High Thermoelectric Figure of Merit Achieved in Cu2S1- xTe x Alloys Synthesized by Mechanical Alloying and Spark Plasma Sintering

Rapid microwave-assisted synthesis and characterization of a novel CuCoTe nanocomposite material for optoelectronic and dielectric applications

In the realm of nanomaterial research, copper telluride and cobalt telluride have individually attracted considerable attention owing to their unique properties and potential applications. However, there exists a notable gap in the literature when it comes to the exploration of composite materials derived from these elements. From this point of view, a ternary CuCoTe nanocomposite was prepared using the microwave synthesis method. Various characterizations were performed by varying the power and irradiation time. X-Ray diffraction study and transmission electron microscopy analysis confirmed the polycrystalline nature of the material with Cu2Te and CoTe hexagonal phases. Field emission scanning electron microscopy images reveal nanoparticle-like morphology, which remains unchanged even when the time of irradiation increases. In addition, the nanoparticle size of the material lies in the range of 30-39 nm. The differential scanning calorimetry inferred various exothermic and endothermic peaks. Meanwhile, the optical analysis from the UV-visible study shows the red-shifted absorbance, enabling the material for semiconductor and photovoltaic devices. Furthermore, the optical bandgap of the material varies in the range from 2.45 to 3.61 eV, which reveals the tuneable bandgap desiring the material for various optoelectronic applications. The frequency-temperature-dependent dielectric study gives results for dielectric parameters, conductivity, and impedance behaviour. The material's dielectric constant, dielectric loss, and AC conductivity enhance with the increase in temperature. This behaviour of the material broadens the area of applicability in energy storage devices.

Largely Enhanced Thermoelectric Power Factor of Flexible Cu2-xS Film by Doping Mn

Copper-sulfide-based materials have attracted noteworthy attention as thermoelectric materials due to rich elemental reserves, non-toxicity, low thermal conductivity, and adjustable electrical properties. However, research on the flexible thermoelectrics of copper sulfide has not yet been reported. In this work, we developed a facile method to prepare flexible Mn-doped Cu2-xS films on nylon membranes. First, nano to submicron powders with nominal compositions of Cu2-xMnyS (y = 0, 0.01, 0.03, 0.05, 0.07) were synthesized by a hydrothermal method. Then, the powders were vacuum-filtrated on nylon membranes and finally hot-pressed. Phase composition and microstructure analysis revealed that the films contained both Cu2S and Cu1.96S, and the size of the grains was ~20-300 nm. By Mn doping, there was an increase in carrier concentration and mobility, and ultimately, the electrical properties of Cu2-xS were improved. Eventually, the Cu2-xMn0.05S film showed a maximum power factor of 113.3 μW m-1 K-2 and good flexibility at room temperature. Moreover, an assembled four-leg flexible thermoelectric generator produced a maximum power of 249.48 nW (corresponding power density ~1.23 W m-2) at a temperature difference of 30.1 K, and had good potential for powering low-power-consumption wearable electronics.

Novel Nanoarchitectured Cu2Te as a Photocathodes for Photoelectrochemical Water Splitting Applications

Designing photocathodes with nanostructures has been considered a promising way to improve the photoelectrochemical (PEC) water splitting activity. Cu₂Te is one of the promising semiconducting materials for photoelectrochemical water splitting, the performance of Cu₂Te photocathodes remains poor. In this work, we report the preparation of Cu₂Te nanorods (NRs) and vertical nanosheets (NSs) assembled film on Cu foil through a vapor phase epitaxy (VPE) technique. The obtained nano architectures as photocathodes toward photoelectrochemical (PEC) performance was tested afterwards for the first time. Optimized Cu₂Te NRs and NSs photocathodes showed significant photocurrent density up to 0.53 mA cm^-2 and excellent stability under illumination. Electrochemical impedance spectroscopy and Mott-Schottky analysis were used to analyze in more detail the performance of Cu₂Te NRs and NSs photocathodes. From these analyses, we propose that Cu₂Te NRs and NSs photocathodes are potential candidate materials for use in solar water splitting.

