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  • 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, ISSN 2045-2322, 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.

  • 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: Abstract 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.

  • 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.
    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. 

  • 9.
    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.

  • 10.
    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.

  • 11.
    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.

  • 12.
    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.

  • 13.
    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, 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.

  • 14.
    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.

  • 15.
    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.

  • 16.
    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, ISSN 2046-2069, 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.

  • 17.
    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.

  • 18.
    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.

  • 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. 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.

  • 20. 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.

  • 21.
    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.

  • 22.
    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, ISSN 2041-1723, 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.

  • 23.
    Hu, Guangzhi
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Nitze, Florian
    Chalmers University of Technology.
    Jia, Xueen
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sharifi, Tiva
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Barzegar, Hamid Reza
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Chemistry. Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Reduction free room temperature synthesis of a durable and efficient Pd/ordered mesoporous carbon composite electrocatalyst for alkaline direct alcohols fuel cell2014In: RSC Advances, ISSN 2046-2069, E-ISSN 2046-2069, Vol. 4, no 2, p. 676-682Article in journal (Refereed)
    Abstract [en]

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

  • 24.
    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.

  • 25. 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.

  • 26. 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.

  • 27.
    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.

  • 28.
    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, ISSN 2046-2069, 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.

  • 29.
    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 ReactionManuscript (preprint) (Other academic)
    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.

  • 30.
    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.
    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, China.
    Shchukarev, Andrey
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Ma, Jingyuan
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Yttria stabilized and surface activated platinum (PtxYOy) nanoparticles through rapid microwave assisted synthesis for oxygen reduction reaction2018In: Nano Energy, ISSN 2211-2855, E-ISSN 2211-3282, Vol. 46, p. 141-149Article in journal (Refereed)
    Abstract [en]

    The enhancement of platinum (Pt) based catalysts for the oxygen reduction reaction (ORR) by addition of rare earth metals represents a promising strategy to achieve high activity yet low content of the precious metal and concurrently addresses stability issues experienced by traditional late transition metal doping. Improvement in Pt utilization is essential for vehicular applications where material cost and abundancy is a great concern. Here we report a fast and efficient production route of yttria-stabilized platinum nanoparticles (PtxYOy) using a conventional household microwave oven. ORR performance showed a significant improvement and an optimum activity at a 3:1 Pt:Y ratio outperforming that of commercial Pt-Vulcan with a doubled specific activity. Incorporation of Y is evidenced by extended X-ray absorption fine structure and energy dispersive X-ray analysis, while significant amounts of integrated Y2O3 species are detected by X-ray photoelectron spectroscopy. Density functional theory calculations suggest surface migration and oxidation of Y, forming stable superficial yttrium oxide species with low negative enthalpies of formation. The robustness of PtxYOy is shown experimentally and through theoretical arguments demonstrating that surface yttria acts as a stabilizing agent and promoter of highly active ORR sites on the remaining Pt surface, surpassing even the Pt3Y alloy configuration.

  • 31.
    Sarkar, Anjana
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    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.
    Shchukarev, Andrey
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Mohl, Melinda
    Rautio, Anne-Riikka
    Pitkänen, Olli
    Sharifi, Tiva
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Kordas, Krisztian
    Mikkola, Jyri-Pekka
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Photocatalytic reduction of CO2 with H2O over modified TiO2 nanofibers: Understanding the reduction pathway2016In: Nano Reseach, ISSN 1998-0124, E-ISSN 1998-0000, p. 1-13Article in journal (Refereed)
    Abstract [en]

    Nanosized metal (Pt or Pd)-decorated TiO2 nanofibers (NFs) were synthesized by a wet impregnation method. CdSe quantum dots (QDs) were then anchored onto the metal-decorated TiO2 NFs. The photocatalytic performance of these catalysts was tested for activation and reduction of CO2 under UV-B light. Gas chromatographic analysis indicated the formation of methanol, formic acid, and methyl formate as the primary products. In the absence of CdSe QDs, Pd-decorated TiO2 NFs were found to exhibit enhanced performance compared to Pt-decorated TiO2 NFs for methanol production. However, in the presence of CdSe, Pt-decorated TiO2 NFs exhibited higher selectivity for methanol, typically producing ∼90 ppmg−1·h−1 methanol. The CO2 photoreduction mechanism is proposed to take place via a hydrogenation pathway from first principles calculations, which complement the experimental observations.

