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2022 Volume 38 Issue 8
2022, 38(8): 200602
doi: 10.3866/PKU.WHXB202006029
Abstract:
Polymer films are widely used as biomaterials, electronic devices, food packaging materials and gas separation membranes. In practice, cross-linking is an effective method to enhance their stability and increase the strength of these films. However, conventional cross-linked polymer films cannot degrade under mild conditions. Herein, we fabricated two cross-linkable, yet biodegradable, polymer films of ~0.2 mm via solution casting using cinnamate-grafted polycaprolactones, namely: a poly((α-(cinnamoyloxymethyl)-1, 2, 3-triazol) caprolactone) (PCTCL133) homopolymer and a poly(caprolactone-stat-CTCL) (P(CL156-stat-CTCL28)) copolymer. The successful syntheses of the polymers were confirmed via proton nuclear magnetic resonance (1H NMR) spectroscopy, size exclusion chromatography (SEC), and Fourier transform infrared (FT-IR) spectroscopy. The PCTCL homopolymer appeared as a transparent film, owing to its side groups that impede its crystallinity; in contrast, the copolymer film appeared translucent, owing to its PCL segments that are easily crystallized. The cinnamate groups facilitated the cross-linking of the polymer films when irradiated by ultraviolet (UV) light; this is indicated by its insoluble character in tetrahydrofuran, which is a good solvent for both polymers. SEC analysis indicated that a fraction of the P(CL156-stat-CTCL28) film remained un-cross-linked after irradiation, owing to its crystalline structure. In contrast, UV irradiation caused the PCTCL homopolymer film to become homogeneously cross-linked, which exhibited a cross-linking density of 49% after 2 h as indicated by the 1H NMR results. Thermogravimetric analysis (TGA) indicated that cross-linking of the PCTCL films caused a minimal change in thermal stability. Both the cross-linked polymer films were able to degrade upon the addition of a modest amount of concentrated hydrochloric acid, as confirmed by SEC and 1H NMR. However, the degradation rate significantly decreased after cross-linking, thereby indicating its tunable character that can be altered by varying the cross-linking density. In addition, the rate of degradation can be adjusted upon varying the fraction of cross-linked PCTCL groups in the copolymer. In principle, treating the polymer films with sufficient amounts of acid could form degradation products with molecular weights less than 300 g∙mol−1. To further explore the mechanical properties of such materials, we investigated the correlation between the initial concentration used for solution casting and the Young's modulus of the film by employing molecular dynamics simulations. These results indicate that tougher films are prepared when using more concentrated polymer solutions, owing to a higher degree of chain entanglement. In summary, the prepared films with tunable degradability are promising materials for biomedical applications. In principle, this platform could be utilized in hydrogels and coating materials for a broad scope of applications. ![]()
Polymer films are widely used as biomaterials, electronic devices, food packaging materials and gas separation membranes. In practice, cross-linking is an effective method to enhance their stability and increase the strength of these films. However, conventional cross-linked polymer films cannot degrade under mild conditions. Herein, we fabricated two cross-linkable, yet biodegradable, polymer films of ~0.2 mm via solution casting using cinnamate-grafted polycaprolactones, namely: a poly((α-(cinnamoyloxymethyl)-1, 2, 3-triazol) caprolactone) (PCTCL133) homopolymer and a poly(caprolactone-stat-CTCL) (P(CL156-stat-CTCL28)) copolymer. The successful syntheses of the polymers were confirmed via proton nuclear magnetic resonance (1H NMR) spectroscopy, size exclusion chromatography (SEC), and Fourier transform infrared (FT-IR) spectroscopy. The PCTCL homopolymer appeared as a transparent film, owing to its side groups that impede its crystallinity; in contrast, the copolymer film appeared translucent, owing to its PCL segments that are easily crystallized. The cinnamate groups facilitated the cross-linking of the polymer films when irradiated by ultraviolet (UV) light; this is indicated by its insoluble character in tetrahydrofuran, which is a good solvent for both polymers. SEC analysis indicated that a fraction of the P(CL156-stat-CTCL28) film remained un-cross-linked after irradiation, owing to its crystalline structure. In contrast, UV irradiation caused the PCTCL homopolymer film to become homogeneously cross-linked, which exhibited a cross-linking density of 49% after 2 h as indicated by the 1H NMR results. Thermogravimetric analysis (TGA) indicated that cross-linking of the PCTCL films caused a minimal change in thermal stability. Both the cross-linked polymer films were able to degrade upon the addition of a modest amount of concentrated hydrochloric acid, as confirmed by SEC and 1H NMR. However, the degradation rate significantly decreased after cross-linking, thereby indicating its tunable character that can be altered by varying the cross-linking density. In addition, the rate of degradation can be adjusted upon varying the fraction of cross-linked PCTCL groups in the copolymer. In principle, treating the polymer films with sufficient amounts of acid could form degradation products with molecular weights less than 300 g∙mol−1. To further explore the mechanical properties of such materials, we investigated the correlation between the initial concentration used for solution casting and the Young's modulus of the film by employing molecular dynamics simulations. These results indicate that tougher films are prepared when using more concentrated polymer solutions, owing to a higher degree of chain entanglement. In summary, the prepared films with tunable degradability are promising materials for biomedical applications. In principle, this platform could be utilized in hydrogels and coating materials for a broad scope of applications.
