Géza Tóth
University of Oulu
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Publication
Featured researches published by Géza Tóth.
ACS Nano | 2011
Ming-Chung Wu; Jussi Hiltunen; András Sápi; Anna Avila; William Larsson; Hsueh-Chung Liao; Mika Huuhtanen; Géza Tóth; Andrey Shchukarev; Noémi Laufer; Ákos Kukovecz; Zoltán Kónya; Jyri-Pekka Mikkola; Riitta L. Keiski; Wei-Fang Su; Yang-Fang Chen; Heli Jantunen; Pulickel M. Ajayan; Robert Vajtai; Krisztian Kordas
We report the synthesis of N-doped TiO(2) nanofibers and high photocatalytic efficiency in generating hydrogen from ethanol-water mixtures under UV-A and UV-B irradiation. Titanate nanofibers synthesized by hydrothermal method are annealed in air and/or ammonia to achieve N-doped anatase fibers. Depending on the synthesis route, either interstitial N atoms or new N-Ti bonds appear in the lattice, resulting in slight lattice expansion as shown by XPS and HR-TEM analysis, respectively. These nanofibers were then used as support for Pd and Pt nanoparticles deposited with wet impregnation followed by calcination and reduction. In the hydrogen generation tests, the N-doped samples were clearly outperforming their undoped counterparts, showing remarkable efficiency not only under UV-B but also with UV-A illumination. When 100 mg of catalyst (N-doped TiO(2) nanofiber decorated with Pt nanoparticles) was applied to 1 L of water-ethanol mixture, the H(2) evolution rates were as high as 700 μmol/h (UV-A) and 2250 μmol/h (UV-B) corresponding to photo energy conversion percentages of ∼3.6 and ∼12.3%, respectively.
ACS Nano | 2010
Niina Halonen; Aatto Rautio; Anne Riikka Leino; Teemu Kyllönen; Géza Tóth; Jyrki Lappalainen; Krisztian Kordas; Mika Huuhtanen; Riitta L. Keiski; András Sápi; Mária Szabó; Ákos Kukovecz; Zoltán Kónya; Imre Kiricsi; Pulickel M. Ajayan; Robert Vajtai
Three-dimensional carbon nanotube scaffolds created using micromachined Si/SiO2 templates are used as nanoparticulate filters and support membranes for gas-phase heterogeneous catalysis. The filtering efficiency of better than 99% is shown for the scaffolds in filtering submicrometer particles from air. In the hydrogenation of propene to propane reaction low activation energy of E(a) approximately 27.8 +/- 0.6 kJ x mol(-1), a considerably high turnover rate of approximately 1.1 molecules x Pd site(-1) x s(-1) and durable activity for the reaction are observed with Pd decorated membranes. It is demonstrated that appropriate engineering of macroscopic-ordered nanotube architectures can lead to multifunctional applications.
ACS Nano | 2010
Eduardo Gracia-Espino; Giovanni Sala; Flavio Pino; Niina Halonen; Juho Luomahaara; Jani Mäklin; Géza Tóth; Krisztian Kordas; Heli Jantunen; Mauricio Terrones; Panu Helistö; Heikki Seppä; Pulickel M. Ajayan; Robert Vajtai
The electrical properties of random networks of single-wall carbon nanotubes (SWNTs) obtained by inkjet printing are studied. Water-based stable inks of functionalized SWNTs (carboxylic acid, amide, poly(ethylene glycol), and polyaminobenzene sulfonic acid) were prepared and applied to inkjet deposit microscopic patterns of nanotube films on lithographically defined silicon chips with a back-side gate arrangement. Source-drain transfer characteristics and gate-effect measurements confirm the important role of the chemical functional groups in the electrical behavior of carbon nanotube networks. Considerable nonlinear transport in conjunction with a high channel current on/off ratio of approximately 70 was observed with poly(ethylene glycol)-functionalized nanotubes. The positive temperature coefficient of channel resistance shows the nonmetallic behavior of the inkjet-printed films. Other inkjet-printed field-effect transistors using carboxyl-functionalized nanotubes as source, drain, and gate electrodes, poly(ethylene glycol)-functionalized nanotubes as the channel, and poly(ethylene glycol) as the gate dielectric were also tested and characterized.
Journal of Materials Chemistry | 2015
Jhih Fong Lin; Olli Pitkänen; Jani Mäklin; R. Puskás; Ákos Kukovecz; Aron Dombovari; Géza Tóth; Krisztian Kordas
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.
Scientific Reports | 2015
Tung N Ngoc Pham; Ajaikumar Samikannu; Jarmo Kukkola; Anne-Riikka Rautio; Olli Pitkänen; Aron Dombovari; Gabriela S. Lorite; Teemu Sipola; Géza Tóth; Melinda Mohl; Jyri-Pekka Mikkola; Krisztian Kordas
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.
