Jiri Frydrych

Photoelectrochemical Water Splitting with Mesoporous Hematite Prepared by a Solution-Based Colloidal Approach

Sustainable hydrogen production through photoelectrochemical water splitting using hematite (alpha-Fe(2)O(3)) is a promising approach for the chemical storage of solar energy, but is complicated by the materials nonoptimal optoelectronic properties. Nanostructuring approaches have been shown to increase the performance of hematite, but the ideal nanostructure giving high efficiencies for all absorbed light wavelengths remains elusive. Here, we report for the first time mesoporous hematite photoelectodes prepared by a solution-based colloidal method which yield water-splitting photocurrents of 0.56 mA cm(-2) under standard conditions (AM 1.5G 100 mW cm(-2), 1.23 V vs reversible hydrogen electrode, RHE) and over 1.0 mA cm(-2) before the dark current onset (1.55 V vs RHE). The sintering temperature is found to increase the average particle size, and have a drastic effect on the photoactivity. X-ray photoelectron spectroscopy and magnetic measurements using a SQUID magnetometer link this effect to the diffusion and incorporation of dopant atoms from the transparent conducting substrate. In addition, examining the optical properties of the films reveals a considerable change in the absorption coefficient and onset properties, critical aspects for hematite as a solar energy converter, as a function of the sintering temperature. A detailed investigation into hematites crystal structure using powder X-ray diffraction with Rietveld refinement to account for these effects correlates an increase in a C(3v)-type crystal lattice distortion to the improved optical properties.

Facile fabrication of tin-doped hematite photoelectrodes – effect of doping on magnetic properties and performance for light-induced water splitting

We present a new, easily scalable method for the deposition of nanocrystalline hematite photoelectrodes based on the spin-coating of a mixed solution containing tin(II) and iron(III) chlorides followed by thermal treatment. Our facile approach does not require any additional film-forming organic species and allows simple control of the photoelectrochemical performance of the electrode by adjusting the degree of tin doping. When annealed at 650 °C a strong increase in the water oxidation photocurrent is observed with increasing tin concentration. The maximum performance (0.45 mA cm−2 at 1.43 V vs. RHE) was found at the highest possible tin loading (20 : 100, Sn : Fe). The contrasting performance of electrodes annealed at 650 °C and 800 °C suggests different activation processes for dopant diffusion and activation. The doping of tin into the crystal structure of hematite thin films is directly evidenced by X-ray photoelectron spectroscopy and indirectly by changes in the intrinsic magnetic parameters (Morin temperature, Neel temperature) of the hematite films. The magnetization measurements thus represent a potential technique to quantify doping amounts in hematite.

Universal LabVIEW-powered Mössbauer spectrometer based on USB, PCI or PXI devices

A new design of the universal Mossbauer spectrometer is presented. Hardware solution is based on commercial-available data acquisition devices working on the USB, PCI or PXI platform controlled by the main application running on the personal computer. Final application allows, in addition to Mossbauer spectra accumulation, the detailed analysis of the acquired detector signal in energy and time domains, and also to tune the velocity driving system separately. The experimental results show a high flexibility in various detectors and velocity transducers usage. It is easy to change the way of operation according to the different experimental requirements. This concept can be used with all common spectrometric benches with different velocity transducers, radioactive sources and gamma-ray detectors. This is a new approach in the Mossbauer spectrometer construction.

Digital proportional-integral-derivative velocity controller of a Mössbauer spectrometer

The digital proportional-integral-derivative (PID) velocity controller used in the Mossbauer spectrometer implemented in field programmable gate array (FPGA) is based on the National Instruments CompactRIO embedded system and LabVIEW graphical programming tools. The system works as a remote system accessible via the Ethernet. The digital controller operates in real-time conditions, and the maximum sampling frequency is approximately 227 kS s−1. The system was tested with standard sample measurements of α-Fe and α-57Fe2O3 on two different electromechanical velocity transducers. The nonlinearities of the velocity scales in the relative form are better than 0.2%. The replacement of the standard analog PID controller by the new system brings the possibility of optimizing the control process more precisely.

Development of the Fast Scintillation Detector with Programmable High Voltage Adjustment Suitable for Mössbauer Spectroscopy

This work is focused on a development of a compact fast scintillation detector suitable for Mossbauer spectroscopy (low energy X‐ray/γ‐ray detection) where high counting rates are inevitable. Optimization of this part was necessary for a reliable function, better time resolution and to avoid a detector pulses pile‐up effect. The pile‐up effect decreases the measurement performance, significantly depends on the source activity and also on the pulse duration. Our new detection unit includes a fast scintillation crystal YAP:Ce, an R6095 photomultiplier tube, a high voltage power supply socket C9028‐01 assembly, an AD5252 digital potentiometer with an I2C interface and an AD8000 ultra fast operation preamplifier. The main advantages of this solution lie in a short pulse duration (less than 200 ns), stable operation for high activities, programmable gain of the high voltage supply and compact design in the aluminum housing.

Conversion Electron Detectors for 57Fe Mössbauer Measurements

This review article is intended to help all those, who now begin to work in the field of conversion electron Mossbauer spectroscopy (CEMS) and who have to select a right type of a detector. There exist a large number of specific engineering designs in each of the detector kind, which is impossible to cover in one paper. That is why we only try to outline basic classification and general survey of properties of individual CEMS detector kinds supplemented by references to its more detailed analysis. 57Fe is the most frequently used isotope in CEMS and thereby the paper is devoted to this case.

Investigation of the Mössbauer Spectrum Quality as a Dependence on the Frequency of the Velocity Signal

This paper is focused on a quality characterizing the Mossbauer spectra measured for various frequencies of the velocity signal. Standard electromechanical double‐loudspeaker drive and digital PID velocity controller were used for calibration spectra measurement in the frequency interval from 4 up to 100 Hz. Several parameters were evaluated for recommendation of the suitable velocity signal frequency.