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2020 Volume 36 Issue 1
Fabrication of Polymer-Supported Metal Organic Framework Membrane and Its Gas Separation Performance
2020, 36(1): 190107
doi: 10.3866/PKU.WHXB201901079
Abstract:
The fabrication of compact, continuous, and large-scale metal organic framework (MOF) membranes with high permeability and H2/CO2 selectivity remains challenging because of the wake interaction between the MOF membrane and the substrate. In addition, substrates with smooth and plain surfaces and suitable pore size are required to prepare high-quality MOF membranes because it is difficult to obtain dense and continuous MOF membranes on a substrate with large pores and rough surfaces. To overcome these challenges, numerous MOF membrane growth methods have emerged, including in situ (direct) growth, secondary (seeded) growth, and layer-by-layer growth methods as well as electrostatic spinning and the chemical modification of the substrate. Among these methods, usage of substrates suitable for surface-functionalization is a promising technique. Herein, Al2O3 was selected as the substrate and was coated with PIM-1 (one polymer of intrinsic microporosity), followed by carboxylation of PIM-1 to furnish a large number of carboxyl groups on the surface. In situ growth of the MOF membrane using the interactions between the carboxyl group and the metal yielded two types of compact, continuous, and large-scale polymer-supported MOF membranes (PIM-1-COOH/ZIF-8 and PIM-1-COOH/HKUST-1). Furthermore, the fabricated polymer-supported MOF membrane structures were investigated by powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). Gas separation experiments were performed to explore the gas permeability and selectivity of the prepared MOF membranes. The XRD characterization confirmed the pure phase and high crystallinity of the MOF membranes. The SEM images showed that the MOF membranes were compact and continuous with a tight combination between the MOF crystal membrane and the substrate. Gas separation measurements showed that both MOF membranes exhibited high H2 permeability and selectivity for H2/CO2. For the PIM-1-COOH/ZIF-8 and PIM-1-COOH/HKUST-1 membranes, the 1 : 1 binary mixtures gas separation factors of H2/CO2 calculated as the gas molar ratios in the permeate and retentate side 7.32 and 9.69, respectively, at room temperature and atmospheric pressure. The H2/CO2 mixture separation factors of the two MOF membranes exceeded the corresponding Knudsen constants (4.7), with H2 permeances higher than 3.16 × 10-6 and 1.14 × 10-6 mol·m-2·s-1·Pa-1, respectively. The ideal separation factors of H2/CO2 of both MOF membranes calculated as the ratio of single gas permeances were 7.70 and 12.04, respectively, with the respective H2 permeances of up to 3.73 × 10-6 and 3.86 × 10-6 mol·m-2·s-1·Pa-1. Because of their outstanding characteristics, these novel MOF membranes can be widely used in the fields of H2 purification and separation. ![]()
The fabrication of compact, continuous, and large-scale metal organic framework (MOF) membranes with high permeability and H2/CO2 selectivity remains challenging because of the wake interaction between the MOF membrane and the substrate. In addition, substrates with smooth and plain surfaces and suitable pore size are required to prepare high-quality MOF membranes because it is difficult to obtain dense and continuous MOF membranes on a substrate with large pores and rough surfaces. To overcome these challenges, numerous MOF membrane growth methods have emerged, including in situ (direct) growth, secondary (seeded) growth, and layer-by-layer growth methods as well as electrostatic spinning and the chemical modification of the substrate. Among these methods, usage of substrates suitable for surface-functionalization is a promising technique. Herein, Al2O3 was selected as the substrate and was coated with PIM-1 (one polymer of intrinsic microporosity), followed by carboxylation of PIM-1 to furnish a large number of carboxyl groups on the surface. In situ growth of the MOF membrane using the interactions between the carboxyl group and the metal yielded two types of compact, continuous, and large-scale polymer-supported MOF membranes (PIM-1-COOH/ZIF-8 and PIM-1-COOH/HKUST-1). Furthermore, the fabricated polymer-supported MOF membrane structures were investigated by powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). Gas separation experiments were performed to explore the gas permeability and selectivity of the prepared MOF membranes. The XRD characterization confirmed the pure phase and high crystallinity of the MOF membranes. The SEM images showed that the MOF membranes were compact and continuous with a tight combination between the MOF crystal membrane and the substrate. Gas separation measurements showed that both MOF membranes exhibited high H2 permeability and selectivity for H2/CO2. For the PIM-1-COOH/ZIF-8 and PIM-1-COOH/HKUST-1 membranes, the 1 : 1 binary mixtures gas separation factors of H2/CO2 calculated as the gas molar ratios in the permeate and retentate side 7.32 and 9.69, respectively, at room temperature and atmospheric pressure. The H2/CO2 mixture separation factors of the two MOF membranes exceeded the corresponding Knudsen constants (4.7), with H2 permeances higher than 3.16 × 10-6 and 1.14 × 10-6 mol·m-2·s-1·Pa-1, respectively. The ideal separation factors of H2/CO2 of both MOF membranes calculated as the ratio of single gas permeances were 7.70 and 12.04, respectively, with the respective H2 permeances of up to 3.73 × 10-6 and 3.86 × 10-6 mol·m-2·s-1·Pa-1. Because of their outstanding characteristics, these novel MOF membranes can be widely used in the fields of H2 purification and separation.
2020, 36(1): 190700
doi: 10.3866/PKU.WHXB201907006
Abstract:
An increasing number of recent studies have shown that the binding kinetics of a drug molecule to its target correlates strongly with its efficacy in vivo. Therefore, ligand optimization oriented to improved binding kinetics provides new ideas for rational drug design. Currently, ligand binding kinetics is modeled mainly through extensive molecular dynamics simulations, which limits its application to real-world problems. The present study aimed at obtaining a general-purpose quantitative structure-kinetics relationship (QSKR) model for predicting the dissociation rate constant (koff) of a ligand based on its complex structure. This type of model is expected to be suitable for high-throughput tasks in structure-based drug design. We collected the experimentally measured koff values for 406 ligand molecules from literature, and then constructed a three-dimensional structural model for each protein-ligand complex through molecular modeling. A training set was compiled using 60% of those complexes while the remaining 40% were assigned to two test sets. Based on distance-dependent protein-ligand atom pair descriptors, a random forest algorithm was adopted to derive a QSKR model. Various random forest models were then generated based on the descriptor sets obtained under different conditions, such as distance cutoff, bin width, and feature selection criteria. The cross-validation results of those models were then examined. It was observed that the optimal model was obtained when the distance cutoff was 15 Å (1 Å = 0.1 nm), the bin width was 3 Å, and feature selection variance level was 2. The final QSKR model produced correlation coefficients around 0.62 on the two independent test sets. This level of accuracy is at least comparable to that of the predictive models described in literature, which are typically computationally much more expensive. Our study attempts to address the issue of predicting koff values in drug design. We hope that it can provide inspiration for further studies by other researchers. ![]()
An increasing number of recent studies have shown that the binding kinetics of a drug molecule to its target correlates strongly with its efficacy in vivo. Therefore, ligand optimization oriented to improved binding kinetics provides new ideas for rational drug design. Currently, ligand binding kinetics is modeled mainly through extensive molecular dynamics simulations, which limits its application to real-world problems. The present study aimed at obtaining a general-purpose quantitative structure-kinetics relationship (QSKR) model for predicting the dissociation rate constant (koff) of a ligand based on its complex structure. This type of model is expected to be suitable for high-throughput tasks in structure-based drug design. We collected the experimentally measured koff values for 406 ligand molecules from literature, and then constructed a three-dimensional structural model for each protein-ligand complex through molecular modeling. A training set was compiled using 60% of those complexes while the remaining 40% were assigned to two test sets. Based on distance-dependent protein-ligand atom pair descriptors, a random forest algorithm was adopted to derive a QSKR model. Various random forest models were then generated based on the descriptor sets obtained under different conditions, such as distance cutoff, bin width, and feature selection criteria. The cross-validation results of those models were then examined. It was observed that the optimal model was obtained when the distance cutoff was 15 Å (1 Å = 0.1 nm), the bin width was 3 Å, and feature selection variance level was 2. The final QSKR model produced correlation coefficients around 0.62 on the two independent test sets. This level of accuracy is at least comparable to that of the predictive models described in literature, which are typically computationally much more expensive. Our study attempts to address the issue of predicting koff values in drug design. We hope that it can provide inspiration for further studies by other researchers.
2020, 36(1): 190700
doi: 10.3866/PKU.WHXB201907008
Abstract:
"Unprotected" metal and alloy nanoclusters prepared using the alkaline-ethylene glycol method (AEGM), stabilized by adsorbed solvent molecules and simple ions, have been widely applied in the development of high-performance heterogeneous catalysts and the exploration of the effects of metal particle size and composition, surface ligands of support, and modifiers on the catalytic properties of heterogeneous catalysts. The formation process and mechanism of such unprotected metal nanoclusters need to be further investigated. In this study, the formation process and mechanism of unprotected Pt and Ru nanoclusters prepared with AEGM were investigated by in situ quick X-ray absorption fine spectroscopy (QXAFS), in situ ultraviolet-visible (UV-Vis) absorption spectroscopy, transmission electron microscopy, and dynamic light scattering. It was discovered that during the formation of unprotected Pt nanoclusters, a portion of Pt(Ⅳ) species was reduced to Pt(Ⅱ) species at room temperature. With increasing temperature, Cl- coordinated to Pt ions was gradually replaced with OH- to form intermediate platinum complexes, which further condensated to form colloidal nanoparticles. Obvious scattering signals of the colloidal nanoparticles could be observed in the UV-Vis absorption spectra of the reaction system before the formation of Pt-Pt bonds, as revealed by QXAFS measurements. In situ QXAFS analysis revealed that Pt nanoclusters were derived from the reduction of Pt oxide nanoparticles. The average particle size of the nanoparticles obtained by heating the reaction mixture for 15 min at 80 ℃ was 3.7 nm. High resolution transmission electron microscopy (HRTEM) images showed that the spacing between the crystal planes of the nanoparticles was 0.249 nm, indicating that the intermediate nanoparticles were platinum oxide. As the reaction proceeded, the average size of the nanoparticles decreased to 2.4 nm, and two types of nanoparticles were observed having different contrasts, corresponding to Pt metal nanoclusters standing on the intermediate metal oxide nanoparticles as confirmed by HRTEM images. When the reaction time was further extended, the average size of nanoparticles decreased to 1.4 nm, and the observed lattice spacing of the nanoparticles was the same as that of Pt(111) crystal plane at 0.227 nm, indicating that the final products were Pt metal nanoclusters. In general, when metal oxides are reduced to metal nanoclusters, the density of the nanoparticles will increase, whereas the volume will decrease. Moreover, as shown in this study, the formation of multiple small metal nanoclusters standing on one metal oxide nanoparticle was also observed in TEM photographs. Thus, compared with the size of the initial nanoparticles, the average size of the final metal nanoclusters was significantly reduced. On the other hand, during the formation of unprotected Ru metal nanoclusters, Cl- in RuCl3 was first replaced with OH- to form Ru(OH)63-, which further condensated to form Ru oxide nanoparticles, and unprotected Ru metal nanoclusters were derived from the reduction of Ru oxide nanoparticles by ethylene glycol. Because of the formation of intermediate metal oxide nanoparticles in the reaction process, the subsequent rapid reduction reaction was confined to the nanoparticles, resulting in unprotected metal nanoclusters having a small size and narrow particle size distribution. This study is of significance to the development of high-performance energy conversion catalysts, fine chemical synthesis catalysts, sensors, and other functional systems. ![]()
"Unprotected" metal and alloy nanoclusters prepared using the alkaline-ethylene glycol method (AEGM), stabilized by adsorbed solvent molecules and simple ions, have been widely applied in the development of high-performance heterogeneous catalysts and the exploration of the effects of metal particle size and composition, surface ligands of support, and modifiers on the catalytic properties of heterogeneous catalysts. The formation process and mechanism of such unprotected metal nanoclusters need to be further investigated. In this study, the formation process and mechanism of unprotected Pt and Ru nanoclusters prepared with AEGM were investigated by in situ quick X-ray absorption fine spectroscopy (QXAFS), in situ ultraviolet-visible (UV-Vis) absorption spectroscopy, transmission electron microscopy, and dynamic light scattering. It was discovered that during the formation of unprotected Pt nanoclusters, a portion of Pt(Ⅳ) species was reduced to Pt(Ⅱ) species at room temperature. With increasing temperature, Cl- coordinated to Pt ions was gradually replaced with OH- to form intermediate platinum complexes, which further condensated to form colloidal nanoparticles. Obvious scattering signals of the colloidal nanoparticles could be observed in the UV-Vis absorption spectra of the reaction system before the formation of Pt-Pt bonds, as revealed by QXAFS measurements. In situ QXAFS analysis revealed that Pt nanoclusters were derived from the reduction of Pt oxide nanoparticles. The average particle size of the nanoparticles obtained by heating the reaction mixture for 15 min at 80 ℃ was 3.7 nm. High resolution transmission electron microscopy (HRTEM) images showed that the spacing between the crystal planes of the nanoparticles was 0.249 nm, indicating that the intermediate nanoparticles were platinum oxide. As the reaction proceeded, the average size of the nanoparticles decreased to 2.4 nm, and two types of nanoparticles were observed having different contrasts, corresponding to Pt metal nanoclusters standing on the intermediate metal oxide nanoparticles as confirmed by HRTEM images. When the reaction time was further extended, the average size of nanoparticles decreased to 1.4 nm, and the observed lattice spacing of the nanoparticles was the same as that of Pt(111) crystal plane at 0.227 nm, indicating that the final products were Pt metal nanoclusters. In general, when metal oxides are reduced to metal nanoclusters, the density of the nanoparticles will increase, whereas the volume will decrease. Moreover, as shown in this study, the formation of multiple small metal nanoclusters standing on one metal oxide nanoparticle was also observed in TEM photographs. Thus, compared with the size of the initial nanoparticles, the average size of the final metal nanoclusters was significantly reduced. On the other hand, during the formation of unprotected Ru metal nanoclusters, Cl- in RuCl3 was first replaced with OH- to form Ru(OH)63-, which further condensated to form Ru oxide nanoparticles, and unprotected Ru metal nanoclusters were derived from the reduction of Ru oxide nanoparticles by ethylene glycol. Because of the formation of intermediate metal oxide nanoparticles in the reaction process, the subsequent rapid reduction reaction was confined to the nanoparticles, resulting in unprotected metal nanoclusters having a small size and narrow particle size distribution. This study is of significance to the development of high-performance energy conversion catalysts, fine chemical synthesis catalysts, sensors, and other functional systems.
