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2017 Volume 75 Issue 10
2017, 75(10): 933-942
doi: 10.6023/A17060272
Abstract:
Biomolecular self-assembly plays a significant role for physiological function. Inspired by this, the construction of functional structures and architectures by biomolecular self-assembly has attracted tremendous attentions. Peptides can be assembled into diverse nanostructures, exhibiting important potential for biomedical and green-life technology applications. How to achieve the structural precise regulation of various nanostructures and functionalization by precise control of structures is the two key challenges in the field of peptide self-assembly. As the assembly process is a spontaneous thermodynamic and kinetic driven process, and is determined by the cooperation of various intermolecular non-covalent interactions, including hydrogen-bonding, electrostatic, π-π stacking, hydrophobic, and van der Waals interactions, the reasonable regulation of these non-covalent interactions is a critical pathway to achieve the two goals. To modulate these non-covalent interactions, one of the common used methods is to change the kinetic factors/external environment, including pH, ionic strength, and temperature, etc. These kinetic factors can effectively influence the interactions between peptides and solvents, resulting in dynamic and responsive variations in structures through multiple length scales and ultimate morphologies. However, the fatal disadvantage is the lacking of the precise regulation of assembled structures in the molecular level with consideration of both thermodynamics and kinetics. Compared with changing the external environment, the specific and precise molecular design is more favorable to achieve the structural precise regulation. The molecular structures and the component of building blocks can be rationally designed. For example, one can modulate the interactions between two or more than two building blocks by changing the physicochemical properties of each building block, enabling self-assembly and structural diversity of the final nanostructures. Furthermore, by combining peptides and other functional biomolecules (such as porphyrins), the functionalization of assembled nanostructures and architectures can be achieved more easily and flexibly. In this review, we will focus on the structural precise regulation and the functionalization of assembled peptide nanostructures. It is believed that the precise regulation of nanostructures is promising to promote the development of peptide-based materials towards green-life technology applications.
Biomolecular self-assembly plays a significant role for physiological function. Inspired by this, the construction of functional structures and architectures by biomolecular self-assembly has attracted tremendous attentions. Peptides can be assembled into diverse nanostructures, exhibiting important potential for biomedical and green-life technology applications. How to achieve the structural precise regulation of various nanostructures and functionalization by precise control of structures is the two key challenges in the field of peptide self-assembly. As the assembly process is a spontaneous thermodynamic and kinetic driven process, and is determined by the cooperation of various intermolecular non-covalent interactions, including hydrogen-bonding, electrostatic, π-π stacking, hydrophobic, and van der Waals interactions, the reasonable regulation of these non-covalent interactions is a critical pathway to achieve the two goals. To modulate these non-covalent interactions, one of the common used methods is to change the kinetic factors/external environment, including pH, ionic strength, and temperature, etc. These kinetic factors can effectively influence the interactions between peptides and solvents, resulting in dynamic and responsive variations in structures through multiple length scales and ultimate morphologies. However, the fatal disadvantage is the lacking of the precise regulation of assembled structures in the molecular level with consideration of both thermodynamics and kinetics. Compared with changing the external environment, the specific and precise molecular design is more favorable to achieve the structural precise regulation. The molecular structures and the component of building blocks can be rationally designed. For example, one can modulate the interactions between two or more than two building blocks by changing the physicochemical properties of each building block, enabling self-assembly and structural diversity of the final nanostructures. Furthermore, by combining peptides and other functional biomolecules (such as porphyrins), the functionalization of assembled nanostructures and architectures can be achieved more easily and flexibly. In this review, we will focus on the structural precise regulation and the functionalization of assembled peptide nanostructures. It is believed that the precise regulation of nanostructures is promising to promote the development of peptide-based materials towards green-life technology applications.
