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  • 1.
    Abou-Hamad, Edy
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Goze-Bac, Christophe
    Université Montpellier II, France.
    Nitze, Florian
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Schmid, Michael
    Physikalisches Institut, Universität Stuttgart, Germany.
    Aznar, Robert
    Université Montpellier II, France.
    Mehring, Michael
    Physikalisches Institut, Universität Stuttgart, Germany.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Electronic properties of Cs-intercalated single-walled carbon nanotubes derived from nuclear magnetic resonance2011In: New Journal of Physics, ISSN 1367-2630, E-ISSN 1367-2630, Vol. 13, p. 053045 (1)-(9)Article in journal (Refereed)
    Abstract [en]

    We report on the electronic properties of Cs-intercalated singlewalled carbon nanotubes (SWNTs). A detailed analysis of the 13C and133Cs nuclear magnetic resonance (NMR) spectra reveals an increased metallization of the pristine SWNTs under Cs intercalation. The ‘metallization’ of CsxC materials where x =0–0.144 is evidenced from the increased local electronic density of states (DOS) n(EF)at the Fermi level of the SWNTs as determined from spin–lattice relaxation measurements. In particular, there are two distinct electronic phases called α and β and the transition between these occurs around x = 0.05. The electronic DOS at the Fermi level increases monotonically at low intercalation levels x <0.05 (α-phase), whereas it reaches a plateau in the range 0.05 < x < 0.143 at high intercalation levels (β-phase). The new β-phase is accompanied by a hybridization of Cs(6s) orbitals with C(sp2)orbitals of the SWNTs. In both phases, two types of metallic nanotubes are found with a low and a high local n(EF), corresponding to different local electronic band structures of the SWNTs.

  • 2.
    Barzegar, Hamid R.
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nitze, Florian
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sharifi, Tiva
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ramstedt, Madeleine
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Tai, Cheuk W.
    Malolepszy, Artur
    Stobinski, Leszek
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Simple Dip-Coating Process for the Synthesis of Small Diameter Single-Walled Carbon Nanotubes-Effect of Catalyst Composition and Catalyst Particle Size on Chirality and Diameter2012In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 116, no 22, p. 12232-12239Article in journal (Refereed)
    Abstract [en]

    We report on a dip-coating method to prepare catalyst particles (mixture of iron and cobalt) with a controlled diameter distribution on silicon wafer substrates by changing the solution's concentration and withdrawal velocity. The size and distribution of the prepared catalyst particles were analyzed by atomic force microscopy. Carbon nanotubes were grown by chemical vapor deposition on the substrates with the prepared catalyst particles. By decreasing the catalyst particle size to below 10 nm, the growth of carbon nanotubes can be tuned from few-walled carbon nanotubes, with homogeneous diameter, to highly pure single-walled carbon nanotubes. Analysis of the Raman radial breathing modes, using three different Raman excitation wavelengths (488, 633, and 785 nm), showed a relatively broad diameter distribution (0.8-1.4 nm) of single-walled carbon nanotubes with different chiralities. However, by changing the composition of the catalyst particles while maintaining the growth parameters, the chiralities of single-walled carbon nanotubes were reduced to mainly four different types, (12, 1), (12, 0), (8, 5), and (7, 5), accounting for about 70% of all nanotubes.

  • 3.
    Barzegar, Hamid Reza
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Sharifi, Tiva
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nitze, Florian
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nitrogen Doping Mechanism in Small Diameter Single-Walled Carbon Nanotubes: Impact on Electronic Properties and Growth Selectivity2013In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 117, no 48, p. 25805-25816Article in journal (Refereed)
    Abstract [en]

