was found to have antibacterial properties. Concerning
the additional steps to an initial mechanical step, it is
apparent that the synthesis of nanoparticles requires a
combination of methods.
ii. Method-2
In a modified physical method for generating
nanoparticles of titanium, Al-doped samples of TiO 2
were subjected to milling by a planetary type ball mill
for 90 min (Nadeem et al. 2018). It has to be pointed
out the reasons for doping are not connected to a
physical method of synthesizing nanoparticles, but inter
alia, to reduce the band gap, to increase photocatalytic
activity and inhibit change of phase. Milling was followed by addition of deionized water, as drops,
accompanied by concomitant stirring to yield a solution. The latter was ball-milled for 2 h followed by
annealing, using stepwise increases in temperatures,
between 773 and 1170 K. A similar procedure, without
using Al-doping, gave pure nano-TiO 2.
2.2.2 Sol–Gel Methods
i. Method 1
Variations of the sol–gel technique have been used to
prepare nanoparticles of titanium dioxide (Guo et al.
2016). In principle, this technique is fairly simple and
starting material is generally a metal salt of a metal
alkoxide which is subjected to hydrolysis resulting in
the generation of a colloidal suspension. The latter is
referred to as a sol. When polymerization sets in, the
sol assumes a gelatin-like form which is referred to as a
sol–gel (solid gel). If the sol–gel is placed in a mould
and heated to drive out solvent, a substance with a
porous nature called an aerogel is formed. Some
examples will serve to the variations in the application
of the sol–gel method. The precursor in the following
variation was TiCl 4 with ethanol as solvent (Sabry et al.
2016). Controlled addition of TiCl 4 solution to ethanol
gave a yellow solution. The formation of the sol–gel
was monitored over periods from 1 to 5 days. After
drying at 80 °C, the solid was subjected to calcination
at 500 °C. It transpired that the size of the nanoparticles
was dependent on the temperature used.
ii. Method 2
With tetra-n-butyl orthotitanate as a precursor and
hydrochloric acid as solvent, nanoparticles of titanium
dioxide in anatase and rutile phases obtained using the
sol–gel method (Dalvandi and Ghasemi 2013). Here,
the precursor and solvent were stirred well for 9 h to
make a homogeneous mixture after which added liquid
ammonia. An increase in calcination temperature led to
a change in phase from anatase to rutile. As in the
previous example, the size of the particle depended on
the temperature at which the calcination was done.
iii. Method 3 with combustion
A combination of sol–gel with a combustion method
gave pure anatase phase titanium dioxide nanoparticles
(Jongprateep et al. 2015). The chosen precursor was
either titanium isopropoxide (TTIP) or submicrometre
sized particles of TiO 2 and method was sol–gel or
combustion. The average size of powders by the
combustion method using TTIP and submicroparticles
as precursors, yielded particles with average sizes of 44
and 77 nm, respectively, while average sizes of particles using the sol–gel method for TTIP and submicrometre particles were found to be 48 and 85 nm,
respectively.
2.3 Thermal Methods of Preparing
Nanoparticles of Titanium Dioxide
2.3.1 Thermal Method
These techniques could be hydrothermal or solvothermal.
The following examples should suffice to illustrate how the
method is used. Nano-TiO 2 was produced using TiCl 3 as a
precursor, by three different routes (Zeng and Zeng 2017). In
the first route, TiCl 3 and NH 4 F (as a mineralizer) were mixed
in distilled water as the medium of reaction. As the dopant,
NiCl 2 was added to the above mixture with vigorous stirring
over 15-min of the period. In the second route, TiCl 3 was
mixed with NH 4 F only. For the third route, TiCl 3 was added
to NiCl 2. All three reaction mixtures were subjected to
heating in an autoclave at 180 °C for a whole day and night
to give a precipitate and a solution phase. The solution phase
was pipetted out to leave a precipitate containing the targeted
material. The precipitate is washed with purified water and
alcohol. The dry product was characterized by X-ray
diffraction (XRD), scanning electron microscopy
(SEM) and transmission electron microscopy (TEM). In
summary, the results of these revealed that different types of
crystals were formed from the three different routes used and
that these researchers (Zeng and Zeng 2017) had succeeded
in producing single crystals of anatase titanium dioxide
which had one reactive crystal face.
2.3.2 Solvothermal Method
The solvothermal method was used to synthesize very fine
crystals of TiO 2 targeted for photocatalytic uses (Kim et al.
2003). The precursor was chosen as titanium isopropoxide
(TTIP). It was dissolved in dry toluene together with oleic
acid, under an atmosphere of argon. Varying amounts of TIP
were used to give 5:100; 10:100; 20:100 of TIP to solvent.