  • 32.
    Sharifi, Tiva
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Chen, Anran
    Hu, Guangzhi
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Oxygen Reduction Reactions on Single- or Few-Atom Discrete Active Sites for Heterogeneous Catalysis2019In: Advanced Energy Materials, ISSN 1614-6832, article id 1902084Article in journal (Refereed)
    Abstract [en]

    The oxygen reduction reaction (ORR) is of great importance in energy-converting processes such as fuel cells and in metal-air batteries and is vital to facilitate the transition toward a nonfossil dependent society. The ORR has been associated with expensive noble metal catalysts that facilitate the O-2 adsorption, dissociation, and subsequent electron transfer. Single- or few-atom motifs based on earth-abundant transition metals, such as Fe, Co, and Mo, combined with nonmetallic elements, such as P, S, and N, embedded in a carbon-based matrix represent one of the most promising alternatives. Often these are referred to as single atom catalysts; however, the coordination number of the metal atom as well as the type and nearest neighbor configuration has a strong influence on the function of the active sites, and a more adequate term to describe them is metal-coordinated motifs. Despite intense research, their function and catalytic mechanism still puzzle researchers. They are not molecular systems with discrete energy states; neither can they fully be described by theories that are adapted for heterogeneous bulk catalysts. Here, recent results on single- and few-atom electrocatalyst motifs are reviewed with an emphasis on reports discussing the function and the mechanism of the active sites.

  • 33.
    Sharifi, Tiva
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Jia, Xueen
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sandström, Robin
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Comprehensive study of an earth-abundant bifunctional 3D electrode for efficient water electrolysis in alkaline medium2015In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 7, no 51, p. 28148-28155Article in journal (Refereed)
    Abstract [en]

    We report efficient electrolysis of both water splitting half reactions in the same medium by a bifunctional 3D electrode comprising Co3O4 nanospheres nucleated on the surface of nitrogen-doped carbon nanotubes (NCNTs) that in turn are grown on conductive carbon paper (CP). The resulting electrode exhibits high stability and large electrochemical activity for both oxygen and hydrogen evolution reactions (OER and HER). We obtain a current density of 10 mA/cm(2) in 0.1 M KOH solution at overpotentials of only 0.47 and 0.38 V for OER and HER, respectively. Additionally, the experimental observations are understood and supported by analyzing the Co3O4:NCNT and NCNT:CP interfaces by ab initio calculations. Both the experimental and the theoretical studies indicate that firm and well-established interfaces along the electrode play a crucial role on the stability and electrochemical activity for both OER and HER.

  • 34.
    Sharifi, Tiva
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Larsen, Christian
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wang, Jia
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Kwong, Wai Ling
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Mercier, Guillaume
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Messinger, Johannes
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Edman, Ludvig
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Toward a Low-Cost Artificial Leaf: Driving Carbon-Based and Bifunctional Catalyst Electrodes with Solution-Processed Perovskite Photovoltaics2016In: Advanced Energy Materials, ISSN 1614-6832, Vol. 6, no 20, p. 1-10, article id 1600738Article in journal (Refereed)
    Abstract [en]

