Impact of Plasmonic Photothermal Effects on the Reactivity of Au Nanoparticle Modified Graphene Electrodes Visualized Using Scanning Electrochemical Microscopy

The paradox of thermal vs. non-thermal effects in plasmonic photocatalysis

The debate surrounding the roles of thermal and non-thermal pathways in plasmonic catalysis has captured the attention of researchers and sparked vibrant discussions within the scientific community. In this review, we embark on a thorough exploration of this intriguing discourse, starting from fundamental principles and culminating in a detailed understanding of the divergent viewpoints. We probe into the core of the debate by elucidating the behavior of excited charge carriers in illuminated plasmonic nanostructures, which serves as the foundation for the two opposing schools of thought. We present the key arguments and evidence put forth by proponents of both the non-thermal and thermal pathways, providing a perspective on their respective positions. Beyond the theoretical divide, we discussed the evolving methodologies used to unravel these mechanisms. We discuss the use of Arrhenius equations and their variations, shedding light on the ensuing debates about their applicability. Our review emphasizes the significance of localized surface plasmon resonance (LSPR), investigating its role in collective charge oscillations and the decay dynamics that influence catalytic processes. We also talked about the nuances of activation energy, exploring its relationship with the nonlinearity of temperature and light intensity dependence on reaction rates. Additionally, we address the intricacies of catalyst surface temperature measurements and their implications in understanding light-triggered reaction dynamics. The review further discusses wavelength-dependent reaction rates, kinetic isotope effects, and competitive electron transfer reactions, offering an all-inclusive view of the field. This review not only maps the current landscape of plasmonic photocatalysis but also facilitates future explorations and innovations to unlock the full potential of plasmon-mediated catalysis, where synergistic approaches could lead to different vistas in chemical transformations.

Mass Transport Limitations in Plasmonic Photocatalysis

The interpretation of mechanisms governing hot carrier reactivity on metallic nanostructures is critical, yet elusive, for advancing plasmonic photocatalysis. In this work, we explored the influence of the diffusion of molecules on the hot carrier extraction rate at the solid-liquid interface, which is of fundamental interest for increasing the efficiency of photodevices. Through a spatially defined scanning photoelectrochemical microscopy investigation, we identified a diffusion-controlled regime hindering the plasmon-driven photochemical activity of metallic nanostructures. Using low-power monochromatic illumination (<2 W cm-2), we unveiled the hidden influence of mass transport on the quantum efficiency of plasmonic photocatalysts. The availability of molecules at the solid-liquid interface directly limits the extraction of hot holes, according to their nature and energy, at the reactive spots in Au nanoislands on an ultrathin TiO2 substrate. An intriguing question arises: does the mass transport enhancement caused by thermal effects unlock the reactivity of nonthermal carriers under steady state?

Quantification and description of photothermal heating effects in plasmon-assisted electrochemistry

A growing number of reports have demonstrated plasmon-assisted electrochemical reactions, though debate exists around the mechanisms underlying the enhanced activity. Here we address the impact of plasmonic photothermal heating with cyclic voltammetry measurements and finite-element simulations. We find that plasmonic photothermal heating causes a reduction in the hysteresis of the anodic and cathodic waves of the voltammograms along with an increase in mass-transport limiting current density due to convection induced by a temperature gradient. At slow scan rates, a temperature difference as low as 1 K between the electrode surface and bulk electrolytic solution enhances the current density greater than 100%. Direct interband excitation of Au exclusively enhances current density by photothermal heating, while plasmon excitation leads to photothermal and nonthermal enhancements. Our study reveals the role of temperature gradients in plasmon-assisted electrochemistry and details a simple control experiment to account for photothermal heating.

