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2020 Volume 78 Issue 4
2020, 78(4): 289-298
doi: 10.6023/A20020027
Abstract:
Transition metal-catalyzed direct C-H functionalization is one of the most efficient and powerful tools for the rapid synthesis of organic molecules. The use of functional groups in the molecules or covalently attached coordinating groups as directing groups has been realized as a major strategy to control the selectivity. Noncovalent interactions are of great importance in the field of molecular biology, supramolecular chemistry, material science and drug discovery. More recently, the use of well-designed ligands to enable the site-selective C-H functionalization via noncovalent interactions has emerged as a highly promising yet relatively less explored strategy. In this perspective, recent advances in this cutting-edge area are summarized. The perspective was classified into four sections according to the type of noncovalent interactions, including hydrogen bonding, ion pair, Lewis acid-base interaction and electrostatic interaction. Emphasis is placed on the mode of noncovalent interactions among the transition metals, ligands and substrates. The limitation of current research and the prospect of future work will also be discussed. We anticipate that this strategy might become a promising complementary strategy to control the positional selectivity in C-H functionalization reactions.
Transition metal-catalyzed direct C-H functionalization is one of the most efficient and powerful tools for the rapid synthesis of organic molecules. The use of functional groups in the molecules or covalently attached coordinating groups as directing groups has been realized as a major strategy to control the selectivity. Noncovalent interactions are of great importance in the field of molecular biology, supramolecular chemistry, material science and drug discovery. More recently, the use of well-designed ligands to enable the site-selective C-H functionalization via noncovalent interactions has emerged as a highly promising yet relatively less explored strategy. In this perspective, recent advances in this cutting-edge area are summarized. The perspective was classified into four sections according to the type of noncovalent interactions, including hydrogen bonding, ion pair, Lewis acid-base interaction and electrostatic interaction. Emphasis is placed on the mode of noncovalent interactions among the transition metals, ligands and substrates. The limitation of current research and the prospect of future work will also be discussed. We anticipate that this strategy might become a promising complementary strategy to control the positional selectivity in C-H functionalization reactions.
2020, 78(4): 299-310
doi: 10.6023/A19110412
Abstract:
During the past decade, transition metal-catalyzed dehydrogenative cross-couplings have emerged as an attractive strategy in synthetic chemistry due to its high step- and atom-economy as well as the less functionalized coupling partners. However, such reactions have to always use stoichiometric amount of sacrificial oxidants such as peroxides, high-valent metals (Cu salts, Ag salts, etc.), or iodine(Ⅲ) oxidants, thereby leading to possible generation of toxic wastes and making the process less desirable from a green chemistry perspective. The recently developed photocatalytic CCHE (cross-coupling hydrogen-evolution) reactions are a conceptually new type of reactions enabled by combination of photo-redox catalysis and proton reduction catalysis, wherein the photocatalyst uses light energy as the driving force for the cross-coupling and the hydrogen evolution catalyst may capture electrons and protons from the substrates or reaction intermediates to produce molecular hydrogen (H2). Thus, without use of any sacrificial oxidant and under mild conditions, the dual catalyst system may afford cross-coupling products with excellent yields and an equivalent amount of H2 as the sole byproduct. This kind of cross-coupling delivers a greener synthetic strategy and is particularly useful for reactions that involve species sensitive to traditional oxidants. In CCHE reactions, the raw materials are directly converted into products and hydrogen, the reactions are highly atom economy, environmentally friendly, and have attractive potential industrial application prospects. In this review, recent dramatic developments of photocatalytic and electrochemical CCHE reactions are discussed via the most prominent mechanistic pathways, the types of C-C bond, C-X (heteroatom) bond, or X-X bond formations and specific reaction classes.
