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.
Fluorescent graphene quantum dots
(GQDs) prepared from low-cost
and sustainable precursors are highly desirable for various applications,
including luminescence-based sensing, optoelectronics, and bioimaging.
Among different natural precursors, the unique structural and compositional
variety and the abundance of aromatic carbon in lignin make it a unique
and renewable precursor for the green synthesis of advanced carbon-based
materials including GQDs. However, the inferior photoluminescence
quantum yield of GQDs prepared from natural precursors, including
lignin, limits their practical utility. Here, for the first time,
we demonstrate that the presence of heteroatoms in the innate structure
of lignosulfonate can be leveraged to derive in situ heteroatom-doped
GQDs with excellent photophysical properties. The as-synthesized lignosulfonate-derived
GQDs showed compelling blue fluorescence with a high quantum yield
of 23%, which is attributed to in situ S and N doping as confirmed
by using X-ray photoelectron spectroscopy and Fourier transform infrared
spectroscopy analyses. Assisted by the in situ doping, we further
engineered the lignosulfonate-derived GQDs by incorporating a metal
atom dopant to derive an enhanced quantum yield of 31%, the highest
for any lignin-derived GQDs. Moreover, fundamental photoluminescence
studies reveal the presence of multiple emissive centers, with edge
states acting as dominant emission centers. Finally, we also demonstrate
the applicability of the luminescent, metal- and nonmetal-codoped
lignin-derived GQDs as a highly selective sensor for the sub-nanomolar
level detection of mercuric ions in water.
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