Excitonic Au4Ru2(PPh3)2(SC2H4Ph)8 cluster for light-driven dinitrogen fixation

Unraveling the Photocatalytic Mechanism of N2 Fixation on Single Ruthenium Sites

Photocatalytic N2 fixation offers promise for ammonia synthesis, yet traditional photocatalysts encounter challenges such as low efficiency and short carrier lifetimes. Atomically precise ligand-metal nanoclusters emerge as a solution to address these issues, but the photophysical mechanism remains elusive. Inspired by the synthesis of Au4Ru2 NCs, we investigate the mechanism behind N2 activation on Au4Ru2, focusing on photoactivity and carrier dynamics. Our results reveal that vibration of the Ru-N bond in the low-frequency domain suppresses the deactivation process leading to a long lifetime of the excited N2. By the strategy of isoelectronic substitution, we identify the single Ru sites as the active sites for N2 activation. Furthermore, these ligand-protected M4Ru2 (M = Au, Ag, Cu) NCs show robust thermal stability in explicit solvation and decent photochemical activity for N2 activation and NH3 production. These findings have significant implications for the optimization of catalysts for sustainable ammonia synthesis.

Recent progress in atomically precise metal nanoclusters for photocatalytic application

Photocatalysis is a widely recognized green and sustainable technology that can harness inexhaustible solar energy to carry out chemical reactions, offering the opportunity to mitigate environmental issues and the energy crisis. Photocatalysts with wide spectral response and rapid charge transfer capability are crucial for highly efficient photocatalytic activity. Atomically precise metal nanoclusters (NCs), an emerging atomic-level material, have attracted great interests owing to their ultrasmall size, unique atomic stacking, abundant surface active sites, and quantum confinement effect. In particular, the molecule-like discrete electronic energy level endows them with small-band-gap semiconductor behavior, which allows for photoexcitation in order to generate electrons and holes to participate in the photoredox reaction. In addition, metal NCs exhibit strong light-harvesting ability in the wide spectral UV-near IR region, and the diversity of optical absorption properties can be precisely regulated by the composition and structure. These merits make metal NCs ideal candidates for photocatalysis. In this review, the recent advances in atomically-precise metal NCs for photocatalytic application are summarized, including photocatalytic water splitting, CO2 reduction, organic transformation, photoelectrocatalytic reactions, N2 fixation and H2O2 production. In addition, the strategy for promoting photostability, charge transfer and separation efficiency of metal NCs is highlighted. Finally, a perspective on the challenges and opportunities for NCs-based photocatalysts is provided.

Modelling Photocatalytic N2 Reduction to Ammonia: Where We Stand and Where We Are Going

Artificial ammonia synthesis via the Haber-Bosch process is environmentally problematic due to the high energy consumption and corresponding CO2 ${_2 }$ emissions, produced during the reaction and before hand in hydrogen production upon methane steam reforming. Photocatalytic nitrogen fixation as a greener alternative to the conventional Haber-Bosch process enables us to perform nitrogen reduction reaction (NRR) under mild conditions, harnessing light as the energy source. Herein, we systematically review first-principles calculations used to determine the electronic/optical properties of photocatalysts, N2 adsorption and to expound possible NRR mechanisms. The most commonly studied photocatalysts for nitrogen fixation are usually modified with dopants, defects, co-catalysts and Z-scheme heterojunctions to prevent charge carrier recombination, improve charge separation efficiency and adjust a band gap to for utilizing a broader light spectrum. Most studies at the atomistic level of modeling are grounded upon density functional theory (DFT) calculations, wholly foregoing excitation effects paramount in photocatalysis. Hence, there is a dire need to consider methods beyond DFT to study the excited state properties more accurately. Furthermore, a few studies have been examined, which include higher level kinetics and macroscale simulations. Ultimately, we show there is still ample room for improvement with regard to first principles calculations and their integration in multiscale models.