2020, 36(1): 190707
doi: 10.3866/PKU.WHXB201907078
Abstract:
Triboluminescence is a fascinating luminescence phenomenon induced by mechanical stimuli. Triboluminescent materials have potential applications in lighting, displays, and sensing, owing to their distinctive modes of light generation. However, organic triboluminescent materials are severely limited, and their luminescence mechanism remains unclear. Herein, we found that the luminescent manganese(Ⅱ) complex [BPP]2[MnBr4] displayed interesting triboluminescence performance. A series of green emissive tetrahalomanganese(Ⅱ) complexes was rationally designed and synthesized. The associated single crystal structures revealed that all complexes consisted of one [MnX4]2− (X = Br or Cl) ion and two organic cationic ligands per unit cell, with a tetrahedral geometrical symmetry around the Mn(Ⅱ) ion. In addition, the photophysical properties of tetrahalomanganese(Ⅱ) complexes were easily tuned by varying the organic ligands or halogen ions, which is beneficial for these organic-inorganic hybrid structures. Under UV light irradiation, all tetrahalomanganese(Ⅱ) complexes in the solid state exhibited bright green luminescence and a broad featureless emission band at 450–650 nm. The time-resolved photoluminescent decay curves demonstrated that the emission lifetimes of the prepared tetrahalomanganese(Ⅱ) complexes ranged from 260.5 μs to 1.95 ms, which was attributed to phosphorescence. The long-lived emission was mainly due to the spin-forbidden nature of the metal center d–d (4T1(G)→ 6A1) radiative transition. Thermogravimetric analysis was performed to examine the thermodynamic stabilities of the tetrahalomanganese(Ⅱ) complexes. The thermal stabilities of manganese(Ⅱ) complexes with P-based ligands were higher than those of the complexes containing N-based ligands. Upon applying a force to the crystals, the tetrahalomanganese(Ⅱ) complexes all exhibited prominent triboluminescence that could be observed by the naked eye in the dark. Systematic analysis of the crystals showed that the TL activities of the manganese(Ⅱ) complexes were related to the intra- and inter-molecular C-H···X (X = Br or Cl) interactions. The intra- and inter-molecular C-H···X interactions significantly reduced the possible energy loss caused by molecular vibrations and rotations in the [MnX4]2− unit under mechanical stress, improving TL emission. Moreover, a comparison of photoluminescence and triboluminescence indicated that different excitation sources yielded two distinct luminescence processes: transition of excitons excited by illumination and recombination of electrons and holes on the surface driven by polarization charges. Overall, the results presented herein new opportunities for fundamental research based on the developed class of triboluminescent materials. ![]()
Triboluminescence is a fascinating luminescence phenomenon induced by mechanical stimuli. Triboluminescent materials have potential applications in lighting, displays, and sensing, owing to their distinctive modes of light generation. However, organic triboluminescent materials are severely limited, and their luminescence mechanism remains unclear. Herein, we found that the luminescent manganese(Ⅱ) complex [BPP]2[MnBr4] displayed interesting triboluminescence performance. A series of green emissive tetrahalomanganese(Ⅱ) complexes was rationally designed and synthesized. The associated single crystal structures revealed that all complexes consisted of one [MnX4]2− (X = Br or Cl) ion and two organic cationic ligands per unit cell, with a tetrahedral geometrical symmetry around the Mn(Ⅱ) ion. In addition, the photophysical properties of tetrahalomanganese(Ⅱ) complexes were easily tuned by varying the organic ligands or halogen ions, which is beneficial for these organic-inorganic hybrid structures. Under UV light irradiation, all tetrahalomanganese(Ⅱ) complexes in the solid state exhibited bright green luminescence and a broad featureless emission band at 450–650 nm. The time-resolved photoluminescent decay curves demonstrated that the emission lifetimes of the prepared tetrahalomanganese(Ⅱ) complexes ranged from 260.5 μs to 1.95 ms, which was attributed to phosphorescence. The long-lived emission was mainly due to the spin-forbidden nature of the metal center d–d (4T1(G)→ 6A1) radiative transition. Thermogravimetric analysis was performed to examine the thermodynamic stabilities of the tetrahalomanganese(Ⅱ) complexes. The thermal stabilities of manganese(Ⅱ) complexes with P-based ligands were higher than those of the complexes containing N-based ligands. Upon applying a force to the crystals, the tetrahalomanganese(Ⅱ) complexes all exhibited prominent triboluminescence that could be observed by the naked eye in the dark. Systematic analysis of the crystals showed that the TL activities of the manganese(Ⅱ) complexes were related to the intra- and inter-molecular C-H···X (X = Br or Cl) interactions. The intra- and inter-molecular C-H···X interactions significantly reduced the possible energy loss caused by molecular vibrations and rotations in the [MnX4]2− unit under mechanical stress, improving TL emission. Moreover, a comparison of photoluminescence and triboluminescence indicated that different excitation sources yielded two distinct luminescence processes: transition of excitons excited by illumination and recombination of electrons and holes on the surface driven by polarization charges. Overall, the results presented herein new opportunities for fundamental research based on the developed class of triboluminescent materials.
2020, 36(1): 190402
doi: 10.3866/PKU.WHXB201904026
Abstract:
The reactivity of atomic metal cations toward CH4 has been extensively investigated over the past decades. Closed-shell metal cations in electronically ground states are usually inert with CH4 under thermal collision conditions because of the extremely high stability of methane. With the elevation of collision energies, closed-shell atomic gold cations (Au+) have been reported to react with CH4 under single-collision conditions to produce AuCH2+, AuH+, and AuCH3+ species. Further investigations found that the ion-source-generated AuCH2+ cations can react with CH4 to synthesize C―C coupling products. These previous studies suggested that new products for the reaction of Au+ with CH4 can be identified under multiple-collision conditions with sufficient collision energies. However, the reported ion-molecule reactions involving methane were usually performed under single- or multiple-collision conditions with thermal collision energies. In this study, a new reactor composed of a drift tube and ion funnel is constructed and coupled with a homemade reflectron time-of-flight mass spectrometer. Laser-ablation-generated Au+ ions are injected into the reactor and drift 120 mm to react with methane seeded in the helium drift gas. The reaction products and unreacted Au+ ions are focused through the ion funnel and accumulate through a linear ion trap and are then detected by a mass spectrometer. In the reactor, the pressure is approximately 100 Pa, and the electric field between the drift tube and ion funnel can regulate the collision energies between ions and molecules. The reaction of the closed-shell atomic Au+ cation with CH4 is investigated, and the C―C coupling product AuC2H4+ is observed under multiple-collision conditions with elevated collision energies. Density functional theory calculations are performed to understand the mechanism of the coupling reaction (Au++ 2CH4 → AuC2H4+ + 2H2). Two pathways involving Au―CH2 and Au―CH3 species can separately mediate the C―C coupling process. The activation of the second C―H bond in each process requires additional energy to overcome the relatively high barrier (2.07 and 2.29 eV). Ion-trajectory simulations under multiple-collision conditions are then conducted to determine the collisional energy distribution in the reactor. These simulations confirmed that the electric fields between the drift tube and ion funnel could supply sufficient center-of-mass kinetic energies to facilitate the C―C coupling process to form AuC2H4+. The following catalytic cycle could then be postulated:\begin{document}$\mathrm{AuC}_{2} \mathrm{H}_{4}^{+}+\mathrm{CH}_{4} \stackrel{\Delta}{\longrightarrow} \mathrm{AuCH}_{4}^{+}+\mathrm{C}_{2} \mathrm{H}_{4}, \mathrm{AuCH}_{4}^{+}+\mathrm{CH}_{4} \stackrel{\Delta}{\longrightarrow} \mathrm{AuC}_{2} \mathrm{H}_{4}^{+}+2 \mathrm{H}_{2}$\end{document} \begin{document}$\mathrm{CH}_{4} \stackrel{\mathrm{Au}^{+}, \Delta}{\longrightarrow} \mathrm{C}_{2} \mathrm{H}_{4}+2 \mathrm{H}_{2}$\end{document} ![]()
The reactivity of atomic metal cations toward CH4 has been extensively investigated over the past decades. Closed-shell metal cations in electronically ground states are usually inert with CH4 under thermal collision conditions because of the extremely high stability of methane. With the elevation of collision energies, closed-shell atomic gold cations (Au+) have been reported to react with CH4 under single-collision conditions to produce AuCH2+, AuH+, and AuCH3+ species. Further investigations found that the ion-source-generated AuCH2+ cations can react with CH4 to synthesize C―C coupling products. These previous studies suggested that new products for the reaction of Au+ with CH4 can be identified under multiple-collision conditions with sufficient collision energies. However, the reported ion-molecule reactions involving methane were usually performed under single- or multiple-collision conditions with thermal collision energies. In this study, a new reactor composed of a drift tube and ion funnel is constructed and coupled with a homemade reflectron time-of-flight mass spectrometer. Laser-ablation-generated Au+ ions are injected into the reactor and drift 120 mm to react with methane seeded in the helium drift gas. The reaction products and unreacted Au+ ions are focused through the ion funnel and accumulate through a linear ion trap and are then detected by a mass spectrometer. In the reactor, the pressure is approximately 100 Pa, and the electric field between the drift tube and ion funnel can regulate the collision energies between ions and molecules. The reaction of the closed-shell atomic Au+ cation with CH4 is investigated, and the C―C coupling product AuC2H4+ is observed under multiple-collision conditions with elevated collision energies. Density functional theory calculations are performed to understand the mechanism of the coupling reaction (Au++ 2CH4 → AuC2H4+ + 2H2). Two pathways involving Au―CH2 and Au―CH3 species can separately mediate the C―C coupling process. The activation of the second C―H bond in each process requires additional energy to overcome the relatively high barrier (2.07 and 2.29 eV). Ion-trajectory simulations under multiple-collision conditions are then conducted to determine the collisional energy distribution in the reactor. These simulations confirmed that the electric fields between the drift tube and ion funnel could supply sufficient center-of-mass kinetic energies to facilitate the C―C coupling process to form AuC2H4+. The following catalytic cycle could then be postulated:
2020, 36(1): 190406
doi: 10.3866/PKU.WHXB201904066
Abstract:
Graphitic carbon nitrides (g-C3N4) with different surface areas were prepared by pyrolysis using different precursors including melamine, dicyandiamide, thiourea and urea, and subsequently characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectra (FTIR), X-ray photoelectron spectroscopy (XPS), thermal gravimetric analysis (TGA) and N2 adsorption. Their basicities were measured by temperature-programmed desorption of CO2 (CO2-TPD) and acid-base titration. The catalytic properties for the Knoevenagel condensation of benzaldehyde and malononitrile were investigated in various solvents. In non-polar toluene solution, the benzaldehyde conversions of the g-C3N4 catalysts were low and changed according to their respective surface areas and basicities. However, in polar ethanol solution, the benzaldehyde conversions of all catalysts were similar, and much higher than those in toluene. This could not be explained by the results obtained from either of the two conventional basicity measurements. Further experimental results proved that g-C3N4 catalysts swelled in polar solutions, and more basic sites were exposed on the surface of the swollen catalysts, leading to the imminent increase in catalytic activity. This was proved by the catalyst poisoning data, which showed that the g-C3N4 catalyst lost its activity completely in toluene by adding 40.9 mmol·g-1 benzoic acid, while the same catalyst was still active in ethanol until the added amount exceeded 143.3 m·g-1. Additionally, the reaction tests in various solutions showed that the swelling effect was enhanced according to the polarity of the solvent used. A similar conclusion could be reached for the Knoevenagel condensation of furfural and malononitrile in various solvents. The reusability of g-C3N4 catalyst in Knoevenagel condensation was also studied, which showed that g-C3N4 was stable in liquid-phase reactions, whose activity dropped from 74.2% to 63.8% after three regeneration processes. ![]()
Graphitic carbon nitrides (g-C3N4) with different surface areas were prepared by pyrolysis using different precursors including melamine, dicyandiamide, thiourea and urea, and subsequently characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectra (FTIR), X-ray photoelectron spectroscopy (XPS), thermal gravimetric analysis (TGA) and N2 adsorption. Their basicities were measured by temperature-programmed desorption of CO2 (CO2-TPD) and acid-base titration. The catalytic properties for the Knoevenagel condensation of benzaldehyde and malononitrile were investigated in various solvents. In non-polar toluene solution, the benzaldehyde conversions of the g-C3N4 catalysts were low and changed according to their respective surface areas and basicities. However, in polar ethanol solution, the benzaldehyde conversions of all catalysts were similar, and much higher than those in toluene. This could not be explained by the results obtained from either of the two conventional basicity measurements. Further experimental results proved that g-C3N4 catalysts swelled in polar solutions, and more basic sites were exposed on the surface of the swollen catalysts, leading to the imminent increase in catalytic activity. This was proved by the catalyst poisoning data, which showed that the g-C3N4 catalyst lost its activity completely in toluene by adding 40.9 mmol·g-1 benzoic acid, while the same catalyst was still active in ethanol until the added amount exceeded 143.3 m·g-1. Additionally, the reaction tests in various solutions showed that the swelling effect was enhanced according to the polarity of the solvent used. A similar conclusion could be reached for the Knoevenagel condensation of furfural and malononitrile in various solvents. The reusability of g-C3N4 catalyst in Knoevenagel condensation was also studied, which showed that g-C3N4 was stable in liquid-phase reactions, whose activity dropped from 74.2% to 63.8% after three regeneration processes.