    Nitrogen doping in carbon nanostructures has attracted interest for more than a decade, and recent implementation of such structures in energy conversion systems has boosted the interest even more. Despite numerous studies, the structural conformation and stability of nitrogen functionalities in small diameter single-walled carbon nanotubes (SWNTs), and the impact of these functionalities on the electronic and mechanical properties of the SWNTs, are incomplete. Here we report a detailed study on nitrogen doping in SWNTs with diameters in the range of 0.8?1.0 nm, with well-defined chirality. We show that the introduction of nitrogen in the carbon framework significantly alters the stability of certain tubes, opening for the possibility to selectively grow nitrogen-doped SWNTs with certain chirality and diameter. At low nitrogen concentration, pyridinic functionalities are readily incorporated and the tubular structure is well pertained. At higher concentrations, pyrrolic functionalities are formed, which leads to significant structural deformation of the nanotubes and hence a stop in growth of crystalline SWNTs. Raman spectroscopy is an important tool to understand guest atom doping and electronic charge transfer in SWNTs. By correlating the influence of defined nitrogen functionalities on the electronic properties of SWNTs with different chirality, we make precise interpretation of experimental Raman data. We show that the previous interpretation of the double-resonance G?-peak in many aspects is wrong and instead can be well-correlated to the type of nitrogen doping of SWNTs originating from the p- or n-doping nature of the nitrogen incorporation. Our results are supported by experimental and theoretical data.

  • 4.
    Barzegar, Hamid Reza
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nitze, Florian
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Malolepszy, Artur
    Stobinski, Leszek
    Tai, Cheuk-Wai
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Water assisted growth of C-60 rods and tubes by liquid-liquid interfacial precipitation method2012In: Molecules, ISSN 1420-3049, E-ISSN 1420-3049, Vol. 17, no 6, p. 6840-6853Article in journal (Refereed)
    Abstract [en]

    C-60 nanorods with hexagonal cross sections are grown using a static liquid-liquid interfacial precipitation method in a system of C-60/m-dichlorobenzene solution and ethanol. Adding water to the ethanol phase leads instead to C-60 tubes where both length and diameter of the C-60 tubes can be controlled by the water content in the ethanol. Based on our observations we find that the diameter of the rods/tubes strongly depends on the nucleation step. We propose a liquid-liquid interface growth model of C-60 rods and tubes based on the diffusion rate of the good C-60 containing solvent into the poor solvent as well as on the size of the crystal seeds formed at the interface between the two solvents. The grown rods and tubes exhibit a hexagonal solvate crystal structure with m-dichlorobenzene solvent molecules incorporated into the crystal structure, independent of the water content. An annealing step at 200 degrees C at a pressure <1 kPa transforms the grown structures into a solvent-free face centered cubic structure. Both the hexagonal and the face centered cubic structures are very stable and neither morphology nor structure shows any signs of degradation after three months of storage.

  • 5.
    Hu, Guangzhi
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nitze, Florian
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Barzegar, Hamid Reza
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sharifi, Tiva
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Mikolajczuk, Ania
    Polish Acad Sci, Inst Phys Chem, PL-01224 Warsaw, Poland.
    Tai, Cheuk-Wai
    Stockholm Univ, Arrhenius Lab, Dept Mat & Environm Chem, S-10691 Stockholm, Sweden.
    Borodzinski, Andrzej
    Polish Acad Sci, Inst Phys Chem, PL-01224 Warsaw, Poland.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Palladium nanocrystals supported on helical carbon nanofibers for highly efficient electro-oxidation of formic acid, methanol and ethanol in alkaline electrolytes2012In: Journal of Power Sources, ISSN 0378-7753, E-ISSN 1873-2755, Vol. 209, p. 236-242Article in journal (Refereed)
    Abstract [en]

