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2022 Volume 38 Issue 4
2022, 38(4): 200300
doi: 10.3866/PKU.WHXB202003004
Abstract:
The molecular magnetic tunnel junction (MMTJ) with high tunnel magnetoresistance (TMR) is an important component for devices such as computers and electronic storage. With the rapid development of the modern electronics industry, the decrease of device size and the increase of area density, it is important to improve TMR technology. In addition, the computing process faces huge challenges. As the size of electronic devices decreases, small changes may cause completely different transmission characteristics, therefore the minute details of the device must be carefully controlled. In this paper, in order to find large TMR values and explore the role of symmetry on spin-polarized transport properties, γ-graphyne nanodots (γ-GYND) coupled between ferromagnetic (FM) metallic zigzag graphene nanoribbon (ZGNR) electrodes were used. Depending on the widths of the ZGNR and two types of contact positions between the ZGNR and γ-graphyne nanodots (γ-GYND), eight ZGNR/γ-GYND/ZGNR MMTJs with different symmetries were constructed. By using Keldysh non-equilibrium Green's function (NEGF) and density functional theory (DFT), the I-V curve, the spin-injection efficiency (SIE) and TMR of MMTJs were calculated. We found that the transport properties of these MMTJs differed substantially. For absolute symmetric MMTJs, due to the wave functions corresponding to the band structure near the Fermi energy having different parity, the electron transport between the wave functions with different parity is prohibited, so we can see that the spin-down current is always zero. This implies that these absolutely symmetrical structures have 100% spin injection efficiency over a wide range of bias voltages. In addition, the calculation results also show that these absolutely symmetric structures also have large TMR at low bias, up to 3.7 × 105, indicating that these devices have a large magnetoresistance effect and high magnetic field sensitivity, which can be used in the read head of computer hard disks, MRAM, and various magnetic sensors. However, for these asymmetric MMTJs, since there is no limitation of the wave function parity of the left and right electrodes, the spin-up current and spin-down current fluctuated as the bias voltage increased, so perfect SIE does not appear. In addition, the calculation results showed that the TMR of asymmetric MMTJs were four orders of magnitude smaller than with symmetric MMTJs. Thus the symmetry of MMTJs has a great influence on the spin-polarized transport properties of the device. These absolutely symmetrical MMTJs have spin-polarized transport properties that are far superior to other MMTJs. This is conducive to the manufacture of spin filters, rectifiers, and various magnetic sensors. Finally, these excellent characteristics can be explained by the transmission coefficient, local density of states (LDOS) and band structure. ![]()
The molecular magnetic tunnel junction (MMTJ) with high tunnel magnetoresistance (TMR) is an important component for devices such as computers and electronic storage. With the rapid development of the modern electronics industry, the decrease of device size and the increase of area density, it is important to improve TMR technology. In addition, the computing process faces huge challenges. As the size of electronic devices decreases, small changes may cause completely different transmission characteristics, therefore the minute details of the device must be carefully controlled. In this paper, in order to find large TMR values and explore the role of symmetry on spin-polarized transport properties, γ-graphyne nanodots (γ-GYND) coupled between ferromagnetic (FM) metallic zigzag graphene nanoribbon (ZGNR) electrodes were used. Depending on the widths of the ZGNR and two types of contact positions between the ZGNR and γ-graphyne nanodots (γ-GYND), eight ZGNR/γ-GYND/ZGNR MMTJs with different symmetries were constructed. By using Keldysh non-equilibrium Green's function (NEGF) and density functional theory (DFT), the I-V curve, the spin-injection efficiency (SIE) and TMR of MMTJs were calculated. We found that the transport properties of these MMTJs differed substantially. For absolute symmetric MMTJs, due to the wave functions corresponding to the band structure near the Fermi energy having different parity, the electron transport between the wave functions with different parity is prohibited, so we can see that the spin-down current is always zero. This implies that these absolutely symmetrical structures have 100% spin injection efficiency over a wide range of bias voltages. In addition, the calculation results also show that these absolutely symmetric structures also have large TMR at low bias, up to 3.7 × 105, indicating that these devices have a large magnetoresistance effect and high magnetic field sensitivity, which can be used in the read head of computer hard disks, MRAM, and various magnetic sensors. However, for these asymmetric MMTJs, since there is no limitation of the wave function parity of the left and right electrodes, the spin-up current and spin-down current fluctuated as the bias voltage increased, so perfect SIE does not appear. In addition, the calculation results showed that the TMR of asymmetric MMTJs were four orders of magnitude smaller than with symmetric MMTJs. Thus the symmetry of MMTJs has a great influence on the spin-polarized transport properties of the device. These absolutely symmetrical MMTJs have spin-polarized transport properties that are far superior to other MMTJs. This is conducive to the manufacture of spin filters, rectifiers, and various magnetic sensors. Finally, these excellent characteristics can be explained by the transmission coefficient, local density of states (LDOS) and band structure.
