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2022 Volume 38 Issue 5
2022, 38(5): 200500
doi: 10.3866/PKU.WHXB202005006
Abstract:
Graphene is a 2D nanocomposite that has been gaining popularity in the research community in recent years. It is light weight with high tensile strength and excellent electrical conductivity. While graphene nanosheets are typically assembled into macroscopic nanocomposites, their beneficial properties may degrade from the aggregation of nanosheets owing to the weak interfacial interactions caused by random orientation and many other obstacles. Thus, finding an effective way to assemble the graphene nanosheets while not impacting its intrinsic properties is challenging. In nature, live organisms have always assembled numerous nanomaterials into high-performance nanocomposites. For example, nacre is composed of aragonite nanoplatelets and biopolymers such as protein and chitin. The aragonite nanoplatelets with a 95% volume fraction are stacked into a layered structure and "glued" together by biopolymers based on the "brick-and-mortar" architecture. The fracture toughness is 3000 times higher than natural aragonite minerals owing to the "extrinsic toughening mechanism" from the crack deflection and bridging in the "brick-and-mortar" architecture. We propose a nacre-inspired layered structure in graphene-based nanocomposites with two complementary strategies: constructing nacre-like and inverse nacre-like structures. This paper first introduces the structure and toughening mechanism of nacre and clarify the advantages of a bioinspired strategies. Then, some of the recent work on nacre-inspired graphene-based multifunctional nanocomposites is discussed. To construct the nacre-like structure, we fabricated graphene-based fibers and membranes with graphene as the main component. The nacre-like graphene-based nanocomposites have excellent tensile strength and toughness due to the synergistic effects from interfacial interactions and building blocks. It also demonstrated high electrical conductivity, which makes it suitable for electromagnetic interference shielding or supercapacitors. We also fabricated inverse nacre-like graphene-based nanocomposites with a small amount of graphene. The inverse nacre-like graphene-based nanocomposites has a layered structure and exhibited the "extrinsic toughening mechanism" seen in nacre. Consequently, the inverse nacre-like graphene-based nanocomposites possesses high fracture toughness that pushes the limit of "mixing rule". With the addition of graphene, the inverse nacre-like nanocomposites are suitable for use in many applications such as electrical conductivity, electrical heating, temperature measurement and many other functions. Finally, our study summarizes the strategies to overcome the obstacles we encountered during the assembly process to construct both the nacre-like and inverse nacre-like structures that were based on graphene. Some of the challenges we encountered include the small sample size, the quality of graphene nanosheets and developing hierarchical assembly techniques. The upcoming trends in nacre-inspired graphene-based multifunctional nanocomposites will also be discussed. ![]()
Graphene is a 2D nanocomposite that has been gaining popularity in the research community in recent years. It is light weight with high tensile strength and excellent electrical conductivity. While graphene nanosheets are typically assembled into macroscopic nanocomposites, their beneficial properties may degrade from the aggregation of nanosheets owing to the weak interfacial interactions caused by random orientation and many other obstacles. Thus, finding an effective way to assemble the graphene nanosheets while not impacting its intrinsic properties is challenging. In nature, live organisms have always assembled numerous nanomaterials into high-performance nanocomposites. For example, nacre is composed of aragonite nanoplatelets and biopolymers such as protein and chitin. The aragonite nanoplatelets with a 95% volume fraction are stacked into a layered structure and "glued" together by biopolymers based on the "brick-and-mortar" architecture. The fracture toughness is 3000 times higher than natural aragonite minerals owing to the "extrinsic toughening mechanism" from the crack deflection and bridging in the "brick-and-mortar" architecture. We propose a nacre-inspired layered structure in graphene-based nanocomposites with two complementary strategies: constructing nacre-like and inverse nacre-like structures. This paper first introduces the structure and toughening mechanism of nacre and clarify the advantages of a bioinspired strategies. Then, some of the recent work on nacre-inspired graphene-based multifunctional nanocomposites is discussed. To construct the nacre-like structure, we fabricated graphene-based fibers and membranes with graphene as the main component. The nacre-like graphene-based nanocomposites have excellent tensile strength and toughness due to the synergistic effects from interfacial interactions and building blocks. It also demonstrated high electrical conductivity, which makes it suitable for electromagnetic interference shielding or supercapacitors. We also fabricated inverse nacre-like graphene-based nanocomposites with a small amount of graphene. The inverse nacre-like graphene-based nanocomposites has a layered structure and exhibited the "extrinsic toughening mechanism" seen in nacre. Consequently, the inverse nacre-like graphene-based nanocomposites possesses high fracture toughness that pushes the limit of "mixing rule". With the addition of graphene, the inverse nacre-like nanocomposites are suitable for use in many applications such as electrical conductivity, electrical heating, temperature measurement and many other functions. Finally, our study summarizes the strategies to overcome the obstacles we encountered during the assembly process to construct both the nacre-like and inverse nacre-like structures that were based on graphene. Some of the challenges we encountered include the small sample size, the quality of graphene nanosheets and developing hierarchical assembly techniques. The upcoming trends in nacre-inspired graphene-based multifunctional nanocomposites will also be discussed.
