The Ti 2 Ni and the related η-carbide structure are known to exhibit various intriguing physical ... more The Ti 2 Ni and the related η-carbide structure are known to exhibit various intriguing physical properties. The Ti 2 Ni structure with the cubic space group F d3m is surprisingly complex, consisting of a unit cell with 96 metal atoms. The related η-carbide compounds correspond to a filled version of the Ti 2 Ni structure. Here, we report on the structure and superconductivity in the η-carbide type suboxides Ti 4 M 2 O with M = Co, Rh, Ir. We have successfully synthesized all three compounds in single phase form. We find all three compounds to be type-II bulk superconductors with transition temperatures of T c = 2.7, 2.8, and 5.4 K, and with normalized specific heat jumps of ∆C/γT c = 1.65, 1.28, and 1.80 for Ti 4 Co 2 O, Ti 4 Rh 2 O, and Ti 4 Ir 2 O, respectively. We find that all three superconductors, exhibit high upper-critical fields. Particularly noteworthy is in this regard Ti 4 Ir 2 O with an upper critical field of µ 0 H c2 (0) = 16.06 T, which exceeds by far the weak-coupling Pauli limit-widely consider as the maximal upper critical field-of µ 0 H Pauli = 9.86 T. The role of the void filling light atom X has so far been uncertain for the overall physical properties of these materials. Herein, we have successfully grown single crystals of Ti 2 Co. In contrast to the metallic η-carbide type suboxides Ti 4 M 2 O, we find that Ti 2 Co displays a semimetallic behavior down to 0.75 K. Below 0.75 K we observe a broad decrease in the resistivity, which can most likely be attributed to an onset of a superconducting transition at lower temperatures. Hence, the octahedral void-filling oxygen plays a crucial role for the overall physical properties, even though its effect on the crystal structure is small. Our results indicate that the design of new superconductors by incorporation of electron-acceptor atoms may in the Ti 2 Ni-type structures and other materials with crystallographic void position be a promising future approach. The remarkably high upper critical fields, in this family of compounds, may furthermore spark significant future interest.
In the transition metal dichalcogenide IrTe$_2$, low-temperature charge-ordered phase transitions... more In the transition metal dichalcogenide IrTe$_2$, low-temperature charge-ordered phase transitions lead to the occurrence of stripe phases of different periodicities and nearly degenerate energies. Bulk-sensitive measurements have shown that, upon cooling, IrTe$_2$ undergoes two such first-order phase transitions at T$_{c_1}=270$~K and T$_{c_2}=180$~K. Here, using surface sensitive probes of the electronic structure of IrTe$_2$, we show that, in addition, another \textit{first-order} transition occurs at the surface at T$_{c_3}=165$~K. By performing measurements over a full thermal cycle, we also reveal the complete hysteresis of all these phases.
M. Rumo, ∗ A. Pulkkinen, 2 B. Salzmann, G. Kremer, B. Hildebrand, K.Y. Ma, F.O. von Rohr, C.W. Ni... more M. Rumo, ∗ A. Pulkkinen, 2 B. Salzmann, G. Kremer, B. Hildebrand, K.Y. Ma, F.O. von Rohr, C.W. Nicholson, T. Jaouen, and C. Monney † Département de Physique and Fribourg Center for Nanomaterials, Université de Fribourg, CH-1700 Fribourg, Switzerland School of Engineering Science, LUT University, FI-53850, Lappeenranta, Finland Department of Chemistry, University of Zurich, CH-8057 Zurich, Switzerland Univ Rennes, CNRS, Institut de Physique de Rennes UMR 6251, F-35000 Rennes, France (Dated: July 6, 2021)
IrTe2 undergoes a series of charge-ordered phase transitions below room temperature that are char... more IrTe2 undergoes a series of charge-ordered phase transitions below room temperature that are characterized by the formation of stripes of Ir dimers of different periodicities. Full hemispherical X-ray photoelectron diffraction (XPD) experiments have been performed to investigate the atomic position changes undergone near the surface of 1T−IrTe2 in the first-order phase transition, from the (1 × 1) phase to the (5 × 1) phase. Comparison between experiment and simulation allows us to identify the consequence of the dimerization on the Ir atoms local environment. We report that XPD permits to unveil the break of symmetry of IrTe2 trigonal to a monoclonic unit cell and confirm the occurence of the (5 × 1) reconstruction within the first few layers below the surface with a staircase-like stacking of dimers.
