R. Mohan Sankaran

Linking catalyst composition to chirality distributions of as-grown single-walled carbon nanotubes by tuning Ni x Fe 1− x nanoparticles

Chirally pure single-walled carbon nanotubes (SWCNTs) are required for various applications ranging from nanoelectronics to nanomedicine. Although significant efforts have been directed towards separation of SWCNT mixtures, including density-gradient ultracentrifugation , chromatography and electrophoresis, the initial chirality distribution is determined during growth and must be controlled for non-destructive, scalable and economical production. Here, we show that the chirality distribution of as-grown SWCNTs can be altered by varying the composition of Ni(x)Fe(1-x) nanocatalysts. Precise tuning of the nanocatalyst composition at constant size is achieved by a new gas-phase synthesis route based on an atmospheric-pressure microplasma. The link between the composition-dependent crystal structure of the nanocatalysts and the resulting nanotube chirality supports epitaxial models and is a step towards chiral-selective growth of SWCNTs.

Microplasmas for nanomaterials synthesis

Microplasmas have attracted a tremendous amount of interest from the plasma community because of their small physical size, stable operation at atmospheric pressure, non-thermal characteristics, high electron densities and non-Maxwellian electron energy distributions. These properties make microplasmas suitable for a wide range of materials applications, including the synthesis of nanomaterials. Research has shown that vapour-phase precursors can be injected into a microplasma to homogeneously nucleate nanoparticles in the gas phase. Alternatively, microplasmas have been used to evaporate solid electrodes and form metal or metal-oxide nanostructures of various composition and morphology. Microplasmas have also been coupled with liquids to directly reduce aqueous metal salts and produce colloidal dispersions of nanoparticles. This topical review discusses the unique features of microplasmas that make them advantageous for nanomaterials synthesis, gives an overview of the diverse approaches previously reported in the literature and looks ahead to the potential for scale-up of current microplasma-based processes.

Contact electrification of insulating materials

The electrostatic charge that is generated when two materials are contacted or rubbed and then separated is a well-known physical process that has been studied for more than 2500 years. Contact electrification occurs in many contexts, both natural and technological. For example, in dust storms the collisions between particles lead to electrostatic charging and in extreme cases, extraordinary lightning displays. In electrophotography, toner particles are intentionally charged to guide their deposition in well-defined patterns. Despite such a long history and so many important consequences, a fundamental understanding of the mechanism behind contact electrification remains elusive. An open question is what type of species are transferred between the surfaces to generate charge—experiments suggest various species ranging from electrons to ions to nanoscopic bits of material, and theoretical work suggests that non-equilibrium states may play an important role. Another open question is the contact electrification that occurs when two insulating materials with identical physical properties touch—since there is no apparent driving force, it is not clear why charge transfer occurs. A third open question involves granular systems—models and experiments have shown that a particle-size dependence for the charging often exists. In this review, we discuss the fundamental aspects of contact electrification and highlight recent research efforts aimed at understanding these open questions.

Plasma-liquid electrochemistry: Rapid synthesis of colloidal metal nanoparticles by microplasma reduction of aqueous cations

We demonstrate operation of an aqueous electrochemical cell with an atmospheric-pressure microplasma cathode and a solid metal anode for the rapid production of colloidal metal nanoparticles. Microplasmas are miniaturized versions of low-pressure glow discharges that operate nonthermally at high pressures (∼1atm) and contain energetic electrons. Aqueous metal cations are directly reduced by electrons in the microplasma without the presence of a solid cathode or chemical reducing agents. Kinetic studies performed by UV-visible absorbance spectroscopy suggest that particle nucleation and growth are coupled to redox reactions initiated in the electrochemical cell through plasma-liquid interactions.

Perspectives on atmospheric-pressure plasmas for nanofabrication

Low-pressure, low-temperature plasmas are widely used for materials applications in industries ranging from electronics to medicine. To avoid the high costs associated with vacuum equipment, there has always been a strong motivation to operate plasmas at higher pressures, up to atmospheric. However, high-pressure operation of plasmas often leads to instabilities and gas heating, conditions that are unsuitable for materials applications. The recent development of microscale plasmas (i.e. microplasmas) has helped realize the sustainment of stable, non-thermal plasmas at atmospheric pressure and enable low-cost materials applications. There has also been an unexpected benefit of atmospheric-pressure operation: the potential to fabricate nanoscale materials which is not possible by more conventional, low-pressure plasmas. For example, in a high-pressure environment, nanoparticles can be nucleated in the gas phase from vapour (or solid metal) precursors. Alternatively, non-thermal, atmospheric-pressure plasmas can be coupled with liquids such as water or ethanol to nucleate and modify solution-phase nanoparticles. In this perspective paper, we review some of these recent efforts and provide an outlook for the rapidly emerging field of atmospheric-pressure plasmas for nanofabrication.

