Highly active, stable, and cheap Pt‐free catalysts for the hydrogen evolution reaction (HER) are facing increasing demand as a result of their potential use in future energy‐conversion systems. ...However, the development of HER electrocatalysts with Pt‐like or even superior activity, in particular ones that can function under alkaline conditions, remains a significant challenge. Here, the synthesis of a novel carbon‐loaded ruthenium nanoparticle electrocatalyst (Ru@CQDs) for the HER, using carbon quantum dots (CQDs), is reported. Electrochemical tests reveal that, even under extremely alkaline conditions (1 m KOH), the as‐formed Ru@CQDs exhibits excellent catalytic behavior with an onset overpotential of 0 mV, a Tafel slope of 47 mV decade−1, and good durability. Most importantly, it only requires an overpotential of 10 mV to achieve the current density of 10 mA cm−2. Such catalytic characteristics are superior to the current commercial Pt/C and most noble metals, non‐noble metals, and nonmetallic catalysts under basic conditions. These findings open a new field for the application of CQDs and add to the growing family of metal@CQDs with high HER performance.
The ruthenium@carbon quantum dots (Ru@CQDs) electrocatalyst is very robust for the hydrogen evolution reaction in alkaline media, with an onset overpotential of 0 mV and low overpotentials at 10 mA cm−2 (10 mV). More importantly, after 10 000 cycles, the current density at 10 mA cm−2 of the Ru@CQDs catalyst increases merely 4 mV at 10 mA cm−2.
As an emerging building unit, carbon dots (CDs) have been igniting the revolutionaries in the fields of optoelectronics, biomedicine, and bioimaging. However, the difficulty of synthesizing CDs in ...aqueous solution with full‐spectrum emission severely hinders further investigation of their emission mechanism and their extensive applications in white light emitting diodes (LEDs). Here, the full‐color‐emission CDs with a unique structure consisting of sp3‐hybridized carbon cores with small domains of partially sp2‐hybridized carbon atoms are reported. First‐principle calculations are initially used to predict that the transformation from sp3 to sp2 hybridization redshifts the emission of CDs. Guided by the theoretical predictions, a simple, convenient, and controllable route to hydrothermally prepare CDs in a single reaction system is developed. The prepared CDs have full‐spectrum emission with an unprecedented two‐photon emission across the whole visible color range. These full‐color‐emission CDs can be further nurtured by slight modifications of the reaction conditions (e.g., temperature, pH) to generate the emission color from blue to red. Finally a flexible LEDs with full‐color emission by using epoxy CDs films is developed, indicating that the strategy affords an industry translational potential over traditional fluorophores.
A novel multi‐color‐emissive carbon dots (CDs) with two‐photon fluorescence are developed in aqueous solution for the first time by slight modifications of the reaction conditions. Theoretical calculations combined with experiments show that the photoluminescence mechanism of the multi‐color‐emissive CDs is the size change and the conversion of sp3 to sp2 hybridization.
Fermion portal dark matter Bai, Yang; Berger, Joshua
The journal of high energy physics,
11/2013, Letnik:
2013, Številka:
11
Journal Article
Recenzirano
Odprti dostop
A
bstract
We study a class of simplified dark matter models in which one dark matter particle couples with a mediator and a Standard Model fermion. In such models, collider and direct detection ...searches probe complimentary regions of parameter space. For Majorana dark matter, direct detection covers the region near mediator-dark matter degeneracy, while colliders probe regions with a large dark matter and mediator mass splitting. For Dirac and complex dark matter, direct detection is effective for the entire region above the mass threshold, but colliders provide a strong bound for dark matter lighter than a few GeV. We also point out that dedicated searches for signatures with two jets or a monojet not coming from initial state radiation, along missing transverse energy can cover the remaining parameter space for thermal relic dark matter.
A
bstract
Spherically symmetric magnetic and dyonic black holes with a magnetic charge
Q
= 2 are studied in the Standard Model and general relativity. A magnetically charged black hole with mass ...below 9
.
3 × 10
35
GeV has a “hairy” cloud of electroweak gauge and Higgs fields outside the event horizon with 1
/m
W
in size. An extremal magnetic black hole has a hair mass of 3.6 TeV, while an extremal dyonic black hole has an additional mass of
q
2
× 1
.
6 GeV for a small electric charge
q
≪ 2
π/e
2
. A hairy dyonic black hole with an integer charge is not stable and can decay into a magnetic one plus charged fermions. On the other hand, a hairy magnetic black hole can evolve via Hawking radiation into a nearly extremal one that is cosmologically stable and an interesting object to be searched for.
