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2019 Volume 35 Issue 8
2019, 35(8): 787-791
doi: 10.3866/PKU.WHXB201904035
Abstract:
2019, 35(8): 794-795
doi: 10.3866/PKU.WHXB201810033
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2019, 35(8): 796-797
doi: 10.3866/PKU.WHXB201810066
Abstract:
2019, 35(8): 798-799
doi: 10.3866/PKU.WHXB201812002
Abstract:
2019, 35(8): 800-801
doi: 10.3866/PKU.WHXB201901024
Abstract:
2019, 35(8): 802-803
doi: 10.3866/PKU.WHXB201901030
Abstract:
2019, 35(8): 804-805
doi: 10.3866/PKU.WHXB201903013
Abstract:
2019, 35(8): 806-807
doi: 10.3866/PKU.WHXB201903025
Abstract:
2019, 35(8): 808-815
doi: 10.3866/PKU.WHXB201901035
Abstract:
In situ liquid cell transmission electron microscopy (LCTEM) was used to observe the dynamic self-assembly behavior of gold nanorod (AuNR)/graphene (G) composites in real-time. Many important reactions in chemistry, physics, and biology occur in solution and real-time imaging of the reaction objects in a liquid medium can further our understanding of the reaction at the nanoscale. Observations of liquid samples using transmission electron microscopy (TEM) have historically been challenging due to issues with evaporation and difficulty in forming thin liquid layers that are suitable for election beam transmission. In situ LCTEM, as an emerging technology, provides novel opportunities for the real-time and high-resolution observation of dynamic processes in solution. In this communication, we report the use of in situ LCTEM to study the assembly behavior of graphene and AuNRs. By tracking and recording the changes in the positions and shapes of the AuNRs and graphene over time, novel composite formation mechanisms between AuNRs and graphene were observed. The AuNRs tended to approach the graphene edges tip-first due to charge attraction. After the assembled structures were formed, the AuNRs could rotate with the graphene edges, among which the edge-to-edge structure was more stable, without angle changes between the AuNR and graphene edge. Drifting motions of the self-assembled structures were observed. And compared with smaller self-assembled structures, the larger structures seem more effectively resisted pushing by liquid flow. In addition, the motions of the larger structure were more easily slowed due to the drag from the liquid cell window substrate. Graphene folding structures were also observed with LCTEM, suggesting that the folding structure can open and close in the liquid, causing apparent relative position changes between Au and graphene for a fixed AuNR on the graphene layer. Overall, the self-assembled structures are very stable and did not show any disassembly behavior in the liquid. Moreover, the AuNR/graphene composites were used as catalysts and showed improved catalytic performance compared to that of bare AuNRs in 4-nitrophenol reduction experiments. The self-assembled catalyst with a mass composite 1 : 5 AuNRs/G ratio exhibited the best performance with a kapp value of 0.5570 min−1, 8 times that of the bare AuNRs. This significant improvement is closely related to the optimized and stable structure of the AuNR/graphene composites. In situ LCTEM provided a powerful characterization method for analyzing the complex self-assembly behavior of the composites in a liquid and will be useful for the development of high performance composite catalyst materials. ![]()
In situ liquid cell transmission electron microscopy (LCTEM) was used to observe the dynamic self-assembly behavior of gold nanorod (AuNR)/graphene (G) composites in real-time. Many important reactions in chemistry, physics, and biology occur in solution and real-time imaging of the reaction objects in a liquid medium can further our understanding of the reaction at the nanoscale. Observations of liquid samples using transmission electron microscopy (TEM) have historically been challenging due to issues with evaporation and difficulty in forming thin liquid layers that are suitable for election beam transmission. In situ LCTEM, as an emerging technology, provides novel opportunities for the real-time and high-resolution observation of dynamic processes in solution. In this communication, we report the use of in situ LCTEM to study the assembly behavior of graphene and AuNRs. By tracking and recording the changes in the positions and shapes of the AuNRs and graphene over time, novel composite formation mechanisms between AuNRs and graphene were observed. The AuNRs tended to approach the graphene edges tip-first due to charge attraction. After the assembled structures were formed, the AuNRs could rotate with the graphene edges, among which the edge-to-edge structure was more stable, without angle changes between the AuNR and graphene edge. Drifting motions of the self-assembled structures were observed. And compared with smaller self-assembled structures, the larger structures seem more effectively resisted pushing by liquid flow. In addition, the motions of the larger structure were more easily slowed due to the drag from the liquid cell window substrate. Graphene folding structures were also observed with LCTEM, suggesting that the folding structure can open and close in the liquid, causing apparent relative position changes between Au and graphene for a fixed AuNR on the graphene layer. Overall, the self-assembled structures are very stable and did not show any disassembly behavior in the liquid. Moreover, the AuNR/graphene composites were used as catalysts and showed improved catalytic performance compared to that of bare AuNRs in 4-nitrophenol reduction experiments. The self-assembled catalyst with a mass composite 1 : 5 AuNRs/G ratio exhibited the best performance with a kapp value of 0.5570 min−1, 8 times that of the bare AuNRs. This significant improvement is closely related to the optimized and stable structure of the AuNR/graphene composites. In situ LCTEM provided a powerful characterization method for analyzing the complex self-assembly behavior of the composites in a liquid and will be useful for the development of high performance composite catalyst materials.
