Colloidal syntheses are used worldwide to prepare various nanomaterials for multiple applications. In most cases, the colloidal syntheses proceed in the liquid phase by reduction of a precursor in presence of reducing agents. Typically, various ligands / capping agents / stabilizers are claimed to be needed for successful syntheses leading to stable colloids with size control [1] Unfortunately, those chemicals that interact with the NP surface can be detrimental for further applications where a clean surface would be preferred, for instance in catalysis or medicine.

Rather than removing those chemicals by tedious chemical- and energy-consuming steps, the Nanomaterials Engineering for Sustainable Technologies (NEST) group aims at developing surfactant-free syntheses, i.e. syntheses with a minimal amount of chemicals that lead to readily active materials, for instance for catalysis. A focus is on processes carried out at room or low temperature and using relatively safe chemicals. This approach not only brings new opportunities for new fundamental insights into nanomaterial synthesis, but also facilitates the general production and use of the nanomaterials, while it facilitates the scaling of the related nanotechnologies.

This talk will provide an overview of our achievements related to metal nanomaterials and in particular our recent findings on gold nanoparticles. Highlights will be on:

Sustainability, with the possibility to develop surfactant-free colloidal Pt [2], Ir[2], Os [3], nanoparticles, but also Au [4]. It will be illustrated how the use of additives can actually be detrimental compared to the surfactant-free approach [5].

Fundamental insights, with the example of Ir and Pt nanoparticle formation [6, 7] but also Au [8].

Size control without surfactant, with the example of Au nanoparticles obtained by ethanol-mediated or NaBH₄-mediated synthesis, leveraging on reducing agent / Au molar ratio, using mixtures of reducing agents, water conductivity and/or inducing the synthesis in different ways [8-12].

Stability, the surfactant-free colloids are typically stable for months even for storage at room temperature and the cations have been overlooked knobs to boost stability and to perform syntheses at higher precursor concentrations. The stability increases in the order K⁺<Na⁺<Li⁺ for various colloidal sytnheses [13].

Catalytic activity, with the example of size effects studies performed for the 4-nitrophenol reduction as a model reaction for water treatment and electrochemical energy conversion with the case studies of the ethanol oxidation reaction (EOR) [4, 9, 14, 15] the oxygen evolution reaction (OER) [2] and the oxygen reduction reaction (ORR) [16]. A focus will be on showing the benefits of surfactant-free syntheses to best design and study the corresponding electrocatalysts [1, 15, 16].

The accent will be on showing that despite the absence of surfactants, the syntheses lead to stable nanoparticles with size control even at relatively high concentration of precursors. The talk will open an emerging case studies where the surfactant-free NPs are being investigated as useful model system for instance in self-driving laboratories [17].