthesised by the hydrothermal treatment of a mixture of FeCl3 and NH4H2PO4 solutions. In a typical experimental procedure, 3.20 cm-3 of an aqueous FeCl3 solution (0.5 moldm-3) and 2.88 cm-3 of an aqueous NH4H2PO4 solution (0.02 mol dm-3) are mixed with vigorous stirring, followed by the addition of water to give a final volume of 80 cm-3. The mixture is then hydrothermally treated at 220 °C for 48 h. The product consists almost entirely of nanotubes with outer diameters of 90-110 nm, inner diameters of 40-80 nm and lengths of 250-400 nm.37

Lead titanate nanotubes (PbTiO3 NT) can be obtained using similar methods to those used in the preparation of barium titanates, by the hydrothermal treatment of a TiO2 nanotube array with an aqueous solution of lead acetate (0.001 mol dm-3) at 260 °C for 6 h.38 Lead titanate nanotubes demonstrate good piezoelectric and ferroelectric properties.

Magnesium hydroxide nanotubes (Mg(OH)2 NT) can be prepared using a two-stage method via an intermediate product of Mg10(OH)18Cl2. 5H2O nanofibres.39 The hydrothermal treatment of this intermediate at 180 °C for 6 h in ethylenediamine (without stirring), results in the formation of lamellar nanotubes with an outer diameter of 80-300 nm, a wall thickness of 30-80 nm and a length of several tenths of a micrometre. A recent alternative method allows for the production of a smaller size of nanotubes.34 According to this method, 20 cm3 of an aqueous solution of MgCl2 (0.15 mol dm-3) is slowly mixed with 10 cm3 of a NH3. H2O solution (5 mol dm-3), with vigorous stirring at room temperature. The precipitate is thoroughly washed with water, and hydrothermally treated in 18 cm3 of a water-methanol mixture (1 : 1 v/v) with the addition of 0.3 g NaNO3 at 250 °C for 36 h. The nanotubes formed have a diameter of 10-20 nm and a length of several micrometres.

Manganese(IV) oxide nanotubes (MnO2 NT) can be produced using several methods, each of which results in the formation of nanotubes with a specific morphology and crystal structure. Solid MnO2 can exist in a, p, 8 and g crystallographic structures. The most stable is p-MnO2 which can be obtained with a nanotubular microstructure using the following method. In a typical process, 4.0mmol of MnSO4. H2O is dissolved in 10 cm-3 of distilled water, and 4.5mmol of PVP (K30, polymerization degree 360) is added slowly with vigorous stirring. When the solution becomes transparent, 8 cm-3 of an aqueous solution containing 8.0mmol of NaClO3 is added with continuous stirring. The resulting transparent solution is hydrothermally treated at 160 °C for 10 h. The p-MnO2 nanotubes obtained are usually cylindrical in shape, with a diameter of 200-500 nm and a length of 1-6 mm.40

d-MnO2 nanotubes can be obtained via the disproportion-hydrolysis of a-NaMnO2. In a typical synthesis, 0.3 g of NaMnO2 is dispersed into 30 cm-3 of diluted water, followed by hydrothermal treatment at 120-140 °C for about 4 days.41 The nanotubes are characterized by a multilayer wall structure (inter-layer distance of 0.7 nm), with diameters of 10-20 nm and lengths of several mm (see Figure 1.4a). Similar multilayered MnO2 nanotubes can also be produced using a multistage approach via the formation of single-layer MnO2 nanosheets.42

Nickel hydroxide nanotubes (Ni(OH)2 NT) can be obtained by the hydrothermal treatment of the precipitate formed during the slow mixing of 10 cm3 of NH3. H2O solution (0.2 mol dm-3) with 10 cm3 of Ni(NO3)2 solution (0.025 mol dm-3). A thorough wash of the precipitate with water, followed by hydrothermal treatment in 18 cm3 of a water-methanol mixture (1 : 1 v/v) with addition of 0.3 g of NaNO3 at 250 °C for 24 h, results in the formation of multilayer wall nanotubes with 0.7 nm interlayer spacing, 10-20 nm diameter and several micrometres length.34 The lamellar wall structure of nanotubes is