2019, 35(8): 816-828
doi: 10.3866/PKU.WHXB201810060
Abstract:
Micelles, a kind of surfactant aggregate formed in water, may be swollen to a generally limited extent upon addition of a liquid hydrophobic compound. Swollen micelles have attracted considerable research attention because they can enhance the solubility of the said hydrophobic compound. The development of swollen micelles is of significant interest in terms of both scientific and industrial applications, such as drug delivery, oil recovery, and soil remediation. While there have been many studies focusing on micellar solubilization, several questions remain unanswered: the capacity to quantitatively solubilize the drug in drug delivery, the interaction between micelles and non-polar oil when microemulsions are not formed, and the differences and similarities between swollen micelles and microemulsions. Comprehensive understanding of and insight into swollen micelles will be helpful to tailor surfactants for industrial applications. Herein, we reviewed the recent progress in the field of swollen micelles in terms of solubilization capacity, solubilization site, micellar morphology, etc. First, the UV spectrophotometry results demonstrate that the solubilization capacity of micelles is related to their molecular structures and surfactant properties. The solubilization is also dependent on the composition and nature of the hydrophobic compounds, the presence of electrolytes, temperature, etc. Second, the solubilization site may be located in the micellar core, the palisade layer of the micelle, the micelle surface, or the hydrophilic shell of the micelle, depending on the property of the solubilized compounds and the morphology of the micelles. In general, the micellar aggregation number increases with increasing oil concentration; high concentration of oil causes the formation of spherical micelles, while high concentration of oil results in ellipsoidal micelles. Furthermore, the micellar size increased gradually with increasing oil concentrations. Finally, the differences and similarities between swollen micelles and microemulsions were clarified. It is believed that microemulsions can be considered as swollen micelles, but there has been some strong evidence that differentiates swollen micelles and microemulsions. Based on our results, we believe that microemulsions can be considered as swollen micelles, but all micellar solutions cannot be swollen to the extent of microemulsions, unless the specific structural requirements and conditions are satisfied. Overall, understanding the properties of swollen micelles and how they transform to microemulsions not only provide theoretical support for practical applications of surfactants, but can also be used to design new surfactants. ![]()
Micelles, a kind of surfactant aggregate formed in water, may be swollen to a generally limited extent upon addition of a liquid hydrophobic compound. Swollen micelles have attracted considerable research attention because they can enhance the solubility of the said hydrophobic compound. The development of swollen micelles is of significant interest in terms of both scientific and industrial applications, such as drug delivery, oil recovery, and soil remediation. While there have been many studies focusing on micellar solubilization, several questions remain unanswered: the capacity to quantitatively solubilize the drug in drug delivery, the interaction between micelles and non-polar oil when microemulsions are not formed, and the differences and similarities between swollen micelles and microemulsions. Comprehensive understanding of and insight into swollen micelles will be helpful to tailor surfactants for industrial applications. Herein, we reviewed the recent progress in the field of swollen micelles in terms of solubilization capacity, solubilization site, micellar morphology, etc. First, the UV spectrophotometry results demonstrate that the solubilization capacity of micelles is related to their molecular structures and surfactant properties. The solubilization is also dependent on the composition and nature of the hydrophobic compounds, the presence of electrolytes, temperature, etc. Second, the solubilization site may be located in the micellar core, the palisade layer of the micelle, the micelle surface, or the hydrophilic shell of the micelle, depending on the property of the solubilized compounds and the morphology of the micelles. In general, the micellar aggregation number increases with increasing oil concentration; high concentration of oil causes the formation of spherical micelles, while high concentration of oil results in ellipsoidal micelles. Furthermore, the micellar size increased gradually with increasing oil concentrations. Finally, the differences and similarities between swollen micelles and microemulsions were clarified. It is believed that microemulsions can be considered as swollen micelles, but there has been some strong evidence that differentiates swollen micelles and microemulsions. Based on our results, we believe that microemulsions can be considered as swollen micelles, but all micellar solutions cannot be swollen to the extent of microemulsions, unless the specific structural requirements and conditions are satisfied. Overall, understanding the properties of swollen micelles and how they transform to microemulsions not only provide theoretical support for practical applications of surfactants, but can also be used to design new surfactants.
2019, 35(8): 829-839
doi: 10.3866/PKU.WHXB201811027
Abstract:
Molecular electronics has been the subject of increasing interest since 1974. Although it describes the utilization of single molecules as active components of electrical devices, molecular electronics remains a fundamental subject to date. Considering that the length of a single molecule is typically several nanometers, the electrical characterization of a probe molecule is a significant experimental challenge. A metal/molecule/metal junction can bridge the gap between nanometer-sized molecules and the macroscopic measuring circuit and is, thus, generally considered as the most common prototype in molecular electronics. For the fabrication and characterization of single-molecule junctions, break junction methods, which include the mechanically controllable break junction (MCBJ) technique and the scanning tunneling microscopy-break junction (STM-BJ) technique, were proposed at the turn of the century and have been developed rapidly in recent years. These methods are widely employed in the experimental study of charge transport through single-molecule junctions and provide a platform to investigate the physical and chemical processes at the single-molecule level. In this review, we mainly focus on MCBJ and STM-BJ techniques applicable for single-molecule conductance measurement and highlight the progress of these techniques in the context of identification and modulation of chemical reactions and evaluation of their reaction kinetics at the single-molecule level. We begin by presenting the operation principles of MCBJ and STM-BJ and stating their brief comparison. Subsequently, we summarize the recent advances in modulating single-molecule chemical reactions. In this regard, we introduce several examples that involve changing the environmental solution, applying an external electrical field, and resorting to electrochemical gating. Next, we overview the application of the break junction techniques in the investigation of reaction kinetics at the single-molecule level. In this section, we also present a brief introduction to studies on single-molecule reaction kinetics using graphene-based nanogaps, wherein conventional metallic electrodes were replaced by graphene electrodes. Furthermore, we discuss the combination of break junction techniques and surface-enhanced Raman spectroscopy for detecting single-molecule reactions occurring at nanometer-scale separation. We discuss the historical development of this combined method and present the latest advancement explaining the origin of the low conductance of 1, 4-benzenedithiol, which is a topic of significant concern in single-molecule electronics. Finally, we discuss some future issues in molecular electronics, including the expansion from simple molecules to complex molecular systems and the introduction of multi-physical fields into single-molecule junctions. Moreover, we provide a list of critical characterization tools in molecular electronics and discuss their potential applications.
