Early stages of covalent organic framework formation imaged in operando.

Covalent organic frameworks (COFs) are a functional material class able to harness, convert and store energy. However, after almost 20 years of research, there are no coherent prediction rules for their synthesis conditions. This is partly because of an incomplete picture of nucleation and growth at the early stages of formation. Here we use the optical technique interferometric scattering microscopy (iSCAT)1-3 for in operando studies of COF polymerization and framework formation. We observe liquid-liquid phase separation, pointing to the existence of structured solvents in the form of surfactant-free (micro)emulsions in conventional COF synthesis. Our findings show that the role of solvents extends beyond solubility to being kinetic modulators by compartmentation of reactants and catalyst. Taking advantage of these observations, we develop a synthesis protocol for COFs using room temperature instead of elevated temperatures. This work connects framework synthesis with liquid phase diagrams and thereby enables an active design of the reaction environment, emphasizing that visualization of chemical reactions by means of light-scattering-based techniques can be a powerful approach for advancing rational materials synthesis.

Direct Bottom-Up In Situ Growth: A Paradigm Shift for Studies in Wet-Chemical Synthesis of Gold Nanoparticles.

Plasmonic gold nanoparticles have been used increasingly in solid-state systems because of their applicability in fabricating novel sensors, heterogeneous catalysts, metamaterials, and thermoplasmonic substrates. While bottom-up colloidal syntheses take advantage of the chemical environment to control size, shape, composition, surface chemistry, and crystallography of the nanostructures precisely, it can be challenging to assemble nanoparticles rationally from suspension onto solid supports or within devices. In this Review, we discuss a powerful recent synthetic methodology, bottom-up in situ substrate growth, which circumvents time-consuming batch presynthesis, ligand exchange, and self-assembly steps by applying wet-chemical synthesis to form morphologically controlled nanostructures on supporting materials. First, we briefly introduce the properties of plasmonic nanostructures. Then we comprehensively summarize recent work that adds to the synthetic understanding of in situ geometrical and spatial control (patterning). Next, we briefly discuss applications of plasmonic hybrid materials prepared by in situ growth. Overall, despite the vast potential advantages of in situ growth, the mechanistic understanding of these methodologies remains far from established, providing opportunities and challenges for future research.

Focusing Surface Acoustic Waves with a Plasmonic Hypersonic Lens.

Plasmonic nanoantennas have proven to be efficient transducers of electromagnetic to mechanical energy and vice versa. The sudden thermal expansion of these structures after an ultrafast optical pulsed excitation leads to the emission of hypersonic acoustic waves to the supporting substrate, which can be detected by another antenna that acts as a high-sensitivity mechanical probe due to the strong modulation of its optical response. Here, we propose and experimentally demonstrate a nanoscale acoustic lens comprised of 11 gold nanodisks whose collective oscillation at gigahertz frequencies gives rise to an interference pattern that results in a diffraction-limited surface acoustic beam of about 340 nm width, with an amplitude contrast of 60%. Via spatially decoupled pump-probe experiments, we were able to map the radiated acoustic energy in the proximity of the focal area, obtaining a very good agreement with the continuum elastic theory.

Potential Alignment in Tandem Catalysts Enhances CO2-to-C2H4 Conversion Efficiencies.

The in-tandem catalyst holds great promise for addressing the limitation of low *CO coverage on Cu-based materials for selective C2H4 generation during CO2 electroreduction. However, the potential mismatch between the CO-formation catalyst and the favorable C-C coupling Cu catalyst represents a bottleneck in these types of electrocatalysts, resulting in low tandem efficiencies. In this study, we propose a robust solution to this problem by introducing a wide-CO generation-potential window nickel single atom catalyst (Ni SAC) supported on a Cu catalyst. The selection of Ni SAC was based on theoretical calculations, and its excellent performance was further confirmed by using in situ IR spectroscopy. The facilitated carbon dimerization in our tandem catalyst led to a ∼370 mA/cm2 partial current density of C2H4, corresponding to a faradic efficiency of ∼62%. This performance remained stable and consistent for at least ∼14 h at a high current density of 500 mA/cm2 in a flow-cell reactor, outperforming most tandem catalysts reported so far.

Optical Metasurfaces for Energy Conversion.

