Ultrasmall Clusters Research Articles

Exploring the materials space in the smallest particle size range: from heterogeneous catalysis to electrocatalysis and photocatalysis.

Ultrasmall clusters of subnanometer size can possess unique and even unexpected physical and chemical propensities which make them interesting in various fields of basic science and for potential applications, such as catalysis, photocatalysis, electrocatalysis, and optical and chemical sensors, just to name a few examples. These small particles often offer the tunability of their performance in an atom-by-atom fashion and an economic atom-efficient use of the metal loading. In this paper we review recent progress in the characterization and theory of well-defined subnanometer clusters in catalytic processes, and discuss their optical properties and stability, along with the potential of the size-selected clusters for the understanding of catalytic processes and for the development of new classes of catalysts.

Rational Design of Unique MoSe2-Carbon Nanobowl Particles Endows Superior Alkali Metal-Ion Storage Beyond Lithium.

Attracted by the rich earth abundance and low-cost advantages, alkali metal-ion (Na/K)-based energy storage devices have attracted wide interest as promising candidates for energy economizing in recent years. Unfortunately, the lack of suitable host materials with high capacity and long life span for alkali metal-ion storage has severely impeded their practical application in large-scale energy storage devices. Herein, we present a promising anode candidate composed of ultrasmall MoSe2 clusters embedded in a nitrogen-doped hollow carbon nanobowl substrate to form unique MoSe2-Carbon nanobowl particles (denoted as MoSe2⊂CNB). MoSe2⊂CNB demonstrates exceptional electrochemical properties for alkali metal-ion storage including sodium and potassium. In situ Raman spectroscopy and galvanostatic intermittent titration measurements reveal the possible reason for the high performance of MoSe2⊂CNB. Notably, the assembled potassium-ion hybrid capacitors could manifest an extraordinary energy density of 130.7 W h kg-1 at 0.2 A g-1, a high power density of 13,607 W kg-1, and an enviable cycle life after 6000 cycles, further reflecting the great developmental potential for energy storage devices in practical applications. This work provides a new method to design functional nanostructures for electrode materials to drive the development and application of possible energy storage devices.

Gold Nanoclusters: Imaging, Therapy, and Theranostic Roles in Biomedical Applications.

For the past two decades, atomic gold nanoclusters (AuNCs, ultrasmall clusters of several to 100 gold atoms, having a total diameter of <2 nm) have emerged as promising agents in the diagnosis and treatment of cancer. Owing to their small size, significant quantization occurs to their conduction band, which leads to emergent photonic properties and the disappearance of the plasmonic responses observed in larger gold nanoparticles. For example, AuNCs exhibit native luminescent properties, which have been well-explored in the literature. Using proteins, peptides, or other biomolecules as structural scaffolds or capping ligands, required for the stabilization of AuNCs, improves their biocompatibility, while retaining their distinct optical properties. This paved the way for the use of AuNCs in fluorescent bioimaging, which later developed into multimodal imaging combined with computer tomography and magnetic resonance imaging as examples. The development of AuNC-based systems for diagnostic applications in cancer treatment was then made possible by employing active or passive tumor targeting strategies. Finally, the potential therapeutic applications of AuNCs are extensive, having been used as light-activated and radiotherapy agents, as well as nanocarriers for chemotherapeutic drugs, which can be bound to the capping ligand or directly to the AuNCs via different mechanisms. In this review, we present an overview of the diverse biomedical applications of AuNCs in terms of cancer imaging, therapy, and combinations thereof, as well as highlighting some additional applications relevant to biomedical research.

Polyacids for producing colloidally stable amorphous calcium carbonate clusters in water

AbstractAs an important precursor of crystalline phases, amorphous calcium carbonate (ACC), especially ultrasmall ACC clusters, have been attracting great interest in fundamental research, materials chemistry, as well as industrial applications. However, it is still challenging to synthesize stable ACC clusters in water that can be isolated and concentrated without severe aggregation. Herein, we report a facile dialysis method for producing colloidally stable ACC clusters that are well dispersed in water with the protection of specific polycarboxylic acids, which could be also easily deprotected by small‐molecule acids like sodium citrate. Inherent proto‐calcite short‐range order is found to be present in all the obtained ACC clusters. The present strategy not only allows for the preparation of monodisperse isolated ACC clusters in aqueous solution but also shows great potential in industrial uses like waste water treatment.

