Sebastian Fiechter

Helmholtz-Zentrum Berlin | HZB · Scientific Administration

A thiomolybdate [Mo3S13]2- nanocluster is a promising catalyst for hydrogen evolution reaction (HER) due to the high number of active edge sites. In this work, thiomolybdate cluster films are prepared by spin-coating of a (NH4)2Mo3S13 solution both on FTO glass substrates as hydrogen evolving electrodes and on highly 00.1-textured WSe2 for photoele...

Bismuth vanadate (BiVO4) is a promising photoanode material for photoelectrochemical water splitting due to its relative stability, low‐cost and non‐toxic properties. However, its performance is limited by the large band gap (Eg) of 2.4 eV, and the record photocurrent is already within 90% of its theoretical limit. Further photocurrent enhancement...

There is an urgent need for cheap, stable, and abundant catalyst materials for photoelectrochemical water splitting. Manganese oxide is an interesting candidate as an oxygen evolution reaction (OER) catalyst, but the minimum thickness above which MnO x thin films become OER-active has not yet been established. In this work, ultrathin (<10 nm) manga...

ZnGeP2 crystals and epilayers can be grown by chemical vapor transport using phosphorous of hydrogen chloride as transporting agent. The limiting factor in the transport rate is the volatility of germanium which has a seven order of magnitude smaller partial pressure than elemental zinc. optical absorption measurements of the vapor phase show the p...

Water splitting in photoelectrochemical cells is a promising technology to produce solar hydrogen. Fe2TiO5 pseudobrookite with a bandgap of around 2 eV absorbs the predominant visible range of the solar spectrum and is emerging as a promising photoanode for such cells. Herein, we present Fe2TiO5 pseudobrookite-based films prepared by aerosol-assist...

Pulsed laser deposition (PLD) is an appealing technique to fabricate thin films with specific film orientation, stoichiometry, and morphology through tuning of experimental parameters. Here, we present Fe2TiO5, one of the promising photoanode materials, grown on fluorine-doped tin oxide (FTO) substrates through PLD. The structural and morphological...

Molybdenum sulfide is of interest as a noble metal-free catalyst for the hydrogen evolution reaction (HER). In crystallized form, it shows a typical stacking of planar S–Mo–S layers whereas the catalytically active centers are situated on the edges of these entities characterized by non-saturated bonds of the molybdenum atoms. In this study, 2H-MoS...

The efficient catalysis of the four-electron oxidation of water to molecular oxygen is a central challenge for the development of devices for the production of solar fuels. This is equally true for artificial leaf-type structures and electrolyzer systems. Inspired by the oxygen evolving complex of Photosystem II, the biological catalyst for this re...

In order to identify new solar water splitting photoanodes, Fe–Ti–W–O materials libraries were fabricated by combinatorial reactive co-sputtering and investigated by high-throughput characterization methods to elucidate compositional, thickness, and structural properties. In addition, photoelectrochemical measurements such as potentiodynamic photoc...

Highly (00.1)-textured polycrystalline WSe2 films are developed as hydrogen evolving photoelectrodes and improved through deposition of a thin ammonium thiomolybdate (NH4)2Mo3S13 catalyst film. This semiconducting thiomolybdate forms a heterojunction with the p-type WSe2 film passivating recombination centers of excited electron-hole pairs at the e...

Molybdenum sulfide MoSx is considered as attractive hydrogen evolution catalyst since it is free of noble metals and shows a low overpotential. Especially, amorphous molybdenum sulfide has attracted attention because of its high catalytic activity. However, the catalytic mechanism of the hydrogen evolution reaction is not yet fully understood. Ther...

The surfactant-assisted synthesis of tantalum oxide nanoparticles (Ta 2 O 5), its subsequent nitridation to form tantalum nitride (Ta 3 N 5), and the evaluation of the photoactivity of these nanoparticles, are presented. The surface, optical, and compositional characterization of the in-house synthesized photocatalysts indicate spherical nanopartic...

The surfactant-assisted synthesis of tantalum oxide nanoparticles (Ta2O5), its subsequent nitridation to form tantalum nitride (Ta3N5), and the evaluation of the photoactivity of these nanoparticles, are presented. The surface, optical, and compositional characterization of the in-house synthesized photocatalysts indicate spherical nanoparticles wi...

p> In the present work we demonstrate a record photocurrent density of 5.6 mA cm^-2 at 0 V_RHE for WSe₂ photocathodes coated with an effective/earth abundant catalyst - ammonium thiomolybdate (NH₄)₂Mo₃S₁₃ (ATM). The photocurrent density can be tuned by optimizing the WSe<su...