During the past decade, transition metal-catalyzed dehydrogenative cross-couplings have emerged as an attractive strategy in synthetic chemistry due to its high step- and atom-economy as well as the less functionalized coupling partners. However, such reactions have to always use stoichiometric amount of sacrificial oxidants such as peroxides, high-valent metals (Cu salts, Ag salts, etc.), or iodine(Ⅲ) oxidants, thereby leading to possible generation of toxic wastes and making the process less desirable from a green chemistry perspective. The recently developed photocatalytic CCHE (cross-coupling hydrogen-evolution) reactions are a conceptually new type of reactions enabled by combination of photo-redox catalysis and proton reduction catalysis, wherein the photocatalyst uses light energy as the driving force for the cross-coupling and the hydrogen evolution catalyst may capture electrons and protons from the substrates or reaction intermediates to produce molecular hydrogen (H2). Thus, without use of any sacrificial oxidant and under mild conditions, the dual catalyst system may afford cross-coupling products with excellent yields and an equivalent amount of H2 as the sole byproduct. This kind of cross-coupling delivers a greener synthetic strategy and is particularly useful for reactions that involve species sensitive to traditional oxidants. In CCHE reactions, the raw materials are directly converted into products and hydrogen, the reactions are highly atom economy, environmentally friendly, and have attractive potential industrial application prospects. In this review, recent dramatic developments of photocatalytic and electrochemical CCHE reactions are discussed via the most prominent mechanistic pathways, the types of C-C bond, C-X (heteroatom) bond, or X-X bond formations and specific reaction classes.
2020, 78(4): 311-325
doi: 10.6023/A19120426
Abstract:
Covalent organic framework materials (COFs) are a class of organic porous materials with large specific surface area, high porosity and crystallinity. Owning to their special nature of functional versatility and easy modification, COFs can be designed to be efficient catalysts either embed functional active sites into the skeleton through a "top-down" strategy, or load metal nanoparticles into the framework via a post-modification approach. These studies have laid the foundation for the extension of COF's application in heterogeneous and other catalytic fields. The synthetic strategy and application of COF in different types of catalytic reactions are reviewed in this paper. Moreover, the current research situation of COF catalyst is summarized and prospected. Finally, the remaining challenges in this field are also indicated.
Covalent organic framework materials (COFs) are a class of organic porous materials with large specific surface area, high porosity and crystallinity. Owning to their special nature of functional versatility and easy modification, COFs can be designed to be efficient catalysts either embed functional active sites into the skeleton through a "top-down" strategy, or load metal nanoparticles into the framework via a post-modification approach. These studies have laid the foundation for the extension of COF's application in heterogeneous and other catalytic fields. The synthetic strategy and application of COF in different types of catalytic reactions are reviewed in this paper. Moreover, the current research situation of COF catalyst is summarized and prospected. Finally, the remaining challenges in this field are also indicated.
2020, 78(4): 326-329
doi: 10.6023/A19100369
Abstract:
The partitioning of volatile substances in atmospheric particulates is a hot topic in atmospheric science. Ammonium nitrate (NH4NO3) is an important inorganic component in atmospheric aerosols, which is a salt with relatively high vapor pressure. Particles containing NH4NO3 are equilibrium with gaseous NH3 and HNO3 and the partitioning between particle and gas is a strong function of temperature and relative humidity. Atmospheric aerosols have both natural and anthropogenic sources and consist of both organic and inorganic components, and many of them will likely form in semisolid, glassy, and high viscous state in the atmosphere, which show nonequilibrium kinetic characteristics at low relative humidity conditions. In this research, in order to understand the volatility of ammonium nitrate in ultra-viscous aerosol droplets, optical tweezers coupled with cavity-enhanced Raman spectroscopy were employed to observe the volatility of ammonium nitrate in the mixture of NH4NO3/MgSO4 and NH4NO3/sucrose droplets. Optical tweezers-stimulated Raman spectroscopy, compared with other suspension techniques, can not only suspend droplets, but also obtain