Effect of the annealing atmosphere on crystal phase and thermoelectric properties of copper sulfide

1 Effect of the annealing atmosphere on crystal phase and thermoelectric properties of copper sulfide Mengyao Li,† Yu Liu,*,‡ Yu Zhang,† Ting Zhang, Yong Zuo,† Ke Xiao,† Jordi Arbiol,,¶ Jordi Llorca,§ Maria Ibáñez,‡ Junfeng Liu,*,# Andreu Cabot*, †, ¶ † Catalonia Energy Research Institute - IREC, Sant Adrià de Besòs, 08930 Barcelona, Spain. ‡ Institute of Science and Technology Austria (IST Austria), Am Campus 1, 3400, Klosterneuburg, Austria.  Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Catalonia, Spain. ¶ ICREA, Pg. Lluis Companys 23, 08010 Barcelona, Catalonia, Spain. § Institute of Energy Technologies, Department of Chemical Engineering and Barcelona Research Center in Multiscale Science and Engineering, Universitat Politècnica de Catalunya, EEBE, 08019 Barcelona, Spain # Institute for Energy Research, School of Chemistry and Chemical Engineering, Jiangsu University, 212013 Zhenjiang, P.R. China * E-mails: Y. Liu: [email protected]; J. Liu: [email protected]; A. Cabot: [email protected] 2 ABSTRACT Cu2-xS has become one of the most promising thermoelectric materials for application in the middle-high temperature range. Its advantages include the abundance, low cost and safety of its elements and a high performance at high temperature. However, stability issues limit its implementation in real devices, restraining their operation current and temperature, which calls for the optimization of the material performance in the middle temperature range. Here, we present a synthetic protocol for large scale production of covellite CuS nanoparticles at ambient temperature and atmosphere, and using water as solvent. The crystal phase and stoichiometry of the particles is afterward tuned through annealing at a moderate temperature under an inert or reducing atmosphere. While the annealing in an Ar atmosphere results in Cu1.8S nanopowder with a rhombohedral crystal phase, the annealing in a reducing atmosphere leads to tetragonal Cu1.96S. High temperature x-ray diffraction analysis show the material annealed in argon to transform to the cubic phase at ca. 400 K, while the material annealed in the presence of hydrogen undergoes two phase transitions, first to hexagonal and then to a cubic structure. The annealing atmosphere, temperature and air allows adjusting the density of copper vacancies and thus controlling the material transport properties. In this direction, the material annealed under Ar shows higher electrical conductivities but lower Seebeck coefficients than the material annealed in the presence of hydrogen. By optimizing the annealing time, Cu2-xS with record figures of merit in the middle temperature range, up to 1.31 at 710 K, are obtained. We finally demonstrate that this strategy, based on low cost solution synthesis process, is also suitable for the production of high performance Cu2-xS layers using high throughput and cost effective printing technologies. 3 INTRODUCTION Thermoelectric devices, able to convert ubiquitous temperature gradients into electricity, have countless potential applications, but their high manufacturing and materials costs hamper their cost-effectiveness and limit their widespread implementation. To overcome these drawbacks, thermoelectric devices must be manufactured using high throughput technologies and making use of materials based on low cost and abundant elements. The thermoelectric properties of copper sulfides have been explored for well above a century. However, it was not until this last decade that copper sulfides raised especial attention not only because of the low-cost and abundance of the involved elements, but also because of their outstanding thermoelectric performance in the middle-high temperature range. Copper chalcogenides and particularly copper sulfides are a fascinating family of materials with exceptional characteristics1: i) high element abundance, low cost and minor environmental and health impact; ii) enormous structural and compositional versatility; iii) low energy of formation of defects resulting in high defect densities; iv) chalcogen atoms forming regular lattices