Nanostructured surfaces with designed optical functionalities, such as metasurfaces, allow efficient harvesting of light at the nanoscale, enhancing light-matter interactions for a wide variety of material combinations. Exploiting light-driven matter excitations in these artificial materials opens up a new dimension in the conversion and management of energy at the nanoscale. In this review, we outline the impact, opportunities, applications, and challenges of optical metasurfaces in converting the energy of incoming photons into frequency-shifted photons, phonons, and energetic charge carriers. A myriad of opportunities await for the utilization of the converted energy. Here we cover the most pertinent aspects from a fundamental nanoscopic viewpoint all the way to applications.

Investigating Plasmonic Catalysis Kinetics on Hot-Spot Engineered Nanoantennae.

Strong hot-spots can facilitate photocatalytic reactions potentially providing effective solar-to-chemical energy conversion pathways. Although it is well-known that the local electromagnetic field in plasmonic nanocavities increases as the cavity size reduces, the influence of hot-spots on photocatalytic reactions remains elusive. Herein, we explored hot-spot dependent catalytic behaviors on a highly controlled platform with varying interparticle distances. Plasmon-meditated dehalogenation of 4-iodothiophenol was employed to observe time-resolved catalytic behaviors via in situ surface-enhanced Raman spectroscopy on dimers with 5, 10, 20, and 30 nm interparticle distances. As a result, we show that by reducing the gap from 20 to 10 nm, the reaction rate can be sped up more than 2 times. Further reduction in the interparticle distance did not improve reaction rate significantly although the maximum local-field was ∼2.3-fold stronger. Our combined experimental and theoretical study provides valuable insights in designing novel plasmonic photocatalytic platforms.

Unraveling the Chirality Transfer from Circularly Polarized Light to Single Plasmonic Nanoparticles.

Due to their broken symmetry, chiral plasmonic nanostructures have unique optical properties and numerous applications. However, there is still a lack of comprehension regarding how chirality transfer occurs between circularly polarized light (CPL) and these structures. Here, we thoroughly investigate the plasmon-assisted growth of chiral nanoparticles from achiral Au nanocubes (AuNCs) via CPL without the involvement of any chiral molecule stimulators. We identify the structural chirality of our synthesized chiral plasmonic nanostructures using circular differential scattering (CDS) spectroscopy, which is correlated with scanning electron microscopy imaging at both the single-particle and ensemble levels. Theoretical simulations, including hot-electron surface maps, reveal that the plasmon-induced chirality transfer is mediated by the asymmetric distribution of hot electrons on achiral AuNCs under CPL excitation. Furthermore, we shed light on how this plasmon-induced chirality transfer can also be utilized for chiral growth in bimetallic systems, such as Ag or Pd on AuNCs. The results presented here uncover fundamental aspects of chiral light-matter interaction and have implications for the future design and optimization of chiral sensors and chiral catalysis, among others.

Coupling Nano and Atomic Electric Field Confinement for Robust Alkaline Oxygen Evolution.

The alkaline oxygen evolution reaction (OER) is a promising avenue for producing clean fuels and storing intermittent energy. However, challenges such as excessive OH- consumption and strong adsorption of oxygen-containing intermediates hinder the development of alkaline OER. In this study, we propose a cooperative strategy by leveraging both nano-scale and atomically local electric fields for alkaline OER, demonstrated through the synthesis of Mn single atom doped CoP nanoneedles (Mn SA-CoP NNs). Finite element method simulations and density functional theory calculations predict that the nano-scale local electric field enriches OH- around the catalyst surface, while the atomically local electric field improves *O desorption. Experimental validation using in situ attenuated total reflection infrared and Raman spectroscopy confirms the effectiveness of the nano-scale and atomically electric fields. Mn SA-CoP NNs exhibit an ultra-low overpotential of 189 mV at 10 mA cm-2 and stable operation over 100 hours at ~100 mA cm-2 during alkaline OER. This innovative strategy provides new insights for enhancing catalyst performance in energy conversion reactions.

Lattice Oxygen Activation through Deep Oxidation of Co4N by Jahn-Teller-Active Dopants for Improved Electrocatalytic Oxygen Evolution.