Hydrogen Solubility and Atomic Structure of Graphene Supported Pd Nanoclusters.

We investigated the atomic structure of graphene supported Pd nanoclusters and their interaction with hydrogen up to atmospheric pressures at room temperature by surface X-ray diffraction and scanning tunneling microscopy. We find that Ir seeded Pd nanocluster superlattices with 1.2 nm cluster diameters can be grown on the graphene/Ir(111) moiré template with high structural perfection. The superlattice clusters are anchored through the rehybridized graphene to the Ir support, which superimposes a 2.0% inplane compression onto the clusters. During hydrogen exposure at 10 mbar pressure and room temperature, a significant part of the clusters gets unpinned from the superlattice. The clusters in registry undergo an out-of-plane expansion only, whereas the detached clusters expand in in- and out-of-plane directions. The formation of a hydrogen rich PdHx α' phase was not observed. After exposure to 1 bar, the majority of the clusters are unpinned from superlattice sites, due to their surface interaction with hydrogen and possible spill over to the graphene support. Only minor sintering was observed, which is more pronounced for the unpinned clusters. The results give evidence that ultrasmall Pd clusters on graphene are a stable hydrogen storage system with reduced hydrogen storage hysteresis and maintain a large surface area for hydrogen chemisorption.

Controlled growth of ultrasmall Cu2O clusters on TiO2 nanoparticles by atmospheric-pressure atomic layer deposition for enhanced photocatalytic activity

This work presents a gas-phase approach for the synthesis of Cu2O/TiO2 powder-based photocatalysts using atomic layer deposition (ALD). The process is carried out in a fluidized bed reactor working at atmospheric pressure using (trimethylvinylsilyl)-hexafluoroacetulacetonate copper(I) as the Cu-precursor and H2O vapor as the oxidizer. The saturating regime of the chemical reactions and the linear growth of ALD are achieved. In combination with the unsaturated regime, the ALD approach enables the deposition of ultrasmall Cu2O clusters with average diameters in the range of 1.3–2.0 nm, narrow particle size distributions and tunable Cu2O loadings on P25 TiO2 nanoparticles. The photocatalytic performance of Cu2O/TiO2 photocatalysts is investigated by the degradation of organic dyes, including Rhodamine B (RhB), methyl orange, and methylene blue; the results demonstrate that the surface modification of TiO2 nanoparticles by Cu2O nanoclusters significantly enhances the photocatalytic activity of TiO2. This is attributed to the efficient charge transfer between Cu2O and TiO2 that reduces the charge recombination. The photocatalytic reaction mechanism is further investigated for the degradation of RhB, revealing the dominating role of holes, which contribute to both direct hole oxidation and indirect oxidation (i.e. via the formation of hydroxyl radicals). Our approach provides a fast, scalable and efficient process to deposit ultrasmall Cu2O clusters in a controllable fashion for surface engineering and modification.

Insight into the synergetic effect of photocatalysis and transition metal on sulfite activation: Different mechanisms for carbamazepine and diclofenac degradation

Sulfite [S(IV)] is a promising alternative for sulfate radical-based advanced oxidation processes (SR-AOPs). Transition metal and photocatalysis are generally considered to have a synergetic effect for S(IV) activation. However, the study shows that the synergetic effect is target specific. Herein, an ultra-small Fe2O3 clusters deposited graphitic carbon nitride is synthesized and used for S(IV) activation. For carbamazepine (CBZ) degradation, photogenerated holes can transform S(IV) into sulfate radical and photogenerated electrons can accelerate Fe(II)/Fe(III) cycle, which account for the synergetic effect. In contrast, the degradation of diclofenac (DCF) depends on the excitation of DCF rather than photocatalyst. Instead of radical precursor, S(IV) acts as the electron transfer bridge between excited DCF and photocatalyst. Thus, the deposition of Fe2O3 negatively affects DCF degradation. Density Functional Theory calculation shows that the first excited state rather than the ground state of diclofenac is more suitable for reactive site prediction, which confirms the photosensitization-like degradation mechanism. Moreover, the effects of pH and coexisted anions varies for CBZ and DCF. The study shed light on the synergetic effect of transition metal and photocatalysis for S(IV) activation, and also open an avenue for the study of target specific mechanisms for AOPs.

