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2019 Volume 35 Issue 10
2019, 35(10): 1037-1038
doi: 10.3866/PKU.WHXB201904043
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2019, 35(10): 1039-1040
doi: 10.3866/PKU.WHXB201902032
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2019, 35(10): 1041-1042
doi: 10.3866/PKU.WHXB201905045
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2019, 35(10): 1043-1044
doi: 10.3866/PKU.WHXB201903011
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2019, 35(10): 1045-1046
doi: 10.3866/PKU.WHXB201905058
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2019, 35(10): 1047-1048
doi: 10.3866/PKU.WHXB201904076
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2019, 35(10): 1049-1051
doi: 10.3866/PKU.WHXB201905059
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2019, 35(10): 1052-1057
doi: 10.3866/PKU.WHXB201801262
Abstract:
Since the rediscovery of black phosphorus as a fascinating two-dimensional material, other two-dimensional materials comprising group VA elements have attracted tremendous interest, such as antimonene. Since 2015, besides intensive research efforts on the atomic structures, electronic properties and synthesis methods of antimonene, scientists have conducted applied researches on semiconductor and nonlinear optical devices, molecular adsorption and thermoelectric applications based on antimonene. In addition, antimonene quantum dots (SbQDs) as derivatives of antimonene, have also been studied recently, and their potential applications in photothermal therapy have been reported. To further explore the unique properties and potential applicationsof SbQDs, it is important tosynthesize large amounts of high-quality SbQDs. In this work, antimonene samples were prepared by sonication-assisted liquid exfoliation method. Antimony powders (200 mg) were dispersed in 200 mL water, C2H5OH and 1-methyl-2-pyrrolidone (NMP) solvents separately and sonicated for 10 h at a power of 180 W. Thereafter, the suspensions were centrifuged at 6000 r∙min-1 for 20 min, and the supernatant containing antimonene samples were decanted and characterized. The dispersion concentration of antimonene samples in the three solvents (water, C2H5OH and NMP) were measured as 0.57, 1.04, and 4.27 µg∙mL-1, respectively. However, the antimonene concentrations in water, C2H5OH and NMP dropped by 73.7%, 30.8% and 10.5%, respectively, after standing for 96 h. Thus, antimonene dispersed in NMP demonstrated the highest concentration and best stability, which indicates that NMP is more suitable for antimonene exfoliation. Furthermore, transmission electron microscopy (TEM) studies revealed that only the samples prepared in NMP were morphologically quantum dots, while antimonene samples obtained in the other two solvents were mainly nanosheets. The obtained SbQDs in NMP had a lateral size of approximately 3.0 nm. High-resolution transmission electron microscope (HRTEM) also confirmed the good crystal quality of theobtained SbQDs. In addition, we measured the turbidities of antimonene dispersed in those three solvents at various concentrations. As theoretically predicted, the turbidity of antimonne dispersions linearly depends on the concentraion; thus, the antimonene concentrations can be calculated by measuring the turbidity through an optical method. Thus, this study provides a high-throughput, nondestructive method for determining antimonene dispersion concentration, which will faciliate further research in this area. ![]()
Since the rediscovery of black phosphorus as a fascinating two-dimensional material, other two-dimensional materials comprising group VA elements have attracted tremendous interest, such as antimonene. Since 2015, besides intensive research efforts on the atomic structures, electronic properties and synthesis methods of antimonene, scientists have conducted applied researches on semiconductor and nonlinear optical devices, molecular adsorption and thermoelectric applications based on antimonene. In addition, antimonene quantum dots (SbQDs) as derivatives of antimonene, have also been studied recently, and their potential applications in photothermal therapy have been reported. To further explore the unique properties and potential applicationsof SbQDs, it is important tosynthesize large amounts of high-quality SbQDs. In this work, antimonene samples were prepared by sonication-assisted liquid exfoliation method. Antimony powders (200 mg) were dispersed in 200 mL water, C2H5OH and 1-methyl-2-pyrrolidone (NMP) solvents separately and sonicated for 10 h at a power of 180 W. Thereafter, the suspensions were centrifuged at 6000 r∙min-1 for 20 min, and the supernatant containing antimonene samples were decanted and characterized. The dispersion concentration of antimonene samples in the three solvents (water, C2H5OH and NMP) were measured as 0.57, 1.04, and 4.27 µg∙mL-1, respectively. However, the antimonene concentrations in water, C2H5OH and NMP dropped by 73.7%, 30.8% and 10.5%, respectively, after standing for 96 h. Thus, antimonene dispersed in NMP demonstrated the highest concentration and best stability, which indicates that NMP is more suitable for antimonene exfoliation. Furthermore, transmission electron microscopy (TEM) studies revealed that only the samples prepared in NMP were morphologically quantum dots, while antimonene samples obtained in the other two solvents were mainly nanosheets. The obtained SbQDs in NMP had a lateral size of approximately 3.0 nm. High-resolution transmission electron microscope (HRTEM) also confirmed the good crystal quality of theobtained SbQDs. In addition, we measured the turbidities of antimonene dispersed in those three solvents at various concentrations. As theoretically predicted, the turbidity of antimonne dispersions linearly depends on the concentraion; thus, the antimonene concentrations can be calculated by measuring the turbidity through an optical method. Thus, this study provides a high-throughput, nondestructive method for determining antimonene dispersion concentration, which will faciliate further research in this area.
2019, 35(10): 1058-1077
doi: 10.3866/PKU.WHXB201812020
Abstract:
Two-dimensional (2D) layered materials have garnered increasing interest in the past few years due to their unique structures and novel properties. These 2D layered materials with atomic thicknesses cover metals, semiconductors, and insulators, including graphene, black phosphorus (BP), transition metal dichalcogenides (TMDs) and hexagonal boron nitride (BN). Their bandgaps are usually tunable by changing the number of layers and the thicknesses. These 2D material are also sensitive to changes in the surrounding environment, e. g. changes in temperature, pressure, and illumination. Particularly, most 2D materials have high absorption coefficients. Owing to their excellent performance in electronics and optoelectronics and their potential for further development, many optoelectronic devices based on 2D materials, such as photodetectors, have been manufactured and widely used. In this paper, the latest progress of photodetectors based on 2D materials has been outlined. We introduce some 2D materials and their preparation methods, and the mechanisms of photodetectors based on 2D materials, i. e. photovoltaic effect, photoconductive effect, photogating effect, photothermoelectric effect and bolometric effect, have been discussed. Next, we summarize the parameters used to evaluate the performance of photodetectors, including photoresponsivity, external quantum efficiency, internal quantum efficiency, photoconductive gain, signal-to-noise ratio, noise-equivalent power, response time, cutoff frequency, linear dynamic range, and specific detectivity. We also report some recent studies on photodetectors based on 2D materials; among the 2D materials used in these studies, graphene, TMDs, and BP are the most widely used. Many methods have been proposed to improve the performances of photodetectors based on 2D materials, such as doping, designing novel structures, changing the dielectric layer, modifying the contact between channel and electrodes, controlling the surface and the interface, etc. Compared to single 2D materials, heterostructures composed of different 2D materials are more promising for use in photodetectors because they combine materials with different properties, which makes it possible to obtain photodetectors with desired and enhanced performances. Thus, we present some van der Waals heterojunctions and their applications in photodetectors. Finally, we provide a brief summary of the full article and an outlook for future development. ![]()
Two-dimensional (2D) layered materials have garnered increasing interest in the past few years due to their unique structures and novel properties. These 2D layered materials with atomic thicknesses cover metals, semiconductors, and insulators, including graphene, black phosphorus (BP), transition metal dichalcogenides (TMDs) and hexagonal boron nitride (BN). Their bandgaps are usually tunable by changing the number of layers and the thicknesses. These 2D material are also sensitive to changes in the surrounding environment, e. g. changes in temperature, pressure, and illumination. Particularly, most 2D materials have high absorption coefficients. Owing to their excellent performance in electronics and optoelectronics and their potential for further development, many optoelectronic devices based on 2D materials, such as photodetectors, have been manufactured and widely used. In this paper, the latest progress of photodetectors based on 2D materials has been outlined. We introduce some 2D materials and their preparation methods, and the mechanisms of photodetectors based on 2D materials, i. e. photovoltaic effect, photoconductive effect, photogating effect, photothermoelectric effect and bolometric effect, have been discussed. Next, we summarize the parameters used to evaluate the performance of photodetectors, including photoresponsivity, external quantum efficiency, internal quantum efficiency, photoconductive gain, signal-to-noise ratio, noise-equivalent power, response time, cutoff frequency, linear dynamic range, and specific detectivity. We also report some recent studies on photodetectors based on 2D materials; among the 2D materials used in these studies, graphene, TMDs, and BP are the most widely used. Many methods have been proposed to improve the performances of photodetectors based on 2D materials, such as doping, designing novel structures, changing the dielectric layer, modifying the contact between channel and electrodes, controlling the surface and the interface, etc. Compared to single 2D materials, heterostructures composed of different 2D materials are more promising for use in photodetectors because they combine materials with different properties, which makes it possible to obtain photodetectors with desired and enhanced performances. Thus, we present some van der Waals heterojunctions and their applications in photodetectors. Finally, we provide a brief summary of the full article and an outlook for future development.