2022, 38(8): 200901
doi: 10.3866/PKU.WHXB202009015
Abstract:
2022, 38(8): 201200
doi: 10.3866/PKU.WHXB202012005
Abstract:
2022, 38(8): 201200
doi: 10.3866/PKU.WHXB202012009
Abstract:
2022, 38(8): 201201
doi: 10.3866/PKU.WHXB202012011
Abstract:
2022, 38(8): 201202
doi: 10.3866/PKU.WHXB202012023
Abstract:
2022, 38(8): 201202
doi: 10.3866/PKU.WHXB202012028
Abstract:
2022, 38(8): 200907
doi: 10.3866/PKU.WHXB202009071
Abstract:
The gradual popularization of new energy technologies has led to rapid development in the field of electric transportation. At present, the demand for high-power density batteries is increasing and next-generation higher-energy battery chemistries aimed at replacing current lithium-ion batteries are emerging. The lithium-air batteries (LABs) are thought to be the ultimate energy conversion and storage system, because of their highest theoretical specific energy compared with other known battery systems. Current LABs are operated with pure O2 provided by weighty O2 cylinders instead of the breathing air, and this configuration would greatly undermine LAB's energy density and practicality. However, when the breathing air is used as O2 feed for LABs, CO2, as an inevitable impurity therein, usually leads to severe parasitic reactions and can easily deteriorate the performance of LABs. Specifically, Li2O2 will react with CO2 to form Li2CO3 on the cathode surface. Compared with the desired discharge product Li2O2, the Li2CO3 is an insulating solid, which will accumulate and finally passivate the electrode surface leading to the "sudden death" phenomenon of LABs. Moreover, Li2CO3 is hard to decompose and a high overpotential is required to charge LABs containing Li2CO3 compounds, which not only degrades energy efficiency but also decomposes other battery components (e.g., cathode materials and electrolytes). In recent years, researchers have proposed many strategies to alleviate the negative effects brought about by Li2CO3, such as catalyst engineering, electrolyte design, and so on, in which O2 selective permeable membranes are worth noting. This review summarizes the recent progresses on the understanding of the CO2-related chemistry and electrochemistry in LABs and describes the various strategies to mitigate and even avoid the negative effects of CO2. The perspective of CO2 separation technology using selective permeable membranes/filters in the context of LABs is also discussed. ![]()
The gradual popularization of new energy technologies has led to rapid development in the field of electric transportation. At present, the demand for high-power density batteries is increasing and next-generation higher-energy battery chemistries aimed at replacing current lithium-ion batteries are emerging. The lithium-air batteries (LABs) are thought to be the ultimate energy conversion and storage system, because of their highest theoretical specific energy compared with other known battery systems. Current LABs are operated with pure O2 provided by weighty O2 cylinders instead of the breathing air, and this configuration would greatly undermine LAB's energy density and practicality. However, when the breathing air is used as O2 feed for LABs, CO2, as an inevitable impurity therein, usually leads to severe parasitic reactions and can easily deteriorate the performance of LABs. Specifically, Li2O2 will react with CO2 to form Li2CO3 on the cathode surface. Compared with the desired discharge product Li2O2, the Li2CO3 is an insulating solid, which will accumulate and finally passivate the electrode surface leading to the "sudden death" phenomenon of LABs. Moreover, Li2CO3 is hard to decompose and a high overpotential is required to charge LABs containing Li2CO3 compounds, which not only degrades energy efficiency but also decomposes other battery components (e.g., cathode materials and electrolytes). In recent years, researchers have proposed many strategies to alleviate the negative effects brought about by Li2CO3, such as catalyst engineering, electrolyte design, and so on, in which O2 selective permeable membranes are worth noting. This review summarizes the recent progresses on the understanding of the CO2-related chemistry and electrochemistry in LABs and describes the various strategies to mitigate and even avoid the negative effects of CO2. The perspective of CO2 separation technology using selective permeable membranes/filters in the context of LABs is also discussed.
