Multinary Tetrahedrite (Cu12-x-yMxNySb4S13) Nanoparticles: Tailoring Thermal and Optical Properties with Copper-Site Dopants

Tetrahedrite (Cu12Sb4S13) is an earth-abundant and nontoxic compound with prospective applications in green energy technologies such as thermoelectric waste heat recycling or photovoltaic power generation. A facile, one-pot solution-phase modified polyol method has been developed that produces high-purity nanoscale tetrahedrite products with exceptional stoichiometric and phase control. This modified polyol method is used here to produce phase-pure quaternary and quintenary tetrahedrite nanoparticles doped on the Cu-site with Zn, Fe, Ni, Mn, or Co. This is the first time that Cu-site codoped quintenary tetrahedrite and Mn-doped quaternary tetrahedrite have been produced by a solution-phase method. X-ray diffraction shows phase-pure tetrahedrite, while scanning and transmission electron microscopy show the size and morphology of the nanomaterials. Energy dispersive X-ray spectroscopy confirms nanoparticles have near-stoichiometric elemental compositions. Thermal stability of quintenary codoped tetrahedrite material is analyzed using thermogravimetric analysis, finding that codoping with Mn, Fe, Ni, and Zn increased thermal stability while codoping with cobalt decreased thermal stability. This is the first systematic study of the optical properties of quaternary and quintenary tetrahedrite nanoparticles doped on the Cu-site. Visible-NIR diffuse reflectance spectroscopy reveals that the quaternary and quintenary tetrahedrite nanoparticles have direct optical band gaps ranging from 1.88 to 2.04 eV. Data from thermal and optical characterization support that codoped tetrahedrite nanoparticles are composed of quintenary grains. This research seeks to enhance understanding of the material properties of tetrahedrite, leading to the optimization of sustainable, nontoxic, and high-performance photovoltaic and thermoelectric materials.

Improved Thermoelectric Performance of Cu12Sb4S13 through Gd-Substitution Induced Enhancement of Electronic Density of States and Phonon Scattering

Cu₁₂Sb₄S₁₃ has aroused great interest because of its earth-abundant constituents and intrinsic low thermal conductivity. However, the applications of Cu₁₂Sb₄S₁₃ are hindered by its poor thermoelectric performance. Herein, it is shown that Gd substitution not only causes a significant increase in both electrical conductivity σ and thermopower S but also leads to dramatic drop in lattice thermal conductivity κ_L. Consequently, large ZT reaches 0.94 at 749 K for Cu_11.7Gd_0.3Sb₄S₁₃, which is ∼41% higher than the ZT value of undoped sample. Rietveld refinements of XRD results show that accompanying inhibition of impurity phase Cu₃SbS₄, the number of Cu vacancies increases substantially with substituted content x (x ≤ 0.3), which leads to reduced κ_L owing to intensive phonon scattering by the point defects and increased σ arising from the charged defects (V_Cu^'). Crucially, synchrotron radiation photoelectron spectroscopy reveals substantial increment of electronic density of states at Fermi level upon Gd substitution, which is proven, by our first-principle calculations, to originate from contribution of Gd 4f orbit, resulting in enhancement of S. Our study provides us with a new path to enhance thermoelectric performance of Cu₁₂Sb₄S₁₃.

Effects of Sb Deviation from Its Stoichiometric Ratio on the Micro- and Electronic Structures and Thermoelectric Properties of Cu12Sb4S13

Thermoelectric material tetrahedrite Cu₁₂Sb₄S₁₃ has attracted much attention because of its intrinsic low lattice thermal conductivity, excellent electrical transport property, and environment-friendly constituents. However, its thermoelectric figure merit, ZT, is limited because of the low Seebeck coefficient (S) and power factor (PF). Hence, it is indispensable to enhance its S and PF to increase its ZT. Here, we show that when Sb deviation from its stoichiometric ratio in the Cu₁₂Sb₄S₁₃ band structure is modulated, it gives rise to increased density of states and enhancement of the Seebeck coefficient. Moreover, carrier concentration is tuned by changing sulfur and copper vacancies through controlling the Cu₃SbS₄ phase with an atomic ratio of Sb, leading to increased electrical conductivity. In addition, as large as ∼60% reduction of lattice thermal conductivity is obtained by intensified phonon scattering using an impurity phase/element and vacancy-like defects induced by different Sb contents. As a result, a high ZT = 0.86 is achieved at 723 K for the Cu₁₂Sb_4+δS₁₃ sample with δ = 0.2, which is ∼50% larger than that of stoichiometric Cu₁₂Sb₄S₁₃ studied here, indicating that ZT of Cu₁₂Sb₄S₁₃ can be improved through simple modulation of the Sb stoichiometric ratio.