2020, 36(1): 190601
doi: 10.3866/PKU.WHXB201906014
Abstract:
Artificial photosynthesis is an ideal method for solar-to-chemical energy conversion, wherein solar energy is stored in the form of chemical bonds of solar fuels. In particular, the photocatalytic reduction of CO2 has attracted considerable attention due to its dual benefits of fossil fuel production and CO2 pollution reduction. However, CO2 is a comparatively stable molecule and its photoreduction is thermodynamically and kinetically challenging. Thus, the photocatalytic efficiency of CO2 reduction is far below the level of industrial applications. Therefore, development of low-cost cocatalysts is crucial for significantly decreasing the activation energy of CO2 to achieving efficient photocatalytic CO2 reduction. Herein, we have reported the use of a Ni2P material that can serve as a robust cocatalyst by cooperating with a photosensitizer for the photoconversion of CO2. An effective strategy for engineering Ni2P in an ultrathin layered structure has been proposed to improve the CO2 adsorption capability and decrease the CO2 activation energy, resulting in efficient CO2 reduction. A series of physicochemical characterizations including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and atomic force microscopy (AFM) were used to demonstrate the successful preparation of ultrathin Ni2P nanosheets. The XRD and XPS results confirm the successful synthesis of Ni2P from Ni(OH)2 by a low temperature phosphidation process. According to the TEM images, the prepared Ni2P nanosheets exhibit a 2D and near-transparent sheet-like structure, suggesting their ultrathin thickness. The AFM images further demonstrated this result and also showed that the height of the Ni2P nanosheets is ca 1.5 nm. The photoluminescence (PL) spectroscopy results revealed that the Ni2P material could efficiently promote the separation of the photogenerated electrons and holes in [Ru(bpy)3]Cl2·6H2O. More importantly, the Ni2P nanosheets could more efficiently promote the charge transfer and charge separation rate of [Ru(bpy)3]Cl2·6H2O compared with the Ni2P particles. In addition, the electrochemical experiments revealed that the Ni2P nanosheets, with their high active surface area and charge conductivity, can provide more active centers for CO2 conversion and accelerate the interfacial reaction dynamics. These results strongly suggest that the Ni2P nanosheets are a promising material for photocatalytic CO2 reduction, and can achieve a CO generation rate of 64.8 μmol·h-1, which is 4.4 times higher than that of the Ni2P particles. In addition, the XRD and XPS measurements of the used Ni2P nanosheets after the six cycles of the photocatalytic CO2 reduction reaction demonstrated their high stability. Overall, this study offers a new function for the 2D transition-metal phosphide catalysts in photocatalytic CO2 reduction. ![]()
Artificial photosynthesis is an ideal method for solar-to-chemical energy conversion, wherein solar energy is stored in the form of chemical bonds of solar fuels. In particular, the photocatalytic reduction of CO2 has attracted considerable attention due to its dual benefits of fossil fuel production and CO2 pollution reduction. However, CO2 is a comparatively stable molecule and its photoreduction is thermodynamically and kinetically challenging. Thus, the photocatalytic efficiency of CO2 reduction is far below the level of industrial applications. Therefore, development of low-cost cocatalysts is crucial for significantly decreasing the activation energy of CO2 to achieving efficient photocatalytic CO2 reduction. Herein, we have reported the use of a Ni2P material that can serve as a robust cocatalyst by cooperating with a photosensitizer for the photoconversion of CO2. An effective strategy for engineering Ni2P in an ultrathin layered structure has been proposed to improve the CO2 adsorption capability and decrease the CO2 activation energy, resulting in efficient CO2 reduction. A series of physicochemical characterizations including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and atomic force microscopy (AFM) were used to demonstrate the successful preparation of ultrathin Ni2P nanosheets. The XRD and XPS results confirm the successful synthesis of Ni2P from Ni(OH)2 by a low temperature phosphidation process. According to the TEM images, the prepared Ni2P nanosheets exhibit a 2D and near-transparent sheet-like structure, suggesting their ultrathin thickness. The AFM images further demonstrated this result and also showed that the height of the Ni2P nanosheets is ca 1.5 nm. The photoluminescence (PL) spectroscopy results revealed that the Ni2P material could efficiently promote the separation of the photogenerated electrons and holes in [Ru(bpy)3]Cl2·6H2O. More importantly, the Ni2P nanosheets could more efficiently promote the charge transfer and charge separation rate of [Ru(bpy)3]Cl2·6H2O compared with the Ni2P particles. In addition, the electrochemical experiments revealed that the Ni2P nanosheets, with their high active surface area and charge conductivity, can provide more active centers for CO2 conversion and accelerate the interfacial reaction dynamics. These results strongly suggest that the Ni2P nanosheets are a promising material for photocatalytic CO2 reduction, and can achieve a CO generation rate of 64.8 μmol·h-1, which is 4.4 times higher than that of the Ni2P particles. In addition, the XRD and XPS measurements of the used Ni2P nanosheets after the six cycles of the photocatalytic CO2 reduction reaction demonstrated their high stability. Overall, this study offers a new function for the 2D transition-metal phosphide catalysts in photocatalytic CO2 reduction.
2020, 36(1): 190604
doi: 10.3866/PKU.WHXB201906044
Abstract:
The development of the global economy has been accompanied by frequent oil spills caused by accidental leaks and industrial manufacturing, which have seriously threatened the aquatic environment and human health. Traditional methods for the treatment of oily wastewater include centrifugation, skimming, flotation, oil-absorbing technology, etc., which are limited by low separation efficiency as well as secondary pollution during the post-processing of oil absorption materials. Recently, separation technologies utilizing the special wettabilities of filtration membranes have been developed to enrich and recycle oils from wastewater. Among these, the fabrication of superhydrophilic/underwater superhydrophobic membranes have attracted intensive research interest, which can selectively allow the passage of water through the membrane while blocking the oils. However, microorganisms are more likely to breed on these hydrophilic surfaces, eventually leading to the blockage of the membranes. In this study, ZSM-5 zeolite crystals (MFI topological structure) were coated onto the stainless-steel meshes by means of seeding and secondary hydrothermal growth. Then, 70% of the total Na+ ions in the zeolite channels were substituted by Ag+ ions via an ion exchange process. The resultant membranes (Ag@ZCMFs) were superamphiphilic in air, with both water contact angle and oil contact angle of approximately 0°. However, they became superoleophobic when immersed in water, and the underwater oil contact angle reached 151.27° ± 4.34°. In terms of special wettability, Ag@ZCMF achieved efficient separation for various oil-water mixtures with separation efficiencies above 99%. The water flux and intrusion pressure of Ag@ZCMF depended on the diameter of pinholes in the membrane, which could be modulated by altering the time of secondary hydrothermal growth. For instance, the average diameter of pinholes in Ag@ZCMF with optimum secondary growth time of 14 h (Ag@ZCMF-14) reached approximately 21 μm, giving rise to the water flux and intrusion pressure of 54720 L·m-2·h-1 and 4357 Pa, respectively. The anti-corrosion test and rubbing test confirmed the high chemical and mechanical stability of Ag@ZCMF-14, respectively. The separation efficiency of Ag@ZCMF-14 remained stable during ten purification-regeneration cycles, and no obvious attenuation was observed, proving the high separation stability of Ag@ZCMF-14. Furthermore, the loaded Ag+ ions afforded the membrane excellent anti-biofouling activity, which could effectively inhibit the growth of both alga and bacteria in the operating environment, thus preventing membrane blockage during the oil-water separation process. In particular, the bacteriostatic rate of Ag@ZCMF-14 to Escherichia coli reached to 99.6%. These results demonstrate that Ag@ZCMFs with anti-biofouling activity has promising potential future applications in the removal of oil slicks from oily wastewater. ![]()
The development of the global economy has been accompanied by frequent oil spills caused by accidental leaks and industrial manufacturing, which have seriously threatened the aquatic environment and human health. Traditional methods for the treatment of oily wastewater include centrifugation, skimming, flotation, oil-absorbing technology, etc., which are limited by low separation efficiency as well as secondary pollution during the post-processing of oil absorption materials. Recently, separation technologies utilizing the special wettabilities of filtration membranes have been developed to enrich and recycle oils from wastewater. Among these, the fabrication of superhydrophilic/underwater superhydrophobic membranes have attracted intensive research interest, which can selectively allow the passage of water through the membrane while blocking the oils. However, microorganisms are more likely to breed on these hydrophilic surfaces, eventually leading to the blockage of the membranes. In this study, ZSM-5 zeolite crystals (MFI topological structure) were coated onto the stainless-steel meshes by means of seeding and secondary hydrothermal growth. Then, 70% of the total Na+ ions in the zeolite channels were substituted by Ag+ ions via an ion exchange process. The resultant membranes (Ag@ZCMFs) were superamphiphilic in air, with both water contact angle and oil contact angle of approximately 0°. However, they became superoleophobic when immersed in water, and the underwater oil contact angle reached 151.27° ± 4.34°. In terms of special wettability, Ag@ZCMF achieved efficient separation for various oil-water mixtures with separation efficiencies above 99%. The water flux and intrusion pressure of Ag@ZCMF depended on the diameter of pinholes in the membrane, which could be modulated by altering the time of secondary hydrothermal growth. For instance, the average diameter of pinholes in Ag@ZCMF with optimum secondary growth time of 14 h (Ag@ZCMF-14) reached approximately 21 μm, giving rise to the water flux and intrusion pressure of 54720 L·m-2·h-1 and 4357 Pa, respectively. The anti-corrosion test and rubbing test confirmed the high chemical and mechanical stability of Ag@ZCMF-14, respectively. The separation efficiency of Ag@ZCMF-14 remained stable during ten purification-regeneration cycles, and no obvious attenuation was observed, proving the high separation stability of Ag@ZCMF-14. Furthermore, the loaded Ag+ ions afforded the membrane excellent anti-biofouling activity, which could effectively inhibit the growth of both alga and bacteria in the operating environment, thus preventing membrane blockage during the oil-water separation process. In particular, the bacteriostatic rate of Ag@ZCMF-14 to Escherichia coli reached to 99.6%. These results demonstrate that Ag@ZCMFs with anti-biofouling activity has promising potential future applications in the removal of oil slicks from oily wastewater.
2020, 36(1): 190701
doi: 10.3866/PKU.WHXB201907012
Abstract:
Inorganic-organic or hybrid perovskite materials, which are the complementary counterparts of pure inorganic perovskites, can provide many new opportunities in the researches of phase transitions, critical phenomena, and relevant properties, as they combine the characteristics of inorganic and organic components. Therefore, the hybrid perovskites of ammonium metal formate framework are very promising, and their properties have been found to be strongly dependent on the characteristics of the constituent metal ions and/or ammonium ions. Herein, we used solid solution strategies, borrowed from solid state chemistry, to investigate the anisotropic diluted magnetic hybrid perovskite system of [CH3NH3][CoxZn1-x(HCOO)3], wherein the B-sites are occupied by the mixed metal ions of Co2+ and Zn2+. The solid solution compounds of this series in the range x = 0–1 (or the molar percent Co% = 0–100%) were successfully prepared using conventional solution chemistry methods. The resulting compounds were demonstrated to be iso-structural by using both single-crystal and powder X-ray diffraction analyses. The solid solution crystals belong to the orthorhombic space group Pnma, with the cell parameters being a = 8.3015(2)–8.3207(3) Å, b = 11.6574(4)–11.6811(5) Å, c = 8.1315(3)–8.1427(4) Å, and V = 787.89(5)–790.98(7) Å3. The perovskite structure consists of a simple cubic anionic metal-formate framework and CH3NH3+ cations which are located in the framework cavities, with N―H···O hydrogen bonds formed between the framework and the cation. The members of this series showed negligible changes (< 0.4%) in their respective lattice and structural parameters. Thus, the prepared solid solution compounds constitute good molecule-based examples for the study of magnetic dilution under almost the same structural parameters and molecular geometries. Upon dilution, the magnetization per mole of Co at low temperatures and low fields was suppressed by the magnetic anisotropy of Co2+ and gradual destruction of the large spin canting between coupled Co2+ ions, in contrast to the magnetization enhancement observed in the isotropic diluted system of [CH3NH3][MnxZn1-x(HCOO)3] with the same perovskite structure. The percolation limit was estimated as (Co%)P = 27(1)% (or xP = 0.27(1)) from the magnetic data, which was slightly lower than that predicted by the percolation theory for a simple cubic lattice (31%); this trend was due to the strong magnetic anisotropy of the present system. In addition, rare incommensurate phase transitions were primarily detected below ~120 K for the pure Co and Zn members, which may also affect the magnetic properties of the materials. ![]()
Inorganic-organic or hybrid perovskite materials, which are the complementary counterparts of pure inorganic perovskites, can provide many new opportunities in the researches of phase transitions, critical phenomena, and relevant properties, as they combine the characteristics of inorganic and organic components. Therefore, the hybrid perovskites of ammonium metal formate framework are very promising, and their properties have been found to be strongly dependent on the characteristics of the constituent metal ions and/or ammonium ions. Herein, we used solid solution strategies, borrowed from solid state chemistry, to investigate the anisotropic diluted magnetic hybrid perovskite system of [CH3NH3][CoxZn1-x(HCOO)3], wherein the B-sites are occupied by the mixed metal ions of Co2+ and Zn2+. The solid solution compounds of this series in the range x = 0–1 (or the molar percent Co% = 0–100%) were successfully prepared using conventional solution chemistry methods. The resulting compounds were demonstrated to be iso-structural by using both single-crystal and powder X-ray diffraction analyses. The solid solution crystals belong to the orthorhombic space group Pnma, with the cell parameters being a = 8.3015(2)–8.3207(3) Å, b = 11.6574(4)–11.6811(5) Å, c = 8.1315(3)–8.1427(4) Å, and V = 787.89(5)–790.98(7) Å3. The perovskite structure consists of a simple cubic anionic metal-formate framework and CH3NH3+ cations which are located in the framework cavities, with N―H···O hydrogen bonds formed between the framework and the cation. The members of this series showed negligible changes (< 0.4%) in their respective lattice and structural parameters. Thus, the prepared solid solution compounds constitute good molecule-based examples for the study of magnetic dilution under almost the same structural parameters and molecular geometries. Upon dilution, the magnetization per mole of Co at low temperatures and low fields was suppressed by the magnetic anisotropy of Co2+ and gradual destruction of the large spin canting between coupled Co2+ ions, in contrast to the magnetization enhancement observed in the isotropic diluted system of [CH3NH3][MnxZn1-x(HCOO)3] with the same perovskite structure. The percolation limit was estimated as (Co%)P = 27(1)% (or xP = 0.27(1)) from the magnetic data, which was slightly lower than that predicted by the percolation theory for a simple cubic lattice (31%); this trend was due to the strong magnetic anisotropy of the present system. In addition, rare incommensurate phase transitions were primarily detected below ~120 K for the pure Co and Zn members, which may also affect the magnetic properties of the materials.