2019, 35(10): 1078-1089
doi: 10.3866/PKU.WHXB201812057
Abstract:
With the rapid development of science and technology, various nanomaterials have continually emerged to meet human needs. As a newly emerging class of nanomaterials, two-dimensional (2D) materials have received wide attention recently in energy storage, catalysis, sensing and biomedicine due to their unique features such as good mechanical property, high specific surface area, excellent thermal and electrical conductivity. Biomacromolecules are the special organic molecules with various biological activities which exist extensively in every aspect of human life. When 2D materials meet biomacromolecules to display their own unique advantages, more opportunities and challenges have arisen for the exploitation and fabrication of novel nanomaterials with unique electrical, mechanical, biological properties and specific functions. In recent years, extensive research has been carried out with outstanding achievement thus the combination of 2D materials and biomacromolecules becomes a new hotspot. There were generally two binding interactions between 2D materials and biomacromolecules, namely non-covalent binding (electrostatic interaction, hydrophobic effect, π–π stacking, van der Waals interaction) and covalent binding (special chemical reactions between the functional groups of 2D materials and biomacromolecules). In addition, due to the excellent photothermal conversion performance, 2D materials could exhibit a non-contact interaction to biomacromolecules through the photo-thermal effect which has greatly broadened their applications. Up to now, numerous studies have clearly revealed the binding and effect mechanism and the research will be more focused on expanding the scope and application. Currently, the combination of 2D materials and biomacromolecules has widely involved in many cutting-edge applications such as flexible device, biosensor, smart skin, drug delivery, antibacterial, disease therapy and so on. Although a lot of progress has been made, several highlight open questions still need to be urgently addressed, such as the production cost of 2D materials, biological activity of biomacromolecules, stability and biocompatibility of 2D/biomacromolecule nanomaterials. This review summarizes the interactions between some typical 2D materials (i.e. graphene, graphene oxide, nitrogen-doped graphene, molybdenum disulfide, phosphorene, silylene and germanene) and biomacromolecules (i.e. silk protein, lysozyme, bovine serum albumin, bovine hemoglobin, ovalbumin, villin, bovine fibrinogen, DNA/RNA, glucose oxidase and chitosan) and focuses on the recent progress of some typical applications (i.e. engineering application, disease therapy and antibacterial). The non-covalent and covalent bindings of 2D materials and biomacromolecules are discussed in detail, and the applications of the combination of 2D materials and biomacromolecules in engineering and bioscience have been reviewed. Finally, the challenges for the future development of 2D materials and biomacromolecules are also briefly proposed. ![]()
With the rapid development of science and technology, various nanomaterials have continually emerged to meet human needs. As a newly emerging class of nanomaterials, two-dimensional (2D) materials have received wide attention recently in energy storage, catalysis, sensing and biomedicine due to their unique features such as good mechanical property, high specific surface area, excellent thermal and electrical conductivity. Biomacromolecules are the special organic molecules with various biological activities which exist extensively in every aspect of human life. When 2D materials meet biomacromolecules to display their own unique advantages, more opportunities and challenges have arisen for the exploitation and fabrication of novel nanomaterials with unique electrical, mechanical, biological properties and specific functions. In recent years, extensive research has been carried out with outstanding achievement thus the combination of 2D materials and biomacromolecules becomes a new hotspot. There were generally two binding interactions between 2D materials and biomacromolecules, namely non-covalent binding (electrostatic interaction, hydrophobic effect, π–π stacking, van der Waals interaction) and covalent binding (special chemical reactions between the functional groups of 2D materials and biomacromolecules). In addition, due to the excellent photothermal conversion performance, 2D materials could exhibit a non-contact interaction to biomacromolecules through the photo-thermal effect which has greatly broadened their applications. Up to now, numerous studies have clearly revealed the binding and effect mechanism and the research will be more focused on expanding the scope and application. Currently, the combination of 2D materials and biomacromolecules has widely involved in many cutting-edge applications such as flexible device, biosensor, smart skin, drug delivery, antibacterial, disease therapy and so on. Although a lot of progress has been made, several highlight open questions still need to be urgently addressed, such as the production cost of 2D materials, biological activity of biomacromolecules, stability and biocompatibility of 2D/biomacromolecule nanomaterials. This review summarizes the interactions between some typical 2D materials (i.e. graphene, graphene oxide, nitrogen-doped graphene, molybdenum disulfide, phosphorene, silylene and germanene) and biomacromolecules (i.e. silk protein, lysozyme, bovine serum albumin, bovine hemoglobin, ovalbumin, villin, bovine fibrinogen, DNA/RNA, glucose oxidase and chitosan) and focuses on the recent progress of some typical applications (i.e. engineering application, disease therapy and antibacterial). The non-covalent and covalent bindings of 2D materials and biomacromolecules are discussed in detail, and the applications of the combination of 2D materials and biomacromolecules in engineering and bioscience have been reviewed. Finally, the challenges for the future development of 2D materials and biomacromolecules are also briefly proposed.