    We present the synthesis of palladium nanocrystals self-assembled on helical carbon nanofibers functionalized with benzyl mercaptan (Pd-S-HCNFs) and their electrocatalytic activity toward the oxidation of formic acid, methanol and ethanol. Helical carbon nanofibers (HCNFs) were first functionalized with benzyl mercaptan based on the pi-pi interactions between phenyl rings and the graphitic surface of HCNFs. Palladium nano crystals (PdNC) were fixed on the surface of functionalized HCNF by Pd-S bonds in a simple self-assembly method. The as-prepared materials were characterized by high resolution transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX), cyclic voltammetry (CV), and fuel cell tests. CV characterization of the as-prepared materials shows a very high electrocatalytic activity for oxidation of formic acid, ethanol and methanol in strong alkaline electrolyte. In comparison to commercial catalyst Vulcan XC-72 decorated with Pd nanoparticles, the proposed Pd-S-HCNFs nano composite material shows oxidation currents for formic acid, ethanol and methanol at the Pd-S-HCNF-modified electrode that are higher than that at the Pd/XC-72 modified electrode with a factor of 2.0, 1.5, and 2.3, respectively. In a formic acid fuel cell the Pd-S-HCNF modified electrode yields equal power density as commercial Pd/XC-72 catalyst. Our results show that Pd-decorated helical carbon nanofibers with diameters around 40-60 nm have very high potential as active material in fuel cells, electrocatalysts and sensors. (C) 2012 Elsevier B.V All rights reserved.

  • 6.
    Hu, Guangzhi
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nitze, Florian
    Chalmers University of Technology.
    Jia, Xueen
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sharifi, Tiva
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Barzegar, Hamid Reza
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Chemistry. Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Reduction free room temperature synthesis of a durable and efficient Pd/ordered mesoporous carbon composite electrocatalyst for alkaline direct alcohols fuel cell2014In: RSC Advances, ISSN 2046-2069, E-ISSN 2046-2069, Vol. 4, no 2, p. 676-682Article in journal (Refereed)
    Abstract [en]

    The development of easy and environmentally benign synthesis methods of efficient electrocatalysts for use in energy conversion applications motivates researchers all over the world. Here we report a novel and versatile method to synthesize well-dispersed palladium-functionalized ordered mesoporous carbons (Pd/OMCs) at room temperature without any reducing agent by one-pot mixing of tri(dibenzylideneacetone)palladium(0) (Pd2DBA3) and OMCs together in a common N,N-dimethylformamide (DMF) solution. The formation of Pd nanoparticles and their crystallization on the OMC is catalyzed by protons in the solution and can thus be controlled by the solution pH. The complete process and the as-prepared nanocomposite was characterized by UV-spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (HTEM), X-ray photoelectron spectrum (XPS), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). The electrocatalytic property of the as-decorated material was examined with cyclic voltammetry (CV). The Pd/OMC composite shows up to two times higher electrocatalytic ability with a significantly better durability towards ethanol and methanol oxidation in alkaline media compared to commercial high surface area conductive carbon black Vulcan XC-72 decorated with equivalent Pd nanoparticles. Our described method provides new insight for the development of highly efficient carbon based nanocatalysts by simple and environmentally sound methods.

  • 7.
    Hu, Guangzhi
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nitze, Florian
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sharifi, Tiva
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Barzegar, Hamid Reza
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Self-assembled palladium nanocrystals on helical carbon nanofibers as enhanced electrocatalysts for electro-oxidation of small molecules2012In: Journal of Materials Chemistry, ISSN 0959-9428, E-ISSN 1364-5501, Vol. 22, no 17, p. 8541-8548Article in journal (Refereed)
    Abstract [en]

    We present a novel approach to prepare helical carbon nanofibers homogeneously functionalized with single crystal palladium nanoparticles via a phase-transfer method. The materials were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDX), thermogravimetric analysis (TGA) and electrochemical measurements. We find that homogeneous and small single-crystal Pd nanoparticles can be easily functionalized with phenyl mercaptan, transferred into the toluene phase from the dimethyl sulfoxide (DMSO) phase and then non-covalently self-assembled onto the surface of helical carbon nanofibers with a very good dispersion and homogeneous diameters of 4.5 +/- 0.6 nm. The palladium-helical carbon nanofiber composite exhibits significantly higher electrochemical active area and electrocatalytic activity towards the electrooxidation of formic acid, ethanol and methanol than the commercial electrocatalyst Pd/Vulcan XC-72. Our results show that the prepared material can be potentially used as an advanced nano-electrocatalyst in a direct alkaline fuel cell system.

