Cuboidal Nanocrystals Research Articles

Size-Controlled Cu3VSe4 Nanocrystals as Cathode Material in Platinum-Free Dye-Sensitized Solar Cells.

In this work, we report the first single-step, size-controlled synthesis of Cu3VSe4 cuboidal nanocrystals, with the longest dimension ranging from 9 to 36 nm, and their use in replacing the platinum counter electrode in dye-sensitized solar cells. Cu3VSe4, a ternary semiconductor from the class of sulvanites, is theoretically predicted to have good hole mobility, making it a promising candidate for charge transport in solar photovoltaic devices. The identity and crystalline purity of the Cu3VSe4 nanocrystals were validated by X-ray powder diffraction (XRD) and Raman spectroscopy. The particle size was determined from the XRD data using the Williamson-Hall equation and was found in agreement with the transmission electron microscopy imaging. Based on the electrochemical activity of the Cu3VSe4 nanocrystals, studied by cyclic voltammetry, the nanomaterials were further employed for fabricating counter electrodes (CEs) in Pt-free dye-sensitized solar cells. The counter electrodes were prepared from Cu3VSe4 nanocrystals as thin films, and the charge transfer kinetics were studied by electrochemical impedance spectroscopy. The work demonstrates that Cu3VSe4 counter electrodes successfully replace platinum in DSSCs. CEs fabricated with the Cu3VSe4 nanocrystals having an average particle size of 31.6 nm outperformed Pt, leading to DSSCs with the highest power conversion efficiency (5.93%) when compared with those fabricated with the Pt CE (5.85%).

Confinement and Exciton Binding Energy Effects on Hot Carrier Cooling in Lead Halide Perovskite Nanomaterials.

The relaxation of the above-gap ("hot") carriers in lead halide perovskites (LHPs) is important for applications in photovoltaics and offers insights into carrier-carrier and carrier-phonon interactions. However, the role of quantum confinement in the hot carrier dynamics of nanosystems is still disputed. Here, we devise a single approach, ultrafast pump-push-probe spectroscopy, to study carrier cooling in six different size-controlled LHP nanomaterials. In cuboidal nanocrystals, we observe only a weak size effect on the cooling dynamics. In contrast, two-dimensional systems show suppression of the hot phonon bottleneck effect common in bulk perovskites. The proposed kinetic model describes the intrinsic and density-dependent cooling times accurately in all studied perovskite systems using only carrier-carrier, carrier-phonon, and excitonic coupling constants. This highlights the impact of exciton formation on carrier cooling and promotes dimensional confinement as a tool for engineering carrier-phonon and carrier-carrier interactions in LHP optoelectronic materials.

Kinetic Control for Continuously Tunable Lattice Parameters, Size, and Composition during CsPbX3 (X = Cl, Br, I) Nanorod Synthesis.

The fast kinetics of all-inorganic CsPbX3 (X = Cl, Br, or I) nanocrystal growth entail that many synthetic strategies for structural control established in other semiconductor systems do not apply. Rather, products are often determined by thermodynamic factors, limiting the range of synthetic outcomes and functionality. In this study, we show how reaction kinetics are significantly slowed if nanocrystals are prepared using a dual injection strategy that moderates the crucial interaction between cesium and halide during nucleation and growth. The result is highly uniform nanorod or cuboid nanocrystals with a controllable size and aspect ratio across the quantum confinement regime, obtainable for both pure and mixed halide compositions. Further, the crystal lattice is continuously tunable between the tetragonal (I4/mcm) and orthorhombic (Pbnm) phases, independent of the overall nanorod morphology, enabling significantly more sophisticated structure-property relationships that can be tailored during this kinetically controlled synthesis.

Ferroelectric monodisperse Ln-doped barium titanate cuboidal nanocrystals prepared by a solvothermal route

We demonstrate here that the dielectric, electrical and optical properties of BaTiO3colloidal nanocrystals with a cuboidal shape can be modulated upon aliovalent doping with Ln3+rare earth ions (Ln = La3+, Ce3+, Nd3+, Sm3+, Gd3+, Dy3+, and Tm3+).

Control of Monodomain Polymer-Stabilized Cuboidal Nanocrystals of Chiral Nematics by Confinement.

Liquid crystals are important components of optical technologies. Cuboidal crystals consisting of chiral liquid crystals-the so-called blue phases (BPs), are of particular interest due to their crystalline structures and fast response times, but it is critical that control be gained over their phase behavior as well as the underlying dislocations and grain boundaries that arise in such systems. Blue phases exhibit cubic crystalline symmetries with lattice parameters in the 100 nm range and a network of disclination lines that can be polymerized to widen the range of temperatures over which they occur. Here, we introduce the concept of strain-controlled polymerization of BPs under confinement, which enables formation of strain-correlated stabilized morphologies that, under some circumstances, can adopt perfect single-crystal monodomain structures and undergo reversible crystal-to-crystal transformations, even if their disclination lines are polymerized. We have used super-resolution laser confocal microscopy to reveal the periodic structure and the lattice planes of the strain and polymerization stabilized BPs in 3D real space. Our experimental observations are supported and interpreted by relying on theory and computational simulations in terms of a free energy functional for a tensorial order parameter. Simulations are used to determine the orientation of the lattice planes unambiguously. The findings presented here offer opportunities for engineering optical devices based on single-crystal, polymer-stabilized BPs whose inherent liquid nature, fast dynamics, and long-range crystalline order can be fully exploited.

