Synthetic Nanosheets of Natural van der Waals Heterostructures

Creation of new van der Waals heterostructures by stacking different two dimensional (2D) crystals on top of each other in a chosen sequence is the next challenge after the discovery of graphene, mono/few layer of h ‐BN, and transition‐metal dichalcogenides. However, chemical syntheses of van der Waals heterostructures are rarer than the physical preparation techniques. Herein, we demonstrate the kinetic stabilization of 2D ultrathin heterostructure (ca. 1.13–2.35 nm thick) nanosheets of layered intergrowth SnBi₂Te₄, SnBi₄Te₇, and SnBi₆Te₁₀, which belong to the Sn_m Bi_2n Te_{3n +m} homologous series, by a simple solution based synthesis. Few‐layer nanosheets exhibit ultralow lattice thermal conductivity (κ _lat) of 0.3–0.5 W m⁻¹ K⁻¹ and semiconducting electron‐transport properties with high carrier mobility.     

Creating Zipper‐Like van der Waals Gap Discontinuity in Low‐Temperature‐Processed Nanostructured PbBi2nTe1+3n: Enhanced Phonon Scattering and Improved Thermoelectric Performance

Nanoengineered materials can embody distinct atomic structures which deviate from that of the bulk‐grain counterpart and induce significantly modified electronic structures and physical/chemical properties. The phonon structure and thermal properties, which can also be potentially modulated by the modified atomic structure in nanostructured materials, however, are seldom investigated. Employed here is a mild approach to fabricate nanostructured PbBi_2nTe_1+3n using a solution‐synthesized PbTe‐Bi₂Te₃ nano‐heterostructure as a precursor. The as‐obtained monoliths have unprecedented atomic structure, differing from that of the bulk counterpart, especially the zipper‐like van der Waals gap discontinuity and the random arrangement of septuple‐quintuple layers. These structural motifs break the lattice periodicity and coherence of phonon transport, leading to ultralow thermal conductivity and excellent thermoelectric z T.     

Time‐Resolved Assembly of Cluster‐in‐Cluster {Ag12}‐in‐{W76} Polyoxometalates under Supramolecular Control

We report the time‐resolved supramolecular assembly of a series of nanoscale polyoxometalate clusters (from the same one‐pot reaction) of the form: [H_(10+m)Ag₁₈Cl(Te₃W₃₈O₁₃₄)₂]_n, where n=1 and m=0 for compound 1 (after 4 days), n=2 and m=3 for compound 2 (after 10 days), and n=∞ and m=5 for compound 3 (after 14 days). The reaction is based upon the self‐organization of two {Te₃W₃₈} units around a single chloride template and the formation of a {Ag₁₂} cluster, giving a {Ag₁₂}‐in‐{W₇₆} cluster‐in‐cluster in compound 1 , which further aggregates to cluster compounds 2 and 3 by supramolecular Ag‐POM interactions. The proposed mechanism for the formation of the clusters has been studied by ESI‐MS. Further, control experiments demonstrate the crucial role that TeO₃^2?, Cl^?, and Ag⁺ play in the self‐assembly of compounds 1 – 3 .     

Phase equilibria in the PbSe-Bi<Subscript>2</Subscript>Se<Subscript>3</Subscript>-Se system and thermodynamic properties of intermediate phases

The Pb-Bi-Se system in the PbSe-Bi₂Se₃-Se-Se composition region was studied by measurement of concentration circuits of the type (−) PbSe(solid) liquid electrolyte, Pb²⁺(Pb-Bi-Se)(solid) (+) in the temperature range 300–430 K and by X-ray powder diffraction. A solid-phase equilibrium diagram was constructed, and the formation was confirmed for the ternary compounds Pb₅Bi₆Se₁₄, Pb₅Bi₁₂Se₂₃, and Pb₅Bi₁₈Se₃₂, which belong to the homologous series [(PbSe)₅] _m · [(Bi₂Se₃)₃] _n . From the emf versus temperature equations, the partial thermodynamic functions [`(DG)]\overline {\Delta G}, [`(DH)]\overline {\Delta H}, [`(DS)]\overline {\Delta S} of PbSe in alloys were calculated. Based on the solid-phase equilibrium diagram from these partial molar quantities using the corresponding data for PbSe and Bi₂Se₃, the standard thermodynamic functions of formation and standard entropies of the above ternary compounds were calculated.     