2022, 38(5): 200601
doi: 10.3866/PKU.WHXB202006016
Abstract:
HfO2-based ferroelectric capacitors, particularly TiN/HfxZr1-xO2/TiN metal insulator metal (MIM) capacitors, have attracted considerable attention as promising candidates in the new generation of nonvolatile memory applications, because of their excellent stability, high performance, and complementary metal oxide semiconductor (CMOS) compatibility. At the electrode interface of TiN/HfxZr1-xO2/TiN MIM ferroelectric devices, the existence of the TiOxNy layer, which was formed during HfxZr1-xO2 film crystallization and TiN oxidization, can affect interface/grain boundary energy, film stress, and conduction band offset at the TiN/HfxZr1-xO2 interface, thereby affecting the ferroelectric device performance. Because the electrical performance of TiN/HfxZr1-xO2/TiN capacitors depends on both the ferroelectric HfxZr1-xO2 thin films and electrode TiN/insulator HfxZr1-xO2 interface, it is essential to control the fabrication of the TiN/HfxZr1-xO2/TiN heterostructure. Herein, we report a new method for preparing HfxZr1-xO2 ferroelectric thin films, sandwiched between TiN electrodes, by atomic layer deposition (ALD) and using ultra high vacuum (UHV) sputtering equipment interconnected with an ultra-high vacuum system. The quasi in situ characterization by transmission electron microscopy (TEM), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and other analytical methods conducted in our study indicates that the surface of the bottom TiN electrode does not contain oxygen. Moreover, a flat signal for impurities at the interface suggests that the superior ferroelectric performance of HfxZr1-xO2-based device is mainly attributed to the pristine HfxZr1-xO2/TiN interface. Furthermore, the ferroelectric properties of TiN/HfxZr1-xO2/TiN heterostructures on silicon can be modulated by varying ZrO2 doping concentration and rapid thermal annealing (RTA) temperature, which can be well monitored and controlled by the interconnected system. We also investigate the ferroelectric properties of TiN/HfxZr1-xO2/TiN capacitors with different ZrO2 doping concentrations (30%–60% (x)) at room temperature by changing the ALD pulsing ratio within the vacuum interconnected system. Three identical 10 nm-thick Hf0.5Zr0.5O2 samples sandwiched between TiN electrodes are annealed in N2 ambient at 400, 450 and 600 ℃ for 5 min to investigate the effect of RTA on device performance. The evolution of P-E hysteresis at different applied voltages and RTA temperatures reveals that the saturation of P-E hysteresis and remanent polarization increase with RTA temperature. This increase is especially evident at low applied voltages such as 1.5 V. A higher remanent polarization of 21.5 μC·cm-2 than the previously reported value and a low coercive voltage of 1.35 V were achieved for the electric field of 3 MV·cm-1 by doping 50% (molar fraction, x) ZrO2 in HfO2 through RTA at 600 ℃ for film crystallization. ![]()
HfO2-based ferroelectric capacitors, particularly TiN/HfxZr1-xO2/TiN metal insulator metal (MIM) capacitors, have attracted considerable attention as promising candidates in the new generation of nonvolatile memory applications, because of their excellent stability, high performance, and complementary metal oxide semiconductor (CMOS) compatibility. At the electrode interface of TiN/HfxZr1-xO2/TiN MIM ferroelectric devices, the existence of the TiOxNy layer, which was formed during HfxZr1-xO2 film crystallization and TiN oxidization, can affect interface/grain boundary energy, film stress, and conduction band offset at the TiN/HfxZr1-xO2 interface, thereby affecting the ferroelectric device performance. Because the electrical performance of TiN/HfxZr1-xO2/TiN capacitors depends on both the ferroelectric HfxZr1-xO2 thin films and electrode TiN/insulator HfxZr1-xO2 interface, it is essential to control the fabrication of the TiN/HfxZr1-xO2/TiN heterostructure. Herein, we report a new method for preparing HfxZr1-xO2 ferroelectric thin films, sandwiched between TiN electrodes, by atomic layer deposition (ALD) and using ultra high vacuum (UHV) sputtering equipment interconnected with an ultra-high vacuum system. The quasi in situ characterization by transmission electron microscopy (TEM), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and other analytical methods conducted in our study indicates that the surface of the bottom TiN electrode does not contain oxygen. Moreover, a flat signal for impurities at the interface suggests that the superior ferroelectric performance of HfxZr1-xO2-based device is mainly attributed to the pristine HfxZr1-xO2/TiN interface. Furthermore, the ferroelectric properties of TiN/HfxZr1-xO2/TiN heterostructures on silicon can be modulated by varying ZrO2 doping concentration and rapid thermal annealing (RTA) temperature, which can be well monitored and controlled by the interconnected system. We also investigate the ferroelectric properties of TiN/HfxZr1-xO2/TiN capacitors with different ZrO2 doping concentrations (30%–60% (x)) at room temperature by changing the ALD pulsing ratio within the vacuum interconnected system. Three identical 10 nm-thick Hf0.5Zr0.5O2 samples sandwiched between TiN electrodes are annealed in N2 ambient at 400, 450 and 600 ℃ for 5 min to investigate the effect of RTA on device performance. The evolution of P-E hysteresis at different applied voltages and RTA temperatures reveals that the saturation of P-E hysteresis and remanent polarization increase with RTA temperature. This increase is especially evident at low applied voltages such as 1.5 V. A higher remanent polarization of 21.5 μC·cm-2 than the previously reported value and a low coercive voltage of 1.35 V were achieved for the electric field of 3 MV·cm-1 by doping 50% (molar fraction, x) ZrO2 in HfO2 through RTA at 600 ℃ for film crystallization.
2022, 38(5): 200605
doi: 10.3866/PKU.WHXB202006059
Abstract:
Rechargeable aqueous Zinc-ion batteries (ZIBs) have emerged as potential energy storage devices due to their high energy density, low cost, and safety. To date, numerous cathodes based on manganese dioxide, vanadium dioxide, and polyanionic compounds have been reported. Among them, MnO2 cathodes are particularly desirable candidates for commercialization owing to their tunnel structure and affordability. In particular, the parasitic reaction of Mn-based cathodes in alkaline batteries can be suppressed in mild aqueous electrolytes, resulting in enthusiasm for the development of rechargeable Zn||MnO2 batteries. Even though various MnO2 phases have been reported as hosts for Zn2+/H+ insertion, MnO2 crystal structures undergo significant, irreversible transformations during cycling, which is a major challenge in Zn||MnO2 batteries. In addition, the tunnel structure can collapse under the insertion of the hydrated cation resulting in Mn2+ dissolution into the electrolyte and significant loss in capacity over long cycling periods. The MnO2 cathode also has low intrinsic electronic conductivity due to the large charge transfer resistance, which limits the diffusivity of divalent ions. Despite the achievements made in the field of ZIBs so far, designing active materials and ZIBs systems to meet commercial requirements is a significant challenge. In this study, we report the preparation of polypyrrole-wrapped MnO2/carbon nanotubes (PPy@MnO2/CNT) as composite cathodes for aqueous ZIBs. A combination of design strategies was used to increase structural stability and improve electronic conductivity, including increased electrode/electrolyte interaction by using nano-sized structures, shortened diffusion pathways through multistage composites, and enhanced electrical conductivity with conductive composites. The three-dimensional (3D) structured PPy/CNT network can facilitate mass and charge transport during the charge and discharge processes. The structure of MnO2 wrapped by polypyrrole effectively prevents the dissolution of MnO2. Thus, the assembled Zn||MnO2 batteries, using PPy@MnO2/CNT composite cathodes, exhibit a high capacity of 210 mAh·g-1 at 1 A·g-1, and achieve 85.7% capacity retention after 1000 charge/discharge cycles. Moreover, a high specific capacity of 100 mAh·g-1 could be maintained at 2 A·g-1, exhibiting excellent kinetic performance. The assembled quasi-solid Zn//MnO2 battery, benefiting from the xanthan gum electrolyte and flexible CNT film, possesses intrinsic safety, bending resistance, and high potential in wearable applications. ![]()
