Preparation technology and photoluminescence properties of CdTe nanocrystals in colloidal solutions and polymeric matrices

An analysis of physical mechanisms responsible for the influence of stabilizer type and ambient material on CdTe nanocrystal growth rate and passivation effectiveness of surface dangling bonds has been carried out. The possibility to change properties of nanocrystals by changing the stabilizer type has been shown

Light-Induced Defect Formation and Pt Single Atoms Synergistically Boost Photocatalytic H2 Production in 2D TiO2-Bronze Nanosheets ?

Ultrathin two-dimensional (2D) semiconductor nanosheets decorated with single atomic species (SAs) have recently attracted increasing attention due to their abundant surface-exposed reactive sites and maximum SAs binding capabilities thus lowering the catalyst cost, without sacrificing high performance for photocatalytic hydrogen (H2) production from water. Here, we present a strategy to prepare titanium dioxide-bronze nanosheets (TiO2-BNS) and H2-reduced TiO2 nanosheets (TiO2- HRNS) synthesized, characterized, and applied for photocatalytic H2 production. Surprisingly, black TiO2-HRNS show complete photo inactivity, while the TiO2-BNS-Pt0.05 nanohybrid shows excellent H2 production rate with a very low loading of 0.05 wt % Pt. TiO2-BNS-Pt0.05 presents around 10 and 99 times higher photocatalytic rate than pristine TiO2-BNS under solar and 365 nm UV-LED light irradiation, respectively. Due to the 2D morphology and the presence of abundant coordinating sites, the successful formation of widely dispersed Pt SAs was achieved. Most excitingly, the in situ formation of surface-exposed defect sites (Ti3+) was observed for TiO2-BNS under light illumination, suggesting their significant role in enhancing the H2 production rate. This self-activation and amplification behavior of TiO2-BNS can be extended to other 2D systems and applied to other photocatalytic reactions, thus providing a facile approach for fully utilizing noble metal catalysts via the successful formation of SAs

Physico-chemical study of steroids from different matureness corn silk material

This study shows an ultrasonic assisted extraction procedure of steroids from corn silk (CS). The total steroids contents were positively correlated with the ultrasonic assisted extraction time. The extracted steroids contents varied according to the different maturity stages of CS. There were three tested CS maturity stages: silking stage (CS-S), milky stage (CS-M) and mature stage (CS-MS). The β - sitosterol standardization method with 530 nm wavelength colorimetric measurements were applied to determine the content of extracted steroids. Measured steroids concentrations range were from 38.3 × 10-3 mg.mL-1 to 368.9 × 10-3 mg.mL-1 in different extraction time and CS maturity stages. The highest concentration of steroids, 368.9 × 10-3 mg.mL-1 was found in CS-MS sample with the 75 minutes ultrasonic extraction time. The fluorescence mapping techniques were used to confirm the existence of steroids. The thermal analysis illustrated a typical multistep decomposition process for the CS-S, CS-M and CS-MS samples. Two endothermic peaks were found: The first one was 54.3 °C for CS-S and CS-MS, 60.2 °C for CS-M, the second one, 397 °C (CS-MS), 415.1 °C (CS-M) and 419.7 °C (CS-S) attributed to the total thermal decomposition. The observed exotheric process found at 524 °C corresponded to CS-MS sample decomposition. The optimal ultrasonic-assisted extraction time for all samples under study CS-S, CS-M and CS-MS was about 75 minutes and the optimal steroids extraction contents obtained were 92.8 × 10-3 mg.mL-1 (CS-S), 124.2 × 10-3 mg.mL-1 (CS-M) and 368.9 × 10-3 mg.mL-1 (CS-MS) respectively. © 2019 Potravinarstvo Slovak Journal of Food Sciences

Physico-chemical study of flavonoids from different matureness corn silk material

There was tested a simple extraction procedure of flavonoids separation from the original corn silk (CS) material. It was found, that the total flavonoids content differs with the extraction time and extraction temperature. There were found different flavonoids contents in extracts prepared from different maturity stages of the original corn silk material (silking stage (CS-S), milky stage (CS-M)). Extracted flavonoids content was quantified by the lutin standardization method by means of colorimetry at 510 nm wavelength. Obsreved flavonoids concentration was ranging from 2×10-3 mg.mL-1 to 7 ×10-3 mg.mL-1 dependent on the extraction time period and extraction temperature. The highest flavonoids concentration of 7.5×10-3 mg.mL-1 was found for CS-M after 20 minutes extraction time and 80 °C extraction temperature. There was confirmed the presence of flavonoids by fluorescence mapping experiments. There was found a typical multistep decomposition process for both CS-S and CS-M materials by TG analysis. There was found a melting temperature of flavonoids of 54.3 °C for corn silk silking stage material exhibiting 58.9 J.g-1 heat of fusion and 60.2 °C for corn silk milky stage material with 112.9 J.g-1 heat of fusion. The optimal conditions of corn silk flavonoids extraction were 40 °C, 50 minutes for CS-S, the optimal flavonoids extraction content was (6.8 ±2.1)×10-3 mg.mL-1, 80°C, 20 minutes for CS-M and the optimal extraction content was (7.2 ±0.3)×10-3 mg.mL-1. © 2018 Potravinarstvo Slovak Journal of Food Sciences

