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 ∼3.8%. Our dual sensitization strategy comprises self-assembly of Type-II CdTe/CdS core/shell QDs on porous TiO₂ 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 TiO₂ 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_0.71Sr_0.29MnO₃ Nanoparticles Synthesized by the Nonaqueous Sol–Gel 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 ∼20 nm La_0.71Sr_0.29MnO₃ particles synthesized by the nonaqueous sol–gel 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 ∼1.3 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 ∼2 μm 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 metal–organic framework as an efficient bifunctional catalyst for overall water-splitting. The NiFe-NCs with ∼200 nm side length have a Ni/Fe molar ratio of 3:2 and is a composite of NiO and α/γ-Fe₂O₃. The NCs demonstrate overpotentials of 271 and 197 mV for OER and HER, respectively, in 1 M KOH at 10 mA cm^–2, 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^–2. 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 Mn²⁺ ions to create long-lived charge carriers and reduce the electron–hole recombination. The <i>in situ</i> deposition of QDs although offers good surface coverage and better charge separation; in “doped” QDSCs the type of doping responsible for higher efficiencies cannot be ascertained. In the electrophoretically deposited Mn:CdS QDSCs, highest lattice doping of Mn²⁺ ions in presynthesized colloidal CdS QDs results in the highest efficiency, whereas exchange coupled Mn²⁺–Mn²⁺ pairs increase nonradiative electron–hole 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 9–13 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 PrMnO₃ 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 sol–gel techniques, the undoped PrMnO₃ nanoparticles with oxygen nonstoichiometry were rendered with exceptional ferromagnetic character. While ∼40 nm sized nanoparticles had magnetization of 84 emu/g and coercivity of 885 Oe with 50 kOe sweeping field, the bulk 2 μm 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 μm 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 ∼100 nm thick CaCO₃ nanosheets, and 3d transition metals (Mn and Fe) gave MnCO₃ microcubes and Fe₃O₄ tetragonal bipyramids. The lanthanides (La, Ce, and Pr) presented hierarchical flower-shaped structures with self-assembled 85–250 nm thick nanosheets comprising of La₂O₂CO₃ and LaOHCO₃ mixed phases, CeO₂ and Pr₂O₂CO₃, respectively. The Gibb’s free energy of formation (Δ<i>G</i>_f^o) of the oxide or decomposition of the carbonates at elevated temperatures >700 °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 °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 Mn₃O₄ surface layer, the binding chemistry of the surfactants plays an essential role. For example, the Mn₃O₄ 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 Mn₃O₄ shell on the antiferromagnetic MnO core provides an exchange coupling at the core–shell interface. Thicker the Mn₃O₄ shell, higher is the exchange-biased hysteresis loop shift

Maneuvering the Physical Properties and Spin States To Enhance the Activity of La–Sr–Co–Fe–O Perovskite Oxide Nanoparticles in Electrochemical Water Oxidation

Perovskite oxides have attracted considerable attention as durable electrocatalysts for metal–air 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_1–<i>x</i>Sr_<i>x</i>Co_1–<i>y</i>Fe_<i>y</i>O_3−δ (LSCF; <i>x</i> = 0.301, <i>y</i> = 0.298, and δ = 0.05–0.11) toward electrochemical water oxidation (OER) by altering the calcination temperature of the nonaqueous sol–gel 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_g orbital occupancy of 1.26, the interconnected ∼90 nm LSCF NPs prepared at 975 °C (LSCF-975) outperforms the other distinguishable LSCF morphologies, requiring 440 mV overpotential to achieve 10 mA/cm², 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 Co³⁺ 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_0.9O₃ (ABO₃) 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 Mn–O–Mn bond angles and tolerance factor leads to FM ordering below ∼100 K in usually AFM PrMn_0.9O₃ NPs. The concurrent decrease of lattice cell volume and Mn–O–Mn bond angles near the AFM to FM transition temperature (<i>T</i>_c) suggests that the AFM character increases just above <i>T</i>_c due to atomic deformations and reduced Mn–Mn 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 structure–property parallelism was cross-checked with the A-site vacant Pr_0.9MnO_3.2 NPs