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  • 1.
    Abou-Hamad, Edy
    et al.
    Universite Montpellier II.
    Kim, Y
    University of Pennsylvania.
    Talyzin, Alexandr
    Umeå University, Faculty of Science and Technology, Physics.
    Goze-Bac, Christophe
    Universite Montpellier II.
    Luzzi, David
    University of Pennsylvania.
    Rubio, Angelo
    University of Basque Country.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Physics.
    Hydrogenation of C-60 in Peapods: Physical Chemistry in Nano Vessels2009In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 113, no 20, p. 8583-8587Article in journal (Refereed)
    Abstract [en]

    Hydrogenation of C-60 molecules inside SWNT was achieved by direct reaction with hydrogen gas at elevated pressure and temperature. Evidence for the C-60 hydrogenation in peapods is provided by isotopic engineering with specific enrichment of encapsulated species and high resolution C-13 and H-1 NMR spectroscopy with the observation of characteristic diamagnetic and paramagnetic shifts of the NMR lines and the appearance of sp(3) carbon resonances. We estimate that approximately 78% of the C-60 molecules inside SWNTs are hydrogenated to an average degree of 14 hydrogen atoms per C-60 molecule. As a consequence, the rotational dynamics of the encapsulated C60Hx molecules is clearly hindered. Our successful hydrogenation experiments open completely new roads to understand and control confined chemical reactions at the nano scale

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

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

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

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

  • 4.
    Boily, Jean-Francois
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Water Structure and Hydrogen Bonding at Goethite/Water Interfaces: Implications for Proton Affinities2012In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 116, no 7, p. 4714-4724Article in journal (Refereed)
    Abstract [en]

    Molecular dynamics simulations of four crystallographic terminations of goethite (alpha-FeOOH) in contact with liquid water (300 K, 1 bar) were performed to resolve interfacial water structures in the vicinity of surface (hydr)oxo groups. Interfacial water molecules adopted highly surface-specific configurations on (010), 100), (110), and (021) planes of goethite. Water molecules generally had weaker hydrogen bond numbers and strengths, as well as smaller self-diffusion coefficients, than their bulk liquid counterparts. Relaxed surface Fe-O and H acceptor distances and populations were used to estimate proton affinity constants of singly-, and triply-coordinated (hydr)oxo groups using the multisite complexation model. These calculations confirmed that singly coordinated groups are mainly responsible for charge uptake under normal environmental conditions. However, revised proton affinity constants showed that protonation of doubly-coordinated hydroxo groups and one type of triply-coordinated oxo group may be favored in the presence of strongly binding negatively charged ligands. These calculations should facilitate elucidation of surface complexation mechanisms on this environmentally important material.

  • 5.
    Boily, Jean-Francois
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Lins, Roberto D
    Pacific Northwest National Laboratory, Richland Washington, USA .
    Electrostatic cooperativity of hydroxyl groups at metal oxide surfaces2009In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 113, no 38, p. 16568-16570Article in journal (Refereed)
    Abstract [en]

    The O−H bond distribution of hydroxyl groups at the {110} goethite (α-FeOOH) surface was investigated by molecular dynamics. This distribution was strongly affected by electrostatic interactions with neighboring oxo and hydroxo groups. The effects of proton surface loading, simulated by emplacing two protons at different distances of separation, were diverse and generated several sets of O−H bond distributions. DFT calculations of a representative molecular cluster were also carried out to demonstrate the impact of these effects on the orientation of oxygen lone pairs in neighboring oxo groups. These effects should have strong repercussions on O−H stretching vibrations of metal oxide surfaces.

  • 6.
    Boily, Jean-Francois
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Shchukarev, Andrey
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    X-ray photoelectron spectroscopy of fast-frozen hematite colloids in aqueous solutions. 2. tracing the relationship between surface charge and electrolyte adsorption2010In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 114, no 6, p. 2613-2616Article in journal (Refereed)
    Abstract [en]

    Colloidal-sized hematite spheroids exposed to aqueous NaCl solutions were investigated by X-ray photoelectron spectroscopy using the fast-frozen technique. The O 1s region provided evidence for (de)protonation reactions of surface (hydr)oxo groups of OH-enriched/O-depleted hematite Surfaces. These results were also correlated to changes in sodium (Na 1s) and chloride (Cl 2p) contents with pH. Electrolyte ion surface loadings were successfully predicted using a classic thermodynamic adsorption model normalized for surface site density. These efforts pointed to ion-specific inner-Helmholtz plane capacitances.

  • 7. Chumakova, Natalia A.
    et al.
    Rebrikova, Anastasya T.
    Talyzin, Aleksandr V.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Paramonov, Nikita A.
    Vorobiev, Andrey Kh
    Korobov, Mikhail, V
    Properties of Graphite Oxide Powders and Membranes as Revealed by Electron Paramagnetic Resonance Spectroscopy2018In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 122, no 39, p. 22750-22759Article in journal (Refereed)
    Abstract [en]

    The spin probe technique was used to study graphite oxide (GO) powders swelled in polar liquids (CH3CN, CH3OH, and H2O) and liquid-free GO membranes (GOM). The nitroxide radicals TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl) and TEMPOL (4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl) readily penetrated into the interplane space of GO from the solution. Electron paramagnetic resonance (EPR) spectra of these radical probes were sensitive to molecular mobility and orientation ordering within the internal space of GO. The radicals embedded in swelled GO were in two states with different rotational mobilities. The small fraction of radicals located in the interplane space of GO and detected in the broad range of temperatures was in the state of fast rotation, similar to the same radicals dissolved in bulk liquids, thus providing experimental evidence of formation of a liquid-like media within the interplane space of GO. Such mobile media may be responsible for the unusual permeation properties of GOM, which is reported in the literature. Second, less-mobile fraction of radicals was found to be immobilized at the internal surface of GO and was sensitive to phase transformations in the swelled GO structures. The transformations were detected as anomalies at temperature dependences of rotational mobility of radicals. The detected dependence of EPR spectra of probe radicals on orientation of GOM, relative to the direction of magnetic field in the EPR spectrometer, was used for quantitative characterization of orientation alignment of GO planes within the membranes. Such an approach may serve as an elegant method to estimate the relative quality of membranes and other GO-layered structures.

  • 8.
    Ding, Xiangbin
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Song, Xiaowei
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Boily, Jean-Francois
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Identification of fluoride and phosphate binding sites at FeOOH surfaces2012In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 116, no 41, p. 21939-21947Article in journal (Refereed)
    Abstract [en]

    Iron oxyhydroxide minerals occur widely in nature and play important roles in environmental and industrial processes. Owing to their high reactivity, these minerals can act as sinks and/or transformation centers for a variety of inorganic and organic ions. Interfacial reactions are often mediated by surface (hydr)oxo groups. These groups can be singly, doubly, or triply coordinated with respect to underlying Fe atoms. In order to investigate the reactivity of these differently coordinated groups, Fourier transform infrared (FTIR) spectroscopy was used to examine adsorption products formed on iron oxyhydroxide surfaces. The absence of water was required to probe the O-H stretching region after initial reactions in aqueous media. This work was specifically focused on synthetic, submicrometer-sized lepidocrocite and goethite particles reacted with aqueous solutions of sodium fluoride and monosodium phosphate. Langmuir-Freundlich adsorption isotherms were calibrated on adsorption data in aqueous media at various pH values to obtain the maximum sorption densities for these ions under these conditions. FTIR measurements of the resulting solids dried under N-2(g) show that fluoride and phosphate ions preferentially exchange with singly coordinated hydroxyls. Doubly coordinated groups can, however, be exchanged with fluoride ions at relatively high loading densities. Triply coordinated groups remain, in contrast, resilient to exchange. They may, however, stabilize phosphate species by hydrogen bonding. These findings add further constraints to our understanding of adsorption reactions and to the formulation of molecularly adequate thermodynamic models.

