Electrochemical Synthesis of Unique Nanomaterials in Ionic Liquids

The review considers the features of the processes of the electrochemical synthesis of nanostructures in ionic liquids (ILs), including the production of carbon nanomaterials, silicon and germanium nanoparticles, metallic nanoparticles, nanomaterials and surface nanostructures based on oxides. In addition, the analysis of works on the synthesis of nanoscale polymer films of conductive polymers prepared using ionic liquids by electrochemical methods is given. The purpose of the review is to dwell upon an aspect of the applicability of ILs that is usually not fully reflected in modern literature, the synthesis of nanostructures (including unique ones that cannot be obtained in other electrolytes). The current underestimation of ILs as an electrochemical medium for the synthesis of nanomaterials may limit our understanding and the scope of their potential application. Another purpose of our review is to expand their possible application and to show the relative simplicity of the experimental part of the work.

Pyrrolidinium Containing Ionic Liquid Electrolytes for Li-Based Batteries

Ionic liquids are potential alternative electrolytes to the more conventional solid-state options under investigation for future energy storage solutions. This review addresses the utilization of IL electrolytes in energy storage devices, particularly pyrrolidinium-based ILs. These ILs offer favorable properties, such as high ionic conductivity and the potential for high power drain, low volatility and wide electrochemical stability windows (ESW). The cation/anion combination utilized significantly influences their physical and electrochemical properties, therefore a thorough discussion of different combinations is outlined. Compatibility with a wide array of cathode and anode materials such as LFP, V₂O₅, Ge and Sn is exhibited, whereby thin-films and nanostructured materials are investigated for micro energy applications. Polymer gel electrolytes suitable for layer-by-layer fabrication are discussed for the various pyrrolidinium cations, and their compatibility with electrode materials assessed. Recent advancements regarding the modification of typical cations such a 1-butyl-1-methylpyrrolidinium, to produce ether-functionalized or symmetrical cations is discussed.

Growth of Homogeneous Luminescent Silicon-Terbium Nanowires by One-Step Electrodeposition in Ionic Liquids

An electrodeposition method for the growth of homogeneous silicon-terbium nanowires (NWs) with green light emission is described. The method involves template-assisted electrochemical co-deposition of Si/Tb NWs with 90-nm diameter from an electrolyte bath containing Si and Tb precursors in an ionic liquid (IL). This method of deposition is advantageous over other conventional techniques as it is relatively simple and cost-effective and avoids harsh deposition conditions. The deposited NWs are of uniform dimensions with homogeneous composition incorporating 10% of Tb and exhibit intense room temperature (RT) luminescence in the visible range due to Tb emission. These results were confirmed by combining classical characterization such as scanning electron microscopy (SEM) and photoluminescence (PL) performed on an assembly of NWs with spatially resolved experiments such as transmission electron microscopy (TEM) and cathodoluminescence (CL). This electrodeposition method provides an alternative and extremely simple approach for depositing silicon-rare earth nanostructures for optical and sensing applications.

Quantifying the Short-Range Order in Amorphous Silicon by Raman Scattering

Quantification of the short-range order in amorphous silicon has been formulized using Raman scattering by taking into account established frameworks for studying the spectral line-shape and size dependent Raman peak shift. A theoretical line-shape function has been proposed for representing the observed Raman scattering spectrum from amorphous-Si-based on modified phonon confinement model framework. While analyzing modified phonon confinement model, the term "confinement size" used in the context of nanocrystalline Si was found analogous to the short-range order distance in a-Si thus enabling one to quantify the same using Raman scattering. Additionally, an empirical formula has been proposed using bond polarizability model for estimating the short-range order making one capable to quantify the distance of short-range order by looking at the Raman peak position alone. Both the proposals have been validated using three different data sets reported by three different research groups from a-Si samples prepared by three different methods making the analysis universal.

