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2019 Volume 35 Issue 6
2019, 35(6): 555-556
doi: 10.3866/PKU.WHXB201808032
Abstract:
2019, 35(6): 557-558
doi: 10.3866/PKU.WHXB201809010
Abstract:
2019, 35(6): 559-560
doi: 10.3866/PKU.WHXB201809016
Abstract:
2019, 35(6): 561-562
doi: 10.3866/PKU.WHXB201809022
Abstract:
2019, 35(6): 563-564
doi: 10.3866/PKU.WHXB201809054
Abstract:
2019, 35(6): 565-571
doi: 10.3866/PKU.WHXB201805080
Abstract:
Borophene, a boron analogue of graphene, exhibits a rich variety of chemical and physical properties. Here, we provide an intensive overview of recent progress in theoretical modeling and experimental synthesis of borophene. In particular, we analyze the influence of substrate, growth temperature, and precursor on the selectivity of boron nucleation. While three-dimensional (3D) bulk boron is more stable than a two-dimensional (2D) boron sheet, the nucleation barrier determined by the growth process controls the formation of the material and it depends on the specific growth environment. Theoretical studies have shown that a metal substrate can play an important role in stabilizing 2D boron clusters over their 3D form, resulting in the kinetically favored growth of 2D boron on the substrate even though the 2D boron clusters will be overwhelmingly less stable than the 3D form with increasing cluster size. Ag and Cu substrates have proven to be particularly suitable for achieving this preference. Guided by theoretical works and perhaps original insights, experimentalists from two independent groups have successfully synthesized 2D boron sheets on silver substrates by depositing ultra-high purity boron onto a clean Ag (111) surface under high vacuum conditions. Moreover, the borophene samples were found to exhibit the same atomic structure previously predicted to be preferred on this substrate. Besides the substrate, the growth temperature is also key to the final product. When the temperature is too low, boron growth cannot overcome the nucleation barrier of the 2D structure. As a result, boron clusters or amorphous boron structures are likely to be formed. In contrast, an excessively high growth temperature will steer the growth to overcome the nucleation barrier of 3D boron, possibly yielding boron nanofilms with finite thickness. Therefore, the growth temperature needs to be carefully controlled, so that the free energy of boron growth will be located between the nucleation barriers of the 3D and 2D forms. Some impurity elements found in synthetic source materials, such as hydrogen and oxygen, can also impact boron nucleation. The existence of these elements may alter the competition between 2D and 3D structures during the nucleation process. More importantly, hydrogen and oxygen can passivate the dangling bonds on the surface of a 3D boron structure, lowering its surface energy, and therefore, impairing the nucleation of 2D boron structures. At present, molecular beam epitaxy (MBE) is the only method with which borophene has been successfully synthesized. Yet this method is very expensive, suffers from low yield, and is constrained to small sample sizes. Thus, exploring the growth of borophene via chemical vapor deposition (CVD) on different substrates is critically important for realizing the great potential of borophene in various applications. By discussing possible growth conditions and atomistic mechanisms of borophene nucleation as well as theoretical methods for modeling and simulations, we suggest prospects for chemical vapor deposition growth of borophene on selected substrates. This work aims to offer useful guidance for chemical synthesis of large-area, high-quality borophenes and promote their practical applications. ![]()
Borophene, a boron analogue of graphene, exhibits a rich variety of chemical and physical properties. Here, we provide an intensive overview of recent progress in theoretical modeling and experimental synthesis of borophene. In particular, we analyze the influence of substrate, growth temperature, and precursor on the selectivity of boron nucleation. While three-dimensional (3D) bulk boron is more stable than a two-dimensional (2D) boron sheet, the nucleation barrier determined by the growth process controls the formation of the material and it depends on the specific growth environment. Theoretical studies have shown that a metal substrate can play an important role in stabilizing 2D boron clusters over their 3D form, resulting in the kinetically favored growth of 2D boron on the substrate even though the 2D boron clusters will be overwhelmingly less stable than the 3D form with increasing cluster size. Ag and Cu substrates have proven to be particularly suitable for achieving this preference. Guided by theoretical works and perhaps original insights, experimentalists from two independent groups have successfully synthesized 2D boron sheets on silver substrates by depositing ultra-high purity boron onto a clean Ag (111) surface under high vacuum conditions. Moreover, the borophene samples were found to exhibit the same atomic structure previously predicted to be preferred on this substrate. Besides the substrate, the growth temperature is also key to the final product. When the temperature is too low, boron growth cannot overcome the nucleation barrier of the 2D structure. As a result, boron clusters or amorphous boron structures are likely to be formed. In contrast, an excessively high growth temperature will steer the growth to overcome the nucleation barrier of 3D boron, possibly yielding boron nanofilms with finite thickness. Therefore, the growth temperature needs to be carefully controlled, so that the free energy of boron growth will be located between the nucleation barriers of the 3D and 2D forms. Some impurity elements found in synthetic source materials, such as hydrogen and oxygen, can also impact boron nucleation. The existence of these elements may alter the competition between 2D and 3D structures during the nucleation process. More importantly, hydrogen and oxygen can passivate the dangling bonds on the surface of a 3D boron structure, lowering its surface energy, and therefore, impairing the nucleation of 2D boron structures. At present, molecular beam epitaxy (MBE) is the only method with which borophene has been successfully synthesized. Yet this method is very expensive, suffers from low yield, and is constrained to small sample sizes. Thus, exploring the growth of borophene via chemical vapor deposition (CVD) on different substrates is critically important for realizing the great potential of borophene in various applications. By discussing possible growth conditions and atomistic mechanisms of borophene nucleation as well as theoretical methods for modeling and simulations, we suggest prospects for chemical vapor deposition growth of borophene on selected substrates. This work aims to offer useful guidance for chemical synthesis of large-area, high-quality borophenes and promote their practical applications.