the chemical composition, structure of droplets and other information according to the conventional Raman scattering spectra of droplets. The radius and refractive index of droplets can be calculated according to stimulated Raman-Mie scattering resonance. The advantages of optical tweezers stimulated Raman spectroscopy are that particle radius can be accurately measured, chemical composition, phase and shape can be controlled, and long-term observation can be realized. Here, the radii and refractive indexes of the levitated droplets were determined in real time using the wavelength positions of stimulated Raman spectra, and the effective vapor pressures of ammonium nitrate at different relative humidity (RH) were obtained according to Maxwell equation. For the droplets with ammonium nitrate/sucrose molar ratio of 1:1, the effective vapor pressure of ammonium nitrate decreased with the decrease of RH. When the RH decreased from 70% to 20%, the effective vapor pressure of ammonium nitrate decreased from (3.577±0.82)×10-5 to (6.55±1.36)×10-6 Pa, compared to the vapor pressure of pure ammonium nitrate (1.67±0.24)×10-3 to (6.64±0.3)×10-3 Pa, the evaporation of ammonium nitrate in the mixed droplet was suppressed by sucrose, especially at low RH. For the mixed droplets with ammonium nitrate/magnesium sulfate molar ratio of 1:1, a similar phenomenon was observed. When the RH decreased from 70% to 40%, the effective vapor pressure of ammonium nitrate decreased from (4.38±0.21)×10-3 to (8.13±2.34)×10-5 Pa. The results showed that the effective saturated vapor pressures of ammonium nitrate in ultra-viscous droplets at low humidity were 1~3 orders lower than those of pure ammonium nitrate. Obviously, the volatilization of ammonium nitrate in droplets was inhibited at low relative humidity.
The partitioning of volatile substances in atmospheric particulates is a hot topic in atmospheric science. Ammonium nitrate (NH4NO3) is an important inorganic component in atmospheric aerosols, which is a salt with relatively high vapor pressure. Particles containing NH4NO3 are equilibrium with gaseous NH3 and HNO3 and the partitioning between particle and gas is a strong function of temperature and relative humidity. Atmospheric aerosols have both natural and anthropogenic sources and consist of both organic and inorganic components, and many of them will likely form in semisolid, glassy, and high viscous state in the atmosphere, which show nonequilibrium kinetic characteristics at low relative humidity conditions. In this research, in order to understand the volatility of ammonium nitrate in ultra-viscous aerosol droplets, optical tweezers coupled with cavity-enhanced Raman spectroscopy were employed to observe the volatility of ammonium nitrate in the mixture of NH4NO3/MgSO4 and NH4NO3/sucrose droplets. Optical tweezers-stimulated Raman spectroscopy, compared with other suspension techniques, can not only suspend droplets, but also obtain the chemical composition, structure of droplets and other information according to the conventional Raman scattering spectra of droplets. The radius and refractive index of droplets can be calculated according to stimulated Raman-Mie scattering resonance. The advantages of optical tweezers stimulated Raman spectroscopy are that particle radius can be accurately measured, chemical composition, phase and shape can be controlled, and long-term observation can be realized. Here, the radii and refractive indexes of the levitated droplets were determined in real time using the wavelength positions of stimulated Raman spectra, and the effective vapor pressures of ammonium nitrate at different relative humidity (RH) were obtained according to Maxwell equation. For the droplets with ammonium nitrate/sucrose molar ratio of 1:1, the effective vapor pressure of ammonium nitrate decreased with the decrease of RH. When the RH decreased from 70% to 20%, the effective vapor pressure of ammonium nitrate decreased from (3.577±0.82)×10-5 to (6.55±1.36)×10-6 Pa, compared to the vapor pressure of pure ammonium nitrate (1.67±0.24)×10-3 to (6.64±0.3)×10-3 Pa, the evaporation of ammonium nitrate in the mixed droplet was suppressed by sucrose, especially at low RH. For the mixed droplets with ammonium nitrate/magnesium sulfate molar ratio of 1:1, a similar phenomenon was observed. When the RH decreased from 70% to 40%, the effective vapor pressure of ammonium nitrate decreased from (4.38±0.21)×10-3 to (8.13±2.34)×10-5 Pa. The results showed that the effective saturated vapor pressures of ammonium nitrate in ultra-viscous droplets at low humidity were 1~3 orders lower than those of pure ammonium nitrate. Obviously, the volatilization of ammonium nitrate in droplets was inhibited at low relative humidity.