Electrochemical Scanning Tunneling Microscopic Study of the Potential Dependence of Germanene Growth on Au(111) at pH 9.0

Germanene is a 2D material whose structure and properties are of great interest for integration with Si technology. Preparation of germanene experimentally remains a challenge because, unlike graphene, bulk germanene does not exist. Thus, germanene cannot be directly exfoliated and is mostly grown in ultrahigh vacuum. The present report uses electrodeposition in an aqueous HGeO₃^- solution at pH 9. Germanene deposition has been limited to 2-3 monolayers, thus greatly restricting many applicable characterization methods. The in situ technique of electrochemical scanning tunneling microscopy was used to follow Ge deposition on Au(111) as a function of potential. Previous work by this group at pH 4.5 suggested germanene growth, but no buffer was used, resulting in change in surface pH. The addition of borate buffer to create pH 9.0 solution has reduced hydrogen formation and stabilized the surface pH, allowing systematic characterization of germanene growth versus potential. Initial germanene nucleated at defects in the Au(111) herringbone (HB) reconstruction. Subsequent growth proceeded down the face-centered cubic troughs, slowly relaxing the HB. The resulting honeycomb (HC) structure displayed an average lattice constant of 0.41 ± 0.06 nm. Continued growth resulted in the addition of a second layer on top, formed initially by nucleating around small islands and subsequent lateral 2D growth. Near atomic resolution of the germanene layers displayed small coherent domains, 2-3 nm, of the HC structure composed of six-membered rings. Domain walls were based on defective, five- and seven-membered rings, which resulted in small rotations between adjacent HC domains.

Ionic liquid electrodeposition of strain-released Germanium nanowires as stable anodes for lithium ion batteries

With the growing demand for portable and wearable electronic devices, it is imperative to develop high performance Li-ion batteries with long life times. Germanium-based materials have recently demonstrated excellent lithium-ion storage ability and are being considered as the most promising candidates for the anodes of lithium-ion batteries. Nevertheless, the practical implementation of Ge-based materials to Li-ion batteries is greatly hampered by the poor cycling ability that resulted from the huge volume variation during lithiation/delithiation processes. Herein, we develop a simple and efficient method for the preparation of Ge nanowires without catalyst nanoparticles and templates, using ionic liquid electrodeposition with subsequent annealing treatment. The Ge nanowire anode shows improved electrochemical performance compared with the Ge dense film anode. A capacity of ∼1200 mA h g^-1 after 200 cycles at 0.1 C is obtained, with an initial coulombic efficiency of 81.3%. In addition, the Ge nanowire anode demonstrates superior rate capability with excellent capacity retention and stability (producing highly stable discharge capacities of about 620 mA h g^-1 at 5 C). The improved electrochemical performance is the result of the enhanced electron migration and electron transport paths of the nanowires, and sufficient elasticity to buffer the volume expansion. This approach encompasses a low energy processing method where all the material is electrochemically active and binder free. The improved cycling stability and rate performance characteristics make these anodes highly attractive for the most demanding lithium-ion applications.

Template-based syntheses for shape controlled nanostructures

A variety of nanostructured materials are produced through template-based synthesis methods, including zero-dimensional, one-dimensional, and two-dimensional structures. These span different forms such as nanoparticles, nanowires, nanotubes, nanoflakes, and nanosheets. Many physical characteristics of these materials such as the shape and size can be finely controlled through template selection and as a result, their properties as well. Reviewed here are several examples of these nanomaterials, with emphasis specifically on the templates and synthesis routes used to produce the final nanostructures. In the first section, the templates have been discussed while in the second section, their corresponding synthesis methods have been briefly reviewed, and lastly in the third section, applications of the materials themselves are highlighted. Some examples of the templates frequently encountered are organic structure directing agents, surfactants, polymers, carbon frameworks, colloidal sol-gels, inorganic frameworks, and nanoporous membranes. Synthesis methods that adopt these templates include emulsion-based routes and template-filling approaches, such as self-assembly, electrodeposition, electroless deposition, vapor deposition, and other methods including layer-by-layer and lithography. Template-based synthesized nanomaterials are frequently encountered in select fields such as solar energy, thermoelectric materials, catalysis, biomedical applications, and magnetowetting of surfaces.

Synthesis of Functional Nanomaterials in Ionic Liquids

Utilization of ionic liquids (ILs) in material synthesis is a promising field. The unusual properties of ILs provide new opportunities for the design of functional materials, and much excellent work has been reported. Here, the progress in material design and synthesis using ILs, especially nanomaterials, is discussed, including the unitization of ILs as synthetic media, templates, precursors, or components in the synthesis of various categories of nanomaterials. The challenges and opportunities in this interesting and rapid developing area are also discussed.

Bio-templated germanium photonic crystals by a facile liquid phase deposition process

Ge photonic crystals were synthesized by replicating butterfly wings with GeO₂via a facile liquid phase deposition followed by a reducing process.

