110
M. I. Qadir et al.
The metallic nanoparticles in ILs are solvated preferentially by the charged
moieties of the ions, with an interface layer that is one ion thick. Therefore, both
cations and anions are present in contact with the metal. Pensando et al. used
density functional theory (DFT) methods to study the mechanism of solvation and
stabilisation of Ru NPs through the interactions between 1,3-dimethylimidazolium
bis(trifluoromethanesulfonyl)amide (MMIm.NTf 2 ), and the Ru (001) surface of a
cluster of metal atoms [14]. It was observed that the alkyl side chain of the imidazolium cation is found further away from the NPs, while the orientation of the cations
with respect to the NP surface shows that the imidazolium ring most probably lies
perpendicular to the NP surface, with the CH 3 group attached to N3 closer to the
metal. This is a result of the stronger attractive interaction of the charged moieties
with the metal surface.
The presence of interaction between the ILs and NPs has been extensively
studied by TEM, SAX, XRD, XPS and EXAFS analyses. Dupont et al. reported
that XAS analysis demonstrated the formation of an IL protective layer surrounding
Pt and Ir NP surfaces with an extended molecular length of around 2.8–4.0 nm
depending on the type of anion present [17, 18]. This suggests the presence of
semi-organised anionic species composed of supramolecular aggregates of the type
[(BMI) x−n (X) x )]
n− . This multilayer is probably composed of anions located immediately adjacent to the NP surface providing the Coulombic repulsion and countercations that provide the charge balance, i.e., quite close to DLVO-type stabilisation.
Moreover, XPS analysis of the isolated Pt, Ir, Co, Ru and Rh NPs also indicates the
presence of an IL layer on the surface of NPs [19]. Recently, the association of F1s
of NTf 2 with Ru/Fe NPs prepared in hydrophobic BMIm.NTf 2 IL was observed. The
F1s signal showed a peak associated with uncoordinated NTf 2 (688.6 eV) and a new
component appeared at 684.8 eV [20], which was attributed to the IL interaction with
Ru/Fe NPs, as is observed in Au NPs [21].
4.3 Preparation of Soluble NPs by Chemical Methods
To prepare ‘soluble’ transition-metal NPs in ILs, simple reduction/hydrogenation of
organometallic complexes and metal salts in ILs by a chemical method is the most
investigated and used method (Table 4.1). The most frequent reducing agents are
H 2 gas, NaBH 4 , ascorbic acid, sodium citrate and SnCl 2 [22]. However, hydride
sources are not likely to be used in ILs, due their basic character, deprotonating
the imidazolium cation and generating carbenes that may bind to the metal surface.
On the other hand, these reducing agents also produce various by-products such as
sodium (Na) and boron (B) compounds, which are difficult to remove from the IL
and stick to the surface of the formed NPs [22]. Hydrogen gas can easily reduce
the metal complexes and salts into their respective ‘surface-clean’ NPs in ILs under
elevated reaction conditions. In this regard, Dupont et al. were the first to prepare Ir
NPs (2.0 ± 0.4 nm) in BMIm.PF 6 ILs by the simple reduction of [Ir(cod)Cl] 2 using
M. I. Qadir et al.
The metallic nanoparticles in ILs are solvated preferentially by the charged
moieties of the ions, with an interface layer that is one ion thick. Therefore, both
cations and anions are present in contact with the metal. Pensando et al. used
density functional theory (DFT) methods to study the mechanism of solvation and
stabilisation of Ru NPs through the interactions between 1,3-dimethylimidazolium
bis(trifluoromethanesulfonyl)amide (MMIm.NTf 2 ), and the Ru (001) surface of a
cluster of metal atoms [14]. It was observed that the alkyl side chain of the imidazolium cation is found further away from the NPs, while the orientation of the cations
with respect to the NP surface shows that the imidazolium ring most probably lies
perpendicular to the NP surface, with the CH 3 group attached to N3 closer to the
metal. This is a result of the stronger attractive interaction of the charged moieties
with the metal surface.
The presence of interaction between the ILs and NPs has been extensively
studied by TEM, SAX, XRD, XPS and EXAFS analyses. Dupont et al. reported
that XAS analysis demonstrated the formation of an IL protective layer surrounding
Pt and Ir NP surfaces with an extended molecular length of around 2.8–4.0 nm
depending on the type of anion present [17, 18]. This suggests the presence of
semi-organised anionic species composed of supramolecular aggregates of the type
[(BMI) x−n (X) x )]
n− . This multilayer is probably composed of anions located immediately adjacent to the NP surface providing the Coulombic repulsion and countercations that provide the charge balance, i.e., quite close to DLVO-type stabilisation.
Moreover, XPS analysis of the isolated Pt, Ir, Co, Ru and Rh NPs also indicates the
presence of an IL layer on the surface of NPs [19]. Recently, the association of F1s
of NTf 2 with Ru/Fe NPs prepared in hydrophobic BMIm.NTf 2 IL was observed. The
F1s signal showed a peak associated with uncoordinated NTf 2 (688.6 eV) and a new
component appeared at 684.8 eV [20], which was attributed to the IL interaction with
Ru/Fe NPs, as is observed in Au NPs [21].
4.3 Preparation of Soluble NPs by Chemical Methods
To prepare ‘soluble’ transition-metal NPs in ILs, simple reduction/hydrogenation of
organometallic complexes and metal salts in ILs by a chemical method is the most
investigated and used method (Table 4.1). The most frequent reducing agents are
H 2 gas, NaBH 4 , ascorbic acid, sodium citrate and SnCl 2 [22]. However, hydride
sources are not likely to be used in ILs, due their basic character, deprotonating
the imidazolium cation and generating carbenes that may bind to the metal surface.
On the other hand, these reducing agents also produce various by-products such as
sodium (Na) and boron (B) compounds, which are difficult to remove from the IL
and stick to the surface of the formed NPs [22]. Hydrogen gas can easily reduce
the metal complexes and salts into their respective ‘surface-clean’ NPs in ILs under
elevated reaction conditions. In this regard, Dupont et al. were the first to prepare Ir
NPs (2.0 ± 0.4 nm) in BMIm.PF 6 ILs by the simple reduction of [Ir(cod)Cl] 2 using