Au@Rh Nanoflowers toward plasmon-enhanced electrochemical reactions

Author(s)

Maria Paula de Souza Rodrigues

• Document type: Doctoral Thesis
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Conjunto das Químicas (IQ e FCF) (CQ/DBDCQ)
• Defense date: 2022-10-25

Examining board members

Susana Inês Cordoba de Torresi; Ana Flavia Nogueira; Germano Tremiliosi Filho

• Advisor: Susana Inês Cordoba de Torresi

Abstract

Nanoelectrochemistry brings the advantage of using nanoparticles with unique properties when compared to their bulk counterparts. Among those properties, there is the localized surface plasmon resonance (LSPR), which gives rise to valuable physical effects such as hot-carrier generation and local heating effect. However, not every metal presents the LSPR on the visible range of the spectrum and, thus, requires specific and costly apparatus to optimize LSPR outcomes. However, it makes a valuable challenge to tune the LSPR extinction band toward the visible spectrum through the rational design of nanoparticles. One strategy is to combine different metals with catalytic and plasmonic properties in hybrid nanostructures, allowing an improved energy harvesting and catalytic activity toward electrochemical reactions upon excitation at the visible spectrum. This thesis systematically investigated the synthesis of gold-rhodium core-shell nanoflowers (Au@Rh NFs), their formation mechanism, and their application in nanoelectrochemistry. The nanostructures exhibited the LSPR excitation in two regions of the visible range of the spectrum owing to the Au LSPR extinction band (548 nm) and, the combination of Rh LSPR extinction band and Au interband transitions (~ 420 nm). Distinct electrochemical reactions were chosen to evaluate the LSPR excitation impact on the catalysts activity and/or selectivity. The results showed that each electrochemical reaction was affected differently by the nanoflowers properties and LSPR excitation. A linear dependency was in activity with increasing rhodium content was observed in the case of hydrogen evolution reaction (HER). The NFs best performance was upon 533 nm laser incidence, wavelength that matches the materials LSPR extinction band. The reaction overpotential was reduced in 40 mV, and no significant change was observed upon irradiation of a wavelength outside the materials plasmonic range. Additionally, the activity of Au@Rh NFs were remarkably higher when compared to its monometallic counterparts owing to its stronger adsorption of icelike interfacial water conformation. For the electrocatalytical tests for Ethanol oxidation reaction (EOR) and CO2 reduction reaction (CO2RR), the nanostructures showed a distinct behavior to the catalyst metallic ratio. The sample with the intermediate rhodium amount presented the highest performance due to its higher resistance to CO poisoning when compared to the catalyst with the highest Rh content. The EOR was deeply investigated by electrochemical methods both in dark and light conditions, indicating no significant difference in the reaction mechanism upon LSPR excitation. This observation demonstrates that the LSPR excitation on EOR might facilitate the reactions kinetics, rather than change the limiting reaction step. The best performance was obtained upon the 533 nm laser incidence, with an increase of 352 and 36 % in the catalyst activity and selectivity, respectively. On the other hand, the nanoflowers best performance toward CO2RR was obtained with the 405 nm laser, which only matches partially the Au@Rhs LSPR extinction band. Although surprisingly, this observation is related to the facilitated CO desorption upon the 405 nm light incidence. Electrochemical impedance spectroscopy confirmed that the improved LSPR activity owes to the lower energetic barrier induced by light, which reflects in lower charge transfer resistance under LSPR excitation. This thesis provided a deeper inside into the control parameters to achieve optimized electrocatalysts, as well as into the mechanism of plasmon-enhanced electrochemical reactions. (AU)

• FAPESP's process: 18/16846-0 - Metal oxide nanowires decorated with gold nanoparticles for heterogeneous catalysis application
• Grantee: Maria Paula de Souza Rodrigues
• Support Opportunities: Scholarships in Brazil - Doctorate (Direct)