Topics in Current Chemistry (2020) 378:2
1 3
whiskers and nanorods was based on the sonication of synthesized titania nanoparticles in strongly basic solution (NaOH, 10 M), following by washing with dilute
HNO 3 (0.1 M) and deionized water and vacuum drying. Compared to other methods
used for the synthesis of 1-D structured titania (template synthesis, supra-molecular
assembles, hydrothermal synthesis, and inductive synthesis), this synthetic approach
taking place in a one-pot synthesis is faster, while avoiding the use and removal step
of the templates and the need of calcination for crystallization as the last step.
The used synthesized TiO 2 nanoparticles as precursors were prepared by hydrolysis of titanium butoxide, followed by calcination at 650 °C for 1 h. The average size
of them was around 20 nm, while the crystallographic composition was 17% anatase
and 83% rutile. For the synthesis of the whiskers, synthesized titanium oxide nanoparticles were dispersed in the basic aqueous solution inside a Teflon vessel. The
mixture was ultrasonicated for 80 min (direct immersion of Ti-horn, 560 W elec. , frequency not specified but probably in the low-frequency range, 20–80 kHz). The temperature during the synthesis was 80 °C.
Then the mixture after sonication was washed with diluted HNO 3 for 2 h and
with deionized water for 6 h. The obtained particles had a slender sheet structure of
a 60-nm diameter and a length around 1 μm. The interesting outcome arises from
the elemental stoichiometry analysis, which was found to be H 3 Ti 3 O 7.5 . The bands
at ~ 3400 and ~ 1630 cm
−1
at the IR spectrum were linked to the stretching vibrations of the O–H bond and to bending vibration of H–O–H, revealing the presence
of water. Since the XRD pattern matched with that of H 2 Ti 3 O 7 [113], and taking
into consideration the thermogravimetric results, the product was assigned from the
authors as H 2 Ti 3 O 7 .0.5H 2 O. Further washing of the product with water for 8 h led to
nano-whisker arrays of a 5-nm diameter. The X-ray diffractogram revealed that the
crystallographic phase changed to TiO 2 (B) [114] (Fig. 13).
For the preparation of the NTBs, the mixture was treated with half the US power
(280 W elec. ) for 60 min, and afterward, the Teflon vessel was maintained in an oil
bath at 110 °C for 4 h. The washing was with HNO 3 (0.1 M, 2 h) and deionized
water (14 h). The obtained NTBs had a 5-nm diameter and 200–300-nm length. The
XRD analysis revealed that the crystallographic phase was an intermediate between
H 2 Ti 3 O 7 ·0.5H 2 O and TiO 2 (B). No Na was detected at the elemental analysis, while
the ratio of Ti to O was 1:2.
The proposed mechanism of the whisker formation was based initially on the
US-assisted reaction of the base that leads to the cleavage of some Ti–O–Ti bonds.
The formed layered titanate lattices have octahedral form with alkali metal ions
to occupy the interlayered regions. During the washing with acid and water, ion
exchange and dehydration occur, resulting to H 2 Ti 3 O 7 ·0.5H 2 O. Extended dehydration by water washing promotes the transformation to titanate bronze. The role of
US is vital since it promotes the reaction between the raw nanoparticles and the
base, as well as controls the oriented growth. The synthesis is faster by the application of US compared to the reported hydrothermal methods of nanorod formation.
It is worth mentioning that without ultrasonication, no whiskers were obtained. A
lower US irradiation power and the hydrothermal treatment promotes the formation
of bigger titanate sheets and the exfoliation of nanosheets. The latter roll into NTBs
during the washing due to the removal of the ions and, as a result, to alterations
48
Reprinted from the journal
1 3
whiskers and nanorods was based on the sonication of synthesized titania nanoparticles in strongly basic solution (NaOH, 10 M), following by washing with dilute
HNO 3 (0.1 M) and deionized water and vacuum drying. Compared to other methods
used for the synthesis of 1-D structured titania (template synthesis, supra-molecular
assembles, hydrothermal synthesis, and inductive synthesis), this synthetic approach
taking place in a one-pot synthesis is faster, while avoiding the use and removal step
of the templates and the need of calcination for crystallization as the last step.
The used synthesized TiO 2 nanoparticles as precursors were prepared by hydrolysis of titanium butoxide, followed by calcination at 650 °C for 1 h. The average size
of them was around 20 nm, while the crystallographic composition was 17% anatase
and 83% rutile. For the synthesis of the whiskers, synthesized titanium oxide nanoparticles were dispersed in the basic aqueous solution inside a Teflon vessel. The
mixture was ultrasonicated for 80 min (direct immersion of Ti-horn, 560 W elec. , frequency not specified but probably in the low-frequency range, 20–80 kHz). The temperature during the synthesis was 80 °C.
Then the mixture after sonication was washed with diluted HNO 3 for 2 h and
with deionized water for 6 h. The obtained particles had a slender sheet structure of
a 60-nm diameter and a length around 1 μm. The interesting outcome arises from
the elemental stoichiometry analysis, which was found to be H 3 Ti 3 O 7.5 . The bands
at ~ 3400 and ~ 1630 cm
−1
at the IR spectrum were linked to the stretching vibrations of the O–H bond and to bending vibration of H–O–H, revealing the presence
of water. Since the XRD pattern matched with that of H 2 Ti 3 O 7 [113], and taking
into consideration the thermogravimetric results, the product was assigned from the
authors as H 2 Ti 3 O 7 .0.5H 2 O. Further washing of the product with water for 8 h led to
nano-whisker arrays of a 5-nm diameter. The X-ray diffractogram revealed that the
crystallographic phase changed to TiO 2 (B) [114] (Fig. 13).
For the preparation of the NTBs, the mixture was treated with half the US power
(280 W elec. ) for 60 min, and afterward, the Teflon vessel was maintained in an oil
bath at 110 °C for 4 h. The washing was with HNO 3 (0.1 M, 2 h) and deionized
water (14 h). The obtained NTBs had a 5-nm diameter and 200–300-nm length. The
XRD analysis revealed that the crystallographic phase was an intermediate between
H 2 Ti 3 O 7 ·0.5H 2 O and TiO 2 (B). No Na was detected at the elemental analysis, while
the ratio of Ti to O was 1:2.
The proposed mechanism of the whisker formation was based initially on the
US-assisted reaction of the base that leads to the cleavage of some Ti–O–Ti bonds.
The formed layered titanate lattices have octahedral form with alkali metal ions
to occupy the interlayered regions. During the washing with acid and water, ion
exchange and dehydration occur, resulting to H 2 Ti 3 O 7 ·0.5H 2 O. Extended dehydration by water washing promotes the transformation to titanate bronze. The role of
US is vital since it promotes the reaction between the raw nanoparticles and the
base, as well as controls the oriented growth. The synthesis is faster by the application of US compared to the reported hydrothermal methods of nanorod formation.
It is worth mentioning that without ultrasonication, no whiskers were obtained. A
lower US irradiation power and the hydrothermal treatment promotes the formation
of bigger titanate sheets and the exfoliation of nanosheets. The latter roll into NTBs
during the washing due to the removal of the ions and, as a result, to alterations
48
Reprinted from the journal