Tackling the conflict between the optical and electronic properties of the ultrathin optoelectronic films, is of critical importance for the development of highly efficient photo-energy conversion devices. We show in this report a proof of concept of designing ultrathin photoelectrode film that compensates intrinsic low absorption coefficients and...

Metal oxide photoelectrodes typically suffer from poor carrier transport properties and extensive carrier recombination, which is caused by the presence of intrinsic or extrinsic defects in the material. Here, the influence of annealing temperature and atmosphere on the formation and suppression of defects in BiVO4—one of the best performing metal...

Due to its high energy density, hydrogen could play an important role to store chemical energy in GW scale. To produce it in large quantities, “artificial leaf”-type structures can be used to convert solar light into hydrogen by photoelectrochemical splitting of water. Since this process is most efficiently working under acidic conditions, acid-sta...

Photoemission with a twist Attosecond time-resolved spectroscopy provides the ability to probe the fastest electronic processes in atoms and solids. Yet the photoemission process from solids is not fully understood. Siek et al. studied photoemission from the layered van der Waals material WSe ² and found that electron emission occurs as a sequence...

In this study we demonstrate that the PEC performance of BiVO4 photoanodes can be improved by the deposition of a MnPi catalyst layer. We investigated the electronic structure of the layer using in situ soft X-ray Absorption spectroscopy (XAS) at the Mn L-edge upon varying the applied potentials and the illumination conditions. Using the linear com...

The recent climate change agreement in Paris highlights the imperative to aggressively decarbonize the energy economy and develop new technologies, especially for the generation of electrical energy that are environmentally clean. This challenge can only be addressed by a multi-pronged approach to research and education of the next generation of sc...

α-Mn2O3 is of interest as a low-cost and environmentally benign electrocatalyst for the Oxygen Evolution Reaction (OER) in the process of water splitting. Mechanically stable α-Mn2O3 electrodes are prepared by annealing of galvanostatically deposited MnOOHx layers on F:SnO2-coated glass. The overpotential η to achieve a current density of j = 10 mA...

Ag nanoparticles have attracted interest for plasmonic absorption enhancement of solar cells. For this purpose, well-defined particle sizes and densities as well as very low deposition temperatures are required. Thus, we report here a new spray chemical vapour deposition method for producing Ag NP films with independent size and density control at...

Intermediate band semiconductors have raised interest as materials to both enhance photovoltaics’ efficiency and promote photocatalytic activity driven by visible light. The present work shows the synthesis of In2S3 doped with four different ratios of V using ILGAR technique. This nebulize-spray based technique allows the deposition of In2(V)S3 thi...

Solar powered electrolysis of water is a promising way to harvest and store solar energy in the form of large quantities of hydrogen. The photovoltage of semiconducting materials is used in photoelectrochemical cells or photovoltaic devices to provide the required thermodynamic potential for water splitting (1.23V) and the over-potentials for the o...

Although Cu(In,Ga)(S,Se)2 (CIGSe) based thin film solar cells have reached efficiencies exceeding 22% based on vacuum processed CIGSSe absorbers, the supply of indium and gallium might become an issue if CIGSSe thin-film solar cells are produced in very large volumes. It is therefore mandatory to reduce the wastage of indium and gallium during the...

The development of energy converting devices, which are able to light-induced split water into hydrogen and oxygen, has attracted increasing notice in the last two decades leading to numerous research activities worldwide addressing fundamental questions in the better understanding of photosynthesis as well as in the design of bioreactors and inorg...

The generation of fuels from sunlight and water is considered as a task of paramount importance for a sustainable energy supply in the future. Decomposition of water by a photoelectrochemical process is a possibility to store solar energy in the form of hydrogen on a large scale. For this purpose, a suitable membrane can be immersed in an aqueous e...

The development of highly active and stable non-noble metal catalysts (NNMC) for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEM-FC) becomes of importance in order to enable cost reduction. In this work, we discuss the structural composition as derived from Fe-57 Mößbauer spectroscopy and X-ray diffraction, catalytic...

Due to the high energy density of hydrogen it could play an important role as energy storage medium replacing fossil fuels in the future. To produce it in large quantities “artificial leaf”-type structures could be used converting solar light into this fuel by electrochemical decomposition of water. Since this process is most efficient working unde...

