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Two new chalcohalides La6Cd0.75Ga2Q11.5Cl2.5 (Q = S and Se) with mixed-anionic functional groups were synthesized through a high temperature reaction using LaCl3 as reaction-flux. Both compounds crystalize in the non-centrosymmetric space group P63 with highly oriented tetrahedron [GaQ4] and one-dimension chain [CdQ/Cl6] (Q = S and Se). The La6Cd0.75Ga2S11.5Cl2.5 shows a large laser-induced damage threshold (LIDT) (18.6 × AgGaS2) and strong second harmonic generation (SHG) response (0.8 × AgGaS2). These results indicate that the introduction of halogens into chalcogenides is beneficial to achieve large LIDTs and SHG response.

2M-WS2 as a new phase of transition metal dichalcogenides possesses many novel physical properties, such as superconductivity and topological surface states. The effect of n-type doping on the superconductivity of this material has been studied. However, p-type doping has not been studied, because it is difficult to implement p-type doping in metastable 2M-WS2. In this paper, p-type doping was achieved in 2M-WS2 for the first time by using Mo. With the increase of the Mo content, the carrier concentration rises slightly from 1.42 × 1021 cm-1 to 1.56 × 1021 cm-1. Meanwhile, the superconducting transition temperature decreases monotonously with the increase of Mo doping and reaches a minimum value of 4.37 K at the doping limit of x = 0.6 in W1-xMoxS2. Combining the data of n-type doped 2M-WS2 from our previous research, we summarize the carrier concentration and superconducting transition temperature in a phase diagram, which shows a typical dome-like shape. These results uncover the relationship between the carrier concentration and electronic state of 2M-WS2.

Chevrel-phase superconducting compounds AyMo6Q8 (A = Pb, Mg, etc. and Q = S, Se, Te) have attracted much attention due to their ultrahigh upper critical magnetic fields (Hc2). However, the binary c...

A new mid-infrared nonlinear optical oxyselenide compound, Sr4Pb1.5Sb5O5Se8, was successfully synthesized by traditional solid-state reaction. It crystallizes in the noncentrosymmetric space group Cm (No. 8) of the monoclinic system. The crystal structure is composed of edge-sharing quasi-2D 2∞[Pb1.5Sb3Se8]4− zigzag layers with Sr2+ ions and anionic 1∞[SbO2.5]2− pseudo-chains separating the adjacent layers. The compound has good thermal stability up to approximately 1000 K which is beneficial for nonlinear optical applications. The sharp down signal of the second harmonic generation (SHG) response measured with a 2.09 μm laser in the reflection mode reveals its nonlinear optical properties. The new compound Sr4Pb1.5Sb5O5Se8 provides some guidance to design new mid-infrared nonlinear optical materials.

Transition metal dichalcogenides and carbonitrides (TMDs and MXenes) have attracted great attention in electrochemistry due to their tunable electronic structures. Herein, a new compound of Nb2Se2C is designed as a "TMD-MXene"-like material. It exhibits better oxygen evolution reaction performance than most other reported TMDs, MXenes, and commercial electrocatalysts due to the enriched active sites and excellent conductivity.

Black TiO2−x is an attractive material due to its adjustable band structure, useful in different applications, and has been heavily investigated and developed. The current reduction of TiO2 involves a process from surface to center: thus, the issue of either insufficient or excessive reduction seems inevitable. To the best of our knowledge, it is rarely reported that a homogeneous defect distribution can be facilely achieved inside black TiO2−x. In this study, a bulk-reduced rutile TiO2−x was obtained via a circuitous two-step approach, with an intermediate Magneli phase (TinO2n−1). With the decomposition of a solid atmosphere creator (e.g. KClO4 grains) at a high temperature, the missing oxygen ions in Ti4O7 could be replenished quantitatively. The TEM and SAED results reveal that the oxidation process is not just a surface reaction, but it involves reverse displacement or structural rearrangement inside the crystal. In particular, the periodic variation of crystallographic sheer planes was the direct evidence of the above bulk reaction, i.e. unified long period for Ti4O7 and varied long period for critical ratio sample. The as-prepared samples showed different band gaps and colors, based on their oxygen content. KClO4, with a critical mixing ratio of around 17.5 mol%, could almost oxidize Ti4O7 to black rutile TiO2−x. Furthermore, the sample formed at the critical ratio showed a considerable photo-thermal response speed of 1.7 °C s−1 and an equilibrium temperature of up to 70 °C under irradiation with light above 400 nm, which can be an evidence for the enhancement of non-intrinsic absorption. In short, this study provides a new route for the controllable preparation of black TiO2−x, and a possibility for further development of other solid atmosphere creators.

