124 research outputs found
The investigation into highly efficient heptazine-based polymeric photocatalysts for visible light-driven solar fuel synthesis
Artificial photosynthesis has been regarded as a promising method to generate fuels in a much greener way by utilising inexhaustible solar energy via water splitting and CO2 conversion. Polymeric semiconductors have been recently identified as promising photocatalysts due to their comparatively low cost and ease modification of the electronic structure. However, the majority only respond to a limited wavelength region ( 420 nm), leading to a 10.3% apparent quantum yield (λ = 420 nm). Both theoretical calculations and spectroscopies have attributed such superior performance to enhanced charge separation and narrow bandgap. Such new polymer was then coupled with an inorganic photocatalyst to construct a Z-scheme system, which successfully splits water into both H2 and O2 in a stoichiometry ratio. Further, an efficient strategy was demonstrated to stepwise tailor the bandgap of polymeric photocatalysts from 2.7 to 1.9 eV by carefully manipulating the O/N linker/terminal atoms in the heptazine chains. These polymers work stably and efficiently for both H2 and O2 evolution (420 nm < λ < 710 nm), exhibiting nearly 20 times higher activity compared to g-C3N4 with high AQYs under visible light irradiation. Experimental and theoretical results have attributed the narrowed band gap and enhanced charge separation to the oxygen incorporation into the linker/terminal position. Based on this success, a more challenging multi-electron photochemical process of visible light-driven CO2 reduction in water was investigated using junctions consisting of the novel polymers and two kinds of carbon quantum dots (CQD) cocatalysts. The novel CQD was synthesised via a microwave-assisted method while the other CQD fabricated via sonication of glucose was reported as reduction cocatalysts (redCQD). In CO2 reduction reactions, the novel CQD/polymer junctions selectively produce methanol and O2 while the redCQD/polymer junction generates CO only. Ultrafast spectroscopies revealed that novel CQD works as a hole acceptor in the junctions, different from the redCQD as an electron acceptor. Electrons reach the surface of polymers to reduce CO2 to produce methanol while holes accumulate on CQD to oxidise water. Microwave-assisted CQD shows more favourable water adsorption instead of methanol adsorption compared with polymers, thus facilitating methanol production instead of CO. Therefore, the function of CQD is a key reason for such high selectivity
Insight on Reaction Pathways of Photocatalytic CO2 Conversion
Photocatalytic CO2 conversion to value-added chemicals is a promising solution to mitigate the current energy and environmental issues but is a challenging process. The main obstacles include the inertness of CO2 molecule, the sluggish multi-electron process, the unfavorable thermodynamics, and the selectivity control to preferable products. Furthermore, the lack of fundamental understanding of the reaction pathways accounts for the very moderate performance in the field. Therefore, in this Perspective, we attempt to discuss the possible reaction mechanisms toward all C1 and C2 value-added products, taking into account the experimental evidence and theoretical calculation on the surface adsorption, proton and electron transfer, and products desorption. Finally, the remaining challenges in the field, including mechanistic understanding, reactor design, economic consideration, and potential solutions, are critically discussed by us
Electronic Noise of a Single Skyrmion
To enable the practical use of skyrmion-based devices, it is essential to
achieve a balance between energy efficiency and thermal stability, while also
ensuring reliable electrical detection against noise. Understanding how a
skyrmion interacts with material disorder and external perturbations is thus
essential. Here we investigate the electronic noise of a single skyrmion under
the influence of thermal fluctuations and spin currents in a magnetic thin
film. We detect the thermally induced noise with a 1/f signature in the strong
pinning regime but a random telegraph noise in the intermediate pinning regime.
Both the thermally dominated and current-induced telegraph-like signals are
detected in the weak pinning regime. Our results provide a comprehensive
electronic noise picture of a single skyrmion, demonstrating the potential of
noise fluctuation as a valuable tool for characterizing the pinning condition
of a skyrmion. These insights could also aid in the development of low-noise
and reliable skyrmion-based devices
New modeling of the Vostok ice flow line and implication for the glaciological chronology of the Vostok ice core
International audienceWe have used new spaceborne (elevation) and airborne (ice thickness) data to constrain a 2D1/2 model of snow accumulation and ice flow along the Ridge B‐Vostok station ice flow line (East Antarctica). We show that new evaluations of the ice flow line geometry (from the surface elevation), ice thickness (from low‐frequency radar data), and basal melting and sliding change significantly the chronology of the Vostok ice core. This new Vostok dating model reconciles orbital and glaciological timescales and is in good agreement with the Dome Fuji glaciological timescale. At the same time, the new model shows significantly older ages than the previous GT4 timescale for the last glacial part, being thus in better agreement with the GRIP and GISP2 chronologies
Data-Adaptive Wavelets and Multi-Scale Singular Spectrum Analysis
Using multi-scale ideas from wavelet analysis, we extend singular-spectrum
analysis (SSA) to the study of nonstationary time series of length whose
intermittency can give rise to the divergence of their variance. SSA relies on
the construction of the lag-covariance matrix C on M lagged copies of the time
series over a fixed window width W to detect the regular part of the
variability in that window in terms of the minimal number of oscillatory
components; here W = M Dt, with Dt the time step. The proposed multi-scale SSA
is a local SSA analysis within a moving window of width M <= W <= N.