with Cu atoms occupying not well-defined interstitial sites; v) strong influence of Cu 3d states in the chalcogenide electronic properties; vi) singular and highly tunable functional properties, including low direct band gaps, plasmonic properties, high charge carrier concentrations, high ionic conductivity, and low thermal conductivity. Even the simplest binary Cu-S system exhibits a particularly rich phase diagram with many possible stoichiometric compositions and crystallographic phases, some of them with quite complex atomic arrangements. The main crystal phases in the Cu-S system at ambient temperature are the monoclinic chalcocite (Cu2.0-1.997S), monoclinic djurleite (Cu1.97S-Cu1.93S), triclinic roxbyite (Cu58S32 or Cu1.81S), cubic digenite (Cu9S5 or Cu1.8S), orthorhombic anilite (Cu7S4 or Cu1.75S), and hexagonal covellite (CuS). These crystal structures are based upon approximately hexagonal (djurlite, chalcocite, roxbyite and covellite) or cubic (anilite and digenite) close-packing of the sulfur atoms, with Cu atoms occupying various interstitial sites with different statistical probabilities that depend on temperature.2,3 Chalcocite Cu2S displays a monoclinic P21/c crystal structure. This is essentially a 4 superstructure of a hexagonal lattice with slight distortions where the 36 crystallographically distinct Wyckoff sites are fully occupied with 12 S atoms and 24 Cu atoms.3,4 Stoichiometric Cu2S is considered to be intrinsically unstable at ambient conditions owing to a thermodynamic tendency towards loosing Cu through oxidation in the surface.5 Thus chalcocite nanoparticles degrade quickly into copper deficient Cu2-xS phases, most notably djurleite Cu1.97S, which is considered the thermodynamically stable stoichiometry under ambient conditions. Djurleite Cu1.93-1.97S also displays a monoclinic structure, but with a slightly reduced Cu concentration. The unit cell content of djurleite corresponds to Cu62S32 or Cu1.93S. If an extra Cu atom is added, the unit cell content becomes Cu63S32 or Cu1.97S, thus explaining the lower and higher limit of Cu1.93-1.97S for djurleite. Both chalcocite and djurleite are very common phases at ambient temperature and are often intermixed or intergrown.6 Roxbyite Cu58S32 or Cu1.81S is closely related to chalcocite and djurletite, but it has a triclinic structure consisting of a distorted hexagonal framework of 32 S atoms with 58 independent Cu atoms, all having triangular coordination.7 Roxbyite is uncommon in bulk, but can be readily synthesized in the form of nanocrystals.8 Digenite Cu1.8S has a cubic, but still complex structure with space group Fd3m. Sulfur atoms form a fcc structure and the copper atoms are statistically distributed over several sites. It is sometimes referred to as Cu9S5 but since Cu atoms are statistically distributed, the formula Cu1.8S is preferred.9 Anitite Cu7S4 or Cu1.75S has an orthorhombic crystal phase, space group Pnma, in which the S atoms approximate an fcc arrangement and the Cu atoms are ordered in the interstices.10,11 Finally, covellite CuS has a peculiar layered structure for such a simple stoichiometry. It is also a hexagonal phase with alternating layers of CuS and Cu-S2-Cu.12–14 This unusual structure is explained by the fact that it is more energetically favorable for the S atoms to covalently bond to each other than to bond with Cu atoms.12 The Cu-S phase diagram is actually much more complex, with additional well-defined phases/minerals, including geerite (Cu1.6S), spionkopite (Cu1.39S) and yarrowite (Cu1.12S). Furthermore, several metastable phases also exist and are commonly obtained at ambient temperature, such as a tetragonal Cu1.96S phase 6,15–17 and a rhombohedral Cu1.8S structure.18,19 At relatively low temperatures, just above 70-100 ºC depending on stoichiometry, Cu atoms 5 become highly mobile, which makes it difficult to determine their exact positions. Thus only a statistical distribution of Cu atoms over the various Wyckoff positions in the lattice can be determined and the symmetry of the observed crystal structures increases.20,21 The ambient temperature monoclinic chalcocite phase, known as low chalcocite, transforms into a hexagonal