2017, 75(10): 943-966
doi: 10.6023/A17040183
Abstract:
Proton exchange membrane fuel cells (PEMFCs) that directly convert chemical energy into electrical energy can be applied to portable power and fuel cell electric vehicles, due to their advantages such as environment-friendliness, high power density and high convert efficiency. However, the high loading of Pt-based catalysts on the cathode oxygen reduction reaction (ORR) hinder the commercial application of PEMFCs for the high price, resource shortage and easy poisoning of Pt. Thus, developing inexpensive, high performance and durability non-noble metal cathode catalysts will promote the large-scale commercialization of PEMFCs. As the most likely alternative to Pt, carbon-based non-noble ORR catalysts have been widely studied. In this review, firstly, the electrocatalytic mechanism for ORR is simply introduced. Secondly, the carbon-based non-noble ORR catalysts are divided into transition metal-nitrogen-carbon compounds (M-N-C) and non-metal heteroatom-doped carbon catalysts; the researches of material preparations and active sites are summarized and discussed. Thirdly, the applications of carbon-based non-noble ORR catalysts in PEMFC are reviewed. Although great progress has been achieved in this area of research and development, there are still some challenges for carbon-based non-noble ORR catalysts. Firstly, the ORR electrocatalytic mechanism isn't clear, especially carbon-based non-noble catalysts. Secondly, the ORR active sites of carbon-based non-noble catalysts remain controversial, which can be mainly divided into the transition metal coordination compounds, the doped heteroatom, the filled metal and the defect sites. Thirdly, the actual activity and stability of carbon-based non-noble catalysts are still below the PEMFC target. In summary, the future research directions on carbon-based non-noble catalysts for PEMFC applications would be proposed as follows:(1) fundamentally understanding the ORR mechanisms and their relationship with catalyst active site structures and composition using both theoretical calculations and experimental approaches; (2) improving catalyst activity and stability to satisfy the practical application of PEMFC.
Proton exchange membrane fuel cells (PEMFCs) that directly convert chemical energy into electrical energy can be applied to portable power and fuel cell electric vehicles, due to their advantages such as environment-friendliness, high power density and high convert efficiency. However, the high loading of Pt-based catalysts on the cathode oxygen reduction reaction (ORR) hinder the commercial application of PEMFCs for the high price, resource shortage and easy poisoning of Pt. Thus, developing inexpensive, high performance and durability non-noble metal cathode catalysts will promote the large-scale commercialization of PEMFCs. As the most likely alternative to Pt, carbon-based non-noble ORR catalysts have been widely studied. In this review, firstly, the electrocatalytic mechanism for ORR is simply introduced. Secondly, the carbon-based non-noble ORR catalysts are divided into transition metal-nitrogen-carbon compounds (M-N-C) and non-metal heteroatom-doped carbon catalysts; the researches of material preparations and active sites are summarized and discussed. Thirdly, the applications of carbon-based non-noble ORR catalysts in PEMFC are reviewed. Although great progress has been achieved in this area of research and development, there are still some challenges for carbon-based non-noble ORR catalysts. Firstly, the ORR electrocatalytic mechanism isn't clear, especially carbon-based non-noble catalysts. Secondly, the ORR active sites of carbon-based non-noble catalysts remain controversial, which can be mainly divided into the transition metal coordination compounds, the doped heteroatom, the filled metal and the defect sites. Thirdly, the actual activity and stability of carbon-based non-noble catalysts are still below the PEMFC target. In summary, the future research directions on carbon-based non-noble catalysts for PEMFC applications would be proposed as follows:(1) fundamentally understanding the ORR mechanisms and their relationship with catalyst active site structures and composition using both theoretical calculations and experimental approaches; (2) improving catalyst activity and stability to satisfy the practical application of PEMFC.
