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1.
Nature ; 579(7799): 368-374, 2020 03.
Artigo em Inglês | MEDLINE | ID: mdl-32188941

RESUMO

Two-dimensional van der Waals heterostructures (vdWHs) have attracted considerable interest1-4. However, most vdWHs reported so far  are created by an arduous micromechanical exfoliation and manual restacking process5, which-although versatile for proof-of-concept demonstrations6-16 and fundamental studies17-30-is clearly not scalable for practical technologies. Here we report a general synthetic strategy for two-dimensional vdWH arrays between metallic transition-metal dichalcogenides (m-TMDs) and semiconducting TMDs (s-TMDs). By selectively patterning nucleation sites on monolayer or bilayer s-TMDs, we precisely control the nucleation and growth of diverse m-TMDs with designable periodic arrangements and tunable lateral dimensions at the predesignated spatial locations, producing a series of vdWH arrays, including VSe2/WSe2, NiTe2/WSe2, CoTe2/WSe2, NbTe2/WSe2, VS2/WSe2, VSe2/MoS2 and VSe2/WS2. Systematic scanning transmission electron microscopy studies reveal nearly ideal vdW interfaces with widely tunable moiré superlattices. With the atomically clean vdW interface, we further show that the m-TMDs function as highly reliable synthetic vdW contacts for the underlying WSe2 with excellent device performance and yield, delivering a high ON-current density of up to 900 microamperes per micrometre in bilayer WSe2 transistors. This general synthesis of diverse two-dimensional vdWH arrays provides a versatile material platform for exploring exotic physics and promises a scalable pathway to high-performance devices.

2.
Adv Mater ; : e1907112, 2020 Feb 05.
Artigo em Inglês | MEDLINE | ID: mdl-32020715

RESUMO

Electrochemical conversion of nitrogen (N2 ) into value-added ammonia (NH3 ) is highly desirable yet formidably challenging due to the extreme inertness of the N2 molecule, which makes the development of a robust electrocatalyst prerequisite. Herein, a new class of bullet-like M-Te (M = Ru, Rh, Ir) glassy porous nanorods (PNRs) is reported as excellent electrocatalysts for N2 reduction reaction (NRR). The optimized IrTe4 PNRs present superior activity with the highest NH3 yield rate (51.1 µg h-1 mg-1 cat. ) and Faraday efficiency (15.3%), as well as long-term stability of up to 20 consecutive cycles, making them among the most active NRR electrocatalysts reported to date. Both the N2 temperature-programmed desorption and valence band X-ray photoelectron spectroscopy data show that the strong chemical adsorption of N2 is the key for enhancing the NRR and suppressing the hydrogen evolution reaction of IrTe4 PNRs. Density functional theory calculations comprehensively identify that the superior adsorption strength of IrTe4 adsorptions originates from the synergistic collaboration between electron-rich Ir and the highly electroactive surrounding Te atoms. The optimal adsorption of both N2 and H2 O in alkaline media guarantees the superior consecutive NRR process. This work opens a new avenue for designing high-performance NRR electrocatalysts based on glassy materials.

3.
Nat Commun ; 10(1): 5190, 2019 11 15.
Artigo em Inglês | MEDLINE | ID: mdl-31729389

RESUMO

As one of next-generation semiconductors, hybrid halide perovskites with tailorable optoelectronic properties are promising for photovoltaics, lighting, and displaying. This tunability lies on variable crystal structures, wherein the spatial arrangement of halide octahedra is essential to determine the assembly behavior and materials properties. Herein, we report to manipulate their assembling behavior and crystal dimensionality by locally collective hydrogen bonding effects. Specifically, a unique urea-amide cation is employed to form corrugated 1D crystals by interacting with bromide atoms in lead octahedra via multiple hydrogen bonds. Further tuning the stoichiometry, cations are bonded with water molecules to create a larger spacer that isolates individual lead bromide octahedra. It leads to zero-dimension (0D) single crystals, which exhibit broadband 'warm' white emission with photoluminescence quantum efficiency 5 times higher than 1D counterpart. This work suggests a feasible strategy to modulate the connectivity of octahedra and consequent crystal dimensionality for the enhancement of their optoelectronic properties.

