Decoupling Plasmonic Hot Carrier from Thermal Catalysis via Electrode Engineering.

Increased attention has been directed toward generating nonequilibrium hot carriers resulting from the decay of collective electronic oscillations on metal known as surface plasmons. Despite numerous experimental endeavors, demonstrating hot carrier-mediated photocatalysis without a heating contribution has proven challenging, particularly for single electron transfer reactions where the thermal contribution is generally detrimental. An innovative engineering solution is proposed to enable single electron transfer reactions with plasmonics. It consists of a photoelectrode designed as an energy filter and photocatalysis performed with light function modulation instead of continuously. The photoelectrode, consisting of FTO/TiO2 amorphous (10 nm)/Au nanoparticles, with TiO2 acting as a step-shape energy filter to enhance hot electron extraction and charge-separated state lifetime. The extracted hot electrons were directed toward the counter electrode, while the hot holes performed a single electron transfer oxidation reaction. Light modulation prevented local heat accumulation, effectively decoupling hot carrier catalysis from the thermal contribution.

Can photoluminescence quenching be a predictor for perovskite solar cell efficiencies?

Bromide-based perovskites have large bandgaps, making them attractive for tandem solar cells developed to overcome the Shockley-Queisser limit. A perovskite solar cell architecture employs transporting layers to improve charge extraction and transport. Due to the wide variety of materials and preparation methods, it is critical to devise fast screening methods to rank transporting layers. Herein, we evaluate perovskite fluorescence quenching followed by time- and energy-resolved photoluminescence (TER-PL) and analyse the intensity dependence as a potential method to qualify charge-transporting layers rapidly. The capability of the technique was evaluated with TiO2/FAPbBr3 and SnO2/FAPbBr3, the most commonly used electron transporting layers, which were prepared using standard protocols to make best-performing devices. The results revealed that TiO2 is the most effective quencher due to the higher density of states in the conduction band, consistent with Marcus-Gerischer's theory. However, record-performance devices use SnO2 as the electron transport layer. This shows that the relationship between photoluminescence quenching and device performance is not bidirectional. Therefore, additional measurements like conductivity are also needed to provide reliable feedback for device performance.

Effect of Synthesis Method on Reaction Mechanism for Hydrogen Evolution over CuxOy/TiO2 Photocatalysts: A Kinetic Analysis.

The existing literature survey reports rare and conflicting studies on the effect of the preparation method of metal-based semiconductor photocatalysts on structural/morphological features, electronic properties, and kinetics regulating the photocatalytic H2 generation reaction. In this investigation, we compare the different copper/titania-based photocatalysts for H2 generation synthesized via distinct methods (i.e., photodeposition and impregnation). Our study aims to establish a stringent correlation between physicochemical/electronic properties and photocatalytic performances for H2 generation based on material characterization and kinetic modeling of the experimental outcomes. Estimating unknown kinetic parameters, such as charge recombination rate and quantum yield, suggests a mechanism regulating charge carrier lifetime depending on copper distribution on the TiO2 surface. We demonstrate that H2 generation photoefficiency recorded over impregnated CuxOy/TiO2 is related to an even distribution of Cu(0)/Cu(I) on TiO2, and the formation of an Ohmic junction concertedly extended charge carrier lifetime and separation. The outcomes of the kinetic analysis and the related modeling investigation underpin photocatalyst physicochemical and electronic properties. Overall, the present study lays the groundwork for the future design of metal-based semiconductor photocatalysts with high photoefficiencies for H2 evolution.

Nano-Impact Single-Entity Electrochemistry Enables Plasmon-Enhanced Electrocatalysis.

Plasmon-enhanced electrocatalysis (PEEC), based on a combination of localized surface plasmon resonance excitation and an electrochemical bias applied to a plasmonic material, can result in improved electrical-to-chemical energy conversion compared to conventional electrocatalysis. Here, we demonstrate the advantages of nano-impact single-entity electrochemistry (SEE) for investigating the intrinsic activity of plasmonic catalysts at the single-particle level using glucose electrooxidation and oxygen reduction on gold nanoparticles as model reactions. We show that in conventional ensemble measurements, plasmonic effects have minimal impact on photocurrents. We suggest that this is due to the continuous equilibration of the Fermi level (EF ) of the deposited gold nanoparticles with the EF of the working electrode, leading to fast neutralization of hot carriers by the measuring circuit. The photocurrents detected in the ensemble measurements are primarily caused by photo-induced heating of the supporting electrode material. In SEE, the EF of suspended gold nanoparticles is unaffected by the working electrode potential. As a result, plasmonic effects are the dominant source of photocurrents under SEE experimental conditions.