Molecular electronics has been the subject of increasing interest since 1974. Although it describes the utilization of single molecules as active components of electrical devices, molecular electronics remains a fundamental subject to date. Considering that the length of a single molecule is typically several nanometers, the electrical characterization of a probe molecule is a significant experimental challenge. A metal/molecule/metal junction can bridge the gap between nanometer-sized molecules and the macroscopic measuring circuit and is, thus, generally considered as the most common prototype in molecular electronics. For the fabrication and characterization of single-molecule junctions, break junction methods, which include the mechanically controllable break junction (MCBJ) technique and the scanning tunneling microscopy-break junction (STM-BJ) technique, were proposed at the turn of the century and have been developed rapidly in recent years. These methods are widely employed in the experimental study of charge transport through single-molecule junctions and provide a platform to investigate the physical and chemical processes at the single-molecule level. In this review, we mainly focus on MCBJ and STM-BJ techniques applicable for single-molecule conductance measurement and highlight the progress of these techniques in the context of identification and modulation of chemical reactions and evaluation of their reaction kinetics at the single-molecule level. We begin by presenting the operation principles of MCBJ and STM-BJ and stating their brief comparison. Subsequently, we summarize the recent advances in modulating single-molecule chemical reactions. In this regard, we introduce several examples that involve changing the environmental solution, applying an external electrical field, and resorting to electrochemical gating. Next, we overview the application of the break junction techniques in the investigation of reaction kinetics at the single-molecule level. In this section, we also present a brief introduction to studies on single-molecule reaction kinetics using graphene-based nanogaps, wherein conventional metallic electrodes were replaced by graphene electrodes. Furthermore, we discuss the combination of break junction techniques and surface-enhanced Raman spectroscopy for detecting single-molecule reactions occurring at nanometer-scale separation. We discuss the historical development of this combined method and present the latest advancement explaining the origin of the low conductance of 1, 4-benzenedithiol, which is a topic of significant concern in single-molecule electronics. Finally, we discuss some future issues in molecular electronics, including the expansion from simple molecules to complex molecular systems and the introduction of multi-physical fields into single-molecule junctions. Moreover, we provide a list of critical characterization tools in molecular electronics and discuss their potential applications.
2019, 35(8): 840-849
doi: 10.3866/PKU.WHXB201811016
Abstract:
Ca2+ and Mg2+ ions are the main divalent cations in living cells and play vital roles in the structure and function of biological membranes. To date, the differences in the effects of these two ions on the Escherichia coli (E. coli) inner membrane at various concentrations remain unknown. Here, the effects of Ca2+ and Mg2+ ions on a mixed lipid bilayer composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG) in a 3 : 1 ratio (mol/mol), which mimics the E. coli inner membrane, were quantitatively differentiated at different concentrations by dynamic light scattering (DLS), zeta potential measurements and all-atom molecular dynamics (AA-MD) simulations. The DLS results demonstrated that the POPE/POPG liposomes were homogeneous and monodisperse in solutions with Ca2+ or Mg2+ ion concentrations of 0 and 1 mmol∙L-1. As the Ca2+ or Mg2+ ion concentration was increased to 5-100 mmol∙L-1, lipid aggregation or the fusion of unilamellar liposomes occurred in the ion solutions. The zeta potential measurements showed that both the Ca2+ and Mg2+ ions had overcharging effects on the negatively charged POPE/POPG liposomes. The AA-MD simulation results indicated that the Ca2+ ions irreversibly adsorbed on the membranes when the simulation time was longer than 100 ns, while the Mg2+ ions were observed to dynamically adsorb on and desorb from the membranes at various concentrations. These results are consistent with the DLS and zeta potential experiments. The average numbers of Ca2+ and Mg2+ ions in the first coordination shell of the oxygen atoms of the phosphate, carbonyl and hydroxyl groups of POPE and POPG (i.e., the first coordination numbers) in the pure membrane and membranes containing 5 and 100 mmol∙L-1 ions were calculated from the radial distribution functions. The results indicated that the primary binding site of these two ions on POPE and POPG at the concentrations studied was the negatively charged phosphate group. Thus, these results might explain the overcharging effects of both the Ca2+ and Mg2+ ions on the POPE/POPG liposomes. Moreover, as the Ca2+ concentration increased, the area per lipid of the lipid bilayers decreased, and the membrane thickness increased, while the Mg2+ ions had negligible effects on these membrane parameters. In addition, these ions had different effects on the orientation of the lipid head groups. These simulation results may be used to provide the possible explanations for the differences between Ca2+ and Mg2+ ions in DLS and zeta potential measurements at the atomic level. The experimental results and MD simulations provide insight into various biological processes regulated by divalent cations, such as membrane fusion. ![]()
Ca2+ and Mg2+ ions are the main divalent cations in living cells and play vital roles in the structure and function of biological membranes. To date, the differences in the effects of these two ions on the Escherichia coli (E. coli) inner membrane at various concentrations remain unknown. Here, the effects of Ca2+ and Mg2+ ions on a mixed lipid bilayer composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG) in a 3 : 1 ratio (mol/mol), which mimics the E. coli inner membrane, were quantitatively differentiated at different concentrations by dynamic light scattering (DLS), zeta potential measurements and all-atom molecular dynamics (AA-MD) simulations. The DLS results demonstrated that the POPE/POPG liposomes were homogeneous and monodisperse in solutions with Ca2+ or Mg2+ ion concentrations of 0 and 1 mmol∙L-1. As the Ca2+ or Mg2+ ion concentration was increased to 5-100 mmol∙L-1, lipid aggregation or the fusion of unilamellar liposomes occurred in the ion solutions. The zeta potential measurements showed that both the Ca2+ and Mg2+ ions had overcharging effects on the negatively charged POPE/POPG liposomes. The AA-MD simulation results indicated that the Ca2+ ions irreversibly adsorbed on the membranes when the simulation time was longer than 100 ns, while the Mg2+ ions were observed to dynamically adsorb on and desorb from the membranes at various concentrations. These results are consistent with the DLS and zeta potential experiments. The average numbers of Ca2+ and Mg2+ ions in the first coordination shell of the oxygen atoms of the phosphate, carbonyl and hydroxyl groups of POPE and POPG (i.e., the first coordination numbers) in the pure membrane and membranes containing 5 and 100 mmol∙L-1 ions were calculated from the radial distribution functions. The results indicated that the primary binding site of these two ions on POPE and POPG at the concentrations studied was the negatively charged phosphate group. Thus, these results might explain the overcharging effects of both the Ca2+ and Mg2+ ions on the POPE/POPG liposomes. Moreover, as the Ca2+ concentration increased, the area per lipid of the lipid bilayers decreased, and the membrane thickness increased, while the Mg2+ ions had negligible effects on these membrane parameters. In addition, these ions had different effects on the orientation of the lipid head groups. These simulation results may be used to provide the possible explanations for the differences between Ca2+ and Mg2+ ions in DLS and zeta potential measurements at the atomic level. The experimental results and MD simulations provide insight into various biological processes regulated by divalent cations, such as membrane fusion.