2020, 36(1): 190704
doi: 10.3866/PKU.WHXB201907043
Abstract:
Pure organic radical molecules on metal surfaces are of great significance in exploration of the electron spin behavior. However, only a few of them are investigated in surface studies due to their poor thermal stability. The adsorption and conformational switching of two verdazyl radical molecules, namely, 1, 5-biisopropyl-3-(benzo[b]benzo[4,5]thieno[2, 3-d]thiophen-2-yl)-6-oxoverdazyl (B2P) and 1, 5-biisopropyl-3-(benzo[b]benzo[4,5]thieno[2, 3-d]thiophen-4-yl)-6-oxoverdazyl (B4P), are studied by scanning tunneling microscopy (STM) and density functional theory (DFT). The adsorbed B2P molecules on Au(111) form dimers, trimers and tetramers without any ordered assembly structure in which two distinct appearances of B2P in STM images are observed and assigned to be its "P" and "T" conformations. The "P" conformation molecules appear in the STM image with a large elliptical protrusion and two small ones of equal size, while the "T" ones appear with a large protrusion and two small ones of different size. Likewise, the B4P molecules on Au(111) form dimers at low coverage, strip structure at medium coverage and assembled structure at high coverage which also consists of above-mentioned two conformations. Both B2P molecules and B4P molecules are held together by weak intermolecular interaction rather than chemical bond. STM tip induced conformational switching of both verdayzl radicals is observed at the bias voltage of +2.0 V. The "T" conformation of B2P can be switched to the "P" while the "P" conformation of B4P can be switched to the "T" one. For both molecules, such a conformational switching is irreversible. The DFT calculations with Perdew-Burke-Ernzerhof version exchange-correlation functional are used to optimize the model structure and simulate the STM images. STM images of several possible molecular conformations with different isopropyl orientation and different tilt angle between verdazyl radical and Au(111) surface are simulated. For conformations with different isopropyl orientation, the STM simulated images are similar, while different tilt angles of verdazyl radical lead to significantly different STM simulated images. Combined STM experiments and DFT simulations reveal that the conformational switching originates from the change of tilting angle between the verdazyl radical and Au(111) surface. The tilt angles in "P" and "T" conformations are 0° and 50°, respectively. In this study, two different adsorption conformations of verdazyl radicals on the Au(111) surface are presented and their exact adsorption structures are identified. This study provides a possible way to study the relationship between the electron spin and configuration conversion of pure organic radical molecules and a reference for designing more conformational switchable radical molecules that can be employed as interesting molecular switches. ![]()
Pure organic radical molecules on metal surfaces are of great significance in exploration of the electron spin behavior. However, only a few of them are investigated in surface studies due to their poor thermal stability. The adsorption and conformational switching of two verdazyl radical molecules, namely, 1, 5-biisopropyl-3-(benzo[b]benzo[4,5]thieno[2, 3-d]thiophen-2-yl)-6-oxoverdazyl (B2P) and 1, 5-biisopropyl-3-(benzo[b]benzo[4,5]thieno[2, 3-d]thiophen-4-yl)-6-oxoverdazyl (B4P), are studied by scanning tunneling microscopy (STM) and density functional theory (DFT). The adsorbed B2P molecules on Au(111) form dimers, trimers and tetramers without any ordered assembly structure in which two distinct appearances of B2P in STM images are observed and assigned to be its "P" and "T" conformations. The "P" conformation molecules appear in the STM image with a large elliptical protrusion and two small ones of equal size, while the "T" ones appear with a large protrusion and two small ones of different size. Likewise, the B4P molecules on Au(111) form dimers at low coverage, strip structure at medium coverage and assembled structure at high coverage which also consists of above-mentioned two conformations. Both B2P molecules and B4P molecules are held together by weak intermolecular interaction rather than chemical bond. STM tip induced conformational switching of both verdayzl radicals is observed at the bias voltage of +2.0 V. The "T" conformation of B2P can be switched to the "P" while the "P" conformation of B4P can be switched to the "T" one. For both molecules, such a conformational switching is irreversible. The DFT calculations with Perdew-Burke-Ernzerhof version exchange-correlation functional are used to optimize the model structure and simulate the STM images. STM images of several possible molecular conformations with different isopropyl orientation and different tilt angle between verdazyl radical and Au(111) surface are simulated. For conformations with different isopropyl orientation, the STM simulated images are similar, while different tilt angles of verdazyl radical lead to significantly different STM simulated images. Combined STM experiments and DFT simulations reveal that the conformational switching originates from the change of tilting angle between the verdazyl radical and Au(111) surface. The tilt angles in "P" and "T" conformations are 0° and 50°, respectively. In this study, two different adsorption conformations of verdazyl radicals on the Au(111) surface are presented and their exact adsorption structures are identified. This study provides a possible way to study the relationship between the electron spin and configuration conversion of pure organic radical molecules and a reference for designing more conformational switchable radical molecules that can be employed as interesting molecular switches.
2020, 36(1): 190803
doi: 10.3866/PKU.WHXB201908038
Abstract:
Nanostructured bismuth oxyselenide (Bi2O2Se) semiconductor, a two-dimensional (2D) materials with high-mobility, air-stability, and tunable bandgap, has recently emerged as a candidate of channel material for future digital (electronic and optoelectronic) applications. In terms of material morphology, some basic issues will be addressed when a two-dimensional layered crystal is shaped into a one-dimensional (1D) geometry due to size effect; these include the space-confined transport in a plane, which leads to dramatic changes in electronic, optical, and thermal properties. These novel 1D nanostructures with unique properties are an optimal choice for fabricating next-generation integrated circuits and functional devices within the nanometer scale such as gate-all-around field-effect transistors, single-electron transistors, chemical sensors, and THz detectors. As one of the high-mobility 2D semiconductor, 1D high-quality Bi2O2Se nanoribbons could be promising for applications in high-performance transistors; however, their synthesis has not been completely developed yet. In our study, we report on the facile growth of Bi2O2Se nanoribbons on mica substrates via a bismuth-catalyzed vapor-liquid-solid (VLS) mechanism. The preparation of Bi2O2Se nanoribbons is based on a previous work that emphasized on the oxidation of Bi2Se3 in a chemical vapor deposition (CVD) system and the use of bismuth (Bi) particles as the precursor of Bi catalysis. The morphology, composition, and structure of the as-grown Bi2O2Se nanoribbons were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman spectroscopy, transmission electron microscopy (TEM), as well as other methods. For a Bi mediated VLS growth process, the growth of Bi2O2Se nanoribbons can be self-assembled; further, in this process, as-grown epitaxial Bi2O2Se nanoribbons are free-standing with out-of-plane morphology on the mica substrate. Additionally, combining the spherical aberration corrected transmission electron microscope (ACTEM) and selected electron diffraction (SAED) methods, we discovered that the as-synthesized Bi2O2Se nanoribbons were single crystalline with high quality. We further investigated the controllable growth for domain size by optimizing the growth temperature of the Bi2O2Se nanoribbons. As-synthesized single-crystal Bi2O2Se nanoribbons have widths in the range of 100 nm to 20 μm and lengths in the sub-millimeter range. By employing a polymer poly(methyl methacrylate) (PMMA) assisted clean transfer method with the assistance of deionized water, the Bi2O2Se nanoribbons can be easily transferred onto a SiO2/Si substrate. Fabricated into the top-gated field-effect device, the Bi2O2Se nanoribbon sample (transferred to the SiO2/Si substrate) exhibited high electronic performances; these included a high electron mobility of ∼220 cm2∙V−1∙s−1 at room temperature, good switching behavior with on/off ratio of > 106, and high on current density of ∼42 μA∙μm−1 at a channel length of 10 μm. Therefore, Bi2O2Se nanoribbons are expected to be a promising materials for building high-performance transistors in the future. ![]()
Nanostructured bismuth oxyselenide (Bi2O2Se) semiconductor, a two-dimensional (2D) materials with high-mobility, air-stability, and tunable bandgap, has recently emerged as a candidate of channel material for future digital (electronic and optoelectronic) applications. In terms of material morphology, some basic issues will be addressed when a two-dimensional layered crystal is shaped into a one-dimensional (1D) geometry due to size effect; these include the space-confined transport in a plane, which leads to dramatic changes in electronic, optical, and thermal properties. These novel 1D nanostructures with unique properties are an optimal choice for fabricating next-generation integrated circuits and functional devices within the nanometer scale such as gate-all-around field-effect transistors, single-electron transistors, chemical sensors, and THz detectors. As one of the high-mobility 2D semiconductor, 1D high-quality Bi2O2Se nanoribbons could be promising for applications in high-performance transistors; however, their synthesis has not been completely developed yet. In our study, we report on the facile growth of Bi2O2Se nanoribbons on mica substrates via a bismuth-catalyzed vapor-liquid-solid (VLS) mechanism. The preparation of Bi2O2Se nanoribbons is based on a previous work that emphasized on the oxidation of Bi2Se3 in a chemical vapor deposition (CVD) system and the use of bismuth (Bi) particles as the precursor of Bi catalysis. The morphology, composition, and structure of the as-grown Bi2O2Se nanoribbons were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman spectroscopy, transmission electron microscopy (TEM), as well as other methods. For a Bi mediated VLS growth process, the growth of Bi2O2Se nanoribbons can be self-assembled; further, in this process, as-grown epitaxial Bi2O2Se nanoribbons are free-standing with out-of-plane morphology on the mica substrate. Additionally, combining the spherical aberration corrected transmission electron microscope (ACTEM) and selected electron diffraction (SAED) methods, we discovered that the as-synthesized Bi2O2Se nanoribbons were single crystalline with high quality. We further investigated the controllable growth for domain size by optimizing the growth temperature of the Bi2O2Se nanoribbons. As-synthesized single-crystal Bi2O2Se nanoribbons have widths in the range of 100 nm to 20 μm and lengths in the sub-millimeter range. By employing a polymer poly(methyl methacrylate) (PMMA) assisted clean transfer method with the assistance of deionized water, the Bi2O2Se nanoribbons can be easily transferred onto a SiO2/Si substrate. Fabricated into the top-gated field-effect device, the Bi2O2Se nanoribbon sample (transferred to the SiO2/Si substrate) exhibited high electronic performances; these included a high electron mobility of ∼220 cm2∙V−1∙s−1 at room temperature, good switching behavior with on/off ratio of > 106, and high on current density of ∼42 μA∙μm−1 at a channel length of 10 μm. Therefore, Bi2O2Se nanoribbons are expected to be a promising materials for building high-performance transistors in the future.