  • 9. Du, Mingrun
    et al.
    Dong, Jiajun
    Zhang, Ying
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Andersson, Ove
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Liu, Bingbing
    Wang, Mingchao
    Li, Zepeng
    Wei, Tong
    Zhou, Qingjun
    Yang, Xiong
    High pressure and high temperature induced polymerization of C60 solvates: The effect of intercalated aromatic solvents2021In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 125, no 31, p. 17155-17163Article in journal (Refereed)
    Abstract [en]

    The polymerization of three typical aromatic solvent-doped fullerene materials with similar hexagonal closest packed (hcp) structures (mesitylene/C60, m-dichlorobenzene/C60 and m-xylene/C60 solvates) is studied under high pressure and high temperature (HPHT, 1.5 GPa, 573 K and 2 GPa, 700 K, respectively). Raman and photoluminescence spectroscopies reveal that the intercalated aromatic solvents play a crucial role in tailoring the extent of polymerization of C60 molecules. In the solvates, the solvents confine formation of covalent bonds between C60 molecules to the 001 direction and the (001) plane of the hcp lattices, leading to the formation of mixed polymeric phases of monomers, dimers, one-dimensional (1D) chainlike oligomers, and two-dimensional (2D) tetragonal phase polymers under suitable HPHT conditions. The type and number of substituent groups of the aromatic solvents are found to have significant influence, determining the amounts and types of polymeric phases formed. Our studies enrich the understanding of the formation mechanisms for controllably fabricating polymeric fullerenes and facilitate targeted design and synthesis of unique fullerene-based carbon materials.

  • 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.
    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.

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  • 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 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.

  • 13.
    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.

  • 14. Gustafsson, Håkan
    et al.
    Ahrén, Maria
    Söderlind, Fredrik
    Córdoba Gallego, José M
    Käll, Per-Olov
    Nordblad, Per
    Westlund, Per-Olof
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Uvdal, Kajsa
    Engström, Maria
    Magnetic and Electron Spin Relaxation Properties of (GdxY1−x)2O3 (0 ≤ x ≤ 1) Nanoparticles Synthesized by the Combustion Method. Increased Electron Spin Relaxation Times with Increasing Yttrium Content2011In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 115, no 13, p. 5469-5477Article in journal (Refereed)
    Abstract [en]

    The performance of a magnetic resonance imaging contrast agent (CA) depends on several factors, including the relaxation times of the unpaired electrons in the CA. The electron spin relaxation time may be a key factor for the performance of new CAs, such as nanosized Gd2O3 particles. The aim of this work is, therefore, to study changes in the magnetic susceptibility and the electron spin relaxation time of paramagnetic Gd2O3 nanoparticles diluted with increasing amounts of diamagnetic Y2O3. Nanoparticles of (GdxY1−x)2O3 (0 ≤ x ≤ 1) were prepared by the combustion method and thoroughly characterized (by X-ray diffraction, transmission electron microscopy, thermogravimetry coupled with mass spectroscopy, photoelectron spectroscopy, Fourier transform infrared spectroscopy, and magnetic susceptibility measurements). Changes in the electron spin relaxation time were estimated by observations of the signal line width in electron paramagnetic resonance spectroscopy, and it was found that the line width was dependent on the concentration of yttrium, indicating that diamagnetic Y2O3 may increase the electron spin relaxation time of Gd2O3 nanoparticles.

  • 15. Holmboe, Michael
    et al.
    Bourg, Ian C.
    Molecular Dynamics Simulations of Water and Sodium Diffusion in Smectite Interlayer Nanopores as a Function of Pore Size and Temperature2014In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 118, no 2, p. 1001-1013Article in journal (Refereed)
    Abstract [en]

    The diffusion coefficients (D) of water and solutes in nanoporousNa-smectite clay barriers have been widely studied because of their importancein high-level radioactive waste (HLRW) management and in the isolation of contaminated sites. However, few measurements have been carried out at the high temperatures that are expected to occur in HLRW repositories. We address this knowledge gap by using molecular dynamics (MD) simulations to predict the temperature dependence of diffusion in clay interlayer nanopores, expressed as a pore scale activation energy of diffusion (Ea). Our sensitivity analysis shows that accurate prediction of pore scale Dand Eavalues requires careful consideration of the influence of pore size, simulation cell size, and clay structure flexibility on MD simulation results. We find that predicted Dvalues in clay interlayer nanopores are insensitive to the size of the simulation cell (contrary to the behavior observed in simulation of bulk liquid water) but sensitive to the vibrational motions of clay atoms (particularly in the smallest pores investigated here, the one-, two-, and three-layer hydrates). Our predicted DandEavalues are consistent with experimental data. They reveal,for both water and Na+, that Eaincreases by∼6 kJ mol−1with increasing confinement, when going from bulk liquid water to theone-layer hydrate of Na-montmorillonite.

  • 16. Holmboe, Michael
    et al.
    Jonsson, Mats
    Wold, Susanna
    Influence of γ-radiation on the reactivity of Montmorillonite towards H2O22012In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 81:2, s. 190-194, p. 1001-1013Article in journal (Other academic)
    Abstract [en]

    Compacted and water saturated bentonite will be used as an engineered barrier in deep geological repositories for radioactive waste in many countries. Due to the high dose rate of ionizing radiation outside the canisters holding the nuclear waste, radiolysis of the interlayer and pore water in the compacted bentonite is unavoidable. Upon reaction with the oxidizing and reducing species formed by water radiolysis (OH•, e-(aq), H•, H202, H2, H02•, H30+), the overall redox properties in the bentonite barrier may change. In this study the influence of γ-radiation on the structural Fe(II)/Fe(III) content in montmorillonite and its reactivity towards hydrogen peroxide (H2O2) was investigated in parallel experiments. The results show that under anoxic conditions the structural Fe(II)/FeTot ratio of dispersed montmorillonite are increased from ≤ 3 to 25-30% after γ-doses comparable to repository conditions. Furthermore, a strong correlation between the structural Fe(II)/FeTot ratio and the H2O2 decomposition rate in montmorillonite dispersions was found. This correlation was further verified in experiments with consecutive H2O2 additions, since the structural Fe(II)/FeTot ratio was seen to decrease concordantly. This work shows that the structural iron in montmorillonite could be a sink for one of the major oxidants formed upon water radiolysis in the bentonite barrier, H2O2.

  • 17.
    Hubarevich, Aliaksandr
    et al.
    Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy.
    Huang, Jian-An
    Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy.
    Giovannini, Giorgia
    Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy;Empa, Swiss Federal Laboratories for Materials Science and Technology, Lerchenfeldstrasse 5, St. Gallen CH-9014, Switzerland.
    Schirato, Andrea
    Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy;Department of Physics, Politecnico di Milano, Piazza Leonardo da Vinci 32, I-20133 Milan, Italy.
    Zhao, Yingqi
    Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy.
    Maccaferri, Nicolò
    Department of Physics and Materials Science, University of Luxembourg, 162a, avenue de la Faïencerie, L-1511 Luxembourg, Luxembourg.
    De Angelis, Francesco
    Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy.
    Alabastri, Alessandro
    Department of Electrical and Computer Engineering, Rice University, 6100 Main Street MS-378, Houston, Texas 77005, United States.
    Garoli, Denis
    Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy;Faculty of Science and Technology Free University of Bozen, Piazza Università 5, 39100 Bolzano, Italy.
    λ-DNA through Porous Materials—Surface-Enhanced Raman Scattering in a Simple Plasmonic Nanopore2020In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 124, no 41, p. 22663-22670Article in journal (Refereed)
    Abstract [en]

    Engineered electromagnetic fields in plasmonic nanopores enable enhanced optical detection for single-molecule sensing and sequencing. Here, a plasmonic nanopore prepared in a thick nanoporous film is used to investigate, by means of surface-enhanced Raman spectroscopy, the interaction between the metallic surface of the pore and a long-chain double-strand DNA molecule free to diffuse through the pore. We discuss how the matrix of the porous material can interact with the molecule thanks to: (i) transient aspecific interactions between the porous surface and DNA; (ii) diffusion; and (iii) thermal and optical forces exerted by the localized field in a metallic nanostructure on the DNA molecule. An interaction time up to tens of milliseconds enables us to collect high signal-to-noise Raman signatures, allowing an easy label-free reading of information from the DNA molecule. Moreover, to increase the rate of detection, we tested a polymeric porous hydrogel placed beneath the solid-state membrane. The hydrogel enables a slowdown of the molecule diffusion time, thus increasing the number of detected interaction events by a factor 20. The analysis of the observed Raman peaks and their relative intensities, combined with theoretical simulations, allows us to get further information on the process of translocation and the folding state and orientation of the translocating molecule. Our results demonstrate temporary adsorption of the DNA molecule on the porous material during the translocation due to the diffusion force. Finally, we provide a qualitative evaluation of the nucleotides' contents in the different groups of collected signal. The proposed approach can find interesting applications not only in DNA sensing and sequencing but also on generic nanopore spectroscopy.