2022, 38(8): 200907
doi: 10.3866/PKU.WHXB202009073
Abstract:
With the development of clean energy sources such as solar and wind power, large-scale energy storage technologies will play a significant role in the rational utilization of clean energy. Sodium ion batteries have garnered considerable attention for large-scale energy storage owing to their low cost and the presence of abundant sodium resources. It is particularly crucial to develop electrode materials for sodium battery with good rate capability and long cycle life. Orthogonal-phase niobium oxide (T-Nb2O5) exhibits good potential to be used as anode material for sodium-ion batteries owing to its high theoretical specific capacity (200 mAh·g−1) and high ionic diffusion coefficient. Furthermore, it demonstrates a better performance than that of graphite and exhibits a higher specific capacity than that of Li4TiO4 when used in sodium-ion batteries. However, its poor electrical conductivity has hindered its practical application. Recently, effective strategies such as coating with carbon materials or metal conductive particles have been developed to overcome this issue. Although the electrochemical performance of T-Nb2O5 has been improved, the sodiation mechanism of T-Nb2O5 is still unclear. It is considered to be similar to the lithium mechanism wherein lithium ions diffuse rapidly on the (001) planes, but exhibit difficulty in diffusing across the (001) planes. In this study, the electrochemical sodiation behaviors along the (001) lattice planes and the [001] direction of the T-Nb2O5 nanosheet are studied by in situ transmission electron microscopy (TEM). The results indicate that there are a large number of dislocations and domain boundaries in nanocrystals. Furthermore, it was observed that, sodium ions can diffuse across the (001) lattice planes through these defects, and then diffuse rapidly on the (001) planes. Meanwhile, we found a modulation structure in the [001] direction of the original nanosheet, in which alternating compressive and tensile strains were observed. These strain distributions can be regulated by the insertion of sodium ions, while the modulation structure is maintained. Moreover, the in situ TEM method used in this work can be applied to various energy materials. ![]()
With the development of clean energy sources such as solar and wind power, large-scale energy storage technologies will play a significant role in the rational utilization of clean energy. Sodium ion batteries have garnered considerable attention for large-scale energy storage owing to their low cost and the presence of abundant sodium resources. It is particularly crucial to develop electrode materials for sodium battery with good rate capability and long cycle life. Orthogonal-phase niobium oxide (T-Nb2O5) exhibits good potential to be used as anode material for sodium-ion batteries owing to its high theoretical specific capacity (200 mAh·g−1) and high ionic diffusion coefficient. Furthermore, it demonstrates a better performance than that of graphite and exhibits a higher specific capacity than that of Li4TiO4 when used in sodium-ion batteries. However, its poor electrical conductivity has hindered its practical application. Recently, effective strategies such as coating with carbon materials or metal conductive particles have been developed to overcome this issue. Although the electrochemical performance of T-Nb2O5 has been improved, the sodiation mechanism of T-Nb2O5 is still unclear. It is considered to be similar to the lithium mechanism wherein lithium ions diffuse rapidly on the (001) planes, but exhibit difficulty in diffusing across the (001) planes. In this study, the electrochemical sodiation behaviors along the (001) lattice planes and the [001] direction of the T-Nb2O5 nanosheet are studied by in situ transmission electron microscopy (TEM). The results indicate that there are a large number of dislocations and domain boundaries in nanocrystals. Furthermore, it was observed that, sodium ions can diffuse across the (001) lattice planes through these defects, and then diffuse rapidly on the (001) planes. Meanwhile, we found a modulation structure in the [001] direction of the original nanosheet, in which alternating compressive and tensile strains were observed. These strain distributions can be regulated by the insertion of sodium ions, while the modulation structure is maintained. Moreover, the in situ TEM method used in this work can be applied to various energy materials.
2022, 38(8): 201100
doi: 10.3866/PKU.WHXB202011009
Abstract:
Solid oxide fuel cell (SOFC) with high energy conversion efficiency, low pollutant emission, and good fuel adaptability has witnessed rapid development in recent years. However, the commercialization of SOFC remains limited by constraints of performance and stability. Electrochemical impedance spectroscopy (EIS) can distinguish ohmic impedance caused by ion transport from polarization impedance related to electrode reaction; it has been widely used in the research of performance and stability as an efficient on-line characterization technology. The physical/chemical processes involved in EIS overlap significantly and can be decomposed by the distribution of relaxation times (DRT) method which does not depend on prior assumptions. Since industrial large-size SOFC is vulnerable to the influence of inductance and disturbance when testing EIS, its EIS analysis is rarely studied and mostly based on the research results of cells with smaller electrode active area. To further elucidate the impedance spectrum of industrial large-size SOFC under actual working conditions, the EIS of industrial-size (10 cm × 10 cm) anode-supported planar SOFC was systematically tested over a broad temperature and anode/cathode gas composition range. First, the quality of the impedance data was examined by performing a Kramers-Kronig test. The residuals of real and imaginary data were within the range of ±1%, indicating good data quality. Then, the DRT method was adopted to parse the EIS data. By comparing and analyzing the DRT results under different conditions, the corresponding relationships between each characteristic peak in the DRT results and the specific electrode process in the SOFC were revealed. The characteristic frequencies were separated into 0.5-1, 1-30, 10-30, 1 × 102-1 × 103, and 1 × 104-3 × 104 Hz regions, corresponding to gas conversion within the anode, gas diffusion within the anode, oxygen surface exchange reaction within the cathode, charge-transfer reaction within the anode, and oxygen ionic transport process, respectively. In this study, the identification of each electrode process in industrial large-size SOFC is realized, indicate that the gas conversion process in large-size SOFC with larger active area and smaller flows cannot be ignored compared with the cells with smaller electrode active area. The method followed and the results obtained have a universal quality and can be applied to the in situ characterization, online monitoring, and degradation mechanism research of SOFC, thus laying a foundation for the optimization of the performance and stability. ![]()
Solid oxide fuel cell (SOFC) with high energy conversion efficiency, low pollutant emission, and good fuel adaptability has witnessed rapid development in recent years. However, the commercialization of SOFC remains limited by constraints of performance and stability. Electrochemical impedance spectroscopy (EIS) can distinguish ohmic impedance caused by ion transport from polarization impedance related to electrode reaction; it has been widely used in the research of performance and stability as an efficient on-line characterization technology. The physical/chemical processes involved in EIS overlap significantly and can be decomposed by the distribution of relaxation times (DRT) method which does not depend on prior assumptions. Since industrial large-size SOFC is vulnerable to the influence of inductance and disturbance when testing EIS, its EIS analysis is rarely studied and mostly based on the research results of cells with smaller electrode active area. To further elucidate the impedance spectrum of industrial large-size SOFC under actual working conditions, the EIS of industrial-size (10 cm × 10 cm) anode-supported planar SOFC was systematically tested over a broad temperature and anode/cathode gas composition range. First, the quality of the impedance data was examined by performing a Kramers-Kronig test. The residuals of real and imaginary data were within the range of ±1%, indicating good data quality. Then, the DRT method was adopted to parse the EIS data. By comparing and analyzing the DRT results under different conditions, the corresponding relationships between each characteristic peak in the DRT results and the specific electrode process in the SOFC were revealed. The characteristic frequencies were separated into 0.5-1, 1-30, 10-30, 1 × 102-1 × 103, and 1 × 104-3 × 104 Hz regions, corresponding to gas conversion within the anode, gas diffusion within the anode, oxygen surface exchange reaction within the cathode, charge-transfer reaction within the anode, and oxygen ionic transport process, respectively. In this study, the identification of each electrode process in industrial large-size SOFC is realized, indicate that the gas conversion process in large-size SOFC with larger active area and smaller flows cannot be ignored compared with the cells with smaller electrode active area. The method followed and the results obtained have a universal quality and can be applied to the in situ characterization, online monitoring, and degradation mechanism research of SOFC, thus laying a foundation for the optimization of the performance and stability.
2022, 38(8): 201201
doi: 10.3866/PKU.WHXB202012019
Abstract:
Surfactants are widely applied for promoting miscibility and reducing interfacial tension between oil and water phases because of their remarkable amphiphilic morphology. Along with development and popularization of tertiary oil recovery techniques, surfactants play a significant role in crude oil exploitation. Among the various tertiary oil recovery techniques, supercritical CO2-enhanced oil recovery is a promising method for improving oil recovery. However, the establishment of CO2-enhanced oil recovery brought new requirements and challenges to traditional surfactant research and development, especially for molecular design. In this method, the reduction of the minimum miscibility pressure between supercritical CO2 and crude oil is required to achieve miscible flooding—an important means to enhance oil recovery. Therefore, a novel miscible flooding agent that exhibits oil-water miscibility analogous to conventional surfactants is desirable for this method. Meanwhile, a conspicuous difference of polarity matching the high polarity of H2O molecule against low polarity of alkane molecule, which is the essential feature of traditional surfactant, won't suit this case well due to a medium polarity of CO2 molecule. According to previous work done in our laboratory, surfactants with multiple ester groups considerably reduce the minimum miscibility pressure between supercritical CO2 and crude oil. Therefore, inspired by the "oil-water-amphiphilic molecules" design, herein, we replaced the hydrophilic moiety with multiple ester groups and designed a new type of "oil-CO2 amphiphilic molecule" as a miscible flooding agent, which is composed of an alkane tail and multiple ester groups as the lipophilic and CO2-philic groups, respectively. In the strategy based on the proposed agent, the number of ester groups and the length of the alkane tail are the main parameters. In addition, we optimized the molecular structure of the proposed agent, CAA8-X, which comprises cetyl and acetyl sucrose esters as the lipophilic and CO2-philic groups, respectively. We verified that the as-synthesized agent can remarkably reduce the minimum miscibility pressure between supercritical CO2 and various types of oil samples, including kerosene, white oil, and crude oil from the Changqing region. The crude oil-CO2 minimum miscibility pressure reduction ratio was 20.5% as measured by the vanishing interfacial tension method and the slim tube test. In this study, we also established a method called the rising height method to measure the minimum miscibility pressure with significantly reduced time and equipment cost. Furthermore, to demonstrate the mechanism of this miscible flooding agent for CO2-enhanced oil recovery, the affinity between the CO2-philic group and molecular CO2 was investigated via molecular dynamics simulation. The results indicated that the "oil-CO2 amphiphilic molecule" can reduce oil-CO2 interfacial tension because of lower affinity potential energy between the CO2-philic group and molecular CO2. ![]()
Surfactants are widely applied for promoting miscibility and reducing interfacial tension between oil and water phases because of their remarkable amphiphilic morphology. Along with development and popularization of tertiary oil recovery techniques, surfactants play a significant role in crude oil exploitation. Among the various tertiary oil recovery techniques, supercritical CO2-enhanced oil recovery is a promising method for improving oil recovery. However, the establishment of CO2-enhanced oil recovery brought new requirements and challenges to traditional surfactant research and development, especially for molecular design. In this method, the reduction of the minimum miscibility pressure between supercritical CO2 and crude oil is required to achieve miscible flooding—an important means to enhance oil recovery. Therefore, a novel miscible flooding agent that exhibits oil-water miscibility analogous to conventional surfactants is desirable for this method. Meanwhile, a conspicuous difference of polarity matching the high polarity of H2O molecule against low polarity of alkane molecule, which is the essential feature of traditional surfactant, won't suit this case well due to a medium polarity of CO2 molecule. According to previous work done in our laboratory, surfactants with multiple ester groups considerably reduce the minimum miscibility pressure between supercritical CO2 and crude oil. Therefore, inspired by the "oil-water-amphiphilic molecules" design, herein, we replaced the hydrophilic moiety with multiple ester groups and designed a new type of "oil-CO2 amphiphilic molecule" as a miscible flooding agent, which is composed of an alkane tail and multiple ester groups as the lipophilic and CO2-philic groups, respectively. In the strategy based on the proposed agent, the number of ester groups and the length of the alkane tail are the main parameters. In addition, we optimized the molecular structure of the proposed agent, CAA8-X, which comprises cetyl and acetyl sucrose esters as the lipophilic and CO2-philic groups, respectively. We verified that the as-synthesized agent can remarkably reduce the minimum miscibility pressure between supercritical CO2 and various types of oil samples, including kerosene, white oil, and crude oil from the Changqing region. The crude oil-CO2 minimum miscibility pressure reduction ratio was 20.5% as measured by the vanishing interfacial tension method and the slim tube test. In this study, we also established a method called the rising height method to measure the minimum miscibility pressure with significantly reduced time and equipment cost. Furthermore, to demonstrate the mechanism of this miscible flooding agent for CO2-enhanced oil recovery, the affinity between the CO2-philic group and molecular CO2 was investigated via molecular dynamics simulation. The results indicated that the "oil-CO2 amphiphilic molecule" can reduce oil-CO2 interfacial tension because of lower affinity potential energy between the CO2-philic group and molecular CO2.
2022, 38(8): 210105
doi: 10.3866/PKU.WHXB202101055
Abstract:
The diameter-controlled growth of single-walled carbon nanotubes (SWNTs) is one of the key issues of SWNT synthesis and application. To guarantee that SWNTs grow with desired diameters, it is necessary to control catalyst size and modulate growth conditions. SWNTs with diameters of 0.9–1.2 nm are highly desirable for near-infrared fluorescence bioimaging and serving as effective single-photon sources for the development of quantum devices. Herein, we used an FeCo/MgO catalyst to grow bulk SWNTs with diameters in the range and studied the influence of catalysts and chemical vapor deposition (CVD) growth conditions on the diameter of SWNTs. The preparation of catalyst precursors is a key step in obtaining catalyst nanoparticles of small size. In the impregnation process, we used three different types of metal salts, namely, sulfates, acetates, and nitrates, to prepare the catalysts. The metal sulfates, which exhibit the weakest hydrolysis ability, were found to grow SWNTs with the smallest diameters. Lowering the immersion pH, which suppresses the hydrolysis of metal ions, was also favorable for growing smaller SWNTs. Moreover, the addition of complexing agent molecules such as ethylenediaminetetraacetic acid during the impregnation process, which inhibits the hydrolysis of metal ions as well, further confined the diameter distribution of the resultant SWNTs. During the solution drying process, metal salts hydrolyze into metal hydroxides and oxides. Under mild hydrolysis conditions, the produced hydroxide and oxide particles are smaller and more likely to be uniformly distributed on the surface of the supports. Therefore, it is more favorable to produce catalysts with controlled sizes under mild hydrolysis conditions, which are preferred for diameter control of the resultant SWNTs. In the CVD growth process, we used either ethanol or methane as the carbon source and found that, under our experimental conditions, the SWNTs grown from ethanol had smaller diameters than those from methane. The hydrogen content in the CVD process also affects diameter distribution of SWNTs. As the carbon-to-hydrogen ratio decreased, SWNTs with larger diameters disappeared, and the number of SWNTs with smaller diameters increased. During the CVD process, the carbon-to-hydrogen ratio determines the carbon feeding rate to the catalysts. At a low carbon feeding rate, catalysts of large sizes are underfed and unable to grow SWNTs, whereas smaller catalysts are in a favorable condition for growth. Therefore, the average diameter of the SWNTs decreased as the carbon-to-hydrogen ratio decreased. ![]()
The diameter-controlled growth of single-walled carbon nanotubes (SWNTs) is one of the key issues of SWNT synthesis and application. To guarantee that SWNTs grow with desired diameters, it is necessary to control catalyst size and modulate growth conditions. SWNTs with diameters of 0.9–1.2 nm are highly desirable for near-infrared fluorescence bioimaging and serving as effective single-photon sources for the development of quantum devices. Herein, we used an FeCo/MgO catalyst to grow bulk SWNTs with diameters in the range and studied the influence of catalysts and chemical vapor deposition (CVD) growth conditions on the diameter of SWNTs. The preparation of catalyst precursors is a key step in obtaining catalyst nanoparticles of small size. In the impregnation process, we used three different types of metal salts, namely, sulfates, acetates, and nitrates, to prepare the catalysts. The metal sulfates, which exhibit the weakest hydrolysis ability, were found to grow SWNTs with the smallest diameters. Lowering the immersion pH, which suppresses the hydrolysis of metal ions, was also favorable for growing smaller SWNTs. Moreover, the addition of complexing agent molecules such as ethylenediaminetetraacetic acid during the impregnation process, which inhibits the hydrolysis of metal ions as well, further confined the diameter distribution of the resultant SWNTs. During the solution drying process, metal salts hydrolyze into metal hydroxides and oxides. Under mild hydrolysis conditions, the produced hydroxide and oxide particles are smaller and more likely to be uniformly distributed on the surface of the supports. Therefore, it is more favorable to produce catalysts with controlled sizes under mild hydrolysis conditions, which are preferred for diameter control of the resultant SWNTs. In the CVD growth process, we used either ethanol or methane as the carbon source and found that, under our experimental conditions, the SWNTs grown from ethanol had smaller diameters than those from methane. The hydrogen content in the CVD process also affects diameter distribution of SWNTs. As the carbon-to-hydrogen ratio decreased, SWNTs with larger diameters disappeared, and the number of SWNTs with smaller diameters increased. During the CVD process, the carbon-to-hydrogen ratio determines the carbon feeding rate to the catalysts. At a low carbon feeding rate, catalysts of large sizes are underfed and unable to grow SWNTs, whereas smaller catalysts are in a favorable condition for growth. Therefore, the average diameter of the SWNTs decreased as the carbon-to-hydrogen ratio decreased.
2022, 38(8): 210403
doi: 10.3866/PKU.WHXB202104030
Abstract:
Quantum dot light-emitting diodes (QLEDs) constitute the next-generation display technology because of their wide color gamut, narrow emission spectrum, adjustable emission wavelength, and ease of solution processability. With the development of novel material and device preparation techniques, the QLEDs not only show an external quantum efficiency (EQE) of more than 20% in red, green, and blue (primary color) devices, but also achieve 100% Rec.2020 (recommendation standard for ultrahigh-resolution display) color gamut coverage. However, the future commercialization of QLEDs is still a challenge. The T95 lifetime (defined as 95% time for the luminance to decay to the initial value L0 = 1000 cd·m-2) of red, green, and blue QLED devices is significantly lower than that of commercially available organic light-emitting diodes (OLEDs). This is ascribed to the lacking of understanding and argument to hypothesis of degradation mechanisms. A QLED is a sandwich structure composed of a quantum dot (QD) emitter layer, carrier transport layer, and electrode layer. The QLED works on the principle of electroluminescence: electrons and holes injected from the electrodes on both sides of the device cross multiple interfaces and reach the QD emitter layer to undergo radiation recombination. Generally, the QD emitter layer adopts the structure of a wide-band gap shell wrapped around a narrow band-gap core. Because of the deep valence band maximum, the hole injection barrier is higher, and the hole injection efficiency is reduced. This not only disturbs the injection balance but also leads to the accumulation of interfacial holes, which is one of the important factors affecting the efficiency and life of the device. Past studies have attempted to understand charge accumulation behavior in QLEDs by predicting the interfacial energy band structure, and there are very few reports on the direct measurement of charge accumulation. In this work, we built a charge extraction circuit to investigate the charge accumulation behavior before and after aging in a prototype red QLED. In the fresh red QLEDs, the number of accumulated charges gradually increased with the driving current density and tended to saturate above turn-on current density. In the aged red QLEDs, the accumulated charges increased with a decrease in luminance. Our method to investigate the charge accumulation behavior developed can be extended to various kinds of LEDs, such as OLEDs and perovskite LEDs, thus providing insight into their working mechanism. ![]()
Quantum dot light-emitting diodes (QLEDs) constitute the next-generation display technology because of their wide color gamut, narrow emission spectrum, adjustable emission wavelength, and ease of solution processability. With the development of novel material and device preparation techniques, the QLEDs not only show an external quantum efficiency (EQE) of more than 20% in red, green, and blue (primary color) devices, but also achieve 100% Rec.2020 (recommendation standard for ultrahigh-resolution display) color gamut coverage. However, the future commercialization of QLEDs is still a challenge. The T95 lifetime (defined as 95% time for the luminance to decay to the initial value L0 = 1000 cd·m-2) of red, green, and blue QLED devices is significantly lower than that of commercially available organic light-emitting diodes (OLEDs). This is ascribed to the lacking of understanding and argument to hypothesis of degradation mechanisms. A QLED is a sandwich structure composed of a quantum dot (QD) emitter layer, carrier transport layer, and electrode layer. The QLED works on the principle of electroluminescence: electrons and holes injected from the electrodes on both sides of the device cross multiple interfaces and reach the QD emitter layer to undergo radiation recombination. Generally, the QD emitter layer adopts the structure of a wide-band gap shell wrapped around a narrow band-gap core. Because of the deep valence band maximum, the hole injection barrier is higher, and the hole injection efficiency is reduced. This not only disturbs the injection balance but also leads to the accumulation of interfacial holes, which is one of the important factors affecting the efficiency and life of the device. Past studies have attempted to understand charge accumulation behavior in QLEDs by predicting the interfacial energy band structure, and there are very few reports on the direct measurement of charge accumulation. In this work, we built a charge extraction circuit to investigate the charge accumulation behavior before and after aging in a prototype red QLED. In the fresh red QLEDs, the number of accumulated charges gradually increased with the driving current density and tended to saturate above turn-on current density. In the aged red QLEDs, the accumulated charges increased with a decrease in luminance. Our method to investigate the charge accumulation behavior developed can be extended to various kinds of LEDs, such as OLEDs and perovskite LEDs, thus providing insight into their working mechanism.
