Phase Conversion from Hexagonal CuS_<i>y</i>Se_1–<i>y</i> to Cubic Cu_2–<i>x</i>S_<i>y</i>Se_1–<i>y</i>: Composition Variation, Morphology Evolution, Optical Tuning, and Solar Cell Applications

In this work, we report a simple and low-temperature approach for the controllable synthesis of ternary Cu–S–Se alloys featuring tunable crystal structures, compositions, morphologies, and optical properties. Hexagonal CuS_<i>y</i>Se_1–<i>y</i> nanoplates and face centered cubic (fcc) Cu_2–<i>x</i>S_<i>y</i>Se_1–<i>y</i> single-crystal-like stacked nanoplate assemblies are synthesized, and their phase conversion mechanism is well investigated. It is found that both copper content and chalcogen composition (S/Se atomic ratio) of the Cu–S–Se alloys are tunable during the phase conversion process. Formation of the unique single-crystal-like stacked nanoplate assemblies is resulted from oriented stacking coupled with the Ostwald ripening effect. Remarkably, optical tuning for continuous red shifts of both the band-gap absorption and the near-infrared localized surface plasmon resonance are achieved. Furthermore, the novel Cu–S–Se alloys are utilized for the first time as highly efficient counter electrodes (CEs) in quantum dot sensitized solar cells (QDSSCs), showing outstanding electrocatalytic activity for polysulfide electrolyte regeneration and yielding a 135% enhancement in power conversion efficiency (PCE) as compared to the noble metal Pt counter electrode

Identification of Multifunctional Graphene–Gold Nanocomposite for Environment-Friendly Enriching, Separating, and Detecting Hg²⁺ Simultaneously

By virtue of the specific amalgam of mercury with gold and high specific area of a graphene scaffold, an environment-friendly multifunctional graphene–gold nanocomposite (G-AuNPs) has been identified and prepared by a simple one-pot redox reaction. The resultant G-AuNPs can reversibly enrich about 94% of Hg²⁺ in water samples, which can be further separated by only a simple filtration. Importantly, the color of the G-AuNPs suspension exclusively changes from purple–red to light brown upon the addition of Hg²⁺ in the presence of ascorbic acid, which can be applied for colorimetric detection of Hg²⁺ with a detection limit (3σ, <i>n</i> = 20) of 1.6 × 10^–8 mol·L^–1. Furthermore, using ascorbic acid as reducing agents, both the preparation process and the resultant nanocomposite are nontoxic. To the best of our knowledge, this is the first report to enrich, separate and detect Hg²⁺ contaminant simultaneously without causing any secondary pollution

Cu₂ZnSnS₄ Hierarchical Microspheres as an Effective Counter Electrode Material for Quantum Dot Sensitized Solar Cells

We explore the application of Cu₂ZnSnS₄ (CZTS) microspheres as an effective counter electrode material for low-cost and high-efficiency quantum dot sensitized solar cells (QDSSCs). Nearly monodisperse Cu₂ZnSnS₄ (CZTS) hierarchical microspheres with diameters of ∼2 μm built from nanoflakes have been synthesized via a facile solvothermal approach. The nanoflakes are assembled from CZTS quantum dots with 3–5 nm, showing a three-tiered organization of hierarchical microspheres. The morphology, crystal structure, composition, and optical properties of the CZTS microspheres are characterized by SEM, HRTEM, XRD, XPS, EDS, EELS, Raman, and UV–vis analysis. Chemical conversion and phase transformation from hexagonal CuS to tetragonal CZTS have been systematically investigated to reveal formation mechanism of the CZTS microspheres. These CZTS microspheres are used as an effective counter electrode material in QDSSCs for the first time to show high electrocatalytic activity for catalyzing reduction of polysulfide (S^2–/S_<i>n</i>^2–) electrolyte, contributing to significant improvement in short current density (<i>J</i>_SC) and fill factor (FF). A solar cell using the CZTS microspheres-coated FTO (SnO₂:F) glass substrate as a counter electrode achieves a power conversion efficiency (PCE) of 3.73% under AM 1.5G illumination with an intensity of 100 mW cm^–2, which is much higher than that (0.33%) of the cell using the bare FTO glass substrate as a counter electrode and is also higher than that (2.27%) of the cell using the noble Pt-coated FTO glass substrate as a counter electrode