Effect of Sn Substitution on the Thermoelectric Properties of Synthetic Tetrahedrite

The present study reports the effect of Sn substitution on the structural and thermoelectric properties of synthetic tetrahedrite (Cu₁₂Sb₄S₁₃) system. The samples were prepared with the intended compositions of Cu₁₂Sb_{4- x}Sn _xS₁₃ ( x = 0.25, 0.35, 0.5, 1) and sintered using spark plasma sintering. A detailed structural characterization of the samples revealed tetrahedrite phase as the main phase with Sn substituting at both Cu and Sb sites instead of only Sb site. The theoretical calculations using density functional theory revealed that Sn at Cu(1) 12d or Cu(2) 12e site moves the Fermi level ( E_F) toward the band gap, whereas Sn at Sb 8c site introduces hybridized hole states near E_F. Consequently, a relatively high optimum power factor of 1.3 mW/mK² was achieved by the x = 0.35 sample. The Sn-substituted samples exhibited a significant decrease in the total thermal conductivity (κ_T) compared to the pristine composition (Cu₁₂Sb₄S₁₃), primarily because of reduced electronic thermal conductivity. Due to an optimum power factor (1.3 mW/mK²) and reduced thermal conductivity (0.9 W/mK), a maximum zT of 0.96 at 673 K was achieved for x = 0.35 sample, which is nearly 40% increment compared to that of the pristine (Cu₁₂Sb₄S₁₃) sample.

Thermoelectric properties of the tetrahedrite-tennantite solid solutions Cu12Sb4-xAsxS13 and Cu10Co2Sb4-yAsyS13 (0 ≤ x, y ≤ 4)

Tetrahedrites, a class of copper- and sulfur-rich minerals, exhibit inherently very low lattice thermal conductivity and adjustable electronic properties that make them interesting candidates for thermoelectric applications. Here, we investigate the influence of isovalent As substitution on the Sb site on the structural and transport properties (5-700 K) of the two solid solutions Cu12Sb4-xAsxS13 and Cu10Co2Sb4-yAsyS13 (0 ≤ x, y ≤ 4). Electronic band structure calculations predict that As has only a weak influence on the valence bands and hence, on the p-type metallic character of Cu12Sb4S13. In agreement with these predictions, all the samples of the series Cu12Sb4-xAsxS13 exhibit p-type metallic behavior with relatively low electrical resistivity and moderate thermopower values that only slightly evolve with the As content. In contrast, the substitution of Co for Cu in As-rich samples seems less favorable as suggested by a decrease in the Co concentration with increasing the As content. This trend leads to a concomitant increase in the electrical resistivity and thermopower leaving the ZT values practically unchanged with respect to purely Cu-based samples. As a result, peak ZT values ranging between 0.60 and 0.75 are achieved at 700 K for both series. The lack of significant variations in the ZT values confirms the robustness of the thermoelectric performances of tetrahedrites with respect to variations in the Sb-to-As ratio.

Recent advances in inorganic material thermoelectrics

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

Electronic and thermoelectric properties of Zn and Se double substituted tetrahedrite

The influence of Zn and Se double substitution on the electronic and thermoelectric properties of tetrahedrite is investigated in this study.

Powder metallurgically synthesized Cu12Sb4S13tetrahedrites: phase transition and high thermoelectricity

A powder metallurgical process was used to synthesise Cu₁₂Sb₄S_13−xcompounds as natural powders for use as high performance thermoelectric materials.

Effect of Ni, Bi and Se on the tetrahedrite formation

Tetrahedrite formation is influenced positively by selenium and negatively by bismuth and nickel. However, selenium decreases the skinnerite formation temperature.