Taking a different road: following Ag25 and Au25 cluster activation via in situ differential pair distribution function analysis.

Synchrotron X-ray total scattering measurements and accompanying pair distribution function (PDF) analyses are an excellent characterization technique to complement both transmission electron microscopy (TEM) and extended X-ray absorption fine structure (EXAFS) spectroscopy methods for detailed structural studies of atom-precise metal clusters. Herein, we study the thermal activation of Au25(SR)18- and Ag25(SR)18- clusters on alumina supports via in situ differential PDF (dPDF) analyses to compare structural changes in the metal clusters upon thermal activation in air. The metal-metal interatomic distances in Au25(SR)18- and Ag25(SR)18- clusters as measured by the dPDF method are comparable with those measured via single-crystal crystallographic and EXAFS methods. Compared to EXAFS analysis, in situ dPDF data can also provide high-temperature, non-element specific, longer range structural information with excellent temporal resolution. TEM and dPDF results show that Ag25(SR)18 systems behave significantly differently than analogous Au25(SR)18 systems upon thermal activation. Atom-precise Au clusters on alumina supports show continuous growth in particle size with increasing activation temperature due to particle coalescence upon thermal deprotection, and grow to an average size of 11.2 ± 2.1 nm for samples thermally activated at 650 °C. Conversely, analogous Ag clusters on alumina supports show particle size growth to mid-sized particles (3.2 ± 0.4 nm) at activation temperatures of 450 °C, beyond which the Ag particles then undergo thermal degradation to give smaller Ag clusters with an average size of 1.4 ± 0.2 nm for samples thermally activated at 650 °C. The significant difference in the behaviours of atom-precise, thiolate-protected Au and Ag clusters upon thermal activation emphasizes the development of distinct activation protocols for different metal cluster systems.

Au Cluster-Cored Dendrimers Fabricated by Direct Synthesis and Post-functionalization Routes.

The use of dendrimers and dendrons as stabilizing agents for metal nanoparticles and nanoclusters has captured interest in both the biomedicine and catalysis fields. Herein, we describe the synthesis of Au cluster-cored dendrimers by either direct synthesis or multi-step functionalization pathways. Direct synthesis of Au cluster-cored dendrimers was performed by the Brust-Schiffrin method using cystamine core poly(amidoamine) (PAMAM) dendrons as capping agents. Alternatively, a divergent approach to make nanoclusters with dendritic branching groups by functionalizing glycine-cystamine Au clusters was also carried out. This synthesis involved sequential Michael addition reactions of methyl acrylate followed by a subsequent amide coupling reaction with ethylenediamine on amine-terminated Au nanoclusters to form dendritic architectures around the Au core. The chemical structure of the ligands was confirmed by proton nuclear magnetic resonance after each functionalization reaction, and the cluster size was characterized by transmission electron microscopy. Au cluster-cored dendrimers with amine or ester terminal groups on the surface were produced. The resulting amine- and ester-terminated Au cluster-cored dendrimers synthesized by the divergent method are stable in solution and in the presence of excess reducing agent. In contrast, amine-terminated Au cluster-cored dendrimers synthesized by direct synthesis undergo aggregation in solution over time as a result of the high reactivity of the surface, while ester-terminated Au cluster-cored dendrimers formed by direct synthesis have much larger core sizes than seen using the divergent approach. Finally, the catalytic activities of these clusters for 4-nitrophenol reductions have been investigated. Cluster-cored dendrimers formed by direct synthesis had larger core sizes and higher catalytic activities than those formed by the divergent approach, which is likely due to the poor passivation of the Au surface for the directly synthesized clusters. Furthermore, Au cluster-cored dendrimers with less sterically bulky dendrons showed higher catalytic activities.

Size-Controlled Synthesis of Modifiable Glycine-Terminated Au Nanoclusters as a Platform for Further Functionalization.