  • 18. Jiang, Linhai
    et al.
    Yao, Mingguang
    Liu, Bo
    Li, Quanjun
    Liu, Ran
    Lv, Hang
    Lu, Shuangchen
    Gong, Chen
    Zou, Bo
    Cui, Tian
    Liu, Bingbing
    Hu, Guangzhi
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Controlled Synthesis of CeO2/Graphene Nanocomposites with Highly Enhanced Optical and Catalytic Properties2012In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 116, no 21, p. 11741-11745Article in journal (Refereed)
    Abstract [en]

    In this paper, CeO2 nanocubes with the (200)-terminated surface/graphene sheet composites have been prepared successfully by a simple hydrothermal method. It is found that the CeO2 nanocubes with high crystallinity and specific exposed surface are well dispersed on well-exfoliated graphene surface. The (200)-terminated surface/graphene sheet composites modified electrode showed much higher sensitivity and excellent selectivity in its catalytic performance compared to a CeO2 nanoparticle-modified electrode. The photoluminescence intensity of the CeO2 anchored on graphene is about 30 times higher than that of pristine CeO2 crystals in air. The higher oxygen vacancy concentration in CeO2 is supposed to be an important cause for the higher photoluminescence and better electrochemical catalytic performance observed in the (200)-terminated surface/graphene sheet composites. Such ingenious design of supported well-dispersed catalysts in nanostructured ceria catalysts, synthesized in one step with an exposed high-activity surface, is important for technical applications and theoretical investigations.

  • 19.
    Klechikov, Alexey
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sun, Jinhua
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Vorobiev, Alexei
    Talyzin, Aleksandr V.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Swelling of Thin Graphene Oxide Films Studied by in Situ Neutron Reflectivity2018In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 122, no 24, p. 13106-13116Article in journal (Refereed)
    Abstract [en]

    Permeation of multilayered graphene oxide (GO) membranes by polar solvents is known to correlate with their swelling properties and amount of sorbed solvent. However, quantitative estimation of sorption using standard (e.g., gravimetric) methods is technically challenging for few nanometers thick GO membranes/films exposed to solvent vapors. Neutron reflectivity (NR) was used here to evaluate the amount of solvents intercalated into the film which consists of only ∼31.5 layers of GO. Analysis of NR data recorded from the GO film exposed to vapors of polar solvents provides information about change of film thickness due to swelling, amount of intercalated solvent, and selectivity in sorption of solvents from binary mixtures. A quantitative study of GO film sorption was performed for D2O, d-methanol, ethanol, dimethyl sulfoxide (DMSO), acetonitrile, dimethylformamide (DMF), and acetone. Using isotopic contrast, we estimated selectivity in sorption of ethanol/d-methanol mixtures by the GO film. Estimation of sorption selectivity was also performed for D2O/DMF, D2O/DMSO, and D2O/acetonitrile binary mixtures. Sorption of polar solvents was compared for the thin GO film, micrometer thick free standing GO membranes, and graphite oxide powders.

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  • 20.
    Kozin, Philipp A
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Boily, Jean-François
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Proton Binding and Ion Exchange at the Akaganéite/Water Interface2013In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 117, no 12, p. 6409-6419Article in journal (Refereed)
    Abstract [en]

    Proton exchange in nanosized synthetic akaganéite particles suspended in aqueous media and in ionic strengths of 3–100 mM NaCl and NaClO4 was monitored by high precision potentiometry at 25 °C. Proton budgets in the pH 3–10 range pertain to simultaneously occurring surface complexation and bulk ionic exchange reactions. Surface complexation reactions involve proton binding to (hydr)oxo groups of the dominant crystallographic planes of the particles. These are responsible for the colloidal attributes of the akaganéite particles, as confirmed by electrophoretic mobility measurements. Bulk ionic exchange involves the codiffusion of protons and chloride ions through the tunnel structure of the hollandite-type akaganéite bulk. Chloride ions migrate to bulk complexation sites that are ideally defined by eight surrounding hydroxyl groups, ≡(OH)8. Protons are in turn considered to be bound to neighboring oxo groups, ≡O. Collectively, the complexes are referred as [≡(OH)8···Cl······HO≡]. A thermodynamic model accounting for these two processes was developed to predict the pH (3–9), ionic strength (3–100 mM), and ionic medium (NaCl, NaClO4) dependence of the potentiometric data. This model is supported by new zeta potential data pointing to an isoelectric point of 9.6–10.3 for pristine akaganéite particles and by Fourier transform infrared spectra showing the impact of pH and ionic medium on bulk proton-chloride loadings. Our proposed stoichiometry for a chloride-rich solid of β-FeOOH·(HCl)0.192 corresponds to a maximal occupancy of 75% for chloride ions in the [≡(OH)8···Cl······HO≡] bulk complexation sites. Samples equilibrated in pure aqueous solutions should have a composition of β-FeOOH·(HCl)0.151, corresponding to a 60% occupancy for chloride ions due to a partial exchange of HCl. Our model can be used to predict compositional changes in the akaganéite bulk and surfaces upon any variations in pH and ionic media considered in this work.

  • 21.
    Kwong, Wai Ling
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Lee, Cheng Choo
    Umeå University, Faculty of Science and Technology, Department of Plant Physiology.
    Messinger, Johannes
    Umeå University, Faculty of Science and Technology, Department of Chemistry. Department of Chemistry-Ångström Laboratory, Uppsala University, S-75120 Uppsala, Sweden.
    Scalable Two-Step Synthesis of Nickel Iron Phosphide Electrodes for Stable and Efficient Electrocatalytic Hydrogen Evolution2017In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 121, no 1, p. 284-292Article in journal (Refereed)
    Abstract [en]

    The development of efficient, durable, and inexpensive hydrogen evolution electrodes remains a key challenge for realizing a sustainable H-2 fuel production via electrocatalytic water splitting. Herein, nickel-iron phosphide porous films with precisely controlled metal content were synthesized on Ti foil using a simple and scalable two-step strategy of spray-pyrolysis deposition followed by low-temperature phosphidation. The nickel-iron phosphide of an optimized Ni:Fe ratio of 1:4 demonstrated excellent overall catalytic activity for hydrogen evolution reaction (HER) in 0.5 M H2SO4, achieving current densities of -10 and -30 mA cm(-2) at overpoteritials of 101 and 123 mV, respectively, with a Tafel slope of 43 mV dec(-1). Detailed analysis obtained by X-ray diffraction, electron microscopy, electrochemistry, and X-ray photoelectron spectroscopy revealed that the superior overall HER activity of nickel iron phosphide as compared to nickel phosphide and iron phosphide was a combined effect of differences in the morphology (real surface area) and the intrinsic catalytic properties (electronic structure). Together with a long-term stability and a near-100% Faradaic efficiency, the nickel-iron phosphide electrodes produced in this study provide blueprints for large-scale H-2 production.

