Crystal structure across the β to α phase transition in thermoelectric Cu2-x Se

Chemical-Vapor-Deposition-Synthesized Two-Dimensional Non-Stoichiometric Copper Selenide (β-Cu2-xSe) for Ultra-Fast Tetracycline Hydrochloride Degradation under Solar Light

The high concentration of antibiotics in aquatic environments is a serious environmental issue. In response, researchers have explored photocatalytic degradation as a potential solution. Through chemical vapor deposition (CVD), we synthesized copper selenide (β-Cu2-xSe) and found it an effective catalyst for degrading tetracycline hydrochloride (TC-HCl). The catalyst demonstrated an impressive degradation efficiency of approximately 98% and a reaction rate constant of 3.14 × 10-2 min-1. Its layered structure, which exposes reactive sites, contributes to excellent stability, interfacial charge transfer efficiency, and visible light absorption capacity. Our investigations confirmed that the principal active species produced by the catalyst comprises O2- radicals, which we verified through trapping experiments and electron paramagnetic resonance (EPR). We also verified the TC-HCl degradation mechanism using high-performance liquid chromatography-mass spectrometry (LC-MS). Our results provide valuable insights into developing the β-Cu2-xSe catalyst using CVD and its potential applications in environmental remediation.

Modeling critical thermoelectric transports driven by band broadening and phonon softening

Critical phenomena are one of the most captivating areas of modern physics, whereas the relevant experimental and theoretical studies are still very challenging. Particularly, the underlying mechanism behind the anomalous thermoelectric properties during critical phase transitions remains elusive, i.e., the current theoretical models for critical electrical transports are either qualitative or solely focused on a specific transport parameter. Herein, we develop a quantitative theory to model the electrical transports during critical phase transitions by incorporating both the band broadening effect and carrier-soft TO phonon interactions. It is found that the band-broadening effect contributes an additional term to Seebeck coefficient, while the carrier-soft TO phonon interactions greatly affects both electrical resistivity and Seebeck coefficient. The universality and validity of our model are well confirmed by experimental data. Furthermore, the features of critical phase transitions are effectively tuned. For example, alloying S in Cu2Se can reduce the phase transition temperature but increase the phase transition parameter b. The maximum thermoelectric figure of merit zT is pushed to a high value of 1.3 at the critical point (377 K), which is at least twice as large as those of normal static phases. This work not only provides a clear picture of the critical electrical transports but also presents new guidelines for future studies in this exciting area.

Strain-Induced Defect Evolution for the Construction of Porous Cu2-xSe with Enhanced Thermoelectric Performance

Superionic Cu2-xSe, with disordered and even liquid-like Cu ions, has been extensively studied as a high efficiency thermoelectric material. However, the relationship between lattice stability and microstructure evolution in Cu2-xSe under strain, which is crucial for its application, has seldom been explored in previous research. In this study, we investigate the impacts of hydrostatic compression strain on the microstructural evolution and, consequently, its implications for thermoelectric performance. Molecular dynamics (MD) simulations show that high hydrostatic compression strain could induce local diffusion of Cu ions and Se twin evolution, resulting in the breaking and reforming of Cu-Se dynamic bonds and the unstable Se sublattice. The subsequent annealing process of the destabilized structure promoted Se evaporation from the sublattice and resulted in lotus-seedpod-like pores. The reduced sound velocity and intensified phonon scattering, due to pores, lead to a reduction in the lattice thermal conductivity from 0.44 W m-1 K-1 to 0.24 W m-1 K-1 at 800 K, a decrease of approximately 45%, in the porous Cu1.92Se sample. These findings reveal the relationship between stability and defect evolution in Cu2-xSe under high hydrostatic compression, offering a straightforward strategy of defect engineering for designing unique microstructures by leveraging the instability in superionic conductor materials.