  • 8.
    Hu, Guangzhi
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sharifi, Tiva
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nitze, Florian
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Barzegar, Hamid Reza
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Tai, Cheuk-Wai
    Stockholm Univ, Dept Mat & Environm Chem, Stockholm, Sweden.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Phase-transfer synthesis of amorphous palladium nanoparticle-functionalized 3D helical carbon nanofibers and its highly catalytic performance towards hydrazine oxidation2012In: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 543, p. 96-100Article in journal (Refereed)
    Abstract [en]

    Amorphous palladium nanoparticles functionalized helical carbon nanofibers (ApPd-HCNFs) were synthesized using a phase-transfer method. Palladium nanoparticles (Pd-NP) were first prepared using n-dodecyl sulfide as reducing agent and stabilizing ligands in ethanol. The Pd-NPs were then modified with benzyl mercaptan and transferred into a toluene solution with HCNFs which were decorated with amorphous palladium. The materials were characterized with high-resolution transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy and cyclic voltammetry showing that amorphous palladium nanoparticles were uniformly anchored at the HCNFs surface and that the ApPd-HCNFs exhibit high electrocatalytic activity towards hydrazine oxidation. (C) 2012 Elsevier B.V. All rights reserved.

  • 9.
    Jia, Xueen
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hu, Guangzhi
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nitze, Florian
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Barzegar, Hamid Reza
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sharifi, Tiva
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Tai, Cheuk-Wai
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Synthesis of Palladium/Helical Carbon Nanofiber Hybrid Nanostructures and Their Application for Hydrogen Peroxide and Glucose Detection2013In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 5, no 22, p. 12017-12022Article in journal (Refereed)
    Abstract [en]

    We report on a novel sensing platform for H2O2 and glucose based on immobilization of palladium-helical carbon nanofiber (Pd-HCNF) hybrid nanostnictures and glucose oxidase (GOx) with Nafion on a glassy carbon electrode (GCE). HCNFs were synthesized by a chemical vapor deposition process on a C-60-supported Pd catalyst. Pd-HCNF nanocomposites were prepared by a one-step reduction free method in dimethylformamide (DMF). The prepared materials were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), scanning electron microscopy (SEM), and Raman spectroscopy. The Nafion/Pd-HCNF/GCE sensor exhibits excellent electrocatalytic sensitivity toward H2O2 (315 mA M-1 cm(-2)) as probed by cyclic voltammetry (CV) and chronoamperometry. We show that Pd-HCNF-modified electrodes significantly reduce the overpotential and enhance the electron transfer rate. A linear range from 5.0 mu M to 2.1 mM with a detection limit of 3.0 mu M (based on the S/N = 3) and good reproducibility were obtained. Furthermore, a sensing platform for glucose was prepared by immobilizing the Pd-HCNFs and glucose oxidase (GOx) with Nafion on a glassy carbon electrode. The resulting biosensor exhibits a good response to glucose with a wide linear range (0.06-6.0 mM) with a detection limit of 0.03 mM and a sensitivity of 13 mA M-1 cm(-2). We show that small size and homogeneous distribution of the Pd nanoparticles in combination with good conductivity and large surface area of the HCNFs lead to a H2O2 and glucose sensing platform that performs in the top range of the herein reported sensor platforms.