Strain is ubiquitous in solid-state materials, but despite its fundamental importance and technol... more Strain is ubiquitous in solid-state materials, but despite its fundamental importance and technological relevance, leveraging externally applied strain to gain control over material properties is still in its infancy. In particular, strain control over the diverse phase transitions and topological states in two-dimensional transition metal dichalcogenides remains an open challenge. Here, we exploit uniaxial strain to stabilize the long-debated structural ground state of the 2D topological semimetal IrTe2, which is hidden in unstrained samples. Combined angle-resolved photoemission spectroscopy and scanning tunneling microscopy data reveal the strain-stabilized phase has a 6 × 1 periodicity and undergoes a Lifshitz transition, granting unprecedented spectroscopic access to previously inaccessible type-II topological Dirac states that dominate the modified inter-layer hopping. Supported by density functional theory calculations, we show that strain induces an Ir to Te charge transfer ...
We report on the synthesis and the superconductivity of Zr 4 Rh 2 O x (x = 0.4, 0.5, 0.6, 0.7, 1.... more We report on the synthesis and the superconductivity of Zr 4 Rh 2 O x (x = 0.4, 0.5, 0.6, 0.7, 1.0). These compounds crystallize in the η-carbide structure, which is a filled version of the complex intermetallic Ti 2 Ni structure. We find that in the system Zr 4 Rh 2 O x , already a small amount (x ≥ 0.4) of oxygen addition stabilizes the η-carbide structure over the more common intermetallic CuAl 2 structure-type, in which Zr 2 Rh crystallizes. We show that Zr 4 Rh 2 O 0.7 and Zr 4 Rh 2 O are bulk superconductors with critical temperatures of T c ≈ 2.8 K and 4.7 K in the resistivity, respectively. Our analysis of the superconducting properties reveal both compounds to be strongly type-II superconductors with critical fields up to µ 0 H c1 (0) ≈ 8.8 mT and µ 0 H c2 (0) ≈ 6.08 T. Our results support that the η-carbides are a versatile family of compounds for the investigation of the interplay of interstitial doping on physical properties, especially for superconductivity.
Transition metal compounds containing nickel and copper have been investigated as electrode mater... more Transition metal compounds containing nickel and copper have been investigated as electrode materials for energy storage. (Ni,Cu)(OH) 2 CO 3 and a-Ni(OH) 2 with different Ni/Cu ratios are synthesized by a simple hydrothermal method. All the samples show the similar dandelion-like morphology. It is demonstrated that the mole ratio of Ni to Cu is a key factor to influence the phase and electrochemical performances of the products. Carbonate anions will be intercalated into nickel copper layered double hydroxides with a hydrotalcite structure when the sample is nickel-rich, which will lead to much better electrochemical properties compared with the copper-rich samples with crystalline (Ni,Cu)(OH) 2 CO 3 structure. Among all the materials, sample (Ni 0.89 Cu 0.11) 2 (OH) 2 CO 3 can deliver the highest specific capacitance of 1017.3 F g À1 at 1 A g À1 and retain 68.5% of the original value after 4000 cycles at 5 A g À1. Meanwhile, an asymmetric capacitor which is assembled by using the optimized material as a positive electrode and activated carbon as a negative electrode exhibits a high energy density of 38.56 Wh kg À1 at a power density of 850.01 W kg À1 and a high power density of 8407.4 W kg À1 at an energy density of 21.7 Wh kg À1. Based on the above results, nickel copper carbonate hydroxides are of potential application for energy storage.
Highlights • Co-Fe LDH@NiO composite with shell-core structure was deposited on Ni foam. • The sh... more Highlights • Co-Fe LDH@NiO composite with shell-core structure was deposited on Ni foam. • The shell-core structure will generate high resistance between the interface of core and shell. • Electrochemical performance of the hybrid was improved at a low current density.
