Topics in Current Chemistry (2020) 378:2
1 3
in detail. When TPT was hydrolyzed under sonication at pH 0.7, the obtained sample of limited mass had a mix of rutile and anatase phases. Increasing the pH of the
supernatant of the above synthesis to 8.6 and further sonication for 3 h led to a pure
anatase phase. Based on all the above, it is obvious that even though US irradiation
and temperature play a key role by promoting the crystallization, the pH can determine the finally crystallographic phase. Another outcome derived by the authors
was that the hydrolysis of TPT in water is slower compared to TTC, resulting in a
more homogeneous and partly condensed gel. The formation of a hotspot due to US
waves inside the gel phase promotes the polycondensation of the Ti–OH species and
the formation of a large number of seed nuclei, leading to smaller nanoparticles.
In 2001, Yu et al. [85] studied the effect of US irradiation (cleaner bath, 47 kHz,
120 W elec. ) as well as the role of the ethanol-to-water ratio during the hydrolysis
upon precipitation of titanium tetraisopropoxide in pure water or mixed EtOH–H 2 O
solution at different ratios, followed by in-air calcination at 500 °C for 1 h. The ratio
of ethanol to water was found to play a key role in the crystallinity of the final powder, and, as a result, in the photocatalytic reactivity. While in pure aqueous solution,
the obtained material had a mix of anatase and brookite phases (in a ratio of around
80:20); the addition of methanol led to the elimination of the brookite phase. The
materials obtained without using methanol were found to possess a higher photoactivity against the oxidation of acetone in air compared to P25. On the contrary, the
material with a solely anatase phase showed the lowest oxidative performance. The
authors linked this to the fact that the presence of two crystallographic phases has a
positive impact on the photocatalytic activity, by decreasing the combination of the
photogenerated e
−
/h
+
pairs. In 2010, Ghows et al. synthesized nanosized TiO 2 by
hydrolysis of titanium tetra-isopropoxide in a solution of ethanol/water under lowintensity and high-frequency (500 kHz) sonication [86], although they did not study
their photocatalytic properties. The crystalline phase and particle size were dependent on the ethanol-to-water ratio, US irradiation time, and temperature.
2.1.3 Altering the Surface Chemical Features and Bandgap
In 2011, Chen et al. [87] reported that the distortion and doping of the outer surface of TiO 2 nanoparticles by high-pressure and high-temperature hydrogenation
led to an enhancement of the visible light absorption. Interestingly, and for the
first time, the obtained TiO 2 powder did not have the characteristic white color,
but a deep dark one. The reported synthesis of this material was conducted in two
phases. In the first phase, titanium dioxide nanocrystals of an ~8-nm diameter
were synthesized by a sol–gel method, using an organic template and acid (pluronic F127). The white powder obtained after calcination (500 °C, 6 h) underwent hydrogenation under a high-pressure (20 bars) H 2 atmosphere at ~ 200 °C
for 5 days, resulting in a black powder, stable even after 1 year.
The HRTEM analysis revealed no shape alteration upon hydrogenation; however, an outer disordered layer around 1 nm in thickness appeared. The X-ray diffractogram of both white and black samples revealed the characteristic peaks of
the anatase structure. The Raman spectrum of the white sample showed the six
typical Raman-active modes of the anatase structure. The Raman spectrum of the
40
Reprinted from the journal
1 3
in detail. When TPT was hydrolyzed under sonication at pH 0.7, the obtained sample of limited mass had a mix of rutile and anatase phases. Increasing the pH of the
supernatant of the above synthesis to 8.6 and further sonication for 3 h led to a pure
anatase phase. Based on all the above, it is obvious that even though US irradiation
and temperature play a key role by promoting the crystallization, the pH can determine the finally crystallographic phase. Another outcome derived by the authors
was that the hydrolysis of TPT in water is slower compared to TTC, resulting in a
more homogeneous and partly condensed gel. The formation of a hotspot due to US
waves inside the gel phase promotes the polycondensation of the Ti–OH species and
the formation of a large number of seed nuclei, leading to smaller nanoparticles.
In 2001, Yu et al. [85] studied the effect of US irradiation (cleaner bath, 47 kHz,
120 W elec. ) as well as the role of the ethanol-to-water ratio during the hydrolysis
upon precipitation of titanium tetraisopropoxide in pure water or mixed EtOH–H 2 O
solution at different ratios, followed by in-air calcination at 500 °C for 1 h. The ratio
of ethanol to water was found to play a key role in the crystallinity of the final powder, and, as a result, in the photocatalytic reactivity. While in pure aqueous solution,
the obtained material had a mix of anatase and brookite phases (in a ratio of around
80:20); the addition of methanol led to the elimination of the brookite phase. The
materials obtained without using methanol were found to possess a higher photoactivity against the oxidation of acetone in air compared to P25. On the contrary, the
material with a solely anatase phase showed the lowest oxidative performance. The
authors linked this to the fact that the presence of two crystallographic phases has a
positive impact on the photocatalytic activity, by decreasing the combination of the
photogenerated e
−
/h
+
pairs. In 2010, Ghows et al. synthesized nanosized TiO 2 by
hydrolysis of titanium tetra-isopropoxide in a solution of ethanol/water under lowintensity and high-frequency (500 kHz) sonication [86], although they did not study
their photocatalytic properties. The crystalline phase and particle size were dependent on the ethanol-to-water ratio, US irradiation time, and temperature.
2.1.3 Altering the Surface Chemical Features and Bandgap
In 2011, Chen et al. [87] reported that the distortion and doping of the outer surface of TiO 2 nanoparticles by high-pressure and high-temperature hydrogenation
led to an enhancement of the visible light absorption. Interestingly, and for the
first time, the obtained TiO 2 powder did not have the characteristic white color,
but a deep dark one. The reported synthesis of this material was conducted in two
phases. In the first phase, titanium dioxide nanocrystals of an ~8-nm diameter
were synthesized by a sol–gel method, using an organic template and acid (pluronic F127). The white powder obtained after calcination (500 °C, 6 h) underwent hydrogenation under a high-pressure (20 bars) H 2 atmosphere at ~ 200 °C
for 5 days, resulting in a black powder, stable even after 1 year.
The HRTEM analysis revealed no shape alteration upon hydrogenation; however, an outer disordered layer around 1 nm in thickness appeared. The X-ray diffractogram of both white and black samples revealed the characteristic peaks of
the anatase structure. The Raman spectrum of the white sample showed the six
typical Raman-active modes of the anatase structure. The Raman spectrum of the
40
Reprinted from the journal