  • 22.
    Kwong, Wai Ling
    et al.
    Umeå University, Faculty of Science and Technology, Department of Plant Physiology.
    Lee, Cheng Choo
    Umeå University, Faculty of Science and Technology, Department of Plant Physiology.
    Messinger, Johannes
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Transparent Nanoparticulate FeOOH Improves the Performance of a WO3 Photoanode in a Tandem Water-Splitting Device2016In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 120, no 20, p. 10941-10950Article in journal (Refereed)
    Abstract [en]

    Oxygen evolution catalysts (OEC) are often employed on the surface of photoactive, semiconducting photoanodes to boost their kinetics and stability during photoelectrochemical water oxidation. However, the necessity of using optically transparent OEC to avoid parasitic light absorption by the OEC under front-side illumination is often neglected. Here, we show that furnishing the surface of a WO3 photoanode with suitable loading of FeOOH as a transparent OEC improved the photocurrent density by 300% at 1 V versus RHE and the initial photocurrent-to-O-2 Faradaic efficiency from similar to 70 to similar to 100%. The data from the photo-voltammetry, electrochemical impedance, and gas evolution measurements these improvements were a combined result of reduced hole-transfer resistance for water oxidation, minimized surface recombination of charge carriers, and improved stability against photocorrosion of WO3. We demonstrate the utility of transparent FeOOH-coated W(O)3 in a solar-powered, tandem water-splitting device by combining it with a double-junction Si solar cell and a Ni-Mo hydrogen evolution catalyst. This device performed at a solar-to-hydrogen conversion efficiency of 1.8% in near-neutral K2SO4 electrolyte.

  • 23. Liepuoniute, Ieva
    et al.
    Huynh, Chau Minh
    Umeå University, Faculty of Science and Technology, Department of Chemistry. Department of Chemistry and Biochemistry, University of California, Los Angeles, USA.
    Perez-Estrada, Salvador
    Wang, Yangyang
    Khan, Saeed
    Houk, K. N.
    Garcia-Garibay, Miguel A.
    Enhanced Rotation by Ground State Destabilization in Amphidynamic Crystals of a Dipolar 2,3-Difluorophenylene Rotator as Established by Solid State 2H NMR and Dielectric Spectroscopy2020In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 124, no 28, p. 15391-15398Article in journal (Refereed)
    Abstract [en]

    We report the synthesis and rotational dynamics of the pillared metal–organic framework Zn2(F2BDC)2(DABCO) where F2BDC = 2,3-difluorobenzene-1,4-dicarboxylate acts as a rotating dipolar linker and DABCO = 1,4-diazabicyclo[2.2.2]octane acts as a spacer (F2MOF 1). The pillared structure of F2MOF 1 was confirmed by X-ray diffraction and CP-MAS 13C NMR analyses. Using variable temperature solid state 2H NMR and broadband dielectric spectroscopy, we characterized the rotational dynamics of the dipolar F2BDC linker in the solid state. Variable temperature (VT) quadrupolar echo 2H NMR measurements revealed a rotational activation energy of Ea = 6.8 ± 0.1 kcal/mol, which agreed well with temperature- and frequency-dependent dielectric measurements, indicating a barrier of Ea = 7.1 ± 0.5 kcal/mol. Structural data from single crystal X-ray diffraction and quantum mechanical calculations (DFT) suggest that the rotational potential is determined by steric interactions between the dipolar rotator and the stator linkers such that fluorine atoms in the F2BDC linker reduce the activation energy by destabilization of the coplanar BDC ground state.

  • 24. Lin, Hu
    et al.
    Iakunkov, Artem
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Severin, Nikolai
    Talyzin, Aleksandr V.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Rabe, Jürgen P.
    Rapid aging of bilayer graphene oxide2022In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 126, no 48, p. 20658-20667Article in journal (Refereed)
    Abstract [en]

    Atomic force microscopy (AFM) imaging was used to study swelling of individual bilayer graphene oxide (GO) flakes in water and ethanol vapors. We found that within 5 days after sample deposition the swelling of bilayered Hummers GO (HGO) in ethanol vapors disappears nearly completely, whereas the swelling in water remains not affected. Swelling can therefore be used as a sensitive indicator of rapid aging of bilayered HGO, which occurs both in air and under inert gas. The surprising ability of 5-days-aged bilayered HGO to swell in water but not in ethanol fits to the effects observed in μm-thick GO membranes after several years of aging. Remarkably, bilayered Brodie GO (BGO) maintains swelling in ethanol after storing it under the same conditions as HGO. Moreover, we demonstrate that AFM can be used to detect hole defects in individual GO sheets. The BGO bilayer swelling in ethanol vapors starts either on the flake edges or in very few points, forming ∼2 Å height fronts, which propagate slowly within interlayer spaces in all directions. The increase of the average distance between HGO sheets occurs simultaneously all over the flake, demonstrating a high abundance of hole defects. Our results imply that the permeation path length in HGO membranes must be significantly shorter compared to BGO, which is important to take into account in the modeling of membrane permeation.

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  • 25. Liu, Dedi
    et al.
    Yao, Mingguang
    Wang, Lin
    Li, Quanjun
    Cui, Wen
    Liu, Bo
    Lui, Ran
    Zou, Bo
    Cui, Tian
    Liu, Bingbing
    Liu, Jing
    Sundqvist, Bertil
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Wågberg, Thomas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Pressure-induced phase transitions of C70 nanotubes2011In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 115, no 18, p. 8918-8922Article in journal (Refereed)
    Abstract [en]

    Single crystalline C70 nanotubes having a face-centered-cubic (fcc) structure with diameters on a nanometer scale were synthesized by a facile solution method. In situ high pressure Raman spectroscopy and X-ray diffraction have been employed to study the structural stability and phase transitions of the pristine sample. We show that the molecular orientation-related phase transition from the fcc structure to a rhombohedral structure occurs at about 1.5 GPa, which is 1 GPa higher than in bulk C70. Also, the C70 molecules themselves are more stable in the nanotubes than in bulk crystals, manifested by a partial amorphization at 20 GPa. The crystal structure of C70 nanotubes could partially return to the initial structure after a pressure cycle above 30.8 GPa, and the C70 molecules were intact up to 43 GPa. The bulk modulus of C70 nanotubes is measured to be 50 GPa, which is twice larger than that of bulk C70.

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  • 26.
    Lucas, Marie
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Boily, Jean-François
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Electrochemical Response of Bound Electrolyte Ions at Oriented Hematite Surfaces: A Local Electrochemical Impedance Spectroscopy Study2017In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 121, no 50, p. 27976-27982Article in journal (Refereed)
    Abstract [en]

    The electrochemical response of millimeter-sized hematite (α-Fe2O3) electrode surfaces to bound ions of NaCl, NH4Cl, and NaHCO3 salts was monitored by alternating current scanning electrochemical microscopy (AC-SECM). Local electrochemical impedance spectroscopy (LEIS) measurements along 100 μm lines on the (001) and (012) faces of hematite were used to extract capacitance and resistance parameters affected by bound inorganic ions. Equivalent circuit modeling was used to suggest that (1) double layer capacitances are affected by the spatial distribution of ions, and that (2) compact plane capacitance and resistance are affected by the closeness of association of ions to surface hydroxo groups. This study confirms the sensitivity of the technique to electrolyte ion binding, and provides new and key insight into the micrometer-scale electrochemical properties of iron oxides exposed to environmentally relevant conditions.

  • 27.
    Luzan, Serhiy
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Cataldo, Franco
    INAF—Osservatorio Astrofisico di Catania, Catania, Italy.
    Tsybin, Yury
    Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland.
    Talyzin, Alexandr
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Thermal decomposition of C60H182009In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 113, no 30, p. 13133-13138Article in journal (Refereed)
    Abstract [en]

    Products of thermal dehydrogenation of C 60H18(which mainly occurs at 450-600°C) were studied by XRD, Raman, IR and mass spectrometry. IR spectra indicate that dehydrogenation resulted in partial recovery of pristine C 60. XRD data indicate that the cell parameter of the face-centered cubic structure, which is higher for C 60H18(14.55 Å) than for C60(14.17 Å), remained higher following heat treatment, and heating at>500° C caused further expansion (to 14.78 Å). The increase in the cell parameter correlates with the beginning of partial fullerene cage collapse (corroborated by IR, Raman and MS data) and is suggested to result from “self-doping”.