2019, 35(8): 850-857
doi: 10.3866/PKU.WHXB201811040
Abstract:
As a unique two-dimensional material, graphitic carbon nitride (g-C3N4) has received significant attention for its particular electronic structure and chemical performance. Its instinctive defect can provide a stable anchoring site for metals, potentially improving the surface reactivity. Ni-based catalysts are economical but their activity for CO2 methanation is lower than that of noble metal catalysts. Ni nanoparticles (NPs) supported on a substrate can further enhance the stability and activity of catalysts. Based on the principles of strong metal-support interaction (SMSI) and the synergistic effect on an alloy, MNi12/g-C3N4 composites as novel catalysts are expected to improve stability and catalytic performance of Ni-based catalysts. The configurations are established with core-shell structures of MNi12 (M = Fe, Co, Cu, Zn) nanoparticles (NPs) supported on g-C3N4 in this work. In the CO2 methanation reaction, the reactivity of CO on slab (ECO) is a critical factor, which is relative to the catalytic activity. Thus, the catalytic reactivity of these complexes via CO adsorption were explored using density functional theory (DFT). The values of cohesive energy (Ecoh) for MNi12 NPs range from -39.90 eV to -34.82 eV, suggesting that the formation of these NPs is favored as per thermodynamics, and Ecoh and partial density of state (PDOS) reveal that the central M atom with the less filled d-shell interacts more strongly with surface Ni atoms. Therefore, ZnNi12 is the most unstable structure among all the studied alloy, and the synergistic effect is also the weakest among them. When MNi12 NPs are supported on the g-C3N4 substrate, the binding energies (Eb) vary from -9.40 eV to -8.39 eV, indicating that g-C3N4 is indeed a good material for stabilizing these NPs. The PDOS analysis of pure g-C3N4 suggests the sp2 dangling bonds of N atoms in g-C3N4 can stabilize these transition metal NPs. Furthermore, the results of CO adsorbed on MNi12 NPs and MNi12/g-C3N4 composites show that ECO and dCO reduced with the introduction of g-C3N4. According to the results of the analysis of the Hirshfeld charges and electrostatic potential (ESP), the reason is that CO obtains less electrons from MNi12 NPs after deposition on the g-C3N4 substrate, which lowers the reactivity of CO on catalysts. Additionally, the deformation charge density is analyzed to investigate the interaction between the NPs and g-C3N4. With the introduction of g-C3N4, charge redistribution indicates the strong metal-support interaction, which further reduces the CO adsorption energy. In summary, MNi12 supported on g-C3N4 exhibit not only high stability but also tunable reactivity in CO2 methanation. These changes are beneficial for CO2 methanation reaction. ![]()
As a unique two-dimensional material, graphitic carbon nitride (g-C3N4) has received significant attention for its particular electronic structure and chemical performance. Its instinctive defect can provide a stable anchoring site for metals, potentially improving the surface reactivity. Ni-based catalysts are economical but their activity for CO2 methanation is lower than that of noble metal catalysts. Ni nanoparticles (NPs) supported on a substrate can further enhance the stability and activity of catalysts. Based on the principles of strong metal-support interaction (SMSI) and the synergistic effect on an alloy, MNi12/g-C3N4 composites as novel catalysts are expected to improve stability and catalytic performance of Ni-based catalysts. The configurations are established with core-shell structures of MNi12 (M = Fe, Co, Cu, Zn) nanoparticles (NPs) supported on g-C3N4 in this work. In the CO2 methanation reaction, the reactivity of CO on slab (ECO) is a critical factor, which is relative to the catalytic activity. Thus, the catalytic reactivity of these complexes via CO adsorption were explored using density functional theory (DFT). The values of cohesive energy (Ecoh) for MNi12 NPs range from -39.90 eV to -34.82 eV, suggesting that the formation of these NPs is favored as per thermodynamics, and Ecoh and partial density of state (PDOS) reveal that the central M atom with the less filled d-shell interacts more strongly with surface Ni atoms. Therefore, ZnNi12 is the most unstable structure among all the studied alloy, and the synergistic effect is also the weakest among them. When MNi12 NPs are supported on the g-C3N4 substrate, the binding energies (Eb) vary from -9.40 eV to -8.39 eV, indicating that g-C3N4 is indeed a good material for stabilizing these NPs. The PDOS analysis of pure g-C3N4 suggests the sp2 dangling bonds of N atoms in g-C3N4 can stabilize these transition metal NPs. Furthermore, the results of CO adsorbed on MNi12 NPs and MNi12/g-C3N4 composites show that ECO and dCO reduced with the introduction of g-C3N4. According to the results of the analysis of the Hirshfeld charges and electrostatic potential (ESP), the reason is that CO obtains less electrons from MNi12 NPs after deposition on the g-C3N4 substrate, which lowers the reactivity of CO on catalysts. Additionally, the deformation charge density is analyzed to investigate the interaction between the NPs and g-C3N4. With the introduction of g-C3N4, charge redistribution indicates the strong metal-support interaction, which further reduces the CO adsorption energy. In summary, MNi12 supported on g-C3N4 exhibit not only high stability but also tunable reactivity in CO2 methanation. These changes are beneficial for CO2 methanation reaction.
