Sub-nanometer Particles Research Articles

A Supramolecular‐Nanocage‐Based Framework Stabilized by π‐π Stacking Interactions with Enhanced Photocatalysis

Abstractπ frameworks, defined as a type of porous supramolecular materials weaved from conjugated molecular units by π‐π stacking interactions, provide a new direction in photocatalysis. However, such examples are rarely reported. Herein, we report a supramolecular‐nanocage‐based π framework constructed from a photoactive Cu(I) complex unit. Structurally, 24 Cu(I) complex units stack together through π‐π stacking interactions, forming a truncated octahedral nanocage with sodalite topology. The inner diameter of the nanocage is 2.8 nm. By sharing four open faces, each nanocage connects with four equivalent ones, forming a 3D porous π framework (π‐2). π‐2 shows good thermal and chemical stability, which can adsorb CO2, iodine, and methyl orange molecules. More importantly, π‐2 can serve as a photocatalyst for hydrogen evolution reaction. With ultrafine Pt subnanometer particles (0.9±0.1 nm) incorporated into the nanocages as a co‐catalyst, the hydrogen evolution rate reaches a record‐high value of 524012 μmol/gPt/h in the absence of any additional photosensitizers. The high photocatalytic activity can be ascribed to the ultrafine size of the Pt particles, as well as the fast electron transfer from π‐2 to the highly active Pt upon illumination. π‐2 represents the unique stable supramolecular‐cage‐based π framework with excellent photocatalytic activity.

A Supramolecular-Nanocage-Based Framework Stabilized by π-π Stacking Interactions with Enhanced Photocatalysis.

π frameworks, defined as a type of porous supramolecular materials weaved from conjugated molecular units by π-π stacking interactions, provide a new direction in photocatalysis. However, such examples are rarely reported. Herein, we report a supramolecular-nanocage-based π framework constructed from a photoactive Cu(I) complex unit. Structurally, 24 Cu(I) complex units stack together through π-π stacking interactions, forming a truncated octahedral nanocage with sodalite topology. The inner diameter of the nanocage is 2.8 nm. By sharing four open faces, each nanocage connects with four equivalent ones, forming a 3D porous π framework (π-2). π-2 shows good thermal and chemical stability, which can adsorb CO2, iodine, and methyl orange molecules. More importantly, π-2 can serve as a photocatalyst for hydrogen evolution reaction. With ultrafine Pt subnanometer particles (0.9±0.1 nm) incorporated into the nanocages as a co-catalyst, the hydrogen evolution rate reaches a record-high value of 524012 μmol/gPt/h in the absence of any additional photosensitizers. The high photocatalytic activity can be ascribed to the ultrafine size of the Pt particles, as well as the fast electron transfer from π-2 to the highly active Pt upon illumination. π-2 represents the unique stable supramolecular-cage-based π framework with excellent photocatalytic activity.

L-arginine-etched nickel-silver electrocatalyst for low-potential hydrogen evolution

The oxidation of formaldehyde at low potentials to achieve anodic hydrogen production is a crucial focus of current research. Copper-based alloys are prominent electrocatalysts in these reactions but face long-term stability challenges and are susceptible to CO intermediate poisoning. A novel approach that utilizes L-arginine in the surface modification of nickel foam substrates has been developed, enabling the highly dispersed deposition of silver catalysts on the substrates as sub-nanometer particles. This modified electrocatalyst allows formaldehyde oxidation and water electrolysis at a minimal potential of 0.32 VRHE and bipolar hydrogen production with near 100% Faradaic efficiency. This surface modification of substrate material provides a new approach to the design of electrodes.

Identification and localization of Pt species in Pt-NaA zeolite catalysts prepared by direct synthesis

Several characterization techniques were employed to elucidate the Pt species present in the structure of Pt-NaA catalysts. Three types of Pt particles were identified: nanoparticles located at the surface of the crystals (with a higher size and flattened form), 1–2 nm particles inside the structure, and sub-nanometric particles, lower than 1 nm, confined in the pores. The low PtPt coordination number (EXAFS) is compatible with both flattened dihedral-like morphology, as well as the presence of sub-nanometric particles. The Pt0.6_NaA catalyst exhibited higher activity (in the WGS) than the other catalysts, which could be related to the presence of small particles.