2017, 75(10): 967-978
doi: 10.6023/A17070302
Abstract:
In recent years, the wettability of colloidal PCs has attracted much interest from researchers due to potential applications in printing, sensor, microfluidics and so on. In this paper, we present two kinds of research work related to PCs' wettability. On the one hand, the functional colloidal PCs have been fabricated from the modification of its wettability. Where, the wettability of PCs can be modified from superhydrophilic, superhydrophobic, amphiphilic, gradient wettability, controllable wettability and patterned wettability. Wettability is an important property of solid surface and can be generally controlled mainly by its surface chemical composition and surface topographic structure. Surface chemical composition determines surface free energy (i.e., hydrophilicity/hydrophobicity), while the surface topographic structure can amplify hydrophilicity or hydrophobicity, based on the Wenzel and modified Cassie equation. Thus, PCs with specific wettability have been fabricated based on their intrinsic, well-ordered surface topographic structure, and chemical composition. The superhydrophilic and superhydrophobic PCs have been achieved based on the amplification effect of the surface well-ordered topographic structure. The gradient PCs have been fabricated by changing the topographic structure. The PCs with controllable wettability can be obtained when introducing a responsive group onto PCs' surface. The underwater oil-adhesion properties of PCs have been controlled by varying the latex from spherical or cauliflower-like to single cavity. On the other hand, functional PCs are fabricated from the substrate with specific wettability. Typically, high-quality and crack free PCs are achieved from superhydrophobic substrate, pattern PCs from the hydrophilic-hydrophobic substrate, PC dome with excellent wide-angle property is fabricated from hydrophobic substrate. Otherwise, gas-liquid or liquid-liquid interface has also been included as a special substrate for the fabrication of functional PCs, such as flower-shape or cake-shaped Janus PCs. Colloidal photonic crystals (PCs), the periodic arrangement of monodispersed latex spheres, have attracted much interest from researchers due to their unique light manipulation properties. The combination of the special wettability and light manipulation properties of PCs will bring many novel properties and promising applications. Finally, the outlook and challenges for colloidal photonic crystals with special wettability are discussed. The work is of importance for the creation of novel functional materials.
In recent years, the wettability of colloidal PCs has attracted much interest from researchers due to potential applications in printing, sensor, microfluidics and so on. In this paper, we present two kinds of research work related to PCs' wettability. On the one hand, the functional colloidal PCs have been fabricated from the modification of its wettability. Where, the wettability of PCs can be modified from superhydrophilic, superhydrophobic, amphiphilic, gradient wettability, controllable wettability and patterned wettability. Wettability is an important property of solid surface and can be generally controlled mainly by its surface chemical composition and surface topographic structure. Surface chemical composition determines surface free energy (i.e., hydrophilicity/hydrophobicity), while the surface topographic structure can amplify hydrophilicity or hydrophobicity, based on the Wenzel and modified Cassie equation. Thus, PCs with specific wettability have been fabricated based on their intrinsic, well-ordered surface topographic structure, and chemical composition. The superhydrophilic and superhydrophobic PCs have been achieved based on the amplification effect of the surface well-ordered topographic structure. The gradient PCs have been fabricated by changing the topographic structure. The PCs with controllable wettability can be obtained when introducing a responsive group onto PCs' surface. The underwater oil-adhesion properties of PCs have been controlled by varying the latex from spherical or cauliflower-like to single cavity. On the other hand, functional PCs are fabricated from the substrate with specific wettability. Typically, high-quality and crack free PCs are achieved from superhydrophobic substrate, pattern PCs from the hydrophilic-hydrophobic substrate, PC dome with excellent wide-angle property is fabricated from hydrophobic substrate. Otherwise, gas-liquid or liquid-liquid interface has also been included as a special substrate for the fabrication of functional PCs, such as flower-shape or cake-shaped Janus PCs. Colloidal photonic crystals (PCs), the periodic arrangement of monodispersed latex spheres, have attracted much interest from researchers due to their unique light manipulation properties. The combination of the special wettability and light manipulation properties of PCs will bring many novel properties and promising applications. Finally, the outlook and challenges for colloidal photonic crystals with special wettability are discussed. The work is of importance for the creation of novel functional materials.
