In recent years, faceted semiconducting nanoparticles emerge as a very promising material in the context of photocatalytic energy conversion with augmented degree of energy conversion efficiency. The surface charge and the geometry of these materials as well as the surrounding fluid composition play a very important role in photocatalysis. While these systems have been studied quite extensively in experiments using AFM force-distance spectroscopy measurements, the fundamental understanding of the charge distribution in these materials and the charge transfer process to the surrounding fluid is lacking. For example, when an AFM tip is approaching one of the side facets of the nanoparticle instead of the horizontally oriented facet (the schematic of the same is depicted in Figure 1), one expects a significant functional dependence of the inclination angle between the AFM tip and the particle in the resulting interaction force. Exploring the same is not only important from fundamental viewpoint but also has the potential in constructing a new paradigm towards designing efficient faceted nanoparticles with optimum performance.
Figure 1. (a) Sketch of an AFM tip approaching a faceted BiVO4. Here, the AFM tip is approaching one of the side facets, making an angle θ with the transverse co-ordinate. (b) Experimental image consisting of 2-D charge map of (100) and (110) facets of SrTiO3 showing opposite charge (red: positive charge, blue: negative charge).
The big question of this Bachelor assignment is to identify how the inclination angle between the AFM tip and a faceted nanoparticle alters the potential distribution and the resulting tip-sample interaction force. To this end, you will perform numerical simulations using the finite element based commercial tool COMSOL Multiphysics for analyzing the scenario of an AFM tip approaching horizontal and side facets of a BiVO4 nanoparticle. Simulating the potential distribution will be the first step the knowledge of which will be utilized later in determining the force-distance curves that one typically measures in AFM spectroscopy experiments. Not only the angle of inclination, but also the tip geometry as well as tip-sample distance can play crucial role in altering the interaction force and thus, will be examined. Apart from these geometrical constraints, the effect of fluid composition, like the pH and the concentration of the electrolyte, type of electrolyte (symmetric or asymmetric), the rate of protonation-deprotonation reaction, etc. can strongly influence the charge distribution and thus, the interaction force. Hence, examining the role of the fluid composition will be the final task.
In addition to the standard learning objectives for a Bachelor’s/Master’s project (research planning, academic writing, data presenting, how to work in a lab environment, etc.), you will:
· Obtain knowledge on electrokinetics, charge distribution in different scenarios.
· Learn about the different interaction forces that can typically arise between two charged entities
separated by a finite distance.
· Have basic training on numerical simulations using the framework of COMSOL.
· Depending on your interests, may extend the simulation set up for the case of a
semiconducting nanoparticle which will necessitate the coupling between the semiconductor
physics and electrokinetics.
Daily Supervision: Dr. Siddhartha Mukherjee
Second Daily Supervision: Dr. Igor Siretanu
Supervision: Prof. Dr. Frieder Mugele