2019, 35(10): 1090-1098
doi: 10.3866/PKU.WHXB201810059
Abstract:
Two-dimensional (2D) materials, led by graphene, have emerged as nano-building blocks to develop high-performance membranes. The atom-level thickness of nanosheets makes a membrane as thin as possible, thereby minimizing the transport resistance and maximizing the permeation flux. Meanwhile, the sieving channels can be precisely manipulated within sub-nanometer size for molecular separation, such as gas separation. For instance, graphene oxide (GO) channels with an interlayer height of about 0.4 nm assembled by external forces exhibited excellent H2/CO2 sieving performance compared to commercial membranes. Cross-linking was also employed to fabricate ultrathin (< 20 nm) GO-facilitated transport membranes for efficient CO2 capture. A borate-crosslinked membrane exhibited a high CO2 permeance of 650 GPU (gas permeation unit), and a CO2/CH4 selectivity of 75, which is currently the best performance reported for GO-based composite membranes. The CO2-facilitated transport membrane with piperazine as the carrier also exhibited excellent separation performance under simulated flue gas conditions with CO2 permeance of 1020 GPU and CO2/N2 selectivity as high as 680. In addition, metal-organic frameworks (MOFs) with layered structures, if successfully exfoliated, can serve as diverse sources for MOF nanosheets that can be fabricated into high-performance membranes. It is challenging to maintain the structural and morphological integrity of nanosheets. Poly[Zn2(benzimidazole)4] (Zn2(bim)4) was firstly exfoliated into 1-nm-thick nanosheets and assembled into ultrathin membranes possessing both high permeance and excellent molecular sieving properties for H2/CO2 separation. Interestingly, reversed thermo-switchable molecular sieving was also demonstrated in membranes composed of 2D MOF nanosheets. Besides, researchers employed layered double hydroxides (LDHs) to prepare molecular-sieving membranes via in situ growth, and the as-prepared membranes showed a remarkable selectivity of ~80 for H2-CH4 mixture. They concluded that the amount of CO2 in the precursor solution contributed to LDH membranes with various preferred orientations and thicknesses. Apart from these 2D materials, MXenes also show great potential in selective gas permeation. Lamellar stacked MXene membranes with aligned and regular sub-nanometer channels exhibited excellent gas separation performance. Moreover, our ultrathin (20 nm) MXene nanofilms showed outstanding molecular sieving property for the preferential transport of H2, with H2 permeance as high as 1584 GPU and H2/CO2 selectivity of 27. The originally H2-selective MXene membranes could be transformed into membranes selectively permeating CO2 by chemical tuning of the MXene nanochannels. This paper briefly reviews the latest groundbreaking studies in 2D-material membranes for gas separation, with a focus on sub-nanometer 2D channels, exfoliation of 2D nanosheets with structural integrity, and tunable gas transport property. Challenges, in terms of the mass production of 2D nanosheets, scale-up of lab-level membranes and a thorough understanding of the transport mechanism, and the potential of 2D-material membranes for wide implementation are briefly discussed. ![]()
Two-dimensional (2D) materials, led by graphene, have emerged as nano-building blocks to develop high-performance membranes. The atom-level thickness of nanosheets makes a membrane as thin as possible, thereby minimizing the transport resistance and maximizing the permeation flux. Meanwhile, the sieving channels can be precisely manipulated within sub-nanometer size for molecular separation, such as gas separation. For instance, graphene oxide (GO) channels with an interlayer height of about 0.4 nm assembled by external forces exhibited excellent H2/CO2 sieving performance compared to commercial membranes. Cross-linking was also employed to fabricate ultrathin (< 20 nm) GO-facilitated transport membranes for efficient CO2 capture. A borate-crosslinked membrane exhibited a high CO2 permeance of 650 GPU (gas permeation unit), and a CO2/CH4 selectivity of 75, which is currently the best performance reported for GO-based composite membranes. The CO2-facilitated transport membrane with piperazine as the carrier also exhibited excellent separation performance under simulated flue gas conditions with CO2 permeance of 1020 GPU and CO2/N2 selectivity as high as 680. In addition, metal-organic frameworks (MOFs) with layered structures, if successfully exfoliated, can serve as diverse sources for MOF nanosheets that can be fabricated into high-performance membranes. It is challenging to maintain the structural and morphological integrity of nanosheets. Poly[Zn2(benzimidazole)4] (Zn2(bim)4) was firstly exfoliated into 1-nm-thick nanosheets and assembled into ultrathin membranes possessing both high permeance and excellent molecular sieving properties for H2/CO2 separation. Interestingly, reversed thermo-switchable molecular sieving was also demonstrated in membranes composed of 2D MOF nanosheets. Besides, researchers employed layered double hydroxides (LDHs) to prepare molecular-sieving membranes via in situ growth, and the as-prepared membranes showed a remarkable selectivity of ~80 for H2-CH4 mixture. They concluded that the amount of CO2 in the precursor solution contributed to LDH membranes with various preferred orientations and thicknesses. Apart from these 2D materials, MXenes also show great potential in selective gas permeation. Lamellar stacked MXene membranes with aligned and regular sub-nanometer channels exhibited excellent gas separation performance. Moreover, our ultrathin (20 nm) MXene nanofilms showed outstanding molecular sieving property for the preferential transport of H2, with H2 permeance as high as 1584 GPU and H2/CO2 selectivity of 27. The originally H2-selective MXene membranes could be transformed into membranes selectively permeating CO2 by chemical tuning of the MXene nanochannels. This paper briefly reviews the latest groundbreaking studies in 2D-material membranes for gas separation, with a focus on sub-nanometer 2D channels, exfoliation of 2D nanosheets with structural integrity, and tunable gas transport property. Challenges, in terms of the mass production of 2D nanosheets, scale-up of lab-level membranes and a thorough understanding of the transport mechanism, and the potential of 2D-material membranes for wide implementation are briefly discussed.
2019, 35(10): 1099-1111
doi: 10.3866/PKU.WHXB201811005
Abstract:
The research in two-dimensional (2D) materials, such as graphene, transition metal dichalcogenides (TMDs) and black phosphorus, has been further flourished with the recent emergence of heterostructures composed of dissimilar 2D materials. The interfacing/coupling between different constituent components in a heterostructure has given rise to interesting phenomena and useful properties. For example, depending on the type of 2D materials, the distance and the kind of bonding between them, as well as the crystalline property of the hetero-interface, the interface may provide charge traps, exciton recombination centers, or bridges for effective charge/energy transfer. It has also been found that the spatial arrangement in addition to the composition of the constituents is an important factor influencing the overall properties of the heterostructures. Although many methods, such as dry transfer and vapor-phased growth are able to yield heterostructures from pristine or highly crystalline 2D crystals with spatial control, such as vertical heterostructures and lateral heterostructures, these methods are generally not scalable, which has restricted the use of the obtained heterostructures mostly to fundamental studies. The solution-phased synthesis methods, such as solvothermal/hydrothermal synthesis, electrochemical deposition and hot-injection method, may be more suitable for mass production of functional heterostructures despite the relatively low product quality. In the past couple of years, a diverse kinds of hetero/hybrid structures of 2D materials have been prepared successfully in wet-chemical processes. However, precise control over the geometric arrangement of the constituent components has been challenging in solution. Currently, four types of heterostructures including 2D crystals grown on a larger 2D template, vertical heterostructures, lateral heterostructures, and core-shell heterostructures have been prepared in solution. For the first type, flexible 2D nanosheets such as graphene and monolayer TMDs are used as synthesis templates to support the nucleation and growth of other 2D crystals. For vertical heterostructures, relatively rigid nanoplates are used to allow continuous deposition of 2D layers of other materials to form sandwich-like structures. The formation of lateral heterostructures requires edge growth on existing 2D materials without basal deposition, and therefore other methods such as cation exchange can be used as alternative routes. The preparation of core-shell 2D heterostructures generally involves both epitaxial edge growth and basal deposition and has been realized in both metallic and semiconductor structures. In this review, these kinds of heterostructures based on 2D materials will be discussed in terms of their synthesis methods, properties and possible applications. In addition, we will discuss the challenges and possible opportunities in this research direction. ![]()