2017, 75(10): 979-990
doi: 10.6023/A17060282
Abstract:
Two-dimensional (2D) materials have received great attentions in recent years, including BN, transition metal dichalcogenides, transition metal oxides and black phosphorus. Among them, graphene-like transition metal dichalcogenides (TMDCs), such as MoS2, WS2, MoSe2, TiS2, are emerging as key materials in electronics and chemical industry because of their excellent physical and chemical properties. Because of the quantum confinement and surface effects, the 2D nanomaterials exhibit completely different properties from their bulk, leading to a new field in material science and technology. The ability to prepare high quality and large scale TMDCs is the foundation for their practical applications. Until now, many methods have been employed to prepare various morphologies of TMDCs, including mechanical cleavage, intercalation-exfoliation, ultrasonic-assisted liquid-phase exfoliation, chemical vapor deposition and hydrothermal synthesis. In this paper, the authors introduce the crystal structures and electronic properties of TMDCs briefly. The dimension from bulk to single or few layers leads to changes of these nanomaterials, showing novel properties in electronic transfer rate, catalytic activity, etc. Then the top-down and bottom-up preparation methods are summarized, and the advantages and disadvantages of these methods are discussed. At present, the challenge is that there are no proper ways to prepare TMDCs in large scale with controlled thickness and general application. As every single material has its performance limitation, the hotpot in preparation lies in the hybridization with other materials to create functional composites, aiming to improve their electronic and optical properties for special devices, and the most commonly used components are graphene and other 2D materials. And the authors also introduce the research progress in applications systematically, with emphasis on electronic devices, optoelectronic devices, sensing platforms, energy storage devices and catalyst, showing a wide range of applications. In addition, the authors also give some perspectives on the challenges and prospects in this field.
Two-dimensional (2D) materials have received great attentions in recent years, including BN, transition metal dichalcogenides, transition metal oxides and black phosphorus. Among them, graphene-like transition metal dichalcogenides (TMDCs), such as MoS2, WS2, MoSe2, TiS2, are emerging as key materials in electronics and chemical industry because of their excellent physical and chemical properties. Because of the quantum confinement and surface effects, the 2D nanomaterials exhibit completely different properties from their bulk, leading to a new field in material science and technology. The ability to prepare high quality and large scale TMDCs is the foundation for their practical applications. Until now, many methods have been employed to prepare various morphologies of TMDCs, including mechanical cleavage, intercalation-exfoliation, ultrasonic-assisted liquid-phase exfoliation, chemical vapor deposition and hydrothermal synthesis. In this paper, the authors introduce the crystal structures and electronic properties of TMDCs briefly. The dimension from bulk to single or few layers leads to changes of these nanomaterials, showing novel properties in electronic transfer rate, catalytic activity, etc. Then the top-down and bottom-up preparation methods are summarized, and the advantages and disadvantages of these methods are discussed. At present, the challenge is that there are no proper ways to prepare TMDCs in large scale with controlled thickness and general application. As every single material has its performance limitation, the hotpot in preparation lies in the hybridization with other materials to create functional composites, aiming to improve their electronic and optical properties for special devices, and the most commonly used components are graphene and other 2D materials. And the authors also introduce the research progress in applications systematically, with emphasis on electronic devices, optoelectronic devices, sensing platforms, energy storage devices and catalyst, showing a wide range of applications. In addition, the authors also give some perspectives on the challenges and prospects in this field.
2017, 75(10): 991-997
doi: 10.6023/A17070345
Abstract:
Increasing evidence suggests that nanoscale zero-valent iron (nZVI) is an effective nanomaterial for the enrichment and separation of heavy metals from water, especially for recovering precious metals such as gold and silver from trace level sources. In this work, a nano-iron reactor, consisting of reaction zone, separation zone and reuse facilities, is applied to recovery of silver from aqueous solution using nZVI. We demonstrate that nZVI could sequester Ag+ (ca. 1 mg/L) and be transformed into high-grade (32.0 mg/g) silver solids ("ore") as nZVI is recycled in this "reaction-separation-reuse" system. Besides, increasing hydraulic retention time (HRT), from 10 min to 60 min, could enhance the enrichment efficiency and finally improve silver content in solid phase. We further demonstrate that there is a positive correlation between solution oxidation-reduction potential in reaction zone and Ag+ concentration in effluent, and this relationship can be used to regulate the reaction kinetics and separation efficiency. Data from oxidation-reduction potential regulating experiment are presented and a mathematic formula is provided, proving this system is reliable and controllable. Solid phase characterizations with X-ray diffraction and X-ray photoelectron spectroscopy confirm that Ag+ is reduced to metallic silver (Ag0). Images acquired via high-resolution transmission electron microscopy reveal that Ag0 ( < 10 nm) is deposited on the surface of nZVI (Ag-nZVI). Pure silver nanoparticles (AgNPs, 9~32 nm) could be acquired by simply processing Ag-nZVI with sulfuric acid and polyvinyl pyrrolidone. Batch experiments confirm that nZVI is far more efficient and less pH-dependent, comparing to other materials (e.g., mZVI, α-Fe2O3, nTiO2). 99% Ag+ (1000 mg/L) could be sequestrated in less than 15 s with 1 g/L nZVI. And the separation coefficient of nZVI for Ag+ reaches 3.2×104, which is several orders of magnitude higher than that of conventional adsorbents and reductants (102~741). This study demonstrates that nZVI is a powerful candidate to recover Ag from water (e.g., industrial wastewater, groundwater) with trace level silver and produce valuable AgNPs.