  • 28.
    Luzan, Serhiy
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Talyzin, Alexandr
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hydration of graphite oxide in electrolyte and non-electrolyte solutions2011In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 115, no 50, p. 24611-24614Article in journal (Refereed)
    Abstract [en]

    Pressure induced insertion of liquid media was studied for graphite oxide (GO) immersed in excess amounts of aqueous copper acetate and sucrose solutions and compared to previous experiments with GO immersed in solute-free water media. Compression of GO in copper acetate solution resulted in significant enhancement of high pressure anomaly compared to pure water: interlayer distance reached 17.4 Å at 2.3 GPa while for pure water the maximal observed layer separation was 13.08 Å. Compression of GO in sucrose solution was found to be very similar to compression in solute-free water. These results confirm that copper ions can be pressure-inserted into GO structure while the expansion of structure is attributed to osmotic swelling. Sucrose dissolves in water in molecular form (nonelectrolyte) which results in weaker absorption into the GO structure and the absence of osmotic swelling. Pressure induced insertion of various solutions into the GO structure could possibly be promising for synthesis of new graphite intercalation materials or graphene-related composites.

  • 29.
    Luzan, Serhiy
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Tsybin, Yury O.
    Biomolecular Mass Spectrometry Laboratory, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland.
    Talyzin, Alexandr
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Reaction of C60 with Hydrogen gas: In Situ monitoring and pathways2011In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 115, no 23, p. 11484-11492Article in journal (Refereed)
    Abstract [en]

    Kinetics and pathways of C60 reaction with hydrogen gas were studied in a broad temperature interval and over extended periods of time. Specifically, hydrogenation was monitored in situ at high temperature and high hydrogen pressure conditions using the gravimetric method. The shape of gravimetric curve was found to depend on hydrogenation temperature: at 350–400 °C saturation of the sample weight was achieved, whereas at 420–440 °C the sample weight reached the maximum and decreased upon prolonged hydrogenation. The weight decrease is due to fullerene cage fragmentation with formation of light hydrocarbons evaporating from the sample. Hydrogenation products were studied by X-ray diffraction, MALDI TOF and APPI FT-ICR mass spectrometry, liquid chromatography, and elemental analysis. Hydrogenation pathways (from C60H18 up to C60H56) and possible mechanisms of hydrogenation-induced fragmentation of fulleranes are discussed.

  • 30.
    Mercier, Guillaume
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Klechikov, Alexey
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Hedenstrom, Mattias
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Johnels, Dan
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Baburin, Igor A.
    Seifert, Gotthard
    Mysyk, Roman
    Talyzin, Alexandr V.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Porous Graphene Oxide/Diboronic Acid Materials: Structure and Hydrogen Sorption2015In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 119, no 49, p. 27179-27191Article in journal (Refereed)
    Abstract [en]

    Solvothermal reaction of graphite oxide (GO) with benzene-1,4-diboronic acid (DBA) was reported previously to result in formation of graphene oxide framework (GOP) materials. The theoretical structure of GOFs consists of graphene layers separated by benzene-diboronic "pillars" with similar to 1 nm slit pores thus providing the opportunity to use it as a model material to verify the effect of a small pore size on hydrogen adsorption. A set of samples with specific surface area (SSA) in the range of similar to 50-1000 m(2)/g were prepared using variations of synthesis conditions and GO/DBA proportions. Hydrogen storage properties of GOF samples evaluated at 293 and 77 K were found to be similar to other nanocarbon trends in relation to SSA values. Structural characterization of GO/DBA samples showed all typical features reported as evidence for formation of a framework structure such as expanded interlayer distance, increased temperature of thermal exfoliation, typical features in FTIR spectra, etc. However, the samples also exhibited reversible swelling in polar solvents which is not compatible with the idealized GOF structure linked by benzenediboronic molecular pillars. Therefore, possible alternative nonframework models of structures with pillars parallel and perpendicular to GO planes are considered.

  • 31.
    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.

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  • 32.
    Modestov, Mikhail
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Bychkov, Vitaly
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Valiev, Damir
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Marklund, Mattias
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Internal structure of planar electrochemical doping fronts in organic semiconductors2011In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 115, no 44, p. 21915-21926Article in journal (Refereed)
    Abstract [en]

    The internal structure of electrochemical doping fronts in organic semiconductors is investigated using an extended drift-diffusion model for ions, electrons, and holes. The model also involves the injection barriers for electrons and holes in the partially doped regions in the form of the Nernst equation, together with a strong dependence of the electron and hole mobility on concentrations. It is shown that the internal structure of the doping fronts is controlled by a balance between the diffusion and mobility processes. The asymptotic behavior of the concentrations and the electric field is studied analytically inside the doping fronts. The numerical solution for the front structure confirms the most important findings of the analytical theory: a sharp head of the front in the undoped region, a smooth relaxation tail in the doped region, and a plateau at the critical point of transition from doped to undoped regions. The theoretically predicted complex structure of the doping fronts is in agreement with the previous experimental data. The acceleration of the p- and n-fronts toward each other in light-emitting electrochemical cells is described. The theoretical predictions for the planar front acceleration are in a good quantitative agreement with the experimental measurements for the backside of the curved doping fronts.

  • 33. Mohl, Melinda
    et al.
    Dobo, Dorina
    Kukovecz, Akos
    Konya, Zoltan
    Kordas, Krisztian
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Wei, Jinquan
    Vajtai, Robert
    Ajayan, Pulickel M.
    Formation of CuPd and CuPt Bimetallic Nanotubes by Galvanic Replacement Reaction2011In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 115, no 19, p. 9403-9409Article in journal (Refereed)
    Abstract [en]

    A galvanic replacement reaction has been successfully applied to prepare CuPd and CuPt bimetallic nanotubes. The nanotubes were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM) combined with energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) techniques. Ultralong, single crystalline copper nanowires (NWs) with a diameter of similar to 64 nm and a length of several micrometers were used as template material. By controlling the amount of noble metal salt added, nanotubes with different compositions were obtained. After the replacement of Cu with Pt, nanotubes composed of a PtCu alloy were formed. EDS analysis revealed that the Pt content increased until about 66%. No further increase in the molar ratio resulted in any additional Pt incorporation into the alloy. As for the replacement of Cu with Pd, the thickening of the nanotubes was observed indicating that nanotubes composed of Pd nanoparticles were formed. Bacicscattered electron imaging and SEM-EDS revealed CuPd nanotubes with approximately 2.3% Cu content. These remarks indicate different evolution mechanism for the nanotubes in the two systems.

  • 34.
    Mäkie, Peter
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Persson, Per
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Österlund, Lars
    Solar Light Degradation of Trimethyl Phosphate and Triethyl Phosphate on Dry and Water-Precovered Hematite and Goethite Nanoparticles2012In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 116, no 28, p. 14917-14929Article in journal (Refereed)
    Abstract [en]

    We report on the solar-light-mediated degradation of trimethyl phosphate (TMP) and triethyl phosphate (TEP) on hematite and goethite nanoparticles in synthetic air. Adsorption and photoreactions of TMP and TEP were studied by in situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) on dry and water-precovered nanoparticles in dark and under simulated solar light irradiation. Two-dimensional correlation analysis of infrared spectra was used to identify surface products as a function of reaction time. The optical properties of the hematite and goethite nanoparticles were investigated with optical spectrophotometry. The optical band gap was determined by analysis of the Tauc relationship around the band gap energy, E-g, yielding band gap energies of 2.14 and 2.28 eV for hematite and goethite nanoparticles, respectively. It is found that both TMP and TEP are readily photodegraded upon solar light irradiation (employing AM1.5 filters with 1735 W m(-2)), yielding surface orthophosphate as the final product. The first step in the dissociation of TMP and TEP is displacement of the methoxy and ethoxy groups, respectively, yielding adsorbed dimethyl phosphate (DMP) and methoxy, and diethyl phosphate (DEP) and ethoxy intermediates. Further photodegradation displaces additional methoxy and ethoxy groups with adsorbed orthophosphate as final reaction product. Methoxy and ethoxy fragments are simultaneously oxidized to carboxylates and carbonates. Photodegradation of TMP and TEP is promoted by OH radicals, which is evidenced by the higher photodegradation rate on water-precovered surfaces. The rate of TMP degradation is higher than that for TEP contrary to what is expected from their corresponding bulk hydrolysis rates, but consistent with their surface reactivity in dark, where TMP is observed to dissociate at room temperature but not TEP (or only very slowly). The photodegradation rate is higher on the goethite nanoparticles than the hematite nanoparticles on both dry and water precovered surfaces. The TMP and TEP photodegradation rate constants are found to be 0.025 (0.058) and 0.008 (0.023) min(-1), respectively, on water-precovered hematite (goethite) nanoparticles.

