Synthesis and Characterization of Ultra-Small Gold Nanoparticles in the Ionic Liquid 1-Ethyl-3-methylimidazolium Dicyanamide, [Emim][DCA]

We report on gold clusters with around 62 gold atoms and a diameter of 1.15±0.10 nm. Dispersions of the clusters are long-term stable for two years at ambient conditions. The synthesis was performed by mixing tetrachloroauric acid (HAuCl4  ⋅ 3 H2 O) with the ionic liquid 1-ethyl-3-methylimidazolium dicyanamide ([Emim][DCA]) at temperatures of 20 to 80 °C. Characterization was performed with small-angle X-ray scattering (SAXS), UV-Vis spectroscopy, and MALDI-TOF mass spectrometry. A three-stage model is proposed for the formation of the clusters, in which cluster growth from gold nuclei takes place according to the Lifshitz-Slyozov-Wagner (LSW) model followed by oriented attachment to form colloidal stable clusters.

Time-resolved in situ nanoparticle size evolution during magnetron sputtering onto liquids

Despite extensive research since 1996, there are still open questions regarding the primary location of the nucleation process, the growth mechanism of the nanoparticles (NPs), and the influence of the liquid properties on the ultimate size of the NPs for the magnetron sputtering of metals onto liquids. Hence, for the first time to the authors' knowledge, the particle size evolution is in situ and in real-time examined during and after the sputtering of the silver atoms onto silicone oil, i.e., Sputtering onto Liquids (SoL) process. The particle size distribution (PSD) is measured via the Light Extinction Spectroscopy (LES) technique, and the deposition rate and stirring speed effects on the PSDs are analyzed. Based on De Brouckere mean diameters, the size evolution of silver nanoparticles (Ag NPs) over time is monitored. Ag NPs bigger than 20 nm are detected, and the PSDs are shown to be poly-disperse, which is also supported by the ex situ TEM measurements and in situ time-resolved absorption spectra. Moreover, it is shown that aggregation and growth of Ag NPs occur both at the plasma-liquid interface and inside the silicone oil during and after the magnetron sputtering. Despite the same amount of deposited silver, the growth kinetics of Ag NPs in silicone oil vary at different deposition rates. In particular, at higher deposition rates, larger NPs are formed. Stirring is seen to help disaggregate the particle lumps. Faster stirring does not substantially influence the final size but promotes the formation of smaller NPs (<20 nm). Also, low colloidal stability of Ag NPs in silicone oil is observed.

A benevolent direction to environmental suitability: ionic liquid immobilized MoO3 nanoparticles used in the efficient visible light-driven photocatalytic degradation of antibiotics

This paper summarizes the synthesis of a novel MoO₃-bonded imidazolium sulfonic acid chloride (MoO₃-IL) using a facile precipitation method and its application in the degradation of two organic pollutants.

A microwave assisted ionic liquid route to prepare bivalent Mn5O8 nanoplates for 5-hydroxymethylfurfural oxidation

In order to develop highly active non-precious metal catalysts for the selective oxidation of the platform compound 5-hydroxymethylfurfural (HMF) to the value-added bio-chemical 2,5-diformylfuran (DFF), we prepared high purity bivalent Mn₅O₈ nanoplates by a microwave-assisted ionic liquid route. The precursor of bivalent Mn₅O₈ nanoplates was formed through π-π stacking between imidazolium rings of the ionic liquid 1-butyl-3-methyl-imidazolium chloride and extending hydrogen bonds between Cl anions and hydrohausmannite. An oriented aggregation growth occurred on the basis of the Ostwald ripening under microwave heating. The high purity bivalent Mn₅O₈ nanoplates obtained through calcination at 550 °C for 2 h exhibited high HMF conversion (51%) and DFF selectivity (94%) at 5 bar of oxygen pressure in 2 h. The high concentration of Mn⁴⁺ on the exterior surfaces of Mn₅O₈ nanoplates as active sites coupled with good crystallinity played key roles for desirable mass and heat transfer, and for fast desorption avoiding over-oxidation. The reaction process over the Mn₅O₈ nanoplates was proposed based on the understanding of Mn⁴⁺ active centers and lattice oxygen via a Mn⁴⁺/Mn²⁺ two-electron cycle to enhance their catalytic performance. Furthermore, the Mn₅O₈ nanoplates could be readily recovered and reused without loss of catalytic activity. Thus, the high purity Mn₅O₈ nanoplates with good catalytic performance raises the prospect of using the type of sole metal oxide for practical applications.

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.