2019, 35(6): 572-590
doi: 10.3866/PKU.WHXB201806060
Abstract:
Carbon atoms can bond together in different molecular configurations leading to different carbon allotropes including diamond, fullerene, carbon nanotubes, graphene, and graphdiyne that are widely used or explored in a number of fields. Carbon dots (CDs), which are generally surface-passivated carbon nanoparticles less than 10 nm in size, are other new members of carbon allotropes. CDs were serendipitously discovered in 2004 during the electrophoresis purification of single-walled carbon nanotubes. Similar to their popular older cousins, fullerenes, carbon nanotubes, and graphene, CDs have drawn much attention in the past decade and have gradually become a rising star because of the advantages of chemical inertness, high abundance, good biocompatibility, and low toxicity. Interestingly, CDs typically display excitation-energy- and size-dependent fluorescent behavior. Depending on their structures, the fluorescence from CDs is either attributed to the quantum-confinement effect and conjugated π-domains of the carbogenic core (intrinsic states), or determined by the hybridization of the carbon skeleton and the connected chemical groups (surface states). Compared with the traditional semiconductors, quantum dots, and their organic dye counterparts, fluorescent CDs possess not only excellent optical properties and small-size effect, but also the advantages of low-cost synthesis, good photo-bleaching resistance, tunable band gaps, and surface functionalities. For these reasons, CDs are considered to be emergent nanolights for bio-imaging, sensing, and optoelectronic devices. Additionally, CDs feature abundant structural defects at their surface and edges, excellent light-harvesting capability, and photo-induced electron-transfer ability, thus facilitating their applications in photocatalysis and energy storage and conversions. To date, remarkable progress has been achieved in terms of synthetic approaches, properties, and applications of CDs. This review aims to classify the different types of CDs, based on the structures of their carbogenic cores, and to describe their structural characteristics in terms of synthesis approaches. Two well-established strategies for synthesizing CDs, the top-down and bottom-up routes, are highlighted. The diverse potential applications, in the bio-imaging and diagnosis, sensing, catalysis, optoelectronics, and energy-storage fields, of CDs with different structures and physicochemical properties, are summarized, covering the issues of surface modification, heteroatom doping, and hybrids made by combining CDs with other species such as metals, metal oxides, and biological molecules. The challenges and opportunities for the future development of CDs are also briefly outlined. ![]()
Carbon atoms can bond together in different molecular configurations leading to different carbon allotropes including diamond, fullerene, carbon nanotubes, graphene, and graphdiyne that are widely used or explored in a number of fields. Carbon dots (CDs), which are generally surface-passivated carbon nanoparticles less than 10 nm in size, are other new members of carbon allotropes. CDs were serendipitously discovered in 2004 during the electrophoresis purification of single-walled carbon nanotubes. Similar to their popular older cousins, fullerenes, carbon nanotubes, and graphene, CDs have drawn much attention in the past decade and have gradually become a rising star because of the advantages of chemical inertness, high abundance, good biocompatibility, and low toxicity. Interestingly, CDs typically display excitation-energy- and size-dependent fluorescent behavior. Depending on their structures, the fluorescence from CDs is either attributed to the quantum-confinement effect and conjugated π-domains of the carbogenic core (intrinsic states), or determined by the hybridization of the carbon skeleton and the connected chemical groups (surface states). Compared with the traditional semiconductors, quantum dots, and their organic dye counterparts, fluorescent CDs possess not only excellent optical properties and small-size effect, but also the advantages of low-cost synthesis, good photo-bleaching resistance, tunable band gaps, and surface functionalities. For these reasons, CDs are considered to be emergent nanolights for bio-imaging, sensing, and optoelectronic devices. Additionally, CDs feature abundant structural defects at their surface and edges, excellent light-harvesting capability, and photo-induced electron-transfer ability, thus facilitating their applications in photocatalysis and energy storage and conversions. To date, remarkable progress has been achieved in terms of synthetic approaches, properties, and applications of CDs. This review aims to classify the different types of CDs, based on the structures of their carbogenic cores, and to describe their structural characteristics in terms of synthesis approaches. Two well-established strategies for synthesizing CDs, the top-down and bottom-up routes, are highlighted. The diverse potential applications, in the bio-imaging and diagnosis, sensing, catalysis, optoelectronics, and energy-storage fields, of CDs with different structures and physicochemical properties, are summarized, covering the issues of surface modification, heteroatom doping, and hybrids made by combining CDs with other species such as metals, metal oxides, and biological molecules. The challenges and opportunities for the future development of CDs are also briefly outlined.
2019, 35(6): 591-597
doi: 10.3866/PKU.WHXB201806042
Abstract:
Kerosene is an ideal endothermic hydrocarbon. Its pyrolysis plays a significant role in the thermal protection for high-speed aircraft. Before it reacts, kerosene experiences thermal decomposition in the heat exchanger and produces cracked products. Thus, to use cracked kerosene instead of pure kerosene, knowledge of their ignition properties is needed. In this study, ignition delay times of cracked kerosene/air and kerosene/air were measured in a heated shock tube at temperatures of 657–1333 K, an equivalence ratio of 1.0, and pressures of 1.01 × 105–10.10 × 105 Pa. Ignition delay time was defined as the time interval between the arrival of the reflected shock and the occurrence of the steepest rise of excited-state CH species (CH*) emission at the sidewall measurement location. Pure helium was used as the driver gas for high-temperature measurements in which test times needed to be shorter than 1.5 ms, and tailored mixtures of He/Ar were used when test times could reach up to 15 ms. Arrhenius-type formulas for the relationship between ignition delay time and ignition conditions (temperature and pressure) were obtained by correlating the measured high-temperature data of both fuels. The results reveal that the ignition delay times of both fuels are close, and an increase in the pressure or temperature causes a decrease in the ignition delay time in the high-temperature region (> 1000 K). Both fuels exhibit similar high-temperature ignition delay properties, because they have close pressure exponents (cracked kerosene: τign∝P-0.85; kerosene:τign∝P-0.83) and global activation energies (cracked kerosene: Ea = 143.37 kJ·mol-1; kerosene: Ea = 144.29 kJ·mol-1). However, in the low-temperature region (< 1000 K), ignition delay characteristics are quite different. For cracked kerosene/air, while the decrease in the temperature still results in an increase in the ignition delay time, the negative temperature coefficient (NTC) of ignition delay does not occur, and the low-temperature ignition data still can be correlated by an Arrhenius-type formula with a much smaller global activation energy compared to that at high temperatures. However, for kerosene/air, this NTC phenomenon was observed, and the Arrhenius-type formula fails to correlate its low-temperature ignition data. At temperatures ranging from 830 to 1000 K, the cracked kerosene ignites faster than the kerosene; at temperatures below 830 K, kerosene ignition delay times become much shorter than those of cracked kerosene. Surrogates for cracked kerosene and kerosene are proposed based on the H/C ratio and average molecular weight in order to simulate ignition delay times for cracked kerosene/air and kerosene/air. The simulation results are in fairly good agreement with current experimental data for the two fuels at high temperatures (> 1000 K). However, in the low-temperature NTC region, the results are in very good agreement with kerosene ignition delay data but disagree with cracked kerosene ignition delay data. The comparison between experimental data and model predictions indicates that refinement of the reaction mechanisms for cracked kerosene and kerosene is needed. These test results are helpful to understand ignition properties of cracked kerosene in developing regenerative cooling technology for high-speed aircraft. ![]()
Kerosene is an ideal endothermic hydrocarbon. Its pyrolysis plays a significant role in the thermal protection for high-speed aircraft. Before it reacts, kerosene experiences thermal decomposition in the heat exchanger and produces cracked products. Thus, to use cracked kerosene instead of pure kerosene, knowledge of their ignition properties is needed. In this study, ignition delay times of cracked kerosene/air and kerosene/air were measured in a heated shock tube at temperatures of 657–1333 K, an equivalence ratio of 1.0, and pressures of 1.01 × 105–10.10 × 105 Pa. Ignition delay time was defined as the time interval between the arrival of the reflected shock and the occurrence of the steepest rise of excited-state CH species (CH*) emission at the sidewall measurement location. Pure helium was used as the driver gas for high-temperature measurements in which test times needed to be shorter than 1.5 ms, and tailored mixtures of He/Ar were used when test times could reach up to 15 ms. Arrhenius-type formulas for the relationship between ignition delay time and ignition conditions (temperature and pressure) were obtained by correlating the measured high-temperature data of both fuels. The results reveal that the ignition delay times of both fuels are close, and an increase in the pressure or temperature causes a decrease in the ignition delay time in the high-temperature region (> 1000 K). Both fuels exhibit similar high-temperature ignition delay properties, because they have close pressure exponents (cracked kerosene: τign∝P-0.85; kerosene:τign∝P-0.83) and global activation energies (cracked kerosene: Ea = 143.37 kJ·mol-1; kerosene: Ea = 144.29 kJ·mol-1). However, in the low-temperature region (< 1000 K), ignition delay characteristics are quite different. For cracked kerosene/air, while the decrease in the temperature still results in an increase in the ignition delay time, the negative temperature coefficient (NTC) of ignition delay does not occur, and the low-temperature ignition data still can be correlated by an Arrhenius-type formula with a much smaller global activation energy compared to that at high temperatures. However, for kerosene/air, this NTC phenomenon was observed, and the Arrhenius-type formula fails to correlate its low-temperature ignition data. At temperatures ranging from 830 to 1000 K, the cracked kerosene ignites faster than the kerosene; at temperatures below 830 K, kerosene ignition delay times become much shorter than those of cracked kerosene. Surrogates for cracked kerosene and kerosene are proposed based on the H/C ratio and average molecular weight in order to simulate ignition delay times for cracked kerosene/air and kerosene/air. The simulation results are in fairly good agreement with current experimental data for the two fuels at high temperatures (> 1000 K). However, in the low-temperature NTC region, the results are in very good agreement with kerosene ignition delay data but disagree with cracked kerosene ignition delay data. The comparison between experimental data and model predictions indicates that refinement of the reaction mechanisms for cracked kerosene and kerosene is needed. These test results are helpful to understand ignition properties of cracked kerosene in developing regenerative cooling technology for high-speed aircraft.