2020, 36(1): 190507
doi: 10.3866/PKU.WHXB201905076
Abstract:
The biosafety of nanoparticles is gaining extensive attention due to their dichotomous effects in fields of biomedicine and atmospheric chemistry. A number of studies have been carried out focusing on the cytotoxicity of nanoparticles and their interactions with cells. However, the mechanism of nanoparticle–cell interactions remains unclear. Here, we review the latest progress in the study of nanoparticle-cell interactions from a cellular chemo-mechanical perspective. Cell mechanics play an important role in cell differentiation, proliferation, apoptosis, polarization, adhesion, and migration. An understanding of the effects of nanoparticles on cell mechanics is therefore needed in order to enhance comprehension of nanoparticle–cell interactions. Firstly, the main molecules and signal pathways related to mechanical chemistry are introduced from three perspectives: cell surface adhesion receptors, the cytoskeleton, and the nucleus. Specifically, integrins and cadherins play a critical role in sensing both the external mechanical force and the force of cell transmission. Actin and microtubules, which are two components of the cytoskeletal network, act as a bridge in mechanical conduction. The nucleus can also be mechanically stressed by the surrounding cytoskeleton through the contraction of the matrix. The nuclear envelope also plays important roles in sensing mechanical signals and in adjusting the morphology and function of the nucleus. We summarize the major nanoparticle-based tools used in the laboratory for the study of cell mechanics, which includes traction force microscopy, atomic force microscopy, optical tweezers, magnetic manipulation, micropillars, and force-induced remnant magnetization spectroscopy. In addition, we discuss the effects that nanoparticles have on cell mechanics. Nanoparticles interact with the adhesion of molecules on the cell membrane surface and on cell cytoskeletal proteins, which further affects the mechanical properties involved in cell stiffness, cell adhesion, and cell migration. Overall, the general conclusions regarding the effects of nanoparticles on cell mechanics are as follows: (1) Nanoparticles can affect cell adhesion by disrupting tight and adherent junctions, and by regulating cell-extracellular matrix adhesion; (2) Nanoparticles can interact with cytoskeletal proteins (actins and tubulins) leading to structural reorganization or disruption of microtubules and F-actin; (3) Cell stiffness changes with the structural reorganization of the cytoskeleton; (4) Cell migration ability can be affected through changes in the cytoskeleton, cell adhesion, and the expression of cell migration-related proteins/molecules. To develop the nano-biosafety evaluation system, future studies should attempt to gain a better understanding of the molecular mechanisms involved with regards to nanoparticles and cell mechanics. Ultimately, further development of new methods and technologies based on nano-mechanical chemistry for diagnosis and treatment purposes are expected, given the wide application of nanomaterials in the biomedical field. ![]()
The biosafety of nanoparticles is gaining extensive attention due to their dichotomous effects in fields of biomedicine and atmospheric chemistry. A number of studies have been carried out focusing on the cytotoxicity of nanoparticles and their interactions with cells. However, the mechanism of nanoparticle–cell interactions remains unclear. Here, we review the latest progress in the study of nanoparticle-cell interactions from a cellular chemo-mechanical perspective. Cell mechanics play an important role in cell differentiation, proliferation, apoptosis, polarization, adhesion, and migration. An understanding of the effects of nanoparticles on cell mechanics is therefore needed in order to enhance comprehension of nanoparticle–cell interactions. Firstly, the main molecules and signal pathways related to mechanical chemistry are introduced from three perspectives: cell surface adhesion receptors, the cytoskeleton, and the nucleus. Specifically, integrins and cadherins play a critical role in sensing both the external mechanical force and the force of cell transmission. Actin and microtubules, which are two components of the cytoskeletal network, act as a bridge in mechanical conduction. The nucleus can also be mechanically stressed by the surrounding cytoskeleton through the contraction of the matrix. The nuclear envelope also plays important roles in sensing mechanical signals and in adjusting the morphology and function of the nucleus. We summarize the major nanoparticle-based tools used in the laboratory for the study of cell mechanics, which includes traction force microscopy, atomic force microscopy, optical tweezers, magnetic manipulation, micropillars, and force-induced remnant magnetization spectroscopy. In addition, we discuss the effects that nanoparticles have on cell mechanics. Nanoparticles interact with the adhesion of molecules on the cell membrane surface and on cell cytoskeletal proteins, which further affects the mechanical properties involved in cell stiffness, cell adhesion, and cell migration. Overall, the general conclusions regarding the effects of nanoparticles on cell mechanics are as follows: (1) Nanoparticles can affect cell adhesion by disrupting tight and adherent junctions, and by regulating cell-extracellular matrix adhesion; (2) Nanoparticles can interact with cytoskeletal proteins (actins and tubulins) leading to structural reorganization or disruption of microtubules and F-actin; (3) Cell stiffness changes with the structural reorganization of the cytoskeleton; (4) Cell migration ability can be affected through changes in the cytoskeleton, cell adhesion, and the expression of cell migration-related proteins/molecules. To develop the nano-biosafety evaluation system, future studies should attempt to gain a better understanding of the molecular mechanisms involved with regards to nanoparticles and cell mechanics. Ultimately, further development of new methods and technologies based on nano-mechanical chemistry for diagnosis and treatment purposes are expected, given the wide application of nanomaterials in the biomedical field.
Recent Advances in Polyoxometalates for Applications in Electrocatalytic Hydrogen Evolution Reaction
2020, 36(1): 190606
doi: 10.3866/PKU.WHXB201906063
Abstract:
Hydrogen (H2), a clean and sustainable energy carrier, is regarded as one of the most promising alternatives to carbon-based fuels. Hydrogen can be generated in a more sustainable way from renewable energy sources via electrocatalytic water splitting. However, the high cost and low abundance of the benchmarking platinum-based hydrogen evolution reaction (HER) catalysts hinder their widespread applications. Thus, developing highly efficient, stable, and low-cost electrocatalysts to replace platinum for HER is imperative, but remains a challenging task. Recently, efforts have been devoted to developing non-noble HER electrocatalysts, including transition metal carbides, oxides, phosphides, and sulfides. However, traditional synthetic strategies cannot effectively control active sites and the catalysts tend to aggregate under high temperature. Recently, polyoxometalates (POMs) have been applied as precursors for the preparation of non-noble HER electrocatalysts as they contain discrete metal-oxygen clusters with well-defined structures. POMs are typically composed of oxygen ligands and high-valent metal ions such as Ⅴ(Ⅴ), Mo(Ⅵ), and W(Ⅵ), which can serve as Ⅴ, Mo, and W sources to produce the corresponding metal carbides, oxides, phosphides, and sulfides by pyrolysis at high temperature. Some POMs may also contain a series of redox-active heteroatoms, which are usually named hetero-polyoxometalates. These can serve as precursors to electrocatalysts with uniform heteroatom doping. Moreover, direct applications of POMs as molecular catalysts in HER have, in recent years, received rapidly growing attention. This is because POMs not only serve as mediators or molecular catalysts to facilitate the HER, but can also be deposited on the electrode surface to catalyze the HER. However, the interpretation that HER catalytic activity enhancement is due to the intrinsic catalytic properties of the electrodeposited polyoxometalate or the deposition of small amounts of platinum has been highly debated. Reviewing these studies may help us understand the intrinsic active sites as well the intrinsic HER mechanism of POMs and POMs-derived catalysts, and thus design more efficient HER catalysts. This review, therefore, focuses on recent progress in the applications of POMs and their derivatives in electrocatalytic HER. Firstly, basic HER mechanisms for common metal catalysts and POMs molecular catalysts are discussed along with challenges in the field of HER. Next, applications of POMs molecular catalysts and POMs-derived catalysts in HER are summarized. Finally, some perspectives of POMs-based catalysts/pre-catalysts for electrocatalytic HER are proposed. ![]()
Hydrogen (H2), a clean and sustainable energy carrier, is regarded as one of the most promising alternatives to carbon-based fuels. Hydrogen can be generated in a more sustainable way from renewable energy sources via electrocatalytic water splitting. However, the high cost and low abundance of the benchmarking platinum-based hydrogen evolution reaction (HER) catalysts hinder their widespread applications. Thus, developing highly efficient, stable, and low-cost electrocatalysts to replace platinum for HER is imperative, but remains a challenging task. Recently, efforts have been devoted to developing non-noble HER electrocatalysts, including transition metal carbides, oxides, phosphides, and sulfides. However, traditional synthetic strategies cannot effectively control active sites and the catalysts tend to aggregate under high temperature. Recently, polyoxometalates (POMs) have been applied as precursors for the preparation of non-noble HER electrocatalysts as they contain discrete metal-oxygen clusters with well-defined structures. POMs are typically composed of oxygen ligands and high-valent metal ions such as Ⅴ(Ⅴ), Mo(Ⅵ), and W(Ⅵ), which can serve as Ⅴ, Mo, and W sources to produce the corresponding metal carbides, oxides, phosphides, and sulfides by pyrolysis at high temperature. Some POMs may also contain a series of redox-active heteroatoms, which are usually named hetero-polyoxometalates. These can serve as precursors to electrocatalysts with uniform heteroatom doping. Moreover, direct applications of POMs as molecular catalysts in HER have, in recent years, received rapidly growing attention. This is because POMs not only serve as mediators or molecular catalysts to facilitate the HER, but can also be deposited on the electrode surface to catalyze the HER. However, the interpretation that HER catalytic activity enhancement is due to the intrinsic catalytic properties of the electrodeposited polyoxometalate or the deposition of small amounts of platinum has been highly debated. Reviewing these studies may help us understand the intrinsic active sites as well the intrinsic HER mechanism of POMs and POMs-derived catalysts, and thus design more efficient HER catalysts. This review, therefore, focuses on recent progress in the applications of POMs and their derivatives in electrocatalytic HER. Firstly, basic HER mechanisms for common metal catalysts and POMs molecular catalysts are discussed along with challenges in the field of HER. Next, applications of POMs molecular catalysts and POMs-derived catalysts in HER are summarized. Finally, some perspectives of POMs-based catalysts/pre-catalysts for electrocatalytic HER are proposed.
2020, 36(1): 190608
doi: 10.3866/PKU.WHXB201906087
Abstract:
Iron carbides, especially Hägg carbide (χ-Fe5C2), have become a topic of significant research interest due to their potential application in various fields over the past decades. For Fischer-Tr psch (F-T) synthesis, χ-Fe5C2 has been confirmed as an active phase. In addition, this well-known catalytic material is a candidate for potential application in electrochemistry, magnetic imaging, and various therapies. The physical chemistry, including structure, stability, and catalytic properties of χ-Fe5C2 has been studied since its discovery. The C2/c crystal structure of Hägg carbide was initially resolved in the 1960s. Because various iron oxides and carbides always co-exist in the synthesized χ-Fe5C2 samples, the structure model still faces challenges. The crystal structure is being revised with high-purity samples using modern characterization techniques and theoretical methods. However, it is very difficult to obtain the pure phase of χ-Fe5C2 via traditional preparation methods owing to the metastable phase of χ-Fe5C2. Hence, tremendous efforts have been devoted to the synthesis of χ-Fe5C2. Recently, some processes to prepare single-phase and structure-controlled χ-Fe5C2 nanostructures have been reported. Many iron and carbon precursors can be used to prepare Hägg carbide. Carburization in solid-solid, solid-gas, and solid-liquid phases can be adopted to synthesize χ-Fe5C2 of various sizes and morphologies. The success of synthetic chemistry has provided novel insights into the mechanism of phase transformation in χ-Fe5C2. More details regarding the formation of the χ-Fe5C2 structure in the solid-gas and solid-liquid phases have been revealed via in situ characterization methods. The formation and crystallization of an Fe-C amorphous composite is likely the key step. The application of χ-Fe5C2 in catalysis has also benefited from novel synthesis strategies. With the development of these preparation methods, tuning the activity and selectivity of χ-Fe5C2 has become possible. A heterostructure of small Co/χ-Fe5C2 with low cobalt loading showed an unexpectedly high CO conversion rate at low temperature. Beyond classical F-T synthesis, χ-Fe5C2 is a promising catalyst for the production of light olefins, long chain α-olefins, aromatics, and alcohol synthesis by modification with other elements. Combining density functional theory (DFT) calculations and kinetic analysis, the roles of promoters and interaction with χ-Fe5C2 have been evaluated to some extent. Herein, the recent progress in the synthesis, structural analysis, formation mechanisms, and catalytic performance of χ-Fe5C2 is summarized. A collection of synthesis methods is presented, and novel methods for regulating catalytic properties are reviewed. We believe that advanced synthesis methods are key to a deeper understanding and better utilization of this material. ![]()
Iron carbides, especially Hägg carbide (χ-Fe5C2), have become a topic of significant research interest due to their potential application in various fields over the past decades. For Fischer-Tr psch (F-T) synthesis, χ-Fe5C2 has been confirmed as an active phase. In addition, this well-known catalytic material is a candidate for potential application in electrochemistry, magnetic imaging, and various therapies. The physical chemistry, including structure, stability, and catalytic properties of χ-Fe5C2 has been studied since its discovery. The C2/c crystal structure of Hägg carbide was initially resolved in the 1960s. Because various iron oxides and carbides always co-exist in the synthesized χ-Fe5C2 samples, the structure model still faces challenges. The crystal structure is being revised with high-purity samples using modern characterization techniques and theoretical methods. However, it is very difficult to obtain the pure phase of χ-Fe5C2 via traditional preparation methods owing to the metastable phase of χ-Fe5C2. Hence, tremendous efforts have been devoted to the synthesis of χ-Fe5C2. Recently, some processes to prepare single-phase and structure-controlled χ-Fe5C2 nanostructures have been reported. Many iron and carbon precursors can be used to prepare Hägg carbide. Carburization in solid-solid, solid-gas, and solid-liquid phases can be adopted to synthesize χ-Fe5C2 of various sizes and morphologies. The success of synthetic chemistry has provided novel insights into the mechanism of phase transformation in χ-Fe5C2. More details regarding the formation of the χ-Fe5C2 structure in the solid-gas and solid-liquid phases have been revealed via in situ characterization methods. The formation and crystallization of an Fe-C amorphous composite is likely the key step. The application of χ-Fe5C2 in catalysis has also benefited from novel synthesis strategies. With the development of these preparation methods, tuning the activity and selectivity of χ-Fe5C2 has become possible. A heterostructure of small Co/χ-Fe5C2 with low cobalt loading showed an unexpectedly high CO conversion rate at low temperature. Beyond classical F-T synthesis, χ-Fe5C2 is a promising catalyst for the production of light olefins, long chain α-olefins, aromatics, and alcohol synthesis by modification with other elements. Combining density functional theory (DFT) calculations and kinetic analysis, the roles of promoters and interaction with χ-Fe5C2 have been evaluated to some extent. Herein, the recent progress in the synthesis, structural analysis, formation mechanisms, and catalytic performance of χ-Fe5C2 is summarized. A collection of synthesis methods is presented, and novel methods for regulating catalytic properties are reviewed. We believe that advanced synthesis methods are key to a deeper understanding and better utilization of this material.