The mixtures were stirred for 24 h, then placed in an
76
K. G. Moodley and V. Arumugam
the additional steps to an initial mechanical step, it is
apparent that the synthesis of nanoparticles requires a
combination of methods.
ii. Method-2
In a modified physical method for generating
nanoparticles of titanium, Al-doped samples of TiO 2
were subjected to milling by a planetary type ball mill
for 90 min (Nadeem et al. 2018). It has to be pointed
out the reasons for doping are not connected to a
physical method of synthesizing nanoparticles, but inter
alia, to reduce the band gap, to increase photocatalytic
activity and inhibit change of phase. Milling was followed by addition of deionized water, as drops,
accompanied by concomitant stirring to yield a solution. The latter was ball-milled for 2 h followed by
annealing, using stepwise increases in temperatures,
between 773 and 1170 K. A similar procedure, without
using Al-doping, gave pure nano-TiO 2.
2.2.2 Sol–Gel Methods
i. Method 1
Variations of the sol–gel technique have been used to
prepare nanoparticles of titanium dioxide (Guo et al.
2016). In principle, this technique is fairly simple and
starting material is generally a metal salt of a metal
alkoxide which is subjected to hydrolysis resulting in
the generation of a colloidal suspension. The latter is
referred to as a sol. When polymerization sets in, the
sol assumes a gelatin-like form which is referred to as a
sol–gel (solid gel). If the sol–gel is placed in a mould
and heated to drive out solvent, a substance with a
porous nature called an aerogel is formed. Some
examples will serve to the variations in the application
of the sol–gel method. The precursor in the following
variation was TiCl 4 with ethanol as solvent (Sabry et al.
2016). Controlled addition of TiCl 4 solution to ethanol
gave a yellow solution. The formation of the sol–gel
was monitored over periods from 1 to 5 days. After
drying at 80 °C, the solid was subjected to calcination
at 500 °C. It transpired that the size of the nanoparticles
was dependent on the temperature used.
ii. Method 2
With tetra-n-butyl orthotitanate as a precursor and
hydrochloric acid as solvent, nanoparticles of titanium
dioxide in anatase and rutile phases obtained using the
sol–gel method (Dalvandi and Ghasemi 2013). Here,
the precursor and solvent were stirred well for 9 h to
make a homogeneous mixture after which added liquid
ammonia. An increase in calcination temperature led to
a change in phase from anatase to rutile. As in the
previous example, the size of the particle depended on
the temperature at which the calcination was done.
iii. Method 3 with combustion
A combination of sol–gel with a combustion method
gave pure anatase phase titanium dioxide nanoparticles
(Jongprateep et al. 2015). The chosen precursor was
either titanium isopropoxide (TTIP) or submicrometre
sized particles of TiO 2 and method was sol–gel or
combustion. The average size of powders by the
combustion method using TTIP and submicroparticles
as precursors, yielded particles with average sizes of 44
and 77 nm, respectively, while average sizes of particles using the sol–gel method for TTIP and submicrometre particles were found to be 48 and 85 nm,
respectively.
2.3 Thermal Methods of Preparing
Nanoparticles of Titanium Dioxide
2.3.1 Thermal Method
These techniques could be hydrothermal or solvothermal.
The following examples should suffice to illustrate how the
method is used. Nano-TiO 2 was produced using TiCl 3 as a
precursor, by three different routes (Zeng and Zeng 2017). In
the first route, TiCl 3 and NH 4 F (as a mineralizer) were mixed
in distilled water as the medium of reaction. As the dopant,
NiCl 2 was added to the above mixture with vigorous stirring
over 15-min of the period. In the second route, TiCl 3 was
mixed with NH 4 F only. For the third route, TiCl 3 was added
to NiCl 2. All three reaction mixtures were subjected to
heating in an autoclave at 180 °C for a whole day and night
to give a precipitate and a solution phase. The solution phase
was pipetted out to leave a precipitate containing the targeted
material. The precipitate is washed with purified water and
alcohol. The dry product was characterized by X-ray
diffraction (XRD), scanning electron microscopy
(SEM) and transmission electron microscopy (TEM). In
summary, the results of these revealed that different types of
crystals were formed from the three different routes used and
that these researchers (Zeng and Zeng 2017) had succeeded
in producing single crystals of anatase titanium dioxide
which had one reactive crystal face.
2.3.2 Solvothermal Method
The solvothermal method was used to synthesize very fine
crystals of TiO 2 targeted for photocatalytic uses (Kim et al.
2003). The precursor was chosen as titanium isopropoxide
(TTIP). It was dissolved in dry toluene together with oleic
acid, under an atmosphere of argon. Varying amounts of TIP
were used to give 5:100; 10:100; 20:100 of TIP to solvent.
The mixtures were stirred for 24 h, then placed in an
76
K. G. Moodley and V. Arumugam