Polymer carbon dots (PCDs) are proposed as a new class of room‐temperature phosphorescence (RTP) materials. The abundant energy levels in PCDs increase the probability of intersystem crossing (ISC) ...and their covalently crosslinked framework structures greatly suppress the nonradiative transitions. The efficient methods allow the manufacture of PCDs with unique RTP properties in air without additional metal complexation or complicated matrix composition. They thus provide a route towards the rational design of metal‐free RTP materials that may be synthesized easily. Furthermore, we find that RTP is associated with a crosslink‐enhanced emission (CEE) effect, which provides further routes to design improved PCDs with diverse RTP performance. Our results show the potential of PCDs as a universal route to achieve effective metal‐free RTP.
Room‐temperature phosphorescence: Polymer carbon dots (PCDs) showing metal‐free room‐temperature phosphorescence (RTP) have been constructed by using a facile method. The contribution of the cross‐link‐enhanced emission effect to the generation of RTP is verified and proposed as a guideline to forecast and synthesize a series of PCDs with diverse RTP performance (ISC=intersystem crossing).
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.
We present an in situ powder X-ray diffraction study on the phase stability and polymorphism of the metal–organic framework ZIF-4, Zn(imidazolate)2, at simultaneous high pressure and high ...temperature, up to 8 GPa and 600 °C. The resulting pressure–temperature phase diagram reveals four, previously unknown, high-pressure–high-temperature ZIF phases. The crystal structures of two new phasesZIF-4-cp-II and ZIF-hPT-IIwere solved by powder diffraction methods. The total energy of ZIF-4-cp-II was evaluated using density functional theory calculations and was found to lie in between that of ZIF-4 and the most thermodynamically stable polymorph, ZIF-zni. ZIF-hPT-II was found to possess a doubly interpenetrated diamondoid topology and is isostructural with previously reported Cd(Imidazolate)2 and Hg(Imidazolate)2 phases. This phase exhibited extreme resistance to both temperature and pressure. The other two new phases could be assigned with a unit cell and space group, although their structures remain unknown. The pressure–temperature phase diagram of ZIF-4 is strikingly complicated when compared with that of the previously investigated, closely related ZIF-62 and demonstrates the ability to traverse complex energy landscapes of metal–organic systems using the combined application of pressure and temperature.
Crystalline solids dominate the field of metal–organic frameworks (MOFs), with access to the liquid and glass states of matter usually prohibited by relatively low temperatures of thermal ...decomposition. In this work, we give due consideration to framework chemistry and topology to expand the phenomenon of the melting of 3D MOFs, linking crystal chemistry to framework melting temperature and kinetic fragility of the glass-forming liquids. Here we show that melting temperatures can be lowered by altering the chemistry of the crystalline MOF state, which provides a route to facilitate the melting of other MOFs. The glasses formed upon vitrification are chemically and structurally distinct from the three other existing categories of melt-quenched glasses (inorganic nonmetallic, organic, and metallic), and retain the basic metal–ligand connectivity of crystalline MOFs, which connects their mechanical properties to their starting chemical composition. The transfer of functionality from crystal to glass points toward new routes to tunable, functional hybrid glasses.
Abstract
We estimate the causal contributions of spatiotemporal changes in temperature (
T
) and precipitation (
Pr
) to changes in Earth’s atmospheric methane concentration (
C
CH4
) and its isotope ...ratio
δ
13
C
H
4
over the last four decades. We identify oscillations between positive and negative feedbacks, showing that both contribute to increasing
C
CH4
. Interannually, increased emissions via positive feedbacks (e.g. wetland emissions and wildfires) with higher land surface air temperature (
LSAT
) are often followed by increasing
C
CH4
due to weakened methane sink via atmospheric
•
OH, via negative feedbacks with lowered sea surface temperatures (
SST
), especially in the tropics. Over decadal time scales, we find alternating rate-limiting factors for methane oxidation: when
C
CH4
is limiting, positive methane-climate feedback via direct oceanic emissions dominates; when
•
OH is limiting, negative feedback is favoured. Incorporating the interannually increasing
C
CH4
via negative feedbacks gives historical methane-climate feedback sensitivity ≈ 0.08 W m
−2
°C
−1
, much higher than the IPCC AR6 estimate.
The concept of the memristor, a resistor with memory, was proposed by Chua in 1971 as the fourth basic element of electric circuitry. Despite a significant amount of effort devoted to the ...understanding of memristor theory, our understanding of the nonpinched current–voltage (I–V) hysteresis loop in memristors remains incomplete. Here we propose a physical model of a memristor, with a capacitor connected in parallel, which explains how the nonpinched I–V hysteresis behavior originates from the capacitive-coupled memristive effect. Our model replicates eight types of characteristic nonlinear I–V behavior, which explains all observed nonpinched I–V curves seen in experiments. Furthermore, a reversible transition from a nonpinched I–V hysteresis loop to an ideal pinched I–V hysteresis loop is found, which explains the experimental data obtained in C15H11O6-based devices when subjected to an external stimulus (e.g., voltage, moisture, or temperature). Our results provide the vital physics models and materials insights for elucidating the origins of nonpinched I–V hysteresis loops ascribed to capacitive-coupled memristive behavior.
Metal halide perovskites (MHPs) are of great interest for optoelectronics because of their high quantum efficiency in solar cells and light-emitting devices. However, exploring an effective strategy ...to further improve their optical activities remains a considerable challenge. Here, we report that nanocrystals (NCs) of the initially nonfluorescent zero-dimensional (0D) cesium lead halide perovskite Cs
PbBr
exhibit a distinct emission under a high pressure of 3.01 GPa. Subsequently, the emission intensity of Cs
PbBr
NCs experiences a significant increase upon further compression. Joint experimental and theoretical analyses indicate that such pressure-induced emission (PIE) may be ascribed to the enhanced optical activity and the increased binding energy of self-trapped excitons upon compression. This phenomenon is a result of the large distortion of PbBr
octahedral motifs resulting from a structural phase transition. Our findings demonstrate that high pressure can be a robust tool to boost the photoluminescence efficiency and provide insights into the relationship between the structure and optical properties of 0D MHPs under extreme conditions.
The recent discovery of H3S and LaH10 superconductors with record high superconducting transition temperatures Tc at high pressure has fueled the search for room-temperature superconductivity in the ...compressed superhydrides. Here we introduce a new class of high Tc hydrides with a novel structure and unusual properties. We predict the existence of an unprecedented hexagonal HfH10, with remarkably high value of T c (around 213–234 K) at 250 GPa. As concerns the novel structure, the H ions in HfH10 are arranged in clusters to form a planar "pentagraphenelike" sublattice. The layered arrangement of these planar units is entirely different from the covalent sixfold cubic structure in H3S and clathratelike structure in LaH10. The Hf atom acts as a precompressor and electron donor to the hydrogen sublattice. This pentagraphenelike H10 structure is also found in ZrH10, ScH10, and LuH10 at high pressure, each material showing a high Tc ranging from 134 to 220 K. Our study of dense superhydrides with pentagraphenelike layered structures opens the door to the exploration of a new class of high T c superconductors.
Earth's deep carbon cycle affects atmospheric CO
, climate, and habitability. Owing to the extreme solubility of CaCO
, aqueous fluids released from the subducting slab could extract all carbon from ...the slab. However, recycling efficiency is estimated at only around 40%. Data from carbonate inclusions, petrology, and Mg isotope systematics indicate Ca
in carbonates is replaced by Mg
and other cations during subduction. Here we determined the solubility of dolomite CaMg(CO
)
and rhodochrosite (MnCO
), and put an upper limit on that of magnesite (MgCO
) under subduction zone conditions. Solubility decreases at least two orders of magnitude as carbonates become Mg-rich. This decreased solubility, coupled with heterogeneity of carbon and water subduction, may explain discrepancies in carbon recycling estimates. Over a range of slab settings, we find aqueous dissolution responsible for mobilizing 10 to 92% of slab carbon. Globally, aqueous fluids mobilise Formula: see text% (Formula: see text Mt/yr) of subducted carbon from subducting slabs.
PDF