phase known us high chalcocite at around 376 K, and to a cubic phase at about 709 K, known as cubic chalcocite. Monoclinic djurlite is stable up to about 363 K. At this temperature it transforms into a combination of hexagonal chalcocite and cubic dijenite phases, with the hexagonal phase disappearing as increasing temperature.16 Dijenite undergoes complete phase transition into a higher symmetry cubic phase at even lower temperature, 346 K.9,18 The high temperature dijenite phase is known as high digenite, and it is identical to the cubic chalcocite.22 Above 710 K only high-digenite with Cu2S composition or cubic chalcocite, in equilibrium with copper, can be obtained.9,22 Anilite is also stable only to 345 K,22,23 transforming into the cubic high digenite phase at higher temperatures. To make things even more complex, phase and stoichiometry do not always go together. For instance, hexagonal digenite has been reported especially in nanoparticle form, and a cubic high chalcocite has been also found. To minimize confusion, it is wise to associate the mineral names to the crystal phase regardless of the exact composition, thus hexagonal digenite should be better referred as a Cu-deficient high chalcocite and the cubic high chalcocite as a Cu-rich high digenite.3 Phase transitions strongly depends on several parameters including the initial phase, pressure and composition. The lower the Cu content, the lower the transition temperature from monoclinic to hexagonal and cubic phase for instance. Besides, the phase transition temperature also strongly depends on the crystal domain size. A good example is observed for chalcocite Cu2S nanocrystals, where the smaller the size, the smaller the transition temperature.24 Actually, small enough nanocrystals are reported to maintain the hexagonal high chalcocite phase even at ambient temperature.25 The atmosphere composition, and particularly the sulfur partial pressure is another parameter that strongly influences the phase transition.9 The electronic structure and functional properties of Cu2-xS depend on both stoichiometry and crystal phase. In Cu chalcogenides, the top of the valence band has a strong contribution from the chalcogenide p orbitals and the bottom of the conduction band mainly has contributions 6 from Cu 4s and 4p orbitals.1,26 It is assumed that each Cu atom contributes to bonding with one 4s electron and each chalcogen atom contributes with six p electrons. In the fully stoichiometric Cu2S material, the valence band is completely filled and the material would behave as an intrinsic semiconductor. However, when Cu vacancies are created, holes are generated in the top of the valence band. The Cu deficiency mainly affects the valency of the chalcogen, whereas the valency of Cu remains close to +1 in all phases and stoichiometries. All Cu2-xS exhibit p-type electronic character with a stoichiometry-dependent hole concentration that increases with the number of Cu vacancies (x). Besides, the type of sulfur arrangement in the crystal structure also plays an important role on the electronic transport properties of the material.27 On the other hand, the Cu disorder provides the material not only with a high ionic conductivity but also an ultralow thermal conductivity. Overall, thermoelectric properties are very sensitive to the Cu deficiency, i.e. the Cu2-xS phase and stoichiometry. Thus, phase tuning at the synthesis step or through post-synthesis treatments should be a very effective strategy to optimize thermoelectric properties of copper sulfide. In the present work, we demonstrate a very simple and low-cost room temperature aqueous phase method for the large-scale preparation of copper sulfide. Additionally, we analyze the possibilities of crystallographic phase tuning of nanocrystalline Cu2-xS by means of thermal annealing at moderate temperatures under inert or reducing gas atmospheres. Finally, we analyze the thermoelectric properties of the obtained compounds as a function of the annealing temperature and explain the obtained results taking into account the phase transitions undergone by the materials when heated. 7 EXPERIMENTAL Chemicals. Copper (II) nitrate trihydrate (Cu(NO3)2·3H2O) and ammonium sulfide solution ((NH4)2S 20% in H2O) were purchased from Fisher. Chemicals were used as received without further purification. Copper sulfide nanoparticles. The reaction took place at ambient atmosphere and temperature inside a fume hood. To obtain around 1 g of nanoparticles, 10 mmol Cu(NO3)2·3H2O was dissolved in 35 ml of deionized water in a centrifuge tube using ultrasounds until obtaining a clear light-blue solution. Then, 2 ml (NH4)2S was injected into the centrifuge tube, which was strongly shaken to force a proper mixing of the reactants. Upon (NH4)2S injection, the solution turned black owing to the nucleation of copper sulfide through the reaction: (NH4)2S + Cu(NO3)2 → CuS + 2NH4NO3 After 1 min reaction, particles were recovered by centrifugation. Then, they were purified using 5 dispersion/precipitation cycles using deionized water and ethanol. Finally, nanoparticles were dried under vacuum overnight at room temperature and stored in an Ar-filled glovebox. Nanomaterial consolidation. Dried nanoparticles were annealed at 450 ℃ for 180 min under gas flow inside a tube furnace located inside the glovebox. Two different gas atmospheres were studied: argon and 5% hydrogen in argon (Ar/H2). Then, the annealed nanopowders were loaded into a graphite die and hot-pressed at 420 ℃ and 60 MPa for 4 min using a custom-made hot press inside the Ar-filled glovebox.28–32 The relative densities of the obtained pellets were measured by the Archimedes’ method to be ca. 93% of the theoretical value. Structural and chemical characterization. X-ray diffraction (XRD, 2θ angle: 20ºto 80º; scanning rate: 1.5º/min) analyses were carried out on a Bruker AXS D8 ADVANCE X-ray diffractometer with Cu−Kα radiation (λ = 1.5406 Å). Size and morphology of nanoparticles were examined by transmission electron microscopy (TEM) using a ZEISS LIBRA 120, operating at 120 kV, and field-emission scanning electron microscopy (SEM) on an Auriga Zeiss operated at 5.0 kV. The material composition was analyzed using an Oxford energy dispersive X-ray spectrometer (EDX) attached to a Zeiss Auriga SEM at 20.0 kV. Crystallographic structure and chemical composition were analyzed by high resolution TEM 8 (HRTEM) and electron energy loss spectroscopy (EELS), respectively, using a Tecnai F20 fieldemission gun microscope at 200 keV with an embedded Gatan QUANTUM image filter. X-ray photoelectron spectroscopy (XPS) was carried out on a Specs system equipped with a Mg anode XR50 source operating at 250 W and a Phoibos 150 MCD-9 detector (Specs GmbH). The pressure in the analysis chamber was kept below 10-7 Pa. Data processing was performed with the CasaXPS program (Casa Software Ltd.). Thermoelectric Property Measurements. Seebeck coefficients were measured using a static DC method. Electrical resistivity data was obtained by a standard four-probe method. Both the Seebeck coefficient and the electrical resistivity were measured simultaneously in an LSR-3 LINSEIS system in the temperature range between room temperature and 710 K under helium atmosphere. All samples were measured at least 3 consecutive times during heating up to around 710 K. Taking into account the system accuracy and the measurement precision, an error of ca. 4% in the measurement of the electrical conductivity and Seebeck coefficient was estimated. Thermal conductivities (κtotal) were obtained by multiplying the thermal diffusivity (λ), the constant pressure heat capacity (Cp) and the density of the material (ρ), where κtotal= λCpρ. A Xenon Flash Apparatus XFA600 was used to determine the thermal diffusivities of the samples with an estimated error of ca. 5 %. The constant pressure heat capacity (Cp) was estimated from empirical formulas by the Dulong–Petit limit (3R law). The density values were measured using the Archimedes’ method. To avoid cluttering the plots, error bars were not included in the figures. Hall charge carrier concentrations (nн) and mobilities (μн) at room temperature were measured with the Van der Pauw method using a magnetic field of 0.6 T (ezHEMS, NanoMagnetics). 