Triggering the lattice oxygen oxidation mechanism is crucial for improving oxygen evolution reaction (OER) performance, because it could bypass the scaling relation limitation associated with the conventional adsorbate evolution mechanism through the direct formation of oxygen-oxygen bond. High-valence transition metal sites are favorable for activating the lattice oxygen, but the deep oxidation of pre-catalysts suffers from a high thermodynamic barrier. Here, taking advantage of the Jahn-Teller (J-T) distortion induced structural instability, we incorporate high-spin Mn3+ ( t 2 g 3 e g 1 ${{t}_{2g}^{3}{e}_{g}^{1}}$ ) dopant into Co4N. Mn dopants enable a surface structural transformation from Co4N to CoOOH, and finally to CoO2, as observed by various in situ spectroscopic investigations. Furthermore, the reconstructed surface on Mn-doped Co4N triggers the lattice oxygen activation, as evidenced experimentally by pH-dependent OER, tetramethylammonium cation adsorption and online electrochemical mass spectrometry measurements of 18O-labelled catalysts. In general, this work not only offers the introducing J-T effect approach to regulate the structural transition, but also provides an understanding about the influence of the catalyst's electronic configuration on determining the reaction route, which may inspire the design of more efficient catalysts with activated lattice oxygen.

Highly Selective Monomethylation of Amines with CO2 /H2 via Ag/Al2 O3 as a Catalyst.

The selective synthesis of monomethylated amines with CO2 is particularly challenging because the formation of tertiary amines is thermodynamically more favorable. Herein, a new strategy for the controllable synthesis of N-monomethylated amines from primary amines and CO2 /H2 is explored. First-principle calculations reveal that the dissociation of H2 via an heterolytic route reduces the reactivity of methylated amines and thus inhibit successive methylation. In situ DRIFTS proves the process of formation and decomposition of ammonium salt by secondary amine reversible binding with H+ on the Ag/Al2 O3 catalyst, thereby reducing its reactivity. Meanwhile, the energy barrier for the rate-determining step of monomethylation was much lower than that of overmethylation (0.34 eV vs. 0.58 eV) means amines monomethylation in preference to successive methylation. Under optimal reaction conditions, a variety of amines were converted to the corresponding monomethylated amines in good to excellent yields, and more than 90 % yield of product was obtained.

Photo-induced enhanced Raman spectroscopy as a probe for photocatalytic surfaces.

Photo-induced enhanced Raman spectroscopy (PIERS) has emerged as a highly sensitive surface-enhanced Raman spectroscopy (SERS) technique for the detection of ultra-low concentrations of organic molecules. The PIERS mechanism has been largely attributed to UV-induced formation of surface oxygen vacancies (Vo) in semiconductor materials, although alternative interpretations have been suggested. Very recently, PIERS has been proposed as a surface probe for photocatalytic materials, following Vo formation and healing kinetics. This work establishes comparison between PIERS and Vo-induced SERS approaches in defected noble-metal-free titanium dioxide (TiO2-x) films to further confirm the role of Vo in PIERS. Upon application of three post-treatment methods (namely UV-induction, vacuum annealing and argon etching), correlation of Vo kinetics and distribution could be established. A proposed mechanism and further discussion on PIERS as a probe to explore photocatalytic materials are also presented. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 2)'.

Ordered Ag Nanoneedle Arrays with Enhanced Electrocatalytic CO2 Reduction via Structure-Induced Inhibition of Hydrogen Evolution.

Silver is an attractive catalyst for converting CO2 into CO. However, the high CO2 activation barrier and the hydrogen evolution side reaction seriously limit its practical application and industrial perspective. Here, an ordered Ag nanoneedle array (Ag-NNAs) was prepared by template-assisted vacuum thermal-evaporation for CO2 electroreduction into CO. The nanoneedle array structure induces a strong local electric field at the tips, which not only reduces the activation barrier for CO2 electroreduction but also increases the energy barrier for the hydrogen evolution reaction (HER). Moreover, the array structure endows a high surface hydrophobicity, which can regulate the adsorption of water molecules at the interface and thus dynamically inhibit the competitive HER. As a result, the optimal Ag-NNAs exhibits 91.4% Faradaic efficiency (FE) of CO for over 700 min at -1.0 V vs RHE. This work provides a new concept for the application of nanoneedle array structures in electrocatalytic CO2 reduction reactions.

