applications of the final titanium oxide-based nanomaterials (Grindi et al. 2014;
Voepel and Smarsly 2017). Recently, interesting results have been published in this
field by using 1-ethyl-3-methylimidazolium for the preparation of titania nanoparticles
for ammonia detection (Sali et al. 2017) or 1-hexyl-3-methylimidazolium and 1-octyl3-methylimidazolium for the synthesis of titanium oxide nanofluids with applications
in acaricides detection (Wu et al. 2017).
Modification of Titanium Oxide by Different Synthetic Methods
As explained before, the most important disadvantage of the use of titanium oxide as
photocatalyst is its poor absorption of visible light; therefore, for an efficient use of
its photocatalytic properties, UV excitation energy is needed, which leads to a
higher-energy consumption, and subsequently higher costs, for the UV/TiO 2 system
(Janczarek et al. 2007).
To overcome the limitation associated with the energy consumption and to
improve the photocatalytic applicability of the titanium oxide-based materials, an
increase of the porosity and surface area is beneficial for the system. In addition, a
reduction of the band-gap is especially desirable to improve the absorption capacity
of titanium oxide materials. Therefore, the modification of titanium oxide absorption
spectrum toward visible light has been extensively studied (Janczarek et al. 2007;
Sánchez-Muñoz et al. 2013). For example, different methods such as stabilization
with dyes (e.g., tionine), doping with metallic ions (using transition metals such as
Cu, Co, Ni, Cr, Mn, Mo, Nb, V, Fe, Ru, Zn, Au, Ag, or Pt) or nonmetallic elements
(e.g., N, S, C, B, P, I, and F), and coupling with other semiconductors of lower bandgap (e.g., CdS nanoparticles) usually lead to a lower band-gap of the final
nanosystem (Gupta and Tripathi 2011; Ni et al. 2007; Zaleska 2008).
It has been experimentally observed that doping with transition metals of the 3d
block (one element, various elements, or one element and a co-catalyst) is an
efficient and simple method to modulate the band-gap values of the semiconductor
due to their ability to act as electron acceptors which minimizes the charge recombination. However, doping is effective only when using a low number of ions as
when the optimal quantity is exceeded, the photocatalytic activity may decrease due
to the formation of recombination electron-hole centers (Torres-Martínez and RuizGómez 2011). A consequence of the doping, in the ideal amount, is the modification
of the chemical structure of titanium oxide, with a displacement of the absorption
maximum to higher wavelengths lowering the band-gap (Janczarek et al. 2007;
Sánchez-Muñoz et al. 2013; Calatayud et al. 2012).
In addition, the transition anatase–rutile may be modified due to the presence of
certain ions in the growing step of the titanium oxide nanoparticles. It has been
established that certain dopants with +4 charge increase the temperature of the
anatase–rutile transition, while other dopants with different or variable charge
(except Al
3+ ) decrease the temperature of transition anatase–rutile (Torres-Martínez
and Ruiz-Gómez 2011). Various authors have studied the doping with interesting
7 Titanium Oxide-Based Nanomaterials with Photocatalytic Applications. . .
221
Voepel and Smarsly 2017). Recently, interesting results have been published in this
field by using 1-ethyl-3-methylimidazolium for the preparation of titania nanoparticles
for ammonia detection (Sali et al. 2017) or 1-hexyl-3-methylimidazolium and 1-octyl3-methylimidazolium for the synthesis of titanium oxide nanofluids with applications
in acaricides detection (Wu et al. 2017).
Modification of Titanium Oxide by Different Synthetic Methods
As explained before, the most important disadvantage of the use of titanium oxide as
photocatalyst is its poor absorption of visible light; therefore, for an efficient use of
its photocatalytic properties, UV excitation energy is needed, which leads to a
higher-energy consumption, and subsequently higher costs, for the UV/TiO 2 system
(Janczarek et al. 2007).
To overcome the limitation associated with the energy consumption and to
improve the photocatalytic applicability of the titanium oxide-based materials, an
increase of the porosity and surface area is beneficial for the system. In addition, a
reduction of the band-gap is especially desirable to improve the absorption capacity
of titanium oxide materials. Therefore, the modification of titanium oxide absorption
spectrum toward visible light has been extensively studied (Janczarek et al. 2007;
Sánchez-Muñoz et al. 2013). For example, different methods such as stabilization
with dyes (e.g., tionine), doping with metallic ions (using transition metals such as
Cu, Co, Ni, Cr, Mn, Mo, Nb, V, Fe, Ru, Zn, Au, Ag, or Pt) or nonmetallic elements
(e.g., N, S, C, B, P, I, and F), and coupling with other semiconductors of lower bandgap (e.g., CdS nanoparticles) usually lead to a lower band-gap of the final
nanosystem (Gupta and Tripathi 2011; Ni et al. 2007; Zaleska 2008).
It has been experimentally observed that doping with transition metals of the 3d
block (one element, various elements, or one element and a co-catalyst) is an
efficient and simple method to modulate the band-gap values of the semiconductor
due to their ability to act as electron acceptors which minimizes the charge recombination. However, doping is effective only when using a low number of ions as
when the optimal quantity is exceeded, the photocatalytic activity may decrease due
to the formation of recombination electron-hole centers (Torres-Martínez and RuizGómez 2011). A consequence of the doping, in the ideal amount, is the modification
of the chemical structure of titanium oxide, with a displacement of the absorption
maximum to higher wavelengths lowering the band-gap (Janczarek et al. 2007;
Sánchez-Muñoz et al. 2013; Calatayud et al. 2012).
In addition, the transition anatase–rutile may be modified due to the presence of
certain ions in the growing step of the titanium oxide nanoparticles. It has been
established that certain dopants with +4 charge increase the temperature of the
anatase–rutile transition, while other dopants with different or variable charge
(except Al
3+ ) decrease the temperature of transition anatase–rutile (Torres-Martínez
and Ruiz-Gómez 2011). Various authors have studied the doping with interesting
7 Titanium Oxide-Based Nanomaterials with Photocatalytic Applications. . .
221