Pulsed laser deposition of porous N-carbon supported cobalt (oxide) thin films for highly efficient oxygen evolution

Identification of efficient non-precious metal catalysts for the oxygen evolution reaction (OER) remains a great challenge. Here we report robust cobalt (oxide) nanoparticles deposited on a porous nitrogendoped carbon (N-carbon) film prepared by pulsed laser deposition under a reactive background gas, which exhibit highly efficient OER performance with a low overpotential and high stability

Highly active nickel–cobalt/nanocarbon thin films as efficient water splitting electrodes

Developing low cost, highly active and stable electrocatalysts for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) using the same electrolyte has remained a major challenge. Herein, we report a novel and robust material comprised of Nickel-Cobalt nanoparticles coated on a porous nitrogen-doped carbon (NC) thin film synthesized via a two-step pulsed laser deposition technique. The optimized sample (Ni0.5Co0.5/NC) achieved lowest overpotentials of 176 mV and 300 mV at a current density of 10 mAcm-2 for HER and OER, respectively. The optimized OER activity might be attributed to the available metal oxide nanoparticles with effective electronic structure configuration and enhanced mass/charge transport capability. At the same time, the porous nitrogen doped carbon incorporated with cobalt and nickel species can serve as an excellent HER catalyst. As a result, the newly developed electrocatalysts manifest high current densities and strong electrochemical stability in overall water splitting, outperforming most of the previously reported non-precious metal-based catalysts

Effects of inorganic seed promoters on MoS2 few-layers grown via chemical vapor deposition

In the last years, transition metal dichalcogenides (TMDs), especially at the two-dimensional (2D) limit, gained a large interest due to their unique optical and electronic properties. Among them, MoS2 received great attention from the scientific community due to its versatility, workability, and applicability in a large number of fields such as electronics, optoelectronics and electrocatalysis. To open the possibility of 2D-MoS2 exploitation, its synthesis over large macroscopic areas using cost-effective methods is fundamental. In this study, we report a method for the synthesis of large-area (~ cm2) few-layers MoS2 via liquid precursor CVD (L-CVD), where the Mo precursor (i.e. ammonium heptamolybdate AHM) is provided via a solution that is spin-coated over the substrate. Given the capability of organic and inorganic molecules, such as alkaline salts, to enhance MoS2 growth, we investigated the action of different inorganic salts as seed promoters. In particular, by using visible Raman spectroscopy, we focused on the effect of Na(OH), KCl, KI, and Li(OH) on the thickness, morphology, uniformity and degree of coverage of the grown MoS2. We optimized the process tuning parameters such as the volume of spin-coated solution, the growth temperature, and the seed promoter concentration, to synthesise the lowest possible thickness which resulted to be 2 layers (2L) of the highest quality. We witnessed that the addition of an inorganic seed promoter in the solution improves the extension of the grown MoS2 promoting lateral growth front, and therefore the degree of coverage. From this study, we conclude that, amongst the investigated seed promoters, K-based salts proved to grant the growth of high-quality two-layer MoS2 with optimal and uniform coverage of the SiO2/Si substrate surface

Raman spectroscopy as a tool to investigate the structure and electronic properties of carbon-atom wires

Graphene, nanotubes and other carbon nanostructures have shown potential as candidates for advanced technological applications due to the different coordination of carbon atoms and to the possibility of π-conjugation. In this context, atomic-scale wires comprised of sp-hybridized carbon atoms represent ideal 1D systems to potentially downscale devices to the atomic level. Carbon-atom wires (CAWs) can be arranged in two possible structures: a sequence of double bonds (cumulenes), resulting in a 1D metal, or an alternating sequence of single–triple bonds (polyynes), expected to show semiconducting properties. The electronic and optical properties of CAWs can be finely tuned by controlling the wire length (i.e., the number of carbon atoms) and the type of termination (e.g., atom, molecular group or nanostructure). Although linear, sp-hybridized carbon systems are still considered elusive and unstable materials, a number of nanostructures consisting of sp-carbon wires have been produced and characterized to date. In this short review, we present the main CAW synthesis techniques and stabilization strategies and we discuss the current status of the understanding of their structural, electronic and vibrational properties with particular attention to how these properties are related to one another. We focus on the use of vibrational spectroscopy to provide information on the structural and electronic properties of the system (e.g., determination of wire length). Moreover, by employing Raman spectroscopy and surface enhanced Raman scattering in combination with the support of first principles calculations, we show that a detailed understanding of the charge transfer between CAWs and metal nanoparticles may open the possibility to tune the electronic structure from alternating to equalized bonds

