90
5 Synthesis of Three-Dimensional Nanomaterials
from both the Co 3 O 4 and CoMoO 4 . Zhai et al. reported an effective and simple
strategy to prepare large areal mass loading of MnO 2 on porous graphene gel/Ni
foam (denoted as MnO 2 /G-gel/NF) for supercapacitors (SCs) (Zhai et al. 2013).
Graphene hydrogel/nickel foam (G-gel/NF) was simply obtained by immersing a
piece of nickel foam (2 cm × cm, 1 × mm thick) in a suspension of GO (3 mg mL
−1 )
and subsequently heating at 180 °C for 12 h. During the process, GO nanosheets
were reduced into G-gel and coated on Ni foam. The color of Ni foam became dark
after hydrothermal reaction. Moreover, an asymmetric supercapacitor (ASC) on the
basis of MnO 2 /G-gel/NF (MnO 2 mass: 6.1 mg cm
−2 ) used as the positive electrode
and G-gel/NF as the negative electrode achieved a remarkable energy density of
0.72 mW h cm
−3 . Rational design and synthesis of binder-free hybrid electrodes
with hierarchical core-shell structures has been regarded as an effective strategy to
improve the electrochemical performance of the supercapacitors by Xu et al. (Wu
et al. 2017). In this work, Co 3 O 4 @NiCo 2 O 4 core-shell structures with high yield are
successfully fabricated on flexible carbon cloth using a facile hydrothermal method
for ultra-long Co 3 O 4 nanowires and a chemical bath for NiCo 2 O 4 nanoflakes.
5.4 Solvothermal Method
Instead of water as a medium for organic solvent, the method, similar to the
hydrothermal synthesis principle for preparing nanometer material, is called
Solvothermal Method. Replacing water with non-aqueous solvent not only expands
the application scope of hydrothermal technology, but also realizes the reactions
which cannot be achieved under normal conditions, including preparing materials
with metastable structure.
As Pang et al. emphasized the significance of morphology, Wang et al. developed
a simple morphology-controlled synthesis for hierarchical α-Ni(OH) 2 microspheres
(Wang et al. 2018; Zhang et al. 2016a, b). Three kinds of α-Ni(OH) 2 microspheres
with different structures were prepared by one-step surfactant-assisted solvothermal
method in Fig. 5.6A, using water, ethanol, and their mixture as solvent, respectively.
First, Ni(NO 3 ) 2 · 6H 2 O and sodium dodecyl sulfate (SDS) were dissolved in 50 mL
deionized water, 50 mL ethanol and their mixed solution (25 mL deionized water
and 25 ml ethanol), respectively, followed by addition of 3.0 g urea with continuous stirring. Then the solutions were transferred into 100 mL Teflon-lined stainless
autoclaves and held at 110 °C for 15 h. After cooled down to room temperature naturally, the precipitates were collected by centrifugation and washed several times with
deionized water and absolute ethanol. Finally, the products were dried to constant
weight at 60 °C. The samples obtained from three different solvents were denoted as
Ni(OH) 2 -a, Ni(OH) 2 -b, and Ni(OH) 2 -c correspondingly. The effects of solvent on the
structure, morphology, and capacitive performance of α-Ni(OH) 2 were investigated.
Among the as-prepared samples, Ni(OH) 2 -c microspheres, obtained from the mixed
solvent (ethanol solution), are composed of hierarchical flower-like nanosheets and
possess the roughest surface and the largest specific surface area (101.3 m
2 g
−1 ).
5 Synthesis of Three-Dimensional Nanomaterials
from both the Co 3 O 4 and CoMoO 4 . Zhai et al. reported an effective and simple
strategy to prepare large areal mass loading of MnO 2 on porous graphene gel/Ni
foam (denoted as MnO 2 /G-gel/NF) for supercapacitors (SCs) (Zhai et al. 2013).
Graphene hydrogel/nickel foam (G-gel/NF) was simply obtained by immersing a
piece of nickel foam (2 cm × cm, 1 × mm thick) in a suspension of GO (3 mg mL
−1 )
and subsequently heating at 180 °C for 12 h. During the process, GO nanosheets
were reduced into G-gel and coated on Ni foam. The color of Ni foam became dark
after hydrothermal reaction. Moreover, an asymmetric supercapacitor (ASC) on the
basis of MnO 2 /G-gel/NF (MnO 2 mass: 6.1 mg cm
−2 ) used as the positive electrode
and G-gel/NF as the negative electrode achieved a remarkable energy density of
0.72 mW h cm
−3 . Rational design and synthesis of binder-free hybrid electrodes
with hierarchical core-shell structures has been regarded as an effective strategy to
improve the electrochemical performance of the supercapacitors by Xu et al. (Wu
et al. 2017). In this work, Co 3 O 4 @NiCo 2 O 4 core-shell structures with high yield are
successfully fabricated on flexible carbon cloth using a facile hydrothermal method
for ultra-long Co 3 O 4 nanowires and a chemical bath for NiCo 2 O 4 nanoflakes.
5.4 Solvothermal Method
Instead of water as a medium for organic solvent, the method, similar to the
hydrothermal synthesis principle for preparing nanometer material, is called
Solvothermal Method. Replacing water with non-aqueous solvent not only expands
the application scope of hydrothermal technology, but also realizes the reactions
which cannot be achieved under normal conditions, including preparing materials
with metastable structure.
As Pang et al. emphasized the significance of morphology, Wang et al. developed
a simple morphology-controlled synthesis for hierarchical α-Ni(OH) 2 microspheres
(Wang et al. 2018; Zhang et al. 2016a, b). Three kinds of α-Ni(OH) 2 microspheres
with different structures were prepared by one-step surfactant-assisted solvothermal
method in Fig. 5.6A, using water, ethanol, and their mixture as solvent, respectively.
First, Ni(NO 3 ) 2 · 6H 2 O and sodium dodecyl sulfate (SDS) were dissolved in 50 mL
deionized water, 50 mL ethanol and their mixed solution (25 mL deionized water
and 25 ml ethanol), respectively, followed by addition of 3.0 g urea with continuous stirring. Then the solutions were transferred into 100 mL Teflon-lined stainless
autoclaves and held at 110 °C for 15 h. After cooled down to room temperature naturally, the precipitates were collected by centrifugation and washed several times with
deionized water and absolute ethanol. Finally, the products were dried to constant
weight at 60 °C. The samples obtained from three different solvents were denoted as
Ni(OH) 2 -a, Ni(OH) 2 -b, and Ni(OH) 2 -c correspondingly. The effects of solvent on the
structure, morphology, and capacitive performance of α-Ni(OH) 2 were investigated.
Among the as-prepared samples, Ni(OH) 2 -c microspheres, obtained from the mixed
solvent (ethanol solution), are composed of hierarchical flower-like nanosheets and
possess the roughest surface and the largest specific surface area (101.3 m
2 g
−1 ).