deposition processes). On the other hand, those that are focused on chemical
reactions are usually named CVP (chemical vapor deposition processes) (Chen
and Mao 2007).
Among the vapor-phase methods, the flame direct oxidation method is probably
the most widely used for the preparation of submicronic anatase particles. This
process is based on the combustion (with oxygen) of a mixture of hydrogen and
titanium(IV) chloride in a flame. Using this method, the particle size is normally
between 10 and 100 nm. This method is usually time-dependent, as long periods of
time in the flame give higher particle size.
There are some other methods in vapor phase, such as electric arc-induced
plasma, laser- and aerosol-induced methods, (Kho et al. 2011) and electron beams,
although all these methods are less extended than the flame oxidation.
During the last 5 years, additional thermal plasma methods have also been applied
for the preparation of different phases of titanium oxides with very interesting
results, which open up new directions in the search of novel more controlled
synthetic methods for the preparation of titanium oxide-based materials (Arif et al.
2017).
Synthetic Methods in Liquid Phase
Liquid-phase synthetic methods offer many advantages compared with the vaporphase methods, such as a strict control of the stoichiometric conditions, formation of
complex forms, production of homogeneous materials, and possibility of preparation
of composite materials. However, some disadvantages are also present when using
liquid-phase methods, like higher costs of the chemical precursors, presence of
carbon as impurities, and longer reaction times (Gupta and Tripathi 2011). In most
of the cases, these methods usually have, as an objective, the preparation of
nanometric particles with narrow particle size distributions and of extremely high
external surface area.
There are a high number of synthetic methods in liquid phase such as hydrothermal, solvothermal, electrodeposition, sonochemical, micellar, inverted micellar, and
microwave methods, among others (Chen and Mao 2007). However, the procedures
based on the sol–gel technique (Fig. 7.2) are the ones most extended due to their
versatility, reproducibility, and ease of the control of the synthetic parameters
(Pizarro 2005; Chen and Mao 2007).
Sol–gel methods are based on a physical transition from a system in liquid state
(usually a colloidal suspension of nanosized particles, namely, a sol) to a solid phase,
namely, a gel (which is a solid constituted of at least two phases with liquid phases
trapped and immobilized by the solid phase) (Macwan et al. 2011). The precursors
that are usually employed in the preparation of the sol are metallic inorganic salts,
such as titanium alkoxides. In a typical sol–gel method (Fig. 7.2), the precursor is
hydrolyzed, and a subsequent polycondensation step leads to a colloidal suspension
(sol), normally using a base or an acid as catalyst of the reaction. In the system, some
very small chains of solid particles are formed in the liquid (organic or aqueous
7 Titanium Oxide-Based Nanomaterials with Photocatalytic Applications. . .
219
reactions are usually named CVP (chemical vapor deposition processes) (Chen
and Mao 2007).
Among the vapor-phase methods, the flame direct oxidation method is probably
the most widely used for the preparation of submicronic anatase particles. This
process is based on the combustion (with oxygen) of a mixture of hydrogen and
titanium(IV) chloride in a flame. Using this method, the particle size is normally
between 10 and 100 nm. This method is usually time-dependent, as long periods of
time in the flame give higher particle size.
There are some other methods in vapor phase, such as electric arc-induced
plasma, laser- and aerosol-induced methods, (Kho et al. 2011) and electron beams,
although all these methods are less extended than the flame oxidation.
During the last 5 years, additional thermal plasma methods have also been applied
for the preparation of different phases of titanium oxides with very interesting
results, which open up new directions in the search of novel more controlled
synthetic methods for the preparation of titanium oxide-based materials (Arif et al.
2017).
Synthetic Methods in Liquid Phase
Liquid-phase synthetic methods offer many advantages compared with the vaporphase methods, such as a strict control of the stoichiometric conditions, formation of
complex forms, production of homogeneous materials, and possibility of preparation
of composite materials. However, some disadvantages are also present when using
liquid-phase methods, like higher costs of the chemical precursors, presence of
carbon as impurities, and longer reaction times (Gupta and Tripathi 2011). In most
of the cases, these methods usually have, as an objective, the preparation of
nanometric particles with narrow particle size distributions and of extremely high
external surface area.
There are a high number of synthetic methods in liquid phase such as hydrothermal, solvothermal, electrodeposition, sonochemical, micellar, inverted micellar, and
microwave methods, among others (Chen and Mao 2007). However, the procedures
based on the sol–gel technique (Fig. 7.2) are the ones most extended due to their
versatility, reproducibility, and ease of the control of the synthetic parameters
(Pizarro 2005; Chen and Mao 2007).
Sol–gel methods are based on a physical transition from a system in liquid state
(usually a colloidal suspension of nanosized particles, namely, a sol) to a solid phase,
namely, a gel (which is a solid constituted of at least two phases with liquid phases
trapped and immobilized by the solid phase) (Macwan et al. 2011). The precursors
that are usually employed in the preparation of the sol are metallic inorganic salts,
such as titanium alkoxides. In a typical sol–gel method (Fig. 7.2), the precursor is
hydrolyzed, and a subsequent polycondensation step leads to a colloidal suspension
(sol), normally using a base or an acid as catalyst of the reaction. In the system, some
very small chains of solid particles are formed in the liquid (organic or aqueous
7 Titanium Oxide-Based Nanomaterials with Photocatalytic Applications. . .
219