Phonon-Assisted Hot Carrier Generation in Plasmonic Semiconductor Systems.

Plasmonic materials have optical cross sections that exceed by 10-fold their geometric sizes, making them uniquely suitable to convert light into electrical charges. Harvesting plasmon-generated hot carriers is of interest for the broad fields of photovoltaics and photocatalysis; however, their direct utilization is limited by their ultrafast thermalization in metals. To prolong the lifetime of hot carriers, one can place acceptor materials, such as semiconductors, in direct contact with the plasmonic system. Herein, we report the effect of operating temperature on hot electron generation and transfer to a suitable semiconductor. We found that an increase in the operation temperature improves hot electron harvesting in a plasmonic semiconductor hybrid system, contrasting what is observed on photodriven processes in nonplasmonic systems. The effect appears to be related to an enhancement in hot carrier generation due to phonon coupling. This discovery provides a new strategy for optimization of photodriven energy production and chemical synthesis.

Ultrafast hot-hole injection modifies hot-electron dynamics in Au/p-GaN heterostructures.

A fundamental understanding of hot-carrier dynamics in photo-excited metal nanostructures is needed to unlock their potential for photodetection and photocatalysis. Despite numerous studies on the ultrafast dynamics of hot electrons, so far, the temporal evolution of hot holes in metal-semiconductor heterostructures remains unknown. Here, we report ultrafast (t < 200 fs) hot-hole injection from Au nanoparticles into the valence band of p-type GaN. The removal of hot holes from below the Au Fermi level is observed to substantially alter the thermalization dynamics of hot electrons, reducing the peak electronic temperature and the electron-phonon coupling time of the Au nanoparticles. First-principles calculations reveal that hot-hole injection modifies the relaxation dynamics of hot electrons in Au nanoparticles by modulating the electronic structure of the metal on timescales commensurate with electron-electron scattering. These results advance our understanding of hot-hole dynamics in metal-semiconductor heterostructures and offer additional strategies for manipulating the dynamics of hot carriers on ultrafast timescales.

Reduction Mechanisms of Anticancer Osmium(VI) Complexes Revealed by Atomic Telemetry and Theoretical Calculations.

Resonant X-ray emission spectroscopy (RXES) has developed in the past decade as a powerful tool to probe the chemical state of a metal center and in situ study chemical reactions. We have used it to monitor spectral changes associated with the reduction of osmium(VI) nitrido complexes to the osmium(III) ammine state by the biologically relevant reducing agent, glutathione. RXES difference maps are consistent with the proposed DFT mechanism and the formation of two stable osmium(IV) intermediates, thereby supporting the overall pathway for the reduction of these high-valent anticancer metal complexes for which reduction by thiols within cells may be essential to the antiproliferative activity.

Direct Observation of a Plasmon-Induced Hot Electron Flow in a Multimetallic Nanostructure.

Plasmon hot carriers are interesting for photoredox chemical synthesis but their direct utilization is limited by their ultrafast thermalization. Therefore, they are often transferred to suitable accepting materials that expedite their lifetime. Solid-state photocatalysts are technologically more suitable than their molecular counterparts, but their photophysical processes are harder to follow due to the absence of clear optical fingerprints. Herein, the journey of hot electrons in a solid-state multimetallic photocatalyst is revealed by a combination of ultrafast visible and infrared spectroscopy. Dynamics showed that electrons formed upon silver plasmonic excitation reach the gold catalytic site within 700 fs and the electron flow could also be reversed. Gold is the preferred site until saturation of its 5d band occurs. Silver-plasmon hot electrons increased the rate of nitrophenol reduction 16-fold, confirming the preponderant role of hot electrons in the overall catalytic activity and the importance to follow hot carriers' journeys in solid-state photosystems.

Comparative study of the around-Fermi electronic structure of 5d metals and metal-oxides by means of high-resolution X-ray emission and absorption spectroscopies.