4.
Angew Chem Int Ed Engl ; 58(39): 13983-13988, 2019 Sep 23.
Artigo em Inglês | MEDLINE | ID: mdl-31342633

RESUMO

Channel-rich RuCu snowflake-like nanosheets (NSs) composed of crystallized Ru and amorphous Cu were used as efficient electrocatalysts for oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and overall water splitting in pH-universal electrolytes. The optimized RuCu NSs/C-350 °C and RuCu NSs/C-250 °C show attractive activities of OER and HER with low overpotentials and small Tafel slopes, respectively. When applied to overall water splitting, the optimized RuCu NSs/C can reach 10 mA cm-2 at cell voltages of only 1.49, 1.55, 1.49 and 1.50 V in 1 m KOH, 0.1 m KOH, 0.5 m H2 SO4 and 0.05 m H2 SO4 , respectively, much lower than those of commercial Ir/C∥Pt/C. The optimized electrolyzer exhibits superior durability with small potential change after up to 45 h in 1 m KOH, showing a class of efficient functional electrocatalysts for overall water splitting.

5.
Angew Chem Int Ed Engl ; 58(33): 11491-11496, 2019 Aug 12.
Artigo em Inglês | MEDLINE | ID: mdl-31206953

RESUMO

Oxygen vacancies are usually considered to be beneficial in catalytic conversion of polysulfides in lithium-sulfur batteries. Now it is demonstrated that the conversion of polysulfides was hindered by oxygen vacancies on ultrathin niobic acid. The inferior performance induced by the oxygen vacancy was mainly attributed to the decreased electric conductivity as well as the weakened adsorption of polysulfides on the catalyst surface. This work shows that the care should be taken when designing a new catalyst for the lithium-sulfur battery using a defect-engineering strategy.

6.
Nat Commun ; 10(1): 1112, 2019 03 07.
Artigo em Inglês | MEDLINE | ID: mdl-30846692

RESUMO

Further minimizing the defect state density in the semiconducting absorber is vital to boost the power conversion efficiency of solar cells approaching Shockley-Queisser limit. However, it lacks a general strategy to control the precursor chemistry for defects density reduction in the family of iodine based perovskite. Here the alkaline environment in precursor solution is carefully investigated as an effective parameter to suppress the incident iodine and affects the crystallization kinetics during film fabrication, via rationale adjustment of the alkalinity of additives. Especially, a 'residual free' weak alkaline is proposed not only to shrink the bandgap of the absorber by modulating the stoichiometry of organic cation, but also to improve the open circuit voltage in the resultant device. Consequently, the certified efficiency of 20.87% (Newport) is achieved with one of the smallest voltage deficits of 413 mV in the planar heterojunction perovskite solar cell.

7.
iScience ; 11: 492-504, 2019 Jan 25.
Artigo em Inglês | MEDLINE | ID: mdl-30684494

RESUMO

Although electrochemical water splitting is an effective and green approach to produce oxygen and hydrogen, the realization of efficient bifunctional catalysts that are stable in variable electrolytes is still a significant challenge. Herein, we report a three-dimensional hierarchical assembly structure composed of an ultrathin Ru shell and a Ru-Ni alloy core as a catalyst functioning under universal pH conditions. Compared with the typical Ir/C-Pt/C system, superior catalytic performances and excellent durability of the overall water splitting under universal pH have been demonstrated. The introduction of Ni downshifts the d-band center of the Ru-Ni electrocatalysts, modulating the surface electronic environment. Density functional theory results reveal that the mutually restrictive d-band interaction lowers the binding of (Ru, Ni) and (H, O) for easier O-O and H-H formation. The structure-induced eg-dz2 misalignment leads to minimization of surface Coulomb repulsion to achieve a barrier-free water-splitting process.

8.
Org Biomol Chem ; 17(5): 1191-1201, 2019 01 31.
Artigo em Inglês | MEDLINE | ID: mdl-30648165

RESUMO

Here we describe the Cp*Rh(iii)-catalyzed cascade arylfluorination reactions of α-diazoketoesters with arylboronic acids and N-fluorobenzenesulfonimide for one-pot C(sp3)-C(aryl) and C(sp3)-F bond formation. The arylfluorination reaction can be accomplished with remarkable chemo- and regioselectivity. Our mechanistic investigation showed that the Rh-catalyzed arylfluorination of diazoacetates occurred by (1) transmetalation of arylboronic acids to form an arylrhodium(iii) complex, (2) coupling of diazomalonates with the arylrhodium(iii) complex to form carbene-rhodium, (3) migratory carbene insertion to form a diketonato-rhodium(iii) complex - probably via rearrangement of the putative σ-alkylrhodium(iii) complex, and (4) electrophilic fluorination of the diketonato-rhodium to form the α-aryl-α-fluoromalonates.