Rechargeable aqueous Zinc-ion batteries (ZIBs) have emerged as potential energy storage devices due to their high energy density, low cost, and safety. To date, numerous cathodes based on manganese dioxide, vanadium dioxide, and polyanionic compounds have been reported. Among them, MnO2 cathodes are particularly desirable candidates for commercialization owing to their tunnel structure and affordability. In particular, the parasitic reaction of Mn-based cathodes in alkaline batteries can be suppressed in mild aqueous electrolytes, resulting in enthusiasm for the development of rechargeable Zn||MnO2 batteries. Even though various MnO2 phases have been reported as hosts for Zn2+/H+ insertion, MnO2 crystal structures undergo significant, irreversible transformations during cycling, which is a major challenge in Zn||MnO2 batteries. In addition, the tunnel structure can collapse under the insertion of the hydrated cation resulting in Mn2+ dissolution into the electrolyte and significant loss in capacity over long cycling periods. The MnO2 cathode also has low intrinsic electronic conductivity due to the large charge transfer resistance, which limits the diffusivity of divalent ions. Despite the achievements made in the field of ZIBs so far, designing active materials and ZIBs systems to meet commercial requirements is a significant challenge. In this study, we report the preparation of polypyrrole-wrapped MnO2/carbon nanotubes (PPy@MnO2/CNT) as composite cathodes for aqueous ZIBs. A combination of design strategies was used to increase structural stability and improve electronic conductivity, including increased electrode/electrolyte interaction by using nano-sized structures, shortened diffusion pathways through multistage composites, and enhanced electrical conductivity with conductive composites. The three-dimensional (3D) structured PPy/CNT network can facilitate mass and charge transport during the charge and discharge processes. The structure of MnO2 wrapped by polypyrrole effectively prevents the dissolution of MnO2. Thus, the assembled Zn||MnO2 batteries, using PPy@MnO2/CNT composite cathodes, exhibit a high capacity of 210 mAh·g-1 at 1 A·g-1, and achieve 85.7% capacity retention after 1000 charge/discharge cycles. Moreover, a high specific capacity of 100 mAh·g-1 could be maintained at 2 A·g-1, exhibiting excellent kinetic performance. The assembled quasi-solid Zn//MnO2 battery, benefiting from the xanthan gum electrolyte and flexible CNT film, possesses intrinsic safety, bending resistance, and high potential in wearable applications.
2022, 38(5): 200606
doi: 10.3866/PKU.WHXB202006061
Abstract:
The interfacial mass transfer characteristics of the gas-oil miscibility process are important in gas flooding technology to improve oil recovery. In this study, the process of gas flooding with actual components of Jilin oilfield is investigated by using molecular dynamics simulation method. We have chosen several alkane molecules based specifically on the actual components of crucial oil as the model oil phase for our study. The pressure of the gas phase is adjusted by changing the number of gas molecules while keeping the oil phase constant in the simulation. After the simulation, we analyze the variations of density in the gas-oil phase and interfacial characteristics to obtain the minimum miscibility pressure (MMP) for different displacement gases. The results show that the density of the gas phase increases while the density of the oil phase decreases with an increase in the displacement gas pressure, resulting in efficient mixing between the gas phase and the oil phase. At higher gas pressures, the thickness of the interface between the gas and oil phases is higher while the interfacial tension is lower. At the same time, we observed that the higher the CO2 content in the displacement phase, the thicker the oil-gas interface becomes and the better the oil-gas mixing is under the same gas pressure. In this work, the gas-oil miscibility is studied with pure CO2, pure N2, and the mixture of these two gases, and it is found that the minimum miscibility pressure for pure CO2 flooding (22.3 MPa) is much lower than that for pure N2 flooding (119.0 MPa). When these two gases are mixed in 1 : 1 ratio, the MMP (50.7 MPa) is between the MMPs of the two pure gases. Moreover, the pressure required with CO2 is lower than that required with N2 to achieve the same displacement effect. Finally, we explain the mechanisms of the different miscibility processes for different gas pressure and different displacement gases from the perspective of the total energy of the system and the potential of the mean force between the gas and the oil. The total energy of the system increases with the pressure of the gas phase, implying that the number of collisions between the oil and gas molecules increases and the gas-oil miscibility is enhanced. In addition, by analyzing the potential of mean force profiles, it can be concluded that the force of attraction between the oil-phase molecules and CO2 molecules is greater than that between the oil-phase molecules and N2 molecules; thus, the CO2 molecules easily mix with oil, and the effect of displacement is more obvious. These results are of great significance for understanding the interfacial mass transfer characteristics of the gas-oil miscibility process and for guiding the optimization and design of enhanced oil recovery technology by gas flooding. ![]()
The interfacial mass transfer characteristics of the gas-oil miscibility process are important in gas flooding technology to improve oil recovery. In this study, the process of gas flooding with actual components of Jilin oilfield is investigated by using molecular dynamics simulation method. We have chosen several alkane molecules based specifically on the actual components of crucial oil as the model oil phase for our study. The pressure of the gas phase is adjusted by changing the number of gas molecules while keeping the oil phase constant in the simulation. After the simulation, we analyze the variations of density in the gas-oil phase and interfacial characteristics to obtain the minimum miscibility pressure (MMP) for different displacement gases. The results show that the density of the gas phase increases while the density of the oil phase decreases with an increase in the displacement gas pressure, resulting in efficient mixing between the gas phase and the oil phase. At higher gas pressures, the thickness of the interface between the gas and oil phases is higher while the interfacial tension is lower. At the same time, we observed that the higher the CO2 content in the displacement phase, the thicker the oil-gas interface becomes and the better the oil-gas mixing is under the same gas pressure. In this work, the gas-oil miscibility is studied with pure CO2, pure N2, and the mixture of these two gases, and it is found that the minimum miscibility pressure for pure CO2 flooding (22.3 MPa) is much lower than that for pure N2 flooding (119.0 MPa). When these two gases are mixed in 1 : 1 ratio, the MMP (50.7 MPa) is between the MMPs of the two pure gases. Moreover, the pressure required with CO2 is lower than that required with N2 to achieve the same displacement effect. Finally, we explain the mechanisms of the different miscibility processes for different gas pressure and different displacement gases from the perspective of the total energy of the system and the potential of the mean force between the gas and the oil. The total energy of the system increases with the pressure of the gas phase, implying that the number of collisions between the oil and gas molecules increases and the gas-oil miscibility is enhanced. In addition, by analyzing the potential of mean force profiles, it can be concluded that the force of attraction between the oil-phase molecules and CO2 molecules is greater than that between the oil-phase molecules and N2 molecules; thus, the CO2 molecules easily mix with oil, and the effect of displacement is more obvious. These results are of great significance for understanding the interfacial mass transfer characteristics of the gas-oil miscibility process and for guiding the optimization and design of enhanced oil recovery technology by gas flooding.