Au/Cu Bimetallic Nanoparticles via Double-Target Sputtering onto a Liquid Polymer

Alloy nanoparticles (NPs) of a bimetal system, Au/Cu, that form intermetallic compounds in a bulk state have been successfully produced using a double-target sputtering technique onto a low-cost and biocompatible liquid polymer (polyethylene glycol, PEG). The formation of an Au/Cu solid solution alloy in individual NPs was revealed by scanning transmission electron microscopy-energy-dispersive X-ray elemental mapping analysis. Altering the sputter currents for Au and Cu targets resulted in a tailored NP composition, but the particle sizes did not significantly vary. We found similar structures, sizes, and optical properties of Au/Cu NPs obtained by double-head sputtering on carbon-coated transmission electron microscopy grids or PEG and by Au/Cu alloy target sputtering. Random alloy formation occurred in matrix sputtering using double-target heads. This method is advantageous for manipulating the alloy composition through highly independent control of sputter parameters for each metal target.

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.

In-Situ XPS Monitoring and Characterization of Electrochemically Prepared Au Nanoparticles in an Ionic Liquid

Gold nanoparticles (Au NPs) have been electrochemically prepared in situ and in vacuo using two different electrochemical device configurations, containing an ionic liquid (IL), N-N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide, that serves both as reaction and as stabilizing media for the NPs. It was observed in both devices that Au NPs were created using an anodically triggered route. The created Au NPs are relatively small (3-7 nm) and reside within the IL medium. X-ray photoelectron spectroscopy is utilized to follow not only the formation of the NPs but also their charging/discharging properties, by monitoring the charging shifts of the Au4f peak representing the electrodes and also the Au NPs as well as the F1s peak of the IL after polarizing one of the electrodes. Accordingly, DC polarization across the electrodes leads to a uniform binding energy shift of F1s of the IL along with that of Au4f of the NPs within. Moreover, this shift corresponds to only half of the applied potential. AC polarization brings out another dimension for demonstrating further the harmony between the charging/discharging property of the IL medium and the Au NPs in temporally and laterally resolved fashions. Polarization of the electrodes result in perfect spectral separation of the Au4f peaks of the NPs from those of the metal in both static (DC) and in time- and position-dependent (AC) modes.

Synthesis of Metal Nanoparticles and Metal Fluoride Nanoparticles from Metal Amidinate Precursors in 1-Butyl-3-Methylimidazolium Ionic Liquids and Propylene Carbonate

Decomposition of transition-metal amidinates [M{MeC(NiPr)2} n ] [M(AMD) n ; M=MnII, FeII, CoII, NiII, n=2; CuI, n=1) induced by microwave heating in the ionic liquids (ILs) 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF4]), 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIm][PF6]), 1-butyl-3-methylimidazolium trifluoromethanesulfonate (triflate) ([BMIm][TfO]), and 1-butyl-3-methylimidazolium tosylate ([BMIm][Tos]) or in propylene carbonate (PC) gives transition-metal nanoparticles (M-NPs) in non-fluorous media (e.g. [BMIm][Tos] and PC) or metal fluoride nanoparticles (MF2-NPs) for M=Mn, Fe, and Co in [BMIm][BF4]. FeF2-NPs can be prepared upon Fe(AMD)2 decomposition in [BMIm][BF4], [BMIm][PF6], and [BMIm][TfO]. The nanoparticles are stable in the absence of capping ligands (surfactants) for more than 6 weeks. The crystalline phases of the metal or metal fluoride synthesized in [BMIm][BF4] were identified by powder X-ray diffraction (PXRD) to exclusively Ni- and Cu-NPs or to solely MF2-NPs for M=Mn, Fe, and Co. The size and size dispersion of the nanoparticles were determined by transmission electron microscopy (TEM) to an average diameter of 2(±2) to 14(±4) nm for the M-NPs, except for the Cu-NPs in PC, which were 51(±8) nm. The MF2-NPs from [BMIm][BF4] were 15(±4) to 65(±18) nm. The average diameter from TEM is in fair agreement with the size evaluated from PXRD with the Scherrer equation. The characterization was complemented by energy-dispersive X-ray spectroscopy (EDX). Electrochemical investigations of the CoF2-NPs as cathode materials for lithium-ion batteries were simply evaluated by galvanostatic charge/discharge profiles, and the results indicated that the reversible capacity of the CoF2-NPs was much lower than the theoretical value, which may have originated from the complex conversion reaction mechanism and residue on the surface of the nanoparticles.