Solution-Processed Self-Stratifying Layer with Controllable Dielectric Polarization for High-Luminance Organic Light-Emitting Diodes

Spin-coated poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) layers are well known to show a PSS-rich surface layer. Such a self-stratifying PEDOT:PSS layer has been applied for improving maximum external quantum efficiency (EQE) of organic light-emitting diodes (OLEDs). However, such devices typically show much reduced high-luminance performance affecting practical applications of such a self-stratifying interlayer (SSL). In this work, we demonstrate that a simple ionization process can eliminate the adverse effects at high luminance while maintaining high maximum EQE. It is shown that ions of the salt can interact with hydroxyl groups of the PSS polymer and thus disorder the orientation polarization. This implies that the ionization process enables active tuning of the dielectric properties of the PEDOT:PSS layer. It reduces carrier accumulation caused by orientation polarization of the SSL and thus suppresses both exciton annihilation and electric stress across the emitting layer in OLEDs. With this strategy, the device using the self-stratifying PEDOT:PSS layer shows a wide window of operating current density which is nearly 6 times compared with that of the corresponding device without the treatment. This enables 5 times of luminance and operation lifetime enhancements

Ionic Charge Transfer Complex Induced Visible Light Harvesting and Photocharge Generation in Perovskite

Organometal trihalide perovskite has recently emerged as a new class of promising material for high efficiency solar cells applications. While excess ions in perovskites are recently getting a great deal of attention, there is so far no clear understanding on both their formation and relating ions interaction to the photocharge generation in perovskite. Herein, we showed that tremendous ions indeed form during the initial stage of perovskite formation when the organic methylammonium halide (MAX^a, X^a = Br and I) meets the inorganic PbX^b₂ (X^b = Cl, Br, I). The strong charge exchanges between the Pb²⁺ cations and X^a‑ anions result in formation of ionic charge transfer complexes (iCTC). MAX^a parties induce empty valence electronic states within the forbidden bandgap of PbX^b₂. The strong surface dipole provide sufficient driving force for sub-bandgap electron transition with energy identical to the optical bandgap of forming perovskites. Evidences from XPS/UPS and photoluminescence studies showed that the light absorption, exciton dissociation, and photocharge generation of the perovskites are closely related to the strong ionic charge transfer interactions between Pb²⁺ and X^a‑ ions in the perovskite lattices. Our results shed light on mechanisms of light harvesting and subsequent free carrier generation in perovskites

Preparation and Size Control of Sub-100 nm Pure Nanodrugs

Pure nanodrugs (PNDs), nanoparticles consisting entirely of drug molecules, have been considered as promising candidates for next-generation nanodrugs. However, the traditional preparation method via reprecipitation faces critical challenges including low production rates, relatively large particle sizes, and batch-to-batch variations. Here, for the first time, we successfully developed a novel, versatile, and controllable strategy for preparing PNDs via an anodized aluminum oxide (AAO) template-assisted method. With this approach, we prepared PNDs of an anticancer drug (VM-26) with precisely controlled sizes reaching the sub-20 nm range. This template-assisted approach has much higher feasibility for mass production comparing to the conventional reprecipitation method and is beneficial for future clinical translation. The present method is further demonstrated to be easily applicable for a wide range of hydrophobic biomolecules without the need of custom molecular modifications and can be extended for preparing all-in-one nanostructures with different functional agents

Effects of Small Polar Molecules (MA⁺ and H₂O) on Degradation Processes of Perovskite Solar Cells