  • 10.
    Larsen, Christian
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Barzegar, Hamid Reza
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nitze, Florian
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    On the fabrication of crystalline C-60 nanorod transistors from solution2012In: Nanotechnology, ISSN 0957-4484, E-ISSN 1361-6528, Vol. 23, no 34, p. 344015-Article in journal (Refereed)
    Abstract [en]

    Flexible and high-aspect-ratio C-60 nanorods are synthesized using a liquid-liquid interfacial precipitation process. As-grown nanorods are shown to exhibit a hexagonal close-packed single-crystal structure, with m-dichlorobenzene solvent molecules incorporated into the crystalline structure in a C-60:m-dichlorobenzene ratio of 3.2. An annealing step at 200 degrees C transforms the nanorods into a solvent-free face-centred-cubic polycrystalline structure. The nanorods are deposited onto field-effect transistor structures using two solvent-based techniques: drop-casting and dip-coating. We find that dip-coating deposition results in a preferred alignment of non-bundled nanorods and a satisfying transistor performance. The latter is quantified by the attainment of an electron mobility of 0.08 cm(2) V-1 s(-1) and an on/off ratio of >10(4) for a single-crystal nanorod transistor, fabricated with a solution-based and low-temperature process that is compatible with flexible substrates.

  • 11.
    Nitze, Florian
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Synthesis and characterization of palladium based carbon nanostructure-composites and their clean-energy application2013Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Carbon nanostructures are a wide field with many applications. The use of carbon nanostructures as support in heterogeneous catalysis is a key development that led together with the use of nanoparticles to a significant cost reduction of catalysts. Catalysts designed in this way are widely applied in fuel cell technologies. For portable devices especially low temperature fuel cells are desirable with low hazards for the user. One technology which fulfills these requirements is the direct formic acid fuel cell (DFAFC). DFAFC have many promising characteristics, such as high electromotive force and easy fuel handling. However, they still suffer from too low power output and lifetime for commercialization.

    This thesis focusses on two main aspects: the synthesis of carbon nanostructures by chemical vapor deposition (CVD) and their application as catalyst support. The materials are investigated by many different techniques ranging from transmission electron microscopy (TEM) to fuel cell tests.

    Different carbon nanostructures could be synthesized by catalytic CVD on palladium (Pd) nanoparticles. Multi-walled carbon nanotubes (MWCNTs), carbon nanofibers (CNFs) and helical carbon nanofibers (HCNFs) were grown, selectively, dependent on temperature, using acetylene as carbon precursor. Especially HCNF raised further interest due to their unique structure. A growth model for HCNFs was developed based on an anisotropic extrusion model. The synthesis conditions for HCNFs were optimized until an almost 100 % purity with very high efficiency was obtained.

    The unique helical but fiber-like structure made the material very interesting as support for heterogeneous catalysis. Several catalysts based on Pd nanoparticle decorated HCNFs were developed. The synthesis methods ranged from standard methods like the polyol method to phase-transfer methods. The catalysts showed very promising results for the electro-oxidation of methanol, ethanol and formic acid. This makes them highly attractive for fuel cell applications. The catalysts were tested in DFAFC. The superiority of HCNF-based catalysts is attributed to the good attachment of nanoparticles to the defect-rich and easy to functionalize surface of HCNFs in combination with adequate film forming properties during electrode preparation.

  • 12.
    Nitze, Florian
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Abou-Hamad, Edy
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Carbon nanotubes and helical carbon nanofibers grown by chemical vapour deposition on C60 fullerene supported Pd nanoparticles2011In: Carbon, ISSN 0008-6223, E-ISSN 1873-3891, Vol. 49, no 4, p. 1101-1107Article in journal (Refereed)
    Abstract [en]

    Chemical vapour deposition (CVD) represents a cheap and versatile method to produce carbon nanostructures. Here we present how we by using a standard CVD setup together with Pd nano particles as a catalyst can produce helical fibers with very periodic pitch, helicity, and narrow diameter distribution. The C60 supported Pd catalyst particles are produced by a wet chemistry process and applied to silicon substrates. By raising the growth temperature from 550 °C to 800 °C we can tune the growth products from helical carbon fibers to straight hollow carbon fibers and finally to carbon nanotubes at the highest temperatures. In the intermediate temperature region of 650 °C a mixture of all three components appears. At 550 °C the efficiency of the process is optimized by the amount of water during the growth. Different from most previous studies we can detect most of the catalyst particles embedded in the grown structures. In all fibers the catalyst particles are situated exactly in the middle of the fibers suggesting a two-directional growth. From the shape of the catalyst particles and by adopting a simple model we conclude that the fibers coil due to blocked carbon diffusion pathways on or through the catalyst particles.