Nanotechnology | 2006
Krisztian Kordas; Géza Tóth; J. Levoska; Mika Huuhtanen; Riitta L. Keiski; Matti Härkönen; Thomas F. George; Jouko Vähäkangas
Palladium nanoparticles are deposited in the pores of freestanding, mesoporous anodic aluminium oxide (AAO) templates via a chemical route at room temperature. In the precursor, Pd2+ ions are complexed with ammonia and reduced to Pd0 by adding an excess amount of formaldehyde. To enhance the infiltration of pores with the precursor, 2-propanol is added to the solution. After a few hours of plating at room temperature, crystalline Pd0 nanoparticles with an average size of ~4 nm form on the pore walls of the AAO templates. The effect of heat treatment on the structure of the AAO/Pd system is also studied. The specimens are investigated using FESEM, EFTEM/EDX, and electron and x-ray diffraction techniques. Preliminary catalyst activity analyses reveal good selectivity of ~90%, but low conversion of ~1.2% of carbon monoxide to methane in hydrogen flow at 330 °C on AAO/Pd. The oxidized form of the catalyst shows instantaneous oxidation with ~100% selectivity and conversion of carbon monoxide to carbon dioxide in oxygen flow at 140 °C.
Journal of Nanoparticle Research | 2014
Ming-Chung Wu; Hsueh Chung Liao; Yu Cheng Cho; Che Pu Hsu; Ting Han Lin; Wei-Fang Su; András Sápi; Ákos Kukovecz; Zoltán Kónya; Andrey Shchukarev; Anjana Sarkar; William Larsson; Jyri-Pekka Mikkola; Melinda Mohl; Géza Tóth; Heli Jantunen; Anna Valtanen; Mika Huuhtanen; Riitta L. Keiski; Krisztian Kordas
In this study, a set of nitrogen-doped TiO2-based nanomaterials demonstrating photocatalytic activity was developed by combining the efforts of lattice doping and metal nanoparticle decoration and tested for photo-degradation of methylene blue dye by applying solar simulator irradiation. The surface potential shifts of these TiO2-based photocatalytic nanomaterials measured by Kelvin probe force microscope have been used to study the degree of electron generation of the photocatalysts after irradiation and were well correlated with the photocatalytic activity. The nitrogen-doped TiO2 nanowires decorated with Pt nanoparticles can induce obvious electron accumulation and result in a large shift of surface potential. The analysis shows a clear correlation between the surface potential shift and the photodegradation activity. Furthermore, a thorough comparative photocatalytic activity study combined with X-ray photoelectron spectroscopy analysis of the materials—doped with nitrogen under various conditions—reveals that the photocatalytic efficiency of the catalysts is maintained even if the lattice doping is leached e.g., by thermal treatments after doping.Graphical AbstractBy monitoring the surface potential shifts of various TiO2-based photocatalysts by photo-assisted Kelvin probe force microscopy, we obtain a useful tool for developing novel materials with high photocatalytic activity.
Nanotechnology | 2012
Li Song; Géza Tóth; Jinquan Wei; Zheng Liu; Wei Gao; Lijie Ci; Robert Vajtai; Morinobu Endo; Pulickel M. Ajayan
We report on the current-carrying capability and the high-current-induced thermal burnout failure modes of 5-20 μm diameter double-walled carbon nanotube (DWNT) fibers made by an improved dry-spinning method. It is found that the electrical conductivity and maximum current-carrying capability for these DWNT fibers can reach up to 5.9 × 10(5) S m(-1) and over 1 × 10(5) A cm(-2) in air. In comparison, we observed that standard carbon fiber tended to be oxidized and burnt out into cheese-like morphology when the maximum current was reached, while DWNT fiber showed a much slower breakdown behavior due to the gradual burnout in individual nanotubes. The electron microscopy observations further confirmed that the failure process of DWNT fibers occurs at localized positions, and while the individual nanotubes burn they also get aligned due to local high temperature and electrostatic field. In addition a finite element model was constructed to gain better understanding of the failure behavior of DWNT fibers.
Scientific Reports | 2016
Georgies Alene Asres; Aron Dombovari; Teemu Sipola; R. Puskás; Ákos Kukovecz; Zoltán Kónya; Alexey P. Popov; Jhih Fong Lin; Gabriela S. Lorite; Melinda Mohl; Géza Tóth; Anita Lloyd Spetz; Krisztian Kordas
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.
Nanotechnology | 2010
Jarmo Kukkola; Aatto Rautio; Giovanni Sala; Flavio Pino; Géza Tóth; Anne-Riikka Leino; Jani Mäklin; Heli Jantunen; Antti Uusimäki; Krisztian Kordas; Eduardo Gracia; Mauricio Terrones; Andrey Shchukarev; Jyri-Pekka Mikkola
Aluminum foils were anodized in sulfuric acid solution to form thick porous anodic aluminum oxide (AAO) films of thickness approximately 6 microm. Electrodes of carboxyl-functionalized single-wall carbon nanotube (SWCNT) thin films were inkjet printed on the anodic oxide layer and the electrical characteristics of the as-obtained SWCNT-AAO-Al structures were studied. Nonlinear current-voltage transport and strong temperature dependence of conduction through the structure was measured. The microstructure and chemical composition of the anodic oxide layer was analyzed using transmission and scanning electron microscopy as well as x-ray photoelectron spectroscopy. Schottky emission at the SWCNT-AAO and AAO-Al interfaces allowed by impurity states in the anodic aluminum oxide film together with ionic surface conduction on the pore walls of AAO gives a reasonable explanation for the measured electrical conduction. Calcined AAO is proposed as a dielectric material for SWCNT-field effect transistors.