Impact of D₂O/H₂O solvent exchange on the emission of HgTe and CdTe quantum dots: Polaron and energy transfer effects

We have studied light emission kinetics and analyzed carrier recombination channels in HgTe quantum dots that were initially grown in H2O. When the solvent is replaced by D2O, the nonradiative recombination rate changes highlight the role of the vibrational degrees of freedom in the medium surrounding the dots, including both solvent and ligands. The contributing energy loss mechanisms have been evaluated by developing quantitative models for the nonradiative recombination via (i) polaron states formed by strong coupling of ligand vibration modes to a surface trap state (nonresonant channel) and (ii) resonant energy transfer to vibration modes in the solvent. We conclude that channel (i) is more important than (ii) for HgTe dots in either solution. When some of these modes are removed from the relevant spectral range by the H2O to D2O replacement, the polaron effect becomes weaker and the nonradiative lifetime increases. Comparisons with CdTe quantum dots (QDs) served as a reference where the resonant energy loss (ii) a priori was not a factor, also confirmed by our experiments. The solvent exchange (H2O to D2O), however, is found to slightly increase the overall quantum yield of CdTe samples, probably by increasing the fraction of bright dots in the ensemble. The fundamental study reported here can serve as the foundation for the design and optimization principles of narrow bandgap quantum dots aimed at applications in long wavelength colloidal materials forinfrared light emitting diodes and photodetectors.We acknowledge financial support by the grant from the Research Grants Council of the Hong Kong S.A.R., China (project CityU 11302114). MIV acknowledges financial support from the FCT (Portugal)

Defect engineering over anisotropic brookite toward substrate-specific photo-oxidation of alcohols

Generally adopted strategies for enhancing the photocatalytic activity are aimed at tuning the visible light response, the exposed crystal facets, and the nanocrystal shape. Here, we present a different approach for designing efficient photocatalysts displaying a substrate-specific reactivity upon defect engineering. The platinized, defective anisotropic brookite TiO2 photocatalysts are tested for alcohol photoreforming showing up to an 11-fold increase in methanol oxidation rate, compared with the pristine one, while presenting much lower ethanol or isopropanol specific oxidation rates. We demonstrate that the substrate- specific alcohol oxidation and hydrogen evolution reactions are tightly related, and when the former is increased, the latter is boosted. The reduced anisotropic brookite shows up to 18-fold higher specific photoactivity with respect to anatase and brookite with isotropic nanocrystals. Advanced in situ characterizations and theoretical investigations reveal that controlled engineering over oxygen vacancies and lattice strain produces large electron polarons hosting the substratespecific active sites for alcohol photo-oxidation

Influence of the dispersion environment nature on photoluminescence properties of CdTe nanocrystals in colloidal solutions

The physico-chemical properties of low-dimensional structures based on CdTe obtained in the course of colloidal synthesis have been investigated. Analyzed have been the main photoluminescence characteristics of CdTe nanocrystals, which are stabilized by thioglycolic acid and obtained using the deionized water and aqueous solutions of ethylene glycol and glycerol with different concentrations as dispersion environment. It has been shown that stability of colloidal solutions of CdTe nanocrystals depends on the nature of dispersion environment and concentration of stabilizer

Liquid-Liquid Diffusion-Assisted Crystallization: A Fast and Versatile Approach Toward High Quality Mixed Quantum Dot-Salt Crystals

Cataloged from PDF version of article.Here, a new, fast, and versatile method for the incorporation of colloidal quantum dots (QDs) into ionic matrices enabled by liquid-liquid diffusion is demonstrated. QDs bear a huge potential for numerous applications thanks to their unique chemical and physical properties. However, stability and processability are essential for their successful use in these applications. Incorporating QDs into a tight and chemically robust ionic matrix is one possible approach to increase both their stability and processability. With the proposed liquid-liquid diffusion-assisted crystallization (LLDC), substantially accelerated ionic crystallization of the QDs is shown, reducing the crystallization time needed by one order of magnitude. This fast process allows to incorporate even the less stable colloids including initially oil-based ligand-exchanged QDs into salt matrices. Furthermore, in a modified two-step approach, the seed-mediated LLDC provides the ability to incorporate oil-based QDs directly into ionic matrices without a prior phase transfer. Finally, making use of their processability, a proof-of-concept white light emitting diode with LLDC-based mixed QD-salt films as an excellent color-conversion layer is demonstrated. These findings suggest that the LLDC offers a robust, adaptable, and rapid technique for obtaining high quality QD-salts

Ultrafast exciton dynamics in CdxHg(1-x)Te alloy quantum dots

Ultrafast transient absorption spectroscopy is used to investigate sub-nanosecond exciton dynamics in CdxHg(1−x)Te alloy colloidal quantum dots. A bleach was observed at the band gap due to state-filling, the mono-exponential decay of which had a characteristic lifetime of 91 ± 1 ps and was attributed to biexciton recombination; no evidence of surface-related trapping was observed. The rise time of the bleach, which is determined by the rate at which hot electrons cool to the band-edge, ranged between 1 and 5 ps depending on the pump photon energy. Measuring the magnitude of the bleach decay for different pump fluences and wavelengths allowed the quantum yield of multiple exciton generation to be determined, and was 115 ± 1% for pump photons with energy equivalent to 2.6 times the band gap