Increasing evidence suggests that nanoscale zero-valent iron (nZVI) is an effective nanomaterial for the enrichment and separation of heavy metals from water, especially for recovering precious metals such as gold and silver from trace level sources. In this work, a nano-iron reactor, consisting of reaction zone, separation zone and reuse facilities, is applied to recovery of silver from aqueous solution using nZVI. We demonstrate that nZVI could sequester Ag+ (ca. 1 mg/L) and be transformed into high-grade (32.0 mg/g) silver solids ("ore") as nZVI is recycled in this "reaction-separation-reuse" system. Besides, increasing hydraulic retention time (HRT), from 10 min to 60 min, could enhance the enrichment efficiency and finally improve silver content in solid phase. We further demonstrate that there is a positive correlation between solution oxidation-reduction potential in reaction zone and Ag+ concentration in effluent, and this relationship can be used to regulate the reaction kinetics and separation efficiency. Data from oxidation-reduction potential regulating experiment are presented and a mathematic formula is provided, proving this system is reliable and controllable. Solid phase characterizations with X-ray diffraction and X-ray photoelectron spectroscopy confirm that Ag+ is reduced to metallic silver (Ag0). Images acquired via high-resolution transmission electron microscopy reveal that Ag0 ( < 10 nm) is deposited on the surface of nZVI (Ag-nZVI). Pure silver nanoparticles (AgNPs, 9~32 nm) could be acquired by simply processing Ag-nZVI with sulfuric acid and polyvinyl pyrrolidone. Batch experiments confirm that nZVI is far more efficient and less pH-dependent, comparing to other materials (e.g., mZVI, α-Fe2O3, nTiO2). 99% Ag+ (1000 mg/L) could be sequestrated in less than 15 s with 1 g/L nZVI. And the separation coefficient of nZVI for Ag+ reaches 3.2×104, which is several orders of magnitude higher than that of conventional adsorbents and reductants (102~741). This study demonstrates that nZVI is a powerful candidate to recover Ag from water (e.g., industrial wastewater, groundwater) with trace level silver and produce valuable AgNPs.