2022, 38(5): 200606
doi: 10.3866/PKU.WHXB202006064
Abstract:
Black phosphorus (BP) is a promising candidate for photovoltaic and optoelectronic applications owing to its excellent electronic and optical properties. It is believed that defects generally accelerate non-radiative electron-hole recombination in BP and hinder improvement of device performance. Experiments defy this expectation. Using state-of-the-art ab initio time-dependent density functional theory combined with non-adiabatic molecular dynamics, we investigate the non-radiative electron-hole recombination in monolayer (MBP) and MBP containing nanopore defects (MBP-ND). We demonstrate that non-radiative electron-hole recombination is promoted by the P-P stretching vibrations, and the recombination time of MBP-ND is approximately 5.5 times longer than that of the MBP system. This is mainly attributed to the following three factors: First, the nanopore creates no mid-gap state when increasing the bandgap by 0.22 eV owing to the downshift of the valence band maximum, caused by the decrease in the inter-layer P-P bond length, thereby weakening the antibonding interaction. Second, the nanopore reduces the overlap of electron and hole wave functions by diminishing the charge densities near the defect. Simultaneously, the nanopore significantly inhibits the thermal-driven atomic fluctuations. The increased bandgap correlated with the decreased wave function overlap and slowed thermal motions of the nuclei in the MBP-ND system reduces the non-adiabatic coupling by a factor of approximately 2 with respect to the pristine system. Third, the slow atomic motions weaken the electron-vibrational interaction and decrease the intensity of the major vibration mode at 440 cm−1, which is the main source for creating non-adiabatic coupling, leading to loss of coherence formed between a pair of electronic states via non-adiabatic coupling and causing electron-hole recombination that results in a 1.5-fold increase in the coherence time in the MBP-ND system with respect to the MBP system. Consequently, the increased bandgap and decreased non-adiabatic coupling compete successfully with the prolonged coherence time, extending the excited-state lifetime to 2.74 ns in the system containing nanopore defects, which is only 480 ps in the pristine system. These phenomena arise owing to a complex interplay of the unusual chemical, structural, electrostatic, and quantum properties of BP with and without nanopore defects. This study is of great significance for understanding the excited-state properties of BP. The detailed mechanistic understanding of the prolonged charge carriers lifetime of MBP decorated with nanopore defects provides key insights for defect engineering in BP and other 2-dimensional materials for a broad range of solar and electro-optic applications by reducing the non-radiative charge and energy losses. ![]()
Black phosphorus (BP) is a promising candidate for photovoltaic and optoelectronic applications owing to its excellent electronic and optical properties. It is believed that defects generally accelerate non-radiative electron-hole recombination in BP and hinder improvement of device performance. Experiments defy this expectation. Using state-of-the-art ab initio time-dependent density functional theory combined with non-adiabatic molecular dynamics, we investigate the non-radiative electron-hole recombination in monolayer (MBP) and MBP containing nanopore defects (MBP-ND). We demonstrate that non-radiative electron-hole recombination is promoted by the P-P stretching vibrations, and the recombination time of MBP-ND is approximately 5.5 times longer than that of the MBP system. This is mainly attributed to the following three factors: First, the nanopore creates no mid-gap state when increasing the bandgap by 0.22 eV owing to the downshift of the valence band maximum, caused by the decrease in the inter-layer P-P bond length, thereby weakening the antibonding interaction. Second, the nanopore reduces the overlap of electron and hole wave functions by diminishing the charge densities near the defect. Simultaneously, the nanopore significantly inhibits the thermal-driven atomic fluctuations. The increased bandgap correlated with the decreased wave function overlap and slowed thermal motions of the nuclei in the MBP-ND system reduces the non-adiabatic coupling by a factor of approximately 2 with respect to the pristine system. Third, the slow atomic motions weaken the electron-vibrational interaction and decrease the intensity of the major vibration mode at 440 cm−1, which is the main source for creating non-adiabatic coupling, leading to loss of coherence formed between a pair of electronic states via non-adiabatic coupling and causing electron-hole recombination that results in a 1.5-fold increase in the coherence time in the MBP-ND system with respect to the MBP system. Consequently, the increased bandgap and decreased non-adiabatic coupling compete successfully with the prolonged coherence time, extending the excited-state lifetime to 2.74 ns in the system containing nanopore defects, which is only 480 ps in the pristine system. These phenomena arise owing to a complex interplay of the unusual chemical, structural, electrostatic, and quantum properties of BP with and without nanopore defects. This study is of great significance for understanding the excited-state properties of BP. The detailed mechanistic understanding of the prolonged charge carriers lifetime of MBP decorated with nanopore defects provides key insights for defect engineering in BP and other 2-dimensional materials for a broad range of solar and electro-optic applications by reducing the non-radiative charge and energy losses.
