PNNL

Abstract

This work for the first time unfurls the fundamental mechanisms and set the stage for an approach to derive electrocatalytic activity, which is otherwise not possible, in a traditionally known wide band gap oxide material. Specifically, we report on the tunable optical properties, in terms of wide spectral selectivity and red-shifted band gap, and electrocatalytic behavior of iron (Fe) doped gallium oxide (ß-Ga2O3) model system. X-ray diffraction (XRD) studies of sintered Ga2-xFexO3 (GFO) (0.0=x=0.3) compounds provide evidence for the Fe3+ substitution at Ga3+ site without any secondary phase formation. Rietveld refinement of XRD patterns reveal that the GFO compounds crystallize in monoclinic crystal symmetry with C2/m space group. The electronic structure of the GFO compounds probed using X-ray photoelectron spectroscopy data reveals that, at lower concentrations of Fe exhibits mixed chemical valence states (Fe3+, Fe2+), whereas single chemical valence state (Fe3+) is evident for higher Fe content (x=0.20-0.30). The optical absorption spectra reveal a significant red shift in the optical band gap with Fe doping. The origin of the significant red shift even at low concentrations of Fe (x=0.05) is attributed to the strong sp-d exchange interaction originated from the 3d5 electrons of Fe3+. The optical absorption edge observed at ˜450 nm with lower intensity is the characteristic of Fe doped compounds associated with Fe3+- Fe3+ double excitation process. Coupled with optical band gap red shift, electrocatalytic studies of GFO compounds reveals that, interestingly, Fe doped Ga2O3 compound exhibits electrocatalytic activity in contrast to intrinsic Ga2O3. Fe doped samples (GFO) demonstrated appreciable electrocatalytic activity towards the generation of H2 through electrocatalytic water splitting. An onset potential and tafel slope of GFO compounds includes: ~900 mV, ~210 mVdec-1 (x=0.15) and ~1036 mV, ~290 mVdec-1 (x=0.30). The electrocatalytic activity of Fe doped Ga-oxide compounds is attributed to cumulative effect of different mechanisms such as; doping resulted new catalytic centers, enhanced conductivity and electron mobility. Hence, in this report, for the first time, we explored a new pathway; electrocatalytic behavior of Fe doped Ga2O3 resulted due to Fe chemical states, red shift in optical band gap. The implications derived from this work may be applicable to a large class of compounds and further options may be available to design functional materials for electrocatalytic energy production.