Aron Dombovari

Synthesis of tungsten carbide and tungsten disulfide on vertically aligned multi-walled carbon nanotube forests and their application as non-Pt electrocatalysts for the hydrogen evolution reaction

Direct growth of tungsten carbide and sulfide on vertically aligned carbon nanotubes (CNTs) is demonstrated for a facile construction of three-dimensional electrocatalyst nanostructures. A thin layer of tungsten was first deposited on free-standing CNT forests followed by annealing in an inert atmosphere or in the vapor of sulfur for obtaining tungsten carbide or disulfide, respectively. Besides structural characterization of the composites by means of scanning and transmission electron microscopy, Raman spectroscopy and X-ray diffraction, the produced CNT–catalyst composites are also evaluated in the hydrogen evolution reaction and compared to Pt as well as to the original and W coated CNT forests. The results indicate that the template growth of catalyst nanostructures on vertically aligned CNT forests is a suitable approach for catalytic electrode fabrication, which for example may find use in the hydrogen evolution reaction.

Self-assembled large scale metal alloy grid patterns as flexible transparent conductive layers

The development of scalable synthesis techniques for optically transparent, electrically conductive coatings is in great demand due to the constantly increasing market price and limited resources of indium for indium tin oxide (ITO) materials currently applied in most of the optoelectronic devices. This work pioneers the scalable synthesis of transparent conductive films (TCFs) by exploiting the coffee-ring effect deposition coupled with reactive inkjet printing and subsequent chemical copper plating. Here we report two different promising alternatives to replace ITO, palladium-copper (PdCu) grid patterns and silver-copper (AgCu) fish scale like structures printed on flexible poly(ethylene terephthalate) (PET) substrates, achieving sheet resistance values as low as 8.1 and 4.9 Ω/sq, with corresponding optical transmittance of 79% and 65% at 500 nm, respectively. Both films show excellent adhesion and also preserve their structural integrity and good contact with the substrate for severe bending showing less than 4% decrease of conductivity even after 105 cycles. Transparent conductive films for capacitive touch screens and pixels of microscopic resistive electrodes are demonstrated.

Industrially benign super-compressible piezoresistive carbon foams with predefined wetting properties: from environmental to electrical applications

In the present work electrically conductive, flexible, lightweight carbon sponge materials derived from open-pore structure melamine foams are studied and explored. Hydrophobic and hydrophilic surface properties - depending on the chosen treatment conditions - allow the separation and storage of liquid chemical compounds. Activation of the carbonaceous structures substantially increases the specific surface area from ~4 m2g−1 to ~345 m2g−1, while retaining the original three-dimensional, open-pore structure suitable for hosting, for example, Ni catalyst nanoparticles. In turn the structure is rendered suitable for hydrogenating acetone to 2-propanol and methyl isobutyl ketone as well for growing hierarchical carbon nanotube structures used as electric double-layer capacitor electrodes with specific capacitance of ~40 F/g. Mechanical stress-strain analysis indicates the materials are super-compressible (>70% volume reduction) and viscoelastic with excellent damping behavior (loss of 0.69 ± 0.07), while piezoresistive measurements show very high gauge factors (from ~20 to 50) over a large range of deformations. The cost-effective, robust and scalable synthesis - in conjunction with their fascinating multifunctional utility - makes the demonstrated carbon foams remarkable competitors with other three-dimensional carbon materials typically based on pyrolyzed biopolymers or on covalently bonded graphene and carbon nanotube frameworks.

A novel WS2 nanowire-nanoflake hybrid material synthesized from WO3 nanowires in sulfur vapor

In this work, WS2 nanowire-nanoflake hybrids are synthesized by the sulfurization of hydrothermally grown WO3 nanowires. The influence of temperature on the formation of products is optimized to grow WS2 nanowires covered with nanoflakes. Current-voltage and resistance-temperature measurements carried out on random networks of the nanostructures show nonlinear characteristics and negative temperature coefficient of resistance indicating that the hybrids are of semiconducting nature. Bottom gated field effect transistor structures based on random networks of the hybrids show only minor modulation of the channel conductance upon applied gate voltage, which indicates poor electrical transport between the nanowires in the random films. On the other hand, the photo response of channel current holds promise for cost-efficient solution process fabrication of photodetector devices working in the visible spectral range.

On-chip integrated vertically aligned carbon nanotube based super- and pseudocapacitors

On-chip energy storage and management will have transformative impacts in developing advanced electronic platforms with built-in energy needs for operation of integrated circuits driving a microprocessor. Though success in growing stand-alone energy storage elements such as electrochemical capacitors (super and pseusocapacitors) on a variety of substrates is a promising step towards this direction. In this work, on-chip energy storage is demonstrated using architectures of highly aligned vertical carbon nanotubes (CNTs) acting as supercapacitors, capable of providing large device capacitances. The efficiency of these structures is further increased by incorporating electrochemically active nanoparticles such as MnOx to form pseudocapacitive architectures thus enhancing device capacitance areal specific capacitance of 37 mF/cm2. The demonstrated on-chip integration is up and down-scalable, compatible with standard CMOS processes, and offers lightweight energy storage what is vital for portable and autonomous device operation with numerous advantages as compared to electronics built from discrete components.

Ultrasensitive H2S gas sensors based on p-type WS2 hybrid materials

Owing to their higher intrinsic electrical conductivity and chemical stability with respect to their oxide counterparts, nanostructured metal sulfides are expected to revive materials for resistive chemical sensor applications. Herein, we explore the gas sensing behavior of WS2 nanowire-nanoflake hybrid materials and demonstrate their excellent sensitivity (0.043 ppm-1) as well as high selectivity towards H2S relative to CO, NH3, H2, and NO (with corresponding sensitivities of 0.002, 0.0074, 0.0002, and 0.0046 ppm-1, respectively). Gas response measurements, complemented with the results of X-ray photoelectron spectroscopy analysis and first-principles calculations based on density functional theory, suggest that the intrinsic electronic properties of pristine WS2 alone are not sufficient to explain the observed high sensitivity towards H2S. A major role in this behavior is also played by O doping in the S sites of the WS2 lattice. The results of the present study open up new avenues for the use of transition metal disulfide nanomaterials as effective alternatives to metal oxides in future applications for industrial process control, security, and health and environmental safety.

Size-Dependent H2 Sensing Over Supported Pt Nanoparticles

Catalyst size affects the overall kinetics and mechanism of almost all heterogeneous chemical reactions. Since the functional sensing materials in resistive chemical sensors are practically the very same nanomaterials as the catalysts in heterogeneous chemistry, a plausible question arises: Is there any effect of the catalyst size on the sensor properties? Our study attempts to give an insight into the problem by analyzing the response and sensitivity of resistive H₂ sensors based on WO₃ nanowire supported Pt nanoparticles having size of 1.5±0.4 nm, 6.2±0.8 nm, 3.7±0.5 nm and 8.3±1.3 nm. The results show that Pt nanoparticles of larger size are more active in H₂ sensing than their smaller counterparts and indicate that the detection mechanism is more complex than just considering the number of surface atoms of the catalyst.

High photoresponse of individual WS2 nanowire-nanoflake hybrid materials

van der Waals solids have been recognized as highly photosensitive materials that compete conventional Si and compound semiconductor based devices. While 2-dimensional nanosheets of single and mult ...