Dieter Schmeißer

Unification of Catalytic Water Oxidation and Oxygen Reduction Reactions: Amorphous Beat Crystalline Cobalt Iron Oxides

Catalytic water splitting to hydrogen and oxygen is considered as one of the convenient routes for the sustainable energy conversion. Bifunctional catalysts for the electrocatalytic oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are pivotal for the energy conversion and storage, and alternatively, the photochemical water oxidation in biomimetic fashion is also considered as the most useful way to convert solar energy into chemical energy. Here we present a facile solvothermal route to control the synthesis of amorphous and crystalline cobalt iron oxides by controlling the crystallinity of the materials with changing solvent and reaction time and further utilize these materials as multifunctional catalysts for the unification of photochemical and electrochemical water oxidation as well as for the oxygen reduction reaction. Notably, the amorphous cobalt iron oxide produces superior catalytic activity over the crystalline one under photochemical and electrochemical water oxidation and oxygen reduction conditions.

Experimental electronic structure of In2O3 and Ga2O3

Transparent conducting oxides (TCOs) pose a number of serious challenges. In addition to the pursuit of high-quality single crystals and thin films, their application has to be preceded by a thorough understanding of their peculiar electronic structure. It is of fundamental interest to understand why these materials, transparent up to the UV spectral regime, behave also as conductors. Here we investigate In2O3 and Ga2O3, two binary oxides, which show the smallest and largest optical gaps among conventional n-type TCOs. The investigations on the electronic structure were performed on high-quality n-type single crystals showing carrier densities of ~1019?cm?3 (In2O3) and ~1017?cm?3 (Ga2O3). The subjects addressed for both materials are: the determination of the band structure along high-symmetry directions and fundamental gaps by angular resolved photoemission (ARPES). We also address the orbital character of the valence- and conduction-band regions by exploiting photoemission cross sections in x-ray photoemission (XPS) and by x-ray absorption (XAS). The observations are discussed with reference to calculations of the electronic structure and the experimental results on thin films.

Novel corrosion-resistant films for Mg alloys

Abstract A novel method for the corrosion inhibition of Mg alloys, based on the formation of a thin polyacrylic–polypyrrole film, is presented. The film was created after simple immersion of the samples in a specially formulated emulsion of polyacrylic and polypyrrole polymers. The synthesis conditions were studied, as well as the resulting corrosion resistance of coated magnesium foils (AZ31) in terms of corrosion rate in neutral aqueous NaCl solutions. The relative ratio of polyacrylic polymer to polypyrrole was found to be important for the film properties. Furthermore, only in situ polymerisation of pyrrole in the polyacrylic emulsion can provide advanced corrosion properties in relatively thin films (2–5 μm). Blending of polypyrrole powder with the polyacrylic emulsion gives rise to thick films (20 μm) with reduced corrosion-inhibition properties. The corrosion rate was only weakly dependent on the film thickness of these novel coatings. We argue that the mechanism of corrosion inhibition is related to the formation of a thin passive layer of chemisorbed polypyrrole polymers or oligomers on the native magnesium oxide of the surface. The structure is further stabilised by polyacrylic polymer chains interacting as dopants of polypyrrole. The method is environmentally friendly: non-toxic chemicals are used, no rinsing is required, etc. The advanced corrosion-inhibition properties of the films, as well as the simple application process on the metal surface, provide significant potential for the industrial application of the method.

Electronic and chemical passivation of hexagonal 6H–SiC surfaces by hydrogen termination

Hydrogenation of 6H–SiC (0001) and (0001) is achieved by high-temperature hydrogen treatment. Both surfaces show a low-energy electron diffraction pattern representative of unreconstructed surfaces of extremely high crystallographic order. On SiC(0001), hydrogenation is confirmed by the observation of sharp Si–H stretching modes. The absence of surface band bending for n- and p-type samples is indicative of electronically passivated surfaces with densities of charged surface states in the gap of below 7×1010 cm−2 for p-type and 1.7×1012 cm−2 for n- type samples, respectively. Even after two days in air, the surfaces show no sign of surface oxide in x-ray photoelectron spectroscopy.

Self-Poled Transparent and Flexible UV Light-Emitting Cerium Complex–PVDF Composite: A High-Performance Nanogenerator

Cerium(III)-N,N-dimethylformamide-bisulfate [Ce(DMF)(HSO4)3] complex is doped into poly(vinylidene fluoride) (PVDF) to induce a higher yield (99%) of the electroactive phases (β- and γ-phases) of PVDF. A remarkable enhancement of the output voltage (∼32 V) of a nanogenerator (NG) based on a nonelectrically poled cerium(III) complex containing PVDF composite film is achieved by simple repeated human finger imparting, whereas neat PVDF does not show this kind of behavior. This high electrical output resembles the generation of self-poled electroactive β-phase in PVDF due to the electrostatic interactions between the fluoride of PVDF and the surface-active positive charge cloud of the cerium complex via H-bonding and/or bipolar interaction among the opposite poles of cerium complex and PVDF, respectively. The capacitor charging capability of the flexible NG promises its applicability as piezoelectric-based energy harvester. The cerium(III) complex doped PVDF composite film exhibit an intense photoluminescence in the UV region, which might be due to a participation of electron cloud from negative pole of bipolarized PVDF. This fact may open a new area for prospective development of high-performance energy-saving flexible solid-state UV light emitters.