Skeleton-Sn anchoring isolated Pt site to confine subnanometric clusters within *BEA topology

Subnanometric clusters (single atoms and ultrasmall clusters) encapsulated inside the pores or cages of zeolite crystals can remarkably enhance their stability and then broaden the catalytic applications. We herein utilized the framework-Sn in Beta zeolite to anchor the guest platinum precursors in the impregnation process, forming Sn-Beta-confined Pt subnanometric clusters (Pt@Sn-Beta) in the following direct reduction. The solid proof for the anchoring effect by forming monodispersed precursor of (Si-O)3(H2O)2Sn(IV)-(O)-[PtCl5]− and final active site of (Si-O)2Sn(II)-(O)-Ptn(n=1~15) was firstly provided in the present study by 119Sn NMR, XAFS and in-situ XPS spectra. The restrictions given by zeolite pores and electronic effect of Sn inhibited the coke formation and sintering of ultrafine Pt clusters, thereby achieving an excellent stability in propane dehydrogenation by maintaining 90% of the maximum propane conversion over 250 h at 99% propene selectivity. This finding has profound significance for understanding the mechanism of heteroatom-containing zeolites as a novel support for highly dispersed metal catalysts.

How Does Temperature Affect the Infrared Vibrational Spectra of Nanosized Silicate Dust?

Magnesium-rich silicates are pervasive throughout the universe and are found in many astronomical environments as small dust grains. Most of the information we have about such silicate dust comes from infrared (IR) observations, whereby the prominent ∼9.7 μm Si-O stretching and 18–20 μm O–Si–O bending vibrational bands are used as identifying signatures. During the initial stages of silicate dust nucleation around aging stars, the nascent grains are nanosized and heated to 800–1000 K. Later, the larger fully formed grains are ejected into the cold interstellar medium and are subsequently heavily processed (e.g., by shattering and sputtering), likely leading to an abundant population of nanosilicate grains. Such ultrasmall grains can eventually form a dust component of protoplanetary disks, where they can be again heated to a few hundred degrees. Although nanosilicate grains are astrochemically important and are likely carriers of the ubiquitous anomalous microwave emission, they have yet to be unambiguously identified through observation. The advent of the James Webb Space Telescope, with IR instruments of unrivalled sensitivity covering the 5-28 μm wavelength range, promises to open a window on these elusive, but presumably abundant, species. Here, we examine Mg-rich ultrasmall silicate clusters with both olivinic (Mg2SiO4) and pyroxenic (MgSiO3) stoichiometries and accurately calculate their IR spectra at temperatures between 0 and 800 K. Although this temperature range is below that required to anneal bulk magnesium silicates (∼1000 K), we show that nanosized grains can exhibit significant anharmonic motions and premelted structural conformational fluxionality even at 400 K. As a result, we demonstrate that the IR spectra of nanosilicates are highly sensitive to temperature, which should thus be considered when attempting to identify these species.

Dual-Functional Template-Induced In Situ Polymerization Process Enables the Hierarchical Carbonaceous Nanotubes with Simultaneous Sn Cluster Incorporation and Nitrogen-Doping for Superior Potassium-Ion Storage.