Vertical Cu Nanoneedle Arrays Enhance the Local Electric Field Promoting C2 Hydrocarbons in the CO2 Electroreduction.

Electrocatalytic reduction of CO2 to multicarbon products is a potential strategy to solve the energy crisis while achieving carbon neutrality. To improve the efficiency of multicarbon products in Cu-based catalysts, optimizing the *CO adsorption and reducing the energy barrier for carbon-carbon (C-C) coupling are essential features. In this work, a strong local electric field is obtained by regulating the arrangement of Cu nanoneedle arrays (CuNNAs). CO2 reduction performance tests indicate that an ordered nanoneedle array reaches a 59% Faraday efficiency for multicarbon products (FEC2) at -1.2 V (vs RHE), compared to a FEC2 of 20% for a disordered nanoneedle array (CuNNs). As such, the very high and local electric fields achieved by an ordered Cu nanoneedle array leads to the accumulation of K+ ions, which benefit both *CO adsorption and C-C coupling. Our results contribute to the design of highly efficient catalysts for multicarbon products.

Highly Efficient Decomposition of Perfluorocarbons for over 1000 Hours via Active Site Regeneration.

Tetrafluoromethane (CF4 ), the simplest perfluorocarbon (PFC), has the potential to exacerbate global warming. Catalytic hydrolysis is a viable method to degrade CF4 , but fluorine poisoning severely restricts both the catalytic performance and catalyst lifetime. In this study, Ga is introduced to effectively assists the defluorination of poisoned Al active sites, leading to highly efficient CF4 decomposition at 600 °C with a catalytic lifetime exceeding 1,000 hours. 27 Al and 71 Ga magic-angle spinning nuclear magnetic resonance spectroscopy (MAS NMR) showed that the introduced Ga exists as tetracoordinated Ga sites (GaIV ), which readily dissociate water to form Ga-OH. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and density function theory (DFT) calculations confirmed that Ga-OH assists the defluorination of poisoned Al active sites via a dehydration-like process. As a result, the Ga/Al2 O3 catalyst achieved 100 % CF4 decomposition keeping an ultra-long catalytic lifetime and outperforming reported results. This work proposes a new approach for efficient and long-term CF4 decomposition by promoting the regeneration of active sites.

Breaking K+ Concentration Limit on Cu Nanoneedles for Acidic Electrocatalytic CO2 Reduction to Multi-Carbon Products.

Electrocatalytic CO2 reduction reaction (CO2 RR) to multi-carbon products (C2+ ) in acidic electrolyte is one of the most advanced routes for tackling our current climate and energy crisis. However, the competing hydrogen evolution reaction (HER) and the poor selectivity towards the valuable C2+ products are the major obstacles for the upscaling of these technologies. High local potassium ions (K+ ) concentration at the cathode's surface can inhibit proton-diffusion and accelerate the desirable carbon-carbon (C-C) coupling process. However, the solubility limit of potassium salts in bulk solution constrains the maximum achievable K+ concentration at the reaction sites and thus the overall acidic CO2 RR performance of most electrocatalysts. In this work, we demonstrate that Cu nanoneedles induce ultrahigh local K+ concentrations (4.22 M) - thus breaking the K+ solubility limit (3.5 M) - which enables a highly efficient CO2 RR in 3 M KCl at pH=1. As a result, a Faradaic efficiency of 90.69±2.15 % for C2+ (FEC2+ ) can be achieved at 1400 mA.cm-2 , simultaneous with a single pass carbon efficiency (SPCE) of 25.49±0.82 % at a CO2 flow rate of 7 sccm.

Subsurface Engineering Induced Fermi Level De-pinning in Metal Oxide Semiconductors for Photoelectrochemical Water Splitting.

Photoelectrochemical (PEC) water splitting is a promising approach for renewable solar light conversion. However, surface Fermi level pinning (FLP), caused by surface trap states, severely restricts the PEC activities. Theoretical calculations indicate subsurface oxygen vacancy (sub-Ov ) could release the FLP and retain the active structure. A series of metal oxide semiconductors with sub-Ov were prepared through precisely regulated spin-coating and calcination. Etching X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), and electron energy loss spectra (EELS) demonstrated Ov located at sub ∼2-5 nm region. Mott-Schottky and open circuit photovoltage results confirmed the surface trap states elimination and Fermi level de-pinning. Thus, superior PEC performances of 5.1, 3.4, and 2.1 mA cm-2 at 1.23 V vs. RHE were achieved on BiVO4 , Bi2 O3 , TiO2 with outstanding stability for 72 h, outperforming most reported works under the identical conditions.

