Umeå University's logo

umu.sePublications
Change search
Refine search result
12 1 - 50 of 63
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Rows per page
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sort
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
Select
The maximal number of hits you can export is 250. When you want to export more records please use the Create feeds function.
  • 1.
    Annamalai, Alagappan
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Division of Biotechnology, Advanced Institute of Environmental and Bioscience, College of Environmental and Bioresource Sciences, Chonbuk National University, Republic of Korea.
    Lee, Hyun Hwi
    Choi, Sun Hee
    Lee, Su Yong
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Subramanian, Arunprabaharan
    Park, Jaedeuk
    Kong, Ki-jeong
    Jang, Jum Suk
    Sn/Be Sequentially co-doped Hematite Photoanodes for Enhanced Photoelectrochemical Water Oxidation: Effect of Be2+ as co-dopant2016In: Scientific Reports, E-ISSN 2045-2322, Vol. 6, article id 23183Article in journal (Refereed)
    Abstract [en]

    For ex-situ co-doping methods, sintering at high temperatures enables rapid diffusion of Sn4+ and Be2+ dopants into hematite (alpha-Fe2O3) lattices, without altering the nanorod morphology or damaging their crystallinity. Sn/Be co-doping results in a remarkable enhancement in photocurrent (1.7 mA/cm(2)) compared to pristine alpha-Fe2O3 (0.7 mA/cm(2)), and Sn4+ mono-doped alpha-Fe2O3 photoanodes (1.0 mA/cm(2)). From first-principles calculations, we found that Sn4+ doping induced a shallow donor level below the conduction band minimum, which does not contribute to increase electrical conductivity and photocurrent because of its localized nature. Additionally, Sn4+-doping induce local micro-strain and a decreased Fe-O bond ordering. When Be2+ was co-doped with Sn4+-doped alpha-Fe2O3 photoanodes, the conduction band recovered its original state, without localized impurities peaks, also a reduction in micro-strain and increased Fe-O bond ordering is observed. Also the sequence in which the ex-situ co-doping is carried out is very crucial, as Be/Sn co-doping sequence induces many under-coordinated O atoms resulting in a higher micro-strain and lower charge separation efficiency resulting undesired electron recombination. Here, we perform a detailed systematic characterization using XRD, FESEM, XPS and comprehensive electrochemical and photoelectrochemical studies, along with sophisticated synchrotron diffraction studies and extended X-ray absorption fine structure.

    Download full text (pdf)
    fulltext
  • 2.
    Annamalai, Alagappan
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sandström, Robin
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Boulanger, Nicolas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Boily, Jean-Francois
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Muehlbacher, Inge
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Double donor Sb5+doped hematite (Fe3+) photoanodes for surface-enhanced PEC water splitting2018In: Abstracts of Papers of the American Chemical Society, ISSN 0065-7727, Vol. 256Article in journal (Other academic)
  • 3.
    Annamalai, Alagappan
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sandström, Robin
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Boulanger, Nicolas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Boily, Jean-Francois
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Mühlbacher, Inge
    Shchukarev, Andrey
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Influence of Sb5+ as a Double Donor on Hematite (Fe3+) Photoanodes for Surface-Enhanced Photoelectrochemical Water Oxidation2018In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 10, no 19, p. 16467-16473Article in journal (Refereed)
    Abstract [en]

    To exploit the full potential of hematite (α-Fe2O3) as an efficient photoanode for water oxidation, the redox processes occurring at the Fe2O3/electrolyte interface need to be studied in greater detail. Ex situ doping is an excellent technique to introduce dopants onto the photoanode surface and to modify the photoanode/electrolyte interface. In this context, we selected antimony (Sb5+) as the ex situ dopant because it is an effective electron donor and reduces recombination effects and concurrently utilize the possibility to tuning the surface charge and wettability. In the presence of Sb5+ states in Sb-doped Fe2O3 photoanodes, as confirmed by X-ray photoelectron spectroscopy, we observed a 10-fold increase in carrier concentration (1.1 × 1020 vs 1.3 × 1019 cm–3) and decreased photoanode/electrolyte charge transfer resistance (∼990 vs ∼3700 Ω). Furthermore, a broad range of surface characterization techniques such as Fourier-transform infrared spectroscopy, ζ-potential, and contact angle measurements reveal that changes in the surface hydroxyl groups following the ex situ doping also have an effect on the water splitting capability. Theoretical calculations suggest that Sb5+ can activate multiple Fe3+ ions simultaneously, in addition to increasing the surface charge and enhancing the electron/hole transport properties. To a greater extent, the Sb5+- surface-doped determines the interfacial properties of electrochemical charge transfer, leading to an efficient water oxidation mechanism.

  • 4.
    Barzegar, Hamid Reza
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Univ Calif Berkeley, Dept Phys, Berkeley, CA 94720 USA.
    Gracia Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    C60/Collapsed Carbon Nanotube Hybrids: A Variant of Peapods2015In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 15, no 2, p. 829-834Article in journal (Refereed)
    Abstract [en]

    We examine a variant of so-called carbon nanotube peapods by packing C60 molecules inside the open edge ducts of collapsed carbon nanotubes. C60 insertion is accomplished through a facile single-step solution-based process. Theoretical modeling is used to evaluate favorable low-energy structural configurations. Overfilling of the collapsed tubes allows infiltration of C60 over the full cross-section of the tubes and consequent partial or complete reinflation, yielding few-wall, large diameter cylindrical nanotubes packed with crystalline C60 solid cores.

    Download full text (pdf)
    fulltext
  • 5.
    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.

  • 6.
    Barzegar, Hamid Reza
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Univ Calif Berkeley, Dept Phys, Berkeley, CA 94720 USA; Lawrence Berkeley Natl Lab, Div Mat Sci, Berkeley, CA 94720 USA; Univ Calif Berkeley, Kavli Energy NanoSci Inst, Berkeley, CA 94720 USA; Lawrence Berkeley Natl Lab, Berkeley, CA 94720 USA.
    Yan, Aiming
    Coh, Sinisa
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Dunn, Gabriel
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Louie, Steven G.
    Cohen, Marvin L.
    Zettl, Alex
    Electrostatically Driven Nanoballoon Actuator2016In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 16, no 11, p. 6787-6791Article in journal (Refereed)
    Abstract [en]

    We demonstrate an inflatable nanoballoon actuator based on geometrical transitions between the inflated (cylindrical) and collapsed (flattened) forms of a carbon nanotube. In situ transmission electron microscopy experiments employing a nanoelectromechanical manipulator show that a collapsed carbon nanotube can be reinflated by electrically charging the nanotube, thus realizing an electrostatically driven nanoballoon actuator. We find that the tube actuator can be reliably cycled with only modest control voltages (few volts) with no apparent wear or fatigue. A complementary theoretical analysis identifies critical parameters for nanotube nanoballoon actuation.

  • 7.
    Barzegar, Hamid Reza
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Department of Physics, University of California, Berkeley, CA 94720, USA; Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA; Kavli Energy NanoSciences Institute at the University of California, Berkeley and the Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
    Yan, Aiming
    Coh, Sinisa
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ojeda-Aristizabal, Claudia
    Dunn, Gabriel
    Cohen, Marvin L.
    Louie, Steven G.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Zettl, Alex
    Spontaneous twisting of a collapsed carbon nanotube2017In: Nano Reseach, ISSN 1998-0124, E-ISSN 1998-0000, Vol. 10, no 6, p. 1942-1949Article in journal (Refereed)
    Abstract [en]

    We study the collapsing and subsequent spontaneous twisting of a carbon nanotube by in situ transmission electron microscopy (TEM). A custom-sized nanotube is first created in the microscope by selectively extracting shells from a parent multi-walled tube. The few-walled, large-diameter daughter nanotube is driven to collapse via mechanical stimulation, after which the ribbon-like collapsed tube spontaneously twists along its long axis. In situ diffraction experiments fully characterize the uncollapsed and collapsed tubes. The experimental observations and associated theoretical analysis indicate that the origin of the twisting is compressive strain.

  • 8.
    Ekeroth, Sebastian
    et al.
    Department of Physics, Chemistry and Biology, Linköping University, Linköping, Sweden.
    Ekspong, Joakim
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Perivoliotis, Dimitrios K.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sharma, Sachin
    Department of Physics, Chemistry and Biology, Linköping University, Linköping, Sweden.
    Boyd, Robert
    Department of Physics, Chemistry and Biology, Linköping University, Linköping, Sweden.
    Brenning, Nils
    Department of Physics, Chemistry and Biology, Linköping University, Linköping, Sweden; Division of Space and Plasma Physics, School of Electrical Engineering, KTH Royal Institute of Technology, Stockholm, Sweden.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Helmersson, Ulf
    Department of Physics, Chemistry and Biology, Linköping University, Linköping, Sweden.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Magnetically Collected Platinum/Nickel Alloy Nanoparticles as Catalysts for Hydrogen Evolution2021In: ACS Applied Nano Materials, E-ISSN 2574-0970, Vol. 4, no 12, p. 12957-12965Article in journal (Refereed)
    Abstract [en]

    The hydrogen evolution reaction (HER) is a key process in electrochemical water splitting. To lower the cost and environmental impact of this process, it is highly motivated to develop electrocatalysts with low or no content of noble metals. Here, we report on an ingenious synthesis of hybrid PtxNi1-x electrocatalysts in the form of a nanoparticle-nanonetwork structure with very low noble metal content. The structure possesses important features such as good electrical conductivity, high surface area, strong interlinking, and substrate adhesion, which render an excellent HER activity. Specifically, the best performing Pt0.05Ni0.95 sample demonstrates a Tafel slope of 30 mV dec-1 in 0.5 M H2SO4 and an overpotential of 20 mV at a current density of 10 mA cm-2 with high stability. The impressive catalytic performance is further rationalized in a theoretical study, which provides insight into the mechanism on how such small platinum content can allow for close-to-optimal adsorption energies for hydrogen.

