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IrTe$_2$ 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 $1T-$IrTe$_2$ in the first-order phase transition, from the $(1\times1)$ phase to the $(5\times1)$ 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 IrTe$_2$ trigonal to a monoclonic unit cell and confirm the occurence of the $(5\times1)$ reconstruction within the first few layers below the surface with a staircase-like stacking of dimers., Comment: This is the version of the article before peer review or editing, as submitted by M. Rumo to J. Phys.: Condens. Matter. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at DOI

The Ti$_2$Ni and the related $\eta$-carbide structure are known to exhibit various intriguing physical properties. The Ti$_2$Ni structure with the cubic space group $Fd\bar{3}m$ is surprisingly complex, consisting of a unit cell with 96 metal atoms. The related $\eta$-carbide compounds correspond to a filled version of the Ti$_2$Ni structure. Here, we report on the structure and superconductivity in the $\eta$-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 $T_{\rm c}$ = 2.7, 2.8, and 5.4 K, and with normalized specific heat jumps of $\Delta C/\gamma T_{\rm c}$ = 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 Ti$_4$Ir$_2$O with an upper critical field of $\mu_0 H_{\rm c2}{\rm (0)}$ =~16.06~T, which exceeds by far the weak-coupling Pauli limit of 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 $\eta$-carbide type suboxides Ti$_4$M$_2$O, we find that Ti$_2$Co displays a semimetallic behavior. 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., Comment: Submitted: 3. August 2021, published: 1. November 2021, Supporting Information at publisher's page: https://pubs.acs.org/doi/suppl/10.1021/acs.chemmater.1c02683/suppl_file/cm1c02683_si_001.pdf

IrTe

We present a combined angle-resolved photoemission spectroscopy and low-energy electron diffraction (LEED) study of the prominent transition metal dichalcogenide ${\mathrm{IrTe}}_{2}$ upon potassium (K) deposition on its surface. Pristine ${\mathrm{IrTe}}_{2}$ 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. Supported by density functional theory calculations, we first show that the K atoms dope the topmost ${\mathrm{IrTe}}_{2}$ layer with electrons, therefore strongly decreasing the work function and shifting only the electronic surface states towards higher binding energy. We then follow the evolution of its electronic structure as a function of temperature across the charge-ordered phase transitions and observe that their critical temperatures are unchanged for K coverages of 0.13 and 0.21 monolayer. Using LEED we also confirm that the periodicity of the related stripe phases is unaffected by the K doping. We surmise that the charge-ordered phase transitions of ${\mathrm{IrTe}}_{2}$ are robust against electron surface doping, because of its metallic nature at all temperatures, and due to the importance of structural effects in stabilizing charge order in ${\mathrm{IrTe}}_{2}$.

Pyrite, also known as fool's gold is the thermodynamic stable polymorph of FeS2. It is widely considered as a promising d-band semiconductor for various applications due to its intriguing physical properties. Marcasite is the other naturally occurring polymorph of FeS2. Measurements on natural crystals have shown that it has similarly promising electronic, mechanical, and optical properties as pyrite. However, it has been only scarcely investigated so far, because the laboratory-based synthesis of phase-pure samples or high quality marcasite single crystal has been a challenge until now. Here, we report the targeted phase formation via hydrothermal synthesis of marcasite and pyrite. The formation condition and phase purity of the FeS2 polymorphs are systematically studied in the form of a comprehensive synthesis map. We, furthermore, report on a detailed analysis of marcasite single crystal growth by a space-separated hydrothermal synthesis. We observe that single phase product of marcasite forms only on the surface under the involvement of H2S and sulphur vapor. The availability of high-quality crystals of marcasite allows us to measure the fundamental physical properties, including an allowed direct optical bandgap of 0.76 eV, temperature independent diamagnetism, an electronic transport gap of 0.11 eV, and a room-temperature carrier concentration of 4.14 × 1018 cm−3. X-ray absorption/emission spectroscopy are employed to measure the band gap of the two FeS2 phases. We find marcasite has a band gap of 0.73 eV, while pyrite has a band gap of 0.87 eV. Our results indicate that marcasite – that is now synthetically available in a straightforward fashion – is as equally promising as pyrite as candidate for various semiconductor applications based on earth abundant elements., Pyrite, also known as fool's gold is the thermodynamic stable polymorph of FeS2.