2022, 38(8): 201106
doi: 10.3866/PKU.WHXB202011060
Abstract:
Metal-organic nanostructures on surfaces have attracted considerable attention owing to their structural stability and potential applications. In metal-organic nanostructures, metal atoms are derived from the externally deposited metals or native surface atoms. Externally deposited metals are of rich diversity and depend on the targeted nanostructures. However, native surface atoms are restricted to single-crystalline metal surfaces of gold, silver, and copper, which are usually used in surface science. Metal-organic nanostructures mostly consist of Au- or Cu-coordinated motifs, while only a few consist of surface Ag atoms. Further investigation into the molecule-metal interactions is beneficial for the accurately controlled fabrication of the desired nanostructure. As for the building blocks, organic molecules coordinate with native surface atoms by M―C, M―N, and M―O bonds. The reactions of terminal alkynes or Ullmann couplings could realize the formation of C―M―C linkages. Cu and Au atoms could coordinate with molecules with terminal cyano or pyridyl groups to form N―M―N bonds. In addition, surface Ag adatoms could coordinate with phthalocyanine through Ag―N bonds. However, a comprehensive study of M―O coordination bonds is still lacking. Thus, we used hydroxyl-terminated molecules to coordinate with Ag adatoms to form metal-organic coordination nanostructures and study the O―Ag linkages. In this case, we successfully built and investigated a series of ordered structures by depositing 4, 4'-dihydroxy-1, 1': 3', 1''-terphenyl (H3PH) molecules on the Ag(111) surface by scanning tunneling microscopy. At room temperature, a close-packed ordered structure was formed by H3PH molecules through cyclic hydrogen bonds, and the repeat unit of such nanostructures contained eight H3PH molecules. Upon increasing the annealing temperature, the formation of O―Ag bond led to a change in the self-assembly pattern. When the annealing temperature was increased to 330 K, a new ordered nanostructure occurred due to the combination of O―Ag coordination and hydrogen bonds. Upon further increasing the annealing temperature to 420 K, a honeycomb structure and coexisting two-fold coordination chains appeared, which only consisted of O―Ag―O linkages. Density functional theory calculations were carried out to analyze the metal-molecule reaction pathways and energy barriers of the O―Ag―O bonds. The energy barrier of the O―Ag bond is 1.41 eV, which is less than that of the O―Ag―O linkage calculated to be 1.85 eV. The low energy barrier of the O―Ag bond and large coordination energy of the O―Ag―O linkage can be attributed to the formation of the hierarchical metal-organic nanostructure. The results obtained herein provide an effective approach for designing and building two-dimensional hierarchical structures with organic small molecules and metal adatoms on single-crystalline metal surfaces. ![]()
Metal-organic nanostructures on surfaces have attracted considerable attention owing to their structural stability and potential applications. In metal-organic nanostructures, metal atoms are derived from the externally deposited metals or native surface atoms. Externally deposited metals are of rich diversity and depend on the targeted nanostructures. However, native surface atoms are restricted to single-crystalline metal surfaces of gold, silver, and copper, which are usually used in surface science. Metal-organic nanostructures mostly consist of Au- or Cu-coordinated motifs, while only a few consist of surface Ag atoms. Further investigation into the molecule-metal interactions is beneficial for the accurately controlled fabrication of the desired nanostructure. As for the building blocks, organic molecules coordinate with native surface atoms by M―C, M―N, and M―O bonds. The reactions of terminal alkynes or Ullmann couplings could realize the formation of C―M―C linkages. Cu and Au atoms could coordinate with molecules with terminal cyano or pyridyl groups to form N―M―N bonds. In addition, surface Ag adatoms could coordinate with phthalocyanine through Ag―N bonds. However, a comprehensive study of M―O coordination bonds is still lacking. Thus, we used hydroxyl-terminated molecules to coordinate with Ag adatoms to form metal-organic coordination nanostructures and study the O―Ag linkages. In this case, we successfully built and investigated a series of ordered structures by depositing 4, 4'-dihydroxy-1, 1': 3', 1''-terphenyl (H3PH) molecules on the Ag(111) surface by scanning tunneling microscopy. At room temperature, a close-packed ordered structure was formed by H3PH molecules through cyclic hydrogen bonds, and the repeat unit of such nanostructures contained eight H3PH molecules. Upon increasing the annealing temperature, the formation of O―Ag bond led to a change in the self-assembly pattern. When the annealing temperature was increased to 330 K, a new ordered nanostructure occurred due to the combination of O―Ag coordination and hydrogen bonds. Upon further increasing the annealing temperature to 420 K, a honeycomb structure and coexisting two-fold coordination chains appeared, which only consisted of O―Ag―O linkages. Density functional theory calculations were carried out to analyze the metal-molecule reaction pathways and energy barriers of the O―Ag―O bonds. The energy barrier of the O―Ag bond is 1.41 eV, which is less than that of the O―Ag―O linkage calculated to be 1.85 eV. The low energy barrier of the O―Ag bond and large coordination energy of the O―Ag―O linkage can be attributed to the formation of the hierarchical metal-organic nanostructure. The results obtained herein provide an effective approach for designing and building two-dimensional hierarchical structures with organic small molecules and metal adatoms on single-crystalline metal surfaces.