    Molecular hydrogen can be generated renewably by water splitting with an artificial-leaf device, which essentially comprises two electrocatalyst electrodes immersed in water and powered by photovoltaics. Ideally, this device should operate efficiently and be fabricated with cost-efficient means using earth-abundant materials. Here, a lightweight electrocatalyst electrode, comprising large surface-area NiCo2O4 nanorods that are firmly anchored onto a carbon-paper current collector via a dense network of nitrogen-doped carbon nanotubes is presented. This electrocatalyst electrode is bifunctional in that it can efficiently operate as both anode and cathode in the same alkaline solution, as quantified by a delivered current density of 10 mA cm(-2) at an overpotential of 400 mV for each of the oxygen and hydrogen evolution reactions. By driving two such identical electrodes with a solution-processed thin-film perovskite photovoltaic assembly, a wired artificial-leaf device is obtained that features a Faradaic H-2 evolution efficiency of 100%, and a solar-to-hydrogen conversion efficiency of 6.2%. A detailed cost analysis is presented, which implies that the material-payback time of this device is of the order of 100 days.

  • 35. Shen, Hangjia
    et al.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ma, Jingyuan
    Tang, Haodong
    Mamat, Xamxikamar
    Wagberg, Thomas
    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.
    Guo, Shaojun
    Atomically FeN2 moieties dispersed on mesoporous carbon: A new atomic catalyst for efficient oxygen reduction catalysis2017In: Nano Energy, ISSN 2211-2855, E-ISSN 2211-3282, Vol. 35, p. 9-16Article in journal (Refereed)
    Abstract [en]

    Earth-abundant materials with Fe-N-C centers have been identified as promising catalysts for oxygen reduction reaction (ORR), but these alternatives for Pt catalysts are usually the porphyrin-like FeN4 configuration. The density functional theory (DFT) calculations reveal that FeN2 outperforms FeN4 due to its lower interaction with *O-2 and *OH intermediates and enhanced electron transport, however, achieving an optimum design of these earth-abundant materials with the enriched FeN2 catalytic centers is still a great challenge. Here, we report an intriguing template casting strategy to introduce a mass of atomically dispersed FeN2 moieties onto the surface of N-doped ordered mesoporous carbon for boosting ORR electrocatalysis. One of unique parts herein is to pre anchor Fe precursor on the surface of template (SBA-15) during catalyst synthesis, preventing Fe from penetrating into the carbon skeleton and facilitating the removal of excessive Fe-based particles during silica elimination by HF etching, resulting in a desirable model structure comprising only highly active atomically dispersed FeN2 sites, as confirmed by high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM), extended X-ray absorption fine structure (EXAFS) and Mossbauer spectroscopy analysis. The well-defined structure prompts us to understand the nature of the catalytic active sites, and to demonstrate that the catalyst activity is linearly proportional to the concentration of FeN2 sites. The obtained atomic electrocatalyst exhibits superior electrocatalytic performance for ORR with a more positive half-wave potential than that of Pt/C catalyst. We further establish a kinetic model to predict the ORR activity of these single-atom dispersed catalysts. The present work elaborates on a profound understanding for designing low-cost, highly efficient FeN2-based electrocatalyst for boosting ORR.

  • 36. Shen, Hangjia
    et al.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Ma, Jingyuan
    Zang, Ketao
    Luo, Jun
    Wang, Le
    Gao, Sanshuang
    Mamat, Xamxikamar
    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.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Guo, Shaojun
    Synergistic Effects between Atomically Dispersed Fe-N-C and C-S-C for the Oxygen Reduction Reaction in Acidic Media2017In: Angewandte Chemie International Edition, ISSN 1433-7851, E-ISSN 1521-3773, Vol. 56, no 44, p. 13800-13804Article in journal (Refereed)
    Abstract [en]

    Various advanced catalysts based on sulfur-doped Fe/N/C materials have recently been designed for the oxygen reduction reaction (ORR); however, the enhanced activity is still controversial and usually attributed to differences in the surface area, improved conductivity, or uncertain synergistic effects. Herein, a sulfur-doped Fe/N/C catalyst (denoted as Fe/SNC) was obtained by a template-sacrificing method. The incorporated sulfur gives a thiophene-like structure (C-S-C), reduces the electron localization around the Fe centers, improves the interaction with oxygenated species, and therefore facilitates the complete 4e(-) ORR in acidic solution. Owing to these synergistic effects, the Fe/SNC catalyst exhibits much better ORR activity than the sulfur-free variant (Fe/NC) in 0.5m H2SO4.

