Surfactant Adsorption on Pt Nanoparticles and its Impact on Catalysis

Licentiatavhandling, 2026

The optical and catalytic properties of metallic nanoparticles depend on composition and surface functionalisation. In practical environments, nanoparticle surfaces are rarely pristine but instead frequently covered by organic molecular ligands originating from synthesis or deliberate functionalisation. An experimental challenge in colloidal synthesis is that any change in ligand identity or coverage is accompanied by subsequent variations in the nanoparticle size, morphology, or surface structure. Therefore, changes in catalytic activity measured with the different nanoparticles obtained cannot be solely attributed to the effects of the ligands.

This licentiate thesis investigates ligand–nanoparticle interactions through controlled and reversible adsorption–desorption cycles on lithographically fabricated Pt nanoparticles supported on a surface and with well-defined geometry. By employing initially ligand-free Pt nanoparticles, the influence of adsorbed molecular layers is examined without the shape variability typically encountered in colloidal systems. Cetyltrimethylammonium bromide (CTAB) is used as a model surfactant due to its well-characterized amphiphilic structure and concentration-dependent interfacial organization. A mild chemical reduction protocol enables repeated ligand removal while preserving nanoparticle integrity, allowing systematic comparison of multiple surface coverages on the same nanoparticle ensemble.

Nanoplasmonic sensing is utilized to monitor CTAB adsorption and re-arrangement in real time, exploiting the sensitivity of localized surface plasmon resonances to changes in the surrounding dielectric environment. Ensemble measurements constitute the primary experimental approach and reveal reproducible trends in adsorption kinetics and layer stability. The catalytic oxidation of ascorbic acid is employed as a model reaction to evaluate how ligand coverage influences reaction rate. A distinct impact of ligand coverage on catalytic activity is revealed. A preliminary single nanoparticle experiment is also presented, demonstrating the feasibility of resolving ligand adsorption at the level of individual nanoparticles. Together, the results show that ligand shells influence access to the nanoparticle surface and catalytic reaction rates, and that these effects can be investigated by using nanoplasmonic sensing protocols.