Characterisation of the solid electrolyte interface during lithiation/delithiation of germanium in an ionic liquid

The characterisation of the SEI layer revealed that LiTFSI–[Py_1,4] is a relatively good ionic liquid based electrolyte for lithium batteries. However modifications in the electrolyte or a different anion might be necessary to improve the stability and composition of the SEI layer.

Electrodeposition in Ionic Liquids

Due to their attractive physico-chemical properties, ionic liquids (ILs) are increasingly used as deposition electrolytes. This review summarizes recent advances in electrodeposition in ILs and focuses on its similarities and differences with that in aqueous solutions. The electrodeposition in ILs is divided into direct and template-assisted deposition. We detail the direct deposition of metals, alloys and semiconductors in five types of ILs, including halometallate ILs, air- and water-stable ILs, deep eutectic solvents (DESs), ILs with metal-containing cations, and protic ILs. Template-assisted deposition of nanostructures and macroporous structures in ILs is also presented. The effects of modulating factors such as deposition conditions (current density, current density mode, deposition time, temperature) and electrolyte components (cation, anion, metal salts, additives, water content) on the morphology, compositions, microstructures and properties of the prepared materials are highlighted.

Preparation of Ge nanotube arrays from an ionic liquid for lithium ion battery anodes with improved cycling stability

Ge nanotube array anodes are prepared by template-assisted electrodeposition from an ionic liquid to obtain superior cycling performance. They show remarkable cycling ability at 0.2 C, with a very high initial discharge capacity of 1641 mA h g(-1) and a charge capacity of 1260 mA h g(-1). After 250 cycles the capacity retention is 98% relative to that at the 50th cycle.

Application of ionic liquids in hydrometallurgy

Ionic liquids, low temperature molten salts, have various advantages manifesting themselves as durable and environmentally friendly solvents. Their application is expanding into various fields including hydrometallurgy due to their unique properties such as non-volatility, inflammability, low toxicity, good ionic conductivity, and wide electrochemical potential window. This paper reviews previous literatures and our recent results adopting ionic liquids in extraction, synthesis and processing of metals with an emphasis on the electrolysis of active/light, rare earth, and platinum group metals. Because the research and development of ionic liquids in this area are still emerging, various, more fundamental approaches are expected to popularize ionic liquids in the metal manufacturing industry.

Halometallate complexes of germanium(II) and (IV): probing the role of cation, oxidation state and halide on the structural and electrochemical properties

The Ge(IV) chlorometallate complexes, [EMIM]2 [GeCl6 ], [EDMIM]2 [GeCl6 ] and [PYRR]2 [GeCl6 ] (EMIM=1-ethyl-3-methylimidazolium; EDMIM=2,3-dimethyl-1-ethylimidazolium; PYRR=N-butyl-N-methylpyrrolidinium) have been synthesised and fully characterised; the first two also by single-crystal X-ray diffraction. The imidazolium chlorometallates exhibited significant CH⋅⋅⋅Cl hydrogen bonds, resulting in extended supramolecular assemblies in the solid state. Solution (1) H NMR data also showed cation-anion association. The synthesis and characterisation of Ge(II) halometallate salts [EMIM][GeX3 ] (X=Cl, Br, I) and [PYRR][GeCl3 ], including single-crystal X-ray analyses for the homologous series of imidazolium salts, are reported. In these complexes, the intermolecular interactions are much weaker in the solid state and they appear not to be significantly associated in solution. Cyclic-voltammetry experiments on the Ge(IV) species in CH2 Cl2 solution showed two distinct, irreversible reduction waves attributed to Ge(IV) -Ge(II) and Ge(II) -Ge(0) , whereas the Ge(II) species exhibited one irreversible reduction wave. The potential for the Ge(II) -Ge(0) reduction was unaffected by changing the cation, although altering the oxidation state of the precursor from Ge(IV) to Ge(II) does have an effect; for a given cation, reduction from the [GeCl3 ](-) salts occurred at a less cathodic potential. The nature of the halide co-ligand also has a marked influence on the reduction potential for the Ge(II) -Ge(0) couple, such that the reduction potentials for the [GeX3 ](-) salts become significantly less cathodic when the halide (X) is changed Cl→Br→I.