    Download full text (pdf)
    fulltext
  • 9.
    Ekspong, Joakim
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Boulanger, Nicolas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Surface activation of graphene nanoribbons for oxygen reduction reaction by nitrogen doping and defect engineering: An ab initio study2018In: Carbon, ISSN 0008-6223, E-ISSN 1873-3891, Vol. 137, p. 349-357Article in journal (Refereed)
    Abstract [en]

    Introducing heteroatoms and creating structural defects on graphene is a common and rather successful strategy to transform its inert basal plane into an efficient metal-free electrocatalyst for oxygen reduction reaction (ORR). However, the intricate atomic configuration of defective graphenes difficult their optimization as ORR electrocatalysts, where not only a large density of active sites is desirable, but also excellent electrical conductivity is required. Therefore, we used density functional theory to investigate the current-voltage characteristics and the catalytic active sites towards ORR of nitrogen-doped and defective graphene by using 8 zig-zag graphene nanoribbons as model systems. Detailed ORR catalytic activity maps are created for ten different systems showing the distribution of catalytic hot spots generated by each defect. Subsequently, the use of both current-voltage characteristics and catalytic activity maps allow to exclude inefficient systems that exhibit either low electrical conductivity or have adsorption energies far from optimal. Our study highlights the importance of considering not only the interaction energy of reaction intermediates to design electrocatalysts, but also the electrical conductivity of such configurations. We believe that this work is important for future experimental studies by providing insights on the use of graphene as a catalyst towards the ORR reaction. 

  • 10.
    Ekspong, Joakim
    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.
    Theoretical Analysis of Surface Active Sites in Defective 2H and 1T ' MoS2 Polymorphs for Hydrogen Evolution Reaction: Quantifying the Total Activity of Point Defects2020In: Advanced Theory and Simulations, E-ISSN 2513-0390, Vol. 3, no 3, article id 1900213Article in journal (Refereed)
    Abstract [en]

    Defect engineering is a common and promising strategy to improve the catalytic activity of layered structures such as MoS2, where in particular the 2H and 1T ' polymorphs have been under intense study for their activity toward the hydrogen evolution reaction. However, the large variety of defects, each with its own distinct and usually unknown effects, complicates the design and optimization of such defective materials. Therefore, it is relevant to characterize in detail the effect of individual defects and to be able to combine these observations to describe more complex materials, such as those seen experimentally. Therefore, nine point defects (antisites defects and vacancies) are theoretically studied on single layer 1T, 1T ', and 2H MoS2 polymorphs, and the variation and spatial distribution in the active sites are identified. It is found that all defective 1T ' monolayers exhibit an increase in the exchange current density of at least 2.3 times when compared to pristine 1T ' MoS2, even if a reduced number of active sites are observed. The results are later used to propose a methodology to study materials containing a mixture of crystal phases, or other alterations that cause inhomogeneous changes in the activity of catalytic sites.

  • 11.
    Ekspong, Joakim
    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.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hydrogen Evolution Reaction Activity of Heterogeneous Materials: A Theoretical Model2020In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 124, no 38, p. 20911-20921Article in journal (Refereed)
    Abstract [en]

    In this study, we present a new comprehensive methodology to quantify the catalytic activity of heterogeneous materials for the hydrogen evolution reaction (HER) using ab initio simulations. The model is composed of two parts. First, the equilibrium hydrogen coverage is obtained by an iterative evaluation of the hydrogen adsorption free energies (ΔGH) using density functional theory calculations. Afterward, the ΔGH are used in a microkinetic model to provide detailed characterizations of the entire HER considering all three elementary steps, i.e., the discharge, atom + ion, and combination reactions, without any prior assumptions of rate-determining steps. The microkinetic model takes the equilibrium and potential-dependent characteristics into account, and thus both exchange current densities and Tafel slopes are evaluated. The model is tested on several systems, from polycrystalline metals to heterogeneous molybdenum disulfide (MoS2), and by comparing to experimental data, we verify that our model accurately predicts their experimental exchange current densities and Tafel slopes. Finally, we present an extended volcano plot that correlates the electrical current densities of each elementary reaction step to the coverage-dependent ΔGH.

    Download full text (pdf)
    fulltext
  • 12.
    Ekspong, Joakim
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sandström, Robin
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rajukumar, Lakshmy Pulickal
    Terrones, Mauricio
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Stable Sulfur‐Intercalated 1T′ MoS2 on Graphitic Nanoribbons as Hydrogen Evolution Electrocatalyst2018In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 28, no 46, article id 1802744Article in journal (Refereed)
    Abstract [en]

    The metastable 1T′ polymorph of molybdenum disulfide (MoS2) has shown excellent catalytic activity toward the hydrogen evolution reaction (HER) in water‐splitting applications. Its basal plane exhibits high catalytic activity comparable to the edges in 2H MoS2 and noble metal platinum. However, the production and application of this polymorph are limited by its lower energetic stability compared to the semiconducting 2H MoS2 phase. Here, the production of stable intercalated 1T′ MoS2 nanosheets attached on graphitic nanoribbons is reported. The intercalated 1T′ MoS2 exhibits a stoichiometric S:Mo ratio of 2.3 (±0.1):1 with an expanded interlayer distance of 10 Å caused by a sulfur‐rich intercalation agent and is stable at room temperature for several months even after drying. The composition, structure, and catalytic activity toward HER are investigated both experimentally and theoretically. It is concluded that the 1T′ MoS2 phase is stabilized by the intercalated agents, which further improves the basal planes′ catalytic activity toward HER.

  • 13.
    Ekspong, Joakim
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sharifi, Tiva
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Shchukarev, Andrey
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Klechikov, Alexey
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Stabilizing Active Edge Sites in Semicrystalline Molybdenum Sulfide by Anchorage on Nitrogen-Doped Carbon Nanotubes for Hydrogen Evolution Reaction2016In: Advanced Functional Materials, ISSN 1616-301X, E-ISSN 1616-3028, Vol. 26, no 37, p. 6766-6776Article in journal (Refereed)
    Abstract [en]

    Finding an abundant and cost-effective electrocatalyst for the hydrogen evolu-tion reaction (HER) is crucial for a global production of hydrogen from water electrolysis. This work reports an exceptionally large surface area hybrid catalyst electrode comprising semicrystalline molybdenum sulfi de (MoS 2+ x) catalystattached on a substrate based on nitrogen-doped carbon nanotubes (N-CNTs), which are directly grown on carbon fiber paper (CP). It is shown here that nitrogen-doping of the carbon nanotubes improves the anchoring of MoS 2+ xcatalyst compared to undoped carbon nanotubes and concurrently stabilizes a semicrystalline structure of MoS 2+ x with a high exposure of active sites for HER. The well-connected constituents of the hybrid catalyst are shown to facilitate electron transport and as a result of the good attributes, the MoS 2+ x/N-CNT/CPelectrode exhibits an onset potential of −135 mV for HER in 0.5 M H2SO4, a Tafel slope of 36 mV dec −1, and high stability at a current density of −10 mA cm −2.

  • 14.
    Enevold, Jenny
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Dahlberg, Tobias
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Stangner, Tim
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Tang, Shi
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Lindh, E. Mattias
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Andersson, Magnus
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Tunable two-dimensional patterning of a semiconducting Nanometer-Thin C60 fullerene film using a spatial light modulator2020In: ACS Applied Nano Materials, ISSN 2574-0970, Vol. 3, no 6, p. 2574-0970Article in journal (Other academic)
    Abstract [en]

    The photochemical coupling of fullerene molecules into covalently connected oligomeric or polymeric structures can result in drastically lowered solubility in common solvents with retained semiconductor properties. Here, we exploit this combination of properties for the utilization of fullerenes as a negative photoresist material with electronic functionality. Specifically, we develop an easily tunable exposure system, essentially comprising a laser and a computer-controlled spatial light modulator (SLM) featuring >8 million independently controlled pixels, for the spatially selective photochemical transformation of nanometer-thin C60 fullerene films. With a carefully designed laser-SLM-exposure/solvent-development cycle, we are able to realize well-resolved two-dimensional hexagonal or square patterns of circular C60 microdots with a center-to-center distance of 1–5 μm and a maximum thickness of 20–35 nm over several square-millimeter-sized areas on a substrate. The functionality of such a hexagonal C60 pattern was demonstrated by its inclusion in between the transparent electrode and the active material in a light-emitting electrochemical cell, which featured an enhanced light output by >50% in comparison to a reference device void of the patterned C60 layer.