9.
Science ; 363(6424): 265-270, 2019 01 18.
Artigo em Inglês | MEDLINE | ID: mdl-30655439

RESUMO

The components with soft nature in the metal halide perovskite absorber usually generate lead (Pb)0 and iodine (I)0 defects during device fabrication and operation. These defects serve as not only recombination centers to deteriorate device efficiency but also degradation initiators to hamper device lifetimes. We show that the europium ion pair Eu3+-Eu2+ acts as the "redox shuttle" that selectively oxidized Pb0 and reduced I0 defects simultaneously in a cyclical transition. The resultant device achieves a power conversion efficiency (PCE) of 21.52% (certified 20.52%) with substantially improved long-term durability. The devices retained 92% and 89% of the peak PCE under 1-sun continuous illumination or heating at 85°C for 1500 hours and 91% of the original stable PCE after maximum power point tracking for 500 hours, respectively.

10.
Nanoscale ; 9(46): 18490-18497, 2017 Nov 30.
Artigo em Inglês | MEDLINE | ID: mdl-29160328

RESUMO

Unravelling upconversion (UC) energy transfer mechanisms is significant for designing novel efficient anti-Stokes phosphors. We have studied the correlation of different lanthanide dopants within Er3+-self-sensitized core@shell upconversion nanoparticles (UCNPs). Here, our focus will be on high-concentration dopants that are able to sufficiently produce the clustering effect, especially within the interplay between Er3+ and Yb3+. We demonstrate that whatever the amount of the self-sensitizer (e.g., Er3+), abnormal absorption enhancement will occur as long as Yb3+ clusters are present. This effect originates from the substantial energy transfer between Yb3+-Yb3+ clusters despite the increased energy transfer from Yb3+ to Er3+. Therefore, the energy transfer efficiency is still constrained. However, we conversely used one of the aforementioned quench-paths of UC energy transfer to easily transfer the energy from the in-shell shell layer to the in-core area with the assistance of the energy potential reservoir, which was given by the homogeneous core@shell band offset at the interface region. Indirectly, we actualize the Er3+ UC luminescence with self-sensitization through an extended energy transfer path. This work provides a solid support and analytic theory for unraveling the energy transfer mechanism from recent works on Er3+ self-sensitized UC luminescence.

11.
Inorg Chem ; 56(14): 7975-7984, 2017 Jul 17.
Artigo em Inglês | MEDLINE | ID: mdl-28682062

RESUMO

The excellent ion conductivities of pyrochlore-type materials are believed to be based on oxygen anion transportations caused by the intrinsic defects, in which the anion Frenkel (a-Fr) pair (VO+IO) defect is the most stable one that lacks detailed study. The partially disordered pyrochlore with formation of the a-Fr pair defect will result in more disorder in local pyrochlore structure and increase number of possible migration paths for oxygen anions, which could further improve the ion conductivities of materials. Hence, we studied the formation of a-Fr defect pairs in La2Hf2O7 as a representative pyrochlore structure by density functional theory (DFT) calculations. Three types of defect migration sites were discovered with the ability to incorporate interstitial oxygen atoms from 48f sites and form a-Fr defect pairs (IO+VO (48f)). Besides the most stable vacant 8a sites with lowest defect formation energy of 3.49 eV/pair, two other novel migration sites have been first reported with ability to form a-Fr pair defect with formation energies of 6.53 and 8.49 eV/pair, respectively. These two new types of migration path, as intermediate sites, could construct a diffuse channel with vacant 8a site for interstitial oxygen anions diffusion in the lattice and significantly decrease the distance and barrier of each jump for oxygen atoms. In contrast with the oxygen interstitial defects, the formation of a-Fr pair defect shows higher priority because of much lower formation energies. Since oxygen anions could be easier to generate and diffuse in the pyrochlore structure, the a-Fr pair defect can be explained as the origin of excellent ion conductivities of pyrochlore materials. This work provides a detailed understanding of relationship between intrinsic defects and electronic properties, which enable us to predict electronic properties of other pyrochlore-type materials in the future study.

12.
Phys Chem Chem Phys ; 19(14): 9457-9469, 2017 Apr 05.
Artigo em Inglês | MEDLINE | ID: mdl-28333170

RESUMO

An energy conversion model has been established for the intrinsic persistent luminescence in solids related to the native point defect levels, formations, and transitions. In this study, we showed how the recombination of charge carriers between different defect levels along the zero phonon line (ZPL) can lead to energy conversions supporting the intrinsic persistent phosphorescence in solids. This suggests that the key driving force for this optical phenomenon is the pair of electrons hopping between different charged defects with negative-Ueff. Such a negative correlation energy will provide a sustainable energy source for electron-holes to further recombine in a new cycle with a specific quantum yield. This will help us to understand the intrinsic persistent luminescence with respect to native point defect levels as well as the correlations of electronics and energetics.

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