Synthesis and properties enhancement of metal nanoclusters templated on a biological molecule/ionic liquids complex

Two biological template molecules and two short-chain imidazolium ionic liquids with amphipathy were selected to synthesize four different types of metal nanoclusters, which were templated on a biological molecule/imidazolium ionic liquids complex.

Ionic liquids for nano- and microstructures preparation. Part 2: Application in synthesis

Ionic liquids (ILs) are widely applied to prepare metal nanoparticles and 3D semiconductor microparticles. Generally, they serve as a structuring agent or reaction medium (solvent), however it was also demonstrated that ILs can play a role of a co-solvent, metal precursor, reducing as well as surface modifying agent. The crucial role and possible types of interactions between ILs and growing particles have been presented in the Part 1 of this review paper. Part 2 of the paper gives a comprehensive overview of recent experimental studies dealing with application of ionic liquids for preparation of metal and semiconductor based nano- and microparticles. A wide spectrum of preparation routes using ionic liquids is presented, including precipitation, sol-gel technique, hydrothermal method, nanocasting and ray-mediated methods (microwave, ultrasound, UV-radiation and γ-radiation). It was found that ionic liquids formed of a 1-butyl-3-methylimidazolium [BMIM] combined with tetrafluoroborate [BF4], hexafluorophosphate [PF6], and bis(trifluoromethanesulfonyl)imide [Tf2N] are the most often used ILs in the synthesis of nano- and microparticles, due to their low melting temperature, low viscosity and good transportation properties. Nevertheless, examples of other IL classes with intrinsic nanoparticles stabilizing abilities such as phosphonium and ammonium derivatives are also presented. Experimental data revealed that structure of ILs (both anion and cation type) affects the size and shape of formed metal particles, and in some cases may even determine possibility of particles formation. The nature of the metal precursor determines its affinity to polar or nonpolar domains of ionic liquid, and therefore, the size of the nanoparticles depends on the size of these regions. Ability of ionic liquids to form varied extended interactions with particle precursor as well as other compounds presented in the reaction media (water, organic solvents etc.) provides nano- and microstructures with different morphologies (0D nanoparticles, 1D nanowires, rods, 2D layers, sheets, and 3D features of molecules). ILs interact efficiently with microwave irradiation, thus even small amount of IL can be employed to increase the dielectric constant of nonpolar solvents used in the synthesis. Thus, combining the advantages of ionic liquids and ray-mediated methods resulted in the development of new ionic liquid-assisted synthesis routes. One of the recently proposed approaches of semiconductor particles preparation is based on the adsorption of semiconductor precursor molecules at the surface of micelles built of ionic liquid molecules playing a role of a soft template for growing microparticles.

Nanoporous ionic organic networks: stabilizing and supporting gold nanoparticles for catalysis

Nanoporous ionic organic networks (PIONs) with a high ionic density (three cation-anion pairs per unit) have been synthesized by a facile SN2 nucleophilic substitution reaction. Owing to the electrostatic and steric effect, those ionic networks with porous channels can stabilize and support gold (Au) nanoparticles (NPs) in 1-2 nm. The Au@PION hybrid materials used as a heterogeneous catalyst were highly active, selective, and stable in the aerobic oxidation of saturated alcohols.

Sonochemistry in the service of SOFC research

Decoration of SOFC anode cermets with metal nanoparticles (NPs) enchance their ability and stability in natural gas to hydrogen reform. A novel sonoelectrochemical approach of Au-NPs synthesis (mean 12.31±2.69nm) is suggested, according to which the sonication is held constant while the electrochemical activity is either pulsed or continuous. The gold colloidal solution is cosonicated with state of the art cermet powder to yield particles decorated with Au-NPs. Nevertheless sonochemical routes of mixed molybdenum, rhenium or tungsten mixed oxides synthesis are utilized in order to decorate SOFC anode cermets. The decoration loading achieved spanned from 0.1 to 10.0wt.%.

Colloidal nickel/gallium nanoalloys obtained from organometallic precursors in conventional organic solvents and in ionic liquids: noble-metal-free alkyne semihydrogenation catalysts