  • 13.
    Nitze, Florian
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Andersson, Britt
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ammonia assisted growth of multiwalled carbon nanotubes2009In: Physica status solidi. B, Basic research, ISSN 0370-1972, E-ISSN 1521-3951, Vol. 246, no 11-12, p. 2440-2443Article in journal (Refereed)
    Abstract [en]

    Bamboo structured nitrogen-containing carbon nanotubes are grown on e-beam deposited thin metal films (cobalt/iron) by chemical vapor deposition. The approach uses ammonia as supporting gas to form nano catalyst particles in the pretreatment phase. Ammonia is also used as nitrogen source. The effect of ammonia on the grown structures at different temperatures (720 and 810 °C) is investigated by SEM, TEM and Raman spectroscopy. We show that ammonia promotes growth of vertically aligned CNT carpets on thicker metal films (5 nm) than usually used. At the same time ammonia is inducing a high amount of defects but this effect changes significantly with temperature. At 720 °C defects are clearly ammonia induced while at 810 °C the defects seem to be induced by a different mechanism. Furthermore, the presence of ammonia in both pretreatment phase and growth phase reduces the outer diameter of the grown nanotubes.

  • 14.
    Nitze, Florian
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Andersson, Britt
    Umeå University, Faculty of Science and Technology, Department of Applied Physics and Electronics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ammonia Induced CNT Growth on Metal Films2009In: Symposium K: Nanotubes and Related Nanostructures, 2009Conference paper (Other academic)
  • 15.
    Nitze, Florian
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Barzegar, Hamid Reza
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wagberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Easy synthesis of Pd fullerene polymer structures from the molten state of tris(dibenzylideneacetone)dipalladium(0)2012In: Physica status solidi. B, Basic research, ISSN 0370-1972, E-ISSN 1521-3951, Vol. 249, no 12, p. 2588-2591Article in journal (Refereed)
    Abstract [en]

    Pd fullerene composites were first synthesized and studied in the early 90s by for example Nagashima et al. In this study we present a novel and rapid approach to synthesize Pd fullerides based on direct reaction of C60 with Pd2dba3. We show that the Pd fullerene polymer phase forms at temperatures around the melting point of Pd2dba3 (150 degrees C) and that it proceeds upon further annealing while releasing dba. The synthesis reactions were studied in TGA/DSC. TEM revealed that the material easily collapses under the electron beam into nanoparticles. Under very low doses almost no particles can be found. Similarly, Raman spectroscopy confirmed the formation of Pd fulleride polymers but also supported the collapse of the Pd fulleride phase when irradiated by high laser power. CVD experiments have been conducted on directly coated Si substrates showing similar results to previous reports, namely that Pd2C60 is an efficient catalysts for the growth of helical carbon nanofibers. Our study gives both insights into the formation of nanoparticles as well as the synthesis of C60 polymers. The method is also compatible with direct coating processes making it useful for a broad spectrum of CVD and catalysis applications. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

  • 16.
    Nitze, Florian
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Mazurkiewicz, Marta
    Malolepszy, Artur
    Mikolajczuk, Anna
    Kedzierzawski, Piotr
    Tai, Cheuk-Wai
    Hu, Guangzhi
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Kurzydlowski, Krzysztof Jan
    Stobinski, Leszek
    Borodzinski, Andrzej
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Synthesis of palladium nanoparticles decorated helical carbon nanofiber as highly active anodic catalyst for direct formic acid fuel cells2012In: Electrochimica Acta, ISSN 0013-4686, E-ISSN 1873-3859, Vol. 63, p. 323-328Article in journal (Refereed)
    Abstract [en]

