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Advances in Single Entity Electrochemistry for Semiconducting Nanocrystal Studies
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The research in this thesis focuses on the electrochemical and photoelectrochemical behavior of semiconducting nanoparticles (NPs) as single entities. The primary goal of this work is to develop methods to detect individual NP collisions—specifically semiconducting NPs—and to characterize their electrons transfer processes. Semiconductors are relevant in photocatalytic applications, either as a direct solar energy harvester or in water and solvent remediation. Most of the current applications are related to these two general modes. In this work, the photooxidation of methanol on titanium dioxide NPs was our model of study. This photocatalytic reaction has applications as a solar energy harvester and water remediation. The reaction is also of fundamental interest. Moreover, the single entity study under photocatalytic conditions is an approach to the mesoscopic domain, which could improve our understanding of the properties of connectivity, assembly, aggregation, and mass transfer of electrons through a film of semiconductor material for its application in solar cells. Furthermore, this work discusses relevant topics in stochastic electrochemistry, such as mass transport of NPs, filtering effects, and photocatalysis processes on single entities. The main discussion focuses on the stochastic event signal, i.e., the nano-impact and the subsequent electron transfer, i.e., the faradaic processes. We investigated filtering frequencies (digital and instrumental), noise, frequency of collisions, and final nano transients (high-resolution experiments) to discuss signal processing effects. We used ZnO NPs as a model to understand NP mass transport and the relative contribution of migration and diffusion for metal oxides. We modeled this process with the well-known Nernst-Plank equation and a subsequent development on migration by Amatore et al. While observing Zn electrolysis on a ZnO NP, we discussed the rate of electrolysis of Zn into a mercury amalgam and the effect of the instrumentation on these measurements. In the last chapter, we reported our advance in TiO2 methanol photooxidation through a HeCd laser irradiation. Firstly, we discussed the model developed with the combination of Mie theory on spherical particles and photoelectrochemistry. Secondly, we described nano-collisions in different experimental setups, leading to different types of transitions—homogenized and spiked. Homogenized suspension, TiO2, NPs are suspended in the cell with a fixed concentration before any alignment or electrochemical measurement. In a spiked suspension, a concentrated colloidal suspension of TiO2 NPs is injected at a certain time in the i-t transient measurement after the alignment of the laser. We fully characterize two systems for the spiking technique but not for the homogenized suspension. Thirdly, we described and analyzed the electrochemical response on Pt/TiO2 decorated electrodes. Fourthly, we discussed the implications of correlating the Mie theory and the electrochemical transients, viz., the step current and their statistical distributions. Finally, this works studies a NP captured on 600 nm Pt electrodes to confirm the photocurrent delivered by a single NP with high-resolution experiments. These experiments provide insight into mechanisms and NP properties that have not been reported previously to the best of our knowledge.