2020, 78(4): 330-336
doi: 10.6023/A19110400
Abstract:
Improving the performance of electrocatalytic formic acid oxidation is the key issue to develop high-performance direct formic acid fuel cells (DFAFC). Pt-based and Pd-based materials are the important electrocatalysts for formic acid oxidation. Micro/nano-porous metal materials are widely concerned in the electrochemistry field due to the high specific electrode-surface area. The dynamic hydrogen bubble template (DHBT) method has been widely used for preparing the three-dimensional honeycomb-like porous nano-metals (3DHPNMs). However, as far as we know, the use of a sacrificial metal template to prepare the 3DHPNMs with improved performance for the electrocatalytic oxidation of small organic molecules has not been reported. Herein, a three-dimensional honeycomb-like porous nano-AuPtCu (3DHPN-AuPtCu) composite was electrodeposited on a gold-plated glassy carbon electrode (Aupla/GCE) by the DHBT method, followed by anodic stripping of Cu to yield a 3DHPN-AuPtCu/Aupla/GCE. The relevant modified electrodes were characterized by cyclic voltammetry (CV), metallographic microscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy and inductively coupled plasma-atomic emission spectrometry. The SEM results clearly revealed that the use of the sacrificial Cu template can modulate the metal-honeycomb structure, and the 3DHPN-AuPtCu/Aupla/GCE can thus possess the better micro/nano-porous structure and the improved electrocatalytic performance than a Cu-template-free 3DHPN-AuPt/Aupla/GCE. In our opinion, the simultaneous electrodeposition of Cu can intervene in the electrodeposition of Au and Pt, and thus a new structure with more active sites exposed and the electrocatalysis performance improved can be obtained after the anodic stripping of electrodeposited Cu. As a result, the 3DHPN-AuPtCu/Aupla/GCE exhibited high anti-poisoning nature and high stability, because many discontinuous Pt atoms on this electrode can suppress the formation of adsorption-state COads during the electrocatalytic oxidation of formic acid. The electrocatalytic oxidation peak current density on 3DHPN-AuPtCu/Aupla/GCE in 0.5 mol/L aqueous H2SO4 containing 0.2 mol/L HCOOH was 12.5 mA·cmPt-2 (CV, -0.3~1.0 V, 50 mV/s), which is superior to the control electrodes and many reported Pt-based electrocatalysis electrodes. The suggested double- template method for preparing honeycomb-structured micro/nano-porous metal materials with improved performance has the potential for wider electrocatalysis and electroanalysis applications.
Improving the performance of electrocatalytic formic acid oxidation is the key issue to develop high-performance direct formic acid fuel cells (DFAFC). Pt-based and Pd-based materials are the important electrocatalysts for formic acid oxidation. Micro/nano-porous metal materials are widely concerned in the electrochemistry field due to the high specific electrode-surface area. The dynamic hydrogen bubble template (DHBT) method has been widely used for preparing the three-dimensional honeycomb-like porous nano-metals (3DHPNMs). However, as far as we know, the use of a sacrificial metal template to prepare the 3DHPNMs with improved performance for the electrocatalytic oxidation of small organic molecules has not been reported. Herein, a three-dimensional honeycomb-like porous nano-AuPtCu (3DHPN-AuPtCu) composite was electrodeposited on a gold-plated glassy carbon electrode (Aupla/GCE) by the DHBT method, followed by anodic stripping of Cu to yield a 3DHPN-AuPtCu/Aupla/GCE. The relevant modified electrodes were characterized by cyclic voltammetry (CV), metallographic microscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy and inductively coupled plasma-atomic emission spectrometry. The SEM results clearly revealed that the use of the sacrificial Cu template can modulate the metal-honeycomb structure, and the 3DHPN-AuPtCu/Aupla/GCE can thus possess the better micro/nano-porous structure and the improved electrocatalytic performance than a Cu-template-free 3DHPN-AuPt/Aupla/GCE. In our opinion, the simultaneous electrodeposition of Cu can intervene in the electrodeposition of Au and Pt, and thus a new structure with more active sites exposed and the electrocatalysis performance improved can be obtained after the anodic stripping of electrodeposited Cu. As a result, the 3DHPN-AuPtCu/Aupla/GCE exhibited high anti-poisoning nature and high stability, because many discontinuous Pt atoms on this electrode can suppress the formation of adsorption-state COads during the electrocatalytic oxidation of formic acid. The electrocatalytic oxidation peak current density on 3DHPN-AuPtCu/Aupla/GCE in 0.5 mol/L aqueous H2SO4 containing 0.2 mol/L HCOOH was 12.5 mA·cmPt-2 (CV, -0.3~1.0 V, 50 mV/s), which is superior to the control electrodes and many reported Pt-based electrocatalysis electrodes. The suggested double- template method for preparing honeycomb-structured micro/nano-porous metal materials with improved performance has the potential for wider electrocatalysis and electroanalysis applications.