2022, 38(4): 200303
doi: 10.3866/PKU.WHXB202003033
Abstract:
Corrosion protection of reinforcing steel in concrete is an urgent task in modern society. Use of corrosion inhibitors in concrete is an effective, simple, and economical method for protecting reinforcing steel from corrosion. Mixed corrosion inhibitors usually perform better than a single inhibitor in actual reinforced concrete systems because of their synergistic inhibition effects. In recent years, environmentally friendly corrosion inhibitors have attracted increasing attention from corrosion researchers. Diisooctyl sebacate and sodium D-gluconate are environmentally friendly organic corrosion inhibitors, and ZnSO4 is an inorganic cathodic inhibitor, they may form an innovative, nontoxic, and pollution-free mixed corrosion inhibitor to control reinforcing steel corrosion. Additionally, diisooctyl sebacate and sodium D-gluconate serve as absorption-type inhibitors, and ZnSO4 acts as a precipitation-type inhibitor. We hypothesized that their combination might show a good synergistic corrosion inhibition effect on reinforcing steel. In this study, we developed a diisooctyl sebacate-based mixed corrosion inhibitor that includes D-gluconate and ZnSO4 and investigated its synergistic inhibition effects on reinforcing steel (Q235 steel) corrosion in a simulated polluted concrete pore solution. The reinforcing steel corrosion behavior and the properties of the mixed corrosion inhibitor were studied by polarization curve measurements, electrochemical impedance spectroscopy tests, and surface analysis methods (scanning electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy). The results indicated that the reinforcing steel in the simulated polluted concrete pore solution (pH 11.00, 0.5 mol·L-1 NaCl) was in an active dissolving state and that localized corrosion took place. The mixed corrosion inhibitor, consisting of diisooctyl sebacate (59 mmol·L-1), sodium D-gluconate (0.5 mmol·L-1), and ZnSO4 (1.5 mmol·L-1), had an obvious and powerful inhibition effect. Its inhibition efficiency reached 96.8% and 90.0% in the simulated polluted concrete pore solution and the cement mortar, respectively. The mixture of diisooctyl sebacate with sodium D-gluconate and ZnSO4 acted as a mixed-type inhibitor and effectively controlled both anodic and cathodic reactions of the steel corrosion. ![]()
Corrosion protection of reinforcing steel in concrete is an urgent task in modern society. Use of corrosion inhibitors in concrete is an effective, simple, and economical method for protecting reinforcing steel from corrosion. Mixed corrosion inhibitors usually perform better than a single inhibitor in actual reinforced concrete systems because of their synergistic inhibition effects. In recent years, environmentally friendly corrosion inhibitors have attracted increasing attention from corrosion researchers. Diisooctyl sebacate and sodium D-gluconate are environmentally friendly organic corrosion inhibitors, and ZnSO4 is an inorganic cathodic inhibitor, they may form an innovative, nontoxic, and pollution-free mixed corrosion inhibitor to control reinforcing steel corrosion. Additionally, diisooctyl sebacate and sodium D-gluconate serve as absorption-type inhibitors, and ZnSO4 acts as a precipitation-type inhibitor. We hypothesized that their combination might show a good synergistic corrosion inhibition effect on reinforcing steel. In this study, we developed a diisooctyl sebacate-based mixed corrosion inhibitor that includes D-gluconate and ZnSO4 and investigated its synergistic inhibition effects on reinforcing steel (Q235 steel) corrosion in a simulated polluted concrete pore solution. The reinforcing steel corrosion behavior and the properties of the mixed corrosion inhibitor were studied by polarization curve measurements, electrochemical impedance spectroscopy tests, and surface analysis methods (scanning electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy). The results indicated that the reinforcing steel in the simulated polluted concrete pore solution (pH 11.00, 0.5 mol·L-1 NaCl) was in an active dissolving state and that localized corrosion took place. The mixed corrosion inhibitor, consisting of diisooctyl sebacate (59 mmol·L-1), sodium D-gluconate (0.5 mmol·L-1), and ZnSO4 (1.5 mmol·L-1), had an obvious and powerful inhibition effect. Its inhibition efficiency reached 96.8% and 90.0% in the simulated polluted concrete pore solution and the cement mortar, respectively. The mixture of diisooctyl sebacate with sodium D-gluconate and ZnSO4 acted as a mixed-type inhibitor and effectively controlled both anodic and cathodic reactions of the steel corrosion.
2022, 38(4): 200304
doi: 10.3866/PKU.WHXB202003046
Abstract:
The acid properties of SAPO-34 molecular sieves (MSs), including the strength and density of Brönsted acids, have attracted enormous attention in past decades because of the excellent performance of SAPO-34 in industrial processes such as the methanol-to-olefins (MTO) process and the selective catalytic reduction of NOx with NH3 (NH3-SCR). Currently, pure-phase SAPO-34 MSs with different Si contents can be easily obtained by utilizing multifarious organic structure-directing agents (OSDAs). However, the resulting SAPO-34 MSs have different acid properties, which may affect their catalytic performance. Hence, correlating the acid properties with the OSDAs and Si contents is of significance to synthesize SAPO-34 MSs with the desired properties. Herein, the acid properties of four series of SAPO-34 MSs with varying Si contents synthesized using tetraethylammonium hydroxide (TEAOH), diisopropylamine (DIPA), n-butylamine (nBA), and morpholine (MOR) as the OSDAs were probed in detail by thermogravimetry (TG), Rietveld refinement, and solid-state nuclear magnetic resonance (ss-NMR) analyses. The strength and acid density were systematically investigated by exploring the host-guest interactions between the probed molecule CD3CN and the framework using 1H magic angle spinning (MAS) NMR spectroscopy, and the local environments of Si were studied by 29Si MAS NMR spectroscopy. The results of TG and Rietveld refinement showed that the SAPO-34 MSs templated by TEAOH and DIPA have only one OSDA per cha (one of the composite building units) cage in the longitudinal configuration, while those templated by nBA and MOR possess two OSDAs occluded in the cha cage in an up-and-down arrangement. Interestingly, the acid strength of SAPO-34 templated by TEAOH increased with increasing Si content, while the acid density remained almost unchanged. In contrast, the acid density of SAPO-34 templated by DIPA decreased evidently with an increase in the Si content, while the acid strength showed only a small variation. Among the other two samples, SAPO-34 templated by MOR has the most amounts of acid densities compared to SAPO-34 templated by nBA, while the strength is not superior. Thus, we conclude that the acid density is associated with the number of OSDAs in each cha cage and their protonation ability, while the difference in acid strength is attributed to the number of Si atoms at the edges of the Si islands. The findings of this study will provide insight into the acid properties of related crystalline porous materials. ![]()
The acid properties of SAPO-34 molecular sieves (MSs), including the strength and density of Brönsted acids, have attracted enormous attention in past decades because of the excellent performance of SAPO-34 in industrial processes such as the methanol-to-olefins (MTO) process and the selective catalytic reduction of NOx with NH3 (NH3-SCR). Currently, pure-phase SAPO-34 MSs with different Si contents can be easily obtained by utilizing multifarious organic structure-directing agents (OSDAs). However, the resulting SAPO-34 MSs have different acid properties, which may affect their catalytic performance. Hence, correlating the acid properties with the OSDAs and Si contents is of significance to synthesize SAPO-34 MSs with the desired properties. Herein, the acid properties of four series of SAPO-34 MSs with varying Si contents synthesized using tetraethylammonium hydroxide (TEAOH), diisopropylamine (DIPA), n-butylamine (nBA), and morpholine (MOR) as the OSDAs were probed in detail by thermogravimetry (TG), Rietveld refinement, and solid-state nuclear magnetic resonance (ss-NMR) analyses. The strength and acid density were systematically investigated by exploring the host-guest interactions between the probed molecule CD3CN and the framework using 1H magic angle spinning (MAS) NMR spectroscopy, and the local environments of Si were studied by 29Si MAS NMR spectroscopy. The results of TG and Rietveld refinement showed that the SAPO-34 MSs templated by TEAOH and DIPA have only one OSDA per cha (one of the composite building units) cage in the longitudinal configuration, while those templated by nBA and MOR possess two OSDAs occluded in the cha cage in an up-and-down arrangement. Interestingly, the acid strength of SAPO-34 templated by TEAOH increased with increasing Si content, while the acid density remained almost unchanged. In contrast, the acid density of SAPO-34 templated by DIPA decreased evidently with an increase in the Si content, while the acid strength showed only a small variation. Among the other two samples, SAPO-34 templated by MOR has the most amounts of acid densities compared to SAPO-34 templated by nBA, while the strength is not superior. Thus, we conclude that the acid density is associated with the number of OSDAs in each cha cage and their protonation ability, while the difference in acid strength is attributed to the number of Si atoms at the edges of the Si islands. The findings of this study will provide insight into the acid properties of related crystalline porous materials.