2019, 35(6): 624-629
doi: 10.3866/PKU.WHXB201807035
Abstract:
Room temperature ionic liquids (ILs) that can exhibit a colorimetric response to moisture in the air are rarely reported in the literature. In this study, an azophenolic IL solution exhibited a spontaneous a colorimetric response, driven by the formation of hydrogen bonding between the [PhN＝NPhO] anion and moisture in the air. This phenomenon was clearly understood using ultraviolet-visible (UV-Vis) absorption spectroscopy, nuclear magnetic resonance (NMR) spectra, experimental data, and theoretical calculations. Specifically, in the UV-Vis absorption spectra, absorption around 455 nm decreased, while the band around 343 nm increased in the IL CHCl3 solution as time progressed; this was accompanied by a color change from orange to faint yellow. This spontaneous, self-responsive process was further observed using 1H NMR data. When the IL solution was placed with sufficient time, all the 1H NMR peaks of the azophenolic anion shifted downfield, but no new signals appeared in the upfield region. The reason for this was easily identified as the stimuli in the air, such as CO2 and moisture. When pure CO2 was bubbled through the IL CHCl3 solution, the solution color changed from its original orange to light orange, but could not change further to faint yellow, which ruled out CO2 gas as a stimulus. When a small amount of water was gradually added to the IL solution (MeCN solvent), the absorption band around 474 nm decreased, coupled with an increase in the absorption band around 347 nm. This was accompanied by a color change from orange to faint yellow, which was almost identical to the self-responsive process in CHCl3 and CCl4. Moreover, two cuvettes of IL CHCl3 solution were placed under relative humidities of 28% and 100%, respectively; the IL CHCl3 solution required a much longer time to exhibit a complete color change from orange to faint yellow under a lower relative humidity, demonstrating that moisture is the most likely stimulus triggering the self-responsive color change of the IL solution. As revealed by the Gaussian 09 program at the B3LYP/6-31++G(p, d) level, the distance between the oxygen atom on the azophenolic anion and the hydrogen atom on the H2O molecule was 0.174 nm, and the corresponding angle was 171.12°. Furthermore, the atomic dipole moment corrected Hirshfeld (ADCH) charge of the oxygen atom on the azophenolic anion was −0.52, and it increased to −0.62 after the azophenolic anion interacted with the H2O. Reduced density gradient analysis revealed that the spike corresponding to O∙∙∙H―O for the IL-H2O complex was located at around −0.04 a.u.. All the above data indicate that the presence of hydrogen bonding rendered the IL solution responsive to the moisture stimulus, and this response was accompanied by a color change that was visible to the naked eye. To the best of our knowledge, this is the first demonstration of a colorimetric change in an IL solution in response to moisture. We hope this work can help us to gain insight into some seemingly abnormal phenomena that occur during the research process. ![]()
Room temperature ionic liquids (ILs) that can exhibit a colorimetric response to moisture in the air are rarely reported in the literature. In this study, an azophenolic IL solution exhibited a spontaneous a colorimetric response, driven by the formation of hydrogen bonding between the [PhN＝NPhO] anion and moisture in the air. This phenomenon was clearly understood using ultraviolet-visible (UV-Vis) absorption spectroscopy, nuclear magnetic resonance (NMR) spectra, experimental data, and theoretical calculations. Specifically, in the UV-Vis absorption spectra, absorption around 455 nm decreased, while the band around 343 nm increased in the IL CHCl3 solution as time progressed; this was accompanied by a color change from orange to faint yellow. This spontaneous, self-responsive process was further observed using 1H NMR data. When the IL solution was placed with sufficient time, all the 1H NMR peaks of the azophenolic anion shifted downfield, but no new signals appeared in the upfield region. The reason for this was easily identified as the stimuli in the air, such as CO2 and moisture. When pure CO2 was bubbled through the IL CHCl3 solution, the solution color changed from its original orange to light orange, but could not change further to faint yellow, which ruled out CO2 gas as a stimulus. When a small amount of water was gradually added to the IL solution (MeCN solvent), the absorption band around 474 nm decreased, coupled with an increase in the absorption band around 347 nm. This was accompanied by a color change from orange to faint yellow, which was almost identical to the self-responsive process in CHCl3 and CCl4. Moreover, two cuvettes of IL CHCl3 solution were placed under relative humidities of 28% and 100%, respectively; the IL CHCl3 solution required a much longer time to exhibit a complete color change from orange to faint yellow under a lower relative humidity, demonstrating that moisture is the most likely stimulus triggering the self-responsive color change of the IL solution. As revealed by the Gaussian 09 program at the B3LYP/6-31++G(p, d) level, the distance between the oxygen atom on the azophenolic anion and the hydrogen atom on the H2O molecule was 0.174 nm, and the corresponding angle was 171.12°. Furthermore, the atomic dipole moment corrected Hirshfeld (ADCH) charge of the oxygen atom on the azophenolic anion was −0.52, and it increased to −0.62 after the azophenolic anion interacted with the H2O. Reduced density gradient analysis revealed that the spike corresponding to O∙∙∙H―O for the IL-H2O complex was located at around −0.04 a.u.. All the above data indicate that the presence of hydrogen bonding rendered the IL solution responsive to the moisture stimulus, and this response was accompanied by a color change that was visible to the naked eye. To the best of our knowledge, this is the first demonstration of a colorimetric change in an IL solution in response to moisture. We hope this work can help us to gain insight into some seemingly abnormal phenomena that occur during the research process.