2019, 35(8): 858-867
doi: 10.3866/PKU.WHXB201812011
Abstract:
CL-20 exhibits high energy density, but its high sensitivity limits its use in various applications. A high-energy and low-sensitivity co-crystal high explosive prepared around CL-20 has the potential to widen the application scope of CL-20 single crystals. The initial physical and chemical responses along different lattice vectors of the energetic co-crystal CL-20/HMX impacted by 4-10 km·s-1 of steady shock waves were simulated using the ReaxFF molecular dynamics method combined with the multiscale shock technique (MSST). The temperature, pressure, density, particle velocity, initial decomposition paths, final stable reaction products, and shock Hugoniot curves were obtained. The results show that after application of the shock wave, the energetic co-crystals successively undergo an induction period, fast compression, slow compression, and expansion processes. The fast and slow compression processes correspond to the fast and slow decomposition of the reactants, respectively. An exponential function was adopted to fit the decay curve of the reactants and the decay rates of CL-20 and HMX were compared. Overall, with increasing shock wave velocity, the response time of the reactants was gradually advanced and that of CL-20 molecular decomposition in the co-crystal occurred earlier than that of HMX after the shock wave incident along each lattice vector. The decay rate of CL-20 was highest during the fast decomposition stage, followed by that of HMX. However, the decay rate of the reactants during the slow decomposition stage was similar. The initial reaction path of the energetic co-crystal involves the dimerization of CL-20, while the initial reaction path of the shock-induced co-crystal decomposition involves fracturing of the N-NO2 bond in CL-20 to form NO2. Then, small intermediate molecules such as N2O, NO, HONO, OH, and H are formed. The final stable products are N2, H2O, CO2, CO, and H2. The shock sensitivities of the lattice vector in the b and c directions were the same, but lower than that of the lattice vector a direction. The minimum velocities (us) of the shock wave inducing CL-20 and HMX decomposition were 6 and 7 km·s-1, respectively. Moreover, the particle velocities behind the shock waves on the three lattice vectors showed only minor differences. The shock-induced initiation pressures of CL-20/HMX along lattice vectors a, b, and c were 16.52, 17.41, and 17.41 GPa, respectively, as determined by the shock Hugoniot relation. The detonation pressure ranged from 36.75 to 47.43 GPa. ![]()
CL-20 exhibits high energy density, but its high sensitivity limits its use in various applications. A high-energy and low-sensitivity co-crystal high explosive prepared around CL-20 has the potential to widen the application scope of CL-20 single crystals. The initial physical and chemical responses along different lattice vectors of the energetic co-crystal CL-20/HMX impacted by 4-10 km·s-1 of steady shock waves were simulated using the ReaxFF molecular dynamics method combined with the multiscale shock technique (MSST). The temperature, pressure, density, particle velocity, initial decomposition paths, final stable reaction products, and shock Hugoniot curves were obtained. The results show that after application of the shock wave, the energetic co-crystals successively undergo an induction period, fast compression, slow compression, and expansion processes. The fast and slow compression processes correspond to the fast and slow decomposition of the reactants, respectively. An exponential function was adopted to fit the decay curve of the reactants and the decay rates of CL-20 and HMX were compared. Overall, with increasing shock wave velocity, the response time of the reactants was gradually advanced and that of CL-20 molecular decomposition in the co-crystal occurred earlier than that of HMX after the shock wave incident along each lattice vector. The decay rate of CL-20 was highest during the fast decomposition stage, followed by that of HMX. However, the decay rate of the reactants during the slow decomposition stage was similar. The initial reaction path of the energetic co-crystal involves the dimerization of CL-20, while the initial reaction path of the shock-induced co-crystal decomposition involves fracturing of the N-NO2 bond in CL-20 to form NO2. Then, small intermediate molecules such as N2O, NO, HONO, OH, and H are formed. The final stable products are N2, H2O, CO2, CO, and H2. The shock sensitivities of the lattice vector in the b and c directions were the same, but lower than that of the lattice vector a direction. The minimum velocities (us) of the shock wave inducing CL-20 and HMX decomposition were 6 and 7 km·s-1, respectively. Moreover, the particle velocities behind the shock waves on the three lattice vectors showed only minor differences. The shock-induced initiation pressures of CL-20/HMX along lattice vectors a, b, and c were 16.52, 17.41, and 17.41 GPa, respectively, as determined by the shock Hugoniot relation. The detonation pressure ranged from 36.75 to 47.43 GPa.
2019, 35(8): 868-875
doi: 10.3866/PKU.WHXB201811033
Abstract:
Driven by the wide-scale implementation of intermittent renewable energy generating technologies, such as wind and solar, sodium-ion batteries have recently attracted attention as an inexpensive energy storage system due to the abundance, low cost, and relatively low redox potential of sodium. However, in comparison with lithium-ion batteries, which are known for long cycle life, sodium-ion batteries usually suffer from significant capacity fading during long-term cycling due to the large volume expansion/contraction of the electrode active materials caused by insertion/extraction of the large sodium ion. In recent years, intense effort has been focused on the search for high performance electrode materials and electrolytes to improve the cyclability of sodium-ion batteries, and some progress has been achieved. The incorporation of additives into the electrolyte is a simple and efficient method of improving the cycle stability of sodium-ion batteries. Fluoroethylene carbonate (FEC) is generally considered to be a suitable additive for the formation of the anode solid electrolyte interphase (SEI), due to a relatively low-lying lowest unoccupied molecular orbital (LUMO). However, it is suggested that FEC it will not be oxidized on the cathode since it also has a relatively low highest occupied molecular orbital (HOMO). In this study, we investigated the effect of FEC as an additive on the cycle life of a sodium-ion battery with a P2-NaxCo0.7Mn0.3O2 (x ≈ 1) layered sodium transition metal oxide as the cathode active material, a sodium metal foil anode, a glass fiber separator, and an electrolyte composed of NaClO4 and a varying mass content of FEC dissolved in propylene carbonate (PC). We analyzed the effect of the FEC additive on the morphology and chemical composition of the separator and cathode electrode surface using scanning electron microscopy (SEM), transmission electron microscopy (TEM), infrared spectroscopy, and X-ray photoelectron spectroscopy (XPS), and studied the evolution of the crystalline structure of the cathode active material during charge and discharge using in situ X-ray diffraction (XRD). We found that an appropriate amount of FEC additive significantly suppressed the decomposition of the PC solvent, and assisted the formation of a NaF-rich protective layer on the cathode surface, which helped to maintain the structural stability of the cathode material, thereby improving the cycle stability of the sodium-ion battery. Density functional theory (DFT) calculations showed that FEC coordinates more readily with the ClO4- anion on the cathode surface than does the PC solvent. This drives the formation of the NaF-rich protective layer on the cathode surface. We believe these results could provide inspiration in the design of electrolyte additives for protection of the sodium cathode during cycling, thus improving the cycling performance of sodium-ion batteries. ![]()
Driven by the wide-scale implementation of intermittent renewable energy generating technologies, such as wind and solar, sodium-ion batteries have recently attracted attention as an inexpensive energy storage system due to the abundance, low cost, and relatively low redox potential of sodium. However, in comparison with lithium-ion batteries, which are known for long cycle life, sodium-ion batteries usually suffer from significant capacity fading during long-term cycling due to the large volume expansion/contraction of the electrode active materials caused by insertion/extraction of the large sodium ion. In recent years, intense effort has been focused on the search for high performance electrode materials and electrolytes to improve the cyclability of sodium-ion batteries, and some progress has been achieved. The incorporation of additives into the electrolyte is a simple and efficient method of improving the cycle stability of sodium-ion batteries. Fluoroethylene carbonate (FEC) is generally considered to be a suitable additive for the formation of the anode solid electrolyte interphase (SEI), due to a relatively low-lying lowest unoccupied molecular orbital (LUMO). However, it is suggested that FEC it will not be oxidized on the cathode since it also has a relatively low highest occupied molecular orbital (HOMO). In this study, we investigated the effect of FEC as an additive on the cycle life of a sodium-ion battery with a P2-NaxCo0.7Mn0.3O2 (x ≈ 1) layered sodium transition metal oxide as the cathode active material, a sodium metal foil anode, a glass fiber separator, and an electrolyte composed of NaClO4 and a varying mass content of FEC dissolved in propylene carbonate (PC). We analyzed the effect of the FEC additive on the morphology and chemical composition of the separator and cathode electrode surface using scanning electron microscopy (SEM), transmission electron microscopy (TEM), infrared spectroscopy, and X-ray photoelectron spectroscopy (XPS), and studied the evolution of the crystalline structure of the cathode active material during charge and discharge using in situ X-ray diffraction (XRD). We found that an appropriate amount of FEC additive significantly suppressed the decomposition of the PC solvent, and assisted the formation of a NaF-rich protective layer on the cathode surface, which helped to maintain the structural stability of the cathode material, thereby improving the cycle stability of the sodium-ion battery. Density functional theory (DFT) calculations showed that FEC coordinates more readily with the ClO4- anion on the cathode surface than does the PC solvent. This drives the formation of the NaF-rich protective layer on the cathode surface. We believe these results could provide inspiration in the design of electrolyte additives for protection of the sodium cathode during cycling, thus improving the cycling performance of sodium-ion batteries.
2019, 35(8): 876-884
doi: 10.3866/PKU.WHXB201901019
Abstract:
A copolymer P(MEO2MAm-co-OEGMAn)-b-PDPAp (poly[di(ethylene glycol) methyl ether methacrylate-co-oligo(ethylene glycol) methacrylate]-b-poly[2-(diisopropylamino) ethyl methacrylate]) had been designed to have good stimuli-response for developing temperature/pH dual-responsive drug delivery systems. In this study, the copolymer was synthesized in two steps: atom transfer radical polymerization (ATRP) method, followed by the continuous ATRP method for P(MEO2MAm-co-OEGMAn) and P(MEO2MAm-co-OEGMAn)-b-PDPAp, respectively. The data of proton nuclear magnetic resonance spectroscopy (1H NMR) and gel permeation chromatography (GPC) showed that the chemical compositions of the two copolymers could be precisely controlled. The aqueous solution properties of the copolymers were investigated using a UV-visible spectrophotometer. Results showed that the former random copolymers had reversible temperature response, while the end product exhibited temperature/pH dual-responsive behaviors. Their lower critical solution temperatures (LCSTs) were found to be dependent on the ratios of the monomers. Firstly, the relationship between the monomer ratio and the low critical solution temperature (LCST) of P(MEO2MAm-co-OEGMAn) was investigated. It was found that within the range of the experimental study (m + n = 100, 0 < n < 30), the LCST of the copolymer showed a linear relationship with the number of OEGMA units in each polymer chain. Then, the proportion of the two monomers (MEO2MAm and OEGMA) was fixed (m = 90, n = 10) and the random terpolymer P(MEO2MAm-co-OEGMAn)-b-PDPAp was synthesized. The LCSTs of P(MEO2MAm-co-OEGMAn)-b-PDPAp were found to be highly dependent on the DPA unit numbers in the range of 15 < p < 30 while their pH trigger points were not related to the chemical composition of the P(MEO2MAm-co-OEGMAn)-b-PDPAp. Finally, the copolymer P(MEO2MA90-co-OEGMA10)-b-PDPA22 with a LCST of 43 ℃ and a pH trigger point of 6.5 was carefully selected for preparing micelles, as a nanocarrier of doxorubicin (DOX) for an in vitro release study. The in vitro release kinetics of copolymer micelles were studied under different conditions. At pH 7.4, which mimics the normal physiological condition, the total release amount of DOX at 37 ℃ was about 25%, which was much higher than that at a higher temperature of 45 ℃ (approximately 5%). However, at pH 5.0, which mimics the intracellular environment, the cumulative release of the DOX quickly reached 95% at 37 ℃, while the cumulative release of the drug at 45 ℃ was only about 65% in the same period of time. For the latter, a slow and sustained release was observed and the cumulative drug release amount reached about 85% in 30 h. These results showed that the environmental stimuli response of the copolymer determined the drug release behavior of the micelles. This copolymer-based drug carrier could be expected to undergo bursting and sustained drug release in response to different conditions. Therefore, the results show potential for applications in the design and preparation of controllable drug transportation systems. ![]()
A copolymer P(MEO2MAm-co-OEGMAn)-b-PDPAp (poly[di(ethylene glycol) methyl ether methacrylate-co-oligo(ethylene glycol) methacrylate]-b-poly[2-(diisopropylamino) ethyl methacrylate]) had been designed to have good stimuli-response for developing temperature/pH dual-responsive drug delivery systems. In this study, the copolymer was synthesized in two steps: atom transfer radical polymerization (ATRP) method, followed by the continuous ATRP method for P(MEO2MAm-co-OEGMAn) and P(MEO2MAm-co-OEGMAn)-b-PDPAp, respectively. The data of proton nuclear magnetic resonance spectroscopy (1H NMR) and gel permeation chromatography (GPC) showed that the chemical compositions of the two copolymers could be precisely controlled. The aqueous solution properties of the copolymers were investigated using a UV-visible spectrophotometer. Results showed that the former random copolymers had reversible temperature response, while the end product exhibited temperature/pH dual-responsive behaviors. Their lower critical solution temperatures (LCSTs) were found to be dependent on the ratios of the monomers. Firstly, the relationship between the monomer ratio and the low critical solution temperature (LCST) of P(MEO2MAm-co-OEGMAn) was investigated. It was found that within the range of the experimental study (m + n = 100, 0 < n < 30), the LCST of the copolymer showed a linear relationship with the number of OEGMA units in each polymer chain. Then, the proportion of the two monomers (MEO2MAm and OEGMA) was fixed (m = 90, n = 10) and the random terpolymer P(MEO2MAm-co-OEGMAn)-b-PDPAp was synthesized. The LCSTs of P(MEO2MAm-co-OEGMAn)-b-PDPAp were found to be highly dependent on the DPA unit numbers in the range of 15 < p < 30 while their pH trigger points were not related to the chemical composition of the P(MEO2MAm-co-OEGMAn)-b-PDPAp. Finally, the copolymer P(MEO2MA90-co-OEGMA10)-b-PDPA22 with a LCST of 43 ℃ and a pH trigger point of 6.5 was carefully selected for preparing micelles, as a nanocarrier of doxorubicin (DOX) for an in vitro release study. The in vitro release kinetics of copolymer micelles were studied under different conditions. At pH 7.4, which mimics the normal physiological condition, the total release amount of DOX at 37 ℃ was about 25%, which was much higher than that at a higher temperature of 45 ℃ (approximately 5%). However, at pH 5.0, which mimics the intracellular environment, the cumulative release of the DOX quickly reached 95% at 37 ℃, while the cumulative release of the drug at 45 ℃ was only about 65% in the same period of time. For the latter, a slow and sustained release was observed and the cumulative drug release amount reached about 85% in 30 h. These results showed that the environmental stimuli response of the copolymer determined the drug release behavior of the micelles. This copolymer-based drug carrier could be expected to undergo bursting and sustained drug release in response to different conditions. Therefore, the results show potential for applications in the design and preparation of controllable drug transportation systems.
2019, 35(8): 896-902
doi: 10.3866/PKU.WHXB201810064
Abstract:
Multilayer phosphorescent organic lighting-emitting diodes (PHOLEDs) with complicated device configurations have greatly increased the complexity of manufacturing and the fabrication cost. Therefore, there is strong incentive to develop simplified OLEDs, such as a single-layer device that has the structure of anode/hole injection layer (HIL)/emissive layer/electron injection layer/cathode. However, because of the absence of a carrier transport layer, the single-layer device suffers from severe charge injection difficulties and unbalanced carrier transport. Hence, the performances of single-layer devices reported so far have not been satisfactory. It has been proved that the modification of the electrode/organic interface could influence carrier injection to improve the device performance in multilayer PHOLEDs. Modification of the electrode/organic interface is more essential for achieving high-performance single-layer OLEDs. In this work, efficient green phosphorescent single-layer OLEDs based on the structure of indium tin oxide (ITO)/C60 (1.2 nm):MoO3 (0.4 nm)/1, 3, 5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi):fac-tris(2-phenylpyridine)iridium [Ir(ppy)3]/LiF (0.7 nm)/Al (120 nm) were fabricated. C60, MoO3, and C60:MoO3 were applied as the HILs, respectively, for comparison. The layer of TPBi played a dual role of host and electron-transporting material within the emission layer. Thus, the properties of the HILs play an important role in the adjustment of electron/hole injection to attain transport balance of the charge carriers in single-layer OLEDs with electron-transporting hosts. It is found that appropriate adjustment of the HIL is a key factor to achieve high-efficiency single-layer OLEDs. The large affinity of MoO3 (6.37 eV), inducing electron transfer from the highest occupied molecular orbital of C60 to MoO3, results in the formation of C60 cations and induces the decrease of the valence from Mo+6 to Mo+5; therefore, C60:MoO3 can adjust the hole injection properties well. Finally, a single-layer OLED with a maximum current efficiency of 35.88 cd∙A−1 was achieved. Compared with devices with MoO3 (28.99 cd∙A−1) or C60 (10.46 cd∙A−1) as HILs, the device performance was improved by 24% and 243%, respectively. Overall, a novel and effective method of using different mixed ratios of C60 and MoO3 as the HIL to realize effective charge carrier regulation is proposed, and it is of great significance for fabricating high-performance single-layer OLEDs. ![]()
Multilayer phosphorescent organic lighting-emitting diodes (PHOLEDs) with complicated device configurations have greatly increased the complexity of manufacturing and the fabrication cost. Therefore, there is strong incentive to develop simplified OLEDs, such as a single-layer device that has the structure of anode/hole injection layer (HIL)/emissive layer/electron injection layer/cathode. However, because of the absence of a carrier transport layer, the single-layer device suffers from severe charge injection difficulties and unbalanced carrier transport. Hence, the performances of single-layer devices reported so far have not been satisfactory. It has been proved that the modification of the electrode/organic interface could influence carrier injection to improve the device performance in multilayer PHOLEDs. Modification of the electrode/organic interface is more essential for achieving high-performance single-layer OLEDs. In this work, efficient green phosphorescent single-layer OLEDs based on the structure of indium tin oxide (ITO)/C60 (1.2 nm):MoO3 (0.4 nm)/1, 3, 5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi):fac-tris(2-phenylpyridine)iridium [Ir(ppy)3]/LiF (0.7 nm)/Al (120 nm) were fabricated. C60, MoO3, and C60:MoO3 were applied as the HILs, respectively, for comparison. The layer of TPBi played a dual role of host and electron-transporting material within the emission layer. Thus, the properties of the HILs play an important role in the adjustment of electron/hole injection to attain transport balance of the charge carriers in single-layer OLEDs with electron-transporting hosts. It is found that appropriate adjustment of the HIL is a key factor to achieve high-efficiency single-layer OLEDs. The large affinity of MoO3 (6.37 eV), inducing electron transfer from the highest occupied molecular orbital of C60 to MoO3, results in the formation of C60 cations and induces the decrease of the valence from Mo+6 to Mo+5; therefore, C60:MoO3 can adjust the hole injection properties well. Finally, a single-layer OLED with a maximum current efficiency of 35.88 cd∙A−1 was achieved. Compared with devices with MoO3 (28.99 cd∙A−1) or C60 (10.46 cd∙A−1) as HILs, the device performance was improved by 24% and 243%, respectively. Overall, a novel and effective method of using different mixed ratios of C60 and MoO3 as the HIL to realize effective charge carrier regulation is proposed, and it is of great significance for fabricating high-performance single-layer OLEDs.
Preparation of Defective TiO2-x Hollow Microspheres for Photocatalytic Degradation of Methylene Blue
2019, 35(8): 885-895
doi: 10.3866/PKU.WHXB201812022
Abstract:
In recent years, photocatalytic degradation of organic pollutants has attracted considerable attention because of its potential application for solving environmental problems. Among various semiconductor photocatalysts, TiO2 is considered a promising candidate due to its excellent structural stability. Many researchers have focused on improving the visible-light catalytic efficiency of TiO2, because the large band gap of TiO2 limits its utilization of visible light energy. Recently, it has been proved that intrinsic defects like oxygen vacancies in TiO2 can trigger the visible light activity. TiO2 hollow microspheres with large surface areas have shown high photocatalytic efficiencies in the degradation of organic pollutants. To date, the photocatalytic performance of TiO2-x hollow microspheres has not been investigated. The kinetics of photocatalytic degradation of organic dyes is usually depicted by the pseudo-first-order kinetic equation. However, a few studies have demonstrated the impact of light absorption by the dye itself on photocatalytic performance in terms of the rate equation. In this study, defective TiO2-x hollow microspheres were prepared by the hydrogen reduction process to effectively promote photocatalytic activity under visible light irradiation. The structure and properties were characterized by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) analysis, X-ray photoelectron spectroscopy (XPS), electron spin-resonance (ESR), Raman spectrometry, ultraviolet-visible diffuse-reflectance spectroscopy (UV-Vis DRS), and electrochemical tests. The photocatalytic performance was evaluated based on the photocatalytic degradation of methylene blue (MB) solution under visible light irradiation. The mechanism underlying the enhancement of photocatalytic activity was also discussed. The results show that the visible-light photocatalytic activity of TiO2-x, and TiO2-x hollow microsphere benefit from the presence of oxygen vacancies on the surface. The photocatalytic activity of TiO2-x hollow microspheres is better than that of TiO2-x, attributed to the formation of hollow structures with higher specific surface areas. The mechanism of MB degradation occurring on the TiO2-x hollow microsphere surface was also investigated. The results show that the MB molecules are photodegraded by the photogenerated hole (h+), reactive superoxide radical (•O2-), and hydroxyl radicals (•OH), and that the •OH radicals, produced only by photogenerated holes, play an essential role in the degradation of MB. Based on the discussion of the effect of initial concentration of MB on the degradation process, a new kinetic model was proposed for the photocatalytic degradation of dye, considering the effect of visible light absorbed by MB molecules, because the data estimated by pseudo-first-order kinetic equation do not fit well with the experimental data. The Runge-Kutta method was used to obtain the numerical solution of the kinetic model. The results show that the kinetic model proposed for photocatalytic dye degradation gives a more realistic description of the photocatalytic degradation of MB because the calculated results fit better with the experimental data. The rate constant (kapp) of the pseudo-first-order kinetic equation decreases with increasing initial concentration of MB, indicating that kapp is affected by the light absorption properties of MB, because an increase in the initial concentration of MB will lead to increased absorption of visible light by MB molecules rather than by TiO2-x hollow microsphere. Unlike the rate constant kapp, the rate constant ka in the proposed model describes the process of photocatalytic dye degradation more effectively because it does not depend on the initial dye concentration. ![]()
In recent years, photocatalytic degradation of organic pollutants has attracted considerable attention because of its potential application for solving environmental problems. Among various semiconductor photocatalysts, TiO2 is considered a promising candidate due to its excellent structural stability. Many researchers have focused on improving the visible-light catalytic efficiency of TiO2, because the large band gap of TiO2 limits its utilization of visible light energy. Recently, it has been proved that intrinsic defects like oxygen vacancies in TiO2 can trigger the visible light activity. TiO2 hollow microspheres with large surface areas have shown high photocatalytic efficiencies in the degradation of organic pollutants. To date, the photocatalytic performance of TiO2-x hollow microspheres has not been investigated. The kinetics of photocatalytic degradation of organic dyes is usually depicted by the pseudo-first-order kinetic equation. However, a few studies have demonstrated the impact of light absorption by the dye itself on photocatalytic performance in terms of the rate equation. In this study, defective TiO2-x hollow microspheres were prepared by the hydrogen reduction process to effectively promote photocatalytic activity under visible light irradiation. The structure and properties were characterized by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) analysis, X-ray photoelectron spectroscopy (XPS), electron spin-resonance (ESR), Raman spectrometry, ultraviolet-visible diffuse-reflectance spectroscopy (UV-Vis DRS), and electrochemical tests. The photocatalytic performance was evaluated based on the photocatalytic degradation of methylene blue (MB) solution under visible light irradiation. The mechanism underlying the enhancement of photocatalytic activity was also discussed. The results show that the visible-light photocatalytic activity of TiO2-x, and TiO2-x hollow microsphere benefit from the presence of oxygen vacancies on the surface. The photocatalytic activity of TiO2-x hollow microspheres is better than that of TiO2-x, attributed to the formation of hollow structures with higher specific surface areas. The mechanism of MB degradation occurring on the TiO2-x hollow microsphere surface was also investigated. The results show that the MB molecules are photodegraded by the photogenerated hole (h+), reactive superoxide radical (•O2-), and hydroxyl radicals (•OH), and that the •OH radicals, produced only by photogenerated holes, play an essential role in the degradation of MB. Based on the discussion of the effect of initial concentration of MB on the degradation process, a new kinetic model was proposed for the photocatalytic degradation of dye, considering the effect of visible light absorbed by MB molecules, because the data estimated by pseudo-first-order kinetic equation do not fit well with the experimental data. The Runge-Kutta method was used to obtain the numerical solution of the kinetic model. The results show that the kinetic model proposed for photocatalytic dye degradation gives a more realistic description of the photocatalytic degradation of MB because the calculated results fit better with the experimental data. The rate constant (kapp) of the pseudo-first-order kinetic equation decreases with increasing initial concentration of MB, indicating that kapp is affected by the light absorption properties of MB, because an increase in the initial concentration of MB will lead to increased absorption of visible light by MB molecules rather than by TiO2-x hollow microsphere. Unlike the rate constant kapp, the rate constant ka in the proposed model describes the process of photocatalytic dye degradation more effectively because it does not depend on the initial dye concentration.