α-Form Co x Ni 1-x hydroxides with different Co/Ni ratios were synthesized by a chemical co-preci... more α-Form Co x Ni 1-x hydroxides with different Co/Ni ratios were synthesized by a chemical co-precipitation method under mild conditions. The effects of Co/Ni ratio on the structure, morphology, and supercapacitive properties of α-form Co x Ni 1-x hydroxides were investigated in detail. α-Form Co x Ni 1-x hydroxides had structures similar to those of hydrotalcite and α-Co(OH) 2 , and they exhibited better electrochemical performance than α-Co(OH) 2 on testing by XRD,
Supercapacitor with metal hydroxide nanosheets as electrode can have high capacitance. However, t... more Supercapacitor with metal hydroxide nanosheets as electrode can have high capacitance. However, the cycling stability and high rate capacity is low due to the low electrical conductivity. Here, the exfoliated α-Co(OH)2 nanosheets with high capacitance has been assembled on few-layer graphene with high electric conductivity by a facile yet effective and scalable solution method. Exfoliated hydrotalcite-like α-Co(OH)2 nanosheets and few-layer graphene suspensions were prepared by a simple ultrasonication in formamide and N-methyl-2-pyrrolidone, respectively. Subsequently, a hybrid was made by self-assembly of α-Co(OH)2 and few-layer graphene when the two dispersions were mixed at room temperature. The hybrid material provided a high specific capacitance of 567.1F/g at 1A/g, while a better rate capability and better stability were achieved compared to that mad of pristine and single exfoliated α-Co(OH)2. When the hybrid nanocomposite was used as a positive electrode and activated carbon was applied as negative electrode to assembly an asymmetric capacitor, an energy density of 21.2Wh/kg at a power density of 0.41kW/kg within a potential of 1.65V was delivered. The high electrochemical performance and facile solution-based synthesis method suggested that the hybrid of exfoliated α-Co(OH)2/few-layer graphene could be a potential electrode material for electrochemical capacitor.
Abstract Co-Fe layered double hydroxides (LDHs) with different Co/(Fe 2+ , Fe 3+ ) atomic ratios ... more Abstract Co-Fe layered double hydroxides (LDHs) with different Co/(Fe 2+ , Fe 3+ ) atomic ratios are fabricated via co-precipitation followed by an I 2 partial oxidation process. The initial Co/Fe ratio in solution is changed in a wide range and has a significant influence on the phase and structure of the product. Structural analysis confirms that Co-Fe LDHs with four Co/Fe molar ratios (2.35, 1.07, 0.58 and 0.24) shows a typical hydrotalcite-like structure and layered plate-like morphology. A high Fe content will destroy the hydrotalcite structure and lead to the formation of magnetite particles. Electrochemical data demonstrate that the specific capacitance of Co-Fe LDHs is strongly dependent on Co/Fe ratio. The Co-Fe LDHs with a high Co/Fe ratio of 2.35 can deliver the highest capacitance of 728 F g −1 at 1 A g −1 among them and when assembled into asymmetric supercapacitor with activate carbon as negative material, it can exhibit an energy density of 27.3 Wh kg −1 at a power density of 823.5 W kg −1 . The incorporation of Fe not only prevents the formation of β-Co(OH) 2 but also delivers a high capacitance. But, too much Fe is not preferred because it will lead to unstable structures and poor electrochemical performance due to the generation of iron compounds.
Nickel hydroxide nanosheet films are prepared via a hydrothermal treatment of nickel foam at a lo... more Nickel hydroxide nanosheet films are prepared via a hydrothermal treatment of nickel foam at a low temperature using Fe(NO 3) 3 as an oxidant without any templates and nickel salts. The as-formed Ni(OH) 2 nanosheets are vertically self-grown on the Ni foam substrate. They are interconnected and freestanding to form a homogeneously porous coating on the substrate with a thickness about 1 mm. The mass loading of Ni(OH) 2 film on nickel foam can be precisely determined by thermogravimetric analysis on base of phase transformation from nickel hydroxide to oxide. The mass loading of Ni(OH) 2 is strongly related with the concentration of Fe(NO 3) 3. The Ni(OH) 2 film grown on nickel foam can directly serve as additive-free electrodes and display a much high specific capacitance, 1100 F g À1 at 0.5 A g À1 , excellent rate-capability performance owing to fast electron and ion transport. The Ni(OH) 2 film can transform into NiO by annealing under a moderate temperature. However, the NiO film shows a low specific capacitance of 79 F g À1 at 0.5 A g À1. The preparation method concerns limited chemicals and produces little residual, implying a green chemical process to fabricate Ni(OH) 2 and NiO films on Ni substrate.