This work begins with a summary of the role of reduced graphene oxide (RGO) in promoting the photoactivity of oxides and chalcogenides. Initial indicators suggest that RGO is a good material for promoting charge speration over short time scales. However the story over long term illumination is different. This presentation delineates the results of...

In photosynthesis nature uses transition metal complexes as catalysts to evolve oxygen and hydrogen from water. In this process the catalytic centers are separated from the light capturing and absorbing co-factors in photosystem I and II integrated in the thylakoid membrane. To develop bio-inspired catalysts and to mimic e.g. the Mn 3 CaO 3 MnO com...

An integrated water-splitting device based on a triple-junction silicon-based solar cell (a-Si:H/a-Si:H/μc-Si:H; a-Si=amorphous silicon, μc-Si=microcrystalline) in superstrate configuration modified with catalysts at the back and front contacts is described. In this configuration, the solar cell is illuminated by the glass substrate, while the back...

This work delineates the results of a systematic analysis of the photoelectrochemical (PEC) and optical responses of reduced graphene oxide (RGO) combined with a representative multi-metal oxide - bismuth titanate (BTO) on films. We investigated the role of RGO as a promoter of charge separation and transport, by suppressing recombination loss of c...

Today, most metal and nitrogen doped carbon catalysts for ORR reveal a rather heterogeneous composition. This can be reasoned by a non-optimized precursor composition and different steps in the preparation process to get the required active material. The significant presence of inorganic metal species interfere the assignment of descriptors related...

In this study, a promising alternative method for addressing grain boundary issues in hematite (α-Fe2O3)-based photoanodes is presented. The porous α-Fe2O3 films are prepared by dip coating a polymerizable precursor onto fluorine-doped tin oxide (FTO) substrates. The photoelectrochemical (PEC) performance of α-Fe2O3 photoanodes is characterized and...

A typical procedure based on sol-gel aggregation for the fabrication of mesoporous WO3 photoanodes was modified and used produce modified tungsten oxide films by utilizing the tungstic acid sol admixed with Keggin-type borotungstic acid (H5BW12O40) or the borotungstic acid-stabilized hematite (Fe2O3). Physicochemical properties (structure, morpholo...

A procedure based on sol-gel aggregation for the fabrication of mesoporous WO 3 photoanaodes was adopted to produce the doped oxide films by admixing the tungstic acid sol with Keggin-type borotungstic acid (H 5 BW 12 O 40 ) or the borotungstic acid–stabilized hematite (Fe 2 O 3 ). Such physicochemical properties as structure, morphology and spectr...

Cooperative photoelectrochemical (PEC) conversion of solar energy into chemical fuels is considered as one of the most promising solutions to the sustainable energy needs of mankind, considering the intermittent and spatial fluctuations in the availability of sunlight on earth. The development of synthetic visible-light-driven semiconductor catalys...

Ultra-thin α-Fe2O3 (hematite) films have been deposited by radio frequency (RF) sputtering technique and photoelectrochemically investigated towards their ability to oxidize water. By varying the deposition power and time as well as the sputter gas flow (argon), the microstructure and morphology of the film were optimized. It was found that the inc...

Hydrogen is expected to become a commonly used energy carrier on the global scale in the near future. However, hydrogen as a fuel is environmentally friendly only when generated from water using renewable energy, such as solar energy. Therefore, intensive research aims to develop a new generation of solar materials, which may be used for the produc...

In the effort to develop an efficient and cost effective photo-electrochemical device for water splitting driven by sunlight only, transition metal oxides are promising candidates to catalyze the oxygen evolution reaction (OER) at the anode. We used X-ray photoelectron spectroscopy (XPS) to characterize very active manganese and cobalt oxide thin f...

The compound semiconductor RuS2, known as mineral laurite, has been investigated as a potential (photo)electrochemically active anode material for the oxygen evolution in the process of (photo)electrolytic water splitting. The contribution describes for the first time the preparation of RuS2 thin films deposited on (100)- and (111)-oriented FeS2 (p...

Sunlight is a nearly inexhaustible energy source, but its large-scale use requires an efficient storage solution. One promising storage concept is the direct conversion of light into chemical energy, for example by producing hydrogen through solar-powered water electrolysis. In order to provide the required minimum photovoltage of 1.23 V, photoelec...

Different manganese oxide phases were prepared as thin films to elucidate their structure–function relationship with respect to oxygen evolution in the process of water splitting. For this purpose, amorphous MnOx films anodically deposited on F:SnO2/glass and annealed at different temperatures (to improve film adherence and crystallinity) were test...

