ABSTRACT
Solar water splitting is a potentially scalable method to store solar energy in the form of renewable hydrogen gas. In this study, we demonstrate that the photoelectrochemical (PEC) performance of hematite photoanodes can be improved by modification with the oxygen evolution catalyst CoBi. The current density at 1.23 V of the pristine hematite under one sun is 0.88 mA cm-2 and it increases to 1.12 mA cm-2 after CoBi modification (â¼27% improvement). The presence of a CoBi cocatalayst layer is proposed to improve the oxygen evolution reaction (OER) kinetics and also to prevent electron-hole recombination at the surface via passivating surface defects as well as suppressing the tunneling of electrons from the hematite core, thus improving the photocurrents and resulting in a negative shift of photocurrent onset potentials. These effects of CoBi modification are supported by experimental data obtained by performing electrochemical impedance spectroscopy (EIS), PEC and incident photon-to-current efficiency (IPCE) measurements. To investigate the electronic structure of the CoBi cocatalyst deposited on hematite, XPS and in situ X-ray absorption spectroscopy (XAS) are employed. Co K-edge spectra at different potentials and light conditions are recorded. This makes the present work different from most of the previous studies. Using a quantitative analysis method, information on the mean oxidation state of Co in the CoBi film under applied potential and illumination is revealed. We also compare different methods for determining the oxidation state from the edge position and find that the integral method and half height methods are most suitable. In summary, the present work underlines the improvement of the semiconductor/cocatalyst interface of oxygen evolving photoanodes and strengthens the importance of in situ XAS spectroscopy when studying catalysts. This study is the first report so far combining the studies of the PEC performance of a CoBi modified hematite nanorod array photoanode and in situ XAS at the Co K-edge.
ABSTRACT
High-throughput characterization by soft X-ray absorption spectroscopy (XAS) and electrochemical characterization is used to establish a correlation between electronic structure and catalytic activity of oxygen evolution reaction (OER) catalysts. As a model system a quasi-ternary materials library of Ni1-y-zFeyCrzOx was synthesized by combinatorial reactive magnetron sputtering, characterized by XAS, and an automated scanning droplet cell. The presence of Cr was found to increase the OER activity in the investigated compositional range. The electronic structure of NiII and CrIII remains unchanged over the investigated composition spread. At the Fe L-edge a linear combination of two spectra was observed. These spectra were assigned to FeIII in Oh symmetry and FeIII in Td symmetry. The ratio of FeIII Oh to FeIII Td increases with the amount of Cr and a correlation between the presence of the FeIII Oh and a high OER activity is found.
ABSTRACT
A new cell for operando soft X-ray absorption spectroscopy in transmission mode is presented. Developed for investigations on solar water-splitting catalysts, the cell allows the study of solid films in direct contact with electrolyte solution while applying voltage and visible light. The design is optimized for fast sample exchange and the simultaneous measurement of fluorescence and transmission signal. The capability of the cell is presented on a manganese oxide (MnOx) film, where electronic structure changes are monitored during forward and backward potential changes. Detailed information about the varying contributions of several Mn oxidation states during this process was revealed.
ABSTRACT
Two manganese porphyrin complexes, manganese tetraphenylporphyrin chloride (MnTPP-Cl) and manganese octaethylporphyrin chloride (MnOEP-Cl), exhibit distinctive spectral features of metal-to-ligand charge-transfer (MLCT) when dissolved in dichloromethane, characterized by resonant inelastic X-ray scattering at the Mn L-edge and N K-edge. The metal-ligand orbital mixing that mediates the MLCT is analyzed with the help of density functional theory/restricted open-shell configuration interaction singles calculations. On the basis of experimental and theoretical analyses, the distinctive MLCT is argued to originate from alteration of the porphyrin outer ligands: phenyl groups in MnTPP-Cl and ethyl groups in MnOEP-Cl.
ABSTRACT
The effect of monovalent cations (Li(+), K(+), NH4 (+), Na(+)) on the water structure in aqueous chloride and acetate solutions was characterized by oxygen K-edge X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy, and resonant inelastic X-ray scattering (RIXS) of a liquid microjet. We show ion- and counterion dependent effects on the emission spectra of the oxygen K-edge, which we attribute to modifications of the hydrogen bond network of water. For acetates, ion pairing with carboxylates was also probed selectively by XAS and RIXS. We correlate our experimental results to speciation data and to the salting-out properties of the cations.
ABSTRACT
Third harmonic generation (THG) of femtosecond laser pulses in sputtered nanocrystalline TiO2 thin films is investigated. Using layers of graded thickness, the dependence of THG on the film parameters is studied. The maximum THG signal is observed at a thickness of 180 nm. The corresponding conversion efficiency is 26 times larger compared to THG at the air-glass interface. For a demonstration of the capabilities of such a highly nonlinear material for pulse characterization, third-order autocorrelation and interferometric frequency-resolved optical gating (IFROG) traces are recorded with unamplified nanojoule pulses directly from a broadband femtosecond laser oscillator.