2020, 78(4): 337-343
doi: 10.6023/A19120419
Abstract:
In ammonium perchlorate (AP) based composite propellants, α-Fe2O3 nanoparticles and hydroxyl terminated polybutadiene (HTPB) are commonly used as catalyst and binder respectively. Their properties and dispersion significantly affect the combustion performance of the composite propellants. However, α-Fe2O3 nanoparticles are hard to disperse uniformly in the binder HTPB owing to the high viscosity, which will decrease their catalytic activity. Directly composite processing of α-Fe2O3 nanoparticles with other main components of composite propellants may be an effective strategy to prevent from aggregation without introducing other components to the solid propellant at the same time. In this paper, the α-Fe2O3/ (IPDI-HTPB) composite nanoparticles were prepared by choosing curing agent isophorone diisocyanate (IPDI) as grafting bridge. The typical procedure was as follows: (1) The first step was to synthesize IPDI-HTPB. In order to adjudged the reaction terminal point of IPDI and HTPB, the reaction kinetics of HTPB and IPDI were first researched and the proper reaction conditions were chosen as follows:molar ratio of HTPB and IPDI is 1:1, reaction temperature is under 80℃ and reaction time is 2 h. (2) The second step was to synthesize α-Fe2O3/(IPDI-HTPB) composite nanoparticles. Firstly, the stoichiometric α-Fe2O3 nanoparticles were dispersed in toluene under ultrasound for 10 min. Secondly, the mixture were added in the above IPDI-HTPB solution and the reaction kept for 4 h. Thirdly, the α-Fe2O3/(IPDI-HTPB) composite nanoparticles were centrifuged and washed with toluene and ether for several times. Finally, α-Fe2O3/(IPDI-HTPB) composite nanoparticles were dried in the oven at 80℃ for 12 h. (3) The structure of α-Fe2O3/(IPDI-HTPB) composite nanoparticles were characterized by X-ray diffractometer (XRD), transmittance electron microscopy (TEM), Fourier transform infrared spectrometer (FTIR) and thermogravimetric analysis (TGA). It was observed that HTPB could be chemically coated on the surface of the α-Fe2O3 nanoparticles by the grafting activity of IPDI. The depth of the IPDI-HTPB was nearly 5 nm. The α-Fe2O3/(IPDI-HTPB) composite nanoparticles showed hydrophobicity after the composite process. Compare with the pure α-Fe2O3 nanoparticles, α-Fe2O3 nanoparticles in α-Fe2O3/IPDI-HTPB composite nanoparticles showed better catalytic activity on the thermal decomposition of AP.