    Download full text (pdf)
    fulltext
  • 15.
    Fan, Junpeng
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ekspong, Joakim
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ashok, Anumol
    Koroidov, Sergey
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Solid-state synthesis of few-layer cobalt-doped MoS2 with CoMoS phase on nitrogen-doped graphene driven by microwave irradiation for hydrogen electrocatalysis2020In: RSC Advances, E-ISSN 2046-2069, Vol. 10, no 56, p. 34323-34332Article in journal (Refereed)
    Abstract [en]

    The high catalytic activity of cobalt-doped MoS2 (Co–MoS2) observed in several chemical reactions such as hydrogen evolution and hydrodesulfurization, among others, is mainly attributed to the formation of the CoMoS phase, in which Co occupies the edge-sites of MoS2. Unfortunately, its production represents a challenge due to limited cobalt incorporation and considerable segregation into sulfides and sulfates. We, therefore, developed a fast and efficient solid-state microwave irradiation synthesis process suitable for producing thin Co–MoS2 flakes (∼3–8 layers) attached on nitrogen-doped reduced graphene oxide. The CoMoS phase is predominant in samples with up to 15 at% of cobalt, and only a slight segregation into cobalt sulfides/sulfates is noticed at larger Co content. The Co–MoS2 flakes exhibit a large number of defects resulting in wavy sheets with significant variations in interlayer distance. The catalytic performance was investigated by evaluating the activity towards the hydrogen evolution reaction (HER), and a gradual improvement with increased amount of Co was observed, reaching a maximum at 15 at% with an overpotential of 197 mV at −10 mA cm−2, and a Tafel slope of 61 mV dec−1. The Co doping had little effect on the HER mechanism, but a reduced onset potential and charge transfer resistance contributed to the improved activity. Our results demonstrate the feasibility of using a rapid microwave irradiation process to produce highly doped Co–MoS2 with predominant CoMoS phase, excellent HER activity, and operational stability.

    Download full text (pdf)
    fulltext
  • 16.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Behind the Synergistic Effect Observed on Phosphorus Nitrogen Codoped Graphene during the Oxygen Reduction Reaction2016In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 120, no 49, p. 27849-27857Article in journal (Refereed)
    Abstract [en]

    Ab initio calculations are performed to investigate how the simultaneous introduction of phosphorus and nitrogen into graphene modifies the availability and spatial distribution of catalytic active sites for an oxygen reduction reaction (ORR). A phosphoryl group (R-3-P=0) is selected as a representative for the phosphorus doping, and the ORR is studied under alkaline conditions where a 4e(-) mechanism is used to determine the limiting step and overpotential (eta(ORR)) along the entire graphene surface. A scanning procedure is used to construct eta(ORR) maps for pristine-, N-, P-, and diverse PN codoped graphenes. The results indicate that a single N (P) atom activates up to 17 (3) C atoms, while the simultaneous introduction of P and N activates up to 55 C atoms equivalent to 57% of the surface. Additionally, PN codoped graphenes reveals that the relative location of both dopants has significant effects on the ORR performance, where a P N separation distance of at least 4 angstrom minimizes the localization of electronic states on the neighboring C atoms and improves the quantity and distribution of active sites. The results shows the importance of designing synthesis procedures to control the dopant concentration and spatial distribution to maximize the number of active sites. Furthermore, the eta(ORR) maps reveal features that could be obtained by scanning tunneling microscopy allowing us to experimentally identify and possibly quantify the catalytic active sites on carbon-based materials.

  • 17.
    Gracia-Espino, Eduardo
    et al.
    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.
    Sharifi, Tiva
    Yan, Aiming
    Zettl, Alex
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Fabrication of One-Dimensional Zigzag [6,6]-Phenyl-C-61-Butyric Acid Methyl Ester Nanoribbons from Two-Dimensional Nanosheets2015In: ACS Nano, ISSN 1936-0851, E-ISSN 1936-086X, Vol. 9, no 10, p. 10516-10522Article in journal (Refereed)
    Abstract [en]

    One-dimensional (10) zigzag [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) nanoribbons are produced by folding two-dimensional ultrathin PCBM nanosheets in a simple solvent process. The unique 1D PCBM nanostructures exhibit uniform width of 3.8 +/- 0.3 nm, equivalent to four PCBM molecules, and lengths of 20-400 nm. These nanoribbons show well-defined crystalline structure, comprising PCBM molecules in a hexagonal arrangement without trapped solvent molecules. First-principle calculations and detailed experimental characterization provide an insight into the structure and formation mechanism of the 1D PCBM nanoribbons. Given their dimensions and physical properties, we foresee that these nanostructures should be ideal as acceptor material in organic solar cells.

    Download full text (pdf)
    fulltext
  • 18.
    Gracia-Espino, Eduardo
    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. Department of Physics, University of California, Berkeley, California 94720, United States.
    Zettl, Alex
    Coronene-based graphene nanoribbons insulated by boron nitride nanotubes: electronic properties of the hybrid structure2018In: ACS Omega, E-ISSN 2470-1343, Vol. 3, no 10, p. 12930-12935Article in journal (Refereed)
    Abstract [en]

    We present a theoretical study on the formation of graphene nanoribbons-via polymerization of coronene molecules-inside the inner cavity of boron nitride nanotubes. We examine the electronic property of the hybrid system, and we show that the boron nitride nanotube does not significantly alter the electronic properties of the encapsulated graphene nanoribbon. Motivated by previous experimental works, we examine graphene nanoribbons with two different widths and investigate probable scenarios for defect formation and/or twisting of the resulting graphene nanoribbons and their effect on the electronic properties of the hybrid system.

  • 19.
    Gracia-Espino, Eduardo
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Hu, Guangzhi
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Shchukarev, Andrey
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Understanding the Interface of Six-Shell Cuboctahedral and Icosahedral Palladium Clusters on Reduced Graphene Oxide: Experimental and Theoretical Study2014In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 136, no 18, p. 6626-6633Article in journal (Refereed)
    Abstract [en]

    Studies on noble-metal-decorated carbon nanostructures are reported almost on a daily basis, but detailed studies on the nanoscale interactions for well-defined systems are very rare. Here we report a study of reduced graphene oxide (rGOx) homogeneously decorated with palladium (Pd) nanoclusters with well-defined shape and size (2.3 +/- 0.3 nm). The rGOx was modified with benzyl mercaptan (BnSH) to improve the interaction with Pd clusters, and N,N-dimethylformamide was used as solvent and capping agent during the decoration process. The resulting Pd nanoparticles anchored to the rGOx-surface exhibit high crystallinity and are fully consistent with six-shell cuboctahedral and icosahedral clusters containing similar to 600 Pd atoms, where 45% of these are located at the surface. According to X-ray photoelectron spectroscopy analysis, the Pd clusters exhibit an oxidized surface forming a PdOx shell. Given the well-defined experimental system, as verified by electron microscopy data and theoretical simulations, we performed ab initio simulations using 10 functionalized graphenes (with vacancies or pyridine, amine, hydroxyl, carboxyl, or epoxy groups) to understand the adsorption process of BnSH, their further role in the Pd cluster formation, and the electronic properties of the graphene-nanoparticle hybrid system. Both the experimental and theoretical results suggest that Pd clusters interact with fiinctionalized graphene by a sulfur bridge while the remaining Pd surface is oxidized. Our study is of significant importance for all work related to anchoring of nanoparticles on nanocarbon-based supports, which are used in a variety of applications.

  • 20.
    Gracia-Espino, Eduardo
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry. Umeå University, Faculty of Science and Technology, Department of Physics.
    Jia, Xueen
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Improved oxygen reduction performance of Pt–Ni nanoparticles by adhesion on nitrogen-doped graphene2014In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 118, no 5, p. 2804-2811Article in journal (Refereed)
    Abstract [en]

    Graphene and its derivatives hold great potential as support for nanocatalyst in various energy applications, such as fuel cells, batteries, and capacitors. In this work, we used density functional theory to analyze substrate effect on the electrocatalytic activity of Pt–Ni bimetallic nanoparticles for oxygen reduction reaction (ORR). The dissociative mechanism is used to evaluate the ORR performance (energy barrier for O2 dissociation, free energy of intermediates, d-band center, overpotential, and electrochemical activity) for a Pt–Ni core–shell-like nanoparticle (PtNiCS) deposited on nondefective graphene (GS) or nitrogen-doped graphene (N-GS). The electronic and catalytic properties of PtNiCS on N-GS designate N-doped graphene as the best substrate to use for ORR, showing better interaction with the bimetallic cluster, improved charge transfer between constitutes, and a superior ORR performance when compared to PtNiCS on GS. The N-GS has a significant effect in reducing the energy barrier for O2 dissociation and decrease the energetic stability of HO* intermediates, resulting in enhanced ORR activity compared with the PtNiCS on GS. In addition, the strong interaction between PtNiCS cluster and N-GS substrate may lead to an improved long-term stability of the catalytic particle during ORR cycles.

  • 21.
    Gracia-Espino, Eduardo
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Chemistry. Advanced Materials Department, IPICYT, Camino a la Presa San José 2055, Col. Lomas 4a sección, San Luis Potosí S.L.P., 78216, México. E-mail: flo@ipicyt.edu.mx.
    López-Urias, F.
    Terrones, H.
    Terrones, M.
    Electron transport study on functionalized armchair graphene nanoribbons: DFT calculations2016In: RSC Advances, E-ISSN 2046-2069, Vol. 6, no 26, p. 21954-21960Article in journal (Refereed)
    Abstract [en]

    Quantum transport studies are performed on doped and functionalized 8- and 11-armchair graphene nanoribbons (aGNRs) by means of density functional theory. Substitutional doping is performed by introducing boron, nitrogen, oxygen, silicon, phosphorus, and sulfur atoms within the lattice of the aGNRs. Other functional groups such as borane, amine, hydroxyl, thiol, silane, silene, phosphine, and phosphorane groups are also introduced at the nanoribbon's edge. The dopant position and the nanoribbon's width strongly influence the current-voltage characteristics, and generally, the narrow 8-aGNRs and edge-doped 11-aGNRs show deteriorated transport properties, mainly due to the formation of irregular edges that create highly localized states disrupting several conducting bands. On the other hand, the inside-doped 11-aGNRs are barely affected, mainly because these systems preserve the edge's structure, thus edge conduction bands still contribute to the electron transport. Our results suggest that wider graphene nanoribbons could be functionalized at the inner sections without significantly compromising their transport characteristics while retaining the chemical reactivity that characterize doped nanocarbons. Such characteristics are highly desirable in fuel cells where doped graphene is used as a catalyst support or as a metal-free catalyst.

