Highly luminescent bulk two-dimensional covalent organic frameworks(COFs)attract much attention recently.Origin of their luminescence and their large Stokes shift is an open question.After first-principles calculation...Highly luminescent bulk two-dimensional covalent organic frameworks(COFs)attract much attention recently.Origin of their luminescence and their large Stokes shift is an open question.After first-principles calculations on two kinds of COFs using the GW method and Bethe-Salpeter equation,we find that monolayer COF has a direct band gap,while bulk COF is an indirect band-gap material.The calculated optical gap and optical absorption spectrum for the direct excitons of bulk COF agree with the experiment.However,the calculated energy of the indirect exciton,in which the photoelectron and the hole locate at the conduction band minimum and the valence band maximum of bulk COF respectively,is too low compared to the fluorescence spectrum in experiment.This may exclude the possible assistance of phonons in the luminescence of bulk COF.Luminescence of bulk COF might result from exciton recombination at the defects sites.The indirect band-gap character of bulk COF originates from its AA-stacked conformation.If the conformation is changed to the AB-stacked one,the band gap of COF becomes direct which may enhance the luminescence.展开更多
The rational design and construction of inexpensive and highly active electrocatalysts for hydrogen evolution reaction(HER)is of great importance for water splitting.Herein,we develop a facile approach for preparation...The rational design and construction of inexpensive and highly active electrocatalysts for hydrogen evolution reaction(HER)is of great importance for water splitting.Herein,we develop a facile approach for preparation of porous carbon-confined Ru-doped Cu nanoparticles(denoted as Ru-Cu@C)by direct pyrolysis of the Ru-exchanged Cu-BTC metal–organic framework.When served as the electrocatalyst for HER,strikingly,the obtained Ru-Cu@C catalyst exhibits an ultralow overpotential(only 20 mV at 10 mA cm^(-2))with a small Tafel slope of 37 m V dec^(-1)in alkaline electrolyte.The excellent performance is comparable or even superior to that of commercial Pt/C catalyst.Density functional theory(DFT)calculations confirm that introducing Ru atoms into Cu nanocrystals can significantly alter the desorption of H_(2) to achieve a close-to-zero hydrogen adsorption energy and thereby boost the HER process.This strategy gives a fresh impetus to explore low-cost and high-performance catalysts for HER in alkaline media.展开更多
Co_(2)VO_(4) with Co tetrahedrons and octahedrons of transition metal oxides has achieved progress in electrocatalysts and batteries.However,high metal-metal interactions make it challenging to maintain high reactivit...Co_(2)VO_(4) with Co tetrahedrons and octahedrons of transition metal oxides has achieved progress in electrocatalysts and batteries.However,high metal-metal interactions make it challenging to maintain high reactivity as well as increase the conductivity and stability of supercapacitors.In this work,spinel-structured CoZn_(0.5)V_(1.5)O_(4) with a high specific surface area was synthesized through an ion-exchange process from the metal-organic frameworks of zinc-cobalt.Density functional theory calculations indicate that the replacement of transition metal by Zn can decrease the interaction between the transition metals,leading to a downshift in the π^(∗)-orbitals(V-O)and half-filled a_(1g) orbitals near the Fermi level,thus increasing the conductivity and stability of CoZn_(0.5)V_(1.5)O_(4).As a supercapacitor electrode,CoZn_(0.5)V_(1.5)O_(4) exhibits high cycling durability(99.4% capacitance retention after 18,000 cycles)and specific capacitance(1100mFcm^(-2) at 1mAcm^(-2)).This work provides the possibility of designing octahedral and tetrahedral sites in transition metal oxides to improve their electrochemical performance.展开更多
基金supported by the National Natural Science Foundation of China(No.21833004,No.21573131and No.21433006)the Natural Science Foundation of Shandong Province(No.JQ201603)。
文摘Highly luminescent bulk two-dimensional covalent organic frameworks(COFs)attract much attention recently.Origin of their luminescence and their large Stokes shift is an open question.After first-principles calculations on two kinds of COFs using the GW method and Bethe-Salpeter equation,we find that monolayer COF has a direct band gap,while bulk COF is an indirect band-gap material.The calculated optical gap and optical absorption spectrum for the direct excitons of bulk COF agree with the experiment.However,the calculated energy of the indirect exciton,in which the photoelectron and the hole locate at the conduction band minimum and the valence band maximum of bulk COF respectively,is too low compared to the fluorescence spectrum in experiment.This may exclude the possible assistance of phonons in the luminescence of bulk COF.Luminescence of bulk COF might result from exciton recombination at the defects sites.The indirect band-gap character of bulk COF originates from its AA-stacked conformation.If the conformation is changed to the AB-stacked one,the band gap of COF becomes direct which may enhance the luminescence.
基金the National Key R&D Program of China(2018YFB0605700)the National Natural Science Foundation of China(51778570,51879230,21725101,21871244,21521001,and 21703145)+1 种基金China Postdoctoral Science Foundation(2019TQ0298,2019M660151)Fujian Institute of Innovation(CAS)。
文摘The rational design and construction of inexpensive and highly active electrocatalysts for hydrogen evolution reaction(HER)is of great importance for water splitting.Herein,we develop a facile approach for preparation of porous carbon-confined Ru-doped Cu nanoparticles(denoted as Ru-Cu@C)by direct pyrolysis of the Ru-exchanged Cu-BTC metal–organic framework.When served as the electrocatalyst for HER,strikingly,the obtained Ru-Cu@C catalyst exhibits an ultralow overpotential(only 20 mV at 10 mA cm^(-2))with a small Tafel slope of 37 m V dec^(-1)in alkaline electrolyte.The excellent performance is comparable or even superior to that of commercial Pt/C catalyst.Density functional theory(DFT)calculations confirm that introducing Ru atoms into Cu nanocrystals can significantly alter the desorption of H_(2) to achieve a close-to-zero hydrogen adsorption energy and thereby boost the HER process.This strategy gives a fresh impetus to explore low-cost and high-performance catalysts for HER in alkaline media.
基金supported by the National Natural Science Foundation of China(51872204,52072261 and 22011540379)the National Key Research and Development Program of China(2017YFA0204600)+1 种基金Shanghai Social Development Science and Technology Project(20dz1201800)Shanghai Sailing Program(21YF1430900).
文摘Co_(2)VO_(4) with Co tetrahedrons and octahedrons of transition metal oxides has achieved progress in electrocatalysts and batteries.However,high metal-metal interactions make it challenging to maintain high reactivity as well as increase the conductivity and stability of supercapacitors.In this work,spinel-structured CoZn_(0.5)V_(1.5)O_(4) with a high specific surface area was synthesized through an ion-exchange process from the metal-organic frameworks of zinc-cobalt.Density functional theory calculations indicate that the replacement of transition metal by Zn can decrease the interaction between the transition metals,leading to a downshift in the π^(∗)-orbitals(V-O)and half-filled a_(1g) orbitals near the Fermi level,thus increasing the conductivity and stability of CoZn_(0.5)V_(1.5)O_(4).As a supercapacitor electrode,CoZn_(0.5)V_(1.5)O_(4) exhibits high cycling durability(99.4% capacitance retention after 18,000 cycles)and specific capacitance(1100mFcm^(-2) at 1mAcm^(-2)).This work provides the possibility of designing octahedral and tetrahedral sites in transition metal oxides to improve their electrochemical performance.