Room-temperature epitaxial electrodeposition of single-crystalline germanium nanowires at the wafer scale from an aqueous solution

Direct epitaxial growth of single-crystalline germanium (Ge) nanowires at room temperature has been performed through an electrodeposition process on conductive wafers immersed in an aqueous bath. The crystal growth is based on an electrochemical liquid-liquid-solid (ec-LLS) process involving the electroreduction of dissolved GeO2(aq) in water at isolated liquid gallium (Ga) nanodroplet electrodes resting on single-crystalline Ge or Si supports. Ge nanowires were electrodeposited on the wafer scale (>10 cm(2)) using only common glassware and a digital potentiostat. High-resolution electron micrographs and electron diffraction patterns collected from cross sections of individual substrate-nanowire contacts in addition to scanning electron micrographs of the orientation of nanowires across entire films on substrates with different crystalline orientations, supported the notion of epitaxial nanowire growth. Energy dispersive spectroscopic elemental mapping of single nanowires indicated the Ga(l) nanodroplet remains affixed to the tip of the growing nanowire throughout the nanowire electrodeposition process. Current-voltage responses measured across many individual nanowires yielded reproducible resistance values. The presented data cumulatively show epitaxial growth of covalent group IV nanowires is possible from the reduction of a dissolved oxide under purely benchtop conditions.

Synthesis of Silicon and Germanium Nanowire Assemblies by Template-Assisted Electrodeposition from an Ionic Liquid

We report on the template-assisted synthesis of silicon and germanium nanowires from the air- and water-stable ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([Py1,4] TFSA). The synthesis was done by electrochemical deposition in the pores of a commercial track-etched polycarbonate membrane. After chemical dissolution of the polycarbonate membrane in dichloromethane, nanowire assemblies with a regular arrangement were obtained. Different lengths of nanowires can be obtained by varying the applied potential and the time of deposition. The nanowire assemblies were characterised by scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM/EDX). Our results show that the template-assisted electrochemical deposition approach in ionic liquids has the potential to easily synthesise germanium and silicon nanowire assemblies.

Characterization and properties of micro- and nanowires of controlled size, composition, and geometry fabricated by electrodeposition and ion-track technology

The combination of electrodeposition and polymeric templates created by heavy-ion irradiation followed by chemical track etching provides a large variety of poly- and single-crystalline nanowires of controlled size, geometry, composition, and surface morphology. Recent results obtained by our group on the fabrication, characterization and size-dependent properties of nanowires synthesized by this technique are reviewed, including investigations on electrical resistivity, surface plasmon resonances, and thermal instability.

Template-free preparation of crystalline Ge nanowire film electrodes via an electrochemical liquid-liquid-solid process in water at ambient pressure and temperature for energy storage

The direct electrodeposition of crystalline germanium (Ge) nanowire film electrodes from an aqueous solution of dissolved GeO(2) using discrete 'flux' nanoparticles capable of dissolving Ge(s) has been demonstrated. Electrodeposition of Ge at inert electrode substrates decorated with small (<100 nm), discrete indium (In) nanoparticles resulted in crystalline Ge nanowire films with definable nanowire diameters and densities without the need for a physical or chemical template. The Ge nanowires exhibited strong polycrystalline character as-deposited, with approximate crystallite dimensions of 20 nm and a mixed orientation of the crystallites along the length of the nanowire. Energy dispersive spectroscopic elemental mapping of individual Ge nanowires showed that the In nanoparticles remained at the base of each nanowire, indicating good electrical communication between the Ge nanowire and the underlying conductive support. As-deposited Ge nanowire films prepared on Cu supports were used without further processing as Li(+) battery anodes. Cycling studies performed at 1 C (1624 mA g(-1)) indicated the native Ge nanowire films supported stable discharge capacities at the level of 973 mA h g(-1), higher than analogous Ge nanowire film electrodes prepared through an energy-intensive vapor-liquid-solid nanowire growth process. The cumulative data show that ec-LLS is a viable method for directly preparing a functional, high-activity nanomaterials-based device component. The work presented here is a step toward the realization of simple processes that make fully functional energy conversion/storage technologies based on crystalline inorganic semiconductors entirely through benchtop, aqueous chemistry and electrochemistry without time- or energy-intensive process steps.

Growth of Ge nanofilms using electrochemical atomic layer deposition, with a "bait and switch" surface-limited reaction

Ge nanofilms were deposited from aqueous solutions using the electrochemical analog of atomic layer deposition (ALD). Direct electrodeposition of Ge from an aqueous solution is self-limited to a few monolayers, depending on the pH. This report describes an E-ALD process for the growth of Ge films from aqueous solutions. The E-ALD cycle involved inducing a Ge atomic layer to deposit on a Te atomic layer formed on Ge, via underpotential deposition (UPD). The Te atomic layer was then reductively stripped from the deposit, leaving the Ge and completing the cycle. The Te atomic layer was bait for Ge deposition, after which the Te was switched out, reduced to a soluble telluride, leaving the Ge (one "bait and switch" cycle). Deposit thickness was a linear function of the number of cycles. Raman spectra indicated formation of an amorphous Ge film, consistent with the absence of a XRD pattern. Films were more stable and homogeneous when formed on Cu substrates, than on Au, due to a larger hydrogen overpotential, and the corresponding lower tendency to form bubbles.