  • 35.
    Nordenström, Andreas
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Boulanger, Nicolas
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Iakunkov, Artem
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Baburin, Igor
    Theoretische Chemie, Technische Universitat Dresden, Dresden, Germany.
    Klechikov, Alexey
    Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden.
    Vorobiev, Alexei
    Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden.
    Talyzin, Alexandr V.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Intercalation of dyes in graphene oxide thin films and membranes2021In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 125, no 12, p. 6877-6885Article in journal (Refereed)
    Abstract [en]

    Intercalation of dyes into thin multilayered graphene oxide (GO) films was studied by neutron reflectivity and X-ray diffraction. Methylene blue (MB) penetrates the interlayer space of GO in ethanol solution and remains intercalated after the solvent evaporation, as revealed by the expansion of the interlayer lattice and change in chemical composition. The sorption of MB by thin GO films is found to be significantly stronger compared to the sorption of Crystal violet (CV) and Rose bengal (RB). This effect is attributed to the difference in the geometrical shape of planar MB and essentially nonflat CV and RB molecules. Graphite oxides and restacked GO films are found to exhibit different methylene blue (MB) sorptions. MB sorption by precursor graphite oxide and thin spin-coated films of GO is significantly stronger compared to freestanding micrometer-thick membranes prepared by vacuum filtration. Nevertheless, the sorption capacity of GO membranes is sufficient to remove a significant part of the MB from diluted solutions tested for permeation in several earlier studies. High sorption capacity results in strong modification of the GO structure, which is likely to affect permeation properties of GO membranes. Therefore, MB is not suitable for testing size exclusion effects in the permeation of GO membranes. It is not only hydration or solvation diameter but also the exact geometrical shape of molecules that needs to be taken into account considering size effects for penetration of molecules between GO layers in membrane applications.

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  • 36.
    Norén, Katarina
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Loring, John S
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Bargar, John R
    Persson, Per
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Adsorption mechanisms of EDTA at the water−iron oxide interface: implications for dissolution2009In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 113, no 18, p. 7762-7771Article in journal (Refereed)
    Abstract [en]

    The interactions between chelating agents and metal oxide particles play important roles for the distribution and availability of metal ions in aquatic environments. In this work, the adsorption of ethylenediaminetetraacetate (EDTA) onto goethite (α-FeOOH) was studied as a function of pH, time, and background electrolyte concentration at 25.0 °C, and the molecular structures of the surface complexes formed were analyzed by means of infrared spectroscopy using the attenuated total reflectance sampling technique. The collective infrared spectroscopic results of this study show that two surface complexes consisting of HEDTA3− and H2EDTA2− predominate at the water−goethite interface within the pH range of 3−9. No direct interactions of these complexes with surface Fe(III) ions were detected; hence, most likely the surface complexes are stabilized at the interface by electrostatic and hydrogen-bonding forces. The formation of the EDTA surface complexes is fast (time scale of minutes), but a slower (time scale of hours to days) dissolution reaction also occurs. The dissolved iron in solution is in the form of the highly stable FeEDTA solution complex, and the experimental evidence presented indicates that this complex can readsorb to the mineral surface. As dissolution proceeds, the concentration of FeEDTA in the solution phase increases, and this in turn leads to a buildup of readsorbed FeEDTA onto goethite. In the pH range of 4−7, this dissolution and readsorption process increases the total EDTA concentration at the surface. Under the experimental conditions in the present study, it is primarily the presence of uncomplexed EDTA in solution that drives the dissolution of goethite resulting in the subsequent readsorption of FeEDTA, while the HEDTA3− and H2EDTA2− surface complexes are stable during this process.

  • 37.
    Norén, Katarina
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Persson, Per
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Co-adsorption of Ga(III) and EDTA at the water/goethite interface: spectroscopic evidence for the formation of ternary surface complexes2010In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 114, no 39, p. 16547-16555Article in journal (Other academic)
    Abstract [en]

    Co-adsorption reactions between metal ions and anionic ligands play important roles in controlling availability and transport of chemical species in natural aquatic environments as well as in industrial processes. A molecular understanding of the properties of the surface species formed provides means to model these reactions in a predictive manner and to exploit them in synthetic routes of modified surfaces. In this study, we have used EXAFS and infrared spectroscopies in combination with quantitative adsorption measurements to investigate the coadsorption of Ga(III) and EDTA on α-FeOOH (goethite) as a function of pH. The quantitative results showed a 1:1 stoichiometry between adsorbed Ga(III) and EDTA and a maximum in total adsorption around pH 5. EXAFS and infrared data showed that the molecular structures displayed pH-dependent characteristics, and within the studied pH range, these results were concurrent and indicated that Ga(III)EDTA formed ternary surface complexes on goethite. The collective results were fully consistent with the occurrence of both outer sphere Ga(III)EDTA and inner sphere ternary surface complexes of type A (i.e., a surface−Ga(III)−EDTA structure), where the latter was favored by increasing pH. This study showed that despite a macroscopic adsorption behavior that was seemingly ligand-like, a substantial fraction of Ga(III) may bond directly to surface hydroxyl groups.

  • 38. Rautio, Anne-Riikka
    et al.
    Pitkänen, Olli
    Järvinen, Topias
    Samikannu, Ajaikumar
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Halonen, Niina
    Mohl, Melinda
    Mikkola, Jyri-Pekka
    Umeå University, Faculty of Science and Technology, Department of Chemistry. Laboratory of Industrial Chemistry and Reaction Engineering, Department of Chemical Engineering, Process Chemistry Centre Åbo Akademi University, Åbo, Finland.
    Kordas, Krisztian
    Electric double-layer capacitors based on multiwalled carbon nanotubes: can nanostructuring of the nanotubes enhance performance?2015In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 119, no 7, p. 3538-3544Article in journal (Refereed)
    Abstract [en]

    Supercapacitors prepared from chemically modified and vacuum filtered buckypapers were studied. The aim was to evaluate how its pore structure impacts the specific capacitance, energy, and power density in different electrolytes. The specific capacitance varies in a linear fashion with the specific surface area for nanotubes modified by the means of catalytic, low-temperature partial catalytic oxidation using cobalt oxide nanoparticles decorating the nanotubes. In contrast, electrodes composed of nanotubes preactivated in CO2 demonstrated only a minor increase in their specific capacitance, despite the observed significant increase in specific surface area. The radically improved surface area was a result of emergence and deposition of soot on the nanotubes during the activation process, as revealed by transmission electron microscopy. Among six different types of electrode materials, the CoOx decorated materials proved to have the highest specific capacitance (similar to 25 F/g in aqueous KOH and similar to 15 F/g in triethylsulfonium bis(trifluoromethylsulfonyl)imide ionic liquid). Thus, highly structured carbon nanotubes giving rise to energy and power storage densities comparable with commercial and other multiwalled carbon nanotube based electric double-layer capacitor devices were obtained.

  • 39. Rebrikova, Anastasiya T.
    et al.
    Klechikov, Alexey
    Umeå University, Faculty of Science and Technology, Department of Physics. Department of Physics and Astronomy, Materials Physics, Uppsala University, Ångströmlaboratoriet, Uppsalla, Sweden.
    Iakunkov, Artem
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Sun, Jinhua
    Umeå University, Faculty of Science and Technology, Department of Physics. Department of Industrial and Materials Science, Chalmers Tekniska Högskola, Sweden.
    Talyzin, Aleksandr V.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Avramenko, Natalya, V
    Korobov, Mikhail
    Swollen Structures of Brodie Graphite Oxide as Solid Solvates2020In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 124, no 42, p. 23410-23418Article in journal (Refereed)
    Abstract [en]

    Swelling of Brodie graphite oxide (B-GO) was studied for a series of normal alcohols from methanol to 1-nonanol. Isopiestic, X-ray diffraction (XRD), thermogravimetric, and differential scanning calorimetry data demonstrated that sorption of polar liquids into GO lamellas formed the set of regular swollen structures, simple binary “solid solvates”, characterized by the distance between the GO planes and the value of sorption. Temperature–composition behavior of the swollen structures was adequately described by conventional binary phase diagrams. Phase transformation of the low-temperature swollen structure of B-GO with 1-nonanol gave a clear example of incongruent melting transition typical for the binary solvates. A discreet set of the interplane distances observed by XRD and the stepwise equilibrium desorption pointed to the layered arrangement of solvent molecules in the swollen structures. The swollen structures with one to five parallel layers were observed for a series of normal alcohols with B-GO. The average volume of one layer, 0.36 ± 0.06 cm3 g–1 B-GO, was almost the same for rather different organic liquids and was possibly restricted by the internal geometry of B-GO. This internal volume available for the sorption of the first layer was reasonably estimated from geometrical parameters of B-GO.