Preparation and properties of porous BaTiO3 nanostructured ceramics produced from cuboidal nanocrystals

The preparation and properties of BaTiO3 nanostructured ceramics with porosity level in the range of percolation limit (33% and 37% porosity) produced by partial sintering of cubic nanoparticles are presented. Hydrothermally synthesized cuboid-like particles were produced by using Field-Assisted Sintering Technique facility in which temperature and pressure were selected to ensure the consolidation of mechanically stable porous nanoceramics, while preserving as much as possible the starting grain shape. Nanosized grains in the range of (10–40) nm and multiscale porosity ranging from a few nm to hundreds of nm were observed in the sintered ceramics. The dielectric constant of porous nanoceramics assumes low values of ~280–320 and shows a flat thermal response typical to nanostructured ceramics, without a net ferroelectric-paraelectric peak, followed by a Curie-Weiss dependence in the paraelectric state, with negative Curie Weiss temperatures and lowered Curie constant, as result of porosity and ultrafine grain size. A strong conductivity relaxation around room temperature related to air-ceramic interface phenomena indicated a possible sensitivity of these ceramics for gas sensing. Preliminary qualitative tests with saturated acetone vapours have shown a good response of both resistive and reactive components of such porous BaTiO3 nanoceramics and possible gas sensing interface-related mechanisms were discussed.

The Surface Chemistry and Structure of Colloidal Lead Halide Perovskite Nanocrystals.

ConspectusSince the initial discovery of colloidal lead halide perovskite nanocrystals, there has been significant interest placed on these semiconductors because of their remarkable optoelectronic properties, including very high photoluminescence quantum yields, narrow size- and composition-tunable emission over a wide color gamut, defect tolerance, and suppressed blinking. These material attributes have made them attractive components for next-generation solar cells, light emitting diodes, low-threshold lasers, single photon emitters, and X-ray scintillators. While a great deal of research has gone into the various applications of colloidal lead halide perovskite nanocrystals, comparatively little work has focused on the fundamental surface chemistry of these materials. While the surface chemistry of colloidal semiconductor nanocrystals is generally affected by their particle morphology, surface stoichiometry, and organic ligands that contribute to the first coordination sphere of their surface atoms, these attributes are markedly different in lead halide perovskite nanocrystals because of their ionicity.In this Account, emerging work on the surface chemistry of lead halide perovskite nanocrystals is highlighted, with a particular focus placed on the most-studied composition of CsPbBr3. We begin with an in-depth exploration of the native surface chemistry of as-prepared, 0-D cuboidal CsPbBr3 nanocrystals, including an atomistic description of their surface termini, vacancies, and ionic bonding with ligands. We then proceed to discuss various post-synthetic surface treatments that have been developed to increase the photoluminescence quantum yields and stability of CsPbBr3 nanocrystals, including the use of tetraalkylammonium bromides, metal bromides, zwitterions, and phosphonic acids, and how these various ligands are known to bind to the nanocrystal surface. To underscore the effect of post-synthetic surface treatments on the application of these materials, we focus on lead halide perovskite nanocrystal-based light emitting diodes, and the positive effect of various surface treatments on external quantum efficiencies. We also discuss the current state-of-the-art in the surface chemistry of 1-D nanowires and 2-D nanoplatelets of CsPbBr3, which are more quantum confined than the corresponding cuboidal nanocrystals but also generally possess a higher defect density because of their increased surface area-to-volume ratios.

Mesoporous Assembly of Cuboid Anatase Nanocrystals into Hollow Spheres: Realizing Enhanced Photoactivity of High Energy {001} Facets

Herein, we showcase a unique one-step synthetic strategy to obtain a mesoporous assembly of cuboid anatase nanocrystals into hollow spheres that not only offers high surface area but also predominantly tenders the reactive high energy {001} facets. The detailed experimental studies, parameter optimization, and characterizations reveal the crucial role played by the shape and structure directing agents during nucleation and growth. Concentration of F–-ions plays a determining role in stabilizing the size and shape at the initial stage of formation while an optimal balance of SO42– anions is critical in generating the hollow porous assemblies while retaining the morphology of the primary nanocuboid subunits. The single crystalline anatase TiO2 nanocuboids and their hollow spherical assemblies; both show substantial enhancement in photocatalytic and photoelectrochemical activity when compared with commercial P25. Photoelectrodes fabricated using cuboid nanocrystals demonstrate a superior DSSC efficiency when...

