10 research outputs found
Dual Sensitization Strategy for High-Performance Core/Shell/<i>Quasi-shell</i> Quantum Dot Solar Cells
The potential of quantum-dot sensitized
solar cells (QDSCs), a
promising candidate for third-generation photovoltaics, has not been
fully realized with the corresponding power conversion efficiencies
(PCE) still hovering below 9%. In this context, we demonstrate an
optimized dual sensitization strategy that combines the linker-assisted
self-assembly of QDs and successive ionic layer adsorption and reaction
(SILAR) approach to assemble high-efficiency QDSCs. CdTe/CdS core/shell
QDSC is chosen as the model system whose PCE, so far, has been reported
at ā¼3.8%. Our dual sensitization strategy comprises self-assembly
of Type-II CdTe/CdS core/shell QDs on porous TiO<sub>2</sub> followed
by deposition of an additional CdS quasi-shell through SILAR. The
highest QD surface coverage was optimized by systematic pH variation,
whereby PCE improved from 2.04(1)% (pH 11) to 3.696(5)% (pH 13). It
was observed that while the epitaxial shell passivates the core surface
traps, the nonepitaxial quasi-shell passivates the TiO<sub>2</sub> surface states. Thus, for core/shell, core/<i>quasi-shell</i> and core/shell/<i>quasi-shell</i> sensitized devices,
PCE increased as 1.5(1)%, 3.6(4)%, and 5.69(2)%, respectively. The
thickness of CdS shell and quasi-shell were optimized to achieve the
PCE of CdTe/CdS/<i>CdS</i> core/shell/<i>quasi-shell</i> QDSCs as high as 6.32(9)% (6.41% for the champion cell), which notably
is the highest for any aqueous processed QDSC
Enhanced Low-Field Magnetoresistance in La<sub>0.71</sub>Sr<sub>0.29</sub>MnO<sub>3</sub> Nanoparticles Synthesized by the Nonaqueous SolāGel Route
In colossal magnetoresistive (CMR)
materials, magnetic fields of
several tesla are typically required to exhibit large changes in electrical
resistance, and hence, materials should be engineered to provide a
more sensitive MR response at lower fields for their viability in
practical applications. Enhanced low-field magnetoresistance (LFMR)
was observed in highly ferromagnetic ā¼20 nm La<sub>0.71</sub>Sr<sub>0.29</sub>MnO<sub>3</sub> particles synthesized by the nonaqueous
solāgel route. The enhanced LFMR of the nanoparticles (NPs)
reaches 29.8% at 30 K with 50 mT, and the high-field magnetoresistance
was 56% with a 5000 mT applied field. The large LFMR effect can be
attributed to the spin-polarized tunneling across the ā¼1.3
nm thick natural grain boundaries. The weaker MR effect below 30 K
was attributed to the re-entrant spin glass at the core of the NPs
and surface spin glass phase that could be eliminated with lower and
higher applied fields, respectively. The magnetic and MR properties
of the NPs were compared to those of the corresponding ā¼2 Ī¼m
bulk material with the same elemental composition. These results provide
insight into the role of particle size, grain boundaries, and spin
glass phases on the MR properties, and the consequence of this finding
is useful for the potential fabrication of LFMR devices
Porous NiFe-Oxide Nanocubes as Bifunctional Electrocatalysts for Efficient Water-Splitting
Electrocatalytic
water-splitting, a combination of oxygen and hydrogen evolution reactions
(OER and HER), is highly attractive in clean energy technologies,
especially for high-purity hydrogen production, whereas developing
stable, earth-abundant, bifunctional catalysts has continued to pose
major challenges. Herein, a mesoporous NiFe-oxide nanocube (NiFe-NC)
system is developed from a NiFe Prussian blue analog metalāorganic
framework as an efficient bifunctional catalyst for overall water-splitting.
The NiFe-NCs with ā¼200 nm side length have a Ni/Fe molar ratio
of 3:2 and is a composite of NiO and Ī±/Ī³-Fe<sub>2</sub>O<sub>3</sub>. The NCs demonstrate overpotentials of 271 and 197
mV for OER and HER, respectively, in 1 M KOH at 10 mA cm<sup>ā2</sup>, which outperform those of 339 and 347 mV for the spherical NiFe-oxide
nanoparticles having a similar composition. The electrolyzer constructed
using NiFe-NCs requires an impressive cell voltage of 1.67 V to deliver
a current density of 10 mA cm<sup>ā2</sup>. Along with a mesoporous
structure with a broad pore size distribution, the NiFe-NCs demonstrate
the qualities of a desired corrosion-resistant water-splitting catalyst
with long-term stability. The exposure of active sites at the edges
and vertices of the NCs was validated to play a crucial role in their
overall catalytic performance
Plight of Mn Doping in Colloidal CdS Quantum Dots To Boost the Efficiency of Solar Cells
One of the strategies to boost the
efficiency of highly promising
yet struggling quantum dot solar cells (QDSCs) is to dope the QDs
with optically active Mn<sup>2+</sup> ions to create long-lived charge
carriers and reduce the electronāhole recombination. The <i>in situ</i> deposition of QDs although offers good surface coverage
and better charge separation; in ādopedā QDSCs the type
of doping responsible for higher efficiencies cannot be ascertained.
In the electrophoretically deposited Mn:CdS QDSCs, highest lattice
doping of Mn<sup>2+</sup> ions in presynthesized colloidal CdS QDs
results in the highest efficiency, whereas exchange coupled Mn<sup>2+</sup>āMn<sup>2+</sup> pairs increase nonradiative electronāhole
recombination and decrease the efficiency of QDSCs. With Mn:Cd at.
% of 1.44, the efficiency could be increased to 2.1% as compared to
1.3% for CdS QDSC. The increase in efficiency by 66.4% is due to slower
charge recombination in the photoanode and the electrolyte interface
and higher electron lifetime in the doped QDSCs
Chemical Modifications of Porous Carbon Nanospheres Obtained from Ubiquitous Precursors for Targeted Drug Delivery and Live Cell Imaging
Cost-effective anti-cancer drug delivery vehicles that can ensure controlled
and targeted transportation of drug molecules are pertinent to modern
day biomedical applications. Minimally toxic 9ā13 nm diameter
porous carbon nanospheres (PNs) were synthesized by oxidative cutting
of porous carbon matrices (PCs) obtained by carbonization of pasture
grass, human hair and sucrose. Among them, the grass-derived PNs (PN-G)
with superior surface area, porosity and graphitic content demonstrate
a significant loading of the drug both by chemical binding and physisorption.
Polyethylenimine (PEI) and folic acid (FA) functionalization maintain
therapeutic efficacy of the drug doxorubicin (DOX) to the targeted
folate receptor (FR) overexpressed human cervical cancer cells (HeLa)
and human breast cancer cells (MDA-MB-231) through receptor mediated
endocytosis whereas FR deficient normal cells (human embryonic kidney
293) exhibit substantially lower endocytosis under identical conditions.
Moreover, upon loading cell-impermeable propidium iodide (PI), the
PNs display superior activity toward near-infrared (NIR) live cell
imaging in HeLa cells whereby due to a higher binding affinity of
PI with the nucleic acids, the PI-to-PN energy transfer quenched fluorescence
is recovered. This dual functionality of controlled and targeted drug
delivery and photobleaching resistant live cell imaging by the cost-effective
PNs has larger implications in nanomedicine research and technology.
Porous carbon nanospheres derived from abundant resources act as highly
efficient and cost-effective nanocarriers for targeted anticancer
drug delivery and live cell imaging
Coexistence of High Magnetization and Anisotropy with Non-monotonic Particle Size Effect in Ferromagnetic PrMnO<sub>3</sub> Nanoparticles
Instances
of the coexistence of high ferromagnetic magnetization
with large anisotropy are scarce in the rare-earth manganite family.
In manganites, high magnetizations are compromised with small coercivity
and vice versa. Using nonaqueous solāgel techniques, the undoped
PrMnO<sub>3</sub> nanoparticles with oxygen nonstoichiometry were
rendered with exceptional ferromagnetic character. While ā¼40
nm sized nanoparticles had magnetization of 84 emu/g and coercivity
of 885 Oe with 50 kOe sweeping field, the bulk 2 Ī¼m sized particles
showed a magnetization of 51 emu/g albeit with a higher coercivity
of 2000 Oe. These parameters are so far the highest among manganite
systems with similarly sized particles. The competition between the
ferromagnetic and antiferromagnetic phases both at the particle core
and at the grain boundaries resulted in a non-monotonous trend of
magnetic properties between 20, 40, and 2 Ī¼m particles. The
sudden increase of coercivity toward lower temperatures was a result
of the freezing of random spins at the surface of the strongly interacting
nanoparticles which also increased the magnetic anisotropy. These
results are of prime significance since the coexistence of such a
large magnetization with high coercivity was rarely observed in pristine
or doped manganites
Direct Correlation of the Morphologies of Metal Carbonates, Oxycarbonates, and Oxides Synthesized by Dry Autoclaving to the Intrinsic Properties of the Metals
Dry
autoclaving of the metal acetates under similar conditions
resulted in various phases and morphologies of metal carbonates, oxycarbonates,
and oxides. The acetates of calcium (Group IIA) gave ā¼100 nm
thick CaCO<sub>3</sub> nanosheets, and 3d transition metals (Mn and
Fe) gave MnCO<sub>3</sub> microcubes and Fe<sub>3</sub>O<sub>4</sub> tetragonal bipyramids. The lanthanides (La, Ce, and Pr) presented
hierarchical flower-shaped structures with self-assembled 85ā250
nm thick nanosheets comprising of La<sub>2</sub>O<sub>2</sub>CO<sub>3</sub> and LaOHCO<sub>3</sub> mixed phases, CeO<sub>2</sub> and
Pr<sub>2</sub>O<sub>2</sub>CO<sub>3</sub>, respectively. The Gibbās
free energy of formation (Ī<i>G</i><sub>f</sub><sup>o</sup>) of the oxide or decomposition
of the carbonates at elevated temperatures >700 Ā°C controls
the
final phase. Elemental line scans showed carbon coating on the nanosheets,
whereas carbon existed as separate microspheres whenever the micron-sized
morphologies were obtained. The solidification kinetics of the supercritical
metal intermediates and carbon were comparable when the freezing point
(FP) of the metals is <1000 Ā°C such that once the nanosheets
formed from stacking of the solidified nuclei, carbon could wrap and
stabilize the nanosheets. The autoclaved products were air heated
to obtain the metal oxide phases. Defect-related emissions were observed
from the rare-earth oxides
Surfactant-Mediated Resistance to Surface Oxidation in MnO Nanostructures
The intrinsic physical properties
of nanostructures of metals and
their oxides are altered when they are prone to surface oxidation
in ambient atmosphere. To overcome this limitation, novel synthesis
methodologies are required. In this study, solid octahedral shapes
of MnO limit the inward oxygen diffusion compared to that of the MnO-nanoparticle-assembled
octahedra. In addition to morphology control, which restricts the
thickness of the Mn<sub>3</sub>O<sub>4</sub> surface layer, the binding
chemistry of the surfactants plays an essential role. For example,
the Mn<sub>3</sub>O<sub>4</sub> surface layer is 0.4 nm thinner with
trioctylphosphine oxide than with trioctylamine as the surfactant.
The nanostructures were prepared by varying the surfactants, surfactant-to-precursor
molar ratio, accelerating agent, and reaction heating rate. The surface
oxidation of MnO nano-octahedra was probed by Rietveld analysis of
X-ray diffraction patterns and X-ray photoelectron spectroscopy and
characterized by magnetic measurements, as the presence of ferrimagnetic
Mn<sub>3</sub>O<sub>4</sub> shell on the antiferromagnetic MnO core
provides an exchange coupling at the coreāshell interface.
Thicker the Mn<sub>3</sub>O<sub>4</sub> shell, higher is the exchange-biased
hysteresis loop shift
Maneuvering the Physical Properties and Spin States To Enhance the Activity of LaāSrāCoāFeāO Perovskite Oxide Nanoparticles in Electrochemical Water Oxidation
Perovskite
oxides have attracted considerable attention as durable electrocatalysts
for metalāair batteries and fuel cells due to their precedence
in oxygen electrocatalysis in spite of the complexities involved with
their crystal structure, spin states, and physical properties. Here
we report optimization of the activity of a model perovskite system
La<sub>1ā<i>x</i></sub>Sr<sub><i>x</i></sub>Co<sub>1ā<i>y</i></sub>Fe<sub><i>y</i></sub>O<sub>3āĪ“</sub> (LSCF; <i>x</i> = 0.301, <i>y</i> = 0.298, and Ī“ = 0.05ā0.11) toward electrochemical
water oxidation (OER) by altering the calcination temperature of the
nonaqueous solāgel synthesized nanoparticles (NPs). Our results
show that improved OER activity is the result of a synergism between
its morphology, surface area, electrical conductivity, and spin state
of the active transition metal site. With an e<sub>g</sub> orbital
occupancy of 1.26, the interconnected ā¼90 nm LSCF NPs prepared
at 975 Ā°C (LSCF-975) outperforms the other distinguishable LSCF
morphologies, requiring 440 mV overpotential to achieve 10 mA/cm<sup>2</sup>, a performance comparable to the best-performing perovskite
oxide electrocatalysts. While the interconnected NP morphology increases
the propensity of electronic conduction across crystalline grain boundaries,
the morphology-tuned high spin Co<sup>3+</sup> ions increases the
probability of binding reaction intermediates at the available surface
sites. Density functional theory based work function modeling further
demonstrates that LSCF-975 is the most favorable OER catalyst among
others in terms of a moderate work function and Fermi energy level
facilitating the adsorption and desorption of reaction intermediates
Extensive Parallelism between Crystal Parameters and Magnetic Phase Transitions of Unusually Ferromagnetic Praseodymium Manganite Nanoparticles
The alterations in
physical property across different space groups of the same material
are sometimes conveniently reflected by the crystal structure as a
function of temperature. However, mirroring the physical property
and crystal parameters over a wide range of temperatures within the
same space group is quite unusual. Remarkably, Rietveld analyses of
the X-ray diffraction patterns of PrMn<sub>0.9</sub>O<sub>3</sub> (ABO<sub>3</sub>) nanoparticles (NPs) with a constant <i>Pnma</i> space group from 300 to 10 K could successfully predict the four
magnetic phases, viz. paramagnetic, antiferromagnetic (AFM), ferromagnetic
(FM), and spin-glass-like ordering. The increase in MnāOāMn
bond angles and tolerance factor leads to FM ordering below ā¼100
K in usually AFM PrMn<sub>0.9</sub>O<sub>3</sub> NPs. The concurrent
decrease of lattice cell volume and MnāOāMn bond angles
near the AFM to FM transition temperature (<i>T</i><sub>c</sub>) suggests that the AFM character increases just above <i>T</i><sub>c</sub> due to atomic deformations and reduced MnāMn
separation. The predictions from crystal structure refinement were
successfully verified from the cooling path of the temperature-dependent
field-cooled magnetization measurements. A mechanism involving incoherent
spin reversal due to competition between the neighboring spins undergoing
antiparallel to parallel spin rotations was suggested. The structureāproperty
parallelism was cross-checked with the A-site vacant Pr<sub>0.9</sub>MnO<sub>3.2</sub> NPs