Interfacial States in Au/Reduced TiO2 Plasmonic Photocatalysts Quench Hot-Carrier Photoactivity

Understanding the interface of plasmonic nanostructures is essential for improving the performance of photocatalysts. Surface defects in semiconductors modify the dynamics of charge carriers, which are not well understood yet. Here, we take advantage of scanning photoelectrochemical microscopy (SPECM) as a fast and effective tool for detecting the impact of surface defects on the photoactivity of plasmonic hybrid nanostructures. We evidenced a significant photoactivity activation of TiO2 ultrathin films under visible light upon mild reduction treatment. Through Au nanoparticle (NP) arrays deposited on different reduced TiO2 films, the plasmonic photoactivity mapping revealed the effect of interfacial defects on hot charge carriers, which quenched the plasmonic activity by (i) increasing the recombination rate between hot charge carriers and (ii) leaking electrons (injected and generated in TiO2) into the Au NPs. Our results show that the catalyst's photoactivity depends on the concentration of surface defects and the population distribution of Au NPs. The present study unlocks the fast and simple detection of the surface engineering effect on the photocatalytic activity of plasmonic semiconductor systems.

Photothermal Nanomaterials: A Powerful Light-to-Heat Converter

All forms of energy follow the law of conservation of energy, by which they can be neither created nor destroyed. Light-to-heat conversion as a traditional yet constantly evolving means of converting light into thermal energy has been of enduring appeal to researchers and the public. With the continuous development of advanced nanotechnologies, a variety of photothermal nanomaterials have been endowed with excellent light harvesting and photothermal conversion capabilities for exploring fascinating and prospective applications. Herein we review the latest progresses on photothermal nanomaterials, with a focus on their underlying mechanisms as powerful light-to-heat converters. We present an extensive catalogue of nanostructured photothermal materials, including metallic/semiconductor structures, carbon materials, organic polymers, and two-dimensional materials. The proper material selection and rational structural design for improving the photothermal performance are then discussed. We also provide a representative overview of the latest techniques for probing photothermally generated heat at the nanoscale. We finally review the recent significant developments of photothermal applications and give a brief outlook on the current challenges and future directions of photothermal nanomaterials.

Inducing SERS activity at graphitic carbon using graphene-covered Ag nanoparticle substrates: Spectroelectrochemical analysis of a redox-active adsorbed anthraquinone

Graphitic carbon electrodes are central to many electrochemical energy storage and conversion technologies. Probing the behavior of molecular species at the electrochemical interfaces they form is paramount to understanding redox reaction mechanisms. Combining surface-enhanced Raman scattering (SERS) with electrochemical methods offers a powerful way to explore such mechanisms, but carbon itself is not a SERS activating substrate. Here, we report on a hybrid substrate consisting of single- or few-layer graphene sheets deposited over immobilized silver nanoparticles, which allows for simultaneous SERS and electrochemical investigation. To demonstrate the viability of our substrate, we adsorbed anthraquinone-2,6-disulfonate to graphene and studied its redox response simultaneously using SERS and cyclic voltammetry in acidic solutions. We identified spectral changes consistent with the reversible redox of the quinone/hydroquinone pair. The SERS intensities on bare silver and hybrid substrates were of the same order of magnitude, while no discernible signals were observed over bare graphene, confirming the SERS effect on adsorbed molecules. This work provides new prospects for exploring and understanding electrochemical processes in situ at graphitic carbon electrodes.

Transparent Ultramicroelectrodes for Studying Interfacial Charge-Transfer Kinetics of Photoelectrochemical Water Oxidation at TiO2 Nanorods with Scanning Electrochemical Microscopy

Scanning electrochemical microscopy (SECM) has been extensively applied to the electrochemical analysis of the surfaces and interfaces of a photoelectrochemical (PEC) system. A semiconductor photoelectrode with a well-defined geometry and active surface area comparable to SECM's tip is highly desired for accurately quantifying interfacial charge-transfer activities and photoelectrochemically generated redox species, where the broadening effects due to the mass transfer gradient and nonlocal electron transfer at a planar semiconductor surface can be minimized. Here, we present a newly developed platform as a SECM substrate for investigating semiconductor PEC activities, which is based on a transparent ultramicroelectrode (UME) fabricated by using two-step photolithographic patterning and ion milling methods. This transparent UME with a 25 μm recessed disk shape is fully characterized with SECM for quantifying the interfacial charge-transfer rates of IrCl₆^2-/IrCl₆^3- by comparing with theoretical results from finite element simulations in COMSOL Multiphysics. When coated with TiO₂ nanorods as a model semiconductor material, the transparent UME can be used to quantify the catalytic PEC water oxidation in a feedback mode of SECM by sampling tip and substrate current signals simultaneously. This transparent UME-SECM study provides insights into the potential-dependent PEC water oxidation reaction mechanism and the quantitative analysis of photocurrent contributions from water oxidation and the SECM tip-generated redox mediator. The transparent UME-SECM method can be potentially expanded to other SECM operation modes such as surface interrogation for understanding the dynamics of the electrode surfaces and interfaces of a PEC system.

