Andrea Baldi

Traditional heterogeneous catalysis is constrained by kinetic and thermodynamic limits, such as the Sabatier principle and reaction equilibrium. Dynamic and resonant catalysts hold promise to overcome these limitations by actively oscillating a catalyst’s physical or electronic structure at the time scale of the catalytic cycle, allowing programmable control over reaction pathways, and leading to improved rate and selectivity. External stimuli such as temperature swing, mechanical strain, electric charge, and light can perturb catalyst surfaces in different ways, altering adsorbate coverage, binding energies, and transition states beyond what steady-state catalysis allows. This work surveys the current state of dynamic catalysis, introduces the concept of “stimulando” characterization for observing transient dynamics, and outlines key modeling, mechanistic, and benchmarking strategies to advance the field toward improved chemical transformation.

Measuring nanoscale temperature gradients is critical for understanding and controlling photothermal processes in plasmonics, with implications for catalysis, optoelectronics, and nanomedicine. However, accurately probing the local heating of individual nanoparticles remains challenging due to their subwavelength size and the harsh conditions of many applications. Here, we demonstrate a sensitive and robust Raman thermometry method that maps photothermal heating of single nanoparticles with diverse geometries, compositions, and excitation wavelengths. Our approach relies on the temperature-dependent Raman shift of a thermally evaporated copper phthalocyanine (CuPc) layer deposited onto the nanoparticles. The straightforward sample preparation and the known thermal, chemical, and optical stability of CuPc make this method suitable for a wide range of experimental conditions. We apply the technique to lithographically fabricated silver nanodisks and nanobars, as well as colloidally synthesized gold nanorods, and resolve their photothermal response at the single-particle level. For particles larger than the diffraction limit, the measured temperature profiles reveal their morphology and heating behavior. For sub-diffraction particles, the measurements capture clear and reproducible trends in photothermal response, although the extracted temperature reflects an apparent, averaged value. Based on these results, we predict that for plasmonic nanostructures with sufficiently high SERS enhancement it will be possible to retrieve the true nanoparticle temperature even in the deeply sub-diffraction regime. This work establishes Raman thermometry as a practical, robust, and versatile method for investigating nanoscale heat transport at the single-particle level, enabling studies in challenging environments such as photochemical reactors and optoelectronic devices.

Light absorption in plasmonic gold nanoparticles (AuNPs) generates short-lived nonthermalized electrons and holes that can drive redox reactions, making them attractive for photocatalytic applications. However, characterizing these processes and disentangling competing effects due to photothermal heating or plasmonic field enhancement remain challenging. Here, we exploit the light-driven synthesis of silver shells around AuNPs as a model reaction to study the role of nonthermalized carriers in plasmonic gold photochemistry. By coupling in situ extinction spectroscopy with Mie theory calculations, we extract internal quantum efficiencies (IQEs) for AuNPs with different diameters. The size dependence of the IQE scales with the probability of light absorption within 2-3 nm of the Au nanoparticle surface, highlighting the critical role of short-lived d-band holes, which recombine on ultrafast time scales, in driving our chemistry. More broadly, our approach provides a simple yet powerful framework to characterize activation mechanisms and quantify carrier dynamics in plasmonic photocatalysis.

Traditional heterogeneous catalysis is constrained by kinetic and thermodynamic limits, such as the Sabatier principle and reaction equilibrium. Dynamic and resonant catalysts hold promise to overcome these limitations by actively oscillating a catalyst's physical or electronic structure at the timescale of the catalytic cycle, allowing programmable control over reaction pathways, and leading to improved rate and selectivity. External stimuli like temperature swing, mechanical strain, electric charge, and light can perturb catalyst surfaces in different ways, altering adsorbate coverage, binding energies, and transition states beyond what steady-state catalysis allows. This work surveys the current state of dynamic catalysis, introduces the concept of “stimulando” characterization for observing transient dynamics, and outlines key modeling, mechanistic, and benchmarking strategies to advance the field toward sustainable chemical transformation.

Controlling and understanding thermal energy at the nanoscale is a key challenge in nanoscience, with broad implications for semiconductor technology, photocatalysis, and light-activated medicine. Plasmonic nanostructures offer a powerful approach to achieve sub-wavelength photothermal confinement by converting light into localized heat. In this review, we first examine the fundamental mechanisms of plasmonic light-to-heat conversion and how both classical and non-Fourier heat transfer models describe the spatiotemporal evolution of temperature. We then explore how advances in material science, nanophotonic architectures, and micro-environmental engineering have opened up new opportunities for achieving faster and more localized thermal gradients. Finally, we briefly assess the capabilities and limitations of current nanothermometry techniques for resolving temperature with nanometer precision and at ultrafast timescales.

Light absorption in plasmonic gold nanoparticles (AuNPs) generates short-lived non-thermalized electrons and holes that can drive redox reactions, making them attractive for photocatalytic applications. However, characterizing these processes and disentangling competing effects due to photothermal heating or plasmonic field enhancement remains challenging. Here, we exploit the light-driven synthesis of silver shells around AuNPs as a model reaction to study the role of non-thermalized carriers in plasmonic gold photochemistry. By coupling in-situ extinction spectroscopy with Mie theory calculations, we extract internal quantum efficiencies (IQEs) for AuNPs with different diameters. The size dependence of the IQE scales with the probability of light absorption within 2-3 nm from the Au nanoparticle surface, highlighting the critical role of short-lived d-band holes, which recombine on ultrafast timescales, in driving our chemistry. More broadly, our approach provides a simple yet powerful framework to characterize activation mechanisms and quantify carrier dynamics in plasmonic photocatalysis.

A novel colloidal synthesis approach for fabricating Au@ZnO core@shell nanoparticles with tunable core size, shell thickness, and Al dopant concentration.

Solar photocatalysis appears as a viable approach for the production of value-added chemicals from CO₂. However, up to now, there is no information on the influence of the light intensity on the product distribution of CO₂ hydrogenation and the modeling of the actual local temperature at the catalytic sites for typical nanoparticulate photocatalysts. Herein, it is shown that for a photothermal catalyst containing a high density of homogeneously distributed Ru nanoparticles, the collective heating prevails, resulting in a homogeneous temperature distribution in the material that should be relatively close to that of the support and that can be measured macroscopically. Moreover, light intensity has a clear influence on product distribution due to the differences in the local temperature, and therefore, attention should be paid to stable operating conditions, temperature, and CO₂ conversion that can result in remarkable differences in product selectivity for the same catalyst as a function of light intensity.

Plasmonic materials, such as gold nanoparticles (AuNPs), exhibit significant extinction and near-field enhancement across the visible and near-infrared spectrum, attributable to localized surface plasmon resonances (LSPRs). Epsilon-near-zero (ENZ) materials, such as aluminum doped zinc oxide (AZO) are known in non-linear optics for their ability to generate and manipulate light-matter interactions through processes like higher harmonic generation. Combining doped ZnO with plasmonic materials therefore holds promise for enhancing non-linear efficiencies and tuning their operational wavelengths. To date, however, only top-down structures based on plasmonically decorated thin ENZ films have been realized, and no colloidal and scalable route to obtain these hybrid materials has been reported yet. Here, we introduce a novel colloidal synthesis approach for fabricating Au@AZO core@shell nanoparticles with tunable core size, shell thickness, and dopant concentration, allowing for the spectral alignment of the LSPRs of the AuNPs with the non-linear optical properties of the AZO shells. Our method involves the colloidal synthesis of gold cores followed by a controlled sol-gel process to deposit ZnO and AZO shells, resulting in core diameters ranging from 25 to 69 nm, shell thicknesses from 16 to 47 nm, and aluminum doping levels between 0 and 4 at.%. Our procedure widens the range of hybrid plasmonic nanostructures that can be colloidally synthesized, opening new possibilities for the large scale fabrication of high-performance nanomaterials for integration in photonic, photocatalytic, and sensing applications.