2022, 38(5): 200701
doi: 10.3866/PKU.WHXB202007017
Abstract:
High-performance rechargeable lithium ion batteries have been widely applied in electrochemical energy storage fields, such as, energy storage grids, portable electronic devices, and electric vehicles (EVs). However, the energy density of lithium ion batteries needs to be increased, and the cost of battery materials could be further reduced for wider commercial applications. An Ni-rich cathode, LiNixMnyCo1-x-yO2 (x > 0.8), with high specific capacity is the most promising material for next-generation Li-ion batteries. LiNixMnyCo1-x-yO2 (x > 0.8) contains three transition metal elements, Ni, Mn, and Co, respectively. The role of Ni2+ is to provide high capacity for recharge The role of Mn4+ is to stabilize the lattice structure during charging-discharging cycling. Crucially, the role of Co3+ in Ni-rich materials is to improve the electrical conductivity and inhibit cation disorder in the lattice during electrochemical cycling. However, Co is both in shortage and expensive, which limits its worldwide commercial application. This work investigates substituting Co with other abundant and cheap transition metals. Transition metal ions Cr3+, Cd2+, and Zr4+ can replace Co3+ in Ni-rich cathode materials. LiNi0.8Cr0.1Mn0.1O2, LiNi0.8Cd0.1Mn0.1O2, and LiNi0.8Zr0.1Mn0.1O2 were synthesized by a co-precipitation method. Zr was found to be the best candidate for replacing Co in Ni-rich cathode materials. This study investigated Zr4+-doped Co-free Ni-rich materials. Initially, a carbonate co-precipitation process was used to synthesize Ni0.8Zr0.1Mn0.1CO3. This is due to that Zr3+/Zr4+ ions are not precipitated in the strong alkali solution, and the pH during hydroxide co-precipitation and carbonate co-precipitation processes are approximately 11 and 8, respectively. Therefore, the carbonate co-precipitation synthesis method was chosen. Ni0.8Zr0.1Mn0.1CO3 was synthesized by carbonate co-precipitation at pH = 7.6, 7.8, 8.0, and 8.2. After electrochemical analysis, pH = 7.8 was identified as the optimal value. The next stage of the research involved completing an electrochemical performance comparison on two lithium sources. The following lithium sources were added to the precursor; LiOH·2O, and a 1:1 mixture of LiOH·2O and Li2CO3. The lithium source with the 1:1 mixture, exhibited better performance for the Ni-rich cathode, LiNi0.8Zr0.1Mn0.1O2. In this study, the ideal doping amount of Zr in Ni-rich materials was 0.05. In conclusion, by careful control of co-precipitation pH and Li source, the Zr doped cobalt free Ni-rich cathode LiNi0.85Mn0.1Zr0.05O2 delivered a discharge capacity of 179.9 mAh·g-1 at 0.2C. This was achieved between the voltage range of 2.75-4.3 V, with an 80 cycle capacity retention of 96.52%. ![]()
High-performance rechargeable lithium ion batteries have been widely applied in electrochemical energy storage fields, such as, energy storage grids, portable electronic devices, and electric vehicles (EVs). However, the energy density of lithium ion batteries needs to be increased, and the cost of battery materials could be further reduced for wider commercial applications. An Ni-rich cathode, LiNixMnyCo1-x-yO2 (x > 0.8), with high specific capacity is the most promising material for next-generation Li-ion batteries. LiNixMnyCo1-x-yO2 (x > 0.8) contains three transition metal elements, Ni, Mn, and Co, respectively. The role of Ni2+ is to provide high capacity for recharge The role of Mn4+ is to stabilize the lattice structure during charging-discharging cycling. Crucially, the role of Co3+ in Ni-rich materials is to improve the electrical conductivity and inhibit cation disorder in the lattice during electrochemical cycling. However, Co is both in shortage and expensive, which limits its worldwide commercial application. This work investigates substituting Co with other abundant and cheap transition metals. Transition metal ions Cr3+, Cd2+, and Zr4+ can replace Co3+ in Ni-rich cathode materials. LiNi0.8Cr0.1Mn0.1O2, LiNi0.8Cd0.1Mn0.1O2, and LiNi0.8Zr0.1Mn0.1O2 were synthesized by a co-precipitation method. Zr was found to be the best candidate for replacing Co in Ni-rich cathode materials. This study investigated Zr4+-doped Co-free Ni-rich materials. Initially, a carbonate co-precipitation process was used to synthesize Ni0.8Zr0.1Mn0.1CO3. This is due to that Zr3+/Zr4+ ions are not precipitated in the strong alkali solution, and the pH during hydroxide co-precipitation and carbonate co-precipitation processes are approximately 11 and 8, respectively. Therefore, the carbonate co-precipitation synthesis method was chosen. Ni0.8Zr0.1Mn0.1CO3 was synthesized by carbonate co-precipitation at pH = 7.6, 7.8, 8.0, and 8.2. After electrochemical analysis, pH = 7.8 was identified as the optimal value. The next stage of the research involved completing an electrochemical performance comparison on two lithium sources. The following lithium sources were added to the precursor; LiOH·2O, and a 1:1 mixture of LiOH·2O and Li2CO3. The lithium source with the 1:1 mixture, exhibited better performance for the Ni-rich cathode, LiNi0.8Zr0.1Mn0.1O2. In this study, the ideal doping amount of Zr in Ni-rich materials was 0.05. In conclusion, by careful control of co-precipitation pH and Li source, the Zr doped cobalt free Ni-rich cathode LiNi0.85Mn0.1Zr0.05O2 delivered a discharge capacity of 179.9 mAh·g-1 at 0.2C. This was achieved between the voltage range of 2.75-4.3 V, with an 80 cycle capacity retention of 96.52%.