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  • 40.
    Shchukarev, Andrey
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Boily, Jean-François
    Pacific Northwest National Laboratory, Richland, WA, United States.
    Felmy, Andrew R
    XPS of Fast-Frozen Hematite Colloids in NaCl Aqueous Solutions: I. Evidence for the Formation of Multiple Layers of Hydrated Sodium and Chloride Ions Induced by the {001} Basal Plane2007In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 111, no 49, p. 18307-18316Article in journal (Refereed)
    Abstract [en]

    The composition of fast-frozen wet pastes of hematite particles of different morphologies equilibrated in NaCl aqueous solutions was investigated by X-ray photoelectron spectroscopy. Two hematite preparations consisted of micrometer-sized platelets with 42% (HEM-1) and 82% (HEM-8) of the surface terminated by the {001} basal plane and a third of spheroids with no recognizable crystal plane (HEM-control). All hematite samples responded to changes in pH (4 and 9) and ionic strength (0, 10, 100 mM), showing that acid/base reactions of surface hydroxyl groups impact the composition of the paste. The HEM-1 and HEM-8 sample exhibited unusually large Na, Cl, and water contents at the highest ionic strength (100 mM) compared to HEM-control and all other minerals studied with this technique previously. The Na 1s and Cl 2p spectra occurred at binding energies typical of hydrated Na+ and Cl- ions and possessed energy-loss features, suggesting a three-dimensional distribution of these ions in the paste. An approximate stochiometric Na/Cl/H2O ratio of 1:1:2 was obtained in all samples in 100 mM NaCl as well as a strong correlation between the these compounds and the fraction of the {001} basal plane present in the hematite particles. The basal plane of hematite is proposed to induce the formation of a hydrated NaCl structure in the fast-frozen pastes, one that is compositionally reminiscent of hydrohalite (NaCl·2H2O), by stabilizing multiple layers of hydrated Na+ and Cl- ions prior to freezing.

  • 41.
    Shimizu, Kenichi
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Boily, Jean-Francois
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Electrochemical Signatures of Crystallographic Orientation and Counterion Binding at the Hematite/Water Interface2015In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 119, no 11, p. 5988-5994Article in journal (Refereed)
    Abstract [en]

    The interfacial electrochemistry of hematite (α-Fe2O3) is a key aspect for understanding the behavior of this important mineral phase in photocatalytic water-splitting devices as well as in terrestrial and atmospheric systems. Nano- to microsized particles are often multifaceted and exhibit terminations of varied crystallographic orientations and structures. As structure often controls reactivity, this study was devised to identify the impact of crystallographic orientation on the electrochemical response of hematite electrode surfaces contacted with technologically, geochemically, and environmentally important solutions of inorganic ions (NaCl, NaHCO3, and NH4Cl). Electrochemical impedance spectroscopy (EIS) measurements of single hematite crystals oriented along the (001) and (012) faces were used for this purpose. The EIS responses of the electrodes were described in terms of an equivalent electrical circuit that accounts for fast bulk and slower interfacial processes. Capacitance and resistance values for the bulk processes confirmed the anisotropic conductivity attributes of hematite and supported the use of the EIS data for interpreting the crystallographic orientation dependence of interfacial processes. These efforts extracted diffuse (C-dl) and compact (T-ad) layer capacitances and resistance (R-ad), as well as relaxation times pertaining to the re-equilibration of interfacial species during EIS. Capacitance values confirmed the greater charge-storing capability of the (012) face (C-dl = 110 mu F.cm(-2); T-ad = 335 mu F.cm(-2).s(-phi)) compared to the (001) face (C-dl = 0.20.6 mu F.cm(-2); T-ad = 0.20.6 mu F.cm(-2).s(-phi)). This was also confirmed through the resistance values pertaining to the transfer of charge carriers across the compact plane, which were lower (R-ad = 0.00.8 M Omega.cm(-2)) on the (012) face than on the (001) face (R-ad = 14 M Omega.cm(-2)). Binding of chloride and (bi)carbonate on the (012) face under acidic conditions was associated with an increase in capacitance values and relaxation times. The lowest capacitances and relaxation times occurred in the pH 8-9 region, which correspond to a likely point of zero charge. The capacitance values in NH4Cl were considerably lower than in NaCl and NaHCO3, owing to hydrogen bonding between the NH4+/NH3 species and surface (hydr)oxo groups. Such interactions can block protonation reactions and can be translated to negligible relaxation times for this system. Collectively, these findings underpin the interdependency of the hematite electrode surface orientation on its electrochemical signatures for important inorganic ions of direct relevance to technological and natural systems.

  • 42.
    Shimizu, Kenichi
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Shchukarev, Andrey
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Boily, Jean-François
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    X-ray photoelectron spectroscopy of fast-frozen hematite colloids in aqueous solutions: 3. stabilization of ammonium species by surface (hydr)oxo groups2011In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 115, no 14, p. 6796-6801Article in journal (Refereed)
    Abstract [en]

    The speciation of ammonium at the hematite/water interface was probed by cryogenic X-ray photoelectron spectroscopy. Wet pastes of colloidal hematite spheroids equilibrated in aqueous solutions of 50 mM NH4Cl exhibit distinctive pH-sensitive N 1s peaks for both NH4+ (401.7 eV) and NH3 (400.1 eV), yet total N/Fe ratios remain relatively invariant (0.029 ± 0.006) throughout the pH 2.2−10.5 range. Both NH4+ and NH3 species coexist throughout most of the tested pH range. NH4+ is most likely stabilized at the interface by hydrogen bonding with surface (hydr)oxo groups. A cationic sorption edge for NH3 is driven by proton abstraction of NH4+ by (hydr)oxo groups, forming surface complexes of the type ≡Fe−OH···NH3. These interactions shift the NH4+/NH3 equilibrium from pKa = 9.3 in water to 8.4 at the interface. Removal of excess water by vacuum dehydration induces, on the other hand, formation of NH2 directly bound to surface Fe atoms. These results underscore distinct ammonium species in contact with mineral surfaces and should be considered in understanding environmental and catalytic reactions in this medium.

  • 43.
    Simanova, Anna A.
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Loring, John S.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Persson, Per
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Formation of ternary metal-oxalate surface complexes on alpha-FeOOH particles2011In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 115, no 43, p. 21191-21198Article in journal (Refereed)
    Abstract [en]

    Processes at the aqueous interfaces of metal (hydr)oxide particles greatly influence the mobility, bioavailability, and reactivity of metal ions and ligands. Here we investigated the time-dependent reactions of oxalate or Me(C(2)O(4))(3)(3-) (Me = Fe(III), Al(III), Ga(III), Co(III)) with goethite in aqueous suspension at pH 4 using attenuated total reflectance infrared (ATR-IR) and extended X-ray absorption fine structure (EXAFS) spectroscopy. The data indicate four coordination modes for oxalate and Fe(C(2)O(4))(3)(3-) adsorbed at the goethite surface: (1) outer-spherically with a hydration shell similar to aqueous ligand; (2) outer-spherically but hydrogen bonded to a surface site; (3) inner-spherically to surface iron; (4) inner-spherically within a ternary type A surface complex. In the presence of oxalate, the two outer-sphere complexes form rapidly, but with time these species are partially consumed and the ternary inner-sphere complex is formed as a result of a dissolution-readsorption process. We propose that iron in these ternary complexes is more labile than iron that is mostly embedded in the lattice. Thus, ternary complexation may play an important role in iron bioavailabilty in the environment. For goethite reacted with Al(C(2)O(4))(3)(3-) or Ga(C(2)O(4))(3)(3-), these four surface complexes are accompanied by an additional Al(III) or Ga(III) ternary oxalate surface complex.