The composition of occupied and unoccupied electronic states in the vicinity of Fermi energies is vital for all materials and relates to their physical, chemical and mechanical properties. This work demonstrates how the combination of resonant and non-resonant X-ray emission spectroscopies supplemented with theoretical modelling allows for quantitative analysis of electronic states in 5d transition metal and metal-oxide materials. Application of X-rays provides element selectivity that, in combination with the penetrating properties of hard X-rays, allows determination of the composition of electronic states under working conditions, i.e. non-vacuum environment. Tungsten metal and tungsten oxide are evaluated to show the capability to simultaneously assess composition of around-band-gap electronic states as well as the character and magnitude of the crystal field splitting.

In situ observation of charge transfer and crystal field formation via high energy resolution X-ray spectroscopy during temperature programmed oxidation.

Herein, it has been demonstrated how resonant X-ray emission spectroscopy can be employed to study the charge transfer dynamics in real-time during the temperature-induced oxidation of metallic tungsten. Application of high energy resolution schemes allowed distinguishing charge transfer to separate orbitals resulting from crystal field splitting. Based on the time-resolved studies, it was possible to determine the corresponding charge transfer rates. From the experimental data, we determined that the electron transfer during the thermal oxidation of the metal dominates in the temperature range of 470-570 °C, reaching a maximum of 0.036 electrons per °C.

The influence of nitrogen doping on the electronic structure of the valence and conduction band in TiO2.

X-ray emission spectroscopy (XES) and X-ray absorption spectroscopy (XAS) provide a unique opportunity to probe both the highest occupied and the lowest unoccupied states in matter with bulk sensitivity. In this work, a combination of valence-to-core XES and pre-edge XAS techniques are used to determine changes induced in the electronic structure of titanium dioxide doped with nitrogen atoms. Based on the experimental data it is shown that N-doping leads to incorporation of the p-states on the occupied electronic site. For the conduction band, a decrease in population of the lowest unoccupied d-localized orbitals with respect to the d-delocalized orbitals is observed. As confirmed by theoretical calculations, the N p-states in TiO2 structure are characterized by higher binding energy than the O p-states which gives a smaller value of the band-gap energy for the doped material.

Investigating DNA Radiation Damage Using X-Ray Absorption Spectroscopy.

The biological influence of radiation on living matter has been studied for years; however, several questions about the detailed mechanism of radiation damage formation remain largely unanswered. Among all biomolecules exposed to radiation, DNA plays an important role because any damage to its molecular structure can affect the whole cell and may lead to chromosomal rearrangements resulting in genomic instability or cell death. To identify and characterize damage induced in the DNA sugar-phosphate backbone, in this work we performed x-ray absorption spectroscopy at the P K-edge on DNA irradiated with either UVA light or protons. By combining the experimental results with theoretical calculations, we were able to establish the types and relative ratio of lesions produced by both UVA and protons around the phosphorus atoms in DNA.

Resonant X-ray emission spectroscopy of platinum(II) anticancer complexes.

Platinum-based drugs are commonly used in cancer treatment. The biological activity of a metallodrug is obviously closely related to its chemical and stereochemical characteristics. An overlooked aspect is the effect of the ligand to the electronic structure of the metal atom (coordinated atom). We report herein a Resonant X-ray Emission Spectroscopy (RXES) study on the chemical speciation of chiral platinum complexes in which diastereomers are distinguished on the basis of their metal electronic configuration. This demonstrates RXES high chemical speciation capabilities, a necessary property to further investigate the reactivity of the Pt atom towards nucleophiles or bionucleophiles, and an important complement the previously reported RXES abilities, namely that it can be employed for in situ studies at physiological concentrations.

Depth-Resolved X-ray Absorption Spectroscopy by Means of Grazing Emission X-ray Fluorescence.

Grazing emission X-ray fluorescence (GEXRF) is well suited for nondestructive elemental-sensitive depth-profiling measurements on samples with nanometer-sized features. By varying the grazing emission angle under which the X-ray fluorescence signal is detected, the probed depth range can be tuned from a few to several hundred nanometers. The dependence of the XRF intensity on the grazing emission angle can be assessed in a sequence of measurements or in a scanning-free approach using a position-sensitive area detector. Hereafter, we will show that the combination of scanning-free GEXRF and fluorescence detected X-ray absorption spectroscopy (XAS) allows for depth-resolved chemical speciation measurements with nanometer-scale accuracy. While the conventional grazing emission geometry is advantageous to minimize self-absorption effects, the use of a scanning-free setup makes the sequential scanning of the grazing emission angles obsolete and paves the way toward time-resolved depth-sensitive XAS measurements. The presented experimental approach was applied to study the surface oxidation of an Fe layer on the top of bulk Si and of a Ge bulk sample. Thanks to the penetrating properties and the insensitivity toward the electric conduction properties of the incident and emitted X-rays, the presented experimental approach is well suited for in situ sample surface studies in the nanometer regime.