An improved and simple synthetic method for producing stable narrow-sized glycine-cystamine (Gly-CSA)-functionalized Au nanoclusters (NCs) from protected Fmoc-glycine-cystamine (Fmoc-Gly-CSA)-functionalized Au NCs is demonstrated in this study. The NC size and size distribution can be controlled directly as a function of reducing agent concentration with the formation of smaller NC core diameters at higher concentrations of NaBH4. Furthermore, when using 0.30 M NaBH4, three UV-vis absorption peaks at 690, 440, and 390 nm were seen, which are consistent with the formation of Fmoc-Gly-CSA-functionalized Au25L18 NCs. After deprotection of the Gly-CSA-functionalized Au NCs, the reactivity of the primary amine groups was investigated. Methyl acrylate-glycine-cystamine (MA-Gly-CSA)-functionalized Au NCs with terminal acetyl groups were formed via the Michael addition reaction of terminal amine groups with methyl acrylate. This reaction resulted in the formation of ester-terminated Au NCs including atom-precise MA-Gly-CSA Au25(SR)18 NCs. The functionalization of the ligand was confirmed by 1H NMR and UV-vis spectra, and TEM images of MA-Gly-CSA- and Gly-CSA-functionalized Au NCs showed that the size of the NCs remained unchanged after the reaction. With controllable NC size and facile functionalization of the Gly-CSA-functionalized Au NCs, these clusters have promising potential as scaffolds for biomedical applications.

Unveiling the Surface and the Ultrastructure of Palladized Fungal Biotemplates.

In this paper, two biosystems based on filamentous fungi and Pd nanoparticles (NPs) were synthesized and structurally characterized. In the first case, results concerning the integration and distribution of Pd-NPs on Phialomyces macrosporus revealed that nanoparticles are accumulated on the cell wall, keeping the cytoplasm isolated from abiotic particles. However, the Penicillium sp. species showed an unexpected internalization of Pd-NPs in the fungal cytosol, becoming a promising biosystem to further studies of in vivo catalytic reactions. Next, we report a new solution-based strategy to prepare palladized biohybrids through sequential reduction of Pd2+ ions over previously harvested fungus/Au-NP composites. The chemical composition and the morphology of the biohybrid surface were characterized using a combination of scanning electron microscopy, transmission electron microscopy, and photoelectron spectroscopy. The deposition of Pd0 over the fungal surface produced biohybrids with a combination of Au and Pd in the NPs. Interestingly, other chemical species such as Au+ and Pd2+ are also observed on the outermost wall of microorganisms. Finally, the application of A. niger/AuPd-NP biohybrids in the 3-methyl-2-buten-1-ol hydrogenation reaction is presented for the first time. Biohybrids with a high fraction of Pd0 are active for this catalytic reaction.

Exploring the structure of atom-precise silver-palladium bimetallic clusters prepared via improved single-pot co-reduction synthesis protocol.

Designing atom-precise bimetallic clusters with a relatively cost-effective and more abundant metal than Au (i.e., Ag) is desirable for the development of heterogeneous bimetallic cluster catalysts for industrial applications. Atom-precise Ag-based bimetallic clusters, which are analogs of the well-studied Au based clusters, are yet to be fully explored as catalysts. Establishing the Pd loading limit and the position of the Pd dopant in AgPd bimetallic clusters will further give an insight into the structure-activity relationships for these atom-precise AgPd heterogeneous catalysts. In this study, an improved single-pot co-reduction strategy was employed to prepare the bimetallic clusters, which were then characterized by mass spectrometry, x-ray photoelectron spectroscopy (XPS), and x-ray absorption spectroscopy (XAS) to identify the loading and position of the dopant metal. Our results show that only a single dopant Pd atom can be incorporated, and in comparison with monometallic Ag25 clusters, the absorption peaks of Ag24Pd1(SPhMe2)18 2- bimetallic clusters are blue shifted due to the incorporation of Pd. The XPS and XAS results show that the Ag24Pd1(SPhMe2)18 2- bimetallic clusters have multivalent Ag(0) and Ag(I) atoms and surprisingly show Pd(II) species with significant Pd-S bonding, despite the prevailing wisdom that the Pd dopant should be in the center of the cluster. The XAS results show that the singly doped Pd atom predominantly occupies the staple position, albeit we cannot unambiguously rule out the Pd atom in an icosahedral surface position in some clusters. We discuss the ramifications of these results in terms of possible kinetically vs thermodynamically controlled cluster formation.

