Anomalous Emission Shift of CdSe/CdS/ZnS Quantum Dots at Cryogenic Temperatures

The band-gap energy of most bulk semiconductors tends to increase as the temperature decreases. However, non-monotonic temperature dependence of the emission energy has been observed in semiconductor quantum dots (QDs) at cryogenic temperatures. Here, using stable and highly efficient CdSe/CdS/ZnS QDs as the model system, we quantitatively reveal the origins of the anomalous emission red-shift (∼8 meV) below 40 K by correlating ensemble and single QD spectroscopy measurements. About one-quarter of the anomalous red-shift (∼2.2 meV) is caused by the temperature-dependent population of the band-edge exciton fine levels. The enhancement of electron-optical phonon coupling caused by the increasing population of dark excitons with temperature decreases contributes an ∼3.4 meV red-shift. The remaining ∼2.4 meV red-shift is attributed to temperature-dependent electron-acoustic phonon coupling

Water Effects on Colloidal Semiconductor Nanocrystals: Correlation of Photophysics and Photochemistry

With high-quality CdSe/CdS core/shell nanocrystals as the main model system and under a controlled atmosphere, responses of photoexcited semiconductor nanocrystals to two active species (water and/or oxygen) in an ambient environment are studied systematically. Under photoexcitation, although high-quality semiconductor nanocrystals in either thin solid films or various solutions have a near-unity photoluminescence quantum yield, there is still a small probability (∼10–5 per photon absorbed) to be photoreduced by the water molecules efficiently accumulated in the highly hydrophilic nanocrystal–ligands interface. The resulting negatively charged nanocrystals are the starting point of most photophysical variations, and the hydroxyl radicalkey photo-oxidation product of waterplays the main role for initiating various photochemical processes. Depending on the supplementation of water to the interface, accessibility to oxygen, photoirradiation power, type of matrices, type of measurement schemes, and solubility of nanocrystals in the solution, various photophysical/photochemical phenomenaeither reported or not reported in the literatureare reproducibly observed. Results confirm that photophysical properties and photochemical reactions can be well-correlated, offering a unified and unique basis for fundamental studies and the design of processing techniques in industry

Tailoring Defect Density in UiO-66 Frameworks for Enhanced Pb(II) Adsorption

Defect engineering of metal organic frameworks offers potential prospects for tuning their features toward particular applications. Herein, two series of defective UiO-66 frameworks were synthesized via changing the concentration of the linker and synthetic temperature of the reaction. These defective materials showed a significant improvement in the capability of Pb­(II) removal from wastewater. This strategy for defect engineering not only created additional active sites, more open framework, and enhanced porosity but also exposed more oxygen groups, which served as the adsorption sites to improve Pb­(II) adsorption. A relationship among degree of defects, texture features, and performances for Pb­(II) removal was successfully developed as a proof-of-concept, highlighting the importance of defect engineering in heavy metal remediation. To investigate the kinetic and adsorption isotherms, we performed adsorption experiments influenced by the time and concentration of the adsorbate, respectively. For the practicality of the materials, the most significant parameters such as pH, temperature, adsorbent concentration, selectivity, and recyclability as well as simulated natural surface water were also examined. This study provides a clue for the researchers to design other advanced defective materials for the enhancement of adsorption performance by tuning the defect engineering

Selective and Light-Enhanced Au(III) Recovery by a Porphyrin-Based Metal–Organic Framework: Performance and Underlying Mechanisms

Recovering gold from unconventional sources, such as electronic waste, offers significant environmental and economic benefits. Exploiting materials and methods with high efficiency and selectivity is demanding. Herein, we reported a novel light-enhanced Au(III) recovery process using a porphyrin-based metal–organic framework (PCN-224). Our results showed that PCN-224 exhibited a remarkable Au(III) recovery capacity of up to 2613 mg/g when exposed to visible light irradiation, which was 3 times higher than that in the dark. Furthermore, light irradiation also improved the Au selectivity of PCN-224 against coexisting ions, including Zn2+, Mg2+, Cd2+, Ni2+, Hg2+, Cu2+, Pb2+, Al3+, and Fe3+. Based on characterization and kinetic analysis, an adsorption–reduction mechanism was proposed for the light-enhanced Au recovery, and porphyrin linkers played an essential role as active sites for both adsorption and reduction. To further protect the porphyrin linkers in PCN-224, acetic acid was introduced as a representative electron donor molecule in electronic waste, which could further enhance the Au(III) recovery capacity to 4946 mg/g. In addition, we demonstrated that PCN-224 and its light-enhanced feature also performed effectively in the actual leaching solution of waste electrical and electronic equipment, and the framework was successfully reused for at least six cycles. Overall, our discoveries could inspire the design of more outstanding materials and the artful use of clean energy to recover precious metals while minimizing the environmental impact

Unraveling Mechanisms of Highly Efficient Yet Stable Electrochemiluminescence from Quantum Dots