Efforts to replace noble-metal catalysts by low-cost alternatives are of constant interest. The organometallic, non-aqueous wet-chemical synthesis of various hitherto unknown nanocrystalline Ni/Ga intermetallic materials and the use of NiGa for the selective semihydrogenation of alkynes to alkenes are reported. Thermal co-hydrogenolysis of the all-hydrocarbon precursors [Ni(COD)2] (COD = 1,5-cyclooctadiene) and GaCp* (Cp* = pentamethylcyclopentadienyl) in high-boiling organic solvents mesitylene and n-decane in molar ratios of 1 : 1, 2 : 3 and 3 : 1 yields the nano-crystalline powder materials of the over-all compositions NiGa, Ni2Ga3 and Ni3Ga, respectively. Microwave induced co-pyrolysis of the same precursors without additional hydrogen in the ionic liquid [BMIm][BF4] (BMIm = 1-butyl-3-methyl-imidazolium) selectively yields the intermetallic phases NiGa and Ni3Ga from the respective 1 : 1 and 3 : 1 molar ratios of the precursors. The obtained materials are characterized by transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), IR, powder X-ray diffraction (PXRD) and atomic absorption spectroscopy (AAS). The single-source precursor [Ni(GaCp*)(PMe3)3] with a fixed Ni : Ga stoichiometry of 1 : 1 was employed as well. In comparison with the co-hydrogenolytic dual precursor source approach it turned out to be less practical due to inefficient nickel incorporation caused by the parasitic formation of stable [Ni(PMe3)4]. The use of ionic liquid [BMIm][BF4] as a non-conventional solvent to control the reaction and stabilize the nanoparticles proved to be particularly advantageous and stable colloids of the nanoalloys NiGa and Ni3Ga were obtained. A phase-selective Ni/Ga colloid synthesis in conventional solvents and in the presence of surfactants such as hexadecylamine (HDA) was not feasible due to the undesired reactivity of HDA with GaCp* leading to inefficient gallium incorporation. Recyclable NiGa nanoparticles selectively semihydrogenate 1-octyne and diphenylacetylene (tolan) to 1-octene and diphenylethylene, respectively, with a yield of about 90% and selectivities of up to 94 and 87%. Ni-NPs yield alkanes with a selectivity of 97 or 78%, respectively, under the same conditions.

Synthesis of Cu, Zn and Cu/Zn brass alloy nanoparticles from metal amidinate precursors in ionic liquids or propylene carbonate with relevance to methanol synthesis

Microwave-induced decomposition of the transition metal amidinates {[Me(C(N(i)Pr)2)]Cu}2 (1) and [Me(C(N(i)Pr)2)]2Zn (2) in the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF4]) or in propylene carbonate (PC) gives copper and zinc nanoparticles which are stable in the absence of capping ligands (surfactants) for more than six weeks. Co-decomposition of 1 and 2 yields the intermetallic nano-brass phases β-CuZn and γ-Cu3Zn depending on the chosen molar ratios of the precursors. Nanoparticles were characterized by high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM), dynamic light scattering and powder X-ray diffractometry. Microstructure characterizations were complemented by STEM with spatially resolved energy-dispersive X-ray spectrometry and X-ray photoelectron spectroscopy. Synthesis in ILs yields significantly smaller nanoparticles than in PC. β-CuZn alloy nanoparticles are precursors to catalysts for methanol synthesis from the synthesis gas H2/CO/CO2 with a productivity of 10.7 mol(MeOH) (kg(Cu) h)(-1).

Solvation and stabilization of palladium nanoparticles in phosphonium-based ionic liquids: a combined infrared spectroscopic and density functional theory study

Pd nanoparticles interact with both the anions and cations of tri-tert-butyl-R-phosphonium-based ionic liquids. Dispersion forces dominate the cation–nanoparticle interactions.

The halogen analogs of thiolated gold nanoclusters

Is it possible to replace all the thiolates in a thiolated gold nanocluster with halogens while still maintaining the geometry and the electronic structure? In this work, we show from density functional theory that such halogen analogs of thiolated gold nanoclusters are highly likely. Using Au(25)X(18)(-) as an example, where X = F, Cl, Br, or I replaces -SR, we find that Au(25)Cl(18)(-) demonstrates a high similarity to Au(25)(SR)(18)(-) by showing Au-Cl distances, Cl-Au-Cl angles, band gap, and frontier orbitals similar to those in Au(25)(SR)(18)(-). DFT-based global minimization also indicates the energetic preference of staple formation for the Au(25)Cl(18)(-) cluster. The similarity between Au(m)(SR)(n) and Au(m)X(n) could be exploited to make viable Au(m)X(n) clusters and to predict structures for Au(m)(SR)(n).

