Tobias Grewe

A crystal structure analysis and magnetic investigation on highly ordered mesoporous Cr2O3

A series of highly ordered mesoporous Cr(2)O(3) were prepared through the nanocasting pathway from decomposition of chromium(VI) oxide using KIT-6 as a hard template. The effects of the calcination temperature on the crystal structure, textural parameters and magnetic properties of the material were investigated. It was found that with increasing calcination temperature, surface area and pore volume of the mesoporous Cr(2)O(3) decreased slightly. Unpredictably, increasing calcination temperature also influences the lattice parameters of the Cr(2)O(3) crystal, and this rearrangement in the lattice parameter leads to changes in the value of the Néel temperature. A spin-flop transition has been observed at a magnetic field smaller than that of bulk material.

Nanocatalysts for Solar Water Splitting and a Perspective on Hydrogen Economy

In this review article, nanocatalysts for solar hydrogen production are the focus of discussion as they can contribute to the development of sustainable hydrogen production in order to meet future energy demands. Achieving this task is subject of scientific aspirations in the field of photo- and photoelectrocatalysis for solar water splitting where systems of single catalysts or tandem configurations are being investigated. In search of a suitable catalyst, a number of crucial parameters are laid out which need to be considered for material design, in particular for nanostructured materials that provide exceptional physical and chemical properties in comparison to their bulk counterparts. Apart from synthetic approaches for nanocatalysts, key parameters and properties of nanostructured photocatalysts such as light absorption, charge carrier generation, charge transport, separation and recombination, and other events that affect nanoscale catalysts are discussed. To provide a deeper understanding of these key parameters and properties, their contribution towards existing catalyst systems is evaluated for photo- and photoelectrocatalytic solar hydrogen evolution. Finally, an insight into hydrogen production processes is given, stressing the current development of sustainable hydrogen sources and presenting a perspective towards a hydrogen-based economy.

Alkali metals incorporated ordered mesoporous tantalum oxide with enhanced photocatalytic activity for water splitting

Herein, we report a novel synthetic approach for the preparation of alkali (Na, K) metal incorporated ordered mesoporous tantalate composites and their photocatalytic performance for water splitting. With the main focus on sodium based composite materials, a series of samples with ordered mesoporosity and high surface area (108–120 m2 g−1) was prepared by a variation of the Ta/Na ratios through a soft templating route. The structural parameters and properties of the samples were analyzed by low angle XRD, N2-physisorption, TEM and STEM analysis, EDX, XPS, Raman and diffuse reflectance UV-Vis spectroscopy. The incorporation of alkali metals resulted in ordered mesoporous tantalate composites. Furthermore, the addition of alkaline earth (Ca, Ba, Sr) metals to the precursor solution of ordered mesoporous tantalum oxide was attempted. However, alkaline earth metals gave unordered tantalate composites. Photocatalytic investigations for water splitting, by using methanol as a sacrificial agent, indicated that the incorporation of alkali metals enhanced the hydrogen production rate of the photocatalyst whereas addition of alkaline earth metals decreased the hydrogen production. Among the sodium based samples, a Ta/Na ratio of 9 showed the best performance. The efficiency of this sample was further improved through decorated NiOx as co-catalyst. A 2.5 wt% NiOx loading was found to be the optimal loading amount, generating 64 μmol h−1 H2 and 31 μmol h−1 O2 when tested for overall water splitting.

Amorphous and Crystalline Sodium Tantalate Composites for Photocatalytic Water Splitting

A facile hydrothermal synthesis protocol for the fabrication of sodium tantalates for photocatalytic water splitting is presented. Mixtures of tantalum and sodium ethoxide precursors were dispersed in ethanol, and ammonium hydroxide solution was used as mineralizer. By adjusting the amount of mineralizer, a variety of sodium tantalates with various morphologies, textural parameters, band gaps, crystal phases, and degrees of crystallinity were fabricated. The reaction was carefully monitored with a pressure sensor inside the autoclave reactor, and the obtained samples were characterized using X-ray diffraction, transmission electron microscopy, N2-physisorption, and ultraviolet-visible light spectroscopy. Among the series, the amorphous sample and the composite sample that consists of amorphous and crystalline phases showed superior activity toward photocatalytic hydrogen production than highly crystalline samples. Particularly, an amorphous sodium tantalate with a small fraction of crystalline nanoparticles with perovskite structure was found to be the most active sample, reaching a hydrogen rate of 3.6 mmol h(-1) from water/methanol without the use of any cocatalyst. Despite its amorphous nature, this photocatalyst gave an apparent photocatalyst activity of 1200 μmol g(-1) L(-1) h(-1) W(1-), which is 4.5-fold higher than highly crystalline NaTaO3. In addition, the most active sample gave promising activity for overall water splitting with a hydrogen production rate of 94 μmol h(-1), which is superior to highly crystalline NaTaO3 prepared by conventional solid-solid state route.