A novel strategy for stabilization of sub-nanometric Pd colloids on kryptofix functionalized MCM-41: nanoengineered material for Stille coupling transformation

The stabilization of sub-nanometric metal particles (< 1 nm) with suitable distribution remained challenging in the catalytic arena. Herein, an intelligent strategy was described to anchoring and stabilizing sub-nanometric Pd colloids with an average size of 0.88 nm onto Kryptofix 23 functionalized MCM-41. Then, the catalytic activity of Pd@Kryf/MCM-41 was developed in Stille coupling reaction with a turnover frequency (TOF) value of 247 h−1. The findings demonstrate that porous MCM-41 structure and high-affinity Kryptofix 23 ligand toward adsorption of Pd colloids has a vital role in stabilizing the sub-nanometric particles and subsequent catalytic activity. Overall, these results suggest that Pd@Kryf/MCM-41 is a greener, more suitable option for large-scale applications and provides new insights into the stabilization of sub-nanometric metal particles.

Multifunctionalized Gold Sub-Nanometer Particles for Sensitizing Radiotherapy against Glioblastoma.

Glioblastoma is the most common lethal malignant intracranial tumor with a low 5-year survival rate. Currently, the maximal safe surgical resection, followed by high-dose radiotherapy (RT), is a standard treatment for glioblastoma. However, high-dose radiation to the brain is associated with brain injury and results in a high fatality rate. Here, integrated pharmaceutics (named D-iGSNPs) composed of gold sub-nanometer particles (GSNPs), blood-brain barrier (BBB) penetration peptide iRGD, and cell cycle regulator α-difluoromethylornithine is designed. In both simulated BBB and orthotopic murine GL261 glioblastoma models, D-iGSNPs are proved to have a beneficial effect on the BBB penetration and tumor targeting. Meanwhile, data from cell and animal experiments reveal that D-iGSNPs are able to sensitize RT. More importantly, the synergy of D-iGSNPs with low-dose RT can exhibit an almost equal therapeutic effect with that of high-dose RT. This study demonstrates the therapeutic advantages of D-iGSNPs in boosting RT, and may provide a facile approach to update the current treatment of glioblastoma.

Cobalt Oxide-Supported Pt Electrocatalysts: Intimate Correlation between Particle Size, Electronic Metal-Support Interaction and Stability.

Oxide supports can modify and stabilize platinum nanoparticles (NPs) in electrocatalytic materials. We studied related phenomena on model systems consisting of Pt NPs on atomically defined Co3O4(111) thin films. Chemical states and dissolution behavior of model catalysts were investigated as a function of the particle size and the electrochemical potential by ex situ emersion synchrotron radiation photoelectron spectroscopy and by online inductively coupled plasma mass spectrometry. Electronic metal-support interaction (EMSI) yields partially oxidized Ptδ+ species at the metal/support interface of metallic nanometer-sized Pt NPs. In contrast, subnanometer particles form Ptδ+ aggregates that are exclusively accompanied by subsurface Pt4+ species. Dissolution of Cox+ ions is strongly coupled to the presence of Ptδ+ and the reduction of subsurface Pt4+ species. Our findings suggest that EMSI directly affects the integrity of oxide-based electrocatalysts and may be employed to stabilize Pt NPs against sintering and dissolution.

Confinement of Cobalt Species in Mesoporous N-Doped Carbons and the Impact on Nitroarene Hydrogenation

A general strategy to synthesize mesoporous and macroporous carbon and N-doped mesoporous carbon is presented. The immobilization of cobalt catalysts with dispersed subnanometer particles in mesopo...