2022, 38(4): 200404
doi: 10.3866/PKU.WHXB202004046
Abstract:
Photocatalytic hydrogen evolution is a scalable pathway to generate hydrogen fuels while mitigating environmental crisis. Strategies based on modification of the host photocatalyst surface are key to improve the adsorption/activation ability of the reaction molecules and the efficiency of charge transport, so that high-efficiency photocatalytic systems can be realized. Cadmium sulfide (CdS), a visible light-responsive semiconductor material, is widely used in photocatalysis because of its simple synthesis, low cost, abundant raw materials, and appropriate bandgap structure. Many researchers have focused on improving the photocatalytic efficiency of CdS because the rapid charge recombination in this material limits its applications. Among the various strategies proposed in this regard, surface modification is an effective and simple method used to improve the photocatalytic performance of materials. In this work, polyvinyl pyrrolidone (PVP)-capped CdS (denoted as P-CdS) nanopopcorns with hexagonal wurtzite (WZ)-cubic zinc blende (ZB) homojunctions were fabricated via one-step gamma-ray radiation-induced reduction under ambient conditions. Subsequent alkaline treatment under ambient conditions led to a dramatic improvement in the activity of the alkalized PVP-capped CdS (MP-CdS) photocatalyst. The structure and properties of the photocatalyst were determined by X-ray diffraction (XRD) analysis, field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) analysis, Brunauer-Emmett-Teller (BET) specific surface area measurements, and photoelectric tests. The photocatalytic performance was evaluated based on the photocatalytic H2 evolution under visible-light irradiation. The mechanism underlying the enhancement of the photocatalytic activity is also discussed. The results showed that after the alkaline treatment, the crystal structure of CdS with WZ-ZB homojunctions was preserved, but PVP on the surface of CdS hydrolyzed to form PVP hydrolysis product (MPVP) with carboxyl and amino groups. Owing to the increased alkaline solubility, a portion of MPVP dissolved into the solution and was removed from the surface of MP-CdS, exposing a greater number of active sites of the WZ-ZB phase junctions with a larger specific surface area. On the other hand, the carboxyl groups in MPVP coordinated with CdS could affect the bandgap and valence band position of CdS to facilitate the photocatalysis. Because of the synergistic effects of the exposure of WZ-ZB phase junctions and band structure engineering, the alkalized samples at a 1 mol·L-1 concentration of NaOH showed a H2 evolution rate of 477 μmol·g-1·h-1 under visible-light illumination, which was twice that obtained for the pristine P-CdS photocatalysts. This simple and low-cost post-synthesis strategy can be extended to the preparation of diverse functional photocatalysts. The present work is expected to contribute to the practical application of sulfide photocatalysts. ![]()
Photocatalytic hydrogen evolution is a scalable pathway to generate hydrogen fuels while mitigating environmental crisis. Strategies based on modification of the host photocatalyst surface are key to improve the adsorption/activation ability of the reaction molecules and the efficiency of charge transport, so that high-efficiency photocatalytic systems can be realized. Cadmium sulfide (CdS), a visible light-responsive semiconductor material, is widely used in photocatalysis because of its simple synthesis, low cost, abundant raw materials, and appropriate bandgap structure. Many researchers have focused on improving the photocatalytic efficiency of CdS because the rapid charge recombination in this material limits its applications. Among the various strategies proposed in this regard, surface modification is an effective and simple method used to improve the photocatalytic performance of materials. In this work, polyvinyl pyrrolidone (PVP)-capped CdS (denoted as P-CdS) nanopopcorns with hexagonal wurtzite (WZ)-cubic zinc blende (ZB) homojunctions were fabricated via one-step gamma-ray radiation-induced reduction under ambient conditions. Subsequent alkaline treatment under ambient conditions led to a dramatic improvement in the activity of the alkalized PVP-capped CdS (MP-CdS) photocatalyst. The structure and properties of the photocatalyst were determined by X-ray diffraction (XRD) analysis, field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) analysis, Brunauer-Emmett-Teller (BET) specific surface area measurements, and photoelectric tests. The photocatalytic performance was evaluated based on the photocatalytic H2 evolution under visible-light irradiation. The mechanism underlying the enhancement of the photocatalytic activity is also discussed. The results showed that after the alkaline treatment, the crystal structure of CdS with WZ-ZB homojunctions was preserved, but PVP on the surface of CdS hydrolyzed to form PVP hydrolysis product (MPVP) with carboxyl and amino groups. Owing to the increased alkaline solubility, a portion of MPVP dissolved into the solution and was removed from the surface of MP-CdS, exposing a greater number of active sites of the WZ-ZB phase junctions with a larger specific surface area. On the other hand, the carboxyl groups in MPVP coordinated with CdS could affect the bandgap and valence band position of CdS to facilitate the photocatalysis. Because of the synergistic effects of the exposure of WZ-ZB phase junctions and band structure engineering, the alkalized samples at a 1 mol·L-1 concentration of NaOH showed a H2 evolution rate of 477 μmol·g-1·h-1 under visible-light illumination, which was twice that obtained for the pristine P-CdS photocatalysts. This simple and low-cost post-synthesis strategy can be extended to the preparation of diverse functional photocatalysts. The present work is expected to contribute to the practical application of sulfide photocatalysts.