Potassium ion-based energy storage devices have received extensive attention for grid-level applications due to their abundant natural resources and low cost. However, the large ionic radius of K+ leads to inferior capacities and cyclic stability, which hinders their practical application. Herein, hierarchical carbonaceous nanotubes with simultaneous ultrasmall Sn cluster incorporation and nitrogen doping (denoted as u-Sn@NCNTs) are fabricated using MnO2 nanowires as a dual-functional template (in situ polymerization and shape-directing agents) and subsequent carbonization treatment. The u-Sn@NCNTs exhibit a superior K+ storage capability with a high reversible capacity (220.1 mA h g-1 at 0.1 A g-1) and long cycling stability (149.9 mA h g-1 at 1 A g-1 after 4000 cycles). Besides, the u-Sn@NCNTs exhibit superior cycling stability up to 10000 cycles at 5 A g-1 for Na+ storage. The potassium storage mechanism and kinetics are investigated based on ex situ X-ray photoelectron spectroscopy, in situ Raman spectrum, and galvanostatic intermittent titration technique. More importantly, u-Sn@NCNTs can be used as the anode for potassium ion hybrid capacitors, achieving a superior energy density of 181.4 W h kg-1 at a power density of 185 W kg-1 with excellent cycling capability. This work could push forward the development and application of carbonaceous-based anode materials for next-generation high-performance rechargeable batteries.

Self-assembly of highly-dispersed phosphotungstic acid clusters onto graphitic carbon nitride nanosheets as fascinating molecular-scale Z-scheme heterojunctions for photocatalytic solar-to-fuels conversion

Rational assembly of small-sized photosensitizers onto 2D semiconductors can effectively promote the photocatalytic activity of the formed 2D heterojunction photocatalyst due to the hetero-interfacial charge-transfer process. However, the achievement of the 2D semiconductor-based heterojunction photocatalyst with the exposures of both abundant light-harvesting and catalytic sites at the hetero-interface region is still a huge challenge. Herein, we synthesized phosphotungstic acid/graphitic carbon nitride (HPW/g-C3N4) heterojunction nanosheets (NSs) with the highly-dispersed distribution of ultra-small HPW clusters (1∼2 nm) by using a facile self-assembly method based on the static adsorption-deposition process. The formed molecular-scale hetero-interface between HPW clusters and g-C3N4 NSs resulted in the exposure of abundant active-sites on the highly-dispersed HPW/g-C3N4 hetero-interface. By combining the steady-state and transient photoluminescence spectra with the wavelength-controlled experiments, we demonstrated that the hetero-interfacial charge-transfer process occurring in the HPW/g-C3N4 heterojunction NSs obeyed the Z-scheme mechanism rather than the common “type-II” heterojunction mechanism. In this way, the lifetimes of photoinduced electrons on the conduction band of g-C3N4 could be prolonged for initiating the photocatalytic solar-to-fuels conversion. Upon interband excitations of both the two hetero-components in the HPW/g-C3N4 heterojunction NSs, the photocatalytic activities of H2 generation and CO2 reduction could be enhanced by ∼2.2 and ∼6.7 times as compared to the pure g-C3N4 NSs, even though the HPW component was photocatalytic-inert for either H2 generation or CO2 reduction upon UV–vis light irradiation.

Ultrasmall Ni-ZnO/SiO2 Synergistic Catalyst for Highly Efficient Hydrogenation of NaHCO3 to Formic Acid.

Conversion of CO2 into fuels and chemicals has been considered to be an important strategy to reduce greenhouse gas emissions and alleviate the energy crisis. Bicarbonate as a CO2 source is convenient for experimental operation. Herein, based on the synergistic effect of Ni and ZnO benefitting from the electronic transfer, ultrasmall Ni-ZnO clusters (∼2 nm) stabilized by microporous silica nanoparticles were designed and prepared for catalyzing the hydrogenation of sodium bicarbonate to formic acid. The yield of formic acid reached up to 97.0% at 260 °C/3 MPa for 2 h, which is higher than those reported non-noble metal-based catalysts. The good performance of Ni-ZnO/SiO2 can be attributed to the ultrasmall active component size and the synergy effect based on electron transfer between Ni and ZnO.