Compound clusters of heavy post-transition elements

Neutral binary clusters of Pb-As, Pb-Se, Pb-Te, Bi-In and Bi-Te are generated by inert gas condensation in a double oven source and probed by electron impact. Cluster ions corresponding to the Zintl polyanions Pb ₅ ^2? and Pb ₉ ^4? with respect to atom and valence electron number, are strongly enriched by electron induced dissociation ((Pb₂As₃)⁺, (Bi₄In)⁺, (Pb₄As₅)⁺, (Bi₇In₂)⁺). For the corresponding systems, no other compound cluster ions are enriched in a comparable manner. Enhanced stability is found for (Pb_n?1As)⁺ (n=7, 10, 13) and (Bi₃Te)⁺, which are isoelectronic with neutral ‘magic’ Pb_n clusters and the very stable Bi₄ molecule, respectively.     

Tuning Optimum Temperature Range of Bi2Te3‐Based Thermoelectric Materials by Defect Engineering

Bi₂Te₃‐based solid solutions, which have been widely used as thermoelectric (TE) materials for the room temperature TE refrigeration, are also the potential candidates for the power generators with medium and low‐temperature heat sources. Therefore, depending on the applications, Bi₂Te₃‐based materials are expected to exhibit excellent TE properties in different temperature ranges. Manipulating the point defects in Bi₂Te₃‐based materials is an effective and important method to realize this purpose. In this review, we focus on how to optimize the TE properties of Bi₂Te₃‐based TE materials in different temperature ranges by defect engineering. Our calculation results of two‐band model revel that tuning the carrier concentration and band gap, which is easily realized by defects engineering, can obtain better TE properties at different temperatures. Then, the typical paradigms about optimizing the TE properties at different temperatures for n‐type and p‐type Bi₂Te₃‐based ZM ingots and polycrystals are discussed in the perspective of defects engineering. This review can provide the guidance to improve the TE properties of Bi₂Te₃‐based materials at different temperatures by defects engineering.     

Intrinsically Low Thermal Conductivity and High Carrier Mobility in Dual Topological Quantum Material,n‐Type BiTe

A challenge in thermoelectrics is to achieve intrinsically low thermal conductivity in crystalline solids while maintaining a high carrier mobility (μ). Topological quantum materials, such as the topological insulator (TI) or topological crystalline insulator (TCI) can exhibit high μ. Weak topological insulators (WTI) are of interest because of their layered hetero‐structural nature which has a low lattice thermal conductivity (κ_lat). BiTe, a unique member of the (Bi₂)_m(Bi₂Te₃)_n homologous series (m:n=1:2), has both the quantum states, TCI and WTI, which is distinct from the conventional strong TI, Bi₂Te₃ (where m:n=0:1). Herein, we report intrinsically low κ_lat of 0.47–0.8 W m^?1 K^?1 in the 300–650 K range in BiTe resulting from low energy optical phonon branches which originate primarily from the localized vibrations of Bi bilayer. It has high μ≈516 cm² V^?1 s^?1 and 707 cm² V^?1 s^?1 along parallel and perpendicular to the spark plasma sintering (SPS) directions, respectively, at room temperature.     