  • 22.
    Gracia-Espino, Eduardo
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry. Umeå University, Faculty of Science and Technology, Department of Physics.
    López-Urías, F
    Kim, YA
    Hayashi, T
    Muramatsu, H
    Endo, M
    Terrones, H
    Terrones, Mauricio
    Dresselhaus, Mildred S
    Novel carbon-based nanomaterials: graphene and graphitic nanoribbons2013In: Handbook of advanced ceramics: materials; applications; processing; and properties; second edition / [ed] Shigeyuki Somiya, Oxford: Elsevier, 2013, p. 61-87Chapter in book (Other academic)
    Abstract [en]

    The fascinating characteristic of carbon atoms to create multiple orbital hybridizations (e.g., sp, sp2, or sp3) provides the possibility to synthesize one-, two-, and three-dimensional carbon nanostructures with unique physical–chemical properties. In this way, the two-dimensional (2D) carbon-atomic layered crystal (graphene) and graphitic nanoribbons have attracted the attention of several scientific groups around the world due to their novel and unusual physicochemical properties. The relative simplicity of the Novoselov–Geim method to extract a single graphene layer along with the fascinating properties of graphene, such as the linear E(k) electronic structure in monolayer graphene, has stimulated extensive experimental and theoretical studies. This chapter reviews experimental and theoretical work on graphene with special attention to graphene nanoribbons. We focus on the role of topological defects, edge chirality, and chemical doping on the electronic, transport, and structural properties of graphene and graphene nanoribbons. We also review different synthesis techniques, such as chemical vapor deposition, chemical routes, and nanotube exfoliation, to obtain carbon nanoribbons. We also summarize common characterization techniques used for graphene materials, such as scanning electron microscopy, high resolution electron microscopy, scanning tunneling spectroscopy, near edge X-ray absorption fine structure, electron spin resonance, and Raman spectroscopy techniques. Edge-state characterization and the special magnetic properties of edges are also reviewed. In addition, first-principles density functional theory calculations of the electronic and transport properties of doped armchair nanoribbons are described. Finally, we discuss the future perspectives of these graphene-like materials, including applications in electronic devices, composites, catalysts, and energy storage devices.

  • 23.
    Gracia-Espino, Eduardo
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Chemistry. Advanced Materials Department, IPICYT, México.
    López-Urías, Florentino
    Terrones, Humberto
    Terrones, Mauricio
    Self-assembly synthesis of decorated nitrogen-doped carbon nanotubes with ZnO nanoparticles: anchoring mechanism and the effects of sulfur2015In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 119, no 1, p. 741-747Article in journal (Refereed)
    Abstract [en]

    Hybrid systems consisting of ZnO nanoparticles (ZnO-NPs) anchored on the surface of nitrogen-doped multiwalled carbon nanotubes (CNX-MWNTs) have been synthesized. The anchoring process consists of a self-assembly method involving the mixing of CNX-MWNTs in a solution with N,N-dimethylformamide, zinc acetylacetonate, and thiophene. Thiophene is used as a capping agent for controlling the size and distribution of ZnO-NPs, as well as an anchoring element between the NPs and the nanotube walls. Scanning and transmission electron microscopy characterization revealed that the ZnO-NPs are homogeneously deposited on the surface of CNX-MWNTs. X-ray powder diffraction analysis demonstrated that the ZnO-NPs exhibit a Wurtzite-type crystal structure with an average particle diameter of 5 nm. We also show that the ZnO-NPs do not exhibit a preferential growth direction with respect to the nanotube surface, and their formation is simply controlled by the concentration of the passivating agent. Density functional theory (DFT) calculations confirm that sulfur (from thiophene) is an effective passivating agent for ZnO by preferentially binding low-coordinated Zn atoms. However, the ZnO-NPs could be chemically bonded to the nanotubes through oxygen atoms close to the nitrogenated sites of the tubes. Our results also demonstrate that isolated and sulfur passivated ZnO-NPs become magnetic and exhibit half-metallicity (electronic states with only one spin component are present at the Fermi level). Sulfur-passivated ZnO retains these properties even after forming ZnO/CNX-MWNT hybrid materials.

  • 24.
    Gracia-Espino, Eduardo
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Chemistry. Advanced Materials Department, IPICYT, Mexico.
    Rebollo-Plata, Bernabe
    Martinez-Gutierrez, Hugo
    Munoz-Sandoval, Emilio
    Lopez-Urias, Florentino
    Endo, Morinobu
    Terrones, Humberto
    Terrones, Mauricio
    Temperature Dependence of Sensors Based on Silver-Decorated Nitrogen-Doped Multiwalled Carbon Nanotubes2016In: Journal of Sensors, ISSN 1687-725X, E-ISSN 1687-7268, article id 4319498Article in journal (Refereed)
    Abstract [en]

    Vapor sensors are easily fabricated onto alumina substrates using foils of silver-decorated nitrogen-doped multiwalled carbon nanotubes (CNX-MWNTs-Ag) as active sensing material. The vapor sensors are tested using carbon disulfide, acetone, ethanol, and chloroform vapors. The CNX-MWNTs are produced by chemical vapor deposition process and then decorated with 14 nm Ag nanoparticles (Ag-NPs). The samples are characterized using scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. Our results demonstrate that Ag-decorated CNX-MWNTs exhibit a better response and sensitivity when compared with pristine CNX-MWNTs based sensors, making them promising candidates for air-pollutants environmental monitoring. The temperature effect on the sensor performance is also studied; we found that the detection mechanism could be tuned from physisorption, at room temperature, to chemisorption at higher working temperature. Finally, first-principles density functional calculations are carried out to understand the interactions between the systems involved in the sensors, finding good agreement between experimental results and the theoretical approach.

    Download full text (pdf)
    fulltext
  • 25. Han, Xin-Bao
    et al.
    Tang, Xing-Yan
    Lin, Yue
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Liu, San-Gui
    Liang, Hai-Wei
    Hu, Guangzhi
    Umeå University, Faculty of Science and Technology, Department of Physics. Key Laboratory of Chemistry of Plant Resources in Arid Regions, State Key Laboratory Basis of Xinjiang Indigenous Medicinal Plants Resource Utilization, Xinjiang Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Urumqi 830011, China.
    Zhao, Xin-Jing
    Liao, Hong-Gang
    Tan, Yuan-Zhi
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Xie, Su-Yuan
    Zheng, Lan-Sun
    Ultrasmall Abundant Metal-Based Clusters as Oxygen-Evolving Catalysts2019In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 141, no 1, p. 232-239Article in journal (Refereed)
    Abstract [en]

    The oxygen evolution reaction is a crucial step in water electrolysis to develop clean and renewable energy. Although noble metal-based catalysts have demonstrated high activity for the oxygen evolution reaction, their application is limited by their high cost and low availability. Here we report the use of a molecule-to-cluster strategy for preparing ultrasmall trimetallic clusters by using the polyoxometalate molecule as a precursor. Ultrafine (0.8 nm) transition-metal clusters with controllable chemical composition are obtained. The transition-metal clusters enable highly efficient oxygen evolution through water electrolysis in alkaline media, manifested by an overpotential of 192 mV at 10 mA cm–2, a low Tafel slope of 36 mV dec–1, and long-term stability for 30 h of electrolysis. We note, however, that besides the excellent performance as an oxygen evolution catalyst, our molecule-to-cluster strategy provides a means to achieve well-defined transition-metal clusters in the subnanometer regime, which potentially can have an impact on several other applications.

  • 26. Han, Xin-Bao
    et al.
    Wang, Dong-Xue
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Luo, Yu-Hui
    Tan, Yuan-Zhi
    Lu, Dong-Fei
    Li, Yang-Guang
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wang, En-Bo
    Zheng, Lan-Sun
    Fe-substituted cobalt-phosphate polyoxometalates as enhanced oxygen evolution catalysts in acidic media2020In: Cuihuà xuébào, ISSN 0253-9837, E-ISSN 1872-2067, Vol. 41, no 5, p. 853-857Article in journal (Refereed)
    Abstract [en]

    All-inorganic and earth-abundant bi-/trimetallic hydr(oxy)oxides are widely used as oxygen evolution electrocatalysts owing to their remarkable performance. However, their atomically precise structures remain undefined, complicating their optimization and limiting the understanding of their enhanced performance. Here, the underlying structure-property correlation is explored by using a well-defined cobalt-phosphate polyoxometalate cluster [{Co-4(OH)(3)(PO4)}(4)(SiW9O34)(4)](32-) (1), which may serve as a molecular model of multimetal hydr(oxy)oxides. The catalytic activity is enhanced upon replacing Co by Fe in 1, resulting in a reduced overpotential (385 mV) for oxygen evolution (by 66 mV) compared to that of the parent 1 at 10 mA cm(-2) in an acidic medium; this overpotential is comparable to that for the IrO2 catalyst. These abundant-metal-based polyoxometalates exhibit high stability, with no evidence of degradation even after 24 h of operation.

