Soft, Wet-Chemical Synthesis of Metastable Superparamagnetic Hexagonal Close-Packed Nickel Nanoparticles in Different Ionic Liquids

Ionic Liquids in Metal, Photo-, Electro-, and (Bio) Catalysis

Ionic liquids (ILs) have unique physicochemical properties that make them advantageous for catalysis, such as low vapor pressure, non-flammability, high thermal and chemical stabilities, and the ability to enhance the activity and stability of (bio)catalysts. ILs can improve the efficiency, selectivity, and sustainability of bio(transformations) by acting as activators of enzymes, selectively dissolving substrates and products, and reducing toxicity. They can also be recycled and reused multiple times without losing their effectiveness. ILs based on imidazolium cation are preferred for structural organization aspects, with a semiorganized layer surrounding the catalyst. ILs act as a container, providing a confined space that allows modulation of electronic and geometric effects, miscibility of reactants and products, and residence time of species. ILs can stabilize ionic and radical species and control the catalytic activity of dynamic processes. Supported IL phase (SILP) derivatives and polymeric ILs (PILs) are good options for molecular engineering of greener catalytic processes. The major factors governing metal, photo-, electro-, and biocatalysts in ILs are discussed in detail based on the vast literature available over the past two and a half decades. Catalytic reactions, ranging from hydrogenation and cross-coupling to oxidations, promoted by homogeneous and heterogeneous catalysts in both single and multiphase conditions, are extensively reviewed and discussed considering the knowledge accumulated until now.

The Phase Control of Transition Metallic Elements via Facile Chemical and Physical Syntheses

The crystal phases of metals are important factors to tune the properties of metals, and therefore received extensive attention. Traditionally, phase control is performed within limited numbers of elements by harsh conditions, such as face-centered cubic Fe by high temperature. This review summarizes most reports in metal phase control area, including elements of Fe, Co, Ni, Cu, Ru, Pd, Rh, Os and Au. For every metallic element, the facile phase control methods are systematically introduced, such as epitaxial growth, ball milling, chemical reduction, etc. Their corresponding applications and the mechanisms for phase control are thoroughly discussed.

Nickel nanoparticle-decorated reduced graphene oxide/WO3 nanocomposite - a promising candidate for gas sensing

We report for the first time the combination of WO3 sensing elements with a non-noble metal-carbon composite, namely a nickel metal nanoparticle-carbon composite (Ni@rGO). Previous work with WO3 had used either NiO (as part of the WO3 lattice), solely carbon, Pd-surface decorated WO3 (Pd@WO3), or Pd or Pt@carbon@WO3. We demonstrate the gas response for pure WO3, rGO/WO3 and Ni@rGO/WO3 sensing elements towards NO2 and acetone in air as well as towards CO in N2. The addition of 0.35 wt % Ni@rGO composite to WO3 enables the increase of the sensory response by more than 1.6 times for NO2 vapors. The gas response towards acetone using 0.35 wt % Ni@rGO/WO3 composite was 1.5 times greater for 3500 ppm than for 35,000 ppm acetone. For 0.35 wt % Ni@rGO/WO3 composite and CO gas, a response time (T res) of 7 min and a recovery time (T rec) of 2 min was determined.

Nickel nanoparticles supported on a covalent triazine framework as electrocatalyst for oxygen evolution reaction and oxygen reduction reactions

Covalent triazine frameworks (CTFs) are little investigated, albeit they are promising candidates for electrocatalysis, especially for the oxygen evolution reaction (OER). In this work, nickel nanoparticles (from Ni(COD)2) were supported on CTF-1 materials, which were synthesized from 1,4-dicyanobenzene at 400 °C and 600 °C by the ionothermal method. CTF-1-600 and Ni/CTF-1-600 show high catalytic activity towards OER and a clear activity for the electrochemical oxygen reduction reaction (ORR). Ni/CTF-1-600 requires 374 mV overpotential in OER to reach 10 mA/cm2, which outperforms the benchmark RuO2 catalyst, which requires 403 mV under the same conditions. Ni/CTF-1-600 displays an OER catalytic activity comparable with many nickel-based electrocatalysts and is a potential candidate for OER. The same Ni/CTF-1-600 material shows a half-wave potential of 0.775 V for ORR, which is slightly lower than that of commercial Pt/C (0.890 V). Additionally, after accelerated durability tests of 2000 cycles, the material showed only a slight decrease in activity towards both OER and ORR, demonstrating its superior stability.