    We present a single metal approach to produce highly active catalyst materials based on Pd-decorated helical carbon nanofibers. Helical carbon fibers are synthesized by a chemical vapor deposition process on a C-60 supported Pd catalyst and the obtained fibers are functionalized by H2O2 followed by a decoration with Pd nanoparticles. Although transmission electron microscopy images show that the decoration is relatively inhomogeneous the electrocatalytic activity for formic acid oxidation is very high. Cyclic voltammetry measurements (CV) show that the generated current peak value for Pd-decorated helical carbon nanofibers is 300 mA/mg(Pd) for a scan rate of 10 mV/s. This is significantly higher than the corresponding value of a reference sample of multiwalled carbon nanotubes decorated with Pd nanoparticles by the same process. Fuel cell tests for our Pd-decorated helical carbon nanofibers also displayed a high power density, although not as superior to Pd-decorated multiwalled nanotubes as measured by CV. Our results show that helical carbon nanofibers have several good properties, such as a rigid anchoring of catalyst nanoparticles and a suitable structure for creating functionalization defects which make them an interesting candidate for electrochemical applications. (C) 2012 Elsevier Ltd. All rights reserved.

  • 17.
    Nitze, Florian
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Department of Chemical and Biological Engineering, Chalmers University of Technology, Sweden.
    Sandström, Robin
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Barzegar, Hamid Reza
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hu, Guangzhi
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Mazurkiewicz, Marta
    Malolepszy, Artur
    Stobinski, Leszek
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Direct support mixture painting, using Pd(0) organo-metallic compounds - an easy and environmentally sound approach to combine decoration and electrode preparation for fuel cells2014In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 2, no 48, p. 20973-20979Article in journal (Refereed)
    Abstract [en]

    An inventive, fast and straight-forward approach for the direct preparation of fuel cell electrodes has been developed and tested. Our approach avoids long catalyst preparation and post-synthesis treatment. It reduces the use of chemicals and thereby concomitantly lowers the environmental impact and improves cost efficiency. It combines decoration of the support by palladium nanoparticles with electrode preparation through a simple one-step ink-painting and annealing process. Composites have been investigated by high resolution transmission electron microscopy, scanning electron microscopy, and Xray diffraction. Crystalline particles are well-attached and well-distributed on the support. Particles are of few nanometers in size and spherical for decorated Vulcan whereas they are larger and irregularly shaped for decorated helical carbon nanofibers (HCNFs). Electrodes with a metal loading of 0.8 mg cm(-2) have been tested in a direct formic acid fuel cell. Both the Vulcan and the HCNF electrodes show a similar and high power output of up to 120 mW mg(-1). They also show similar performances in deactivation experiments conducted at 200 mA cm(-2) even when using only high purity grade formic acid. After deactivation the electrodes show no structural damage, making them superior to most commercial catalysts. The electrodes can be completely regenerated to initial activity by simple treatment with water. The easy regeneration process indicates that CO-adsorption on the fuel cell anode catalyst is not the main poisoning mechanism responsible for electrode degeneration.

  • 18.
    Sharifi, Tiva
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Barzegar, Hamid Reza
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Jia, Xueen
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nitze, Florian
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hu, Guangzhi
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nordblad, Per
    Department of Engineering Sciences, Uppsala University, Box 534, 751 21 Uppsala, Sweden.
    Tai, Cheuk-Wai
    Department of Materials and Environmental Chemistry, Arrhenius Laboratory, Stockholm University, S-106 91 Stockholm, Sweden.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Formation of nitrogen-doped graphene nanoscrolls by adsorption of magnetic gamma-Fe2O3 nanoparticles2013In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 4, p. 2319-Article in journal (Refereed)
    Abstract [en]