The research in two-dimensional (2D) materials, such as graphene, transition metal dichalcogenides (TMDs) and black phosphorus, has been further flourished with the recent emergence of heterostructures composed of dissimilar 2D materials. The interfacing/coupling between different constituent components in a heterostructure has given rise to interesting phenomena and useful properties. For example, depending on the type of 2D materials, the distance and the kind of bonding between them, as well as the crystalline property of the hetero-interface, the interface may provide charge traps, exciton recombination centers, or bridges for effective charge/energy transfer. It has also been found that the spatial arrangement in addition to the composition of the constituents is an important factor influencing the overall properties of the heterostructures. Although many methods, such as dry transfer and vapor-phased growth are able to yield heterostructures from pristine or highly crystalline 2D crystals with spatial control, such as vertical heterostructures and lateral heterostructures, these methods are generally not scalable, which has restricted the use of the obtained heterostructures mostly to fundamental studies. The solution-phased synthesis methods, such as solvothermal/hydrothermal synthesis, electrochemical deposition and hot-injection method, may be more suitable for mass production of functional heterostructures despite the relatively low product quality. In the past couple of years, a diverse kinds of hetero/hybrid structures of 2D materials have been prepared successfully in wet-chemical processes. However, precise control over the geometric arrangement of the constituent components has been challenging in solution. Currently, four types of heterostructures including 2D crystals grown on a larger 2D template, vertical heterostructures, lateral heterostructures, and core-shell heterostructures have been prepared in solution. For the first type, flexible 2D nanosheets such as graphene and monolayer TMDs are used as synthesis templates to support the nucleation and growth of other 2D crystals. For vertical heterostructures, relatively rigid nanoplates are used to allow continuous deposition of 2D layers of other materials to form sandwich-like structures. The formation of lateral heterostructures requires edge growth on existing 2D materials without basal deposition, and therefore other methods such as cation exchange can be used as alternative routes. The preparation of core-shell 2D heterostructures generally involves both epitaxial edge growth and basal deposition and has been realized in both metallic and semiconductor structures. In this review, these kinds of heterostructures based on 2D materials will be discussed in terms of their synthesis methods, properties and possible applications. In addition, we will discuss the challenges and possible opportunities in this research direction.
2019, 35(10): 1112-1118
doi: 10.3866/PKU.WHXB201805032
Abstract:
As a new 2D material with excellent chemical stability, good electric conductivity, and high specific surface area, graphene has been widely used in energy storage and conversion devices. However, 2D graphene layers are easily stacked, which may significantly reduce the surface area and degrade the excellent electrical properties of graphene. To avoid this, one of the most effective methods is to construct 3D graphene (3DG) with specific porous microstructures. Chemical vapor deposition (CVD) is an important method for the synthesis of high-quality 3DG, where templates play a defining role in controlling the structure and cost of 3DG. Metallic materials with 3D microstructures, such as nickel foam, have proven to be useful as substrates for the growth of high-quality 3DG. However, metal substrates are usually expensive, and the pickling solution generated after etching may cause environmental problems. Therefore, non-metallic substrate materials with lower costs have been investigated for the preparation of 3DG. Herein, we developed a novel template material, mammal bone ashes, for the CVD preparation of 3DG. Mammal bone ash is an inexpensive and abundant biomass hydroxyapatite. During the high-temperature CVD reaction, the bone ash powders were slightly sintered to form a continuous porous structure with graphene coating. The morphology of 3DG is inherited from the microstructure of bone ash templates. After removing the bone ash template with hydrochloric acid, the template-grown 3DG was obtained with a unique bicontinuous structure, i.e. both the graphene framework and the void space were continuous. In addition, the pickling solution of the bone ash templates after etching was exactly the same as that for the raw materials for the production of phosphoric acid to achieve high atom utilization. We further optimized the graphitization degrees, layer number, and porous morphology of 3DGs. The microstructure evolution of 3DG is highly relevant to the layer thickness and uniformity of graphene layers. A short growth time would lead to a non-uniform and thin layer of graphene, which is not able to support a complex 3D porous structure. In contrast, a uniform graphene layer with proper thickness is capable of forming a robust 3D architecture. In addition, the facile CVD method can be extended to a series of metal phosphate templates, including tricalcium phosphate [Ca3(PO4)2], trimagnesium phosphate [Mg3(PO4)2], and aluminum phosphate [AlPO4]. 3DG with bicontinuous morphology is promising as a conductive frame material in electrochemical energy storage devices. As an illustration, high-performance Li-S batteries were fabricated by the uniform composition of an S cathode on 3DG. In comparison with heavily stacked 2D graphene sheets in reduced graphene oxide / S composite, the non-flat structure of 3DGs remained unchanged even after the harsh melt-diffusion process of high-viscosity liquid sulfur. The resulting 3DG/S cathode delivered a high specific capacity of ~550 mAh∙g-1 at a high current rate (2C). Our work opens an avenue to the low-cost and high-utility production of 3D graphene, which could be integrated with the well-developed phosphorus chemical industry. ![]()
As a new 2D material with excellent chemical stability, good electric conductivity, and high specific surface area, graphene has been widely used in energy storage and conversion devices. However, 2D graphene layers are easily stacked, which may significantly reduce the surface area and degrade the excellent electrical properties of graphene. To avoid this, one of the most effective methods is to construct 3D graphene (3DG) with specific porous microstructures. Chemical vapor deposition (CVD) is an important method for the synthesis of high-quality 3DG, where templates play a defining role in controlling the structure and cost of 3DG. Metallic materials with 3D microstructures, such as nickel foam, have proven to be useful as substrates for the growth of high-quality 3DG. However, metal substrates are usually expensive, and the pickling solution generated after etching may cause environmental problems. Therefore, non-metallic substrate materials with lower costs have been investigated for the preparation of 3DG. Herein, we developed a novel template material, mammal bone ashes, for the CVD preparation of 3DG. Mammal bone ash is an inexpensive and abundant biomass hydroxyapatite. During the high-temperature CVD reaction, the bone ash powders were slightly sintered to form a continuous porous structure with graphene coating. The morphology of 3DG is inherited from the microstructure of bone ash templates. After removing the bone ash template with hydrochloric acid, the template-grown 3DG was obtained with a unique bicontinuous structure, i.e. both the graphene framework and the void space were continuous. In addition, the pickling solution of the bone ash templates after etching was exactly the same as that for the raw materials for the production of phosphoric acid to achieve high atom utilization. We further optimized the graphitization degrees, layer number, and porous morphology of 3DGs. The microstructure evolution of 3DG is highly relevant to the layer thickness and uniformity of graphene layers. A short growth time would lead to a non-uniform and thin layer of graphene, which is not able to support a complex 3D porous structure. In contrast, a uniform graphene layer with proper thickness is capable of forming a robust 3D architecture. In addition, the facile CVD method can be extended to a series of metal phosphate templates, including tricalcium phosphate [Ca3(PO4)2], trimagnesium phosphate [Mg3(PO4)2], and aluminum phosphate [AlPO4]. 3DG with bicontinuous morphology is promising as a conductive frame material in electrochemical energy storage devices. As an illustration, high-performance Li-S batteries were fabricated by the uniform composition of an S cathode on 3DG. In comparison with heavily stacked 2D graphene sheets in reduced graphene oxide / S composite, the non-flat structure of 3DGs remained unchanged even after the harsh melt-diffusion process of high-viscosity liquid sulfur. The resulting 3DG/S cathode delivered a high specific capacity of ~550 mAh∙g-1 at a high current rate (2C). Our work opens an avenue to the low-cost and high-utility production of 3D graphene, which could be integrated with the well-developed phosphorus chemical industry.