Insights into the structure-activity relationships of chiral 1,2-diaminophenylalkane platinum(II) anticancer derivatives.

The structure-activity relationships of chiral 1,2-diaminophenylalkane platinum(II) anticancer derivatives are studied, including interactions with telomeric- and genomic-like DNA sequences, the pKa of their diaqua species, structural properties obtained from DFT calculations and resonant X-ray emission spectroscopy. The binding modes of the compounds to telomeric sequences were elucidated, showing no major differences with conventional cis-platinum(II) complexes like cisplatin, supporting that the cis-square planar geometry governs the binding of small Pt(II) complexes to G4 structures. Double-stranded DNA platination kinetics and acid-base constants of the diaqua species of the compounds were measured and compared, highlighting a strong steric dependence of the DNA-binding kinetics, but independent to stereoisomerism. Structural features of the compounds are discussed on the basis of dispersion-corrected DFT, showing that the most active series presents conformers for which the platinum atom is well devoid of steric hindrance. If reactivity indices derived from conceptual DFT do not show evidences for different reactivity between the compounds, RXES experiments provide new insight into the availability of platinum orbitals for binding to nucleophiles.

Study of the reactivity of silica supported tantalum catalysts with oxygen followed by in situ HEROS.

We report on the reactivity of grafted tantalum organometallic catalysts with molecular oxygen. The changes in the local Ta electronic structure were followed by in situ high-energy resolution off-resonant spectroscopy (HEROS). The results revealed agglomeration and formation of Ta dimers, which cannot be reversed. The process occurs independently of starting grafted complex.

The use of Resonant X-ray Emission Spectroscopy (RXES) for the electronic analysis of metal complexes and their interactions with biomolecules.

This review presents a new application of Resonant X-ray Emission Spectroscopy (RXES) to study the mechanism of action of metal containing anticancer derivatives and in particular platinum in situ and in vivo. The technique is an example of a photon-in photon-out X-ray spectroscopic approach, which enables chemical speciation of drugs to be determined and therefore to derive action mechanisms, and to determine drug binding rates under physiological conditions and therapeutic concentrations. This is made feasible due to the atomic specificity and high penetration depth of RXES. The review presents examples of the three main types of information that can be obtained by RXES and establishes an experimental protocol to perfect the measurements within cells.

Dye-injected electron trapping in TiO2 determined by broadband transient infrared spectroscopy.

We report the dynamics of electrons injected into TiO2 due to the excitation of Ru-N719 dye at 532 nm. The synchrotron based broadband transient mid-IR spectroscopy revealed that the injected electrons are quickly confined to a trap state with an average energy of ca. 240 meV below the conduction band. The average energy of the trapping states did not change with the increase of the delay time, suggesting a singular electronic identity of the trap states.

Temperature-programmed reduction of NiO nanoparticles followed by time-resolved RIXS.

The electronic structure of nano-NiO was determined using resonant inelastic X-ray scattering (RIXS) spectroscopy. The nanosized NiO particles were reduced in situ, leading to the formation of metallic Ni in a single step. Time-resolved RIXS elucidated in real time the changes on the occupied and unoccupied electronic structure of the material, which are dramatically affected by the reduction process.

Nanomaterials as a Service (NaaS) concept: on-demand protocols for volume synthesis of nanomaterials.

Establishing scalable nanomaterials synthesis protocols remains a bottleneck towards their commercialisation and, thus, a topic of intense research and development. Herein, we present an automated machine-learning microfluidic platform capable of synthesising optically active nanomaterials from target spectra originating from prior experience, theorised or published. Implementing unsupervised Bayesian optimisation with Gaussian processes reduces the optimisation time and the need for prior knowledge to initiate the process. Using PTFE tubing and connectors enables facile change in reactor design. Ultimately, the platform substitutes the labour-intensive trial-and-error synthesis and provides a pathway to standardisation and volume synthesis, slowing down the translation and commercialisation of high-quality nanomaterials. As a proof-of-concept, Ag nanoplates and Prussian-blue nanoparticle protocols were optimised and validated for volume production.