Amorphous Copper-Based Nanoparticles with Clusterization-Triggered Phosphorescence for Ultrasensing 2,4,6-Trinitrotoluene

Amorphous metal-based nanostructures have attracted great attention recently due to their facilitative electron transfer and abundant reactive sites, whereas it remains enigmatic as to whether amorphous copper-based nanoparticles (CuNPs) can be achieved. Here, for synthesizing amorphous CuNPs, glutathione is adopted as a ligand to inhibit the nucleation and crystallization process via its electrostatic repulsion. By subtly tailoring the solvent polarity, not only can amorphous glutathione-functionalized CuNPs (GSH-CuNPs) with phosphorescent performance be achieved after transferring the non-conjugation of GSH ligand to through-space conjugation, namely clusterization-triggered emission, but also the phosphorescence-off of GSH-CuNPs toward 2,4,6-trinitrotoluene (TNT) can be realized by the photoinduced electron-transfer process through the hydrogen bond channel, which is established between carboxyl and amino groups of GSH-CuNPs with the nitryl group of TNT. Benefitting from the intrinsic superiorities of the amorphous CuNPs, desired phosphorescence and detection performances of GSH-CuNPs toward airborne TNT microparticulates are undoubtedly realized, including high quantum yield (13.22%), excellent specificity in 33 potential interferents, instantaneous response, and ultralow detection limit (1.56 pg). The present GSH-CuNPs are expected to stretch amorphous metal-based nanostructures and deepen the insights into amorphous materials for optical detection.

A Revealing Insight into Gold Cluster Photocatalysts: Visible versus (Vacuum) Ultraviolet Light

[Au₂₅(PPh₃)₁₀(SC₂H₄Ph)₅Cl₂]²⁺ (Au25) supported on TiO₂ (P25) exhibited distinct photocatalytic behaviors in the oxidation of amines using visible or ultraviolet light. The activity under visible light (455 nm) was superior to that under ultraviolet light. To gain insight into the origin of this difference, we investigated the photoreaction pathways of Au25 isolated in the gas phase upon irradiation with a pulsed laser with wavelengths of 455, 193, and 154 nm. High-resolution mass spectrometry revealed photon energy-dependent pathways for Au25: dissociation of the PPh₃ ligands and PPh₃AuCl units at 455 nm, dissociation into small [Au_nS_m]⁺ ions (n = 3-20; m = 0-4) at 193 nm, and ionization affording the triply charged state at 154 nm. These results were substantiated by density functional theory simulations. On the basis of these results, we proposed that the inferior photocatalytic activity of Au25/P25 under ultraviolet light is mainly due to the poor photostability of Au25.

Cu(I) complexes with aggregation-induced emission for enhanced photodynamic antibacterial application

This communication reports the design of aggregation-induced emission (AIE)-featured PEG-condensed Cu(I)-p-MBA aggregates (PCuA). Benefiting from the AIE trait and intrinsic antibacterial property of Cu species, the as-developed PCuA exhibits enhanced photodynamic antibacterial activities against broad-spectrum bacteria, providing a paradigm in the design of novel antibacterial agents.

Isomerism effects in relaxation dynamics of Au24(SR)16thiolate-protected gold nanoclusters

Understanding the excited state behavior of isomeric structures of thiolate-protected gold nanoclusters is still a challenging task. In this paper, based on grand unified model and ring model for describing thiolate-protected gold nanoclusters, we have predicted four isomers of Au₂₄(SR)₁₆nanoclusters. Density functional theory calculations show that the total energy of one of the predicted isomers is 0.1 eV lower in energy than previously crystallized isomer. The nonradiative relaxation dynamics simulations of Au₂₄(SH)₁₆isomers are performed to reveal the effects of structural isomerism on relaxation process of the lowest energy states, in which that most of the low-excited states consist of core states. In addition, crystallized isomer possesses the shorter e-h recombination time, whereas the most stable isomer has the longer recombination time, which may be attributed to the synergistic effect of nonadiabatic coupling and decoherence time. Our results could provide practical guidance to predict new gold nanoclusters for future experimental synthesis, and stimulate the exploration of atomic structures of same sized gold nanoclusters for photovoltaic and optoelectronic devices.

