Synthesis and Characterisation of Hemihydrate Gypsum-Polyacrylamide Composite: A Novel Inorganic/Organic Cementitious Material.

This study combined inorganic α-hemihydrate gypsum (α-HHG) with organic polyacrylamide (PAM) hydrogel to create a novel α-HHG/PAM composite material. Through this facile composite strategy, this fabricated material exhibited a significantly longer initial setting time and higher mechanical strength compared to α-HHG. The effects of the addition amount and the concentration of PAM precursor solution on the flowability of the α-HHG/PAM composite material slurry, initial setting time, and mechanical properties of the hardened specimens were investigated. The structural characteristics of the composite material were examined using XRD, FE-SEM, and TGA. The results showed that the initial setting time of the α-HHG/PAM composite material was 25.7 min, which is an extension of 127.43% compared to that of α-HHG. The flexural strength and compressive strength of the oven-dried specimens were 23.4 MPa and 58.6 MPa, respectively, representing increases of 34.73% and 84.86% over values for α-HHG. The XRD, FE-SEM, and TGA results all indicated that the hydration of α-HHG in the composite material was incomplete. The incompleteness is caused by the competition between the hydration process of inorganic α-HHG and the gelation process of the acrylamide molecules for water, which hinders some α-HHG from entirely reacting with water. The enhanced mechanical strength of the α-HHG/PAM composite material results from the tight interweaving and integrating of organic and inorganic networks. This study provides a concise and efficient approach to the modification research of hemihydrate gypsum.

Electronic and Transport Properties of Strained and Unstrained Ge2Sb2Te5: A DFT Investigation.

In recent years, layered chalcogenides have attracted interest for their appealing thermoelectric properties. We investigated the Ge2Sb2Te5 compound in two different stacking sequences, named stacking 1 (S1) and stacking 2 (S2), wherein the Ge and Sb atomic positions can be interchanged in the structure. The compound unit cell, comprising nine atoms, is made of two layers separated by a gap. We show, using the quantum theory of atoms in molecules, that the bonding across the layers has characteristics of transit region bonding, though with a close resemblance to closed-shell bonding. Both S1 and S2 are shown to bear a similar small gap. The full determination of their thermoelectric properties, including the Seebeck coefficient, electrical conductivity and electronic and lattice thermal conductivities, was carried out by solving the Boltzmann transport equation. We show that stacking 1 exhibits a larger Seebeck coefficient and smaller electrical conductivity than stacking 2, which is related to their small electronic gap difference, and that S1 is more suitable for thermoelectric application than S2. Moreover, under certain conditions of temperature and doping level, it could be possible to use S1-Ge2Sb2Te5 as both a p and n leg in a thermoelectric converter. Under biaxial, tensile and compressive strains, we observe that the thermoelectric properties are improved for both S1 and S2. Furthermore, the increase in the power factor of S1 in the cross-plane direction, namely perpendicular to the gap between the layers, shows that strains can counteract the electronic transport hindrance due to the gap.

Lychee-like TiO2@Fe2O3 Core-Shell Nanostructures with Improved Lithium Storage Properties as Anode Materials for Lithium-Ion Batteries.

In this study, lychee-like TiO2@Fe2O3 microspheres with a core-shell structure have been prepared by coating Fe2O3 on the surface of TiO2 mesoporous microspheres using the homogeneous precipitation method. The structural and micromorphological characterization of TiO2@Fe2O3 microspheres has been carried out using XRD, FE-SEM, and Raman, and the results show that hematite Fe2O3 particles (7.05% of the total mass) are uniformly coated on the surface of anatase TiO2 microspheres, and the specific surface area of this material is 14.72 m2 g-1. The electrochemical performance test results show that after 200 cycles at 0.2 C current density, the specific capacity of TiO2@Fe2O3 anode material increases by 219.3% compared with anatase TiO2, reaching 591.5 mAh g-1; after 500 cycles at 2 C current density, the discharge specific capacity of TiO2@Fe2O3 reaches 273.1 mAh g-1, and its discharge specific capacity, cycle stability, and multiplicity performance are superior to those of commercial graphite. In comparison with anatase TiO2 and hematite Fe2O3, TiO2@Fe2O3 has higher conductivity and lithium-ion diffusion rate, thereby enhancing its rate performance. The electron density of states (DOS) of TiO2@Fe2O3 shows its metallic nature by DFT calculations, revealing the essential reason for the high electronic conductivity of TiO2@Fe2O3. This study presents a novel strategy for identifying suitable anode materials for commercial lithium-ion batteries.

