Electrochemically Determining Electronic Structure of ZnO Quantum Dots with Different Surface Ligands.

The electronic structure of quantum dots (QDs) including band edges and possible trap states is an important physical property for optoelectronic applications. The reliable determination of the energy levels of QDs remains a big challenge. Herein we employ cyclic voltammetry (CV) to determine the energy levels of three types of ZnO QDs with different surface ligands. Coupled with spectroscopic techniques, it is found that the onset potential of the first reductive wave is likely related to the conduction band edges while the first oxidative wave originates from the trap states. The determined specific energy levels in CV further demonstrates that the ZnO QDs without surface ligands mainly have oxygen interstitial defects whilst the ZnO QDs covered with ligands contain oxygen vacancies. The present electrochemical method offers a powerful and effective way to determine the energy levels of wide bandgap ZnO QDs, which will boost their device performance.

Electrochemically Quantifying the Phase Transition of Cesium Lead Halide Perovskite Quantum Dots in Purification.

A good purification strategy for obtaining high-quality and low-cost perovskite QDs ink requires a complete removal of the impurities but with a minimal phase transition of QDs from the perovskite phases to the nonperovskite δ-phase. This pioneering work reports the electrochemical quantification on the phase transition level of CsPbI3 QDs in purification. Cyclic voltammetry of the purified QDs evidenced the formation of a new product in the purification process, which was demonstrated to be the undesired nonperovskite δ-phase by independent structural analysis. The developed electrochemical methodology further enabled the quantification of the extent of the phase transition of the QDs purified using different strategies by simply analyzing the charge associated with the relevant peaks and allowing optimization of the purification. The latter is of vital importance for commercialization and is an essential step for boosting their device performance.

Probing Dynamics of Lead(II) Sulfide Quantum Dots in Solution Ligand Exchange by Voltammetry.

The ligand/quantum dots (QDs) ratio is crucial for the liquid state ligand exchange process to ensure a high-quality surface passivation and stable QDs ink. Herein we report an electrochemical method to investigate the ligand exchanged PbS-PbI2 QDs. It is found that the shell and core Pb(II) are distinguished by their reduction peak position in the cyclic voltammogram and the peak charge ratio gives the shell/core composition of the QDs. Combined with XPS analysis and UV-vis spectroscopy, it is further indicated that the shell/core ratio of PbS-PbI2 QDs varies as the ligand PbI2 concentration changes. Specifically, below a certain concentration, more PbI2 binds to the QD surface, leading to better passivation when the PbI2 concentration increases; however, beyond that concentration, decomposition of QDs likely occurs via an anion exchange process. The presented electrochemical method provides a new and powerful tool to investigate and optimize QD surface chemistry for boosting the scale up applications of QD devices.

Preparation and characterization of a self-suspending ultra-low density proppant.

A self-suspending ultra-low density proppant (UDP) was developed based on the polymerization of the unsaturated carbon double bond. Its performance was characterized by FT-IR and SEM, and the sphericity and roundness, diameter distribution, density, mechanical properties, the conductivity of the propped fracture, and mass loss of different fluids were measured. The test results indicated that the UDP no longer contained the unsaturated carbon double bond and the polymerization took place in the raw material. The fracture surface of UDP is compact and it is not easy to produce debris after compression failure. The sphericity and roundness of UDP were above 0.9, and the high sphericity and roundness provided high conductivity. The stirring speed has a great influence on the diameter of UDP, and the UDP with different sizes could be used to prop the hydraulic fracture to different widths. The average apparent density of UDP is as low as 1.044 g cm-3, and it can be suspended in the fracturing fluid for a long time. The strain in the UDP is higher than that in the ceramsite and quartz sand, but its crushing ratio is far below theirs; therefore, the conductivity of the fracture propped by UDP was higher than that of quartz sand and ceramsite. The solubility of UDP in kerosene, reservoir water, and hydrochloric acid is below 1%, indicating that the UDP is also suitable for acid fracturing with proppant. All the experimental results proved that the self-suspending ultra-low density proppant has great potential use in hydraulic fracturing and acid fracturing.

A reservoir-damage-free encapsulated acid dually controlled by hydrogen ion concentration and temperature.

