Material jetting of carbon nano onions for printed electronics.

As an additive manufacturing process, material jetting techniques allow to selectively deposit droplets of materials in liquid or powder form through a small-diameter aperture, such as a nozzle of a print head. For the fabrication of printed electronics, a variety of inks and dispersions of functional materials can be deposited by drop-on-demand printing on rigid and flexible substrates. In this work, zero-dimensional multi-layer shell-structured fullerene material, also known as carbon nano-onion (CNO) or onion-like carbon, is printed on polyethylene terephthalate substrates using drop-on-demand inkjet printing. CNOs are produced using a low-cost flame synthesis technique and characterized by electron microscopy, Raman, x-ray photoelectron spectroscopy, and specific surface area and pore size measurements. The produced CNO material has an average diameter of ∼33 nm, pore diameter in the range ∼2-40 nm and a specific surface area of 160 m2.g-1. The CNO dispersions in ethanol have a reduced viscosity (∼1.2 mPa.s) and are compatible with commercial piezoelectric inkjet heads. The jetting parameters are optimized to avoid satellite drops and to obtain a reduced drop volume (52 pL), resulting in optimal resolution (220µm) and line continuity. A multi-step process is implemented without inter-layer curing and a fine control over the CNO layer thickness is achieved (∼180 nm thick layer after 10 printing passes). The printed CNO structures show an electrical resistivity of ∼600 Ω.m, a high negative temperature coefficient of resistance (-4.35 × 10-2°C-1) and a marked dependency on relative humidity (-1.29 × 10-2RH%-1). The high sensitivity to temperature and humidity, combined to the large specific area of the CNOs, make this material and the corresponding ink a viable prospect for inkjet-printed technologies, such as environmental and gas sensors.

Green Solvents for the Liquid Phase Exfoliation Production of Graphene: The Promising Case of Cyrene.

The liquid phase exfoliation (LPE) of graphite has allowed to produce graphene materials on a large scale and at a reasonable cost. By this method, stable dispersions, inks and liquid suspensions containing atomic-thick graphene flakes with tailored concentrations can be produced, opening up applications in a wide range of cutting-edge technologies such as functional coatings, printed and flexible electronics, and composites. However, currently established LPE techniques raise several health and environmental risks, since unsafe and toxic solvents (such as NMP, DMF, and DMSO) are often regarded as the most effective liquid media for the process. Therefore, it appears necessary to unlock eco-friendly and sustainable methods for the production of graphene at an industrial scale. This review focuses on the latest developments in terms of green solvents for LPE production of graphene. We highlight the use of a new green solvent, Cyrene, and its performance when compared to conventional solvents.

Selective transport of water molecules through interlayer spaces in graphite.

Interlayer space in graphite is impermeable to ions and molecules, including protons. Its controlled expansion would find several applications in desalination, gas purification, high-density batteries, etc. In the past, metal intercalation has been used to modify graphitic interlayer spaces; however, resultant intercalation compounds are unstable in water. Here, we successfully expanded graphite interlayer spaces by intercalating aqueous KCl ions electrochemically. Our spectroscopy studies provide clear evidence for cation-π interactions explaining the stability of the devices, though weak anion-π interactions were also detectable. The water conductivity shows several orders of enhancement when compared to unintercalated graphite. Water evaporation experiments further confirm the high permeation rate. There is weak ion permeation through interlayer spaces, up to the highest chloride concentration of 1 M, an indication of sterically limited transport. In these very few transported ions, we observe hydration energy-dependent selectivity between salt ions. These strongly suggest a soft ball model of steric exclusion, which is rarely reported. These findings improve our understanding of molecular and ionic transport at the atomic scale.

Carbon nano-onion-powered optically transparent and economical dye-sensitized solar cells.

The integration of dye-sensitized solar cells (DSSCs) with building roof panels, windows, and various decorative outdoor installations is presently an important research topic for their immediate commercialization potential because of their power generation capability, sustainability, and aesthetic appearance. For industrial applications, Pt counter electrodes (CEs) need to be replaced with Pt-free CEs because of their limited sources and cost. An ideal CE should be economical, abundant, and have excellent electrochemical stability and activity, with easy processing in bulk. As an alternative practical CE, we introduce for the first time a carbon nano-onion (CNO)-based CE for transparent DSSCs. We developed a simple, energy-efficient one-step synthesis technique for fabricating high-quality CNOs in bulk quantities without any sophisticated instrumentation and expensive nanodiamond precursors. CNO CEs proved to be promising in terms of optical transparency, reasonable electrochemical redox activity for the I-/I3- redox couple, and exchange current density comparable to Pt CEs. CNO-powered DSSCs demonstrated an optical transparency of >55% with a significant solar energy conversion efficiency of 5.17%. The intrinsic hydrophilicity of the as-synthesized CNO eliminates the use of a binder or an additive, unlike in the case of other carbon allotropes. Our results demonstrate the possibility of using CNO-based CEs as promising substitutes for scarce and expensive Pt-based CEs for low-cost DSSCs.

Liquid phase high shear exfoliated graphene nanoplatelets as counter electrode material for dye-sensitized solar cells.

Graphene nanoplatelets (GNPs) are prepared from natural graphite by a simple and low-cost liquid phase high shear exfoliation method. The as-prepared GNPs are used as a counter electrode (CE) material for dye-sensitized solar cells (DSSCs). To confirm the Exfoliated GNPs, structural and morphological studies are carried out using X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and selected area electron diffraction (SAED) studies. The electrochemical behaviour of GNPs as a counter electrode material is evaluated and compared with standard Platinum (Pt) electrode using cyclic-voltammetry (CV) and electrochemical impedance spectroscopy (EIS). These studies indicated that electrocatalytic activity towards I-/I3- redox mediator exhibited by the GNPs based electrode is comparable to standard Pt counter electrodes. DSSCs are fabricated using the counter electrodes made of GNPs and the photo-conversion efficiency is found to be 6.23% under standard test conditions, which is comparable to Pt based DSSCs proving them as potential alternative materials for counter electrodes.

A simple route to making counter electrode for dye sensitized solar cells (DSSCs) using sucrose as carbon precursor.

Dye sensitized solar cells (DSSCs) have attracted much attention in recent years due to low cost fabrication as compared to silicon-based and thin film solar cells. Though, platinum is an excellent catalytic material for use in preparation of counter electrodes (CEs) for DSSCs it is expensive. Alternatives to replacement of platinum (Pt) that have been examined are carbon materials, conductive polymers and hybrids. In this work, counter electrode for DSSCs was fabricated using carbon material obtained from graphitization of sucrose at high temperature. A slurry of the carbon produced from sucrose graphitization was made with polyvinylpyrrolidone (PVP) as a surfactant and a coating was obtained by doctor blading the slurry over the FTO glass substrate. The current density (Jsc) and open circuit voltage (V(OC)) of fabricated cell (area 0.25 cm(2)) was 10.28 mAc m(-2) and 0.76 V respectively. The efficiency of the cell was 4.33% which was just slightly lower than that obtained for similar cells using platinum based counter electrode.