Synthesis of plasmonic Fe/Al nanoparticles in ionic liquids

Bottom-up and top-down approaches are described for the challenging synthesis of Fe/Al nanoparticles (NPs) in ionic liquids (ILs) under mild conditions. The crystalline phase and morphology of the metal nanoparticles synthesized in three different ionic liquids were identified by powder X-ray diffractometry (PXRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), selected-area electron diffraction (SAED) and fast Fourier transform (FFT) of high-resolution TEM images. Characterization was completed by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) for the analysis of the element composition of the whole sample consisting of the NPs and the amorphous background. The bottom-up approaches resulted in crystalline FeAl NPs on an amorphous background. The top-down approach revealed small NPs and could be identified as Fe4Al13 NPs which in the IL [OPy][NTf2] yield two absorption bands in the green-blue to green spectral region at 475 and 520 nm which give rise to a complementary red color, akin to appropriate Au NPs.

Synthesis of nickel/gallium nanoalloys using a dual-source approach in 1-alkyl-3-methylimidazole ionic liquids

NiGa is a catalyst for the semihydrogenation of alkynes. Here we show the influence of different dispersion times before microwave-induced decomposition of the precursors on the phase purity, as well as the influence of the time of microwave-induced decomposition on the crystallinity of the NiGa nanoparticles. Microwave-induced co-decomposition of all-hydrocarbon precursors [Ni(COD)2] (COD = 1,5-cyclooctadiene) and GaCp* (Cp* = pentamethylcyclopentadienyl) in the ionic liquid [BMIm][NTf2] selectively yields small intermetallic Ni/Ga nanocrystals of 5 ± 1 nm as derived from transmission electron microscopy (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and supported by energy-dispersive X-ray spectrometry (EDX), selected-area energy diffraction (SAED) and X-ray photoelectron spectroscopy (XPS). NiGa@[BMIm][NTf2] catalyze the semihydrogenation of 4-octyne to 4-octene with 100% selectivity towards (E)-4-octene over five runs, but with poor conversion values. IL-free, precipitated NiGa nanoparticles achieve conversion values of over 90% and selectivity of 100% towards alkene over three runs.

Aggregation control of Ru and Ir nanoparticles by tunable aryl alkyl imidazolium ionic liquids

Metal-nanoparticles (M-NPs) were synthesized in a wet-chemical synthesis route in tunable aryl alkyl ionic liquids (TAAILs) based on the 1-aryl-3-alkyl-substituted imidazolium motif from Ru₃(CO)₁₂ and Ir₄(CO)₁₂ by microwave-heating induced thermal decomposition. The size and size dispersion of the NPs were determined by transmission electron microscopy (TEM) to an average diameter of 2.2(±0.1) to 3.9(±0.3) nm for Ru-NPs and to an average diameter of 1.4(±0.1) to 2.4(±0.1) nm for Ir-NPs. The TAAILs used contain the same bis(trifluoromethylsulfonyl)imide anion but differ in the substituents on the 1-aryl ring, e.g. 2-methyl-, 4-methoxy- and 2,4-dimethyl groups and in the 3-alkyl chain lengths (C₄H₉, C₅H₁₁, C₈H₁₇, C₉H₁₉, C₁₁H₂₃). All used TAAILs are suitable for the stabilization of Ru- and Ir-NPs over months in the IL dispersion. Different from all other investigations on M-NP/IL systems which we are aware of the particle separation properties of the TAAILs vary strongly as a function of the aryl substituent. Good NP separation can be achieved with the 4-methoxyphenyl- and 2,4-dimethylphenyl-substituted ILs, irrespective of the 3-alkyl chain lengths. Significant aggregation can be observed for 2-methylphenyl-substituted ILs. The good NP separation can be correlated with a negative electrostatic potential at the 4-methoxyphenyl or 4-methylphenyl substituent that is in the para-position of the aryl ring, whereas the 2-(ortho-)methylphenyl group assumes no negative potential. ε-ePC-SAFT calculations were used to validate that the interactions between ILs and the washing agents (required for TEM analyses) do not cause the observed aggregation/separation behaviour of the M-NPs. Ru-NPs were investigated as catalysts for the solvent-free hydrogenation of benzene to cyclohexane under mild conditions (70 °C, 10 bar) with activities up to 760 (mol cyclohexane) (mol Ru)^-1 h^-1 and over 95% conversion in ten consecutive runs for Ru-NPs. No significant loss of catalytic activity could be observed.