2019, 35(6): 598-606
doi: 10.3866/PKU.WHXB201806034
Abstract:
Leucine zipper-functionalized liposomes are promising drug carriers for cancer treatment because of their unique thermosensitivity. The leucine zippers, which consist of two α-helical polypeptides that dimerize in parallel, have characteristic heptad repeats (represented by [abcdefg]n). A leucine residue was observed periodically at site "d" to stabilize the dimerization of the two polypeptides through inter-chain hydrophobic interactions. As the temperature increased, the inter-chain hydrophobic interactions became weaker, eventually triggering the dissociation of the leucine zippers. Due to the unique nature of the temperature response, leucine zippers are useful for developing novel lipid-peptide vesicles for drug delivery because they allow for better control and optimization of drug release under mild hyperthermia. The base sequence of the leucine zipper peptides used in our lab for the functionalize liposomal carrier is [VAQLEVK-VAQLESK-VSKLESK-VSSLESK]. Our recent experiments revealed that modifying this peptide at the N-terminus with distinct functional groups can change the physicochemical properties of the lipopeptides, and eventually affect the liposomes' phase transition behaviors. Four leucine zipper-structured lipopeptides with distinct head groups, viz. ALA, C3CO, C5CO, and POCH, were studied computationally to examine the influence of the molecular structures on the phase transition behaviors of lipopeptides. A series of computational techniques including quantum mechanics (QM) calculations, implicit solvation replica exchange molecular dynamics (REMD) simulations, dihedral principal component analysis (dPCA), and dictionary of protein secondary structure (DSSP) methods, and the conventional explicit solvation molecular dynamics (MD) simulations were applied in this work. First, QM calculations were conducted to obtain the partial charges of some modified head groups. Implicit-solvent REMD simulations were then performed to study the effect of temperature on the folded conformations of the leucine zipper peptides. The dPCA method was used to simulate trajectories to identify representative structures of the peptides at various temperatures, and the DSSP method was used to determine conformation transitions of the four lipopeptides ALA, C3CO, C5CO, and POCH at 324.8, 312.1, 319.1, and 319.4 K, respectively. The thermostability of the lipopeptide dimers in the lipid DPPC bilayer was studied in the conventional explicit solvent MD simulations. Finally, we conducted a deep analysis on the area per lipid and the electron-density profile for the DPPC bilayer to explore the folding and unfolding processes of the lipopeptides in the liposomes to better understand the underlying phase transition mechanisms of the thermosensitive liposomes. On this basis, we could further improve the thermosensitivity of the leucine zipper-structured lipopeptides, thereby facilitating the development of liposomal drug delivery techniques in the future. ![]()
Leucine zipper-functionalized liposomes are promising drug carriers for cancer treatment because of their unique thermosensitivity. The leucine zippers, which consist of two α-helical polypeptides that dimerize in parallel, have characteristic heptad repeats (represented by [abcdefg]n). A leucine residue was observed periodically at site "d" to stabilize the dimerization of the two polypeptides through inter-chain hydrophobic interactions. As the temperature increased, the inter-chain hydrophobic interactions became weaker, eventually triggering the dissociation of the leucine zippers. Due to the unique nature of the temperature response, leucine zippers are useful for developing novel lipid-peptide vesicles for drug delivery because they allow for better control and optimization of drug release under mild hyperthermia. The base sequence of the leucine zipper peptides used in our lab for the functionalize liposomal carrier is [VAQLEVK-VAQLESK-VSKLESK-VSSLESK]. Our recent experiments revealed that modifying this peptide at the N-terminus with distinct functional groups can change the physicochemical properties of the lipopeptides, and eventually affect the liposomes' phase transition behaviors. Four leucine zipper-structured lipopeptides with distinct head groups, viz. ALA, C3CO, C5CO, and POCH, were studied computationally to examine the influence of the molecular structures on the phase transition behaviors of lipopeptides. A series of computational techniques including quantum mechanics (QM) calculations, implicit solvation replica exchange molecular dynamics (REMD) simulations, dihedral principal component analysis (dPCA), and dictionary of protein secondary structure (DSSP) methods, and the conventional explicit solvation molecular dynamics (MD) simulations were applied in this work. First, QM calculations were conducted to obtain the partial charges of some modified head groups. Implicit-solvent REMD simulations were then performed to study the effect of temperature on the folded conformations of the leucine zipper peptides. The dPCA method was used to simulate trajectories to identify representative structures of the peptides at various temperatures, and the DSSP method was used to determine conformation transitions of the four lipopeptides ALA, C3CO, C5CO, and POCH at 324.8, 312.1, 319.1, and 319.4 K, respectively. The thermostability of the lipopeptide dimers in the lipid DPPC bilayer was studied in the conventional explicit solvent MD simulations. Finally, we conducted a deep analysis on the area per lipid and the electron-density profile for the DPPC bilayer to explore the folding and unfolding processes of the lipopeptides in the liposomes to better understand the underlying phase transition mechanisms of the thermosensitive liposomes. On this basis, we could further improve the thermosensitivity of the leucine zipper-structured lipopeptides, thereby facilitating the development of liposomal drug delivery techniques in the future.