2022, 38(4): 200405
doi: 10.3866/PKU.WHXB202004054
Abstract:
Electroreduction of CO2 is one of the most promising CO2 conversion pathways because of its moderate reaction conditions, controllable product composition, and environment-friendliness. However, most of the current CO2 electroreduction technologies have not reached the techno-economic threshold for a competitively profitable electrochemical process. Based on a simple two-electron transfer process, the electroreduction of CO2 to CO, which is further processed into syngas with the reduction of H2O to H2, is postulated to be the most promising pathway for a profitable electrochemical process. Such a process urgently requires nonprecious electrocatalysts that can precisely control the CO/H2 ratio. Herein, we present a tailored synthesis of bifunctional electrocatalysts with high activity, which can realize the preparation of syngas with controlled compositions via molecular engineering of a ternary nanocomposite. Specifically, a mixture of melamine, triphenylphosphine, and nickel acetate was milled and dissolved in ethanol; the ternary nanocomposite was obtained after rotary evaporation of the mixture. We prepared the catalysts by pyrolyzing the obtained composites at 850 ℃ for 2 h. The synthesis strategy was facile and easy to scale. The specific surface area and pore volume of the bifunctional electrocatalyst were both significantly enhanced upon increasing the concentration of the phosphorus source, triphenylphosphine, during the precursor preparation. The obtained bifunctional electrocatalysts had hierarchically porous structures, which had well-dispersed active sites and could promote mass transport. Raman spectra revealed higher degrees of disorder with higher P/Ni ratios in the precursor. X-ray photoelectron spectroscopy verified the presence of Ni-Px and Ni-Nx functionalities, which were the active sites for hydrogen evolution and CO2 reduction, respectively. Hence, the electrocatalytic performance of this series of bifunctional electrocatalysts can be tuned from CO-dominant to H2-dominant. The electrochemical performance was evaluated using a CO2-saturated 0.5 mol·L-1 KHCO3 aqueous solution at ambient temperature by linear sweep voltammetry and potentiostatic electrolysis. Through these experiments, we determined that the activity of the catalysts was influenced by the surface phosphorus/Ni-Nx site ratio. The highest CO faradaic efficiency (91%) was achieved at -0.8 V (vs a reversible hydrogen electrode, RHE) with Ni-N-C in the absence of Ni-P. The CO/H2 molar ratio in the syngas stream was tunable from 2 : 5 to 10 : 1 in the potential range from -0.7 to -1.1 V (vs RHE) with a total faradic efficiency of 100%. The syngas composition directly links to the molar ratio of the two integrated components, nickel phosphide and Ni-N-C. Additionally, the stability of the optimized bifunctional electrocatalyst at -0.7 V for 8 h was tested, in which the CO/H2 ratio was maintained between 1.2 and 1.3, indicating excellent stability. This study provides a new perspective for the engineering of bifunctional electrocatalysts for the conversion of abundant CO2 and water into syngas with tailorable CO/H2 ratios. ![]()
Electroreduction of CO2 is one of the most promising CO2 conversion pathways because of its moderate reaction conditions, controllable product composition, and environment-friendliness. However, most of the current CO2 electroreduction technologies have not reached the techno-economic threshold for a competitively profitable electrochemical process. Based on a simple two-electron transfer process, the electroreduction of CO2 to CO, which is further processed into syngas with the reduction of H2O to H2, is postulated to be the most promising pathway for a profitable electrochemical process. Such a process urgently requires nonprecious electrocatalysts that can precisely control the CO/H2 ratio. Herein, we present a tailored synthesis of bifunctional electrocatalysts with high activity, which can realize the preparation of syngas with controlled compositions via molecular engineering of a ternary nanocomposite. Specifically, a mixture of melamine, triphenylphosphine, and nickel acetate was milled and dissolved in ethanol; the ternary nanocomposite was obtained after rotary evaporation of the mixture. We prepared the catalysts by pyrolyzing the obtained composites at 850 ℃ for 2 h. The synthesis strategy was facile and easy to scale. The specific surface area and pore volume of the bifunctional electrocatalyst were both significantly enhanced upon increasing the concentration of the phosphorus source, triphenylphosphine, during the precursor preparation. The obtained bifunctional electrocatalysts had hierarchically porous structures, which had well-dispersed active sites and could promote mass transport. Raman spectra revealed higher degrees of disorder with higher P/Ni ratios in the precursor. X-ray photoelectron spectroscopy verified the presence of Ni-Px and Ni-Nx functionalities, which were the active sites for hydrogen evolution and CO2 reduction, respectively. Hence, the electrocatalytic performance of this series of bifunctional electrocatalysts can be tuned from CO-dominant to H2-dominant. The electrochemical performance was evaluated using a CO2-saturated 0.5 mol·L-1 KHCO3 aqueous solution at ambient temperature by linear sweep voltammetry and potentiostatic electrolysis. Through these experiments, we determined that the activity of the catalysts was influenced by the surface phosphorus/Ni-Nx site ratio. The highest CO faradaic efficiency (91%) was achieved at -0.8 V (vs a reversible hydrogen electrode, RHE) with Ni-N-C in the absence of Ni-P. The CO/H2 molar ratio in the syngas stream was tunable from 2 : 5 to 10 : 1 in the potential range from -0.7 to -1.1 V (vs RHE) with a total faradic efficiency of 100%. The syngas composition directly links to the molar ratio of the two integrated components, nickel phosphide and Ni-N-C. Additionally, the stability of the optimized bifunctional electrocatalyst at -0.7 V for 8 h was tested, in which the CO/H2 ratio was maintained between 1.2 and 1.3, indicating excellent stability. This study provides a new perspective for the engineering of bifunctional electrocatalysts for the conversion of abundant CO2 and water into syngas with tailorable CO/H2 ratios.