One-dimensional germanium nanostructures--formation and their electron field emission properties

Ge nanostructures were synthesized by reduction of GeO(2) in H(2) atmosphere at various temperatures. Entangled and straight Ge nanowires with oxide shells were grown at high temperatures. Ge nanowires with various numbers of nodules were obtained at low temperatures. Ge nanowires without nodules exhibited remarkable field emission properties with a turn-on field of 4.6 V µm(-1) and field enhancement factor of 1242.

New frontiers in materials science opened by ionic liquids

Ionic liquids (ILs) including ambient-temperature molten salts, which exist in the liquid state even at room temperature, have a long research history. However, their applications were once limited because ILs were considered as highly moisture-sensitive solvents that should be handled in a glove box. After the first synthesis of moisture-stable ILs in 1992, their unique physicochemical properties became known in all scientific fields. ILs are composed solely of ions and exhibit several specific liquid-like properties, e.g., some ILs enable dissolution of insoluble bio-related materials and the use as tailor-made lubricants in industrial applications under extreme physicochemical conditions. Hybridization of ILs and other materials provides quasi-solid materials, which can be used to fabricate highly functional devices. ILs are also used as reaction media for electrochemical and chemical synthesis of nanomaterials. In addition, the negligible vapor pressure of ILs allows the fabrication of electrochemical devices that are operated under ambient conditions, and many liquid-vacuum technologies, such as X-ray photoelectron spectroscopy (XPS) analysis of liquids, electron microscopy of liquids, and sputtering and physical vapor deposition onto liquids. In this article, we review recent studies on ILs that are employed as functional advanced materials, advanced mediums for materials production, and components for preparing highly functional materials.

Aqueous electrodeposition of Ge monolayers

The electrodeposition of germanium on Au(111) in aqueous solutions has been investigated by means of cyclic voltammetry, Auger electron spectroscopy, and in situ scanning tunneling microscopy (STM). The data yield a picture of germanium deposition, which starts with the formation of two well-ordered hydroxide phases, with 1/3 ML and 4/9 ML coverages upon initial reduction of the Ge(IV) species (probably H(2)GeO(3) at pH 4.7). Those structures appear to result from a three-electron reduction to form surface-limited structures with (square root(3) x square root(3))R30 degrees or (3 x 3) unit cells, respectively. Further reduction, probably in a two-electron process from the hydroxide structures, resulted in a germanium hydride structure, again surface-limited, with a coverage of close to 0.8 ML. The hydride structure is very flat, though with the periodic modulation characteristic of a Moiré pattern. Longer deposition times and lower potentials resulted in increased coverage of Ge in some cases, but with apparently limited coverage as a function of pH. The maximum Ge coverage, about 4 ML, was observed using a pH 9.32 deposition solution. At potentials negative of the Moiré pattern, about -850 mV versus Ag/AgCl, a "corruption" of the smooth Moiré pattern occurred. This roughening appears to mark the initial formation of a Au-Ge alloy, accounting for the observation of coverage in excess of that needed to form the Moiré pattern at some pH values.

Ionic-liquid materials for the electrochemical challenges of the future

Ionic liquids are room-temperature molten salts, composed mostly of organic ions that may undergo almost unlimited structural variations. This review covers the newest aspects of ionic liquids in applications where their ion conductivity is exploited; as electrochemical solvents for metal/semiconductor electrodeposition, and as batteries and fuel cells where conventional media, organic solvents (in batteries) or water (in polymer-electrolyte-membrane fuel cells), fail. Biology and biomimetic processes in ionic liquids are also discussed. In these decidedly different materials, some enzymes show activity that is not exhibited in more traditional systems, creating huge potential for bioinspired catalysis and biofuel cells. Our goal in this review is to survey the recent key developments and issues within ionic-liquid research in these areas. As well as informing materials scientists, we hope to generate interest in the wider community and encourage others to make use of ionic liquids in tackling scientific challenges.