Atomically Local Electric Field Induced Interface Water Reorientation for Alkaline Hydrogen Evolution Reaction.

The slow water dissociation process in alkaline electrolyte severely limits the kinetics of HER. The orientation of H2 O is well known to affect the dissociation process, but H2 O orientation is hard to control because of its random distribution. Herein, an atomically asymmetric local electric field was designed by IrRu dizygotic single-atom sites (IrRu DSACs) to tune the H2 O adsorption configuration and orientation, thus optimizing its dissociation process. The electric field intensity of IrRu DSACs is over 4.00×1010  N/C. The ab initio molecular dynamics simulations combined with in situ Raman spectroscopy analysis on the adsorption behavior of H2 O show that the M-H bond length (M=active site) is shortened at the interface due to the strong local electric field gradient and the optimized water orientation promotes the dissociation process of interfacial water. This work provides a new way to explore the role of single atomic sites in alkaline hydrogen evolution reaction.

Accelerating CO2 Electroreduction to Multicarbon Products via Synergistic Electric-Thermal Field on Copper Nanoneedles.

Electrochemical CO2 reduction is a promising way to mitigate CO2 emissions and close the anthropogenic carbon cycle. Among products from CO2RR, multicarbon chemicals, such as ethylene and ethanol with high energy density, are more valuable. However, the selectivity and reaction rate of C2 production are unsatisfactory due to the sluggish thermodynamics and kinetics of C-C coupling. The electric field and thermal field have been studied and utilized to promote catalytic reactions, as they can regulate the thermodynamic and kinetic barriers of reactions. Either raising the potential or heating the electrolyte can enhance C-C coupling, but these come at the cost of increasing side reactions, such as the hydrogen evolution reaction. Here, we present a generic strategy to enhance the local electric field and temperature simultaneously and dramatically improve the electric-thermal synergy desired in electrocatalysis. A conformal coating of ∼5 nm of polytetrafluoroethylene significantly improves the catalytic ability of copper nanoneedles (∼7-fold electric field and ∼40 K temperature enhancement at the tips compared with bare copper nanoneedles experimentally), resulting in an improved C2 Faradaic efficiency of over 86% at a partial current density of more than 250 mA cm-2 and a record-high C2 turnover frequency of 11.5 ± 0.3 s-1 Cu site-1. Combined with its low cost and scalability, the electric-thermal strategy for a state-of-the-art catalyst not only offers new insight into improving activity and selectivity of value-added C2 products as we demonstrated but also inspires advances in efficiency and/or selectivity of other valuable electro-/photocatalysis such as hydrogen evolution, nitrogen reduction, and hydrogen peroxide electrosynthesis.

Mechanistic Insights into OC-COH Coupling in CO2 Electroreduction on Fragmented Copper.

The carbon-carbon (C-C) bond formation is essential for the electroconversion of CO2 into high-energy-density C2+ products, and the precise coupling pathways remain controversial. Although recent computational investigations have proposed that the OC-COH coupling pathway is more favorable in specific reaction conditions than the well-known CO dimerization pathway, the experimental evidence is still lacking, partly due to the separated catalyst design and mechanistic/spectroscopic exploration. Here, we employ density functional theory calculations to show that on low-coordinated copper sites, the *CO bindings are strengthened, and the adsorbed *CO coupling with their hydrogenation species, *COH, receives precedence over CO dimerization. Experimentally, we construct a fragmented Cu catalyst with abundant low-coordinated sites, exhibiting a 77.8% Faradaic efficiency for C2+ products at 300 mA cm-2. With a suite of in situ spectroscopic studies, we capture an *OCCOH intermediate on the fragmented Cu surfaces, providing direct evidence to support the OC-COH coupling pathway. The mechanistic insights of this research elucidate how to design materials in favor of OC-COH coupling toward efficient C2+ production from CO2 reduction.