Laia Castilla Amorós
The need for efficient and selective catalysts, capable of driving important conversions to build a more sustainable society, encourages the development of synthetic approaches towards new nanomaterials. Cu-based bimetallic nanoparticles (NPs) promise to fulfill this gap by selectively producing energy-dense products via CO2 electroreduction (CO2RR). However, as small variations in its composition and configuration can enormously affect the catalyst performance, the vast diversity of bimetallic NPs challenges systematic catalyst evaluation. Colloidal bimetallic NPs serve as an ideal platform for the exploration of composition-configuration-property relationships, necessary for rational catalyst design. Although an extensive library of noble metal bimetallic NPs exists, much less has been done regarding non-noble Cu-based bimetallic NPs despite their potential as catalysts for CO2RR. The aim of this thesis is to advance in the synthesis of Cu-based NPs, with focus on Cu-Ga and Cu-In, which have been predicted or demonstrated to possess interesting catalytic properties.The thesis starts with an introduction on bimetallic NPs and the different colloidal approaches to synthesize them. Galvanic replacement reactions (GRRs) are discussed more in details, as they are the synthetic approach of choice for the Cu-Ga and Cu-In NPs. Ga and In belong to a family of liquid metals; thus, information on their general properties is also provided. The first two experimental chapters focus on the reactivity and surface chemistry of Ga NPs. Chapter 3 demonstrates that Ga NPs react with a copper-amine complex to form anisotropic Cu-Ga nanodimers (NDs). Mechanistic studies reveal that a GRR takes place. Yet, the ND morphology differs from the more typical hollow core@shell structures obtained from GRRs. This unusual morphology is attributed to the liquid nature of Ga NPs and to the presence of a native oxide shell. This discovery fostered a follow up study, which is discussed in Chapter 4. The effect of the chemical nature of the capping ligand on the oxide thickness and reactivity of the Ga NPs is investigated. The thickness of the oxide skin is found to greatly depend on the ligands. Specifically, amines and carboxylic acids promote thicker oxide shells while thiols and phosphines hinder its growth. The reaction between Ga NPs with different oxide thickness and the copper precursor leads to a surprising result, while thicker oxides cause the formation of NDs, isolated Cu NPs form with thinner oxides.Chapter 5 turns towards Cu-In NPs. Following the same synthetic scheme, In NPs with different sizes are exploited as sacrificial seeds for the GRR. Cu-In NPs with an unprecedented variety of morphologies and elemental distributions are obtained: spherical Cu11In9 intermetallic and patchy phase-segregated Cu-In NPs, as well as Cu-Cu11In9 and Cu-In NDs. Segregation of the two metals occurs as the GRR progresses, with time or with higher copper precursor concentration. Comparing the obtained results with the Cu-In phase diagram reveals that the bigger seeds stabilize the bulk-like Cu-Cu11In9 configuration before their complete segregation into Cu-In NDs. Aiding in the development of more predictive synthetic approaches.Finally, Chapter 6 concludes by summarizing the significance of the achievements, which advance the knowledge on underexplored bimetallic NPs and on the GRR when applied beyond noble metals. An outlook on future developments is also provided.
EPFL2022