Figure 1.4 Electron microscopy images of MnO2: a) VOx, b) ZrO2, c) WS2, d) Ni3Si2O5(OH)4 and e) Mg3Si2O5(OH)4 nanotubes. (Images a), b), c), d) and e) were kindly taken with permission from ref. 41, 47, 50, 55, 61 respectively).

similar to that of Ni(SO4)0 3(OH)14 nanofibres obtained under similar hydrothermal conditions.43

Niobate nanotubes (e.g. K4Nb6O17 NT) can be fabricated using a multistage approach similar to that used for MnO2 nanotubes. In the first stage, potassium ions from bulk multilayered K4Nb6O17 are ion-exchanged to protons using strong inorganic acids (e.g. HCl). In the second stage, niobate single-layer nanosheets are exfoliated from the bulk crystal using TBAOH, which intercalates between the layers. In the third stage, the colloidal solution of single layer nanosheets is coagulated by the addition of KCl or NaCl solution forming nanotubes of alkaline metal niobates.42,44 The nanotubes that are obtained are characterised by a multilayered structure, with an external diameter ranging from 15 to 30 nm and a length of 0.1 to 1 mm.

Vanadium(V) oxide nanotubes (VOx NT, where x e 2.45) were originally synthesised using the hydrolysis of vanadium(V) alkoxide in the presence of an amine, followed by hydrothermal treatment. In a typical reaction procedure, a solution of vanadium(v) triisopropoxide (15.75 mmol) and hexadecylamine (7.87 mmol) in a molar ratio of 2 : 1 in ethanol (5 cm3) is stirred in an inert atmosphere for 1 hour, followed by the addition of 15 cm3 of water. After a few hours, a yellow, lamellar-structured composite of surfactant and a hydrolyzed vanadium oxide component are formed. The hydrothermal reaction of the mixture at 180 °C for about a week, leads to the formation of black, isolated or star-like grown-together nanotubes with open ends as the main product.45

VOx-nanotubes are characterized by multilayer wall structures with an interlayer spacing varying in range from 1.7 to 3.8 nm (depending on the size of the amine additive used in synthesis) as seen in Figure 1.4b. The outer diameter of the nanotubes ranges from 15 to 150 nm, with a length of 0.5-15 mm.46,47 A more recent, alternative method of nanotube synthesis uses another precursor. A suspension of V2O5 (15mmol) and a primary amine CnH2n+1NH2 (11 <n<20; molar ratio 1 : 1) in 5 cm3 ofethanol is stirred for 2 h, followed by the addition of 15 cm3 of water. The mixture is stirred at room temperature for 48 h. The hydrothermal treatment of the resulting composite in an autoclave at 180 °C for 7 days generates a black product containing VOx nanotubes.48

Zinc oxide nanotubes (ZnO NT) can be prepared by hydrolysis of Zn(NH3)42+ complexes under hydrothermal conditions. In a typical procedure, a mixture of aqueous Zn(NH3)42+ solution (adjusted to pH 12 with ammonia) and ethanol (1 : 11 v/v) is hydrothermally treated with constant stirring at 180 °C for 13 h.49 The resultant white precipitate can be collected, then washed several times with absolute ethanol and distilled water. The yield of tubular ZnO is about 20% polycrystalline nanotubes of approximately 450 nm in diameter and 4 mm in length.

Zirconium oxide nanotube (ZrO2 NT) arrays can be fabricated by anodic processing of pure zirconium metal in a fluoride-containing electrolyte. According to one method, a degreased and clean, flat zirconium metal electrode is electrochemically oxidized in an aqueous electrolyte containing (NH4)2SO4 solution (1 moldm-3) and 0.5 wt% NH4F at room temperature, at a cell voltage of 20 V.50 ZrO2 nanotubes are characterised by a 50 nm internal diameter and are approximately 17 mm in length (see Figure 1.4c).

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