    Graphene nanoscrolls are Archimedean-type spirals formed by rolling single-layer graphene sheets. Their unique structure makes them conceptually interesting and understanding their formation gives important information on the manipulation and characteristics of various carbon nanostructures. Here we report a 100% efficient process to transform nitrogen-doped reduced graphene oxide sheets into homogeneous nanoscrolls by decoration with magnetic gamma-Fe2O3 nanoparticles. Through a large number of control experiments, magnetic characterization of the decorated nanoparticles, and ab initio calculations, we conclude that the rolling is initiated by the strong adsorption of maghemite nanoparticles at nitrogen defects in the graphene lattice and their mutual magnetic interaction. The nanoscroll formation is fully reversible and upon removal of the maghemite nanoparticles, the nanoscrolls return to open sheets. Besides supplying information on the rolling mechanism of graphene nanoscrolls, our results also provide important information on the stabilization of iron oxide nanoparticles.

  • 19.
    Sharifi, Tiva
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nitze, Florian
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Barzegar, Hamid Reza
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Tai, Cheuk-Wai
    Mazurkiewicz, Marta
    Malolepszy, Artur
    Stobinski, Leszek
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nitrogen doped multi walled carbon nanotubes produced by CVD-correlating XPS and Raman spectroscopy for the study of nitrogen inclusion2012In: Carbon, ISSN 0008-6223, E-ISSN 1873-3891, Vol. 50, no 10, p. 3535-3541Article in journal (Refereed)
    Abstract [en]

    High purity aligned nitrogen doped multi walled carbon nanotubes were synthesized by the catalytic chemical vapor deposition method using pyridine and Fe/Co (2:1 volume ratio) as the single C/N precursor and catalyst material. The average diameter of the synthesized tubes ranges between 29 nm and 57 nm and the nitrogen content of the tubes reaches a maximum of 9.2 (at.)% nitrogen. The effect of nitrogen doping on the Raman scattering of doped tubes and its correlation with X-ray photoelectron spectra (XPS) was investigated. The analysis is based on the investigation of the I-D/I-G (integrated area ratio), other nitrogen characteristic Raman modes and the type of nitrogen inclusion interpreted from the N 1s electron bonding energies in XPS. At doping levels higher than 5% the nitrogen inclusion takes place through another mechanism than at low nitrogen doping levels. Most significant is that pyridinic defects are relatively readily incorporated at low nitrogen doping levels while at nitrogen content higher than 5% the major incorporation mechanism is dominated by pyridinic and pyrrolic defects on an equal basis. Our study gives further insight into nitrogen doping effects and the relation between type of nitrogen inclusion and nitrogen doping levels. (C) 2012 Elsevier Ltd. All rights reserved.

  • 20.
    Yao, Mingguang
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Stenmark, Patrik
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Abou-Hamad, Edy
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nitze, Florian
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Qin, Jian
    Royal Institute of Technology, SE-16440 Stockholm, Sweden.
    Goze-Bac, Christophe
    Centre National de la Recherche Scientifique, Université Montpellier 2, F-34079 Montpellier, France.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Confined adamantane molecules assembled to one dimension in carbon nanotubes2011In: Carbon, ISSN 0008-6223, E-ISSN 1873-3891, Vol. 49, no 4, p. 1159-1166Article in journal (Refereed)
    Abstract [en]

    We have encapsulated adamantane (C10H16) in single- and multi-walled carbon nanotubes. Adamantane is a high symmetry cage like molecule with point group symmetry Td and can be considered as a hydrogen-terminated diamond fragment. We confirmed and identified the successful filling by high resolution transmission electron microscopy, 13C nuclear magnetic resonance, infrared and Raman spectroscopy. 13C nuclear magnetic resonance of the adamantane filled nanotubes reveals that the adamantane molecules stop rotating after encapsulation. A blue-shift of the Raman active radial breathing modes of the carbon nanotubes supports this and suggests a significant interaction between encapsulated adamantane molecules and the single wall nanotubes. The encapsulated adamantane molecules exhibit red shifted infrared C–H vibration modes which we assign to a slight elongation of the C–H bonds. We observe both a nanotube diameter dependence of the adamantane filling ratio and a release rate of adamantane from the CNTs that depends on the CNT diameters.

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