A New Strategy for Realizing the Conversion of “Homo–Hetero–Homo” Heteroepitaxial Growth in Bi2Te3 and the Thermoelectric Performance

Uncovering the reason for structure‐dependent thermoelectric performance still remains a big challenge. A low‐temperature and easily scalable strategy for synthesizing Bi₂Te₃ nanostring hierarchical structures through solution‐phase reactions, during which there is the conversion of “homo–hetero–homo” in Bi₂Te₃ heteroepitaxial growth, is reported. Bi₂Te₃ nanostrings are obtained through the transformation from pure Bi₂Te₃ hexagonal nanosheets followed by Te?Bi₂Te₃ “nanotop” heterostructures to Bi₂Te₃ nanostrings. The growth of Bi₂Te₃ nanostrings appears to be a self‐assembly process through a wavy competition process generated from Te and Bi³⁺. The conversion of homo–hetero–homo opens up new platforms to investigate the wet chemistry of Bi₂Te₃ nanomaterials. Furthermore, to study the effect of morphologies and hetero/homo structures, especially with the same origin and uniform conditions on their thermoelectric properties, the thermoelectric properties of Bi₂Te₃ nanostrings and Te?Bi₂Te₃ heterostructured pellets fabricated by spark plasma sintering have been investigated separately.     

Intrinsically Low Thermal Conductivity and High Carrier Mobility in Dual Topological Quantum Material,n-Type BiTe

A challenge in thermoelectrics is to achieve intrinsically low thermal conductivity in crystalline solids while maintaining a high carrier mobility (μ). Topological quantum materials, such as the topological insulator (TI) or topological crystalline insulator (TCI) can exhibit high μ. Weak topological insulators (WTI) are of interest because of their layered hetero-structural nature which has a low lattice thermal conductivity (κ_lat). BiTe, a unique member of the (Bi₂)_m(Bi₂Te₃)_n homologous series (m:n=1:2), has both the quantum states, TCI and WTI, which is distinct from the conventional strong TI, Bi₂Te₃ (where m:n=0:1). Herein, we report intrinsically low κ_lat of 0.47–0.8 W m⁻¹ K⁻¹ in the 300–650 K range in BiTe resulting from low energy optical phonon branches which originate primarily from the localized vibrations of Bi bilayer. It has high μ≈516 cm² V⁻¹ s⁻¹ and 707 cm² V⁻¹ s⁻¹ along parallel and perpendicular to the spark plasma sintering (SPS) directions, respectively, at room temperature.     

Synthesis and characterization of quaternary selenides Sn2Pb5Bi4Se13 and Sn8.65Pb0.35Bi4Se15

Quaternary selenides Sn₂Pb₅Bi₄Se₁₃ and Sn_8.65Pb_0.35Bi₄Se₁₅ were synthesized from the elements in sealed silica tubes; their crystal structures were determined by single-crystal and powder X-ray diffraction. Both compounds crystallize in monoclinic space group C2/m (No.12), with lattice parameters of Sn₂Pb₅Bi₄Se₁₃: a = 14.001(6) Å, b = 4.234(2) Å, c = 23.471(8) Å, V = 1376.2(1) Å³, R₁/wR₂ = 0.0584/0.1477, and GOF = 1.023; Sn_8.65Pb_0.35Bi₄Se₁₅: a = 13.872(3) Å, b = 4.2021(8) (4) Å, c = 26.855(5) Å, V = 1557.1(5) Å³, R₁/wR₂ = 0.0506/0.1227, and GOF = 1.425. These compounds exhibit tropochemical cell-twinning of NaCl-type structures with lillianite homologous series L(4, 5) and L(4, 7) for Sn₂Pb₅Bi₄Se₁₃ and Sn_8.65Pb_0.35Bi₄Se₁₅, respectively. Measurements of electrical conductivity indicate that these materials are semiconductors with narrow band gaps; Sn₂Pb₅Bi₄Se₁₃ is n-type, whereas Sn_8.65Pb_0.35Bi₄Se₁₅ is a p-type semiconductor with Seebeck coefficients −80(5) and 178(7) μV/K at 300 K, respectively.     

