Influence of light intensity on the steady-state kinetics in tungsten trioxide particulate photoanode studied by intensity-modulated photocurrent spectroscopy

Iron Oxychalcogenides and Their Photocurrent Responses

We report here the results of an experimental investigation of the electronic properties and photocurrent responses of the CaFeOQ and La2O2Fe2OQ2 phases and a computational study of the electronic structure of polar CaFeOSe. We find that both CaFeOQ (Q = S and Se) have band gaps and conduction band edge positions compatible with light-driven photocatalytic water splitting, although the oxysulfide suffers from degradation due to the oxidation of Fe2+ sites. The higher O/Q ratio in the Fe2+ coordination environment in CaFeOSe increases its stability without increasing the band gap beyond the visible range. The photocurrent CaFeOSe shows fast electron-hole separation, consistent with calculated carrier effective masses. These results suggest that these iron oxychalcogenides warrant further study to optimize their stability and morphology for photocatalytic and other photoactive applications.

Anodic Oxidation of Tungsten under Illumination-Multi-Method Characterization and Modeling at the Molecular Level

Tungsten oxide has received considerable attention as photo-anode in photo-assisted water splitting due to its considerable advantages such as significant light absorption in the visible region, good catalytic properties, and stability in acidic and oxidative conditions. The present paper is a first step in a detailed study of the mechanism of porous WO3 growth via anodic oxidation. In-situ electrochemical impedance spectroscopy (EIS) and intensity modulated photocurrent spectroscopy (IMPS) during oxidation of W illuminated with UV and visible light are employed to study the ionic and electronic processes in slightly acidic sulfate-fluoride electrolytes and a range of potentials 4-10 V. The respective responses are discussed in terms of the influence of fluoride addition on ionic and electronic process rates. A kinetic model is proposed and parameterized via regression of experimental data to the EIS and IMPS transfer functions.

Improved Photocatalytic Water Splitting Activity of Highly Porous WO3 Photoanodes by Electrochemical H+ Intercalation

WO3 photoanodes are widely used in photoelectrochemical catalysis, but typically the as-synthesized material is annealed before application. It is therefore desirable to explore less energy-intensive treatments. In this study, WO3 films of up to 3.9 μm thickness were obtained by galvanostatic anodization of tungsten foil in a neutral-pH Na2SO4 and NaF electrolyte, also containing a NaH2PO2 additive (to suppress O2 accumulation on the pore walls). Additionally, the WO3 photoanodes were modified by applying a cathodic reduction (H+ intercalation) and anodic activation treatment in-situ. XPS spectra revealed that intercalation modifies WO3 films; the amount of W5+-O and O-vacancy bonds was increased. Furthermore, subsequent activation leads to a decrease of the W5+ signal, but the amount of O-vacancy bonds remains elevated. The as-prepared and reduced (intercalated & activated) films were tested as OER photoanodes in acidic 0.1 M Na2SO4 media, under illumination with a 365 nm wavelength LED. It was observed that thinner films generated larger photocurrents. The peculiarities detected by XPS for reduced films correlate well with their improved photocatalytic activity. Photo-electrochemical impedance and intensity modulated photocurrent spectroscopies were combined with steady-state measurements in order to elucidate the effects of H+ intercalation on photoelectrochemical performance. The reduction results in films with enhanced photoexcited charge carrier generation/separation, improved conductivity, and possibly even suppressed bulk recombination. Thus, the intercalation & activation adopted in this study can be reliably used to improve the overall activity of as-synthesized WO3 photoanodes, and particularly of those that are initially poorly photoactive.