2022, 38(4): 200500
doi: 10.3866/PKU.WHXB202005009
Abstract:
Pt-based catalysts are widely used in diesel oxidation catalyst (DOC) units, primarily to oxidize the harmful HC, CO, and NO emissions. Notably, NO2 produced from NO oxidation is beneficial for low-temperature activity in NH3-SCR and promotes soot oxidation in diesel particulate filters (DPF). Thus, the conversion of NO is an important parameter for determining the performance of DOCs. Considering the increasingly stringent emission regulations and the economic effectiveness, preparation of low-cost and highly active Pt-based catalysts is indispensable. Generally, the Pt0 content is crucial as it is an active component of DOCs. Small Pt size is beneficial for improving the catalytic activity. In this study, we applied a modified alcohol reduction-impregnation (MARI) method to synthesize highly active 1% (w, mass fraction) Pt/SiO2-Al2O3 (denoted as MA-Pt/SA) catalyst. Meanwhile, using the conventional impregnation method, we prepared the Pt/SiO2-Al2O3 catalyst with the same Pt loading (denoted as C-Pt/SA) as a reference sample. X-ray photoelectron spectroscopy (XPS) and hydrogen temperature program reduction (H2-TPR) analyses proved that the MARI method could produce Pt catalysts with higher Pt0 content. Pt0 content in MA-Pt/SA was ~60.3% while that in C-Pt/SA was only ~23.1%. X-ray diffraction (XRD), CO-diffuse reflectance infrared Fourier transform spectroscopy (CO-DRIFTS), and transmission electron microscopy (TEM) characterization confirmed that the Pt particle size is much smaller over MA-Pt/SA as compared to that over C-Pt/SA. Performance evaluation of MA-Pt/SA and C-Pt/SA was conducted in a simulated diesel atmosphere. The results showed that the maximum NO conversion into NO2 over MA-Pt/SA is 74% and 68% in the absence and presence of H2O, respectively, which were much higher than those over C-Pt/SA (42% and 51% NO conversion with and without H2O, respectively). Furthermore, the temperature for 30% NO conversion over MA-Pt/SA (218 ℃) markedly decreased as compared to that over C-Pt/SA (248 ℃), indicating the excellent low temperature activity. After the aging treatment with reaction gas at high temperatures, aged MA-Pt/SA maintained 69% NO conversion while aged C-Pt/SA showed only 41% NO conversion. In addition, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of NO + O2 co-adsorption suggested that higher Pt dispersion and higher Pt0 content over MA-Pt/SA could facilitate the formation of bridging nitrates as intermediate species in NO oxidation at lower temperatures and could also facilitate their rapid decomposition (or desorption) at higher temperatures, thus imparting a high catalytic activity. Furthermore, a decrease in the Pt loading to 0.5% (w) resulted in a maximum NO conversion of 64% via the MARI method, suggesting a higher catalytic activity compared to that of C-Pt/SA with 1% (w) Pt loading. This work provides a method to prepare highly active Pt-based catalysts with low noble loading. ![]()
Pt-based catalysts are widely used in diesel oxidation catalyst (DOC) units, primarily to oxidize the harmful HC, CO, and NO emissions. Notably, NO2 produced from NO oxidation is beneficial for low-temperature activity in NH3-SCR and promotes soot oxidation in diesel particulate filters (DPF). Thus, the conversion of NO is an important parameter for determining the performance of DOCs. Considering the increasingly stringent emission regulations and the economic effectiveness, preparation of low-cost and highly active Pt-based catalysts is indispensable. Generally, the Pt0 content is crucial as it is an active component of DOCs. Small Pt size is beneficial for improving the catalytic activity. In this study, we applied a modified alcohol reduction-impregnation (MARI) method to synthesize highly active 1% (w, mass fraction) Pt/SiO2-Al2O3 (denoted as MA-Pt/SA) catalyst. Meanwhile, using the conventional impregnation method, we prepared the Pt/SiO2-Al2O3 catalyst with the same Pt loading (denoted as C-Pt/SA) as a reference sample. X-ray photoelectron spectroscopy (XPS) and hydrogen temperature program reduction (H2-TPR) analyses proved that the MARI method could produce Pt catalysts with higher Pt0 content. Pt0 content in MA-Pt/SA was ~60.3% while that in C-Pt/SA was only ~23.1%. X-ray diffraction (XRD), CO-diffuse reflectance infrared Fourier transform spectroscopy (CO-DRIFTS), and transmission electron microscopy (TEM) characterization confirmed that the Pt particle size is much smaller over MA-Pt/SA as compared to that over C-Pt/SA. Performance evaluation of MA-Pt/SA and C-Pt/SA was conducted in a simulated diesel atmosphere. The results showed that the maximum NO conversion into NO2 over MA-Pt/SA is 74% and 68% in the absence and presence of H2O, respectively, which were much higher than those over C-Pt/SA (42% and 51% NO conversion with and without H2O, respectively). Furthermore, the temperature for 30% NO conversion over MA-Pt/SA (218 ℃) markedly decreased as compared to that over C-Pt/SA (248 ℃), indicating the excellent low temperature activity. After the aging treatment with reaction gas at high temperatures, aged MA-Pt/SA maintained 69% NO conversion while aged C-Pt/SA showed only 41% NO conversion. In addition, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of NO + O2 co-adsorption suggested that higher Pt dispersion and higher Pt0 content over MA-Pt/SA could facilitate the formation of bridging nitrates as intermediate species in NO oxidation at lower temperatures and could also facilitate their rapid decomposition (or desorption) at higher temperatures, thus imparting a high catalytic activity. Furthermore, a decrease in the Pt loading to 0.5% (w) resulted in a maximum NO conversion of 64% via the MARI method, suggesting a higher catalytic activity compared to that of C-Pt/SA with 1% (w) Pt loading. This work provides a method to prepare highly active Pt-based catalysts with low noble loading.
