18
2 Water Splitting Reactions and Nanotechnology
due to their chemical inertness, photostability, and low cost. In a study, it is found that
crystalline nanotube of TiO 2 shows increased generation of hydrogen by PEC water
splitting as compared to the amorphous TiO 2 nanotube. However, the amorphous
nanotubes can be converted to the crystalline form at high temperatures like 300 °C.
Highly ordered TiO 2 nanotubes show increased hydrogen generation by PEC water
splitting. In order to obtain these nanotubes, the anodization of 0.1-mm-thick titanium
foils was performed in ethylene glycol containing NH 4 F, H 2 O in the presence of Pt
counterelectrode, and 50 V potential difference. The experiment took 5 h. Afterward,
the newly formed nanomaterials were ultrasonicated with acetone so as to separate the
anodized TiO 2 nanotubular from the titanium substrate. After rinsing with DI water,
second anodization was conducted on the titanium substrate at the same temperature
for the additional growth of nanotubes and the rest of the procedure was repeated.
After repeating the whole process again, the obtained nanotubes were dried in the
stream of nitrogen. The annealing of the nanotubes was carried out at 300–600 °C.
The nanotubes were characterized SEM and XRD [15].
Niobium-doped TiO 2 has also been reported to aid the water splitting reaction
by PEC. These doped nanotubes are obtained by growing them on the Nb-Ti alloy.
These doped nanotubes depict 2.5 times higher current as compared to the undoped
TiO 2 nanotubes [16]. In another study, carbon-doped nanotubes have been developed.
These nanotubes depict 20 times more higher photocurrent densities, as compared
to the undoped TiO 2 under visible light illumination [17].
In nickel–cobalt bimetal phosphides, nanotubes are also found important for solardriven water splitting reactions. These nanotubes were prepared by two steps solidstate reaction, where first being the oxidation reaction occurring at 350 °C in normal
air atmosphere (for the development of Co x Ni y O) from metal organic framework
(MOF)-74 cobalt–nickel substrate and the second reaction involves the phosphorization and calcination at 300 °C with NaH 2 PO 2 in a pure N 2 atmosphere to afford (for
the development of Co x Ni y P). Distinctive phases of bimetal phosphides were attained
from altering the Co/Ni ratios in the precursor of MOF. SEM was used to determine
the phosphorization of the MOF-74. It was observed that despite phosphorization and
calcination the MOF retained its original nanorod morphology. However, the images
showed that the modified MOF has the rough surface as compared to the original
substrate. From the transmission electron microscope (TEM) images of Co 4 Ni 1 P, a
tubular arcade was found running across the nanorods forming morphology was
observed that, usual nanotube structure. The Co 4 Ni 1 P nanotubes have depicted
remarkable hydrogen emission reaction (HER) and oxygen emission reaction (OER)
through water splitting. In alkaline solution, the catalyst was found to achieve 10 m
A cm
−2 current density at a voltage of 1.59 V for both electrodes [18].
Non-metallic nanotubes are also proven to improve efficiency of HER from water
splitting. In a study N, S-co-doped carbon nanotubes (CNT) were prepared in situ
by using polydopamine. The prepared N, S-CNT were simultaneously applied as
the electrocatalysts for HER and OER. These nanotubes depicted excellent activity
both for the HER and OER from splitting of water. For the fabrication of N, S-CNT,
dopamine (DA) was mixed with oxidized CNT dispersion and afterward dissolved
into the water and subjected to sonication. Then, phosphate buffered saline was added
2 Water Splitting Reactions and Nanotechnology
due to their chemical inertness, photostability, and low cost. In a study, it is found that
crystalline nanotube of TiO 2 shows increased generation of hydrogen by PEC water
splitting as compared to the amorphous TiO 2 nanotube. However, the amorphous
nanotubes can be converted to the crystalline form at high temperatures like 300 °C.
Highly ordered TiO 2 nanotubes show increased hydrogen generation by PEC water
splitting. In order to obtain these nanotubes, the anodization of 0.1-mm-thick titanium
foils was performed in ethylene glycol containing NH 4 F, H 2 O in the presence of Pt
counterelectrode, and 50 V potential difference. The experiment took 5 h. Afterward,
the newly formed nanomaterials were ultrasonicated with acetone so as to separate the
anodized TiO 2 nanotubular from the titanium substrate. After rinsing with DI water,
second anodization was conducted on the titanium substrate at the same temperature
for the additional growth of nanotubes and the rest of the procedure was repeated.
After repeating the whole process again, the obtained nanotubes were dried in the
stream of nitrogen. The annealing of the nanotubes was carried out at 300–600 °C.
The nanotubes were characterized SEM and XRD [15].
Niobium-doped TiO 2 has also been reported to aid the water splitting reaction
by PEC. These doped nanotubes are obtained by growing them on the Nb-Ti alloy.
These doped nanotubes depict 2.5 times higher current as compared to the undoped
TiO 2 nanotubes [16]. In another study, carbon-doped nanotubes have been developed.
These nanotubes depict 20 times more higher photocurrent densities, as compared
to the undoped TiO 2 under visible light illumination [17].
In nickel–cobalt bimetal phosphides, nanotubes are also found important for solardriven water splitting reactions. These nanotubes were prepared by two steps solidstate reaction, where first being the oxidation reaction occurring at 350 °C in normal
air atmosphere (for the development of Co x Ni y O) from metal organic framework
(MOF)-74 cobalt–nickel substrate and the second reaction involves the phosphorization and calcination at 300 °C with NaH 2 PO 2 in a pure N 2 atmosphere to afford (for
the development of Co x Ni y P). Distinctive phases of bimetal phosphides were attained
from altering the Co/Ni ratios in the precursor of MOF. SEM was used to determine
the phosphorization of the MOF-74. It was observed that despite phosphorization and
calcination the MOF retained its original nanorod morphology. However, the images
showed that the modified MOF has the rough surface as compared to the original
substrate. From the transmission electron microscope (TEM) images of Co 4 Ni 1 P, a
tubular arcade was found running across the nanorods forming morphology was
observed that, usual nanotube structure. The Co 4 Ni 1 P nanotubes have depicted
remarkable hydrogen emission reaction (HER) and oxygen emission reaction (OER)
through water splitting. In alkaline solution, the catalyst was found to achieve 10 m
A cm
−2 current density at a voltage of 1.59 V for both electrodes [18].
Non-metallic nanotubes are also proven to improve efficiency of HER from water
splitting. In a study N, S-co-doped carbon nanotubes (CNT) were prepared in situ
by using polydopamine. The prepared N, S-CNT were simultaneously applied as
the electrocatalysts for HER and OER. These nanotubes depicted excellent activity
both for the HER and OER from splitting of water. For the fabrication of N, S-CNT,
dopamine (DA) was mixed with oxidized CNT dispersion and afterward dissolved
into the water and subjected to sonication. Then, phosphate buffered saline was added