Probing the Thermal Stability of (3-Mercaptopropyl)-trimethoxysilane-Protected Au25 Clusters by In Situ Transmission Electron Microscopy.

High-surface-area gold catalysts are promising catalysts for a number of selective oxidation and reduction reactions but typically suffer catalyst deactivation at higher temperatures. The major reason for catalyst deactivation is sintering, which can be triggered via two mechanisms: particle migration and coalescence, and Ostwald ripening. Herein, a direct method to synthesize Au25 clusters stabilized with 3-mercaptopropyltrimethoxysilane (MPTS) ligands is discussed. The sintering of Au25 (MPTS)18 clusters on mesoporous silica (SBA-15) is monitored by using an environmental in situ transmission electron microscopy (TEM) technique. Results show that agglomeration of smaller particles is accelerated by increased mobility of particles during heat treatment, while growth of immobile particles occurs via diffusion of atomic species from smaller particles. The mobility of the Au clusters can be alleviated by fabricating overlayers of silica around the clusters. The resulting materials show tremendous sinter-resistance at temperatures up to 650 °C as shown by in situ TEM and extended X-ray absorption fine structure analysis.

Preserving the Exposed Facets of Pt3Sn Intermetallic Nanocubes During an Order to Disorder Transition Allows the Elucidation of the Effect of the Degree of Alloy Ordering on Electrocatalysis.

Controlling which facets are exposed in nanocrystals is crucial to understanding different activity between ordered and disordered alloy electrocatalysts. We modify the degree of ordering of Pt3Sn nanocubes, while maintaining the shape and size, to enable a direct evaluation of the effect of the order on ORR catalytic activity. We demonstrate a 2.3-fold enhancement in specific activity by 60- and 30%-ordered Pt3Sn nanocubes compared to 95%-ordered. This was shown to be likely due to surface vacancies in the less-ordered particles. The greater order, however, results in higher stability of the electrocatalyst, with the more disordered nanoparticles showing the dissolution of tin and platinum species during electrocatalysis.

Activation of atom-precise clusters for catalysis.

The use of atom-precise, ligand-protected metal clusters has exceptional promise towards the fabrication of model supported-nanoparticle heterogeneous catalysts which have controlled sizes and compositions. One major challenge in the field involves the ease at which metallic clusters sinter upon removal of protected ligands, thus destroying the structural integrity of the model system. This review focuses on methods used to activate atom-precise thiolate-stabilized clusters for heterogeneous catalysis, and strategies that can be used to mitigate sintering. Thermal activation is the most commonly employed approach to activate atom-precise metal clusters, though a variety of chemical and photochemical activation strategies have also been reported. Material chemistry methods that can mitigate sintering are also explored, which include overcoating of clusters with metal oxide supports fabricated by sol-gel chemistry or atomic layer deposition of thin oxide films or encapsulating clusters within porous supports. In addition to focusing on the preservation of the size and morphology of deprotected metal clusters, the fate of the removed ligands is also explored, because detached and/or oxidized ligands can also greatly influence the overall properties of the catalyst systems. We also show that modern characterization techniques such as X-ray absorption spectroscopy and high-resolution electron microscopy have the capacity to enable careful monitoring of particle sintering upon activation of metal clusters.

Activation of atomically precise silver clusters on carbon supports for styrene oxidation reactions.