2020, 36(1): 190700
doi: 10.3866/PKU.WHXB201907002
Abstract:
The protein folding problem is regarded as the second genetic code which has yet to be deciphered. To date, Anfinsen's thermodynamic hypothesis, i.e., the native structure of a protein is its most stable state, is the only generally accepted theory for protein folding, although exceptions have been reported. However, this hypothesis is a simple overall statement, with no information regarding where or how a protein is folded. The mechanism underlying protein folding has not yet been elucidated, and it is still not clear how the overall sequence (context) determines the structure of a protein. Based on our recent study, we propose a "Confined Lowest Energy Structure Fragments" (CLESFs) hypothesis. This hypothesis states that proteins are CLESFs joined together by a small number of strong constraints (key long-range interactions). Although the native structure of a protein contains various long-range interactions between amino acids that are far apart in the sequence, only a few strong interactions, such as disulfide bonds, hydrophobic packing, structural ion coordination as in zinc fingers, and hydrogen-bonding networks within beta-sheets, are critical. These key long-range interactions serve as a form of punctuation in the "language" of protein sequence and divide the protein sequence into different "sentences, " i.e., fragments (CLESFs). The local native structures of these CLESFs are the lowest energy structures under the confinements of those key long-range interactions, but the overall protein structure is not necessarily the global minimum as Anfinsen hypothesized. The same fragment may adopt different native structures in different proteins. Each native structure of the same fragment in a different protein is a local minimum for the free fragment and the "global minimum" for the fragment under the specific confinement in the specific protein. Essentially, the native local structures of the CLESFs have an enthalpic advantage (local minimum) which serves as a driving force to form the key long-range interactions; the key long-range interactions stabilize the native local structures with entropy effects by excluding enormous amount of random conformations possible for the fragments. Our CLESFs hypothesis suggests that the protein folding code is not as mysterious as previously thought. Only a few critical long-range interactions have principal influence on the local structures of protein fragments. This is why protein fragments can be grafted onto different proteins, and even more notably, can be grafted onto gold nanoparticles to form a "goldbody". Given that short peptides are generally flexible, and flexible peptides are usually unstable and inactive, it is still a mystery how proteins, i.e., peptides that are long enough to fold into unique structures, evolved in the first place. The CLESFs hypothesis implies that prior to the appearance of the first protein that was long enough to fold into a unique stable structure, there might have been a "Stone Age" during prebiotic protein evolution. At that time, short peptides that could not fold by themselves might have been able to adopt active conformations with a few strong anchors to the surface of "stones", such as rocks, solid particles, or vesicles in the primitive soup, forming CLESFs and gaining an evolutionary advantage against degradation. Later, multiple CLESFs on the same "stone" might have assembled in certain ways to perform more complicated functions, and finally, the first protein might have emerged when individual CLESFs joined together and left the "stone". ![]()
The protein folding problem is regarded as the second genetic code which has yet to be deciphered. To date, Anfinsen's thermodynamic hypothesis, i.e., the native structure of a protein is its most stable state, is the only generally accepted theory for protein folding, although exceptions have been reported. However, this hypothesis is a simple overall statement, with no information regarding where or how a protein is folded. The mechanism underlying protein folding has not yet been elucidated, and it is still not clear how the overall sequence (context) determines the structure of a protein. Based on our recent study, we propose a "Confined Lowest Energy Structure Fragments" (CLESFs) hypothesis. This hypothesis states that proteins are CLESFs joined together by a small number of strong constraints (key long-range interactions). Although the native structure of a protein contains various long-range interactions between amino acids that are far apart in the sequence, only a few strong interactions, such as disulfide bonds, hydrophobic packing, structural ion coordination as in zinc fingers, and hydrogen-bonding networks within beta-sheets, are critical. These key long-range interactions serve as a form of punctuation in the "language" of protein sequence and divide the protein sequence into different "sentences, " i.e., fragments (CLESFs). The local native structures of these CLESFs are the lowest energy structures under the confinements of those key long-range interactions, but the overall protein structure is not necessarily the global minimum as Anfinsen hypothesized. The same fragment may adopt different native structures in different proteins. Each native structure of the same fragment in a different protein is a local minimum for the free fragment and the "global minimum" for the fragment under the specific confinement in the specific protein. Essentially, the native local structures of the CLESFs have an enthalpic advantage (local minimum) which serves as a driving force to form the key long-range interactions; the key long-range interactions stabilize the native local structures with entropy effects by excluding enormous amount of random conformations possible for the fragments. Our CLESFs hypothesis suggests that the protein folding code is not as mysterious as previously thought. Only a few critical long-range interactions have principal influence on the local structures of protein fragments. This is why protein fragments can be grafted onto different proteins, and even more notably, can be grafted onto gold nanoparticles to form a "goldbody". Given that short peptides are generally flexible, and flexible peptides are usually unstable and inactive, it is still a mystery how proteins, i.e., peptides that are long enough to fold into unique structures, evolved in the first place. The CLESFs hypothesis implies that prior to the appearance of the first protein that was long enough to fold into a unique stable structure, there might have been a "Stone Age" during prebiotic protein evolution. At that time, short peptides that could not fold by themselves might have been able to adopt active conformations with a few strong anchors to the surface of "stones", such as rocks, solid particles, or vesicles in the primitive soup, forming CLESFs and gaining an evolutionary advantage against degradation. Later, multiple CLESFs on the same "stone" might have assembled in certain ways to perform more complicated functions, and finally, the first protein might have emerged when individual CLESFs joined together and left the "stone".
2020, 36(1): 190700
doi: 10.3866/PKU.WHXB201907004
Abstract:
Group-Ⅲ nitride (Ⅲ-N) films have numerous applications in LEDs, lasers, and high-power/high-frequency electronic devices because of their direct wide band gap, high breakdown voltage, high saturation velocity of electrons, and high stability. Commercial Ⅲ-N films are usually heteroepitaxially grown on c-sapphire substrate by metal-organic chemical vapor deposition (MOCVD). However, relatively large mismatches occur in the in-plane lattice and thermal expansion between the Ⅲ-N films and sapphire substrates, which lead to high stress and high dislocation density in epilayers that reduce the performance of the LED. Moreover, the poor thermal conductivity of sapphire substrate also hinders many applications. Recently, graphene was used as a buffer layer to overcome the mismatch between Ⅲ-N films and substrates by utilizing van der Waals epitaxy and improving heat dissipation. In this review article, we consider the recent progress in the development of a new type of epitaxial substrate, the so-called "graphene/sapphire substrate" for Ⅲ-N film growth and LED applications. The growth mechanisms are summarized and future prospects are proposed. The article is divided into three parts.1. The synthesis of graphene/sapphire substrate. High-quality monolayer graphene is directly synthesized on sapphire substrates (flat substrate and nanopatterned substrate) by metal-catalyst-free CVD method. The method does not depend on the metal catalyst nor involve a complex and highly technical transfer process, and is compatible with the MOCVD and molecular beam epitaxy process.2. Growth of high-quality Ⅲ-N films on graphene/sapphire substrates. The nucleation of Ⅲ-N on graphene can be tuned by the density of defects in the graphene film. N2 plasma treatment of the graphene/sapphire substrate can increase the nucleation sites for Ⅲ-N growth by introducing pyrrolic nitrogen doping. Epitaxial lateral overgrowth of the Ⅲ-N is promoted on the graphene/sapphire substrate owing to the relatively lower diffusion barrier of atoms on graphene. Consequently, the biaxial stress in group-Ⅲ nitride is significantly decreased while the dislocation density is reduced even without a low-temperature buffer layer. Moreover, vertically-oriented graphene nanowalls can effectively improve the heat dissipation in AlN films.3. High-performance LEDs on graphene/sapphire substrate. High-quality Ⅲ-N films obtained on graphene/sapphire substrates enable LED fabrication. The as-fabricated LEDs on graphene/sapphire substrate deliver much higher light output power compared with that on bare sapphire substrate. The as-fabricated LEDs have low turn-on voltage, high output power, and good reliability. Graphene can also be utilized as transfer medium or transparent conductive electrode to boost LED performance. ![]()
Group-Ⅲ nitride (Ⅲ-N) films have numerous applications in LEDs, lasers, and high-power/high-frequency electronic devices because of their direct wide band gap, high breakdown voltage, high saturation velocity of electrons, and high stability. Commercial Ⅲ-N films are usually heteroepitaxially grown on c-sapphire substrate by metal-organic chemical vapor deposition (MOCVD). However, relatively large mismatches occur in the in-plane lattice and thermal expansion between the Ⅲ-N films and sapphire substrates, which lead to high stress and high dislocation density in epilayers that reduce the performance of the LED. Moreover, the poor thermal conductivity of sapphire substrate also hinders many applications. Recently, graphene was used as a buffer layer to overcome the mismatch between Ⅲ-N films and substrates by utilizing van der Waals epitaxy and improving heat dissipation. In this review article, we consider the recent progress in the development of a new type of epitaxial substrate, the so-called "graphene/sapphire substrate" for Ⅲ-N film growth and LED applications. The growth mechanisms are summarized and future prospects are proposed. The article is divided into three parts.1. The synthesis of graphene/sapphire substrate. High-quality monolayer graphene is directly synthesized on sapphire substrates (flat substrate and nanopatterned substrate) by metal-catalyst-free CVD method. The method does not depend on the metal catalyst nor involve a complex and highly technical transfer process, and is compatible with the MOCVD and molecular beam epitaxy process.2. Growth of high-quality Ⅲ-N films on graphene/sapphire substrates. The nucleation of Ⅲ-N on graphene can be tuned by the density of defects in the graphene film. N2 plasma treatment of the graphene/sapphire substrate can increase the nucleation sites for Ⅲ-N growth by introducing pyrrolic nitrogen doping. Epitaxial lateral overgrowth of the Ⅲ-N is promoted on the graphene/sapphire substrate owing to the relatively lower diffusion barrier of atoms on graphene. Consequently, the biaxial stress in group-Ⅲ nitride is significantly decreased while the dislocation density is reduced even without a low-temperature buffer layer. Moreover, vertically-oriented graphene nanowalls can effectively improve the heat dissipation in AlN films.3. High-performance LEDs on graphene/sapphire substrate. High-quality Ⅲ-N films obtained on graphene/sapphire substrates enable LED fabrication. The as-fabricated LEDs on graphene/sapphire substrate deliver much higher light output power compared with that on bare sapphire substrate. The as-fabricated LEDs have low turn-on voltage, high output power, and good reliability. Graphene can also be utilized as transfer medium or transparent conductive electrode to boost LED performance.