Autogenous Production and Stabilization of Highly Loaded Sub-Nanometric Particles within Multishell Hollow Metal-Organic Frameworks and Their Utilization for High Performance in Li-O2 Batteries.

Sub‐nanometric particles (SNPs) of atomic cluster sizes have shown great promise in many fields such as full atom‐to‐atom utilization, but their precise production and stabilization at high mass loadings remain a great challenge. As a solution to overcome this challenge, a strategy allowing synthesis and preservation of SNPs at high mass loadings within multishell hollow metal–organic frameworks (MOFs) is demonstrated. First, alternating water‐decomposable and water‐stable MOFs are stacked in succession to build multilayer MOFs. Next, using controlled hydrogen bonding affinity, isolated water molecules are selectively sieved through the hydrophobic nanocages of water‐stable MOFs and transferred one by one to water‐decomposable MOFs. The transmission of water molecules via controlled hydrogen bonding affinity through the water‐stable MOF layers is a key step to realize SNPs from various types of alternating water‐decomposable and water‐stable layers. This process transforms multilayer MOFs into SNP‐embedded multishell hollow MOFs. Additionally, the multishell stabilizes SNPs by π‐backbonding allowing high conductivity to be achieved via the hopping mechanism, and hollow interspaces minimize transport resistance. These features, as demonstrated using SNP‐embedded multishell hollow MOFs with up to five shells, lead to high electrochemical performances including high volumetric capacities and low overpotentials in Li–O2 batteries.

Atomic Scale Insight into the Formation, Size, and Location of Platinum Nanoparticles Supported on γ-Alumina

A clear description of the morphology and location, with respect to the support, of metallic subnanometric particles remains a current strenuous experimental challenge in numerous catalytic applica...

Tremolite-actinolite fiber coatings of sub-nanometer silica-rich particles in lungs from deceased Quebec miners.

Tremolite-actinolite (TA) fibers from the lungs of deceased former Quebec mine workers were found to be coated with sub-nanometer particles. Qualitative chemical analyses were performed on the particles indicating that they were composed of silicon and oxygen. The crystal lattice structure of all amphibole minerals, including the TA series, is arranged as pairs of linear chains of SiO4 tetrahedra that are linked together to form double chains. Our observations of the TA fibers from miner's lungs, made using a high-resolution transmission electron microscope, indicated that the tetrahedral silica chains were progressively split, forming dispersed sub-nanometer particles. The non-tetrahedral sites were removed at the surface of the TA fibers, presumably by the oxidation process involved in attempted phagocytosis, which also resulted in fragmentation of the tetrahedral chains. It was found that the silicon-rich particles (SRPs) were variable in diameter, consistent with fragments formed from the splitting of the tetrahedral chains. The TA fibers from lungs displayed coatings and linear interior zones of SRP parallel to the planes of longitudinal fiber splitting. The literature on very small nanoparticles is consistent with deep penetration of SRP into cell DNA interiors, oxidative stress, and carcinogenesis.

Synthesis and characterization of a supported Pd complex on carbon nanofibers for the selective decarbonylation of stearic acid to 1-heptadecene: the importance of subnanometric Pd dispersion

Evaluation of the dispersion of Pd active sites on the catalyst performance during fatty acids decarbonylation to α-olefins was explored in this work. Pd subnanometric particles, clusters and aggregates were found to modulate the catalyst activity.

Noble Metal Particles Confined in Zeolites: Synthesis, Characterization, and Applications.