2022, 38(4): 200401
doi: 10.3866/PKU.WHXB202004017
Abstract:
G-rich DNA sequences can transform into G-quadruplexes (G4s) in the presence of metal ions. Based on the structural switches, G4 has been recognized as an attractive signal-transducing element for constructing colorimetric, electrochemical, and fluorescent sensing platforms capable of recognizing ions, small biological molecules, proteins, and even cells. For fluorescent sensing platforms, fluorescent small molecules (FSMs) specifically binding with G4s, such as crystal violet (CV), protoporphyrin IX (PPIX), zinc protoporphyrin IX (ZnPPIX), and Thioflavin T (ThT), are usually applied as fluorescent signal readout probes. It was noticed that the binding affinity of FSM with G4 is highly dependent on G4 morphologies because G-rich DNA sequences can fold into multiple G4 conformations, such as parallel, antiparallel, or hybrid. For example, CV only binds with antiparallel G4, PPIX or ZnPPIX preferentially interacts with parallel G4, and ThT displays high affinity for hybrid G4. Furthermore, the binding affinity of FSMs with G4 is also dependent on co-existing ions and ion concentrations, especially elevated Na+ level (140 mmol·L-1). It is the reason why the performance of G4-based sensors in biological and environmental samples is decreased with different extents. Therefore, how to design G-rich DNA sequences to generally achieve FSMs specifically binding with G4, which is independent of G4 morphologies and co-existing Na+ and Na+ concentrations remains a challenge. In this study, a simple G-rich DNA sequence (thrombin binding aptamer, TBA) flanked by 10-mer single-stranded DNA at the 3' and 5' termini (TBA-10 bp) is designed. In the presence of K+, TBA transforms into antiparallel G4 (K+-TBA) and TBA-10 bp transforms into antiparallel K+-TBA flanked by fully hybridized double-stranded DNA (ds-DNA) (K+-TBA-10 bp). Actually, ThT cannot effectively bind with antiparallel K+-TBA. Compared with K+-TBA, upon K+-TBA-10 bp binding with ThT, ThT emission fluorescence increased by 100-fold. Importantly, the binding affinity improved by 1000-fold, which is independent of co-existing Na+ and Na+ concentrations (5-140 mmol·L-1). Integrated with UV-Vis spectroscopy, fluorescent spectroscopy, and circular dichroism spectroscopy, it is believed that ThT can specifically and efficiently imbed in the junction between K+-TBA and ds-DNA. To corroborate the binding mode, TBA in TBA-10 bp is substituted by other G-rich DNA sequences transforming into parallel and antiparallel G4 in the presence of K+, respectively. The resulting improved ThT emission fluorescence indicated that such a specific binding mode generally improved the binding affinity of FSMs with G4. Our findings provide new insights into the improvement of the binding affinity of FSMs and G4, and reveal potential biochemical and bioanalytical applications of G4. ![]()
G-rich DNA sequences can transform into G-quadruplexes (G4s) in the presence of metal ions. Based on the structural switches, G4 has been recognized as an attractive signal-transducing element for constructing colorimetric, electrochemical, and fluorescent sensing platforms capable of recognizing ions, small biological molecules, proteins, and even cells. For fluorescent sensing platforms, fluorescent small molecules (FSMs) specifically binding with G4s, such as crystal violet (CV), protoporphyrin IX (PPIX), zinc protoporphyrin IX (ZnPPIX), and Thioflavin T (ThT), are usually applied as fluorescent signal readout probes. It was noticed that the binding affinity of FSM with G4 is highly dependent on G4 morphologies because G-rich DNA sequences can fold into multiple G4 conformations, such as parallel, antiparallel, or hybrid. For example, CV only binds with antiparallel G4, PPIX or ZnPPIX preferentially interacts with parallel G4, and ThT displays high affinity for hybrid G4. Furthermore, the binding affinity of FSMs with G4 is also dependent on co-existing ions and ion concentrations, especially elevated Na+ level (140 mmol·L-1). It is the reason why the performance of G4-based sensors in biological and environmental samples is decreased with different extents. Therefore, how to design G-rich DNA sequences to generally achieve FSMs specifically binding with G4, which is independent of G4 morphologies and co-existing Na+ and Na+ concentrations remains a challenge. In this study, a simple G-rich DNA sequence (thrombin binding aptamer, TBA) flanked by 10-mer single-stranded DNA at the 3' and 5' termini (TBA-10 bp) is designed. In the presence of K+, TBA transforms into antiparallel G4 (K+-TBA) and TBA-10 bp transforms into antiparallel K+-TBA flanked by fully hybridized double-stranded DNA (ds-DNA) (K+-TBA-10 bp). Actually, ThT cannot effectively bind with antiparallel K+-TBA. Compared with K+-TBA, upon K+-TBA-10 bp binding with ThT, ThT emission fluorescence increased by 100-fold. Importantly, the binding affinity improved by 1000-fold, which is independent of co-existing Na+ and Na+ concentrations (5-140 mmol·L-1). Integrated with UV-Vis spectroscopy, fluorescent spectroscopy, and circular dichroism spectroscopy, it is believed that ThT can specifically and efficiently imbed in the junction between K+-TBA and ds-DNA. To corroborate the binding mode, TBA in TBA-10 bp is substituted by other G-rich DNA sequences transforming into parallel and antiparallel G4 in the presence of K+, respectively. The resulting improved ThT emission fluorescence indicated that such a specific binding mode generally improved the binding affinity of FSMs with G4. Our findings provide new insights into the improvement of the binding affinity of FSMs and G4, and reveal potential biochemical and bioanalytical applications of G4.
