Crystal Chemistry, Band-Gap Red Shift, and Electrocatalytic Activity of Iron-Doped Gallium Oxide Ceramics.

This work for the first time unfurls the fundamental mechanisms and sets 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-x Fe x O3 (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 reveals that the GFO compounds crystallize in monoclinic crystal symmetry with a C2/m space group. The electronic structure of the GFO compounds probed using X-ray photoelectron spectroscopy data reveals that at lower concentrations, 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 an optical band-gap red shift, electrocatalytic studies of GFO compounds reveal that, interestingly, Fe-doped Ga2O3 compound exhibits electrocatalytic activity in contrast to intrinsic Ga2O3. Fe-doped samples (GFO) demonstrated appreciable electrocatalytic activity toward the generation of H2 through electrocatalytic water splitting. An onset potential and Tafel slope of GFO compounds include ∼900 mV, ∼210 mV dec-1 (x = 0.15) and ∼1036 mV, ∼290 mV dec-1 (x = 0.30), respectively. The electrocatalytic activity of Fe-doped Ga-oxide compounds is attributed to the cumulative effect of different mechanisms such as doping resulting in new catalytic centers, enhanced conductivity, and electron mobility. Hence, in this report, for the first time, we explored a new pathway; the electrocatalytic behavior of Fe-doped Ga2O3 resulted due to Fe chemical states and red shift in the 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.

Interplay between Solubility Limit, Structure, and Optical Properties of Tungsten-Doped Ga2O3 Compounds Synthesized by a Two-Step Calcination Process.

This work unfolds the fundamental mechanisms and demonstrates the tunable optical properties derived via chemical composition tailoring in tungsten (W)-doped gallium oxide (Ga2O3) compounds. On the basis of the detailed investigation, the solubility limits of tungsten (W6+) ion and associated effects on the crystal structure, morphology, and optical properties of W-doped Ga2O3 (Ga2-2 xW xO3, 0.00 ≤ x ≤ 0.25, GWO) compounds are reported. GWO materials were synthesized via a conventional solid-state reaction route, where a two-step calcination is adopted to produce materials with a high structural and chemical quality. X-ray diffraction analyses of sintered GWO compounds reveal the formation of a solid solution of GWO compounds at lower concentrations W ( x ≤ 0.10), while unreacted WO3 secondary phase formation occurs at higher concentrations ( x>0.10). Insolubility of W at higher concentrations ( x ≥ 0.15) is attributed to the difference in formation enthalpies of respective oxides, i.e., Ga2O3 and WO3. GWO compounds exhibit an interesting trend in morphology evolution as a function of W content. While intrinsic Ga2O3 exhibits rod-shaped morphology, W-doped Ga2O3 compounds exhibit nearly spherical-shaped grain morphology. Increasing W content ( x ≥ 0.10) induces morphology transformation from spherical to faceted grains with different facets (square and hexagonal). Relatively larger grain sizes in GWO compounds might be attributed to vacancy assisted enhanced mass transport due to W incorporation and/or WO3 induced liquid phase sintering. Our findings demonstrate a substantial red shift in band gap ( Eg), which is evident from the optical absorption spectra, enabling the wide spectral selectivity of GWO compounds. W 5d orbitals induced sp- d exchange interaction between valence band and conduction band electrons accounts for the substantial red shift in Eg of GWO compounds. Also, with increasing W, Eg decreases linearly, obeying Vegard law up to x = 0.15 and, at this point, an abrupt Eg drop prevails. The nonlinearity ( bowing effect) behavior in Eg beyond x = 0.15 is due to insolubility of W at higher concentrations. The fundamental scientific understanding of the interdependence of synthetic conditions, structure, chemistry, and band gap could be useful to optimize GWO materials for optical, optoelectronic, and photocatalytic device applications.