Noble metal nanoparticles or subnanometric particles confined in zeolites, that is, metal@zeolite, represent an important type of functional materials with typical core–shell structure. This type of material is known for decades and recently became a research hotspot due to their emerging applications in various fields. Remarkable achievements are made dealing with the synthesis, characterization, and applications of noble metal particles confined in zeolites. Here, the most representative research progress in metal@zeolites is briefly reviewed, aiming to boost further research on this topic. For the synthesis of metal@zeolites, various strategies, such as direct synthesis from inorganic or ligand‐assisted noble metal precursors, multistep postsynthesis encapsulation and ion‐exchange followed by reduction, are introduced and compared. For the characterization of metal@zeolites, several most useful techniques, such as electron microscopy, X‐ray based spectroscopy, infrared and fluorescence emission spectroscopy, are recommended to check the successful confinement of noble metal particles in zeolite matrix and their unique physiochemical properties. For the applications of metal@zeolites, catalysis and optics are involved with an emphasis on catalytic applications including the size‐dependent catalytic properties, the sintering‐resistance properties, the substrate shape‐selective catalysis, and catalysis modulation by zeolite microenvironment.

Pentameric PdAu and PdPt nanoparticles on the MgO(1 0 0) surface and their CO and O2 adsorption properties

The surface mode of the Birmingham Cluster Genetic Algorithm (S-BCGA), which performs an unbiased global optimisation search for clusters adsorbed on a surface, has been employed for the global optimisation of noble metal pentamers on an MgO(1 0 0) substrate. The effect of element identity and alloying in surface-bound neutral subnanometre particles is calculated by energetic analysis of all compositions of supported 5-atom PdAu and PdPt clusters. Our results show that the binding strengths of the component elements to the surface are in the order Pt > Pd > Au. In addition, alloying Pd with Au and Pt is favorable for this size since excess energy calculations show a preference for bimetallic clusters for both cases. Furthermore, the electronic behaviour, which is intermediate between molecular systems and bulk metals allows tuning of the characteristics of particles in the subnanometre size range. The adsorption of CO and O2 probe molecules are also modelled and it is found that CO and O2 adsorption leads to a weakening of the cluster–surface interaction.

Redox-mediated conversion of atomically dispersed platinum to sub-nanometer particles

The concentration of Ce3+ centers in Pt–CeO2 films determines the onset of reduction of atomically dispersed platinum species.

On thermophoresis modeling in inert nanofluids

Thermophoresis plays an important role in forced and natural convection in channels and enclosures when nanofluids are used instead of pure fluids. One objective of this work, therefore, is to investigate whether reliable expressions exist for the estimation of thermophoresis data. As the second objective of this work we will show similarities between thermophoresis of nanoparticles and macromolecule dispersed in a base fluid with thermodiffusion of species in binary mixtures. To this end, a nonequilibrium thermodynamics-based expression primarily developed for the estimation of thermodiffusion factor in binary mixtures is extended and applied to thermophoresis in nanofluids. A hydrodynamics-based expression and the nonequilibrium thermodynamic-based expression developed here, are used to estimate the thermophoretic velocity in nanofluids. Validation results suggest that the general form of the hydrodynamics-based equation is valid for thermophoresis of nano-sized and even sub-nanometer particles in liquids; however, the correct prediction of the matching parameter is still unresolved. Also, the nonequilibrium thermodynamics combined with the concept of activation energy of viscous is somewhat capable of estimating thermophoresis coefficient of inert particles and macromolecules of about 1 nm or smaller. The agreement, however, is qualitative.

Impact of co-adsorbed oxygen on crotonaldehyde adsorption over gold nanoclusters: a computational study

Crotonaldehyde (2-butenal) adsorption over gold sub-nanometer particles, and the influence of co-adsorbed oxygen, has been systematically investigated by computational methods. Using density functional theory, the adsorption energetics of crotonaldehyde on bare and oxidised gold clusters (Au13, d = 0.8 nm) were determined as a function of oxygen coverage and coordination geometry. At low oxygen coverage, sites are available for which crotonaldehyde adsorption is enhanced relative to bare Au clusters by 10 kJ mol(-1). At higher oxygen coverage, crotonaldehyde is forced to adsorb in close proximity to oxygen weakening adsorption by up to 60 kJ mol(-1) relative to bare Au. Bonding geometries, density of states plots and Bader analysis, are used to elucidate crotonaldehyde bonding to gold nanoparticles in terms of partial electron transfer from Au to crotonaldehyde, and note that donation to gold from crotonaldehyde also becomes significant following metal oxidation. At high oxygen coverage we find that all molecular adsorption sites have a neighbouring, destabilising, oxygen adatom so that despite enhanced donation, crotonaldehyde adsorption is always weakened by steric interactions. For a larger cluster (Au38, d = 1.1 nm) crotonaldehyde adsorption is destabilized in this way even at a low oxygen coverage. These findings provide a quantitative framework to underpin the experimentally observed influence of oxygen on the selective oxidation of crotyl alcohol to crotonaldehyde over gold and gold-palladium alloys.