Synthetic pyrite crystals doped with halogen (CI, Br) or manganese (Mn) were investigated by ESR at 78 K with an X-band spectrometer and a TE102 cavity with 100 kc field modulation. The crystals were preferably measured with the directions 〈100〉, 〈110〉, and 〈111〉 parallel to the static magnetic field HO. The ESR spectrum of Mn-doped crystals showed...

In many synthetic pyrite single crystals of high quality and purity we could detect Cr3 + and Ni2 + by EPR at 78 K as unwanted impurities. The assignment was possible by means of the isotopes Cr53 and Ni61, occurring in natural abundance. The paramagnetic centers exhibit unusual g-values. In case of Ni2 + a ligand hyperfine interaction could be obs...

The successful preparation of a nanohybrid electrode made from Ta3N5 nanoparticle powders interlinked by Nafion is described. The basic photoelectrochemical properties as anode for water photolysis are investigated and gas evolution as well as corrosion reactions are determined by differential electrochemical mass spectroscopy (DEMS) and X-ray phot...

The oxygen evolution reaction (OER) represents a bottle neck in the development of sustainable light-driven production of hydrogen from water using cheap and abundant electro-catalysts. Even though metal oxide catalysts have been used for decades, the mechanism of the OER has not been fully understood yet. We investigated Cobalt and Manganese Oxide...

Owing to the increasingly apparent climate change, it becomes imperative to use renewable energy in the production of fuel that is environmentally friendly. At the same time, there is a need to introduce the related education programs to develop the skills of the technical staff working at the front line of rapidly developing renewable energy techn...

Attosecond time-resolved XUV streaking experiments are reported for cleaved WSe2 surfaces. The photoemission from Se 3d and W 4f core levels occurs delayed by 50 attoseconds with respect to the valence band emission.

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 diff...

Solar radiation can be used as a nearly inexhaustible energy source to convert light into chemical energy. In this way fossil fuels can be replaced and a sustainable society established in the future. One promising concept is the production of hydrogen by light powered water electrolysis. In order to provide the required high photo-voltage suitable...

Hydrogen generated by conversion of sunlight into chemical energy is considered as a fuel of the future replacing fossil carbon and hydrocarbons. A sustainable and environmentally acceptable way to produce H 2 is by the photoelectrolysis of water, i.e. the conversion of solar into chemical energy by water splitting. For this purpose we modified a t...

We investigated the photoelectrochemical properties of both n- and p-type (In,Ga)N nanowires (NWs) for water splitting by in-situ electrochemical mass spectroscopy (EMS). All NWs were prepared by plasma-assisted molecular beam epitaxy. Under illumination, the n-(In,Ga)N NWs exhibited an anodic photocurrent, however, no O2 but only N2 evolution was...

According to the present invention, the catalyst performance of a chelate catalyst comprising a complex of a macrocyclic compound such as a porphyrin derivative is improved. Also, the following method is provided: a method for preparing a fuel cell electrode catalyst comprising a nitrogen-containing metal complex in which a metallic element is coor...

alpha-Fe2O3 (hematite) photoanodes for the oxygen evolution reaction (OER) were prepared by a cost-efficient sol-gel procedure. We show that annealing and plasma post-treatments improve significantly the photoelectrochemical properties for the OER. Annealing steps lead to a reorganization of the crystal structure and thereby to a pronounced shift o...

We report for the first time the demonstration of high intensity sonication treatment as a simple and effective way to fundamentally improve the performance of nanoparticles for photoelectrochemical (PEC) water splitting. The capability of making highly photoactive nanoparticles by high intensity sonication is highly appreciated to open up new oppo...

On the path to an energy transition away from fossil fuels to sustainable sources, the European Union is for the moment keeping pace with the objectives of the Strategic Energy Technology-Plan. For this trend to continue after 2020, scientific breakthroughs must be achieved. One main objective is to produce solar fuels from solar energy and water i...

Thin layers of ruthenium sulfide (RuS2 ± x, laurite) were deposited on a heated substrate by reactive magnetron sputtering using a metallic ruthenium target and H2S as reactive gas. The balanced magnetron plasmas were excited by a direct-current (DC) or a radio-frequency (RF, 27 MHz) power supply, respectively. To comprehensively study the influenc...

More efficient energy conversion systems may help to reduce the use of fossil fuels and the emission of greenhouse gases. Polymer Electrolyte Membrane Fuel Cells are such devices which are of particular interest for automotive applications. Unfortunately, cost issues are still limiting the application of this technology in a highly competitive mark...

