Impact of Hot Carrier Dynamics on Photoelectrocatalytic Activity on Au@Pd Antenna-Reactor Nanoparticles.

ConspectusDuring surface plasmon-mediated light-matter interactions, external energies on plasmonic metal nanostructures undergo energy dissipation via elastic e-e scattering, radiative luminescence, and nonradiative processes such as thermal relaxation (phonon) and electronic excitation (electron-hole pairs). In this process, surface plasmon decays dominantly through nonradiative recombination when the metal is smaller than 25 nm, forming hot carriers, including hot electrons and hot holes, with high kinetic energy of 1-3 eV. Although the ultrafast dynamics of hot carriers are on time scales ranging from femtoseconds to picoseconds, these fast-disappearing hot carriers can be collected as the steady-state photocurrent or chemicurrent by adopting the metal-semiconductor (M-S)-based platform for detecting hot carrier flow. Plasmonic hot carriers, especially as they convert to an electrochemical signal, are a promising topic, and their energy conversion mechanisms are being actively studied in the fields of renewable energy, optoelectronics, and photocatalysis. Recent studies have demonstrated that these hot carriers can both improve the performance of solar energy conversion and control the catalytic activity or selectivity by directly participating in the photoelectrochemical (PEC) reaction.In this Account, we describe the inherent relationship between hot carriers and surface plasmon as well as what role hot carriers play throughout the catalytic reaction. The recent experimental work and the theoretical analysis of in situ hot carrier generation on Au nanostructures were conducted with photoconductive atomic force microscopy and finite-difference time-domain (FDTD) simulations, respectively. We highlight the recent nanoscale visualization of hot carrier flow occurring through light-matter interactions and that the localized surface plasmon field surrounding the Au nanostructure leads to boosted hot carrier generation. In addition, we highlight the recent demonstration that plasmonic hot carriers prolong the lifetime of photoexcited carriers in the MAPbI3/Au/TiO2 hybrid nanodiode by the synergistic effect between plasmonic Au and perovskites. From this work, the solar-to-electron conversion performance of this nanodiode significantly increases due to the amplification of light absorption, which helps to design hybrid platforms for efficient hot carrier photovoltaics. We discuss the application of surface plasmon-driven hot electron generation, including hot electron-based photovoltaic devices and photocatalysts. We highlight the recent photoelectrochemical measurements on the Au/p-GaN heterostructures that are controlled by participating plasmonic hot carriers in the water splitting reaction. Furthermore, controlling the flow of both hot electrons and holes by developing hybrid platform configurations for hot carrier applications has promising opportunities for regulating the catalytic activities of hot carrier-based photocatalysis and improving the photoconversion efficiency of hot carrier-based optoelectronic devices.

The dynamics of hot carriers at the interface between plasmonic nanostructures and semiconductor quantum dots presents an innovative way for modulating light-matter interactions at the nanoscale. Utilizing real-time time-dependent density functional theory simulations, this study elucidates the mechanisms underlying the decay of localized surface plasmons and the dynamics of hot carriers within a composite structure comprising a gold nanorod and a Cd33Se33 quantum dot. Our results demonstrate that the interface formation leads to the broadening and red-shift of the localized surface plasmon (LSP) of the gold nanorod. In particular, this LSP is capable of extending into the Cd33Se33 QD. A significant finding of this study is the energy-dependent segregation of hot electrons and holes. At an initial 5.0 fs, hot carriers are absent, yet they commence to appear at 10.0 fs. By 20 fs and 30 fs, the distributions of hot carriers remain largely unchanged. Through LSP decay and the dynamics of hot carriers, our research offers novel insights into hot carrier dynamics of heterostructures.