Hollow cathode sustained plasma microjets: Characterization and application to diamond deposition

Extending the principle of operation of hollow cathode microdischarges to a tube geometry has allowed the formation of stable, high-pressure plasma microjets in a variety of gases including Ar, He, and H2. Direct current discharges are ignited between stainless steel capillary tubes (d = 178 µm) which are operated as the cathode and a metal grid or plate that serves as the anode. Argon plasma microjets can be sustained in ambient air with plasma voltages as low as 260 V for cathode-anode gaps of 0.5 mm. At larger operating voltage, this gap can be extended up to several millimeters. Using a heated molybdenum substrate as the anode, plasma microjets in CH4/H2 mixtures have been used to deposit diamond crystals and polycrystalline films. Micro-Raman spectroscopy of these films shows mainly sp3 carbon content with slight shifting of the diamond peak due to internal stresses. Optical emission spectroscopy of the discharges used in the diamond growth experiments confirms the presence of atomic hydrogen and CH radicals.

Continuous-flow, atmospheric-pressure microplasmas: a versatile source for metal nanoparticle synthesis in the gas or liquid phase

Continuous-flow, atmospheric-pressure microplamas are a unique class of plasmas that are highly suitable for emerging nanomaterials applications. Here, we present two schemes for the preparation of metal nanoparticles based on these plasma sources. Nanoparticles are synthesized in the gas phase by non-thermal dissociation of vapor precursors in a microplasma reactor. Monometallic Ni and Fe nanoparticles, as well as compositionally controlled NiFe bimetallic nanoparticles, can be grown with tunable mean diameters between 1 and 5 nm and narrow size distributions. Alternatively, colloidal metal nanoparticles are produced directly in aqueous solutions. Metal cations generated from anodic dissolution of a bulk metal or present in the form of metal salt are reduced by the microplasma to form nanoparticles and capped by a stabilizer. Both approaches are low cost, scalable and general and should allow a wide range of nanoparticle materials to be synthesized in the gas or liquid phase.

The solvation of electrons by an atmospheric-pressure plasma

Solvated electrons are typically generated by radiolysis or photoionization of solutes. While plasmas containing free electrons have been brought into contact with liquids in studies dating back centuries, there has been little evidence that electrons are solvated by this approach. Here we report direct measurements of solvated electrons generated by an atmospheric-pressure plasma in contact with the surface of an aqueous solution. The electrons are measured by their optical absorbance using a total internal reflection geometry. The measured absorption spectrum is unexpectedly blue shifted, which is potentially due to the intense electric field in the interfacial Debye layer. We estimate an average penetration depth of 2.5±1.0 nm, indicating that the electrons fully solvate before reacting through second-order recombination. Reactions with various electron scavengers including H+, NO2−, NO3− and H2O2 show that the kinetics are similar, but not identical, to those for solvated electrons formed in bulk water by radiolysis.

Plasma-Assisted Reduction of Graphene Oxide at Low Temperature and Atmospheric Pressure for Flexible Conductor Applications.

Reduction of graphene oxide (GO) at low temperature and atmospheric pressure via plasma-assisted chemistry is demonstrated. Hydrogen gas is continuously dissociated in a microplasma to generate atomic hydrogen, which flows from the remote plasma to thin films of GO deposited on a substrate. Direct interaction with ions and other energetic species is avoided to mitigate ion-induced sputter removal or damage. The residual oxygen content and structure of the GO films after plasma treatment is systematically characterized at different temperatures and correlated to the conductivity of the films. For example, at 150 °C, we find that the plasma-reduced GO contains less than 12.5% oxygen and exhibits a sheet resistance of 4.77 × 10(4) Ω/sq, as compared with thermal reduction alone, which results in 22.9% oxygen and a sheet resistance of 2.14 × 10(6) Ω/sq. Overall, the effective removal of oxygen functional groups by atomic hydrogen enables large-scale applications of GO as flexible conductors to be realized.

Electron-transfer reactions at the plasma-liquid interface.

Electrochemical reactions are normally initiated in solution by metal electrodes such as Pt, which are expensive and limited in supply. In this Communication, we demonstrate that an atmospheric-pressure microplasma can act as a gaseous, metal-free electrode to mediate electron-transfer reactions in aqueous solutions. Ferricyanide is reduced to ferrocyanide by plasma electrons, and the reduction rate is found to depend on discharge current. The ability to initiate and control electrochemical reactions at the plasma-liquid interface opens a new direction for electrochemistry based on interactions between gas-phase electrons and ionic solutions.