Hierarchical nickel sulfide (NiSx) hollow microspheres can be successfully synthesized with a tem... more Hierarchical nickel sulfide (NiSx) hollow microspheres can be successfully synthesized with a template-free method using α-Ni(OH)2 microspheres as a precursor by calcination and sulfidation. The intermediate Ni hollow spheres, formed at different calcination temperatures (300°C and 400°C) under H2/N2 atmosphere, can be easily transformed into NiSx with similar morphology and mixed phases of Ni3S2 and NiS during the followed sulfidation process. The formation processes for the hollow structure of NiSx are also discussed in this work. Particularly, the NiSx prepared from Ni intermediate spheres at 300°C show a high specific capacity of 153.6mAhg-1 at 0.5Ag-1 at high mass loading due to its small crystal size, hierarchically porous structure and high electrical conductivity. A hybrid capacitor was assembled by using it as positive electrode and activated carbon as negative electrode to examine their practical applications in a full-cell configuration. The hybrid capacitor exhibited excellent comprehensive performances in 1.6V. The hybrid capacitor also showed good cycling stability, with 81.25% of the initial specific capacitance after 1000 cycles at 2Ag-1. Above results indicate the great potential of the NiSx hollow spheres as a promising electrode material for supercapacitor applications.
The upper critical field is a fundamental measure of the strength of superconductivity in a mater... more The upper critical field is a fundamental measure of the strength of superconductivity in a material. It is also a cornerstone for the realization of superconducting magnet applications. The critical field arises because of the Cooper pair breaking at a limiting field, which is due to the Pauli paramagnetism of the electrons. The maximal possible magnetic field strength for this effect is commonly known as the Pauli paramagnetic limit given as μ 0 H Pauli ≈ 1.86[T/K]•T c for a weakcoupling Bardeen−Schrieffer−Cooper (BCS) superconductor. The violation of this limit is only rarely observed. Exceptions include some low-temperature heavy Fermion and some strongly anisotropic superconductors. Here, we report on the superconductivity at 9.75 K in the centrosymmetric, cubic η-carbidetype compound Nb 4 Rh 2 C 1−δ , with a normalized specific heat jump of ΔC/γT c = 1.64. We find that this material has a remarkably high upper critical field of μ 0 H c2 (0) = 28.5 T, which is exceeding by far its weak-coupling BCS Pauli paramagnetic limit of μ 0 H Pauli = 18.1 T. Determination of the origin and consequences of this effect will represent a significant new direction in the study of critical fields in superconductors.
The Ti 2 Ni and the related η-carbide structure are known to exhibit various intriguing physical ... more The Ti 2 Ni and the related η-carbide structure are known to exhibit various intriguing physical properties. The Ti 2 Ni structure with the cubic space group F d3m is surprisingly complex, consisting of a unit cell with 96 metal atoms. The related η-carbide compounds correspond to a filled version of the Ti 2 Ni structure. Here, we report on the structure and superconductivity in the η-carbide type suboxides Ti 4 M 2 O with M = Co, Rh, Ir. We have successfully synthesized all three compounds in single phase form. We find all three compounds to be type-II bulk superconductors with transition temperatures of T c = 2.7, 2.8, and 5.4 K, and with normalized specific heat jumps of ∆C/γT c = 1.65, 1.28, and 1.80 for Ti 4 Co 2 O, Ti 4 Rh 2 O, and Ti 4 Ir 2 O, respectively. We find that all three superconductors, exhibit high upper-critical fields. Particularly noteworthy is in this regard Ti 4 Ir 2 O with an upper critical field of µ 0 H c2 (0) = 16.06 T, which exceeds by far the weak-coupling Pauli limit-widely consider as the maximal upper critical field-of µ 0 H Pauli = 9.86 T. The role of the void filling light atom X has so far been uncertain for the overall physical properties of these materials. Herein, we have successfully grown single crystals of Ti 2 Co. In contrast to the metallic η-carbide type suboxides Ti 4 M 2 O, we find that Ti 2 Co displays a semimetallic behavior down to 0.75 K. Below 0.75 K we observe a broad decrease in the resistivity, which can most likely be attributed to an onset of a superconducting transition at lower temperatures. Hence, the octahedral void-filling oxygen plays a crucial role for the overall physical properties, even though its effect on the crystal structure is small. Our results indicate that the design of new superconductors by incorporation of electron-acceptor atoms may in the Ti 2 Ni-type structures and other materials with crystallographic void position be a promising future approach. The remarkably high upper critical fields, in this family of compounds, may furthermore spark significant future interest.