Multi-scale SSA varies W, while keeping a fixed W/M ratio, and uses the
eigenvectors of the corresponding lag-covariance matrix C_M as a data-adaptive
wavelets; successive eigenvectors of C_M correspond approximately to successive
derivatives of the first mother wavelet in standard wavelet analysis.
Multi-scale SSA thus solves objectively the delicate problem of optimizing the
analyzing wavelet in the time-frequency domain, by a suitable localization of
the signal's covariance matrix. We present several examples of application to
synthetic signals with fractal or power-law behavior which mimic selected
features of certain climatic and geophysical time series. A real application is
to the Southern Oscillation index (SOI) monthly values for 1933-1996. Our
methodology highlights an abrupt periodicity shift in the SOI near 1960. This
abrupt shift between 4 and 3 years supports the Devil's staircase scenario for
the El Nino/Southern Oscillation phenomenon.Comment: 24 pages, 19 figure
Interfacial Manganese-Doping in CsPbBr3 Nanoplatelets by Employing a Molecular Shuttle
Mn-doping in cesium lead halide perovskite nanoplatelets (NPls) is of particular importance where strong quantum confinement plays a significant role towards the exciton-dopant coupling. In this work, we report an immiscible bi-phasic strategy for post-synthetic Mn-doping of CsPbX3 (X=Br, Cl) NPls. A systematic study shows that electron-donating oleylamine acts as a shuttle ligand to transport MnX2 through the water-hexane interface and deliver it to the NPls. The halide anion also plays an essential role in maintaining an appropriate radius of Mn2+ and thus fulfilling the octahedral factor required for the formation of perovskite crystals. By varying the thickness of parent NPls, we can tune the dopant incorporation and, consequently, the exciton-to-dopant energy transfer process in doped NPls. Time-resolved optical measurements offer a detailed insight into the exciton-to-dopant energy transfer process. This new approach for post-synthetic cation doping paves a way towards exploring the cation exchange process in several other halide perovskites at the polar-nonpolar interface
Unique hole-accepting carbon-dots promoting selective carbon dioxide reduction nearly 100% to methanol by pure water
X.L., M.K.B., J.T. acknowledge EPSRC (EP/S018204/2/1), Leverhulme Trust (Grant No: RPG-2017-122) and Newton Advanced Fellowship grant ((NA170422 and NAF\R1\191163).). X.H., Z.X.G. and S.A.S. acknowledge UCL Grace High-Performance Computing Facility (Grace@UCL) and EPSRC (EP/K021192/1, EP/L018330/1). R.G. thanks the FRQNT for postdoctoral funding and NSERC for operational funding. Y.W., J.C. and C.J., acknowledge CSC Scholarship. R.G., J.F.T. and J.R.D. acknowledge ERC AdG Intersolar grant (291482). J.F.T. acknowledges EPSRC CDT (EP/L015277/1). W.Z. thanks EPSRC for Titan Themis S/TEM microscope (EP/L017008/01). We also thank Dr. Jijia Xie for constructive comments on experimental design.Solar-driven CO2 reduction by abundant water to alcohols can supply sustainable liquid fuels and alleviate global warming. However, the sluggish water oxidation reaction has been hardly reported to be efficient and selective in CO2 conversion due to fast charge recombination. Here, using transient absorption spectroscopy, we demonstrate that microwave-synthesised carbon-dots (mCD) possess unique hole-accepting nature, prolonging the electron lifetime (t50%) of carbon nitride (CN) by six folds, favouring a six-electron product. mCD-decorated CN stably produces stoichiometric oxygen and methanol from water and CO2 with nearly 100% selectivity to methanol and internal quantum efficiency of 2.1% in the visible region, further confirmed by isotopic labelling. Such mCD rapidly extracts holes from CN and prevents the surface adsorption of methanol, favourably oxidising water over methanol and enhancing the selective CO2 reduction to alcohols. This work provides a unique strategy for efficient and highly selective CO2 reduction by water to high-value chemicals.Publisher PDFPeer reviewe
- …