2017, 75(10): 998-1002
doi: 10.6023/A17060277
Abstract:
Direct asymmetric vinylogous Mannich reaction is an efficient and powerful method for the synthesis of δ-amino-α, β-unsaturated carbonyl compounds; however, the nucleophiles are generally limited to γ-butenolides and α, α-dicyanoolefins. Therefore, it is highly desirable to design new vinylogous nucleophiles and develop the related asymmetric reactions. Recently, we disclosed a new type of nitrones derived from isatins and N-benzyl hydroxylamines, which could easily generate nitrone ylide species in the presence of a tertiary amine, and undergo asymmetric formal[3+2] cycloadditions with α, β-unsaturated aldehydes via iminium ion catalysis of a chiral secondary amine. Subsequently, we found that such nitrone ylides could isomerize to more interesting aza-dienolate-type intermediates, and engage in direct stereoselective aza-vinylogous Michael reactions with nitroalkenes under the catalysis of a bifunctional thiourea-tertiary amine, delivering chiral nitrone derivatives with extended carbon skeletons without subsequent cyclization. In this case, the same type of nitrones are employed as nucleophilic precursors under the catalysis of a cinchona alkaloid-based thiourea substance, and effectively assembled with isatins-derived ketimines to accomplish the direct asymmetric aza-vinylogous-type Mannich reactions. A series of densely functionalized nitrones with vicinal tertiary-quaternary stereogenic centers are furnished in high yields (70%~97%) with good to excellent stereoselectivity (83%~99% ee, >19:1 dr). Moreover, subsequent[3+2] dipolar cycloaddition reactions between the chiral nitrones and activated alkenes can be realized in exclusive diastereoselectivity, producing complex spirocyclic indolenine architectures incorporating a hydrogenated isoxazole ring. These nitrones, as a new type of aza-vinylogous nucleophiles, may have a wide range of applications in asymmetric synthesis in the future. A representative procedure for the asymmetric aza-vinylogous-type Mannich reaction is as follows:nitrone 1 (0.1 mmol), ketimine 2 (0.11 mmol), catalyst C5 (0.01 or 0.02 mmol) are added into an oven-dried vial equipped with a magnetic stirbar. Xylene (1.0 mL) is added and the mixture is stirred at 50℃ and monitored by TLC. After completion, the residue is purified by flash column chromatography on silica gel eluting with petroleum ether/ethyl acetate (15:1 to 5:1) to afford the product 3.
Direct asymmetric vinylogous Mannich reaction is an efficient and powerful method for the synthesis of δ-amino-α, β-unsaturated carbonyl compounds; however, the nucleophiles are generally limited to γ-butenolides and α, α-dicyanoolefins. Therefore, it is highly desirable to design new vinylogous nucleophiles and develop the related asymmetric reactions. Recently, we disclosed a new type of nitrones derived from isatins and N-benzyl hydroxylamines, which could easily generate nitrone ylide species in the presence of a tertiary amine, and undergo asymmetric formal[3+2] cycloadditions with α, β-unsaturated aldehydes via iminium ion catalysis of a chiral secondary amine. Subsequently, we found that such nitrone ylides could isomerize to more interesting aza-dienolate-type intermediates, and engage in direct stereoselective aza-vinylogous Michael reactions with nitroalkenes under the catalysis of a bifunctional thiourea-tertiary amine, delivering chiral nitrone derivatives with extended carbon skeletons without subsequent cyclization. In this case, the same type of nitrones are employed as nucleophilic precursors under the catalysis of a cinchona alkaloid-based thiourea substance, and effectively assembled with isatins-derived ketimines to accomplish the direct asymmetric aza-vinylogous-type Mannich reactions. A series of densely functionalized nitrones with vicinal tertiary-quaternary stereogenic centers are furnished in high yields (70%~97%) with good to excellent stereoselectivity (83%~99% ee, >19:1 dr). Moreover, subsequent[3+2] dipolar cycloaddition reactions between the chiral nitrones and activated alkenes can be realized in exclusive diastereoselectivity, producing complex spirocyclic indolenine architectures incorporating a hydrogenated isoxazole ring. These nitrones, as a new type of aza-vinylogous nucleophiles, may have a wide range of applications in asymmetric synthesis in the future. A representative procedure for the asymmetric aza-vinylogous-type Mannich reaction is as follows:nitrone 1 (0.1 mmol), ketimine 2 (0.11 mmol), catalyst C5 (0.01 or 0.02 mmol) are added into an oven-dried vial equipped with a magnetic stirbar. Xylene (1.0 mL) is added and the mixture is stirred at 50℃ and monitored by TLC. After completion, the residue is purified by flash column chromatography on silica gel eluting with petroleum ether/ethyl acetate (15:1 to 5:1) to afford the product 3.