2022, 38(4): 200402
doi: 10.3866/PKU.WHXB202004025
Abstract:
Graphene oxide (GO) possesses a large number of oxygen-containing functional groups on its basal planes and edges, enabling it to disperse well in water and other aqueous media. This property facilitates the processing of GO by various wet-processing methods. Because of its interesting properties and useful intermediate role in preparing graphene derivatives, GO has potential applications in many fields, including composites, separators, sensors, actuators, and energy storage and conversion. At high concentrations, strong, competitive interactions occur in GO aqueous dispersions that significantly impact the rheological behavior of these dispersions. In a liquid medium, the dispersed GO nanosheets form a unique colloidal system, in which solvation, electrostatic interactions, hydrogen bonding, and the lyophilic effect play important roles. The aromatic domains preserved from precursor graphite show attractive van der Waals interaction and π–π stacking between GO sheets. In this study, the effects of pH, temperature, and different organic solvents on the rheological behavior of GO dispersions were investigated through steady and dynamic rheological tests and theoretical analysis. The results showed that enhancing acidity, increasing the temperature within a certain range, and adding organic solvents such as pyridine promote transition of the GO aqueous dispersion from a viscoelastic liquid to a gel state, which shows different rheological properties. GO sheets in dispersion interact through negative charges originating from the many ionizable groups in the nanosheets and electrical double layers. Analysis using the Deryagin-Landau-Verwey-Overbeek (DLVO) theory showed that, under the conditions described above, these interactions were remarkably altered with consequent effects on the rheological properties. Weakened electric double-layer interaction disrupted the GO colloidal dispersion state and resulted in the association of GO nanosheets to form gel. Based on the above understanding, the yield stress of the GO dispersions affected by the volume fraction was analyzed by population balance equation (PBE) modeling. Through creep and relaxation experiments, the structure and rheological properties of GO dispersions at high concentrations were found to be similar in many respects to those of polymers. Therefore, the viscoelastic behavior of GO dispersions can be well described by the Poynting-Thomson model, which can provide theoretical support and advance the study of complex GO dispersions. These results shed new light on the rheological behavior of GO dispersions and can be used to optimize the processing conditions for future applications. ![]()
Graphene oxide (GO) possesses a large number of oxygen-containing functional groups on its basal planes and edges, enabling it to disperse well in water and other aqueous media. This property facilitates the processing of GO by various wet-processing methods. Because of its interesting properties and useful intermediate role in preparing graphene derivatives, GO has potential applications in many fields, including composites, separators, sensors, actuators, and energy storage and conversion. At high concentrations, strong, competitive interactions occur in GO aqueous dispersions that significantly impact the rheological behavior of these dispersions. In a liquid medium, the dispersed GO nanosheets form a unique colloidal system, in which solvation, electrostatic interactions, hydrogen bonding, and the lyophilic effect play important roles. The aromatic domains preserved from precursor graphite show attractive van der Waals interaction and π–π stacking between GO sheets. In this study, the effects of pH, temperature, and different organic solvents on the rheological behavior of GO dispersions were investigated through steady and dynamic rheological tests and theoretical analysis. The results showed that enhancing acidity, increasing the temperature within a certain range, and adding organic solvents such as pyridine promote transition of the GO aqueous dispersion from a viscoelastic liquid to a gel state, which shows different rheological properties. GO sheets in dispersion interact through negative charges originating from the many ionizable groups in the nanosheets and electrical double layers. Analysis using the Deryagin-Landau-Verwey-Overbeek (DLVO) theory showed that, under the conditions described above, these interactions were remarkably altered with consequent effects on the rheological properties. Weakened electric double-layer interaction disrupted the GO colloidal dispersion state and resulted in the association of GO nanosheets to form gel. Based on the above understanding, the yield stress of the GO dispersions affected by the volume fraction was analyzed by population balance equation (PBE) modeling. Through creep and relaxation experiments, the structure and rheological properties of GO dispersions at high concentrations were found to be similar in many respects to those of polymers. Therefore, the viscoelastic behavior of GO dispersions can be well described by the Poynting-Thomson model, which can provide theoretical support and advance the study of complex GO dispersions. These results shed new light on the rheological behavior of GO dispersions and can be used to optimize the processing conditions for future applications.
2022, 38(4): 200500
doi: 10.3866/PKU.WHXB202005007
Abstract:
Inorganic perovskite materials have gained considerable attention owing to their good thermal stability, high absorption coefficient, adjustable bandgap, and simple preparation. However, most inorganic perovskites are sensitive to water and need to be prepared under inert environments in a glove box, which increases their preparation cost. In this study, we used a simple one-step spin coating anti-solvent process to prepare CsPbI2Br, which was then annealed in humid air (relative humidity < 35%) at 300 ℃ for 5 min with isopropanol as the anti-solvent. An inorganic perovskite solar cell with fluorine-doped tin dioxide/compact TiO2/mesoporous TiO2/CsPbI2Br/hole transport materials/Ag structure was prepared. By varying the concentration of the mesoporous precursor, we controlled the thickness of mesoporous TiO2 in order to investigate its effect on the properties of the perovskite films and devices. The X-ray diffraction (XRD) results confirmed the successful synthesis of CsPbI2Br in humid air. Moreover, the thickness of the substrate affected the crystal growth orientation. The scanning electron microscopy results revealed that the thickness of the mesoporous titanium dioxide substrate affected the crystallization processing of CsPbI2Br, resulting in the formation of compounds with different morphologies and phases. The ultraviolet-visible (UV-Vis) and photoluminescence spectra of the perovskite materials revealed that the substrate thickness affected their optical properties. With a decrease in the thickness of the mesoporous TiO2 substrate, the bandgap of CsPbI2Br increased slightly. At the substrate thickness of 145 nm, the defect density of state of CsPbI2Br increased. At the optimum mesoporous titanium dioxide substrate thickness of 732 nm, the device showed the best power conversion efficiency of 8.16%. The electrochemical impedance spectroscopy measurements revealed that the devices prepared on thicker mesoporous layers showed better carrier extraction and transmission capabilities but higher interfacial charge recombination resistance, leading to a lower open-circuit voltage but higher current density. Thus, an increase in the thickness of the mesoporous substrate improved the photovoltaic performance of the devices. The stability of the CsPbI2Br perovskite film improved with an increase in the mesoporous substrate thickness. The stability test results along with the UV-Vis and XRD analysis results showed that the perovskite film prepared on the 732 nm-thick substrate showed no significant structure change after being placed in humid air for 144 h. The stability of the perovskite solar cells was also investigated. The device with the 732 nm-thick substrate could maintain its original efficiency of 73% after exposure to air with relative humidity less than 35% for 72 h. Thus, inorganic perovskite solar cells could be successfully prepared in the humid air environment. ![]()
Inorganic perovskite materials have gained considerable attention owing to their good thermal stability, high absorption coefficient, adjustable bandgap, and simple preparation. However, most inorganic perovskites are sensitive to water and need to be prepared under inert environments in a glove box, which increases their preparation cost. In this study, we used a simple one-step spin coating anti-solvent process to prepare CsPbI2Br, which was then annealed in humid air (relative humidity < 35%) at 300 ℃ for 5 min with isopropanol as the anti-solvent. An inorganic perovskite solar cell with fluorine-doped tin dioxide/compact TiO2/mesoporous TiO2/CsPbI2Br/hole transport materials/Ag structure was prepared. By varying the concentration of the mesoporous precursor, we controlled the thickness of mesoporous TiO2 in order to investigate its effect on the properties of the perovskite films and devices. The X-ray diffraction (XRD) results confirmed the successful synthesis of CsPbI2Br in humid air. Moreover, the thickness of the substrate affected the crystal growth orientation. The scanning electron microscopy results revealed that the thickness of the mesoporous titanium dioxide substrate affected the crystallization processing of CsPbI2Br, resulting in the formation of compounds with different morphologies and phases. The ultraviolet-visible (UV-Vis) and photoluminescence spectra of the perovskite materials revealed that the substrate thickness affected their optical properties. With a decrease in the thickness of the mesoporous TiO2 substrate, the bandgap of CsPbI2Br increased slightly. At the substrate thickness of 145 nm, the defect density of state of CsPbI2Br increased. At the optimum mesoporous titanium dioxide substrate thickness of 732 nm, the device showed the best power conversion efficiency of 8.16%. The electrochemical impedance spectroscopy measurements revealed that the devices prepared on thicker mesoporous layers showed better carrier extraction and transmission capabilities but higher interfacial charge recombination resistance, leading to a lower open-circuit voltage but higher current density. Thus, an increase in the thickness of the mesoporous substrate improved the photovoltaic performance of the devices. The stability of the CsPbI2Br perovskite film improved with an increase in the mesoporous substrate thickness. The stability test results along with the UV-Vis and XRD analysis results showed that the perovskite film prepared on the 732 nm-thick substrate showed no significant structure change after being placed in humid air for 144 h. The stability of the perovskite solar cells was also investigated. The device with the 732 nm-thick substrate could maintain its original efficiency of 73% after exposure to air with relative humidity less than 35% for 72 h. Thus, inorganic perovskite solar cells could be successfully prepared in the humid air environment.