Structural Modularization of Cu2 Te Leading to High Thermoelectric Performance near the Mott-Ioffe-Regel Limit

To date, thermoelectric materials research stays focused on optimizing the material's band edge details and disfavors low mobility. Here, the paradigm is shifted from the band edge to the mobility edge, exploring high thermoelectricity near the border of band conduction and hopping. Through coalloying iodine and sulfur, the plain crystal structure is modularized of liquid-like thermoelectric material Cu₂ Te with mosaic nanograins and the highly size mismatched S/Te sublattice that chemically quenches the Cu sublattice and drives the electronic states from itinerant to localized. A state-of-the-art figure of merit of 1.4 is obtained at 850 K for Cu₂ (S_0.4 I_0.1 Te_0.5 ); and remarkably, it is achieved near the Mott-Ioffe-Regel limit unlike mainstream thermoelectric materials that are band conductors. Broadly, pairing structural modularization with the high performance near the Mott-Ioffe-Regel limit paves an important new path towards the rational design of high-performance thermoelectric materials.

Achievement of Excellent Thermoelectric Properties in Cu-Se-S Compounds via In Situ Phase Separation

In this study, Cu₂Se_1-xS_x (x = 0.1, 0.3, and 0.5) alloy powders were prepared by the hydrothermal synthesis method. In the subsequent sintering process, the spontaneous in situ phase separation process of the sample forms a two-phase hybrid structure. The generated Cu₂S precipitates in the Cu₂Se matrix noticeably enhance phonon scattering, which is beneficial for low thermal conductivity without significantly affecting the electrical transport performance. Ultimately, an optimized thermoelectric performance was obtained in Cu₂Se_0.9S_0.1, reaching a peak zT value of 1.43 at 773 K, the optimum value among the Cu-Se-S systems at this temperature.

Phase Tuning for Enhancing the Thermoelectric Performance of Solution-Synthesized Cu2-xS

Cu₂S, consisting of Earth-abundant and eco-friendly elements, is a promising candidate for thermoelectric energy conversion. The stoichiometric Cu₂S has intrinsically low thermal conductivity together with meagre electrical conductivity. Herein, in order to improve the electrical properties, we develop a facile solvothermal method for synthesizing a series of Cu_2-xS with tunable phase compositions. With reduction in the CuCl/Na₂S molar ratio in the solution precursors from 2:1 to 1.75:1, the solvothermal products transform from single-phase Cu₂S into Cu₂S-Cu_1.96S composites, which are preserved in the hot-pressed pellets. The decreased CuCl/Na₂S molar ratio results in increased room-temperature Hall carrier concentration, leading to enhanced electrical conductivity. Specifically, the pellet corresponding to the 1.75:1 CuCl/Na₂S precursor ratio obtains a power factor of 8.04 μW cm^-1 K^-2 at 815 K, which is ∼735% of the value obtained by stoichiometric Cu₂S. Along with the moderate thermal conductivity, this sample achieves an enhanced zT value of 0.87 at 815 K, representing a ∼250% increase when compared to stoichiometric Cu₂S. This study demonstrates an effective strategy in enhancing the thermoelectric performance of solution-synthesized Cu₂S-based materials by phase tuning.

Enhanced Stability and Thermoelectric Performance in Cu1.85Se-Based Compounds

Liquid-like copper selenium compounds have attracted considerable interest in recent years for their excellent thermoelectric performance, abundant element reserves, and low toxicity. However, the related applications are still limited due to the phase transition and precipitation of Cu under an external field. Here, the cubic Cu_1.85Se-based compounds with suppressed phase transition and improved critical voltage (V_c) are first studied. In particular, Li/Bi co-doping effectively optimizes hole concentration and the ZTs are substantially improved from 0.2 in Cu_1.85Se to 0.7 in Li_0.03Cu_1.81Bi_0.04Se at 760 K. Meanwhile, the latter shows an outstanding V_c above 0.22 V at 750 K, which is the highest value in Cu_2-xSe thermoelectric compounds to date. Moreover, S is alloyed in Li_0.03Cu_1.81Bi_0.04Se to greatly reduce the thermal conductivity and the ZT is further enhanced to 0.9 for Li_0.03Cu_1.81Bi_0.04Se_0.9S_0.1 at 760 K. Our work sheds light on a new strategy to realize good stability and enhanced thermoelectric performance, which provides a new direction for further research.