The Influence of Nanoscale Heterostructures on the Thermoelectric Properties of Bi‐substituted Tl5Te3

Tl_4.5Bi_0.5Te₃ crystallizes in a distorted variant of the Tl₅Te₃ structure type in the space group I4/m. The symmetry reduction compared to Tl₅Te₃ (space group I4/mcm) is a consequence of cation ordering as shown by resonant X‐ray scattering using synchrotron radiation. Tl and Bi predominantly occupy one Wyckoff site each. This ordering is accompanied by displacements of Te atoms. The influence of nanostructuring on the thermoelectric performance of Tl_4.5Bi_0.5Te₃ was investigated for the new composite model system Tl_4.5Bi_0.5Te₃ – TlInTe₂. For the nominal composition (Tl_4.5Bi_0.5Te₃)_0.6(TlInTe₂)_0.4, the thermoelectric Figure of merit ZT reaches 0.8 at 325 °C. Nanoscaled precipitates with sizes of about 100–200 nm probably have beneficial influence on the thermal conductivity at this temperature.     

Ionic Ferrocenyl Coordination Compounds Derived from Imidazole and 1,2,4‐Triazole Ligands and Their Catalytic Effects During Combustion

Alkylferrocene‐based burning‐rate catalysts (BRCs) show conspicuous migration tendency and volatility during prolonged storage and fabrication process of a composite solid propellant. To enhance anti‐migration ability of the BRCs, forty novel ionic coordination compounds, [M(L)₄(H₂O)₂]_mX_n (M = Mn²⁺, Co²⁺, Cu²⁺, Ni²⁺, Zn²⁺, Fe²⁺, Pb²⁺, Cr³⁺, Bi³⁺, or Cd²⁺; L = ferrocenylmethyl imidazole or ferrocenylmethyl‐1,2,4‐triazole; X = picrate or trinitroresorcinolate), were synthesized and characterized by FT‐IR, UV/Vis, and elementary analysis. Additionally, the crystal structures of six compounds were confirmed by single‐crystal X‐ray diffraction. The TG analyses revealed that the new compounds show high thermal stability. Cyclic voltammetry studies suggested that theyare irreversible redox systems. Their catalytic activities in the thermal degradation of ammonium perchlorate (AP), 1,3,5‐trinitro‐1,3,5‐triazacyclo‐hexane (RDX) and 1,2,5,7‐tetranitro‐1,3,5,7‐tetraazacyclooctane (HMX) were examined by DSC technique. The results indicated that all the new compounds exert great effects on the thermal decomposition of AP and RDX, among them some compounds are more active than catocene. Compound 26 has good catalytic ability in the thermal decomposition of HMX, representing a rare example of the reported ferrocene‐based BRCs which show catalytic activity during combustion of HMX.     

High n-value phases in the complex bismuth oxides with layered structure,Bi2CaNan−2NbnO3n+3

Complex bismuth oxides with layered structure are prepared with a series of compositions in the system Bi₂CaNb₂O₉-NaNbO₃. It is found by X-ray powder diffraction that each compound is composed of more than two phases, which are described by a formula Bi₂CaNa_n?2Nb_nO_3n+3, e.g., in the sample with the nominal composition Bi₂CaNb₂O₉ · 8NaNbO₃, the phases with n = 6 to 8 appear predominantly. These phases are closely intergrown to each other. Moreover, high-resolution electron microscopy reveals that microsyntactic intergrowth frequently occurs in the phases with n > 5. The occurrence of the latter intergrowth is explained in terms of the bond length obtained.     

Homologous series of layered tetradymite-like compounds in Bi-Te and GeTe-Bi2Te3 systems

Formation of the homologous series of layered compounds nBi₂-mBi₂Te₃ and nGeTe mBi₂Te₃ is analyzed. For both series, the crystal structure contains two-layer (Bi2 or GeTe) and five-layer (Bi₂Te₃) packets perpendicular to the c axis. Unlike the Bi₂ packets, the GeTe packets do not retain their individuality during crystal structure formation; they are linked to the Bi₂Te₃ five-layer packets to form new multilayer packets leading to a considerable change in the character of interatomic interactions. Translated from Zhumal Struktumoi Khimii, Vol. 41, No. 1, pp. 97-105, January–February, 2000.     

Phase equilibria in the Tl2Te-PbTe-Bi2Te3 system

Phase equilibria in the Tl₂Te-PbTe-Bi₂Te₃ system were studied by differential thermal analysis, X-ray powder diffraction, and microhardness measurements. Some polythermal sections and isothermal (at 600 and 800 K) sections of the phase diagram and a projection of the liquidus surface were constructed. It was shown that the system is characterized by the formation of solid solutions with the Tl₅Te₃ structure (??) and solid solutions based on PbTe (??₁), Tl₂Te (??), Bi₂Te₃ (??), and two TlBiTe₂ (??₂ and ?á?₂) phases. Their homogeneity regions were determined. The liquidus surface consists of the primary crystallization fields of the ??-, ??₁-, ?á?₂-, and ??-phases and the compounds PbBi₂Te₄ and PbBi₄Te₇. The liquidus of the ?? phase is degenerate. The primary crystallization fields of the phases were determined, and the types and coordinates of in- and monovariant equilibria were found.     

Synthesis and characterization of new homologue multinary selenides,M2Sb5Bi5Se17 (M = Sn,Pb)

New quaternary selenides M₂Sb₅Bi₅Se₁₇ (M = Sn, Pb) were synthesized using solid-state sintering reactions that crystallize in the monoclinic system with C2/m (No. 12) space group with lattice parameters a = 27.914(7) Å, b = 4.0804(11) Å, c = 15.512(4) Å, and β = 114.881(9)° for M = Sn, and a = 27.987(3) Å, b = 4.1062(5) Å, c = 15.6372(19) Å, and β = 115.318(3)° for M = Pb, respectively. The crystal structure is related to a homologous series [A⁺²_2x−4B⁺³₄ Se⁻²_2x−2][B⁺³_2y−2Se⁻²_3y−3] with (x, y) = (3, 4) that contains building units of two-dimensional slabs of NaCl¹¹¹-type [Sb₂Bi₄Se₁₁] separated by 1D ribbons NaCl¹⁰⁰-type [Pb₂Sb₃BiSe₆]. The NaCl¹¹¹ unit contains edge-shared octahedra filled with Sb³⁺ and Bi³⁺ cations, which are parallel and overlapped to form a step-layer 2D network stacking alone [001]. The NaCl¹⁰⁰ type ribbons containing Pb²⁺ and Sb³⁺ in square or trigonal pyramidal environments with the general formula [M₆Se₆] filled in the space between 2D layers of NaCl¹¹¹ units. The conductivity measurement revealed semiconducting property with band gaps of ~0.1 eV. Pb₂Sb₅Bi₅Se₁₇ exhibits low thermal conductivity 3,000 μW cm⁻¹ K⁻¹ in a temperature range of 300–480 K.     

A Facile Route for the Synthesis of Polycationic Tellurium Cluster Compounds: Synthesis in Ionic Liquid Media and Characterization by Single‐Crystal X‐ray Crystallography and Magnetic Susceptibility

An innovative soft chemical approach was applied, using ionic liquids as an alternative reaction medium for the synthesis of tellurium polycationic cluster compounds at room temperature. [Mo₂Te₁₂]I₆, Te₆[WOCl₄]₂, and Te₄[AlCl₄]₂ were isolated from the ionic liquid [BMIM]Cl/AlCl₃ ([BMIM]⁺: 1‐n‐butyl‐3‐methylimidazolium) and characterized. Black, cube‐shaped crystals of [Mo₂Te₁₂]I₆, which is not accessible by conventional chemical transport reaction, were obtained by reaction of the elements at room temperature in [BMIM]Cl/AlCl₃. The monoclinic structure (P2₁/n, a = 1138.92(2) pm, b = 1628.13(2) pm, c = 1611.05(2) pm, β = 105.88(1) °) is homeotypic to the triclinic bromide [Mo₂Te₁₂]Br₆. In the binulear complex [Mo₂Te₁₂]⁶⁺, the molybdenum(III) atoms are η⁴‐coordinated by terminal Te₄²⁺ rings and two bridging η²‐Te₂^2– dumbbells. Despite the short Mo···Mo distance of 297.16(5) pm, coupling of the magnetic moments is not observed. The paramagnetic moment of 3.53 μ_B per molybdenum(III) atom corresponds to an electron count of seventeen. Black crystals of monoclinic Te₆[WOCl₄]₂ are obtained by the oxidation of tellurium with WOCl₄ in [BMIM]Cl/AlCl₃. Tellurium and tellurium(IV) synproportionate in the ionic liquid at room temperature yielding violet crystals of orthorhombic Te₄[AlCl₄]₂.     

Thermodynamic properties of germanium and lead tellurites

The temperature dependence of the molar heat capacities of the tellurites PbTeO₃, Pb₂Te₃O₈ and Ge(TeO₃)₂ are determined. By statistical manipulation of the values obtained, the parameters in the equations for the corresponding compounds showing this dependence are determined using the least-squares method. These equations and the standard molar entropies are used to determine the thermodynamic functions Δ₀ ^T S _m ⁰ , Δ_T ^T H _m ⁰ and (Φ_m ⁰+Δ₀ ^T H _m ⁰/T) for T'=298.15 K.     

The Second Spacer Cation Assisted Growth of a 2D Perovskite Film with Oriented Large Grain for Highly Efficient and Stable Solar Cells

The fabrication of high‐quality film with large grains oriented along the direction of film thickness is important for 2D Ruddlesden–Popper perovskite‐based solar cells (PVSCs). High‐quality 2D BA₂MA_n?1Pb_nI_3n+1 (BA⁺=butylammonium, MA⁺=methylammonium, n=5) perovskite films were fabricated with a grain size of over 1 μm and preferential orientation growth by introducing a second spacer cation (SSC⁺) into the precursor solution. Dynamic light scattering showed that SSC⁺ addition can induce aggregation in the precursor solution. The precursor aggregates are favorable for the formation of large crystal grains by inducing nucleation and decreasing the nucleation sites. Applying phenylethylammonium as SSC⁺, the optimized inverted planar PVSCs presented a maximum PCE of 14.09 %, which is the highest value of the 2D BA₂MA_n?1Pb_nI_3n+1 (n=5) PVSCs. The unsealed device shows good moisture stability by maintaining around 90 % of its initially efficiency after 1000 h exposure to air (Hr=25±5 %).     

The Effect of pH Values on the Synthesis,Microstructure and Photocatalytic Activity of Ce‐Bi2O3 by a Two‐Step Hydrothermal Method

In this study, the effect of pH values on the microstructure and photocatalytic activity of Ce‐Bi₂O₃ under visible light irradiation was investigated in detail. In alkaline condition (e.g. pH = 9), the as‐prepared Ce‐Bi₂O₃ exhibited an agglomerated status and mesoporous structures without a long‐range order. While in weak acid condition (e.g. pH = 5), the Ce‐Bi₂O₃ exhibited a best morphology with irregular nanosheets. Correspondingly, it possessed largest surface area (24.641 m² g^?1) and pore volume (9.825E‐02 cm³ g^?1). These unique nanosheets can offer an attachment for pollutant molecules and reduce the distance of electron immigration from inner to surface, thus facilitating the separation of photoelectron and hole pairs. Compared with the pure Bi₂O₃, the band gap of Ce‐Bi₂O₃ prepared at different pH was much lower. Among them, the band gap of Ce‐Bi₂O₃ (pH of 5) was lowest (2.61 eV). Ce‐Bi₂O₃ (pH of 5) exhibited as tetragonal crystal with the bismuth oxide in the form of the composites, which could reduce the band gap width or suppress the charge‐carrier recombination, subsequently possessing great photocatalytic activity for acid orange II under visible light irradiation. After 2 h degradation under visible light, the degradation rate of acid Orange II was up to 96.44% by Ce‐Bi₂O₃ prepared at pH 5. Overall, it can be concluded that the pH values had effects on the microstructure and photocatalytic activity of Ce‐Bi₂O₃ catalysts.