Topics in Current Chemistry (2020) 378:2
1 3
2 PART A—Sonication‑Assisted Approaches
2.1 0‑D Particles
2.1.1 Increasing the Porosity
The supramolecular assembly sol–gel method using surfactant molecules as a template/structure-directing agent for the synthesis of mesoporous titania was reported
firstly in 1995 by Antonelli and Ying [82]. The main drawback of the obtained hexagonally packed mesostructured TiO 2 was the presence of residual phosphorous
from the alkyl phosphate surfactant. From then, different long-chain organic molecules were studied as phosphorus-free surfactants. In 2000, Wang et al. [83] reported
a novel synthesis of mesoporous nanostructured titanate of a high porosity, by simultaneous ultrasonication during the synthesis (1.13 cm in diameter Ti horn, 20 kHz,
100 W/cm
2
). An ethanolic solution of the organic amine and titanium isopropoxide
was added slowly to a doubly distilled water, followed by aging for 6 h. The addition and the aging occurred under high-intensity ultrasonication, with the maximum
temperature reaching 80 °C. The removal of the surfactant from the obtained powder by centrifugation was achieved by dilute ethanolic HNO 3 solution and washing
with ethanol. The dried powder was also calcinated in a vacuum at 350 °C (8 h) or
450 °C (4 h). Three different long-chain organic amines (decylamine, dodecylamine,
and octadecylamine) were studied as the structure-directing agents.
The result was spherical or globular particles between 50 and 200 nm as an
aggregation of very small nanoparticles, as can be seen from the high-resolution
transmission electron microscopy(HRTEM) image (Fig. 4). The X-ray diffraction
(XRD) analysis showed an amorphous nature even after calcination at 350 °C, but
the rise of the calcination temperature to 450 °C led to an anatase crystallinity. The
surface areas after extraction, and calcination at 350 °C and 450 °C, were 853, 467,
and 79 m
2
/g, respectively. These values are high for metal oxides and higher than
analogous titanate hexagonal mesoporous framework structures synthesized by
hydrothermal and then thermal treatment with dodecylamine as a structure-directing
agent (710 m
2
/g) [83, 84]. The most interesting outcome was the that the obtained
nanoparticles had a structure of disordered wormhole framework, rather than a longranged hexagonal structure. This kind of channel motif and the high surface area
are ultimately important for catalytic application, due to the improved diffusion and
the availability of the active reaction sites. Compared to various other reports for
the synthesis of mesoporous TiO 2 nanoparticles, the benefits of this sonochemical
method is the simplicity and rapid rate of synthesis, that leads also to nanoparticles
of a high porosity. The authors linked the role of US irradiation to the accelerated
condensation/polymerization of titanium hydroxide at the interface of the gas phase
of the hotspots and the bulk solution.
38
Reprinted from the journal
1 3
2 PART A—Sonication‑Assisted Approaches
2.1 0‑D Particles
2.1.1 Increasing the Porosity
The supramolecular assembly sol–gel method using surfactant molecules as a template/structure-directing agent for the synthesis of mesoporous titania was reported
firstly in 1995 by Antonelli and Ying [82]. The main drawback of the obtained hexagonally packed mesostructured TiO 2 was the presence of residual phosphorous
from the alkyl phosphate surfactant. From then, different long-chain organic molecules were studied as phosphorus-free surfactants. In 2000, Wang et al. [83] reported
a novel synthesis of mesoporous nanostructured titanate of a high porosity, by simultaneous ultrasonication during the synthesis (1.13 cm in diameter Ti horn, 20 kHz,
100 W/cm
2
). An ethanolic solution of the organic amine and titanium isopropoxide
was added slowly to a doubly distilled water, followed by aging for 6 h. The addition and the aging occurred under high-intensity ultrasonication, with the maximum
temperature reaching 80 °C. The removal of the surfactant from the obtained powder by centrifugation was achieved by dilute ethanolic HNO 3 solution and washing
with ethanol. The dried powder was also calcinated in a vacuum at 350 °C (8 h) or
450 °C (4 h). Three different long-chain organic amines (decylamine, dodecylamine,
and octadecylamine) were studied as the structure-directing agents.
The result was spherical or globular particles between 50 and 200 nm as an
aggregation of very small nanoparticles, as can be seen from the high-resolution
transmission electron microscopy(HRTEM) image (Fig. 4). The X-ray diffraction
(XRD) analysis showed an amorphous nature even after calcination at 350 °C, but
the rise of the calcination temperature to 450 °C led to an anatase crystallinity. The
surface areas after extraction, and calcination at 350 °C and 450 °C, were 853, 467,
and 79 m
2
/g, respectively. These values are high for metal oxides and higher than
analogous titanate hexagonal mesoporous framework structures synthesized by
hydrothermal and then thermal treatment with dodecylamine as a structure-directing
agent (710 m
2
/g) [83, 84]. The most interesting outcome was the that the obtained
nanoparticles had a structure of disordered wormhole framework, rather than a longranged hexagonal structure. This kind of channel motif and the high surface area
are ultimately important for catalytic application, due to the improved diffusion and
the availability of the active reaction sites. Compared to various other reports for
the synthesis of mesoporous TiO 2 nanoparticles, the benefits of this sonochemical
method is the simplicity and rapid rate of synthesis, that leads also to nanoparticles
of a high porosity. The authors linked the role of US irradiation to the accelerated
condensation/polymerization of titanium hydroxide at the interface of the gas phase
of the hotspots and the bulk solution.
38
Reprinted from the journal