Composition and phase controlled colloidal synthesis of I-II-V-VI and I-V-VI₂ chalcogenide nanocrystals

This thesis contributes to the development of synthesis protocols for I-V-VI₂ and I-II-V-VI (I= Cu, Na, K, and Li, II= Ni, Co and Zn, V= Sb and VI= S) based colloidal NCs. The impact of ligands, precursors, and temperature as separate components in the synthesis of NCs with composition and phase control is illustrated. The synthesized NCs shows promising properties for thermoelectric application and as an anode material for Li ion batteries.

Chapter 3 of this thesis explores the growth mechanisms of three distinct tetrahedrite substituted NCs (Cu₁₀Zn₂Sb₄S₁₃, Cu₁₀Co2Sb4S₁₃, and Cu₁₀Ni₁.₅Sb₄S1₃). The study shows that the thiophilicity between the Zn, Ni and Co precursors is critical for generating pure-phase NCs with controlled size and shape. While all of the synthesized crystal phases exhibit very low thermal conductivity, the Cu₁₀.5Sb₄Ni₁.₅S₁₃ system has the highest electrical conductivity compared to Cu₁₀Zn₂Sb₄S₁₃ and Cu₁₀Co₂Sb₄S₁₃. This study highlights an effective synthesis strategy for the growth of complex quaternary nanocrystals and their high potential for application in thermoelectrics.

The simple, low-temperature, size-tunable (50-90 nm) colloidal hot injection method for the formation of NaSbS₂-based mixed ionic and electronic semiconductor (MIECs) using readily accessible, non-toxic precursors is described in chapter 4. To control the size and shape of the NaSbS₂ nanocrystals, the key synthetic factors that were explored include temperature, cationic precursor, and ligand. FTIR analysis revealed that carboxylate ligands are coordinated to the surface of the produced NaSbS2 NCs. The ionic and electronic conductivities of the produced NaSbS₂ nanocrystals are 3.31x10-¹⁰(e-) and 1.9 x10^-5 (Na+) S cm^-1, respectively, demonstrating their capacity to compete with the ionic and electronic conductivities of perovskite materials created via solid-state reaction. This study offered a mechanistic understanding and a post-synthetic examination of parameters influencing the development of sodium antimony chalcogenide.

Chapter 5 describes the phase-controlled colloidal production of NaSbS₂ nanocrystals. Colloidal hot injection method was used to investigate the effect of various synthesis factors (such as reaction time and temperature, as well as the sodium precursor) on the phase-controlled synthesis of NaSbS2 NCs with sizes ranging from 30 to 50 nm. The reactivity of the sodium source and reaction time are key parameters in the phase-controlled synthesis of NaSbS2 NCs. The NCs synthesized with a more reactive sodium source (NaOH) result in cubic nanocrystal formation, while a less reactive sodium source (CH3COONa) results in phase transformation from cubic to monoclinic over a period of 2hr reaction time. The coordination of carboxylate functionality to the surface of the NCs was shown by FTIR spectroscopy and solid-state NMR. Furthermore, the NaSbS₂ NCs was tested as an anode material for Li-ion batteries (LIBs).The KSbS2 electrode at a current density of 200 mA ^-1 gives a specific capacity of 565 mA hg^-1

In chapter 6, the synthesis of phase and compositionally tunable AxSbySz NCs (A= Li and K) by colloidal hot injection approach is demonstrated. The approach utilizes less toxic and environmentally friendly precursors compared to solid-state reactions. The reaction temperature was found to be the key parameter in the synthesis of KxSbySz NCs with phase and compositional control. NCs synthesised using KOH as a potassium source at 250 ᵒC reaction temperature results KSbS2 (monoclinic) NCs while, heating reaction mixture at 310 ᵒC results K3SbS₄ (orthorhombic) NCs. However, Li8Sb6S₁₄ NCs were formed using Li-isopropoxide as a Li source at 250 ᵒC reaction temperature for 30 min. The synthesized NCs show promising ionic conductivities. Among them, K₃SbS₄ has a room temperature conductivity of 3.7 x 10-6 Scm^-1 , which is among the best recorded for a sulfide-based solid-state electrolyte for potassium ion batteries