Symmetry-breaking synthesis of Janus Au/CeO2 nanostructures for visible-light nitrogen photofixation

Precise manipulation of the reactive site spatial distribution in plasmonic metal/semiconductor photocatalysts is crucial to their photocatalytic performance, but the construction of Janus nanostructures through symmetry-breaking synthesis remains a significant challenge. Here we demonstrate a synthetic strategy for the selective growth of a CeO2 semi-shell on Au nanospheres (NSs) to fabricate Janus Au NS/CeO2 nanostructures with the assistance of a SiO2 hard template and autoredox reaction between Ag+ ions and a ceria precursor. The obtained Janus nanostructures possess a spatially separated architecture and exhibit excellent photocatalytic performance toward N2 photofixation under visible-light illumination. In this scenario, N2 molecules are reduced by hot electrons on the CeO2 semi-shell, while hole scavengers are consumed by hot holes on the exposed Au NS surface, greatly promoting the charge carrier separation. Moreover, the exposed Au NS surface in the Janus structures offers an additional opportunity for the fabrication of ternary Janus noble metal/Au NS/CeO2 nanostructures. This work highlights the genuine superiority of the spatially separated nanoarchitectures in the photocatalytic reaction, offering instructive guidance for the design and construction of novel plasmonic photocatalysts.

Tuning photoelectron dynamic behavior of thiolate-protected MAu24 nanoclusters via heteroatom substitution

Heteroatom substitution of gold nanoclusters enables precise tuning of their physicochemical properties at the single-atom level, which has a significant impact on the applications related to excited states including photovoltaics, photocatalysis and photo-luminescence. To this end, understanding the effect of metal exchange on the structures, electronic properties and photoexcited dynamic behavior of nanoclusters is imperative. Combining density functional theory with time-domain nonadiabatic molecular dynamics simulations, herein we explored the effect of metal replacement on the electronic and vibrational properties as well as excited-state dynamics of ligand-protected MAu₂₄(SR)₁₈ (M = Pd, Pt, Cd, and Hg) nanoclusters. At the atomistic level, we elucidate hot carrier relaxation and recombination dynamic behavior with various doping atoms. Such distinct excited-state behavior of MAu₂₄(SR)₁₈ nanoclusters is attributed to different energy gaps and electron-phonon coupling between the donor and acceptor energy levels, owing to the perturbation of nanoclusters by a single foreign atom. The specific phonon modes involved in excited-state dynamics have been identified, which are associated with the MAu₁₂ core and ligand rings. This time-dependent excited-state dynamic study fills the gap between structure/composition and excited-state dynamic behavior of MAu₂₄(SR)₁₈ nanoclusters, which would stimulate the exploration of their applications in photoenergy storage and conversion.

A Review of State of the Art in Phosphine Ligated Gold Clusters and Application in Catalysis

Atomically precise gold clusters are highly desirable due to their well-defined structure which allows the study of structure-property relationships. In addition, they have potential in technological applications such as nanoscale catalysis. The structural, chemical, electronic, and optical properties of ligated gold clusters are strongly defined by the metal-ligand interaction and type of ligands. This critical feature renders gold-phosphine clusters unique and distinct from other ligand-protected gold clusters. The use of multidentate phosphines enables preparation of varying core sizes and exotic structures beyond regular polyhedrons. Weak gold-phosphorous (Au-P) bonding is advantageous for ligand exchange and removal for specific applications, such as catalysis, without agglomeration. The aim of this review is to provide a unified view of gold-phosphine clusters and to present an in-depth discussion on recent advances and key developments for these clusters. This review features the unique chemistry, structural, electronic, and optical properties of gold-phosphine clusters. Advanced characterization techniques, including synchrotron-based spectroscopy, have unraveled substantial effects of Au-P interaction on the composition-, structure-, and size-dependent properties. State-of-the-art theoretical calculations that reveal insights into experimental findings are also discussed. Finally, a discussion of the application of gold-phosphine clusters in catalysis is presented.

