Ligand Pyrolysis during Air-Free Inorganic Nanocrystal Synthesis

Abstract

Oleylamine (OLA) is a ubiquitous organic ligand used in the synthesis of inorganic nanocrystals (NCs) due to its ability to reduce metal precursors and sterically stabilize NCs. However, attempts to thermally remove the ligand from deposited OLA-capped NC films result in ligand pyrolysis, where the organic residue is ultimately embedded into the active material as a carbon defect. Herein, we investigated the thermal decomposition and restructuring of OLA ligands using a combination of Raman spectroscopy and X-ray photoelectron spectroscopy (XPS), seeking to determine the specific nature of OLA’s decomposition and reasons for its ultimate incorporation into a carbon-rich fine-grain layer observed in Cu2ZnSn­(S1–x Se x )4 (CZTSSe) systems. While it has historically been expected that OLA pyrolyzes during thermal treatments, this work identifies that OLA decomposes into nanostructured graphitic carbon, which is first detectable immediately after the NC synthesis at 225 °C, well before the employment of thermal treatments. These graphitic flakes are further identified as disordered graphene oxide, which segregates from OLA-capped NCs during deposition to assemble into the carbon-rich layer. In identifying that the carbon-rich fine-grain layer originates prior to thermal treatments, we introduce a strategy to partially reduce the presence of this layer by isolating the graphene oxide flakes immediately after the CZTS NC synthesis.

Chemical Considerations for Colloidal Nanocrystal Synthesis

“…The ligand oleylamine is often involved in precursor conversion chemistry; see also the previous section. In addition, oleylamine can pyrolyze into graphitic carbon during the synthesis of Cu 2 ZnSnS 4 nanocrystals . In general, one cannot assume that the ligands added to the reaction are also the ligands ending up on the final nanocrystal surface.…”

“…In addition, oleylamine can pyrolyze into graphitic carbon during the synthesis of Cu 2 ZnSnS 4 nanocrystals. 62 In general, one cannot assume that the ligands added to the reaction are also the ligands ending up on the final nanocrystal surface. Ligands are not per definition inert, but ligand transformations have received a lot less attention than precursor conversion.…”

Ligand Decomposition during Nanoparticle Synthesis: Influence of Ligand Structure and Precursor Selection

“…Ligand decomposition was first observed during the synthesis of kesterite Cu 2 ZnSnS 4 (CZTS) with the commonly used monounsaturated oleylamine (OLA) ligand. 28 To screen ligands for decomposition during NP synthesis, we first synthesized Cu 2 ZnSnS 4 (CZTS) NPs with the saturated and unsaturated amine and carboxylic acid ligands, as shown in Figure 1 : DDA (saturated amine), OLA (monounsaturated amine), LOA (polyunsaturated amine), SA (saturated carboxylic acid), OA (monounsaturated carboxylic acid), and LA (polyunsaturated carboxylic acid).…”

“… 32 − 39 In our previous work, during the synthesis of Cu 2 ZnSnS 4 (CZTS) NPs with elemental sulfur and the OLA ligand, graphitic and amorphous carbon byproducts formed. 28 …”

“…32−39 In our previous work, during the synthesis of Cu 2 ZnSnS 4 (CZTS) NPs with elemental sulfur and the OLA ligand, graphitic and amorphous carbon byproducts formed. 28 While the polymerization and aromatization of olefins have been widely studied in the field of polymer science, ligand decomposition (i.e., polymerization and graphitization) during NP synthesis is an emerging observation. Traditional polymerization of olefins is most commonly achieved through the use of Ziegler−Natta and metallocene catalysts and co-catalysts, 40−45 cationic reactions with acidic reagents, 46 anionic reactions with nucleophilic reagents, 45−47 or radical reactions.…”