We report on the synthesis and the superconductivity in 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 c 1 (0) ≈ 8.8 mT and μ 0 H c 2 (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 materials for energy storage. (Ni,Cu)(OH)2CO3 and α-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)2CO3 structure. Among all the materials, sample (Ni0.89Cu0.11)2(OH)2CO3 can deliver the highest specific capacitance of 1017.3 F g−1 at 1 A g−1 and retain 68.5% of the original value after 4000 cycles at 5 A g−1. 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−1 at a power density of 850.01 W kg−1 and a high power density of 8407.4 W kg−1 at an energy density of 21.7 Wh kg−1. Based on the above results, nickel copper carbonate hydroxides are of potential application for energy storage.

Pyrite, also known as fool's gold is the thermodynamic stable polymorph of FeS$_2$. It is widely considered as a promising d-band semiconductor for various applications due to its intriguing physical properties. Marcasite is the other naturally occurring polymorph of FeS$_2$. Measurements on natural crystals have shown that it has similarly promising electronic, mechanical, and optical properties as pyrite. However, it has been only scarcely investigated so far, because the laboratory-based synthesis of phase-pure samples or high-quality marcasite single crystal has been a challenge until now. Here, we report the targeted phase formation via hydrothermal synthesis of marcasite and pyrite. The formation condition and phase purity of the FeS$_2$ polymorphs are systematically studied in the form of a comprehensive synthesis map. We, furthermore, report on a detailed analysis of marcasite single crystal growth by a space-separated hydrothermal synthesis. We observe that single phase product of marcasite forms only on the surface under the involvement of H$_2$S and sulphur vapor. The availability of high-quality crystals of marcasite allows us to measure the fundamental physical properties, including an allowed direct optical bandgap of 0.76 eV, temperature independent diamagnetism, an electronic transport gap of 0.11 eV, and a room-temperature carrier concentration of 4.14 $\times$ 10$^{18}$ cm$^{-3}$. X-ray absorption/emission spectroscopy are employed to measure the band gap of the two FeS$_2$ phases. We find marcasite has a band gap of 0.73 eV, while pyrite has a band gap of 0.87 eV. Our results indicate that marcasite -- that is now synthetically available in a straightforward fashion -- is as equally promising as pyrite as candidate for various semiconductor applications based on earth abundant elements., Comment: Supporting Information available at publisher's page: https://www.rsc.org/suppdata/d1/sc/d1sc03026d/d1sc03026d1.pdf

CoO microspheres and metallic Co could be successfully synthesized by simply reacting cobalt acetate with a mixture solvent of ethylene glycol and deionized water in a hydrothermal process for different times. As the reaction proceeded, α-Co(OH)2, CoO and metallic Co were produced. To understand the phase evolution processes from α-Co(OH)2 to CoO and then metallic Co, a range of time-dependent experiments were carried out, and the intermediate products obtained at different reaction times were investigated in detail. The investigation revealed that CoO microspheres were actually evolved from α-Co(OH)2 as a precursor. Just elongating the reaction time, CoO microspheres could be further reduced to metallic Co. With a pure ethylene glycol medium for the same reaction, only α-Co(OH)2 could be generated, indicating an important role of water. When the obtained CoO microspheres were used as anode materials for lithium-ion batteries, they delivered a specific capacity of 803 mA h g−1 at 0.1 A g−1 with a retention of 453 mA h g−1 after 70 cycles. Meanwhile, the magnetic properties of the obtained CoO microspheres and metallic Co were investigated, with the CoO microspheres showing an antiferromagnetic behavior and the metallic Co exhibiting ferromagnetic characteristics. This study suggested a novel method for synthesizing CoO with a uniform microsphere morphology and bulk metallic Co easily.

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.

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.