Marrying luminescent Au nanoclusters to TiO2 for visible-light-driven antibacterial application

Long-lasting yet visible-light-driven bacterial inhibition is highly desired for environmental protection and public health maintenance. However, conventional semiconductors such as titanium dioxide (TiO₂) are impotent for such antibacterial application due to their low utilization rate for visible light. Herein we report the design of a long-lasting yet visible-light-driven antibacterial agent based on marrying luminescent Au nanoclusters (Au NCs for short) to TiO₂ (TiO₂-NH₂@Au NCs). The as-obtained TiO₂-NH₂@Au NC antibacterial agent not only possesses superior utilization for visible light due to the participation of Au NCs as a good photosensitizer, but also has excellent separation efficacy of photogenerated carriers, thereby efficiently enhancing the generation of reactive oxygen species (ROS) for killing bacteria. Consequently, the TiO₂-NH₂@Au NCs display excellent antibacterial activity with good durability against both Gram-positive and Gram-negative bacteria such as Staphylococcus aureus (99.37%) and Escherichia coli (99.92%) under visible-light irradiation (λ ≥ 400 nm). This study is interesting because it provides a paradigm change in the design of long-lasting yet visible-light-driven NC-based antibacterial agents for diversified bactericidal applications.

On the photocatalysis evolution of heteroatom-doped Ag4M2 nanoclusters

Atomically precise metal nanoclusters doped with one or more heteroatom of other metals have exhibited extraordinary catalytic properties. Here we report a series of thiolate-protected Ag4M2 (M is dopant Ni, Pd and Pt) nanoclusters that adopt a similar structural framework like a distorted hexahedron, in which four Ag atoms are located at the midpoints of four side edges and two metal heteroatoms reside on the centres of the top and the bottom planes. The opposite orders of the catalytic performances of the three catalysts for the photocatalytic degradation of the methyl orange and rhodamine B dyes are found, which is attributed to two different types of inter-molecular recombination mechanisms. In both photocatalytic systems, both the catalyst and the dye are visible-light active, and the inter-molecular recombination of the photo-excited hole in the catalyst and the photo-excited electron in the dye leads to charge separation across the system comprising the catalyst and the dye. The study represents an important step towards developing the precise tailoring of the composition and structure to control the physicochemical properties of metal nanoclusters.

Efficient Photoexcited Charge Separation at the Interface of a Novel 0D/2D Heterojunction: A Time-Dependent Ultrafast Dynamic Study

To achieve efficient conversion and avoid loss of solar energy, ultrafast charge separation and slow electron-hole recombination are desired. Combining time-dependent density functional theory (TD-DFT) with nonadiabatic molecular dynamics, Au₉(PH₃)₈/MoS₂, as a prototype for zero-dimensional/two-dimensional (0D/2D) heterojunction, has been demonstrated to present excellent light absorption capacity and effective charge separation characteristics. In the heterojunction, photoexcitation of the Au₉(PH₃)₈ nanocluster drives an ultrafast electron transfer from Au₉(PH₃)₈ to MoS₂ within 20 fs, whereas photoexcitation of the MoS₂ nanosheet leads to hole transfer from MoS₂ to Au₉(PH₃)₈ within 680 fs. The strong nonadiabatic coupling and prominent density overlap are responsible for the faster electron separation relative to hole separation. In competition with the charge separation, electron-hole recombination requires 205 ns, ensuring an effective carrier separation. Our atomistic TD-DFT simulation provides valuable insights into the photocarrier dynamics at the Au₉(PH₃)₈/MoS₂ interface, which would stimulate the exploration of 0D/2D hybrid materials for photovoltaic and optoelectronic devices.

Distinct structure assembly driven by metal-ligand binding in Au23 nanoclusters and its relation to photocatalysis

Here, we introduce two Au₂₃ nanoclusters to unveil the significance of metal-ligand binding-induced assembly. The Au₂₃ cluster protected by the thiolate ligand is packed in the shell-by-shell arrangement, while the Au₂₃ cluster capped by dual ligands of thiolate and PPh₃ is constructed from the assembly of Au₄ tetrahedra. Furthermore Au₂₃ from Au₄ tetrahedron-based assembly is capable of converting absorbed visible light into more excitons, compared to Au₂₃ from shell-by-shell assembly, thus exhibiting more efficient photocatalysis.