Formation and Transformation of Cu2-xSe1-yTey Nanoparticles Synthesized by Tellurium Anion Exchange of Copper Selenide

Ion exchange reactions of colloidal nanoparticles post-synthetically modify the composition while maintaining the morphology and crystal structure and therefore are important for tuning properties and producing otherwise inaccessible and/or metastable materials. Reactions involving anion exchange of metal chalcogenides are particularly interesting, as they involve the replacement of the sublattice that defines the structure while also requiring high temperatures that can be disruptive. Here, we show that the tellurium anion exchange of weissite Cu_2-xSe nanoparticles using a trioctylphosphine-tellurium complex (TOP═Te) yields weissite Cu_2-xSe_1-yTe_y solid solutions, rather than complete exchange to weissite Cu_2-xTe, with compositions that are tunable based on the amount of TOP═Te used. Upon storage at room temperature in either solvent or air, tellurium-rich Cu_2-xSe_1-yTe_y solid solution nanoparticles transform, over the span of several days, to a selenium-rich Cu_2-xSe_1-yTe_y composition. The tellurium that is expelled from the solid solution during this process migrates to the surface and forms a tellurium oxide shell, which correlates with the onset of particle agglomeration due to the change in surface chemistry. Collectively, this study demonstrates tunable composition during tellurium anion exchange of copper selenide nanoparticles along with unusual post-exchange reactivity that transforms the composition, surface chemistry, and colloidal dispersibility due to the apparent metastable nature of the solid solution product.

Dynamic correlations and possible diffusion pathway in the superionic conductor Cu2-xSe

The superionic conductor Cu_2-xSe has regained interest as a thermoelectric material owing to its low thermal conductivity, suggested to arise from a liquid-like Cu substructure, and the material has been coined a phonon-liquid electron-crystal. Using high-quality three-dimensional X-ray scattering data measured up to large scattering vectors, accurate analysis of both the average crystal structure as well as the local correlations is carried out to shed light on the Cu movements. The Cu ions show large vibrations with extreme anharmonicity and mainly move within a tetrahedron-shaped volume in the structure. From the analysis of weak features in the observed electron density, the possible diffusion pathway of Cu is identified, and it is clear from its low density that jumps between sites are infrequent compared with the time the Cu ions spend vibrating around each site. These findings support the conclusions drawn from recent quasi-elastic neutron scattering data, casting doubt on the phonon-liquid picture. Although there is diffusion of Cu ions in the structure, making it a superionic conductor, the jumps are infrequent and probably not the origin of the low thermal conductivity. From three-dimensional difference pair distribution function analysis of the diffuse scattering data, strongly correlated movements are identified, showing atomic motions which conserve interatomic distances at the cost of large changes in angles.

Emerging 2D Copper-Based Materials for Energy Storage and Conversion: A Review and Perspective

2D materials have shown great potential as electrode materials that determine the performance of a range of electrochemical energy technologies. Among these, 2D copper-based materials, such as Cu-O, Cu-S, Cu-Se, Cu-N, and Cu-P, have attracted tremendous research interest, because of the combination of remarkable properties, such as low cost, excellent chemical stability, facile fabrication, and significant electrochemical properties. Herein, the recent advances in the emerging 2D copper-based materials are summarized. A brief summary of the crystal structures and synthetic methods is started, and innovative strategies for improving electrochemical performances of 2D copper-based materials are described in detail through defect engineering, heterostructure construction, and surface functionalization. Furthermore, their state-of-the-art applications in electrochemical energy storage including supercapacitors (SCs), alkali (Li, Na, and K)-ion batteries, multivalent metal (Mg and Al)-ion batteries, and hybrid Mg/Li-ion batteries are described. In addition, the electrocatalysis applications of 2D copper-based materials in metal-air batteries, water-splitting, and CO₂ reduction reaction (CO₂ RR) are also discussed. This review also discusses the charge storage mechanisms of 2D copper-based materials by various advanced characterization techniques. The review with a perspective of the current challenges and research outlook of such 2D copper-based materials for high-performance energy storage and conversion applications is concluded.

Atomic-Scale Observation of Grain Boundary Dominated Unsynchronized Phase Transition in Polycrystalline Cu2 Se

Phase transition is a physical phenomenon that attracts great interest of researchers. Although the theory of second-order phase transitions is well-established, their atomic-scale dynamics in polycrystalline materials remains elusive. In this work, second-order phase transitions in polycrystalline Cu₂ Se at the transition temperature are directly observed by in situ aberration-corrected transmission electron microscopy. Phase transitions in microcrystalline Cu₂ Se start at the grain boundaries and extend inside the grains. This phenomenon is more pronounced in nanosized grains. Analysis of phase transitions in nanocrystalline Cu₂ Se with different grain boundaries demonstrates that grain boundary energy dominates unsynchronized phase transition behavior. This suggests that the energy of grain boundaries is the key factor influencing the energetic barrier for initiation of phase transition. The findings advance atomic-scale understanding of second-order phase transitions, which is crucial for the control of this process in polycrystalline materials.

Charge-Compensation in a Displacement Mg2+ Storage Cathode through Polyselenide-Mediated Anion Redox

Chalcogenides have been viewed as important conversion-type Mg²⁺ -storage cathodes to fulfill the high volumetric energy density promise of magnesium (Mg) batteries. However, the low initial Columbic efficiency and the rapid capacity degradation remain challenges for the chalcogenide cathodes, as the clear Mg²⁺ -storage mechanism has yet to be clarified. Herein, we illustrate that the charge storage mechanism of the Cu_2-x Se cathode is a reversible displacement reaction along with a polyselenide (PSe) mediated solution process of anion-compensation. The unique anion redox improves charge storage, while the dissolution of PSe also leads to performance degradation. To address this issue, we introduce Mo₆ S₈ into the Cu_2-x Se cathode to immobilize PSe, which significantly improves performance, especially the reversible capacity (from 140 mAh g^-1 to 220 mAh g^-1 ). This work provides inspiration for the modification of the Mg²⁺ -storage cathode, which is a milestone for high-performance Mg batteries.

Insights into Low Thermal Conductivity in Inorganic Materials for Thermoelectrics

Efficient manipulation of thermal conductivity and fundamental understanding of the microscopic mechanisms of phonon scattering in crystalline solids are crucial to achieve high thermoelectric performance. Thermoelectric energy conversion directly and reversibly converts between heat and electricity and is a promising renewable technology to generate electricity by recovering waste heat and improve solid-state refrigeration. However, a unique challenge in thermal transport needs to be addressed to achieve high thermoelectric performance: the requirement of crystalline materials with ultralow lattice thermal conductivity (κ_L). A plethora of strategies have been developed to lower κ_L in crystalline solids by means of nanostructural modifications, introduction of intrinsic or extrinsic phonon scattering centers with tailored shape and dimension, and manipulation of defects and disorder. Recently, intrinsic local lattice distortion and lattice anharmonicity originating from various mechanisms such as rattling, bonding heterogeneity, and ferroelectric instability have found popularity. In this Perspective, we outline the role of manipulation of chemical bonding and structural chemistry on thermal transport in various high-performance thermoelectric materials. We first briefly outline the fundamental aspects of κ_L and discuss the current status of the popular phonon scattering mechanisms in brief. Then we discuss emerging new ideas with examples of crystal structure and lattice dynamics in exemplary materials. Finally, we present an outlook for focus areas of experimental and theoretical challenges, possible new directions, and integrations of novel techniques to achieve low κ_L in order to realize high-performance thermoelectric materials.