2017, 75(10): 1003-1009
doi: 10.6023/A17070298
Abstract:
Fuel cells which use hydrogen peroxide as oxidant have been widely studied and presents good development foreground. As a liquid fuel, H2O2 possesses advantages of easily storage and transportation which make it can be widely used in underwater and space as a power source. At present, the most widely used catalysts for H2O2 electroreduction are noble metal catalysts. Compared with noble metals, transition metal oxides possess advantages of low cost and extensive sources. However, the catalytic activity of transition metal oxides is still much lower than noble metals. Therefore, many efforts should be made to improve the electrochemical performance of transition metal oxides. In this work, rGO is used as an additive to improve the electrochemcial performance of MnO2. An original electrode of MnO2 in-situ supported on reduced graphene oxide modified Ni foam (MnO2/rGO@Ni foam) is prepared through two-step hydrothermal methods. Primarily, the novel current collector of rGO@Ni foam is obtained with larger surface area which is beneficial to the next loading of MnO2. Secondly, MnO2 is grown on the rGO@Ni foam also by a hydrothermal treatment. Besides large surface area, the addition of rGO can provide more channels for electron transfer and then accelerate the reaction rate of H2O2 reduction. The morphology and phase composition of the as-prepared electrode are investigated by measurements of X-ray diffractometer (XRD), scanning electron microscopy (SEM) and transmission electron microscope (TEM). It can be concluded from SEM and TEM images, both rGO and MnO2 exhibit sheet-like structure and there are many gaps existing between these sheets. Especially, the as-prepared MnO2 nanosheets builds a honeycomb structure which makes positive effects on the contact between H2O2 and catalyst. And XRD and HRTEM results show that MnO2 and rGO are successfully prepared on Ni foam. The electrochemical performance of the MnO2/rGO@Ni foam electrode toward H2O2 reduction is investigated by cyclic voltammetry and chronoamperometry in a three-electrode system in solutions of NaOH and H2O2. Results reveal that the reduction current density of H2O2 reduction on the MnO2/rGO@Ni foam electrode reaches 240 mA/cm2 in a solution of 1.0 mol/L H2O2 and 3 mol/L NaOH at -0.8 V which is much higher than that on MnO2 directly supported on Ni foam (MnO2@Ni foam). At the same time, a better stability is also achieved on the MnO2/rGO@Ni foam electrode. Generally speaking, the addition of rGO highly improves the electrocatalytic activity and stability of the as-prepared electrode indicating great application prospect in the future.
Fuel cells which use hydrogen peroxide as oxidant have been widely studied and presents good development foreground. As a liquid fuel, H2O2 possesses advantages of easily storage and transportation which make it can be widely used in underwater and space as a power source. At present, the most widely used catalysts for H2O2 electroreduction are noble metal catalysts. Compared with noble metals, transition metal oxides possess advantages of low cost and extensive sources. However, the catalytic activity of transition metal oxides is still much lower than noble metals. Therefore, many efforts should be made to improve the electrochemical performance of transition metal oxides. In this work, rGO is used as an additive to improve the electrochemcial performance of MnO2. An original electrode of MnO2 in-situ supported on reduced graphene oxide modified Ni foam (MnO2/rGO@Ni foam) is prepared through two-step hydrothermal methods. Primarily, the novel current collector of rGO@Ni foam is obtained with larger surface area which is beneficial to the next loading of MnO2. Secondly, MnO2 is grown on the rGO@Ni foam also by a hydrothermal treatment. Besides large surface area, the addition of rGO can provide more channels for electron transfer and then accelerate the reaction rate of H2O2 reduction. The morphology and phase composition of the as-prepared electrode are investigated by measurements of X-ray diffractometer (XRD), scanning electron microscopy (SEM) and transmission electron microscope (TEM). It can be concluded from SEM and TEM images, both rGO and MnO2 exhibit sheet-like structure and there are many gaps existing between these sheets. Especially, the as-prepared MnO2 nanosheets builds a honeycomb structure which makes positive effects on the contact between H2O2 and catalyst. And XRD and HRTEM results show that MnO2 and rGO are successfully prepared on Ni foam. The electrochemical performance of the MnO2/rGO@Ni foam electrode toward H2O2 reduction is investigated by cyclic voltammetry and chronoamperometry in a three-electrode system in solutions of NaOH and H2O2. Results reveal that the reduction current density of H2O2 reduction on the MnO2/rGO@Ni foam electrode reaches 240 mA/cm2 in a solution of 1.0 mol/L H2O2 and 3 mol/L NaOH at -0.8 V which is much higher than that on MnO2 directly supported on Ni foam (MnO2@Ni foam). At the same time, a better stability is also achieved on the MnO2/rGO@Ni foam electrode. Generally speaking, the addition of rGO highly improves the electrocatalytic activity and stability of the as-prepared electrode indicating great application prospect in the future.