In ammonium perchlorate (AP) based composite propellants, α-Fe2O3 nanoparticles and hydroxyl terminated polybutadiene (HTPB) are commonly used as catalyst and binder respectively. Their properties and dispersion significantly affect the combustion performance of the composite propellants. However, α-Fe2O3 nanoparticles are hard to disperse uniformly in the binder HTPB owing to the high viscosity, which will decrease their catalytic activity. Directly composite processing of α-Fe2O3 nanoparticles with other main components of composite propellants may be an effective strategy to prevent from aggregation without introducing other components to the solid propellant at the same time. In this paper, the α-Fe2O3/ (IPDI-HTPB) composite nanoparticles were prepared by choosing curing agent isophorone diisocyanate (IPDI) as grafting bridge. The typical procedure was as follows: (1) The first step was to synthesize IPDI-HTPB. In order to adjudged the reaction terminal point of IPDI and HTPB, the reaction kinetics of HTPB and IPDI were first researched and the proper reaction conditions were chosen as follows:molar ratio of HTPB and IPDI is 1:1, reaction temperature is under 80℃ and reaction time is 2 h. (2) The second step was to synthesize α-Fe2O3/(IPDI-HTPB) composite nanoparticles. Firstly, the stoichiometric α-Fe2O3 nanoparticles were dispersed in toluene under ultrasound for 10 min. Secondly, the mixture were added in the above IPDI-HTPB solution and the reaction kept for 4 h. Thirdly, the α-Fe2O3/(IPDI-HTPB) composite nanoparticles were centrifuged and washed with toluene and ether for several times. Finally, α-Fe2O3/(IPDI-HTPB) composite nanoparticles were dried in the oven at 80℃ for 12 h. (3) The structure of α-Fe2O3/(IPDI-HTPB) composite nanoparticles were characterized by X-ray diffractometer (XRD), transmittance electron microscopy (TEM), Fourier transform infrared spectrometer (FTIR) and thermogravimetric analysis (TGA). It was observed that HTPB could be chemically coated on the surface of the α-Fe2O3 nanoparticles by the grafting activity of IPDI. The depth of the IPDI-HTPB was nearly 5 nm. The α-Fe2O3/(IPDI-HTPB) composite nanoparticles showed hydrophobicity after the composite process. Compare with the pure α-Fe2O3 nanoparticles, α-Fe2O3 nanoparticles in α-Fe2O3/IPDI-HTPB composite nanoparticles showed better catalytic activity on the thermal decomposition of AP.
2020, 78(4): 344-354
doi: 10.6023/A19120455
Abstract:
As a new member of the two-dimensional nanomaterial family, borophene is regarded as a potential material platform for nanoscale electronic devices. Especially, borophene-based electrodes have potential application values in light-emitting diodes, organic light-emitting diodes, organic solar cells and field emitters. Therefore, the work function modulation (to an optimal value) of borophene is highly important to maximize the energy conversion efficiency and performance of the device. Based on the first-principles density functional theory, the effects of Li adsorption on the structure, electronic properties and work function of double-layer α-borophene (DBBP) are studied. The calculation results show that Li adsorption can effectively adjust the work function of DBBP from 4.65 eV to 1.96~4.46 eV with different Li contents. This engineering range is superior to what are reported in the literatures for Li-adsorbed monolayer BBP (modified from 4.16 eV to 2.31~3.67 eV), and double-layer graphene with intercalated Li (3.4~3.9 eV) and K (3.3~3.8 eV). The work functions of Li2(D)/DBBP (3.73 eV) and Li3(D)/DBBP (2.91 eV) are close to the commonly used electrode materials Mg and Ca, respectively, while the work function of Li4(D)/DBBP is even lower than Ca. In addition, the factors that affect the work function reduction of Lin/DBBP relative to DBBP, such as configuration, substrate deformation, binding energy, electron transfer, charge rearrangement, electrostatic potential, vacuum and Fermi level, are systematically studied. The results demonstrate that the decrease in the Lin/DBBP work function is mainly due to the change in Fermi level, while the change in vacuum level only plays a minor role. Apart from that, the deformation of the substrate does not have a positive effect on the reduction of the Lin/DBBP work function, but the electron transfer from the adsorbed atoms to the matrix (charge redistribution caused by chemical effects) is the inherent reason for the decrease in the Lin/DBBP work function. This study shows that Li adsorption is a simple and effective method to reduce the work function of DBBP. Due to its metallic character and extremely low work function, Li-adsorbed DBBP nanomaterials can be utilized as cathode materials in electronic devices.