Surface-Enhanced Raman Spectroscopy-Scanning Electrochemical Microscopy: Observation of Real-Time Surface pH Perturbations

Understanding and controlling chemical dynamics at electrode interfaces is key to electrochemical applications in sensing, electrocatalysis, and energy storage. Here, we introduce colocalized surface-enhanced Raman scattering-scanning electrochemical microscopy (SERS-SECM) as a multimodal tool able to simultaneously probe and affect electrochemical interfaces in real time. As a model system to demonstrate SERS-SECM, we used a self-assembled monolayer of 4-mercaptopyridine (4MPy), a pH sensitive Raman indicator, anchored to silver nanoparticles as a substrate. We modulated the local pH at the surface with chronoamperometry, inducing the hydrogen evolution reaction (HER) at the SECM tip and observed subsequent Raman peak height changes in the 4MPy. We then performed cyclic voltammetry of HER at the SECM tip while measuring SERS spectra every 200 ms to highlight the technique's real-time capabilities. Our results show the capability to sensitively interrogate and trigger chemical/electrochemical dynamic surface phenomena. We hope SERS-SECM will provide insight on the link between heterogeneous and homogeneous reactivity at electrochemical interfaces.

Plasmonic metal nanostructures: concepts, challenges and opportunities in photo-mediated chemical transformations

Plasmonic metal nanostructures (PMNs) are characterized by the plasmon oscillation of conduction band electron in response to external radiation, enabling strong light absorption and scattering capacities and near-field amplification. Owing to these enhanced light-matter interactions, PMNs have garnered extensive research interest in the past decades. Notably, a growingly large number of reports show that the energetics and kinetics of chemical transformations on PMNs can be modified upon photoexcitation of their plasmons, giving rise to a new paradigm of manipulating the reaction rate and selectivity of chemical reactions. On the other hand, there is urgent need to achieve clear understanding of the mechanism underlying the photo-mediated chemical transformations on PMNs for unleashing their full potential in converting solar energy to chemicals. In this perspective, we review current fundamental concepts of photo-mediated chemical transformations executed at PMNs. Three pivotal mechanistic questions, i.e., thermal and nonthermal effects, direct and indirect charge transfer processes, and the specific impacts of plasmon-induced potentials, are explored based on recent studies. We highlight the critical aspects in which major advancements should be made to facilitate the rational design and optimization of photo-mediated chemical transformations on PMNs in the future.

Simple experimental procedures to distinguish photothermal from hot-carrier processes in plasmonics

Light absorption and scattering of plasmonic metal nanoparticles can lead to non-equilibrium charge carriers, intense electromagnetic near-fields, and heat generation, with promising applications in a vast range of fields, from chemical and physical sensing to nanomedicine and photocatalysis for the sustainable production of fuels and chemicals. Disentangling the relative contribution of thermal and non-thermal contributions in plasmon-driven processes is, however, difficult. Nanoscale temperature measurements are technically challenging, and macroscale experiments are often characterized by collective heating effects, which tend to make the actual temperature increase unpredictable. This work is intended to help the reader experimentally detect and quantify photothermal effects in plasmon-driven chemical reactions, to discriminate their contribution from that due to photochemical processes and to cast a critical eye on the current literature. To this aim, we review, and in some cases propose, seven simple experimental procedures that do not require the use of complex or expensive thermal microscopy techniques. These proposed procedures are adaptable to a wide range of experiments and fields of research where photothermal effects need to be assessed, such as plasmonic-assisted chemistry, heterogeneous catalysis, photovoltaics, biosensing, and enhanced molecular spectroscopy.