Effect of iron-carbide formation on the number of active sites in Fe–N–C catalysts for the oxygen reduction reaction in acidic media

In this work Fe–N–C catalysts were prepared by the oxalate-supported pyrolysis of FeTMPPCl or H2TMPP either in the presence or absence of sulfur. The well-known enhancing effect of sulfur-addition on the oxygen reduction activity was confirmed for these porphyrin precursors. The pyrolysis process was monitored in situ by high-temperature X-ray diffraction under synchrotron radiation (HT-XRD) and thermogravimetry coupled with mass-spectroscopy (TG-MS). It was found that the beneficial effect of sulfur could be attributed to the prevention of iron-carbide formation during the heat-treatment process. In the case of pyrolysis of the sulfur-free precursors an excessive iron-carbide formation leads to disintegration of FeN4-centers, hence limiting the number of ORR active sites on the final catalyst. Physical characterization of the catalysts by bulk elemental analysis, X-ray diffraction (XRD), Raman and 57Fe Mosbauer spectroscopy confirmed the outcome from HT-XRD and TG-MS. It could be shown that the avoidance of carbide formation during pyrolysis represents a promising way to enhance the density of ORR active sites on those catalysts. This can be done either by sulfur-addition or the performance of an intermediate acid leaching. As iron carbide is often found as a by-product in the preparation of Fe–N–C catalysts this work gives some general strategies for enhancing the density of active sites enabling higher current densities.

Ellipsometry and XPS comparative studies of thermal and plasma enhanced atomic layer deposited Al2O3-films

Summary We report on results on the preparation of thin (<100 nm) aluminum oxide (Al2O3) films on silicon substrates using thermal atomic layer deposition (T-ALD) and plasma enhanced atomic layer deposition (PE-ALD) in the SENTECH SI ALD LL system. The T-ALD Al2O3 layers were deposited at 200 °C, for the PE-ALD films we varied the substrate temperature range between room temperature (rt) and 200 °C. We show data from spectroscopic ellipsometry (thickness, refractive index, growth rate) over 4” wafers and correlate them to X-ray photoelectron spectroscopy (XPS) results. The 200 °C T-ALD and PE-ALD processes yield films with similar refractive indices and with oxygen to aluminum elemental ratios very close to the stoichiometric value of 1.5. However, in both also fragments of the precursor are integrated into the film. The PE-ALD films show an increased growth rate and lower carbon contaminations. Reducing the deposition temperature down to rt leads to a higher content of carbon and CH-species. We also find a decrease of the refractive index and of the oxygen to aluminum elemental ratio as well as an increase of the growth rate whereas the homogeneity of the film growth is not influenced significantly. Initial state energy shifts in all PE-ALD samples are observed which we attribute to a net negative charge within the films.

Improved performance of a polymer nanogenerator based on silver nanoparticles doped electrospun P(VDF–HFP) nanofibers

We report on the electrospinning of poly(vinylidene fluoride-hexafluoropropylene) [P(VDF-HFP)] nanofibers doped with silver nanoparticles for the preparation of a polymer based nanogenerator (PNG). It has been found that the yield of the piezoelectric phase is increased by the addition of silver nanoparticles. Furthermore, defects in the P(VDF-HFP) electrospun fibers are removed resulting in a significant enhancement in the output power of the PNG. A maximum generated PNG output voltage of 3 V with a current density of 0.9 μA cm(-2) is achieved.

Selective polypyrrole electrodes for quartz microbalances: NO2 and gas flux sensitivities

Abstract Quartz microbalances were produced which have polypyrrole as a conductive polymer electrode. The polymer layers were operated as both electrode and sensitive material to detect the concentration of toxic gases. Tests with NO2 demonstrated detection limits below the MAK values at room temperature. It was also investigated the response to some other noxious compounds and to the total gas flux. The possible mechanisms associated with the sensor properties are pointed out and discussed.

Silicate layer formation at Pr2O3∕Si(001) interfaces

We studied Pr2O3∕Si(001) interfaces by synchrotron radiation photoelectron spectroscopy and by ab initio calculations. We show that the interface formed during molecular-beam epitaxy under the oxygen partial pressure above 1×10−8mbar consists of a mixed Si–Pr oxide, such as (Pr2O3)(SiO)x(SiO2)y. Neither an interfacial SiO2 nor an interfacial silicide is formed. The silicate formation is driven by a low energy of O in a PrOSi bond and by the strain in the subsurface SiOx layer. We expect that this natural interfacial Pr silicate will facilitate the integration of the high-k dielectric Pr2O3 into future complementary metal–oxide–semiconductor technologies.