Bimetallic Co/Al nanoparticles in an ionic liquid: synthesis and application in alkyne hydrogenation

CoAl- and Co₃Al-NPs are effective catalysts for alkyne-to-alkane hydrogenation with DIBAL-H as a co-catalyst under mild conditions (2 bar H₂, 30 °C).

Thermodynamic properties of selenoether-functionalized ionic liquids and their use for the synthesis of zinc selenide nanoparticles

Three selenoether-functionalized ionic liquids (ILs) of N-[(phenylseleno)methylene]pyridinium (1), N-(methyl)- (2) and N-(butyl)-N'-[(phenylseleno)methylene]imidazolium (3) with bis(trifluoromethanesulfonyl)imide anions ([NTf2]) were prepared from pyridine, N-methylimidazole and N-butylimidazole with in situ obtained phenylselenomethyl chloride, followed by ion exchange to give the desired compounds. The crystal structures of the bromide and tetraphenylborate salts of the above cations (1-Br, 2-BPh4 and 3-BPh4) confirm the formation of the desired cations and indicate a multitude of different supramolecular interactions besides the dominating Coulomb interactions between the cations and anions. The vaporization enthalpies of the synthesized [NTf2]-containing ILs were determined by means of a quartz-crystal microbalance method (QCM) and their densities were measured with an oscillating U-tube. These thermodynamic data have been used to develop a method for assessment of miscibility of conventional solvents in the selenium-containing ILs by using Hildebrandt solubility parameters, as well as for modeling with the electrolyte perturbed-chain statistical associating fluid theory (ePC-SAFT) method. Furthermore, structure-property relations between selenoether-functionalized and similarly shaped corresponding aryl-substituted imidazolium- and pyridinium-based ILs were analyzed and showed that the contribution of the selenium moiety to the enthalpy of vaporization of an IL is equal to the contribution of a methylene (CH2) group. An incremental approach to predict vaporization enthalpies of ILs by a group contribution method has been developed. The reaction of these ILs with zinc acetate dihydrate under microwave irradiation led to ZnSe nanoparticles of an average diameter between 4 and 10 nm, depending on the reaction conditions.

Heterobimetallic triple-decker complexes derived from a dianionic aromatic stannole ligand

A neutral heterobimetallic triple-decker stannole complex was prepared by the reaction of an anionic ruthenocene bearing a stannole dianionic ligand with [Rh(cod)Cl]₂ (cod = 1,5-cyclooctadiene), and the resulting Ru–Rh complex exhibits an electronic property different from those of the corresponding Ru–Ru and Rh–Rh complexes.

Tuning the structure and magnetic behavior of Ni–Ir-based nanoparticles in ionic liquids

We report on a simple preparation of extremely small diameter (ca. 2 nm) Ni–Ir-based NPs with either core–shell like or alloy-like microstructures.

Metal Nanoparticles in Ionic Liquids

During the last years ionic liquids (ILs) were increasingly used and investigated as reaction media, hydrogen sources, catalysts, templating agents and stabilizers for the synthesis of (monometallic and bimetallic) metal nanoparticles (M-NPs). Especially ILs with 1,3-dialkyl-imidazolium cations featured prominently in the formation and stabilization of M-NPs. This chapter summarizes studies which focused on the interdependencies of the IL with the metal nanoparticle and tried to elucidate, for example, influences of the IL-cation, -anion and alkyl chain length. Qualitatively, the size of M-NPs was found to increase with the size of the IL-anion. The influence of the size of imidazolium-cation is less clear. The M-NP size was both found to increase and to decrease with increasing chain lengths of the 1,3-dialkyl-imidazolium cation. It is evident from such reports on cation and anion effects of ILs that the interaction between an IL and a (growing) metal nanoparticle is far from understood. Factors like IL-viscosity, hydrogen-bonding capability and the relative ratio of polar and non-polar domains of ILs may also influence the stability of nanoparticles in ionic liquids and an improved understanding of the IL-nanoparticle interaction would be needed for a more rational design of nanomaterials in ILs. Furthermore, thiol-, ether-, carboxylic acid-, amino- and hydroxyl-functionalized ILs add to the complexity by acting also as coordinating capping ligands. In addition imidazolium cations are precursors to N-heterocyclic carbenes, NHCs which form from imidazolium-based ionic liquids by in situ deprotonation at the acidic C2-H ring position as intermediate species during the nanoparticle seeding and growth process or as surface coordinating ligand for the stabilization of the metal nanoparticle.