As a new member of the two-dimensional nanomaterial family, borophene is regarded as a potential material platform for nanoscale electronic devices. Especially, borophene-based electrodes have potential application values in light-emitting diodes, organic light-emitting diodes, organic solar cells and field emitters. Therefore, the work function modulation (to an optimal value) of borophene is highly important to maximize the energy conversion efficiency and performance of the device. Based on the first-principles density functional theory, the effects of Li adsorption on the structure, electronic properties and work function of double-layer α-borophene (DBBP) are studied. The calculation results show that Li adsorption can effectively adjust the work function of DBBP from 4.65 eV to 1.96~4.46 eV with different Li contents. This engineering range is superior to what are reported in the literatures for Li-adsorbed monolayer BBP (modified from 4.16 eV to 2.31~3.67 eV), and double-layer graphene with intercalated Li (3.4~3.9 eV) and K (3.3~3.8 eV). The work functions of Li2(D)/DBBP (3.73 eV) and Li3(D)/DBBP (2.91 eV) are close to the commonly used electrode materials Mg and Ca, respectively, while the work function of Li4(D)/DBBP is even lower than Ca. In addition, the factors that affect the work function reduction of Lin/DBBP relative to DBBP, such as configuration, substrate deformation, binding energy, electron transfer, charge rearrangement, electrostatic potential, vacuum and Fermi level, are systematically studied. The results demonstrate that the decrease in the Lin/DBBP work function is mainly due to the change in Fermi level, while the change in vacuum level only plays a minor role. Apart from that, the deformation of the substrate does not have a positive effect on the reduction of the Lin/DBBP work function, but the electron transfer from the adsorbed atoms to the matrix (charge redistribution caused by chemical effects) is the inherent reason for the decrease in the Lin/DBBP work function. This study shows that Li adsorption is a simple and effective method to reduce the work function of DBBP. Due to its metallic character and extremely low work function, Li-adsorbed DBBP nanomaterials can be utilized as cathode materials in electronic devices.
2020, 78(4): 355-362
doi: 10.6023/A20010017
Abstract:
Oxygen evolution reaction (OER) is a crucial half reaction of electrochemical water splitting and metal-air batteries. But its sluggish four-electron reaction leads to a high overpotential. Current commercial OER catalysts are mainly noble metal-based materials, but their high cost restricts their broad application. Therefore, extensive efforts have been devoted to exploring low-cost and efficient OER catalysts. Nonprecious metal-based materials have been regarded as promising OER catalyst candidates, due to their abundancy on the earth, controllable morphologies and tunable chemical states. Among various nonprecious metal-based materials, metal-organic frameworks (MOFs) have attracted much attention, because of their large specific surface area and rich metal centers. However, their poor electrochemical activities, stabilities and conductivities severely affect their application in OER catalysis. To improve the activities of MOFs, several methods have been adopted, such as synthesizing ultrathin nanosheets, growing MOFs on nickel foam or carbon cloth, doping heteroatoms, and introducing synergistic interactions between two materials. In 1970, Wagner proposed a space-charge theory, which indicates that the carrier property can be tuned through adjusting interface. Inspired by this theory, constructing metal oxide-catalyst interface seems to be a promising strategy to improve activities of catalysts. CeO2 is a well-known cocatalyst due to its reversible Ce3+/Ce4+ redox. Previous works have demonstrated that OER performance can be effectively improved through introducing CeO2 since it can speed up the electron mobility and induce strong interaction between CeO2 and metal sites. In this work, an efficient OER catalyst was achieved through introducing CeO2 into FeNi MOF catalyst. FeNi MOF nanosheet arrays grown on nickel foam was firstly prepared via a solvothermal process. Then CeO2 nanoclusters (5 nm) were coated onto FeNi MOF surface by electrodeposition. A series of characterizations were employed to study the morphology, structure and surface electronic state information of the as-obtained CeO2/FeNi MOF. From X-ray photoelectron spectroscopic analysis, the doping of CeO2 clusters and the strong electronic interaction between CeO2 clusters and FeNi MOF induce the formation of Fe/Ni-O-Ce bonds and optimize the electronic structures of Fe/Ni sites, which will enhance OER activities. The OER performance tests confirm that CeO2/FeNi MOF indeed exhibits a superior OER activity than FeNi MOF alone. The hybrid catalyst delivers a higher mass activity (235.4 A·g-1) and a faster turnover frequency (0.065 s-1) than those of FeNi MOF (43.8 A·g-1, 0.018 s-1). Compared with FeNi MOF, CeO2/FeNi MOF also shows better OER kinetics, as evidenced by a decreased Tafel slope, a reduced charge transfer resistance. Besides, CeO2/FeNi MOF presents an outstanding stability (50 h, 50 mA·cm-2). All these features make our CeO2/FeNi MOF a potential catalyst in the future application. The interfacial strategy through introducing CeO2 to modulate Fe and Ni active sites may open a door for developing high-performance OER catalysts in future.