Mechanistic Study of Pt–Re/γ-Al2O3 Catalyst Deactivation by Chemical Imaging of Carbonaceous Deposits Using Advanced X-ray Detection in Scanning Transmission Electron Microscopy

Accumulation of carbonaceous deposits (also known industrially as “coke”) on catalytic active sites derived from process conditions has been a long-standing problem in industrial catalysis. Over accumulation of the deposits ultimately lead to deactivation; thus, characterization of coke evolution becomes critical toward developing catalytic materials with improved stability. This study focuses on extending an understanding of how and where coke forms on Pt–Re/γ-Al2O3, an industrial catalyst used in reforming processes. With the use of spherical aberration-corrected scanning transmission electron microscopy (Cs-STEM) and enhanced energy dispersive X-ray spectroscopy (EDX), we characterize the Pt–Re metal sites and show, to our knowledge, the first examples of chemically imaged subnanometer particles and also the first to use chemical maps to identify coke location on a catalyst. A two-step mechanism is reported based on the results whereby coke appears to stream from the metal function of the catalyst resu...

A density functional global optimisation study of neutral 8-atom Cu-Ag and Cu-Au clusters

The effect of doping on the energetics and dimensionality of eight atom coinage metal subnanometre particles is fully resolved using a genetic algorithm in tandem with on the fly density functional theory calculations to determine the global minima (GM) for CunAg(8−n) and CunAu(8−n) clusters. Comparisons are made to previous ab initio work on mono- and bimetallic clusters, with excellent agreement found. Charge transfer and geometric arguments are considered to rationalise the stability of the particular permutational isomers found. An interesting transition between three dimensional and two dimensional GM structures is observed for copper-gold clusters, which is sharper and appears earlier in the doping series than is known for gold-silver particles.

The formation and evolution of oxide particles in oxide-dispersion-strengthened ferritic steels during processing

The fabrication of oxide-dispersion-strengthened (ODS) steels is a multi-stage process involving powder ball milling, degassing and consolidation by hot isostatic pressing. Y is introduced by mechanical alloying (MA) with either Y2O3 or Fe2Y so a high density of Y–Ti–O-based oxide nanoparticles is formed. The evolution of ∼2nm oxide nanoparticles and larger >5nm grain boundary oxides has been studied at each step of the processing. The nanoparticle dispersions produced in material MA with Fe2Y were comparable to those produced by alloying with Y2O3. Hence the majority of oxygen which forms the nanoparticles must be incorporated from the atmosphere during MA, rather than being introduced via the alloying additions. During mechanical alloying, a high density of subnanometer particles are formed (2.5±0.5×1024m−3). The oxygen content of the nanoparticles decreases slightly on annealing, and then the composition of the nanoparticles remains constant throughout subsequent processing stages. The nanoparticle size increases during processing up to ∼2nm radius, while the number density decreases to 4±0.5×1023m−3. Grain boundary oxides (>5nm) have a Ti–Cr–O-rich shell, and contain no Y before consolidation, but have similar core composition to the matrix nanoparticles after consolidation. The formation of the larger grain boundary oxides is shown to take place during the degassing and consolidation stages, and this occurs at the expense of the nanoparticles in the matrix. This work provides a mechanistic understanding of the importance of controlling the oxygen content in the powder during MA, and the resulting impact on the formation of the ODS microstructure.