Organic carbonates as stabilizing solvents for transition-metal nanoparticles

Biodegradable, non-toxic, "green" and inexpensive propylene carbonate (PC) solvent is shown to function as a stabilizing medium for the synthesis of weakly-coordinated transition-metal nanoparticles. Kinetically stable nanoparticles (M-NPs) with a small and uniform particle size (typically <5 ± 1 nm) have been reproducibly obtained by easy, rapid (3 min) and energy-saving 50 W microwave irradiation under an argon atmosphere from their metal-carbonyl precursors in PC. The M-NP/PC dispersions are stable for up to three weeks according to repeated TEM studies over this time period. The rhodium nanoparticle/PC dispersion is a highly active catalyst for the biphasic liquid-liquid hydrogenation of cyclohexene to cyclohexane with activities of up to and 1875 (mol product) (mol Rh)(-1) h(-1) and near quantitative conversion at 4 to 10 bar H(2) and 90 °C. From the PC dispersion the M-NPs can be coated with organic capping ligands such as 3-mercaptopropionic acid or trioctylphosphine oxide for further stabilization.

Microwave synthesis and inherent stabilization of metal nanoparticles in 1-methyl-3-(3-carboxyethyl)-imidazolium tetrafluoroborate

The synthesis of Co-NPs and Mn-NPs by microwave-induced decomposition of the metal carbonyls Co(2)(CO)(8) and Mn(2)(CO)(10), respectively, yields smaller and better separated particles in the functionalized IL 1-methyl-3-(3-carboxyethyl)-imidazolium tetrafluoroborate [EmimCO(2)H][BF(4)] (1.6 ± 0.3 nm and 4.3 ± 1.0 nm, respectively) than in the non-functionalized IL 1-n-butyl-3-methylimidazolium tetrafluoroborate [Bmim][BF(4)]. The particles are stable in the absence of capping ligands (surfactants) for more than six months although some variation in particle size could be observed by TEM.

Growth of sputter-deposited gold nanoparticles in ionic liquids

The growth of gold nanoparticles (NPs) synthesized by sputter deposition on an ionic liquid surface is studied in situ in the bulk phase of the ionic liquids (ILs) 1-butyl-3-methylimidazolium dicyanamide [C(1)C(4)Im][N(CN)(2)], 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide [C(1)C(4)Im][Tf(2)N], 1-butyl-3-methylimidazolium tetrafluoroborate [C(1)C(4)Im][BF(4)], 1-butyl-3-methylimidazolium hexafluorophosphate [C(1)C(4)Im][PF(6)] and 1-butyl-3-methylimidazolium triflate [C(1)C(4)Im][TfO]. It is found that primary nanoparticles with a diameter smaller than 2.5 nm are present in the sample immediately after sputtering. Growth of these primary particles proceeds after the end of the sputtering process and stops when the nanoparticles reach a certain size. Depending on the viscosity of the ionic liquid this growth process can proceed several hours to several days. The growth speed is fastest for the least viscous ionic liquid and follows the trend [C(1)C(4)Im][N(CN)(2)] > [C(1)C(4)Im][Tf(2)N] > [C(1)C(4)Im][TfO] > [C(1)C(4)Im][BF(4)] > [C(1)C(4)Im][PF(6)]. It is also found that a higher concentration of sputtered gold results in faster growth of the gold nanoparticles. A discussion on the growth mechanism of sputtered gold NPs is included.

Gold nanowire networks: synthesis, characterization, and catalytic activity

Gold nanowire networks (AuNWNs) with average widths of 17.74 nm (AuNWN(1)) or 23.54 nm (AuNWN(2)) were synthesized by direct reduction of HAuCl(4) with sodium borohydride powder in deep eutectic solvents, such as ethaline or reline, at 40 °C. Their width and length were dependent on the type of solvent and the NaBH(4)/HAuCl(4) molar ratio (32 in ethaline and 5.2 in reline). High resolution transmission electron microscopy (HR-TEM) analysis of the gold nanowire networks showed clear lattice fringes of polycrystalline nanopowder of d = 2.36, 2.04, 1.44, and 1.23 Å corresponding to the (111), (200), (220), or (311) crystallographic planes of face centered cubic gold. The purified AuNWNs were used as catalysts for the chemical reduction of p-nitroaniline to diaminophenylene with sodium borohydride in aqueous solution. The reaction was monitored in real time by UV-vis spectroscopy. The results show that the reduction process is six times faster in the presence of gold nanowire networks stabilized by urea from the reline (AuNWN(2)) than in the presence of gold nanowire networks stabilized by ethylene glycol from ethaline (AuNWN(1)). This is due to a higher number of corners and edges on the gold nanowires synthesized in reline than on those synthesized in ethaline as proven by X-ray diffraction (XRD) patterns recorded for both types of gold nanowire networks. Nevertheless, both types of nanomaterials determined short times of reaction and high conversion of p-nitroaniline to diaminophenylene. These gold nanomaterials represent a new addition to a new generation of catalysts: gold based catalysts.