  • 27.
    Hu, Guangzhi
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. Laboratory of Environmental Science and Technology, The Xinjiang Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Urumqi 830011, China.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sandström, Robin
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sharifi, Tiva
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Cheng, Shaodong
    Shen, Hangjia
    Wang, Chuanyi
    Guo, Shaojun
    Yang, Guang
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Atomistic understanding of the origin of high oxygen reduction electrocatalytic activity of cuboctahedral Pt3Co-Pt core-shell nanoparticles2016In: Catalysis Science & Technology, ISSN 2044-4753, E-ISSN 2044-4761, Vol. 6, no 5, p. 1393-1401Article in journal (Refereed)
    Abstract [en]

    PtM-based core-shell nanoparticles are a new class of active and stable nanocatalysts for promoting oxygen reduction reaction (ORR); however, the understanding of their high electrocatalytic performance for ORR at the atomistic level is still a great challenge. Herein, we report the synthesis of highly ordered and homogeneous truncated cuboctahedral Pt3Co-Pt core-shell nanoparticles (cs-Pt3Co). By combining atomic resolution electron microscopy, X-ray photoelectron spectroscopy, extensive first-principles calculations, and many other characterization techniques, we conclude that the cs-Pt3Co nanoparticles are composed of a complete or nearly complete Pt monolayer skin, followed by a secondary shell containing 5-6 layers with similar to 78 at% of Pt, in a Pt3Co configuration, and finally a Co-rich core with 64 at% of Pt. Only this particular structure is consistent with the very high electrocatalytic activity of cs-Pt3Co nanoparticles for ORR, which is about 6 times higher than commercial 30%-Pt/Vulcan and 5 times more active than non-faceted (spherical) alloy Pt3Co nanoparticles. Our study gives an important insight into the atomistic design and understanding of advanced bimetallic nanoparticles for ORR catalysis and other important industrial catalytic applications.

  • 28.
    Hu, Guangzhi
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nitze, Florian
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Ma, Jingyuan
    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.
    Jia, Xueen
    Shchukarev, Andrey
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Lu, Lu
    Ma, Chuansheng
    Yang, Guang
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Small palladium islands embedded in palladium-tungsten bimetallic nanoparticles form catalytic hotspots for oxygen reduction2014In: Nature Communications, E-ISSN 2041-1723, Vol. 5, p. Article number: 5253-Article in journal (Refereed)
    Abstract [en]

    The sluggish kinetics of the oxygen reduction reaction at the cathode side of proton exchange membrane fuel cells is one major technical challenge for realizing sustainable solutions for the transportation sector. Finding efficient yet cheap electrocatalysts to speed up this reaction therefore motivates researchers all over the world. Here we demonstrate an efficient synthesis of palladium-tungsten bimetallic nanoparticles supported on ordered mesoporous carbon. Despite a very low percentage of noble metal (palladium: tungsten = 1:8), the hybrid catalyst material exhibits a performance equal to commercial 60% platinum/Vulcan for the oxygen reduction process. The high catalytic efficiency is explained by the formation of small palladium islands embedded at the surface of the palladium-tungsten bimetallic nanoparticles, generating catalytic hotspots. The palladium islands are similar to 1 nm in diameter, and contain 10-20 palladium atoms that are segregated at the surface. Our results may provide insight into the formation, stabilization and performance of bimetallic nanoparticles for catalytic reactions.

  • 29.
    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, 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.

  • 30.
    Inaba, Akira
    et al.
    Osaka, Japan.
    Miyazaki, Yuji
    Osaka, Japan.
    Michalowski, Pawel P.
    Warsaw, Poland.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Calorimetric measurements on Li4C60 and Na4C602015In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 142, no 16, article id 164706Article in journal (Refereed)
    Abstract [en]

    We show specific heat data for Na4C60 and Li4C60 in the range 0.4-350 K for samples characterized by Raman spectroscopy and X-ray diffraction. At high temperatures, the two different polymer structures have very similar specific heats both in absolute values and in general trend. The specific heat data are compared with data for undoped polymeric and pristine C60. At high temperatures, a difference in specific heat between the intercalated and undoped C60 polymers of 100 J K−1 mol−1 is observed, in agreement with the Dulong-Petit law. At low temperatures, the specific heat data for Li4C60 and Na4C60 are modified by the stiffening of vibrational and librational molecular motion induced by the polymer bonds. The covalent twin bonds in Li4C60 affect these motions to a somewhat higher degree than the single intermolecular bonds in Na4C60. Below 1 K, the specific heats ofboth materials become linear in temperature, as expected from the effective dimensionality of the structure. The contribution to the total specific heat from the inserted metal ions can be well described by Einstein functions with TE = 386 K for Li4C60 and TE = 120 K for Na4C60, but for both materials we also observe a Schottky-type contribution corresponding to a first approximation to a two-level system with ΔE = 9.3 meV for Li4C60 and 3.1 meV for Na4C60, probably associated with jumps between closely spaced energy levels inside “octahedral-type” ionic sites. Static magnetic fields up to 9 T had very small effects on the specific heat below 10 K.

    Download full text (pdf)
    fulltext
  • 31. Jehova Gonzalez, Viviana
    et al.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics. Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Morelos-Gomez, Aaron
    Lopez-Urias, Florentino
    Terrones, Humberto
    Terrones, Mauricio
    Biotin molecules on nitrogen-doped carbon nanotubes enhance the uniform anchoring and formation of Ag nanoparticles2015In: Carbon, ISSN 0008-6223, E-ISSN 1873-3891, Vol. 88, p. 51-59Article in journal (Refereed)
    Abstract [en]

    An efficient method for anchoring silver nanoparticles (Ag-NPs) on the surface of nitrogendoped multi-walled carbon nanotubes (CNx-MWCNTs) is reported. The process involves the attachment of biotin molecules on the surface of CNx-MWCNTs (both, pristine and acid treated) that act as a reducing agent for AgNO3, thus generating an efficient and homogeneous coating of Ag-NPs (similar to 3 nm in diameter). The reduction of AgNO3 on either pristine CNx-MWCNTs or acid treated CNx-MWCNTs (without biotin) results in Ag-NPs of large diameters and size distribution, in addition to a low anchoring efficiency. We confirmed that the use of biotin substantially improves the Ag-NPs anchoring efficiency, especially on acid treated CNx-MWCNTs. In order to elucidate the mechanism whereby Ag-NPs strongly bind to the surface of CNx-MWCNTs, density functional theory (DFT) calculations were carried out. These revealed the existence of covalent bonds established between one side of the biotin molecule and the CNx-MWCNT surface through oxygen atoms, leaving accessible the exposed sulfur atoms at the other end, which further provided an excellent interaction with the Ag-NPs via S Ag bonds. Finally, we demonstrate that these Ag-NPs coated CNx-MWCNTs could be used as efficient sensors of CS2. (c) 2015 Elsevier Ltd. All rights reserved.

  • 32.
    Kagkoura, Antonia
    et al.
    Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, Athens, Greece.
    Karamoschos, Nikolaos
    Department of Chemistry, University of Ioannina, Ioannina, Greece.
    Perivoliotis, Dimitrios K.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Piñeiro-García, Alexis
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Tasis, Dimitrios
    Department of Chemistry, University of Ioannina, Ioannina, Greece; Foundation of Research and Technology, Hellas – Institute of Chemical Engineering Sciences, FORTH/ICEHT, Patras, Greece; Institute of Materials Science and Computing, University Research Center of Ioannina (URCI), Ioannina, Greece.
    Tagmatarchis, Nikos
    Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, Athens, Greece.
    Bifunctional nanostructured palladium/MoSx electrocatalyst for cathode hydrogen evolution reaction PEM water electrolysis and oxygen reduction reaction2023In: Advanced Sustainable Systems, E-ISSN 2366-7486, Vol. 7, no 5, article id 2200518Article in journal (Refereed)
    Abstract [en]

    The creation of effective Pd-based architectures with numerous electrocatalytic active sites and efficient charge transfer is of key importance for improving the electrocatalytic performance in water electrolyzer and fuel cell applications. On the other hand, MoS2, possessing multiple electrocatalytic active sites, can act both as support and booster to Pd-based electrocatalytic structures. Herein, MoSx@Pd hybrids were successfully synthesized by using a one-pot liquid phase solvothermal strategy with stoichiometric excess of Pd. The optimized MoSx@Pd proves to be an excellent bifunctional electrocatalyst for both hydrogen evolution reaction and oxygen reduction reaction (ORR). Optimized MoSx@Pd operates the process for hydrogen evolution at the same potential as Pt/C and achieves a low overpotential of 76 mV at −10 mA cm−2 due to improved reaction kinetics and charge transfer processes between Pd and MoS2. On top of that, MoSx@Pd exhibits excellent performance and stability as cathode electrocatalyst in a polymer electrolyte membrane water electrolyzer. Simultaneously, the bifunctional electrocatalyst shows enhanced electrocatalytic ORR activity and stability by maintaining 93% of its initial activity outperforming commercial Pt/C. Finally, rotating ring disk electrode analysis reveals that ORR proceeds through the energy efficient 4e− pathway, with water being the main product, rendering MoSx@Pd a promising component for fuel cells.