Metal clusters have distinct features such as large surface area, low-coordination-atom enriched surfaces, and discrete energy levels that influence their behavior during catalytic reactions. Atomically-precise Ag clusters, which are analogues of more well-studied Au clusters, are yet to be fully explored as catalysts for various chemical reactions. 2,4-Dimethylbenzenethiol-protected Ag25 clusters were prepared and deposited onto carbon supports followed by calcination. Results from X-ray absorption fine structure (EXAFS) spectroscopy measurements and other characterization techniques indicated that thermal activation of carbon-supported Ag25 clusters resulted in dethiolation of Ag clusters at 250 °C and beyond, and consequently mild growth in particle sizes of Ag clusters on carbon supports was seen with increasing activation temperatures. Both as-prepared and activated Ag25 clusters were active for styrene oxidation reactions, with high selectivity towards styrene oxide, without using any promoter. Results show that mild activation at 250 °C yields the most active catalysts, and higher activation temperatures lead to decreased activities and slightly poorer selectivity to styrene oxidation as a result of cluster sintering. EXAFS data shows the resulting activated clusters are composed of Ag metal and that all thiols are removed from the Ag cluster surfaces, though XPS data shows that thiol oxidation products are still present in the sample.

Platinum Inhibits Low-Temperature Dry Lean Methane Combustion through Palladium Reduction in Pd-Pt/Al2 O3 : An In†Situ X-ray Absorption Study.

Palladium-platinum bimetallic catalysts supported on alumina with palladium/platinum molar ratios ranging from 0.25 to 4 are studied in dry lean methane combustion in the temperature range of 200 to 500 °C. Platinum addition decreases the catalyst activity, which cannot be explained by the decrease in dispersion or the structure sensitivity of the reaction. In situ X-ray absorption near-edge structure and extended X-ray absorption fine structure spectroscopy measurements have been conducted for monometallic Pd, Pt, and 2:1 Pd-Pt catalysts. Monometallic palladium is fully oxidized in the full temperature range, whereas platinum addition promotes palladium reduction, even in a reactive oxidizing environment. The Pd/PdO weight ratio in bimetallic Pd-Pt 2:1 catalysts decreases from 98/2 to 10/90 in the 200-500 °C temperature range under the reaction conditions. Thus, platinum promotes the formation of the reduced palladium phase with a significantly lower activity than that of oxidized palladium. The study sheds light on the effect of platinum on the state of the active palladium surface under low-temperature dry lean methane combustion conditions, which is important for methane-emission control devices.

Solving local structure around dopants in metal nanoparticles with ab initio modeling of X-ray absorption near edge structure.

We adopted ab initio X-ray absorption near edge structure (XANES) modeling for structural refinement of local environments around metal impurities in a large variety of materials. Our method enables both direct modeling, where the candidate structures are known, and the inverse modeling, where the unknown structural motifs are deciphered from the experimental spectra. We present also estimates of systematic errors, and their influence on the stability and accuracy of the obtained results. We illustrate our approach by revealing the evolution of local environment of palladium atoms in palladium-doped gold thiolate clusters upon chemical and thermal treatments.

Thermal degradation mechanism of triangular Ag@SiO2 nanoparticles.

Triangular silver nanoparticles are promising materials for light harvesting applications because of their strong plasmon bands; these absorption bands are highly tunable, and can be varied over the entire visible range based on the particle size. A general concern with these materials is that they are unstable at elevated temperatures. When thermally annealed, they suffer from changes to the particle morphology, which in turn affects their optical properties. Because of this stability issue, these materials cannot be used in applications requiring elevated temperatures. In order to address this problem, it is important to first understand the degradation mechanism. Here, we measure the changes in particle morphology, oxidation state, and coordination environment of Ag@SiO2 nanotriangles caused by thermal annealing. UV-vis spectroscopy and TEM reveal that upon annealing the Ag@SiO2 nanotriangles in air, the triangular cores are truncated and smaller nanoparticles are formed. Ag K-edge X-ray absorption spectroscopy (XANES and EXAFS) shows that the small particles consist of Ag(0), and that there is a decrease in the Ag-Ag coordination number with an increase in the annealing temperature. We hypothesize that upon annealing Ag in air, it is first oxidized to AgxO, after which it subsequently decomposes back to well-dispersed Ag(0) nanoparticles. In contrast, when the Ag@SiO2 nanotriangles are annealed in N2, since there is no possibility of oxidation, no small particles are formed. Instead, the triangular core rearranges to form a disc-like shape.

Effect of relative humidity on crystal growth, device performance and hysteresis in planar heterojunction perovskite solar cells.

Due to the hygroscopic nature of organolead halide perovskites, humidity is one of the most important factors affecting the efficiency and longevity of perovskite solar cells. Although humidity has a long term detrimental effect on device performance, it also plays a key role during the initial growth of perovskite crystals. Here we demonstrate that atmospheric relative humidity (RH) plays a key role during the formation of perovskite thin films via the sequential deposition technique. Our results indicate that the RH has a substantial impact on the crystallization process, and hence on device performance. SEM and pXRD analysis show an increase in crystallite size with increasing humidity. At low RH, the formation of small cubic crystallites with large gaps between them is observed. The presence of these voids adversely affects device performance and leads to substantial hysteresis in the device. At higher RH, the perovskite crystals are larger in size, with better connectivity between the crystallites. This produced efficient planar heterojunction solar cells with low hysteresis. By careful control of the RH during the cell fabrication process, efficiencies of up to 12.2% are reached using P3HT as the hole-transport material.

Isolation of carboxylic acid-protected Au25 clusters using a borohydride purification strategy.

We report the synthesis of 11-mercaptoundecanoic acid (11-MUA) and 16-mercaptohexadecanoic acid (16-MHA) protected Au25 clusters with moderate yields (∼15%) using a NaBH4 purification strategy. UV-vis spectroscopy, transmission electron microscopy (TEM), and matrix-assisted laser desorption/ionization (MALDI) mass spectrometry were employed to study the entire process of the isolation of 11-MUA-protected Au25 clusters from a polydisperse Au cluster solution. UV-vis and TEM data clearly show the formation of a polydisperse mixture, which upon the addition of NaBH4 leads to the growth and precipitation of non-Au25 clusters, leaving the Au25 clusters behind. MALDI MS shows the molecular ion peak for the 11-MUA-protected Au25 cluster. 11-MUA-protected Au25 clusters in THF were purified by slowly increasing the pH of the solution, which leads to the complete deprotonation of carboxyl groups on the surface and eventually precipitation of Au25 clusters. Further protonation of these clusters by acetic acid leads to their solubilization in THF. These results show that, owing to the inherent stability of Au25 clusters, a NaBH4 purification strategy can be used to isolate Au25 clusters with surface carboxylic acid functionalities from a polydisperse Au cluster solution.

Spectroscopic and photophysical study of the demetallation of a zinc porphyrin and the aggregation of its free base in a tetraalkylphosphonium ionic liquid.

Dissolving zinc tetraphenylporphyrin in the tetraalkylphosphonium chloride ionic liquid P4448Cl results in progressive demetallation of the solute and quantitative production of the free base porphyrin. Aggregation of the free base occurs in which the monomer and J aggregates are in fully reversible thermal equilibrium in the ionic liquid. The thermodynamic, kinetic and spectroscopic behaviour of this system is described based on absorption, emission and excited state lifetime measurements. Both the thermodynamics of the ground state aggregation and the kinetics of the excited state relaxation processes are unusual due to the particular role played by the ionic liquid solvent.

Panchromatic enhancement of light-harvesting efficiency in dye-sensitized solar cells using thermally annealed Au@SiO2 triangular nanoprisms.

Plasmonic enhancement is an attractive method for improving the efficiency of dye-sensitized solar cells (DSSCs). Plasmonic materials with sharp features, such as triangular metal nanoparticles, show stronger plasmonic effects than their spherical analogues; however, these nanoparticles are also often thermally unstable. In this work, we investigated the thermal stability of Au@SiO2 triangular nanoprisms by annealing at different temperatures. Morphological changes were observed at temperatures greater than 250 °C, which resulted in a blue shift of the localized surface plasmon resonance (LSPR). Annealing at 450 °C led to a further blue shift; however, this resulted in better overlap of the LSPR with the absorption spectrum of black dye. By introducing 0.05% (w/w) Au@SiO2 nanoprisms into DSSCs, we were able to achieve a panchromatic enhancement of the light-harvesting efficiency. This led to a 15% increase in the power conversion efficiency from 3.9 ± 0.6% to 4.4 ± 0.4%.

Plasmonic Enhancement of Dye Sensitized Solar Cells in the Red-to-near-Infrared Region using Triangular Core-Shell Ag@SiO2 Nanoparticles.

Recently, plasmonic metal nanoparticles have been shown to be very effective in increasing the light harvesting efficiency (LHE) of dye-sensitized solar cells (DSSCs). Most commonly, spherical nanoparticles composed of silver or gold are used for this application; however, the localized surface plasmon resonances of these isotropic particles have maxima in the 400-550 nm range, limiting any plasmonic enhancements to wavelengths below 600 nm. Herein, we demonstrate that the incorporation of anisotropic, triangular silver nanoprisms in the photoanode of DSSCs can dramatically increase the LHE in the red and near-infrared regions. Core-shell Ag@SiO2 nanoprisms were synthesized and incorporated in various quantities into the titania pastes used to prepare the photoanodes. This optimization led to an overall 32 ± 17% increase in the power conversion efficiency (PCE) of cells made using 0.05% (w/w) of the Ag@SiO2 composite. Measurements of the incident photon-to-current efficiency provided further evidence that this increase is a result of improved light harvesting in the red and near-infrared regions. The effect of shell thickness on nanoparticle stability was also investigated, and it was found that thick (30 nm) silica shells provide the best protection against corrosion by the triiodide-containing electrolyte, while still enabling large improvements in PCE to be realized.

Redispersion of transition metal nanoparticle catalysts in tetraalkylphosphonium ionic liquids.

Despite the fact that particle sintering is one of the most common events leading to the deactivation of metal nanoparticle (NP) catalysts, there is a paucity of studies investigating potential routes for the regeneration of smaller, catalytically active nanoparticles from larger particles formed after repeated catalytic cycles. Here, we reveal a simple yet elegant technique for the 'redispersion' of sintered NPs in tetraalkylphosphonium halide ionic liquids (ILs). The procedure described can use environmentally benign oxidants, be carried out at mild temperatures, and is shown to be applicable to a large number of catalytically important transition metals. TEM and UV-Vis spectroscopy reveal that this methodology can indeed regenerate smaller NPs from sintered systems. A sample catalytic reaction reveals that the redispersed NPs are as catalytically active as they were prior to sintering.

Stable and recyclable Au25 clusters for the reduction of 4-nitrophenol.

Thiol-stabilized Au(25)L(18) monolayer protected clusters (MPCs) were found to be active for the reduction of 4-nitrophenol. Results suggest that these MPCs are stable catalysts and do not lose their structural integrity during the catalytic process. High stability under the reaction conditions enables the recyclability of these MPCs.

Highly stable noble-metal nanoparticles in tetraalkylphosphonium ionic liquids for in situ catalysis.

Gold and palladium nanoparticles were prepared by lithium borohydride reduction of the metal salt precursors in tetraalkylphosphonium halide ionic liquids in the absence of any organic solvents or external nanoparticle stabilizers. These colloidal suspensions remained stable and showed no nanoparticle agglomeration over many months. A combination of electrostatic interactions between the coordinatively unsaturated metal nanoparticle surface and the ionic-liquid anions, bolstered by steric protection offered by the bulky alkylated phosphonium cations, is likely to be the reason behind such stabilization. The halide anion strongly absorbs to the nanoparticle surface, leading to exceptional nanoparticle stability in halide ionic liquids; other tetraalkylphosphonium ionic liquids with non-coordinating anions, such as tosylate and hexafluorophosphate, show considerably lower affinities towards the stabilization of nanoparticles. Palladium nanoparticles stabilized in the tetraalkylphosphonium halide ionic liquid were stable, efficient, and recyclable catalysts for a variety of hydrogenation reactions at ambient pressures with sustained activity. Aerial oxidation of the metal nanoparticles occurred over time and was readily reversed by re-reduction of oxidized metal salts.