Hybrid materials with three-dimensional hierarchical architectures have drawn considerable attention in the field of energy storage and conversion owing to the improved electrolyte-accessible surface area and shortened ion transfer paths. In this work, a novel hierarchical Co-Fe LDH@NiO composite with shell-core rod arrays grown on the surface of Ni foam is presented for the use of electrode material without any organic binders and conductive agents. It is found that the expected good conductivity, large electrolyte-accessible surface area and sufficient active sites cannot be well achieved at the same time. Owing to the facile electrolyte diffusion paths and large exposed surface area, Co-Fe LDH@NiO with 46.06% Co-Fe LDH has a specific capacity of 361 C g −1 at 1 A g −1 . However, at high current densities, its performance decreases quite dramatically due to the poor electrical conductivity of NiO support and the high interface contact resistance between NiO microrods and Co-Fe LDH nanosheets. The assembled hybrid supercapacitor based on Co-Fe LDH@NiO - Ni and activated carbon electrodes can deliver an energy density of 22 Wh kg −1 at the power density of 800 W kg −1 , showing a promising potential in energy storage and conversion.

We demonstrate the fabrication of a core-shell structure composite of Co–Fe layered double hydroxides (LDHs) on the surface of carbon fiber cloth (CFC). The Co–Fe LDHs nanoplates are vertically anchored onto the surface of electrical conductive CFC via a simple hydrothermal process followed by an I2 oxidation process. The hexagonal Co–Fe LDHs nanoplates on the surfaces of carbon fibers form a porous network film. The carbon fibers with high conductivity can act as conductive cores to support a large number of Co–Fe LDHs nanoplates to deliver a high specific capacitance of 774 F g−1 at the current density of 1 A g−1. An assembled asymmetric capacitor by using Co–Fe LDHs-CFC as positive electrode material and activated carbon as negative electrode material can deliver an energy density of 16.1 Wh kg−1 at the power density of 399.7 W kg−1 with a good cycling stability. This study provides a way to combine Co–Fe LDHs with carbon materials to obtain a composite that can deliver an improved performance for electrochemical capacitor application.

Nickel hydroxide nanosheet films are prepared via a hydrothermal treatment of nickel foam at a low temperature using Fe(NO3)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 inter-connected and free-standing to form a homogeneously porous coating on the substrate with a thickness about 1 μm. 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(NO3)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−1 at 0.5 A g−1, 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−1 at 0.5 A g−1. 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.

CoO microspheres and metallic Co could be successfully synthesized by simply reacting cobalt acetate with a mixture solvent of ethylene glycol and deionized water in a hydrothermal process for different times. As the reaction proceeded, α-Co(OH)

α-Form CoxNi1–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 CoxNi1–x hydroxides were investigated in detail. α-Form CoxNi1–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, FTIR, and electrochemical performance (when x ≠ 0). The highest specific capacitance at 1 A g–1 for α-form CoxNi1–x hydroxides (for x = 0.2) is 1696.7 F g–1. An asymmetric supercapacitor packaged with Co0.6Ni0.4 hydroxide and activated carbon as positive and negative material, respectively, had an energy density of 23.5 Wh kg–1 at a power density of 412.5 W kg–1.

A novel hydrothermal process is demonstrated to prepare acetate anions intercalated α-Co(OH)2 that can be delaminated in water without any additional anion exchange processes. Positively charged Co(OH)2 nanosheets with lateral size of hundreds of nanometers and thickness less than 2 nm can be obtained by dispersing the as-obtained α-Co(OH)2 into water followed by sonication. The exfoliated Co(OH)2 nanosheets can be restacked into its original structure with different interlayer d-spacings. A flexible free-standing film with stacking Co(OH)2 nanosheets and graphene oxide (GO) layers can be obtained through flocculation of the Co(OH)2 nanosheets with GO nanosheets suspensions followed by a vacuum filtration, but the content of Co(OH)2 has to be kept under a low value so as to obtain films with flexible nature. Electrochemical tests show that this kind of film is not suitable to be used as electrode material for supercapacitor and lithium ion battery, because the content of active material is not high and the compacted junction between opposite charged nanosheets will prevent the electrolyte from diffusing into the interlayer space.