2022, 38(4): 200505
doi: 10.3866/PKU.WHXB202005054
Abstract:
Supercapacitors that can withstand extremely low temperatures have become desirable in applications including portable electronic devices, hybrid electric vehicles, and renewable energy conversion systems. Graphene is considered as a promising electrode material for supercapacitors owing to its high specific surface area (up to 2675 m2·g-1) and electrical conductivity (approximately 2 × 102 S·m-1). However, the restacking of graphene sheets decreases the accessible surface area, reduces the ion diffusion rate and prolongs the ion transport pathways, thereby limiting the energy storage performance at low temperatures (typically < 100 F·g-1 at sub-zero temperatures). Herein, we fabricate a supercapacitor based on holey graphene and mixed-solvent organic electrolyte for ultra-low-temperature applications (e.g., -60 ℃). Reduced holey graphene oxide (rHGO) was synthesized as the electrode material via an oxidative-etching process with H2O2. Methyl formate was mixed with propylene carbonate to improve the electrolyte conductivity at temperatures ranging from -60 to 25 ℃. The as-fabricated supercapacitor showed a high room-temperature capacitance of 150.5 F·g-1 at 1 A·g-1, which was almost 1.5 times greater than that of the supercapacitor using untreated reduced graphene oxide (rGO; 101.4 F·g-1). The improved capacitance could be attributed to the increased accessible surface rendered by the abundant mesopores and macropores on the holey surface. As the temperature decreased to -60 ℃, the rHGO supercapacitor still delivered a high capacitance of 106.2 F·g-1 with a retention of 70.6%, which was superior to other state-of-the-art graphene-based supercapacitors. Electrochemical impedance spectra tests revealed that the ion diffusion resistance in rHGO was significantly smaller than that in rGO and less influenced by temperature with a lower activation energy. This was because the holey morphology can provide transport pathways for ions and reduce the ion diffusion length during charging/discharging, consequently diminishing the diffusion resistance at low temperatures. Specifically, at -60 ℃, the energy density of supercapacitor reached up to 26.9 Wh·kg-1 at 1 A·g-1 with a maximum power density of 18.7 kW·kg-1 at 20 A·g-1, surpassing the low-temperature performance of conventional carbon-based supercapacitors. Moreover, after 10000 cycles at -60 ℃ with a current density of 5 A·g-1, 89.1% of capacitance was retained, suggesting the stable and reliable power output of the current supercapacitor at extremely low temperatures. ![]()
Supercapacitors that can withstand extremely low temperatures have become desirable in applications including portable electronic devices, hybrid electric vehicles, and renewable energy conversion systems. Graphene is considered as a promising electrode material for supercapacitors owing to its high specific surface area (up to 2675 m2·g-1) and electrical conductivity (approximately 2 × 102 S·m-1). However, the restacking of graphene sheets decreases the accessible surface area, reduces the ion diffusion rate and prolongs the ion transport pathways, thereby limiting the energy storage performance at low temperatures (typically < 100 F·g-1 at sub-zero temperatures). Herein, we fabricate a supercapacitor based on holey graphene and mixed-solvent organic electrolyte for ultra-low-temperature applications (e.g., -60 ℃). Reduced holey graphene oxide (rHGO) was synthesized as the electrode material via an oxidative-etching process with H2O2. Methyl formate was mixed with propylene carbonate to improve the electrolyte conductivity at temperatures ranging from -60 to 25 ℃. The as-fabricated supercapacitor showed a high room-temperature capacitance of 150.5 F·g-1 at 1 A·g-1, which was almost 1.5 times greater than that of the supercapacitor using untreated reduced graphene oxide (rGO; 101.4 F·g-1). The improved capacitance could be attributed to the increased accessible surface rendered by the abundant mesopores and macropores on the holey surface. As the temperature decreased to -60 ℃, the rHGO supercapacitor still delivered a high capacitance of 106.2 F·g-1 with a retention of 70.6%, which was superior to other state-of-the-art graphene-based supercapacitors. Electrochemical impedance spectra tests revealed that the ion diffusion resistance in rHGO was significantly smaller than that in rGO and less influenced by temperature with a lower activation energy. This was because the holey morphology can provide transport pathways for ions and reduce the ion diffusion length during charging/discharging, consequently diminishing the diffusion resistance at low temperatures. Specifically, at -60 ℃, the energy density of supercapacitor reached up to 26.9 Wh·kg-1 at 1 A·g-1 with a maximum power density of 18.7 kW·kg-1 at 20 A·g-1, surpassing the low-temperature performance of conventional carbon-based supercapacitors. Moreover, after 10000 cycles at -60 ℃ with a current density of 5 A·g-1, 89.1% of capacitance was retained, suggesting the stable and reliable power output of the current supercapacitor at extremely low temperatures.
2022, 38(4): 210403
doi: 10.3866/PKU.WHXB202104032
Abstract:
2022, 38(4): 210500
doi: 10.3866/PKU.WHXB202105006
Abstract:
2022, 38(4): 210501
doi: 10.3866/PKU.WHXB202105010
Abstract:
