depending on the starting anatase (Mohammadi et al. 2008; Gupta and Tripathi
2011; Sánchez-Muñoz et al. 2013). Therefore, on raising the calcination temperature, the crystallinity increases (lowering the band-gap); however, the surface area
decreases (as the particle size increases and the pores usually collapse).
The resulting photocatalytic activity is dominated by an equilibrium and compensation of crystallinity and surface area. Usually, a high degree of crystallinity is
more efficient than a high surface area, because the electron-hole recombination is
smaller in more crystalline materials. Indeed, the electron-hole recombination is the
biggest problem in most of the photocatalytic reactions in which products have more
energy than the reagents (Kudo and Miseki 2009). In addition, the hydroxyl concentration on the external surface area may have a high influence on the
photocatalytic activity (Oosawa and Grätzel 1988).
Some authors have recently investigated the phase transformation steps and their
relationship with the photocatalytic activity, finding that a higher photocatalytic
activity is usually found in crystalline mixtures of rutile and anatase or brookite
and anatase when compared with pure anatase (Gupta and Tripathi 2011; Luis et al.
2011).
7.2.1 Synthetic Methods for the Preparation
of Nanostructures of TiO 2
There are several methods for the preparation of titanium dioxide, and usually the
properties of the obtained materials are dependent on the synthetic route and the
calcination parameters (Luis et al. 2011). Indeed, following the same procedure, the
particle size may change if a different quantity of starting material is used
(Thamaphat et al. 2008).
The photoinduced reactions usually occur on the photocatalyst surface. Therefore, a high surface area is beneficial for the photocatalytic activity, making the use
of nanometric titanium oxide interesting (Souvereyns et al. 2013). In recent years,
the development of new synthetic methods for an effective reduction of the particle
size has been very significant, obtaining mesoporous or nanoporous materials of
titanium oxide in the form of nanocrystals, nanopowders, thin films, nanotubes, and
nanorods using sol–gel methods either with or without organic surfactants. In
addition, homogeneous precipitation, hydrothermal methods, melted salts, or chemical methods in vapor phase have also been recently used (Sánchez-Muñoz et al.
2013; Thamaphat et al. 2008).
Synthetic Methods in Vapor Phase
The synthetic methods in vapor phase are referred to each process in which the
materials are condensed in a vacuum chamber to form a solid phase. This kind of
processes, without chemical reactions involved, are known as PVD (physical vapor
218
A. Boudjemaa and S. Gómez-Ruiz
2011; Sánchez-Muñoz et al. 2013). Therefore, on raising the calcination temperature, the crystallinity increases (lowering the band-gap); however, the surface area
decreases (as the particle size increases and the pores usually collapse).
The resulting photocatalytic activity is dominated by an equilibrium and compensation of crystallinity and surface area. Usually, a high degree of crystallinity is
more efficient than a high surface area, because the electron-hole recombination is
smaller in more crystalline materials. Indeed, the electron-hole recombination is the
biggest problem in most of the photocatalytic reactions in which products have more
energy than the reagents (Kudo and Miseki 2009). In addition, the hydroxyl concentration on the external surface area may have a high influence on the
photocatalytic activity (Oosawa and Grätzel 1988).
Some authors have recently investigated the phase transformation steps and their
relationship with the photocatalytic activity, finding that a higher photocatalytic
activity is usually found in crystalline mixtures of rutile and anatase or brookite
and anatase when compared with pure anatase (Gupta and Tripathi 2011; Luis et al.
2011).
7.2.1 Synthetic Methods for the Preparation
of Nanostructures of TiO 2
There are several methods for the preparation of titanium dioxide, and usually the
properties of the obtained materials are dependent on the synthetic route and the
calcination parameters (Luis et al. 2011). Indeed, following the same procedure, the
particle size may change if a different quantity of starting material is used
(Thamaphat et al. 2008).
The photoinduced reactions usually occur on the photocatalyst surface. Therefore, a high surface area is beneficial for the photocatalytic activity, making the use
of nanometric titanium oxide interesting (Souvereyns et al. 2013). In recent years,
the development of new synthetic methods for an effective reduction of the particle
size has been very significant, obtaining mesoporous or nanoporous materials of
titanium oxide in the form of nanocrystals, nanopowders, thin films, nanotubes, and
nanorods using sol–gel methods either with or without organic surfactants. In
addition, homogeneous precipitation, hydrothermal methods, melted salts, or chemical methods in vapor phase have also been recently used (Sánchez-Muñoz et al.
2013; Thamaphat et al. 2008).
Synthetic Methods in Vapor Phase
The synthetic methods in vapor phase are referred to each process in which the
materials are condensed in a vacuum chamber to form a solid phase. This kind of
processes, without chemical reactions involved, are known as PVD (physical vapor
218
A. Boudjemaa and S. Gómez-Ruiz