2017, 75(10): 1010-1016
doi: 10.6023/A17050236
Abstract:
Recently, there has been a significant interest in utilizing well-ordered, porous inverse-opal films for applications in optical, electronic and (bio)chemical fields. However, uncontrolled defects are always formed during their preparation process, which limit their practical applications. In this work, we examine the feasibility of using template/matrix co-assembly strategies to fabricate crack-free inverse opal thin films. Polystyrene spheres (PS) are chosen as a colloidal template, and two matrix precursors[tetraethoxysilane (TEOS) precursor and regenerated silk fibroin solution] are used for the current study. Our scanning electron microscope (SEM) and optical spectrum results show that, for the TEOS-based system, the resulting silica gel due to the sol-gel transition of TEOS can effectively fill the gap between particles, but cannot affect the self-assembly of PS colloidal particles. After selective removal of the PS template, centimeter-scale crack-free and well-ordered inverse opal films can be obtained. In addition, for a constant concentration of TEOS, the film thickness and order degree of structure can be simply tuned by adjusting the concentrations of colloidal spheres. In comparison with indirect approach through template self-assembly and liquid infiltration, such a co-assembly approach can effectively minimize the associated cracking and avoid the need for matrix infiltration into the preassembled colloidal spheres. On the other hand, macro-molecule silk fibroin has a relatively strong interaction with PS colloidal particles, which is demonstrated by SEM and confocal images. Due to their interaction, silk fibroin molecules are preferably adsorbed on the surface of PS spheres, which can restrain the self-assembly of colloidal particles. As a result, it cannot form well-ordered silk film based on such co-assembly strategy. That is to say, the co-assembly approach is not suitable for systems that matrices have strong interactions with templates. These findings pave the way to use the template/matrix co-assembly strategy for rationally designing and developing crack-free inverse opal films and to apply such well-ordered and porous materials in a variety of fields.
Recently, there has been a significant interest in utilizing well-ordered, porous inverse-opal films for applications in optical, electronic and (bio)chemical fields. However, uncontrolled defects are always formed during their preparation process, which limit their practical applications. In this work, we examine the feasibility of using template/matrix co-assembly strategies to fabricate crack-free inverse opal thin films. Polystyrene spheres (PS) are chosen as a colloidal template, and two matrix precursors[tetraethoxysilane (TEOS) precursor and regenerated silk fibroin solution] are used for the current study. Our scanning electron microscope (SEM) and optical spectrum results show that, for the TEOS-based system, the resulting silica gel due to the sol-gel transition of TEOS can effectively fill the gap between particles, but cannot affect the self-assembly of PS colloidal particles. After selective removal of the PS template, centimeter-scale crack-free and well-ordered inverse opal films can be obtained. In addition, for a constant concentration of TEOS, the film thickness and order degree of structure can be simply tuned by adjusting the concentrations of colloidal spheres. In comparison with indirect approach through template self-assembly and liquid infiltration, such a co-assembly approach can effectively minimize the associated cracking and avoid the need for matrix infiltration into the preassembled colloidal spheres. On the other hand, macro-molecule silk fibroin has a relatively strong interaction with PS colloidal particles, which is demonstrated by SEM and confocal images. Due to their interaction, silk fibroin molecules are preferably adsorbed on the surface of PS spheres, which can restrain the self-assembly of colloidal particles. As a result, it cannot form well-ordered silk film based on such co-assembly strategy. That is to say, the co-assembly approach is not suitable for systems that matrices have strong interactions with templates. These findings pave the way to use the template/matrix co-assembly strategy for rationally designing and developing crack-free inverse opal films and to apply such well-ordered and porous materials in a variety of fields.