Electron-electron (e-e) interaction is known as a source of logarithmic renormalizations for Dirac fermions in quantum field theory. The renormalization of electron-optical phonon coupling (EPC) by e-e interaction, which plays a pivotal role in hot carrier and phonon dynamics, has been discussed since the discovery of graphene. We investigate hot carrier dynamics and EPC strength using time-resolved ultrabroadband terahertz (THz) spectroscopy combined with numerical simulation based on the Boltzmann transport equation and a comprehensive temperature model. The numerical simulation demonstrates that the extrinsic carrier scatterings by the Coulomb potential of the charged impurity and surface polar phonons are significantly suppressed by the carrier screening effect and have negligible contributions to the THz photoconductivity in heavily doped graphene on polyethylene terephthalate (PET) substrate. The large negative photoconductivity and the non-Drude behavior of THz conductivity spectra appear under high pump fluence and can be attributed to the temporal variation of the hot carrier distribution and scattering rate. The transient reflectivity well reflects the EPC strength and temporal evolution of the hot carrier and optical phonon dynamics. We successfully estimate the EPC matrix element of the ${A}_{1}^{\ensuremath{'}}$ optical phonon mode near the $\mathbf{K}$ point as ${\ensuremath{\langle}{D}_{\mathbf{K}}^{2}\ensuremath{\rangle}}_{\mathrm{F}}\ensuremath{\approx}450$ ${(\mathrm{eV}/\AA{})}^{2}$ from the fitting of THz conductivity spectra and temporal evolution of transient THz reflectivity. The corresponding dimensionless EPC constant ${\ensuremath{\lambda}}_{\mathbf{K}}\ensuremath{\approx}0.09$ at Fermi energy ${\ensuremath{\varepsilon}}_{\mathrm{F}}=0.43\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ is slightly larger than the prediction of the renormalization group approach including the dielectric screening effect of the PET substrate. This leads to a significant difference in hot carrier and phonon dynamics compared with those without the renormalization effect by the e-e interaction. This approach can provide a quantitative understanding of hot carrier and optical phonon dynamics and support the development of future graphene optoelectronic devices.

Atomically precise Au25(GSH)18 nanoclusters versus plasmonic Au nanocrystals: Evaluating charge impetus in solar water oxidation

Hot carriers are energetic photoexcited carriers driving a large range of chemicophysical mechanisms. At the nanoscale, an efficient generation of these carriers is facilitated by illuminating plasmonic antennas. However, the ultrafast relaxation rate severally impedes their deployment in future hot-carrier based devices. In this paper, we report on the picosecond relaxation dynamics of hot carriers in plasmonic monocrystalline gold nanoantennas. The ultrafast dynamics of the hot carriers is experimentally investigated by interrogating the nonlinear photoluminescence response of the antenna [1]. From this investigation, we reveal some leverages to control the dynamics of such hot carriers within nano antenna. In particular, an increase by a factor up to five of this dynamics (from 0.5 ps to 2.5 ps) is observed for resonant nanoantenna compared to off-resonance antenna and when excitation power increases. By a two temperature model we model quantitatively the dynamics of hot carriers and we demonstrate the nonlinear generation of these carriers. The control over the carrier dynamics should allow to employ their energy more effieciently within physico-chemical processes. In a second part, we investigate the hot carrier dynamics with a spectrally resolved two-pulse correlation configuration, and demonstrate that the relaxation of the photoexcited carriers depends of their energies relative to the Fermi level. We find a 60% variation in the relaxation rate for electron−hole pair energies ranging from ca. 0.2 to 1.8 eV. The quantitative relationship between hot-carrier energy and relaxation dynamics is an important finding for optimizing hot-carrier-assisted processes and shed new light on the intricacy of nonlinear photoluminescence in plasmonic [2]. [1] O. Demichel et al, ACS Photonics 3, 791 (2016) [2] R. Mejard et al, ACS Photonics 3, 1482 (2016)

Hot holes behind the improvement in ultraviolet photoresponse of Au coated ZnO nanorods