Oil and gas exploration and development extends from medium-low temperatures to high and ultra-high temperatures with the development of the oil and gas industry. High-temperature deep carbonate reservoir acid fracturing has introduced more stringent requirements, a slower chemical reaction rate and excellent dissolution performance of acid systems, which means that the acid system should still have a certain dissolution ability above 135 °C. A novel water-soluble encapsulated acid (EA), dual controlled by hydrogen ion concentration and temperature, was developed to exploit ultra-high-temperature carbonate reservoirs. The encapsulating material was insoluble and isolated the internal solid acid at high H+ ion concentrations and low temperatures, but the solid acid was released as the encapsulating material was dissolved at low H+ ion concentrations and high temperatures. This unique performance was characterized by ESEM, TGA, FTIR, NMR, mechanical performance, solubility, etching performance, and etching fracture conductivity. All the scientific results show that this EA can be applied as a long-distance etching acid controlled by H+ ion concentration and temperature, without the need for a thickener and emulsifier to reduce the reaction between the rock and the acid near the wellbore. The test results demonstrated that the solid acid had good thermal stability at 135 °C, the encapsulation material was almost insoluble in high acid concentrations (>14%) at any temperature, and the solid acid began to release when the concentration of hydrochloric acid was less than 14% and the temperature was higher than 95 °C. The rock etching and dissolution behavior was better than that of HCl with the same concentration and the etching fracture conductivity was improved by supplementing the consumption of H+ ions when etching rock. The encapsulating material is completely dissolved after acid fracturing, avoiding reservoir damage by the residue. The described EA is a promising approach for application in acid fracturing of carbonate reservoirs at ultra-high temperature (>135 °C).

Narrow Band Gap Lead Sulfide Hole Transport Layers for Quantum Dot Photovoltaics.

The band structure of colloidal quantum dot (CQD) bilayer heterojunction solar cells is optimized using a combination of ligand modification and QD band gap control. Solar cells with power conversion efficiencies of up to 9.33 ± 0.50% are demonstrated by aligning the absorber and hole transport layers (HTL). Key to achieving high efficiencies is optimizing the relative position of both the valence band and Fermi energy at the CQD bilayer interface. By comparing different band gap CQDs with different ligands, we find that a smaller band gap CQD HTL in combination with a more p-type-inducing CQD ligand is found to enhance hole extraction and hence device performance. We postulate that the efficiency improvements observed are largely due to the synergistic effects of narrower band gap QDs, causing an upshift of valence band position due to 1,2-ethanedithiol (EDT) ligands and a lowering of the Fermi level due to oxidation.

High Performance PbS Quantum Dot/Graphene Hybrid Solar Cell with Efficient Charge Extraction.

Hybrid colloidal quantum dot (CQD) solar cells are fabricated from multilayer stacks of lead sulfide (PbS) CQD and single layer graphene (SG). The inclusion of graphene interlayers is shown to increase power conversion efficiency by 9.18%. It is shown that the inclusion of conductive graphene enhances charge extraction in devices. Photoluminescence shows that graphene quenches emission from the quantum dot suggesting spontaneous charge transfer to graphene. CQD photodetectors exhibit increased photoresponse and improved transport properties. We propose that the CQD/SG hybrid structure is a route to make CQD thin films with improved charge extraction, therefore resulting in improved solar cell efficiency.

Poly(3-hexylthiophene-2,5-diyl) as a Hole Transport Layer for Colloidal Quantum Dot Solar Cells.

Lead sulfide colloidal quantum dot (CQD) solar cells demonstrate extremely high short-circuit currents (Jsc) and are making decent progress in power conversion efficiencies. However, the low fill factors (FF) and open-circuit voltages have to be addressed with urgency to prevent the stalling of efficiency improvements. This paper highlights the importance of improving hole extraction, which received much less attention as compared to the electron-accepting component of the device architecture (e.g., TiO2 or ZnO). Here, we show the use of semiconducting polymer poly(3-hexylthiophene-2,5-diyl) to create efficient CQD devices by improving hole transport, removing interfacial barriers, and minimizing shunt pathways, thus resulting in an overall improvement in device performance stemming from better Jsc and FF.

Nano-intermetallic AuCu3 catalyst for oxygen reduction reaction: performance and mechanism.