2022, 38(5): 200708
doi: 10.3866/PKU.WHXB202007088
Abstract:
With the development of photovoltaic devices, organic-inorganic hybrid perovskite solar cells (PSCs) have been promising devices that have attracted significant attention in the fields of industrial and scientific research. Currently, the photoelectric conversion efficiency (PCE) of PSCs has been improved to 25.2%, and they are considered to be the primary alternative to silicon-based solar cells. However, the environmental stability of PSCs is unsatisfactory; they are prone to degradation under exposure to moisture, oxygen, elevated temperature, or even light illumination, which restricts their wide application in industrial production. Previous studies have elucidated that understanding the ultraviolet (UV)-induced degradation mechanism of organic-inorganic PSCs is of great importance for the improvement of light stability in PSCs. However, until now, there has been almost no comprehensive investigation on the decay process of PSCs under UV light illumination nor on the corresponding evolution of their microstructure. In this study, focused ion beam scanning electron microscopy (FIB-SEM) and aberration-corrected transmission electron microscopy (TEM) were used to comprehensively study changes in the performance and the evolution of the microstructure of PSC devices. The experimental results show that a built-in electric field developed under UV light illumination, which drove the diffusion of iodide ions (I-) from the CH3NH3PbI3 (MAPbI3) layer to the hole transfer layer (HTL, Spiro-OMeTAD). Together with the photo-excited holes in the HTL, the I- ions reacted with the Au electrode, and the Au atoms were oxidized into Au+ ions. Furthermore, Au+ ions preferred to diffuse across the HTL and the perovskite layer into the interface between the SnO2 and MAPbI3 layers. SnO2 is known to be a good electron transfer layer (ETL), which should collect the photo-excited electrons to reduce the Au+ ions into metallic Au clusters; this is why the Au electrode was destroyed and Au clusters aggregated at the SnO2-MAPbI3 interface under the UV light illumination. Meanwhile, the Au clusters would accelerate the degradation of the perovskite. In addition, as the PSC performance declined (as determined by the PCE, open-circuit voltage (Voc), and short-circuit current (Jsc)), the decomposition of tetragonal MAPbI3 into hexagonal PbI2 was observed at the interface between Spiro-OMeTAD and MAPbI3, along with a widening of the grain boundaries in the perovskite layer. All of these factors play critical roles in the UV-induced degradation of PSCs. This is the first study to elucidate the light-induced migration of Au from the metal electrode to the interface between SnO2/MAPbI3, which reveals that the UV-induced degradation of PSCs may be mitigated by finding new ways to restrain the interdiffusion of Au+ and I- ions. ![]()
With the development of photovoltaic devices, organic-inorganic hybrid perovskite solar cells (PSCs) have been promising devices that have attracted significant attention in the fields of industrial and scientific research. Currently, the photoelectric conversion efficiency (PCE) of PSCs has been improved to 25.2%, and they are considered to be the primary alternative to silicon-based solar cells. However, the environmental stability of PSCs is unsatisfactory; they are prone to degradation under exposure to moisture, oxygen, elevated temperature, or even light illumination, which restricts their wide application in industrial production. Previous studies have elucidated that understanding the ultraviolet (UV)-induced degradation mechanism of organic-inorganic PSCs is of great importance for the improvement of light stability in PSCs. However, until now, there has been almost no comprehensive investigation on the decay process of PSCs under UV light illumination nor on the corresponding evolution of their microstructure. In this study, focused ion beam scanning electron microscopy (FIB-SEM) and aberration-corrected transmission electron microscopy (TEM) were used to comprehensively study changes in the performance and the evolution of the microstructure of PSC devices. The experimental results show that a built-in electric field developed under UV light illumination, which drove the diffusion of iodide ions (I-) from the CH3NH3PbI3 (MAPbI3) layer to the hole transfer layer (HTL, Spiro-OMeTAD). Together with the photo-excited holes in the HTL, the I- ions reacted with the Au electrode, and the Au atoms were oxidized into Au+ ions. Furthermore, Au+ ions preferred to diffuse across the HTL and the perovskite layer into the interface between the SnO2 and MAPbI3 layers. SnO2 is known to be a good electron transfer layer (ETL), which should collect the photo-excited electrons to reduce the Au+ ions into metallic Au clusters; this is why the Au electrode was destroyed and Au clusters aggregated at the SnO2-MAPbI3 interface under the UV light illumination. Meanwhile, the Au clusters would accelerate the degradation of the perovskite. In addition, as the PSC performance declined (as determined by the PCE, open-circuit voltage (Voc), and short-circuit current (Jsc)), the decomposition of tetragonal MAPbI3 into hexagonal PbI2 was observed at the interface between Spiro-OMeTAD and MAPbI3, along with a widening of the grain boundaries in the perovskite layer. All of these factors play critical roles in the UV-induced degradation of PSCs. This is the first study to elucidate the light-induced migration of Au from the metal electrode to the interface between SnO2/MAPbI3, which reveals that the UV-induced degradation of PSCs may be mitigated by finding new ways to restrain the interdiffusion of Au+ and I- ions.
2022, 38(5): 200803
doi: 10.3866/PKU.WHXB202008037
Abstract:
A unique mixed-dimensional van der Waals heterostructure can be formed by integrating one-dimensional (1D) and two-dimensional (2D) materials. Such a 1D/2D mixed-dimensional heterostructure will not only inherit the unique properties of 2D/2D heterostructures, but also has a variety of stacking configurations, offering a new platform to adjust its structure and properties. The combination of p-type 1D single-walled carbon nanotubes (SWCNTs) and n-type 2D molybdenum disulfide (MoS2) is one such example, possessing tunable properties. In situ chemical vapor deposition (CVD) is one of the most effective methods to construct 1D SWCNT/2D MoS2 mixed-dimensional heterostructures. There are several reports of successfully grown SWCNT/MoS2 heterostructures. The reports indicate that these heterostructures exhibit strong electrical and mechanical couplings between the SWCNTs and MoS2, making it suitable for the construction of high-performance electronic and optoelectronic devices. However, there are still several problems associated with the in situ CVD growth of SWCNT/MoS2 heterostructures. First, the growth mechanism of the 1D SWCNT/2D MoS2 heterostructure is unclear. We still do not know how the existence of small-diameter SWCNTs will affect the nucleation and growth process of MoS2. It is undetermined whether MoS2 flakes will grow above the preexisting SWCNTs or under them. Second, current studies all report the growth of MoS2 on a substrate sparsely covered by SWCNTs, which have a wide chirality distribution. Since the chirality of SWCNTs determines their physical properties and the density of SWCNTs significantly affects its performance in electronic devices, both the low density and wide chirality distribution of SWCNTs reported in these studies impose negative impacts on the interface behavior of SWCNT/MoS2 heterostructures and their performance in devices. Herein, we report the preparation of high-quality 1D SWCNT/2D MoS2 heterostructures by directly growing MoS2 on dense and narrow-chirality distributed SWCNTs on a silicon substrate. To achieve this goal, high-purity semiconducting SWCNTs with narrow chirality distributions were sorted from the raw arc-discharged SWCNTs, and then high-density SWCNT arrays or networks were formed on a silicon substrate by dip-coating. Through in-depth analyses of the surface morphology and structure of the nuclei, we found that MoS2 may prefer to grow under the SWCNTs and will grow much faster in the grooves between the SWCNTs to form a growth front. Therefore, an interesting "absorption-diffusion-absorption" growth mechanism has been proposed to explain the nucleation and growth process of SWCNT/MoS2 heterostructures. In addition, we confirm the presence of strong charge coupling in the mixed-dimensional heterostructure through Raman analysis. Carriers can be quickly transferred through the interface between the SWCNTs and MoS2, paving a way for the future design and fabrication of novel electronic and optoelectronic devices based on 1D/2D heterostructures. ![]()