Oxygen evolution reaction (OER) is a crucial half reaction of electrochemical water splitting and metal-air batteries. But its sluggish four-electron reaction leads to a high overpotential. Current commercial OER catalysts are mainly noble metal-based materials, but their high cost restricts their broad application. Therefore, extensive efforts have been devoted to exploring low-cost and efficient OER catalysts. Nonprecious metal-based materials have been regarded as promising OER catalyst candidates, due to their abundancy on the earth, controllable morphologies and tunable chemical states. Among various nonprecious metal-based materials, metal-organic frameworks (MOFs) have attracted much attention, because of their large specific surface area and rich metal centers. However, their poor electrochemical activities, stabilities and conductivities severely affect their application in OER catalysis. To improve the activities of MOFs, several methods have been adopted, such as synthesizing ultrathin nanosheets, growing MOFs on nickel foam or carbon cloth, doping heteroatoms, and introducing synergistic interactions between two materials. In 1970, Wagner proposed a space-charge theory, which indicates that the carrier property can be tuned through adjusting interface. Inspired by this theory, constructing metal oxide-catalyst interface seems to be a promising strategy to improve activities of catalysts. CeO2 is a well-known cocatalyst due to its reversible Ce3+/Ce4+ redox. Previous works have demonstrated that OER performance can be effectively improved through introducing CeO2 since it can speed up the electron mobility and induce strong interaction between CeO2 and metal sites. In this work, an efficient OER catalyst was achieved through introducing CeO2 into FeNi MOF catalyst. FeNi MOF nanosheet arrays grown on nickel foam was firstly prepared via a solvothermal process. Then CeO2 nanoclusters (5 nm) were coated onto FeNi MOF surface by electrodeposition. A series of characterizations were employed to study the morphology, structure and surface electronic state information of the as-obtained CeO2/FeNi MOF. From X-ray photoelectron spectroscopic analysis, the doping of CeO2 clusters and the strong electronic interaction between CeO2 clusters and FeNi MOF induce the formation of Fe/Ni-O-Ce bonds and optimize the electronic structures of Fe/Ni sites, which will enhance OER activities. The OER performance tests confirm that CeO2/FeNi MOF indeed exhibits a superior OER activity than FeNi MOF alone. The hybrid catalyst delivers a higher mass activity (235.4 A·g-1) and a faster turnover frequency (0.065 s-1) than those of FeNi MOF (43.8 A·g-1, 0.018 s-1). Compared with FeNi MOF, CeO2/FeNi MOF also shows better OER kinetics, as evidenced by a decreased Tafel slope, a reduced charge transfer resistance. Besides, CeO2/FeNi MOF presents an outstanding stability (50 h, 50 mA·cm-2). All these features make our CeO2/FeNi MOF a potential catalyst in the future application. The interfacial strategy through introducing CeO2 to modulate Fe and Ni active sites may open a door for developing high-performance OER catalysts in future.
