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 (β-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.
This work unfolds the fundamental mechanisms and demonstrates the tunable optical properties derived via chemical composition tailoring in tungsten (W)-doped gallium oxide (Ga 2 O 3 ) compounds. On the basis of the detailed investigation, the solubility limits of tungsten (W 6+ ) ion and associated effects on the crystal structure, morphology, and optical properties of W-doped Ga 2 O 3 (Ga 2−2x W x O 3 , 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 WO 3 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., Ga 2 O 3 and WO 3 . GWO compounds exhibit an interesting trend in morphology evolution as a function of W content. While intrinsic Ga 2 O 3 exhibits rod-shaped morphology, W-doped Ga 2 O 3 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 WO 3 induced liquid phase sintering. Our findings demonstrate a substantial red shift in band gap (E g ), 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 E g of GWO compounds. Also, with increasing W, E g decreases linearly, obeying Vegard law up to x = 0.15 and, at this point, an abrupt E g drop prevails. The nonlinearity (bowing ef fect) behavior in E g 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.
Tailoring the optical and electronic properties of wide band gap β-Ga2O3 has been of tremendous importance to utilize the full potential of the material in current and emerging technological applications in electronics, optics, and optoelectronics. In the present work, we report the effect of Ti-dopant insolubility driven chemical inhomogeneity on the structural, morphological, chemical bonding, electronic structure, and band gap red shift characteristics in Ga2O3 polycrystalline compounds. Ga2–2x Ti x O3 (GTO; 0 ≤ x ≤ 0.20) compounds were synthesized using a conventional high-temperature solid state reaction route under variable calcination temperatures (1050–1250 °C) while sintering was performed at 1350 °C. X-ray diffraction analysis of GTO samples reveals that the formation of single-phase compounds occurs only at a very low concentration of Ti doping (<5 at. %), whereas higher Ti doping results in composite formation with a significant undissolved TiO2 rutile phase. However, in sintered samples, fraction of undissolved rutile phase transformed into monoclinic TiO2. Rietveld refinement of intrinsic Ga2O3 and single-phase Ti-doped compound (x = 0.05) confirms that samples are stabilized in monoclinic symmetry with C2/m space group. Surface morphologies of samples reveal that intrinsic Ga2O3 exhibits rod shaped morphology, while Ti-doped compounds exhibit spherical morphology. Moreover, in doped compounds with abnormal grain growth, lattice twinning induced striations were noted in contrast to intrinsic Ga2O3. High-resolution X-ray photoelectron spectroscopic analysis of Ga 2p shows a positive shift compared to metallic Ga due to interaction between the electron cloud of adjacent ions. Ti 2p1/2 spectra show anomalous broadening due to the Coster–Kronig effect. First-principles calculations using hybrid density functional theory show that Ti preferentially substitutes on octahedral Ga sites and that it behaves as a deep donor in Ga2O3. From the optical absorption spectra, a red shift in the optical band gap is observed. Absorption within the band gap of Ga2O3 is attributed to the inclusion of undissolved TiO2, as TiO2 has a type I alignment within the gap of Ga2O3. In addition, the electrocatalytic behavior of GTO compounds was examined. From electrocatalytic studies it is evident that doped compounds exhibit appreciable electrocatalytic activity in contrast to intrinsic Ga2O3.
Iron (Fe)-doped gallium oxide (Ga2O3) compounds (Ga2–x Fe x O3; x = 0.0–0.3; referred to GFO) were synthesized by the standard high-temperature solid-state chemical reaction method. X-ray diffraction analyses confirmed that the sintered GFO compounds stabilized in monoclinic crystal structure with C2/m space group. Local structure and chemical bonding analyses using XANES revealed that the Fe occupies octahedral and tetrahedral sites similar to Ga in parent Ga2O3 lattice without considerable changes in the local symmetry. Morphology of the GFO compounds is characterized by the presence of rod-shaped particle (from around 2.0–3.5 μm) features. The energy dispersive X-ray spectroscopy confirmed the chemical stoichiometry of the GFO compounds, where the atomic ratio of the constituted elements is in accordance with the calculated concentration values. The frequency- and temperature-dependent dielectric properties of the GFO compounds exhibited the traditional dielectric dispersion behavior. Relatively high dielectric constant at lower frequencies is attributed to Maxwell–Wagner type of dielectric relaxation, which primarily originated from uncompensated charges at electrode material interface. On the basis of the results and analyses, the effect of Fe content on the crystal structure, chemical bonding and local structure, and dielectric properties of Ga2–x Fe x O3 compounds is established.
Ba(Fe 0.7 Ta 0.3 )O 3-δ (BFTO) compounds were synthesized using a conventional, high-temperature solid-state ceramic reaction method by varying the sintering temperature (T s = 1200−1350 °C). The crystal structure, electronic structure, and electrocatalytic activity of BFTO compounds were evaluated. The processing temperature induced phase transformations and structural quality influences the electronic structure and electrocatalytic activity of BFTO compounds. At T s = 1200 °C, Ba(Fe 0.7 Ta 0.3 )O 3−δ stabilizes as a mixture of the orthorhombic + rhombohedral phase (Amm2 + R3m). With increasing T s (≥1250 °C), Ba(Fe 0.7 Ta 0.3 )O 3−δ ceramics stabilize in tetragonal + rhombohedral [P4mm + R3m] mixed phase with a variation in the number of respective phases. Highresolution X-ray photoelectron spectroscopy (XPS) of constituent elements, namely, Ba 3d, Fe 2p, Ta 4f, and O 1s levels reveals the electronic structure changes due to changes in the chemical environment resulted from structural transformation. The XPS analyses indicate that the processing temperature significantly influences the chemical environment of Fe and Ta cations in BFTO. The Ba 3d 5/2 core-level XPS spectra indicate that the perovskite phase gradually increases with increasing sintering temperature. The presence of absorption bands that are exclusively due to MO 6 stretching vibration that is connected to Ba ion as well as stretching of M−O bonds in infrared spectroscopy data indicate the structural and chemical quality of the BFTO compounds. The electrocatalytic activity of BFTO was evaluated toward hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR). Though all of the samples demonstrated appreciable electrocatalytic properties, the best electrochemical catalytic activity was shown by BFTO samples sintered at 1350 °C. BFTO-1350 °C showed an onset potential of −0.690 V vs reversible hydrogen electrode (RHE) for HER and an onset potential of 0.73 V vs RHE for ORR indicating its significant electrocatalytic performance. A general increase in activity with sintering temperature is potentially due to the improved structural quality of the BFTO ceramics. In addition to offering the fundamental insights into perovskite materials based on co-doped BaTiO 3 for electrocatalysis, the present work may contribute to the design and development of materials using co-doping of different chemical valence cations for other energy-related applications.
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