We present petrological investigations and mineral chemistry of several Tethyan ophiolites to reveal the occurrence, origin, and fate of water in podiform chromitites. The results show that ...clinopyroxene and olivine in chromitites have H2O contents of 801-366 and 53-17 ppm, respectively. The highest water contents of olivine occur in massive chromitite and the lowest always in the clinopyroxene-bearing ores because much of the available hydrous fluids was taken up by the clinopyroxene during crystallization. The major and trace elemental and Li isotopic compositions of clinopyroxene associated with chromite and olivine in podiform chromitites indicate formation from a mixture of surface hydrous fluids on chromite grains and evolved melts from which olivine crystallized. The hydrous fluids initially originated from dehydration of a subducting slab as revealed by Li isotopic compositions of clinopyroxene and olivine in the chromitites. High fluid/rock ratios facilitated concentration of chromite to form chromitite, suppressing crystallization of olivine. The hydrous fluids that were collected on the chromite grain surface during crystallization allowed chromite grains to rise via decreasing density in the form of bubbles, thus promoting their gathering and concentration. The fate of these hydrous fluids depends on ambient physical and chemical conditions. Mostly they hydrate adjacent olivine grains in the chromitite or penetrate the surrounding dunite envelope. In some cases, the fluids dissolve into silicate melts to produce water-bearing clinopyroxene and/or hydrous minerals, such as amphibole, or infiltrate silicate and chromite grains to form inclusions, which may exsolve later in the form of mineral lamellae. Our investigations provide direct natural evidence for the presence and importance of water in the formation and evolution of chromite deposits, as inferred by earlier experimental studies.
Carbon dots (CDots), an emerging class of photoluminescent materials, exhibit excellent optical, electrical and chemical properties with many potential applications. Hetero-atom doping in CDots ...represents a robust and versatile technique to further improve the chemical composition and structure properties of CDots. In this review, we provide an updated roadmap of the hetero-atom-doped CDots. Various up-to-date synthetic strategies, starting with different raw materials to synthetic methods to doping elements, are surveyed. Furthermore, the catalysis, optical properties, biological properties and the potential applications of hetero-atom doped CDots, including nano-probes, optoelectronic devices, catalysis, and biomedicine are also reviewed. We further provide an outlook towards future developments of the hetero-atom-doped CDots. This review will provide a summary of the latest progress of hetero-atom-doped CDots and is expected to inspire further developments in this exciting class of materials.
The stability of perovskite solar cells has shown a huge variation with respect to the film process and film morphology, while the underlining mechanism for the morphology-dependent degradation of ...the perovskite film has remained elusive. Herein, we report a scaling behavior of moisture-induced grain degradation in polycrystalline CH3NH3PbI3 films. The degradation rates of CH3NH3PbI3 films in moisture were shown to be sensitive to the grain sizes. The duration that was needed for different films to degrade by the same percent showed a linear relationship with the grain size, despite the fact that the films were formed by five different deposition methods. This scaling behavior can be explained by the degradation along the in-plane direction, which is initiated at the grain boundary (GB). The GBs of CH3NH3PbI3 films consist of an amorphous intergranular layer, which allows quick diffusion of moisture into the perovskite films. It was found that thermal annealing induced surface self-passivation plays a critical role in stabilizing the surfaces of thin films and single crystals by reducing the moisture-sensitive methylammonium ions at the surface. The determination of the scaling behavior of grain degradation highlights the importance of stabilizing the GBs to improve the stability of perovskite solar cells.
A
bstract
For a class of macroscopic dark matter with a large interaction strength with Standard Model particles, a nucleus could be captured by the dense, heavy dark matter as it traverses ordinary ...material. The radiated photon carries most of the binding energy and is a characteristic signature for dark matter detection. We develop analytic formulas and present numerical results for this radiative capture process in the low energy, non-dipole limit. Large-volume neutrino detectors like NO
ν
A, JUNO, DUNE and Super(Hyper)-K may detect multi-hit or single-hit radiative capture events and can search for dark matter up to one gram in mass.
Halide perovskites exhibit diverse properties depending on their compositions. However, integrating desired properties into one material is still challenging. Here, a facile solution‐processed ...epitaxial growth method to grow 2D perovskite single crystal on top of 3D perovskite single crystal, which can passivate the surface defects for improved device performance is reported. Short formamidine (FA+) ions are replaced by long organic cations, which can fully align and cover the single crystal surface to prevent the ions migration or short FA+ ions volatilization. The thickness of epitaxial layer can be finely adjusted by controlling the growth time. The defect density of single crystals heterojunction is only 3.18 × 109 cm−3, and the carrier mobility is 80.43 cm2 V−1 s−1, which is greater than that of the control 3D perovskite single crystal. This study for the first time realized large area 3D/2D perovskite single crystals heterojunction, which suppressed ions migration and exhibited advanced performance in hard X‐rays detection applications. This strategy also provides a way to grow large area 2D perovskite single crystal from solution processes.
The 2D epitaxial layer of the 3D/2D perovskite single crystals heterojunction can passivate the surface defects and suppress ion migration or short formamidine FA+ ion volatilization, which boost its optoelectric properties. The 3D/2D heterojunction X‐ray detector exhibits stable response to 120 kVp hard X‐rays, with the lowest detectable dose rate of 55 nGyairs−1.
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