  • 33. Kukkola, Jarmo
    et al.
    Rautio, Aatto
    Sala, Giovanni
    Pino, Flavio
    Tóth, Géza
    Leino, Anne-Riikka
    Mäklin, Jani
    Jantunen, Heli
    Uusimäki, Antti
    Kordás, Krisztián
    Gracia-Espino, Eduardo
    Terrones, Mauricio
    Shchukarev, Andrey
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Mikkola, Jyri-Pekka
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Electrical transport through single-wall carbon nanotube-anodic aluminum oxide-aluminum heterostructures2010In: Nanotechnology, ISSN 0957-4484, E-ISSN 1361-6528, Vol. 21, no 3, p. 35707-Article in journal (Refereed)
    Abstract [en]

    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.

  • 34.
    Kwong, Wai Ling
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry. Kemiskt Biologiskt Centrum (KBC) ; Department of Chemistry-Ångström Laboratory Molecular Biomimetics, Uppsala University.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Lee, Cheng Choo
    Umeå University, Faculty of Science and Technology, Department of Plant Physiology.
    Sandström, Robin
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Messinger, Johannes
    Umeå University, Faculty of Science and Technology, Department of Chemistry. Kemiskt Biologiskt Centrum (KBC) ; Department of Chemistry-Ångström Laboratory Molecular Biomimetics, Uppsala University.
    Cationic Vacancy Defects in Iron Phosphide: A Promising Route toward Efficient and Stable Hydrogen Evolution by Electrochemical Water Splitting2017In: ChemSusChem, ISSN 1864-5631, E-ISSN 1864-564X, Vol. 10, no 22, p. 4544-4551Article in journal (Refereed)
    Abstract [en]

    Engineering the electronic properties of transition metal phosphides has shown great effectiveness in improving their intrinsic catalytic activity for the hydrogen evolution reaction (HER) in water splitting applications. Herein, we report for the first time, the creation of Fe vacancies as an approach to modulate the electronic structure of iron phosphide (FeP). The Fe vacancies were produced by chemical leaching of Mg that was introduced into FeP as "sacrificial dopant". The obtained Fe-vacancy-rich FeP nanoparticulate films, which were deposited on Ti foil, show excellent HER activity compared to pristine FeP and Mg-doped FeP, achieving a current density of 10 mAcm(-2) at overpotentials of 108 mV in 1 m KOH and 65 mV in 0.5 m H2SO4, with a near-100% Faradaic efficiency. Our theoretical and experimental analyses reveal that the improved HER activity originates from the presence of Fe vacancies, which lead to a synergistic modulation of the structural and electronic properties that result in a near-optimal hydrogen adsorption free energy and enhanced proton trapping. The success in catalytic improvement through the introduction of cationic vacancy defects has not only demonstrated the potential of Fe-vacancy-rich FeP as highly efficient, earth abundant HER catalyst, but also opens up an exciting pathway for activating other promising catalysts for electrochemical water splitting.

    Download full text (pdf)
    fulltext
  • 35.
    Liu, Yong-feng
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Tang, Shi
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Fan, Junpeng
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Yang, Jinpeng
    College of Physical Science and Technology, Yangzhou University, Jiangsu 225009, China.
    Liu, Xianjie
    Kera, Satoshi
    Department of Photo-Molecular Science, Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan; .
    Fahlman, Mats
    Laboratory for Organic Electronics, ITN, Linköping University, Norrköping SE-60174, Sweden.
    Larsen, Christian
    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.
    Wang, Jia
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Highly Soluble CsPbBr3 Perovskite Quantum Dots for Solution-Processed Light-Emission Devices2021In: ACS Applied Nano Materials, E-ISSN 2574-0970, p. 1162-1174Article in journal (Refereed)
    Abstract [en]

    We report on the synthesis of CsPbBr3 perovskite quantum dots (PeQDs) with a high solubility of 75 g/L in toluene and a good film-forming property, as enabled by a dense layer of didodecyldimethylammonium bromide and octanoic acid surface ligands. The crystalline and monodisperse PeQDs feature a cubic-like shape, with an edge length of 10.1 nm, and a high photoluminescence quantum yield of greater than 90% in toluene solution and 36% as a thin film. We find that the PeQDs are n-type doped following the synthesis but also that they can be p-type and additionally n-type doped by in situ electrochemistry. These combined properties render the PeQDs interesting for the emitter in solution-processed light-emitting electrochemical cells (LECs), and we report a PeQD-LEC with air-stabile electrodes that emits with a narrow emission spectrum (λpeak = 514 nm, full width at half-maximum = 24 nm) and a luminance of 250 cd/m2 at 4 V and a luminance of 1090 cd/m2 at 6.8 V. To reach this performance, it was critical to include a thin solution-processed layer comprising p-type poly(vinyl carbazole) and a tetrahexylammonium tetrafluoroborate ionic liquid between the PeQD emission layer and the anode in order to compensate for the as-synthesized n-type doping of the PeQDs.

    Download full text (pdf)
    fulltext
  • 36.
    Liu, Yong-feng
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics. College of Physical Science and Technology, Yangzhou University, Yangzhou, China.
    Tang, Shi
    Umeå University, Faculty of Science and Technology, Department of Physics. LunaLEC AB, Umeå, Sweden.
    Wu, Xiuyu
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Boulanger, Nicolas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gracia-Espino, Eduardo
    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. LunaLEC AB, Umeå, Sweden.
    Wang, Jia
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Carbon nanodots: a metal-free, easy-to-synthesize, and benign emitter for light-emitting electrochemical cells2022In: Nano Reseach, ISSN 1998-0124, E-ISSN 1998-0000, Vol. 15, no 6, p. 5610-5618Article in journal (Refereed)
    Abstract [en]

    Light-emitting electrochemical cells (LECs) can be fabricated with cost-efficient printing and coating methods, but a current drawback is that the LEC emitter is commonly either a rare-metal complex or an expensive-to-synthesize conjugated polymer. Here, we address this issue through the pioneering employment of metal-free and facile-to-synthesize carbon nanodots (CNDs) as the emitter in functional LEC devices. Circular-shaped (average diameter = 4.4 nm) and hydrophilic CNDs, which exhibit narrow cyan photoluminescence (peak = 485 nm, full width at half maximum = 30 nm) with a high quantum yield of 77% in dilute ethanol solution, were synthesized with a catalyst-free, one-step solvothermal process using low-cost and benign phloroglucinol as the sole starting material. The propensity of the planar CNDs to form emission-quenching aggregates in the solid state was inhibited by the inclusion of a compatible 2,7-bis(diphenylphosphoryl)-9,9′-spirobifluorene host compound, and we demonstrate that such pristine host-guest CND-LECs turn on to a peak luminance of 118 cd·m−2 within 5 s during constant current-density driving at 77 mA·cm−2.

  • 37.
    Miranda la Hera, Vladimir
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Mena Gómez, Josué
    Umeå University, Faculty of Science and Technology, Department of Physics. Departament de Química Física i Inorganica, Universitat Rovira i Virgili, Tarragona, Spain.
    Canto-Aguilar, Esdras
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Barzegar, Hamid Reza
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Carvajal, Joan J.
    Departament de Química Física i Inorganica, Universitat Rovira i Virgili, Tarragona, Spain.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Electronic properties of hexagonal v-shaped gallium nitride pits2023In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 127, no 51, p. 24658-24665Article in journal (Refereed)
    Abstract [en]

    In this work, the morphology, surface composition, and electronic properties of porous GaN films containing hexagonal V-shaped pits were studied. The V-pits are orientated along the [0001] direction of GaN, and we observed a clear relation between the growth time with the surface composition, film thickness, and pit morphology, which in turn had a significant impact on the band gap, valence band maximum, and the work function. The effect on the position of the valence band maximum and work function is explained by the formation of superficial oxygen-rich phases such as Ga2O3 and nonstoichiometric GaNxOy as supported by X-ray photoelectron spectroscopy and density functional theory (DFT). We further show a change in the optical band gap with the thickness of the porous films explained by a change in the tensile strain caused by open-core screw dislocations that gives rise to the formation of V-pits. The correlation between strain and the band gap is supported by DFT calculations. Our study provides insights into the intricate relation between surface states and electronic properties of semiconducting materials and offers directions for designing GaN heterojunctions with specific optical and electronic properties.

    Download full text (pdf)
    fulltext
  • 38.
    Miranda la Hera, Vladimir
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wu, Xiuyu
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Mena, Josué
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Barzegar, Hamid Reza
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ashok, Anumol
    Department of Materials and Environmental Chemistry, Stockholm University, Stockholm, Sweden.
    Koroidov, Sergey
    Department of Physics, Stockholm University, Stockholm, Sweden.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Controlled synthesis of tellurium nanowires by physical vapor deposition2022In: Nanomaterials, E-ISSN 2079-4991, Vol. 12, no 23, article id 4137Article in journal (Refereed)
    Abstract [en]

    One-dimensional tellurium nanostructures can exhibit distinct electronic properties from those seen in bulk Te. The electronic properties of nanostructured Te are highly dependent on their morphology, and thus controlled synthesis processes are required. Here, highly crystalline tellurium nanowires were produced via physical vapour deposition. We used growth temperature, heating rate, flow of the carrier gas, and growth time to control the degree of supersaturation in the region where Te nanostructures are grown. The latter leads to a control in the nucleation and morphology of Te nanostructures. We observed that Te nanowires grow via the vapour–solid mechanism where a Te particle acts as a seed. Transmission electron microscopy (TEM) and electron diffraction studies revealed that Te nanowires have a trigonal crystal structure and grow along the (0001) direction. Their diameter can be tuned from 26 to 200 nm with lengths from 8.5 to 22 μm, where the highest aspect ratio of 327 was obtained for wires measuring 26 nm in diameter and 8.5 μm in length. We investigated the use of bismuth as an additive to reduce the formation of tellurium oxides, and we discuss the effect of other growth parameters.