2019, 35(6): 607-615
doi: 10.3866/PKU.WHXB201805054
Abstract:
With the discovery and large-scale exploitation of natural gas resources such as shale gas and combustible ice, which are mainly composed of methane, their effective utilization has become a national strategic interest. Partial oxidation of methane (POM) to synthesis gas is one of the important methods for the utilization of natural gas and shale gas resources. The commonly used Ni/SiO2 catalyst for POM easily deactivates due to carbon deposition on the surface. To solve this problem, a urea precipitation method was employed in this work to prepare Ni-based catalysts modified with different amounts of tungsten (at W/Ni molar ratios of 0, 0.01, 0.03, 0.05, 0.07, and 0.10), and the catalyst stability in POM as well as the role of W were investigated. From characterizations such as X-ray diffraction (XRD), transmission electron microscopy (TEM), temperature-programmed reduction (H2-TPR), and X-ray photoelectron spectroscopy (XPS), we obtained the following results. The amount of W added to the Ni-based catalysts has a significant influence on their catalytic performances in POM and their physicochemical properties. The particle size of Ni in the catalysts decreases with W addition, and the Ni particle distribution on the support surfaces becomes more uniformed; however, the catalytic activity for POM is not significantly influenced. However, W-modified Ni-based catalysts show an increasing improvement in their stability in POM with increasing W/Ni molar ratio, with an optimum at the W/Ni molar ratio of 0.07; at the W/Ni molar ratio of 0.10, they exhibit a rapid deactivation in POM in a short time. Although interactions between Ni and SiO2 in the as-prepared catalysts are weak, the presence of adequate tungsten (W/Ni molar ratio of 0.05 and above) in the Ni-based catalysts can reduce the Ni particle size to some extent, and lead to the formation of strong interactions between Ni and W, which leads to an improvement in the dispersion of Ni on the support surface and imparts resistance for Ni particle growth in the POM reaction. The increased interaction between Ni and W changes the chemical state or oxygen affinity of Ni particles on the catalyst surfaces, and some of the partially oxidized Ni species (Niδ+) on the catalyst surfaces coexist with reduced Ni species (Ni0) during POM. Using an adequate amount of W-modified Ni catalysts results in almost no carbon deposition on the surfaces during POM, but using only a moderate amount results in good catalytic behavior and stability in POM. This finding suggests that the presence of W can not only enhance the anti-coking ability of the Ni-based catalysts and sustain their good stability in POM if the W content is low (i.e., W/Ni molar ratio of 0.07 and below), but also lead to the deactivation of W-modified catalysts in POM if the W content is high (i.e., W/Ni molar ratio of 0.10 and above), due to high oxygen affinity or the presence of more Ni species in oxidized form. In addition, α-WC (tungsten carbide) was identified using XRD to be formed on the surface of the moderate-amount W-modified Ni catalysts after POM, and it could inhibit or eliminate carbon deposition on the Ni-based catalyst surfaces. The catalytic performance evaluation of the catalysts in POM under a long time period confirmed that α-WC is stable. ![]()
With the discovery and large-scale exploitation of natural gas resources such as shale gas and combustible ice, which are mainly composed of methane, their effective utilization has become a national strategic interest. Partial oxidation of methane (POM) to synthesis gas is one of the important methods for the utilization of natural gas and shale gas resources. The commonly used Ni/SiO2 catalyst for POM easily deactivates due to carbon deposition on the surface. To solve this problem, a urea precipitation method was employed in this work to prepare Ni-based catalysts modified with different amounts of tungsten (at W/Ni molar ratios of 0, 0.01, 0.03, 0.05, 0.07, and 0.10), and the catalyst stability in POM as well as the role of W were investigated. From characterizations such as X-ray diffraction (XRD), transmission electron microscopy (TEM), temperature-programmed reduction (H2-TPR), and X-ray photoelectron spectroscopy (XPS), we obtained the following results. The amount of W added to the Ni-based catalysts has a significant influence on their catalytic performances in POM and their physicochemical properties. The particle size of Ni in the catalysts decreases with W addition, and the Ni particle distribution on the support surfaces becomes more uniformed; however, the catalytic activity for POM is not significantly influenced. However, W-modified Ni-based catalysts show an increasing improvement in their stability in POM with increasing W/Ni molar ratio, with an optimum at the W/Ni molar ratio of 0.07; at the W/Ni molar ratio of 0.10, they exhibit a rapid deactivation in POM in a short time. Although interactions between Ni and SiO2 in the as-prepared catalysts are weak, the presence of adequate tungsten (W/Ni molar ratio of 0.05 and above) in the Ni-based catalysts can reduce the Ni particle size to some extent, and lead to the formation of strong interactions between Ni and W, which leads to an improvement in the dispersion of Ni on the support surface and imparts resistance for Ni particle growth in the POM reaction. The increased interaction between Ni and W changes the chemical state or oxygen affinity of Ni particles on the catalyst surfaces, and some of the partially oxidized Ni species (Niδ+) on the catalyst surfaces coexist with reduced Ni species (Ni0) during POM. Using an adequate amount of W-modified Ni catalysts results in almost no carbon deposition on the surfaces during POM, but using only a moderate amount results in good catalytic behavior and stability in POM. This finding suggests that the presence of W can not only enhance the anti-coking ability of the Ni-based catalysts and sustain their good stability in POM if the W content is low (i.e., W/Ni molar ratio of 0.07 and below), but also lead to the deactivation of W-modified catalysts in POM if the W content is high (i.e., W/Ni molar ratio of 0.10 and above), due to high oxygen affinity or the presence of more Ni species in oxidized form. In addition, α-WC (tungsten carbide) was identified using XRD to be formed on the surface of the moderate-amount W-modified Ni catalysts after POM, and it could inhibit or eliminate carbon deposition on the Ni-based catalyst surfaces. The catalytic performance evaluation of the catalysts in POM under a long time period confirmed that α-WC is stable.
2019, 35(6): 616-623
doi: 10.3866/PKU.WHXB201805082
Abstract:
TiO2 nanotube arrays (NTAs) have high photocatalytic activity; however, their weak visible light absorption limits their solar energy utilization and environmental application. Perovskite (ABO3)-type oxides with a narrow band gap can absorb visible light in a wide wavelength range and have excellent stability; however, their photocatalytic activity is relatively low. Coupling TiO2 NTAs with ABO3 to form heterojunctions is one of the most promising approaches to extend the optical absorption of TiO2 NTAs into the visible-light range and promote the separation rate of photogenerated electron–hole pairs. However, to date, constructing ABO3-TiO2 NTA heterostructured composites has been extremely challenging owing to the different crystallization temperatures of anatase TiO2 NTAs and ABO3. In this work, LaCoO3 nanoparticles were first synthesized using a sol-gel method. The as-prepared LaCoO3 nanoparticles were then modified on the surface of the TiO2 NTAs using an electrophoretic deposition technique, and a series of LaCoO3-TiO2 NTAs photocatalysts were thus constructed by controlling the deposition time. Results of the scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) demonstrated that the nanoparticles prepared through the sol-gel method were LaCoO3 with a uniform size and high crystallization. The average diameter of the LaCoO3 nanoparticles was 100 nm. The binding strength between the LaCoO3 nanoparticles and the TiO2 NTAs was strong. The UV-visible absorption spectra (diffuse reflectance spectroscopy; DRS) demonstrated that the absorption band edge of the LaCoO3-TiO2 NTAs was gradually red-shifted into the visible light region with the increase in electrophoretic time. The LaCoO3-TiO2 NTAs prepared by the electrophoretic deposition technique for 15 min exhibited a strong light absorption in the wide wavelength range from 250 to 700 nm, which was the same as that of the LaCoO3 nanoparticles loaded on a Ti foil. The results of the photocatalytic degradation of methyl orange (MO) under visible light irradiation demonstrated that the photocatalytic degradation rate of MO over LaCoO3-TiO2 NTAs was considerably higher than those of TiO2 NTAs and LaCoO3 nanoparticles loaded on a Ti foil. The LaCoO3-TiO2 NTAs prepared by the electrophoretic deposition technique for 15 min showed the highest photocatalytic degradation rate of MO, which was a four-fold enhancement compared to that of TiO2 NTs under the same conditions. The p-n heterojunctions between the LaCoO3 nanoparticles and the TiO2 nanotubes were responsible for the enhanced visible light photocatalytic activity. The results of the electrochemical impedance spectroscopy (EIS) and photoluminescence spectroscopy (PL) tests demonstrated that the loading of the LaCoO3 nanoparticles effectively promoted the separation and transport of photogenerated charges, thereby enhancing the visible light photocatalytic activity of the TiO2 NTAs. ![]()