Electrically Tunable Antiferroelectric to Paraelectric Switching in a Semiconductor

The monoclinic α-Cu₂Se phase is the first multipolar antiferroelectric semiconductor identified recently by electron microscopy. As a semiconductor, although there are no delocalized electrons to form a static macroscopic polarization, a spontaneous and localized antiferroelectric polarization was found along multiple directions. In conventional ferroelectrics, the polarity can be switched by an applied electric field, and a ferroelectric to paraelectric transition can be modulated by temperature. Here, we show that a reversible and robust antiferroelectric to paraelectric switching in a Cu₂Se semiconductor can be tuned electrically by low-voltage and high-frequency electric pulses, and the structural transformations are imaged directly by transmission electron microscopy (TEM). The atomic mechanism of the transformation was assigned to an electrically triggered cation rearrangement with a low-energy barrier. Due to differences of the antiferroelectric and paraelectric phases regarding their electrical, mechanical, and optical properties, such an electrically tunable transformation has a great potential in various applications in microelectronics.

Thermoelectric performance enhancement of eco-friendly Cu2Se through incorporating CB4

We have prepared Cu2Se + x wt% CB4 composites with x = 0, 0.1, 0.3, 0.5, and 0.7 by a hydrothermal method and hot-pressing technique. The structural and compositional analysis indicates that pure phase Cu2Se powders were synthesized and the densified layered bulk samples were obtained. Electrical properties testing showed that the sample with x = 0.5 has the high power factor of 0.886 mW m-1 K-2 due to its high Seebeck coefficient. Meanwhile, the thermal conductivity was suppressed to 0.6 W m-1 K-1 at 773 K. As a result, the final optimized ZT value of 1.46 at 773 K was achieved. These results suggest that CB4 could be an alternative inclusion to improve effectively the thermoelectric performance of Cu2Se.

Sample preparation induced phase transitions in solution deposited copper selenide thin films

Thin films of CuSe were deposited onto GaAs substrate. XRD showed that the as-deposited films were of the Klockmannite (CuSe – P6₃/mmc 194) phase with lattice parameters a₀ = b₀ = 0.3939 nm, c₀ = 1.7250 nm; however, electron diffraction in the TEM surprisingly indicated the β-Cu_2−xSe phase (Cu_1.95Se – R3̄m 166) with lattice parameters a₀ = b₀ = 0.412 nm, c₀ = 2.045 nm. The discrepancy originated from the specimen preparation method, where the energy of the focused ion beam resulted in loss of selenium which drives a phase transition to β-Cu_2−xSe in this system. The same phase transition was observed also upon thermal treatment in vacuum, as well as when the 200 keV electron beam was focused on a powder sample in the TEM. The initial phase can be controlled to some extent by changing the composition of the reactants in solution, resulting in thin films of the cubic α-Cu_2−xSe (Cu_1.95Se – Fm3̄m) phase co-existing together with the β-Cu_2−xSe phase.

Ion beam irradiation causes Klockmannite CuSe to lose Se and transform into β-Cu₂Se. Caution must be taken when using the dual beam FIB for preparing TEM specimen.

Electroresistance in multipolar antiferroelectric Cu2Se semiconductor

Electric field-induced changes in the electrical resistance of a material are considered essential and enabling processes for future efficient large-scale computations. However, the underlying physical mechanisms of electroresistance are currently remain largely unknown. Herein, an electrically reversible resistance change has been observed in the thermoelectric α-Cu2Se. The spontaneous electric dipoles formed by Cu+ ions displaced from their positions at the centers of Se-tetrahedrons in the ordered α-Cu2Se phase are examined, and α-Cu2Se phase is identified to be a multipolar antiferroelectric semiconductor. When exposed to the applied voltage, a reversible switching of crystalline domains aligned parallel to the polar axis results in an observed reversible resistance change. The study expands on opportunities for semiconductors with localized polar symmetry as the hardware basis for future computational architectures.