  • 44.
    Song, Xiaowei
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Boily, Jean-Francois
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Surface Hydroxyl Identity and Reactivity in Akaganeite2011In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 115, no 34, p. 17036-17045Article in journal (Refereed)
    Abstract [en]

    Hydroxyl groups on surfaces of well-defined akaganeite (beta-FeOOH) particles were identified by Fourier transform infrared spectroscopy. These efforts, assisted by molecular dynamics simulations, enabled the extraction of spectral signatures for these groups of the dominant (100), (001), and (010) crystallographic planes. Band assignments were supported by spectral variations induced by proton and chloride adsorption as well as temperature-programmed desorption. Molecular dynamics simulations were used to determine patterns and free energies of formation of hydrogen bonds. Surface Fe-O distances as well as hydrogen-bond numbers were also used to predict proton affinities. All spectral component concentrations display highly comparable responses to proton loadings with those of other FeOOH minerals previously studied with our coupled experimental-theoretical approach. These similarities underpin common thermodynamic stabilities for hydroxyls of a given Fe nuclearity on different planes of different minerals.

  • 45.
    Song, Xiaowei
    et al.
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Boily, Jean-Francois
    Umeå University, Faculty of Science and Technology, Department of Chemistry.
    Variable Hydrogen Bond Strength in Akaganeite2012In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 116, no 3, p. 2303-2312Article in journal (Refereed)
    Abstract [en]

    Akaganeite (beta-FeOOH) is a chloride-bearing iron oxy-hydroxide with a hollandite-type structure. This high specific surface area mineral has been the object of numerous studies given its high reactivity and involvement in natural and industrial processes. The important ion exchange attributes of this mineral involve similar to 0.4 x 0.4 nm wide channels in which chloride ions are stabilized by hydrogen bonding from bulk OH groups. This work provides further details on the relationship between bulk chloride ion loadings and hydrogen bond strengths. Molecular dynamics calculations were first carried out on chloride-free and bearing lattices to build a conceptual model for possible interactions in the akaganeite bulk. Experimental work was thereafter carried out on synthetic acicular particles (7 x 80 to 11 x 110 nm) reacted to aqueous solutions of HCl, then dried under dry N-2(g). These samples were studied by Fourier transform infrared spectroscopy, temperature-programmed desorption, X-ray photoelectron spectroscopy, and X-ray powder diffraction as well as transmission electron spectroscopy. Results collectively show that Cl/Fe molar ratios increasing from 0.169 up to 0.442 induce important changes in the hydrogen bonding environment of bulk hydroxyls. This can specifically be seen through shifts in bulk O-H stretching frequencies from 3496/3395 to 3470/3350 cm(-1). These changes are associated with a substantial shortening of particle lengths (97 to 45 nm), expansion of crystallographic lattice size (up to 0.9%), and increases in median thermal dehydroxylation temperatures (260 to 305 degrees C). Our work thereby highlights important variations in physicochemical attributes of akaganeite particles reacted with HCl. Such variations should consequently be considered in settings involving submicrometer-sized akaganeite particles.

  • 46.
    Talyzin, Alexandr
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Luzan, Serhiy
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Pressure-induced insertion of liquid acetone into the graphite oxide structure2010In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 114, no 11, p. 7004-7006Article in journal (Refereed)
    Abstract [en]

    An expansion of the graphite oxide structure due to pressure-induced insertion of acetone solvent was observed upon compression at 0.9 GPa. However, conversion into the expanded high-pressure phase is not complete, with the high-pressure and the ambient-pressure phases coexisting even at pressures up to 4.5 GPa. The phase transformation is not correlated with the solidification point of acetone, a behavior similar to that of the graphite oxide/alcohol systems and unlike that of the previously studied graphite oxide/water system. It can be concluded that pressure-induced insertion of solvent into the graphite oxide structure occurs not only for protic solvents (water, methanol, ethanol) but also for aprotic polar solvents (acetone).

  • 47.
    Talyzin, Alexandr
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Luzan, Serhiy
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Leifer, Klaus
    Akhtar, S.
    Fetzer, John
    Cataldo, Franco
    Tsybin, Yury
    Coronene fusion by heat treatment: road to nanographenes2011In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 115, no 27, p. 13207-13214Article in journal (Refereed)
    Abstract [en]

    The reactions of coronene dehydrogenation and fusion upon heat treatment in the temperature range of 500–700 °C were studied using XRD, TEM, Raman, IR, and NEXAFS spectroscopy. The formation of a coronene dimer (dicoronylene) was observed at temperatures 530–550 °C; dicoronylene can easily be separated using sublimation with a temperature gradient. An insoluble and not sublimable black precipitate was found to form at higher temperatures. Analysis of the data shows that dimerization of coronene is followed at 550–600 °C by oligomerization into larger molecules. Above 600 °C amorphization of the material and formation of graphitic nanoparticles was observed. Coronene fusion by annealing is proposed as a road to synthesis of larger polycyclic aromatic hydrocarbons and nanographenes.

  • 48.
    Talyzin, Alexandr V.
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Luzan, Serhiy
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Anoshkin, Ilya
    Nasibulin, Albert
    Esko, Kauppinen
    Dzwilewski, Andrzej
    Kreta, Ahmed
    Hassanien, Abdou
    Lundstedt, Anna
    Grennberg, Helena
    Hydrogen-Driven Cage Unzipping of C60 into Nano-Graphenes2014In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 118, no 12, p. 6504-6513Article in journal (Refereed)
  • 49.
    Talyzin, Alexandr V.
    et al.
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Luzan, Serhiy
    Umeå University, Faculty of Science and Technology, Department of Physics.
    Anoshkin, Ilya V.
    Nasibulin, Albert G.
    Kauppinnen, Esko I.
    Dzwilewski, Andrzej
    Kreta, Ahmed
    Jamnik, Janko
    Hassanien, Abdou
    Lundstedt, Anna
    Grennberg, Helena
    Hydrogen-Driven Cage Unzipping of C-60 into Nano-Graphenes2014In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 118, no 12, p. 6504-6513Article in journal (Refereed)
    Abstract [en]

    Annealing of C-60 in hydrogen at temperatures above the stability limit of C H bonds in C60Hx (500-550 degrees C) is found to result in direct collapse of the cage structure, evaporation of light hydrocarbons, and formation of solid mixture composed of larger hydrocarbons and few-layered graphene sheets. Only a minor part of this mixture is soluble; this was analyzed using matrix-assisted laser desorption/ionization MS, Fourier transform infrared (FTIR), and nuclear magnetic resonance spectroscopy and found to be a rather complex mixture of hydrocarbon molecules composed of at least tens of different compounds. The sequence of most abundant peaks observed in MS, which corresponds to C2H2 mass difference, suggests a stepwise breakup of the fullerene cage into progressively smaller molecular fragments edge-terminated by hydrogen. A simple model of hydrogen-driven C-60 unzipping is proposed to explain the observed sequence of fragmentation products. The insoluble part of the product mixture consists of large planar polycyclic aromatic hydrocarbons, as evidenced by FTIR and Raman spectroscopy, and some larger sheets composed of few-layered graphene, as observed by transmission electron microscopy. Hydrogen annealing of C-60 thin films showed a thickness-dependent results with reaction products significantly different for the thinnest films compared to bulk powders. Hydrogen annealing of C-60 films with the thickness below 10 nm was found to result in formation of nanosized islands with Raman spectra very similar to the spectra of coronene oligomers and conductivity typical for graphene.

  • 50.
    Talyzin, Alexandr V.
    et al.
    Umeå University, Faculty of Science and Technology, Physics.
    Szabó, Tamás
    Dékány, Imre
    Langenhorst, Falko
    Sokolov, Petr S.
    Solozhenko, Vladimir L.
    Nanocarbons by High-Temperature Decomposition of Graphite Oxide at Various Pressures2009In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 113, no 26, p. 11279-11284Article in journal (Refereed)
12 1 - 50 of 52
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