TiO2 nanotube arrays (NTAs) have high photocatalytic activity; however, their weak visible light absorption limits their solar energy utilization and environmental application. Perovskite (ABO3)-type oxides with a narrow band gap can absorb visible light in a wide wavelength range and have excellent stability; however, their photocatalytic activity is relatively low. Coupling TiO2 NTAs with ABO3 to form heterojunctions is one of the most promising approaches to extend the optical absorption of TiO2 NTAs into the visible-light range and promote the separation rate of photogenerated electron–hole pairs. However, to date, constructing ABO3-TiO2 NTA heterostructured composites has been extremely challenging owing to the different crystallization temperatures of anatase TiO2 NTAs and ABO3. In this work, LaCoO3 nanoparticles were first synthesized using a sol-gel method. The as-prepared LaCoO3 nanoparticles were then modified on the surface of the TiO2 NTAs using an electrophoretic deposition technique, and a series of LaCoO3-TiO2 NTAs photocatalysts were thus constructed by controlling the deposition time. Results of the scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) demonstrated that the nanoparticles prepared through the sol-gel method were LaCoO3 with a uniform size and high crystallization. The average diameter of the LaCoO3 nanoparticles was 100 nm. The binding strength between the LaCoO3 nanoparticles and the TiO2 NTAs was strong. The UV-visible absorption spectra (diffuse reflectance spectroscopy; DRS) demonstrated that the absorption band edge of the LaCoO3-TiO2 NTAs was gradually red-shifted into the visible light region with the increase in electrophoretic time. The LaCoO3-TiO2 NTAs prepared by the electrophoretic deposition technique for 15 min exhibited a strong light absorption in the wide wavelength range from 250 to 700 nm, which was the same as that of the LaCoO3 nanoparticles loaded on a Ti foil. The results of the photocatalytic degradation of methyl orange (MO) under visible light irradiation demonstrated that the photocatalytic degradation rate of MO over LaCoO3-TiO2 NTAs was considerably higher than those of TiO2 NTAs and LaCoO3 nanoparticles loaded on a Ti foil. The LaCoO3-TiO2 NTAs prepared by the electrophoretic deposition technique for 15 min showed the highest photocatalytic degradation rate of MO, which was a four-fold enhancement compared to that of TiO2 NTs under the same conditions. The p-n heterojunctions between the LaCoO3 nanoparticles and the TiO2 nanotubes were responsible for the enhanced visible light photocatalytic activity. The results of the electrochemical impedance spectroscopy (EIS) and photoluminescence spectroscopy (PL) tests demonstrated that the loading of the LaCoO3 nanoparticles effectively promoted the separation and transport of photogenerated charges, thereby enhancing the visible light photocatalytic activity of the TiO2 NTAs.
Gold-Silver Alloy Film Based Spectral Surface Plasmon Resonance Imaging Sensor with High Sensitivity
2019, 35(6): 630-636
doi: 10.3866/PKU.WHXB201806082
Abstract:
This paper reports, for the first time, a gold-silver alloy film based broadband spectral surface plasmon resonance imaging (SPRI) sensor that enables in situ quantitative detection of chemical and biological molecules adsorbed on the partial or entire surface of the alloy film. The use of the gold-silver alloy film as the sensing layer makes the SPRI sensor lower in detection cost and higher in detection sensitivity as compared with the conventional sensor with a pure gold film. The gold-silver alloy films of ~50 nm thicknesses were deposited on glass substrates using a sputtering target made of gold (50%)-silver (50%, w, mass fraction) alloy. Both the SPR spectra and SPR color images for the gold-silver alloy films covered with pure water were measured at different incident angles using the laboratory-made Krestchmann-type multifunctional platform. The two-dimensional (2D) hue profile and the average hue for each SPR color image were obtained by calculation with the hue algorithm. Using the average hue as the sensitivity parameter, the spectral SPRI sensor enables quantitative detection. The spectral range in which the average hue is most sensitive to refractive index (RI) changes of bulk solution and to molecular adsorption was determined to be between 595 and 610 nm. In this narrow spectral range the average hue is linearly dependent on the resonant wavelength and its slope (representing the hue variation induced by per unit change in resonant wavelength) is Δhue/ΔλR = 7.52 nm-1, implying that the hue-based RI sensitivity is 7.52 times as high as the wavelength-based RI sensitivity. This implication was experimentally demonstrated in this work. After setting the initial resonant wavelength of the sensor in the hue-sensitive spectral range, the hue-based RI sensitivity of the SPRI sensor was measured to be S = 29879 RIU-1, which is 8 times higher than that obtained with the gold-film SPR chip under the same conditions (S = 3658 RIU-1 for the gold-film SPR chip). Nonspecific adsorption of bovine serum albumin (BSA) molecules on the gold-silver alloy film was monitored in real time by the time-resolved spectral SPRI method, and the temporal change in the average hue was obtained. The time required for BSA adsorption to reach equilibrium is determined to be about 15 min. This study illustrates that the gold-silver alloy film based SPRI sensor has the powerful capability of quantitative detection of sub-monomolecular adsorption of proteins. ![]()
This paper reports, for the first time, a gold-silver alloy film based broadband spectral surface plasmon resonance imaging (SPRI) sensor that enables in situ quantitative detection of chemical and biological molecules adsorbed on the partial or entire surface of the alloy film. The use of the gold-silver alloy film as the sensing layer makes the SPRI sensor lower in detection cost and higher in detection sensitivity as compared with the conventional sensor with a pure gold film. The gold-silver alloy films of ~50 nm thicknesses were deposited on glass substrates using a sputtering target made of gold (50%)-silver (50%, w, mass fraction) alloy. Both the SPR spectra and SPR color images for the gold-silver alloy films covered with pure water were measured at different incident angles using the laboratory-made Krestchmann-type multifunctional platform. The two-dimensional (2D) hue profile and the average hue for each SPR color image were obtained by calculation with the hue algorithm. Using the average hue as the sensitivity parameter, the spectral SPRI sensor enables quantitative detection. The spectral range in which the average hue is most sensitive to refractive index (RI) changes of bulk solution and to molecular adsorption was determined to be between 595 and 610 nm. In this narrow spectral range the average hue is linearly dependent on the resonant wavelength and its slope (representing the hue variation induced by per unit change in resonant wavelength) is Δhue/ΔλR = 7.52 nm-1, implying that the hue-based RI sensitivity is 7.52 times as high as the wavelength-based RI sensitivity. This implication was experimentally demonstrated in this work. After setting the initial resonant wavelength of the sensor in the hue-sensitive spectral range, the hue-based RI sensitivity of the SPRI sensor was measured to be S = 29879 RIU-1, which is 8 times higher than that obtained with the gold-film SPR chip under the same conditions (S = 3658 RIU-1 for the gold-film SPR chip). Nonspecific adsorption of bovine serum albumin (BSA) molecules on the gold-silver alloy film was monitored in real time by the time-resolved spectral SPRI method, and the temporal change in the average hue was obtained. The time required for BSA adsorption to reach equilibrium is determined to be about 15 min. This study illustrates that the gold-silver alloy film based SPRI sensor has the powerful capability of quantitative detection of sub-monomolecular adsorption of proteins.