In the transition metal dichalcogenide IrTe$_2$, low-temperature charge-ordered phase transitions... more In the transition metal dichalcogenide IrTe$_2$, low-temperature charge-ordered phase transitions lead to the occurrence of stripe phases of different periodicities and nearly degenerate energies. Bulk-sensitive measurements have shown that, upon cooling, IrTe$_2$ undergoes two such first-order phase transitions at T$_{c_1}=270$~K and T$_{c_2}=180$~K. Here, using surface sensitive probes of the electronic structure of IrTe$_2$, we show that, in addition, another \textit{first-order} transition occurs at the surface at T$_{c_3}=165$~K. By performing measurements over a full thermal cycle, we also reveal the complete hysteresis of all these phases.
M. Rumo, ∗ A. Pulkkinen, 2 B. Salzmann, G. Kremer, B. Hildebrand, K.Y. Ma, F.O. von Rohr, C.W. Ni... more M. Rumo, ∗ A. Pulkkinen, 2 B. Salzmann, G. Kremer, B. Hildebrand, K.Y. Ma, F.O. von Rohr, C.W. Nicholson, T. Jaouen, and C. Monney † Département de Physique and Fribourg Center for Nanomaterials, Université de Fribourg, CH-1700 Fribourg, Switzerland School of Engineering Science, LUT University, FI-53850, Lappeenranta, Finland Department of Chemistry, University of Zurich, CH-8057 Zurich, Switzerland Univ Rennes, CNRS, Institut de Physique de Rennes UMR 6251, F-35000 Rennes, France (Dated: July 6, 2021)
IrTe2 undergoes a series of charge-ordered phase transitions below room temperature that are char... more IrTe2 undergoes a series of charge-ordered phase transitions below room temperature that are characterized by the formation of stripes of Ir dimers of different periodicities. Full hemispherical X-ray photoelectron diffraction (XPD) experiments have been performed to investigate the atomic position changes undergone near the surface of 1T−IrTe2 in the first-order phase transition, from the (1 × 1) phase to the (5 × 1) phase. Comparison between experiment and simulation allows us to identify the consequence of the dimerization on the Ir atoms local environment. We report that XPD permits to unveil the break of symmetry of IrTe2 trigonal to a monoclonic unit cell and confirm the occurence of the (5 × 1) reconstruction within the first few layers below the surface with a staircase-like stacking of dimers.
Strain is ubiquitous in solid-state materials, but despite its fundamental importance and technol... more Strain is ubiquitous in solid-state materials, but despite its fundamental importance and technological relevance, leveraging externally applied strain to gain control over material properties is still in its infancy. In particular, strain control over the diverse phase transitions and topological states in two-dimensional transition metal dichalcogenides remains an open challenge. Here, we exploit uniaxial strain to stabilize the long-debated structural ground state of the 2D topological semimetal IrTe2, which is hidden in unstrained samples. Combined angle-resolved photoemission spectroscopy and scanning tunneling microscopy data reveal the strain-stabilized phase has a 6 × 1 periodicity and undergoes a Lifshitz transition, granting unprecedented spectroscopic access to previously inaccessible type-II topological Dirac states that dominate the modified inter-layer hopping. Supported by density functional theory calculations, we show that strain induces an Ir to Te charge transfer ...
We report on the synthesis and the superconductivity of Zr 4 Rh 2 O x (x = 0.4, 0.5, 0.6, 0.7, 1.... more We report on the synthesis and the superconductivity of Zr 4 Rh 2 O x (x = 0.4, 0.5, 0.6, 0.7, 1.0). These compounds crystallize in the η-carbide structure, which is a filled version of the complex intermetallic Ti 2 Ni structure. We find that in the system Zr 4 Rh 2 O x , already a small amount (x ≥ 0.4) of oxygen addition stabilizes the η-carbide structure over the more common intermetallic CuAl 2 structure-type, in which Zr 2 Rh crystallizes. We show that Zr 4 Rh 2 O 0.7 and Zr 4 Rh 2 O are bulk superconductors with critical temperatures of T c ≈ 2.8 K and 4.7 K in the resistivity, respectively. Our analysis of the superconducting properties reveal both compounds to be strongly type-II superconductors with critical fields up to µ 0 H c1 (0) ≈ 8.8 mT and µ 0 H c2 (0) ≈ 6.08 T. Our results support that the η-carbides are a versatile family of compounds for the investigation of the interplay of interstitial doping on physical properties, especially for superconductivity.
