Ramona Gutkowski

Photodeposition of copper and chromia on gallium oxide: the role of co-catalysts in photocatalytic water splitting.

Split second: The photocatalytic activity of gallium oxide (β-Ga2 O3) depends strongly on the co-catalysts CuOx and chromia, which can be efficiently deposited in a stepwise manner by photoreduction of Cu(2+) and CrO4 (2-). The water-splitting activity can be tuned by varying the Cu loading in the range 0.025-1.5 wt %, whereas the Cr loading is not affecting the rate as long as small amounts (such as 0.05 wt %) are present. Chromia is identified as highly efficient co-catalyst in the presence of CuOx : it is essential for the oxidation of water.

High-Resolution Analysis of Photoanodes for Water Splitting by Means of Scanning Photoelectrochemical Microscopy

In pursuance of efficient tools for the local analysis and characterization of novel photoelectrocatalytic materials, several SECM-based techniques have been developed, aiming on the combined benefit of a local irradiation of the analyzed sample and a microelectrode probe for the localized electrochemical analysis of the surface. We present the development and application of scanning photoelectrochemical microscopy (SPECM) for the laterally resolved characterization of photoelectrocatalytic materials. Particularly, the system was developed for the photoelectrochemical characterization of n-type semiconductor-based photoanodes for water splitting. By using the tip microelectrode simultaneously for local irradiation and as an electrochemical probe, SPECM was capable to simultaneously provide information about the local photocurrent generated at the sample under irradiation and to detect the photoelectrocatalytically evolved oxygen at the microelectrode. In combination with a novel means of irradiation of the interrogated sample, local analysis of semiconductor materials for light-induced water splitting with improved lateral resolution is achieved.

Improved photoelectrochemical performance of electrodeposited metal-doped BiVO4 on Pt-nanoparticle modified FTO surfaces

The recombination of photogenerated electron–hole pairs is one of the main limiting factors of photoelectrocatalysts absorbing in the visible part of the solar spectrum. Especially for BiVO4 the slow electron transport to the back contact facilitates charge recombination. Hence, thin layers have to be used to obtain higher photocurrents which are concomitantly only allow low absorption of the incident light. To address this limitation we have modified FTO substrates with Pt-nanoparticles before electrodepositing BiVO4. The Pt-nanoparticles decrease the overpotential for the electrodeposition of BiVO4, but more importantly they provide the basis for decreased charge recombination. Electrodeposited Mo-doped BiVO4 on Pt-nanoparticle modified FTO exhibits a substantially decreased recombination of photogenerated charge carriers during frontside illumination. Simultaneous co-doping of BiVO4 with two different metals leads to a substantial enhancement of the incident-photon-to-current efficiency (IPCE) during light driven oxygen evolution reaction. Highest IPCE (>30% at 1.2 V vs. RHE) values were obtained for Mo/Zn- and Mo/B-doped BiVO4.

New materials for the light-induced hydrogen evolution reaction from the Cu–Si–Ti–O system

Cu-containing photocathodes are generally limited by fast photocorrosion under working conditions. Hence stabilization of these materials is a key factor in their potential application for the light-induced hydrogen evolution reaction (HER). In order to identify new materials, oxidized Cu–Si–Ti metallic thin film precursor materials libraries were evaluated using a combinatorial approach. High-throughput photoelectrochemical characterization using an automated optical scanning droplet cell was performed on a material library to analyze doping and alloying effects on the light-induced HER. The results revealed that compositions near Ti-doped CuSiO3 (dioptase and copper-polysilicate) and Si-doped Cu3TiOx act as comparatively stable and highly active materials for HER.

Unraveling compositional effects on the light-induced oxygen evolution in Bi(V–Mo–X)O4 material libraries

The influence of co-deposited transition metals X (X = Ta, W, Nb) with various relative concentrations on the photoelectrochemical performance of BiVO4 is investigated. Thin film material libraries with well-defined composition gradients of Bi, V and two transition metals are fabricated by combinatorial sputter co-deposition. Materials with the highest photoelectrochemical performance are identified by high-throughput characterization of the Bi(V–Mo–X)O4 material libraries using an optical scanning droplet cell. Bi(V–Mo–W)O4 and Bi(V–Mo–Nb)O4 material libraries show the highest improvement in the photocurrent, with ten times higher photocurrents of up to 1 mA cm−2 compared to a BiVO4 reference material library. Deviations from the V : Bi equiatomic ratio lead to a decrease in the photocurrent for pristine monoclinic BiVO4. By the addition of transition metals this effect is minimized and no significant decrease in the photocurrent occurs up to 10 at% variation from the equiatomic V : Bi ratio. Excellent photoelectrochemical performance is reached under these conditions in regions with a V : Bi atomic ratio of 70 : 30 and co-deposited Nb concentrations of >10 at%. Scanning photoelectrochemical microscopy allows the evaluation of the correlation between the generated oxygen at a photoanode and the measured photocurrent.

Combinatorial Synthesis and High-Throughput Characterization of Fe–V–O Thin-Film Materials Libraries for Solar Water Splitting

The search for suitable materials for solar water splitting is addressed with combinatorial material science methods. Thin film Fe-V-O materials libraries were synthesized using combinatorial reactive magnetron cosputtering and subsequent annealing in air. The design of the libraries comprises a combination of large compositional gradients (from Fe10V90O x to Fe79V21O x) and thickness gradients (from 140 to 425 nm). These material libraries were investigated by high-throughput characterization techniques in terms of composition, structure, optical, and photoelectrochemical properties to establish correlations between composition, thickness, crystallinity, microstructure, and photocurrent density. Results show the presence of the Fe2V4O13 phase from ∼11 to 42 at. % Fe (toward low-Fe region) and the FeVO4 phase from ∼37 to 79 at. % Fe (toward Fe-rich region). However, as a third phase, Fe2O3 is present throughout the compositional gradients (from low-Fe to Fe-rich region). Material compositions with increasing crystallinity of the FeVO4 phase show enhanced photocurrent densities (∼160 to 190 μA/cm2) throughout the thickness gradients whereas compositions with the Fe2V4O13 phase show comparatively low photocurrent densities (∼28 μA/cm2). The band gap energies of Fe-V-O films were inferred from Tauc plots. The highest photocurrent density of ∼190 μA/cm2 was obtained for films with ∼54 to 66 at. % Fe for the FeVO4 phase with ∼2.04 eV for the indirect and ∼2.80 eV for the direct band gap energies.