Investigating Cu-Site Doped Cu-Sb-S Nanoparticles Using Photoelectron and Electron Paramagnetic Resonance Spectroscopy

Abstract

Tetrahedrite (Cu₁₂Sb₄S₁₃) and famatinite (Cu₃SbS₄) are good candidates for green energy applications because they possess promising thermoelectric and photovoltaic properties as well as contain earth-abundant and nontoxic constituents. Herein, X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and electron paramagnetic resonance spectroscopy (EPR) methods examined inherent electronic properties and interatomic magnetic interactions of Cu-site doped tetrahedrite and famatinite nanomaterials. An energy-efficient modified polyol method was utilized for the synthesis of tetrahedrite and famatinite nanoparticles doped on the Cu-site with Zn, Fe, Ni, Mn, and Co. This is the first parallel study of tetrahedrite and famatinite nanomaterials with XPS, UPS, and EPR methods alongside a systematic analysis of dopant-dependent effects on the electronic structure and magnetic interactions for each material. XPS showed that the Cu and Sb species in tetrahedrite and famatinite possess different oxidation states, while UPS characterization reveals larger dopant-dependent shifts in the work function for tetrahedrite nanoparticles (4.21 to 4.79 eV) than for famatinite nanoparticles (4.57 to 4.77 eV). Finally, all famatinite nanoparticles display an EPR signal, indicating trace amounts of paramagnetic Cu(II) present below the detection limit of XPS. For tetrahedrite, EPR signatures were observed only for the Zn-doped and Mn-doped nanoparticles, suggesting signal broadening from Cu-Cu spin exchange or spin-lattice relaxation. This study demonstrates the complementary nature of XPS and EPR techniques for studying the oxidation states of metals in solid-state nanomaterials. Comparing the electronic and magnetic properties of tetrahedrite and famatinite while studying the impact of dopant incorporation will guide future endeavors in designing sustainable, high-performance materials for renewable energy applications.

Figure 1

Unit cells of (a) tetrahedrite (Cu₁₂Sb₄S₁₃) and (b) famatinite (Cu₃SbS₄). In the tetrahedrite unit cell (a), the tetrahedral Cu site is labeled in red and the trigonal Cu site in orange. In the famatinite unit cell (b), all Cu atoms are colored red.

Figure 2

(a) Powder XRD patterns for undoped (Cu₁₂Sb₄S₁₃) and doped (Cu₁₁M₁Sb₄S₁₃, M = Zn, Fe, Ni, Mn, or Co) tetrahedrite nanoparticles with associated reference pattern and (b) powder XRD patterns for undoped (Cu₃SbS₄) and doped (Cu_2.7M_0.3SbS₄, M = Zn, Fe, Ni, Mn, and Co) famatinite nanoparticles with associated reference pattern. Patterns are labeled according to the identity of the dopant (M) and the four most intense peaks are indexed on each reference.

Figure 3

XPS spectra of the Cu 2p, Sb 3d, and S 2p regions for tetrahedrite (Cu₁₁M₁Sb₄S_13, M = Zn, Fe, Ni, Mn, Co) nanoparticles (a, c, e) and famatinite (Cu_2.7M_0.3SbS₄, M = Zn, Fe, Ni, Mn, Co) nanoparticles (b, d, f). The legend identifies samples based on the identity of the dopant species (M), i.e., “Zn Doped” refers to the Cu₁₁ZnSb₄S₁₃ tetrahedrite sample or the Cu_2.7Zn_0.3SbS₄ famatinite sample. The undoped samples (black lines) are double the size of other lines for reference purposes.

Figure 4

UPS spectra for (a) tetrahedrite (Cu₁₁M₁Sb₄S_13, M = Zn, Fe, Ni, Mn, Co) nanoparticles and (b) famatinite (Cu_2.7M_0.3SbS₄, M = Zn, Fe, Ni, Mn, Co) nanoparticles. The insets in (a) and (b) display the secondary electron cutoff of the tetrahedrite and famatinite nanomaterials, respectively. The legend identifies samples based on the identity of the dopant species (M), i.e., “Zn Doped” refers to the Cu₁₁ZnSb₄S₁₃ tetrahedrite sample or the Cu_2.7Zn_0.3SbS₄ famatinite sample.

Figure 5

Electron paramagnetic resonance spectra for (a) tetrahedrite (Cu₁₁M₁Sb₄S_13, M = Zn, Fe, Ni, Mn, Co) nanoparticles and (b) famatinite (Cu_2.7M_0.3SbS₄, M = Zn, Fe, Ni, Mn, Co) nanoparticles. The inset in (b) displays a magnified view of the signal found for the Fe-doped and Co-doped famatinite samples. The Mn-doped signal in (a) is reduced by a factor of 3. The legend identifies samples based on the identity of the dopant species (M), i.e., “Zn Doped” refers to the Cu₁₁ZnSb₄S₁₃ tetrahedrite sample or the Cu_2.7Zn_0.3SbS₄ famatinite sample.