5.44 Chemical Message
The above-discussed results suggest that the main goal for developing of new
photocatalysts is to separate the positive charges (holes) from the negative ones
(electrons). Both of them are able to singularly interact with the substrate. The
charge separation has constituted the basis for optimizing the catalyst materials, and
the strategies ranged from the use of particles with high aspect ratio
(shape-controlled) to the use of doped oxides or mixed oxides with different band
gaps, to the use of non-innocent supports. These aspects will be described in next
chapters.
5.45 The Case of Shape-Controlled TiO 2 Nanoparticles
The photocatalytic activity resulted is strictly related to the exposed surfaces;
however, a clear assessment of the role of the crystal faces in the photocatalytic
processes appears necessary [23, 24]. To this aim, the properties of the photogenerated defects and their dependence on the exposed faces of different anatase
crystals have been investigated. Spherical particles, nanobars, and rhombic and
rhombic elongated nanocrystal were synthesized by specific solvothermal procedures. The charge trapping centers active in irradiated particles were Ti
3+ O
− O 2
− ,
all detectable by electron spin resonance. The quantification of these defects in the
various shape-controlled particles gives an indication of which defects are prevailing on a given face.
5.46 Discussion of the Case
Figure 5.32 shows the TEM and HRTEM images of the different materials and
indicates their nick names. The differences in shape are well evident as well as the
location of the different facets.
Details of the morphology are given in Table 5.5.
It appears from the t 1/2 values that the rhombic-shaped particles are the most
catalytically active. Thus, the photocatalytic activity is greatly dependent on the
particle shape. Interestingly, the best performance occurs for R-shaped particles
which also display a relatively high SSA BET . Despite having the lowest SSA BET ,
RE nanocrystals show relevant activity too. Finally despite the very high SSA BET ,
NBs show the worst photoefficacy. These results suggest that the difference in
surface area is not representative of the photocatalytic properties. Instead, the
exposed crystal faces and their relative SSA BET play the major role. Specifically,
126
5 The Symmetry Properties Describe the Electronic Structure …
The above-discussed results suggest that the main goal for developing of new
photocatalysts is to separate the positive charges (holes) from the negative ones
(electrons). Both of them are able to singularly interact with the substrate. The
charge separation has constituted the basis for optimizing the catalyst materials, and
the strategies ranged from the use of particles with high aspect ratio
(shape-controlled) to the use of doped oxides or mixed oxides with different band
gaps, to the use of non-innocent supports. These aspects will be described in next
chapters.
5.45 The Case of Shape-Controlled TiO 2 Nanoparticles
The photocatalytic activity resulted is strictly related to the exposed surfaces;
however, a clear assessment of the role of the crystal faces in the photocatalytic
processes appears necessary [23, 24]. To this aim, the properties of the photogenerated defects and their dependence on the exposed faces of different anatase
crystals have been investigated. Spherical particles, nanobars, and rhombic and
rhombic elongated nanocrystal were synthesized by specific solvothermal procedures. The charge trapping centers active in irradiated particles were Ti
3+ O
− O 2
− ,
all detectable by electron spin resonance. The quantification of these defects in the
various shape-controlled particles gives an indication of which defects are prevailing on a given face.
5.46 Discussion of the Case
Figure 5.32 shows the TEM and HRTEM images of the different materials and
indicates their nick names. The differences in shape are well evident as well as the
location of the different facets.
Details of the morphology are given in Table 5.5.
It appears from the t 1/2 values that the rhombic-shaped particles are the most
catalytically active. Thus, the photocatalytic activity is greatly dependent on the
particle shape. Interestingly, the best performance occurs for R-shaped particles
which also display a relatively high SSA BET . Despite having the lowest SSA BET ,
RE nanocrystals show relevant activity too. Finally despite the very high SSA BET ,
NBs show the worst photoefficacy. These results suggest that the difference in
surface area is not representative of the photocatalytic properties. Instead, the
exposed crystal faces and their relative SSA BET play the major role. Specifically,
126
5 The Symmetry Properties Describe the Electronic Structure …