Publications by authors named "Gunther Wittstock"

75 Publications

Effect of a Cocatalyst on a Photoanode in Water Splitting: A Study of Scanning Electrochemical Microscopy.

Anal Chem 2021 Sep 30;93(36):12221-12229. Epub 2021 Aug 30.

School of Mathematics and Science, Chemistry Department, Carlvon Ossietzky University of Oldenburg, D-26111 Oldenburg, Germany.

With a proper band gap of ∼2.4 eV for solar light absorption and suitable valence band edge position for oxygen evolution, scheelite-monoclinic bismuth vanadate (BiVO) has become one of the most attractive photocatalysts for efficient visible-light-driven photoelectrochemical (PEC) water splitting. Several studies have indicated that surface modification of BiVO with a cocatalyst such as NiFe layered double hydroxide (LDH) can significantly increase the PEC water splitting performance of the catalyst. Herein, we experimentally investigated the charge transfer dynamics and charge carrier recombination processes by scanning electrochemical microscopy (SECM) with the feedback mode on the surface of BiVO and BiVO/NiFe-LDH as model samples. The ratio of rate constants for photogenerated hole () to electron () via the photocatalyst of BiVO/NiFe-LDH reacting with the redox couple is found to be five times larger than that of BiVO under illumination. In this case, the ratio of the rate constants / stands for the interfacial charge recombination process. This implies the cocatalyst NiFe-LDH suppresses the electron back transfer greatly and finally reduces the surface recombination. Control experiments with cocatalysts CoPi and RuO onto BiVO further verify this conclusion. Therefore, the SECM characterization allows us to make an overall analysis on the function of cocatalysts in the PEC water splitting system.
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http://dx.doi.org/10.1021/acs.analchem.1c01235DOI Listing
September 2021

An Electrically Conducting Three-Dimensional Iron-Catecholate Porous Framework.

Angew Chem Int Ed Engl 2021 Aug 1;60(33):18065-18072. Epub 2021 Jul 1.

Department of Chemistry, Ludwig-Maximilians-Universität (LMU), & Center for NanoScience (CeNS), Butenandtstrasse 11, 81377, Munich, Germany.

We report the synthesis of a unique cubic metal-organic framework (MOF), Fe-HHTP-MOF, comprising hexahydroxytriphenylene (HHTP) supertetrahedral units and Fe ions, arranged in a diamond topology. The MOF is synthesized under solvothermal conditions, yielding a highly crystalline, deep black powder, with crystallites of 300-500 nm size and tetrahedral morphology. Nitrogen sorption analysis indicates a highly porous material with a surface area exceeding 1400 m  g . Furthermore, Fe-HHTP-MOF shows broadband absorption from 475 up to 1900 nm with excellent absorption capability of 98.5 % of the incoming light over the visible spectral region. Electrical conductivity measurements of pressed pellets reveal a high intrinsic electrical conductivity of up to 10  S cm . Quantum mechanical calculations predict Fe-HHTP-MOF to be an efficient electron conductor, exhibiting continuous charge-carrier pathways throughout the structure.
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http://dx.doi.org/10.1002/anie.202102670DOI Listing
August 2021

Impact of the Crystalline LiSi Phase on the Self-Discharge Mechanism of Silicon Negative Electrodes in Organic Electrolytes.

ACS Appl Mater Interfaces 2020 Dec 1;12(50):55903-55912. Epub 2020 Dec 1.

School of Mathematics and Sciences, Chemistry Department, Carl von Ossietzky University of Oldenburg, D-26111 Oldenburg, Germany.

Because of their high specific capacity and rather low operating potential, silicon-based negative electrode materials for lithium-ion batteries have been the subject of extensive research over the past 2 decades. Although the understanding of the (de)lithiation behavior of silicon has significantly increased, several major challenges have not been solved yet, hindering its broad commercial application. One major issue is the low initial Coulombic efficiency and the ever-present self-discharge of silicon electrodes. Self-discharge itself affects the long-term stability of electrochemical storage systems and, additionally, must be taken into consideration for inevitable prelithiation approaches. The impact of the crystalline LiSi phase is of great interest as the phase transformation between crystalline () and amorphous () phases not only increases the specific surface area but also causes huge polarization. Moreover, there is the possibility for electrochemical over-lithiation toward the LiSi phase because of the electron-deficient LiSi phase, which can be highly reactive toward the electrolyte. This poses the question about the impact of the -LiSi phase on the self-discharge behavior in comparison to its amorphous counterpart. Here, silicon thin films used as model electrodes are lithiated to cut-off potentials of 10 mV and 50 mV Li|Li ( and ) in order to systematically investigate their self-discharge mechanism open-circuit potential () measurements and to visualize the solid electrolyte interphase (SEI) growth by means of scanning electrochemical microscopy. We show that the -LiSi phase is formed for the electrode, while it is not found for the electrode. In turn, the electrode displays an almost linear self-discharge behavior, whereas the electrode reaches a plateau at 380 mV Li|Li, which is due to the phase transition from -LiSi to the -LiSi phase. At this plateau potential, the phase transformation at the Si|electrolyte interface results in an electronically more insulating and more uniform SEI ( electrode), while the electrode displays a less uniform SEI layer. In summary, the self-discharge mechanism of silicon electrodes and, hence, the irreversible decomposition of the electrolyte and the corresponding SEI formation process heavily depend on the structural nature of the underlying lithium-silicon phase.
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http://dx.doi.org/10.1021/acsami.0c16742DOI Listing
December 2020

Electrochemical Activation of Self-Assembled Monolayers for the Binding of Effectors.

Langmuir 2020 12 24;36(48):14623-14632. Epub 2020 Nov 24.

School of Mathematics and Science, Chemistry Department, Carl von Ossietzky University of Oldenburg, D-26111 Oldenburg, Germany.

A self-assembled monolayer (SAM) on gold was prepared from a diaminoterephthalate (DAT) derivative as functional molecule and 1-decanthiol as a backfiller. The DAT derivative is -protected by a -butyloxycarbonyl (Boc) group and is anchored to the gold surface via a liponic acid as a stable anchor group. The terminal DAT moiety exhibits interesting effector properties such as fluorescence and electrochemical activity. Irreversible oxidation of the monolayer at 0.4 V (Hg|HgSO) in 0.1 M HClO triggers deprotection of the DAT group and subsequent chemical reactions, during which 10% of the DAT groups of the original SAM are transformed to a new surface-bound, quasi-reversible redox couple with a formal potential of 0.0 V (Hg|HgSO) and a standard rate constant of 8 s in 0.1 M HClO. Immersion of the mixed SAM in 0.1 M HClO at open circuit potential or oxidation in 0.1 M HSO did not produce this surface-bound redox couple. The monolayers were thoroughly characterized by X-ray photoelectron spectroscopy (XPS) and polarization modulation infrared reflection absorption spectroscopy (PM IRRAS) after the different preparation steps indicating only minor changes in the overall composition of the monolayer, in particular, the preservation of the heteroatoms. The new redox couple is likely a diimine, in agreement with its ability to bind nucleophiles such as anilines by conjugate addition that could be followed by multicycle voltammetry and XPS. The DAT effector group is especially interesting because it can also report the binding reaction by changed electrochemical and fluorescence signals.
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http://dx.doi.org/10.1021/acs.langmuir.0c02426DOI Listing
December 2020

Electronic Transitions in Different Redox States of Trinuclear 5,6,11,12,17,18-Hexaazatrinaphthylene-Bridged Titanium Complexes: Spectroelectrochemistry and Quantum Chemistry.

Chemphyschem 2020 Nov 16;21(22):2506-2514. Epub 2020 Oct 16.

School of Mathematics and Science, Chemistry Department, Carl von Ossietzky University of Oldenburg, 26111, Oldenburg, Germany.

Multinuclear transition metal complexes bridged by ligands with extended π-electronic systems show a variety of complex electronic transitions and electron transfer reactions. While a systematic understanding of the photochemistry and electrochemistry has been attained for binuclear complexes, much less is known about trinuclear complexes such as hexaphenyl-5,6,11,12,17,18-hexaazatrinaphthylene-tristitanocene [(Cp Ti) HATN(Ph) ]. The voltammogram of [(Cp Ti) HATN(Ph) ] shows six oxidation and three reduction waves. Solution spectra of [(Cp Ti) HATN(Ph) ] and of the electrochemically formed oxidation products show electronic transitions in the UV, visible and the NIR ranges. Density functional theory (DFT) and linear response time-dependent DFT show that the three formally titanium(II) centers transfer an electron to the HATN ligand in the ground state. The optically excited transitions occur exclusively between ligand-centered orbitals. The charged titanium centers only provide an electrostatic frame to the extended π-electronic system. Complete active self-consistent field (CASSCF) calculation on a structurally simplified model compound, which considers the multi-reference character imposed by the three titanium centers, can provide an interpretation of the experimentally observed temperature-dependent magnetic behavior of the different redox states of the title compound in full consistency with the interpretation of the electronic spectra.
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http://dx.doi.org/10.1002/cphc.202000547DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7756296PMC
November 2020

Catalytic Activity of Alkali Metal Cations for the Chemical Oxygen Reduction Reaction in a Biphasic Liquid System Probed by Scanning Electrochemical Microscopy.

Chemistry 2020 Aug 23;26(47):10882-10890. Epub 2020 Jul 23.

Carl von Ossietzky University of Oldenburg, Chemistry Department, 261111, Oldenburg, Germany.

Chemical reduction of dioxygen in organic solvents for the production of reactive oxygen species or the concomitant oxidation of organic substrates can be enhanced by the separation of products and educts in biphasic liquid systems. Here, the coupled electron and ion transfer processes is studied as well as reagent fluxes across the liquid|liquid interface for the chemical reduction of dioxygen by decamethylferrocene (DMFc) in a dichloroethane-based organic electrolyte forming an interface with an aqueous electrolyte containing alkali metal ions. This interface is stabilized at the orifice of a pipette, across which a Galvani potential difference is externally applied and precisely adjusted to enforce the transfer of different alkali metal ions from the aqueous to the organic electrolyte. The oxygen reduction is followed by H O detection in the aqueous phase close to the interface by a microelectrode of a scanning electrochemical microscope (SECM). The results prove a strong catalytic effect of hydrated alkali metal ions on the formation rate of H O , which varies systematically with the acidity of the transferred alkali metal ions in the organic phase.
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http://dx.doi.org/10.1002/chem.202001967DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7496973PMC
August 2020

A highly crystalline anthracene-based MOF-74 series featuring electrical conductivity and luminescence.

Nanoscale 2019 Nov 29;11(43):20949-20955. Epub 2019 Oct 29.

Department of Chemistry, Ludwig-Maximilians-Universität (LMU), Butenandtstr. 5-13 (E), 81377 Munich, Germany. and Nanosystems Initiative Munich (NIM) and Center for NanoScience (CeNS), Schellingstr. 4, 80799 Munich, Germany.

Recently, a small group of metal-organic frameworks (MOFs) has been discovered featuring substantial charge transport properties and electrical conductivity, hence promising to broaden the scope of potential MOF applications in fields such as batteries, fuel cells and supercapacitors. In combination with light emission, electroactive MOFs are intriguing candidates for chemical sensing and optoelectronic applications. Here, we incorporated anthracene-based building blocks into the MOF-74 topology with five different divalent metal ions, that is, Zn, Mg, Ni, Co and Mn, resulting in a series of highly crystalline MOFs, coined ANMOF-74(M). This series of MOFs features substantial photoluminescence, with ANMOF-74(Zn) emitting across the whole visible spectrum. The materials moreover combine this photoluminescence with high surface areas and electrical conductivity. Compared to the original MOF-74 materials constructed from 2,5-dihydroxy terephthalic acid and the same metal ions Zn, Mg, Ni, Co and Mn, we observed a conductivity enhancement of up to six orders of magnitude. Our results point towards the importance of building block design and the careful choice of the embedded MOF topology for obtaining materials with desired properties such as photoluminescence and electrical conductivity.
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http://dx.doi.org/10.1039/c9nr05431fDOI Listing
November 2019

Electron Transfer and Electron Excitation Processes in 2,5-Diaminoterephthalate Derivatives with Broad Scope for Functionalization.

ChemistryOpen 2019 Sep 2;8(9):1176-1182. Epub 2019 Jul 2.

Carl von Ossietzky University Oldenburg School of Mathematics and Science, Chemistry Department D-26111 Oldenburg Germany.

Derivatives of 2,5-diaminoterephthalate (DAT) are efficient fluorescence dyes that are also redox-active, thus allowing for the electrochemical manipulation of spectral properties. The electrochemical behaviour of seven DAT derivatives was studied by cyclic voltammetry in dichloromethane. In the absence of a proton donor, DATs should be oxidized in two one-electron steps. The first step is usually quasi-reversible while the second step is either quasi-reversible or irreversible. Some electrochemical properties such as the formal potentials and the ratio between the anodic and the cathodic current were determined from the cyclic voltammograms. Correlation between the formal potential of first oxidation and the absorption or the fluorescence emission wavelengths are established for this specific type of dyes. These correlations were confirmed with density functional theory calculations.
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http://dx.doi.org/10.1002/open.201900138DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6718077PMC
September 2019

Diverging surface reactions at TiO- or ZnO-based photoanodes in dye-sensitized solar cells.

Phys Chem Chem Phys 2019 Jun 6;21(24):13047-13057. Epub 2019 Jun 6.

Institute of Applied Physics and Center for Materials Research, Justus-Liebig-University Giessen, Heinrich-Buff-Ring 16, D-35392 Giessen, Germany.

Fast recombination of electrons from semiconductors with the oxidized redox species in the electrolyte represents a major bottleneck in the improvement of ZnO-based dye-sensitized solar cells (DSSCs). While processes at the semiconductor-electrolyte interface are well studied on TiO electrodes, the interactions of the ZnO surface with the electrolyte solution in DSSCs are less explored. This work aims at clarifying the different impact of the two contrasting redox couples I/I or [Co(bpy-pz)] (bpy-pz = bis(6-(1H-pyrazol-1-yl)-2,2'-bipyridine)) in electrolytes containing either no additives or Li ions and/or 4-tert-butlypyridine (TBP) in DSSCs using screen-printed nanoparticulate TiO (NP-TiO) or electrodeposited ZnO (ED-ZnO) photoanodes sensitized with the indoline dye DN216. A detailed photoelectrochemical study is performed to investigate light absorption, charge transfer and mass transport in these cells. We demonstrate that the chemical nature of the semiconductor directly influences the affinity of adsorbates. This drastically influences the energy levels and recombination kinetics at the semiconductor-electrolyte interface, electron and ion transport in the porous system as well as light absorption of dye molecules by the Stark effect. The present investigation reveals the origin of major performance losses in DSSCs based on ED-ZnO photoanodes as well as the relevance of ionic interactions with NP-TiO photoanodes that can both serve as the starting point for rationally guided improvement of their conversion efficiencies.
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http://dx.doi.org/10.1039/c9cp01215jDOI Listing
June 2019

Diaminoterephthalate-α-lipoic acid conjugates with fluorinated residues.

Beilstein J Org Chem 2019 26;15:981-991. Epub 2019 Apr 26.

Institut für Chemie, Universität Oldenburg, Carl von Ossietzky-Str. 9-11, D-26129 Oldenburg, Germany.

Two bifunctional diaminoterephthalate (DAT) fluorescence dyes were prepared in a three-step sequence including one deprotection reaction. One functional unit is α-lipoic acid (ALA) for binding the dye to gold surfaces. It was introduced to the DAT scaffold by an amidation reaction. The other functional unit is a -(trifluoromethyl)benzyl group for facile detection of the surface-bound material by X-ray photoelectron spectroscopy (XPS). This residue was introduced by reductive amination of the DAT scaffold with the respective benzaldehyde derivative. In one compound (60% yield over three steps) the ALA unit is directly bound to the DAT as a relatively electron-withdrawing amide. In solution (CHCl), this material shows strong fluorescence (quantum yield 57% with emission at 495 nm, absorption maximum at 420 nm). The other compound (57% yield over three steps) possesses a propylene spacer between the ALA and the DAT units for electronic decoupling, thus, bathochromic shifts are observed (absorption at 514 nm, emission at 566 nm). The quantum yield is, however, lower (4%). Self-assembled monolayers on a gold surface of both compounds were prepared and characterized by high-resolution XPS of the C 1s, O 1s, S 2p, N 1s and F 1s emissions. The high signal-to-noise ratios of the F 1s peaks indicated that trifluoromethylation is an excellent tool for the detection of surface-bound materials by XPS.
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http://dx.doi.org/10.3762/bjoc.15.96DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6541350PMC
April 2019

Vectorial near-field coupling.

Nat Nanotechnol 2019 07 13;14(7):698-704. Epub 2019 May 13.

Institute of Physics and Center of Interface Science, Carl von Ossietzky University, Oldenburg, Germany.

The coherent exchange of optical near fields between two neighbouring dipoles plays an essential role in the optical properties, quantum dynamics and thus the function of many naturally occurring and artificial nanosystems. These interactions are challenging to quantify experimentally. They extend over only a few nanometres and depend sensitively on the detuning, dephasing and relative orientation (that is, the vectorial properties) of the coupled dipoles. Here, we introduce plasmonic nanofocusing spectroscopy to record coherent light scattering spectra with 5 nm spatial resolution from the apex of a conical gold nanotaper. The apex is excited solely by evanescent fields and coupled to plasmon resonances in a single gold nanorod. We resolve resonance energy shifts and line broadenings as a function of dipole distance and relative orientation. We demonstrate how these phenomena arise from mode couplings between different vectorial components of the interacting optical near fields, specifically from the coupling of the nanorod to both transverse and longitudinal polarizabilities of the taper apex.
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http://dx.doi.org/10.1038/s41565-019-0441-yDOI Listing
July 2019

Correlations of Crystal and Electronic Structure via NMR and X-ray Photoelectron Spectroscopies in the RETMAl (RE = Sc, Y, La-Nd, Sm, Gd-Tm, Lu; TM = Ni, Pd, Pt) Series.

Inorg Chem 2019 May 7;58(10):7010-7025. Epub 2019 May 7.

Institut für Anorganische und Analytische Chemie , Universität Münster , Corrensstrasse 30 , 48149 Münster , Germany.

A total of 35 intermetallic aluminum compounds have been synthesized from the elements via arc melting and characterized by powder X-ray diffraction. A total of 15 of them have been previously reported; however, detailed property investigations were missing. Compounds of the RETMAl (rare earth metal RE = Sc, Y, La-Nd, Sm, Gd-Tm, Lu) series with transition metal TM = Ni, Pd, and Pt crystallize isostructurally in the orthorhombic MgCuAl type structure ( Cmcm, oC16, fc). Single-crystal X-ray diffraction investigations were conducted on YNiAl, LaNiAl, YPdAl, ScPtAl, and YPtAl. The TM and Al atoms form a [TMAl] polyanion, the RE atoms reside in cavities within the framework. While the Sc, Y, La, and Lu compounds exhibit Pauli-paramagnetic behavior, consistent with all atoms being closed shell, the other RETMAl compounds show paramagnetism along with magnetic ordering at low temperatures, in line with an open-shell trivalent oxidation state for the RE atoms. Solid-state Al NMR investigations were carried out on the Pauli-paramagnetic samples, all showing only a single central transition, in line with one crystallographic site for the respective atoms. The observed quadrupolar coupling constants and electric-field-gradient asymmetry parameters were found to be in good agreement with the density-functional-theory-calculated values. Isotropic resonance shifts are dominated by the Fermi-contact interactions with s-conduction electron densities at the Fermi edge (Knight shifts). The bonding characteristics mirror the electronic density of states and crystal chemistry of the family of intermetallic compounds under consideration. Both the Knight shifts and quadrupolar coupling constants can be predicted based on element-specific increments.
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http://dx.doi.org/10.1021/acs.inorgchem.9b00648DOI Listing
May 2019

Synthesis, Crystal Structure, and Selected Properties of [Au(S CNH ) ]SCN: A Precursor for Gold Macro-Needles Consisting of Gold Nanoparticles Glued by Graphitic Carbon Nitride.

Chemistry 2019 May 30;25(27):6763-6772. Epub 2019 Apr 30.

Institute of Inorganic Chemistry, Christian-Albrechts-Universität zu Kiel, Max-Eyth-Str. 2, 24118, Kiel, Germany.

A new preparation route is developed for the synthesis of needle-like crystals of [Au(S CNH ) ]SCN, which avoids disproportionation of the Au salt used as a starting material. In the crystal structure, the two crystallographically independent Au centers are in a square-planar environment of two S CNH ligands. The Hirshfeld surface analysis reveals the presence of noncovalent intermolecular S⋅⋅⋅S interactions, which are essential for the spatial arrangement of the molecules. Density functional theory (DFT) calculations including dispersion and damping corrections result in a unit cell volume very close to the value determined experimentally. Thermal decomposition in an inert atmosphere generates black needles with lengths of up to 500 μm. X-ray powder diffraction and pair distribution function analyses demonstrate that the needles are composed of nanosized crystals with a volume-weighted average domain size of 20(1) nm. According to results of X-ray photoemission experiments, the black needles are covered by a nitrogen-rich carbon nitride with composition near (CN) N. C solid-state NMR investigations indicate that two different carbon species are present, with signals corresponding well to heptazine units as in melon and triazine units as in poly(triazin imide) type compounds. Scanning transmission electron microscopy tomography evidences that the needles are composed of slightly elongated nanoparticles.
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http://dx.doi.org/10.1002/chem.201805913DOI Listing
May 2019

Independent control over residual silver content of nanoporous gold by galvanodynamically controlled dealloying.

Nanoscale 2018 Sep;10(36):17166-17173

Institute of Applied and Physical Chemistry and Center for Environmental Research and Sustainable Technology, University Bremen, Leobener Str. UFT, 28359 Bremen, Germany.

A new procedure was developed and characterized for the galvanodynamically controlled dealloying (GCD) of AuxAg100-x alloys to obtain nanoporous gold (npAu) mainly as an unsupported catalyst material for partial oxidation of alcohols. Such catalysts require residual Ag content of less than 1 at%. GCD was compared to the preparation of npAu by potentiostatically controlled deallyoing (PCD) and free corrosion (FC). The main advantage of GCD is the ability to obtain npAu with a predetermined residual Ag content including residual Ag contents below 1 at% while retarding the coarsening of the ligaments. For PCD and FC, there is a strong increase of ligament size with decreasing residual Ag content because the longer times required for dealloying unavoidably lead to coarsening of the npAu structure. On the other hand, GCD also prevented too high initial current density that leads to cracking of the samples and prevents formation of mechanically stable monoliths. GCD tolerates different compositions of the starting alloy for AuxAg100-x within the tested composition range (20 at% ≤ xAu ≤ 30 at%). The samples obtained by GCD were tested for methanol and ethanol oxidation and showed favorable characteristics for partial oxidation of methanol to methyl formate and of ethanol to ethyl acetate.
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http://dx.doi.org/10.1039/c8nr03699cDOI Listing
September 2018

Printing with Satellite Droplets.

Small 2018 Sep 3;14(39):e1802583. Epub 2018 Sep 3.

Carl von Ossietzky University of Oldenburg, School of Mathematics and Science, Center of Interface Sciences, Institute of Chemistry, D-26111, Oldenburg, Germany.

Despite their specific methodologies, all current noncontact printing techniques such as inkjet printing (IJP), involve the break-up of a liquid meniscus during the separation of the ink droplet from the bulk ink reservoir. Often, the break-up of a liquid meniscus results in the formation of one or more satellite droplet whose volumes are several orders of magnitude smaller than the primary droplet. Many attempts are directed to suppress or control the formation of satellite droplets because they blur the printing result. For the first time, a simple mechanism by which a single satellite droplet is exclusively formed and directed to the substrate by a gas stream while the primary droplet remains attached to a metal rod used for controlling the formation and break-up of the meniscus is reported. High printing resolution is demonstrated by satellite droplets printing (SDP) without the need for small orifices which are prone to clogging. Furthermore, the droplet generation from a large orifice enables SDP to handle viscous inks which has remained challenging for traditional IJP.
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http://dx.doi.org/10.1002/smll.201802583DOI Listing
September 2018

Synthesis, Crystal Structures, and Magnetic and Electrochemical Properties of Highly Phenyl Substituted Trinuclear 5,6,11,12,17,18-Hexaazatrinaphthylene (HATNPh)-Bridged Titanium Complexes.

Inorg Chem 2018 Sep 10;57(17):11165-11174. Epub 2018 Aug 10.

Institut für Chemie , Carl von Ossietzky Universität Oldenburg , D-26111 Oldenburg , Germany.

Trinuclear titanium complexes coordinated by one ligand with three coordination sites have shown properties of mixed valency and a high number of reversible redox steps. Herein we report on the hexaphenyl-substituted derivative (CpTi)(μ-HATNPh) (2). On reaction of 2 with the ferrocenium salt [CpFe]BF, the cationic complexes [(CpTi)(μ-HATNPh)] ( n = 1-3; 3-5) become available in a selective way. Cyclic voltammograms show 10 reversible redox states of the trinuclear species 2 without decomposition. In order to classify the degree of electronic communication between the titanium centers, comproportionation constants K, IVCT bands in NIR spectra, and magnetic measurements were analyzed. These parameters show strong coupling effects between the titanium centers but no full delocalization. In addition, single-crystal X-ray analysis of the neutral complex 2 and its oxidation products (1+ (3), 2+ (4), and 3+ (5)) revealed the geometric structure of the molecule in the solid state. For the cationic species anion-π interactions between the electron-deficient central ring of the HATNPh ligand and BF counterions were found.
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http://dx.doi.org/10.1021/acs.inorgchem.8b01728DOI Listing
September 2018

Inkjet Printing in Liquid Environments.

Small 2018 Jul 28;14(27):e1801212. Epub 2018 May 28.

School of Mathematics and Science, Center of Interface Sciences, Institute of Chemistry, Carl von Ossietzky University of Oldenburg, D-26111, Oldenburg, Germany.

Inkjet printing (IJP) is an old but still vivifying technique for flexible and cost-effective printing of various kinds of functional inks. Normally, IJP can only work in gaseous environments. Here, it is shown that traditional piezoelectric IJP can be performed in liquid environments with a totally different droplet dispensing and manipulating mechanism. With the same piezoelectric nozzle, the volume of the droplets printed in a carrier liquid can be thousands of times smaller than those printed in air. Therefore, this work demonstrates a working mode of traditional IJP with a highly improved resolution opening possibilities for novel applications of the IJP technique.
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http://dx.doi.org/10.1002/smll.201801212DOI Listing
July 2018

Photocatalytic degradation of the herbicide chloridazon on mesoporous titania/zirconia nanopowders.

Environ Sci Pollut Res Int 2018 Dec 20;25(35):34873-34883. Epub 2017 Dec 20.

Institute of Chemistry, Carl von Ossietzky University Oldenburg, 26129, Oldenburg, Germany.

Advanced oxidation processes using semiconducting photocatalysts for the degradation of organic pollutants are a promising approach for the remediation of pesticide-contaminated wastewater. High photodegradation efficiency and stability of the photocatalyst are of key importance for practical application of the semiconductor. In this study, mesoporous TiO/ZrO nanopowders were synthesized via two techniques; evaporation-induced self-assembly (EISA) and sol-gel using triblock copolymers Pluronic P123 and F127. The photodegradation activities of the composites were determined by employing the herbicide chloridazon as a model compound. Due to well-developed mesoporosity, the TiO/ZrO nanocomposite synthesized by EISA displays high surface area and small crystallite sizes leading to higher photocatalytic activity than pristine TiO prepared under similar condition and commercial Degussa P25 nanopowder. The optimum amount of zirconium required for the highest activities was identified and found to be 0.14 and 0.05 mol% for the EISA and sol-gel-prepared samples, respectively. Systematic studies of the post-thermal treatment step for both samples show that Zr inhibits an anatase-to-rutile phase transition only up to 600 °C, at higher temperature phase separation occurs. Samples synthesized by EISA method showed enhanced degradation activity than sol-gel-synthesized samples.
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http://dx.doi.org/10.1007/s11356-017-1023-xDOI Listing
December 2018

Electrocatalytic methanol oxidation with nanoporous gold: microstructure and selectivity.

Nanoscale 2017 Nov;9(45):17839-17848

Institute of Materials Physics and Technology, Hamburg University of Technology, Hamburg, Germany.

The properties of Nanoporous Gold (NPG) obtained by the selective dissolution of Ag from an Au-Ag alloy can be tuned by the details of its fabrication, and specifically the residual Ag content is correlated to the ligament size of the material. We link this correlation to methanol electro-oxidation. Specifically, two different NPG types (obtained by potentiostatic dealloying) are compared with one obtained by free corrosion. They show remarkable differences in activity. Quantitative product analysis reveals that NPG shows nearly selective oxidation of CHOH to HCOO when NPG is used as an active electrode in contrast to planar Au. This trend can further be enhanced when applying finer nanoporous structures that are linked to a higher Ag content. X-ray photoelectron spectroscopy (XPS) reveals changes in the nature of residual Ag from which we conclude that Ag is not a passive component in the methanol oxidation process.
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http://dx.doi.org/10.1039/c7nr05124gDOI Listing
November 2017

Redox titration of gold and platinum surface oxides at porous microelectrodes.

Phys Chem Chem Phys 2017 Aug;19(34):22915-22925

Carl von Ossietzky University of Oldenburg, Institute of Chemistry, Carl-von-Ossietzky-Str. 9-11, 26129 Oldenburg, Germany.

The extension of the surface interrogation mode of scanning electrochemical microscopy (SI-SECM) is demonstrated for porous electrode materials. These materials are often high surface area powders which are very important electrocatalysts for instance in fuel cells or water electrolyzers. The powdered electrocatalyst material is filled into a cavity-microelectrode which is then operated as the sample electrode in SECM. After a surface oxide generation step, the oxides on the porous sample are reduced by [Ru(NH)] formed at the microelectrode probe of the SECM while the sample is at open circuit potential. Such porous electrodes pose the difficulty to cope with unavoidable variations in the filling of the cavity and to access the entire surface by the mediator. The electrochemically active surface area is used to compensate the variation in filling. It can also be used for calculating coverages of surface oxides for a better comparison between different electrodes. We found a complete and fast accessibility for all investigated porous electrodes which is based on electron transfer. Therefore, we propose a "vertical feedback" mechanism analogous to SECM feedback experiments on extended flat samples at open circuit potential. Moreover, the current transients indicate that distinctive oxide species with different kinetics are present. Taken together, these measures ensure consistent determination of oxide coverages for nanoporous gold and carbon-supported platinum nanoparticles.
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http://dx.doi.org/10.1039/c7cp04589aDOI Listing
August 2017

A Platform for Electric Field Aided and Wire-Guided Droplet Manipulation.

Small 2017 02 14;13(5). Epub 2016 Nov 14.

Carl von Ossietzky University of Oldenburg, Faculty of Mathematics and Science, Center of Interface Sciences, Institute of Chemistry, D-26111, Oldenburg, Germany.

Small droplets can be manipulated based on controlling the adhesion work to a hydrophobic wire. The wire can be used to pick up, transport, and lay down droplets with volumes between picoliters to microliters avoiding the sliding of droplets over chip surfaces. Handling of droplets on surfaces with large steps such as engraved wells or channels is possible.
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http://dx.doi.org/10.1002/smll.201601691DOI Listing
February 2017

A polarized liquid-liquid interface meets visible light-driven catalytic water oxidation.

Chem Commun (Camb) 2016 Sep;52(76):11382-11385

Institute of Chemistry, Carl von Ossietzky University of Oldenburg, D-26111 Oldenburg, Germany.

Hyperbranched nanostructured bismuth vanadate at a chemically polarized water/organic interface is applied for efficient visible light-driven catalytic oxidation of water in the presence of [Co(bpy)](PF) as an organic soluble electron acceptor. The photocurrent response originating from the transfer of photo-excited electrons in BiVO to [Co(bpy)] is measured by scanning electrochemical microscopy.
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http://dx.doi.org/10.1039/c6cc04275aDOI Listing
September 2016

Hydrodynamic dispensing and electrical manipulation of attolitre droplets.

Nat Commun 2016 08 12;7:12424. Epub 2016 Aug 12.

Institute of Chemistry, Center of Interface Sciences, Faculty of Mathematics and Science, Carl von Ossietzky University of Oldenburg, D-26111 Oldenburg, Germany.

Dispensing and manipulation of small droplets is important in bioassays, chemical analysis and patterning of functional inks. So far, dispensing of small droplets has been achieved by squeezing the liquid out of a small orifice similar in size to the droplets. Here we report that instead of squeezing the liquid out, small droplets can also be dispensed advantageously from large orifices by draining the liquid out of a drop suspended from a nozzle. The droplet volume is adjustable from attolitre to microlitre. More importantly, the method can handle suspensions and liquids with viscosities as high as thousands mPa s markedly increasing the range of applicable liquids for controlled dispensing. Furthermore, the movement of the dispensed droplets is controllable by the direction and the strength of an electric field potentially allowing the use of the droplet for extracting analytes from small sample volume or placing a droplet onto a pre-patterned surface.
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http://dx.doi.org/10.1038/ncomms12424DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4990644PMC
August 2016

Speciation of nanoscale objects by nanoparticle imprinted matrices.

Nanoscale 2016 Aug 9;8(29):13934-43. Epub 2016 Mar 9.

Institute of Chemistry, the Hebrew University of Jerusalem, Jerusalem 9190401, Israel.

The toxicity of nanoparticles is not only a function of the constituting material but depends largely on their size, shape and stabilizing shell. Hence, the speciation of nanoscale objects, namely, their detection and separation based on the different species, similarly to heavy metals, is of outmost importance. Here we demonstrate the speciation of gold nanoparticles (AuNPs) and their electrochemical detection using the concept of "nanoparticles imprinted matrices" (NAIM). Negatively charged AuNPs are adsorbed as templates on a conducting surface previously modified with polyethylenimine (PEI). The selective matrix is formed by the adsorption of either oleic acid (OA) or poly(acrylic acid) (PAA) on the non-occupied areas. The AuNPs are removed by electrooxidation to form complementary voids. These voids are able to recognize the AuNPs selectively based on their size. Furthermore, the selectivity could be improved by adsorbing an additional layer of 1-hexadecylamine, which deepened the voids. Interestingly, silver nanoparticles (AgNPs) were also recognized if their size matched those of the template AuNPs. The steps in assembling the NAIMs and the reuptake of the nanoparticles were characterized carefully. The prospects for the analytical use of NAIMs, which are simple, of small dimension, cost-efficient and portable, are in the sensing and separation of nanoobjects.
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http://dx.doi.org/10.1039/c6nr01106cDOI Listing
August 2016

Reactive oxygen species formed in organic lithium-oxygen batteries.

Phys Chem Chem Phys 2016 Apr;18(16):10774-80

Carl von Ossietzky University, Faculty of Mathematics and Natural Sciences, Center of Interface Science, Institute of Chemistry, D-26111 Oldenburg, Germany.

Li-oxygen batteries with organic electrolytes are of general interest because of their theoretically high gravimetric energy density. Among the great challenges for this storage technology is the generation of reactive oxygen species such as superoxides and peroxides that may react with the organic solvent molecules and other cell components. The generation of such species has been assumed to occur during the charging reaction. Here we show that superoxide is formed also during the discharge reaction in lithium ion-containing dimethyl sulfoxide electrolytes and is released into the solution. This is shown independently by fluorescence microscopy after reaction with the selective reagent 4-chloro-7-nitrobenzo-2-oxa-1,3-diazole and by local detection using a microelectrode of a scanning electrochemcial microscope positioned in a defined distance of 10 to 90 μm above the gas diffusion electrode.
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http://dx.doi.org/10.1039/c5cp07145cDOI Listing
April 2016

Voltammetric pH Nanosensor.

Anal Chem 2015 Dec 10;87(23):11641-5. Epub 2015 Nov 10.

Institute of Physical Chemistry, Polish Academy of Sciences , Kasprzaka 44/52, PL-01224 Warsaw, Poland.

Nanoscale pH evaluation is a prerequisite for understanding the processes and phenomena occurring at solid-liquid, liquid-liquid, and liquid-gas interfaces, e.g., heterogeneous catalysis, extraction, partitioning, and corrosion. Research on the homogeneous processes within small volumes such as intracellular fluids, microdroplets, and microfluidic chips also requires nanometer scale pH assessment. Due to the opacity of numerous systems, optical methods are useless and, if applicable, require addition of a pH-sensitive dye. Potentiometric probes suffer from many drawbacks such as potential drift and lack of selectivity. Here, we present a voltammetric nanosensor for reliable pH assessment between pH 2 and 12 with high spatial resolution. It consists of a pyrolytic carbon nanoelectrode obtained by chemical vapor deposition (CVD) inside a quartz nanopipette. The carbon is modified by adsorption of syringaldazine from its ethanolic solution. It exhibits a stable quasi-reversible cyclic voltammogram with nearly Nernstian dependency of midpeak potentials (-54 mV/pH). This sensor was applied as a probe for scanning electrochemical microscopy (SECM) in order to map pH over a platinum ultramicroelectrode (UME), generating hydroxide ions (OH(-)) by the oxygen reduction reaction (ORR) at a diffusion-controlled rate in aerated phosphate buffered saline (PBS). The results reveal the alkalization of the electrolyte close to the oxygen reducing electrode, showing the insufficient buffer capacity of PBS to maintain a stable pH at the given conditions.
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http://dx.doi.org/10.1021/acs.analchem.5b03482DOI Listing
December 2015

Microelectrospotting as a new method for electrosynthesis of surface-imprinted polymer microarrays for protein recognition.

Biosens Bioelectron 2015 Nov 27;73:123-129. Epub 2015 May 27.

Department of Inorganic and Analytical Chemistry, Budapest University of Technology and Economics, Szent Gellért tér 4, H-1111 Budapest, Hungary; MTA-BME "Lendület" Chemical Nanosensors Research Group, Department of Inorganic and Analytical Chemistry, Budapest University of Technology and Economics, Szent Gellért tér 4, H-1111 Budapest, Hungary. Electronic address:

Here we introduce microelectrospotting as a new approach for preparation of protein-selective molecularly imprinted polymer microarrays on bare gold SPR imaging chips. During electrospotting both the gold chip and the spotting tip are electrically connected to a potentiostat as working and counter electrodes, respectively. The spotting pin encloses the monomer-template protein cocktail that upon contacting the gold surface is in-situ electropolymerized resulting in surface confined polymer spots of ca. 500 µm diameter. By repeating this procedure at preprogrammed locations for various composition monomer-template mixtures microarrays of nanometer-thin surface-imprinted films are generated in a controlled manner. We show that the removal and rebinding kinetics of the template and various potential interferents to such microarrays can be monitored in real-time and multiplexed manner by SPR imaging. The proof of principle for microelectrospotting of electrically insulating surface-imprinted films is made by using scopoletin as monomer and ferritin as protein template. It is shown that microelectrospotting in combination with SPR imaging can offer a versatile platform for label-free and enhanced throughput optimization of the molecularly imprinted polymers for protein recognition and for their analytical application.
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http://dx.doi.org/10.1016/j.bios.2015.05.049DOI Listing
November 2015

Catalysis at the room temperature ionic liquid|water interface: H2O2 generation.

Chem Commun (Camb) 2015 Apr;51(31):6851-3

Institute of Physical Chemistry, Polish Academy of Sciences, ul.Kasprzaka 44/52, 01-224, Warszawa, Poland.

H2O2 is produced at the interface between a room-temperature ionic liquid with decamethylferrocene as an electron donor and an acidic aqueous solution. The electron donor can be regenerated electrochemically.
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http://dx.doi.org/10.1039/c5cc01480hDOI Listing
April 2015

Spatiotemporal changes of the solid electrolyte interphase in lithium-ion batteries detected by scanning electrochemical microscopy.

Angew Chem Int Ed Engl 2014 Sep 30;53(39):10531-5. Epub 2014 Jul 30.

Department of Chemistry, Carl v. Ossietzky University of Oldenburg, 26111 Oldenburg (Germany).

The solid electrolyte interphase (SEI) in lithium-ion batteries separates the highly reductive lithiated graphite from reducible electrolyte components. It is critical for the performance, durability, and safe operation of batteries. In situ imaging of the SEI is demonstrated using the feedback mode of scanning electrochemical microscopy (SECM) with 2,5-di-tert-butyl-1,4-dimethoxy benzene as mediator. The formation of the SEI is indicated by a decrease of the mediator regeneration rate. Prolonged imaging of the same region revealed fluctuation of the passivating properties on time scales between 2 min and 20 h with an inhomogeneous distribution over the sample. The implications of the approach for in situ assessment of local SEI properties on graphite electrodes are discussed with respect to studying the influence of mechanical stress on SEI reliability and the mode of action of electrolyte additives aiming at improving SEI properties.
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http://dx.doi.org/10.1002/anie.201403935DOI Listing
September 2014

Nanoparticle-imprinted polymers for size-selective recognition of nanoparticles.

Angew Chem Int Ed Engl 2014 Jan 4;53(1):294-8. Epub 2013 Nov 4.

Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904 (Israel) http://chem.ch.huji.ac.il/mandler.

Citrate-stabilized gold nanoparticles 15 nm and 33 nm in diameter were transferred concomitantly with a monolayer of positively charged polyaniline by Langmuir-Blodgett transfer at pH 5 onto a conducting indium-doped tin oxide (ITO) support. Films consisting of one to three layers of polyaniline with thicknesses of 1-3 nm were prepared and characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy. After electro-oxidation of the Au nanoparticles in 0.1 M KCl, cavities were left behind in the film that could be analyzed by SEM. These cavities were able to recapture analyte nanoparticles from a solution of pH 10 and showed size-exclusion properties. The amount of nanoparticles taken up by the cavities was conveniently analyzed by measuring the charge associated with the electro-oxidation of these particles in 0.1 M KCl after the film had been rinsed with water. The size-exclusion properties improved with the number of Langmuir-Blodgett layers transferred.
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http://dx.doi.org/10.1002/anie.201305962DOI Listing
January 2014
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