This paper introduces a new approach for catalyst design using the non-precious metal Cu as one of the catalytic active centers. This differs from previous studies that considered precious metals to be responsible for the catalytic reaction in precious alloys. Intermetallic AuCu3/C nanoparticles with a diameter of 3 nm were developed for the first time, with uniform dispersion and a narrow size distribution. The ca. 3 nm as-synthesised AuCu3/C showed superior catalytic performance for oxygen reduction reactions (ORR) in alkaline solutions, with comparable half-wave potential and 1.5 times mass current density of commercial Pt/C at 0.80 V (vs. reversible hydrogen electrode (RHE)). The advanced catalytic activities are mainly attributed to the synergetic effects of electro-active atomic Au and Cu on the particle surface, in which Cu helps to activate the O2 molecule and Au benefits OH(-) desorption. The excellent durability and methanol tolerance exhibited in alkaline solutions provide another advantage for AuCu3/C to be considered as a potential alternative cathode catalyst in alkaline fuel cells.

Durability enhancement of intermetallics electrocatalysts via N-anchor effect for fuel cells.

Insufficient durability and catalytic activity of oxygen reduction reaction (ORR) electrocatalyst are key issues that have to be solved for the practical application of low temperature fuel cell. This paper introduces a new catalyst design strategy using N-anchor to promote the corrosion resistance of electrocatalyst. The as-synthesized N-Pt3Fe1/C shows a high electrocatalytic activity and a superior durability towards ORR. The kinetic current density of N-Pt3Fe1/C as normalized by ECSA is still as high as 0.145 mA cm(-2) and only 7% loss after 20,000 potential cycles from 0.6 to 1.2 V (vs. NHE) in O2-bubbling perchloric acid solution, whereas Pt3Fe1/C shows 49% loss under the same tests. The N-anchor approach offers novel opportunities for the development of ORR catalyst with excellent electrochemical properties.

Mechanism of oxygen reduction reaction catalyzed by Fe(Co)-Nx/C.

Fe(Co)-Nx/C is an important candidate catalyst for the next generation proton exchange membrane fuel cells (PEMFC), but the relationship between the structure and the oxygen reduction activity is still unclear. In this work, the different active site structures of Fe(Co)-Nx/C are explored and the oxygen reduction catalytic mechanisms are studied by means of density functional theory (DFT). Different kinds of Me-Nx/C motifs, including the edge site around the graphene sheet and the internal site in the graphene sheet (as well as in the graphyne sheet), are constructed and investigated. The calculated results suggest that for the edge active sites, high O2 adsorption strength may result in direct oxidation of metal ions thus losing their catalytic activity. The internal active sites are stable in acidic solution and display catalytic ability of oxygen reduction. The catalytic activity of the internal site is affected by three factors: the kind of internal metal ion, the bonded nitrogen or carbon atoms with metal ions and the size of the graphene sheet.

[Effects of oxymatrine on serum levels of Th1/Th2 type cytokines in HBsAg transgenic mice].

OBJECTIVE: To explore the effects of oxymatrine on serum levels of Th1/Th2 cytokines in HBsAg transgenic mice. METHODS: HBsAg transgenic mice were divided into oxymatrine group and control group. Each mouse was injected with either oxymatrine 200 mg/kg 0.2 ml or 0.9% NaCl 0.2 ml intraperitoneally once a day for 30 days. Serum IFN-gamma, IL-2 and IL-4, IL-10 were quantitated before and after different treatment. RESULTS: There was no significant difference on the levels of IFN-gamma and IL-4 before and after treatment in control group. While in oxymatrine group, the levels of IFN-gamma before and after treatment were (3.108+/-3.172) pg/ml and (11.059+/-6.971) pg/ml; those of IL-4 were (29.045+/-13.235) pg/ml and (13.024+/-9.002) pg/ml (both P less than 0.001). After treatment, the levels of IL-2 in control and oxymatrine group were (1.070+/-0.447) pg/ml and (5.537+/-2.887) pg/ml (P less than 0.000 1); and those of IL-10 were (97.226+/-73.306) pg/ml and (33.607+/-23.154) pg/ml (P less than 0.01). CONCLUSION: After injection of oxymatrine to HBsAg transgenic mice, the serum concentration of Th1 cytokines increased while the Th2 cytokines decreased. This can help us understand more better on the mechanisms of anti-HBV effect of oxymatrine.