2019, 35(6): 651-656
doi: 10.3866/PKU.WHXB201805037
Abstract:
The preparation of plasmonic metal-based substrates has been a hot research topic during the past decades in the area of surface-enhanced Raman spectroscopy (SERS). The localized surface plasmon resonance effect of plasmonic metal nanostructures enhances the electromagnetic field for SERS analysis, thereby making SERS an extremely sensitive detection technique. However, commonly developed plasmonic metal substrates exhibit poor stability and reproducibility. Since the separation of graphene from graphite, graphene has been widely used in various fields because of its unique physical, chemical, electronic, and optical properties. In the field of SERS, graphene has been used for graphene-enhanced Raman scattering, which makes use of the chemical enhancement mechanism in SERS. In addition, it has capabilities of surface molecular enrichment, quenching fluorescence, surface homogenization, and has strong chemical stability. Due to these characteristics of graphene, SERS substrates based on graphene-metal nanocapsules have attracted the attention of researchers. In this work, a small size gold-graphitic nanocapsules (Au@G) was prepared by chemical vapor deposition (CVD). The material exhibits a core-shell structure consisting of a graphitized carbon layer coated on Au nanoparticles (Au NPs). The Au NP core of the Au@G provides a major enhancement factor for Raman analysis, and the external graphitized carbon shell ensures strong chemical stability of the material. The Au@G exhibits a uniform particle size with diameter ~17 nm. In order to control the size of the Au@G, tetraethyl orthosilicate (TEOS) and tetraethylorthotrimethylammonium bromide were used as the raw material and template, respectively, a 45 nm-thick layer of mesoporous silica was coated on the synthesized Au NPs. The presence of the mesoporous silica capping layer prevented aggregation and particle size growth of the Au NPs during high-temperature CVD. At the same time, we studied the effect of TEOS concentration on the growth of the graphitized carbon layer during CVD. The results revealed that a decrease of the TEOS concentration is conducive for obtaining a high graphitic Au@G, and the concentration of TEOS does not affect the particle size of the Au@G. Raman detection of crystal violet molecules using Au@G demonstrated the latter's good Raman enhancement effect. The Au@G prepared by high-temperature CVD exhibits a clean surface with no impurities. It is an SERS substrate with both physical and chemical enhancement. The unique Raman spectral peaks and small size of Au@G ensure its great potential for use in the fields of molecular detection and cell imaging analysis. ![]()
The preparation of plasmonic metal-based substrates has been a hot research topic during the past decades in the area of surface-enhanced Raman spectroscopy (SERS). The localized surface plasmon resonance effect of plasmonic metal nanostructures enhances the electromagnetic field for SERS analysis, thereby making SERS an extremely sensitive detection technique. However, commonly developed plasmonic metal substrates exhibit poor stability and reproducibility. Since the separation of graphene from graphite, graphene has been widely used in various fields because of its unique physical, chemical, electronic, and optical properties. In the field of SERS, graphene has been used for graphene-enhanced Raman scattering, which makes use of the chemical enhancement mechanism in SERS. In addition, it has capabilities of surface molecular enrichment, quenching fluorescence, surface homogenization, and has strong chemical stability. Due to these characteristics of graphene, SERS substrates based on graphene-metal nanocapsules have attracted the attention of researchers. In this work, a small size gold-graphitic nanocapsules (Au@G) was prepared by chemical vapor deposition (CVD). The material exhibits a core-shell structure consisting of a graphitized carbon layer coated on Au nanoparticles (Au NPs). The Au NP core of the Au@G provides a major enhancement factor for Raman analysis, and the external graphitized carbon shell ensures strong chemical stability of the material. The Au@G exhibits a uniform particle size with diameter ~17 nm. In order to control the size of the Au@G, tetraethyl orthosilicate (TEOS) and tetraethylorthotrimethylammonium bromide were used as the raw material and template, respectively, a 45 nm-thick layer of mesoporous silica was coated on the synthesized Au NPs. The presence of the mesoporous silica capping layer prevented aggregation and particle size growth of the Au NPs during high-temperature CVD. At the same time, we studied the effect of TEOS concentration on the growth of the graphitized carbon layer during CVD. The results revealed that a decrease of the TEOS concentration is conducive for obtaining a high graphitic Au@G, and the concentration of TEOS does not affect the particle size of the Au@G. Raman detection of crystal violet molecules using Au@G demonstrated the latter's good Raman enhancement effect. The Au@G prepared by high-temperature CVD exhibits a clean surface with no impurities. It is an SERS substrate with both physical and chemical enhancement. The unique Raman spectral peaks and small size of Au@G ensure its great potential for use in the fields of molecular detection and cell imaging analysis.
2019, 35(6): 637-643
doi: 10.3866/PKU.WHXB201807004
Abstract:
The E/Z isomerization reaction is found extensively in most organic molecules containing double bond unit. This limits their practical application as luminescent materials partly, especially under photoirradiation. Therefore, it is important to obtain E/Z isomers with stable configuration in the excited state after photoirradiation. It is well known that cyanostilbene and its analogues play an important role in the development of organic opto/electronic materials. The substituted cyano group on C＝C double bonds has strong electron-withdrawing ability and large steric hindrance, which benefits the formation of donor-acceptor (D-A) structures and formation of intramolecular charge transfer. In our previous work, we reported a triphenylamine-cyanostilbene molecule (TPNCF) formed by modifying the cyanostilbene structure with triphenylamine, which maintained a stable E/Z configuration as a film and in high polar solvents. According to solvatochromism mechanisms and the results of theoretical calculations, we proposed that the charge transfer (CT) excited state between the triphenylamine donor and cyanostilbene acceptor groups induced the stable configuration of the E- and Z- isomers under photoirradiation. Under irradiation, the E/Z isomerization process occurring at a higher energy locally excited (LE) state was suppressed by a rapid internal conversion process from the LE to CT state. This work inspired us to provide a universal and effective molecular design strategy by modifying D-A substituents on double bonds that can successfully stabilize E/Z isomers. To further confirm that the CT excited state induced stable E- and Z- isomers in the cyanostilbene structure under photoirradiation, we designed and synthesized a donor-acceptor phenoxazine-cyanostilbene molecule (PZNCF) and successfully characterized its two E/Z isomers. In comparison with the reported TPNCF molecule, the in-situ NMR and UV spectra of E- and Z- isomers of PZNCF demonstrated that the E/Z isomerization rate became slower under photoirradiation, which indicated that the stronger electron-donating group of phenoxazine substituted in the cyanostilbene structure induced a more stable E/Z isomer configuration in its excited state. DFT calculations and photophysical results indicated that a stronger CT state was generated in both E- and Z- isomers of PZNCF. This further confirmed our hypothesized mechanism where the stable E/Z configuration can be obtained under photoirradiation by forming a suitable donor-acceptor structure to suppress the E/Z isomerization reaction in the LE state by a rapid internal crossing process from the LE to CT state. This molecular design strategy is of great significance to organic photochemistry and photoelectronics for molecules with double bond units. ![]()