A facile sulfur-assisted method to synthesize porous alveolate Fe/g-C3N4 catalysts with ultra-small cluster and atomically dispersed Fe sites

Heterogeneous catalysts with ultra-small clusters and atomically dispersed (USCAD) active sites have gained increasing attention in recent years. However, developing USCAD catalysts with high-density metal sites anchored in porous nanomaterials is still challenging. Here, through the template-free S-assisted pyrolysis of low-cost Fe-salts with melamine (MA), porous alveolate Fe/g-C3N4 catalysts with high-density (Fe loading up to 17.7 wt%) and increased USCAD Fe sites were synthesized. The presence of a certain amount of S species in the Fe-salts/MA system plays an important role in the formation of USCAD S-Fe-salt/CN catalysts; the S species act as a “sacrificial carrier” to increase the dispersion of Fe species through Fe-S coordination and generate porous alveolate structure by escaping in the form of SO2 during pyrolysis. The S-Fe-salt/CN catalysts exhibit greatly promoted activity and reusability for degrading various organic pollutants in advanced oxidation processes compared to the corresponding Fe-salt/CN catalysts, due to the promoted accessibility of USCAD Fe sites by the porous alveolate structure. This S-assisted method exhibits good feasibility in a large variety of S species (thiourea, S powder, and NH4SCN) and Fe salts, providing a new avenue for the low-cost and large-scale synthesis of high-density USCAD metal/g-C3N4 catalysts.

Carbon Monoxide Promotes the Catalytic Hydrogenation on Metal Cluster Catalysts.

Size effect plays a crucial role in catalytic hydrogenation. The highly dispersed ultrasmall clusters with a limited number of metal atoms are one candidate of the next generation catalysts that bridge the single-atom metal catalysts and metal nanoparticles. However, for the unfavorable electronic property and their interaction with the substrates, they usually exhibit sluggish activity. Taking advantage of the small size, their catalytic property would be mediated by surface binding species. The combination of metal cluster coordination chemistry brings new opportunity. CO poisoning is notorious for Pt group metal catalysts as the strong adsorption of CO would block the active centers. In this work, we will demonstrate that CO could serve as a promoter for the catalytic hydrogenation when ultrasmall Pd clusters are employed. By means of DFT calculations, we show that Pdn (n = 2‐147) clusters display sluggish activity for hydrogenation due to the too strong binding of hydrogen atom and reaction intermediates thereon, whereas introducing CO would reduce the binding energies of vicinal sites, thus enhancing the hydrogenation reaction. Experimentally, supported Pd2CO catalysts are fabricated by depositing preestablished [Pd2(μ-CO)2Cl4]2- clusters on oxides and demonstrated as an outstanding catalyst for the hydrogenation of styrene. The promoting effect of CO is further verified experimentally by removing and reintroducing a proper amount of CO on the Pd cluster catalysts.

Ligand-Mediated Nanocluster Formation with Classical and Autocatalytic Growth

We present a systematic study of ligand-mediated nanocluster (NC) formation using a kinetic model, which provides atomic insight into sub-nanometer cluster (S-NC) and NC formation. Our model describes the role of ligand-mediated nucleation and growth in obtaining monodisperse NCs. Nucleation includes metal ion reduction, reversible ligand association to the metal ion/atom, and formation of dimer nuclei. Growth can occur through autocatalytic surface growth and ligand-associated monomer addition to the cluster, depending on the rate of metal ion to neutral metal atom conversion. Furthermore, we studied the effect of the initial concentration of metal ions on NC formation using fast and slow reducing agents in the presence of slowly and rapidly binding ligands. The model shows that fast nucleation, slow growth, and a high molar ratio of rapidly binding ligand to metal ions promote the formation of S-NCs and NCs. Our results can guide experiments in the synthesis of ultrasmall clusters.

Synthesis and Growth Mechanism of Stable Prenucleated (≈0.8 nm Diameter) PbS Quantum Dots by Medium Energy Ion Scattering Spectroscopy.

In the current work, stable prenucleated PbS quantum dots (QDs) with a sub-nanometer (0.8 nm) size have been successfully synthesized via a systematically designed experiment. A detailed analysis of critical nucleation, growth, and stability for such ultrasmall prenucleated clusters is done. The experimental strategy is based on controlled concentration, temperature and injection of respective precursors, thus enabling us to control nucleation rate and separation of stable sub-nanometer PbS QDs with size 0.8 nm. Significantly, by providing additional thermal energy to sub-nanometer PbS QDs, we achieved the fully nucleated cubic crystalline structure of PbS with size of around 1.5 nm. The size and composition of the prenucleated QDs are investigated by sophisticated tools like X-ray photoelectron spectroscopy (XPS) and medium energy ion scattering (MEIS) spectroscopy which confirms the synthesis of PbS with Pb2+ rich surface while the UV-Vis spectroscopy and X-ray diffraction (XRD) data suggests an alternative crystallization path. Non-classical nucleation theory is employed to substantiate the growth mechanism of prenucleated PbS QDs.