Intercalation of p-Aminopyridine and p-Ethylenediamine Molecules into Orthorhombic In1.2Ga0.8S3 Single Crystals.

A single crystalline layered semiconductor In1.2Ga0.8S3 phase was grown, and by intercalating p-aminopyridine (NH2-C5H4N or p-AP) molecules into this crystal, a new intercalation compound, In1.2Ga0.8S3·0.5(NH2-C5H4N), was synthesized. Further, by substituting p-AP molecules with p-ethylenediamine (NH2-CH2-CH2-NH2 or p-EDA) in this intercalation compound, another new intercalated compound-In1.2Ga0.8S3·0.5(NH2-CH2-CH2-NH2) was synthesized. It was found that the single crystallinity of the initial In1.2Ga0.8S3 samples was retained after their intercalation despite a strong deterioration in quality. The thermal peculiarities of both the intercalation and deintercalation of the title crystal were determined. Furthermore, the unit cell parameters of the intercalation compounds were determined from X-ray diffraction data (XRD). It was found that increasing the c parameter corresponded to the dimension of the intercalated molecule. In addition to the intercalation phases' experimental characterization, the lattice dynamical properties and the electronic and bonding features of the stoichiometric GaInS3 were calculated using the Density Functional Theory within the Generalized Gradient Approximations (DFT-GGA). Nine Raman-active modes were observed and identified for this compound. The electronic gap was found to be an indirect one and the topological analysis of the electron density revealed that the interlayer bonding is rather weak, thus enabling the intercalation of organic molecules.

Preparation and Chromaticity Control of CoTiO3/NiTiO3 Co-Coated TiO2 Composite Pigments.

In this study, home-made amorphous TiO2 microspheres with good mono-dispersity and large numbers of mesopores on the surface were used as substrates. The intermediate microspheres were obtained by adding Co/Ni sources with different Co/Ni molar ratios in a water bath and making them react by water bath heating. By calcining the intermediate microspheres deposited on the TiO2 ones, a core-shell structured spherical CoTiO3/NiTiO3 inorganic composite pigment was prepared. The synthesized pigments were characterized by X-ray diffraction (XRD), Raman spectroscopy, field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectrometry (EDS), laser particle size (LPS) analysis and colorimetry. The results show that when the calcination temperature is 800 °C and the Co/Ni molar ratio is 0.5:0.5, the pigments consist of a TiO2 core and outer ilmenite CoTiO3/NiTiO3 shell. Moreover, the surface of the product microspheres is smooth, and the particles are of regular sphericity with a uniform particle size of about 1.8 µm. The colorimetric analysis from the samples calcined at 800 °C shows color changes from yellow-green to dark green as the Co/Ni molar ratio increases (0.1:0.9 to 0.9:0.1). A Co/Ni molar ratio that is too high or too low results in the formation of by-products such as Co3O4 or NiO, respectively, which adhere to the product surface and affect the chromaticity of the product. This work has enabled the chromatic modulation of yellow-green inorganic pigments, providing a solution for the preparation of spherical inorganic pigments that are more suitable for industrial inkjet printing.

Structural and Electrochemical Properties of Li2O-V2O5-B2O3-Bi2O3 Glass and Glass-Ceramic Cathodes for Lithium-Ion Batteries.

In this study, 20Li2O-60V2O5-(20 - x)B2O3-xBi2O3 (x = 5, 7.5, 10 mol%) glass materials have been prepared by the melt-quenching method, and the structure and morphology of the glass materials have been characterized by XRD, FTIR, Raman, and FE-SEM. The results show that the disordered network of the glass is mainly composed of structural motifs, such as VO4, BO3, BiO3, and BiO6. The electrochemical properties of the glass cathode material have been investigated by the galvanostatic charge-discharge method and cyclic voltammetry, and the results show that with the increases of Bi2O3 molar content, the amount of the VO4 group increases, and the network structure of the glass becomes more stable. To further enhance the electrochemical properties, glass-ceramic materials have been obtained by heat treatment, and the effect of the heat treatment temperature on the structure and electrochemical properties of the glass has been studied. The results show that the initial discharge capacity of the glass-ceramic cathode obtained by heat treatment at 280 °C at a current density of 50 mA·g-1 is 333.4 mAh·g-1. In addition, after several cycles of charging and discharging at a high current density of 1000 mA·g-1 and then 10 cycles at 50 mA·g-1, its discharge capacity remains at approximately 300 mAh·g-1 with a capacity retention rate of approximately 90.0%. The results indicate that a proper heat treatment temperature is crucial to improving the electrochemical properties of glass materials. This study provides an approach for the development of new glass cathode materials for lithium-ion batteries.

Investigation of PbSnTeSe High-Entropy Thermoelectric Alloy: A DFT Approach.

Thermoelectric materials have attracted extensive attention because they can directly convert waste heat into electric energy. As a brand-new method of alloying, high-entropy alloys (HEAs) have attracted much attention in the fields of materials science and engineering. Recent researches have found that HEAs could be potentially good thermoelectric (TE) materials. In this study, special quasi-random structures (SQS) of PbSnTeSe high-entropy alloys consisting of 64 atoms have been generated. The thermoelectric transport properties of the highest-entropy PbSnTeSe-optimized structure were investigated by combining calculations from first-principles density-functional theory and on-the-fly machine learning with the semiclassical Boltzmann transport theory and Green-Kubo theory. The results demonstrate that PbSnTeSe HEA has a very low lattice thermal conductivity. The electrical conductivity, thermal electronic conductivity and Seebeck coefficient have been evaluated for both n-type and p-type doping. N-type PbSnTeSe exhibits better power factor (PF = S2σ) than p-type PbSnTeSe because of larger electrical conductivity for n-type doping. Despite high electrical thermal conductivities, the calculated ZT are satisfactory. The maximum ZT (about 1.1) is found at 500 K for n-type doping. These results confirm that PbSnTeSe HEA is a promising thermoelectric material.

First-Principle Investigations on the Electronic and Transport Properties of PbBi2Te2X2 (X = S/Se/Te) Monolayers.

This paper reports first-principles calculations on PbBi2Te2S2, PbBi2Te2Se2 and PbBi2Te4 monolayers. The strain effects on their electronic and thermoelectric properties as well as on their stability have been investigated. Without strain, the PbBi2Te4 monolayer exhibits highest Seebeck coefficient with a maximum value of 671 µV/K. Under tensile strain the highest power factor are 12.38×1011 Wm-1K-2s-1, 10.74×1011 Wm-1K-2s-1 and 6.51×1011 Wm-1K-2s-1 for PbBi2Te2S2, PbBi2Te2Se2 and PbBi2Te4 at 3%, 2% and 1% tensile strains, respectively. These values are 85.9%, 55.0% and 3.3% larger than those of the unstrained structures.

Structure-Property Relationships in Transition Metal Dichalcogenide Bilayers under Biaxial Strains.

This paper reports a Density Functional Theory (DFT) investigation of the electron density and optoelectronic properties of two-dimensional (2D) MX2 (M = Mo, W and X = S, Se, Te) subjected to biaxial strains. Upon strains ranging from -4% (compressive strain) to +4% (tensile strain), MX2 bilayers keep the same bandgap type but undergo a non-symmetrical evolution of bandgap energies and corresponding effective masses of charge carriers (m*). Despite a consistency regarding the electronic properties of Mo- and WX2 for a given X, the strain-induced bandgap shrinkage and m* lowering are strong enough to alter the strain-free sequence MTe2, MSe2, MS2, thus tailoring the photovoltaic properties, which are found to be direction dependent. Based on the quantum theory of atoms in molecules, the bond degree (BD) at the bond critical points was determined. Under strain, the X-X BD decreases linearly as X atomic number increases. However, the kinetic energy per electron G/ρ at the bond critical point is independent of strains with the lowest values for X = Te, which can be related to the highest polarizability evidenced from the dielectric properties. A cubic relationship between the absolute BD summation of M-X and X-X bonds and the static relative permittivity was observed. The dominant position of X-X bond participating in this cubic relationship in the absence of strain was substantially reinforced in the presence of strain, yielding the leading role of the X-X bond instead of the M-X one in the photovoltaic response of 2D MX2 material.

Strain Effects on the Electronic and Thermoelectric Properties of n(PbTe)-m(Bi2Te3) System Compounds.

Owing to their low lattice thermal conductivity, many compounds of the n(PbTe)-m(Bi2Te3) homologous series have been reported in the literature with thermoelectric (TE) properties that still need improvement. For this purpose, in this work, we have implemented the band engineering approach by applying biaxial tensile and compressive strains using the density functional theory (DFT) on various compounds of this series, namely Bi2Te3, PbBi2Te4, PbBi4Te7 and Pb2Bi2Te5. All the fully relaxed Bi2Te3, PbBi2Te4, PbBi4Te7 and Pb2Bi2Te5 compounds are narrow band-gap semiconductors. When applying strains, a semiconductor-to-metal transition occurs for all the compounds. Within the range of open-gap, the electrical conductivity decreases as the compressive strain increases. We also found that compressive strains cause larger Seebeck coefficients than tensile ones, with the maximum Seebeck coefficient being located at -2%, -6%, -3% and 0% strain for p-type Bi2Te3, PbBi2Te4, PbBi4Te7 and Pb2Bi2Te5, respectively. The use of the quantum theory of atoms in molecules (QTAIM) as a complementary tool has shown that the van der Waals interactions located between the structure slabs evolve with strains as well as the topological properties of Bi2Te3 and PbBi2Te4. This study shows that the TE performance of the n(PbTe)-m(Bi2Te3) compounds is modified under strains.

Influence of the stacking sequence on layered-chalcogenide properties: first principles investigation of Pb2Bi2Te5.

The Pb2Bi2Te5 compound has been reported in the literature with two stacking sequences -Te-Pb-Te-Bi-Te-Bi-Te-Pb-Te- and -Te-Bi-Te-Pb-Te-Pb-Te-Bi-Te- labelled in this work as A and B, respectively. The electronic and the thermoelectric properties of the Pb2Bi2Te5 compound with the 2 different stacking sequences have been determined from a series of first principles calculations using density functional theory (DFT). The related compounds PbTe and Bi2Te3 have also been investigated for comparison. Different exchange-correlation functionals have been tested, without spin-orbit coupling, which has been found to have important effects. The elastic moduli, dielectric constants, Born effective charges, and phonon dispersion within the quasi-harmonic approximation have also been calculated and based on these calculations results, the thermal conductivity has been determined by solving the Boltzmann transport equation. Additionally, the QTAIM theory was employed to explain the differences in the properties of the 2 stackings. The most interesting compound for thermoelectric applications has been found to be Pb2Bi2Te5 with the stacking B sequence. The highest zT values have been found to be 4.02 in the a-axis direction and 2.26 in the c-axis one.

Structure-Property Relationships of 2D Ga/In Chalcogenides.

Two-dimensional MX (M = Ga, In; X = S, Se, Te) homo- and heterostructures are of interest in electronics and optoelectronics. Structural, electronic and optical properties of bulk and layered MX and GaX/InX heterostructures have been investigated comprehensively using density functional theory (DFT) calculations. Based on the quantum theory of atoms in molecules, topological analyses of bond degree (BD), bond length (BL) and bond angle (BA) have been detailed for interpreting interatomic interactions, hence the structure-property relationship. The X-X BD correlates linearly with the ratio of local potential and kinetic energy, and decreases as X goes from S to Te. For van der Waals (vdW) homo- and heterostructures of GaX and InX, a cubic relationship between microscopic interatomic interaction and macroscopic electromagnetic behavior has been established firstly relating to weighted absolute BD summation and static dielectric constant. A decisive role of vdW interaction in layer-dependent properties has been identified. The GaX/InX heterostructures have bandgaps in the range 0.23-1.49 eV, absorption coefficients over 10-5 cm-1 and maximum conversion efficiency over 27%. Under strain, discordant BD evolutions are responsible for the exclusively distributed electrons and holes in sublayers of GaX/InX. Meanwhile, the interlayer BA adjustment with lattice mismatch explains the constraint-free lattice of the vdW heterostructure.

Electron Density and Its Relation with Electronic and Optical Properties in 2D Mo/W Dichalcogenides.

Two-dimensional MX2 (M = Mo, W; X = S, Se, Te) homo- and heterostructures have attracted extensive attention in electronics and optoelectronics due to their unique structures and properties. In this work, the layer-dependent electronic and optical properties have been studied by varying layer thickness and stacking order. Based on the quantum theory of atoms in molecules, topological analyses on interatomic interactions of layered MX2 and WX2/MoX2, including bond degree (BD), bond length (BL), and bond angle (BA), have been detailed to probe structure-property relationships. Results show that M-X and X-X bonds are strengthened and weakened in layered MX2 compared to the counterparts in bulks. X-X and M-Se/Te are weakened at compressive strain while strengthened at tensile strain and are more responsive to the former than the latter. Discordant BD variation of individual parts of WX2/MoX2 accounts for exclusively distributed electrons and holes, yielding type-II band offsets. X-X BL correlates positively to binding energy (Eb), while X-X BA correlates negatively to lattice mismatch (lm). The resulting interlayer distance limitation evidences constraint-free lattice of vdW structure. Finally, the connection between microscopic interatomic interaction and macroscopic electromagnetic behavior has been quantified firstly by a cubic equation relating to weighted BD summation and static dielectric constant.

Thermoelectric Properties of Sb-S System Compounds from DFT Calculations.

By combining density functional theory, quantum theory of atoms in molecules and transport properties calculations, we evaluated the thermoelectric properties of Sb-S system compounds and shed light on their relationships with electronic structures. The results show that, for Sb2S3, the large density of states (DOS) variation induces a large Seebeck coefficient. Taking into account the long-range weak bonds distribution, Sb2S3 should exhibit low lattice thermal conductivity. Therefore, Sb2S3 is promising for thermoelectric applications. The insertion of Be atoms into the Sb2S3 interstitial sites demonstrates the electrical properties and Seebeck coefficient anisotropy and sheds light on the understanding of the role of quasi-one-dimensional structure in the electron transport. The large interstitial sites existing in SbS2 are at the origin of phonons anharmonicity which counteracts the thermal transport. The introduction of Zn and Ga atoms into these interstitial sites could result in an enhancement of all the thermoelectric properties.

Ultra-Thin Platinum Deposits by Surface-Limited Redox Replacement of Tellurium.

Platinum is the most employed electrocatalyst for the reactions taking place in energy converters, such as the oxygen reduction reaction in proton exchange membrane fuel cells, despite being a very low abundant element in the earth's crust and thus extremely expensive. The search for more active electrocatalysts with ultra-low Pt loading is thus a very active field of investigation. Here, surface-limited redox replacement (SLRR) that utilizes the monolayer-limited nature of underpotential deposition (UPD) was used to prepare ultrathin deposits of Pt, using Te as sacrificial metal. Cyclic voltammetry and anodic potentiodynamic scanning experiments have been performed to determine the optimal deposition conditions. Physicochemical and electrochemical characterization of the deposited Pt was carried out. The deposit comprises a series of contiguous Pt islands that form along the grain interfaces of the Au substrate. The electrochemical surface area (ECSA) of the Pt deposit obtained after 5 replacements, estimated to be 18 m²/g, is in agreement with the ECSA of extended surface catalysts on flat surfaces.

Influence of the modified Becke-Johnson exchange potential on thermoelectric properties: application to Mg2Si.

The electronic and transport properties of Mg(2)Si are calculated using the density-functional and semi-classical Boltzmann transport theories. A detailed comparison of the efficiency of the Perdew, Burke, and Ernzerhof (PBE) and the modified Becke-Johnson (m-BJ) exchange potentials on the prediction of these properties is presented. We show that the m-BJ potential is able to predict the Mg(2)Si thermoelectric properties with better agreement with the experimental data than the PBE potential. These findings are correlated with a small modification of the valence orbitals of the materials and the opening of the gap.