Morphology Controlled Synthesis of ZnS(en)<sub>0.5 </sub>Nanostructures via a Novel Mixed-Solvent Solvothermal Process

By using a mixed-solvent solvothermal process, we report a simple and general approach for the morphology controlled synthesis of well-defined ZnS(en)0.5(en=ethylenediamine) rectangular nanosheets. The structure, and morphology of the as-synthesized products were investigated by using XRD, SEM, and TEM. By continuously adjusting the ratio between ethylenediamine and ethylene glycol, cuboid nanocrystals, nanosheets, short nanorods and nanoparticles have been successfully achieved. This conclusive evidence shows that the ratio of solvents played a key role in the morphology-controlled synthesis process. The UV-vis absorption spectrum shows that the morphology evolution provides us an opportunity to structurally modulate physical property, i.e. the energy gap. This simple approach of modulating the physical property promises future application in applied optics, optoelectronic and luminescent instruments.

Controlled antisolvent precipitation of spironolactone nanoparticles by impingement mixing

Continuous antisolvent precipitation of spironolactone nanoparticles were performed by impingement mixing in this work. In the range of Reynolds numbers (Re) 2108–6325 for the antisolvent water stream and 1771–5313 for the solvent stream, i.e. acetonic drug solution, 302–360 nm drug nanoparticles were achieved. Increasing drug concentration from 25 to 50 and 100 mg/ml led to a significant size increase from 279.0 ± 2.6 to 302.7 ± 4.9 and 446.0 ± 17.3 nm, respectively. “Two-step crystallization” was first observed for spironolactone in the water/acetone system: the drug was precipitated initially as spherical cluster, which rearranged into ordered cuboidal nanocrystals finally. The nanoformulation showed faster dissolution rate in comparison with the raw drug. By combining the impingement mixing and an on-line spray drying, a fully continuous process may be developed for mass-production of dried drug nanoparticles.

Oleic acid assisted glycothermal synthesis of cuboidal Ba0.6Sr0.4TiO3 nanocrystals and their ordered architectures via self-assembly

The size-controllable and highly monodispersed cuboidal barium strontium titanate (BST) nanocrystals have been successfully synthesized through a glycothermal process with no mineralizers but oleic acid as growth-directing agent. The synthesized BST nanocrystals under different glycothermal conditions were structurally characterized by XRD, IR, FESEM, TEM and HRTEM and investigated with respect to the effects of key influencing factors including the amount of oleic acid, duration of glycothermal process and 1,4-butanediol/water volume ratio in the reaction media on the formation of BST nanocrystals and their size and morphological evolution. It has been found that the oleic acid incorporated into the glycothermal system plays a decisive role in promoting the formation of cuboidal nanocrystals. It allows the BST nanocrystals to form via a nucleation-growth mechanism instead of in situ reactions and the selective chemical adsorption of oleic acid molecules on the facets with lower plane indices of newly-built BST nuclei directs them to grow into uniform cuboidal BST nanocrystals. The duration of glycothermal reactions and the polarity of reaction media can remarkably affect the dynamic process of the formation of BST nanocrystals. These regularly-shaped and highly monodispersed nanocrystals show a spontaneity of self-assembling into 2D ordered architectures when they were dispersed in organic solvents like cyclohexane and droped onto a hydrophobic surface of substrates, which creates a chance for the ferroelectric oxide nanocrystals to self-assemble into nanoscale electronic devices.

Seed‐Assisted Synthesis of BaCrO4 Nanoparticles and Nanostructures in Water‐in‐Oil Microemulsions

AbstractIsooctane dispersions of discrete isometric BaCrO4 nanoparticles or self‐assembled linear chains of prismatic BaCrO4 nanoparticles were added as surfactant‐coated seed crystals/nanostructures to Na2CrO4/NaAOT/Ba(AOT)2/isooctane microemulsion reaction solutions prepared at w = 10 with molar ratios favoring the de novo synthesis of either nanoparticle chains ([Ba2+]/ [CrO42–] = 1:1) or isolated nanoparticles ([Ba2+]/[CrO42–] = 1:5.5). Addition of BaCrO4 nanoparticles or chains under particle‐ or chain‐producing conditions, respectively, resulted in preferential growth of the seeds with retention of particle morphology and nanostructure architecture. In contrast, addition of linear chains to microemulsion reaction solutions under particle‐producing conditions resulted in disruption of the seed nanostructure and overgrowth of the released prismatic nanoparticles to produce discrete oval‐shaped or cuboidal nanocrystals depending on the seed concentration used. Discrete faceted nanoparticles were also produced by seed‐assisted synthesis when isometric nanoparticles were added at relatively high concentrations to chain‐producing microemulsion reaction solutions; however, decreasing the seed population resulted in intact self‐assembled linear chains and superlattices that consisted of interlinked prismatic nanoparticles with end‐capped pseudo‐hexagonal morphology. Growth of the seeds and their assembly/disassembly was consistent with a model of coupled synthesis and self‐organization based on the strength of electrostatic interactions at the surfactant‐crystal interface. The results suggest that microemulsion‐based processes could be of general importance for controlling the secondary growth of pre‐organized nanoparticle‐based superstructures, as well as the morphological refinement of their constituent building blocks.