Structural patterns in carbon chemisorption on ultrasmall iron clusters: A first-principles study

To get a better understanding of the catalyst chemistry of iron clusters, four representative Fen (n = 1–4) clusters were chosen to systematically investigate the carbonization processes through subsequently adding carbon atoms by using density functional theory calculations. With respect to the quantum results, the analysis of structures, stabilities, and magnetic moments were conducted. It is found that there are three typical peaks of the chemisorption energies for all the clusters studied during the carbonization processes. The third type of peaks is the adsorption energies of critical structures that indicate the formations of the typical long carbon chains. These structures contain 10, 13, 14, and 17 carbon atoms corresponding to Fe1, Fe2, Fe3, and Fe4 pure clusters, respectively, which generate different numbers of carbon chains with the varying lengths. Furthermore, the total magnetic moments of four pure iron clusters would be reduced by the formations of FeC bonds, and their values would change with the adding carbon atoms.

Silver Quantum Clusters: Effective Nanofillers for Improving the Fracture Toughness of Epoxy Composites

Abstract Novel optical, electronic, and chemical properties emerge when few metal atoms held together by metal-metal bonds, these ultra-small pieces of metals are also called quantum clusters (QCs).Silver quantum clusters (AgQCs) protected by thiolates, the diameter of one QC is around 1 nm, termed as one of the ultra-small sized materials belongs to the size category of fullerenes. In the present research, we presented the effectiveness of these ultra-small silver clusters for enhancing the fracture toughness (KIC) property of bisphenol A diglycidyl ether (DGEBA) epoxy composite. Enhancing the fracture toughness play an important role in composite material’s designs. The little amount of QCs (0.06 wt%) addition to epoxy polymer substantially enhanced the KIC of DGEBA epoxy composite by 22%. Strong reinforcing influences of these ultra-small nanofillers on the microstructure of epoxy nanocomposites is clearly evident by the X-ray diffraction (XRD) and scanning electron microscopy (SEM) examinations.

Nanoassemblies of ultrasmall clusters with remarkable activity in carbon dioxide conversion into C1 fuels

Cu nanoassemblies formed transiently during reaction from size-selected subnanometer Cu4 clusters supported on amorphous OH-terminated alumina convert CO2 into methanol and hydrocarbons under near-atmospheric pressure at rates considerably higher than those of individually standing Cu4 clusters. An in situ characterization reveals that the clusters self-assemble into 2D nanoassemblies at higher temperatures which then disintegrate upon cooling down to room temperature. DFT calculations postulate a formation mechanism of these nanoassemblies by hydrogen-bond bridges between the clusters and H2O molecules, which keep the building blocks together while preventing their coalescence.

Size-controlled, high optical quality ZnO nanowires grown using colloidal Au nanoparticles and ultra-small cluster catalysts

Size-controlled ZnO nanowires were grown by eclipse pulsed laser deposition using Au colloids and ultra-small (sub 2 nm) Au101 and Au9 clusters as catalysts, tethered to c-plane sapphire substrates using aminopropyltrimethoxysilane (APTMS). A strong correlation was observed between ZnO nanowire tip diameter and Au catalyst size, with no apparent lower limit to the size of Au nanoparticles able to catalyze nanowire growth. All ZnO nanowires produced intense ultraviolet photoluminescence with almost no visible defect band emission that is commonly observed in ZnO-based materials, including nanowires. A correlation was also observed between the quality of the ultraviolet photoluminescence (in terms of decreasing exciton line widths and surface-related emission) and decreasing ZnO nanowire diameter, with the highest quality emission obtained using Au9 atomically precise cluster catalysts, used here as catalysts for ZnO growth for the first time.