Colloidal synthesis and photophysical characterization of silicon-compatible Ge1−xSnx alloy quantum dots with composition-tunable near-infrared absorption and photoluminescence is reported.
Tin phosphides make up a class of
materials that have received
a noteworthy amount of interest in photocatalysis, charge storage,
and thermoelectric devices. Dual stable oxidation states of tin (Sn2+ and Sn4+) allow tin phosphides to exhibit different
stoichiometries and crystal phases. However, the synthesis of such
nanostructures with control over morphology and crystal structure
has proven to be a challenging task. Herein, we report the first colloidal
synthesis of size-, shape-, and phase-controlled, narrowly disperse
rhombohedral Sn4P3, hexagonal SnP, and trigonal
Sn3P4 nanoparticles (NPs) displaying tunable
morphologies and size-dependent physical properties. The control over
NP morphology and crystal phase was achieved by tuning the nucleation/growth
temperature, Sn/P molar ratio, and incorporation of additional coordinating
solvents (alkylphosphines). The absorption spectra of Sn3P4 NPs (3.0 ± 0.4 to 8.6 ± 1.8 nm) exhibit size-dependent
blue shifts in energy gaps (1.38–0.88 eV) compared to the theoretical
value of bulk Sn3P4 (0.83 eV), consistent with
quantum confinement effects. The trigonal Sn3P4 NPs adopt rhombohedral Sn4P3 and hexagonal
SnP crystal structures at 180 and 250 °C, respectively. Structural
and surface analysis indicates consistent bond energies for phosphorus
across different crystal phases, whereas the rhombohedral Sn4P3 NPs demonstrate Sn oxidation states distinctive from
those of the hexagonal and trigonal phases because of the complex
chemical structure. All phases exhibit N(1s) and ν(N–H) energies suggestive of alkylamine surface functionalization and
are devoid of tetragonal Sn impurities.
Electrocatalytic
water splitting presents an exciting opportunity
to produce environmentally benign fuel to power human activities and
reduce reliance on fossil fuels. Transition metal nanoparticles (NPs)
and their alloys are emerging as promising candidates to replace expensive
platinum group metal (PGM) catalysts. Herein, we report the synthesis
of distinct crystal phases and compositions of Ni1–x
Mo
x
alloy NPs as low-cost,
earth-abundant electrocatalysts for the hydrogen evolution reaction
(HER) in alkaline medium. Phase-pure cubic and hexagonal Ni and Ni1–x
Mo
x
alloy
NPs, with sizes ranging from 18 to 43 nm and varying Mo composition
(∼0–11.4%), were produced by a low-temperature colloidal
chemistry method. As-synthesized NPs show spherical to polygonal morphologies
and a systematic shifting of Bragg reflections to lower 2θ angles
with increasing Mo, suggesting the growth of homogeneous alloys. XPS
analysis indicates the dominance of metallic Ni(0) and Mo(0) species
in the core of the alloy NPs as well as the presence of higher valent
Ni
n+ and Mo
n+ surface species, stabilized by surfactant ligands. The cubic alloys
exhibit significantly higher HER activity in comparison to the hexagonal
alloys. For a current density of −10 mA/cm2, the
cubic alloys demonstrate overpotentials of −62 to −177
mV compared to −162 to −242 mV for the hexagonal alloys.
The overpotentials of cubic alloys are comparable to the commercial
Pt-based electrocatalysts for which the overpotentials range from
−68 to −129 mV at −10 mA/cm2. In general,
a decrease in overpotential and an increase in HER activity were observed
with increasing concentration of Mo (up to 6.6%) for the cubic alloys.
The cubic Ni0.934Mo0.066 alloy NPs exhibit the
highest activity as alkaline HER electrocatalysts.
Using hybrid functional calculations
and experimental characterization,
we analyze optical properties of 2–3 nm Ge1–x
Sn
x
alloy quantum dots,
synthesized by colloidal chemistry methods. Hybrid functional theory,
tuned to yield experimental bulk band structure of germanium, reproduces
directly measured properties of Ge1–x
Sn
x
quantum dots, such as lattice
constants, energy gaps, and absorption spectra. Time-dependent hybrid
functional calculations yield optical absorption in good agreement
with experiments, and allow probing the nature of the dark excitons
in quantum dots. Calculations suggest a spin-forbidden dark exciton
ground state, which is supported by the changes in the photoluminescence
lifetimes with temperature and tin concentrations. The synthesis and
theoretical understanding of Ge1–x
Sn
x
alloy quantum dots will add to the
overall toolbox of low to nontoxic, silicon-compatible group IV semiconductors
with potential application in visible to near-infrared optoelectronics.
Narrowly disperse rhombohedral Sn4P3 (I), hexagonal SnP (II), and trigonal Sn3P4 (III) nanoparticles (NPs) are prepared by size‐, shape‐, and phase‐controlled colloidal synthesis.
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