Designing Photocatalysts for Hydrogen Evolution: Are Complex Preparation Strategies Necessary to Produce Active Catalysts?

A facile synthetic route for the preparation of highly active photocatalysts was developed. The protocol involves the preparation of a photocatalyst through the direct injection of metal alkoxide precursors into solutions in a photoreactor. As a proof of concept, a tantalum oxide based photocatalyst was chosen as a model system. Tantalum ethoxide [Ta(OEt)5 ] was injected rapidly into a photoreactor filled with a water/methanol mixture, and a TaOx (OH)y composite formed and was able to produce hydrogen under light illumination. Compared to commercial and mesostructured Ta2 O5 and NaTaO3 materials, TaOx (OH)y produced by direct injection shows superior hydrogen production activity. Notably, the samples prepared by direct injection are amorphous; however, their photocatalytic performance is much higher than those of their crystalline equivalents. If Ta(OEt)5 was dispersed in methanol before injection, an amorphous framework with higher surface area and larger pore volume was formed, and the hydrogen production rate increased further. The addition of a sodium precursor during the injection further boosted the photocatalytic activity. Furthermore, this concept has also been applied to a titanium-based photocatalyst, and a much better hydrogen production rate has been obtained in comparison with that of commercial TiO2 (P25-Degussa); therefore, the direct-injection synthesis is a flexible method that opens the door to the facile preparation of highly active nanostructured photocatalysts for hydrogen production.

A Study on the Growth of Cr2O3 in Ordered Mesoporous Silica and Its Replication

A systematic study on the growth of Cr2O3 in three-dimensional cubic ordered mesoporous silica (KIT-6) and its replication through nanocasting is reported. By changing the loading time and amount of precursor, the size and shape of the obtained replica could be controlled to some extent. More interestingly, in contrast to previously published studies, when KIT-6 with an aging temperature of 100 °C, which has a high degree of interconnectivity, was used as a hard template, a cubic ordered mesoporous Cr2O3 replica with an open uncoupled subframework structure and reduced symmetry was obtained. Formation of a replica with different symmetry and uncoupled subframework structure is not only related to the degree of interconnectivity of the parent, but also strongly depends on the type of metal oxide and its growth mechanism in the silica template. Nanocasting of Cr2O3 with a low loading results in a replica with monomodal pore size distribution that has same symmetry as the hard template, whereas increasing the loading amount alters the symmetry of the replica and yields a replica with bimodal distribution.

Hyperbranched potassium lanthanum titanate perovskite photocatalysts for hydrogen generation

Semiconductors with hierarchical nanostructured morphologies may be promising as high surface area photocatalysts for producing hydrogen from water. However, there are few scalable synthesis methods that can achieve such morphologies in metal oxide semiconductors such as titanates. Here, hydrothermal methods were used to synthesize nanostructured potassium lanthanum titanate (KLTO) perovskite without using templates or structure-directing agents. The obtained materials were octahedral particles composed of orthogonal hyperbranched nanowires, a morphology that is usually obtained using catalyst-mediated vapor phase methods. Several fundamental materials properties of KLTO were determined for the first time, including the bandgap (3.3 eV), semiconductor type (n-type), flat band potential, and conduction band maximum (−0.265 V and −0.835 V vs. NHE, respectively). The KLTO hyperbranched structures were also investigated as UV-photocatalysts for H2 production and displayed higher activities than P25 TiO2 and KLTO nanoparticles. The H2 production rate for KLTO decorated with 1 wt% Pt using thermal decomposition of K2PtCl4 reached ca. 2.5 mmol h−1 and was stable for 20 h of irradiation.