A unique mixed-dimensional van der Waals heterostructure can be formed by integrating one-dimensional (1D) and two-dimensional (2D) materials. Such a 1D/2D mixed-dimensional heterostructure will not only inherit the unique properties of 2D/2D heterostructures, but also has a variety of stacking configurations, offering a new platform to adjust its structure and properties. The combination of p-type 1D single-walled carbon nanotubes (SWCNTs) and n-type 2D molybdenum disulfide (MoS2) is one such example, possessing tunable properties. In situ chemical vapor deposition (CVD) is one of the most effective methods to construct 1D SWCNT/2D MoS2 mixed-dimensional heterostructures. There are several reports of successfully grown SWCNT/MoS2 heterostructures. The reports indicate that these heterostructures exhibit strong electrical and mechanical couplings between the SWCNTs and MoS2, making it suitable for the construction of high-performance electronic and optoelectronic devices. However, there are still several problems associated with the in situ CVD growth of SWCNT/MoS2 heterostructures. First, the growth mechanism of the 1D SWCNT/2D MoS2 heterostructure is unclear. We still do not know how the existence of small-diameter SWCNTs will affect the nucleation and growth process of MoS2. It is undetermined whether MoS2 flakes will grow above the preexisting SWCNTs or under them. Second, current studies all report the growth of MoS2 on a substrate sparsely covered by SWCNTs, which have a wide chirality distribution. Since the chirality of SWCNTs determines their physical properties and the density of SWCNTs significantly affects its performance in electronic devices, both the low density and wide chirality distribution of SWCNTs reported in these studies impose negative impacts on the interface behavior of SWCNT/MoS2 heterostructures and their performance in devices. Herein, we report the preparation of high-quality 1D SWCNT/2D MoS2 heterostructures by directly growing MoS2 on dense and narrow-chirality distributed SWCNTs on a silicon substrate. To achieve this goal, high-purity semiconducting SWCNTs with narrow chirality distributions were sorted from the raw arc-discharged SWCNTs, and then high-density SWCNT arrays or networks were formed on a silicon substrate by dip-coating. Through in-depth analyses of the surface morphology and structure of the nuclei, we found that MoS2 may prefer to grow under the SWCNTs and will grow much faster in the grooves between the SWCNTs to form a growth front. Therefore, an interesting "absorption-diffusion-absorption" growth mechanism has been proposed to explain the nucleation and growth process of SWCNT/MoS2 heterostructures. In addition, we confirm the presence of strong charge coupling in the mixed-dimensional heterostructure through Raman analysis. Carriers can be quickly transferred through the interface between the SWCNTs and MoS2, paving a way for the future design and fabrication of novel electronic and optoelectronic devices based on 1D/2D heterostructures.
2022, 38(5): 200603
doi: 10.3866/PKU.WHXB202006037
Abstract:
Sustainable freshwater supply is a grave challenge to the society because of the severe water scarcity and global pollution. Seawater is an inexhaustible source of industrial and potable water. The relevant desalination technologies with a high market share include reverse osmosis and thermal distillation, which are energy-intensive. Capacitive deionization (CDI) is a desalination technology that is gaining extensive attention because of its low energy consumption and low chemical intensity. In CDI, charged species are removed from the aqueous environment via applying a voltage onto the anode and cathode. For desalination, Na+ and Cl- ions are removed by the cathode and anode, respectively. With the boom in electrode materials for rechargeable batteries, the Na+ removal electrode (cathode) has evolved from a carbon-based electrode to a faradaic electrode, and the desalination performance of CDI has also been significantly enhanced. A conventional carbon-based electrode captures ions in the electrical double layer (EDL) and suffers from low charge efficiency, thus being unsuitable for use in water with high salinity. On the other hand, a faradaic electrode stores Na+ ions through a reversible redox process or intercalation, leading to high desalination capacity.However, the Cl- removal electrode (anode) has not yet seen notable development. Most research groups employ activated carbon to remove Cl-, and therefore, summarizing Cl- storage electrodes for CDI is necessary to guide the design of electrode systems with better desalination performance. First, this review outlines the evolution of CDI configuration based on the electrode materials, suggesting that the anode and cathode are of equal importance in CDI. Second, a systematic summary of the anode materials used in CDI and a comparison of the characteristics of different electrodes, including those based on Ag/AgCl, Bi/BiOCl, 2-dimensional (2D) materials (layered double hydroxide (LDH) and MXene), redox polymers, and electrolytes, are presented. Then, the underlying mechanism for Cl- storage is refined. Similar to the case of Na+ storage, traditional carbon electrodes store Cl- via electrosorption based on the EDL. Ag/AgCl and Bi/BiOCl remove Cl- through a conversion reaction, i.e., phase transformation during the reaction with Cl-. 2D materials store Cl- in the space between adjacent layers, a process referred as ion intercalation, with layered double hydroxide (LDH) and MXene showing higher Cl- storage potential. Redox polymers and electrolytes allow for Cl- storage via redox reactions. Among all the materials mentioned above, Bi/BiOCl and LDH are the most promising for the construction of CDI anodes because of their high capacity and low cost. Finally, to spur the development of novel anodes for CDI, the electrodes applied in a chlorine ion battery are introduced. This is the first paper to comb through reports on the development of anode materials for CDI, thus laying the theoretical foundation for future materials design. ![]()
Sustainable freshwater supply is a grave challenge to the society because of the severe water scarcity and global pollution. Seawater is an inexhaustible source of industrial and potable water. The relevant desalination technologies with a high market share include reverse osmosis and thermal distillation, which are energy-intensive. Capacitive deionization (CDI) is a desalination technology that is gaining extensive attention because of its low energy consumption and low chemical intensity. In CDI, charged species are removed from the aqueous environment via applying a voltage onto the anode and cathode. For desalination, Na+ and Cl- ions are removed by the cathode and anode, respectively. With the boom in electrode materials for rechargeable batteries, the Na+ removal electrode (cathode) has evolved from a carbon-based electrode to a faradaic electrode, and the desalination performance of CDI has also been significantly enhanced. A conventional carbon-based electrode captures ions in the electrical double layer (EDL) and suffers from low charge efficiency, thus being unsuitable for use in water with high salinity. On the other hand, a faradaic electrode stores Na+ ions through a reversible redox process or intercalation, leading to high desalination capacity.However, the Cl- removal electrode (anode) has not yet seen notable development. Most research groups employ activated carbon to remove Cl-, and therefore, summarizing Cl- storage electrodes for CDI is necessary to guide the design of electrode systems with better desalination performance. First, this review outlines the evolution of CDI configuration based on the electrode materials, suggesting that the anode and cathode are of equal importance in CDI. Second, a systematic summary of the anode materials used in CDI and a comparison of the characteristics of different electrodes, including those based on Ag/AgCl, Bi/BiOCl, 2-dimensional (2D) materials (layered double hydroxide (LDH) and MXene), redox polymers, and electrolytes, are presented. Then, the underlying mechanism for Cl- storage is refined. Similar to the case of Na+ storage, traditional carbon electrodes store Cl- via electrosorption based on the EDL. Ag/AgCl and Bi/BiOCl remove Cl- through a conversion reaction, i.e., phase transformation during the reaction with Cl-. 2D materials store Cl- in the space between adjacent layers, a process referred as ion intercalation, with layered double hydroxide (LDH) and MXene showing higher Cl- storage potential. Redox polymers and electrolytes allow for Cl- storage via redox reactions. Among all the materials mentioned above, Bi/BiOCl and LDH are the most promising for the construction of CDI anodes because of their high capacity and low cost. Finally, to spur the development of novel anodes for CDI, the electrodes applied in a chlorine ion battery are introduced. This is the first paper to comb through reports on the development of anode materials for CDI, thus laying the theoretical foundation for future materials design.