9 RESULTS AND DISCUSSIONS A rapid, ambient temperature and surfactant-free synthetic protocol was developed to produce copper sulfide nanoparticles using (NH4)2S and Cu(NO3)2·3H2O as sulfur and copper precursors, respectively (Figure 1a, see details in the experimental section). Figure 1b,c shows representative SEM and TEM images of the reaction product that consisted in ca. 60 nm nanoparticles with flat morphology (Figure 1c). XRD analysis showed the crystal structure of the obtained nanoparticles to match the covellite CuS phase (JCPDS No. 01-079-2321). Figure 1. a) Schematic illustration of the three-step process used to produce copper sulfide pellets that involves the synthesis of nanoparticles at ambient pressure and temperature, the annealing of the particles at 723 K in Ar or Ar/H2 atmosphere, and the hot press of the materials at 693 K and 60 MPa. b) SEM micrograph of CuS nanoparticles. c) TEM micrographs of CuS nanoparticles. d) Histogram of the particle size distribution. e) XRD pattern obtained from CuS nanoparticles. 16 Figure 7. Thermoelectric properties of Cu2-xS pellets obtained from nanoparticles annealed under Ar (black solid symbols) and Ar/H2 (blue open symbols): a) thermal conductivity, κtotal; b) lattice thermal conductivity, κL; c) thermoelectric figure of merit, ZT; Comparison of ZT values at 710 K for Cu2-xS-based materials.36–50 The excellent performances measured from materials initially produced in nanoparticle form demonstrates the high suitability of the presented material processing strategy. We believe that the synthesis of copper sulfide in the form of nanoparticles is key both to reduce production cost, as no large amounts of energy and time are invested in organizing atoms in large crystal domains, and to control composition by means of a mild thermal annealing in a controlled atmosphere. Besides, the availability of the material in the form of nanoparticles suspended in a low-cost and safe solvent such as water allows producing devices using printing technologies. To advance in this direction, we prepared inks using the annealed nanoparticles and printed them on a mica substrate (Figure 8). Then we hot-pressed the films in the same conditions as the pellets. Figure 8b-d displays the thermoelectric properties of the films obtained from Cu2- 17 xS nanoparticles annealed under Ar or Ar/H2 flow for 3h. The results obtained from the films were very similar to those obtained from pellets, which demonstrates the potential of these nanoparticle-based material to produce thermoelectric devices using cost-effective and highthroughput printing technologies. Figure 8. Schematic illustration of devices of copper sulfide produced by printing. Thermoelectric properties of the Cu2-xS films: a) electrical conductivity, σ; b) Seebeck coefficient, S; c) power factor, PF. 18 CONCLUSIONS We reported a facile, scalable and low-cost method for the synthesis of covellite CuS nanoparticles in aqueous media and at room temperature. The produced covellite nanoparticles lose sulfur when heated under inert or reducing atmosphere, and change their crystal phase to metastable rhombohedral Cu1.8S or tetragonal Cu1.96S. The annealing atmosphere and time allowed tuning the amount of sulfur lost and thus the stoichiometry and crystal structure of the final material. The sample stoichiometry determines the density of copper vacancies formed, thus the hole concentration and consequently the transport properties of the material. Comparing the material annealed in Ar with the annealed in Ar/H2, we observe the former to be characterized by much higher charge carrier concentrations and electrical conductivities, but much lower Seebeck coefficients. Overall, the material annealed in Ar/H2 for 3h was characterized with the highest power factors and thermoelectric figures of merit, up to 1.31 at 710 K, which is the highest ZT reported for Cu2-xS at this temperature. The availability of the material in the form of nanoparticles also allowed the direct printing of the material in the form of films. The printed films were characterized by similar thermoelectric performances as those obtained from the pellets. ACKNOWLEDGEMENT This work was supported by the European Regional Development Funds. 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