    Download full text (pdf)
    fulltext
  • 39.
    Perivoliotis, Dimitrios K.
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ekspong, Joakim
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Zhao, Xue
    College of Chemistry and Engineering, Yunnan Normal University, Kunming, China.
    Hu, Guangzhi
    Umeå University, Faculty of Science and Technology, Department of Physics. Institute for Ecological Research and Pollution Control of Plateau Lakes, School of Ecology and Environmental Science, Yunnan University, Kunming, China.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Recent progress on defect-rich electrocatalysts for hydrogen and oxygen evolution reactions2023In: Nano Today, ISSN 1748-0132, E-ISSN 1878-044X, Vol. 50, article id 101883Article, review/survey (Refereed)
    Abstract [en]

    To meet the demanding requirements for clean energy production, the need to develop advanced electrocatalysts for efficiently catalysing the water splitting reactions attracts a continuously increased attention. However, to meet the anticipated expansion in green hydrogen production from renewable energy sources, the catalysts used for the water splitting reaction not only need to satisfy the required figures of merit but should concurrently be based mainly on abundant, non-critical materials with low environmental impact. In last decades, non-noble metal catalysts, based on transition metals, rare-earth metals, dichalcogenides, and light elements such as phosphorus, nitrogen, and sulphur have shown improved performance. Moreover, in recent years increased interest has been focused on variations of such materials, more specifically on the introduction of defects to further boost their catalytic performance. Through the many studies performed over the last years, it is now possible to summarize, understand and describe the role of these defects for the water splitting reactions, namely the hydrogen and oxygen evolution reactions, and thereby to suggest strategies in the development of next generation electrocatalysts. This is the goal of the current review; we critically summarize the latest progress on the role of introduced defects for catalytic electrolysis applications by scrutinizing the structure–performance correlation as well as the specific catalytic activity. A broad class of nanomaterials is covered, comprising transition metal dichalcogenides, transition metal oxides and carbides, carbon-based materials as well as metal–organic frameworks (MOFs). Finally, the main challenges and future strategies and perspectives in this rapidly evolving field are provided at the end of the review.

    Download full text (pdf)
    fulltext
  • 40.
    Pinedo-Escobar, José Alfonso
    et al.
    Unidad Académica de Ciencias Químicas, Universidad Autónoma de Zacatecas, Campus Universitario Siglo XXI, km. 6 Carr. Zacatecas-Guadalajara s/n Ejido La Escondida, Zacatecas, Mexico.
    Fan, Junpeng
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Moctezuma, Edgar
    Facultad de Ciencias Químicas, Universidad Autnoma de San Luis Potosí, Av. Manuel Nava #6, San Luis Potosí, Mexico.
    Gomez-Solís, Christian
    División de Ciencias e Ingenieŕa, Universidad de Guanajuato, León, Guanajuato, Mexico.
    Carrillo Martinez, Cristina Jared
    Unidad Académica de Ciencias Químicas, Universidad Autónoma de Zacatecas, Campus Universitario Siglo XXI, km. 6 Carr. Zacatecas-Guadalajara s/n Ejido La Escondida, Zacatecas, Mexico.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nanoparticulate Double-Heterojunction Photocatalysts Comprising TiO2(Anatase)/WO3/TiO2(Rutile) with Enhanced Photocatalytic Activity toward the Degradation of Methyl Orange under Near-Ultraviolet and Visible Light2021In: ACS Omega, E-ISSN 2470-1343, Vol. 6, no 18, p. 11840-11848Article in journal (Refereed)
    Abstract [en]

    Nanoparticulate double-heterojunction photocatalysts comprising TiO2(Anatase)/WO3/TiO2(Rutile) were produced by a sol–gel method. The resulting photocatalysts exhibit clear synergistic effects when tested toward the degradation of methyl orange under both UV and visible light. Kinetic studies indicate that the degradation rate on the best double-heterojunction photocatalyst (10 wt % WO3-TiO2) depends mainly on the amount of dye concentration, contrary to pure oxides in which the degradation rate is limited by diffusion-controlled processes. The synergistic effects were confirmed through systematic and careful studies including holes and OH radical formation, X-ray diffraction, electron microscopy, elemental analysis, UV–vis diffuse reflectance spectroscopy, and surface area analysis. Our results indicate that the successful formation of a double heterojunction in the TiO2(Anatase)/WO3/TiO2(Rutile) system leads to enhanced photoactivity when compared to individual oxides and commercial TiO2 P25.

    Download full text (pdf)
    fulltext
  • 41.
    Piñeiro-García, Alexis
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Perivoliotis, Dimitrios K.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wu, Xiuyu
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Benchmarking molybdenum-based materials as cathode electrocatalysts for proton exchange membrane water electrolysis: can these compete with Pt?2023In: ACS Sustainable Chemistry and Engineering, E-ISSN 2168-0485, Vol. 11, no 20, p. 7641-7654Article in journal (Refereed)
    Abstract [en]

    Proton exchange membrane water electrolysis (PEMWE) is a promising technology to produce high-purity renewable hydrogen gas. However, its operation efficiency is highly dependent on the usage of expensive noble metals as electrocatalysts. Replacing, decreasing, or simply extending the operational lifetime of these precious metals have a positive impact on the hydrogen economy. Mo-based electrocatalysts are often praised as potential materials to replace the Pt used at the cathode to catalyse the hydrogen evolution reaction (HER). Most electrocatalytic studies are performed in traditional three-electrode cells with different operational conditions than those seen in PEM systems, making it difficult to predict the expected material’s performance under industrially relevant conditions. Therefore, we investigated the viability of using three selected Mo-based nanomaterials (1T′-MoS2, Co-MoS2, and β-Mo2C) as HER electrocatalysts in PEMWE systems. We investigated the effects of replacing Pt on the catalyst loading, charge transfer resistance, kinetics, operational stability, and hydrogen production efficiency during the PEMWE operation. In addition, we developed a methodology to identify the individual contribution of the anode and cathode kinetics in a PEMWE system, allowing to detect the cause behind the performance drop when using Mo-based electrocatalysts. Our results indicate that the electrochemical performance in three-electrode cells might not strictly predict the performance that could be achieved in PEMWE cells due to differences in interfaces and porosity of the macroscopic catalyst layers. Among the catalysts studied, 1T′-MoS2 is truly an excellent candidate to replace Pt as an HER electrocatalyst due to its low overpotential, low charge transfer resistance, and excellent durability, reaching a high efficiency of ∼75% at 1 A cm-2 and 1.94 V. Our study highlights the importance of a continuous development of efficient noble-metal free HER electrocatalysts suitable for PEMWE systems.

    Download full text (pdf)
    fulltext
  • 42.
    Piñeiro-García, Alexis
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wu, Xiuyu
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rafei, Mouna
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Mörk, Paul Jonathan
    Umeå University, Faculty of Science and Technology, Department of Physics. Faculty of physics and astronomy, Julius-Maximilians-Universität Würzburg, Würzburg, Germany.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    A Quaternary mixed oxide protective scaffold for ruthenium during oxygen evolution reaction in acidic media2023In: Communications Engineering, E-ISSN 2731-3395, Vol. 2, no 1, article id 28Article in journal (Refereed)
    Abstract [en]

    Proton exchange membrane water electrolysis is widely used in hydrogen production, but its application is limited by significant electrocatalyst dissolution at the anode during the oxygen evolution reaction (OER). The best performing electrocatalysts to date are based on ruthenium and iridium oxides, but these experience degradation even at moderate cell potentials. Here we investigate a quaternary Sn-Sb-Mo-W mixed oxide as a protective scaffold for ruthenium oxide. The acid-stable mixed oxide consists of an interconnected network of nanostructured oxides capable of stabilizing ruthenium into the matrix (Ru-MO). In combination with titanium fibre felt, we observed a lower degradation in the oxygen evolution reaction activity compared to unprotected ruthenium oxide after the electrochemical stress test. The superior stability of Ru-MO@Ti is attributed to the presence of MO which hinders the formation of reactive higher valence ruthenium (Ru+8). Our work demonstrates the potential of multi-metal oxides to extend the lifetime of the OER active metal and the titanium support.

    Download full text (pdf)
    fulltext
  • 43.
    Rafei, Mouna
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Miranda la Hera, Vladimir
    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 Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Study on the electronic and structural properties of copper oxide filmsManuscript (preprint) (Other academic)
  • 44.
    Rafei, Mouna
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Miranda la Hera, Vladimir
    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 Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Study on the electronic and structural properties of oxidized copper films2022In: AIP Advances, E-ISSN 2158-3226, Vol. 12, no 10, article id 105203Article in journal (Refereed)
    Abstract [en]

    A detailed study of the oxidation of Cu substrates was carried out under controlled conditions by regulating the pressure, atmosphere composition, process time, and temperature. By tuning the synthesis conditions, the formation of cuprous oxide (Cu2O) or cupric oxide (CuO) could be preferentially promoted. The oxidation temperature was varied from 400 to 1050 °C, and a gradual oxidation of metallic Cu to Cu2O was achieved at mild oxidation conditions (400-600 °C), while the formation of CuO was only observed at higher temperatures (≥900 °C). The surface morphology was also affected changing from a highly granular texture (400 °C) with grain sizes between 0.59 ± 0.15 μm to smooth large crystallites (≥900 °C) with a size within 2.76 ± 0.97 μm. We also show that by controlling the oxidation temperature (400-1050 °C), it is possible to tune the work function and the ionization potential of the resulting Cu2O/CuO film, properties that are important for various optoelectronic applications.