Electronic origin of the enhanced thermoelectric efficiency of Cu2Se

Thermoelectric materials (TMs) can uniquely convert waste heat into electricity, which provides a potential solution for the global energy crisis that is increasingly severe. Bulk Cu₂Se, with ionic conductivity of Cu ions, exhibits a significant enhancement of its thermoelectric figure of merit zT by a factor of ~3 near its structural transition around 400 K. Here, we show a systematic study of the electronic structure of Cu₂Se and its temperature evolution using high-resolution angle-resolved photoemission spectroscopy. Upon heating across the structural transition, the electronic states near the corner of the Brillouin zone gradually disappear, while the bands near the centre of Brillouin zone shift abruptly towards high binding energies and develop an energy gap. Interestingly, the observed band reconstruction well reproduces the temperature evolution of the Seebeck coefficient of Cu₂Se, providing an electronic origin for the drastic enhancement of the thermoelectric performance near 400 K. The current results not only bridge among structural phase transition, electronic structures and thermoelectric properties in a condensed matter system, but also provide valuable insights into the search and design of new generation of thermoelectric materials.

The order-disorder transition in Cu2Se and medium-range ordering in the high-temperature phase

The high thermoelectric performance of cuprous selenide (Cu₂Se) arises from its specific structures consisting of two independent sublattices, i.e. the rigid face-centered cubic (f.c.c.) Se sublattice and the flexible Cu sublattice showing a variety of ordered configurations at numerous interstitial sites. Upon increasing the temperature, the Cu sublattice undergoes an order-to-disorder transition but the details of the structural evolution have not been fully elucidated. Here, in situ transmission electron microscopy (TEM) is used to investigate the thermally induced structural changes of Cu₂Se in both real and reciprocal spaces. Order-disorder transition was found to proceed in nanoblocks accompanied by the structural fluctuations between low-temperature and high-temperature phases. Electron diffraction revealed the emergence of medium-range ordering of Cu atoms in the high-temperature f.c.c. phase. By referring to the Coulomb interaction evaluations, the superstructures for the medium-range ordering were constructed. Such medium-range atomic ordering was sustained over a wide temperature range (from the phase transition temperature to over 800 K in the TEM) but gradually changed to short-range ordering as indicated by the appearance of diffuse scattering rings.

High-performance thermoelectric silver selenide thin films cation exchanged from a copper selenide template

Over the past decade, Ag₂Se has attracted increasing attention due to its potentially excellent thermoelectric (TE) performance as an n-type semiconductor. It has been considered a promising alternative to Bi-Te alloys and other commonly used yet toxic and/or expensive TE materials. To optimize the TE performance of Ag₂Se, recent research has focused on fabricating nanosized Ag₂Se. However, synthesizing Ag₂Se nanoparticles involves energy-intensive and time-consuming techniques with poor yield of final product. In this work, we report a low-cost, solution-processed approach that enables the formation of Ag₂Se thin films from Cu_2-x Se template films via cation exchange at room temperature. Our simple two-step method involves fabricating Cu_2-x Se thin films by the thiol-amine dissolution of bulk Cu₂Se, followed by soaking Cu_2-x Se films in AgNO₃ solution and annealing to form Ag₂Se. We report an average power factor (PF) of 617 ± 82 μW m^-1 K^-2 and a corresponding ZT value of 0.35 at room temperature. We obtained a maximum PF of 825 μW m^-1 K^-2 and a ZT value of 0.46 at room temperature for our best-performing Ag₂Se thin-film after soaking for 5 minutes. These high PFs have been achieved via full solution processing without hot-pressing.