Mechanistic insights into the two-phase synthesis of heteroleptic Au nanoclusters

A mechanistic study on the two-phase synthesis of heteroleptic Au nanoclusters (NCs) is reported here. First, the effects of binary ligands on controlling the size of Au NCs were examined: (1) the binary ligands could exhibit an eclectic effect on the size control of Au NCs if the binding affinities of such hetero-ligands with Au are comparable and (2) the binary ligands could exhibit a competitive effect on the size control of Au NCs, and the size of the Au NCs could be determined by the ligand with stronger binding affinity to Au. This finding is interesting and can shed some light on the design of new functional metal NCs. Secondly, the formation mechanism of the heteroleptic Au NCs that originated from the complex precursors was unprecedentedly studied. The complex precursors of the heteroleptic Au NCs were identified to be the predominant hybridized ligand_#1-Au(i)-ligand_#2 species, which is helpful for understanding the synthetic mechanisms in depth. Moreover, the growth processes of the heteroleptic Au NCs were also monitored, and some fundamental perceptions about the growth pathway and the structures of the Au NCs were obtained.

Towards H2O catalyzed N2-fixation over TiO2 doped Run clusters (n = 5, 6): a mechanistic and kinetic approach

H2O driven N2 fixation is known as the best alternative pathway to synthesise NH3 under ambient conditions. The thermodynamic non-spontaneous reaction can be accomplished by a photocatalytic water splitting reaction over a TiO2 supported surface with oxygen vacancies. Previous experiments have also shown N2 activation over a neutral Ru cluster whose catalytic activity was remarkably enhanced by TiO2 doping. In this article, we have investigated the detailed mechanism and kinetics of the H2O catalyzed nitrogen reduction reaction (NRR) over bare and TiO2 doped Ru5 clusters in conjunction with DFT and TST calculations. The lack of photochemical activity of the small model cluster provoked us to explore an alternative route of NH3 formation via H2O catalysis. For this, we have considered H2 as co-reactant. The partial reduction of N2 into NH3 or N2H4 could be achieved by a H2O oxidation reaction, however, catalytic regeneration requires additional H2 which effectively makes the overall reaction catalyzed by H2O. Above all, the present investigation suggests that NH3 is most favorably produced through the distal mechanism. Analysis of the rate constants demonstrates that the doping with TiO2 accelerates the kinetics of NRR by a few orders of magnitude. Furthermore, an increase of the size of the metal cluster would not significantly enhance the overall performance of NRR.

Ligand-protected Au4Ru2 and Au5Ru2 nanoclusters: distinct structures and implications for site-cooperation catalysis

We report two ligand-protected Au₄Ru₂ and Au₅Ru₂ nanoclusters with distinct atomic-packing modes and electronic structures, both of which act as ideal model catalysts for identifying the catalytically active sites of catalysts on the nanoclusters. Au₅Ru₂ exhibits superior catalytic performances to Au₄Ru₂ for N-methylation of N-methylaniline to N-methylformanili, which is likely due to the site-cooperation catalysis of Au₅Ru₂.

Precisely modulating the surface sites on atomically monodispersed gold-based nanoclusters for controlling their catalytic performances

Atomically precise gold nanoclusters protected by ligands are being intensely investigated in current catalysis science, due to the definitive correlation between the catalytic properties and structures at an atomic level. By solving the crystal structures of the nanoclusters, coupled with in situ and ex situ spectroscopy, a very fundamental understanding can be achieved to learn what controls the catalytic activation, active site structure, and catalytic mechanism. Herein, we mainly focus on the recent progress in catalysis controlled by precisely modulating the surface structures of the nanoclusters, including the alteration of the surface motifs, the doping of heterogeneous atoms in the surface of the nanoclusters, and the surface ligand engineering. The article is expected to help not only gain deep insight into the crucial roles of surface motifs of the nanoclusters in regulating the catalytic properties, but also explore the wide catalytic applications of atomically precise nanoclusters by elaborately tailoring the surface of the nanoclusters.