Elastic properties of graphene suspended on a polymer substrate by e-beam exposure

A method for fabricating multiple free-standing structures on the same sheet of graphene is demonstrated. Mechanically exfoliated mono- and bilayer graphene sheets were sandwiched between two layers of polymethyl-methacrylate. Suspended areas were defined by e-beam exposure allowing precise control over their shape and position. Mechanical characterization of suspended graphene sheets was performed by nanoindentation with an atomic force microscopy tip. The obtained built-in tensions of 12 nN are significantly lower than those in suspended graphene exfoliated on an SiO2 substrate, and therefore permit access to the intrinsic properties of this material system

Solvent-dependent termination, size and stability in polyynes synthesis by laser ablation in liquids

In recent years there has been a growing interest in sp-carbon chains as possible novel nanostructures. An example of sp-carbon chains are the so-called polyynes, characterized by the alternation of single and triple bonds that can be synthesized by pulsed laser ablation in liquid (PLAL) of a graphite target. In this work, by exploiting different solvents in the PLAL process, e.g. water, acetonitrile, methanol, ethanol, and isopropanol, we systematically investigate the solvent role in polyyne formation and stability. The presence of methyland cyano-groups in the solutions influences the termination of polyynes, allowing to detect, in addition to hydrogen-capped polyynes up to HC22H, methyl-capped polyynes up to 18 carbon atoms (i.e. HCnCH3) and cyanopolyynes up to HC12CN. The assignment of each species was done by UV-Vis spectroscopy and supported by density functional theory simulations of vibronic spectra. In addition, surface-enhanced Raman spectroscopy allowed to observe differences, due to different terminations (hydrogen, methyl-and cyano group), in the shape and positions of the characteristic Raman bands of the size-selected polyynes. The evolution in time of each polyyne has been investigated evaluating the chromatographic peak area, and the effect of size, terminations and solvents on polyynes stability has been individuated.Comment: 13 pages, 5 figures. Supporting Information of this article is available in the end of this manuscrip

Semiconductor-to-Metal Transition in Carbon-Atom Wires Driven by sp2 Conjugated End Groups

Bis(biphenyl)-capped polyynes are investigated to unveil the modulation of the electronic and optical properties of sp-hybridized carbon-atom wires (CAWs) capped with π-conjugated sp2 end groups. Raman and surface enhanced Raman spectroscopy experiments and density functional theory (DFT) calculations reveal structural changes from polyyne-like with alternating single–triple bonds toward cumulene-like with more equalized bonds as a consequence of the charge transfer occurring when wires interact with metallic nanoparticles. While polyynes have semiconducting electronic properties, a more equalized system tends to a cumulene-like structure characterized by a nearly metallic behavior. The effect of different sp2 end groups in driving a semiconductor-to-metal transition is investigated by DFT calculations on a series of CAWs capped with different terminations. We discuss how the modulation of the structural, electronic, and vibrational properties of the sp-carbon chain toward the metallic wire is not trivial and requires a suitable chemical design of the end group and control of charge transfer. These results provide a guideline for the design of novel sp–sp2 hybrid carbon nanosystems with tunable properties, where graphene-like and polyyne-like domains are closely interconnected. The capability to tune the final electronic or optical response of the material makes these hybrid sp–sp2 systems appealing for a future all-carbon-based science and technology

Tuning electrical properties of hierarchically assembled Al-doped ZnO nanoforests by room temperature Pulsed Laser Deposition

Large surface area, 3D structured transparent electrodes with effective light management capability may represent a key component in the development of new generation optoelectronic and energy harvesting devices. We present an approach to obtain forest-like nanoporous/hierarchical Al-doped ZnO conducting layers with tunable transparency and light scattering properties, by means of room temperature Pulsed Laser Deposition in a mixed Ar:O2 atmosphere. The composition of the background atmosphere during deposition can be varied to modify stoichiometry-related defects, and therefore achieve control of electrical and optical properties, while the total background pressure controls the material morphology at the nano- and mesoscale and thus the light scattering properties. This approach allows to tune electrical resistivity over a very wide range (10^-1 - 10^6 Ohm*cm), both in the in-plane and cross-plane directions. Optical transparency and haze can also be tuned by varying the stoichiometry and thickness of the nano-forests