α-Fe(2)O(3) (hematite) photoanodes for the oxygen evolution reaction (OER) were prepared by a cost-efficient sol-gel procedure. Due to low active photoelectrochemical properties observed, it is assumed that the sol-gel procedure leads to hematite films with defects and surface states on which generated charge carriers are recombined or immobilized...

Crystalline RuS(2) layers were prepared on titanium sheets by reactive magnetron sputtering using a metallic ruthenium target and a H(2)S-Ar mixture as process gas. The ability of these layers for the electrooxidation of water (OER) was investigated by differential electrochemical mass spectrometry (DEMS) in 0.5 M H(2)SO(4) electrolyte. It was obse...

Characterization of facets of particles is a common problem. In this paper an algorithm is presented which allows automated quantitative 3D analysis of facets of many particles within tomographic datasets. The algorithm is based on the analysis of probability distributions of the orientations of triangle normals of mesh representations. The result...

Fe-based catalytic sites for the reduction of oxygen in acidic medium have been identified by (57)Fe Mössbauer spectroscopy of Fe/N/C catalysts containing 0.03 to 1.55 wt% Fe, which were prepared by impregnation of iron acetate on carbon black followed by heat-treatment in NH(3) at 950 °C. Four different Fe-species were detected at all iron concent...

Nanostructured hematite films prepared by a sol-gel procedure were evaluated for the photo-induced oxygen evolution reaction (OER). Surface treatments such as plasma and post-annealing at 500 degrees C in oxygen gas flow were used in order to condition the electrode surface. Electrochemical measurements of the plasma treated samples revealed that t...

Based on the oxygen evolving μ-oxido-Mn4Ca cluster unit in photosynthesis II, Ca2Mn3O8 films prepared by dip-coating and screen-printing were tested as oxygen evolving catalysts. Structural analysis confirmed that smooth 100 nm films can be grown from a Ca2MnO4 sol–gel solution showing a high orientation in (1 0 0) direction when deposited on F:SnO...

The development of novel oxides, nanocomposites and architectures is in demand for the direct conversion of solar energy into other forms of energy, such as chemical and electrical energy. Especially, those oxides and devices, which can be used for photolysis of water (hydrogen and oxygen evolution) and for water purification, are of interest. So f...

Titanium dioxide (rutile) is known as n-type semiconductor. Recent studies show that prolonged oxidation of pure n-type TiO2 may lead to its conversion into a p-type semiconductor. It has been documented that the conversion is associated with the formation of titanium vacancies. The present work derives the defect disorder model of TiO2, which expl...

The high cost of proton-exchange-membrane fuel cells would be considerably reduced if platinumbased catalysts were replaced by iron-based substitutes, which have recently demonstrated comparable activity for oxygen reduction, but whose cause of activity decay in acidic medium has been elusive. Here, we reveal that the activity of Fe/N/C-catalysts p...

For the first time, the effect on structure and composition by a second heat treatment in ammonia was studied in detail for Fe–N–C catalysts prepared by oxalate-supported pyrolysis of chloroirontetrametoxyphenylporphyrin (Fe(TMPP)Cl). The oxygen reduction reaction (ORR) activity was determined by rotating ring-disk electrode (RRDE) measurements in...

Platinum electrocatalysts for the oxygen reduction reaction have been prepared by impregnation of platinum acetylacetonate on pristine and NH3 treated Black Pearls. The subsequent heat treatment leads to electrochemical active platinum particles in nm-scale (2 - 3 nm). By electrochemical measurements (CV and RDE), the platinum surface properties an...

Presently our institute pursues the aim to realize a monolithic structure to split water from sunlight by immerging a membrane in an electrolyte consisting of photo-electrocatalytic electrode surfaces to evolve hydrogen and oxygen from water as demonstrated for the first time by Khaselev and Turner in 1998 [1]. For this type of device alternative p...

We present transmission electron microscope (TEM) tomography investigations of ruthenium-based fuel cell catalyst materials as employed in direct methanol fuel cells (DMFC). The digital three-dimensional representation of the samples not only enables detailed studies on number, size, and shape but also on the local orientation of the ruthenium part...

Powder of carbon nitride has been investigated by surface photovoltage spectroscopy at temperatures between 30 °C and 150 °C. Photo-generated holes were preferentially separated towards the external surface. Electronic states below the optical band gap from which charge separation may be possible have not been observed. The band gap of the investig...