  • 37. Shen, Hangjia
    et al.
    Gracia-Espino, Eduardo
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wang, Le
    Qin, Danfeng
    Gao, Sanshuang
    Mamat, Xamxikamar
    Ren, Wei
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hu, Guangzhi
    Umeå University, Faculty of Science and Technology, Department of Physics. Chinese Acad Sci, Xinjiang Tech Inst Phys & Chem, Urumqi 830011, Peoples R China.
    Microwave-assisted synthesis of multimetal oxygen-evolving catalysts2017In: Electrochemistry communications, ISSN 1388-2481, E-ISSN 1873-1902, Vol. 81, p. 116-119Article in journal (Refereed)
    Abstract [en]

    Oxygen evolution reaction (OER) plays a pivotal role in water-splitting. Here, we report a facile method to synthesize multimetal supported on commercial carbon black via a time-saving microwave process. Crystalline FeNi3 nanoparticles homogeneously doped with Mo are formed via a microwave treatment and activated to metal oxyhydroxide in-situ during cyclic voltammetry test with overpotential of only 280 mV at 10 mA cm(-2) for OER in alkaline electrolyte, outperforming RuO2. Our synthesis methodology is a promising alternative for large-scale production, delivering a valuable contribution to catalyst preparation and electrocatalytic water oxidation research.

  • 38. Subramanian, Arunprabaharan
    et al.
    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. Division of Biotechnology, Safety, Environment and Life Science Institute, College of Environmental and Bioresource Sciences, Chonbuk National University, Iksan 54596, Republic of Korea.
    Lee, Hyun Hwi
    Lee, Su Yong
    Choi, Sun Hee
    Jang, Jum Suk
    Effect of tetravalent dopants on hematite nanostructure for enhanced photoelectrochemical water splitting2018In: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 427, p. 1203-1212Article in journal (Refereed)
    Abstract [en]

    In this paper, the influence of tetravalent dopants such as Si4+, Sn4+, Ti4+, and Zr4+ on the hematite (alpha-Fe2O3) nanostructure for enhanced photoelectrochemical (PEC) water splitting are reported. The tetravalent doping was performed on hydrothermally grown akaganeite (beta-FeOOH) nanorods on FTO (fluorine-doped tin-oxide) substrates via a simple dipping method for which the respective metal-precursor solution was used, followed by a high-temperature (800 degrees C) sintering in a box furnace. The photocurrent density for the pristine (hematite) photoanode is similar to 0.81 mA/cm(2) at 1.23 V-RHE, with an onset potential of 0.72 V-RHE; however, the tetravalent dopants on the hematite nanostructures alter the properties of the pristine photoanode. The Si4+-doped hematite photoanode showed a slight photocurrent increment without a changing of the onset potential of the pristine photoanode. The Sn4+- and Ti4+-doped hematite photoanodes, however, showed an anodic shift of the onset potential with the photocurrent increment at a higher applied potential. Interestingly, the Zr4+-doped hematite photoanode exhibited an onset potential that is similar to those of the pristine and Si4+-doped hematite, but a larger photocurrent density that is similar to those of the Sn4+- and Ti4+-doped photoanodes was recorded. The photoactivity of the doped photoanodes at 1.23 V-RHE follows the order Zr > Sn > Ti > Si. The onset-potential shifts of the doped photoanodes were investigated using the Ab initio calculations that are well correlated with the experimental data. X-ray diffraction (XRD) and scanning-electron microscopy (FESEM) revealed that both the crystalline phase of the hematite and the nanorod morphology were preserved after the doping procedure. X-ray photoelectron spectroscopy (XPS) confirmed the presence of the tetravalent dopants on the hematite nanostructure. The charge-transfer resistance at the various interfaces of the doped photoanodes was studied using impedance spectroscopy. The doping on the hematite photoanodes was confirmed using the Mott-Schottky (MS) analysis. 

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