2019, 35(10): 1119-1127
doi: 10.3866/PKU.WHXB201812023
Abstract:
Controllable synthesis of MoS2 with desired number of layers via chemical vapor deposition (CVD) remains challenging. Hence, it is highly desirable to develop a theoretical model that can be used to predict the single- and multilayer growth of MoS2 quantitatively, and provide guidelines for experimental fabrication. Herein we have established a kinetic Monte Carlo (kMC) model to predict the CVD growth of mono- and bilayer MoS2. First, we proposed that the growth rates of layer 1 and layer 2 were governed by the distribution of the adatom concentration, and the growth kinetics of compact triangular MoS2 followed the kink nucleation-propagation mechanism. The adatom concentration was formulated in terms of adatom flux, effective lifetime of adatoms, growth temperature, binding energies, edge energies, and nucleation criterion. The kink nucleation and propagation were determined by energy barriers of the adatom attachments to the zigzag and armchair edges. We then employed an analytic thermodynamic criterion to extract these parameters. Using the calibrated model, we found that the growth rate of layer 2 strongly depended on the size of layer 1 and decreased monotonically with increasing size of layer 1, and might even become prohibited at the maximum size of layer 1. Furthermore, we analyzed the size and morphology evolutions of bilayer MoS2 at different growth temperatures and adatom fluxes. Throughout the growth processes of bilayer MoS2, the morphologies of layers 1 and 2 maintained triangular shapes with compact edges, consistent with the kink nucleation-propagation growth mechanism. Our simulations revealed that the growth of bilayer MoS2 was promoted by increasing the growth temperature or decreasing the adatom flux, which corroborated the experimental observations. The increase in growth temperature led to reduced adatom concentration at the edge of layer 2 in accordance with the adatom concentration far from the edge of layer 2, resulting in a consistent difference in the adatom concentration to promote the growth of bilayer MoS2. Similarly, the decrease in adatom flux lowered the difference between the adatom concentrations far from the edge and at the edge of layer 1, decelerating the growth of layer 1. The decelerated growth of layer 1 reduced the difference between the adatom concentrations far from the edge and at the edge of layer 2 to zero, permitting the growth of bilayer MoS2. To guide the experimental synthesis, we constructed a phase diagram to delineate the permitted or prohibited growth of bilayer MoS2 at different growth temperatures and adatom fluxes. Hence, this work not only unveils the conditions for the growth of mono- and bi-layer MoS2, but also provides guidelines for controllable synthesis of MoS2 with the desired number of layers. ![]()
Controllable synthesis of MoS2 with desired number of layers via chemical vapor deposition (CVD) remains challenging. Hence, it is highly desirable to develop a theoretical model that can be used to predict the single- and multilayer growth of MoS2 quantitatively, and provide guidelines for experimental fabrication. Herein we have established a kinetic Monte Carlo (kMC) model to predict the CVD growth of mono- and bilayer MoS2. First, we proposed that the growth rates of layer 1 and layer 2 were governed by the distribution of the adatom concentration, and the growth kinetics of compact triangular MoS2 followed the kink nucleation-propagation mechanism. The adatom concentration was formulated in terms of adatom flux, effective lifetime of adatoms, growth temperature, binding energies, edge energies, and nucleation criterion. The kink nucleation and propagation were determined by energy barriers of the adatom attachments to the zigzag and armchair edges. We then employed an analytic thermodynamic criterion to extract these parameters. Using the calibrated model, we found that the growth rate of layer 2 strongly depended on the size of layer 1 and decreased monotonically with increasing size of layer 1, and might even become prohibited at the maximum size of layer 1. Furthermore, we analyzed the size and morphology evolutions of bilayer MoS2 at different growth temperatures and adatom fluxes. Throughout the growth processes of bilayer MoS2, the morphologies of layers 1 and 2 maintained triangular shapes with compact edges, consistent with the kink nucleation-propagation growth mechanism. Our simulations revealed that the growth of bilayer MoS2 was promoted by increasing the growth temperature or decreasing the adatom flux, which corroborated the experimental observations. The increase in growth temperature led to reduced adatom concentration at the edge of layer 2 in accordance with the adatom concentration far from the edge of layer 2, resulting in a consistent difference in the adatom concentration to promote the growth of bilayer MoS2. Similarly, the decrease in adatom flux lowered the difference between the adatom concentrations far from the edge and at the edge of layer 1, decelerating the growth of layer 1. The decelerated growth of layer 1 reduced the difference between the adatom concentrations far from the edge and at the edge of layer 2 to zero, permitting the growth of bilayer MoS2. To guide the experimental synthesis, we constructed a phase diagram to delineate the permitted or prohibited growth of bilayer MoS2 at different growth temperatures and adatom fluxes. Hence, this work not only unveils the conditions for the growth of mono- and bi-layer MoS2, but also provides guidelines for controllable synthesis of MoS2 with the desired number of layers.
2019, 35(10): 1128-1133
doi: 10.3866/PKU.WHXB201812037
Abstract:
Recently, ferroelectric materials have attracted considerable research attention. In particular, two dimensional (2D) ferroelectric materials have been considered as most crucial for next-generation circuit designs because of their application as novel electric memory devices. However, a 2D ferroelectric material is very rare. The ferroelectric materials with the form ABP2X6 (A = Ag, Cu; B = Bi, In; X = S, Se) are of interest because of their ferroelectric property maintained in their ultrathin structures. Within the ABP2X6 monolayer, the P―P bonds form the pillars that hold the top and bottom X planes, while the off-center A―B atoms between the X layers induce a spontaneous ferroelectric polarization. If the two off-center A―B sites are equally aligned, this would lead to the appearance of the paraelectric state. Such intriguing structures must impart novel mechanical properties to the materials. Until now, there has been no report on the mechanical properties of monolayer ABP2X6. Based on first-principles calculations, we studied the structural, electronic, mechanical as well as the electromechanical coupling properties of monolayer ABP2X6 (A = Ag, Cu; B = Bi, In; X = S, Se). We found that they are all semiconductors with wide bandgaps of 2.73, 2.17, 3.00, and 2.31 eV for CuInP2Se6, CuBiP2Se6, AgBiP2S6, and AgBiP2Se6, respectively, which are calculated based on the Heyd-Scuseria-Ernzerhof (HSE) exchange correlation functional model. The conduction band minimum is mainly from p orbitals of X and B atoms, whereas the valence band maximum is due to the hybridization of the p orbital of X atoms and the d orbital of A atoms. Moreover, there are three short and three long A/B―X bonds due to the A―B off-center displacement. Together with the d-p orbital hybridization, the main reason for the distorted ferroelectric structure in ABP2X6 monolayers is the Jahn-Teller effect. ABP2X6 monolayers are predicted to be a new class of auxetic materials with an out-of-plane negative Poisson's ratio, i.e., the values of the negative Poisson's ratio are in the order AgBiP2S6 (−0.805) < AgBiP2Se6 (−0.778) < CuBiP2Se6 (−0.670) < CuInP2S6 (−0.060). This is mainly due to the tensile strain applied in the x/y direction enlarging the angle between P―P bonds and top layer X atoms, thereby enhancing the bucking height of monolayer ABP2X6. Moreover, external strain has a significant impact on the A―B off-center displacement, rendering an out-of-plane piezoelectric polarization. The values of e13 for CuInP2S6, CuBiP2Se6, AgBiP2S6
Recently, ferroelectric materials have attracted considerable research attention. In particular, two dimensional (2D) ferroelectric materials have been considered as most crucial for next-generation circuit designs because of their application as novel electric memory devices. However, a 2D ferroelectric material is very rare. The ferroelectric materials with the form ABP2X6 (A = Ag, Cu; B = Bi, In; X = S, Se) are of interest because of their ferroelectric property maintained in their ultrathin structures. Within the ABP2X6 monolayer, the P―P bonds form the pillars that hold the top and bottom X planes, while the off-center A―B atoms between the X layers induce a spontaneous ferroelectric polarization. If the two off-center A―B sites are equally aligned, this would lead to the appearance of the paraelectric state. Such intriguing structures must impart novel mechanical properties to the materials. Until now, there has been no report on the mechanical properties of monolayer ABP2X6. Based on first-principles calculations, we studied the structural, electronic, mechanical as well as the electromechanical coupling properties of monolayer ABP2X6 (A = Ag, Cu; B = Bi, In; X = S, Se). We found that they are all semiconductors with wide bandgaps of 2.73, 2.17, 3.00, and 2.31 eV for CuInP2Se6, CuBiP2Se6, AgBiP2S6, and AgBiP2Se6, respectively, which are calculated based on the Heyd-Scuseria-Ernzerhof (HSE) exchange correlation functional model. The conduction band minimum is mainly from p orbitals of X and B atoms, whereas the valence band maximum is due to the hybridization of the p orbital of X atoms and the d orbital of A atoms. Moreover, there are three short and three long A/B―X bonds due to the A―B off-center displacement. Together with the d-p orbital hybridization, the main reason for the distorted ferroelectric structure in ABP2X6 monolayers is the Jahn-Teller effect. ABP2X6 monolayers are predicted to be a new class of auxetic materials with an out-of-plane negative Poisson's ratio, i.e., the values of the negative Poisson's ratio are in the order AgBiP2S6 (−0.805) < AgBiP2Se6 (−0.778) < CuBiP2Se6 (−0.670) < CuInP2S6 (−0.060). This is mainly due to the tensile strain applied in the x/y direction enlarging the angle between P―P bonds and top layer X atoms, thereby enhancing the bucking height of monolayer ABP2X6. Moreover, external strain has a significant impact on the A―B off-center displacement, rendering an out-of-plane piezoelectric polarization. The values of e13 for CuInP2S6, CuBiP2Se6, AgBiP2S6
2019, 35(10): 1134-1141
doi: 10.3866/PKU.WHXB201809013
Abstract:
Two-dimensional transition metal disulfides (TMDs) have recently attracted significant research attention due to their rich physical and chemical properties. Graphene has also been studied intensively due to its high electron mobility of ~200000 cm2·V−1·s−1. Since there is no band gap, it is difficult for a graphene-based device to achieve high current on/off ratio. For TMDs, such as MoS2, MoSe2, WSe2, and WS2, the band gaps of these materials can be adjusted according to the number of layers. Since TMD has the advantage of suppressing source-drain tunneling current in an ultra-short transistor and offering superior immunity to short-channel effects, it is also attractive for use as a channel material in Si complementary metal oxide semiconductor (CMOS) devices larger than 22 nm. Among them, MoS2 in single-layer and multi-layer films have been intensively researched for many years. MoS2-based field effect transistors (FETs) with excellent electrical properties have been reported. WS2 has lower in-plane electronic mass than MoS2, MoSe2, and MoTe2, and therefore has potential for higher carrier mobility or higher output current for WS2-based FETs. Experimental research on WS2 is limited compared to MoS2, and more work is needed to further exploit the full potential of WS2-based FETs. Therefore, the electron-phonon interaction and vibration properties of WS2 used in nano-electronic applications and FETs must be investigated. To this end, mono-layer (1L), few-layer (FL), and bulk WS2 films were prepared using mechanical exfoliation from a WS2 crystal. 3M scotch-tape was used for transferring the WS2 films. Detailed temperature-dependent Raman study on 1L, FL, and bulk WS2 films has been conducted using a 514-nm excitation laser. Raman spectroscopy, as an effective and non-destructive approach for phonon vibration study, has been used to evaluate TMDs. The Raman spectra reveal much useful information on the test sample in terms of peak position and spectral shape change. With the film thickness increasing to bulk, the A1g(Γ) and E2g1(Γ) modes show blue-shift and red-shift, respectively, with respect to 1L WS2. Moreover, when the dominant Raman vibration modes swaps between E2g1(Γ) and A1g(Γ), the "cross-over" temperature was identified for 1L, FL, and bulk WS2 films. WS2 shows smaller frequency change Δ between the E2g1(Γ) and A1g(Γ) modes than MoS2, with varying film thickness. The temperature coefficient of the Raman peak position was one magnitude lower for WS2 than MoS2, implying that WS2 has better thermal stability than MoS2. The results of this systematic study provide a physical guidance for WS2-based device design.
Two-dimensional transition metal disulfides (TMDs) have recently attracted significant research attention due to their rich physical and chemical properties. Graphene has also been studied intensively due to its high electron mobility of ~200000 cm2·V−1·s−1. Since there is no band gap, it is difficult for a graphene-based device to achieve high current on/off ratio. For TMDs, such as MoS2, MoSe2, WSe2, and WS2, the band gaps of these materials can be adjusted according to the number of layers. Since TMD has the advantage of suppressing source-drain tunneling current in an ultra-short transistor and offering superior immunity to short-channel effects, it is also attractive for use as a channel material in Si complementary metal oxide semiconductor (CMOS) devices larger than 22 nm. Among them, MoS2 in single-layer and multi-layer films have been intensively researched for many years. MoS2-based field effect transistors (FETs) with excellent electrical properties have been reported. WS2 has lower in-plane electronic mass than MoS2, MoSe2, and MoTe2, and therefore has potential for higher carrier mobility or higher output current for WS2-based FETs. Experimental research on WS2 is limited compared to MoS2, and more work is needed to further exploit the full potential of WS2-based FETs. Therefore, the electron-phonon interaction and vibration properties of WS2 used in nano-electronic applications and FETs must be investigated. To this end, mono-layer (1L), few-layer (FL), and bulk WS2 films were prepared using mechanical exfoliation from a WS2 crystal. 3M scotch-tape was used for transferring the WS2 films. Detailed temperature-dependent Raman study on 1L, FL, and bulk WS2 films has been conducted using a 514-nm excitation laser. Raman spectroscopy, as an effective and non-destructive approach for phonon vibration study, has been used to evaluate TMDs. The Raman spectra reveal much useful information on the test sample in terms of peak position and spectral shape change. With the film thickness increasing to bulk, the A1g(Γ) and E2g1(Γ) modes show blue-shift and red-shift, respectively, with respect to 1L WS2. Moreover, when the dominant Raman vibration modes swaps between E2g1(Γ) and A1g(Γ), the "cross-over" temperature was identified for 1L, FL, and bulk WS2 films. WS2 shows smaller frequency change Δ between the E2g1(Γ) and A1g(Γ) modes than MoS2, with varying film thickness. The temperature coefficient of the Raman peak position was one magnitude lower for WS2 than MoS2, implying that WS2 has better thermal stability than MoS2. The results of this systematic study provide a physical guidance for WS2-based device design.