The high efficiency chalcopyrite tandem cell with two solar absorbers of CuGaS2 and CuInSe2 remains difficult to be fabricated, but intermediate band chalcopyrite is practical to be developed. Herein, the Fe incorporation into the Ga site of chalcopyrite is successful to induce obvious solar multiband absorption. Three intermediate bands are evidenced by the UV–Vis–NIR absorption spectra, which is confirmed by first principle density functional theory (DFT) calculations. These intermediates bands improve solar energy conversion by absorbing more photons in several solar energy ranges including 2.21–1.48 eV, 1.48–0.73 eV, and around 0.57 eV to enhance solar thermal collection and photocatalytic degradation. The consistent nature of the intermediate bands suggested by theoretical analyses mainly are composed of the nondegenerate the eg and t2g states of Fe-3d orbitals.

Black TiO2-x attracts enormous attention due to its large solar absorption and improved conductivity. In this work, a novel structure of TiO2-x with conductive TiO layer, performing full-spectrum absorption, was synthesized in one step by the unforeseen dismutation reaction of titanium sub-oxides (Tin O2n-1 ) in ammonium halide atmosphere. For this new reaction, a possible mechanism of decomposition-etching-disproportionation-rehydrolysis process was proposed. The vital intermediate reactant TiCl4 , which verifies the assumption, has been captured in the form of (NH4 )2 TiCl6 , especially where Ti2 O3 is the reactant. Furthermore, this work not only can nominate TiO as an alternative for noble metals or carbon materials in the aim to improve the electron conductivity and solar absorption of black TiO2-x , which are important in electrochemistry and optoelectronics fields, but also can be a new route to synthesize special structures for other multivalent transition metals.

Chemical vapor deposition (CVD) method generally used for the fabrication of single-crystal graphene is still imperfect in the suppression of nucleation density, which is detrimental to the domain size of single-crystal graphene to a great extent. Herein, we have successfully developed a new strategy to realize the controllable nucleation by the plasma treatment of Cu foil, and carry out the rapid growth of single-crystal graphene, combined with the program heating and concentration gradient growth method during the CVD-grown stage. The plasma treatment can effectively remove impurities of Cu foil surface, which reduces the active sites for single-crystal graphene nucleation. Meanwhile, CuO nanoparticles are formed on the Cu foil surface by the plasma treatment, which can be reduced and produce oxygen after annealing at high temperature in H2 atmosphere to promote the rapid growth of graphene and suppress graphene nucleation. Eventually, we have achieved the synthesis of hexagon-shaped single-crystal graphene with the low defects, large-size of ∼5 mm, fast growth rate of 2.5 mm h−1, high uniformity (monolayer coverage of ≥90%) and high field-effect mobility of ≥12,500 cm2 V−1 s−1. Our work provides a facile and effective method for the fabrication of large-size single-crystal graphene, paving the way towards future potential applications of single-crystal graphene in the industrial level.

2M-WS

Transition metal dichalcogenides and carbonitrides (TMDs and MXenes) have attracted great attention in electrochemistry due to their tunable electronic structures. Herein, a new compound of Nb2Se2C is designed as a "TMD-MXene"-like material. It exhibits better oxygen evolution reaction performance than most other reported TMDs, MXenes, and commercial electrocatalysts due to the enriched active sites and excellent conductivity.

Chevrel-phase superconducting compounds A

Multiband materials have received increasing attention due to their superior solar absorption properties. Here we design a new multiband compound, namely Ba5Bi2Co2S10, which crystallizes in the space group C22h-P21/m (No. 11) of the monoclinic system. Ba5Bi2Co2S10 is composed of one-dimensional 1∞[Bi2Co2S10]10- chains along the a axis. The adjacent chains are separated by Ba2+ ions. The optical band gap of the compound is 1.05 eV and 0.74 eV, presenting typical multi-absorption characteristics. First-principles calculations, which are perfectly consistent with the experimental results, show that the hybrid coupling effect between Co and S gives rise to multiband characteristics. Evident optoelectronic conversion properties were observed under visible light illumination with a photocurrent density of 4.0 mA cm-2 at 1 V.

The 2H molybdenum disulfide (MoS2 ), as a stable hexagonal phase, has been one of the most studied transition metal dichalcogenides over the past decades. In the last five years, the metastable phases of MoS2 (1T, 1T', 1T'', and 1T''') have seen a revival of interests. Different from the edge-sharing [MoS6 ] trigonal prisms in the 2H MoS2 phase, these metastable phases are composed of the edge-sharing [MoS6 ] octahedra, in which the neighboring Mo-Mo distances differ. Due to the various crystal structures and different electronic configurations of the building [MoS6 ] motifs, these metastable polytypes are endowed with intriguing physical properties and potential applications in diverse fields. In this Review, the recent research progress on metastable MoS2 is summarized, especially with an emphasis on the diverse synthetic approaches and the newly uncovered physical properties. The phase structures and electronic band structures are also outlined. In the end, a perspective of the future investigation on metastable MoS2 is discussed.

Heat capacities of TiO2(B), 50 nm in diameter and several micrometers in length, containing given amounts of anatase were measured using a Quantum Design Physical Property Measurement System (PPMS) over a temperature range from 1.9 to 302 K. After eliminating the contribution from anatase, the heat capacities for TiO2(B) were fitted to a theoretical model in the low temperature range (T 65 K). The standard molar heat capacity, molar entropy, and molar enthalpy for TiO2(B) at T = 298.15 K were determined to be (58.18 ± 0.81) J·K−1·mol−1, (52.31 ± 0.69) J·K−1·mol−1, and (9.03 ± 0.12) kJ·mol−1, respectively, leading to a Gibbs free energy of −(6.56 ± 0.09) kJ·mol−1. The heat capacity of TiO2(B) is found to be similar to brookite, but significantly higher than those of anatase and rutile when temperature is above 200 K.

The phase transition of SbSI from non-centrosymmetry to centrosymmetry occurs at 292K, which is too low for its practical applications such as ferroelectricity and nonlinear optics. Here, the substitution of Sb by Bi atoms is proposed to improve its phase transition temperature. A new chalcohalide, Bi0.33Sb0.67SI, has been successfully synthesized by self-flux method. It crystallizes in the non-centrosymmetric space group Pmc21 (No. 26) with a ​= ​4.1190 (9) A, b ​= ​10.225 (2) A, c ​= ​8.5318 (18) A. Different from the crystal structure of SbSI, Bi0.33Sb0.67SI is composed of two kinds of edge-sharing distorted square pyramid [MS3I2]5- (M ​= ​Bi or Sb), which form [ ∞ 1 M2S4I2]4- chains along a axis. The compound exhibits a moderate powder second harmonic generation signal at 2.09 ​μm, which was consistent with its non-centrosymmetric structure.

In this work, we report on a comprehensive study about the bridge role of Tb 3 + in broadband excited Sr 3 Y(PO 4 ) 3 :Ce 3 + , Tb 3 + , Sm 3 + phosphors. With Tb 3 + acting as an energy transfer bridge, Ce 3 + → (Tb 3 + ) n → Sm 3 + energy transfer process was utilized to circumvent Ce 3 + -Sm 3 + metal-metal charge transfer (MMCT) quenching in Sr 3 Y(PO 4 ) 3 host. Sm 3 + was efficiently sensitized by the broad absorption band (4f 1 → 5d 1 transition) of Ce 3 + in near-ultraviolet spectral region. The color tones of Sr 3 Y(PO 4 ) 3 :Ce 3 + , yTb 3 + , zSm 3 + phosphors were tuned from blue through green and finally to orange with increasing Tb 3 + /Sm 3 + doping concentration. Furthermore, energy transfer mechanisms from Ce 3 + to Tb 3 + (dipole-dipole mechanism) and Tb 3 + to Sm 3 + (exchange interaction) as well as the corresponding energy transfer efficiencies (higher than 90%) are systematically investigated. Sr 3 Y(PO 4 ) 3 :0.02Ce 3 + , 0.90Tb 3 + , 0.02Sm 3 + phosphor shows a thermal stability up to 493 K, superior to that in analogous reports. The quantum efficiency of Sr 3 Y(PO 4 ) 3 :0.02Ce 3 + , 0.90Tb 3 + , 0.02Sm 3 + phosphor with 315 nm excitation was calculated to be 60%.

Multiband materials have received increasing attention due to their superior solar absorption properties. Here we design a new multiband compound, namely Ba5Bi2Co2S10, which crystallizes in the space group C22h-P21/m (No. 11) of the monoclinic system. Ba5Bi2Co2S10 is composed of one-dimensional 1∞[Bi2Co2S10]10- chains along the a axis. The adjacent chains are separated by Ba2+ ions. The optical band gap of the compound is 1.05 eV and 0.74 eV, presenting typical multi-absorption characteristics. First-principles calculations, which are perfectly consistent with the experimental results, show that the hybrid coupling effect between Co and S gives rise to multiband characteristics. Evident optoelectronic conversion properties were observed under visible light illumination with a photocurrent density of 4.0 mA cm-2 at 1 V.

Black TiO

Ultrathin cation-exchanged layered metal oxides are promising for many applications, while such substances are barely successfully synthesized to show several atomic layer thickness, owing to the strong electrostatic force between the adjacent layers. Herein, we took LiCoO2, a prototype cation-exchanged layered metal oxide, as an example to study. By developing a simple synthetic route, we synthesized LiCoO2 nanosheets with 5–6 cobalt oxide layers, which are the thinnest ever reported. Ultrathin nanosheets thus prepared showed a surprising coexistence of increased oxidation state of cobalt ions and oxygen vacancy, as demonstrated by magnetic susceptibility, X-ray photoelectron, electron paramagnetic resonance, and X-ray absorption fine spectra. This unique feature enables a higher electronic conduction and electrophilicity to the adsorbed oxygen than the bulk. Consequently ultrathin LiCoO2 nanosheets provided a current density of 10 mA cm–2 at a small overpotential of a mere 0.41 V and a small Tafel slope...

2 M WS2 is a newly discovered metastable transition metal dichalcogenide (TMDs) material with the highest instinct superconducting transition temperature (Tc) of 8.8 K among all TMDs superconductors. Here we report the structural regulation of 2 M WS2 by Se-doping and careful investigation on the doping effect on superconducting property. The results show that cell volume expands from 217.66 A3 to 231.74 A3 as the ratio of Se/(S + Se) increasing from 0 to 1. At the same time, Tc decreases from 8.75 K to 2.92 K as the ratio increases from 0 to 0.4 along with the carrier concentration dropping from 1.42 × 1021 cm−3 to 8.06 × 1020 cm−3. The entire phase diagram over a large carrier density range is drawn. These results provide insight into the relationship between the structure and superconducting state in 2 M WS2 and can further guide to modify electronic states of other TMDs materials.

A new platinum-based ternary selenide, Pt3Pb2Se2 was successfully synthesized at 1123 K by using a traditional solid-state method. It crystallizes in a orthorhombic structure with the Cmcm space group, and consists of quasi-2D [Pt3Pb2Se2] layers connected by inherent Pt–Pb bonds. It is related to the parkerite structures with half transition metal occupation. Therefore, it belongs to the half anti-perovskite family structure. The R-T curve reveals its metallic behavior and shows high conductivity (RRR = 20.5) at room temperature. The X-ray photoelectron and ultraviolet photoelectron spectroscopy spectrums were collected. It has low work function (ϕ = −4.02 eV). First principles calculations were consistent with the experimental results.

This work represents the first report on the heat capacity of brookite TiO2, which was measured using a Quantum Design physical property measurement system (PPMS) in the temperature range of 2 K to 300 K. The experimental data were fitted using a series of theoretical functions for low temperatures (T 75 K), from which the thermodynamic functions of brookite TiO2 were calculated. The standard molar entropy and molar enthalpy for brookite TiO2 at T = 298.15 K were determined to be 52.61 ± 0.53 J · K−1 · mol−1 and 9.03 ± 0.09 kJ · mol−1, respectively, leading to a Gibbs free energy of −6.65 ± 0.18 kJ · mol−1. Strikingly, this Gibbs free energy was found to be slightly less than those reported for rutile TiO2 (−6.377 kJ · mol−1) and anatase TiO2 (−6.198 kJ · mol−1) (Smith et al., 2009). Further data analyses demonstrated that the thermal stability of TiO2 polymorphs follows a sequence of anatase

Series of perovskite PrCo1-xNixO3-δ (x = 0-0.4) were prepared and carefully investigated to understand the spin state transition driven by hole doping and further to reveal the effect of spin state transition on electronic conduction. It is shown that with increasing doping level, the transition temperature Ts for Co(3+) ions from low-spin (LS) to intermediate-spin (IS) reduces from 211.9 K for x = 0 to 190.5 K for x = 0.4. XPS and FT-IR spectra demonstrate that hole doping promoted this transition due to a larger Jahn-Teller distortion. Moreover, a thermal activation of spin disorder caused by thermal population of the spin states for Co ions has a great impact on the electrical transport of these perovskite samples. This work may shed light on the comprehension of spin transition in cobalt oxides through hole doping, which is promising for finding new strategies of enhancing electronic conduction, especially for energy and catalysis applications.

The 2H molybdenum disulfide (MoS

2H MoS2 has been intensively studied because of layer-dependent electronic structures and novel physical properties. Though the metastable 1T MoS2 with the [MoS6] octahedron was observed from the microscopic area, the true crystal structure of 1T phase has not been determined strictly. Moreover, the true physical properties have not been demonstrated from experiments due to the challenge for the preparation of pure 1T MoS2 crystals. Here, we successfully synthesized the 1T MoS2 single crystals and re-determined the crystal structure of 1T MoS2 from single-crystal X-ray diffraction. 1T MoS2 crystalizes in space group P-3m1 with a cell of a = b = 3.190(3) {\AA} and c = 5.945(6) {\AA}. The individual MoS2 layer consists of MoS6 octahedron sharing edge with each other. More surprisingly, the bulk 1T MoS2 crystals undergo a superconducting transition of Tc = 4 K, which is the first observation of superconductivity in pure 1T MoS2 phase.

2H MoS

Recently, metastable polymorphs of MoS2 (1T, 1T’, 1T‴) have raised much attention due to their rich crystal structures and intriguing physicochemical properties. They are often derived from alkali metal intercalated compounds of MoS2, Ax(H2O)yMoS2 (A = alkali metal), by topotactic reaction. In order to make clear the deintercalation process, it is demanded to elucidate the compositions and crystal structures of Ax(H2O)yMoS2. Here, we report the synthesis of high-quality Ax(H2O)yMoS2 (A = K, Rb, Cs) single crystals, determine their crystal structures and investigate the physical properties. The three new quasi two-dimensional compounds crystallize in the identical monoclinic P21/m (No.11) space group determined by single crystal X-ray diffraction. Their structures consist of ordered MoS2 layers, which are intercalated by disordered hydrated A+ cations. Each layer contains edge-sharing [MoS6] octahedral, forming one-dimensional Mo zigzag chains along the b-axis. The resistivity of Ax(H2O)yMoS2 increases with the decrease of temperature, which is the typical semiconducting behavior. The apparent difference in the resistivity of the three compounds is mainly related to the number of electrons injected by alkali metal atoms. Magnetic measurements indicated that the three compounds are dominantly paramagnetic accompanied by weak ferromagnetic contributions.

Ultrathin cation-exchanged layered metal oxides are promising for many applications, while such substances are barely successfully synthesized to show several atomic layer thickness, owing to the strong electrostatic force between the adjacent layers. Herein, we took LiCoO

Nickel-substituted double-perovskites, LnBaCo1.6Ni0.4O5+δ (LnBCN, Ln = Pr, Nd and Sm), are investigated as potential cathode materials for intermediate-temperature solid-oxide fuel cells based on the Ce0.8Sm0.2O1.9 (SDC) electrolyte. The LnBCN materials exhibit high chemical compatibility with the SDC electrolyte at 950 °C for 2 h. Substitution of Ni for Co decreases the thermal expansion coefficient and the electrical conductivity, as compared with the nickel-free samples while retaining desirable electrochemical performance. The area-specific resistances of the LnBCN samples on the SDC electrolyte where Ln = Pr, Nd, and Sm at 700 °C are 0.056, 0.077, and 0.11 Ω cm2, respectively. For Ln = Pr, Nd, and Sm cathodes, the maximum power densities of the electrolyte-supported Ni-SDC/SDC/LnBCN single cells with a 0.3 mm thick SDC electrolyte reach 732, 714, and 572 mW cm−2 at 800 °C, respectively. These results show that nickel-substituted LnBCN double-perovskites are potential cathode candidates for use in IT-SOFCs.

This work reports on spin state and exchange integral of cobalt ions in stoichiometric ZnCo2O4 nanoparticles with varying particle size from about 24 to 105 nm. Cobalt ions in ZnCo2O4 nanoparticles are present as trivalence in mixed spin state. The effective magnetic moment is distributed in the range of 2.1 ∼ 1.31 μB at room temperature with coarsening of nanoparticles. Further, it is demonstrated that stoichiometric ZnCo2O4 undergoes a magnetic transition from paramagnetism to antiferromagnetism with decrease of temperature, showing a transition temperature of about 5 K. The standard molar entropy and enthalpy for 24 nm ZnCo2O4 are 170.6 ± 1.7 J K−1 mol−1 and 28.2 ± 0.3 kJ mol−1 at 298.15 K, respectively. Based on the heat capacity data, the exchange integral is determined to be 4.16 × 10−22 J. The results report here are really important for further understanding the magnetic and electronic properties of spinel oxides.