The E/Z isomerization reaction is found extensively in most organic molecules containing double bond unit. This limits their practical application as luminescent materials partly, especially under photoirradiation. Therefore, it is important to obtain E/Z isomers with stable configuration in the excited state after photoirradiation. It is well known that cyanostilbene and its analogues play an important role in the development of organic opto/electronic materials. The substituted cyano group on C＝C double bonds has strong electron-withdrawing ability and large steric hindrance, which benefits the formation of donor-acceptor (D-A) structures and formation of intramolecular charge transfer. In our previous work, we reported a triphenylamine-cyanostilbene molecule (TPNCF) formed by modifying the cyanostilbene structure with triphenylamine, which maintained a stable E/Z configuration as a film and in high polar solvents. According to solvatochromism mechanisms and the results of theoretical calculations, we proposed that the charge transfer (CT) excited state between the triphenylamine donor and cyanostilbene acceptor groups induced the stable configuration of the E- and Z- isomers under photoirradiation. Under irradiation, the E/Z isomerization process occurring at a higher energy locally excited (LE) state was suppressed by a rapid internal conversion process from the LE to CT state. This work inspired us to provide a universal and effective molecular design strategy by modifying D-A substituents on double bonds that can successfully stabilize E/Z isomers. To further confirm that the CT excited state induced stable E- and Z- isomers in the cyanostilbene structure under photoirradiation, we designed and synthesized a donor-acceptor phenoxazine-cyanostilbene molecule (PZNCF) and successfully characterized its two E/Z isomers. In comparison with the reported TPNCF molecule, the in-situ NMR and UV spectra of E- and Z- isomers of PZNCF demonstrated that the E/Z isomerization rate became slower under photoirradiation, which indicated that the stronger electron-donating group of phenoxazine substituted in the cyanostilbene structure induced a more stable E/Z isomer configuration in its excited state. DFT calculations and photophysical results indicated that a stronger CT state was generated in both E- and Z- isomers of PZNCF. This further confirmed our hypothesized mechanism where the stable E/Z configuration can be obtained under photoirradiation by forming a suitable donor-acceptor structure to suppress the E/Z isomerization reaction in the LE state by a rapid internal crossing process from the LE to CT state. This molecular design strategy is of great significance to organic photochemistry and photoelectronics for molecules with double bond units.
2019, 35(6): 644-650
doi: 10.3866/PKU.WHXB201805068
Abstract:
In the emerging field of nanoscience, tubular structures have been attracting remarkable interest due to their well-defined geometry, high specific area, and exceptional physical and chemical properties. Among them, oriented ZnO tubular arrays are regarded as promising candidates for various applications such as optoelectronics, solar cells, sensors, field emission, piezoelectrics, and catalysis. Although template-directed and selective dissolution synthesizing strategies are commonly used to prepare ZnO nanotubes, repeatability and large scale preparation are still challenging. In this study, ZnO nanotube arrays were controllably prepared by tuning the hydrothermal parameters, without the use of any additives. The mechanism underlying the self-conversion of ZnO nanorods to nanotubes was comprehensively studied based on the surface energy theory. It has been proved that the metastable top surface of the ZnO nanorods dissolves preferentially to reach a stable state during the hydrothermal growth. The specific surface energy of different crystal faces of ZnO nanorods was calculated using molecular dynamics simulation. The top surface of the ZnO nanorod, the Zn-terminated [0001] face, demonstrated much higher surface free energy than did the lateral faces, which indicated that the self-dissolution of top face (002) is energetically favorable. The self-conversion behavior of ZnO nanorod arrays with different diameters was specifically investigated by adjusting the initial precursor concentration, density of the crystal seed layers, and growth time. The dissolution-crystallization equilibrium concentration, determined by crystal surface energy, was found to be a key factor for the formation of the tubular structure. Notably, the critical equilibrium conditions for the self-conversion of ZnO nanorods to nanotubes, including zinc ion concentration and pH, have been identified by studying parameters corresponding to the dissolution-crystallization equilibrium for the metastable top surface of the ZnO nanorods. The preparation of the ZnO nanotube arrays was successfully accelerated and simplified via two-step procedure: (1) preparation of ZnO nanorod arrays and (2) self-conversion of ZnO nanorods to nanotubes. The preparation method based on the self-conversion mechanism from rods to tubes for polar oxides is simpler and more easily controllable as compared to the reported methods involving variety of additives. Because of the advantages of adaptability to a wide range of substrates, excellent conducting properties, and filling ability, the prepared ZnO nanotube array films were used in encapsulating phase-change materials. The encapsulated phase-change material exhibited excellent heat storage/release properties and heat conductivities. This indicates the potential application of precision devices for temperature control. ![]()
In the emerging field of nanoscience, tubular structures have been attracting remarkable interest due to their well-defined geometry, high specific area, and exceptional physical and chemical properties. Among them, oriented ZnO tubular arrays are regarded as promising candidates for various applications such as optoelectronics, solar cells, sensors, field emission, piezoelectrics, and catalysis. Although template-directed and selective dissolution synthesizing strategies are commonly used to prepare ZnO nanotubes, repeatability and large scale preparation are still challenging. In this study, ZnO nanotube arrays were controllably prepared by tuning the hydrothermal parameters, without the use of any additives. The mechanism underlying the self-conversion of ZnO nanorods to nanotubes was comprehensively studied based on the surface energy theory. It has been proved that the metastable top surface of the ZnO nanorods dissolves preferentially to reach a stable state during the hydrothermal growth. The specific surface energy of different crystal faces of ZnO nanorods was calculated using molecular dynamics simulation. The top surface of the ZnO nanorod, the Zn-terminated [0001] face, demonstrated much higher surface free energy than did the lateral faces, which indicated that the self-dissolution of top face (002) is energetically favorable. The self-conversion behavior of ZnO nanorod arrays with different diameters was specifically investigated by adjusting the initial precursor concentration, density of the crystal seed layers, and growth time. The dissolution-crystallization equilibrium concentration, determined by crystal surface energy, was found to be a key factor for the formation of the tubular structure. Notably, the critical equilibrium conditions for the self-conversion of ZnO nanorods to nanotubes, including zinc ion concentration and pH, have been identified by studying parameters corresponding to the dissolution-crystallization equilibrium for the metastable top surface of the ZnO nanorods. The preparation of the ZnO nanotube arrays was successfully accelerated and simplified via two-step procedure: (1) preparation of ZnO nanorod arrays and (2) self-conversion of ZnO nanorods to nanotubes. The preparation method based on the self-conversion mechanism from rods to tubes for polar oxides is simpler and more easily controllable as compared to the reported methods involving variety of additives. Because of the advantages of adaptability to a wide range of substrates, excellent conducting properties, and filling ability, the prepared ZnO nanotube array films were used in encapsulating phase-change materials. The encapsulated phase-change material exhibited excellent heat storage/release properties and heat conductivities. This indicates the potential application of precision devices for temperature control.