Transition metal compounds containing nickel and copper have been investigated as electrode mater... more Transition metal compounds containing nickel and copper have been investigated as electrode materials for energy storage. (Ni,Cu)(OH) 2 CO 3 and a-Ni(OH) 2 with different Ni/Cu ratios are synthesized by a simple hydrothermal method. All the samples show the similar dandelion-like morphology. It is demonstrated that the mole ratio of Ni to Cu is a key factor to influence the phase and electrochemical performances of the products. Carbonate anions will be intercalated into nickel copper layered double hydroxides with a hydrotalcite structure when the sample is nickel-rich, which will lead to much better electrochemical properties compared with the copper-rich samples with crystalline (Ni,Cu)(OH) 2 CO 3 structure. Among all the materials, sample (Ni 0.89 Cu 0.11) 2 (OH) 2 CO 3 can deliver the highest specific capacitance of 1017.3 F g À1 at 1 A g À1 and retain 68.5% of the original value after 4000 cycles at 5 A g À1. Meanwhile, an asymmetric capacitor which is assembled by using the optimized material as a positive electrode and activated carbon as a negative electrode exhibits a high energy density of 38.56 Wh kg À1 at a power density of 850.01 W kg À1 and a high power density of 8407.4 W kg À1 at an energy density of 21.7 Wh kg À1. Based on the above results, nickel copper carbonate hydroxides are of potential application for energy storage.
Highlights • Co-Fe LDH@NiO composite with shell-core structure was deposited on Ni foam. • The sh... more Highlights • Co-Fe LDH@NiO composite with shell-core structure was deposited on Ni foam. • The shell-core structure will generate high resistance between the interface of core and shell. • Electrochemical performance of the hybrid was improved at a low current density.
α-Form Co x Ni 1-x hydroxides with different Co/Ni ratios were synthesized by a chemical co-preci... more α-Form Co x Ni 1-x hydroxides with different Co/Ni ratios were synthesized by a chemical co-precipitation method under mild conditions. The effects of Co/Ni ratio on the structure, morphology, and supercapacitive properties of α-form Co x Ni 1-x hydroxides were investigated in detail. α-Form Co x Ni 1-x hydroxides had structures similar to those of hydrotalcite and α-Co(OH) 2 , and they exhibited better electrochemical performance than α-Co(OH) 2 on testing by XRD,
Supercapacitor with metal hydroxide nanosheets as electrode can have high capacitance. However, t... more Supercapacitor with metal hydroxide nanosheets as electrode can have high capacitance. However, the cycling stability and high rate capacity is low due to the low electrical conductivity. Here, the exfoliated α-Co(OH)2 nanosheets with high capacitance has been assembled on few-layer graphene with high electric conductivity by a facile yet effective and scalable solution method. Exfoliated hydrotalcite-like α-Co(OH)2 nanosheets and few-layer graphene suspensions were prepared by a simple ultrasonication in formamide and N-methyl-2-pyrrolidone, respectively. Subsequently, a hybrid was made by self-assembly of α-Co(OH)2 and few-layer graphene when the two dispersions were mixed at room temperature. The hybrid material provided a high specific capacitance of 567.1F/g at 1A/g, while a better rate capability and better stability were achieved compared to that mad of pristine and single exfoliated α-Co(OH)2. When the hybrid nanocomposite was used as a positive electrode and activated carbon was applied as negative electrode to assembly an asymmetric capacitor, an energy density of 21.2Wh/kg at a power density of 0.41kW/kg within a potential of 1.65V was delivered. The high electrochemical performance and facile solution-based synthesis method suggested that the hybrid of exfoliated α-Co(OH)2/few-layer graphene could be a potential electrode material for electrochemical capacitor.
Abstract Co-Fe layered double hydroxides (LDHs) with different Co/(Fe 2+ , Fe 3+ ) atomic ratios ... more Abstract Co-Fe layered double hydroxides (LDHs) with different Co/(Fe 2+ , Fe 3+ ) atomic ratios are fabricated via co-precipitation followed by an I 2 partial oxidation process. The initial Co/Fe ratio in solution is changed in a wide range and has a significant influence on the phase and structure of the product. Structural analysis confirms that Co-Fe LDHs with four Co/Fe molar ratios (2.35, 1.07, 0.58 and 0.24) shows a typical hydrotalcite-like structure and layered plate-like morphology. A high Fe content will destroy the hydrotalcite structure and lead to the formation of magnetite particles. Electrochemical data demonstrate that the specific capacitance of Co-Fe LDHs is strongly dependent on Co/Fe ratio. The Co-Fe LDHs with a high Co/Fe ratio of 2.35 can deliver the highest capacitance of 728 F g −1 at 1 A g −1 among them and when assembled into asymmetric supercapacitor with activate carbon as negative material, it can exhibit an energy density of 27.3 Wh kg −1 at a power density of 823.5 W kg −1 . The incorporation of Fe not only prevents the formation of β-Co(OH) 2 but also delivers a high capacitance. But, too much Fe is not preferred because it will lead to unstable structures and poor electrochemical performance due to the generation of iron compounds.