    Download full text (pdf)
    fulltext
  • 45.
    Rafei, Mouna
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Piñeiro-García, Alexis
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wu, Xiuyu
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Perivoliotis, Dimitrios K.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hydrogen evolution mediated by sulfur vacancies and substitutional Mn in few-layered molybdenum disulfide2024In: Materials Today Energy, ISSN 2468-6069, Vol. 41, article id 101524Article in journal (Refereed)
    Abstract [en]

    MoS2 is widely praised as a promising replacement for Pt as an electrocatalyst for the hydrogen evolution reaction (HER), but even today, it still suffers from low performance. This issue is tackled by using Mn3+ as a surface modifier to trigger sulfur vacancy formation and enhance electron transport in few-layered 2H MoS2. Only 10% of Mn is sufficient to transform the semiconductive MoS2 into an active HER electrocatalyst. The insertion of Mn reduces both HER onset potential and Tafel slope which allows reaching 100 mA/cm2 at an overpotential of 206 mV, ten times larger of what undoped MoS2 can achieve. The enhanced activity arises because Mn3+ introduces electronic states near the conduction band, promotes sulfur vacancies, and increases the hydrogen adsorption. In addition to its facile production and extended shelf-life, Mn–MoS2 exhibits an efficiency of 73% at 800 mA/cm2 and 2.0 V when used in proton exchange membrane water electrolyzers.

    Download full text (pdf)
    fulltext
  • 46.
    Rafei, Mouna
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wu, Xiuyu
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Piñeiro-García, Alexis
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Miranda la Hera, Vladimir
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Non-stoichiometric NiFeMo solid solutions; tuning the hydrogen adsorption energy via molybdenum incorporation2022In: Advanced Materials Interfaces, ISSN 2196-7350, Vol. 9, no 34, article id 2201214Article in journal (Refereed)
    Abstract [en]

    Solution precursor plasma spraying is used to produce catalytic trimetallic coatings containing Ni, Fe and Mo directly onto stainless-steel mesh, Ni foam and carbon paper. The resulting material is mostly comprised of face centered cubic FeNi3 alloy forming a highly porous coating with nanostructured features. The addition of Mo (up to ≈14 at%) generates no new crystal phases but only an increase in the lattice parameter, indicating the formation of FeNi3Mox solid solutions. The FeNi3Mox solid solutions are used as electrocatalyst for the hydrogen evolution reaction (HER) in alkaline media. The addition of Mo increases the HER activity significantly reaching an optimum performance at ≈9 at% Mo (FeNi3Mo0.40) with an overpotential at −10 mA cm−2 of 112 mV and a Tafel slope of 109 mV dec−1. The enhanced HER activity is attributed to the formation of a FeNi3Mox solid solution with an increased work function that is correlated to smaller hydrogen adsorption energies. Theoretical activity maps reveal that sites near superficial Mo atoms forms catalytic hot spots and are responsible for the observed activity.

    Download full text (pdf)
    fulltext
  • 47.
    Ràfols-Ribé, Joan
    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.
    Jenatsch, Sandra
    Lundberg, Petter
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sandström, Andreas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Tang, Shi
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Larsen, Christian
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Elucidating Deviating Temperature Behavior of Organic Light-Emitting Diodes and Light-Emitting Electrochemical Cells2021In: Advanced Optical Materials, ISSN 2162-7568, E-ISSN 2195-1071, Vol. 9, no 1, article id 2001405Article in journal (Refereed)
    Abstract [en]

    Organic light-emitting diodes (OLEDs) and light-emitting electrochemical cells (LECs) exhibit different operational modes that render them attractive for complementary applications, but their dependency on the device temperature has not been systematically compared. Here, the effects of a carefully controlled device temperature on the performance of OLEDs and LECs based on two common emissive organic semiconductors are investigated. It is found that the peak luminance and current efficacy of the two OLEDs are relatively temperature independent, whereas, the corresponding LECs exhibit a significant increase by approximate to 85% when the temperature is changed from 20 to 80 degrees C. A combination of simulations and measurements reveal that this deviating behavior is consistent with a shift of the emission zone from closer to the transparent anode toward the center of the active material for both the OLEDs and the LECs, which in turn can be induced by a stronger positive temperature dependence of the mobility of the holes than the electrons.

    Download full text (pdf)
    fulltext
  • 48.
    Ràfols-Ribé, Joan
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Zhang, Xiaoying
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Larsen, Christian
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Lundberg, Petter
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Lindh, E. Mattias
    RISE Energy Technology Center AB, Piteå, Sweden.
    Mai, Cuc Thu
    Department of Chemistry − Ångström Laboratory, Uppsala University, Uppsala, Sweden.
    Mindemark, Jonas
    Department of Chemistry − Ångström Laboratory, Uppsala University, Uppsala, Sweden.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Controlling the emission zone by additives for improved light‐emitting electrochemical cells2022In: Advanced Materials, ISSN 0935-9648, E-ISSN 1521-4095, Vol. 34, no 8, article id 2107849Article in journal (Refereed)
    Abstract [en]

    The position of the emission zone (EZ) in the active material of a light-emitting electrochemical cell (LEC) has a profound influence on its performance because of microcavity effects and doping- and electrode-induced quenching. Previous attempts of EZ control have focused on the two principal constituents in the active material—the organic semiconductor (OSC) and the mobile ions—but this study demonstrates that it is possible to effectively control the EZ position through the inclusion of an appropriate additive into the active material. More specifically, it is shown that a mere modification of the end group on an added neutral compound, which also functions as an ion transporter, results in a shifted EZ from close to the anode to the center of the active material, which translates into a 60% improvement of the power efficiency. This particular finding is rationalized by a lowering of the effective electron mobility of the OSC through specific additive: OSC interactions, but the more important generic conclusion is that it is possible to control the EZ position, and thereby the LEC performance, by the straightforward inclusion of an easily tuned additive in the active material.

    Download full text (pdf)
    fulltext
  • 49.
    Sandström, Robin
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ekspong, Joakim
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Oxidatively Induced Exposure of Active Surface Area during Microwave Assisted Formation of Pt3Co Nanoparticles for Oxygen Reduction Reaction2019In: RSC Advances, E-ISSN 2046-2069, Vol. 9, no 31, p. 17979-17987Article in journal (Refereed)
    Abstract [en]

    The oxygen reduction reaction (ORR), the rate-limiting reaction in proton exchange membrane fuel cells, can efficiently be facilitated by properly manufactured platinum catalysts alloyed with late 3d transition metals. Herein we synthesize a platinum:cobalt nanoparticulate catalyst with a 3:1 atomic ratio by reduction of a dry organometallic precursor blend within a commercial household microwave oven. The formed nanoparticles are simultaneously anchored to a carbon black support that enables large Pt surface area. Two separate microwave treatment steps were employed, where step one constitutes a fast oxidative treatment for revealing active surface area while a reductive secondary annealing treatment promotes a Pt rich surface. The resulting Pt3Co/C catalyst (~3.4 nm) demonstrate an enhanced ORR activity directly attributed to incorporated Co with a specific and mass activity of 704 μA cm-2Pt and 352 A g-1Pt corresponding to an increase by 279 % and 66 % respectively compared to a commercial Pt/C (~1.8 nm) catalyst measured under identical conditions. The method´s simplicity, scalability and novelty is expected to further assist in Pt-Co development and bring the catalyst one step closer toward commercialization and utility in fuel cells.

    Download full text (pdf)
    fulltext
  • 50.
    Sandström, Robin
    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.
    Annamalai, Alagappan
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Persson, Per
    Linköping University.
    Persson, Ingemar
    Linköping University.
    Ekspong, Joakim
    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.
    Microwave-Induced Structural Ordering of Resilient Nanostructured L10-FePt Catalysts for Oxygen Reduction Reaction2020In: ACS Applied Energy Materials, E-ISSN 2574-0962, Vol. 3, no 10, p. 9785-9791Article in journal (Refereed)
    Abstract [en]

    We show how structurally ordered L10 face-centered tetragonal (fct) FePt nanoparticles are produced by a solid-state microwave-assisted synthesis method. The structural phase as well as the incorporated Fe into the nanoparticles is confirmed by X-ray diffraction and high resolution high-angle annular dark field scanning transmission electron microscopy experiments. The prepared particles exhibit a remarkable resilience toward crystallite growth at high temperatures. Directly correlated to the L10 phase, the best oxygen reduction reaction (ORR) characteristics are achieved for particles with a 1:1 Fe:Pt atomic ratio and an average size of ~2.9 nm where Pt-specific evaluation provided a high mass and specific activity of ~570 A/gPt and ~600 μA/cm2Pt respectively. Our results demonstrate that well-structured catalysts possessing activities vastly exceeding Pt/C (~210 A/gPt & ~250 μA/cm2Pt), can be synthesized through a fast and highly eco-friendly method. We note that the achieved mass activity represent a significant leap toward the theoretical maximum for fully ordered FePt nanoparticles.

    Download full text (pdf)
    fulltext
12 1 - 50 of 63
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf