Ulrich Hasse
University of Greifswald
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Featured researches published by Ulrich Hasse.
Angewandte Chemie | 2010
Anna M. Nowicka; Ulrich Hasse; Michael Hermes; Fritz Scholz
Gold is used for various purposes because of its resistance to oxidation and its electrical, magnetic, optical, and other physical properties. Gold is one of the most important materials in the electronics industry, for optics, and in electrochemistry as an electrode material. The surface smoothness and cleanness of gold is of utmost importance, in particular for optical and electrochemical applications. For surface cleaning and smoothing of gold, a number of physical, chemical, and electrochemical methods have been reported; however, a tool for dissolving only the asperities on a gold surface has not been reported to date. Metallic gold is widely used in medicine as implant material. 12] Such gold implants release gold into the adjacent tissue and it was hypothesized that the release occurs by an immune reaction by oxidation. Herein, we report unexpected experimental results showing that OHC radicals in Fenton s reagent quickly dissolve gold from a mechanically polished gold surface to lead to a much smoother, that is, chemically polished, surface. The OHC radicals preferentially dissolve the gold atoms that are part of small asperities present on the surface, even after careful mechanical polishing. The reaction terminates after dissolution of the asperities. The reported effect can explain the release of gold from medical implants, and it may be used for polishing gold for various applications. Although it is well known that the OHC radicals of Fenton s reagent rapidly dissolve the rather stable self-assembled monolayers of alkyl sulfides on Au electrodes, attack on a bare Au surface was unexpected because gold is known for its stability towards oxidation unless the gold ions are strongly complexed, as, for example, in cyanide solutions. Figure 1a shows an AFM image of the Au surface after mechanical polishing and before attack by OHC radicals. Figure 1b is the seventh AFM image recorded following seven periods of OHC generation in an electrochemical Fenton reaction for 10 seconds between each image recording. Figure 1c shows the 32nd image after a sequence of 25 OHC generations each of 10 seconds duration (see the Experimental Section). Figure 1a–c clearly indicates that the electrode surface is smoothed considerably. The decrease of real surface area was also studied with two independent electrochemical methods, namely a) evaluation of the “gold oxide system”, and b) underpotential deposition (upd) of Pb. The decrease in real surface area of the gold was quantified by CV ( 0.3 V to 1. 5 V (vs. Ag/AgCl)) in 0.1m H2SO4 solution. The “gold oxide system” (method a) was recorded and evaluated with respect to the charge underneath the oxidation and reduction peaks. That charge (and also the peak currents) depends on the real electrode surface and strongly decreased after a series of attacks by Fenton s reagent (see Figure 2). Control experiments have shown that the “gold oxide system” is neither affected by hydrogen peroxide, nor by Fe or Fe ions. The decrease of the oxidation charge is due to the diminished real electrode surface, as is clear from the AFM images. The final real surface area was 37% of the initial surface area. The real surface area of gold (method b) was also determined by upd of Pb on Au (according to reference [15]). The obtained results show that the anodic and cathodic peak charge of underpotential-deposited Pb decreased to 30.4% after OHC attack for 70 minutes, that is, the final real surface area was 30.4% of the initial area. The strongest surface area decrease happened during the first 20 minutes. The electrode was always mechanically polished before attack by Fenton s reagent, that is, the polishing was essentially a roughening compared to the smoothing from Fenton s reagent. To be sure that the surface area decrease was really due to dissolution of Au, the concentration of Au was determined in the Fenton solution as a function of attack time. Figure 3 shows that the attack rate was high at the beginning and very small at the end of the reaction. The reduction in Au dissolution rate does not mean that Au is not further oxidized, as could be proved by the following experiment: the Au electrode was mechanically polished, then electrochemically reduced at 0.2 V for 5 s, then attacked by OHC, and finally a voltammogram was recorded from 1.0 to 0.2 V (the starting potential was chosen so that no gold oxide could be formed electrochemically, and indeed without OHC exposure the Au did not show any reduction peak). After OHC exposure the reduction peak of gold oxide was present, and the peak decreased after subsequent OHC exposure and finally became constant (because of smoothing). The final roughness factor was 1.04, and from the reduction charge it was calculated that 1.08 monolayers of Au were oxidized. This experiment proves that the surface of Au is always oxidized by OHC, but a measurable dissolution takes place only as long as the surface has certain asperities. Our results show that OHC oxidizes the surface of gold, preferentially dissolves the asperities on the metal surface, and thus accomplishes a very effective smoothing of the surface. Since the smooth parts of the gold surface are not measurably dissolved, it is inferred that the reaction of OHC with the more ordered gold atoms leads to a stable gold oxide layer that can be reduced back to gold without any significant [*] Dr. U. Hasse, Dr. M. Hermes, Prof. Dr. F. Scholz Institut f r Biochemie, Universit t Greifswald Felix-Hausdorff-Strasse 4, 17487 Greifswald (Germany) Fax: (+ 49)3834-864-451 E-mail: [email protected] Homepage: http://www.chemie.uni-greifswald.de/~analytik/
Angewandte Chemie | 2010
Anna M. Nowicka; Ulrich Hasse; Gustav Sievers; Mikolaj Donten; Zbigniew Stojek; Stephen Fletcher; Fritz Scholz
It has long been known that defects on a gold surface play an important role in electrocatalysis, but the precise mechanism has always been unclear. This work indicates that the defect sites provide partially filled d-orbitals that stabilize freeradical intermediates. Strong evidence for this hypothesis is that the sites can be selectively knocked out by treatment with OH• radicals generated by Fentons reagent. The knockout effect is demonstrated using oxygen reduction, hydrogen reduction, and the redox electrochemistry of hydroquinone.
Electrochemistry Communications | 2001
Ulrich Hasse; Fritz Scholz
Litharge (α-PbO) crystals with edge lengths between 100 and 500 nm have been deposited on polycrystalline gold electrodes and electrochemically reduced in one step to metallic lead. The reduction was followed by in situ atomic force microscopy. The results reveal that the reduction is an epitactic solid-state reaction with conservation of the initial crystal orientation. It was possible to depict the progressing of the reaction front supporting a simple theoretical model for the reaction mechanism.
Chemsuschem | 2008
Fritz Scholz; Ulrich Hasse
Among all global environmental prob-lems, there is one which dominates overall others: the excessive release ofcarbon dioxide due to burning offossilfuels like coal, oil, and gas. Carbon diox-ide is the main anthropogenic chemicalcompound responsible for global warm-ing (anthropogenic greenhouse effect)and all its devastating effects of globalclimate changes, such as the rise ofocean levels, growth ofdesert areas, andthawing ofpermafrost soil.
Journal of Solid State Electrochemistry | 2016
Chinnaya Jeyabharathi; Paula Ahrens; Ulrich Hasse; Fritz Scholz
AbstractThe electrochemical oxidation of single-crystal gold surfaces has been well studied, and the exposed crystal planes can be reliably distinguished based on the peak potentials of oxide formation. However, the multiple oxidation peaks of polycrystalline gold have not yet been unambiguously related to crystal planes. In this work, we used cyclic voltammetric responses of activated polycrystalline gold electrodes recorded in sulfuric acid solutions to allow constructing relationships between crystal planes and oxide peaks. The studies of oxide formation were complemented by measuring double-layer non-faradaic currents, lead underpotential deposition (Pb-upd), the oxygen reduction reaction (ORR), and the hydrogen evolution reaction (HER). Graphical abstractThe link between three gold oxide current peaks and exposed low index crystal planes, viz. Au(100), Au(110) and Au(111) on polycrystalline gold electrode
Journal of Solid State Electrochemistry | 2014
Chinnaya Jeyabharathi; Ulrich Hasse; Paula Ahrens; Fritz Scholz
The oxygen reduction reaction (ORR) in acid media can be catalyzed on gold electrodes when the surface is activated by mechanical or electrochemical pretreatments. The activation is caused by increased surface roughness and defects, or asperities. After activation, a slow recrystallization of the surface as a function of relaxation time leads to deactivation of the surface for the ORR. After removal of active centers, the surface is not affected over time, which reveals that the surface recrystallization is associated with the deactivation. Experiments using various amounts of Au nanoparticles (AuNPs) immobilized on glassy carbon (GC) show a positive shift of peak potential of oxygen reduction and peroxide oxidation with increasing particle coverage.
Journal of Solid State Electrochemistry | 2012
Ulrich Hasse; Katja Fricke; Daiane Dias; Gustav Sievers; Harm Wulff; Fritz Scholz
Annealed thin layers of gold with large mono-crystalline areas were treated with OH· radicals generated in an electrochemical Fenton reaction. The morphological changes observed with ex situ atomic force microscopy in non-contact mode and grazing incidence X-ray diffractometry show that the grain boundaries, and generally the non-{111} planes, are the loci of highest reactivity, i.e., the places where the gold dissolution is much faster than on the {111} planes.
Journal of Solid State Electrochemistry | 2012
Gustav Sievers; Ulrich Hasse; Fritz Scholz
The nucleation and growth of platinum on polycrystalline gold was studied by chronoamperometry, cyclic voltammetry, and atomic force microscopy before and after treatment of the gold surface with hydroxyl (OH•) radicals. Two different procedures of mechanical polishing of the gold surface (“coarse polish” and “fine polish”) were applied before the treatment with OH• radicals. The nucleation and growth of Pt was much better reproducible on electrodes which underwent a “coarse polish”. The treatment of the Au surface with OH• radicals decreased the number of active sites; however, the nucleation growth mode remained the same (3-D instantaneous). The spontaneous Pt deposition (no externally applied potential) on Au was unaffected by the treatment with OH• radicals. In situ atomic force microscopy experiments showed that the Pt starts to grow only on some of the Au grains, most probably on those which have active sites on their surface. This leads to a roughening of the electrode surface upon Pt deposition. Treatment with OH• radicals did only quantitatively diminish the amount of deposited Pt, but qualitatively the imaging of the Pt growth remained the same. Obviously, the OH• radicals lead to a knockout (decreasing number) of active sites for Pt nucleation, while the nature of the remaining active sites stays unaffected.
Advanced Materials Research | 2013
Sergei Zhuravkov; Evgeny Plotnikov; Dmitry Martemiyanov; Nikolay A. Yavorovsky; Ulrich Hasse; Stefan Zander
The morphological and structural characteristics of nanoscale silver particles obtained by the method of electric spark dispersion of metal granules in the liquid aprotic medium were obtained using atomic force microscopy, transmission electron microscopy, and dynamic light scattering spectroscopy. The specific surface, morphology, structure and the distribution by size of the particles are presented.
Journal of Solid State Electrochemistry | 2018
Dana Thal; Heike Kahlert; Jeyabharathi Chinnaya; Paula Ahrens; Ulrich Hasse
The impact of 1-decanethiol self-assembled monolayer (SAM) formation and removal cycles on polycrystalline Au surfaces and SAM quality was studied with the help of CV, DPV, Pb-UPD, STM, and AFM. The SAM removal was accomplished by dissolution with oxygen radicals generated by UV photolysis of aqueous hydrogen peroxide. During the first Au-SAM formation and removal cycles, the surface roughness decreased. After that, the surface properties remained almost unaffected, indicating that the cyclic treatment removed the most reactive gold surface sites, until a rather stable surface resulted, which guaranteed highly reproducible SAM formation.