Nanotubes titania

Mohapatra SK, Misra M, Mahajan VK et al (2007) A novel method for the synthesis of titania nanotubes using sonoelectrochemical method and its application for photoelectro-chemical splitting of water. J Catal 246 362-369... 

Khan, M.A. and Yang, O.B. (2009) Photocatalytic water splitting for hydrogen production under visible lighton Ir and Co ionized titania nanotube. Catalysis Today, 146 (1-2), 177-182. 

Table 108 Properties and CO conversion rates as a function of temperature over Au catalysts in combination with titania nanotubes using a feed containing 4.494% CO and a steam partial pressure of 31.1 kPa. The catalyst volume was 0.5 cm3 and the space velocity was 2000 h l 495... 

Au/Ti02 Deposition precipitation onto titania nanotubes as synthesized 174 (from 47) 18.4 3.1 2 12 23... 

The main difference between titania nanotube and the ID nanostructures discussed before is the presence of an hollow structure, but which has significant consequences for their use as catalytic materials (i) in the hollow fiber nanoconfinement effects are possible, which can be relevant for enhancing the catalytic performance (ii) due to the curvature, similarly to multi-wall carbon nanotubes, the inner surface in the nanotube is different from that present on the external surface this effect could be also used to develop new catalysts and (iii) different active components can be localized on the external and internal walls to realize spatially separated (on a nanoscale level) multifunctional catalysts. 

In addition, although TEM images are provided, it is unclear where the gold particles are located (inside or outside the Titania nanotubes). Therefore, it is unclear when there is a specific role of Titania nanostructure or instead the Titania nanotubes are simply a high surface area support. There are alternative and economic ways to produce high surface area Titania supports and at least Titania nanofibers would be preferable to nanotubes, if only the external surface of the nanotubes has to be used. 

Au/Ti02 systems prepared by deposition-precipitation method. Therefore, it is probable that a more relevant effect is the presence of some residual alkali on the surface due to the preparation method for these Titania nanotubes. 

Wang et al240 reported the electrooxidation of MeOH in H2S04 solution using Pd well-dispersed on Ti nanotubes. A similar reaction was studied by Schmuki et al.232 (see above), but using Pt/Ru supported on titania nanotube which appear a preferable catalyst. Only indirect tests (cyclic voltammetry) have been reported and therefore it is difficult to understand the real applicability to direct methanol fuel cell, because several other aspects (three phase boundary to methanol diffusivity, etc.) determines the performance. 

In conclusion, these data do not allow concluding whether or not Titania nanotubes form better catalysts due to their intrinsic nanostructure, and not simply because they have a high geometrical surface area and provide a good dispersion of supported catalysts. These properties may be found in other Titania based catalysts not having a ID nanostructure. On the other hand, it is also clear from above comments that most of the studies up to now were justified essentially from the curiosity to use a novel support more than from the rational design of advanced catalysts, which use the metal oxide nanostructure as a key component to develop... 

The preparation of Titania nanocoils has been yet not investigated in literature. However, quite recent results258 show that the effective structure of Titania nanotube likely produced by controlled anodization process is that of a helical (compressed) nanocoil. Fig. 11 shows this concept. It was also demonstrated that the formation of these helical nanocoils improves the photo-generated current compared to samples after short anodization where only a Titania layer is formed. 

Fig. 3.23 Efficiency under near UV illumination of a photoelectrochemical cell comprised of a titania nanotube array photoanode and Pt counter electrode. For the calculation of efficiency using equation (3.6.13), a two electrode geometry was used while for the calculation using equations (3.6.15a) and (3.6.16), a three electrode geometry was used.

Fig. 3.24 Incident photon to current efficiency (IPCE) spectrum of a titania nanotube array photoelectrode.

Fig. 3.25 (a) The solar photocurrent spectrum of a titania nanotube array obtained using data from Fig. 3.19 and Fig. 3.24. (b) The total solar photocurrent obtained by integrated the photocurrent of (a). 

With Vbias= 0.51V, the solar photoconversion efficiency of titania nanotube (6 pm length) array photoelectrodes was calculated as 0.6 %. 

Metal oxide nanotubes have been synthesized by a diverse variety of fabrication routes. For example titania nanotubes, and nanotube arrays, have been produced by deposition into a nanoporous alumina template [48-51], sol-gel transcription using organo-gelators as templates [52,53], seeded growth [54], hydrothermal processes [55-57] and anodic oxidation [58-65]. 

The following mechanism has been proposed for titania nanotube formation by Bavykin and co-workers [94] ... 

Different from the formation mechanism of titania nanotubes, Fe203 nanotubes are formed by a coordination-assisted dissolution process [95]. The presence of phosphate ions is the crucial factor that induces the formation of a tubular structure, which results from the selective adsorption of phosphate ions on the surfaces of hematite particles and their ability to coordinate with ferric ions. 

Fabrication of titania nanotube arrays via anodic oxidation of titanium foil in fluoride based solutions was first reported in 2001 by Gong and co-workers [58]. Further studies focused on precise control and extension of the nanotube morphology [21], length and pore size [22], and wall thickness [3]. Electrolyte composition plays a critical role in determining the resultant nanotube array architecture and, potentially, its chemical composition. Electrolyte composition determines both the rate of nanotube array formation, as well as the rate at which the resultant oxide is dissolved. In most cases, a fluoride ion containing electrolyte is needed for nanotube array formation. In an effort to shift the band gap of the titania... 

Fig. 5.6 FESEM images of 10 V titania nanotube arrays anodized at (a) 5°C, and (b) SOX. The pore size is nearly 22 nm for all samples. In (a) the average wall thickness is 34 nm, and in (b) is 9 nm.

Fabrication of Titania Nanotube Arrays by Anodization 283 Dimethyl Sulfoxide Electrolytes... 

The titania nanotube arrays were observed in the anodization voltage range 20 to 65V in the NH4F concentration range (0.1 - 0.5 wt%) and H2O (1% - 4%) in ethylene glycol (EG) [24,27]. Figure 5.13 depict the self alignment exhibited by the nanotube arrays, in bottom... 

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