Nickel hydroxide nanosheet films are prepared via a hydrothermal treatment of nickel foam at a lo... more Nickel hydroxide nanosheet films are prepared via a hydrothermal treatment of nickel foam at a low temperature using Fe(NO 3) 3 as an oxidant without any templates and nickel salts. The as-formed Ni(OH) 2 nanosheets are vertically self-grown on the Ni foam substrate. They are interconnected and freestanding to form a homogeneously porous coating on the substrate with a thickness about 1 mm. The mass loading of Ni(OH) 2 film on nickel foam can be precisely determined by thermogravimetric analysis on base of phase transformation from nickel hydroxide to oxide. The mass loading of Ni(OH) 2 is strongly related with the concentration of Fe(NO 3) 3. The Ni(OH) 2 film grown on nickel foam can directly serve as additive-free electrodes and display a much high specific capacitance, 1100 F g À1 at 0.5 A g À1 , excellent rate-capability performance owing to fast electron and ion transport. The Ni(OH) 2 film can transform into NiO by annealing under a moderate temperature. However, the NiO film shows a low specific capacitance of 79 F g À1 at 0.5 A g À1. The preparation method concerns limited chemicals and produces little residual, implying a green chemical process to fabricate Ni(OH) 2 and NiO films on Ni substrate.
Hierarchical nickel sulfide (NiSx) hollow microspheres can be successfully synthesized with a tem... more Hierarchical nickel sulfide (NiSx) hollow microspheres can be successfully synthesized with a template-free method using α-Ni(OH)2 microspheres as a precursor by calcination and sulfidation. The intermediate Ni hollow spheres, formed at different calcination temperatures (300°C and 400°C) under H2/N2 atmosphere, can be easily transformed into NiSx with similar morphology and mixed phases of Ni3S2 and NiS during the followed sulfidation process. The formation processes for the hollow structure of NiSx are also discussed in this work. Particularly, the NiSx prepared from Ni intermediate spheres at 300°C show a high specific capacity of 153.6mAhg-1 at 0.5Ag-1 at high mass loading due to its small crystal size, hierarchically porous structure and high electrical conductivity. A hybrid capacitor was assembled by using it as positive electrode and activated carbon as negative electrode to examine their practical applications in a full-cell configuration. The hybrid capacitor exhibited excellent comprehensive performances in 1.6V. The hybrid capacitor also showed good cycling stability, with 81.25% of the initial specific capacitance after 1000 cycles at 2Ag-1. Above results indicate the great potential of the NiSx hollow spheres as a promising electrode material for supercapacitor applications.
The upper critical field is a fundamental measure of the strength of superconductivity in a mater... more The upper critical field is a fundamental measure of the strength of superconductivity in a material. It is also a cornerstone for the realization of superconducting magnet applications. The critical field arises because of the Cooper pair breaking at a limiting field, which is due to the Pauli paramagnetism of the electrons. The maximal possible magnetic field strength for this effect is commonly known as the Pauli paramagnetic limit given as μ 0 H Pauli ≈ 1.86[T/K]•T c for a weakcoupling Bardeen−Schrieffer−Cooper (BCS) superconductor. The violation of this limit is only rarely observed. Exceptions include some low-temperature heavy Fermion and some strongly anisotropic superconductors. Here, we report on the superconductivity at 9.75 K in the centrosymmetric, cubic η-carbidetype compound Nb 4 Rh 2 C 1−δ , with a normalized specific heat jump of ΔC/γT c = 1.64. We find that this material has a remarkably high upper critical field of μ 0 H c2 (0) = 28.5 T, which is exceeding by far its weak-coupling BCS Pauli paramagnetic limit of μ 0 H Pauli = 18.1 T. Determination of the origin and consequences of this effect will represent a significant new direction in the study of critical fields in superconductors.
Uploads
Papers by Keyuan Ma