Solving the disordered structure of β-Cu2-xSe using the three-dimensional difference pair distribution function

High-performing thermoelectric materials such as Zn₄Sb₃ and clathrates have atomic disorder as the root to their favorable properties. This makes it extremely difficult to understand and model their properties at a quantitative level, and thus effective structure-property relations are challenging to obtain. Cu_2-xSe is an intensely studied, cheap and non-toxic high performance thermoelectric, which exhibits highly peculiar transport properties, especially near the β-to-α phase transition around 400 K, which must be related to the detailed nature of the crystal structure. Attempts to solve the crystal structure of the low-temperature phase, β-Cu_2-xSe, have been unsuccessful since 1936. So far, all studies have assumed that β-Cu_2-xSe has a three-dimensional periodic structure, but here we show that the structure is ordered only in two dimensions while it is disordered in the third dimension. Using the three-dimensional difference pair distribution function (3D-ΔPDF) analysis method for diffuse single-crystal X-ray scattering, the structure of the ordered layer is solved and it is shown that there are two modes of stacking disorder present which give rise to an average structure with higher symmetry. The present approach allows for a direct solution of structures with disorder in some dimensions and order in others, and can be thought of as a generalization of the crystallographic Patterson method. The local and extended structure of a solid determines its properties and Cu_2-xSe represents an example of a high-performing thermoelectric material where the local atomic structure differs significantly from the average periodic structure observed from Bragg crystallography.

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.

Thermal Conductivity during Phase Transitions

Thermal conductivity is a very basic property that determines how fast a material conducts heat, which plays an important and sometimes a dominant role in many fields. However, because materials with phase transitions have been widely used recently, understanding and measuring temperature-dependent thermal conductivity during phase transitions are important and sometimes even questionable. Here, the thermal transport equation is corrected by including heat absorption due to phase transitions to reveal how a phase transition affects the measured thermal conductivity. In addition to the enhanced heat capacity that is well known, it is found that thermal diffusivity can be abnormally lowered from the true value, which is also dependent on the speed of phase transitions. The extraction of the true thermal conductivity requires removing the contributions from both altered heat capacity and thermal diffusivity during phase transitions, which is well demonstrated in four selected kinds of phase transition materials (Cu₂ Se, Cu₂ S, Ag₂ S, and Ag₂ Se) in experiment. This study also explains the lowered abnormal thermal diffusivity during phase transitions in other materials and thus provides a novel strategy to engineer thermal conductivity for various applications.

Discovery of colossal Seebeck effect in metallic Cu2Se

Both electrical conductivity σ and Seebeck coefficient S are functions of carrier concentration being correlated with each other, and the value of power factor S2σ is generally limited to less than 0.01 W m-1 K-2. Here we report that, under the temperature gradient applied simultaneously to both parallel and perpendicular directions of measurement, a metallic copper selenide, Cu2Se, shows two sign reversals and colossal values of S exceeding ±2 mV K-1 in a narrow temperature range, 340 K < T < 400 K, where a structure phase transition takes place. The metallic behavior of σ possessing larger magnitude exceeding 600 S cm-1 leads to a colossal value of S2σ = 2.3 W m-1 K-2. The small thermal conductivity less than 2 W m-1 K-1 results in a huge dimensionless figure of merit exceeding 400. This unusual behavior is brought about by the self-tuning carrier concentration effect in the low-temperature phase assisted by the high-temperature phase.

Nanoscale Behavior and Manipulation of the Phase Transition in Single-Crystal Cu2 Se

Phase transition is a fundamental physical phenomenon that has been widely studied both theoretically and experimentally. According to the Landau theory, the coexistence of high- and low-temperature phases is thermodynamically impossible during a second-order phase transition in a bulk single crystal. Here, the coexistence of two (α and β) phases in wedge-shaped nanosized single-crystal Cu₂ Se over a large temperature range are demonstrated. By considering the surface free-energy difference between the two phases and the shape effect, a thermodynamic model is established, which explicitly explains their coexistence. Intriguingly, it is found that with a precise control of the heating temperature, the phase boundary can be manipulated at atomic level. These discoveries extend the understanding of phase transitions to the nanoscale and shed light on rational manipulation of phase transitions in nanomaterials.

Recent advances in inorganic material thermoelectrics

Time line of representative inorganic bulk thermoelectric materials from 1960s to the present.