2019, 35(10): 1142-1149
doi: 10.3866/PKU.WHXB201810040
Abstract:
Graphene is one of the most promising materials in nanotechnology and has attracted worldwide attention and research interest owing to its high electrical conductivity, good thermal stability, and excellent mechanical strength. Perfect graphene samples exhibit outstanding electrical and mechanical properties. However, point defects are commonly observed during fabrication which deteriorate the performance of graphene based-devices. The transport properties of graphene with point defects essentially depend on the imperfection of the hexagonal carbon atom network and the scattering of carriers by localized states. Furthermore, an in-depth understanding of the effect of specific point defects on the electronic and transport properties of graphene is crucial for specific applications. In this work, we employed density functional theory calculations and the non-equilibrium Green's function method to systematically elucidate the effects of various point defects on the electrical transport properties of graphene, including Stone-Waals and inverse Stone-Waals defects; and single and double vacancies. The electrical conductance highly depends on the type and concentration of point defects in graphene. Low concentrations of Stone-Waals, inverse Stone-Waals, and single-vacancy defects do not noticeably degrade electron transport. In comparison, DV585 induces a moderate reduction of 25%–34%, and DV55577 and DV5555-6-7777 induce significant suppression of 51%–62% in graphene. As the defect concentration increases, the electrical conductance reduces by a factor of 2–3 compared to the case of graphene monolayers with a low concentration of point defects. These distinct electrical transport behaviors are attributed to the variation of the graphene band structure; the point defects induce localized states near the Fermi level and result in energy splitting at the Dirac point due to the breaking of the intrinsic symmetry of the graphene honeycomb lattice. Double vacancies with larger defect concentrations exhibit more flat bands near the Fermi energy and more localized states in the defective region, resulting in the presence of resonant peaks close to the Fermi energy in the local density of states. This may cause resonant scattering of the carriers and a corresponding reduction of the conductance of graphene. Moreover, the partial charge densities for double vacancies and point defects with larger concentrations exhibit enhanced localization in the defective region that hinder the charge carriers. The electrical conductance shows an exponential decay as the defect concentration and energy splitting increase. These theoretical results provide important insights into the electrical transport properties of realistic graphene monolayers and will assist in the fabrication of high-performance graphene-based devices.
Graphene is one of the most promising materials in nanotechnology and has attracted worldwide attention and research interest owing to its high electrical conductivity, good thermal stability, and excellent mechanical strength. Perfect graphene samples exhibit outstanding electrical and mechanical properties. However, point defects are commonly observed during fabrication which deteriorate the performance of graphene based-devices. The transport properties of graphene with point defects essentially depend on the imperfection of the hexagonal carbon atom network and the scattering of carriers by localized states. Furthermore, an in-depth understanding of the effect of specific point defects on the electronic and transport properties of graphene is crucial for specific applications. In this work, we employed density functional theory calculations and the non-equilibrium Green's function method to systematically elucidate the effects of various point defects on the electrical transport properties of graphene, including Stone-Waals and inverse Stone-Waals defects; and single and double vacancies. The electrical conductance highly depends on the type and concentration of point defects in graphene. Low concentrations of Stone-Waals, inverse Stone-Waals, and single-vacancy defects do not noticeably degrade electron transport. In comparison, DV585 induces a moderate reduction of 25%–34%, and DV55577 and DV5555-6-7777 induce significant suppression of 51%–62% in graphene. As the defect concentration increases, the electrical conductance reduces by a factor of 2–3 compared to the case of graphene monolayers with a low concentration of point defects. These distinct electrical transport behaviors are attributed to the variation of the graphene band structure; the point defects induce localized states near the Fermi level and result in energy splitting at the Dirac point due to the breaking of the intrinsic symmetry of the graphene honeycomb lattice. Double vacancies with larger defect concentrations exhibit more flat bands near the Fermi energy and more localized states in the defective region, resulting in the presence of resonant peaks close to the Fermi energy in the local density of states. This may cause resonant scattering of the carriers and a corresponding reduction of the conductance of graphene. Moreover, the partial charge densities for double vacancies and point defects with larger concentrations exhibit enhanced localization in the defective region that hinder the charge carriers. The electrical conductance shows an exponential decay as the defect concentration and energy splitting increase. These theoretical results provide important insights into the electrical transport properties of realistic graphene monolayers and will assist in the fabrication of high-performance graphene-based devices.
2019, 35(10): 1150-1156
doi: 10.3866/PKU.WHXB201901002
Abstract:
Polymers are widely used advanced materials composed of macromolecular chains, which can be found in materials used in our daily life. Polymer materials have been employed in many energy and electronic applications such as energy harvesting devices, energy storage devices, light emitting and sensing devices, and flexible energy and electronic devices. The microscopic morphologies and electrical properties of the polymer materials can be tuned by molecular engineering, which could improve the device performances in terms of both the energy conversion efficiency and stability. Traditional polymers are usually considered to be thermal insulators owing to their amorphous molecular chains. Graphene-based polymeric materials have garnered significant attention due to the excellent thermal conductivity of graphene. Advanced polymeric composites with high thermal conductivity exhibit great potential in many applications. Therefore, research on the thermal transport behaviors in graphene-based nanocomposites becomes critical. Vacancy defects in graphene are commonly observed during its fabrication. In this work, the effects of vacancy defects in graphene on thermal transport properties of the graphene-polyethylene nanocomposite are comprehensively investigated using molecular dynamics (MD) simulation. Based on the non-equilibrium molecular dynamics (NEMD) method, the interfacial thermal conductance and the overall thermal conductance of the nanocomposite are taken into consideration simultaneously. It is found that vacancy defects in graphene facilitate the interfacial thermal conductance between graphene and polyethylene. By removing various proportions of carbon atoms in pristine graphene, the density of vacancy defects varies from 0% to 20% and the interfacial thermal conductance increases from 75.6 MW·m−2·K−1 to 85.9 MW·m−2·K−1. The distinct enhancement in the interfacial thermal transport is attributed to the enhanced thermal coupling between graphene and polyethylene. A higher number of broken sp2 bonds in the defective graphene lead to a decrease in the structure rigidity with more low-frequency (< 15 THz) phonons. The improved overlap of vibrational density states between graphene and polyethylene at a low frequency results in better interfacial thermal conductance. Moreover, the increase in the interfacial thermal conductance induced by vacancy defects have a significant effect on the overall thermal conductance (from 40.8 MW·m−2·K−1 to 45.6 MW·m−2·K−1). In addition, when filled with the graphene layer, the local density of polyethylene increases on both sides of the graphene. The concentrated layers provide more aligned molecular arrangement, which result in better thermal conductance in polyethylene. Further, the higher local density of the polymer near the interface provides more atoms for interaction with the graphene, which leads to stronger effective interactions. The relative concentration is insensitive to the density of vacancy defects. The reported results on the thermal transport behavior of graphene-polyethylene composites provide reasonable guidance for using graphene as fillers to tune the thermal conduction of polymeric composites.
Polymers are widely used advanced materials composed of macromolecular chains, which can be found in materials used in our daily life. Polymer materials have been employed in many energy and electronic applications such as energy harvesting devices, energy storage devices, light emitting and sensing devices, and flexible energy and electronic devices. The microscopic morphologies and electrical properties of the polymer materials can be tuned by molecular engineering, which could improve the device performances in terms of both the energy conversion efficiency and stability. Traditional polymers are usually considered to be thermal insulators owing to their amorphous molecular chains. Graphene-based polymeric materials have garnered significant attention due to the excellent thermal conductivity of graphene. Advanced polymeric composites with high thermal conductivity exhibit great potential in many applications. Therefore, research on the thermal transport behaviors in graphene-based nanocomposites becomes critical. Vacancy defects in graphene are commonly observed during its fabrication. In this work, the effects of vacancy defects in graphene on thermal transport properties of the graphene-polyethylene nanocomposite are comprehensively investigated using molecular dynamics (MD) simulation. Based on the non-equilibrium molecular dynamics (NEMD) method, the interfacial thermal conductance and the overall thermal conductance of the nanocomposite are taken into consideration simultaneously. It is found that vacancy defects in graphene facilitate the interfacial thermal conductance between graphene and polyethylene. By removing various proportions of carbon atoms in pristine graphene, the density of vacancy defects varies from 0% to 20% and the interfacial thermal conductance increases from 75.6 MW·m−2·K−1 to 85.9 MW·m−2·K−1. The distinct enhancement in the interfacial thermal transport is attributed to the enhanced thermal coupling between graphene and polyethylene. A higher number of broken sp2 bonds in the defective graphene lead to a decrease in the structure rigidity with more low-frequency (< 15 THz) phonons. The improved overlap of vibrational density states between graphene and polyethylene at a low frequency results in better interfacial thermal conductance. Moreover, the increase in the interfacial thermal conductance induced by vacancy defects have a significant effect on the overall thermal conductance (from 40.8 MW·m−2·K−1 to 45.6 MW·m−2·K−1). In addition, when filled with the graphene layer, the local density of polyethylene increases on both sides of the graphene. The concentrated layers provide more aligned molecular arrangement, which result in better thermal conductance in polyethylene. Further, the higher local density of the polymer near the interface provides more atoms for interaction with the graphene, which leads to stronger effective interactions. The relative concentration is insensitive to the density of vacancy defects. The reported results on the thermal transport behavior of graphene-polyethylene composites provide reasonable guidance for using graphene as fillers to tune the thermal conduction of polymeric composites.
2019, 35(10): 1157-1164
doi: 10.3866/PKU.WHXB201812054
Abstract:
Solar energy, which is clean, affordable and reliable, can help alleviate the current environmental pollution and energy crisis efficiently. In the past few decades, great progress has been made in harvesting and converting solar energy into chemical energy. Among various technologies, plasmon-induced photoelectrochemistry has been proposed as a promising alternative for solar energy conversion. The hot electrons generated from plasmon excitation and transfer from metal nanostructures to semiconductors is a potential new paradigm for solar energy conversion. However, the ultrafast decay of the hot carriers is unfavorable for the improvement of photocatalytic efficiency. Therefore, finding more efficient photocatalysts, with enhanced light absorption and a longer carrier lifetime, is of paramount importance for improving the conversion efficiency of solar energy, but their fabrication is challenging. In this work, a plasmonic metal/semiconductor heterostructure based on Ag nanoparticles embedded in two-dimensional (2D) amorphous sub-stoichiometric tungsten trioxide (a-WO3−x), followed by annealing, was successfully fabricated. Firstly, the peculiar nanostructure of 2D a-WO3−x was successfully constructed from WS2 nanosheets with supercritical CO2 (SC CO2) at 200 ℃. Secondly, the Ag/a-WO3−x heterostructure was synthesized using an in situ reduction method. Finally, the obtained 2D heterostructure of Ag/WO3−x was annealed at 400 ℃ in N2 to further improve its stability and conductivity. X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) were used to characterize the structure, morphology, and composition of the material, respectively. UV-Vis spectra were also measured to evaluate light adsorption. Characterization results show that the amorphous structure can effectively anchor metal nanoparticles, and the metal nanoparticles are uniformly dispersed in the amorphous region and have a small size. The as-prepared nanocomposites showed efficient photoelectrochemical (PEC) water splitting when serving as photoelectrode materials, and efficient PEC activity towards photo-oxidation degradation currents under excitation of Ag localized surface plasmon resonance (LSPR). The photocurrent response of the Ag/WO3−x heterostructure was approximately five times greater than that of a-WO3−x. Moreover, the PEC degradation efficiency of Ag/WO3−x reached 96.7% for MO under Vis light illumination (after reaction for 120 min), while the PEC degradation efficiency of WO3−x was only 63.6%. The high PEC performance of the composite photoanode can be ascribed to the local surface plasmon resonance (LSPR) effect of the Ag nanoparticles, which can enhance the light absorption and hot electron transformation. Moreover, the construction of local crystalline-amorphous interfaces can further promote the separation efficiency of the photogenerated electron-hole pairs, and thus increase conductivity. This work provides a positive strategy for the fabrication of advanced photocatalysts, and a new perspective on understanding of the synergistic effects of structural and electronic regulations.
Solar energy, which is clean, affordable and reliable, can help alleviate the current environmental pollution and energy crisis efficiently. In the past few decades, great progress has been made in harvesting and converting solar energy into chemical energy. Among various technologies, plasmon-induced photoelectrochemistry has been proposed as a promising alternative for solar energy conversion. The hot electrons generated from plasmon excitation and transfer from metal nanostructures to semiconductors is a potential new paradigm for solar energy conversion. However, the ultrafast decay of the hot carriers is unfavorable for the improvement of photocatalytic efficiency. Therefore, finding more efficient photocatalysts, with enhanced light absorption and a longer carrier lifetime, is of paramount importance for improving the conversion efficiency of solar energy, but their fabrication is challenging. In this work, a plasmonic metal/semiconductor heterostructure based on Ag nanoparticles embedded in two-dimensional (2D) amorphous sub-stoichiometric tungsten trioxide (a-WO3−x), followed by annealing, was successfully fabricated. Firstly, the peculiar nanostructure of 2D a-WO3−x was successfully constructed from WS2 nanosheets with supercritical CO2 (SC CO2) at 200 ℃. Secondly, the Ag/a-WO3−x heterostructure was synthesized using an in situ reduction method. Finally, the obtained 2D heterostructure of Ag/WO3−x was annealed at 400 ℃ in N2 to further improve its stability and conductivity. X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) were used to characterize the structure, morphology, and composition of the material, respectively. UV-Vis spectra were also measured to evaluate light adsorption. Characterization results show that the amorphous structure can effectively anchor metal nanoparticles, and the metal nanoparticles are uniformly dispersed in the amorphous region and have a small size. The as-prepared nanocomposites showed efficient photoelectrochemical (PEC) water splitting when serving as photoelectrode materials, and efficient PEC activity towards photo-oxidation degradation currents under excitation of Ag localized surface plasmon resonance (LSPR). The photocurrent response of the Ag/WO3−x heterostructure was approximately five times greater than that of a-WO3−x. Moreover, the PEC degradation efficiency of Ag/WO3−x reached 96.7% for MO under Vis light illumination (after reaction for 120 min), while the PEC degradation efficiency of WO3−x was only 63.6%. The high PEC performance of the composite photoanode can be ascribed to the local surface plasmon resonance (LSPR) effect of the Ag nanoparticles, which can enhance the light absorption and hot electron transformation. Moreover, the construction of local crystalline-amorphous interfaces can further promote the separation efficiency of the photogenerated electron-hole pairs, and thus increase conductivity. This work provides a positive strategy for the fabrication of advanced photocatalysts, and a new perspective on understanding of the synergistic effects of structural and electronic regulations.
