Understanding the diffusion-driven colloidal deposition of laser-generated ligand-free nanoparticles on dispersed catalyst supports
Final Report Abstract
The functionalization of support materials with nanoparticles is essential for the development of heterogeneous catalysts. The direct adsorption of colloidal nanoparticles allows for independent control of the nanoparticle loading, size, and composition. In the case of electrostatic attraction, adsorption is quantitative but non-selective. In the case of electrostatic repulsion, the particle adsorption is hindered, but, if the adsorption barrier and the diffusive (thermal) energy of the particles are similar, diffusion-controlled adsorption is still observed. In some cases, however, previous experiments even showed quantitative diffusion-driven adsorption despite a much higher energy barrier (e.g. for gold nanoparticles on commercial TiO2, mixedphase), indicating additional influencing factors. The project aimed to investigate the diffusion-driven adsorption of gold nanoparticles systematically and to understand the influence of local aspects such as point defects, facets, or crystallographic mixed phases. Material-wise, TiO₂, ZnS and nitrogen-doped carbon loaded with gold or platinum nanoparticles were the focus of the study. 1. Point Defects: Our theoretical study showed that point defects can lower the adsorption barrier under electrostatically repulsive conditions (diffusion control) and act as attractive centers in case of a sufficiently high defect density. 2. Surface Defects: A laser-based method generated a TiO₂ series with increasing oxygen defect density. A quantitative titration method developed in the project shows that these defects form acidic centers, which increased the gold nanoparticles' adsorption efficiency only under attractive pH conditions. 3. Facets and Heterojunctions: Experiments with ZnS showed facet-selective adsorption under diffusion-controlled conditions in the case of mixed-phase systems (similar to P25, TiO2). The results strengthen the hypothesis of local charge conditions altered by heterojunctions (between two crystal phases of the same material). 4. Functional Groups: Nitrogen-doped carbon supports showed an increased activity in the oxygen reduction reaction (ORR) with platinum nanoparticles, but the overall performance was limited due to the low surface area of the available materials. Conclusion: The project highlights the importance of knowing the local charge distribution for the general prediction and control of diffusion-controlled nanoparticle adsorption. Point defects, facets, and heterojunctions (in mixed-phase systems) play a crucial role. Predictions via DLVO theory should always be complemented by local analysis of the particle surface, which is available via the fluoride titration developed in the project and newer AFM methods. By controlling the charge distribution on the support and nanoparticles, catalysts with nanoparticles placed on defects or facets in a controlled manner could be synthesized in the future.
Publications
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Differentiating between Acidic and Basic Surface Hydroxyls on Metal Oxides by Fluoride Substitution: A Case Study on Blue TiO2 from Laser Defect Engineering. Angewandte Chemie International Edition, 62(12).
Lau, Kinran; Niemann, Felix; Abdiaziz, Kaltum; Heidelmann, Markus; Yang, Yuke; Tong, Yujin; Fechtelkord, Michael; Schmidt, Torsten C.; Schnegg, Alexander; Campen, R. Kramer; Peng, Baoxiang; Muhler, Martin; Reichenberger, Sven & Barcikowski, Stephan
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Defect-Selective Energy Barrier Crossing during Adsorption of Colloidal Gold Nanoparticles on Zinc Sulfide Crystals under Overall Electrostatic Repulsion. The Journal of Physical Chemistry C, 128(43), 18622-18633.
Matten, Manuel; Lange, Thomas; Rohe, Markus; Mei, Bastian; Reichenberger, Sven & Barcikowski, Stephan
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Fluoride Substitution: Quantifying Surface Hydroxyls of Metal Oxides with Fluoride Ions. Advanced Materials Interfaces, 11(26).
Lau, Kinran; Zerebecki, Swen; Pielsticker, Lukas; Hetaba, Walid; Dhaka, Kapil; Exner, Kai S.; Reichenberger, Sven & Barcikowski, Stephan
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The multivariate interaction between Au and TiO2 colloids: the role of surface potential, concentration, and defects. Nanoscale, 16(5), 2552-2564.
Lau, Kinran; Giera, Brian; Barcikowski, Stephan & Reichenberger, Sven