Solar-to-chemical conversion via photocatalysis is of paramount importance for a sustainable future. Thus, investigating the synergistic effects promoted by light in photocatalytic reactions is crucial. The tandem oxidative coupling of alcohols and amines is an attractive route to synthesize imines. Here, we unravel the performance and underlying reaction pathway in the visible-light-driven tandem oxidative coupling of benzyl alcohol and aniline employing Au/CeO2 nanorods as catalysts. We propose an alternative reaction pathway for this transformation that leads to improved efficiencies relative to individual CeO2 nanorods, in which the localized surface plasmon resonance (LSPR) excitation in Au nanoparticles (NPs) plays an important role. Our data suggests a synergism between the hot electrons and holes generated from the LSPR excitation in Au NPs. While the oxygen vacancies in CeO2 nanorods trap the hot electrons and facilitate their transfer to adsorbed O2 at surface vacancy sites, the hot holes in the Au NPs facilitate the α-H abstraction from the adsorbed benzyl alcohol, evolving into benzaldehyde, which then couples with aniline in the next step to yield the corresponding imine. Finally, cerium-coordinated superoxide species abstract hydrogen from the Au surface, regenerating the catalyst surface.

Direct evidence and enhancement of surface plasmon resonance effect on Ag-loaded TiO2 nanotube arrays for photocatalytic CO2 reduction

We report measurements of photocatalytic water splitting using Au films with and without TiO2 coatings. In these structures, a thin (3–10 nm) film of TiO2 is deposited using atomic layer deposition (ALD) on top of a 100 nm thick Au film. We utilize an AC lock-in technique, which enables us to detect the relatively small photocurrents (~mA) produced by the short-lived hot electrons that are photoexcited in the metal. Under illumination, the bare Au film produces a small AC photocurrent (<1µA) for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) due to hot electrons and hot holes, respectively, that are photoexcited in the Au film. The samples with TiO2 produce a larger AC photocurrent indicating that hot electrons are being injected from the metal into the TiO2 semiconductor where they then reduce hydrogen ions in solution forming H2 (i.e., 2H+ + 2e- → H2). The AC photocurrent exhibits a narrow peak when plotted as a function of reference potential, which is a signature of hot electrons. Here, we photoexcite a monoenergetic source of hot electrons, which produces a peak in the photocurrent, as the electrode potential is swept through the resonance with the redox potential of the desired half-reaction. This stands in contrast to conventional bulk semiconductor photocatalysts, whose AC photocurrent saturates beyond a certain potential (i.e., light limited photocurrent). The photocurrents produced at the metal–liquid interface are smaller than those of the metal–semiconductor system, mainly because, in the metal–semiconductor system, there is a continuum of energy and momentum states that each hot electron can be injected into, while for an ion in solution, the number of energy and momentum states are very small.1 2, 3 We also report plasmon resonant excitation of hot electrons in a metal-based photocatalyst in contact with ferrocene redox couple in acetonitrile solution. Here the photocatalyst consists of a 50nm thick Au film and a top thin (5nm) film of Al2O3 deposited using atomic layer deposition. In this configuration, hot electrons, photo-excited in the metal, jump over the oxide barrier ultimately reducing ferrocenium in solution (i.e., Fe(C5H5)2 + + e- → Fe(C5H5)2) producing a photocurrent. In order to amplify this process, the bottom gold electrode is patterned into a plasmon resonant grating structure with a pitch of 500nm. The photocurrent (i.e., charge transfer rate) is measured as a function of incident angle using 633nm wavelength light. We observe peaks in the photocurrent at incident angles of ±10o from normal when the light is polarized parallel to the incident plane (p-polarization) and perpendicular to the lines on the grating. Based on these peaks, we estimate an overall plasmonic gain (or amplification) factor of 2.3X in the charge transfer rate. At these same angles, we also observe sharp dips in the photo-reflectance, corresponding to the condition when there is wavevector matching between the incident light and the plasmon mode in the grating. No angle dependence is observed in the photocurrent or photoreflectance when the incident light is polarized perpendicular to the incident plane (s-polarization) and parallel to the lines on the grating. Finite difference time domain (FDTD) simulations also predict sharp dips in the photoreflectance at ±10o, and the electric field intensity profiles show clear excitation of a plasmon-resonant mode when illuminated at those angles with p-polarized light.

Separation of hot electrons and holes in Au/LaFeO3 to boost the photocatalytic activities both for water reduction and oxidation

This article discusses the hot charge carriers in semiconductor quantum dots (QDs): their generation, relaxation, extraction and applications in different technologically relevant areas. It has been reported that the most common ways to generate hot charge carriers are photo‐excitation by energy more than the band gap energy. However, recently the other means to generate hot charge carriers such as doping, and semiconductor plasmon interaction have also been evolved and their advantages over the conventional method have been discussed. Relaxation dynamics of hot charge carriers and different processes related to this such as multiple exciton generation, Auger recombination, phonon vibrations and ligand assisted charge carrier relaxation are mentioned. It was shown that the relaxation mechanism of hot charge carriers (both hot electron and hot hole) follow different pathways and either of the mechanism is playing a role depending on the QDs structure and the surrounding conditions. Studies pertaining to interaction of QDs with other semiconductors, metal nanoparticles or with different molecular adsorbates suggest that the hot charge carrier extraction is possible in these heterojunctions with extraction time in sub‐ps scale. Application of hot charge carriers in different fields such as photovoltaics, photo‐catalysis, infrared detectors, and H2 detection have been shown and it was observed that the efficiency of the above‐mentioned processes is much higher when the hot charge carriers are involved, suggesting their importance in these areas.

ConspectusLocalized surface plasmon resonance (LSPR), a distinctive optoelectronic property of plasmonic nanocrystals, arises from the collective oscillation of conduction electrons in resonance with incident light. The excitation of LSPR confines incident light near the surface of plasmonic nanocrystals and amplifies the local electric field. Moreover, the frequency of LSPR is highly tunable in the visible and near-IR regions, allowing plasmonic nanocrystals to efficiently absorb and scatter light across the solar spectrum. Such a property makes plasmonic nanocrystals promising candidates for utilizing solar irradiation to drive chemical reactions, a process known as plasmonic photocatalysis. Upon the resonant excitation of LSPR, energetic hot electrons and holes are generated via the nonradiative decay of LSPR in plasmonic nanocrystals. Those hot carriers can be transferred into the molecular orbitals of adsorbed reactants, enabling chemical transformations at the surface of nanocrystals. However, during the charge transport within plasmonic nanocrystals, hot carriers rapidly relax into lower-energy states. As a result, their energy is often dissipated to the lattice as heat, increasing the local temperature rather than directly contributing to chemical reactions─posing a fundamental challenge to achieving efficient solar-to-chemical energy conversion using plasmonic nanocrystals.To address this challenge, our group has developed multiple strategies to control the lifetime, energy level, and spatial distribution of plasmon-generated hot carriers to enhance the photocatalytic activity of Au nanocrystals. To extend the lifetime of hot carriers to match the slow kinetics of chemical reactions, Au nanocrystals were attached to an n-type semiconductor to form a heterojunction. This structure was found to prolong the lifetime of hot electrons through efficient spatial separation of hot electrons and holes, facilitated by the Schottky barrier at the metal/semiconductor interface. In parallel, decorating Au nanocrystals with redox-active molecules was shown to extend the lifetime of hot holes. Those hot holes were chemically stabilized and trapped within the bonds of the redox-active species, allowing them to participate in subsequent chemical reactions. Furthermore, a direct correlation between the activity of hot-electron-driven reduction reactions and the size of plasmonic nanocrystals, as well as between hot-hole-driven oxidation reactions and the wavelength of incident light, was established. Those observations demonstrated that energy levels of hot carriers involved in chemical reactions can be manipulated by tuning the size of nanocrystals and the wavelength of light. Moreover, positively charged molecules with facet-selective adsorption on Au nanocrystals were found to stabilize the plasmon-generated hot electrons, enabling control over the spatial distribution of hot carriers. Manipulating plasmon-generated hot carriers not only enhances the kinetics of plasmon-driven chemical reactions─such as oxygen evolution, hydrogen evolution, and nanocrystal growth─but also introduces new reaction pathways in those chemical processes, paving the way for highly efficient plasmonic photocatalysis.

For many decades, fossil fuels, including gas, oil, and coal, have acted as the primary contributors to electricity generation [1]. Nevertheless, the combustion of carbon-based fuels causes substantial emissions of greenhouse gases, notably carbon dioxide (CO2), thereby precipitating climate change and adversely affecting human well-being and the environment [2]. Consequently, there is an endeavour to innovate and develop efficient and environmentally acceptable energy sources. The oxygen evolution reaction (OER) constitutes a pivotal half-reaction within diverse renewable energy technologies. Despite its significance, the OER is intrinsically sluggish and energy-intensive, necessitating the advancement of electrocatalysts that are both efficient and stable to facilitate this reaction [3]. In recent years, layered double hydroxides (LDH) have emerged as potential candidates for OERs due to their cost-effectiveness, versatile composition, and favourable electrocatalytic properties [4]. Ni-based LDHs, particularly NiFe-LDH, have been extensively investigated and proven to be effective OER electrocatalysts in alkaline environments [5]. However, its limited electrical conductivity hinders further enhancement of its OER catalytic activity. Meanwhile, Co-based LDHs, such as NiCo-LDH and CoFe-LDH have been proven to have excellent electrocatalytic activity [6]. In contrast to binary LDHs, the ternary LDHs, incorporating diverse transition elements can exhibit higher capacitance and contain more abundant active sites [7]. However, the utilization of LDH electrode materials is limited by their low conductivity and tendency for agglomeration. Graphene serves as an excellent substrate for catalyst immobilisation in electrocatalysis, owing to its superior electrical conductivity, high surface areas, and impressive stability [8]. Therefore, the combination of ternary LDHs, characterised by reversible redox activity, and conductive graphene is expected to represent an efficient approach for fabricating hybrid materials with enhanced oxygen evolution reaction (OER) activity, facilitated by the advantageous interplay between LDHs and graphene.Accordingly, in this study, trimetallic CoNiFe-LDHs were designed and grown on graphene (G) through a one-step hydrothermal approach to obtain a structure that promotes efficient charge transfer, optimizing the OER kinetics. A 2-level full-factorial model was used to determine the effect of Co (1.5, 3 and 4.5 mmol) and graphene (10, 30 and 50 mg) concentrations on the OER onset potential, which was the chosen response parameter. The independent and dependent variables were fitted to the linear model equation, using ANOVA analysis. The F-values, the ratio of noise to response, confirmed that the model is significant (p<0.05). The p-values less than 0.05 indicate the model terms, such as cobalt and graphene concentrations and their interaction, are significant, implying that the OER onset potential is strongly correlated to these parameters. The polarization curves of CoNiFe-LDH/G composites for OER are shown in Figure 1. The OER was run in triplicate using the Co3Ni3Fe3-LDH/G30 (central point) to estimate the variability (0.58%). The comparison study showed that a low onset potential (1.54 V) and overpotential at 10 mA cm-2 (1.58 V) was achieved for Co1.5Ni3Fe3-LDH/G10, demonstrating that a low concentration of cobalt and graphene could make for an ideal electrocatalyst in practical applications.[1] R.F. Hirsh, J.G. Koomey, Electricity Consumption and Economic Growth: A New Relationship with Significant Consequences?, Electricity Journal 28 (2015) 72–84.[2] X.H. Chen, K. Tee, M. Elnahass, R. Ahmed, Assessing the environmental impacts of renewable energy sources: A case study on air pollution and carbon emissions in China, J Environ Manage 345 (2023) 118525.[3] F. Zeng, C. Mebrahtu, L. Liao, A.K. Beine, R. Palkovits, Stability and deactivation of OER electrocatalysts: A review, Journal of Energy Chemistry 69 (2022) 301–329.[4] J. Qian, Y. Zhang, Z. Chen, Y. Du, B.J. Ni, NiCo layered double hydroxides/NiFe layered double hydroxides composite (NiCo-LDH/NiFe-LDH) towards efficient oxygen evolution in different water matrices, Chemosphere 345 (2023) 140472.[5] S. He, R. Yue, W. Liu, J. Ding, X. Zhu, N. Liu, R. Guo, Z. Mo, Nano-NiFe LDH assembled on CNTs by electrostatic action as an efficient and durable electrocatalyst for oxygen evolution, Journal of Electroanalytical Chemistry 946 (2023) 117718.[6] Y. Li, G. Zhou, J. Yin, F. Li, Q. Zou, W. Chen, W. Yan, Q. Li, C. Liu, A. Khataee, L. Zhang, Aboundent oxygen defects in CoFe-LDH derivatives for enhanced photo-thermal synergistic catalytic hydrogen production from NaBH4, Int J Hydrogen Energy 48 (2023) 16745–16755.[7] A. Raja, N. Son, Y. Il Kim, M. Kang, Hybrid ternary NiCoCu layered double hydroxide electrocatalyst for alkaline hydrogen and oxygen evolution reaction, J Colloid Interface Sci 647 (2023) 104–114.[8] W. Gao, D. Havas, S. Gupta, Q. Pan, N. He, H. Zhang, H.L. Wang, G. Wu, Is reduced graphene oxide favorable for nonprecious metal oxygen-reduction catalysts?, Carbon N Y 102 (2016) 346–356. Figure 1

Surface plasmon polaritons are a central concept in nanoplasmonics and have been exploited to develop ultrasensitive chemical detection platforms, as well as imaging and spectroscopic techniques at the nanoscale. Surface plasmons can decay to form highly energetic (or hot) electrons in a process that is usually thought to be parasitic for applications, because it limits the lifetime and propagation length of surface plasmons and therefore has an adverse influence on the functionality of nanoplasmonic devices. Recently, however, it has been shown that hot electrons produced by surface plasmon decay can be harnessed to produce useful work in photodetection, catalysis and solar energy conversion. Nevertheless, the surface-plasmon-to-hot-electron conversion efficiency has been below 1% in all cases. Here we show that adiabatic focusing of surface plasmons on a Schottky diode-terminated tapered tip of nanoscale dimensions allows for a plasmon-to-hot-electron conversion efficiency of ∼30%. We further demonstrate that, with such high efficiency, hot electrons can be used for a new nanoscopy technique based on an atomic force microscopy set-up. We show that this hot-electron nanoscopy preserves the chemical sensitivity of the scanned surface and has a spatial resolution below 50nm, with margins for improvement.

A potential field of study for improving the efficiency of next-generation photovoltaic devices hot carriers in perovskite solar cells is investigated in this review paper. Considering their relevance to hot carrier dynamics, the paper thoroughly studies metal halide perovskites’ essential characteristics and topologies. We review important aspects like carrier excitation, exciton binding energy, phonon coupling, carrier excitation, thermalization, and hot hole and hot electron dynamics. We investigate, in particular, the significance of relaxation mechanisms, including thermalization and the Auger heating effect. Moreover, the bottleneck effect and defect management are discussed with an eye on their impact on device performance and carrier behaviour. A review of experimental methods for their use in investigating hot carrier dynamics, primarily transient photovoltage measurements, is included. Utilizing this thorough investigation, we hope to provide an insightful analysis of the difficulties and techniques for reducing the effect of hot carriers in perovskite solar cells and optimizing their performance.