2020, 36(1): 190700
doi: 10.3866/PKU.WHXB201907009
Abstract:
In the 1960s, Maiman constructed the first laser. Pulsed lasers with high repetition rates and short pulse widths have extensive applications in fiber optics, military applications, spectroscopy, laser ranging, materials processing, medicine, and frequency conversion, etc. For instance, short pulse lasers with high repetition rates are desirable for material processing, in which the processing speed depends upon the repetition rate of the laser source. Electro-optic Q-switching has numerous advantages in many fields because of its better hold-off ability, larger pulse energy, and more controllable repetition rates. In 1961, Franken et al. first applied a ruby laser directly to quartz crystals and observed double-frequency radiation. Afterward, Bloembergen et al. analyzed the principle of nonlinear optical parametric generation theoretically. Since then, nonlinear optics has been playing an increasingly vital role in human society. Mid-infrared (mid-IR) lasers using nonlinear optical (NLO) crystals have essential applications in science as well as in daily life (e.g., infrared remote sensing, biological tissue imaging, environmental monitoring, and minimally invasive medical surgery). For generating mid-IR lasers in the spectral range of 3–20 µm, NLO materials are indispensable for optical parametric oscillation (OPO) or difference frequency generation. It is common for the available wavelength range to be limited by multiphonon absorption in the oxide crystal, and the damage threshold for semiconductors is relatively low. At present, the most widely used NLO crystal materials in the mid-IR band are semiconductor crystals represented by ZnGeP2. However, their laser damage thresholds are low, which limits their application range. Therefore, one of the key issues in the field of NLO materials at present is to explore new mid-IR NLO crystal materials with excellent performance that are applicable to high-power lasers. Langasite materials are famous for their multifunctionality in optoelectronic applications, such as in piezoelectric convertors, electro-optic Q-switched laser generation, and surface acoustic wave devices. Their structure without central symmetry endows the crystal with electro-optic, piezoelectric, and NLO properties, and their laser damage threshold is high because they are oxides. The phonon energy of the crystal is low and the transmission range is wide owing to their composition, which may have important applications for mid-IR high-power lasers. The Langasite family comprise a set of perfect electro-optical crystals with an electro-optical coefficient of 2.3 × 10−12 m∙V−1, a broad transmission spectral range, and a high optical damage threshold of 950 MW∙cm−2. Besides, their small piezoelectric coefficient (6 × 10−12 C∙N−1) reveals the possibility for Q-switching under high repetition rates without a piezoelectric ring effect. In this brief review, three important compounds—La3Ga5SiO14, La3Ga5.5Nb0.5O14, and La3Ga5.5Ta0.5O14—are investigated and analyzed based on available experimental data. The electro-optical Q-switch and mid-IR OPO applications are summarized in detail. Finally, promising search directions for new metal oxides that have good mid-IR NLO performances are discussed. ![]()
In the 1960s, Maiman constructed the first laser. Pulsed lasers with high repetition rates and short pulse widths have extensive applications in fiber optics, military applications, spectroscopy, laser ranging, materials processing, medicine, and frequency conversion, etc. For instance, short pulse lasers with high repetition rates are desirable for material processing, in which the processing speed depends upon the repetition rate of the laser source. Electro-optic Q-switching has numerous advantages in many fields because of its better hold-off ability, larger pulse energy, and more controllable repetition rates. In 1961, Franken et al. first applied a ruby laser directly to quartz crystals and observed double-frequency radiation. Afterward, Bloembergen et al. analyzed the principle of nonlinear optical parametric generation theoretically. Since then, nonlinear optics has been playing an increasingly vital role in human society. Mid-infrared (mid-IR) lasers using nonlinear optical (NLO) crystals have essential applications in science as well as in daily life (e.g., infrared remote sensing, biological tissue imaging, environmental monitoring, and minimally invasive medical surgery). For generating mid-IR lasers in the spectral range of 3–20 µm, NLO materials are indispensable for optical parametric oscillation (OPO) or difference frequency generation. It is common for the available wavelength range to be limited by multiphonon absorption in the oxide crystal, and the damage threshold for semiconductors is relatively low. At present, the most widely used NLO crystal materials in the mid-IR band are semiconductor crystals represented by ZnGeP2. However, their laser damage thresholds are low, which limits their application range. Therefore, one of the key issues in the field of NLO materials at present is to explore new mid-IR NLO crystal materials with excellent performance that are applicable to high-power lasers. Langasite materials are famous for their multifunctionality in optoelectronic applications, such as in piezoelectric convertors, electro-optic Q-switched laser generation, and surface acoustic wave devices. Their structure without central symmetry endows the crystal with electro-optic, piezoelectric, and NLO properties, and their laser damage threshold is high because they are oxides. The phonon energy of the crystal is low and the transmission range is wide owing to their composition, which may have important applications for mid-IR high-power lasers. The Langasite family comprise a set of perfect electro-optical crystals with an electro-optical coefficient of 2.3 × 10−12 m∙V−1, a broad transmission spectral range, and a high optical damage threshold of 950 MW∙cm−2. Besides, their small piezoelectric coefficient (6 × 10−12 C∙N−1) reveals the possibility for Q-switching under high repetition rates without a piezoelectric ring effect. In this brief review, three important compounds—La3Ga5SiO14, La3Ga5.5Nb0.5O14, and La3Ga5.5Ta0.5O14—are investigated and analyzed based on available experimental data. The electro-optical Q-switch and mid-IR OPO applications are summarized in detail. Finally, promising search directions for new metal oxides that have good mid-IR NLO performances are discussed.
2020, 36(1): 190701
doi: 10.3866/PKU.WHXB201907010
Abstract:
The high-order chromatin structure plays a non-negligible role in gene regulation. The formation of chromatin structure and its regulatory mechanisms have been studied intensely. To analyze the high-order chromatin structures, both computational and physical models have been developed, including polymer physics models and molecular crowding models. Over the past few years, the phase separation theory has drawn a lot of research interest, and the effect of heterochromatin and transcriptional factors (TFs) on phase separation has attracted much attention. Existing phase separation models for chromatin focus on multivalent molecules or on epigenetic properties and does not adequately explore the dependence of chromatin structure organization and remodeling on DNA sequence. Genomes of a number of species are highly uneven at multiple scales. It can be divided purely based on sequential properties into two sequentially, epigenetically, and transcriptionally distinct regions, namely forest and prairie domains, demonstrating the intrinsic mosaicity in genome. Compared to prairies, forest domains are on average more gene-rich, accessible, transcriptionally active, higher in open-sea methylation level, and are enriched in RNA polymerase Ⅱ binding sites as well as active histone modifications. Moreover, different structural properties of these two types of sequential domains suggest that sequence may play a role in topologically associated domain (TAD) and compartment formation. The chromatin sequence-structural relationship and functional regulation in different cell types with almost identical sequences are discussed in this review. We try to describe the evolution of chromatin structure in multiple biological processes including early development, differentiation, and senescence in a unified framework. The forest and prairie domains with high and low CGI densities, respectively, show enhanced segregation from each other in development, differentiation, and senescence. Meanwhile the multiscale forest-prairie spatial intermingling is cell-type specific and increases upon differentiation, thereby helping to define cell identity. The consistency between chromatin structure and open-sea methylation level suggests that the latter is a promising indicator of structural segregation, deepening our understanding of epigenetic-structure relation. We further discuss the physical driving forces of phase separation as well as their biological implications. The phase separation of the uneven 1D sequence in 3D space serves as a potential driving force, and together with cell type specific epigenetic marks and transcription factors shapes the chromatin structure in different cell types. Transcriptional complex along with dynamic TFs and epigenetic marks may account for local structure formation and separation, regulating chromatin structure at a smaller spatial-temporal scale based on their sequential environment. Finally, role of physical factors like temperature and sequence unevenness in affecting chromatin structure have also been discussed. ![]()
The high-order chromatin structure plays a non-negligible role in gene regulation. The formation of chromatin structure and its regulatory mechanisms have been studied intensely. To analyze the high-order chromatin structures, both computational and physical models have been developed, including polymer physics models and molecular crowding models. Over the past few years, the phase separation theory has drawn a lot of research interest, and the effect of heterochromatin and transcriptional factors (TFs) on phase separation has attracted much attention. Existing phase separation models for chromatin focus on multivalent molecules or on epigenetic properties and does not adequately explore the dependence of chromatin structure organization and remodeling on DNA sequence. Genomes of a number of species are highly uneven at multiple scales. It can be divided purely based on sequential properties into two sequentially, epigenetically, and transcriptionally distinct regions, namely forest and prairie domains, demonstrating the intrinsic mosaicity in genome. Compared to prairies, forest domains are on average more gene-rich, accessible, transcriptionally active, higher in open-sea methylation level, and are enriched in RNA polymerase Ⅱ binding sites as well as active histone modifications. Moreover, different structural properties of these two types of sequential domains suggest that sequence may play a role in topologically associated domain (TAD) and compartment formation. The chromatin sequence-structural relationship and functional regulation in different cell types with almost identical sequences are discussed in this review. We try to describe the evolution of chromatin structure in multiple biological processes including early development, differentiation, and senescence in a unified framework. The forest and prairie domains with high and low CGI densities, respectively, show enhanced segregation from each other in development, differentiation, and senescence. Meanwhile the multiscale forest-prairie spatial intermingling is cell-type specific and increases upon differentiation, thereby helping to define cell identity. The consistency between chromatin structure and open-sea methylation level suggests that the latter is a promising indicator of structural segregation, deepening our understanding of epigenetic-structure relation. We further discuss the physical driving forces of phase separation as well as their biological implications. The phase separation of the uneven 1D sequence in 3D space serves as a potential driving force, and together with cell type specific epigenetic marks and transcription factors shapes the chromatin structure in different cell types. Transcriptional complex along with dynamic TFs and epigenetic marks may account for local structure formation and separation, regulating chromatin structure at a smaller spatial-temporal scale based on their sequential environment. Finally, role of physical factors like temperature and sequence unevenness in affecting chromatin structure have also been discussed.
2020, 36(1): 190702
doi: 10.3866/PKU.WHXB201907021
Abstract:
high tensile strength, mobility, and thermal conductivity as well as clean surface. Hence, CNTs have been widely investigated for many potential applications, for example, as additives in composites and main components of integrated circuits. However, the former application widely used does not exploit their intrinsic properties, while the latter has only been demonstrated at the level of laboratory prototype devices. As the main factor determining future applications of CNTs is the ability to achieve their structure-controlled synthesis, this review first introduces a classification of CNT structures highlighting the potential difficulties associated with fine CNT structure control due to the similarities between different CNTs. Then, advances in the basic research and industrialization of CNTs in the past decades are summarized, including fine structure control, aggregation synthesis, and scale-up production. Catalysts are crucial for controlling the structure of CNTs, as their lifetime determines the CNT length and size (wall number and diameter), while their state and formation affects CNT chirality. Moreover, as the microscopic properties of individual CNTs often differ from their macroscale performance at industrial-scale production, their aggregation state should be carefully taken into consideration. Therefore, several methods were developed to realize different types of aggregates, such as lattice orientation for obtaining horizontally aligned CNT arrays, the use of catalysts with high density for the synthesis of vertical CNT arrays, direct deposition of CNT films, and even fabrication of very complex three-dimensional (3D) macrostructures. Furthermore, many efforts have been invested to promote CNT industrialization and develop various techniques to increase CNT production, including the fluidized bed method and floating method. Finally, the ideal synthesis of CNTs should combine structure control with scale-up preparation. To this aim, further theoretical understanding of the detailed CNT growth mechanism is still needed to clarify, for example, how CNT caps form at the atomic scale, which is the close matching relationship between CNTs and catalysts, and how the growth model affects the chirality preference of single-walled carbon nanotubes (SWNTs). Experimentally, different methods to grow SWNTs with a uniform structure should be further developed, focusing on catalyst design to increase temperature tolerance and achieve epitaxial growth of SWNT segments. On the other hand, the large-scale synthesis of SWNTs should also be reconsidered, for instance, by improving the growth equipment. In order to identify suitable applications for different CNT products, standards should be established and adopted. In addition to improving CNT synthesis, the driving force of the CNT industry in the future will be finding disruptive applications of CNTs, whose functions and contributions are irreplaceable. In conclusion, still much progress is needed to achieve the complete commercialization of CNTs in the future. Nevertheless, the rapid development and continuous attention given to this field may lead to growth opportunities in the CNT industry. ![]()
high tensile strength, mobility, and thermal conductivity as well as clean surface. Hence, CNTs have been widely investigated for many potential applications, for example, as additives in composites and main components of integrated circuits. However, the former application widely used does not exploit their intrinsic properties, while the latter has only been demonstrated at the level of laboratory prototype devices. As the main factor determining future applications of CNTs is the ability to achieve their structure-controlled synthesis, this review first introduces a classification of CNT structures highlighting the potential difficulties associated with fine CNT structure control due to the similarities between different CNTs. Then, advances in the basic research and industrialization of CNTs in the past decades are summarized, including fine structure control, aggregation synthesis, and scale-up production. Catalysts are crucial for controlling the structure of CNTs, as their lifetime determines the CNT length and size (wall number and diameter), while their state and formation affects CNT chirality. Moreover, as the microscopic properties of individual CNTs often differ from their macroscale performance at industrial-scale production, their aggregation state should be carefully taken into consideration. Therefore, several methods were developed to realize different types of aggregates, such as lattice orientation for obtaining horizontally aligned CNT arrays, the use of catalysts with high density for the synthesis of vertical CNT arrays, direct deposition of CNT films, and even fabrication of very complex three-dimensional (3D) macrostructures. Furthermore, many efforts have been invested to promote CNT industrialization and develop various techniques to increase CNT production, including the fluidized bed method and floating method. Finally, the ideal synthesis of CNTs should combine structure control with scale-up preparation. To this aim, further theoretical understanding of the detailed CNT growth mechanism is still needed to clarify, for example, how CNT caps form at the atomic scale, which is the close matching relationship between CNTs and catalysts, and how the growth model affects the chirality preference of single-walled carbon nanotubes (SWNTs). Experimentally, different methods to grow SWNTs with a uniform structure should be further developed, focusing on catalyst design to increase temperature tolerance and achieve epitaxial growth of SWNT segments. On the other hand, the large-scale synthesis of SWNTs should also be reconsidered, for instance, by improving the growth equipment. In order to identify suitable applications for different CNT products, standards should be established and adopted. In addition to improving CNT synthesis, the driving force of the CNT industry in the future will be finding disruptive applications of CNTs, whose functions and contributions are irreplaceable. In conclusion, still much progress is needed to achieve the complete commercialization of CNTs in the future. Nevertheless, the rapid development and continuous attention given to this field may lead to growth opportunities in the CNT industry.
2020, 36(1): 190705
doi: 10.3866/PKU.WHXB201907052
Abstract:
Inorganic, organic, and biological materials have specific natural properties which mostly depend on their atomic structures. The properties of novel materials can be predicted based solely on knowing the structure fully. Thus, structure determination plays a very important role in chemistry, physics, and materials science. X-ray crystallography, including single-crystal X-ray diffraction (SCXRD) and powder X-ray diffraction (PXRD), remains an important technique for studying structures. However, SCXRD can only be applied to high-quality large single crystals without disorders/defects, whereas PXRD provides only one-dimensional information and reflections with the similar d-values will overlap, which makes it difficult to determine the unit-cell parameters, space groups, and accurate intensities. Another important technique for structural determination is electron crystallography (EC). As the electron is the probe, EC alone can be used for those crystals which are too small to be studied by SCXRD or too complex to be studied by PXRD. Electrons interact much more strongly with matter than X-rays; therefore, both electron diffractions (ED) patterns and high-resolution transmission electron microscopy (HRTEM) images can be obtained from nano-sized crystals. Although electron crystallography started later than X-ray crystallography, it has become a very important technique for structural analysis after several decades of development. Especially, three dimensional (3D) ED techniques have been developed, automated electron diffraction tomography (ADT) and rotation electron diffraction (RED), which allow for automated data collection without requiring considerable expertise on the operation of electron microscopes. In addition, the intensities of 3D ED data can be extracted and used for structure determination using specialized software developed for SCXRD. However, the strong interactions between electrons and materials also result in dynamic effects and beam damage. Although the dynamic effects in 3D electron diffraction techniques (ADT and RED) can be significantly reduced, some structures still pose problems for obtaining an initial model due to beam damage. Therefore, EC and X-ray crystallography have significant limitations. For many complicated crystals, a single technique is insufficient to solve the crystal structure and different techniques that supply complementary structural information must be used to obtain a complete structural determination. Herein, the application of X-ray crystallography combined with EC for the analysis of complex inorganic crystal structures will be introduced, covering issues associated with peak overlap, impurities, pseudo-symmetry and twinning, disordered frameworks, location guests, and aperiodic structures. ![]()
Inorganic, organic, and biological materials have specific natural properties which mostly depend on their atomic structures. The properties of novel materials can be predicted based solely on knowing the structure fully. Thus, structure determination plays a very important role in chemistry, physics, and materials science. X-ray crystallography, including single-crystal X-ray diffraction (SCXRD) and powder X-ray diffraction (PXRD), remains an important technique for studying structures. However, SCXRD can only be applied to high-quality large single crystals without disorders/defects, whereas PXRD provides only one-dimensional information and reflections with the similar d-values will overlap, which makes it difficult to determine the unit-cell parameters, space groups, and accurate intensities. Another important technique for structural determination is electron crystallography (EC). As the electron is the probe, EC alone can be used for those crystals which are too small to be studied by SCXRD or too complex to be studied by PXRD. Electrons interact much more strongly with matter than X-rays; therefore, both electron diffractions (ED) patterns and high-resolution transmission electron microscopy (HRTEM) images can be obtained from nano-sized crystals. Although electron crystallography started later than X-ray crystallography, it has become a very important technique for structural analysis after several decades of development. Especially, three dimensional (3D) ED techniques have been developed, automated electron diffraction tomography (ADT) and rotation electron diffraction (RED), which allow for automated data collection without requiring considerable expertise on the operation of electron microscopes. In addition, the intensities of 3D ED data can be extracted and used for structure determination using specialized software developed for SCXRD. However, the strong interactions between electrons and materials also result in dynamic effects and beam damage. Although the dynamic effects in 3D electron diffraction techniques (ADT and RED) can be significantly reduced, some structures still pose problems for obtaining an initial model due to beam damage. Therefore, EC and X-ray crystallography have significant limitations. For many complicated crystals, a single technique is insufficient to solve the crystal structure and different techniques that supply complementary structural information must be used to obtain a complete structural determination. Herein, the application of X-ray crystallography combined with EC for the analysis of complex inorganic crystal structures will be introduced, covering issues associated with peak overlap, impurities, pseudo-symmetry and twinning, disordered frameworks, location guests, and aperiodic structures.
2020, 36(1): 190705
doi: 10.3866/PKU.WHXB201907053
Abstract:
The discoveries about the functions of biomolecular liquid-liquid phase separation in cell have been increased rapidly in the past decade. Condensates produced by phase separation play key roles in many cellular curial events. These biological functions are based on the physicochemical properties of phase separation. This review discusses the recent progress in understanding the physical and chemical mechanisms of biological liquid-liquid phase separation. (1) We summarized the basic properties and experimental characterization methods of phase separation droplets, including the morphology, fusion, and wetting, along with the dynamic properties of molecules in droplets, which are usually described by diffusion coefficients or viscosity and permeability. (2) We discussed the conditions affecting the liquid-liquid phase separation of biological molecules, including concentration, temperature, ionic strength, pH, and crowding effects. A database for liquid-liquid phase separation, LLPSDB, was introduced, and three types of nucleic acid concentration effects on the phase separation of protein molecules are discussed. These effects depend on the relative interaction strengths of protein-nucleic acid and protein-protein interactions. The major driving force of phase separation is multivalent interactions, and molecular flexibility is necessary for the dynamic properties. We summarized the diverse sources of multivalence, including multiple tandem repetitive domains, regular oligomerization, low-complexity domains (usually intrinsically disordered with repeat motifs for binding), and nucleic acid molecules via the main chain phosphates or repeat sequences. (3) We reviewed the statistical thermodynamics theories for describing the macromolecular liquid-liquid separation, including the Flory-Huggins theory, Overbeek-Voorn correction, random phase approximation method, and field theory simulation method. We discussed the experimental and simulation methods for studying the physiochemical mechanism of liquid-liquid phase separation. Model systems with simplified sequences for experimental studies were listed, including systems for studying the effects of charge properties, residue types, sequence length, and other properties. Molecular simulation methods can provide detailed information regarding the liquid-liquid phase separation process. We introduced two coarse-grain methods, the slab molecular dynamic simulation and Monte Carlo simulation using the lattice model. (4) The physiochemical properties of liquid-liquid phase separation govern the diverse functions of reversible phase transitions in a cell. We collected and analyzed important cases of biomolecular phase separation in cell activities. These biological functions were classified into five categories, including enrichment, sequestration, biological switching cooperation, localization, and mechanical force generation. We linked these functions with the physiochemical properties of liquid-liquid phase separation. To understand the specific phase-separation processes in biological activities, three types of related molecules must be studied: scaffold molecules mainly contributing to aggregate formation, recruited functional client molecules, and molecules that regulate the formation and disassembly of aggregates. We reviewed four regulation methods for the phase separation process, including changing the charge distribution by post-translational modification, changing the molecular concentration by gene expression or degradation regulation, changing the oligomerization state, and changing the cell solution environment (such as pH). Designing compounds for phase separation regulation has attracted significant attention for treating related diseases. Methods for discovering molecules that can regulate post-translational modifications or inhibit interactions in the droplets are emerging. The recently discovered phase separation phenomena and molecules in living organisms represent only the tip of the iceberg. In the future, it will be necessary to systematically examine liquid-liquid phase separation events and related molecules in all phases of biological processes. ![]()
The discoveries about the functions of biomolecular liquid-liquid phase separation in cell have been increased rapidly in the past decade. Condensates produced by phase separation play key roles in many cellular curial events. These biological functions are based on the physicochemical properties of phase separation. This review discusses the recent progress in understanding the physical and chemical mechanisms of biological liquid-liquid phase separation. (1) We summarized the basic properties and experimental characterization methods of phase separation droplets, including the morphology, fusion, and wetting, along with the dynamic properties of molecules in droplets, which are usually described by diffusion coefficients or viscosity and permeability. (2) We discussed the conditions affecting the liquid-liquid phase separation of biological molecules, including concentration, temperature, ionic strength, pH, and crowding effects. A database for liquid-liquid phase separation, LLPSDB, was introduced, and three types of nucleic acid concentration effects on the phase separation of protein molecules are discussed. These effects depend on the relative interaction strengths of protein-nucleic acid and protein-protein interactions. The major driving force of phase separation is multivalent interactions, and molecular flexibility is necessary for the dynamic properties. We summarized the diverse sources of multivalence, including multiple tandem repetitive domains, regular oligomerization, low-complexity domains (usually intrinsically disordered with repeat motifs for binding), and nucleic acid molecules via the main chain phosphates or repeat sequences. (3) We reviewed the statistical thermodynamics theories for describing the macromolecular liquid-liquid separation, including the Flory-Huggins theory, Overbeek-Voorn correction, random phase approximation method, and field theory simulation method. We discussed the experimental and simulation methods for studying the physiochemical mechanism of liquid-liquid phase separation. Model systems with simplified sequences for experimental studies were listed, including systems for studying the effects of charge properties, residue types, sequence length, and other properties. Molecular simulation methods can provide detailed information regarding the liquid-liquid phase separation process. We introduced two coarse-grain methods, the slab molecular dynamic simulation and Monte Carlo simulation using the lattice model. (4) The physiochemical properties of liquid-liquid phase separation govern the diverse functions of reversible phase transitions in a cell. We collected and analyzed important cases of biomolecular phase separation in cell activities. These biological functions were classified into five categories, including enrichment, sequestration, biological switching cooperation, localization, and mechanical force generation. We linked these functions with the physiochemical properties of liquid-liquid phase separation. To understand the specific phase-separation processes in biological activities, three types of related molecules must be studied: scaffold molecules mainly contributing to aggregate formation, recruited functional client molecules, and molecules that regulate the formation and disassembly of aggregates. We reviewed four regulation methods for the phase separation process, including changing the charge distribution by post-translational modification, changing the molecular concentration by gene expression or degradation regulation, changing the oligomerization state, and changing the cell solution environment (such as pH). Designing compounds for phase separation regulation has attracted significant attention for treating related diseases. Methods for discovering molecules that can regulate post-translational modifications or inhibit interactions in the droplets are emerging. The recently discovered phase separation phenomena and molecules in living organisms represent only the tip of the iceberg. In the future, it will be necessary to systematically examine liquid-liquid phase separation events and related molecules in all phases of biological processes.
2020, 36(1): 190608
doi: 10.3866/PKU.WHXB201906085
Abstract:
Over the past decades, advances in science and technology have greatly benefitted the society. However, the exploitation of fossil fuels and excessive emissions of polluting gases have disturbed the balance of the normal carbon cycle, causing serious environmental issues and energy crises. Global warming caused by heavy CO2 emissions is driving new attempts to mitigate the increase in the concentration of atmospheric CO2. Significant efforts have been devoted for CO2 conversion. To date, the electroreduction of CO2, which is highly efficient and offers a promising strategy for both storing energy and managing the global carbon balance, has attracted great attention. In addition, the electrosynthesis of value-added C2+ products from CO2 addresses the need for the long-term storage of renewable energy. Therefore, developing catalysts that function under ambient conditions to produce C2 selectively over C1 products will increase the utility of renewable feedstocks in industrial chemistry applications. Recently, great progress has been made in the development of materials for electrocatalytic CO2 reduction (ECR) toward C2+ products; however, some issues (e.g., low selectivity, low current efficiency, and poor durability) remain to be addressed. In addition, the elementary reaction mechanism of each C2+ product remains unclear, contributing to the blindness of catalyst design. In this regard, the development of proposed mechanisms of ECR toward C2+ products is summarized herein. The key to generating C2+ products is improving the chances of C―C coupling. Test conditions significantly influence the reaction path of the catalyst. Thus, three different paths that that are most likely to occur during ECR to C2+ products are proposed, including the CO, CO-COH, and CO-CO paths. In addition, typical material regulatory strategies and technical designs for ECR toward C2+ products (e.g. crystal facet modulation, defect engineering, size effect, confinement effects, electrolyzer design, and electrolyte pH) are introduced, focusing on their effects on the selectivity, current efficiency, and durability. The four strategies for catalyst design (crystal facet modulation, defect engineering, size effect, and confinement effect) primarily affect the selectivity of the ECR via adjustment of the adsorption of reaction intermediates. The last two strategies for technique design (electrolyzer design and electrolyte pH) contributing greatly toward improving the current efficiency than selectivity. Finally, the challenges and perspectives for ECR toward C2+ products and their future prospects are discussed herein. Therefore, breakthroughs in the promising field of ECR toward the generation of C2+ products are possible when these catalyst design strategies and mechanisms are applied and novel designs are developed. ![]()
Over the past decades, advances in science and technology have greatly benefitted the society. However, the exploitation of fossil fuels and excessive emissions of polluting gases have disturbed the balance of the normal carbon cycle, causing serious environmental issues and energy crises. Global warming caused by heavy CO2 emissions is driving new attempts to mitigate the increase in the concentration of atmospheric CO2. Significant efforts have been devoted for CO2 conversion. To date, the electroreduction of CO2, which is highly efficient and offers a promising strategy for both storing energy and managing the global carbon balance, has attracted great attention. In addition, the electrosynthesis of value-added C2+ products from CO2 addresses the need for the long-term storage of renewable energy. Therefore, developing catalysts that function under ambient conditions to produce C2 selectively over C1 products will increase the utility of renewable feedstocks in industrial chemistry applications. Recently, great progress has been made in the development of materials for electrocatalytic CO2 reduction (ECR) toward C2+ products; however, some issues (e.g., low selectivity, low current efficiency, and poor durability) remain to be addressed. In addition, the elementary reaction mechanism of each C2+ product remains unclear, contributing to the blindness of catalyst design. In this regard, the development of proposed mechanisms of ECR toward C2+ products is summarized herein. The key to generating C2+ products is improving the chances of C―C coupling. Test conditions significantly influence the reaction path of the catalyst. Thus, three different paths that that are most likely to occur during ECR to C2+ products are proposed, including the CO, CO-COH, and CO-CO paths. In addition, typical material regulatory strategies and technical designs for ECR toward C2+ products (e.g. crystal facet modulation, defect engineering, size effect, confinement effects, electrolyzer design, and electrolyte pH) are introduced, focusing on their effects on the selectivity, current efficiency, and durability. The four strategies for catalyst design (crystal facet modulation, defect engineering, size effect, and confinement effect) primarily affect the selectivity of the ECR via adjustment of the adsorption of reaction intermediates. The last two strategies for technique design (electrolyzer design and electrolyte pH) contributing greatly toward improving the current efficiency than selectivity. Finally, the challenges and perspectives for ECR toward C2+ products and their future prospects are discussed herein. Therefore, breakthroughs in the promising field of ECR toward the generation of C2+ products are possible when these catalyst design strategies and mechanisms are applied and novel designs are developed.
