68
4 Synthesis of Two-Dimensional (2D) Nanomaterials
stability) than bulk LDHs using the typical co-precipitation method. For example,
the NiFe-LDH nanosheets exhibited lower overpotential and higher stability, even
compared the most IrO 2 catalyst (Fig. 4.3j). Owing to the rapid synthesis and ultrathin nanosheets, the unique 2D nanostructures exhibited more surface defects, which
could serve as active sites to achieve efficient catalysis of the OER.
A basic understanding of the electronic structures of the different LDHs could
be used by density functional theory (DFT) calculations, and it can confirm the
higher electronic conductivity and spin-polarization characteristics for Fe-based
LDH nanosheets, as well. Therefore, this work not only presents a facile strategy
to obtain 2D atomic-thick nanosheets toward highly enhanced OER activity but also
supplies a detailed understanding of the electronic structures of LDH nanostructures
from a theoretical perspective.
4.4 MXenes
Recently, a new family of 2D early transition metal carbides, nitrides, and carbonitrides, commonly known as MXenes, has been found (Naguib et al. 2011, 2012,
2014). Generally, MXenes are fabricated from MAX phases consisting of layered
ternary carbides with the formula M n+1 AX n , where M is an early transition metal (Sc,
Ti, V, Cr, Zr, Nb, Mo, Hf, or Ta), A is an element from groups 12–16 (Cd, Al, Si, P, S,
Ga, Ge, As, In, Sn, Tl, Pb, or S), and X is carbon and/or nitrogen (Ma et al. 2016; Zhao
et al. 2012; Ghidiu et al. 2014; Khazaei et al. 2014). The surface-exposed transition
metal sheet, MXenes, was synthesized by selecting corrosion element A from MAX
phase by strong acid and stripping (Gao et al. 2017). At room temperature, after
replacing element A with HF aqueous solution as the etchant, the relatively strong
metal bonds of M and A in M n+1 AX n phase are replaced by OH, O or F weak hydrogen
bonds (Xiao et al. 2016a). Therefore, the etching and stripping processes usually give
MXene samples hydroxyl, oxygen, and fluorine groups. The first principle calculation shows that the electron structure of the generated MXene is obviously different
from that of its parent MAX phase. For example, functionalization with OH, O, and
F leads to a transition from metal to semiconductor (Lei et al. 2015). MXenes, with
both metallic conductivity and hydrophilic behavior, have demonstrated their potential as potential electrode materials for electrochemical energy storage devices, such
as lithium-ion batteries (Ren et al. 2016; Chen et al. 2017a), Li-S batteries (Liang
et al. 2015b; Zhao et al. 2015c), sodium-ion batteries (Xie et al. 2016; Lian et al.
2017) and supercapacitors (Lukatskaya et al. 2013; Zhao et al. 2014b).
4.4.1 Transition Metal Carbides
Ti 3 C 2 is one of the most widely studied members of this family. Particularly, Ti 3 C 2
is a promising electrode material for ultracapacitor due to its characteristics of metal
4 Synthesis of Two-Dimensional (2D) Nanomaterials
stability) than bulk LDHs using the typical co-precipitation method. For example,
the NiFe-LDH nanosheets exhibited lower overpotential and higher stability, even
compared the most IrO 2 catalyst (Fig. 4.3j). Owing to the rapid synthesis and ultrathin nanosheets, the unique 2D nanostructures exhibited more surface defects, which
could serve as active sites to achieve efficient catalysis of the OER.
A basic understanding of the electronic structures of the different LDHs could
be used by density functional theory (DFT) calculations, and it can confirm the
higher electronic conductivity and spin-polarization characteristics for Fe-based
LDH nanosheets, as well. Therefore, this work not only presents a facile strategy
to obtain 2D atomic-thick nanosheets toward highly enhanced OER activity but also
supplies a detailed understanding of the electronic structures of LDH nanostructures
from a theoretical perspective.
4.4 MXenes
Recently, a new family of 2D early transition metal carbides, nitrides, and carbonitrides, commonly known as MXenes, has been found (Naguib et al. 2011, 2012,
2014). Generally, MXenes are fabricated from MAX phases consisting of layered
ternary carbides with the formula M n+1 AX n , where M is an early transition metal (Sc,
Ti, V, Cr, Zr, Nb, Mo, Hf, or Ta), A is an element from groups 12–16 (Cd, Al, Si, P, S,
Ga, Ge, As, In, Sn, Tl, Pb, or S), and X is carbon and/or nitrogen (Ma et al. 2016; Zhao
et al. 2012; Ghidiu et al. 2014; Khazaei et al. 2014). The surface-exposed transition
metal sheet, MXenes, was synthesized by selecting corrosion element A from MAX
phase by strong acid and stripping (Gao et al. 2017). At room temperature, after
replacing element A with HF aqueous solution as the etchant, the relatively strong
metal bonds of M and A in M n+1 AX n phase are replaced by OH, O or F weak hydrogen
bonds (Xiao et al. 2016a). Therefore, the etching and stripping processes usually give
MXene samples hydroxyl, oxygen, and fluorine groups. The first principle calculation shows that the electron structure of the generated MXene is obviously different
from that of its parent MAX phase. For example, functionalization with OH, O, and
F leads to a transition from metal to semiconductor (Lei et al. 2015). MXenes, with
both metallic conductivity and hydrophilic behavior, have demonstrated their potential as potential electrode materials for electrochemical energy storage devices, such
as lithium-ion batteries (Ren et al. 2016; Chen et al. 2017a), Li-S batteries (Liang
et al. 2015b; Zhao et al. 2015c), sodium-ion batteries (Xie et al. 2016; Lian et al.
2017) and supercapacitors (Lukatskaya et al. 2013; Zhao et al. 2014b).
4.4.1 Transition Metal Carbides
Ti 3 C 2 is one of the most widely studied members of this family. Particularly, Ti 3 C 2
is a promising electrode material for ultracapacitor due to its characteristics of metal