Amide-Assisted Synthesis of Iron Germanium Sulfide (Fe₂GeS₄) Nanostars: The Effect of LiN(SiMe₃)₂ on Precursor Reactivity for Favoring Nanoparticle Nucleation or Growth

Abstract

Olivine Fe2GeS4 has been identified as a promising photovoltaic absorber material introduced as an alternate candidate to iron pyrite, FeS2. The compounds share similar benefits in terms of elemental abundance and relative nontoxicity, but Fe2GeS4 was predicted to have higher stability with respect to decomposition to alternate phases and, therefore, more optimal device performance. Our initial report of the nanoparticle (NP) synthesis for Fe2GeS4 was not well understood and required an inefficient 24 h growth to dissolve an iron sulfide impurity. Here, we report an amide-assisted Fe2GeS4 NP synthesis that directly forms the phase-pure product in minutes. This significant advance was achieved by the replacement of the poorly understood hexamethyldisilazane (HMDS) additive and TMS2S by the conjugate base, lithium bis­(trimethylsilyl)­amide (LiN­(SiMe3)2), and elemental S, respectively. We hypothesized that fragments of both TMS2S and HMDS had carried out the roles that Brønsted bases play in amide-assisted NP syntheses and were necessary for Ge incorporation. Convolution of this role with the supply of S in TMS2S caused the iron sulfide impurities. Separating these effects in the use of LiN­(SiMe3)2 and elemental S resulted in synthetic control over the ternary phase. Herein we explore the Fe–Ge–S reaction landscape and the role of the base. Its concentration was found to increase the reactivities of the Fe, Ge, and S precursors, and we discuss possible metal-amide intermediates. This affords tunability in two areas: favorability of NP nucleation versus growth and phase formation. The phase-purity of Fe2GeS4 depends on the molar ratios of the cations, base, and amine as well as the Fe:Ge:S molar ratios. The resultant Fe2GeS4 NPs exhibit an interesting star anise-like morphology with stacks of nanoplates that intersect along a 6-fold rotation axis. The optical properties of the Fe2GeS4 NPs are consistent with previously published measurements showing a measured band gap of 1.48 eV.

Tailored growth of single-crystalline InP tetrapods

“…Similarly, we anticipate that InCl 3 will turn to more reactive metal-amide complexes, as evidenced by the observation of In metal (XRD data for the reaction residue shown in Supplementary Fig. 6 ) 32 – 35 . Supplementary Fig.…”

“…To suppress the continued growth of InP tetrahedrons, we additionally injected lithium bis(trimethylsilyl)amide (LiHMDS) to accelerate their precursor-conversion. LiHMDS is a well-known strong base (pKa of 30), allowing metal precursors to react with long-chain primary amines, which rapidly deprotonate RNH 2 into RNHLi 32 . Similarly, we anticipate that InCl 3 will turn to more reactive metal-amide complexes, as evidenced by the observation of In metal (XRD data for the reaction residue shown in Supplementary Fig.…”

“…In the presence of Cl and methylamine (MA), the (111) surface is appreciably stabilized by Cl and Cl/MA passivation while the ( ) surface shows a relatively high surface energy despite InCl passivation. Under our reaction conditions, InCl could be generated from InCl 3 reduction by reducing agents such as aminosphosphine or amines 32 , 45 – 47 (more details can be found in Supplementary Figs. 11 and 12 ).…”

Olivine Crystal Structure-Directed Twinning in Iron Germanium Sulfide (Fe2GeS4) Nanoparticles

“…still remains a challenge, especially for multinary compounds and complex crystal structures. Our group initially published a solution-phase synthesis of Fe 2 GeS 4 NPs in 2013, and recently, we reported a thorough redesign of the synthesis accessing the direct formation of phase-pure Fe 2 GeS 4 NPs by replacing the poorly understood additive, hexamethyldisilazane, with its conjugate base, lithium bis­(trimethylsilyl)­amide, and substituting elemental sulfur for the highly reactive hexamethyldisilathiane (TMS 2 S). Amide-assisted syntheses exist for a number of compounds, and the presence of the amide has been found to increase the reactivity of metal cations allowing control over the formation of multinary NPs in which cation precursors of differing reactivity must be balanced. − In nanoparticle reactions with alkylamine solvents, the amide has been found to act as a Brønsted base, allowing highly reactive metal alkylamides to form as intermediates. , This high precursor reactivity provided the opportunity for Fe 2 GeS 4 to form in the unusual morphology of twinned NPs.…”

“…Characterization of NP Products. All NP products were examined via powder X-ray diffraction (PXRD) for phase-purity and matched the pattern referenced in our synthetic report, 50 ICSD Collection Code #333. 85 Diffraction was performed on a Bruker D8 Discover X-ray diffractometer using a Cu Kα radiation source (λ = 1.54 Å) and a Lynxeye XE-T positron-sensitive detector.…”