Degradation mechanisms of methylammonium lead halide perovskite solar cells (PSCs) have drawn much attention recently. Herein, the bulk and surface degradation processes of the perovskite were differentiated for the first time by employing combinational studies using electrochemical impedance spectroscopy (EIS), capacitance frequency (CF), and X-ray diffraction (XRD) studies with particular attention on the roles of small polar molecules (MA⁺ and H₂O). CF study shows that short-circuit current density of the PSCs is increased by H₂O at the beginning of the degradation process coupled with an increased surface capacitance. On the basis of EIS and XRD analysis, we show that the bulk degradation of PSCs involves a lattice expansion process, which facilitates MA⁺ ion diffusion by creating more efficient channels. These results provide a better understanding of the roles of small polar molecules on degradation processes in the bulk and on the surface of the perovskite film

Heat Treatment for Regenerating Degraded Low-Dimensional Perovskite Solar Cells

Organolead halide perovskite devices are reported to be susceptible to thermal degradation, which results from heat-induced fast ion diffusion and structural decomposition. In this work, it is found that the performances of degraded low-dimensional perovskite solar cells can be considerably improved (e.g., power conversion efficiency shows ∼10% increase over the fresh device) by a short-time heat treatment (85 °C, 3 min). Capacitance–frequency, X-ray diffraction, and ionic diffusion calculation results suggest that heat treatment can enhance the crystallinity of the degraded low-dimensional perovskite and minimize the detrimental effects caused by water molecules, leading to improved performances. Our results indicate that the heat treatment does not necessarily lead to the accelerated degradation but can also regenerate the degraded low-dimensional perovskite

Polarity-Free Epitaxial Growth of Heterostructured ZnO/ZnS Core/Shell Nanobelts

Surface-polarity-induced formation of ZnO/ZnS heterojunctions has a common characteristic that ZnS (or ZnO) is exclusively decorated on a Zn-terminated (0001) surface of ZnO (or ZnS) due to its comparatively chemically active nature to an O (or S)-terminated (000–1) surface. Here, we report a polarity-free and symmetrical growth of ZnS on both ZnO±(0001) surfaces to form a new heterostructured ZnO/ZnS core/shell nanobelt via a thermal evaporation method. Remarkably, the ZnS shell is single-crystalline and preserves the structure and orientation of the inner ZnO nanobelt with an epitaxial relationship of (0001)_ZnO//(0001)_ZnS; [2–1–10]_ZnO//[2–1–10]_ZnS. Through this case, we demonstrate that an anion-terminated polar surface could also drive the nucleation and growth of nanostructures as the cation-terminated surface by controlling the growth kinetics. Considering high-performance devices based on ZnO/ZnS heterojunctions, the current ZnO/ZnS nanobelt is advantageous for optoelectronic applications due to its single-crystalline nature and relatively more efficient charge separation along 3D heterointerfaces

Enhanced Tolerance to Stretch-Induced Performance Degradation of Stretchable MnO₂‑Based Supercapacitors

The performance of many stretchable electronics, such as energy storage devices and strain sensors, is highly limited by the structural breakdown arising from the stretch imposed. In this article, we focus on a detailed study on materials matching between functional materials and their conductive substrate, as well as enhancement of the tolerance to stretch-induced performance degradation of stretchable supercapacitors, which are essential for the design of a stretchable device. It is revealed that, being widely utilized as the electrode material of the stretchable supercapacitor, metal oxides such as MnO₂ nanosheets have serious strain-induced performance degradation due to their rigid structure. In comparison, with conducting polymers like a polypyrrole (PPy) film as the electrochemically active material, the performance of stretchable supercapacitors can be well preserved under strain. Therefore, a smart design is to combine PPy with MnO₂ nanosheets to achieve enhanced tolerance to strain-induced performance degradation of MnO₂-based supercapacitors, which is realized by fabricating an electrode of PPy-penetrated MnO₂ nanosheets. The composite electrodes exhibit a remarkable enhanced tolerance to strain-induced performance degradation with well-preserved performance over 93% under strain. The detailed morphology and electrochemical impedance variations are investigated for the mechanism analyses. Our work presents a systematic investigation on the selection and matching of electrode materials for stretchable supercapacitors to achieve high performance and great tolerance to strain, which may guide the selection of functional materials and their substrate materials for the next-generation of stretchable electronics