With CdSe/CdS/ZnS core/shell/shell quantum dots (QDs) as the model system, time- and potential-resolved spectroelectrochemical measurements are successfully applied for studying the general mechanisms and kinetics of electrochemiluminescence (ECL) generation. The rate constant of electron injection from the cathode into a QD to form a negatively charged QD (QD–) increases monotonically from −0.88 V to −1.2 V (vs Ag/AgCl). Mainly due to the deep LUMO of the QDs, the resulting QD– as the key intermediate for ECL generation is structurally stable and possesses very slow spontaneous deionization channels. The latter (the main non-ECL channels) are usually 3–4 orders of magnitude slower than the rate constant of the successive hole injection from an active co-reactant into a QD–. The kinetic studies quantify the internal ECL quantum yield of ideal QD ECL emitters to be nearly identical to that of photoluminescence, which is near unity for the current system. Identification of the key intermediate, discovery of the related elementary steps, and determination of all rate constants not only establish a general framework for understanding ECL generation but also offer basic design rules for ECL emitters

Synergistic Effect of Metal Cations and Visible Light on 2D MoS₂ Nanosheet Aggregation

Aggregation significantly influences the transport, transformation, and bioavailability of engineered nanomaterials. Two–dimensional MoS2 nanosheets are one of the most well-studied transition-metal dichalcogenide nanomaterials. Nonetheless, the aggregation behavior of this material under environmental conditions is not well understood. Here, we investigated the aggregation of single-layer MoS2 (SL-MoS2) nanosheets under a variety of conditions. Trends in the aggregation of SL-MoS2 are consistent with classical Derjaguin–Landau–Verwey–Overbeek (DLVO) colloidal theory, and the critical coagulation concentrations of cations follow the order of trivalent (Cr3+) 2+, Mg2+, Cd2+) +, K+). Notably, Pb2+ and Ag+ destabilize MoS2 nanosheet suspensions much more strongly than do their divalent and monovalent counterparts. This effect is attributable to Lewis soft acid–base interactions of cations with MoS2. Visible light irradiation synergistically promotes the aggregation of SL-MoS2 nanosheets in the presence of cations, which was evident even in the presence of natural organic matter. The light-accelerated aggregation was ascribed to dipole–dipole interactions due to transient surface plasmon oscillation of electrons in the metallic 1T phase, which decrease the aggregation energy barrier. These results reveal the phase-dependent aggregation behaviors of engineered MoS2 nanosheets with important implications for environmental fate and risk

Redispersion Behavior of 2D MoS₂ Nanosheets: Unique Dependence on the Intervention Timing of Natural Organic Matter

The aggregation–redispersion behavior of nanomaterials determines their transport, transformation, and toxicity, which could be largely influenced by the ubiquitous natural organic matter (NOM). Nonetheless, the interaction mechanisms of two-dimensional (2D) MoS2 and NOM and the subsequent influences on the redispersion behavior are not well understood. Herein, we investigated the redispersion of single-layer MoS2 (SL-MoS2) nanosheets as influenced by Suwannee River NOM (SRNOM). It was found that SRNOM played a decisive role on the redispersion of MoS2 2D nanosheets that varied distinctly from the 3D nanoparticles. Compared to the poor redispersion of MoS2 aggregates in the absence or post-addition of SRNOM to the aggregates, co-occurrence of SRNOM in the dispersion could largely enhance the redispersion and mobility of MoS2 by intercalating into the nanosheets. Upon adsorption to SL-MoS2, SRNOM enhanced the hydration force and weakened the van der Waals forces between nanosheets, leading to the redispersion of the aggregates. The SRNOM fractions with higher molecular mass imparted better dispersity due to the preferable sorption of the large molecules onto SL-MoS2 surfaces. This comprehensive study advances current understanding on the transport and fate of nanomaterials in the water system and provides fresh insights into the interaction mechanisms between NOM and 2D nanomaterials

Enhancing the Permselectivity of Thin-Film Composite Membranes Interlayered with MoS₂ Nanosheets via Precise Thickness Control

The demand for highly permeable and selective thin-film composite (TFC) nanofiltration membranes, which are essential for seawater and brackish water softening and resource recovery, is growing rapidly. However, improving and tuning membrane permeability and selectivity simultaneously remain highly challenging owing to the lack of thickness control in polyamide films. In this study, we fabricated a high-performance interlayered TFC membrane through classical interfacial polymerization on a MoS2-coated polyethersulfone substrate. Due to the enhanced confinement effect on the interface degassing and the improved adsorption of the amine monomer by the MoS2 interlayer, the MoS2-interlayered TFC membrane exhibited enhanced roughness and crosslinking. Compared to the control TFC membrane, MoS2-interlayered TFC membranes have a thinner polyamide layer, with thickness ranging from 60 to 85 nm, which can be tuned by altering the MoS2 interlayer thickness. A multilayer permeation model was developed to delineate and analyze the transport resistance and permeability of the MoS2 interlayer and polyamide film through the regression of experimental data. The optimized MoS2-interlayered TFC membrane (0.3-inter) had a 96.8% Na2SO4 rejection combined with an excellent permeability of 15.9 L m–2 h–1 bar–1 (LMH/bar), approximately 2.4 times that of the control membrane (6.6 LMH/bar). This research provides a feasible strategy for the rational design of tunable, high-performance NF membranes for environmental applications