Boosting High Thermoelectric Performance of Ni-Doped Cu1.9S by Significantly Reducing Thermal Conductivity

At present, copper sulfide materials have been predicted as promising thermoelectric materials due to their inexpensiveness and nontoxicity property. Most researches on copper sulfide are focused on Cu₂S and Cu_1.8S because they are more easily synthesized into a single phase; however, the improper electrical conductivity greatly hindered their thermoelectric properties. In this work, a series of high-performance Cu_1.9-xNi_xS (x = 0, 0.01, 0.015, and 0.02) bulk samples were fabricated by accurately manipulating the ratio of Cu/S with appropriate Ni-doping. The thermoelectric properties of Ni-doped Cu_1.9S were explored in detail for the first time. It can be found that carrier thermal conductivity and lattice thermal conductivity of Cu_1.9-xNi_xS were effectively reduced via Ni-doping, simultaneously, without the great influence on the power factor. Here, the carrier thermal conductivity (κ_e) was reduced due to the extreme reduction of Hall carrier concentration. In addition, amounts of nanopores introduced by Ni-doping and complex crystal structure from the phase transition of the second phase strengthen the phonon scattering and reduce lattice thermal conductivity (κ_l) remarkably. As a consequence, the lowest carrier thermal conductivity and lattice thermal conductivity reach 0.006 and 1.08 W m^-1 K^-1 for Cu_1.88Ni_0.02S at 773 K, and the average ZT is about 0.39 from 323 to 773 K (the ZT_max is about 0.9 at 773 K). This work demonstrates that low-cost and easily fabricated Ni-doped Cu_1.9S is a pleasurable candidate for thermoelectric application despite it usually being treated as an ion conductor.

Promising and Eco-Friendly Cu2 X-Based Thermoelectric Materials: Progress and Applications

Due to the nature of their liquid-like behavior and high dimensionless figure of merit, Cu₂ X (X = Te, Se, and S)-based thermoelectric materials have attracted extensive attention. The superionicity and Cu disorder at the high temperature can dramatically affect the electronic structure of Cu₂ X and in turn result in temperature-dependent carrier-transport properties. Here, the effective strategies in enhancing the thermoelectric performance of Cu₂ X-based thermoelectric materials are summarized, in which the proper optimization of carrier concentration and minimization of the lattice thermal conductivity are the main focus. Then, the stabilities, mechanical properties, and module assembly of Cu₂ X-based thermoelectric materials are investigated. Finally, the future directions for further improving the energy conversion efficiency of Cu₂ X-based thermoelectric materials are highlighted.

Enhanced Thermoelectric Performance of Quaternary Cu2-2 xAg2 xSe1- xS x Liquid-like Chalcogenides

Liquid-like binary Cu_2-δX (X = S, Se, and Te) chalcogenides and their ternary solid solutions have gained notable attention in thermoelectrics due to their interesting and abnormal thermal and electrical transport properties. However, previous studies mainly focus on a single element alloying at either an anion or cation site whereas the investigation on cation/anion co-alloying is very rare so far. Here, a series of quaternary Cu_{2-2 x}Ag_{2 x}Se_{1- x}S _x ( x = 0.01, 0.03, 0.05, 0.1, 0.15) liquid-like copper chalcogenide materials have been fabricated and the effects of Ag/S co-alloying on the thermoelectric properties of Cu₂Se have been systematically studied. It is found that all compounds are mixed phases at room temperature but single cubic phase at high temperatures. The introduction of Ag and S in Cu₂Se brings about a large mass fluctuation rather than strain field fluctuation that effectively suppresses the lattice thermal conductivity. Furthermore, on increasing the Ag and S contents, the high electrical conductivity of pristine Cu₂Se is well tuned to the optimal range derived from the single parabolic band model analysis. Consequently, a peak zT of 1.6 at 900 K is achieved in Cu_1.8Ag_0.2Se_0.9S_0.1, which is about 33% higher than that of binary Cu₂Se.