2022, 38(5): 200801
doi: 10.3866/PKU.WHXB202008018
Abstract:
Metal oxide semiconductor (MOS) gas sensors have been widely used in military and scientific research, as well as various industries; this is because of the unique advantages of MOS gas sensors including their small size, low power consumption, high sensitivity, and good silicon chip compatibility. However, the poor selectivity of MOS sensors has restricted their potential application in the Internet of Things (IoT) era. In this paper, progress in the research addressing the selectivity issues of MOS sensors is reviewed, and three strategies for selective MOS sensors, and performance improvements of MOS, e-nose, and thermal modulation, are introduced. Research on the performance improvements of MOS-sensitive materials provides an important guarantee for fast and accurate identification of trace gas molecules. The e-nose system adopts an array of sensors with distinct surface chemical properties; more "features" of volatile organic compound (VOC) molecules can be extracted by enlarging the number of sensor arrays, providing a "many-to-one" or "many-to-many" approach to discriminate VOC gas molecules via pattern recognition/machine learning algorithms. For thermal modulation technology, the working temperature of the sensor is intentionally swept during one measurement cycle, and the dynamic response signals of the sensor to different VOC gases under a given temperature mode are tested. Combined with signal processing and pattern recognition/machine learning, the "one-to-many" recognition of VOC gas molecules is realized by a single MOS sensor. Principal component analysis (PCA), linear discriminant analysis (LDA), and neural network (NN) pattern recognition/machine learning algorithms are compared in this review. Among them, the LDA algorithm based on supervised learning can be used as a signal dimension reduction or pattern recognition method. It is mainly applicable to the gas identification and classification of small datasets of VOC gas molecules. LDA is superior to PCA (based on unsupervised learning) in identifying and classifying VOC gas molecules. Compared with the LDA algorithm, an artificial neural network (ANN) based on the back-propagation algorithm, as a highly robust machine learning classification model, has the potential to process large datasets and realize the classification and identification of multiple kinds of VOC gases. Finally, the deep learning algorithm of convolutional neural networks (CNNs), with the performance of data dimension reduction, feature extraction, and robust identification, is expected to be applied in the field of VOC gas identification. Based on the performance improvement of MOS, a combination of multiple modulation methods and array technology, as well as the latest developments of deep learning algorithms in the artificial intelligence (AI) field, will greatly enhance the VOC molecular recognition capability of nonselective MOS sensors. ![]()
Metal oxide semiconductor (MOS) gas sensors have been widely used in military and scientific research, as well as various industries; this is because of the unique advantages of MOS gas sensors including their small size, low power consumption, high sensitivity, and good silicon chip compatibility. However, the poor selectivity of MOS sensors has restricted their potential application in the Internet of Things (IoT) era. In this paper, progress in the research addressing the selectivity issues of MOS sensors is reviewed, and three strategies for selective MOS sensors, and performance improvements of MOS, e-nose, and thermal modulation, are introduced. Research on the performance improvements of MOS-sensitive materials provides an important guarantee for fast and accurate identification of trace gas molecules. The e-nose system adopts an array of sensors with distinct surface chemical properties; more "features" of volatile organic compound (VOC) molecules can be extracted by enlarging the number of sensor arrays, providing a "many-to-one" or "many-to-many" approach to discriminate VOC gas molecules via pattern recognition/machine learning algorithms. For thermal modulation technology, the working temperature of the sensor is intentionally swept during one measurement cycle, and the dynamic response signals of the sensor to different VOC gases under a given temperature mode are tested. Combined with signal processing and pattern recognition/machine learning, the "one-to-many" recognition of VOC gas molecules is realized by a single MOS sensor. Principal component analysis (PCA), linear discriminant analysis (LDA), and neural network (NN) pattern recognition/machine learning algorithms are compared in this review. Among them, the LDA algorithm based on supervised learning can be used as a signal dimension reduction or pattern recognition method. It is mainly applicable to the gas identification and classification of small datasets of VOC gas molecules. LDA is superior to PCA (based on unsupervised learning) in identifying and classifying VOC gas molecules. Compared with the LDA algorithm, an artificial neural network (ANN) based on the back-propagation algorithm, as a highly robust machine learning classification model, has the potential to process large datasets and realize the classification and identification of multiple kinds of VOC gases. Finally, the deep learning algorithm of convolutional neural networks (CNNs), with the performance of data dimension reduction, feature extraction, and robust identification, is expected to be applied in the field of VOC gas identification. Based on the performance improvement of MOS, a combination of multiple modulation methods and array technology, as well as the latest developments of deep learning algorithms in the artificial intelligence (AI) field, will greatly enhance the VOC molecular recognition capability of nonselective MOS sensors.
